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Energy Procedía 34 (2013) 439 - 448
10th Eco-Energy and Materials Science and Engineering (EMSES2012)
Electrochemical Performance of Ni1-xCox-GDC Cermet
Anodes for SOFCs
Jiratchaya Ayawannaa*, Darunee Wattanasiriwecha, Suthee Wattanasiriwecha and Kazunori Satob
aDepartment of Materials Science, School of Science, Mae Fah Luang University, Muang, Chiang Rai 57100, Thailand bDepartment of Environmental Engineering, Faculty of Engineering, Nagaoka University of Technology, Nagaoka, _Niigata 940-2188, Japan_
Abstract
Electrochemical performance of Ni1-xCox-GDC (Gd01Ce0.9O195) cermet was studied for the use as a novel anode for solid oxide fuel cells (SOFCs). Ni1-xCoxO (x=0.25, 0.5, 0.75) was prepared from NiO and Co3O4 compounds via a solid state calcination at 1000°C for 10 h in air. A progressive increase in lattice parameters of the Ni1-xCoxO as x was increased suggested the extent of solid solution between NiO and Co3O4. The trace of free Co3O4 was found only in the Ni0.25Co0.75O calcined powder indicating incomplete dissolution between NiO and Co3O4 at this condition. After reduction at 800°C for 2 h in H2, the interfacial polarization resistance and current-voltage (I-V) of the Ni1-xCox-GDC//YSZ//LSM-YSZ cells were measured at the temperatures between 650 and 800°C. Grain growth and coarsening of the Ni1-xCox-GDC cermet was generally observed when the amount of Co atomic fraction was increased especially at x = 0.75. The Ni0 75Co025-GDC anode exhibited the better uniform micro structure, as compared to other composite anodes, resulting in enhanced electrochemical performance of the cells. When the grain coarsening occurred, suppression of the overall electrochemical performance of the cells was obtained.
© 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT)
Keywords: Electrochemical performance; Interfacial polarization resistance; Nii-xCox-GDC anodes; Solid oxide fuel cells
* Corresponding author. Tel.: +66-82-180-7870. E-mail address: jiratchaya.jjj@gmail.com.
1876-6102 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology
Thanyaburi (RMUTT)
doi: 10.1016/j.egypro.2013.06.772
1. Introduction
Nickel-gadolinia doped ceria (Ni-GDC) cermets have long been used as an anode material for hydrogen oxidation in solid oxide fuel cells (SOFCs). The catalytic activity and the degradation of Ni-GDC anode in operating conditions play an important role in determining SOFC performance [1,2]. Coarsening of Ni particles, resulting in a decrease of the triple phase boundary length of the anode, has been reported as one of the main causes for electrical degradation of the cells [1,3]. Alloying Ni with another metal has been considered as an alternative method used for suppression of Ni particle coarsening. Some metal alloys also showed additional effects on improvement of the electrical conductivity, oxidation resistance and thermal properties of the alloyed anode [1,2,4,5]. The desired properties of the alloyed anodes are determined by high surface area, high thermal stability and excellent fuel-oxidation ability. A study by Ringuede et al. on (Ni,M)-YSZ cermet anodes (M=Cu, Fe, Co) showed that when M was Cu (mp = 1083°C) particle coarsening and a decrease of specific surface area of the anodes were obtained [6]. Degradation of the YSZ electrolyte due to inward diffusion of Cu was also observed. The specific surface area of (Ni,Fe)-YSZ and (Ni,Co)-YSZ cermets, however, were comparable to that of Ni-YSZ cermet because the melting temperatures of these metallic Ni, Fe and Co and their oxides were >1400°C, which were higher than typical firing temperatures of the cermet anode (1250-1350°C). Coarsening problem thus could be avoided by using (Ni,Fe)-YSZ or (Ni,Co)-YSZ cermet anodes. The polarization resistances of (Ni,Cu)-YSZ was thus higher than that of the (Ni-Fe)-YSZ and (Ni,Co)-YSZ anode [7]. Effects of transition metal additions (Mn, Ag, Fe and Co, 10 mol%) to Ni-SDC anode, using LSGM electrolyte, were studied by Ishihara et al. [8]. They found that the activation energy for anodic reaction in both Ni-Mn and Ni-Ag alloyed anodes was higher than that of Ni-Fe or Ni-Co alloyed anode. This indicated that the activity of reaction sites in the Ni-SDC might be decreased by the addition of Mn or Ag. An increase of anodic overpotential was thus found in both Ni-Mn and Ni-Ag alloyed anodes, resulting in decreased power density of SOFCs. A decrease in the electrical conductivity of Ni1-xFex-GDC (x=0-0.15) anode with an increase of Fe content was observed by Cho et al. [9]. The disruption of Ni atom ensembles by Fe atoms decreased number of reaction sites of the anode. The electrochemical performance of the cells in hydrogen atmosphere was thus degraded with the use of Ni1-xFex-GDC anode.
In case of cobalt addition, the melting temperatures of both cobalt oxide (1935°C) and cobalt metal (1495°C) are higher than that of Ni. The significant positive effects as a result of Co addition with different contents on microstructure and performance of the Ni-YSZ anode and overall cell have been reported by many researchers. A decrease in anodic polarization resistance was achieved for the Ni0.5Co0.5-YSZ and Ni0.3Co0.7-YSZ anode which attributed to an enlarged reaction area or triple phase boundary [1]. Similarly, Ishihara et al. has reported a decrease in anodic overpotential (rj) and IR losses of Ni anode, resulting in an increase of the overall cell performance when 10 mol% of Co was introduced to this Ni anode [8]. Using the same Co content, Ni-Co alloy hydrogen electrode for an alkaline fuel cell (ACE) was studied by Chatterjee et al. [5]. They reported that Co could reduce the reduction potential for hydrogen either by improving electrocatalytic behavior of Ni or by decreasing the resistance of Ni-electrode. A decrease in the particle size together with an improvement of electrical conductivity of Ni-electrode was found by addition of 10 mol% Co. This associated with an enhancement of surface area for chemisorption of hydrogen which further improved the catalytic activity of hydrogen oxidation of Ni-electrode when Co was incorporated. Improved performance of anode and overall cell was also reported by Grgicak et al. after 8 mol% of Co was added into the Ni-YSZ anode [4]. This was attributed to larger surface area but smaller particle size of the Nia92Co0.08-YSZ cermet as compared to that of Ni-YSZ. With the use of Ni-GDC anode, Cho et al. recently reported that progressive alloying Ni with Co (10-20 mol%) in Ni-Ce0.8Gdo.2Oi.9 anode had a negative effect on the anode conductivity and polarization resistance under hydrogen atmosphere due to enhanced electron scattering that was cause by the solid solution effect based on Nordheim's rule [10]. The effect of Co alloying in Ni-GDC anode was likely different from the
study in Ni-YSZ anode. However, the report for the effect of Co addition on the electrochemical performance of Ni-GDC anode and overall cell still rarely observed in a wide range of Co content. Ni-Co alloys were thus selected to study the applicability as an alternative to conventional Ni anode material for SOFCs in this work. Nii-xCoxO-GDC anode composites with the Co fraction (x) of 0, 0.25, 0.5 and 0.75 were explored. The selected contents of Co were extended from above reviews in order to further investigate a change in anode microstructure and a trend of anode performance. Study of the overall cell performance with the use of the Ni1-xCox-GDC cermet anodes was also addressed.
2. Experimental
2.1. Powder preparation and characterization
10 mol% gadolinia-doped ceria (GDC) powder was prepared from cerium nitrate hexahydrates (Ce(NO3)3-6H2O, 99.99%) and gadolinium nitrate hexahydrates (Gd(NO3)3-6H2O, 99.99%) through the sol-gel combustion method. The mixture of metal nitrate precursors in a stoichiometric amount was dissolved in distillated water and mixed with citric acid monohydrate (C6H8O7-H2O) solution in a molar ratio of 2:1. The solution was heated until the yellowish ash-like powder of GDC powder was formed. The powder was consequently calcined at 700°C for 2 h in air to remove remaining carbon residue. Ni^ xCoxO (x=0.25, 0.5, 0.75) compounds were prepared from nickel oxide powder (NiO) (99%, American Elements) and cobalt oxide powder (Co3O4) (99%, Nacalai Tesque, Japan) in the ratio of 75/25, 50/50 and 25/75 w/w through the solid solution method. The powders were mixed in acetone using an agate mortar. The mixed powders were calcined at 1000°C for 10 h under surrounding air to obtain the Ni1-xCoxO powders with desired compositions.
Phase identification was carried out for all calcined powders using the X-ray diffractometer (Rigaku RAD-3A) powered at 40 kV and 30 mA with Cu-Ka radiation and graphite monochromator. Crystallite size of all powders was determined using Debye-Scherrer equation (Eq. 1) from the line broadening of the peak at around 20=28.6° for the GDC phase and 20 range from 36.8° to 37.3° for the Ni1-xCoxO (x=0-0.75) phase.
0-M. (1)
/?cos0
where D is crystallite size in nm, A is the radiation wavelength (0.1540598 nm with Cu target), 9 is the diffraction angle at the line broadening peak, and is the corrected line width (in radian unit) at half peak intensity. The lattice parameter ( a ) of all powders was calculated using the relationship as shown in Eq. 2.
= d (V h2 + k2 +12) (2)
where d is the spacing of any particular set of lattices, hkl are the miller indices of the planes of diffraction.
2.2. Cell preparation and testing
A Ni1-xCoxO-GDC anode//8YSZ electrolyte//LSM-YSZ cathode single cell were fabricated using a screen printing method. The anode powder consisting of 60 wt% of Ni1-xCoxO (x= 0, 0.25, 0.5, 0.75) and
40 wt% of GDC were prepared by mixing and grinding up the mixtures in acetone using an agate mortar. The Nii_xCoxO-GDC composite powders were mixed with the glycerin binder to form anode slurries. The slurries were applied on the yttria-stabilized zirconia (8YSZ) electrolyte disks (15 mm diameter, 0.5 mm thick) and sintered at 1300°C for 3 h in air. The composite cathode powder was prepared using Lao.85Sro.i5MnO3 (LSM) (SEIMI, Japan) and 8YSZ (Tosho, Japan) at a ratio of 70/30 w/w. Glycerin binder was mixed with the LSM-YSZ cathode powder to obtain a cathode slurry which was then coated onto the other side of the YSZ disk and then sintered at 1200°C for 3 h in air. Pt paste (TR-7070, Tanaka Kikinzoku International, Japan) was painted on the anode side as a reference electrode. Pt wire was used as a current collector. The anode and cathode sides were fed with H2/O2 as fuel/oxidant. Current CD-Voltage (V) characteristics of the cells from 700 to 800°C were measured using the current interrupted method. The impedance analysis was performed using a frequency respond analyzer (FRA5097) under the frequency range of 0.1 Hz-500 kHz in the temperature range of 700-800°C. Microstructures of the tested anodes were examined using an electron microprobe analyzer (Shimadzu EPMA-1600).
3. Results and discussion
3.1. Phase identification
The calcined GDC powder exhibited a cubic fluorite structure corresponding to the Gd0.1Ce0.9O1.95 phase (ICSD 01-075-0161). The Ni1-xCoxO (x=0.25-0.75) powders exhibited a cubic structure similar to the starting NiO powder (ICSD 01-073-1519) (Figure 1). Crystallite size of the calcined GDC powder calculated with the Debye-Scherrer equation was around 41 nm, while the crystallite size of the calcined Ni1-xCoxO (x=0.25-0.75) powders was in the range of 53-58 nm. A downward shift of 20 positions together with an increase of lattice parameters of NiO were observed when the Co atomic fraction was increased suggesting the dissolution of the larger Co (roo2+ = 0.74 A [4]) into the smaller Ni sites (rM2+ = 0.69 A [4]) (Figure 2). However, segregation of Co3O4 phase was detected when Co atomic fraction was 0.75, indicating incomplete dissolution between NiO and Co3O4 at this condition.
V 1 « Jt* r x=0.75 V 1 i v V ». « K K
V i ! . x=0.5 V [ I Y Y
XI x=0.25 ! 1 Y Y
0 JL x=0 0 1 o c il h ki
30 40 50 60 70
26 (Cu-Ka)/degrees
0.25 0.5 0.75 Cobalt Atom Fraction (x)
Fig. 1. XRD patterns of M^Co^O powders calcined at 1000°C Fig. 2. Lattice parameter of Ni^C^O (x=0-0.75)
for 10 h (O ) NiO (V ) Ni1.xCoxO and ($) after calcined at 1000°C for 10 h
• « I: ^Uw-'i*** ¡•i x=0 • $ 75 'S.
• HI' , 1.....1 „ 1... • 1 ^ ' u x=0 • ir 1 .1 5
• 1 A * i I • 1 < x=0,25
• i i • 1 • x=0 T 3Li
.......1' ........1......
20 30 40 50 60 70 29 (Cu-Ka)/degrees
Fig. 3. XRD patterns of Nii.xCoxO-GDC composite powders calcined at 1300°C for 3 h ( ( 0 GDC
and (■&) Ni1-xCoxO
Fig. 4. Lattice parameter of GDC phase as a function of Co Fig. 5. Lattice parameter of Nii-xCoxO phase (x=0-0.75) in content in Ni1-xCoxO-GDC composite powder Nii-xCoxO-GDC composite powder
calcined at 1300°C calcined at 1300°C
The Ni1-xCoxO (x=0-0.75) powders were mixed with the GDC powder in acetone using an agate mortar. The mixtures were subsequently calcined at 1300°C for 3 h in air, which is the same firing condition of anode layer coating, to obtain the Ni1-xCoxO (x=0-0.75)-GDC anode powders. Phase transformation of these Ni1-xCoxO-GDC composite powders was examined and the result is shown in Figure 3. The result did not show any second reaction phase, indicating that there was no interaction between Ni1-xCoxO and GDC phase at this firing temperature. The XRD patterns of Nio.75Co0.25O-GDC and Ni05Co05O-GDC powders were similar to that of the NiO-GDC powder, while peak shift to the lower degree two theta positions was observed. The lattice parameter (a) of GDC remained unchanged when
Co atomic fraction was increased (Figure 4), while that of NiO phase in Ni1-xCoxO-GDC anode mixture showed a similar trend as in the pure Ni1-xCoxO powder (Figure 5).
3.2. Microstructure analysis
Microstructure of the Ni1-xCox-GDC (x=0-0.75) cermets after reduction in H2 is shown in Figure 6. Although pore formers were not used, all cermet anodes had enough pores to allow for gas permeation. There was no significant change in porosity and grain size of the Ni-GDC cermet when Co with atomic fraction of 0.25 was incorporated (Figure 6b). However, coarsening of the Ni grains was slightly reduced and spherical GDC grains were obtained for the Ni0.75Co0.25-GDC cermet anode. A significant change of microstructure was not found after further increase of Co atomic fraction to 0.5 (Figure 6c). Large grain clusters as a result of significant coarsening were observed with further increase of Co atomic fraction to 0.75 (Figure 6d). This result was in good agreement with the morphology change of Ni-Co alloys studied by Srivastava et al. [11]. The decomposition of free Co3O4 to CoO compound at the temperature higher than 950°C has been reported [12]. Our previous study also showed that the coexistence of Co3O4 and CoO was observed when Co3O4 phase was heated to 1400°C in air [13]. In addition, the eutectic temperature of the GDC-CoO system was found to occur at around 1200°C [14]. The CoO could possibly be dissociated from the free Co3O4 phase and acted as a liquid sintering aid so grain coarsening was general observed in the Ni0.25Co0.75-GDC.
Fig. 6. Microstructure of Ni1_xCox-GDC cermets (a) x=0, (b) x=0.25, (c) x=0.5, (d) x=0.75
3.3. Electrochemical performance
After reduction at 800°C for 2 h in hydrogen in order to obtain the metallic phases of Ni or Ni1-xCox in the cermets, the voltage (E) and power density (P) as a function of the current density (J) for the single cells were measured and compared in Figure 7. The open circuit voltage (OCV) measured at 800°C for the cells with Ni1-xCox-GDC (x=0-0.75) anodes were comparable. The current-voltage curve (I-V) showed a declining trend in the whole current ranges reflecting electrochemical and ohmic polarization. Since the electrolyte and cathode of these cells were prepared similarly, the difference in the slopes of these I-V curves was consequently mainly caused by the difference in anodic polarization resistances (Rp) of the cermet anodes.
The highest power density (62 mW-cm-2 at 800°C) was achieved for the single cell with Ni075Co0 25-GDC anode (Figure 7). The lowest anodic polarization resistance was also obtained for this anode (Figure 8). The anode reaction of the SOFC under hydrogen atmosphere was generally considered to be a multiphase, including the catalytic activity, adsorption/desorption of the hydrogen molecule, gas diffusion and the electronic conductivity of anode materials which were critical for maximizing an electrochemical performance of the cells [9]. An enhanced cell performance with the use of Ni075Co0 25-GDC anode was probably due to more uniformity of grain connectivity as a result of less-grain coarsening. In addition, a characteristic of Ni-Co alloy which exhibited better electrical conductivity than Ni [5] could probably partly contribute to the enhancement of the cell performance. The cell performance was largely degraded for the Ni0.5Co0.5-GDC and especially Ni0.25Co0.75-GDC anode (Figure 7). The microstructure of Ni0.5Co0.5-GDC, however, was not clear to explain this phenomenon since a significant difference of microstructure was not observed compared to that of the Ni075Co025-GDC (Figure 6), and further careful investigation is needed to clarify this point. In the Ni0.25Co0.75-GDC anode largely degraded overall cell performance was believed due to a decrease in the anode reaction area caused by dramatic grain coarsening after increasing Co atomic fraction to 0.75 (Figure 6d).
Fig. 7. Cell voltage (open symbols) and power density (solid symbols) vs. current density plots at 800°C for the cells
with Nii_xCox-GDC cermet anodes
Fig. 8. Arrhenius plot of Rp'' for Nii_xCox-GDC anodes in H2
The impedance spectra for the single cells using the Ni1-xCox-GDC (x=0-0.75) cermet anodes obtained under an open circuit condition at 800°C are shown in Figure 9. The intercept on the real axis at high frequencies determined as the cell ohmic resistance (Rn), which includes electrode and electrolyte resistances and the resistance due to the contact between the working electrode and the electrolyte. The width of impedance arc on the real axis refers to the electrode interfacial resistance (RE) which related to the polarization performance of the electrodes. The intercept at low frequencies on the real-axis is determined as the total cell resistance (Rn + RE). [15-16].
A Ni-GDC
o Nio.75Coo.25-GDC
□ Nio.5Ccoj-GDC
8 H3 XN1025C0075-GDC
1.3H, 10 H2 ^
■ tf ■—m—1—'-*-
Re(Z)/£2rm
Fig. 9. Impedance spectra at 800°C for the cells with Ni1-xCox-GDC cermet anodes
From Figure 9, impedance spectra showed a similar trend to the cell performance. The total cell resistance (22 i2-cm2) and ohmic resistance Q were lowest in the cell with Nia75Co025-GDC anode. This suggested that the cell performance was related to the interfacial resistance of the anodes which in turn was dependent on both the electrical properties and microstructure of anode material.
However, the thickness of the YSZ electrolyte was kept relatively constant at 0.5 mm and preparation of the cathode was identical, the electrolyte resistance and cathodic interfacial resistances for these cells
were presumably the same. The minimum Rn value was thus mainly attributed to a decrease in anodic resistance due to an improved electrical conduction of Ni after Co alloying, the electrical contact between the Ni-Co alloy grains and also the contact resistance between the Ni0.75Co0.25-GDC cermet anode and the YSZ electrolyte [16].
Overpotential (/7) and Rn values was reported to be useful parameters to differentiate the overall electrode performance of anodes prepared from different conditions [16]. However, there was significant difference in the Rn values between various anodes in this study, the electrode performance of Ni1-xCox-GDC (x=0-0.75) anodes was thus characterized by those two parameters and also RE values, which are
graphically compared in Figure 10. The result showed the lowest values of /7 , Rn and RE for the Ni075Co0 25-GDC anode, indicating the better electrode performance of Ni075Co0 25-GDC anode compared to the others. This might be related to the uniform microstructure and the lesser degree of particles coarsening that enhanced the triple phase boundary length and thus increase in the electrode/electrolyte contact area in Ni0 75Co0 25-GDC anode as indicated by the decreased RE .
Fig. 10. A comparison of electrode performance of Nii_xCOx-GDC cermet anodes
From the above results, it can be implied that the uniform microstructure with minimum Ni grain coarsening was obtained when 25 wt% of Co3O4 was incorporated into the Ni-GDC cermet anodes (Nia75Co0 25-GDC). An improved electrical properties and microstructure, indicated by reduced Rn of the Ni0.75Co0.25-GDC anode, led to an increase in (i) electrical contact areas between the anode-anode grains and also (ii) the contact areas between the anode-electrolyte grains resulting in an extension of the triple phase boundary lengths. This suggested by a decrease in RE and total cell resistance, leading to an increase in power density of the cell when the Ni075Co025-GDC anode was employed.
4. Conclusion
Alloying conventional NiO with Co3O4 at the weight fraction of 25 % prior to mixing with GDC (Ni075Co025-GDC) resulted in an improvement of cell performance of the Ni-GDC anode due to an increase in electrical contacts between anode-anode and anode-electrolyte. Progressive degradation, without yet a clear reason, of overall performance was found for the Ni-GDC anode when Co3O4 was increased to 50 wt% (Ni05Co05-GDC). Some segregation of the free Co3O4 was observed with further increase the Co3O4 content up to 75 wt% (Ni0.25Co0.75-GDC), suggesting that the solubility of Co3O4 in NiO lattice was well below this level. CoO phase dissociated from free Co3O4 could possibly aid particle densification resulted in grain growth and large grain coarsening in the Ni0.25Co0.75-GDC porous cermets after sintering at 1300°C. A decrease in the surface area for anode reaction resulted in the degradation of the overall cell performance.
Acknowledgements
The financial supports from the Royal Golden Jubilee Ph.D. Program of Thailand Research fund, Thailand and Mae Fah Lunag University are highly appreciated. Environmental Materials Science Laboratory, Department of Environmental Engineering, Nagaoka University of Technology, Japan is gratefully acknowledged.
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