Abstract: A redox flow battery (“CRB”) performs as an energy storage system and has a negative electrode that directly utilizes CO2 in the battery charge step as an active species instead of metals. The CRB also has a positive electrode utilizing a metallic or non-metallic redox species, and a cation exchange membrane in between the negative and positive electrodes. The negative electrode comprises a porous base layer, a porous intermediate layer containing a metal oxide and a bi-functional catalyst layer for electrochemical reduction of CO2 or carbonate to formate and for formate oxidation to either carbonate or CO2. The bi-functional catalyst can be a PdSn based catalyst, such as PdSn, PdSnIn, and PdSnPb. The metal oxide in the intermediate layer acts as a catalyst support and can be a non-Platinum group metal (PGM) oxide, such as LaCoO3 or LaNiO3.