Critical evaluation of cobalt-based anodes for proton exchange membrane water electrolysis

Proton-exchange membrane water electrolysis (PEMWE) is a leading technology for green hydrogen production. However, its large-scale deployment is constrained by the reliance on scarce and expensive iridium oxides as anodes for the low-pH oxygen evolution reaction. Cobalt-based anodes have been widely proposed as cheap, earth-abundant alternatives, yet their true viability under realistic PEMWE conditions remains unclear. In this review, we present a critical evaluation of cobalt-based anodes, combining thermodynamic analysis, mechanistic insights, device-level performance, and techno-economic considerations. The thermodynamic analysis reveals that cobalt oxides are intrinsically unstable in the highly oxidising, low-pH environment, where dissolution to Co2+ is thermodynamically favoured. We review recent mechanistic studies that reveal extensive potential-dependent dissolution and apparent “self-healing” behaviour that relies on dissolved species in the electrolyte. Case studies of state-of-the-art cobalt-based electrocatalysts tested in PEMWEs show that apparent stability is generally achieved at high cell voltages, excessive loadings, and water recirculation mode. A simple techno-economic analysis demonstrates that the voltage penalty, combined with rapid degradation, leads to a substantial increase in electricity cost per kilogram of H2 that overcomes any savings from replacing iridium. Overall, the persistent stability challenges associated with currently reported cobalt-based anodes suggest that they face substantial difficulty in satisfying the durability and efficiency benchmarks required for industrial PEMWE operation under realistic conditions, highlighting the need to prioritise the development of thermodynamically stable oxide matrices and more reliable stabilisation strategies.