Hydrogen energy is a kind of renewable energy with high efficiency, wide sources and pollution-free. The hydrogen fuel cells are one of the alternatives for powering vehicles. However, high mass density hydrogen storage is a technical bottleneck restricting the development of hydrogen fuel cell vehicles at present. Hydrogen can be normally stored in gaseous, liquid and solid states. The gaseous hydrogen storage has high mass hydrogen storage density but its volumetric hydrogen storage density is relatively low. Moreover, it is difficult to continuously increase the pressure of storage cylinder when it has reached to 70 MPa. In addition, the high storage pressure increases the energy consumption when the hydrogen compression, and causes certain potential safety risk. The cryogenic liquid hydrogen storage has a relatively high hydrogen storage density but the liquefication process may consume nearly 30% of the stored energy. Also, the liquid hydrogen storage requires special insulated containers, which are generally costly and not suitable for vehicle on-board applications. Compared with gaseous and liquid hydrogen storages, some of the solid materials have advantages in mass density, cost and safety. The volumetric density of solid hydrogen storage is higher than gaseous type, but its mass density is greatly affected by the mass of material. The solid hydrogen storage with high mass density is expected to meet the hydrogen storage targets set by the United States Department of Energy and International Energy Agency, and has potential for on-board applications. The on-board application of solid hydrogen storage is one of the research directions at present. Principle of solid hydrogen storage can be divided into physical adsorption and chemical adsorption according to the form of hydrogen storage in the material. Physical adsorption refers to the adsorption of molecules through van der Waals force, but the weak force between adsorbent and adsorptive leads to a low hydrogen storage density. Chemical adsorption refers to hydrogen atoms occupy the lattice interstitial of metal to form metal hydrides, or form chemical bonds with metal elements to form coordinated metal hydrides. These materials with high theoretical hydrogen storage density and good safety are suitable for on-board hydrogen storage system. At present, the research of hydrogen storage materials focuses on improving the dynamic and thermodynamic properties on the basis of maintaining high hydrogen storage density and some progresses had been made. Magnesium hydride, borohydride, aluminum hydride and amino compounds are common hydrogen storage materials. MgH2 has simple decomposition process, high dehydrogenation efficiency and good reversibility, but it is difficult to be reversibly de/hydrogenation under moderate conditions. LiBH4 is a coordination hydride with a theoretical mass hydrogen storage density of up to 18.5%. However, LiBH4 dehydrogenation is completed in two steps and the dehydrogenation temperature is above 500 ℃. NaAlH4, LiAlH4 and LiNH2 are also coordination hydride with high theoretical mass hydrogen storage density. But they suffer from the shortcomings of high decomposition temperature, slow dehydrogenation kinetics and poor reversibility under moderate conditions. The by-product NH3 from the decomposition of LiNH2 may cause some damage to the fuel cell. The existing solid hydrogen storage materials and their improvements are still undeveloped. The mass hydrogen storage density inevitably declines during the cycling process, and the reversible dehydrogenation and hydrogenation at moderate temperature and pressure are difficult to be used on vehicles. By reviewing the literature, several improved methods were summarized: nanostructures to increase the surface area of materials, doping catalysts to reduce the activation energy of reactions, doping highly electronegative elements to weaken the strong interatomic interaction and adding other reactants to change the reaction pathway. The improved methods could be used to increase the reaction rate of solid hydrogen storage materials, reduce the stability of the system and enhance the performance of de/hydrogenation. Ball milling could reduce the diameter of material particles and form nanostructures on the surface. Nanostructures could increase the contact area between the catalyst and the material, shorten the hydrogen diffusion distance and improve the catalytic efficiency. Therefore, the addition of catalyst and the pre-treatment of materials were often through ball milling. Nano-confinement could encapsulate hydrogen storage materials in inert nanostructured porous materials to inhibit particle agglomeration, improve kinetic and thermodynamic properties, prevent the entry of air and water vapor, and ensure safety of system, such as LiBH4-activated carbon composite system and LiBH4 confined in double-layered carbon nanobowl-confined. The doping of catalysts and other additives could be used to improve the properties of hydrogen storage materials. For example, transition metals and their compound (Nb, Ti, Fe, Zr, Co and ZrCo, TiCl3, ZrCl4, TiO2) could reduce the activation energy and increase the reaction rate. Forming a composite hydrogen storage system by adding other reactants could change the reaction pathway reducing the enthalpy change and the stability of the system. MgH2-AlH3, LiBH4-MgH2, LiNH2-LiH were common composite hydrogen storage systems. Doping highly electronegative materials could also reduce the stability of the system to improve the hydrogen storage performance. For example, Mg, Al and Ti were added to the LiBH4 system to replace the central Li. Doping of elements metal with high electronegativity could weaken the bonding between atoms, reduce the dehydrogenation temperature and improve the thermodynamic properties of the materials. To conclude, by comparing and analyzing of hydrogen storage performance of the above high-density solid hydrogen storage materials and their improvement measures, the development of using lightweight porous support materials and composite materials with efficient lightweight catalysts was an effective way to improve the performance of solid hydrogen storage. © 2022, Youke Publishing Co., Ltd. All right reserved.
