Carbon Nanotube Based Materials and Devices for Energy Storage Application

## **Chapter 6**

## Carbon Nanotubes: Applications to Energy Storage Devices

*Ruhul Amin, Petla Ramesh Kumar and Ilias Belharouak*

## **Abstract**

Carbon nanotubes (CNTs) are an extraordinary discovery in the area of science and technology. Engineering them properly holds the promise of opening new avenues for future development of many other materials for diverse applications. Carbon nanotubes have open structure and enriched chirality, which enable improvements the properties and performances of other materials when CNTs are incorporated in them. Energy storage systems have been using carbon nanotubes either as an additive to improve electronic conductivity of cathode materials or as an active anode component depending upon structural and morphological specifications. Furthermore, they have also been used directly as the electrode material in supercapacitors and fuel cells. Therefore, CNTs demand a huge importance due to their underlying properties and prospective applications in the energy storage research fields. There are different kinds of carbon nanotubes which have been successfully used in batteries, supercapacitors, fuel cells and other energy storage systems. This chapter focuses on the role of CNTs in the different energy storage and conversion systems and impact of their structure and morphology on the electrochemical performances and storage mechanisms.

**Keywords:** carbon nanotube, energy storage, nanocomposite, batteries, fuel cells, supercapacitor

#### **1. Introduction**

Carbon is one of the most important elements on earth and it plays a crucial role in living organisms and modern technological world either as complex compounds or in its elemental form. Carbon has several allotropes (e.g. graphite, diamond, lonsdaleite, Buckyball and amorphous carbon etc.) and different morphological textures (nanotube, nanowire and graphene). Specific applications in devices and other uses are highly specific to the textures and nature of the allotrope of desired properties. Notably, ever since graphite and diamond were discovered for the first time in 1779, their innovative applications have been growing untill the present. Leveraging the benefits of these carbon morphologies, the journey towards innovation and discovery has continued to advance at a steady pace and almost two centuries later, Sumio Iijima discovered for the first time the existence of multiwalled carbon nanotubes (MWCNTs) and in 1992 he observed single-walled CNTs (SWCNTs) [1]. The synthesis and characterization of CNTs is beyond the scope of this chapter. It should be noted that graphite and CNTs have some characteristic properties and features, that enable them to be used in the energy storage and conversion systems. It is worth mentioning that the carbon nanotubes (CNTs), have been envisioned to potentially impact different areas of science and technology due to their unique properties and structural features [2–4]. Specifically, CNTs have very high tensile strength of 60 GPa and high electronic conductivity reported to be 108 Scm−1 and 107 Scm−1 for single-walled and multi-walled carbon nanotubes, respectively [5, 6]. Besides the potential practical applications in chemical and bio sensors [7, 8], field emission materials [9], catalyst [10], electronic devices [11], CNTs have been used in energy storage and conversion systems like, alkali metal ion batteries [12], fuel cells [13], nano-electronic devices [14] supercapacitors [15], and hydrogen storage devices [16]. The extraordinarily high electronic conductivity of CNTs enable CNT and graphite as an additive to composite electrodes and facilitate activation of poorly conducting electrode materials making them electrochemically active. In this chapter, we emphasize the applications of CNTs in four different areas: alkali metal ion (Li, Na and K) batteries, alkali metal air batteries, supercapacitors, and fuel cells. The underlying governing structural features and morphological impact on the electrochemical performances have been discussed and the specific storage mechanisms are also highlighted.

## **2. Structure and properties of carbon nanotubes**

Carbon nanotubes can be either as single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs). Simply a wrapped graphene sheet with a hallow fiber is the single-walled CNT. On the other hand, a combination and collection of SWCNTs is the multi-walled CNTs. It should be noted that carbon nanotubes are designated as one-dimensional (1D) structures because of the long length-to-diameter ratio (aspect ratio) [17]. The electronic properties of CNTs are associated with the geometrical structure of them which is uniquely specified by a pair of indexes called chiral indexes (n, m). There are three typical types of CNTs can be obtained: armchair (n, n), zigzag (n,0), and chiral (n, m), depending on the orientation of the graphene lattice with respect to the tube axis they are twisted [18–20]. The formation of a single-walled CNT is shown in **Figure 1** by rolling a single graphene sheet in different directions. It is worth to mention that the rolling introduces strain into the carbon bonds oriented circumferentially while the single graphene sheet is made into a tube. This strain will be greater for smaller diameters; therefore, the armchair will be more strained than zigzag single-walled CNTs [21].

**Properties of CNTs***:* Importantly, the local electronic character of carbon nanotubes is highly dependent on the carbon framework arrangements either zigzag or armchair. Also, there is a long-range defect which is formed by displacement or disorientation of standard nanocarbon structures, including hybridization of carbons, vacancies creation, and bond rotations (*Stone − Wales)*. These imperfections are responsible for the chemical, mechanical and optoelectronic properties of CNTs. Noting that this imperfection can modify the electronic properties of CNTs by creating Fermi levels variation and the resulted charge diffusion processes can be affected [22, 23].

The resistivity of CNTs resulted from the electrical conductivity is determined by their carbon framework (graphite) and the one-dimensional character which is regulated by the quantum mechanical properties. The resistance of CNTs is independent of the length of the tube and act as a good conductor in which the highest current density can be as high as 109 A cm−2. This important property of CNTs may improve the rate capability of electrochemical devices like batteries and capacitors. The helicity and diameter of CNT determines either it would be metallic or semiconducting in nature [24]. It should be mentioned that the strong C=C double

#### *Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

bonds in the carbon nanotubes makes them having high Young's modulus in its axial direction and highest tensile strength. Of course, the presence of imperfection/defects in the tube wall reduces the Young's modulus and tensile strength remarkably. However, reported experimental data are significantly smaller than the theoretical predictions which is most probably resulted from the high flexibility and aspect ratio [25, 26]. At room temperature the thermal conductivities of individual SWCNTs is reported up to 6600 W/(m K) which is almost double than the pure diamond [27]. Besides these, the CNTs have many others useful properties such as electro-optic effect, saturable absorption and *Kerr effect* etc. [28, 29].

The favorable and beneficial electrical, mechanical and thermal properties of carbon nanotubes are promising for various electrochemical applications like batteries, supercapacitors, fuel cells and hydrogen storage. Some important properties of SWCNTs and MWCNTs are listed in **Table 1**.

#### **Figure 1.**

*Lattice, two off-set triangular sublattices of graphene and graphene sheet rolling vector map. Reproduced from Ref. [21] with permission from the Royal Society of Chemistry.*


#### **Table 1.**

*Properties of single walled and multi walled nanotubes.*

The values of Young's modulus and tensile strength of CNTs are around 1.2 TPa and 160 GPa, respectively. These unique mechanical properties make CNTs one of the toughest materials and play a vital role in protecting electrode integrity during the charge–discharge cycle of alkali-metal ion batteries. Furthermore, CNT based paper can be used as active material and current collector in supercapacitors, which can reduce the contact resistance as well as electrode weight. The thermal stability of CNTs is also an important property, which can help the composite electrode for stable battery operation at high current rates. SWCNTs and DWCNTs are showing a positive thermal expansion coefficient of 1.9 x 10–5 K–1 and 2.1 x 10–5 K–1, respectively, at room temperature. This negligible thermal expansion coefficient makes CNTs feasible for high energy density battery applications.

## **3. Storage mechanism of carbon nanotubes in electrochemical applications**

CNTs have showed high performance as anode materials and cathode additive for alkali metal ion batteries because of their favorable properties (electrical, mechanical, and structural). The battery electrode based on CNTs attracted attention of many research groups around the world. Recently different modifications in the CNTs have been made for the deployment as a promising electrode material regarding alkali metal ion intercalation, adsorption, and diffusion [31]. In Lithium ion Batteries (LIBs), it has been well established that Li+ ions are stored via two mechanisms, one is intercalation and other one is alloying [32]. The lithium ion storage mechanism in CNTs have been investigated by many research groups. First, let us go into detail about intercalation mechanism in pure carbon nanotubes. Because of different morphologies, the amount of Li<sup>+</sup> ion insertion is not limited to LiC6. The capacities (Li ion storage capacity) is highly dependent to the CNT morphology, especially defects and diameter of the carbon nanotubes [33].

*Defects (n-rings)* can be occured naturally or introduced by treatment (nitric acid treatment or ball milling) as shown **Figure 2**. The theoretical studies (DFT total-energy calculations using local-density approximation (LDA) and the

#### **Figure 2.**

*Types of defects (rings of the red dots) in a (5,5) SWCNT. Reproduced from Ref. [34] with permission from the American Physical Society.*

generalized-gradient approximation (GGA)) were employed to investigate the detailed energetics of lithium ion adsorption on the defective single-wall carbon nanotubes [34]. It turned out that the presence of the holes on CNTs wall increase their capacity which is most likely favorable diffusion of lithium ion into the inside of the carbon nanotubes and reduces the diffusion path length [35, 36].

Another important note is that Li<sup>+</sup> ion can also penetrate the CNTs from its ends. Meunier *et al*., adopted *ab initio* simulations for investigating the lithium ion migration through the ends of open-ended carbon nanotubes [37]. It is obvious that the CNTs should be short in size to allow Li+ ions to freely intercalate/de-intercalate. The theoretical studies indicated that the capacity difference between opened and closed carbon nanotubes was almost 120 mAh g−1 [38]. It is also reported that the reversible Li storage capacity increased from LiC6 in closed ended tubes to LiC3 after etching which might be due to the short and highly defective CNTs generation [33]. Once Li+ ion entered CNT either from the end of tube or through defects, it undergoes one 1D random walk in the tube. Provided that if the tube is very large the Li+ ion will be able enter, however, it will be difficult to exit or never exit. It is indirectly proved by Wang et al., showing that capacity of a short (300 nm) CNTs is much higher than the longer CNTs (micro-meter) [39]. On the other hand, Yang *et al*., investigated the impact of length on electrochemical properties of CNTs. It was observed that the small size CNTs exhibit relatively less charge-transfer resistance than longer CNTs. It is not clearly explained why the lithium ion diffusion coefficients (DLi) of both the long and short CNTs reduces as the intercalation is on progress and voltage drops. It might be due to repulsive interactive as lithium concentration increases in the tube. However, in short CNTs the difference between initial and final value of diffusion was smaller than longer CNTs. Therefore, the investigation indicates that shorter the CNTs length better will be the electrochemical performances [36]. In addition, Wang *et al*., developed solid state cutting method to prepare the short CNTs from micro-meter long CNTS using Nickel Oxide (NiO)

#### **Figure 3.**

*The variation of Li/C ratio as a function of tube diameter [White and grey balls represent C and Li atoms]. Reproduced from Ref. [41] with permission from Elsevier.*

particle as a cutter at 900°C. They successfully obtained short CNTs around 200 nm in length and the measured electrochemical reversible capacities increases as the length of CNT is decreases [40]. The same research group used Iron (II) sulfide (FeS) as a catalyst to produce the short CNTs as well as directly grown short CNTs with length of 200–500 nm. They are able to show that the long CNTs exhibited 188 mAh g−1 while short CNTs 502 mAh g−1 capacity [39].

Furthermore, there is significant relationship between the ratio of lithium-carbon (Li/C) and the diameter of tube. If the tube diameter is bigger, the intercalated lithium atoms gravitated to form multi-shell structural feature when the system is at the equilibrium state (**Figure 3**). These structures with a linear chain in the axis will improve the lithium capacity. It was also reported that the interaction potential at the central region is varied with the diameter of the nanotubes and diameter of 4.68 Å has higher interaction energy, that made CNTs better candidate for lithium ion battery anode material [41, 42].

Another important factor for lithium storage in CNTs is conducting nature of CNTs. There are two different types of CNTs, as mention above, one is semiconducting another one is metallic CNTs based on their chirality. The experimental measurements and modeling studies indicated that if the chiral vector is a multiple of 3, the CNT behaves like metallic; otherwise it would be semiconducting. The metallic CNTs is able to store approximately 5 times more lithium ions than semiconducting CNTs [43].

### **4. Electrochemical applications**

#### **4.1 Carbon nanotubes and their composites for alkali metal ion batteries (Li, Na and K) and other batteries**

As it is discussed above, the one-dimensional carbon nanotube can be obtained as single-walled carbon nanotubes and multiwalled carbon nanotubes. Last 20 years, applications of CNTs are emerging in energy storage research on carbon structures and nano composite materials because of their excellent electrochemical properties including lower density, higher tensile strength, and higher rigidity [44].

**Li-Ion Batteries (LIBs)**: Both single walled and multi walled carbon nanotubes are highly investigated in lithium ion battery either as an anode material or as a conductive additive in the composite electrodes. It is worth mentioning here that the one-dimensional CNTs enable to store higher amount of lithium than the conventional graphitic carbon (specific capacity of 372 mAh g−1). The CNTs exhibits reversible capacities range between 300 and 1250 mAh g−1, depending on structure and morphology and defect concentration [44–47]. The SWCNTs show first discharge capacity around 2500 mAh g−1 with a voltage plateau between 1 and 2 V vs. Li/Li+ . However, after first charge–discharge cycle the voltage profile varies based on the quality of CNTs and their pre-treatment [48]. Yang *et al*., prepared unetched SWCNTs by co-pyrolysis method and the measured capacity was 170 mAh g−1 and 266 mAh g−1 for differently treated two samples [36] although the theoretical studies indicates that the reversible capacities should be more than 1116 mAh g−1 (LiC2 stoichiometry) as it is possible for single walled CNTs [49].

Along with SWCNTs, researchers successfully demonstrated the lithium ion intercalation into MWCNTs [50] (**Figure 4**). It is interesting to note that the specific capacities around 8500 mAh g−1 was reported for multi-walled CNTs at slow current rate (0.1 mA cm−2). On the Contrary, however, most of the carbon nanotubes show capacities typically less than 4000 mAh g−1 [44]. A comparative study has been carried out on highly conductive, binder-free, free-standing flexible films *Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

**Figure 4.**

*(a) Schematic representation of the microstructure of nanotube array and energy storage mechanism and (b) cycle performance of carbon nanotube array (CNTA) electrodes. Reproduced from Ref. [50] with permission from Elsevier.*

made from three different types of carbon nanotubes (SWCNTs, DWCNTs and MWCNTs). They were able to show that the free standing MWCNT film was retain its capacity after hundreds cycles, which is better than other CNTs films [51]. Lahiri *et al*., prepared directly deposited MWCNTs on cooper current collector by chemical vapor deposition (CVD). It exhibits better specific capacity, at high current rate of 3C and good cyclic stability over 50 cycles [52].

Charan *et al.,* prepared aligned multiwalled carbon nanotubes (MWNTs) on stainless-steel foil and obtained high stable specific capacity of 460 mAh g−1 for 1200 cycles at 1C rate [53]. Li et al., synthesized stacked multiwall carbon

nanotubes (MWCNTs) by floating catalyst chemical vapor deposition (FC-CVD) method and observed a stable discharge capacity of 310 mAh g−1 at 0.5 C rate for 300 cycles [54]. Brian *et al*., obtained highest capacity for SWCNT electrodes with using 1 M LiPF6 in Ethylene carbonate: Propylene carbonate: Dimethyl carbonate (EC:PC:DMC) as the electrolyte and capacity retention is more than 95% after 10 cycles [55]. Researchers have using different surface functionalization and doping (N, B) processes for getting efficient Li ion storage in CNTs [56] and highly concentrated N doped CNTs was developed and presented reversible capacity of 494 mAh g−1 which is almost double conventional CNTs capacity [57]. On the other hand, when flexible and free-standing pyridin-B-CNTs film was prepared using one-step floating catalyst chemical vapor deposition method, it delivers high specific capacity with excellent cycle stability of 548 mAh g−1 after 300 cycles at 0.1 A g−1 [56].

Up to now discussion was concentrated on the raw CNTs utilization in lithium ion battery as an anode material. Hereafter the discussion will be focused on the collective data for hybrid nanocomposites by incorporating CNTs into Li-storage compounds as new electrode (anode & cathode) materials. In this composite electrode, significance of π-orbital overlap in metallic type CNTs where electrons can transfer with mean free paths along the length of the nanotube (ballistic transport). So, when it is used as an additive, it will increase rate performance, especially combined with the poor electronic conductive cathode materials. Furthermore, CNTs have the mechanical and electrical properties along with a large surface area which is beneficial for lithium ion battery composite electrode [48]. The CNT was employed in silicon based anode consisting of silicon nanowire/graphene sheet (SiNW@G) which was intertwined architectures [58] where CNT can act either as conductive additive or active component depending on the operation voltage of the cell. The molybdenum dioxide was embedded with multiwalled carbon nanotubes (MoO2/MWCNT) by hydrothermal process where hybrid composite consists of spherical flowerlike MoO2 nanostructures interconnected by MWCNTs and exhibits reversible lithium storage capacity of 1143 mAh g−1 at a current density of 100 mA g−1. The zinc oxide was covered by N-doped carbon freestanding membrane electrodes for lithium ion batteries and the hybrid material shows the high performance with a specific capacity (850 mAh g−1at a current density of 100 mA g−1) and excellent cycling stability [59]. The polymer-derived silicon oxy-carbide/carbon nanotube (SiOC/CNT) composites exhibit stable lithium anode material [60].

The application of carbon nanotubes as an additive for anode or cathode has huge advantages compared to other carbon form like amorphous carbon, acetylene black tc.. As discussed above the CNTs have a high electrical conductivity at room temperature and very small amount (0.2% w/w) of CNTs will be able to create a percolation network for electronic conductivity [61] and therefore, could increase orders of magnitude in electrical conductivity of composite electrodes and form better percolation network. CNTs have been employed as an conducting additive for LiCoO2, LiNi0.7Co0.3O2, LiFePO4, LiMnPO4 and LiNi0.5Mn1.5O4 cathodes; showing better in the reversible capacity of the composite electrodes compared to other carbon polymorphs [62–65].

**Lithium Sulfur Batteries (Li-SBs)**: After LIBs, Lithium sulfur batteries are drawing much attention due to the high energy density of lithium-sulfur (Li-S) batteries (2600 Wh kg−1) and is natural abundance of sulfur. Beside the potential advantages, the major challenge is the electronically insulation behavior of sulfur. In addition, during the cycling processes, the polysulfides are formed which are soluble, and discharge intermediate and products migrate towards Li anode. This impacts the columbic efficiency, accelerates battery self-discharge and cycle life. Several research groups are using CNTs for sulfur encapsulation to overcome above mentioned problems [66]. The sulfur is incorporated carbon nanotubes, nano pores and/or in between nano wires for Li-S battery cathode. The electrode delivers discharge capacity of 669 mAh g−1 after 300 cycles with a low capacity fading rate of 0.166% per cycle at 0.1 C rate [67]. Sometimes functional groups were grafted on the modified multi-walled carbon nanotubes which can adsorb the dissolved polysulfides and enhance the redox reaction of lithium polysulfides and in parallel provides the electronic conduction pathway.

**Sodium Ion Batteries (SIBs):** Off significance, CNTs cannot be used as anode for Na ion batteries, like LIBs, because of large radius of Na ion (1.02 Å) which cannot be intercalated comfortably into the layer structure. The Na ion intercalation into graphite is thermodynamically unstable and it cannot form primary stage structures of NaC6 or NaC8. The Pure graphite can deliver a maximum capacity of ~31 mAh g−1 by forming NaC70 [68]. The defect-rich and disordered carbon nanotube structures have been synthesized for enhance the sodium storage as an anode for SIBs, which exhibits reversible capacities over 130 mAh g−1. Very recently, Han et al., prepared high defective and disorder mesoporous carbon nanotubes by ethanol flame method. The electrode displays a remarkable rate capability of 145 mAh g−1 at 1 A g−1, with excellent cyclability [69]. Another approach to obtain defective CNTs is doping of heteroatoms, such as nitrogen, which can also enhance the electrical conductivity of carbon nanotubes [70].

CNTs have been using as an additive for lower electronic conductive electrode materials in SIBs. It was reported that porous FePO4 nanoparticles were electrically connected by single-wall carbon nanotubes synthesized by hydrothermal reaction. The fabricated composite electrode shows discharge capacity of 120 mAh g−1 at a 0.1 C rate with unprecedented cycling stability [71]. The CNTs have been using as a promising additive for polyhedral cathode materials like NaTi2(PO4)3, Na3V2(PO4)2F3, Na2FePO4F, NaVPO4F, Na4VMn(PO4)3, Na4MnCr(PO4)3, Na3V2 (PO4)3, Na2Fe(SO4)2, Na2MnSiO4, Na3V2O2x(PO4)2F3-2x, Na4Co3(PO4)2P2O7, Prussian blue analogues …etc. [72]. Our group published the impact of MWCNT on particle growth as well as electrochemical properties of Na3V2O2x(PO4)2F3-2x cathode. Among three carbon sources (Carbon, MWCNT & rGO), MWCNT is more effective to obtain moderate particle size with enhanced electrochemical properties (**Figure 5**). The prepared Na3V2O2x(PO4)2F3-2x-MWCNT composite delivers the stable capacity of 98 and 89 mAh g−1 in half cell and full cell with NaTi2(PO4)3-MWCNT configurations, respectively [73]. It should be noted that most of the alloying and conversion anode materials lose their electron conducting path due to the pulverization during charge–discharge cycles. In this case, CNT can be used as conductive additive as well as electrode integrity protector. The battery research community has been encapsulated metal based (e.g. Sn) anode with the CNTs to accommodate the volume expansion during Na insertion to avoid the pulverization. The reported results indicate that the carbon encapsulated, Sn@Ndoped, nanotubes is beneficial to get good reversible capacity of 398 mAh g−1 at 100 mA g−1, with capacity retention of 67% over 150 cycles [74, 75]. The ultrathin MoS2 nanosheets was developed on the surfaces of CNTs by a hydrothermal method MoS2/CNTs, which exhibit excellent electrochemical performance as conversion anode materials for SIBs. The MoS2/CNTs, shows a reversible capacity of 504 mAh g−1 at a current rate of 50 mA g−1 over 100 cycles [76]. Many alloying and conversion anode materials have used CNTs as conductive additive, examples TiO2, MoS2, CuS, Fe2O3, & FeO.

**Potassium Ion Batteries (PIBs):** Unlike sodium, potassium ion can be intercalated into graphite structure without requiring a special electrolyte solvent for K-ion batteries (PIBs). It was reported that theoretical capacity of K battery is 279 mAh g−1 (KC8) by stepwise potassiation through KC36 and KC24 phases based on

**Figure 5.**

*(a) Cyclability of Na3V2O2x(PO4)2F3-2x along with three different carbon materials. Charge-discharge curves for the Na3V2O2x(PO4)2F3-2x with (b) carbon, (c) MWCNT, and (d) rGO. Reproduced from Ref. [73] with permission from Springer Nature.*

intercalation/deintercalation mechanism [77]. Noting that the insertion potential of K+ into the graphite structure is little higher than that of Li<sup>+</sup> , which could make more secure battery systems. However, the biggest obstacle is the poor cycle stability of graphite as the anode for PIBs. Battery community have been trying to improve the performances of PIBs and fulfill the requirements of commercialization [78]. Liu *et al*., prepared N-doped bamboo-like carbon nanotubes by simple pyrolysis method and the unique structured material shows a high reversible capacities of 204 mAh g−1 and 186 mAh g−1 at 500 mA g−1 and 1000 mA g−1, respectively [79]. The science behind the better performance is not well understood yet.

The analysis of electron density difference demonstrates the interaction between the K ion and the nitrogen doped CNTs which has strong ionic bonding, and the electron re-distributions between N5 & N6 CNTs. It is shown, in the K ion –N5 CNT systems (**Figure 6A**), the net gain of electronic charge on the pyrrolic N atom plays more significant role than those of the other two pyridinic N atoms. The N6 CNT (**Figure 6B**), the alkali metal atom associates strongly with two pyridinic N atoms, therefore, the overlapping of the corresponding peaks in **Figure 6** (bottom) is seen. The bonding with the third pyridinic N atom is relatively weaker [80]. The theoretical studies predicted that inner carbon of CNT is dense while outer carbon of CNT is loosely bind. The hierarchical carbon nanotubes structures in the inner dense part act as skeleton while the outer loose-CNT effectively accommodates the K-ion accommodation, which are showing a better specific capacity of 232 mAh g−1 and good cyclic stability [81]. Like other electrode systems these carbon nanotubes are expected to act as a conducting additive assuring the electrical percolation in the composite electrode and to protect the integrity of electrode using their mechanical properties [82, 83].

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

**Figure 6.**

*Differential electron densities (A) K-ion on N5 CNT, (B) K-ion on N6 CNT: top, side view; middle, top view; bottom, electron density differences in the plane. Reproduced from Ref. [80] with permission from the Royal Society of Chemistry.*

**Lead acid batteries***:* Lead acid battery is one of the most popular electrochemical storage systems for the last 150-years, however, it has been suffering from poor lifecycle. The limited lifecycle is most probably originated due to the formation of large non-conducting uncontrolled lead sulfate (PbSO4) crystals both the positive and negative plates. The deposition of insulating PbSO4 crystals lower the electrical conductivity and accessibility of electrolytes to active material in both plates [84, 85]. Various research groups studies different amorphous carbons as a sulfation-suppressing additive in negative plates, because the sulfation is more prominent in negative plate than positive plate due to slower kinetic reaction. Recently, Prof. Aurbach and his group used SWCNTs as a suppresser of uncontrolled sulfation processes in lead-acid battery electrodes. The carbon nanotubes additive would be uniformly distributed throughout the composite electrode and capable of boosting charge acceptance at low concentrations [86, 87].

#### **4.2 Metal-air batteries**

**Lithium-Air Batteries (LABs***):* Recently battery community focused on the metal-air battery due to higher theoretical density. It is just an alternative to LIBs. The most popular and promising metal−air batteries are lithium -air and zincair batteries. The energy density of rechargeable lithium-air batteries very high energy (~1700 Wh kg−1) comparable to gasoline and much higher than secondary Li-ion batteries (~160 Wh kg−1). The reaction mechanism of lithium air battery is appeared to be very simple, at discharge state oxygen in air reduced by lithium ions to form lithium peroxides via 2Li<sup>+</sup> + 2e− + O2 ⇔ Li2O2 and/or 4Li+ + 4e− + O2 ⇔ 2Li2O, and formation of lithium and oxygen from decomposition of lithium oxides during charge processes. The thermodynamic equilibrium cell voltage for the discharge reaction in LABs is 2.96 V vs. Li/Li+ [88]. In practical realization, the reported cell voltage is less than 2.96 V which is due to the cell polarization resulted from the oxygen reduction and evolution reaction during discharge and charge processes. However, breaking the discharge products during the charge processes, it requires much more than 2.96 V to drive the reverse electrochemical reaction. Either pure catalyst or carbon -supported catalyst particles have been used to accelerate the electrode reactions [89]. It should be mentioned that most of the time CNTs have been used as conductive supporting materials for metal and metal oxide catalyst particles in the metal-air batteries. The functionalized CNTs can also be used as air electrodes. It was reported that the flexible multiwalled carbon nanotube exhibited very high specific capacity of 34,600 mAh g−1 at a current density of 500 mA g−1 in the Li–O2 batteries [90]. It is indicated that CNTs have huge prospectuses as in the Li-air battery cathode component.

**Zn-Air batteries (ZAB)**: Zinc−air batteries are very safe for electrical vehicles which is fabricated by non-flammable and non-explosive materials. They can be used for other safe applications. As mentioned above, the electrocatalysts is required in air electrode to efficiently accelerate the kinetics of the oxygen reactions [91] and increases the battery performances and efficiencies. It is demonstrated that the nitrogen-doped carbon nanotubes (N-CNTs) promoted notable ORR activity in acid and alkaline solutions. This is because of the inserted heterogeneous nitrogen which might activates the reaction sites and can induce in breaking the O-O bonds of O2 molecules [92].

Another critical role of CNTs in batteries is the current collector. Present, flexible CNTs based carbon papers can be fabricated from all CNTs and used as anode and current collector for aqueous battery systems. Conventional current collectors, such as carbon cloth and metal foils (stainless steel, Titanium), are low surface area and highly corrosive in aqueous media. Also, these CNTs can be used as a pure binder in primary thermal battery electrode fabrication. The electrode with the CNTs binder has better thermal stability than conventional organic binders. The traditional organic binders were decomposed before reaching the operating temperature of 500°C, and its residual material can act as an insulator.

#### **5. Fuel-cells (FCs)**

The reaction mechanism in Li-air battery and fuel cells has great similarity where oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are important for fuel cells efficiency. To enhance the efficiency of the fuel cell, a catalyst is needed. Instead of using expensive Pt as a catalyst, researchers started using a supporter, which can improve the capability of low-cost catalyst. Commonly used catalysts supporters are porous carbon, carbon nanotubes, graphene, and

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

other carbon polymorphs. It was demonstrated, at higher current density, CNT supported FCs, exhibited better electrochemical performances than the carbon black supported FCs [93]. Doping with heteroatoms or loading of transition metal catalysts on CNTs substantially enhance the activity of highly efficient fuel cells. There are few reports on encapsulation of Ag, Fe, Co, CuSe (**Figure 7**) & Ni based compounds in pure CNTs, which are showing the high ORR performance in fuel cells [94]. It is also reported that the higher oxidation state of Ni is very active for OER and inactive to ORR. However, Ni encapsulated N doped CNTs are showing very high ORR activity and less OER active. Several studies are compared the performances of the platinum catalyst with non-noble metal catalysts with the CNT support and they exhibit better catalytic activity and it reduces the cost of whole cell. Furthermore, CNTs can make the fuel cell highly stable and high resistive against corrosion during electrochemical reaction [94, 95]. CNTs not only increase the catalytic activity; enhance the corrosion resistance. Besides, CNTs improve the mass transmission capability of both electrodes in a fuel cell.

#### **5.1 Supercapacitors**

The morphology of electrode materials and fabrication process plays an important role for the performance of a supercapacitor. The capacitance value of a supercapacitor is highly dependent on electrode surface-area and porosity. The basic principle of a capacitor is to store energy by separation of charge at the electrode and electrolyte interface (i.e., double layer capacitance). The ions transfer between the two electrodes is mediated by diffusion across the electrolyte [96]. Supercapacitors exhibits better reversibility, higher power density, and longer cycle life which made it attentive and promising for energy-storage devices. It is worth to mention that supercapacitors exhibit the highest known power capability (2–5 kW kg−1), but they suffer from a moderate energy density (3–6 Wh kg−1).

#### **Figure 7.**

*Carbon nanotubes decorated with copper selenide (CuSe) nanoparticles for microbial fuel cells. Reproduced from Ref. [94] with permission from Elsevier.*

Carbon nanotubes (CNTs) are very promising as supercapacitor electrode materials because of their excellent electrical properties and one-dimensional nanostructures. Noting that defect free or less defect CNTs has smaller surface area and micropore content than conventional activated carbon (AC), which made them insufficient capacitance in CNT-based electrodes. However, it is reported that the formation of defects on surface and open ends by alkaline solution activation increases the surface area of CNTS [97] and exhibits better capacitance value. The SWCNTs show enhanced specific capacitance than those of MWCNTs which results from large surface area of SWCNTs. However, that MWCNTs could generate capacitance twice as high in comparison to SWCNTs which is attributed to the presence of mesopores and entangled tube structure, facilitating the transport of the ions [98]. The flexible aligned SWCNTs with high surface area and better electrical conductivity is beneficial for capacitors applications [99]. It should be mentioned that contact resistance reduces the performance of supercapacitor and therefore, polished metal foils is used as current collectors to grow the carbon nanotubes for lowering contact resistance. The better discharge efficiency can be obtained through the electrodynamics and can result high power density [100]. The cell resistance can be lower either by fabricating carbon nanotubes as thin film electrodes which has coherent structures with highly concentrated colloidal suspension or fabricating CNT based thin film electrodes using an electrophoretic deposition (EPD) method. It is reported that these flexible CNTs films are binder free and forms network with negligible electrode resistance [101]. As we mentioned in above applications, N doped CNTs may contribute to improving the power characteristics of supercapacitors their own way. The doped nitrogen modifies the conduction band and the modified electronic structure which helps to enhance the quantum capacitance and electrical conductivity of CNTs [102]. Recently researchers have started the fabrication of a high-performance wire-type supercapacitor with CNTs to get the high voltage and high energy density (**Figure 8**). It should be noted that the carbon nanotube sheets were wrapped to make a fiber shaped supercapacitors on elastic polymeric fibers with moderate stretch ability [103, 104].

#### **Figure 8.**

*(a) Schematic representation of the wire-type supercapacitor, (b) galvanostatic charge/discharge curves and (c) Comparison plots of areal capacitance versus current density for CF electrodes coated with CNT, CNT-IL, Ppy/CNT-Ionic Liquid, and Ppy/CNT-Ionic Liquid/AuNP. Reproduced from Ref. [104] with permission from Elsevier.*

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

Graphitization and pore size distribution of CNT are also significant factors for supercapacitor application. While heating, the specific surface area increases, but the capacitance decreased due to the average pore diameter decreases and saturated at high temperature. Furthermore, chemically activated of CNTs also shows tubular morphology with defects on the surface that gave a significant increase in pore volume. Aligned CNTs can also significantly improve the capacitance and power density of supercapacitors. It is also reported that the highly packed and aligned CNTs showed higher capacitance and less capacitance drop when compared to other thick CNT based electrodes.

## **6. Conclusions**

One-dimensional carbon nanotubes (CNTs) have been considered as potential candidates for the development of energy storage materials based on their unique chemical and physical properties. The architecture and quality of the CNTs plays a vital role on the electrochemical performances exhibited by both batteries and supercapacitors. It is observed that a slight modification (defects creation, heteroatoms doping & controlling the distribution of pore sizes) in the CNT structure brings out complementary properties that translate to excellent electrochemical performances. Anchored and directly grown aligned structure of CNTs trends to have high stability and fast ion transportation. The composite electrode with incorporated CNTs is being benefited from the high surface area, excellent conductivity, enhanced specific capacity, better cyclability and rate capability. CNTs can be used as an electrochemically active and inactive electrode component in energy storage systems. It turns out that all types of CNTs can serve as flexible supporting materials and can also enable next generation flexible energy storage devices. The future of advanced energy storage systems (either batteries or supercapacitor) can certainly be benefited from the incorporation of CNTs. The extraordinarily high electronic conductivity also enables CNTs and graphite as an additive to the composite electrode and enable to activate poorly conducting electrode materials to make them electrochemically active. Moreover, the structures and morphologies of CNTs are beneficial for supercapacitors and as catalyst support for fuel cells.

### **Acknowledgements**

This manuscript has been supported by Oak Ridge Nation Laboratory (ORNL) managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE).

## **Author details**

Ruhul Amin1 \*, Petla Ramesh Kumar2 and Ilias Belharouak1

1 Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

2 Department of Applied Chemistry, Tokyo University of Science, Tokyo, Japan

\*Address all correspondence to: aminr@ornl.gov

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

## **References**

[1] Iijima S, Ichihashi T, Ando Y. Pentagons, heptagons and negative curvature in graphite microtubule growth. Nature. 1992;356(6372):776-778.

[2] Iijima S. Helical microtubules of graphitic carbon. Nature. 1991;354(6348):56-58.

[3] Iijima S, Ajayan PM, Ichihashi T. Growth model for carbon nanotubes. Phys Rev Lett. 1992;69(21):3100-3103.

[4] Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature. 1993;363(6430):603-605.

[5] Yu M-F, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load. Science. 2000;287(5453):637.

[6] Ando Y, Zhao X, Shimoyama H, Sakai G, Kaneto K. Physical properties of multiwalled carbon nanotubes. Int J Inorg Mater. 1999;1(1):77-82.

[7] Zhang Y, Bunes BR, Wu N, Ansari A, Rajabali S, Zang L. Sensing methamphetamine with chemiresistive sensors based on polythiopheneblended single-walled carbon nanotubes. Sensors and Actuators B: Chemical. 2018;255:1814-1818.

[8] Shobin LR, Manivannan S. Silver nanowires-single walled carbon nanotubes heterostructure chemiresistors. Sensors and Actuators B: Chemical. 2018;256:7-17.

[9] Rakhi RB, Sethupathi K, Ramaprabhu S. Electron field emission properties of conducting polymer coated multi walled carbon nanotubes. Appl Surf Sci. 2008;254(21):6770-6774.

[10] Yan Y, Miao J, Yang Z, Xiao F-X, Yang HB, Liu B, et al. Carbon nanotube catalysts: recent advances in synthesis, characterization and applications. Chem Soc Rev. 2015;44(10):3295-3346.

[11] Saito S. Carbon Nanotubes for Next-Generation Electronics Devices. Science. 1997;278(5335):77.

[12] Landi BJ, Ganter MJ, Cress CD, DiLeo RA, Raffaelle RP. Carbon nanotubes for lithium ion batteries. Energy Environ Sci. 2009;2(6):638-654.

[13] Akbari E, Buntat Z. Benefits of using carbon nanotubes in fuel cells: a review. International Journal of Energy Research. 2017;41(1):92-102.

[14] Collins PG, Zettl A, Bando H, Thess A, Smalley RE. Nanotube Nanodevice. Science. 1997;278(5335):100.

[15] Pan H, Li J, Feng Y. Carbon nanotubes for supercapacitor. Nanoscale research letters. 2010;5(3):654-668.

[16] Cheng H-M, Yang Q-H, Liu C. Hydrogen storage in carbon nanotubes. Carbon. 2001;39(10):1447-1454.

[17] Dresselhaus MS, Dresselhaus G, Saito R. Physics of carbon nanotubes. Carbon. 1995;33(7):883-891.

[18] Monea BF, Ionete EI, Spiridon SI, Ion-Ebrasu D, Petre E. Carbon Nanotubes and Carbon Nanotube Structures Used for Temperature Measurement. Sensors. 2019;19(11).

[19] Amelinckx S, Lucas A, Lambin P. Electron diffraction and microscopy of nanotubes. Rep Prog Phys. 1999;62(11):1471-1524.

[20] Belin T, Epron F. Characterization methods of carbon nanotubes: a review. Materials Science and Engineering: B. 2005;119(2):105-118.

[21] Hodge SA, Bayazit MK, Coleman KS, Shaffer MSP. Unweaving the rainbow: a review of the relationship between single-walled carbon nanotube molecular structures and their chemical reactivity. Chem Soc Rev. 2012;41(12):4409-4429.

[22] Zandiatashbar A, Lee G-H, An SJ, Lee S, Mathew N, Terrones M, et al. Effect of defects on the intrinsic strength and stiffness of graphene. Nature Comm. 2014;5(1):3186.

[23] Terrones H, Lv R, Terrones M, Dresselhaus MS. The role of defects and doping in 2D graphene sheets and 1D nanoribbons. Rep Prog Phys. 2012;75(6):062501.

[24] Wei BQ, Vajtai R, Ajayan PM. Reliability and current carrying capacity of carbon nanotubes. Appl Phys Lett. 2001;79(8):1172-1174.

[25] Pugno NM. The role of defects in the design of space elevator cable: From nanotube to megatube. Acta Mater. 2007;55(15):5269-5279.

[26] Robertson DH, Brenner DW, Mintmire JW. Energetics of nanoscale graphitic tubules. Physical Review B. 1992;45(21):12592-12595.

[27] Hone J, Batlogg B, Benes Z, Johnson AT, Fischer JE. Quantized Phonon Spectrum of Single-Wall Carbon Nanotubes. Science. 2000;289(5485):1730.

[28] Yamashita S. Nonlinear optics in carbon nanotube, graphene, and related 2D materials. APL Photonics. 2018;4(3):034301.

[29] Matsuda K. 1 - Fundamental optical properties of carbon nanotubes and graphene. In: Yamashita S, Saito Y, Choi JH, editors. Carbon Nanotubes and Graphene for Photonic Applications: Woodhead Publishing; 2013. p. 3-25.

[30] Eder D. Carbon Nanotube− Inorganic Hybrids. Chem Rev. 2010;110(3):1348-1385.

[31] Xiong Z, Yun YS, Jin H-J. Applications of Carbon Nanotubes for Lithium Ion Battery Anodes. Materials. 2013;6(3).

[32] de las Casas C, Li W. A review of application of carbon nanotubes for lithium ion battery anode material. J Power Sources. 2012;208:74-85.

[33] Shimoda H, Gao B, Tang XP, Kleinhammes A, Fleming L, Wu Y, et al. Lithium Intercalation into Opened Single-Wall Carbon Nanotubes: Storage Capacity and Electronic Properties. Phys Rev Lett. 2001;88(1):015502.

[34] Nishidate K, Hasegawa M. Energetics of lithium ion adsorption on defective carbon nanotubes. Physical Review B. 2005;71(24):245418.

[35] Mi CH, Cao GS, Zhao XB. A non-GIC mechanism of lithium storage in chemical etched MWNTs. Journal of Electroanalytical Chemistry. 2004;562(2):217-221.

[36] Yang S, Huo J, Song H, Chen X. A comparative study of electrochemical properties of two kinds of carbon nanotubes as anode materials for lithium ion batteries. Electrochim Acta. 2008;53(5):2238-2244.

[37] Meunier V, Kephart J, Roland C, Bernholc J. Ab Initio Investigations of Lithium Diffusion in Carbon Nanotube Systems. Phys Rev Lett. 2002;88(7):075506.

[38] Prem Kumar T, Ramesh R, Lin YY, Fey GT-K. Tin-filled carbon nanotubes as insertion anode materials for lithiumion batteries. Electrochem Commun. 2004;6(6):520-525.

[39] Wang XX, Wang JN, Su LF. Preparation and electrochemical *Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

performance of ultra-short carbon nanotubes. J Power Sources. 2009;186(1):194-200.

[40] Wang XX, Wang JN, Chang H, Zhang YF. Preparation of Short Carbon Nanotubes and Application as an Electrode Material in Li-Ion Batteries. Adv Funct Mater. 2007;17(17):3613-3618.

[41] Garau C, Frontera A, Quiñonero D, Costa A, Ballester P, Deyà PM. Ab initio investigations of lithium diffusion in single-walled carbon nanotubes. Chem Phys. 2004;297(1):85-91.

[42] Zhao M, Xia Y, Liu X, Tan Z, Huang B, Li F, et al. Curvature-induced condensation of lithium confined inside single-walled carbon nanotubes: Firstprinciples calculations. Phys Lett A. 2005;340(5):434-439.

[43] Hosam MS, Martin K. Introductory Chapter: Carbon Nanotubes. 2019.

[44] Carter R, Oakes L, Cohn AP, Holzgrafe J, Zarick HF, Chatterjee S, et al. Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries. J Phys Chem, C. 2014;118(35):20137-20151.

[45] Eom J-Y, Kwon H-S. Effects of the chemical etching of single-walled carbon nanotubes on their lithium storage properties. Mater Chem Phys. 2011;126(1):108-113.

[46] Eom J, Kim D, Kwon H. Effects of ball-milling on lithium insertion into multi-walled carbon nanotubes synthesized by thermal chemical vapour deposition. J Power Sources. 2006;157(1):507-514.

[47] DiLeo RA, Castiglia A, Ganter MJ, Rogers RE, Cress CD, Raffaelle RP, et al. Enhanced Capacity and Rate Capability of Carbon Nanotube Based

Anodes with Titanium Contacts for Lithium Ion Batteries. ACS Nano. 2010;4(10):6121-6131.

[48] Liu X-M, Huang Zd, Oh Sw, Zhang B, Ma P-C, Yuen MMF, et al. Carbon nanotube (CNT)-based composites as electrode material for rechargeable Li-ion batteries: A review. Compos Sci Technol. 2012;72(2):121-144.

[49] Zhao J, Buldum A, Han J, Ping Lu J. First-Principles Study of Li-Intercalated Carbon Nanotube Ropes. Phys Rev Lett. 2000;85(8):1706-1709.

[50] Zhang H, Cao G, Wang Z, Yang Y, Shi Z, Gu Z. Carbon nanotube array anodes for high-rate Li-ion batteries. Electrochim Acta. 2010;55(8):2873-2877.

[51] Chew SY, Ng SH, Wang J, Novák P, Krumeich F, Chou SL, et al. Flexible free-standing carbon nanotube films for model lithium-ion batteries. Carbon. 2009;47(13):2976-2983.

[52] Lahiri I, Oh S-W, Hwang JY, Cho S, Sun Y-K, Banerjee R, et al. High Capacity and Excellent Stability of Lithium Ion Battery Anode Using Interface-Controlled Binder-Free Multiwall Carbon Nanotubes Grown on Copper. ACS Nano. 2010;4(6):3440-3446.

[53] Masarapu C, Subramanian V, Zhu H, Wei B. Long-Cycle Electrochemical Behavior of Multiwall Carbon Nanotubes Synthesized on Stainless Steel in Li Ion Batteries. Adv Funct Mater. 2009;19(7):1008-1014.

[54] Li J, Kaur AP, Meier MS, Cheng Y-T. Stacked-cup-type MWCNTs as highly stable lithium-ion battery anodes. Journal of Applied Electrochemistry. 2014;44(1):179-187.

[55] Landi BJ, Ganter MJ, Schauerman CM, Cress CD, Raffaelle RP. Lithium Ion Capacity of Single Wall Carbon Nanotube

Paper Electrodes. J Phys Chem, C. 2008;112(19):7509-7515.

[56] Wang L, Guo W, Lu P, Zhang T, Hou F, Liang J. A Flexible and Boron-Doped Carbon Nanotube Film for High-Performance Li Storage. Frontiers in Chemistry. 2019;7(832).

[57] Li X, Liu J, Zhang Y, Li Y, Liu H, Meng X, et al. High concentration nitrogen doped carbon nanotube anodes with superior Li+ storage performance for lithium rechargeable battery application. J Power Sources. 2012;197:238-245.

[58] Wang B, Li X, Luo B, Zhang X, Shang Y, Cao A, et al. Intertwined Network of Si/C Nanocables and Carbon Nanotubes as Lithium-Ion Battery Anodes. ACS Appl Mater Interfaces. 2013;5(14):6467-6472.

[59] Zhang H, Wang Y, Zhao W, Zou M, Chen Y, Yang L, et al. MOF-Derived ZnO Nanoparticles Covered by N-Doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes. ACS Appl Mater Interfaces. 2017;9(43):37813-37822.

[60] Bhandavat R, Singh G. Stable and Efficient Li-Ion Battery Anodes Prepared from Polymer-Derived Silicon Oxycarbide–Carbon Nanotube Shell/ Core Composites. J Phys Chem, C. 2013;117(23):11899-11905.

[61] Landi BJ, Raffaelle RP, Heben MJ, Alleman JL, VanDerveer W, Gennett T. Single Wall Carbon Nanotube−Nafion Composite Actuators. Nano Lett. 2002;2(11):1329-1332.

[62] Jin EM, Jin B, Park KH, Gu H-B, Park G-C, Kim K-W. Electrochemical Characteristics of Lithium Iron Phosphate with Multi-Walled Carbon Nanotube for Lithium Polymer Batteries. J Nanosci Nanotech. 2008;8:5057-5061.

[63] Guoping W, Qingtang Z, Zuolong Y, MeiZheng Q. The effect of different kinds of nano-carbon conductive additives in lithium ion batteries on the resistance and electrochemical behavior of the LiCoO2 composite cathodes. Solid State Ionics. 2008;179(7):263-268.

[64] Chiu T-M, Barraza-Fierro JI, Castaneda H. Comprehensive Interfacial Mechanisms of LiMnPO4-MWCNT Composite ratios in Acidic Aqueous Electrolyte. Electrochim Acta. 2017;253:93-103.

[65] Van Le T, Le MLP, Van Tran M, Nguyen NMT, Luu AT, Nguyen HT. Fabrication of Cathode Materials Based on Limn2o4/Cnt and Lini0.5mn1.5o4/ Cnt Nanocomposites for Lithium Ion Batteries Application. Materials Research. 2015;18:1044-1052.

[66] Shi Z, Yang Y, Huang Y, Yue H, Cao Z, Dong H, et al. Organic Alkali Metal Salt Derived Three-Dimensional N-Doped Porous Carbon/Carbon Nanotubes Composites with Superior Li–S Battery Performance. ACS Sustainable Chem Eng. 2019;7(4):3995-4003.

[67] Xu Y-W, Zhang B-H, Li G-R, Liu S, Gao X-P. Covalently Bonded Sulfur Anchored with Thiol-Modified Carbon Nanotube as a Cathode Material for Lithium–Sulfur Batteries. ACS Applied Energy Materials. 2020;3(1):487-494.

[68] Xiao B, Rojo T, Li X. Hard Carbon as Sodium-Ion Battery Anodes: Progress and Challenges. ChemSusChem. 2019;12(1):133-144.

[69] Han W, Zhou Y, Zhu T, Chu H. Combustion synthesis of defect-rich carbon nanotubes as anodes for sodium-ion batteries. Appl Surf Sci. 2020;520:146317.

[70] Li D, Zhang L, Chen H, Ding L-x, Wang S, Wang H. Nitrogen-doped

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

bamboo-like carbon nanotubes: promising anode materials for sodium-ion batteries. Chem Commun. 2015;51(89):16045-16048.

[71] Liu Y, Xu Y, Han X, Pellegrinelli C, Zhu Y, Zhu H, et al. Porous Amorphous FePO4 Nanoparticles Connected by Single-Wall Carbon Nanotubes for Sodium Ion Battery Cathodes. Nano Lett. 2012;12(11):5664-5668.

[72] Kumar PR, Essehli R, Yahia HB, Amin R, Belharouak I. Electrochemical studies of a high voltage Na4Co3(PO4)2P2O7–MWCNT composite through a selected stable electrolyte. RSC Adv. 2020;10(27):15983-15989.

[73] Kumar PR, Jung YH, Kim DK. Influence of carbon polymorphism towards improved sodium storage properties of Na3V2O2x(PO4)2F3-2x. J Solid State Electrochem. 2017;21(1):223-232.

[74] Luu THT, Duong DL, Lee TH, Pham DT, Sahoo R, Han G, et al. Monodispersed SnS nanoparticles anchored on carbon nanotubes for highretention sodium-ion batteries. J Mater Chem A. 2020;8(16):7861-7869.

[75] Ruan B, Guo H-p, Hou Y, Liu Q, Deng Y, Chen G, et al. Carbon-Encapsulated Sn@N-Doped Carbon Nanotubes as Anode Materials for Application in SIBs. ACS Appl Mater Interfaces. 2017;9(43):37682-37693.

[76] Zhang S, Yu X, Yu H, Chen Y, Gao P, Li C, et al. Growth of Ultrathin MoS2 Nanosheets with Expanded Spacing of (002) Plane on Carbon Nanotubes for High-Performance Sodium-Ion Battery Anodes. ACS Appl Mater Interfaces. 2014;6(24):21880-21885.

[77] Jian Z, Luo W, Ji X. Carbon Electrodes for K-Ion Batteries. J Am Chem Soc. 2015;137(36):11566-11569. [78] Fan L, Ma R, Zhang Q, Jia X, Lu B. Graphite Anode for a Potassium-Ion Battery with Unprecedented Performance. Angew Chem Int Ed. 2019;58(31):10500-10505.

[79] Liu Y, Yang C, Pan Q, Li Y, Wang G, Ou X, et al. Nitrogen-doped bamboolike carbon nanotubes as anode material for high performance potassium ion batteries. J Mater Chem A. 2018;6(31):15162-15169.

[80] Zhao C, Lu Y, Liu H, Chen L. Firstprinciples computational investigation of nitrogen-doped carbon nanotubes as anode materials for lithium-ion and potassium-ion batteries. RSC Adv. 2019;9(30):17299-17307.

[81] Wang Y, Wang Z, Chen Y, Zhang H, Yousaf M, Wu H, et al. Hyperporous Sponge Interconnected by Hierarchical Carbon Nanotubes as a High-Performance Potassium-Ion Battery Anode. Adv Mater. 2018;30(32):1802074.

[82] Zeng S, Zhou X, Wang B, Feng Y, Xu R, Zhang H, et al. Freestanding CNT-modified graphitic carbon foam as a flexible anode for potassium ion batteries. J Mater Chem A. 2019;7(26):15774-15781.

[83] Gabaudan V, Touja J, Cot D, Flahaut E, Stievano L, Monconduit L. Double-walled carbon nanotubes, a performing additive to enhance capacity retention of antimony anode in potassium-ion batteries. Electrochem Commun. 2019;105:106493.

[84] Marom R, Ziv B, Banerjee A, Cahana B, Luski S, Aurbach D. Enhanced performance of starter lighting ignition type lead-acid batteries with carbon nanotubes as an additive to the active mass. J Power Sources. 2015;296:78-85.

[85] Shapira R, Nessim GD, Zimrin T, Aurbach D. Towards promising electrochemical technology for load leveling applications: extending cycle life of lead acid batteries by the use of carbon nano-tubes (CNTs). Energy Environ Sci. 2013;6(2):587-594.

[86] Swogger SW, Everill P, Dubey DP, Sugumaran N. Discrete carbon nanotubes increase lead acid battery charge acceptance and performance. J Power Sources. 2014;261:55-63.

[87] Banerjee A, Ziv B, Shilina Y, Levi E, Luski S, Aurbach D. Single-Wall Carbon Nanotube Doping in Lead-Acid Batteries: A New Horizon. ACS Appl Mater Interfaces. 2017;9(4):3634-3643.

[88] Geng D, Ding N, Hor TSA, Chien SW, Liu Z, Wuu D, et al. From Lithium-Oxygen to Lithium-Air Batteries: Challenges and Opportunities. Adv Energy Mater. 2016;6(9):1502164.

[89] Zhu Q-C, Du F-H, Xu S-M, Wang Z-K, Wang K-X, Chen J-S. Hydroquinone Resin Induced Carbon Nanotubes on Ni Foam As Binder-Free Cathode for Li–O2 Batteries. ACS Appl Mater Interfaces. 2016;8(6):3868-3873.

[90] Wang C, Xie Z, Zhou Z. Lithiumair batteries: Challenges coexist with opportunities. APL Materials. 2019;7(4):040701.

[91] Lee DU, Park HW, Park MG, Ismayilov V, Chen Z. Synergistic Bifunctional Catalyst Design based on Perovskite Oxide Nanoparticles and Intertwined Carbon Nanotubes for Rechargeable Zinc–Air Battery Applications. ACS Appl Mater Interfaces. 2015;7(1):902-910.

[92] Zhu S, Chen Z, Li B, Higgins D, Wang H, Li H, et al. Nitrogen-doped carbon nanotubes as air cathode catalysts in zinc-air battery. Electrochim Acta. 2011;56(14):5080-5084.

[93] Girishkumar G, Rettker M, Underhile R, Binz D, Vinodgopal K, McGinn P, et al. Single-Wall Carbon Nanotube-Based Proton Exchange Membrane Assembly for Hydrogen Fuel Cells. Langmuir. 2005;21(18):8487-8494.

[94] Tan L, Liu Z-Q, Li N, Zhang J-Y, Zhang L, Chen S. CuSe decorated carbon nanotubes as a high performance cathode catalyst for microbial fuel cells. Electrochim Acta. 2016;213:283-290.

[95] Deng D, Yu L, Chen X, Wang G, Jin L, Pan X, et al. Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction. Angew Chem Int Ed. 2013;52(1):371-375.

[96] Wen L, Liming D. Carbon Nanotube Supercapacitors. 2010.

[97] Yun YS, Yoon G, Kang K, Jin H-J. High-performance supercapacitors based on defect-engineered carbon nanotubes. Carbon. 2014;80:246-254.

[98] Frackowiak E, Jurewicz K, Delpeux S, Béguin F. Nanotubular materials for supercapacitors. J Power Sources. 2001;97-98:822-825.

[99] Futaba DN, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, et al. Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes. Nature materials. 2006;5(12):987-994.

[100] An KH, Kim WS, Park YS, Choi YC, Lee SM, Chung DC, et al. Supercapacitors Using Single-Walled Carbon Nanotube Electrodes. Adv Mater. 2001;13(7):497-500.

[101] Du C, Yeh J, Pan N. High power density supercapacitors using locally aligned carbon nanotube electrodes. Nanotechnology. 2005;16(4):350-353.

[102] Lim SH, Elim HI, Gao XY, Wee ATS, Ji W, Lee JY, et al. Electronic and optical properties of

*Carbon Nanotubes: Applications to Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.94155*

nitrogen-doped multiwalled carbon nanotubes. Physical Review B. 2006;73(4):045402.

[103] Sun J, Huang Y, Fu C, Wang Z, Huang Y, Zhu M, et al. Highperformance stretchable yarn supercapacitor based on PPy@CNTs@ urethane elastic fiber core spun yarn. Nano Energy. 2016;27:230-237.

[104] Song C, Yun J, Keum K, Jeong YR, Park H, Lee H, et al. High performance wire-type supercapacitor with Ppy/ CNT-ionic liquid/AuNP/carbon fiber electrode and ionic liquid based electrolyte. Carbon. 2019;144:639-648.

**Chapter 7**

## Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices

*Maria Tariq, Tajamal Hussain, Adnan Mujahid, Mirza Nadeem Ahmad, Muhammad Imran Din, Azeem Intisar and Muhammad Zahid*

## **Abstract**

With the increasing pressure of population, the energy demand is growing explosively. By 2050, it is expected that the world population may reach to about 9 billion which may result in the increase of energy requirement to about 12.5 trillion watts. Due to increasing pressures of population, industries and technology, concerns to find possibilities to cope with increasing demand of energy resources, arise. Although the renewable energy resources including fossil fuels, wind, water and solar energy have been used for a long time to fulfill the energy requirements, but they need efficient conversions and storage techniques and are responsible for causing environmental pollution due to greenhouse gases as well. It is thus noteworthy to develop methods for the generation and storage of renewable energy devices that can replace the conventional energy resources to meet the requirement of energy consumption. Due to high energy demands, the sustainable energy storage devices have remained the subject of interest for scientists in the history, however, the traditional methods are not efficient enough to fulfill the energy requirements. In the present era, among other variety of advanced treatments, nano-sciences have attracted the attention of the scientists. While talking about nano-science, one cannot move on without admiring the extraordinary features of carbon nanotubes (CNTs) and other carbon based materials. CNTs are on the cutting edge of nano science research and finding enormous applications in energy storage devices. Excellent adsorption capabilities, high surface area, better electrical conductivity, high mechanical strength, corrosion resistance, high aspect ratio and good chemical and physical properties of CNTs have grabbed tremendous attention worldwide. Their charge transfer properties make them favorable for energy conversion applications. The limitation to the laboratory research on CNTs for energy storage techniques due to low specific capacitance and limited electrochemical performance can be overcome by surface functionalization using surface functional groups that can enhance their electrical and dispersion properties. In this chapter, ways CNTs employed to boost the abilities of the existing material used to store and transfer of energy have been discussed critically. Moreover, how anisotropic properties of CNTs play important role in increasing the energy storage capabilities of functional materials. It will also be discussed how various kinds of materials can be combined along CNTs to get better results.

**Keywords:** Energy storage, CNTs, Capacitors, Batteries

## **1. Introduction**

With the increasing pressure of population, the energy demand is growing explosively. By 2050, it is expected that the world population may reach to about 9 billion which may result in the increase of energy requirement to about 12.5 trillion watts. Due to increasing pressures of population, industries and technology, concerns to find possibilities to cope with increasing demand of energy resources arises. Although the renewable energy resources including fossil fuels, wind, water and solar energy have been used for long time to fulfill the energy requirements, but they need efficient conversions and storage techniques and are responsible for causing environmental pollution due to greenhouse gasses as well. It is thus noteworthy to develop methods for generation and storage of renewable energy devices that can replace the conventional energy resources to meet the requirement of energy consumption.

In this chapter, we want to grab attention of the readers towards the applications of CNTs in energy storage devices. The basic principle of energy storage devices is briefly explained. Also role of carbon nanotubes as cathode and anode in different types of energy storage are discussed in this chapter.

There are two fundamental ways of storing electrochemical energy. One is the energy storage via faradic process while the other one is a non-faradic process. In the non-faradic devices, electricity is stored in electrostatic way while the faradic devices store energy electrochemically by redox reactions of active reagents. Pseudocapacitors and batteries are the examples of faradic devices while supercapacitors are the non-faradic energy storage devices.

## **2. Basic principle**

In general, electrochemical energy storage devices involve three main steps:


The energy storage devices usually store energy at the electrode/electrolyte interface in the form of accumulation of charge at the positive and negative electrodes as ions [1]. The ability of energy storage of devices is greatly affected by the electrochemical reaction that occurs at electrode electrolyte interface [1, 2].

#### **2.1 Charging-discharging mechanism**

The basic mechanism of charging and discharging of batteries as well as capacitors are discussed below.

#### *2.1.1 Battery*

Battery is composed of three main components; (i) an anode (ii) a cathode and (iii) an ionic conductor acting as an electrolyte. In order to avoid short circuit, a rigid separating medium is placed between the two electrodes (anode and cathode) *Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

[3]. In a charged cell, movement of ions takes place from cathode to anode and reduction occurs due to ionic conduction. This electrons transportation occurs through an external circuit [4]. When a cell is discharged, oxidation occurs at anode which results in the formation of ionic species. Than these ions travel through the electrolyte and recombine at the cathode. The work is done in the process of ions transport as the ionic species produced at anode are unable to travel through the insulating electrolyte, thus they are conducted through an external circuit towards the cathode [5].

#### *2.1.2 Capacitor*

Electrochemical capacitors are divided into two main categories which are (i) electric double layer capacitor (EDLC) and (ii) pseudocapacitors. Similar to the battery, all electrochemical capacitors have a pair of electrodes which stores electrical energy [6]. An aqueous solution of acid or alkali such as that of sulfuric acid or potassium hydroxide or any other ionic liquid acts as an electrolyte [7].

There is a dielectric medium present between the electrodes of pseudocapacitors. The applied voltage produces dipoles in which electrical charges are stored. On other hand, in EDLC, electrical charges are arranged at the electrodes/ electrolyte boundaries as 'electric double layer' also known as helmholtz plan [8]. The energy is delivered quickly in EDLC because of quick response of materials to the potential change and physical reactions. It is different from the behavior of battery because, the electrode potential is a continuous function of degree of charge, which is different from thermodynamic behavior of reactants of battery. It is more advantageous over battery due to its environmental friendly materials, long life span and rapid charge/discharge ability [9]. Charging-discharging pattern of the super capacitor with the time is shown in **Figure 1**. The EDLC stores charge without chemical reaction thus no heat is generated leading to high efficiency and long life. The energy stored due to fast redox reactions results in faster charging and discharging of capacitor than that of the battery. Nevertheless, due to the confined electrode surface of EDLC, the amount of energy stored in it is limited and much lower as compared to that of pseudocapacitors and batteries [10].

**Figure 1.** *Charging-discharging curve of supercapcitor.*

The electric double layer can be shown in the form of equation as:

$$\mathbf{C} = \mathbf{Q} \mid \mathbb{V} = \mathbf{g}\_{\mathbf{o}} \mathbf{g}\_{r} \mathbf{A} \nmid \mathbf{d} \tag{1}$$

Where,

C = capacitance of electrode Q = charge transferred at potential V Ɛr = dielectric constant of electrolyte Ɛo = dielectric constant of vacuum

d = distance between electrodes A = surface area of electrode

There are three main parameters that affect all the electrochemical energy storage devices. These include (i) specific capacitance, (ii) power, and (iii) energy density.

The total amount of electric charge that can be stored in capacitor is called the capacitance whereas the maximum amount of power that can be supplied per unit mass is called power density. The energy density can be defined as amount of energy stored per unit mass. EDLC possess lower energy densities as compared to batteries but have many advantages like high power density, faster charging and discharging, long life cycle and no change in chemical structure during charging and discharging [11].

#### **3. CNTs for energy storage devices**

Over the past many years, several advancements have been introduced in the primary conception and modification of electrode materials used for energy storage devices. Carbon-based materials, such as activated carbons (ACs), carbon nanotubes (CNTs) and graphenes have proved to be good electrode materials for energy storage devices [12, 13].

CNTs are on the cutting edge of nano science research and finding enormous applications in energy storage devices. Excellent adsorption capabilities, high surface area, better electrical conductivity, high mechanical strength, corrosion resistance, high aspect ratio and good chemical and physical properties of CNTs have grabbed tremendous attention worldwide [14, 15]. Their charge transfer properties make them favorable for energy conversion applications. The limitation to the laboratory research on CNTs for energy storage techniques due to low specific capacitance and poor electrochemical performance can be overcome by surface functionalization using surface functional groups that can enhance their electrical and dispersion properties [16]. Also the use of CNTs for energy storage devices is cheap due to easily available precursor carbon material for synthesis of CNTs. The researches on various energy storage applications of CNTs include Li-ion batteries, hydrogen storage, fuel cells and energy conversions etc.

#### **4. Li-Ion batteries**

Li-ion batteries show high energy density as compare to other rechargeable batteries. They have grabbed attention for various applications extending from electronic portable devices to electronic vehicles [17]. Among many other rechargeable batteries, LIBs have low cost, are safe for use and have least side reactions. They can offer maximum energy, high voltage, good capacity and density [18].

#### *Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

During the charging process, Lithium ions move from cathode to anode through an aqueous electrolyte present between the electrodes. The required driving force for this process is the chemical potential difference of Li between the electrodes. During discharging, reduction occurs at the cathode by intercalating Li-ions, while oxidation occurs at the anode simultaneously. In this way, electric current flows through external circuit to perform the required work [19, 20].

The properties of LIB such as energy density, cycle durability, rate of charging and discharging and flexibility is greatly affected by selection of suitable materials for the anode, cathode and the electrolyte [21]. The use of nanostructured materials adds many advantages over the conventional materials, such as larger contact area with electrolyte, short transport pathway for Li ions insertion and reversible Li intercalation. CNTs have been proved to be most suitable additive materials for Electrodes in LIBs and role of CNTs in LIBs is explained in the **Figure 2**. As compared to conventional LIBs, the maximum energy storing capacity of CNTs based Li-ion batteries is 1000 mAh/g (three times higher than conventional) [22].

### **4.1 CNTs based anode**

An anode can be made of pure CNTs or composite metals, which acts as the negative electrode of the LIB during charging while cathode is composed of Li metal oxides or transition metals oxides that acts as the positive electrode of LIB in discharging. The electrochemical performance of Li ion batteries depends largely on the effective cyclic intercalation of Li ions between the electrodes. The ideal characteristics of the battery include fast charging, higher ionic storage and slow discharge [23].

Normally the metallic Lithium used as an anode in Li ion batteries causes safety issues and they have short lifetime and high cost. Carbon based materials and Li-based alloys can replace metallic Li as anode. Use of these materials reduces the activity of Li as compared to lithium metal thus results in decreasing reactivity with electrolyte, reducing the voltage of cell and improving safety. The unique structure of CNTs allows the rapid movement of Li ions through insertion and de-insertion [24, 25]. LIB anodes can be replaces by single wall carbon nanotubes as well as multiwall carbon nanotubes either by simply their deposition on a current collector or by their direct growth on a catalytically modified current collector. SWCNTs and

**Figure 2.** *Incorporation of CNTs in LIBs.*

MWCNTs possess higher theoretical electrical conductivities (approximately 106 and 105S/m, respectively) and a good elastic strength (»60GPa) [26, 27].

The factors which affect the kinetics of lithium inside CNTs include radius, length, chirality and structure defects. These factors can be optimized to obtain maximum capacity results. The Li insertion capacity of carbon nanotubes in LIBs depends on chirality. The metallic CNTs show higher insertion capacities as compared to semiconductor CNTs [28].

The intercalation capacity of Li in CNT based Li-batteries is directly associated with the morphology of CNTs. Any structural defect in the morphology of CNTs affects its capacity. If there are holes in the side wall of CNTs due to defect, Li ions diffuse into them easily as compared to defect free CNTs. Li ions move randomly inside the nanotubes such that longer the length of nanotubes, slower the effective diffusion [29].

A limitation in the use of CNT anodes in LIBs is the non-reversible loss of charge after first cycle because of formation of a layer of solid electrolyte inter-phase on the CNTs. This issue can be resolved by using CNTs as conducting additives. The CNT composites with Li material have been proved to be very efficient as they resist the agglomeration as well as increase the conductivity of anode [30].

#### **4.2 CNTs based cathode**

In Li-ion batteries the active cathode material play key role in determining their performance. A variety of materials are discovered as the suitable materials for cathode of LIBs, comprising LiCoO2, LiNiO2, LiMnO2, spinel LiMn2O4, LiFePO4, LiMPO4 and elemental sulfur [31–33].

The selection of appropriate cathode material greatly affect the performance of the Li-ion batteries [34]. Carbon nanotubes have been proved to be the most efficient cathode composite materials as they can reduce resistance thus increase the electrochemical performance of composite cathode. The high aspect ratio and geometry of MWCNTs provide continuous conductive network allowing efficient electron transport through material [35]. The large surface are of CNTs provides close contact with active material.

CNTs as additives for cathode materials have been reported by many researchers. Among them most widely used is the nanostructured LiFePO4 with carbon nanocomposites containing monodispersed nanofibers of LiFePO4 electrode [36].

For CNT based cathode, nanoparticles should have firm chemical bonds with the active materials so that CNTs act as the current-collectors for faster transport, better strength and larger surface area. CNTs can be introduced into the active material in a number of ways, including simply adding to the forerunner at the early stage of processing of active materials or by their growth in the active electrode material.

#### **5. Super capacitors**

Electrochemical capacitors, also recognized as super capacitors, are the rechargeable energy storage devices that store charge of thousands of Farads in the electrodeelectrolyte interface. In contrast with other energy storage devices, super capacitors provide high power, low weight and high rate of charging-discharging [37].

Super capacitors are divided into three main types:


### *Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

In all types of SCs, carbon is the most commonly used electrode material because they are easily available, less costly, have larger surface area and possess excellent electrical, electrochemical and mechanical properties [38].

Super capacitors are also differentiated into different types depending upon the charge storage mechanism.


The electrochemical double layer capacitor (EDLC) stores energy in a double layer of ions of electrolyte (helmholtz layer) formed on the surface of electrodes surface. The Helmholtz layer stores the charge physically. Pseudocapacitors contain electrodes of active material that store charge by faradic mechanism. Pseudocapacitors possess double the energy density as compared to EDLCs because it includes the bulk as well as the surface of the electrodes [37].

The performance of supercapacitors can be upgraded by increasing the electrode surface area or using appropriate material for electrodes. Comparison of the different features of EDLC and pseudocapictors is given in **Table 1**.

## **5.1 Electrode material for supercapacitors**

Electrode materials play fundamental role to determine the efficiency of a supercapacitor. CNTs can be used as active materials for electrodes as well as incorporated with other additive materials. Many forms of carbon materials are proved to be effective electrode material for electrochemical capacitors. They help the ions to diffuse at the surface and also help to increase change in volume during charging-discharging.

The mostly used CNT based electrodes for supercapacitors include: [39].


## **5.2 Bare CNT electrode**

CNTs are frequently used as electrode material for EC capacitors due to high surface area. The capacitance of electrochemical capacitors is significantly higher


**Table 1.** *Comparison of EDLC and pseudocapacitor.* than other capacitors; SWCNT electrodes show a capacitance of 180.0 F/g, a power density of 20.0 kW/kg and energy density of 7.0 Wh/kg [40].

CNTs can be modified for fabrication to electrode material by attachment of chemical groups through covalent bond or by wrapping the functional groups noncovalently [41]. To improve the power densities and energy, dopants are also used such as N-CNTs [42]. Furthermore, larger surface area can be obtained by oxidation. However it is difficult for bare CNTs, to obtain high energy density and power density simultaneously because of dependence of storage mechanism on physical process.

#### **5.3 Polymer\CNT composites electrodes**

Conducting polymers are grabbing the attention as supercapacitors electrode materials owing to higher specific capacitance, high conductivity in charged state, thus reduced equivalent resistance and improved power density. The randomly arranged carbon nanotubes with polymer matrix have a synergistic effect on the capacitance [43].

Among the conducting polymers, CNT composites are the most commonly used polymer composites including polyaniline [PAni] [44, 45] and polypyrrole [PPy] [46] and polythiophene (PTh) composites. We have reported in our work, electrical and thermal properties of polymethyl methacrylate CNTs composites with polyaniline-multiwalled carbon nanotubes (PANI-CNTs) as filler. Theoretically calculated percolation threshold was found to be 1.3 wt% [47]. We have also found from research that PANI had lower thermal stability than its composites with MWCNTs and Ag-MWCNTs [48].

These polymer composites exhibit several advantages like flexibility, stability, and lower cost, good electrical conductivity, more stable capacitance, and large scale production. The modification of composite due to added constituents depends upon the factors such as conductivity, accessibility and diffusion distance in electrode [49].

In one of our reported studies, Polystyrene adsorbed multi-walled carbon nanotubes incorporated polymethylmethacrylate composites have been synthesized with alleviated electrical properties. The calculated value of percolation threshold was 0.1 wt% [50].

#### **5.4 Metal oxide\CNT composites electrodes**

Metal oxides are frequently used as electrodes for electrochemical capacitors due to high densities and high strength [51]. Transition metals are more effectively used because they exhibit more than one oxidation states that results in high capacitance [52]. The faradic behavior of metal oxides depends upon the hydration properties and crystalline structure. CNTs are introduced to metal oxides so that when the composite is added to the electrode, it restricts the volume change. Among many metal oxide/CNT composites, the most widely used as electrode material is MnO2. MnO2 possess high theoretical capacitance, found abundantly in nature, and is environmental friendly, easily affordable and easily processed [53, 54]. Ramezani et al. reported the specific capacitance of MnO2-CNT composites at a high scan rate of 20 mV/s, to be 180 F/g and possessed a high rate capacity [55]. Reddy et al. also reported Au doped MnO2-CNT hybrid coaxial composites having capacitance of 68.0 F/g, energy density (4.5 Wh/kg), power density to be 33.0 kW/kg, and the cycle stability up to 1000 cycles. Effect of CNTs based metal oxide composite on the efficiency of the electrode is best explained in **Figure 3** and **Table 2** [59].

*Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

**Figure 3.** *Areal capacitance of CNTs fibers electrodes with different MnO2 coating.*


**Table 2.**

*Data od CNTs based metal oxide composites as electrodes and their efficiency.*

## **6. CNTs as flexible and separate electrodes**

The energy storage devices including LIBs and super-capacitors are weighty, bulky and rigid. Therefore, they are now being replaced by the flexible storage devices due to their distinctive advantages such as less weight, flexibility and diversity of shapes etc. Therefore the flexible energy storage devices are most wanted [60].

The CNTs play an important role due to their manipulating capabilities in making flexible electrodes for flexible storage devices. CNTs play a dual role as current collector as well as active material. The CNTs thin films reduce the electrodes size and also increase flexibility and stability [61].

For the fabrication of CNTs as flexible electrodes, few aspects must be taken in account, such as young modulus of the thin film, to make sure that it may not degrade during bending or expanding. Secondly, during the charging discharging process, heat is released which may cause expansion of the material, effecting the working of the device. Thus it is also important to confirm the thermal stability of the active material [62].

#### **6.1 CNT paper for energy storage**

CNT papers having improved energy storage capabilities, have grabbed the attention for useful applications. CNT thin films are proved to possess excellent electrochemical performance due to having good conductivity, flexibility and fast heat dissipation capability [63]. With the improving technologies, CNT

electrodes are being modified into CNT paper for the energy storage [64]. A number of CNT papers have been reported as electrodes for storage devices. In 2004, Morris et al. reported a free standing single walled CNTs paper electrode and its application in LIBs as initiative. This SWCNT paper is capable of showing energy of 600.0 Wh/kg and power density of nearly 3.0 kW/kg [65]. A CNT bucky-paper was invented by filtration of DWCNTs which was mechanically stable and flexible [66]. Another free flexible SWCNT paper was made by the chemical vapour deposition method, having the specific capacitance (35.0 F/g) and power density (197.3 kW/kg) [67].

The performance of energy storage of CNT paper can be enhanced by adding pseudocapacitance [68]. Xiao et al. utilized vacuum filtration method to prepare a flexible free-standing carbon nanotubes films and also used electro-chemical method in order to join redox functional groups to the CNT films [69]. The active groups containing CNT films revealed high capacitance of 150.0 mF/cm. Yang's group introduced oxygen functional groups to CNTs thin film through an acid treatment. The film showed elevated volumetric energy of approximately 200 Wh/ kg and power of approximately 10 kW/kg [70].

#### **6.2 CNT fibers**

The typical weaving technology is used for making fiber shaped CNT electrodes. The fiber shaped electrodes are highly stretchable and flexible with good integration capability [71, 72]. The prime properties of electrode such as conductance, heat resistance and stability etc. are determined by the core material of the fiber. Thus, it is very important to select an appropriate material for the fiber [73].

Novel approach reported by Lu, Zan, et al. included development of super elastic hybrid CNT/graphene fiber accompanied by electro deposition of polyaniline to obtain high performing fiber supercapacitor. It was observed that the specific capacitance of prepared fiber was increased to 39% [12].

Chen, Tao, et al. invented a CNTs-based wire shaped electrode for batteries and supercapacitors and found excellent electro-chemical performance of prepared wire shaped devices with outstanding mechanical and electric properties of core CNTs [74].

#### **6.3 CNT and polymers composites**

All polymer based energy storing devices are more useful than batteries and supercapacitors due to their environmental friendly nature, flexible, low cost and versatility. In the novel approaches of flexible energy storage devices, many different ways have been used in which the electrode materials include conducting polymers [75–77] or polymers/CNTs composites [78–81].

The significance of using these polymer composite electrodes is the excellent mechanical properties and structural strength along with high tensile strength of electrode. In addition the densities of polymer-based electrodes are equivalent to that of composite electrodes [82].

Many polymer composite materials have been reported having higher electrochemical performance like Poly-pyrrole (PPY) on CoO nanowires [83], Poly-aniline (PANI) hybrid electrode [82], PPY on free CNTs bucky-paper [84] etc. Adding polymers to CNTs to form flexible composites electrode is a promising approach to obtain better electrochemical performance along with flexibility for flexible energy storage devices.

*Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

## **7. Flexible energy storage devices**

There is a great demand of elastic energy storage devices owing to their flexibility, portability and less weight. **Figure 4** shows the importance of such flexible energy storage devices. These energy storage devices are used as wearable devices, soft electronic devices and roll up display [85, 86]. In order to achieve flexible energy storage devices, the main challenge is to select appropriate material having high capacity and conductivity. There are two main types of elastic energy storage devices:


## **7.1 Flexible Li-ion batteries**

In order to design portable electronics such as smart cards, wireless sensor, wearable devices, roll up displays etc. flexible Li ion batteries are required which have high energy density and excellent rate capabilities [87]. Flexible batteries have been developed by many routes including cellulose based batteries [88], polymer batteries [89], soft packing batteries [90], and paper based batteries [91, 92]. The performance of flexible batteries highly depends upon the type of electrode material thus a soft flexible nanostructured material is highly recommended to construct a flexible battery. Carbon nanotubes, owing to their unique properties like extremely flexible and highly conductive, take their top priority to be used as electrode material for flexible batteries [93].

**Figure 4.** *Applications of flexible energy storage devices.*

Ajayan et al. reported porous cellulose paper having CNTs embedded on it used as electrode. The paper was capable of bending, twisting and rolling to any degree [63]. Ren Jingn et al. used MWCNT/LiO2 as electrodes to form a malleable wireshaped Li-ion battery. The battery showed the power-density of 880 W/kg and energy-density of 27 Wh/kg. The prepared wire-shaped batteries were fabricated into low weight, flexible and malleable battery textile to check their application [94].

Fang et al. developed a lithium sulfur battery by twisting a fibrous cathode fabricated by aligned CNTs coated with sulfur and an anode of Li wire. The composite cathode displayed capacity of 1051 mAh/g versus sulfur which retained 600 mAh/g after 100 running cycles, showing good cycling performance [95].

#### **7.2 Flexible supercapaitors**

In the modern era, transportable electronic devices including mobiles, wearable electronics and light weight elastic electronic devices are of great demand. While talking about portable energy storage devices, one cannot ignore supercapacitors. Supercapacitors are having applications in every electronic device because of higher specific capacitance and power density [96–98]. Therefore, flexible supercapacitors are always preferred for elastic electronic devices. CNTs are proved to be excellent electrode material for flexible supercapacitors owing to their high aspect ratio, high conductance and porosity [99, 100].

Wang, Q. et al. reported synthesis of strong flexible CNT-MnO2 nanosheets with excellent capacitance for flexible supercapacitor [101]. In another approach, reduced graphene-oxide and carbon nanotubes were developed as electrodes for flexible supercapacitors. The addition of CNTs provided a dense structure having mesopores in hybrid fiber. The electrode exhibits high tensile strength, high conductance and capacitance of 354.9 F/cm3 [102]. CuO/MWCNTs nanocomposites were synthesized which showed the specific-capacitance of 452.8 F/g and the scan ate of 10 mV/s [103].

Niu et al. prepared stretchable buckled SWCNT films combined with polydimethylsiloxane (PDMS) and used them as electrode fo flexible supercapacitor [104] it showed maximum flexibility and strechability.

### **8. Conclusion**

CNTs are on the cutting edge of nano science research and finding enormous applications in energy storage devices. Excellent adsorption capabilities, high surface area, better electrical conductivity, high mechanical strength, corrosion resistance, high aspect ratio and good chemical and physical properties of CNTs have grabbed tremendous attention worldwide. Among energy storage devices, Li ion batteries, electric double layer capacitors and pseudocapacitors are more commonly used. In Li-ion batteries CNTs are use as cathodes as well as anodes. It is observed that as compared to conventional LIBs, the maximum energy storing capacity of CNTs based Li-ion batteries is 1000 mAh/g i.e. three times higher than conventional. In case of supercapacitors, CNTs based electrodes include bare CNTs, polymer/CNTs electrodes and metal oxide/CNTs electrodes. Carbon nanotubes based flexible electrodes have become popular due to their distinctive advantages such as less weight, flexibility and diversity of shapes etc. Flexible energy storage devices such as flexible lithium ion batteries and flexible super capacitors are used as wearable devices, soft electronic devices and roll up display. In order to achieve flexible energy storage devices, the main challenge of selecting appropriate material having high capacity and conductivity can be achieved by using carbon nanotubes.

*Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

## **Author details**

Maria Tariq1 , Tajamal Hussain1 \*, Adnan Mujahid1 , Mirza Nadeem Ahmad<sup>2</sup> , Muhammad Imran Din1 , Azeem Intisar1 and Muhammad Zahid3

1 Institute of Chemistry, University of the Punjab, Lahore, Pakistan

2 Department of Applied Chemistry, Govt College University Faisalabad, Pakistan

3 Department of Chemistry, Agriculture University of Faisalabad, Faisalabad, Pakistan

\*Address all correspondence to: tajamalhussain.chem@pu.edu.pk

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Liu, J., et al., *Advanced energy storage devices: basic principles, analytical methods, and rational materials design.* Advanced science, 2018. **5**(1): p. 1700322.

[2] Sumboja, A., et al., *Electrochemical energy storage devices for wearable technology: a rationale for materials selection and cell design.* Chemical Society Reviews, 2018. **47**(15): p. 5919-5945.

[3] Mehtab, T., et al., *Metal-organic frameworks for energy storage devices: batteries and supercapacitors.* Journal of Energy Storage, 2019. **21**: p. 632-646.

[4] Li, J., et al., *Studies on the cycle life of commercial lithium ion batteries during rapid charge–discharge cycling.* Journal of Power Sources, 2001. **102**(1-2): p. 294-301.

[5] Gogotsi, Y. and R.M. Penner, *Energy storage in nanomaterials–capacitive, pseudocapacitive, or battery-like?* 2018, ACS Publications.

[6] Eftekhari, A., *Metrics for fast supercapacitors as energy storage devices*. 2018, ACS Publications.

[7] Merlet, C., et al., *On the molecular origin of supercapacitance in nanoporous carbon electrodes.* Nature materials, 2012. **11**(4): p. 306-310.

[8] Boota, M., et al., *Organic-inorganic all-pseudocapacitive asymmetric energy storage devices.* Nano Energy, 2019. **65**: p. 104022.

[9] Miller, E.E., Y. Hua, and F.H. Tezel, *Materials for energy storage: Review of electrode materials and methods of increasing capacitance for supercapacitors.* Journal of Energy Storage, 2018. **20**: p. 30-40.

[10] Zhou, Y., et al., *Ultrahigh-Areal-Capacitance Flexible Supercapacitor* 

*Electrodes Enabled by Conformal P3MT on Horizontally Aligned Carbon-Nanotube Arrays.* Advanced Materials, 2019. **31**(30): p. 1901916.

[11] Allagui, A., et al., *Capacitive behavior and stored energy in supercapacitors at power line frequencies.* Journal of Power Sources, 2018. **390**: p. 142-147.

[12] Lu, Z., et al., *Superelastic hybrid CNT/graphene fibers for wearable energy storage.* Advanced Energy Materials, 2018. **8**(8): p. 1702047.

[13] Wen, L., F. Li, and H.M. Cheng, *Carbon nanotubes and graphene for flexible electrochemical energy storage: from materials to devices.* Advanced Materials, 2016. **28**(22): p. 4306-4337.

[14] De Volder, M.F., et al., *Carbon nanotubes: present and future commercial applications.* science, 2013. **339**(6119): p. 535-539.

[15] Soni, S.K., B. Thomas, and V.R. Kar, *A Comprehensive Review on CNTs and CNT-Reinforced Composites: Syntheses, Characteristics and Applications.* Materials Today Communications, 2020: p. 101546.

[16] Jia, X. and F. Wei, *Advances in production and applications of carbon nanotubes*, in *Single-Walled Carbon Nanotubes*. 2019, Springer. p. 299-333.

[17] Li, H., *Practical evaluation of Li-ion batteries.* Joule, 2019. **3**(4): p. 911-914.

[18] El Kharbachi, A., et al., *Exploits, advances and challenges benefiting beyond Li-ion battery technologies.* Journal of Alloys and Compounds, 2020. **817**: p. 153261.

[19] Liu, S., et al., *Deep-discharging li-ion battery state of charge estimation using a partial adaptive forgetting factors least* 

*Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

*square method.* IEEE Access, 2019. **7**: p. 47339-47352.

[20] He, Y., et al., *A new model for State-of-Charge (SOC) estimation for high-power Li-ion batteries.* Applied Energy, 2013. **101**: p. 808-814.

[21] Lin, C., et al., *Li 4 Ti 5 O 12-based anode materials with low working potentials, high rate capabilities and high cyclability for high-power lithium-ion batteries: A synergistic effect of doping, incorporating a conductive phase and reducing the particle size.* Journal of Materials Chemistry A, 2014. **2**(26): p. 9982-9993.

[22] Liu, J., *Addressing the grand challenges in energy storage.* Advanced Functional Materials, 2013. **23**(8): p. 924-928.

[23] Liu, X.-M., et al., *Carbon nanotube (CNT)-based composites as electrode material for rechargeable Li-ion batteries: a review.* Composites Science and Technology, 2012. **72**(2): p. 121-144.

[24] Chen, Y., et al., *Hollow carbonnanotube/carbon-nanofiber hybrid anodes for Li-ion batteries.* Journal of the American Chemical Society, 2013. **135**(44): p. 16280-16283.

[25] Chen, L., et al., *Porous graphitic carbon nanosheets as a high-rate anode material for lithium-ion batteries.* ACS applied materials & interfaces, 2013. **5**(19): p. 9537-9545.

[26] Goriparti, S., et al., *Review on recent progress of nanostructured anode materials for Li-ion batteries.* Journal of power sources, 2014. **257**: p. 421-443.

[27] Kang, C., et al., *3-dimensional carbon nanotube for Li-ion battery anode.* Journal of Power Sources, 2012. **219**: p. 364-370.

[28] Kawasaki, S., et al., *Metallic and semiconducting single-walled carbon* 

*nanotubes as the anode material of Li ion secondary battery.* Materials Letters, 2008. **62**(17-18): p. 2917-2920.

[29] Bhatt, M.D. and C. O'Dwyer, *Recent progress in theoretical and computational investigations of Li-ion battery materials and electrolytes.* Physical Chemistry Chemical Physics, 2015. **17**(7): p. 4799-4844.

[30] Wang, X.X., et al., *Preparation of short carbon nanotubes and application as an electrode material in Li-ion batteries.* Advanced Functional Materials, 2007. **17**(17): p. 3613-3618.

[31] Park, K., et al., *Electrochemical nature of the cathode interface for a solid-state lithium-ion battery: interface between LiCoO2 and garnet-Li7La3Zr2O12.* Chemistry of Materials, 2016. **28**(21): p. 8051-8059.

[32] Xiong, X., et al., *Role of V2O5 coating on LiNiO2-based materials for lithium ion battery.* Journal of Power Sources, 2014. **245**: p. 183-193.

[33] Yu, H. and H. Zhou, *High-energy cathode materials (Li2MnO3–LiMO2) for lithium-ion batteries.* The journal of physical chemistry letters, 2013. **4**(8): p. 1268-1280.

[34] Li, Q., et al., *Conjugated carbonyl polymer-based flexible cathode for superior lithium-organic batteries.* ACS applied materials & interfaces, 2019. **11**(32): p. 28801-28808.

[35] Diao, G., et al., *Nickel and cobalt effect on properties of MWCNT-based anode for Li-ion batteries.* Applied Nanoscience, 2020: p. 1-7.

[36] Sides, C.R., et al., *A high-rate, nanocomposite LiFePO4∕ carbon cathode.* Electrochemical and Solid State Letters, 2005. **8**(9): p. A484.

[37] Sarno, M., *Nanotechnology in energy storage: the supercapacitors*, in *Studies in* 

*Surface Science and Catalysis*. 2019, Elsevier. p. 431-458.

[38] Li, J., *Review of electrochemical capacitors based on carbon nanotubes and graphene.* Graphene, 2012. **1**(01): p. 1.

[39] Kumar, S., et al., *Carbon nanotubes: A potential material for energy conversion and storage.* Progress in energy and combustion science, 2018. **64**: p. 219-253.

[40] Tashima, D., et al., *Space charge distributions of an electric double layer capacitor with carbon nanotubes electrode.* Thin Solid Films, 2007. **515**(9): p. 4234-4239.

[41] Hiraoka, T., et al., *Compact and Light Supercapacitor Electrodes from a Surface-Only Solid by Opened Carbon Nanotubes with 2 200 m2 g− 1 Surface Area.* Advanced Functional Materials, 2010. **20**(3): p. 422-428.

[42] Sevilla, M., et al., *Surface modification of CNTs with N-doped carbon: an effective way of enhancing their performance in supercapacitors.* ACS Sustainable Chemistry & Engineering, 2014. **2**(4): p. 1049-1055.

[43] Magu, T.O., et al., *A review on conducting polymers-based composites for energy storage application.* Journal of Chemical Reviews, 2019. **1**(1, pp. 1-77.): p. 19-34.

[44] Dong, B., et al., *Preparation and electrochemical characterization of polyaniline/multi-walled carbon nanotubes composites for supercapacitor.* Materials Science and Engineering: B, 2007. **143**(1-3): p. 7-13.

[45] Yazdi, M.K., et al., *PANI-CNT nanocomposites*, in *Fundamentals and Emerging Applications of Polyaniline*. 2019, Elsevier. p. 143-163.

[46] An, K.H., et al., *High-capacitance supercapacitor using a nanocomposite* 

*electrode of single-walled carbon nanotube and polypyrrole.* Journal of the Electrochemical Society, 2002. **149**(8): p. A1058.

[47] Bashir, F., et al., *Tailoring electrical and thermal properties of polymethyl methacrylate-carbon nanotubes composites through polyaniline and dodecyl benzene sulphonic acid impregnation.* Polymer Composites, 2018. **39**(S2): p. E1052-E1059.

[48] Hussain, T., et al., *Polyaniline/silver decorated-MWCNT composites with enhanced electrical and thermal properties.* Polymer Composites, 2018. **39**(S3): p. E1346-E1353.

[49] Wei, C., et al., *Polymer composites with functionalized carbon nanotube and graphene*, in *Polymer Composites with Functionalized Nanoparticles*. 2019, Elsevier. p. 211-248.

[50] Hussain, T., et al., *Polystyrene adsorbed multi-walled carbon nanotubes incorporated polymethylmethacrylate composites with modified percolation phenomena.* MRS Advances, 2018. **3**(1): p. 25-30.

[51] Zhi, M., et al., *Nanostructured carbon–metal oxide composite electrodes for supercapacitors: a review.* Nanoscale, 2013. **5**(1): p. 72-88.

[52] Lee, T.H., et al., *High energy density and enhanced stability of asymmetric supercapacitors with mesoporous MnO2@ CNT and nanodot MoO3@ CNT freestanding films.* Energy Storage Materials, 2018. **12**: p. 223-231.

[53] Wu, P., et al., *Synthesis and characterization of self-standing and highly flexible* δ*-MnO2@ CNTs/CNTs composite films for direct use of supercapacitor electrodes.* ACS applied materials & interfaces, 2016. **8**(36): p. 23721-23728.

[54] Yang, P., et al., *Low-cost highperformance solid-state asymmetric*  *Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

*supercapacitors based on MnO2 nanowires and Fe2O3 nanotubes.* Nano letters, 2014. **14**(2): p. 731-736.

[55] Ramezani, M., M. Fathi, and F. Mahboubi, *Facile synthesis of ternary MnO2/graphene nanosheets/carbon nanotubes composites with high rate capability for supercapacitor applications.* Electrochimica Acta, 2015. **174**: p. 345-355.

[56] Huang, M., et al., *Layered manganese oxides-decorated and nickel foamsupported carbon nanotubes as advanced binder-free supercapacitor electrodes.* Journal of Power Sources, 2014. **269**: p. 760-767.

[57] Zhou, R., et al., *High-performance supercapacitors using a nanoporous current collector made from super-aligned carbon nanotubes.* Nanotechnology, 2010. **21**(34): p. 345701.

[58] Zhang, L., et al., *Flexible hybrid membranes with Ni (OH) 2 nanoplatelets vertically grown on electrospun carbon nanofibers for high-performance supercapacitors.* ACS applied materials & interfaces, 2015. **7**(40): p. 22669-22677.

[59] Reddy, A.L.M., et al., *Multisegmented Au-MnO2/carbon nanotube hybrid coaxial arrays for high-power supercapacitor applications.* The Journal of Physical Chemistry C, 2010. **114**(1): p. 658-663.

[60] Gwon, H., et al., *Recent progress on flexible lithium rechargeable batteries.* Energy & Environmental Science, 2014. **7**(2): p. 538-551.

[61] Xiao, X., et al., *Freestanding mesoporous VN/CNT hybrid electrodes for flexible all-solid-state supercapacitors.* Advanced Materials, 2013. **25**(36): p. 5091-5097.

[62] Utsunomiya, T., et al., *Self-discharge behavior and its temperature dependence of carbon electrodes in lithium-ion* 

*batteries.* Journal of Power Sources, 2011. **196**(20): p. 8598-8603.

[63] Pushparaj, V.L., et al., *Flexible energy storage devices based on nanocomposite paper.* Proceedings of the National Academy of Sciences, 2007. **104**(34): p. 13574-13577.

[64] Hu, L., et al., *Highly conductive paper for energy-storage devices.* Proceedings of the National Academy of Sciences, 2009. **106**(51): p. 21490-21494.

[65] Morris, R.S., et al., *High-energy, rechargeable Li-ion battery based on carbon nanotube technology.* Journal of Power Sources, 2004. **138**(1-2): p. 277-280.

[66] Endo, M., et al., *'Buckypaper'from coaxial nanotubes.* Nature, 2005. **433**(7025): p. 476-476.

[67] Niu, Z., et al., *Compact-designed supercapacitors using free-standing single-walled carbon nanotube films.* Energy & Environmental Science, 2011. **4**(4): p. 1440-1446.

[68] Yao, B., et al., *Paper-based electrodes for flexible energy storage devices.* Advanced Science, 2017. **4**(7): p. 1700107.

[69] Cheng, Y., et al., *Flexible and cross-linked N-doped carbon nanofiber network for high performance freestanding supercapacitor electrode.* Nano energy, 2015. **15**: p. 66-74.

[70] Lee, S.W., et al., *Self-standing positive electrodes of oxidized few-walled carbon nanotubes for light-weight and high-power lithium batteries.* Energy & Environmental Science, 2012. **5**(1): p. 5437-5444.

[71] Zhang, Y., et al., *High-performance lithium–air battery with a coaxial-fiber architecture.* Angewandte Chemie International Edition, 2016. **55**(14): p. 4487-4491.

[72] Wang, B., et al., *Fabricating continuous supercapacitor fibers with high performances by integrating all building materials and steps into one process.* Advanced Materials, 2015. **27**(47): p. 7854-7860.

[73] Pan, S., et al., *Wearable solar cells by stacking textile electrodes.* Angewandte Chemie, 2014. **126**(24): p. 6224-6228.

[74] Chen, T., et al., *Nitrogen-Doped Carbon Nanotube Composite Fiber with a Core–Sheath Structure for Novel Electrodes.* Advanced Materials, 2011. **23**(40): p. 4620-4625.

[75] Wang, J.-Z., et al., *Highly flexible and bendable free-standing thin film polymer for battery application.* Materials Letters, 2009. **63**(27): p. 2352-2354.

[76] Wang, C., et al., *Functionalised polyterthiophenes as anode materials in polymer/polymer batteries.* Synthetic metals, 2010. **160**(1-2): p. 76-82.

[77] Mihranyan, A., et al., *A novel high specific surface area conducting paper material composed of polypyrrole and Cladophora cellulose.* The Journal of Physical Chemistry B, 2008. **112**(39): p. 12249-12255.

[78] Meng, C., C. Liu, and S. Fan, *Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties.* Electrochemistry communications, 2009. **11**(1): p. 186-189.

[79] Xiao, Q. and X. Zhou, *The study of multiwalled carbon nanotube deposited with conducting polymer for supercapacitor.* Electrochimica Acta, 2003. **48**(5): p. 575-580.

[80] Frackowiak, E., et al., *Supercapacitors based on conducting polymers/nanotubes composites.* Journal of Power Sources, 2006. **153**(2): p. 413-418.

[81] Wang, J., et al., *Highly-flexible fibre battery incorporating polypyrrole cathode and carbon nanotubes anode.* Journal of power sources, 2006. **161**(2): p. 1458-1462.

[82] Patil, D.S., et al., *Polyaniline based electrodes for electrochemical supercapacitor: Synergistic effect of silver, activated carbon and polyaniline.* Journal of Electroanalytical Chemistry, 2014. **724**: p. 21-28.

[83] Zhou, C., et al., *Construction of high-capacitance 3D CoO@ polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor.* Nano letters, 2013. **13**(5): p. 2078-2085.

[84] Che, J., P. Chen, and M.B. Chan-Park, *High-strength carbon nanotube buckypaper composites as applied to free-standing electrodes for supercapacitors.* Journal of Materials Chemistry A, 2013. **1**(12): p. 4057-4066.

[85] Gates, B.D., *Flexible electronics.* Science, 2009. **323**(5921): p. 1566-1567.

[86] Bauer, S., *Flexible electronics: sophisticated skin.* Nature materials, 2013. **12**(10): p. 871-872.

[87] Liu, J. and X.W. Liu, *Twodimensional nanoarchitectures for lithium storage.* Advanced materials, 2012. **24**(30): p. 4097-4111.

[88] Jabbour, L., et al., *Cellulose-based Li-ion batteries: a review.* Cellulose, 2013. **20**(4): p. 1523-1545.

[89] Nyholm, L., et al., *Toward flexible polymer and paper-based energy storage devices.* Advanced Materials, 2011. **23**(33): p. 3751-3769.

[90] Choi, K.H., et al., *Thin, deformable, and safety-reinforced plastic crystal polymer electrolytes for high-performance flexible lithium-ion batteries.* Advanced Functional Materials, 2014. **24**(1): p. 44-52.

*Applications of Carbon Based Materials in Developing Advanced Energy Storage Devices DOI: http://dx.doi.org/10.5772/intechopen.97651*

[91] Li, N., et al., *Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates.* Proceedings of the National Academy of Sciences, 2012. **109**(43): p. 17360-17365.

[92] Zhu, H., et al., *Tin anode for sodium-ion batteries using natural wood fiber as a mechanical buffer and electrolyte reservoir.* Nano letters, 2013. **13**(7): p. 3093-3100.

[93] Liu, L., W. Ma, and Z. Zhang, *Macroscopic carbon nanotube assemblies: preparation, properties, and potential applications.* Small, 2011. **7**(11): p. 1504-1520.

[94] Ren, J., et al., *Elastic and wearable wire-shaped lithium-ion battery with high electrochemical performance.* Angewandte Chemie, 2014. **126**(30): p. 7998-8003.

[95] Fang, X., et al., *A cable-shaped lithium sulfur battery.* Advanced materials, 2016. **28**(3): p. 491-496.

[96] Lu, X., et al., *H-TiO2@ MnO2//H-TiO2@ C core–shell nanowires for high performance and flexible asymmetric supercapacitors.* Advanced materials, 2013. **25**(2): p. 267-272.

[97] Zhou, S., et al., *Cellulose Nanofiber@ Conductive Metal–Organic Frameworks for High-Performance Flexible Supercapacitors.* ACS nano, 2019. **13**(8): p. 9578-9586.

[98] Han, Y. and L. Dai, *Conducting polymers for flexible supercapacitors.* Macromolecular Chemistry and Physics, 2019. **220**(3): p. 1800355.

[99] Liu, L., Z. Niu, and J. Chen, *Flexible supercapacitors based on carbon nanotubes.* Chinese Chemical Letters, 2018. **29**(4): p. 571-581.

[100] Zhu, S., J. Ni, and Y. Li, *Carbon nanotube-based electrodes for flexible supercapacitors.* NANO RESEARCH, 2020.

[101] Wang, Q., et al., *Flexible supercapacitors based on carbon nanotube-MnO2 nanocomposite film electrode.* Chemical Engineering Journal, 2019. **371**: p. 145-153.

[102] Xu, T., et al., *Reduced graphene oxide/carbon nanotube hybrid fibers with narrowly distributed mesopores for flexible supercapacitors with high volumetric capacitances and satisfactory durability.* Carbon, 2019. **152**: p. 134-143.

[103] Paulose, R. and M. Raja, *CuO nanoparticles/multi-walled carbon nanotubes (MWCNTs) nanocomposites for flexible supercapacitors.* Journal of nanoscience and nanotechnology, 2019. **19**(12): p. 8151-8156.

[104] Niu, Z., et al., *Highly stretchable, integrated supercapacitors based on single-walled carbon nanotube films with continuous reticulate architecture.* Advanced Materials, 2013. **25**(7): p. 1058-1064.

Section 5
