P-type electronic transport in Ce0.8Gd0.2O2-δ:: The effect of transition metal oxide sintering aids

Small (2 mol%) additions of cobalt, iron and copper oxides into Ce0.8Gd0.2O2-delta considerably improve sinterability of ceria-gadolinia (CGO) solid electrolyte, making it possible to obtain ceramics with 95-99% density and sub-micron grain sizes at 1170-1370 K. The minor dopant additions have no essential effect on the total and ionic conductivity, whilst the p-type conduction in the transition metal-containing materials at 900-1200 K is 8-30 times higher than that in pure CGO. The oxygen ion transference numbers of the Co-, Fe- and Cu-doped ceramics, determined by the modified e.m.f. technique under oxygen/air gradient, are in the range 0.89-0.99. The electron-hole contribution to the total conductivity increases with temperature, as the activation energy for ionic conduction, 78 to 82 kJ/mol, is significantly lower than that for the p-type electronic transport (139-146 kJ/mol). The results show that CGO sintered with such additions can still be used as solid electrolytes for intermediate-temperature electrochemical applications, including solid oxide fuel cells (SOFCs) operating at 770-970 K, but that increasing operation temperature is undesirable due to performance loss.