**8. Hydrogen storage behavior of Mg-containing HEAs**

Amongst the growing need for energy and constant decline in non-renewable energy sources, renewable energy storage devices are gaining popularity. The demand for Hydrogen Storage Devices is also increasing for the same reason. Among many other alternatives of hydrogen storage principles, metal hydrides are considered as some ideal candidates as these do not require cryogenic cooling like liquid hydrogen storages and can absorb decent amount of hydrogen as hydrides due to the high bond strength between metal and hydrogen [54]. Most metal hydrides exhibit exothermic reactions during hydride formation, whereas; the desorption reaction is endothermic in most cases. So, external temperature rise is necessary to continue the storage cycle. The negative enthalpy and entropy values are responsible for the high bond strength between metal ions and hydrogen [40]. The ΔH=-74 kJ.*mol*<sup>1</sup> *<sup>H</sup>*<sup>2</sup> and <sup>Δ</sup><sup>S</sup>-135 J.*<sup>K</sup>*<sup>1</sup> *mol*<sup>1</sup> *H*<sup>2</sup> for *MgH*<sup>2</sup> [40, 54]. The major drawback of single metal hydrides is the low desorption of hydrogen leading to low cyclability [54]. Single metal hydrides exhibit a hydrogen to metal ratio, [H]/[M]= 0.6. To solve this issue, several approaches have been taken e.g. using two or more metals to form hydrides, using non-hydride compositions for hydrogen storage, using boro-hydrides and ultimately, MPEA, commonly known as HEA which are a matter of research now [54].

The hydrogen storage in HEAs is related to a reversible phase transformation upon hydrogen absorption. The complex crystal structure of HEAs can store hydrogen in both tetrahedral and octahedral voids simultaneously, resulting in a higher hydrogen storage capacity than any single or, binary metal hydride. TiVZrNbHf HEA has the hydrogen to metal ratio, [H]/[M]= 2.5 [57]. The hydrogen to metal ratio is 2 for single metal hydrides [41].

Zepon et. al. conducted a research on Mg containing HEA and found the hydrogen absorption capacity to be 1.2 wt% of the alloy. The HEA exhibits a BCC structure while it converts into an FCC structure upon hydrogen absorption [55]. The HEA was produced using high energy ball milling while the hydride was produced using reactive ball milling. The main reason for upgrading to Mg containing HEAs for hydrogen storage is because of the light weight of Mg which might reduce the weight of hydrogen storage devices in light duty fuel cell vehicles. Efficient hydrogen storage requires light weight storage devices for high gravimetric capacity [41].

Marcelo et. al. produced hydrides of MgVCr and MgVTiCrFe alloys using reactive milling and the MgVCr consisted of a BCC phase with presence of β-*MgH*<sup>2</sup> on 72 hours of reactive milling. While the MgVTiCrFe consisted of amorphous phase in this study. The hydrogen storage capacity was measured at 30°C, 150°C and 350°C temperature. The hydrogen storage capacity was observed to be extremely low at 30°C and 150°C while it increased at 350°C. The MgVTiCrFe alloy did not show very promising hydrogen storage characteristics but MgVCr is a promising candidate for this purpose exhibiting a reversible 0.95 wt% hydrogen absorption capacity [56].

Strozi et. al synthesized MgVAlCrNi HEA using high energy ball milling but the equiatomic compound showed a very low hydrogen storage capacity, so two nonequiatomic alloy, Mg28V28Al19Cr19Ni6 and Mg26V31Al31Cr6Ni6 were proposed and studied for hydrogen storage which also didn't show promising results. The nonequiatomic compositions were selected in such a way that the amount of Mg and V is increased, the solubility of hydrogen is increased and so does the lattice parameter because, the increase in lattice parameter indicates that there will be more available interstitial space for hydrogen absorption into the structure. This study prioritizes on the importance of the enthalpy of hydrogen solution on the hydrogen storage capacity of Mg containing HEAs [39]. The positive enthalpy of hydride formation for most of the elements present in these alloys is responsible for this low hydrogen storage behavior.

A very recent study by Montero et al. suggests the improvement in cycling behavior of TiVZrNb HEA upon introduction of Mg to it. After the 12th cycle, the absorption capacity reduces upto 2.41 wt% from the initial 2.8 wt%. The temperature where the maximum desorption occur, is 290°C for Mg10*Ti*10*V*25*Zr*10Nb25, while it is 330°C for *Ti*32*:*5*V*27*:*5*Zr*12*:*5Nb27*:*<sup>5</sup> [41]. As the desorption reaction is endothermic, it requires extermal temperature rise to promote the process. The lower the temperature when the desorption starts, the more economically viable and efficient the storage device will be. This particular composition shows the lowest temperature for the maximum desorption to occur. Observations from this study suggests that the Mg10*Ti*10*V*25*Zr*10Nb25, alloy loses around 11 % of the hydrogen storage capacity in the second cycle, where the storage capacity reduces from 2.7% to


#### **Table 5.** *Hydrogen absorption capacity of HEAs.*

### *Magnesium Containing High Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.98557*

2.41%, whereas, *Ti*32*:*5*V*27*:*5*Zr*12*:*5Nb27*:*<sup>5</sup> loses 28% of the hydrogen storage capacity during the first 4 cycles. This recent study suggests the enhancement in the reversable capacity of Mg containing HEAs than Mg-less HEAs. There are not many studies done on Mg containing HEAs for hydrogen storage applications and so the field is open for further research. But this particular study suggests a high potential of Mg containing MPEAs in this certain application.

The **Table 5** summarizes the findings.
