Reduction behaviors of tin oxides and oxyhydroxides during electrochemical reduction of carbon dioxide in an aqueous solution under neutral conditions†

It is well known that tin oxides and oxyhydroxides show high selectivity for the electrochemical CO₂ reduction reaction (CO₂RR) to form HCOOH in aqueous solutions. Tin oxides and oxyhydroxides are reduced to form metallic Sn during the CO₂RR, and the formed interface between the oxide and metallic Sn plays important roles in the CO₂RR. In this study, reduction behaviors of tin oxides and oxyhydroxide during the CO₂RR were investigated. SnO, SnO₂ and tin oxyhydroxide containing both amorphous and crystalline phases were formed using solvothermal, sol–gel and precipitation methods, respectively. Reduction current densities of SnO₂ and the oxyhydroxide for the CO₂RR and hydrogen evolution reaction at −0.8 V vs. RHE were higher than that of SnO, and the faradaic efficiency of the oxyhydroxides for formation of HCOOH and CO was >90%. Based on high-resolution TEM observation and EDS mappings, it was revealed that metallic Sn nanoparticles with a ∼40 nm diameter were formed from SnO₂ and tin oxyhydroxides during the CO₂RR via a dissolution and reductive deposition process. Aggregates of SnO₂ and the oxyhydroxide were dissolved in a neutral electrolyte solution during the CO₂RR, and subsequently, metallic Sn nanoparticles with highly effective surface areas were formed on carbon electrodes via reductive deposition from dissolved Sn cations, leading to a higher reduction current. The thickness of native oxide layers formed on the surface of the metallic Sn particles in air after the CO₂RR from the oxyhydroxide was greater than those of SnO and SnO₂. Therefore, it is speculated that metallic surfaces of the former ones were more easily formed at the interface between SnO_x and metallic Sn than those of the latter ones, leading to high selectivity for the CO₂RR.