Exploring the thermodynamics of the bromine electrode in concentrated solutions for improved parametrisation of hydrogen–bromine flow battery models

• Published in: Journal of power sources 2021-10, Vol.508, p.230202, Article 230202
• Main Authors: Wlodarczyk, Jakub K., Küttinger, Michael, Friedrich, Andreas K., Schumacher, Jürgen O.
• Format: Article
• Language: English
• Subjects: Activity coefficient; Bromine electrode; hydrogen–bromine redox flow battery; Modelling and simulation; Open circuit potential; Thermodynamics
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2021.230202

Thermodynamic properties of the bromine electrode in an exemplary hydrogen–bromine flow battery (HBFB) are investigated in detail. Open-circuit potential (OCP) measurements of HBRB electrolytes in a liquid junction-free setup and electrolyte Raman spectra are employed to estimate polybromides speciation. An improved mathematical description of the bromine electrode OCP versus state of charge is provided. This paper addresses the phenomenon of polybromides formation at concentrations up to 7.7 mol L-1 HBr and 3.85 mol L-1 Br2 and their significant impact on the OCP. The model takes into account tri-, penta- and heptabromides formation, precisely modelled electrolyte activity coefficients (up to 11-molal HBr), electrolyte density, and temperature. It is elucidated that the polybromide formation constants found in literature treating dilute electrolytes are substantially too low. Newly determined equilibrium constants, applicable over a wider concentration range are provided for 25 and 43 °C together with their standard enthalpy changes. The model is successfully validated in an independent experiment using a real, pilot-scale HBFB. It is concluded that the usage of a simple Nernst-like equation to calculate the OCP of flow battery electrodes containing concentrated electrolytes leads to erroneous results. [Display omitted] •A novel thermodynamic bromine electrode model is described and validated.•It is applied to describe the open-circuit voltage of a pilot-scale flow battery.•Polybromide formation constants are re-evaluated at high electrolyte concentrations.•Mean activity coefficient of aqueous HBr up to 11 m between 0 and 70 °C is modelled.