First-principle investigation for the hydrogen storage properties of NaXH3 (X= Mn,Fe, Co) perovskite type hydrides

In the present study, NaXH₃ (X = Mn, Fe, Co) perovskite type hydrides have been investigated by performing first-principles calculation. The results of the structural optimizations show that all these compounds have negative formation energy implying the thermodynamic stability and synthesisability. The mechanical stability of these compounds has been studied with the elastic constants. Moreover, the polycrystalline properties like bulk modulus, Poisson's ratio, etc. have been obtained using calculated elastic constants of interest compounds. The electronic properties have been studied and band structures have been drawn with the corresponding partial density of states. These plots indicated that NaXH₃ hydrides show metallic characteristics. The charge transfer characteristics in these compounds have been studied with the Bader partial charge analysis. The phonon dispersion curves and corresponding density of states indicated that NaXH₃ compounds are dynamically stable compounds. The investigation on hydrogen storage characteristics of NaXH₃ compounds resulted in hydrogen storage capacities of 3.74, 3.70 and 3.57 wt% for X = Mn, Fe and Co, respectively. The present study is the first investigation of NaXH₃ perovskite type hydrides as known up to date and may provide remarkable contribution to the future researches in hydrogen storage applications.     

MgTiO3Hx and CaTiO3Hx perovskite compounds for hydrogen storage applications

The first principle calculations are used to investigate hydrogen storage properties of MgTiO₃H_x and CaTiO₃H_x (x = 0, 3, 6, and 8) perovskite compounds in cubic phase (Pm3 m). In order to examine the stability of these compounds, formation enthalpies are calculated and all compounds (except MgTiO₃H₆ and MgTiO₃H₈) are found to be stable. The second order elastic constants and related polycrystalline elastic moduli (e.g., shear modulus, Young's modulus, Poisson's ratio, Debye temperature, sound velocities) are determined and the results are discussed in detail. The mechanical stability determination indicates that MgTiO₃, CaTiO₃, and CaTiO₃H₆ compounds are only stable compounds and also MgTiO₃ and CaTiO₃H₆ are ductile while CaTiO₃ is a brittle material. Also, the mechanical anisotropy is discussed via two-dimensional (2D) and three-dimensional (3D) surfaces for mechanical stable compounds and they are found to have anisotropic behaviour (except linear compressibility for MgTiO₃, CaTiO₃). Electronic band structure and corresponding partial density of states (PDOS) and charge density have been plotted. Bader charge analysis have been done. MgTiO₃ has metallic behaviour whereas CaTiO₃ and CaTiO₃H₆ have semiconductor behaviour. Among all compounds, CaTiO₃H₆ is found to be only one that could be used in the hydrogen storage applications. For this compound, the gravimetric hydrogen storage capacity is calculated as 4.27 wt% and the hydrogen desorption temperature is obtained as 827.1 K.     

Geometric structures and hydrogen storage properties of M2B7 (M=Be,Mg, Ca) clusters

Based on the density functional theory, we investigate the electronic properties of the clusters M₂B₇ (M = Be, Mg, Ca) and their hydrogen storage properties systematically in this paper. Extensive global search results show that the global minimal structures of the three systems (Be₂B₇, Mg₂B₇ and Ca₂B₇) are heptagonal biconical structure, and the two alkaline earth metals are located at the top of the biconical. Chemical bonding analyses show that M₂B₇ clusters have 6σ and 6π delocalized electrons, which are doubly aromatic. At the wB97XD level, the three systems have good hydrogen storage capabilities. The hydrogen storage density of Be₂B₇ is as high as 23.03 wt%, while Mg₂B₇ and Ca₂B₇ also far exceed the hydrogen storage target set by the U.S. Department of Energy in 2017. Their average adsorption energies of H₂ molecules all ranged from 0.1 eV/H₂ to 0.48 eV/H₂, which is fall in between physisorption and chemisorption. Extensive Born Oppenheimer molecular dynamics (BOMD) simulations show that the H₂ molecules of the three systems can be completely released at a certain temperature. Therefore, M₂B₇ systems can achieve reversible adsorption of H₂ molecules at normal temperature and pressure. It can be seen that the B₇ clusters modified by alkaline earth metals may become a promising new nano-hydrogen storage material.     

Investigation of structural,electronic and lattice dynamical properties of XNiH3 (X = Li,Na and K) perovskite type hydrides and their hydrogen storage applications

XNiH₃ (X = Li, Na, and K) perovskite type hydrides have been studied by using Density Functional Theory (DFT) and these materials are found to be stable and synthesizable. The X-ray diffraction patterns have been obtained and they indicate that all materials have the polycrystalline structure. The electronic properties have been investigated and it has been found that these structures show metallic character. The Bader partial charge analysis has also been performed. In addition, the elastic constants have been calculated and these materials are found to be mechanically stable. Using these elastic constants, the mechanical properties such as bulk modulus, shear modulus, Poisson's ratio have been obtained. Moreover, the Debye temperatures and thermal conductivities have been studied. The anisotropic elastic properties have been visualized in three dimensions (3D) for Young's modulus, linear compressibility, shear modulus and Poisson's ratio as well as with the calculation of the anisotropic factors. Additionally, the dynamical stability has been investigated and obtained phonon dispersion curves show that these materials are dynamically stable. Also, the thermal properties including free energy, enthalpy, entropy and heat capacity have been studied. The hydrogen storage properties have been examined and the gravimetric hydrogen storage capacities have been calculated as 4.40 wt%, 3.57 wt% and 3.30 wt% for LiNiH₃, NaNiH₃ and KNiH₃, respectively. Furthermore, the hydrogen desorption temperatures have been obtained as 446.3 K, 419.5 K and 367.5 K for LiNiH₃, NaNiH₃ and KNiH₃, respectively.     

The effect of hydrogen on the electronic,mechanical and phonon properties of LaMgNi4 and its hydrides for hydrogen storage applications

Density functional theory calculations are used herein to explore the effect of hydrogen on the electronic, mechanical and phonon properties of LaMgNi₄ and its hydrides. The polycrystalline elastic moduli, Poisson's ratios and Debye temperatures are calculated based on the single-crystal elastic constants and Voigt-Reuss-Hill approximations. It is also found that all these materials are metallic behavior, ductile and anisotropic in nature. The mechanical anisotropy is discussed via several anisotropy indices and three-dimensional (3D) surface constructions. The effect of high temperature on the free energy, entropy, and heat capacity are also studied by using the quasi-harmonic Debye model. LaMgNi₄ and its hydrides are found to be energetically, mechanically and dynamically stable. Also, they are thermodynamically stable and the order of phase stability is LaMgNi₄H₇ > LaMgNi₄H₄ > LaMgNi₄H > LaMgNi₄. In addition, the highest gravimetric hydrogen storage capacity is found to be 1.74 wt% for LaMgNi4H₇.     

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO₃ perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO₃ perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO₃ perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO₃ perovskite has been conducted and hydrogen storage properties of BaYO₃H_x (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO₃ perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO₃ perovskite. Hence, this work could enlighten the possible future studies.     

Lithium metal hydrides (Li2CaH4 and Li2SrH4) for hydrogen storage; mechanical,electronic and optical properties

Hydrogen storage is a critical step for commercialisation of hydrogen consumed energy production. Among other storage methods, solid state storage of hydrogen attracts much attention and requires extensive research. This study rationally and systematically designs novel solid state hydrides; Li₂CaH₄ (GHD is obtained as −6.95 wt %) and Li₂SrH₄ (GHD is obtained as −3.83 wt %) using computational method. As a first step, we suggest and predict crystal structures of solid state Li₂CaH₄ and Li₂SrH₄ hydrides and look for synthesizability. Then, the mechanical stabilities of hydrides are identified using elastic constants. Both hydrides fulfil the well-known Born stability criteria, indicating that both Li₂CaH₄ and Li₂SrH₄ are mechanically stable materials. Several critical parameters, bulk modulus, shear modulus, Cauchy pressures, anisotropy factors of hydrides and bonding characteristics are obtained and evaluated. Furthermore, electronic and optical band structures of hydrides are computed. Both Li₂CaH₄ and Li₂SrH₄ have indirect bands gaps as 0.96 eV (Г-U) and 1.10 eV (Г-R). Thus, both materials are electronically semiconducting. Also, Bader charge analysis of hydrides have been carried out. Charge density distribution suggests an ionic-like (or polarized covalent) bonding interaction between the atoms.     

The reversible hydrogen storage abilities of metal Na (Li,K, Ca,Mg, Sc,Ti, Y) decorated all-boron cage B28

The density functional theory is used to study the hydrogen storage abilities of alkali metal Li (Na, K), alkaline-earth metal Mg (Ca), and transition metal Ti (Ti, Sc, Y) decorated B₂₈, which is the possible smallest all-boron cage and contains one hexagonal hole and two octagonal holes. The most stable structure of B₂₈ explored by the calypso search is as same as that explored by Zhao et al. [Nanoscale 7(2015)15086]. It is calculated that the hollow sites outside of the cavities should be the most stable for all metals except for Ti. The average adsorption energy of H₂ molecules (E_ad) adsorbed by each Na (Ca, K, Mg, Sc, Y and Li) atom outside of the B₂₈ cage are in the range from 0.2 to 0.6 eV, which is suitable for hydrogen storage under near-ambient conditions. However, the largest hydrogen gravimetric density (HGD) for the B₂₈Sc₃-12H₂ structure is smaller than the target of 5.5 wt% by the year 2017 specified by the US Department of Energy (DOE). Therefore, the metal Ti (Sc) decorated all-boron cage B₂₈ should not be good candidates for hydrogen storage. The calculated desorption temperature and the molecular dynamic simulation indicate that the B₂₈M₃-nH₂ (M = Na, Li, Ca, K, Mg, Y) structures are easy to desorb the H₂ molecules at the room temperature (T = 300 k). Furthermore, the B₂₈ cages bridged by the sp²-terminated B₅ chain can hold Na (Li, Ca, K, Mg, Y) atoms to capture hydrogen molecules with moderate E_ad and HGD. These findings suggest a new route to design hydrogen storage materials under the near-ambient conditions.     

A first-principles simulation of the metal borohydride ammonia borane complex (LiBH4)2(NH3BH3) and the decomposition reaction pathway for hydrogen storage

In this work, we predict a range of favorable functional properties of (LiBH₄)₂NH₃BH_3, a relatively new member of the boron-containing metal borohydride ammonia borane family, by means of ab initio calculations, as well as its parent compounds, LiBH₄ and NH₃BH₃, for comparisons. Both the mechanical and dynamical stabilities of this new compound have been demonstrated theoretically for the first time. Results from elastic modulus calculations show that the mechanical properties of (LiBH₄)₂NH₃BH₃ are remarkably improved compared with its parent compounds. Secondly, Electronic structure results show that it remains to an insulator with large band gap typical of the boron-containing hydrogenous family, but the band gap can be tuned by the compositions of NH₃BH₃ and LiBH₄. Charge analysis demonstrates that charge transfers in individual layers of LiBH₄ and NH₃BH₃ are similar to LiBH₄ and NH₃BH₃, respectively. A measurable amount of charge transfers from the LiBH₄ layers to the NH₃BH₃ layers result in enhanced activation properties for hydrogenation and dehydrogenation in (LiBH₄)₂NH₃BH₃. Thirdly, Free energies of six possible dehydrogenation reactions have been calculated from 0 K to 700 K, and the results show that combination of NH₃BH₃ and LiBH₄ can reduce the dehydrogenation energies compared with the parent compounds, a result consistent with recent experiments. Meanwhile, the N–B bond strengths increase and thereby borazine and diborane formation are reduced upon dehydrogenation.     

Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A = Ca,Sr and Ba)

The crystal structure of a photocatalyst generally plays a pivotal role in its electronic structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped perovskite compounds ATiO₃ (M = Ca, Sr and Ba) via a hydrothermal method. Their optical properties and photocatalytic activities were systematically explored from the viewpoint of their dependence on structural variations, i.e. impact of bond length and bond angles. Our results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance and catalytic performance are strongly governed by the TiO bond length and TiOTi bond angle. A long TiO bond and deviation of TiOTi bond angle away from 180° deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic activity belongs to Sr_0.9La_0.1Ti_0.9Cr_0.1O₃ with an average hydrogen production rate ～2.88 μmol/h under visible light illumination (λ ≥ 400 nm), corresponding to apparent quantum efficiency ～ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site.     

Elastic properties of perovskite-type hydrides LiBeH3 and NaBeH3 for hydrogen storage

Density functional theory (DFT) was used to study mechanical properties of orthorhombic and cubic LiBeH₃ and cubic NaBeH₃. The band gaps were estimated using different approximations available in the WIEN2k code. The elastic constants of LiBeH₃ and NaBeH₃ were calculated. Both hydrides were found to be stable mechanically. From elastic constants, the bulk modulus and the linear bulk modulus along crystallographic axes of single crystal were calculated. The calculated values of linear bulk moduli were in good agreement with the reported theoretical values. Young's and Shear moduli, Poisson's ratio and micro-hardness for ideal polycrystalline LiBeH₃ and NaBeH₃ were also calculated. Both these hydrides can be classified as brittle materials according to obtained results. The values of bulk, shear and Young's moduli obtained in our study were higher than those reported for other popular hydrides for hydrogen storage. Shear and elastic anisotropic factors along with Debye temperature are also discussed using theoretical elastic constants.     

Influence of Ti/Hf doping on hydrogen storage performance and mechanical properties of ZrCo compounds: A first principle study

Element doping is an effective way to improve the performance of hydrogen storage. The influences of Ti- and Hf-substituted dopants on the hydrogen storage and mechanical properties in a Zirconium-Cobalt alloy have been investigated by the first principle method. The results show that the Ti and Hf atoms preferentially occupy the Zr atoms to form new Zr₇Co₈Ti and Zr₇Co₈Hf compounds. The cohesive energy of Zr₇Co₈Ti and Zr₇Co₈Hf are ?7.518 eV/atom and ?7.531 eV/atom, thus Zr₇Co₈Hf shows a better structural stability than Zr₇Co₈Ti. The B/G ratio of Zr₇Co₈Ti and Zr₇Co₈Hf are 3.42 and 3.89, respectively, which indicates that Ti doping could increase the ductility of the alloy, thus improving the recycling performance of the alloy. The calculation for the electronic structure, mechanical property and structural stability may provide an effective way and theoretical evidence for the better design and optimization of hydrogen storage materials.     

Structural evolution and hydrogen storage performance of Mg3LaHn (n = 9–20)

The magnesium-lanthanum-hydrogen systems possess the goodish stability and high hydrogen storage capacity, which make them perspective as commercial Mg-based hydrogen storage materials. The exploration of these intriguing properties evolving from small atomic and molecule cluster to bulk phase are, to our knowledge, the longstanding challenge. Here, we perform a theoretical study on the structural and electronic properties of Mg₃LaH_n (9 ≤ n ≤ 20) clusters by using the Crystal Structure AnaLYsis by Particle Swarm Optimization method combined with density functional theory calculations. It is found that Mg₃LaH₁₅ is the most stable cluster in the series, with hydrogen storage capacity of 6.6 wt% and adsorption energy of 2.76 eV. The present results offer new insights for the design and synthesis of novel hydrogen storage materials in the future.     

Role of Mn-substitution towards the enhanced hydrogen storage performance in FeTi

The role of Mn substitution in FeTi towards the hydrogenation kinetics and hydrogen storage capacity was investigated using a combination of experimental and theoretical tools. Pristine and Mn-substituted FeTi was produced by electro-deoxidation of oxide precursors, such as natural ilmenite, titania and manganese dioxide. The produced materials were evaluated for hydrogen storage capacity. Ab-initio density functional theory (DFT) calculations were performed to understand the thermodynamics and kinetics of hydrogen absorption in pristine and doped FeTi. DFT calculations demonstrate that although thermodynamic and kinetic parameters of hydrogen absorption with Mn substitution is similar to those in the pristine FeTi, oxygen affinity of Mn at the surface is higher than Fe or Ti. We conclude that Mn acts as a sacrificial oxidizing element and oxidizes more readily at the surface over Fe or Ti, resulting in easy activation of the FeTi alloy. We show superior cyclic-hydrogen absorption behavior in bulk in FeTi with Mn substitution. After 20 charge-discharge cycles, the measured hydrogen storage capacity of FeTi with Mn substitution was steady ～120 mA h/g (0.8 wt %), which is noticeably higher than that of pristine FeTi. The experimental and theoretical results shows that in case of a practical hydrogen storage scenario, Mn substitution will benefit in reducing absorption fatigue in FeTi. Further, most likely it may not be possible to use pristine FeTi phase.     

Enhancement of hydrogen storage properties of metal-organic framework-5 by substitution (Zn,Cd and Mg) and decoration (Li,Be and Na)

Two strategies of decoration by three elements Z = Li, Be and Na in cyclic site, and substitution of Zn by Mg and Cd in unit cell were used in the framework of functional density theory to tune the hydrogen storage properties of metal-organic framework-5 (MOF-5) based on Zn whose decomposition temperature and initial gravimetric capacity are 300 K and 1.57 wt% respectively.Based on the adsorption of hydrogen molecules in the crystal surface at three different adsorption sites with three orientations of H₂, we show that our system may reach a maximum gravimetric storage capacity of 4.09 wt% for multiple hydrogen molecules. Moreover, the functionalization of Z combined to the substitution, shows an exceptional improvement of hydrogen storage properties. For example, Mg-MOF-5 decorated with Li₂ has a capacity up to 5.41 wt% and 513 K as desorption temperature.     

Effects of NH4+ doping on the hydrogen storage properties of metal hydrides

Doping can modify the properties of metal hydrogen storage materials significantly. Currently, the metal doping is a frequent strategy, while the non-metal cation doping has not been examined extensively so far. In this study, the effects of NH₄⁺ doping on the hydrogen storage properties of different metal hydrides, including TiH₂, Ti_0·25V_0·25Nb_0·25Zr_0·25H₂, Ti_0·5V_0·5H₂ and VH₂, are investigated by first-principles calculations. It is found that the NH₄⁺ presents a good affinity for metal hydrides and the NH₄⁺ incorporation leads to charge redistribution and formation of dihydrogen bond. Furthermore, the NH₄⁺ doping in metal hydrides is favorable for enhancing the hydrogen storage capacity and decreasing the thermal stability simultaneously. The possible reason for the NH₄⁺ doping induced destabilization in metal hydrides is the relatively weak interaction between NH₄⁺ and hydrogen atoms.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

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.     

Hydrogen sorption properties of M0.2Ca0.8MgH4 (M = Li,Na)

The synthesis, thermodynamic destabilisation and hydrogen sorption properties of the M_0.2Ca_0.8MgH₄ hydride system, where (M = Na or Li) have been investigated. Samples were mechanically milled under argon for 5, 10 and 15 h; then characterised by X-ray diffraction (in/ex-situ), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) coupled with a mass spectrometer (MS). On Li and Na substitution into Ca–Mg–H ternary hydride, diffraction peaks shifted to higher angles. 2 and 3 endothermic reactions were observed for the Li_0.2Ca_0.8MgH₄ and Na_0.2Ca_0.8MgH₄. The maximum amount of hydrogen release was achieved for the 5 h milled Na_0.2Ca_0.8MgH₄ hydride from 298 °C to 386 °C accounting for 3.5 wt % of H₂.     

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi₅-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi₅ phase. Secondary phases, including Ca₂Ni₇ and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi₅ phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi₅. Fairly flat plateau regions are retained for all compositions except the CaNi_4.8Mo_0.2 composition where a Ca₂Ni₇ phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi_4.9Mo_0.1 alloy. CaNi_4.9Al_0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi_4.8Mo_0.2 composition.