Electrochemical behavior of mixed-conducting oxide cathodes in contact with apatite-type La10Si5AlO26.5 electrolyte

The electrochemical activity of porous La2Ni0.8Cu0.2O4+δ, La2Ni0.8Cu0.2O4+δ–Ag, La0.8Sr0.2Fe0.8Co0.2O3−δ–Ce0.8Gd0.2O2−δ and La0.7Sr0.3MnO3−δ–Ce0.8Gd0.2O2−δ electrodes in contact with apatite-type La10Si5AlO26.5 solid electrolyte has been appraised at 873–1073 K in air. The polarization resistance of nickelate-based cathodes is substantially higher compared to similar layers applied onto (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ, whilst the corresponding activation energies, 69–74 kJ/mol, are close to the Ea values for ionic conduction in these electrolytes. The relatively low performance is primarily associated with the surface diffusion of silica from La10Si5AlO26.5, which partially blocks the electrochemical reaction zone without formation of secondary phases detectable by X-ray diffraction. The oxygen reduction kinetics is also strongly influenced by the transport properties of solid electrolyte and by the exchange-related processes at the electrode surface. The role of the latter factor becomes evident on increasing current density, and in the cases when ionic conductivity of the electrode materials is low. As for other solid oxide electrolyte cells, the performance of mixed-conducting cathodes applied onto La10Si5AlO26.5 can be improved by incorporating electrocatalytically-active components, such as Ag and PrOx, and by reducing electrode fabrication temperature.