Electrochemical Performance of Three Novel Bromine-Sequestering Agents for Zinc/Bromine Flow Battery Electrolytes

Three different bromine-sequestering agents (BSAs) were proposed and assessed for their fitness-for-utilization in zinc–bromine redox flow batteries (Zn/Br RFBs). These BSAs included the bromide salts of the following cations: 1-ethyl-1-methyl-imidazolium ([C₂MIm]⁺), 1-ethyl-1-hydroxymethyl-imidazolium ([C₂OHMIm]⁺) and 1-ethyl-hydroxypiperidinium ([C₂OHPy]⁺). Half-cell cyclic voltammetry and a.c. impedance spectroscopy as well as full-cell cyclic charge–discharge tests were performed to determine the influence of these agents on both on zinc- and bromine-side electrochemical performance. All three BSAs were found to increase peak currents in both zinc- and bromine-side half-cells. [C₂MIm]⁺ and [C₂OHPy]⁺ performed best for bromine- and zinc-sides, respectively, while [C₂OHMIm]⁺ performed overall poorly compared to the other BSAs. Impedance spectra indicate bromine-side performance is strongly linked to Warburg diffusion limitations. On the other hand, voltammetry results indicate zinc-side half-cell performance improves with decreasing nucleation overpotentials during zinc electrodeposition. [C₂MIm]⁺ gave the highest voltaic and coulombic efficiencies during full-cell cycling, while [C₂OHMIm]⁺ again showed overall relatively poor performance. Scanning electron micrographs indicate a strong relationship between chemical structure of the BSA employed and the crystallinity of zinc electrodeposits. Both [C₂MIm]⁺ and [C₂OHMIm]⁺ were found to improve the quality of zinc plating, with the later producing nanocrystalline needle-like deposits. These findings warrant further investigation of BSAs made up of weakly-coordinating cations carrying diffuse positive charges. While these compounds have been proposed for application in Zn/Br systems, they are also potentially applicable to other types of RFBs which employ the Br₂/Br^– redox couple and require the use of electrolytes containing BSAs.