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.     

Enhanced hydrogen storage properties of LiBH4 modified by NbF5

In this work, the hydriding–dehydriding properties of the LiBH₄–NbF₅ mixtures were investigated. It was found that the dehydrogenation and reversibility properties of LiBH₄ were significantly improved by NbF₅. Temperature-programed dehydrogenation (TPD) showed that 5LiBH₄–NbF₅ sample started releasing hydrogen from as low as 60 °C, and 4 wt.% hydrogen could be obtained below 255 °C. Meanwhile, ∼7 wt.% H₂ could be reached at 400 °C in 20LiBH₄–NbF₅ sample, whereas pristine LiBH₄ only released ∼0.7 wt.% H₂. In addition, reversibility measurement demonstrated that over 4.4 wt.% H₂ could still be released even during the fifth dehydrogenation in 20LiBH₄–NbF₅ sample. The experimental results suggested that a new borohydride possibly formed during ball milling the LiBH₄–NbF₅ mixtures might be the source of the active effect of NbF₅ on LiBH₄.     

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 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     

Significant effects of graphite fragments on hydrogen storage performances of LiBH4: A first-principles approach

The graphite fragments are used to establish the calculated models of LiBH₄ mechanically milled by graphite additive. Density-functional theory calculations have been performed to optimize the crystal structures of LiBH₄ inserted by different graphite fragments. It is found that the unsaturated sites introduced by graphite fragments are occupied by BH₃ complexes and H atoms. The dehydrogenation energies are calculated to investigate the dehydrogenation ability of LiBH₄ with graphite fragments. The results show that BH₃ complexes bonded with graphite fragments have lower dehydrogenation energy than pure LiBH₄, which is an important factor of the improvement of thermodynamic property of LiBH₄ system. The hydrogen diffusion behaviors of graphite fragments doped LiBH₄ are investigated by the nudged elastic band method. It is found that the hydrogen diffusion barriers of β-H (the H atoms of BH₃ complexes bonding with C atoms) diffusing to other sites are small in the LiBH₄/graphite fragment systems. The results show that the existence of β-H type significantly enhances the hydrogen diffusion kinetics of the graphite fragments doped LiBH₄.     

Hydrogen storage performance of LiBH4+1/2MgH2 composites improved by Ce-based additives

LiBH₄+1/2MgH₂ is a promising reactive hydride composite for hydrogen storage. In the present study, three Ce-based additives were used as catalysts to enhance the hydrogen storage performance of LiBH₄+1/2MgH₂ composites. The composites with Ce additives demonstrated significantly improved dehydrogenation kinetics and cyclic stability compared with the pure composite. X-ray diffraction and scanning electron microscopy analyses clearly revealed the phase transitions and morphological evolution during the hydriding-dehydriding cycling. The composites with Ce-based additives displayed stable nanostructures, in contrast to the rapid microstructural deterioration in the uncatalyzed composite. The CeB₆ formed in the composites had a particle size of 10 nm after five cycles. It may act as the nucleus for MgB₂ formation during dehydrogenation and thus account for the structural and performance stability of the composites upon cycling.     

Hydrogen storage performance of 5LiBH4 + Mg2FeH6 composite system

The hydrogen storage properties of 5LiBH₄ + Mg₂FeH₆ reactive hydride composites for reversible hydrogen storage were investigated by comparing with the 2LiBH₄ + MgH₂ composite in the present work. The dehydrogenation pathway and reaction mechanism of 5LiBH₄ + Mg₂FeH₆ composite were also investigated and elucidated. The self-decomposition of Mg₂FeH₆ leads to the in situ formation of Mg and Fe particles on the surface of LiBH₄, resulting in a well dispersion between different reacting phases. The formation of FeB is observed during the dehydrogenation of 5LiBH₄ + Mg₂FeH₆ composite, which might supplies nucleation sites of MgB₂ during the dehydrogenation process, but is not an ascendant catalyst for the self-decomposition of LiBH₄. And FeB can also transform to the LiBH₄ and Fe by reacting with LiH and H₂ during the rehydrogenation process. The dehydrogenation capacity for 5LiBH₄ + Mg₂FeH₆ composite still gets to 6.5 wt% even after four cycles. The X-ray diffraction analyses reveal the phase transitions during the hydriding and dehydriding cycle. The formed FeB in the composite maintains a nanostructure after four hydriding-dehydriding cycles. The loss of hydrogen storage capacity and de-/rehydrogenation kinetics can be attributed to the incomplete generation of Mg₂FeH₆ during the rehydrogenation process.     

Confined LiBH4: Enabling fast hydrogen release at ∼100 °C

LiBH₄ has been attracted tremendous research interest as a hydrogen storage material for mobile applications due to its very high gravimetric hydrogen capacity of 18.6 wt%. However, its real use is heavily hindered by the high operational temperature that is required above 350 °C to release hydrogen with various recent improvements. This is obviously much higher than the ambient temperature of about 100 °C. In this paper, we report the synthesis of LiBH₄ confined by SBA-15 template, which achieves fast hydrogen release of LiBH₄ at ∼100 °C. The confined LiBH₄ system starts to release hydrogen at only 45 °C and can release 8.5 wt% hydrogen (on the basis of LiBH₄ itself) within 10 min at 105 °C, which opens a new window and overcome the most challenging barrier to realize practical hydrogen storage of LiBH₄.     

Pressure-induced phase transitions in LiBH4: Density functional theory calculations

Using pseudopotential density functional theoretical methods, we systematically study the phase stability, structural properties and high-pressure behaviors of LiBH₄. The total-energy calculations show that the orthorhombic structure with Pnma symmetry found by experiments [J. Alloys Compd. 346, 200 (2002)] is more stable than the other proposed structures at 0 K and 0 GPa. The calculated Pnma structural parameters agree well with experimental results. With the pressure extracted directly from first-principles calculations, we predict that the Pnma to Pnma* [Phys. Rev. Lett. 104, 215501 (2010)] and the Pnma* to P-421c structural phase transitions occur at 2.0 and 11.6 GPa respectively, accompanied with volume contractions of 1.02% and 2.78%. It may reduce the volume requirement of hydrogen storage. We find that the Vinet EOS fitting can introduce some errors in predicting structural phase transitions of LiBH₄. A detailed study of the electronic structures reveals the bonding characteristics between B and H and between Li and H as well as the nonmetallic features of Pnma, Pnma* and P-421c structures.     

Enhanced desorption properties of LiBH4 incorporated into mesoporous TiO2

LiBH₄ nano-particles are incorporated into mesoporous TiO₂ scaffolds via a chemical impregnation method. And the enhanced desorption properties of the composite have been investigated. The LiBH₄/TiO₂ sample starts to release hydrogen at 220 °C and the maximal desorption peak occurs at about 330 °C, much lower compared to the bulk LiBH₄. Furthermore, the composite exhibits excellent dehydrogenation kinetics, with 11 wt% of hydrogen liberated from LiBH₄ at 300 °C within 3 h. X-ray diffraction and Fourier transform infrared spectroscopy are used to confirm the nanostructure of LiBH₄ in the TiO₂ scaffold. This work demonstrates that confinement within active porous scaffold host is a promising approach for enhancing hydrogen decomposition properties of light-metal complex hydrides.     

Enhanced dehydrogenation properties of LiBH4 compositing with hydrogenated magnesium-rare earth compounds

LiBH₄ is regarded as a promising hydrogen storage material due to its high hydrogen density. In this study, the dehydrogenation properties of LiBH₄ were remarkably enhanced by doping hydrogenated Mg₃RE compounds (RE denotes La, Ce, Nd rare earth metals), which are composed of nanostructured MgH₂ and REH_2+x (denoted as H − Mg₃RE). For the LiBH₄ + H − Mg₃La mixture, the component LiBH₄ desorbed 6 wt.% hydrogen even at a relatively low temperature of 340 °C, far lower than the desorption temperature of pure LiBH₄ or the 2LiBH₄ + MgH₂ system. This kinetic improvement is attributed to the hydrogen exchange mechanism between the H − Mg₃La and LiBH₄, in the sense that the decomposition of MgH₂ and LaH_2+x catalyzed the dehydrogenation of LiBH₄ through hydrogen exchange effect rather than mutual chemical reaction requiring higher temperature and hydrogen pressure. However, prior to fast hydrogen release, the hydrogen exchange effect suppressed the dehydriding of MgH₂ and elevated its desorption temperature. It is expected to strengthen the hydrogen exchange effect by compositing the LiBH₄ with other nanosized metal hydrides and to obtain better dehydrogenation properties.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

Remarkable irreversible and reversible dehydrogenation of LiBH4 by doping with nanosized cobalt metalloid compounds

Nanosized cobalt sulfide and cobalt boride were synthesized and doped into LiBH₄ to improve the dehydrogenation properties of this important candidate for hydrogen storage. With respect to CoS_x doping, the dehydrogenation temperature (peak temperature observed by mass spectrometry) of pristine LiBH₄ can be reduced from 440 °C to 175 °C with a maximum capacity of 6.7 wt% at 50% doping. Unfortunately, B₂H₆ is liberated and the process is not reversible because the CoS_x dopant reacts with LiBH₄ to form more stable compounds. By changing CoS_x to CoB_x, a reversible dehydrogenation was realized with greatly improved reversibility. The dehydrogenation temperature was reduced to 350 °C with a maximum capacity of 8.4 wt% at 50% doping amount. It is very significant that CoB_x is stable and the release of B₂H₆ is eliminated. A reversible hydrogen desorption of about 5.3 wt% can be achieved with a LiBH₄ + 50% CoB_x mixture under a mild rehydrogenation condition of 400 °C at 10 MPa H₂. It is obvious that CoS_x acts as a reactant even though the dehydrogenation is greatly enhanced, while CoB_x behaves as a catalyst significantly promoting the dehydrogenation and reversibility of LiBH₄.     

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.     

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

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

Destabilization of LiBH4 by SrF2 for reversible hydrogen storage

The de-/rehydrogenation features of the 6LiBH₄/SrF₂ reactive hydride system have been systematically investigated. It was found that the thermal stability of LiBH₄ can be reduced markedly by combining it with SrF₂. Dehydrogenation of the 6LiBH₄/SrF₂ system proceeds via the 6LiBH₄ + SrF₂ → SrB₆ + 2LiF + 4LiH + 10H₂ reaction, which involves SrH₂ as the intermediate product. The dehydrogenation enthalpy change was experimentally determined to be 52 kJ/mol H₂ based on the P–C isotherm analysis. For rehydrogenation, LiBH₄ and SrF₂ were regenerated along with LiSrH₃ at 450 °C under ～8 MPa hydrogen pressure; thus, approximately 5.2 wt% of hydrogen can be released during the second dehydrogenation process.     

Improved hydrogen storage reversibility of LiBH4 destabilized by Y(BH4)3 and YH3

In the present study, the synthesis of two different LiBH₄–Y(BH₄)₃ and LiBH₄–YH₃ composites was performed by mechanochemical processing of the 4LiBH₄–YCl₃ mixture and as-milled 4LiBH₄–YCl₃ plus 3LiH. It was found that Y(BH₄)₃ and YH₃ formed in situ during milling are effective to promote LiBH₄ destabilization but differ substantially from each other in terms of the dehydrogenation pathway. During LiBH₄–Y(BH₄)₃ dehydriding, Y(BH₄)₃ decomposes first generating in situ freshly YH₃ and subsequently, it destabilizes LiBH₄ with the formation of minor amounts of YB₄. About 20% of the theoretical hydrogen storage was obtained via the rehydriding of YB₄–4LiH–3LiCl at 400 °C and 6.5 MPa. As a novel result, a compound containing (B₁₂H₁₂)²⁻ group was identified during dehydriding of Y(BH₄)₃. In the case of 4LiBH₄–YH₃ dehydrogenation, the increase of the hydrogen back pressure favors the formation of crystalline YB_4, whereas a reduction to ≤0.1 MPa induces the formation of minor amounts of Li₂B₁₂H₁₂. Although for hydrogen pressures ≤0.1 MPa direct LiBH₄ decomposition can occur, the main dehydriding pathway of 4LiBH₄–YH₃ composite yields YB₄ and LiH. The nanostructured composite obtained by mechanochemical processing gives good hydrogen storage reversibility (about 80%) regardless of the hydrogen back pressure.     

The effects of Ti-based additives on the kinetics and reactions in LiH/MgB2 hydrogen storage system

In this work we investigated the effect of Ti, TiH₂, TiB₂, TiCl₃, and TiF₃ additives on the hydrogen de/re-sorption kinetics and reaction pathways of LiH/MgB₂ mixture. From high pressure differential scanning calorimeter (HP-DSC) measurements it was found that these additives all effectively decrease the onset temperature of hydrogenation. The isothermal hydrogenation/dehydrogenation measurements suggest that Ti, TiH₂, and TiB₂ can significantly improve the hydrogen sorption kinetics of LiH/MgB₂ mixture. The absorption kinetics of TiF₃ and TiCl₃ doped samples are slower than the baseline (2LiH-MgB₂ without additive), but their desorption kinetics are faster than the baseline and other additives doped systems. X-ray diffraction (XRD) analysis reveals that the additive Ti in LiH/MgB₂ actively participates in both hydrogenation and dehydrogenation process, which can be regarded as an effective additive of this system.