Highly tunable syngas production by electrocatalytic reduction of CO2 using Ag/TiO2 catalysts

The direct conversion of CO2 to syngas with controllable H2/CO ratio has been investigated using electrochemical reduction of CO2 in aqueous media. Despite considerable progress on electrocatalytic syngas production, it remains challenging to generate a stable H2/CO ratio over a wide range of applied potentials. In this study, we investigated Ag/TiO2 catalysts, by which we achieved a stable H2/CO ratio with high Faradaic efficiency (93–100%) and partial current densities (~164 mA·cm−2) for syngas production in a flow cell. The H2/CO ratio was controlled by changing the catalyst properties resulting from oxygen vacancies and phase difference of TiO2. The H2/CO ratio of Ag/TiO2 catalysts was increased by introducing oxygen vacancy defects in the bulk and on the surface of TiO2 anatase. Furthermore, the H2/CO ratio was also increased by changing the TiO2 phases from anatase to rutile, even if the rutile phase possessed fewer oxygen vacancies. The 40 wt% Ag/TiO2 catalysts calcined in different gases (Ag/TiO2 anatase-air, Ag/TiO2 anatase-H2/Ar, and Ag/TiO2 rutile-air) exhibited H2/CO ratios of 0.1–0.3, 0.5–1.1, and 0.5–1.5, respectively, within the range of potential from −0.35 to −0.65 V (vs. RHE).