Abstract
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.
Highlights
As ionic liquids permit a light-weight and flexible thermoelectrical generator (TEG) design without any harmful components, they can significantly contribute to power-supply autonomous systems for wearable applications
For two series of alkylammonium- and cholinebased ionic liquids, the values for SE and power output differ strongly, meaning there is no direct correlation between SE and power
The link between the Seebeck coefficient and power is current extraction, which can be identified as a key factor for TEG efficiency
Summary
As ionic liquids permit a light-weight and flexible thermoelectrical generator (TEG) design without any harmful components, they can significantly contribute to power-supply autonomous systems for wearable applications. For body-heat-based energy harvesting, a good and pleasant contact with the skin is crucial.[1] Solid-state TEGs, for example based on BiTe, use thin-film technologies to fabricate flexible modules.[2] a major disadvantage is the strong decline of the temperature difference between the warm and cold surfaces due to the too high thermal conductivity, and, the output voltage. As ionic liquids have a lower thermal conductivity and higher Seebeck-coefficients (SEs),[3] they might be more suitable despite a still lower power output compared to solid-state BiTe-TEGs. Why is the voltage output that important? Autonomous systems consist in general of an energy management and storage unit to condition the voltage level and to provide the necessary power even if no energy can be harvested. The power management unit* needs a minimum input voltage (Received August 14, 2017; accepted February 23, 2018; published online March 7, 2018)
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