**6.2 Mechanism of hydrogenation in magnesium-based materials**

The pure magnesium, which has a hexagonal structure reacts with hydrogen molecules reversibly to form magnesium hydride in which the parametric details are presented in **Table 6**—the reversible reaction presented in Eq. (1).

$$\text{Hydrogen storage equation: } \text{Mg} + \text{H}\_2 \rightarrow \text{MgH}\_2 \tag{1}$$

The crystal structure of MgH2 transformed into tetragonal β- MgH2. At the same time, the pressure is increased with the high hydrogen pressure at the ambient temperature. The β- MgH2 transformed into a metastable orthorhombic γ- MgH2 phase. The schematic illustration of the mechanisms is presented in **Figure 3**.

The following reaction mechanism is composed of five different intermetallic processes.


5. conversion of metal into metal hydride [29].

*Magnesium Metal Matrix Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.96241*


#### **Table 6.**

*Magnesium and magnesium hydride crystallographic data [27].*

**Figure 3.** *The hydrogen gas ab/desorption mechanisms in magnesium [28].*

#### **6.3 Nanostructuring**

Nanostructuring is also one of the important factors that can enhance the kinetics of magnesium-based materials, and it can destabilize the thermodynamics of magnesium hydride formations. The reduction of particle size into nanoscale will decrease the stability of metal hydride. The nanoparticles significantly contribute to the overall surface energy. The total energy required for dehydrogenations on metal hydrides depends on the radius of particle size r and could be as in Eq. (2) [30].

$$
\Delta \mathbf{G} \left( \mathbf{r} \right) = \Delta \mathbf{G}\_o \left( \mathbf{r} \right) + \mathbf{R} \mathbf{T} \ln \frac{P}{P\_o} + \frac{\mathbf{3} V\_{M\Delta\_{M \to \text{ML1}}(\mathbf{r}, r)}}{r} \tag{2}
$$

### **6.4 Magnesium composites in hydrogen storage application**

#### *6.4.1 Mg-carbon composites in hydrogen storage*

The novel electronic properties of carbon and exciting interaction between hydrogen and carbon atoms, particularly nanostructured materials, exhibit the prominent catalytic effect on the Mg as the hydrogen storage material. Initial time, graphite has been proposed as one of the anti-sticking agents in the ball milling to improve the efficiency of the process with magnesium and examined its catalytic effect on the hydrogenation characteristics. Later on, the studies investigated

#### **Figure 4.**

*Hydrogenation kinetic curves for AZ31- magnesium alloy/carbon materials, (a) Hydrogen absorption, (b) Hydrogen desorption [28].*


#### **Table 7.**

*Hydrogen storage properties of Mg-alloys with different types of carbonaceous materials.*

the advantages of the carbon additives such as single-walled carbon nanotubes (SWNT), activated carbon (AC), carbon black (CB), fullerene (C60), and boron nitrate (BN) nanotubes in the enhancements of practical hydrogenations properties of magnesium and its alloys.

The addition of carbon materials shows significant advantages in hydrogenation kinetics and particularly in the improvement of hydrogenation capacities. Moreover, Huang et al. investigated the hydrogenation kinetics with different carbon materials such as activated carbon (AC), carbon black (CB), and graphene nanosheets. The AZ31- magnesium alloy added with graphene sheets reached the maximum storage capacity of 6.83 wt.%. The AZ31-magnesium alloy reached its maximum capacity in less than 15 minutes and released entire hydrogen in less than 3 minutes, and its kinetic curves presented in **Figure 4**. It clearly shows that carbon

*Magnesium Metal Matrix Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.96241*

and magnesium-based materials as composites (Mg alloys-carbon composites) produce the prominent catalytic effect for hydrogenation. Moreover, the results illiterate the carbon effects, which are strongly related to the unique electronic-π properties and sheet morphology of nanographene, which can make the high porosity with the high surface area. The sheet-like morphology acts as the nucleation site for the hydrogen molecules penetration into the materials and could increase the hydrogenation kinetics [28].

The theoretical capacities of different materials were calculated by the elemental molecular equation as follows

*H H Mg Al Zn c xM xM xM yM zM tM* <sup>=</sup> + + + + +… 2 2 C (3)

Where C is the theoretical capacity (wt.%); MH2 is the molecular weight of hydrogen; MMg is the molecular weight of Mg; MAl is the molecular weight of Al; MZn is the molecular weight of Zn; MC is the molecular weight of C with its corresponding weightage of the quantity in the reaction (x, y, z, and t). The hydrogen storage properties of Mg-Alloys with different carbonaceous materials were presented in **Table 7**.
