The Influence of Inorganic Salt Additives in the Electrolyte on Iron–Chromium Flow Batteries at Room Temperature

The iron–chromium flow battery (ICFB) is one of the most promising candidates for energy storage, but the high temperature of 65 °C causes serious engineering problems for large-scale industrial applications. In this study, we explore the ICFBs' performance at room temperature and optimize the electrolyte by introducing suitable inorganic salt additives. The results show that the introduction of chlorine-containing inorganic salts (LiCl, NaCl, and KCl) increases the concentration of active Cr(H2O)4Cl2+ and Cr(H2O)5Cl2+ ions and improves the battery performance at room temperature. The content of active Cr(H2O)4Cl2+ and Cr(H2O)5Cl2+ ions is analyzed by ultraviolet–visible (UV–vis) absorption spectroscopy. And the bonding function between Cr3+ and Cl– is verified by Fourier transform Raman spectroscopy. Herein, LiCl is the most effective additive and an ICFB with 0.2 M LiCl shows the best battery performance at room temperature. The battery achieves an energy efficiency (EE) of 83.07% at a current density of 40 mA/cm2, which is 4.61% higher than that of the initial electrolyte (78.46%). The work would stimulate the study of ICFBs at room temperature and promote the commercialization of ICFBs.