A highly active electrolyte for high-capacity iron‑chromium flow batteries

Iron‑chromium flow battery (ICFB) is the one of the most promising flow batteries due to its low cost. However, the serious capacity loss of ICFBs limit its further development. Herein, we analyze the capacity loss mechanism of ICFBs. The capacity loss is due to inactive Cr(H 2 O) 6 3+ ions result in the mismatched content of active ions in catholyte and anolyte. Thus, a simple method is proposed to raise the content of active Cr 3+ species (Cr(H 2 O) 5 Cl 2+ and Cr(H 2 O) 4 Cl 2 + ), achieving the match of Fe 2+ /Fe 3+ and Cr 3+ /Cr 2+ redox couples and reducing the capacity loss of ICFBs. Meanwhile, the increasing content of active Cr 3+ ions was proved by the quantitative analysis of the UV–vis absorption spectra. As a result, the ICFB with the improved electrolyte of 1 M FeCl 2  + 1.3 M CrCl 3  + 3 M HCl (E-1.3Cr) exhibits an energy efficiency (EE) of 84.51%, which is much higher than the original one of 1 M FeCl 2  + 1 M CrCl 3  + 3 M HCl (E-1Cr) (82.32%) at a current density of 80 mA cm −2 . Most importantly, the ICFB with E-1.3Cr (7.44 mAh/cycle) shows the much slower capacity decay rate than that with E-1Cr (9.16 mAh/cycle). This work provides an effective way to accelerate engineering applications of ICFBs.