In the face of more and more serious environmental pollution and energy crisis, the urgent task facing mankind is to develop non-polluting and renewable new energy. Therefore, as a kind of clean energy, the utilization of hydrogen energy is paid special attention. The discovery and application of magnesium hydrogen storage materials create possible conditions for the utilization of hydrogen energy. Magnesium can reversibly stores and releases 717wt% of its own weight of hydrogen. Such high hydrogen storage capacity, coupled with low cost, indicates that magnesium has great potential in hydrogen storage systems. In addition, magnesium-based hydrogen storage alloy is a kind of material that can store a large amount of hydrogen in the gap of crystal. Moreover, magnesium is the lightest metal of structural materials and is the sixth most abundant in the earth's crust. Hence, magnesium-based materials form a unique group of materials from the perspective of hydrogen storage.
- Mg2Ni series alloy: Among transition metals, Ni is considered to be the best alloying element. The reason is that hydrogen storage alloy is best composed of a strong hydride forming element and a weak hydride forming element according to Miedema's rule. The bonding force between Ni and hydrogen is weak, and the formation enthalpy of hydrogen is low. Mg2Ni forms Mg2NiH4 after hydrogen absorption, and the formation enthalpy is lower than MgH2. Moreover, Ni has catalytic activity for hydrogen molecules, and excessive Ni also has the ability to resist anodic oxidation in the process of hydrogen storage. In addition, the structure of Mg2Ni changes greatly after hydrogenation, and the hexagonal cell expands and recombines into the high temperature phase of fluorite structure, while the low temperature phase is formed by slight distortion of the high temperature phase.
Figure 1. An example of hydrogen storage by Mg2Ni alloy.
- Rare earth element-Mg-Ni allo: This kind of magnesium series materials are ternary alloys, most of which have LaMg2Ni9 crystal phase. Initially, this material was sintered by a research group through LaNi5 and MgNi2 powder. Later, a research team added appropriate alloying elements to LaNi3 alloy to obtain ternary or quaternary alloys of structure type AB2C9. Based on the AB2C9 alloy system, many other hydrogen storage alloy phase have developed, including La7.0Mg75.5Ni17.5, La2MgNi9, La5Mg2Ni23, LaNi4Mg, La3MgNi14 and others.
Figure 2. The P-C-T curves of the La7.0Mg75.5Ni17.5 alloy at different temperatures.
- Mg-rare earth element alloy: Both Mg and rare earth elements are strong hydride forming elements. The formation enthalpy of rare earth element hydrides is higher than that of magnesium hydrides, so unstable intermetallic compounds are usually formed between Mg and rare earth element, and it is difficult to form hydrides when reacting with hydrogen. However, the properties of Mg-rare earth element alloys can be significantly improved after the improvement of preparation methods. For example, a research team synthesized Mg-rare earth element alloy series alloy by induction melting method, and found that its highest hydrogen absorption is about 3.0%. The disproportionation reaction occurs after hydrogen absorption, and the reaction products are MgH2 and LaH3. LaH3 has a high decomposition temperature and remains stable in the hydrogen absorption and discharge cycle.
- Yang X, Hou Q, Yu L, et al. Improvement of hydrogen storage characteristcs of MgH2 with flake Ni nano-catalyst composite[J]. Dalton Transactions, 2021.
- Hu, Lin, Nan, et al. Phase Transformation and Hydrogen Storage Properties of an La7.0Mg75.5Ni17.5 Hydrogen Storage Alloy[J]. Crystals, 2017.