An Aqueous Redox Flow Battery Using CO₂ as an Active Material with a Homogeneous Ir Catalyst**

Abstract

For the application of CO2 as an energy storage material, a H2 storage system has been proposed based on the interconversion of CO2 and formic acid (or formate). However, energy losses are inevitable in the conversion of electrical energy to H2 as chemical energy (~70% electrical efficiency) and H2 to electrical energy (~40% electrical efficiency). To overcome these significant energy losses, we developed a system based on the interconversion of CO2 and formate for the direct storage and generation of electricity. In this paper, we report an aqueous redox flow battery system using homogeneous Ir catalysts with CO2–formate redox pair. The system exhibited a maximum discharge capacity of 10.5 mAh (1.5 Ah L‐1), capacity decay of 0.2% per cycle, and total turnover number of 2550 after 50 cycles. During charging–discharging, in situ fluorescence X‐ray absorption fine structure spectroscopy based on an online setup indicated that the active species was in a high valence state of Ir4+.