The Br–/Br2 redox couple in aqueous solution has been often employed for redox flow batteries along with N-methyl-N-ethyl pyrrolidinium bromide (MEPBr) as a bromine-complexing agent, which forms insoluble organic droplets of MEPBr3 complexes during electro-oxidation of Br–. We, for the first time, report the electrochemistry of Br– electro-oxidation in electrochemically generated single droplets of MEPBr3 using the current transient method on an ultramicroelectrode (UME). Current spikes were observed in the chronoamperogram of the aqueous solutions containing more than 32 mM of MEPBr, and they correspond to electro-oxidation of Br– in MEPBr3. The voltammetric behavior of Br– electro-oxidation in single droplets of MEPBr3 was similar to that in the aqueous phase. The maximum concentration of Br– in the MEPBr3 droplets was estimated to be ∼7.5 M by fitting the observed current transient curves to the simulation using a bulk electrolysis model. Our study reveals that MEPBr3 also plays a vital role as an electrochemical reaction medium for Br– electro-oxidation in the Br–/Br2 redox system.

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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.

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The faradaic reaction at the insulator is counterintuitive. For this reason, electroorganic reactions at the dielectric layer have been scarcely investigated despite their interesting aspects and opportunities. In particular, the cathodic reaction at a silicon oxide surface under a negative potential bias remains unexplored. In this study, we utilize defective 200-nm-thick n+-Si/SiO2 as a dielectric electrode for electrolysis in an H-type divided cell to demonstrate the cathodic electroorganic reaction of anthracene and its derivatives. Intriguingly, the oxidized products are generated at the cathode. The experiments under various conditions provide consistent evidence supporting that the electrochemically generated hydrogen species, supposedly the hydrogen atom, is responsible for this phenomenon. The electrogenerated hydrogen species at the dielectric layer suggests a synthetic strategy for organic molecules.

As a novel approach to the in situ real-time investigation of an ITO electrode during the wet etching process, step-excitation Fourier-transform electrochemical impedance spectroscopy (FT-EIS) was implemented. The equivalent circuit parameters (e.g., R ct, C dl) continuously obtained by the FT-EIS measurements during the entire etching process showed an electrode activation at the initial period as well as the completion of etching. The FT-EIS results were further validated by cyclic voltammograms and impedance measurements of partially etched ITO films using ferri- and ferrocyanide solution in combination with FESEM imaging, EDS, XRD analyses, and COMSOL simulation. We also demonstrated that this technique can be further utilized to obtain intact interdigitated array (IDA) electrodes in a reproducible manner, which is generally considered to be quite tricky due to delicacy of the pattern. Given that the FT-EIS allows for instantaneous snapshots of the electrode at every moment, this work may hold promise for in situ real-time examination of structural, electrokinetic, or mass transfer-related information on electrochemical systems undergoing constantly changing, transient processes including etching, which would be impossible with conventional electroanalytical techniques.

Ultrafast transport of Br2 in a polybromide redox-active ionic liquid allows electron transfer-limited voltammograms of Br2 reduction. The reorganization energy at the inner-Helmholtz plane can be determined based on the Marcus–Hush–Chidsey model.

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