Designing catalyst layer morphology for high-performance water electrolysis using synchrotron X-ray nanotomography

The limited availability of iridium in the Earth’s crust poses severe challenges to establishing gigawatt-scale electrolyzers that are needed for energy storage; this problem urgently calls for reduced iridium loadings. Reducing iridium loadings requires catalyst structure optimization, but to date, little attention has been paid to the characterization of electron, proton, and mass transport in the catalyst layer, particularly at the nanoscale. We present the 3D nanoscale pore structure of iridium-based catalyst layers via synchrotron full-field transmission X-ray microscopy (TXM) and perform pore network modeling to determine effective transport properties in water electrolyzers. We observe a wide range of pore sizes in the catalyst layer, constituting pathways that facilitate mass transport. Increasing the thickness of the ionomer layer that covers the catalyst particles significantly increases protonic conductivity at the cost of reducing the open pore space and electrical conductivity, both of which are detrimental to electrolyzer performance.