Catalytic effect of Al2TiO5 on the dehydrogenation properties of LiAlH4

Lithium alanate (LiAlH₄) is considered as a promising material for storing hydrogen (H₂) in solid-state form for onboard applications due to its advantage of high gravimetric H₂ capacity. LiAlH₄ could release H₂ ～7.9 wt.% when heated up to ～250 °C. Nevertheless, the high desorption temperature, sluggish desorption kinetics, and irreversibility hamper the application of LiAlH₄ for solid-state H₂ storage materials. Therefore, in this study, we have used aluminum titanate (Al₂TiO₅) as an additive to diminish the desorption temperature and enhance the desorption kinetics of LiAlH₄. The addition of a small amount of Al₂TiO₅ (5 wt.%) into LiAlH₄ significantly decreased the decomposition temperature and enhanced the desorption kinetics, in which Al₂TiO₅-doped LiAlH₄ started to release H₂ at ～90 °C and was able to desorb H₂ as much as ～3.5 wt.% at 90 °C within 1 h. Without the catalyst, pure LiAlH₄ starts to release H₂ at ～145 °C and only desorbs H₂ as low as 0.3 wt.% at 90 °C within 1 h. The activation energies for H₂ release in the two-step desorption process of LiAlH₄ were reduced after catalysis with Al₂TiO₅. The activation energies of as-milled LiAlH₄ were 80 kJ/mol and 91 kJ/mol, respectively, as calculated by the Arrhenius plot. The activation energies were lowered to 68 kJ/mol and 79 kJ/mol after milling with Al₂TiO₅. The scanning electron microscopy images revealed that the LiAlH₄ particles became smaller and less agglomerated when Al₂TiO₅ was added. It is believed that the in-situ formation of active species during the desorption process and reduction in particles size play a vital role in improving the dehydrogenation properties of the Al₂TiO₅-doped LiAlH₄ system.     

Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

Study on the catalytic roles of MgFe₂O₄ on the dehydrogenation performance of LiAlH₄ was carried out for the first time. Notable improvement on the dehydrogenation of LiAlH₄–MgFe₂O₄ compound was observed. The initial decomposition temperatures for the catalyzed LiAlH₄ were decreased to 95 °C and 145 °C for the first and second stage reactions, which were 48 °C and 28 °C lower than the milled LiAlH₄. As for the desorption kinetics performance, the MgFe₂O₄ doped-LiAlH₄ sample was able to desorb faster with a value of 3.5 wt% of hydrogen in 30 min (90 °C) while the undoped LiAlH₄ was only able to desorb 0.1 wt% of hydrogen. The activation energy determined from the Kissinger analysis for the first two desorption reactions were 73 kJ/mol and 97 kJ/mol; which were 31 and 17 kJ/mol lower as compared to the milled LiAlH₄. The X-ray diffraction result suggested that the MgFe₂O₄ had promoted significant improvements by reducing the LiAlH₄ decomposition temperature and faster desorption kinetics through the formation of active species of Fe, LiFeO₂ and MgO that were formed during the heating process.     

Mechanochemical synthesis and dehydrogenation properties of Yb(AlH4)3

The facile synthesis of ytterbium tetrahydroaluminate Yb(AlH₄)₃ is conducted by a mechanochemical procedure under hydrogen atmosphere for the first time. Results show that the synthesized Yb(AlH₄)₃ remains as an amorphous state. The thermal decomposition of Yb(AlH₄)₃ goes through a four-stage pathway with several amorphous intermediate phases during the process. The first dehydrogenation step of Yb(AlH₄)₃ presents a relatively low apparent activation energy of 99.6 kJ mol^?1, and ninety percent of the hydrogen from this stage can be liberated within 20 min at 160 ^°C. Rehydrogenation tests above 160 ^°C and 14 MPa hydrogen pressure demonstrate the unsuccessful rehydrogenations of the first decomposition step due to the formation of a thermodynamically more stable compound YbHCl.     

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.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

On the dehydrogenation of LiAlH4 enhanced by Ti salts and cryogenic ball-milling

Several mixtures of LiAlH₄ and Ti salts (TiH₂, TiF₃, and TiCl₄) were produced using short milling times and cryogenic (liquid nitrogen) cooling. The stoichiometric (2:1) and 5 mol% mixtures LiAlH₄/TiH₂ demonstrated minor improvements on the dehydrogenation temperature of LiAlH₄. Conversely, an enhancement of the dehydrogenation reaction was observed in the LiAlH₄ added with 5 mol% of TiCl₄ and in the stoichiometric mixture 3LiAlH₄ + TiF₃. In these mixtures, an important reduction of the dehydrogenation temperature was observed (37 °C and 55 °C on-set temperature, respectively). This improvement was promoted by the use of cryogenic ball milling and careful control of the energy added to the mixtures during ball milling.     

Ultra-fast dehydrogenation behavior at low temperature of LiAlH4 modified by fluorographite

LiAlH₄ modified by different weight ratios of fluorographite (FGi) can be synthesized through mechanical ball-milling and their dehydrogenation behaviors were investigated. LiAlH₄ particles distributed on the FGi surface with greatly decreased sizes are achieved, comparing with ball-milled pristine LiAlH₄. Greatly reduced dehydrogenation temperatures are discovered in LiAlH₄-FGi composites. Among these composites, LiAlH₄-40FGi composite exhibits an ultra-fast hydrogen release at very low temperature as 61.2 °C, and 5.7 wt% hydrogen is liberated in seconds. Besides, the released hydrogen is of high purity according to MS test. Furthermore, XRD analysis on the dehydrogenated products proves that FGi changes the dehydrogenation reaction pathway of LiAlH₄, through which the dehydrogenation reaction enthalpy change is remarkably reduced, leading to greatly improved hydrogen desorption properties. Such investigations have discovered the potential of solid-state way of producing hydrogen under ambient temperatures.     

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.     

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.     

Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Recent advances in catalyst-enhanced LiAlH4 for solid-state hydrogen storage: A review

LiAlH₄ is regarded as a potential material for solid-state hydrogen storage because of its high hydrogen content (10.5 wt%). However, its high decomposition temperature, slow dehydrogenation kinetics and irreversibility under moderate condition hamper its wider applications. Mechanical milling treatment and doping with a catalyst or additive has drawn significant ways to improve hydrogen storage properties of LiAlH₄. Microstructure or nanostructure materials were developed by using a ball milling technique and doping with various types of catalysts or additives which had dramatically improved the efficiency of LiAlH₄. However, the state-of-the-art technologies is still far from meeting the expected goal for the applications. In this paper, the overview of the recent advances in catalyst-enhanced LiAlH₄ for solid-state hydrogen storage is detailed. The remaining challenges and the future prospect of LiAlH₄–catalyst system is also discussed. This paper is the first systematic review that focuses on catalyst-enhanced LiAlH₄ for solid-state hydrogen storage.     

Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2

In this study, the hydrogen storage properties of MgH₂ with the addition of K₂TiF₆ were investigated for the first time. The temperature-programmed desorption results showed that the addition of 10 wt% K₂TiF₆ to the MgH₂ exhibited a lower onset desorption temperature of 245 °C, which was a decrease of about 105 °C and 205 °C compared with the as-milled and as-received MgH₂, respectively. The dehydrogenation and rehydrogenation kinetics of 10 wt% K₂TiF₆-doped MgH₂ were also significantly improved compared to the un-doped MgH₂. The results of the Arrhenius plot showed that the activation energy for the hydrogen desorption of MgH₂ was reduced from 164 kJ/mol to 132 kJ/mol after the addition of 10 wt% K₂TiF₆. Meanwhile, the X-ray diffraction analysis showed the formation of a new phase of potassium hydride and titanium hydride together with magnesium fluoride and titanium in the doped MgH₂ after the dehydrogenation and rehydrogenation process. It is reasonable to conclude that the K₂TiF₆ additive doped with MgH₂ played a catalytic role through the formation of active species of KH, TiH₂, MgF₂ and Ti during the ball milling or heating process. It is therefore proposed that this newly developed product works as a real catalyst for improving the hydrogen sorption properties of MgH₂.     

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.     

Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene

The investigation of thermally induced dehydrogenation of LiBH₄ reveals that LiBH₄ doped with the graphene catalysts shows superior dehydrogenation and rehydrogenation performance to that of Vulcan XC-72, carbon nanotube and BP2000 doped LiBH₄. For doping with 20 wt.% graphene, thermal dehydrogenation of LiBH₄ is found to start at ca. 230 °C and a total weight loss of 11.4 wt.% can be obtained below 700 °C. With increased loading of graphene within a LiBH₄ sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH₄ are found to decrease while the hydrogen release amount is found to increase. Moreover, variation of the equilibrium pressure obtained from isotherms measured at 350–450 °C indicate the dehydrogenation enthalpy is reduced from 74 kJ mol⁻¹ H₂ for pure LiBH₄ to ca. 40 kJ mol⁻¹ H₂ for 20 wt.% graphene doped LiBH₄. Importantly, the reversible dehydrogenation/rehydrogenation process was achieved under 3 MPa H₂ at 400 °C for 10 h, with a capacity of ca. 4.0 wt.% in the tenth cycle. Especially, LiBH₄ is reformed and new species, Li₂B₁₀H₁₀, is detected after the rehydrogenation process.     

Role of particle size,grain size,microstrain and lattice distortion in improved dehydrogenation properties of the ball-milled Mg(AlH4)2

The decreased dehydrogenation temperature and improved dehydrogenation kinetics were achieved by high-energy ball milling Mg(AlH₄)₂. The particle size, grain size, microstrain and lattice distortion of the post-milled samples, i.e., from macro- to micro-scale, were systematically characterized by means of SEM and XRD measurements. The results indicated that the high-energy ball milling process led to not only a decrease in the particle size and grain size but also an increase in the microstrain and lattice distortion, which provides a synergetic effect of the thermodynamics and kinetics on lowering the dehydrogenation temperatures of the post-milled Mg(AlH₄)₂ samples. From the kinetic point of view, the refinement of the particles and grains shortens the diffusion distance, and the increase of the microstrain and lattice distortion enhances the diffusivity, which work together to decrease the apparent activation energy for hydrogen desorption. Besides, the presence of microstrain and lattice distortion increased the free energy of the post-milled samples, which was released by recovery and recrystallization processes upon heating. This offers more heat release during the first-step dehydrogenation, consequently leading to thermodynamically decline in dehydrogenation temperatures of the post-milled samples. Such a finding provides insights into the mechanistic understanding on decreased dehydrogenation temperature and improved dehydrogenation kinetics of the post-milled metal hydrides as hydrogen storage materials.     

Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH₄)₂ were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH₄ units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH₄)₂ in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (Ti_iA) and substitute for the Al atom (Ti_Al), to form a high-coordination structure TiH_n (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in Ti_Al and with the Al 3p electrons in Ti_iA, which weakens the Al–H bond of adjacent AlH₄ units and facilitates the hydrogen dissociation. A TiAl₃H₁₃ intermediate in Ti_iA is inferred as the precursor of Mg(AlH₄)₂ dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH₄ tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH₄)₂ because of its abundant activated hydrogen atoms and low hydrogen removal energy.     

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.     

Understanding the dehydrogenation properties of MgH2 catalysed by Na3AlF6

The catalytic effect of Na₃AlF₆ on the dehydrogenation properties of the MgH₂ with X wt% (X = 5, 10, 20 and 50) have been investigated by ball milling technique. Based on the temperature-programme-desorption result, the addition of 10 wt% Na₃AlF₆ to the MgH₂ has demonstrated the best dehydrogenation properties performance. The dehydrogenation temperature of the un-doped MgH₂ has experienced a reduction for about 60 °C after doped with 10 wt% Na₃AlF₆. The dehydrogenation kinetics also has been improved with the addition of 10 wt% Na₃AlF₆. Based on the Kissinger analysis, it was observed that the apparent activation energy of MgH₂ desorption is remarkably decreased from 158 kJ/mol to 129 kJ/mol with the addition of 10 wt% Na₃AlF₆. Meanwhile, the formations of new species, the NaMgF₃, the NaF and the AlF₃ in the doped composite after the de/rehydrogenation processes are found in the X-ray diffraction analysis. These new species are expected to act as the active species that probably contributes to enhance the dehydrogenation properties of MgH₂.     

Low-temperature hydrogen release through LiAlH4 and NH4F react in Et2O

The application of hydrogen energy urgently requires a high-capacity hydrogen storage technology that can release hydrogen at low temperature. The composite of LiAlH₄ and NH₄F has a hydrogen storage capacity of up to 8.06 wt%, but the release of hydrogen requires a reaction temperature of about 170 °C, and the reaction is difficult to control. In this work, the reaction between LiAlH₄ and NH₄F is proposed to be carried out in diethyl ether to improve its hydrogen release performance. It exhibits good hydrogen release performance over a wide temperature range of −40–25 °C, and the hydrogen release capacity at −40 °C, −20 °C, 0 °C and 25 °C can reach 4.41 wt%, 6.79 wt%, 6.85 wt% and 7.78 wt%, respectively. The activation energy of the reaction is 38.41 kJ mol⁻¹, which is much lower than many previously reported catalytic hydrolysis systems that can release hydrogen at room temperature. Our study demonstrates a high-performance hydrogen storage system with very low operating temperature, which may lay the foundation for the development of practical mobile/portable hydrogen source in the north and the Arctic.     

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.