# 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
#### 3.1.3 The Mg-Fe/Mg\({}_{2}\)FeH\({}_{6}\) System  
Mg\({}_{2}\)FeH\({}_{6}\) is one of the suitable materials for high temperature TES system. [PERSON] et al. [58] firstly discovered Mg\({}_{2}\)FeH\({}_{6}\) in 1984 by heating the mixture of Mg and Fe powders with an atomic ratio of 2:1 under H\({}_{2}\) atmosphere. Later, Mg\({}_{2}\)FeH\({}_{6}\) could be prepared by reactive mechanical alloying of Mg and Fe metal powders under the hydrogen atmosphere at room temperature [59; 60; 61]. Compared with MgH\({}_{2}\), as a heat storage material, Mg\({}_{2}\)FeH\({}_{6}\) exhibited much higher stability at high temperatures and lower hydrogen dissociation pressure. With the cycle number increasing, the MgH\({}_{2}\) system was going to have a hydrogen capacity loss while the Mg\({}_{2}\)FeH\({}_{6}\) system showed a steady trend of good maintenance. Moreover, Mg\({}_{2}\)FeH\({}_{6}\) system even retained about 5 wt % hydrogen capacity after 600 cycles and presented the heat storage density of \(\sim\)1921 kJ/kg with a reaction enthalpy about 77 kJ/mol. Since the good cycle stability of Mg\({}_{2}\)FeH\({}_{6}\), they added Mg\({}_{2}\)FeH\({}_{6}\) into the Mg/MgH\({}_{2}\) system to inhibit capacity degradation by co-cyclization. The authors even investigated different ratios of Mg: Fe in the mixed Mg\({}_{2}\)FeH\({}_{6}\)-MgH\({}_{2}\) systems and drew a conclusion that the ratios should lie in less than 20:1 for a good hydrogen capacity cycle stability [62]. They indicated that the possible reason for the good cycle stability of Mg\({}_{2}\)FeH\({}_{6}\) was the separation of metals in two immiscible phases on hydrogen desorption and merging to a compound upon hydrogen absorption [19]. It required a desorption  
Figure 4: Illustrations of different proposed mechanisms for the evolution of uncoated Mg and coated Mg samples, (**a**) prior to cycling, (**b**) the onset of hydrogenation, and (**c**) during dehydrogenation. Reproduced with permission from [57], Copyright Elsevier, 2017.  
temperature higher than 450 \({}^{\circ}\)C due to the outstanding stability at high temperature conditions and even when the temperature was up to 590 \({}^{\circ}\)C, the hydrogen desorption cycle stability was not affected.  
A test rig was designed and assembled recently in Max Planck Institute [63] (Figure 5). The most significant part of this heat storage test rig was the heat storage reactor, which was mainly made up of 13-tube bundle reactor (Figure 6a). Figure 6b shows the tube bundle reactor without the shell. Molten salt was used as heat transfer during heat storage and heat collection. Moreover, it surrounded tubes and flowed as Figure 6c present due to the design of eight baffles. Sealing strips were used to prevent the stream from by-pass at the inner wall of the reactor shell. In this TES system demonstration, 5 kg Mg\({}_{2}\)FeH\({}_{6}\) was filled in the heat storage tank and molten salt served as heat transport medium due to the high working temperature (higher than 400 \({}^{\circ}\)C). This test rig demonstration could release 1.6 kWh and store 1.5 kWh heat in the first test. The system did not maintain a stable operating condition during repeated experiments and further improvements of this system were under investigation [63].