Common hydrogen storage materials for hydrogen storage

Currently, hydrogen storage materials include metal hydrides, carbon fiber carbon nanotubes, non-carbon nanotubes, glass hydrogen storage microspheres, complex hydrogen storage materials, and organic liquid hydrides. Here are only three aspects of alloys, organic liquids and nano-materials for hydrogen storage materials are introduced.

A, alloy hydrogen storage materials

Hydrogen storage alloy refers to a certain temperature and hydrogen pressure, can be reversible in a large number of absorption, storage and release of hydrogen intermetallic compounds, the principle is that the metal and hydrogen to form, such as ionic compounds, *** valence of metal hydrides, metal-phase hydrides - intermetallic compounds and other combinations, and under certain conditions can be released from the hydrogen. Alloy as a hydrogen storage material to meet certain requirements, first of all its hydride generation heat to be appropriate, if the heat of generation is too high, the generation of hydride is too stable, the release of hydrogen will require a higher temperature. If the heat of generation is too low, it is not easy to absorb hydrogen. Secondly, the equilibrium pressure for the formation of hydrides should be appropriate, preferably only a few atmospheres near room temperature, to facilitate the absorption and release of hydrogen, and to absorb and release fast, so as to meet the needs of practical applications. In addition, the alloys and their hydrides are less sensitive to impurities such as water, oxygen and carbon dioxide, so that the material properties do not deteriorate when hydrogen is repeatedly absorbed and released. Moreover, the hydrides of hydrogen storage materials should also meet the conditions of reliable performance, safety, harmlessness and chemical stability during storage and transportation. Now has been studied and meet the above requirements are magnesium, rare earth system, titanium and zirconium system.

In the above hydrogen storage materials, magnesium hydrogen storage alloy has a higher hydrogen storage capacity, and absorb and release hydrogen platform, abundant resources, low price, the application prospects are very attractive. Magnesium can be directly reacted with hydrogen, in 300-400 ℃ and higher pressure, the reaction generates Mg and H2 reaction generates MgH2: Mg + H2= MgH2?△H=-74.6kJ/mol. MgH2 theoretical hydrogen content of up to 7.6%, has a rutile structure, performance is more stable, in the 287 ℃ when the decomposition pressure is 101.3kPa. Due to the poor kinetics of pure magnesium hydrogen absorption and discharge reaction and the high temperature of hydrogen absorption and discharge, pure magnesium is seldom used to store hydrogen directly, for this reason, people began to study magnesium-based hydrogen storage alloy materials. So far, more than 300 kinds of important magnesium-based hydrogen storage alloy materials have been studied.

Two, liquid organic hydrogen storage materials

Organic liquid hydride hydrogen storage is with the help of unsaturated liquid organics and hydrogen of a pair of reversible reactions, that is, hydrogenation and dehydrogenation reaction to realize. Hydrogen storage in hydrogenation reaction, dehydrogenation reaction when the release of hydrogen, organic liquids as a hydrogen carrier to achieve the purpose of storage and transportation of hydrogen. Olefins, alkynes, aromatics and other unsaturated organic liquids can be used as hydrogen storage materials, but from the energy consumption of the hydrogen storage process, the amount of hydrogen storage, hydrogen storage agent, physical and other considerations, to aromatic hydrocarbons, especially monocyclic aromatic hydrocarbons as a hydrogen storage agent for the best, commonly used organic hydrogen carriers are benzene, toluene, methylcyclohexane, naphthalene and so on. The hydrogen storage technology using these organic liquid hydrides as the hydrogen storage agent is a new type of hydrogen storage technology developed in the 1980's. In 1980, Taube et al. analyzed and demonstrated the possibility of using methylcyclohexane as the hydrogen carrier to store hydrogen to provide fuel for automobiles. Subsequently, many scholars have carried out a lot of fruitful research and development on the technology of providing fuel for automobiles, and the storage and transportation of catalytic hydrogenation and dehydrogenation have been widely developed. Organic liquid hydride hydrogen storage as a new type of hydrogen storage material, its hydrogen storage characteristics are: organic liquid storage, transportation safety and convenience, can use the existing storage and transportation equipment, is conducive to long-distance mass transportation, large hydrogen storage capacity, benzene and toluene theoretical hydrogen storage capacity of 7.19 (wt) % and 6.18 (wt) %, respectively, than the existing metal hydrogen storage capacity is much higher, the cost of the storage agent is low and can be recycled many times. The cost of hydrogen storage is low and can be recycled many times. The hydrogenation reaction releases a lot of heat, which can be utilized, and the dehydrogenation reaction can utilize waste heat. At present, the main problem is that the dehydrogenation temperature of organic hydrogen carrier is high, and the actual hydrogen release efficiency is low. Therefore, the development of low-temperature and high-efficiency catalysts for dehydrogenation of organic hydrogen carriers and the adoption of membrane-catalyzed dehydrogenation technology are of great significance in improving the efficiency of the process.

Three, nano-materials for hydrogen storage

The nano-materials for hydrogen storage can be divided into two ways, one is to nano-materialize the original hydrogen storage materials, and the other is to develop new nano-materials as hydrogen storage materials.

Hydrogen storage alloys nanosized to improve the hydrogen storage properties are mainly manifested in the following reasons. (1) For nano-sized metal particles, the continuous energy band splits into discrete energy levels, and the average spacing between the energy levels increases, making it easy for hydrogen atoms to obtain the energy needed for dissociation, which is manifested in the reduction of the activation energy of hydrogen storage alloys and the activation temperature. (2) nanoparticles have a huge specific surface area, the transport of electrons will be scattered by the surface of the particles, the interface between the particles to form a high potential barrier to electron scattering, the accumulation of interfacial charge to produce interfacial polarization, and the greater the electronegativity difference between the elements, the more negative the enthalpy of generation of the alloy, and the more stable the alloy hydride. The metal hydride can be generated in large quantities, and the mass of hydrogen absorbed per unit volume is significantly larger than that of macroscopic particles. (3) Nano hydrogen storage alloy has large specific surface area, high surface energy, the effective adsorption area of hydrogen atoms increases significantly, the hydrogen diffusion resistance decreases, and the hydrogen decomposition reaction is catalyzed by the alloy nanocrystals, the nanocrystals have a high proportion of surface-active atoms, which is conducive to the adsorption of the reactant on its surface, effectively reducing the activation energy of the hydrogen adsorption on the surface of the electrode, and thus has a high electrocatalytic performance. In addition, because the nanocrystalline grains are quite fine, resulting in the increase of crystal boundaries and lattice defects, and the atoms at the crystal defects and dislocations have higher energy can be regarded as the active center of the reaction, which reduces the overpotential of hydrogen precipitation. (4) The refinement of the grain hardness increases, the overall strength of the hydrogen storage alloy with the increase in grain size and enhance the resistance to acid and alkali and cyclic charging and discharging pulverization, as well as resistance to charge and discharge formation of the oxygen pressure on the impact of the hydrogen storage substrate, and significantly improve the corrosion resistance of the hydrogen storage alloy.