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 OHads 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.