Electrochemical Anilinium Reduction: Identifying Metastable Surface Intermediate on Pt and Its Voltage-Driven Decomposition to Hydrogen Evolution

Abstract

Organic Brønsted acids have attracted attention as green electrolytes for the hydrogen evolution reaction (HER) and CO2 reduction in aqueous media. Although previous research has attempted to understand the electrochemical reduction of organic acids and their potential intermediates, the underlying mechanisms and the nature of the surface intermediate(s) remain largely unexplored. In this study, we demonstrated that anilinium undergoes direct electrochemical reduction on a Pt electrode, leading to the formation of a stable, reduced anilinium surface intermediate. Electrochemical quartz crystal microbalance (EQCM) analysis revealed that the intermediate molecules were adsorbed to form multilayers, whereas they were unstable and decomposed to H2 and aniline in bulk solution, as verified by in situ hydrogen gas chromatographic analysis. Voltammetric investigations of a Pt ultramicroelectrode (UME) showed the voltage-driven, stochastic, heterogeneous reductive decomposition of the reduced anilinium intermediate layer and the subsequent formation of a critical hydrogen bubble. From voltammetric analysis, the stability constant of the adsorbed intermediate against its heterogeneous dimerization on a Pt electrode was determined to be ~105. Density functional theory (DFT) simulations strongly supported the reduction of the anilinium intermediate molecules with multiple layers adsorbed on PtH. These findings would provide mechanistic insights into the electrochemical reduction of anilinium for HER and its potential application to other electrocatalytic processes.