# Reversible Hydrogen Storage Using Nanocomposites
## 2 Nanocomposite Materials for Hydrogen Storage
### Mg\({}_{2}\)FeH\({}_{6}\) Complex Hydride  
The nanocatalysts (e.g., nano-Ni) lead to lower hydrogen decomposition temperature characteristics of micro/nanocrystalline MgH\({}_{2}\) as demonstrated in the previous section. In this section, a novel transition metal complex hydride involving the combination of MgH\({}_{2}\) and Fe ball milled under hydrogen ambient thus exhibit reversible hydrogen storage capacity by volume, 150 kg\(\cdot\)m\({}^{-3}\)[50], which is greater than the liquid hydrogen storage capacity [51] at cryogenic temperature. Moreover, these Mg\({}_{2}\)FeH\({}_{6}\) complex hydride possesses a storage capacity by weight basis, 5.0 wt.% at 400 \({}^{\circ}\)C, which is very comparable to plain MgH\({}_{2}\) capacity of 7.6 wt.%. The Mg\({}_{2}\)FeH\({}_{6}\) hydrides having [FeH\({}_{6}\)]\({}^{4-}\) complex anion sites that are octahedrally coordinated the central Mg atoms in an octahedral crystal lattice [52]. In a similar way, the MgH\({}_{2}\) will react with other transition metals such as Co and Ni to form ternary and quaternary structures, such as Mg\({}_{2}\)CoH\({}_{5}\) and Mg\({}_{2}\)NiH\({}_{4}\) thus leads to obtain reversible hydrogen storage systems [53]. The reaction kinetics of the above Mg-based transition metal complexes can be improved by reacting MgH\({}_{2}\) in the nanoform with nano Fe, nano Co, or nano Ni [46; 54]. It has been very recently shown that the combination of nano Fe and nano Ti transition metals with the carbon nanovariant such as carbon nanotube on the MgH\({}_{2}\) exhibited faster hydrogen dissociation and effective hydrogen atom transport between the MgH\({}_{2}\) grains [55; 56]. Unlike Mg\({}_{2}\)Ni intermetallic equilibrium phase for efficient reversible hydrogen storage (Mg\({}_{2}\)NiH\({}_{4}\)) at low temperature (\(<\)300 \({}^{\circ}\)C), the Mg\({}_{2}\)Fe does not exist as an intermetallic stable compound phase. However, under hydrogen gas pressure Mg/MgH\({}_{2}\) and Fe creates a stable Mg\({}_{2}\)FeH\({}_{6}\) complex hydride [50] per the following chemical reactions,  
\[2\mathrm{Mg}+\mathrm{Fe}+3\mathrm{H}_{2}\leftrightarrow\mathrm{Mg}_{2}\mathrm{ FeH}_{6} \tag{1}\]  
\[2\mathrm{MgH}_{2}+\mathrm{Fe}+\mathrm{H}_{2}\leftrightarrow\mathrm{Mg}_{2} \mathrm{FeH}_{6} \tag{2}\]  
The synthesis of nanocomplex hydride Mg\({}_{2}\)FeH\({}_{6}\) was done in solid-state milling process based on the above reactions with either Mg or MgH\({}_{2}\) as precursor with the appropriate crystallite size. The hydrogen gas pressure is adjusted to optimize the milling parameters such as the time duration, ball to powder ratio, etc., to obtain a single phase Mg\({}_{2}\)FeH\({}_{6}\). The X-ray diffraction patterns of the bulk materials before and after milling are depicted in Figure 6. The bulk Mg and Fe show sharp crystalline Bragg reflection before ball milling, whereas, the nanocrystalline Mg\({}_{2}\)FeH\({}_{6}\) phase forms after ball milling processes as revealed from the widening of the XRD peak around 44\({}^{\circ}\). The DSC profiles of Mg\({}_{2}\)FeH\({}_{6}\) in its bulk and nanocrystalline forms are shown in Figure 7. The bulk Mg\({}_{2}\)FeH\({}_{6}\) shows an endothermic transition due to hydrogen decomposition at peak temperature of 380 \({}^{\circ}\)C and a shoulder inflection transition at 310 \({}^{\circ}\)C reflects the secondary phase decomposition. In comparison to the bulk, the nanocrystalline samples showed a sharp endothermic transition temperature at a lower peak temperature of 280 \({}^{\circ}\)C; additionally, on-set temperature of 200 \({}^{\circ}\)C is also clear from Figure 7 Mg\({}_{2}\)FeH\({}_{6}\) behaves very similar to nano-Ni doped micro/nanocrystalline MgH\({}_{2}\) examined in the previous section.