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

Flowless Zn–Br2 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–Br2 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 O2 evolution contributes to mitigation of inhomogeneous plating and vertical growth of Zn metal at the Zn anode. Consequently, a flowless Zn–Br2 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.