Bimetallic Two-Dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts

Obtaining acidic bifunctional oxygen electrocatalysts that simultaneously provide high activity and high stability for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains a significant challenge and need for unitized regenerative fuel cells and metal-air batteries. We report that bimetallic nickel-platinum/cobalt-iridium two-dimensional (2D) nanoframes provide significantly higher ORR and OER activities compared with platinum-iridium oxide (Pt-IrO2). The metallic alloy 2D nanoframes (NiPt and CoIr) were synthesized by thermal reduction of noble-metal-decorated transition-metal hydroxide nanosheets followed by chemical leaching. The 2D nanoframes utilize noble metal (Pt, Ir)-non-noble metal (Ni, Co) interactions to alter the atomic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional (3D) accessibility to the catalytically active sites. Within the cyclic voltammograms, hydrogen adsorption/desorption features on Pt were suppressed within Pt-IrO2 but were clearly observed within NiPt-CoIr which is attributed to the different size and shape of the nanoframes relative to Pt and IrO2 that result in less interparticle interaction within NiPt-CoIr compared with commercial Pt-IrO2. Rotating disk electrode testing showed that the 2D nanoframe bifunctional oxygen electrocatalysts showed significantly higher ORR mass activity, OER mass activity, and round-trip efficiency compared with Pt-IrO2. Over repeated mode switching between ORR and OER potential ranges using an accelerated durability testing protocol, the NiPt-CoIr 2D nanoframe electrocatalysts showed lower ORR stability but improved OER stability compared with Pt-IrO2. Bimetallic 3D structures with controlled size, shape, surface structure, and morphology provide the opportunity to design bifunctional catalysts with improved activity and stability.