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.     

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.     

Two-dimensional C@TiO2/Ti3C2 composite with superior catalytic performance for NaAlH4

Carbon coated titanium dioxide supported on two-dimensional titanium carbide (C@TiO₂/Ti₃C₂) is synthesized by simple annealing under a flowing acetylene (C₂H₂) atmosphere, and applied to improve the hydriding/dehydriding behavior of sodium alanate (NaAlH₄). The results indicate that as-prepared C@TiO₂/Ti₃C₂ composite exhibits excellent catalytic activity. The initial temperature for hydrogen desorption is reduced by 70 °C compared with the pristine sample. About 4.0 wt% hydrogen is released in 13 min at 140 °C. The apparent activation energies (E_a) of 10 wt% C@TiO₂/Ti₃C₂ catalyzing NaAlH₄ for the first two-steps dehydrogenation are 72.41 and 64.27 kJ mol⁻¹ respectively. The structural analyses reveal that C@TiO₂/Ti₃C₂ interacts with NaAlH₄ by using ball milling and decomposes to form Ti-species which works in combination with carbon to improve the dehydrogenation performance of NaAlH₄. This result provides an important progress in the hydrogen storage of NaAlH₄ catalyzed by MXene.     

Improving the hydrogen storage performance of lithium borohydride by Ti3C2 MXene

Two-dimensional layered material of Ti₃C₂ has been used to improve the hydrogen desorption properties of LiBH₄. The results of temperature-programmed dehydrogenation (TPD) and isotherm dehydrogenation (TD) demonstrate that adding the Ti₃C₂ contributes to the hydrogen storage performance of LiBH₄. The dehydrogenation temperature decreases and the dehydrogenation rate increases with increasing the adding amounts of Ti₃C₂. The onset dehydrogenation temperature of LiBH₄ + 40 wt% Ti₃C₂ composite is 120 °C and approximately 5.37 wt% hydrogen is liberated within 1 h at 350 °C. Furthermore, the activation energy of LiBH₄ + wt.% Ti₃C₂ is also greatly reduced to 70.3 kJ/mol, much lower than that of pure LiBH₄. The remarkable dehydrogenation property of the LiBH₄+ 40 wt% Ti₃C₂ may be due to the layered active Ti-containing Ti₃C₂ and the high surface area of MXene.     

In-situ synthesis of amorphous Mg(BH4)2 and chloride composite modified by NbF5 for superior reversible hydrogen storage properties

The solvent-free amorphous Mg(BH₄)₂ composite was in-situ synthesized by ball milling LiBH₄ and MgCl₂. It is found that the onset dehydrogenation temperature of the as-synthesized composite is 126.9 °C, which is roughly 156 °C lower than that of pristine Mg(BH₄)₂. The activation energy of the amorphous Mg(BH₄)₂ and pristine Mg(BH₄)₂ for the first dehydrogenation step was calculated as 120.01 kJ/mol and 487.99 kJ/mol, respectively. Hence the kinetics improvement is certified by the lower E_a value of the dehydrogenation process. When adding NbF₅ into the composite, the catalyzed composite exhibits better hydrogen storage properties compared to pristine and amorphous Mg(BH₄)₂. The catalyzed composite starts to release hydrogen at proximately 120 °C with a total capacity of 10.04 wt%. The reversibility of the catalyzed composite is also improved. The capacity of the catalyzed composite at the second cycle is 5.5 wt%. For the third and fourth cycles the catalyzed composite can still liberate 4 wt% H₂. Besides, the onset hydrogen desorption temperature during four cycles are extremely lower than those of pristine and amorphous Mg(BH₄)₂. The peaks of the intermediate MgB₁₂H₁₂ is detected by FTIR as the regenerated hydrogenation product in the catalyzed composite. It can be speculated from the detailed analysis that there are mainly three reasons for the improved properties. Firstly, the additive NbF₅ is favorable to enhance the hydrogen storage properties by modifying the dehydrogenation path and producing MgF₂ and NbB₂ as new products. Secondly, the in-situ formation of amorphous Mg(BH₄)₂ is likely to improve the dehydrogenation properties of the samples due to its different reactivity comparing to crystal ones. Finally, LiCl can serve as buffer in the composite and thus improve the dehydrogenation properties.     

Dehydrogenation properties of two phases of LiNH2BH3

Lithium amidoborane (LiNH₂BH₃) is known as one of the most prospective hydrogen storage materials. In this paper, the differences between two allotropes (α-LiNH₂BH₃ and β-LiNH₂BH₃) of LiNH₂BH₃ in the dehydrogenation properties was reported for the first time. A series of mixtures of α-LiNH₂BH₃/β-LiNH₂BH₃ with different mass ratios were prepared by ball milling for different time and the contents of two phases in samples were determined with Rietveld's method. The thermal decomposition behaviors of samples were investigated by DSC. It shows that the initial dehydrogenation temperature of samples decreases with the content of α-LiNH₂BH₃ phase increasing. The initial dehydrogenation temperature of α-LiNH₂BH₃ is about 61 °C, which is approximately 15 °C lower than that of β-LiNH₂BH₃. Dehydrogenation kinetic analysis shows that α-LiNH₂BH₃ has the lower activation energy (157 kJ mol⁻¹) and higher rate (k = 1.422 × 10¹ min⁻¹) than that of β-LiNH₂BH₃ (272 kJ mol⁻¹ and 1.023 × 10⁻¹ min⁻¹, respectively). It is suggested that α-LiNH₂BH₃ is more supportive for hydrogen desorption. It gives a critical clue on exploring the dehydrogenation mechanism of lithium amidoborane. Moreover, the significant decrease of desorption temperature will shine a light on on-board hydrogen storage systems.     

Enhancement of the hydrogen release of Mg(BH4)2 by concomitant effects of nano-confinement and catalysis

Magnesium borohydride, Mg(BH₄)₂, is an interesting material for hydrogen storage due to its high hydrogen content (14.9 wt.% of H₂). Unfortunately, a temperature of at least 350 °C is needed for releasing its hydrogen and the rehydrogenation process is only feasible under harsh conditions (950 bar H₂ and 300 °C). In order to improve the performances of this compound, we analyze in this study the concomitant effects of nano-confinement into mesoporous carbons and addition of NiPt catalysts. This study uses different characterization tools to determine the effects of both nano-confinement and catalysts onto the pathway of decomposition. Usually, bulk Mg(BH₄)₂ decomposes in several steps passing through intermediate species for which activation energies are high. In this study, we show that the confinement and catalyst addition on Mg(BH₄)₂ result in a single step of hydrogen release and an activation energy below that of the bulk material with a value of 178 ± 14 kJ mol⁻¹ as determined by the Kissinger's method. Interestingly, the hydrogen release is fully completed, i.e. 8H atoms per Mg(BH₄)₂ formula unit are released, in less than 2 h at 350 °C.     

Achievement of excellent hydrogen sorption through swift hydrogen transport in 1:2 Mg(NH2)2–LiH catalyzed by Li4BH4(NH2)3and carbon nanostructures

The present studies deal with the catalytic character of carbon nanostructure (Graphene (Gr) and single-wall carbon nanotubes (SWNTs), and their composite versions) on the hydrogen sorption behavior of 1:2 Mg(NH₂)₂–LiH/Li₄BH₄(NH₂)₃. The inclusion of an optimal quantity of 2 wt% SWNTs in Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ resulted in superior hydrogen sorption over 2 wt% Gr and 2 wt% of (Gr and SWNT) composite. The onset desorption temperature for SWNTs catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ is 108 °C which is 32 °C, 44 °C lower compared to Gr catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ and uncatalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ respectively. The de/re-hydrogenation kinetics of the SWNT catalyzed sample has been found to be 4.02 wt% and 4.63 wt% within 15min at 170 °C and 7 MPa H₂ pressure, correspondingly. The activation energy for SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ has been found to be 69.75 kJ/mol. The SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ shows good cyclic stability (almost no degradation) up to 10 cycles. The better hydrogen sorption for SWNTs is attributed to the ballistic transport of hydrogen atoms within and across the amide/hydride matrix. In contrast, Gr sheets agglomerate, which adversely affects hydrogen sorption from Gr and Gr+SWNT composites. A hydrogen sorption mechanism has been proposed based on structural, microstructural, Fourier-transform infrared spectroscopy, and Raman characterization results.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

The Mg/MAX-phase composite for hydrogen storage

The Mg/MAX-phase composite materials are synthesized by reactive ball milling (RBM) in a hydrogen gas atmosphere, and phase composition and dehydrogenation performance of the composites are investigated. The Ti₃AlC₂ MAX-phase markedly reduces the dehydrogenation temperature of the MgH₂ to 246 °C for the sample with 5 wt% of Ti₃AlC₂ MAX-phase and to 236 °C for the sample with 7 %wt. of Ti₃AlC₂ MAX-phase. The highest hydrogen capacity of 5.6 wt% was achieved for the Mg+7 wt% MAX-phase composite. The kinetic mechanism of the dehydrogenation of the composites is investigated by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) technique.     

NbF5 additive improves hydrogen release from magnesium borohydride

The dehydrogenation properties of Mg(BH₄)₂ with various additives (SiO₂, VCl₃, CoCl₂ and NbF₅) were investigated. The addition of NbF₅ significantly improved the extent of hydrogen release as well as the kinetics. While neat Mg(BH₄)₂ starts to release hydrogen >270 °C, Mg(BH₄)₂ with NbF₅ begins hydrogen release ∼75 °C, as confirmed by mass spectrometry and thermogravimetry. The maximum hydrogen yield of Mg(BH₄)₂, obtained in the presence of 15 wt% NbF₅, was 3.7, 7.4, 10.0, 11.4 wt% for 150, 250, 300 and 350 °C, respectively. Using pXRD, we confirmed that the final crystalline product at 300 °C from Mg(BH₄)₂ + 15 wt% NbF₅ was Mg, while it was MgH₂ for neat Mg(BH₄)₂. Solid state ¹¹B NMR analysis of Mg(BH₄)₂ with 15 wt% NbF₅ at 300 °C showed significant selectivity toward the formation of Mg(B₁₂H₁₂) as intermediate, while neat Mg(BH₄)₂ showed β-Mg(BH₄)₂, Mg(B₂H₆) as well as some Mg(B₁₂H₁₂). Our results demonstrate that NbF₅ is a promising additive to provide high hydrogen yield values from Mg(BH₄)₂ at moderate temperatures <300 °C.     

Study on the dehydrogenation properties and reversibility of Mg(BH4)2AlH3 composite under moderate conditions

Mg(BH₄)₂ has been considered as one of the promising light metal complex hydrides due to its high hydrogen capacity and low cost. But its higher thermal stability (dehydrogenation at above 300 °C) needs to be improved for the practical application. In this study, the aluminum hydride AlH₃ was introduced into complex borohydride Mg(BH₄)₂ to synthesize a new Mg(BH₄)₂AlH₃ composite by ball milling method. It is found that the active Al¹ formed from the self-decomposition of AlH₃ can effectively improve the dehydrogenation properties of Mg(BH₄)₂, the Mg(BH₄)₂AlH₃ composite starts to release hydrogen at 130.8 °C with a total hydrogen capacity of 11.9 wt.%. The dehydrogenated products of the composite is composed of Mg₂Al₃ and B at 350 °C, resulting in the improved hydrogen desorption properties of Mg(BH₄)₂AlH₃ composite. The Mg₂Al₃ and B products would be further transformed into MgAlB₄ and Al at 500 °C. Moreover, the Mg₂Al₃ and B dehydrogenated products show better reversible hydrogen storage property than that of the MgAlB₄ and Al products. This research shows a way to alter hydrogen de/hydrogenation route and reversibility of Mg(BH₄)₂ complex hydride by compositing with AlH₃ and controlling the dehydrogenation temperature.     

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH₄–Mg(BH₄)₂ (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH₄, Mg(BH₄)₂ or pristine LiBH₄–Mg(BH₄)₂, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co⁺ which may serve as the functional species in catalyzing the dehydrogenation of the composite.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

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.     

Boosting the dehydrogenation efficiency of dodecahydro-N-ethylcarbazole by assembling Pt nanoparticles on the single-layer Ti3C2Tx MXene

As the candidates for large-scale hydrogen storage, liquid organic hydrogen carriers (LOHCs) exhibit evident advantages in hydrogen storage density and convenience of storage and transportation. Among them, NECZ (N-ethylcarbazole)/12H-NECZ (dodecahydro-N-ethylcarbazole) is considered as a typical system with the lower hydrogenation/dehydrogenation temperature. However, the low dehydrogenation efficiency restrict its commercial applications. In this work, the single-layer Ti₃C₂T_x MXene was employed as the support to load the Pt nanoparticles for the 12H-NECZ dehydrogenation reaction. The effect of transition metals, loading amounts and morphologies of catalysts were analyzed. It was found that the 3 wt% Pt/S–Ti₃C₂T_x catalyst exhibited the best catalytic performance with 100% conversion, 91.55% selectivity of NECZ and 5.62 wt% hydrogen release amount at 453 K, 101.325 kPa for 7 h. The product distributions and kinetics analysis suggested that the elementary reaction from 4H-NECZ to NECZ was the rate-limiting step. The selectivity of NECZ is sensitive to the dehydrogenation temperature. Combined with the XRD, SEM, HRTEM, XPS, BET and FT-IR results, it could be indicated that the special two-dimension structure of S–Ti₃C₂T_x and electronic effect between Pt and S–Ti₃C₂T_x enhanced the dehydrogenation efficiency of 12H-NECZ. The measurements of cyclic dehydrogenation indicated that the Pt/S–Ti₃C₂T_x catalyst exhibited good stability after 42 h. This work brought a new strategy for the design of efficient catalysts using two-dimensional materials in the applications of the liquid organic storage hydrogen technology.     

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.     

Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.