**3. Applications of conducting polymers and 1-D nanostructures composites**

Many of the targeted applications for 1-D nanostructures require their incorporation into conducting polymers. Therefore, such nanocomposites are expected to find applications in nanoelectronic devices, sensors, catalysis or electrocatalysis and energy.

#### **3.1 Energy conversion and energy storage applications**

Incorporating 1-D nanostructure into conducting polymers play an important role in fabricating materials in energy conversion devices such as solar cells, and fuel cells, and energy storage devices such as lithium-ion batteries and supercapacitors. These materials have improved conductivity, cycleability, mechanical stability, processability, and specific capacitance.

Solar cells are energy conversion devices that convert sun light to electric energy. In comparison to nanoparticles, 1-D nanostructures are excellent candidates for the

#### *Conducting Polymer 1-D Composites: Formation, Structure and Application DOI: http://dx.doi.org/10.5772/intechopen.102484*

preparation of solar cells because they provide high electron mobility along these nanostructures. Their hole-transporting properties may contribute to the improvement of the photovoltaic efficiency performance. Solar cell devices fabricated with aligned ZnO/P3TH and ZnO/didodecylquaterthiophene (QT) composites exhibit well-resolved characteristics with the efficiency of 0.036%, a circuit current density (*J*sc) of 0.32 mA cm<sup>2</sup> , and a fill factor (FF) of 0.28 [49]. Additional enhancement of photovoltaic properties of ZnO/P3TH-based solar cells can be achieved by incorporation of TiO2 into the composite structure. Yang et al. [8] demonstrated that solar cells based on ZnO-TiO2/P3TH composites exhibited an efficiency of 0.29% for devices stored in air for 1 month. While, these devices without TiO2 layer had an efficient of only 0.04%. CNTs/PEDOT-PSS (polystyrenesulfonate) nanotubes were also used as dye-sensitized solar cells (DSCs), which exhibit very high performance with the energy conversion efficiency of 6.5%, *J*sc = 15.5 cm<sup>2</sup> , and FF = 0.63 [50].

Fuel cells, which convert the chemical energy of a fuel directly into electricity by electrochemical reactions, have attracted attention for applications in electric vehicles [51]. Also in this case, the introduction of 1-D composites containing polymers can significantly improve the performance of such systems. For example, Co/PPY/MWCNTs nanotube composites were used as the cathode electrocatalysts for the reduction of oxygen in polymer electrolyte fuel cells (PEMFCs), direct ethanol fuel cells (DEFCs), and direct methanol fuel cells (DMFCs) [52]. The stability of these composites for the reduction of oxygen was excellent without any noticeable loss in performance over long PEMFC operating time, which is shown in **Figure 10**.

Rechargeable batteries are widely used in daily life such as in cell phones, laptop computers, and electric vehicles. 1-D nanofibers composite of V2O5/PANI has been used as cathode materials in ion-Li batteries [53]. The composite showed enhanced capacitance properties in comparison to vanadium oxide nanotubes. The charge capacity of V2O5/PANI composite nanofibers was about 150 Ah kg<sup>1</sup> during the 10

#### **Figure 10.**

*Cell performance showing the stability of PEMFC at 90°C with cathode catalyst containing Co/PPY/MWCNTs and anode catalyst containing Pt/Ru/MWCNTs at ambient pressure (reproduced with permission from Ref. [52]).*

initial charge/discharge cycles, while a charge capacity of 100 Ah kg<sup>1</sup> was obtained for V2O5 nanotubes. The composite also exhibits much better cyclability in comparison to V2O5 nanotubes. A significant enhancement in electrochemical performance has been also found for the silver vanadium oxides (SVO)/PANI triaxial nanowires [54]. It has been observed that the SVO/PANI triaxial nanowires exhibited a much higher current density than that of *β*-AgVO3 nanowires, due to faster kinetics of charge transfers and higher capacity. Therefore, these composites can be used as cathode material for Li<sup>+</sup> ion batteries with high specific capacity and good cycle performance [54]. The composite of polypyrrole@CNTs has been used as pseudocapacitive cathodes and Fe3O4@carbon as anode for nonaqueous lithium-ion capacitor applications [55]. Due to the synergistic effect of the remarkable pseudocapacity of the polypyrrole and the high electrical conductivity of CNTs, the polypyrrole@CNTs composite exhibited enhanced capacitive properties and cycle life in comparison to the pristine CNTs. The rechargeable sulfate- and sodium-ion batteries based on MWCNTs-polypyrrole core-shell nanowire anode and a Na0.44MnO2 nanorod cathode were also studied [56]. This MWCNTs-polypyrrole core-shell nanowire//Na0.44MnO2 nanorod full cell delivered discharge capacities of 99.2 mAh g<sup>1</sup> and 87.2 mAh g<sup>1</sup> with a high voltage of 1.6 V at the charge-discharge current densities of 100 mA g<sup>1</sup> and 3000 mA g<sup>1</sup> , respectively, making it suitable for large scale energy storage application [56].

Supercapacitors also called electrochemical capacitors with high specific power, exceptional long cycle life compared with rechargeable batteries, and higher specific energy compared to conventional capacitors are of great interest for their potential applications in portable electronics, hybrid electronic vehicles, memory protection of computer electronics and renewable energy system [57]. Active electrode materials used in supercapacitors can be classified into three main categories: carbon, transition metal or metal oxide, and conducting polymer. Among these third groups, carbon has a relatively low specific capacitance usually under 200 F g<sup>1</sup> , metal oxides are either expensive, for example, ruthenium oxide, or poor conductors, for examples MnO2, NiO, etc., while conducting polymers have a high specific capacitance, but their cyclic stability is poor. Therefore, nanocomposites can be useful for the construction of electrochemical capacitors with both high capacitance and good cyclic stability. PANI/CNFs composite was tested as an electrochemically active component for supercapacitors [38, 39]. The specific capacitance of PANI layer-coated CNFs showed a maximum value of 264 F g<sup>1</sup> at 20 nm thickness of PANI, whereas that of pure CNFs were as 100 F g<sup>1</sup> [38]. The specific capacitance of 557 F g<sup>1</sup> and good cycling stability were reported for CNFs/PANI by Kotal et al. [39]. The electrochemical measurements of graphene nanoribbons with PPY showed the highest specific capacitance of 2066 F g<sup>1</sup> . Therefore, these nanocomposites could be used as an electrode material for the fabrication of high-capacity supercapacitors [40]. The nanocomposite of metal wires and conducting polymers can be also used in supercapacitors [12, 13]. In the case of 3-D Ag nanowires/PPY the maximum specific capacitance, maximum power and energy density 509 F g<sup>1</sup> , 60.7 W kg<sup>1</sup> , and 4.27 Wh kg<sup>1</sup> was reported, respectively. The fabricated supercapacitors showed excellent stability of almost 90% after 10,000 charge/discharge cycles [13]. The capacitance performance of a variety of conducting polymer and 1-D carbon nanostructure composites is summarized in **Table 3**. In addition to CNTs, nanocomposites containing metal, metal oxide or metal hydroxide and conducting polymers have also been intensively investigated as building components in supercapacitors [62–65].

*Conducting Polymer 1-D Composites: Formation, Structure and Application DOI: http://dx.doi.org/10.5772/intechopen.102484*


#### **Table 3.**

*Capacitance performance of the composites of conducting polymers with 1-D carbon nanostructures.*

#### **3.2 Electronic nanodevices**

It is generally accepted that 1-D nanostructures provide a good system to investigate the dependence of electrical transport or mechanical properties on dimensionality and size reduction. Most conducting polymers are suited for the construction of electronic devices because of their high electrical conductivity, and mechanical flexibility. Therefore, materials combined of 1-D nanostructures and conducting polymers can be potentially applicable in diodes, memory, transistors, and photovoltaic devices.

Woo et al. [66] reported the fabrication of organic light-emitting diode (OLED) using a conjugated emissive copolymer, poly(3,6-*N*-2-ethylhexyl carbazolyl cyanoterephthalidence) (PECCP) and SWCNTs dispersed in a hole conducting PEDOT in the buffer. The schematic of this device construction is shown in **Figure 11**. This composite deposited on the ITO served as an anode in LED. A cathode was a bi-layer consisting of a LiF and Al.

**Figure 11.** *Schematic of OLED construction.*

By a combination of electrochemical polymerization of pyrrole and electrophoretic deposition of CNTs, new composite material has been prepared and tested for application in a triode-type field emission array (FEA) [67]. This triode-type FEA showed an emission current of 35 mA at an anode voltage of 1000 V and the gate voltage of 60 V. The emission current of the FEA was modulated by the gate voltage of 30 V. For photovoltaic applications, nanocomposite material consisting of CNTs and PANI as highly conductive and transparent has also been prepared [68]. Organic photovoltaic cells were built using this film as an anode in flexible ITO-free devices. These results indicated that novel ITOfree optoelectronic devices can be optimized with very high performance using transparent films of conjugated polymers and carbon 1-D nanomaterials.

#### **3.3 Sensors**

Conducting polymers are good candidates for chemical and biological sensors because the interactions with various analytes may influence the redox and doping states of these materials. Adding a second nanocomponent, such as carbon nanotubes, metal and metal oxide nanostructures, and biological materials, into conducting polymer is another way to increase the charge mobility of conducting polymers or to change the affinity of these composites.

Because of the large specific surface areas, these nanocomposites are good candidates for gas sensors. For example, CNTs/conducting polymer nanocomposites exhibit high sensitivity in NH3 detection. Ammonia is one of the important industrial exhaust gases with high toxicity. Liu et al. [69] demonstrated a simple and effective method of NH3 detection by at sensors based on MWCNTs/PANI nanocomposites. The results showed that MWCNTs/PANI had high sensitivity and quick sensor response, good reproducibility and repeatability for NH3 detection. The mechanism of the enhanced sensitivity may be attributed to the increased surface area of PANI, providing more active sites for the adsorption of NH3 molecules. Similar properties were also observed for MWCNTs/Au/PANI nanocomposites [70]. However, the high cost discourages its extensive application. The sensing properties of the CNFs/PPY coaxial nanocables for toxic gases, such as NH3 and HCl detection were also studied [71]. These materials were fabricated by one-step vapor deposition polymerization. This simple process allowed the formation of ultrathin and uniform PPY layer on the CNFs surface, which thickness was dependent on the loaded amount of the monomer. The responses of the CNFs/PPY coaxial nanocables after interaction with NH3 and HCl were dependent on the thickness of the PPY layer on CNFs and exhibited reversible and reproducible performance (**Figure 12**). The resistance change of the CNFs/PPY coaxial nanocables was negligible when the thickness of the PPY layer was smaller than 10 nm. The sensitivity of these nanocables increased significantly with increasing the PPY thickness and then stopped increasing when the PPY layer thickness was larger than the growth limit thickness point (22 nm) [71].

In recent years, conducting polymers with 1-D nanostructured composites have also been used to construct a variety of biosensors because of their large surface area and unique electronic, chemical, and mechanical properties. In biosensors, the detection of H2O2 is important because it is often a product in enzymatic reactions. The sensing performance of coaxial nanowires consisting of a layer of PPY uniformly coated onto aligned CNTs in H2O2 detection makes it attractive for the fabrication of oxidase-based glucose biosensors, because H2O2 is generated in the reaction between glucose and oxygen in the presence of glucose oxidase (GOX) [72]. In these materials the aligned structure of CNTs plays a significant part in

*Conducting Polymer 1-D Composites: Formation, Structure and Application DOI: http://dx.doi.org/10.5772/intechopen.102484*

#### **Figure 12.**

*Variation in normalized resistance change (absolute value) of the CNFs/PPY nanocable sensors after exposure to (a) NH3 (20 ppm), and (b) HCl (20 ppm) vapors as a function of the PPY layer thickness. Inset TEM image of the CNFs/PPY nanocable (reproduced with permission from Ref. [71]).*

#### **Figure 13.**

*The experiment setup of the electrochemical polymerization of porypyrrole propilic acid/anti-rabbit IgG immobilized TiO2 nanowires immunosensor system (reproduced with permission from Ref. [73]).*

glucose determination, shifting its oxidation potential toward less positive values and enhancing the sensitivity of glucose determination. The immunosensor was also constructed based on an antibody/conducting polymer/TiO2 nanowires film [73]. First, TiO2 nanowires were made by hydrothermal synthesis and spin-coated on Au/Ti microelectrodes surface patterned Si/SiO2 substrate. Next, polypyrrole propylic acid (PPA) and antibody composite films were immobilized on the surface of TiO2 nanowires by electrochemically polymerized using pyrrole propylic acid (PA) and anti-rabbit IgG (1o AB) mixture solution, as illustrated in **Figure 13**. The devices designed in these studies showed a linear concentration range of antigen determination between 11.2 μg/mL to 112 μg/mL. The detection sensitivity of these immunosensors was 0.64 A/(g/mL) for the 5 V of the applied voltage and the sensitivity for this voltage was better than that of 6 V and 7 V [73].

#### **3.4 Catalysis**

Catalytic materials are important for the industry and the development of various sensors. Therefore, composites of conducting polymer and 1-D nanostructures also have been studied in this area of research.

The nanowires consisting of gold-coated PANI film exhibit excellent catalytic behavior for the chemical reduction of organic dyes such as methylene blue (MB) and rhodamine B (RhB) [74]. Most of these dyes are not biodegradable and persist in the environment, but they can be disposed of by chemical reduction using a strong reducing agent as an economical route. Unfortunately, the chemical reduction of dyes is a very slow process under ambient conditions. Therefore, these Au/PANI nanowires are used as catalysts for the reduction of MB and RhB dyes in the presence of NaBH4. A total catalytical reduction of MB and RhB was observed. The catalytic activity of composite was much better in comparison to the catalytic performance of individual components [75, 76].

Several 1-D semiconductor materials such as TiO2, ZnO, MnO2, CdS, etc., have also been used as semiconductor photocatalysists. However, their wide band gap and the low quantum yield largely limited the overall photocatalytic efficiency. The photocatalytic performance of this semiconductor can be significantly improved by incorporating them into 1-D structures with conducting polymers. For example, a novel photocatalyst, polypyrrole coated Ag/TiO2 nanofibers, was synthesized using an electrospinning technique, followed by a surfactant *in-situ* chemical polymerization method [77]. This photocatalyst showed obvious visible-light photocatalytic activity in the decomposition of gaseous acetone. The 1.0 wt% PPY/Ag/TiO2 sample provided the optimum photocatalytic activity. In **Figure 14**, photocurrent transient responses of PPY/Ag/TiO2, PPY/TiO2, Ag/TiO2 are compared. The photocurrent followed the order: PPY/Ag/TiO2 > PPY/TiO2 > Ag/TiO2 > pure TiO2. The enhancement of PPY/Ag/TiO2 in photocurrent indicates smaller recombination and more efficient separation of photogenerated electron-hole pairs at its interface. Besides, the recycling test revealed that the PPY/Ag/TiO2 nanofibers were stable and effective for the removal of organic pollutants [77]. The high photoactivity of the PPY/Ag/TiO2 nanofibers can be attributed to the synergistic effect originating from the excited-state electrons in PPY, which can be readily injected into the TiO2 conduction band and next transported to the Fermi level of Ag (**Figure 15**) [77]. Therefore, the combination of conducting polymers with semiconducting materials is an effective strategy for improving photocatalytic activity.

The composites consisting of two or more components containing CNTs and the conducting polymers can be also used as electrocatalysts for hydrogen and alcohol fuel cells [52], as well as bio and microbial fuel cells [78]. Using these materials as a cathode or anode catalysts is an important step in reducing the use of high-cost

#### **Figure 14.**

*(a) Photocurrent transient responses and (b) photocatalytical activity of PPY/Ag/TiO2 and of single and twocomponent samples (reproduced with permission from Ref. [77]).*

*Conducting Polymer 1-D Composites: Formation, Structure and Application DOI: http://dx.doi.org/10.5772/intechopen.102484*

**Figure 15.**

*Postulate mechanism of the visible-light-induced photodegradation of acetone with PPY/Ag/TiO2 nanofibers (reproduced with permission from Ref. [77]).*

platinum and platinum-based electrocatalysts to promote practical applications. Additional optimization of the catalyst structure and stability may improve catalysts performance and reduce the total cost.

### **4. Summary**

This paper reviews study of the formation, properties and applications of conducting polymer 1-D nanocomposites. These materials have attracted much attention due to their unique physical, mechanical, chemical and electrochemical properties that provide nanostructures formed from them with multi-functionality. As described in this review, many synthetic approaches have been developed, including *in-situ* and *ex-situ* chemical and electrochemical method, template and template-free method, vapor-phase polymerization, emulsion polymerization, hydrothermal reaction, redox reaction, electrospinning, and so on, for the fabrication of conducting polymer nanocomposites with carbon nanotubes and nanofibers, 1-D metals, metal oxides, metal complexes, semiconductors, and porphyrins. Nanocomposites synthesized by these methods exhibit many exceptional properties. The size, shape and structure morphology of these composites can be controlled by the conditions of composite formation. 1-D structures exhibit high real surface area. The organization of these structures on the substrate surface provides unique conditions for reactant transport within the 1-D composite film. These composites usually exhibit good conductivity. Such properties enable wide practical applications of 1-D composites. They can be used for charge storage devices construction and as a component of electronic devices. They are applying in a chemical, biochemical and electrochemical sensors. Their catalytic and photocatalytic properties allow the use of these systems to remove environmental pollutants. So, the improvement of the syntheses is sought, which would lead to: (i) highly conducting, (ii) macroscopically uniform materials, (iii) produced at reasonable reaction time, and (iv) in high yield.

Therefore, conducting polymer 1-D nanostructures are useful materials in basic research and technology. Important of synthesis conditions and the production of new 1-D structures can have a significant impact on scientific development. Studies for the preparation of such materials are still desired. It is believed, that the combined chemical, physical, and mechanical properties of these nanocomposite materials are crucial in the development in the future more discoveries will be made in this field.

*Nanocomposite Materials for Biomedical and Energy Storage Applications*
