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

Abstract

Bifunctional oxygen electrocatalysts that facilitate the acidic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the same electrode are critical components of unitized regenerative fuel cells (URFCs) and metal-air batteries. Obtaining bifunctional oxygen catalysts that simultaneously provide high ORR and OER activity, high stability, and lower precious metal content remains a significant challenge. Our recent work has explored bimetallic two-dimensional (2D) nanoframes as an approach to obtain carbon-free, unsupported nanostructures and demonstrated nickel-platinum and nickel-iridium 2D nanoframes function as high activity, separate ORR and OER catalysts respectively.1,2 To obtain high activity bifunctional oxygen catalysts, we investigated the combination of NiPt and CoIr 2D nanoframes. Bimetallic 2D nanoframes were prepared by microwave-assisted synthesis of noble metal-decorated transition metal hydroxide nanosheets followed by thermal reduction and chemical leaching steps. The 2D nanoframes utilize noble metal (Pt,Ir)-transition metal (Ni, Co) interactions to alter surface electronic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional accessibility to the catalytically active sites. Electrochemical testing using a rotating disk electrode configuration showed the two-component 2D nanoframe bifunctional oxygen electrocatalysts have higher ORR mass activity, OER mass activity, and round-trip efficiency compared with baseline Pt-IrO2 catalysts. Accelerated durability testing consisting of repeated voltage cycles over ORR/OER potential ranges showed that the 2D nanoframe catalysts exhibit similar ORR stability and improved OER stability compared with Pt-IrO2. The ability to combine highly catalytically active surfaces within a carbon-free 3D nanoarchitecture provides the opportunity to design bifunctional catalysts with improved activity and stability. References Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171