The milled LiBH4/h-BN composites exhibiting unexpected hydrogen storage kinetics and reversibility

The effect of nanoscale h-BN addition by milling on the de-/re-hydrogenation of LiBH₄ was investigated. With the increasing h-BN ratio, the milled LiBH₄/h-BN composites showed lower dehydrogenation temperature. For the LiBH₄-3BN composite (mole ratio 1:3), the on-set dehydrogenation temperature was reduced from 290 °C for the milled pure LiBH₄ down to 175 °C, and the initial dehydrogenation capacity could reach 3.1 wt.% (equivalent to 13.7 wt.% of the component LiBH₄) within ～2 h at 400 °C. Under moderate rehydrogenation conditions of 400 °C and 10 MPa H₂ pressure, the 2nd and 3th cyclic dehydrogenation capacity of LiBH₄-3BN composite almost remained unchanged, indicating remarkably improved rehydrogenation reversibility in comparison to milled pure LiBH₄. FTIR analysis reveals specific interaction between h-BN and LiBH₄ probably originating from the polar mechanism between polarizable B–H bond and B–N bond, which should be responsible for the enhanced dehydrogenation kinetics and reversibility. This work demonstrates the specific catalytic role of nanoscale h-BN and its potential for reversible hydrogen storage by compositing with high-capacity borohydrides.     

A comparative study of LiBH4-based composites with metal hydrides and fluorides for hydrogen storage

A comparative study was performed on four LiBH₄-based hydrogen storage composites 2LiBH₄ + MgX₂ and 6LiBH₄ + CaX₂ (X = H and F). The composites with fluorides and those with corresponding hydrides exhibited similar hydrogen storage properties. The dehydrogenation of 2LiBH₄ + MgF₂ demonstrated a strong dependence on the hydrogen back pressure, similar to that of 2LiBH₄ + MgH₂. The reversible hydrogen storage of 2LiBH₄ + MgF₂ was achieved under a back pressure of 5 bar at 450 °C. Dehydrogenation under lower H₂ pressures resulted in the production of Mg and thus a partial reversibility. In contrast, both 6LiBH₄ + CaH₂ and 6LiBH₄ + CaF₂ revealed reversible hydrogen storage properties regardless of the hydrogen back pressure. The structural difference between MgB₂ and CaB₆ may account for the observed differences in hydrogen storage properties of the Mg- and Ca-containing LiBH₄ reactive composites.     

Theoretical discovery of high capacity hydrogen storage metal tetrahydrides

Hydrogen energy is attractive energy carrier due to its high energy density, abundant, environmentally friendly and renewable etc. However, the search for the high capacity hydrogen storage material is still a great challenge. In addition, the hydrogen storage materials should have excellent catalytic activity and superior mechanical properties to meet dehydrogenation and transportation. Here, we report on a novel metal tetrahydride that can effectively improve the hydrogen storage capacity. We obtain two novel metal tetrahydrides: TiH₄ and VH₄ based on the phonon dispersion and thermodynamically, respectively. In particular, those metal tetrahydrides not only exhibit good dehydrogenation behavior but also show superior mechanical properties. We demonstrate that the high hydrogen storage capacity of those tetrahydrides derives from the alternative stacking of metal layer and hydrogen layer. However, the excellent dehydrogenation process is attributed to the van der Waals interaction between hydrogen layers. Finally, the thermodynamic properties of TiH₄ and VH₄ are discussed.     

Recent progress on the development of high entropy alloys (HEAs) for solid hydrogen storage: A review

High entropy alloys (HEA) represent a kind of materials with unique structural and functional properties, and have attracted wide attentions in many fields including hydrogen storage. Due to the huge diversity in the composition of HEAs, novel hydrogen storage materials with superior comprehensive performance are expected to be developed following the concept, with some notable progress made in the past decade. In this study, the present research status in HEAs for hydrogen storage is summarized from the aspects of theoretical guide, composition and preparation, microstructure and hydrogen storage properties. Moreover, the key issues in future development and possible application scenarios are analyzed.     

Perspectives of high entropy alloys as hydrogen storage materials

Storage is a challenging issue that cuts across distribution, delivery, and safe end-uses of hydrogen as fuel. All the fuel cell vehicles are equipped with inefficient and unsafe high-pressure hydrogen cylinders. It is well known that storing such a highly flammable gas at high pressure is not safe. Only hydrogen can be stored safely as a form of metal hydrides, and all the investigated metal hydrides are inefficient in one way or another. Four essential hydrogen parameters for solid-state storage for fuel cell applications are high volumetric storage capacity, excellent heat transfer, and recharge time and feasible charging discharging temperatures. The available metal tanks have good gravimetric storage capacity but did not satisfy the prescribed criterion for good volumetric capacity necessary for mobile applications. Recently, some promising reports are published on the hydrogen storage properties of newly discovered High Entropy Alloys (HEAs). HEAs provide vast composition selection freedom for the formation of favorable simple solid solution phase for hydrogen storage. The four core effects of these alloys may also play a vital role in hydrogen storage properties. Here we reviewed and summarized the published results on hydrogen storage properties of HEAs to date. We underlined different essential aspects for the future development of HEAs as hydrogen storage materials. This review article discusses and describes the perspectives of HEAs in regards to the hydrogen storage applications of these alloys and will provide insight into the future development of hydrogen storage HEAs.     

Recent advances in catalyst-enhanced LiAlH4 for solid-state hydrogen storage: A review

LiAlH₄ is regarded as a potential material for solid-state hydrogen storage because of its high hydrogen content (10.5 wt%). However, its high decomposition temperature, slow dehydrogenation kinetics and irreversibility under moderate condition hamper its wider applications. Mechanical milling treatment and doping with a catalyst or additive has drawn significant ways to improve hydrogen storage properties of LiAlH₄. Microstructure or nanostructure materials were developed by using a ball milling technique and doping with various types of catalysts or additives which had dramatically improved the efficiency of LiAlH₄. However, the state-of-the-art technologies is still far from meeting the expected goal for the applications. In this paper, the overview of the recent advances in catalyst-enhanced LiAlH₄ for solid-state hydrogen storage is detailed. The remaining challenges and the future prospect of LiAlH₄–catalyst system is also discussed. This paper is the first systematic review that focuses on catalyst-enhanced LiAlH₄ for solid-state hydrogen storage.     

Modification of NaAlH4 properties using catalysts for solid-state hydrogen storage: A review

Energy is an essential requirement in our daily lives. Currently, most of our energy demands are fulfilled by fossil fuels. After 20 years, non-renewable fossil fuels are estimated to plummet rapidly. The world will face energy shortage and will seek for a new environmental method of energy generation for transportation, economy and application. Hydrogen is a fascinating energy carrier that is considered as ‘hydrogen economy’ for the future. The key challenge in developing the hydrogen economy is the context of hydrogen storage. Storing hydrogen via the solid-state method has received special attention and consideration because of its safety and larger storage capacity. A light complex hydride, NaAlH₄, is considered as an attractive material for solid-state hydrogen storage owing to its high hydrogen capacity, bulk in availability and low cost. Sluggish sorption kinetics and poor reversibility have driven research into various catalysts to enhance its hydrogen storage properties. This review article examines the development of different catalysts and their effects on the hydrogen storage properties of NaAlH₄. The addition of catalyst offers synergistic catalytic effect on the dehydrogenation performance of NaAlH₄. Doping NaAlH₄ with catalyst promote promising results such as lower decomposition temperature, improved kinetics and reduced activation energy. Superior performance on the dehydrogenation performance of NaAlH₄ doping with the catalyst may be due to the nanosized catalyst particle and in situ formed active species that may serve as nucleation sites at the surface of the NaAlH₄ matrix and benefiting the kinetics properties of NaAlH₄.     

Interactions of Y and Cu on Mg2Ni type hydrogen storage alloys: A study based on experiments and density functional theory calculation

Evolution of microstructure and hydrogen storage performances were studied in a Y substituted Mg₂₄Ni₁₀Cu₂ hydrogen storage alloy. Interactions of Y and Cu on the phase structure and hydrogen storage properties were explore. Substitution by Y refined the microstructure and yield existence of YMgNi₄. Furthermore, Y addition promoted the replacement of Cu for Ni in the Mg₂Ni.The study of the alloy's dehydrogenation performance and mechanism showed that the addition of Y did not alter the mechanism of random nucleation and subsequent growth, but reduced the activation energy of the dehydrogenation of the alloy from 77.4 kJ/mol to 67.6 kJ/mol. The thermodynamic energy of the dehydrogenation was also improved, and the enthalpy change (ΔH) and entropy change (ΔS) of the Mg₂NiH₄ phase decreased from 67.1 J/K/mol H₂ and 123.1 J/K/mol H₂ to 61.1 J/K/mol H₂ and 115.4 J/K/mol H₂, respectively. Furthermore, the density functional theory calculation showed that the addition of Y promoted the substitution of Cu for Ni, further reduced the stability of the main hydride Mg₂NiH₄, facilitated the release of hydrogen, and reduced the ΔH and ΔS of the hydride dehydrogenation.     

Effect of Fe substitution on first hydrogenation kinetics of TiFe-based hydrogen storage alloys after air exposure

This study investigated how Fe substitution with Ni, Co, Cu, Mn, and Cr affected the first hydrogenation behavior of air-exposed TiFe-based hydrogen storage alloys. The alloy ingots were crushed into powders and exposed to air for 1 h to analyze the first hydrogenation kinetics. Although Fe was substituted with up to 30% of Ni, Co, and Cu, the alloys had a single TiFe phase. In addition, the TiFe_0·7Ni_0·2Co_0.1 and TiFe_0·7Co_0·2Ni_0.1 alloys also had a single TiFe phase in spite of the simultaneous substitution. The composition of the oxide layer changed by the addition of Ni, Co, and Cu, but the alloys did not absorb hydrogen. In the TiFe_0·8Mn_0.2 and TiFe_0·8Cr_0.2 alloys, a dual-phase microstructure consisting of TiFe and Mn/Cr-rich C14 Laves phase was formed, with a larger amount in TiFe_0·8Cr_0.2. Both samples absorbed hydrogen after air exposure without any thermal activation process. Comparing the first hydrogenation kinetics, TiFe_0·8Cr_0.2 had a shorter incubation time and faster hydrogen absorption rate than TiFe_0·8Mn_0.2.     

New insights into the electrochemical and thermodynamic properties of AB-type ZrNi hydrogen storage alloys by native defects and H-doping: Computational experiments

In this study, we perform a computational experiment to inspect the impact of native Zr/Ni defects and H-doping atoms on the electrochemical and thermodynamic properties of the AB-type ZrNi alloys. The Korringa-Kohn-Rostoker (KKR) method integrated with the coherent potential approximation (CPA) was employed to execute the calculations. The results revealed that native Zr/Ni defects and hydrogen doping have a beneficial effect on the hydrogen storage properties of the studied compounds by decreasing the stability and decomposition temperature. In particular, we find that with an optimal concentration of native Zr/Ni defects and H-doping, the obtained values of the decomposition temperature are in accordance with the required values for the practical use of nickel-metal hydride (Ni-MH) batteries as a negative electrodes (253 to 318 K) as well as powering proton exchange membrane (PEM) fuel cells (289 to 393 K). Using the density of states (DOS), this decrease can be explained by the diminution of the number of Zr and Ni atoms that establish strong bonds with H atoms and by the shift of the total DOS toward the higher energies. The electrochemical capacity of Zr_1-x-yNiH_3+y and ZrNi_1-x-yH_3+y compounds increases to reach values of 550 and 540 mAh/g, respectively. These values are almost twice higher compared to the compounds currently used in the market based on the AB₅-type alloy LaNi₅ (300 mAh/g). These findings of enhanced electrochemical and thermodynamic properties could provide useful clues for the development of better ZrNi-based materials for Ni-MH batteries, PEM fuel cells and other related areas.     

Nanomaterials for on-board solid-state hydrogen storage applications

Hydrogen has the highest gravimetric density (energy density per unit mass) of any fuel. The combustion of hydrogen releases energy in the form of heat. When hydrogen reacts with oxygen in a fuel cell, the reaction releases energy in the form of electricity. Unlike hydrocarbon-based fuels, the generation of energy from either the combustion of hydrogen or the reaction of hydrogen with oxygen in a fuel cell is not accompanied by the emission of greenhouse gases. This makes hydrogen a promising solution to solve global warming issues. However, hydrogen has a low volumetric density (low energy density per unit volume) which makes storing or transporting hydrogen extremely difficult and expensive. To accelerate the utilization of hydrogen as an energy carrier, it is necessary to develop advanced hydrogen storage methods that have the potential to have a higher energy density.The hydrogen storage market is segmented by application into: (1) Stationary power: stored hydrogen is consumed for example in a fuel cell for use in backup power stations, refueling stations, power stations; (2) Portable power: hydrogen storage applications for electronic devices such as mobile phones, flash lights, and portable generators; and (3) Transportation: industries including automobiles, aerospace, unmanned aerial systems, and hydrogen tanks used throughout the hydrogen supply chain. The increasing development of light and heavy fuel cell vehicles is expected to drive the development of on-board solid-state hydrogen technologies.A large number of research groups worldwide for many years have been trying to develop materials having the right set of thermodynamic and kinetic properties, along with all of the physical properties (high gravimetric density, high volumetric density, etc.) to allow for low-pressure storage system in ambient conditions. However, to date, no material has been found that satisfies all the desired properties to be viably used in many applications. Even if we consider only three parameters namely gravimetric density, volumetric density, and system cost, no materials that can meet the ultimate targets of the U.S. Department of Energy (DOE) or the 2030 targets of the European Union's Fuel Cells and Hydrogen Joint Undertaking (FCH JU) and the New Energy and Industrial Technology Development Organization (NEDO) in Japan.The present article reviews advances in solid-state hydrogen storage technology and compares the opportunities and challenges of selected materials. The materials reviewed in this article have a wider spectrum than the materials reviewed in other existing articles, including carbon nanotubes (CNTs), metal–organic frameworks (MOFs), graphene, boron nitride (BN), fullerene, silicon, amorphous manganese hydride molecular sieve, and metal hydrides. Pioneering works, important breakthroughs, as well as the latest developments for promising materials are also reviewed.In addition, for the first time the targets set by several official regulatory agencies for solid-state hydrogen storage are summarized. Achievements in academic and industrial research are compared against these targets.The future prospects of promising materials are analyzed based on how its practical application can be implemented according to market needs.     

Understanding crystal structure roles towards developing high-performance V-free BCC hydrogen storage alloys

Current bottlenecks in the supply and high cost of V have negatively impacted their application. There is great interest in developing V-based hydrogen storage alloys that use less or free V. Here, we investigate the role of V in deliberately designed V-based alloys. Our results affirm that V plays an undeniable role in enhancing hydrogen storage properties. It is found that V maintains the stable single BCC structure but leads to more residual hydrogen (1.4 wt%) because of the high stability of the dihydride and smaller hydriding rate because of the small lattice parameter, which offers unexpected but encouraging perspectives towards reducing the need of V in such alloys. Mo substitution for V effectively alleviates the higher residual hydrogen to achieve a high dehydriding capacity of 2.5 wt%. Moreover, the suction-cast (Ti_0.46Cr_0.54)_97.5Mo_2.5 alloy, which keeps BCC structure after suction-cast process and contains a low-Mo content, also exhibits dehydriding capacity of 2.3 wt%. The enthalpy change as well as dehydriding capacity of V-Free alloys obtained were similar to those reported V-based alloys. These findings are attractive for developing new V-free BCC hydrogen storage alloys and higher hydrogen capacity.     

Dual-tuning the thermodynamics and kinetics: Magnesium-naphthalocyanine nanocomposite for low temperature hydrogen cycling

2,11,20,29-Tetra-tert-butyl-2,3-naphthalocyanine (TTBNc) was used as an alternative host to support magnesium (Mg) nanoparticles. After deposition and decomposition of dibutylmagnesium on TTBNc, Mg nanoparticles of around 4 nm supported on TTBNc were observed by TEM. These TTBNc stabilized Mg nanoparticles were found to absorb hydrogen at 100 °C and release hydrogen from 75 °C. The Mg-TTBNc material showed good hydrogen cycling properties and structural stability. Kinetic measurements showed fast hydrogen absorption within 2 min at 150 °C. The hydrogen desorption kinetics were slower at the same temperature but faster at 250 °C with 80% of the hydrogen desorbed within 1 h. Enthalpy and entropy for hydrogen uptake and release in Mg-TTBNc determined from PCT measurements were found to be of 52.7 ± 4.9 kJ mol^?1 H₂ and 107.8 ± 9.4 J K^?1 mol^?1 H_2, respectively. These values are much lower than those of bulk Mg.     

Effect of catalysts on microstructure,hydrogen storage thermodynamics,and kinetics performance of La5Mg85Ni10 alloy

The effects of the type and amount of transition metal catalyst on the microstructure and hydrogen storage performance of La₅Mg₈₅Ni₁₀ + x wt.% M (x = 1, 3, 5, 7; M = TiF₃, NbF₅, Cr₂O₃) alloys milled for 10 h have been investigated. The evolution of microstructure and phase of catalyzed alloys in the absorption/desorption process have been characterized by XRD and HRTEM. The results show that the hydrogen storage capacity of the alloy decreases as the catalyst increases. On the one hand, the catalytic effects of different amount of catalyst TiF₃ were studied. TiF₃ exists in form of MgF₂ and TiH₂ phases and Ea decreases firstly and then increases as the amount of TiF₃ increases. When 5 wt.% TiF₃ is added, the hydrogen desorption activation energy shows the lowest Ea = 45.2 kJ/mol. On the other hand, the catalytic effects of TiF₃, Cr₂O₃ and NbF₅ are compared in detail. It was found that TiF₃ has better catalytic effect than Cr₂O₃ and NbF₅ due to TiF₃ nanoparticles can refine the grains better, provide hydrogen diffusion channels and reduce the nucleation driving force of the alloys.     

Exploration of Ti substitution in AB2-type YZrFe based hydrogen storage alloys

To improve the hydrogen storage properties of YZrFe alloys, the alloying with Ti was carried out to obtain Y_0.7Zr_(0.3-x)Ti_xFe₂ (x = 0.03, 0.09, 0.1, 0.2) alloys by different processes. It was expected that Ti would substitute Zr and decrease the lattice constant of YFe₂-based C15 Laves phase. All YZrTiFe quaternary alloys consist of the main Y(Zr)Fe₂ phase and the minor YFe₃ phase. Despite the large solubility of Ti in Zr or Zr in Y, the Ti incorporation into YZrFe alloys results in the inhomogeneity of Y and the segregation of Ti, and thus decreases the hydrogen storage capacity. Only the alloy Y_0.7Zr_0.27Ti_0.03Fe₂ containing very few Ti shows the substitution of Ti to Zr and the resultant improvement in the dehydriding equilibrium pressure.     

Calcium hydrazinidoborane: Synthesis,characterization, and promises for hydrogen storage

Viewing calcium hydrazinidoborane Ca(N₂H₃BH₃)₂ (9.3 wt% H) as a potential hydrogen storage material, we long sought to synthesize it by solid-solid reaction of calcium hydride CaH₂ and hydrazine borane N₂H₄BH₃. However, it was elusive because of unsuitable experimental conditions. In situ synchrotron thermodiffraction helped us to identify the key role played by the temperature in the formation of the new phase. From 45 °C, new diffraction peaks appear, and the DSC analysis shows an exothermic signal. Thermal activation is thus required to make solid-state CaH₂ react with melted (liquid-state) N₂H₄BH₃. The XRD pattern can be indexed using a mixture of two phases: (i) unreacted CaH₂ as a minor phase (29 wt%) and (ii) the hitherto elusive Ca(N₂H₃BH₃)₂ (71 wt%). The as-formed Ca(N₂H₃BH₃)₂ crystallizes in a monoclinic Ic (No. 9) unit cell where the intermolecular interactions form chains (layers) along the a axis, resulting in intra-chain and inter-chain Ca⋅⋅⋅Ca distances as short as 4.39 and 7.04 Å respectively. Beyond 90 °C, Ca(N₂H₃BH₃)₂ decomposes, as evidenced by the diffraction peaks fading, an exothermic signal revealed by DSC, a weight loss (5.3 wt% at 200 °C) observed by TGA, and a gas release (H₂, and some N₂, NH₃, N₂H₄) monitored by MS. The as-formed thermolytic residue is amorphous and of complex polymeric composition. These results and the next challenges, are discussed herein.     

Improvement of Mg–Al alloys for hydrogen storage applications

In the context of energy carrier, storage of hydrogen is one of the key challenges for research today. The group of Mg-based hydrides stands as a promising candidate for competitive hydrogen storage with high reversible hydrogen capacity.     

Design of hydrogen storage alloys in view of chemical bond between atoms

The chemical interactions operating in hydrogen storage alloys are simulated by the DV-Xα molecular orbital method, using tetrahedral or octahedral model clusters. It is found that hydrogen interacts more strongly with hydride non-forming elements, B (e.g., Ni, Fe) than hydride forming elements, A (e.g., La, Ti, Mg), in agreement with our previous calculations of hydrogen storage alloys (e.g., LaNi₅, TiFe, Mg₂Ni). However, it is noted that such a B–H interaction is not dominant unless A elements exist in the neighborhood, so that both A and B are indeed essential elements for hydrogen storage alloys. Also, it is shown that the A/B compositional ratio of hydrogen storage alloys can be understood in terms of a simple parameter, 2 Bo(A–B)/[Bo(A–A)+Bo(B–B)], where the Bo(A–B), Bo(A–A) and the Bo(B–B) are the bond strengths between atoms given in the parentheses.     

Mg-based metastable nano alloys for hydrogen storage

Mg-based materials have been widely researched for hydrogen storage development due to the low price of Mg, abundant resources of Mg element in the earth's crust and the high hydrogen capacity (ca. 7.7 mass% for MgH₂). However, the challenges of poor kinetics, unsuitable thermodynamic properties, large volume change during hydrogen sorption cycles have greatly hindered the practical applications. Here in this review, our recent achievements of a new research direction on Mg-based metastable nano alloys with a Body-Centered Cubic (BCC) lattice structure are summarized. Different with other metals/alloys/complex hydrides etc. which involve significant lattice structure and volume change from hydrogen introduction and release, one unique nature of this kind of metastable nano alloys is that the lattice structure does not change obviously with hydrogen absorption and desorption, which brings interesting phenomenon in microstructure properties and hydrogen storage performances (outstanding kinetics at low temperature and super high hydrogen capacity potential). The synthesis results, morphology and microstructure characterization, formation evolution mechanisms, hydrogen storage performances and geometrical effect of these metastable nano alloys are discussed. The nanostructure, fresh surface from ball milling process and fast hydrogen diffusion rate in BCC lattice structure, as well as the unique nature of maintaining original BCC metal lattice during hydrogenation result in outstanding hydrogen storage performances for Mg-based metastable nano alloys. This work may open a new sight to develop new generation hydrogen storage materials.     

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.