Platinum has been known for decades to have a unique combination of activity and stability for the hydrogen evolution half water electrolysis (the HER). Clearly understanding of the HER mechanism on Pt might be expected to help guide the search for earth-abundant electrocatalysts with similar properties, yet, despite decades of effort, well-established experimental trends, e.g. the dependence of HER on Pt surface structure, cannot be rationalized by any well understood mechanism.

One such perplexing observation is why, exactly, does the onset of H₂ formation occur on Pt(111) so close to 0 V RHE (i.e. ≈ 0.66 ML of adsorbed H and the thermodynamic limit). Recent computational studies offer two important observations. (i) As H adsorbs on Pt(111) at potentials above the onset of H₂ formation it displaces chemisorbed water. As a result above coverages of ≈ 0.6 ML of adsorbed H the surface dipole abruptly increases inducing H desorption. (ii) The structure of adsorbed H on Pt(111), from 0 - 0.5 ML, is the same in UHV and in contact with liquid H₂O. At all coverages both on-top and fcc-hollow adsorption sites are filled.

Here we address these observations — and show how they are the result of the same underlying physical chemistry — by conducting vibrationally resonant sum frequency generation (VSFG) at a Pt(111) working electrode as a function of applied bias and an H₂ dosed Pt(111) single crystal in UHV as a function of H coverage. Steady-state VSFG measurements show the presence of top-adsorbed H at all surface coverages both under electrochemical control and in UHV (at temperatures above ≈ 200 K). At the electrochemical interface Au pump - VSFG probe measurements suggest that the onset of H₂ formation is characterized by greatly enhanced mobility of a sub-population of top-adsorbed H on ultrafast timescales. Time-averaged low energy electron diffraction (LEED) and temperature programmed desorption (TPD) measurements of adsorbed H in UHV show that a characteristic p(2x2) adsorption superstructure exists at all surface coverages that saturates at 0.75 ML. Taken together these results suggest that the onset of H₂ formation on Pt(111) occurs because lowering potential induces H adsorption that destabilizes the surface structure of adsorbed H both in the plane of the surface and along the surface normal, abruptly, near surface coverages of 0.66 ML.

Yujin Tong

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