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.     

Evaluation of hydrogen storage performance of ZrTiVNiCrFe in electrochemical and gas-solid reactions

In the present study, the hydrogen storage performance of multi-principal-component ZrTiVNiCrFe alloy produced through rapid solidification has been examined by electrochemical methods and gas-solid reactions. XRD and EBSD analyses reveal the hexagonal Laves phase structure (type C14) with average grain size of 300 nm and root-mean-square microstrain of 0.19%. Cyclic voltammetry and electrochemical impedance spectroscopy analyses in the hydrogen sorption/desorption region give insight to the sorption/desorption kinetics and the change in the desorption charge in terms of the applied potential. The pressure-composition isotherms measured in course of gas-solid reaction confirm the hydrogen storage capacity reaching 1.6 wt% at the first hydrogenation at room temperature, then reducing to 1.3–1.4% during subsequent cycling. According to the calorimetric titration study, there is a significant hysteresis primarily caused by the non-equilibrium character of the hydrogenation process.     

Fast hydrogen absorption/desorption kinetics in reactive milled Mg-8 mol% Fe nanocomposites

This study aims to better understand the Fe role in the hydrogen sorption kinetics of Mg–Fe composites. Mg-8 mol% Fe nanocomposites produced by high energy reactive milling (RM) for 10 h resulted in MgH₂ mixed with free Fe and a low fraction of Mg₂FeH₆. Increasing milling time to 24 h allowed formation of a high fraction of Mg₂FeH₆ mixed with MgH₂. The hydrogen absorption/desorption behavior of the nanocomposites reactive milled for 10 and 24 h was investigated by in-situ synchrotron X-ray diffraction, thermal analyses and kinetics measurements in Sieverts-type apparatus. It was found that both 10 and 24 h milled nanocomposites presents extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions, i.e., 300–350 °C under 10 bar H₂ for absorption and 0.13 bar H₂ for desorption. Nanocomposites with MgH₂, low Fe fraction and no Mg₂FeH₆ are suggested to be the most appropriate solution for hydrogen storage under the mild conditions studied.     

Mechanisms of hydrides’ nucleation and the effect of hydrogen pressure induced driving force on de-/hydrogenation kinetics of Mg-based nanocrystalline alloys

Aiming to gain insight on the hydrogen storage properties of Mg-based alloys, partial hydrogenation and hydrogen pressure related de-/hydrogenation kinetics of Mg–Ni–La alloys have been investigated. The results indicate that the phase boundaries, such as Mg/Mg₂Ni and Mg/Mg₁₇La₂, distributed within the eutectics can act as preferential nucleation sites for β-MgH₂ and apparently promote the hydrogenation process. For bulk alloy, it is observed that the hydrogenation region gradually grows from the fine Mg–Ni–La eutectic to primary Mg region with the extension of reaction time. After high-energy ball milling, the nanocrystalline powders with crystallite size of 12～20 nm exhibit ameliorated hydrogen absorption/desorption performance, which can absorb 2.58 wt% H₂ at 368 K within 50 min and begin to desorb hydrogen from ～508 K. On the other side, variation of hydrogen pressure induced driving force significantly affects the reaction kinetics. As the hydrogenation/dehydrogenation driving forces increase, the hydrogen absorption/desorption kinetics is markedly accelerated. The dehydrogenation mechanisms have also been revealed by fitting different theoretical kinetics models, which demonstrate that the rate-limiting steps change obviously with the variation of driving forces.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

Composition dependent microstructure evolution,activation and de-/hydrogenation properties of Mg–Ni–La alloys

In this work, in order to elucidate the effect of different alloying elements on the microstructure, activation and the de-/hydrogenation kinetics performance, the Mg–20La, Mg–20Ni and Mg–10Ni–10La (wt.%) alloys have been prepared by near equilibrium solidification combined with high-energy ball milling treatment to realize the internal optimization as well as particle refinement. The results show that the microstructures of the prepared alloys are significantly refined by forming different types and sizes of intermetallic compounds. Meanwhile, the effects of LaH₃ and Mg₂Ni within the activated samples on de-/hydrogenation kinetics are also discussed. It is observed that the alloy containing LaH₃ preserves stable hydrogenation behavior between 573 and 623 K, while the hydrogenation properties of the alloy containing Mg₂Ni is susceptible to temperature. A preferable hydrogenation performance is observed in Mg–10Ni–10La alloy, which can absorb as high as 5.86 wt% hydrogen within 15 min at 623 K and 3.0 MPa hydrogen pressure. Moreover, the desorption kinetics and the desorption activation energies are evaluated to illustrate the mechanism based on improved dehydrogenation performance. The addition of proper alloying elements Ni and La in combination with reasonable processing is an effective strategy to improve the de-/hydrogenation performance of Mg-based alloys.     

Improving the hydrogenation properties of AZ31-Mg alloys with different carbonaceous additives by high energy ball milling (HEBM) and equal channel angular pressing (ECAP)

In this study, the hydrogen storage performance of commercial AZ31-Mg alloys combined with various allotropes of carbon was investigated and the microstructural modifications with respect to plastic deformation and high energy milling techniques investigated, with the aim of obtaining enhanced hydrogen storage efficiency. The hydrogen storage performance of alloys prepared with different weight ratios of carbonaceous materials as a catalyst was monitored in order to explore the effective improvement in hydrogen storage performance through microstructural modification. Additionally, the effects of different processing methods such as equal channel angular pressing (ECAP) and high energy ball milling (HEBM) were also observed. AZ31 Mg based composites with various carbon additives were produced through gravity resistance casting and their micrographic structures examined through optical Microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The average particle size distributions of the sample powders were also measured. The rate of hydrogenation kinetics was calculated by a Sievert's type apparatus. Significant enhancement of the hydrogenation performance was obtained with the addition of carbonaceous materials. Overall, the hydrogen storage performance after ECAP deformation of the AZ31-3CB (carbon Black) composite showed a gain in the maximum capacity of 6.72 ± 0.05 wt%. Similar, after milling of the AZ31-3G (Graphene) composite materials, a maximum potential capacity of 6.83 ± 0.04 wt% was attained within 792 ± 144.34 s, with desorption of the entire H₂ content in 143.2 ± 26.09 s. The obtained results revealed significant improvement in the hydrogen storage capacity of AZ31-Mg alloys with the addition of carbon materials and with respect to plastic deformation and milling techniques.     

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.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

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.     

Synthesis by reactive ball milling and cycling properties of MgH2–TiH2 nanocomposites: Kinetics and isotopic effects

MgH₂, MgH₂–TiH₂ nanocomposites and their deuterated analogues have been obtained by reactive ball milling and their kinetic and cycling hydrogenation properties have been analysed by isotope measurements and high-pressure differential scanning calorimetry (HP-DSC). Kinetics of material synthesis depends on both Ti-content and the isotopic nature of the gas. For pure Mg, the synthesis is controlled by isotope diffusion in Mg and therefore MgH₂ forms faster than MgD₂. For the MgH₂–TiH₂ nanocomposites, the synthesis is controlled by the efficiency of milling. Kinetics of reversible hydrogen/deuterium sorption in nanocomposites have been studied at 548 K. The rate limiting step is isotope diffusion for absorption and Mg/MgH₂ interface displacement for desorption. HP-DSC measurements demonstrate that the TiH₂ phase acts as a gateway for hydrogen sorption even in presence of MgO and provides abundant nucleation sites for Mg and MgH₂ phases. The 0.7MgH₂–0.3TiH₂ nanocomposite exhibits steady hydrogen storage capacity after 100 cycles of absorption–desorption.     

Non-alloy Mg anode for Ni-MH batteries: Multiple approaches towards a stable cycling performance

Mg attracts much research interest as anode material for Ni-MH batteries thanks to its lightweight, cost-effectiveness and high theoretical capacity (2200 mA h g^?1). However, its practical application is tremendously challenged by the poor hydrogen sorption kinetics, passivation from aggressive aqueous electrolytes, and insulating nature of MgH₂. Mg-based alloys exhibit enhanced hydrogen sorption kinetics and electrical conductivity, but significant amount of costly transition metal elements are required. In this work, we have, for the first time, utilized non-alloyed but catalyzed Mg as anode for Ni-MH batteries. 5 mol.% TiF₃ was added to nanosized Mg for accelerating the hydrogen sorption kinetics. Several strategies for preventing the problematic passivation of Mg have been studied, including protective encapsulation of the electrode and utilizing room-temperature/high-temperature ionic liquids and an alkaline polymer membrane as working electrolyte. Promising electrochemical performance has been achieved in this Mg–TiF₃ composite anode based Ni-MH batteries with room for further improvements.     

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.     

Metal hydride hydrogen compressors: A review

Metal hydride (MH) thermal sorption compression is an efficient and reliable method allowing a conversion of energy from heat into a compressed hydrogen gas. The most important component of such a thermal engine – the metal hydride material itself – should possess several material features in order to achieve an efficient performance in the hydrogen compression. Apart from the hydrogen storage characteristics important for every solid H storage material (e.g. gravimetric and volumetric efficiency of H storage, hydrogen sorption kinetics and effective thermal conductivity), the thermodynamics of the metal–hydrogen systems is of primary importance resulting in a temperature dependence of the absorption/desorption pressures). Several specific features should be optimised to govern the performance of the MH-compressors including synchronisation of the pressure plateaus for multi-stage compressors, reduction of slope of the isotherms and hysteresis, increase of cycling stability and life time, together with challenges in system design associated with volume expansion of the metal matrix during the hydrogenation.     

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.     

Microstructural evolution and hydrogen storage properties of a Ni-modified Mg15Al alloy

On the basis of modification of transition metals on Mg-Al hydrogen storage alloys, Mg15Al5Ni alloy with Ni content of 5 wt% has been prepared by high energy ball mill. The results show that Ni particles uniformly distribute on the surface of particles, while several Ni particles are embedded inside alloy particles. These Ni particles tend to redistribute after hydrogenation. The phase composition analysis reveals the formation of stable Al₃Ni₂ phase in Ni-modified alloy after hydrogenation. The hydrogen absorption performance of Mg15Al5Ni alloy has been improved by introducing Ni, which can absorb 4.36 wt% hydrogen within 5 min at 350 °C. Meanwhile, the activation properties of Mg15Al5Ni alloy can be obviously deteriorated due to the addition of Ni. However, uniformly distributed Al₃Ni₂ nanocrystals with grain sizes around 10 nm hinder grain growth of hydrides, ameliorating hydrogenation kinetics of Mg15Al5Ni alloy. Besides, the modified effect of Ni on hydrogenation kinetics of Mg15Al5Ni alloy has been also discussed in this work.     

Development of polymer nanocomposites with sodium alanate for hydrogen storage

The development of materials based on polymer nanocomposites for hydrogen storage with lower temperature of desorption might contribute to the consolidation of the use of hydrogen as a sustainable energy. The purpose of this work was to develop hybrid porous materials consisting of polyaniline or sulfonated polyetherimide as polymer matrices and a potential hydride for hydrogen storage – sodium alanate. Multiwall carbon nanotubes and titanium dioxide were also added in order to improve the hydrogen absorption capacity of the sodium alanate. The nanocomposites were prepared via solution mixing and analyzed by differential scanning calorimetry, thermogravimetric analysis, transmission and scanning electron microscopy and kinetic of hydrogen sorption. The nanoparticles had some influence on the polymers structures, modifying its thermal properties, such as glass transition temperature and the onset temperature of degradation. Microscope analyses showed that not all the particles were always well dispersed and distributed through the matrices. However, kinetics of hydrogen sorption tests indicated a significant amount of hydrogen (up to 1.2 wt%) in the nanocomposites after 12 h at relatively low temperature (120 °C) and 32 bar.     

High-performance La–Mg–Ni-based alloys prepared with low cost raw materials

La–Mg–Ni-based hydrogen storage alloys showed good application prospects owing to their high hydrogen storage capacity. However, the poor cycling stability was a key problem. In order to improve the cycling stability, low cost YFe_0.85 master alloy was used as raw material to prepare La–Mg–Ni-based La_0.8-xY_xMg_0.2Ni_3-0.85xFe_0.85x (x = 0.50, 0.55, 0.60) hydrogen storage alloys by powder sintering method. The alloys were mainly composed of PuNi₃ phase and MgCu₄Sn phase. With the increase of Y and Fe, the cell parameters of PuNi₃ phase decreased. Lower mismatch coefficient promoted the cycling stability. As the case of x = 0.60, the capacity retention rate rose up to 95.45%. Aside from the cycling stability, appropriate substitution content contributed to higher capacity and satisfactory kinetics. As the case of x = 0.55, the hydrogen storage capacity reached 1.529 wt%, and hydriding time for the x = 0.60 alloy shrank to 76.7% of that for alloys without Y and Fe at 303 K.     

Enhancement of hydrogen storage capacity in polyaniline-vanadium pentoxide nanocomposites

The high-pressure H₂ sorption isotherms of polyaniline–vanadium pentoxide nanocomposites (PANI–VONC) have been performed at liquid nitrogen temperature. A large increment of hydrogen storage capacity occurs in PANI–VONC (∼1.8 wt%) in contrast to that in pristine vanadium pentoxide (∼0.2 wt%) and in non-treated polyaniline (∼0.2 wt%). It is considered that the reason for the enhancement of hydrogen storage in PANI–VONC is the intercalation of polyaniline (PANI) into the vanadium pentoxide layers. The intercalation of PANI decreases the interlayer distance from 1.1 nm between vanadium pentoxide layers to 0.72 nm between vanadium pentoxide layer and polyaniline layer.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.