Use of Deep Eutectic Solvents (DESs) for the Preparation of Thermoelectric Materials

Global energy consumption was reported at 4.1 x 10²⁰ J (equivalent to 13 terawatts) in 2005, expected to more than triple by the end of the century based on projected population and economic growth. These rapidly increasing energy demands coupled with concerns over climate change as well as depletion of fossil fuel supplies has led to intensive efforts to increase the efficiency of current processes of energy production, conversion and recovery. Thermoelectric materials have in recent years emerged as a potential solution to this problem. Thermoelectric devices use temperature gradients to generate electrical energy due to the potential difference across a cell. The cell potentials for most materials are generally quite small and so semiconductor materials have to be used to optimise the cell potential. These materials are generally quite difficult to produce and this thesis examines two novel methods of creating semiconductor materials on a conducting surface. Both techniques make use of deep eutectic solvents (DESs), a novel type of solvent formed by mixing a quaternary ammonium salt with a hydrogen bond donor. This study will focus on simple metal sulfides as they are inexpensive to prepare and could be used for large surface area devices. The first technique oxidises a metal electrochemically in a DES containing sodium sulfide to produce a metal sulfide on the metal surface. The second technique uses powders of the metal sulfide on an electrode surface and by a process of oxidation and reduction it attempts to fuse the powder onto the electrode surface. It is shown that both techniques are successful and a thermoelectric device could be constructed but the layers produced are relatively thin.