Optimisation of Perovskite Materials for Fuel Electrodes

Abstract

Excellent thermal and mechanical stability, physical compatibility with electrolyte materials and relatively low cost have attracted interest in the application of perovskite oxide materials as fuel electrodes in SOFC designs. The present fuel electrode of choice is a nickel/YSZ cermet; the nickel acts as a fuel oxidation catalyst and provides electronic conductivity, whilst the YSZ provides oxygen ion conductivity. The challenge is to develop a single phase oxide material which provides all of the above and at the same time reduces the problems of coking, nickel sintering and sulphur poisoning sometimes encountered with the nickel/YSZ cermet. A rationale for the application of perovskites as fuel electrodes is given and the defect chemistry of perovskites is then reviewed. Attention is given to reducible transition metal ions, doping, non-stoichiometry and the effects these have on chemical stability and both the magnitude and mechanism of conduction. Findings are summarised and the need for an optimal doping strategy is identified. We report a nickel-free solid oxide fuel cell (SOFC) anode, La0.75Sr0.25Cr0.5Mn0.5O3, with comparable electrochemical performance to Ni/YSZ cermets. The electrode polarisation resistance approaches 0.2ohmcm2 at 900°C in 97%H2/3%H2O. Very good performance is achieved for methane oxidation without using excess steam. The anode is stable in both fuel and air conditions and shows stable electrode performance in methane. Thus both redox stability and operation in low steam hydrocarbons have been demonstrated, overcoming two of the major limitations of the current generation of nickel zirconia cermet SOFC anodes.