# A Comparative Study of Choline Chloride Deep Eutectic Electrolytes: Towards Sustainable Supercapacitors

## TL;DR

This paper compares three choline chloride-based deep eutectic solvents for use in sustainable supercapacitors, finding ethaline to be the most promising.

## Contribution

A comparative evaluation of ethaline, glyceline, and reline as sustainable electrolytes for supercapacitors.

## Key findings

- All three DESs remained liquid at low temperatures and showed amorphous behavior.
- Ethaline exhibited the lowest resistivity and highest capacity among the tested DESs.

## Abstract

Over the past few decades, ionic liquids (ILs) have gained attention as electrolytes, although concerns about their environmental persistence and toxicity challenge their status as green solvents. In this framework, choline chloride (ChCl) offers a more sustainable alternative due to its low toxicity, biodegradability, and cost-effectiveness. Although ChCl has a high melting point, its combination with hydrogen bond donor compounds (HBDs) can result in liquid mixtures at much lower temperatures, known as deep eutectic solvents (DESs). This study presents a comparative evaluation of three ChCl-based DESs, glyceline, ethaline, and reline (obtained from mixtures of ChCl and glycerol, ethylene glycol, and urea), with a focus specifically on their potential as electrolyte candidates for supercapacitors. Using differential scanning calorimetry (DSC), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and nuclear magnetic resonance (NMR), we assess their thermal, electrochemical, and structural properties. All DESs displayed amorphous behavior and a strong tendency to remain liquid even at very low temperatures. Among them, ethaline showed the most promising electrochemical performance, exhibiting the lowest resistivity and the highest capacity.