Hydrogen absorption of catalyzed magnesium below room temperature

Hydrogen absorption of magnesium (Mg) catalyzed by 1 mol% niobium oxide (Nb₂O₅) was demonstrated under the low temperature condition even at −50 °C. The kinetic and thermodynamic properties were examined for MgH₂ with and without Nb₂O₅. By considering the remarkable absorption features at such low temperature, the essential hydrogen absorption properties were investigated under accurate isothermal conditions. As the results, the activation energy of hydrogen absorption for the catalyzed Mg was evaluated to be 38 kJ/mol, which was significantly smaller than that of MgH₂ without the catalyst. The kinetic improvement was also found on the hydrogen desorption process. On the other hand, thermodynamic properties were not changed by the catalyst as a matter of course. Therefore, the Nb₂O₅ addition mainly affects the reaction rates between Mg and hydrogen and shows the excellent catalytic effects.     

Sorption kinetics in metal hydrides by leaky coating

The 3D geometry of a hydrogen absorbing metal grain (Pd) is mimicked by a membrane made of the metal with identical properties, which is sealed on one side with a hydrogen semi-impermeable surface (Cu). The hydrogen loss through the sealed membrane surface is negligible, i.e., the hydrogen uptake measurement is that of a bulk material (Sieverts measurement), but the surface desorbs sufficient hydrogen to be detected by a mass spectrometer. With this, two independent spatial and temporal kinetic properties are defined which allow the reconstruction of the time dependent hydrogen distribution inside the membrane. As proof of concept, the mechanism of hydride formation in Pd is analyzed, corroborating the formation and growth of incoherent interfaces during hydrogen sorption.     

Hydrogen absorption kinetics of V4Cr4Ti alloy prepared by aluminothermy

The hydrogen absorption kinetics of V4Cr4Ti alloy, synthesized by aluminothermy process has been investigated in the temperature range of 373–773 K. The obtained hydrogen absorption kinetic curves were linearly fitted using a series of mechanism function to reveal the kinetics parameter and reaction mechanism. Nucleation and growth, one dimensional diffusion and three-dimensional diffusion processes are the intrinsic rate limiting steps of hydrogen absorption at 373 K. It was found that nucleation and growth processes disappear between 413 K–473 K. However at higher temperatures (>473 K), nucleation and growth as well as one dimensional diffusion process disappear. In the temperature ranges investigated (473 K–773 K), three-dimensional diffusion process was the intrinsic rate limiting step. The apparent activation energy was calculated using Arrhenius equation and found to be 6.1 kJ/mol. This value appears to be relatively higher which can be attributed to the presence of aluminium, which has blocked the absorption sites and increased the activation energy.     

Hydrogen absorption in Ni-catalyzed Mg: A model for measurements in the low temperature range

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this article we continued the work presented in our previous paper by modeling the hydrogen absorption in Ni-catalyzed magnesium in the range of pressures of 500 kPa–5000 kPa and temperatures from 423 K to 468 K. A new model based in the Ginstling–Brounshtein diffusion equation was proposed for the hydrogen absorption kinetics. It adds the contribution of the pressure of the gaseous phase and the enthalpy of reaction to the previously mentioned diffusive model. An activation energy for the process was estimated and the value obtained (112 kJ/mol) was concordant with previous values reported in the literature.     

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.     

Hydrogen storage property of sandwiched magnesium hydride nanoparticle thin film

Hydrogen sorption property of magnesium (Mg) in the form of sandwiched Pd/Mg/Pd films is investigated. Pulsed laser deposition method was applied to deposit the samples consisting of films of nanoparticles. The enthalpy of formation of MgH₂ was found to be −68 kJ/mol H₂ for films with nanoparticle size on the order of 50 nm, which is smaller than the value for bulk MgH₂ and may be explained by the concept of excess volume.     

Hydrogen absorption/desorption kinetics of magnesium nano-nickel composites synthesized by dry particle coating technique

Dry particle coating technique was utilized for coating nano-nickel on magnesium particles. The objectives of this study were to evaluate the effects of the nanocatalyst (Ni) on the hydrogen absorption and desorption kinetics of the Mg-based composite (Mg–Ni). Hydrogen absorption curves plotted as a function of time showed that composites processed for longer periods of time exhibited significantly higher hydrogen absorption rates. With increased coating time, the catalyst was more evenly distributed over the Mg surface, resulting in the formation of a composite with increased hydrogen capacity and kinetics. Meanwhile, no significant morphology change in the composites was observed. A change in the hydrogen absorption rate as a function of time was observed. This change in rate implies a change in the rate limiting mechanism from chemical absorption on the Mg particle to diffusion into the Mg particle. TGA results showed that lower coating speeds resulted in lower initial desorption temperatures as well as steeper desorption rates. Higher heating rates resulted in faster reaction rates and therefore reduced hydrogen absorption time and increased hydrogen storage capacity.     

Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles

Commercial metal nanoparticles of Fe, Co, Ni, Cu, Zn were added to MgH₂ by ball-milling to improve the kinetics of hydrogen release and the reversibility during successive absorption/desorption cycles. metal nanoparticles were well dispersed into the MgH₂ matrix without formation of any ternary metal hydrides, nor binary compounds. Activation energy values were determined for the various samples by temperature programmed desorption experiments while the hydride formation enthalpy was deduced from Van't Hoff equation starting from high pressure volumetric isotherms acquired at different temperatures. The presence of transient effect during the absorption process was excluded by comparing successive hydrogenation/dehydrogenation cycles recorded at 350 °C on Ni and Fe-containing samples. Information about hydrogen absorption kinetics was also obtained. Promisingly, the Ni, Fe, and Co containing samples have shown a good stability, enhanced catalytic performance, and high rate of hydrogen absorption while Zn and Cu nanoparticles worked more like inhibitors than activators.     

Hydrogen storage behaviors of magnesium hydride catalyzed by transition metal carbides

In this study, some transitional metal carbides (Ti₃C₂, Ni₃C, Mo₂C, Cr₃C₂ and NbC) were prepared to enhance the hydrogen storage behaviors of magnesium-based materials. The carbides with a weight ratio of 5 wt% were introduced into magnesium hydride (MgH₂) by mechanical ball milling, and the microstructure, phase composition and hydrogen storage properties of the composites were studied in detail. The phase compositions of Ni₃C, Mo₂C, Cr₃C₂ and NbC in the ball-milled composites have not changed during hydrogen absorption and desorption cycles. However, Ti₃C₂ decompose into multivalent Ti during hydrogenation process. All of these metal carbides can enhance the hydrogen absorption and desorption kinetics of MgH₂. Among them, Ti₃C₂ shows the best catalytic effect on dehydrogenation kinetic properties of MgH₂, followed by the Ni₃C, NbC, Mo₂C and Cr₃C₂.     

Pd/Mg/Pd thin films prepared by pulsed laser deposition under different helium pressures: Structure and electrochemical hydriding properties

Three-layered Pd/Mg/Pd thin films were prepared by pulsed laser deposition in the presence of helium gas. For Pd layer deposition, the He pressure was fixed at 200 mTorr whereas different pressures of He were used for Mg layer deposition (50, 200 and 600 mTorr). The degree of crystallinity and of (001) texture in the Mg layer increase with increasing He pressure. In addition, the increase in He pressure upon Mg deposition greatly accentuates the roughness of the Mg layer, which induces an extension of the outer Pd/Mg interface region. In contrast, the inner Pd/Mg interface is sharp for all the Pd/Mg/Pd films. The electrochemical hydrogen sorption properties of the Pd/Mg/Pd films are improved by increasing the He pressure for Mg layer deposition. However, the maximum H-solubility in the Mg layer remains low (H/Mg ∼0.26) and is not significantly increased by the presence of the inner Pd layer, indicating that Mg hydride phase is confined in the outer Pd/Mg interface region.     

Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

The hydrogen absorption and desorption properties of a MgH₂ – 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH₂. Nb ethoxide reacts with MgH₂ during heating, releasing C₂H₆ and H₂, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH₂ without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H₂ to 94 kJ/mol H₂, and from 176 kJ/mol H₂ to 75 kJ/mol H₂, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbH_x achieved by the dispersion of the liquid additive.     

Large scale magnesium hydride tank coupled with an external heat source

The experimental study of a large scale magnesium hydride tank (10 kg) is presented. In order to enhance thermal exchanges and improve storage time, the MgH₂ powder has been compacted with 10 wt.% of Expanded Natural Graphite. An efficient heat exchanger has been designed to transfer the heat from the endo/exothermic sorption reactions to an external heat source/sink through a high temperature heat transfer fluid. An improvement of the loading and discharging time along the first hydrogenation cycles is observed owing to material thermal conductivity modifications. The properties of the cycled material have been studied in order to understand this evolution of the tank behavior.     

Hydrogen storage systems for fuel cells: Comparison between high and low-temperature metal hydrides

The need for the enhancement of alternative energy sources is increasingly recognised and, in this perspective, the achievement of hydrogen economy seems to be fundamental. In this regard, fuel cells represent an interesting option for small and medium scale distributed renewable generation; however, these systems are inextricably linked with the concept of hydrogen storage. Research on metal hydrides revealed the opportunity to use these materials as basic elements in hydrogen storage devices, called MH systems. This means that interest exists in investigating the behaviour of metal hydrides: in fact, MH system operation is based on the hydriding/dehydriding reactions hydrides undergo, and, with the aim of evaluating the performance of such devices, these processes must be discussed and modelled.In the light of this, a simple numerical model to study hydride-based storage systems and their integration with fuel cells was developed: two low-temperature hydrides (LaNi₅, LaNi_4·8Al_0.2) and two high-temperature hydrides (Mg, Mg₂Ni) were selected and their behaviours in a MH system were simulated and compared with the help of such a model. This is an essential step in identifying the hydrides more suited to the application in question. Results showed that the choice is the trade off between encumbrance and reaction times; this implies that low-temperature hydrides are preferable because their encumbrance is limited and their reaction temperature range grants a greater versatility in small scale generation.     

Numerical modeling of hydrogen absorption measurements in Ni-catalyzed Mg

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this work, hydrogen absorption in Ni-catalyzed magnesium was measured in a wide range of pressure (500 kPa–5000 kPa) and temperature (498 K–573 K). Using this information, a model for the absorption kinetics and thermal behavior of the hydrogen storage system was proposed. This model could be used in the design of Ni-catalyzed magnesium storage tanks and other applications. It considers the independent contribution of three variables: temperature, pressure and reacted fraction to estimate the hydrogen absorption rate. An activation energy for the process was estimated and the value obtained (92 kJ/mol) was concordant with previous values reported in the literature.     

Kinetics of isothermal hydrogenation of magnesium with TiH2 additive

Ball-milled magnesium hydride with titanium hydride as a catalytic additive has been demonstrated to have excellent hydrogenation and dehydrogenation kinetics in recent studies, and is considered to be a promising material for hydrogen storage and thermal energy storage applications. The present work investigated the hydrogenation kinetics of this material across a wide temperature range, from room temperature to 200 °C using a Sieverts type apparatus. The kinetics tests were conducted under a methodically designed isothermal condition to minimize the thermal gradient effect, which is often neglected in the literature. It was found that the hydrogenation kinetics under isothermal conditions were significantly different from those under non-isothermal conditions. Additionally, it was determined that the hydrogenation kinetics under isothermal conditions were numerically best fit by the Johnson–Mehl–Arrami model.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C,which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Hydrogen desorption kinetics of the destabilized LiBH4AlH3 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.     

Nanostructured hydrogen storage materials prepared by high-energy reactive ball milling of magnesium and ferrovanadium

Hydrogen storage nanocomposites prepared by high energy reactive ball milling of magnesium and vanadium alloys in hydrogen (HRBM) are characterised by exceptionally fast hydrogenation rates and a significantly decreased hydride decomposition temperature. Replacement of vanadium in these materials with vanadium-rich Ferrovanadium (FeV, V80Fe20) is very cost efficient and is suggested as a durable way towards large scale applications of Mg-based hydrogen storage materials. The current work presents the results of the experimental study of Mg–(FeV) hydrogen storage nanocomposites prepared by HRBM of Mg powder and FeV (0–50 mol.%). The additives of FeV were shown to improve hydrogen sorption performance of Mg including facilitation of the hydrogenation during the HRBM and improvements of the dehydrogenation/re-hydrogenation kinetics. The improvements resemble the behaviour of pure vanadium metal, and the Mg–(FeV) nanocomposites exhibited a good stability of the hydrogen sorption performance during hydrogen absorption – desorption cycling at T = 350 °C caused by a stability of the cycling performance of the nanostructured FeV acting as a catalyst. Further improvement of the cycle stability including the increase of the reversible hydrogen storage capacity and acceleration of H₂ absorption kinetics during the cycling was observed for the composites containing carbon additives (activated carbon, graphite or multi-walled carbon nanotubes; 5 wt%), with the best performance achieved for activated carbon.