Nanomaterials for Hydrogen Storage
It is generally known that pristine MgH2 theoretically can release ∼7.6 wt.% H2 at decomposition temperatures >300°C at a H2 pressure of ∼1 bar.1-3 However, so far, MgH2 based materials have limited practical applications because both hydrogenation and dehydrogenation reactions are very slow, and hence, relatively high temperatures are required.4 The phenomenon of mechanical milling helps to pulverize the particles of MgH2 into micro-or nanocrystalline phases and thus leads to lowering the activation energy of desorption. Without using catalysts, the activation energy of absorption corresponds to the activation barrier for the dissociation of the H2 molecule and the formation of hydrogen atoms. The activation energies of the H2 sorption for the bulk MgH2, mechanically milled MgH2, and nanocatalyst-doped MgH2 are 162, 144, and 71 kJ/mol, respectively. It is undoubtedly seen that the activation barrier has been drastically lowered by nanocatalyst doping. Figure 22.1 represents the thermogravimetric proles of bulk MgH2, which exhibits weight loss due to H2 decomposition at 415°C.