Proton-Conducting, Vacancy-Rich HIrO 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 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'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(NaIr)O and then exfoliation to produce cation vacancy-rich, 1 nm-thick iridium oxide nanosheets (labeled as □-HIrO). Besides being a proton conductor, the □-HIrO 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 □-HIrO 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.