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

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 %.