Understanding how electrocatalysts evolve during operation is critical to optimizing their performance. The oxygen evolution reaction (OER) exposes electrocatalytic materials to harsh conditions that lead to significant transformations. A discovery that revolutionized OER catalysis was the dramatic improvement in activity due to the intentional, or incidental, in situ incorporation of Fe into Ni and Co oxyhydroxides. This finding raised questions about the true active phase in OER catalysts. While Fe incorporation is now widely acknowledged, there is still significant room for improvement in comprehending the in situ incorporation of trace metals and its effects on the electrochemical and structural properties of OER electrocatalysts.
We report a comprehensive investigation of in situ metal incorporation on nickel oxyhydroxide OER electrocatalysts encompassing four multivalent cations: Fe, Co, Mn, and Cu. We found that adding trace amounts of these to alkaline electrolytes alters the electrocatalytic and energy storage properties of NiO_xH_y films. As opposed to the increase in OER activity induced by Fe incorporation (Figure 1a), in situ incorporation of trace Co and Mn increases the redox capacitance (Figure 1b), while Cu incorporation does not proceed. Depth profiling measurements reveal that metal incorporation is confined to the film's surface (Figure 1c-d), resulting in an interstratified structure that partially retains the more active, disordered phase. We describe four factors determining the occurrence of in situ metal incorporation, underscoring its nature as a cation exchange reaction. By providing a better understanding of metal incorporation, our results underscore its potential as a strategy for manipulating the surface chemical composition.

Figure 1. (a) Linear sweep voltammograms and (b) galvanostatic charge-discharge profiles showing the effects of in situ incorporation of trace metal cations added to KOH electrolyte; (c) 2D maps and 3D depth profile renderings showing the spatial distribution of Ni^- and Fe^- markers, and (d) depth profiles of secondary-ion fragments across the NiOOH film.

Jay Bender

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