The electrochemical reduction of CO2 is a promising way to reduce CO2 emission and store chemical energy. However, the competition between CO2 reduction and hydrogen evolution reaction hinders the conversion efficiency of CO2. CO2 and bicarbonate are often considered as sources of carbon to be reduced to CO and other products in CO2 reduction. Recent studies by the Koper and co-workers also indicate that the H2CO3 and HCO3- can be the primary proton donor for HER at certain potentials. In this study, we try to understand the mechanism of the hydrogen evolution reaction (HER) in Ar and CO2 purged Na2SO4 solution on Au using sum frequency generation vibrational spectroscopy. The vibrational spectra at an Ar purged Na2SO4 solution/gold interface show a strong peak around 3630 cm-1 which is assigned to OH-, a product of water reduction. On the contrary, this peak disappears for the CO2 purged Na2SO4 solution/gold interface. Instead, a peak around 3400 cm-1, which is assigned to the Na+/CO32- ion association, appears at catalytic potential. The disappearance of OH- peak indicates that H2CO3 and/or HCO3- as opposed to H2O serves as the proton source for HER in CO2 purged Na2SO4 electrolyte. This may be either through direct H2CO3/HCO3- reduction or by buffering the interfacial pH. Voltammetric measurements also reveal a significant increase in the HER rate for CO2 purged Na2SO4 solution. H2CO3/HCO3- and water reduction are shown to have intrinsically different spectral features, indicating that the vibrational sum frequency spectra of interfacial water may be crucial to understand the competing mechanisms of HER and CO2 reduction.

Spectroscopic observation of HER at the Na2SO4 solution/gold interface using sum frequency generation vibrational spectroscopy

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