Hydrogen Desorption Kinetics Research Articles

Hydrogen desorption kinetics of CeCl3-doped sodium aluminum hydride compacts measured by parallel in situ FTIR-ATR-spectroscopy and gravimetry

In this work, in situ FTIR-ATR-spectroscopy and gravimetry were used in parallel to determine the dehydrogenation reaction kinetics of 2 mol% CeCl3-doped NaAlH4 compacts. Dehydrogenation measurements were performed under isothermal and isobaric conditions at different temperatures. Subsequently spectroscopic and gravimetric data were fitted by Johnson-Mehl-Avrami and Arrhenius equation to obtain the rate constants of the dehydrogenation and activation energy of the NaAlH4–Na3AlH6 phase change. On the one hand the measurements showed that cycling of the powder compacts enhances the dehydrogenation reaction kinetics of the NaAlH4 phase, whereby a decrease of the activation energy by 3.4 kJ/mol was measured. On the other hand an influence on the calibration of the spectroscopic signal to the mass of desorbed hydrogen was observed.

Tailoring the hydrogen desorption thermodynamics of V2H by alloying additives

Vanadium could be a potential candidate for on board hydrogen storage application because of its high gravimetric hydrogen storage capacity (∼3.8mass%) which is even better then the most widely explored AB5, AB2 & AB intermetallic compounds. Hydrogen absorption of vanadium takes place at ambient temperature and pressure with fast kinetics. The vanadium hydride (VH2) releases hydrogen in two steps: (1) VH2(γ)(s)↔½ V2H(β)(s) and (2) V2H(β)(s)↔2V(s)+½ H2(g). First step is achievable at the ambient temperature and pressure conditions while, the second step requires high temperature (590K). Thus only half of the total hydrogen storage capacity is available for use on subsequent absorption–desorption cycles at the ambient temperature. The usable hydrogen storage capacity of VH2 at ambient conditions could be enhanced by tailoring the thermodynamics and kinetics of second step of hydrogen desorption reaction. This could be possible by selecting suitable alloy additives. The present study deals with the selection criteria of alloy additives based on the electronic consideration to tailor the hydrogen desorption thermodynamics and kinetics of V2H.

Spatially and Kinetically Resolved Mapping of Hydrogen in a Twinning-Induced Plasticity Steel by Use of Scanning Kelvin Probe Force Microscopy

The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1–2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption.

Remarkable hydrogen desorption properties and mechanisms of the Mg2FeH6@MgH2 core–shell nanostructure

Mg2FeH6@MgH2 dual-metal hydride with core-shell nanostructure starts to release hydrogen at 220 ºC with fast hydrogen desorption kinetics.

Modelling and evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride

The ‘shrinking core’ model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride.

Improvement on Hydrogen Desorption Performance of Calcium Borohydride Diammoniate Doped with Transition Metal Chlorides

Calcium borohydride diammoniate with a molecular formula of Ca(BH4)(2).2NH(3) is a novel complex hydride for hydrogen storage. However, it suffers from high temperature and sluggish kinetics for hydrogen desorption. In this work, the temperature and kinetics for hydrogen desorption of calcium borohydride diammoniate were effectively improved through doping transition metal chlorides (such as CoCl2, NiCl2, and FeCl3) in a closed vessel. Three additives could effectively decrease the temperature for hydrogen desorption of Ca(BH4)(2).2NH3. Among them, CoCl2-doped Ca(BH4)(2).2NH3 could desorb hydrogen even at a low temperature of 170 degrees C, and at 200 degrees C, 7.6 wt % hydrogen with high purity is released, which shows superior performance for hydrogen desorption than that of pristine Ca(BH4)(2).2NH(3.) For CoCl2-doped Ca(BH4)(2).2NH(3), X-ray absorption fine structure (XAFS) revealed that the improvements could be ascribed to the catalytic function of well-dispersed cobalt particles, which results in a much lower activation energy (86.1 kJ/mol) for hydrogen desorption of calcium borohydride diammoniate.

Tuning the Thermodynamic Properties of MgH2 at the Nanoscale via a Catalyst or Destabilizing Element Coating Strategy

Magnesium nanoparticles as small as 7 nm have been synthesized by reducing di-n-buytlmagnesium with lithium and individually coated with elements including Co, Fe, Ni, Si, or Ti for further tuning of their thermodynamic and kinetic properties. Coating was achieved via a transmetalation process whereby precursors were directly deposited at the surface of the magnesium nanoparticles acting as a reducing agent. Studies of hydrogen storage properties showed that these coated magnesium nanoparticles were capable of absorbing hydrogen at low temperatures <140 °C and partially formed transition metal complex hydrides at temperatures above 350 °C. These complex hydrides, assumed to form a shell around the magnesium core, were found to control hydrogen desorption kinetics to some extent. More remarkably, thermodynamics of nanosize magnesium were significantly altered through this coating process. In particular, upon Ni coating a significant decrease of the plateau pressure occurred while achieving fast kinetics. This demonstrates the possibility of tuning both the thermodynamic and kinetic properties of magnesium at the nanoscale and provides a new tool for achieving enhanced hydrogen storage properties for nanosize magnesium.

Effects of Ti-based catalysts on hydrogen desorption kinetics of nanostructured magnesium hydride

In the present work, the synergetic effect of Ti-based catalysts (TiH2 and TiO2 particles) on hydrogen desorption kinetics of nanostructured magnesium hydride was investigated. Nanostructured 84 mol% MgH2–10%mol TiH2–6%mol TiO2 nanocomposite powder was prepared by high-energy ball milling and subjected to thermal analyses. Evaluation of the absorption/desorption properties revealed that the addition of the Ti-based catalysts significantly improved the hydrogen storage performance of MgH2. A decrease in the decomposition temperature (as high as 100 °C) was attained after co-milling of MgH2 with the Ti-based catalysts. Meanwhile, solid-state chemical reactions between MgH2 and TiO2 nanoparticles during co-milling slightly decreased the maximum hydrogen capacity. It was also found that formation of micro-cracks at the particle surfaces during thermal cycling enhanced the H-kinetics. Isothermal and non-isothermal thermal analysis revealed that the addition of Ti-catalysts reduced the decomposition activation energy of MgH2 by 20–30 kJ/mol.

Effects of compaction pressure and graphite content on hydrogen storage properties of Mg(NH2)2–2LiH hydride

Preparation of hydride–graphite compacts serves as an effective method to improve the volumetric hydrogen storage density and the effective thermal conductivity for light complex hydrides. This paper presents the effects of compaction pressure and expanded natural graphite (ENG) content on the hydrogen storage properties of the Mg(NH2)2–2LiH–0.07KOH compacts. The results show that the hydrogen desorption kinetics of the 1st sorption cycle decreases with the increase of the compaction pressure. However, the compacts exhibit the similar hydrogen desorption kinetics and capacities from the 2nd sorption cycles on regardless of the compaction pressure. The ENG addition significantly enhances the desorption kinetics because of the improvement of the heat transfer performance of the hydride. Furthermore, the volumetric hydrogen storage density of the hydride reaches 47 g/L after the compaction at 365 MPa, but it reduces by increasing the ENG content.

Catalytic effect of melt-spun Ni3FeMn alloy on hydrogen desorption properties of nanocrystalline MgH2 synthesized by mechanical alloying

To improve hydrogen desorption properties of magnesium hydride, a composite material with composition of MgH2-5 at% Ni3FeMn has been prepared by co-milling MgH2 powder with Ni3FeMn alloy either in the form of as-cast (sample A) or melt-spun ribbon (sample B). The study has shown that the addition of Ni3FeMn alloy to magnesium hydride can yield a finer particle size after mechanical alloying (MA). As a consequence, the desorption temperature of mechanically activated MgH2 for 30 h has decreased from 319 °C to 307 °C for sample A and to 290 °C for sample B. Furthermore, some favorable effects of Ni3FeMn alloy on hydrogen desorption kinetics have been observed. Further improvement in the hydrogen desorption of melt-spun containing composite can be related to higher hardness value of the melt-spun powder compared to the as-cast alloy, and probably a more homogeneous distribution of the alloyed elements.

Hydrogen desorption kinetics of MgH2 synthesized from modified waste magnesium

Abstract In the present study, hydrogen desorption properties of magnesium hydride (MgH2) synthesized from modified waste magnesium chips (WMC) were investigated. MgH2 was synthesized by hydrogenation of modified waste magnesium at 320 °C for 90 min under a pressure of 6 × 106 Pa. The modified waste magnesium was prepared by mixing waste magnesium with tetrahydrofuran (THF) and NaCl additions, applying mechanical milling. Next, it was investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) techniques in order to characterize its structural properties. Hydrogen desorption properties were determined by differential scanning calorimetry (DSC) under nitrogen atmosphere at different heating rates (5, 10, and 15 °C/min). Doyle and Kissenger non-isothermal kinetic models were applied to calculate energy (Ea) values, which were found equal to 254.68 kJ/mol and 255.88 kJ/mol, respectively.

Investigation of Effect of Milling Atmosphere and Starting Composition on Mg2FeH6 Formation

In this study we investigated the synthesis and the hydrogen storage properties of Mg2FeH6. The complex hydride was prepared by ball milling under argon and hydrogen atmosphere from 2Mg + Fe and 2MgH2 + Fe compositions. The samples were characterized by X-ray powder diffraction and scanning electron microcopy. Kinetics of hydrogen absorption and desorption were measured in a Sievert’s apparatus. We found that the milling atmosphere plays a more important role on Mg2FeH6 synthesis than the starting compositions. Ball milling under hydrogen pressure resulted in smaller particles sizes and doubled the yield of Mg2FeH6 formation. Despite the microstructural differences after ball milling, all samples had similar hydrogen absorption and desorption kinetics. Loss of capacity was observed after only five cycles of hydrogen absorption/desorption.

Microstructures and Hydrogen Desorption Properties of the MgH2–AlH3 Composite with NbF5 Addition

Niobium fluoride (NbF5) is introduced into the MgH2 + 1/4AlH3 hydride composite by ball milling to improve the hydrogen desorption properties of the Mg–Al–H system. It is found that after being ball milled with 1 mol % of NbF5, AlH3 in the composite has almost fully decomposed and forms metallic Al, which indicates that NbF5 can significantly destabilize AlH3. DSC results show that NbF5 addition also helps reduce the peak desorption temperature of MgH2 in the composite from 324 to 280 °C. Isothermal desorption measurements demonstrate that MgH2 in the doped composite can rapidly release 98% of its hydrogen after desorption at 300 °C for 1 h, while the valued is only 46% for the undoped composite. The activation energy for hydrogen desorption of MgH2 in the doped composite is calculated to be 104.5 kJ/mol, much lower than that in the undoped composite (127.4 kJ/mol). These results suggest that NbF5 addition dramatically improves the hydrogen desorption kinetics of the MgH2 + 1/4AlH3 composite. Dehydrogenat...

Desorption and sublimation kinetics for fluorinated aluminum nitride surfaces

The adsorption and desorption of halogen and other gaseous species from surfaces is a key fundamental process for both wet chemical and dry plasma etch and clean processes utilized in nanoelectronic fabrication processes. Therefore, to increase the fundamental understanding of these processes with regard to aluminum nitride (AlN) surfaces, temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) have been utilized to investigate the desorption kinetics of water (H2O), fluorine (F2), hydrogen (H2), hydrogen fluoride (HF), and other related species from aluminum nitride thin film surfaces treated with an aqueous solution of buffered hydrogen fluoride (BHF) diluted in methanol (CH3OH). Pre-TPD XPS measurements of the CH3OH:BHF treated AlN surfaces showed the presence of a variety of Al-F, N-F, Al-O, Al-OH, C-H, and C-O surfaces species in addition to Al-N bonding from the AlN thin film. The primary species observed desorbing from these same surfaces during TPD measurements included H2, H2O, HF, F2, and CH3OH with some evidence for nitrogen (N2) and ammonia (NH3) desorption as well. For H2O, two desorption peaks with second order kinetics were observed at 195 and 460 °C with activation energies (Ed) of 51 ± 3 and 87 ± 5 kJ/mol, respectively. Desorption of HF similarly exhibited second order kinetics with a peak temperature of 475 °C and Ed of 110 ± 5 kJ/mol. The TPD spectra for F2 exhibited two peaks at 485 and 585 °C with second order kinetics and Ed of 62 ± 3 and 270 ± 10 kJ/mol, respectively. These values are in excellent agreement with previous Ed measurements for desorption of H2O from SiO2 and AlFx from AlN surfaces, respectively. The F2 desorption is therefore attributed to fragmentation of AlFx species in the mass spectrometer ionizer. H2 desorption exhibited an additional high temperature peak at 910 °C with Ed = 370 ± 10 kJ/mol that is consistent with both the dehydrogenation of surface AlOH species and H2 assisted sublimation of AlN. Similarly, N2 exhibited a similar higher temperature desorption peak with Ed = 535 ± 40 kJ/mol that is consistent with the activation energy for direct sublimation of AlN.

Effect of CO on hydrogen storage performance of KF doped 2LiNH2 + MgH2 material

(2LiNH2+MgH2) material is one of the most promising hydrogen storage materials. In applications, CO impurity in hydrogen has an effect on the hydrogen storage performance of (2LiNH2+MgH2) material. In this work, the effect of CO on the hydrogen storage properties of KF doped (2LiNH2+MgH2) material was investigated by using hydrogen containing 1mol% CO as the hydrogenation gas source. The results indicate that the hydrogen desorption capacity of the hydride decreases from 4.73wt.% to 3.88wt.% after 5 hydrogenation cycles. The decrease of the capacity is attributed to the permanent loss of the NH2 caused by the formation of Li2CN2 and KCN. Moreover, the hydrogen desorption kinetics declines obviously, and the dehydrogenation activation energy increases from 122.1kJ/mol to 132.0kJ/mol. Furthermore, it is found that the hydrogen desorption kinetics cannot be recovered to the initial level after re-milling. The main reason for these is that the catalytic effect on (Mg(NH2)2+2LiH+0.05KF) system disappears due to the transformation from KH to KCN.

Improved kinetics of the Mg(NH2)2–2LiH system by addition of lithium halides

The Mg(NH2)2–2LiH composite is a promising on-board hydrogen storage material due to its high reversible hydrogen capacity and suitable thermodynamic properties. However, the severe kinetic barrier inhibits its low temperature operation. In the present work, the additive effects of lithium halides on the Mg(NH2)2–2LiH system were studied systematically. Experimental results showed that, among all those lithium halides, the LiBr doped Mg(NH2)2–2LiH composite exhibited the best dehydrogenation performance. The hydrogen sorption and desorption rates of the Mg(NH2)2–2LiH–0.2LiBr sample are ∼3 and 2 times, respectively, faster than that of the pristine sample at 140 °C. At the same time, enhanced kinetics for hydrogen desorption was observed from an activation energy (Ea) of ca. 92 ± 9 kJ mol−1 which was significantly decreased by 35 kJ mol−1 compared with the pristine sample. Subsequently, a plausible mechanism for the modified dehydrogenation/re-hydrogenation process was proposed.

Hydrogen Desorption/Absorption Kinetics of MgH&lt;sub&gt;2&lt;/sub&gt; Catalyzed with TiO&lt;sub&gt;2&lt;/sub&gt;

We report on the preparation and hydrogen desorption/absorption kinetics of nanocrystalline magnesium hydride (MgH2) added commercial TiO2by high-energy ball milling. The phase and composition of the as-milled powders are characterized by X-ray diffraction (XRD). The results show that the milled sample contained MgH2phase, small amount of Mg and various phases of TiO2such as tetragonal and orthorhombic structure. The effect of the milling time (10, 20 and 30 h) on the hydrogen desorption property of MgH2has been investigated and found that the milling time of 20 h has excellent dehydrogenation properties, which can release 3.3 wt% H2within 60 min at 300oC, which indicates that the kinetics of hydrogen desorption of MgH2-TiO2composite has been greatly enhanced compared to the pure MgH2. Moreover, hydrogen absorption kinetics of the sample milled 20 h has been studied and the hydrogen content is 0.7, 0.8 and 1.2 wt% H2at 250, 280 and 300oC within 60 min, respectively.

The Role of Cr in H Desorption Kinetics in Rapidly Solidified Al

Hydrogen desorption kinetics for rapidly solidified high purity Al and Al-Cr alloy foils containing 1.0, 1.5 and 3.0 at % Cr were investigated by means of thermal desorption analysis (TDA) at a heating rate of 3.3°C/min. For the first time, it was found that oxide inclusions of Al2O3 are dominant high-temperature hydrogen traps compared with pores and secondary phase precipitates resulted in rapid solidification of Al and its alloys. The correspondent high-temperature evolution rate peak was identified to be positioned at 600°C for high purity Al and shifted to 630°C for Al-Cr alloys. Amount of hydrogen trapped by dislocations increases in the alloys depending on Cr content. Microstructural hydrogen trapping behaviour in low-and intermediate temperature regions observed here was in coincidence with previous data obtained for RS materials using thermal desorption spectroscopy (TDS). The present results on hydrogen thermal desorption evolution indicate that the effect of oxide surface layers becomes remarkable in TDA measurements and show advantages in combinations of both desorption analysis methods to investigate hydrogen desorption kinetics in materials.

The Influence of Morphology on the Hydrogen Storage Properties of Magnesium Based Materials Processed by Cold Rolling

In this communication we report the effect of macro and microstructure on the hydrogen storage properties of magnesium based materials. Magnesium hydride is an attractive material for hydrogen storage applications since it has a high hydrogen volumetric density. Furthermore, the high enthalpy of hydride formation makes it attractive for thermal energy storage applications. Besides, magnesium is an abundant and low cost material. However, the Mg/MgH2 system requires high operating temperatures due to its thermodynamic stability and slow hydrogen absorption and desorption kinetics. Magnesium’s first hydrogenation is a very long and costly process. This work aims to ameliorate this process which would effectively reduce the cost of MgH2. Commercial pure magnesium samples were processed by cold rolling. After processing, the samples presented limited hydrogen absorption due to their small surface area to volume ratio. To overcome this problem the samples were then reduced to powder using a bastard file. The samples were characterized by scanning electron microscopy and presented different morphology. Hydrogen storage properties and morphology are discussed and correlated. Results show an important improvement on the hydrogen absorption and desorption kinetics for the comminuted samples.

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH2 + MgH2) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO2, O2, N2 and CH4 are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH2 + MgH2) system. The results indicate that the hydrogen capacity of the (Mg(NH2)2 + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li2CN2 and MgO appear after (2LiNH2 + MgH2) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH2−, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.