Surface Process of Doped Ceria Reduction by Electrical Conductivity Relaxation

Abstract

Ceria and doped ceria materials are often used as the oxide components for solid oxide fuel cell (SOFC) anodes to enhance their catalytic activity, especially those using Cu as the electronic conductors. In this work, the surface process of doped ceria reduction, i.e. the chemical surface exchange process in reduced atmospheres, is studied using electrical conductivity relaxation (ECR) technique to characterize their catalytic activity for fuel oxidation. The oxygen surface exchange coefficient of Gd0.1Ce0.9O2-δ is comparable to that obtained by thermogravimetric measurement, demonstrating the feasibility of ECR method in determining the oxygen transport in doped ceria under reducing conditions. The relaxation process is limited by the surface exchange step and almost independent on the bulk oxygen ion diffusion kinetics. Among various materials of R0.2Ce0.8O2-δ (R = Y, Gd, Sm, La) and SmxCe1-xO2-δ (x = 0, 0.05, 0.1, 0.2, 0.3), Sm0.2Ce0.8O2-δ exhibits the highest surface exchange coefficient, thus should be promise as the anode component. Moreover, it is found that, at temperature below 700°C, surface exchange kinetics at the grain boundary is significantly faster than on the grain, suggesting additional advantage of developing SOFCs by low-temperature sintering.