# Reversible Hydrogen Storage Using Nanocomposites
## 2 Nanocomposite Materials for Hydrogen Storage
### MgH\({}_{2}\)  
Bulk magnesium hydride, MgH\({}_{2}\), shows promising hydrogen storage capacity of 7.6 wt.% at temperatures above 300 \({}^{\circ}\)C and at a hydrogen pressure of 1 atm [46; 47; 48]. However, MgH\({}_{2}\) have limited practical applications because of slow kinetics and high temperatures (>200 \({}^{\circ}\)C) required for de-/hydrogenation reactions. [49]. In bulk MgH\({}_{2}\), high activation energy (162 kJ-mol\({}^{-1}\)) is required for the dissociation of hydrogen molecule to its atoms. Mechanical pulverization/milling of bulk MgH\({}_{2}\) creates micro/nanocrystalline phases, the surface to volume ratio increases, which leads to lower activation energy of desorption, 144 kJ-mol\({}^{-1}\), hence on-set temperature for hydrogen dissociation also decreases. Further reduction of the activation energy, as low as 71 kJ-mol\({}^{-1}\)[21], can be achieved by nanocatalyst doping of micro/nanocrystalline MgH\({}_{2}\). Figures 2 and 3 demonstrate the thermogravimetric (TGA) and differential scanning calorimetric (DSC) curves of hydrogen decomposition from bulk MgH\({}_{2}\), micro/nanocrystalline MgH\({}_{2}\) and nano-Ni doped micro/nanocrystalline MgH\({}_{2}\). The bulk MgH\({}_{2}\) shows a sharp weight loss due to hydrogen release in the endothermic reactions at around 410 \({}^{\circ}\)C. The TGA (Figure 2) of micro/nanocrystalline MgH\({}_{2}\) obtained by 9 and 12 h ball milling exhibit weight loss at reduced temperatures of 345 and 340 \({}^{\circ}\)C, respectively; whereas the DSC (Figure 3) of 10.5 h ball milled MgH\({}_{2}\) shows two endothermic transition at 390 and 375 \({}^{\circ}\)C. For the 10 mol% nano-Ni catalyzed micro/nanocrystalline MgH\({}_{2}\), thermal decomposition temperature is further reduced to 245 \({}^{\circ}\)C (Figure 2), which is at least 170 \({}^{\circ}\)C lower than the bulk counterpart. The DSC experiments were carried out for the different concentrations of nano-Ni in micro/nanocrystalline MgH\({}_{2}\) as depicted in Figure 3. For the low 1 mol% nano-Ni concentration, there is a wide endothermic transition that occurs due to decomposition of hydrogen from micro/nanocrystalline MgH\({}_{2}\) at around 295 \({}^{\circ}\)C, however for higher concentrations, 2-4 mol%, a sharp endothermic peak is predominant at a temperature around 230 \({}^{\circ}\)C. Therefore, by comparing both TGA and DSC results (Figures 2 and 3), one can derive the conclusion that at least 45-70 \({}^{\circ}\)C reduction in temperature is possible by mechanical pulverization of bulk MgH\({}_{2}\), and further 100-140 \({}^{\circ}\)C reduction in hydrogen decomposition temperature was observed for mechanically pulverized MgH\({}_{2}\) doped with metal nanocatalysts, nano-Ni. The enthalpy of hydrogen decomposition from MgH\({}_{2}\) metal hydrides were calculated and tabulated in Table 2.  
The structural characterization of the bulk, micro/nanocrystalline and nanocatalyst doped MgH\({}_{2}\) was carried out using X-ray diffraction (XRD) and is shown in Figure 4. The Bragg reflection of bulk MgH\({}_{2}\) shows sharp crystalline peaks. However, in the micro/nanocrystalline MgH\({}_{2}\), the crystallite size was reduced to the nano dimension (5 times reduced X-ray intensity in terms of counts) as observed from broadening of full width at half maximum (FWHM) of the crystalline peaks. No additional peaks corresponding to MgO or any other impurities were observed in the ball milled samples. For the nanocatalyst doped MgH\({}_{2}\), a small inflection peak at an angle of 45\({}^{\circ}\) represents the nano-Ni. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Sample** & **T\({}_{\textbf{dec}}\) (\({}^{\circ}\)C)** & **\(\Delta\)H\({}_{\textbf{dec}}\) (kJ-mol\({}^{-1}\) H\({}_{2}\))** \\ \hline Commercial MgH\({}_{2}\) & 415 & 55.98 \(\pm\) 0.50 \\ MgH\({}_{2}\) (MC) & 375 & 46.35 \(\pm\) 0.50 \\ MgH\({}_{2}\) (MC)-nano-Ni & 225 & 36.50 \(\pm\) 0.50 \\ \hline \hline \end{tabular}
\end{table}