Applications of Nanomaterials in Energy Storage and Electronics

Carbon Nanotube Based Nanomaterials for Solar Energy Storage Devices

Ravi Prakash¹, Sunil Kumar¹, ², Pralay Maiti¹, *

¹ School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi 221005, India

² Department of Chemistry, L.N.T. College (B.R.A. Bihar University), Muzaffarpur-842002, India

Abstract

Carbon nanotubes (CNTs) and their nanocomposites are used in various products and technologies due to their unique characteristics. For their future implementation, the manufacturing of CNTs with appropriate specifications has gained momentum in the area of nanoscience and technology. Conventional phase change materials used in solar thermal energy storage have low thermal conductivity. CNTs are used to prepare phase change materials with high thermal conductivity to solve this issue. This chapter addresses the synthesis, structure, and properties of CNTs. The different varieties of solar energy storage systems used to store solar radiation are also discussed. Further, we explain the phase change materials (PCMs) as suitable solar thermal energy storage systems and discuss the methods to prepare CNT-based nanomaterials for use as a heat transfer fluid (HTF) after using the CNTs based PCMs in solar storage systems. CNT based nanomaterials as a heat transfer fluid significantly increase the effective receiving efficiency, thermal conductivity, and absorption coefficient of such storage systems.

Keywords: Arc discharge, Carbon nanotubes, Chemical vapour deposition, Electrolysis, Graphene, Heat transfer fluid, Laser ablation, Multi-walled carbon nanotubes, Nanomaterials, Nanotechnology, Phase change materials, Photovoltaic, Single walled carbon nanotubes, Solar cells, Solar energy storage devices, Solar radiation, Thermal conductivity, Sonochemical, Specific heat capacity.

* Corresponding author Pralay Maiti: School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi 221005, India; E-mail: pmaiti.mst@itbhu.ac.in

INTRODUCTION

The production of renewable energy, consumption, and storage are major global challenges for researchers [1-3]. Solar energy devices that convert solar energy directly into electrical energy are called solar cells [4, 5]. There are significant

research and development efforts underway to improve the device efficiency and lower the fabrication cost [3, 4, 6-8]. Photovoltaic devices are already used in the current era, but the devices suffer from insufficient durability and higher expense for fabrication. Further, solar energy production is mainly dependent on weather conditions; that is why solar power generation is irregular and unpredictable. Moreover, the energy requirement is considerably high in the daytime, and solar energy is available only for a small number of hours, creating the problem of maintaining a balance between the requirement and supply [9]. These two factors are the driving force behind the development of efficient solar energy storage devices, which may help reduce the fluctuation arising from the generation side and provide the possibility of performing auxiliary services. Energy storage systems are thus increasingly reducing the mismatch between need and supply and improving the work capability and reliability of energy systems which play a crucial role in conserving the produced energy [10, 11]. The conventional mechanism of the solar energy storage device is to convert the solar energy into electrical energy through solar panels and then store it in batteries, but it suffers from the issue of high manufacturing costs. In recent years, some research groups have introduced phase change materials (PCMs) as an alternative method for solar energy storage [12]. Since then, the developments of phase-change materials have become a hot research topic. The solar thermal energy storage devices are fabricated using PCMs due to their excellent stable form during phase transition.

In this method, the solar energy is converted into thermal energy using the PCMs and stored in a storage tank, which acts as a thermal battery [12-14]. Nanotechnology is an important field in the development of modern technology and attracts researchers in all fields. Carbon nanotubes are prime members of the research and development in nanotechnology. CNTs are classified into two categories (Fig. 1) based on a number of layers present in the structure; (1) single-walled carbon nanotubes (SWCNTs) and (2) multi-walled carbon nanotubes (MWCNTs). The physicochemical properties are shown in Table 1. CNTs possess high thermal, mechanical, and electrical characteristics, making them suitable for developing smart composite materials used in energy storage devices, field emitters, sensors, and so on [15]. This chapter deals with CNT-based nanomaterials, their synthesis, properties, and applications in solar energy storage devices. CNTs have been used in various technologies, depending on their attractive electrical, mechanical, and thermal properties [16]. They are primarily used in electronics [19], transistors, and display technologies, owing to their electrical properties [20, 21].

Fig. (1))

Conceptual diagram of SWCNTs and MWCNTs; Reprinted with copyright permission from Ref [17, 18].

CARBON NANOTUBES

Carbon nanotubes contain sp² hybridisation and assume different structures with graphite as a well-known example. Graphene is a 2-dimensional (2D) single layer of graphite in the list of carbon nanomaterials. Graphene is stronger than diamond because it contains sp² hybridisation, which is stronger than sp³ hybridisation in a diamond.

The sp² hybridized carbon can form open and closed cages with honeycomb structures [22] and Kroto et al. [23] discovered such kinds of structures. Carbon nanotubes are large molecules of pure carbon that are long, thin and tube-like, about 1-3 nanometres in diameter, and hundreds to thousands of nanometres long.

Table 1 Physiochemical properties of SWCNTs and MWCNTs.

The various types of carbon cage structures were studied long back when Iijima [24] first observed the tubular carbon (tube-like structure) structure in 1991. In carbon nanotubes, the carbon molecules are cylindrical and have unique electrical, thermal and mechanical properties that make carbon nanotubes suitable for use in different areas. The carbon nanotubes consist of up to several tens of graphite shells collectively known as multi-walled carbon nanotubes (MWCNTs) with a large length/diameter ratio. The diameter of carbon nanotubes being ~ 1.0 nm and the separation between the two adjacent shells being ~ 0.34 nm. After two years, Iijima, Ichihashi [25] and Bethune et al. [26] synthesised the single-walled carbon nanotubes (SWNTs). The CNTs are classified into three types on the basis of chirality; (1) armchair structure, (2) zigzag structure, and (3) chiral structure (Fig. 2).

Fig. (2))

Schematics diagrams of CNTs. (a) armchair structure. (b) zigzag structure, and (c) chiral structure. Reprinted with copyright permission from Ref [27].

The important structure of SWCNTs and MWCNTs are shown in Fig. (3). The SWCNTs are cylindrical-shaped, and MWCNTs consist of several concentric SWCNTs. The structures of both CNTs are different, and hence, their properties are also different [28, 29].

Fig. (3))

(a) Schematic diagram of how grapheme sheets are rolled up to form CNTs; (b) representation of the three types of SWNTs structure obtained with the pair (n, m) from the chiral vector. Reprinted with copyright permission from Ref [28]. (c) Electronic structure of armchair (metallic); and (d) Electronic structure of zig-zag (semiconductor). Reprinted with copyright permission from Ref [29].

PROPERTIES OF CARBON NANOTUBES

The properties of CNTs depend upon their atomic arrangement, tube length, diameter, and morphology [28-30]. SWCNT properties are governed by structure formation by the bonds between the carbon atoms of graphene sheets [28, 29]. The structure of SWCNTs depends upon the chirality of the tube, which refers to a chirality vector and chirality angle (Ѳ), which are shown in Fig. (3). The chirality vector is the linear combination of a1 and a2 of the simple hexagonal through the following relationship:

where, n and m are integers and the single-cell vectors of two-dimensional matrix formed by the graphene sheet in which the direction of the nanotubes axis is perpendicular to the chiral vectors shown in Fig. (3a). The graphene sheets are rolled in the direction indicated by the chiral vector pair (n, m), and the values of the chiral vectors’ pair allow three types of arrangement for SWCNTs, (1) zig-zag, (2) arm chair, and (3) chiral, which are shown in Fig. (3b). The mechanical, electrical, and optical properties and the nanotube’s chirality are determined by the chiral vector pair (n, m) [28, 29]. If the value of the pair (n, m) is a multiple of 3, then the structure of nanotubes looks like an arm chair, has metallic behaviour, and its Fermi level is partially filled (Fig. 3c). When the value of the pair (n, m) is a multiple of 3, the structure is zig-zag and has the behaviour of a semiconductor (Fig. 3d). Carbon nanotubes have unique structures and properties like high aspect ratio, which gives them good electrical, thermo-mechanical properties [31], high tensile strength [28, 31, 32] (~50-500 GPa), very low density (~1.3 g/cm³) and very high Young’s modulus (~1500 GPa) [32, 33]. Due to these unique and excellent properties, CNTs are stronger and, at the same time, lighter than steel [31]. The perfect arrangement of carbon-carbon covalent bonds along the axis of nanotubes makes them very strong, with an excellent strength-to-weight ratio [31]. The MWCNTs have a broad range [34, 35] of UV-vis light absorbance [36, 37], which yields good light-thermal [38, 39] conversion capability [40, 41]. CNTs have very high thermal conductivity and are stable up to 2800 °C in vacuum [42, 43].

SYNTHESIS OF CARBON NANOTUBES

The MWNTs and SWNTs are synthesised by various methods: arc-discharge, electrolysis, laser-ablation, sonochemical or hydrothermal and chemical vapour deposition, etc. The various synthetic methods are shown in Fig. (4), which can be used to produce the CNTs in large quantities. The first production method of CNTs was high temperature preparation techniques like arch discharge or laser ablation were used, but recently, such methods have been replaced by low chemical vapour technique deposition technique (< 800 ⁰C), since the alignment, orientation, length diameter, purity and density of CNTs can precisely be controlled in the latter method [44].

Fig. (4))

Schematic diagram of various synthesis methods conventionally used for the preparation of CNTs.

Arc Discharge Method

In the arc discharge method using a high temperature, preferably above 1700 oC, CNTs are synthesised with a lower structural defects in comparison to other methods. Arc discharge method was used to synthesise the SWCNTs, MWCNTs, and double walled carbon nanotubes (DWCNTs).

Bethune et al. produced CNTs with a small diameter (1.2 nm) through the co-evaporation of carbon with cobalt in an arc generator [26]. Likewise, Ajayan et al. also synthesised the SWCNTs (1-2 nm diameter) through the arc discharge method with cobalt used in the helium atmosphere [45]. The most utilized catalyst in the synthesis of SWCNTs is nickel and Seraphin et al. have performed detailed studies on the catalytic activity of various catalysts like Ni, Pd, and Pt in the synthesis of carbon nanoclusters using the DC arc discharge method with operating conditions of 28 V and 70 A under the 550 Torr helium atmospheric pressure. They observed that the Ni filled anode stimulated the growth of SWCNTs [46]. Similarly, Saitio et al. also reported the rapid growth of SWCNTs by using the fine Ni particles and Zhou et al. reported radial growth of SWCNTs by using the Yttrium carbide deposited anode.

The MWCNTs synthesis through the arc discharge method is very simple. Shimotani et al. [47] synthesized MWCNTs by using the arc discharge method under helium, acetone, ethanol and hexane atmosphere at different pressures from 150 to 500 Torr, and concluded that the arc discharge method produces MWCNTs three times more in ethanol, acetone and hexane as compared to helium atmospheric condition. This can be explained as follows: acetone, ethanol, and hexane may be ionized. The molecules may decompose to carbon and hydrogen atoms. These ionized species may contribute to the synthesis of MWCNTs, resulting in higher yields MWCNTs. MWCNTs yield increases in the presence of an organic atmosphere with increasing pressures up to 400 Torr [47]. The arc discharge deposition is usually promising with DC arc discharge and pulsed technique. Jung et al. synthesised high yield MWCNTs by arc discharge method using liquid nitrogen, and they concluded that the arc discharge method can be used to synthesize MWCNTs at a large scale with high purity. Montoro et al. [47, 48] synthesised high-quality SWCNTs and MWCNTs through arc discharge method using pure graphite electrode using the H3VO3 aqueous solution. The DWCNTs have been synthesized through the arc discharge method, but the process is not easy though some research groups have reported the successful synthesis of DWCNTs. Hutchison et al. first reported the successful synthesis of DWCNTs through arc discharge method under the mixture of hydrogen and argon atmosphere and SWCNTs were obtained as a by-product during the synthesis of the DWCNTs using arc discharge methods. Sugai et al. reported a novel synthesis of DWCNTs with high temperature pulsed arc discharge method using the Y/Ni alloy catalyst [49]. Qiu et al. [50] also reported their successful synthesis from coal through arc discharge method in hydrogen free atmosphere.

Laser Ablation

This method has been used to synthesize high-quality and high-purity SWCNTs. In 1995 [51], Smalley’s group first used this method, which has similar principles and mechanism as of arc discharge, but laser hitting of a graphite pellet containing nickel or cobalt [52] catalyst was used to produce energy. Almost all lasers used for the ablation has been Nd:YAG and CO2. Zhang et al. used continuous CO2 laser ablation without applying any additional heat to the target for the preparation of SWCNTs, and they observed that the average diameter of prepared SWCNTs through CO2 laser was increased with the increase of laser power [53-55].

Chemical Vapour Deposition

In this process, CNTs are prepared by heating the catalyst materials in a furnace (relatively lower temperature 500–1000 °C) under the flowing hydrocarbon through the tube reactor for a certain period of time. The catalysts are transition-metal nanoparticles having a high surface area and the catalysts serve as a seed to the growth of nanotubes.

SOLAR ENERGY STORAGE DEVICES

Solar energy storage systems are classified in different ways. The solar energy storage systems are classified as: (1) thermal storage, (2) electrical storage, (3) chemical storage, (4) mechanical storage, and (5) electromagnetic storage. The different solar storage systems used to store the solar energy are shown in Fig. (5). The solar thermal energy storage devices are traditionally classified into three major categories [56] depending on the physical principles used for energy conversion and storage [11]. The first method relates with general property of matter to experience bulk heating, and the energy storage is proportional to the specific heat capacity of the energy absorbing materials, which is often called sensible heat [11, 57]. The second method involves the heat absorption/release (latent heat) properties of the materials during the phase transition [58, 59]. In the third method, the chemical reaction process requires to produce chemical compounds having high energy chemical bonds, and which release their energy upon disruption [11, 60].

Fig. (5))

Schematic diagrams of various solar energy storage systems.

Another method was also added in this series which is based on the absorption of a photon and the generation of electron hole pairs in energy storage devices such as lithium ion rechargeable batteries [11, 61] which is also combined with the generation of hydrogen. The above method can also be classified in the high energy (the light is utilized in the visible range) domain and another in low energy (IR light) domain of solar energy, which is converted as stored energy [11]. The electronic excitation in absorber materials involves absorbing the photons in the visible region of the solar radiation, and the electrical energy is produced by the photovoltaic cell, which may store energy in the form of chemical energy via a photochemical reaction. This method is referred to as the photonics conversion method and these are electrochemical batteries fed by photovoltaic cells. Photosynthesis and electrochemical energy storage devices directly absorb the photons by photo-electrode and produce a chemical fuel. The materials absorb the IR light of solar radiation, which excites the photons and produces heat that can be stored, and the method is conventionally called the thermal conversion method. The method is based on heat-induced chemical reactions and is classified into sensible and latent heat-based.

CNT NANOCOMPOSITES IN SOLAR ENERGY STORAGE DEVICES

CNTs are a good candidate as a filler in composite materials to enhance thermal transport due to their high thermal conductivity [62, 63]. Many research groups have reported enhanced thermal conductivity of the PCMs with the addition of CNTs [64-68]. Ji et al. reported the functionalised MWNTs/plasmatic acid composite materials to prepare PCMs in thermal storage systems [62]. Solar energy collector is an important part of solar energy storage device, which converts the solar energy directly into heat [69]. In 1970, Minradi et al. proposed an idea about the direct absorption solar collectors (DASC) [70]. The DASC system directly absorbs the solar energy radiation through heat transfer fluid and decreases the radiation heat loss [71] by avoiding the heat resistance between medium and absorption surface. The photo-absorption and thermo-physical properties of heat transfer fluids have affected the receiving efficiency of the DASC system [72]. Hence, to increase the receiving efficiency, it has been proposed to use novel slurry with fine optical and thermo-physical properties. Recently, some research groups have enhanced the optical absorption properties of working heat transfer fluid with the dispersion of the metal nanoparticles [73] and carbon-based composites [74]. However, the working fluids have very low specific heat capacity, which makes it impossible for them to store more energy, and hence, the reception efficiency is insignificant. Since PCMs have large specific heat capacity, it has been found that when the PCMs are dispersed with heat transfer carrying fluid, the latent functional heat of fluid can enhance the thermal energy storage capacity of the working fluid [75, 76]. Ma et al. used CNTs effectively to prepare PCMs for utilization in solar energy storage materials [69].

The optimized amount of CNTs has been previously incorporated in PCMs, which enhanced the thermal conductivity, solar radiation absorption efficiency, solar heat storage capacity and specific heat capacity of the heat transfer working fluid. The effective receiving efficiency of the heat transfer fluid is determined by using the following equation (2) [77]:

where, η is the receiver efficiency, mass of the heat transfer slurry is m, Cp is the specific heat capacity, Gs is the intensity of solar radiation, T represents the real time temperature of the slurry, A is the area of energy received by the light source, the radiation time of the sample is t. The GsA t in the equation (2) represents the heat received by the simulated light source/collector from the sun, and the m ∫ Cp (T) dT represents the heat actually absorbed by heat transfer fluid through the irradiation to increase its temperature. The ratio of these two provides the so called receiving efficiency [69]. Ma et al. calculated the effective receiving efficiency from equation (2) for water, 5 wt.% paraffin@SnO2 and paraffin@SnO2/CNTs, which are shown in Fig. (6a) as a function of temperature. The CNTs mixed slurry was observed to exhibit higher effective receiving efficiency as compared to water and 5 wt.% paraffin@SnO2, and also high photo-conversion efficiency or absorption as compared to the base fluid. UV-vis spectroscopy was used to determine the optical properties of prepared slurry, and from such measurements, it has been observed that the absorption of paraffin@SnO2/CNTs composites exhibits three times more efficiency as compared to paraffin@SnO2 slurry. Tong et al. [78] also used CNTs in solar radiation collectors, using different concentrations of MWNTs in MWNTs/water nano-fluid employed in u-tube solar collectors. When MWNTs with 0.24% volume concentration were employed in the nano-fluid, they enhanced the heat transfer coefficient by 8% between the tube and working fluid as compared to the base nano-fluid (without MWNTs).

Fig. (6))

(a) Effective receiving efficiency of 5 wt.% paraffin@SnO2 microcapsules and paraffin@SnO2/ CNTs dispersion slurry; and (b) UV-vis reflection spectra of paraffin@SnO2 microcapsules and paraffin@SnO2/CNTs dispersion slurry. Reprinted with copyright permission from Ref [69].

CONCLUSION

CNTs are an important class of nanomaterials derived from carbon with unique properties, which makes them more attractive for use in various technologies. Although their scope of application is extremely wide and exhaustive, in this chapter, we have specifically dealt with the synthesis and application of CNTs based nanomaterials in solar energy storage devices. An optimum amount of CNTs needs to be added, for instance, in PCMs, to enhance the thermal conductivity and solar heat storage capacity of the materials (heat transfer working fluid in such a case) resulting in an increase in the efficiency of the storage device. A good number of important recent research studies are available in this field. Therefore in this chapter, we have focused on the important synthesis methods (laser ablation, arc discharge, CVD, etc.) and various types of CNTs, namely SWCNTs, DWCNTs and MWCNTs, characteristic properties and application of the CNTs, which are responsible for the boosting of nanotechnology applied to solar energy storage.

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTERESTS

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

The author (Ravi Prakash) acknowledges the Institutes (IIT-BHU) internship for carrying out this work.

REFERENCE