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

The electrochemical conversion of CO represents a promising way to simultaneously reduce CO emissions and store chemical energy. However, the competition between CO reduction (COR) and the H evolution reaction (HER) hinders the efficient conversion of CO in aqueous solution. In water, CO is in dynamic equilibrium with HCO, HCO , and CO . While CO and its associated carbonate species represent carbon sources for COR, recent studies by Koper and co-workers indicate that HCO and HCO also act as proton sources during HER ( 2020, , 4154-4161, 2021, , 4936-4945, 2022, , 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/COR on Au electrodes. The vibrational spectra at the Ar-purged NaSO solution/Au interface, where only HER occurs, show a strong peak around 3650 cm, which appears at the HER onset potential and is assigned to OH. Notably, this species is absent for the CO-purged NaSO solution/gold interface; instead, a peak around 3400 cm appears at catalytic potential, which is assigned to CO in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH product) and HCO reduction (CO product). Monitoring the relative intensities of these features as a function of potential in NaHCO electrolyte reveals that the proton donor switches from HCO at low overpotential to HO 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 COR in aqueous solution.