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.

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.

Graphene electrodes and deep eutectic solvents (DES) are two emerging materials systems that have individually shown highly promising properties in electrochemical applications. To date, however, it has not been tested whether the combination of graphene and DES can yield synergistic effects in electrochemistry. We therefore study the electrochemical behavior of a defined chemical vapor deposited (CVD) graphene monolayer of centimeter-scale, transferred onto insulating SiO2/Si supports, in the common DES choline chloride/ethylene glycol (12CE) under typical electrochemical conditions. We measure the graphene potential window in 12CE and estimate the apparent electron transfer kinetics of an outer-sphere redox couple. We further explore the applicability of the 12CE electrolyte to fabricate nanostructured metal (Zn) and metalloid (Ge) hybrids with graphene by electrodeposition. By comparing our graphene electrodes with common bulk glassy carbon electrodes, a key finding we make is that the two-dimensional nature of the graphene electrodes has a clear impact on DES-based electrochemistry. Thereby we provide a first framework towards rational optimization of graphene-DES systems for electrochemical applications.

The electrodeposition of tin, bismuth, and tin–bismuth alloys from Sn^II and Bi^III chlorometalate salts in the choline chloride/ethylene glycol (1:2 molar ratio) deep eutectic solvent was studied on glassy carbon and gold by cyclic voltammetry, rotating disc voltammetry, and chronoamperometry. The Sn^II‐containing electrolyte showed one voltammetric redox process corresponding to Sn^II/Sn⁰. The diffusion coefficient of [SnCl₃]⁻, detected as the dominating species by Raman spectroscopy, was determined from Levich and Cottrell analyses. The Bi^III‐containing electrolyte showed two voltammetric reduction processes, both attributed to Bi^III/Bi⁰. Dimensionless current/time transients revealed that the electrodeposition of both Sn and Bi on glassy carbon proceeded by 3D‐progressive nucleation at a low overpotential and changed to instantaneous at higher overpotentials. The nucleation rate of Bi on glassy carbon was considerably smaller than that of Sn. Elemental Sn and Bi were electrodeposited on Au‐coated glass slides from their respective salt solutions, as were Sn–Bi alloys from a 2:1 Sn^II/Bi^III solution. The biphasic Sn–Bi alloys changed from a Bi‐rich composition to a Sn‐rich composition by making the deposition potential more negative.

Understanding ion transport in porous carbon electrodes is crucial for enhancing the performance of electrochemical energy storage devices. However, for systems using carbon electrodes and water-in-salt electrolytes, this is not generally understood. Here, two salts with different ionic interaction in water, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or choline chloride (ChCl), were utilized at concentrations up to 20 mol∙kg^-1 to explore the ion transport behavior. We report a new method for calculating the ion diffusion coefficient in carbon pores, considering the diffusivity of the bulk electrolyte, as well as the tortuosity and porosity of the carbon electrode. Accuracy is validated by comparing data with bulk electrolyte diffusivity reported from PFG-NMR in the literature, which is further used together with porosity estimated with nitrogen gas adsorption, and tortuosity from electrochemical impedance spectroscopy. This technique effectively distinguishes between the tortuosity in micro- and mesopores by considering their volume and surface area. The different ion hydration patterns of ChCl and LiTFSI at concentrations above 10 mol∙kg^-1 influence the ion transport and the tortuosity to a different extent. This is confirmed by changes in hydrogen bonding observed in the Raman water bands. Lastly, we introduce a relationship between the tortuosity, in-pore ion diffusivity and capacitance to distinguish the charge distribution within micro- and mesopores at open circuit as well as under applied bias voltages. Our findings reveal the degree of ion dissociation in concentrated aqueous electrolytes as a key parameter determining the charging/ discharging rate performance of carbon electrode based capacitor. This study is helping to develop carbon materials and compatible electrolytes to ensure the capacitor meets the desired performance criteria while being reliable and efficient.