# 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  
For a specific chemical reaction applied for TES, the progress of the chemical reaction is much faster than the rate of thermal conductivity, as a result, the rate-limiting step of this chemical heat  
Figure 7: **(a)** Hydrogen desorption PCI curves for NaMgH\({}_{3}\)\(\rightarrow\) NaH + Mg + H\({}_{2}\) performed at different temperatures. Hydrogen desorption equilibrium data points below \(\sim\)3 wt % desorption have been truncated for clarity while dashed lines indicate inferred data. **(b)** Kinetic H\({}_{2}\) desorption data of NaMgH\({}_{3}\)\(\rightarrow\) NaH + Mg + H\({}_{2}\) performed at 418.8 \({}^{\circ}\)C. **(c)** van’t Hoff plot of H\({}_{2}\) desorption equilibrium pressures and the linear fit to the data. Reproduced with the permission from [70], Copyright American Chemical Society, 2011.  
storage system is the rate of thermal conductivity [19]. Besides the heat transfer ability of the storage tank and internal heat exchanger, the most significant issue is the thermal conductivity of materials. In PNNL's project [33], they used open cell Duocel copper foam with interstitial spaces, which was filled with metal hydride powder to enhance thermal conductivity in both radial and axial directions. [PERSON] et al. [78] studied and summarized different types of Mg-based materials with various structures (Figure 8) and evaluated their thermal and kinetics properties (Table 4). In their work, samples such as micrometer scale Mg powders, Mg nanoparticles, single crystal Mg, nanocrystalline Mg\({}_{50}\)Co\({}_{50}\) body centered cubic (bcc) alloy, and Mg thin film were investigated. Due to the much more scattering of electron/phonon by more boundaries in nanostructured materials, the Mg nanoparticles and Mg\({}_{50}\)Co\({}_{50}\) alloy delivered poor thermal conductivity and relatively low heat capacity. However, compared with other microscale materials (325 mesh Mg, Mg single crystal), the nanostructured materials showed an outstanding hydrogen absorption kinetics. On the one hand, nanoscale materials could optimize the hydrogen absorption kinetics. On the other hand, it would sacrifice the thermal conductivity. A similar phenomenon was observed between the Ti-catalyzed nanoscale MgH\({}_{2}\) and commercial MgH\({}_{2}\) (Figure 9) [34], Ti-catalyzed nanoscale MgH\({}_{2}\) performed much faster hydrogen absorption kinetics than commercial MgH\({}_{2}\) under the same conditions. The inset of Figure 9 also suggested the huge difference in hydrogen absorption rate based on the simulation of Jander diffusion Equation [79]. Also, [PERSON] et al. [80] found the thermal conductivity decreased with the increasing of critical thickness in silicones with different morphologies (film, nanocrystalline, nanowire) at 300 K. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Heat Capacity** & **Thermal Conductivity** & **Hydrogen** \\  & **(kJ/kg/K)** & **(W/m/K)** & **Absorption Kinetics** \\ \hline
325 mesh Mg & 1.046 & 10.42 & poor \\ Mg single crystal (0001) & 1.082 & 168.0 & poor \\ Mg nanoparticles & 0.954 & 4.985 & good \\ Mg\({}_{50}\)Co\({}_{50}\) bcc alloy & 0.594 & 0.432 & superior \\ Pd capped Mg thin film & 1.020 & 82.0 & superior \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thermal and kinetic properties of several types of Mg-based materials. Reprinted with the permission from [78], Copyright Elsevier, 2014. Figure 8: SEM images of (**a**) 325 mesh Mg, (**b**) Mg nanoparticle sample, (**c**) milled Mg\({}_{50}\)Co\({}_{50}\) BCC alloy, (**d**) cross-section observation of Mg thin film without Pd layer, (**e**) top view of Mg thin film without Pd layer and (**f**) top view of Pd capped Mg thin film. Reproduced with the permission from [78], Copyright Elsevier, 2014. However, it seems that [PERSON] et al. recently have found a solution [81].