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

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 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.