Significantly improved dehydrogenation of LiAlH4 destabilized by K2TiF6

The effects of K₂TiF₆ on the dehydrogenation properties of LiAlH₄ were investigated by solid-state ball milling. The onset decomposition temperature of 0.8 mol% K₂TiF₆ doped LiAlH₄ is as low as 65 °C that 85 °C lower than that of pristine LiAlH₄. Isothermal dehydrogenation properties of the doped LiAlH₄ were studied by PCT (pressure–composition–temperature). The results show that, for the 0.8 mol% K₂TiF₆ doped LiAlH₄ that dehydrogenated at 90 °C, 4.4 wt% and 6.0 wt% of hydrogen can be released in 60 min and 300 min, respectively. When temperature was increased to 120 °C, the doped LiAlH₄ can finish its first two dehydrogenation steps in 170 min. DSC results show that the apparent activation energy (E_a) for the first two dehydrogenation steps of LiAlH₄ are both reduced, and XRD results suggest that TiH₂, Al₃Ti, LiF and KH are in situ formed, which are responsible for the improved dehydrogenation properties of LiAlH₄.     

LiAlH4–ZrCl4 mixtures for hydrogen release at near room temperature

The dehydrogenation temperature of LiAlH₄ was significantly reduced by the production of mixtures with ZrCl₄. Stoichiometric 4:1, and 5 mol % mixtures of LiAlH₄ and ZrCl₄ were produced by ball milling at room temperature and ?196 °C, and tested for dehydrogenation at low temperature. Cryogenic ball-milling resulted in an effective way to produce reactive mixtures for hydrogen release; because of achieving small aggregates size (5–20 μm) in 10 min of cryomilling while preventing substantial decomposition during preparation. Dehydrogenation reaction in the mixtures LiAlH₄/ZrCl₄ started around 31–47 °C under different heating rates. Partial dehydrogenation was proved at 70 °C: 4.4 wt % for the 5 mol% ZrCl₄–LiAlH₄ mixture, and 3.4 wt % for the best 4:1 stoichiometric mixture. Complete dehydrogenation up to 250 °C released 6.4 wt% and 4.1 wt%, respectively. Dehydrogenation reactions are exothermic, and the LiAlH₄/ZrCl₄ mixtures are unstable and difficult to handle. The activation energy of the exothermic reactions was estimated as 113.5 ± 9.8 kJ/mol and 40.6 ± 6.6 kJ/mol for 4LiAlH₄+ZrCl₄ and 5%mol ZrCl₄+LiAlH₄ samples milled in cryogenic conditions, respectively. The dehydrogenation pathway was changed in the LiAlH₄/ZrCl₄ mixtures as compared to pure LiAlH₄. Dehydrogenation reaction is proposed to form Al, LiCl, Zr, and H₂ as main products. Modification of the dehydrogenation reaction of LiAlH₄ was achieved at the cost of reducing the total hydrogen release capacity.     

Effect of ball milling time on the hydrogen storage properties of TiF3-doped LiAlH4

In the present work, the catalytic effect of TiF₃ on the dehydrogenation properties of LiAlH₄ has been investigated. Decomposition of LiAlH₄ occurs during ball milling in the presence of 4 mol% TiF₃. Different ball milling times have been used, from 0.5 h to 18 h. With ball milling time increasing, the crystallite sizes of LiAlH₄ get smaller (from 69 nm to 43 nm) and the dehydrogenation temperature becomes lower (from 80 °C to 60 °C). Half an hour ball milling makes the initial dehydrogenation temperature of doped LiAlH₄ reduce to 80 °C, which is 70 °C lower than as-received LiAlH₄. About 5.0 wt.% H₂ can be released from TiF₃-doped LiAlH₄ after 18 h ball milling in the range of 60 °C–145 °C (heating rate 2 °C min⁻¹). TiF₃ probably reacts with LiAlH₄ to form the catalyst, TiAl₃. The mechanochemical and thermochemical reactions have been clarified. However, the rehydrogenation of LiAlH₄/Li₃AlH₆ can not be realized under 95 bar H₂ in the presence of TiF₃ because of their thermodynamic properties.     

LiBH4–VCl3 and LiAlH4–VCl3 mixtures prepared in soft milling conditions for hydrogen release at low temperature

Mixtures of LiBH₄/VCl₃ and LiAlH₄/VCl₃ in 5:1, 3:1, and 5% mol stoichiometries were prepared and tested for hydrogen release. The mixtures were prepared in 10 min of ball milling at room temperature or with cryogenic (N₂-liquid) cooling. The mixtures demonstrated diverse hydrogen release levels, but all of them started releasing hydrogen at low temperatures (33–66 °C) with a change in the reaction pathway as compared to pure LiBH₄ or LiAlH₄. The driving force for that is the formation of the stable salt LiCl. The best material was the 5% mol VCl₃ + LiAlH₄ cryogenic mixture because of the low-temperature dehydrogenation onset of 34 °C; and the dehydrogenation level of 5.1 wt.%, and 6.4 wt.% that was achieved upon heating at 100 °C and 250 °C, respectively.     

Improved reversible hydrogen storage of LiAlH4 by nano-sized TiH2

Transition metal halides are mostly used as dopants to improve the hydrogen storage properties of LiAlH₄, but they will cause hydrogen capacity loss because of their relatively high molecular weights and reactions with LiAlH₄. To overcome these drawbacks, active nano-sized TiH₂ (TiH₂^nano) prepared by reactive ball milling is used to dope LiAlH₄. It shows superior catalytic effect on the dehydrogenation of LiAlH₄ compared to commercial TiH₂. TiH₂^nano-doped LiAlH₄ starts to release hydrogen at 75 °C, which is 80 °C lower than the onset dehydrogenation temperature of commercial LiAlH₄. About 6.3 wt.% H₂ can be released isothermally at 100 °C (800 min) or at 120 °C (150 min). The apparent activation energies of the first two dehydrogenation reactions of LiAlH₄ are reduced by about 20 and 24 kJ mol⁻¹, respectively. Meanwhile, the regeneration of LiAlH₄ is realized through extracting the solvent from LiAlH₄·4THF, which is obtained by ball milling the dehydrogenated products of TiH₂^nano-doped LiAlH₄ in the presence of THF and 5 MPa H₂. This suggests that TiH₂ is also an effective catalyst for the formation of LiAlH₄·4THF.     

Hydrogen release from LiAlH4/FeCl2 and LiBH4/FeCl2 mixtures prepared in cryogenic conditions

Mixtures of LiAlH₄/FeCl₂ and LiBH₄/FeCl₂ were produced in several conditions of stoichiometry, time, and vial-oscillation frequency at cryogenic conditions (−196 °C) by ball milling. The best production conditions were milling for 30 min at −196 °C and 15 Hz of the oscillation frequency of the vial. Temperature-programmed desorption and differential scanning calorimetry experiments indicated that hydrogen release in LiAlH₄/FeCl₂ mixtures begins at approximately 60 °C and ends at 160 °C. Meanwhile, the LiBH₄/FeCl₂ mixtures presented dehydrogenation on-set at 40 °C, finishing at about 300 °C. These materials behave as reactive mixtures, where the LiCl formation is considered the driving force of the decomposition reactions. Cryogenic ball-milling has a good effect on the production of the mixtures and low-temperature dehydrogenation of the studied samples.     

Comparative study on the dehydrogenation properties of TiCl4-doped LiAlH4 using different doping techniques

Lithium aluminum hydride (LiAlH₄) is an attractive hydrogen storage material because of its comparatively high gravimetric hydrogen storage capacity. In this study, titanium tetrachloride (TiCl₄), which is liquid at room temperature, was chosen as dopant because of its high catalytic efficiency regarding the dehydrogenation of LiAlH₄. Three low-energy doping methods (additive dispersion via ball milling at low rotation speed, magnetic stirring and magnetic stirring in ethyl ether) with different TiCl₄ concentrations were compared in order to obtain optimum dehydrogenation properties of LiAlH₄. At 80 °C, TiCl₄-doped LiAlH₄ can release up to 6.5 wt.%-H₂, which opens the way to use of exhaust heat of PEM fuel cells to trigger the hydrogen release from LiAlH₄.     

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.     

Preparation of a new Ti catalyst for improved performance of NaAlH4

Three effective Ti catalysts for NaAlH₄ were made by stoichiometrically reacting TiCl₃ with LiAlH₄ in tetrahydrofuran (THF), NaAlH₄ in THF, and LiAlH₄ in diethyl ether (Et₂O). The solid products produced after drying were named ex situ catalysts and designated respectively as Ti(Li)T, Ti(Na)T and Ti(Li)E. NaAlH₄ was dry doped with 2 mol% of these ex situ catalysts, and for comparison, NaAlH₄ was conventionally wet doped with 2 mol% TiCl₃ in THF that made in situ catalyst (designated as TiCl₃). All four doped samples were dry ball milled, and hydrogenation and dehydrogenation studies were carried out over five cycles. Temperature programmed desorption, constant temperature desorption, and constant temperature cycling curves showed that the effectiveness of these catalysts decreased as Ti(Li)T > Ti(Na)T > TiCl₃ > Ti(Li)E. Ti(Li)T ex situ catalyst, being the best Ti catalyst, markedly decreased the dehydrogenation temperature, improved both the hydrogenation and dehydrogenation kinetics with sustained rates over cycling, and exhibited the least loss of hydrogen storage capacity over cycling. Ti(Li)T ex situ catalyst exhibited properties commensurate with some of the best NaAlH₄ catalysts to date, such as CeCl₃, ScCl₃ and Ti nanocluster. It is easy to make, readily available and relatively inexpensive.     

Effects of TiCl3-decorated MWCNTs addition on the dehydrogenation behavior and stability of LiAlH4

A complex catalyst for enhancing the dehydrogenation kinetics of LiAlH₄ was developed by using an impregnation process to decorate TiCl₃ on multiwall carbon nanotubes (MWCNTs). The effects of these composite catalysts on the dehydrogenation behavior were investigated by using thermal gravimetric analysis (TGA) and in-situ synchrotron X-ray diffraction (XRD) technique. The experimental results showed that the initial dehydrogenation temperature could be lowered by adding the appropriate amount of TiCl₃–MWCNTs composite. LiAlH₄ became unstable and decomposed even at room temperature when 20 wt% TiCl₃–MWCNTs was added.     

Catalytic effects of FGO-Ni addition on the dehydrogenation properties of LiAlH4

LiAlH₄ is an ideal hydrogen storage material with a theoretical hydrogen storage capacity of 10.6 wt%. In order to reduce the hydrogen release temperature and increase the hydrogen release amount of LiAlH₄, multilayer graphene oxide and nickel (FGO-Ni) composite catalyst were prepared by physical ball milling and doped into LiAlH₄. The effect of FGO-Ni composite catalyst on the dehydrogenation performance of LiAlH₄ was studied by pressure-composition-temperature apparatus, scanning electron microscope (SEM) and X-ray diffractometer. The results show that, compared with pure LiAlH₄, the hydrogen release time of LiAlH₄ doped with 9 wt%FGO-3wt%Ni is obviously shortened about 90min at 150 °C and the hydrogen release amount of LiAlH₄ doped with 9 wt%FGO-3wt%Ni also increased 1.8 wt%. Importantly, the dehydrogenation amount of LiAlH₄ (9 wt%FGO)-3wt% could reach 4 wt% at 135 °C which was 4 times higher than that of the pure LiAlH₄. At the same temperature, the hydrogen release of pure LiAlH₄ was only 0.84 wt%. In contrast, doping FGO-Ni composite catalyst reduces the hydrogen release temperature of LiAlH₄ and weakens the hydrogen release barrier. Forthermore, SEM results showed that doping FGO-Ni reduced the agglomeration between LiAlH₄ particles and increased the specific surface area of the sample, which improving the hydrogen release properties of LiAlH₄.     

The dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive

For Li₃AlH₆ prepared by mechanical milling method, the dissociation reaction enthalpy and activation energy are calculated to be 22.1 kJ mol⁻¹ H₂ and 133.7 ± 2.7 kJ mol⁻¹, respectively. The dehydrogenation performance of Li₃AlH₆ is greatly enhanced by TiF₃ additive, especially in the kinetic behaviors. For the Li₃AlH₆ + 10 mol% TiF₃ sample, the starting temperature of dehydrogenation is obviously decreased by 60 °C from that of pure Li₃AlH₆ (190 °C), and 3.0 wt.% H₂ may be released within 1000 s at 120 °C under an initial vacuum. With the amount of TiF₃ increasing, the starting temperature decreases and the kinetics improves due to the decrease in the activation energy. The X-ray diffraction (XRD) together with thermogravimetric analysis (TGA) results show that there are three mechanochemical reactions involved during milling: i) Li₃AlH₆ + TiF₃ → 3 LiF + Al + Ti + 3H₂, ii) Ti + H₂ → TiH₂, iii) 3 Al + Ti → Al₃Ti. The in-situ formed Ti species (TiH₂ and Al₃Ti) co-catalyze the thermal dehydrogenation of Li₃AlH₆.     

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH4), its self-discharge at low temperatures and rehydrogenation

LiAlH₄ containing 5 wt.% of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H₂) during high energy ball milling reaching ∼3.5 wt.% H₂ after 5 h of milling. In contrast, no H₂ desorption is observed during low energy milling of LiAlH₄ containing n-Fe. Similarly, no H₂ desorption occurs during high energy ball milling for LiAlH₄ containing micrometric Fe (μ-Fe) and, for comparison, both the micrometric and nanometric Ni (μ-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH₄ for both the Fe and Ni additives. A volumetric lattice expansion larger than 1% results in the profound destabilization of LiAlH₄ accompanied by continuous H₂ desorption during milling according to reaction: LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH₄ by the action of mechanical energy, replacing the Al ions and forming a substitutional solid solution. The quantity of dissolved metal ions depends primarily on the total energy of milling per unit mass of powder generated within a prescribed milling time, the type of additive ion e.g. Fe vs. Ni and on the particle size (micrometric vs. nanometric) of metal additive. For thermal dehydrogenation the average apparent activation energy of Stage I (LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂) is reduced from the range 76 to 96 kJ/mol for the μ-Fe additive to about 60 kJ/mol for the n-Fe additive. For Stage II dehydrogenation (1/3Li₃AlH₆ → LiH+1/3Al + 0.5H₂) the average apparent activation energy is within the range 77–93 kJ/mol, regardless of the particle size of the Fe additive (μ-Fe vs. n-Fe). The n-Fe and n-Ni additives, the latter used for comparison, provide nearly identical enhancement of dehydrogenation rate during isothermal dehydrogenation at 100 °C. Ball milled (LiAlH₄ + 5 wt.% n-Fe) slowly self-discharges up to ∼5 wt.% H₂ during storage at room temperature (RT), 40 and 80 °C. Fully dehydrogenated (LiAlH₄ + 5 wt.% n-Fe) has been partially rehydrogenated up to 0.5 wt.% H₂ under 100 bar/160°C/24 h. However, the rehydrogenation parameters are not optimized yet.     

Reinforce the dehydrogenation process of LiAlH4 by accumulating porous activated carbon

Herein, it is reported that activated carbon (AC) alters the hydrogen storage behavior of lithium alanate (LiAlH₄) prepared by the ball milling technique. Notable improvements in onset decomposition temperature and desorption kinetics are attained for LiAlH₄ added 10 wt.% of AC composite compared to as-received and as-milled LiAlH₄. The onset decomposition temperature of LiAlH₄-10 wt.% AC dropped to 100 °C and 160 °C for the first and second steps. The composite also released 3.4 wt.% of hydrogen after 90 min compared to as-received and as-milled which is less than 0.2 wt.% of hydrogen within the same period. The XRD result discovered an additional peak of the Li₃AlH₆ and Al compounds appeared after the milling process, concluding that LiAlH₄ becomes unstable after the addition of AC. FTIR measurement has verified the presence of the Li₃AlH₆ and carbon bonding in the LiAlH₄-10 wt.% AC composite. The composite's activation energy (E_a) for the first and second steps is 70 kJ/mol and 85 kJ/mol, respectively. These values decrease from as-milled LiAlH₄ for both steps, demonstrating the catalytic effect of AC in this system. FESEM images illustrate that after ball milling, the particle size of LiAlH₄-10 wt.% AC composite decreases. The considerable improvement in the hydrogen storage characteristic of the LiAlH₄-10 wt.% AC composite is thought to be the collaborative role of amorphous carbon.     

TiF4-doped Mg(AlH4)2 with significantly improved dehydrogenation properties

A significant decrease in the dehydrogenation temperature of Mg(AlH₄)₂ was achieved by low-energy ball milling with TiF₄. Approximately 8.0 wt% of hydrogen was released from the Mg(AlH₄)₂-0.025TiF₄ sample with an on-set temperature of 40 °C, which represents a decrease of 75 °C relative to pristine Mg(AlH₄)₂. In contrast to the three-step reaction for pristine Mg(AlH₄)₂, hydrogen desorption from the TiF₄-doped sample involves a two-step process because the Ti-based species participates in the dehydrogenation reaction. The presence of TiF₄ alters the nucleation and growth of the dehydrogenation product, significantly decreasing the activation energy barrier of the first step in the dehydrogenation of Mg(AlH₄)₂. Further hydrogenation measurements revealed that the presence of the Ti-based species was also advantageous for hydrogen uptake, as the on-set hydrogenation temperature was only 100 °C for the dehydrogenated TiF₄-doped sample, compared with 130 °C for the additive-free sample.     

Desorption properties of LiAlH4 doped with LaFeO3 catalyst

The addition of a catalyst and ball milling process was found to be one of the efficient method to reduce the decomposition temperature and improve the desorption kinetics of lithium aluminium hydride (LiAlH₄). In this paper, a transition metal oxide, LaFeO₃ was used as a catalyst. Decomposition temperature of the 10 wt% of LaFeO₃-doped LiAlH₄ system was found to be lowered from 143 °C to 103 °C (first step) and from 175 °C to 153 °C (second step), respectively. In isothermal desorption kinetics, the amount of hydrogen released of the doped sample was improved to 3.9 wt% in 2.5 h at 90 °C. Meanwhile, the undoped sample had released less than 1.0 wt% of hydrogen under the same condition. The activation energy of the LaFeO₃-doped LiAlH₄ sample was measured to be 73 kJ/mol and 90 kJ/mol for the first two dehydrogenation reactions compared to 107 kJ/mol and 119 kJ/mol for the undoped sample. The improvements of desorption properties were the results from the formation of LiFeO₂, Fe and La or La-containing phase during the heating process.     

Enhancing the dehydrogenation properties of LiAlH4 using K2NiF6 as additive

Lithium alanate (LiAlH₄) is a material that can be potentially used for solid-state hydrogen storage due to its high hydrogen content (10.5 wt%). Nevertheless, a high desorption temperature, slow desorption kinetic, and irreversibility have restricted the application of LiAlH₄ as a solid-state hydrogen storage material. Hence, to lower the decomposition temperature and to boost the dehydrogenation kinetic, in this study, we applied K₂NiF₆ as an additive to LiAlH₄. The addition of K₂NiF₆ showed an excellent improvement of the LiAlH₄ dehydrogenation properties. After adding 10 wt% K₂NiF₆, the initial decomposition temperature of LiAlH₄ within the first two dehydrogenation steps was lowered to 90 °C and 156 °C, respectively, that is 50 °C and 27 °C lower than that of the аs-milled LiAlH₄. In terms of dehydrogenation kinetics, the dehydrogenation rate of K₂NiF₆-doped LiAlH₄ sample was significantly higher as compared to аs-milled LiAlH₄. The K₂NiF₆-doped LiAlH₄ sample can release 3.07 wt% hydrogen within 90 min, while the milled LiAlH₄ merely release 0.19 wt% hydrogen during the same period. According to the Arrhenius plot, the apparent activation energies for the desorption process of K₂NiF₆-doped LiAlH₄ are 75.0 kJ/mol for the first stage and 88.0 kJ/mol for the second stage. These activation energies are lower compared to the undoped LiAlH₄. The morphology study showed that the LiAlH₄ particles become smaller and less agglomerated when K₂NiF₆ is added. The in situ formation of new phases of AlNi and LiF during the dehydrogenation process, as well as a reduction in particle size, is believed to be essential contributors in improving the LiAlH₄ dehydrogenation characteristics.     

The effects of the nanometric interstitial compounds TiC,ZrC and TiN on the mechanical and thermal dehydrogenation and rehydrogenation of the nanocomposite lithium alanate (LiAlH4) hydride

Profuse mechanical dehydrogenation occurs during controlled high energy ball milling of LiAlH₄ containing 5 wt.% of the nanometric interstitial compounds such as n-TiC, n-TiN and n-ZrC which involves a gradual decomposition of LiAlH₄ to the mixture of Li₃AlH₆ and Al (Stage I) followed by a further decomposition of Li₃AlH₆ to the mixture of Al and LiH (Stage II). XRD reveals that the interstitial compounds remain stable in the hydride matrix during entire ball milling duration. The effectiveness of the nanometric interstitial compound additives for mechanical dehydrogenation increases on the order of n-TiN > n-TiC > n-ZrC. X-ray diffraction (XRD) reveals that there is no measurable change in a unit cell volume of LiAlH₄ after ball milling which indicates that an accelerated mechanical dehydrogenation of LiAlH₄ containing the nanometric interstitial compounds is unrelated to the lattice expansion as we have already reported for the nanometric metal Fe (n-Fe). In addition, the observed strong catalytic activity of the nanometric interstitial compounds for mechanical dehydrogenation is not related to their valence electron concentration (VEC) number. However, the n-TiN additive, which is the most effective one for mechanical dehydrogenation, has the smallest average particle size of 20 nm and the largest Specific Surface Area (SSA > 80 m²/g). For thermal dehydrogenation in Stage I the average apparent activation energy, E_A, for the interstitial compound additives is within the range of 87–96 kJ/mol whereas, for comparison, the nanometric metallic additives, n-Fe and n-Ni, exhibit drastically smaller apparent activation energy on the order of 55–70 kJ/mol. The average apparent activation energy for thermal dehydrogenation in Stage II is in the range of 63–80 kJ/mol in the order of E_A(n-ZrC) < E_A(n-Ti = n-TiC) and is lower than that for the nanometric metal additives n-Ni and n-Fe. In summary, the nanometric interstitial compounds do not substantially affect the apparent activation energy of Stage I but are able to reduce the apparent activation energy of thermal dehydrogenation in Stage II. XRD reveals that the interstitial compounds remain stable in the hydride matrix up to the dehydrogenation temperature of at least 165 °C. Ball milled LiAlH₄ containing 5 wt.% n-TiC, n-TiN and n-ZrC is able to slowly discharge large quantities of H₂ up to 5–6 wt.% during storage at 40 °C. Unfortunately, the results of rehydrogenation at 165 °C under 95 bar for 5 h indicate that LiAlH₄ containing the nanometric interstitial compounds exhibits no rehydrogenation.     

Significant effect of TiF3 on the performance of 2NaAlH4+Ca(BH4)2 hydrogen storage properties

Titanium fluoride (TiF₃) is doped into the reactive hydride composite of 2NaAlH₄ + Ca(BH₄)₂ by ball milling to enhance the hydrogen storage properties of the composite system. NaAlH₄ and Ca(BH₄)₂ phases were fully transformed to Ca(AlH₄)₂ and NaBH₄ phases after the ball-milling process (6 h). Four major stages were discovered in the undoped and TiF₃-doped system, which is corresponding to; (i) Ca(AlH₄)₂, (ii) CaAlH₅, (iii) CaH₂ and (iv) NaBH₄, respectively. The addition of TiF₃ to the studied composite resulted in both reduced decomposition temperature and enhanced sorption kinetics compared with the undoped composite. The onset desorption temperature was reduced from 125 °C to 60 °C for the first stage in the TiF₃-doped composite, compared with the undoped composite. From differential scanning calorimetry analysis, the decomposition temperature for all stages has shifted to a lower temperature after doping with TiF₃. The activation energy has greatly reduced by 63.6 and 21.9 kJ/mol for CaAlH₅ and NaBH₄ stages, respectively, as compared with the undoped 2NaAlH₄ + Ca(BH₄)₂ composite. During the dehydrogenation process, the formation of new active species of Al₃Ti together with CaF₂ played a vital role in accelerating the reactions in 5 wt% TiF₃ doped to the studied composite system.