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

Abstract

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 CO₃²⁻. 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 CO₃²⁻ in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH⁻ product) and HCO₃⁻ reduction (CO₃²⁻ 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.