Hydrogen storage in Mg2Ni(Fe)H4 nano particles synthesized from coarse-grained Mg and nano sized Ni(Fe) precursor

In this work, Mg2Ni(Fe)H4 was synthesized using precursors of nano Ni(Fe) composite powder prepared through arc plasma method and coarse-grained Mg powder. The microstructure, composition, phase components and the hydrogen storage properties of the Mg–Ni(Fe) composite were carefully investigated. It is observed that the Mg2Ni(Fe)H4 particles formed from the Mg–Ni(Fe) composite have a diameter of 100–240 nm and a portion of Fe in the Ni(Fe) nano particles transformed into α-Fe nano particles with the diameter of 40–120 nm. DSC measurements showed that the peak desorption temperature of the Mg2Ni(Fe)H4 was reduced to 501 K and the apparent activation energy for hydrogen desorption of the Mg2Ni(Fe)H4 was 97.2 kJ mol−1 H2. The formation enthalpy of Mg2Ni(Fe)H4 was measured to be −53.1 kJ mol−1 H2. The improvements in hydrogen sorption kinetics and thermodynamics can be attributed to the catalytic effect from α-Fe nano particles and the destabilization of Mg2NiH4 caused by the partial substitution of Ni by Fe, respectively.


Introduction
Magnesium hydride has a relatively high hydrogen storage capacity (7.6 wt%), an environmentally friendly nature and low cost, which meets some basic requirements for onboard and stationary applications set by the US DOE. However, the high thermodynamic stability of MgH 2 (DH ¼ À75 kJ mol À1 H 2 ) is a big hurdle for lowering the hydrogen desorption temperature of Mg-based hydrides. 1,2 It has been established that when Mg alloys contain a non-hydride forming element, the value of hydrogenation enthalpy can be decreased. Therefore, alloying is a traditional and effective strategy for altering the thermodynamics of Mg-based alloys for hydrogen storage.
One of the typical examples is Mg 2 Ni, which can react with H 2 to form Mg 2 NiH 4 . It should be noted that the formation enthalpy of Mg 2 NiH 4 (DH ¼ À64.5 kJ mol À1 H 2 ) is lower than that of MgH 2 which allows the former to desorb at lower temperatures than MgH 2 . 3 Morinaga et al. found that hydrogen interacted more strongly with Ni atoms rather than Mg atoms in Mg 2 NiH 4 . 4 This existence of Ni in Mg 2 NiH 4 weakened the Mg-H bond as compared to MgH 2 , and leads to a lower formation enthalpy of Mg 2 NiH 4 . The crystallographic and hydrogen storage properties of Mg 2 NiH 4 were rstly reported by Reilly and Wiswall. 3 Since single phase Mg 2 Ni cannot be simply obtained by casting as phase separation occurs during solidication, 5,6 reactive mechanical alloying (RMA), mechanical milling (MM) and high-energy ball milling (HEBM) have been employed to synthesize Mg 2 Ni or Mg 2 NiH 4 . 7-11 However, it usually takes a long time to prepare Mg 2 Ni or Mg 2 NiH 4 via these methods. Therefore, the size of the raw material was reduced to nanoscale to promote the formation of Mg 2 Ni or Mg 2 NiH 4 . 12, 13 Shao et al. prepared Mg 2 Ni from Mg and Ni nanoparticles produced by hydrogen plasma-metal reaction. 13 It was established that at 553 K and 3 MPa hydrogen pressure, Mg 2 NiH 4 could be generated.
More recently a number of research groups focused on various synthesis methods for doping with additives to further improve the kinetic properties of Mg 2 Ni. 14 16 The alloy has higher reversible hydrogen storage capacity of 3.3 wt% and the minimum dehydrogenation temperature is reduced to 493 K.
Iron is considered as a good catalytic/alloying element to improve kinetics/thermodynamics of Mg-based hydrogen storage materials. 18,19 In addition, the hydride of Fe is highly unstable, which may be benecial for the structural reversibility of Mg 2 Ni in dehydriding. 18 In the present work, the Ni(Fe) nano particles were prepared through an arc plasma method described in previous works. 13,20 The formation of the Mg 2 Ni(Fe) H 4 by using Ni(Fe) precursor and its hydrogen sorption kinetic and thermodynamic properties were carefully investigated. Based on the experimental investigations, the mechanisms of the formation and hydrogen sorption behaviors of the Mg 2 -Ni(Fe)H 4 were proposed.

Sample preparation
In this work, the Ni(Fe) composite nano powder was prepared by an arc plasma evaporation apparatus. 20 The purities of commercially available pure Ni and Fe powders are over 99.5% and the particle size ranges from 50 to 150 mm. The preparation of the powder involves several steps. First, pure Ni and Fe powders were mixed homogeneously with a Ni to Fe weight ratio of 3 : 1. Then the mixture was compressed to cylinders with a diameter of 10 mm and height of 7 mm under a uniaxial pressure of 15 MPa at room temperature. These cylinders were put into the reaction chamber of arc plasma evaporation apparatus lled with mixed gases of 0.01 MPa H 2 and 0.05 MPa Ar. The Ni(Fe) composite nano powder was produced using arc evaporation with the current set at 150 A. Aer evaporation and condensation, the powder was passivated in mixed argon and air (7 : 1) for 12 h. About 2.0 g mixture of the commercially available Mg powder and Ni(Fe) composite nano powder in a 2 : 1 molar ratio was ball milled for 4 h in a planetary ball miller. The Mg powder has a purity of 99.5% and its particle size ranges from 50 to 150 mm. The ball milling was carried out in a 50 ml stainless steel vessel under 0.1 MPa Ar atmosphere at 200 rpm with a ball-to-powder weight ratio of 30 : 1. Then these samples were further heated up to 623 K and 673 K in hydrogen under a pressure of 3.7 MPa. Aer hydrogenation, samples were cooled down to room temperature and the sample container was evacuated to 10 À3 Pa.

Characterization
The compositions of the Ni(Fe) and Mg-Ni(Fe) powders were analyzed by using inductive coupled plasma emission spectrometer (ICP). The phase identications of as-milled and hydrogenated composite powders were carried out by X-ray diffraction (XRD) using a D/max 2550VL/PCX apparatus equipped with a Cu Ka radiation source. The morphology and microstructure of the powders were characterized by a JEM-2100F transmission electron microscope (TEM) equipped with an energy dispersive spectrometer (EDS) micro-analysis and a backscattered electron detector. The hydrogen sorption isotherms at various temperatures were obtained using a Sievert type pressure-composition-temperature (PCT) volumetric apparatus provided by Shanghai institute of microsystem and information technology. The dehydriding behaviors of hydrogenated samples were investigated by using differential scanning calorimetry (DSC, Netzsch STA449F3 Jupiter) under 0.1 MPa Ar atmosphere at heating rates of 3, 5 and 10 K min À1 from room temperature to 773 K.

Results and discussions
3.1 Formation of Mg 2 Ni(Fe)H 4 from Mg and Ni(Fe) powders Fig. 1 shows the XRD pattern of the as prepared Ni(Fe) powder. These peaks correspond to Ni phase but shi to low angles, suggesting the increased lattice parameter. Based on the XRD patterns, the lattice constant of this phase is determined to be: a ¼ 0.3548 nm, which is higher than that of Ni (a ¼ 0.3535 nm). Such a lattice expansion of Ni can be only attributed to the partial substitution of Ni by Fe. Based on the Scherrer's equation, 21 the average grain size of Ni(Fe) is determined to be 80 nm.
ICP analysis revealed that Ni content in the as prepared Fe(Ni) composite powder is 61.2 wt%, lower than that in the original mixture (75 wt%). According to the Ohno's model, the vapor generation rate of a metal through hydrogen plasma reaction method is proportional to its reaction parameter (R p ), which can be expressed by the following equation: 22 where DH r is the reaction enthalpy between hydrogen and metal, L s is the vaporization heat of the metal at temperature T, N H 2 (T) and N H 2 (273) refer to the densities of molecular hydrogen in metal at temperature T and 273 K, respectively. According to eqn (1), the vapor generation rate mainly depends on the hydrogen affinity to metals (DH r ) and vaporization heat (L s ). The reaction enthalpy between H and Fe is quite similar to the one between H and Ni ($20 kJ mol À1 H 2 ) while the heat of vaporization for Ni (370.4 kJ mol À1 ) is higher than that for Fe (349.6 kJ mol À1 ). Therefore, the evaporation rate of Ni is lower than that of Fe. This is consistent with the ICP result for which the Ni content in the Ni(Fe) powder is lower than that in the mixed powder before arc evaporation. TEM observations have been done on the as prepared Ni(Fe) composite powder. As seen from Fig. 2a, the shapes of the particles in the Ni(Fe) composite powder are mostly spherical Fig. 1 The X-ray diffraction (XRD) pattern of the Ni(Fe) composite powder prepared using arc plasma method. and the particle size ranges from 50 nm to 260 nm with an average size of about 80 nm. Such morphology and size distribution were also observed in pure Ni powder produced by arc plasma method. 23,24 The corresponding elemental distribution maps of Ni and Fe (Fig. 2b and c) conrm the fairly homogeneous distributions of Ni and Fe in the Ni(Fe) powder.
Furthermore, EDS analyses also reveal that the Ni to Fe weight ratio is 59.2 : 40.8, which is in good agreement with the result obtained by the ICP measurement.  Fig. 4b and c. Based on the RIR analysis of those XRD patterns, the phase components in the hydrogenated powders were calculated and the results were given in Table 1. Aer hydrogenation at 623 K under 3.7 MPa H 2 for 48 h, peaks from Mg 2 NiH 4 and a-Fe appear on the XRD pattern of the Mg-Ni(Fe)-H powder (Fig. 4b). Meanwhile, Ni(Fe) content reduces from 57.8 wt% in as milled Mg-Ni(Fe) powder to 2.0 wt% in the hydrogenated one. Therefore, with the presence of Ni(Fe) in the Mg-Ni(Fe) powder, Mg 2 NiH 4 is able to be synthesized and then most of Ni(Fe) have been transformed into a-Fe. As the temperature increases from 623 K to 673 K, the diffraction peaks of b-MgH 2 become weaker and Ni(Fe) could hardly be detected on the XRD pattern of Mg-Fe(Ni)-H powder.     the simulated ring patterns of Mg 2 NiH 4 . The majority of the diffraction pattern can be indexed with Mg 2 NiH 4 phase. In addition, the HRTEM observation was further carried out to study the microstructure of nano particles in more detail, as shown in Fig. 5c. It is clearly shown that the bigger bright particles are Mg 2 NiH 4 and the smaller dark particles are a-Fe, according to the measurements of inter-planar spacing. The corresponding EDS maps of Mg, Ni and Fe (Fig. 6d-f) conrm the segregation of Fe element in the smaller spherical particles and the uniform distributions of Ni and Mg elements in the bigger irregular ones. Therefore, aer hydrogenation at 673 K under a 3.7 MPa hydrogen pressure for 48 h, Ni atoms diffused from Ni(Fe) nano particles to the coarse-grained Mg particles to form Mg 2 NiH 4 and then a portion of Fe atoms in the Ni(Fe) nano particles transformed into a-Fe nano particles with smaller particle size. The shapes of the a-Fe nano particles are mostly spherical and the particle size ranges from 40 nm to 120 nm with an average size of about 60 nm. In addition, a portion of Fe atoms have also diffused with Ni atoms to form Mg 2 Ni(Fe)H 4 , as conned by the EDS analyses.
The formation of Mg 2 NiH 4 requires a long-distance diffusion of not only small H atoms but also the large metallic species Mg and Ni. Owing to the low diffusion rates of Mg and Ni, the formation of the Mg 2 NiH 4 generally suffers from poor kinetics. In systems based on Mg and Ni that are tailored mainly toward fast sorption kinetics, MgH 2 would rather form prior to Mg 2 NiH 4 . 13,25 Fig. 6 shows a sketch illustrating the formation of Mg 2 Ni(Fe)H 4 nano particles. Firstly, the mixed Mg powders in micron scale and Ni(Fe) nano particles are heated in hydrogen, as shown in Fig. 6a. At the beginning of the absorption, because of the catalytic effect from nickel and iron on the dissociation of gaseous hydrogen, 26 MgH 2 will form through hydrogen spill over mechanism while Ni(Fe) particles still exists, as seen in Fig. 6b. Then Mg 2 NiH 4 will nucleate at the phase boundaries between MgH 2 and Ni(Fe) (Fig. 6c) Fig. 6d).  Table 2. During H 2 absorption, two plateaus are clearly visible   on each prole. According to the XRD results given above, two types of hydrides, MgH 2 and Mg 2 Ni(Fe)H 4 , formed aer hydrogenation. It is worth noting that Mg 2 NiH 4 has higher equilibrium pressure than MgH 2 due to its lower thermodynamic stability. 29,30 The maximum hydrogen absorption capacities at 598, 623, 648 and 673 K are 3.0, 3.1, 3.6 and 3.9 wt%, respectively. It is noticed that the capacities at 598 and 623 K are lower than the theoretical value of 3.6 wt% for Mg 2 Ni. This is due to the existence of MgO, the residual Mg in the hydride Mg-Ni(Fe) composites, as shown in Fig. 4b. However, when the hydrogenated temperature is above 648 K, the capacity is higher than 3.6 wt%. Since the original molar ratio of Mg and Ni(Fe) in the Mg-Ni(Fe) is 2 : 1 and the ICP analysis revealed that Ni content in the Fe(Ni) powder is 61.2 wt%, the molar ratio of Mg to Ni in the Mg-Ni(Fe) is actually 3.3 : 1, higher than 2 : 1. Therefore, aer complete hydrogenation, the maximum hydrogen absorption capacities of the Mg-Ni(Fe) can be higher than 3.6 wt%. The van't Hoff plot (ln P versus 1/T) is used to estimate the thermal stability of hydrides, as shown in Fig. 7b. Table 3 presents the value of the formation enthalpy (DH) for the Mg 2 -Ni(Fe)H 4 in this work and the ones for the Mg 2 NiH 4 reported in literature. According to the linear ttings of ln P versus 1000/T in Fig. 8b, the van't Hoff equations for the hydrogenation are ln(P low ) ¼ À6.392 Â 10 3 /T + 10.57 for MgH 2 and ln(P high ) ¼ À8.138 Â 10 3 /T + 12.81 for Mg 2 Ni(Fe)H 4 . The hydrogenation enthalpies (DH ab ) for MgH 2 and Mg 2 Ni(Fe)H 4 are therefore calculated to be À67.7 and À53.1 kJ mol À1 H 2 , respectively. In contrast, Pourabdoli et al. reported that the integral heat of H 2 desorption for the MgH 2 -10 wt% (9Ni-2Mg-Y) nano-composite was about 78 kJ mol À1 H 2 measured by using adsorption microcalorimetry. 31 Therefore, the formation enthalpy of Mg 2 Ni(Fe) H 4 is obviously higher than those reported in literature. Van Setten et al. have studied the inuence of transition metals (Cu, Fe and Co) on the structural and hydrogen sorption properties of Mg 2 NiH 4 using rst-principle based calculations. 18 Doping with Co or Cu leads to octahedron is distorted. The hydrogen tetrahedra around such Ni atoms are distorted with Ni-H distances from 1.51 to 1.80Å, whereas in undoped Mg 2 NiH 4 they are between 1.56 and 1.58Å. Therefore, the highest hydriding enthalpy, À55.5 kJ mol À1 H 2 , is found for the Fe doped Mg 2 NiH 4 . 18 This value is higher than the simulated value for Mg 2 NiH 4 (À63.5 kJ mol À1 H 2 ). Therefore, the results present in this work also conrm that doping with Fe would remarkably destabilize Mg 2 NiH 4 through increasing the formation enthalpy. In addition, the thermodynamics of marginally stable compounds, whereas doping with Fe leads to an unstable compound. In the Fe doped Mg 2 NiH 4 , the hydrogenation of MgH 2 in the Mg-Ni(Fe) is also slightly improved by the addition of Ni(Fe) nanoparticles, which is in accordance with recent experimental results and theoretical calculations. For instance, the hydriding enthalpies of Mg-Ni nanocomposite coprecipitated from solution and MgH 2 -Ni/Ti prepared by ball milling were reduced to À70.0 and À67.8 kJ mol À1 H 2 . 32, 33 Dai and Shevlin carried out theoretical calculations and found that the signicant electronic density donation from the H À ions to an empty d-state of the Ni dopant was able to reduce the H À anion charge to weaken the hydrogen bonding. 34,35 Thus, doping with Ni would also remarkably destabilize MgH 2 to improve the thermodynamics of MgH 2 . Fig. 8a shows DSC curves obtained by heating the Mg-Ni(Fe)-H powder up to 723 K at heating rates of 3, 5 and 10 K min À1 under Ar. These curves suggest a two-step desorption behavior by showing two endothermic peaks. The rst one is a strong and broad endothermic peak in the lower temperature range, while the second one is a relatively weaker peak in the higher temperature range. Clearly, the strong peak corresponds well to the decomposition of Mg 2 Ni(Fe)H 4 while the weak one results from the decomposition of the MgH 2 . The peak dehydrogenation temperatures of the Mg 2 Ni(Fe)H 4 in the Mg-Ni(Fe)-H powder at heating rates of 3, 5 and 10 K min À1 are 501.3, 510.6 and 526.1 K, respectively. Zou et al. prepared the Mg-rich Mg-Ni ultrane particles through the arc plasma method. 27 DSC analyses showed that there are three endothermic peaks appeared at 513, 643 and 668 K, which correspond to the phase transformation of Mg 2 NiH 4 from its low temperature form to the high temperature form, the dehydriding of Mg 2 NiH 4 and MgH 2 , respectively. Therefore, the

Hydrogen sorption behaviors of the Mg-Ni(Fe) powder
where b is the heating rate, T p is the peak temperature, and R is the gas constant. Just as expected, the peaks of dehydrogenation shi toward higher temperature as the heating rate increases. According to eqn (4), the E de value of the

Conclusions
In the present work, the formation of Mg 2 Ni(Fe)H 4 in the mixed precursors of the coarse-grained Mg powder and the Ni(Fe) nano powder prepared through arc plasma method was investigated. The microstructure, composition, phase components and hydrogen storage properties of the Mg-Ni(Fe) powder were characterized and the main results are as follows.
(1) Aer sintering of the Mg-Ni(Fe) powder at 673 K under 3.7 MPa H 2 for 48 h, the yield of Mg 2 Ni(Fe)H 4 reached 61.2 wt% and a portion of Fe atoms in the Ni(Fe) nano particles transformed into a-Fe nano particles.
(2) Mg 2 Ni(Fe)H 4 shows lower hydrogen desorption temperature and better desorption kinetics when compared to the Mg 2 NiH 4 obtained from arc plasma evaporated Mg-Ni powder. The peak dehydrogenation temperature of Mg 2 Ni(Fe)H 4 is reduced to 501.3 K and the dehydrogenation activation energy is reduced to 97.2 kJ mol À1 H 2 .
(3) The formation enthalpy of Mg 2 Ni(Fe)H 4 is determined to be À53.1 kJ mol À1 H 2 , higher than the values reported in literature for Mg 2 NiH 4 . The improvements in hydrogen sorption kinetics and thermodynamics can be attributed to the catalytic effect of a-Fe nano particles surrounding the Mg 2 Ni(Fe)H 4 and the destabilization effect caused by Fe substitution for Ni in the Mg 2 Ni(Fe)H 4 , respectively.

Conflicts of interest
There are no conicts to declare.