•Al promoted hydriding kinetics of Mg greatly during hydriding combustion synthesis.•The Mg-10 at% Al composite absorbed 6.34 wt% H2 under given conditions.•Al breaks the surface oxide layers of Mg by forming Al12Mg17 alloy.Mg-based alloys are widely considered as promising materials for hydrogen storage. However, owing to the high chemical activity of Mg, the surface of Mg powder is always covered with a layer of close packed MgO, which can retard the hydrogenation of Mg. In this paper, 10 at% Al was added to accelerate the hydriding kinetics of Mg through tabletting and grinding treatments. After heating to 420 °C and subsequent holding at 340 °C for 120 min under 1.5 MPa hydrogen pressure, the Mg-10 at% Al composite could absorb 6.34 wt% H2 in the process of hydriding combustion synthesis (HCS) while the same treated pure Mg powder was barely hydrided. Upon heating from room temperature to 420 °C, Al alloyed with Mg to form an Al12Mg17 phase on the surface of Mg particles and the newly formed alloy was oxide-free, which would react with H2 when the temperature decreased to 340 °C. Then hydrogen was able to get into the inner part of the Mg particle from the former Al12Mg17 sites where the MgO shell had been broken in the alloying period to further react with unhydrided Mg.

Metal hydrides (MHs) have recently been designed for hydrogen sensors, switchable mirrors, rechargeable batteries, and other energy-storage and conversion-related applications. The demands of MHs, particular fast hydrogen absorption/desorption kinetics, have brought their sizes to nanoscale. However, the nanostructured MHs generally suffer from surface passivation and low aggregation-resisting structural stability upon absorption/desorption. This study reports a novel strategy named microencapsulated nanoconfinement to realize local synthesis of nano-MHs, which possess ultrahigh structural stability and superior desorption kinetics. Monodispersed Mg2NiH4 single crystal nanoparticles (NPs) are in situ encapsulated on the surface of graphene sheets (GS) through facile gas–solid reactions. This well-defined MgO coating layer with a thickness of ≈3 nm efficiently separates the NPs from each other to prevent aggregation during hydrogen absorption/desorption cycles, leading to excellent thermal and mechanical stability. More interestingly, the MgO layer shows superior gas-selective permeability to prevent further oxidation of Mg2NiH4 meanwhile accessible for hydrogen absorption/desorption. As a result, an extremely low activation energy (31.2 kJ mol–1) for the dehydrogenation reaction is achieved. This study provides alternative insights into designing nanosized MHs with both excellent hydrogen storage activity and thermal/mechanical stability exempting surface modification by agents.

In this work, Mg-based hydrogen storage composites with an initial 100-x: x (x=25, 32.3, 50, 66.7) of Mg:Ni molar ratio were prepared by HCS+MM and their phase compositions and electrochemical performances were investigated in detail. The results show that the composites with desirable constituents can be achieved by adjusting the molar ratio of the starting materials in the HCS process. Particularly, the HCS product of Mg67.7Ni32.3 consists of the principal phase Mg2NiH4 and minor phase Mg2NiH0.3. The dominate phase varies from Mg2NiH0.3 and MgH2 for the Mg enriched sample (x<32.3) to MgNi2 and Ni for the Ni enriched sample (x>32.3). The MM modification not only brings about grain refinement of the alloys, but also leads to phase transformation of part Mg2NiH4 to Mg2NiH0.3 in the Mg67.7Ni32.3 sample. Electrochemical tests indicate that each sample can reach its maximum discharge capacity at the first cycle. Mg67.7Ni32.3 displays the highest discharge capacity as well as a superior electrochemical kinetics owing to its excellent H atom diffusion ability and lower charge-transfer resistance. The Mg67.7Ni32.3 provides the most optimized Mg/Ni atomic ratio considering the comprehensive electrochemical properties of all samples.

•A novel 4LiAlH4 + Mg2NiH4 composite was synthesized.•The synergistic hydrogen desorption effect has been discovered for the first time.•Mg2NiH4 promotes decomposition of LiAlH4 in MM and thermal desorption process.•In situ formed Al can destabilize Mg2NiH4 via a new reaction pathway.•The composite shows ultrafast re-hydrogenation kinetics in initial three cycles.Mg2NiH4 was prepared by hydriding combustion synthesis (HCS) method firstly. Then, the as prepared Mg2NiH4 was mechanically milled with LiAlH4 to form a novel composite of 4LiAlH4 + Mg2NiH4. Hydrogen storage properties and reaction mechanism of the 4LiAlH4 + Mg2NiH4 composite during hydrogenation/dehydrogenation have been investigated systematically by pressure-composition-temperature (PCT), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements. The microstructure of the composite has been investigated by scanning electron microscopy (SEM). The experimental results show that there is a mutual destabilization effect between Mg2NiH4 and LiAlH4 during hydrogen desorption. Mg2NiH4 can promote the decomposition of LiAlH4 in both of the ball milling process and the thermal desorption process. Conversely, the in situ formed Al from the decomposition of LiAlH4 can destabilize Mg2NiH4 via a new reaction pathway with lower activation energy. Moreover, the 4LiAlH4 + Mg2NiH4 composite shows ultrafast re-hydrogenation kinetics, and the re-hydrogenation mechanism has been revealed.

•2LiBH4-MgH2 expressed 10 times higher conductivity than LiBH4.•2LiBH4-MgH2 possessed a window voltage of 4 V.•The lithium ionic mobility is contributed by grain boundary based on HRTEM.2LiBH4-MgH2 was employed as fast ionic conductor for the first time. The present work found that 2LiBH4-MgH2 composite offers σ of 10− 2 S cm− 1 at a temperature over 373 K, and possesses σ value 10 times higher than that of LiBH4 at a temperature below 373 K. A window voltage of 4 V can be achieved for 2LiBH4-MgH2 according to the cyclic voltammetry (CV for 5 cycles) tests. X-ray diffraction (XRD) and Fourier Transform infrared spectroscopy (FTIR) measurements on 2LiBH4-MgH2 samples milled for different time demonstrate that original phases are well preserved during ball milling and temperature ramping even after CV tests. Their chemical stabilities were also demonstrated by differential scanning calorimetry (DSC) measurements. The enhanced lithium mobility at low temperature in 2LiBH4-MgH2 should be ascribed to the favorable boundaries, in which defects and amorphous phases were clearly observed by means of high-resolution transmission electron microscope (HRTEM).2LiBH4-MgH2 was synthesized by ball-milling and employed as lithium ionic conductor. It offers conductivity of 10− 2 S cm− 1 at a temperature over 373 K, and 10 times higher than that of LiBH4 at a temperature below 373 K.Download high-res image (169KB)Download full-size image

•Polyaniline (PANI) was used to modify the surface of Mg3MnNi2 alloy firstly.•PANI has been successfully polymerized on the surface of Mg3MnNi2 alloy particles.•Discharge capacity retention (R20) was increased obviously after PANI modification.•Modification with PANI can favor the charge-transfer reaction on the alloy surface.In this paper, polyaniline (PANI) has been adopted to modify the surface of Mg3MnNi2 hydrogen storage electrode alloy prepared by induction melting via chemical deposition. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) tests were performed to investigate the phase composition and microstructure. The comprehensive analysis based on the results of XRD, SEM, TEM and FT-IR indicates that PANI with a thickness of about 10 nm has been successfully polymerized on the surface of Mg3MnNi2 alloy. The discharge capacity retention (R20) of the Mg3MnNi2 alloy has been increased from 13.5% to 36.2% after modification with PANI, indicating better cycling stability. The Tafel polarization curves reveal that the surface modification of Mg3MnNi2 alloy with PANI can enhance its anti-corrosion ability. The increase of the exchange current density (Io) indicates that the surface activity of Mg3MnNi2 alloy can be improved through the surface modification with PANI. However, the high rate discharge ability (HRD) test shows that the kinetic property of Mg3MnNi2 alloy after the surface modification is declined, which is mainly due to the decline of the hydrogen diffusion in PANI, evidenced by the decreased limit current density (IL) obtained from the anodic polarization curves.

•Ni-based catalysts with different morphologies were fabricated in a controlled way.•Desorption performances of MgH2 were enhanced depending on the morphology of Ni.•Ni6GS4 showed the best catalytic effect due to the desirable microstructure of Ni.•The superior sorption kinetics was well maintained during hydrogen sorption cycles.Searching for effective catalysts is one of the most attractive subjects to enhance the hydrogen storage properties, especially the sorption kinetics of the metal-based hydrides. The effect of the Ni morphology (including size and shape) on the desorption performance of MgH2 was studied in the paper, in which a modified wet chemical route was used to control the morphology of Ni. By introducing the as-synthesized Ni-based catalysts into MgH2, a well-distinguished desorption behavior was found, evidencing the existence of the morphology-dependent effect of the catalytic phase. Among them, graphene sheets (GS) supported Ni (Ni6GS4) exhibited the best catalytic performance with a lowered MgH2 onset desorption temperature of 225 °C (the undoped MgH2 initiated to desorb hydrogen at 308 °C) and a saturated hydrogen desorption capacity of 6.74 wt%. The significant enhancement of the hydrogen storage kinetics was ascribed to not only the relative small particle size of the initial Ni active phase but also its well-dispersed distribution state and further refinement during ball milling. In addition, we believe that the good dispersive ability of GS was of particular importance in separating the Ni nanoparticles from each other and preventing aggregating of the MgH2 particles, leading to a desirable cyclic performance. Our results have experimentally shown the influence of shape and size of metal-based catalysts on the MgH2 system, providing guideline or strategy designing nanostructured catalysts with light weight, ultra-fine particle size, well-formed distribution and high activity.

•LiBH4-Li3N possessed higher conductivity than LiBH4 by nearly 100 times.•LiBH4-Li3N presents a window voltage of 3 V, much higher than Li3N.•The lithium ionic mobility was mainly contributed by Li3N according to 7Li NMR.The x LiBH4-Li3N (x = 1, 2, 4) composites were investigated on their lithium ionic conductivities. They maintained the original phase structures of orthorhombic LiBH4 and hexagonal β-Li3N during ball milling according to the analyses by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Electrochemical impedance spectra (EIS) tests showed that LiBH4-Li3N composites lift the lithium ionic conductivities from that of bulk LiBH4 by nearly 100 times at low temperature. LiBH4-Li3N offers 1.06 × 10− 5 S cm− 1 of conductivity while bulk LiBH4 possesses only 2.05 × 10− 7 S cm− 1 at 323 K. Cyclic voltammetry (CV) tests indicated that the voltage window of composites could attain 3 V, which is much more stable than Li3N (< 1 V). No thermodynamic incident appears in DSC curves except for the orthorhombic-hexagonal transition endothermic peak of LiBH4 in the x LiBH4-Li3N composites. It confirms the phases stabilities of the x LiBH4-Li3N (x = 1, 2, 4) composites before and after CV tests. In-situ solid state Nuclear Magnetic Resonance (NMR) tests on LiBH4-Li3N illustrated that the high conductivity in low temperature should not come from high-temperature phase of LiBH4. Therefore, the enhancement of lithium ionic mobility of the composite might be attributed to both Li3N and the interfaces between small crystallites where transmission electron microscope (TEM) morphology showed the appearance of amorphous grain boundary.

•LixNa3-xAlH6 compounds have been successfully prepared by grinding mixtures of Li3AlH6 and Na3AlH6.•The x range in LixNa3-xAlH6 compounds is 0.9–1.3.•Both ΔHde and Ea values of Li1.3Na1.7AlH6 are the lowest in LixNa3-xAlH6 compounds.Mixed alkali alanates LixNa3-xAlH6 have been successfully synthesized by means of grinding mixtures of Li3AlH6 and Na3AlH6 in specific molar ratios. Non-stoichiometric LixNa3-xAlH6 compounds with single perovskite-type structures (space group Fm-3m) can be formed only within the composition range of x = 0.9–1.3. Li1.3Na1.7AlH6 exhibits superior hydrogen storage properties over other LixNa3-xAlH6 compounds. Its onset dehydrogenation temperature (∼423 K) was lowered by more than 40 K from other samples in temperature programmed dehydrogenation (TPD) curves. Also, the dehydrogenation capacity of Li1.3Na1.7AlH6 (3.45 wt.%) is the highest among the compounds. The dehydrogenation enthalpy values of LixNa3-xAlH6 decreased as x increased from 0.9 to 1.3 according to the results by isothermal pressure-composition (PCI) curves and van't Hoff plots. It shows that the dehydrogenation Li1.3Na1.7AlH6 (49.7 kJ mol H2−1) was greatly destabilized from that of LiNa2AlH6 (68.1 kJ mol H2−1). Furthermore, the apparent activation energy of dehydrogenation for Li1.3Na1.7AlH6 (138.1 kJ mol−1) was remarkably lowered from that of LiNa2AlH6. This illustrates that Li1.3Na1.7AlH6 exhibits enhanced dehydrogenation kinetics from that of LiNa2AlH6.

Magnesium hydride is considered as an ideal candidate for effective hydrogen storage due to its high gravimetric hydrogen capacity and accessibility. But its use as a commercial material is hindered by its relatively high operating temperatures and slow release/uptake kinetics. To solve this, we first synthesized Ni decorated graphene nanoplate (Ni/Gn) catalysts with highly dispersed metal nano-particles (NPs) via a facile method, then the as-prepared Ni/Gn catalysts were introduced by using the hydriding combustion synthesis and mechanical milling (HCS + MM) method to obtain Mg-based composites. Remarkable enhancement of hydrogen sorption rates has been found for these composites in the presence of Ni/Gn additives, especially for the Mg@Ni8Gn2 sample: a hydrogen absorption amount of 6.28 wt% within 100 s at 373 K and a hydrogen desorption amount of 5.73 wt% within 1800 s at 523 K. A rather low activation energy (71.8 kJ mol−1) for the dehydrogenation of MgH2 was determined in the same sample, indicating that relatively moderate temperatures are required to absorb/desorb hydrogen. The excellent hydrogen sorption rates of the composites are thought to be associated with the high dispersity of in situ formed nanometric Mg2NiH4 particles during the HCS + MM process. In addition, a microstrain-induced synergetic hydrogen sorption mechanism is proposed, being correlated by the local introduction of a Mg2Ni nano-catalyst into the Mg matrix.

•The Mg64Pd3Co33 enhances discharge capacity of 624 mAh g−1 from Mg67Co33.•Reaction kinetics, corrosion resistance and cyclic stability were improved by Pd.•Hydroxides, surface passivation, and dissolution of Mg affected cyclic stability.The ternary Mg67−xPdxCo33 (x = 1, 3, 5, 7) alloys were prepared and served as anode materials for the Ni-MH battery system. Pd facilitates the formation of a full body-centered cubic (BCC) phase in binary Mg67Co33. All Mg67−xPdxCo33 (x = 1, 3, 5, 7) alloys possess BCC structure in nano-crystalline, which were observed by XRD and TEM analyses. In addition, their lattice parameters increase with the augmentation of Pd content. The charge–discharge experiments show that Mg64Pd3Co33 owns the maximum discharge capacity of 624 mAh g−1 among Mg67−xPdxCo33 (x = 1, 3, 5, 7) electrodes, which was greatly enhanced from our previously studied binary Mg–Co and ternary Mg–Co–Pd electrodes. All electrochemical kinetics e.g. exchange current density, hydrogen atomic diffusion capability were improved by substituting Pd for Mg, which were also relevant with the increment of Pd amount in the alloys. X-ray photoelectric spectroscopy (XPS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) demonstrated that Pd relieved the severe corrosions and capacity degradations of the electrodes.

•The onset temperature of hydrogen release was decreased to 54 °C in 4MgH2LiAlH4TiH2 composite.•Additive TiH2 reduced the de/re-hydrogenation activation energies.•TiH2 was not involved in the reactions, but served as catalyst for composite's dehydrogenation.The hydrogen storage performance of a 4MgH2LiAlH4 composite system was greatly improved by adding TiH2. The temperature-programmed release curve of the 4MgH2LiAlH4TiH2 composite reflected that the onset temperature of dehydrogenation decreased remarkably to 54 °C from that of 4MgH2LiAlH4 (100 °C). X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses indicated that TiH2 was not involved in the decomposition of either LiAlH4 or MgH2 or their interactions. In the ternary composite, TiH2 can be regarded as an effective catalyst for LiAlH4, Li3AlH6 and MgH2, of which the activation energies of dehydrogenation were reduced by 23.3 kJ/mol, 29.1 kJ/mol and 45.5 kJ/mol from those of 4MgH2LiAlH4, respectively. The rehydrogenation of those products at 350 °C cannot be fully integrated into their original phases, and the reversible capacity was ascribed only to the formation of MgH2. The activation energy of rehydrogenation of MgH2 was greatly decreased to 108.9 kJ/mol from 158.8 kJ/mol of 4MgH2LiAlH4. Isothermal hydrogenation curves and fitted lines from the Arrhenius equation demonstrated the enhancement of hydrogenation kinetics.

•The nanosized Ni/MWCNTs catalyst was successfully prepared.•Ni/MWCNTs shows superior catalytic effect on H absorption/desorption of Mg.•Mg85-(Ni/MWCNTs)15 composite shows the best hydrogen storage properties.•Ni/MWCNTs coupling with TiF3 improves the hydriding/dehydriding properties largely.Multi-wall carbon nanotubes supported nano-nickel (Ni/MWCNTs) with superior catalytic effects was introduced to magnesium hydride by the process of hydriding combustion synthesis (HCS) and mechanical milling (MM). The effect of different Ni/MWCNTs contents (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%) on the hydrogenation and dehydrogenation properties of the composite was investigated systematically. It is revealed that Mg85-(Ni/MWCNTs)15 composite shows the best comprehensive hydrogen storage properties, which absorbs 5.68 wt.% hydrogen within 100 s at 373 K and releases 4.31 wt.% hydrogen within 1800 s at 523 K under initial hydrogen pressures of 3.0 and 0.005 MPa, respectively. The in situ formed nano-Mg2Ni and MWCNTs have excellent catalytic effect on the hydrogenation and dehydrogenation performances of MgH2. To further improve the hydrogen absorption/desorption properties, TiF3 was added to the Mg–Ni/MWCNTs system. The result shows that TiF3 addition has little influence on the thermodynamic performance, but affects greatly the kinetic properties. The Mg85-(Ni/MWCNTs)15-TiF3 composite exhibits an appreciably enhanced hydrogen desorption performance at low temperature, and the hydrogen desorption capacity within 1800 s at 473 K for the TiF3-added composite is approximately four times the capacity of Mg85-(Ni/MWCNTs)15 under the same condition. The catalytic effects during hydrogenation and dehydrogenation have been discussed in the study.

•Binary MgxCo100–x alloys (x = 40, 45, 50, 55, 60, 63) with BCC structure can reversibly charge–discharge in Ni-MH battery system.•With the increase of Mg content in the alloys, electrodes discharge kinetics will be increased accordingly.•The Pd-doped ternary alloys drastically increase the discharge capacities (280–500 mAh g−1) from binary ones.•The additional Pd improves cycle stabilities, kinetics, and corrosion resistances of the alloys.In this work, a serial of Mg–Co binary alloys with body-centered cubic (BCC) phase were prepared for anode materials of Ni-MH hydrogen storage battery system. TEM/SAED analyses on binary Mg–Co alloys demonstrated that their grains were all in nano-size (∼5 nm) with BCC structure. Electrochemical tests found that, with increase of the concentration of Mg in alloys, theoretical charge–discharge capacities would increase accordingly, discharge kinetics (exchange current densities and hydrogen diffusion abilities) were improved as well. However, the tested capacity will attain a maximum value (325 mAh g−1) at the point of Mg55Co45 and deteriorate subsequently with continuous increase of Mg content, which was possibly due to the optimized lattice parameter value and Mg corrosion. The increase of Co concentration in the binary alloys effectively inhibited the corrosion of the electrodes. On the basis of Mg–Co binary alloys, the Pd-doped ternary alloys with BCC structure were also prepared by means of ball milling. These alloys possess greatly enhanced discharge capacities (280–500 mAh g−1) from binary ones. It shows that with increase of the Mg content, the discharge capacities of Mg–Co–Pd ternary alloys will increase monotonously. The additional Pd also improved cycle stabilities, kinetics, and corrosion potentials of the alloys, which was beneficial to improving the properties of Mg–Co hydrogen storage electrodes.

•Mg50Co50-based BCC alloys exhibit reversible charge–discharge capacities.•The Mg50Co45Pd5 alloy possesses the maximum discharge capacity of 458 mAh g−1.•Additive Pd enhances the discharge kinetics the Mg50Co50-based alloys.The Mg50Co50 and Mg50Pd5Co45 alloys with nano-crystalline body-centered cubic (BCC) structure were confirmed by means of XRD and TEM/SAED analyses. They are available in the anode of Ni–MH battery system, which can be reversibly charged and discharged with maximum capacity of 458 mAh g−1. They exhibit greatly enhanced reversible hydrogen storage, differing from their tough dehydrogenation performances in solid–gas system [Y. Zhang et al., Journal of Alloys and Compounds 393 (2005) 147–153]. This phenomenon can be ascribed to the structural stability of Mg50Co50-based binary and ternary BCC alloys in the electrochemical reactions, and also the electro-catalysis activity. Introducing Pd into Mg50Co50 alloy lifts not only the initial discharge capacity, but also the high-rate discharge-ability. The exchange current density and hydrogen diffusion mobility can also be improved by the Pd additive in the ternary electrode alloys.

•TiH2 and Pd synergistically enhanced the dehydrogenation performances of MgH2.•The co-additives improved the release kinetics more markedly than individual one.•Pd slightly destabilized the ternary phase system by yielding Mg6Pd.In the present work, co-additives of Pd and TiH2 synergistically enhance the dehydrogenation performance of MgH2. The volumetric release measurements on 2MgH2–TiH2–0.1Pd composite revealed that the onset temperature of the dehydrogenation (150 °C) was greatly lowered by 150 °C from that of pristine MgH2. X-ray diffraction (XRD) analyses and high-resolution transmission electron microscopy (HR-TEM) observations found that Pd decomposed the MgH2 through yielding the Mg6Pd phase. The destabilizations of MgH2 in 2MgH2–0.1Pd and 2MgH2–TiH2–0.1Pd were confirmed by calculating the slopes of Van't Hoff plots of those composites based on pressure–composition isotherms. Those composites contain two significant plateaus in P–C isotherms of dehydrogenation, one represents the 2MgH2–0.1Pd release in initial stage, the other belongs to the self-decomposition of MgH2. TiH2 did not involve in the destabilization of MgH2 in the composite. However, it remarkably reduced the activation energy of the ternary system in dehydrogenation and improved its kinetics along with Pd. The activation energy value (77 kJ mol−1) of 2MgH2–TiH2–0.1Pd was found drastically lower than those of 2MgH2–0.1Pd (254.6 kJ mol−1) and 2MgH2–TiH2 (154.2 kJ mol−1). It exhibits that the synergistic effect by co-additives of TiH2 and Pd on dehydrogenation kinetics are more remarkable than any individual one.

•Mg45Pd5Ni45Zr5 alloy was prepared for the anode of Ni-MH battery.•The cycle stability of Mg45Pd5Ni45Zr5 is remarkably enhanced from that of Mg50Ni50.•The composite film on Mg45Pd5Ni45Zr5 surface can effectively suppress corrosion.The Mg50Ni50–based ternary and quaternary alloys partially substituted by Pd and/or Zr have been intensively studied on their structures and electrochemical hydrogen storage performances. The Mg45Pd5Ni50 and Mg45Pd5Ni45Zr5 alloys contained both amorphous phase and body-centered cubic phase, differing from most previously studied amorphous Mg50Ni50–based alloys. The detected lattice parameters of nano-crystalline BCC phase in Mg45Pd5Ni50 and Mg45Pd5Ni45Zr5 alloys were 0.2916 nm and 0.2933 nm, respectively. Charge-discharge tests indicated that Pd facilitates lifting the cyclic retention rate in 20 cycles (C20/Cmax) from 13.2% of Mg50Ni50 alloy to 47.3% of Mg45Pd5Ni50 alloy. The partial substitution of Zr for Ni in Mg50Ni50 alloy improved the initial capacity by 12% from that of Mg50Ni50 alloy (445 mAh g−1). Synergetic substitution of Pd and Zr in Mg50Ni50 can greatly inhibit the corrosion of Mg45Pd5Ni45Zr5 alloy. According to XPS study, the Mg, Pd, Ni and Zr on the surface of Mg45Pd5Ni45Zr5 alloy would be oxidized into MgO, PdO2, Ni2O3 and ZrO2 during charge/discharge cycles. They formed a passive composite film and therefore improved the cyclic stability.

•Nano-nickel addition facilitates nanocrystallization and amorphization of Mg2NiH4.•Mg2NiH4 – 2 nano-nickel reaches the maximum discharge capacity of 896 mAh/g.•The cycle stability is improved with increasing the nano-nickel coating amount.•The kinetic property of Mg2NiH4 is also enhanced with nano-nickel addition.In this paper, we succeed to modify the Mg2NiH4 with nano-nickel coating via mechanical milling, denoted by Mg2NiH4 – x nano-nickel (x = 0, 1, 2, 3). Effect of different nano-nickel coating amount on the structural and electrochemical properties of Mg2NiH4 has been investigated in detail. X-ray diffraction (XRD) analyses show that the crystalline Mg2NiH4 transforms into the nanocrystalline and amorphous state by mechanical milling with nano-nickel. High resolution transmission electron microscopy (HRTEM) and corresponding selected area electron diffraction (SAED) analyses confirm the existence of nanocrystalline and amorphous structure. Electrochemical measurements reveal that the maximum discharge capacity of Mg2NiH4 first increases to 896 mAh/g when x = 2 then decreases with increasing the nano-nickel coating amount. Furthermore, the positive shift of the corrosion potential demonstrates the improvement of anti-corrosion ability during charging/discharging cycles. High rate dischargeability (HRD) test shows that the kinetic property of Mg2NiH4 is obviously enhanced with increasing the nano-nickel coating amount. The exchange current density (I0) increases and the charge-transfer resistance (Rct) decreases with the increase of nano-nickel content, indicating that the charge-transfer rate on the electrode surface is increased. Meanwhile, the hydrogen diffusion ability inside the hydride bulk is also accelerated by nano-nickel coating.

•A novel Pd/MWCNTs catalyst was synthesized by a solution chemical reduction method.•The Mg–Pd/MWCNTs composites were prepared for the first time by HCS + MM.•Pd/MWCNTs significantly increase hydrogenation degree of Mg during the HCS process.•There is a synergistic effect of MWCNTs and Pd on the hydrogen storage properties.•Both hydriding/dehydriding properties of the Mg–Pd/MWCNTs composites are improved.To improve the hydrogen storage performance of magnesium hydride, multi-wall carbon nanotubes supported palladium (Pd/MWCNTs) was introduced to the magnesium-based materials. Pd/MWCNTs catalysts with different amounts of Pd (20 wt.%, 40 wt.%, 60 wt.%, 80 wt.%) were synthesized by a solution chemical reduction method. Afterwards, Mg95–Pdm/MWCNTs5−m (m = 0, 1, 2, 3, 4, 5) were prepared for the first time by hydriding combustion synthesis (HCS) and mechanical milling (MM). It is determined by X-ray diffraction (XRD) analysis that Pd/MWCNTs can significantly increase the hydrogenation degree of magnesium during the HCS process. The microstructures of the composites obtained by transmission electron microscope (TEM) and field emission scanning electronic microscopy (FESEM) analyses show that Pd nanoparticles are well supported on the surface of carbon nanotubes and the Pd/MWCNTs are dispersed uniformly on the surface of MgH2 particles. Moreover, it is revealed that there is a synergistic effect of MWCNTs and Pd on the hydrogen storage properties of the composites. The Mg95–Pd3/MWCNTs2 shows the optimal hydriding/dehydriding properties, requiring only 100 s to reach its saturated hydrogen absorption capacity of 6.67 wt.% at 473 K, and desorbing 6.66 wt.% hydrogen within 1200 s at 573 K. Additionally, the dehydrogenation activation energy of MgH2 in this system is decreased to 78.6 kJ/mol H2, much lower than that of as-received MgH2.

Magnesium hydride is considered as an ideal candidate for effective hydrogen storage due to its high gravimetric hydrogen capacity and accessibility. But its use as a commercial material is hindered by its relatively high operating temperatures and slow release/uptake kinetics. To solve this, we first synthesized Ni decorated graphene nanoplate (Ni/Gn) catalysts with highly dispersed metal nano-particles (NPs) via a facile method, then the as-prepared Ni/Gn catalysts were introduced by using the hydriding combustion synthesis and mechanical milling (HCS + MM) method to obtain Mg-based composites. Remarkable enhancement of hydrogen sorption rates has been found for these composites in the presence of Ni/Gn additives, especially for the Mg@Ni8Gn2 sample: a hydrogen absorption amount of 6.28 wt% within 100 s at 373 K and a hydrogen desorption amount of 5.73 wt% within 1800 s at 523 K. A rather low activation energy (71.8 kJ mol−1) for the dehydrogenation of MgH2 was determined in the same sample, indicating that relatively moderate temperatures are required to absorb/desorb hydrogen. The excellent hydrogen sorption rates of the composites are thought to be associated with the high dispersity of in situ formed nanometric Mg2NiH4 particles during the HCS + MM process. In addition, a microstrain-induced synergetic hydrogen sorption mechanism is proposed, being correlated by the local introduction of a Mg2Ni nano-catalyst into the Mg matrix.