Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

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 storage in carbon/Mg nanocomposites synthesized by ball milling

Carbon nanocomposites obtained by ball milling of graphite and magnesium with organic additives (benzene or cyclohexane) under different conditions have been studied with the aim of preparing novel hydrogen storage materials. It has been proved by thermal desorption spectrometry (TDS) and neutron diffraction measurements that the hydrogen taken up by the nanocomposites exists in at least two states; the one is the hydrogen strongly associated with the carbon component and the other the hydride in the magnesium component. The ball milling resulted in the generation of large amounts of dangling carbon bonds in graphite, which acted as active sites to take up the hydrogen. When _D2${\mathrm{D}}_2$ gas was brought into contact with such composites, the isotope exchange reaction with the hydrogen in the magnesium hydride occurred at 453 K, and not with the hydrogen associated with the carbon. The properties of such hydrogen taken up were also discussed from the standpoint of isotope effects.     

Hydrogen storage behavior of magnesium catalyzed by nickel-graphene nanocomposites

In present study nanocomposites of Graphene Like Material (GLM) and nickel containing 5–60 wt % Ni were prepared by a co-reduction of graphite oxide and Ni²⁺ ions. These nanocomposites served as effective catalysts of hydrogenation-dehydrogenation of magnesium based materials and showed a high stability on cycling. Composites of magnesium hydride with Ni/GLM were prepared by high-energy ball milling in hydrogen. The microstructures and phase compositions of the studied materials were characterized by XRD, SEM and TEM showing that Ni nanoparticles have size of 2–5 nm and are uniformly distributed in the composites. The kinetic curves of hydrogen absorption and desorption by the composites were measured using a Sievert's type laboratory setup and were analyzed using the Avraami – Erofeev approach. The re-hydrogenation rate constants and the Avraami exponents fitting the kinetic equations for the Mg/MgH₂+Ni/GLM composites show significant changes as compared to the Mg/MgH₂ prepared at the same conditions and this difference has been assigned to the changes in the mechanism of nucleation and growth and alteration of the rate-limiting steps of the hydrogenation reaction. The composites of Mg with Ni/GLM have a high reversible hydrogen capacity exceeding 6.5 wt % H and also show high rates of hydrogen absorption and desorption and thus belong to the promising hydrogen storage materials.     

Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

Hydrogen production via hydrolysis reaction from ball-milled Mg-based materials

Hydrogen generation by hydrolysis of Mg and MgH₂${}_2$ has been investigated in pure water and 1 M KCl. It has been found that hydrolysis reaction of Mg and Mg–Ni composite, both obtained by high-energy ball milling, is faster and extensive when they are immersed in 1 M KCl. In contrast, milled Mg and Mg–Ni composite in pure water, MgH₂${}_2$ and MgH₂${}_2$–Ni composites in pure water and in 1 M KCl show low yield and reactivity. Hydrolysis kinetics and yield are maximum with Mg–10 at% Ni composite milled for 30 min, so reaction is fully completed within an hour in the presence of chloride ions. It is related to the creation of micro-galvanic cells between Mg and dispersed Ni elements, accentuating greatly Mg corrosion in highly conductive aqueous media. A significant increase of the H₂${}_2$ production is also observed with 30 min milled Mg sample, likely because of the accentuation in the pitting corrosion resulting from the creation of numerous defects and fresh surfaces through the milling process. On the other hand, intensive ball milling of pure magnesium has no effect on the Mg reactivity in pure water. Ball milling effect is likely masked by the significant Mg passivation in pure water. A correlation is established between the conversion yield of ball-milled MgH₂${}_2$ powder in pure water and its effective surface area, which is increased by the milling process. Ni addition has no effect on the hydrolysis reaction in nonconductive media (i.e. pure water) and with nonconductive material (i.e. MgH₂${}_2$).     

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

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

Hydrogen storage and thermal conductivity properties of Mg-based materials with different structures

Mg-based hydrogen storage materials can be very promising candidates for stationary energy storage application due to the high energy density and low cost of Mg. Hydrogen storage kinetics and thermal conductivity are two important factors for the material development for this kind of application. Here we studied several types of Mg-based materials with different structure-micrometer scale Mg powders, Mg nanoparticles, single crystal Mg, nanocrystalline Mg₅₀Co₅₀ BCC alloy and Mg thin film samples. It seems the Mg materials with good kinetics usually are the ones with nanostructure and tend to show poor thermal conductivity due to electron/phonon scattering resulting from more interfaces and boundaries in nanomaterials. Based on this work, good crystallinity Mg phase incorporated in carbon nano framework could be one promising option for energy storage.     

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.     

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 storage in binary and ternary Mg-based alloys: A comprehensive experimental study

This study focused on hydrogen sorption properties of 1.5 μm thick Mg-based films with Al, Fe and Ti as alloying elements. The binary alloys are used to establish as baseline case for the ternary Mg–Al–Ti, Mg–Fe–Ti and Mg–Al–Fe compositions. We show that the ternary alloys in particular display remarkable sorption behavior: at 200 °C the films are capable of absorbing 4–6 wt% hydrogen in seconds, and desorbing in minutes. Furthermore, this sorption behavior is stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys. Even after 100 absorption/desorption cycles, no degradation in capacity or kinetics is observed. For Mg–Al–Fe, the properties are clearly worse compared to the other ternary combinations. These differences are explained by considering the properties of all the different phases present during cycling in terms of their hydrogen affinity and catalytic activity. Based on these considerations, some general design principles for Mg-based hydrogen storage alloys are suggested.     

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.     

Gd3Fe5O12/MgAl-LDH nanocomposites: Sonochemical synthesis,optimization and electrochemical hydrogen storage studies

The development of an eco-friendly and pollution-free hydrogen storage cell power system has received considerable research attention in recent years. Several prominent developments in energy storage mechanisms have been made during the last decade, influencing innovation, exploration, and the probable path for improving energy storage knowledge. We propose a hydrogen energy storage system based on novel electrode materials and electrochemical methods. A series of nanocomposites based on MgAl-LDH and Gd₃Fe₅O₁₂ garnet were designed as active materials. Ultrasonic radiation was used for the synthesis of Gd₃Fe₅O₁₂/MgAl-LDH nanocomposites. Structures of Gd₃Fe₅O₁₂ without any impurities were achieved by sonication power of 90 W/cm² while the synthetic sample without sonication power led to the synthesis of Gd₃Fe₅O₁₂ in the presence of GdFeO₃ phase. The hydrogen storage capacity for pristine MgAl-LDH and Gd₃Fe₅O₁₂ was measured at 213 and 388 mAhg⁻¹ after 15 cycles, respectively. Then, capacity for Gd₃Fe₅O₁₂/MgAl-LDH nanocomposites increased to 316 mAhg⁻¹ at current of 1 mA in 15th cycles. Newly developed electrode materials such as Gd₃Fe₅O₁₂/MgAl-LDH with mechanisms such as spillover, redox and physical adsorption are excellent candidates for energy storage power systems.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

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.     

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.     

Optimization of TiH2 content for fast and efficient hydrogen cycling of MgH2-TiH2 nanocomposites

Magnesium hydride is extensively examined as a hydrogen store due to its high hydrogen content and low cost. However, high thermodynamic stability and sluggish kinetics hinder its practical application. To overcome this last drawback, different Ti amounts (y = 0, 0.025, 0.05, 0.1, 0.2 and 0.3) were added to magnesium to form (1-y)MgH₂+yTiH₂ nanocomposites (NC) by reactive ball milling under hydrogen gas. Thermodynamic stability of the MgH₂ phase in NCs was determined using a manometric Sieverts rig. Reversible hydrogen capacity and reaction kinetics were determined at 573 K over 20 sorption cycles under a limited reaction time of 15 min. On increasing Ti amount, reaction kinetics are enhanced both in absorption and desorption leading to a higher reversibility for hydrogen storage with the MgH₂ phase. However, titanium increases the molar weight of NCs and forms irreversible titanium hydride. The highest reversible capacity (4.9 wt% H) was obtained for the lowest here studied TiH₂ content (y = 0.025).     

Hydrogen storage measurements in novel Mg-based nanostructured alloys produced via rapid solidification and devitrification

Nanostructured materials for hydrogen storage with a composition of Mg₈₅Ni_15−xM_x (M = Y or La, x = 0 or 5) are formed by devitrification of amorphous and amorphous-nanocrystalline precursors produced by melt-spinning. All three compositions exhibit a maximum storage capacity of about 5 mass % H at 573 K. When ball-milled for 30 min in hexanes, the binary alloy can be activated (first-cycle hydrogen absorption) at 473 K. DSC experiments indicate that desorption in this sample begins at 525 K, compared to 560 K when the material is activated at 573 K; which indicates an improvement in the hydride reaction thermodynamics due to capillarity effects. Additions of Y and La improve the degradation in storage capacity observed during cycling of the binary alloy by slowing microstructural coarsening. Alloying with La also shows a decrease of about 8 kJ/mol and 5 kJ/mol in the enthalpy of reaction for MgH₂ and Mg₂NiH₄ formation, respectively, compared to the binary alloy; resulting in some desorption of H₂ at 473 K. The improved thermodynamics are discussed in terms of destabilization of the hydrides relative to new equilibrium phases introduced by alloying additions. The proposed hydriding reaction for the La-containing material is in agreement with previously reported experimental results.     

Hydrogen generation via alcoholysis reaction using ball-milled Mg-based materials

An investigation on the hydrogen generation by reacting ball-milled Mg-based materials in different alcoholic solutions (methanol, ethanol, 2-propanol) was performed. The MgH₂ reactivity in methanol is very low (maximum conversion yield <10%$<10\%$) and no improvement is induced by the ball milling treatment. In contrast, the ball milling affects greatly the Mg reactivity in methanol. The Mg powder milled for 30 min displays a maximum conversion yield of 47% compared to 3% for unmilled Mg powder. Its high reactivity is ascribed to the creation of numerous defects and fresh surfaces during the initial stage of the milling process, favoring the corrosion of Mg in methanol. In addition, the presence of water in the methanol solution inhibits drastically the alcoholysis reaction despite its low amount (0.3 vol%). The higher hydrogen production is obtained with the composite Mg-10 at% Ni milled for 30 min leading to a conversion yield of 70% after 45 min of reaction in methanol, which corresponds to a hydrogen gravimetric yield of 4.5 wt% (including Ni mass and excluding methanol mass). The positive effect of Ni addition on the yield and kinetics of the alcoholysis reaction is explained by the creation of micro-galvanic cells between Mg and Ni components. No hydrogen is released from the decomposition of milled Mg powder in ethanol and 2-propanol solutions.     

Hydrogen storage in destabilized borohydride materials

Several new destabilized borohydride systems were prepared by mechanochemical synthesis and characterized to determine their suitability for hydrogen storage. The mixtures included: Mg(BH₄)₂/Ca(BH₄)₂; Mg(BH₄)₂/CaH₂/3NaH; and Mg(BH₄)₂/CaH₂; systems as well as a double cation hydride MnLi(BH₄)₃. Temperature programmed desorption, TPD, analyses showed that the desorption temperature of Mg(BH₄)₂ can be lowered by ball milling it with Ca(BH₄)₂. The resulting mixture absorbed and released hydrogen with the pressure composition temperature, PCT, isotherm displaying a well-defined plateau region. The other two systems; Mg(BH₄)₂/CaH₂ and Mg(BH₄)₂/CaH₂/NaH, can also absorb and release hydrogen. The desorption enthalpies are all in the 84–88 kJ/mol range. These systems, however, are only partially reversible and lose some of their hydrogen-holding capacity after the initial desorption. A plausible explanation for this is that the mechanisms involve the formation of a (B₁₂H₁₂)⁻²-containing intermediate which has a high kinetic barrier to re-hydrogenation. TPD analysis also showed that the double cation material, MnLi(BH₄)₃ can release hydrogen in the range of 130 °C but the process is irreversible. A Kissinger analysis of the first decomposition step in the differential thermal analysis, DTA, data showed that the activation energies for all the Mg(BH₄)₂-based borohydrides range from 115 to 167 kJ/mol.