An organic imidazolium derivative additive inducing fast and highly reversible redox reactions in zinc-bromine flow batteries

In zinc-bromine redox flow batteries (ZBBs), the weak molecular structure and stability of bromine-complexing agent (BCA) can sometime negatively affect battery's performance. To address this issue, this paper introduces a 1,2-dimethyl-3-ethylimidazolium bromide (DMEIm center dot Br, C7H13BrN2), comprising planar molecular structure with strong molecular-polarizability and low steric hindrance. The effectiveness of the DMEIm center dot Br is compared and verified with those of two popular BCAs through various electrochemical experiments including full-cell tests for 200 cycles. Experimental results show that the DMEIm center dot Br significantly contributes to apparently enhancing reaction kinetics and reversibility of Zn2+/Zn((s) )and Br-/Br-2 redox couples by inducing highly reversible zinc-plating/stripping (by strong electrostatic shielding effect) and bromine-capture/release (along with strong bromine-binding strength) in anolyte and catholyte solutions, respectively. The superior chemical and electro-chemical properties are clearly demonstrated by the fact that the DMEIm center dot Br-supported solution in ZBBs exhibits 5.53 (24.19) and 7.29 (16.99) % higher current and voltaic efficiencies than the pristine solution at the tem-perature of 25 (60) ?C, respectively. It also exhibits remarkably improved discharge-capacity retention of averagely 99.17% for 200 cycles along with a slight discharge-capacity loss of only 1.46% vs. 1st cycle at 200th cycle, even at the high temperature of 60 ?C.

Automated summary

In this paper, a new bromine-complexing agent (DMEIm center dot Br) is introduced and its effectiveness in improving the performance of zinc-bromine redox flow batteries is demonstrated. The DMEIm center dot Br enhances the reaction kinetics and reversibility of the Zn2+/Zn((s)) and Br-/Br-2 redox couples, leading to higher current and voltaic efficiencies as well as improved discharge-capacity retention.