Optimization of an Aqueous Zn-Mn Redox Flow Battery System

Zinc-based negative electrodes are chosen for their cost-effectiveness, low equilibrium potential, and high electrochemical reversibility, making them widely utilized in redox flow batteries. To achieve a high cell voltage, zinc electrodes are usually paired with positive half-cells featuring high equilibrium potentials, such as Zn-air and Zn-Br systems [1]. The work investigates the feasibility of an alkaline redox flow battery with high energy density and operating voltage involving Zn/ Zn(OH)₄ 2- and MnO₄⁻/MnO₄²⁻ redox couples. The study focuses on the less-explored permanganate half-cell, which offers a high equilibrium potential (+0.55 V vs SHE) and excellent solubility in alkaline conditions (~3.6 M). These properties make it a promising candidate for positive electrolytes with high energy density [2]. In the study, the electrolytes of the two cells were optimized separately to increase conductivity and avoid side reactions. Cyclic voltammetry experiments at different scan rates confirm the excellent reversibility of electrochemical reactions in both the zinc and permanganate-based electrolytes, characterized by high faradic efficiency. Subsequently, a prototype flow battery is assembled and subjected to galvanostatic charge/discharge tests to assess stability and performance. The cell is evaluated at varying catholyte concentrations and different current densities to determine rate performance. The galvanostatic tests demonstrate remarkable cycling stability and notable cell potential compared to other aqueous systems. The battery exhibits high energetic efficiency (EE > 75%) and Coulombic efficiency (CE) and voltage efficiency (VE) both around 85%. Capacity tests at full charge reveal stable electrochemical reactions throughout the charging and discharging processes. Additionally, ex-situ UV-vis spectroscopy is employed to investigate the mechanism of the manganate/permanganate redox reaction and evaluate the permeability of permanganate ions through membranes during cycling. Overall, the study provides significant insights into the development of high-voltage aqueous batteries. The alkaline Zn-MnO₄⁻ flow battery described in this work emerges as a promising candidate for energy storage applications, offering enhanced stability, high energy efficiency, and energy density. References: [1] J. Ren, Z. Wang, B. Liu, al -, J.D. Milshtein, J.L. Barton, T.J. Carney, G. Wang, H. Zou, X. Zhu, M. Ding, C. Jia, Recent progress in zinc-based redox flow batteries: a review, J Phys D Appl Phys 55 (2021) 163001. https://doi.org/10.1088/1361-6463/AC4182. [2] A.N. Colli, P. Peljo, H.H. Girault, High energy density MnO 4 - /MnO 4 2− redox couple for alkaline redox flow batteries, Chemical Communications 52 (2016) 14039–14042. https://doi.org/10.1039/C6CC08070G.