Time Transient Electrochemical Monitoring of Tetraalkylammonium Polybromide Solid Particle Formation: Observation of Ionic Liquid-to-Solid Transitions

Energy storage systems (ESSs) using a Br^–/Br₂ redox reaction such as a Zn/Br redox flow battery (RFB) or a redox-enhanced electrochemical capacitor (Redox-EC) suffer from self-discharge reactions resulting in significant Coulombic loss. To inhibit the self-discharge, quaternary ammonium (Q⁺) and tetraalkylammonium (T⁺) bromide are added to form ionic liquid (QBr_2n+1) and solid (TBr₃) polybromides during the ESS charging process. The electrochemical formation of liquid QBr_2n+1 and its electrochemical properties have been examined. The detailed mechanisms of ionic solid TBr₃ formation, however, have not yet been explored. In this article, we analyzed the ionic liquid-to-solid phase transition of TBr₃ particles using a time transient electrochemical method. We suggest the formation of ionic solid TBr₃ particles via hydrated TBr₃ droplets as an intermediate phase, which are generated by electro-oxidation of Br^– in an aqueous TBr solution. We found the phase transition time of TBr₃ particles is strongly dependent on the chemical structure of T⁺ and the concentration of TBr in an aqueous solution.