Speakers
Description
In recent years, nickel-oxyhydroxide has come into focus as a low-cost and efficient material in the Oxygen Evolution Reaction (OER) via electrochemical water splitting. It is a prototypical active and stable catalyst for alkaline OER, however, its precise reaction mechanism is not yet fully understood. We coupled pulse voltammetry, operando X-ray absorption spectroscopy (XAS) and density functional theory (DFT) simulations to characterize the nature of the active surface and elucidate the OER mechanism on nominally Fe-free NiOOH. Electrochemical and XAS data show the Ni(OH)2 transformation to NiOOH in the known cyclic voltammetry peak before the OER onset and the material stays NiOOH during OER. Nevertheless, oxidative charge accumulates with more and more anodic bias and this charge correlates with the logarithm of OER current. As opposed to the case of IrOx, surface oxyl formation on NiOOH is thermodynamically prohibitive. Instead, surface water deprotonates to surface hydroxyl in a broad potential window relevant to OER, and its coverage increases with increasing anodic bias. Although hydroperoxo and especially superoxo are thermodynamically favoured, their formation is kinetically inhibited, in line with XAS data indicating their coverages being below detection limit. This suggests that the rate determining step is likely O-O bond formation. We find that O-O bond formation proceeds through the involvement of 3 hydroxyl sites; one site participates in O-O bond formation and the other two sites are hydrogen acceptors. Moreover, as the hydroxyl coverage increases, the barrier of this O-O bond formation step decreases linearly via a Brønsted relationship. A simple kinetic model based on this relationship provides reaction rate predictions that are in good agreement with the electrochemical results.
Hence, in strong analogy to Ir-based materials under acidic conditions, we found the potential dependent activity of NiOOH under alkaline conditions is driven by oxidative charge accumulation, rather than direct action of the electrochemical bias on the reaction coordinate. In the case of NiOOH, however, adsorbed hydroxyl is active in water nucleophilic attack.
