Hydrogen Absorption Desorption Kinetics Research Articles

One electron is enough: A chemical bonding approach on the hydrogen induced structural transformation in MPd3 (M = Mg, In, Tl) intermetallic compounds

It is crucial to understand the nature of the metal-hydrogen interactions in the intermetallic hydrides in order to develop new hydrogen storage materials with optimum metal-H interactions for desired hydrogen absorption-desorption kinetics. The MPd3 intermetallic compounds (M = Mg, In, Tl) transform to cubic anti-perovskite type (AuCu3-type with hydrogen in the octahedral intersect) structure from ZrAl3-type four-fold ccp-superstructure up on hydrogenation. According to the DFT-based orbital space electronic structure and chemical bonding analysis, this structural transformation is driven by maximizing the coordination of hydrogen surrounded by Pd, forming the Pd (4d) – H (1s), and … Pd–H–Pd … multicenter interactions. These interactions provide sufficient thermodynamic stability for the hydrides to form, with negative binding energies of hydrogen. The binding energies are found to be significantly low, with the possibility of the ease of hydrogen desorption within limited reaction conditions.

Destabilization of the LiBH4–NaBH4 Eutectic Mixture through Pore Confinement for Hydrogen Storage

Both LiBH4 and NaBH4 are well known for having high hydrogen contents, but also high decomposition temperatures and slow hydrogen absorption–desorption kinetics, preventing their use for hydrogen storage applications. The low melting temperature (219 °C) of their eutectic mixture 0.71 LiBH4–0.29 NaBH4 allowed the synthesis of a new composite material through the melt infiltration of the hydrides into the ~5 nm diameter pores of a CMK-3 type carbon. A composite of 0.71 LiBH4–0.29 NaBH4 and non-porous graphitic carbon discs was also prepared by similar methods for comparison. Both composites showed improved kinetics and a partial reversibility of the dehydrogenation/rehydrogenation reactions. However, the best results were observed for the CMK-3 nanoconfined hydrides; a consistent uptake of about 3.5 wt.% H2 was recorded after five hydrogenation/dehydrogenation cycles for an otherwise non-reversible system. The improved hydrogen release kinetics are attributed to carbon–hydride surface interactions rather than nanoconfinement, while enhanced heat transfer due to the carbon support may also play a role. Likewise, the carbon–hydride contact proved beneficial in terms of reversibility, without, however, ruling out the potential positive effect of pore confinement.

In situ incorporation of highly dispersed nickel and vanadium trioxide nanoparticles in nanoporous carbon for the hydrogen storage performance enhancement of magnesium hydride

Slow hydrogen absorption-desorption kinetics and high thermal stability hinder the industrial application of Mg/MgH2 systems. In order to overcome these drawbacks, the in situ incorporation of highly dispersed nickel and vanadium trioxide nanoparticles in nanoporous carbon ((Ni-V2O3)@C) was introduced to enhance the hydrogen storage performance of Mg/MgH2. It was found that the 10 wt% (Ni-V2O3)@C-containing sample demonstrated excellent hydrogen storage performance. The 10 wt% (Ni-V2O3)@C-containing sample could uptake hydrogen at room temperature, and its initial hydrogenation temperature was reduced by 100 °C as compared to that of pristine MgH2. Moreover, this system could absorb 5.50 wt% of H2 at 25 °C and release 6.05 wt% of H2 at 275 °C in 10 min. In addition, the hydrogenation and dehydrogenation activation energies of the 10 wt% (Ni-V2O3)@C-containing sample were calculated as 42.1 kJ·mol−1 and 84.6 kJ·mol−1, respectively, which were 46.8 kJ·mol−1 and 51.6 kJ·mol−1 lower than those of pristine MgH2, respectively. Mechanism analysis results revealed that V2O3 was partially converted into VO during milling with MgH2, and Ni reacted with Mg to form in situ Mg2Ni after the first dehydrogenation. The in situ Mg2Ni/Mg2NiH4 coating around Mg/MgH2 acted as a “hydrogen pump” to drive hydrogen diffusion and dissociation, and the presence of C inhibited the agglomeration of Mg/MgH2 particles. Therefore, the addition of (Ni-V2O3)@C was found to be a good strategy to improve the hydrogen storage performance of MgH2.

Graphene and Carbon Nanotubes Fibrous Composite Decorated with PdMg Alloy Nanoparticles with Enhanced Absorption-Desorption Kinetics for Hydrogen Storage Application.

We describe a graphene and fibrous multiwall carbon nanotubes (f-MWCNT) composite film prepared by plasma-enhanced chemical vapor deposition for use as a suitable and possible candidate of hydrogen storage materials. A high storage capacity of 5.53 wt% has been obtained with improved kinetics. The addition of binary PdMg alloy nanoparticles to the surface of graphene-fibrous nanotubes composite films raised the storage capacity by 53% compared to the film without PdMg decorated nanoparticles. Additionally, the graphene/f-MWCNT composite film decorated with PdMg nanoparticles exhibited an enhanced hydrogen absorption–desorption kinetics. The fibrous structure of the MWCNTs, alongside graphene sheets within the film, creates an enormous active region site for hydrogen reaction. The addition of PdMg nanoparticles enhanced the reaction kinetics due to the catalytic nature of Pd, and increased the hydrogen content due to the high absorption capacity of Mg nanoparticles. The combination of Pd and Mg in a binary alloy nanoparticle enhanced the hydrogen capacity and absorption–desorption kinetics.

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH2-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO2, and Ti4Fe2Ox suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH2 and hydrogenation of nano-Ti and Ti4Fe2Ox. The possible reduction of TiO2 during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO2 compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH2 by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH2 down to 74 and 67 kJ/mol for MgH2 modified with nano-TiO2 and Ti4Fe2O0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH2. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti4Fe2O0.3 nanocomposite, ΔHdes = 76 kJ/mol H2 and ΔSdes = 138 J/K·mol H2, correspond to the reported literature values for pure polycrystalline MgH2. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO2 and Ti4Fe2Ox additives. Cycling stability of Mg/Ti4Fe2Ox was substantially improved by introduction of 3 wt% graphite into the composite.

Effect of annealing temperature on the microstructure of Pr2Co7 alloy and its hydrogen absorption-desorption kinetics

In this work, we studied the effect of annealing temperature on the kinetics of hydrogen absorption/desorption in nanocrystalline Pr2Co7 alloy. The pressure-composition (P–C) isotherm measurements revealed that the plateau pressure of Pr2Co7 alloy after annealing becomes comparatively flat. The maximum hydrogen absorption content of as-milled alloy and the annealed one reached 1.01 and 1.2 H/M (H/M: hydrogen per metal), respectively. The derived formation enthalpy ΔH and Gibbs free energy ΔG of the hydride in the annealed alloy are higher than those of the as-milled compound, indicating that the hydrides stability increases after annealing. The mechanism of hydrogen absorption-desorption process in the as-milled and annealed alloys was further investigated according to the Johnson-Mehl-Avrami formula. The hydrogenation of the as-milled and annealed Pr2Co7 alloys was found to be controlled by the three-dimensional-interface process. The activation energies Ea were determined and indicate that annealing is a very beneficial way to improve hydrogen absorption-desorption kinetics of the nanocrystalline Pr2Co7 alloy. The latter has a very fast absorption kinetic at 298 K. Throughout the study, it has been shown that Pr2Co7 alloy exhibits excellent hydrogenation properties which open a new perspective to be a new candidate for important uses in the field of hydrogen storage such as negative electrodes for nickel metal hydride (Ni-MH) batteries, mobile and short range vehicular applications.

Tailoring the hydrogen absorption desorption's dynamics of Mg[sbnd]MgH2 system by titanium suboxide doping

Titanium suboxide (TiO) is one of the best catalysts which improved the hydrogen absorption-desorption property of MgH2 Mg system. The TiO catalyzed Mg MgH2 have shown a remarkably reduced apparent activation energy and enhanced the hydrogen absorption-desorption kinetics. The X-ray photoelectron spectroscopy (XPS) analysis has indicated that the oxidation state of Ti in TiO remains unchanged during ball milling and hydrogen absorption-desorption of TiO-doped-MgH2. The X-ray diffraction (XRD) analysis further confirms the XPS result. The TiO has shown the excellent catalytic effect on the MgMgH2 system which remarkably reduced the hydrogen absorption-desorption temperatures.

Improving the hydrogenation properties of MgH2 at room temperature by doping with nano-size ZrO2 catalyst

Recent studies have shown that the hydrogen absorption–desorption kinetics in magnesium can be improved by milling magnesium hydride (MgH2) with transition metal oxides. Accordingly, the present study investigates the particle size, crystalline structure and hydrogen absorption properties of MgH2 composite samples containing zirconium dioxide (ZrO2) nano-powder in molar fractions ranging from 1 to 5%. Hydrogen absorption kinetics isotherms are presented for temperatures in the range of 25–150 °C. The hydriding processes and kinetics of MgH2–ZrO2 alloy powers at different temperatures have also been studied. By theoretical analysis of the experimental data of the pyrolytic hydriding reaction, and fitting this with the Johnson–Mehl–Avrami (JMA) equation, the kinetic equation and the corresponding kinetic parameters, including the reaction order, the reaction rate constant, and the reaction activation energy, have been obtained. In general, the results show that ZrO2 acts as a refining agent, and is thus beneficial in decreasing the MgH2 activation energy and reducing the crystalline size during the milling process.

Crystallization behavior and hydrogen storage kinetics of amorphous Mg11Y2Ni2 alloy

The crystallization behavior and hydrogen storage kinetics of the amorphous Mg11Y2Ni2 alloy prepared by melt-spinning technique were investigated in detail. It was found that the amorphous Mg11Y2Ni2 alloy transforms into the crystalline Mg11Y2Ni2 in metastable state at 279°C and subsequently decomposes into Mg, YMgNi4 and Y(Mg) solid solution in stable state at 366°C. All the amorphous, metastable and stable Mg11Y2Ni2 alloys can be hydrogenated into MgH2, Mg2NiH4 and YH2/YH3. However, the stable Mg11Y2Ni2 alloy has slightly slower hydrogen absorption–desorption kinetics due to the worse dispersivity of YH2/YH3 and Mg2NiH4.

New Mg–Mn–Ni alloys as efficient hydrogen storage materials

New ternary Mg–Mn–Ni alloys synthesised by high energy ball milling have been studied as hydrogen storage materials. A new Mg 3MnNi 2 ternary intermetallic compound of the Ti 2Ni type has been identified, its structure was studied by neutron diffraction. It has been shown that in the Mg–Mn–Ni system this intermetallic coexists with Mg, forming two-phase Mg–Mg 3MnNi 2 alloys. The Mg 3MnNi 2 compound reversibly absorbs about 1 wt.% of hydrogen already at room temperature and forms interstitial type Mg 3MnNi 2H ∼3 hydride. Substantially higher hydrogenation capacity has been observed in Mg–Mg 3MnNi 2 alloys with high Mg content, e.g. 5.4 wt.% for the Mg 88Mn 4Ni 8 alloy at 250–350 °C. Hydrogen absorption–desorption kinetics of these alloys are drastically improved compared to undoped magnesium due to catalytic effect of the Mg 3MnNi 2 phase.

Hydriding behavior in Zr-based AB 2 alloy by gas atomization process

The hydriding characteristics of Zr-based AB 2 alloy produced by gas atomization have been investigated during its absorption–desorption reaction with hydrogen gas. Its gas-phase hydrogenation properties are different from those of specimens prepared by conventional methods. For the particle morphology of the as-cast and gas-atomized powders, it can be seen that the mechanically crushed powders are irregular, while the atomized powder particles are spherical. In PCT (Pressure–Composition–Temperature) measurements, for the gas-atomized particles smaller than 50 μm, the hydrogen storage capacity is dramatically decreased and the hysteresis loop becomes larger than that of the gas-atomized particles larger than 50 μm. In addition, the increase of jet pressure of gas atomization results in the decrease of hydrogen storage capacity and the slope of plateau pressure significantly increases. TEM and EDS studies showed the increase of jet pressure in the atomization process accelerated the phase separation within grain of the gas-atomized alloy, which brought about a poor hydrogenation property. In the measurements of hydrogen absorption–desorption kinetics, the improvement of desorption kinetics of gas-atomized AB 2 alloys was mainly caused by the higher plateau pressure, which is attributed to the smaller grain size and higher site energy for hydrogen in the gas-atomized alloys.

Kinetic investigation of the catalytic effect of a body centered cubic-alloy TiV 1.1Mn 0.9 (BCC) on hydriding/dehydriding properties of magnesium

In this work, the morphology, crystal structure and hydrogen absorption–desorption kinetics of ball-milled Mg–TiV 1.1Mn 0.9 composites were investigated. It was found that a low proportion of BCC-catalyst has a drastic effect on the sorption kinetics of MgH 2. Hydrogen sorption kinetics and capacities of the composite are correlated with milling time. The composite MgH 2–2 mol% BCC milled for 40 h can absorb 6.2 wt% hydrogen within 2 min, and desorbs the same amount in about 7 min at 573 K. Extending the milling period to 80 h reduces the storage capacity of the composite. The absorption behavior is best described by a three-dimensional growth of the hydride phase from the surface to the bulk with hydrogen diffusing through the hydride phase, whereas desorption kinetics are controlled by the two-dimensional growth of magnesium (in the bulk and surface) with a constant velocity of the Mg/MgH 2 interface. BCC alloy catalyst is particularly effective for desorption kinetics when the composite is milled for a long period of time.

Effect of Nb 2O 5 on MgH 2 properties during mechanical milling

Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH 2) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb 2O 5 when added in larger volume fraction. The effect of Nb 2O 5 on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH 2 can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb 2O 5 on MgH 2 sorption properties are rationalised in a new approach considering Nb 2O 5 as a dispersing agent, which helps reduce MgH 2 particle size during milling.

Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride

It has recently been discovered that energetic ball milling of hydrides can improve their hydrogen sorption properties significantly. In this work, we present a systematic study of structural modifications and hydrogen absorption–desorption kinetics of ball-milled magnesium hydride. Structural investigations showed that after only 2 h of milling, a metastable orthorhombic (γ) magnesium hydride phase is formed. A Rietveld analysis of the X-ray diffraction spectrum of the 20 h milled sample gave a proportion of 74 wt.% MgH2, 18 wt.% γ MgH2 and 8 wt.% MgO. The hydrogen capacity and sorption kinetics were measured before and after milling. We found that the sorption kinetics are much faster for the milled sample compared to the unmilled one. This explains the fact that the hydrogen desorption temperature of the ball-milled sample as measured by pressured differential scanning calorimetry (PDSC), is reduced by 64 K compared to the unmilled sample. There is no significant change of the storage capacity upon milling and the absorption plateau pressure does not change. From the desorption curves, the activation energy was deduced. The milling also increased the specific surface area. This was confirmed by SEM micrographs and BET measurements. Possible mechanisms explaining the improved kinetics are presented.

Hydrogen absorption-desorption kinetics of MmNi 5 and related compounds

A study was made to determine the effects of aluminum and iron substitution on the reaction kinetics of MmNi 5 ( Mm = mischmetal). Each sample used in this study i.e. MmNi 5, MmNi 4.5 A1 0.5 and MmNi 4.85Fe 0.15, was mixed with a heat buffer so that nearly isothermal conditions could be maintained. Experiments were also designed so that the ratio of the equilibrium plateau pressure to the opposing pressure was the same for each sample. This was done to ensure that the thermodynamic driving force acting on each sample was the same. In addition, a sufficiently large number of absorption-desorption cycles were initially done on each sample for reproducible results to be obtained. The relative reaction rates among the various samples studied were found to change with the temperatures and pressure ratios that were chosen. Thus it is very important that any comparison of the reaction kinetics of metal hydrides should also include information about the temperature intervals used and the activation energies of each sample, as well as quantitative expressions showing how the reaction rates change with the imposed pressure ratio. At 25 °C and a pressure ratio of 2, reaction rates were found to be in the following order: MmNi 5 > MmNi 4.5 Al 0.5 ≈ LaNi 5 > MmNi 4.85 Fe 0.15 Thus it was concluded that aluminum and iron substitution has a deleterious effect on the reaction kinetics of MmNi 5 hydride. Of the materials studied, MmNi 4.5Al 0.5 seems to be the most practical material for hydrogen storage. This is because it has thermodynamic and kinetic properties similar to those of LaNi 5 but at a cheaper price.

Hydrogen absorption and desorption properties of FeTi 1.14O 0.03 in impure hydrogen containing CO, CO 2 and oxygen

The hydrogen absorption and desorption properties of FeTi 1.14O 0.03 in impure hydrogen containing CO, CO 2 or oxygen were studied using either an automated apparatus for hydrogen absorption-desorption cycles combined with a mass spectrometer or gas chromatography. It was found that the effect of CO or CO 2 on the hydrogen absorption capacity and the hydrogen absorption-desorption kinetics of the alloy were more deleterious than that of oxygen. The hydrogen absorption capacity degraded after cycling in hydrogen containing CO or CO 2 was rapidly restored by cycling in pure hydrogen. However, restoration by cycling in pure hydrogen was more difficult after cycling in hydrogen containing oxygen. Gas analysis revealed that CO and CO 2 react with hydrogen to form CH 4 and C 2H 4, and that oxygen reacts with the alloy. It was concluded that the poisoning of oxygen is different from that of CO or CO 2. The mechanism of “surface poisoning” by these impurities is discussed.