Utilization of CO2 for production of various chemicals and fuels is one of the most intense areas of research covering a wide range of thermo-, bio-, and electro-catalytic variants. Additionally, a more recent development is the utilization of CO2 for energy generation and storage in carbonnegative electrochemical systems. Two classes of batteries have been proposed: metal (e.g., Li, Na, Zn) / CO2 [1] and non-metal CO2 redox flow batteries [2], respectively. Among the first category, Li / CO2 has the highest theoretical specific energy (1876 Wh kg−1 ) and nominal discharge voltage (~2.6 V). However, the discharge capacities are rather low at present, approximately 2 mA h cm−2 [3]. A non-rechargeable Zn / CO2 battery producing CH4 was operated at a maximum power density of only 1 mW cm−2 [4]. Much higher power densities, up to 100 mW cm−2 , could be obtained with the non-metal CO2/formate − Br2/Br− rechargeable (redox) flow battery. A crucial component of the latter technology is the effectiveness of the bifunctional (or bidirectional) electrocatalyst on the negative electrode for catalyzing the CO2 reduction to formate during charge, and formate oxidation to carbonate during discharge [5]. A comparative analysis of the various CO2 based battery technologies is provided. The negative and positive electrode properties and different electrolyte options are discussed in the context of prospects for scale-up and commercialization.
