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.     

Enhanced hydrogen sorption properties of Ni and Co-catalyzed MgH2

MgH₂ is one of the most promising hydrogen storage materials due to its high capacity and low cost. In an effort to develop MgH₂ with a low dehydriding temperature and fast sorption kinetics, doping MgH₂ with NiCl₂ and CoCl₂ has been investigated in this paper. Both the dehydrogenation temperature and the absorption/desorption kinetics have been improved by adding either NiCl₂ or CoCl₂, and a significant enhancement was obtained in the case of the NiCl₂ doped sample. For example, a hydrogen absorption capacity of 5.17 wt% was reached at 300 °C in 60 s for the MgH₂/NiCl₂ sample. In contrast, the ball-milled MgH₂ just absorbed 3.51 wt% hydrogen at 300 °C in 400 s. An activation energy of 102.6 kJ/mol for the MgH₂/NiCl₂ sample has been obtained from the desorption data, 18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH₂/CoCl₂, which also exhibits an enhanced kinetics, and of the pure MgH₂ sample, respectively. In addition, the enhanced kinetics was observed to persist even after 9 cycles in the case of the NiCl₂ doped MgH₂ sample. Further kinetic investigation indicated that the hydrogen desorption from the milled MgH₂ is controlled by a slow, random nucleation and growth process, which is transformed into two-dimensional growth after NiCl₂ or CoCl₂ doping, suggesting that the additives reduced the barrier and lowered the driving forces for nucleation.     

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.     

Effect of LiBH4 on hydrogen storage property of MgH2

The effect of lithium borohydride (LiBH₄) on the hydriding/dehydriding kinetics and thermodynamics of magnesium hydride (MgH₂) was investigated. It was found that LiBH₄ played both positive and negative effects on the hydrogen sorption of MgH₂. With 10 mol.% LiBH₄ content, MgH₂–10 mol.% LiBH₄ had superior hydrogen absorption/desorption properties, which could absorb 6.8 wt.% H within 1300 s at 200 °C under 3 MPa H₂ and completed desorption within 740 s at 350 °C. However, with the increasing amount of LiBH₄, the hydrogenation/dehydrogenation kinetics deteriorated, and the starting desorption temperature increased and the hysteresis of the pressure-composition isotherm (PCI) became larger. Our results showed that the positive effect of LiBH₄ was mainly attributed to the more uniform powder mixture with smaller particle size, while the negative effect of LiBH₄ might be caused by the H–H exchange between LiBH₄ and MgH₂.     

Improving hydrogen storage properties of MgH2 by addition of alkali hydroxides

In this study, alkali hydroxides (NaOH, LiOH, KOH) were attempted as new-type catalytic additives to improve the hydrogen storage properties of MgH₂ by ball milling. It is shown that the additives readily react with the MgH₂ to form perovskite-structured NaMgH₃ and KMgH₃, but not the LiMgH₃. Although the in-situ formed perovskite hydrides remain stable during hydriding/dehydriding cycles, both NaMgH₃ and KMgH₃ show predominant catalytic role on the hydriding and dehydriding of MgH₂. In contrast, the LiOH modified MgH₂ presents much inferior sorption kinetics because of the absence of LiMgH₃. The catalytic mechanism of perovskite hydrides can be explained by the high hydrogen mobility in the perovskite structure. This study shows that such cheap hydroxides could be used as efficient catalysts to improve the hydrogen storage properties of high-capacity metal hydrides and complex hydrides.     

Site density effect of Ni particles on hydrogen desorption of MgH2

Fine Ni particles are effective in catalyzing hydrogen sorption of MgH₂, but there is confusion about the extent of this effect in relation to Ni particle size and content. Here, effects of Ni particles of different sizes on hydrogen desorption of MgH₂ were comparatively investigated. MgH₂ mixed with only 2 at% of fine Ni particles rapidly desorbs hydrogen up to 6.5 wt% around 200–340 °C, but there is no significant difference in the desorption temperature of the mixture when Ni particles vary from 90 to 200 nm. Increasing the content of Ni to 4 at%, or a combined (2 at% Ni + 2 at% Fe), leads to hydrogen desorption starting from 160 °C. Further analyses of the literature suggest that the effectiveness of Ni catalysis largely depends on its site density over MgH₂ surface, i.e., an optimal site density of catalytic particles is important in balancing the sorption properties of MgH₂. The projected trend suggests that MgH₂ can desorb hydrogen from 100 °C, the targeted temperature for fuel cells, if the number of catalyst sites is around 4 × 10¹⁴ per m² of MgH₂, or the number ratio of Ni to MgH₂ particles is about a million to one.     

Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

Hydrogen desorption properties of MgH2 catalysed with NaNH2

To improve hydrogen desorption properties of MgH₂, mechanical milling of MgH₂ with low concentration (2 and 5%) of NaNH₂ has been performed. Pre-milling of MgH₂ for 10 h has been done and then six samples have been synthesised with different milling times from 15 to 60 min. Microstructural characterisation has been performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and laser scattering measurements (PSD), and correlated to desorption properties examined using Differential Scanning Calorimetry (DSC) and Hydrogen Sorption Analyser (HSA). Thermal analysis shows that desorption temperatures are shifted towards lower values. It also highlights the significance of milling time and additive concentration on desorption behaviour.     

Desorption properties of MgH2 composite with a composition of 40MgH2 + 37LiBH4 + 18MgF2 + 5Ti isopropoxide

In order to increase the hydrogen storage capacity of Mg-based materials, a mixture with a composition of 2LiBH₄ + MgF₂ and LiBH₄, which has a hydrogen storage capacity of 18.4 wt%, were added to MgH₂. Ti isopropoxide was also added to MgH₂ as a catalyst. A MgH₂ composite with a composition of 40 wt%MgH₂ + 25 wt%LiBH₄ + 30 wt% (2LiBH₄ + MgF₂) + 5 wt%Ti isopropoxide (corresponding to 40 wt%MgH₂ + 37 wt%LiBH₄ + 18 wt%MgF₂ + 5 wt%Ti isopropoxide) was prepared by reactive mechanical grinding. The hydrogen storage properties of the sample were then examined. Hydrogen content vs. desorption time curves for consecutive 1st desorptions of 40 wt%MgH₂ + 37 wt%LiBH₄ + 18 wt%MgF₂ + 5 wt%Ti isopropoxide from room temperature to 823 K showed that the total desorbed hydrogen quantity for consecutive 1st desorptions was 8.30 wt%.     

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.     

Comparative studies of reaction rates of NH3 with MgH2 and LiH

The reaction rate of MgH₂ with NH₃ is studied using a two-layered structure containing a top MgH₂ layer and a bottom LiNH₂ layer. Quantification of the effluent gas composition from the two-layered structure indicates substantial NH₃ emission, while the X-ray diffraction analysis reveals little formation of the reaction products between MgH₂ and NH₃. In contrast, the study of the two-layered structure containing a top LiH layer and a bottom LiNH₂ layer reveals that the reaction between LiH and NH₃ is much faster than that between MgH₂ and NH₃.     

Hydrogen sorption properties of MgH2/NaBH4 composites

The hydrogen sorption properties of magnesium hydride–sodium borohydride composites prepared by means of high-energy ball milling under Ar atmosphere were investigated. Mutual influence of milling time and the content of NaBH₄ were studied. Microstructural and morphological analyses were carried out using X-ray Diffraction (XRD), laser scattering measurements and Scanning Electron Microscopy (SEM), while kinetic analysis and cycling were performed in a Sievert's volumetric apparatus. It has been shown that low content of NaBH₄ and short milling time are beneficial for hydrogen sorption kinetics.     

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.     

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

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

Effects of Ti-based catalysts and synergistic effect of SWCNTs-TiF3 on hydrogen uptake and release from MgH2

The present investigations are focused on the effect of different Ti-based catalysts (Ti, TiO₂, TiCl₃ and TiF₃) on de/re-hydrogenation characteristics of nanocrystalline MgH₂. Desorption temperature of milled MgH₂ lowers from 380 to 350, 340, 310 and 260 °C with the addition of Ti, TiO_2, TiCl₃ and TiF₃ respectively. The rehydrogenation characteristics are also improved through the deployment of Ti-based catalysts. Among all Ti based additives, TiF₃ is found to be the most effective catalyst for hydrogen sorption from nano MgH₂. The better catalytic effect of TiF₃ over other Ti-based catalyst can be explained on the basis of temperature programmed reduction (TPR) studies. TPR experiments performed for different Ti additives, reveals that there is no oxidation/reduction reaction below 400 °C except for TiF₃. The TPR profile of TiF₃ shows some oxidation/reduction reaction exhibits at 200 °C. In order to further improve the sorption characteristics and cyclability of TiF₃ catalyzed nano MgH₂, we have investigated the effect of SWCNTs in MgH₂+TiF₃ sample. De/rehydrogenation characteristics reveal the synergistic effect of SWCNTs and TiF₃ in MgH₂+TiF₃ sample. The details of the improvement in sorption behavior of MgH₂–TiF₃ in presence of SWCNTs are described and discussed.     

Changes in kinetic parameters of decomposition of MgH2 destabilized by irradiation with C ions

In attempt to improve desorption behaviour of MgH₂, the influence of well-defined structural changes induced within a thin surface layer of MgH₂ have been investigated. The defects were induced by 30 keV C²⁺ ions irradiation using different fluencies ranging from 10¹²–10¹⁶ ions/cm². The hydrogen desorption properties were investigated by thermal desorption spectroscopy analysis (TDS), while kinetics parameters were deduced using non-isothermal kinetic approach. The existence of multiple TDS peaks and different curve shapes indicate difference in desorption mechanism. To understand changes in the rate limiting step, shapes of all desorption peaks have been analyzed using different kinetic models. Regarding the irradiated sample, the function based on Avrami–Erofeev model with n=4$n=4$ gives the best fit over θ   range from 0.3 to 0.8 while for untreated sample the best fit is obtained for Avrami–Erofeev model with n=3$n=3$. The change in mechanism can be attributed to the different way of nuclei growth.     

Assessment of changes in desorption mechanism of MgH2 after ion bombardment induced destabilization

This paper deals with non-isothermal kinetics models of hydrogen desorption from MgH₂ altered by ion bombardment and stresses the importance of the MgH₂ surface during its decomposition. In the case of argon-irradiated samples, where defects are induced in the near-surface region, the Avrami Erofeev mechanism with parameter n = 2 can be adopted while in the case of boron-irradiated samples, where defects are created deeper in the bulk, the desorption mechanism is the same with n = 3. The difference is possibly related to the concentration and good dispersion of defects in near-surface region in the samples.     

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.     

The effect of a Ti-V-based BCC alloy as a catalyst on the hydrogen storage properties of MgH2

The effect of Ti_0.4Cr_0.15Mn_0.15V_0.3 (termed BCC due to the body centered cubic structure) alloy on the hydrogen storage properties of MgH₂ was investigated. It was found that the hydrogenated BCC alloy showed superior catalysis properties compared to the quenched and ingot samples. As an example, the 1 h milled MgH₂ + 20 wt.% hydrogenated BCC shows a peak temperature of dehydrogenation of about 294 °C. This is 16, 27 and 74 °C lower than those of MgH₂ ball milled with quenched BCC, ingot BCC and an uncatalysed MgH₂ sample, respectively. The hydrogenated BCC alloy is much easier to crush into small particles, and embed in MgH₂ aggregates as revealed by X-ray diffraction and scanning electron microscope results. The BCC not only increases the hydrogen atomic diffusivity in the bulk Mg but also promotes the dissociation and recombination of hydrogen. The activation energy, E_a, for the dehydrogenation of the MgH₂/hydrogenated BCC mixture was found to be 71.2 ± 5 kJ mol H₂⁻¹ using the Kissinger method. This represents a significant decrease compared to the pure MgH₂ (179.7 ± 5 kJ mol H₂⁻¹), suggesting that the catalytic effect of the BCC alloy significantly decreases the activation energy of MgH₂ for dehydrogenation by surface activation.     

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.