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.     

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.     

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.     

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.     

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.     

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.     

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.     

Investigation of surface and near-surface effects on hydrogen desorption kinetics of MgH2

Desorption of hydrogen atoms from the (110) surface of rutile magnesium hydride (MgH₂) was investigated using density functional theory (DFT) and pseudopotential method. System was represented by (110) (2×2) slab MgH₂ supercell with 12 atomic layers along the z-axis. The H-desorption was modeled by the successive release of the four two-fold bonded H atoms from the (110) surface of MgH₂. Dependence of the H-desorption energy on number and configuration of remaining surface hydrogen atoms has been determined. The features of the H atoms diffusion from the bulk towards the surface have been investigated, too. The results suggest that decrease in number of surface H atoms generally lowers the H-desorption energy in each desorption step and that both the H–H and the Mg–H interatomic interactions strongly influence the H-desorption process. The hydrogen vacancy formation energy in the first three sub-surface layers also exhibits a pronounced dependence on concentration. These findings lead to the conclusion that tendency of the MgH₂ (110) surface to preserve a maximum possible surface H concentration in its most stable configuration is the limiting factor for the H-desorption kinetics. In principle, the obtained results allow us to determine preferred paths of surface and sub-surface H-diffusion for a wide range of H concentrations and the principle features of the MgH₂ dehydrogenation process, at least for the H-rich region. Being rather comprehensive, the approach is applicable for other metal hydrides, as well.     

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.     

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.     

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.     

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 hydrogen storage properties and reaction mechanism of the MgH2-NaAlH4 composite system

In this study, we report the hydrogen absorption/desorption properties and reaction mechanism of the MgH₂-NaAlH₄ (4:1) composite system. This composite system showed improved dehydrogenation performance compared with that of as-milled NaAlH₄ and MgH₂ alone. The dehydrogenation process in the MgH₂-NaAlH₄ composite can be divided into four stages: NaAlH₄ is first reacted with MgH₂ to form a perovskite-type hydride, NaMgH₃ and Al. In the second dehydrogenation stage, the Al phase reacts with MgH₂ to form Mg₁₇Al₁₂ phase accompanied with the self-decomposition of the excessive MgH₂. NaMgH₃ goes on to decompose to NaH during the third dehydrogenation stage, and the last stage is the decomposition of NaH. Kissinger analysis indicated that the apparent activation energy, E_A, for the MgH₂-relevent decomposition in MgH₂-NaAlH₄ composite was 148 kJ/mol, which is 20 kJ/mol less than for as-milled MgH₂ (168 kJ/mol). X-ray diffraction patterns indicate that the second, third, and fourth stages are fully reversible. It is believed that the formation of Al₁₂Mg₁₇ phase during the dehydrogenation process alters the reaction pathway of the MgH₂-NaAlH₄ (4:1) composite system and improves its thermodynamic properties.     

Enhancement of hydrogen-storage performance of MgH2 by Mg2Ni formation and hydride-forming Ti addition

MgH₂, rather than Mg, was used as a starting material in this work. A sample with a composition of MgH₂–10Ni–4Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. At n = 1, the sample desorbed 2.53 wt% H for 10 min, 3.99 wt% H for 20 min, 4.58 wt% H for 30 min, and 4.68 wt% H for 60 min at 593 K under 1.0 bar H₂. At n = 2, the sample absorbed 3.59 wt% H for 5 min, 4.55 wt% H for 25 min, and 4.60 wt% H for 45 min at 593 K under 12 bar H₂. The inverse dependence of the hydriding rate on the temperature in the initial stage and the normal dependence of the hydriding rate on the temperature in the later stage were discussed. The rate-controlling step for the dehydriding reaction of activated MgH₂–10Ni–4Ti was analyzed as the chemical reaction at the hydride/α-solid solution interface.     

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.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Catalytic effect of Gd2O3 and Nd2O3 on hydrogen desorption behavior of NaAlH4

This study investigated the effect of Nd₂O₃ and Gd₂O₃ as catalyst on hydrogen desorption behavior of NaAlH₄. Pressure-content-temperature (PCT) equipment measurement proved that both two oxides enhanced the dehydrogenation kinetics distinctly and increasing Nd₂O₃ and Gd₂O₃ from 0.5 mol% to 5 mol% caused a similar effect trend that the dehydrogenation amount and average dehydrogenation rate increased firstly and then decreased under the same conditions. 1 mol% Gd₂O₃–NaAlH₄ presented the largest hydrogen desorption amount of 5.94 wt% while 1 mol% Nd₂O₃–NaAlH₄ exerted the fastest dehydrogenation rate. Scanning Electron microscopy (SEM) analysis revealed that Gd₂O₃–NaAlH₄ samples displayed uniform surface morphology that was bulky, uneven and flocculent. The difference of Nd₂O₃–NaAlH₄ was that with the increasing of Nd₂O₃ content, the particles turned more and more big. Compared to dehydrogenation behavior, this phenomenon demonstrated that small particles structure were beneficial to hydrogen desorption. Besides, the further study found that different catalysts and addition amounts had different effects on the microstructure of NaAlH_4.