Dimensional Regulation of Monoclinic Phase Iridium Dioxide Switches the Mechanism for Acid Water Oxidation

Abstract Electrochemical water splitting represents a promising strategy for hydrogen production, with oxygen evolution reaction (OER) being a critical half‐reaction. Iridium dioxide (IrO ₂ ) is an advanced OER catalyst owing to excellent stability. However, regulating key intermediates and decreasing reaction barriers remain great challenges. Here, 2D metastable‐phase IrO ₂ nanosheet (M‐IrO ₂ NS) by a mixed molten salt method is reported. M‐IrO ₂ NS exhibits a 3% compressive strain along the a ‐axis ( a = 4.30 Å vs 4.43 Å) compared to 1D metastable‐phase IrO ₂ nanoribbon (M‐IrO ₂ NR). In 0.5 M H ₂ SO ₄ , M‐IrO ₂ NS exhibits a low overpotential of 186 mV at 10 mA cm ⁻² and a high mass activity of 2071.8 mA mg _Ir ⁻¹ at 1.5 V versus reversible hydrogen electrode (RHE). When integrated into a proton exchange membrane water electrolysis (PEMWE), M‐IrO ₂ NS maintains a current density of 2.83 A cm ⁻² at 1.8 V for 1600 h without degradation. Mechanistic investigations reveal a transition from adsorbate evolution mechanism (AEM) in M‐IrO ₂ NR to oxide pathway mechanism (OPM) in M‐IrO ₂ NS, confirmed by the Fourier transform infrared measurements, density‐functional‐theory calculations and mass spectrometry measurements. This study demonstrates the impact of dimensional regulation on optimizing the OER and provides a new platform for electrocatalyst development.