**2.1 Dry spinning**

*Science, Technology and Advanced Application of Supercapacitors*

used [4–6].

[17–22].

mentioned properties.

ion diffusion paths [35–37].

tive electrodes.

far higher than that of EDLCs.

operate for extended periods of time, often millions of cycles without losing their energy storage capacities giving them an edge over batteries in how long they can be

Supercapacitors have two main classifications that are based on their charge storage mechanism and the type of electrode materials [4]. The first one, electric double layer capacitor (EDLC), stores charges electrostatically on the electrodeelectrolyte interfaces of the high surface area carbon materials. This process involves physical adsorption of ions at the electrode and electrolyte interface [2]. The second one, pseudocapacitor, on the other hand, stores charges within the electrodes in response to fast surface and near-surface redox reactions [5, 7]. The electrodes are derived from transition metal oxides and conducting polymers. Due to these redox reactions, pseudocapacitors have been reported with energy densities

With the emergence of flexible electronics such as foldable displays [8], soft photo-detectors [9] and bendable field effect transistors [10], flexible supercapacitors have become more popular than ever. They have been found to be suitable in powering portable and flexible electronic devices, and several have been fabricated with lightweight, flexible and possessing high power and energy densities [11–14]. Planar format supercapacitors have been found to have larger volumes and structural limitations which impede their use in lighter, smaller and omnidirectional flexible electronic devices [15, 16]. To solve these problems, lightweight and high energy density fiber-shaped supercapacitors have been explored and fabricated

Fiber electrodes for supercapacitors have been made from active materials with

Polyaniline (PANI) is probably the most widely studied of the conductive polymers because of its high electronic conductivity, redox and ion exchange properties, excellent environmental stability and ease of preparation [32–34]. It has, therefore, been extensively explored in energy storage devices fabricated with pseudocapaci-

Bulk PANI, however, due to its low accessible surface area is not ideal for energy storage device electrodes. The workaround to this has been to fabricate nanostructured PANI materials. These structures have typically been made using a carbon template thereby producing materials with a large area to volume ratio and shorter

In this chapter, we report our high energy density fiber supercapacitors based on CNT-PANI fiber composites. A chemical oxidation polymerization technique is employed to deposit PANI on the surface of the CNT fibers. This composite material gives superior performance as supercapacitor electrodes due to the fast redox reactions between the PANI and the electrolytes used. To create our CNT fibers, we employ a technique that involves dry spinning of multi-walled carbon nanotube (MWCNT) fibers from vertically aligned MWCNT arrays grown by chemical vapor deposition (CVD) as described in a previous publication by our research group [38]. This technique is used to spin continuous fiber at industrial rates from MWCNT arrays of 3 cm width and 4.25 cm length, resulting in fibers with diameters of approximately 55 μm and up to 40 m in length. Next, these fibers underwent atmospheric pressure oxygen plasma functionalization to create oxygen plasma functionalized CNT (OPFCNT) fibers as the base structure for the PANI deposition.

nanostructures, such as CNTs [23–25], graphene [17, 26, 27] and metal oxides [28–30]. However, the most widely studied ones have been CNT fiber electrodes and their composites. This is attributed to CNT's inherent flexibility, high surface area and high electrical conductivity [31]. In their fiber formats, they are highly aligned and have excellent mechanical durability while maintaining all their afore-

**40**

CNT fibers are dry spun from vertically aligned CNT arrays. In our work, thin films of Fe and Co were sputtered on a silicon wafer, overlaid with approximately 5 nm Al2O3 as a buffer layer, by means of physical vapor deposition (PVD). The created structure serves as a catalyst for the growth of aligned CNT arrays on the silicon wafer. This surface-treated substrate was then diced up into required pieces and then exposed to a CVD environment in a FirstNano ET3000 reactor. The resulting CNT array was drawable and spinnable and, by means of twisting and pulling. A homemade setup was used to fabricate highly aligned fibers [38], as shown in **Figure 1**. These pristine CNT fibers, when used to form EDLCs, produce quite low energy densities necessitating the deposition of PANI on them to increase the energy density.

## **2.2 Oxygen plasma functionalization of fibers**

After the fibers were spun, they underwent an atmospheric pressure oxygen plasma functionalization process to improve the wettability of the fibers. This is necessary since carbon-based materials are naturally hydrophobic and need improved wettability to increase the deposition of PANI on the surface of the fiber during the oxidative polymerization process [39–42]. In previous publications [43, 44], carbon-based materials were treated with acids to functionalize them and thereby improve the wettability before polymerization. These involve wet chemistry and as such mostly require multi-step reactions and involve strong chemicals, which affect the bulk properties of the CNT structures. The plasma functionalization process employed in this work is continuous, effective and can be used industrially for extensive lengths of fibers.

Oxygen plasma functionalization was generated by systematically pulling the CNT fiber through a plasma head with a chamber for tubular structures (Surfx Atomflo 400 system). The set-up is shown below in **Figure 2**. The pristine CNT fiber was threaded through the plasma head and affixed to the collector bobbin with double-sided tape. The fiber was pulled through the plasma head at a speed of 0.206 cm/s using the collector bobbin on the motor. This processing led to the functionalization of the fiber with the following plasma parameters: 60 W power, 0.1 L/min oxygen and 15 L/min helium. These parameters were chosen since they ensured the fibers to be functionalized had minimum destruction, checked by Raman spectroscopy.

**Figure 1.** *Carbon nanotube fiber spinning process by twisting and pulling.*

**Figure 2.** *Oxygen plasma functionalization set-up for CNT fibers.*

### **2.3 Chemical oxidation polymerization**

The oxidation of aniline in an acidic aqueous medium using ammonium peroxydisulfate (APS) as an oxidant is widely used and reported in the literature [45, 46]. Emeraldine salt (green color) is the form of PANI obtained during this process [32, 47]. PANI can exist in three oxidation states: leucoemeraldine (fully reduced), emeraldine (partially oxidized) and pernigraniline (fully oxidized) [32, 45–47].

At a pH of less than 2.5, the oxidative polymerization of aniline is a chain reaction [48]. The growth of the chains proceeds by the addition of the monomeric aniline molecules to the active chain ends. The chain growth is terminated after at least one of the reactants in the polymerization runs out. If there is an excess of the APS (oxidant), the resulting polymer remains in the pernigraniline form [49], especially at molar ratios of APS to aniline of over 1.5. If the rate of APS to aniline is equal to 1.25 [50] or aniline is in excess, pernigraniline is reduced to emeraldine at the end of the reaction while aniline is oxidized at the same time to emeraldine [48, 51]. We, therefore, ensured in all our tests that we had excess aniline to promote emeraldine growth, the most thermally and environmentally stable form of PANI [52–54].

The oxygen plasma functionalized CNT (OPFCNT) fibers were cut into 7.5 cm portions and affixed to copper tapes with fast drying silver paint (TedPella Inc.). The copper tapes served as the leads used to connect the devices for electrochemical testing. These electrodes were then placed into 10 ml beakers and put into an ice bath. Aniline monomer dissolved in 1 mol/L HCl and the ammonium persulphate (APS) solution also dissolved in 1 mol/L HCl were then put in the various beakers with fibers at different ratios of aniline to APS. The amount of PANI formed on the fibers was controlled by the ratio of aniline to APS used as well as the time the solution was allowed to polymerize. The fibers were taken out after the polymerization time and rinsed in a beaker with deionized water to wash off the excess PANI.

#### **2.4 Electrode and device fabrication**

Fiber supercapacitors were created using poly (vinyl alcohol) and sulfuric acid (PVA-H2SO4), as well as polyvinylidene fluoride-co-hexafluoropropylene and 1-ethyl-3-ethylimidazolium (PVDF-EMIMBF4) gel electrolytes. The PVA-H2SO4 was made with 10 ml DI water, 2 ml H2SO4 and 1 g PVA. The PVDF-EMIMBF4 gel electrolyte was prepared with 15 ml acetone, 1.5 g PVDF, and 3 ml EMIMBF4. The PVA-H2SO4 was operated at a 1 V window, while the PVDF-EMIMBF4 was operated at a 3.2 V window. The larger voltage window the PVDF-EMIMBF4 allowed enabled

**43**

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites*

us to reach larger energy densities. Devices were made from these fibers by coating them with the gel electrolyte (PVA-H2SO4 or PVDF-EMIMBF4). The fibers were then placed parallel to each other on a weighing sheet, with more electrolyte and

Electrochemical measurements were carried out with an electrochemical workstation (Gamry, Interface 1000) at room temperature. The electrochemical characteristics of the electrodes and devices were evaluated by cyclic voltammetry at various scan rates, galvanostatic charge-discharge tests, and electrochemical

amplitude of 10 mV at the open circuit potential. In a three-electrode configuration test, Ag/AgCl was used as the reference electrode, platinum served as the counter

The capacitance (*C*) of the electrodes and fiber supercapacitors was calculated from the galvanostatic discharge curves at different current densities by using the equation: *C=IΔt/ΔV.* The gravimetric capacitance (*Cm*) and areal capacitance (*CA*) were calculated by the following formula: *Cm = C/m* and *CA = C/A,* respectively. The gravimetric energy density (*Em*) and power density (*Pm*) were calculated by the

expressions: *Em = 1/2(Cm(ΔV)2)/3.6* and *Pm = 3600Em/t*.*Pm* <sup>=</sup> <sup>3600</sup>\_\_\_

and *A* refer to the mass and volume of the device, respectively [40, 55].

copy (Renishaw inVia, 514 nm Ar-ion laser with a laser spot of ~1μm<sup>2</sup>

2 <sup>∗</sup> *Cv* (*V*)2 \_\_\_\_\_\_\_

taken for each sample on at least two different locations.

0.776 to 1.195, signifying the destruction of the carbon sp2

energy density (*EA*) and power density (*PA*) were calculated by the expressions:

the discharge current, *t* is the discharge time, *ΔV* is the operating voltage window, *m*

Scanning electron microscopy (SEM) (FEI XL30, 5 kV) and Raman spectros-

characterize the CNT-PANI. The masses of the fibers were taken on a Sartorius SE2 ultra-microbalance. X-ray photoelectron spectroscopy (XPS) data were obtained using a VG Thermo-Scientific MultiLab 3000 ultra-high vacuum surface analysis

energy. The high-resolution scans for carbon and low-resolution survey scans were

The plasma functionalization of the fiber was confirmed by Raman data **Figure 3a** and XPS data **Figure 3b–d**. From the Raman spectra in **Figure 3a**, we observe an increment in the ratio of intensities between the D and G peak, from

functionalization. In **Figure 3b**, there is a documented increase in the atomic weight percent of oxygen from 9.1% in the pristine state to 28.17% for the oxygen plasma functionalized thread. **Figure 3c** and **d** is deconvoluted high-resolution C1s and O1s peaks from the XPS data, showing the various oxygen functional groups found on the surface of the fiber which is in close agreement with data

PANI-CNT composite fibers were created from four ratios of aniline to APS

(1:1, 2:1, 5:1 and 10:1). The OPFCNT fibers were placed with the chemicals as they polymerized for an hour. From our electrochemical half-cell tests, we observed that a 2:1 aniline to APS ratio gave the best specific capacitance, as seen in **Figure 4a**. Further testing of OPFCNT fibers with varying durations (10 minutes to 6 hours) of polymerization revealed that the composite fibers that

to 10<sup>−</sup><sup>1</sup>

3.6 and *PA = 3600EA***/t***, PV* <sup>=</sup> <sup>3600</sup>\_\_\_

Torr base pressure using an Al Kα source of 1486.6 eV excitation

Hz using sinusoidal voltage

*Em*

*EV*

bonds during plasma

*<sup>t</sup>* The areal

*<sup>t</sup>* where *I* is

) were used to

*DOI: http://dx.doi.org/10.5772/intechopen.80487*

**2.5 Electrode and device characterization**

impedance spectroscopy measurements from 106

electrode and the experiments were run in 1 M Na2SO4.

sealed with Kapton tape.

*EA = 1/2(CA(ΔV)2)/3.6 EV* <sup>=</sup> \_1

system, with ~10<sup>−</sup><sup>9</sup>

**3. Results and discussion**

reported in the literature [39, 56].

us to reach larger energy densities. Devices were made from these fibers by coating them with the gel electrolyte (PVA-H2SO4 or PVDF-EMIMBF4). The fibers were then placed parallel to each other on a weighing sheet, with more electrolyte and sealed with Kapton tape.
