A Highly Soluble Iron‐Based Posolyte Species with High Redox Potential for Aqueous Redox Flow Batteries

Abstract A novel iron‐based posolyte redox species are presented for an aqueous redox flow battery, (Tetrakis(2‐pyridylmethyl)ethylenediamine)iron(II) dichloride, which is obtained by a simple synthetic route, shows a high redox potential of 0.788 V versus SHE, and exhibits exceptional aqueous solubility of 1.46 M. Paired with bis(3‐trimethylammonio)propyl viologen tetrachloride at neutral pH, the battery demonstrates an open‐circuit voltage of 1.19 V and delivers good cycling performance, with a capacity fade rate of 0.28% per day and coulombic efficiency of 99.3%. Postmortem chemical and electrochemical analyses of the posolyte species suggest future routes for stabilization of the complex. Among all the iron complexes with a redox potential above 0.4 V versus SHE, this compound exhibits the highest solubility. These results offer valuable insights that can be applied to the development of future posolyte species for sustainable energy storage solutions.