# Reversible Hydrogen Storage Using Nanocomposites
## 2 Nanocomposite Materials for Hydrogen Storage
### Carbon based Nanocvariants  
The physisorption of hydrogen are phenomenally superior in carbon nanovariant types of materials such as Buckminster Fullerenes, single of multiwalled carbon nanotubes, nanofibers, nanoshells, nanobells etc., because of their unique characteristics such as a high surface to volume ratio,  
Figure 12: TGA/DSC profiles of pristine (or undoped) and nano-Ni doped Zn(BH\({}_{4}\))\({}_{2}\).  
Figure 13: GC–MS profiles (B\({}_{x}\)H\({}_{y}\) ion counts vs. retention time) of undoped and nano-Ni doped Zn(BH\({}_{4}\))\({}_{2}\).  
highly porous microstructure, lighter weight, and low heat or thermodynamic values. Though there were some discrepancies in terms of hydrogen storage that was reported for these carbon nanovariants, the physical understanding and mechanistic approach of the hydrogen-solid surface interaction is emerging. The physical adsorption occurs while interacting molecular hydrogen on the surface of the nanovariant material because of weak van der Walls forces of attraction. Additionally, the highest occupied molecular orbital of carbon atom overlaps with the occupied electronic wave function of the hydrogen electron. The chemisorption on the other hand related to overcoming the activation energy barrier in the process of dissociation of hydrogen reversibly. The major difference between the physisorption and chemisorption of hydrogen is that for the physisorbed process, one or less than one hydrogen atom is coordinated with at least two carbon atoms, whereas the chemisorbed process, at least two hydrogen atoms coordinated with one carbon atom [72; 73; 74]. The binding energy required for the physisorbed hydrogen is on the order of 0.1 eV, while the chemisorbed hydrogen requires greater than 2-3 eV.  
Carbon nanotubes, an allotrope of carbon played a vital role in hydrogen storage [75] and this first study created waves in preparing such nanovariants experimentally and understanding them in the theoretical framework. Densifying hydrogen on both the surface and bulk of the carbon nanostructures has proven to be an ideal solution than the compressed hydrogen in gas cylinders. A storage density of at least 10% could be stored in single wall carbon nanotubes [76] and was demonstrated using thermal programed desorption at NREL, USA. Fullerenes, on the other hand, are a new form of carbon with close-caged molecular structure, which were first reported by [PERSON] in 1985 [77]. It is a potential hydrogen storage material based on the ability to react with hydrogen via hydrogenation of carbon-carbon double bonds. Theory predicts that a maximum of 60 hydrogen atoms can be attached to both the inside (endohedrally) and outside (exohedrally) of the fullerene spherical surface. Thus, a stable C\({}_{60}\)H\({}_{60}\) isomer can be formed with a theoretical hydrogen content of 7.7 wt.%. It seems that the fullerene hydride reaction is reversible at high temperatures. The 100% conversion of C\({}_{60}\)H\({}_{60}\) indicates that 30 mol of H\({}_{2}\) gas will be released from each mole of fullerene hydride compound. However, this reaction requires a temperature of about 823-873 K [78]. Solid C\({}_{60}\) has an FCC lattice at room temperature and a molecular density of 1.69 g\(\cdot\)cm\({}^{-3}\). The molecules can rotate due to the weak molecular interaction. Fullerene is an allotropic of carbon. Fullerene molecules have five-fold and six-fold symmetries, whose vertexes contain carbon atoms. Fullerene, C\({}_{60}\) is the smallest, highly symmetric and an equilibrium structure for gas storage applications.