First-Principles Studies Of Hydrogen-Storage Materials

Development of hydrogen-storage materials with high performance is one of research focuses today. The operations as doping and metal-coating with carbon nanotubes, fullerenes and other carbon-based materials can improve their performance in the storage of hydrogen, which makes them to be one of the most promising hydrogen-storage materials.The hydrogen-storage properties of C6o which is coated by lithium atoms or doped by boron atoms are studies based on the first-principles calculations. The C48B12coated by lithium atoms with the symmetry of S6are discussed firstly, founding that lithium atoms has six stable adsorption sites on the surface of C48B12and at most five hydrogen molecules can be adsorbed by each lithium atom in molecular form, which four hydrogen molecules are adsorbed by chemical adsorption and induced-dipole adsorption as well as one hydrogen molecule is adsorbed by induced-dipole adsorption. There are two different forms of dense-coating on C48B12that eight lithium atoms evenly adsorb above eight six-membered ring which is Li8C48B12, twelve lithium atoms evenly adsorb above twelve five-membered ring which is Li12C48B12. It is shown that the performance of lithium atom on the storage of hydrogen is not affected in both of the dense-coating forms. Forty hydrogen molecules are adsorbed on Li8C48B12and the theoretical mass ratio of hydrogen's content is up to9.5wt%, sixty hydrogen molecules are adsorbed on Li12C48B12and the theoretical mass ratio of hydrogen's content is up to13.6wt%, both of which are beyond the standard drafted by United States Department of Energy that hydrogen as vehicle power should reach6.5wt%on the mass ratio of hydrogen's content for hydrogen-storage materials. Li12C48B12performances better on the storage of hydrogen than Li8C48B12, though lithium atom has better adsorption ability on the six-member ring composed of two boron atoms and four carbon atoms than on the five-membered ring composed of one boron atom and four carbon atoms. In general, Li8C48B12has priority in the production. At the same time, lithium atoms adsorb more stable on Li8C48B12than on L112C48B12with only0.49eV/Li, which means, in practice, it is difficult for us to obtain Li12C48B12with higher hydrogen-storage ability.To solve the problem that the transition metals as titanium atoms cluster on the surface of C6o, this paper will first try to do multiple replacement and dope titanium atoms to C6o. It is found that, after the second replacement, one of the two titanium atoms and one carbon atom of Ti2C58tend to form TiC fragment and release itself, while another titanium atom move to the center of the12-membered ring, forming a novel structures with the symmetry of C2v, TiC58. On the base of calculation, it is shown that the first and second hydrogen molecule is catalyzed by Ti and disintegrated into four hydrogen atoms with low dissociation energy barrier. Then, there are another four hydrogen molecules, in the form of molecular, adsorbed on the surrounding of Ti through Kubas interactions. That means, titanium atom in the TiC58can adsorb six hydrogen molecules altogether,50%more than four hydrogen molecules at most adsorbed by titanium atom on carbon nanotubes surface which is reported in the literature. Also, it is shown that each titanium atom of Ti6C4g has the similar hydrogen-storage properties to titanium atom of TiC58, adsorbing up to twenty-four hydrogen atoms and twenty-four hydrogen molecules in total, whose theoretical mass ratio of hydrogen's content reaches7.7wt%.It is found that lithium atoms adsorbs stably above the six-membered carbon rings of Ti6C48to get the compound, Li8Ti6C48, rather than forms a cluster with and lithium atoms. It is shown that the catalyst of titanium atoms of Li8Ti6C48to hydrogen molecules is weaken, however, each titanium atom can adsorb four hydrogen molecules in the molecular form, which is similar to the action mode of titanium atoms to hydrogen molecule in Ti6C48, each lithium atom of Li8Ti6C48can adsorb up to five hydrogen molecules, which is similar to the action mode of Li coated on the C48B12surface to hydrogen molecules. System can absorb sixty-four hydrogen molecules in total, whose theoretical mass ratio of hydrogen's content reaches to12.2wt%.Hydrogen atom is hard to diffuse in magnesium, which limits the application of magnesium in storing hydrogen. However, the presence of vacancies within metal can greatly improve its performance in diffusion dynamic. By examining the formation energy of mono-vacancies in the surface of Mg (0001), three layers under the surface and bulk and the stability of di-vacancies across two different layers, the possible diffusion path of hydrogen atom in magnesium is studied in this paper. It is shown that the second layer under the surface has the highest formation energy of mono-vacancies and the least number of vacancies, making it the main barrier in the diffusion of the hydrogen. The di-vacancy which crosses the second and third layer with nearest distance is stable.