# Mg-Based Hydrogen Absorbing Materials for Thermal Energy Storage--A Review
## 3 Different Types of Mg-Based Materials for Thermal Energy Storage
### Different Heat Storage Systems of Mg-Based Materials
#### 3.1.2 Doped Mg/MgH\({}_{2}\) System
Besides the Nb\({}_{2}\)O\({}_{5}\), [PERSON] and collaborators also fabricated V-doped Mg by dehydrogenation sample obtained from ball milling the mixture of 1 g MgH\({}_{2}\) and 5 wt % V\({}_{2}\)O\({}_{5}\)[56]. The XPS analysis (Figure 3a) determined the presence of MgO and V. SEM-EDS results (Figure 3a-d) revealed that vanadium was distributed over the surface of Mg particle. In addition, the uneven area (circled) in the mapping results could be due to the incomplete reduction of V\({}_{2}\)O\({}_{5}\) in that area during ball milling. From the inset curves of Figure 3e, we could see that the peak of Mg2\(s_{1/2}\) was exactly consistent with the Mg2\(s_{1/2}\) peak position of MgO at 88.4 eV. Moreover, the peaks located in 517.2 eV and 513.0 eV referred to V\({}^{0}\), the shift of V peak might be due to chemical interaction and the doping of V over Mg surface. Importantly, the thermodynamic calculation found that V\({}_{2}\)O\({}_{5}\) could be reduced by MgH\({}_{2}\), producing MgO, metallic V, and hydrogen even at room temperature. This as-prepared sample could rise in temperature up to 192.5 \({}^{\circ}\)C within 40 s and showed ultrafast hydrogenation kinetics and remarkably enhanced dehydrogenation kinetics. [PERSON] et al. performed elevated temperature cycling studies on commercial gas atomized Mg powder with various catalysts (chromium, iron, vanadium, and stainless steel) prepared by magnetron sputter method [57]. At 350 \({}^{\circ}\)C, the faster reaction kinetics and successive cycling up to 90 times were obtained due to the presence of surface catalysts that boosted the fast nucleation stage of hydrogenation/dehydrogenation. The authors proposed suggested mechanisms for the uncoated Mg and coated Mg samples for hydrogenation/dehydrogenation (Figure 4). They mentioned that particle sintering competed with particle fragmentation during cycling at 350 \({}^{\circ}\)C. The coated Mg sample possessed rapid hydrogen diffusion during hydrogenation and quick hydrogen dissociation during dehydrogenation, which was due to the existence of catalysts. The enhancement of hydrogenation  
Figure 3: The SEM-EDS analysis of nano-engineered Mg: (**a**) microstructure of the portion of the sample of which EDS mapping has been done (**b**) magnesium mapping, (**c**) oxygen mapping, and (**d**) vanadium mapping. (**e**) The XPS analysis of vanadium in the composite to confirm the chemical state. The inset is the XPS analysis of Mg2s electron to ratify the chemical of Mg at the surface. Reproduced with permission from [56], Copyright Elsevier, 2017.  
kinetics might be attributed to the presence of the catalyst, which increased density of nucleation sites and prevented the sintering phenomena between the Mg particles and the decreipitation of particles during cycling, enabling the catalysts to combine with freshly exposed Mg surface. The mechanism they proposed is beneficial to the detrimental effects on activation and kinetics aspects, which provides a route to develop the TES material with fast kinetics.