Metal hydrides are compounds composed of alkali metal elements, alkali earth metal elements except beryllium, part of d zone elements, part of f zone elements and hydrogen elements, which have been widely used in the field of hydrogen storage. Metal hydrides hydrogen storage has the advantages of safety and reliability, low energy consumption, high storage capacity, relatively mature preparation technology and process. In addition, metal hydride hydrogen storage has the function of hydrogen purification and compression. Therefore, metal hydride is the most widely used hydrogen storage material at present. Moreover, metal hydrides are closely related to hydrogen storage alloys. During hydrogen storage, hydrogen storage alloys absorb a large amount of hydrogen to obtain metal hydrides under certain temperature and pressure conditions. Heat is given off as metal hydrides are formed. Then, when the resulting metal hydrides are heated, they break down again, releasing the hydrogen trapped inside.
- Rare earth metal hydrides: Rare earth metal hydrides are obtained by rare earth hydrogen storage alloys absorbing hydrogen. The typical representative of rare earth hydrogen storage alloy is LaNi5.The alloy is CaCu5 hexagonal structure, which has the advantages of easy activation, moderate and flat platform pressure, small equilibrium pressure difference between hydrogen absorption and hydrogen desorption, excellent kinetic performance and not easy to be poisoned. At 25 degree centigrade and 0.2MPa, the hydrogen storage capacity of this alloy is about 1.4% and the heat of decomposition is 30kJ/mol, so hydrogen storage can be achieved at room temperature. In addition, the alloy also has the characteristics of high purity of hydrogen absorption and desorption, so it can be used as a way to prepare high purity hydrogen.
Figure 1. An example of rare earth metal hydride used in hydrogen storage.
- Magnesium metal hydride: Magnesium metal hydrides are obtained by magnesium hydrogen storage alloys absorbing hydrogen. The typical magnesium hydrogen storage alloy is Mg2Ni. Magnesium alloys are characterized by low cost, light weight and high hydrogen storage capacity. Therefore, magnesium alloy is considered as the most promising alloy material. In recent years, in order to improve the hydrogen storage capacity of magnesium hydrogen storage alloys, they are prepared into thin films. The experimental results show that the thin film of magnesium hydrogen storage alloy has the advantages of fast hydrogen absorption and desorption speed, strong anti-powder ability, high thermal conductivity and relatively easy surface treatment.
- Titanium metal hydride: Titanium metal hydrides are obtained by titanium hydrogen storage alloys absorbing hydrogen. The typical representative of titanium hydrogen storage alloy is TiFe. Titanium hydrogen storage alloy has the advantages of good hydrogen storage performance, low temperature and moderate cost. In order to further improve the performance of titanium hydrogen storage alloys, many scholars use Ni and other metals to partially replace Fe, so as to form ternary alloys to realize normal temperature activation, so that it has higher practical value.
Figure 2. An example of titanium metal hydrides used in hydrogen storage.
- Others: In addition to the above categories, other metal hydrides that can be used for hydrogen storage include vanadium metal hydrides, zirconium metal hydrides and others. For example, a typical zirconium hydrogen storage alloy is ZrMn2, which has the advantages of large amount of hydrogen absorption and desorption, long cycle life, easy activation and small thermal effect.
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- Park, Choi, Na, et al. Oxygen reduction behavior of HDH TiH2 powder during dehydrogenation reaction[J]. Metals - Open Access Metallurgy Journal, 2019, 9(11):1154.