Electrochemical behaviors of mixed conducting oxide anodes for solid oxide fuel cell

To clarify guidelines for a high-performance mixed conducting oxide anode, electrochemical behaviors of mixed conducting oxide anodes were studied on the oxides, La0.9 Ca0.1 Cr0.8 Al0.2 O3, La0.9 Ca0.1 Cr0.2 Al0.8 O3, Sr0.9 La0.1 TiO3, Sr0.8 La0.2 TiO3, SrTi0.97 Nb0.03 O3, Ce0.9 Gd0.1 O1.95, and Ce0.992 Nb0.008 O2. The hydrogen oxidation on the oxide anodes is studied by ac impedance and steady-state polarization measurements. In the ac impedance measurements, a slow relaxation process of the order of 10-2 Hz was observed with the CeO2 -based and La0.9 Ca0.1 Cr0.2 Al0.8 O3 anodes. The corresponding pseudocapacitances are 104 - 106 μF cm-2. These pseudocapacitances are identified as the chemical capacitance due to the variation of the nonstoichiometric oxygen content of the electrode material. At the same electrode potential, the CeO2 -based anodes showed a far higher steady-state current than the LaCrO3 - and the SrTiO3 -based anodes. The extension of the reaction zone beyond the three phase boundary is estimated from our experimental results. The reaction zone of the Ce0.9 Gd0.1 O1.95 anode extends from the three-phase boundary to the electrode/gas interface. To estimate what determines the electrode performance of the oxide anodes, the effect of the material property and the electrode microstructure was studied. The effect of the material property is much larger than that of the electrode microstructure. High ionic conductivity and catalytic activity with a certain level of electronic conductivity is required for a high-performance oxide anode.