The crossover behavior of bromine species in the metal-free flow battery

Abstract: The Br₂/Br⁻ oxidation–reduction couple is employed as the positive electrolyte in some redox flow batteries, such as zinc–bromine, vanadium–bromine, and hydrogen–bromine batteries. However, the Br₂ and Br⁻ crossover behavior with the proton exchange membrane is detrimental in these flow batteries. To explore the mechanism of the Br₂ and Br⁻ crossover, a spectrophotometer and a special reagent were developed to detect the accumulative penetration of the Br₂ and Br⁻ in the negative electrolyte. In addition, a laboratory-designed quinone–bromine flow battery was developed to investigate the effect of Br⁻ and Br₂ crossover behavior on cell performance. What’s more, molecular dynamics simulations of Br⁻ and Br₂ passing through a membrane were performed to analyze Br⁻ and Br₂ crossover behavior. It can be concluded that Br₂ crossover was the key factor contributing to the current inefficiency. The coulombic efficiencies increased from 72.5 to 82.5 to 90% as the current density increased from 0.025 to 0.075 A cm⁻² which occurred due to Br species crossover. The cell performance factors are strongly related to the amount of Br₂ crossover; hence, these factors will vary as the bulk aqueous phase Br species concentration changes. Graphical Abstract: The Br₂ and Br⁻ crossover mechanism with the proton exchange membrane in a quinone-bromine flow battery was detected through a spectrophotometer with a special reagent and the molecular dynamics (MD) simulations, and the related cell performance was also investigated, which is beneficial to other redox flow batteries with the employment of Br₂/Br⁻ oxidation-reduction couple.[Figure not available: see fulltext.]