Formation and hydrogen storage behavior of nanostructured Mg2FeH6 in a compressed 2MgH2–Fe composite

The present study focuses on enhancing the yield of Mg₂FeH₆ and its hydrogen storage performances through a novel high-pressure compression approach. For which, MgH₂ and Fe powders are first mechanically milled in a molar ratio of 2:1 and subsequently compressed to a cylindrical pellet. Due to the compression, the yield of Mg₂FeH₆ in the compressed 2MgH₂–Fe pellet (90%) has been increased by 24% as compared to the reference ball-milled powder (66%). The thermodynamic destabilization of Mg₂FeH₆ in the pelletized sample is observed through measuring the pressure-composition isotherms, resulting in the reduced ab/desorption enthalpy for the pellet sample (−68.34 and 75.61 kJ/mol H₂, respectively). The hydrogen uptake and release kinetics of Mg₂FeH₆ is remarkably fast, and it can store/release about 5 wt% H in less than 2.5 min at 400 °C. The faster hydrogen ab/desorption kinetics corresponds to the lower activation energies (36 and 95 kJ/mol H₂, respectively). The excellent yield of Mg₂FeH₆ and its improved hydrogen storage properties for the compressed pellet are primarily attributed to the microstructural modifications upon high-pressure compression, and also the obtained results for Mg₂FeH₆ ternary hydride are linked to the literature data based on theoretical calculations.     

Mg/MgH2 hydrogen storage system destabilized by recyclable AlH3–NaBH4 composite

While borohydrides, such as NaBH₄, were often used as supplements to improve hydrogen storage properties of Mg/MgH₂ systems, they have long suffered from high decomposition temperature and irreversible dehydrogenation process. Here, we report that NaBH₄ can reversibly serve as a hydrogen storage host and reactant for Mg/MgH₂ systems under mild reaction conditions with the help of Al/AlH₃. 90 wt%MgH₂–5 wt.%AlH₃–5 wt.%NaBH₄ (M-5AB) has been successfully synthesized using the conventional mechanical alloying technique. The dehydrogenation activation energy and enthalpy are 20% and 9% reduced than those of pure Mg/MgH₂. After 10 hydrogen absorption and desorption cycles, the hydrogen storage capacity of M-5AB can reach 6.35 wt%. The X-ray diffraction (XRD) and the transmission electron microscope (TEM) measurements revealed that the interface of additives and Mg/MgH₂ decompose to Mg₁₇Al₁₂, MgAlB₄ and NaH phases. The Mg₁₇Al₁₂ and MgAlB₄ phases reduces the barrier of free energies of hydrogenated and dehydrogenated states, helping NaBH₄ to recover after rehydrogenation. These discoveries indicate that Al species can boost the decomposition and reformation of NaBH₄, providing a wider degree of freedom for the material design of Mg-based hydrogen storage materials.     

Structural characterization and hydrogen storage properties of MgH2–Mg2CoH5 nanocomposites

Mixtures of XMg–Co containing different amounts of Mg (X = 2, 3 and 7) were reactive milled under hydrogen atmosphere. 2Mg–Co only formed the Mg₂CoH₅ complex hydride, while the mixtures 3Mg–Co and 7Mg–Co formed different contents of Mg₂CoH₅ and MgH₂. Their structural features and hydrogen storage properties were analyzed by different techniques. In-situ synchrotron X-ray diffraction, combined with thermal analysis techniques, (differential scanning calorimetry, thermal gravimetric analysis and quadrupole mass spectrometer) was carried out to observe the behavior of the MgH₂–Mg₂CoH₅ mixtures during the first H-desorption. It was found that the presence of the Mg₂CoH₅ complex hydride has a beneficial effect on the first H-desorption of the MgH₂. Additionally, after first desorption, conventional hydrogenation under high pressure and high temperature of 3Mg–Co and 7Mg–Co samples led to the formation of the Mg₆Co₂H₁₁ complex hydride. The presence of Mg₆Co₂H₁₁ considerably impaired the desorption properties of the nanocomposites.     

200 NL H2 hydrogen storage tank using MgH2–TiH2–C nanocomposite as H storage material

MgH₂-based hydrogen storage materials are promising candidates for solid-state hydrogen storage allowing efficient thermal management in energy systems integrating metal hydride hydrogen store with a solid oxide fuel cell (SOFC) providing dissipated heat at temperatures between 400 and 600 °C. Recently, we have shown that graphite-modified composite of TiH₂ and MgH₂ prepared by high-energy reactive ball milling in hydrogen (HRBM), demonstrates a high reversible gravimetric H storage capacity exceeding 5 wt % H, fast hydrogenation/dehydrogenation kinetics and excellent cycle stability. In present study, 0.9 MgH₂ + 0.1 TiH₂ +5 wt %C nanocomposite with a maximum hydrogen storage capacity of 6.3 wt% H was prepared by HRBM preceded by a short homogenizing pre-milling in inert gas. 300 g of the composite was loaded into a storage tank accommodating an air-heated stainless steel metal hydride (MH) container equipped with transversal internal (copper) and external (aluminium) fins. Tests of the tank were carried out in a temperature range from 150 °C (H₂ absorption) to 370 °C (H₂ desorption) and showed its ability to deliver up to 185 NL H₂ corresponding to a reversible H storage capacity of the MH material of appr. 5 wt% H. No significant deterioration of the reversible H storage capacity was observed during 20 heating/cooling H₂ discharge/charge cycles. It was found that H₂ desorption performance can be tailored by selecting appropriate thermal management conditions and an optimal operational regime has been proposed.     

Development of Ca–Mg–H2–ZrCl4 composite for hydrogen storage applications

Both CaH₂ and MgH₂ are good candidate for the development of hydrogen storage materials because of their high hydrogen storage capacity. However, both the hydrides are quite stable thermodynamically and required high temperature for hydrogen sorption process. The MgH₂–CaH₂ composite could show the favourable hydrogen sorption reaction because of Ca–Mg intermetallic formation. The idea motivated to perform the experiments starting with these metal hydrides. It has been found that the hydrogen sorption reaction kinetics improved substantially. The dihydrogen product has shown a few intermetallic of magnesium and calcium. The hydrogen sorption temperature and pressure of the alloy was remarkably improved by the doping with ZrCl₄ as a catalyst. The activation energy and the thermodynamic parameters of un-catalyzed and catalyzed alloy were studied. Present studied indicated that the CaH₂–MgH₂–ZrCl₄ could be a potential candidate for the mobile hydrogen storage system.     

MgH2–TiF4-MWCNTs based hydrogen storage tank with central tube heat exchanger

Improvement of hydrogen sorption kinetics of MgH₂–TiF₄-MWCNTs based tank by addition of central tube heat exchanger and enhancement of hydrogen diffusion is proposed. After doping with TiF₄ and MWCNTs, dehydrogenation temperature of MgH₂ decreases significantly (ΔT = up to 90 °C). Superior hydrogen permeability, favoring hydrogen sorption kinetics is detected at hydrogen supply side to the middle of the tank, while effective heat transfer during exothermic hydrogenation is assured by the temperature increment of heat exchanger fluid (compressed air at room temperature). Hydrogen desorption and absorption can be completed within 120–150 and 25 min, respectively, up to twice as fast as the tank without heat exchanger from the previous studies. Due to fast hydrogenation rate resulting in short reaction time at high equilibrium temperature (up to 390 °C), particle agglomeration and/or sintering of MgH₂ upon cycling are prevented. Enhanced de/rehydrogenation rates and suppression of MgH₂ particle growth during cycling yield to considerable reversibility upon 20 de/rehydrogenation cycles with storage capacity up to 5.60 wt % H₂ (82% theoretical value). By increasing operating temperature to 330–335 °C, hydrogen released with constant flow rate of 0.30 standard L/min is prolonged up to three times, favoring electrical power production of PEMFC stack. Electrical performances obtained from PEMFC stack (13 single cells) supplied with hydrogen gas from MgH₂-based tank are also investigated.     

Remarkable hydrogen storage properties and mechanisms of the shell–core MgH2@carbon aerogel microspheres

Carbon aerogel (CA) microspheres with highly crumpled graphene–like sheets surface and network internal structure have been successfully prepared by an inverse emulsion polymerization routine, subsequently ball milled with Mg powder to fabricate Mg@CA. The Mg change into MgH₂ phases, decorating on the surface of the CA forming MgH₂@CA microspheres composite after the hydrogenation process at 400 °C. The MgH₂@CA microspheres composite displays MgH₂–CA shell–core structure and shows enhanced hydrogenation and dehydrogenation rates. It can quickly uptake 6.2 wt% H₂ within 5 min at 275 °C and release 4.9 wt% H₂ within 100 min at 350 °C, and the apparent activation energy for the dehydrogenation is decreased to 114.8 kJ mol^?1. The enhanced sorption kinetics of the composite is attributed to the effects of the in situ formed MgH₂ NPs during the hydrogenation process and the presence of CA. The nanosized MgH₂ could reduce the hydrogen diffusion distance, and the CA provides the sites for nucleation and prevents the grains from agglomerating. This novel method of in situ producing MgH₂ NPs on zero–dimensional architecture can offer a new horizon for obtaining high performance materials in the hydrogen energy storage field.     

Effect of Mo–Ni treatment on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloys

In order to improve kinetic properties of La–Mg–Ni-based hydrogen storage alloys, Mo–Ni treatment was applied to La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. FESEM results showed that after Mo–Ni treatment some network-shaped substance with nano-size formed on the surface of the alloy particles. The EDS results revealed increase in Ni content and emerge of Mo element. EIS and Linear polarization showed that charge-transfer resistance decreased and exchange current density increased for the treated alloy electrode, and the high rate dischargeability (HRD) was consequently improved. HRD at 1500 mA/g increased from 22.5% to 39.5%. Mo- and Ni-single treatments were performed compared with the Mo–Ni treatment, and the results showed that the single treatment improved HRD slightly, far less than the Mo–Ni treatment.     

Enhanced hydrogen storage properties of TiN–LiAlH4 composite

The hydrogen storage properties of LiAlH₄ doped efficient TiN catalyst were systematically investigated. We observe that TiN catalyst enhances the dehydrogenation kinetics and decreases the dehydrogenation temperature of LiAlH₄. The dehydrogenation behaviors of 2%TiN–LiAlH₄ are investigated using temperature programmed desorption (TPD), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Interestingly, the onset hydrogen desorption temperature of 2%TiN–LiAlH₄ sample gets lowered from 151.0 °C to 90.0 °C with a faster kinetics, and the dehydrogenation rate reached a maximum value at 137.2 °C. By adding a small amount of as-prepared TiN, approximately 7.1 wt% of hydrogen can be released from the LiAlH₄ at 130 °C. Interestingly, the result of the FTIR indicates that the 2%TiN–LiAlH₄ maybe restore hydrogen under 5.5 MPa hydrogen. Moreover, 2%TiN–LiAlH₄ displayed a substantially reduced activation energy for LiAlH₄ dehydrogenation.     

Characterization of CoB–silica nanochains hydrogen storage composite prepared by in-situ reduction

The CoB–silica nanochains hydrogen storage composite was prepared by in-situ reduction of cobalt salt on the surface of amine-modified silica nanospheres. The structure and morphology of the sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The valence state of atoms was characterized by X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the sample were also investigated. The results demonstrated that the CoB–silica nanochains hydrogen storage composite possessed amorphous nanochains structure by a series of nanospheres connecting in one-dimension. In addition, the material as electroactive negative electrodes showed high reversible discharge capacity (about 500 mAh/g in the first cycle) and good cycling stability. A properly electrochemical reaction mechanism was constructed primarily.     

Synthesis and characterization of g/Ni–SiO2 composite for enhanced hydrogen storage applications

Hydrogen is considered as one of the most important clean energy carriers for the future. Many experimental and theoretical investigations have focused on the adsorption and activation of H₂ on the metal surfaces. Metal oxides and semiconductors are suitable materials for this purpose. Gelatin assisted Ni loaded SiO₂ (g/Ni–SiO₂) was prepared and its structural properties, morphology, composition and surface properties were analyzed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM), Elemental mapping and energy dispersive spectrum (EDS), High resolution transmission electron microscopy (HR-TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. The prepared material was effectively utilized for H₂ storage applications at room temperature. The H₂ storage capacity of g/Ni–SiO₂ was twice that of pristine SiO₂. This may be due to large change in pore volume and pore diameter of g/Ni–SiO₂, which may enhances the H₂ storage capacity of the sample. The H₂ storage capacity of other materials such as ZnO, anatase TiO₂, g/TiO₂, g/ZnO, g/TiO₂–SO₄^2-, Sb doped TiO₂, Ag₂S/TiO₂, Sb₂O₃, CdS and SiO₂/CdS also studied and compared with g-Ni/SiO₂.     

Improved hydrogen storage kinetics and thermodynamics of RE-Mg-based alloy by co-doping Ce–Y

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg₉₀Ce₅Y₅ alloy, which has high capacity and high stability, was prepared by vacuum induction melting. The XRD, SEM, TEM, PCI, and DSC were used to characterize the microstructure and phase transformation of alloy as well as hydrogen storage property. The results indicate that the Mg₉₀Ce₅Y₅ alloy consists of multiphase structure, including the CeMg₁₂, Y₅Mg₂₄, Ce₂Mg₁₇ as well as residual Mg phase, besides, part Y dissolved in both Mg and CeMg₁₂/Ce₂Mg₁₇ phase to form solid solutions. After hydrogen absorption, these phases transform into the MgH₂, CeH_2.73 and YH₂ phase, while after hydrogen desorption, the MgH₂ transforms into the Mg phase, but the rare earth hydride phase was not changed. There is another reversible transformation between the CeH_2.73 and CeO₂ phase, which is beneficial for the cyclic stability of the alloy. The alloy has the reversible hydrogen capacity of about 6.0 wt% H₂ as well as the activation energy of 114.3 kJ/mol, and also exhibits enhanced kinetics compared with the pure MgH₂ sample, as a result of the synergistic effect of rare earth hydride phase. Meanwhile, it is also noted that the Mg₉₀Ce₅Y₅ alloy begins to release hydrogen below 250 °C and the rate of hydrogen desorption is mainly dominated by surface controlled.     

Remarkable decrease in dehydrogenation temperature of Li–B–N–H hydrogen storage system with CoO additive

Cobalt monoxide (CoO) was introduced into the Li–B–N–H system as a catalyst precursor, and the hydrogen desorption behavior of the LiBH₄–2LiNH₂–xCoO (x = 0–0.20) composites was investigated. It was observed that the majority of hydrogen desorption from the CoO-added sample occurred simultaneously with the melting of α-Li₄BN₃H₁₀. Moreover, the 0.05CoO-added sample exhibited optimized dehydrogenation properties, desorbing 9.9 wt% hydrogen completely with an onset temperature of 100 °C and exhibiting a decrease of more than 120 °C in the onset dehydrogenation temperature with respect to that of the additive-free sample. The activation energy of hydrogen desorption for the 0.05CoO-added sample was reduced by 30%. XAFS measurements showed that the CoO additive was first reduced chemically to metallic Co during the initial stage of thermal dehydrogenation, and the newly produced metallic Co acted as the catalytic active species in favor of the creation of B–N bonding. More importantly, approximately 1.1 wt% of hydrogen could be recharged into the fully dehydrogenated 0.05CoO-added sample at 350 °C and a hydrogen pressure of 110 atm, which represents much better performance than that exhibited by the pristine sample.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Influence of adding graphene on the hydrogen storage thermodynamics and kinetics of as-milled CeMg12–Ni alloy

Graphene can been used to improve the hydrogen storage performance of magnesium based materials because of its outstanding electrical conductivity, high specific surface area, and good mechanical qualities. In this paper, mechanical ball milling method was utilized to prepare the CeMg₁₂/Ni alloy combined with graphene in order to investigate the impact of graphene on microstructure, kinetics, and thermodynamic properties of alloy. The results reveal that the addition of graphene can refine the grains of alloy, increase the surface defects of alloy particles and promote Ni to be dissolved in the main phase of the alloy. The absolute value of enthalpy drops from 65.27 kJ/mol to 60.3 kJ/mol when Graphene is added at a higher concentration. The addition of graphene improves the dehydrogenation performance of alloy hydride by lowering its thermal stability. Because of a minimum dehydrogenation activation energy of 85.23 kJ/mol and dehydrogenation characteristic time of 1090s, the alloy with 6 wt% Graphene exhibits the best dehydrogenation kinetic performances.     

Superior hydrogen storage properties of MgH2–10 wt.% TiC composite

The hydrogen storage performance of MgH₂–10 wt.% TiC composite was investigated. The additive TiC nanoparticle led to a pronounced improvement in the de/hydrogenation kinetics of MgH₂. The composite could dehydrogenate 6.3 wt.% at 573 K while the milled MgH₂ only released 4.9 wt.% of hydrogen at the same condition. The improvement came from that the activation energy of dehydrogenation was decreased from 191.27 kJ mol⁻¹ to 144.62 kJ mol⁻¹ with the TiC additive. The MgH₂–10 wt.% TiC composite also absorbed 6.01 wt.% (or 5.1 wt.%) of hydrogen under 1 MPa H₂ at 573 K (or 473 K) in 3000 s. Even at 1 MPa H₂ and 373 K, it could absorb 4.1 wt.% of hydrogen, but milled MgH₂ could not absorb hydrogen at this condition. Additionally, the composite had good cycling stability, and its hydrogen capacity only decreased 3.3% of the first run after 10 de/hydrogenation cycles. The improved hydrogen storage properties were explained to the TiC particles embedded in the MgH₂, which provided the pathways for the hydrogen diffusion into the MgH₂–10 wt.% TiC composite.     

Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH₄ + MgH₂ system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl₅ additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB₂ composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB₂ composite can be significantly enhanced with the increase of NbCl₅ content. The 3 mol% NbCl₅ doped 2LiH–MgB₂ composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H₂ at 350 °C and release 8.5 wt% H₂ at 400 °C, respectively. In contrast, the undoped 2LiH–MgB₂ sample uptakes 6.2 wt% H₂ and releases 3.1 wt% H₂ under identical measurement conditions. Moreover, the 3 mol% NbCl₅ doped 2LiH–MgB₂ composite can release more than 9.0 wt% H₂ within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl₅ additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.     

The electrochemical performances of Ti–V-based hydrogen storage composite electrodes prepared by ball milling method

In order to overcome the inherent disadvantages of Ti–V-based hydrogen storage alloys, such as poor activation behavior and low high-rate dischargeability, the novel composites Ti_0.17Zr_0.08V_0.35Cr_0.1Ni_0.3–x wt.% La_0.7Mg_0.3Ni_2.75Co_0.75 (x = 0, 5, 10 and 20) were successfully synthesized by ball milling method in the present study. And the structure and overall electrochemical properties of as-prepared composites are investigated systemically. The electrochemical studies show that the maximum discharge capacity of the composite electrodes displays no variation with the increase of La_0.7Mg_0.3Ni_2.75Co_0.75 content, whereas the high-rate dischargeability (HRD) and the activation behavior are distinctly improved with increasing x. The electrochemical hydrogen kinetics of composite electrodes is also studied by means of electrochemical impedance spectroscopy (EIS), linear polarization (LP), anodic polarization (AP) and potential-step measurements. It is found that the charge-transfer reaction resistance R_ct is decreased with increasing the amount of La_0.7Mg_0.3Ni_2.75Co_0.75 while exchange current density I₀, limiting current density I_L and hydrogen diffusion coefficient D are all increased with increasing the amount of La_0.7Mg_0.3Ni_2.75Co_0.75. These results suggest that the formation of composite with La_0.7Mg_0.3Ni_2.75Co_0.75 alloy is a promising strategy for improving the HRD, activation behavior and electrochemical kinetics of Ti–V-based alloy electrodes.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.