Mixed-Conducting Perovskites as Cathode Materials for Protonic Ceramic Fuel Cells: Understanding the Trends in Proton Uptake

The proton uptake of 18 compositions in the perovskite family (Ba,Sr,La)(Fe,Co,Zn,Y)O3-delta, perovskites, which are potential cathode materials for protonic ceramic fuel cells (PCFCs), is investigated by thermogravimetry. Hydration enthalpies and entropies are derived, and the doping trends are explored. The uptake is found to be largely determined by the basicity of the oxide ions. Partial substitution of Zn on the B-site strongly enhances proton uptake, while Co substitution has the opposite effect. The proton concentration in Ba0.95La0.05Fe0.8Zn0.2O3-delta is found to be 10% per formula unit at 250 degrees C, 5.5% at 400 degrees C, and 2.3% at 500 degrees C, which are the highest values reported so far for a mixed-conducting perovskite exhibiting hole, proton, and oxygen vacancy transport. A comprehensive set of thermodynamic data for proton uptake in (Ba,Sr,La)(Fe,Co,Zn,Y)O3-delta is determined. Defect interactions between protons and holes partially delocalized from the B-site transition metal to the adjacent oxide ions decrease the proton uptake. From these results, guidelines for the optimization of PCFC cathode materials are derived.