Promotional effect of surface hydroxyls on electrochemical reduction of CO2 over SnOx/Sn electrode

Abstract

Tin oxide (SnOx) formation on tin-based electrode surfaces during CO2 electrochemical reduction can have a significant impact on the activity and selectivity of the reaction. In the present study, density functional theory (DFT) calculations have been performed to understand the role of SnOx in CO2 reduction using a SnO monolayer on the Sn(112) surface as a model for SnOx. Water molecules have been treated explicitly and considered actively participating in the reaction. The results showed that H2O dissociates on the perfect SnO monolayer into two hydroxyl groups symmetrically on the surface. CO2 energetically prefers to react with the hydroxyl, forming a bicarbonate (HCO3(t)∗) intermediate, which can then be reduced to either formate (HCOO∗) by hydrogenating the carbon atom or carboxyl (COOH∗) by protonating the oxygen atom. Both steps involve a simultaneous CO bond breaking. Further reduction of HCOO∗ species leads to the formation of formic acid in the acidic solution at pH<4, while the COOH∗ will decompose to CO and H2O via protonation. Whereas the oxygen vacancy (VO) in the oxide monolayer maybe formed by the reduction, it can be recovered by H2O dissociation, resulting in two embedded hydroxyl groups. The results show that the hydroxylated surface with two symmetric hydroxyls is energetically more favorable for CO2 reduction than the hydroxylated VO surface with two embedded hydroxyls. The reduction potential for the former has a limiting-potential of −0.20V (RHE), lower than that for the latter (−0.74V (RHE)). Compared to the pure Sn electrode, the formation of SnOx monolayer on the electrode under the operating conditions promotes CO2 reduction more effectively by forming surface hydroxyls, thereby providing a new channel via COOH∗ to the CO formation, although formic acid is still the major reduction product.