# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 2 D-Conducting Polymers and Their Thermoelectric Properties
### Pani  
PANI is another important conducting polymer due to its low cost, good stability, simple synthetic method and adjustable electronic properties for a wide range of applications [74; 75; 76; 77; 78]. In order to enhance the transport properties, various protonic acids (such as H\({}_{2}\)SO\({}_{4}\), HCl, HClO\({}_{4}\), polyacrylic acid,  
Figure 6: (**a**,**b**) Photographs of the flexible free-standing PPy nanotube film. SEM images of the free-standing (**c**) PPy-1 films and (**d**) PPy-2 films. TEM images of (**e**) PPy-1 nanotubes and (**f**) PPy-2 nanotubes. The temperature dependence of (**g**) electrical conductivity and (**h**) Seebeck coefficient of PPy-1 and PPy-2 films. Readapted with permission from Elsevier B.V. [71].  
Figure 7: (**a**) Schematic illustration of the formation procedure of PPy nanoparticles (PTs), nanotubes (NTs) and nanowires (NWs). TEM images of (**b**) PPy-PTs, (**c**) PPy-NTs and (**d**) PPy-NWs. (**e**) Thermoelectric power factor of PPy-PTs, PPy-NTs and PPy-NWs. Readapted with permission from MDPI [72].  
sulfosalicylic acid, p-toluenesulfonic, etc.) have been used as dopants for PANI and the conductivities are greatly influenced by the doping level and dopants. In 2010, [PERSON] et al. prepared \(\beta\)-naphthalene sulfonic acid (\(\beta\)-NSA) doped PANI nanotubes using a template-free method through oxidation polymerization of aniline monomer using APS as the oxidizing agent with \(\beta\)-NSA as dopant and soft template [79]. The length of the tubes ranges from 5 to 8 \(\upmu\)m with an average diameter of 200 nm (Figure 8a,b). The \(\beta\)-NSA doped PANI without nanostructure was also synthesized and used as reference to study the thermoelectric properties (Figure 8c,d). Different doping levels were investigated, and the PANI nanotubes synthesized with an aniline/NSA ratio of 4:1 exhibit an optimum Seebeck coefficient of 212.4 \(\upmu\)V K\({}^{-1}\) at 300 K, which is greater than the reference sample by seven folds. Additionally, electrical conductivity increased by almost two folds from 0.0045 to 0.0077 S cm\({}^{-1}\), while thermal conductivity decreased from 0.29 to 0.21 W m\({}^{-1}\) K\({}^{-1}\). The PANI nanotubes structures are strongly twisted and entangled, thereby increasing the phonon scattering at the boundary during heat transfer. As a consequence, the phonons mean free path significantly decreases then nanostructures are introduced into the system.  
In 2014, [PERSON] et al. also reported the synthesis and thermoelectric performance study of PANI nanowires or nanorods doped with different acids (Figure 9) [80]. The 1D nanostructured PANI was synthesized using the soft template method with APS as oxidizing agent, CTAB as soft template and different acids (HCl, acetic acid and p-toluenesulfonic (p-TSA)) as dopants. The effects of doping agents and doping levels on the thermoelectric properties were systematically studied. The p-TSA was found to be the best dopant between the three acids as the bulky p-toluenesulfonic anions helps to orientate the polymer chains better. Also with the same doping concentration, p-TSA-doped nanowires with reduced diameters and more consistent nanostructures (70 nm in diameter and 1-2 m in length) produced a higher Seebeck coefficient and lower thermal conductivity compared to p-TSA-doped nanorods (150 nm in diameter and 0.2 to 1 m in length), which is due to the improved charge carrier mobility as a result of more ordered arrangement of the polymer chains.  
Figure 8: (**a**,**b**) SEM images of PANI-NSA nanotubes. (**c**) Electrical conductivities (\(\square\)) and Seebeck coefficients (\(\Delta\)) of PANI nanotubes with different doping ratio. (**d**) Thermal conductivities (\(\square\)) and ZT value (\(\Delta\)) of PANI nanotubes changed with the doping ratio. Readapted with permission from Elsevier B.V. [79].