# EXAFS and Rotating Disc Electrode Study into the Thermochromic Behavior of Nickel Salts in Deep Eutectic Solvents

## TL;DR

This paper studies how nickel chloride changes color and deposits on electrodes in different deep eutectic solvents when heated.

## Contribution

The study reveals how solvent composition affects nickel speciation and electrochemical performance at elevated temperatures.

## Key findings

- In ethylene glycol DES, nickel coordination changes reversibly between octahedral and tetrahedral structures between 90 and 100°C.
- Urea DES shows irreversible color change above 100°C due to solvent decomposition into ammonia-like species.
- Higher temperatures correlate with improved electrochemical behavior and faster electron transfer in nickel chloride solutions.

## Abstract

The thermochromic
and electrodeposition behavior of nickel
chloride
was investigated in two choline chloride-based deep eutectic solvents
(DES), with either ethylene glycol or urea as the hydrogen bond donors.
In the ethylene glycol DES, thermochromism was found to be reversible,
with ligand exchange resulting in the main structural change from
octahedral to tetrahedral coordination taking place between 90 and
100 °C. In the urea DES, a change in color only took place above
100 °C, at which point a suspected ammonia species was irreversibly
formed from decomposition of the solvent. The speciation effects were
studied by using UV–vis and EXAFS spectroscopies, together
with the electrochemical methods of cyclic voltammetry and rotating
disk voltammetry. The observed speciation changes evolving at higher
temperatures were seen to correlate with more well-defined electrochemical
behavior together with faster electron-transfer kinetics and higher
Coulombic efficiency.