Real-time measurement of desorption temperature and kinetics of magnesium hydride powder sample based on optical reflection

We demonstrate the proof-of-principle that interaction between visible light and a magnesium hydride sample in reflective mode can be used to determine the desorption temperature and kinetics of magnesium hydride in powder form. The demonstrated optical technique requires only milligrams of sample and can potentially be used to measure the de/absorption temperature and kinetics of magnesium nanostructures, which are often fabricated via the physical vapor deposition method inside an optically transparent quartz tube. This would help to eliminate the common problem of oxidation associated with removal and transport of the freshly fabricated nanostructures into an inert protective environment. This optical technique could be applied to any hydrogen-storage material in the form of powder which shows a significant difference in its optical absorption between the hydride and the non-hydride phase.     

In-situ neutron diffraction study of magnesium amide/lithium hydride stoichiometric mixtures with lithium hydride excess

The hydrogen sorption of mixtures of magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) with different molecular ratios have been investigated using in-situ neutron diffraction; the experiments were performed at D20/ILL and SPODI/FRMII. The results reveal a common reaction pathway for 1:2, 3:8 and 1:4 magnesium amide: lithium hydride mixtures. Intermediate reaction steps are observed in both ab- and desorption. The thermodynamic properties of the system at 200 °C are not changed by the addition of excess lithium hydride. This finding has important implications for the tailoring the characteristics of this promising hydrogen storage material.     

Effect of powder characteristics for a magnesium based metal hydride store

The particle size and shape of commercially available MgH₂ have been characterised, together with those of ball milled MgH₂. These parameters have then been compared with those obtained for the hydrogenated and dehydrogenated state. In addition, thermal conductivity measurements of each specimen have been made. The results show that significant changes occur upon dehydrogenation, which are contrary to expectation. These changes include an increase in average particle size of 28% post dehydrogenation for as received MgH₂, rather than the theorised decrease, and an increase of approximately 300% post dehydrogenation in the ball milled MgH₂ sample. The change in thermal conductivity between the two material states is also higher than expected with an average variation of 14% between the samples in the hydrogenated and dehydrogenated states.     

Correlation between kinetics and chemical bonding state of catalyst surface in catalyzed magnesium hydride

We reported on the hydrogen desorption properties, microstructure, kinetics, and chemical bonding state of catalyst surface for composites of MgH₂ and 1 mol% Nb₂O₅ ball-milled for 0.02 h, 0.2 h, 2 h, 20 h under 1 MPa H₂ atmosphere, as well as hand-mixed (HM) one. Hydrogen desorption properties were significantly improved by ball-milling with Nb₂O₅. Then, we estimated by Kissinger Method the activation energy (E_a) of hydrogen desorption reaction that decreased with the increase of ball-milling time. Especially, E_a of the sample ball-milled for 0.2 h was drastically decreased, compared with that of the sample ball-milled for 0.02 h. TEM observations revealed that the distribution of Nb₂O₅ in MgH₂ was gradually improved during ball-milling. On the other hand, we confirmed by XPS that in the sample ball-milled for 0.2 h, Nb₂O_5-x phase(s) existed at least on the surface. It can be suggested that these deoxidized Nb₂O_5-x phases eventually decrease E_a as substantial catalyst rather than Nb₂O₅ itself.     

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.     

An investigation on the thermodynamics and kinetics of magnesium hydride decomposition based on isotope effects

This work investigates the thermodynamics and kinetics of magnesium hydride decomposition by analyzing isotope effects in hydride and deuteride samples. Complete pressure composition desorption isotherm measurements of MgD₂ are reported for the first time. Deuterium desorption enthalpy and entropy obtained from the van’t Hoff plot of the middle plateau fugacities are 73.8 ± 0.4 kJ/mol and 135.5 ± 0.6 J/mol K, respectively, which are in good accordance with the values obtained more than fifty years ago from plateau pressure measurements. This result reveals that the enthalpy of desorption of MgD₂ is slightly lower than that of MgH₂, whereas the entropy change is higher for the deuteride than for the hydride. Although the differences in the enthalpy and entropy of both isotopes are weak, the synergy of both effects is capable of explaining the higher equilibrium pressures for the deuteride than for the hydride.On the other hand, kinetics of magnesium hydride decomposition has been investigated by simultaneous H and D desorption experiments from mixed hydride-deuteride samples. The obtained results reveal that that decomposition is controlled by the nucleation and growth of the Mg phase. Because this reaction step is not affected by the isotopic replacement of H for D no isotope effect is observed in the kinetics of magnesium hydride decomposition. On the contrary, a marked isotope effect is observed in the kinetics of H₂(D₂) absorption by magnesium. In this case, the lighter isotope shows faster kinetics than the heavier one, what has been related to the fact that absorption is rate limited by H(D) diffusion through the hydride(deuteride) phase.     

Desorption kinetics of lithium amide/magnesium hydride systems at constant pressure thermodynamic driving forces

Lithium amide and magnesium hydride are lightweight materials with high hydrogen-holding capacities and thus they are of interest for hydrogen storage. In the present work mixtures with initial molar compositions of (LiNH₂ + MgH₂) and (2LiNH₂ + MgH₂) were ball milled with and without the presence of 3.3 mol% potassium hydride dopant. Temperature programmed desorption, TPD, analyses of the mixtures showed that the potassium hydride doped samples had lower onset temperatures than their corresponding pristine samples. The dehydrogenation kinetics of the doped and pristine mixtures was compared at 210 °C. In each case a constant pressure thermodynamic driving force was applied in which the ratio of the plateau pressure to the applied hydrogen pressure was set at 10. Under equivalent conditions, the (LiNH₂ + MgH₂) mixture desorbed hydrogen about 4 times faster than the (2LiNH₂ + MgH₂) mixture. The addition of potassium hydride dopant was found to have a 25-fold increase on the desorption rates of the (2LiNH₂ + MgH₂) mixture, however it had almost no effect on the desorption rates of the (LiNH₂ + MgH₂) mixture. Activation energies were determined by the Kissinger method. Results showed the potassium hydride doped mixtures to have lower activation energies than the pristine mixtures.     

Effect of admixing different carbon structural variants on the decomposition and hydrogen sorption kinetics of magnesium hydride

The effect of admixing catalysts comprised of carbon nanostructures, specifically planar, helical and twisted carbon nanofibers, spherical carbon particles and multi-walled carbon nanotubes, on the hydrogen storage properties of magnesium hydride has been investigated. Optimum results were achieved with the mixture containing twisted carbon nanofibers (TCNF) synthesized by Ni catalyst derived by oxidative dissociation of catalyst precursor LaNi₅. The desorption temperature of 2 wt.% TCNF admixed MgH₂ is ∼65 K lower than that of pristine MgH₂ milled for the same duration. The enhancement in hydrogen absorption capacity of MgH₂ admixed with 2 wt.% TCNF has been found to be two-fold in the first 10 minutes at 573 K and under a hydrogen pressure of 2 MPa, i.e. 4.8wt% as compared to 2.5 wt% for MgH₂ alone. The increase in capacity by a factor of about two within the first 10 minutes as a result of the catalytic activity of TCNF is one of the exciting results obtained for hydrogen absorption in catalyzed MgH₂.     

Calculated X-ray powder diffraction data for phases encountered in lead/acid battery plates

Calculated X-ray powder diffraction data are presented, in both graphical and numerical form, for ten phases relevant to the positive and negative plates of the lead/acid battery. The data have been derived from recently published, accurate, single crystal (X-ray) or powder (neutron) diffraction crystal structure studies, and are therefore free from the effects of preferred orientation, phase impurity, micro-absorption, and variation in particle size and shape. Moreover, since the intensifies of all possible Bragg reflections consistent with the space group symmetry are calculated, there are no ambiguities in the assignments of the Miller indices of the peaks. The data are given in the form of the complete, simulated Cu Kα diffraction profile from 1 to 100° 2θ (or the first 512 reflections), and also as the integrated intensities relative to a maximum value of 100 for the range 1–60° 2θ in each pattern. Calculated Reference Intensity Ratios for the strongest peak in the patterns relative to the (022) reflection of CaF₂ (fluorite) and the (113) reflection of α-Al₂O₃ (corundum), are also presented for use in quantitative X-ray diffraction phase analysis of mixtures of these compounds in lead/acid battery plates.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Isothermal curves and kinetics in a metal hydride

The isotherms for the alloy DB 5800 between — 196 °C (77 K) and 40 °C (313 K) are measured. The pressure range is 0 bar up to 2 bar. A new reactor was developed and constructed for these conditions. An equation to describe the isotherms is derivated. Isotherm measurements are possible at temperatures below 130 °C (143 K). Kinetic measurements are carried out, an equation for the storing rate is derivated, and the kinetic parameters are determined.     

Experimental and numerical study of a magnesium hydride tank

A small-scale experimental magnesium hydride tank was designed and tested to illustrate the feasibility of hydrogen storage in magnesium hydride. A prototype of the tank was filled with 123 g of previously ball-milled and doped MgH₂. About 80 nl of hydrogen can be reversibly stored at a pressure less than 1 MPa. However, owing to the fact that the heat of a reaction limits the absorption and desorption processes, these latter are slowed down. To have a better understanding of the heat and mass transfers in the tank, a numerical model was developed using the Fluent software. The numerical simulations of hydrogen sorption are found to be in good agreement with the experimental results. During desorption, as an example, the reaction occurs locally and progresses from the tank walls towards the core.     

Hydrogenation properties of pure magnesium and magnesium–aluminium thin films

We have studied the hydrogenation/dehydrogenation behaviour of multilayered stacks of Pd/Mg/Pd and Pd–Fe(Ti)–Mg–Al–Mg–Fe(Ti)–Pd grown by electron beam physical vapour deposition. The palladium coating was deposited at both sides of the structure to ensure a fast dissociation rate and good transport properties for hydrogen as well as to avoid oxidation of magnesium either from atmosphere as from the substrate surface. Fe and Ti layers were included in the stack composition in order to assess their possible catalyst effect as well as to prevent the formation of Mg_xPd_y intermetallics during the thermal treatments. We have studied the structure evolution after thermal treatments as well as after the hydrogenation and dehydrogenation processes using XRD. We have also followed the reactions kinetics by resistometry and differential scanning calorimetry. The nanostructured Mg films have been hydrogenated at temperature as low as 50 °C in few minutes. Adding aluminium to magnesium has improved its hydrogenation capacity. We have also observed that the formation of an Mg_xAl_y intermetallic before hydrogenation improves the storage capacity. We have confirmed that titanium is a better catalyst for the hydrogenation/dehydrogenation of the Mg films.     

Recent progress in magnesium hydride modified through catalysis and nanoconfinement

Hydrogen is an ideal energy carrier because of its high chemical energy, environmental friendliness and renewability. In order to realize the safe, efficient and compact hydrogen storage, various solid-state hydrogen storage materials based on the physisorption or chemisorption of hydrogen have been developed over the past decades. Among them, magnesium hydride, MgH₂, is identified as one of the most promising candidates due to its high hydrogen storage density, low cost and abundance of Mg element. However, the sluggish kinetics and high thermodynamic stability of MgH₂ result in its high operation temperature and low hydrogen sorption rate, impeding its practical application. In this article, the recent progress in catalysis and nanoconfinement effects on the hydrogen storage properties of MgH₂ is comprehensively reviewed. In particular, the synergetic roles of catalysis and nanoconfinement in MgH₂ are highlighted. Furthermore, the future challenges and prospects of emerging research for MgH₂ are discussed. It is suggested that the nonmetal-doped porous carbon materials could be a class of ideal additives to enhance the hydrogen storage properties of MgH₂ by the synergetic effects of catalysis and nanoconfinement.     

In situ X-ray diffraction of prototype sodium metal halide cells: Time and space electrochemical profiling

The feasibility of using energy dispersive X-ray diffraction to characterize full size battery cells is demonstrated by unprecedented in situ measurements of the electrochemical processes taking place inside high temperature sodium metal halide (Na/MCl₂, M = Ni and/or Fe) cells during charge/discharge cycling. Diffraction data provide phase information either via line scans across the 5 cm wide cells or via fixed location scans as a function of time. The data confirm the propagation of a well-defined chemical reaction front, as a function of charge/discharge time, beginning at the ceramic separator and proceeding inward. Measurement of the temporal evolution of the phase abundances yields mechanistic understanding and reaction rates as a function of charge/discharge state. In the case where M includes Fe, the data also clearly show the appearance of an intermediate phase, Na₆FeCl₈, during charging, thereby underscoring the power of this technique to reveal subtle mechanistic information. A number of additional detailed electrochemical kinetic effects are also discussed. This study shows that in situ high energy X-ray diffraction characterization of advanced battery cells in space and time is eminently feasible on a routine basis, and has great potential to advance the understanding of “buried” chemical processes.     

Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride

In present paper, different sizes of CeO₂ nanoparticles were synthesized by ball milling and their effect on the absorption kinetics and decomposition temperature of MgH₂ was studied. It was found that a small amount of admixing of the above said catalysts with MgH₂ exhibits improved hydrogen storage properties. Among these different sizes of CeO₂ nanoparticles, 2 weight % admixed CeO₂ with a particle size of ∼10–15 nm led to decrease in desorption temperature by ∼50 K. Moreover, it also shows 1.5 times better absorption kinetics with respect to pure MgH₂. The samples were characterized using SEM, TEM and XRD techniques. The hydrogenation/dehydrogenation properties were measured by gas reaction controller.     

Dehydrogenation kinetics and catalysis of organic heteroaromatics for hydrogen storage

The complete recovery of the H₂ stored on dodecahydro-N-ethylcarbazole was achieved at 443 K and 101 kPa using Pd catalysts prepared by incipient wetness impregnation and calcination in He rather than air. Over a 4 wt% Pd/SiO₂ catalyst, the reaction proceeded to complete conversion within 22 min and complete H₂ recovery (5.8 wt%) within 1.6 h. The dehydrogenation rate of dodecahydro-N-ethylcarbazole and selectivity to the completely dehydrogenated product, N-ethylcarbazole, were dependent upon the Pd particle size. The dehydrogenation rate of dodecahydro-N-ethylcarbazole was compared to that of dodecahydrocarbazole and dodecahydrofluorene. The lower turn-over frequency (TOF) for dodecahydrocarbazole was attributed to a strong adsorption of the dehydrogenated products to Pd through the N atom, whereas the ethyl group in dodecahydro-N-ethylcarbazole prevented a strong N interaction with the surface. Density functional theory (DFT) results showed that dodecahydrocarbazole and dodecahydrofluorene were more strongly adsorbed on Pd than dodecahydro-N-ethylcarbazole leading to a significant decrease in their TOFs for H₂ recovery.     

Improvement of reaction kinetics by metal chloride on ammonia and lithium hydride system

Ammonia NH₃ and lithium hydride LiH system releases hydrogen even at room temperature to form lithium amide LiNH₂. LiNH₂ is recycled back to NH₃ and LiH below 300 °C under hydrogen H₂ flow condition. However, the reaction rate of the system is slow for a practical application. In this work, various kinds of transition metal chlorides were examined as a potential catalyst to improve the kinetics. For hydrogen desorption reaction, the reaction kinetics of titanium chloride TiCl₃ dispersing LiH was about 8 times faster than the raw LiH, suggesting that TiCl₃ possessed an excellent catalytic effect. In the case of the regeneration reaction, the reaction kinetics was also improved by the addition of TiCl₃. It was mainly caused by physical effects in contrast to the hydrogen desorption process, in other words, the small crystallite and/or particle were formed by the milling with the additive.     

Experimental study of ignition of magnesium powder by electrostatic discharge

Ignition of non-aerosolized powders by electrostatic discharge (ESD) is investigated. A spherical powder of Mg, for which thermal ignition kinetics was described in the literature, was used in experiments. The experimental setup was built based on a commercially available apparatus for ESD ignition sensitivity testing. Additional diagnostics enabled measurements of electrical current, voltage, and spark and ignited powder emission in real time. The spark duration was of the order of a few μs. The spark current and voltage were always observed to have significant AC components. The electrical impedance of the spark discharge was determined experimentally using the recorded current traces and assuming that the spark and powder could be represented as a series LRC circuit. The optical emission was filtered to separate the signals produced by the spark plasma and by the heated and igniting powder. The radiation signal produced by the igniting powder was always delayed after the spark. The delay time decreased from about 3.5 to 0.5 ms as the spark energy increased from 10 to 60 mJ; the delay remained nearly constant when the spark energy continued to increase to over 100 mJ. For experiments where the powder volume decreased or where binder was introduced delay times were reduced. For the powder with a small amount of binder, the ignition delay continued to decrease for spark energies exceeding 60 mJ. Interpretation of the obtained experimental data suggests that the ignition is primarily due to direct Joule heating of the powder by the spark current.