Abstract
Low-grade thermal energy harvesting presents great challenges to traditional thermoelectric systems based on the Seebeck effect, the thermogalvanic effect, and the Soret effect due to fixed temperature gradient and low voltage output. In this study, we report an ionic thermoelectric system, essentially a supercapacitor (SC) containing an ionic liquid (IL) electrolyte and activated carbon electrodes, which works on the thermocapacitive effect and does not require any fixed temperature gradient, rather it works in a homogeneously changing temperature. A systematic investigation is carried out on SCs containing two different ILs, 1-Ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl), EMIm TFSI, and 1-Ethyl-3-methylimidazolium acetate, EMIm OAc. A high voltage output of 176 mV is achieved for EMIm TFSI containing SC by exposing just to 60 °C environment. Moreover, a large voltage of 502 mV is recovered from the SC upon subjecting to heat after one electrical charge/discharge cycle. A system containing two SCs in series demonstrates a significant voltage of 947 mV. The observed performance difference between the two ILs is rationalized in terms of the extent of asymmetry in the interfaces of the electrical double layer that essentially originates from different diffusivity of individual ions. The mechanism can be applied to a plethora of ILs to exploit low-grade heat to store electricity without a fixed temperature gradient, opening up the possibility to merge different scientific communities and enrich this rising research field.
Highlights
Thermal energy is an omnipresent energy resource that is generated as a byproduct of various industrial processes as well as non-industrial sources such as power plants and heat pumps
A systematic investigation is carried out to explore the thermal charging of SCs containing two different ionic liquid (IL), EMIm TFSI, and EMIm OAc, with activated carbon electrodes, as an ionic thermoelectric system working in the low-grade heat regime (60 °C)
The study reveals that a significant voltage output can be achieved without any fixed temperature gradient but rather with a homogeneous change in temperature, due to asymmetry in the two electrical double layer interfaces
Summary
Thermal energy is an omnipresent energy resource that is generated as a byproduct of various industrial processes as well as non-industrial sources such as power plants and heat pumps. SCs containing porous carbon electrodes and ionic liquid (IL) electrolytes with different properties of cations and anions have a great potential of utilizing low-grade waste heat to store electrical energy in a homogeneously varying temperature. Owing to these unique features ILs are utilized in polymer electrolytes to overcome their intrinsic low conductivity in a wide range of temperatures where ILs act as plasticizer [22,23] Most importantly, as they are solvent-free unlike organic or aqueous electrolytes, they are perfect candidates to study the ionic interactions that result in asymmetry in the electrical double layers, which is necessarily responsible for dissimilar interfacial properties that facilitate voltage output in an ionic thermoelectric system. When the devices are subjected to heat at the end of the electrical charge/discharge cycle, a significant increase of voltage rise can be observed, which opens the possibility to assign a parallel life to SCs
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