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.