Unravelling the role of particle size and nanostructuring on the oxygen evolution activity of Fe-doped NiO

Nickel-based oxides and oxyhydroxide catalysts exhibit state-of-the-art activity for the sluggish oxygen evolution reaction (OER) in alkaline conditions. A widely employed strategy to increase the gravimetric activity of the catalyst is to increase the active surface area via nanostructuring or decreasing the particle size. However, fundamental understanding about how tuning these parameters influences the density of oxidized species and their reaction kinetics remains unclear. Here, we use solution combustion synthesis, a low-cost and scalable approach, to synthesize a series of Fe0.1Ni0.9O samples from different precursor salts. Based on the precursor salt, the nanoparticle size can be changed significantly from ~2.5 nm to ~37 nm. The OER activity in pH 13 trends inversely with the particle size. Using operando, time-resolved optical spectroscopy, we quantify the density of oxidized species as a function of potential and demonstrate that the OER kinetics exhibits a second order dependence on the density of these species, suggesting that the OER mechanism relies on O-O coupling between neighbouring oxidized species. With decreasing particle size, the density of species accumulated is found to increase, with a decrease in their intrinsic reactivity for OER, attributed to a stronger binding of *O species (i.e. a cathodic shift of species energetics). This signifies that the high apparent OER activity per geometric area of the smaller nanoparticles is driven by their ability to accumulate a larger density of oxidized species. This study not only experimentally disentangles the influence of the density of oxidized species and intrinsic kinetics on the overall rate of OER, but also highlights the importance of tuning these parameters independently to develop more active OER catalysts.