Redox reactions in deep eutectic solvents: characterisation and application

Ionic liquids are a field of intense chemical research activity due to their potential to facilitate a wide range of chemical innovations. Due to the cost of ionic liquids alternative materials which exhibit similar properties, but are less expensive, are also the subject of interest and deep eutectic solvents are a prime example. Deep eutectic solvents are commonly based on quaternary ammonium chloride salts and therefore allow the ready dissolution of a range of metal chloride salts to form transition metal complexes. The electrolytes formed in this manner have been proposed as the basis for a range of novel processes to electrodeposit metals. This thesis presents the first application of a range of electroanalytical techniques to quantitatively characterize the properties of electrochemical reactions involving copper and iron chloro complexes in deep eutectic solvents. The methods used are based on cyclic voltammetry, electrochemical impedance spectroscopy and potential steps. Methods based on both static and hydrodynamic electrodes are considered. The reactions are found to be equally kinetically facile as in aqueous solvents, however the relatively viscous nature of the solvent is reflected in the transport properties of the complexes formed. The successful application of these techniques required the development of a suitably responsive reference electrode system. A system employing a quasireference electrode and reference probe in parallel was found to provide both a well-defined reference potential and suitably low impedance. The application of deep eutectic solvents in redox flow batteries is also reported for the first time and shown to allow the simple application of transition metal chlorocomplexes. However, the inferior transport properties of the electrolyte is shown to currently be a limiting factor within the temperature range studied. This requires the application of current densities at least an order of magnitude lower than those typically used in redox flow batteries. Application of deep eutectic solvents as the media for redox flow batteries required the development of a novel separator material based on jellification of the electrolyte using polyethylene glycol, this material was found to possess comparable conductivity to the pure electrolyte