| Hydrogen is widely viewed as the next generation of energycarrier to replace the fossil fuels due to its abundance, high chemicalenergy, and pollution-free burning. However, hydrogen storage is the“bottleneck”for the on-board application of hydrogen as energycarrier. Materials suitable for hydrogen storage of vehicularapplications must meet some rigid requirements, such as highvolumetric and gravimetric densities, fast kinetics for adsorption aswell as desorption of molecular hydrogen at ambient conditions,recyclability. To achieve the reversible hydrogen uptaking andreleasing at near ambient conditions, the H2binding energy should besomewhat intermediate between that of physisorption andchemisorption where the hydrogen is stored mainly in molecular form.Several ways have been investigated and developed to store hydrogengas, involving its compression, liquefaction, and adsorption in severalmetals and metal alloys and so on. Unfortunately, none of thesetechnologies is good enough to satisfy the on-board application ofhydrogen energy, even though each way possesses desirablecharacteristics in certain areas.In this paper, inspired by our previous foundation of a B12core in B12CO12, we propose metal atoms doped B12cluster, the metals weused in the paper are Li atoms （The Alkali Metal） and Ti, Sc atoms. Allthe isomers are optimized at the level of density functional theory（DFT） with Becke’s three-parameter exchange functional andLee-Yang-Parr correlation functional using the Gaussian 03 program.The standard split valence basis set 6-31G（d, p） is employed todescribe the orbital of all atoms involved. The geometry optimizationsare done with no symmetry restriction. Frequency calculations forselected structures are carried out to identify it an energy minima onthe potential energy surface. All the population analysis is also basedon the data obtained at B3LYP/6-31G（d, p） level. The average bindingenergy per H2（ABE/ H2） is defined asEa= {E [B12X4-n H2]-E [B12X4]–n E [H2]}/n （X=Li, Ti, Sc）Where, E [B12X4], E [H2] and E [B12X4-n H2] are the electronic energyof relaxed B12X4, H2and B12X4-n H2, respectively; and n is the number ofH2molecule. Many investigations demonstrate that MP2 method ismore efficient for calculating the weak interactions. So, to get theaccuracy ABE/H2, single point energy calculations for all B12X4-n H2,the most stable B12X4and H2are performed at MP2/6-311G（d, p） level.The basis set superposition error （BSSE） has been corrected usingthe full counterpoise method for all the B12X4-n H2complexes atMP2/6-311G（d, p） level.The density functional theory （DFT） has been performed toinvestigate the structures and stabilities of B12Li4clusters. The Liatoms prefer the hexagon sites. Once the metals are absorbed on B12,each can bring up to four hydrogen molecules with an average bindingenergy of 0.033-0.089 eV, corresponding to 13.32 wt%. The NBO analysis indicates that the charge of Li atom in B12Li4is 0.727|e|, andinductive interaction is important for Li to adsorb the H2molecules inB12Li4.We investigated hydrogen adsorption on Ti-doped B12nanostructures. The results show that the lowest-energystructure（D2d） and the lower-lying structure（Td） of B12Ti4can avoidingthe notorious clustering problem. As for the B12Ti4with Tdsymmetry ,Ti can bind strongly to the hexagons of B12to form regular tetrahedronstructure, thus it can store up to 16 H2molecules via the Kubasinteraction with an average binding energy of 0.434 eV/H2,corresponding to a gravimetric density of hydrogen storage of 9.125wt%. If each Ti adsorbs 3 H2, the average adsorption energy is 0.26ev/H2, corresponding to a gravimetric density of hydrogen storage of6.952 wt%, while B12Ti4（D2d） can only host 8 H2molecules at mostwith an average binding energy of 0.521 eV per H2. We found that theKupas interaction dominates the attracting between Ti atoms andhydrogen molecules.The structure and hydrogen adsorption property of a B12Sc4wasinvestigated with the first principle calculations. Like the B12Ti4（D2d）,in B12Sc4, Metal atoms prefer binding to B atoms other than forming Ticlusters. The B12Sc4can bind up to 12 H2molecules with an averagebinding energy of 0.108 eV per H2, It is very interesting that we findthat in B12Sc4, every B3rings can be regarded as a super atom with s-,p- and d-like orbitals.This work was supported by National Basic Research （973）Program of China （No. 210CB635110） and National Natural ScienceFoundation of China （21031003）. |
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