Grain Boundary-Derived\nLocal Amorphization Enhances\nAcidic OER

IrO<sub><i>x</i></sub> of the amorphous phase\nhas long\nbeen recognized to exhibit higher catalytic activity than crystalline\nanalogues toward oxygen evolution reaction (OER) but always at the\nexpense of reduced stability. Here, we report an ultrathin Ir surface\nwith high-density grain boundaries (GBs), which transforms into locally\nstabilized amorphous IrO<sub><i>x</i></sub> by forming an\nIr/IrO<sub><i>x</i></sub> interface under OER conditions.\nThe catalyst displays a low overpotential of 263 mV at 10 mA cm<sup>–2</sup> and a mass activity (5.8 A mg<sub>Ir</sub><sup>–1</sup> at 1.53 V) of over 90-fold higher than that of commercial IrO<sub>2</sub>, along with long-term stability for over 350 h. The activity\nenhancement arises from the stronger binding strength of *OOH on the\namorphous GBs relative to the crystalline region, thus breaking the\nscaling relationship between *OH and *OOH and reducing the energy\nbarrier for the potential determining step of the OER. Proton exchange\nmembrane water electrolysis using this catalyst achieves 2.7 A cm<sup>–2</sup> at 2 V cell voltage and operates stably at 1 A cm<sup>–2</sup> for over 200 h. The stabilization of the amorphous\nIrO<sub><i>x</i></sub> phase at GBs may accelerate the development\nof more active and robust acidic OER electrocatalysts.