B-Site High-Entropy Stabilized A-Site Deficient Perovskite Air Electrode for Efficient and Durable Protonic Ceramic Electrolysis

Protonic ceramic electrolysis cells (PCECs) offer a low‑temperature route for efficient hydrogen production, but their performance is often limited by poor catalytic activity and insufficient stability of air electrodes. Herein, we report a synergistic strategy combining A‑site deficiency with B‑site high‑entropy design to simultaneously enhance triple ionic-electronic conductivity (e⁻/O²⁻/H⁺⁾ and structural stability. The resulting perovskite, Ba_0.8Co_0.2Fe_0.2Zr_0.2Y_0.2Ni_0.2O_3−δ (B_0.8CFZYN), exhibits abundant oxygen vacancies, favorable hydration thermodynamics, and expanded electrochemically active sites from the triple‑phase boundary to the entire electrode bulk. The electrode achieves an exceptionally low polarization resistance of 0.146 Ω cm² at 650°C in humid air. In a single PCEC, current densities reach 1278.9, 873.5, and 531.7 mA cm⁻² at 1.3 V and 650°C, 600°C, and 550°C, respectively. Moreover, the cell shows negligible degradation over 250 h at 600°C. This work demonstrates that high‑entropy defect engineering provides a powerful platform for designing highly active and robust electrocatalysts for intermediate-temperature PCECs.