Mechanochemical methods for the synthesis of new magnesium-based composite materials for hydrogen accumulation

Abstract

We present a brief overview of works on the synthesis of magnesium hydrides and alloys by traditional methods and analyze mechanical methods for synthesis of these materials. Advantages of reactive milling for the preparation of new efficient hydrogen sorbents based on magnesium are discussed. It is shown that the reaction rate of the mechanochemical synthesis of MgH 2 increases four times as a result of the introduction of additives of intermetallic compounds based on Ti and Zr. In view of the depletability of fossil fuel and greenhouse effects, which affect negatively the ecological situation of the planet, the development of new approaches to the solution of a global power problem becomes particularly urgent. Hydrogen power is one of the most promising alternative trends. The calorific value of hydrogen is 142 MJ / kg, and in terms of this index, it is three times more efficient than that of gasoline. Since the sole combustion product of hydrogen is water, it is an absolutely pollution-free fuel. However, on our planet, hydrogen practically does not occur naturally in the pure form. The main source of the starting material for hydrogen is water, the resources of which are practically unlimited (though hydrogen is also obtained as a by-product in a number of industrial chemical, metallurgical, and other processes) on the Earth. The extracted gaseous hydrogen must be stored, transported, and burned, efficiently transforming and using the released energy. Compact hydrogen storage is one of the most important problems. In this case, the method of chemical binding of hydrogen in metal hydrides is efficient and safe (in contrast to using compressed hydrogen). In these compounds, large volumes of hydrogen can be accumulated and released back under insignificant changes in temperature and pressure. A substantial disadvantage of metal hydride materials base on alloys of titanium, zirconium, and rare-earth metals (REMs) is an insignificant hydrogen capacity by weight, which is no larger than 2 wt. %, whereas, according to assessments of experts, for hydrogen storage systems in transport, a hydrogen capacity of at least 5 wt. % is required. This is why, in the world, the interest in hydrides of light metals, namely lithium, aluminum, magnesium, and other compositions based on them, has increased. Magnesium hydride, which has a reverse hydrogen capacity of 7.6 wt. % and is much cheaper than other metal hydride materials, is of particular interest. However, the practical use of magnesium hydride in hydrogen storage systems is impossible because of a number of substantial disadvantages. These are high temperatures/pressures (> 300°C and 5 – 10 MPa, respectively) and low sorption–desorption rates, complexity of activation, and degradation of properties in cycling. Investigations performed in the last decade showed that mechanical treatment of magnesium and its hydride in high-energy ball mills is a promising method for the activation and acceleration of hydrogenation–dehydrogenation processes. The use of hydrogen as a process atmosphere for the mechanochemical synthesis of hydride ma