Multidecker (C_6-nB_nH₆) M Complexes For Hydrogen Storage: An Ab Initio Investigation

In recent years, Hydrogen has been recognized as an ideal energycarrier and has the potential to reduce our dependence on fossil fuelswhich are not only limited but also not friendly to the environment.Hydrogen energy is considered as one of the most important newenergies in the21st century. However, one of the bottlenecks of usinghydrogen for large-scale commercial application, is the lack of properonboard hydrogen storage materials. To be an ideal hydrogen storagematerial, the materials must be characterized by large gravimetricand volumetric density of hydrogen storage, fast hydrogen uptake andrelease kinetics, structural stability during repeated hydrogencharging and releasing cycling, and cost effectiveness. An extensivesearch for efficient hydrogen storage materials led to the finding ofseveral promising candidates. Unfortunately, none of these materialsachieve the target set by the U.S. Department of Energy （DOE）. Thesematerials can not be applied to the practical hydrogen storage. In thisthesis, the structure and hydrogen storage properties of Multidecker(C_6-nB_nH₆)M Complexes （M=Sc, Ti, n=0⁴） were investigated basedon ab initio calculations. Calculations based on ab initio methodsdemonstrate that boron doping can increase the binding energy oftransition metal atoms to C_6-nB_nH₆rings. The average Sc-B distancedecreases with the increase of boron atom monotonically, while the average Sc-C distance decreases in a zigzag style. We showed thatthe substitution of carbon with boron atoms in benzene will enhancethe interaction between transition metal atoms and the benzene-likecyclic compounds. The boron substitution has a pronounced effect onthe charge of the transition atoms. The doping of B atoms makes thenatural charge of transition metal atoms in (C_6-nB_nH₆)M on thepositively charged. Taking into account clustering of the transitionmetal atoms, we choose the material whose binding energies aregreater than or equal to the cohesive energy of bulk Sc. The largemetal binding energy is favorable for overcoming the metal clusteringproblem.The results show that transition metal atoms can enhance theinteraction with hydrogen molecules. Transition metal atom canadsorb hydrogen molecule through Kubas interaction. The bondlength of the H2molecules which adsorbed on transition metal atomshas somewhat elongation to the isolated H2molecule bond length. For(C_6-nB_nH₆)Sc isomers, we only discuss the hydrogen adsorptionproperties of this three isomers, namely, that of isomer （C4B2H6）Sc,（C3B3H6）Sc and （C2B4H6）Sc. Further investigations show that（C4B2H6）Sc,（C3B3H6）Sc can absorb five and four molecular H2, whichresponds to the hydrogen storage capacity of7.7and6.3wt%,respectively. The average adsorption energy of these two isomers isabout12-14kJ/mol, which promise them the potential building blocksto construct hydrogen storage materials with high storage capacity atroom temperature and modest pressure. For (C_6-nB_nH₆)Ti isomers, weselected （C4B2H6）Ti,（C3B3H6）Ti and （C2B4H6）Ti to be discussed. Theresults show that（C4B2H6）Ti,（C3B3H6）Ti and （C2B4H6）Ti can adsorbed5, 4and5H₂molecule, which responds to the hydrogen storage capacityof7.5wt%,6.2wt%and7.7wt%, respectively. Meanwhile, electronicstructure analysis is performed to interpret why boron dopingenhancing the binding of Sc to C_6-nB_nH₆rings and why (C_6-nB_nH₆)Mcomplexes absorbing different numbers of H₂molecules.Based on our results, we believe that (C_6-nB_nH₆)M isomers can beused as the basic unit of the design of hydrogen storage materials.These materials can store hydrogen reversibly to uptake and releaseunder ambient conditions, and can reach the required gravimetric andvolumetric density. Based on this result, we expect to use theseisomers as building blocks to construct hydrogen materials with highstorage capacity at room temperature and modest pressure. Theseresults can provide some guidance for the designing and futureexperiments of the transition metal atoms doping nanostructuredhydrogen storage materials.