Human production and life cannot do without energy, it is the material basis of all human activities. Moreover, hydrogen energy, as the most ideal green energy, has become a hot spot of research and development all over the world. Hydrogen energy storage is the most critical problem to be solved in practical applications. In addition, many types of materials, including alloy materials, metal organic frame materials, covalent organic frame materials and others, have been developed. When it comes to improve the performance of hydrogen storage materials, catalysts and modifiers are indispensable.
- Catalysts: Adding catalysts can effectively improve the performance of hydrogen storage materials. Transition metals, metal oxides, metal halides are the mainstream of the existing catalysts. When catalyst is added into the hydrogen storage material, the kinetics of hydrogen absorption and discharge can be improved and the temperature of hydrogen absorption and discharge can be reduced. The following three aspects will affect the catalytic performance of the catalyst, which requires special attention in the design of the catalyst. First, the chemical nature will affect the catalytic effect of the catalyst. In general, the catalytic activity of polyvalent catalysts is better than that of single valence catalysts, while the catalytic activity of transition metal halides is generally higher than that of metal oxides and single metals. Second, the amount and process of catalyst addition will affect the catalytic effect of catalyst. For example, in the mechanical ball milling process, the ball milling time has an important effect on the catalyst effect. Third, the size and specific surface area of catalyst will affect the catalytic effect. Generally, the catalytic effect of nano-catalyst particles is better than that of micron particles. The higher the specific surface area of catalyst particles, the more obvious the catalytic effect.
Figure 1. An example of catalyst used in hydrogen storage.
- Modifiers: In addition to using catalysts to improve the hydrogen storage performance of the materials, it can also be improved by adding modifiers into the material. Common modifiers include intermetallic compounds, carbon materials and others. Common intermetallic compounds used to improve the hydrogen storage performance of materials include LaNi5, FeTi and others. In addition, carbon material has large specific surface area, low cost, and can store hydrogen at low temperature, so it is an ideal modifier of hydrogen storage materials. For example, for magnesium based hydrogen storage materials, adding an appropriate amount of carbon is beneficial to improve the density of hydrogen storage materials. There are two reasons for this. Firstly, the long range disordered and short range ordered structure of microcrystalline carbon determines its good grindability, brittleness, dispersity and lubricity, which is easy to be pulverized by ball grinding and can quickly realize the nanocrystallization. Second, the mutual peritectic between magnesium and carbon atoms can provide a carrier for the catalyst, so that the catalyst can be evenly distributed in the hydrogen storage material.
Figure 2. Modified hydrogen storage material with carbon nanotubes.
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