Effect of Surface Reforming via O₃ Treatment on the Electrochemical CO₂ Reduction Activity of a Ag Cathode

We investigated the effect of O3 oxidation treatment on the electrochemical CO2 reduction activity of a Ag cathode. The surface structure of the Ag electrode was altered using a AgO precursor, which allowed for effective and selective CO2 reduction with high current density. The total current density and faradaic efficiency to CO evolution of the Ag electrode treated using O3 (rO3_Ag electrode) were 4.0 mA cm–2 and 96%, respectively, at an applied voltage of −1.6 V vs Ag/AgCl, whereas those of an untreated Ag foil electrode were 1.3 mA cm–2 and 62%, respectively. Despite enlargement of the electrochemical surface area of the active Ag species by O3 treatment, the turnover frequency of CO evolution using the rO3_Ag electrode was twofold greater than that obtained using the untreated Ag foil electrode. Grazing-incidence X-ray diffraction revealed that the Ag particles grew anisotropically along the [111] direction on the electrode surface due to the reduction of AgO species, which were formed during O3 treatment; this result indicated that the obtained Ag surface was favorable for the electrochemical reduction of CO2 while suppressing H2 production. This simple surface reforming technique enhanced the activity for the electrochemical reduction of CO2 to CO and could be applied in the design of future electrodes for recycling CO2 into feedstocks.