Active and Stable Bifunctional Oxygen Electrocatalysts and Catalyst Layers for Unitized Regenerative Fuel Cells

Abstract

Proton exchange membrane unitized regenerative fuel cells (PEM-URFCs) that can function in both fuel cell and electrolysis modes provide potential weight and cost advantages over discrete systems; however, improved catalysts and catalyst layers are needed to improve performance and stability. PEM-URFCs require bifunctional oxygen electrocatalysts that facilitate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and significant challenges remain to increase catalyst activity, reduce precious metal loading, and obtain long-term stability over the wide potential voltage ranges that occur switching between fuel cell and electrolyzer modes. We have recently shown that bimetallic nanoframes, formed from thermal treatment of noble-metal decorated nanosheets, provide bifunctional oxygen electrocatalysts with significantly higher activity compared with monometallic structures, evaluated using a rotating disk electrode configuration.1 The nanostructured bimetallic alloys utilize noble metal-non-noble transition metal interactions to alter the surface electronic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional accessibility to the catalytically active sites. In addition to design of the catalytically active site, the catalyst layer has important influences on mass transfer and flooding within PEM-URFCs. From our preliminary evaluations, the oxygen catalyst nanostructure, catalyst loading, and catalyst layer composition have significant effects on the performance of PEM-URFC membrane electrode assemblies (MEAs) that operate in both fuel cell and electrolyzer modes. References Godínez-Salomón, F.; Albiter, L.; Mendoza-Cruz, R.; Rhodes, C.P. Bimetallic Two-dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts, ACS Applied Energy Materials , 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051