Kinetic parameters influencing the performance of IT-SOFC composite electrodes

Composite electrodes have made important contributions to the development of intermediate temperature solid oxide fuel cells (IT-SOFC). At a macroscopic level the separate electronic and ionic components can usefully be considered as a combined mixed conductor. This allows available kinetic data for oxygen self-diffusion (D* cm2 s−1) and surface exchange (k cm s−1) coefficients to be evaluated so that possible rate-determining processes can be identified. For LSM/YSZ and LSCF/CGO composite electrodes it appears that the k values for YSZ and CGO have to be enhanced by at least two orders of magnitude to account for observed cathode ASR values. It is suggested that ‘spillover’ reactions involving the electronic component could be responsible for the increased rate of O2− injection into YSZ and CGO. Additional strategies to further enhance the surface exchange rate for CGO at 500°C, such as doping with redox cations (e.g. Tb4+/Tb3+, Nb5+/Nb4+), changing the reduction enthalpy for Ce4+⇒Ce3+, and promotion with noble metal catalysts, are also discussed.