**5. Graphene nanocomposites reaction mechanism for hydrogen storage**

Various authors have synthesized the graphene nanocomposites and discussed the kinetics thereof. Ngqalakwezi et al. synthesized a Ca/graphene nanocomposite using the solution mixing method for hydrogen storage purposes [10]. The calcium clusters in the nanocomposite bind hydrogen molecules without dissociation through the Kubas interaction and through the unidirectional polarization. Furthermore, they realized the Ca atoms on the graphene matrix increased the binding energy and therefore catalyzed the adsorption of the hydrogen molecule because of the polarization of the molecule [10]. The diagram below depicts the kinetic of the hydrogen storage into the graphene matrix (**Figure 3**).

Zhou et al. synthesized Pd/graphene nanocomposite for hydrogen storage [9]. In this work, four hydrogen storage performances were hypothesized for the uptake of hydrogen in the Pd/graphene nanocomposite: hydrogen molecules bind to the Pd atoms on the graphene matrix, hydrogen dissociates and forms a palladium hydride (PdHx), atomic hydrogen binds at the unsaturated sites of graphene or the hydrogen molecule is trapped within the graphene matrix [9]. The presence of the Pd atoms on the graphene matrix also elevate the binding energy and thus speeding up the adsorption of the hydrogen molecule due to the polarization of the hydrogen molecule orbitals with the Pd orbitals. The sluggish kinetics of Pd result in the dissociation of the hydrogen molecule on the surface of the Pd atom. Some of the dissociated hydrogen atoms are ejected into the gas phase and some diffuse into the Pd lattice and form the palladium hydride (PdHx) [9]. The diagram below is a conceptual figure of the reaction mechanism for the Pd/graphene nanocomposite. Zhou et al. further synthesized a Ni/Graphene nanocomposite and hypothesized the same reaction mechanism as the Pd/graphene nanocomposite [18] (**Figure 4**).

**Figure 3.** *Reaction mechanism of Ca/Graphene nanocomposite.*

**Figure 4.**

*Reaction mechanism of Pd/Graphene nanocomposite (adapted from [18]).*
