2020
Liquid structure and transport properties of the deep eutectic solvent ethaline
Yong Zhang, Derrick Poe, Luke Heroux, Hency Squire, Brian W. Doherty, Zhuoran Long, Mark Dadmun, Burcu Gurkan, Mark E. Tuckerman and Edward J. Maginn
Journal of Physical Chemistry B, 124, 5251-5264, 2020; https://doi.org/10.1021/acs.jpcb.0c04058
Ethaline, comprised of a 1:2 molar ratio of choline chloride (ChCl) and ethylene glycol (EG), is one of the most widely studied deep eutectic solvents (DES). This work reported a comprehensive study of the structure and dynamics of ethaline using a range of experimental and simulation techniques. Experimentally measured densities and viscosities match previously reported values very well. Classical and ab initio molecular dynamics simulations both capture experimental liquid structure, as determined by neutron scattering measurements. Computed diffusivities and viscosities also agree well with experiments. The simulations suggest that the solvation environment in ethaline is more complicated than what may be assumed based on its stoichiometric composition. Addition of ChCl into pure EG breaks the hydrogen bond (HB) network in EG and multiple types of HBs are formed. In particular, the HBs between Cl and EG hydroxyl groups were found to be stronger than those hydroxyl HBs in pure EG, resulting in higher viscosity and slower dynamics in ethaline compared to pure EG. The complex HBs in ethaline also cause complexity in dynamics. Rotational relaxation of choline is much slower than ethylene glycol. Dynamic heterogeneities persist for over 10 ns in ethaline, with the chloride ion experiencing the most dynamical heterogeneity due to its hydrogen bonding interactions. Based on MD simulations, significant insights into coordination probability, contact pair fraction, and hydrogen bond dynamics, were obtained. These analyses provided great insight into the structure and dynamics of ethaline and revealed the direct connection between local hydrogen bonding structure and transport properties. These fundamental understandings will guide the study and design of other DES mixtures, and the development of electrolytes with optimum properties for flow battery applications.
Mechanism of Electrochemical Oxidation of Nitroxide Radicals in Ethaline Deep Eutectic Solvent
Nora A. Shaheen, M. Ijjada, Miomir B. Vukmirovic, Rohan Akolkar
Journal of Electrochemical Society, 167, 143505, 2020
https://doi.org/10.1149/1945-7111/abc439
In this manuscript, an “Adsorption – Desorption” mechanism is proposed to explain unusual values of the anodic charge transfer coefficient 
associated with the electrochemical oxidation of two nitroxide-containing organics: TEMPO and 4-hydroxy-TEMPO. These values were extracted in both aqueous and deep eutectic solvent electrolytes, and were found to approach unity under a variety of conditions. In this mechanism, the nitroxide radical undergoes (step i) fast adsorption and charge transfer, followed by (step ii) slow desorption of the oxidized product. This impact of this desorption-limited oxidation was further investigated on transient cyclic voltammograms. Through a numerical model, the irreversibility observed at high concentrations was captured. Understanding mechanisms governing charge transfer of redox-active organics is critical for designing high energy density organic flow batteries, particularly when limited by surface coverage of the redox-active organic.
Feasibility of TEMPO-functionalized imidazolium, ammonium and pyridinium salts as redox-active carriers in ethaline deep eutectic solvent for energy storage
Brian Chen, Sarah Mitchell, Nicholas Sinclair, Jesse Wainright, Emily Pentzer, Burcu Gurkan
Molecular Systems Design & Engineering, 5, 1147-1157, 2020; https://doi.org/10.1039/D0ME00038H
TEMPO is a common redox active specie that is studied widely in redox flow batteries. However, we found that its solubility is limited in deep eutectic solvent ethaline. This study derivatized imidazolium, ammonium, and pyridinium salts with the nitroxy radical functionality to incorporate into ethaline. While the functionalization of the salts improved the solubility of TEMPO, its reactivity in the presence of the alcohol group of ethylene glycol led to redox irreversibility. The redox flow battery experiments demonstrate the feasibility of these salts as redox active species in deep eutectic solvents for energy storage.
Electroanalytical Investigation of the Electrode–Electrolyte Interface of Quaternary Ammonium Ionic Liquids: Impact of Alkyl Chain Length and Ether Functionality
Jeffrey M Klein, Henry Squire, Burcu Gurkan
Journal of Physical Chemistry C, 124, 10, 5613–5623, 2020; https://doi.org/10.1021/acs.jpcc.9b08016
This study developed a combined surface enhanced Raman spectroscopy (SERS) and electrochemical impedance spectroscopy (EIS) method to probe electrode-electrolyte interfaces to identify the surface species of ionic liquids (ILs) as the electrode potential is swept. This method can be applied more broadly to other complex electrolytes. The study shows that long alkyl chains of IL cations buckle to compact with increased polarization (a phenomenon that has not been reported before).
From Salt in Solution to Solely Ions: Solvation of Methyl Viologen in Deep Eutectic Solvents and Ionic Liquids
Jeffrey M Klein, Henry Squire, William Dean, Burcu E Gurkan
Journal of Physical Chemistry B, 124, 29, 6348–6357, 2020; https://doi.org/10.1021/acs.jpcb.0c03296
Solvation of redox species in hydrogen bonded concentrated electrolytes such as deep eutectic solvents (DESs) is important to understand as it impacts redox potentials and redox reversibility in energy storage. Raman spectroscopy suggests that the redox specie methyl viologen dichloride (MVCl2) is strongly solvated by ethylene glycol in choline bis(trifluorosulfonyl)imide (TFSI) and ethylene glycol mixtures whereas it is interstitially accommodated in holes of ethaline (choline chloride : ethylene glycol, 1:2) mixtures and the pyrrolidinium TFSI ionic liquid. The unhindered redox center of MVCl2 in DES ethaline leads to its robust redox reversibility.
2019
Metal-Free Deep Eutectic Solvents: Preparation, Physical Properties, and Significance
Burcu Gurkan, Henry Squire, Emily Pentzer
Journal of Physical Chemistry Letters, 10, 24, 7956–7964, 2019
https://doi.org/10.1021/acs.jpclett.9b01980
This perspective provides an overview of metal-free deep eutectic solvents (DESs) with the goal of standardizing sample preparation and characterization in the field. Specific properties of interest for various applications including energy storage and characterization methods are summarized for the newcomers to the field in order to engage other researchers and help advance the field.
Understanding the Role of Complexation in the Charge-Transfer Kinetics of the Cu2+ + e ↔ Cu1+ Redox Reaction in Ethaline Deep Eutectic Solvent
Dai Shen, Miomir B Vukmirovic, Rohan Akolkar
Journal of Electrochemical Society, 166 E526, 2019; https://doi.org/10.1149/2.1211915jes