Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source...

The electrochemical conversion of CO₂ represents a promising way to simultaneously reduce CO₂ emissions and store chemical energy. However, the competition between CO₂ reduction (CO₂R) and the H₂ evolution reaction (HER) hinders the efficient conversion of CO₂ in aqueous solution. In water, CO₂ is in dynamic equilibrium with H₂CO₃, HCO₃ ^-, and CO₃ ^2-. While CO₂ and its associated carbonate species represent carbon sources for CO₂R, recent studies by Koper and co-workers indicate that H₂CO₃ and HCO₃ ^- also act as proton sources during HER (J. Am. Chem. Soc. 2020, 142, 4154-4161, ACS Catal. 2021, 11, 4936-4945, J. Catal. 2022, 405, 346-354), which can favorably compete with water at certain potentials. However, accurately distinguishing between competing reaction mechanisms as a function of potential requires direct observation of the non-equilibrium product distribution present at the electrode/electrolyte interface. In this study, we employ vibrational sum frequency generation (VSFG) spectroscopy to directly probe the interfacial species produced during competing HER/CO₂R on Au electrodes. The vibrational spectra at the Ar-purged Na₂SO₄ solution/Au interface, where only HER occurs, show a strong peak around 3650 cm^-1, which appears at the HER onset potential and is assigned to OH^-. Notably, this species is absent for the CO₂-purged Na₂SO₄ solution/gold interface; instead, a peak around 3400 cm^-1 appears at catalytic potential, which is assigned to CO₃ ^2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH^- product) and HCO₃ ^- reduction (CO₃ ^2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO₃ electrolyte reveals that the proton donor switches from HCO₃ ^- at low overpotential to H₂O at higher overpotential. This work represents the first direct detection of OH^- on a metal electrode produced during HER and provides important insights into the surface reactions that mediate selectivity between HER and CO₂R in aqueous solution.