Abstract
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 Br 2 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. • 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.
Highlights
An economically viable redox flow battery (RFB) system is characterised by low cost of energy storage, which can be achieved by increasing the battery energy density [1,2]
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 Open-circuit potential (OCP)
The goal of this paper is to (1) provide a framework for scrutinising thermodynamic phenomena occurring in real flow batteries employing the bromine electrode, especially the hydrogen–bromine flow battery (HBFB), (2) explain by means of a mathematical model the dramatic impact of these phenomena on the cell open-circuit voltage, OCV, (3) provide a tool for predicting bromine RFB electrode half-cell potential as a function of broad range of electrolyte compositions and temperatures (4) discuss the relevance of cell equilibrium thermodynamics as well as thermodynamics-related polarisation effects such as concentration polarisation for the practical operation of bromine-based (HBFB, V–Br, Zn–Br) and other halogen-based RFBs
Summary
An economically viable redox flow battery (RFB) system is characterised by low cost of energy storage, which can be achieved by increasing the battery energy density (e.g. augmenting electrolyte concentrations) [1,2]. An attempt to quantify the thermodynamic phenomena occurring in highly concentrated bromine half cell electrolytes (posolytes in HBFB) is made in order to steer clear of using complex empirical formulae, formal potentials or fitting, and to increase versatility and predictivity of the HBFB or other bromine-based cell models. The goal of this paper is to (1) provide a framework for scrutinising thermodynamic phenomena occurring in real flow batteries employing the bromine electrode, especially the HBFB, (2) explain by means of a mathematical model the dramatic impact of these phenomena on the cell open-circuit voltage, OCV, (the versatile model was validated in a real HBFB pilot-scale system) (3) provide a tool for predicting bromine RFB electrode half-cell potential as a function of broad range of electrolyte compositions and temperatures (4) discuss the relevance of cell equilibrium thermodynamics as well as thermodynamics-related polarisation effects such as concentration polarisation for the practical operation of bromine-based (HBFB, V–Br, Zn–Br) and other halogen-based RFBs
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