Understanding compositionally complex electrocatalysts using epitaxial films and correlative multi-scale characterization for structure-activity mapping.
Compositionally complex solid solutions (frequently referred to as high entropy alloys) provide a unique route for designing high-performance electrocatalysts, where the polyelemental surface composition can be seamlessly tuned to optimize activity, selectivity, and stability. However, the mechanistic understanding of these electrocatalysts remains limited by the lack of a model system with a crystallographically defined surface that is compatible with correlative, multi-scale characterization. Here, we present epitaxial films as a model platform for studying compositionally complex electrocatalysts. Using magnetron sputtering, we realize (111) epitaxial Ir-Pd-Pt-Rh-Ru films on a (0001) sapphire substrate via a (111) Pt buffer layer, confirmed via X-ray diffraction and transmission electron microscopy. The growth approach is applicable across a broad composition range and produces smooth surfaces (root mean square roughness <1 nm) with micrometer-sized grains in the nanoscale films. For these films, we demonstrate direct structure-activity mapping at the nanoscale through precise co-localization using micro-indents and performing correlative atomic force microscopy, electron backscatter diffraction, and scanning electrochemical cell microscopy. Our work establishes a model platform for fundamental scale-bridging characterization and paves the way for rational design of compositionally complex electrocatalysts.