NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

The significantly enhanced dehydrogenation performance of binary complex system, NH₃BH₃/LiBH₄·NH₃, were achieved through a chemical modification of LiH to form ternary composites of x (LiH–NH₃BH₃)/LiBH₄·NH₃. Among the studied composites, 3LiH–3NH₃BH₃/LiBH₄·NH₃ released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 °C with fast kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile byproducts, were released from 3NH₃BH₃/LiBH₄·NH₃ at the same conditions. Further investigations revealed that the hydrogen emission from x (LiH–NH₃BH₃)/LiBH₄·NH₃ composites is based on the combination mechanism of H^δ+ and H^δ− through the interaction between LiH–NH₃BH₃ and NH₃ group in LiBH₄·NH₃, in which the controllable protic hydrogen source from the stabilized NH₃ group played a crucial role in providing optimal stoichiometric ratio of H^δ+ and H^δ−, and thus leading to the significant improvement of dehydrogenation capacity and purity.     

Nanoconfinement of LiBH4·NH3 towards enhanced hydrogen generation

Successful synthesis of LiBH₄·NH₃ confined in nanoporous silicon dioxide (LiBH₄·NH₃@SiO₂) was achieved via a new “ammonia-deliquescence” method, which avoids the involvement of any solvents during the process of synthesis. Compared to the pure LiBH₄·NH₃, the confined LiBH₄·NH₃@SiO₂ exhibited significantly improved dehydrogenation properties, which not only suppressed the emission of NH₃, but also decreased the onset dehydrogenation temperature to 60 °C, thus leading to an enhanced conversion of NH₃ to H₂. In the temperature range of 60–300 °C, the mole ratio of H₂ release for the confined LiBH₄·NH₃@SiO₂ is 85 mol % of the total gas evolved, compared to 2.66 mol % for the pristine LiBH₄·NH₃. Isothermal dehydrogenation results showed that the LiBH₄·NH₃@SiO₂ is able to release about 1.26, 2.09, and 2.35 equiv. of hydrogen, at 150 °C, 200 °C, and 250 °C, respectively. From analysis of the Fourier transform infrared, Raman, and nuclear magnetic resonance spectra of the confined LiBH₄·NH₃@SiO₂ sample heated to various temperatures, as well as its dehydrogenation product under NH₃ atmosphere, it is proposed that the improved dehydrogenation of LiBH₄·NH₃@SiO₂ is mainly attributable to two crucial factors resulting from the nanoconfinement: (1) stabilization of the NH₃ in the nanopores of SiO₂, and (2) enhanced combination of LiBH₄ and NH₃ groups, leading to fast dehydrogenation at low temperature.     

Effects of Mg and Al doping on the electronic structure and dehydrogenation of LiBH4·NH3

The electronic structures and bonding characters, the occupation energies of dopants, as well as the formation energies of Frenkel defects in pure LiBH₄·NH₃ and in Mg- and Al-substituted LiBH₄·NH₃ were investigated by using first-principles calculations. The occupation energies show that the substitutions with Mg and Al destabilize LiBH₄·NH₃ and that Mg substitution is easier than Al substitution. Substitution with Mg or Al partly reduced interactions between B–H and N–H atoms, thus improving the dehydrogenation property of LiBH₄·NH₃. At the same time, substitution with Mg or Al increases the interactions between metal and N atoms, which stabilize the NH₃ group and inhibit the release of NH₃ during dehydrogenation. The formation energy of Frenkel defects indicates that Mg or Al doping facilitates the formation of Frenkel defects. Our theoretical studies show that Mg and Al are good candidates but Al is better than Mg for improving the dehydrogenation property of LiBH₄·NH₃.     

LiBH4·NH3BH3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties

Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results in the formation of a new complex, LiBH₄·NH₃BH₃. Its structure was successfully determined using combined X-ray diffraction and first-principles calculations. LiBH₄·NH₃BH₃ was carefully studied in terms of its decomposition behavior and reversible dehydrogenation property, particularly in comparison with the component phases. In parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding structure changes in the reaction process. Our study found that LiBH₄·NH₃BH₃ first disproportionates into (LiBH₄)₂·NH₃BH₃ and NH₃BH₃, and the resulting mixture exhibits a three-step decomposition behavior upon heating to 450 °C, totally yielding ∼15.7 wt% hydrogen. Interestingly, it was found that h-BN was formed at such a moderate temperature. And owing to the in situ formation of h-BN, LiBH₄·NH₃BH₃ exhibits significantly improved reversible dehydrogenation properties in comparison with the LiBH₄ phase.     

Hydrogen generation from noncatalytic hydrolysis of LiBH4/NH3BH3 mixture for fuel cell applications

Ammonia borane (NH₃BH₃) and lithium borohydride (LiBH₄) are promising hydrides as they contain 19.6 wt.% and 18.5 wt.% hydrogen respectively. However, hydrolysis of NH₃BH₃ needs catalysts or high temperature to initiate the release of hydrogen. On the other hand, hydrolysis of LiBH₄ is incomplete, because the agglomeration of LiBH₄ and its products limits its full utilization. In the present work, hydrolysis performance of LiBH₄/NH₃BH₃ mixture was investigated. The results show that LiBH₄/NH₃BH₃ mixture can fully release its theoretical amount of hydrogen at room temperature without catalysts. In the presence of LiBH₄, NH₃BH₃ can be fully hydrolyzed at room temperature. In return, in the presence of NH₃BH₃, the agglomeration can be avoided resulting in a complete hydrolysis process. Our results indicate that the improvements are attributed to the intermolecular electron migration between LiBH₄ and NH₃BH₃, which changes the reactivity of these compounds. Hydrolytic heat of LiBH₄ also contributes to the promoted hydrolysis of NH₃BH₃. Our results present a novel strategy for noncatalytic hydrolysis of NH₃BH₃ and LiBH₄ for proton exchange membrane fuel cell applications.     

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.     

Reversible hydrogen storage from 6LiBH4-MCl3 (M = Ce, Gd) composites by in-situ formation of MH2

Different destabilized LiBH₄ systems with several interacting components are being explored for hydrogen storage applications. In this study, hydrogen sorption properties of as-milled 6LiBH₄-MCl₃ composites (M = Ce, Gd) are investigated by X-ray diffraction, differential scanning calorimetry and thermovolumetric measurements. The chemical interaction between metal halides and LiBH₄ decreases the dehydrogenation temperature in comparison with as-milled LiBH₄. Hydrogen release starts at 220 °C from the decomposition of M(BH₄)₃ formed during milling and proceeds through destabilization of LiBH₄ by in-situ formed MH₂. The dehydrogenation products CeB₆-LiH and GdB₄-LiH can be rehydrided under moderate conditions, i.e 400 °C and 6.0 MPa of hydrogen pressure for 2 h without catalyst. A new 6LiBH₄-CeCl₃-3LiH composite shows promissory hydrogen storage properties via the formation by milling of CeH_2+x. Our study is the first work about reversible hydrogen storage in LiBH₄-MCl₃ composites destabilized by in-situ formed MH₂.     

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.     

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.     

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.     

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.     

The reactions in LiBH4-NaNH2 hydrogen storage system

Stepwise reactions were observed in the ball milling and heating process of the LiBH₄-NaNH₂ system by means of X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR). During the ball milling process, two concurrent reactions take place in the mixture: 3LiBH₄ + 4NaNH₂ → Li₃Na(NH₂)₄ + 3NaBH₄ and LiBH₄ + NaNH₂ → LiNH₂ + NaBH₄. The heating process from 50 °C to 400 °C is mainly the concurrent reactions of Li₃Na(NH₂)₄ + 3LiBH₄ → 2Li₃BN₂ + NaBH₄ + 8H₂ and 2LiNH₂ + LiBH₄ → Li₃BN₂H₈ → Li₃BN₂ + 4H₂, where the intermediate phases Li₃Na(NH₂)₄ and LiNH₂ serve as the reagents to decompose LiBH₄. The merged equations for the mechanochemical and the heating reactions below 400 °C can be denoted as 3LiBH₄ + 2NaNH₂ → Li₃BN₂ + 2NaBH₄ + 4H₂. The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H₂, which agrees well with the theoretical capacity (5.5 wt.% H₂) of the merged equation and thus demonstrates the suggested pathway. The subsequent step consists of the decompositions of NaBH₄ and Li₃Na(NH₂)₄ within the temperature range of 400 °C-600 °C. The apparent activation energies of the two steps are 114.8 and 123.5 kJ/mol, respectively. They are all lower than that of our previously obtained bulk LiBH₄.     

Hydrogen desorption from LiBH4 destabilized by chlorides of transition metal Fe,Co, and Ni

The dehydrogenation temperature of LiBH₄ was considerably decreased to 230 °C–300 °C when it was manually mixed with FeCl₂, CoCl₂ and NiCl₂ in a molar ratio of 2:1. Mixing LiBH₄ with NiCl₂ or FeCl₂ led to complete hydrogen desorption from LiBH₄, i.e. 18.3 wt% hydrogen was achieved with respect to the weight of LiBH₄. However, the CoCl₂ addition resulted in less hydrogen release due to the formation of diborane. Ball milling treatment for the mixtures of LiBH₄ and these chlorides further decreased hydrogen desorption temperatures. The results indicate that LiBH₄ could be effectively destabilized by the chlorides of Fe, Co and Ni. Small doping of these chlorides into LiBH₄ was found to be effective in enhancing dehydrogenation kinetics of the remaining LiBH₄ due to the formation of metal borides.     

Hydrogen generation from the ball milled composites of sodium and lithium borohydride (NaBH4/LiBH4) and magnesium hydroxide (Mg(OH)2) without and with the nanometric nickel (Ni) additive

The composites of (NaBH₄+2Mg(OH)₂) and (LiBH₄+2Mg(OH)₂) without and with nanometric Ni (n-Ni) added as a potential catalyst were synthesized by high energy ball milling. The ball milled NaBH₄-based composite desorbs hydrogen in one exothermic reaction in contrast to its LiBH₄-based counterpart which dehydrogenates in two reactions: an exothermic and endothermic. The NaBH₄-based composite starts desorbing hydrogen at 240 °C. Its ball milled LiBH₄-based counterpart starts desorbing at 200 °C. The latter initially desorbs hydrogen rapidly but then the rate of desorption suddenly decelerates. The estimated apparent activation energy for the NaBH₄-based composite without and with n-Ni is equal to 152 ± 2.2 and 157 ± 0.9 kJ/mol, respectively. In contrast, the apparent activation energy for the initial rapid dehydrogenation for the LiBH₄-based composite is very low being equal to 47 ± 2 and 38 ± 9 kJ/mol for the composite without and with the n-Ni additive, respectively. XRD phase studies after volumetric isothermal dehydrogenation tests show the presence of NaBO₂ and MgO for the NaBH₄-based composite. For the LiBH₄-based composite phases such as MgO, Li₃BO₃, MgB₂, MgB₆ are the products of the first exothermic reaction which has a theoretical H₂ capacity of 8.1 wt.%. However, for reasons which are not quite clear, the first reaction never goes to full completion even at 300 °C desorbing ∼4.5 wt.% H₂ at this temperature. The products of the second endothermic reaction for the LiBH₄-based composite are MgO, MgB₆, B and LiMgBO₃ and the reaction has a theoretical H₂ capacity of 2.26 wt.%. The effect of the addition of 5 wt.% nanometric Ni on the dehydrogenation behavior of both the NaBH₄-and LiBH₄-based composites is rather negligible. The n-Ni additive may not be the optimal catalyst for these hydride composite systems although more tests are required since only one n-Ni content was examined.     

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.     

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.     

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.     

Promoted hydrogen release from 3LiBH4/MnF2 composite by doping LiNH2: Elimination of diborane release and reduction of decomposition temperature

The evolution of diborane accompanying H₂ release during the decomposition of transition metal borohydrides reduces the purity of evolved hydrogen and results in capacity loss during cycling. To solve the problem, a small amount of LiNH₂ is doped into a 3LiBH₄/MnF₂ composite and the decomposition properties are investigated. The results show that after doping LiNH₂, the formation of diborane during decomposition is effectively suppressed meanwhile the decomposition temperature is significantly reduced. Around 5 wt.% pure hydrogen can be released at 95–140 °C from 5 wt.% LiNH₂-doped 3LiBH₄/MnF₂ composite. These improvements in the decomposition performance are mainly attributed to the prevention of the formation of B–H–B bonds for B₂H₆ and the destabilization of B–H bonds in borohydrides by the interaction of BH₄⁻ and NH₂⁻.