Ab initio calculations of MgH2, MgH2:Ti and MgH2:Co compounds

The understanding of hydrogen bonding in magnesium and magnesium based alloys is an important step toward its prospective use. In the present study, a density functional theory (DFT) based, full-potential augmented plane waves method of calculation, extended with local orbitals (FP-APW+lo), was used to investigate the stability of MgH₂ and MgH₂:TM (TM = Ti and Co) 10 wt % alloys and the influence of this alloying on hydrogen storage properties of MgH₂ compound. Effects of a possible spin polarisation induced in the system by transition metal (TM) ions were considered too. It has been found that TM-H bonding is stronger than the Mg–H bond, but at the same time it weakens other bonds in the second and third coordination around a TM atom, which leads to overall destabilization of the MgH₂ compound. Due to a higher number of d-electrons, this effect is more pronounced for Co alloying, where in addition, the spin polarisation has a noticeable and stabilising influence on the compound structure.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Influence of the method of synthesis on hydrogen adsorption properties of mesoporous binary B2O3/Al2O3 gel systems

Three series of binary oxide systems B₂O₃/Al₂O₃ were prepared and the effect of alumina on dispersion of boron (B₂O₃) component was investigated. The aim of the study was to achieve a maximum dispersion of B₂O₃ in the Al₂O₃ a gel matrix that would lead to increased sorption capacity on boron oxide. Many attempts were made to establish the preparation conditions that would lead to a maximum dispersion of B₂O₃ in the Al₂O₃ gel matrix needed to increase the hydrogen sorption capacity on boron oxide. The systems were characterized by X-ray diffraction, SEM, TEM and low temperature nitrogen adsorption. Hydrogen adsorption was tested in the volumetric system.Results of the study showed that the amount of hydrogen adsorbed on B₂O₃ depended not only on the surface area of the system but also on the separation of B₂O₃ domains in Al₂O₃ gel network. Irrespective of the method of synthesis of the binary oxide system, the dispersion of B₂O₃ phase reflected in the amount of hydrogen adsorbed was the highest for the systems of the lowest B/Al molar ratios studied, i.e. for B/Al = 0.25.     

Theoretical investigation of adsorption and dissociation of H2 on (ZrO2)n (n=1-6) clusters

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_nO_2nH₂ (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_nO_2nH₂ 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.     

Intrinsic mechanisms on enhancement of hydrogen desorption from MgH2 by (001) surface doping

A first principle study was carried out to investigate the dehydrogenation properties of metal (001) surface doped MgH₂. Site preference of dopants was identified and dehydrogenation properties of the doped systems were analyzed based on the total energy and electronic structure calculations. It was shown that Al and Ti prefer to substitute for Mg atoms, whereas Mn and Ni prefer to occupy interstitial sites. The mechanisms that dopants improve the dehydrogenation properties of the considered systems were clarified. Al weakens the interactions between the Mg and the H atoms and has high potential to drive a formation of an Al-Mg cluster, and therefore improves the dehydrogenation performance of the Al doped system. Ti strongly interacts with its neighboring H atoms, distorts their positions, and could potentially generate a TiH₂ phase by attracting two H atoms. Mn greatly distorts the surface structure and causes a dramatic reduction on the dehydrogenation energy in the Mn interstitially doped system. A Ni-H tetrahedral cluster is observed, which acts as a seed to form Mg₂NiH₄ phase, in the Ni doped MgH₂ (001) surface. Therefore, the improvement of the dehydrogenation properties of Ni doped system is expectable due to the formation of thermodynamically less stable Mg₂NiH₄ phase.     

Atomistic study of LaNbO4; surface properties and hydrogen adsorption

We have calculated fundamental properties of pure and hydrogen-covered (010), (101), (100) and (001) surfaces of the low temperature monoclinic phase of LaNbO₄ (LN). The (010) surface was the most stable one, exhibiting electronic structure and local geometric configurations similar to bulk. As the first stage of proton migration into the electrolyte, the ability of LN surfaces to split H₂ molecules was probed indirectly by calculating the adsorption energy of H atoms on two of the LN surfaces. H adsorption on the (010) surface was found to be strongly endothermic, and thus cannot contribute much in splitting H₂. The adsorption energy on the relatively unstable (101) surface was on the other hand approximately −0.6 eV, in the right range for surface H₂ to be catalyzed beneficially. H adsorption on this surface was induced by surface states in the band gap of the clean surface. Since the unstable (101) surface is not abundant, the rate of dissociative adsorption of H₂ on the LN surface can be anticipated to be very low. Application of the energies to simple adsorption isotherm calculations for typical proton conducting fuel cells (PCFCs) operating temperatures correspondingly showed very low H coverage, and it is not expected that LaNbO₄ surfaces can contribute much to the H₂ activation reaction of a PCFC anode.     

Hydrogen storage properties of the pseudo binary laves phase (Sc1−xZrx)(Co1−yNiy)2 system

The (Sc_1−xZr_x)(Co_1−yNi_y)₂-H_z system has been studied using both experimental techniques and ab initio calculations. The material was synthesised through high temperature synthesis and characterised using powder XRD. Hydrogen absorption and desorption was studied in-situ using synchrotron radiation. Maximal storage capacity increased when Co replaced Ni and substitution of Sc for Zr increased the equilibrium pressure. Density functional based calculations reproduce the experimental trends in terms of cell parameters both for the non-hydrogenated systems as well as for the hydrogenated systems, and helped to quantitatively understand the observed hydrogen uptake properties.     

Ab initio study of hydrogen adsorption on Zn2(NDC)2(diPyTz) metal-organic framework decorated with alkali and alkaline earth metal cations

We have studied effect of alkali and alkaline earth metal cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺) decoration on hydrogen adsorption of the organic linker of Zn₂(NDC)₂(diPyTz) by employing three cluster models: diPyTz:mLi⁺ (m = 1–4), diPyTz:mLi⁺:nH₂ (m = 0,1,2 and n = 1–5) and diPyTz:1M⁺:1H₂ (M⁺ = Na⁺, K⁺, Be²⁺, Mg²⁺) complexes, using density functional theory (DFT) and second-order Moller–Plesset perturbation theory (MP2). The calculated binding energies show that decoration of the organic linker with alkali and alkaline earth metal cations enhanced H₂ interaction with diPyTz when compared with the pristine diPyTz. The atomic charges were derived by Mulliken, ChelpG and ESP methods. Finally, the atoms in molecules theory (AIM) were also applied to get more insight into the nature of the interaction of diPyTz and Li⁺. Results of AIM analysis show that N–Li⁺ bond in diPyTz organic linker's complex appears as shared electron interaction.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

Electronic structure and stability of new FCC magnesium hydrides Mg7MH16 and Mg6MH16 (M = Ti,V, Nb): An ab initio study

MgH₂ is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH₂ with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg₇TiH_x, Mg_6.5NbH_x and Mg₆VH_x of Ca₇Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH₂. Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self-consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties.     

Hydrogen adsorption properties of Ag decorated TiO2 nanomaterials

In this work, we present the synthesis of Ag doped TiO₂ materials. The products are characterized by powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption, and hydrogen adsorption. The Ag/TiO₂ materials exhibit 3.65 times higher in hydrogen adsorption capability compared with the non-doped TiO₂ materials thank to the existence of Ti³⁺ species, which are Kubas-type hydrogen adsorption centers, and the Ag nanoparticles which provide spillover effects. We believe that this is the first time that both Kubas-type adsorption and spillover are exploited in the design of novel hydrogen storage materials.     

Effect of catalyst LaCl3 on hydrogen storage properties of lithium alanate (LiAlH4)

LiAlH₄ doped with LaCl₃ additives were studied by PCT experiment and X-ray diffraction (XRD). Doping with 1 mol% LaCl₃ resulted in a great decrease in the amount of hydrogen release associated with the first and second dehydrogenation steps. Increasing the amount of LaCl₃ from 1 to 6 mol% caused such marked changes in behavior that the total amount of hydrogen release increased first and then decreased. The study on the first rehydrogenation cycle showed that LiAlH₄ doped with 2 mol% LaCl₃ presents the greatest absorption of hydrogen of about 0.8 wt%. The results from XRD spectra analysis indicated the reversibility of Li₃AlH₆.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

An ab initio study of dissociative adsorption of H2 on FeTi surfaces

Dissociative adsorption of H₂ on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H₂ molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H₂ molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti)_L–Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti)_S–Fe, are the most stable sites for H₂ and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H₂ molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H₂ relaxation and climbing image nudged elastic band method. It is found that H₂ dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H₂ molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.     

High-capacity hydrogen release through hydrolysis of NaB3H8

NaB₃H₈ has advantages over NaBH₄ and NH₃BH₃, two most widely studied chemical hydrides for hydrogen storage via hydrolysis. NaB₃H₈ has an extraordinary high solubility in water and thus possesses a high theoretical capacity of 10.5 wt% H via hydrolysis, in contrast to 7.5 wt% for NaBH₄ and 5.1 wt% for NH₃BH₃. NaB₃H₈ is reasonably stable in water which makes it unnecessary to add corrosive NaOH as a stabilizer as the case for NaBH₄. Furthermore, hydrolysis of NaB₃H₈ can be catalyzed by a Co-based catalyst with fast kinetics that is comparable to Ru-based catalysts. Therefore, cost-effective hydrolysis of NaB₃H₈ is possible for practical applications. A high capacity of 7.4 wt% H was achieved when water was included in the materials weight.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.     

Theoretical study of hydrogen adsorption on nitrogen doped graphene decorated with palladium clusters

Density functional theory was used to study the adsorption of hydrogen on small palladium clusters (Pd_n, n = 1–4) supported on pyridine-like nitrogen doped graphene. Charge transfer and strong binding (up to four times higher than binding energy of Pd cluster supported on graphene) between graphene–nitrogen and palladium clusters prevent detachment of clusters and leads to three types of adsorption states of hydrogen. The first type is a molecular hydrogen physisorbed, the second one is an activated state of H₂ without adsorption barriers where H–H bond is relaxed and the third type is dissociated state. In dissociated cases, we found barriers below 0.56 eV. This means the process might occur spontaneously at room temperature. We also show that metal–N–vacancy complexes are very stable and involve nitrogen sp² and p_z, carbon p_z and palladium d orbitals. Besides, these systems exhibit an interesting magnetic behavior.     

The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage

Based on density functional theory, the capacities of FeC₅H₅, Fe₂C₅H₅ and one-dimensional (FeC₅H₅)_∞ nanowire as hydrogen storage media were investigated. The results show that FeC₅H₅ and Fe₂C₅H₅ can adsorb five and ten H₂ molecules, respectively, and form stable FeC₅H₅(H₂)₅ and Fe₂C₅H₅(H₂)₁₀ systems. The hydrogen storage capacities of the two systems are 7.63 wt% and 10.15 wt%, while the average adsorption energies are 0.49 and 0.73 eV/H², indicating that FeC₅H₅ and Fe₂C₅H₅ are excellent hydrogen storage media. In addition, (FeC₅H₅)_∞ nanowire can also adsorb H₂ molecules (1.62 wt%). Most importantly, the magnetic and electrical properties of the nanowire are sensitive to the additional H₂, thus (FeC₅H₅)_∞ can be used for selecting and detecting H₂ molecules.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.