Apparent Activation Energy Of Dehydrogenation Research Articles

Achievement in modifying dehydrogenation kinetics for Sc-containing Mg matrix in Mg–Y–Zn alloy

To eliminate the adverse effects of coarse α-Mg phase on the hydrogen storage kinetics of Mg-rich alloys, element Sc with high solid solubility in α-Mg is introduced into Mg–Y–Zn alloy initiatively, which shows positive effects to enhance the hydrogen storage performance. The Sc-doping effectively enhances the dehydrogenation kinetics by motivating the formation of abundant stacking faults. The stacking faults provide the hydrogen atoms with diffusion channels to accelerate the mutual phase transformation of metal/hydride. With 1 at.% Sc substituting for Y, the apparent activation energy for dehydrogenation obtains a 11% reduction compared with Sc-free alloy. Besides, Sc-doping facilities the decomposition of YH3, which provides “electron transfer” effect to accelerate the decomposition of MgH2. Sc-doping affects the hydrogenation behavior by weakening the H2 sorption but ensuring the H atoms diffusion in Mg matrix. With the gradually substitution of Sc for Y, the developed dendrite arms make it difficult for hydrogen to diffuse deeper into the samples, resulting in large ratio of unhydrided Mg. It can be concluded that the quantity of defects within Mg and catalytic sites outside the Mg matrix are important for achieving rapid hydrogen absorption and desorption simultaneously.

Anchoring Mo single atoms on N-CNTs synchronizes hydrogenation/dehydrogenation property of Mg/MgH2

MgH2 has attracted substantial consideration mainly due to its sufficient content and high gravimetric density etc. However, its high thermodynamic stability and poor dynamics hinder its practical application. N-doping carbon nanotubes (N-CNTs) supported Mo single atoms (MoSA-N-CNTs) was constructed to facilitate the hydrogenation/dehydrogenation properties of the Mg/MgH2 system. According to the synergistic effect of N-CNTs nanoconfinement and MoSA catalysts, Mg@MoSA-N-CNTs can absorb 7.37 wt.% H2 during the milling within 15 h and the corresponding activation energy for hydrogenation can be reduced to 21.2 kJ·mol−1. MgH2 @MoSA-N-CNTs releases 7.23 wt.% H2 at 325 ℃ for 5 min, nearly 3 times pristine MgH2. Meanwhile, its initial dehydriding temperature decreases from 265.7°C to 204.6°C, and its apparent activation energy for dehydrogenation also greatly decreases from 108.6 kJ/mol to 68.4 kJ/mol. The above values are much lower than the pure MgH2. The synergistic effect mechanism of N-CNTs and MoSA on hydrogenation/dehydrogenation properties of Mg/MgH2 is proposed systemically. It is revealed that the MoSA and the N-CNTs not only accelerate H2 dissociation and diffusion but also effectively prevent the Mg/MgH2 nanoparticles agglomeration. The above results highlight the great promise of Mg/MgH2 @MoSA-N-CNTs composite for high-performance Mg-based hydrogen storage materials.

Nanoscale microstructures and hydrogenation properties of an as-cast vanadium-based medium-entropy alloy

Vanadium-based hydrogen storage alloys have been widely investigated; however, alloys in the cast state are typically coarse-grained. In this study, an as-cast V45Fe15Ti20Cr20 medium-entropy alloy was prepared by arc melting, and microstructural analysis revealed that the alloy was composed of nanocrystals. The initial pretreatment temperature of the alloy was approximately 100 K lower than that of the as-cast coarse-grained alloy. At room temperature, the time required for the alloy to reach 90% saturation was only 140 s, indicating excellent hydrogen absorption kinetics. The alloy is fully activated after two hydrogen absorption/desorption cycles. The phase transformation of the alloy in the early hydrogenation stage was investigated using X-ray diffraction, and the results showed that the BCC phase was completely transformed into the BCT phase when hydrogen uptake was performed for 6 s. Furthermore, the apparent activation energy of dehydrogenation in the present alloy calculated using the Kissinger method was 69.8 ± 0.8 kJ/mol. The pressure-composition-isotherms tests showed that the hydrogen absorption capacity of the alloy at 295 K was 2.12 wt%. The hydrogenation/dehydrogenation enthalpy change of the alloy was calculated by the Van't Hoff equation, which was 30.90 ± 1.47 and 33.95 ± 0.41 kJ/mol, respectively. The present work demonstrates that nanostructured vanadium-based hydrogen storage alloys can be fabricated using traditional casting techniques. Our study also enriches the understanding of the microstructures of medium-entropy alloys, which may provide positive guidance for the design of novel vanadium-based hydrogen storage alloys.

Improved MgH2 kinetics and cyclic stability by fibrous spherical NiMoO4 and rGO

BackgroundThe synergistic catalysis has a good improvement on the hydrogen storage properties of MgH2. MethodsIn this work, fibrous spherical NiMoO4 bimetallic catalysts were prepared by hydrothermal and sintering methods, and introduced into the MgH2 system by ball milling to improve their kinetic performance. In addition, rGO (reduced graphene oxide) was also introduced to improve the cycling stability of MgH2. Significant findingsIt is observed that NiMoO4 has a significant improvement in the kinetics of MgH2 absorption and dehydrogenation. MgH2 + 10 wt% NiMoO4 can start dehydrogenation at 191 °C. After complete dehydrogenation, it starts to absorb hydrogen below 50 °C. The apparent activation energy of dehydrogenation and hydrogen absorption of MgH2 + 10 wt% NiMoO4 composites were reduced by 35.43 kJ mol−1 and 30.10 kJ mol−1, respectively, compared with pure MgH2. The results of characterization indicated that the uniformly dispersed NiMoO4 produced Mg2Ni and elemental Mo in situ after the first hydrogen release reaction. It is believed that the synergistic effect of Mo and Ni elements accelerates the hydrogen absorption and desorption performance of hydrogen storage materials. Cycling performance tests showed that the addition of rGO improved the cycling stability of the hydrogen storage material.

Graphene supported Pt–Ni bimetallic nanoparticles for efficient hydrogen generation from KBH4/NH3BH3 hydrolysis

Developing highly efficient and stable supported bimetallic nanoparticles catalysts via a facile strategy is one of the most admirable methods for sustainable hydrogen production from borohydride hydrolysis. Herein, we developed a facile technology for rapidly and straightforwardly manufacturing Pt–Ni bimetallic nanoparticles (BNPs) supported by partially reduced graphene oxide (prGO) with excellent catalytic activity and outstanding durability for hydrogen production from KBH 4 and NH 3 BH 3 alkaline solution. The uniformly dispersed Pt 40 –Ni 60 BNPs with a statistical size of around 2.6 nm exhibited a surprising catalytic activity of 23,460 mol-H 2 ·h −1 ·mol-Pt −1 at 308 K, moreover, whose activity was high up to 80% of the first time even after 30 runs, demonstrating an outstanding stability. The apparent activation energy for dehydrogenation of KBH 4 and NH 3 BH 3 were respectively about 27.8 and 33.6 k J /mol for the prepared Pt 40 –Ni 60 /prGO catalyst. The extraordinary catalytic activity of the Pt 40 –Ni 60 /prGO catalyst owing to the strong charge transfer effect between Pt–Ni BNPs and graphene. • The Pt 40 –Ni 60 /prGO exhibited a high activity of 23,460 mol-H 2 ·h −1 ·mol-Pt −1 at 308 K. • The E a of KBH 4 and NH 3 BH 3 hydrolysis were respectively 27.8 and 33.6 k J /mol. • The activity of Pt 40 –Ni 60 /prGO reached up to 80% of the 1st run after 30 cycles for KBH 4 hydrolysis.

Remarkable catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen sorption properties of MgH2

In the present study, the catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen absorption and desorption properties of MgH2 has been investigated. The MgH2 nanocomposites were prepared by high-energy ball-milling. The morphology, phase structure, thermal behavior, and hydrogen storage properties of the materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), and the pressure-composition temperature (PCT) methods. ZrO2 and Ni nanoparticles were homogenously dispersed into the MgH2 matrix. The calculated apparent activation energy for dehydrogenation was 63.4 kJ/mol, which was decreased by 80.1 kJ/mol compared to that of as-milled MgH2. As a result, MgH2+5 wt.%Ni+5 wt.%ZrO2 demonstrated improved dehydrogenation and hydrogenation kinetics at 310 °C. The MgH2+5 wt.%Ni+5 wt.%ZrO2 sample released about 6.83 wt.% and absorbed about 6.10 wt.% in less than 30 min. Therefore, the co-catalysis of Ni and ZrO2 significantly enhances the hydrogenation and dehydrogenation properties of MgH2.

Effect of milling duration on hydrogen storage thermodynamics and kinetics of Mg-based alloy

Mechanical milling is widely recognized as the best method to prepare nano-structured magnesium based hydrogen storage materials. The composites La7Sm3Mg80Ni10 + 5 wt% TiO2 (named La7Sm3Mg80Ni10–5TiO2) whose structures are nano-crystal and amorphous accompanied by great hydrogen absorption and desorption properties were fabricated by mechanical milling. The research focuses on the effect of milling duration on the thermodynamics and dynamics. The instruments of researching the gaseous hydrogen storing performances include Sievert apparatus, DSC and TGA. The calculation of dehydrogenation activation energy was realized by applying Arrhenius and Kissinger formulas. The calculation results show the specimen milled for 10 h exhibits the optimal activation performance and hydrogenation and dehydrogenation kinetics. Extending or shrinking the milling duration will lead to the degradation of hydrogen storage performances. The as-milled (10 h) alloy at the full activated state can absorb 4 wt% hydrogen in 87 s at 473 K and 3 MPa and release 3 wt% H2 in 288 s at 573 K and 1 × 10−4 MPa. The changed milling durations have little impact on the thermodynamic properties of experimental samples and the enthalpy change (ΔH) of the alloy milled for 10 h is 74.23 kJ/mol. Moreover, it is found that the as-milled (10 h) alloy displays the minimum apparent activation energy of dehydrogenation (59.1 kJ/mol), suggesting the optimal hydrogen storing property of the as-milled (10 h) alloy.

Magnesium Nanoparticles With Pd Decoration for Hydrogen Storage.

In this work, Magnesium nanoparticles with Pd decoration, ranging from 40 to 70 nm, were successfully coprecipitated from tetrahydrofuran (THF) solution, assigned as the Mg–Pd nanocomposite. The Mg–Pd nanocomposite exhibits superior hydrogen storage properties. For the hydrogenated Mg–Pd nanocomposite at 150°C, the onset dehydrogenation temperature is significantly reduced to 216.8°C, with a lower apparent activation energy for dehydrogenation of 93.8 kJ/mol H2. High-content γ-MgH2 formed during the hydrogenation process, along with PH0.706, contributes to the enhancing of desorption kinetics. The Mg–Pd nanocomposite can take up 3.0 wt% hydrogen in 2 h at a temperature as low as 50°C. During lower hydrogenation temperatures, Pd can dissociate hydrogen and create a hydrogen diffusion pathway for the Mg nanoparticles, leading to the decrease of the hydrogenation apparent activation energy (44.3 kJ/mol H2). In addition, the Mg–Pd alloy formed during the hydrogenation/dehydrogenation process can play an active role in the reversible metal hydride transformation, destabilizing the MgH2.

Nano-inducement of Ni for low-temperature dominant dehydrogenation of Mg–Al alloy prepared by HCS+MM

Magnesium hydride is one of the most ideal solid hydrogen storage materials (HSMs). However, its sluggish absorption/desorption kinetics and undesirable thermodynamics create a burden for industrial-scale applications. In this paper, the effects of micro-Ni and nano-Ni on the dehydrogenation properties of Mg–Al alloys were compared and studied. All the samples were prepared by hydriding combustion synthesis (HCS) and mechanical milling (MM). Temperature programmed desorption (TPD) and differential scanning calorimeter (DSC) analyses showed that the catalytic effect of nano-Ni was vastly superior to that of micro-Ni and the catalytic effect of Ni was more excellent than Al3Ni2 on the dehydrogenation of Mg–Al alloy. Mg90Al10-5nanoNi-MM exhibited the best dehydrogenation property, in which, two dehydrogenation peaks at 260 °C and 343 °C in the DSC curve (10 °C/min) were observed and the main dehydrogenation occurred at the lower temperature. The apparent activation energy for dehydrogenation of MgH2 was reduced from 144.8 kJ/mol to 88.8 and 65.5 kJ/mol for the two dehydrogenation peaks. Ex-situ X-ray differential (XRD) and high resolution transmission electron microscope (HRTEM) were conducted to understand the microstructure evolution during dehydrogenation. Nano-inducement of nano-Ni dramatically promotes the regulation of Al on MgH2 thus improving the dehydrogenation performance.

New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF3. A nanocomposite of Mg0.95In0.05-5 wt% CeF3 was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg0.95In0.05 solid solution is changed by the addition of CeF3. Mg0.95In0.05-5 wt% CeF3 nanocomposite transforms to MgH2, MgF2 and intermetallic compounds of MgIn and CeIn3 by hydrogenation. Upon dehydrogenation, MgH2 reacts with the intermetallic compounds of MgIn and CeIn3 forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn3 impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg0.95In0.05-5 wt% CeF3 nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol−1⋅H2, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol−1⋅H2, a reduction of ～73 kJ⋅mol−1⋅H2 relative to that for Mg0.95In0.05 solid solution. As a result, Mg0.95In0.05-5 wt% CeF3 nanocomposite can release ～57% H2 in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.

Excellent catalytic activity of a two-dimensional Nb4C3Tx (MXene) on hydrogen storage of MgH2

A two-dimensional Nb4C3Tx (MXene) prepared via a chemical exfoliation was applied into MgH2 by ball milling. The activated MgH2-5 wt%Nb4C3Tx composite exhibits superior hydrogen storage kinetics properties. The onset temperature for the activated MgH2-5 wt%Nb4C3Tx composite is significantly reduced from 296.5 °C (the balled pure MgH2) to 150.6 °C, representing a 145.9 °C reduction, and which can release completely within 800 s at 250 °C, with a larger rate constant of 1.09164 wt% min−1. Additionally, the apparent activation energy for dehydrogenation of the activated MgH2- 5wt%Nb4C3Tx composite is 81.2 kJ/mol H2 much lower than that of the balled pure Mg (153.8 kJ/mol H2). After dehydrogenation, the sample can absorb approximately 3.50 wt% hydrogen within 2 h at temperatures as low as 50 °C, and the apparent activation energy for hydrogenation decreases to 27.8 kJ/mol H2 much lower than that of the dehydrogenated pure Mg (82.1 kJ/mol H2). Such excellent hydrogen storage kinetics and lower energy barriers explain high catalytic activity derived from Nb4C3Tx towards the hydrogen storage reaction of MgH2, which are likely due to the unique layered structure and in situ formed NbHx. This research extends the knowledge of designing efficient catalyst via the two-dimensional MXene in the hydrogen storage materials.

Catalytic effect of in situ formed nano-Mg2Ni and Mg2Cu on the hydrogen storage properties of Mg-Y hydride composites

Two hydrides nanocomposites, consisting of MgH2-Mg2NiH4-YH2 and MgH2-MgCu2-YH2 phases, were prepared by hydrogenation upon the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 amorphous alloys, respectively. The catalytic effect of nano-Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 particles on hydrogen storage properties of Mg-Y hydride composites have been studied. It is shown that the activated melt-spun Mg80Y5Ni15 alloy exhibits higher hydrogen absorption capacity and faster hydriding/dehydriding kinetics than those of the activated melt-spun Mg80Y5Cu15 alloy. The activated melt-spun Mg80Y5Ni15 alloy absorbs about 4.2 wt% hydrogen in 10 min at 200 °C, and almost fully desorbs hydrogen within 5 min at 280 °C. In contrast, the activated melt-spun Mg80Y5Cu15 alloy absorbs about 4 wt % of hydrogen in 2 h at 280 °C, and desorbs 3.7 wt% of hydrogen in 80 min at 300 °C. The apparent activation energy of dehydrogenation for the activated melt-spun Mg80Y5Ni15 alloy was calculated to be 83.9 kJ/mol, which is significantly lower than that of 132.3 kJ/mol for the activated melt-spun Mg80Y5Cu15 alloy. The excellent hydrogen storage properties of the activated melt-spun Mg80Y5Ni15 alloy can be attributed to the multiple cracks and refined particle size generated during hydrogenation, and the uniform distributed catalytic Mg2Ni/Mg2NiH4 nanoparticles in the Mg/MgH2 matrix. In contrast, a large amount of Mg2Cu/MgCu2 phase significantly aggregations, and the separation between Mg2Cu/MgCu2 and Mg/MgH2 matrix in the activated melt-spun Mg80Y5Cu15 alloy, which greatly decrease the catalytic effect of Mg2Cu for the activated melt-spun Mg80Y5Cu15 alloy.

Enhanced dehydrogenation kinetic properties and hydrogen storage reversibility of LiBH4 confined in activated charcoal

Abstract LiBH4 was confined into activated charcoal (AC) by melt infiltration method (MI), and its effects on the hydrogen sorption properties were investigated. The N2 adsorption results reveal that melt infiltration method can effectively incorporated LiBH4 into AC. It can maintain the structural integrity of the scaffold and ensure the confinement effect. The nano-confined LiBH4/AC starts to release hydrogen at around 190 °C, which is 160 °C lower than that of pure LiBH4, and reaches a hydrogen desorption capacity of 13.6% at 400 °C. When rehydrogenated under the condition of 6 MPa H2 and 350 °C, it has a reversible hydrogen storage capacity of 6%, while pure LiBH4 shows almost no reversible hydrogen storage capacity under the same condition. Mass spectrometry analysis (MS) results suggest that no diborane or other impurity gases are released in the decomposition process. The apparent activation energy of dehydrogenation of LiBH4 after confinement into AC decreases from 156.0 to 121.1 kJ/mol, which leads to the eminent enhancement of dehydrogenation kinetics of LiBH4.

Preparations and de/re-hydrogenation properties of LixNa3-xAlH6 (x=0.9–1.3) non-stoichiometric compounds

Mixed alkali alanates LixNa3-xAlH6 have been successfully synthesized by means of grinding mixtures of Li3AlH6 and Na3AlH6 in specific molar ratios. Non-stoichiometric LixNa3-xAlH6 compounds with single perovskite-type structures (space group Fm-3m) can be formed only within the composition range of x = 0.9–1.3. Li1.3Na1.7AlH6 exhibits superior hydrogen storage properties over other LixNa3-xAlH6 compounds. Its onset dehydrogenation temperature (∼423 K) was lowered by more than 40 K from other samples in temperature programmed dehydrogenation (TPD) curves. Also, the dehydrogenation capacity of Li1.3Na1.7AlH6 (3.45 wt.%) is the highest among the compounds. The dehydrogenation enthalpy values of LixNa3-xAlH6 decreased as x increased from 0.9 to 1.3 according to the results by isothermal pressure-composition (PCI) curves and van't Hoff plots. It shows that the dehydrogenation Li1.3Na1.7AlH6 (49.7 kJ mol H2−1) was greatly destabilized from that of LiNa2AlH6 (68.1 kJ mol H2−1). Furthermore, the apparent activation energy of dehydrogenation for Li1.3Na1.7AlH6 (138.1 kJ mol−1) was remarkably lowered from that of LiNa2AlH6. This illustrates that Li1.3Na1.7AlH6 exhibits enhanced dehydrogenation kinetics from that of LiNa2AlH6.

Remarkably improved dehydrogenation of ZrCl4 doped NaAlH4 for hydrogen storage application

Thermal dehydrogenation kinetics of sodium alanate (NaAlH4) has been studied with respect to ZrCl4 additive, and the results were compared with pure NaAlH4. The FTIR analysis has shown insignificant effects of ZrCl4 on the structural integrity of AlH4− anion after ball milling. Partial reduction of ZrCl4 has been observed during ball milling with NaAlH4. The in-situ reduction favors to homogeneous and adherent dropping of ZrCl4 over the NaAlH4 surface which leads to remarkably improved dehydrogenation process. The dehydrogenation of ZrCl4 doped NaAlH4 occurred in three steps similar to pure NaAlH4. The dehydrogenation temperatures of all the steps were substantially decreased as compared to pure NaAlH4. The apparent activation energy of dehydrogenation of ZrCl4 doped NaAlH4 was evaluated for all the dehydrogenation steps and found to be significantly less with respect to pure NaAlH4.

Thermodynamics and kinetics of nano-engineered Mg-MgH2 system for reversible hydrogen storage application

Thermodynamics and kinetics of hydrogenation-dehydrogenation of nanometric iron (nFe) doped Mg-MgH2 system have been studied. The nFe-doped Mg could be hydrogenated even at 0°C up to 45% of the theoretical hydrogen storage capacity within an hour. The dehydrogenation of nFe doped MgH2 starts below 150°C. The remarkably improved hydrogenation-dehydrogenation kinetics could be attributed to the nano-engineered surface of MgH2 by nFe. The enthalpies of hydrogenation-dehydrogenation were found to be 76kJ/mol, and 77kJ/mol respectively. The activation energy of hydrogenation was evaluated as 41±2kJ/mol which is same as the diffusion barrier of hydrogen in Mg matrix. The apparent activation energy of dehydrogenation of nFe-doped MgH2 was found to be 74±1kJ/mol which is same as the enthalpy of dehydrogenation. The nFe-doped Mg-MgH2 system has shown cyclic stability up to 50 cycles without significant changes in the kinetics and hydrogen storage capacity. Three-dimensional diffusion seems to be controlling the dehydrogenation process.

Development of Mg[sbnd]Li[sbnd]B based advanced material for onboard hydrogen storage solution

Lithium borohydride-magnesium hydride (LiBH4–MgH2) reactive composite has been considered as one of the potential candidate for energy storage in the form of hydrogen because of its high hydrogen storage capacity. The composite requires high temperature (>500 °C) to dehydrogenate above 80% of its theoretical hydrogen storage capacity due to kinetic constraints. Therefore, a suitable catalyst is required to lower the dehydrogenation temperature. In this regard, the catalytic effect of ZrCl4 on the dehydrogenation of LiBH4–MgH2 reactive composite has been studied. It was found that the apparent activation energy of dehydrogenation decreases considerably followed by reduction in onset dehydrogenation temperature. The X-ray diffraction analysis of dehydrogenated composite has shown two phase of MgLi intermetallics. The formation of MgLi intermetallic phases eventually increases the amount of hydrogen release and, it could be beneficial during rehydrogenation. Moreover, dehydrogenation of catalyzed MgH2–LiBH4 favors the formation of MgB2 under mild conditions.

The effect of milling energy input during mechano-chemical activation synthesis (MCAS) of the nanocrystalline manganese borohydride (Mn(BH4)2) on its thermal dehydrogenation properties

Abstract A manganese borohydride, Mn(BH4)2, co-existing with a nanocrystalline LiCl salt, which is a reaction “dead-weight” byproduct, was successfully synthesized by the mechano-chemical activation synthesis (MCAS) during ball milling the (nLiBH4 + MnCl2) mixtures having the molar ratios n = 2 and 3, using the total milling energy input, QTR, from 36.4 to 364 kJ/g. The crystallite (grain) size of the synthesized nanocrystalline Mn(BH4)2 hydride attains 21 ± 5.0 nm for the energy input QTR = 36.4 kJ/g and then it is further reduced to 18 ± 1.0 nm for QTR = 145.6 kJ/g and finally to 14 ± 0.5 nm for QTR = 364 kJ/g. The crystallite (grain) size of LiCl is very close to 30 nm regardless of the milling energy input, QTR. During continuous heating in a Differential Scanning Calorimeter (DSC), Mn(BH4)2 decomposes in endothermic reaction releasing H2 and forming amorphous Mn and B in the process. The synthesized nanocrystalline Mn(BH4)2 hydride, co-existing with a nanocrystalline LiCl salt, is capable of desorbing up to ∼ 4.5 wt.% at 100 °C. The values of the apparent activation energy for dehydrogenation obtained in the present work are very low. The apparent activation energy for the n = 3 nanocomposite decreases monotonically from ∼70 to ∼59 kJ/mol with increasing milling energy input whereas the apparent activation energy for the n = 2 nanocomposite decreases from about 65 kJ/mol for QTR = 36.4 kJ/g to about 53 kJ/mol for QTR = 145.6 kJ/g and then again increases to ∼59 kJ/mol for the QTR = 364 kJ/g. These changes closely follow the variations in the average powder particle size obtained with the varying milling energy input. For the milling energy input QTR = 36.4 and 145.6 kJ/g the average powder particle size decreases to 14.9 ± 6.6 and 7.5 ± 2.6 μm, respectively, and subsequently increases reaching the average size of 16.1 ± 6.3 μm for the milling energy input QTR = 364 kJ/g. On the other hand, the apparent activation energy for dehydrogenation doesn't depend on the average crystallite (grain) size. The amorphous Mn and B elements are also formed after isothermal dehydrogenation. The synthesized Mn(BH4)2 hydride is very stable and doesn't excessively release H2 during a long-term storage at room temperature for over 120 days under a slight overpressure of argon.

Size effect on hydrogen storage properties of NaAlH4 confined in uniform porous carbons

Uniform porous carbons with pore size of 200nm, 60nm, 30nm and 4nm were separately synthesized using hard-template method, and nanoparticles of NaAlH4 have been infiltrated into the above porous carbons. A correlation between pore size and hydrogen desorption kinetics of NaAlH4 is established. In contrast with bulk NaAlH4, all of the confined NaAlH4 exhibit a single-step dehydrogenation process to form NaH/Al product. However, the onset and peak hydrogen desorption temperatures are significantly influenced by the pore size of carbon. As the pore size is reduced to 30nm, NaAlH4 starts to decompose at temperature of about 100°C, with a dehydrogenation peak of 172°C. The apparent activation energy for dehydrogenation is estimated to be 84.9 and 69.7kJ/mol for the NaAlH4 confined in 30 and 4nm porous carbons, reduced by 43 and 58kJ/mol contrast to the first step of bulk NaAlH4, respectively. Besides, by confining NaAlH4 into porous carbon with pore size≤30nm, the in situ generated NaH can decompose at a temperature below 250°C, i.e. reduction of the temperature for 100°C compared with that generated by decomposition of bulk NaAlH4. Thus all of hydrogen in NaAlH4 can be released and utilized at a relatively moderate condition without doping additives. Cycling investigation shows that smaller size exerts a favorable effect on the hydrogen storage capacity retention.

Mechano-chemical synthesis of manganese borohydride (Mn(BH4)2) and inverse cubic spinel (Li2MnCl4) in the (nLiBH4 + MnCl2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

Manganese borohydride (Mn(BH4)2) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH4) and manganese chloride (MnCl2) by applying high energy ball milling for 30 min. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH4 + MnCl2) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H2 that increases with the molar ratio n but does not exceed ∼0.2 wt.% for n = 23. However, longer milling duration leads to more H2 released. For the equimolar ratio n = 1 the principal phases synthesized are Li2MnCl4, an inverse cubic spinel phase, and the Mn(BH4)2 borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH4)2. With the n increasing from 3 to 23 LiBH4 is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH4)2. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 °C shows the release of hydrogen as a principal gas with a maximum intensity around 130–150 °C accompanied by a miniscule quantity of borane B2H6. The intensity of the B2H6 peak is 200–600 times smaller than the intensity of the corresponding H2 peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH4)2 up to about 270–280 °C. At 100 °C under 1 bar H2 pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ∼4 wt.% H2 which is a very large amount of H2 considering that the mixture also contains 2 mol of LiCl salt. The H2 quantities experimentally desorbed at 100 and 200 °C do not exceed the maximum theoretical quantities of H2 expected to be desorbed from Mn(BH4)2 for various molar ratios n. It clearly confirms that the contribution from B2H6 evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100–200 °C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, EA ∼ 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H2 during storage at room temperature and 40 °C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m2/g substantially enhances the rate of discharge at 40 °C.