Hydrogen Desorption Kinetics Research Articles

Confinement Effects for Lithium Borohydride: Comparing Silica and Carbon Scaffolds.

LiBH4 is a promising material for hydrogen storage and as a solid-state electrolyte for Li ion batteries. Confining LiBH4 in porous scaffolds improves its hydrogen desorption kinetics, reversibility, and Li+ conductivity, but little is known about the influence of the chemical nature of the scaffold. Here, quasielastic neutron scattering and calorimetric measurements were used to study support effects for LiBH4 confined in nanoporous silica and carbon scaffolds. Pore radii were varied from 8 Å to 20 nm, with increasing confinement effects observed with decreasing pore size. For similar pore sizes, the confinement effects were more pronounced for silica than for carbon scaffolds. The shift in the solid–solid phase transition temperature is much larger in silica than in carbon scaffolds with similar pore sizes. A LiBH4 layer near the pore walls shows profoundly different phase behavior than crystalline LiBH4. This layer thickness was 1.94 ± 0.13 nm for the silica and 1.41 ± 0.16 nm for the carbon scaffolds. Quasi-elastic neutron scattering confirmed that the fraction of LiBH4 with high hydrogen mobility is larger for the silica than for the carbon nanoscaffold. These results clearly show that in addition to the pore size the chemical nature of the scaffold also plays a significant role in determining the hydrogen mobility and interfacial layer thickness in nanoconfined metal hydrides.

Significantly enhanced hydrogen desorption properties of Mg(AlH4)2 nanoparticles synthesized using solvent free strategy

Mg(AlH4)2 nanoparticles with a particle size less than 10nm have been successfully synthesized by mechanochemical method using LiAlH4 and MgCl2 as raw materials together with LiCl buffering additive. In comparison to Mg(AlH4)2 microparticles, Mg(AlH4)2 nanoparticles exhibit a faster hydrogen desorption kinetics and lower desorption temperature. The hydrogen desorption temperatures of the first and second dehydrogenation steps are 80 and 220°C for the Mg(AlH4)2 nanoparticles, which are about 65 and 60°C, respectively, lower than those of Mg(AlH4)2 microparticles. The decomposition activation energy is reduced from 135kJ/mol for Mg(AlH4)2 microparticles to 105.3kJ/mol for Mg(AlH4)2 nanoparticles. It is proposed that the shortened diffusion distance and enhanced diffusivity of Mg(AlH4)2/MgH2 nanoparticles provide an energy destabilization for lowering the dehydrogenation temperature, and thus being the key factor for promoting the hydrogen desorption kinetics. More importantly, it is demonstrated that the dehydrided nano MgH2 hydride with a particle size below 10nm can be formed after rehydrogenation process, resulting in the good cycling hydrogen desorption performance of nano MgH2.

Hydrogen desorption kinetics of the destabilized LiBH4[sbnd]AlH3 composites

LiBH4 can be destabilized by AlH3 addition. In this work, the hydrogen desorption kinetics of the destabilized LiBH4AlH3 composites were investigated. Isothermal hydrogen desorption studies show that the LiBH4 + 0.5AlH3 composite releases about 11.0 wt% of hydrogen at 450 °C for 6 h and behaves better kinetic properties than either the pure LiBH4 or the LiBH4 + 0.5Al composite. The apparent activation energy for the LiBH4 decomposition in the LiBH4 + 0.5AlH3 composite estimated by Kissinger's method is remarkably lowered to 122.0 kJ mol−1 compared with the pure LiBH4 (169.8 kJ mol−1). Besides, AlH3 also improves the reversibility of LiBH4 in the LiBH4 + 0.5AlH3 composite. For the LiBH4 + xAlH3 (x = 0.5, 1.0, 2.0) composites, the decomposition kinetics of LiBH4 are enhanced as the AlH3 content increases. The sample LiBH4 + 2.0AlH3 can release 82% of the hydrogen capacity of LiBH4 in 29 min at 450 °C, while only 67% is obtained for the LiBH4 + 0.5AlH3 composite in 110 min. Johnson−Mehl−Avrami (JMA) kinetic studies indicate that the reaction LiBH4 + Al → ‘LiAlB’ + AlB2 + H2 is controlled by the precipitation and subsequently growth of AlB2 and LiAlB compounds with an increasing nucleation rate.

Hydrogen storage thermodynamics and dynamics of La–Mg–Ni-based LaMg12-type alloys synthesized by mechanical milling

Nanocrystalline/amorphous LaMg12-type alloy–Ni composites with a nominal composition of LaMg11Ni + x wt% Ni (x = 100, 200) were synthesized by mechanical milling. Effects of Ni content and milling time on the gaseous hydrogen storage thermodynamics and dynamics of alloys were systematically investigated. The hydrogen desorption properties were studied by Sievert apparatus and a differential scanning calorimeter (DSC). Thermodynamic parameters (ΔH and ΔS) for the hydrogen absorption and desorption of alloys were calculated by Van’t Hoff equation. Hydrogen desorption activation energy of alloy hydride was estimated by Arrhenius and Kissinger methods. The increase in Ni content has a slight effect on the thermodynamic properties of alloys, but it significantly enhances the hydrogen absorption and desorption kinetics performance of alloys. Moreover, variation of milling time clearly affects the hydrogen storage properties of alloys. Hydrogen absorption capacity ($$C_{ 1 0 0}^{\text{a}}$$) and hydrogen absorption saturation ratio ($$R_{ 1 0}^{\text{a}}$$) (a ratio of the hydrogen absorption capacity at 10 min to the saturated hydrogen absorption capacity) have maximum values with milling time varying. But hydrogen desorption ratio ($$R_{ 2 0}^{\text{d}}$$) (a ratio of the hydrogen desorption capacity at 20 min to the saturated hydrogen absorption capacity) always increases with milling time prolonging. Particularly, prolonging milling time from 5 to 60 h makes $$R_{ 2 0}^{\text{d}}$$ increase from 10.89% to 16.36% for the x = 100 alloy and from 13.93% to 21.68% for the x = 200 alloy, respectively.

Effects of VF4 on the hydriding cycling at 373 K and dehydriding of Mg99Ni prepared by hydriding combustion synthesis and mechanical milling (HCS+MM)

The catalytic effects of VF4 on the hydriding cycling properties at low temperature (373 K) and dehydrogenation of Mg99Ni prepared by hydriding combustion synthesis (HCS) and mechanical milling (MM) have been systematically investigated. The onset dehydrogenation temperature of the HCS+MM-Mg99Ni+VF4 composite was around 50 K lower than that of the HCS+MM-Mg99Ni composite. The hydrogen desorption kinetics of the HCS+MM-Mg99Ni+VF4 composite was largely accelerated, desorbing 5.26 wt% hydrogen within 1800 s at 523 K. In contrast, only 2.71 wt% hydrogen was desorbed under the same condition for the HCS+MM-Mg99Ni. Moreover, the HCS+MM-Mg99Ni+VF4 composites could desorb completely 6.50 wt% hydrogen at 533 K in 1800 s and at 543 K in 1300 s, respectively. The hydriding cycling property of the HCS+MM-Mg99Ni at low temperature was studied and it was improved obviously by the additive VF4, so that the composite could reach its saturated hydriding capacity of 5.57 wt% with a high activity at 373 K within 50 s at the fifth hydriding cycle. Additionally, the dehydrogenation activation energy of the HCS+MM-Mg99Ni+VF4 composite was determined to be 95.62 kJ/mol H2, 39.1% less than that of as-received MgH2. Mechanism analysis indicated that the VH0.91 phase probably formed from the mechanical milling contributed to the enhanced dehydrogenation and hydriding cycling properties of HCS+MM-Mg99Ni.

Influence of Titanium and Iron Additives to Magnesium on Hydrogen-Sorption Properties, Thermal Stability, and Kinetics of Hydrogen Desorption from MgH2 Phase of Mechanical Alloy

We report on studies of the process of cyclic hydriding–dehydriding (eight cycles) of a mechanical alloy synthesized in a ball mill by grinding powder mixture Mg + 10 wt.% TiH2 + 10 wt.% Fe in argon medium. Hydrogen-sorption properties and thermal stability of the mechanical alloy are studied employing thermodesorption spectroscopy at hydrogen pressure of 0.1 MPa. Hydrogen capacity of the mechanical alloy is determined to be 4.9 wt.%, and onset temperature of hydrogen desorption from the alloy under this study equals 220°C. Kinetics of the process of hydrogen desorption from the hydride phases of the mechanical alloy is examined. The present studies have revealed that mechanical treatment of the magnesium powder in Ar medium with both Fe and Ti additives causes a greater effect on improving the kinetics of hydrogen desorption from hydride phase MgH2 of the alloy and decreasing its thermal stability in comparison with that caused by mechanical treatment of the same powder with either Fe or Ti additives in hydrogen medium.

Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system

In this paper, the influence of multi-walled carbon nanotube (MWCNT)-doping on the hydrogen storage properties of the Li3N system was systematically investigated. Compared with the pure Li3N sample, the MWCNT-doped Li3N sample shows faster hydrogen absorption and desorption kinetics and a drastically improved cycling stability. Along with increasing MWCNT content, the hydrogen storage improvement becomes more apparent. When the MWCNT doping level reaches 10 mol%, the enhancement effect is significant. The improved hydrogen storage properties of the Li3N system by MWCNT-doping can be reduced to the physical effect of ball milling, the increased specific surface area and large pores as well as the good thermal conductivity of MWCNTs.

Nitrogen modified templated carbons for energy application: Effect of templates and nitrogen precursors

Nitrogen modified templated carbons were prepared by carbonization at 923 K with dwelling time of 3 h. The effects of nitrogen precursors and templates on the surface area, pore structure and nitrogen loading of modified carbons were studied. Acetonitrile and aniline were used as nitrogen precursors, while zeolite and silica gel as inorganic templates. The surface areas of nitrogen modified carbons were reduced as compared to that of unmodified carbons. The effect is more significant for aniline precursor and zeolite template. Irrespective of nitrogen precursors, zeolite and silica gel templates resulted in mainly microporous and mesoporous carbons respectively. The nitrogen modified templated carbons were tested for hydrogen uptake. The hydrogen uptake of silica gel templated carbons increased on incorporation of nitrogen in spite of decrease in surface area. The high uptake of silica gel templated modified carbon can be attributed to higher activation of hydrogen by nitrogen centers present in the carbon network. However, drop in hydrogen uptake for zeolite templated modified carbons may be attributed to significant decrease in surface area. The hydrogen desorption kinetics was observed to depend on porous structure. Presence of micropores offered higher diffusion resistance, making desorption of hydrogen slower.

Hydrogen desorption kinetics from H‐Si (111) surfaces studied by optical sum frequency generation and second harmonic generation

We have studied hydrogen desorption from a flat H‐Si (111)1 × 1 surface at 711 K by observing sum frequency generation (SFG) and second harmonic generation (SHG) spectra. Flat H‐Si (111) surfaces were prepared by dosing hydrogen molecules in an ultra‐high vacuum chamber with a base pressure of 〜10−8 Pa. Combining the SFG and SHG methods, the desorption order has been clarified over the whole hydrogen coverage range from 1 monolayer (ML) to 0 ML. The hydrogen desorption was assigned as second order in the high coverage range of 1 ML–0.18 ML by using SFG spectroscopy and as first order in the coverage range of 0.18 ML–0.0 ML by using SHG spectroscopy. Copyright © 2016 John Wiley & Sons, Ltd.

An investigation on hydrogen storage thermodynamics and kinetics of Pr–Mg–Ni-based PrMg12-type alloys synthesized by mechanical milling

Mechanical ball milling technology was utilized to synthesize the nanocrystalline and amorphous PrMg11Ni + x wt% Ni (x = 100, 200) alloys. The effect of Ni content and milling time on thermodynamics and kinetics of the alloys were systematically investigated. The hydrogen absorption and desorption properties were analyzed by Sievert's apparatus and differential scanning calorimetry (DSC). Results show that increasing Ni content brings on a slight reduction in thermodynamic parameters (ΔH and ΔS) of the alloys, and it dramatically enhances the hydrogen absorption and desorption kinetics of the alloys. Varying milling time greatly affects the hydrogen storage properties of the alloys. The hydrogen absorption capacity of the alloys first increases and then deceases with the increase of milling time, and the maximum capacity of the alloy is approximately 5.8 wt% and 6.0 wt% for the x = 100 and x = 200 alloys. Moreover, the hydrogen desorption activation energy obviously decreases with increasing the Ni content and ball milling time.

An investigation on hydrogen storage thermodynamics and kinetics of Nd–Mg–Ni-based alloys synthesized by mechanical milling

In order to improve the hydrogen storage kinetics of NdMg12-type alloy, the Mg in the alloy was partially substituted by Ni, and mechanical milling technology was used to fabricate the nanocrystalline and amorphous NdMg11Ni + x wt.% Ni (x = 100, 200) alloys. Effects of Ni content and milling time on the gaseous hydrogen storage thermodynamics and kinetics of alloys were investigated systematically. The structures of as-cast and milled alloys were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscope (HRTEM). The pressure-composition isotherms (P-C-T) and the hydrogen absorption and desorption kinetics of the as-milled alloys were measured by an automatically controlled Sievert's apparatus and a differential scanning calorimetry (DSC) (SDT-Q600) with a H2 detector. Thermodynamic parameters (ΔH and ΔS) for the hydrogen absorption and desorption of alloys were calculated from Van't Hoff equation. Hydrogen desorption activation energy of alloy hydride was also estimated by using Arrhenius and Kissinger methods. It is found that the variation of Ni content brings on a slight effect on the thermodynamic properties of the alloys, but it significantly improves the hydrogen absorption and desorption kinetics. The improved gaseous hydrogen storage kinetics of alloys are considered to be ascribed to a decrease in hydrogen desorption activation energy caused by increasing Ni content and prolonging milling time.

Hydrogen storage in filed magnesium

In this paper we report an investigation on the processing of magnesium samples by filing and the hydrogen storage properties of the obtained chips. Filing of Mg was performed manually and mechanized. The effect of the filing tool design on the morphology and hydrogen storage properties of Mg chips was evaluated. The hydrogen absorption and desorption kinetics of manual filed Mg is strongly influenced by the geometry of the filing tool. Chips obtained by using a fine tool have larger surface areas and faster hydrogenation and dehydrogenation kinetics. Moreover, these chips show high resistance to air contamination and can absorb and release hydrogen even after long periods of exposition to air. Chips obtained by mechanized filing also showed suitable hydrogen storage properties and a slight increase in incubation period for hydrogen absorption. After this initial delay, the reaction kinetics of hydrogen absorption was similar for both mechanized and manually filed chips. The hydrogen storage properties of the chips are not affected by the machining speed adopted during mechanized filing. These results clearly indicate the feasibility of adopting filing processing to produce Mg for hydrogen storage and open possibilities to scale-up the production of such material using a simple and economical approach.

Improvement in hydrogen desorption performances of magnesium based metal hydride reactor by incorporating helical coil heat exchanger

Mg-based metal hydride has been viewed as one of the most promising materials for hydrogen storage. However, large amount of reaction heat during the hydrogen absorption/desorption cycles negatively affects the heat and mass transfer, thus leading to poor performances. In this work, helical coil heat exchanger (HCHE) is proposed to be inserted into Mg-based metal hydride reactor for improving the heat and mass transfer. The models on pressure-composition-temperature properties and hydrogen desorption kinetics of Mg-based reactor are firstly established for the simulation through experiments. The comparison of hydrogen desorption performances for the reactors incorporating the conventional straight tube, finned tube and new helical coil heat exchangers is conducted based on numerical simulation. The comparison results show that the HCHE has the best effects on improving the heat and mass transfer characteristics of reactor among the three heat exchangers, due to its secondary circulation. Besides, it is suggested that the influence of hydrogen pressure gradient should be considered for the prediction of the performance of Mg-based metal hydride reactor with HCHE.

Hydrogen storage and thermal transport properties of pelletized porous Mg-2 wt.% multiwall carbon nanotubes and Mg-2 wt.% graphite composites

We synthesized pelletized porous composites of Mg admixed with 2 wt.% of either multiwall carbon nanotubes or graphite. The composites were prepared by high energy ball-milling of Mg powder with carbonaceous additives, followed by uniaxial compression and sintering in hydrogen environment under mechanical constraint. The correlations between ball-milling conditions, composite microstructure, hydrogenation kinetics, and thermal conductivity of the pellets were established. The presence and condition of carbon additives controls the morphology of Mg particles and, consequently, the mechanical stability of the pellet upon hydrogenation cycling. The best combination of hydrogen desorption kinetics, thermal conductivity, and mechanical stability was obtained for the pellets synthesized from the mixture of Mg with 2 wt.% of carbon nanotubes processed by 4 h of co-milling. The milling transformed carbon nanotubes into carbon nano-particles/nano-onions. These carbonaceous species promote metal nucleation from the hydride phase and allow formation of Mg-Mg bonds between the Mg particles contributing to mechanical stability of the pellet.

Hydrogen Storage Thermodynamics and Dynamics of Nd–Mg–Ni-Based NdMg12-Type Alloys Synthesized by Mechanical Milling

Nanocrystalline and amorphous NdMg12-type NdMg11Ni + x wt% Ni (x = 100, 200) hydrogen storage alloys were synthesized by mechanical milling. The effects of Ni content and milling time on hydrogen storage thermodynamics and dynamics of the alloys were systematically investigated. The gaseous hydrogen absorption and desorption properties were investigated by Sieverts apparatus and differential scanning calorimeter connected with a H2 detector. Results show that increasing Ni content significantly improves hydrogen absorption and desorption kinetics of the alloys. Furthermore, varying milling time has an obvious effect on the hydrogen storage properties of the alloys. Hydrogen absorption saturation ratio (\(R_{ 1 0}^{\text{a}}\); a ratio of the hydrogen absorption capacity in 10 min to the saturated hydrogen absorption capacity) of the alloys obtains the maximum value with varying milling time. Hydrogen desorption ratio (\(R_{ 2 0}^{\text{d}}\), a ratio of the hydrogen desorption capacity in 20 min to the saturated hydrogen absorption capacity) of the alloys always increases with extending milling time. The improved hydrogen desorption kinetics of the alloys are considered to be ascribed to the decreased hydrogen desorption activation energy caused by increasing Ni content and milling time.

Hydrogen diffusion kinetics and structural integrity of superhigh pressure Mg-5 wt%Ni alloys with dendrite interface

A strategy of low-angle orientation dendrite interface-high index planes-which prepared by super-high pressure (SHP) technique, is proposed and successfully improved the hydrogen storage properties of Mg based materials for the first time, wherein a simple binary Mg-Ni alloy is used as a sample to elucidate its related mechanisms. The phase composition, morphology variation and hydrogen storage properties of the as-cast and SHP Mg-5Ni alloys in the temperature range of 1100–1600 °C are systemically investigated. The reversible hydrogen storage capacities and plateau hydrogen pressure of the as-cast and SHP alloys are close due to the same hydrogen storage phases (Mg and Mg2Ni). Note that although SHP treatment at 1600 °C has a large weight fraction of Mg6Ni compound, it still effectively reduces the onset temperature of dehydriding (∼262 °C), as well as improves the hydrogen desorption kinetics at low temperatures and structural integrity. The low onset temperature and outstanding hydrogen sorption/desorption kinetics are mainly associated with the formation of a large number of dendrite interface, in which the hydrogen atoms readily occur “zig-zag” jumps along {11−20} prismatic planes. This dendrite interface of high index planes which prepared by SHP technique paves a new pathway to enhance the hydrogen storage performances of magnesium based alloys.

Hydrogen behavior in Al[sbnd]Cr alloys: Synchrotron-based photoelectron microscopy of the rapidly solidified structure

This work reports recent advances in characterization of the composition at submicron length scales of Al-based rapidly solidified alloys, which are correlated to their interactions with hydrogen. The hydrogen desorption kinetics and the evolution of the surface and subsurface structure and chemical state of high-purity Al and Al-3.0 at.% Cr alloy have been studied by thermal desorption analysis and scanning photoelectron microscopy. The alloy foil structure consisted of Cr-rich and Cr-depleted surface regions exhibits Cr segregation beneath the surface. The obtained results reveal how inclusions of aluminum oxide species can become very strong hydrogen traps at the highest temperatures of 600–630 °C, representing irreversible trapping in Al and its alloy. The effect of strong reversible and irreversible hydrogen traps on hydrogen desorption from aluminum alloys is discussed, suggesting that their direct identification in the microstructure provides further insights into hydrogen embrittlement mechanisms for aluminum materials in energy technologies.

Point-defect kinetics in α- and γ-MgH2

The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH2, desorption rates may be higher in the crystalline γ phase compared to the equilibrium bulk α phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856–3865] that this effect is responsible for enhanced desorption from ball-milled MgH2, since smaller particles contain a higher proportion of the metastable γ phase. We investigate hydrogen transport kinetics in these phases of MgH2 by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in γ-MgH2 is smaller by 0.032 eV compared to α-MgH2. Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in γ-MgH2 is 0.02 eV lower than in α-MgH2. We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the γ phase than in the α phase, thus explaining the faster dehydrogenation kinetics of γ-MgH2.

The Temperature-Programmed Desorption of H2 from Cu/ZrO2

The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters $$A_{des}$$ and $$E_{des}$$ were determined to be 1.24 × 109 s−1 and 68 kJ mol−1, respectively. As the modeling with constant values of $$A_{des}$$ and $$E_{des}$$ yielded signals which were too narrow, dependence of $$E_{des}$$ on coverage was introduced applying $$E_{des} - K (\Theta _H)^n$$ . By application of the “full-analysis” method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol−1 − (6.25 kJ mol−1 × $$\Theta _H$$ ) for $$E_{des}$$ was determined.

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.