# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 3 Strategies to form 1D Conducting Polymer Nanocomposite Thermoelectric Materials  
### In Situ Polymerization of Conducting Polymers on 1D Nanofilers  
Currently, surface coating of conducting polymers via in situ polymerization on inorganic or carbon nanomaterials is the most commonly used strategy to fabricate composite thermoelectric materials. To attain high thermoelectric performance, it is essential to construct the coating polymer with ordered orientation and extended chain conformation, which can reduce the energy needed for inter-chain and intra-chain hopping, offering improved carrier mobility of the polymer. The in situ polymerization method can make monomer molecules polymerize and extend in a systematic mode on the surface of the nanof fillers as a result of the interfacial interactions (conjugation, hydrogen-bonding, van der Waals forces, etc.) between polymer and nanofillers. Moreover, the thickness of the polymer coating can be easily altered by varying the weight ratios of the monomers to nanofillers. A series of 1D nanocomposite materials with enhanced thermoelectric performance have been successfully developed, such as PEDOT/Te nanowires [81; 82; 83], PEDOT/CNTs [84; 85], PANI/Bi\({}_{2}\)Te\({}_{3}\) nanowires [86], PANI/CNTs [87; 88; 89; 90; 91; 92; 93; 94], PANI/ZnO nanorods [95], PPy/CNTs [96; 97; 98], etc. To obtain highly aligned polymer chains at molecular level, [PERSON] et al. prepared PANI/CNT composite nanofibers by in situ polymerization combined with electrospinning process [87]. As shown in Figure 10b, PANI was coated on multi-walled CNTs via in situ polymerization of aniline using APS as oxidant. The prepared composite powders were then re-dispersed in chloroform and subjected to electrospinning using rotating drum collector to generate aligned nanofibers with diameters around 150-200 nm, which were finally pressed into compacted pellets for thermoelectric performance measurement. Electrospun fibers from directly mixed CNTs and PANI composite (DM-CNT/PANI) were also prepared as a comparison. It was found highly ordered polymer chains were grown on the CNTs via in situ polymerization as a result of strong conjugation interactions between PANI and CNTs. Furthermore, during the electrospinning process, the high voltage electrical field is also effective for aligning both CNTs and PANI polymer chains due to their anisotropic electric dipole moment. As shown in the SEM and TEM images (Figure 10c,d), all the nanotubes were evenly enclosed by PANI and oriented along the fiber axis in the CNT/PANI samples. The DM-CNT/PANI fibers have a similar structure but less homogeneity due to the existence of isolated PANI particles and bare parts of PANI coating layer. The CNT/PANI composite fibers were found to with significantly greater electrical conductivity and power factor (75% improvement compared to DM-CNT/PANI samples) in the polymer chain oriented direction. Figure 9: SEM images of (**a**) PANI nanorods prepared in an ice bath; (**b**) PANI nanowires prepared at room temperature. (**c**) The temperature (T) dependence of figure-of-merit ZT of different acid doped PANI nanowires. (**d**) The temperature (T) dependence of figure-of-merit ZT of p-TSA doped PANI nanowires and nanorods. Readapted with permission from Elsevier B.V. [80]. Flexible, stretchable and mechanically stable thermoelectric materials are most promising for next-generation wearable electronic devices [99]. In 2016, [PERSON] et al. reported the facile preparation of a super flexible and mechanically stretchable thermoelectric films with tunable thickness via in situ polymerization of PPy on single-walled CNTs (SWCNTs) followed by a vacuum filtration process (Figure 11a,b) [96]. The CNTs were fully exfoliated by using sodium dodecyl sulfate (SDS) in water with or without ethanol, and then pyrrole monomers were added and polymerized on the surface of CNTs using iron sulfate as oxidant. The thickness of the PPy coating layers are easily altered by tuning the SWCNT: Py mass ratios.