Half-Cell Electrochemical Performance of Hybridized Ionic Liquid Additives for Zinc/Bromine Flow Battery Applications

Abstract

The effects of employing hybrid ionic liquid (IL) mixtures as bromine sequestration agents in zinc/bromine (Zn/Br) flow batteries are investigated. The ILs 1-ethyl-3-methylimidazolium bromide ([C2MIm]Br) and 1-ethyl-1-methylpyrrolidinium bromide ([C2MPyrr]Br) are studied within Zn/Br electrolytes in different mixing proportions at glassy carbon electrodes. Cyclic voltammetry showed beneficial performance in the Zn and Br half-cells when utilizing pure [C2MIm]Br and [C2MPyrr]Br, respectively. Consequently, it is recommended that electrolytes containing single IL types are better suited for Zn/Br systems compared to those utilizing hybrid mixtures. Electrochemical impedance spectroscopy showed that low diffusion limitations and charge-transfer resistance in both half-cells, as well as low Zn-side polarization resistance, contribute towards beneficial voltammetry performance. Scanning electron microscopy, X-ray diffraction and Williamson–Hall analysis of zinc electrodeposited during battery charging showed that the type and proportion of IL present influences crystal morphology and size. These findings warrant further investigation into full-cell testing, as well fundamental studies on the behavior and influence of other hybrid IL formulations for Zn/Br system applications. The methods and findings presented in this work are also applicable to electrodeposition and redox flow batteries which utilize pure or hybrid ILs for bromine sequestration purposes.