Narrow Results

Transition metals such as titanium are known to attract (molecular) H₂ in some complexes through Kubas interaction. This interaction involves coordination of sigma bonds of the H2 with the transition metal, and it is comparatively strong, allowing promising hydrogen storage applications. In this work, the impact of structural geometry on this coordination is investigated using a model titan–carbide structure (TiC₄H₄) by performing density functional theory calculations.

In this paper, the nanocrystalling composite Mg-Ni-TiO₂-CNTs was made by mechanical ball milling under hydrogen pressure of 5.0×10^-5Pa, The hydrogen storage properties and operation temperatures were investigated. It was found that the composite can absorb 7.5wt.% hydrogen in 60s under 5.0×10^-5Pa hydrogen pressure and desorb 6.5wt% hydrogen in 600s at 260°C under 5.0×10^-5Pa hydrogen pressure. The composite has very high absorption, hydrogen capacity and remarkable kinetic properties in hydriding /dehydeiding process. TiO₂, Ni and CNTs were used as additive to improve the hydrogen storage performance. They can be utilized as an effective catalyst to improve hydriding/dehydriding performance of hydrogen storage material and lead to pronounced enhancement on the hydrogen storage property of Mg. The results showed that the composite has potential application in the future.

Carbon-based materials, due to their low cost and weight, have long been considered as suitable physisorption substrates for the reversible storage of hydrogen. Nanoporous carbons can be engineered to achieve exceptional storage capacities: gravimetric excess adsorption of 0.073 ± 0.003 kgH₂/kg carbon, gravimetric storage capacity of 0.106 ± 0.005 kgH₂/kg carbon, and volumetric storage capacity of 0.040 ± 0.002 kgH₂/liter carbon, at 80 K and 50 bar. The nanopores generage high storage capacity by having a very high surface are, by generating a high H₂-wall interaction potential, and by allowing multi-layer adsorption of H₂ (at cryogenic temperatures). In this paper we show how the experimental adsorption isotherms can be understood from basic theoretical considerations and computational simulations of the adsorption in a bimodal distribution of narrow and wide pore spaces. We also analyze the possibility of multi-layer adsorption, and the effects of hypothetical larger adsorption energies. Finally, we present the results of coupled ab initio calculations and Monte Carlo simulations showing that partial substitution of carbon atoms in nanoporous matrix with boron results in significant increases of the adsorption energy and storage capacity.

The density functional theory (DFT) is used to investigate the adsorption of H₂ molecules on the Al-adsorbed single-walled carbon nanotube (Al-SWCNT) and the hydrogen storage capacity. The most stable position for the Al atoms adsorbed on the outer surface of SWCNT is confirmed with the binding energy of -2.510 eV. The research results indicate that the hydrogen storage capacity can be greatly improved when the adsorbed H₂ molecules are arranged into a layered structure, and the analyzing of electronic density also supports this point. It is found that the H₂ molecules can be absorbed in six layers at most with a suitable average binding energy of -0.131 eV/H₂, and the hydrogen storage capacity of Al-(7, 7) SWCNT (Al₇C₇₀) is approximately 28.0 wt% which is much higher than the DOE's target.

A comprehensive study of structural, morphological, magnetic and hydrogen absorption properties of LaNi₅–H system was investigated. The X-ray diffraction patterns show that as-synthesized LaNi₅ alloy is single phase with CaCu₅-type structure while some weak peaks of elemental nickel also appeared after several hydrogenation/dehydrogenation (H/D) cycling. The presence of pure Ni was also followed using the room temperature magnetic measurements. After H/D cycling, the particle size decreases and particle size distribution was found nearly uniform compared to noncycled alloy. The pressure-composition isotherms (PCIs) of the hydrogen absorption reaction were determined in the temperature range 20–80°C using a homemade Sievert's type experimental apparatus, and then the enthalpy and entropy of hydride formation were calculated. The hydriding kinetic mechanism of LaNi₅ was evaluated using the different fitting models: Jander diffusion model (JDM), Johnson–Mehl–Avrami (JMA) and Chou models. All employed models confirm an increase in the hydriding reaction rate with temperature. However, the calculated results using JMA model show a better agreement with the experimental data and hence we believe that diffusion along with nucleation and growth is the rate-controlling step for the hydriding reaction. The values of activation energy for hydriding reaction were also obtained by JD and JMA models.

The structural, stability and electronic properties of C15-AB₂ (A = Ti, Zr; B = Cr) isomeric intermetallic compounds were systematically investigated by using density functional theory (DFT) and plane-wave pseudo-potential (PW-PP) method. The macroscopic properties including the lattice constant, bulk modulus and stability for these compounds were studied before and after hydrogenation. For parent compounds, the enthalpy of formation was evaluated with regard to their bulk modules and electronic structures. After hydrogenation of compounds at different interstitial tetrahedral sites (A₂B₂, A₁B₃, B₄), a volume expansion was found for hydrides. The stability properties of hydrides characterized the A₂B₂ sites as the site preference of hydrogen atoms for both compounds. The Miedema's "reverse stability" rule is also satisfied in these compounds as lower the enthalpy of formation for the host compound, the more stable the hydride. Analysis of microscopic properties (electronic structures) after hydrogenation at more stable interstitial site (A₂B₂) shows that the H atoms interact stronger with the weaker (or non) hydride forming element B (Cr) than the hydride forming element A (Ti/Zr). A correlation was also found between the stability of the hydrides and their electronic structure: the deeper the hydrogen band, the less stable the hydride.

A comprehensive study of structural, morphological, hydrogen absorption and magnetic properties of MmNi_4.22 Co_0.48Mn_0.15Al_0.15 alloy as a promising hydrogen storage media was investigated. The X-ray diffraction (XRD) profiles show that the alloy maintains its crystal structure (hexagonal LaNi₅-type) even after 30 hydrogenation/dehydrogenation (H/D) cycles. However, the XRD peaks are found to be slightly broadened after cycling. SEM images reveal that particles size of the cycled sample decreases, with more uniform particle size distribution compared to noncycled ones. The pressure-composition (PC) isotherms and kinetics curves of hydrogen absorption reaction were obtained at different working temperatures by using a homemade Sievert apparatus. The enthalpy and entropy of hydride formation of the alloy were evaluated. Furthermore, the Jander diffusion and Johnson–Mehl–Avrami models as the fitting models were employed to study the kinetic mechanism of hydriding reaction and its activation energy. The room temperature magnetic measurements indicate that the milling and H/D cycling change the magnetic properties of the as-annealed alloy.

The theoretical calculations indicate that the metal-doped boron nitride (BN) sheets are potential materials to store the hydrogen and tune the bandgap. It is all known that the BN sheet is a nonmagnetic wide-bandgap semiconductor. Using density function theory (DFT), the lattice parameters of Cr-doped BN sheets are optimized, which are still kept on two-dimensional (2D) planar geometry, and the bandgap and ${\mathrm{\text{H}}}_2$ storage are studied. The simulation results show that the ${\mathrm{\text{H}}}_2$ molecule can be easily absorbed by Cr-doped N in BN sheet. As the adsorption energy was greatly decreasing with the increasing number of Cr-doped N, B had an affinity for adsorption of ${\mathrm{\text{H}}}_2$. With the increase of Cr doping, the bandgap of Cr-doped BN sheet is decreasing. The bandgap decreases from 4.705 eV to 0.08 eV. So Cr-doped BN sheet is a promising material in storing ${\mathrm{\text{H}}}_2$ and tuning the bandgap.

In this study, we investigated stable structures for a transition metal atom–boron (CrB) wheel-like clusters and compared them with their corresponding 3D counterparts by means of density functional theory (DFT). In addition, hydrogen storage capability of the wheel-like system was investigated. All simulations were performed at the B3LYP/TZVP level of theory. We set out a complete route to the formation of CrB wheel-like clusters. Our results showed that, some of the clusters, investigated in this work (CrB_n; n = 4, 6, 7, 8), either prefer to be in a 3D geometry rather than 2D quasi-planar or planar geometry. However, hydrogen doping has an interesting effect on both 2D quasi-planar and 3D geometries of this system. Simply it transforms the 3D structure, first, into a 2D quasi-planar, then a planar geometry. Furthermore, our results show that H–cluster interaction is too high for reversible hydrogen storage for these clusters.

Using a density functional approach calculation, the structural and electronic properties of Mg₈Ni₅, Mg₈Ni₄Al, Mg₈Ni₄Ca and Mg₈Ni₄Mn clusters are investigated. The Al-, Ca-, Mn-doping in MgNi clusters lengthen $d_{\mathrm{\text{(Mg–Ni)}}}$, also increase the Mulliken charge, and improve magnetic moment on most shell atoms, so that the doping is helpful for hydrogen storage. The reaction heats of hydrogenation, the enthalpies of formation, the desorption energies show that the Mg₈Ni₄Mn, Mg₈Ni₅, Mg₈Ni₄Al clusters have a good property of hydrogen storage materials with relatively higher reaction heat of hydrogenation, and that their hydrides have a smaller desorption energy. So the Mg₈Ni₄M (M = Ni, Al, Ca, Mn) clusters are some preferred candidates of H storage. These results indicate the advantage of small size and suitable metal-doped MgNi clusters on hydrogenation and dehydrogenation.

We use the first-principles calculation based on density functional theory (DFT) to investigate the hydrogen storage of Li decorated coronene graphene. Our result indicates that single Li atom can adsorb three H₂ molecules and the adsorption energy per H₂ is −0.224 eV. When four Li atoms doped, the largest hydrogen gravimetric density is 6.82 wt.% and this is higher than the 2017 target by the US department of energy (DOE). Meanwhile, the adsorption energy per H₂ is −0.220 eV, which is suitable for H₂ molecules to store. Therefore, Li decorated coronene graphene will be a candidate for hydrogen storage materials in the future.

The geometries, structural stability, electrical and magnetic characteristics of pure and multiple palladium (Pd)-adsorbed graphene, followed by hydrogen adsorption, are investigated using first-principles calculations with the density functional theory. In the DFT-D2 technique, first-principles computations with the van der Waals interaction are done using the generalized gradient approximation. In a $4\times 4$ supercell, the adsorption energy per Pd atom is found to be 1.20 eV in the optimal adsorption shape. The bandgap of 51 meV has opened in multiple Pd-decorated graphene, according to band calculations. This band’s opening is ascribed to a symmetry break. The binding energy for hydrogen adsorption in optimal double Pd-decorated graphene was determined to be in the range of (0.14–0.73) eV per hydrogen molecule, indicating that Pd-decorated graphene might be used as a hydrogen storage material.

In this paper, the hydrogen storage performance of Sc (Ti, Li)-modified $\mathrm{\Psi }$-graphene ($\mathrm{\Psi }$-g) is investigated by density functional theory (DFT). Sc and Ti can be stably adsorbed on $\mathrm{\Psi }$-g with the binding energies of 4.78 eV and 4.83 eV, respectively. Li atoms can also be stably absorbed on B-doped $\mathrm{\Psi }$-g with the binding energy of 2.51 eV. In addition, Sc and Ti atoms attached on $\mathrm{\Psi }$-g can adsorb up to ten and eight hydrogen molecules, and the hydrogen gravimetric storage capacity reaches 7.93 wt.% and 6.20 wt.%, respectively. Li atoms attached on B-doped $\mathrm{\Psi }$-g can adsorb up to eight hydrogen molecules with the hydrogen gravimetric storage capacity 9.32 wt.%, all meeting DOE standards. On the whole, Bader charge analysis shows that the charge is transferred from C atoms to H atoms, which is more conducive to hydrogen adsorption. All the researches prove that the Sc, Ti-decorated $\mathrm{\Psi }$-g and Li-decorated B-doped $\mathrm{\Psi }$-g are potential hydrogen storage materials.

Hydrogen is a promising candidate for green energy sources for future endeavors because of its abundance on Earth. Although its storage is a major challenge for the researchers of this era because of its unsafe and highly explosive nature. The structural, optoelectronic, thermoelectric, vibrational, thermodynamic properties and hydrogen storage capacity of XMgH₃ ($\mathrm{\text{X}}=\mathrm{\text{Sr}}$, Ba) are carried out by using the full potential linearized augmented plane wave (FP-LAPW) method in the DFT framework. The theoretical study about these magnesium-based metal hydride perovskites, i.e., SrMgH₃ and BaMgH₃, declares them structurally stable compounds in space group Pm-3m. The optimization graph for SrMgH₃ and BaMgH₃ reflects the lowest ground state energy, i.e., −6759Ry and −16683Ry, respectively. Comparatively, BaMgH₃ seems to be more stable. The electronic band structures and density of states declare them pure metallic due to zero band gap and overlapping of electronic states of the valence and the conduction bands. The electrical conductivity of BaMgH3 increases up to $4.5\times 10^{20}{(\mathrm{\Omega }\cdot \mathrm{\text{m}}\cdot \mathrm{\text{s}})}^{-1}$ and thermal conductivity $1.25\times 10^{16}{(\mathrm{\Omega }\cdot \mathrm{\text{m}}\cdot \mathrm{\text{s}})}^{-1}$ in the temperature range 100K to 1000K revealing the good metallic character of BaMgH₃. The optical analysis portrays the absorption of compounds in the visible range along with valance shell electrons to the weak bond of hydrogen and dissociates hydrogen molecules at a certain intensity of light. BaMgH₃ compound shows minimum scattering and maximum absorption of light in the visible region up to 3eV. The reflectivity peaks in the visible region 3.0eV show that 40% of light energy is absorbed due to the opaque nature of BaMgH₃. Both these compounds are declared thermodynamically stable due to negative free energy such as −1.20eV for SrMgH₃ and −1.50eV energy for BaMgH₃ at 1000K, respectively. Moreover, the three acoustic modes showing zero imaginary phonon frequencies at $\mathrm{\Gamma }$ symmetry points predict these compounds’ structural and thermodynamical stability. The gravimetric hydrogen storage concentration of SrMgH₃ and BaMgH₃ is determined as 2.637% and 1.836%, respectively.

The hydrogen-rich compound (H₂)_nCH₄ (for n = 1, 2, 3, 4) or for short (H₂)_nM is one of the most promising hydrogen storage materials. The (H₂)₄M molecule is the best hydrogen-rich compound among the (H₂)_nM structures and it can reach the hydrogen storage capacity of 50.2 wt.%. However, the (H₂)_nM always requires a certain pressure to remain stable. In this work, we first investigated the binding energy of the different structures in (H₂)_nM and energy barrier of H₂ rotation under different pressures at ambient temperature, applying ab initio methods based on van der Waals density functional (vdW-DF). It was found that at 0 GPa, the (H₂)_nM is not stable, while at 5.8 GPa, the stability of (H₂)_nM strongly depends on its structure. We further investigate the Raman spectra of (H₂)_nM structures at 5.8 GPa and found the results were consistent with experiments. Excitingly, we found that boron nitride nanotubes (BNNTs) and graphite and hexagonal boron nitride (h-BN) can be used to store (H₂)₄M, which give insights into hydrogen storage practical applications.

In this paper, we report the synthesis of polymer coated palladium (Pd) nanoparticles through a single stage reduction of Pd²⁺ ions by ethylene glycol. Polyvinyl pyrrolidone (PVP, MW 25,000) is used as a stabilizer. Self-assembled Pd nanoparticles (10–40 nm) were used in hydrogen adsorption studies. Gravimetric adsorption measurements were carried out in a pressure range of 0–26 bar at 293, 324, 364 and 392 K. Saturation for all isotherms was obtained within a few bars of pressure at all temperatures. Maximum hydrogen storage capacity observed was 0.58 wt.% at 324 K and 20 bar. Net adsorption calculations indicated that required tank volume (for storing a particular amount of hydrogen) can be significantly reduced by using a tank filled with Pd nanoparticle.

Effect of light alkali metal (Li and Na) decorated on the C₅₉B heterofullerene for hydrogen storage is considered using DFT-MPW1PW91 method. Results show that Li and Na atoms strongly prefer to adsorb on top of five-member and six-member ring where a carbon atom is replaced by a boron atom. Significant charge transfer from the alkali metal to the C₅₉B compensates for the electron deficiency of C₅₉B and makes the latter aromatic in nature. Corrected binding energies of hydrogen molecule on the alkali-doped C₅₉B using counterpoise method, structural properties and NBO analysis indicate that first hydrogen molecule is adsorbed physically and does not support minimal conditions of DOE requirement. Finally, positive values of binding energies for the adsorption of a second hydrogen molecule show that alkali doped C₅₉B are capable of storing a maximum of one hydrogen molecule.

Inorganic fullerene-like MoS₂ nanotubes synthesized by a gas–solid reaction were used as the electrode materials of hydrogen and lithium storage in aqueous and nonaqueous electrolytes, respectively. Very interesting results such as reversible hydrogen reduction/oxidation and lithium insertion have been obtained.

We have synthesized a set of nanostructured carbon samples including a variety of carbon nanotubes and carbonaceous particles, by catalytic thermal decomposition of CH₄ on catalyst LaNi₅ powder with different reaction temperatures. Products obtained at reaction temperatures 550~900°C were characterized by means of HR-TEM, SEM and Raman Scattering. In addition, electrochemical charge–discharge cycling method was carried out at room temperature to measure the reversible hydrogen capacity in pressed electrodes containing mixture of catalyst, nanostructured carbon samples and carbonaceous particles. Results showed that the abundance ratio of well-crystallized graphite to amorphous carbon in each product increases with increasing reaction temperatures. This preliminary study showed also that the hydrogen storage capacity of synthesis products measured in an electrochemical half-cell at room temperature correlates with the nanostructure and morphology of the variety of nanostructured carbon samples. Additionally, the hydrogen adsorption capacity against specific surface area (BET) for synthesis products produced at temperatures higher than 670°C is ranging from 14 to 25 wt.%/(1000 m²/g).

We investigated the stability of aminoborane with electron withdrawing groups such as –CF₃ and –CN using density functional theory calculations. For the electron withdrawing groups of –CF₃ and –CN, we observed similar tendency of B–N bond strength: electron withdrawal from the N atom reduces the strength of B–N bonds. And we find that the electron density localized on B and N is also reduced through the charge analysis. These electron withdrawing substituents make cyclic dimers and trimers stabilized less than electron donating ones.