B-site high valence cation co-doping boosts fast oxygen kinetics in a cobalt-free perovskite air electrode for reversible solid oxide cells

Abstract

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 Bi_0.5Sr_0.5FeO_3−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 Bi_0.5Sr_0.5Fe_0.8Nb_0.1Ta_0.1O_3−δ (BSFN_0.1T_0.1) demonstrates exceptional oxygen ion diffusivity (D_chem = 4.378 × 10⁻⁴ cm² s⁻¹) and exchange kinetics (K_chem = 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 BSFN_0.1T_0.1 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.