Insight into Improving Energy Efficiency of Electrochemical CO2 Reduction to Formate in Divided H-type Cell

The conversion of carbon dioxide (CO2) into formate by the intermittent generation of renewable energy is of great interest for many applications. Most studies have so far focused on how to improve the faradaic efficiency but ignored the importance of energy efficiency. In this study, the electrochemical reduction of CO2 (ERC) was carried out in a divided H-type electrolyzer. The improvement of energy efficiency was studied in the context of three aspects: (1) contrasting the cathodic catalysts, i.e., traditional Sn foil with highly active nano-Bi (produced in situ by reduction from nano-Bi2O3), (2) comparing anodic catalysts, i.e., traditional Pt plate and the as-prepared Ir/C, and (3) contrasting anodic electrolytes, i.e., aqueous solutions of KHCO3 and KOH. The energy efficiency of the ERC to formate can reach 38.2% with the r-Bi2O3 cathodic catalyst, and Ir/C anodic catalyst were used in 1.0 M KOH anolyte, at a low cell potential (2.06 V). This work concluded that KOH anolyte is more suitable for the ERC to formate. Furthermore, the method of calculating the energy efficiency for the ERC to formate was also systematically reviewed.