The effect of A-site and B-site substitution on BaFeO_3-δ: An investigation as a cathode material for intermediate-temperature solid oxide fuel cells

This work systematically investigates the effects of single A-site dopant (5 mol% La³⁺, Sm³⁺ and Gd³⁺) and single B-site dopant (5 mol% Zr⁴⁺ and Ce⁴⁺) on the structure and oxygen reduction reaction of BaFeO_3-δ (BFO) used as a cathode for solid oxide fuel cells. The materials are prepared by solid-state method and their structural, electronic, electrocatalytic properties are characterized and compared. X-ray diffraction reveals 5 mol% A-site or B-site dopant is sufficient to stabilize the cubic phase of BFO, as predicted by the lattice calculation. X-ray photoelectron spectroscopy and iodometric titration demonstrates that neither of the two doping sites has obvious advantage over the other towards the formation of additional oxygen vacancies. B-site doped BFO shows a lower electrical conductivity than A-site doped ones, however, they have much quicker response to electrical conductivity relaxation, likely originating from the expanded lattice size. With the largest oxygen vacancy concentrations, Ba_0.95La_0.05FeO_3-δ and BaFe_0.95Zr_0.05O_3-δ stand out from the A-site and B-site doped BFO, respectively, and polarization resistances of 0.029 Ω cm² and 0.020 Ω cm² are achieved at 700 °C, PO₂=0.2atm. With a similar amount of oxygen vacancies, B-site doping is more advantageous for enhancing oxygen bulk diffusion kinetics, and thus ORR activity.