Cycle behaviour of hydrogen bromine redox flow battery cells with bromine complexing agents

Bromine complexing agents (BCA) are used to improve the safety of aqueous bromine electrolytes versus bromine outgassing in bromine electrolytes. In this work, cycling performance of hydrogen-bromine redox flow battery cells with 1-ethylpyridin-1-ium bromide ([C2Py]Br) as BCA in a bromine electrolyte with a theoretical capacity of 179.6 A h L−1 is investigated for the first time. The BCA leads to increased ohmic overvoltages. One cause of the ohmic drop can be attributed to [C2Py]+ cation interaction with the perfluorosulfonic acid (PFSA) membrane, which results in a drop of its conductivity. The BCA also interacts with bromine in the cell, by forming a non-aqueous fused salt second phase which exhibits a ten times lower conductivity compared to the aqueous electrolyte. A steep rise in cell voltage at the beginning of the charge curve followed by a regeneration of the cell voltage is attributed to this effect. Electrolyte crossover leads to an accumulation of [C2Py]+ in the electrolyte solution and intensifies both adverse processes. Under this condition only 30% of the theoretical electrolyte capacity of 179.6 A h L−1 is available under long term cycle conditions. However, electrolyte capacity is high enough to compete with other flow battery technologies. [Display omitted] •[C2Py]Br as bromine complexing agent in safe posolytes for H2/Br2-RFB.•Two-phase electrolyte: bromine-poor aqueous phase and bromine-rich fused salt.•Bromine half cell performance limitation due to formation of [C2Py]-fused salt.•Membrane performance limitation due to interaction with [C2Py]+ cations.•Br2 crossover intensifies both effects due to release of BCA from fused salt.