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The relationship between the electrochemical performance of an anode in a solid oxide fuel cell and the distribution of yttrium-doped barium zirconate (BZY) in the electrochemically active zone of nickel (Ni)/yttria-stabilized zirconia (YSZ) anodes was investigated to clarify the promoting effect of BZY on electrochemical reaction. After BZY was injected into Ni/YSZ anodes using an ink-jet technique for control of the distribution of BZY, the anode performance improved in both humidified H 2 and dry CH 4 fuels. The performance of the BZY-infiltrated Ni/YSZ anodes depended on the distribution of BZY; an increasing gradient of BZY from the electrolyte side to the anode surface side was particularly effective in improving the performance. These results suggest that the optimum amount of infiltrated BZY to enhance the electrochemical reaction depended on the distribution of the oxygen chemical potential in the Ni/YSZ anodes. According to the competitive adsorption equilibrium of reactants and the Langmuir-type reaction on the surface of Ni at the triple-phase boundary, the oxygen coverage was a critical factor in the electrochemical reaction and depended on the oxygen chemical potential. In conclusion, the enhanced electrochemical reaction in the BZY-infiltrated Ni/YSZ anodes was therefore due to the increased oxygen coverage.

Electrochemical performance and feasibility of proton-conducting solid oxide fuel cells (SOFCs) using methane (CH4) fuel were investigated. At high fuel utilization, higher protonic transport number of electrolytes enables higher open-circuit voltage (OCV) when hydrogen (H2) is the fuel. In contrast, a high oxide ionic transport number is needed when CH4 is the fuel. Conductivities of each charge carrier of an yttrium-doped barium zirconate (BZY) electrolyte were therefore estimated by measuring the OCV under various partial pressures. A dense BZY electrolyte was obtained by the addition of 0.6 mol% nickel (Ni). This electrolyte exhibited protonic- and oxide ionic-mixed conductivity; in particular, these conductivities were similar to each other at 800–900°C, indicating high efficiency. A Ni/BZY anode enabled stable operation of a proton-conducting SOFC using 3% humidified CH4 fuel. The maximum power density of this SOFC was 2.35 mW cm−2 at 900°C and 1.69 mW cm−2 at 800°C, which were higher than that using a platinum anode. The activation energy for electrode polarization of the Ni/BZY anode using 3% humidified CH4 fuel was similar to that using 3% humidified H2 fuel, indicating that H2 via decomposition of CH4 was the main reactant on the Ni/BZY anode.

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