Solid-state synthesis of amorphous TiB2 nanoparticles on graphene nanosheets with enhanced catalytic dehydrogenation of MgH2

A bi-component catalyst TiB₂/GNSs (GNSs is the abbreviation of graphene nanosheets) is synthesized by a solid-state method. Microstructural characterizations based on SEM (scanning electron microscopy), TEM (transmission electron microscopy) and N₂ physisorption show that the size of TiB₂/GNSs catalyst is at nanoscale (20–30 nm) with a surface area of 84.69 m² g⁻¹. The TiB₂/GNSs nanoparticles ball milled with MgH₂ and exhibit enhanced catalytic effects on the dehydrogenation properties of MgH₂ compares to TiB₂ and GNSs individually. DSC (differential scanning calorimetry) measurements confirm that the peak desorption temperature of MgH₂-5 wt%TiB₂/GNSs composites can be lowered more than 44 °C than the pure as-milled MgH₂. And the dehydrogenation kinetics of TiB₂/GNSs-doped MgH₂ is severalfold acceleration compares to the pure as-milled MgH₂. It is proposed that the TiB₂/GNSs nanoparticles could significantly enhance the intimate interface between TiB₂/GNSs and hydride, therefore, provide more active “catalytic sites” and H “diffusion channels” to reduce the dehydrogenation temperature and improve the dehydrogenation kinetics of MgH₂. The synergistic effect of nano-GNSs and TiB₂ nanoparticles contributes to the highly efficient for dehydrogenation of MgH₂-5wt%TiB₂/GNSs composites.     

Improved reversible dehydrogenation of LiBH4–MgH2 composite by the synergistic effects of Al and MgO

In this paper, we report a novel method of improving the reversible dehydrogenation properties of the 2LiBH₄–MgH₂ composite. Our study found that mechanically milling with small amount of Al powder can markedly shorten or even eliminate the problematic incubation period that interrupts the dehydrogenation steps of the 2LiBH₄–MgH₂ composite. But the resulting composite showed serious kinetics degradation upon cycling. In an effort to solve this problem, we found that combined usage of small amounts of Al and MgO enabled the 2LiBH₄–MgH₂ composite to rapidly and reversibly deliver around 9 wt% hydrogen at 400 °C under 0.3 MPa H₂, which compares favorably with the dehydrogenation performance of the composites with transition-metal additives. A combination of phase/microstructural analyses and series of control experiments has been conducted to gain insight into the promoting effects of Al and MgO. It was found that Al and MgO additives act as precursor and promoter for the formation of AlB₂ heterogeneous nucleation sites, respectively.     

Improved reversible dehydrogenation properties of 2LiBH4–MgH2 composite by milling with graphitic carbon nitride

The 2LiBH₄–MgH₂ reactive hydride composite is a promising hydrogen storage system due to the combined high hydrogen capacity and relatively moderate reaction enthalpy. However, the sluggish de/rehydrogenation kinetics severely impedes its practical applications. In this study, graphitic carbon nitride (C₃N₄) as a metal-free additive was added to the 2LiBH₄–MgH₂ composite and examined with respect to the promoting effect on the hydrogen storage properties of the composite. Our study found that mechanically milling with small amount of C₃N₄ additive can eliminate the incubation period between two dehydrogenation steps and thus markedly enhance the dehydrogenation kinetics of the LiBH₄–MgH₂ composite. Further cyclic study found that the composite with C₃N₄ additive exhibits improved cyclic dehydrogenation property although it also shows capacity loss upon cycling, particularly in the second cycle. Combined dehydrogenation property, phase analysis and a series of designed experiments suggested that the C₃N₄ additive could react with both LiBH₄ and MgH₂ in heating process, and the resulting products may improve the reversible dehydrogenation property of the composite system.     

Influence of different amounts of FeCl3 on decomposition and hydrogen sorption kinetics of MgH2

In the present work, the hydrogen storage properties of MgH₂-X wt.% FeCl₃ (X = 5, 10, 15 and 20) are investigated experimentally. It is found that the MgH₂ + 10 wt.% FeCl₃ sample exhibits the best comprehensive hydrogen storage properties, in terms of the onset dehydrogenation temperature, the hydrogen amounts de/reabsorbed as well as the relative de/rehydrogenation rates. The onset dehydrogenation temperature of the 10 wt.% FeCl₃-doped MgH₂ sample is reduced by about 90 °C compared to the as-milled MgH₂, and the sorption kinetics measurements indicate that the FeCl₃-doped sample displays an average dehydrogenation rate 5–6 times faster than that of the undoped MgH₂ sample. Higher levels of doping introduce negative effects, such as lower capacity and slower absorption/desorption rates compared to samples with lower FeCl₃ doping levels. The apparent activation energy for hydrogen desorption is decreased from 166 kJ•mol⁻¹ for as-milled MgH₂ to 130 kJ•mol⁻¹ by the addition of 10 wt.% FeCl₃. It is believed that the improvement of the MgH₂ sorption properties in the MgH₂/FeCl₃ composite is due to the catalytic effects of the in-situ generated Fe species and MgCl₂ that are formed during the heating process.     

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.     

Improved dehydrogenation of MgH2-Li3AlH6 mixture with TiF3 addition

MgH₂-Li₃AlH₆ mixture shows a mutual activation effect between the components. But the dehydrogenation kinetics is still slow, especially at temperature as low as 250 °C. Hereby, an additive (TiF₃) was introduced into the mixture in the present study. The reaction mechanisms were studied by the combined analyses of X-ray diffraction (XRD), thermogravimetric analysis (TGA), as well as thermodynamic calculations. A two-step ball milling method could reduce the mechanical decomposition of Li₃AlH₆ effectively and was adopted. During milling, Li₃AlH₆ reacts with TiF₃ and produces Al₃Ti while MgH₂ remains stable. All the species are well mixed after milling and the grain size is as small as 100 nm. During TGA test, all the reactions occur at lower temperatures compared with undoped mixture, especially the dehydrogenation of MgH₂, which shows a decrease of 60 °C. Its activation energy is reduced by 32.0 kJ mol⁻¹. The first three isothermal (250 °C) cycles indicate that the kinetics of dehydrogenation has been greatly enhanced, showing a reversible capacity of 4.5 wt.% H₂. The time needed for the 1st dehydrogenation has been shortened to 3600 s from 8000 s for the undoped mixture. These improvements are mainly attributed to the catalytic effect of the in-situ formed Al₃Ti. But there is no influence on the rehydrogenation kinetics and the enthalpy of the dehydrogenation of MgH₂ is unchanged.     

A study of the ZrF4, NbF5, TaF5, and TiCl3 influences on the MgH2 sorption properties

Magnesium hydride with 7 wt.% of various metal halide additives (ZrF₄, TaF₅, NbF₅ and TiCl₃) were ball milled, and the influence of these dopants on the kinetics of absorption and desorption was studied. The pressure-composition-temperature isotherms (P-C-T) measured by Sieverts’ apparatus did not show thermodynamic changes in the studied materials. Moreover, XPS studies demonstrated that the metal halides used in this study (except ZrF₄) took part in the partial and full disproportionation reactions directly after milling and the first desorption/absorption cycle. The catalytic effect of metal halides on the Mg hydrogenation/dehydrogenation process was caused by the formation of pure transition metal and/or the MgF₂ phase, which led to the influence of two simultaneous factors on the sorption properties of the MgH₂.     

Improvement of hydrogen storage properties in MgH2 catalysed by K2NbF7

The catalytic effects of K₂NbF₇ on the hydrogen storage properties of MgH₂ have been studied for the first time. MgH₂ + 5 wt% K₂NbF₇ has reduced the onset dehydrogenation temperature to 255 °C, which is 75 °C lower than the as-milled MgH₂. For the rehydrogenation kinetic, at 150 °C, MgH₂ + 5 wt% K₂NbF₇ absorbs 4.7 wt% of hydrogen in 30 min whereas the as-milled MgH₂ only absorbs 0.7 wt% of hydrogen under similar condition. For the dehydrogenation kinetic, at 320 °C, the MgH₂ + 5 wt% K₂NbF₇ is able to release 5.2 wt% of hydrogen in 5.6 min as compared to 0.3 wt% by the as-milled MgH₂ under similar condition. Comparatively, the E_a value of MgH₂ + 5 wt% K₂NbF₇ is 96.3 kJ/mol, which is 39 kJ/mol lower compared to the as-milled MgH₂. The MgF₂, the KH and the Nb that are found after the heating process are believed to be the active species that have improved the system properties. It is concluded that the K₂NbF₇ is a good catalyst to improve the hydrogen storage properties of MgH₂.     

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.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

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.     

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.     

Combined effects of hydrogen back-pressure and NbF5 addition on the dehydrogenation and rehydrogenation kinetics of the LiBH4–MgH2 composite system

It is well known that the dehydrogenation pathway of the LiBH₄–MgH₂ composite system is highly reliant on whether decomposition is performed under vacuum or a hydrogen back-pressure. In this work, the effects of hydrogen back-pressure and NbF₅ addition on the dehydrogenation kinetics of the LiBH₄–MgH₂ system are studied under either vacuum or hydrogen back-pressure, as well as the subsequent rehydrogenation and cycling. For the pristine sample, faster desorption kinetics was obtained under vacuum, but the performance is compromised by slow absorption kinetics. In contrast, hydrogen back-pressure remarkably promotes the absorption kinetics and increases the reversible hydrogen storage capacity, but with the penalty of much slower desorption kinetics. These drawbacks were overcome after doping with NbF₅, with which the dehydrogenation and rehydrogenation kinetics was significantly improved. In particular, the enhanced kinetics was observed to persist well, even after 9 cycles, in the case of the NbF₅ doped sample under hydrogen back-pressure, as well as the suppression of forming Li₂B₁₂H₁₂. Furthermore, the mechanism that is behind these effects of NbF₅ additive on the reversible dehydrogenation reaction of the LiBH₄–MgH₂ system is discussed.     

In operando study of TiVCr additive in MgH2 composites

We report an in operando study of the hydrogenation and dehydrogenation of MgH₂–TiVCr composites. The experiment was performed by means of in situ synchrotron XRD in order to get insights on the influence of the TiVCr additive on the sorption properties of the MgH₂ based composite. Sequential Rietveld refinement analysis was performed to investigate the structural changes of MgH₂ and of the additive during hydrogenation and dehydrogenation processes. Significant non-monotonic changes in the lattice volume of the TiVCrH_x solid solution were observed concomitantly to the MgH₂ formation or decomposition. These volume changes are assigned to the variation of the hydrogen content in TiVCrH_x. These results provide evidence of cooperative effects between the H₂ storage material and the additive.     

Improved reversible dehydrogenation properties of LiBH4–MgH2 composite by tailoring nanophase structure using activated carbon

In this study, activated carbon (AC) was added to the 2LiBH₄–MgH₂ composite and examined with respect to its effect on the hydrogen storage properties of the system. Our study found that AC is an effective additive for promoting the reversible dehydrogenation of the 2LiBH₄–MgH₂ composite. A series of control experiments were carried out to optimize the sample preparation method, milling time and addition amount of AC. In comparison with the neat LiBH₄–MgH₂ system, the LiBH₄–MgH₂–AC composite prepared under optimized conditions exhibits enhanced dehydrogenation kinetics, improved cyclic stability and particularly, eliminated incubation period between the two dehydrogenation stages. A combination of phase/microstructure/chemical state analyses has been conducted to gain insight into the promoting effect of AC on the reversible dehydrogenation of the 2LiBH₄–MgH₂ system. Our study found that AC exerts its promoting effect via tailoring nanophase structure of the 2LiBH₄–MgH₂ composite.     

Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles

2LiBH₄/MgH₂ system is a representative and promising reactive hydride composite for hydrogen storage. However, the high desorption temperature and sluggish desorption kinetics hamper its practical application. In our present report, we successfully introduce CoNiB nanoparticles as catalysts to improve the dehydrogenation performances of the 2LiBH₄/MgH₂ composite. The sample with CoNiB additives shows a significant desorption property. Temperature programmed desorption (TPD) measurement demonstrates that the peak decomposition temperatures of MgH₂ and LiBH₄ are lowered to be 315 °C and 417 °C for the CoNiB-doped 2LiBH₄/MgH₂. Isothermal dehydrogenation analysis demonstrates that approximately 10.2 wt% hydrogen can be released within 360 min at 400 °C. In addition, this study gives a preliminary evidence for understanding the CoNiB catalytic mechanism of 2LiBH₄/MgH₂     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

A novel complex oxide TiVO3.5 as a highly active catalytic precursor for improving the hydrogen storage properties of MgH2

Herein, we demonstrate the successful preparation of a novel complex transition metal oxide (TiVO_3.5) by oxidizing a solid-solution MXene (Ti_0.5V_0.5)₃C₂ at 300 °C and its high activity as a catalyst precursor in the hydrogen storage reaction of MgH₂. The prepared TiVO_3.5 inherits the layered morphology of its MXene precursor, but the layer surface becomes very coarse because of the presence of numerous nanoparticles. Adding a minor amount of TiVO_3.5 remarkably reduces the dehydrogenation and hydrogenation temperatures of MgH₂ and enhances the reaction kinetics. The 10 wt% TiVO_3.5-containing sample exhibits optimal hydrogen storage properties, as it desorbs approximately 5.0 wt% H₂ in 10 min at 250 °C and re-absorbs 3.9 wt% H₂ in 5 s at 100 °C and under 50 bar of hydrogen pressure. The apparent activation energy is calculated to be approximately 62.4 kJ/mol for the MgH₂-10 wt% TiVO_3.5 sample, representing a 59% reduction in comparison with pristine MgH₂ (153.8 kJ/mol), which reasonably explains the remarkably reduced dehydrogenation operating temperature. Metallic Ti and V are detected after ball milling with MgH₂; they are uniformly dispersed on the MgH₂ matrix and act as actual catalytic species for the improvement of the hydrogen storage 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.     

Hydrogen sorption kinetics of MgH2 catalyzed with titanium compounds

Identification of effective catalyst is a subject of great interest in developing MgH₂ system as a potential hydrogen storage medium. In this work, the effects of typical titanium compounds (TiF₃, TiCl₃, TiO₂, TiN and TiH₂) on MgH₂ were systematically investigated with regard to hydrogen sorption kinetics. Among them, adding TiF₃ leads to the most pronounced improvement on both absorption and desorption rates. Comparative studies indicate that the TiH₂ and MgF₂ phases in situ introduced by TiF₃ fail to explain the superior catalytic activity. However, a positive interaction between TiH₂ and MgF₂ is observed. Detailed comparison between the effect of TiF₃ and TiCl₃ additive suggests the catalytic role of F anion. XPS examination reveals that new bonding state(s) of F anion is formed in the MgH₂ + TiF₃ system. On the basis of these results, we propose that the substantial participation of F anion in the catalytic function contributes to the superior activity of TiF₃.