Modified viologen-assisted reversible bromine capture and release in flowless zinc-bromine batteries

Abstract

Flowless Zn-Br-2 batteries exhibit considerable potential for energy storage system applications, which require the principal features of high safety, low cost, and long-term cycle stability. However, central challenges such as uncontrolled bromine crossover to anodes and aqueous electrolyte decomposition producing gases lead to a low cycle performance of batteries. Herein, we demonstrate that the introduction of bis(2-trimethylammonio) propyl viologen tetrabromide (PV(Br)(4)) onto a graphite felt (GF) electrode (PV(Br)(4)/GF) improves the cycle stability of flowless Zn-Br-2 batteries comprising a 2.5 M aqueous ZnBr2 electrolyte as the Zn and Br sources. During charging, PV(Br)(4) entraps corrosive and volatile Br2 formed inside the GF electrode via favorable interactions with the four Br- anions of PV(Br)(4), while polybromide anions are produced via an electrochemical-chemical growth mechanism. Furthermore, the PV(Br)(4) on the GF electrode reversibly releases Br- into the electrolyte through the electrochemical reduction of entrapped polybromide anions during discharging. In addition, the spatially anchoring PV(Br)(4) on a GF electrode suppresses undesired oxidative decomposition of water by minimizing the physical contact with the electrode, thereby mitigating the depletion of the electrolyte during cycling. Suppression of O-2 evolution contributes to mitigation of inhomogeneous plating and vertical growth of Zn metal at the Zn anode. Consequently, a flowless Zn-Br-2 battery with a PV(Br)(4)/GF electrode exhibits a high Coulombic efficiency of 95.6 % over 400 cycles with a current density of 10 mA cm(-2) and high areal capacity of 24.3 mAh cm(- 2).