Mechanism of electrochemical carbon dioxide reduction to formate on tin electrode

Carbon dioxide can be electrochemically reduced to formate on tin electrodes. Formate is one of the most economically viable products of CO2 reduction reaction (CO2RR). While CO2 is reduced to formate, hydrogen evolution reaction (HER) occurs concomitantly. The kinetics of CO2RR and HER on the tin electrode was investigated. Potentiodynamic polarization studies were conducted in CO2 saturated and N2 saturated 0.1 M KHCO3 solutions, in the potential range from -0.014 V to -1.59 V vs. RHE. A faradaic efficiency of 93.95 % towards formate production was obtained at -1.09 V vs. RHE. The mass transfer effects and bicarbonate dissociation equilibrium were used to estimate the concentration of the reactants at the electrode surface. Density functional theory calculations indicate that -OCHO intermediate species is thermodynamically favoured, and a four -step reaction with two intermediates is proposed. The proposed mechanism captures the major features of the polarization data. CO2 reduction occurs via two intermediate species, viz. adsorbed H and OCHO species, while HER occurs via Volmer-Heyrovsky steps. The model predicts that in N2 saturated solutions, the fractional surface coverage of adsorbed H reaches a maximum of -0.48 at a potential of -0.82 V vs. RHE while in CO2 saturated solutions, the corresponding value is -0.29. In addition, the maximum fractional surface coverage of adsorbed OCHO is predicted to be -0.12 in CO2 saturated solutions.