Understanding the mass-transfer of Br species in an aqueous and quaternary ammonium polybromide biphasic system via particle-impact electrochemical analysis

Abstract

In this article, we present our studies on the mass-transfer of Br species existing in an acidic aqueous solution containing quaternary ammonium bromide (QBr) during electro-oxidation of Br-. We found that the mass-transfer of quaternary ammonium polybromide (QBr(2n)(+1)) droplets toward a Pt ultra-microelectrode (UME) are mainly governed by diffusion, although their migration is not negligible. We further revealed that the migration of Br- in a QBr(2n)(+1) droplet significantly affects the current associated with the electro-oxidation of Br- during the collision of the droplet on a Pt UME. Finally, we observed current spikes in which the current decayed and reached a certain steady state with a non-zero value. These were interpreted based on Br--transfer at the interface between water and a QBr(2n)(+1) droplet (water vertical bar QBr(2n+1)) using finite element analysis. The presented electrochemical analyses would be potentially important to understand the operating mechanism of redox flow batteries (RFBs) using Br-/Br-2 as a half redox reaction to inhibit the self-discharge that occurs during the charging process.