Dehydriding Reaction Research Articles

Systematic study of first-row transition metals chlorides as reaction accelerators for Mg/ MgH2 for hydrogen storage application

Mg has been proposed as a possible hydrogen storage material. However, Mg necessarily requires the addition of a suitable material with catalytic activity to improve hydriding and dehydriding reactions. Some chlorides of transition metals have been tested as improvers of hydriding/dehydriding reactions in Mg/MgH2. However, experimental preparation and tested conditions are dissimilar among different published reports. A systematic study of first-row transition metal chlorides as additives is presented here to understand the various factors affecting the hydriding/dehydriding reactions in Mg/MgH2 prepared and tested under the same conditions. The added transition metal chlorides (TMClx, x = oxidation state) include metals in oxidation states +4, +3, and +2 whenever commercially available. Additional chlorides, such as ZrCl4 and AlCl3, were included for comparison. The mixtures of Mg-TMClx were produced employing cryogenic ball milled and characterized by i) temperature-programmed hydriding and dehydriding, ii) X-ray photoelectron spectroscopy at Mg, transition metals, and chlorine 2p edges, iii) X-ray diffraction, and iv) scanning electron microscope. The best additives are VCl3>TiCl4>NiCl2>AlCl3. In contrast, the worst additives among the first-row transition metal chlorides were CrCl2, ZnCl2, and CuCl2. They do not improve at all the hydrogen uptake kinetics of pure Mg. Chemical state, side reactions, crystal structure, and morphology were discussed as factors affecting the behavior of the hydriding and dehydriding reactions. A complex interrelationship among these factors is observed. However, the electronegativity of the transition metal and an interaction of the type Mg-TM-Cl, are proposed as the main factors for the observed improvement of hydriding and dehydriding reactions.

Fast Hydrogen Sorption Kinetics in Mg-VCl3 Produced by Cryogenic Ball-Milling.

Hydrogen storage in Mg/MgH2 materials is still an active research topic. In this work, a mixture of Mg-15wt.% VCl3 was produced by cryogenic ball milling and tested for hydrogen storage. Short milling time (1 h), liquid N2 cooling, and the use of VCl3 as an additive produced micro-flaked particles approximately 2.5-5.0 µm thick. The Mg-15wt.% VCl3 mixture demonstrated hydrogen uptake even at near room-temperature (50 °C). Mg-15wt.% VCl3 achieved ~5 wt.% hydrogen in 1 min at 300 °C/26 bar. The fast hydriding kinetics is attributed to a reduction of the activation energy of the hydriding reaction (Ea hydriding = 63.8 ± 5.6 kJ/mol). The dehydriding reaction occurred at high temperatures (300-350 °C) and 0.8-1 bar hydrogen pressure. The activation energy of the dehydriding reaction is 123.11 ± 0.6 kJ/mol. Cryomilling and VCl3 drastically improved the hydriding/dehydriding of Mg/MgH2.

Thermodynamics and kinetics of hydriding and dehydriding reactions in Mg-based hydrogen storage materials

Mg-based materials are one of the most promising hydrogen storage candidates due to their high hydrogen storage capacity, environmental benignity, and high Clarke number characteristics. However, the limited thermodynamics and kinetic properties pose major challenges for their engineering applications. Herein, we review the recent progress in improving their thermodynamics and kinetics, with an emphasis on the models and the influence of various parameters in the calculated models. Subsequently, the impact of alloying, composite, and nano-crystallization on both thermodynamics and dynamics are discussed in detail. In particular, the correlation between various modification strategies and the hydrogen capacity, dehydrogenation enthalpy and temperature, hydriding/dehydriding rates are summarized. In addition, the mechanism of hydrogen storage processes of Mg-based materials is discussed from the aspect of classical kinetic theories and microscope hydrogen transferring behavior. This review concludes with an outlook on the remaining challenge issues and prospects.

Microstructure and hydrogen storage kinetics of Mg89RE11 (RE = Pr, Nd, Sm) binary alloys.

The present work reports the fabrication of Mg89RE11 (RE = Pr, Sm, Nd) alloys by a vacuum induction furnace. The phase analysis, structure characterization and microstructure observation of Mg89RE11 alloys were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). There exhibits a multiphase microstructure in the as-cast Mg89RE11 (RE = Pr, Nd, Sm) alloys from the comprehensive analysis made from XRD and SEM, which are containing the major phase (RE5Mg41) and several secondary phases (REMg3 and REMg12). The detections of XRD and TEM reveal that these experimental alloys turn into a MgH2 nanocrystalline composite with equably distributed RE hydride nanoparticles after hydriding and this MgH2 major phase turns into a Mg nanocrystalline after dehydriding. The determination results of the hydrogen storage kinetics show that adding the rare earth element (Pr, Sm and Nd) ameliorates the hydriding and dehydriding kinetics of the Mg-based alloys dramatically. The hydrogen desorption activation energy Ea(de) of Mg89Pr11, Mg89Nd11, Mg89Sm11 are 140.595, 139.191, 135.280 kJ mol−1 H2, respectively. Specially, the hydrogen storage capacity (wt%) of Mg89Sm11 alloy that added Sm element can reached 5 wt%. The improvement of the hydrogen storage performance of Mg89RE11 alloys can be principally ascribed to the RE hydride nanoparticles facilitating the hydriding and dehydriding reactions.

Effect of Al* generated in situ in hydriding on the dehydriding properties of Mg-Al alloys prepared by hydriding combustion synthesis and mechanical milling

In this work, the effects of Al* generated in situ in hydriding on the dehydriding of Mg-Al alloys prepared by Hydriding Combustion Synthesis (HCS) and Mechanical Milling (MM) were studied. Thermal analysis showed that the peak dehydriding temperature of MgH2 was reduced by 25 °C (or 11 °C) when 20 at% Al was added in HCS (or MM) process, even though, the same dehydriding reaction of 17MgH2 + 12Al*/Al → Al12Mg17 + 17H2 was observed. Isothermal dehydriding at 300 °C for 3 h demonstrated that it took 81 min for HCS+MM-MgH2 to desorb 50% of hydrogen, while only 24 min was required for HCS+MM-Mg80Al*20. The apparent activation energy for dehydriding of MgH2 was reduced from 144.3 ± 7.19 kJ/mol for HCS+MM-MgH2 to 118.8 ± 6.73 kJ/mol for HCS+MM-Mg90Al*10. The scanning electron microscopy/energy dispersive spectroscopy and X-ray photoelectron spectroscopy analyses revealed that the Al* in situ generated from the hydriding of Al12Mg17 in HCS would significantly decrease dehydriding temperature and improve dehydriding kinetics of the MgH2-Al composites because of a uniform distribution, the shortening reaction diffusion length, providing unoxidized surfaces to react with MgH2 and the high chemical activity of Al*.

Thermodynamics Study of Dehydriding Reaction on the PuO2(110) Surface from First Principles

Using the first-principles atomistic thermodynamics method in combination with the DFT + U theory, we calculate the pressure–temperature phase diagrams of the dehydriding reactions on the hydrogenated PuO2(110) surface and determine the thermodynamic boundaries that control the feasibility of reaction. Effects of hydrogen coverage on the surface dehydriding processes are investigated. Compared to the dehydriding in the form of water molecule accompanied by oxygen vacancy on the surface, desorption of hydrogen molecule is more energetically favorable. However, the relative hydrogen and water vapor pressure could modify the feasibility of two types of dehydriding processes, which may lead to water desorption superior to hydrogen one, even at 300 K.

Nucleation and growth behaviors of hydriding and dehydriding reactions of Mg2Ni

Mg2Ni has drawn attention due to its high reaction rates, good reversibility, and possibility to be applied as a negative electrode in the Ni-MH (metal hydride) batteries. Mg2Ni samples were prepared by sintering under Ar in a stainless-steel crucible at 823K. Johnson-Mehl equation for nucleation and growth mechanism was applied to the behaviors of hydriding and dehydriding reactions of Mg2Ni. At 571K under 30bar H2, the activation of the Mg2Ni sample was completed at the number of cycles of three (n=3). At n=3, hydrided fractions of the sample were 0.51 (1.92wt% H) at 0.50min and 0.90 (3.38wt% H) at 2.00min. Reverse temperature dependences of the initial hydriding rate and the quantity of hydrogen absorbed for 60min appeared from 571K to 620K. Hydriding and dehydriding reactions progressed by the nucleation and growth mechanism and could be expressed by the Johnson-Mehl equation.

Thermophysicochemical Reaction of ZrCo–Hydrogen–Helium System

Nuclear fusion energy, which is clean and infinite, has been studied for more than half a century. Efforts are in progress worldwide for the demonstration and validation of nuclear fusion energy. Korea has been developing hydrogen isotope storage and delivery system (SDS) technologies including a basic scientific study on a hydrogen storage medium. An SDS bed, which is a key component of the SDS, is used for storing hydrogen isotopes in a metal hydride form and supplying them to a tokamak. Thermophysicochemical properties of the ZrCo–H $$_{2}$$ –He system are investigated for the practical utilization of a hydriding alloy system. The hydriding reaction, in which $$\hbox {ZrCoH}_{\mathrm{x}}$$ is composed as ZrCo absorbing hydrogen, is exothermic. The dehydriding reaction, in which $$\hbox {ZrCoH}_{\mathrm{x}}$$ decomposes into ZrCo and hydrogen, is endothermic. The heat generated through the hydriding reaction interrupts the hydriding progress. The heat loss by a dehydriding reaction impedes the dehydriding progress. The tritium decay product, helium-3, covers the ZrCo and keeps the hydrogen from contact with ZrCo in the SDS bed. In this study, we designed and fabricated a ZrCo bed and its performance test rig. The helium blanketing effect on a ZrCo hydrogen reaction with 0 % to 20 % helium content in a gaseous phase and a helium blanket removal method were studied experimentally. In addition, the volumetric flow rates and temperature at the beginning of a ZrCo hydrogen reaction in a hydrogen or helium atmosphere, and the cooling of the SDS bed by radiation only and by both radiation and natural convection related to the reuse cycle, were obtained.

New dehydrogenation pathway of LiBH4 + MgH2 mixtures enabled by nanoscale LiBH4

This study reports a new dehydrogenation pathway for the LiBH4 + MgH2 system. We find that in the presence of nanoscale LiBH4, the first dehydriding reaction is the decomposition of LiBH4 and the product from the LiBH4 decomposition in turn causes the hydrogen release from MgH2. The new reaction pathway has resulted in a large amount of hydrogen release from MgH2 below 150 °C. The discovery made in this study confirms the ab initio density functional theory calculations reported in the literature and a previous study showing that the new reaction pathway observed in this study can be achieved during high-energy ball milling of MgH2 at ambient temperature in conjunction with aerosol spraying of LiBH4 solution. The establishment of this new reaction pathway opens the door to make LiBH4 + MgH2 as a viable system for reversible hydrogen storage applications near 150 °C in the future.

Hydriding and dehydriding in air-exposed Mg[sbnd]Fe powder mixtures

MgFe mixtures, with a nominal Fe content of 3 wt.% and 6 wt.%, hereafter MgFe (3 wt.%) and MgFe (6 wt.%), were produced by ball milling and exposed to air for 12 h. After a proper activation procedure, which consisted in heating in vacuum and three hydriding/dehydriding cycles; their hydriding and dehydriding reactions were characterized by a gravimetric method. The hydrogen uptakes were 5.3 wt. %, 5.6 wt. % and 6.2 wt. % for the air-exposed Mg, MgFe (3 wt.%) and MgFe (6 wt.%) mixtures respectively at 350 °C and 30 bar. Pressure-Composition-Temperature (PCT) curves demonstrated a reduction of the hysteresis of the MgFe mixtures as compared to pure Mg. Powder X-ray diffraction of the studied materials confirmed the formation of fine mixtures of Mg and Fe, without evidence of solid solution or mixed hydride formation.

Thermodynamic and Kinetic Properties of La20.5MgNi78.5 and La15.5Mg6Ni78.5 Hydrogen Storage Alloys: The Theoretical Models and Their Verifications

In this paper, we present a statistic thermodynamic model quantitatively describing the pressurecomposition- isotherms (PCI) curve, which consists of hydrogen storage capacity, temperature, and equilibrium pressure in the hydriding and dehydriding (H/D) reactions, and a theoretical kinetic model to clarify the H/D kinetic mechanism of hydrogen storage alloys. The results of the calculations are well agreed with the experiments performed with La20.5MgNi78.5 and La15.5Mg6Ni78.5 alloys, their maximum hydrogen storage capacities and the hydride formation enthalpies at 303~333 K are 1.41 wt.% H2 and -31.64 kJ/mol H2 for La20.5MgNi78.5 and 1.31 wt.% H2 and -27.23 kJ/mol H2 for La15.5Mg6Ni78.5, respectively. A new diffusion kinetic model is proposed with the consideration of Pilling–Bedworth Ratio, i.e. hydrogen-induced volume change used for studying the hydrogen absorption reaction kinetics. The activation energies are calculated to be 31.25 kJ/mol H2 for La20.5MgNi78.5 and 24.24 kJ/mol H2 for La15.5Mg6Ni78.5, respectively.

Enhancement in dehydriding performance of magnesium hydride by iron incorporation: A combined experimental and theoretical investigation

Structural change and dehydriding mechanism of MgH2 with atomic Fe incorporation from reactive ball milling are characterized and simulated by first-principles calculation. Two kinds of hydrides β- and γ-MgH2 are formed from Mg powders under hydrogen atmosphere by 3.0 h of milling with pretreated anthracite as milling aid. Experimental studies suggest that the atomic Fe can be incorporated onto MgH2 surface by the shearing effect of Fe-based milling balls on Mg/MgH2 particles. The incorporated Fe has a high dispersity on MgH2 surface and can form atomic clusters FeH4/FeH2 by combining with H anions. The dehydriding reaction of the Fe-incorporated MgH2 begins at hydride surface and shows an enhanced performance with apparent activation energy of 110.3 kJ mol−1. Theoretical studies suggest that the incorporated Fe can act as a bridge that contributes to electron transfer from H anion to Mg cation before H2 molecule formation. The intrinsic reason of atomic Fe in catalyzing dehydriding reaction of MgH2 lies in its moderate strength of electron attraction.

Structure, hydrogen storage kinetics and thermodynamics of Mg-base Sm5Mg41 alloy

The composition, phase components, and microstructure of Mg-based Sm5Mg41 alloy prepared by vacuum induction melting technique were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and high resolution transmission electron microscopy (HRTEM). The gaseous hydrogen ab/desorption properties of the alloy were measured by an automatically controlled Sieverts apparatus. The results indicate that the as-cast alloy consists of two phases, major phase Sm5Mg41 and secondary phase SmMg3. The MgH2 and Sm3H7 phases form after hydrogen absorption, while Mg and Sm3H7 phases exist after hydrogen desorption at 340 °C. Field Emission Transmission Electron Microscopy (FETEM) observation reveals the microstructure and phase distribution of Mg-based Sm5Mg41 alloy before and after hydrogen absorption and the hydriding and dehydriding reaction pathways as follow: Sm5Mg41 + SmMg3 + H2 → Sm3H7 + MgH2 ↔ Sm3H7 + Mg + H2. The hydrogen storage thermodynamics and kinetics of the Mg-based Sm5Mg41 alloy are improved as a result of the formation of the Sm3H7 nanoparticles. Consequently, the starting dehydrogenation temperature of the alloy hydride is about 270 °C. The dehydrogenation and hydrogenation activation energies of the alloy are estimated to be 135.28 and 77.802 kJ/mol, which suggests that the Sm3H7 nanoparticles play a beneficial role to reduce the total potential barrier that the hydrogen absorption or desorption reaction must overcome. The hydrogenation enthalpy of the alloy was determined to be −76.52 kJ/mol H2, indicating that adding Sm can slightly alter the hydrogen absorption thermodynamic property of Mg-based alloy. The desorption property improved by alloying Sm is attribute to the enhanced kinetics rather than the variation in the thermodynamics.

Effect of microwave irradiation on hydrogen sorption properties of hand mixed MgH2 – 10 wt.% carbon fibers

The effect of microwave (MW) irradiation on the hydrogen sorption properties of magnesium powder is explored in the present work. MgH2 – 10 wt.% CFs (CFs = Carbons Fibers) was prepared by hand mixing, dehydrogenated under microwave irradiation for 20 s and then hydrogenated/dehydrogenated at about 300 °C – 1 MPa and 330 °C–0.03 MPa to investigate the effect of microwave irradiation on the solid/gas sorption properties. It has to be noted that the hydrogen absorption capacity and sorption kinetics of the MgH2 – 10 wt.% CFs mixture increased after dehydriding under MW irradiation. The MgH2 – 10 wt.% CFs mixture dehydrogenated by microwave irradiation can absorb about 5.8 wt.% and 5.3 wt.% H at 330 and 300 °C, respectively, within 2 h while the as-prepared MgH2 – 10 wt.% CFs mixture absorb only 4.6 wt.% H within the same duration. It is also demonstrated that MgH2 – 10 wt.% CFs mixture dehydrogenated by microwave irradiation exhibited good hydrogen desorption properties and, as an example, a microwave irradiated sample could release 5.8 wt.% H within 1 h at 330 °C in comparison to the as-prepared MgH2 – 10 wt.% CFs mixture which desorbed 4.4 wt.% H within 3 h. Scanning electron microscopy (SEM) images revealed that the particle sizes of the MW dehydrogenated mixture decreased after several solid/gas sorption cycles. This contribute to the improvement of hydrogen storage properties of the microwaves dehydrogenated MgH2 – 10 wt.% CFs mixture. In addition, the hydrogenated MgH2 – 10 wt.% CFs mixture show reproducible and better microwave-assisted dehydriding reaction during second microwaves cycle.

Kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni alloys investigated by the modified Chou model

The phenomenon of volume change is very important in the hydriding and dehydriding reactions to clarify their kinetic mechanisms. However, few of the theoretical models take this factor into account in the study of kinetic mechanisms. In the present work, the modified Chou model with considering the volume change by Pilling-Bedworth Ratio (PBR) β is applied to interpret the kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni ternary hydrogen storage alloys. The calculation results agree well with the experimental data, indicating that the modified Chou model can well describe the kinetic behaviors of La–Mg–Ni alloys. Diffusion is determined to be the rate-controlling step of hydriding and dehydriding for the examples in this work. The activation energies are calculated to be 39.4 kJ/mol for hydriding reaction (β = 1.29) and 93.1 kJ/mol for dehydriding reaction (β = 0.78) of Mg–10.6La–3.5Ni nanoparticles at 453–623 K. For the hydrogen absorption kinetics of La2Mg16Ni alloy under 0.5–3 MPa at 598 K, β is 1.25 and the equilibrium hydrogen pressure is calculated to be 0.332 MPa. For the hydrogen desorption kinetics of Mgx(LaNi3)100−x alloys at 623 K, the characteristic times increase in the order of tc(x = 70,β= 0.79) < tc(x= 60, β= 0.79) < tc(x= 40, β= 0.97) < tc(x= 50, β = 0.85). The value of β is 0.77 and the hydrogen desorption rate of Mg–5La–10Ni alloy prepared by the ultrahigh pressure (UHP) method is faster than that of the alloy prepared by the as-cast method, whose characteristic times are 7428 s and 4651 s, respectively. Furthermore, the calculation accuracy can also be improved by the modified Chou model compared with the original Chou model after taking PBR into consideration.

Hydrogen desorption behaviour and microstructure evolution of a γ-AlH3/MgCl2 nano-composite during dehydriding

A deep insight into the mechanism of the dehydriding reaction of γ-AlH3 nano-composite.

Phase equilibria of Ce–Mg–Ni ternary system at 673 K and hydrogen storage properties of selected alloy

The isothermal section of Ce–Mg–Ni ternary system at 673 K was constructed through thermodynamic calculation coupled with experimental verification. Under guidance of the phase diagram in Mg-rich corner, Ce2.92Mg91.84Ni5.24 was designed to investigate its hydrogen storage properties. Moreover, the mechanism of α-to-β (α: diluted phase and β: hydride) phase transformation in the Ce2.92Mg91.84Ni5.24–H2 system was elucidated by in situ synchrotron powder X-ray diffraction (SR-PXRD). The alloy showed a good activated ability and displayed two plateaus in the pressure–composition–temperature curves. The kinetic mechanisms of hydriding and dehydriding reactions were analyzed by Chou model. The rate-controlling steps were the diffusion for the initial stage in hydriding reaction and the surface penetration for dehydriding reaction. SR-PXRD patterns indicated that the activated Ce2.92Mg91.84Ni5.24 gradually transformed into CeH2.51, (Mg) (with solid solution of H atoms) and Mg2NiH0.3, finally transformed into CeH2.51, MgH2 and Mg2NiH4 during the first hydriding process.

A Study of the Pressure-Composition-Temperature Curve of Mg(BH4)2 by Sievert’s Type Apparatus

The Pressure-Composition-Temperature(PCT) curve of Mg(BH4)2 was studied by Sievert`` type apparatus. In order to understand the slope of the first plateau pressure, a thermal analysis technique was introduced. The PCT curves of the dehydriding reaction for Mg(BH4)2 are measured at 573, 598, 623, and 653 K, respectively. Under every temperature measured, the first plateau pressure shows slope but the second plateau pressure of MgH2 is clearly observed. The apparent enthalpy for the first plateau pressure from Mg(BH4)2 to MgH2 is roughly estimated. The equilibrium pressure at each temperature is determined to be the first plateau pressure where the hydrogen content is -5wt%. The apparent enthalpy for the first reaction from Mg(BH4)2 was estimated to be △H = 51.1 kJ mol-1 H2, and the apparent enthalpy for the dehydriding reaction from MgH2 was estimated to be △H = 90.0 kJ mol-1H2. The result of the thermal analysis shows three peaks. This means that it decomposes at least through the pathway of three reactions.

Hydrogen-Storage Properties of Ni and LiBH4-Added MgH2.

In this work, MgH2 was employed as a starting material instead of Mg used in our previous work. Ni and LiBH4, which can absorb 18.4 wt% of hydrogen, were added. A sample with a composition of 86 wt% MgH2 + 10 wt% Ni + 4 wt% LiBH4 (named MgH2-10Ni-4LiBH4) was prepared by milling under hydrogen (reaction-involved milling) and its hydrogen-storage properties were examined. In addition, the rate-limiting step for the dehydriding reaction of the sample at the first cycle was analyzed. The activation of MgH2-10Ni-4LiBH4 for hydriding and dehydriding reactions was not required. The as-milled sample absorbed and released nearly 5 wt% H at 623 K for 60 min; it absorbed 4.90 wt% H under 12 bar H2 for 20 min and released 4.94 wt% H under 1.0 bar H2 for 60 min. The hydriding rate exhibited an inverse dependence on temperature. This is due to a decrease in the driving force for the hydriding reaction (the difference between the applied hydrogen pressure and the equilibrium plateau pressure) with the increase in temperature.

Enhancement of Hydrogen-Storage Characteristics of Magnesium Hydride via Reaction-Involved Milling with Nickel and Lithium Borohydride

Ni and LiBH4 with a high hydrogen-storage capacity of 18.4 wt% were added to MgH2. A sample with a composition of 86 wt% MgH2-10 wt% Ni-4 wt% LiBH4 (named MgH2-10Ni-4LiBH4) was prepared by milling under H2 (reaction-involved milling, RIM), and its hydriding and dehydriding properties were then examined. The activation of MgH2-10Ni-4LiBH4 for hydriding and dehydriding reactions was not required. The as-milled sample absorbed 2.54 wt% H for 5 min, 3.72 wt% H for 10 min, 4.90 wt% H for 20 min, and 4.90 wt% H for 60 min at 623 K under 12 bar H2, absorbing nearly 5 wt% H for 60 min. The as-milled sample desorbed 2.86 wt% H for 5 min, 4.78 wt% H for 15 min, and 4.94 wt% H for 60 min at 623 K under 1.0 bar H2. The inverse dependence of the hydriding rate on temperature is due to a decrease in the driving force for the hydriding reaction (the difference between applied hydrogen pressure and equilibrium plateau pressure) as the temperature increases.