Effect of Br2 Complexation on a Hydrogen-Bromine Flow Battery Performance

Abstract

Hydrogen-bromine flow batteries offer a viable solution for storing energy from the electrical grid or directly from renewable energy sources such as wind and solar due to its high roundtrip efficiency and relatively low cost. During charge, a hydrogen-bromine flow battery stores energy in the form of hydrogen (H2) and bromine (Br2). During discharge, hydrobromic acid (HBr) is produced. One of the major issues with the hydrogen-bromine flow battery system is the difficulty of safely storing bromine. Bromine has a high vapor pressure and is both toxic and very corrosive. Past research has shown that complexing Br2 with organic molecules is an effective way to reduce the amount of free bromine in the solution, thus reducing its vapor pressure and corrosivity. For example, Eustace demonstrated how quarternary ammonium salts could be complexed with bromine in zinc-bromine flow batteries.2 Kosek et al previously studied the complexation of bromine using polyethylene glycol (PEG).3 Kosek et al performed kinetic studies of PEG-1000 in concentration ranges of 1-5M and temperatures of 24-43oC, observing the highest dissociation rates occurred at 3M HBr and room temperature.3 Cho et al demonstrated high performance of a hydrogen-bromine flow battery using high surface area porous carbon electrodes for the bromine reaction.1 Other research conducted by Kosek et al and Masud et al demonstrates high performance and good stability (compared to Pt) when using non-Platinum catalysts at the hydrogen electrode such as rhodium sulfide or ruthenium/iridium at the hydrogen electrode.3,4 In this study, we will use different types of complexing agents at various concentrations to determine the most suitable conditions for reducing the concentration of free bromine while also maintaining high bromine electroactivity. Electrochemical measurement techniques using a rotating disc electrode will be used to acquire the kinetic reaction parameters of interest, including the exchange current density and charge transfer coefficients. The effect of using complexing agents in the HBr/Br2 electrolyte will then be tested in an actual fuel cell to obtain polarization curves. This information is expected to determine the viability of using a complexing agent in an actual hydrogen-bromine fuel cell system in order to reduce the concentration of free Br2 while also enabling high performance.