New hybrid redox flow battery with high energy density using V–Mn/V–Mn multiple redox couples

Abstract

A new hybrid redox flow V–Mn/V–Mn battery is introduced for enhancing the energy density of a V/V system. The energy density of the V–Mn/V–Mn system is high because the system has multiple redox reactions involving both V and Mn ions, and the operating cell potential increases owing to the high standard potential of the Mn(II)/Mn(III) reaction. Consequently, the discharge energy density is increased by 66.8% compared with that of the V/V system. Additionally, because the V–Mn/V–Mn system contains the same components in the negative and positive electrolytes, a reduction in energy density due to ion crossover with cycling can be achieved through a rebalancing process. The V–Mn/V–Mn system is not simply a mixture of V and Mn electrolytes but has a unique advantage. The presence of V ions reduces the particle size of the MnO2 produced by the disproportionation reaction during the charging process. The small MnO2 particles can flow into the cell together with the electrolyte and can be reduced to Mn ions. As a result, the reversibility and durability of the system are greatly improved. Therefore, the V–Mn/V–Mn system can complement the low energy density of the V/V system.