Progress of graphene and loaded transition metals on Mg-based hydrogen storage alloys

Mg-based hydrogen storage alloys have become a research hotspot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, its high thermodynamic stability and slow kinetics limit its application, so the modification of Mg-based hydrogen storage alloys has become the development direction of Mg-based alloys. Transition metals can be used as catalysts for the dehydrogenation of hydrogen storage alloys due to their excellent structural, electrical, and magnetic properties. Graphene, because of its unique sp² hybrid structure, excellent chemical stability, and a specific surface area of up to 2600 m²/g, can be used as a support for transition metal catalysts. In this paper, the internal mechanism of graphene as a catalyst for the catalysis of Mg-based hydrogen storage alloys was analyzed, and the hydrogen storage properties of graphene-catalyzed Mg-based hydrogen storage alloys were reviewed. The effects of graphene-supported different catalysts (transition metal, transition metal oxides, and transition metal compounds) on the hydrogen storage properties of Mg-based hydrogen storage alloys were also reviewed. The results showed that graphene played the roles of catalysis, co-catalysis, and inhibition of grain aggregation and growth in Mg-based hydrogen storage materials.     

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.     

Geometrical effect in Mg-based metastable nano alloys with BCC structure for hydrogen storage

Mg-based materials are thought to be promising candidates for future hydrogen storage applications due to the low cost, abundant resources and large hydrogen storage capacity. However, they suffer from the challenges of sluggish kinetics and large volume change after hydriding/dehydriding (H/D) process. In order to address the problems, we successfully synthesized the Mg-based Body-Centered Cubic (BCC) metastable nano alloys with much improved kinetics while almost no obvious structure change after H/D process. In this work, the obtained Mg₅₅Co₄₅ metastable alloy with BCC structure can reach a hydrogen storage capacity of 3.24 wt% (hydrogen per metal or H/M = 1.28, H/Mg = 2.33) at −15 °C and this absorption temperature in Mg-based BCC structure is the lowest temperature reported for Mg-based materials to absorb hydrogen. Importantly, the BCC structure is maintained without obvious metal lattice change after H/D process. Nevertheless, the potential uptake of about 20 wt% theoretical hydrogen capacity (H/M = 9) for this unique BCC structure cannot be reached up to now. Herein, we discuss the mechanism from the geometrical effect aspect to figure out the difference between the experimental hydrogen storage capacity (H/M = 1.28) and the theoretical one (H/M = 9).     

Metal hydride materials for solid hydrogen storage: A review

Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides.     

Hydrogen storage by Mg-based nanocomposites

Hydrogen storage materials research is entered to a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities and the electrochemical discharge capacities of the Mg₂(Ni, Cu)-, LaNi₅-, ZrV₂-type nanocrystalline alloys and Mg₂Ni/LaNi₅-, Mg₂Ni/ZrV₂-type nanocomposites have been measured. The electronic properties of the Mg₂Ni_1-xCu_x, LaNi₅ and ZrV₂ alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)₂Ni (50 wt%)-La(Ni,Mn,Al,Co)₅ (50 wt%) and (Mg,Mn)₂Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)_2.4 (25 wt%) materials releases 1.65 wt% and 1.38 wt% hydrogen at 25 °C, respectively. The strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence hydrogenation properties of Mg-based nanocomposities.     

The effect of Na addition on the first hydrogen absorption kinetics of cast hypoeutectic Mg–La alloys

With superior properties of Mg such as high hydrogen storage capacity (7.6 wt% H/MgH₂), low price, and low density, Mg has been widely studied as a promising candidate for solid-state hydrogen storage systems. However, a harsh activation procedure, slow hydrogenation/dehydrogenation process, and a high temperature for dehydrogenation prevent the use of Mg-based metal hydrides for practical applications. For these reasons, Mg-based alloys for hydrogen storage systems are generally alloyed with other elements to improve hydrogen sorption properties. In this article, we have added Na to cast Mg–La alloys and achieved a significant improvement in hydrogen absorption kinetics during the first activation cycle. The role of Na in Mg–La has been discussed based on the findings from microstructural observations, crystallography, and first principles calculations based on density functional theory. From our results in this study, we have found that the Na doped surface of Mg–La alloy systems have a lower adsorption energy for H₂ compared to Na-free surfaces which facilitates adsorption and dissociation of hydrogen molecules leading to improvement of absorption kinetic. The effect of Na on the microstructure of these alloys, such as eutectic refinement and a density of twins is not highly correlated with absorption kinetics.     

Catalysis and evolution on cycling of nano-structured magnesium multilayer thin films

This paper explores the hydrogen cycling properties of Mg/Cr and Mg/V multilayer thin films and studies the effect of chromium and vanadium transition metal catalysts on the cycling properties of thick magnesium coatings. Two transition-metal catalysed magnesium-based multilayer PVD coatings are compared with a non-catalysed magnesium control sample. The (micro-)structural evolution of the thin film coatings into fine, flakey powders is studied in-depth using XRD, SEM and TEM and the hydrogen storage properties of all three materials are assessed using volumetric, gravimetric and calorimetric methods focussing on the effect of the microstructure and composition of the coatings on the hydrogen storage kinetics. It was found that the chromium-catalysed coating had the most favourable hydrogen storage kinetics with an activation energy for the dehydrogenation reaction of 65.7 ± 2.5 kJ mol⁻¹ and a hydrogen capacity of 6.1 ± 0.3 wt%. The mechanism of the dehydrogenation reaction of the catalysed samples was studied using the CV and JMAK kinetic models and it was found that the catalyst material influenced not only the hydrogen storage kinetics but also the mechanism of the reaction.     

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.     

Development of new type Ni-Mg texture-like structure hydrogen absorber

This study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by X-ray diffraction (XRD) and the formed hydride tetragonal-MgH₂ was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H₂ at 25 °C. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding-dehydriding kinetics.     

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.     

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.     

The study of amorphous La@Mg catalyst for high efficiency hydrogen storage

Amorphous catalysts have a large number of catalytic active sites. Here, we report a magnesium composite trace lanthanum catalyst (La@Mg), in which La and Mg layers form amorphous Mg–La compound on the surface of layered Mg. The test shows this La@Mg has hydrogen storage capacity of about 7.6 wt% and hydrogen desorption of 7.2 wt%, higher than that of crystalline La@Mg and sole Mg, rapid absorption/desorption kinetic and stable reversible absorption/desorption cycles. La@Mg exhibits an optimistic hydrogen storage performance than Mg-based materials previously reported in the literature. Combined with theoretical calculations, it is shown that the amorphous Mg–La has an catalysis on hydrogen storage performance of La@Mg system, which contributing to the dispersion of Mg and providing channels for hydrogen diffusion, facilitating hydrogenation by accelerating H atoms diffuse between the subsurface and the surface. This work provides experiment and mechanism guidance for the development of efficient hydrogen storage materials.     

Hydriding behavior of Mg-50 wt% ZrCrFe composite Prepared by high energy ball milling

Magnesium hydride has a high theoretically storage capacity, which amounts to 7.6 wt%. It is therefore a promising candidate for hydrogen storage applications. However, its major drawback is its high desorption temperature of well over 300 °C, which is related to the high stability of the Mg–H bonds and expressed in the high enthalpy of hydride formation (77 kJ/mol). The preparation of Mg composites with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties of Mg. Thus we prepared Mg-50 wt% ZrCrFe alloy composite by high energy ball milling under argon atmosphere. X-ray diffraction (XRD) studies on the composite before and after hydriding cycles suggest no intermetalic phase is formed between Mg and the elements of the alloy. The morphological studies carried on by Scanning Electron Microscope (SEM) technique suggest that the alloy particles are homogeneously distributed throughout the Mg surface. A particle reduction after hydrogenation is also visible. Hydriding/dehydriding properties of the composites are investigated by PCT measurements using a dynamic system. The maximum hydrogen capacity for this composite is found to be 4.5 wt%. The reaction kinetics have also been recorded in a temperature range from RT to 300 °C and the thermodynamic parameters calculated from Van’t Hoff plot. From the results it is found that the alloy reacts with hydrogen also when cooled to room temperature while at higher temperature it works as catalyst.     

Hydrolysis of Mg-based alloys and their hydrides for efficient hydrogen generation

The hydrolysis of Mg-based alloys and their hydrides with high abundance on the earth and low cost could produce hydrogen with high theoretical capacity and the formation of by-products that have no pollution to the environment. Hence, it has been regarded as one of the most promising way for hydrogen generation. Particularly, a gravimetric capacity of 6.4 wt% and 3.4 wt% H₂ could be produced from the hydrolysis of pure Mg and MgH₂, respectively, even when stoichiometric water is included for calculation. The formation of passive magnesium hydroxides with dense structure, however, could immediately interrupt the hydrolysis reaction of Mg/MgH₂, which leads to ultralow yield and sluggish hydrogen generation rate. Recent studies have demonstrated that the hydrolysis reaction of Mg/MgH₂ could be effectively enhanced in terms of both yield and kinetics by the formation of Mg-based alloys and their hydrides. This review aims to summarize the recent progress in the hydrolysis of Mg-based alloys and their hydrides and the involved hydrolysis mechanisms.     

The role of spark plasma sintering on the improvement of hydrogen storage properties of Mg-based composites

Spark plasma sintering (SPS) is a newly developed material preparation technology and is very suitable for the multi-component and/or dissimilar materials preparation. In this paper, Mg–V_77.8Zr_7.4Ti_7.4Ni_7.4, Mg–V₃₈Zr₂₅Ti₁₅Ni₂₂ and Mg–ZrMn₂ composites were synthesized by SPS method and their hydrogen storage properties were evaluated. The results showed that with the addition of the second alloys, the hydrogen desorption temperature of pure Mg decreased apparently, with the reversible hydrogen storage capacity increased from nearly 0 of pure Mg to near 95% of its total absorption at 573 K. The hydrogen ab/desorption kinetics were also greatly improved, with the hydrogen absorption mechanism changed from surface reaction of pure Mg to three-dimension diffusion of the composite. TEM observation indicated that a thin transition zone of nanocrystalline Mg was produced at the sintering interface during SPS, which may be responsible for the improvement of hydrogen storage properties of these Mg-based composites.     

Achieving superior hydrogen storage properties via in-situ formed nanostructures: A high-capacity Mg–Ni alloy with La microalloying

Mg-based hydride is a promising hydrogen storage material, but its capacity is hindered by the kinetic properties. In this study, Mg–Mg₂Ni–LaH_x nanocomposite is formed from the H-induced decomposition of Mg₉₈Ni_1·67La_0.33 alloy. The hydrogen capacity of 7.19 wt % is reached at 325 °C under 3 MPa H₂, attributed to the ultrahigh hydrogenation capacity in Stage I. The hydrogen capacity of 5.59 wt % is achieved at 175 °C under 1 MPa H₂. The apparent activation energies for hydrogen absorption and desorption are calculated as 57.99 and 107.26 kJ/mol, which are owing to the modified microstructure with LaH_x and Mg₂Ni nanophases embedding in eutectic, and tubular nanostructure adjacent to eutectic. The LaH_2.49 nanophase can catalyze H₂ molecules to dissociate and H atoms to permeate due to its stronger affinity with H atoms. The interfaces of these nanophases provide preferential nucleation sites and alleviate the “blocking effect” together with tubular nanostructure by providing H atoms diffusion paths after the impingement of MgH₂ colonies. Therefore, the superior hydrogenation properties are achieved because of the rapid absorption process of Stage I. The efficient synthesis of nano-catalysts and corresponding mechanisms for improving hydrogen storage properties have important reference to related researches.     

Mg-based composites for enhanced hydrogen storage performance

Hydrogen storage in solids of hydrides is advantageous in comparison to gaseous or liquid storage. Magnesium based materials are being studies for solid-state hydrogen storage due to their advantages of high volumetric and gravimetric hydrogen storage capacity. However, unfavorable thermodynamic and kinetic barriers hinder its practical application. In this work, we presented that kinetics of Mg-based composites were significantly improved during high energy ball milling in presence of various types of carbon, including plasma carbon produced by plasma-reforming of hydrocarbons, activated carbon, and carbon nanotubes. The improvement of the kinetics and de-/re-hydrogenation performance of MgH₂ and TiC-catalysed MgH₂ by introduction of carbon are strongly dependent on the milling time, amount of carbon and carbon structure. The lowest dehydrogenation temperature was observed at 180 °C by the plasma carbon–modified MgH₂/TiC. We found that nanoconfinement of carbon structures stabilised Mg-based nanocomposites and hinders the nanoparticles growth and agglomeration. Plasma carbon was found to show better effects than the other two carbon structures because the plasma carbon contained both few layer graphene sheets that served as an active dispersion matrix and amorphous activated carbons that promoted the spill-over effect of TiC catalysed MgH₂. The strategy in enhancing the kinetics and thermodynamics of Mg-based composites is leading to a better design of metal hydride composites for hydrogen storage.     

Structural evolution and hydrogen storage performance of Mg3LaHn (n = 9–20)

The magnesium-lanthanum-hydrogen systems possess the goodish stability and high hydrogen storage capacity, which make them perspective as commercial Mg-based hydrogen storage materials. The exploration of these intriguing properties evolving from small atomic and molecule cluster to bulk phase are, to our knowledge, the longstanding challenge. Here, we perform a theoretical study on the structural and electronic properties of Mg₃LaH_n (9 ≤ n ≤ 20) clusters by using the Crystal Structure AnaLYsis by Particle Swarm Optimization method combined with density functional theory calculations. It is found that Mg₃LaH₁₅ is the most stable cluster in the series, with hydrogen storage capacity of 6.6 wt% and adsorption energy of 2.76 eV. The present results offer new insights for the design and synthesis of novel hydrogen storage materials in the future.     

Hydrogen storage in Mg: A most promising material

In the last one decade hydrogen has attracted worldwide interest as an energy carrier. This has generated comprehensive investigations on the technology involved and how to solve the problems of production, storage and applications of hydrogen. The interest in hydrogen as energy of the future is due to it being a clean energy, most abundant element in the universe, the lightest fuel and richest in energy per unit mass. It will provide, Cheap Electricity, Cook Food, Drive Car, Run Factories, Jet Planes, Hydrogen Village and for all our domestic energy requirements. In short hydrogen shows the solution and also allows the progressive and non-traumatic transition of today's energy sources, towards feasible safe reliable and complete sustainable energy chains. The present article deals with the hydrogen storage in metal hydrides with particular interest in Mg as it has potential to become one of the most promising storage materials. Many metals combine chemically with Hydrogen to form a class of compounds known as Hydrides. These hydrides can discharge hydrogen as and when needed by raising their temperature or decreasing the external pressure. An optimum hydrogen-storage material is required to have various properties viz. high hydrogen capacity per unit mass and unit volume which determines the amount of available energy, low dissociation temperature, moderate dissociation pressure, low heat of formation in order to minimize the energy necessary for hydrogen release, low heat dissipation during the exothermic hydride formation, reversibility, limited energy loss during charge and discharge of hydrogen, fast kinetics, high stability against O₂ and moisture for long cycle life, cyclibility, low cost of recycling and charging infrastructures and high safety. So far the most of hydrogen storage alloys such as LaNi₅, TiFe, TiMn₂, have hydrogen storage capacities, not more than 2 wt% which is not satisfactory for practical application as per DOE Goal. A group of Mg based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity upto 7.6 wt% for on board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system as well as by ball milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower desorption temperatures, however, the state of the art materials are still far from meeting the aimed target for their transport applications. Therefore further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides. In the present article the possibility of commercialization of Mg based alloys has been discussed.     

Improved hydrogen cycling kinetics of nano-structured magnesium/transition metal multilayer thin films

A magnesium-based nano-structured multilayer material was developed by magnetron-assisted physical vapour deposition with promising hydrogen storage properties. The material has a reversible capacity of 4.6 wt% at temperatures between 250 and 350 °C and hydrogenates in <10 min at 250 °C. An activation energy of the dehydrogenation reaction of E_a = 71.6 kJ mol⁻¹ was measured by differential scanning calorimetry. Structural analysis by TEM and SEM showed that the thin magnesium layers of 16.5 nm thickness interspersed with 2.5 nm of an amorphous, nickel-rich transition metal mix resulted in a favourable nanostructure after hydrogen cycling at up to 350 °C. The material also retained its fast kinetics and capacity for the 50 cycles that the material underwent. Comparison of XRD data with TEM shows that the layer thickness of such nano-structured, directional Mg layers in PVD multilayers can be reliably estimated by XRD. In addition the XRD texture relates to the microstructural evolution of the multilayered structure pre- and post-cycling.