Ni nanoclusters as oxygen evolution catalysts on porous supports for electro- and photocatalysis

The efficiency of green hydrogen production via water splitting is typically hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Here we investigate the performance of various nickel nanoclusters, deposited via a binder-free gas-phase method, as OER catalysts on two distinct porous platforms: commercial gas diffusion layers (GDL) for electrocatalysis and mesoporous TiO 2 thin films for photoelectrocatalysis. For dark electrocatalysis on GDL, we find a non-linear relationship between catalyst loading and activity, where the lowest Ni loadings exhibited the highest specific activity. Trace iron impurities in the electrolyte dramatically enhanced the performance, leading to a 120-fold increase in specific current for the lowest loading samples through the in situ formation of highly active NiFe oxyhydroxide species. When integrated as co-catalysts on mesoporous TiO₂ photoanodes, Ni nanoclusters significantly improved photocurrents, with an optimal loading of 0.27-0.89 μg/cm 2 . While Fe impurities also boosted photoelectrochemical performance at low Ni coverages, the effect was less pronounced and became detrimental at higher loadings. These findings underscore that the precise control of the catalyst loading and composition is decisive for designing scalable and highly efficient systems for water oxidation.