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.

基于溴化物/多溴化物的离子液体（bromide/polybromide-based ionic liquids）近来作为储能器件受到广泛关注，因其可同时充当溶剂与氧化还原对（redox pair），兼具双重功能。然而该类离子液体的氧化还原反应会在电极表面生成乳液，使得其电化学机制的研究极具挑战性。我们针对1-乙基-1-甲基吡咯烷鎓多溴化物（1-ethyl-1-methylpyrrolidinium polybromide，缩写为MEPBr₂ₙ₊₁）单液滴的电流放大效应展开了研究，该体系属于Br⁻/Br₂ₙ₊₁⁻型离子液体。Br⁻/Br₃⁻在电极表面的非均相电子转移（heterogeneous electron transfer）速率极快，但仍是限制该电化学体系整体反应的关键瓶颈。假设电荷传输遵循自由扩散（free diffusion）模式，计算得到的Br⁻/Br₃⁻在MEPBr₂ₙ₊₁液滴中的扩散系数（diffusion coefficient）十分出乎意料，其数值较遵循格罗图斯（Grotthuss）类跳跃机制的质子传导还要高出一个数量级。原位拉曼光谱（in situ Raman spectroscopy）证实，电解过程中液滴内的多溴化物组成会发生动态变化，并将快速的电荷传播与MEPBr₂ₙ₊₁相中的多溴化物网络建立了关联。