Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Tuning the reaction mechanism and hydrogenation/dehydrogenation properties of 6Mg(NH2)29LiH system by adding LiBH4

The hydrogen storage properties of 6Mg(NH₂)₂9LiH-x(LiBH₄) (x = 0, 0.5, 1, 2) system and the role of LiBH₄ on the kinetic behaviour and the dehydrogenation/hydrogenation reaction mechanism were herein systematically investigated. Among the studied compositions, 6Mg(NH₂)₂9LiH2LiBH₄ showed the best hydrogen storage properties. The presence of 2 mol of LiBH₄ improved the thermal behaviour of the 6Mg(NH₂)₂9LiH by lowering the dehydrogenation peak temperature nearly 25 °C and by reducing the apparent dehydrogenation activation energy of about 40 kJ/mol. Furthermore, this material exhibited fast dehydrogenation (10 min) and hydrogenation kinetics (3 min) and excellent cycling stability with a reversible hydrogen capacity of 3.5 wt % at isothermal 180 °C. Investigations on the reaction pathway indicated that the observed superior kinetic behaviour likely related to the formation of Li₄(BH₄)(NH₂)₃. Studies on the rate-limiting steps hinted that the sluggish kinetic behaviour of the 6Mg(NH₂)₂9LiH pristine material are attributed to an interface-controlled mechanism. On the contrary, LiBH₄-containing samples show a diffusion-controlled mechanism. During the first dehydrogenation reaction, the possible formation of Li₄(BH₄)(NH₂)₃ accelerates the reaction rates at the interface. Upon hydrogenation, this ‘liquid like’ of Li₄(BH₄)(NH₂)₃ phase assists the diffusion of small ions into the interfaces of the amide-hydride matrix.     

Releasing 9.6 wt% of H2 from Mg(NH2)2–3LiH–NH3BH3 through mechanochemical reaction

Ball milling the mixture of Mg(NH₂)₂, LiH and NH₃BH₃ in a molar ratio of 1:3:1 results in the direct liberation of 9.6 wt% H₂ (11 equiv. H), which is superior to binary systems such as LiH–AB (6 equiv. H), AB–Mg(NH₂)₂ (No H₂ release) and LiH–Mg(NH₂)₂ (4 equiv. H), respectively. The overall dehydrogenation is a three-step process in which LiH firstly reacts with AB to yield LiNH₂BH₃ and LiNH₂BH₃ further reacts with Mg(NH₂)₂ to form LiMgBN₃H₃. LiMgBN₃H₃ subsequently interacts with additional 2 equivalents of LiH to form Li₃BN₂ and MgNH as well as hydrogen.     

Synthesis and dehydrogenation of LiCa(NH2)3(BH3)2

We report the synthesis of a new hydrogen storage material with a composition of LiCa(NH₂)₃(BH₃)₂. The theoretical hydrogen capacity of LiCa(NH₂)₃(BH₃)₂ is 9.85 wt.%. It can be prepared by ball milling the mixture of calcium amidoborane (Ca(NH₂BH₃)₂) and lithium amide (LiNH₂) in a molar ratio of 1:1. The experimental results show that this material starts to release hydrogen at a temperature as low as ca. 50 °C, which is ca. 70 °C lower than that of pure Ca(NH₂BH₃)₂ possibly resulting from the active interaction of NH₂⁻ in LiNH₂ with BH₃ in Ca(NH₂BH₃)₂. ca. 4.1 equiv. or 6.8 wt.% hydrogen can be released at 300 °C. The dehydrogenation is a mildly exothermic process forming stable nitride products.     

The dehydrogenation kinetics and reversibility improvements of Mg(BH4)2 doped with Ti nano-particles under mild conditions

Magnesium borohydride, Mg(BH₄)₂, is ball-milled with Ti nano-particles. Such catalyzed Mg(BH₄)₂ releases more hydrogen than pristine Mg(BH₄)₂ does during isothermal dehydrogenation at 270, 280, and 290 °C. The catalyzed Mg(BH₄)₂ also exhibits better dehydrogenation kinetics than the pristine Mg(BH₄)₂. Based on kinetics model fitting, the activation energy (E_a) of the catalyzed Mg(BH₄)₂ is calculated to be lower than pristine Mg(BH₄)₂. During partial dehydrogenation, the catalyzed Mg(BH₄)₂ releases 4.23 wt % (wt%) H₂ for the second dehydrogenation at 270 °C, comparing to 4.05, and 3.75 wt% H₂ at 280, and 290 °C. The reversibility of 4.23 wt% capacity is also one of the highest for Mg(BH₄)₂ dehydrogenation under mild conditions such as 270 °C as reported. 4 cycles of Mg(BH₄)₂ dehydrogenation are conducted at 270 °C. The capacities degrade during 4 cycles and tend to be stable at about 3.0 wt% for the last two cycles. By analyzing the hydrogen de/absorption products of the catalyzed sample, Mg(BH₄)₂ is found to be regenerated after rehydrogenation according to Fourier Transform Infrared (FTIR) spectroscopy. Ti nano-particles can react with Mg(BH₄)₂ during ball-milling and de/rehydrogenation. The products include TiH_1.924, TiB, and TiB₂, which can improve the dehydrogenation properties of Mg(BH₄)₂ from a multiple aspect.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Synthesis and crystal structure analysis of complex hydride Mg(BH4)(NH2)

Complex hydride Mg(BH₄)(NH₂), which consists of double anion BH₄⁻ and NH₂⁻, was synthesized and the crystal structure was analyzed by synchrotron X-ray diffraction. The mixture sample of Mg(BH₄)₂ + Mg(NH₂)₂ prepared by ball milling was reacted and crystallized to Mg(BH₄)(NH₂) by heating at about 453 K. This crystal phase transforms into amorphous phase above 473 K and subsequently the dehydrogenation begins. The crystal structure of Mg(BH₄)(NH₂) was determined from measurement data at 453 K (chemical formula: Mg_0.94(BH₄)₁(NH₂)_0.88, crystal system: tetragonal, space group: I4₁ (No.80), Z = 8, lattice constants: a = 5.814(1), c = 20.450(4) Å at 453 K). Mg(BH₄)(NH₂) is ionic crystal which the cation (Mg²⁺) and the anions (BH₄⁻ and NH₂⁻) are stacking alternately along the c-axis direction. Two BH₄⁻ and two NH₂⁻ tetrahedrally coordinate around Mg²⁺ ion.     

Improved dehydrogenation properties of the combined Mg(BH4)2·6NH3–nNH3BH3 system

A combined strategy via mixing Mg(BH₄)₂·6NH₃ with ammonia borane (AB) is employed to improve the dehydrogenation properties of Mg(BH₄)₂·6NH₃. The combined system shows a mutual dehydrogenation improvement in terms of dehydrogenation temperature and hydrogen purity compared to the individual components. A further improved hydrogen liberation from the Mg(BH₄)₂·6NH₃–6AB is achieved with the assistance of ZnCl₂, which plays a crucial role in stabilizing the NH₃ groups and promoting the recombination of NH^δ+?HB^δ−. Specifically, the Mg(BH₄)₂·6NH₃–6AB/ZnCl₂ (with a mole ratio of 1:0.5) composite is shown to release over 7 wt.% high-pure hydrogen (>99 mol%) at 95 °C within 10 min, thereby making the combined system a promising candidate for solid hydrogen storage.     

Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.     

Effect of LiCe(BH4)3Cl with a high Li ion conductivity on the hydrogen storage properties of LiMgNH system

The effect of LiCe(BH₄)₃Cl on the hydrogen storage properties of Mg(NH₂)₂2LiH system was studied systematically, which has a high Li ion conductivity. The hydrogen desorption temperatures for LiMgNH system shift to lower temperatures by 0.05LiCe(BH₄)₃Cl doping with the onset temperature of dehydrogenation decreasing by 40 °C and the peak temperature decreasing by 30 °C. The Mg(NH₂)₂2LiH–0.03LiCe(BH₄)₃Cl composite exhibits an improved comprehensive hydrogen storage properties, which can reversibly store about 5.0 wt% hydrogen at 160 °C, and released hydrogen as much as 8.6 times faster than that of the Mg(NH₂)₂2LiH composite at 160 °C. The results indicated that the LiCe(BH₄)₃Cl-containing sample exhibited much better cycling properties than that of Mg(NH₂)₂2LiH sample. XRD and FTIR results show that the structure of LiCe(BH₄)₃Cl does not change before and after hydrogen absorption/desorption, indicating it plays the catalytic effect. The hydrogen desorption activation energy of Mg(NH₂)₂2LiH doped with 0.03LiCe(BH₄)₃Cl was reduced by 37.5%. The rate-controlling step of desorption shifted from the diffusion to the chemical reaction by the addition of LiCe(BH₄)₃Cl, indicating that the diffusion rates of small ions like H⁺, Li⁺ and Mg²⁺ in LiMgNH system are significantly enhanced, which could be well explained by the improved ionic conductivity of LiCe(BH₄)₃Cl doped sample.     

Synthesis of ammine dual-metal (V,Mg) borohydrides with enhanced dehydrogenation properties

The penta-ammine vanadium (III) borohydride, i.e. V(BH₄)₃·5NH₃, was successfully synthesized via ball-milling of VCl₃·5NH₃ and LiBH₄ in a molar ratio of 1:3. This compound was shown to release 11.5 wt% hydrogen with a H₂-purity of 85 mol% by 350 °C. To improve the dehydrogenation purity of V(BH₄)₃·5NH₃, Mg(BH₄)₂ with various molar ratios was mixed with V(BH₄)₃·5NH₃ to synthesize expected ammine metal-mixed borohydrides, among which the formed VMg(BH₄)₅·5NH₃ was indexed to be a monoclinic unit cell with lattice parameters of a = 19.611 Å, b = 14.468 Å, c = 6.261 Å, β = 93.678° and V = 1772.75 Å³. Dehydrogenation results revealed that the Mg(BH₄)₂ modified V(BH₄)₃·5NH₃ system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH₄)₃·5NH₃/2Mg(BH₄)₂ sample, 12.4 wt% pure hydrogen can be released upon heating to 300 °C. Further investigation on the dehydrogenation mechanism of the VMg(BH₄)₅·5NH₃ system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at 75 °C) as complementary to the prime combination of BH···HN.     

Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH4)2 composited with 2D MXene

Mg(BH₄)₂ occupies a large hydrogen storage capacity of 14.7 wt%, and has been widely recognized to be one of the potential candidates for hydrogen storage. In this work, 2D MXene Ti₃C₂ was introduced into Mg(BH₄)₂ by a facile ball-milling method in order to improve its dehydrogenation properties. After milling with Ti₃C₂, Mg(BH₄)₂–Ti₃C₂ composites exhibit a novel “layered cake” structure. Mg(BH₄)₂ with greatly reduced particle sizes are found to disperse uniformly on Ti₃C₂ layered structure. The initial dehydrogenation temperature of Mg(BH₄)₂ has been decreased to 124.6 °C with Ti₃C₂ additive and the hydrogen liberation process can be fully accomplished below 400 °C. Besides, more than 10.8 wt% H₂ is able to be liberated from Mg(BH₄)₂–40Ti₃C₂ composite at 330 °C within 15 min, while pristine Mg(BH₄)₂ merely releases 5.3 wt% hydrogen. Moreover, the improved dehydrogenation kinetics can be retained during the subsequent second and third cycles. Detailed investigations reveal that not only Ti₃C₂ keeps Mg(BH₄)₂ particles from aggregation during de/rehydrogenation, but also the metallic Ti formed in-situ serves as the active sites to catalyze the decomposition of Mg(BH₄)₂ by destabilizing the B–H covalent bonds. This synergistic effect of size reduction and catalysis actually contributes to the greatly advanced hydrogen storage characteristics of Mg(BH₄)₂.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Improvement of desorption performance of Mg(BH4)2 by two-dimensional Ti3C2 MXene addition

Magnesium borohydride (Mg(BH₄)₂) is an attractive materials for solid-state hydrogen storage due to its high hydrogen content (14.9 wt%). In the present work, the dehydrogenation performance of Mg(BH₄)₂ by adding different amounts (10, 20, 40, 60 wt%) of two-dimensional layered Ti₃C₂ MXene is studied. The Mg(BH₄)₂-40 wt% Ti₃C₂ composite releases 7.5 wt% hydrogen at 260 °C, whereas the pristine Mg(BH₄)₂ only releases 2.9 wt% hydrogen under identical conditions, and the onset desorption temperature decreases from 210 °C to a relative lower temperature of 82 °C. The special layered structure of Ti₃C₂ MXene and fluorine plays an important role in dehydrogenation process especially at temperatures below 200 °C. The main dehydrogenation reaction is divided into two steps, and activation energy of the Mg(BH₄)₂-40 wt% Ti₃C₂ composite is 151.3 kJ mol⁻¹ and 178.0 kJ mol⁻¹, respectively, which is much lower than that of pure Mg(BH₄)₂.