Optimizing the hydrogen storage in boron nitride nanotubes by defect engineering

We use ab initio density functional theory calculations to study the interaction of hydrogen with vacancies in boron nitride nanotubes to optimize the hydrogen storage capacity through defect engineering. The vacancies reconstruct by forming B–B and N–N bonds across the defect site, which are not as favorable as heteronuclear B–N bonds. Our total energy and structure optimization results indicate that the hydrogen cleaves these reconstructing bonds to form more stable atomic structures. The hydrogenated defects offer smaller charge densities that allow hydrogen molecule to pass through the nanotube wall for storing hydrogen inside the nanotubes. Our optimum reaction pathway search revealed that hydrogen molecules could indeed go through a hydrogenated defect site with relatively small energy barriers compared to the pristine nanotube wall. The calculated activation energies for different diameters suggest a preferential diameter range for optimum hydrogen storage in defective boron nitride nanotubes.     

Boron substituted carbon nanotubes—How appropriate are they for hydrogen storage?

The storage of hydrogen in carbon nanotubes requires appropriate chemical activators in suitable geometry. In this study, the role of boron substitution in carbon nanotubes is demonstrated for activation and storage of hydrogen.     

Hydrogen storage on superalkali NLi4 decorated β12-borophene: A first principles insights

The hydrogen storage properties of superalkali NLi₄ decorated β₁₂-borophene have been comprehensively investigated based on first-principles density functional calculations (DFT). It is found that the NLi₄ cluster can be stably anchored on the surface of β₁₂-borophene because of its large binding energy. The calculated Bader charge population indicates that the charges are transferred from Li atoms to the original monolayer and causes the NLi₄ steady adsorbs onto the surface of β₁₂-borophene. For H₂ storage, two sides of NLi₄ decorated β₁₂-borophene can adsorb up to 24H₂ molecules with an ideal H₂ adsorption energy of ?0.176 eV/H_2. Meanwhile, the hydrogen uptake density achieves 7.66 wt% and surpasses the target of 6.5 wt% from U.S. Department of Energy (DOE). In addition, the adsorption reasons of H₂ molecules include the orbital hybridization between H₂ and β₁₂-borophene from the calculated projected density of states (PDOS) and the polarization effect of electrostatic field from the calculated charge density difference. We hope this theoretical study can encourage the experimental fabrication for hydrogen storage applications in the near future.     

DFT study of Al doped cage B12Hn clusters

Density Functional Theory (DFT) with B3LYP/6-311++g** level has been performed to investigate the electronic structures of cage B₁₂H_n for up to n ≤ 12 and AlB₁₂H_n for up to n ≤ 13. Moreover, the computations has been extended to the charged clusters of [B₁₂H₁₂]^q, [AlB₁₂H₁₂]^q and [AlB₁₂H₁₃]^q where (q = ±1 and ±2). Their energetics are calculated and structural analysis have been carried out. Cage form of the B₁₂ remains stable against to hydrogen adsorptions.     

Lithium-functionalized boron phosphide nanotubes (BPNTs) as an efficient hydrogen storage carrier

In this work we have carried out extensive Density Functional Theory (DFT) and ab-initio Molecular Dynamics (AIMD) simulations to study the structural and electronic properties, thermal stability, and the adsorption/desorption processes of hydrogen H₂ molecules on Lithium (Li) functionalized one-dimensional boron phosphide nanotubes (BPNTs), for possible use as materials of H₂-storage media. Our results show that Li atoms can be adsorbed on the hollow sites of (7,7)-BPNT with binding energy ranging from 1.69 eV, for one Li, to 1.65 eV/Li for 14 Li atoms adsorbed on (7,7)-BPNT. These large energies of Li prevent the formation of clusters on the nanotube sidewall. The investigation of the electronic behavior showed that (7.7)-BPNT semiconductor turns metallic upon the Li-adsorption. Furthermore, the average binding energy of H₂-molecules adsorbed on nLi@BPNT(mH₂) systems (with n = 1, 2, 4, 6, 8, 14 and m = 1, 2, 3, 4, with m the number of H₂ for each Li) lies within a range of 0.13–0.20 eV/H₂ which is compatible to the required range for adsorption/desorption of H₂-molecules at room conditions. A H₂-storage gravimetric capacity up to 4.63% was found for 14Li@BPNT(4H₂) system. In addition, AIMD simulation strongly indicates that given adequate monitoring of the temperature, the charge/release process of H₂-molecules can be controlled. Our findings suggest that Li-functionalized (7,7) boron phosphide nanotubes can provide a valuable underlying material for H₂-storage technologies and therefore must certainly be the subject of further experimental exploration.     

Characterization and the hydrogen storage capacity of titania-coated electrospun boron nitride nanofibers

In the present work, titania-coated (TiO₂) boron nitride nanofibers were produced by the electrospinning method, and the effect of heat treatment on the nanofibers was studied. Electrospinning method is often adopted for the synthesis of one-dimensional nanofibers due to high productivity, simplicity, and cost-effectiveness. In this study, boric oxide was deposited on co-electrospun polyacrylonitrile and TiO₂. TiO₂-coated boron nitride nanofibers, with a diameter of 100 nm, were obtained after heat treatment and nitridation. The effects of heat treatment on the morphology, surface area and hydrogen storage capacity were studied extensively. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) showed long, bead-free nanofibers and the presence of TiO₂ nanoparticles on the nanofibers. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy depicted hexagonal structures of boron nitride. The hydrogen uptake capacities of the nanofibers were investigated by pressure composition isotherm (PCI) in the pressure range of 1–70 bar at room temperature.     

A novel hydrogen storage medium of Ca-coated B40: First principles study

Using first principles study, we have investigated the hydrogen storage capacity of Ca-coated B₄₀. Our result shows that Ca prefers to adsorb on the top hollow center of heptagonal ring of B₄₀ due to the large binding energy of ?2.820 eV. Bader charges calculation indicates that charges transfer from Ca to B₄₀ result in an induced electric field so that H₂ molecules are polarized and adsorbed onto the surface of B₄₀ without dissociation. The Ca₆B₄₀ complex can adsorb up to 30 H₂ molecules with average adsorption energy of ?0.177 eV/H₂ and the hydrogen storage gravimetric density reaches up to 8.11 wt.%, higher than the goal from DOE by the year 2020. These findings will suggest a new and potential structure for hydrogen storage in the future.     

Mixed-metal Li3N-based systems for hydrogen storage: Li3AlN2 and Li3FeN2

The hydrogen storage systems Li₃AlN₂ and Li₃FeN₂ were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li₃MN₂ (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li₃AlN₂ prepared from hydrogenous starting materials (LiNH₂ and LiAlH₄) performed better than that synthesized from non-hydrogenous materials (Li₃N and AlN). For both Li₃AlN₂ materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li₃AlN₂ synthesized from LiNH₂ and LiAlH₄ showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li₃FeN₂ synthesized from Li₃N and Fe displayed similar kinetics to that synthesized from Li₃N and Fe_xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li₃FeN₂ displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li₃AlN₂ and Li₃FeN₂, doping with TiCl₃ resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.     

Hydrogen adsorption on Ce/BNNT systems: A DFT study

The adsorption of H₂ on Ce-doped boron nitride nanotubes (BNNT) is investigated by using density functional theory. For the Ce/BNNT system, it is found that Ce preferentially occupies the hollow site over the BN hexagon. The results indicate that seven H₂ per Ce can be adsorbed and 5.68 wt% H₂ can be stored in Ce₃/BNNT system. Among nanotubes doped with metals, Ce exhibits the most favorable hydrogen adsorption characteristics in terms of the adsorption energy and the uptake capacity. Both hybridization of the Ce-5d orbital with the H-1s orbital and the polarization of the H₂ molecules contribute to the hydrogen adsorption. Ce clustering can be suppressed by preferential binding of Ce atoms on BNNT, which denotes that BNNT as a hydrogen storage substrate is better than CNT due to its heteropolar binding nature.     

A first principles study on H-atom interaction with bcc metals

Solute hydrogen trapping has long been proposed as one of the mechanisms for hydrogen embrittlement in steel. It has been reported that the maximum hydrogen trapping energy of metallic solutes ranged from ?0.7 eV to ?0.9 eV. In this work, the mechanism of metal-H interaction in Cr-Mo steels was investigated with first principles calculations by modelling the binary alloy Fe-X (X = C, Si, Mn, Cr, Mo) system with reference to the chemical composition of Cr-Mo steels. The formation of hydrogen bonds in the case of H atoms located at different sites in Fe-X crystals was analyzed. Results indicated that various atomic doping had different roles in hydrogen effect in the steel, with C, Si and Mo doping making the solid solution of hydrogen in Fe crystals easier, while Mn and Cr doping was rather more difficult. In Fe-Mn and Fe-Cr crystals, the repulsion between Fe lattices was insignificant when H atoms were located in tetrahedral sites, which considerably reduced the binding energy in the crystal. When H atoms were dissolved into the crystal, the interatomic bonding interactions in Fe-X crystals were weakened, resulting in higher charge density fluctuations. The current work extends the understanding of H-atom diffusion and migration in steel from the microscopic scale to the atomic and electronic scales, which underpins the physics for tailoring chemical elements of bcc metals towards higher resistance to hydrogen embrittlement.     

First-principles study on hydrogen storage by graphitic carbon nitride nanotubes

First-principles calculations based on density functional theory were carried out to investigate the hydrogen storage capacity of graphitic carbon nitride nanotubes. Graphitic carbon nitride nanotubes could be attractive hydrogen sorbent for two reasons: firstly, its porous structure allows easy access of hydrogen into the interior of the nanotubes; and secondly, the doubly bonded nitrogen at its pore edges provides active sites for either the adsorption of hydrogen (chemically and physically), or functionalization with metal catalysts. Our calculations show that an isolated nanotube can uptake up to 4.66 wt. % hydrogen, with an average overall hydrogen adsorption energy of about −0.22 eV per H atom. In the form of a bulk bundle, the hydrogen storage capacity is enhanced due to the increased availability of space among the tubes. We predict that the hydrogen storage capacity in the bundle is at least 5.45 wt. %. Importantly, hydrogen molecules can easily access the tube’s interior due to the low energy barrier (∼0.54 eV) for their passage through the pores, indicating a fast uptake rate at relatively low pressure and temperature. Our findings show that graphitic carbon nitride nanotubes should be applicable to practical hydrogen storage because of the high gravimetric capacity and fast uptake rate.     

HfNi and its hydrides – First principles calculations

Hydrogen induced modifications to the structural, electronic and bonding properties of HfNi are investigated by performing first principles calculations. The full-potential linearized augmented plane waves (FP-LAPW) code based on the density functional theory (DFT) was used. The charge transfer and bonding between the constituent atoms is examined by means of the Bader's atoms in molecule (AIM) theory. The calculated enthalpies of formation of HfNi, HfNiH and HfNiH₃ are −53.5 kJ/mol atom, −17.3 kJ/molH and −34.6 kJ/molH. They are found to be in a good agreement with the experimental and semi-empirical values. The calculated stability of the hydrides is in agreement with their hydrogen absorption ability.