Origin of the Universal Potential-Dependent Organic Oxidation on Nickel Oxyhydroxide

Abstract

The paired electrolysis via anodic organic oxidation and cathodic hydrogen evolution has been an emerging hydrogen production technology with high efficiency and high values. Nickel oxyhydroxide is a special electrocatalyst that is capable of selectively oxidizing various alcohols, aldehydes, and amines to the corresponding carboxylates and nitriles at more favorable potentials compared to the oxygen evolution reaction. However, its detailed molecular-level mechanism is still in debate, especially for the potential-dependent behavior of some organic substrates. In this study, we revealed that the nickel oxyhydroxide can be dissected into two functional regions with a more facilely oxidized surface for facilitated substrate adsorption and a relatively inert bulk phase for accelerated electron transfer via probe-assisted kinetics and in situ surface-enhanced Raman spectroscopy. The prerequisite of the two regions conceals a universal potential-dependent oxidation behavior for almost all organic substrates. Further combining with the computational investigation unravels the origin of this potential dependence from the exothermic O/N-centered adsorption process on the surface and the rate-limiting proton-coupled electron-transfer (PCET) steps for the C–H bond breaking that demands bulk electron conductivity. This provides a rationale for designing more conductive underlayers to break the intrinsic limitations of nickel oxyhydroxide toward more efficient organic electrooxidation processes.