Towards ionic liquid-based thermoelectrochemical cells for the harvesting of thermal energy

Liquid-state thermoelectrochemical cells offer an alternative design to traditional semiconductor-based thermoelectric devices for the harvesting of thermal energy. They are capable of continuous and cheap operation at moderate temperatures, are low maintenance and produce zero carbon emissions. The good thermal and electrochemical stability, non-volatility and non-flammability of ionic liquids (ILs) makes them promising electrolytes for these devices, especially for applications involving low grade thermal energy available at temperatures in the 100-200 °C range. Power generation characteristics have been determined for a number of such ionic liquid-based devices using the iodide/triiodide (I−/I3−) redox couple. Power densities as high as 29 mW/m2 were measured in unoptimised devices operating with a hot side at 130 °C. The performance of these devices is a complex function of thermodynamic and transport properties of the electrolyte. An adjusted thermoelectric figure of merit (ZT*) was designed to compare physical properties of these liquid electrolytes that determine their viability as electrolytes in thermoelectric energy conversion. This comparison requires evaluation of the diffusivity of the redox couple, the thermal conductivity of the electrolyte, and the Seebeck coefficient (Se) of the redox electrolyte. These parameters were determined independently and then combined in the figure of merit relationship to analyse device performance using a range of different electrolytes.