Hydrogen storage alloys are intermetallic compounds that can absorb, store and release hydrogen in large quantities reversibly. Hydrogen storage alloys have the characteristics of large hydrogen storage, no pollution, safety and reliability. During hydrogen absorption and discharge of hydrogen storage alloys, thermal effect occurs. Heat is given off by hydrogen absorption and absorbed by dehydrogenation. Hydrogen storage alloys absorb hydrogen when temperature is lowered or pressure is increased. Conversely, when temperature is increased or pressure is lowered, hydrogen is released.
- Rare earth series hydrogen storage alloys: The rare earth series hydrogen storage alloys can be expressed by general formula AB5 and has CaCu5 hexagonal structure. LaNi5 is a typical example, which was discovered by Philips Laboratory in 1969. The hydrogen storage capacity of LaNi5 alloy can reach to 1.37wt% and the dehydrogenation does not need high temperature and high pressure. LaNi5 cannot meet the large demand of industrial production because of the high price of pure rare earth metals. In order to reduce the cost, people use mixed rare earth metal elements to replace part of La in LaNi5, and replace Ni with Co, Al, Mn, Fe, Cr, Cu, Si, Sn to improve the performance. Therefore, multi-mixed rare earth hydrogen storage alloys are developed.
Figure 1. The crystal structure of LaNi5 alloy.
- Titanium series hydrogen storage alloys: At present, a variety of titanium hydrogen storage alloys have been developed, such as ferrotitanium, titanium-manganese, titanium-chromium, titanium-zirconium, titanium-nickel, titanium-copper and others. Among them, ferrotitanium is AB type, and the others are AB2 type alloys. Among titanium hydrogen storage alloys, ferrotitanium and manganese hydrogen storage alloys are the most practical. FeTi alloy is a typical AB type hydrogen storage alloy with CsCl structure. As a kind of hydrogen storage material, the hydrogen storage capacity of FeTi alloy is even slightly higher than LaNi5. TiMnx is also a promising type of hydrogen storage alloys. This type alloy has a hydrogen absorption capacity of 1.89wt %, and is easily activated at room temperature.
- Magnesium series hydrogen storage alloys: MgNi2 is a typical magnesium material, which has great potential for light and high energy hydrogen storage materials. Both in terms of material price and theoretical hydrogen storage capacity, it is superior to rare earth alloy and titanium alloy, and its theoretical capacity is up to 1000mAh/ g, which is about 2.7 times that of LaNi5 alloy. However, MgNi2 alloy can only absorb and release hydrogen at low temperature. Furthermore, the reaction speed is very slow, and it is difficult to activate, which makes its practical application difficult.
Figure 2. An example of magnesium series hydrogen storage alloys.
- Zirconium series hydrogen storage alloys: Zirconium series alloys are represented by ZrV2, ZrCr2, ZrMn2 and others, which can be expressed by the general formula AB2. Zirconium series alloys have the advantages of large hydrogen absorption, fast reaction with hydrogen, easy activation and no lag effect, so they are promising new hydrogen storage materials.
- Liu Y, Pan H, Gao M, et al. Advanced hydrogen storage alloys for Ni/MH rechargeable batteries [J]. Journal of Materials Chemistry, 2011, 21.
- Edalati K, Uehiro R, Ikeda Y, et al. Design and synthesis of a magnesium alloy for room temperature hydrogen storage[J]. Acta Materialia, 2018:88-96.