Defect-Engineered Mn1–XCo2O4 Spinel Oxides as Highly Active and Durable Cathodes for Proton-Conducting Solid Oxide Fuel Cells

Developing sustainable and robust proton-conducting solid oxide fuel cells (H-SOFCs) requires overcoming the sluggish oxygen reduction reaction (ORR) and degradation of traditional rare-earth-containing perovskite cathodes. To address this, we propose a rare-earth-free metal-cation-deficient spinel structure to modulate the defect chemistry and surface reaction kinetics of the cathode. A series of Mn-deficient manganese-cobalt spinels, Mn 1– X Co 2 O 4 ( X = 0–0.15), were synthesized. Combined experimental characterizations and density functional theory (DFT) calculations systematically reveal the influence of cation vacancies on the synergistic transport of electrons, protons, and oxygen ions, alongside overall ORR activity. Results indicate that moderate Mn deficiency preserves the spinel structure while inducing a higher concentration of oxygen vacancies. This structural evolution enhances small-polaron hopping conductivity, significantly promotes H 2 O adsorption and hydration reactions, and lowers both oxygen vacancy formation energy and proton migration barriers. Consequently, the optimized Mn 0.9 Co 2 O 4 cathode achieves a remarkably low polarization resistance of 0.123 Ω cm 2 at 700 ° C. Furthermore, an anode-supported single cell integrated with this cathode delivers a peak power density of 980 mW/cm 2 at 700 ° C and maintains stable operation over an 80-h test. Therefore, constructing a metal-deficient spinel structure serves as a defect engineering strategy to elevate the electrocatalytic activity and durability of H-SOFC cathodes.