Ionic liquid electrolytes in thermoelectrochemical cells

The ability to harvest heat as a sustainable energy source would make a significant contribution to the reduction of our reliance on fossil fuels. Industrial and geothermal processes provide a massive source of heat that is readily available for energy conversion. Currently, these extensive and often continuous energy sources are severely underutilized, with one of the primarily limitations being the low efficiency of the thermoelectric technologies presently capable of converting heat into useful electrical energy. Thermoelectrochemical cells (TECs) utilise an alternative design to traditional semiconductor-based thermoelectric devices and offer the promise of low maintenance, operation at moderate temperatures and with no carbon emissions. The performance of a TEC is governed by the thermodynamic and transport properties of the electrolyte and the kinetics of the redox reaction at the electrode. This work studied various ionic liquid-redox couple combinations, and different electrode materials, to gather fundamental understanding of the conversion of thermal energy into electrical energy in these systems. The TECs described here utilise ionic liquid-redox couple electrolytes that represent a promising development in the field of thermal energy conversion technologies. The unique physical properties of ionic liquids (ILs) make them ideal for application as electrolytes in TECs. ILs exhibit negligible vapor pressure and therefore offer the potential for long-lasting devices as no significant electrolyte losses due to evaporation will occur. ILs can also have high thermal stability and a broad liquid temperature range, allowing for their use at higher temperatures (>100 °C) than for aqueous-based devices. Finally, ILs have low thermal conductivity, which is essential for large temperature gradients to be maintained across the TEC. For traditional semiconductor-based systems, the potential of a thermoelectric material to offer utility in a device is governed by the “figure of merit”, ZT. ZT = Se^2 Tσ/κ This is composed of the Seebeck coefficient, Se, the electrical conductivity, σ, the thermal conductivity, κ, and the absolute temperature, T. A value of ZT ≥ 1 is generally required for a material to be considered to have promise for use in a thermoelectric device. For redox couples in an electrolyte, the figure of merit does not adequately represent their performance limiting properties. Therefore, an adjusted figure of merit, ZT*, was developed in this work. This incorporates the diffusivity and concentration of the redox species in the electrolyte, rather than the electrical conductivity, to more properly assess the suitability of the redox electrolyte for TECs. These properties were investigated for a series of different redox species, I-/I3-, Fe(CN)64-/3-, and Co(bpy)32+/3+, in a range of different IL and organic solvent environments. This thesis reports the first published studies of the different physical properties that make up ZT* for these electrolyte solutions. To assess both the transport properties of the electrolyte and the kinetics of the electrode, a custom-designed TEC tailored specifically for IL electrolytes was developed. The first device measurements of power-potential difference and current-potential difference relationships using IL electrolytes were reported. The magnitude of output power and current density from the TECs allowed for comparison of different electrolyte and electrode material combinations. However, understanding the limitations that these materials impose on the TEC performance required a more fundamental investigation and thus a theoretical simulator was also developed. The simulator is a mathematical model used to investigate the influence of different parameters on the diffusion-limiting current, which is electrolyte-dependent, and the limiting exchange current, which is dependent on the electrode kinetics, to identify the limiting factors in any given device. From analysis of the physical properties of the electrolytes to measurement of the TEC output parameters using a series of electrolyte-electrode combinations, this work demonstrates the utility of ILs for thermoelectrochemical applications. The continuing development of these and other thermoelectric energy conversion technologies can help alleviate our reliance on fossil fuels, which will become increasingly important in the future.