A review of cobalt-free positrodes for protonic ceramic cells: bridging fundamental insights and technological development

Abstract Protonic ceramic cells are emerging as a next-generation technology for sustainable energy conversion, operating efficiently at 400 °C–600 °C in fuel cell protonic ceramic fuel cell (PCFC) and protonic ceramic electrolysis cell (PCEC) modes. The positrode (positive electrode) governs performance in both directions, catalyzing oxygen reduction to water in PCFC and oxygen evolution/water-splitting in PCEC, with the former typically rate-limiting at intermediate temperatures and the latter penalized under high steam activity. Concerns over volatility, thermal-expansion mismatch, and supply risk motivate a shift from cobalt-based to cobalt-free positrodes. This review critically assesses such materials, ranging from single perovskites, double perovskites, Ruddlesden–Popper phases, triple-conducting oxides, and infiltrated composites, through structure-function correlations linking B-site chemistry, defect equilibria, hydration, proton transport, and surface exchange. We evaluate synthesis and fabrication routes for scalability and interfacial control, survey advanced characterization, and analyze degradation spanning chemical, interfacial, and mechanical modes. Sustainability is benchmarked via supply-risk and life-cycle metrics. We conclude with research priorities and design principles for high-performance cobalt-free positrodes in both PCFC and PCEC operations.