Structure-Activity Relationships in Ni-Fe Oxyhydroxide Oxygen Evolution Electrocatalysts

Abstract

The oxygen evolution reaction (OER) is kinetically slow and hence a significant efficiency loss in electricity-driven water electrolysis. Understanding the relationships between architecture, composition, and activity in high-performing catalyst systems are critical for the development of better catalysts.1 To reach the relevant operating conditions of industrial electrolyzers, high mass loadings are required even when current state of the art catalysts are employed. Several challenges arise when studying high loading films which complicate determination of intrinsic OER activity. For example, to accurately report turnover frequencies the number of metal centers participating in the OER must be known. Furthermore, diminished electronic and mass transport through a thick catalyst film could result in active sites located in different parts of the film generating oxygen at different rates. We compare several methods of determining the number of electrochemically active metal centers, and report the loading dependent behavior of Ni-Fe oxyhydroxide films, the fastest known water oxidation catalyst in alkaline media.2We find that commonly employed methods used to determine the electrochemically active surface area could result in errors of several orders of magnitude due to the potential dependent conductivity of many transition metal-based catalyst systems. We discovered that the method of electrodeposition strongly affects the intrinsic Ni(Fe)OOH activity, as well as the practical electrode performance.3Pulse electrodeposited films increase in turnover frequency with loading, while continuously deposited films decrease in activity with loading. The improvements stemming from this pulsed deposition method are wide reaching, affecting both the architecture and the composition of the catalyst film. The role of these effects on intrinsic and geometric OER activities will be discussed in the context of the design principles for high-performance electrode architectures. (1) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549-7558. (2) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261. (3) Batchellor, A. S.; Boettcher, S. W. Pulse-Electrodeposited Ni–Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts with High Geometric and Intrinsic Activities at Large Mass Loadings. ACS Catalysis 2015, 5, 6680-6689.