LiBH4 has been loaded into a highly ordered mesoporous carbon scaffold containing dispersed NbF5 nanoparticles to investigate the possible synergetic effect of nanoconfinement and nanocatalysis on the reversible hydrogen storage performance of LiBH4. A careful study shows that the onset desorption temperature for nanoconfined LiBH4@MC-NbF5 system is reduced to 150 °C, 225 °C lower than that of the bulk LiBH4. The activation energy of hydrogen desorption is reduced from 189.4 kJ mol–1 for bulk LiBH4 to 97.8 kJ mol–1 for LiBH4@MC-NbF5 sample. Furthermore, rehydrogenation of LiBH4 is achieved under mild conditions (200 °C and 60 bar of H2). These results are attributed to the active Nb-containing species (NbHx and NbB2) and the function of F anions as well as the nanosized particles of LiBH4 and high specific surface area of the MC scaffold. The combination of nanoconfinement and nanocatalysis may develop to become an important strategy within the nanotechnology for improving reversible hydrogen storage properties of various complex hydrides.

TiC as a novel catalyst was used in preparing TiC-doped sodium aluminum hydride by ball-milling NaH/Al mixture or NaAlH4 with TiC powder under a hydrogen atmosphere. It is found that TiC-doped NaH/Al composite absorbs 4.77 wt % hydrogen at 120 °C, desorbs more than 80% hydrogen of its initial hydrogen capacity at 155 °C with a stable cycling dehydriding rate and capacity, and exhibits better reversible hydrogen storage properties than those of TiC-doped NaAlH4 composite or Ti-doped NaH/Al composite. The catalytic mechanism of TiC for reversible hydrogen storage behavior of TiC-doped sodium aluminum has been studied through XRD and SEM-EDS analyses. The experiment led us believe that the refined TiC particles inlaid on the surface of a larger hydride matrix act not only as the catalytic active sites for the redox reaction of hydrogen, and act as the hydrogen spillover for hydrogen diffusion, but also prevent the growth in size of small spherical alanate, resulting in the improvement of hydriding/dehydriding properties of the sodium alanate system.

Magnesium hydride (MgH2) exhibits long-term stability and has recently been developed as a safe alternative to store hydrogen in the solid state, due to its high capacity of 7.6 wt% H2 and low cost compared to other metal hydrides. However, the high activation energy and poor kinetics of MgH2 lead to inadequate hydrogen storage properties, resulting in low energy efficiency. Nano-catalysis is deemed to be the most effective strategy in improving the kinetics performance of hydrogen storage materials. In this work, robust and efficient architectures of carbon-wrapped transition metal (Co/C, Ni/C) nanoparticles (8–16 nm) were prepared and used as catalysts in the MgH2 system via ball milling to improve its de/rehydrogenation kinetics. Between the two kinds of nano-catalysts, the Ni/C nanoparticles exhibit a better catalytic efficiency. MgH2 doped with 6% Ni/C (MgH2-6%Ni/C) exhibits a peak dehydrogenation temperature of 275.7 °C, which is 142.7, 54.2 and 32.5 °C lower than that of commercial MgH2, milled MgH2 and MgH2 doped with 6% Co/C (MgH2-6%Co/C), respectively. MgH2 doped with 6% Ni/C can release about 6.1 wt% H2 at 250 °C. More importantly, the dehydrogenated MgH2-6%Ni/C is even able to uptake 5.0 wt% H2 at 100 °C within 20 s. Moreover, a cycling test of MgH2 doped with 8% Ni/C demonstrates its excellent hydrogen absorption/desorption stability with respect to both capacity (up to 6.5 wt%) and kinetics (within 8 min at 275 °C for dehydrogenation and within 10 s at 200 °C for rehydrogenation). Mechanistic research reveals that the in situ formed Mg2Ni and Mg2NiH4 nanoparticles can be regarded as advanced catalytically active species in the MgH2-Ni/C system. Meanwhile, the carbon attached around the surface of transition metal nanoparticles can successfully inhibit the aggregation of the catalysts and achieve the steadily, prompting de/rehydrogenation during the subsequent cycling process. The intrinsic catalytic effects and the uniform distributions of Mg2Ni and Mg2NiH4 result in a favorable catalytic efficiency and cycling stability. Nano-catalysts with this kind of morphology can also be applied to other metal hydrides to improve their kinetics performance and cycling stability.

Nanoscale CeAl4 was directly synthesized by the thermal reaction between CeH2 and nano-aluminum at 300 °C. Then nano CeAl4-doped sodium alanate (NaAlH4) was synthesized by ball milling NaH/Al with 0.04CeAl4 under hydrogen atmosphere at room temperature, and the catalytic efficiency of nanoscale CeAl4 for hydrogen storage of NaAlH4 was systematically investigated. It is shown that CeAl4 can effectively improve the dehydrogenation properties of sodium alanate system. The 0.04CeAl4-doped NaAlH4 system starts to release hydrogen below 80 °C, completes dehydrogenation within 10 min at 170 °C, and exhibits good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). Apparent activation energy of the dehydrogenation of NaAlH4 can be effectively reduced by addition of CeAl4, resulting in the decrease in desorption temperatures. Moreover, by analyzing the reaction kinetics of nano CeAl4-doped NaAlH4 sample, both of the decomposition steps are conformed to a two-dimensional phase-boundary growth mechanism. The mechanistic investigations gained here can help to understand the de-/rehydrogenation behaviors of catalyzed complex metal hydride systems.

•The effects of Mn substitution for Co in the ZrCo1−xMnx were investigated.•The initial activation behavior of ZrCo1−xMnx is improved with increasing Mn.•Co substituted with Mn results in the disproportionation during the activation.ZrCo1−xMnx (x = 0–0.1) alloys for tritium storage were prepared by induction levitation melting under an argon atmosphere. The effect of Mn substitution for Co on the alloy microstructure, initial activation behavior, hydrogen storage kinetics and thermodynamics was investigated. The results show that the ZrCo1−xMnx (x = 0, 0.025, 0.05) alloys have a single phase of ZrCo, while ZrCo1−xMnx (x = 0.075, 0.1) alloys consist of a main phase of ZrCo and a secondary phase of ZrMn2. It is observed that the initial activation time (uptake to 95% of saturated hydrogen capacity) decreases from 63.73 h to 0.24 h as the Mn content increases from x = 0 to x = 0.1. However, Mn substituted for Co is determined to result in disproportionation during activation and a loss in hydrogen capacity. For increasing Mn content in the alloy, the plateau width for Pressure-Composition-Temperature (P-C-T) curve is shortened, while the plateau pressure remains mostly unchanged. DSC measurements were also performed to investigate the thermal stability of the ZrCo1−xMnx system.

In this paper, we present a new method to synthesize a dual-cation (Li+, Mg2+) borohydride. It is found that Li–Mg–B–H is formed by mechanical milling a mixture of LiBH4 and MgCl2 with a molar ratio of 3 : 1 in diethyl ether (Et2O) and a subsequent heating process. The morphology and structure of the as-prepared Li–Mg–B–H compound are studied by SEM, XRD, FTIR and NMR measurements. Further experiments testify that Li–Mg–B–H can release approximately 12.3 wt% of hydrogen under 4 bar initial hydrogen pressure from room temperature to 500 °C and reach a maximum desorption rate of 13.80 wt% per h at 375 °C, which is 30 times faster than that of pristine LiBH4. Thermal analysis indicates that the decomposition process of the new compound involves three steps: (1) Li–Mg–B–H first decomposes into LiBH4 and MgH2 and synchronously releases a number of H2 molecules; (2) MgH2 decomposes to Mg and H2; (3) LiBH4 reacts with Mg, generating H2, MgB2 and LiH. Moreover, Li–Mg–B–H is proved to be partially reversible, which can release 5.3 wt% hydrogen in the second dehydrogenation process. The strategy of altering the χp of metal ions in borohydrides may shed light on designing dual-cation borohydrides with better hydrogen storage performance.

Na2Ti3O7 nanotubes (NTs) with a uniform diameter of 10 nm and Na2Ti3O7 nanorods (NRs) with a diameter of 100–500 nm were synthesized via a hydrothermal method and a solid-state method, respectively, and then introduced into MgH2 by ball milling to catalyze the hydrogenation/dehydrogenation process. The MgH2–Na2Ti3O7 NT and MgH2–Na2Ti3O7 NR composites can desorb 6.5 wt% H2 within 6 min and 16 min at 300 °C, respectively, while the bulk MgH2 hardly releases any hydrogen even over a much longer time. In addition, isothermal rehydrogenation measurements show that the MgH2–Na2Ti3O7 NT composite can absorb 6.0 wt% H2 within 60 s at 275 °C and can even absorb 1.5 wt% H2 within 30 min at a temperature as low as 50 °C. TEM and HRTEM analyses indicate that the Na2Ti3O7 NTs are homogeneously distributed in MgH2, which catalyze the de-/rehydrogenation of MgH2 and meanwhile offer numerous diffusion channels to significantly accelerate the transportation of hydrogen atoms. Moreover, compared with bulk MgH2 and the MgH2–Na2Ti3O7 NR composite, the activation energy of the MgH2–Na2Ti3O7 NT composite is significantly decreased to 70.43 kJ mol−1. Such Na2Ti3O7 NTs with a unique morphology of the catalyst being distributed as nanotubes in MgH2 are believed to pave the way for the future design of hydrogen storage materials with excellent hydrogen storage performances.

Mg(AlH4)2 nanoparticles with a particle size less than 10 nm have been successfully synthesized by mechanochemical method using LiAlH4 and MgCl2 as raw materials together with LiCl buffering additive. In comparison to Mg(AlH4)2 microparticles, Mg(AlH4)2 nanoparticles exhibit a faster hydrogen desorption kinetics and lower desorption temperature. The hydrogen desorption temperatures of the first and second dehydrogenation steps are 80 and 220 °C for the Mg(AlH4)2 nanoparticles, which are about 65 and 60 °C, respectively, lower than those of Mg(AlH4)2 microparticles. The decomposition activation energy is reduced from 135 kJ/mol for Mg(AlH4)2 microparticles to 105.3 kJ/mol for Mg(AlH4)2 nanoparticles. It is proposed that the shortened diffusion distance and enhanced diffusivity of Mg(AlH4)2/MgH2 nanoparticles provide an energy destabilization for lowering the dehydrogenation temperature, and thus being the key factor for promoting the hydrogen desorption kinetics. More importantly, it is demonstrated that the dehydrided nano MgH2 hydride with a particle size below 10 nm can be formed after rehydrogenation process, resulting in the good cycling hydrogen desorption performance of nano MgH2.

•Au0.3Pd0.7-(La2O3)0.6 NPs supported on CNTs have been successfully fabricated.•The catalyst exhibits remarkable catalytic activity with TOF of 589 h−1 at 50 °C.•The catalyst exhibits a solo catalysis without any CO generation.•The present system can also be extended on the catalysis for other applications.Formic acid (FA, HCOOH), a convenient and safe hydrogen storage material, has the great potential for fuel cell applications. However, hydrogen generation of FA is inefficient in the presence of heterogeneous catalysts at relatively low temperatures, which remains a big challenge. Herein, La2O3-modified highly dispersed AuPd alloy nanoparticles (AuPdLa2O3) with small particle size have been successfully anchored on carbon nanotubes (CNTs) by a facile co-reduction route. Moreover, the catalyst exhibits excellent catalytic activity and 100% hydrogen selectivity for hydrogen generation in the formic acid/sodium formate (FA/SF) system with the initial turnover frequency (TOF) value of 589 mol H2 mol−1 catalyst h−1 at 50 °C and 280 mol H2 mol−1 catalyst h−1 even at room temperature (25 °C). The present Au0.3Pd0.7-(La2O3)0.6/CNTs with superior catalysis on FA dehydrogenation without any CO generation at room temperature can not only pave the way for practical application of hydrogen storage system, but also can be extended to other catalysis system.Download high-res image (200KB)Download full-size image

•CoTiO3/graphene composite were firstly synthesized for anode of Li-ion battery.•CoTiO3/graphene composite shows better electrochemical performances than bare CoTiO3.•CoTiO3 nanoparticles could uniformly distribute on the surface of graphene.A novel perovskite CoTiO3/graphene composite with a particle size of 100 nm was synthesized by ball-milling and high temperature solid method and utilized as anode materials for lithium-ion batteries. The structure morphology and electrochemical performance of CoTiO3/graphene nanocomposite were investigated by X-ray diffraction (XRD), Raman spectroscopy measurements, scanning electron microscopy (SEM), galvanostatic charge-discharge tests and electrochemical impedance spectroscopy (EIS) tests. The CoTiO3/graphene nanocomposite delivers a higher stable discharge capacity (∼500 mAh g−1) and better cycling stability (86.2%), as well as improved rate capability over 80 cycles, compared with those of bare CoTiO3. Moreover, this CoTiO3/graphene nanocomposite exhibits stable discharge capacity (∼350 mAh g−1) after 3000 cycles at high current density (1 A g−1). The improved electrochemical properties of CoTiO3/graphene can be attributed to the good conductivity and large surface of graphene and the combination novel nanostructure of CoTiO3 and graphene.Download high-res image (210KB)Download full-size image

•Firstly synthesized and investigated Mg(BH4)2AlH3 complex hydride.•Mg(BH4)2AlH3 composite starts to decompose from 130.8 °C.•Mechanism of AlH3 improving dehydrogenation of Mg(BH4)2 was studied.•Altering dehydrogenation temperature can control Mg(BH4)2AlH3 reversibility.Mg(BH4)2 has been considered as one of the promising light metal complex hydrides due to its high hydrogen capacity and low cost. But its higher thermal stability (dehydrogenation at above 300 °C) needs to be improved for the practical application. In this study, the aluminum hydride AlH3 was introduced into complex borohydride Mg(BH4)2 to synthesize a new Mg(BH4)2AlH3 composite by ball milling method. It is found that the active Al∗ formed from the self-decomposition of AlH3 can effectively improve the dehydrogenation properties of Mg(BH4)2, the Mg(BH4)2AlH3 composite starts to release hydrogen at 130.8 °C with a total hydrogen capacity of 11.9 wt.%. The dehydrogenated products of the composite is composed of Mg2Al3 and B at 350 °C, resulting in the improved hydrogen desorption properties of Mg(BH4)2AlH3 composite. The Mg2Al3 and B products would be further transformed into MgAlB4 and Al at 500 °C. Moreover, the Mg2Al3 and B dehydrogenated products show better reversible hydrogen storage property than that of the MgAlB4 and Al products. This research shows a way to alter hydrogen de/hydrogenation route and reversibility of Mg(BH4)2 complex hydride by compositing with AlH3 and controlling the dehydrogenation temperature.

Correction for ‘Enhanced hydrogen storage properties of MgH2 with numerous hydrogen diffusion channels provided by Na2Ti3O7 nanotubes’ by Liuting Zhang et al., J. Mater. Chem. A, 2017, 5, 6178–6185.

•Quantum dots SnO2/nitrogen-doped graphene oxide composite were synthesized for Li-ion battery.•SnO2@NRGO electrode show superior electrochemical performance to SnO2@RGO electrode and SnO2 electrode.•SnO2@NRGO electrode exhibits excellent reversible capacity of 1333.5 mAh g−1 over 450 cycles.SnO2 is considered as one of the anode material for Li-ion batteries in terms of its superiority in high theoretical capacity (1494 mAh g−1), low cost and environmental friendly. However, it is suffered from several issues such as rapid capacity deterioration, undesirable aggregation of tin particles and pesky expansion of volume. To conquer these shortcomings, a novel composite of ultrasmall SnO2 quantum dots with an average particle size of 4–5 nm anchored on nitrogen-doped reduced graphene oxide (SnO2@NRGO) was first in situ synthesized By means of hydrothermal method. The results show that as-prepared SnO2@NRGO electrode exhibits a greater enhancement in its initial discharge capacity (1678.4 mAh g−1) and reversible capacity (1333.5 mAh g−1 after 450 cycles) at a current density of 500 mA g−1, implying a long cycle life. Furthermore, the high rate capability of SnO2@NRGO is superior to SnO2@RGO and SnO2 electrodes. The excellent electrochemical reversibility of SnO2@NRGO electrode can be ascribed to the great conductivity, ultrahigh specific surface area and the synergetic effect between ultrasmall SnO2 quantum dots and NRGO.

•A new strategy to improve hydrogen desorption properties of LiBH4 is achieved by compositing with fluorographene.•LiBH4–50FG composite can release hydrogen at low temperature of 148.1 °C with a capacity of 8.2 wt.% H2.•A remarkably enhanced reversible hydrogen desorption capacity of 7.1 wt.% is obtained in LiBH4–40FG composite.LiBH4 is a promising hydrogen storage material for its large capacity. However, high desorption temperature, sluggish kinetics and demanding rehydrogenation severely hinder its practical use. Surface functional groups of graphene in many cases are treated as effective approaches to obtain some kinds of excellent properties of energy storage materials. In the current work, a new facile and effective strategy to improve the reversible hydrogen desorption properties of LiBH4 is proposed by composing with functionalized graphene to form the LiBH4–fluorographene composite. The fluorographene (FG) nanosheets are successfully exfoliated from fluorographite (FGi) and composed with LiBH4. It is demonstrated that the FG can remarkably improve the hydrogen desorption thermodynamics, kinetics and reversibility of LiBH4 via reactant destabilization method. An extremely fast hydrogen desorption process with a high capacity of 8.2 wt.% at 148.1 °C is achieved in the LiBH4–50FG composite. Further research reveals that the enhancement actually roots in the strengthened interfacial interaction between LiBH4 and exfoliated FG. Moreover, it is confirmed that the LiBH4–40FG composite exhibits a significantly enhanced reversible hydrogen desorption capacity of 7.2 wt.% and LiBH4 is regenerated. Such enhanced reversible hydrogen desorption properties are ascribed to the strengthened interfacial interactions between LiBH4 and FG with large surface, as well as the formation of LiHxF1−x phase.

Nanoscale catalyst doping is regarded as one of the most effective strategies to improve the kinetics performance of hydrogen storage materials, but the agglomeration of nanoparticles is usually unavoidable during the repeated de/rehydrogenation processes. Herein, hierarchically structured catalysts (Fe/C, Co/C and Ni/C) were designed and fabricated to overcome the agglomeration issue of nanocatalysts applied to the 2LiBH4-MgH2 system for the first time. Uniform transition metal (TM) nanoparticles (∼10 nm) wrapped by few layers of carbon are synthesized by pyrolysis of the corresponding metal–organic frameworks (MOFs), and introduced into the 2LiBH4-MgH2 reactive hydride composites (RHCs) by ball milling. The particular features of the carbon-wrapped architecture effectively avoid the agglomeration of the TM nanoparticles during hydrogen storage cycling, and high catalysis is maintained during the subsequent de/rehydrogenation processes. After de/rehydrogenation cycling, FeB, CoB and MgNi3B2 can be formed as the catalytically active components with a particle size of 5–15 nm, which show a homogeneous distribution in the hydride matrix. Among the three catalysts, in situ-formed MgNi3B2 shows the best catalytic efficiency. The incubation period of the Fe/C, Co/C and Ni/C-doped 2LiBH4-MgH2 system between the two dehydrogenation steps was reduced to about 8 h, 4 h and 2 h, respectively, which is about 8 h, 12 h and 14 h shorter than that of the undoped 2LiBH4-MgH2 sample. In addition, the two-step dehydrogenation peak temperatures of the Ni/C-doped 2LiBH4-MgH2 system drop to 323.4 °C and 410.6 °C, meanwhile, the apparent activation energies of dehydrogenated MgH2 and LiBH4 decrease by 58 kJ mol−1 and 71 kJ mol−1, respectively. In particular, the cycling hydrogen desorption of the Ni/C-doped 2LiBH4-MgH2 sample exhibits very good stability compared with the undoped sample. The present approach, which ideally addresses the agglomeration of nanoparticles with efficient catalysis on the RHCs, provides a new inspiration to practical hydrogen storage application for high performance complex hydrides.

Highly dispersed AgPd hollow spheres anchored on graphene (denoted as AgPd-Hs/G) were successfully synthesized through a facile one-pot hydrothermal route for the first time. The fabrication strategy was efficient and green by using L-ascorbic acid (L-AA) as the reductant and trisodium citrate dihydrate as the stabilizer, without employing any seed, surfactant, organic solvent, template, stabilizing agent, or complicated apparatus. The as-synthesized AgPd-Hs/G catalyst exhibits a sphere-shaped hollow structure with an average diameter of about 18 nm and a thin wall of about 5 nm. The hollow architecture with a thin wall and excellent dispersion on the graphene ensure that most of the atoms are located on the surface or sub-surface, which provides reactive catalytic sites for the dehydrogenation of formic acid. Therefore, a superior catalytic effect was achieved compared with other catalysts such as Pd/G and AgPd/C. The as-synthesized AgPd-Hs/G exhibits a catalytic activity with an initial turnover frequency (TOF) value as high as 333 mol H2 mol−1 catalyst h−1 even at room temperature (25 °C) toward the decomposition of formic acid. The present AgPd-Hs/G with efficient catalysis on the dehydrogenation of formic acid without any CO generation at room temperature can pave the way for a practical liquid hydrogen storage system and therefore promote the application of formic acid in fuel cell systems.

•The hydrogen desorption properties of LiBH4–Ca(BH4)2–NbF5@CMK-3 were investigated.•The modified sample releases H2 at 120 °C, lower by 80 °C than primitive sample.•13.3 wt% H2 is released within 250 min for LiBH4–Ca(BH4)2–NbF5@CMK-3.NbF5 and eutectic 0.68LiBH4–0.32Ca(BH4)2 were successively infiltrated into highly ordered mesoporous carbon (CMK-3) to obtain a LiBH4–Ca(BH4)2–NbF5@CMK-3 composite. A synergetic effect of nanoconfinement and catalysis on hydrogen desorption properties of the composite was achieved. The onset desorption temperature of LiBH4–Ca(BH4)2–NbF5@CMK-3 is dramatically lower than 120 °C compared with that of as-milled LiBH4–Ca(BH4)2 or LiBH4–Ca(BH4)2@CMK-3. A hydrogen amount as high as 13.3 wt% can be released within 250 min for LiBH4–Ca(BH4)2–NbF5@CMK-3, compared with only 10.4 wt% for pure LiBH4–Ca(BH4)2, in which nanoconfinement and catalysis both make some contribution. The cycling hydrogen desorption performance of the composite becomes better likewise due to the addition of NbF5. Further microstructure analysis reveals that F− tended to bind with Ca2+ to form CaF2, whereas Nb compound is believed to catalyze the whole system in the form of Nb-based boride (NbB2).

Mg2FeH6@MgH2 dual-metal hydrides with a core–shell nanostructure were synthesized via ball-milling and heat treatment methods using Mg and Fe as raw materials assisted by diethyl ether addition. Systematic investigations of the association between the microstructure and hydrogen desorption properties of the Mg2FeH6@MgH2 core–shell hydride were performed. It is found that the as-synthesized Mg2FeH6@MgH2 is comprised of the Mg2FeH6-core with a particle size of 40–60 nm and the MgH2-shell with a thickness of 5 nm. The hydrogen desorption of the Mg2FeH6@MgH2 core–shell nanoparticle starts at 220 °C, which is ∼45 °C lower than that of the Mg2FeH6/MgH2 micrometer particle. Compared to the as-synthesized Mg2FeH6/MgH2 micrometer particle, the Mg2FeH6@MgH2 core–shell sample exhibited faster hydrogen desorption kinetics, which released more than 5.0 wt% H2 within 50 min at 280 °C. The desorption activation energy of the core–shell Mg2FeH6@MgH2 was reduced to 115.7 kJ mol−1 H2, while the desorption reaction enthalpy and entropy were calculated to be −80.6 ± 7.4 kJ mol−1 H2 and −140.0 ± 11.9 J K−1 mol−1 H2, respectively. It is proposed that the improvements of both hydrogen desorption kinetics and thermodynamics are due to the special core–shell nanostructure of Mg2FeH6@MgH2. More remarkably, it is demonstrated that the core–shell nanostructure could be recovered after rehydrogenation, leading to excellent cycling hydrogen desorption properties of Mg2FeH6@MgH2. In addition, the suggested dehydrogenation mechanism involves the dehydrogenation of the MgH2-shell followed by the decomposition of the Mg2FeH6-core into Mg and Fe according to the three-dimensional phase-boundary process.

•Annealing reduces the hydrogen absorption pressure and the desorption enthalpy.•Prolonging annealing time flattens the hydrogen desorption plateau of the alloy.•Prolonging annealing time enhances the hydrogen desorption plateau pressure.•Ti1.02Cr1.1Mn0.3Fe0.6 annealed at 1123 K for 5 h shows the best overall performance.The as-cast Ti1.02Cr1.1Mn0.3Fe0.6 alloy for hybrid hydrogen storage vessel application was annealed at different temperatures (873 K, 973 K, 1123 K, 1173 K) for 2 h, and annealed at 1123 K for different time (2, 5, 8 h) respectively, and their microstructure and hydrogen storage properties were investigated systematically. The results show that the as-cast alloy has a single C14 Laves phase, and all annealed alloys consist of a C14 Laves main phase and a secondary phase. After annealing at different temperatures for 2 h, the hydrogen absorption pressure at 298 K decreases, however, the maximum hydrogen storage capacity and desorption pressures at 318 K decrease slightly too. As the annealing time extends, the hydrogen absorption plateau pressure at 298 K and hydrogen desorption plateau pressure at 318 K increase, and the hydrogen desorption capacity increases first and then decreases, which reaches the highest desorption capacity of 1.721 wt.% at the annealing time of 5 h. Among the studied alloys, the alloy annealed at 1123 K for 5 h has the best overall properties for hybrid hydrogen storage application, its hydrogen absorption plateau at 298 K is 29.09 MPa, its hydrogen desorption plateau pressure at 318 K is 45.12 MPa, its hydrogen storage capacity is 1.721 wt.% and its dissociation enthalpy (ΔHd) is 17.78 kJ/mol H2.

•Novel synergistic effects of NbCl5 and h-BN on improving the dehydrogenation kinetics properties of LiBH4.•Dehydrogenation of NbCl5/h-BN co-doped LiBH4 is two-dimensional diffusion controlled kinetics mechanism.•The in situ formed nano-NbH@BN probably acts as the active species during the dehydrogenation.LiBH4 is an attractive material for hydrogen storage owing to its high hydrogen capacity of 13.8 wt% capacities. However, its high thermodynamic stability and sluggish kinetics limit its practical application as an onboard hydrogen storage medium. In this work, a synergetic effect of NbCl5 and hexagonal BN (h-BN) on notably improving the dehydrogenation properties of LiBH4 was investigated. It is found that the addition of NbCl5 and h-BN co-dopants can significantly enhance the dehydrogenation kinetics of LiBH4, and the catalytic effect of co-dopant is better than that of NbCl5 or h-BN dopant separately. The NbCl5/h-BN co-doped LiBH4 can release 10.78 wt% hydrogen, which is about 13 times and 5 times more than that of the NbCl5 doped LiBH4 and h-BN doped LiBH4 within 10 min at 400 °C, respectively. The major dehydrogenation temperature of NbCl5/h-BN co-doped LiBH4 is reduced to 377 °C, much lower than that of ball-milled LiBH4 (464 °C). The apparent activation energy (Ea) of hydrogen desorption is reduced from 195.81 kJ/mol of LiBH4 to 122.75 kJ/mol of NbCl5/h-BN co-doped LiBH4. The microstructural results reveal that the catalytic effect of NbCl5/h-BN co-dopant on improving the dehydrogenation kinetics of LiBH4 could be ascribed to the in situ formed nano NbH@h-BN, which serves as the heterogeneous nucleation site to reduce the decomposition activation energy barrier of LiBH4 and shortens the distance of the solid–liquid phase boundary movement of LiBH4 decomposition.

The remarkable hydrogen de/absorption properties of lithium borohydride are achieved by mechanically milling LiBH4 with hexagonal boron nitride (h-BN). It is found that the dehydrogenation properties of LiBH4 are improved with increasing the amount of h-BN. The 30 mol% h-BN doped LiBH4 composite starts to release hydrogen from just 180 °C, which is 100 °C lower than the onset dehydrogenation temperature of ball milled LiBH4. Moreover, the 30 mol% h-BN doped LiBH4 composite can release 12.6 wt% hydrogen in 2 h at 400 °C, while only 0.98 wt% H2 is gained from ball milled LiBH4. The apparent activation energy (Ea) of hydrogen desorption had been reduced from 198.31 kJ mol−1 for ball milled LiBH4 to 155.8 kJ mol−1 for 30 mol% h-BN doped LiBH4. In addition, the rehydrogenation of the composite is achieved under 400 °C and 10 MPa of H2. These remarkable results are largely attributed to the lone pair electrons of nitrogen induced destabilization of LiBH4 and their heterogeneous nucleation.

Magnesium hydride is widely investigated because of its high hydrogen storage capacity. However, the unfavorable thermodynamic and kinetic barriers hinder its practical application. To ease these problems, three kinds of NbHx nanoparticles were prepared by wet-chemical methods and then introduced into MgH2 for catalytically enhancing its hydrogen storage properties in this work. The results show that all the NbHx nanoparticles are effective in promoting the de-/rehydrogenation kinetics of MgH2, and the three NbHx doped MgH2 composites can desorb 7.0 wt % H2 within 9 min at 300 °C while ball milled MgH2 only releases 0.2 wt % H2 in 9 min and 4.1 wt % H2 even in 200 min. Interestingly, the significant hydrogen absorption by NbHx doped MgH2 under lower temperature ranging from 50 to 100 °C was observed; thus, MgH2/c-NbHx sample can uptake about 4.0 wt % H2 at 100 °C. It is found that the more disordered the structure and smaller the size of the NbHx particles, the better is the catalytic effect on hydrogen storage performances of MgH2. Analyses of XRD, XPS, and TEM results indicate that the NbHx remains stable in the ball milling and following de-/rehydrogenation process and act as active catalytic species in improving hydrogen storage performance of MgH2. Moreover, a mechanism is proposed to understand how the nanosized NbHx acted as charge transfer between Mg2+ and H–, which contributes to the significantly improved hydrogen storage performances of MgH2. It is believed that the use of Nb-based nanoparticles as catalysts would greatly promote the development of the practical applications of MgH2 for hydrogen storage.

•We firstly introduce a new scaffold with high mechanical stability in confined system.•Dehydrogenation temperature of densified composite is significantly reduced by 181 °C.•Rehydrogenation of LiBH4 achieves under mild condition with improved cycle stability.•Catalysis arises from the interface/surface effect between hydrides and scaffolds.•Succeeded in increasing the gravimetric and volumetric capacity of the confined system.Nanoconfining hydrogen storage material inside nanopores has emerged as an intriguing strategy to influence the material characteristics but also sacrifices part of system gravimetric and volumetric hydrogen capacity. Herein, we tackle these two challenges by nanoconfining LiBH4 into a new scaffold, zeolite-templated carbon (ZTC), with high porosity and excellent mechanical stability to form a densified nanoconfinement system. After nanoconfinement of LiBH4 and 750 MPa densification, the nanocomposite begins to release hydrogen at 194 °C, 181 °C lower than that of the bulk LiBH4. The rehydrogenation of LiBH4 achieves under mild conditions with improved cycle stability. Moreover, the activation energy of hydrogen desorption is dramatically reduced by 60.4 kJ mol−1, coupled with the foaming effect of desorption almost eliminated. More importantly, the over-infiltrated LiBH4@ZTC systems are capable of maintaining their good hydrogen storage performances, which is attributed to the interface/surface effect between hydrides and scaffolds. With ultra-high pressure densification and high uploading amount of LiBH4, the nanoconfined composite achieves an exceptional gravimetric capacity of 6.92 wt% and volumetric capacity of 75.43 g L−1. These findings add new insights in the development of nanoconfinement systems with enhanced hydrogen storage capacity.

SnO2 has high capacity but poor cycling stability for Li-ion batteries due to pulverization and aggregation. Herein, we tackle these two challenges by uniformly dispersing carbon coated nanoSnO2 into a micro-sized porous carbon matrix to form a nano-SnO2/C composite anode using a facile and scalable in situ synthesis strategy. The SnO2@C nanocomposite exhibits a capacity of 640 mA h g−1 at 500 mA g−1 in the initial 150 cycles and then increases to 720 mA h g−1 and maintains this capacity for 420 cycles. The superior electrochemical performance with long cycle lifetimes of the carbon foam–SnO2 nanocomposites could be attributed to their unique carbon microstructures: the network of carbon sheets provides favorable electron transport, while the interconnected micro-/mesopores can serve as the effective channels of lithium ion transport, thereby supplying short lithium ion diffusion pathways. Meanwhile, these pores surrounding the active species of nanoSnO2 along with flexible carbon nanosheets can accommodate the severe volume variations during prolonged electrochemical cycling and mitigate the Sn aggregation. The present study provides a large-scale synthesis route to synthesize SnO2-based anode materials with superior electrochemical performance for lithium ion batteries.

As anode materials for lithium ion batteries, metal oxides have large storage capacity. However, their cycle life and rate capability are still not suitable for commercial applications. Herein, 3D hierarchical Fe3O4 spheres associated with a 5–10 nm carbon shell were designed and fabricated. In the constructed architecture, the thin carbon shells can avoid the direct exposure of encapsulated Fe3O4 to the electrolyte and preserve the structural and electrochemical integrity of spheres as well as inhibit the aggregation of pulverized Fe3O4 during electrochemical cycling. The hierarchical structure formed by the bottom-up self-assembly approach can efficiently accommodate the mechanical stress induced by the severe volume variation of Fe3O4 during lithiation–delithiation processes. Moreover, the carbon shell together with the structure integrity and durability endows the favorable high conductivity and efficient ion transport. All these features are critical for high-performance anodes, therefore enabling an outstanding lithium storage performance with a long cycle lifespan. For instance, such an electrode could deliver a capacity of 910 mA h g−1 even after 600 cycles with a discharge–charge rate of 1 A g−1. In addition, this effective strategy may be readily extended to construct many other classes of hybrid electrode materials for high-performance lithium-ion batteries.

•Co-doping CeH2 + KH can improve the hydrogen storage property of NaAlH4 markedly.•0.98NaH/Al + 0.02CeCl3/0.02KH composite shows the best hydrogen storage property.•Thermodynamics and kinetics of NaAlH4 are improved by Co-doping with CeH2 + KH.•Dehydrogenation of composite is conformed to phase-boundary growth mechanism.Sodium aluminum hydride (NaAlH4) was directly synthesized by ball milling NaH/Al co-doped with CeCl3 + KH under a hydrogen pressure of 3 MPa at room temperature. Out of various samples corresponding to xNaH/Al + 0.02CeCl3 + yKH (x + y = 1; y = 0, 0.02, 0.04 mol%) composites, the composite with y = 0.02 exhibits the optimum de/hydrogenation properties. It shows that the addition of KH can effectively improve the dehydrogenation properties of second step reaction of NaAIH4 system. The composite with y = 0.02 starts to release hydrogen from 87 °C and completes dehydrogenation within 20 min at 170 °C, with good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). After ball milling, the CeCl3 precursor can be changed into CeH2 catalytic active component in the first several de/hydrogenation cycles. Apparent activation energy of the second decomposition step of NaAIH4 system can be effectively decreased by addition of KH, resulting in the decrease of desorption temperatures. Based on the microstructure analyses combined with hydrogen storage performances, the improved dehydrogenation properties of sodium aluminum hydride system are ascribed to the lattice volume expansion of Na3AlH6 during the dehydrogenation process resulted from the addition of KH. Moreover, by analyzing the reaction kinetics of CeCl3 + KH co-doped sample, both of the decomposition steps of composite with y = 0.02 were conformed to the two-dimension phase-boundary growth mechanism. The mechanistic investigations gained here could help to understand the de/rehydrogenation behaviors of catalyzed complex metal hydride systems.Sodium aluminum hydride (NaAlH4) was directly synthesized by ball milling NaH/Al co-doped with CeCl3 + KH under a hydrogen pressure of 3 MPa at room temperature. It was found that the addition of KH can effectively improve the dehydrogenation properties of second step reaction. Among different composites, 0.98NaH/Al + 0.02CeCl3/0.02KH shows the best de/hydrogenation properties, and both of the dehydrogenation steps of 0.98NaH/Al + 0.02CeCl3/0.02KH composite were conformed to two-dimension phase-boundary growth mechanism.

•NbCl5 doped 2LiH/MgB2 with superior de/hydrogenation properties was observed for the first time.•The 2LiH–MgB2–0.03NbF5 composite absorbs 9.3 wt% H2 within 150 min.•Its average hydrogenation rate is four times faster than that of undoped sample.•The 2LiH–MgB2–0.03NbF5 composite can release more than 9.0 wt% H2 within 300 min at 400 °C.•The well-distributed NbH plays an important role in the improved of de/hydrogenation properties.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 3 mol% NbCl5 doped 2LiH–MgB2 composite exhibits the superior reversible hydrogen storage performance. This composite can release more than 9.0 wt % H2 within 300 min at 400 ºC without obvious degradation of capacity over the first 10 cycles. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system.

•Superior dehydrogenation property is achieved for nano-sized LiBH4 adsorbed on surface of FGi.•The LiBH4–FGi composite starts to release hydrogen without impurity gas at around 180 °C.•About 7.4 wt% hydrogen is released within seconds below 200 °C for the LiBH4–FGi composite.•Partial reversibility of the LiBH4–FGi composite has been demonstrated.A significant enhancement in the dehydrogenation performance of LiBH4 is achieved by modifying with fluorographite (FGi). In-depth investigations show that the dehydrogenation thermodynamics and kinetics of LiBH4 are strongly improved by ball milling LiBH4 with FGi. The ball-milled LiBH4–FGi (mass ratio of 1:1) composite starts to release hydrogen without impurity gas at around 180 °C, and obtains a hydrogen desorption capacity of 7.2 wt% below 200 °C in seconds, which is improved dramatically compared with pristine ball-milled LiBH4. Microscopic morphology indicates that numerous ∼90 nm spots formed on the surface of FGi. Based on the microstructure analyses combined with hydrogen storage performances, the prominent effect of FGi is largely attributed to the nano-modifying effect and the exothermic reaction between LiBH4 and FGi during the dehydrogenation process. Furthermore, partial reversibility of the LiBH4–FGi composite has been demonstrated and the mechanism underlying the cycling capacity loss is discussed. The use of FGi may shed light on future study on searching for new strategies to improve both the thermodynamics and kinetics of light-metal complex hydrides.Superior hydrogenation performance is achieved for the nano-sized LiBH4 adsorbed on the surface of FGi, in which a capacity close to 7.4 wt% is realized within seconds. Furthermore, hydrogen can be released at a temperature as low as 180 °C, which can fulfill the operation temperature range of PEM fuel cells. This method is highly promising for monitoring of dehydrogenation properties of metal complex borohydrides and could lead new solid-state hydrogen storage materials to higher performances.

•We study the critical addition amount of NbF5 to emit catalytic characteristic.•We reveal a possible catalytic mechanism of NbF5 in 2LiBH4–MgH2 system.•2LiBH4–MgH2 doped with NbF5 in weight ratios of 40:4 exhibit superior properties.•Excellent reversibility as high as 20 cycles without capacity loss is achieved.•The remarkable catalytic effects of NbF5 are derived from the reaction with LiBH4.In the present work, the role of NbF5 addition amount in affecting the comprehensive hydrogen storage properties (dehydrogenation, rehydrogenation, cycling performance, hydrogen capacity) of 2LiBH4–MgH2 system as well as the catalytic mechanism of NbF5 have been systematically studied. It is found that increasing the addition amount of NbF5 to the 2LiBH4–MgH2 system not only results in dehydrogenation temperature reduction and hydriding–dehydriding kinetics enhancement but also leads to the de/rehydrogenation capacity loss. Compared with other samples, 2LiBH4–MgH2 doping with NbF5 in weight ratios of 40:4 exhibits superior comprehensive hydrogen storage properties, which can stably release ∼8.31 wt.% hydrogen within 2.5 h under 4 bar H2 and absorb ∼8.79 wt.% hydrogen within 10 min under 65 bar H2 at 400 °C even up to 20 cycling. As far as we know, this is the first time that excellent reversibility as high as 20 cycles without obvious degradation tendency in both of hydrogen capacity and reaction rate has been achieved in the 2LiBH4–MgH2 system. The further experimental study reveals that the highly catalytic effects of NbF5 on the 2LiBH4–MgH2 system are derived from the reaction between NbF5 and LiBH4, which provides a fundamental insight into the catalytic mechanism of NbF5.

•The dehydrogenation and reversibility of LiBH4 were significantly improved by NbF5.•LiBH4–NbF5 samples started releasing hydrogen from as low as 60 °C.•4 wt.% hydrogen could be obtained below 255 °C in 5LiBH4–NbF5.•Over 4.4 wt.% H2 could still be released even for the fifth cycle in 20LiBH4–NbF5.In this work, the hydriding–dehydriding properties of the LiBH4–NbF5 mixtures were investigated. It was found that the dehydrogenation and reversibility properties of LiBH4 were significantly improved by NbF5. Temperature-programed dehydrogenation (TPD) showed that 5LiBH4–NbF5 sample started releasing hydrogen from as low as 60 °C, and 4 wt.% hydrogen could be obtained below 255 °C. Meanwhile, ∼7 wt.% H2 could be reached at 400 °C in 20LiBH4–NbF5 sample, whereas pristine LiBH4 only released ∼0.7 wt.% H2. In addition, reversibility measurement demonstrated that over 4.4 wt.% H2 could still be released even during the fifth dehydrogenation in 20LiBH4–NbF5 sample. The experimental results suggested that a new borohydride possibly formed during ball milling the LiBH4–NbF5 mixtures might be the source of the active effect of NbF5 on LiBH4.

A simple approach to dramatically enhance the dehydrogenation properties of sodium borohydride is achieved by ball milling NaBH4 with fluorographite (FGi). It was found that the ball-milled NaBH4–FGi composite starts to release hydrogen without impurity gas at a lower temperature of 125 °C, and obtains a hydrogen desorption capacity of ca. 4.8 wt% below 130 °C in seconds, which is improved markedly compared to the ball-milled pristine NaBH4. The significant thermodynamic and kinetic improvement of the NaBH4–FGi composite can be ascribed to the reaction between NaBH4 and FGi as well as the formation of micro-scale NaBH4. Moreover, since the dehydrogenation process of NaBH4–FGi composite is exothermal, the fully reverse reaction is not feasible. In-depth investigations show that the partial rehydrogenation is due to the formation of Na2B12H12 and another new borohydride.

•SnLi4.4@C core–shell hierarchical composite was successfully fabricated.•The composite exhibits a reversible capacity of 680 mA h g−1 after 200 cycles at 200 mA g−1.•The present study effectively circumvents the low initial Coulombic efficiency of the Sn-related nanocomposites.•The present study provides a protocol for pairing lithium-free cathodes to make the next-generation high energy LIBs.Induction melting associated with simple ball-milling is utilized to synthesize a SnLi4.4@C core–shell hierarchical composite in which nanometer-sized SnLi4.4 particles are uniformly dispersed and encapsulated by carbon matrix. When evaluated as anode materials for lithium ion batteries, the composite exhibits a reversible capacity of 680 mA h g−1 after 200 cycles at 200 mA g−1. A capacity of 310 mA h g−1 is obtained even at a high rate of 5000 mA g−1. The superior electrochemical performance is ascribed to the fact that the prelithiated SnLi4.4 will not exert any expansion stress on the carbon matrix during the subsequent delithiation and lithiation processes, therefore guarantee the sustainable integrity of the composite in the prolonged cycling. The carbon matrix offers continuous transport paths for Li+ ions and electrons inside the composite. Meanwhile the carbon can sufficiently prevent the disintegration and aggregation of Sn nanoparticles upon prolonged cycling. The present study effectively circumvents the low initial Coulombic efficiency of the Sn-related nanocomposites and provides a protocol for pairing lithium-free cathodes to make the next-generation high energy lithium ion batteries.

A system of 2LiH–MgB2 and its hydrogenated 2LiBH4–MgH2 is an attractive candidate for hydrogen storage. However, its hydriding–dehydriding kinetics have to be further improved for practical application. In the present work, three kinds of Ni–B nanoparticles with different crystalline states and particle sizes were prepared by wet-chemical reduction and mechanochemical methods, and then introduced into a 2LiH–MgB2 composite for catalytic enhancement. The catalytic roles of Ni–B nanoparticles on the hydriding–dehydriding properties were investigated systematically. The results show that all of the Ni–B nanoparticles can significantly enhance the hydriding–dehydriding kinetics of the 2LiH–MgB2 composite, resulting in no incubation period for the formation of MgB2 during dehydrogenation. The more disordered the amorphous structure and the smaller sized the Ni–B particles are, the better the catalytic effect that is obtained. Microstructure analyses clearly reveal the formation of the MgNi3B2 phase in the dehydriding process, which acts as the nucleation agent for MgB2 formation determined by an edge-to-edge model. Directly doping with Ni–B nanoparticles in the 2LiH–MgB2 system shows a higher hydrogen desorption capacity of 9.4 wt% and obtains a better catalytic efficiency than doping with NiCl2.

NaAlH4, a prototypical high energy density complex hydride, possesses a favorable thermodynamics and high hydrogen storage capacity. However, the poor kinetics and degradation of cycling stability retard its practical application. To ease these problems, CeB6, CeF3 and CeO2 nanoparticles with a size of about 10 nm are synthesized by the wet-chemistry method and introduced into NaAlH4 systems as additives in this work. The results show that all of the nanoparticles are effective in improving the hydriding–dehydriding kinetics of NaAlH4, and nano-CeB6 possesses the highest catalytic activity. The rehydrogenation of dehydrogenated NaAlH4 doped with nano-CeB6 can be accomplished in less than 20 min with a high capacity of 4.9 wt%, which shows a 20% increase in capacity compared to that of chloride-doped NaAlH4. Due to the structural stability and good dispersion of nano-CeB6 and nano-CeF3, a favorable cycling stability with high capacity retention is achieved for their doped samples. Moreover, hydrogen can be released from the hydrogenated sample doped with nano-CeB6 at a temperature as low as 75 °C, fulfilling the operation temperature of a PEM fuel cell. In the nano-CeO2 doped NaAlH4 system, CeO2 is first reduced to CeH2.51. In the subsequent cycles, the formed CeH2.51 gradually transforms into Ce–Al, and simultaneously the kinetics of the doped system is further enhanced. It is believed that the utilization of Ce-based nanoparticles as catalysts would substantially improve the practical applications of NaAlH4 for hydrogen storage.

LiBH4–MgH2 system in a 2:1 molar ratio constitutes a representative reactive hydride composite (RHC) for hydrogen storage. However, sluggish kinetics and poor reversibility hinder the practical applications. To ease these problems, amorphous TiB2 and NbB2 nanoparticles were synthesized and employed as catalysts for the 2LiBH4–MgH2 system. Isothermal de-/rehydrogenation and temperature programmed mass spectrometry (MS) measurements show that amorphous TiB2 and NbB2 nanoparticles can significantly improve the hydrogen storage performance of the 2LiBH4–MgH2 system. 9 wt% hydrogen can be released within only 6 min for nanoTiB2-doped 2LiBH4–MgH2, while for the undoped composite limited hydrogen of 3.9 wt% is released in 300 min at 400 °C. The dehydrogenation activation energies for the first and second steps are dramatically reduced by 40.4 kJ mol−1 and 35.2 kJ mol−1 after doping with nanoTiB2. It is believed that TiB2 and NbB2 nanoparticles can first catalyze the dehydrogenation of MgH2, and then induce the decomposition of LiBH4 and meanwhile act as nucleation agents for MgB2, thereby greatly enhancing the kinetics of dehydrogenation. The present study gives clear evidence for the significant performance of transition metal boride species in doped RHCs, which is critically important for understanding the mechanism and further improving the hydrogen storage properties of RHCs.

•Ti–Cr–Mn–Fe based alloys have been developed for hybrid hydrogen storage application.•Partial substitution of Cr with Fe or Mn improves the hydrogen desorption pressure.•Ti super-stoichiometry in Ti–Cr–Mn–Fe alloys improves the hydrogen storage capacity.•Ti1.02Cr1.1Mn0.3Fe0.6 shows the best overall properties for hybrid hydrogen storage.Three series of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption plateau pressures for hybrid hydrogen storage vessel application were prepared by induction levitation melting, as well as their crystallographic characteristics and hydrogen storage properties were investigated. The results show that all of the alloys were determined as a single phase of C14-type Laves structure. As the Fe content in the TiCr1.9−xMn0.1Fex (x = 0.4–0.6) alloys increases, the hydrogen absorption and desorption plateau pressures increase, and the hydrogen storage capacity and plateau slope factor decrease respectively. The same trends are observed when increasing the Mn content in the TiCr1.4−yMnyFe0.6 (y = 0.1–0.3) alloys, except for the plateau slope factor. Compared with the stoichiometric TiCr1.1Mn0.3Fe0.6 alloy, the titanium super-stoichiometric Ti1+zCr1.1Mn0.3Fe0.6 (z = 0.02, 0.04) alloys have larger hydrogen storage capacities and lower hydrogen desorption plateau pressures. Among the studied alloys, Ti1.02Cr1.1Mn0.3Fe0.6 has the best overall properties for hybrid hydrogen storage application. Its hydrogen desorption pressure at 318 K is 41.28 MPa, its hydrogen storage capacity is 1.78 wt.% and its dissociation enthalpy (ΔHd) is 16.24 kJ/mol H2.Ti–Cr–Mn–Fe based alloys with high hydrogen desorption plateau pressures have been developed for hybrid hydrogen storage vessel application. Among the studied alloys, Ti1.02Cr1.1Mn0.3Fe0.6 shows the best overall performance for hybrid hydrogen storage. Its hydrogen desorption plateau pressure at 318 K is 41.28 MPa, its hydrogen storage capacity is 1.78 wt.% and its dissociation enthalpy is 16.24 kJ/mol H2.

•A novel method for the synthesis of amorphous Ni–B nanoparticles has been developed.•The as-synthesized Ni–B nanoparticles are uniform with size of 5 nm.•The amorphous Ni–B has larger specific surface area and higher thermal stability.•This work provides a novel method for preparation of other amorphous nanoparticles.A novel method for the synthesis of amorphous Ni–B nanoparticles has been developed using a mechanochemical displacement reaction by high energy ball milling. Based on various characterizations including XRD, ICP, BET, DSC, SEM, TEM and XPS, the synthesized Ni–B amorphous nanoparticles are present in the form of uniform and homogeneously distributed ultrafine nanoparticles with the size around 5 nm. Larger specific surface area and higher thermal stability can be obtained by such mechanochemical method than the conventional wet-chemical reduction method. The present work provides a novel method for preparation of other amorphous nanoparticles, and greatly expands the industrial application space of Ni–B amorphous nanoparticles.

•NaAlH4 was infiltrated into porous carbons with pore size of 200, 60, 30 and 4 nm, respectively.•Ea of dehydrogenation decreased by 43 and 58 kJ/mol for NaAlH4 in carbon with pore size of 30 and 4 nm.•Decomposition temperature of in situ formed NaH in carbon with pore size≤30 nm was reduced by 100 °C.•Small pore size exerts a favorable effect on the cycling capacity retention of confined NaAlH4.Uniform porous carbons with pore size of 200 nm, 60 nm, 30 nm and 4 nm were separately synthesized using hard-template method, and nanoparticles of NaAlH4 have been infiltrated into the above porous carbons. A correlation between pore size and hydrogen desorption kinetics of NaAlH4 is established. In contrast with bulk NaAlH4, all of the confined NaAlH4 exhibit a single-step dehydrogenation process to form NaH/Al product. However, the onset and peak hydrogen desorption temperatures are significantly influenced by the pore size of carbon. As the pore size is reduced to 30 nm, NaAlH4 starts to decompose at temperature of about 100 °C, with a dehydrogenation peak of 172 °C. The apparent activation energy for dehydrogenation is estimated to be 84.9 and 69.7 kJ/mol for the NaAlH4 confined in 30 and 4 nm porous carbons, reduced by 43 and 58 kJ/mol contrast to the first step of bulk NaAlH4, respectively. Besides, by confining NaAlH4 into porous carbon with pore size≤30 nm, the in situ generated NaH can decompose at a temperature below 250 °C, i.e. reduction of the temperature for 100 °C compared with that generated by decomposition of bulk NaAlH4. Thus all of hydrogen in NaAlH4 can be released and utilized at a relatively moderate condition without doping additives. Cycling investigation shows that smaller size exerts a favorable effect on the hydrogen storage capacity retention.Graphical abstract

A significant improvement in hydrogenation/dehydrogenation properties of 2LiH/MgB2 can be achieved by adding NbF5. The results show that the NbF5 additive is effective for enhancing the de/hydrogenation kinetics of the Li–Mg–B–H system and reducing the desorption temperatures of MgH2 and LiBH4. For the 2LiH–MgB2–0.03NbF5 sample, About 9.0 wt % hydrogen capacity is obtained rapidly under cyclic conditions of rehydrogenation within 20 min at 350 °C and dehydrogenation within 20 min at 400 °C; thus, catalytic improvement persists well in the subsequent reversible dehydrogenation cycles. Moreover, the sample could reversibly reabsorb and release more than 9.0 wt % hydrogen even at 250 and 375 °C, respectively. Microstructure analyses reveal that the NbF5 additive in improving the de/hydrogenation properties of Li–Mg–B–H system could be ascribed to the synergistic effect of in situ formed nano NbH particles acting as “active gateways” facilitating the diffusion of hydrogen, and the “favorable thermodynamic destabilization” from the reversible transition of LiH1–xFx caused by functionality of F-anion substitution. This fundamental understanding provides us with insights into the design and optimization of the catalytic method and species for the catalyzed Li–Mg–B–H system.

2LiBH4–MgH2 is an attractive system for hydrogen storage. However, its dehydriding and rehydriding kinetics have to be further improved for practical applications. To solve these problems, three transition metal chlorides (FeCl2, CoCl2 and NiCl2) were introduced into the 2LiBH4–MgH2 system and their catalytic roles in the dehydrogenation–rehydrogenation properties were investigated systematically. The results show that all three chlorides can significantly enhance the dehydriding and rehydriding kinetics of the 2LiBH4–MgH2 system with/without hydrogen back pressure, and NiCl2 is the best modifier among them. The NiCl2-doped sample exhibits no incubation period for generating MgB2 during dehydrogenation. X-Ray diffraction and scanning electron microscopy analyses clearly reveal the phase formation of MgNi3B2, which may act as the nucleation site for MgB2 formation, and thus the incubation period shrinks. A highly homogeneous distribution of the in situ formed MgNi3B2 nanoclusters can significantly improve the dehydriding–rehydriding kinetics. Single-phase MgNi3B2 was successfully synthesized and a similar catalytic effect is obtained by directly doping identical MgNi3B2 into the 2LiBH4–MgH2 system, which results in a higher hydrogen storage capacity of 9.4 wt%.

The as-cast Ti10V77Cr6Fe6Zr alloy was heat-treated at 1373 K for 8 h or 1523 K for 5 min and then quenched in water. The influence of heat treatment on the microstructure and hydrogen storage properties of Ti10V77Cr6Fe6Zr alloy was investigated systematically. The results show that all of the as-cast and heat-treated alloys consist of BCC main phase and C14 Laves secondary phase. After heat treatment, the phase abundance of BCC enhances and the plateau region of P–C–T curve is flattened, but the hydrogen absorption capacity is decreased. However, the alloy heat-treated at 1523 K for 5 min achieves a enhanced hydrogen desorption capacity of 1.82 wt.% at 333 K against 0.1 MPa, which is higher than 1.44 wt.% hydrogen desorption capacity of the as-cast alloy.Highlights► The as-cast Ti10V77Cr6Fe6Zr alloy with a BCC main phase and a little C14 Laves secondary phase was prepared by levitation induction melting. ► The as-cast Ti10V77Cr6Fe6Zr alloy was heat-treated at 1373 K for 8 or 1523 K for 5 min and then quenched in water. ► After heat treatment, the content of C14 Laves phase decreases and the hydrogen desorption plateau is flattened distinctly. ► The sample heat-treated at 1573 K for 5 min has the best overall hydrogen storage properties.

The hydrogen storage properties of 5LiBH4 + Mg2FeH6 reactive hydride composites for reversible hydrogen storage were investigated by comparing with the 2LiBH4 + MgH2 composite in the present work. The dehydrogenation pathway and reaction mechanism of 5LiBH4 + Mg2FeH6 composite were also investigated and elucidated. The self-decomposition of Mg2FeH6 leads to the in situ formation of Mg and Fe particles on the surface of LiBH4, resulting in a well dispersion between different reacting phases. The formation of FeB is observed during the dehydrogenation of 5LiBH4 + Mg2FeH6 composite, which might supplies nucleation sites of MgB2 during the dehydrogenation process, but is not an ascendant catalyst for the self-decomposition of LiBH4. And FeB can also transform to the LiBH4 and Fe by reacting with LiH and H2 during the rehydrogenation process. The dehydrogenation capacity for 5LiBH4 + Mg2FeH6 composite still gets to 6.5 wt% even after four cycles. The X-ray diffraction analyses reveal the phase transitions during the hydriding and dehydriding cycle. The formed FeB in the composite maintains a nanostructure after four hydriding-dehydriding cycles. The loss of hydrogen storage capacity and de-/rehydrogenation kinetics can be attributed to the incomplete generation of Mg2FeH6 during the rehydrogenation process.Highlights► MgH2 and Fe is added to LiBH4 by the addition of Mg2FeH6 as a precursor. ► A total hydrogen release capacity of 8 wt% is obtained for the composite. ► The composite still gets to 6.5 wt% even after the four cycles. ► The formation of FeB might supplies nucleation sites of MgB2. ► The loss of cycle performance is attributed to the incomplete generation of Mg2FeH6.

2LiBH4–Li3AlH6 composite samples doped with or without 5 wt % metal fluorides (CeF3, NiF2 and TiF3) were prepared by ball milling, and the effects of fluoride additives on the hydrogen storage performance of the samples were comparatively investigated. It is found that the undoped 2LiBH4–Li3AlH6 system presents a favorable destabilized dehydrogenation performance as compared with the as-milled pure LiBH4. The three fluorides can enhance the dehydriding kinetics of the 2LiBH4–Li3AlH6 destabilized system to some extent. The TiF3-doped composite exhibits the most prominent behavior in terms of the low dehydrogenation temperature and fast dehydriding rate. The activation energy for the decomposition of LiBH4 was measured by differential scanning calorimetry; that of the TiF3 doped composite was calculated to be 118.3 kJ/mol, which is much lower than that of the undoped composite (197.6 kJ/mol). In addition, the experimental results show that the reversibility of the 2LiBH4–Li3AlH6 composite is improved by the doping of TiF3, which plays a catalytic role, strengthens the interaction between LiBH4 and Li3AlH6, and thus further improves the de/rehydrogenation performance.

By directly introducing LaCl3, La3Al11, SmCl3, SmAl3 into NaAlH4 system using one-step synthesis method, the effects of these additives on NaAlH4 were systematically investigated with regard to hydriding and dehydriding properties. Results showed that the materials doped with aluminide exhibit similar kinetics to the chloride-doped NaAlH4. The apparent activation energy Ea of doped NaAlH4 were calculated to be 86.4–93.0 kJ/mol and 96.1–99.3 kJ/mol for the first and second dehydrogenation step respectively by using Kissinger’s approach, much lower than those of pristine NaAlH4. A reversible hydrogen capacity of 4.8 wt% can be achieved for the La3Al11- and SmAl3-doped NaAlH4, which is 10–20% higher than chloride-doped NaAlH4. Investigations on the phase evolvement and microstructure in the cycling in LaCl3- and La3Al11-doped NaAlH4 clearly demonstrate that La species is presented as the form of La-Al nanoclusters in the materials. The combination of hydrogen storage properties and the microstructures unequivocally reveal that the in situ formed rare-earth-Al species play a crucial rule in catalyzing the chloride-doped NaAlH4.Highlights► LaCl3, SmCl3, La3Al11, SmAl3-doped NaAlH4 were prepared by direct synthesis method. ► Similar kinetics were obtained in the chloride and aluminide-doped NaAlH4. ► The materials doped with aluminide exhibit higher hydrogen storage capacities. ► XRD and SEM/EDS indicate that RE-Al species play a crucial rule in catalyzing NaAlH4.

TiF3-doped NaH/Al mixture was hydrogenated into Na3AlH6 and NaAlH4 complex hydrides by reactive ball-milling at room temperature through the optimization of milling duration and hydrogen pressure. The analysis of the preparation of NaAlH4 samples during reactive ball-milling process has been performed by XRD and TG/DSC. It has been found that Na3AlH6 was formed under 0.5 MPa hydrogen pressure and 30 h milling duration, while NaAlH4 was formed under 0.8 MPa hydrogen pressure and 45 h milling duration. The process of preparing NaAlH4 by ball-milling was found accomplished via two reaction steps, namely: (1) NaH + Al + H2 → Na3AlH6 and (2) Na3AlH6 + Al + H2 → NaAlH4. As the hydrogen pressure and milling duration increase, the synthetic yield of NaAlH4 and its corresponding dehydriding capacity are both increased. With increased hydrogen pressure (0.8–3 MPa) and milling duration (45–60 h), the cell volume of Na3AlH6 decreases while that of NaAlH4 increases gradually. The abundance of Na3AlH6 phase decreases from 57.76 (x = 0.8, y = 45) to 8.69 wt.% (x = 3, y = 60), and the abundance of NaAlH4 phase increases from 20.63 (x = 0.8, y = 45) to 86.50 wt.% (x = 3, y = 60). All the samples prepared in this way have fairly good activation behavior and fast hydriding/dehydriding reaction kinetics, which are capable of absorbing 4.26 wt.% hydrogen at 120 °C and desorbing 4.12 wt.% hydrogen at 150 °C, respectively. The improvement of hydriding/dehydriding properties is ascribed to the favorable microstructure and ultrafine particle features of nanosized NaAlH4 formed during ball-milling at the optimum synthetic condition.

One of the major questions in a catalytically enhanced NaAlH4 system used for hydrogen storage that remains is where catalysts like Ti/Ce reside and present as what form improving the kinetics and reversible hydrogen storage performance. In the present study, by directly introducing Ce−Al species with a structure of CeAl2 into NaAlH4, a dramatic enhancement in the hydrogen release and uptake kinetics of NaAlH4 was achieved. CeAl2-doped NaAlH4 can be reloaded 4.9 wt % hydrogen at moderate conditions in 20 min, which is among the highest values ever reported for NaAlH4. Besides, the material exhibits an exceptional performance under low pressures. For example, a capacity of more than 4.0 wt % hydrogen can be achieved at a hydrogen pressure as low as 4.0 MPa. The apparent activation energy of NaAlH4 doped with 2 mol % CeAl2 is estimated to be 72.3−90.4 kJ/mol and 93.6−98.9 kJ/mol for the first and the second dehydrogenation step respectively by using Kissinger’s approach, much lower than those of pristine NaAlH4. After prolonged cycling, the Ce−Al species transforms to a more stable species of CeAl4. On the basis of these findings and the previous investigations, the active species and mechanism of catalysis in doped NaAlH4 were discussed.

The synthesis and hydrogen-storage properties of the mixed alkali metal and alkaline-earth metal borohydride are systematically investigated. It is found that mechanical milling a mixture of LiBH4 and CaCl2 with molar ratio of 3:1 in tetrahydrofuran (THF) forms a new LiBH4·Ca(BH4)2·2THF compound. Thermal analysis indicates that the decomposition process of the new compound involves four steps: (1) LiBH4·Ca(BH4)2·2THF decomposes first into high temperature phase of LiBH4, CaB6, and an intermediate phase Ca–B–H–Cl hydride and synchronously releases a large number of H2 and THF gas together with less B2H6; (2) the eutectic melting of LiBH4 and Ca–B–H–Cl hydride; (3) Ca–B–H–Cl hydride decomposes into CaHCl, CaB6, and a little H2; (4) LiBH4 reacts with CaHCl, generating H2, CaH2, CaB6, LiH, and LiCl. The first dehydrogenation step starts at ca. 70 °C, which is much lower than that of the pristine LiBH4 and Ca(BH4)2. According to Kissenger’s equation, the activation energies of the different dehydrogenation steps are respectively 291.95, 132.06, and 117.13 kJ/mol, except the second decomposition step. Moreover, this new compound LiBH4·Ca(BH4)2·2THF is proved to be partially reversible, which can release 5.63 wt % hydrogen in the second rehydrogenation/dehydrogenation cycle.

The CeAl4-doped NaAlH4 has been synthesized by mechanical milling NaH/Al mixture with 4 mol % CeAl4 as catalyst under hydrogen pressure of 3 MPa. The hydrogen desorption thermodynamics and kinetics of as-synthesized NaAlH4 were systematically investigated. The enthalpies for the first and second dehydrogenation steps of CeAl4-doped NaAlH4 system are estimated to be 40.56 ± 1.62 and 51.48 ± 1.92 kJ/mol H2, respectively. By regulating the desorption temperatures, the two dehydrogenation steps were studied separately under a constant hydrogen backpressure of 0.1 MPa. The apparent activation energy, Ea, for the first and second step is estimated to be 87.9 and 103.6 kJ/mol, respectively, by using Arrhenius equation. Isothermal dehydrogenation measurements show that no induction period is observed in the first step or the second step under the measuring conditions. Both of the decomposition steps conform to the Johnson–Mehl–Avrami (JMA) formalism with Avrami exponent n ≈ 1, indicating that the nucleation of decomposition process is of the site saturation type. Detailed modeling study presents that the first-step dehydrogenation kinetics is most likely controlled by the reaction at a moving boundary, whereas the second-step decomposition follows the first-order reaction mechanism. Change in the dehydrogenation temperature does not alter the nature of decomposition mechanism.

Nanocrystalline Na2LiAlH6 was directly synthesized by mechanical milling 2NaH/LiH/Al mixture with TiF3 catalyst under hydrogen pressure of 3.0 MPa. The synthesized Na2LiAlH6 exhibits a dehydriding capacity of 3.09 wt% in the first cycle, which is higher than that of Na3AlH6. Because of the complexity of mass transfer, the rehydrogenation process of the dehydrided Na2LiAlH6 is more intricate than that of the dehydrided sodium alanate, causing the formation of Na3AlH6 and the reduction of rehydriding capacity in the following cycles. As temperature increases from 70 to 120 °C, hydrogen absorption kinetics is extremely enhanced. The dehydrided material can reabsorb 80% of the reversible hydrogen capacity within 5 min when the temperature is above 100 °C with an initial hydrogen pressure of 4 MPa. The scanning electron microscopy and energy dispersive X-ray spectroscopy show that the as-synthesized Na2LiAlH6 along with the catalyst form a much homogeneous composite with a spherical particle size of 200 nm–2 μm.

The influence of Fe content on the microstructure and hydrogen storage properties of Ti16Zr5Cr22V57−xFex (x = 2–8) alloys was investigated systematically. The results show that all alloys consist of a BCC main phase and a small amount of C14 Laves secondary phase. The crystal lattice parameters of the BCC main phase in the alloys decrease with the increase of the Fe content. Under moderate conditions, all the alloys have good activation behaviors and hydriding/dehydriding kinetics. As the x increases, the hydrogen desorption plateau pressure of the alloys increases consequently. Among the studied alloys, Ti16Zr5Cr22V55Fe2 alloy has suitable hydrogen desorption plateau pressures indicated by the middle value of pressure range. (0.1–1 MPa) at 298 K and the best overall hydrogen storage properties.

The nanocrystalline Mg + x wt% LaMg2Ni (x = 0, 5, 10, 20, 30) composites were prepared by reactive ball-milling, their microstructure and hydrogen storage characteristics were investigated. The results show that the addition of LaMg2Ni improves the hydriding rate and capacity. The hydriding capacity of the Mg + x wt% LaMg2Ni (x = 5, 10, 20, 30) composites are all above 4.1 wt% at 120 °C and above 4.3 wt% at 180 °C within 6000 s. Moreover, the addition of LaMg2Ni also improves the dehydriding performance of the composites. The main reason for the improvement of hydriding/dehydriding properties investigated by XRD and SEM shows that the synergistic effect among the multiphase nanocrystalline Mg-based structures make hydrogen easily absorbed/desorbed on the interface of the matrix.

By directly doping CeAl4 into sodium aluminium hydride, which probably serves as the active species in the hydriding and dehydriding processes of CeCl3-doped NaAlH4, a high reversible hydrogen capacity of 4.77–4.92 wt% (close to expected capacity of 5.13 wt%) can be achieved in less than 20 min under moderate conditions.

The amorphous Mg–Al–Ni composites were prepared by mechanical ball-milling of Mg17Al12 with x wt.% Ni (x = 0, 50, 100, 150, 200). The effects of Ni addition and ball-milling parameters on the electrochemical hydrogen storage properties and microstructures of the prepared composites have been investigated systematically. For the Mg17Al12 ball-milled without Ni powder, its particle size decreases but the crystal structure does not change even the ball-milling time extending to 120 h, and its discharge capacity is less than 15 mAh g−1. The Ni addition is advantageous for the formation of Mg–Al–Ni amorphous structure and for the improvement of the electrochemical characteristics of the composites. With the Ni content x increasing, the composites exhibit higher degree of amorphorization. Moreover, the discharge capacity of the composite increases from 41.3 mAh g−1 (x = 50) to 658.2 mAh g−1 (x = 200) gradually, and the exchange current density I0 increases from 67.1 mA g−1 (x = 50) to 263.8 mA g−1 (x = 200), which is consistent with the variation of high-rate dischargeability (HRD). The ball-milled Mg17Al12 + 200 wt.% Ni composite has the highest cycling discharge capacity in the first 50 cycles.

V2.1TiNi0.4Zr0.06Mn0.05 alloy samples were heat-treated respectively at 1173 K, 1223 K and 1273 K for 18 h, then quenched in water. The phase structure and electrochemical properties of the as-cast and quenched samples were comparatively investigated. The results show that all the alloy samples consist of a V-based solid solution main phase and a C14-type Laves secondary phase, the secondary phase precipitates along the grain boundaries of the main phase. Moreover, after quenching treatment, the lattice parameters increase and the crystal grains of the main phase grow further. It is found that after quenching treatment at 1173 K, the quenched alloy have a satisfied maximum discharge capacity of 452 mAh g−1, while the capacity retention, high-rate dischargeability and exchange current density were improved significantly.

The synergistic effect of metallic Ti and Zr as co-dopants on the reversible hydrogen storage properties of NaAlH4NaAlH4 was investigated systematically. Metallic Ti and Zr powders were used directly and separately as dopants, as well as used as co-dopants together in the preparation of NaAlH4NaAlH4 by hydrogenation of ball-milled mixtures of NaH/Al. The hydriding/dehydriding properties of the composites were then investigated. It was found that the addition of Ti and Zr powders together as co-dopants on hydriding/dehydriding properties is superior to doping with Ti or Zr alone. The highest reversible hydrogen capacity of the hydride doped with Ti and Zr together as co-dopants is 4.34 wt% at 160∘C. The hydriding rate increases with the hydriding pressure increasing from 7.5 to 11.5 MPa. The dehydriding kinetics is improved with the dehydriding temperature increasing from 90 to 130∘C, and the dehydriding rate of the composite doped with Ti and Zr as co-dopants is 1.6 and 2.0 times that of the sample doped with Ti and Zr alone at 130∘C. Microstructure analysis reveals that the improvement of the hydriding/dehydriding properties of NaH/Al (NaAlH4NaAlH4) can be partially ascribed to the in situ interaction of active titanium-hydride and zirconium-hydrides formed in the ball-milling process and subsequently acted as the catalytic active sites on the surface of hydride matrix. The effect of lattice expansion on enthalpy change indicates that the further improved dehydriding property of the composite can also be attributed to a favorable thermodynamic modification of the bulk composite hydride co-doped with Ti–Zr powders. In a word, the synergistic effect on the improvement of hydriding/dehydriding properties by introducing the addition of Ti–Zr together as co-dopants can be ascribed to both the “superficial catalytic process” and the “favorable thermodynamic modification” of the composite.

A Ti-doped sodium aluminum hydride was prepared by ball-milling NaH/Al mixture with 10 mol% Ti powder under hydrogen for 12 h. The hydriding/dehydriding behaviors of the ball-milled sample under different temperatures (85–160 °C) and hydrogen pressures (7.5–13.5 MPa) were investigated. The results show that the hydriding/dehydriding temperature and hydrogen pressure affect the hydrogen storage behavior noticeably. During hydriding, under 13.5 MPa hydrogen pressure, as the hydriding temperature increases from 85 to 140 °C, the hydrogen absorption rate increases first and then decreases, and reaches the highest value at 120 °C. And the hydrogen absorption rate increases with the increase of hydrogen pressure all the way from 7.5 to 13.5 MPa. Moreover, the hydrogen desorption rate over the hydrogen pressure of 0.1 MPa increases noticeably with increasing dehydriding temperature. On cycling, the hydriding/dehydriding capacity increases first and then decreases, reaching a maximum value at the fourth cycle. X-ray diffraction (XRD) analyses show that the hydrogen storage process of the system is governed by the slow reaction kinetics and incomplete reaction of Na3AlH6Na3AlH6 in the hydriding/dehydriding processes.

The microstructure and electrochemical behavior of V2.1TiNi0.4Zr0.06Cr0.152 hydrogen storage electrode alloy have been investigated in comparison with V2.1TiNi0.4Zr0.06 alloy. The results show that V2.1TiNi0.4Zr0.06Cr0.152 alloy consists of a V-based solid solution main phase and a C14-type Laves secondary phase in the form of three-dimensional network, being similar to V2.1TiNi0.4Zr0.06 alloy, the secondary phase precipitates along the grain boundaries of the main phase. As compared with V2.1TiNi0.4Zr0.06 alloy, the unit cell volume of each phase in the V2.1TiNi0.4Zr0.06Cr0.152 alloy contracts. It is found that adding Cr restricts the dissolution of vanadium and titanium into the KOH electrolyte, and improves the corrosion resistance of the alloy, thus the cycling stability after 30 cycles increases from 22.34% (V2.1TiNi0.4Zr0.06) to 77.96% (V2.1TiNi0.4Zr0.06Cr0.152). Furthermore, V2.1TiNi0.4Zr0.06Cr0.152 alloy has a better high-rate dischargeability and higher exchange current density compared with V2.1TiNi0.4Zr0.06 alloy, but its maximum discharge capacity decreases.

The hydrogen storage properties and microstructures of Ti-doped NaAlH4NaAlH4 complex hydrides prepared by hydrogenation of ball-milled NaH/Al mixture with xx mol% Ti powder (x=0,4,6,10)(x=0,4,6,10) were investigated. It is found that hydrogen as ball-milling atmosphere is better than argon. The reversible hydrogen storage properties improve with increasing Ti content. As the milling time (t)t) extends from 1 to 24 h, the hydrogen desorption capacity increases first and then decreases, and reaches a maximum capacity of 4.25 wt% at t=16t=16. The catalytic mechanism for hydrogen storage behavior of Ti-doped NaAlH4NaAlH4 is attributed to the presence of active small TiH1.924TiH1.924 and TiAl particles, which are scattered on the surface of much larger NaAlH4NaAlH4 (NaH/Al) globelets, acting as the catalytic active sites for the complex compound and playing an important catalytic role in the hydriding-dehydriding process.

The phase structures and electrochemical properties of the V2.1TiNi0.4Zrx (x=0-0.06) hydrogen storage electrode alloys have been investigated. It is found that all the alloys consist of a main phase of V-based solid solution with a bcc structure and a secondary phase with a three-dimensional network structure. The secondary phase precipitates along the grain boundaries of the main phase. For the alloys with Zr content x≤0.02, the secondary phase is the TiNi-based phase. As x reaches 0.04, the secondary phase changes into the C14-type Laves phase. Moreover, both the unit cell of the main phase and the secondary phase expand with increasing Zr content. Electrochemical measurements show that the activation behavior and maximum discharge capacities of the Zr-added alloy are better than those of the V2.1TiNi0.4 alloy. As the Zr content in the alloy increases, its high-rate dischargeability is improved significantly, but its cycle stability degrades gradually. For the alloy with the Zr content of x=0.04, the best overall electrochemical performance is obtained.

The structural and electrochemical characteristics of V2.1TiNi0.5Hf0.05M0.152 (M = Co, Cr) hydrogen storage electrode alloys have been investigated. It is found that all the alloys consist of two phases indexed as a main phase of V-based solid solution with a bcc structure and a secondary phase of C14-type Laves phase. The addition of Co or Cr into the V2.1TiNi0.5Hf0.05 alloy decreases the amount of the main phase and increases the amount of the secondary phase. After adding Co or Cr into the V2.1TiNi0.5Hf0.05 alloy, the unit cell of the main phase contracts and that of the secondary phase expands. Electrochemical measurements show that the maximum discharge capacities of the Co- and Cr-added alloys are much less than those of the V2.1TiNi0.5Hf0.05 alloy, but their cycle stability and high-rate dischargeability are markedly improved. The results show that the addition of Co or Cr into the V2.1TiNi0.5Hf0.05 alloy is very helpful in improving the cycling stability.

Mg2FeH6@MgH2 dual-metal hydrides with a core–shell nanostructure were synthesized via ball-milling and heat treatment methods using Mg and Fe as raw materials assisted by diethyl ether addition. Systematic investigations of the association between the microstructure and hydrogen desorption properties of the Mg2FeH6@MgH2 core–shell hydride were performed. It is found that the as-synthesized Mg2FeH6@MgH2 is comprised of the Mg2FeH6-core with a particle size of 40–60 nm and the MgH2-shell with a thickness of 5 nm. The hydrogen desorption of the Mg2FeH6@MgH2 core–shell nanoparticle starts at 220 °C, which is ∼45 °C lower than that of the Mg2FeH6/MgH2 micrometer particle. Compared to the as-synthesized Mg2FeH6/MgH2 micrometer particle, the Mg2FeH6@MgH2 core–shell sample exhibited faster hydrogen desorption kinetics, which released more than 5.0 wt% H2 within 50 min at 280 °C. The desorption activation energy of the core–shell Mg2FeH6@MgH2 was reduced to 115.7 kJ mol−1 H2, while the desorption reaction enthalpy and entropy were calculated to be −80.6 ± 7.4 kJ mol−1 H2 and −140.0 ± 11.9 J K−1 mol−1 H2, respectively. It is proposed that the improvements of both hydrogen desorption kinetics and thermodynamics are due to the special core–shell nanostructure of Mg2FeH6@MgH2. More remarkably, it is demonstrated that the core–shell nanostructure could be recovered after rehydrogenation, leading to excellent cycling hydrogen desorption properties of Mg2FeH6@MgH2. In addition, the suggested dehydrogenation mechanism involves the dehydrogenation of the MgH2-shell followed by the decomposition of the Mg2FeH6-core into Mg and Fe according to the three-dimensional phase-boundary process.

Na2Ti3O7 nanotubes (NTs) with a uniform diameter of 10 nm and Na2Ti3O7 nanorods (NRs) with a diameter of 100–500 nm were synthesized via a hydrothermal method and a solid-state method, respectively, and then introduced into MgH2 by ball milling to catalyze the hydrogenation/dehydrogenation process. The MgH2–Na2Ti3O7 NT and MgH2–Na2Ti3O7 NR composites can desorb 6.5 wt% H2 within 6 min and 16 min at 300 °C, respectively, while the bulk MgH2 hardly releases any hydrogen even over a much longer time. In addition, isothermal rehydrogenation measurements show that the MgH2–Na2Ti3O7 NT composite can absorb 6.0 wt% H2 within 60 s at 275 °C and can even absorb 1.5 wt% H2 within 30 min at a temperature as low as 50 °C. TEM and HRTEM analyses indicate that the Na2Ti3O7 NTs are homogeneously distributed in MgH2, which catalyze the de-/rehydrogenation of MgH2 and meanwhile offer numerous diffusion channels to significantly accelerate the transportation of hydrogen atoms. Moreover, compared with bulk MgH2 and the MgH2–Na2Ti3O7 NR composite, the activation energy of the MgH2–Na2Ti3O7 NT composite is significantly decreased to 70.43 kJ mol−1. Such Na2Ti3O7 NTs with a unique morphology of the catalyst being distributed as nanotubes in MgH2 are believed to pave the way for the future design of hydrogen storage materials with excellent hydrogen storage performances.

Magnesium hydride (MgH2) exhibits long-term stability and has recently been developed as a safe alternative to store hydrogen in the solid state, due to its high capacity of 7.6 wt% H2 and low cost compared to other metal hydrides. However, the high activation energy and poor kinetics of MgH2 lead to inadequate hydrogen storage properties, resulting in low energy efficiency. Nano-catalysis is deemed to be the most effective strategy in improving the kinetics performance of hydrogen storage materials. In this work, robust and efficient architectures of carbon-wrapped transition metal (Co/C, Ni/C) nanoparticles (8–16 nm) were prepared and used as catalysts in the MgH2 system via ball milling to improve its de/rehydrogenation kinetics. Between the two kinds of nano-catalysts, the Ni/C nanoparticles exhibit a better catalytic efficiency. MgH2 doped with 6% Ni/C (MgH2-6%Ni/C) exhibits a peak dehydrogenation temperature of 275.7 °C, which is 142.7, 54.2 and 32.5 °C lower than that of commercial MgH2, milled MgH2 and MgH2 doped with 6% Co/C (MgH2-6%Co/C), respectively. MgH2 doped with 6% Ni/C can release about 6.1 wt% H2 at 250 °C. More importantly, the dehydrogenated MgH2-6%Ni/C is even able to uptake 5.0 wt% H2 at 100 °C within 20 s. Moreover, a cycling test of MgH2 doped with 8% Ni/C demonstrates its excellent hydrogen absorption/desorption stability with respect to both capacity (up to 6.5 wt%) and kinetics (within 8 min at 275 °C for dehydrogenation and within 10 s at 200 °C for rehydrogenation). Mechanistic research reveals that the in situ formed Mg2Ni and Mg2NiH4 nanoparticles can be regarded as advanced catalytically active species in the MgH2-Ni/C system. Meanwhile, the carbon attached around the surface of transition metal nanoparticles can successfully inhibit the aggregation of the catalysts and achieve the steadily, prompting de/rehydrogenation during the subsequent cycling process. The intrinsic catalytic effects and the uniform distributions of Mg2Ni and Mg2NiH4 result in a favorable catalytic efficiency and cycling stability. Nano-catalysts with this kind of morphology can also be applied to other metal hydrides to improve their kinetics performance and cycling stability.

SnO2 has high capacity but poor cycling stability for Li-ion batteries due to pulverization and aggregation. Herein, we tackle these two challenges by uniformly dispersing carbon coated nanoSnO2 into a micro-sized porous carbon matrix to form a nano-SnO2/C composite anode using a facile and scalable in situ synthesis strategy. The SnO2@C nanocomposite exhibits a capacity of 640 mA h g−1 at 500 mA g−1 in the initial 150 cycles and then increases to 720 mA h g−1 and maintains this capacity for 420 cycles. The superior electrochemical performance with long cycle lifetimes of the carbon foam–SnO2 nanocomposites could be attributed to their unique carbon microstructures: the network of carbon sheets provides favorable electron transport, while the interconnected micro-/mesopores can serve as the effective channels of lithium ion transport, thereby supplying short lithium ion diffusion pathways. Meanwhile, these pores surrounding the active species of nanoSnO2 along with flexible carbon nanosheets can accommodate the severe volume variations during prolonged electrochemical cycling and mitigate the Sn aggregation. The present study provides a large-scale synthesis route to synthesize SnO2-based anode materials with superior electrochemical performance for lithium ion batteries.

As anode materials for lithium ion batteries, metal oxides have large storage capacity. However, their cycle life and rate capability are still not suitable for commercial applications. Herein, 3D hierarchical Fe3O4 spheres associated with a 5–10 nm carbon shell were designed and fabricated. In the constructed architecture, the thin carbon shells can avoid the direct exposure of encapsulated Fe3O4 to the electrolyte and preserve the structural and electrochemical integrity of spheres as well as inhibit the aggregation of pulverized Fe3O4 during electrochemical cycling. The hierarchical structure formed by the bottom-up self-assembly approach can efficiently accommodate the mechanical stress induced by the severe volume variation of Fe3O4 during lithiation–delithiation processes. Moreover, the carbon shell together with the structure integrity and durability endows the favorable high conductivity and efficient ion transport. All these features are critical for high-performance anodes, therefore enabling an outstanding lithium storage performance with a long cycle lifespan. For instance, such an electrode could deliver a capacity of 910 mA h g−1 even after 600 cycles with a discharge–charge rate of 1 A g−1. In addition, this effective strategy may be readily extended to construct many other classes of hybrid electrode materials for high-performance lithium-ion batteries.

LiBH4–MgH2 system in a 2:1 molar ratio constitutes a representative reactive hydride composite (RHC) for hydrogen storage. However, sluggish kinetics and poor reversibility hinder the practical applications. To ease these problems, amorphous TiB2 and NbB2 nanoparticles were synthesized and employed as catalysts for the 2LiBH4–MgH2 system. Isothermal de-/rehydrogenation and temperature programmed mass spectrometry (MS) measurements show that amorphous TiB2 and NbB2 nanoparticles can significantly improve the hydrogen storage performance of the 2LiBH4–MgH2 system. 9 wt% hydrogen can be released within only 6 min for nanoTiB2-doped 2LiBH4–MgH2, while for the undoped composite limited hydrogen of 3.9 wt% is released in 300 min at 400 °C. The dehydrogenation activation energies for the first and second steps are dramatically reduced by 40.4 kJ mol−1 and 35.2 kJ mol−1 after doping with nanoTiB2. It is believed that TiB2 and NbB2 nanoparticles can first catalyze the dehydrogenation of MgH2, and then induce the decomposition of LiBH4 and meanwhile act as nucleation agents for MgB2, thereby greatly enhancing the kinetics of dehydrogenation. The present study gives clear evidence for the significant performance of transition metal boride species in doped RHCs, which is critically important for understanding the mechanism and further improving the hydrogen storage properties of RHCs.

By directly doping CeAl4 into sodium aluminium hydride, which probably serves as the active species in the hydriding and dehydriding processes of CeCl3-doped NaAlH4, a high reversible hydrogen capacity of 4.77–4.92 wt% (close to expected capacity of 5.13 wt%) can be achieved in less than 20 min under moderate conditions.

A system of 2LiH–MgB2 and its hydrogenated 2LiBH4–MgH2 is an attractive candidate for hydrogen storage. However, its hydriding–dehydriding kinetics have to be further improved for practical application. In the present work, three kinds of Ni–B nanoparticles with different crystalline states and particle sizes were prepared by wet-chemical reduction and mechanochemical methods, and then introduced into a 2LiH–MgB2 composite for catalytic enhancement. The catalytic roles of Ni–B nanoparticles on the hydriding–dehydriding properties were investigated systematically. The results show that all of the Ni–B nanoparticles can significantly enhance the hydriding–dehydriding kinetics of the 2LiH–MgB2 composite, resulting in no incubation period for the formation of MgB2 during dehydrogenation. The more disordered the amorphous structure and the smaller sized the Ni–B particles are, the better the catalytic effect that is obtained. Microstructure analyses clearly reveal the formation of the MgNi3B2 phase in the dehydriding process, which acts as the nucleation agent for MgB2 formation determined by an edge-to-edge model. Directly doping with Ni–B nanoparticles in the 2LiH–MgB2 system shows a higher hydrogen desorption capacity of 9.4 wt% and obtains a better catalytic efficiency than doping with NiCl2.

2LiBH4–MgH2 is an attractive system for hydrogen storage. However, its dehydriding and rehydriding kinetics have to be further improved for practical applications. To solve these problems, three transition metal chlorides (FeCl2, CoCl2 and NiCl2) were introduced into the 2LiBH4–MgH2 system and their catalytic roles in the dehydrogenation–rehydrogenation properties were investigated systematically. The results show that all three chlorides can significantly enhance the dehydriding and rehydriding kinetics of the 2LiBH4–MgH2 system with/without hydrogen back pressure, and NiCl2 is the best modifier among them. The NiCl2-doped sample exhibits no incubation period for generating MgB2 during dehydrogenation. X-Ray diffraction and scanning electron microscopy analyses clearly reveal the phase formation of MgNi3B2, which may act as the nucleation site for MgB2 formation, and thus the incubation period shrinks. A highly homogeneous distribution of the in situ formed MgNi3B2 nanoclusters can significantly improve the dehydriding–rehydriding kinetics. Single-phase MgNi3B2 was successfully synthesized and a similar catalytic effect is obtained by directly doping identical MgNi3B2 into the 2LiBH4–MgH2 system, which results in a higher hydrogen storage capacity of 9.4 wt%.

NaAlH4, a prototypical high energy density complex hydride, possesses a favorable thermodynamics and high hydrogen storage capacity. However, the poor kinetics and degradation of cycling stability retard its practical application. To ease these problems, CeB6, CeF3 and CeO2 nanoparticles with a size of about 10 nm are synthesized by the wet-chemistry method and introduced into NaAlH4 systems as additives in this work. The results show that all of the nanoparticles are effective in improving the hydriding–dehydriding kinetics of NaAlH4, and nano-CeB6 possesses the highest catalytic activity. The rehydrogenation of dehydrogenated NaAlH4 doped with nano-CeB6 can be accomplished in less than 20 min with a high capacity of 4.9 wt%, which shows a 20% increase in capacity compared to that of chloride-doped NaAlH4. Due to the structural stability and good dispersion of nano-CeB6 and nano-CeF3, a favorable cycling stability with high capacity retention is achieved for their doped samples. Moreover, hydrogen can be released from the hydrogenated sample doped with nano-CeB6 at a temperature as low as 75 °C, fulfilling the operation temperature of a PEM fuel cell. In the nano-CeO2 doped NaAlH4 system, CeO2 is first reduced to CeH2.51. In the subsequent cycles, the formed CeH2.51 gradually transforms into Ce–Al, and simultaneously the kinetics of the doped system is further enhanced. It is believed that the utilization of Ce-based nanoparticles as catalysts would substantially improve the practical applications of NaAlH4 for hydrogen storage.

LiBH4 has been loaded into a highly ordered mesoporous carbon scaffold containing dispersed NbF5 nanoparticles to investigate the possible synergetic effect of nanoconfinement and nanocatalysis on the reversible hydrogen storage performance of LiBH4. A careful study shows that the onset desorption temperature for nanoconfined LiBH4@MC-NbF5 system is reduced to 150 °C, 225 °C lower than that of the bulk LiBH4. The activation energy of hydrogen desorption is reduced from 189.4 kJ mol–1 for bulk LiBH4 to 97.8 kJ mol–1 for LiBH4@MC-NbF5 sample. Furthermore, rehydrogenation of LiBH4 is achieved under mild conditions (200 °C and 60 bar of H2). These results are attributed to the active Nb-containing species (NbHx and NbB2) and the function of F anions as well as the nanosized particles of LiBH4 and high specific surface area of the MC scaffold. The combination of nanoconfinement and nanocatalysis may develop to become an important strategy within the nanotechnology for improving reversible hydrogen storage properties of various complex hydrides.

Highly dispersed AgPd hollow spheres anchored on graphene (denoted as AgPd-Hs/G) were successfully synthesized through a facile one-pot hydrothermal route for the first time. The fabrication strategy was efficient and green by using L-ascorbic acid (L-AA) as the reductant and trisodium citrate dihydrate as the stabilizer, without employing any seed, surfactant, organic solvent, template, stabilizing agent, or complicated apparatus. The as-synthesized AgPd-Hs/G catalyst exhibits a sphere-shaped hollow structure with an average diameter of about 18 nm and a thin wall of about 5 nm. The hollow architecture with a thin wall and excellent dispersion on the graphene ensure that most of the atoms are located on the surface or sub-surface, which provides reactive catalytic sites for the dehydrogenation of formic acid. Therefore, a superior catalytic effect was achieved compared with other catalysts such as Pd/G and AgPd/C. The as-synthesized AgPd-Hs/G exhibits a catalytic activity with an initial turnover frequency (TOF) value as high as 333 mol H2 mol−1 catalyst h−1 even at room temperature (25 °C) toward the decomposition of formic acid. The present AgPd-Hs/G with efficient catalysis on the dehydrogenation of formic acid without any CO generation at room temperature can pave the way for a practical liquid hydrogen storage system and therefore promote the application of formic acid in fuel cell systems.