Scholarly article on topic 'Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers'

Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers Academic research paper on "Chemical sciences"

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Electrochimica Acta
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{"electrocatalyst support materials" / "oxygen reduction" / "hydrogen oxidation" / electrolysis / "N-doped carbon" / "graphitic carbon nitride"}

Abstract of research paper on Chemical sciences, author of scientific article — Noramalina Mansor, Thomas S. Miller, Ishanka Dedigama, Ana Belen Jorge, Jingjing Jia, et al.

Abstract Electrochemical power sources, such as polymer electrolyte membrane fuel cells (PEMFCs), require the use of precious metal catalysts which are deposited as nanoparticles onto supports in order to minimize their mass loading and therefore cost. State-of-the-art/commercial supports are based on forms of carbon black. However, carbon supports present disadvantages including corrosion in the operating fuel cell environment and loss of catalyst activity. Here we review recent work examining the potential of different varieties of graphitic carbon nitride (gCN) as catalyst supports, highlighting their likely benefits, as well as the challenges associated with their implementation. The performance of gCN and hybrid gCN-carbon materials as PEMFC electrodes is discussed, as well as their potential for use in alkaline systems and water electrolyzers. We illustrate the discussion with examples taken from our own recent studies.

Academic research paper on topic "Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers"

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Title: Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers

Author: Noramalina Mansor Thomas S. Miller Ishanka Dedigama Ana Belen Jorge Jingjing Jia Veronika Brazdova Cecilia Mattevi Chris Gibbs David Hodgson Paul R. Shearing Christopher A. Howard Furio Cora Milo Shaffer Daniel J.L. Brett Paul F. McMillan

Electrochimica Acta

PII: DOI:

Reference:

S0013-4686(16)32339-8

http://dx.doi.Org/doi:10.1016/j.electacta.2016.11.008 EA 28298

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Electrochimica Acta

Received date: Revised date: Accepted date:

13-7-2016 31-10-2016 2-11-2016

Please cite this article as: Noramalina Mansor, Thomas S.Miller, Ishanka Dedigama, Ana Belen Jorge, Jingjing Jia, Veronika Brazdova, Cecilia Mattevi, Chris Gibbs, David Hodgson, Paul R.Shearing, Christopher A.Howard, Furio Cora, Milo Shaffer, Daniel J.L.Brett, Paul F.McMillan, Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.11.008

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Review Article

Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers

Noramalina Mansora^, Thomas S. Miller1^, Ishanka Dedigamaa, Ana Belen Jorgec, Jingjing Jiad, Veronika Brazdovab, Cecilia Mattevid, Chris Gibbse David Hodgsonf, Paul R. Shearinga, Christopher A. Howardg, Furio Corab, Milo Shafferd, Daniel J. L. Bretta *, Paul F. McMillanb *

a Electrochemical Innovation Lab, Department of Chemical Engineering, University College London, London WC1E 7JE; b Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ;c Materials Research Institute, School of Engineering and Materials Science, Queen Mary University of London, London El 4NS; d Department of Chemistry, Imperial College London, London SW7 2AZ;e UCL Businessplc, The Network Building 97 Tottenham Court Road, London W1T 4TP;f Amalyst Ltd, Faraday Wharf, Innovation Birmingham Campus, Birmingham Science Park Aston, Birmingham B7 4BB, g Department of Physics & Astronomy, University College London, London, WC1E 6BT, United Kingdom.

* PFM:p.f.mcmillan@ucl.ac.uk, DJLB: d.brett@ucl.ac.uk

^ NM and TSM have contributed equally to this work.

Graphical abstract

Highlights

• Graphitic carbon nitride (gCN) describes many materials with different structures

• gCNs can exhibit excellent mechanical, chemical and thermal resistance

• A major obstacle for pure gCN catalyst supports is limited electronic conductivity

• Composite/Hybrid gCN structures show excellent performance as catalyst supports

• gCNs have great potential for use in fuel calls and water electrolyzers

Abstract

Electrochemical power sources such as polymer electrolyte membrane fuel cells (PEMFCs) require

the use of precious metal catalysts, which are deposited as nanoparticles onto supports in order to

minimize their mass loading and therefore cost. State-of-the-art/commercial supports are based on

forms of carbon black. However, carbon supports present disadvantages including corrosion in the

operating fuel cell environment and loss of catalyst activity. Here we review recent work examining

the potential of different varieties of graphitic carbon nitride (gCN) as catalyst supports,

highlighting their likely benefits, as well as the challenges associated with their implementation. The performance of gCN and hybrid gCN-carbon materials as PEMFC electrodes is discussed, as well as their potential for use in alkaline systems and water electrolyzers. We illustrate the discussion with examples taken from our own recent studies.

Keywords: electrocatalyst support materials; oxygen reduction; hydrogen oxidation; electrolysis; N-doped carbon, graphitic carbon nitride

1. Introduction

The increasing demand for, but reducing supply of, traditional fossil fuels has driven efforts to develop cheap, abundant and environmentally friendly technologies for energy production, conversion and storage [1]. This has, in turn, led to dramatic improvements in the performance of hydrogen or methanol fuel cells and also in hydrogen-producing water electrolyzers. Despite these advances, current 'state-of-the-art' systems are still largely dependent on metals with low abundancies in the earth's crust, such as Pt, to provide the active catalyst components which can amount to as much as 42% of the total system cost [2, 3]. A long-term goal is to reduce the use of these scarce metal catalysts, while simultaneously improving the lifetime and efficiency of the energy conversion devices. Most operating fuel cells incorporate a polymer electrolyte membrane (PEM) and operate in acidic electrolyte media. Using an alkaline electrolyte is one option for achieving cost reduction, as the oxygen reduction reaction (ORR) occurs more readily in alkaline media, it permits the use of non-precious metal cathode catalysts such as Ni, or lower Pt loading [4]. However, a commercial alkaline anion exchange membrane with comparable performance and durability to Nafion® (the leading acid based PEM material), has not yet been developed. Currently, intense research efforts are being directed at (i) lowering the Pt loading in PEM fuel cell (PEMFC) electrodes and improving their efficiency and durability, and (ii) seeking alternative catalyst systems. It has been shown that carbon nitride materials can play a key role in both of these target areas.

The catalysts used in fuel cells and electrolyzers are typically in the form of high surface area nanoparticles (NPs) with a narrow size distribution, which are deposited and dispersed uniformly on a high surface area support (~200-1000 m2 g-1), usually commercially available carbon black. The effect NP size and shape has on catalytic activity has been well documented in the literature, with studies reporting an ideal particle size of ~3 nm for maximum activity. This particle size has been shown to provide an optimal adsorption energy between the catalyst and reactants [57]. Making supported NP catalyst structures maximizes the electrochemically active surface area (ECSA) of the NPs while the amount of metal used is minimized, for greater efficiency at lower cost. Incorporation of a proton-conducting ionomer into this catalyst layer facilitates the formation of three-phase boundary regions, where electrochemical reactions occur in contact between the catalyst, fuel and electrolyte [8].

Although most research has focused on development of the electrocatalysts (ECs), the nature of the support material and its interaction with the NPs are also crucial for effective electrode operation. The catalyst support determines the porosity, tortuosity and wettability of the electrode. It also influences the size distribution and stability of the deposited NPs, the ability to produce and infiltrate inks for deposition in membrane electrode assemblies (MEA), and the capacity to control the efficiency of electronic and chemical catalyst-support interactions [9-11]. An ideal catalyst support should (i) allow an even distribution of NPs across its surface for maximum ESCA to volume ratio; (ii) have a high electronic conductivity to contribute to the electrochemical functioning of the electrode; (iii) have a porous network structure over multiple length scales that enhances reactant access; (iv) exhibit high stability under both the operating and transient conditions of the specific application, which can periodically lead to potential excursions as high as 1.5 V, causing severe carbon corrosion on the electrode; (v) require minimal processing via chemical or thermal activation; (vi) be compatible with a range of catalyst fabrication processes and (vii) be manufactured by a low-cost process that is environmentally benign. These are a stringent set of conditions which must be met simultaneously.

Carbon black is used as the catalyst support material of choice in PEMFCs due to its ready availability, low cost, minimal environmental impact and favorable physical properties. However, carbon is well known to undergo electrochemical oxidation under fuel cell operating conditions and loss of catalytic activity during repeated cycling is known to occur [12, 13]. This is because at the cathode the cell voltage typically changes from 0.55 to 0.90 V during transient operating conditions [14] and a PEMFC system goes through an estimated 38,500 start-stop cycles over its lifetime [14]. Additionally, during the shutdown and start-up steps, a H2/air front is created at the anode, which drives the potential at the cathode above 1 V. Conversely, during operation, fuel starvation may occur in a localized area of the anode, leading to an increase in anode potential until water electrolysis occurs in order to sustain the cell current [15]. Loss of catalyst activity can take place through several mechanisms, including particle aggregation (Ostwald ripening), crystal migration, dissolution/precipitation, and carbon corrosion [16, 17]. The catalyst/support interaction plays a major role in determining the extent of these processes. For example, Pt is known to act as a catalyst for carbon corrosion [18-21]; this means that Pt particles are more likely to become detached or 'orphaned' from the support because corrosion is enhanced at the interface between the two. The support is also responsible for determining the agglomerate structure and microstructure of the catalyst layer. An effective microstructure will create a well-connected, highly active ECSA and ensure efficient mass transport of reactants to the triple phase boundary (TPB) where the

electrolyte, catalyst and fuel meet at a reactive site. Degradation of the support not only leads to a reduction in the ECSA, but also to a lowering of the rate of mass-transport, by blocking pores and modifying the surface roughness. Because of these issues there has been a drive to find a next generation of catalyst supports that exhibit high stability to corrosion and that interact beneficially with the catalyst particles. Alternative support materials should also offer the potential for improved intrinsic catalyst stability, but few materials simultaneously meet these requirements. For example, certain metal alloys that possess high catalytic activity, exhibit poor stability on conventional carbon supports. However, these metal-alloy catalysts may be feasible on an alternative support which 'anchors' the NPs more effectively, or that results in an electronically stabilizing effect.

A wide range of alternative catalyst supports have been tested, including advanced carbon materials such as carbon nanotubes, aerogels and graphene [22, 23], non-carbon supports including tantalum oxyphosphate, titania, tin or indium oxides, tungsten carbide [24, 25], as well as carbon doped with boron [26-29], sulphur [30, 31], or phosphorous [32-34]. However, N-doped carbon supports are by far the most widely researched, particularly as spatial correlations have been shown to exist between regions of high N content and stabilized NPs [35, 36]. N-doping is usually carried out post-synthesis, via treatment with NH3 at elevated temperatures, resulting in N-doped carbons containing a few atomic percent N, randomly located throughout the material [35]. This approach provides little or no control over the nitrogen functionalities being introduced, which can be problematic as certain functionalities contribute in specific ways to the enhancement of catalytic effects [37]. For example, pyridinic N is suggested to form charge-transfer complexes with Pt [37], enhancing the kinetics of the ORR and methanol oxidation reaction (MOR), and preventing metal agglomeration during NP formation [36]. However the mechanistic details of these processes are yet to be fully understood. Furthermore, a strong NP attachment to the catalyst support is of little use if the surrounding, purely carbonaceous, matrix still suffers from corrosion [13]. To this end, there has been a move towards the development of more stable supports, including graphitic carbon nitrides (gCNs) that, in contrast with N-doped carbons, usually consist of a backbone of C-N bonds and contain a high concentration of N sites, located at defined positions within the layers [38-40]. Here we present a summary of recent results and future perspectives for using gCNs as catalyst support materials for fuel cells and water electrolyzers.

2. Graphitic carbon nitrides: material types and structures

Graphitic carbon nitride is a generic term applied to a large family of materials with different chemical and structural characteristics. In recent publications these have been increasingly

referred to as "g-C3N4" compounds, but that is a misnomer: almost none of the materials concerned occur with a 3:4 C:N ratio, they generally contain substantial concentrations of H and other elements (especially O), the "graphitic" layers are likely to be far from complete, and the sheet-like domains are unlikely to be planar [41, 42]. Several separate classes of gCN materials have been identified. The first concerns graphitic carbon, graphene-like carbons or carbon nanotubes prepared with a degree of N incorporation which extends up to a few atomic per cent [43, 44] and possess metallic properties [44, 45]. They are most usefully termed N-doped carbons rather than gCNs. The use of this type of material as EC supports began in the 1980s [46, 47], where carbonaceous materials pyrolyzed together with nitrogen-containing precursors were utilized. Since then, various other synthesis approaches have been shown to provide support materials that were found to improve the performance of the transition metal catalyst-support combinations [35, 36, 48-50].

The largest class of true carbon nitride compounds include a range of "graphitic" or polymeric structures with compositions extending along, or near, a tie-line between C2N3H and C3N4 in a ternary C-N-H phase diagram [42]. They are formed by condensation reactions from readily available precursors such as dicyandiamide, melamine or urea following thermal treatments in the 500-700 oC range [51]. These are most accurately termed gCNH compounds and their structures are usually based on linked heptazine (tri-s-triazine, C6N7) units. At lower synthetic temperatures, these polyheptazines (PHs) form poorly organized ribbon-like structures, such as those found in Liebig's melon [52], which are terminated laterally by -NH- and -NH2 groups (Figure 1a). As the processing temperature is raised, the degree of condensation increases with loss of NH3 components, and sheet-like units begin to form "graphitic" structures. Theoretically, this process can be extended to the g-C3N4 composition (Figure 1b), but this has not been observed in practice [41, 51]. Instead, the limiting stoichiometries recorded so far among gCNH materials have been close to the C2N3H composition. A further class of carbon nitride structures is based on independent triazine (C3N3) rings linked together by -N= or -NH- groups. These polytriazine imide (PTI) layers provide an alternative solution to forming infinite graphitic sheets with the C3N4 composition (Figure 1c,d). The first of these was prepared in nanocrystalline form using chemical vapor deposition (CVD) techniques [53, 54]. Later, a crystalline triazine-based graphitic carbon nitride (TGCN, Figure 1c) was discovered during a molten salt synthesis, deposited on the walls of the reaction vessel or at the surface of the melt phase [55]. In this material the layers are reported to contain C6N6 voids that are stacked according to an AB or ABC pattern [55]. The bandgap of this material was estimated to be 2.7 eV, although this does not explain the dark color of the synthesized material. The PTI family of crystalline gCN materials is extended by inclusion of heteroatoms such

as H, Li, Br and Cl within the structure, which are introduced during the synthesis in a molten salt (e.g., LiCl:KBr) or by high pressure-high temperature routes [56-60]. These materials contain larger (C12N12) ring voids within the layers, with Cl- or Br- ions residing in the interlayer spaces or occupying the void sites. The triazine units are linked by -NH- groups that decorate the interior of the void sites, and the hydrogens can be fully or partially replaced by Li+ cations. Additional Li+ species can occur between the layers [58, 59] (Figure 1d, e).

3. gCN, gCNH and PTI materials as PEM fuel cell catalyst supports

gCNH and PTI materials are reported to be semiconductors with bandgaps in the 2.5-2.8 eV range [57]. That property leads to visible light absorption and their yellow to brown coloration. gCN compounds alone, or materials decorated with Pt, RuO2 or related metal/metal oxide NPs as co-catalysts, have been reported to act as photocatalysts capable of splitting water into H2 and O2, as well as driving other redox reactions under simulated solar irradiation [61-63]. In their pure and doped form they have also been shown to exhibit redox catalytic behavior [64] and they have been tested for energy storage applications and as Li+ battery anodes [65, 66]. As these highly nitrided materials also have excellent mechanical, chemical and thermal resistance [61, 63] they have the ability to survive even the harshest PEM fuel cell operating conditions. In addition, their structures typically contain a high concentration of N-H functional groups, as well as lone pairs present on the N atoms of the triazine or heptazine units, providing Bransted acid and Lewis base sites and an abundance of tethering sites for the metal NPs. Combined, their intrinsic catalytic activity, their high stability, their unique termination chemistry and high surface area [63], has led to gCNH and PTI materials being developed as components in EC systems.

Insights into the improved chemical resistance of gCNH and PTI materials to corrosion, compared to carbon, can be obtained from simple band structure considerations (Figure 2). The allowed energy states in conductive carbons span across the H+/H2 and O2/H2O potentials, and thus provide competitive redox sites accessible under both hydrolysis and fuel cell conditions. In contrast, the C atoms in gCN, gCNH and PTI compounds are bonded to N atoms with fully satisfied valence states and are thus fully oxidized. This chemistry opens a band gap that brackets the H+/H2 and O2/H2O potentials and thus corrosion of the C-N backbone is thermodynamically unfavorable under electrochemical operation. It should, however, be remembered that most fuel cell electrode processes occur at potentials that do not match the theoretical thermodynamic values and that activation overpotentials play a significant role in the real-world fuel cell processes.

An early study of a gCNH material relevant to its behavior as a fuel cell EC, was carried out by Lyth et al., who investigated its intrinsic catalytic activity for the ORR [67]. Using material synthesized from cyanuric chloride, they demonstrated that the electrochemical ORR was efficiently catalyzed on gCNH when compared to carbon black. Unfortunately, the current density achieved was too low for use in practical applications. This was attributed to the low surface area and wide band gap of the material, which limited the activity of the electrode-reactant-electrolyte TPB. Upon blending with high surface area carbon, the performance improved as a result of the increased surface area and electronic conductivity. Aside from this intrinsic catalytic activity and enhanced durability, the abundant pyridinic sites within gCNH have been shown to provide anchoring sites for metal NPs, reducing aggregation and potentially enhancing the catalytic activity. Kim et al. showed that in a direct methanol fuel cell (DMFC), PtRu supported on gCNH exhibited 78-83% higher power density than the same catalyst deposited on carbon black [40]. In another study, Mansor et al. investigated gCNH and PTI materials as supports for chemically deposited Pt NPs [38]. Both materials displayed greater electrochemical stability than carbon black during accelerated corrosion testing. In particular, Pt supported on the highly crystalline PTI.LiCl showed excellent durability, as defined by changes in the ECSA under potential cycling, and this catalyst-support combination was found to have superior intrinsic methanol oxidation activity [38]. It is not yet known if the Li and Cl incorporated within the PTI materials played an active role in the EC, similar to that played by boron, which acts as a Lewis acid in B-doped carbon nitrides by reducing the electron density on C atoms [68]. It is possible their role was entirely passive and the increased crystallinity of the PTI was purely responsible for the observed improvement in durability.

A major obstacle to the use of metal-free gCNs as EC catalysts or as catalyst supports for

metal NPs, is their poor conductivity. As discussed above, these gCN materials are semiconductors

with band gaps up to 2.8 eV [57], which severely limits their intrinsic electrical conductivity and

can negatively impact their practical application. For example, Miller et al. showed that in gCNH

based lithium-ion battery anodes, increased resistivity negatively impacted Li storage capacities

[65]. However, when these materials are employed as an anodic catalyst support it has been found

that the fast reaction kinetics of the hydrogen evolution reaction (HOR) compensate for the

limitations of reduced conductivity (vide infra). On the other hand, the cathodic ORR is a slow

process and ECs supported purely on gCN materials exhibit poor ORR performance. It is only when

the gCNs are closely contacted with conductive materials (e.g. graphene) that efficient oxygen

reduction is observed. As shown in the study by Kim et al. [40], gCN materials can be combined in a composite or hybrid electrode system along with conductive forms of carbon.

4. gCN/C composite/hybrid electrocatalysts and catalyst supports

4.1 Mixed gCN/C hybrid electrocatalysts and catalyst supports

First-principles theoretical studies using pure melem units (C6N10H6,) as a model for the behavior of gCNH, have shown that these materials have poor electron-transfer efficiency for the ORR, preferring the 2-electron over the direct 4-electron pathway and therefore producing an accumulation of poisoning intermediates, such as H2O2 [69]. The addition of a conductive substrate increases the number of electrons accumulated on the melem surface, and hence participating in the reaction, thereby facilitating the 4-electron ORR process (Figure 3). Thusly, several studies have focused on testing the performance of gCN-carbon composite materials as metal-free catalysts and catalyst supports for fuel cell electrodes [70, 71].

Graphene and the related material reduced graphene oxide (rGO) are popular choices of conducting substrate for gCNHs, due to their high surface area and electrical conductivity [72-74]. Furthermore, density functional theory (DFT) calculations for a PH model of a g-C3N4 material reveal strong electronic coupling and charge transfer at the interface with graphene, resulting in enhanced electronic conductivity of the gCN [75]. Early experimental reports on metal-free gCNH-graphene composite ECs show excellent ORR performance, especially in alkaline electrolytes [70, 76, 77]. It was also documented that metals such as Pd and PtRu supported on gCNH/rGO exhibited very high electrocatalytic activity for formic acid, methanol and ethanol oxidation reactions, as well as enhanced durability and alcohol tolerance [78, 79]. These studies highlighted the importance of optimizing the gCN to graphene ratio in the composites to maintain the catalytic activity while improving the conductivity.

In our own recent work we have been examinining the performance of carbon nitride-rGO composites as an EC support for the ORR and HOR [80]. Here hybrid gCNH-rGO materials were synthesized from melamine and dicyandiamide dissolved in a GO solution, which was pyrolyzed under N2 following water removal by freeze-drying (Figure 4). From relative peak intensities in the X-ray photoelectron spectrum (Figure 4a) we determined that the composite only contained ~ 30% graphene (rGO), with the remainder gCNH. Examination by scanning electron microscopy (SEM. Figure 4b) showed a well-integrated composite structure, with rGO layers closely associated with gCNH to provide the necessary conductive backbone. Pt was subsequently deposited via a modified

NaBH4 assisted ethylene glycol reduction method, resulting in an excellent dispersion of Pt NPs on the support, with average particle size ~2.8 nm (Figure 4d), as shown by transmission electron microscopy (TEM) images (Figure 4c). The ECSA of Pt supported on the gCNH/rGO remained lower than commercial Pt/C, but it was three times higher than Pt on gCNH alone (Figure 4e), indicating that the incorporation of rGO improves the electronic conductivity of the support and allowed electrical contact to a greater number of NPs.

Figure 5 shows the ORR polarization curves, specific activity (SA) and mass activity (MA) of the material described above, other gCN supports and a commercial Pt/C at 0.90 V. These data were obtained from ex situ rotating disk electrode tests, where the SA and MA values for commercial Pt/C were comparable to those reported in the literature [81]. It was found that Pt on the gCNH-rGO composite had improved ORR activity over Pt/gCNH or Pt/rGO, despite the higher ECSA of the latter (Figure 4e). The gCN containing materials also demonstrated SA at 0.90 V comparable to that of commercial Pt/C (~500 |iA cm-2). Although the overall MA was lower than commercial Pt/C, due to the lowered ECSA, these systems have considerable scope for optimization.

Although the incorporation of rGO into the composite resulted in an improvement in the electronic conductivity, the layered morphology of the graphene component limited the number of active sites and mass transport access for reactants, negatively affecting the ORR performance. The HOR performance of Pt/gCNH-rGO measured in situ was, however, comparable to commercial Pt/C, as demonstrated in the fuel cell polarization curve in Figure 6a. As the HOR is a very fast reaction [82], the lowered ECSA has less impact on the catalytic performance here.

As discussed above, fuel cell anodes often suffer from durability issues that result from normal fuel cell operation, which over time, corrode the electrode. However, long-term potential cycling durability tests show that our Pt/gCNH-rGO had improved stability over commercial Pt/C, as shown in Figure 6b. This work therefore shows that gCNH-rGO hybrid materials are attractive as alternative, durable, anode catalyst support for PEM fuel cells. Further investigations are ongoing to further test the performance parameters, including electrode durability, in a real fuel cell configuration.

4.2 Three-dimensional (3D) gCN/C hybrid electrocatalysts and catalyst supports

Several routes for the fabrication of ordered 3D architectures of gCNH with a robust porous structure, high surface area, abundant N active sites and good electrical conductivity have been

investigated. For example, 3D gCNH materials have been produced by thermal polycondensation of precursors deposited on a framework structure made from commercial melamine sponge [83] (Figure 7). The resultant microporous structure exhibited a 3D interconnected network composed of 2D gCNH nanosheets with hierarchical pores ranging from 30 - 170 nm. The BET surface area was however low (40 m2 g-1). Other 3D frameworks incorporating gCNH, rGO or carbon nanotubes have been obtained via hydrothermal synthesis, followed by freeze-drying/supercritical drying [84, 85], or by chemical crosslinking approaches [86]. Generally, in these methods, exfoliated gCNH nanosheets are homogeneously distributed in solution by mixing with GO and a 3D network is formed during a hydrothermal process, due to the partial removal of oxygen-containing groups from GO and restoration of van der Waals forces [85-88]. The dispersed gCNH nanosheets adhere onto the rGO surfaces and are thus incorporated in the 3D porous network [85, 88]. One such 3D hybrid monolith produced by Huang et al. showed a BET surface area of up to 376 m2 g-1, with a hierarchical pore distribution ranging from sub-micrometer to several micrometers, along with a homogeneous dispersion of ultrafine Pt NPs (Figure 8) [84]. This interconnected and porous catalyst system showed an excellent EC activity, high tolerance to poisoning, and reliable stability when used as anode ECs for DMFCs (Figure 8d).

Recently we have also developed a 3D hybrid material, based upon a highly crystalline PTI rather than an amorphous gCNH [89]. Pt was deposited onto this 3D material via ethylene glycol reduction, yielding NPs with ~5 nm average size (Figure 9). Preliminary tests using a rotating disk electrode showed that the Pt/PTI-rGO hybrid construct exhibited enhanced ORR activity, with at least ten-fold higher current density than Pt on pristine PTI at 0.90 V (Figure 10). The material also displayed higher current density than Pt on rGO at 0.90 V, indicating that the combination of PTI and rGO improved the overall performance. Although the current density was lower than that of commercial Pt/C, these initial results are encouraging, considering the high durability of PTI. Further exploration of routes to enhance the interaction between gCNs and the conducting scaffold, the development of novel exfoliation pathways for gCNH and the improvement of methods to increase overall surface area and therefore access to N-containing catalytic sites, will significantly benefit applications for fuel cells.

4.3 Other gCN/C hybrid electrocatalysts and catalyst supports

Non-noble metal catalysts such as Fe and Co, supported on gCNH mixed/hybrids, have also been considered as alternative catalysts for the ORR and HOR [90-93]. Liu et al. reported excellent ORR electrocatalytic activity, comparable to commercial Pt/C with enhanced durability and

methanol tolerance, for a Co-doped gCNH/graphene based catalyst under alkaline conditions [91]. They attributed this to the formation of Co-N moieties, which acted as active sites and promoted rapid charge transfer at the Co-gCNH/graphene interfaces. They further modified the catalyst and developed a novel core-shell structure with CoO as the core and Co-doped gCNH/graphene as the shell. The resulting hybrid displayed similar ORR catalytic activity, but with an improved stability due to the 'self-healing' ability of the oxide core that functioned as a source of cobalt, slowly releasing metal atoms or ions to heal the damaged active surface sites [92].

The use of composite materials involving gCNH materials doped with, for example, P and S as EC supports represents another field of investigation. For example, although its overall performance was still below that of commercial Pt/C, P-doped gCNH grown on flexible carbon fibre paper (CFP) has been shown to exhibit enhanced ORR activity compared to its phosphorous-free counterpart, and comparable to Pt supported on CFP [94]. 2D nanohybrid structures based on graphene quantum dots decorated onto sulphur-doped gCNH have also been developed. This unique S-gCN@GQD hybrid material has shown enhanced ORR catalytic activity, which was comparable to those of well-developed graphene and GQD based catalysts [95]. Such heteroatom-doped gCN hybrid materials are still far from commercial application, however they do present an interesting path for constructing new types of gCNH-based materials for energy conversion applications.

5. Integrated catalysts containing carbon nitride components

Di Noto et al. have defined "carbon nitride-based EC" (CN-based EC) supports as being composed of a carbon-based matrix into which nitrogen atoms have been embedded through synthesis reactions [45]. This family therefore includes all of the N-doped carbons, which have been extensively reviewed elsewhere [36] and that form structures ranging from N-doped mesoporous carbon [96] to N-doped carbon nanotubes [43]. These materials generally have N contents <5 at% and they typically exhibit significantly different catalytic behavior to the gCN materials discussed above. CN-based ECs are often prepared by a one-pot synthesis, whereby solvated metal complexes and precursor molecules with high nitrogen content (either as one or multiple species) are adsorbed onto a carbon or carbonaceous precursor and the resultant composite material is pyrolyzed. The pyrolysis step acts to form the metal NPs and incorporate nitrogen into the carbon matrix [46, 47]. In its simplest form this methodology has produced effective bulk electrocatalysts [46], but these materials have poorly defined structures and intrinsic irreproducibility, which has a significant impact on their overall efficacy and lifetime performance [45].

More recently, a new class of CN-based ECs has emerged in which there is a much more ordered and highly localized concentration of N atoms. This has been achieved by devising strategies to intelligently incorporate catalyst precursors within carbonaceous matrices [97-99]. Through the development of 3D-crosslinked inorganic-organic networks, Di Noto et al. were able to coordinate metal ions with macromolecular N-containing ligands, which, through the use of a two-step 'soft' pyrolysis technique, was than graphitized to form a foam-like conductive carbon matrix in which NPs were held in stabilizing 'N coordination nests' [45, 97, 98]. This method has been refined further through the introduction of a core-shell approach [100, 101], whereby the metal and its surrounding N-containing matrix are constructed onto a conductive core. Through the use of these methods it was shown that the standard Pt NPs could be replaced with Pd in ORR ECs [102, 103] or a bimetallic system [104], while still achieving a reduction in ORR overpotential vs Pt/C [105]. High Pt loading core-shell structures have also been described by Wu et al. [99]. These ECs showed improved activity for methanol oxidation over Pt/C due to higher Pt utilization and more rapid charge transfer, although here the Pt was deposited after the N-containing shell was constructed.

6. Carbon nitrides as catalyst supports for PEM water electrolyzers (PEMWEs)

PEMWEs are a promising technology for producing hydrogen from renewable, intermittent energy sources such as wind and photovoltaics [1, 106, 107]. PEMWEs have many advantages over the traditional alkaline electrolyzer systems, for example, they are capable of achieving current densities over 2 A cm-2, reducing both operational and other costs of electrolysis [108]. In addition, PEMWE technologies provide very high purity hydrogen, giving the opportunity to obtain compressed gases directly from an installation. Furthermore, PWMEWs are ecologically clean, safer than other alternatives [109] and have a low gas crossover rate, allowing them to work over a wide range of power inputs [110]. However, current state-of-the-art PEMWEs also face significant challenges, mainly the need for relatively high loadings of precious metal ECs on both anode and cathode due to the corrosive acidic regime provided by the proton exchange membrane. These ECs must not only resist the corrosive low pH conditions (pH ~2), but must also sustain high applied overpotentials (~2 V), especially at high current densities [110].

Only a few materials, generally Pt group metals and their compounds, will perform effectively in operational PEMWE environments. Amongst those typically considered, the highest catalytic activity for the oxygen evolution reaction (OER) is exhibited by RuO2. Unfortunately, this material corrodes at an appreciable rate with oxygen evolution [111] and hence must be stabilized

by introduction of iridium or other elements. IrO2 can be used as an alternative EC [112, 113]. As with fuel cells, the high costs and low abundance of the catalytically active metals, has driven research into reducing their loading by dispersing them on low-cost supports that remain stable under operating conditions. Most approaches to-date have focused on supporting IrO2 on materials such as TiO2 [114], P-SiC [115], SnO2 [116], Ta2O5 [117], Nb2O5 [118] and Sb2O5 [119]. Because gCN and gCNH materials have been attracting attention as fuel cell catalyst supports with potentially superior properties, it is of interest to study their performance in a PEMWE environment.

Jorge Sobrido et al. presented preliminary data for a bench scale, gCN-based, PEMWE operating at 50 °C and atmospheric pressure [120]. The results demonstrated that use of gCNH as a catalyst support at the anode improves the OER charge transfer kinetics and therefore the electrocatalytic activity (Figure 11). Polarization measurements in this study showed high operating potentials for the gCNH supported EC, relative to unsupported IrO2. However, the impedance data indicated that the gCNH supported IrO2 has significantly smaller charge transfer resistance than the unsupported IrO2 at higher current density, with the most significant loss associated with higher Ohmic resistance. This implies that there may be an avenue to optimize a gCNH-based electrode with reduced catalyst loading, although steps will need to be taken to improve the electronic conductivity of the catalyst.

7. Conclusion and outlook

Given the clear advantages associated with the incorporation of nitrogen into EC supports, there is great promise for future catalyst supports based on gCN materials. gCN supports have the potential to better anchor Pt and other metal NPs, improve the catalyst dispersion and durability, reduce the catalyst loading, and lead to improved catalyst performance. The results reviewed above demonstrate the progress already made with this this unique family of materials for EC development for fuel cell and electrolyzer applications. Now it will be necessary to cultivate a deeper understanding of the catalyst/support interactions and the role of the gCN or gCNH phases in determining the OER and ORR reaction kinetics. It is also essential to optimize the gCN structure and chemistry, including the CxNyHz stoichiometry, and work on the incorporation of these materials in hybrid electrocatalysts and catalyst support combinations with high intrinsic catalytic activity, surface area and electrical conductivity. The continued development of new technological inks and 3D porous networks based on gCN materials is, likewise, an area for future development,

as is the preparation of new families of integrated NP catalysts, where the active metal sites and protective N-rich regions are incorporated along with conductive graphene. As these new materials are investigated and efficient catalyst-support combinations identified and tested, it will be critically important to advance efficient, low-cost and scalable processes for their synthesis and commercial production, especially via drop-in technologies that can enter the marketplace rapidly.

Acknowledgements

This project has received funding from the European Union's Graphene Flagship under Horizon 2020 research and innovation programme grant agreement No. 696656 - GrapheneCore1 and from the EPSRC EP/L017091/1. C.M. acknowledges the award of a Royal Society University Research Fellowship by the UK Royal Society.

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Figure 1. Structural motifs found in graphitic carbon nitrides. (a) Liebig's melon ([C6N7(NH2)(NH)]n) contains zig-zag chains of heptazine (tri-s-triazine) units linked by bridging nitrogens and decorated on their edges by N-H groups, (b) fully condensed C3N4 layer based on heptazine units, (c) fully condensed C3N4 layer based on triazine units, (d) PTI backbone based on triazine ring units, linked by N-H bridges and e) Side elevation of fully occupied PTI.LiCl.

Figure 2. Calculated electronic density of states for conductive C (graphene) and a selection of three representative layered gCN or gCNH structures including polyheptazine (PH) and polytriazine (PT) varieties of C2N3H, and one PT layer of g-C3N4. The calculations were carried out using density functional theory and the PBE functional under periodic boundary conditions. Vertical lines indicate the standard H+/H2 and O2/H2O potentials.

Figure 3. (a) Free energy plots of ORR and geometry optimized configurations of adsorbed species on a model gCNH surface with zero (path I), two (path II), and four electron (path III) participations. Grey, blue, red and white represent C, N, O, H atoms, respectively. (b-d) Schematic pathway of ORR on gCNH with zero electron, gCNH with two electron participation, and gCNH -conductive support composite with four electron participation, respectively. Red areas represent the catalytically active sites. Reprinted and modified with permission from [69]. Copyright 2011 American Chemical Society.

Figure 4. gCNH-rGO composite structures (a) XPS spectrum of the C1s region of a gCNH-rGO composite prepared via simultaneous rGO reduction and gCNH formation reactions. (b) SEM image of gCNH-rGO composite synthesized by simultaneous rGO reduction and gCNH formation. (c) TEM image of Pt dispersion on gCNH-rGO composite. (d) Average particle size and size distribution of Pt nanoparticles. (e) ECSA of Pt on supports obtained from cyclic voltammetry in deoxygenated 0.1 M HClO4.

Figure 5. (a) The ORR polarization curves of Pt/gCNH, Pt/rGO, Pt/C (Alfa Aesar), and Pt/gCNH-rGO at 1600 rpm in 0.1 M HClO4. (b) Detail of the ORR polarization curves highlighting the overpotential region. (c) Specific activity and (d) Mass activity of the materials obtained at 0.90 V.

Figure 6. (a) Fuel cell polarization curves of the Pt/C, Pt/gCNH-rGO and Pt/rGO as anode in H2/O2 cell at 80°C. (d) Change in electrochemical surface area with respect to potential cycling (1.0 - 1.6 V) in 0.1 M HClO4 at room temperature.

■ 0.6 -

"c 0.4

—•— Pt/carbon

—«— Pt/gCN-rGO

—Pt/rGO

200 400 600

Current Density / mA cm

0.0-Vr

Number of cycles

Figure 7. 3D gCN structures (a) Schematic illustration of the preparation of a 3D porous gCN. (b) SEM and TEM images of porous g-C3N4 monolith. (c) N2 sorption isotherms of the monolith. (d) Photographs of monolith showing that it could support a 100 g weight. Reprinted with permission from [83]. Copyright John Wiley and Sons.

Figure 8. 3D hybrid gCN structures. (a) Schematic illustration of 3D porous rGO-carbon nitride (CN) architectures prepared by hydrothermal reaction, Pt NPs are on the surfaces of both graphene and g-C3N4 nanosheets. (b, c) TEM morphological and structural analysis of a Pt on graphene-CN hybrid (3:7 ratio, Pt/G3-(CN>), Insets in c: HRTEM image and Pt NP size distribution. (d) Cyclic voltammograms of the Pt/G-CN architecture (Pt/G3-(CN)?), Pt/G, Pt/CN and Pt/C in 1 M H2SO4 and 2 M methanol solution at a scan rate of 20 mV s-1, reflecting the highest electrocatalytic activity of Pt/G3-(CN)7. Reprinted with permission from [84]. Copyright John Wiley and Sons.

Figure 9. Structure of PTI-rGO 3D hybrid aerogel catalyst support (a) SEM image of PTI-rGO 3D hybrid aerogel (b) TEM image of Pt dispersion on PTI-rGO hybrid aerogel. (c) Average particle size and size distribution of Pt nanoparticles.

Figure 10. Electrochemical testing of PTI and 3D PTI-rGO hybrid catalyst supports (a) ORR polarization curves of Pt supported on PTI, and rGO, and PTI-rGO hybrid, in comparison to commercial Pt/C (Alfa Aesar), at 1600 rpm in 0.1 M HClO4. (b) Detail highlighting the ORR overpotential region.

0.0 0.2 0.4 0.6 0.8 1.0 0.80 0.85 0.90 0.95 1.00

Potential vs RHE / V Potential vs RHE / V

Figure 11. Electrochemical data for unsupported IrO2 (40%) and gCN supported IrO2 (40%) as anode catalysts (both with 1.6 mg cm-2 metal loading) for a PEMWE. (a) Cell potential (V) vs current density measurements and (b) Nyquist plots of imaginary vs real parts of the complex impedance function (Z) for PEMWE tested with IrO2 nanoparticles supported on gCN (IrO2(40%)-gCNH) and pure IrO2 nanoparticles (IrO2(40%)-Unsupported) both of which were applied onto a Nafion membrane and used as anode electrocatalysts operating at 80 °C, ambient pressure.