Improved Electrochemical Properties of an Ni-Based YSZ Cermet Anode for the Direct Supply of Methane by Co Alloying with an Impregnation Method

: To avoid the proneness to degradation due to coking in the operation of solid oxide fuel cells (SOFCs) directly running on methane (CH 4 ) fuels, a modiﬁed porous anode of the Ni 1 − X Co X / YSZ (yttria-stabilized zirconia) cermet prepared by an impregnation method is presented. The inﬂuence of the Co alloying content on the cermet microstructure, SOFC characteristics, and prolonged cell performance stability has been studied. Co was incorporated into Ni and formed a solid solution of Ni 1 − X Co X alloy connected with the YSZ as the cermet anode. The porous microstructure of the Ni 1 − X Co X / YSZ cermet anode formed by sintering exhibited a grain growth with an increase in the Co alloying content. The electrochemical performance of the cells consisting of the Ni 1 − X Co X / YSZ cermet anode, the YSZ electrolyte, and the LSM (La 0.8 Sr 0.2 MnO 3 ) cathode showed an enhancement by the Ni 1 − X Co X impregnation treatment for the respective supply of H 2 and CH 4 to the anode. The cell using the Ni 0.75 Co 0.25 / YSZ cermet anode (the Ni 0.75 Co 0.25 cell) showed the highest cell performance among the cells tested. In particular, the performance enhancement of this cell was found to be more signiﬁcant for CH 4 than that for H 2 ; a 45% increase in the maximum power density for CH 4 and a 17% increase for H 2 at 750 ◦ C compared with the performance of the cell using the Ni / YSZ cermet anode. Furthermore, the prolonged cell performance stability with a continuous CH 4 supply was found for the Ni 0.85 Co 0.15 and Ni 0.75 Co 0.25 cells at least for 60 h at 750 ◦ C. These enhancement e ﬀ ects were caused by the optimum porous microstructure of the cermet anode with the low anodic polarization resistance.


Introduction
Solid oxide fuel cells (SOFCs) have been attracting much interest as a role of the alternative electric power supply system. The main feature of SOFCs is high power generation efficiency due to the direct conversion of chemical energy into electricity, which is not limited by the Carnot efficiency, and thus solution with the composition of 60% Ni 1−X Co X + 40%YSZ (wt %) under continuously vigorous stirring at room temperature. Each suspension was stirred for 2 h and the temperature was subsequently raised up to 180 • C for dehydration. The dehydrated powder was calcined at 800 • C for 5 h in air.
The calcined powder was manually ground in an agate mortar with a pestle for 1 h in ethanol, and the dried ground powder was mixed with glycerol to prepare slurries. The slurry was painted as the anode at one face of the YSZ electrolyte disk (15 mm diameter, 0.23 ± 0.01 mm thick and surface polished, Nikkato, Osaka, Japan). The slurry-painted YSZ disk was heated at 1300 • C for 3 h in air. The La 0.8 Sr 0.2 MnO 3 -YSZ composite was used as the cathode material; it was prepared by painting a slurry of the 70%La 0.8 Sr 0.2 MnO 3 (LSM, Seimi Chemical, Chigasaki, Kanagawa Prefecture, Japan) + 30%YSZ (wt %) powder mixture with glycerol to the other face of the YSZ disk and heated at 1200 • C for 3 h in air to form the La 0.8 Sr 0.2 MnO 3 -YSZ (denoted as LSM-YSZ) cathode. The geometrical area of both the anode and cathode was 0.28 cm 2 with approximately 70 µm thick. As the reference electrode a Pt paste (TR-7603, Tanaka Kikinzoku Kogyo, Tokyo, Japan) was painted as a small circle close to the anode with a small gap at least 2 mm away from the edge of the anode circle. A circle with a 3 mm diameter of Pt mesh (# 100 mesh), spot-welded with a 0.3 mm thick Pt wire, was respectively used as a current collector for the anode and cathode.
The prepared cell, denoted as the Ni 1−X Co X cell, was fired at 850 • C for 30 min to soften Pyrex ® glass rings which were used to respectively seal the outer edge of the anode face and that of the cathode each with an alumina tube (15 mm outer diameter and 13 mm inner diameter) edge. The temperature of the cell was decreased to 800 • C and the anode face was exposed to an H 2 atmosphere for 2 h to obtain the metallic Ni 1−X Co X prior to the measurement of the single cell performance at 750 • C. The flow rate of H 2 (99.9% purity) was fixed at 20 cm 3 min −1 throughout the measurement. Oxygen gas was fed to the cathode at a flow rate of 20 cm 3 min −1 . The cell performance was evaluated by the current-voltage characteristics. Electrochemical impedance spectra were recorded with a frequency response analyzer (FRA5097, nF) under the open circuit condition. The anodic overvoltage was measured with a digital oscilloscope (TDS2012, Tektronix, Beaverton, OR, USA) by the current interruption method using a current pulse generator (NCPG-101, Nikko Keisoku, Akashi-Shi, Japan). After the cell performance measurement by supplying H 2 to the anode, the fuel gas was switched to a mixture of 20 vol%CH 4 (99.0% purity) and 80 vol% He (99.995% purity) fed at a total flow rate of 20 cm 3 min −1 . The electrochemical measurement was made between 550 and 750 • C. Subsequently, the operating temperature was kept at 750 • C, and the terminal voltage and impedance spectra were recorded at 80 mA to investigate the performance stability for 60 h. Separately, the symmetrical cells using the same YSZ electrolyte disk sandwiched on both sides with the Ni 1−X Co X (x = 0, 0.25)/YSZ cermet electrodes were prepared under the same method described above. The electrode interface conductivity was determined by the electrochemical impedance spectroscopy in a CH 4 atmosphere for the prepared symmetrical cells, which were treated in the same manner as prepared for the anode. The reproducibility of cell performance, including the prolonged stability test was confirmed at least two times for the Ni cell and the Ni 0.75 Co 0.25 cell.
Microstructural observations of the Ni 1−X Co X /YSZ cermet anodes were made by scanning electron microscopy (SEM, SU8000 and TM3000, Hitachi High-Tech, Tokyo, Japan). The carbon deposition to the surface of the Ni 1−X Co X /YSZ cermet anode after the performance stability test supplied with CH 4 was investigated with the SU8000 combined with an energy-dispersive X-ray spectrometer (EDX, X-Max N 80, Oxford Instruments, Abingdon, UK). The Ni 1−X Co X O-YSZ powders for phase identification were prepared in the same method as described above and reduced under the same condition to obtain the Ni 1−X Co X /YSZ cermet powders. The phase identification of theses powders was made by X-ray diffraction (XRD, HF-2100, Rigaku, Tokyo, Japan) with the monochoromated Cu-Kα emission powered at 40 kV and 30 mA.  Figure 1 shows the XRD patterns of the Ni 1−X Co X /YSZ cermet powders. The diffraction peaks both from the Ni 1−X Co X and YSZ phases were indexed as the face-centered cubic (fcc). Increasing the Co content resulted in a decrease in the peak intensities of the Ni 1−X Co X phase with a shift in the peaks to the low diffraction angle side due to the incorporation of Co atoms having the larger atomic size (1.52 Å) than that of Ni (1.49 Å) into the fcc Ni lattice [24]. The observed peak-intensity decrease of the Ni 1−X Co X phase with the increase in the Co content corresponded with the similar decrease of the as-heated Ni 1−X Co X O phase at 1300 • C, as shown in Figure S1. This intensity decrease appears to be connected with an insufficient crystallization of the Ni 1−X Co X O solid solution on heating. In contrast, the peak intensities of the YSZ phase showed an increase in accordance with an appearance of the sharp peak pattern with the increase in the Co content, for example, the 311 reflection exhibiting well resolved Kα 1 and Kα 2 peaks, as shown in Figure 1. Heating the dehydrated Ni 0.5 Co 0.5 O/YSZ precursor at 800 • C led to an appearance of the Co 3 O 4 phase, as shown in Figure S2. The Co 3 O 4 phase disappeared after subsequent heating at 1300 • C in air. Since Co 3 O 4 melts at 895 • C and decomposes into CoO at about 950 • C, Co 3 O 4 can serve as a sintering aid for the mixed Ni 1−X Co X O and YSZ powders. Although the Co 3 O 4 phase was not identified for the Ni 1−X Co X O-YSZ (x = 0.05, 0.15 and 0.25) precursor powders heated at 800 • C, the changes in the peak shapes and intensities with the Co content suggest that the formation of Co 3 O 4 as a by-product leads to an enhancement of the sintering of the Ni 1−X Co X O-YSZ powders. The observed gradual changes in the diffraction peaks from the Ni 1−X Co X and YSZ phases in the XRD profiles support this.

Phase Identification
Ceramics 2020, 3 FOR PEER REVIEW 4 the Ni1-XCoX phase with the increase in the Co content corresponded with the similar decrease of the as-heated Ni1-XCoXO phase at 1300 °C, as shown in Figure S1. This intensity decrease appears to be connected with an insufficient crystallization of the Ni1-XCoXO solid solution on heating. In contrast, the peak intensities of the YSZ phase showed an increase in accordance with an appearance of the sharp peak pattern with the increase in the Co content, for example, the 311 reflection exhibiting well resolved Kα1 and Kα2 peaks, as shown in Figure 1. Heating the dehydrated Ni0.5Co0.5O/YSZ precursor at 800 °C led to an appearance of the Co3O4 phase, as shown in Figure S2. The Co3O4 phase disappeared after subsequent heating at 1300 °C in air. Since Co3O4 melts at 895 °C and decomposes into CoO at about 950 °C, Co3O4 can serve as a sintering aid for the mixed Ni1-XCoXO and YSZ powders. Although the Co3O4 phase was not identified for the Ni1-XCoXO-YSZ (x = 0.05, 0.15 and 0. 25) precursor powders heated at 800 °C, the changes in the peak shapes and intensities with the Co content suggest that the formation of Co3O4 as a by-product leads to an enhancement of the sintering of the Ni1-XCoXO-YSZ powders. The observed gradual changes in the diffraction peaks from the Ni1-XCoX and YSZ phases in the XRD profiles support this.

Microstructure of the Anode
Figure 2a-e show backscattered electron images of fractured cross-sections of the Ni1-XCoX/YSZ cermet anodes. These porous anodes comprise bright dense YSZ grains and dark sponge-like Ni1-XCoX grains, and both of the grains were well connected by sintering. The average size of the YSZ grains increased with the Co content, which was induced by a promoting effect of the impregnated mixture of the Ni1-XCoXO and YSZ powders on the sintering in air. This effect corresponds to promoting the effect of cobalt oxide addition to NiO and YSZ on their sintering [24,25]. Figure 2f shows a cross section of the anode and electrolyte of the Ni0.75Co0.25 cell, which revealed that the anode and electrolyte were well bonded and the anode layer thickness was constant.

Microstructure of the Anode
Figure 2a-e show backscattered electron images of fractured cross-sections of the Ni 1−X Co X /YSZ cermet anodes. These porous anodes comprise bright dense YSZ grains and dark sponge-like Ni 1−X Co X grains, and both of the grains were well connected by sintering. The average size of the YSZ grains increased with the Co content, which was induced by a promoting effect of the impregnated mixture of the Ni 1−X Co X O and YSZ powders on the sintering in air. This effect corresponds to promoting the effect of cobalt oxide addition to NiO and YSZ on their sintering [24,25]. Figure 2f shows a cross section of the anode and electrolyte of the Ni 0.75 Co 0.25 cell, which revealed that the anode and electrolyte were well bonded and the anode layer thickness was constant.  Figure 3a shows the cell voltage (E) and power density (P) as a function of the current density (J) of the Ni1-XCoX cells supplied with H2 at 750 °C. The open circuit voltage (OCV) was 1.4 V, which agreed with the theoretically calculated value provided that the equilibrium partial pressure of oxygen (PO2) is determined by the fixed partial pressure ratio of water vapor pressure (PH2O) in the supplied H2 gas and its total pressure (PH2) as PH2O/PH2 = 0.0001. The performance of the Ni1-XCoX cells was determined by the Co content; the Ni0.75Co0.25 cell showed the highest maximum power density of 156 mW/cm 2 . The performance of the Ni1-XCoX cells increased in the order, x = 0 < x = 0.05 < x = 0.15, x = 0.5 < x = 0.25. This result implies that the Ni0.75Co0.25/YSZ anode has the most optimum porous structure for the transport of supplied H2 to the reaction sites and the lowest electrical resistance due to the well-connected YSZ and Ni0.75Co0.25 grains.  Figure 3a shows the cell voltage (E) and power density (P) as a function of the current density (J) of the Ni 1−X Co X cells supplied with H 2 at 750 • C. The open circuit voltage (OCV) was 1.4 V, which agreed with the theoretically calculated value provided that the equilibrium partial pressure of oxygen (P O 2 ) is determined by the fixed partial pressure ratio of water vapor pressure (P H2O ) in the supplied H 2 gas and its total pressure (P H2 ) as P H 2 O /P H 2 = 0.0001. The performance of the Ni 1−X Co X cells was determined by the Co content; the Ni 0.75 Co 0.25 cell showed the highest maximum power density of 156 mW/cm 2 . The performance of the Ni 1−X Co X cells increased in the order, x = 0 < x = 0.05 < x = 0.15, x = 0.5 < x = 0.25. This result implies that the Ni 0.75 Co 0.25 /YSZ anode has the most optimum porous structure for the transport of supplied H 2 to the reaction sites and the lowest electrical resistance due to the well-connected YSZ and Ni 0.75 Co 0.25 grains. Ceramics 2020, 3 FOR PEER REVIEW 6  Figure 3b shows the electrochemical impedance spectra of the Ni1-XCoX cells. The real axis intercept at the high frequency region represents the ohmic resistance (RΩ) of the cell, which includes the overall contact resistance among each of the cell components; the electrode, electrolyte, and current collector. The real axis intercept at the low frequency region represents the total resistance (Rtot), which includes the ohmic resistance combined with the polarization resistance (Rp). The polarization resistance (Rp) was determined by subtracting the ohmic resistance from the total resistance. Figure 3c shows that the Ni0.75Co0.25 cell showed the lowest RΩ and Rp and the highest cell voltage at a constant current, which agreed with the results shown in Figure 3a. Figure 3d shows a behavior of the anodic overvoltage (η) for the Ni1-XCoX cells as a function of J. At the low current density range, the initial slope of the curve represents the activation overvoltage shown in the inset of Figure 3d, while at the higher current density range (> 10 mA cm -2 ), the diffusion overvoltage is dominant in the anodic overvoltage. Both the activation and diffusion overvoltages of the Ni1-XCoX cells decreased with an increase in the Co content (x) and showed the lowest values at x = 0.15, whereas they turned to increase with x > 0.15. This result indicates that the optimum Co alloying into the Ni matrix provides a high activity for the H2 oxidation and a favorable microstructure as the cermet anode, which accorded with the variation of the impedance spectra of the Ni1-XCoX cells.

Cell Performance for a H 2 Supply
As shown in Figure 2, the grain growth of YSZ particles during sintering occurs significantly with an increase of x, which promoted the grain connectivity between the Ni1-XCoX metal and YSZ with a compensation for the decrease of their contact area, resulting in an electrical resistance increase. Venkataramanan et al. reported that a theoretical calculation of an Ni cluster alloyed with 20 mol%  Figure 3b shows the electrochemical impedance spectra of the Ni 1−X Co X cells. The real axis intercept at the high frequency region represents the ohmic resistance (R Ω ) of the cell, which includes the overall contact resistance among each of the cell components; the electrode, electrolyte, and current collector. The real axis intercept at the low frequency region represents the total resistance (R tot ), which includes the ohmic resistance combined with the polarization resistance (R p ). The polarization resistance (R p ) was determined by subtracting the ohmic resistance from the total resistance. Figure 3c shows that the Ni 0.75 Co 0.25 cell showed the lowest R Ω and R p and the highest cell voltage at a constant current, which agreed with the results shown in Figure 3a. Figure 3d shows a behavior of the anodic overvoltage (η) for the Ni 1−X Co X cells as a function of J. At the low current density range, the initial slope of the curve represents the activation overvoltage shown in the inset of Figure 3d, while at the higher current density range (>10 mA cm −2 ), the diffusion overvoltage is dominant in the anodic overvoltage. Both the activation and diffusion overvoltages of the Ni 1−X Co X cells decreased with an increase in the Co content (x) and showed the lowest values at x = 0.15, whereas they turned to increase with x > 0.15. This result indicates that the optimum Co alloying into the Ni matrix provides a high activity for the H 2 oxidation and a favorable microstructure as the cermet anode, which accorded with the variation of the impedance spectra of the Ni 1−X Co X cells.
As shown in Figure 2, the grain growth of YSZ particles during sintering occurs significantly with an increase of x, which promoted the grain connectivity between the Ni 1−X Co X metal and YSZ with a compensation for the decrease of their contact area, resulting in an electrical resistance increase.
Venkataramanan et al. reported that a theoretical calculation of an Ni cluster alloyed with 20 mol% Co using the quantum chemical method predicts the chemisorption energy of hydrogen on the metal cluster (0.541 eV/hydrogen atom), which was lower than that on a pure Ni cluster (0.689 eV/hydrogen atom). They also showed that the H-H bond length at a transition state of hydrogen dissociation process of the Ni cluster was extended from 1.534 to 1.621 Å [26]. These results suggest that Co alloying facilitates the dissociation of H 2 molecules on the Ni 1−X Co X surface, resulting in promoting the electrochemical oxidation of hydrogen at the Ni 1−X Co X /YSZ anode. However, the overvoltage of the Ni 1−X Co X cells increased at x > 0.15, which was caused by a decrease in the TPB length resulting in an increase of the polarization resistance with a decrease of the cell performance. Furthermore, the electrical conductivity of Ni is known to decrease by Co alloying due to electron scattering enhancement [23], which can be a factor for lowering the cell performance. Figure 4a shows J-E and J-P curves of the Ni 1−X Co X cells supplied with CH 4 at 750 • C. The performance of the Ni 1−X Co X cell increased in a similar way to that shown in Figure 3a;

Cell Performance for a CH 4 Supply
The maximum power density of the Ni cell was 94 mW/cm 2 and that of the Ni 0.75 Co 0.25 cell was 136 mW/cm 2 ; a 45% increase was identified. By comparison, for the H 2 supply, the Ni 0.75 Co 0.25 cell showed a 17% increase compared with the Ni cell, as shown in Figure 3a. Figure 4b shows the impedance spectra of the Ni 1−X Co X cells, and Figure 4c shows R Ω and R p as a function of x. The Ni 0.75 Co 0.25 cell exhibited the lowest R Ω and R p as well as the highest terminal voltage at a constant current. Figure 4d shows η as a function of J. The inset shows the initial linear slope range indicating the activation overvoltages of the Ni 1−X Co X cells. The Ni cell exhibited the highest η and the Ni 0.85 Co 0.15 cell exhibited the lowest one. The Ni 0.75 Co 0.25 cell also exhibited a low η. The polarization resistance determined from the initial linear slope for CH 4 was found to be four to five times higher than that for H 2 . This indicates that the electrochemical oxidation of CH 4 requires a higher voltage drop than that of H 2 . Figure 5 shows the Arrhenius plot for the apparent electrode interface conductivity, which contains a contribution from the YSZ electrolyte. Since we used YSZ disks with the same size and the polished surfaces, the contribution can be ignored to compare the relative difference in the impedance among the measured samples. The impedance spectra were collected for both the symmetrical Ni and Ni 0.75 Co 0.25 cells in a CH 4 atmosphere in the temperature range of 550 (or 600)-750 • C. The resistance was determined from the difference between the high-frequency arc and low-frequency arc intercept, as shown in Figures S3 and S4. The reciprocal resistance was regarded as the apparent electrode interface conductivity, which corresponds to the charge transfer occurring at the interface between the Ni 1−X Co X and YSZ grains involving adsorbed CH 4 molecules. The activation energies obtained from the Arrhenius plot were 98 kJ mol −1 for the Ni cell and 96 kJ mol −1 below 650 • C and 39 kJ mol −1 above 650 • C for the Ni 0.75 Co 0.25 cell. The activation energies for the Ni cell between 600 and 750 • C, and that for the Ni 0.75 Co 0.25 cell below 650 • C were almost the same, whereas that for the Ni 0.85 Co 0.25 cell above 650 • C exhibited a much lower energy. Thermal equilibrium of the CH 4 decomposition reaction is known to be product (C and H 2 ) favorable at temperatures greater than about 700 • C. A possible interpretation of the above result is that the thermal dissociation of CH 4 on the Ni 0.85 Co 0.25 surface is more facile and favorable for the following electrochemical steps of the partial or complete oxidation of CH 4 accompanying the charge transfer from the oxide ions to electrons at the interface between Ni 1−X Co X and YSZ and desorption of the products, CO, H 2 , CO 2 , and H 2 O. Li et al., reported a computation calculation of the activation energy of the CH 4 dissociation on Ni and NiCo. The calculation revealed that the activation energy (Ea) of CH 4 dissociation on the Ni(111) surface (1.36 eV (= 131 kJ mol −1 )) was higher than that of the NiCo(111) surface (1.29 eV (= 124 kJ mol −1 )) and the Co(111) (1.25 eV(= 121 kJ mol −1 )) [27,28]. Although their calculation was done on the well-defined single crystal surface, lowering the Ea by Co alloying into the Ni matrix can promote the rate of CH 4 dissociation and result in reducing the anodic polarization resistance for the electrochemical oxidation of CH 4 , provided that the rate-determining step of the anodic oxidation of CH 4 is the dissociation of the adsorbed CH 4 . The Co alloying seems to be effective to control the well-known strong adsorptive property of Ni for CO; the suppression of CO poisoning at the Ni/YSZ-based cermet anode is expected to enhance the cell performance for the direct oxidation of CH 4 . An enhancement of the cell performance was also reported for the direct oxidation of CH 4 by using a Ni 0.5 Co 0.5 /YSZ anode combined with an ScSZ electrolyte [29]. An appropriate amount of Co alloying in the Ni matrix was found to be effective to reduce the anodic activation overvoltage above about 700 • C. electrical conductivity of Ni is known to decrease by Co alloying due to electron scattering enhancement [23], which can be a factor for lowering the cell performance. Figure 4a shows J-E and J-P curves of the Ni1-XCoX cells supplied with CH4 at 750 °C. The performance of the Ni1-XCoX cell increased in a similar way to that shown in Figure 3a; x = 0 < x = 0.05 < x = 0.5 < x = 0.15 < x = 0.25. The maximum power density of the Ni cell was 94 mW/cm 2 and that of the Ni0.75Co0.25 cell was 136 mW/cm 2 ; a 45% increase was identified. By comparison, for the H2 supply, the Ni0.75Co0.25 cell showed a 17% increase compared with the Ni cell, as shown in Figure 3a. Figure  4b shows the impedance spectra of the Ni1-XCoX cells, and Figure 4c shows RΩ and Rp as a function of x. The Ni0.75Co0.25 cell exhibited the lowest RΩ and Rp as well as the highest terminal voltage at a constant current. Figure 4d shows η as a function of J. The inset shows the initial linear slope range indicating the activation overvoltages of the Ni1-XCoX cells. The Ni cell exhibited the highest η and the Ni0.85Co0.15 cell exhibited the lowest one. The Ni0.75Co0.25 cell also exhibited a low η. The polarization resistance determined from the initial linear slope for CH4 was found to be four to five times higher than that for H2. This indicates that the electrochemical oxidation of CH4 requires a higher voltage drop than that of H2.

Prolonged Stability in CH4
The Ni0.85Co0.15 and Ni0.75Co0.25 cells were chosen to investigate the prolonged cell performance stability for CH4 to compare with the Ni cell, which is based on the results that the Ni0.85Co0.15 cell exhibited the lowest anodic overvoltage and the Ni0.75Co0.25 cell exhibited the lowest ohmic resistance. Figure 6a shows changes in the total resistance and cell voltage at a constant current of 80 mA at 750 °C as a function of operating time for the Ni, Ni0.85Co0.15, and Ni0.75Co0.25 cells. The cell voltage of the Ni cell significantly decreased from 0.62 to 0.45 V (about 27% decrease) after operation for 60 h. This decrease corresponded to the increase of the total resistance (= Rp + RΩ) determined from the impedance spectra under the OCV condition, which is shown in Figure 6b. In contrast, the cell voltage of the Ni0.85Co0.15 cell changed from 0.69 to 0.67 V (about 3% decrease) and that of the Ni0.75Co0.25 cell changed from 0.73 to 0.70 V (about 4% decrease); the cell voltage drop was almost insignificant at least up to 60 h. Figure 6b shows a comparison of the impedance spectra before and after the prolonged stability test. The Ni cell showed a larger increase in the impedance arc with a shift to the high value of the real axis, Re(Z), than the Ni0.85Co0.15 and Ni0.75Co0.25 cells with slight shifts of their impedance arcs. The significant increase of Rp + RΩ for the Ni cell is probably caused by the thermal grain growth of Ni, which reduces the contract area between the Ni and YSZ, and the deactivation of the nickel surface with a concurrent carbon deposition. On the other hand, the increases in Rp + RΩ for the Ni0.85Co0.15 and Ni0.75Co0.25 cells were small, which indicates no significant grain growth of Ni as well as the sufficiently active surface state against an exposure of CH4.
After this prolonged stability test, the anode surface morphologies of the Ni and Ni0.85Co0.15 cells were investigated by SEM. Elemental analysis was also made for these anode surfaces. Figure 7a-d show the surface morphologies and corresponding area analysis by EDX. The carbon peak that appeared in the Ni cell is much stronger than that in the Ni0.85Co0.15 cell. This result indicates that the proper Co alloying for the Ni-based/YSZ cermet anode by the impregnation treatment delivers excellent cell performance for the direct supply of CH4 because of the favorable cermet microstructure having low electrical resistance and the surface state resistant to the deactivation by adsorbed CH4. The formation of carbon on an SOFC anode directly exposed to CH4 can easily occur via the pyrolysis

Prolonged Stability in CH 4
The Ni 0.85 Co 0.15 and Ni 0.75 Co 0.25 cells were chosen to investigate the prolonged cell performance stability for CH 4 to compare with the Ni cell, which is based on the results that the Ni 0.85 Co 0.15 cell exhibited the lowest anodic overvoltage and the Ni 0.75 Co 0.25 cell exhibited the lowest ohmic resistance. Figure 6a shows changes in the total resistance and cell voltage at a constant current of 80 mA at 750 • C as a function of operating time for the Ni, Ni 0.85 Co 0.15 , and Ni 0.75 Co 0.25 cells. The cell voltage of the Ni cell significantly decreased from 0.62 to 0.45 V (about 27% decrease) after operation for 60 h. This decrease corresponded to the increase of the total resistance (= R p + R Ω ) determined from the impedance spectra under the OCV condition, which is shown in Figure 6b. In contrast, the cell voltage of the Ni 0.85 Co 0.15 cell changed from 0.69 to 0.67 V (about 3% decrease) and that of the Ni 0.75 Co 0.25 cell changed from 0.73 to 0.70 V (about 4% decrease); the cell voltage drop was almost insignificant at least up to 60 h. Figure 6b shows a comparison of the impedance spectra before and after the prolonged stability test. The Ni cell showed a larger increase in the impedance arc with a shift to the high value of the real axis, Re(Z), than the Ni 0.85 Co 0.15 and Ni 0.75 Co 0.25 cells with slight shifts of their impedance arcs. The significant increase of R p + R Ω for the Ni cell is probably caused by the thermal grain growth of Ni, which reduces the contract area between the Ni and YSZ, and the deactivation of the nickel surface with a concurrent carbon deposition. On the other hand, the increases in R p + R Ω for the Ni 0.85 Co 0.15 and Ni 0.75 Co 0.25 cells were small, which indicates no significant grain growth of Ni as well as the sufficiently active surface state against an exposure of CH 4 .
Since the desorption energy of CO from the Co metal surface (161 kJ mol -1 ) was found to be lower than that from the Ni metal surface (173 kJ mol -1 ) [30], the CO desorption from the Co-alloyed Ni/YSZ cermet can be more facile than from the Ni/YSZ anode, which results in a decrease in carbon deposition on the anode surface and provides stable electrochemical performance.  Since the desorption energy of CO from the Co metal surface (161 kJ mol -1 ) was found to be lower than that from the Ni metal surface (173 kJ mol -1 ) [30], the CO desorption from the Co-alloyed Ni/YSZ cermet can be more facile than from the Ni/YSZ anode, which results in a decrease in carbon deposition on the anode surface and provides stable electrochemical performance. : 0 h, Since the desorption energy of CO from the Co metal surface (161 kJ mol -1 ) was found to be lower than that from the Ni metal surface (173 kJ mol -1 ) [30], the CO desorption from the Co-alloyed Ni/YSZ cermet can be more facile than from the Ni/YSZ anode, which results in a decrease in carbon deposition on the anode surface and provides stable electrochemical performance. : 60 h).
After this prolonged stability test, the anode surface morphologies of the Ni and Ni 0.85 Co 0.15 cells were investigated by SEM. Elemental analysis was also made for these anode surfaces. Figure 7a-d show the surface morphologies and corresponding area analysis by EDX. The carbon peak that appeared in the Ni cell is much stronger than that in the Ni 0.85 Co 0.15 cell. This result indicates that the proper Co alloying for the Ni-based/YSZ cermet anode by the impregnation treatment delivers excellent cell performance for the direct supply of CH 4 because of the favorable cermet microstructure having low electrical resistance and the surface state resistant to the deactivation by adsorbed CH 4 . The formation of carbon on an SOFC anode directly exposed to CH 4 can easily occur via the pyrolysis reaction (Equation (1)) at temperatures higher than about 700 • C. The thermally dissociate carbon can be consumed by the electrochemical oxidation (Equation (2)) with the supply of O 2 − ions through the electrolyte when the product of adsorbed CO is removed from the Ni-based metal surface.

Conclusions
A modified porous anode of the Ni1-XCoX/YSZ (yttria-stabilized zirconia) cermet was prepared by an impregnation method and was applied to the SOFC operation with direct CH4 supply. The cermet microstructure was improved owing to the promoting effect of the Co3O4 phase as a sintering aid on the fabrication of the Ni-based cermet anode. The appropriate Co alloying content, x = 0.15-0.25, was found to increase the cell performance with an excellent prolonged cell performance stability as well as minimizing the carbon deposition thermally from the dissociated CH4. Co was incorporated into Ni and formed a solid solution of Ni1-XCoX alloy connected with the YSZ as the cermet anode. The performance enhancement was found to be more significant for CH4 than that for H2 compared with the performance of the cell using the Ni/YSZ cermet anode (an improvement of 17% in H2 fuel and 45% in CH4 fuel for x = 0.25). This enhancement effect for the direct CH4 supply was probably caused by the optimum porous microstructure of the cermet anode with the low anodic polarization resistance, which is specific to an operating temperature higher than about 700 °C. Since the desorption energy of CO from the Co metal surface (161 kJ mol −1 ) was found to be lower than that from the Ni metal surface (173 kJ mol −1 ) [30], the CO desorption from the Co-alloyed Ni/YSZ cermet can be more facile than from the Ni/YSZ anode, which results in a decrease in carbon deposition on the anode surface and provides stable electrochemical performance.

Conclusions
A modified porous anode of the Ni 1−X Co X /YSZ (yttria-stabilized zirconia) cermet was prepared by an impregnation method and was applied to the SOFC operation with direct CH 4 supply. The cermet microstructure was improved owing to the promoting effect of the Co 3 O 4 phase as a sintering aid on the fabrication of the Ni-based cermet anode. The appropriate Co alloying content, x = 0.15-0.25, was found to increase the cell performance with an excellent prolonged cell performance stability as well as minimizing the carbon deposition thermally from the dissociated CH 4 . Co was incorporated into Ni and formed a solid solution of Ni 1−X Co X alloy connected with the YSZ as the cermet anode. The performance enhancement was found to be more significant for CH 4 than that for H 2 compared with the performance of the cell using the Ni/YSZ cermet anode (an improvement of 17% in H 2 fuel and 45% in CH 4 fuel for x = 0.25). This enhancement effect for the direct CH 4 supply was probably caused by the optimum porous microstructure of the cermet anode with the low anodic polarization resistance, which is specific to an operating temperature higher than about 700 • C.