Nitrogen content governs thermoelectric performance in TPU/SWCNT composites

The use of the thermoelectric effect is a sustainable form of energy generation, as it allows waste heat to be harnessed to produce electrical energy. This study contributes to the replacement of the expensive and geopolitically risky metals currently used on an industrial scale with inexpensive and more environmentally friendly polymer-based composites. Composites based on singlewalled carbon nanotubes (SWCNTs) are being investigated, in which the polymer - here thermoplastic polyurethane (TPU) - modifies the thermoelectric properties of the SWCNTs. Three different types of TPU were selected by the Shore hardness, and the composition of the soft segments. The quantitative characterization of the TPU composition was performed using infrared spectroscopy (IR). This allowed the urethane, ester, and ether content to be determined. SWCNT contents ranging from 1 to 5 wt% were incorporated into the TPU grades via melt-mixing. It can be shown for the first time that an increasing proportion of urethane groups in the TPU leads to a reduction in the Seebeck coefficient from around 40 µV K^-1 up to 10 µV K^-1, which indicates an n-type doping effect. Furthermore, it has been shown that the contents of ether and ester groups have a minor effect on thermoelectric properties. A power factor of 0.1 µW m^-1 K^-2 could be achieved, which is significantly higher than the power factor previously reported for TPU/MWCNT composites.