# 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  
#### 3.1.1 Mg/MgH\({}_{2}\) System  
Theoretically, the pure Mg/MgH\({}_{2}\) system could possess a thermal energy density as high as 2814 kJ/kg [41]. Compared with one of the largest solar thermal power plants in Granada, Spain, which contains about 28,000 tons of 60% NaNO\({}_{3}\) + 40% KNO\({}_{3}\) to store concentrated thermal energy, while only 1100 tons of Mg metal will be enough to the same size of heat storage [19]. The particle size of Mg should be controlled in the range of 25-100 \(\upmu\)m in order to complete a full conversion to MgH\({}_{2}\) and get a faster reaction kinetics at a minimum temperature of 350 \({}^{\circ}\)C under normal pressure [41]. With the temperature increases, the Mg metal particles are going to agglomerate and fuse with each other after a certain cycle time, resulting in bad reaction kinetics and an incomplete reaction between Mg particles and hydrogen. It is noted that the high vapor pressure (0.133 bar at 500 \({}^{\circ}\)C) of Mg particles and comparatively low melting point (Tm = 650 \({}^{\circ}\)C) lead to the poor cycle stability [41]. Indeed, it is of fundamental significance to know the function of H\({}_{2}\) dissociation equilibrium pressure against operation temperature for the better controlling of the Mg/MgH\({}_{2}\) heat storage system. Figure 2 indicates the clear relationship between hydrogen dissociation pressure and temperature. At a given hydrogen pressure, we could get the lowest temperature for H\({}_{2}\) liberation and the highest temperature for the heat release. In other words, based on Figure 2, the direction of the chemical reaction could be adjusted by altering the H\({}_{2}\) pressure. On the other hand, it is also feasible to adjust the temperature range to regulate the progress of the irreversible chemical reaction under a certain H\({}_{2}\) pressure.  
The poor cycling stability of Mg/MgH\({}_{2}\) above 400 \({}^{\circ}\)C could not meet the requirements in practical application due to the sintering of Mg particles and the high hydrogen equilibrium pressure of MgH\({}_{2}\) at high temperature. According to this, many studies were developed and focused on increasing the operation temperature of the Mg/MgH\({}_{2}\) system [47; 48; 49]. [PERSON] et al. synthesized a series of Mg (H\({}_{x}\)F\({}_{1-x}\))\({}_{2}\) (x = 1, 0.95, 0.85, 0.70, 0.50, 0) compounds by ball milling and annealing under hydrogen atmosphere [50]. They adopted the partial fluoride substitution for hydrogen to tune the thermodynamic properties. As the F content increased, the thermal stability increased while the hydrogen absorption/desorption kinetics decreased. The thermal analysis results presented the maximum rate of H\({}_{2}\) desorption at 434 \({}^{\circ}\)C with Mg (H\({}_{0.85}\)F\({}_{0.15}\))\({}_{2}\). Moreover, the pressure-composition-isothermal (PCI) results of Mg (H\({}_{0.85}\)F\({}_{0.15}\))\({}_{2}\) showed the enthalpy (\(\Delta\)H\({}_{\text{des}}\))  
Figure 2: Dissociation pressure curve of MgH\({}_{2}\). The inset is the concept of heat storage based on Mg/MgH\({}_{2}\) material for excess solar energy storage.  
of 73.6 \(\pm\) 0.2 kJ/mol H\({}_{2}\) and entropy (\(\Delta\)S\({}_{\text{des}}\)) of 131.2 \(\pm\) 0.2 J/k/mol H\({}_{2}\) with a hydrogen capacity of 4.6 \(\pm\) 0.2 wt % H\({}_{2}\), which were slightly lower than those of pure MgH\({}_{2}\). In addition, the cycle performance investigated at the temperature range from 420 to 480 \({}^{\circ}\)C over six cycles showed the increasing thermal operating temperature for TES.