Jian Wang1

1, Hong Kong University of Science and Technology, Sai Kung, , Hong Kong

The sluggish electrode reaction rates, especially in the cathode part which occupies the biggest portion of polarization resistance, restricts the practical use of solid oxide fuel cells (SOFCs). Mixed ionic and electronic perovskite oxides (MIECs) with the formula of ABO3-δ used as SOFC cathodes are gaining intensive research interest, due to their low-cost, potentially high catalytic activity, and compositional diversity. In this report, we focus on one most promising parent MIEC, i.e., BaFeO3-δ (BFO), and systematically investigate the effect of doping on BFO as cobalt-free cathode materials. The common lanthanide ions including La3+, Sm3+ and Gd3+ were chosen to partially substitute the A-site of BFO to obtain Ba0.95La0.05FeO3-δ (BLF), Ba0.95Sm0.05FeO3-δ (BSF), and Ba0.95Gd0.05FeO3-δ (BGF), while the less reducible metal ions Zr4+and Ce4+ were doped into the B-site, and BaFe0.95Zr0.05O3-δ (BFZ) and BaFe0.95Ce0.05O3-δ (BFC) were achieved. The materials are prepared carefully, and their structural, electronic, electrocatalytic properties are characterized and compared. XRD reveals 5 mol% single A-site or B-site dopant is sufficient to stabilize the cubic phase of BFO, as predicted by the lattice calculation. XPS 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, Ba0.95La0.05FeO3-δ and BaFe0.95Zr0.05O3-δ stand out from the A-site and B-site doped BFO, respectively. With a similar amount of oxygen vacancies, B-site doping is more advantageous for enhancing oxygen bulk diffusion kinetics, and thus ORR activity. Even though introducing high-valence single dopant can improve the structural stability, it reduces the oxygen vacancy concentration. As a result, doping with either isovalent or lower-valence elements is preferred. We found that 5 mol% isovalent Ca2+ doping in the Ba2+ site and 5 mol% lower-valence In3+ in the Fe3+/Fe4+ site is found to successfully achieve cubic BaFeO3-δ. This is in contrast with the typical approach of substituting elements of higher valence. However, without resorting to co-doping strategy, the phase of BaFe0.95In0.05O3-δ (BFI) is rhombohedral, while Ba0.95Ca0.05FeO3-δ (BCF) is a mixture of the cubic phase together with BaFe2O4 impurities. Thanks to the large oxygen vacancy concentration and fast oxygen mobility, the novel Ba0.95Ca0.05Fe0.95In0.05O3-δ exhibits a favorable ORR activity. The significantly enhanced performance, compared with BFI and BCF, is attributed to the presence of the cubic phase and the large oxygen vacancies brought by the isovalent substitution in the A-site and lower-valence doping in the B-site.