Half-Cell Electrochemical Performance and Cost-Benefit Analysis of Utilizing Hybrid Ionic Liquids in Zinc/Bromine Flow Batteries

Abstract

Ionic liquids (ILs) are typically employed as bromine sequestration agents (BSAs) in zinc/bromine (Zn/Br) flow batteries to facilitate charge/discharge operations. The present work builds upon previous investigations into alternative BSAs, and explores the hybrid utilization of 1-ethyl-3-methylimidazolium bromide ([C2MIm]Br) and the conventionally employed 1-ethyl-1-methylpyrrolidinium bromide ([C2MPyrr]Br) ILs in 5 different molar proportions (1:0, 3:1, 1:1, 1:3 and 0:1, respectively) within the Zn/Br electrolyte. The electrochemical performance and behavior of each solution in the zinc and bromine half-cells was studied via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Preliminary results from electrochemical investigations and a normalized cost-benefit analysis are presented, together with a discussion on the feasibility of using hybrid IL solutions in Zn/Br systems. On the basis of charge and current densities during forward and reverse CV scans (i.e. charging and discharging phases) for a system utilizing glassy carbon electrodes, pure [C2MPyrr]Br and [C2MIm]Br offer the best performance, including efficiencies, for Br and Zn half-cells, respectively. This trend is generally maintained during cost analysis for each half-cell. EIS showed that low charge-transfer resistance is a primary reason for good performance in both half-cells. Furthermore, lower double-layer capacitance and Warburg diffusion limitations are potentially linked to better performance in the Zn and Br half-cells, respectively. Hybrid IL mixtures generally produced higher impedances compared to solutions containing a single type of IL. Interestingly, a preliminary whole-system analysis suggests that pure [C2MPyrr]Br is most cost-effective during charging while pure [C2MIm]Br offers the best cost-benefit during discharge. The 1:1 mixture generally gave the worst electrochemical performance and lowest cost-benefit in both half-cells, suggesting that non-equimolar mixtures of these ILs might be better suited for Zn/Br systems. These findings warrant further investigation of the behavior and influence of hybrid IL formulations for BSA applications in the bulk electrolyte and at the electrode-electrolyte interface, as well as full-cell charge/discharge cycle testing. While this work focuses on Zn/Br batteries, the methods and findings presented are also potentially applicable to other types of RFBs which utilize pure or hybrid ILs for bromine sequestration.