Improvement of safe bromine electrolytes and their cell performance in H$_{2}$/Br$_{2}$ flow batteries caused by tuning the bromine complexation equilibrium

Hydrogen bromine redox flow batteries utilize bromine electrolytes in their positive half cell, offering capacities larger than 100 Ah L$^{-1}$. Addition of quaternary ammonium compounds, so-called bromine complexing agents (BCA), may increase safety as they reduce the vapour pressure of bromine in the posolyte. However, they have not been applied so far. They (a) interact with perfluorosulfonic acid membranes leading to significant reduction of membrane conductivity and (b) they form a low conductive ionic liquid with polybromides, leading to high overvoltage if the formation happens at the electrode. In this work a solution to this problem is proposed by an excess addition of Br$_{2}$ to these electrolytes. The excess bromine leads to a permanent bromine fused salt phase in the tank. Bromine formed in the cell stays in the aqueous phase and bromine transfer between the two phases happens in the tank. Transfer of Br2 without the transfer of [BCA]$^{+}$ cations exists between the phases, while [C2Py]$^{+}$ cations remain in the fused salt and do not influence cell performance. For the first time a posolyte capacity of 179.6 Ah L$^{-1}$ based on 7.7 M hydrobromic acid with BCA is achieved compared to previous investigations with e.g. ... mehr53.9 Ah L$^{-1}$.