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

Energy storage systems (ESSs) using a Br – /Br 2 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 2 n +1 ) and solid (TBr 3 ) polybromides during the ESS charging process. The electrochemical formation of liquid QBr 2 n +1 and its electrochemical properties have been examined. The detailed mechanisms of ionic solid TBr 3 formation, however, have not yet been explored. In this article, we analyzed the ionic liquid-to-solid phase transition of TBr 3 particles using a time transient electrochemical method. We suggest the formation of ionic solid TBr 3 particles via hydrated TBr 3 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 TBr 3 particles is strongly dependent on the chemical structure of T + and the concentration of TBr in an aqueous solution.