# 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
#### 3.1.5 The NaMgH\({}_{3}\) System  
NaMgH\({}_{3}\) is a perovskite-type hydride and which could decompose via a two-step process [69]  
\[\text{NaMgH}_{3}\rightarrow\text{NaH}+\text{Mg}+\text{H}_{2}\text{ (4.0 wt \% H}_{2}) \tag{5}\]  
\[\text{NaH}+\text{Mg}\rightarrow\text{Na(I)}+\text{Mg}+\frac{1}{2}\text{H}_{2} \text{ (2.0 wt \% H}_{2}) \tag{6}\]  
Theoretically, there is 6.0 wt % H\({}_{2}\) release in total in the two reactions. As a result, NaMgH\({}_{3}\) has been considered as a potential hydrogen storage material in recent years. On the other hand, it demonstrated a high thermal stability, a lower hydrogen dissociation pressure, and comparatively low price, which urged this material to be a promising candidate for TES. NaMgH\({}_{3}\) was prepared by milling NaH and MgH\({}_{2}\) in a 1:1 molar ratio performed cryogenically (77 K) with subsequent annealing under 50 bar of H\({}_{2}\) atmosphere at 300 \({}^{\circ}\)C [70]. Researchers are always willing to investigate the thermodynamic properties of materials, especially in the area of hydrogen storage and thermal energy storage. There are several publications [71; 72; 73; 74; 70] which studied the enthalpy of NaMgH\({}_{3}\) for the first decomposition step with the method of pressure-composition-isotherms (PCI). Among them, [PERSON] et al. [71] gave an enthalpy value of 93.9 kJ/mol firstly, [PERSON] et al. [72] reported a close value of 94 kJ/mol and [PERSON] et al. [74] also gave a similar enthalpy value of 92 kJ/mol. Differently, [PERSON] et al. [70] reported a value of 86.6 \(\pm\) 1.0 kJ/mol in their work. In [PERSON] et al. 's work [70], hydrogen desorption PCI curves for NaMgH\({}_{3}\rightarrow\) NaH + Mg + H\({}_{2}\) (Figure 7a) at different temperatures showed 3.29 wt % hydrogen storage capacity experimentally at 398.3 \({}^{\circ}\)C. In the subsequent high-temperature measurements, the hydrogen capacity slightly decreased due to the possibly incomplete rehydrogenation. Point A in Figure 7a,b was the equilibrium state of full desorption and the kinetics was slowed down when approached this equilibrium state such that more than 3 h was needed to reach that at the temperature of 418.8 \({}^{\circ}\)C. From the van't Hoff plots (Figure 7c), they calculated the desorption enthalpy for the first step's decomposition and showed a slightly lower value of 86.6 \(\pm\) 1.0 kJ/mol H\({}_{2}\) at 418.6 \({}^{\circ}\)C. They also measured the absorption kinetics after the sample was fully decomposed to Na (I) and Mg. They found that the absorption kinetics were remarkably slowed down after 75% rehydrogenation. However, the absorption kinetics did not start to decrease to 90% rehydrogenation during measuring the hydrogen absorption performance of NaH and Mg. It was probably some problems related to agglomeration of liquid Na metal which led to the slow kinetics in overall decomposition case [74]. A potential solution was utilization of the first step of decomposition to minimize the kinetic and capacity degradation. What is more, the hydrogen dissociation pressure of NaMgH\({}_{3}\) was favorable for the thermal energy storage (11 bar at 500 \({}^{\circ}\)C) compared to the higher hydrogen dissociation pressure at a high temperature in Mg/MgH\({}_{2}\) (92 bar at 500 \({}^{\circ}\)C) and Mg\({}_{2}\)FeH\({}_{6}\) (66 bar at 500 \({}^{\circ}\)C) systems. Overall, the high enthalpy, low dissociation pressure at high temperature and the wide, flat plateau make NaMgH\({}_{3}\) as suitable TES material. Besides what has been mentioned above, there are a number of other Mg-based materials aiming at the application for TES. Mg\({}_{2}\)NiH\({}_{4}\) has a reaction enthalpy change of \(\sim\)63 kJ/mol [3; 75] but it performs unstable compared to MgH\({}_{2}\). Results present that the Mg\({}_{2}\)NiH\({}_{4}\) uptakes the hydrogen capacity of \(\sim\)3 wt % and shows an lower energy density of 916 kJ/kg than the theoretical value of 1117 kJ/kg because of low formation enthalpy and the heavy weight of materials [41]. [PERSON] et al.