Electrochemical Decomposition of Ethylene Glycol-Choline Chloride Deep Eutectic Solvent
Authors: Sinclair Nicholas , Shen Xiaochen , Wainright Jesse S. 
Published: July 2021

Deep eutectic solvents (DES) are a form of ionic liquid that can offer a larger electrochemical window than water while remaining relatively conductive as compared to other non-aqueous options. DES are made from the eutectic combination of a variety of hydrogen bond donors and acceptors. At a molar ratio of donor and acceptor specific to each DES, the melting point of the mixture is significantly depressed resulting in a room temperature liquid with a high boiling point. As a result, they are considerably less volatile and therefore a safer alternative to traditional organic solvents such as acetonitrile. The low vapor pressure associated with a deep eutectic composition allows for operation at elevated temperatures for faster kinetics, increased conductivity and lower viscosity. DES can also contain a greater concentration of supporting charge carries than many organic solvents (up to 8 M) and many redox active organic compounds are also highly soluble in DES. These factors make DES a promising electrolyte for redox flow batteries, with the potential for higher energy densities than can be achieved in aqueous electrolytes.
One of the most commonly studied DES is a 1:2 molar ratio mixture of choline chloride and ethylene glycol respectively. This mixture, commonly called ‘ethaline’ is one of the most conductive, lowest viscosity DES. While these are desirable characteristics, the stable electrochemical window is only ≈2 V which is only somewhat larger than the usable window of an alkaline aqueous electrolyte. The objective of this effort is to investigate the electrochemical decomposition of ethaline at the anodic and cathodic limits.
Relatively little literature exists on the decomposition of ethaline. It has been claimed that at the anode, several different chlorinated organics are created by various mechanisms and that hydrogen is evolved at the cathode from water contained in the electrolyte.1 While the oxidation of ethylene glycol (EG) is well studied2
,
3, the oxidation of ethaline and the reduction of EG and ethaline both are somewhat unknown. A series of electrolysis experiments using a divided cell were carried out using dry ethaline prepared in a glove box with a water vapor content below 5 ppm. The water content in the electrolyte was determined by Karl Fischer titration to be less than 300 ppm. The products of these experiments were analyzed using mass spectroscopy (of both the anodic and cathodic electrolytes and of the headspace above each) and FTIR. These results show that the positive decomposition involves the oxidation of ethylene glycol to glycolaldehyde, and subsequent formation of the glycolaldehyde dimer in solution. At the negative limit, hydrogen is evolved in direct proportion to the current passed. However, it can be conclusively shown that the reduction current can exceed the maximum possible current due to the reduction of water, implying that the reduction of the DES components is necessary. GC-MS analysis of the negative electrolyte showed the presence of higher molecular weight fragments consistent with the reaction scheme shown in Fig 1 below. In this scheme, the reduction of ethylene glycol results in the generation of H2 gas as observed, and the formation of ethylene oxide. The ethylene oxide produced then can react chemically with ethylene glycol to yield dimers and trimers of polyethylene glycol whose molecular masses are consistent with the peaks observed in the GC-MS analysis.
The identification of these breakdown pathways will provide insight into safety concerns related to overcharge of ethaline electrolytes and will allow for the development of alternative formulations that can extend the potential limits to allow for the incorporation of higher voltage redox couples in redox flow batteries based on DES electrolytes.
References:

Haerens, K., Matthijs, E., Binnemans, K. & Van Der Bruggen, B. Electrochemical decomposition of choline chloride based ionic liquid analogues. (2009) doi:10.1039/b906318h.

Pierre, G., Ziade, A. & El Kordi, M. The oxidation of glyoxal and ethylene glycol on platinum in aqueous acid mediums containing some metal salts. Electrochim. Acta 32, 601–606 (1987).

Livshits, V., Philosoph, M. & Peled, E. Direct ethylene glycol fuel-cell stack—Study of oxidation intermediate products. J. Power Sources 178, 687–691 (2008).

Figure 1. Proposed electrochemical reaction at cathode and subsequent polymerization.

Figure 1