• Energy Research

Looking at energy harvesting using body or waste heat for portable electronic or on-board devices, Ionic liquids are interesting candidates as thermoactive materials in thermoelectric generators (TEGs) because of their outstanding properties. Two different kinds of ionic liquid, with alkylammonium and choline as cations, were studied, whereby different anions and redox couples were combined. This study focussed on the intention to find non-hazardous and environmentally friendly ionic liquids for TEGs to be selected among the thousands that can potentially be used. Seebeck coefficients (SEs) as high as − 15 mV/K were measured, in a particular case for an electrode temperature difference of 20 K. The bottleneck of our TEG device is still the abundance of negative SE liquids matching the internal resistance with the existing positive SE-liquids at series connections. In this paper, we show further progress in finding increased negative SE liquids. For current extraction from the TEG, the ionic liquid must be blended with a redox couple, allowing carrier exchange in a cyclic process under a voltage which is incuced by the asymmetry of the generator in terms of hot and cold electrodes. In our study, two types of redox pairs were tested. It was observed that a high SE of an ionic liquid/redox blend is not a sufficient condition for high power output. It appears that more complex effects between the ionic liquid and the electrode determine the magnitude of the final current/power output. The physico-chemical understanding of such a TEG cell is not yet available.

Thermo-electrochemical cells (or thermocells) represent a promising technology to convert waste heat energy into electrical energy, generating power with minimal material consumption and a limited carbon footprint. Recently, the adoption of ionic liquids has pushed both the operational temperature range and the power output of thermocells. This research discusses the design challenges and the key performance limitations that need to be addressed to deploy the thermocells in real-world applications. For this purpose, a unique up-scaled design of a thermocell is proposed, in which the materials are selected according to the techno-economic standpoint. Specifically, the electrolyte is composed of EMI-TFSI ionic liquid supplemented by [Co(ppy)]3+/2+ redox couples characterized by a positive Seebeck coefficient (1.5 mV/K), while the electrodes consist of carbon-based materials characterized by a high surface area. Such electrodes, adopted to increase the rate of the electrode reactions, lead to a thermoelectric performance one order of magnitude greater than the Pt electrode-based counterpart. However, the practical applications of thermocells are still limited by the low power density and low voltage that can be generated.