Scholarly article on topic 'Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis'

Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis Academic research paper on "Chemical engineering"

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Academic research paper on topic "Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis"



Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis

Paul Boldrin,*'^ Enrique Ruiz-Trejo,^ Joshua Mermelstein,^ José Miguel Bermudez Menendez,§ Tomas Ramirez Reina," and Nigel P. Brandon^

^Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom *The Boeing Company, 5301 Bolsa Ave., Huntington Beach, CA 92647, United States §Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom "Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom

ABSTRACT: Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels, and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review, we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies toward carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells.


1. Introduction to Solid Oxide Fuel Cells 13634

2. Scope of the Review 13634

3. Fundamentals of Carbon Poisoning 13635 3.1. Theoretical Studies on Carbon Deposition in

Catalysts and Fuel Cell Anodes 13637

4. Fundamentals of Sulfur Poisoning 13638 4.1. Theoretical Studies on Sulfur Poisoning of

Catalysts and SOFC Anodes 13639

5. Systems Approaches to Carbon and Sulfur Tolerance 13641

6. Materials Design Strategies for Carbon Tolerance

in SOFC Anodes 13642

6.1. Ni/YSZ Cermets 13642

6.2. Alloying with Noble Metals 13643

6.3. Alloying or Replacement of Nickel with Base Metals 13644

6.4. Replacement of Nickel with Nonmetal Electronic Conductors 13645

6.5. Increasing Alkalinity 13646

6.6. Use of Ceria and Other Oxygen Storage Materials 13647

6.7. Replacement of Cermets with Mixed Ionic-Electronic Conductors (MIECs) 13649

6.7.1. Single Phase MIECs 13649

6.7.2. Addition of Catalytic Metal Nanopar-

ticles to MIECs 13650

6.8. Regeneration of SOFC Anodes Deactivated

by Carbon 13651

7. Materials Design Strategies for Sulfur Tolerance

in SOFC Anodes 13652

7.1. Replacement of YSZ with Ceria 13652

7.2. All-Ceramic Anodes 13654

7.3. Alloying of Nickel with Other Metals 13656

8. Strategies from Conventional Catalysis 13659

8.1. Carbon Tolerance in Conventional Catalysis 13659

8.1.1. Sulfur Passivation 13659

8.1.2. Alloying and Bimetallic Systems 13659

8.1.3. Promoters 13660

8.1.4. Regeneration of Catalysts Deactivated

by Carbon Deposition 13661

8.2. Strategies against Sulfur Poisoning 13663

8.2.1. Noble Metal-Based Catalysts 13663

8.2.2. Alloys, Bimetallic, and Promoters 13664

8.2.3. Support and Structural Modifications 13665

8.2.4. Regeneration of Sulfur-Poisoned Catalysts 13667

9. Conclusions and Perspectives 13668

9.1. Alloying of Nickel 13668

9.2. Alkaline Promoters and Supports 13669

9.3. Ceria, Doped Ceria, and Oxygen Storage 13669

9.4. Preferential Sulfur Binding Sites 13669

Received: May 4, 2016 Published: November 9, 2016

ACS Publications ©2016 American Chemical Society

9.5. Nonmetal Electronic Conductors 13669

9.6. Infiltration of Nanoparticles 13670

9.7. Regeneration 13670

9.8. Theoretical and Computational Studies 13670

9.9. Reflections on Experimental Work 13670

Author Information 13671

Corresponding Author 13671

Notes 13671

Biographies 13671

Acknowledgments 13672

References 13672

0.13%/1000 h,2 while Bloom Energy, based in California, has a commercially available SOFC capable of generating 100-200 kW aimed at the commercial market, especially data centers, with an installed base of over 30 MW.3 Figure 1 shows a diagram of a combined cycle SOFC with integrated gas turbine.


Solid oxide fuel cells (SOFCs) are electrochemical devices for the direct conversion of fuels into electricity. They operate by the conduction of oxide ions and are capable of using a wide variety of fuels, including hydrocarbons, syngas, biogas, and ammonia, as well as hydrogen. The oxidation of fuel takes place at the anode, which needs to be active for electrochemical oxidation of the fuel species and possess both electronic and ionic conductivity. Typically anodes are made either from ceramic-metallic composites (cermets), where each component provides one aspect of the conductivity, or from a mixed ionic-electronic conductor (MIEC), a ceramic which provides both ionic and electronic conductivity.

There are a number of other properties that any materials to be used in SOFC anodes need to possess, including stability toward high temperatures and highly reducing conditions, chemical compatibility with other materials such as electrolytes and interconnect materials, and thermal expansion coefficients matched to the other components during operation and manufacture. The need for these properties places a limitation on which materials can be used. For example, there are materials with high ionic conductivity which are not stable in reducing atmospheres, or which have a large thermal expansion mismatch compared to common electrolyte materials. As well as the direct electrochemical oxidation of fuel species, other relevant reactions which take place in an SOFC anode are water-gas shift, steam reforming, dry reforming, Boudouard reaction, methanation, and hydrocarbon decomposition and cracking, among others.

The development of SOFCs has reached an important phase, with rapid technological advancement over the past decade resulting in multiple programs run by governments and/or companies testing systems greater than 100 kW, and installed commercial products in the low kW range combined heat and power market. An initial understanding of the recent progress of multi-kW-scale SOFC development can be gained by studying the U.S. Department of Energy's SOFC program (Solid State Conversion Alliance, SECA) which is interested in systems of 100 kW and upward operating on syngas from coal or natural gas. For the period of 2005-2007, the SOFC targets were for 1500 h tests on fuel cell stacks with performance degradation targets at steady state of <4%/1000 h, while the latest target is for >25 kW stacks with >4 years lifetime and degradation of <0.2%/1000 h, with cumulative operation times of between 5000 and 10000 h for this generation of SOFCs by 2020. The financial year 2015 funding round supported Fuel Cell Energy and Versa Power to produce a 400 kW system.1 Other large projects include Mitsubishi Heavy Industries demonstrating a 200 kW combined SOFC-gas turbine system operating on syngas at 900 °C, with a degradation rate of

Figure 1. Diagram of a combined cycle SOFC system with integrated gas turbine.

The focus on degradation rates clearly seen above in the large scale and commercial programs is a reflection that one of the key issues facing SOFCs is degradation and its effect on lifetime costs. Performance degradation can be caused by thermal gradients or thermal cycling, oxidation cycling, and long-term incompatibility of components. For SOFCs to continue to become successful commercially, they will need to operate on carbonaceous fuels and be tolerant to common contaminants in those fuels. Two of the most common poisons are carbon and sulfur, and current anodes based on composites of yttrium-doped zirconia (yttria-stabilised zirconia, YSZ) and nickel are not tolerant to them, resulting in long-term degradation and a need for regeneration, which has additional effects on degradation relating to thermal and/or oxidation cycling. For this reason, carbon and sulfur tolerance is a vital area of research for the next generation of SOFCs to compete with conventional power plants at the grid scale and with boilers and combustion engines at smaller scales.

The application of catalysis in fuels processing has been an important research topic for several decades. In catalytic processes involving fuels, carbon (from the fuel itself) and sulfur (present as a contaminant) critically affect the performance of the catalyst. Under certain conditions, this effect can be extremely important and the catalyst is deactivated quickly, leading to unpractical and/or costly processes. For these reasons, huge research efforts have focused on the design of catalysts resistant to carbon deposition and sulfur poisoning. As a result of this, a vast knowledge of possible alternatives to address these issues has been generated. This literature could provide insights into improving the carbon and sulfur tolerance in SOFC materials.


This review discusses all aspects of carbon and sulfur tolerance in SOFC anodes, from mechanistic and theoretical studies to strategies for materials design. In addition, we have studied the catalysis literature, focusing on fundamental studies and catalysts used in reactions under conditions similar to those

in an SOFC anode (e.g., steam reforming and partial oxidation). Since, in the end, poisoning by carbon and sulfur may be inevitable, we have also included sections on regeneration. We have chosen to put these at the end of the relevant materials design section (e.g., regeneration of SOFCs after carbon deposition is at the end of the carbon tolerant SOFC section). In the final section, we have summarized the various strategies used in catalysts and SOFCs to provide carbon and sulfur tolerance with lessons learned from each. Certain aspects of this review have been covered in other reviews in the past decade: Ni-based anodes in hydrocarbon fuels,4 sulfur poisoning of Ni-based anodes and catalysts,5,6 anode performance in hydrocarbons,7,8 catalyst deactivation and regeneration,9 steam reforming for fuel cells,10 internal reforming in fuel cells,11 and sulfar tolerance in hydrogen production catalysts.12


Deposition of carbon-containing species on metal catalysts is one of the main causes of catalyst deactivation and is virtually


inevitable in any reaction involving hydrocarbons. should be clarified that carbon and coke, although often used interchangeably, refer to different species. Carbon refers to the product of CO disproportionation, whereas coke is produced by decomposition or condensation of hydrocarbons.9,16,17 However, for the sake of clarity and readability, only the term carbon will be used in this work.

In reactions involving carbon-containing fuels, the principal reactions leading to carbon deposition can be summarized as follows:13

2CO(g) ^ C(s) + CO2(g) C„Hm(g) ^ nC(s) + m/2H2(g) CO(g) + H2(g) ^ C(s) + H2O(g)

The first reaction is the disproportionation of carbon monoxide and is commonly known as the Boudouard reaction, after its discoverer Octave Leopold Boudouard, a French chemist of the late 19th and early 20th century. It is exothermic at all temperatures but due to the reduction in entropy becomes more favorable at lower temperatures. The second reaction is the decomposition of hydrocarbons and conversely is endothermic with an increase in entropy, so it is favored at high temperatures. The final reaction is the reverse of the original "water-gas reaction" used to produce "water gas" (now known as syngas) from coke using steam. It is distinct from the water-gas shift reaction, which was originally used to reduce (or shift) the carbon monoxide content of the water gas, so that it could be more safely used. It has similar thermodynamics to the Boudouard reaction and so is more favored at lower temperatures.

Carbon deposition is strongly affected by the presence of sulfur and aromatic compounds in the fuel. , Sulfur deactivation can either promote or reduce carbon deposition depending on the conditions,19 and the ability of sulfur to potentially improve carbon tolerance is discussed later on in section 8.1.1. The presence of aromatics in the fuel tends to increase carbon deposition far more than would be expected from their concentration in the fuel. This is likely because carbon deposition is thought to proceed through a mechanism involving the formation of aromatics. Once formed, these aromatics are less reactive than other compounds in the fuel

and serve as nucleation sites for the formation of polynuclear carbon compounds.9,13 The mechanism of carbon formation varies with material (e.g., if it is a metal or metal oxide/ sulfide).9,16 This is important because the effect of the structure and location of carbon on deactivation can be more relevant than the total quantity of carbon deposited on the catalyst. , In the case of metals, the rate of carbon deposition is a function of the type of metal, the crystal size, the promoters, and the interaction between the metal and the support.9,21-29

Formation of solid carbon is favored thermodynamically in a large proportion of the potential operating space of SOFCs.30 Figure 2 shows the region in which carbon deposition is

Figure 2. Carbon deposition limit lines in the C—H—O phase diagram. Reproduced with permission from ref 30. Copyright 2003 The Electrochemical Society.

favored at different temperatures, showing that all common carbon containing fuels are in the carbon deposition region below 1000 °C, including CO and CH3OH. This indicates that oxygen-containing species need to be added to make carbon deposition thermodynamically unfavorable. Factors which increase the thermodynamic favorability of carbon deposition include lower temperatures, higher carbon:oxygen ratios, and low oxygen fluxes. In addition to this, carbon deposition is strongly influenced both inside and outside this thermodynamic window by kinetic factors, especially the relative rates of the forward and reverse Boudouard and methane decomposition reactions, and the presence of aromatic and polyaromatic compounds.

When carbon deposition takes places on metal particles, several situations can lead to deactivation (Figure 3):9 (1) Strong chemisorption as a monolayer or physical adsorption in multilayers blocking access to metal surface sites. (2) Encapsulation of metal particles, deactivating them completely. (3) Plugging of micro- and mesopores blocking access to the active sites inside them. (4) Growth of carbon filaments (whisker carbon) that can stress and fracture the support or push the metal particles off the support. In the case of SOFC anodes, the growth of this carbon can destroy the structure of the fuel cell. (5) Dissolution of carbon atoms into the metal, causing a volume expansion. This is mainly a problem for SOFC anodes, where the metal may have a structural role and therefore these volume changes can destroy the structure of the anode.

Figure 3. Different situations in which carbon deposits can lead to deactivation: (a) carbon layers chemisorbed on metal particles. Reprinted with permission from ref 123. Copyright 2010 Elsevier. (b) Encapsulation of metal particles by carbon deposits. Reprinted from ref 333. Copyright 2006 American Chemical Society. (c) Growth of carbon nanofilaments that push metal particles off the support. Reprinted with permission from ref 401. Copyright 2014 Elsevier. (d) Pore blockage by carbon deposits. Reprinted from with permission from ref 358. Copyright 2010 Elsevier.

By blocking active sites for catalytic and electrocatalytic reactions, carbon can reduce the performance of both catalysts and SOFCs. This type of deactivation can occur even at low levels of carbon deposition but is generally fully reversible by oxidation of the carbon. Techniques for achieving this are discussed in sections 6.8 (for SOFC anodes) and 8.1.4 (for catalysts). Structural deactivation, where carbon deposition causes structural failure, tends to be the most serious problem caused by carbon poisoning in SOFCs. This mode of deactivation is caused by longer term running under conditions favorable to carbon deposition or when using materials such as nickel which catalyze carbon deposition. In SOFCs, because the metal component can have some structural role, failure can also occur by dissolution of carbon into components of the anode, causing a volume expansion which can result in "dusting", where the anode becomes pulverized. This tends to occur when carbon is repeatedly dissolved and removed from the anode materials.

Different types of carbon can be formed in these reactions.9,13 These types of carbon have different reactivities and morphologies, which affect their potential for deactivation. In addition, they can react to be transformed in a different type of carbon, thus varying during the reaction their potential to deactivate the catalyst. ' '

In the case of metal oxides and sulfides, the formation of carbon is the result of cracking reactions catalyzed by acid sites. The rate of carbon deposition depends on the acidity of the catalyst and its porous structure. In this case, deactivation can be caused by chemical or physical effects. In the case of chemical deactivation, carbon can strongly adsorb on the acidic sites while physical deactivation is the result of the pore plugging which blocks access to some catalytic sites.

An ideal carbon tolerant cell would be able to run on hydrocarbons without any added oxidant and would therefore not require high temperatures, steam generators, or other extra modules which are currently used to mitigate carbon poisoning in SOFC-based power generators.

Generally, there are two ways for suppressing (or at least minimizing) the rate of carbon deposition: changing process conditions, such as increasing steam to carbon ratio or increasing temperature, or developing carbon-resistant materials. ' ' ' ' The rate of deactivation is related to the balance between the rates of formation and gasification/oxidation of the carbon, which are strongly influenced by the reaction conditions and the catalytic activity of the materials toward the different reactions involved.9'13'15'31

In catalysis, the range of variation of the reaction conditions is often quite limited, since the conditions need to be designed to optimize the yield of the desired product rather than protect the catalyst. In SOFCs, there is more scope to alter reaction conditions, with compromises made to cost, power, and flexibility. Since Ni is such an effective catalyst for hydrocarbon decomposition, use of reforming to convert hydrocarbons into syngas can be effective. This reforming can be done internally or externally. External reforming requires the extra cost of a separate reforming unit but has the advantage that the reformer has a protective effect on the fuel cell. Internal reforming with steam, CO2, or O2 can be effective due to the high activity of Ni for reforming reactions, but certain conditions such as periods at open circuit voltage (OCV) and low oxygen:carbon ratios can result in carbon deposition.33,34 Alternatively, separate reforming layers have been investigated, but these could complicate fabrication and would need to be composed of a carbon-tolerant catalyst.35-38 Both internal and external reforming have problems with the separation of endothermic reforming and exothermic oxidation reactions; in external reforming there is a need for heat exchangers, while in internal reforming the proximity of exothermic and endothermic reactions causes thermal gradients. Both types of reforming reduce the power output of the cell.

Because of how SOFCs operate, increasing the current density also mitigates against carbon deposition (at least in sulfur-free fuels), due to the increased flow of oxygen into the anode side of the cell. This has the advantage of encouraging reforming reactions without reducing the power output of the cell. Because of the protective effect of oxygen flow across the electrolyte, SOFCs can be started up under hydrogen with the carbon-containing fuel being switched on once the cell is already under load, if it is feasible to have a dedicated hydrogen supply for this purpose.

Running the fuel cell at high temperature can move the conditions outside the region where carbon deposition is thermodynamically favored, although this does not guarantee there will be no carbon deposition. The higher temperatures increase the cost of components other than the anode, which need to be designed to withstand higher temperatures, for example above 800 °C, the most suitable alloys for

interconnectedness have high levels of chromium, which can cause problems with formation of resistive phases39 and cathode degradation.40 The higher temperatures may also reduce the overall lifetime of the system. Alternatively, with Ni-YSZ anodes, it has been shown that decreasing the temperature reduces carbon deposition in a cell operating under load in humidified methane as it slows the methane cracking reaction more than the electrocatalytic oxidation, although high currents and thus high oxygen fluxes into the anode were still required to eliminate carbon deposition entirely.33 3.1. Theoretical Studies on Carbon Deposition in Catalysts and Fuel Cell Anodes

The formation of carbon deposits in catalytic reactions involving hydrocarbons is the consequence of the dehydrogen-ation of these hydrocarbons. Methane is the simplest hydrocarbon and therefore provides the simplest model for understanding the fundamentals of carbon deposition. The dissociation of CH4 over metal surfaces occurs in four

4. 41,42

steps: CH4(g)a

~ *CH3

*CH3a ^ *CH2a + *Ha

*CH2a ~ *CHa + *Ha

*Ca + *Ha

Considering Ni as the active metal surface, the dissociation of methane can take place on two different kinds of active sites: those associated with the planar surfaces (or terraces) and those associated with stepped and defect sites on the metal surfaces.41,42 Considering the planar sites, theoretical studies have shown results that can be surprising at a first view, as can be seen in Figure 4.41 The most stable intermediate in planar

Figure 4. Thermodynamic pathway for the dissociation of methane (CH4) on planar (111) and stepped (211) Ni surfaces. Reprinted with permission from 41. Copyright 2007 Elsevier.

surfaces is *CH and the last step of methane dissociation from

*CH to produce carbon is an endothermic process with high activation energy (Table 1).41-43 These data suggest that carbon deposition should not take place on those Ni surfaces, something that contradicts what has been widely reported experimentally. However, observing the results from the stepped sites, the phenomenon of carbon deposition is easily explained. Stepped surfaces are more reactive than planar, due to electronic and geometrical defects that take place in these low-coordinated surface geometries.41,42,44-46 As a conse-

quence of this, the production of carbon on stepped surfaces is exothermic and thermodynamically favored, creating the driving force for the formation of graphitic carbon deposits.42 A similar situation occurs in the case of other metals and alloys, as can be seen in Table 1. In all the cases, the formation of carbon is thermodynamically more feasible on stepped than planar surfaces.

The process starts with the activation of the first C-H bond in methane. As can be seen in Figure 5, in both cases (planar and stepped surfaces), this takes place over the top of a surface Ni atom. However, in the case of planar surfaces, the energy barrier is higher than in the case of stepped (Table 1). This is due to the higher stability of the adsorbed CH3 on the stepped surface, which gives rise to a stronger bond.44 Similarly, the subsequent steps of the dissociation of methane give rise to species that are more stable on stepped than on planar surfaces. Finally, whereas CH is the most stable species in planar surfaces, C is the most stable species on the stepped, favoring its deposition in these sites.41

Once carbon has been deposited in the metallic sites, two different processes that lead to carbon deposits formation can take place. Either C-C bonds can be formed and then graphitic planes grow parallel to the planar surfaces of the Ni. Graphene is more stable on planar surfaces because carbon atoms are organized in hexagonal structures that can lie parallel to the Ni atoms.41,42,45-48 Alternatively, those isolated atom carbons, once adsorbed, can dissolve into the bulk Ni forming carbides (Figure 6). As a result of this diffusion, carbon atoms can reach facets on the support side of the metal particle. These facets are suitable for the eventual growth of carbon nanotubes.42,47,49

However, as stated by Abild-Petersen et al.,44 the fact that stepped surfaces are more active than planar ones does not mean that steps control the activity of the catalyst. So, if the steps could be blocked, side reactions like carbon deposition would be eliminated, while only moderately reducing the activity of the catalysts for methane processing. This can explain the effect that the addition of Au to Ni catalysts has on coke deposition. Au preferentially binds to low-coordinated Ni sites (like those present on steps). Consequently, it increases the effective coordination number of adjacent Ni atoms and lowers the Ni surface energy due to electronic interaction with


Another strategy for decreasing carbon deposition is to increase the reaction rate of C-O bond formation relative to the C-C bond formation. C atoms can be removed from the surface of the catalyst by oxidation to form CO and CO2.49,50 Thus, if carbon diffusion and C-C formation rates are decreased and oxidation rate increased, carbon deposition can be avoided.49

Following these ideas, the use of different promoters,41,51 partial passivation,41,45 and alloys41,43,49,50,52 have been proposed. Table 1 shows that in all cases alloys have higher thermodynamic barriers to carbon deposition than the metals which make them up, meaning they are obvious targets. Two clear examples of this can be the effect of alloying Ni with Rh or Sn.49,50 The studies by Guo et al.49 and Nikolla et al.50 showed that when Ni is alloyed with Rh or Sn both carbon and oxygen diffusion in the metal lattice and the C-C and C-O bond formation are hindered, but to a different extent, as shown in Table 2 and Figure 7. Consequently, the overall carbon deposition rate was diminished. These studies have supported their theoretical studies with experimental findings that point in the same direction as the DFT results.

Table 1. Activation Barriers (Ea) and Reaction Energies (AE) of the Different Steps in the Dissociation of Methane and Adsorption Energies (Eadsorption) of the Different Species Involved in the Process Reported on Different Metal Surfaces

CH4 ~ *CH3 + *H *CH3 ~ *CH2 + *H *CH2 ~ *CH + *H *CH ~ *C + *H

Ea ae ea ae ea ae ea ae

Ni (1 1 1)38 100a 54a 75« 17« 29« —29«c 130« 63«

Ni (1 1 1)40 113.9 12.5 74.3 9.6 35.7 —25.1c 131.2 53.1

Ni (2 1 1)38b 84a 42« 88« 8« 42« —33«c 88« —29«c

Cu (1 1 1)38 188 96 138 92 113 46 205 130

Cu (1 1 1)40 181.4 83.0 141.8 67.5 101.3 30.9 213.2 115.8

Cu (2 1 1)38b 138 33 134 79 184 13 176 75

Fe (1 1 1)40 98.4 —60.8c 56.0 —30.9c 12.5 —66.6c 100.3 3.9

Co (1 1 1)40 110.0 4.8 66.6 10.6 30.9 —21.2c 120.6 54.0

Cu—Ni (1 1 1)40 105.2 6.8 63.7 4.8 34.7 —34.7c 132.2 49.2

Cu—Ni (2 1 1)38b n.a. 29 n.a. 63 n.a. —54c n.a. 21

Fe—Ni (1 1 1)40 120.6 —29.9c 68.5 — 19.3c 30.9 —56.0c 111.9 4.8

Co—Ni (1 1 1)40 124.5 4.8 73.3 20.3 32.8 — 14.5c 121.6 61.8

Rh (1 1 1)44 332 47.3 220 —42.7c 26.4 —234c 452 224

Rh(1 1 0)44b 282 —54.8c 127 —46.9c 465 —51.0c 207 —72.0c

Rh(1 0 0)44b 261 —53.6c 136 —39.7c 14.2 —250c 284 —93.3c

approximate values extracted from Figure 2 in ref 41. bStepped surface rows. All values are in kJ/mol. cExothermic steps. All values are in kJ/mol.

Figure 5. From left to right, initial, transition, and final state for the dissociation of methane on: (a) Ni (1 1 1) and (b) Ni(2 1 1). C, H, and Ni atoms are represented by dark gray, black, and white colors, respectively. Reprinted with permission from ref 44. Copyright 2005 Elsevier.

Figure 6. (a) Transition and (b) product states of the diffusion of one C atom from an fcc hollow site to a sublayer octahedral site of the Ni(111) surface. Green (light) spheres represent Ni atoms and black (dark) spheres represent C atoms. Reprinted from ref 49. Copyright 2012 American Chemical Society.

Table 2. Activation Barriers (Ea) and Reaction Energies (AE) of C—C bond and C—O Formation over Different Surfaces of Ni(1 1 1) and Ni—Rh (1 1 1)a

C—C formation

C—O formation

Ni(l 1 1) Ni2Rh1(l 1 1) Rh(l 1 1)

Ea —0.90 —0.69 0.19

ae 0.63 0.75 1.34

-1.75 1.66 1.43

ae 1.18 1.16 1.37

«Al energies are shown in eV.

DFT studies on ceria and doped ceria show that carbon deposition should be extremely unfavorable on a ceria surface as long as there are oxygen ions available to react with the carbon atom, which will desorb as CO or CO2. The presence of Ni does not affect the favorability of this process, indicating that the activity of the ceria in cermets should be similar to the activity of pure ceria.56 DFT studies on Ce2O3 show that surface vacancy formation is as energetically unfavorable as on YSZ, indicating a low activity toward oxidation reactions. Combined with experimental measurements showing that the ceria surface was more active in a more reduced state, this indicates that Ce2O3 is not formed at the surface.57 In fact, DFT modeling shows that it is energetically favorable for CeO2 to have two oxygen vacancies, providing the explanation for these results and the high oxidation activity of ceria.

A study on BaCeO3 perovskites found that CeO2-terminated surfaces had much stronger interactions with CH4 than BaO-terminated surfaces, although they did not link this to carbon deposition but to methane oxidation.58 Unfortunately, the thermodynamically favored termination under SOFC anode conditions is BaO, meaning that BaCeO3 should be inactive for methane oxidation.

While the metal particles are regarded as the main sites for carbon deposition, this is also possible on oxide surfaces; for example, both CO and CH4 will form carbon on Y2O3, YSZ, and ZrO2, with the amount of carbon decreasing in that order,53,54 so clearly there is a mechanism for carbon deposition on oxides which is controlled by the surface chemistry.


In addition to the tolerance to carbon, tolerance to sulfur is required to make an SOFC or a catalyst flexible to fuels. The interaction of sulfur with anodes in SOFC has been reviewed in the past few years.5,59,60 Sulfur, contained in all fuels originated from natural sources (fossil or biogas), can be minimized but

Figure 7. (a) DFT-calculated potential energy surfaces for C-C bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the attachment of a C atom to a carbon nucleation center (modeled as a chain of carbon atoms) on the two surfaces shown in the inset. (b) C-O bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the two surfaces shown in the inset. Ni is depicted as large blue (light) atom, Sn as a large purple atom, and carbon chain as a chain of small black atoms. Reprinted with permission from ref 50. Copyright 2008 Elsevier.

will always be present in a wide range of concentrations, for example from 15 to 5000 ppmw for diesel.61 If degradation is unavoidable, at least a certain degree of regeneration must exist in order to guarantee long-term operation.

Several studies have addressed the influence of sulfur poisoni-ng mainly on Ni/YSZ anodes operating on H2/

H2O.62-68 In recent years, the interest has grown to include carbon fuels and H2S, again mainly on Ni/YSZ.69-75 The reactivity of sulfur is related to the number of electron pairs available for bonding, therefore, from the chemical point of view, toxicity decreases in the order of H2S, SO2, and SO42-.76 Other compounds of sulfur may exist in the fuels, but it is expected that in the majority of conditions occurring in an SOFC anode, all sulfur compounds are transformed into H2S.77 Following the notion of chemical reactivity and electrons available for bonding, a non-noble metal with electrons available for bonding will be more affected by sulfur than a


In terms of SOFCs, H2S itself is a fuel that can be oxidized electrochemically and the obvious choice to oxidize the sulfur is the oxygen ion that is being transported through the electrolyte. This happens in the same way that hydrogen is oxidized, but it should be noted that three times more electrons are being used per mole in the electrochemical oxidation of H2S.

H2S + 3O2

H2 + O2

H2O + SO2 + 6e-

H2O + 2e-

Examples of the use of SOFCs with H2S as a fuel have been given in the literature but are rather limited.79-86 H2S can and has been used as a fuel, and it has been shown that SO2 is the product of utilization in a fuel cell either with Pt as the catalyst85,87 or a highly conductive and catalytically active thiospinel.86 It is not clear, however, if these thiospinels could operate in hydrogen-rich fuels, are ionically conductive or are resistant to redox cycling. To facilitate this electrochemical reaction, the supply of electrons and oxygen ions must be a continuous process, and therefore, as in the case of hydrogen oxidation in a classic Ni/YSZ anode, the triple phase boundary (TPB) (i.e., the interface between Ni, YSZ, and gas phase) is critical to the performance. Anything that hinders or slows down this supply of oxygen and electrons to the TPB will have a detrimental effect; examples of hindrance are carbon deposition or agglomeration of the Ni phase. Similarly, anything that blocks the reaction sites for hydrogen oxidation

or internal reforming will be equally detrimental. In the case of nickel anodes, sulfur poisoning is one of the reasons for the decreased electrochemical activity. In what follows, the scope is more concentrated on the presence of H2S in the fuels as pollutant rather than as fuel.

4.1. Theoretical Studies on Sulfur Poisoning of Catalysts and SOFC Anodes

Considering H2S as the source of sulfur, the depletion of the anode performance under H2S containing gas mixtures at elevated temperatures originates from H2S dissociation leading to the adsorption of atomic sulfur (S*) on the anode surface (i.e., adsorbed on Ni atoms when a model anode Ni/YSZ is considered).8^89 The strongly adsorbed S* species block the active sites of the anode surface, decreasing the electrochemical oxidation performance. Experimental studies have shown that sulfur coverage fits a Temkin isotherm on nickel surfaces in catalysts, where the enthalpy of adsorption of sulfur varies linearly with coverage.90 In solid oxide fuel cell anodes, performance degradation is proportional to sulfur coverage at constant current density.91 Figure 8 shows the coverage of sulfur on Ni and phase equilibria, highlighting that sulfur coverage is high even at low sulfur concentrations, with a very

Figure 8. Chemisorption equilibria plotted in the chemical potential diagram for the Ni-S-H system, log[p(H2S)/p(H2)] vs 1/T plot. Dotted and dashed lines for 0s = 0.6 and 0.8, respectively, are isocoverage lines calculated from the equation given in literature.91 Reproduced with permission from ref 92. Copyright 2010 The Electrochemical Society.

(a) (b) (c)

Figure 9. (a) Schematic representation of a slab model with a proper vacuum space for periodic DFT calculations. (b) Four active sites on a (111) plane. (c) Schematic energy profile of gas-phase H2S dissociation on Ni (111) forming atomic S* and H*. "*" denotes surface species. TS1 and TS2 are the transition states. Extracted with permission from ref 105. Copyright 2011 The Royal Society of Chemistry.

Table 3. Activation Barriers (Ea) and Reaction Energies (AE) for the Elementary Steps in a H2S Dissociative Adsorption Process and Adsorption Energies (Eads) of Sulfur Species (S*, HS*, and H2S*)a

metal e 6 Ea1 a e1b ea2C AE2C eadsS* eadsHS* eads^S*

Pt(111)93 0.02 -0.90 0.04 -1.19 5.14 3.00 0.90

Pd(l1l)94 0.37 -1.25 0.04 -0.73 5.15 3.02 0.71

Rh(21l)95 0.01 -1.50 0.32 -1.50 6.0 3.69 1.00

Ni(l00)88 0.29 -1.56 0.45 -1.05 5.96 3.72 0.83

Ni(l1l)88 0.15 -0.98 0.11 -0.86 5.14 2.95 0.67

aAll values in eV. bEa1 and AEl correspond to H2S* ^ HS* + H*. cEa2 and AE2 correspond to HS* ^ H* + S*.

№ ni cu/% cu

Figure 10. (a) Supercell models of homogeneous Ni1-xCux as a function of the alloy composition. Ni and Cu are in gray and in brown, respectively. (B) Comparison of the predicted adsorption energies of atomic sulfur on Ni1-xCux(1 1 1) at PAW-GGA-DFT (•) and GGA-DFT (O). Adapted with permission from ref 96. Copyright 2007 Elsevier.

strong dependence on temperature. Bulk sulfidation of Ni does not occur until much higher sulfur concentrations.92

DFT calculations clearly illustrate the situation.60 Figure 9 pictures a Ni-based anode built as infinite slabs with an adequate vacuum space (around 15 A). Under these circumstances, four types of active sites can be imagined, including atop, bridge, and 3-fold fcc- and hcp-hollow sites. As schematically illustrated in Figure 9c, the mechanism of S* formation could be described as an interfacial reaction of adsorbed H2S* with the Ni surface via two elementary steps of S-H bond cleavages (i.e., H2S* /HS* + H* and HS*/H* + S*).

The associated energy barriers for the subsequent steps of dissociation and adsorption of H2S are presented in Table 3. For sake of comparison, Table 3 includes analogue calculations for several noble metals. The calculated energies evidence that sulfur adsorption is clearly a favorable process on Ni surfaces

with large exothermic reaction energies (AE) and low activation energies Ea. Furthermore, these computational results suggest that replacing Ni with noble metals is not a viable solution to mitigate sulfur poisoning since energy-wise H2S dissociation and S* adsorption also take place on noble metal surfaces.60 The adsorption energies summarized in the table also show that H2S* and HS* bind to metallic surfaces weaker than S*. Hence in principle, greater sulfur resistance could be achieved by avoiding H2S dissociation on the anode surface, although the latter is difficult to achieve given that a stronger S* adsorption energy involves a redistribution of the electronic density that reduces the energy demand for H-S bond breaking.

Alternatively to noble metals, alloying Ni with base metals such as Cu may result in an improved sulfur tolerance.96 Indeed, DFT calculations evidenced that Cu-based anode materials display better tolerance to carbon deposition and

sulfur than Ni-based anodes. Figure 10 shows the evolution of sulfur adsorption energies with the Ni—Cu alloy composition. It seems very clear that the alloying approach increases Ni resistance to sulfur poisoning but the bimetallic system never reaches lower sulfur adsorption energy than monometallic Cu.

The smaller adsorption energy exhibited by the Ni-Cu alloy can be explained in terms of the density of states (DOS) analysis, as detailed in Norskov's d-band theory.98,99 As shown in Figure 11, the antibonding orbitals in Ni—S are higher

under dry hydrogen).1

However, further DFT calcula-

Figure 11. (a) DOS analysis of S* on Ni(111) and Cu(111) in red and blue curves, respectively. A circle represents the antibonding states around the Fermi level. (b) A scheme of the energies of bonding and antibonding states corresponded to those of metal d bands. Adapted with permission from ref 60. Copyright 2011 The Royal Society of Chemistry.

energy than the Cu-S antibonding orbitals, meaning it is easier to excite electrons into the Cu-S antibonding orbital to break the Cu-S bond. This favorable situation has motivated a number of studies focusing on Cu, , , Ni-Cu alloys, , and other alloys such as Ni-Sn,102,103 targeting weaker sulfur interaction with the fuel cell anode. Nevertheless, the main problem of these alternative materials is their poor catalytic activity for the hydrogen oxidation reaction (HOR). Simultaneously improving activity for the HOR and reducing poisoning by H2S is difficult as both are related to the affinity of the material toward hydrogen-containing species.104 In other words, the alloys can effectively enhance the tolerance toward sulfur poisoning but cell performance is sacrificed in turn.

In accordance with thermodynamics, sulfur poisoning of traditional Ni-based anodes is largely unavoidable under a wide range of conditions at very low concentrations of H2S (e.g., below 0.1 ppm of H2S at 800 °C and below 10 ppm at 1000 °C

tions have demonstrated that Ni anodes could be regenerated through a two-step treatment: (1) addition of H2 to reduce sulfur coverage and (2) oxidation with oxygen realizing S as SO2.

Galea et al. described the sulfur removal pathways via oxidation.106 They described a two-step mechanism. In the first step, sulfur concentration is reduced from 0.5 to 0.25 monolayers and in the final stage surface cleaning from 0.25 monolayers of sulfur to complete sulfur removal is achieved (Figure 12).

Although this oxidative treatment is effective, it has an associated drawback which is a high likelihood for Ni oxidation. Therefore, ideally this approach could be improved if the oxygen is supplied by oxygen ion flux through the electrolyte and interacts selectively with sulfur. In response to this problem, YSZ could be total or partially substituted by other ceramic phases with higher oxygen conductivity as CGO showing greater sulfur tolerance.60 The presence of a highly oxygen conductive phase in the anode permits a certain degree of electrochemical oxidation of S* to SO2 facilitating sulfur removal. This strategy of using mixed oxides with high oxygen mobility seems to mitigate (but not fully eliminate) sulfur poisoning in both SOFCs and catalytic processes.


As discussed above, carbon and sulfur represent a technical challenge for SOFC technology. Although this review focuses on anode materials design strategies, currently the main method for mitigating carbon and sulfur poisoning is processing of the fuel externally to the SOFC stack, so it is worthwhile briefly reviewing these aspects of SOFC-based power generation systems which are intended to achieve carbon and sulfur tolerance. Haldor Tops0e have been involved in gas cleaning for SOFCs for many years, and John B0gild Hansen has very helpfully reviewed the company's experience in this

The main strategy used in relatively clean fuel (e.g., consumer grade natural gas, LPG, etc.) is fuel reforming. This converts most of the hydrocarbons to H2 and CO. The reformer can be provided with oxidizing gas as fresh steam or as recycled anode gas. CO2 can also be used in so-called dry reforming. The use of reformers has been demonstrated

Figure 12. Energy profiles of the regeneration process via sulfur oxidation. (A) Gibbs free energy (AG at 800 oC, black line) and enthalpy (AH, red line) profile illustrating relative thermodynamic energy and kinetic pathways of O2 adsorption and SO2 desorption on S-Ni(111) surface with initial coverage 0s = 0.50 ML B) Gibbs free energy (AG at 800 oC, black line) and enthalpy (AH, blue line) profile illustrating relative thermodynamic energy and kinetic pathway of O2 adsorption and SO2 desorption on S-Ni(111) surface with initial coverage 0s = 0.25 ML. Adapted with permission from ref 106. Copyright 2009 Elsevier.

practically in a number of systems. Reformers can add significantly to the cost of the system, with the cost reported as being similar to the fuel cell module itself.108

For dirtier fuels, such as gasified biomass or coal, or biogas, fuel processing becomes more complicated and hence expensive.109 The feedstock may contain up to several percent of sulfur compounds as well as other contaminants such as alkali metals, halides, and phosphorus compounds. For solid fuels, the gasification process which converts the feedstock into a gaseous form suitable for fuel cells can produce significant amounts of aromatic compounds, including smaller molecules such as toluene and larger polyaromatic compounds which can cause carbon deposition in SOFCs. All of the feedstocks mentioned above contain methane and/or short chain hydrocarbons, which again can cause carbon deposition. These feedstocks need several layers of treatment, from desulfurisation to particulate filtering, although most of this is not exclusive to SOFCs, so may not impact the economics of the process compared to competing technologies.

For SOFC-based systems using these fuels, the level of desulfurisation required is crucial to the cost and complexity of the system. For example, to reach levels below 10 ppm, deep desulfurisation is needed, which is normally carried out at 40 bar of pressure or higher,110,m necessitating gas compression and increasing safety issues. More recent work has reduced sulfur below 1 ppm at 10 bar, but this pressure is still too high.112

A final intermediate case is provided by liquid transport fuels, which may be an important market for SOFCs in the future. These have largely been cleaned of contaminants such as tars and metallic impurities but may still contain varying levels of sulfur. In general, the level of sulfur in these fuels is being driven downward due to legislation. Ultralow sulfur diesel (ULSD) standards are normally around 10—50 ppm, although the actual content of sulfur may be as low as 2—3 ppm. For aviation fuels, the sulfur levels are up to 3000 ppm, with an average of around 600 ppm.

As discussed above, some of these levels of sulfur may be too high for Ni/YSZ anodes, although there are examples of SOFC stacks being run on reformed ULSD without desulfurisation. A Tops0e SOFC stack was run on steam reformed ULSD (<10 ppm of S) for 1200 h.113 After an initial 150 h period of rapid degradation, there was only 0.2%/1000 h voltage degradation over the rest of the test. Delphi tested a 5-cell stack with simulated reformate containing 2.5 ppm sulfur and also found a rapid initial degradation followed by stable performance,114 indicating that SOFCs may be able to operate stably with ULSD reformate without desulfurisation, albeit with a performance drop caused by sulfur poisoning. If SOFCs can tolerate ULSD-levels of sulfur, they should be economically attractive for truck auxiliary power units (APUs),115 and higher sulfur tolerance would allow them to be used in aircraft APUs.

Crucially, both the examples above had no hydrocarbons in the reformate. Even low levels of compounds such as ethylene are capable of causing carbon deposition on Ni, even in thermodynamic regimes which do not encourage carbon formation.116 In the first example above, a secondary reformer was used to remove the low levels of hydrocarbons produced by the first reformer,117 while the second example, in common with most studies, used a simulated reformate without these problematic molecules.

From this summary, several points relating to fuel processing become clear. The first is that since producing a suitable feed

gas for an SOFC from almost any starting material is technically feasible, then the driver for carbon and sulfur tolerance in the anode itself is almost entirely economic. The costs of reforming and desulfurisation are each of a similar order of magnitude to the cost of the fuel cell itself and become more important for lower power and/or more portable systems. That being the case, it becomes clear that the key targets for carbon and sulfur tolerance in SOFC anodes are related to either eliminating or reducing the specifications for the reforming and desulfuriza-tion units. So for carbon tolerance, some important targets could be to be able to operate directly on methane, propane, ethanol, or biogas (methane-carbon dioxide mixtures), preferably without steam generation or off-gas recycling or to be able to tolerate low levels of species such as ethene and tars. Meanwhile, for sulfur tolerance, important targets are tolerance to the low sulfur levels in ULSD or natural gas (<10 ppm); for fuels with higher levels of sulfur, tolerance to the levels of sulfur after hydrodesulfurisation catalysis (i.e., without deep desulfur-isation at high pressure, or ZnO or other sorbents); and then finally tolerance to the levels in those fuels themselves.


6.1. Ni/YSZ Cermets

Cermet-based anodes are the most widely used anodes in SOFCs. Traditionally they have the advantage that the best oxide ion conductors can be used, while the metal can provide the catalytic activity and the electronic conductivity. The industry standard material is yttria-stabilized zirconia (YSZ), so-called because the addition of yttrium ions to the zirconia stabilizes the cubic form of the material under a wide range of temperatures. The presence of the Y3+ ions also creates oxygen vacancies, which allows oxygen ion transport, with the maximum conductivity being with 8 mol % of yttria added [known as 8YSZ, (ZrO2)0.92(Y2O3)0.08]. YSZ is very stable toward high temperatures and a wide range of oxidizing conditions. It is also the most widely used electrolyte, having extremely low electronic conductivity, meaning that issues of compatibility between the anode and electrolyte are eliminated by using YSZ in both the anode and electrolyte. The industry standard metal is nickel. Nickel is relatively cheap and highly active toward various reactions involving carbon, as well as being active toward electrochemical oxidation. It is also more stable than other base metals toward high temperatures and unreactive toward common electrolytes such as YSZ.

Both components of Ni/YSZ have problems relating to carbon and sulfur tolerance. Nickel easily dissolves both carbon and sulfur, leading to volume expansions which can cause structural failure of the anode. Nickel is also an extremely good catalyst for solid carbon formation, meaning that carbon filaments can be formed, potentially destroying the structure of the anode and blocking gas diffusion pathways as discussed in section 3. As well as causing failure of the cell, this propensity toward carbon formation also renders nickel a poor catalyst for direct oxidation of hydrocarbons, meaning that high quantities of steam need to be used for cells running on methane or higher hydrocarbons. It also makes nickel susceptible to poisoning by aromatic or polyaromatic compounds which may be present in gasified coal or biomass.118— 24

The problems of YSZ relate to its inertness and consequent inability to mitigate any of the failings of nickel. It has little activity toward electrochemical oxidation or any of the other

Figure 13. Schematics of the most common materials strategies to improve carbon tolerance. The diagram shows a strategy and does not imply a specific mechanism.

important catalytic reactions and possesses extremely low electronic conductivity, meaning that once the nickel has deactivated the cell is useless. It also has no oxygen storage capacity and no ability to absorb sulfur, either of which could help improve carbon or sulfur tolerance. Strategies to mitigate the issues with Ni and YSZ are described schematically in Figure 13.

As shown above, the propensity of Ni/YSZ anodes toward carbon deposition is largely a function of nickel's ability to catalyze carbon formation. Thus, it is natural to look at partially or entirely replacing the nickel. Since nickel is an exceptional electrocatalyst, many efforts to replace this have focused on substituting some other potentially active material for some of the nickel rather than replacing the nickel entirely. The rationale behind this is 2-fold: first, heteroatoms could break up large continuous nickel surfaces which are predisposed toward carbon deposition, and second, to enhance the rates of reactions which compete with carbon deposition, such as carbon oxidation and steam reforming.

6.2. Alloying with Noble Metals

The so-called noble metals (roughly the second and third row transition metals in groups 8-11) may offer enhanced catalysis as well as reducing carbon deposition and are known from conventional catalysis to be active in very small quantities. The earliest example was gold, which causes a reduction in carbon deposition under oxygen-methane mixtures, at the expense of methane reforming activity. Carbon deposition was reduced by up to 8000 times with one-fifth of the nickel replaced with gold.125'126 SOFCs using Au doping have been tested in dry and humidified methane atmospheres, where they showed no carbon deposition after 200 h. A stabilization of CHx surface species leading to a reduction in the rate of graphite formation was found to be responsible.128

Impregnation of Pd into Ni/YSZ anodes showed a marked decrease in polarization resistance in hydrogen, methane, and ethanol, with suppression but not elimination of carbon deposition under the carbonaceous fuels.129 The same was found for impregnation of Pd into Ni on Ce0.9Gd01O195 (CGO).130 Carbon deposition was primarily found to occur in Pd-poor regions. Likewise impregnation of Ru, also into Ni/ CGO anodes, was found to improve stability under methane, ethane, and propane under load and short periods at open circuit voltage (OCV), with the caveat that a 25-40 ^m CGO electrolyte was used, CGO possesses a relatively high electronic conduction under reducing conditions, meaning that there would be a significant oxygen flux even at OCV. Carbon deposition was not seen, as measured from carbon balance analysis. This study also noted one of the problems with the use

of Ru, which is its tendency toward vaporization during synthesis.

A comparative study looking at Ru, Pt, Pd, and Rh on Ni/ YSZ found that Ru, Pt, and Pd suppressed carbon deposition under dry methane compared to the unpromoted material, while Rh actually increased carbon deposition.132 In addition, Ru and Pt improved the power density in fuel cell tests. Rh has however been shown in other tests to reduce carbon deposition on Ni/CGO in microreactors and give more stable performance in button cells in humidified butane, although the butane used in this paper contained sulfur compounds, so the improved performance may be due to improved sulfur tolerance.133 A further difference could be due to the high activity of Rh-ceria for the water-gas shift compared to Rh on

other supports.

Silver has been shown to be a good catalyst for CO oxidation, with no propensity toward carbon deposition.135 Co-doping of Ni/YSZ with Ag and Cu was found to reduce carbon deposition by a factor of 3 or 4 relative to samples doped with Cu or Co, with the carbon deposited being more amorphous.136 Silver can also be deposited electrolessly, and this appears to reduce carbon deposition in dry methane and ethane.137 Cells produced in this way were stable over a period of 100 h in dry methane.138

Noble metals are also used in so-called catalyst or barrier layers in anodes, where a layer is placed between the active anode and the gas supply. This serves to reduce the hydrocarbon content in the anode by blocking hydrocarbons from entering or water and CO2 from leaving. If reforming catalysts are used they can also increase the reforming rate. This is at the cost of power density, due to the increased resistance to diffusion to the electrochemically active layer. In the original work showing this effect, Ru supported on CeO2 was used in a catalytic reforming layer over a Ni/YSZ anode which showed good stability in iso-octane-air-CO2 and propane-air mix-tures.139 Ir/CGO has also been used successfully,140 but more recent work has shown that barrier layers made from materials which show less reforming activity such as Ni-Cu on Zr-doped ceria,141 Ni-doped ceria,142 La0.75Sr025Cr0.5Mn05O3 (LSCM)-CeO2,143 partially stabilized zirconia and zirconium-doped ceria,144 and even Ni/Al2O3145 can also give low or no carbon deposition in the Ni-based anodes underneath, indicating that the main effect may be the barrier layer effect rather than the reforming activity. A further drawback to practical use of barrier layers is their nonconductivity, which may hinder current collection. This could be combatted by incorporation of reforming catalysts into a mainly metallic composite.146 Nevertheless, further work on barrier layers may

Figure 14. Polarization curves and power densities of (a) Ni/CGO and (b) Sn-Ni/CGO anode-supported single cell SOFCs operating at 650 °C with H2 and CH4 and (c) their voltage variations measured at 650 °C in CH4 as a function of time. Reproduced with permission from ref 166. Copyright 2014 The Royal Society of Chemistry.

be informed by section 7, which discusses sulfur and carbon tolerant catalyst materials.

6.3. Alloying or Replacement of Nickel with Base Metals

A similar rationale is behind the use of top row transition metals Co, Cu, and Fe, which also act to break up the continuous nickel sheets. In comparative studies with Ni/YSZ-based anodes, all of these elements, while reducing the carbon deposition also reduce the electrocatalytic activity compared to pure Ni/YSZ.147 Despite this, the benefits of reduced carbon deposition may outweigh the reduced performance, so these systems have been extensively studied, including using different fabrication techniques such as impregnation, microwave irradiation,148 and electroless deposition.149 Impregnation was used to produce a series of Ni-Cu alloys on a porous YSZ substrate which was also impregnated with CeO2.150 While no carbon was detected on the pure Cu anode, anodes with a Cu-Ni ratio of 9:1 and lower displayed significant weight gain due to carbon deposition, although the deposition seemed to be self-limiting at 4:1 and higher, and the cell structure was not destroyed. Interestingly, a higher reduction temperature resulted in lower carbon deposition, and it was suggested that this is caused by copper enrichment at the surface of the alloy. Cell tests on the 4:1 Cu:Ni anode showed a large increase in performance caused by carbon deposition improving electronic percolation. A catalytic study of Ni-Cu/YSZ+CeO2 with the Ni, Cu, and CeO2 impregnated into the YSZ also found significant carbon deposition in the 50:50 Ni:Cu sample after exposure to a 2:1 CH4:O2 mixture at 800 °C for 20 h.151 The amount of carbon was also not reduced by addition of Pd to the composite. A sample with a 25:75 Ni:Cu mixture in contrast showed no carbon deposition.

Electroless deposition149 produced an inhomogeneous distribution of copper, leading to carbon deposition in copper-poor areas. When microwave irradiation was used to deposit copper nanoparticles on a Ni-YSZ anode, the effect was similar to cells produced using impregnation of a Ni-Cu solution, indicating that alloying during synthesis may not be necessary to reduce carbon deposition.148

Tests using copper alone have shown very low activities compared to nickel, with Cu/YSZ anodes showing very low OCV with a dry methane fuel, indicating that it has little activity toward methane oxidation. , , This highlights the importance of the ceria used in a number of the above studies, which will be discussed later.

Iron has also been tested. In a series of studies, it was found that iron could reduce carbon deposition in quantities as low as

10% in both Ni-Fe/La0.9Sr01Ga0.8Mg02O3 (LSGM)154 and Ni-Fe/CGO155 anodes. One study compared Ni and Ni0.9Fe0.1 as supports in metal-supported cells under humidified methane at 650 °C.156 They found that while carbon was deposited in both supports, the carbon in the Ni-Fe support was amorphous, did not retard the rate of the methane reforming reaction in the support, and prevented carbon deposition in the Ni/CGO anode layer (from SEM). In contrast, carbon on the Ni support was highly graphitic, completely deactivated the reforming reaction, and led to cell failure due to carbon deposition in the anode in less than 10 h.

Cobalt, similarly to nickel, has known catalytic activity toward carbon-containing compounds, so it has been investigated in anodes. It seems promising for electro-oxidation of CO, with alloys with Cu producing higher performance in syngas than an equivalent Ni or Cu only cell157 and Ni alloys with Co producing higher exchange current densities in syngas than in hydrogen.158 Cobalt is expected to have less tendency to carbon deposit than nickel, but in tests where nickel is entirely replaced with cobalt in a YSZ cell, carbon deposition was still observed after 15 h in dry methane. No performance loss was observed, however, indicating that the carbon is not poisoning the activity of the Co/YSZ cell, although it could still eventually cause structural failure.159 Under syngas, cells based on Ni-Co alloys became completely delaminated in CO:H2 ratios above 60:40, indicating that Ni-Co alloys are still vulnerable to carbon deposition.158

In a catalytic study, Co-Cu/YSZ+CeO2 with a 50:50 mix of Co and Cu produced by impregnation into YSZ showed very little carbon deposition after exposure to a 2:1 CH4:O2 mixture for 20 h at 800 °C, much lower than a comparative Ni-Cu

sample.151 A similar study conducted with dry butane found that the amount of carbon deposition increased with increasing metal loading, indicating that the metal is still encouraging the formation of carbon, despite the lack of nickel.160 This carbon was amorphous, and did not cause any short-term degradation of the anode performance, although metal particles were seen encapsulated in the carbon fibers formed, indicating that the carbon deposition would cause long-term disruption of the anode structure.

Tin is another metal which has been used to reduce the tendency of nickel to form carbon. Tin has the advantage that it alloys easily with nickel, and the tin segregates to the surface of the particles, meaning that a large improvement in the stability in dry methane and isooctane while under load can be achieved with only 1% of tin with respect to nickel.161 The effect of 1%

0 0.1 OJ 0.3 0,4 05 0J 0.7 0 02 0.4 06 0.8 1 1 2

Current Dtntrty. A/cm* Currant Density, A/cm*

Figure 15. Potentials (open symbols) and power densities (closed symbols) as a function of current density at 973 K for H2 (diamonds), n-butane (triangles), and CH4 (circles). In (A), the cell had a C-ceria-YSZ anode; in (B), the anode also contained 1 wt% Pd. Reproduced with permission from ref 168. Copyright 2003 The Electrochemical Society.

tin in reducing carbon deposition was also seen in ethanol-fueled SOFCs.

There has been some debate about the role and effect of tin. One study replicated some of the testing conditions in ref 161 as well as other conditions with dry and wet methane at different temperatures, but it failed to observe improvements in carbon tolerance under most conditions.163 They ascribed this to their use of electrolyte-supported cells (compared to anode-supported cells in ref 161). Further work by the group suggested that the tin appears to cause the formation of less stable carbon species, meaning that any carbon deposited in the electrochemical region is oxidized, but that carbon can still form outside of this region.164 TPO experiments agree that the stability is due to a reduced rate of graphitic carbon formation rather than total elimination of carbon formation and that keeping the cell under load is still necessary.165 Another paper suggests that hydroxyl groups formed at the tin atoms on the surface are responsible for the effect (Figure 14),166 while a study using DFT and microreactor tests showed that the effect is due to an increase in formation energy of carbon nucleation sites with no increase in energy for CO formation.167 One further study failed to show any improvement when using 1% tin, with increased carbon deposition on Ni/CGO in microreactor tests on humidified butane, which they ascribed to a low operating temperature of 600 °C.133

6.4. Replacement of Nickel with Nonmetal Electronic Conductors

It is also possible to use electronically conducting nonmetals, and these should have intrinsically less tendency toward carbon deposition. They can also have the advantage that nano-structured catalysts, especially precious metal catalysts, can be used without the loss of activity or function caused by alloying with base metals like copper. The carbon deposited due to hydrocarbon cracking is conductive, and one study exploited this. A porous YSZ scaffold was exposed to dry butane, depositing a conductive carbonaceous layer. This was then impregnated with ceria and/or noble metals to improve the catalysis. Pd showed the best activity out of Pd, Pt, or Rh in these cells168 (Figure 15). The cells' performances in butane showed a much smaller improvement through adding a catalytic metal, which was suggested to be due to saturation of carbon on the active metal surfaces.169

A longer term test of the Pt/C-CeO2-YSZ cell in dry methane showed a 15% drop in performance over 100 h. The

impedance spectra showed an increase in the Ohmic resistance, so the loss in performance was attributed to a loss in carbon. An earlier paper by the same group had shown that for a Cu/C-CeO2-YSZ cell, the OCV in C4H10 settles over time to a value of 0.85 V, implying that an equilibrium is reached between partial oxidation products.170 The authors also observed gradual changes in the performance under load, implying changes in the carbonaceous layer over time. These results taken together suggest that the carbonaceous layer will reach an equilibrium over time depending on the fuel, oxygen flux, presence of catalytic metals, and other factors.

A combined thermodynamic and experimental investigation looked at the stability of a range of electronically conducting carbides, borides, nitrides, and silicides in humidified hydrogen with a partial pressure of CO of either 10-1 or 10-6 at 950 °C.64 Of these, only the tungsten carbides and molybdenum carbides were stable and then only at the higher concentration of CO. Since carbides should have an intrinsic carbon tolerance, as well as having been investigated in catalysis for various reforming reactions, this marks them out as potential anode materials. Despite this, tungsten carbide has only recently been investigated in actual anodes, in a WC/YSZ anode. The performance with pure WC/YSZ was poor but could be improved several times by impregnation of a Ru/CeO2 catalyst. The cell was stable under dry methane, with low carbon deposition which was not detrimental to the performance, but careful balancing of the fuel utilization is required to prevent oxidation of the WC. A follow-up study tested fuel cells in humidified methane and methane-hydrogen mixtures, with maximum power densities of ~80 and ~250 mW/cm2, respectively, at 900 °C with a 300 ^m YSZ electrolyte.172 In a further study, the Ru was replaced with Ni, and this cell showed stable performance over a week under humidified methane at 850 °C with no evidence of carbon deposition from SEM.173 Removal of the ceria reduced the performance, but the high stability remained, indicating that it was the tungsten carbide which was preventing carbon deposition on the nickel particles.

Molybdenum carbide has been tested in a proton-conducting cell with a BaCe0.7Zr01Y0.2O3_^ (BCZY) electrolyte operating on ethane.174 The carbide was stable and showed very low levels of carbon deposition both in the cell and when exposed to ethane as a powder as assessed by thermogravimetry and XPS. Under hydrogen at 0.55 V, there was degradation of 5%

over 100 h which was attributed to reduction of the carbide to metallic molybdenum.

One study looking at molybdenum as a dopant in a Ni/YSZ anode observed extremely good performance under humidified methane in steam reforming activity, low carbon deposition, and fuel cell tests, especially in materials which were reduced in the humidified methane rather than in hydrogen. This was suggested to be due to the formation of highly active molybdenum carbide, but no further investigations were conducted to test this hypothesis.125

Tungsten and molybdenum have also simply been used as promoters in a similar way as the other base metals described above. In a combined mass spectrometry-thermogravimetric study, Mo-Cu, W-Cu, and Cu-doped Ni/YSZ were exposed to dry methane at 800, 650, and 500 °C. The samples were produced so as to retain Mo and W in their metallic state. The Mo and W doped samples showed improved tolerance to carbon deposition, which may have been due to the formation

of carbides.175

6.5. Increasing Alkalinity

A third strategy involves increasing the basicity of the material, especially by using an alkaline earth. This strategy is known to reduce carbon deposition in conventional catalysis. All of the alkaline earths have been tested except Be, which can be highly toxic. They have strong basicity, and this modifies the electronic state of nearby nickel to make it less active for carbon deposition.176 In the particular case of MgO, (Ni,Mg)O solid solutions are formed, from which the nickel can be reversibly exsolved.177 This may help when regeneration is required.

Microreactor tests using Ni/YSZ cermets doped with small amounts of MgO, CaO, and SrO (0.2, 1, and 2% of anode mass) showed that CaO and SrO suppressed carbon deposition even at the lowest loadings, while MgO increased the rate of carbon deposition.176 A separate study looking at the same elements plus BaO and La2O3 made similar findings but also noted large changes in microstructure and conductivity depending on which promoters were used, highlighting the need to take into account all the effects of promoters.178 They found that CaO-promoted cells had the highest performance in humidified methane, due to a combination of good steam reforming activity, high conductivity, and low carbon deposition.

The alkaline earths can also dissociatively uptake water, which is then able to oxidize nearby carbon. First-principles studies indicate that BaO is the best alkaline earth for water adsorption,58 and its effect is shown by a study on the effect of Ba on Ni/YSZ anodes fueled with dry propane.179 Using thermogravimetic analysis (TGA) and Raman spectroscopy, they observed water incorporation (weight gain and O—H stretching modes) in the anode. This water uptake may assist the oxidation of carbon on the Ni particle and was further evidenced using DFT calculations.179

This type of carbon elimination process occurs preferentially at the BaO/Ni interfaces. The catalyst works synergistically: the water splitting takes place on barium oxide, the carbon deposition occurs on Ni sites of BaO/Ni, and the subsequent steps occur near the BaO/Ni interfaces. Figure 16 summarizes the proposed mechanism for carbon mitigation in a BaO/Ni-YSZ composite anode of a fuel cell fed with propane. A combined microreactor and fuel cell study on Ni-Cu/CGO anodes doped or not doped with Ba showed that the Ba does not reduce the rate of carbon deposition in microreactor tests

Figure 16. Proposed mechanism for water-mediated carbon removal on the anode with BaO/Ni interfaces. Large balls in bright blue, green, red, blue-gray, and purple are Ni, Ba, O of BaO or YSZ, Zr and Y, respectively, whereas small balls in red, white, and gray are O from H2O, H, and C, respectively. D1 is the dissociative adsorption of H2O, whereas D2 is the dehydrogenation of hydrocarbons or the CO disproportionation reaction. TPB is the triple phase boundary. Reprinted with permission from ref 179. Copyright 2011 MacMillan Publishers Ltd.

in dry methane but does reduce the rate of carbon deposition in fuel cell tests under load.180 It was suggested that Ba assists in oxygen transfer from the electrolyte to the metal surface, although other explanations were not ruled out. Impregnating Ni/CGO with BaO also greatly reduces carbon deposition in humidified CO.181

Because of the mechanisms of carbon suppression of the alkaline earths, the nanostructure of the anode plays a large role in the results for these elements as the oxide must be very near to the nickel without completely covering it. Incorporation of CaO by solid state methods was found to decrease the performance of the cell,176 while doping with Ba by impregnation was found to eliminate carbon deposition while only lowering power density by around 10%.182 Experiments using vapor deposition of Ba on Ni/YSZ showed remarkable stability in dry C3H8 with a sustained power output of 0.4 W/ cm2 over 100 h compared to complete deactivation after less than 1 h for a cell without Ba.179

An expansion of this technique has been to use Ba-containing proton conductors, which have the dual ability to store water and provide some ionic and electronic conductivity. Impregnation of yttrium-doped barium zirconate (BYZ) reduced carbon deposition at the same time as improving the performance, but only if the BYZ was present in the electrochemically active zone. This indicates that the improvement may be due to the increased electrochemical oxidation activity. A DFT study on Ni on yttrium-doped barium cerate (BYC) or YSZ indicated that the termination of the surface of the oxide is important: BaO-terminated surfaces adsorb water much more strongly than ZrO2 or CeO2-terminated surfaces and are thus more able to oxidize carbon at the triple phase


The water storage capacity of one material: Ni/ BaZr0.4Ce0.4Y0.2O3 (BZCY) was actually measured and found to be four to five times higher than a selection of other anode and catalyst materials.184 These materials included Ni/BaZrO3, indicating that the water storage capacity is not solely due to the barium but may also have some contribution from the proton or electron conductivity, the other elements present, or as mentioned above, differences in the preferred surface termination. The Ni/BZCY showed much lower levels of

carbon deposited in microreactor tests at all temperatures and ethanol-steam mixtures, and anodes based on Ni/BZCY were stable under the ethanol-steam for 180 h at 750 °C, in contrast to Ni/CSO (cerium samarium oxide, CexSm1-xO2-s) and Ni/ YSZ anodes which failed after less than 2 h.

BaZr01Ce0.7Y0.1Yb01O3-s (BZCYYb) is a MIEC with proton conductivity, but on its own, it has poor performance due to low electronic conductivity.185 When impregnated with metals the performance improves markedly, and with Ni-Cu impregnated, there is no carbon deposition as measured by Raman under dry methane at OCV at 750 °C, while Ni-impregnated cells showed no carbon deposition under humidified methane at OCV at 750 °C. Ni/BZYYb composites have also been used with ethanol as the fuel; in this case, there was significant carbon deposition, but it was limited to the outside of the anode and was amorphous in nature, indicating that these materials may also reduce the amount of graphitic carbon when carbon deposition does occur.186

Lastly, an in situ Raman study on BaO and barium zirconates showed that, as well as the water adsorption effect on carbon deposition, there was also a reverse Boudouard effect in the barium zirconates, where CO2 adsorbed as CO32- ions were able to react directly with deposited carbon at the triple phase boundary. This effect was not seen in BaO, as the BaCO3

formed was too stable.187

In theory, the alkali metals should also reduce carbon deposition, and this has been shown for Li in reforming layers in SOFC anodes, where doping with Li or codoping with Li and La reduced carbon deposition in the Ni/Al2O3 layer under an 11.5:1 CH4:O2 mixture. It should be noted that in this study, Ca was more effective at reducing carbon deposition, but Li (on its own or combined with La) also showed an improvement in the methane reforming reaction.188 The main concern with use of alkali metals is their volatility. One study used a Li-ion conducting material, Li0.33La0.56TiO3, as the support rather than Al2O3 and found that this improved long-term stability.189 The authors measured the lithium loss and found that the lithium content reduced from 4.68 to 4.63 wt % after 100 h at 800 °C in air, compared to 0.42 to 0.20 wt % for a sample of Li-doped Al2O3. The fact that this study was carried out at 800 °C indicates that the volatility at lower temperatures may be less of a problem.

Other than the oxides of alkali metals and alkaline earths, there are a few other basic oxides which have been tested. Under the conditions of an SOFC anode Mn occurs in the form of MnO which is a basic oxide. Under wet methane at 800 °C, Ni/YSZ doped with 2 or 5% of the NiO replaced with MnO, the cells lasted less than 1 h, similar to the performance of an undoped cell. With 10% MnO, the cell showed dramatically improved performance, with no degradation over 40 h.190 Microreactor tests showed that the amount of carbon decreased by over 150 times compared to undoped Ni/YSZ, and this was shown to be due to a relative decrease in the rate of methane cracking compared to steam reforming.

Conversely, increasing the acidity can worsen carbon deposition. Adding 2.7% aluminum oxide, an acidic oxide, to a Ni/YSZ anode reduced the amount of carbon deposition in a simulated CH4-CO2 biogas mixture, due to an improvement in the dry reforming rate.191 However, when the amount of aluminum oxide was increased to 10%, the carbon deposition was increased due to the increased acidity.

6.6. Use of Ceria and Other Oxygen Storage Materials

The concept of oxygen storage materials was first used in three-way catalysts, where a partially reducible oxide is able to supply oxide ions during periods of fuel-rich conditions and is reoxidized during fuel-lean conditions.192 In SOFCs, this may help to prevent carbon deposition by increasing the rate of supply of oxide ions for oxidizing carbon on the surface of the anode. In reality, for SOFCs the only oxygen storage material of note is ceria and doped ceria. Although there are potentially other oxygen storage materials relevant to anode conditions, only one, MnO,190 has been used (as discussed above), and the effect of its oxygen storage capacity was implied rather than confirmed through experiments.

Ceria is particularly attractive due to its high ionic conductivity arising from the creation of oxygen vacancies on its fluorite lattice when exposed to reductive atmospheres becoming a mixed conductor.193-195 These structural defects are known to improve the oxygen mobility of surface and bulk oxygen of ceria resulting in an enhanced oxygen storage capacity (OSC) which at the same time benefits the oxidation processes.196 These boosted redox features might be useful to eliminate C and S adsorbed species via oxidation and release them as CO2 or SO2.

The oxygen vacancies population and consequently the ionic

conductivity of ceria may be enhanced using promoters.196-198

In particular, acceptor-dopants (e.g., Sm2O3 or Gd2O3) are used to substitute some cerium ions in the fluorite structure, resulting in the formation of oxide-ion vacancy sites to compensate the charge-balance. Furthermore, the well-known activity of ceria-based catalysts for soot combustion in automobiles makes this material interesting for SOFC anodes.194^199 For instance, CGO was employed for methane and hydrogen oxidation exhibiting high current density and good tolerance toward carbon deposition.200

Initial work used impregnation to introduce ceria into porous Ni/YSZ anodes. It was reported by some papers to eliminate carbon deposition while using dry methane,201 but others disagree, showing deactivation over only 30 min.152 Doped cerias can also be impregnated. Doped cerias are oxide ion conductors and have some electronic conductivity under the conditions in a fuel cell anode. This has the effect of extending the triple phase boundary region, which outweighs the fact that pure ceria is a better catalyst for direct oxidation than doped ceria.153,202 Impregnation of CSO nanoparticles into a Ni/YSZ electrode produced a cell with stable performance under dry methane over 1000 h, which was attributed to suppression of nickel sintering and carbon deposition observed in separate catalytic reactions with methane-air mixtures.203 Impregnation of CGO into anodes of nickel/scandia-stabilized zirconia (Ni/ ScSZ) showed relatively stable performance under humidified methane, although carbon deposition could still be observed and had the effect of improving the performance initially (due to formation of conducting carbon networks), before eventually degrading.204 It should be noted that Ni/ScSZ cells without CGO also showed relatively stable but inferior performance, indicating that the main effect of the CGO was to improve performance rather than reduce carbon deposition.

The ceria-zirconia system is well-known in catalysis for its high oxygen storage capacity as the seven coordinate zirconium ions serve to stabilize Ce3+. Ce0.9Zr01O2 was impregnated into Ni-YSZ anodes and was found to greatly reduce carbon deposition in methanol at OCV and almost eliminate it under load, as measured by EDX and TPO.205

Cu/CeO2-YSZ anodes show performance in cell tests under dry methane close to that of Ni/CeO2-YSZ, but with no carbon deposition.152 Importantly, the ceria needs to be impregnated before the Cu, showing that the catalyzed step is the oxidation of hydrocarbons on ceria using oxide ions from the electro-lyte.206 The noncatalytic nature of the copper was reinforced by a study which showed that the replacement of copper with gold showed very little change in performance.207

These results indicate that ceria is active toward electrochemical oxidation, while copper simply acts as a current collector, meaning that the use of ceria allows the complete replacement of Ni with Cu. The advantage of using copper rather than nickel is that copper does not catalyze carbon formation, but the low melting point of copper oxide (1326 °C) means that traditional electrode fabrication methods cannot be used. The above papers use impregnation of copper nitrate into porous YSZ substrates. To improve the conductivity of this type of cell, Fe was added to Cu/CeO2-YSZ anodes, causing carbon deposition, which initially improved the performance by improving the conductivity before causing it to decline slowly.208

Cells based on Cu produced by impregnation may be limited by their electronic conductivity at low Cu contents. In this case, small (<2 wt %) amounts of carbon deposited from exposure of the anode to dry butane were found to improve performance, again because of an increase in electronic conductivity.170 The rate of carbon deposition was the same on YSZ and Cu/YSZ-CGO, implying that Cu and CGO are not catalyzing the carbon deposition. In addition, oxidation and rereduction returned the cell to its original performance, suggesting that the carbon deposits caused no permanent changes in the structure of the anode.

Further improvements to these Cu/CeO2-YSZ anodes can be made by using CSO rather than pure ceria.209 The developed system was suitable for several types of fuels and conserves high power densities after switching from one fuel to another. Figure 17 presents the effect of switching fuel type on

Figure 17. Effect of switching fuel type on the cell with the Cu-(doped ceria) composite anode at 973 K. The power density is shown as a function of time. The fuels were n-butane (C4H10), toluene (C7H8), n-butane, methane (CH4), ethane (C2H6), and 1-butene (C4H8). Reprinted with permission from ref 209. Copyright 2000 Nature Publishing Group.

the cell with the Cu-(doped ceria) composite anode at 973 K. As shown in the plot, 1-butene and ethane leads to the higher power density while toluene generates a current-sensitive drop in power. Toluene as an aromatic compound increases C formation; however, they observed that the anode was self-cleaning upon switching to n-butane. Use of a porous-doped

ceria interlayer can also reduce carbon deposition with humidified methane as fuel.210

Since doped cerias are oxide ion conductors in their own right, it is possible to dispense with the YSZ altogether. Cells produced using Ni/CGO synthesized via a Pechini method showed no carbon deposition from Raman under humidified methane at 600 °C for 50 h; although, it should be noted that the cells, which used a 20 ¡m CGO electrolyte, showed extremely low OCVs, and no attempt was made to find out whether this low OCV was due to oxygen leaks into the anode or to the nonzero electronic conductivity of CGO.211 High levels of carbon deposition were still observed under humidified propane. A Ni/CSO anode was operated on dry methane at 600 °C for 72 h under a current load of 300 mA with very low levels of carbon detected by FIB-SEM and TPO post-test analysis, although again thin CSO electrolytes were used and OCVs of -0.9 V obtained.212 Ni on Mo-doped ceria showed less than 0.04 wt % carbon deposition after exposure to a methane-oxygen mixture (5:2 molar ratio).213 Cells based on this material using 400 ¡m LSGM electrolytes showed reasonable stability over 10 h under load and wet methane, although unfortunately the amount of carbon deposition was not quantified.

There is strong evidence that CSO has high enough electronic conductivity under hydrogen that the limiting factor is not the triple phase boundary length but the surface area of the CSO, intimating that an optimal strategy may be to optimize the surface area of the ceria and improving the catalysis for hydrocarbon oxidation, utilizing the minimum amount of current collecting metal necessary.214 This electronic conductivity, along with the reforming activity of CGO, has been used in the current-collecting layer, where CGO-coated Ni was used at the top of the anode to reduce exposure of Ni to unreformed methane. The cells were stable in dry methane at 610 °C over 1000 h, compared to cells without the CGO-coated Ni layer which failed after <200 h.215

While work on metal-ceria composites has understandably focused on the doped cerias with the highest ionic conductivities (CSO, CGO, etc.), some work has been done on materials with higher oxygen storage capacities. Ce09Zr01O2-based (CZO) impregnated anodes were found by EDX to reduce carbon deposition in humidified methane compared to CeO2-based anodes.216 A larger effect was seen by replacing Ni with Cu, but unlike replacement of CeO2 with CZO, this had a large negative effect on performance for total replacement. A partial replacement of Ni with Cu on CZO was found to be the best compromise between carbon tolerance and performance.

A further advantage of ceria and doped ceria is that the methane cracking reaction is extremely slow. Undoped ceria or ceria doped with varying amounts of Nb or Gd showed between 0.07 and 0.9 monolayer coverage of carbon after 150 min exposure to methane at 900 °C, compared to 142 monolayers deposited on Ni/YSZ at the same temperature.218 The electrochemical oxidation of hydrocarbons over doped ceria is still relatively low, however,219 with higher activities caused by the activity of either Pt current collectors or Ni toward steam reforming. In theory, cells based on Cu and ceria could be doped with noble metals to improve their activity, but the noble metals alloy with Cu forming less active phases.169

6.7. Replacement of Cermets with Mixed Ionic-Electronic Conductors (MIECs)

6.7.1. Single Phase MIECs. At the time of the resurgence in interest in SOFCs in the late 1980s, a concurrent area of interest was direct hydrocarbon oxidation catalysts, for removal of hydrocarbons from car and power plant exhausts. It had been established that a number of oxides were active toward this reaction, and they came to the attention of groups working on SOFCs, with particular attention paid to the perovskite family of oxides.

Perovskites are defined as a family of materials, which present the same structure as the face-centered cubic calcium titanium oxide CaTiO3. The structure of these compounds of general formula ABO3 may be described as a combination of the oxygen and A-site cations that form the cubic close-packed (ccp) framework, the oxygen atoms occupy three-quarters of the sites of the cubic close packed layer and the A-site cation and the larger one, the remaining quarter. The B-site cations occupy one-fourth of the octahedral holes of the ccp arrangement. This structure can also be viewed as the B-site cations occupying the center of the cubic structure while A and O ions are located at the corners and half edges, respectively. Perovskites have a high degree of structural and electronic flexibility, with many different elements and oxidation states able to be incorporated into the structure. The A-site cation can be a low valence rare earth, alkali, or alkaline earth ion; for example, La, Na, Ca, Sr, or Ba, while the B-site is a transition metal, such as Ti, Zr, Fe, Co, Ni, or Cu. Both sites are able to accept multiple different ions simultaneously, and this produces

possibilities for variable oxidation states.220,221 In addition, if there is more than one different element occupying the B-site, these can become ordered, and perovskites displaying this behavior are known as double perovskites (materials with only one B-site element or two or more unordered B-site elements are single perovskites). Their defect chemistry gives the potential for them to exhibit MIEC properties under a wide range of partial pressures of O2 at elevated temperatures.199

Early work centered on perovskites of lanthanum with top row transition metals. One example is La08Sr02FeO3 (LSF), nowadays more familiar as a cathode material, which, while it showed better activity than Pt electrodes and no carbon deposition under dry methane, was not stable under relevant anode overpotentials (less than 0.3 v).222,22:3 Attention quickly focused on substituted lanthanum chromites which were already used as interconnects in SOFCs due to their stability in very low pO2,224 despite the fact that the base material exhibited one of the lowest activities for methane oxidation.225

While lanthanum chromites are not expected to catalyze carbon formation to the same extent as nickel, carbon deposition has been observed. When exposed to dry methane in a fixed bed catalytic reactor, at temperatures above 600 °C, calcium-doped lanthanum chromite was observed to catalyze methane decomposition, resulting in an average of half a monolayer coverage of carbon compared to 112 for Ni/YSZ under the same conditions.226 While this amount of carbon is small, it was found to have a deleterious effect on the catalytic reactions. Addition of 3% steam to the 5% methane feed prevented this carbon build-up. A study of various strontium-and manganese-doped lanthanum chromites (LSCM) containing varying amounts of Cr and Mn found that larger amounts of carbon deposition for Cr-rich compounds and/or exposure to methane at higher temperatures were linked to lower

selectivities toward the total oxidation of methane relative to partial oxidation.227

Due to the low activity of lanthanum chromites, the immediate focus for improvement was on the activity rather than on further reducing carbon deposition. Notwithstanding this, several authors did measure the tendency of doped lanthanum chromites toward carbon deposition. One such study tested various first row transition metal dopants to improve the activity as well as alkaline earth dopants to improve the conductivity. All dopants produced carbon deposition of less than four monolayers at 800 °C. The exception was the Fe-doped material under conditions representing internal reforming, which produced 69.4 monolayers of carbon,228 which is similar to levels which would be expected from a nickel-based cermet.218 Materials which produced no carbon under any conditions were the Sr and Mg double-doped material and the Co-doped material. The two Ni-doped materials (singly doped and codoped with calcium) surprisingly showed no extra carbon deposition compared to most other dopants but did produce considerably higher conversions: between 3 and 5 times higher than any other materials for the reactions representing partial oxidation and dry reforming and 1-2 orders of magnitude higher for the reaction representing steam reforming. In fact, although not known at the time, it is likely that the nickel-doped samples were producing nickel metal nanoparticles under reducing conditions, which helps explain the vastly improved catalysis.229 This is discussed further below in section 6.7.2.

Composites of LSCM with doped ceria show better activity, as well as increased carbon deposition. In one study, carbon deposition after exposure to dry methane for 6 h at 750 °C increased from less than 0.1 wt % in pure LSCM to 1.5 wt % in 33 wt % LSCM:67 wt % lanthanum-doped ceria.230 An increase in the amount of the doped ceria improved performance in fuel cell tests in methane, although above 50 wt % ceria the performance dropped, probably due to lower electronic conduction. Doping lanthanum chromites with ceria may also help. Iron-doped lanthanum chromite codoped with 5% Ce showed much lower carbon deposition at 800 °C in syngas in a microreactor, while symmetrical cells showed less drop in performance under the same conditions and were able to be regenerated by 24 h under load in H2.231

Other perovskite-based anode materials have been tested, for

example, lanthanum aluminates232 and barium titanate,233 but

the most studied single perovskite other than lanthanum chromite is strontium titanate, which is stable and when doped is a MIEC under reducing conditions. To induce electronic conductivity, the base material can be doped with La3+ on the A-site (known as LST)234,2:5 or Nb5+ on the B-site,236 with the stoichiometry controlled to produce either Ti3+, Nb4+, or oxygen deficiency or excess, meaning that this system is compositionally very flexible. A possible hindrance to using this material is the high temperature (>1000 °C) reduction needed to induce a suitable degree of electronic conductivity, and the fact that this conductivity is lost under oxidation. LST does possess very low propensity toward carbon deposition, with less than 1 wt % of carbon deposited after 6 h under dry methane at 800 °C in a microreactor.237 Composites of LST and CGO in the same study showed increased carbon deposition, although still less than 2 wt % carbon with 40 vol % CGO. The increase in carbon deposition was related to the greater degree of interaction between CGO and methane, but in fuel cells the carbon was not shown to have any detrimental effect on the

performance, with a direct correlation found between polarization resistance in the impedance spectrum and propensity toward carbon deposition in the microreactor tests.

More recently, Sr2MMoO6 (where M is a small 2+ cation such as Mg or Ni) double perovskites (perovskites in which the B-site cations are ordered) have also been used as SOFC anodes. The ordering occurs as a consequence of the very different charges on the B-site cations, but currently, no advantage for carbon or sulfur tolerance of using a double perovskite rather than a single perovskite related specifically to this ordering has been suggested. Sr2MgMoO6 showed good activity for CH4 oxidation and stability under short-term testing of 15 h.238 The power density dropped under wet CH4 compared to dry CH4, indicating that direct oxidation was the main route for CH4 conversion. Materials where the Mg was partially or fully replaced with Mn performed worse, with a power density of 838 mW/cm2 for the pure Mg sample reducing to 650 mW/cm2 for the pure Mn sample. Materials using Co or Ni rather than Mn showed similar performance decreases compared to the Mg sample.239 Co was seen to exsolve from the perovskite as Co metal, although initial performance was similar. Co and Ni showed different catalytic behaviors: Co acted mainly through steam reforming, with low performance in dry CH4, while Ni showed no steam reforming activity. It is important to note that all the above studies were carried out with Pt current collectors and a doped ceria barrier layer, which later work has suggested could be responsible for most of the methane oxidation.240 A study doping the Mo site with Nb which did not use a barrier layer or Pt current collector agreed with this poor activity toward methane oxidation, and suggested that, similar to other MIECs studied, the catalyzed reactions between the methane and the MIEC were likely to be limiting in pure MIEC-based systems.241 The study did find that the amount of carbon deposited was very low; however, this would be expected from a system with poor activity toward methane conversion.

Due to the structural flexibility of perovskites, they are able to form reduced compounds while maintaining the perovskite structure, and one promising material which illustrates this is the A-site layered double perovskite PrBaMn2O5+s (PBMO).242 This material is stable across a good pO2 and temperature range and, unlike many of the perovskites described above, appears to have some activity toward hydrocarbon oxidation (with silver used for current collection). Although this material was not tested for carbon deposition or stability under operation in hydrocarbons, a calcium-doped version, PrBa0.8Ca0.2Mn2O5 (PBCMO), was and was stable for 50 h in humidified isooctane followed by 150 h in humidified propane, with currents of 0.2-0.3 A/cm2 achieved at 0.6 V at 700 °C.243

6.7.2. Addition of Catalytic Metal Nanoparticles to MIECs. Since MIECs by definition are electronically percolating, a percolating metal phase is not necessary, but dispersed metals can still be added to promote the catalysis. However, these metal nanoparticles can also be prone to carbon deposition. Impregnation of metal salts (typically nitrates) into the anode is a technique borrowed from catalysis, where it is an extremely widely used method for producing catalysts. Ni, Pd, and Ni-Pd were added to Sr-doped LaCrO3, with Ni-Pd showing a synergistic effect for methane oxidation in dry methane, with little or no carbon deposition observed using a carbon balance approach during testing at 0.5 V and 800 °C. 44 A short period at OCV was sufficient to completely deactivate the electrode toward methane decomposition, while returning

the cell to 0.5 V could only recover 60% of the activity. Addition of hydrogen to the cell was necessary to fully reactivate the cell through methanation of the carbon.

Further work on Ni-Pd and Pd nanoparticles dispersed on an LSCr-CSO anode suggested that the reaction mechanism for the oxidation is fundamentally different comparing Ni-Pd alloys with pure Pd particles.245 The work suggested that the reaction on Pd was close to direct electrochemical oxidation, while the reaction on Ni-Pd alloys was likely through methane cracking followed by electrochemical oxidation of hydrogen, steam reforming of carbon, and electrochemical oxidation of the CO produced. These alloys were found to be resistant to carbon deposition, and it was proposed that doping of Ce, Sr, La, or Sm into the alloy was preventing the formation of carbon fibers, as highlighted by the fact that the Ni-Pd nanoparticles contained trace amounts of these elements.246

Improvements to the stability of Pd nanoparticles can be achieved by impregnation of Pd-core/CeO2-shell nanoparticles, which are able to operate on dry methane without carbon deposition and survive heat treatments in air up to 900 °C with only 9% loss in performance compared to 40% loss in performance with impregnation of just Pd.247

These results highlight some advantages of using a MIEC combined with dispersed metal particles compared to cermets. (1) Although the cell can still be deactivated through carbon deposition under certain conditions, since the metal is not load bearing, complete structural failure does not occur and the cell can be regenerated. (2) In cells based on cermets, the only economically feasible method for adding expensive elements such as Pd is via impregnation into an already formed cermet anode. This results in segregated Pd and Ni particles, which rules out this synergistic alloying effect and does not prevent carbon deposition in Pd poor regions.130

An interesting approach toward decoration of MIECs with catalytic nanoparticles is exsolution, where a reducible metal is incorporated into the oxide structure during synthesis and exsolved forming remarkably stable nanoparticles under reducing conditions.248 A feature of these systems is that the nanoparticles can be cyclically readsorbed and exsolved from the structure. This method has the advantage compared to traditional impregnation that it produces stronger particle— support interactions and so less sintering occurs and the particles are more stable. There are two main disadvantages compared to impregnation: higher reduction temperatures and less control over the composition of the particles; currently there are no reports of alloy nanoparticles deliberately produced by exsolution.

Exsolution was first (deliberately) tested in SOFC anodes with Ru-doped LSCM.249 The Ru exsolved forming particles up to 5 nm in diameter over 50 h under hydrogen at 800 °C, doubling the cell performance and reducing the polarization resistance by a factor of 3. Only 15% of the Ru was found to have exsolved, and the authors suggest that this is due to a combination of slow diffusion and energetic barriers toward removal of too much Ru from the perovskite structure. Later studies found that the exsolved Ru particles acted to hinder carbon deposition on the LSCM.250 Aside from improving the performance of cells running on dry ethanol, it was found in fixed bed tests that carbon deposition was eliminated compared to around 1 wt % for the Ru-free material.250 Co particles can also be exsolved, and these showed <1% weight gain due to carbon deposition when exposed to dry methane for 4 h,

compared to >100% weight gain for Co/CeO2 prepared by impregnation.251

As mentioned previously, nickel doped into LSCM can exsolve out as nickel nanoparticles, and depending on the particle size produced, these can be resistant to carbon deposition. Pulse reaction studies on Ni-doped LSCM indicated that essentially all the methane was converted to carbon dioxide until oxygen stoichiometries were below 2.7, where the methane conversion continues to increase despite CO2 conversion reducing, indicating that methane decomposition (and consequent carbon deposition) was taking place.229 This was considered to be due to the greater degree of nickel exsolution implied by lower oxygen stoichiometries and potentially larger particle size. Co particles were also found to exsolve from Co-doped LSCM, and it was found that exsolved Ni and Co particles have a large effect on the methane oxidation rate with only a small increase in the rate of carbon deposition compared to LSCM. Exsolved Ni showed far better carbon resistance than impregnated Ni252 (Figure 18).

Figure 18. Carbon production rates averaged per pulse for LSCM, LSCMCo, LSCMFe, LSCMNi, and LSCM+Ni. Note that the latter is shown on the right axis. Reprinted from ref 252. Copyright 2010 American Chemical Society.

While exsolved Ni and Co can still show raised levels of carbon deposition, addition of Cu to form nanoalloys can mitigate this. Ni and Co exsolved from Ce08(Co,Ni)02VO3 showed significant amounts of carbon deposition on exposure to dry methane at 700 °C, with 10% weight gain caused by carbon deposition for Co and 27% for Ni compared to <1% for the undoped material.253 However, double doping Cu and Co reduced the weight gain to 2%.254 Double doping Cu and Ni did not reduce the amount of carbon deposition, probably because the 50:50 mix of Cu and Ni used is still prone to carbon deposition as discussed in section 6.3.

Likewise, LSC double-doped with Ni and Fe showed less carbon deposition after exposure to syngas at 850 °C than singly doped Ni-LSC. Singly doped Fe-LSC showed less carbon deposition than either but performed worse in fuel cell tests, while Ni-Fe doubly doped cells performed best.255 XRD and SEM analysis showed that the exsolved Ni and Fe formed alloy particles of around 25-30 nm. The promising symmetrical electrode material Pr04Sr0.6Co0.2Fe0.7Nb01O3-5 (PSCFN) forms Co-Fe nanoalloys at 900 °C under hydrogen, with stable performance for 50 h under dry methane and 100 h under dry butane at 800 °C.256 Microreactor tests on reduced PSCFN

showed considerable carbon deposition (30 wt %) under methane at 850 °C and also high activity for methane cracking.257 The stable performance under methane could be explained by the fact that on initial exposure to methane, CO2 (and presumably water) is produced rather than hydrogen, indicating that there are species active for methane oxidation. In addition, the carbon was able to be oxidized at 450 °C, implying that it was dispersed and amorphous and, therefore, may be oxidized by oxygen flux under SOFC anode conditions.

The exsolution of nanoparticles could be limited by energetic barriers toward removing B-site cations from a stoichiometric perovskite. This can be combatted by synthesis of A-site deficient materials, which allow B-site cations to be removed much more efficiently, allowing even metals such as iron to be exsolved from LST.2 8 Control of the stoichiometry in this way also allowed Ni metal and CeO2 to be exsolved from lanthanum cerium titanate.259 Exsolution of Ni from Ni and Ce double-doped LST was found to greatly reduce the amount and difficulty of removal of deposited carbon in microreactor tests in methane compared to Ni-doped LST.260

SrMoO3 is a MIEC stable under anode conditions, but doping with Ca allowed Mo nanoparticles to exsolve under reducing conditions. These particles had a small beneficial effect under hydrogen, but under methane the Ca-doped materials allowed carbon deposition, in contrast to the undoped material which did not.261 While the formation of carbon implies a greater ability to interact with methane, strangely both the undoped and doped materials showed very low OCVs, which indicates a lack of ability to convert methane.

Nanoparticles of oxides can also be produced through exsolution-type processes. LSCF impregnated with nickel was used as an anode, where it decomposed into strontium cobalt iron oxide perovskite with La2NiO4 finely dispersed over the surface.262 The La2NiO4 was presumed to be the electro-catalytically active phase, and the cells exhibited good performance in dry propane with only a few carbon whiskers observed in the SEM after 100 h of use.

6.8. Regeneration of SOFC Anodes Deactivated by Carbon

As can be seen above, carbon deposition can occur, to varying degrees, on all materials so far studied. In some circumstances, the carbon deposition is not detrimental to performance, or it can even be positive in small amounts as it can improve the electronic conductivity of the anode. As the amount of carbon increases in an SOFC operating over potentially tens of thousands of hours, deleterious effects such as pore blocking and risk of structural failure will inevitably increase, so it may be desirable from time-to-time to remove this carbon. Clearly, it is always possible to remove carbon by heating the cell to high temperatures in air, but a number of studies have investigated the possibilities for removing carbon without damaging the cell.

Kirtley et al. studied carbon removal from Ni/YSZ using 3% H2O, 10% O2, or 11% CO2 in nitrogen and found that the carbon was removed fastest in H2O and slowest in CO2, with times ranging from 10-125 s.263 Through examining the OCV and in situ Raman, the authors were able to identify the stages of carbon removal. First the OCV increased to -0.99 accompanied by the disappearance of carbon peaks in the Raman. This was attributed to the formation of a CO/CO2 gas mixture. This is followed by the appearance of NiO peaks in the Raman and an OCV reflecting the thermodynamic equilibrium of the Ni/NiO couple in the regenerating gas. O2 leads to complete oxidation of Ni to NiO, while H2O and CO2 lead to

partial oxidation. The above study induced carbon deposition from dry methane at OCV, but carbon induced using diesel reformate under load was able to be removed and the cell fully regenerated using dry and wet hydrogen, albeit over a time period of 44 h.264 Regeneration via this method was not possible under conditions where the cathode had also degraded, indicating that the carbon is removed largely by oxygen flux through the electrolyte.265

It is theoretically possible to regain performance without changing the gas mixture by moving from an operating regime where carbon deposition is favored to one where it is not. Ni/ YSZ cells were found to completely regain their initial performance after 24 h under load at 850 °C in a simulated partial oxidation reformate feed, having previously had carbon deposited in the same gas mixture at 650 °C under OCV.266

Symmetrical cells (where the electrode material used during fabrication is the same for anode and cathode) offer interesting theoretical potential for regeneration, given that if carbon deposition occurs they can simply be reversed, whereupon the deposited carbon will be exposed to air and thus oxidized. In a recent review on symmetrical electrode materials, the authors conclude that little work has been done on regeneration of these materials after carbon deposition.267 Table 4 summarizes important examples of the different strategies used to produce carbon tolerant SOFCs


There seems to be sufficient consensus in the literature that sulfur will be adsorbed at the surface of nickel blocking the

reaction sites for oxidation or reforming reactions;72,269

although initially an unwelcome feature, it can be used as an advantage to minimize carbon deposition.2^71 It is also accepted that absorption is more dramatic at lower temperatures and at higher concentration of sulfur.62,66 It is also known that two stages of sulfur poisoning have been observed, one is the surface absorption of sulfur that blocks the reaction sites but that can be reversed and a second one related to an in-depth formation of nickel sulfide that changes the microstructure of nickel and is therefore irreversible. 1,270 Figure 19 shows possible mechanisms for sulfur poisoning in hydrogen and carbon fuel environments.62 There is, however, no consensus in the effect of the current densities on sulfur poisoning. Some authors reported that increasing current densities leads to a decrease in sulfur coverage because of its conversion to SO2.63,65,271 On the contrary, other authors have indicated that sulfur coverage increases with current den-

• . I


A very good agreement with the latter view is

concluded in the recent modeling work of Riegraf et al., where the model involves all gas and solid chemical reactions coupled with electrochemistry. When operating in methane, the adsorbed sulfur suppresses the reforming reaction by blocking the catalytically active sites, and these sites become available if sufficient hydrogen is present to unblock the sites.274

Again there is agreement that full recovery can be achieved if H2S is removed completely from the fuel stream, but there is a limit beyond which damage is irreversible. Concentration and temperatures where recovery is possible vary from article to article but reversibility has been reported independently. This may be related to desorption of sulfur from the nickel surface and reaction with H2 from the clean stream. Some good examples of this recovery are the work of Rasmussen and Hagen,68 Sasaki62 and Zha.63 It is also generally accepted that

the whole surface of nickel is covered and not only the TPB


Whatever change takes place in the anode during poisoning, it must be reversible and provided that oxygen is migrating to the anode via the electrolyte, it is of paramount importance that this process is not stopped and that oxidation or removal of adsorbed sulfur is favorable. To provide sulfur tolerance, the materials and structure of the anode should therefore be capable of adsorbing sulfur and then react with any of the gaseous species present H2, H-C, or even O2- to form SO2.

From the point of view of the materials modification, the strategies more frequently used for the development of sulfur tolerant anodes can be summarized as follows: (l) high oxygen transport to increase sulfur oxidation (Figure 20a). In similar conditions, ScSZ working under H2S/H2 atmospheres shows a higher tolerance to H2S than YSZ, indicating the importance of a higher oxygen supply through the electrolyte.62 (2) Incorporation of additives or partial substitution of nickel (Figure 20b). Substituting Ni for a more sulfur-tolerant metal without compromising H2 activity has been behind much of the work on alloys.89,96 Some of the earlier work attempted copper,97,27S while the most recent use of additives has been aiming at using these as preferential sites of sulfur incorporation,276 and this is reinforced by a thermodynamics studies showing that oxides such as BaO and CeO2 reduce the coverage of sulfur on Ni,277 which is strongly linked to performance.9l Catalytic activity for hydrogen oxidation reaction and H2S dissociation seem to follow analogous trends, maintaining the catalytic activity while simultaneously improving sulfur tolerance difficult via this route.104 (3) Use of all-ceramic anodes (Figure 20c). Perovskites are favored as they can be tailored on the A and B site to improve ionic conductivity, electronic conductivity, catalytic activity, and resistance (e.g., Mg2+ more resistance to sulfide formation than Cr3+ or Mn2+).278 Additionally, the reactivity of a ceramic material is expected to be lower than that of a metal surface.78

7.1. Replacement of YSZ with Ceria

A few papers have compared Ni/YSZ and Ni/CGO electrodes, and for example, Zhang has shown that degradation in Ni/ CGO is lower than in the Ni/YSZ under similar conditions of operation.279 This may not be surprising considering that sulfur can also accumulate in the surface of the CGO forming Ce0xSy-type phases which can react with O2- to produce SO2. Recent studies on the adsorption and removal of H2S from fuel streams by rare earth oxides again suggest that CGO is one of the most promising anodes for operation under H2S-poisoned fuels. Elimination of the adsorbed sulfur can take place in ceria and other rare earth oxides using a reducing, oxidizing, inert gas, or even steam.280,281 This may explain the tolerance to H2S of an anode that has been infiltrated with ceria282 and the minimized potential drop in anodes with lanthanides as additives.62 The tolerance of ceria-based materials to sulfur environments has

been known for some years.283

Donor-dopants of ceria have been studied to a lesser extent compared to acceptor species. Nevertheless some of them are interesting for improving sulfur tolerance, for example Mo. This dopant is especially desirable for sulfur tolerance goals since it can trap S forming MoS2. In this sense, Li et al. investigated the electrical properties of the Mo-doped CeO2 (CMO) as potential anodes for SOFCs. Mo and rare-earth-co-doped Ce0.9_x RExMo0.iO2i-c.5x (x = 0.2, 0.3) (CRMO) oxides were found to retain their fluorite-type structure under H2 at elevated

Table 4. Selected Papers Reporting Improved Carbon Tolerance in SOFC Anodes through Materials Strategies

noble metals

metal fabrication performance ref comments

n' Si. 30

rt>" s

1.5 mol % Au deposited onto Ni/ CGO powder, screen-printed onto electrolyte 0—0.15 mg/cm2 Pd impregnated into

slurry-painted Ni/CGO electrode 0-9 wt % Ru02 mixed into Ni/CGO, formed into a pellet with ~30 //m CGO electrolyte 0.9—2.5 wt % Ag impregnated into Ni/YSZ anode supports, 20 /(m YSZ electrolyte

tested under CH4-H20, 850 °C, compared to no Au: 0.15 V higher OCV, 0.2 V higher at 500 mA/cm2 (both S/C = 3/2); no V degradation under dry methane vs 0.3 mV/h degradation under S/C = 1/2 (no Au); less C deposition (visual) tested in wet CH4 and EtOH, 800 °C, OCV, compared to no Pd: Rp decreases by 2X in CH4 and 4X

in EtOH in loadings above 0.07 mg/cm2; C deposition still observed (SEM, EDX) tested in wet CH^ 600 °C, compared to no Ru: current density increased by 2X at 0.4 V; also, stable performance for 20 h, no C deposition (from carbon balance) (not compared to the no Ru example)

tested in dry CH4, 750 °C, 0.3 A/cm2, compared to no Ag (at 0.6 A/cm2): 0.9 and 2.5 wt % Ag failed at 12 and 81 h, respectively, 1.6% Ag showed no degradation to 100 h; control failed at 5 h. Very little carbon observed by EDX

127 micro reactor and mechanistic studies reinforce effect of gold "

129 micro reactor tests suggest carbon suppression effect

131 another paper agrees that Ru has a beneficial effect,132 but there are few papers on Ru due to problems with its oxides' volatilities; Ru doped in ceramic anodes may be more feasible"49'"1'0 138 a further paper by the same group showed similar results for C2H6, l v while

microreactor results also show C tolerance; ^600 °C and above must be noted

1 the high mobility of Ag at

base metals

metal fabrication performance ref comments

Cu 0-100% Cu impregnated with Ni (Cu + Ni = 20 wt %) and Ce02 (10 wt %) into 400 fim porous YSZ support with 60 pim YSZ electrolyte

Fe anode supports were prepared from Fe203, NiO, and CGO, powders (Fe:Ni 0:100—50:50 w/w), CGO electrolyte

Co Ni/YSZ and Co/YSZ were prepared by coprecipitation then coated on a 500 pim electrolyte support

Sn 1% Sn was impregnated into Ni/YSZ anode-supported cells with a 20 pim YSZ electrolyte

powders and cells tested in dry CH4: at 700 °C, powders with 150 Cu:Ni of 9:1 or 10:0 showed no C deposition. 4:1 gave <0.1 g C/g; in a cell at 800 °C, performance of 4:1 improved from ~0.1 A/cm2 to >0.6 A/cm2 over 500 h due to C deposition

cells tested in dry CH4, 650 °C, 0.2 A/cm2: cells Fe:Ni up to 155 30:70 gave similar power densities 0.3 A/cm2), 50:50 gave <0.2 A/cm2; all Fe-containing cells were stable over 50 h at 0.2 A/cm2, Ni only cell stopped after ~12 h; no carbon observed on Fe:Ni 10:90 by SEM after test cells tested in dry CH4, 850 °C, OCV: anodic overpotential 159 remained stable over 15 h in Co anode, Ni anode failed; C deposition still observed in Co anode by SEM cells tested in dry CH4 and C8H18-air mixtures at 740 °C and 161 0.6 and 0.5 V, respectively; stable performance was obtained in both fuels over 6 h (CH4) or 13 h (C8H18). Ni-only cells completely deactivated

there are many papers on Cu with widespread agreement that it reduces C deposition; activity is poor so normally Ce02 or doped ceria is usedlio

there is some evidence that Ni:Fe alloys at 10% Fe are more active for methane oxidation155 and reforming;11,6 this level of Fe gives stable cells with a variety of oxide ion conductors154

other papers confirm that Co is less vulnerable to C deposition but still vulnerable;158'160 activity for CO oxidation looks promising157'158 despite some papers showing little impact of tin,133'163 the bulk of gamers studying performance and mechanisms in SOFCs 2,16+~ and catalysts ( see section 8.1.2) suggest that the effect of tin is real; several papers have examined 1% and 5% loading, with 1% being the best

nonmetal conductors

phase fabrication performance ref comments

C porous YSZ scaffold impregnated with 10 wt % Ce02 and tested in dry CH4 and C4H10 at 700 °C: maximum power densities of 0.1 W/cm2 in C4H10 and 0.02 W/cm2 in 168 several papers in the early 2000's looked

optionally 1 wt % Pt, Pd or Rh. ~4 wt % carbon is then CH4, similar to Cu/Ce02-YSZ cells; performance in all fuels was greatly increased by adding 1% Pd; a 100 h promising for this technique;168-1 0 however,

deposited in dry C4H10 at 700 °C, 100 fim YSZ electrolyte test of Pt/C-Ce02-YSZ in CH4 showed large increase in Ohmic resistance due to loss of carbon there have been no papers since by this group

or others

WC porous YSZ scaffold impregnated with 25 vol % WC, and tested in humidified CH4 at 850 °C at OCV: no carbon observed visually after 36 h; at 0.7 V stable 173 this strategy is quite unexplored, but the ability of

then 5 wt % Ce02 and 5 wt % Ni performance of 50 mW/cm2 over 24 h with no carbon observed visually WC to protect Ni from C deposition could be


So a g increasing alkalinity

=0 ~ P phase fabrication performance ref comments

to ° O ^ ^ 5" BaO vapor deposition of BaO onto an anode-supported NiO/YSZ cell with 15 fi m YSZ electrolyte tested in dry C3H8 at 750 °C at 0.5 A/cm2: cell voltage stable at 0.8 V for over 100 h compared to BaO-free cell which failed after <1 h; similar results for wet CO and gasified carbon; no C deposition observed by SEM 179 while CaO and SrO also reduce C deposition,176'178 BaO appears to be the best prospect; micro structure appears to be vitally important

?mrev.6b00284 , 13633-13684 BZCY sol—gel synthesis of NiO/BZCY composites copressed with CSO to form an anode-supported cell with 20 pim electrolyte tested in wet C2H5OH at 600 °C at 0.3 A/cm2; voltage stable at 0.75 V for 180 h compared to Ni/YSZ and Ni/CSO which failed after <2 h due to C deposition; no carbon detected or morphology changes detected by SEM after testing 184 numerous Ba-based perovskites have now been tested, including BYZ,1Sj BYC/8 and BZCYYb;185 the efficacy of these perovskites seems clear, and microreactor and modeling studies both confirm this and elucidate the mechanisms

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temperatures. The same team demonstrated the remarkable stability of these Mo-doped CeO2 anodes in wet H2 and wet CH4 mixtures.285 As mentioned above, Mo is a key element to incorporate sulfur resilience. In a recent publication, Chen and co-workers developed a sulfur-resistant SOFC anode by impregnation of Mo01Ce0.9O2+s into a typical Ni/YSZ materi-al.286 Figure 21 shows the successful performance of this

material when submitted to 50 ppm of H2S.

The system allows power densities of 440 and 420 mW cm-2, using H2 with 50 ppm of H2S and methane as fuel, respectively, under a current density of 0.60 A cm-2 at 750 ° C.

7.2. All-Ceramic Anodes

A number of all-cera

electrodes have shown


performance in SOFCs or SOECs.258,259,287,288 As mentioned

before, it is expected that an oxide is less prone to adsorb sulfur than a metal. The classic perovskite SrTiO3 can be doped both in the A and B sites or even have A site deficiency.278 Some work has been performed on Y-doped SrTiO3 doped with Ru and CeO2, showing a limited tolerance to H2S (up to 40 ppm) and especially reversibility when the H2S stream is removed.289 The Sr0.6La0.4TiO3/YSZ (50/50 wt %) anode showed no degradation in the presence of up to 5000 ppm of H2S in a hydrogen fuel,290 and it has even been suggested that the presence of H2S can promote the oxidation of methane.291-293 In general, it seems that perovskites are stable toward operation in sulfur, with many examples being reported, including double

perovskites242 and lithium-ion conducting perovskites.294

It is generally recognized that perovskite-based materials lack the catalytic activity of nickel. As discussed in section 6.7.2, one method to improve the catalytic activity has been to dope the perovskite with transition metals which then exsolve out as catalytically active nanoparticles on reduction. Little work has been done on the tolerance of these nanoparticles toward sulfur, but one study showed that Fe nanoparticles ex-solved out of Sr2Fe15Mo0 5O6 forms FeS under 50 ppm of H2S in H2, with a decline in activity of around 20% from around 0.1 to around 0.08 W/cm2 at 600 °C over a period of 46 h, followed

by stable operation for a further 200 h.295

It has also been reported that the presence of H2S improves the performance of the fuel cell when methane is used as the fuel for La04.Sr0.6Ba0.1TiO3-d,291,2M but the oxidation of H2S to SO2 does not seem to be the main reaction as suggested previously269 in SOFCs but rather a gas reaction with methane and potentially an increase in the conductivity of the perovskite by some as yet unclear mechanism.296 Although doped SrTiO3

has been independently shown to be stable289,291 in H2S, the high concentrations used need to be independently confirmed.

Barium-based perovskites have also shown promise for sulfur tolerance. For instance, Kan et al. prepared the proton-conducting Ba3CaNb2O9 doped with Mn, Fe, and Co and checked the stability of these materials toward H2S.297 They used a 5000 ppm of H2S/H2 stream to evaluate whether the investigated samples can be used as electrodes in contaminated fuels (e.g., natural gas with ppm levels of H2S). Their XRD study indicated that the samples preserved the double-perovskite structure at 600 °C for 12 h. No secondary phase was detected due to the formation of sulfides such as MnSx, FeSx, or CoSx. The SEM study also confirmed that the particle sizes and shapes did not change after H2S treatment. This result suggests that their materials are physically and chemically stable in the SOFC working environments. The same group reported enhanced stability of perovskite-type BaZr01Ce0.7Y0.1M0.1O3-$

H2+0^iti20+2e H2+6^20+2e-

Figure 19. Possible mechanisms of degradation by sulfur poisoning. Reprinted with permission from ref 72. Copyright 2011 Elsevier.

Figure 20. Schematics of the most common material strategies to improve sulfur tolerance. The diagram shows a strategy and does not imply a specific mechanism of desulfurization.

(M = Fe, Mn, and Co) with a substantial chemical stability in

30 ppm of H2S/H2 at elevated temperature during 24 h.

Another Ba-based perovskite prepared by Yang et al. seems to be a promising material.276 In a very complete paper, they reported outstanding sulfur and coking resistance of a barium zirconate-cerate codoped with Y and Yb (BaZr0.1Ce0.7Y02-xYbxO3-5) anode. The terminal voltages of the same cells (with BZCYYb and CSO as electrolyte) at 750 °C were recorded as a function of time when the fuel was contaminated with different concentrations of H2S. The Ni/ BZCYYb anodes for both cells showed no observable change in power output as the fuel was switched from clean hydrogen to hydrogen contaminated with 10, 20, or 30 ppm of H2S. XRD data corroborated the chemical stability of the designed anodes. A study on Ni/BZCY anodes featured even higher levels of H2S (up to 1000 ppm), used Electrochemical Impedance Spectroscopy (EIS) to show that as well as reducing the anode polarization losses compared to Ni/CSO, the BZCY-based anodes showed little increase in Ohmic losses even at 200 ppm of H2S, while the Ni/CSO cell showed severe increases in Ohmic resistance at 100 ppm of H2S (Figure 22). Post-test

Figure 21. Sulfur tolerance test for a CMO-impregnated cell under a current density of 0.60 A cm-2 at 750 °C using H2 and H2 with 50 ppm of H2S as the fuel, respectively. Reprinted with permission from ref 286. Copyright 2012 Elsevier.

EDX showed large decreases in Ni content in the Ni/CSO anode, which were not seen in the Ni/BZCY anode. This indicates that these materials may be hindering restructuring of the Ni at high sulfur levels.

The role of barium in the improved tolerance may be related to the reduction of sulfur chemisorption on nickel. Da Silva and Heck have calculated that the incorporation of oxides, in particular BaO, reduces the sulfur chemisorption on Ni by minimizing the sulfur chemical potential and favoring the formation of BaS. This sulfide can be reconverted to BaO in the presence of water and additional BaO, leading to an in situ regeneration.299 It was also predicted that the addition of BaO enables the anode to tolerate 100 ppm in humidified H2.

Figure 22. i-V, i-P curves, and EIS for the fuel cells with the (a and c) Ni+SDC and (b and d) Ni+BZCY anodes, operating on different fuels at 600 °C. Reproduced from ref 298. Copyright 2014 American Chemical Society.

7.3. Alloying of Nickel with Other Metals

The incorporation of additives or secondary phases has been known in metallurgy for many years. The extraction of nickel (or cobalt) metal from ores involve roasting or oxidation of the sulfide to the oxide followed by in situ reduction with CO. It should be noted that all the key elements necessary for oxidation and elimination of sulfur used in metallurgical processes are present in a fuel cell anode and the analysis of roasting may provide the key to achieve tolerance to sulfur in SOFC. In roasting, the sulfide minerals are treated with very hot air, and the sulfide is converted to an oxide while sulfur is released as sulfur dioxide, typical examples being ZnS, FeS2, PbS2, and Cu2S. Roasting is usually carried out between 500 and 1000 °C,300 the same range of operation of SOFC. Improvement of the roasting process is achieved by adding pyrite (FeS) with the highest rest potential among sulfide minerals, therefore acting as a cathode which accelerates the oxidation of the other sulfides. Finally, reduction with CO leads to the formation of the metal, although a few metals can be obtained directly by oxidation of their sulfides since their oxide is less stable than SO2, well-known examples being: Cu, Ag, and Hg.

Finally, it is worth mentioning the idea of decomposing H2S into hydrogen and sulfur, both valuable products; several routes have been explored in the past.301 The most straightforward suggestion of relevance to SOFC is that H2S be decomposed thermally according to

2H2S ^ 2H2 + 1/4S8, AH = 79.5 kJ/mol

This decomposition has been performed in the presence of MoS2 between 500 and 800 °C, and more recently, it has been reported H2S can be decomposed in the 700-1000 °C temperature range using the perovskite oxide LaSr0.5Mo0.5O3.303 The presence of Mo and a possible decomposition of H2S may be behind the activity and the reported stability of Sr2Mg1-xMnxMoO6-$ in these complex

perovskite anodes.238,304 Mo-containing catalysts are commonly

used in hydrodesulfurization processes.305 The interaction of molybdenum with sulfur can be modified with the presence of a second metal with direct consequences for the hydro-desulfurisation properties. In particular, the synergistic effect of Ni-Mo bonding has been proved to be active toward the hydrodesulfurization.306 In contrast, the effects of Zn, Cu, and Fe on the Mo-S interactions and hydrodesulfurization activity are less pronounced. The Ni-Mo and Ni-S-Mo interactions increase the electron density on Mo, making it more chemically active in two key steps for the reactions: the adsorption of S-containing molecules and the dissociation of H. It is therefore not unthinkable that molybdenum plays a key role in the sulfur tolerance in anodes containing this metal.

Most studies within the literature regarding the tolerance to sulfur report the effect of sulfur poisoning on the electrochemical properties as it is the most direct way of in situ degradation analysis. Therefore, cell voltage changes, power output, and area specific resistances are commonly used to describe the changes to the anode upon a modification. Comparison between the different reports is difficult, and therefore, we shall provide the overall change observed in the

Table 5. Selected Papers Reporting Improved Sulfur Tolerance in SOFC Anodes through Materials Strategies

cell modification

Ni-CGO in YSZ scaffold anode

ScSZ electrolyte (La0.6Sr0.+)0.99CoO3_s cathode ferritic (FeCr) stainless steel support

high porosity (not quantified) low porosity (not quantified)

sulfur tolerance and figures of merit

two stage degradation with area-specific resistance of 0.35 Q cm at 650 °C, 0.25

A cm-2; full regeneration possible two stage degradation area specific resistance of 0.70 Q cm2 at 650 °C, 0.25 A cm~ full regeneration possible

ref 307

n' Si.

Ni/8YSZ and Ni/CGO anodes

3YSZ electrolyte support LSM cathode

Ni/CGO anode

Ni/8YSZ anode

two stage poisoning; stack voltage decrease only to 98.7% of initial value upon 308

addition of 2 ppm of H2S to fuel at 850 °C, 0.225 A cm~2 after 15 h; fuel mixture: 43.8% H2, 6.2% H20, and 50% N2 one stage poisoning; stack voltage decrease to 86.5% of initial value upon addition of 2 ppm of H2S to fuel at 850 °C, 0.319 A cm"2 after 15 h; fuel mixture: 43.8% H2, 6.2% H,0, and 50% N,

Ni/YSZ and Ni/CGO anodes

YSZ or CGO electrolyte support Pt cathode

Ni/CGO anode Ni/YSZ anode

polarization resistance of anode is 4.3 Q cm2 in 700 ppm of H2S in H2, 200 mA/cm2 at

800 °C after 2 h; regeneration possible polarization resistance of anode is 1.2 Q cm2 in 700 ppm of H2S in H2, 200 mA/cm2 at 800 °C after 2 h

Ni/CGO anode support CGO electrolyte

NdBa075Ca02t,Co2O5+^CGO cathode

Ni/CGO+ BaCe09Yb0103_(5 Ni/CGO

cell voltage 0.74 V in pure H2 at 650 °C with 640 mA/cm2 goes immediately to 0.7 V stable over 20 h upon introduction of 500 ppm of H2S; full regeneration possible cell voltage 0.63 V in pure H2 at 650 °C with 640 mA/cm2 goes immediately to 0.61 V and decreases continuously for 6 h; regeneration not possible

Ni^CoyYSZ anodes

YSZ electrolyte support LSM cathode

Ni„.«Coa310/YSZ Ni/YSZ

current exchange density 0.024 A/cm2 in pure H2 improving to 0.094 A/cm2 in 10%

H2S in CH4 after 15 h at 850 °C current exchange density 0.018 A/cm2 in pure H2 improving to 0.032 A/cm2 in 10% H,S in CH4 after 15 h at 850 °C

Ni/YSZ or Sr1_vCevCo02Fe08O3_(5 anodes

ScSZ electrolyte support LSM cathode



current density is 0.088 A/cm2 in H2 at 0.9 V, lowered to 88% of initial value upon

addition of 20 ppm of H2S at 800 °C after 500 min at constant 0.6 V current density is 0.080 A/cm2 in H2 at 0.9 V; immediate drop in current; after 500 min, current lowered to 81% initial normalized value in 50 ppm of H2S at 800 °C at constant 0.6 V

Ni/YSZ anode

YSZ electrolyte support

CSO/LSCF cathode

Ni/YSZ + infiltrated BaZr01Ce0 7Y01Yb01O3_^ (BZCYYb) cell voltage remains above 0.72 V even upon addition of 30 ppm of H2S to pure H2, at 311

700 °C, 0.054 A/cm2; slow degradation


cell voltage from 0.72 V in H2 to 0.62 V to 20 ppm of H2S in H2 within a few minutes at 700 °C, 0.054 A/cm2

Ni/ScSZ anode support ScSZ electrolyte

LSM cathode

Ni/ScSZ anodes ScSZ electrolyte

Ni/YSZ annode YSZ electrolyte

two stage poisoning mechanisms, one fast, reversible and one slow and irreversible; poisoning of methane reforming cell voltage 0.7 V in 2 ppm of H2S, 13% H2, 58% H20, 29% CH4, 850 °C, 1 A/cm2 after 500 h cell voltage 0.45 V in 2 ppm of H2S, 13% H2, 58% H20, 29% CH4, 850 °C, 1 A/cm2 after 500 h two stage poisoning mechanisms; poisoning of methane reforming

Ni/ScSZ anode

Ni/ScSZ anode

cell voltage: 0.55 V at 200 mA/cm2, 800 °C, 100 ppm of H2S in H2 after 1000 s with stable performance

Table 5. continued

cell modification

YSZ electrolyte support Ni/YSZ anode LSM cathode

Ni/YSZ anode Ni/YSZ anode

ScSZ electrolyte

ScSZ electrolyte Ni/YSZ anode

ScSZ electrolyte

LSM cathode

Ni/YSZ anode-support ceria-modified


YSZ electrolyte Ni/YSZ anode LSM cathode

Ni/BaZr04Ce04Y0 2O3_l5 (BZCY) and Ni/Sm0 2Ce0 8OL9 (CSO) anode support Ni/BZCY

CSO electrolyte Ni/CSO Ba0 sSr0 sCo„ sFe0 203_,5 (BSCF), and Sm0 sSr„ 5Co03_s (SSC) cathodes

Ni/YSZ anode support bimetallic coating Ni-Cu/Co/Fe on anode

YSZ electrolyte no extra coating LSCF-GDC cathode

NiSn/YSZ anode support infiltrated NiSn + reformer NiSn/Al203

YSZ electrolyte no infiltration and without reformer LSM-YSZ cathode

sulfur tolerance and figures of merit ref —

cell voltage: 0.18 V 200 mA/cm2, 800 °C, 20 ppm of H2S in H2 after 1000 s; null voltage after 1650 s, two stage poisoning: initial one is fast and reversible, second is ^

slow and irreversible ^

voltage drop 0.12 V, 200 mA/cm2 in 5 ppm of H2S in H2 at 850 °C 62

voltage drop 0.52 V, 200 mA/cm2 in 5 ppm of H2S in H2 at 850 °C

cell voltage = 0.6 V at 0.3 A/cm2 in H2 + 200 ppm of H2S at 700 °C 313

cell voltage =0.4 V at 0.3 A/cm2, 700 °C in H2 + 200 ppm of H2S

OCV = 1.01 V in pure H2, stable 148 mW/cm2 in 100 ppm of H2S at 200 mA/cm2, 298 600 °C for 700 min

OCV = 0.709 V in pure H2 From 137 mW/cm2 to 81 mW/cm2 in 100 ppm of H2S at 200 mA/cm2, 600 °C for 150 min

peak power densities ~1.4 W/cm2 in H2; decreases to 1.0 W/cm2 in 500 ppm of H ,S- 141

H2, enhanced dry reforming of methane peak power densities ~1.4 W/cm2 in H2

cell voltage decreases from 0.72 V in pure H2 to 0.63 V on addition of 500 ppm of H2S 314

at at 850 °C and 1.25 A/cm2; complete regeneration cell voltage decreases continuously from 0.5 to 0.45 V in 48 h at 1.25 A/cm2, 850 °C, 200 ppm in C02:CH4

very same paper when the anodes are modified with the intention to improve sulfur tolerance. Table 5 presents results from selected papers where there has been a variation in the anode with the intention to improve the tolerance to sulfur.


The problems faced by fuel cell anodes regarding carbon and sulfur poisoning are similar in many ways to those faced by conventional catalysts. In fact, in one very important respect carbon and sulfur tolerance is more challenging in conventional catalysis: there is no equivalent of the oxygen flux through the electrolyte which occurs in SOFCs, which tends to reduce the problems with carbon and sulfur. Because of this, it is instructive to look at solutions for tolerant catalysts. This issue has been studied over a much longer period of time, and more intensively, than for SOFCs, and many of the findings have not yet been incorporated into SOFC research. 8.1. Carbon Tolerance in Conventional Catalysis

Several strategies have been studied for minimizing the carbon deposition in catalyst used in reactions involving hydrocarbons, such as steam reforming, dry reforming, partial oxidation, or water gas shift. The use of noble metals, instead of Ni, as the active phase is the best option in terms of carbon resistance. However, similarly to SOFCs, the high cost and low availability of noble metals mean that Ni-based catalysts are favored, and strategies for minimizing carbon deposition in these catalysts

have been developed in the last decades.16,315

8.1.1. Sulfur Passivation. Sulfur passivation was one of the first strategies developed to diminish carbon deposition. The first published works, in steam and dry reforming of methane, appeared in the mid-80s.18,:316,317 The approach consists of partially passivating the active centers of Ni catalysts with sulfur, normally using H2S.18,316-319 Lately, the use of alkanethiols for the passivation has also been proposed with promising

i, 320,321


Hydrogen sulfide chemisorbs on the nickel surface and blocks access to the catalytic centers. This blockage decreases the carbon deposition rate more than the methane reforming rate.316 At complete coverage, carbon atoms cannot be dissolved into the nickel crystal and the whisker growth mechanism is inhibited. However, the complete coverage of the nickel surface with sulfur results in total deactivation. With the use of coverage ratios of around 0.7, it is possible to diminish carbon deposition without compromising reforming activity.316-318 At this coverage ratio, it is not possible to inhibit carbon formation. Nevertheless, the usual whisker structure is replaced by more amorphous structures, which are less deactivating.316,317,319 Hence, at this coverage, the reaction still takes place. This is due to the number of active nickel surface sites needed for each process. Carbon nucleation needs larger sites, which are almost completely blocked at high coverage rates, whereas methane reforming reactions can

proceed in the smaller sites which are still available.18,316-319 However, reforming of larger molecules, like toluene and tars, requires large active sites, so the sulfur passivation can deactivate reforming reactions as well.318 This technology, in the case of the dry reforming of CH4, has been industrially implemented by Haldor Tops0e in the SPARG process.21,32,322

Sulfur-passivated catalysts have been also applied to the dehydrogenation of isobutane, with sulfur passivation improving both the selectivity of the process and inhibiting carbon


8.1.2. Alloying and Bimetallic Systems. The introduction of additional metals that can modify the ability of carbon to assemble or to dissolve in the bulk metal of the catalyst can drastically reduce the potential for carbon deposition. A vast number of bimetallic combinations can be found in the literature. Focusing only on Ni catalysts, bimetallic systems like

Ni-Co,324-328 Ni-Fe,324 Ni-Cu,324,325,329-332 Ni-Mn,324 Ni-Sn,167,325,333,334 and Ni-NM (being NM a noble metal: Rh,

Pt, Pd, Ir, Ru, Au, and Ag)9,49,325,3 ,335-341 have been studied, showing in some cases very promising results.

When one or more other metals are introduced into the system, different structures can be originated depending on the metals' properties, interactions with the support, atmosphere,

temperature, etc.325,336 A schematic representation of these

structures can be seen in Figure 23. Among these structures,

Figure 23. Possible structures shown by bimetallic nanoparticles: (a) core-shell, (b) heterostructure, (c) nanoalloy, (d) segregation, and (e) ensembles. Adapted with permission from ref 325. Copyright 2012 Elsevier.

interest in alloys is increasing. The use of alloys of different metals as the active phase has been deeply developed in recent years. This is due to the superior performance of alloys in terms of conversion and resistance to carbon deposition. ,

The interaction between two or more metals can give rise to geometric and electronic effects, which could affect carbon deposition.325 The geometric effect is the result of the dilution of the atoms of one metal in the other. Thus, surface ensembles are reduced in size. This can dramatically affect the catalyst performance, since many reactions depend on the size of the ensembles, as was explained for the partial sulfur passiva-

tion.325,341 The electronic effect is the result of the difference in

electronic affinity between the metals that can produce an electronic density increase or a decrease in the main metal, depending on whether the secondary metal has a lower or higher electronic affinity. These modifications in the electronic density alter phenomena such as adsorption or desorption of species during the reaction process, affecting activity and


Noble metals are well-known to be more resistant to carbon deposition than Ni, as well as possessing other features such as improved catalytic activity, suppression of Ni oxidation, or sustainability in daily start—stop operations.324,325,340,343 Among the noble metals, the most common used in bimetallic systems with Ni is Rh. In this type of catalyst, Rh atoms enrich the Ni surface, forming a surface alloy Ni-Rh, rather than dissolving into Ni particles and forming a bulk alloy. However, the formation of the alloy strongly depends on the support used

and its interaction with the metallic particles.324,340 In addition,

preparation conditions can also affect the carbon resistance of the bimetallic system. Thus, if the catalysts are calcined in oxygen at high temperatures, metal segregation can occur, giving rise to lower carbon resistance.339 The presence of Rh increases the energy barriers of C diffusion and C-C bond formation, whereas the O diffusion and C-O bond formation are not significantly affected. As a consequence, the global rate

of carbon deposition is decreased.49,325 Similar behavior has

been found in the case of the Ni-Pt systems.325,336 The presence of Pt has been found to promote the formation of small NiO crystals, which facilitates the reduction to Ni0 and improves Ni dispersion.340 The versatility of Ni-Pt system allows the creation of different surface structures (core-shell, monolayers, and alloys) that need to be controlled to minimize

carbon deposition.336,340

Although less studied, Au and Ag have given rise to interesting results in terms of carbon resistance. , Particularly in the case of the Ni-Au alloys, it has been found that the presence of a small amount of gold on a supported nickel catalyst can induce a significant effect on the carbon formation process during the steam reforming of methane.5 Au makes the diffusion of the CHx species (intermediates in carbon growth) significantly difficult, preventing carbon nucleation.330 In the dry reforming of methane, the presence of gold promotes the formation of carbonaceous species which have high reactivity with CO2, thus facilitating gasification.340

However, the elevated cost of noble metals makes it more practical from the industr-ial point of view to develop noble metal-free catalysts. , - , Ni-Co might be a more affordable option. Ni-Co bimetallic catalysts show a synergetic effect that makes the catalyst more active and resistant to carbon deposition than Ni and Co monometallic cata-

lysts.324,326,327 Ni and Co benefit from the electronic effects

that appear in bimetallic systems. They present different oxidation states depending on whether they are used in monometallic or in bimetallic catalysts, indicating an electronic transfer between Ni and Co in the bimetallic catalyst.324 This protects the metal from oxidation during the reaction and confirms the near-distance interaction between the two metal atoms, making it easier to form Ni-Co alloy on the catalyst surface.324 In addition to the synergetic effect, the formation of various spinel-type solid solutions with the supports improves the metal-support interaction and therefore the carbon

resistance.324,327 Cu-Ni system stability has been found to be dependent on temperature and Cu/Ni ratio.329,331 Copper

seems to stabilize the structure of the active site on the Ni surface, thus preventing sintering or loss of nickel crystallites. Adding Cu into the Ni catalyst system can fine-tune the catalytic activity, so that carbon formation and removal can be balanced, preventing deactivation by carbon accumula' ^ 1—1 Auraira

K 22,29,346—350 ^g 15,17,22,346,351—359 ca 346,351,360,361 ^22,26,27 25,326,335,337,362—369 29,348 anj ^29,348,370 '

tion.329,330,332 However, an excessive load of Cu could g

rise to a Cu-rich alloy that can increase carbon deposition.

Sn/Ni alloys seem to be the most promising alternative, not only for their high resistance to carbon deposition but also for

the low price of Sn compared to noble metals.167,325,333,334 Sn/

Ni alloys have shown huge potential for improving the carbon resistance in steam reforming processes by modifying the relative kinetics of C-O and C-C bond formation. Once again, in this case, the formation of the surface alloy is favored over the bulk alloy, especially at low Sn loadings.167,333 DFT studies showed that the presence of Sn, which is mainly located at the surface of the alloy particles, imposes an important barrier to carbon diffusion in Ni crystallites, thus hindering carbides formation and the subsequent nucleation of carbon.167,334 These theoretical results were confirmed in steam reforming of

various hydrocarbons.167,334

8.1.3. Promoters. The addition of different promoters can affect the interaction between the metal and the support or the acid-basic nature of the support, which modifies its tendency to give rise to carbon deposition.2^29 The use of several promoters

can be found in the literature, including Li,22 Na,345,346

Alkali (Li, Na, and K)



or alkali earths (Mg and

are usually introduced in catalyst for-

mulations with the aim of accelerating carbon removal from the catalyst surface due to their basic nature.371 K2O can reduce the carbon deposition rate in reforming processes, but occasionally it also compromises the catalytic activity.22,29,347-35° This reduction of the carbon deposition is a consequence of an improvement of the gasification rate of the carbon deposits, thus improving the stability of the catalyst. However, the interaction of K with Ni gives rise to large NiO crystalline particles. This species has high mobility, thus promoting aggregation of particles and decreasing the activity, while larger

Ni particles also promote carbon deposition.21-24,31 K2O has

also been used as a promoter in the Ni catalysts for the watergas shift reaction345 and in bimetallic Ni-Mo catalysts for the dry reforming of propane,372 presenting in these cases improvements both in carbon resistance and catalytic activity. Similar behavior to K, although less pronounced has been

U J • U. r, Ji-r-v 346,351,360,361,373-375 T

observed in the cases of Na2O and CaO. In

the case of CaO, some researchers have suggested that while the amount of carbon deposits increases, the reactivity of the deposits also increases, leading to a higher stability of the


The use of Mg has also been shown as an interesting option for minimizing carbon deposition, although in this case the preparation of the catalyst and the interaction of Mg with the

support play a critical role.17,22,345,352,354-356,358,359 If Mg is

used as a promoter of Ni/Al2O3, it interacts with Ni, leading to a NiO-MgO solid solution, but when Mg is used as a dopant of the Al2O3 support, it can react to produce a MgAl2O4 spinel. In both cases, Ni sintering is prevented and the carbon deposition is reduced whereas the catalytic activity increases, but the effect achieved by the spinel has been shown to be quantitatively better than that from the solid solution.

Mn is used as a promoter, especially in dry reforming of methane, to reduce carbon deposition, both in Ni- and Co-based catalysts.364,365,375-378 MnOx forms patches that partially cover the Ni surface giving a similar effect to that from sulfur

passivation.375-377 Moreover, Ni dispersion is improved,376,378

and the moderate basicity of the MnOx improves CO2 adsorption and increases carbon gasification rate by forming

reactive carbonate species.376,377

Lanthanide oxides have also been thoroughly studied, with

La and Ce oxides the most promising promoters.379-381 La2O3

has been found to affect positively both activity and resistance

to carbon deposition.22,26,27 Two different effects are promoted

by the presence of La2O3. On one hand, its basicity promotes the absorption of CO2, giving rise to lanthanum oxy-


carbonates. These species play a role in conserving

the stability of the catalyst, since they promote the CO2 decomposition to CO and O, which can increase the carbon gasification rate.22,27 On the other hand, the interaction between La and Ni forms a mixed oxide (NiLa2O4) in the same way as happens with Al2O3. This phase prevents the sintering of Ni particles, thus reducing carbon deposition.27,382 It has also been found that the performance can be improved by the addition of alkaline oxides384 or other lanthanides like Ce or

Pr.383,385 In the

case of the combination of La-Ce, it has been found that after reduction of the catalysts, particles of a mixed oxide appear on top of nickel particles. This decoration of Ni particles reduces the ensemble of Ni similarly as in the

case of sulfur passivation, thus lowering the probability of the nucleation of carbon precursors.383

CeO2 constitutes another interesting promoter for minimizing and even suppressing carbon deposition.29,348 It has been

used as a support as well, but the conversions were lower probably due to the strong metal—support interaction. However, the results showed that its use as a promoter is much better, giving rise to high conversions and resistance to carbon deposition.370,386 It should be noted that the amount of

CeO2 used as a promoter should not exceed a certain value to

avoid compromising the catalytic activity.338,387 The high

resistance to carbon deposition comes from the oxygen storage capacity and oxygen mobility that ceria presents.192,388 CeO2 can store and release reversibly a large amount of oxygen, thus increasing its availability for gasifying the carbon depos-

its.29,348,353 As discussed in section 6.6, CeO2 exhibits excellent

redox properties with a Ce3+-Ce4+ equilibrium and the coexistence of CeO2 and Ce3O4.192,353,388 This behavior can influence the oxidation state of atoms on the surface of the active metal particles (for example Rh0/Rhs+), favoring the

activation of reacting molecules.338,386,389 Other features that

can improve catalyst performance are that CeO2 gives rise to a better dispersion of the active phase, enhancing the catalyst performance and inhibiting the transition of the Y-Al2O3 used as support to the low-surface-area a-Al2O3 at high temperatures.387 When CeO2 is used as a dopant of Al2O3, CeAlO3 species are formed. This species completely inhibits the growth of filamentous carbon, although amorphous carbon is still deposited (Figure 24). , , It has been suggested that

Figure 24. Carbon deposition models in the steam reforming of hydrocarbons over Ni/Al2O3 and Ni-Ce/Al2O3 catalyst: (a) not doped and (b) doped with Ce. Adapted with permission from ref 380. Copyright 2005 Elsevier.

these species suppress the growth of this filamentous carbon by chemical blocking rather than by gasifying them after they have been deposited in the catalyst.387

Finally, ZrO2 is also able to enhance carbon deposition

• . 25,335,362,364—366,368 r^,. «.I.

resistance. This promoter enhances the

dissociation of CO2, forming oxygen intermediates near the contact between ZrO2 and Ni. These intermediates increase the rate of gasification of the carbon deposits.362 In addition, ZrO2 has both basic and weak acidic sites, which improves its

resistance to carbon deposition.326,367 However, the main

interest in ZrO2 seems to be its use in combination with CeO2, since ZrO2 enhances the oxygen storage capacity and oxygen ion mobility of CeO2. This enhancement of CeO2 properties results in an improvement of the resistance against carbon



8.1.4. Regeneration of Catalysts Deactivated by Carbon Deposition. Under certain conditions, catalyst

deactivation due to carbon deposition can be inevitable even in the most resistant catalysts. For this reason, the regeneration of the catalysts is extremely important to maximize the benefit

obtained from them.9,13 Catalysts deactivated by carbon deposition can be regenerated using different gasifying agents (in order of gasification rate): oxygen, steam, carbon dioxide, or hydrogen. The reactions involved in the regeneration processes with these gasifying agents are shown below.9,13,391—393

C + O2

C + H2O ^ H2 + CO

C + CO2

C + 2H2

As discussed above, different types of carbon can deposit on the catalyst surface.13 Thus, they will behave differently during the regeneration. The carbons formed on Ni catalysts involved in reactions with hydrocarbons can be monatomic carbon, polymeric amorphous films, vermicular fibers or whiskers, nickel carbide, and graphitic films.9,13 Both preparation of the

catalyst (metal loading, calcination temperature, particle size, and use of promoters) and reaction conditions (temperature, H/C ratio, and presence of carbon precursors) can affect the type and amount of carbon deposited.

Due to the differences in reactivity between the different types of carbon deposits, different conditions should be applied. Thus, less ordered and more reactive carbons (monatomic carbon or amorphous) need lower temperatures and weak gasifying agents (about 400 °C in H2 or H2O), whereas graphitic carbon needs higher temperatures and strong gasifying agents (700—900 °C in air).9,392 However, as O2 is the strongest and cheapest gasifying agent, in industry catalysts are usually regenerated in air at about 600 °C.9

Although the catalytic activity can be recovered almost completely under certain conditions, catalysts lose activity after

each recovery cycle due to different reasons.13,394 For example,

regeneration in air is a very exothermic process that can lead to hot spots. These hot spots can lead to metal reorganization or sintering, thus deactivating the catalysts in the attempt of recovering the catalytic activity lost due to carbon deposi-

tion.9,13,392 The main reasons for losing catalytic activity during

regeneration are (1) loss of metal particles that were pulled out from the support due to the formation of carbon filaments,9 (2) oxidation of the metals;9,13,26,390,395 Although it can be reversed by subsequent reduction of the catalyst, it sometimes gives rise to the irreversible formation of different structures that can be

13,396—398 9,26,393,398,399

inactive, (3) sintering , and (4) damage of


the support.

However, in the same way that the addition of promoters or the formation of alloys can enhance catalytic activity and resistance against carbon deposition, the regeneration can be

positively affected by the presence of these promoters27,361 and


alloys. Thus, the presence of small amounts of

noble metals can improve reducibility of the main

metal.324,325,340 Moreover, in some cases, after a few

regeneration cycles the performance of the catalyst can be enhanced.325 These strategies for improving carbon resistance can help to facilitate carbon removal during regeneration of the spent catalyst. Table 6 summarizes the different strategies and catalyst modifications used to produce carbon tolerant catalysts.

Table 6. Strategies to Minimize Carbon Deposition in Catalysts

strategy-sulfur passivation

bimetallic catalysts



Ni(S)/Al2O3 Ni(S)/MgAl2O4



























Ni-I^O/A^ Ni/La2O3-Al2O3

Ni/CeO2-ZrO2-Al2O3 Ni/CeO2-La2O3-Al2O3 Ni-Li2O/Al2O3 Ni-MgO/Al2O3

Ni-MnO/Al2O3 Ni-Na2O/Al2O3 Ni/ZrO2-Al2O3 Ni/CeO2-ZrO2

Ni-CaO/La2O3 Ni-SrO/La2O3

Ni-MgO/SiO2 Ni-MnO/SiO2 Ni-ZrO2/SiO2 Ni-K-Ca/NaZSM-5


steam reforming of CH4 CO2 reforming of CH4 CO2 reforming of CH4 dehydrogenation of isobutane

steam reforming of n-butane steam reforming of glycerol CO2 reforming of CH4 CO2 reforming of CH4 CO2 reforming of CH4 CO2 reforming of CH4 CO2 reforming of CH4 steam reforming of ethanol steam reforming of ethanol steam reforming of methanol steam reforming of ethanol CO2 reforming of CH4 steam reforming of gasoline CO2 reforming of propane Steam reforming of gasoline CO2 reforming of CH4 steam reforming of gasoline CO2 reforming of CH4 CO2 reforming of CH4 steam reforming of ethanol methanation of carbon dioxide steam reforming of methane steam reforming of propane steam reforming of isooctane steam reforming of ethanol oxidative steam-reforming of ethanol oxidative steam-reforming of ethanol

water gas shift

CO2 reforming of CH4

CO2 reforming of CH4

steam gasification of polypropylene

steam reforming of propane

oxidative reforming of hexadecane

CO2 reforming of CH4

CO2 reforming of CH4

steam reforming of ethanol

steam reforming of propane

steam reforming of CH4

oxidative reforming of hexadecane

CO2 reforming of CH4

CO2 reforming of CH4

steam reforming of ethanol

steam gasification of polypropylene

CO2 reforming of CH4

CO2 reforming of CH4

CO2 reforming of CH4

methanation of carbon dioxide

autothermal reforming of isooctane

CO2 reforming of CH4

CO2 reforming of CH4

Partial oxidation of CH4

CO2 reforming of CH4

CO2 reforming of CH4

CO2 reforming of CH4

CO2 reforming of CH4

320 and 321

328 and 393

324 and 327 326

329 358

332 332 324 344 372 344

344 339

49 and 338 335

167,333, and 334

167,333, and 334

167,333, and 334

346,351,360, and 374 348,353,370,387, and 389 357

380 383

22, and 346-350 22 and 27 401 380

369 and 397

383 22

22 and 346 351-355

348,365,376, and 377

346 362

337 and 363 390

384 384 367

359 378 378 361

Table 6. continued

strategy catalysts process ref

Ni-MgO/zeolite HY CO2 reforming of CH4 375

Ni-MnO/zeolite HY CO2 reforming of CH4 375

Ni-Co/CeO2-Al2O3 CO2 reforming of CH4 379

Ni-Co/MgAl2O4 CO2 reforming of CH4 324 and 327

Ni-Co/MgO-ZrO2 CO2 reforming of CH4 326

Ni-Cu-MgO/SiO2 steam reforming of ethanol 358

Ni-Cu-CaO/SiO2 steam reforming of ethanol 358

Ni-K/CeO2-Al2O3 CO2 reforming of CH4 29

Ni-Mo-K2O/Al2O3 CO2 reforming of propane 372

Ni-Rh/CeO2-Al2O3 CO2 reforming of CH4 49 and 338

Ni-Rh/CeO2-ZrO2 methanation of carbon dioxide 337

steam reforming of ethanol 335

Co/CeO2-ZrO2 CO2 reforming of CH4 364

Co-Rh/CeO2-ZrO2 steam reforming of ethanol 335

Pt/CeO2-Al2O3 oxidative reforming of hexadecane 383

Pt/ZrO2-Al2O3 CO2 reforming of CH4 368

partial oxidation of CH4 368

Pt/CeO2-ZrO2 partial oxidation of n-tetradecane 366

Pt/CeO2-La2O3-Al2O3 oxidative reforming of hexadecane 383

Rh/CeO2-ZrO2 partial oxidation of n-tetradecane 366

8.2. Strategies against Sulfur Poisoning

Sulfur-containing molecules are frequent impurities in fuels and oil-derived feedstock. These impurities, even at very low concentrations, are responsible for heterogeneous catalyst deactivation. Millions of dollars are lost in chemical and oil

industries as a result of sulfur poisoning.402,403

Generally speaking, two different approaches have been intensively studied to face this problem. The first consists of the sulfur removal from the fuel via hydrodesulfurization and involves a thoughtful catalyst design to achieve high efficiencies (see section 5). In industrial reactors, sulfur is removed to levels below 0.1 ppm by a multiple step process, finishing with

adsorbents normally based on ZnO.404,405 However, a balance

has to be struck among cost, convenience, and effectiveness, and significant savings can be made if higher levels of sulfur can be tolerated. In this sense, the second strategy is to develop sulfur-tolerant catalysts able to operate in sulfur-rich reaction mixtures.78 This second approach is in line with the aim of this review, which is to provide an overview of the current status in carbon- and sulfur-tolerant systems. At this point, a brief reiteration of the fundamental basis of sulfur poisoning given in detail in section 4 may help to understand the developed strategies.

Sulfur poisoning takes place due to sulfidation of the active catalytic species, namely metallic particles and/or metal oxides.13 In the case of a metallic particle (Me) and considering H2S as the source of sulfur, the process could be simplified as follows:

Me0 + H2S ^ MeS + H2

At high T, its effect should decrease because sulfidation is thermodynamically unfavored. However, its kinetics is favored and the result can be different to the expected, depending on the metal used (for example, with Ni AG is not positive even at 1000 °C). Sulfur as a poison causes a multifold effect in the catalytic activity. First, sulfur adsorption physically blocks the catalyst active sites limiting accessibility for the reactants and reducing the probability of reactant molecules encountering each other. Second, by virtue of its strong chemical bond it

electronically modifies the neighbor metal atoms thus modulating their ability to adsorb and/or dissociate reactant molecules.9 In addition, the catalyst surface could be reconstructed due to the strong chemical adsorption. Finally, the presence of strongly bonded sulfur species on the surface of the catalyst hinders the diffusion of both product and reactant species. Figure 25 schematizes the multiple effects caused by sulfur in a metal-supported catalyst.

Figure 25. Simplified representation of the multifold poisoning effect due to sulfur chemisorption (M represents an active metal, A and B represent reactant molecules, and C the reaction product).

In this scenario, catalyst deactivation must be overcome and/ or the poisoned catalysts must be regenerated. It must always be kept in mind that the degree of poisoning depends on the studied reaction, process conditions and the involved catalysts, among other factors. Consequently, a specific catalyst and/or strategy is required for each process. In response to these needs, intensive research has been carried out in the field of heterogeneous catalysis in the last decades generating a wide variety of multicomponent catalysts with different natures and different features aimed at sulfur poison mitigation. Herein, a summary of the most conventional approaches and proposed materials are discussed.

8.2.1. Noble Metal-Based Catalysts. Nickel-based catalysts are still the most preferred materials for reforming reactions due to their good performance, low cost, relatively

simple preparation, and wide availability.406-408 However, apart from the well-known Ni deactivation due to sintering and carbon deposition, this metal is one of the most sensitive active

phases toward sulfur poisoning.409,410 The chemical equilibrium

of sulfidation at 900 °C for Ni is much more favorable

compared to the values obtained for Ru, Pt, Rh, or Co, underlining that Ni is the most sulfur-sensitive metal among the conventional reforming active phases.13

In this sense, the use of noble metals based catalysts is a good choice from an activity and sulfur tolerance point of view, although the cost must be considered.41^412 Mono and bimetallic Pt-based catalysts developed by Farrauto et al. were stable under continuous operation when exposed to sulfur-containing streams in reforming reactions.4 1 Pt/CGO was successfully employed in the steam reforming of isooctane to produce hydrogen demonstrating complete sulfur tolerance.413 In this study, the authors compared the performance of this material with a similar Ni/CGO and a conventional Pt/Al2O3. Only Pt supported on ceria resisted the effect of sulfur. The latter indicates that not only the active phase matters but also the support plays a crucial role in sulfur resistance. Apparently, the Pt atoms in the Pt/CGO are more electron-deficient than Pt atoms in Pt/Al2O3, limiting the interaction with S species.413 However, Pt tolerance toward S poisoning also depends on the considered reaction. For example, in the WGS reaction, many high performance Pt-based catalysts suffer from severe deactivation when exposed to sulfur.9,414-419 Furthermore, this adverse effect seems to be proportional to the amount of sulfur. For example, for a Pt/ZrO2 catalyst, Xue and co-workers reported that the conversion went from 44% in the absence of H2S to 25% (50 ppm of H2S) to 14% (200 ppm of H2S) and

finally 12% when 1000 ppm of H2S were introduced into the

reactant mixture.

Not only the considered reaction but also the nature of the noble metal influences the sulfur tolerance capacity of the catalysts. In other words, not all the noble metals exhibit the same sulfur resistance. For example, Azad et al. and McCoy et al. demonstrated in different papers that Rh is remarkably less sensitive than Pd toward sulfur poisoning.420-422 The combination of metals (Rh-Pd) enhanced the tolerance, conserving high and stable conversion during 12 h of reaction (50 ppm of H2S were used as a sulfur source). The unsuitability of Pd for sulfur tolerance was also shown in the work of Goud et al.25 Their results show the deactivation of a Pd/ZrO2 catalyst on the reforming of hexadecane after a few hours of operation. The inefficiency of Pd was also evidenced in the WGS, this time using ceria as a support and SO2 as a source of

S.423 2

As mentioned above, sulfur poisoning can be envisaged as a steric and an electronic effect. From the electronic point of view, sulfur ligands withdraw electron density from the metals. For instance, the differences among Rh, Pt, and Pd can be explained in terms of electronic effects. Theoretical calculations for model clusters S/M12 (M = Rh, Pt, and Pd) indicate that the tendency of a metal to lose d electrons increases in the following order: Rh < Pt < Pd. , , This agrees well with the relative occupancy of the d shell in the isolated elements: Rh: d8s1 < Pt d9s1 < Pd d10s0. This tendency correlates with the decrease of density of states around the Fermi level for these elements (25% reduction for Rh, 50% for Pt, and approximately 55% for Pd). In accordance with the latter, and strictly considering electronic effects, Pd is the most vulnerable to sulfur poisoning among the three mentioned noble species, in good agreement with the observed behavior in reforming reactions.425

In summary, noble metals may constitute an alternative to mitigate the sulfur poisoning effects in heterogeneous catalysts. Nevertheless, there is no guarantee that these precious metals

will completely tolerate sulfur, and indeed, they frequently fail depending on the reaction conditions and the sulfur concentration. In addition, the nature of the noble metal is a factor to take into account. In this sense, Rh seems to be one of the most promising.

8.2.2. Alloys, Bimetallic, and Promoters. Many efforts have been made aiming to improve the sulfur tolerance capacity of the traditional Ni-based catalysts for reforming reactions.12 The use of promoters and bimetallic combinations (whether alloys or not) have been a frequent strategy in recent years. For example, Xie et al. investigated the behavior of Ni, Rh, and Ni-Rh supported on CeO2-Al2O3 catalysts in the steam reforming of hydrocarbons, introducing sulfur into the reactant mixture.426,427 None of the Ni-containing catalysts was stable to sulfur-laden mixtures, although the Ni-Rh catalyst requires more time before deactivation, over 60 h on-stream. Moreover enhanced carbon deposition due to sulfur was observed, especially for Ni-based materials, but also noble metal combinations, for example in a commercial Pt-Rh/ZrO2 catalyst for the steam reforming of ethanol/gasolines.428 A small amount of sulfur (5 ppm) was enough to deactivate this catalyst after 22 h on stream.

Other Ni-based bimetallic combinations have been tried. For example, Wang et al. carried out screening of catalysts for liquid hydrocarbon reforming using Ni-Mo, Ni-Co, and Ni-Re supported on Al2O3 and introducing 20 ppm of sulfur in the reactant mixture.429 All the bimetallic samples exhibit superior performance to the primary monometallic Ni with Ni-Re/Al2O3 being the most active sample. Indeed, this Ni-Re/Al2O3 sample showed an outstanding performance maintaining hydrocarbon conversions around 90% during a 300 h test in a sulfur-containing stream and at relatively low reforming temperatures (580 °C). A similar positive effect due to the addition of Re in a Ni/zeolite ZSM5 system was reported elsewhere highlighting the ability of Re to mitigate sulfur poisoning.430 In addition, Re can be employed not only as a part of bimetallic systems but also as a promoter of an already active catalyst. For example, Murata et al. developed a very active Ni/Sr/ZrO2 catalyst but with poor sulfur tolerance.431 In order to improve sulfur resistance, a series of dopants were added, including Re, La, Nd, Sm, Ce, Yb, Eu, and Mo. Among all the dopants only La and especially Re enhanced sulfur tolerance. Actually the best sample in this study was Ni-Sr/ZrO2 with 5 wt % Re, which was able to remain stable during 30 h processing a commercial premium gasoline. It can be argued that rhenium seems to be the most promising metal to diminish Ni sulfur poisoning with the extra benefit of enhanced catalytic activity, although the exact mechanism ascribed (sulfur tolerant alloy formation or sacrificial phase) varies between different studies.

Some other traditional bimetallic systems are Ni-Mo and Ni-W. As indicated in the paper of Gonzalez et al., the addition of Mo and W to Ni-based catalysts reduces deactivation in steam reforming.432 The idea is to use Mo as a sacrificial agent given its facility to be sulfidized. In the presence of any sulfur species, Mo would tend to form MoS2 and Ni atoms would not be affected, and so in principle the active sites should be available. Indeed, the electronic interaction between Ni and Mo in the Ni-Mo ensemble increases Mo electron density, easing its interaction with electronegative ligands such as S.78 In other words, Ni promotes the formation of MoS2, and in some particular applications, for example hydrodesulfurization reactions, Ni is considered a promoter while Mo is the metal that carries out the sulfur removal. In reforming reactions, the

classic paper of Bartholomew proved that a Ni-catalyst doped with Mo was more sulfur resistant than the Ni catalyst alone in a feed containing 10 ppm sulfur.433

The combination of an active metal for reforming reactions such as Ni or Pt with Sn is another widely explored alternative.16,434-437 Dumesic et al. obtained very promising results in hydrogen production from biomass reforming using Ni-Sn catalysts.434 In principle, bimetallic Ni-Sn phases were designed to avoid Ni deactivation due to C deposition. As proposed by Trimm, the similar electronic structure of carbon and elements of groups IV and V of the periodic system may favor the interaction of these metals with Ni 3d electrons, thereby reducing the chance of nickel interactions to carbon.16 Further, as explained by Rodriguez and Hrbek, the addition of tin to platinum is a good strategy to prevent sulfur poisoning.78 Tin may act as a site blocker to platinum, avoiding the noble metal interaction with sulfur and improving the stability of the reforming catalysts.78 Tin and platinum form well-defined alloys that are very stable.103 When compared to pure Sn and Pt, these alloys exhibit a lower chemical reactivity toward sulfur-containing species such as SO2, H2S, S2, and thiophene.438,4:39 Figure 26 adapted from Rodriguez's paper underlines the superiority of the Pt-Sn alloy in terms of sulfur uptake compared to the monometallic systems.

0123456789 10

S02 exposure (L)

Figure 26. Total sulfur uptake for the adsorption of SO2 on polycrystalline Sn, Pt(111), and a Sn/Pt(111) alloy. Adapted from ref 78. Copyright 1999 American Chemical Society.

Among the typical site blockers (Cu, Au, Ag, Zn, and Sn) tin is the best choice to promote sulfur tolerance of Pt-based catalysts.78 The electronic perturbations arising from the Pt-Sn bond produce a system which has remarkably low reactivity toward sulfur poisoning.78

Other types of bimetallic systems and alloys involving noble metals have been proposed, aiming to gain sulfur resist-ance.440-442 Bimetallic Pt-Pd and Pt-Ni catalysts were significantly higher sulfur tolerant compared to the monometallic Pt-based catalysts during 50 h of a stability test.440 A commercial catalyst from BASF based on Pt-Rh was also tested for the ATR of JP8.442 The addition of 125 ppm of sulfur in the stream slightly deactivated the catalysts on the first 250 h of operation. A more demanding stability test based on startup/ shutdown operations strongly affected the catalysts' performance with these series of start/stop cycles, the main reason for the catalysts' deactivation.

As mentioned in the previous section, among the noble metals, Pd seems to be the least sulfur-tolerant. Nevertheless, bimetallic combinations also open up a route to improve Pd-based catalysts' sulfur resistance.78 Metal-metal interactions reduce the electron donor capacity of Pd, limiting its tendency to form strong bonds with sulfurlike ligands.443 In particular, Pd-Rh, Pd-Ni, and Pd-Mn may present a good catalytic behavior and be notably less sensitive to the presence of sulfur-containing molecules in the reactant mixtures than pure Pd.78

Briefly, it can be concluded that most of the bimetallic systems proposed in the literature exhibit superior performance (higher catalytic activity and enhanced carbon and sulfur resistance) compared to their individual counterparts. Several reasons account for the positive results obtained with the bimetallic materials: (i) a change in the number of active sites (cooperative effects); (ii) the sacrificial role played by one of the species forming the bimetallic system leaving free and available the second metal; and (iii) an electronic effect coming from the metal-metal interactions resulting in less sensitive materials toward sulfur poisoning (the bimetallic bonding modifies the chemical reactivity of the metal toward sulfur-containing molecules, "ligand effect").

The addition of promoters is an alternative to the bimetallic systems. Special attention has been devoted to alkali metals in this regard. Due to their electropositive behavior, they can easily donate electrons to sulfur ligands, thus shielding the interaction between sulfur species and the actual active phase of the catalyst. Apart from the electronic effect, these types of promoters may act as a site blocker species, physically hindering the arrival of sulfur to the catalytic active center. Ferrandon and co-workers demonstrated that the addition of potassium to a Rh/Al2O3 catalyst in gasoline steam reforming appreciably increased sulfur tolerance.444 They pointed out that sulfur adsorption on the Rh/Al2O3 was limited due to site blockage attributed to K. In turn, they found a drawback: alkali inclusion increased the temperature in the catalyst bed by inhibition of the endothermic steam reforming reaction more than the partial oxidation processes. At the same time, this effect enhanced the sulfur tolerance beyond the initial expectations when K was intended to be a mere sorbent since the stability of sulfide species decreases with temperature.

8.2.3. Support and Structural Modifications. So far all the discussed approaches are focused on the metallic active phase of the catalysts. However, similarly to SOFC anodes, conventional catalysts are composed of metal/oxide mixtures and therefore the role of the support and its behavior toward sulfur poisoning should not be disregarded. Indeed, on the surface of a metal oxide, sulfur can interact with the metal oxygen sites, producing species that have different electronic properties (i.e., sulfides and sulfates) and may be responsible for catalyst deactivation.

In this sense, one of the most widely used strategies to alleviate sulfur poisoning is to select supports with high oxygen mobility.13 It is well established that oxygen mobility mitigates the carbon deposition which can accompany sulfur poison-ing445-450 and presumably helps avoid the formation of inactive metal sulfides. As mentioned in previous sections, ceria is one of the most desirable supports when oxygen mobility is required.45^452 In this way, a rather sulfur-tolerant catalyst was developed by Xue et al. using Pt supported on alumina impregnated with ceria and gadolinia.450 In this report, the Pt/ CGO-alumina catalysts were compared versus a conventional Pt/Al2O3 sample. Only the ceria-based materials resulted in

Table 7. Strategies to Minimize Sulfur Poisoning in Catalysts



bimetallic catalysts







Pd/Gd2O3-ZrO2- CeO2 steam reforming of toluene 421

Ni-Co/MgO- Al2O3 partial oxidation reforming of isooctane 409 Pd-Y2O3/Gd2O3- ZrO2-CeO2 steam reforming of toluene 421

Ni-Co/Al2O3 steam reforming of liquid methylcyclohexane 429 and 432 Pd-CuO/Gd2O3- ZrO2-CeO2 steam reforming of toluene 421

Ni-Mo/Al2O3 steam reforming of liquid methylcyclohexane 429 Rh/Gd2O3-CeO2 steam reforming of toluene 420 and 422

Ni-Re/Al2O3 steam reforming of liquid methylcyclohexane 429 Rh/ZrO2-CeO2 steam reforming of toluene 420 and 422

steam reforming of gasoline 430 Rh-CuO/Gd2O3-CeO2 steam reforming of toluene 420 and 422

Ni-Fe/MgO-Al2O3 partial oxidation reforming of isooctane 409 Rh-CuO/ZrO2- CeO2 2 steam reforming of toluene 420 and 422

Ni-Rh/CeO2-Al2O3 2 steam reforming of liquid hydrocarbons 426 Rh-Pt/Gd2O3-CeO2 steam reforming of toluene 420 and 422

Ni-Sn/MgO-Al2O3 steam reforming of glycerol 407 Rh-Pt/ZrO2-CeO2 steam reforming of toluene 420 and 422

Ni-Sn/CeO2-MgO-Al2O3 steam reforming of glycerol 406 Rh-Pd/Gd2O3-CeO2 steam reforming of toluene 422

Rh-Pd/Gd2O3-CeO2 steam reforming of toluene 422 autothermal reforming of liquid hydrocarbons 441

Rh-Pd/ZrO2-CeO2 steam reforming of toluene 422 Rh-Pd/ZrO2-CeO2 steam reforming of toluene 422

Rh-Pt/ZrO2 steam reforming of ethanol/gasoline 428 Ni-Rh/CeO2- Al2O3 2 steam reforming of liquid hydrocarbons 426

Rh-Pt/Gd2O3-CeO2 steam reforming of toluene 420 Ni/CeO2-Al2O3 steam reforming of liquid hydrocarbons 426 and 427

Rh-Pt/ZrO2-CeO2 steam reforming of toluene 420 Rh/CeO2-Al2O3 steam reforming of liquid hydrocarbons 426 and 427

autothermal reforming of liquid hydrocarbons 441 Ni/CeO2-ZSM-5 steam reforming of liquid hydrocarbons 430

Ni-Re/SrO-ZrO2 steam reforming of liquid hydrocarbons 431 Ni/SrO-ZrO2 steam reforming of liquid hydrocarbons 431

Pt-Pd/Al2O3 autothermal reforming of liquid hydrocarbons 440 Ni-Re/SrO-ZrO2 steam reforming of liquid hydrocarbons 431

Pt-Rh/Al2O3 autothermal reforming of liquid hydrocarbons 441 Ni/La2O3-SrO-ZrO22 3 steam reforming of liquid hydrocarbons 431

Pt-Rh/La2O3- Al2O3 2 3 autothermal reforming of liquid hydrocarbons 441 Pt-Rh/La2O3- Al2O3 2 3 autothermal reforming of liquid hydrocarbons 441

Rh-Pt/SiO2 autothermal reforming of liquid hydrocarbons 441 Rh-Pt/ZrO2-CeO2-SiO22 autothermal reforming of liquid hydrocarbons 441

Rh-Pt/ZrO2-CeO2-SiO2 autothermal reforming of liquid hydrocarbons 441 Rh-Pt/MgO-TiO2 autothermal reforming of liquid hydrocarbons 441

Rh-Pt/TiO2 autothermal reforming of liquid hydrocarbons 441 support modifica-

Rh-Pt/MgO-TiO2 autothermal reforming of liquid hydrocarbons 441 tion Pt/CeO2-Gd2O3 commercial-gasoline 450

Ni-Co/MgO-Al2O3 partial oxidation reforming of isooctane 409 Ni-Fe/MgO-Al2O3-CGO steam reforming of biomass tar 454

Ni-Fe/MgO-Al2O3 partial oxidation reforming of isooctane 409 Pd/CeO2-MOx (M = Cu and Y) jet fuel reforming 420

Ni-Sn/MgO-Al2O3 steam reforming of glycerol 407 Ni/hexaaluminates partial oxidation of methylnaphthalene 458

Ni-Sn/CeO2- steam reforming of 406 La/Sr/Zr/Ni- several reforming 459

MgO-Al2O3 glycerol pyrochlore processes

Pt/Gd2O3-CeO2 steam reforming of isooctane 413

immunity to sulfur attack, with significant differences depending on the order of addition of ceria and gadolinia. Interestingly, the sample where the ceria was impregnated first was the most stable, which the authors ascribe to an improved Pt-CeO2 interaction. This catalyst presented good activity in commercial-gasoline reforming with relatively high sulfur concentration (100-500 ppm provided by thiophene). The authors argue that Pt possesses different electronic

properties when supported on bare alumina compared to the ceria-alumina-based support. Pt metallic sites in alumina are unable to resist sulfur poisoning. A valuable point of this paper is the redox mechanism that the authors proposed for sulfur elimination. Under steam reforming conditions, thiophene was transformed to H2S, which is released and eliminated from the cycle via reduction and reoxidation of the ceria-doped support.450 Azad and Duran also obtained some interesting

results using Rh/CeO2-based materials.420 In this work, the presence of 50 ppm of H2S "activates" the catalysts increasing H2 yields in the steam reforming of toluene. They suggested that such positive effect could be due to the formation of Ce2O2S, which presumably promotes the activity of the supported Rh. Actually, in this particular situation ceria is acting as a sulfur sorbent, and the redox properties of ceria are useful since the reduced oxide (Ce2O3) is more prone to trap sulfur.

It is well-known that the redox properties of ceria can be boosted by the use of promoters resulting in materials with enhanced oxygen storage capacity.196,198,453 Laosiripojana et al. investigated the steam reforming of biomass tar using Ni-Fe supported on MgO-Al2O3, coated with CGO.454 The results indicated that the formation of various Ce-O-S phases influences the catalytic activity with the sulfates having a positive effect in the oxygen mobility and therefore increasing the activity and the sulfides producing an activity drop. Some other examples using Pd/CeO2 samples and CuO and Y2O3 as metal oxide additives benefit the reforming performance. These dopants increase H2 yield due to an increase in metal surface area available for reaction. In addition, CuO increased the stability against sulfur poisoning due to the oxide acting as a sacrificial sulfidation site, taking the sulfur species away from the active metal and/or the ceria support.421

Some groups proposed other types of support modifications in order to improve sulfur tolerance. For instance, incorporation of the active metal into the crystal structure of the oxide phase, followed by exsolution of metal particles on reduction with the aim of stabilizing the particles and at the same time increasing metal dispersion. Smaller, more stable particles should improve sulfur tolerance since the sintering of Ni particles leads to larger crystallites that are more easily

poisoned.76,455 For example, Ni particles can be stabilized on hexaaluminate structures.456-458 Smith et al. prepared nickel

hexaaluminate dispersed on zirconia-doped ceria catalysts obtaining rather good sulfur tolerance in the partial oxidation

of methylnaphthalene.458

Pyrochlore-like structures are also interesting to avoid sulfur poisoning. Pyrochlores are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with a general formula of A2B2O7. An important property of these materials is that catalytically active noble metals can be substituted isomorphically on the B site to form a crystalline catalyst.457 In particular, metals like Ru, Rh, and Pt can be introduced into the B site of the pyrochlore structure because they meet ionic radius constraints and have the required oxidation state. In this situation, the metal is included in the solid network and somehow protected toward sulfur species. The group of Spivey has done intensive research on these types of materi-

als.457,459-462 For example, they found that a La/Sr/Zr/Ni-

pyrochlore loses some activity with 50 ppm of dibenzothio-phene at the initial stages of the reforming reactions. However, the deactivation was not continuous with time on stream. This suggests that the poisoning species are adsorbed on the catalyst surface in the initial steps but are not accumulated on the surface itself. In addition, almost complete activity was recuperated when sulfur was removed from the stream. Similar results were obtained when Rh instead of Ni was introduced in the pyrochlore lattice.462 The pyrochlore structure, although it experienced some deactivation, was more tolerant to sulfur compared to a reference Rh/Al2O3 catalyst.

In a similar way to the carbon tolerance section, Table 7 summarizes the developed catalytic formulations following a given strategy to mitigate sulfur poisoning.

8.2.4. Regeneration of Sulfur-Poisoned Catalysts.

Since most of catalysts are expensive and industrial productivity in many cases depends on the catalysts' performance, there is a need to reactivate or regenerate them. Although the regeneration method is rather catalyst-specific, generally, they involve thermal treatments in oxygen, hydrogen, or steam atmospheres.

As indicated by Bartholomew, the toxicity of the sulfur species depends on how many electron pairs are available for the interaction with the metals.9 In general, toxicity decreases as follows: H2S > SO2 > SO42-, etc. in the order of increased shielding by oxygen. Therefore, oxidation treatment to eliminate sulfides is an alternative to recover activity. Ideally, the main goal of oxygen treatments is to remove all the sulfur species as SO2 (SsoM + O2 gas ^ SO2 gas) at high temperature. For example, Choudhary et al. managed complete activity recovery of a Ni-ceria-based catalyst after being exposed to 7400 ppm of thiophene by thermal treatment at 800 °C in an O2/N2 50:50 mixture.4 3 An inherent drawback of this procedure is the oxidation of the active phase (Ni) during the recovery, thus making necessary a reduction step before rerunning the reaction.

Apart from the active phase oxidation, this type of oxidative treatment involves other disadvantages that limit its application, and therefore, it cannot be considered as a general regeneration procedure. More precisely, the exothermicity associated with this process may produce irreversible catalyst deactivation by thermal degradation and/or phase transformation of the active components.13 For instance, some authors reported irreversible formation of the inactive NiAl2O4 spinel when they tried to reactivate a Ni/Al2O3 reforming catalyst using diluted oxygen at high temperatures.396 In this scenario, oxidative treatment is only useful in some specific cases when the oxidation at high temperatures would not risk modifying the catalyst structure.

Alternatively to oxygen, thermal treatments in steam can be applied to reactivate sulfur-contaminated catalysts. One of the seminal works in this area was carried out by Rostrup-Nielsen dealing with Ni-based catalysts deactivated upon H2S exposure.464 This indicated that steam can remove sulfur as hydrogen sulfide via:

Ni—S + H2O ^ NiO + H2S

Although H2O may produce also some oxidation of reduced Ni:

Ni + H2O ^ Ni—O + H2

At temperatures between 800 and 900 °C, up to 90% of the sulfur can be removed from the catalyst surface. In the same paper, the positive role of alkali promoters such as Ca and Mg in the steam regeneration was discovered. The catalysts doped with small amounts of calcium and magnesium were easier to reactivate. In turn, some other dopants like K or Na did not improve the regeneration process, most likely because sulfur is converted into a form that is retained in the catalysts in the presence of K and Na.

Complete recuperation of reforming activity for bulk Ni catalysts was found by Oudghiri-Hassini et al. using Ar/steam mixtures.465 Regenerated catalysts were characterized by means of XPS, indicating complete sulfur removal from the catalyst surface after the steam treatment. Nevertheless, as indicated

above, Ni oxidation occurred and some oxidized Ni species were identified by infrared spectroscopy, underlining again the risk of altering the catalysts structure when an oxidative treatment is applied.

Reducing atmospheres do not present the catalyst oxidation drawback observed when the spent samples are treated with steam or oxygen. In this sense, this alternative is currently viewed as the most desirable way to remove sulfur from catalysts. Typically, sulfur is released as H2S by the direct reaction of adsorbed sulfur species and H2 (Ssolid + H2 gas ^ H2S gas). According to the thermodynamics of sulfide formation, this process is essentially reversing the metal sulfide equilibrium formation.13

Cheekatamarla et al. observed complete regeneration of a molybdenum carbide catalyst deactivated upon exposure to 500 ppm of benzothiophene using a sequential thermal treatment: first 1 h in He and later 1 h in hydrogen at 900 °C for both processes.466 The authors claimed that the heating step in He may remove weakly adsorbed sulfur species, while for the chemisorbed species (likely forming metal sulfides) heating in hydrogen was required.

The effectiveness of the hydrogen thermal treatment depends as expected on the sulfur concentration used in the catalytic test. For example, Hepola et al.318'467 demonstrated through temperature-programmed hydrogenation (TPH) that complete sulfur removal from commercial Ni-based catalysts was achieved when 500 ppm of H2S was used. On the contrary, when the H2S concentration was increased up to 2000 ppm, the hydrogen treatment was not sufficient to eliminate all the chemisorbed sulfur. Figure 27 represents the TPH profiles

500 ppm H2S / \ 2000 ppm H2S /

300 400 500 600 700 800 900 1000 Temperature (°C)

Figure 27. TPH profiles (70% Ar/30% H2) of Ni-based catalysts after exposure to 500 and 2000 ppm of H2S in N2 at 2 MPa, 900 C, and 4— 6 h. Reprinted with permission from ref 318. Copyright 1997 Elsevier.

discussed in ref 318. It is clear that 2000 ppm provoked a strong adsorption of sulfur on the catalyst's surface, making complete sulfide removal almost impossible.

Other reducing mixtures have been successfully employed to regenerate sulfur-poisoned catalysts. For example, Arosio et al. demonstrated that CH4-reductive pulses can partially recuperate a sulfur-contaminated Pd/Al2O3 catalyst spent in methane combustion.468 A small increase of the temperature up to 600 °C using short time pulses (2 min) gave almost complete catalyst regeneration. Such a treatment combines extensive sulfate decomposition with a PdO reduction/oxidation cycle. The authors claimed that the reductive regeneration of sulfur-poisoned catalysts with CH4-containing atmospheres could be more effective than the analogous treatment in H2, possibly due

to the milder reducing action resulting in minor formation of sulfide species on the catalyst surface.468

In summary, there are routes to regenerate sulfur-poisoned catalysts via thermal treatments in different atmospheres. However, the success of this process depends on several factors, such as sulfur concentration, strength of sulfur interaction with the catalyst surface, catalyst composition and its susceptibility to be affected by the recovery treatment, etc. This complex situation makes necessary a careful choice of the thermal treatment and may not ensure complete activity recuperation.


Solid oxide fuel cells (SOFCs) generate electricity and heat electrochemically from hydrogen and/or carbon-based fuels. The electrodes in SOFCs need to exhibit electronic conductivity, oxygen ion conductivity, and catalytic activity. The fuel oxidation takes place at the anode, where the deactivation by carbon and/or sulfur is one of the key challenges in SOFC technology.

We have reviewed the approaches used in catalysis to prevent or minimize the effects of carbon or sulfur on catalysts. Carbon and sulfur poisoning are much more challenging in conventional catalysis since, as opposed to SOFCs, normally there is no oxygen flux that could help to minimize their deleterious effect. Some strategies have been shown to work in both catalysis and SOFCs; we can have confidence that the effect is real and the basic knowledge is in place to expand or refine those strategies.

It is clear that the search for carbon and sulfur tolerance in catalysts and solid oxide fuel cells is exemplified by the properties of one element, nickel. Its unrivalled propensity to catalyze carbon—carbon bond formation is matched by superiority to other base metals in a variety of other useful reactions. It is also extremely vulnerable to the electron-withdrawing effects of sulfur. The search, then, has focused on two different goals: first to mitigate carbon deposition and sulfur poisoning in nickel-based catalysts, and second, to find catalysts which approach the activity (and cost) of nickel without vulnerability toward sulfur poisoning or catalytic activity toward carbon formation. Several strategies to achieve these goals have emerged in the SOFC and catalysis literature.

9.1. Alloying of Nickel

Alloying of nickel is a strategy that can have an effect on both carbon and sulfur tolerance. Alloying can improve carbon tolerance by reducing the rate of carbon-carbon bond formation, reducing the amount of the most destructive and deactivating graphitic carbon, and/or increasing the rate of competing reactions, such as carbon oxidation. For sulfur tolerance, nickel is the element most vulnerable to sulfidation, so alloying with almost anything improves sulfur tolerance. Conversely, since nickel is an excellent catalyst for many of the reactions in an SOFC anode, alloying may reduce the activity for these reactions.

This strategy has been implemented in a number of different ways in both catalysis and SOFC studies. The classic example in catalysis is addition of noble metals such as Rh and Au, and these have been used in SOFCs with some success. The use of noble metals in SOFCs is complicated by the larger total amount of metal, meaning that proportionally more of the expensive noble metals need to be used. For this group of elements, the developments in MIECs and nonmetal electronic

conductors for SOFC anodes may allow more realistic amounts of these metals to be used, and nanoalloys of Ni with Au, Rh, or Re may be promising for carbon and sulfur tolerance.

In both fields, the issue of cost has driven a search for cheaper alternatives. For obvious reasons, the top row transition metals from Fe through to Cu have been explored extensively. These seem to be effective in reducing the overall carbon deposition and decreasing the amount of graphitic carbon. In the case of these promoters, research could switch to other issues affecting SOFCs, for example, tolerance to redox cycling or compatibility with electrolytes and other components, as their ability to mitigate carbon deposition seems largely agreed upon.

Outside of the top row transition metals, there are some other candidates for carbon and sulfur tolerance, the most promising of which is tin. Tin has been trialed in both catalysis and SOFC anodes and appears to confer both carbon and sulfur tolerance. A further advantage of tin is that the mechanism by which it works is reasonably well-known, meaning this is a good target for further testing in terms of long-term stability and compatibility. Another promising element is molybdenum, which is widely used in catalysis. It has complicated chemistry, with different carbide, sulfide, and oxide phases being stable under possible regimes in an SOFC anode, meaning that further work to clarify its behavior is needed. However, it is potentially a promising electrocatalyst in its own right, so further investigation may be fruitful.

9.2. Alkaline Promoters and Supports

It is well-known in catalysis that basic oxides reduce carbon deposition by increasing the carbon oxidation rate. This is thought to work as the basic sites act as stores for highly reactive hydroxyl radicals. The strategy has found use in the SOFC literature, with elements such as Ba and La looking the most promising for further investigations. The use of alkali metals is underexplored compared to catalyst science, due to the higher mobility of these elements, and also their potential for poisoning the catalytic reactions. The vapor pressures of their oxides approaches that of Ni at 1000 °C (~10-10 bar) at temperatures ranging from ~800 °C for Li2O down to ~500 °C for K2O, while the melting points of Na2O (1132 °C) and K2O (740 °C) are also a concern. The move to intermediate temperature fuel cells may bring at least Na and Li into play.

One interesting strategy which currently appears to be unique to the SOFC literature is the use of basic cationic conductors such as Li+ and H+ conductors for carbon tolerance (although the latter have recently begun to be used as catalysts for the reverse water-gas shift reaction, the link to carbon tolerance has not been made469,470). These maintain the basicity of the materials promoted solely with simple nonconducting alkali and alkaline earth oxides but add in some extra conductivity to improve both the carbon tolerance and electrochemical performance. In the case of Li+ conductors, the Li is also stabilized so less volatile.

One aspect which needs exploration regarding these basic promoters is their sulfur tolerance, especially regarding their ability to mitigate carbon deposition in a sulfur-containing gas feed.

9.3. Ceria, Doped Ceria, and Oxygen Storage

It is fair to say that the discovery of the redox properties of ceria and doped cerias has revolutionized both catalysis and SOFC science, especially given the oxide ion and electronic conductivity of doped cerias. These materials work both by

acting as a store for oxygen, which is then able to react with carbon species, and by their ability to trap sulfur species. It has also been shown conclusively that doped cerias are both electrocatalysts and catalysts in their own right for important reactions such as electrooxidation of hydrogen and reforming of hydrocarbons. It is clear that doped cerias will continue to be incorporated into the current and next generations of SOFC anodes.

With such a useful and varied class of materials, there are obviously many fruitful avenues for research. One of the most obvious is the use of the extremely high oxygen storage capacity materials found in three-way catalysts and other catalytic systems. The earliest of these, the Ce-Zr system, has been somewhat investigated, but it does not appear that other ceria-based systems have been used at all in SOFCs. It is also worth noting that although ceria-based oxygen storage materials are favored because of their relative structural stability on redox cycling, the use of impregnation and MIECs may allow the use of less stable oxygen storage materials.

Other possible routes for further investigation include trying to improve the catalytic activity of doped cerias. Current work in SOFCs has focused on the doped cerias with the highest ionic conductivity, but a focus on the activity toward electrooxidation and reforming of hydrocarbons may be useful, especially where ceria is not the only ionic conducting species. Work to improve the sulfur storage capacity could also be important.

9.4. Preferential Sulfur Binding Sites

Phases which preferentially bind sulfur, and thereby lower the sulfur coverage on Ni or other active metals, have been used in both SOFCs and catalysis. In catalysis species such as Cu, Zn and Mo are known to act as sulfur sorbents in preference to Ni, while in SOFCs this effect has been noted in ceria- and Ba-containing compounds. In addition, there is literature on sulfur sorbents for gas cleaning which may be useful.405 The deposit of a barrier layer (i.e., the first point of contact with the fuel) that protects the most electrochemically active area of the anode (i.e., close to the electrolyte layer) is known to protect against carbon deposition in SOFCs but has not yet been investigated for sulfur poisoning.

9.5. Nonmetal Electronic Conductors

Removal of the metal electronic-conducting phase solves many of worst effects of carbon and sulfur poisoning, and there are two possible solutions for this. Nonmetal conductors in a cermet such as carbides or carbon retain the benefits of cermets, such as the ability to independently optimize the electronic and oxide conducting phases, as well as the disadvantages, such as having to match thermal expansion coefficients and more complicated microstructure optimization. MIECs lose both the advantages and disadvantages of cermets. Both nonmetal conductors and MIECs as potential solutions have deficiencies in different areas. Nonmetal conductors are generally under-researched, and in particular, more work on stability is needed. MIECs are lacking in either electronic or ionic conductivity, and some of the more widely used materials, such as the strontium titanates, require high processing temperatures and are difficult to fabricate into anodes. Both solutions are lacking in catalytic activity and will likely require a further catalytic phase.

9.6. Infiltration of Nanoparticles

In catalysis, infiltration of porous structures with metal nanoparticles is a common practice to maximize the active surface while simultaneously hindering carbon deposition by decreasing the area of graphitic growth. In SOFCs, this approach was first used because the low melting point of copper oxide meant that Cu/YSZ anodes could not be produced by the conventional solid state route. Since then it has been used to add a variety of metals (including nickel) and now has been proved that it can improve important parameters such as the triple phase boundary length.

There are many issues to be resolved with infiltration, especially relating to long-term stability and feasibility of scaling up the process to industrial-sized anodes, but the reason it is interesting for carbon and sulfur tolerance is that it allows much greater control over the chemistry and structure of the electrode. The exploration of the possibilities in SOFC anodes is only just beginning, but already we have seen that the infiltration of barium or ceria allows fine dispersion of the promoter over the surface of the material, enhancing carbon tolerance by ensuring that any given nickel particle is close to a particle of the promoter.

In the future, we could see more complex oxygen storage materials or more advanced catalysts incorporated into the anode structure by this method. The advances in MIECs and possibilities of nonmetal conducting phases such as carbon and carbides should allow designed catalyst nanoparticles (whether containing nickel or not) to be added without their effect being destroyed by alloying into the percolating metal phase.

9.7. Regeneration

Catalysts deactivated by carbon deposition are commonly regenerated by stopping the process and then passing a stream of cleaning gas (hydrogen, steam, carbon dioxide, or oxygen). All of these gases are eventually present in a SOFC anode: as fuel, as a product of oxidation, as a permeant gas, etc. The literature on regeneration of SOFC anodes is very sparse, and modifications of the anode to aid regeneration are nonexistent. Nevertheless, it has already been shown that it is possible to remove carbon deposits from Ni/YSZ anodes by a variety of gases and also by oxygen flux through the electrolyte.

Ideally, anodes would be designed so that they can be regenerated without use of alternative feedstocks or extensive downtime, but failing this, they need to be designed to be regenerated at the minimum temperature for as short a time as possible and be able to withstand any changes which take place during regeneration. From the catalysis literature, it is probable that many promoters which prevent carbon deposition in the first place are also effective in aiding regeneration, whereas for sulfur tolerance where sacrificial phases are used, these might bind more strongly to sulfur, requiring harsher conditions for regeneration.

Exsolution of nanoparticles from MIECs and symmetrical SOFCs also provide interesting alternatives to conventional cermet anodes in terms of regeneration. While this potential benefit has been noted in the literature on these materials, there is little published experimental work proving it.

9.8. Theoretical and Computational Studies

It has become clear in the last five years that theoretical and computational chemistry is finally becoming able to provide accurate insights into the chemical behavior of materials and even interfaces.471 Very recently, accurate predictions have even been made as to the structure and properties of previously

unknown materials relevant to SOFCs.472 A historical problem in SOFC research (and scientific research in general) is the lack of coordination between groups working in different fields. Groups working in fields such as materials chemistry and detailed in situ and ex situ characterization would surely benefit from incorporating insights from theoretical chemistry in the future and making sure their work is relevant to the challenges in SOFCs.

9.9. Reflections on Experimental Work

Many in the literature have claimed experimental results of tolerance against carbon and sulfur. However, caution needs to be taken in the techniques used to analyze these results, and during the research and writing of this review we have noted some points relating to this. (1) Claiming carbon tolerance by lack of performance degradation. It is certainly true that an anode is carbon tolerant if it maintains performance over a long period of time regardless of whether or not carbon is actually present. However, it cannot be claimed that a lack of degradation means that there is no carbon, as it has been shown many times that cells can operate without performance degradation for significant periods despite carbon being deposited. Certainly a testing period of a day or a week as used in many papers is not long enough to claim carbon tolerance in the absence of other data proving that either there is no carbon or that the carbon has reached some kind of steady state. (2) Claiming carbon tolerance by lack of carbon in SEM. While SEM is clearly a useful technique for assessing microstructure, the lack of carbon whiskers in an SEM image is not proof of a lack of carbon. Even EDX needs careful sample preparation for accurate quantitative analysis, for example polishing. (3) Low measured OCVs. This applies to two different systems, thin doped ceria electrolytes and air leakage into the anode chamber. It has been shown that leaks causing an increase in measured OCV of less than 0.02 V from 1.223 to 1.239 V in dry methane results in a decrease in implied water content (as calculated from the H2/O2 equilibrium) of 30% from 0.24% to 0.17%. This is enough to result in a dramatic decrease by over half in the amount of carbon deposition over a period of 1 h. Likewise, many groups using 10-50 ^m CGO and CSO electrolytes report OCVs below 0.9 V, which implies a substantial oxygen flux at OCV through the electrolyte caused by the electronic conductivity of doped ceria (or through a slightly permeable electrolyte). This oxygen flux or leakage should be extremely effective at preventing carbon deposition and improving sulfur tolerance, but if the aim is to study the carbon and sulfur tolerance of the electrode materials then care should be taken to account for this. (4) Current collectors. The paper referred to above also showed that the coverage of silver current collector paste can have a large effect on carbon deposition, even completely preventing it if the electrode is completely covered. Presumably, this works in a similar way to the barrier layer concept discussed in section 6.2. There has also been considerable controversy over the use of platinum current collectors, with claimed high power densities in dry methane for some MIECs being shown to be almost entirely due to the use of platinum current collectors and doped ceria electrolytes.240 (5) Humidification. As shown above, the exact level of humidification can have a profound effect on carbon and sulfur tolerance. Many studies report 3% humidification levels, which if using a bubbler, implies a water temperature of just under 25 °C (25 °C would actually be 3.1%). This seems quite warm for a lab temperature, although the authors of this

review are based in Britain so they may be used to cooler temperatures than many. The humidity level at 20 °C is 2.3% and at 16 °C is 1.8%, so care should be taken that the correct humidity levels are being reported. It should also be borne in mind that these are 100% relative humidity levels, whereas it is known that bubblers are not necessarily effective in reaching 100% relative humidity.474 (6) Interaction between carbon and sulfur. Many papers claim carbon and sulfur tolerance, with separate experiments done to prove each using model gas feeds (e.g., one experiment with dry methane and another with H2S in H2). However, it is well-known from both catalysis and SOFC literature that there is strong interaction between carbon and sulfur, with each having the possibility of hindering or promoting the other under different operating conditions. While there are undoubtedly advantages in simplifying the system by separately studying carbon and sulfur tolerance, it does not necessarily follow that a system which is separately carbon and sulfur tolerant will be simultaneously carbon and sulfur tolerant. This could especially be the case where sulfur may react with promoters which are present to reduce carbon deposition, for example with BaO. More work needs to be done on sulfur-containing carbonaceous fuel in SOFC anodes.

In this review, we have seen the different materials solutions to carbon and sulfur tolerance in catalysis and solid oxide fuel cells, but there is also an aspect of different experimental techniques in the two fields. The foundational techniques of catalysis are well-established over decades, with a focus on gas phase techniques such as chemisorption measurements and temperature-programmed reactions. Some of these have started to be incorporated into SOFC studies, for example, temperature-programmed oxidation. In SOFCs, electrochemical impedance spectroscopy (EIS) has long been a key feature of the investigations, and although it has not been used in catalysis, with advances in impedance analysers, analysis techniques such as distribution of relaxation times (DRT) and modeling, it is possible that EIS could start to be used in model catalysis studies.

Because of the more widespread nature of catalysis science, newer techniques have generally been adopted in catalysis first and later in solid oxide fuel cells. An example of this is in situ Raman, which has been known in catalysis since the early 1990s but is now being used to investigate SOFCs. Other techniques such as high-resolution TEM, XPS, and XRD are following a similar path. A notable counter-example is FIB-SEM and tomography in general, which appears to have been far more enthusiastically adopted in the SOFC community than in catalysis.

As can be seen, the experience gained in the field of catalysis has had an increasing influence in the research paths in SOFC, and hopefully in the near future, this inspiration may be reciprocated as catalysis can profit from the experience of SOFCs. Although electro-catalysis at low temperatures is common, the high temperature regime is still an area of opportunity for catalysis as it features the unique capability of supplying/extracting O2— or H+ to reactant species under an electric bias. A few examples are the synthesis of ammonia at atmospheric pressure,475 the non-Faradaic electrochemical modification of catalytic activity,87 and the electrochemical reduction of CO2 and H2O.476,477

It is clear that the SOFC community has made great strides toward carbon and sulfur tolerance over the past decade. Going forward, the strategies already implemented at lab scale need to be incorporated into more commercially focused devices, while

at lab scale the learnings from catalysis should be used to develop materials which are carbon and sulfur tolerant, especially at lower temperatures. We hope that this review is able to help with both of these.

AUTHOR INFORMATION Corresponding Author


The authors declare no competing financial interest. Biographies

Paul Boldrin received a Ph.D. in materials science from Queen Mary University of London working on continuous hydrothermal synthesis of nanomaterials for catalysis. This was followed by postdoctoral work in chemistry at the University of Liverpool working initially on high throughput discovery of catalysts as a research associate and later as a research coordinator. Currently, he is a postdoctoral research associate at Imperial College London working on solid oxide fuel cells. His research interests include characterisation of the catalytic and electrocatalytic processes occurring in solid oxide cells and membrane reactors and the use of nanomaterials in those devices.

Enrique Ruiz-Trejo obtained his Ph.D. in Materials from Imperial College and immediately after was appointed lecturer at Universidad Nacional Autónoma de Mexico. He was then awarded a Humboldt scholarship at the Max Planck Institute for Solid State Research. In 2009, he moved to Denmark as Senior Scientist at Risoe National Laboratories for Sustainable Energy followed by a position as Research Fellow at the University of St. Andrews. Since 2012, he has held the position of Research Associate in Fuel Cells and Materials Processing at Imperial College. His areas of interest include materials for energy applications and gas separation membranes, the development of electrodes for fuel cells, and the manufacture of metal-ceramic composites.

Joshua Mermelstein is a fuel cell systems engineer at The Boeing Company in Huntington Beach, CA, with an expertise in the solid oxide fuel cell (SOFC) and the proton-exchange membrane (PEM) fuel cell systems. He is currently the lead scientist for fuel cell system development within Boeing's Electronic and Information Solutions Advanced Technology Programs (ATP). Joshua is currently leading efforts as the chief engineer for Boeing's development of a 50 kW reversible solid oxide fuel cell (RSOFC) system used for microgrid energy storage. Joshua also provides technical support for the development of other SOFC- and PEM-based fuel cell systems throughout Boeing. Joshua earned his Bachelor's degree in Chemical Engineering from the University of Arizona in 1999, Masters from the University of Southern California in 2000, and Ph.D. from Imperial College of London in 2010 with the Department of Chemical Engineering and Fuel Cell Research Group of the Energy Futures Lab, after working in industry as a chemical engineer for 7 years. His research at Imperial College focused on the impact and mitigation of carbon formation on SOFC anodes arising from biomass gasification tars through steam reforming, partial oxidation, and dry-reforming technologies. Joshua has published over 10 publications related to his work in this field. His career background spans 10+ years of industry experience in chemical/process engineering, cryogenic and compressed gases, hydrogen and fuel cell technology, fuel cell electric vehicles, plug-in hybrids and BEVs, alternative and renewable energy for stationary power, hydrogen production, and combined heat and power (CHP) for energy efficiency.

Jose Miguel Bermudez Menendez graduated in Chemical Engineering (2008) and obtained a MSc in Process and Environmental Engineering (2010) from the University of Oviedo, Spain. He obtained his Ph.D. in Chemical Engineering (2013) from the same university. His Ph.D. Thesis deals with the CO2 reforming of coke oven gases to produce syngas for methanol synthesis and was developed in the National Institute of Coal-CSIC (Spain). He worked in this research centre for more than 5 years, where he was involved in the development of microwave-assisted processes in the field of energy, mainly focusing on pyrolysis, gasification, and catalytic heterogeneous reactions. He gained a postdoctoral position in Imperial College London in 2014, where he is working on the thermochemical stability of mixed ionic-electronic conductors for oxygen transport membranes. He is also involved in the development of thermochemical processes like supercritical water upgrading or catalytic hydrocracking of heavy oils and biomass. He has coauthored more than 30 peer-reviewed papers and 2 patents on these topics and has been a finalist of the Best Young Researcher Award 2015 of the Spanish Group of Coal.

Tomas Ramirez Reina received his Ph.D. in Chemistry from the University of Seville (Spain) in 2014 under the supervision of Prof. Odriozola and Dr. Ivanova. For his Ph.D. work, he was awarded "best Ph.D. thesis 2014" by the Spanish Society of Catalysis (SECAT). He worked as a visiting researcher in 2011 in the Brookhaven National Laboratory (New York, United States) and in 2012 in the Institute of Chemical Engineering ICE-HT (Patras, Greece). In 2014, he moved to the U.K. as a Research Associate in the Chemical Engineering Department at Imperial College London. Currently, Dr. Reina is a lecturer in the Department of Chemical and Process Engineering in the University of Surrey. His research interests include the development of advanced heterogeneous catalysts for energy and sustainability. In particular, his work is focused on clean hydrogen production, selective oxidation, and hydrocarbon upgrading.

Nigel P. Brandon's research is focused on electrochemical devices for energy applications, with a particular focus on fuel cells, electrolysers, and batteries. He is Director of the U.K. Research Council Energy programme funded Hydrogen and Fuel Cells SUPERGEN Hub and Co-Director of the SUPERGEN Energy Storage Hub. He was the founding Director of the Energy Futures Lab at Imperial College, and a founder of Ceres Power, an AIM listed fuel cell company spun out from Imperial College in 2000. In 2014, he was appointed to the BG Chair in Sustainable Gas and as the founder, Director of the Sustainable Gas Institute at Imperial College.


Funding for this effort has been provided by Boeing Research & Technology and EPSRC grant no. EP/J016454/1.


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