Energy storage properties of graphene nanofillers

C H A P T E R

9 Energy storage properties of graphene nanofillers Jean Mulopo, Jibril Abdulsalam Sustainable Energy and Environment Research Group, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa

1. Introduction Global energy requirement is expected to grow as the world’s population increases. The energy sector is at the verge of a complete evolution as stakeholders are moving away from the traditional fossil-based energy to renewables. Energy storage is emerging to be the most crucial technology option that will enable the widespread adoption and integration of renewables. The impact of energy storage is far-reaching, as it enables renewable energy’s penetration, addresses the increasing energy demand of rural areas, and strengthens the value chain proposition of renewable energy. The huge prospects of energy storage have led to widespread interest among many stakeholders in the energy sector. The improvement seen in the economics of energy storage can be said to be a result of nanotechnology which has brought about huge development in material science advancements [1]. Nanoscale dimensions provide a huge increase in the physical interactions and physiochemical and chemical interfaces in materials. The acquired morphologies for the nanocomposites and the capacity to restructure the interfaces are important in maximizing the material properties and in particular, the variety of available combinations between nanofillers provides huge prospects for material property improvement [2]. To improve the capacity of the storage devices as regards energy and power density, considerable efforts have been geared toward the development of cutting-edge electrode materials. Accordingly, developing nanostructured materials into a well-conducting matrix provides a desirable performance with the potential to excellently store energy [3e7]. Graphene, a 2D sp2-bonded carbon atom laid out in a honeycomb crystal lattice, has drawn huge research interest in materials development [8]. It exhibits features such as large specific surface area, adaptability, chemically stable, exceptional electric, and thermal conductivity [9,10].

Graphene-Based Nanotechnologies for Energy and Environmental Applications https://doi.org/10.1016/B978-0-12-815811-1.00009-0

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9. Energy storage properties of graphene nanofillers

This chapter examines the unique features of graphene and graphene nanofillers. Experimental and theoretical investigations of these properties toward improving the storage capacity of graphene are well outlined. We also provided an overview of lithium-ion (Liion) batteries and supercapacitors as devices for storing energy. Developing a Li-ion battery with high power capacity largely depends on materials with high electrical conductivity to boost the movement of electrons and large specific surface area [7]. Graphene with its remarkable electrical conductivity, mechanical adaptability, exceptional chemical stability, and large surface area will be crucial in the evolvement of such materials [7,11e13]. The merits and limitations in the use of graphene as a material in energy storage, as well as its most promising results and applications to date are reviewed in this chapter. Finally, the challenges and future outlook for graphene nanofillers for energy storage applications are presented.

2. The economics of energy storage Most primary energy sources (coal, crude oil, gas) are stored easily. However, the storage of electricity in large quantity requires a conversion in other intermediate and storable forms of energy (thermal, chemical, kinetics) and recovery at usage points. The following key issues characterize the global energy landscape: a) Electrical energy usage will strongly increase in the next few decades due to the simplicity and flexibility it provides to users, causing global reduction in fossil fuel consumption; it also provides numerous potentials for long-distance transportation. b) The contribution of renewable energies in electrical energy production is also bound to increase if global targets of greenhouse emission reductions are to be achieved. Among these renewable energies, solar and wind energies are intermittent, whereas hydraulic, geothermal, and biomass energies may in most cases cover on spot production needs. c) Electrical energy demand should also increase due to the increase in global population and needs for countries’ growth and living standards that raises growing concerns over energy security and the protection of environmental pollution. The permanent equilibrium between offer and demand, which is required for the stability of electrical distribution networks, makes it indispensable that one integrates storage systems within renewable energy production and distribution networks. The storage systems may assume a buffer role between an intermittent production and an evolving demand whose fluctuations are not always in tune with the production periods. For a long time, electricity producers have been able to adapt, more or less effectively, their offer to consumer demand. The task was more or less achievable with a centralized production scheme, a relatively flexible means of production, and available fossil fuel stocks. The real challenges arise with the recent considerable development of renewable energy production [14]. Although predictable to some extent, these sources of energy are intermittent and not fully programmable and, moreover, their production is generally not centralized. Therefore, it seems essential to develop storage solutions in order to continue to ensure a balance between supply and demand or/and for combination through smart grids.

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Therefore, a major global challenge is how to achieve energy security coupled with a clean sustainable energy production and storage for subsequent consumption. The productive use of available energy resources will therefore largely depend on the accessibility to the energy as at when due, which is why energy storage becomes necessary. The storage of electrical energy in order to maintain equilibrium between the production and consumption of intermittent renewable energies at different levels of the network and within acceptable technical and economic boundaries is therefore an unavoidable prerequisite for the sustainable development of renewables energies. On a global level, energy storage will contribute to the following stakes: a. Environmental benefits linked to the potential of large-scale deployment of intermittent renewable energies. b. Capacity to tailor centralized or decentralized responses to local community constraints. c. Foster both technical and economic independence toward fossil fuels. Presently, there is a general understanding [15] that energy storage is key to influencing the energy sector by enabling extensive adoption and usage. The impact of energy storage is far-reaching. From the perspective of the community, energy storage can address the energy need of the rural communities thereby allowing consumers to tailor their energy consumption to their need hence strengthening the widespread adoption of renewable energy. Advances in materials and production would greatly improve the economics of energy storage. Conventional energy storage methods have limited applications and are fast giving way to emerging technologies, in particular batteries, capacitors, and fuel cells.

3. Graphene nanofillers Graphene is a carbon allotrope, arranged in a honeycomb crystal lattice of sp2-bonded carbon atoms [16,17]. The word graphene originated from Hans-Peter Boehm in 1962 using the combination of graphite and the suffix -ene [18]. To form graphite, graphene sheets are stacked with interplanar spacing of about 0.335 nm. For example, three million graphene sheets is about 1 mm thickness [19]. The span of graphene CeC is 0.142 nm [19]. Graphene forms key structural elements of carbon allotropes, namely fullerenes, carbon nanotubes (CNTs), charcoal, and graphite (Fig. 9.1). In a simple definition, nanofillers are nanoparticles or materials added to a material to reduce the use of a costly binder material or to improve on some properties of the mixed material. Graphene with high elastic modulus and excellent electrical conductivity is a good candidate for use as filler materials [20]. Graphene nanofillers have emerged as an attractive alternative to conventional nanofillers such as nanoclays, nano-oxides, CNTs, etc., but their characterizations and influence on the overall efficiency of the material into which they are incorporated are yet to be fully exploited. Nanostructures provide outstanding interactions (physical and chemical) in materials. Nanocomposites’ ability to modify existing properties of a material is crucial in improving the performance of such materials. The variety of available combinations and mixing between nanofillers enhance huge prospects of material improvement in terms of specifications, for

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FIGURE 9.1

Some carbon allotropes [16].

instance mechanical, thermal, electrical, as well as other energy storage properties. Development and continuous improvement in the quality of nanofillers will lead to increasing utilization of nanocomposites in a variety of applications. (Fig. 9.2) Graphene nanofillers’ emergence has attracted huge attention in materials development. The impact of the properties of graphene nanofillers on a material is usually focused on improving the overall performance and reducing cost of production. Nanofillers can significantly enhance or modify the different properties of the materials into which they are incorporated, such as the energy storage properties. Graphene is a suitable nanofiller in improving material properties such as physical, mechanical, and energy storage [22] (Tables 9.1 and 9.2).

FIGURE 9.2 Nano-objects used for nanocomposites, according to ISO/TS27687:2008 [21].

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TABLE 9.1

Properties of graphene and other carbon allotropes [23].

Dimensions

Graphene

Carbon nanotube Fullerene

2

1 2

Graphite

0 2

3 Sp2

Hybridization

Sp

Mainly Sp

Hardness

Highest

High

High

High

Flexible, elastic

Flexible, elastic

Elastic

Flexible, nonelastic

w1500

w1300

80e90

w10 e 20

Electrical conductivity (S cmL1)

w2000

Dependent on structure

10e10

Anisotropic: 2e3
104a, 6b

Thermal conductivity (W mL1KL1)

4840e5300

3500

0.4

Anisotropic:1500e2000a, 5e10b

Firmness 2

Surface area (m g experiments

)dfrom

L1

Mainly Sp

2

a

a direction. c direction.

b

TABLE 9.2

Specifications of some graphene nano fillers from literature. Dimensions

Materials

Synthesis method

Sizes

Thickness

References

xGnP

Thermal exfoliation of intercalated graphite

15 mm

<10 nm

[24]

Graphene

Thermal exfoliation of intercalated graphite

N/A

3e4 layers

[25]

xGnP-1

Thermal exfoliation of intercalated graphite

1 mm

xGnP-15 R-GN

Thermal exfoliation of intercalated graphite

[26]

15 mm

10 nm

[27]

N/A

30e100 nm

[28]

O-GN xGnP, exfoliated graphite nanoplatelets; xGnP-1 and xGnP-15, xGnP with an average size of 1 and 15 mm, respectively; R-GN and O-GN, graphite nanosheets with random and oriented distributions, respectively.

Graphene of different sheet sizes can be prepared using any of the following methods: mechanical cleavage, chemical vapor deposition, electrochemical exfoliation, as well as epitaxial growth [29e35]. Graphene is easily doped with nitrogen or boron, or altered using polymers, organic or inorganic components. Graphene-based materials derived from such doping or alteration are suitable for energy storage in devices like supercapacitors and batteries [36].

4. Graphene energy storage properties 4.1 Large surface area Surface area is a major property worth considering in order to enhance performance of graphene in storage devices. The electric double-layer capacitance is proportional to the

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effective specific surface area of an electrode material [36]. An increment in graphene’s surface area will significantly improve the storage capacity of electrochemical capacitors (ECs). By improving the surface area of a chemically reduced graphene oxide (rGO) electrode, Stoller et al. [37] got specific capacitances of 135,000 F kg1 and 99,000 F kg1 in aqueous and organic electrolytes [37]. Another innovative study to enlarge surface area was reported by Zhang et al. [38] in which the exfoliation and reduction of graphite oxide were carried out with the use of a microwave. This resulted in capacitance of 191 F g1 with potassium hydroxide as the electrolyte. Zhu et al. [14] reported BET surface area of 3100 m2g1 [39]. Sp2-bonded carbon structure of graphene contains a steady thick wall in a three-dimensional network, which leads to the development of pores of 0.6e5 nm in diameter. Increase in the surface area led to an improved performance of the material in electric double-layer capacitors (EDLCs) with specific capacitance of 165, 166, and 166 F g1 at current densities of 1.4, 2.8, and 5.7 A g1 [39]. 1654 m2 g1 in specific surface area was obtained when graphene of one to two nano mesh-layered structure was synthesized on a porous magnesium oxide layer [40]. The resultant large surface area led to an excellent capacitance performance in EDLC with 255 F g1 specific capacitance [40]. A larger surface area as a result of pore development in graphene nanosheet (GNS) yielded an enhanced capacitance of the capacitor [38]. The presence of pores increases electrode resistance thereby limiting the movement of electrons and ions, Miller et al. [41] proposed a solution with a demonstration using electrodes which were made from a vertically oriented GNS to filter 120 Hz current resulting in a minimized electronic and ionic resistance thereby greatly improving charge storage [41]. Study on complete surface area utilization of graphene was carried out by preparing curved graphene sheets, in which the sheets are stacked back to back, resulting in an improved storage performance [42]. Enlarging graphene surface area enhances the storage capacity of Li-ion battery. Graphene doped with nitrogen/boron was used as an anode material to prevent the reduction of coulombic efficiency because of increase in surface area [43]. The doped graphene achieved a capacity of 235 mAh g1. The enhanced capacity can be ascribed to the large surface area of graphene and the additional active dopants [43].

4.2 Electrical conductivity Electrical conductivity can be described as the property of a material that determines the extent to which the material can conduct electricity. Electrical conductivity also measures the capacity of the material to transmit electrical current. Electrical conductivity is an inherent property of a material. Electrical conductivity (s) is the reciprocal of the electrical resistivity (r): s ¼ 1=r

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(9.1)

4. Graphene energy storage properties

161

where resistivity for a material with a uniform cross-section is: r ¼ RA=l

(9.2)

where R is electrical resistance; A is cross-sectional area; l is length of material. Graphene exploits the influence of electrical double-layer capacitance based on its large surface area and electrical conductivity. Hybrid ECs with superior power and energy density will be dependent on developing novel electrode components, through cautious choice and combination of material that supports high energy storage in addition to showing excellent electrical conductivity with electrochemical stability; for example, graphene combined with transition metal oxides to construct nanostructures in high-performance ECs [44]. Procedures such as chemical doping, hybridization, and use of large-size GO sheets are used to enhance the electrical conductivities of graphene-based materials [45]. Chemical doping is proven to be efficient in modifying graphene’s electrical properties [46]. Hybridization is the bridging of the intersheet junctions between graphene or rGO sheets using highly conducting nanofillers, facilitating the restoration of graphene’s inherent electrical conductivity [47]. Reactive electrode materials and graphene combined enhances the cycle efficiency of Li-ion batteries [48]. Deng et al. [49] synthesized manganese oxide (Mn3O4) nanoparticle/rGO hybrid. The hybrid material exhibited significant capacity up to 900 mAh g1 with a stable cycle. Manganese (III) oxide (Mn3O4) nanoparticles exhibited poor performance when synthesized without graphene [49]. To improve the conductivity of EDLC, hydrogel used in a synthesized graphene and/or graphene oxide was reduced with hydrazine and hydroiodic acid thus improving specific capacitance to 220 F g1 from 200 F g1 [50,51]. In summary, novel advances in graphene nanostructures such as total utilization of surface area of graphene, enhancing the electrical conductivity which involves asymmetric utilization of graphene as additives in addition to other materials, will be crucial in developing highly efficient storage devices. 4.2.1 For graphene 2D conductivity s ¼ enm

(9.3)

m ¼ 200,000 cm2 V1s1 limited by acoustic phonons at n ¼ 1012 cm2 2D sheet resistivity r ¼ 31 U. 3D conductivity: 0.96
106 U1 cm1 higher than for copper of 0.60
106 U1 cm1 4.2.2 Graphene composites result in improved electrical properties Fc w 0.1vol% percolation threshold for polystyreneegraphene composite and sc w 0.1S m1dsufficient for many electrical applications (Fig. 9.3; Table 9.3)  t ð4  4c Þ sc ¼ sf ð1  4c Þ

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(9.4)

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9. Energy storage properties of graphene nanofillers

FIGURE 9.3 The electrical conductivity (S m1) of polystyreneegraphene composites versus the filler volume fraction (vol.%) [23].

TABLE 9.3

Electrical conductivity of some graphene-based materials based on different preparation methods.

Materials

Electrical conductivity (S m-1)

Reference

Chemical vapor density graphene layer hydrazine reduction of graphene oxide (GO) in DMF/water mixture

5
106 to 64
106

[52]

Hydrazine reduction of GO in ammonia Thermal annealing of GO in Ar/H2 at different temperatures

Air drying: 17
10

3

[53]



4



3



3

Annealing at 150 C: 16
10 w 7.2
10 Annealing at 550 C: 49
10 Annealing at 700 C: 93
10

3

[54] [55]

Annealing at 900 C: 38
104 Annealing at 1100 C: 55
104 Solvothermal GO in N-methyl-2-pyrrolidinone at 200 C in an open system

3.7
102

[56]

Solvothermal GO in propylene carbonate at 150 C

21
103

[27]

Annealing at 250 C: w 52
10

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4.3 Specific heat Specific heat is the measure of heat a unit mass of material requires to increase its temperature by 1 C. At a temperature change of 1K, the change in energy density of a material is referred to as its specific heat, represented by: C ¼ dU=dT

(9.5)

where C ¼ specific heat U ¼ energy density T ¼ absolute temperature The thermal energy stored inside a material can be ascertained from its specific heat and also the rate at which the materials cool or heat, which is its thermal time constant (s), represented as: szRCV

(9.6)

where R is thermal resistance; V is material volume. The thermal time constants for materials at nanoscale are very short [57]. A single sheet graphene has a thermal time constant of 0.1 ns [58]. There has been no indication of a direct measurement of the specific heat of graphene [57]. At room temperature, specific heat of graphite is 0.7 J g1 K1 [59,60]. The specific heat value of graphite is about 30% higher than the specific heat of diamond at ambient temperature [61]. Poor linking among the graphite layers gives a higher density of state at low phonon frequencies [59]. Specific heat of these materials is stored by lattice vibrations (phonons) and materials’ electrons free conduction [60,62]. At room temperature, specific heat behavior for these materials is similar for an isolated graphene sheet [57]. (Fig. 9.4)

FIGURE 9.4

Specific heats of graphite, diamond, and graphene [57].

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9. Energy storage properties of graphene nanofillers

104 Di

am

on

e

103

it ph

)

(II

a

Gr

Graphene (Supported)

MW

102

CN

T

k (W m-1 K-1)

Graphene (Suspended) SW CN T

d

GNR 10

Graphite (┴) c axis

1 10

100 T (K)

1000

FIGURE 9.5 Thermal conductivity of graphene, graphite, and diamond [57]. GNR, graphene nanoribbon; MWCNT, mutliwalled carbon nanotube; SWCNT, single-walled carbon nanotube.

At low temperature, the specific heat value can provide valuable insight about the dimensionality of the material and its phonon dispersion. From Fig. 9.5, it can be inferred that the specific heat at low temperatures is linear for isolated graphene sheet.

4.4 Thermal conductivity Thermal conductivity (k) defines the correlation between heat flux per unit area and temperature gradient as: Q ¼ kVT

(9.7)

where Q ¼ heat flux per unit area, Q (W m2) K ¼ thermal conductivity VT ¼ temperature gradient The negative sign in Eq. (9.7) shows that the flow of heat is from high temperature to low temperature. Eq. (9.8) describes the relationship between thermal conductivity and specific heat as: kzSC v l

(9.8)

where v and l are averaged phonon group velocity and mean free path, respectively [63].

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165

Thermal conductivity of graphene at ambient temperature is 2000e4000 W m1 K1, which is one of the highest for any known material [9,64,65]. Composites with metals, metal oxides, nanoparticles (attaching nanoparticles to graphene or wrapping nanoparticles in graphene) for various applications improves its thermal properties. For example, 25 vol% graphene filler in silicon foam matrix results in 6% increase of thermal conductivity [66]. The following can be summarized on thermal conductivity as an energy storage property: 1. Extrinsic thermal conductivity is limited by defects, impurities, and boundaries 2. The thermal conductivity of amorphous carbon is 0.01 W mK1, supported graphene is 600 W mK1, diamond is 2000 W mK1, CNT is 2300 W mK1, and suspended graphene is 2000 W mK1 [65]. 3. Intrinsic thermal conductivity is limited by phonons only

5. Energy storage devices In response to the need to protect the environment and to meet the demand of the fast developing global economy, a cleaner and efficient renewable energy and storage system becomes inevitable. The efficiency of each storage system or device will largely depend on the properties of its materials. Among energy storage solutions, two categories are particularly promising, namely electrochemical (batteries, fuel cells, hydrogen vectors) and electromagnetic solutions (capacitors, magnetic superconductor).

5.1 Capacitors Capacitors are electrical devices that store energy as electric charge in an electric field between two electrodes [67]. A capacitor is usually made up of two conductive electrodes in which an insulating material called dielectric separates them as shown in (Fig. 9.6). Applied voltage causes electric charge to be gathered on the surface of the electrodes which are isolated by the dielectric layer, hence, generating an electric field. The generated electric field thus allows the capacitor to store energy. (Fig. 9.6)

FIGURE 9.6

Schematic diagram of a capacitor [68].

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9. Energy storage properties of graphene nanofillers

The rate of stored energy discharge in capacitors is quiet high but with low storage capacity. Quantity of stored energy is usually defined in terms of voltage over the capacitor. The following equation further describes the correlation: W ¼

1 Q2 1 ¼ C$V 2 2 V 2

(9.9)

where W is the stored energy on the capacitor (Joules), Q is electric charge stored on the capacitor, C is capacitance, and V, voltage across the capacitor. The equation illustrates that energy stored on a capacitor is largely dependent on the capacitance and voltage of the capacitor. Capacitance, C, is usually defined in respect of electric charge and applied voltage. Eq. (9.10) illustrates this relationship: C ¼

Q V

(9.10)

If the capacitance varies with the voltage, then Eq. (9.10) can be rewritten as: C ¼

dQ dV

(9.11)

The capacitance can therefore be defined as capacitor’s ability to store energy (electric charge). The higher the capacitance of a capacitor, the better and the more energy it is able to store. To improve the capacitance of the capacitors, electrodes of large surface area is required; aside from that, materials (dielectric) that have high permittivity and that can reduce the spacing between the electrodes are required. These requirements can be further illustrated by the following equations: V ¼

Qd εA

(9.12)

V ¼ applied voltage Q ¼ electric charge A ¼ surface area ε ¼ permittivity of the dielectric material separating the electrodes with thickness, d. Substituting Eq. (9.10) into Eq. (9.12) will result in: C ¼

εA d

(9.13)

Carbon materials with excellent conductivity and porous structure like graphene, CNTs, and activated carbon can greatly enhance the capacity of capacitors when used as materials for electrodes [42,69e71]. For an energy storage device such as the capacitor, two key performance indicators are critical: the energy density and power density. These two parameters can be defined as

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167

energy or power per unit mass [67]. The power density of capacitors is usually above 5000 watt kilogram1 (W kg1), and energy density about 0.01e0.05 watt-hour kilogram1 Wh/kg [72]. It can be observed that while the power density of capacitors is high, it does have a low energy density. In differentiating capacitors from batteries, the process of energy storage is key. While batteries store energy through an electrochemical process in a chemical form, capacitors store energy in the form of charge in an electric field. The rate of discharge in capacitors is higher than that of batteries because of the chemical process that takes place in batteries. Capacitors store energy right onto the plates, hence making the rate of discharge dependent on the conduction capacity of the capacitor’s plates. Energy in batteries results in higher energy density defined as the capacity to store energy per mass. Unlike capacitors, the working principle of supercapacitors is anchored on an electrochemical double-layer formation, which is as a result of electrical chargers buildup at the interface between the electrolyte and the electrode. Oxy-reduction reactions do not take place but rather an electrostatic storage occurs through charge separation in a Helmholz double layer at each electrode (Fig. 9.7). The interface between the charges assumes the role of a dielectric. The combination of a high conductive surface and a low dielectric layer allows very high capacity values compared to traditional capacitors, whereas the electrolyte limits the tension of elements to few volts. The electrode is designed to provide the highest surface area using microporous carbon support or more recently graphene in a supercapacitor. This high surface allows the supercapacitor to have higher electrostatic storage more than traditional capacitor and keeps its energy much longer. Despite clear advantages such as faster charge, unlimited cycle life, and quick operation mode, supercapacitors still present major drawbacks, namely low energy density and rapid voltage drop as shown in Fig. 9.8. (Figs. 9.7 and 9.8). A potential way forward lies in the application of pseudocapacitors that accomplish the electrochemical storage through faradaic charge transfer via redox reactions, where the desolvated alkali metal cations pervade the double layer, adsorb on the transition metal oxide layer (RuO2, MnO2, etc.) of the anode, and transfer charge to it to enhance energy density by increasing surfaceevolume ratio of anode for redox reactions. In this regards, remarkable

FIGURE 9.7

Helmholz double layer, formed by the electrode, solvent, and solvated ions of the electrolyte [73].

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9. Energy storage properties of graphene nanofillers

FIGURE 9.8 An adapted Ragone chart showing the energy density of different storage systems [74].

FIGURE 9.9

Working principle of a supercapacitor [76].

advances have been made using graphene oxide electrodes, which gives 154 F g1 capacitance and 85.6 Wh kg1 high energy density [46]. The energy density is much higher compared to using a combination of transition metal-oxide nanowire/SWCNTs and thinfilm of 25.5 Wh kg1 energy density [64] or vertically aligned and electrochemically oxidized CNT electrodes of 53 Wh kg1 energy density [75] (Fig. 9.9). Polymers, carbons, and metal oxides are materials currently used in supercapacitor electrodes [38]. Activated carbon is commonly utilized for its better conductivity in addition to large surface area. One limitation with the use of activated carbon is the fact that not all the pores of the activated carbon can be used in supercapacitors. Pores of diameter (<1 nm) cannot be accessed by the electrolyte [77]. For graphene-based materials, such challenges do not exist. Graphene has emerged as an attractive alternative because of its excellent capacitance and low cost of production. Finding ways to increase graphene’s specific surface area is crucial to enhancing graphene performance in supercapacitors. Thus, graphene with a large surface area coupled with superior conductivity, amenable microstructure, and outstanding thermal and mechanical strength is a great material for supercapacitor electrodes [37,39,78] (Table 9.4).

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5. Energy storage devices

TABLE 9.4

Capacitance of graphene and graphene-based composites for supercapacitors.

Material

Capacitance

Reference

Chemically modified graphene

130 F g1

[37]

1

Hydrazine reduced graphene oxide (rGO)

264 F g

1

Exfoliated GO

120 F g

1

Exfoliated GO

[79] [78] [80]

75 F g

1

Solvated graphene films

156.5 F g

[81]

Microwave-activated GO

165 F g1

[39]

N-doped rGO

282 F g1

[82]

Poly-modified graphene

185 F g1

[75]

Vertically oriented graphene

130 F g1

[7]

Graphene/carbon nanotubes

2.8 mF cm2

[83]

MnO2/rGO

315 F g1

[84]

PANI/graphene

970 F g1

[85]

3D NiO/graphene foam

1100 F g1

[86]

5.2 Batteries Battery stores energy in the form of chemical energy which then converts the stored energy into electricity through a reaction known as the redox reaction. The main components of a battery are anode, cathode, and electrolyte (the chemical medium of which isolate the terminals and ease the movement of electrical charge between the terminals). The redox reaction is a reduction and oxidation reaction process. The reactions are basically a reduction of cations at the cathode terminal and the oxidation of anions at the anode terminal. In discharging the stored charge (in form of electricity), the oxidation reaction involves the release of electrons from the anode to the cathode and ions in electrolyte. The ions move current through the electrolyte, while electrons flow simultaneously in the external circuit, thus generating electric current. Batteries can be classified into disposable and rechargeable batteries based on their charging ability. Disposable batteries transform chemical energy to electrical energy in a nonreversible process. Disposable batteries supply energy (electrical energy) until the stored energy is exhausted. Rechargeable batteries on the other hand restore the battery to its original storage capacity (charge) through a charging process. Alkaline batteries are a typical example of disposable batteries, while Li-ion cells are a typical example of rechargeable batteries. Disposable batteries have higher energy density than rechargeable batteries [87]. The major advantages of electrochemical batteries are their design flexibility and reactivity. Electrochemical batteries are often destined for portable usages. Despite their relatively low power, they have however, a high storage capacity and long discharge time (many hours)

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9. Energy storage properties of graphene nanofillers

and often a high efficiency of about 70%e80%. Li-ion batteries have emerged as one of the best battery cells available having about 120e170 Wh kg energy density [42]. Li-ion batteries consist of a graphite anode, transition metal oxide cathode (LiXO2, X ¼ Mn, Fe, Co, Ni), and an electrolyte (Li salt, such as LiPF6, LiBF4, or LiClO4, in an organic solvent, such as ether). The energy storage is based on a faradaic redox reaction. Upon charging, Li ions permeate the anode and are reduced on it. Upon discharge, Li ions permeate to the cathode and are oxidized on it. Li-ion batteries do have some benefits and these includes high energy density and low self-discharge, however limitations such as special charging and high cost are some of its limitations [88]. The electrochemical activity of Li-ion batteries can be improved by anchoring, mixing, wrapping, or encapsulating the active cathode material particles in graphene, which forms a 3D conducting network (Fig. 9.10). An example is the LieS batteries where increase of electrochemical activity can be obtained by using modified sulfuregraphene oxide nanocomposite as the cathode constituent for safety consideration and cost reduction. Moreover, reduction in deterioration is achieved by using cetyltrimethyl ammonium bromide. This configuration allows Li-S batteries to have twice as much energy density as obtained in Li-ion batteries (Fig. 9.11).

FIGURE 9.10

Diagram of lithium-ion battery showing the active cathode material particles encapsulated in

graphene [89].

FIGURE 9.11 Schematic diagram of sulfuregraphene oxide nanocomposite structure [90]. CTAB, cetyltrimethyl ammonium bromide. II. Energy

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5. Energy storage devices

TABLE 9.5

Batteries storage performance overview [91].

Energy Power Temperature ( C) hE (Wh/I) (Wh/kg) (W kgL1)

Voltage Self-discharge Cycle life (V) (% per month) @80% DoD

30e60

Cost estimation (S kwhL1) ($/kw)

85 50e70

20e40

300

2.1

4e8

200

150

10

NiMH 20e50

80 200

40e60

1300e500

1.2

20

>2500

500

20

Li-ion 20e55

93 150e250 100e200 3000e800

w3.6

1e5

<2500

800

50e75

EDLC 30e65

97 5

w2.5

30

Not applicable

Lead acid

5e20

15,000

2000

50

NB, EDLC stands for electric double-layer capacitor.

TABLE 9.6

Performance of graphene and graphene-based materials as anode electrodes for lithium-ion batteries.

Materials

Specific capacities (mAh gL1)

Current densities (A gL1)

References

Graphene nanosheet (GNS)

540

0.05

[92]

GNS/carbon nanotube

730

0.05

[92]

GNS/C60

784

0.05

[92]

N-doped graphene

1043

0.05

[39]

B-doped graphene

1540

0.05

[39]

Reduced graphene oxide (rGO)/Fe3O4

520

1.75

[93]

Mn3O4/rGO

390

1.6

[94]

rGO/CO3O4

1000

0.074

[95]

3

[96]

Organic molecule/rGO

415

Two-dimensional edge plane site of graphene used as electrode in Li-ion batteries enhances adsorption and diffusion of Li-ion, resulting in a reduction in charge time and improved power output [88]. Storage capacity of 540 Ah kg1 was obtained when graphene sheets were used as electrode material in Li-ion battery; this capacity is higher compared to when graphite material was used [88]. Wu et al. [43] reported higher power and energy density when nitrogen-doped graphene was used. The improved performance was observed to be as a result of fast Li-ion absorption and diffusion (Tables 9.5 and 9.6).

5.3 Fuel cells Fuel cells are energy storage devices that are efficient with no adverse effect on the environment [36]. Just like batteries, energy conversion is from chemical energy of fuel to electrical energy. In place of producing heat by burning fuel, a direct conversion of chemical energy

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e-

ee-

eee-

O2 H+

H+

H2

H+

H+

H+

O2

O2

O2

H2O H2O Cathode

Anode

FIGURE 9.12

Schematic diagram of a fuel cell [36].

to electric energy takes place in fuel cells, with no pollution or discharge of harmful wastes to the environment. Recharging is not required on fuel cells and the accompanying products of fuel cell reaction such as water and heat are not harmful to the environment. Fuel cells are exceptionally reliable storage systems. With adequate and consistent supply of fuel, fuel cells can work uninterrupted (Fig. 9.12). In proton exchange membrane (PEM) fuel cell, the hydrogen enters the cell at the anode, where it is oxidized to ions and electrons; the electrons travel from anode to cathode, the hydrogen ions pass through the PEM to the cathode, where the ions interact with oxygen and are reduced to water. Platinum is used as a catalyst of the redox reactions. The fuel cell technology is an affordable (cheaper fuel), reliable (high energy density), and environmentally friendly source of power. Compared to other energy storage systems, fuel cells are said to have the highest energy density of above 500 Wh kg1 [97]. However, platinum remains very expensive [98]. Recent research has suggested the possibilities of the replacement of bulk Pt with Pt nanoparticles by using graphene as Pt nanoparticles support therefore obtaining higher surface area and lower production costs. For instance, Pt nanoparticles on rGO support have increased catalyst activity by about 80%. Other researchers have also reported nonplatinum catalysts: N-doped graphene [99] and nonmetal catalysts: edge-halogenated (Cl, Br, I) graphene nanoplatelets [100].

5.4 Other important application of graphene: solar cells In solar energy application, the intensity and duration of solar radiation are crucial. Photovoltaic cells made of silicon converts sunlight directly to electricity with about 17% efficiency

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5. Energy storage devices

FIGURE 9.13

173

Efficiency of solar cell [103].

[101]. Conventional solid-state solar cells implement a pen junction between two electrodes; with one of the electrodes being transparent; incident photon creates an electronehole pair, which travels to the electrodes; the emf is equal to the quasi-Fermi level separation (Fig. 9.13). To lower photovoltaic electricity price, some improvements are required which include: -

Enhanced technologies for cell fabrication Low-cost semiconductors, or use of low-cost materials Cost reduction of mounting cells and their support structures Flexible electrodes

However, efficiency of single-junction solar cell is limited to 32% at band gap of 1.1 eV [102]. This efficiency is limited by blackbody radiation losses, radiative recombination, and incident light spectrum losses as shown in Fig. 9.14 below.

FIGURE 9.14

Principle of operation and energy level scheme of the Grätzel cells [105].

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9. Energy storage properties of graphene nanofillers

To resolve these challenges, Grätzel cell (electrochemical solar cell) uses a photosensitizer in which incident photons are absorbed by the photosensitizer, when excited; the excited electrons are injected into the TiO2 electrode, thus oxidizing the photosensitizer; the electrons travel over the circuit to the other electrode, where they reduce by the photosensitizer [104]. Recent research have suggested the use of graphene as indium tin oxide (ITO) replacement in solar cells as illustrated in Figs. 9.15 and 9.16 [106]: The following key points can be summed up from the study carried out by Park et al. [106] as illustrated by Fig. 9.16. Anode: graphene Active layer: EDOT: PSS (absorber and p-type)/C60(n-type) Efficiency: 10%e15% of ITO PEDOT: PSS ¼ poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

FIGURE 9.15

FIGURE 9.16

Schematic diagram of the graphene anode organic photovoltaics architecture [106].

Diagram showing energy levels of materials used in perovskite solar cells [107].

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PEDOT: PEG (PC) ¼ poly(3,4-ethylenedioxythiophene)-block-poly(ethylene glycol) (perchlorate) Super structured perovskite solar cell solution-based fabrication at <150 C. Active layer: Perovskite (absorber and p-type)/graphene-TiO. Efficiency 15.6% FTO ¼ fluorine-doped tin oxide coated glass. Perovskite ¼ Al2O3/CH3NH3PbI3-xClx

6. Conclusions and future direction The emergence of graphene has contributed to the rapid development in energy storage. The inclusion of graphene and/or graphene nanofillers into conventional active materials have given rise to notable developments in energy storage systems. Graphene as a standalone material or when added to other materials to form composites in electrode provides improved capacity and performance in comparison to other electrode materials. Large surface area of graphene improves the transport of electrons in energy storage devices, particularly improving remarkably the capacitance in supercapacitors. Future design and improvement of supercapacitors would focus on developing 3D structures assembled in a similar manner as 2D graphene sheets and hybridized with metal oxides or sulfides. Such material has huge prospects of attaining large surface areas, rapid mass, and electron movement. Large surface area of graphene used as anode material in Li-ion batteries led to the attainment of a storage capacity of 235 mAHg1. In Li-ion battery development, an energy density of 200e250 Whkg1 can be achieved. To achieve this, research prospects have shown the need to encapsulate active cathode material particles in graphene. Future outlook for battery development is now focused on LieO2 and LieS batteries, which resulted in a commendable increase in energy density and overall cost reduction. Further advancement in energy density of these batteries will be dependent on material and its properties. In this regard, the unique graphene properties will be of utmost importance. For instance, efficient combination of graphene and sulfur in LieS battery design will have a huge potential at improved performance. Graphene-based composites in which graphene acts as nanofillers in place of other carbon materials will play a huge role in future energy storage devices with corresponding improvement in energy and power density.

References [1] Liu C, Li F, Ma LP, Cheng HM. Advanced materials for energy storage. Adv Mater 2010;22(8):E28e62. [2] Schaefer DW, Justice RS. How nano are nanocomposites? Macromolecules 2007;40(24):8501e17. [3] Cabana J, Monconduit L, Larcher D, Palacin MR. Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions. Adv Mater 2010;22(35):E170e92. [4] Lee JS, Tai Kim S, Cao R, Choi NS, Liu M, Lee KT, et al. Metaleair batteries with high energy density: Lieair versus Zneair. Adv Eng Mater 2011;1(1):34e50. [5] Zhou H, Upreti S, Chernova NA, Hautier G, Ceder G, Whittingham MS. Iron and manganese pyrophosphates as cathodes for lithium-ion batteries. Chem Mater 2010;23(2):293e300. [6] Peng Z, Freunberger SA, Chen Y, Bruce PG. A reversible and higher-rate Li-O2 battery. Science 2012:1223985.

II. Energy

176

9. Energy storage properties of graphene nanofillers

[7] Zhu J, Yang D, Yin Z, Yan Q, Zhang H. Graphene and graphene-based materials for energy storage applications. Small 2014;10(17):3480e98. [8] Blake P, Hill E, Castro Neto A, Novoselov K, Jiang D, Yang R, et al. Making graphene visible. Appl Phys Lett 2007;91(6):063124. [9] Balandin AA. Thermal properties of graphene and nanostructured carbon materials. Nat Mater 2011;10(8):569. [10] Shahil KM, Balandin AA. Thermal properties of graphene and multilayer graphene: applications in thermal interface materials. Solid State Commun 2012;152(15):1331e40. [11] Geim AK. Graphene: status and prospects. Science 2009;324(5934):1530e4. [12] Neto AC, Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, Geim AK. Rev Mod Phys 2009;81:109. [13] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009;457(7230):706. [14] Large scale carbon capture projects. Global CCS Institute; 2016. Available from: http://www. globalccsinstitute.com/projects/large-scale-ccs-projects. [15] Deloitte. Energy storage: tracking the technologies that will transform the power sector. United States: Deloitte; 2018. [16] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6(3):183. [17] Gharekhanlou B, Khorasani S. An overview of tight-binding method for two-dimensional carbon structures. Graph Prop Synth Appl 2011:1e37. [18] Ferrari AC, Bonaccorso F, Fal’Ko V, Novoselov KS, Roche S, Bøggild P, et al. Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale 2015;7(11):4598e810. [19] Saati H. Topological index of some carbon nanotubes and the symmetry group for nanotubes and unit cells in solids. J Math Sci 2015;211(1):58e126. [20] Tjong SC. Polymer composites with graphene nanofillers: electrical properties and applications. J Nanosci Nanotechnol 2014;14(2):1154e68. [21] Marquis DM, Guillaume E, Chivas-Joly C. Properties of nanofillers in polymer. Nanocomposites and polymers with analytical methods. InTech; 2011. [22] Anwar Z, Kausar A, Muhammad B. Polymer and graphite-derived nanofiller composite: an overview of functional applications. Polym Plast Technol Eng 2016;55(16):1765e84. [23] Stankovich S, Dikin DA, Dommett GH, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442(7100):282. [24] Kim S, Drzal LT. High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets. Sol Energy Mater Sol Cells 2009;93(1):136e42. [25] Yavari F, Fard HR, Pashayi K, Rafiee MA, Zamiri A, Yu Z, et al. Enhanced thermal conductivity in a nanostructured phase change composite due to low concentration graphene additives. J Phys Chem C 2011;115(17):8753e8. [26] Xiang J, Drzal LT. Investigation of exfoliated graphite nanoplatelets (xGnP) in improving thermal conductivity of paraffin wax-based phase change material. Sol Energy Mater Sol Cells 2011;95(7):1811e8. [27] Zhu Y, Murali S, Cai W, Li X, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22(35):3906e24. [28] Yoo E, Kim J, Hosono E, Zhou H-s, Kudo T, Honma I. Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries. Nano Lett 2008;8(8):2277e82. [29] Odahara G, Ishikawa T, Otani S, Oshima C. Self-standing graphene sheets prepared with chemical vapor deposition and chemical etching. Appl Surf Sci 2009;7:837e40. [30] Lee S, Lim S, Lim E, Lee KK. Synthesis of aqueous dispersion of graphenes via reduction of graphite oxide in the solution of conductive polymer. J Phys Chem Solids 2010;71(4):483e6. [31] De Arco LG, Zhang Y, Zhou C. Large scale graphene by chemical vapor deposition: synthesis, characterization and applications. Graphene-synthesis, characterization, properties and applications. InTech; 2011. [32] Li X, Magnuson CW, Venugopal A, An J, Suk JW, Han B, et al. Graphene films with large domain size by a twostep chemical vapor deposition process. Nano Lett 2010;10(11):4328e34. [33] Jayasena B, Subbiah S. A novel mechanical cleavage method for synthesizing few-layer graphenes. Nano Res Lett 2011;6(1):95. [34] Su C-Y, Lu A-Y, Xu Y, Chen F-R, Khlobystov AN, Li L-J. High-quality thin graphene films from fast electrochemical exfoliation. ACS Nano 2011;5(3):2332e9.

II. Energy

References

177

[35] Yang W, Chen G, Shi Z, Liu C-C, Zhang L, Xie G, et al. Epitaxial growth of single-domain graphene on hexagonal boron nitride. Nat Mater 2013;12(9):792. [36] Liu J, Xue Y, Zhang M, Dai L. Graphene-based materials for energy applications. MRS Bull 2012;37(12):1265e72. [37] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Letters 2008;8(10):3498e502. [38] Zhang X, Wang B, Sunarso J, Liu S, Zhi L. Graphene nanostructures toward clean energy technology applications. Wiley interdiscip Rev Energy Environ 2012;1(3):317e36. [39] Zhu Y, Murali S, Stoller MD, Ganesh K, Cai W, Ferreira PJ, et al. Carbon-based supercapacitors produced by activation of graphene. Science 2011;332(6037):1537e41. [40] Ning G, Fan Z, Wang G, Gao J, Qian W, Wei F. Gram-scale synthesis of nanomesh graphene with high surface area and its application in supercapacitor electrodes. Chem Commun 2011;47(21):5976e8. [41] Miller JR, Outlaw R, Holloway B. Graphene double-layer capacitor with ac line-filtering performance. Science 2010;329(5999):1637e9. [42] Liu C, Yu Z, Neff D, Zhamu A, Jang BZ. Graphene-based supercapacitor with an ultrahigh energy density. Nano Letters 2010;10(12):4863e8. [43] Wu Z-S, Ren W, Xu L, Li F, Cheng H-M. Doped graphene sheets as anode materials with superhigh rate and large capacity for lithium ion batteries. ACS Nano 2011;5(7):5463e71. [44] Bayramoglu EÇ. Thermal properties and stability of n-octadecane based composites containing multiwalled carbon nanotubes. Polym Compos 2011;32(6):904e9. [45] Zheng Q, Kim J-K. Graphene for transparent conductors: synthesis, properties and applications. Springer; 2015. [46] Liu H, Liu Y, Zhu D. Chemical doping of graphene. J Mater Chem 2011;21(10):3335e45. [47] Zheng Q, Zhang B, Lin X, Shen X, Yousefi N, Huang Z-D, et al. Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by LangmuireBlodgett assembly. J Mater Chem 2012;22(48):25072e82. [48] Qu B, Ma C, Ji G, Xu C, Xu J, Meng YS, et al. Layered SnS2-reduced graphene oxide compositeeA high-capacity, high-rate, and long-cycle life sodium-ion battery anode material. Adv Mater 2014;26(23):3854e9. [49] Deng L, Zhu G, Wang J, Kang L, Liu Z-H, Yang Z, et al. GrapheneeMnO2 and graphene asymmetrical electrochemical capacitor with a high energy density in aqueous electrolyte. J Power Sources 2011;196(24):10782e7. [50] Shaikh S, Lafdi K, Hallinan K. Carbon nanoadditives to enhance latent energy storage of phase change materials. J Appl Phys 2008;103(9):094302. [51] Sanusi O, Warzoha R, Fleischer AS. Energy storage and solidification of paraffin phase change material embedded with graphite nanofibers. Int J Heat Mass Transf 2011;54(19e20):4429e36. [52] Krupka J, Strupinski W. Measurements of the sheet resistance and conductivity of thin epitaxial graphene and SiC films. Appl Phys Lett 2010;96(8):082101. [53] Watcharotone S, Dikin DA, Stankovich S, Piner R, Jung I, Dommett GH, et al. Graphene silica composite thin films as transparent conductors. Nano Lett 2007;7(7):1888e92. [54] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558e65. [55] Wang X, Zhi L, Müllen K. Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Lett 2008;8(1):323e7. [56] Dubin S, Gilje S, Wang K, Tung VC, Cha K, Hall AS, et al. A one-step, solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents. ACS Nano 2010;4(7):3845e52. [57] Pop E, Varshney V, Roy AK. Thermal properties of graphene: fundamentals and applications. MRS Bull 2012;37(12):1273e81. [58] Ong Z-Y, Pop E. Frequency and polarization dependence of thermal coupling between carbon nanotubes and SiO2. J Appl Phys 2010;108(10):103502. [59] Tohei T, Kuwabara A, Oba F, Tanaka I. Debye temperature and stiffness of carbon and boron nitride polymorphs from first principles calculations. Phys Rev B 2006;73(6):064304. [60] Nihira T, Iwata T. Temperature dependence of lattice vibrations and analysis of the specific heat of graphite. Phys Rev B 2003;68(13):134305. [61] Hao F, Fang D, Xu Z. Mechanical and thermal transport properties of graphene with defects. Appl Phys Lett 2011;99(4):041901.

II. Energy

178

9. Energy storage properties of graphene nanofillers

[62] Benedict LX, Louie SG, Cohen ML. Heat capacity of carbon nanotubes. Solid State Commun 1996;100(3):177e80. [63] Jeong C, Datta S, Lundstrom M. Full dispersion versus Debye model evaluation of lattice thermal conductivity with a Landauer approach. J Appl Phys 2011;109(7):073718. [64] Chen S, Moore AL, Cai W, Suk JW, An J, Mishra C, et al. Raman measurements of thermal transport in suspended monolayer graphene of variable sizes in vacuum and gaseous environments. ACS Nano 2010;5(1):321e8. [65] Chen S, Wu Q, Mishra C, Kang J, Zhang H, Cho K, et al. Thermal conductivity of isotopically modified graphene. Nat Mater 2012;11(3):203. [66] Verdejo R, Barroso-Bujans F, Rodriguez-Perez MA, De Saja JA, Lopez-Manchado MA. Functionalized graphene sheet filled silicone foam nanocomposites. J Mater Chem 2008;18(19):2221e6. [67] Miller JR, Simon P. Electrochemical capacitors for energy management. Science Magazine 2008;321(5889):651e2. [68] To J. Mesoporous materials for energy storage. 2012. [69] Halper MS, Ellenbogen JC. Supercapacitors: a brief overview. McLean, Virginia, USA: The MITRE Corporation; 2006. p. 1e34. [70] Qu D. Studies of the activated carbons used in double-layer supercapacitors. J Power Sources 2002;109(2):403e11. [71] Fernández J, Morishita T, Toyoda M, Inagaki M, Stoeckli F, Centeno TA. Performance of mesoporous carbons derived from poly (vinyl alcohol) in electrochemical capacitors. J Power Sources 2008;175(1):675e9. [72] Yan J, Wei T, Fan Z, Qian W, Zhang M, Shen X, et al. Preparation of graphene nanosheet/carbon nanotube/ polyaniline composite as electrode material for supercapacitors. J Power Sources 2010;195(9):3041e5. [73] Molina MG. Dynamic modelling and control design of advanced energy storage for power system applications. Dynamic modelling. InTech; 2010. [74] Elcap. Ragone plot showing power density vs. energy density of various capacitors and batteries. 2013. Available from: https://commons.wikimedia.org/wiki/File:Supercapacitors-vs-batteries-chart.png. [75] Kim TY, Lee HW, Stoller M, Dreyer DR, Bielawski CW, Ruoff RS, et al. High-performance supercapacitors based on poly (ionic liquid)-modified graphene electrodes. ACS Nano 2010;5(1):436e42. [76] Yoo JJ, Balakrishnan K, Huang J, Meunier V, Sumpter BG, Srivastava A, et al. Ultrathin planar graphene supercapacitors. Nano Lett 2011;11(4):1423e7. [77] Barbieri O, Hahn M, Herzog A, Kötz R. Capacitance limits of high surface area activated carbons for double layer capacitors. Carbon 2005;43(6):1303e10. [78] Zhu Y, Stoller M, Cai W, Velamakanni A, Piner R, Chen D, et al. ACS Nano 2010;4:1227. PubMed. 253. [79] Lv W, Tang D-M, He Y-B, You C-H, Shi Z-Q, Chen X-C, et al. Low-temperature exfoliated graphenes: vacuumpromoted exfoliation and electrochemical energy storage. ACS Nano 2009;3(11):3730e6. [80] Vivekchand S, Rout CS, Subrahmanyam K, Govindaraj A, Rao C. Graphene-based electrochemical supercapacitors. J Chem Sci 2008;120(1):9e13. [81] Yang X, Zhu J, Qiu L, Li D. Bioinspired effective prevention of restacking in multilayered graphene films: towards the next generation of high-performance supercapacitors. Adv Mater 2011;23(25):2833e8. [82] Jeong HM, Lee JW, Shin WH, Choi YJ, Shin HJ, Kang JK, et al. Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes. Nano Lett 2011;11(6):2472e7. [83] Beidaghi M, Wang C. Micro-supercapacitors based on interdigital electrodes of reduced graphene oxide and carbon nanotube composites with ultrahigh power handling performance. Adv Funct Mater 2012;22(21):4501e10. [84] Yu G, Hu L, Vosgueritchian M, Wang H, Xie X, McDonough JR, et al. Solution-processed graphene/MnO2 nanostructured textiles for high-performance electrochemical capacitors. Nano Letters 2011;11(7):2905e11. [85] Xue M, Li F, Zhu J, Song H, Zhang M, Cao T. Structure-based enhanced capacitance: in situ growth of highly ordered polyaniline nanorods on reduced graphene oxide patterns. Adv Funct Mater 2012;22(6):1284e90. [86] Dong X-C, Xu H, Wang X-W, Huang Y-X, Chan-Park MB, Zhang H, et al. 3D grapheneecobalt oxide electrode for high-performance supercapacitor and enzymeless glucose detection. ACS Nano 2012;6(4):3206e13. [87] Zhou G, Paek E, Hwang GS, Manthiram A. High-performance lithium-sulfur batteries with a self-supported, 3D Li2s-doped graphene aerogel cathodes. Adv Eng Mater 2016;6(2).

II. Energy

References

179

[88] Pan D, Wang S, Zhao B, Wu M, Zhang H, Wang Y, et al. Li storage properties of disordered graphene nanosheets. Chem Mater 2009;21(14):3136e42. [89] Molenda J, Molenda M. Composite cathode material for Li-ion batteries based on LiFePO4 system. Metal, ceramic and polymeric composites for various uses. InTech; 2011. [90] Song M-K, Zhang Y, Cairns EJ. A long-life, high-rate lithium/sulfur cell: a multifaceted approach to enhancing cell performance. Nano Lett 2013;13(12):5891e9. [91] Hannan MA, Lipu M, Hussain A, Mohamed A. A review of lithium-ion battery state of charge estimation and management system in electric vehicle applications: challenges and recommendations. Renew Sustain Energy Rev 2017;78:834e54. [92] Paek S-M, Yoo E, Honma I. Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure. Nano Lett 2008;9(1):72e5. [93] Zhou G, Wang D-W, Li F, Zhang L, Li N, Wu Z-S, et al. Graphene-wrapped Fe3O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries. Chem Mater 2010;22(18):5306e13. [94] Wang H, Cui L-F, Yang Y, Sanchez Casalongue H, Robinson JT, Liang Y, et al. Mn3O4 graphene hybrid as a high-capacity anode material for lithium ion batteries. J Am Chem Soc 2010;132(40):13978e80. [95] Zhou W, Zhu J, Cheng C, Liu J, Yang H, Cong C, et al. A general strategy toward graphene@ metal oxide coree shell nanostructures for high-performance lithium storage. Energy Environ Sci 2011;4(12):4954e61. [96] Zhu J, Sun K, Sim D, Xu C, Zhang H, Hng HH, et al. Nanohybridization of ferrocene clusters and reduced graphene oxides with enhanced lithium storage capability. Chem Commun 2011;47(37):10383e5. [97] Adánez-Rubio I, Abad A, Gayán P, De Diego L, García-Labiano F, Adánez J. Biomass combustion with CO2 capture by chemical looping with oxygen uncoupling (CLOU). Fuel Process Technol 2014;124:104e14. [98] Zhang C, Shi X, Yu D. The effect of global oil price shocks on China’s precious metals market: a comparative analysis of gold and platinum. J Clean Prod 2018;186:652e61. [99] Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, et al. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011;10(10):780. [100] Jeon NJ, Noh JH, Kim YC, Yang WS, Ryu S, Seok SI. Solvent engineering for high-performance inorganice organic hybrid perovskite solar cells. Nat Mater 2014;13(9):897. [101] Timans P, editor. Advanced gate stack, source/drain, and channel engineering for Si-based CMOS 4: ECS transactions, vol. 13. The Electrochemical Society; 2008. [102] Queisser H. Forward characteristics and efficiencies of silicon solar cells. Solid State Electron 1962;5(1):1e10. [103] Queisser S. The Shockley-Queisser limit for the maximum possible efficiency of a solar cell. 2011. Available from: https://commons.wikimedia.org/wiki/File:ShockleyQueisserBreakdown2_(DE).svg. [104] Mitzi DB, Feild C, Harrison W, Guloy A. Conducting tin halides with a layered organic-based perovskite structure. Nature 1994;369(6480):467. [105] Grätzel M. Dye-sensitized solar cells. J Photochem Photobiol C Photochem Rev 2003;4(2):145e53. [106] Park H, Chang S, Smith M, Gradecak S, Kong J. Interface engineering of graphene for universal applications as both anode and cathode in organic photovoltaics. Sci Rep 2013;3:1581. [107] Acik M, Darling SB. Graphene in perovskite solar cells: device design, characterization and implementation. J Mater Chem A 2016;4(17):6185e235.

II. Energy