# Mg-Based Hydrogen Absorbing Materials for Thermal Energy Storage--A Review
## 3 Different Types of Mg-Based Materials for Thermal Energy Storage
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compound** & **Reaction** & **Material Energy** & **Reaction** \\  & & **Density** & **Temperature (\({}^{\circ}\)C)** \\ \hline Ammonia [30] & NH\({}_{3}\) + AH \(\leftrightarrow\) 1/2m\({}_{2}\) + 3/2H\({}_{2}\) & 67 kJ/mol & 400–500 \\ Methane/water [29] & CH\({}_{4}\) + H\({}_{2}\)O + \(\circ\)C + Mg\({}_{2}\) & n.a. & 500–1000 \\ Hydrorides [29] & Ca(OH)\({}_{2}\) + \(\circ\)C + H\({}_{2}\)O & 3 GJ/m\({}^{3}\) & 500 \\ Calcium carbonate [27] & CaCO\({}_{3}\) \(\leftrightarrow\) CaO + CO\({}_{2}\) & 4.4 GJ/m\({}^{3}\) & 800–900 \\ Iron carbonate [28] & FeCO\({}_{5}\) \(\leftrightarrow\) FeO + CO\({}_{2}\) & 2.6 GJ/m\({}^{3}\) & 180 \\ Metal hydrides [29] & Metal Ar\({}_{2}\) \(\leftrightarrow\) metal \(\mathrm{H}_{2}\) + (\(x-y\)) H\({}_{2}\) & 4 GJ/m\({}^{3}\) & 200–500 \\ Metal oxides (Zn and Fe) [31] & e.g., 2-step water splitting using & n.a. & 200–2500 \\  & Fe\({}_{5}\)O\({}_{4}\)/FeO redox system & n.a. & 210–2300 \\  & Aluminium or alumina [1] & n.a. & n.a. & 200–250 \\  & Methanolation–demethanationation [32] & CH\({}_{3}\)OH + CO + 2H\({}_{2}\) & n.a. & 200–250 \\  & Magnesium oxide [1] & MgO + H\({}_{2}\)O + Mg (OH)\({}_{2}\) & 3.3 GJ/m\({}^{3}\) & 250–400 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Typical chemical storage materials and reactions. Reprinted with permission [1]. temperature metal hydride and stored in the TES system. During the process of extra heat storage, hydrogen is released into the low temperature metal hydride bed and low temperature heat is released. When the sunlight is not available, the low temperature metal hydride is heated and then hydrogen is liberated into high temperature metal hydride bed. The liberated hydrogen reacts with the materials in the high temperature metal hydride bed and provides high temperature heat to the power plant system. As a result, continuous and stable power output could be realized with the function of TES system by storing and releasing thermal energy. Similarly, an innovation based on metal hydride for efficient thermal energy was developed from Pacific Northwest National Laboratory (PNNL) [33]. In their project, the key findings were concluded as follows. The temperature for power generation was 650 \({}^{\circ}\)C, charging time was less than 6 h and volumetric energy density for the whole system was 200 kWh/m\({}^{3}\). Importantly, compared to the state-of-the-art molten salts, the energy density was increased by 10 times. Additionally, the operating pressure was as low as 1 bar H\({}_{2}\) pressure and performed well in the cycle life tests (60 cycles were accomplished). Among large quantities of metal hydrides, Mg-based materials are pretty suitable for the heat storage application. In the hydrogen storage area, Mg-based materials are promising candidates due to the large abundant reserve in the crust, the light weight of Mg element, and high hydrogen storage capacity (7.6 wt % for Mg) [34; 35; 36]. However, the relatively higher hydrogen desorption enthalpy (74.6 kJ/mol, from Sandia National Lab database) becomes a restriction for the future application of Mg-based materials for hydrogen storage [37; 35; 38]. The story is totally different when Mg-based materials are applied to TES. The high hydrogen desorption enthalpy and high hydrogen storage capacity result in a large amount of heat release when the chemical reaction happens at a temperature range from 300 \({}^{\circ}\)C to 500 \({}^{\circ}\)C under a certain hydrogen pressure (1 to 100 bar) [19; 22]. [PERSON] et al. [24] first proposed that Mg/MgH\({}_{2}\) was a potential system for heat storage application due to its favorable thermodynamic properties and many Mg-based materials were employed as TES materials [39; 40; 41; 42; 43; 44; 22]. Moreover, the theoretical simulation methods were also adopted to effectively investigate the thermal storage properties of Mg-based materials [42; 45; 46].