Improved electro-kinetics of new electrolyte composition for realizing high-performance zinc-bromine redox flow battery

Abstract

Aqueous Redox Flow Batteries (ARFB) are the most prominent technology for large-scale energy storage applications. The energy density of the ARFBs is mainly determined by the electrolyte components, which highly influence the flow battery performance. In the present work, we acutely investigated the various electrolyte compositions and optimized the best electrolyte for realizing the high performance of Zn-Br₂ ARFBs. The electrode kinetics, such as rate constant, exchange current density, conductivity, and diffusion coefficient, were analyzed using electrochemical techniques, cyclic voltammetry, and electrochemical impedance analysis. It was observed that zinc bromide (ZnBr₂) + perchloric acid (HClO₄) + 1-Ethyl-1-methylmorpholinium bromide (MEM) + N-ethyl-N-methylpyrrolidinium bromide (MEP) and ZnBr₂ + zinc chloride (ZnCl₂) + MEM + MEP electrolytes showed improved performance, where the redox kinetics of 2Br^-/Br₂ redox couple is greatly enhanced. The presence of perchloric acid unlocks the capacity of full electro-oxidation of bromide (Br^-) to bromine (Br₂) as it involves 1e^- per Br^-, which would be highly beneficial to attain high energy density. Further, Zn-Br₂ RFB adopted with optimized electrolyte formulation ZnBr₂ + HClO₄ + MEM+ MEP shows a better round-trip efficiency and displays a stable long cycling performance over 200 cycles with an energy (EE) and coulombic efficiency (CE) of > 68% and > 92%, respectively.