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For the application of CO 2 as an energy storage material, a H 2 storage system has been proposed based on the interconversion of CO 2 and formic acid (or formate). However, energy losses are inevitable in the conversion of electrical energy to H 2 as chemical energy (≈70 % electrical efficiency) and H 2 to electrical energy (≈40 % electrical efficiency). To overcome these significant energy losses, we developed a system based on the interconversion of CO 2 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 CO 2 -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 Ir IV ., (© 2023 Wiley-VCH GmbH.)