Effect of a Physisorbed Tetrabutylammonium Cation Film on Alkaline Hydrogen Evolution Reaction on Pt Single-Crystal Electrodes

Abstract

The addition of tetrabutylammonium (TBA⁺) to alkaline electrolytes enhances the hydrogen evolution reaction (HER) activity on Pt single-crystal electrodes. The concentration of TBA⁺ significantly influences the HER on Pt(111). Concentrations of ≤1 mM yield no significant effect on HER currents or the coverage of adsorbed hydrogen (H*) but exhibit an interaction with the OH_ads on the surface. Conversely, concentrations of >1 mM result in an apparent site-blocking effect for underpotential-deposited H* caused by the physisorption of the organic cation, which counterintuitively leads to an increase in the HER activity. The physisorption of TBA⁺ is linked to its accumulation in the diffuse layer, as it can be reversibly removed by the addition of nonadsorbing cations such as sodium. Following the previous literature on the TBA⁺ interaction with electrode surfaces, we ascribe this effect to the formation of a two-dimensional TBA⁺ film in the double layer. On stepped Pt single-crystal surfaces, TBA⁺ enhances HER activity at all concentrations, primarily at step sites. Our findings not only highlight the complexities of TBA⁺ accumulation on Pt electrodes but also offer important molecular-level insights for optimizing the HER by organic film formation on various atomic-level electrode structures.