Harnessing Direct Oxo Coupling for Durable Water Oxidation via Atomic-Level Strain Engineering

Iridium oxides are the state-of-the-art oxygen evolution reaction (OER) catalysts in proton exchange membrane water electrolysis (PEMWE). However, its activity is still hampered by the high thermodynamic barrier of *OOH intermediates in the conventional adsorbate evolution mechanism (AEM). To resolve this challenge, we present an atomic-level compressive strain-engineering strategy to modulate reaction pathways by incorporating erbium (Er^{3 +}) into the IrO₂ (Er-IrO_x) framework. The large ionic radius of Er^{3 +} shortens the Ir-Ir distance and optimizes the electronic structure of active sites. This strain-induced reconfiguration shifts the OER pathway from AEM to the direct oxo coupling mechanism (OPM), where O─O formation occurs through radical coupling, bypassing the high-energy *OOH intermediate. The resulting Er-IrO_x catalyst reaches a small Tafel slope of 70.55 mV dec^{- 1} and a remarkably low overpotential of 209 mV at 10 mA cm^-2. More importantly, when configured into a practical PEMWE, it delivers a high current density of 6 A cm^-2 at a low voltage of 1.899 V and maintains durable operation for over 400 h. This work offers a generalized approach for breaking activity-stability trade-offs in Ir-based catalysts, promoting the commercial implementation of green hydrogen production.