Resolving the iridium dilemma in PEM water electrolysis: multiscale design of low-iridium and iridium-free anodes

Proton exchange membrane water electrolysis (PEMWE) is regarded as one of the leading technologies for large-scale green hydrogen production owing to its high efficiency, rapid dynamic response, and compatibility with intermittent renewable energy. However, its large-scale deployment is fundamentally constrained by the iridium dilemma. Although iridium-based catalysts exhibit unparalleled activity and durability for the acidic oxygen evolution reaction (OER), their scarcity, high cost, and limited global supply are incompatible with terawatt-scale deployment. While substantial progress has been achieved in developing low-iridium and iridium-free anodes, practical implementation remains challenging because catalyst activity, durability, interfacial transport, and device performance are intrinsically coupled across multiple length scales, leading to a persistent gap between laboratory catalyst development and practical operation. This Review presents a multiscale framework for resolving the iridium dilemma through the rational design of low-iridium and iridium-free PEMWE anodes. Beginning with the fundamental challenges of activity, durability, and the half-cell-to-device translation, we discuss catalyst-level strategies for maximizing active-site utilisation and balancing activity-stability trade-offs, followed by interface-level approaches for controlling interfacial water structures and proton-transfer processes. At the device level, considerations including scalable catalyst synthesis catalyst-layer-centred anode design, membrane electrode assembly (MEA) design and fabrication, in-cell degradation diagnostics, benchmarking protocols, and techno-economic assessment, are then examined in relation to practical PEMWE operation. Finally, we outline the remaining challenges and future opportunities for bridging fundamental acidic OER catalysis with durable, scalable, and economically viable low-iridium and iridium-free PEM water electrolysers.