Iron based catalyst for the improvement of the sorption properties of KSiH3

Recently, silanides (MSiH₃) have been proposed as the possible hydrogen storage materials due to their hydrogen storage properties. Among these silanides, KSiH₃ has been considered as leading contender due to its high hydrogen storage capacity i.e. 4.3 wt% and suitable thermodynamic parameters. It can absorb and desorb hydrogen reversibly at near ambient temperature, however, a high activation barrier slows down its kinetics. To enhance its kinetic properties, several catalysts have been attempted so far. Nb₂O₅ has been proven as leading catalyst with significant improvement. In the present work, Fe based catalysts were chosen due to their suitability for hydrogen storage materials. Among all the studied catalysts in this work, Fe₂O₃ was found to be the most effective catalyst, reducing the activation energy down to 75 kJ mol⁻¹ from 142 kJ mol⁻¹ for pristine KSi.     

Effects of KNbO3 catalyst on hydrogen sorption kinetics of MgH2

The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ternary compound catalyst such as K₂NiF₆, the influence of KNbO₃ on hydrogen storage properties of MgH₂ has been investigated for the first time. The MgH₂ - KNbO₃ composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH₂ and MgH₂-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH₂ with other Nb-contained catalysts such as Nb, NbO and Nb₂O₃, the desorption rate was almost the same as the one registered for as-milled MgH₂. The analysis of differential scanning calorimetry (DSC) showed that MgH₂-KNbO₃ composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH₂ without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol⁻¹ for this material, while for the as-milled MgH₂ was about 165 ± 2.0 kJ mol⁻¹. It is believed that Nb-ternary oxide catalyst (KNbO₃) showed a good catalytic effect and enhance the sorption kinetics of MgH₂.     

The effects of halide modifiers on the sorption kinetics of the Li-Mg-N-H System

The effects of different transition metal halides (TiCl₃, VCl₃, ScCl₃ and NiCl₂) on the sorption properties of the 1:1 molar ratio of LiNH₂ to MgH₂ are investigated. The modified mixtures were found to contain LiNH₂, MgH₂ and LiCl. TGA results showed that the hydrogen desorption temperature was reduced with the modifier addition in this order: TiCl₃ > ScCl₃ > VCl₃ > NiCl₂. Ammonia release was not significantly reduced resulting in a weight loss greater than the theoretical hydrogen storage capacity of the material. The isothermal sorption kinetics of the modified systems showed little improvement after the first dehydrogenation cycle over the unmodified system but showed drastic improvement in rehydrogenation cycles. X-ray diffraction and Raman spectroscopy identified the cycled material to be composed of LiH, MgH₂, Mg(NH₂)₂ and Mg₃N₂.     

XPS valence band studies of hydrogen storage nanocomposites

In this work, the electronic properties of hydrogen storage LaNi₅/A (A = 10 wt.% C, Cu, Pd, Ni) materials and LaNi₅+Mg_1.5Mn_0.5Ni, LaNi_3.75Mn_0.75Al_0.25Co_0.25 + Mg_1.5Mn_0.5Ni nanocomposites were studied. Results showed that the XPS valence bands measured for mechanically alloyed nanocrystalline alloys and nanocomposites showed a significant broadening compared to those obtained for microcrystalline materials with the same chemical compositions. Furthermore, the surface segregation process of La atoms in LaNi₅/Pd nanocomposites is stronger compared to that observed for the nanocrystalline LaNi₅ alloy thin films.     

Hydrogen sorption kinetics,hydrogen permeability,and thermal properties of compacted 2LiBH4MgH2 doped with activated carbon nanofibers

To improve the packing efficiency in tank scale, hydrides have been compacted into pellet form; however, poor hydrogen permeability through the pellets results in sluggish kinetics. In this work, the hydrogen sorption properties of compacted 2LiBH₄MgH₂ doped with 30 wt % activated carbon nanofibers (ACNF) are investigated. After doping with ACNF, onset dehydrogenation temperature of compacted 2LiBH₄MgH₂ decreases from 350 to 300 °C and hydrogen released content enhances from 55 to 87% of the theoretical capacity. The sample containing ACNF releases hydrogen following a two-step mechanism with reversible hydrogen storage capacities up to 4.5 wt % H₂ and 41.8 gH₂/L, whereas the sample without ACNF shows a single-step decomposition mainly from MgH₂ with only 1.8 wt % H₂ and 15.4 gH₂/L. Significant kinetic improvement observed in the doped system is due to the enhancement of both hydrogen permeability and heat transfer through the pellet.     

Evaluation of hydrogen sorption models for AB5-type metal alloys by employing a gravimetric technique

This paper is concerned with hydrogen absorption and desorption in AB₅-type hydrogen storage metal alloys. We give a brief overview on models which have been proposed for hydrogen sorption in metals over the past decades. We choose three models based on different perspectives, i.e. thermodynamics, reaction kinetics, and mere observation (empiricism), and evaluate their applicability in order to describe the sorption behaviour. Additionally, we propose a model which is based on a cumulative distribution function. In order to evaluate the models, the hydrogen absorption and desorption isotherms of LaNi₅ and LaNi_4.5Co_0.5 are measured by means of a gravimetric technique. A nonlinear regression is performed to fit the models to experimental data. The computed model parameters are compared to values reported in the literature. The emphasis is given to the applicability of the models with respect to describing the non-ideality of the plateau region and the continuity/smoothness of phase transition regions.     

An overview of reactive hydride composite (RHC) for solid-state hydrogen storage materials

Hydrogen storage using the metal hydrides and complex hydrides is the most convenient method because it is safe, enables high hydrogen capacity and requires optimum operating condition. Metal hydrides and complex hydrides offer high gravimetric capacity that allows storage of large amounts of hydrogen. However, the high operating temperature and low reversibility hindered the practical implementation of the metal hydrides and complex hydrides. An approach of combining two or more hydrides, which are called reactive hydride composites (RHCs), was introduced to improve the performance of the metal hydrides and complex hydrides. The RHC system approach has significantly enhanced the hydrogen storage performance of the metal hydrides and complex hydrides by modifying the thermodynamics of the composite system through the metathesis reaction that occurred between the hydrides, hence enhancing the kinetic and reversibility performance of the composite system. In this paper, the overview of the RHC system was presented in detail. The challenges and perspectives of the RHC system are also discussed. This is the first review report on the RHC system for solid-state hydrogen storage.     

Hydrogen sorption kinetics of MgH2 catalyzed with titanium compounds

Identification of effective catalyst is a subject of great interest in developing MgH₂ system as a potential hydrogen storage medium. In this work, the effects of typical titanium compounds (TiF₃, TiCl₃, TiO₂, TiN and TiH₂) on MgH₂ were systematically investigated with regard to hydrogen sorption kinetics. Among them, adding TiF₃ leads to the most pronounced improvement on both absorption and desorption rates. Comparative studies indicate that the TiH₂ and MgF₂ phases in situ introduced by TiF₃ fail to explain the superior catalytic activity. However, a positive interaction between TiH₂ and MgF₂ is observed. Detailed comparison between the effect of TiF₃ and TiCl₃ additive suggests the catalytic role of F anion. XPS examination reveals that new bonding state(s) of F anion is formed in the MgH₂ + TiF₃ system. On the basis of these results, we propose that the substantial participation of F anion in the catalytic function contributes to the superior activity of TiF₃.     

Machine learning analysis of alloying element effects on hydrogen storage properties of AB2 metal hydrides

Zirconium-titanium-based AB₂ is a potential candidate for hydrogen storage alloys and NiMH battery electrodes. Machine learning (ML) has been used to discover and optimize the properties of energy-related materials, including hydrogen storage alloys. This study used ML approaches to analyze the AB₂ metal hydrides dataset. The AB₂ alloy is considered promising owing to its slightly high hydrogen density and commerciality. This study investigates the effect of the alloying elements on the hydrogen storage properties of the AB₂ alloys, i.e., the heat of formation (ΔH), phase abundance, and hydrogen capacity. ML analysis was performed on the 314 pairs collected and data curated from the literature published during 1998–2019, comprising the chemical compositions of alloys and their hydrogen storage properties. The random forest model excellently predicts all hydrogen storage properties for the dataset. Ni provided the most contribution to the change in the enthalpy of the hydride formation but reduced the hydrogen content. Other elements, such as Cr, contribute strongly to the formation of the C14-type Laves phase. Mn significantly affects the hydrogen storage capacity. This study is expected to guide further experimental work to optimize the phase structure of AB₂ and its hydrogen sorption properties.     

Application of hydrides in hydrogen storage and compression: Achievements,outlook and perspectives

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications.With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus.In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized.In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles.In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage”, different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications.     

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.     

Large-vscale hydrogen production and storage technologies: Current status and future directions

Over the past years, hydrogen has been identified as the most promising carrier of clean energy. In a world that aims to replace fossil fuels to mitigate greenhouse emissions and address other environmental concerns, hydrogen generation technologies have become a main player in the energy mix. Since hydrogen is the main working medium in fuel cells and hydrogen-based energy storage systems, integrating these systems with other renewable energy systems is becoming very feasible. For example, the coupling of wind or solar systems hydrogen fuel cells as secondary energy sources is proven to enhance grid stability and secure the reliable energy supply for all times. The current demand for clean energy is unprecedented, and it seems that hydrogen can meet such demand only when produced and stored in large quantities. This paper presents an overview of the main hydrogen production and storage technologies, along with their challenges. They are presented to help identify technologies that have sufficient potential for large-scale energy applications that rely on hydrogen. Producing hydrogen from water and fossil fuels and storing it in underground formations are the best large-scale production and storage technologies. However, the local conditions of a specific region play a key role in determining the most suited production and storage methods, and there might be a need to combine multiple strategies together to allow a significant large-scale production and storage of hydrogen.     

Transition metal doped MgH2: A material to potentially combine fuel-cell and battery technologies

MgH₂ is studied as a negative electrode material for rechargeable batteries on the basis of density functional theory calculations. We calculate the average voltage of the corresponding Li-ion battery, which is in good agreement with the experimental value, and we predict the average voltage for the Na-ion battery. Then, molecular dynamics simulations are used to study the diffusive properties of lithium in MgH₂ clusters. In particular, we dope MgH₂ with transition metals (Fe, Ni, Ti, and V), and analyze the effect on the diffusion of lithium, which is shown to be essentially unaffected. Therefore, we propose that transition metal doped MgH₂ is a material that can be used efficiently in both batteries and fuel-cell technologies.     

Scaled-up production of a promising Mg-based hydride for hydrogen storage

The feasibility of scaling up the production of a Mg-based hydride as material for solid state hydrogen storage is demonstrated in the present work. Magnesium hydride, added with a Zr–Ni alloy as catalyst, was treated in an attritor-type ball mill, suitable to process a quantity of 0.5–1 kg of material. SEM–EDS examination showed that after milling the catalyst was well distributed among the magnesium hydride crystallites. Thermodynamic and kinetic properties determined by a Sievert's type apparatus showed that the semi-industrial product kept the main properties of the material prepared at the laboratory scale. The maximum amount of stored hydrogen reached values between 5.3 and 5.6 wt% and the hydriding and dehydriding times were of the order of few minutes at about 300 °C.     

A novel complex oxide TiVO3.5 as a highly active catalytic precursor for improving the hydrogen storage properties of MgH2

Herein, we demonstrate the successful preparation of a novel complex transition metal oxide (TiVO_3.5) by oxidizing a solid-solution MXene (Ti_0.5V_0.5)₃C₂ at 300 °C and its high activity as a catalyst precursor in the hydrogen storage reaction of MgH₂. The prepared TiVO_3.5 inherits the layered morphology of its MXene precursor, but the layer surface becomes very coarse because of the presence of numerous nanoparticles. Adding a minor amount of TiVO_3.5 remarkably reduces the dehydrogenation and hydrogenation temperatures of MgH₂ and enhances the reaction kinetics. The 10 wt% TiVO_3.5-containing sample exhibits optimal hydrogen storage properties, as it desorbs approximately 5.0 wt% H₂ in 10 min at 250 °C and re-absorbs 3.9 wt% H₂ in 5 s at 100 °C and under 50 bar of hydrogen pressure. The apparent activation energy is calculated to be approximately 62.4 kJ/mol for the MgH₂-10 wt% TiVO_3.5 sample, representing a 59% reduction in comparison with pristine MgH₂ (153.8 kJ/mol), which reasonably explains the remarkably reduced dehydrogenation operating temperature. Metallic Ti and V are detected after ball milling with MgH₂; they are uniformly dispersed on the MgH₂ matrix and act as actual catalytic species for the improvement of the hydrogen storage properties of MgH₂.     

Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling

This work deals with the assessment of the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. Samples are prepared from Mg_xFe (x: 2, 3 and 15) elemental powder mixtures via low energy ball milling under hydrogen atmosphere at room temperature. The highest yield is obtained with Mg₁₅Fe after 150 h of milling (90 wt% of MgH₂). The thermodynamic characterization carried out between 523 and 673 K shows that the obtained Mg–Fe–H hydride systems have similar thermodynamic parameters, i.e. enthalpy and entropy. However, in equilibrium conditions, Mg₁₅Fe has higher hydrogen capacity and small hysteresis. In dynamic conditions, Mg₁₅Fe also shows better hydrogen capacity (4.85 wt% at 623 K absorbed in less than 10 min and after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability in comparison to the other studied compositions. From hydrogen uptake rate measurements performed at 573 and 623 K, the rate-limiting step of the hydrogen uptake reaction is determined by fitting particle kinetic models. According to our results, the hydrogen uptake is diffusion controlled, and this mechanism does not change with the Mg–Fe proportion and temperature.     

Thermodynamics and reaction pathways of hydrogen sorption in Mg6(Pd,TM) (TM = Ag,Cu and Ni) pseudo-binary compounds

Mg₆(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds have been synthesized at the TM solubility limit to determine the influence of TM on the thermodynamics and reaction pathways of the Mg₆Pd–H system. All compounds exhibit a two-plateau pressure behaviour, being the value of the high plateau pressure well above that of the Mg/MgH₂ system. Such destabilization is explained by the formation of different Mg–(Pd,TM) intermetallics and/or Mg₂NiH₄ hydride phases during the hydrogenation reaction. The formation of these phases not only increases the enthalpy of hydrogenation but also enhances disorder leading to a limited destabilization of the hydrogenated state. This compensation effect is characterized by a linear correlation between enthalpy and entropy terms. In addition, this work also provides the assessment at 623 K of the ternary Mg–Pd–Cu phase diagram in the Mg-rich corner.     

Design of a large-scale metal hydride based hydrogen storage reactor: Simulation and heat transfer optimization

An optimized design for a 210 kg alloy, TiMn alloy based hydrogen storage system for stationary application is presented. A majority of the studies on metal hydride hydrogen systems reported in literature are based on system scale less than 10 kg, leaving questions on the design and performance of large-scale systems unanswered. On the basis of sensitivity to various design and operating parameters such as thermal conductivity, porosity, heat transfer coefficient etc., a comprehensive design methodology is suggested. Following a series of performance analyses, a multi-tubular shell and tube type storage system is selected for the present application which completes the absorption process in 900 s and the desorption process in 2000 s at a system gravimetric capacity of 0.7% which is a vast improvement over similar studies. The study also indicates that after fifty percent reaction completion, heat transfer ceases to be the major controlling factor in the reaction. This could help prevent over-designing systems on the basis of heat transfer, and ensure optimum system weight.     

Enhanced hydrogen sorption kinetics of Mg50Ni-LiBH4 composite by CeCl3 addition

Mg₅₀Ni-LiBH₄ and Mg₅₀Ni-LiBH₄-CeCl₃ composites have been prepared by short times of ball milling under argon atmosphere. Combination of HP-DSC and volumetric techniques show that Mg₅₀Ni-LiBH₄-CeCl₃ composite not only uptakes hydrogen faster than Mg₅₀Ni-LiBH₄, but also releases hydrogen at a lower temperature (225 °C). The presence of CeCl₃ has a catalytic role, but it does not modify the thermodynamic properties of the composite which corresponds to MgH₂. Experimental studies on the hydriding/dehydriding mechanisms demonstrate that LiBH₄ and Ni lead to the formation of MgNi₃B₂ in both composites. In addition, XRD/DSC analysis and thermodynamic calculations demonstrate that the addition of CeCl₃ accounts for the enhancement of the hydrogen absorption/desorption kinetics through the interaction with LiBH₄. The in situ formation and subsequent decomposition of Ce(BH₄)₃ provides a uniform distribution of nanosize CeB₄ compound, which plays an important role in improving the kinetic properties of MgH₂.     

Desorption properties of LiAlH4 doped with LaFeO3 catalyst

The addition of a catalyst and ball milling process was found to be one of the efficient method to reduce the decomposition temperature and improve the desorption kinetics of lithium aluminium hydride (LiAlH₄). In this paper, a transition metal oxide, LaFeO₃ was used as a catalyst. Decomposition temperature of the 10 wt% of LaFeO₃-doped LiAlH₄ system was found to be lowered from 143 °C to 103 °C (first step) and from 175 °C to 153 °C (second step), respectively. In isothermal desorption kinetics, the amount of hydrogen released of the doped sample was improved to 3.9 wt% in 2.5 h at 90 °C. Meanwhile, the undoped sample had released less than 1.0 wt% of hydrogen under the same condition. The activation energy of the LaFeO₃-doped LiAlH₄ sample was measured to be 73 kJ/mol and 90 kJ/mol for the first two dehydrogenation reactions compared to 107 kJ/mol and 119 kJ/mol for the undoped sample. The improvements of desorption properties were the results from the formation of LiFeO₂, Fe and La or La-containing phase during the heating process.