Artificial photosynthesis, which would store light energy from the sun in high-energy density chemical bonds, promises a solution for the intermittency of solar power allowing energy use on-demand. One process by which this can be achieved is solar water splitting. The oxygen evolution reaction (OER) is the more kinetically demanding of the two half-reactions of water splitting, and Ni(Fe)-oxo/hydroxide was observed to be one of the most active non-noble catalysts for OER in alkaline. Inclusion of guest metal ions especially Fe enhances OER catalysis and work was directed to understand the role Fe plays in catalytic reactivity. The understanding is complicated by the dynamic nature of the Ni(OH)<sub>2</sub>/NiOOH redox system which undergoes several transformations in the electrochemical medium. Classes of bifunctional electrocatalysts such as nickel selenide and phosphide have emerged whose surfaces transform into the oxo/hydroxide under OER operating conditions. This work probed using electrochemical measurements the roles of Fe included on the surface of Ni(Fe)-based electrocatalysts from the electrolyte versus included in its bulk structure in enhancing OER catalysis and in sustainability of catalytic activity in alkaline medium. In these experiments, films were activated in Fe-containing KOH or Fe-free KOH and OER catalysis was studied in these two media. Electrochemical measurements at anodically-deposited Ni-oxo/hydroxo thin films revealed that low-coordination Fe on the surface (surface-Fe) included from solution post-deposition is responsible for catalytic reactivity and not high-coordination Fe co-deposited in the bulk (bulk-Fe). The active site was dynamic and catalysis at activated Ni(Fe)-oxo/hydroxide was not sustained in the absence of an Fe source in solution. An exchange that results in surface-Fe becoming higher-coordination bulk-Fe was proposed as the deactivation mechanism. OER catalysis at nickel selenide films with and without Fe co-deposited was studied in the presence and absence of Fe in KOH during activation and during catalysis, and electrochemical measurements led to a similar picture emerging regarding the dependence of OER catalysis and its sustainability on solution Fe at electrocatalysts from nickel selenide precursors.

Role of the electrolyte in sustaining catalysis at dynamic active sites of NiFe-based electrocatalysts for the oxygen evolution reaction

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