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

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.