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 IrO₂ under realistic PEMWE conditions remains lacking. Here, we synthesize four crystalline phases of iridium oxide (metastable 1T-, 3R-, Tri-, and conventional Rutile-IrO₂) and demonstrate a strict phase-dependent OER activity trend: 1T-IrO₂ > 3R-IrO₂ > Tri-IrO₂ > Rutile-IrO₂. The 1T-IrO₂ 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 IrO₂ catalysts and highlights the promise of phase engineering for efficient energy conversion.