**3. Graphene nanocomposites**

Nanocomposites, where nanocatalysis and nanostructure are combined, are the next research advancement of high performance of hydrogen storage materials. These materials have distinct functionalities due to the shorten diffusion distance, increased surface area and the multiplied grain boundaries [6]. Graphene, graphene nanocomposites and its derivatives depict a promising potential for different applications such as automotive industry, aerospace, electronics and green energy. This is because graphene has intriguing thermal, mechanical and electrical properties [6, 8, 9].

A lot of studies have been done with graphene and nanocomposites thereof for the practical application in on-board application [7–10]. Graphene nanocomposites can be synthesized using various techniques. These include the self-assembly technique, solution mixing, sol–gel method, hydrothermal or the solvothermal method and other methods [11]. Graphene nanocomposites have been preferred for hydrogen storage application mainly because of their light weight characteristic. Weight of the material is an important factor when considering the practical applications for hybrid cars according to the DoE standards. Graphene alone is a physisorbent material and does not take up a lot of hydrogen however its kinetics are interesting [11–15]. The incorporation of metals on the graphene matrix enhances the functions of the system and improves the hydrogenation of this material to make it more appealing for practical use in hybrid cars [16, 17]. Various metals have been used in this regard to improve the hydrogen uptake of the graphene nanocomposite. Zhou et al., synthesized a Ni/graphene nanocomposite and a Pd/graphene nanocomposite [18]. Ngqalakwezi et al. synthesized a novel Ca/graphene nanocomposite [10]. The graphene nanocomposites can be used and applied in different industries for hydrogen generation in the electrolysis process, in the photo degradation of pollutants, energy storage and other applications.

Furthermore, nanomaterials permit favorable charge, mass and heat transfer, these are added advantages when considering practical application of these materials for on board applications [13, 17, 19]. In addition, nanomaterials assist in dimensional alteration of particular phase transitions and chemical reactions [6]. Nanomaterials not only help and aid in the kinetics of hydrogen storage materials, but they also help to destabilize the thermodynamics of chemisorption materials.

#### **4. Synthesis methods for graphene nanocomposites**

#### **4.1 Self-assembly technique**

The self-assembly process is one of the primary techniques for synthesizing complex materials from molecules in macro, micro and nano scales [20]. In the bottom up techniques in nanotechnology, this method has been considered as one of the most effective methods [21]. In this technique, molecules are utilized as precursor material for synthesizing graphene nanocomposites under environmentally conducive parameters. As such, graphene sheets prepared using the top down approach (mechanical and chemical exfoliation of graphite), can be utilizing as precursor material for the self-assembly technique [20]. The mechanism of selfassembly of graphene can be quite complex and understanding it necessitates the full understanding of the non-covalent and interlayer covalent interactions between graphene derivatives [22]. These interactions include the dipole–dipole interactions, p–p interactions, van Der Waals forces and electrostatic forces. Non- covalent interactions in the self-assembly technique, aid the graphene in producing composites with novel functions and structures [22, 23].

*Hydrogen Storage: Materials, Kinetics and Thermodynamics DOI: http://dx.doi.org/10.5772/intechopen.94300*

These non-covalent interactions are active in various organic solvents and they permit homogenous dispersion for the anticipated self-assembly. Furthermore, graphene molecules allow for functionalization which is double sided and thus creates novel structural architecture with double-sized decoration of functional groups on the graphene sheet. These functional groups in principle, permit layerby-layer coordinated assembly in a supramolecular manner [23].

To take advantage of the characteristics of graphene at nanoscale in macro-sized devices, it is crucial to incorporate the graphene sheets into 3D micro-sized structures with better maneuver of the geometry and dimensionality of the material [24]. Various methods have been reported for the fabrication of 3D porous structures, 2D thin films and 1 dimensional fiber-like molecules [24]. For 3D porous structures these methods have been used; freeze casting self-assembly, breath figure 3D assembly, diffusion driven 3D self-assembly, 3D self-assembly through the hydrothermal process, pickering emulsions for 3D molecules and 3D assembly through chemical reduction [25–28]. For 2D structures these methods have been used; vacuum assisted assembly, Rayleigh and Taylor instability and Marangoni effect self-assembly method, liquid–liquid interfacial 2D assembly method, evaporation induced 2D self-assembly method, electrophoretic method and the Langmuire-Blodgett method [27, 29, 30]. For 1D fibers these methods have employed; 1D self-assembly self-intertwining method, direct drawing self-assembly method, electrophoretic 1D self-assembly and flow directed wet-spinning 1D selfassembly. All of these methods have been successfully employed to synthesize these different structures under various parameters [31–33].

### **4.2 Solution mixing method**

The solution mixing method is one of the easiest method for the synthesis of polymer based graphene nanocomposites. The method comprises of three simple steps which are; the scattering of the filler, the polymer incorporation and the solvent removal through the distillation process or evaporation [34]. This method is also relatively cheap because it does not need the use of expensive equipment or expensive operative protocols, although it has a many steps [35]. Other advantages about this technique are; the method allows for good dispersion of thin particles because of the efficient fragmentation of organoclay agglomerates [35, 36]. This in effect, generates greatly filled key batches that can later be mixed with pristine polymer through the melt compounding process. Furthermore this process allows for the production of highly exfoliated graphene nanocomposites that can be attained through mixing the physical coupling process with chemical reaction.

The synthesis step of this method typically involves dissipating a polymer in a solvent and suspending the filler in a different compatible solvent. Different solvents such as acetone, tetrahydrofuran (THF), toluene, chloroform, dimethylformamide and cyclohexane are utilized in this method [35]. As mentioned above, the solution blending generates ensures excellent exfoliation and dispersion of the filler within the elastomeric matrix [37].

Ultrasonication or high speed shear mixing are employed during the solution blending process to ensure the polymer solution and filler suspension are mixed thoroughly [36]. This technique has been efficient in dispersing nanofillers regardless of the polarity of the polymer. As much as this method has a lot of advantages, it does however have disadvantages as well. The main disadvantage of this technique is the thorough and efficient removal of solvents utilized during the process. Another major disadvantage is the scale up; the solvents utilized are very expensive and scaling up this process would pose financial difficulties and strains [35, 36].

Lastly, the entire process involves a lot of steps and this can influence the subsequent outcome of the process.

A couple of researcher have used this method to synthesize graphene nanocomposites. Wang et al. utilized Cu (OH)2 composite sheets and reduced graphene oxide to synthesize thin micro layered structure of rGo-Cu powder [38]. Tang et al. synthesized graphene nanosheets decorated with Ni nanoparticles utilizing the in situ chemical reduction method [39]. The resulting nanocomposite after wet mixing electrolytic Cu to obtain Ni-graphene nanosheets/Cu nanocomposite, depicted interesting mechanical properties with a yield strength of 268 MPa and a high Young Modulus of 132 GPa [39]. Furthermore, Zeng et al. incorporated Al powder in the GO suspension solution and ultrasonicated the mixture [40]. Algraphene nanocomposite was obtained with a tensile strength of 255 MPa [40]. Li et. al, on the other hand used precursor organic graphite to synthesize graphene oxide using the Hummers method and decorated the graphene with Ni nanoparticles [41]. Ngqalakwezi et al. also synthesized Ca/graphene using the Improved Tours Method and chemical reduction the Ca ions on the surface of the GO [10].

The solution mixing method has been successfully employed to synthesis graphene nanocomposites.

#### **4.3 Sol-gel method**

The sol–gel method is also a simple method that allows the synthesis of a homogenous material that has great compositional controls [42]. This method utilizes metal chlorides or metal alkoxides as the precursor material [11]. The chlorides or alkoxides are treated through a series of condensation and hydrolysis reactions and later the cured composites are dried and calcined [11]. A number of researchers have prepared graphene nanocomposites through the sol–gel method. A SiO2/ graphene nanocomposite was synthesized with good cyclic stability and spectacular specific capacitance of Fg<sup>1</sup> with a current density of 1 Ag<sup>1</sup> by Rezaei et al. [43]. Patil et al. fabricated functional nanographene sheets using cobalt sulphide which had a good rate of cyclic ability and high reversible capacity ranging at about 466 mAhg<sup>1</sup> [44]. Wang et al. fabricated TiC/graphene nanocomposite using the sol gel method [45]. In this work, furfuryl alcohol was utilized as a carbon and this nanocomposite was synthesized for impact or shock absorptions [45]. Furthermore, Sun et al. synthesized various nanoparticles of platinum on sulfonated graphene and used them as anode electrocatalysts in direct ethanol fuel cell using the sol gel method [46]. The particles sized of the nanoparticles on the graphene surface ranged 1.7 nm to 13.9 nm. The sulfonic acid on the graphene improved the adsorption energy of platinum, this was observed through theoretical calculations [46].

The synthesis work done using the sol gel method has proved to be effective in synthesis graphene nanocomposites and thin-film coating materials. This method required decreased reaction temperatures and it is also not complex to follow and conduct although the still challenges in the formation of the sol.

#### **4.4 Hydrothermal method**

The hydrothermal technique has been utilized to fabricate graphene nanocomposites using autoclaves under high pressures and temperatures. The first hydrothermal reaction was reported in 1845 by Schafhautl when he noticed the synthesis of quartz microcrystals from silicic acid [47]. Since then, his method has been developed over the years by many other scientists and other novel synthesis methods have also been reported. The hydrothermal technique is not restricted to the fabrication of common and advanced materials but it also covers a wide range of interdisciplinary subdivisions in the sector of energy storage, simulating biohydrothermal and geothermal process and waste treatment [48]. In principle, the word hydrothermal, was initiated from geological sciences where it refers to a regime of water pressures and high temperatures [49]. Hydrothermal reaction traditionally involves water as catalyst and seldom, as a component of the solid phase during the synthesis at high pressures and temperatures. This technique has many advantages to it such as, excellent dispersion in water, the synthesis using this method is inexpensive due to instrumentation, environmentally friendly, one pot synthesis method, mild operation conditions and the material precursor are also inexpensive [50].

Lee et al. discovered a novel hydrothermal fabrication route to synthesize graphene nanocomposites to improve the photocatalytic activity of TiO2 under visible light. In the method, graphene was decorated or wrapped with TiO2 particles [51]. A high catalytic CeO2/graphene was synthesized by Srivastava using the hydrothermal method [52]. A Ag/graphene nanocomposite with exceptional electroconductivity was synthesized by Yang et al. using the hydrothermal method. In this work, Yang et al. used a hydrazine reductant as a reducing agent and had control over the morphology and the size of Ag nanoparticle on the surface of graphene [53]. Zhang et al. prepared a one pot method for the synthesis of Fe2O3–Ni (OH)2/graphene nanocomposite. The nanocomposite had a good rate capability (at 100% retention), impressive cyclic stability (5000 cycles) and elevated specific capacitance at approximately 857 F/g [54]. A ZnO/graphene nanocomposite was synthesized using the hydrothermal at lower temperature 90 and 80°C [55]. The nanocomposites both displayed good catalytic activity during photocatalytic degradation against rhodamine-B dye [55].
