Structure Design For High-Density Hydrogen Storage Materials And Their Hydrogen Adsorption Mechanism

Hydrogen energy is one of the most promising green and renewable energy to take place of the fossil fuels in the 21 st century because of the high calorific value, abundant reserves, non-toxic and pollution-free. However, hydrogen storage problem is a key constraint of the hydrogen economy, it is important to find a safe, efficient, inexpensive hydrogen storage material for the hydrogen economy development. Gaseous hydrogen storage requires extremely high pressure cylinders with the low mass density and bulk density. Liquid hydrogen storage requires low temperatures with high energy consumption and costs. Neither of them is suitable for widely used in hydrogen storage. On the contrary, Solid-state hydrogen storage is a safe way for hydrogen storage with high energy density and relatively low cost. Low-dimensional material is one of best solid state hydrogen storage materials. The interaction between pure low-dimensional materials and hydrogen molecules is weak, and the adsorption energy is generally less than 0.1eV. The weak physical adsorption effects result in low hydrogen storage and need low temperature to storage. Modification by transition metal atoms is a good way to improve and enhance the hydrogen storage properties of low-dimensional nanomaterials. In the low-dimensional metal-modified nanomaterials, the metal atom plays a role of adsorbed hydrogen molecules, hydrogen storage properties of the material often depends on the hydrogen storage capacity of metal atom, when low-dimensional material with light weight, more holes, large specific surface area and other characteristics, plays a role in dispersing metal atom. And the strong adsorption between substrate and metal is needed to avoid forming metal clusters and lower the hydrogen storage capacity. However, during the studies of metal-modified low-dimensional hydrogen storage material, there are still several unresolved issues: the hydrogen storage properties and hydrogen storage mechanism of metal is not clear, the effect of substrate to metal is not clear, and the design of high-density hydrogen storage materials is also unclear. Based on this, from the reality, the main contents are as follows:1. Based on density functional theory and Dmol~3 package, we study the hydrogen storage capacity and hydrogen storage mechanism of metal. It is found single alkali or alkaline earth metals do not own the ability to adsorb hydrogen molecules, when placed one pair of hydrogen molecule in the vicinity of the metal, after sufficient structural relaxation, hydrogen molecules escape from the metal with a distance more than 4?. The transition metal atom is on the contrary. A part of the transition metal atom such as Sc, Ti, V, Fe, Y, Zr, Nb, Pd, and Pt can adsorb up to eight pairs of hydrogen molecular, and Co, Ni and Mo can adsorb simultaneously four pairs of hydrogen molecules. We make Ti for an example, after calculating the energy level distribution, atoms orbitals and charge density difference of metal adsorb hydrogen molecules, we found that the main adsorption mechanism of transition metal adsorb hydrogen molecules derived from:(i) 4s electron transfers to 3d electrons in metal atom, due to the d-orbital electrons, metals form a petal shape anisotropy electrostatic field and induce hydrogen molecular polarize.(ii) The charge distribution of the polarized hydrogen molecules is located at both ends, easily lead to the hybridization between of s electronic and d electronic, which further enhance the interaction between the metal and the hydrogen molecule.(iii) The charge distribution is located among hydrogen molecules to form a super molecule and further reducing the adsorption energy. The adsorbed sites of hydrogen molecules are depend on the sharp of d orbitals, and locate in the less charge distribution area which inducing a larger electrostatic field. Therefore, the adsorbed structure and the number of hydrogen molecules are depend on d orbitals of Ti atom.2. Based on density functional theory and VASP package, we studied the effect of substrate material to the hydrogen storage properties and hydrogen storage mechanisms of metal. We select four different metals(Ca, Sc, Ti and V) to modify(8,0) carbon nanotubes for an example. It is found Ca atom can capture five hydrogen molecules and remaining three transition metal can absorb up to four pairs molecular hydrogen per metal atom. That is half of the adsorbed number of single metals due to the absence of substrate material which cut off the space for hydrogen molecules to adsorb. By calculating partial wave density of states of metal adsorb hydrogen molecular, we found that the hydrogen storage mechanism of metal@SWCNTs is consistent with the isolated transition metal. In the metal-modified low-dimensional materials, the hydrogen storage capacity mainly rely on metal, and low-dimensional materials only plays a supporting substrate material to disperse metals. It does not hinder the adsorption of hydrogen molecules. There is no essential influence to hydrogen storage properties and mechanisms of metal.3. Based on the above two works, we designed two high-density hydrogen storage materials.(1) Li@ 6,6,12-graphyne composite material. we selected Li, the lightest metal, as the hydrogen adsorbent, and used a porous monolayer 6,6,12-graphyne as the substrate of Li. It is found that Li atoms can be firmly adsorbed on the substrate without the formation of Li clusters. Up to thirty pairs of hydrogen molecules are adsorbed per unit cell, and hydrogen storage capacity is up to 19.3wt%, which far more than the 2015 target of US Department of Energy.(2) Metal@γ-graphyne nanotube hydrogen storage material. Selected transition metals Sc, Ti, V and alkaline earth metals Ca which chemically stable in the environment as hydrogen molecules adsorbent, using γ-graphyne nanotube as a supporting substrate material to disperse metal atoms. We found the adsorption interaction of the metal atoms and nanotube is stronger than the cohesion of bulk metal, and metal is not easy to reunite on the surface of tube. The hydrogen storage capacity of Ca@γ-graphyne nanotube is 5.08wt% and closes to the goal of US Department of Energy.