Chlorine-anion doping enhances the metal-oxygen covalency of Bi0.5Sr0.5FeO3-δ air electrode: achieving superior catalytic activity for reversible solid oxide cells

Reversible solid oxide cells (RSOCs) offer a revolutionary pathway for sustainable energy conversion and storage; however, their commercial viability is severely limited by the suboptimal catalytic capabilities and long-term stability of air electrodes. Herein, this work presents a novel approach to concurrently enhance the catalytic activity, durability, and CO₂ tolerance of the Bi_0.5Sr_0.5FeO_3-δ (BSF) air electrode by substituting oxygen sites with chloride (Cl⁻) anion. Notably, the optimized Bi_0.5Sr_0.5FeO_2.95-δCl_0.05 (BSFCl5) electrode exhibits a remarkable 49 % reduction in polarization resistance (Rp) at 800 °C, while maintaining exceptional CO₂ tolerance—Rp remains unchanged even under 10 % CO₂. In full-cell configurations, BSFCl5 achieves a peak power density of 1.22 W cm⁻² (vs. 0.8 W cm⁻² for BSF) and an electrolysis current density of 2.33 A cm⁻² at 1.5 V in a 70 % CO₂/30 % CO atmosphere, representing a 52.5 % and 72.6 % improvement, respectively. The BSFCl5 half-cell and full-cell exhibit excellent operational stability over 350 h and 150 h, respectively. Combined density functional theory (DFT) simulations and comprehensive experimental characterizations elucidate that Cl doping strengthens the metal-oxygen (MO) covalency, synergistically boosting the oxygen reduction/evolution reaction (ORR/OER) kinetics and stability. This work presents a highly anticipated design approach for the future design of air electrodes for RSOCs with excellent catalytic performance and stability.