The Computation And Design Of Nanostructured Hydrogen Storage Materials

Hydrogen energy, an environment benign and efficient energy carrier, has promising prospect in the filed of on-board vehicles. There are many hydrogen storage technologies and materials, but they do not meet with the practical application due to the inherent limitation of the materials. This leads us to investigate the interaction between the materials and H2. In order to find out feasible and safe materials that can store hydrogen with high gravimetric and volumetric density, and allow hydrogen uptake and release under ambient conditions (-40 to 85℃, and less than 100 atm), in this investigation we analysed the hydrogen storage behavior of several nanomaterials through density functional theory computations.First, we investigated the interaction of H2 with B-doped graphene, nanotube, fullerene and Ca-decorated carbon nanotube. The results show that molecular hydrogen adsorption energy is improved prominently through B doping only in the carbon nanomaterials with large surface curvature. The partly ionic Ca coated on carbon nanotubes can also improve the hydrogen adsorption energy with polarization mechanism. This provides a new clue for designing hydrogen storage carbon nanomaterials. Boron or alkali earth metal atom doped carbon porous nanomaterials may be promising hydrogen storage materials.Then we examined the hydrogen binding energy on MB2 (M=Mg, Sc, Ti, Zr) nanotubes and found that only nanotubes with transition metals have larger H2 binding energies due to the Kubas interation. The Kubas interaction is that theσelectrons of H2 are transferred to the empty d orbitals of transition metals. At the same time some filled d orbitals donate its electrons back toσ* antibonding orbital of H2 to form back-donation bonds. The back-donation bond is crucial for H2 adsorption to transition metals, and the bond length of H2 is enlongated. The degree of enlongation is associated with the intensity of the back-donation bonds. If transition metals donate enough electrons to theσ* antibonding orbital, the H-H bond will be even dissociated. When more hydrogen molecules are added, there are not enough electrons can back donate to the antibonding orbital to destabilize H2, and the intermediate adsorption state can be obtained. Selecting proper transition metals and substrate to construct a composite system other than clustering is a promising idea for designing hydrogen storage nanomaterials.Next, we investigated the hydrogen adsorption and storage in Ca-coated boron fullerene and nanotube. Ca strongly binds to boron fullerene and nanotube surfaces due to charge transfer between Ca and B substrate. Accordingly Ca atoms do not cluster on the surface of boron substrate, while Ti and Sc persist in clustering on B80 surface. B80 fullerene coated with 12 Ca atoms can store up to 60 H2 molecules with a binding energy of 0.12-0.40 eV/H2, corresponding to the gravimetric density of 8.2 wt%, while the hydrogen storage capacity in (9,0) B nanotube is 7.6 wt% with a binding energy of 0.10-0.30 eV/H2. Ca-coated boron fullerenes and nanotubes are favorable for reversible adsorption and desorption of hydrogen at ambient conditions.We also explored the hydrogen storage behavior of metal decorated boron nitride sheet (BNS) with vacancy. The computations show that the binding energy of Sc atom at B vacancy of BNS is much larger than the cohesive energy of Sc bulk. Sc atoms do not cluster on the surface of BNS with B vacancy. Hydrogen molecule is absorbed on the Sc-decorated BNS (Bv) through Kubas interaction, and the hydrogen adsorption energy is 0.19 eV for GGA level.Finally, we studied the catalysis of B-doped graphene, C60 and C36 fullerene for hydrogen dissociation of MAIH4 (M=Li, Na). The results show that B-doped graphene, C60 and C36 fullerene can decrease the hydrogen removal energy of LiAlH4 and NaAlH4. The introduction of B can enhance the catalysis of carbon nanomaterials for hydrogen dissociation of MAIH4. The presense of empty pz orbital makes B electron deficient. Doping B atoms in carbon nanomaterials would decrease the charge transfer from M to AIH4, and weaken the Al-H vovalent bonds.In summury, we combine our investigation and the recent progress to discuss how to find out feasible hydrogen storage nanomaterials that can store hydrogen with high gravimetric and volumetric density, and allow hydrogen uptake and release under ambient conditions. These results can provide some guidance for designing hydrogen storage nanomaterials and future experiment efforts.