Developing advanced oxygen electrodes for reversible protonic ceramic electrochemical cells via controlled bismuth doping on diverse perovskite lattice positions

•BCFT-based perovskites with Bi substitution at different sites are developed as oxygen electrodes for R-PCECs.•Bi doping on the B-site promotes oxygen vacancy formation and oxygen surface kinetics.•DFT calculations reveal improved hydration, proton conduction, and catalytic activity contributed by Bi doping on the B-site.•B-BCFT oxygen electrode exhibits exceptional performance in fuel cell and electrolysis modes.•B-BCFT electrode demonstrates excellent long-term durability and reversible cycling stability. The development of reversible protonic ceramic electrochemical cells (R-PCECs) is hampered by insufficient oxygen electrode activity. This study addresses this challenge by introducing bismuth (Bi) into different lattice sites (A-site, B-site, or both) of Ba(Co0.7Fe0.3)0.85Ta0.15O3-δ (BCFT) to develop novel oxygen electrode materials. Extensive characterizations and density functional theory calculations demonstrate that introducing Bi onto the B-site promotes oxygen vacancy formation and hydration, facilitates proton transportation and improves oxygen exchange ability, leading to the enhanced catalytic activity. In contrast, Bi on the A-site hinders performance. R-PCEC using Ba(Co0.7Fe0.3)0.75Bi0.10Ta0.15O3-δ (B-BCFT) oxygen electrode achieves exceptional performance in fuel cell (1.485 W cm−2 at 650 °C) and electrolysis modes (−1.839 A cm−2 at 1.4 V and 650 °C). Notably, B-BCFT demonstrates outstanding operational stability, lasting for 150 h in both modes and enduring 35 cycles of reversible operation. These results make B-BCFT a promising oxygen electrode material candidate for R-PCECs.