# 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
#### 3.1.2 Doped Mg/MgH\({}_{2}\) System  
To accelerate the chemical reaction, the introduction of catalysts is always a good choice besides increasing reaction temperature and considering the size effect of reaction particles. Ni was used for catalyzing the Mg/MgH\({}_{2}\) system by means of doping in hydrogen storage area. [PERSON] et al. [51] developed a wet chemical method to fabricate a good dispersion of Ni-doped Mg particles. The Ni particles were distributed on the surface of Mg particles (2 wt % Ni) and served as a catalyst for hydrogen absorption and desorption [22; 51]. The Ni and Mg were also ball-milled to get the Ni-doped Mg particles [52]. According to the result, the powder-method need 5 wt % Ni compared to 2 wt % Ni with the chemical method for the realization of the same catalytic efficiency. After around 200 cycles of hydrogen absorption and desorption, the Mg materials remained active and maintained stable. Moreover, the sample still showed a hydrogen capacity of about 6 wt % after 1000 cycles with respect to the long-term test. [PERSON] et al. [53] reported a facile and simple method to synthesize porous Ni nanofibers by a single-nozzle electrospinning technique. Results showed that the as-synthesized Ni nanofibers showed enhanced catalytic effects on the dehydrogenation performance of MgH\({}_{2}\), with the rapid release of 7.02 wt % hydrogen in 11 min at 325 \({}^{\circ}\)C. They also obtained an activation energy of 81.5 kJ/mol for the as-milled MgH\({}_{2}\)-4% Ni nanofibers composite, demonstrating the enhanced kinetics. In addition, they found that the dehydrogenation temperature of MgH\({}_{2}\) significantly decreased when amounts of Ni catalyst increased. i.e., from 340 \({}^{\circ}\)C for pure MgH\({}_{2}\), to 170 \({}^{\circ}\)C for the MgH\({}_{2}\)-2% Ni nanofibers composite, to the lowest value of 143 \({}^{\circ}\)C for the MgH\({}_{2}\)-4% Ni nanofibers sample. The Carnot principle indicates that the higher the heat temperature level, the higher the heat engine's efficiency. In specific, the steam with a higher temperature is more efficient than that with a lower temperature. However, under the high temperature test, both the two systems (chemical method and powder method) had hydrogen capacity loss as the temperature was higher than 440 \({}^{\circ}\)C, which stemmed from the sintering of Mg particles at a temperature higher than 450 \({}^{\circ}\)C [19]. The Max Planck Institute designed and built a prototype of a steam generator. 14.5 kg Ni-doped Mg powder was filled into the isolated cylindrical steel pressure vessel and it could be able to generate 6 kg water vapor per hour at the maximum output power of 4 kW. With a total weight of 40 kg, it could reach the maximum operational steam pressure to 50 bar and temperature to 450 \({}^{\circ}\)C [19]. Besides the Ni catalyst, Nb\({}_{2}\)O\({}_{5}\) was also used for catalyzing the hydrogenation/dehydrogen reaction and improving reaction kinetics [54]. Recently, [PERSON] et al. [55] reported that the mesoporous amorphous Nb\({}_{2}\)O\({}_{5}\) (mNb\({}_{2}\)O\({}_{5}\)) showed remarkable catalytic property on the Mg/MgH\({}_{2}\) system. They ball-milled the mixture of commercial mNb\({}_{2}\)O\({}_{5}\) and MgH\({}_{2}\) to prepare the mNb\({}_{2}\)O\({}_{5}\)-doped MgH\({}_{2}\) composite. Compared with crystalline Nb\({}_{2}\)O\({}_{5}\) (cNb\({}_{2}\)O\({}_{5}\)), the mNb\({}_{2}\)O\({}_{5}\) presented a smaller particle size and a multiple wavy structure, which provided a large specific surface area for the interaction with MgH\({}_{2}\). While the spherical and large particle size of cNb\({}_{2}\)O\({}_{5}\) led to less surface area for the interaction with MgH\({}_{2}\). As a result, the mNb\({}_{2}\)O\({}_{5}\) could remarkably reduce the activation energy of hydrogen dissociation process and be able to increase the temperature up to 175 \({}^{\circ}\)C within a minute during the hydrogenation process. This intensified heat release coupled with a concentrated solar panel had a promising application for a TES system.