Proton‐Conducting, Vacancy‐Rich H <i> _x </i> IrO <i> _y </i> Nanosheets for the Fabrication of Low‐Ionomer‐Dependent Anode Catalyst Layer in PEM Water Electrolyzer

The anode catalyst layer is composed of catalytically functional IrO_x and protonic conducting ionomer and largely dictates catalytic performance of proton exchange membrane water electrolyzer (PEMWE). Here, we report a new type of anode nanocatalyst that possesses both IrO_x's catalytic function and high proton conductivity that traditional anode catalysts lack and demonstrate its ability to construct high-performance, low-ionomer-dependent anode catalyst layer, the interior of which-about 85% of total catalyst layer-is free of ionomers. The proton-conducting anode nanocatalyst is prepared via protonation of layered iridate K_0.5(Na_0.2Ir_0.8)O₂ and then exfoliation to produce cation vacancy-rich, 1 nm-thick iridium oxide nanosheets (labeled as □-H_xIrO_y). Besides being a proton conductor, the □-H_xIrO_y is found to have abundant catalytic active sites for the oxygen evolution reaction due to the optimization of both edge and in-plane iridium sites by multiple cation vacancies. The dual functionality of □-H_xIrO_y allows the fabrication of low-iridium-loading, low-ionomer-dependent anode catalyst layer with enhanced exposure of catalytic sites and reduced electronic contact resistance, in contrast to common fully mixed catalyst/ionomer layers in PEMWE. This work represents an example of realizing the structural innovation in anode catalyst layer through the bifunctionality of anode catalyst.