# Mg-Based Hydrogen Absorbing Materials for Thermal Energy Storage--A Review
## 3 Different Types of Mg-Based Materials for Thermal Energy Storage
### Kinetics and Thermal Conductivity Properties
They prepared the solid porous pellets of Mg-2 wt % multiwall carbon nanotubes (MWCNTs) composites, which were fabricated by high energy ball milling of Mg powder with MWCNTs additives, followed by uniaxial compression and sintering in hydrogen atmosphere under mechanical constraint. From the BSE micrographs of the as-prepared partially hydrogenated samples (Figure 10), we could see that Mg pellets without additive presented nearly circular interconnected MgH\({}_{2}\) (circles marked) and isolated Mg islands (Figure 10a,b). While the pellets with MWCNTs additives showed elongated MgH\({}_{2}\) nuclei (marked by the circle) and interconnected network of metallic Mg (Figure 10c,d). The Figure 10e,f were the morphologies of control pellet using 2 wt % Fe as additive to exclude effect to the growing hydride phase from iron-rich precipitates. The overall microstructure (Figure 10e,f) looked similar to the pure Mg pellets, which indicated that the special morphology observed in the Mg-2 wt % MWCNTs composites was related to the carbon nanotubes. The asymmetrical and anisotropic MgH\({}_{2}\) nuclei indicated that the growth of MgH\({}_{2}\) was much faster along the carbon-rich direction and which means hydrogen diffused much faster along the Mg-carbon interfaces. The proposed mechanism clearly presented us the different MgH\({}_{2}\) nucleus grow process in the Mg-2 wt % MWCNTs sample (Figure 10h) compared to the pure Mg one (Figure 10g). Such anisotropic microstructure had a tendency to solve the incompatibilty problem between thermal conductivity and kinetics. Firstly, the interconnected metallic Mg network enabled the rapid heat transfer over a large distance, which resulted in improved thermal conductivity. According to their results, the thermal conductivity of the pellets with Mg-2 wt % MWCNTs was improved by over 20% than the pure Mg one. Secondly, hydrogen could diffuse more deeply into the particle due to the interconnected metallic Mg network, which ensured the fast hydrogenation kinetics. [PERSON] et al. [82] also developed a general strategy for facile preparation of monodispersed MgH\({}_{2}\) nanoparticles on graphene. Under the structure-directing role of graphene, the as-prepared sample showed high thermal conductivity and good hydrogen storage performance (5.7 wt % H\({}_{2}\) capacity for the 75 wt % MgH\({}_{2}\) loading percentage). [PERSON] et al. [83] fabricated the  
Figure 9: Kinetic performance of Ti-catalyzed and commercial MgH\({}_{2}\). Reproduced with the permission from [34], Copyright Elsevier, 2012.  
cylindrical MgH\({}_{2}\) compacts containing up to 20 wt % graphite flakes with an average size of 20-1200 \(\upmu\)m by the uniaxial compression at 100 MPa. The dispersed graphite flakes were aligned perpendicularly to the compression direction, which had a great enhancement to the effective thermal conductivity. The measured conductivity almost linearly increased with increasing graphite content and could reach 9.3 W m\({}^{-1}\) K\({}^{-1}\) at 20 wt % graphite at 1 bar of hydrogen. They also found the hydrogen pressure and the size of graphite flakes influenced the thermal conductivity. Overall, the kinetics performance of the heat storage system heavily depends on hydrogen absorption kinetics and thermal conductivity of the materials. It seems that the nanoengineering and proper nanocomposites could handle and balance the relationship of hydrogen absorption kinetics and thermal conductivity. Here, we have introduced different TES systems in detail, which include Mg/MgH\({}_{2}\), doped Mg/MgH\({}_{2}\), Mg-Fe/Mg\({}_{2}\)FeH\({}_{6}\), Mg-Co-H, and NaMgH\({}_{3}\) systems. The related properties such as energy density, reaction temperature, cycle stability, kinetics, thermodynamic properties, as well as preparation methods, have been introduced.