A Neutral‐pH Aqueous Redox Flow Battery Based on Sustainable Organic Electrolytes

Abstract Aqueous organic redox flow batteries (AORFBs) have gained increasing attention for large‐scale storage due to the advantages of decoupled energy and power, safe and sustainable chemistry, and tunability of the redox‐active species. Here, we report the development of a neutral‐pH AORFB assembled with a highly water‐soluble ferrocene 1,1‐disulfonic disodium salt (DS−Fc) and two viologen derivatives, 1,1’‐bis(3‐sulfonatopropyl)‐viologen (BSP−Vi) and Bis(3‐trimethylammonium)propyl viologen tetrachloride (BTMAP−Vi). Synthesized electrolytes showed excellent redox potential, good diffusion coefficient, and a good transfer rate constant. In particular, BSP−Vi has a more negative redox potential (‐0.4 V) than BTMAP−Vi (−0.3 V) and faster kinetics; therefore, it was selected to be assembled in an AORFB as anolyte, coupled with DS−Fc as catholyte.The resulting AORFB based on BTMAP−Vi/DS−Fc and BSP−Vi/DS−Fc redox couple had a high cell voltage (1.2 V and 1.3 V, respectively) and theoretical energy density (13 WhL−1 and 14 WhL−1 respectively) and was able to sustain 70 charge‐discharge cycles with energy efficiency as high as 97 %.