**3. Carbon nanotubes**

In addition to their high electrical conductivity [24], unique pore structures, and improved power density in supercapacitors, carbon nanotubes (CNTs) have outstanding thermal stability, superior mechanical qualities, and unique pore structures. Powders made from commercially available CNTs are frequently used as collectors, either in conjunction with other pseudocapacitive materials or on their own, or as pseudocapacitive electrode materials [25]. Van der Waals force is the mechanism that allows CNTs in the electrode to link to one another. This increases the electrode's resistance leading to self-discharge as a consequence of poor adhesion. To address these risks, CNTs are grown on collectors, which can take the form of carbon cloth, graphene, stainless steel mesh, or nickel foam [26]. The following are some instances of research on the significance of CNTs in supercapacitor production.

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNT) and carboxymethylcellulose (CMC) were combined by a hydrothermal process [27]. An N-MWCNT/CMC composite had an ultrasonication-mediated solvothermal reaction to produce the material. The good electrochemical characteristics and rapid redox reaction of the composite electrode in the presence of the PVA/H2SO4 gel electrolyte are deduced from the approximately rectangular shape of the cyclic voltammetry (CV) curves. The N-MWCNT/CMC composite electrode displayed a more significant current than the pure N-MWCNT, demonstrating its superior electrochemical performance and the crucial role of the CMC matrix inclusion on the CNTs in enhancing the electrode's capacitance. According to the galvanostatic charge–discharge (GCD) cyclic stability analysis performed for up to 4000 cycles at a scan rate of 2 Ag<sup>−</sup><sup>1</sup> , the N-MWCNT/CMC nanohybrid composite retained 96% of its initial capacity. The charge transfer (Rct) of the electrodes during the first and the one-thousandth cycles, as determined by the Nyquist plots, is approximately 0.9 and 35, respectively. The steady electrochemical characteristics are influenced as a result of this factor. At low frequencies, it was noticed that the phase angle for the impedance plot of the composite electrodes was greater than 45 degrees; this indicates that the composite electrodes have electrochemical capacitive capabilities.

The closed tips and fewer active sites of CNTs can limit their electrochemical performance. Therefore, Zhang and Xie [28] investigated a successful trial to open the tips of CNTs with oxygen and nitrogen functional groups by an effective chemical acid-etching method. The chemical vapor deposition (CVD) technique was used to perform the acidic treatment on the CNTs fabricated. Li+ -based electrolyte provided

#### *Carbon Nanomaterials Based Supercapacitors: Recent Trends DOI: http://dx.doi.org/10.5772/intechopen.106730*

the best electrochemical performance of the functionalized and tip-open CNTs (FTO-CNTs) compared to the other investigated types of ions as Na<sup>+</sup> and Mg2+.

The areal capacitance obtained from GCD curves for the FTO-CNTs indicates improved electrochemical performance. Compared to CNT growth on carbon cloth (CCC) and carbon cloth (CC), FTO-CNTs have the highest areal capacitance due to their largest CV area as determined by CV curves at a scan rate of 20 mV/s. The functionalization and tip-opening of the CNTs may explain the higher capacitance of FTO-CNTs compared to that of CCC. The higher number of oxidation–reduction reactions is responsible for the greatest charge–charge transfer resistance between ions and electrons (Rct) in FTO–CNTs. The movement of ions from the open tip into the interior of the CNTs may be responsible for the higher diffusion resistances (σ) exhibited by FTO-CNTs compared to those of CCC. Since there were more entrance locations for the diffusion of ions in the FTO-CNTs due to their open tips, a scan rate of 10 mV induced a greater diffusion-controlled capacitance (75%) higher than CCC (65%).

Yang et al. developed an innovative method for dealing with polymer waste and high-value-added recycling of resources [29]. In this study, the researchers investigated a great success in treating polypropylene face mask wastes, a source of environmental pollution, to be useful by carbonizing them into CNTs. Yang et al. proposed employing the manufactured waste face mask CNTs as electrode material in supercapacitors to achieve extra financial benefits. The CNTs were produced using Ni–Fe bimetallic catalysts with varying molar ratios NiFeX (X = 1 to 5 and NiFe/Al = 1).

CV curves of CNT samples appeared in approximately rectangular patterns with broad redox peaks. The development of broad redox peaks explained by the insertion of functional groups comprising nitrogen and oxygen on the surface of carbon nanotubes. The best value of the ratio capacitance was detected in the CNT-NiFe3 sample. For the CNTs sample, the electric double-layer capacitance features were proved by results obtained from capacitance performance (CP) curves which provided an isosceles-like triangle at a current density of 1 A/g and a range from −0.8 V to zero V. After 10,000 cycles, CNT-NiFe3 electrodes have high cycling stability with capacitance retention of 85.41% from the initial value. Also, within a current density of 1 Ag<sup>−</sup><sup>1</sup> , they attain a specific capacitance of 56.04 F/g. Due to the bamboo-like shape of the carbon nanotubes, CNT-NiFe3 can be purified to achieve the maximum specific surface area and N-doped concentration.

A green, simple processing protocol proposed by Bathula et al. [30] utilizing mechanochemical grinding to synthesize hybrid nanostructures of cobalt oxide on nitrogen-doped multiwalled carbon nanotubes (Co3O4-NMWCNT). The NMWCNT in its original form exhibited wire-like geomorphology; however, Co3O4 consists of clusters of pieces, and the NMWCNT-Co3O4 composite includes an interconnected tube structure. The electrochemical properties of symmetric devices made with NMWCNT, and NMWCNT-Co3O4 electrodes were studied. CV curves of both electrodes verified the EDLC behavior and Faradic reaction, respectively. The enclosed area of the CV of the NMWCNT-Co3O4 device is nearly twice that of the NMWCNT device, indicating that the Co3O4 and NMWCNT have a synergy effect. Both materials have remarkable rapid charging and discharging potential. Random CV curves illustrated that the form of CV curves for Co3O4-NMWCNT was maintained across all cycles (indicating exceptional structural stability).

In another article, a composite of polypyrrolopyrrolethieno thiophene (PDPT) and carbon nanotube (CNT) was created by Bathula et al. [31] to test its viability as a hybrid electrode material. The structure of PDPT is based on DPP (π-conjugated

polymer), which includes moieties of both sulfur and nitrogen heterocyclic. DPT accumulates a donor-acceptor (D–A) interface utilizing chemical exfoliation suggested for electron-accepting bulk. Ultrasonic vibrations caused exfoliation in this particular investigation. To obtain an intermolecular hydrogen connection and necessary D–A and p–p packing, the authors investigated a successful mixture between CNTs and bulk DPT nanofibers. Afterwards, a standardized PDPT-CNT composite suspension was produced from the accumulation of the insoluble DPT. The GCD results showed the specific capacitance of PDPT-CNT and PDPT are 126, 90, 60, 30, and 10; and 42, 26, 16, 12, and 5 F/g, detected at current densities of 0.5, 1, 2, 3, and 5 A/g, respectively. Moreover, at a power density of 450 W/kg, the PDPT-CNT device has a maximum energy density of 15.7 W.h/kg.

Zhang et al. [32] constructed a novel wire-shaped coaxial supercapacitor with exceptional performance, made of carbon wires (CW)@MnO2/PVA-KOH/carbon nanotubes (CNTs). For the inner electrode, copper wire was utilized as a current collector to solve the problem of the low electric conductivity of MnO2. However, carbon nanotubes generated via in-situ chemical vapor deposition (CVD) served as the outer electrode, with cobalt-based catalyst particles uniformly dispersed across the surface of SiO2. Then, the device was created by removing the SiO2 layer and filling it with a polyvinyl alcohol-KOH (PVA-KOH) gel electrolyte, simultaneously using the hydrothermal method. At a power density of 37 mW cm<sup>−</sup><sup>3</sup> , the wire-shaped supercapacitor had the highest volumetric energy density of 0.16 mWh cm<sup>−</sup><sup>3</sup> , dropping to 0.12 mWh cm<sup>−</sup><sup>3</sup> at 62.6 mW cm<sup>−</sup><sup>3</sup> power density. The asymmetrical quasirectangle shape of a wire-shaped supercapacitor obtained from CV curves indicates its exceptional electrochemical performance. The observed semicircle in the Nyquist curve of electrochemical impedance spectroscopy (EIS) for the wire-shaped supercapacitor at high frequencies describes the resistance of the charge transfer, which has a value of 1815 W. The equivalent internal resistance value was 22.79 W, deduced from the semicircle's intercept with the real axis at high frequencies. The capacitance retention continued high and stable; after 4000 cycles of charging and discharging, the capacitance of the wire-shaped supercapacitor exhibits excellent retention of over 90.38%. From the Ragone plot, at a power density of 37 mW cm3 , the wire-shaped supercapacitor has the greatest volumetric energy density (0.16 mWh cm3 ). More significantly, it may continue to be 0.12 mWh cm3 even when the power density reaches 62.6 mW cm3 .
