Theoretical Studies On Hydrogen Storage Properties Of Alkali Metal Atoms Decorated Boron Clusters

The development of the world economy and human society cannot do without the development and utilization of energy, while the traditional fossil fuels such as coal, oil and natural gas reserves are limited, and the combustion process will produce CO_x, NO_x, SO_x and other gases, causing serious environmental pollution. The development and utilization of green and clean energy is one of the main ways to solve the energy crisis and environmental problems. Because the hydrogen has rich reserves, high calorific value, and the only combustion product is water which has no pollution to the environment, it has been regarded as an ideal energy carrier. The establishment of hydrogen system mainly include large-scale preparation of hydrogen, efficient storage, safety transportation and reasonable utilization. Among of this, the storage of hydrogen is one of the main technical obstacles that restrict the large-scale application of hydrogen. The methods of hydrogen storage can be divided into physical and chemical storage. The main physical storage methods are compressed gaseous hydrogen, liquid hydrogen and the adsorption of hydrogen of porous materials. Chemical storage is mainly metal hydride hydrogen storage. The gravimetric hydrogen densitiy of physical storage is low, and there is a certain security risk. Most of the hydrogen in chemical storage is in the form of atom, and it is difficult to take off. The ideal hydrogen storage method is the solid hydrogen storage material in which hydrogen is adsorbed in the base material in the molecular form, and the adsorption energy is between the physical and chemical adsorption. Therefore, a new type of high efficient hydrogen storage material has become a research focus in the field of hydrogen storage. Based on the first principles of density functional theory, the hydrogen storage properties of alkali metal atoms doped boron clusters are studied. The main contents are as follows:The all-boron fullerene B₄₀ cluster is second inorganic non metal cage clusters confirmed by theory and experiment following the carbon fullerene C₆₀,and is named as borospherene. The structure is formed by that the top and bottom two mutually staggered B hexagonal rings and the four waist B heptagonal rings, which are mutually fused together with D_2d symmetry. We have studied the electronic properties of Li and Na doped B₄₀ clusters by using density functional theory. When the Li atom or Na atom is bonded to the surface of the B₄₀ cluster respectively, the metal atom is more likely to be distributed at the center of the hexagonal ring and the heptagon ring. The average binding energy of Li atoms is 2.44 eV, which is greater than the cohesive energy of metallic lithium. This makes the Li atoms stably bound to the B₄₀ surface, without agglomeration, and also makes the Li atoms surrounded by enough space to be used for hydrogen storage. This is similar to the case of Ti atoms bound to the surface of the B₄₀ cluster. Next, we calculate the hydrogen storage properties of Li atoms doped B₄₀ cluster. The calculation results show that hydrogen moleculars transfer charges to the Li atoms and the substrate, and the hydrogen molecular bonding electron cloud density decreased. Hydrogen bond is elongated but not destroyed,and hydrogen is still adsorpted by molecular form. The average adsorption energy of hydrogen molecules is 0.12 eV, and the hydrogen storage mass density of Li₆B₄₀ cluster is up to 10.4 wt%.B₁₄ cluster is a cage like cluster which is composed of two opening B heptagonal ring, and has D_2d symmetry. We calculate the electronic properties and hydrogen storage properties of alkali metal atoms Li and Na doped B₁₄ clusters using density functional theory calculations based on the first principle. Li and Na atoms are more inclined to bound to the B-B bridge at the bottom of the seven membered ring, when the two kinds of alki-metal atoms are combined to B₁₄ cluster respectively. The average binding energy of Li atom and Na atom are 1.71 eV and 1.12 eV, respectively, which are larger than the cohesive energy of the corresponding metal block. And this makes the alkali metal atoms Li and Na doped B₁₄ clusters can be stable. Li₈B₁₄ and Na₈B₁₄ clusters can stably adsorb 24 hydrogen molecules, and hydrogen storage mass density are 18.6 wt% and 12.4 wt%, respectively. When hydrogen moleculars is adsorbed, hydrogen molecules transfer the charges to the metal atoms and the substrate material, weakening the interaction between two atoms of hydrogen. But the hydrogen bonds have not been destroyed. The electric field generated around the metal atoms also make hydrogen moleculars polarizability, which makes hydrogen in molecular form stably adsorbed on the surface of Li atoms and Na atoms doped B₁₄ clusters. The average hydrogen adsorption energy are 0.22 eV and 0.12 eV, respectively. Our calculation results show that the hydrogen storage properties of Li atoms doped B₁₄ cluster are better than that of Na atoms.