Influence of strong bromine binding complexing agent in electrolytes on the performance of hydrogen/bromine redox flow batteries

1-n-Hexylpyridin-1-ium bromide [C6Py]Br is investigated in this work as bromine complexing agent (BCA) in aqueous bromine electrolytes on its influence on hydrogen bromine redox flow battery (H2/Br2-RFB) performance. [C6Py]+-cations bind bromine of aqueous polybromide solutions safely in an additional fused salt phase limiting the vapor pressure of Br2. Dissolved in aqueous electrolyte solutions, however, [BCA]+ cations drastically lower PFSA membranes' conductivity in the H2/Br2-RFB. In this work the combination of the very strong bromine-binding [C6Py]+cation and an excess of bromine in the electrolyte lead to an almost complete absorption of 99.6 mol% [C6Py]+ into the fused salt within the electrolyte's operation range. In comparison to similar application of short side chain 1-ethylpyridinium bromide, adverse effects are stronger compensated by use of [C6Py]Br. Increases in membrane resistance of the membrane in contact with [C6Py]+ cations are reversible in contrast for contact with [C2Py]+ cations. Electrolytes are cycled for at least 12 cycles and offer a stable useable capacity of 176.7 Ah L−1 and reach and stable discharge energy densities of 138.91 Wh L−1. Strong bromine binding properties between [C6Py]+ cations and polybromides are confirmed by DFT studies. In contrast to [C2Py]+ electrolytes, the long-chain [C6Py]+ cation in electrolytes has a higher cycling stability or a low decrease in capacity and energy density. [Display omitted] •Use of 1-n-hexylpyridinium bromide in electrolytes in H2/Br2-flow battery cycling•Durable performance of BCA-cation-free aqueous electrolytes in H2/Br2-flow battery•Enhanced PFSA membrane performance with [C6Py]+ vs. [C2Py]+ cations•DFT study: Improved ion pairing of polybromides and [C6Py]+ vs. [C2Py]+ cations•Cycle-stable energy density and capacity of the electrolyte in the H2/Br2-RFB