Fine‐Tunning BaCo _0.4 Fe _0.4 Zr _0.1 Y _0.1 O _3−δ ‐Based Air Electrodes for Reversible Protonic Ceramic Cells via Co‐Engineering A‐site Deficiency and Nickel Content

Abstract Reversible protonic ceramic cells offer the potential for high‐efficiency bidirectional conversion between chemical energy and electricity. However, their widespread adoption is hindered by the absence of air electrodes exhibiting both high catalytic activity and stability. The BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3−δ perovskite, possessing advantageous bifunctional properties, presents a viable option for commercial development. Research indicates that both bulk Ni doping and surface modification with NiO have been shown to further enhance its air electrode performance in reversible protonic ceramic cells (r‐PCCs). However, a systematic optimization of the nickel‐incorporated BCFZY air electrode is still lacking. In this study, we optimize both surface and bulk characteristics of BCFZY‐based electrodes by carefully tuning the A‐site cation deficiency and nickel doping levels in the perovskite precursor. Specifically, a precursor formulated as Ba 0.95 (Co 0.4 Fe 0.4 Zr 0.1 Y 0.1 ) 0.9 Ni 0.1 O 3−δ is synthesized, resulting in a product predominantly composed of a slightly B‐site deficient perovskite phase and surface‐enriched NiO nanoparticles (2.4 wt.%). B‐site deficiency promotes hydration, increasing proton carrier concentration and thus proton conductivity. Simultaneously, surface NiO nanoparticles facilitate surface exchange and oxygen/steam adsorption, improving catalytic activity. As a result, r‐PCCs incorporating this optimized nanocomposite air electrode demonstrate substantial performance obtained in fuel cell and electrolysis operation.