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.