# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 2 D-Conducting Polymers and Their Thermoelectric Properties
### Pedot
Tuning the quantity of oxidizing agent added and the rate of polymerization gave rise to PEDOT with different nanostructures, which were proved to have great effects on the thermoelectric properties (Figure 3f-h). The improvement in thermoelectric properties in terms of electrical conductivity, Seebeck coefficient and power factor follows the order of bulk PEDOT \(<\) globular nanoparticle \(<\) nanorod \(<\) nanotube \(<\) nanofiber. The PEDOT nanofibers exhibit the best electrical conductivity of 71.4 S cm\({}^{-1}\) and Seebeck coefficient of 48.0 \(\upmu\)V K\({}^{-1}\), which is considerably higher than those of the bulk powder samples (0.032 S cm\({}^{-1}\); S = 10.7 \(\upmu\)V K\({}^{-1}\)). The mechanism study shows the oxidation/doping level of PEDOT, the carrier concentration and mobility and the orderly arrangement of the polymer chains all have close relationship with the thermoelectric performance enhancement of the PEDOT nanostructures. Later in 2017, the same research group also reported the relation of thermoelectric performance with post-treatment conditions (acid treatment with H\({}_{2}\)SO\({}_{4}\) and chemical reduction using Na\({}_{2}\)SO\({}_{3}\)) for PEDOT nanorods synthesized by reverse microemulsion polymerization. Through varying the acid concentration and reduction treatment time, the intensity of doping and oxidation, the carrier type transition and orderliness of polymer chain interaction is adjustable. Therefore, the thermoelectric properties of PEDOT nanorods can be modified easily and efficiently [40]. Figure 2: Transmission electron microscopy (TEM) images of (**a**) T-PEDOT nanowires and (**b**) C-PEDOT nanowires. Scanning electron microscopy (SEM) images of (**c**) T-PEDOT nanowires and (**d**) C-PEDOT nanowire-based films. The scale bar is 500 nm for all images. (**e**) S, \(\sigma\), and (**f**) power factor of C-PEDOT nanowire-based film with respect to the hydrazine treatment duration. Readapted with permission from Elsevier Ltd. [39]. Flexible fiber-based thermoelectric generators which are lightweight and 3D deformable are receiving increasing research interest for applications in wearable energy harvesting and mobile electronic systems [41]. Very recently, [PERSON] et al. fabricated a p-type PEDOT:PSS fiber that is highly conductive with via a gelation methodology with the assistance of sulfuric acid in a capillary at an elevated temperature of 90 \({}^{\circ}\)C (Figure 4a) [42]. The hydrogel was then released to a solvent bath to yield a free-standing fiber with the average diameter of 142.5 \(\pm\) 42.2 \(\upmu\)m and a length of more than 20 cm (Figure 4b-d). Post-treatment of the fibers with ethylene glycol or dimethyl sulfoxide successfully improved the electrical conductivity by three times (up to 172.5 S cm\({}^{-1}\)) with only 5% decrease in Seebeck coefficient (14.8 \(\upmu\)V K\({}^{-1}\)), giving rise to an optimum power factor of 4.77 \(\upmu\)W m\({}^{-1}\) K\({}^{-2}\). Furthermore, a p\(-\)n-type thermoelectric generator was assembled with five p-n-type units where the p-type materials were made of PEDOT:PSS hydrogel fibers while n-type material is made of CNT fibers (Figure 4e,f). This fiber-based device showed a satisfactory output voltage of 20.7 mV and a power density of 481.2 \(\upmu\)W cm\({}^{-2}\) when the difference in temperature between the two junctions is \(\sim\)60 K. This is a good example of using organic thermoelectric fibers to harness energy via a wearable device. Figure 3: SEM images of (**a**) bulk PEDOT power; (**b**) globular nanoparticles; (**c**) nanorods or ellipsoidal nanoparticles; (**d**) nanotubes; (**e**) nanofibers; (**f**) Seebeck coefficient and (**g**) power factor of bulk PEDOT and PEDOT nanostructures; (**h**) Carrier mobilities and carrier concentrations of bulk PEDOT and PEDOT nanostructures. Readapted with permission from The Royal Society of Chemistry [35].