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

ABSTRACT 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).