No abstract

Deep eutectic electrolytes have recently been considered as alternatives to classical room-temperature ionic liquids. This work is an initial study of the zinc deposition process from a basic choline chloride/ethylene glycol deep eutectic solvent containing ZnCl2 at 30°C . The system was examined by cyclic voltammetry at static and rotating glassy carbon disk electrodes and by potential step techniques. There was little deposition initially on sweeping or stepping the potential to −0.5 to −0.8 V vs Zn/Zn(II), but more rapid deposition was observed when the potential was subsequently raised to −0.4 to −0.2 V. The role of choline chloride was also studied by comparing with a choline-free electrolyte, which exhibited a more conventional voltammetric response. The formation of a dissolved, intermediate species Z on the cathodic sweep was proposed to account for the observed deposition behavior in the deep eutectic. Furthermore, an observation of the electrodeposition behavior with the addition of sodium ethoxide supports the suggestion that Z is a complex of Zn2+ and deprotonated components of the solvent.

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.

The interface of a 1 : 2 molar choline chloride/ethylene glycol deep eutectic solvent with a glassy carbon electrode has been investigated by polarization modulation reflection-absorption spectroscopy (PM-IRRAS). Temporal spectral changes at open circuit potential show the experiments to be surface sensitive and indicate slow adsorption of electrolyte molecules on the electrode surface. In situ spectroelectrochemical PM-IRRAS measurements reveal characteristic potential-dependent changes of band intensities and wavenumber-shifts in the surface spectra. The potential dependent spectral changes are discussed in terms of adsorption, reduction, desorption and reorientation of choline cations at the interface. Analogies are drawn to the ionic layer structure proposed for the architecture of electrode/ionic liquid interfaces. The results show that in situ PM-IRRAS is generally applicable to glassy carbon electrodes and to electrode interfaces with deep eutectic solvents.