**Abstract**

Flexible and wearable electronic devices are of a high academic and industrial interest. In order to power these devices, there is a need for compatible energy storage units that can exhibit similar mechanical flexibility. Fiber-based devices have thus become increasingly popular since their light-weight, and flexible structure can be easily integrated into textiles. Supercapacitors have garnered a lot of attention due to their excellent cycling durability, fast charge times and superior power density. The primary challenge, however, with electric double layer capacitors (EDLCs), which are part of the supercapacitor family, is that their energy densities are significantly lower compared to those of batteries. Pseudocapacitors, on the other hand, can be designed and created with large energy densities and other outstanding properties typical for supercapacitors. This chapter discusses the fabrication and testing of supercapacitors based on carbon nanotube-polyaniline (PANI) composite fibers. These flexible and light-weight devices are assembled using different electrolytes for comparison. The created in this work PANI-CNT composite devices attain an energy density of 6.16 Wh/kg at a power density of 630 W/kg and retained a capacitance of 88% over 1000 charge-discharge cycles.

**Keywords:** pseudocapacitors, CNT Fibers, PANI, fiber supercapacitors

## **1. Introduction**

Over the last few decades there has been an increase in research into energy storage devices arising from the ever increasing energy demands for applications such as portable electronics and electric transportation [1]. Among portable electronics, wearable electronic devices have created a niche for themselves. Fiber-based devices have become increasingly popular since their thread-type structures can be easily integrated into fabrics and other structures.

Supercapacitors are electrochemical energy storage devices that combine the high-energy-storage capability of conventional batteries with the high-powerdelivery capability of traditional capacitors [2, 3]. Though they have lower stored energies than batteries, they deliver the stored energy in seconds. Supercapacitors 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 used [4–6].

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 far higher than that of EDLCs.

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 [17–22].

Fiber electrodes for supercapacitors have been made from active materials with 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 aforementioned properties.

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 pseudocapacitive electrodes.

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 ion diffusion paths [35–37].

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.

**41**

**Figure 1.**

*Fiber Supercapacitors Based on Carbon Nanotube-PANI Composites*

**2. Electrode and device fabrication and characterization**

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.

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

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

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

**2.2 Oxygen plasma functionalization of fibers**

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

extensive lengths of fibers.

Raman spectroscopy.

**2.1 Dry spinning**
