Effect of firing temperature on the microstructure and performance of PrBaCo₂O_5+δ cathodes on Sm_0.2Ce_0.8O_1.9 electrolytes fabricated by spray deposition-firing processes

The effect of firing temperature on the microstructure and performance of PrBaCo₂O_5+δ cathodes on Sm_0.2Ce_0.8O_1.9 electrolytes fabricated by spray deposition-firing processes is systematically studied by various characterization techniques. The grain size, porosity and particle connection of the electrode as well as the physical contact between the PrBaCo₂O_5+δ and Sm_0.2Ce_0.8O_1.9 layers are influenced differently by the firing temperature. The area specific resistances (ASRs) of the various PrBaCo₂O_5+δ cathodes are measured by electrochemical impedance spectroscopy in both symmetrical two-electrode and three-electrode configurations. The lowest ASR and cathode overpotential are achieved at a firing temperature of 1000 °C. Two main oxygen reduction reaction processes are proposed according to the oxygen partial pressure dependence of the electrode ASR. The rate-determining step is transmitted from a charge-transfer process at low firing temperatures to a non-charge-transfer process at high firing temperatures. A fuel cell with the PrBaCo₂O_5+δ cathode fired at an optimal temperature of 1000 °C delivers the attractive peak power density of 835 mW cm^-2 at 650 °C, while this density is much lower for other firing temperatures. This result suggests the firing temperature of PrBaCo₂O_5+δ electrodes should be carefully optimized for practical applications.