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

The electrochemical conversion of CO 2 represents a promising way to simultaneously reduce CO 2 emissions and store chemical energy. However, the competition between CO 2 reduction (CO 2 R) and the H 2 evolution reaction (HER) hinders the efficient conversion of CO 2 in aqueous solution. In water, CO 2 is in dynamic equilibrium with H 2 CO 3 , HCO 3 - , and CO 3 2- . While CO 2 and its associated carbonate species represent carbon sources for CO 2 R, recent studies by Koper and co-workers indicate that H 2 CO 3 and HCO 3 - 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/CO 2 R on Au electrodes. The vibrational spectra at the Ar-purged Na 2 SO 4 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 CO 2 -purged Na 2 SO 4 solution/gold interface; instead, a peak around 3400 cm -1 appears at catalytic potential, which is assigned to CO 3 2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH - product) and HCO 3 - reduction (CO 3 2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO 3 electrolyte reveals that the proton donor switches from HCO 3 - at low overpotential to H 2 O 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 CO 2 R in aqueous solution.