Transition Metal-Oxygen Covalency: An Important Descriptor for Oxygen Dissociation Activity in Iron-Based PCFC Cathodes

Fe-based perovskites are promising cathode materials for protonic ceramic fuel cells (PCFCs) owing to their competitive electrochemical performance and relatively low cost. However, the optimization principles governing their activity remain unclear due to the fragmented understanding derived from studies conducted by different research groups. Here, a series of Ba 0.95 La 0.05 Fe 0.9 TM 0.1 O 3−δ (TM = Mn, Co, Ni, Cu, and Zn) cathodes are systematically investigated to elucidate the influence of transition-metal dopants on their physicochemical properties and electrochemical performance. The results reveal that TM-O covalency plays a pivotal role in regulating oxygen dissociation and hydration capability, two key factors governing the proton-involved oxygen reduction reaction in PCFC cathodes. Among all compositions, Ba 0.95 La 0.05 Fe 0.9 Cu 0.1 O 3−δ exhibits the lowest polarization resistance of 0.28 Ω·cm 2 at 600 °C. A single cell employing this cathode delivers a peak power density of 1170 mW·cm −2 and demonstrates stable operation for over 200 h at 650 °C. This work establishes a clear structure-property relationship and provides valuable guidance for the rational design of high-performance Fe-based perovskite cathodes for PCFCs.