Improvement of substituting La with Ce on hydrogen storage thermodynamics and kinetics of Mg-based alloys

The rare earth elements are believed to catalyze the reversible reaction between magnesium and hydrogen and reduce the thermal stability of MgH₂ by weakening the Mg–H bond. This study focuses on investigating the effect of Ce partial substitution of La on the comprehensive hydrogen storage performances of La_10-xCe_xMg₈₀Ni₁₀ (x = 0–4) alloys (prepared by vacuum induction melting). The phase composition and microstructure were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The thermodynamics and kinetics of isothermal reactions were measured by the automatic Sievert apparatus. Non-isothermal dehydrogenation performance of the alloys was researched by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). All the experimental alloys have a large capacity of hydrogen absorption and desorption and the kinetics of the Ce containing alloys is better. The additive Ce exists in the solid solution of alloy and results in the refinement of grain, making the stability of the hydride visibly lower, which is the reason for the decline in the initial dehydrogenation temperature and enthalpy (ΔH) of the hydride. Besides, the dehydrogenation activation energy of the alloys is distinctly reduced by composition adjustment, which indicates the improved hydrogen storage performances.     

Improvement on hydrogen storage thermodynamics and kinetics of the as-milled SmMg11Ni alloy by adding MoS2

In this experiment, the Mg-based hydrogen storage alloys SmMg₁₁Ni and SmMg₁₁Ni + 5 wt.% MoS₂ (named SmMg₁₁Ni-5MoS₂) were prepared by mechanical milling. By comparing the structures and hydrogen storage properties of the two alloys, it could be found that the addition of MoS₂ has brought on a slight change in hydrogen storage thermodynamics, an obvious decrease in hydrogen absorption capacity, an obvious catalytic action on hydrogen desorption reaction, and a lowered onset desorption temperature from 557 to 545 K. Additionally, the addition of MoS₂ could dramatically improve the alloy in its hydrogen absorption and desorption kinetics. To be specific, the hydrogen desorption times of 3 wt.% H₂ at 593, 613, 633 and 653 K were measured to be 1488, 683, 390 and 192 s respectively for the SmMg₁₁Ni alloy, which were reduced to 938, 586, 296 and 140 s for the MoS₂ catalyzed SmMg₁₁Ni alloy at identical conditions. The activation energies of the alloys with and without MoS₂ for hydrogen desorption are 87.89 and 100.31 kJ/mol, respectively. The 12.42 kJ/mol decrease is responsible for the ameliorated hydrogen desorption kinetics by adding catalyst MoS₂.     

Improved hydrogen storage kinetics of Mg-based alloys by substituting La with Sm

Element substitution is an effective strategy for improving Mg-based alloys in their hydrogenation/dehydrogenation property. Thereby, in this paper, Sm was selected to partially replace La in a La–Mg-based alloy for improving its hydriding and dehydriding performance. The alloys with the compositions of Mg₈₀Ni₁₀La_10-xSm_x (x = 0–4) were manufactured through vacuum induction melting. Their microstructures and phase compositions were measured by XRD, SEM and HRTEM. The isothermal hydrogen storage property was tested through an automatic Sieverts apparatus. Non-isothermal hydrogen desorption performance was measured through TGA and DSC. Arrhenius and Kissinger methods were adopted to calculate the dehydrogenation activation energy of alloys. The results reveal that all of the experimental alloys can reversibly absorb and release a large amount of H₂ at appropriate temperatures. The substitution of Sm for La ameliorates the hydriding and dehydriding kinetics, but it results in an undesired reduction of hydrogen absorption and desorption capacities. Substituting La by Sm decreases the initial hydrogen release temperature of the hydride visibly. Furthermore, substituting Sm for La engenders the dehydrogenation activation energy decline clearly, which is considered as the main reason for the improved hydrogen desorption kinetics resulted from Sm replacing La.     

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.     

Improved hydrogen storage kinetics of nanocrystalline and amorphous Pr-Mg-Ni-based PrMg12-type alloys synthesized by mechanical milling

In this paper, the nanocrystalline and amorphous PrMg₁₁Ni + x wt.% Ni (x = 100, 200) alloys were synthesized by mechanical milling. The gaseous and electrochemical hydrogen storage performances were studied in detail. The results reveal that increasing Ni content facilitates the glass forming of the alloys, and it significantly improves the gaseous and electrochemical hydrogen storage kinetics performance. Furthermore, milling time varying significantly affects the hydrogen storage properties of the alloys. The hydrogen capacity of the alloys first increases and then decreases with milling time prolonged. The hydriding rate and high-rate discharge ability (HRD) of the as-milled alloys have maximum values with milling time varying. But dehydriding rate always increases with milling time prolonged. The improved gaseous hydrogen storage kinetics of alloys are convinced to be ascribed to a reduction in hydrogen desorption activation energy caused by increasing Ni content and prolonging milling time.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of ball-milled Mg90Y1.5Ce1.5Ni7 alloy

In this work, the Mg₉₀Y_1.5Ce_1.5Ni₇ sample is successfully prepared by combining the vacuum induction melting and the mechanical milling. The phase composition and microstructure characteristics are studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The hydrogenated sample is composed of MgH₂, Mg₂NiH₄, CeH_2.73 phases, whereas only the MgH₂ and Mg₂NiH₄ phases are decomposed during dehydrogenation. The hydrogen storage properties of Mg₉₀Y_1.5Ce_1.5Ni₇ samples are measured by semi-automatic Sievert type apparatus. It is found that the samples could be fully activated within three cycles of absorption and dehydrogenation, with a reversible hydrogen storage capacity of about 5.6 wt%. Also, the “optimal hydrogenation temperature” is reduced to 200 °C, and the dehydrogenation activation energy is calculated to be 68.2 kJ/mol and 65.8 kJ/mol by using the Arrhenius and Kissinger equations, respectively. This work provides a scientific approach to promote the practical application of Mg-based alloy.     

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 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.     

Enhanced electrochemical kinetics of magnesium-based hydrogen storage alloy by mechanical milling with graphite

Magnesium nickel alloy (Mg₂Ni) which used as the negative electrode material in the nickel-metal hydride (Ni/MH) secondary battery is modified by graphite via mechanical milling. The effects of graphite on the Mg₂Ni are systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and a series of electrochemical tests. The results show that the cycle stability of the Mg₂Ni alloy is improved with the addition of 10 wt.% graphite and the discharge capacity at the 20th cycle increase from 116.9 mA g^?1 to 178.5 mA g^?1. The Tafel polarization test indicates better corrosion resistance of the Mg₂Ni/graphite composite. Meanwhile, the results of electrochemical tests indicate that both the charge-transfer reaction rate on the surface of the alloy and the hydrogen diffusion rate inside the bulk of alloy are boosted with the introduction of graphite.     

Preparation and hydrogen storage property of Mg-based hydrogen storage composite embedded by polymethyl methacrylate

A novel embedded Mg-based hydrogen storage nanocomposite was prepared by mechanical milling of hydriding combustion synthesized (HCS) Mg-based hydride and hydrogen permissive/oxygen prohibitive polymer. The Mg-based hydride was mechanically milled with tetrahydrofuran solution of polymethyl methacrylate (PMMA) under argon atmosphere. It is determined by X-ray diffraction (XRD) analysis that the average grain size of all the milled nanocomposites become smaller and the nanocomposites exhibit a good air-stable property. The microstructures of the nanocomposites obtained by Field emission scanning electron microscopy (FESEM) and High-resolution transmission electron microscopy (HRTEM) analyses show that Mg₉₅Ni₅ particles embedded by PMMA have a diameter of smaller than 100 nm, approximately. The nanocomposites show the optimal hydriding/dehydriding properties, requiring 60 min to absorb 3.37 wt.% hydrogen at low temperature of 473 K, and desorbing as high as 1.02 wt.% hydrogen within 120 min at the same temperature. The onset dehydriding temperature of the composites is about 373 K, which is 150 K lower than that of HCS products Mg₉₅Ni₅.     

Effect of Sm content on activation capability and hydrogen storage performances of TiFe alloy

Element substitution is an efficient method to enhance the activation property of TiFe alloys. In this paper, Zr, Mn and Ni were utilized to replace Fe in the alloy partially, and different content rare earth Sm substitute Ti in the alloy. The alloys with nominal compositions of Ti_1.1-xFe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_x (x = 0–0.08) were made through vacuum induction melting. The microstructure, composition and hydrogen storage property of alloys were measured in detail by X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy and automatically Sievert apparatus. The results reveal that the as-cast alloys contain TiFe as major phase and Ti₂Fe as secondary phase. Sm addition refines the grain of alloys obviously. All alloys have good activation properties and can be completely activated without any heat treatment. The activation performance can be further improved by partially replacing Ti with Sm, and the incubation period of activation can be shortened greatly.     

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg₂Ni, Mg₂Co as well as a small amount of Mg₁₂Ce, and convert into the MgH₂–CeH_2.73-Mg₂NiH₄–Mg₂CoH₅ composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH₂, Mg₂Ni/Mg₂NiH₄ and Mg₆Co₂H₁₁/Mg₂CoH₅. Among them, the transformation between Mg₂Ni and Mg₂NiH₄ triggers the “spill-over” effect which promote the decomposition of MgH₂ phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg₆Co₂H₁₁ to Mg₂CoH₅ phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H₂ within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H₂ within 60 min at the temperature of 260 °C.     

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.     

Effects of mechanical milling on hydrogen storage properties of alloy

The hydrogen storage alloy of Ti_0.32Cr_0.43V_0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.     

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).     

Effects of Zr doping on activation capability and hydrogen storage performances of TiFe-based alloy

Activation difficulty is the key problem limiting the application of TiFe-based hydrogen storage alloys. The addition of transition group elements helps to improve the activation properties of TiFe-based hydrogen storage alloy. In our previous work, the Ti_1.08Y_0.02Fe_0.8Mn_0.2 alloy exhibits extremely high hydrogen storage capacity (1.84 wt%) at room temperature with excellent kinetic properties, but it still needs an incubation period of about 1500s. In this study, the composition of Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_x (x = 0, 0.02, 0.04, 0.06, 0.08) alloys was prepared by electromagnetic induction melting. The quantitative analysis of elements by energy dispersive spectrometer shows that in the second phase region containing Zr, the content of Ti element is significantly higher than that of Fe. Meanwhile, the first-principle calculation on Zr-doped TiFe system indicates that Zr is more attractive to substitute Ti than Fe. Therefore, the doping of Zr partially replaces the Ti. The solubility of Zr in TiFe is limited, when x ≤ 0.04, the alloy consists of pure TiFe phase. When x > 0.4, the excess Zr forms precipitates, which reduces the reversible hydrogen absorption and desorption capacity of the TiFe alloy. The addition of Zr significantly shortens the activation time and reduces the plateau pressure of TiFe alloys. The Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_0.04 alloy can be directly activated without the incubation period and its absolute values of enthalpy change (ΔH) and entropy change (ΔS) are minima (ΔH for 23.2 kJ/mol and ΔS for 83.1 J/mol/K).     

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.     

Characterization of microstructure and surface oxide of Ti1.2Fe hydrogen storage alloy

In this study, we investigated the microstructures, hydrogen absorption kinetics, and oxide layers of TiFe and Ti_1.2Fe hydrogen storage alloys. Whereas the TiFe alloy has a single phase, the Ti_1.2Fe alloy is composed of three phases: TiFe, Ti₂Fe, and Ti₄Fe. Under no thermal activation process, the TiFe alloy does not absorb hydrogen, though the Ti_1.2Fe alloy starts to absorb hydrogen after 4 min of incubation time. From the XPS results, it is revealed that the Ti concentration in the oxide layer on the Ti₄Fe phase is higher than that on the TiFe phase, indicating that the Ti concentration in the oxide layer would be important in improving hydrogen absorption kinetics. Based on these results, the hydrogen absorption kinetics could be improved by adjusting composition, enabling the formation of a Ti-rich oxide layer.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Ternary Mg₈₆Y₁₀Ni₄ alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH₂, Mg₂NiH₄, YH₂, and a small amount of YH₃. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg₈₆Y₁₀Ni₄ alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH₂ transformation in the Mg₈₆Y₁₀Ni₄ alloy were slightly higher than that of pure Mg/MgH₂.     

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.