Current Amplification and Ultrafast Charge Transport in a Single Microdroplet of Bromide/Polybromide-Based Ionic Liquid

Bromide/polybromide-based ionic liquids have recently gained attention as energy storage devices because of their dual roles as a solvent and a redox pair. However, their redox reaction is accompanied by the generation of emulsion at the electrode surface, which makes the study of their electrochemical mechanism highly challenging. We investigated the current amplification of a single droplet of 1-ethyl-1-methylpyrrolidinium polybromide (MEPBr2n+1), which is a Br–/Br2n+1–-based ionic liquid. The heterogeneous electron transfer of Br–/Br3– at the electrode is very fast. However, it still limits the overall reaction of this electrochemical system. Assuming free diffusion, the calculated diffusion coefficient of Br–/Br3– in a MEPBr2n+1 droplet is surprising and is an order greater than that of proton conduction following Grotthuss-like hopping. In situ Raman spectroscopy confirmed that the polybromide composition in the droplet varies during electrolysis and correlated the swift charge propagation with the polybromide network in the MEPBr2n+1 phase.