Atomic‐Mesoscale Synergy in Amorphous Iridium Oxide Catalysts for Proton Exchange Membrane Water Electrolysis

Amorphous iridium oxide (IrO_x) is among the most active Ir-based catalysts for the acidic oxygen evolution reaction (OER), yet its stability is severely limited because lattice-oxygen participation often triggers irreversible oxygen loss that leads to iridium dissolution and structural degradation. Here, we present a surfactant-directed synthesis of mesoporous IrO_x electrocatalysts featuring a hollandite-type local structure. This unique structure creates an atomic-mesoscale synergy that enhances OER activity without sacrificing stability and improves high-current-density performance. At the atomic level, the hollandite-type local structure promotes high OER activity and corrosion resistance. In situ spectroscopic and isotopic labeling experiments reveal a reversible cycle of lattice oxygen loss and reformation during OER. This process enables the flexible iridium local structure to transition between an initial six-coordinate state and a low-coordinated active state. At the mesoscale, an interconnected porous network ensures efficient mass transport and maximizes active-site accessibility. As a result, this mesoporous electrocatalyst achieves a low cell voltage (1.75 V @ 2 A cm^-2) and excellent stability for more than 2000 h (@ 2 A cm^-2) in proton exchange membrane water electrolysis (PEMWE).