Vibrational and thermodynamic properties of LiBH4 polymorphs from first-principles calculations

The study of phonons describes the thermodynamic properties behavior of compounds with small atoms because phonons have an important influence on its properties. Lithium borohydride, LiBH₄, is one of the suitable materials for hydrogen storage solid state. Although the transformations of Lithium borohydride LiBH₄ were repeatedly studied by experiments and fundamental side, these transformations are still under discussion. In the present work, the mode vibrational analysis of orthorhombic and hexagonal LiBH₄ structures were considered with ab initio lattice-dynamics based on the quasi-harmonic approximation approach as implemented in Phonopy code. The results show that the orthorhombic structure is thermodynamically stable, while the hexagonal structure is unstable owing to the presence of negative mode frequency. The thermal expansion behavior and various thermodynamic properties stability like heat capacity, entropy and Helmholtz energy were also studied and the obtained results are in good agreement with experiments. This shows a deep connection between stability and strength and helps researchers to estimate accurately the thermodynamic performance of LiBH₄ materials.     

Theoretical study of ZrCoH3 and the anti-disproportionation ability of alloying elements

First-principles calculations were performed to investigate the bonding characteristics of the undoped and doped ZrCoH₃ and predict the anti-disproportionation abilities of alloying elements (i.e. Y, V, Nb, Ta, Zr, Cr, Mn, Ru, Rh, Pd and Zn). The binding between H and Zr (or its substitute elements) shows strong ionic and weak covalent feature, and that between H and Co (or its substitute elements) displays the opposite characteristic. The H diffusion process, the size of the 8e site and the corresponding ZrH distance were calculated, and the substitute elements of Zr (i.e. Nb, Ta and especially V) are predicted to be helpful to improve the ZrCo alloy against the hydrogen-induced disproportionation, and the substitute elements of Co may be not suitable.     

Hydrogen solid storage: First-principles study of ZrNiH3

The ZrNiH₃ compound is a good candidate for hydrogen storage. In this work we used the first-principles calculation to study this compound. The crystal structures, the electronic properties and the optimization of the internal parameters are treated by the FP-LAPW method implanted in the WIEN2K code. The enthalpies of the dehydrogenation of the ZrNiH₃ compound are calculated. We found that the enthalpy is about −42.89 kJ/mol H, greater but similar to the experimental value of −34.3 kJ/mol H. Potential reasons for this discrepancy are discussed.     

Structural,electronic and optical properties of titania nanotubes

Abstract

This review paper describes primarily recent theoretical calculations with some supporting experimental findings on titania nanotubes. Nanotubes with different types and sizes are discussed in detail in terms of existing theoretical and experimental achievements. Both classical and quantum mechanical simulations are focused on. The properties of these nanotubes have been treated within first principle density functional electronic structure simulation methods. In this paper, we pay particular attention to computational aspects, but when appropriate, relationships with experimental results on titania nanostructures will be mentioned. First, the structural properties of titania nanotubes are reviewed, focusing from experimental growth mechanism to possible theoretical stable structure and orientation. Second, the electronic structure of nanotubes is discussed in terms of band gap modifications of titania and photocatalytic efficiencies in photoelectrochemical devices. Finally, current computational limitations and future directions are described with respect to the performances of nanotube titania based photosensitive devices.     

The destabilising effect of alkali metal (Na and K) of hydrazine-borane N2H4BH3 for hydrogen storage: Ab-initio study

In this research work, we have investigated the crystal structure, thermodynamic and electronic properties to elucidate the chemical bonding features of N₂H₄BH₃ and MN₂H₃BH₃ (M = Na and K), using both plane waves and pseudopotentiels methods applied in density functional theory. The as-optimized crystal structures are found to be in a good agreement with the experimental. The bond lengths of these materials have been compared to show the effect of alkali metal (Na and K) in N₂H₄BH₃. The density of states (DOS) indicated that these materials are considered as an insulator with wide band-gap: 5.78, 4.35 and 4.36 eV for N₂H₄BH₃, NaN₂H₃BH₃ and KN₂H₃BH₃, respectively. This indicated that the substitution of one hydrogen atom from N₂H₄BH₃ by alkali atom (Na or K) could reduce the band-gap. The partial DOS, Hirshfeld method and charge density distribution have been used to understand the nature of the chemical bond. It is found that the chemical bonding in each group namely [N₂H₄] and [BH₃] for N₂H₄BH₃ or [N₂H₃] and [BH₃] for MN₂H₃BH₃ (M = Na and K)) have mainly a covalent nature. While, an ionic character between M (Na and K) and the rest of N₂H₃BH₃ group is suggested for MN₂H₃BH₃ (M = Na and K). Interestingly, both N-H and B-H bonds are destabilized in MN₂H₃BH₃ (M = Na and K) with a less protic character of [BH₃] than that of N₂H₄BH₃. Finally, it is observed that each group [BH₃] is bonded covalently to another [N₂H₃] for MN₂H₃BH₃ (M = Na and K) similarly to N₂H₄BH₃ (covalent bonding between [BH₃] and [N₂H₄]). The standard enthalpies of formation of KN₂H₃BH₃ and NaN₂H₃BH₃ are calculated for the first time in this research work.     

The first-principles investigation on the electronic structure and mechanism of LiH + NH3 → LiNH2 + H2 reaction

First-principle density functional theory calculations were used to investigate the electronic structure and mechanism of the LiH + NH₃ → LiNH₂ + H₂ reaction. Along the reaction pathway, intermediate complexes HLi…NH₃ and LiNH₂…H₂ and a transition state can be found. The N-2p electron in the highest occupied molecular orbital (HOMO) of NH₃ transfers to the Li-2s orbital in lowest unoccupied molecular orbital (LUMO) of LiH and forms the initial state HLi…NH₃. In the transition state, H1 of LiH and H2 of NH₃ turn toward each other, resulting in the formation of a H₂ bond. From the transition state to the final state, the geometric configuration changes from C_s to C_2v, and the improvement of geometric configuration symmetry results in a decrease in the energy gap between HOMO and LUMO. The LiH + NH₃ → LiNH₂ + H₂ reaction is exothermic.     

Electronic structure,defect properties,and hydrogen storage capacity of 2H-WS2: A first-principles study

First-principles calculations based on density functional theory (DFT) have been performed for layered 2H tungsten disulphide (WS₂) with the aim to find its possible applications in opto-electronic and hydrogen storage devices. To study opto-electronic features of WS₂, we solve the Bethe-Salpeter-Equation (BSE) on top of the standard self consistent GW quasiparticle (QP) calculations. These calculations capture the excitonic effects which originate near the edge of conduction band, shows good agreement with the available measured data. The suitability of WS₂ as a prospective material for hydrogen storage were predicted by using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. We have explored the effect of interstitial hydrogen and H₂ molecules insertion on the structural stability of WS₂ in detail. The hydrogen atom charge states dependent stability was studied in the context of formation energy. Our calculations suggest that interstitial hydrogen can act as a deep donor whereas H₂ molecule exhibits more stability. The diffusion energy of H₂ molecule from one hollow site to the nearest in-plane hollow site has been calculated using transition state theory. Finally, ab?initio molecular-dynamics (AIMD) calculations are carried out for WS₂ consisting of 16H₂ molecules that ensures its structural stability at temperatures 300, 500, and 1000 K. Present predictions show that this material may be utilize for hydrogen storage due to expected high hydrogen density.     

Structural investigation and hydrogen storage properties of Ca3-xLaxMg2Ni13 alloys

The phase structures and hydrogen storage properties of the Ca_3-xLa_xMg₂Ni₁₃ alloys were investigated. It was found that the La substitution is unfavorable for the formation of the Ca₃Mg₂Ni₁₃-type phase. The La-substituted alloys consist of multiple phases. Increasing La content to x = 2.25 leads to a disappearance of Ca₃Mg₂Ni₁₃-type phase. Among these alloys, the Ca_1.5La_1.5Mg₂Ni₁₃ alloy has highest equilibrium pressures of hydrogen absorption–desorption and a highest hydrogen desorption capacity of 1.34 wt.% at 318 K. The discharge capacity decreases for La-substituted alloys. However, the cycling capacity retention rate (S₃₀) increases from 13.7 to 67.6% when x increases from 0 to 3.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

Structural and reaction pathway analyses of Mg(BH4)2·2NH3 for hydrogen storage : A first-principles study

Mg(BH₄)₂·2NH₃ is a relatively new compound considered for hydrogen storage. The fundamental properties of the compound were comprehensively studied using first-principles calculations, such as crystal structure and electronic structure, reaction Gibbs free energy and possible reaction pathway. The calculated crystal structure is in good agreement with the experimental and other theoretical results. Results from electronic density of states (DOS) and electron localization function (ELF) show the covalent characteristics of the N–H and the B–H bonds, and the weak ionic interactions between the Mg atom and the NH₃ ligands or the (BH₄)⁻ ligands. The reaction Gibbs free energies of several possible decomposition reactions were calculated between 0 and 700 K. All the reactions are exothermic. The most likely reaction pathway of the dehydrogenation reaction was clarified to show five distinct steps.     

Ab initio study of effects of Al and Y co-doping on destabilizing of MgH2

First-principles calculations based on density functional theory (DFT) were performed to study the destabilizing mechanism of co-doped MgH₂ with Al and Y. From the minimization of total electronic energy, the preferential positions of dopants are determined. The calculated formation enthalpy and substitution enthalpy show that incorporation of Al combined with Y atoms into MgH₂ is energetically favorable relative to Al doping alone. Due to strong interaction of the dopant Y with Mg and Al, the hydrogen dissociation energy and the dehydrogenation enthalpy are both reduced, indicating that the synergetic effect of Al and Y on destabilizing the MgH₂ is superior to that of Al doping. The electronic structures show that the breakage of Mg–H bond is much easier in co-doped case, because of the conduction band shift below the Fermi level and the hybridization of dopants with Mg atoms, which effectively decrease the hybridization between Mg and H.     

Mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li2TiO3

Li₂TiO₃ is one of the most promising candidates among solid breeder materials. However, defects introduced into Li₂TiO₃ will act as the strong trapping sites for tritium. In the present study, mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li₂TiO₃ were investigated by means of X-ray diffraction, Raman spectroscopy, electron spin resonance and thermal desorption spectroscopy. The color of samples becomes dark blue and the defects were found to be introduced into Li₂TiO₃ when annealed in vacuum. This color change suggests the change from Ti⁴⁺ to Ti³⁺ due to decrease in oxygen content. The color recovers to white again after annealing in air. X-ray diffraction and Raman spectroscopy results indicate that there are no modifications on Li₂TiO₃ crystal phases, but on crystallinity. The main vacuum-annealing defects are E-centers and no other obvious types of defects were observed from electron spin resonance. Based on the experimental results, the production of defects by annealing in vacuum should be satisfied to the following conditions: (1) Li₂TiO₃ has been exposed in air more than 1 day; (2) Li₂TiO₃ must be annealed at the temperature higher than 300 °C; (3) Li₂TiO₃ should be annealed in vacuum lower than 10 Pa. E-centers formed under vacuum-annealing processes have considerable effects on release behavior of hydrogen isotopes investigated by thermal desorption spectroscopy and further should be considered in future fusion reactor. The present work gives some suggestions for future fusion reactors: (1) Li₂TiO₃ should be preserved in vacuum or kept from water vapor; (2) Li₂TiO₃ should be annealed at high temperature to remove the adsorbed water before loading into the facility, and must be finished within two days to avoid defects coming from reduction; (3) Li₂TiO₃ should be improved by adding more oxygen or other elements to refrain from defects introduced by reduction reaction.     

A study of the ZrF4, NbF5, TaF5, and TiCl3 influences on the MgH2 sorption properties

Magnesium hydride with 7 wt.% of various metal halide additives (ZrF₄, TaF₅, NbF₅ and TiCl₃) were ball milled, and the influence of these dopants on the kinetics of absorption and desorption was studied. The pressure-composition-temperature isotherms (P-C-T) measured by Sieverts’ apparatus did not show thermodynamic changes in the studied materials. Moreover, XPS studies demonstrated that the metal halides used in this study (except ZrF₄) took part in the partial and full disproportionation reactions directly after milling and the first desorption/absorption cycle. The catalytic effect of metal halides on the Mg hydrogenation/dehydrogenation process was caused by the formation of pure transition metal and/or the MgF₂ phase, which led to the influence of two simultaneous factors on the sorption properties of the MgH₂.     

Semiconducting ground-state of three polymorphs of Mg2NiH4 from first-principles calculations

Magnesium nickel hydrides (Mg₂NiH₄) are the prospective candidates for hydrogen storage and switchable mirror. The hydrides exist in two typical crystallographic forms, the low temperature (LT) phase in monoclinic structure, and the high temperature (HT) phase in cubic structure. LT has two modifications–untwinned (LT1) and microtwinned (LT2) structures. The electronic structures of the three polymorphs of Mg₂NiH₄ are investigated using ab initio calculations based on density functional theory. The calculated band gaps of LT1 and HT are in reasonable agreement with experimental observations and other theoretical predications, while the calculated band gap of LT2 is slightly lower than those of LT1 and HT. Electronic-structure analysis shows that strong interactions exist between Ni and H, whereas the interactions between Mg and H are negligible. The strong ionic character between Mg and NiH₄ complex can be viewed as the origin of the semiconducting ground-state.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

Hydrogen storage on calcium-decorated BC7 sheet: A first-principles study

The adsorption behavior of hydrogen molecules on the calcium-decorated BC₇ sheet has been investigated using first-principles calculations. Our calculations demonstrate that the van der Waals interactions are crucial for the hydrogen storage in the calcium-decorated BC₇ sheet. We find that the average adsorption energy per hydrogen molecules decreases with the number of adsorbed hydrogen molecule increasing. When six hydrogen molecules adsorb, the average adsorption energy is 0.26 eV. In this case, the gravimetric density for hydrogen storage on two sides of calcium-decorated BC₇ sheet is about 4.96 wt%. These features indicate that the calcium-decorated BC₇ sheet has potential application in hydrogen storage.     

Pt4, Pd4, Ni4, and Ti4 catalyzed hydrogen spillover on penta-graphene for hydrogen storage: The first-principles and kinetic Monte Carlo study

Penta-graphene is a new 2D allotrope of carbon exclusively consists of pentagons in a planar sheet geometry. In this work, we explored that if it can be a substrate for hydrogen spillover. The density-functional theory (DFT) studies show that the H atom can stably adsorb on sp² carbons. The saturation hydrogen storage density of penta-graphene is estimated to be 5.3 wt%. The Pt₄, Pd₄ Ni₄, and Ti₄ clusters are used as the catalyst for hydrogen spillover, and the migration barriers are 1.25, 1.07, 1.03 and 1.35 eV, respectively. The kinetic Monte Carlo simulations are performed to study the migration process for massive H atoms. The results show that the optimal reaction temperatures are 467, 405, 390, and 504 K for Pt₄, Pd₄, Ni₄, and Ti₄ catalyst, respectively. For Pd₄ and Ni₄ catalysts, the spillover reaction can occur at the appropriate temperature (355 and 340 K, respectively) for onboard hydrogen storage systems applied to light-duty vehicles.     

Electrochemical PCT isotherm study of hydrogen absorption/desorption in AB5 type intermetallic compounds

The enthalpy (ΔH) and entropy (ΔS) of hydride formation/decomposition could be determined either experimentally or theoretically based on models proposed in the literature. The experimental pathway includes gas/solid-phase measurement of pressure–composition–temperature (PCT) isotherms at different temperatures. This measurement is followed by plotting of van't Hoff dependences and evaluation of the ΔH and ΔS from their slopes and intersects, respectively. In this study we demonstrate the applicability of electrochemical PCT isotherm measurements as an advanced method for thermodynamic analysis of hydrogen adsorption/desorption process. Experimentally this is done by electrochemical charging/discharging of an electrode, prepared from AB₅ type alloy with MmNi_4.6Co_0.6Al_0.8 composition (Mm – mischmetal). In addition, the hydride formation as a result of the electrochemical charging is independently confirmed by ex-situ XRD diffraction. Our work demonstrates that not only the electrochemical approach is a viable alternative of PCT gas/solid-phase measurement but it also represents a safer, cost-effective and faster protocol than its hydrogen gas–solid phase equivalent.     

Solid solubility of Mg in Ca2Ni7 and hydrogen storage properties of (Ca2−xMgx)Ni7 alloys

The phase relations and hydrogen storage properties of the (Ca_2−xMg_x)Ni₇ alloys were investigated. It was found that the maximum solid solubility of Mg in the (Ca,Mg)₂Ni₇ phase is about x = 0.5 in the present study. The ‘inter-block-layer’ type stacking faults exist in the (Ca,Mg)₂Ni₇ phase when Mg content is very low. However, the density of stacking faults decreases and the lattice parameters reduce as Mg content increases to its maximum solid solubility. Thus the (Ca_1.5Mg_0.5)Ni₇ alloy has a good reversibility of hydrogen absorption–desorption.     

Effects of partial substitutions of cerium and aluminum on the hydrogenation properties of La(0.65−x)CexCa1.03Mg1.32Ni(9−y)Aly alloy

Hydrogen in metal hydrides could be one of the promising energy storage mediums to address the intermittent nature of renewable energy. To convert the hydrogen energy to electricity, the storage system has to be coupled with a fuel cells system. Hence, it is important to design a hydrogen storage system that meets the operating requirements for a fuel cell system. In this work, the effects of partial substitution of both cerium and aluminum on the hydrogenation properties of La_(0.65−x)Ce_xCa_1.03Mg_1.32Ni_(9−y)Al_y alloys were investigated simultaneously using factorial design. Both Ce and Al additions greatly improved the reversibility of hydrogen storage capacity. However, the maximum hydrogen storage capacity and absorption kinetics can be reduced by the additions. As Ce and Al gave opposite effects on the absorption and desorption plateaus, they could be used to tune the properties of the alloys to the desired operating conditions for fuel cell applications.