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Abstract: Confinement effect on the structural, electronic and thermodynamic properties of LiBH4 is investigated by density functional theory. The thermodynamically and dynamically stable confinement structure is testified to be γ-LiBH4@C31Ti according to the adsorption energy and vibrational frequency calculations. The tridentate structure formed by [BH4]− and Li+ in the unconfined LiBH4 changes into bidentate structure in γ-LiBH4@C31Ti. We observe that both the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of γ-LiBH4@C31Ti shift to high energy level and the splits of DOS peaks occur at the states of H 1s, B 2p, and Li 2p orbitals. Different from the first-step decomposition reaction of LiBH4, the one for γ-LiBH4@C31Ti changes into 2LiBH4@C31Ti → 2LiH + 2B@C31Ti + 3H2. Moreover, the reaction enthalpy for the first-step decomposition reaction of γ-LiBH4@C31Ti decreases to 5.864 eV, which is smaller than that (17.204 eV) of LiBH4. According to the hydrogen removal energy calculations, we observe that the confinement effects make the removal of the first and second hydrogen atoms in γ-LiBH4@C31Ti easy. [Copyright &y& Elsevier]

Abstract: Nonmetal atoms (B, C and Si) are designed to add into Mg2Ni hydrogen storage alloy and its hydride. First-principles density-functional theory calculations have been performed to investigate their crystal structures, electronic and thermodynamic properties. The calculation results present that nonmetal additions in Mg2NiH4 show more effective destabilization than metal additions. Especially for B and C, the decreases of formation enthalpies of Mg2NiH4 reach 0.19 and 0.21 eV/atom. The NiH4 structure near B or C in Mg2NiH4 hydride becomes the tripod-like NiH3 structure. The results show that the thermodynamic stabilities of Mg2Ni and Mg2NiH4 exhibit a nearly linear decrease with the increasing content of nonmetal atoms. The calculated dehydrogenation energies are 59.39, 58.12, 55.84 and 55.30 kJ/mol H2 for Mg2NiH4, Mg2NiB0.5H4, Mg2NiC0.5H4 and Mg2NiSi0.5H4, respectively. It is found that the addition of nonmetal atoms favors the dehydrogenation reaction for Mg2Ni hydrogen storage material. In addition, the effects of nonmetals to the heat capacities and vibrational entropies of Mg2Ni and Mg2NiH4 are also analyzed. [Copyright &y& Elsevier]

Abstract: The structure, vibration, and electronic structure of H2 molecule adsorbed on (ZrO2) n (n = 1–6) clusters were investigated with density functional theory. We found that H2 is easily absorbed on the top Zr atoms of (ZrO2) n (n = 1–6) clusters. The Zr5O10H2 cluster has the lowest binding energies in the Zr n O2n H2 (n = 1–6) clusters. By analyzing vibrational frequency and Mulliken charge, the H–O and Zr–H bonds were found to be formed in different sized Zr n O2n H2 clusters. The dissociation mechanism of H2 shows that the charge transfers from (ZrO2) n cluster to H2 due to the important role of the orbital hybridization between the cluster and H2 molecule. With increasing the number of H2 molecule adsorbed on (ZrO2) n clusters, the adsorption favors to the sites with low coordinate number, and these adsorption modes present a symmetrical tendency. [Copyright &y& Elsevier]

Abstract: Using density functional theory we have investigated the feasibility of bare and Ni decorated Al12N12 cages for hydrogen storage. In the bare Al12N12 cage, each Al atom is capable of adsorption one H2 in molecular form with the average adsorption energy of −0.165 eV. In addition, it is shown that hydrogen prefers to remain inside the Al12N12 cage with molecular form. In the Ni decorated Al12N12 cage, the most stable site for Ni atom is the bridge site over the Al–N bond shared by the six-membered rings (BH site) out of the cage. Ni atom of the NiAl12N12 cage has been found to adsorb up to three hydrogen molecules. It is demonstrated that up to 20 hydrogen molecules can be stored on the exterior surface and inside of the NiAl12N12 cage with total gravimetric density of 6.8 wt%. As the weight percentage hydrogen storage is increasing to 6.5 wt%, the minimum value of the Gibbs free energy becomes positive at 25 K. It indicates that high weight percentage hydrogen storage cannot be achieved in NiAl12N12 cages. [Copyright &y& Elsevier]

Abstract: The graphite fragments are used to establish the calculated models of LiBH4 mechanically milled by graphite additive. Density-functional theory calculations have been performed to optimize the crystal structures of LiBH4 inserted by different graphite fragments. It is found that the unsaturated sites introduced by graphite fragments are occupied by BH3 complexes and H atoms. The dehydrogenation energies are calculated to investigate the dehydrogenation ability of LiBH4 with graphite fragments. The results show that BH3 complexes bonded with graphite fragments have lower dehydrogenation energy than pure LiBH4, which is an important factor of the improvement of thermodynamic property of LiBH4 system. The hydrogen diffusion behaviors of graphite fragments doped LiBH4 are investigated by the nudged elastic band method. It is found that the hydrogen diffusion barriers of β-H (the H atoms of BH3 complexes bonding with C atoms) diffusing to other sites are small in the LiBH4/graphite fragment systems. The results show that the existence of β-H type significantly enhances the hydrogen diffusion kinetics of the graphite fragments doped LiBH4. [Copyright &y& Elsevier]