Study On Hydrogen Storage Properties Of Pure And Alkali Metal Coated B₁₂C₆N₆ Cluster

The extensive consumption of fossil fuels leads to rapid depletion of these resources,it also poses serious environmental pollution and threats our living environment.Hydrogen is a potential ideal energy carrier, as it has the highest heating value per mass of all chemical fuels and its oxidation forms only water. However, a number of key challenges must be overcome before the hydrogen energy economy becomes reality. Besides economic means for large-scale hydrogen production, a safe and efficient storage mediumis also the crucial prerequisite for hydrogen as a future energy carrier can be realized. An ideal storage system should be able to operate under ambient conditions with high recycling capacity and suitable uptake-release kinetics.Up to now, no systems has been found to reach a satisfactory level of performance because hydrogen molecules interact either too weakly or too strongly with the host materials. Solid state storage is the most potential way because of its storage capacity,energy efficiency and safety. Hybrid BCN nanomaterials have high surface area and unique electronic properties, so its have also received considerable attention. We recently investigated the stability and electronic structure of nonstoichiometric B₁₂C₆N₆ cage. B₁₂C₆N₆ is an electron deficient fullerene. In this paper, we investigate adsorption and dissociation of H₂ molecule on B₁₂C₆N₆ cluster and H₂ molecule adsorption on Li-coated B₁₂C₆N₆ cluster. The results show that the adsorption of H₂ molecule on the cage surface is weak physical adsorption; the adsorption energies are about 0.03 eV. The stable dissociation products are determined and the dissociation processes are traced. Molecular orbital compositions show that strong orbital interaction between the hydrogen and the cluster occurs in the process of dissociation and H₂ molecule can be easily dissociated on B₁₂C₆N₆. We present four dissociation paths. The lowest energy barrier is only 0.35 eV, which means the dissociation can take place at moderate temperatures. We investigate the geometry structure, electronic structure and hydrogen storage property of Li-coated B₁₂C₆N₆. Li atoms can be strongly bound to this cage by donating their valance electrons to the virtual 2porbitals of carbon in the cluster. The binding energy（-2.90 eV） is much larger than the cohesive energy（1.63 eV） of bulk Li and it prevents the Li atoms from aggregation.The coated Li atoms have large positive charges and the adsorbed hydrogen molecules can be moderately polarized by the Li⁺ ions. The computation shows that each Li atom coated on B₁₂C₆N₆ can hold 2-3 H₂ molecules with adsorption energies in the range of 0.21-0.24 eV/H₂. The B₁₂C₆N₆Li₈ can adsorb 16 H₂ and achieve a gravimetric hydrogen density of 8.63 wt%. The present results indicate that alkalimetal atoms coated on electron deficient fullerenes can serve as hydrogen storage materials that can operate at ambient temperatures with high recycling storage capacity.