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.     

Ab initio and periodic DFT investigation of hydrogen storage on light metal-decorated MOF-5

The effect of light metal (M = Li, Be, Mg, and Al) decoration on the stability of metal organic framework MOF-5 and its hydrogen adsorption is investigated by ab initio and periodic density functional theory (DFT) calculations by employing models of the form BDC:M₂:nH₂ and MOF-5:M₂:nH₂, where BDC stands for the benzenedicarboxylate organic linker and MOF-5 represents the primitive unit cell. The suitability of the periodic DFT method employing the GGA-PBE functional is tested against MP2/6-311 + G* and MP2/cc-pVTZ molecular calculations. A correlation between the charge transfer and interaction energies is revealed. The metal-MOF-5 interactions are analyzed using the frontier molecular orbital approach. Difference charge density plots show that H₂ molecules get polarized due to the charge generated on the metal atom adsorbed over the BDC linker, resulting in electrostatic guest-host interactions.Our solid state results show that amongst the four metal atoms, Mg and Be decoration does not stabilize the MOF-5 to any significant extent. Li and Al decoration strengthened the H₂-MOF-5 interactions relative to the pure MOF-5 exhibited by the enhanced binding energies. The hydrogen binding energies for the Li- and Al-decorated MOF-5 were found to be sensible for allowing reversible hydrogen storage at ambient temperatures. A high hydrogen uptake of 4.3 wt.% and 3.9 wt.% is also predicted for the Li- and Al-decorated MOF-5, respectively.     

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.     

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.     

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B2H4Li2) and diboryne (B2H2Li2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B₂H₂²⁻ and tetrahydrodiborate dianion, B₂H₄²⁻ which can be considered to be analogues and isoelectronic to acetylene (C₂H₂) and ethelene (C₂H₄) respectively. It is shown that there exists a B-B double bond in B₂H₄Li₂ and a B-B triple bond in B₂H₂Li₂. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B₂H₄Li₂ and ∼24 wt% in B₂H₂Li₂. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C₆H₄B₂Li₂ as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.     

Effects of REF3 (RE = Y,La, Ce) additives on dehydrogenation properties of LiAlH4

The catalytic effects of rare earth fluoride REF₃ (RE = Y, La, Ce) additives on the dehydrogenation properties of LiAlH₄ were carefully investigated in the present work. The results showed that the dehydrogenation behaviors of LiAlH₄ were significantly altered by the addition of 5 mol% REF₃ through ball milling. The destabilization ability of these catalysts on LiAlH₄ has the order: CeF₃>LaF₃>YF₃. For instance, the temperature programmed desorption (TPD) analyses showed that the onset dehydrogenation temperature of CeF₃ doped LiAlH₄ was sharply reduced by 90 °C compared to that of pristine LiAlH₄. Based on differential scanning calorimetry (DSC) analyses, the dehydriding activation energies of the CeF₃ doped LiAlH₄ sample were 40.9 kJ/mol H₂ and 77.2 kJ/mol H₂ for the first and second dehydrogenation stages, respectively, which decreased about 40.0 kJ/mol H₂ and 60.3 kJ/mol H₂ compared with those of pure LiAlH₄. In addition, the sample doped with CeF₃ showed the fastest dehydrogenation rate among the REF₃ doped LiAlH₄ samples at both 125 °C and 150 °C during the isothermal desorption. The phase changes in REF₃ doped LiAlH₄ samples during ball milling and dehydrogenation were examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF₃ were proposed.     

C2H2M (M = Ti,Li) complex: A possible hydrogen storage material

The hydrogen storage capacity of Ti-acetylene (C₂H₂Ti) and Li-acetylene (C₂H₂Li) complex has been tested using second order Møller Plesset method with different basis sets. Single Ti(Li) decorated acetylene complex can adsorb maximum of five(four) hydrogen molecules, which corresponds to the gravimetric hydrogen storage capacity of 12(19.65) wt % and it meets the target of 9 wt % by 2015 specified by US Department of Energy. The hydrogen adsorption energies with zero point energy and Gibbs free energy correction show that hydrogen adsorption on C₂H₂Ti is energetically favourable for a wide range of temperature and that is unfavourable on C₂H₂Li complex even at a very low temperature. Atom centered density matrix propagation molecular dynamics simulations reveal that four H₂ molecules remain adsorbed on C₂H₂Ti complex at 300 K. Though H₂ uptake capacity of C₂H₂Li complex is higher than that of C₂H₂Ti complex, the thermochemistry results favour to C₂H₂Ti complex over C₂H₂Li complex as a possible hydrogen storage media.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

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.     

Development of nanostructured Mg2Ni alloys for hydrogen storage applications

The Mechanically Activated Self-propagating High temperature Synthesis (MASHS) has been employed to obtain nanostructured Mg₂Ni alloys. MASHS process has been further improved by controlling the electrical parameter measurements during the combustion reaction under the thermal explosion mode. The samples were hydrogenated at 20  bar and 300 °C by means of a Pressflow Gas Controller while the dehydrogenation was conducted in a Differential Scanning Calorimetry (DSC) equipped with an H₂ detector of the purged gas. Nanostructured Mg₂Ni demonstrated hydrogen storage capacity around 3.5 wt%. The desorption temperature was about 250 °C at 3 °C/min. The activation energy for dehydrogenation, calculated by the Kissinger method, was about 100  kJ/mol.     

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.     

Effect of Al content on structure and electrochemical properties of LaNi4.4 − xCo0.3Mn0.3Alx hydrogen storage alloys

The structure and electrochemical properties of LaNi_4.4 − xCo_0.3Mn_0.3Al_x hydrogen storage alloys have been investigated by XRD and simulated battery test, including maximum capacity, cyclic stability, self-discharge, high-rate dischargeability (HRD). Samples A, B, C and D were used to represent alloys LaNi_4.4Co_0.3Mn_0.3Al, LaNi_4.3Co_0.3Mn_0.3Al_0.1, LaNi_4.2Co_0.3Mn_0.3Al_0.2 and LaNi_4.1Co_0.3Mn_0.3Al_0.3 respectively. The results indicated that as-prepared LaNi_4.4 − xCo_0.3Mn_0.3Al_x alloys are all single-phase alloys with hexagonal CaCu₅ type structure. The maximum discharge capacity is 330.4 mAh g⁻¹ (Alloy C). With the increase of Al content from A to D, cycle life of alloy electrode has been improved. Higher capacity retention of 89.29% (after 50 charge/discharge cycles) has been observed for electrode D, while with a smaller capacity loss of 12.5% in its self-discharge test. Better high-rate charge/discharge behaviors have been observed in electrode B, and the maximum data is 54.7% at charge current of 900 mA/g) and 68.54% at discharge current of 1800 mA/g). Furthermore, the electrochemical impedance spectroscopy (EIS) analysis shown that the reaction of alloy electrode is controlled by charge-transfer step. The addition of Al results in the formation of protective layer of aluminum oxides on the surface of the alloy electrode, which is good for the improvement of electrode properties in alkaline solution and is detrimental for the charge-transfer process. Therefore, a suitable addition of Al is needed to improve its electrode properties.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.     

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.     

High-capacity hydrogen storage in Li-decorated (AlN)n (n = 12, 24, 36) nanocages

The capability of Li-decorated (AlN)_n (n = 12, 24, 36) nanocages for hydrogen storage has been studied by using density functional theory (DFT) with the generalized gradient approximation (GGA). It is found that each Al atom is capable of binding one H₂ molecule up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, and 0.144 eV/H₂ in the pristine (AlN)_n (n = 12, 24, 36) nanocages, respectively. Further, we find that Li atoms can be preferentially decorated on the top of N atoms in (AlN)_n (n = 12, 24, 36) nanocages without clustering, and up to two H₂ molecules can bind to each Li atom with an average binding energy of 0.145, 0.154, 0.102 eV/H₂ in the Li_n(AlN)_n (n = 12, 24, 36) nanocages, respectively. Both the polarization of the H₂ molecules and the hybridization of the Li-2p orbitals with the H-s orbitals contribute to the H₂ adsorption on the Li atoms. Thus, the Li-decorated (AlN)_n (n = 12, 24, 36) nanocages can store hydrogen up to 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of H₂ molecules lying in the range of 0.1–0.2 eV/H₂ are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. It is also pointed out that when allowed to interact with each other, the agglomeration of Li-decorated (AlN)_n nanocages would lower the hydrogen storage capacity.     

Efficient and highly rapid hydrogen release from ball-milled 3NH3BH3/MMgH3 (M = Na,K, Rb) mixtures at low temperatures

The stoichiometric reactions of ammonia borane (NH₃BH₃, AB) and selected alkali or alkaline-earth metal hydrides produce metal amidoboranes, which possess dehydrogenation property advantages over their parent AB. However, the losses of hydrogen capacity and chemical energy in the preparation process make metal amidoboranes less energy-effective for hydrogen storage application. In the present study, by combining the M⁺–Mg²⁺ double cations remarkably lowers the reactivity of the alkali metal hydrides toward AB. As a result, the starting Mg-based ternary hydrides MMgH₃ (M = Na, K, Rb) and AB phases are largely stable in the mechanical milling process, but transform to the corresponding mixed-cation amidoboranes in the subsequent heating process. Importantly, when the post-milled 3AB/MMgH₃ mixtures are isothermally heated at above 60 °C using water bath, the formation and decomposition processes of the mixed-cation amidoboranes can be favorably combined, giving rise to rapid and efficient dehydrogenation performances at the mild temperatures (60–80 °C). The results acquired may provide a generalized reactions coupling strategy for designing and synthesis other potentially efficient hydrogen storage system.     

Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi2.6−xMnxCo0.9 alloys

ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce)₂Ni₇ phase with a Ce₂Ni₇-type structure, (Pr, Ce)Co₃ phase with a PuNi₃-type structure, and (La, Pr)Ni₅ phase with a CaCu₅-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni₅ phase disappeared with further increasing x (x > 0.45). The hydrogen-storage capacity of the ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and then decreased to 307 mAh/g (x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi_2.15Mn_0.45Co_0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.     

Hydrogen storage properties and microstructures of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys

Hydrogen storage properties, activation performance and thermodynamics of Ti_0.7Zr_0.3(Mn_1−xV_x)₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys and associated microstructures and surface chemical states were investigated by hydrogenation measurements and relevant structure and surface characterization methods. The results showed that the phase composition of the alloy changed from single C14 Laves phase (x ≤ 0.2) to coexistent Laves phase and V-based BCC solid solution phase with increasing V content (x ≥ 0.3). The V in the alloys catalyzed hydrogen dissociation and improved resistivity to oxygen poisoning, so that the alloys could be easily and quickly activated at 293 K even after being exposed in air for a long time. The hydrogen storage capacity of the alloy increased and the plateau pressure decreased with increasing V content. The x   = 0.2 and 0.3 alloys exhibited the best reversible hydrogen storage capacities of above 1.8 wt% at 1 kPa–4 MPa and 293 K. The relative partial molar enthalpy |ΔH|$\left|\Delta H\right|$ increased but the relative partial molar entropy |ΔS|$\left|\Delta S\right|$ decreased with increasing V content, and deviated from the linear relationship for x = 0.4 and 0.5 alloys due to coexisted BCC phase in the alloys.     

The role of magnesium on properties of La3-xMgxNi9 (x=0, 0.5, 1.0, 1.5, 2.0) hydrogen storage alloys from first-principles calculations

The present work gives the electronic structures of La_3-xMg_xNi₉ (x = 0.0–2.0) alloys by first-principles calculations using the generalized gradient approximation of Perdew-Wang 91 (GGA-PW91) method within Cambridge Serial Total Energy Package (CASTEP), aiming at gaining insight into the hydrogen storage mechanism of La_3-xMg_xNi₉ alloys modified by Mg. The results show that the La_3-xMg_xNi₉ alloys consist predominantly of interactions between La-Ni, Ni-Ni or/and Mg-Ni. Among them, La-Ni interaction is the major factor controlling the structural stability of the alloys. Mg substitution increases the La-Ni bonding interactions to achieve stable Mg-containing metal matrices for reversible hydrogen absorption-desorption. This is particularly obvious at high Mg composition, as the La-Ni interactions gradually increase with Mg content. The increase of La-Ni interactions coupled with the decrease of Mg-Ni and Ni-Ni interactions will relieve the hydrogen-induced amorphization and disproportionation, and subsequently enhance the cyclic stability of La_3-xMg_xNi₉ alloys at high Mg content. However, Mg substitution for La leads to a subsequent contraction in cell volume, dramatically reducing the reversible H capacity at high Mg composition such as LaMg₂Ni₉. Suitable Mg content in La-Mg-Ni systems, such as an approximately range x = 1.0–1.4 in La_3-xMg_xNi₉ alloys, is required in trade-off between hydrogen storage capacity and cycle life.     

Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R = La,Nd and Sm) alloys

The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg₁₀NiR (R = La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg₁₀NiNd and Mg₁₀NiSm alloys are in amorphous state but the Mg₁₀NiLa alloy is composed of an amorphous phase and minor crystalline La₂Mg₁₇ after melt-spinning. The alloys can be hydrogenated into MgH₂, Mg₂NiH₄ and a rare earth metal hydride RH_x. The rare earth metal hydride and Mg₂NiH₄ synergistically provide a catalytic effect on the hydrogen absorption–desorption reactions in the Mg−H₂ system. The hydrogen storage kinetics is not influenced by different rare earth metal hydrides but by the particle size of the rare earth metal hydrides.