The Effect of Electrolyte Composition on the Performance of a Single-Cell Iron–Chromium Flow Battery

Flow batteries are promising for large-scale energy storage in intermittent renewable energy technologies. While the iron–chromium redox flow battery (ICRFB) is a low-cost flow battery, it has a lower storage capacity and a higher capacity decay rate than the all-vanadium RFB. Herein, the effect of electrolyte composition (active species and supporting electrolyte concentrations), Fe/Cr molar ratio, and supporting electrolyte type (HCl and H₂SO₄) on the performance (current efficiency (CE), voltage efficiency (VE), energy efficiency, discharge capacity, and capacity decay) of an ICRFB is investigated. The storage capacity of the optimum electrolyte (1.3 m FeCl₂, 1.4 m CrCl₃, 5.0 mm Bi₂O₃ in 1.0 m HCl) is 40% higher (from 17.5 to 24.4 Ah L⁻¹), while the capacity decay rate is tenfold lower (from 3.0 to 0.3% h⁻¹) than the performance of the previously used 1.0 m FeCl₂, 1.0 m CrCl₃ in 3.0 m HCl. At the optimum Fe and Cr concentrations and ratio in 0.5 m HCl, a near constant CE (92.3%), VE (78.7%), and EE (72.6%) are obtained over 50 cycles. The significantly higher capacity decay when using 1.0 m H₂SO₄ (1.6% h⁻¹) compared to 1.0 m HCl (0.3% h⁻¹) confirms that HCl is the more suitable supporting electrolyte.