Grain Boundary-Derived Local Amorphization Enhances Acidic OER

IrO_x of the amorphous phase has long been recognized to exhibit higher catalytic activity than crystalline analogues toward oxygen evolution reaction (OER) but always at the expense of reduced stability. Here, we report an ultrathin Ir surface with high-density grain boundaries (GBs), which transforms into locally stabilized amorphous IrO_x by forming an Ir/IrO_x interface under OER conditions. The catalyst displays a low overpotential of 263 mV at 10 mA cm^–2 and a mass activity (5.8 A mg_Ir^–1 at 1.53 V) of over 90-fold higher than that of commercial IrO₂, along with long-term stability for over 350 h. The activity enhancement arises from the stronger binding strength of *OOH on the amorphous GBs relative to the crystalline region, thus breaking the scaling relationship between *OH and *OOH and reducing the energy barrier for the potential determining step of the OER. Proton exchange membrane water electrolysis using this catalyst achieves 2.7 A cm^–2 at 2 V cell voltage and operates stably at 1 A cm^–2 for over 200 h. The stabilization of the amorphous IrO_x phase at GBs may accelerate the development of more active and robust acidic OER electrocatalysts.