The Mg–Al hydrogen storage alloy was successfully prepared by combustion synthesis (CS) method. The formation of alloy phases during the CS process was studied using X-ray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC). When the time increases from 0, 0.5, 1.0 to 2.0 h at 733 K, the products are Mg and Al; Mg2Al3, Mg and Al; Mg17Al12, Mg2Al3; and Mg; and eventually only Mg17Al12, respectively. Combined with three peaks in the DSC traces, it is concluded that the formation of Mg17Al12 during the CS includes three processes, namely, the formation of Mg2Al3 first; then the unsaturated solid solution, Mg17Al12; and finally the complete Mg17Al12 alloy. The formation of Mg2Al3 prior to Mg17Al12 in this work is different from those prepared by mechanical alloying. This is thought to be related to the instant high temperature during the thermal explosion of CS.

Magnesium chloride efficiently catalyzed the hydrolysis of Mg-based hydride prepared by hydriding combustion synthesis. Hydrogen yield of 1635 mL g−1 was obtained (MgH2), i.e. with 96% conversion in 30 min at 303 K.

Significant improvement of the hydrogen storage property of the magnesium-based materials was achieved by the process of hydriding combustion synthesis (HCS) followed by mechanical milling (MM) and the addition of nanosized Zr0.7Ti0.3Mn2 and MWCNT. Mg95Ni5 doped by 10 wt.% nanosized Zr0.7Ti0.3Mn2 and 3 wt.% MWCNT prepared by the process of HCS + MM absorbed 6.07 wt.% hydrogen within 100 s at 373 K in the first hydriding cycle and desorbed 95.1% hydrogen within 1800 s at 523 K. The high hydriding rate remained well and the hydrogen capacity reached 5.58 wt.% within 100 s at 423 K in the 10th cycle. The dehydrogenation activation energy of this system was 83.7 kJ/mol, which was much lower than that of as-received MgH2. A possible hydrogenation–dehydrogenation mechanism was proposed in terms of the structural features derived from the HCS + MM process and the synergistic catalytic effects of nanosized Zr0.7Ti0.3Mn2 and MWCNT.Highlights► Mg95Ni5 + 10 wt.% nanosized Zr0.7Ti0.3Mn2 + 3 wt.% MWCNT was prepared by HCS + MM. ► The composite exhibits the ultrafast kinetics with a high capacity. ► The composite has an excellent cyclic stability at 423 K. ► A possible hydrogenation–dehydrogenation mechanism was proposed.

Mg2Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various metal elements (Ti, Co, Cr and Y) by mechanical milling. The effects of the modifications on electrochemical properties of the hydride were investigated. Both of the maximum discharge capacity and the high rate dischargeability (HRD) are increased by the modification of Co, while the cycling stability is improved for the hydride modified with Ti, Cr and Y. The Tafel polarization shows that the addition of the metals has a positive effect on improving the anti-corrosion ability. The electrochemical kinetics was also characterized by linear polarization, electrochemical impedance spectroscopy and potentiostatic discharge. The results show that the hydrogen diffusion coefficient and the electrochemical reaction resistance are increased by the modifications, but the exchange current density decreases.

Mg2Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various carbonaceous materials including graphite, multi-walled carbon nanotubes (MWCNTs), carbon aerogels (CAs) and carbon nanofibers (CNFs) by mechanical milling (MM) for 5 h. The structural properties of the modified hydrides were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). All of the modified hydrides show amorphous or nanocrystalline-like phases. The hydride modified with graphite exhibits the most homogenous distribution of particles and the smallest particle size. The effects of the modifications on electrochemical properties of the hydride were investigated by galvanostatic charge/discharge, linear polarization, Tafel polarization, electrochemical impedance spectroscopy and potentiostatic discharge measurements. The results show that the maximum discharge capacity, the high rate dischargeability (HRD), the exchange current density and the hydrogen diffusion ability of the hydride modified with the carbonaceous materials are all increased. Especially, the hydride modified with graphite possesses the highest discharge capacity of 531 mAh/g and the best electrochemical kinetics property.

Mg–Ni–C composite hydrogen storage materials were prepared by first ball milling the powder mixtures of carbon aerogel and nano-Ni, and then mixed with magnesium powder followed by hydriding combustion synthesis (HCS). The HCS product was further treated by mechanical milling for 10 h. The effect of Ni/C ratio on the structures and hydrogen absorption/desorption properties of the materials were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure–composition–temperature (PCT) measurements. It is found that 90Mg–6Ni–4C system shows the best hydriding/dehydriding properties, which absorbs hydrogen at a saturated capacity of 5.23 wt.% within 68 s at 373 K and desorbs 3.74 wt.% hydrogen within 1800 s at 523 K. Moreover, the dehydriding onset temperature of the system is 430 K, which is 45 K lower than that of 90Mg–10Ni system or 95 K lower than that of 90Mg–10C system. The improved hydriding/dehydriding properties are related greatly to the Ni/C ratio and the structures of the composite systems.

The effect of Cu-doping on the hydrogen storage properties of Mg95Ni5Cux (x = 0, 0.5, 1, 2) prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. For dehydriding properties, the dehydriding temperature onset decreases from 450 K for Mg95Ni5 to 420 K for Mg95Ni5Cu2. Additionally, the activation energy for dehydriding decreases from 116 kJ/mol for Mg95Ni5 to 98 kJ/mol for Mg95Ni5Cu2, indicating that the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen absorption capacity of Mg95Ni5Cu2 at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM pretreatment before HCS. The factors for the improvement of the hydrogen storage properties are discussed in this paper.

A Mg–30 wt.% LaNi5 composite was prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM), and the hydriding and dehydriding properties of the HCS + MM product were compared with those of the HCS product and the MM product. The dehydriding temperature onsets of the MM and HCS + MM products were both 470 K, which were lower than that of the HCS product by 100 K. Moreover, the HCS + MM product desorbed faster than the MM product, e.g., the former desorbed completely upon heating to 510 K, whereas the latter did not decompose completely until 590 K. Additionally, the HCS + MM product reached a saturated hydrogen absorption capacity of 3.80 wt.% at 373 K in 50 s, but both the HCS product and the MM product absorbed less than 1.50 wt.% of hydrogen at 373 K in 1800 s. These results suggest the potential of the HCS + MM processing in preparing Mg-based hydrogen storage materials.

The catalytic mechanism of Nb2O5 and NbF5 on the dehydriding property of Mg95Ni5 prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. It was shown that NbF5 was more efficient than Nb2O5 in improving the dehydriding property. In particular, the dehydriding temperature onset decreases from 460 K for Mg95Ni5 to 450 K for Mg95Ni5with 2.0 at.% Nb2O5, whereas it decreases to 410 K for that with 2.0 at.% NbF5. By means of X-ray diffraction and X-ray photoelectron spectroscopy, it was confirmed that the interaction between the Nb ions and the H atoms and that between the anions (O2− or F−) and Mg2+ existed in Mg95Ni5 doped with Nb2O5 or NbF5. Further, the pressure–concentration-isotherms analysis clarified that these interactions destabilized the Mg–H bonding, and that NbF5 had a better effect on the destabilization of the Mg–H bonding than Nb2O5 contributing to the better dehydriding property of (Mg95Ni5)2.0−NbF5.

The composites of Mg–x wt.% CaNi5 (x = 20, 30 and 50) were prepared by hydriding combustion synthesis (HCS) and the phase evolution during HCS as well as the hydriding properties of the products were investigated. It was found that Mg reacted with CaNi5 forming Mg2Ni and Ca during the heating period of HCS. Afterwards, the resultant Mg2Ni and Ca as well as the remnant Mg reacted with hydrogen during the cooling period. The lower platform in the P–C isotherms corresponds to the hydriding of Mg, and the higher one corresponds to Mg2Ni. With the increase of the content of CaNi5 from 20 to 50 wt.%, the hydrogen content of the HCS products increases at first and then decreases. The Mg–30 wt.% CaNi5 composite has the maximum hydrogen capacity of 4.74 wt.%, and it can absorb 3.51 wt.% of hydrogen in the first hydriding process without activation.

The effect of La/Ni ratio on hydrogen storage properties of Mg88.5NixLay (x+y=11.5, y/x  =0, 116, 14, 12 and 21) composites prepared by hydriding combustion synthesis followed by mechanical milling was investigated. It was found that Mg88.5NixLay   (y/x=116) composite showed the largest hydrogen absorption capacity of 4.45 wt% in 30 s at 373 K among the composites. Compared with the other composites, Mg88.5NixLay   (y/x=116) had the rapid dehydriding rate during heating though the initial dehydriding temperatures of all the composites were almost the same and approximately 460 K. XRD analysis indicated that Mg–Ni–La composites contained Mg, MgH2, Mg2NiH0.3, Mg2NiH4 and LaH2. From the analysis of pressure–concentration isotherms, the existence of LaH2 resulted in the increased hydrogen storage capacity in Mg88.5NixLay   (y/x=116) composite. It was demonstrated that the excellent hydrogen storage properties of Mg88.5NixLay   (y/x=116) were attributed to the effective synergetic catalysis of LaH2 and Mg2Ni.

In this paper, a novel method, namely hydriding combustion synthesis (HCS) and subsequent mechanical milling (MM) was used to prepare Mg-based hydrogen storage electrode alloy. The phase structures and electrochemical properties of the alloys before and after MM were characterized by X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. The XRD results showed that the structure of the as-milled alloys was nanocrystallite or amorphous-like state. Electrochemical measurements showed that the discharge capacity was improved greatly for the products of HCS+MM. The HCS product with only 5 h MM showed markedly increased discharge capacity up to 481.5 mAh/g for the first cycle, which was 10 times higher than the HCS product (39.4 mAh/g). The discharge capacity was further increased to 628.3 mAh/g for the HCS product after milling with nickel powder. Besides, the addition of nickel also led to an improved cycling stability of the alloy electrode during cycling in KOH electrolyte. It was indicated that the HCS+MM was promising for preparing Mg-based hydrogen storage electrode alloys.

This paper investigated the effect of the intermediate reaction, Mg + H2 → MgH2 in hydriding combustion synthesis (HCS) process on the structures and hydrogen storage properties of Mg95Ni5 prepared by HCS and subsequent mechanical milling (MM), i.e. HCS + MM. When the MgH2 content in the HCS product was increased from 53 wt.% to 81 wt.%, the hydrogen absorption capacity of our HCS + MM product at 373 K within 100 s was increased from 0.63 wt.% to 4.90 wt.%, and the decomposition temperature onset was decreased approximately from 470 K to 450 K. The improvement in hydrogen storage properties was discussed with respect to the different structures resulted from the different HCS processes by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. Moreover, the HCS product (with 81 wt.% MgH2) milled with 3 wt.% graphite absorbed 5.56 wt.% hydrogen at 373 K in 100 s. The investigation in this study suggested that HCS combined with MM was potential in the preparation of Mg-based materials with excellent hydriding and dehydriding properties.

Hydriding combustion synthesis (HCS) is well known as an innovative processing to produce magnesium-based hydrogen storage alloys, and the product Mg2NiH4Mg2NiH4, as synthesized, possesses excellent activity without any activation process. In this paper, the mechanism of the high activity of Mg2NiH4Mg2NiH4 produced by HCS emphasized mainly on microstructure is discussed based on the analysis of phase composition, particle characteristic, crystal grain size and hydriding reaction by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure composition temperature (PCT). It is indicated that pure composition, loose and porous morphology, a large number of microfissures and small grain size are the intrinsic reasons for the high activity of Mg2NiH4Mg2NiH4 produced by the process of HCS. The result is very helpful for us to control and improve the hydrogen storage properties of HCS product based on the microstructure further.

Hydriding combustion synthesis (HCS) and mechanical milling (MM) are both well-known methods for production of magnesium-based hydrogen storage alloys. The former can produce high active hydride by a simple process, and the latter can synthesis various metastable hydrogen storage materials such as nanocrystalline, amorphous and extended solid solutions phases with excellent hydrogen sorption properties. In the present study, HCS and MM were combined, aiming to decrease sorption temperature for Mg2NiMg2Ni. The high active Mg2NiH4Mg2NiH4, synthesized by HCS, was mechanically milled for 0.5, 6, 40 and 80 h with 5 wt% of graphite under argon atmosphere. The effect of the milling process on the morphology and crystal structural of Mg2NiH4Mg2NiH4 were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The hydrogen storage properties were examined by a Sieverts type apparatus. The nanocrystalline Mg2NiH4Mg2NiH4 milled for 40 h has the best sorption kinetics, which can absorb 2.4 wt% hydrogen at 303 K within 100 s in the first cycle. Several reasons are considered to explain the improvement in hydriding kinetics.

The mechanism of the high activity of Mg2NiH4Mg2NiH4 produced by the process of hydriding combustion synthesis (HCS) was discussed for the first time based mainly on the crystal defects and surface characteristics examined by means of high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A large number of crystal defects (lattice distortions, dislocations, stacking faults and microtwins) observed in the as-dehydrogenated product of Mg2NiH4Mg2NiH4 and an outmost layer of Mg(OH)2Mg(OH)2, instead of MgO determined on the surface of the as-synthesized Mg2NiH4Mg2NiH4, as well as a large amount of fresh surfaces in Mg2NiMg2Ni particles that was generated in the dehydriding process of the as-synthesized Mg2NiH4Mg2NiH4 prior to the first hydrogenation were revealed and considered as the intrinsic reasons contributing to the high activity of Mg2NiH4Mg2NiH4 produced by the process of HCS. These results are helpful to control and improve the hydrogen storage properties of the HCS product further.

We reported the structure and the notable hydrogen storage properties of the composites Mg100−xNix (x = 5, 11.3, 20, 25) prepared from metallic powder mixtures of magnesium and nickel by the process of HCS + MM, i.e., the hydriding combustion synthesis (HCS) followed by mechanical milling (MM). X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that mechanical milling led to drastic pulverization and grain refinement of the composite produced by HCS. All the composites with different compositions showed a remarkable decline in dehydriding temperature comparing with that of the hydride mixtures prepared only by HCS. Furthermore, the hydriding rates of these composites were excellent. At 313 K the composite Mg80Ni20 showed the highest hydrogen capacity of 2.77 wt.% within 600 s among these four composites. The Mg95Ni5 showed maximum capacity of 4.88 wt.% at 373 K and 5.41 wt.% at 473 K within only 100 s. Some factors contributing to the improvement in hydriding rates were discussed in this paper.

Magnesium chloride efficiently catalyzed the hydrolysis of Mg-based hydride prepared by hydriding combustion synthesis. Hydrogen yield of 1635 mL g−1 was obtained (MgH2), i.e. with 96% conversion in 30 min at 303 K.