Pulse-Electrodeposited Ni–Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts with High Geometric and Intrinsic Activities at Large Mass Loadings

One practical metric for electrocatalyst performance is current per geometric area at a given applied overpotential. An obvious route to increase performance is to increase the catalyst mass loadingas long as the intrinsic performance (i.e., specific activity or turnover frequency) of the catalyst is independent of loading, and other electrical, ionic, or mass-transfer resistances are not severe. Here we report the geometric and intrinsic oxygen evolution reaction (OER) activities of Ni­(Fe)­OOH films, the fastest known water oxidation catalyst in basic media, as a function of mass loading from 0 to ∼100 μg cm–2. We discuss practices for measuring and reporting intrinsic activities, highlighting experimental conditions where the film activity on a per-metal-cation basis can be accurately measured and where capacitance measurements of electrochemically active surface area fail. We find that the electrochemical reversibility of the (nominally) Ni2+/3+ redox wave correlates with the apparent intrinsic activity as a function of loading. We report a pulsed-electrodeposition method that dramatically improves the catalyst reversibility and performance at high loading compared to continuous electrodeposition, which we attribute to improved connectivity in the micro/nanostructure and better composition control. Pulse electrodeposited films are shown to have geometric performance similar to a number of advanced composite electrocatalyst structures and to maintain effective per-metal turnover frequencies of >0.4 s–1 at 300 mV overpotential, even for loadings of ∼100 μg cm–2.