Selective In Situ Phase Segregation Enabling Efficient and Stable Protonic Ceramic Fuel Cell Cathode Performance

Abstract Efficient and reliable protonic ceramic fuel cells (PCFCs) necessitate the development of active and durable cathode materials to accelerate the sluggish oxygen reduction reaction (ORR). The most promising PCFC cathode candidates are perovskite‐type structured oxides with mixed oxygen ion, proton, and hole conductivity. However, mixed conductivity often requires materials with alkaline earth elements and the inclusion of these elements in the cathode structure leads to severe degradation in the presence of even small trace amounts of CO ₂ in air. Herein, a new approach is presented to address this challenge by inducing selective in situ phase segregation to engineer the cathode surface and bulk separately. This selective phase segregation is achieved via targeted control of the size mismatch of cations in the perovskite‐type structure, enhancing charge transfer in the bulk while improving CO ₂ resistance at the surface. By co‐incorporating smaller Li ⁺ and larger K ⁺ into the model BaCo _0.4 Fe _0.4 Zr _0.1 Y _0.1 O _3−δ cathode material, it is shown that Li ⁺ segregates to the surface, protecting it from CO ₂ poisoning, while K ⁺ remains in the bulk and accelerates proton transport. Consequently, this in situ restructured cathode can boost the PCFC power output by 30% and improve its CO ₂ tolerance fivefold in the presence of CO ₂ at 600 °C.