Flexible combinatorial ionic/electronic thermoelectric converters to efficiently harvest heat from both temperature gradient and temperature fluctuation

It is of great significance to develop novel heat-harvesting technology due to abundant waste heat on earth. Although thermoelectric generators (TEGs) based on the Seebeck effect under temperature gradient has been studied for more than 200 years, their thermoelectric (TE) performance is still not good enough for large-scale practical application. Ionic TE materials can exhibit much higher thermovoltage than electronic conductors, but they can be used to harvest heat merely from temperature fluctuation. In order to take the advantages of these two types of TE materials, we developed flexible combinatorial TE converters (CTECs) with an ionic TE capacitor (ITEC) made of an ionogel and a TEG consisted of poly (3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), which were connected in parallel, that is, the electrodes of the ITEC and TEG at the hot end are wired together, while those at the cold end are connected. These CTECs can harvest heat from both temperature gradient by the TEG and temperature fluctuation by the ITEC. Their TE performances are sensitive to the factors like heating/cooling rates, temperature gradient profile and internal resistance of the TEG. The specific average power supplied by the CTEC can be up to 4.7 times as that of the control TEG with PEDOT:PSS. Moreover, the TE performance can be further improved by combining an ITEC with a TEG consisted of both p- and n-type legs in series, which can generate a specific average power as 5.8 times as the CTECs with the TEG of only one p-type leg.