2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2

Both commercially available and laboratory-synthesized IrO ₂ catalysts typically possess rutile-type structures and diverse facet orientations. According to the theoretical results from density functional theory calculations, distinct IrO ₂ facets will result in divergent electrocatalytic properties, among which the (101) crystal facet is theoretically predicted as the most energetically favorable for oxygen evolution reaction (OER) owing to its lowest energy barrier. Maintaining a single-unit-cell thickness while exposing a desired facet of 2D IrO ₂ presents a significant opportunity and challenge for the development of high-performance OER anode catalysts. Herein, we develop an ammonia-induced facet engineering for oriented modulation of crystal facets in the ultimate limit of monolayer thickness, and successfully synthesize 2D monolayer IrO ₂ exposing unique (101) facet. At the current density of 10 mA cm ^-2 _geo , an ultralow overpotential of 230 mV has been achieved on the highly activated (101) facet in a three-electrode system. More importantly, in a proton exchange membrane (PEM) electrolyzer, the IrO ₂ anode reaches a low voltage of 1.74 V at an industrial-level current density of 2 A cm ^-2 _geo , much lower than that of all commercial IrO ₂ electrocatalysts. Though facet engineering primarily contributes to modulating the intrinsic activity rather than stability, the as-prepared IrO ₂ (101) monolayer performs over 8,000 hours of PEM water electrolysis (PEMWE) stability at constant 1.5 A cm ^-2 _geo , with a negligible decay rate of 4.0 mV kh ^-1 . Furthermore, even a long-term PEMWE test of 1000 h using the membrane electrode assembly (MEA) with ultra-low Ir loading of 0.2 mg _Ir cm ^-2 _geo under fluctuating operating conditions is performed, E _Cell remains highly electrochemically stable over time at 1.5 A cm ^-2 _geo , without any signs of catalyst degradation. This work proposes that ammonia-induced facet engineering of 2D monolayer IrO ₂ could represent a novel approach to selectively expose the desired (101) facet, thereby enabling unique facet-dependent OER performance and ultrahigh stability in industrial-scale PEM electrolysis, even under voltage fluctuations generated by solar and wind power.