Electrically conductive polymer composites (CPCs) based on insulating polymers filled with carbon nanotubes (CNTs) are being studied as potential thermoelectric (TE) materials to recover waste heat into electrical energy. Even though such composites still have significantly lower thermoelectric performance than traditionally used materials and can only be used in temperature ranges below 240°C for long time, their advantages lie in their availability and cost efficiency, but also in ease of processing, flexibility, low density and intrinsically low thermal conductivity.
In our study, composites based on industrially used polymers were fabricated by melt processing in gram scale and compression molded to thin sheets. The singlewalled CNTs (SWCNTs) used as fillers created an electrically conductive network in this insulating thermoplastic matrix.
In most cases, p-type SWCNTs are also used to fabricate p-type composites, whereas n-type composites are obtained when the polymers contain nitrogen groups, e.g. polyamides or acrylonitrile butadiene styrene (ABS) [1, 2]. The p-type composites can be converted to n-type behavior by adding additives. For example, pre-treatment of SWCNTs with polyethylene glycol (PEG) or ionic liquids led to switching from p- to n-type [3-5].

To prove the concept, modules were built with 3 up to 49 pairs of legs in flat or zigzag geometry, which generated a thermoelectric voltage up to 110 mV at 70 K temperature difference [3]. Even though these TE values are far below industrial requirements, the results show that such polymer composites can in principle be used for TE applications.

References
[1] J. Luo et al. AIMS Materials Science 2016, 3, 1107-1116
[2] B. Krause et al. Journal of Composites Science 2019, 3, 106.
[3] J. Luo et al. Polymer 2017, 108, 513-520.
[4] O. Voigt et al. Journal of Composites Science 2022, 6, 25.
[5] B. Krause; P. Pötschke, Nanomaterials 2022, 12, 3812.
[6] Annual report of IPF Dresden 2021, 68-69.

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