Phase Engineering of Iridium Oxides Enables Direct Coupling of Proton Exchange Membrane Water Electrolysis With Intermittent Electrical Energy

Proton exchange membrane water electrolysis (PEMWE) coupled with intermittent renewables is a leading technology for green hydrogen production, but its large‐scale deployment is impeded by the sluggish kinetics and high iridium cost of the anodic oxygen evolution reaction (OER). Crystal phase regulation offers a rational approach to enhance catalyst intrinsic activity, yet a clear phase‐activity correlation for IrO 2 under realistic PEMWE conditions remains lacking. Here, we synthesize four crystalline phases of iridium oxide (metastable 1T‐, 3R‐, Tri‐, and conventional Rutile‐IrO 2 ) and demonstrate a strict phase‐dependent OER activity trend: 1T‐IrO 2 > 3R‐IrO 2 > Tri‐IrO 2 > Rutile‐IrO 2 . The 1T‐IrO 2 catalyst achieves a PEMWE performance of 3 A cm −2 at only 1.75 V with an Ir loading of 0.4 mg Ir cm −2 , exceeding the U.S. DOE 2026 target. It also shows stable operation for 2000 h at 2 A cm −2 and maintains durability during 1000 h of dynamic current cycling. In situ XANES/EXAFS analyses link the enhanced activity to a higher Ir oxidation state, while in situ Raman spectroscopy identifies the reaction pathway through characteristic Ir‐*OH, Ir‐*O, and Ir‐*OOH intermediates. This work establishes a direct phase‐activity relationship for IrO 2 catalysts and highlights the promise of phase engineering for efficient energy conversion.