Improvement in the Performance of an Fe/FeII Electrode in an All-Iron Redox Flow Battery by the addition of ZnII ions

Redox flow batteries are the most promising large-scale energy storage technologies for solving intermittency issues of renewable energy sources such as wind, solar, etc. They have favorable features over other battery technologies like high energy efficiency, intrinsic safety, independent scaling, and a long lifetime. Among various RFBs, all-Iron redox flow batteries are an attractive choice because iron is the second most abundant metal in earth's crust, is cheap and ecofriendly. However, low charging efficiency, parasitic hydrogen evolution reaction (HER) at the negative Fe/Fe-II electrode, self-discharge by electrolyte cross-over, and poor cycle-life (due to ferric hydroxide precipitation) are the major technical challenges to be overcome for the successful commercialization of all-iron redox flow batteries. Herein, we report an all-iron redox flow battery containing Fe/Fe-II and Fe-III/Fe-II redox couples separated by a self-made anion exchange membrane. We also examined the impact of adding Zn-II ions on the electrochemical performance of the Fe/Fe-II redox couple. The coulombic efficiency, voltage efficiency and energy efficiency of the cell with 0.03 m ZnCl2 was found to be greater (90 %, 70.96 % and 63.86 %) than those of the cell without ZnCl2 (80 %, 62.06 % and 49.64 %). The results reveal that the addition of small amounts of Zn-II ions to the Fe/Fe-II electrode suppresses the hydrogen evolution reaction and increase the cell performance.