Mechanistic Insights into Binding, Configuration, Coverage, and Reaction Rates of Hydrogen on Platinum Surfaces

Hydrogen is a clean alternative to fossil fuels to generate power for heat, industry, electricity, and transportation. Pt is a benchmark electrocatalyst for hydrogen evolution and oxidation reactions (HER/HOR) due to its intrinsic activity. Consequently, Pt-based catalysts have become primary platforms for fundamental studies of HER/HOR mechanisms. Herein, we utilize surface-specific electrochemical sum frequency generation spectroscopy (EC-SFG) to observe HER/HOR intermediates on Pt in situ. We assign two distinct adsorbed hydrogen (*H) resonances as step- and terrace-bound species before quantifying their unique Stark effects and interfacial orientation. We analyze the kinetics of HER; quantifying rate constants for *H generation and H ₂ production through the Heyrovsky or Tafel steps for overpotential- and underpotential-deposited (OPD and UPD) species. We quantify *H surface coverage density as a function of potential to calculate potential-dependent binding energies. Through these analyses, we compose a succinct view of the HER mechanism on Pt electrodes: Under OPD conditions, the Volmer-Tafel mechanism is promoted due to higher surface density and lower binding energy, while UPD conditions promote the Volmer-Heyrovsky mechanism. These findings are supported by our orientational analysis where a smaller Pt–H angle promotes the Tafel step under OPD conditions. The Heyrovsky step is rate-limiting at UPD while the Tafel step is rate-limiting for OPD conditions, with the Volmer step proceeding faster than either H ₂ evolution mechanism. This site-specific understanding of HER/HOR on Pt electrodes will inform mechanisms for other electrode systems and promote EC-SFG as a critical tool in electrochemical mechanistic investigations.