**2. Supercapacitors**

Low grade heat (around 130o

496 Syntheses and Applications of Carbon Nanotubes and Their Composites

energy conversion systems [6].

to 2030 [5]. Reproduced with permission from Elsevier.

**conversion**

devices [10, 11].

C) is a by-product of almost all human activity, especially

when energy conversion is involved. It is also known as "waste heat" because the dissipated heat into the environment is unutilised. Progress in the field of thermal energy conversion can lead to effective use of limited fossil fuels and provide supplemental power to current

**Figure 1.** The United States Department of Energy values and forecasts for energy utilisation in the period from 1980

Nanostructured materials are of great interest in the energy storage and conversion field due to their favourable mechanical, and electrical properties [3, 7]. Carbon nanotubes (CNTs) are one type of nanostructured material that possess these favourable electrical and mechanical properties due to the confinement of one dimension, combined with the surface properties that contribute to the enhanced overall behaviour. The potential of nanostruc‐ tured materials is not only limited to energy storage and conversion devices; but also to nanotransistors [8, 9], actuators [8, 9], electron field emission [8, 9], and biological sensing

The use of carbon-based nanomaterials as electrode materials is practical and economically viable because cheap carbon pre-cursor materials are abundant [12]. As the research into

**1.2. How and why carbon nanotubes can address the issues of energy storage and**

#### **2.1. Background information**

Electrical energy can be stored in two different forms and can best be described when con‐ sidering a battery and a capacitor. In a battery, it is the available chemical energy through the release of charges that performs work when two electroactive species undergo oxidation and reduction [17]; this is termed a Faradaic reaction. In a capacitor, electrostatic forces be‐ tween two oppositely charged plates will separate charge. The generated potential is due to an excess and deficiency of electron charges between the two plates without charge transfer taking place [17]. The current that is observed can be considered as a displacement current due to the rearrangement of charges [2]; this effect is termed as non-Faradaic in nature.

#### *2.1.1. Supercapacitor operation and types*

There are two types of electrochemical capacitors that are referred to as 1) electric double layer capacitors (EDLC) and 2) pseudocapacitors. The construction of these devices can vary, with electrodes being fabricated from porous carbon materials including activated car‐ bons, graphene, carbon nanotubes, templated carbons, metal oxides and conducting poly‐ mers [18, 19]. EDLC or supercapacitors have two electrodes immersed in an electrolyte solution, separated by a semi-permeable dielectric that allows the movement of ions to com‐ plete the circuit but prevents a short circuit from being formed. EDLCs are advantageous as they are able to provide relatively large power densities and larger energy densities than conventional capacitors, and long life cycles compared to that of a battery and ordinary ca‐ pacitor [20]. The performance of supercapacitors is affected by the power density require‐ ments, high electrochemical stability, fast charge/discharge phenomena, and low selfdischarging [21]. Table 1 below shows a comparison between the three types of devices.


**Figure 2.** Schematic diagram of an EDLC supercapacitor with a positive and negative electrode, separator and po‐

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 499

Like EDLC, a pseudocapacitor consists of two porous electrodes with a separator be‐

tween them all immersed in an electrolyte solution [24]. However, the difference is that

the charge is accumulated during Faradaic reactions near to or at the surface of the elec‐

trodes [18], hence non-Faradaic double-layer charging and Faradaic surface processes oc‐

cur simultaneously [25]. The pseudocapacitance arises from a Faradaic reaction when some

of the charge (*q*) passed in an electrode process is related to the electrode potential (*V*)

via thermodynamical considerations [26]. The two principal cases are adsorption pseudo‐

capacitance arising in underpotential deposition processes [26], and homogeneous redox

pseudocapacitance where the reaction is reversible [26, 27]. Pseudocapacitors thus com‐

bine features of both capacitors and batteries [18, 28]. A comparison of energy density

and power density for various electrical energy storage systems is depicted in Figure 3.

Current commercial uses of supercapacitors include personal electronics, mobile telecom‐

munications, back-up power storage, and industrial power and energy management [29,

30]. A recent application is the use of supercapacitors in emergency doors on the Air‐

bus A380, highlighting their safe and reliable performance [30].

rous carbon.

**Table 1.** Comparison of key parameters for a capacitor, supercapacitor and battery [22].

Energy storage is achieved by the build-up and separation of electrical charge that is accu‐ mulated on two oppositely charged electrodes as shown in Figure 2 [12]. As stated previous‐ ly, no charge transfer takes place across the electrode-electrolyte interface and the current that is measured is due to a rearrangement of charges. The electrons involved in the non-Faradaic electrical double layer charging are the conduction band electrons of the electrode. These electrons leave or enter the conduction band state depending on the energy of the least tightly bound electrons or the Fermi level of the system [2]. Supercapacitors exhibit very high energy storage efficiencies exceeding 95 % and are relatively stable for up to104 -105 cycles [4, 5]. The energy given by the equation, E = 0.5CV2 , means that the operat‐ ing voltage is the key in determining the energy characteristics of a supercapacitor. The choice of electrolyte when designing and fabricating a supercapacitor device dictates the op‐ erating voltage [23]. Operating voltages are approximately 1.2V, 2.7V, and 3.5V respectively for aqueous, organic and ionic liquid with all of them having associated advantages and dis‐ advantages [4, 5].

ments, high electrochemical stability, fast charge/discharge phenomena, and low self-

**Parameters Capacitor Supercapacitor Battery** Charge time 10-6 – 10-3 sec 1- 30 sec 0.3 – 3 hrs Discharge time 10-6 – 10-3 sec 1 – 30 sec 1 – 5 hrs Energy Density (Wh/kg) <0.1 1 - 10 20 - 100 Power Density (W/kg) >10 000 1000 - 2000 50 - 200 Cycle life >500 000 > 100 000 500 - 2000

discharging [21]. Table 1 below shows a comparison between the three types of devices.

Charge/discharge efficiency. ≈ 1 0.90 – 0.95 0.7 – 0.85

Energy storage is achieved by the build-up and separation of electrical charge that is accu‐

mulated on two oppositely charged electrodes as shown in Figure 2 [12]. As stated previous‐

ly, no charge transfer takes place across the electrode-electrolyte interface and the current

that is measured is due to a rearrangement of charges. The electrons involved in the non-

Faradaic electrical double layer charging are the conduction band electrons of the electrode.

These electrons leave or enter the conduction band state depending on the energy of the

least tightly bound electrons or the Fermi level of the system [2]. Supercapacitors exhibit

very high energy storage efficiencies exceeding 95 % and are relatively stable for up

ing voltage is the key in determining the energy characteristics of a supercapacitor. The

choice of electrolyte when designing and fabricating a supercapacitor device dictates the op‐

erating voltage [23]. Operating voltages are approximately 1.2V, 2.7V, and 3.5V respectively

for aqueous, organic and ionic liquid with all of them having associated advantages and dis‐

, means that the operat‐

cycles [4, 5]. The energy given by the equation, E = 0.5CV2

**Table 1.** Comparison of key parameters for a capacitor, supercapacitor and battery [22].

498 Syntheses and Applications of Carbon Nanotubes and Their Composites

to104 -105

advantages [4, 5].

**Figure 2.** Schematic diagram of an EDLC supercapacitor with a positive and negative electrode, separator and po‐ rous carbon.

Like EDLC, a pseudocapacitor consists of two porous electrodes with a separator be‐ tween them all immersed in an electrolyte solution [24]. However, the difference is that the charge is accumulated during Faradaic reactions near to or at the surface of the elec‐ trodes [18], hence non-Faradaic double-layer charging and Faradaic surface processes oc‐ cur simultaneously [25]. The pseudocapacitance arises from a Faradaic reaction when some of the charge (*q*) passed in an electrode process is related to the electrode potential (*V*) via thermodynamical considerations [26]. The two principal cases are adsorption pseudo‐ capacitance arising in underpotential deposition processes [26], and homogeneous redox pseudocapacitance where the reaction is reversible [26, 27]. Pseudocapacitors thus com‐ bine features of both capacitors and batteries [18, 28]. A comparison of energy density and power density for various electrical energy storage systems is depicted in Figure 3. Current commercial uses of supercapacitors include personal electronics, mobile telecom‐ munications, back-up power storage, and industrial power and energy management [29, 30]. A recent application is the use of supercapacitors in emergency doors on the Air‐ bus A380, highlighting their safe and reliable performance [30].

*2.2.2. Organic electrolytes*

ion diameter is very small [17].

costly to be produced [4].

Common ions include BF4 -

**2.3. Carbon nanotube powders**

*2.2.3. Ionic liquids*

20 mS cm-1 [34].

and since the stored energy increases as V2

The use of non-aqueous electrolytes in supercapacitors has the main advantage of higher operating voltages compared to aqueous systems. Voltage windows can range up to 2.5 V

densities [32, 34]. It must be noted that to operate at these higher voltages, non-aqueous elec‐ trolytes must be free of water and oxygen which will ensure no evolution of O2 and/or H2O at potential differences above 1.23 V [35]. Salts are added to the system to provide mobile ion movement at the electrode/electrolyte interface. The most common salt used generally consists of lithium ions as these ions move very well under an electric field and the effective

The major disadvantages of non-aqueous systems are the lower conductivity and the high‐ er viscosity resulting in higher equivalent series resistance (ESR) and reduced wettability if the electrode is hydrophilic. A decrease in wettability will effectively reduce the sur‐ face area used by the electrolyte, reducing the energy and power density. Most commer‐ cial systems that use organic electrolytes are manufactured in inert atmospheres and are

Ionic liquids are another class of electrolytes that is proving a great area of research for elec‐ trolytes in supercapacitors. These electrolytes can be considered as molten salts with melting temperatures usually below room temperature where the ionic conductivity is no more than

and quaternary ammonium salts [36]. The physical properties depend on the type of anion and cation and the alkyl chain length [37]. The main advantages are the good solvating properties, relatively high conductivity, non-volatility, low toxicity, large potential window, negligible vapour pressure and good electrochemical stability [37, 38]. Disadvantages in‐ clude high viscosities and low conductivities compared to that of aqueous electrolytes; while some ionic liquid mixtures yield a potential window that is not much greater than that of aqueous systems. Capacitances approaching 100 F/g for activated carbon (AC) electrodes

tions. Balducciet et. al. reported capacitance values of 115 F/g for asymmetric poly(3-methyl‐

Carbon nanotubes (CNTs) were first discovered in 1953 through research in the Soviet Un‐ ion, but the first accessible results were by Sumio Iijima [39], in 1991 as a result of research into buckminster fullerenes. CNTs have a cylindrical shape that can be considered as a gra‐ phene sheet rolled up; either individually as a single-walled carbon nanotube (SWNT), or concentrically as a multi-walled carbon nanotube (MWNT) as depicted in Figure 4and Fig‐ ure 5. However, these sheets can have varying degrees of twist along its length that can lead

thiophene)/AC electrodes using 1-buytl-3-methyl-imidazolium ionic liquids [36, 37].

, CF3SO3 -

, PF6 -, (CF3SO2)2N-

have been reported by Frackowiak et. al. by using (CF3SO2)2N-

, it is possible to attain large energy and power

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 501

as well as imidazolium, pyridinium,

anions and phosphoniumca‐

**Figure 3.** Ragone plot showing specific power against specific energy for various electrical energy storage systems. The times shown are the time constants of the device, which are obtained by dividing the energy density and power density [31].

#### **2.2. Different electrolytes used and their advantages and disadvantages**

The choice in electrolyte is extremely important for supercapacitor design as it influences the performance for energy storage and delivery. The extremely large surface area can allow for enhanced energy and power density as long as the micro-porosity and meso-porosity is tailored to suit the type of electrolyte being used. The electrolyte can be designed to enhance the cyclability, to sustain target power densities during operation, and to have an excellent rate capability (i.e. excellent charge/ discharge behaviour) [32].

#### *2.2.1. Aqueous electrolytes*

For aqueous electrolytes, the maximum operating voltage is theoretically limited by the re‐ duction potential of water (1.23 V at room temperature) [32]. Most aqueous electrolyte sys‐ tems tend to have an electrochemical window of approximately 1 V [32]. Electrolyte conductivity has a significant effect on the equivalent series resistance (esr) of the cell, which determines the power output [4]. Concentrated electrolytes are required to minimise the esr and maximise power capability [32]. In general, strong solutions of acidic electrolytes are much more corrosive than strong basic electrolytes meaning that the electrolyte has to be carefully selected for the particular electrode material.

Aqueous electrolytes tend to have very good kinetic behaviour of the electrolyte ions lead‐ ing to very efficient charge/discharge rates. This behaviour is due to the relatively high con‐ ductivity and low viscosity of the concentrated solutions [4, 33]. For example, the conductivity of 1M H2SO4 is 730 mS cm-1 compared to the much lower value of 10-20 mS cm-1 for organic solutions of lithium salts [34]. Time constants of symmetrical carbon super‐ capacitors using H2SO4 were reported to be 0.1s [34].

#### *2.2.2. Organic electrolytes*

The use of non-aqueous electrolytes in supercapacitors has the main advantage of higher operating voltages compared to aqueous systems. Voltage windows can range up to 2.5 V and since the stored energy increases as V2 , it is possible to attain large energy and power densities [32, 34]. It must be noted that to operate at these higher voltages, non-aqueous elec‐ trolytes must be free of water and oxygen which will ensure no evolution of O2 and/or H2O at potential differences above 1.23 V [35]. Salts are added to the system to provide mobile ion movement at the electrode/electrolyte interface. The most common salt used generally consists of lithium ions as these ions move very well under an electric field and the effective ion diameter is very small [17].

The major disadvantages of non-aqueous systems are the lower conductivity and the high‐ er viscosity resulting in higher equivalent series resistance (ESR) and reduced wettability if the electrode is hydrophilic. A decrease in wettability will effectively reduce the sur‐ face area used by the electrolyte, reducing the energy and power density. Most commer‐ cial systems that use organic electrolytes are manufactured in inert atmospheres and are costly to be produced [4].

#### *2.2.3. Ionic liquids*

**Figure 3.** Ragone plot showing specific power against specific energy for various electrical energy storage systems. The times shown are the time constants of the device, which are obtained by dividing the energy density and power

The choice in electrolyte is extremely important for supercapacitor design as it influences the performance for energy storage and delivery. The extremely large surface area can allow for enhanced energy and power density as long as the micro-porosity and meso-porosity is tailored to suit the type of electrolyte being used. The electrolyte can be designed to enhance the cyclability, to sustain target power densities during operation, and to have an excellent

For aqueous electrolytes, the maximum operating voltage is theoretically limited by the re‐ duction potential of water (1.23 V at room temperature) [32]. Most aqueous electrolyte sys‐ tems tend to have an electrochemical window of approximately 1 V [32]. Electrolyte conductivity has a significant effect on the equivalent series resistance (esr) of the cell, which determines the power output [4]. Concentrated electrolytes are required to minimise the esr and maximise power capability [32]. In general, strong solutions of acidic electrolytes are much more corrosive than strong basic electrolytes meaning that the electrolyte has to be

Aqueous electrolytes tend to have very good kinetic behaviour of the electrolyte ions lead‐ ing to very efficient charge/discharge rates. This behaviour is due to the relatively high con‐ ductivity and low viscosity of the concentrated solutions [4, 33]. For example, the conductivity of 1M H2SO4 is 730 mS cm-1 compared to the much lower value of 10-20 mS cm-1 for organic solutions of lithium salts [34]. Time constants of symmetrical carbon super‐

**2.2. Different electrolytes used and their advantages and disadvantages**

rate capability (i.e. excellent charge/ discharge behaviour) [32].

500 Syntheses and Applications of Carbon Nanotubes and Their Composites

carefully selected for the particular electrode material.

capacitors using H2SO4 were reported to be 0.1s [34].

density [31].

*2.2.1. Aqueous electrolytes*

Ionic liquids are another class of electrolytes that is proving a great area of research for elec‐ trolytes in supercapacitors. These electrolytes can be considered as molten salts with melting temperatures usually below room temperature where the ionic conductivity is no more than 20 mS cm-1 [34].

Common ions include BF4 - , PF6 -, (CF3SO2)2N- , CF3SO3 as well as imidazolium, pyridinium, and quaternary ammonium salts [36]. The physical properties depend on the type of anion and cation and the alkyl chain length [37]. The main advantages are the good solvating properties, relatively high conductivity, non-volatility, low toxicity, large potential window, negligible vapour pressure and good electrochemical stability [37, 38]. Disadvantages in‐ clude high viscosities and low conductivities compared to that of aqueous electrolytes; while some ionic liquid mixtures yield a potential window that is not much greater than that of aqueous systems. Capacitances approaching 100 F/g for activated carbon (AC) electrodes have been reported by Frackowiak et. al. by using (CF3SO2)2N anions and phosphoniumca‐ tions. Balducciet et. al. reported capacitance values of 115 F/g for asymmetric poly(3-methyl‐ thiophene)/AC electrodes using 1-buytl-3-methyl-imidazolium ionic liquids [36, 37].

#### **2.3. Carbon nanotube powders**

Carbon nanotubes (CNTs) were first discovered in 1953 through research in the Soviet Un‐ ion, but the first accessible results were by Sumio Iijima [39], in 1991 as a result of research into buckminster fullerenes. CNTs have a cylindrical shape that can be considered as a gra‐ phene sheet rolled up; either individually as a single-walled carbon nanotube (SWNT), or concentrically as a multi-walled carbon nanotube (MWNT) as depicted in Figure 4and Fig‐ ure 5. However, these sheets can have varying degrees of twist along its length that can lead to the nanotubes to be either metallic or semi-conducting as the change in chiralities induces different orbital overlaps [9]. They exhibit remarkable electrical transport and mechanical properties [7], which is why interest and research into this material has increased over the last two decades. CNT powders have the potential to be tailored to specific energy storage and conversion applications with there being an added advantage that they can be used in all electrolyte environments that encompass aqueous, organic and ionic liquids [40].

*2.3.3. Single walled nanotubes*

A/cm2

produced with permission from Elsevier.

*2.3.4. Multi-walled nanotubes*

efficiency of 98 %.

[52, 53].

sion, annealed then immersed in an PVA/ H3PO4 electrolyte.

capacity of 109

SWNTs have been studied extensively as a supercapacitor and hybrid energy material [4, 50, 51]. The structure of a SWNTis illustrated in Figure 4 with a cylindrical nature apparent as previously stated. Its advantage is that it has very good thermal and conductive properties where the thermal conductivity can exceed 6000 Wm-1K-1 and a potential current carrying

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 503

The maximum reported gravimetric capacitance for SWNT fabricated electrodes (PVA / PVC binder; pressed into pellet) is 180 F/g with an energy density of 7 Wh/kg and a power density of 20 kW/kg using KOH electrolyte [54, 55]. Hu et. al. [56] have recently report‐ ed a solid state paper based SWNT supercapacitor, which has a specific capacitance of 115 F/g, energy density of 48 Wh/kg and a large operating voltage of 3V. The electrode prep‐ aration involved pre-processing where cotton sheets were immersed in the SWNT disper‐

**Figure 4.** (a) Schematic representation of a SWNT[57].(b) FESEM of SWNTs grown onto a Si wafer substrate[58]. Re‐

Like their SWNT counter parts, MWNTs have also been studied extensively as electrode ma‐ terials for supercapacitors [4, 50, 51]. The advantages over SWNTs are their ability to be more easily synthesised on much larger scales, making them more suitable for commercial application. The concentric nature of MWNTs can be observed in the SEM image of Figure 5. The maximum gravimetric capacitance attained for electrodes constructed from MWNTs range between 4-140 F/g with the best available commercial result at 130 F/g from Maxwell's Boost capacitor [59]. Wang et. al. [50] have recently reported partially exfoliated MWNTs on carbon cloth that gave a specific capacitance in the range of 130-165 F/g with a coulombic

#### *2.3.1. CNT synthesis overview*

There are a variety of different methods for making SWNTs and MWNTs that have been developed since CNTs were first discovered. These include laser ablation, arc discharge, chemical vapour deposition (CVD), and high pressure carbon monoxide disproportionation (HiPCO). Recently, work by Harris et. al. has successfully scaled-up the synthesis of CNT using a fluidised bed reactor [41]. All growing conditions for synthesising CNTs require a catalyst to achieve high yields, where the size of the catalyst nanoparticles will determine the diame‐ ter and chirality of the CNT [42]. The CNTs that are formed are generally in a mixture with other carbonaceous product including amorphous carbon and graphitic nanoparticles.

#### *2.3.2. Main synthesis methods for CNT growth*

Both Laser ablation and arc-discharge methods for the growth of CNTs involve the conden‐ sation of carbon atoms generated from the evaporation of carbon sources. High temperature is involved, ranging from 3000 o C - 4000o C [43]. In arc discharge, various gases such as Helium or Hydrogen are induced into plasma by large currents generated at a carbon anode and cathode. This process leads to the evaporation of carbon atoms which produces very high quality MWNTs and SWNTs [44, 45]. Laser ablation also produces very high quality CNTs with a high degree of graphitisation by focusing a CO2 laser (in pulsed or in continuous wave mode) for a period of time onto a rotating carbon target [46]. The HiPCO process utilises clusters of Fe particles as catalysts to create very high quality SWNTs [47]. Catalyst is formed *in situ* by thermal decomposition of iron pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the Fe(CO)5 decomposes into atoms that condense into larger clusters. SWNTs nucleate and grow on these particles in the gas phase [48, 49].

The CVD method usually consists of a furnace, catalyst material, carbon source, a carrier gas, a conditioning gas, and a collection device (usually a substrate). The carrier gas is responsi‐ ble for taking the reacting material onto the substrate where CNT growth occurs at catalyst sites [43]. The components mentioned are essential; however, different groups and research‐ ers have alternative experimental conditions which can contain multiple types of furnaces, and a variety of catalyst and carbon sources. The key advantage of this technique is its capability to directly deposit the CNTs onto the substrate, unlike arc discharge and laser ablation that produces a soot / powder. Recent developments by Harris et. al. [41] has led to the develop‐ ment of a large scale batch process for fabricating MWNTs. Here, a furnace like system called a fluidised bed reactor continuously flows a carrier gas over a porous alumina powder that is impregnated with the catalyst material, leading a to continuous creation of MWNTs where tens of grams can be synthesised in one run.

#### *2.3.3. Single walled nanotubes*

to the nanotubes to be either metallic or semi-conducting as the change in chiralities induces different orbital overlaps [9]. They exhibit remarkable electrical transport and mechanical properties [7], which is why interest and research into this material has increased over the last two decades. CNT powders have the potential to be tailored to specific energy storage and conversion applications with there being an added advantage that they can be used in

There are a variety of different methods for making SWNTs and MWNTs that have been developed since CNTs were first discovered. These include laser ablation, arc discharge, chemical vapour deposition (CVD), and high pressure carbon monoxide disproportionation (HiPCO). Recently, work by Harris et. al. has successfully scaled-up the synthesis of CNT using a fluidised bed reactor [41]. All growing conditions for synthesising CNTs require a catalyst to achieve high yields, where the size of the catalyst nanoparticles will determine the diame‐ ter and chirality of the CNT [42]. The CNTs that are formed are generally in a mixture with other carbonaceous product including amorphous carbon and graphitic nanoparticles.

Both Laser ablation and arc-discharge methods for the growth of CNTs involve the conden‐ sation of carbon atoms generated from the evaporation of carbon sources. High temperature

or Hydrogen are induced into plasma by large currents generated at a carbon anode and cathode. This process leads to the evaporation of carbon atoms which produces very high quality MWNTs and SWNTs [44, 45]. Laser ablation also produces very high quality CNTs with a high degree of graphitisation by focusing a CO2 laser (in pulsed or in continuous wave mode) for a period of time onto a rotating carbon target [46]. The HiPCO process utilises clusters of Fe particles as catalysts to create very high quality SWNTs [47]. Catalyst is formed *in situ* by thermal decomposition of iron pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the Fe(CO)5 decomposes into atoms that condense into larger clusters. SWNTs nucleate and grow on these

The CVD method usually consists of a furnace, catalyst material, carbon source, a carrier gas, a conditioning gas, and a collection device (usually a substrate). The carrier gas is responsi‐ ble for taking the reacting material onto the substrate where CNT growth occurs at catalyst sites [43]. The components mentioned are essential; however, different groups and research‐ ers have alternative experimental conditions which can contain multiple types of furnaces, and a variety of catalyst and carbon sources. The key advantage of this technique is its capability to directly deposit the CNTs onto the substrate, unlike arc discharge and laser ablation that produces a soot / powder. Recent developments by Harris et. al. [41] has led to the develop‐ ment of a large scale batch process for fabricating MWNTs. Here, a furnace like system called a fluidised bed reactor continuously flows a carrier gas over a porous alumina powder that is impregnated with the catalyst material, leading a to continuous creation of MWNTs where

C [43]. In arc discharge, various gases such as Helium

C - 4000o

all electrolyte environments that encompass aqueous, organic and ionic liquids [40].

*2.3.1. CNT synthesis overview*

*2.3.2. Main synthesis methods for CNT growth*

502 Syntheses and Applications of Carbon Nanotubes and Their Composites

is involved, ranging from 3000 o

particles in the gas phase [48, 49].

tens of grams can be synthesised in one run.

SWNTs have been studied extensively as a supercapacitor and hybrid energy material [4, 50, 51]. The structure of a SWNTis illustrated in Figure 4 with a cylindrical nature apparent as previously stated. Its advantage is that it has very good thermal and conductive properties where the thermal conductivity can exceed 6000 Wm-1K-1 and a potential current carrying capacity of 109 A/cm2 [52, 53].

The maximum reported gravimetric capacitance for SWNT fabricated electrodes (PVA / PVC binder; pressed into pellet) is 180 F/g with an energy density of 7 Wh/kg and a power density of 20 kW/kg using KOH electrolyte [54, 55]. Hu et. al. [56] have recently report‐ ed a solid state paper based SWNT supercapacitor, which has a specific capacitance of 115 F/g, energy density of 48 Wh/kg and a large operating voltage of 3V. The electrode prep‐ aration involved pre-processing where cotton sheets were immersed in the SWNT disper‐ sion, annealed then immersed in an PVA/ H3PO4 electrolyte.

**Figure 4.** (a) Schematic representation of a SWNT[57].(b) FESEM of SWNTs grown onto a Si wafer substrate[58]. Re‐ produced with permission from Elsevier.

#### *2.3.4. Multi-walled nanotubes*

Like their SWNT counter parts, MWNTs have also been studied extensively as electrode ma‐ terials for supercapacitors [4, 50, 51]. The advantages over SWNTs are their ability to be more easily synthesised on much larger scales, making them more suitable for commercial application. The concentric nature of MWNTs can be observed in the SEM image of Figure 5. The maximum gravimetric capacitance attained for electrodes constructed from MWNTs range between 4-140 F/g with the best available commercial result at 130 F/g from Maxwell's Boost capacitor [59]. Wang et. al. [50] have recently reported partially exfoliated MWNTs on carbon cloth that gave a specific capacitance in the range of 130-165 F/g with a coulombic efficiency of 98 %.

taining functional groups can lead to an enhanced wettability as well as pseudocapacitance as mentioned above, which maximises the electroactive surface area leading to larger energy densities [8, 23]. It has been proposed that the pseudocapacitative reactions for oxygen func‐

In non-aqueous systems, however, oxygen functionalities are detrimental to device perform‐ ance. Parasitic redox reactions can lead to a degradation of the electrode, as well as adverse effects relating to voltage proofing and increased leakage current [4, 65]. These redox reac‐

Shenet. al. [66] reported in 2011 the effects of changing the carboxylic group concentration on SWNTs. The specific capacitance, power density and energy density 0.5 M H2SO4 electro‐ lyte increased with carboxylic group density reaching a maximum of 149.1 F/g, 304.8 kW/kg, and 20.71 Wh/kg, respectively. The 10 µm film electrodes were fabricated using vacuum fil‐

Nitrogen containing carbons have recently attracted interest due to its n-type behaviour that promotes large pseudocapacitance, which can be obtained even if the surface area of the material is decreased [67]. In some instances, up to 3-fold increase in capacitance have been reported [68]. Typical examples of redox reactions involving nitrogen are described below [69]:

The chosen precursor material affects the types of functionalities that are attached to the car‐ bon backbone. Nitrogen-containing groups may be added via various methods with com‐ pounds containing nitrogen including treatment with urea, melamine, aldehyde resins and polyacylonitrile [4, 70-73]. Surface areas for nitrogen-doped carbon materials are thought to

suggesting that pseudocapacitance makes up a substantial portion of the total capacitance [74, 75]. Y. Zhang et. al. [76] have showed that N-doped MWNTs synthesised via CVD growth exhibited a capacitance of 44.3 F/g, which was more than twice the value obtained than that of the un-doped MWNTs in a 6M KOH electrolyte. K. Lee et. al. [77] have shown that the nitrogen content on vertically aligned CNTs increases the capacitance until a certain point due to an increased donation of an electron by the N (N acts as an n-type dopant) and an enhanced wettability in aqueous systems. Excessive N-doping significantly reduced the conductivity and inhibited charge storage and delivery [77]. The doped and un-doped CNTs

/g [23]. This is much lower than pure SWNTs and pure MWNTs that

/g and 830 m2

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 505

/g respectively;

tions will reduce the cycle life of a device, as well as lowering the operating voltage.

tionalised CNTsinvolve carboxyl groups undergoing electron transfer[64]:

tration to create "bucky papers" onto a mixed cellulose estate membrane.

C−OH − − C=O + H<sup>+</sup> + e<sup>−</sup>

<sup>C</sup> <sup>+</sup> NH <sup>+</sup> 2e<sup>−</sup> <sup>+</sup> 2H+<sup>↔</sup> CH2 <sup>−</sup>NH2

be in excess of 400 m2

*Boron*

CH−NHOH <sup>+</sup> 2e<sup>−</sup> <sup>+</sup> 2H+<sup>↔</sup> CH2 <sup>−</sup>NH2 <sup>+</sup> H2O

have been reported to attain a surface area greater than 1315 m2

were directly grown onto a stainless steel substrate using CVD [77].

C=O + e<sup>−</sup> − − C – O<sup>−</sup>

*Nitrogen*

**Figure 5.** (a) Schematic representation of a SWNT [57]. (b) TEM images of pristine MWNTs [59]. Reproduced with per‐ mission from Elsevier

#### *2.3.5. Surface functionalities*

The presence of surface functionalities such as oxygen, nitrogen, hydrogen, boron and cata‐ lyst nanoparticles (dependent on the synthesis environment and pre-cursor materials) can affect the capacitative behaviour of the electrode through the introduction of Faradaic reac‐ tions [60], changes in electric and ionic conductivity [23], and influencing wettability [61]. A schematic representation of an sp2 hybridised carbon lattice with various dopants is show in Figure 6.

#### *Oxygen*

Carbon materials will have functional groups present on their surface as a result of the precursors and preparation conditions [23]. Most of these functional groups are in the form of –COOH, =CO as well as phenol, quinone and lactone groups [4, 23]. Activation procedures such as post treatment with H2SO4 and / or HNO3 also leads to acid oxygen functionalities [4].

**Figure 6.** Schematic representation of a sp2 hybridised carbon lattice that has been doped with; (a) oxygen functional groups, (b) nitrogen functional groups, (c) boron.

Most of these groups are bonded with carbon atoms at the edge of hexagonal carbon layers where Faradaic reactions (via interactions with the electrolyte) lead to pseudocapacitance such as those developed with transition metal oxides RuO2 and MnO2 [23, 62]. These func‐ tional groups can also be purposely added onto the surface of carbons via oxidation with O2 or acid treatment with HNO3 or H2SO4 [63]. In aqueous systems, the presence of oxygen con‐

taining functional groups can lead to an enhanced wettability as well as pseudocapacitance as mentioned above, which maximises the electroactive surface area leading to larger energy densities [8, 23]. It has been proposed that the pseudocapacitative reactions for oxygen func‐ tionalised CNTsinvolve carboxyl groups undergoing electron transfer[64]:

C−OH − − C=O + H<sup>+</sup> + e<sup>−</sup> C=O + e<sup>−</sup> − − C – O<sup>−</sup>

In non-aqueous systems, however, oxygen functionalities are detrimental to device perform‐ ance. Parasitic redox reactions can lead to a degradation of the electrode, as well as adverse effects relating to voltage proofing and increased leakage current [4, 65]. These redox reac‐ tions will reduce the cycle life of a device, as well as lowering the operating voltage.

Shenet. al. [66] reported in 2011 the effects of changing the carboxylic group concentration on SWNTs. The specific capacitance, power density and energy density 0.5 M H2SO4 electro‐ lyte increased with carboxylic group density reaching a maximum of 149.1 F/g, 304.8 kW/kg, and 20.71 Wh/kg, respectively. The 10 µm film electrodes were fabricated using vacuum fil‐ tration to create "bucky papers" onto a mixed cellulose estate membrane.

#### *Nitrogen*

**Figure 5.** (a) Schematic representation of a SWNT [57]. (b) TEM images of pristine MWNTs [59]. Reproduced with per‐

The presence of surface functionalities such as oxygen, nitrogen, hydrogen, boron and cata‐ lyst nanoparticles (dependent on the synthesis environment and pre-cursor materials) can affect the capacitative behaviour of the electrode through the introduction of Faradaic reac‐ tions [60], changes in electric and ionic conductivity [23], and influencing wettability [61]. A schematic representation of an sp2 hybridised carbon lattice with various dopants is show in

Carbon materials will have functional groups present on their surface as a result of the precursors and preparation conditions [23]. Most of these functional groups are in the form of –COOH, =CO as well as phenol, quinone and lactone groups [4, 23]. Activation procedures such as post

**Figure 6.** Schematic representation of a sp2 hybridised carbon lattice that has been doped with; (a) oxygen functional

Most of these groups are bonded with carbon atoms at the edge of hexagonal carbon layers where Faradaic reactions (via interactions with the electrolyte) lead to pseudocapacitance such as those developed with transition metal oxides RuO2 and MnO2 [23, 62]. These func‐ tional groups can also be purposely added onto the surface of carbons via oxidation with O2 or acid treatment with HNO3 or H2SO4 [63]. In aqueous systems, the presence of oxygen con‐

treatment with H2SO4 and / or HNO3 also leads to acid oxygen functionalities [4].

mission from Elsevier

Figure 6.

*Oxygen*

*2.3.5. Surface functionalities*

504 Syntheses and Applications of Carbon Nanotubes and Their Composites

groups, (b) nitrogen functional groups, (c) boron.

Nitrogen containing carbons have recently attracted interest due to its n-type behaviour that promotes large pseudocapacitance, which can be obtained even if the surface area of the material is decreased [67]. In some instances, up to 3-fold increase in capacitance have been reported [68]. Typical examples of redox reactions involving nitrogen are described below [69]: <sup>C</sup> <sup>+</sup> NH <sup>+</sup> 2e<sup>−</sup> <sup>+</sup> 2H+<sup>↔</sup> CH2 <sup>−</sup>NH2

CH−NHOH <sup>+</sup> 2e<sup>−</sup> <sup>+</sup> 2H+<sup>↔</sup> CH2 <sup>−</sup>NH2 <sup>+</sup> H2O

The chosen precursor material affects the types of functionalities that are attached to the car‐ bon backbone. Nitrogen-containing groups may be added via various methods with com‐ pounds containing nitrogen including treatment with urea, melamine, aldehyde resins and polyacylonitrile [4, 70-73]. Surface areas for nitrogen-doped carbon materials are thought to be in excess of 400 m2 /g [23]. This is much lower than pure SWNTs and pure MWNTs that have been reported to attain a surface area greater than 1315 m2 /g and 830 m2 /g respectively; suggesting that pseudocapacitance makes up a substantial portion of the total capacitance [74, 75]. Y. Zhang et. al. [76] have showed that N-doped MWNTs synthesised via CVD growth exhibited a capacitance of 44.3 F/g, which was more than twice the value obtained than that of the un-doped MWNTs in a 6M KOH electrolyte. K. Lee et. al. [77] have shown that the nitrogen content on vertically aligned CNTs increases the capacitance until a certain point due to an increased donation of an electron by the N (N acts as an n-type dopant) and an enhanced wettability in aqueous systems. Excessive N-doping significantly reduced the conductivity and inhibited charge storage and delivery [77]. The doped and un-doped CNTs were directly grown onto a stainless steel substrate using CVD [77].

*Boron*

Boron is another interesting material for doping CNTs due to its p-type nature which pro‐ motes CNT growth and increases the oxidation temperature of the nanotubes [78]. Howev‐ er, the development of boron doped CNTs for the use as electrodes in supercapacitor devices is not well established [23]. Work by Shiraishi et.al. showed that boron doping MWNTs, increased the capacitance per surface area from 6.5 µF/cm2 to 6.8 µF/cm2 in 0.5M LiBF4/PC [79]. These electrodes were once again synthesised using CVD [79]. Wang et. al. reported in 2008 that interfacial capacitance was increased by 1.5-1.6 times in boron-doped carbon than that in boron-free carbon with alkaline electrolyte (6 M KOH) and/or acid elec‐ trolyte (1 M H2SO4) [80]. The carbon material was made into a slurry using carbon black and PTFE binder and pasted onto a Ni mesh current collector [80].

#### *2.3.6. Advantages, limitations and comparison*

It can be seen that CNTs can be tailored different ways in order to tune (to a degree) the performance of the electrode material. This control has been demonstrated by firstly, vary‐ ing the chiralitiy of the nanotube to produce the single-walled or the multi-walled variety. Both CNT types have associated advantages and disadvantages with SWNTs being able to be synthesised with a high degree of purity; while MWNTs can be synthesised on a larger scale. CNTs can also have functionalities (through addition of oxygen or nitrogen containing groups) added to their structure through treatment in order to change the surface properties and hence wettability of the material. These functionalities enable enhanced compatibility to an electrolyte to maximise electroactive surface area usage and hence performance. Further doping with specific elements such boron and nitrogen can introduce a p-type/n-type be‐ haviour where electrons contribute a Faradaic response to the system and enhance capaci‐ tance and energy density. However, it must be noted that when faradaic processes occur at the electrode/electrolyte interface, irreversible processes increase degradation of the elec‐ trode over time. Specific capacitance of CNTs (three electrode and device testing) is in the order of 5 – 165 F/g with an increase thereafter as a result of doping (i.e due to Faradaic con‐ tribution). It must be pointed out that with electrical energy devices, there is always a tradeoff between energy and power density. Therefore the electrode material has to be tailored to meet the requirements of the specific application.

**Figure 7.** (a) SEM image depicting the growth of templated porous CNTs. (b) The tunability of average pore size distri‐ bution of binary and ternary carbides Al4C3, Ti2AlC,VC,ZrC, Ti3SiC2 by varying the chlorination temperature [29]. Repro‐

Templated porous carbons are of recent great interest in the field of energy storage due to the tunability in porosity, which is necessary to meet the materials application requirements [29, 30]. These templated carbons are commonly known as carbide derived carbons (CDC) as the carbon materials are derived from carbon precursors through physical and/or chemi‐ cal processes [29]. Briefly, the synthesis involves halogenations (usually chlorination) where the carbon is formed by selective extraction of the metal and metalloid atoms, transforming the carbide structure into pure carbon. The carbon layer is formed by inward growth, with retention of the original shape and volume of the precursor [29]. If any metal chlorides are trapped, they can be usually removed by hydrogenation or vacuum annealing [29]. The gen‐

The advantage of forming carbon structures this way is the ability to form a tailored and

Inagaki et. al. in their very comprehensive review of carbon materials for electrochemical ca‐

possible electrode materials with extremely large energy densities and power densities [23].

/g for CDC, which gives rise to

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 507

narrow pore size distribution with a large surface area as can be seen in Figure 7 [29].

duced with permission from John Wiley & Sons

(g)→MCl4

eral reaction scheme is as follows where M = Si, Ti, Zr[23, 29];

pacitors reported a maximum surface area of *S BET* of 2000 m2

(g) + C(s)

**2.4. Templated porous carbons**

MC(s) + 2Cl2

**Figure 7.** (a) SEM image depicting the growth of templated porous CNTs. (b) The tunability of average pore size distri‐ bution of binary and ternary carbides Al4C3, Ti2AlC,VC,ZrC, Ti3SiC2 by varying the chlorination temperature [29]. Repro‐ duced with permission from John Wiley & Sons

#### **2.4. Templated porous carbons**

Boron is another interesting material for doping CNTs due to its p-type nature which pro‐ motes CNT growth and increases the oxidation temperature of the nanotubes [78]. Howev‐ er, the development of boron doped CNTs for the use as electrodes in supercapacitor devices is not well established [23]. Work by Shiraishi et.al. showed that boron doping

LiBF4/PC [79]. These electrodes were once again synthesised using CVD [79]. Wang et. al. reported in 2008 that interfacial capacitance was increased by 1.5-1.6 times in boron-doped carbon than that in boron-free carbon with alkaline electrolyte (6 M KOH) and/or acid elec‐ trolyte (1 M H2SO4) [80]. The carbon material was made into a slurry using carbon black

It can be seen that CNTs can be tailored different ways in order to tune (to a degree) the performance of the electrode material. This control has been demonstrated by firstly, vary‐ ing the chiralitiy of the nanotube to produce the single-walled or the multi-walled variety. Both CNT types have associated advantages and disadvantages with SWNTs being able to be synthesised with a high degree of purity; while MWNTs can be synthesised on a larger scale. CNTs can also have functionalities (through addition of oxygen or nitrogen containing groups) added to their structure through treatment in order to change the surface properties and hence wettability of the material. These functionalities enable enhanced compatibility to an electrolyte to maximise electroactive surface area usage and hence performance. Further doping with specific elements such boron and nitrogen can introduce a p-type/n-type be‐ haviour where electrons contribute a Faradaic response to the system and enhance capaci‐ tance and energy density. However, it must be noted that when faradaic processes occur at the electrode/electrolyte interface, irreversible processes increase degradation of the elec‐ trode over time. Specific capacitance of CNTs (three electrode and device testing) is in the order of 5 – 165 F/g with an increase thereafter as a result of doping (i.e due to Faradaic con‐ tribution). It must be pointed out that with electrical energy devices, there is always a tradeoff between energy and power density. Therefore the electrode material has to be tailored to

to 6.8 µF/cm2

in 0.5M

MWNTs, increased the capacitance per surface area from 6.5 µF/cm2

506 Syntheses and Applications of Carbon Nanotubes and Their Composites

and PTFE binder and pasted onto a Ni mesh current collector [80].

*2.3.6. Advantages, limitations and comparison*

meet the requirements of the specific application.

Templated porous carbons are of recent great interest in the field of energy storage due to the tunability in porosity, which is necessary to meet the materials application requirements [29, 30]. These templated carbons are commonly known as carbide derived carbons (CDC) as the carbon materials are derived from carbon precursors through physical and/or chemi‐ cal processes [29]. Briefly, the synthesis involves halogenations (usually chlorination) where the carbon is formed by selective extraction of the metal and metalloid atoms, transforming the carbide structure into pure carbon. The carbon layer is formed by inward growth, with retention of the original shape and volume of the precursor [29]. If any metal chlorides are trapped, they can be usually removed by hydrogenation or vacuum annealing [29]. The gen‐ eral reaction scheme is as follows where M = Si, Ti, Zr[23, 29]; MC(s) + 2Cl2 (g)→MCl4 (g) + C(s)

The advantage of forming carbon structures this way is the ability to form a tailored and narrow pore size distribution with a large surface area as can be seen in Figure 7 [29].

Inagaki et. al. in their very comprehensive review of carbon materials for electrochemical ca‐ pacitors reported a maximum surface area of *S BET* of 2000 m2 /g for CDC, which gives rise to possible electrode materials with extremely large energy densities and power densities [23]. Gao et. al. have recently reported flexible CDC electrodes fabricated into a device which ob‐ tained a specific capacitance of 135 F/g in 1 M H2SO4 and 120 F/g in 1.5 M tetraethyl ammo‐ nium tetrafluoroborate (TEABF4) [81]. Ordered mesoporous carbon spheres with impregnated NiO, and a maximum surface area of 944 m2 /g yielded a specific capacitance of 205.3 F/g in 2 M KOH [82]. Reported also by Y. Korenblit [83] was a high surface area CDC (2430 m2 /g) with aligned mesopores, which yielded a specific capacitance of 170 F/g and an extremely high capacity retention of 85% at high current densities of 20 A/g.

where a specific capacitance of 1118 F/g was achieved. This electrode was stable with 85% capacity retention after 500 cycles using galvanostatic charge / discharge [90]. Hu et. al. [91] have recently reported a composite electrode materials containing MWNTs coated with pol‐ ypyrrole that achieved a high capacitance of 587 F/g in a 0.1 M NaClO4 / acetonitrile electrolyte.

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 509

**Figure 9.** SEM image of PEDOT/PSS-SWNT composite showing PEDOT/PSS polymer to be integrated with the SWNT

Metal oxides exhibit pseudocapacitative behaviour over small rages of potentials, through redox processes which contribute electron transfer between the electrode / electrolyte inter‐ face (Figure 8). Common materials used in the construction of such devices are oxides of Mn, Ru, Ir, Pt, Rh, Pd, Au, Co and W [22, 26]. By combining metal oxides with CNTs, com‐ posites can be formed that combine both Faradaic and non-Faradic effects enabling a larger energy density to be obtained while still holding reasonable power density. Figure 10 shows

Very recent work on carbon / metal oxide composites can be found in the review by Wang et. al. [50]. Myoungkiet. al. reported recently in their a RuO2 / MWNT, electrode material which achieved a specific capacitance of 628 F/g[92]. The electrode was fabricated by dis‐ persing the mixture in ethanol and casting onto carbon paper [92]. Li. et. al. reported that when MWNT were coated with MnO2, a capacitance of 350 F/g was achieved [93]. More novel materials have been created by incorporating MWNTs and Co3O4,which yielded spe‐ cific capacitances of 200 F/g [94] (acetylene black / PVDF slurry on Ni gauze); while Jaya‐ lakshmi et. al. reported in 2007 V2O5.*x*H2O / CNT film with a specific capacitance of 910 F/

MnO2 particles that have been formed (*insitu*) in the presence of MWNTs.

[89]. Reproduced with permission from The Royal Society of Chemistry.

*2.5.2. CNT / metal oxide*

#### **2.5. Composite electrode materials**

Typical carbonaceous electrode materials (activated carbon, CNTs, graphene, CDC) with high surface areas used in supercapacitors have somewhat reached a limit when it comes energy storage capacity, thus restricting their possible applications [84]. Pseudocapacitor materials that are able to meet the needs of higher energy density are currently being devel‐ oped and combined with carbonaceous materials in order to create composites that when designed into hybrid supercapacitors have advantages of fast rate capability, high storage capacity, and long cyclability[84, 85].

**Figure 8.** Schematic diagram of a reversible redox reaction, as well as EDLC occurring at the electrode/electrolyte in‐ terface leading to pseudocapacitance.

#### *2.5.1. CNT / polymer*

Electrode materials comprised of inherently conducting polymers (ICPs) and CNTs is a promising area of research. The conductive polymer matrix, combined with the network like structure of the CNTs provides an enhanced electronic and ionic conductivity that can con‐ siderably improve charge storage and delivery [86-88].

Antiohos et. al. reported a SWNT / Pedot-PSS composite electrode material that was fabricat‐ ed into a device which had a specific capacitance of 120 F/g (1 M NaNO3 / H2O), coupled with an excellent stability (~90% capacity retention) over 1000 cycles using galvanostatic charge / discharge [89]. The SWNT / Pedot-PSS composite is depicted in Figure 9 where SWNTs are thoroughly dispersed throughout the Pedot-PSS conducting polymer matrix. Kim et. al. re‐ cently fabricated a ternary composite material consisting of MWNTs, graphene, and PANI where a specific capacitance of 1118 F/g was achieved. This electrode was stable with 85% capacity retention after 500 cycles using galvanostatic charge / discharge [90]. Hu et. al. [91] have recently reported a composite electrode materials containing MWNTs coated with pol‐ ypyrrole that achieved a high capacitance of 587 F/g in a 0.1 M NaClO4 / acetonitrile electrolyte.

**Figure 9.** SEM image of PEDOT/PSS-SWNT composite showing PEDOT/PSS polymer to be integrated with the SWNT [89]. Reproduced with permission from The Royal Society of Chemistry.

#### *2.5.2. CNT / metal oxide*

Gao et. al. have recently reported flexible CDC electrodes fabricated into a device which ob‐ tained a specific capacitance of 135 F/g in 1 M H2SO4 and 120 F/g in 1.5 M tetraethyl ammo‐ nium tetrafluoroborate (TEABF4) [81]. Ordered mesoporous carbon spheres with

205.3 F/g in 2 M KOH [82]. Reported also by Y. Korenblit [83] was a high surface area CDC

Typical carbonaceous electrode materials (activated carbon, CNTs, graphene, CDC) with high surface areas used in supercapacitors have somewhat reached a limit when it comes energy storage capacity, thus restricting their possible applications [84]. Pseudocapacitor materials that are able to meet the needs of higher energy density are currently being devel‐ oped and combined with carbonaceous materials in order to create composites that when designed into hybrid supercapacitors have advantages of fast rate capability, high storage

**Figure 8.** Schematic diagram of a reversible redox reaction, as well as EDLC occurring at the electrode/electrolyte in‐

Electrode materials comprised of inherently conducting polymers (ICPs) and CNTs is a promising area of research. The conductive polymer matrix, combined with the network like structure of the CNTs provides an enhanced electronic and ionic conductivity that can con‐

Antiohos et. al. reported a SWNT / Pedot-PSS composite electrode material that was fabricat‐ ed into a device which had a specific capacitance of 120 F/g (1 M NaNO3 / H2O), coupled with an excellent stability (~90% capacity retention) over 1000 cycles using galvanostatic charge / discharge [89]. The SWNT / Pedot-PSS composite is depicted in Figure 9 where SWNTs are thoroughly dispersed throughout the Pedot-PSS conducting polymer matrix. Kim et. al. re‐ cently fabricated a ternary composite material consisting of MWNTs, graphene, and PANI

extremely high capacity retention of 85% at high current densities of 20 A/g.

/g) with aligned mesopores, which yielded a specific capacitance of 170 F/g and an

/g yielded a specific capacitance of

impregnated NiO, and a maximum surface area of 944 m2

508 Syntheses and Applications of Carbon Nanotubes and Their Composites

**2.5. Composite electrode materials**

capacity, and long cyclability[84, 85].

terface leading to pseudocapacitance.

siderably improve charge storage and delivery [86-88].

*2.5.1. CNT / polymer*

(2430 m2

Metal oxides exhibit pseudocapacitative behaviour over small rages of potentials, through redox processes which contribute electron transfer between the electrode / electrolyte inter‐ face (Figure 8). Common materials used in the construction of such devices are oxides of Mn, Ru, Ir, Pt, Rh, Pd, Au, Co and W [22, 26]. By combining metal oxides with CNTs, com‐ posites can be formed that combine both Faradaic and non-Faradic effects enabling a larger energy density to be obtained while still holding reasonable power density. Figure 10 shows MnO2 particles that have been formed (*insitu*) in the presence of MWNTs.

Very recent work on carbon / metal oxide composites can be found in the review by Wang et. al. [50]. Myoungkiet. al. reported recently in their a RuO2 / MWNT, electrode material which achieved a specific capacitance of 628 F/g[92]. The electrode was fabricated by dis‐ persing the mixture in ethanol and casting onto carbon paper [92]. Li. et. al. reported that when MWNT were coated with MnO2, a capacitance of 350 F/g was achieved [93]. More novel materials have been created by incorporating MWNTs and Co3O4,which yielded spe‐ cific capacitances of 200 F/g [94] (acetylene black / PVDF slurry on Ni gauze); while Jaya‐ lakshmi et. al. reported in 2007 V2O5.*x*H2O / CNT film with a specific capacitance of 910 F/ gwith the material being ground into a paste with paraffin and spread onto a graphite elec‐ trode, and tested in 0.1 M KCl [95].

**Figure 10.** Surface cross-section morphology of MnO2 particles being grown (insitu) onto MWNTs[96].Reproduced with permission from Elsevier.

**Figure 11.** SEM image of a reduced graphene oxide / SWNT composite formed into a film.

systems and providing a greater range of applications.

It can be seen that there has been extensive research and development in the use of CNT as electrode materials for energy storage applications. Currently, they provide an excellent platform for devices that require high power density due to the very high surface areas and fast rate capability. Further studies need to be implemented in order to better understand the relationship between electrode porosity and electrolyte. An enhanced understanding of the role of micro and meso-porosity and its effect on system performance is critical. Electro‐ lyte selection is also critical to device performance as it is proportional to the square of the voltage. The main classes of electrolytes are aqueous-based, organic-based and room tem‐ perature ionic liquids. Evolving work has focused on using CNT materials in conjunction with doping of various functional groups such as carboxyls, amines and elements such as boron and nitrogen in order to enhance the electrode performance through increased usage of electrode surface area and / or Faradaic contributions. The most recent work has focused on the creation of composite materials via the combination of CNTs with conducting poly‐ mers or metal oxides. CNT composites have amassed into a prevalent area of research through the search for the discovery of hybrid energy storage devices that are able to have high energy and high power density which are beneficial for creating more energy efficient

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 511

**2.6. Conclusions**

#### *2.5.3. CNT / carbons*

Creating composite materials from CNT and different forms of carbon such as graphene or carbide derived carbons (CDC) can be advantageous due to the fact that the CNTs provide microporosity (large surface area to maximise capacitance and hence energy density); while graphene and CDC can be used to tailor the mesoporosity which improve ions kinetics, en‐ hancing the power density [21]. In Figure 11, a composite of reduced graphene oxide coated with SWNTs is depicted that has been formed into a porous film. The edges of the graphene oxide protrude out with a uniform coating of SWNTs. Recently, Li et. al. fabricated different mass loadings of graphene and CNT composite electrodes by solution casting onto glass, an‐ nealing then peeling off [97]. They reported capacitance ranges of 70-110 F/g at a scan rate of 1 mV/s in 1M H2SO4 [97]. Luet. al. have reported a CNT / graphene composite which was bound together with polypyrrole (through a filtration process) that achieved a specific ca‐ pacitance of 361 F/g at a current density of 0.2 A/g in 1 M KCl. The electrode exhibited excel‐ lent stability with only a 4% capacity loss over 2000 cycles [97]. Dong et. al. have shown that is it possible to form SWNT/graphene oxide core shell structures and spray coat the subse‐ quent material onto a current collector [98]. The performance of these core structures yield‐ ed a material with a specific capacitance of 194 F/g using galvanostatic charge / discharge at a high current density of 0.8 A/g in 1 M KOH [98].

**Figure 11.** SEM image of a reduced graphene oxide / SWNT composite formed into a film.

#### **2.6. Conclusions**

gwith the material being ground into a paste with paraffin and spread onto a graphite elec‐

**Figure 10.** Surface cross-section morphology of MnO2 particles being grown (insitu) onto MWNTs[96].Reproduced

Creating composite materials from CNT and different forms of carbon such as graphene or carbide derived carbons (CDC) can be advantageous due to the fact that the CNTs provide microporosity (large surface area to maximise capacitance and hence energy density); while graphene and CDC can be used to tailor the mesoporosity which improve ions kinetics, en‐ hancing the power density [21]. In Figure 11, a composite of reduced graphene oxide coated with SWNTs is depicted that has been formed into a porous film. The edges of the graphene oxide protrude out with a uniform coating of SWNTs. Recently, Li et. al. fabricated different mass loadings of graphene and CNT composite electrodes by solution casting onto glass, an‐ nealing then peeling off [97]. They reported capacitance ranges of 70-110 F/g at a scan rate of 1 mV/s in 1M H2SO4 [97]. Luet. al. have reported a CNT / graphene composite which was bound together with polypyrrole (through a filtration process) that achieved a specific ca‐ pacitance of 361 F/g at a current density of 0.2 A/g in 1 M KCl. The electrode exhibited excel‐ lent stability with only a 4% capacity loss over 2000 cycles [97]. Dong et. al. have shown that is it possible to form SWNT/graphene oxide core shell structures and spray coat the subse‐ quent material onto a current collector [98]. The performance of these core structures yield‐ ed a material with a specific capacitance of 194 F/g using galvanostatic charge / discharge at

trode, and tested in 0.1 M KCl [95].

510 Syntheses and Applications of Carbon Nanotubes and Their Composites

with permission from Elsevier.

a high current density of 0.8 A/g in 1 M KOH [98].

*2.5.3. CNT / carbons*

It can be seen that there has been extensive research and development in the use of CNT as electrode materials for energy storage applications. Currently, they provide an excellent platform for devices that require high power density due to the very high surface areas and fast rate capability. Further studies need to be implemented in order to better understand the relationship between electrode porosity and electrolyte. An enhanced understanding of the role of micro and meso-porosity and its effect on system performance is critical. Electro‐ lyte selection is also critical to device performance as it is proportional to the square of the voltage. The main classes of electrolytes are aqueous-based, organic-based and room tem‐ perature ionic liquids. Evolving work has focused on using CNT materials in conjunction with doping of various functional groups such as carboxyls, amines and elements such as boron and nitrogen in order to enhance the electrode performance through increased usage of electrode surface area and / or Faradaic contributions. The most recent work has focused on the creation of composite materials via the combination of CNTs with conducting poly‐ mers or metal oxides. CNT composites have amassed into a prevalent area of research through the search for the discovery of hybrid energy storage devices that are able to have high energy and high power density which are beneficial for creating more energy efficient systems and providing a greater range of applications.
