Activation Energy For Hydrogen Desorption Research Articles

Influence of Zr Addition on the Microstructure and Hydrogenation Kinetics of Ti50-xV25Cr25Zrx (x = 0, 5, 7, and 9) Alloys.

Due to the poor activation performance and kinetics of Ti50V25Cr25 alloys, the element Zr was added to improve the phase structure of the alloy and achieve a high-performance hydrogen storage alloy. The Ti50-xV25Cr25Zrx (x = 0, 5, 7, and 9) system alloys were prepared by arc melting. The alloys were analyzed using an X-ray diffractometer (XRD), scanning electron microscope (SEM), and differential scanning calorimeter (DSC). The hydrogen storage capabilities of the alloys were also obtained by the Sievert volumetric method. The results indicated that the alloy with Zr added had a combination of the C15 Laves phase and the BCC phase, whereas the Zr-free alloy had a BCC single phase. The partial replacement of Zr with Ti resulted in an increase in the lattice parameters of the main phase. The hydrogen storage kinetic performance and activation of the alloys both significantly improved with an increasing Zr concentration. The time to reach 90% of the maximum hydrogen storage capacity decreased to 2946 s, 230 s, and 120 s, respectively, with the increases in Zr concentration. The initial hydrogen absorption content of the alloys increased and then decreased after the addition of the element Zr. The second phase expanded with an increasing Zr concentration, which in turn decreased the abundance of the BCC main phase. The Ti43V25Cr25Zr7 alloy showed good cycle stability and hydrogen-desorption performance, and it could absorb 90% of the maximum hydrogen storage capacity in around 230 s. The maximum hydrogen-absorption capacity of the alloy was 2.7 wt%. The diffusion activation energy of hydrogen desorption dropped from 102.67 kJ/mol to 92.62 kJ/mol.

Microstructure and hydrogen storage properties of MgH2/MIL-101(Cr) composite

In this paper, a new hydrogen storage composite based on magnesium hydride and metal-organic framework MIL-101(Cr) (an acronym for Material Institute Lavoisier) has been mechanically synthesized and investigated. The hydrogen sorption and desorption behavior in the composite was investigated for the temperature and pressure ranges of (593−653) K and (0−3) MPa, respectively. According to the pressure-composition-temperature isotherms of hydrogen sorption/desorption, the MgH2–5 wt%MIL-101(Cr) composite starts to absorb hydrogen at a lower pressure at 593 K. In addition, the hydrogen release peak for composite shifts to lower temperatures by about 140 K compared to the pure milled MgH2 during temperature programmed desorption at a heating rate of 6 K/min. The activation energy of hydrogen desorption from the composite is 120 ± 2 kJ/mol, which is 36% lower than the activation energy of hydrogen desorption from magnesium hydride (189 ± 2 kJ/mol). The enthalpy of hydrogen absorption was found to be 73 kJ/mol H2 and 60 kJ/mol H2 for milled MgH2 and MgH2–5 wt%MIL-101(Cr) composites, respectively. It is showed that the MIL-101(Cr) MOF structures decompose during ball milling and the chromium oxide nanoparticles form a core-shell structure with MgH2 particles. Thus, composite has better sorption and desorption properties than pure magnesium/magnesium hydride due to the nanoconfinement of MgH2. The chromium oxide nanoparticles act as active sites on the surface of the magnesium particles and facilitate the dissociative chemisorption or recombinative desorption of hydrogen. The main regularities of the phase transition in the magnesium-hydrogen system for the composite and magnesium hydride during dehydrogenation have been studied for temperatures in the range of 298–750 K. The in situ analysis of the phase transitions during the dehydrogenation of MgH2 and MgH2–5 wt%MIL-101(Cr) composite indicates a pronounced catalytic effect of the addition of MIL-101(Cr) to the Mg/MgH2. Based on the experimental results and ab initio calculations, a mechanism for the sorption and desorption of hydrogen by the composite has been proposed.

Hydrogen embrittlement behavior of two mining chain steels by slow strain rate test

Two mining chain steels 23MnNiMoCr5-4 and 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) with tensile strengths of 1200 MPa and 1250 MPa, respectively, were employed to investigate their hydrogen embrittlement behaviors by slow strain rate tests combined with thermal desorption analyses. It is shown that at initial stage the fracture stress decreases linearly as the hydrogen content increases, and the turning points occur at hydrogen content of 1.2 wppm for 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) and 0.26 wppm for 23MnNiMoCr5-4. The fractured surface observation suggests that the ratio of intergranular fracture area to quasi-cleavage area increases dramatically at the turning points. The maximum hydrogen contents resulting from the corrosive HCl solutions with pHs of 2 are approximately 0.6 wppm 0.3C0.2Si0.3Mn4.2(Cr+Ni+Mo) and 0.11 wppm for 23MnNiMoCr5-4, corresponding to the activation energies for hydrogen desorption are 21.57 kJ/mol and 14.53 kJ/mol, respectively.

(TiVZrNb)83Cr17 high-entropy alloy as catalyst for hydrogen storage in MgH2

High-entropy alloys (HEAs) hold the promise as excellent catalysts to improve the hydrogen storage properties of MgH2 due to their distinctive compositional and structural characteristics and superior room-temperature hydrogen storage performance. In this work, the (TiVZrNb)83Cr17 HEA was successfully designed and synthesized. Moreover, different amounts of the (TiVZrNb)83Cr17 HEA were introduced into MgH2 as catalysts by ball milling to prepare MgH2-x wt% (TiVZrNb)83Cr17 (x = 3, 6, 10) composites. MgH2-6 wt% (TiVZrNb)83Cr17 composite showed excellent hydrogen absorption and desorption properties. At 573 K, the composite can rapidly absorb 5.3 wt% H2 within 5 min, and even at 523 K, it can still absorb 4.0 wt% H2 within 60 min. Also, it can rapidly desorb 6.0 wt% and 4.1 wt% H2 within 60 min at 623 K and 573 K, respectively. In addition, the MgH2-6 wt% (TiVZrNb)83Cr17 composite possessed relatively low activation energy for hydrogen absorption (70.6 kJ/mol H2) and desorption (90.5 kJ/mol H2). Moreover, the capacity retention was as high as 98.5 % after 10 hydrogen de/absorption cycles at 573 K. Interestingly, (TiVZrNb)83Cr17 HEA transformed from the as-prepared biphasic structure (FCC + BCC) to the hydrogenated single-phase structure (FCC) in the hydrogenation process and returned to the initially biphasic structure (FCC + BCC) in the desorption process, which provided more diffusion pathways for H and more nucleation sites for Mg/MgH2. The multi-element (TiVZrNb)83Cr17 HEA catalyst realized the synergistic catalysis of the hydrogen ab/de-sorption of Mg/MgH2. Our findings provide useful insights into the design and development of HEAs catalysts for improving the hydrogen storage properties of MgH2.

Phase evolution, hydrogen storage thermodynamics, and kinetics of ternary Mg98Ho1.5Fe0.5 alloy

Rare earth elements and transition metals have been found to improve the hydrogen storage characteristics of magnesium-based alloys. This study investigated the Mg-Ho-Fe (MHF) ternary alloy prepared using the vacuum induction melting technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), pressure-composition-temperature (PCT), and differential scanning calorimetry (DSC) were used to analyze the alloy's phase transitions, microstructure, thermodynamics, and kinetic properties. The results reveal that the Mg98Ho1.5Fe0.5 alloy forms a solid solution with Ho and Fe in the magnesium matrix. Upon hydrogen absorption, the activated alloy transforms into a mixture of Mg/MgH2 phases and nanoscale HoH2 phases. Notably, only the MgH2 phase decomposes during hydrogen desorption, while the HoH2 phase remains unchanged, exhibiting a positive catalytic effect. The alloy demonstrates excellent hydrogen absorption kinetics, achieving a capacity of 5.56 wt% H2 within 10 min at 360 °C, owing to the combined catalytic effects of Ho and Fe. The activation energy for hydrogen desorption is found to be 135.87 kJ/mol, which is lower than that of the activation energies of pure MgH2 and MgFe alloys, indicating an enhancement in desorption kinetics. Moreover, the enthalpy and entropy changes for hydrogen absorption and desorption are determined to be –70.51 kJ/mol H2, –125.62 J/(K·mol) H2, 72.83 kJ/mol H2, and 128.95 J/(K·mol) H2, respectively. Furthermore, it is worth noting that the thermodynamic properties of the alloy are improved due to the catalytic effect of Ho and Fe.

Hydrogen storage thermodynamics and kinetics of the as-cast and milled Ce-Mg-Ni-based alloy

Aiming at investigating the improvement of hydrogen storage performance of the ternary Ce5Mg85Ni10 alloy after mechanical ball milling, the phase transformation and microstructure evolution of the milled alloys are observed by XRD, SEM and TEM. After ball milling, the alloys mainly exhibit nanocrystalline and amorphous structures and consist of Mg, Mg2Ni and CeMg12 phases. After hydrogen absorption, the milled alloys consist of MgH2, Mg2NiH4 and CeH2.73 phases. Isothermal and non-isothermal reaction curves of hydrogen absorption and desorption of the milled alloys are characterized by an Sievert apparatus, DSC and TGA. The Arrhenius and Kissinger equations are used to calculate the hydrogen desorption activation energy. For the Ce5Mg85Ni10 alloy, its structure and thermodynamics and kinetics performance of hydrogen storage strongly depend on the ball milling time. The results shows that ball milling for 5 h can achieve the optimal activation and kinetics performances but further prolonging the ball milling time lead to the impairment of hydrogen storage performance. Besides, ball milling time has little effect on thermodynamic property. Experiments results show that the 5 h-milled alloy after activation can absorb 4 wt% H2 in 18 s at 473 K and 3 MPa, and can desorb 3 wt% H2 in 906 s at 533 K and 1 × 10−4 MPa. In addition, the hydrogen desorption enthalpy change of the 5 h-milled alloy is 75.28 kJ/mol and the hydrogen desorption apparent activation energy of this alloy is 62.69 kJ/mol.

Improved hydrogen storage thermodynamics and kinetics of La–Ce–Mg–Ni alloy by ball milling

Reducing particle size and modifying surface state by ball milling are recognized as a useful method to improve the hydrogen absorption kinetics of Mg-based alloys. In order to obtain amorphous and nanocrystalline structures, ball milling was used to process the as-cast La7Ce3Mg80Ni10 alloy ingot. The effects of ball milling time (5–30 h) on the structures and the thermodynamics and kinetics of hydrogen absorption/desorption of the as-milled alloys were studied. The XRD, SEM and TEM were applied to investigate the structure of the alloys. The Sievert apparatus, DSC and TGA were applied to study the isothermal and non-isothermal properties of hydrogen absorption/desorption. The Arrhenius and Kissinger methods were used to calculate the activation energy of hydrogen desorption. The results indicate that the 10 h-milled alloy has the optimal activation performance and hydrogen absorption/desorption kinetics. Prolonging or shortening the ball milling time results in the impaired hydrogen storage properties. The 10 h-milled alloy in fully activation state can absorb 4 wt% H2 in 80 s at 473 K and 3 MPa, and it can desorb 3 wt% H2 in 191 s at 573 K and 1 × 10−4 MPa. The hydrogen desorption enthalpy (ΔH) of the 10 h-milled alloy is 74.21 kJ/mol. Moreover, the 10 h-milled alloy has the smallest apparent activation energy for hydrogen desorption (62.9 kJ/mol), which is the reason why it has the optimal hydrogen storage properties.

High catalytic activity derived from TiNbAlC MAX towards improving the hydrogen storage properties of MgH2

MXenes are great catalysts for MgH2 hydrogen storage material. Nevertheless, the synthesis of MXenes needs a large quantity of corrosive HF solution to remove the Al layers from the MAX materials, which is harmful to the environment. In this work, TiNbAlC MAX was employed without HF-treatment to tailor the hydrogen storage of MgH2. The MgH2 + 10 wt% TiNbAlC composite was prepared by ball milling. The addition of 10 wt% TiNbAlC greatly reduces the onset temperature of MgH2 desorption from 332 °C to 197 °C. The composite can desorb 6.2 wt% of H2 in 10 min at a constant temperature of 300 °C. Moreover, the dehydrided MgH2 + 10 wt% TiNbAlC composite can start to absorb hydrogen at room temperature under a H2 pressure of 6 MPa. The hydrogen desorption activation energy of the composite was calculated by Kissinger’s method to be 101 kJ mol−1. Microstructures studies demonstrated that TiNbAlC will partially react with MgH2/H2 to form TiH1.971, which can act as a “hydrogen pump” to facilitate the hydrogen sorption of MgH2. This work suggests that MAX materials are good catalysts for hydrogen storage of MgH2 and provides new insight into the role of MAX phase in tailoring the hydrogen storage of Mg-based hydrogen storage materials.

Hydrogen absorption-desorption properties and hydrolysis performance of MgH2-Zr3V3O0.6Hx and MgH2-Zr3V3O0.6Hx-C composites

In this work, the catalytic effect of additions of η-Zr3V3O0.6 mixed suboxide and the title intermetallic composite with graphite on the properties of MgH2 in the processes of hydrogen storage and hydrogen generation has been studied. The hydride composites were obtained by high-energy reactive ball milling (RBM). The samples were characterized by X-ray diffraction and scanning electron microscopy. The cycling performance of hydrogen desorption and absorption as well as hydrogen generation in the hydrolysis reaction were studied. We found that during the reactive ball milling the studied composites are able in a few minutes to absorb ∼6.5 wt% of hydrogen. The addition of Zr3V3O0.6 and Zr3V3O0.6 + С catalysts not only increases the rates of hydrogen absorption and release, but also lowers the activation energy and the temperature of hydrogen desorption. For a composite containing 10 wt% of suboxide and 3 wt% of graphite, the activation energy of hydrogen desorption determined by the Kissinger method was very low, just 58 kJ/mol, and this value is among the lowest described in the reference publications. We also show that the intermetallic additive forms the Zr3V3O0.6H∼10 hydride, which results in a higher gravimetric capacity of the composite as H storage material. The improved kinetics of hydrogen exchange and increased hydrogenation capacity at modest operating temperatures of 150–200 °C appear to be characteristic for the composites containing suboxide and graphite additions. The reason for the advanced performance is in the different the morphology of the synthesized samples, particularly the graphite-containing composites showing a more developed dispergation of the material (the fraction with a particle size of 1–3 μm is >80 wt%). We furthermore show that the synthesized materials can be used in the hydrolysis process resulting in hydrogen generation. The amount of hydrogen released from the hydride Mg-Zr3V3O0.6-C composite in the hydrolysis reaction reaches 1364 ml/g (the conversion degree is ∼85 % for the process duration of 120 min) when using 0.04 M MgCl2 solutions.

Hierarchical Structure Carbon-Coated CoNi Nanocatalysts Derived from Flower-Like Bimetal MOFs: Enhancing the Hydrogen Storage Performance of MgH2 under Mild Conditions

The construction of transition metal nanocatalysts has become an attractive way to improve the hydrogen storage performance of MgH2. However, it is still a great challenge to obtain multielement transition metal nanocatalysts, improve the hydrogen storage capacity of MgH2 under mild conditions (<573 K), and reduce the apparent activation energy of hydrogen absorption and desorption. In this work, a flower-like CoNi-MOF was successfully synthesized by sacrificing templates. After further pyrolysis, we obtained hierarchical structure flower-like MOF derivatives assembled by CoNi@C nanoparticles with an average particle size of 15.1 nm. Then, MgH2-x wt % CoNi@C nanocomposites are fabricated through the ball milling process. Due to the unique hierarchical flower-like structure of MOF derivatives, the inhibition of an amorphous carbon shell on CoNi alloy growth and agglomeration, and the synergistic catalysis of Mg2Co and Mg2Ni, the MgH2-10 wt % CoNi@C nanocomposite exhibits excellent hydrogen storage capacity under mild conditions and low apparent activation energy: At 473 K, the nanocomposite can quickly absorb 5.0 wt % H2 in 5 min, and even at 373 K, it can still absorb 4.2 wt % H2 in 60 min. At 573 and 548 K, it can release 4.8 and 3.1 wt % H2, respectively. The apparent activation energies for hydrogen absorption and desorption decrease remarkably to 24.9 and 67.3 kJ/mol, respectively, which are significantly better than those for MgH2-10 wt % Co@C (44.7 and 79.8 kJ/mol) and MgH2-10 wt % Ni@C (39.0 and 78.9 kJ/mol) nanocomposites. The first-principles calculation indicated that the hydrogen adsorption energy of the Mg–CoNi model is as low as −5.87 eV. This work provides a new strategy for the design of highly efficient multielement transition metal hydrogen storage material catalysts with a hierarchical structure.

Hydrogen trapping and hydrogen embrittlement (HE) susceptibility of X70 grade high-strength, acid-resistant, submarine pipeline steel with Mg treatment

The hydrogen embrittlement (HE) susceptibility of X70 grade, Acid-resistant, submarine pipeline steel with Mg treatment was studied via constant loading and slow strain rate tension (SSRT). The hydrogen diffusion and trapping behaviour in the tested steels treated with different Mg content was analysed by electrochemical hydrogen permeation testing and thermal desorption spectroscopy (TDS). The results suggested that micro/nanocomposite inclusions with “core-shell” structures obtained via Mg treatment have a greater activation energy of hydrogen desorption. On the one hand, this kind of inclusions can trap more hydrogen and reduce the diffusion coefficient of hydrogen in steel; on the other hand, the internal rigid core of Al–Mg–Ti–O and external soft shell of MnS not only play a role in stress shielding but also hinder the initiation and expansion of stress cracks. For the X70 grade high-strength, acid-resistant submarine pipeline steel, an optimal Mg addition amount is 0.003% (mass fraction).

Quantitative hydrogen trap states on high-angle grain boundaries and at dislocations in iron

Low-temperature thermal desorption spectrometry (L-TDS), by which specimens can be heated from -200 °C, was used to quantitatively evaluate the hydrogen desorption peak temperatures (Tp), the hydrogen desorption activation energies (Ea), and the hydrogen occupation ratios (θ) of high-angle grain boundaries and dislocations in iron. Cold-rolled iron was subjected to recrystallization and grain growth to produce three kinds of specimens with different areas of high-angle grain boundaries, in which low-angle grain boundaries were annihilated. L-TDS spectra of specimens containing dislocations and high-angle grain boundaries displayed two hydrogen desorption peaks, denoted as Peak 1 and Peak 2. Peak 1 corresponded to dislocations and displayed Tp, Ea, and θ values of 23 °C, 28.6 kJ·mol−1, and 1.1 to 2.7 atom·nm−1, respectively. In contrast, Peak 2 corresponded to high-angle grain boundaries and displayed Tp, Ea, and θ values of 85 °C, 43.7 kJ·mol−1, and 2.3 to 2.9 atom·nm−2, respectively.

Microstructural characteristics and hydrogen storage properties of the Mg–Ni–TiS2 nanocomposite prepared by a solution-based method

Magnesium hydride is considered as a promising solid-state hydrogen storage material due to its high hydrogen capacity. How to improve hydrogen desorption kinetics of MgH2 is one of key issues for its practical applications. In this study, we synthesize a Mg–Ni–TiS2 composite through a solution-based synthetic strategy. In the as-prepared composite, the co-precipitated Mg and Ni nanoparticles are highly dispersed on TiS2 nanosheets. As a result, the activation energy for hydrogen desorption decreases to 79.4 kJ mol−1. Meanwhile, the capacity retention rate is kept at the level of 98% and only slight kinetic deterioration is caused after fifty hydrogenation-dehydrogenation cycles. Further investigation indicates that the superior hydrogen desorption kinetics is attributed to the synergistically catalytic effect of the in situ formed Mg2NiH4 and TiH2, and the remained TiS2. The excellent cycle stability is related not only to the inhibition effect of the secondary phases on powder agglomeration and crystallite growth of Mg and MgH2 but also to the prevention effect of MgS and TiS2 on redistribution of catalytic Mg2NiH4 and TiH2 nanoparticles during cycling. This work introduces a feasible approach to develop Mg-based hydrogen storage materials.

Hydrogen Desorption Kinetics of V30Nb10(TixCr1–x)60 High-Entropy Alloys

In recent years, high-entropy alloys (HEAs) have attracted wide attention for their enormous hydrogen storage potential, fast hydrogen absorption kinetics, and a wide range of composition selectivity, and the fact that alloys with body-centered cubic (BCC) structure are considered to possess large capacity. Herein, three V30Nb10(TixCr1–x)60 HEAs with different Ti contents (Ti25, Ti30, Ti35) forming BCC structures were designed using the method of CALPHAD. The microstructure characteristics and the hydrogen storage performances, especially the kinetics of hydrogen desorption, were systematically investigated. The results show that after absorbing ~3.7 wt.% hydrogen at 300 K with 100 bar hydrogen pressure, the studied alloys exhibit similar hydrogen release behaviors at different temperatures. Taking the V30Nb10Ti25Cr35 alloy as an example, it was able to release 1.96 wt.%, 2.21 wt.%, and 2.48 wt.% of hydrogen at 353, 373, and 423 K, respectively. The higher the temperature, the faster the hydrogen desorption kinetics and the more hydrogen released. The hydrogen desorption kinetics of the alloys were successfully fitted with the Ginstling–Brounshtein model, and the main rate-controlling step was diffusion. In addition, the diffusion activation energy of hydrogen desorption decreases with the substitution of Cr content. The present study is expected to provide valuable information for the better development of high-entropy-based hydrogen storage alloys.

Structural features of 18R-type long-period stacking ordered phase in Mg85Zn6Y9 alloy and the Ni doping effect on its hydrogen storage properties

Magnesium-based alloys with 18R-type long-period stacking ordered (LPSO) structures have attracted wide attention for structural and functional applications. To understand hydrogen storage properties of 18R phase, the Mg85Zn6Y9 alloy with 94 wt.% of 18R-type LPSO phase is prepared in this work. The 18R phase has a layered structure where Y-Zn-Mg and Mg layers alternately stack along the c-axis. In the Y-Zn-Mg layers, Y, Zn and partial Mg sites are co-occupied by Y and Mg, Zn and Mg, and Mg and Zn/Y atoms, respectively. Thus the 18R phase is easily decomposed into α-MgH2, γ-MgH2, YH2, YH3, C14-type Laves phase MgZn2 and minor CsCl-type Y(Mg,Zn) during ball milling under hydrogen atmosphere. After further hydrogen absorption-desorption cycling, Y(Mg,Zn) disappears gradually and C14 phase transforms into C15-type Laves phase. By contrast, the Mg85Zn6Y9 alloy has better hydrogen storage kinetics and cycle durability than pure Mg because of the catalytic effect of YH2/YH3 on hydrogen absorption-desorption and inhibition role of Laves phase in Mg crystallite growth. Moreover, the introduction of Ni into Mg85Zn6Y9 sample leads to a further decrease in activation energy of hydrogen desorption from 106.39 to 96.78 kJ mol−1 due to the formation of Mg2Ni. This work not only provides new insights into structural features and hydrogen storage characteristics of 18R phase but offers an effective method for improving hydrogen storage properties.

A comparative study of hydrogen storage properties of AZ31 and AZ91 magnesium alloys processed by different methods

We studied the hydrogen storage properties of commercial AZ31 and AZ91 magnesium alloys with refined microstructure. Various processing techniques, such as equal channel angular pressing (ECAP) and high energy ball milling (HEBM), were utilized to modify the microstructure and phase composition of the alloys. X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical digital microscopy (ODM) were utilized to characterise the microstructure and phase transformations during the hydrogen absorption/desorption processes. The isothermal kinetics are measured by a Sieverts’-type apparatus at the temperature of 350 ℃. In the ECAP process, the AZ31 alloy had a slightly higher storage capacity and faster absorption kinetics, absorbing 6.0 wt% (6.6 wt% - maximum) of hydrogen in less than 12 min. In the HEBM process, AZ91 absorbed 6.5 wt% of hydrogen and desorbed it in 347 ± 13 s, a faster rate than AZ31. The pressure–composition isotherms of all studied alloys at a temperature of 300 °C were determined. The processed alloys exhibited no pressure hysteresis. The microstructure of bulk samples and the morphology of prepared powders were shown to affect their hydrogenation behaviour. The activation energies for hydrogen desorption were 119 ± 2 kJ/mol for AZ31 alloy processed by ECAP and 128 ± 3 kJ/mol for AZ91 alloy processed by HEBM, which are significantly lower than the activation energy values of pure Mg hydride.

Desorption activation energy of hydrogen from the Si (111)1x1: H surface studied by optical sum frequency generation and second harmonic generation

The desorption kinetics of hydrogen from the H-Si (111)1 × 1 surface prepared by molecular hydrogen exposure has been studied by combining optical sum frequency generation (SFG) for higher coverage and second harmonic generation (SHG) for lower coverage at several sample temperatures from 711 K to 770 K. The hydrogen desorption activation energy was found as Ed = 60.1 ± 8.8 kcal/mol (2.60 ± 0.38 eV) for the second order desorption for the coverage above 0.44 ML. For the first order desorption at the coverage below 0.18 ML the desorption activation energy was found as Ed= 32.6 ± 8.1 kcal/mol (1.41±0.35 eV).

A new light-element multi-principal-elements alloy AlMg2TiZn and its potential for hydrogen storage

Hydrogen storage in a new-designed multi-principal-elements alloy (MPEA) AlMg2TiZn was studied. The chemical composition of the alloy ranks the alloy into the so-called light-elements family of MPEAs. The alloy was prepared by high energy ball milling and the measurements were carried out using Sieverts method in temperature interval 310°C–370 °C under the hydrogen-gas pressure up to 25 bar. It was found that hydrogen absorption capacity was about 1.35–1.4 wt% H2. Activation energy of hydrogen desorption, Ea = 148 ± 1 kJ/mol H2, was significantly lower than known value for pure Mg. Hydrogen absorption occurred in two steps, A1 and A2. The values of activation energy Ea = 175 ± 12 kJ/mol H2 for rapid absorption A1 and Ea = 195 ± 8 kJ/mol H2 for subsequent slow absorption A2 were close to that for Mg. It was proposed that A1 was related to formation of αMgH2, the second one, A2, was related to formation of βMgH2. Absolute value of hydride formation enthalpies, ΔH = 75 ± 3 kJ/mol H2 and ΔH = 70 ± 2 kJ/mol H2 obtained for A1 and A2, respectively, as well as value of ΔH = 76 ± 1 kJ/mol H2 for desorption were about the same as that for pure MgH2.

The effect of different Co phase structure (FCC/HCP) on the catalytic action towards the hydrogen storage performance of MgH2

The close contact of fine Co particles with MgH 2 created numerous active sites and provided sufficient channels for hydrogen diffusion, contributing to the accelerated de/rehydrogenation kinetics. High hydrogen desorption temperature and sluggish reaction kinetics are the major limitations for the practical application of MgH 2 . In this study, Co particles with a face centered cubic (FCC) structure and a hexagonal close packed (HCP) structure were prepared facilely and proved to be good catalysts for magnesium hydride. Co particles with FCC structure presented better catalytic effect on MgH 2 than that with HCP structure. Both 7% (mass) Co FCC and HCP particle modified MgH 2 decreased the initial dehydrogenation temperature from 301.3 °C to approximately 195.0 °C, but 7% (mass) Co with FCC structure modified MgH 2 has a faster desorption rate, and around 6.5% (mass) H 2 was desorbed in 10 min at 325 °C. Hydrogen uptake was detected at 70 °C under 3.25 MPa hydrogen pressure and 6.0% (mass) H 2 was recharged in 40 min at 150 °C. The hydrogen desorption and absorption activation energy for 7% (mass) FCC Co modified MgH 2 was significantly decreased to (76.6±8.3) kJ·mol −1 and (68.3±6.0) kJ·mol −1 , respectively. Thermodynamic property was also studied, the plateau pressures of MgH 2 + 7% (mass) FCC Co were determined to be 0.14, 0.28, 0.53 and 0.98 MPa for 300 °C, 325 °C, 350 °C and 375°C. The decomposition enthalpy of hydrogen (Δ H ) for MgH 2 + 7% (mass) FCC Co was (80.6±0.1) kJ·mol −1 , 5.8 kJ·mol −1 lower than that of as-prepared MgH 2 . Moreover, cycling performance for the first 20 cycles revealed that the reaction kinetics and capacity of MgH 2 -FCC Co composite remained almost unchanged. The result of density functional theory calculation demonstrated that cobalt could extract the Mg H bond and reduced the decompose energy of magnesium hydride. Our paper can be presented as a reference for searching highly effective catalysts for hydrogen storage and other energy-related research fields.

Hydrogen storage in TiVCrMo and TiZrNbHf multiprinciple-element alloys and their catalytic effect upon hydrogen storage in Mg

Hydrogen storage performance of nearly equimolar alloys TiVCrMo and TiZrNbH (HEA's) was studied. The alloys were prepared by repeated arc-melting in the form of single-phase alloys and as their multi-phase analogues prepared either by high-energy ball milling (HEBM) of the arc-melted ingots or by HEBM of pure elements. Besides these four-component experimental materials, also their mixtures with Mg were investigated that were prepared by HEBM with Mg (composition in both cases was Mg – 10 wt % HEA). Hydrogen sorption experiments were carried out in temperature interval between 240 °C and 380 °C under hydrogen-gas pressure up to 50 bars. Kinetic curves, temperature-programmed desorption spectra and concentration-pressure isotherms were measured. It was found that activation energy of hydrogen desorption, |Ea|, for materials with Mg was significantly decreased compared to pure Mg. Hydride formation enthalpy, ΔH, observed for all the materials under study was about the same as that for pure MgH2. Hydrogen absorption capacity in pure four-component HEA's was negligible; only hydrogen surface adsorption was detected. The capacity of mixtures of HEA's with Mg was between 3.5 and 6 wt % H2.