Bromine complexing agents (BCA) in aqueous electrolytes for hydrogen bromine flow batteries are used to reduce bromine‘s vapour pressure, while an insoluble and liquid fused salt is formed. The properties (concentrations, composition, conductivity and viscosity) of this fused salt are investigated in this study systematically ex situ by using 7 BCAs at different state of charge in HBr/Br2/H2O electrolytes with a theoretical capacity of 179.6 Ah L−1. Bromine is stored in the fused salt at concentrations up to 13.6 M, reaching theoretical volumetrical capacities up to 730 Ah L−1 in fused salts. The fused salt consists of a pure, bromine‐ and water‐free ionic liquid of organic [BCA]+ cations and polybromides, and its conductivity bases on a hopping mechanism among the polybromides. Alkyl side chain length of the BCAs and distribution of polybromides influence strongly the conductivity and viscosity of the fused salts. 1‐ethylpyridin‐1‐iumbromide results to be favoured BCA for application.

The reversible and fast redox kinetics of bromine/bromide makes it a desirable couple as a catholyte in redox‐flow batteries (RFBs). In principle, the highest possible energy density is obtained with hydrogen‐bromine RFBs. Bromine sequestration agents, also called bromine complexing agents (BCAs), bind bromine in a non‐miscible phase and can, therefore, reduce the vapor pressure of bromine, mitigate its crossover, and result in higher practical range of electrolyte concentration. Therefore, BCAs can enhance the battery's safety and competitivity by significantly decreasing the cost of components. To date, BCAs are commonly used in membrane‐free bromine systems, which cannot provide the high current density demonstrated in hydrogen‐bromine RFBs. Herein, the drastic limitations encountered are shown while operating a hydrogen‐bromine RFB with a standard perfluorinated sulfonic acid membrane due to the strong BCA–perfluorinated sulfonic acid interaction. On the other hand, the benefits of using a polyvinylidene difluoride‐silica (PVDF–SiO2) nanoporous separator are demonstrated, which does not interact with the BCA. In this approach, the hydrogen‐bromine RFB can sustain cycling, albeit at a more moderate current than a BCA‐free battery.