1.1. Energy storage systems
1.1.3. Electrochemical capacitors
1.1.3.2. Shaped carbon-based materials for electrochemical capacitors
Numerous carbon-based materials have been discovered over the years. They have shown potential applications as electrode materials in ECs. The sp2-hybridisation of carbon facilitates the occurrence of numerous shapes [1.42]. Kroto et al. [1.43] discovered the first shaped carbon nanomaterial (SCNM), C60 molecules (Fig. 1.5a) whilst, in 1991, Iijima reported the first tubular nanostructured nanomaterial, multiwalled carbon nanotubes (MWCNTs) (Fig.
1.5b) [1.44,1.45]. Additionally, the first report on synthesis of single walled carbon nanotubes (SWCNTs) was in 1993 [1.46]. Whilst the first report of a successful isolation of graphene was in 2004 (Fig. 1.5c) [1.47].
13
Fig. 1.5. Examples of shaped carbon nanomaterials [1.48].
1.1.3.2.1 Graphene
Graphene (G) is associated with fewer health-related hazards than other carbon allotropes [1.49]. The development of sheets of different structure, functionality and sheet sizes is of extraordinary interest to several researchers [1.50]. This is due to the numerous associated
14
potential applications. Current synthetic methods produce defects such as structural imperfections and chemical impurities randomly distributed within the graphitic framework [1.51]. Use of different starting materials, oxidation methods and reduction processes widen possible materials that can be obtained. In such endeavours, there is a need to control defects and locally induced chemical alterations [1.52]. G can be conveniently synthesised in bulk at low cost [1.53]. The preparation methods include chemical vapour deposition (CVD), solvent thermal reaction, chemical routes from CNTs, exfoliation of graphite through ultra-sonication, graphite intercalation compounds and graphite oxide, and thermal desorption of Si from SiC and even ‘scotch tape’ [1.50,1.52,1.54].
Physical exfoliation, such as micromechanical cleavage [1.55] (‘scotch tape’), is the most preferred in scenarios were the graphene structure and electronic properties need to be preserved [1.56]. However, the procedure produces graphene sheets (GS) of different sizes, shapes, thickness and leds to low GS yields [1.55]. The chemical approach via graphene oxide (GO, Fig. 1.5d), as the initial stage, is a common synthesis method. Practically, it is almost impossible to synthesise G chemically but reduced graphene oxide (RGO) can be produced via synthesis of GO. GO is a non-stoichiometric graphitic carbon material in which the lamellar structure is conserved [1.57]. GO synthesis as a preliminary step to RGO synthesis can be achieved by thermal exfoliation, Brodie’s method and Hummer’s as well as several modified Hummer’s methods [1.50,1.55,1.58].
In fact, GO was first reported by Brodie in 1859 [1.59]. The synthesis steps were time consuming and involved vigorous reaction kinetics such as explosion of potassium chlorate.
The Brodie’s approach involved several treatments of Ceylon graphite using potassium chlorate and fuming nitric acid. Later, Hummer’s method, which was relatively safer, was reported. In the Hummer’s way, H2SO4, NaNO3, graphite flakes and KMnO4 were mixed sequentially followed by slow addition of deionised water (DI) [1.58,1.59].
The thermal reduction approach has several attributes relative to chemical synthesis. These are inclusive of simplicity, ease to perform due to the involvement of simultaneous exfoliation and reduction of graphite oxide [1.50]. Major challenges common to the thermal methods of GS synthesis include large scale production and effective defect control. Oxidation and reduction of G is one of the most effective ways for economical mass production [1.50,1.54]. Yield and size of GS can be controlled by varying crystal structure of parent material and or/graphite exfoliation conditions.
15
G, amongst other graphitic materials, can also be synthesized from graphite via ultra-sonication in organic solvents such N-methyl-2-pyrolidene and dimethyl formamide (DMF). According to Quintana et al. [1.52], ultra-sound waves break the basal structure and produce graphitic carbon fragments of variable sizes and such structures are latter intercalated by solvent molecules. Similar views were shared by Dhakate et al. [1.51]. They highlighted that, typical reactions are more ideal in solvents with surface tensions in the range 40-50 mJ m-2 to avoid enthalpic cost of mixing. According to their report a balance is required because harsh ultra- sonication treatments increase both G yield per mL and defect intensity, and shortens sheet sizes. In their work, they also reported that addition of ferrocene carboxaldehyde to G may result in formation of both graphenated carbon nanotubes and larger GS layers. Graphene nano-ribbons can be synthesised from unzipping of CNTs and depending on the intensity of the procedure, a mixture of starting and product may be obtained [1.60-1.62]. Unzipping of CNTs increases their effective surface area [1.63].
In the CVD method, direct growth of GS on metal substrates has potential to lower contact resistance on current collectors. It also reduces inter-particle resistance, thus ultimately increases power density of electronic devices such as ECs [1.64]. There is a current debate on possibility of epoxy and hydroxyl moieties at the basal plane of GS, and the carboxyl groups at either defect edges or framework in the GO structure. The type and amount of such functionalities can be varied by modifying preparation methods [1.50]. Additionally, mesoporous molecular sieves are preferred in graphene synthesis because of their large surface area, pore volume, narrow pore size distribution and easy surface functionalization. Atchudan et al. [1.65] used Si-MCM-41 enhanced through isomorphous substitution of Si with a transition metal. In their work, transition metal stabilised catalytic sites. Hence, pore sizes were tunable and also growth of graphene balls (GBs) depended on metal particles, catalytic template nature and reaction temperature [1.65].
1.3.2.2. Multiwalled carbon nanotubes
Multiwalled carbon nanotubes can be synthesised by several methods such as laser ablation [1.66], arch discharge [1.67], sol-gel [1.68] and chemical vapour deposition (CVD) [1.69]
methods. The CVD method can easily be scaled to industrial quantities and offers better morphology control [1.70]. MWCNTs, due to their 1-D character and their associated electronic structure, have extraordinary electrical properties. Electrical resistance in MWCNTs
16
occurs when an electron collides with defects in the graphitic structure, i.e. any deflection of an electron from its normal path [1.42,1.71]. Defects in MWCNTs include impurity atoms (doped-MWCNTs, Fig. 1.5e) or an atom vibrating about its position in the graphitic structure but electrons inside MWCNTs are not easily impeded. This is because of their smaller diameters and the higher aspect ratios [1.42]. Additionally, in 1-D materials electrons can only travel either forward or backwards. This infers minimum chances of back scattering, hence, MWCNTs are associated with low electrical resistance. MWCNTs have a Young’s modulus of 1.4 TPa [1.42]. This is facilitated by the carbon-carbon sp2 bonding and their thermal conductivity is general twice as that of diamond [1.68].
1.3.2.3. Surface modifications of carbon nano-materials
Functionalization with organic and inorganic materials offers an alternative way of controlling electronic properties of SCNMs [1.52]. Additionally, SCNMs can be modified by both non- covalent and covalent bonding of moieties. The most common covalent approach is through treatment of MWCNTs by means of oxidative reagents. Oxidizing reagents, such as acid, often used to purify MWCNTs often leaves high oxygen-containing groups on their surfaces, hence, introduces new functionality on the MWCNTs [1.72]. Similarly, the degree of GO oxidation can influence physicochemical properties such as conductivity [1.73] and hydrophilic properties. This often lead to better intercalation chemistry [1.57]. The negatively charged oxygen-containing groups enable chemical integration of metal ions such as titania via electrostatic adsorption [1.36,1.74]. Also, radicals, nitrenes, carbenes and arynes can be used to functionalise G via free radical reaction, CH insertion or cycloaddition. High energy barriers associated with inter-layer conjugation and interlayer van der Waals forces, makes modification of a flat, rigid G structure a challenge [1.75]. This lowers the GS capacitive capability [1.76].
Whereas, in molecular level mixing, carbon nanostructures can act as nucleation sites for metal reduction to form metal oxide (MO) SCNMs suspension [1.70]. Thereby facilitating coating of SCNMs by a MO, hence, promotes a homogeneous distribution. A non-covalent attachment via use of surfactants is a possible way for SCNM modifications that preserves their integrity [1.72]. In this regard, G is soluble in a limited number of solvents such as N-methylpyrolidine and 1,2-dichlorobenzene. Surfactants and polymers are appropriate enhancers of solubility.
17
On the contrary, mechanical integration, via ways such as ball-milling of metal powders and GO, is a better approach to achieve the required dispersion [1.74]. However, they introduce relatively larger amounts of defects which are often detrimental to their suitability in several applications. On the other hand, in colloidal mixing, SCNMs are dispersed with an ultrasonic waterbath, homogeniser, magnetic stirrer or a combination of the strategies. Dispersion in this regard is influenced by both surface area available on the SCNM surface and the solvent used [1.70]. This means variations in this respect can be utilised to alter physicochemical characteristics. Additionally, reaction conditions, such as time spent on the ultrasonic bath influences overall properties, could affect the defect intensity of the graphitic material.