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.     

Effects of single– and co– substitution of Ti and Fe on vacancy formation and dehydrogenation from MgH2 (110) surface: Ab initio study

In this paper, we studied the effect of single- and co-substitution of Ti and Fe on vacancy formation and hydrogen desorption on MgH₂ (110) surface through first principles calculations. In the surface, we calculated and compared total electronic energies for all possible co-substitution systems in order to determine the preferential Ti and Fe co-substitution site. Substitution energy showed that Ti and Fe co-substitution is more energetically favourable than single-substitution of Ti or Fe in the surface. The bond length, DOS and Bader atomic charge were estimated in order to understand bonding mechanism between atoms. We also calculated formation energy for H, Mg and complex Mg-H vacancies and hydrogen desorption energies. More importantly, it was found that improvement in vacancy formation and hydrogen desorption becomes more obvious in Ti and Fe co-substitution system than single-substitution system. Our results indicated that Ti and Fe co-substitution in MgH₂ (110) surface could be more effective method to improve poor hydrogen desorption properties of MgH₂ than Ti or Fe single-substitution.     

DFT study of boron doped MgH2: Bonding mechanism,hydrogen diffusion and desorption

The impact of boron doping on MgH₂ bonding mechanism, hydrogen diffusion and desorption was calculated using density functional theory (DFT). Atomic interactions in doped and non-doped system and its influence on hydrogen and vacancy diffusion were studied in bulk hydride. Slab calculations were performed to study hydrogen desorption energies from (110) boron doped surface and its dependence on the surface configuration and depth position. To study kinetics of hydrogen diffusion in boron vicinity and hydrogen molecule desorption activation energies from boron doped and non-doped (110) MgH₂ surface Nudged Elastic Band (NEB) method was used. Results showed that boron forms stronger, covalent bonds with hydrogen causing the destabilization in its first and second coordination. This leads to lower hydrogen desorption energies and improved hydrogen diffusion, while the impact on the energy barriers for H₂ desorption from hydride (110) surface is less pronounced.     

Synthesis and hydrogen storage characteristics of Mg–B–H compounds by a gas–solid reaction

Magnesium borohydride [Mg(BH₄)₂] is an attractive complex hydride for hydrogen storage. In this study, attempts to synthesize Mg(BH₄)₂ were carried out by a solid–gas reaction through MgH₂ and B₂H₆ in the absence of a liquid medium. The source of B₂H₆ was obtained by heating a mixture of NaBH₄ and ZnCl₂. The profile of pressure versus temperature indicated that the absorption kinetics of B₂H₆ by MgH₂ were slow. Structural analysis confirmed the formation of Mg–B–H compounds. The reaction products presented two-step hydrogen release during heating. A small amount of hydrogen could be released from the as-synthesized Mg–B–H compounds at a low temperature of 215 °C. The slow reaction kinetics were significantly affected by the surface conditions of the MgH₂ powders.     

Electronic structure and charge distribution topology of MgH2 doped with 3d transition metals

Ab initio electronic structure calculations of the Mg₁₅TMH₃₂ (TM – transition metals – 6.25 at.%) systems for the entire 3d TM series have been performed using full-potential (linearized) augmented plane waves with addition of local orbitals (FP-LAPW + LO and APW + lo) as implemented in WIEN2k code and projected augmented waves (PAW) method using Abinit code. Details of bonding and mechanism of the TM impurities influence on destabilization of MgH₂ were established by investigation of changes of electronic structure after the TM impurities insertion into MgH₂ and by using the “atoms in molecules” (AIM) Bader's charge density topology analysis. The obtained trends of all calculated observables show that along the 3d series TMs accomplish different kinds of bonding with nearest and next-nearest neighbor hydrogen atoms that in general weaken related Mg–H bonds and destabilize the surrounding MgH₂ matrix.     

Structural destabilisation of MgH2 obtained by heavy ion irradiation

MgH₂ powder samples have been irradiated with 120 keV Ar⁺⁸ ions with different ion fluencies ranging from 10¹² to 10¹⁶ ions/cm². Irradiation effects are estimated by SRIM calculations, and investigated experimentally using Raman spectroscopy and X-ray diffraction (XRD) analysis. The observed changes of structure and vibrational spectra are elaborated, their consequences on hydrogen bonding in MgH₂ discussed, and influence on H-desorption properties investigated by Temperature Programmed Desorption (TPD) and Differential Scanning Calorimetry (DSC) techniques. It has been established that near-surface defects have a predominant influence on decreasing the H-desorption temperature. Variations of Raman, TPD and DSC spectra with irradiation conditions suggest that there are several mechanisms of dehydriding, and that they depend on defect concentration, interaction and ordering.     

Effect of LiH on hydrogen storage property of MgH2

Recent works showed that the addition of LiBH₄ significantly improves the sorption kinetics of MgH₂, and LiH decomposed from LiBH₄ was supposed to play the catalytic effect on MgH₂. In order to clarify this mechanism, the effect of LiH on the hydriding/dehydriding kinetics and thermodynamics of MgH₂ was systematically investigated. The hydrogenation kinetics of LiH-doped samples, as well as the morphology after several cycles, was similar to those of pure MgH₂, which indicate that Li⁺ had no catalytic effect on the hydrogenation of Mg. Moreover, the addition of LiH strongly retarded the hydrogen desorption of MgH₂ doped with/without Nb₂O₅, and resulted in higher starting temperature of desorption, larger activation energy and larger pressure hysteresis of PCI curves of MgH₂. H₂, HD and D₂ were observed in the desorption products of MgH₂-2LiD, which confirms that H–H exchange indeed occurs between MgH₂ and LiH, hence deteriorate desorption kinetics/thermodynamics of MgH_2. The results implied that the additives containing H⁻ could retard the hydrogen desorption of MgH₂ by H–H exchange effect.     

Improvement of hydrogen-storage properties of MgH2 by Ni,LiBH4, and Ti addition

In this work, differently from our previous work, MgH₂ instead of Mg was used as a starting material. Ni, Ti, and LiBH₄ with a high hydrogen-storage capacity of 18.4 wt% were added. A sample with a composition of MgH₂–10Ni–2LiBH₄–2Ti was prepared by reactive mechanical grinding. MgH₂–10Ni–2LiBH₄–2Ti after reactive mechanical grinding contained MgH₂, Mg, Ni, TiH_1.924, and MgO phases. The activation of MgH₂–10Ni–2LiBH₄–2Ti for hydriding and dehydriding reactions was not required. At the number of cycles, n = 2, MgH₂–10Ni–2LiBH₄–2Ti absorbed 4.09 wt% H for 5 min, 4.25 wt% H for 10 min, and 4.44 wt% H for 60 min at 573 K under 12 bar H₂. At n = 1, MgH₂–10Ni–2LiBH₄–2Ti desorbed 0.13 wt% H for 10 min, 0.54 wt% H for 20 min, 1.07 wt% H for 30 min, and 1.97 wt% H for 60 min at 573 K under 1.0 bar H₂. The PCT (Pressure–Composition–Temperature) curve at 593 K for MgH₂–10Ni–2LiBH₄–2Ti showed that its hydrogen-storage capacity was 5.10 wt%. The inverse dependence of the hydriding rate on temperature is partly due to a decrease in the pressure differential between the applied hydrogen pressure and the equilibrium plateau pressure with the increase in temperature. The rate-controlling step for the dehydriding reaction of the MgH₂–10Ni–2LiBH₄–2Ti at n = 1 was analyzed.     

Hydrogen desorption properties of MgH2/LiAlH4 composites

The hydrogen desorption properties of MgH₂–LiAlH₄ composites obtained by mechanical milling for different milling times have been investigated by Thermal Desorption Spectroscopy (TDS) and correlated to the sample microstructure and morphology analysed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The MgH₂–LiAlH₄ composites show improved hydrogen desorption properties in comparison with both as-received and ball-milled MgH₂. Mixing of MgH₂ with small amount of LiAlH₄ (5 wt.%) using short mechanical milling (15 min) shifts, in fact, the hydrogen desorption peak to lower temperature than those observed with both as-received and milled MgH₂ samples. Longer mixing times of the MgH₂–LiAlH₄ composites (30 and 60 min) reduce the catalytic activity of the LiAlH₄ additive as revealed by the shift of the hydrogen desorption peak to higher temperatures.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Hydrogen absorption–desorption mechanisms for the ball-milled Li3N–MgH2 (1:1) mixture

The Li–Mg–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. In this work, the LiMgN/2LiH was directly synthesized by ball-milling the mixture of Li₃N–MgH₂ at 1:1 molar ratio by a reaction of Li₃N + MgH₂ → LiMgN + 2LiH. The hydrogenation/dehydrogenation properties of the as-prepared LiMgN/2LiH were investigated by a Sieverts'-type apparatus. The mixture of LiMgN/2LiH started to absorb hydrogen at 130 °C, and 2.2 wt%, 3.2 wt% hydrogen were absorbed under a pressure of 5 MPa and 10 MPa, respectively. Powder X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectrometer measurements were used to identify the phase characterizations of the products during the hydrogen absorption–desorption process. The reaction mechanism during the hydrogenation/dehydrogenation process for the Li₃N–MgH₂ system is discussed.     

Effect of carbon addition on hydrogen storage behaviors of Li–Mg–B–H system

Various carbon additives were mechanically milled with LiBH₄/MgH₂ composite and their hydrogen storage behaviors were investigated. It was found that most of the carbon additives exhibited prominent effect on the host material. Among the various carbon additives, purified single-walled carbon nanotubes (SWNTs) exhibited the most prominent effect on the kinetic improvement and cyclic stability of Li–Mg–B–H system. Results show that LiBH₄/MgH₂ composite milled with 10 wt.% purified SWNTs additive can release nearly 10 wt.% hydrogen within 20 min at 450 °C, which is about two times faster than that of the neat LiBH₄/MgH₂ sample. On the basis of hydrogen storage behavior and structure/phase investigations, the possible mechanism involved in the property improvement upon carbon additives was discussed.     

Catalytic mechanisms of TiH2 thin layer on dehydrogenation behavior of fluorite-type MgH2: A first principles study

DFT calculations were carried out to investigate hydrogen release and diffusion behaviors. Results demonstrated that MgH₂/TiH₂ interface is thermodynamically stable with negative adhesion energy of −1.33 J/m² with respect to the individual MgH₂ and TiH₂ slabs. The formation of MgH₂/TiH₂ interface alters the interstice structure and space of the interstitial sites where H atoms located and then significantly lowers the dehydrogenation energy of hydrogen releasing from both the MgH₂ and TiH₂ slabs nearby the interface comparing the bulk MgH₂ and TiH_2. The smallest dehydrogenation energy of 0.06 eV/H could be reached when H releases from MgH₂ side. The study also illustrates that the existence of the MgH₂/TiH₂ interface promotes the diffusion of hydrogen vacancy. The lowest diffusion barrier of hydrogen vacancy in the MgH₂ slab (from the sublayer to the frontier layer to the interface) is estimated as 0.21 eV. Based on the present study, one can deduce that the dehydrogenation of the MgH₂/TiH₂ system will start by H releasing from MgH₂ slab, which generates H vacancies near the interface, then the interior H of MgH₂ migrates to the H vacancies (diffuse of H vacancies in the opposite direction) and releases. The TiH₂ acts as a catalyst promoting the generation and diffusion of H vacancies in MgH₂. Therefore synthesizing of MgH₂/TiH₂/MgH₂ sandwich structure could be an effective approach to promote the dehydrogenation process of MgH₂, and an ideal structure owning geometric hydrogen capacity of 6.45 wt%.     

Influence of transition metals Fe,Ni, and Nb on dehydrogenation characteristics of Mg(BH4)2: Electronic structure mechanisms

First principles calculations on Fe, Ni, and Nb doped Mg(BH₄)₂ were carried out to study the influence of dopants on dehydrogenation properties of Mg(BH₄)₂. It was shown that all dopants considered prefer to substitute for Mg with relatively smaller occupation energies comparing to the B substitution and the interstitial occupation. However, the B substitution shows smaller hydrogen dissociation energy than the Mg substitution. Mechanisms that dopants used to improve dehydrogenation properties of Mg(BH₄)₂ are different. For Mg substitution, Fe strongly interacts with one H atoms of the [BH₄] group, distorts its structural stability and therefore lowers the hydrogen dissociation energy, Ni may attract one particular H atom of the [BH₄] group and weakens the interactions between the B and other H atoms reducing the hydrogen dissociation energy, and the Nb however may drive the formation of NbB₂ and improves the dehydrogenation properties as well. In the B substitution, Fe interacts with the one of H atoms and decreases its structure stability, the Ni will attract its neighbor atoms to form a regular group which is almost identical in structure to that of the NiH₄ group in Mg₂NiH₄, and the NbH₂ and MgH₂ are likely to be generated by Nb doping.     

Stability and bonding mechanism of ternary (Mg,Fe, Ni)H2 hydrides from first principles calculations

The stability and bonding mechanism of ternary magnesium based hydrides (Mg, X, Y)H₂, X or Y = Fe or Ni, were studied by means of electronic structure and total energy calculations using the FP-LAPW method within the GGA. The influence of the selected alloying elements on the stability of the hydride was determined from the difference between the total energy of the alloyed systems and those of the pure metal and the hydride. Full relaxation was carried out against the overall geometry of the supercell and the internal coordinates of the H atoms. The bonding interactions between the alloying atoms and their surrounding H atoms were estimated using the variation of the total energy against the coordinates of H atoms. The alloying elements, Fe and Ni, destabilised MgH₂. This combined with the weak bonds between the alloying elements and H atoms improved the dehydrogenation properties of MgH₂.     

Structural characterization and hydrogen storage properties of MgH2–Mg2CoH5 nanocomposites

Mixtures of XMg–Co containing different amounts of Mg (X = 2, 3 and 7) were reactive milled under hydrogen atmosphere. 2Mg–Co only formed the Mg₂CoH₅ complex hydride, while the mixtures 3Mg–Co and 7Mg–Co formed different contents of Mg₂CoH₅ and MgH₂. Their structural features and hydrogen storage properties were analyzed by different techniques. In-situ synchrotron X-ray diffraction, combined with thermal analysis techniques, (differential scanning calorimetry, thermal gravimetric analysis and quadrupole mass spectrometer) was carried out to observe the behavior of the MgH₂–Mg₂CoH₅ mixtures during the first H-desorption. It was found that the presence of the Mg₂CoH₅ complex hydride has a beneficial effect on the first H-desorption of the MgH₂. Additionally, after first desorption, conventional hydrogenation under high pressure and high temperature of 3Mg–Co and 7Mg–Co samples led to the formation of the Mg₆Co₂H₁₁ complex hydride. The presence of Mg₆Co₂H₁₁ considerably impaired the desorption properties of the nanocomposites.     

Fluorographene nanosheets enhanced hydrogen absorption and desorption performances of magnesium hydride

Fluorographene (FG), which inherits the properties of graphene and fluorographite (FGi), was successfully fabricated through a simple sonochemical exfoliation route in N-methyl-2-pyrrolidone (NMP) and then MgH₂-FG composite was prepared by ball milling. The dehydrogenation and rehydrogenation performances of MgH₂-FG composite were investigated systematically comparing with as-received MgH₂ and MgH₂-G composite. It is found that the as-prepared FG exhibited a significant catalytic effect on the dehydrogenation and rehydrogenation properties of MgH₂. The MgH₂-FG composite can uptake 6.0 wt% H₂ in 5 min and release 5.9 wt% H₂ within 50 min at 300 °C, while the as-received MgH₂ uptakes only 2.0 wt% H₂ in 60 min and hardly releases hydrogen at the same condition. The hydrogen storage cycling kinetics in the first 10 cycles remains almost the same, indicating the excellent reversibility of the MgH₂-FG composite. SEM analysis shows that the particle size of MgH₂-FG composite was ∼200 nm, much smaller than that of as-received MgH₂ (∼20 μm). TEM observations show that MgH₂ particles were embedded in FG layers during ball milling. The dehydrogenation apparent activation energy for the MgH₂ is reduced from 186.3 kJ mol⁻¹ (as-received MgH₂) to 156.2 kJ mol⁻¹ (MgH₂-FG composite). The catalytic mechanism has been proposed that F atoms in FG serve as charge-transfer sites and accelerate the rate of hydrogen incorporation and dissociation, consequently enhance the dehydrogenation and rehydrogenation properties of MgH₂-FG composite. Furthermore, the FG can inhibit the sintering and agglomeration of MgH₂ particle, thus it improves the cycling dehydrogenation and rehydrogenation of MgH₂-FG composite.     

Electronic and dehydrogenation properties of TiB2 cluster-doped NaAlH4 (101) surface: A first-principle approach

The crystal structures, electronic and dehydrogenation properties of TiB₂ cluster-doped NaAlH₄ (101) surface have been investigated by the first-principles density functional theory method. In the TiB₂ cluster-doped NaAlH₄ (101) surface, a Ti-centered TiB₂–Al₂H₈–AlH₅–AlH₃ complex is observed, and the AlH₃ and (AlH₅)²⁻ units in the TiB₂–Al₂H₈–AlH₅–AlH₃ favor the first-step decomposition reaction of NaAlH₄. The calculated electronic properties show that B–Ti bonds are stronger than B–Al and Ti–H bonds, which demonstrates that TiB₂ does not change its configuration in catalyzing the decomposition reaction of NaAlH₄. The results of hydrogen desorption energies imply that the import of TiB₂ makes the strength of Al–H bonds decreases. Therefore, the removal of H atoms, especially the removal of H atoms in the Ti–H–Al bonds is easier in the TiB₂ cluster-doped NaAlH₄ than in pure NaAlH₄.