Equilibrium Properties of a Bromine-Bromide Electrolyte for Flow Batteries

The bromine-bromide redox couple is a promising solution for aqueous cathodes of several flow batteries. In this work, we present an experimental characterization of the system Br2-HBr-H2O at chemical equilibrium, featuring isothermal measurements of Open Circuit Voltage (OCV), density and conductivity in solutions with concentrations up to 2 M Br2 and 4 M HBr. An equilibrium model considering polybromides formation that fits experimental data with an accuracy of 1.8 mV is presented. Information on activity coefficients and equilibrium constants of polybromides can be extrapolated from the model. Furthermore, alternative approaches are considered to effectively fit the measured voltage using algebraic equations, that provide a convenient tool for flow battery modeling at system-level.