Single-faceted IrO2 monolayer enabling high-performing proton exchange membrane water electrolysis beyond 10,000 h stability at 1.5 A cm-2

Both commercial and laboratory-synthesized IrO₂ catalysts typically possess rutile-type structures with multiple facets. Theoretical results predict the (101) facet is the most energetically favorable for oxygen evolution reaction owing to its lowest energy barrier. Achieving monolayer thickness while exposing this desired facet is a significant opportunity for IrO₂. Herein, we develop an ammonia-induced facet engineering for the synthesis of single-faceted IrO₂(101) monolayer. It achieves 230 mV overpotential at 10 mA cm_geo^-2 in a three-electrode system and 1.70 V at 2 A cm_geo^-2 in a proton exchange membrane (PEM) electrolyzer. Though facet engineering primarily contributes to modulating the intrinsic activity rather than stability, single-faceted IrO₂ monolayer performs over 10,000-hour stability at constant 1.5 A cm_geo^-2 (3.95 mV kh^-1 decay) and 1000-hour stability at 0.2 mg_Ir cm_geo^-2 under fluctuating conditions. This work proposes that ammonia-induced facet engineering of IrO₂ monolayer enables facet-dependent oxygen evolution reaction (OER) performance and high stability in industrial-scale PEM electrolysis.