B-site high valence cation co-doping boosts fast oxygen kinetics in a cobalt-free perovskite air electrode for reversible solid oxide cells
Reversible solid oxide cells (RSOCs) represent a promising technology for efficient energy conversion and storage. However, the performance is often limited by the sluggish oxygen kinetics and poor structural stability of the air electrode. The cobalt-free Bi0.5Sr0.5FeO3−δ (BSF) air electrode exhibits a comparatively desirable performance and is expected to be further optimized. Herein, we propose a B-site co-doping strategy by incorporating high valence Nb5+ and Ta5+ into BSF to synergistically optimize the oxygen transport capacity and surface reactivity. The optimized Bi0.5Sr0.5Fe0.8Nb0.1Ta0.1O3−δ (BSFN0.1T0.1) demonstrates exceptional oxygen ion diffusivity (Dchem = 4.378 × 10−4 cm2 s−1) and exchange kinetics (Kchem = 1.330 × 10−3 cm s−1), leading to a 69% reduction in polarization resistance (from 0.195 to 0.06 Ω cm2) at 750 °C. Notably, the BSFN0.1T0.1 air electrode maintains stable performance under 3–10% CO2 atmosphere, demonstrating superior CO2 tolerance. In fuel cell mode, the single cell delivers a peak power density of 616 mW cm−2 (98% enhancement over BSF) at 750 °C. In electrolysis cell mode, a current density of 1370 mA cm−2 is obtained at 750 °C and 1.5 V in 70% CO2/30% CO atmosphere (116% enhancement over BSF). The synergistic effect of Nb5+ and Ta5+ co-doping arises from their similar ionic radii, stable high valence states, and electronegativity differences, which stabilize the perovskite lattice and facilitate oxygen migration, thereby optimizing oxygen reduction/evolution reaction (ORR/OER) activity and electrochemical performance. This work provides a rational design strategy for advanced RSOC air electrodes.
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