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Nanocomposites for Supercapacitor Application

  • P. Anandhi
  • V. Jawahar Senthil Kumar
  • S. HarikrishnanEmail author
Living reference work entry
  • 16 Downloads

Abstract

In this chapter, it is focused to highlight the importance of fabricating nanocomposites as active materials for supercapacitor applications. This chapter begins with a general introduction about the necessity of energy storage, and it presents the types of electrical energy storage devices. Supercapacitor is reckoned to be the best storage device when compared with other storage devices (electrostatic capacitor, battery, fuel cell) due to the following features such as high power density, higher cycle life, high rate capability, and less maintenance. Various electrode materials used in supercapacitors, such as carbon-based materials, metal oxides, and conducting polymers, are discussed. Besides, it emphasizes the use of nanocomposite towards the enhancement of the electrochemical performance of the supercapacitors. Next, various synthesis techniques for preparing nanocomposites are reported, and the chapter ends with applications of the supercapacitor.

Introduction

Renewable energy sources are greatly recommended for the production of energy, mainly to reduce the use of fossil fuels and ensure green environment to the next generation. Albeit renewable energy sources for the generation of electrical energy are suggested, their energy conversion efficiency is poor. Also, continuous generation of electrical energy from them is dubious as the availability of these sources is intermittent and some of them rely on climatic condition. So, in order to ascertain the stable and continuous supply, energy storage is essential so that reasonable, reliable, and efficient energy supply could be achieved in the post-fossil era. Improving energy storage systems is undoubtedly important to store the energy processed for different applications.

Types of Electrical Energy Storage Devices

Capacitor, battery, fuel cell, and supercapacitor are the four main storage devices. The construction and working principle of these devices are discussed in the following sections [50].

Battery

Battery is reckoned to be a commonly used power source for many applications in industrial and domestic electronics. It is a storage device which produces electrical energy from chemical energy by means of redox reaction. It comprises one or more electrochemical cells and each cell contains two electrodes linked electrically by electrolyte with cations and anions. Based on the transport of the ions, the polarity of the cell can be determined. Depending upon the charging capability, batteries are classified into two types and they are as follows: rechargeable battery and disposable battery.

In rechargeable battery, during discharging, the chemical energy is converted into electrical energy, and during charging, the electrical energy is utilized to restore the original chemical composition. Some of the examples for disposable batteries are zinc-carbon battery and alkaline battery. Lead-acid, lithium-ion cell, nickel-cadmium, nickel metal hydride, and nickel-zinc belong to rechargeable battery type. Between these two types, disposable batteries have greater specific energy.

Li-ion battery (shown in Fig. 1) is considered to be the best one in the electrochemical cells as it has certain features like high energy density in the range between 120 and 170 Wh kg−1, reasonable weight, and negligible memory effect [28]. Despite these features, it has two major drawbacks namely, less power density and low charge/discharge rates. Usually, the anode and cathode of Li-ion cell are made up of carbon-based materials and lithium-intercalated compounds like manganese oxide, iron phosphate, nickel oxide, and cobalt oxide, respectively.
Fig. 1

Schematic diagram of Li-ion battery [50]

Lithium ions are migrated between the anode and the cathode. At the time of charging, lithium ions are travelled from the cathode to the anode and during discharging, the ions are travelled back to the cathode from the anode through the connected load. The lithium ion causes hydrogen gas and lithium hydroxide as it reacts with water present in the electrolyte. Hence, the Li-ion batteries are made up of well-sealed container filled with the organic electrolyte and this arrangement would be advantageous to restrain the possibility of hazardous reactions.

Fuel Cell

It is a device which converts the chemical energy derived from the fuel into electrical energy. Unlike batteries, fuel cells do not require recharging and the byproducts derived from the reaction could not cause any kind of harms to the environment. While comparing to thermomechanical techniques, they do not have combustion, which gives rise to maximum energy conversion efficiency of 60%. Owing to these facts, fuel cells are considered to be eco-friendly, cost-effective, and more reliable for power sources. As far as the energy densities of the fuel cells are concerned, they have highest energy density of 500 Wh kg−1 and above. Even though they have high energy density, they suffer due to low power density and because of this, they are recommended for low power applications only.

Based on the electrolytes, fuel cells are classified into ceramic oxide, alkaline solution, and polymer membrane. Among them, much attention is received by the proton exchange membrane (PEM) type. The simplified block diagram of fuel cell representing the construction and working is seen in Fig. 2 [17]. Fuel cell comprises an electrolyte and two electrodes on both sides of the electrolyte layer. In fuel cell, hydrogen fuel is continuously supplied to anode and oxygen is supplied to cathode. Hydrogen at the anode electrode is decomposed into negative and positive ions. Electrolyte membrane of the fuel cell could allow only positive ions to move from anode to cathode and it would never allow negative ions (electrons). These negative ions grouped on other side of the membrane and they would move freely to the cathode via external load.
Fig. 2

Block diagram of PEM fuel cell [50]

Electrostatic Capacitor

It is one of the passive devices in electrical system and it has very simple construction. It consists of two conducting parallel plates separated by a dielectric. It has a tendency to store and release the charge at a faster rate compared with batteries and fuel cells and it has higher power density and on account of this, it can not store higher energy. Figure 3 shows the electrostatic capacitor in which, if an external power source is connected then, positive and negative charges would be accumulated on the two plates separately. The performance of the capacitor can be assessed by means of input voltage and the charge accumulated on the conducting plates. In order to improve the performance of the capacitor, high permittivity of the dielectric materials, shorter distance between the conducting plates, and larger surface area of the plates are required.
Fig. 3

Electrostatic capacitor [50]

Supercapacitor

It has drawn much attention in the field of energy storage device because it is reckoned to be a bridge between battery and capacitor. It is well known that capacitors have tendency to charge and discharge at a faster rate, whereas batteries have the capability of storing greater energy. Supercapacitors have certain significant advantages such as high power density (greater than 10 kW kg−1), greater cycle life (more than 1,000,000 cycles), high rate capability, less maintenance required while working for long time, and low cost. Supercapacitors are used as temporary energy storage device when they are worked with fuel cells or batteries.

According to the energy storage mechanism and transfer of ions from the electrolyte to the electrode surface, supercapacitors can be categorized as electric double layer capacitor (EDLC), pseudocapacitor, and hybrid capacitor.

Electric Double Layer Capacitor (EDLC)

EDLCs are developed using two carbon-based electrode materials, an electrolyte and a separator. EDLCs can store charge either electrostatically or through a non-faradaic method that does not require transfer of charge between the electrode and the electrolyte (shown in Fig. 4). The energy storage concept used by EDLCs is the electrochemical double layer. When voltage is applied to the supercapacitor, ions in the electrolyte diffuse into pores of the electrode of opposite charge. Charge accumulates at the electrode/electrolyte interface, forming two charged layers (double layer) with just an incredibly small distance of separation. This is the distance from the surfaces of the electrode to center of the ion layer. The carbon materials used for these capacitors provides a high surface area with a distance of only a few angstroms (0.1 nm), owing to their porosity. Here, the capacitance is proportional to the surface area and inversely proportional to distance between the two layers. Due to this very small space, high capacitance values could be achieved.
Fig. 4

Electric double layer capacitor (EDLC)

No chemical or compositional variations are involved with non-faradaic operations, because there is no transfer of charge between electrode and electrolyte. For this purpose, charge storage in EDLCs is greatly reversible and allows them to obtain high cycling stability. EDLCs typically operate too many charge/discharge cycles. But, on account of the electrostatic surface charging process, EDLC experience a low energy density. Of late, the researchers have been making efforts on new materials to enhance energy performance and the operating temperature range of EDLC.

Pseudocapacitor

It is another type of supercapacitor between a battery and double-layer capacitor. They store charge through faradaic process that involve charge transfer between electrode and electrolyte. It is also made up of two electrodes separated by electrolyte (shown in Fig. 5). When voltage is given to a pseudocapacitor, reduction and oxidation occur at the electrode, which allows the transfer of charge across the double layer, resulting in faradaic current flowing through the supercapacitor. The charge is stored electrostatically with a chemical reaction at the electrode, without an interaction of the electrode with ions.
Fig. 5

Pseudocapacitor

The capacitance values of pseudocapacitors are higher due to multiple processes involved to store charge. When compared to EDLC, it provides high specific capacitance and energy density due to a faradaic system. The choice of supercapacitor, however, depends on application and availability. Owing to multiple oxidation states, transition metal oxide materials also exhibited high-specific capacitance with pseudocapacitive behavior and are used for many applications. Some other materials used for pseudocapacitors are conducting polymers. Conway stated that many faradaic mechanisms can lead to electrochemical capacitive properties [10].

In the redox pseudocapacitance, the charge transfer from electrode to electrolyte takes place by redox reactions (faradaic reaction). Redox reactions imply reactions of reduction-oxidation. At the time of oxidation and reduction process, the oxidation state of the materials gets changed. Reduction takes place when the electrons are accepted while oxidation state gets reduced. Oxidation means electrons are released and the oxidation state increases. For ruthenium oxide (hydration) with pseudocapacity, redox occurred through the accepting and releasing of electrolyte protons. It also receives electrons and reduces its oxidation state from +4 to +3 during the acceptance of protons [42]. Intercalation in the pseudocapacitor occurs when the ions insertion (or) intercalate in a layer of redox-active material followed by faradaic charge transfer without any crystallographic phase change. An adequate number of electrons could be passed to the host while insertion to ensure an electrical neutrality of electrode. Insertion should be constrained by ion’s ability to diffuse via electrode material.

Hybrid Supercapacitor

As discussed above, EDLCs provided excellent cyclic stable, higher power density while pseudocapacitor exhibited greater specific capacitance. However, EDLC exhibit low energy density, and at the same time, pseudocapacitor has low cyclic stability. So, two factors decide the energy density of SCs: the capacitance of the electrode and the voltage of the cell. An alternative method to increase the capacitance value is to prepare a nanosized and porous material as electrode with improved energy density. Hybrid supercapacitors show better performance by overcoming the disadvantages of both EDLCs and pseudocapacitors.

Hybrid electrode configuration comprises two different electrodes made up of different materials, whereas composite electrodes consist of one type of material which is integrated within the same electrode into another. Hybrid supercapacitors exhibit improved electrochemical behavior when consisting of two different electrodes made from different materials than that of the individual ones.

Hybrid systems were fabricated with energy source of a battery (faradaic) like electrode and a power source of a capacitor (non-faradaic) like electrode (seen in Fig. 6). The combined structure has higher operating potential and gives greater specific capacitance that value is two to three times greater than conventional capacitors, EDLC and pseudocapacitors. It stores charge by both faradaic and non-faradaic process which allowed high energy and power densities than EDLC and greater cyclic stability than pseudocapacitor.
Fig. 6

Hybrid supercapacitor

There are three types of hybrid capacitors namely, asymmetric hybrids, battery-type hybrids and composite hybrids and the electrode configuration of each type has differed. Asymmetric hybrids supercapacitor integrates faradaic and non-faradaic reactions by combining a pseudocapacitive electrode with an EDLC electrode [15]. Generally, the carbon-based materials are used as negative electrode and the pseudocapacitor materials as positive electrode. Battery-type hybrid supercapacitor has made up of integrating a battery electrode with supercapacitor electrode. This type of hybrid capacitors exhibits the improved performance characteristics of both batteries and supercapacitors such as energy density of batteries and recharge time, cycle time and power density of supercapacitors. Composite hybrids combine carbon-based materials with either conducting polymers or metal oxides. The electrode materials could enhance corrosion stability, enhanced the specific capacitance and the working potential.

At the end, various functional parameters of the above discussed energy storage devices are furnished in Table 1.
Table 1

Comparisons between capacitors, batteries, fuel cells, and supercapacitors

Parameters

Capacitors

Batteries

Fuel cells

Supercapacitors

Power density (W/kg)

˃5000

100–3000

1–1000

5000–10,000

Energy density (Wh/kg)

0.01–0.3

30–265

500–2000

0.5–20

Charge/discharge time

1 ps – 1 ms

1–10 h

Not applicable

1 ms – 1 s

Operating voltage range (V)

6–800

1.2–4.2

0.6–0.7

1.0–4.5

Cycle life (cycles)

More than one million

150–2000

Not applicable

Up to one million

Operating temperature range (°C)

−20 to +100

−20 to +65

+50 to +1000

−40 to +85

Cost per Wh

0.10–1

1–2

0.035–0.05

10–20

Electrode Materials

The capacitance and charge storage of supercapacitor depend strongly on the materials used for the electrode. Hence the most important approach to address these challenges is the further development of new materials with high capacity and improved performance compared to existing electrode materials. The capacitance of supercapacitor actually depends on the particular electrode surface area. Since the material contacts with an electrolyte and not all precise surface area are electrochemically accessible, the measured capacitance of the different materials is not linearly increased with the increasing surface area.

Depending on the required energy storage type for the many application and the appropriate capacitance ranges, supercapacitor can be constructed from different materials. Electrode materials of supercapacitor may be divided into three different types based on their use for EDLCs, pseudocapacitors, and hybrid supercapacitors (Table 2). For supercapacitors, a significant number of materials are currently available; carbon is the main commercial material that is generally used and converted into many structures.
Table 2

Types of supercapacitors and their electrode materials

Electric double layer capacitors

Activated carbons

Carbon aerogels

Carbon nanotubes

Carbon fibers

Pseudocapacitors

Metal oxides

Conducting polymers

Hybrid capacitors

 Asymmetric

 Composite

 Battery-type

Carbon materials, metal oxides

Carbon materials, conducting polymers

The metal oxide or conducting polymer used as electrode material for over a certain number of cycles (<10,000) starts to crack and loses capacitance. In order to improve the capacitance, the electrodes of the supercapacitor have to be modified. So, creating a nanocomposite material by incorporating the carbon and metal oxide/conducting polymer or other combinations enhances the capacitance of the supercapacitor.

Carbon Materials

The commonly used electrode materials for EDLC are activated carbon which provides large surface area due to their well porous structure and less expensive than other carbon materials. The highly porous structure can cause some problems. Pores that could be smaller than the electrolyte ions will not help to store the charge. High porosity can also lead to poor conductivity and reduce the maximum power density. Furthermore, this increases the electrolyte resistance which in turn could limit the charge and discharge rates.

In EDLC supercapacitors, typically carbon aerogels are used. They consist of continuous networks of carbon nanoparticles with interspaced mesopores, which enable them to be used without the need for a binder in electrodes. Consequently, carbon aerogel electrodes have lower equivalent series resistance, which helps to increase power density. Graphene is an excellent electronic conductor with a high theoretical specific surface area of 2630 m2 g−1, good chemical stability, and excellent mechanical properties. Graphene’s two-dimensional sheet structure proposes a great building block and ideal conductive platform for accommodating active materials of nano size. The flexible mesopore and macropore graphene structures [47] allow nanoscale materials to fill up the void space while simultaneously providing conduction channels for electrons transport.

Reduced graphene oxide (RGO) has been established as an advanced supercapacitor material due to its high specific surface areas (~2630 mg−1) [52], reasonably good mechanical characteristic, good electrical conductivity, as well as abundant graphite. Low-agglomerated reduced graphene achieved a highest specific capacitance of 250 F/g with the energy density of 28.5 Whkg−1.

Composite Materials

Instead of the materials used for ES electrodes, the combination of various materials form composites is an essential method, since the single materials in the composites can get a synergistic effect by reducing particle size, improving specific surface area, producing porosity, avoiding agglomeration of particles, allowing electron and proton conduction, enhancing active sites, widening the potential window, maintaining active materials against mechanical deterioration, increasing cycling stability, and providing extra pseudocapacity.

Mostly the fabricated composites materials can resolve the limitations of the individual materials and show the benefits of all constituents. High capacitance of 1700 Fg−1 stated based on composite materials [53]. However, it should be noted that reverse effects can also be achieved during the synthesis of composites. However, it is important to point out where the reverse effects can also occur during the composite development process.

Therefore, the synthesis of individual materials and an optimal molar ratio of components for each composite material have to be compromised. In composite materials, the carbon material is combined either with metal oxides or conducting polymers-based materials. Carbon-based materials such as graphene and CNTs are commonly used in the electrode materials, but their specific capacitance is low. On the other hand, the metal oxides and conducting polymers were shown to offer a good pseudocapacitance that produces maximum energy storage.

Besides, they provide low cyclic stability during charge/discharge cycle and as a result restrain their development in realistic supercapacitors applications. Such problems can be reduced by using composite materials as electrodes. This can improve the performance of supercapacitors in order to efficiently meet out the energy storage demand.

Conducting Polymers

The conducing polymers gained much significance as an electrode because of their high energy density, feasibility, ease of manufacturing, etc. The conductive polymers store bulk charges. The energy density of the polymer material is higher than the redox material. The slow ion diffusion in the bulk of the material has an effect on the power density. The low power density of polymer electrodes is a major drawback. Conducting polymers have a higher conductivity than metal oxides which makes them an attractive option. Polyaniline, polyprene, and polythiophene derivatives are the most frequently examined polymers.

The diffusion of the ion in the polymer results in a change in the volume of the polymer causing enormous stress on the structure of the polymer. As cycle number increases, the structure eventually collapses and the polymer’s capacitance is reduced. The polymer degradation also happens more rapidly at high scan rate. The conductive polymers are combined with metal oxides such as ruthenium oxide, manganese oxide, cobalt oxide, nickel oxide, etc. to form a composite. The composite exhibits higher specific capacitance and greater cyclic stability than pure polymer electrode. The ion diffusion in such a composite is still relatively slower, resulting in higher current densities. Ternary composites are tested with various combinations of polymers and metal oxides. Carbon and its derivatives have good conductivity and have good mechanical strength. The intimate relation between electrodes ensures the required conductivity and surface area for the ion to diffuse faster.

Metal Oxide

Metal oxide-based electrode materials were used in supercapacitors because of their high conductivity. Metal oxide electrodes provide large specific capacitance and have a long operating time. While comparing to conventional carbon material, metal oxide materials exhibit a higher energy density and provide greater cyclic stability than polymeric materials. Metal oxide materials not only store the energy similar to carbon material but also reveal faradaic reaction between electrode and ions within suitable potential windows. B.E. Conway [9] stated that metal oxides, for example, RuO2, IrO2, Fe3O4, MnO2, NiO, and Co3O4, etc. have reached high pseudocapacitance.

Ruthenium Oxide

Ruthenium oxide is a material of significant interest in supercapacitor. Ruthenium oxide offers higher capacitance (theoretical capacitance: 1200–2200 Fg−1) due to its hydrous form than conducting polymers and carbon materials. It combines conductivity of metal and reversible redox reactions that occur at the electrode-electrolyte interface as well as in bulk material [25]. Among the transition metal oxides, RuOx was the most widely studied material due to its highly reversible redox reactions, wide potential window (up to 1.4 V), high proton conductivity, three distinct oxidation states accessible within a 1.2 V voltage window, remarkably high specific capacitance, long cycle life, good thermal stability, high rate capability, and metallic type conductivity [38]. The equivalent series resistance of RuO2-based electrode material is much less than other electrode materials. Therefore, it has higher energy density and power density than conducting polymers and carbon supercapacitors, but it is very expensive and shows poor performance at high current densities.

MnO2

Considering cost and toxicity of RuO2, numerous other metal oxides have been examined for replacement. MnO2 was considered an appropriate candidate because its advantages as a high theoretical capacitance of 1370 Fg−1 abundance, a wide operating potential window of approximately 0.9–1.0 V, low cost, and environment friendly [45].

The structural differences of MnO2 found in the 1D, 2D, or the 3D arrangement resulted in different charge storage capacity. There are seven different MnO2 polymorphs known as 1D structure: hollandite, ramsdellite, pyrolusite, Nitodorokite and octahedral molecular sieves (OMS-5), 2D structure birnessite, and 3D spinel. The capacitances of some of the polymorphs are given in Table 3.
Table 3

Specific capacitance of various polymorphs of MnO2

Polymorphs

Electrolyte

Structure

Capacitance

Spinel (λ)

0.5 M K2So4

3D

241

Birnessite(δ)

0.5 M K2So4

2D

225

OMS-5

0.5 M K2So4

1D

217

Nitodorokite

0.5 M K2So4

1D

42

Pyrolusite (β)

0.5 M K2So4

1D

28

Ramsdellite (γ)

0.5 M K2So4

1D

87

Hollandite (α)

0.5 M K2So4

1D

125

The capacitance of different polymorphs has been greatly influenced by microstructure, tunnel size, and ionic conductivity. It was also noticed that the capacitance increased with structural change from 1D to 3D. In addition to nanostructured MnO2, there has also been extensive investigation into the combination of MnO2 with other materials, such as graphite and CNTs. The incorporation of other materials into MnO2 can progress the electron conductivity of the electrode, extend the operating potential, and ensure efficient use of MnO2. As a result, the composite electrodes can display higher capacitances, higher energy and power densities.

For example, ultrathin film (several nanometers) of MnO2 coated on the highly conductive Zn2SnO4 nanowires developed on carbon microfibers revealed maximum capacitance of 642.4 Fg−1 in M Na2SO4, energy density of 36.8 Wh kg−1, and power density of 32 kW kg−1 at 40 Ag−1 with a good long-term cycling stability. The composite of graphene-MnO2 (78 wt% MnO2) fabricated by redox reaction revealed a capacitance of 310 Fg−1at 2 mV s−1, which is almost three times greater than the pure grapheme (104 Fg−1) and MnO2-δ (103 Fg−1) [49].

Cobalt Oxide

Spinal Co3O4 can be considered as one of the best alternate materials to hydrous RuO2 due to its abundance, environmental friendliness, high theoretical capacitance (3560 Fg−1), controllable size and shape, tunable surface and structural properties, good electrochemical performance in alkaline solutions because of its capability to interact with electrolyte ions not only at the surface but also throughout the bulk, low cost, and favorable pseudocapacitive characteristics.

The efficiency of electron and ion transport for charge storage in Co3O4-based pseudocapacitors mostly depends on electrode properties such as surface area, morphology, and alignment of nanocrystalline phase. The cobalt oxide electrode has good efficiency, long-term performance, and corrosion stability. However, the operating potential is low (0.45 V) and, it is concerned with less cyclic stability. In order to extend the operating voltage and to achieve greater cyclic stability, cobalt oxide materials are combined with carbon-based materials. For example, coating of Co3O4 on carbon fiber paper and planar graphitized carbon paper by solvothermal provided the capacitance of 1525 Fg−1 and 1199 Fg−1 for the two electrodes. The cyclic stability achieved after 5000 cycle was 94% and 91%. The use of carbon substrate with Co3O4 showed better cyclic stability [46].

NiO/Ni(OH)2

Nickel oxide is considered to be an alternative electrode material for supercapacitors in alkaline electrolytes because of its fast synthesis, relatively high capacitance (theoretical capacitance of 3750 Fg−1), environmental friendliness, and low cost [47]. The issue of using NiO-based electrode materials for supercapacitor is poor cycle performance and high resistivity (low electrical conductivity). For example, Ni(OH)2 film synthesized by electrodepositing exhibited the capacitance of 578 Fg−1. However, the capacitance value was gradually decreased after 400 cycles, approximately 4.5% because of degradation of the material’s microstructure [55]. To deal these issues, NiO nanocomposite materials were fabricated. In NiO/CNT nanocomposite, the CNT material provides high surface area which increases the redox reaction as well as electrical conductivity of NiO. As a result, NiOx/CNT nanocomposite electrode showed the capacitance of 1000 Fg−1 which is approximately three times higher than the 350 Fg−1 of NiOx thin film electrode material.

It is recognized that nickel oxide electrochemical surface reactivity is strongly dependent on its crystallinity. During synthesis, the calcinations temperature can have significant impact on the crystalline structure of NiOx, the value of x, and the specific capacitance. More crystallization occurs at above 280 °C, and the value of x decreases. Therefore, a maximum capacitance of 696 Fg−1 can be obtained at 250 °C. However, the capacitance decreases significantly when a Ni(OH)2/CNT electrode is heated up to 300 °C. Nickel oxides with a hierarchic porous texture have been developed to improve the unsatisfactory porous Ni oxide structures, which can limit the transport of electrolyte ions and result in slow electrochemical processes in charge storage and delivery [44]. As a result, the nickel oxide capacitance retention ratio improved at a high potential scan rate, as the open hierarchical porous texture allowed ions to easily access the electrode/electrolyte interface.

The preparation process has also been shown to have significant effects on the electrochemical behavior of NiO. For example, NiO with a cubic structure, synthesized through a chemical process, had a maximum specific capacitance of 167 Fg−1, while porous NiO obtained via a sol–gel method showed a specific capacitance of 200–250 Fg−1, and its specific capacitance was 696 Fg−1 after annealing at 250 °C [8]. For further development, a much better performance can be obtained if the microporous structure of this type of material is optimized. For example, a nickel hydroxide material with a mesoporous structure was stated by Kong et al. [22] which exhibited the capacitance of 2055 Fg−1 at 0.625 Ag−1. The charge transfer resistance was greatly reduced after adding mesoporous carbon into the nickel hydroxide to form a composite of nickel hydroxide/mesoporous carbon, resulting in very high capacitance of 2570 Fg−1 [53]. Therefore, both Ni(OH)2 and Co(OH)2–Ni(OH)2 should be appropriate materials for supercapacitors in terms of capacitance. Unfortunately, the operating voltage range of such materials is too narrow.

ZnO

Zinc oxide (ZnO) is used in various fields such as optoelectronics, optics, sensor, solar cell, surface switches, actuators, supercapacitors, and biological imaging. The electrochemical characteristics of ZnO are furthermore analyzed and reported [51]. ZnO used as the electrode material has the potential to achieve improved specific capacitance. But during consecutive cycling, it has the drawback of forming dendrite growth which leads to a decrease in cycle life. Zinc oxide is reckoned to be a suitable electrode material for supercapacitors because of the high theoretical capacitance values, low cost, environmental friendliness, high electrochemical stability, etc.

The low-charge storage capability and limited operating potential window, however, limit their additional applications in SCs. To overcome these limitations, various carbon materials such as graphene, reduced graphene oxides [2], carbon nanotubes (CNTs), graphitic carbon nanofibers, carbon arrays, and activated carbon have been employed to modify ZnO-based electrode materials because of their contributions to additional electric double layer capacitance and expanded working potential window. However, the low adhesive force between carbon materials and ZnO nanocrystals generate electrode pulverization, leading to low cycle stability. Moreover, the highest reported operating voltage range for ZnO/carbon composite material is 1 V, which evidently reduces the energy density. ZnO/rGO composite showed enhanced conductivity, electrochemical stability, and specific capacitance than ZnO.

TiO2

Despite its low cost, natural abundance, and environmentally friendly nature, TiO2 is one of the most promising candidates. Titanium dioxide (TiO2) has been used in various fields, including pigments, self-cleaning, photochromic devices, sensors, dye-sensitized solar cells, lithium ion batteries, supercapacitor, and mainly used as photocatalyst to decompose organic contaminants from wastewater or air treatment. TiO2-B nanotubes were synthesized by solvothermal reaction and the specific capacitance obtained was 17.7 Fg−1 [47]. Salari et al. [37] reported the TiO2 nanotubes and TiO2 nanopowder prepared by anodization and sol–gel technique showed the capacitance of 911 and 181 μF cm−2, respectively.

In addition, pure TiO2 has undesirable capacitance due to its high conduction resistivity. The reliability of the battery type TiO2 electrode material has been restricted in energy storage due to its low electrical conductivity and cyclic stability, particularly several cycling which can lead to loss of electrochemical sites [1]. Furthermore, pure TiO2 has an unacceptable capacitance because of its high resistivity. TiO2 combined with conducting polymer and carbon materials provided the improved performance. Example, in TiO2/rGO nanocomposite, TiO2 material acts as a spacer to inhibit the restack of rGO sheet [35]. In addition, rGO should be considered as great support for TiO2 to improve the cycling stability of fabricated materials and lower the internal resistance of supercapacitors [3].

Electrolytes

Electrolytes used in supercapacitors are categorized into two types: aqueous and nonaqueous. The commercially produced supercapacitors typically consist of organic electrolytes that acquire a wide range of operating voltage. Mostly, the asymmetric hybrid supercapacitors employed aqueous electrolytes with enhanced ionic conductivity. Electrolytes play an important role in the supercapacitor performance. In accordance with Burke, the potential window variation arises owing to the nature of electrolytes and intrinsic resistance [7]. The square of the potential window is proportional to the specific energy and the power capability is inversely proportional to the ionic resistance. At present, for hybrid supercapacitors, three kinds of electrolytes are employed, namely, ionic liquids, organic, and aqueous.

Aqueous electrolytes such as H2SO4, KOH, and KCl are very commonly used electrolytes in supercapacitors because of their abundant and reduced costs. The capacitance varies according to electrolyte choice. Use of aqueous electrolytes provides improved specific capacitance but limit the operating voltage range. Aqua-electrolytes have a voltage window of 1.2 V, and further voltage increase have implications, such as cell damage by building pressures and water decomposition [39]. Qu et al. [33] examined MnO2 nanorods’ fabrication with neutral aqueous electrolytes (e.g., Li2SO4, K2SO4, and Na2SO4). The newly manufactured activated carbon supercapacitor showed a good cycling stability with high power density of 2 KW kg−1 and energy density of 17 Wh kg−1.

Organic electrolytes are generally based on acetonitrile or propylene carbonate that allows higher voltage per cell. The organic electrolytes have a greater operating voltage (0–2.7) than the aqueous electrolytes. This wider operating voltage range raises the energy density significantly higher than that of aqueous electrolytes. The two abovementioned are the most preferred organic electrolytes because of their lower ionic resistivity, although their toxicity and inflammable effects limit their application. The organic electrolytes have low conductivity, low capacitance, expensive, volatile, flammable, and toxic. The organic electrolytes 1 M LiTFSI/CAN used with carbon//V2O5 hybrid composite electrode materials revealed wide operating potential window [54]. In addition, organic electrolytes include complex assembly and purification processes under certain conditions to remove impurities (i.e., humidity) which result in severe self-discharge and performance degradation problems.

The proper selection of electrolyte is important among the main parts of a supercapacitor to maintain the stability of the electrode materials at high temperatures. The organic electrolytes used by the supercapacitors are not suitable for commercial application at the temperatures over 70 °C owing to their detonation temperature and low ignition, the boiling water temperature of aqueous electrolytes are restricted and could not be used actually above 80 °C.

Ionic electrolyte is even used at very high temperatures due to their superior chemical and thermal stability, wide operating voltage in the range of 0–5 V, nonflammability, and marginal vapor pressure [4]. Moreover, ionic liquids have limited ionic conductivity than the aqueous and organic electrolytes. The electrolyte has two different functions. It is not only involved in conductance but also functions as a dissociation path as well. Zarrougui et al. [54] developed six novel ionic liquid electrolytes with low viscosity as supercapacitor. Based on these electrolytes, the electrochemically synthesized method displayed excellent performance, such as large capacitance (135–228 Fg−1), energy density (41–115 Wh g−1), and good cycling stability.

Synthesis Techniques

There are different techniques being employed to fabricate the electrode materials for supercapacitor applications. They are discussed in the following subsections.

Sol–Gel

It is a simple way to create more pure and homogeneous materials. In this method, the solution (sol) combines and integrates microparticles into an integrated network (gel) under the scrupulous conditions. The usefulness of the sol–gel technique is due to the mixing at a much lower temperature of chemicals (precursors) that offers good control of different components at atomic level. The colloidal method and polymeric method differ from each other in the form of precursor used are two basic variants of the sol–gel process. In the two processes, the precursor is dissolved into a solvent. Then as the activated precursor obtained, it reacts to form a network that grows at maximum temperature and time to the container size.

Most of the transition metal oxides were prepared using this method. This method offers the benefit of synthesizing materials for different morphologies. The electrode material provided by this technique has high specific surface area and better electrochemical performance (Goikolea et al. 2016). The reaction time, solvents, surfactants, and temperature are the key factors of this technique to achieve desirable structures with notable electrochemical performance. Using sol–gel technique, NiCo2O4 films were deposited and the specific capacitance of 2157 Fg−1 at current density of 0.133 mAcm−2 was achieved [29]. Yusin and Bannov [53] studied on the fabrication of composite including activated carbon fiber material Ni(OH)2 that shows the specific capacitance of 380 Fg−1. Kim et al. [21] stated the use of the sol–gel technique towards the synthesis of NiO nanoparticles and capacitive characteristics of the electrode materials with respect to different morphologies were determined. Further, it is deduced that the concentration as well as composition of the solution could determine the morphology and mass of the active materials.

Hydrothermal/Solvothermal Technique

The hydrothermal approach is the most famous among the scientists and technologists of various disciplines. It can be described as a superheated, aquatic solution that is environmentally friendly. This technique is superior to other methods, because it is suitable for the preparation of particles (highly sterile, crystalline, qualitative, and regulated physiochemical properties). It is a low-temperature sintering procedure with a fast, easy-to-implement, and scale energy requirement.

However, it has little control over the agglomeration of nanoparticles. In the supercritical process, the solvent properties (e.g., dielectric aggregation, solubility) change drastically. Therefore, supercritical phase provides favorable conditions for particle formation due to improved reaction and high supersaturation. The technique is termed as solvothermic synthesis, if another solvent is employed rather than waters. Many electrode materials are synthesized with this technique, including hexagonal NiCo2O4 nanoparticles [32], CoS2-rGO [43], and CoWO4/Co1-xS rod-like hollow [13], etc.

Coprecipitation Technique

It is an easy approach to produce large-scale powder samples. In order to achieve precipitation, the concentration of one solution has to be higher than solubility limit and also, higher temperature has to be preferred to easily separate it into precipitates. On account of the rapid rate of precipitation, it is tough to control the structure of fabricated samples. Several structures of CoFe2O4 nanocomposite using different precursors [19] were accomplished by this technique. Further, Ni3(PO4)2@GO nanocomposite [26] achieved the specific capacitance of 1329.59 Fg−1 at 0.5 Ag−1 and capacitance retention of the nanocomposite was found to be 88% after 1000 charge/discharge cycles.

In-Situ Polymerization

With the help of ultrasonicator, monomers are suspended in an aqueous solution in this approach. An oxidizing agent is then mixed in order to start the polymerization of the aqueous solution and filtering process extracts the sample from the solution. Through this method, less percentage of nanofibers was obtained and most of the samples were irregular agglomerated samples. However, with little alteration, nanoparticles, nanofibers, and nanorods were studied with improved solution processability. Using in-situ polymerization, PEDOT structures was grown on carbon fiber cloth [34]. When these nanostructures were used for supercapacitor applications, they were able to achieve the specific capacitance of 203 Fg−1, energy density of 4.4 Wh kg−1, and power density of 40.25 kW kg−1. In another work, PANI nanowires were deposited on the surface of MWCNTs by means of in situ electropolymerization. The MWCNTs gave support to polymers and offered a path for the migration of charge. Further, MWCNTs could control the structure in PANI chains during charging/discharging and improve the life span of the structure.

Vacuum Filtration Technique

It is a fast and efficient technique to synthesize the nanocomposites, and it uses the principle of vacuum filtration. Here, the mixture of materials could be modified by changing the mass percentage of each component. Graphene suspension was established by vacuum filtration deposition. Zhang et al. [54] prepared graphene-based Ni foam as electrode material, which exhibits a greater energy density and power density with improved cyclic operations. Using this method, graphene/AC/PPy nanocomposite was fabricated and electrochemical performance of the sample was studied [48]. The specific capacitance of as-fabricated sample was found to be 178 Fg−1 at 0.5 mAcm−2 and after 5000 cyclic operations, it was able to retain only 64.4% of the specific capacitance.

Chemical Vapor Deposition (CVD)

In CVD technique, the substrate is exposed to precursors in the presence of heat or plasma inside the reaction chamber. Owing to chemical reaction or decomposition, the precursors are deposited on the substrate surface and form the thin powder and afterwards, a gas is passed inside the chamber so as to eliminate any volatile by-products. For the fabrication of electrode materials for supercapacitor applications, the CVD technique is also employed, mainly to synthesize CNTs and carbon nanofibers [36]. The preferred catalysts to obtain CNTs growth are iron, nickel, and cobalt [6]. The type and thickness of the catalysts could make an effect on the growth and structure of the CNTs [24].

CVD technique can be suggested to synthesize graphene-type materials. By CVD method, 3D networks of graphene are synthesized using ethanol as the source [5]. These networks are indeed useful to fabricate the composite including graphene and metal oxide and also, the specific capacitance of this composite-based electrode for supercapacitor is 816 Fg−1 at 5 mVs−1 scan rate. Kalam et al. [18] reported that supercapacitors with enhanced electrochemical properties could be prepared via CVD grown graphene/MWCNTs. Lobiak et al. [30] synthesized hybrid carbon materials comprising MWCNTs/graphitic layers, carried out by CVD, over MgO supported metal catalyst.

Electrochemical Deposition Technique

It is the process by which the application of electrical current creates a coating or layer of material on electrode. The layer is formed on the electrode, when the electrode is soaked in a salt solution of the substance to be coated with a negative charge. The salt ions provide a positive charge and these charges are attached to the cathode. When the positive charges are reached to substrate, salt ions receive electrons from the cathode and are reduced to form material. Several metal oxides/hydroxides-based composites as electrode materials were fabricated by means of electrochemical deposition. This method presents exclusive principles and provides flexibility on the control of morphology of the samples; however, it is not possible to prepare samples for large scale.

Three-dimensional NiO film with porous structure was prepared by Liang et al. [27]. The specific capacitance of NiO film was found to be 1670 Fg−1 at a 1 Ag−1 while the as-fabricated nanoporous film divulged better performance during the cycling test. Ni-NiO core-shell nanostructures were fabricated by using deposition of Ni on polystyrene bead template [20]. Then, NiO shell was formed by means of electrodeposition and thermal annealing. This study stated that electrode using higher thickness of NiO layers could achieve greater specific capacitance than that of lower thickness of NiO layers.

Applications

The continuously improved performance of supercapacitors increases their applications constantly. Over the past decade, the rechargeable batteries have dominated the energy storage industry. The demand for improved energy storage for various electronic portability applications and hybrid electric vehicles has been growing rapidly. Need for energy storage is to optimize performance and the costs. The supercapacitor applications are described in the following sections.

Automobiles and Transportation

Well-known requirement for the automotive industry is storage of energy used for multitasking operations such as ignition, start-up, protection, transmission and lighting, etc. Electric cars or hybrid electric vehicles actually require high power over short charging time which producing a power pulsation that can be easily achieved with a supercapacitor. The cars produced by Toyota and Mazda are examples of such automobiles [5]. Hybrid supercapacitors in large electric vehicles such as e-busses launched by Aowei Technology Co. Ltd., Shanghai, China, with fast charging property are another important application [31].

Public Sector

Maxwell trade (supercapacitor manufacturing company) has created the remote controls, operating with supercapacitors in joint project with Celadon. At first, two AAA batteries were used for the remote control whereas supercapacitor initiates a quick charging operation. Supercapacitors are used as stabilizers in wind and photovoltaic power line and are therefore used in renewable and sustainable sources of energy. Here they provide fast energy explosions to wind turbine because of its rapid response to changeable weather conditions [40]. One of the most requirements of all security systems are high power supply in short period of time. In this situation, supercapacitors could provide such power, and its function is comparable to uninterruptible power supply (UPS) working in computers.

The Thames cable cars (cable cars of Emirates Airline) are composed of integrated supercapacitor energy systems for quick lighting and air conditioning. The whole assembly is charged rapidly and has a riding time of 5 min [38]. In 2010, three Japanese companies Nippon-Chemi-Con collaborated on the use of solar and supercapacitors for energy storage to provide environmentally friendly street lamps in Japan.

The system consists of LED lamps with solar plates to absorb solar energy throughout the day and saves that energy in supercapacitor (during charging) while at the night time discharging of the supercapacitor deliver power to the LED lamps. This really offers a more reliable and clean energy system. Computers and memory backup chips in memory backup; the supercapacitor does not really serve as an energy storage device but gives memory backup feature during short power disruption. Supercapacitors find their use as a power supply stabilizer that meets the needs of fluctuating loads such as portable media player, laptops, and so on.

Medical and Industrial Applications

Hybrid supercapacitors can be linked with high voltage pulse distribution applications in the medical field. As such, they are fed to defibrillators which supply 500 J of energy to make the heart function normally [31]. Patients of mental trauma are diagnosed with similar procedures. In addition to dental technology, the ventilator backup could be a new application of supercapacitors. JSR Micro built a supercapacitor to medical imaging machinery in connection with the backup power source [23]. This is the main field for supercapacitor uses, for industrial electronics such as automated meter reading (AMR) to an emergency power backup supply to prevent sever destruction before power is recovered.

FastCAP developed a supercapacitor based power drills due to its greater stability [31]. These drills were used to emulate petroleum and geothermal power sections. These drills provide a feature benefit at higher temperature without transporting an overcharge. Dewalt Power tool Company has introduced drills with supercapacitors for their operation [31].

Defense and Military Applications

Battery-based tools such as sensors, communication tools, and navigators are an open area for supercapacitor uses. A proper supercapacitor assembly could also operate the torpedoes, electromagnetic pulse weapons, radar system, and so on [31]. Tecate Group produced numerous supercapacitors for military application. The applications which require high specific power include airbag exploitation power, avionics display, GPS and missile gadgets, and phased array of radar antenna.

Conclusions and Further Outlook

In this chapter, the development of nanocomposites-based electrodes for supercapacitor is emphasized. The working principles and salient features of electrical energy storage devices were mentioned. The uses of different active materials, namely carbon-based materials, conducting polymers, and metal oxides for electrodes of the supercapacitors, were discussed. Nanocomposites fabricated from various combinations based on metal oxides, carbon-based materials, and conducting polymers were stated in detail. Electrodes using nanocomposites revealed enhanced electrochemical performances. Though the nanocomposites-based electrodes achieved better performance, it requires more investigations to be carried out to find the best combination, which could yield increased specific capacitance, higher energy density, and cyclic stability. The novel nanocomposites to be prepared are required to have the following properties, like morphology, pore size, and wide potential window along with good ionic conductivity.

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Copyright information

© Springer Nature Switzerland AG 2020

Authors and Affiliations

  • P. Anandhi
    • 1
  • V. Jawahar Senthil Kumar
    • 1
  • S. Harikrishnan
    • 2
    Email author
  1. 1.Department of Electronics and Communication Engineering, College of Engineering GuindyAnna UniversityChennaiIndia
  2. 2.Department of Mechanical EngineeringKings Engineering College, IrungattukottaiChennaiIndia

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