Ca-Doped La _0.5 Ba _0.5 FeO _3-δ As an Efficient Cathode Material for Proton-Conducting Direct Ammonia Fuel Cells

Sr or Ca doping on the Ba site of layered perovskites is reported to increase oxygen reduction reaction catalytic activity, but the underlying mechanism of this phenomenon for the protonic cell is not clear. In this research, Ca containing La0.5(Ba0.5–xCax)FeO3-δ (0≤ x ≤0.3) perovskites are explored as cathodes for proton-conducting direct ammonia fuel cells. La0.5Ba0.5FeO3-δ (LBF) has mixed cubic and tetragonal phases with cation ordering, and the Ca doping generally increases the oxide-ion conductivity because it could increase the stability of random cation distribution on the A site and facilitate the formation of oxide ion vacancy in the layer according to the density-function theory (DFT) calculation. The surface exchange coefficient (2.00 × 10–5 cm s–1) and the chemical diffusion coefficient (4.01 × 10-10 cm2 s–1) are enhanced to 8.56 × 10–5 cm s–1 and 7.37 × 10–9 cm2 s–1, respectively, when the Ca content increased from 0 to 30%. For electrochemical tests, the single cell with a cathode of cubic La0.5(Ba0.45Ca0.05)FeO3−δ based on BaZr0.1Ce0.7Y0.2O3 (BZCY) electrolyte shows a polarization resistance of 0.058 Ω cm2 at 700 °C, imparting a peak power density of 682 mW cm–2 under NH3 fuel. The Ca2+ doping above x = 0.05 would decrease the cell output or stability for continuous operation in fuel-cell mode because of the decreased protonic conductivity that could localize the site for water production. This research highlights the effect of isovalent Ca2+ doping on the Ba2+ site and demonstrates the possibility for a proton conducting cathode for direct ammonia fuel cell.