Synergistic Effect of New ZrNi 5 / Nb 2 O 5 Catalytic Agent on Storage Behavior of Nanocrystalline MgH 2 Powders

Due to its availability and high storage capacity, Mg is an ideal material in hydrogen storage applications. In practice, doping Mg/MgH2 with catalyst(s) is necessary in enhancing the de/rehydrogenation kinetics and minimizing both of decomposition temperature and its related apparent activation energy. The present study proposed a new heterogeneous catalytic agent that consisted of intermetallic compound (ZrNi5)/metal oxide (Nb2O5) binary system for using with different concentrations (5−30 wt%) to improve MgH2. Doping MgH2 powders with low concentration (5, 7, 10 wt%) of this new catalytic system led to superior absorption/desorption kinetics, being indexed by the short time that is required to absorb/desorb 4.2−5.6 wt% H2 within 200 s to 300 s. Increasing the doping dose to 15–30 wt% led to better kinetic effect but a significant decrease in the hydrogen storage capacity was seen. The dependent of apparent activation energy and decomposition temperature of MgH2 on the concentration of ZrNi5/Nb2O5 has been investigated. They tended to be linearly decreased with increasing the catalyst concentrations. The results elucidated the crucial role of catalytic additives on the disintegration of MgH2 into ultrafine powders (196 nm to 364 nm diameter). The formation of such nanoparticles enhance the hydrogen diffusion and shorten the time that is required for the hydrogenation/dehydrogenation process. Moreover, this refractory catalytic system acted as a grain growth inhibitor, in which Mg/MgH2 powders maintained their submicron level during the cycle-life-test that was extended to 100 h at 200 ◦C.

Likewise, catalysts are essential substances that are used for doping Mg/MgH 2 materials to modify their behaviors upon deducing the apparent activation energy between reactants (Mg + 2H) and product (MgH 2 ) in many chemical and oil refining applications.Accordingly, the potential energy barrier gap is decreased.Within the last three decades, MgH 2 was targeted by a great number of catalysts that were employed to enhance its behavior.Pure metals, metal alloys and intermetallic compounds, metallic glasses, metal oxides, carbides, and fluorides are some examples of catalysts that were devoted to improve MgH 2 [5,[7][8][9][10].
Among this long list of catalysts, Nb 2 O 5 powders have received great attention due to its capability of enhancing the hydrogenation/dehydrogenation kinetics of MgH 2 and the reduction the activation energy of decomposition [11][12][13].It has been reported that doping MgH 2 with 5 wt% Nb 2 O 5 led to improving the absorption/desorption kinetics to uptake and releasing 6 wt% H 2 at 300 • C within 78 s and 300 s, respectively [14].Hanada et al. reported that the catalyzation of MgH 2 with 14 wt% Nb 2 O 5 led to minimizing the hydrogenation temperature (100 o C) that is required to absorb 4 wt% H 2 under 5 bar within 30 min [14].Decomposition of MgH 2 /14 wt% Nb 2 O 5 required 7 min to desorb 4 wt% H 2 under 0.1 bar/240 • C [14].
Apart from the catalytic metal oxide group, Dehouche et al. published very interesting work related to doping MgH 2 powders with different intermetallic compounds of the Zr 100-x Ni x system [15].They reported that doping MgH 2 powders with eutectoid Zr 47 Ni 53 resulted in the fastest desorption time and the highest initial desorption rate at 250 • C. In addition, they pointed out that the composition of the dispersed Zr 100-x Ni x catalysts had strong influence on the amount of accumulated hydrogen and the desorption rate of Mg-nanocomposite [15].
The present work aims to study the catalytic effect of x-wt% (y-ZrNi 5 /y-Nb 2 O 5 ), x; 5, 7, 10; 15; and, 30, abrasive nanopowders on the behaviors of MgH 2 , as indexed by (i) thermal stability, (ii) apparent activation energy of decomposition, (iii) storage capacity, (iv) de/rehydrogenation kinetics, and (v) cycle-life-time.Moreover, the role of abrasive additives on particle refinement and their influences on hydrogen uptake/release processes were investigated.

Crystal Structure
The X-ray diffraction (XRD) patterns for the starting materials of MgH 2 and ZrNi 5 powders that were obtained after 200 h and 10 h of ball milling are displayed in Figure 1a,b, respectively.MgH 2 powders consisted of β-MgH 2 (PDF file #: 00-012-0697), as indicated by Bragg peaks that appeared at 2θ of 27.97 • , 35.78 • , 39.73 • , and 54.73 • (Figure 1a).The XRD pattern of ZrNi 5 milled for 10 h (Figure 1b) displays Miller-indexed Bragg peaks that matched well with the reported ZrNi 5 -Friauf-Laves phase (PDF#: 00-037-0924).The broadening that was seen in the diffracted lines was attributed to the grain refinement of the powders during the high-energy ball milling process.Figure 1c presents the XRD pattern of MgH 2 that was doped with a mixture of (2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders and then ball-milled for 50 h of ball milling.The Bragg peaks corresponding to MgH 2 and the catalytic agent (ZrNi 5 and Nb 2 O 5 ) became broad, suggesting the formation of nanocomposite powder particles.Neither the reacted phase, such as ZrH 2 , NbH, nor foreign phases could be obtained after this stage of milling, as elucidated in Figure 1c.
The high-resolution transmission electron microscope (HRTEM) image of the MgH 2 -5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) nanocomposite powder that was obtained after the 50 h of milling is displayed in Figure 1d.In general, the sample that was composited of ultrafine nanocrystals of MgH 2 , ZrNi 5 , and Nb 2 O 3 revealed lattice fringes seen from different selected zones in Figure 1d, revealing the existence of single crystalline phases in the sample.The lattice fringes were regularly separated with d spacing of 0.189, 0.234, and 0.205 nm, which agrees well with the (222), (220), and (311) lattice index of ZrNi 5 , respectively.The Moiré-like fringes that are displayed in Figure 1d with a d spacing of 0.197, 0.176, and 0.241 nm match well with the (002), (380), and (181) lattice index of Nb 2 O 5 crystals, respectively.The d spacing of 3 selected β-MgH 2 crystals oriented to (101), (200), and (110) were 0.274, 0.226, and 0.307 nm, respectively, being in excellent agreement with the reported data (PDF file # 00-012-0697).The d spacing of 0.195 nm (Figure 1d) was related to γ-MgH 2 (211) crystal.All of the lattice fringes that were taken from different 25 zones for three individual samples strongly manifested the existence of three phases of pure MgH 2 , ZrNi 5 , and Nb 2 O 5 crystals.The existence of other reacted phases, such as Mg 2 Ni, Zr 100-x Ni x , Nb, NbO, NbH, and Nb crystals in this sample that were obtained after 50 h

EDS Analysis
Local analysis was performed on the powders of the final product while using energy dispersive x-ray spectroscopy (EDS)/HRTEM technique.Figure 2a displays dark field image (DFI) of MgH2/5 wt% (2.5 ZrNi5/2.5 Nb2O5) nanocomposite powder that was obtained after milling for 50 h.The nanocomposite aggregate was virtually divided into 14 zones (indexed in Figure 2a by circular symbols) for conducting EDS analysis, while using a beam of 2 nm in diameter.

EDS Analysis
Local analysis was performed on the powders of the final product while using energy dispersive x-ray spectroscopy (EDS)/HRTEM technique.Figure 2a displays dark field image (DFI) of MgH 2 /5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) nanocomposite powder that was obtained after milling for 50 h.The nanocomposite aggregate was virtually divided into 14 zones (indexed in Figure 2a by circular symbols) for conducting EDS analysis, while using a beam of 2 nm in diameter.

EDS Analysis
Local analysis was performed on the powders of the final product while using energy dispersive x-ray spectroscopy (EDS)/HRTEM technique.Figure 2a displays dark field image (DFI) of MgH2/5 wt% (2.5 ZrNi5/2.5 Nb2O5) nanocomposite powder that was obtained after milling for 50 h.The nanocomposite aggregate was virtually divided into 14 zones (indexed in Figure 2a by circular symbols) for conducting EDS analysis, while using a beam of 2 nm in diameter.1.
Table 1 lists the corresponding EDS results of elemental analysis for the selected zones shown in Figure 2. Based on the EDS analysis, it is concluded that the composition of the end-product for the nanocomposite powders that were obtained after this stage of milling is very homogeneous and does not seriously fluctuate in composition beyond the nanolevel, as shown in Table 1.

Morphology
Figure 3a displays the relationship between the average particle size of nanocomposite powders that were obtained after 50 h of milling and the concentration (wt%) of (x ZrNi 5 /x Nb 2 O 5 ) catalytic agent, in the range between 0 wt% to 30 wt%.MgH 2 powders, which were obtained after 200 h of reactive ball milling (RBM) (before doping) composited of irregular cluster particles that were agglomerated together due to Van der Wales electrostatic attraction to form large aggregates, as shown in Figure 3b.The powder particles that were distributed in a wide range had an average particle size of 1480 nm, as presented in Figure 3a.
When the powders were catalyzed with 5 wt%(2.5 ZrNi 5 /2.5 Nb 2 O 5 ) and milled for 50 h, finer spherical particles were obtained (Figure 3c).In addition, they were distributed in a narrow range to provide an average particle size of 364 nm, as shown in Figure 3a.We should emphasize that the ZrNi 5 /Nb 2 O 5 hard powders played a micro-milling agent role during the milling process.This nanosized lubricant abrasive agent penetrated the rather soft MgH 2 powder matrix and occupied their catalytic sites onto/into the domain powders (Figure 2).The further increasing of catalytic dose to 7, 10, and 15 wt% led to monotonical decreasing on the sizes of nanocomposite powders to 318, 306, and 284 nm, as presented in Figure 3a.
The effect of hard ZrNi 5 /Nb 2 O 5 catalysts became pronounced upon increasing the dose to 20 wt% (10 ZrNi 5 /10 Nb 2 O 5 ) and 30 wt% (15 ZrNi 5 /15 Nb 2 O 5 ).The drastic decreasing of the particle sizes that reached to 203 nm and 196 nm, respectively, implied this (Figure 3a).In addition, the nanocomposite powders had almost spherical-like morphology with minimal agglomeration, as displayed in Figure 3d,e.Using such high molar fraction of catalytic agents led to an increase in the abrasive shear forces that were applied to MgH 2 powders when the effect of applied forces that were generated by the milling balls (10 mm in diameter) on such ultrafine powders (~200 nm) were deduced.Based on the TEM and scanning electron microscope (SEM) results of the present study, it is concluded that increasing the weight percentage of (x ZrNi 5 /x Nb 2 O 5 ) against MgH 2 powders had a beneficial effect on the fabrication of ultrafine nanocomposite powders with uniform spherical shapes (Figure 3c−e) and uniform distribution (Figure 1d).

Thermal Analysis
High-pressure differential scanning calorimetry (HP-DSC) and normal DSC were performed at different heating rates in order to investigate the apparent activation energy (Ea) of the hydrogenation and dehydrogenation process, respectively.

High-Pressure Differential Scanning Calorimetry
The HP-DSC traces that were conducted under 30 bar of hydrogen gas atmosphere with different heating rates of 10, 11, 12, 14, 15 °C/min for the nanocomposite MgH2/5wt% (2.5 ZrNi5/2.5 Nb2O5) powders that were obtained after 50 h of milling are shown together in Figures 4a and 4b.Obviously shown in the figure, all of the scans revealed exothermic peaks (low temperature peaks) that were related to the hydrogenation reaction taking place between hydrogen and metallic Mg.The peak temperature of exothermic peaks appeared in the temperature range between 450 °C and 482 o C, depending on the applied heating rates (Figure 4b).The XRD analysis (not shown here) of the powders that were taken after these exothermic peaks confirmed the formation of MgH2 phase coexisted ZrNi5 and Nb2O5 phases without evidence of the formation of any other reacted phase(s).
The second peaks appeared at a higher temperature side (above 712 °C) that was related to endothermic events, referred to as the decomposition of MgH2 phase (Figure 4a).Where the peak intensity of the exothermic and endothermic peaks increased proportionally with increasing the heating rates, the peak temperatures (Tp) were significantly shifted to the higher temperature side upon increasing the heating rates from 10 °C/min to 15 °C/min, as shown in Figure 4a.The improved hydrogenation kinetics of the nanocomposite MgH2/5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) powders was investigated by calculating the apparent Ea, while using Arrhenius approach.The value Ea of the hydrogenation reaction was determined by measuring the absorption temperature (Tp) peaks with different heating rates (k) and then plotting the ln(k) versus 1/Tp, as shown in Figure 4c.The best fit for the results was calculated by the least-squares method.It follows from Figure 4c that all data points lay closely on the same straight line.

Thermal Analysis
High-pressure differential scanning calorimetry (HP-DSC) and normal DSC were performed at different heating rates in order to investigate the apparent activation energy (E a ) of the hydrogenation and dehydrogenation process, respectively.

High-Pressure Differential Scanning Calorimetry
The HP-DSC traces that were conducted under 30 bar of hydrogen gas atmosphere with different heating rates of 10, 11, 12, 14, 15 • C/min for the nanocomposite MgH 2 /5wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) powders that were obtained after 50 h of milling are shown together in Figure 4a,b.Obviously shown in the figure, all of the scans revealed exothermic peaks (low temperature peaks) that were related to the hydrogenation reaction taking place between hydrogen and metallic Mg.The peak temperature of exothermic peaks appeared in the temperature range between 450 • C and 482 o C, depending on the applied heating rates (Figure 4b).The XRD analysis (not shown here) of the powders that were taken after these exothermic peaks confirmed the formation of MgH 2 phase coexisted ZrNi 5 and Nb 2 O 5 phases without evidence of the formation of any other reacted phase(s).
The second peaks appeared at a higher temperature side (above 712 • C) that was related to endothermic events, referred to as the decomposition of MgH 2 phase (Figure 4a).Where the peak intensity of the exothermic and endothermic peaks increased proportionally with increasing the heating rates, the peak temperatures (T p ) were significantly shifted to the higher temperature side upon increasing the heating rates from 10 • C/min to 15 • C/min, as shown in Figure 4a.The improved hydrogenation kinetics of the nanocomposite MgH 2 /5 wt% (2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders was investigated by calculating the apparent E a , while using Arrhenius approach.The value E a of the hydrogenation reaction was determined by measuring the absorption temperature (T p ) peaks with different heating rates (k) and then plotting the ln(k) versus 1/T p , as shown in Figure 4c.The best fit for the results was calculated by the least-squares method.It follows from Figure 4c that all data points lay closely on the same straight line.
The E a of hydrogenation is obtained from the slope of line (−E/R, where R is the gas constant) and found to be 23.61 kJ/mol.This value, which is far below the one (65 kJ/mol) that was calculated for pure MgH 2 powders [16], indicates a significant improvement on the hydrogenation kinetics upon doping MgH 2 with the present catalytic agent of (ZrNi 5 /Nb 2 O 5 ) powders.
Catalysts 2019, 9, 306 6 of 17 The Ea of hydrogenation is obtained from the slope of line (−E/R, where R is the gas constant) and found to be 23.61 kJ/mol.This value, which is far below the one (65 kJ/mol) that was calculated for pure MgH2 powders [16], indicates a significant improvement on the hydrogenation kinetics upon doping MgH2 with the present catalytic agent of (ZrNi5 / Nb2O5) powders.

Normal-Pressure Differential Scanning Calorimetry
The thermal stabilities and the apparent Ea of decomposition for nanocomposite MgH2/y wt% (x ZrNi5 /x Nb2O5) nanocomposite powders that were obtained after different milling time were investigated with the DSC approach under 1 bar of helium gas atmospheric pressure at different heating rates.Figure 5 shows the DSC thermograms that were conducted at heating rates of 5, 10, 20, 30, and 40 °C/min for the nanocomposite MgH2/5 wt% (2.5 ZrNi5 /2.5 Nb2O5) powders that were obtained after 12.5 h (Figure 5a) and 50 h (Figure 5b) of the milling time.At specific heating rates, the peak temperature of decomposition was significantly decreased with increasing the milling time (Figures 5a and 5b).

Normal-Pressure Differential Scanning Calorimetry
The thermal stabilities and the apparent E a of decomposition for nanocomposite MgH 2 /y wt% (x ZrNi 5 /x Nb 2 O 5 ) nanocomposite powders that were obtained after different milling time were investigated with the DSC approach under 1 bar of helium gas atmospheric pressure at different heating rates.Figure 5 shows the DSC thermograms that were conducted at heating rates of 5, 10, 20, 30, and 40 • C/min for the nanocomposite MgH 2 /5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) powders that were obtained after 12.5 h (Figure 5a) and 50 h (Figure 5b) of the milling time.At specific heating rates, the peak temperature of decomposition was significantly decreased with increasing the milling time (Figure 5a,b).
Catalysts 2019, 9, 306 6 of 17 The Ea of hydrogenation is obtained from the slope of line (−E/R, where R is the gas constant) and found to be 23.61 kJ/mol.This value, which is far below the one (65 kJ/mol) that was calculated for pure MgH2 powders [16], indicates a significant improvement on the hydrogenation kinetics upon doping MgH2 with the present catalytic agent of (ZrNi5 / Nb2O5) powders.

Normal-Pressure Differential Scanning Calorimetry
The thermal stabilities and the apparent Ea of decomposition for nanocomposite MgH2/y wt% (x ZrNi5 /x Nb2O5) nanocomposite powders that were obtained after different milling time were investigated with the DSC approach under 1 bar of helium gas atmospheric pressure at different heating rates.Figure 5 shows the DSC thermograms that were conducted at heating rates of 5, 10, 20, 30, and 40 °C/min for the nanocomposite MgH2/5 wt% (2.5 ZrNi5 /2.5 Nb2O5) powders that were obtained after 12.5 h (Figure 5a) and 50 h (Figure 5b) of the milling time.At specific heating rates, the peak temperature of decomposition was significantly decreased with increasing the milling time (Figures 5a and 5b).The tendency of temperature decrease can be realized from Figure 3, which elucidates the effect of milling time and the additives on powder refining.The destabilization of MgH 2 upon doping with the hard spherical powders of (ZrNi 5 /Nb 2 O 5 ) and milling for long time (50 h) led to a drastic decreasing in the particle size of MgH 2 (Figure 3a), since such abrasive catalysts acted as micro-scaled grinding media to generate lattice defects and imperfections.The existence of these defects raised the free energy of the system, making its boundaries accessible.This is indicated by the significant decreasing of Ea with increasing the milling time (Figure 6a).The tendency of temperature decrease can be realized from Figure 3, which elucidates the effect of milling time and the additives on powder refining.The destabilization of MgH2 upon doping with the hard spherical powders of (ZrNi5/Nb2O5) and milling for long time (50 h) led to a drastic decreasing in the particle size of MgH2 (Figure 3a), since such abrasive catalysts acted as micro-scaled grinding media to generate lattice defects and imperfections.The existence of these defects raised the free energy of the system, making its boundaries accessible.This is indicated by the significant decreasing of Ea with increasing the milling time (Figure 6a).
MgH2/5 wt%(2.5 ZrNi5 + 2.5 Nb2O5) powders that were obtained after 12.5 h of milling had Ea of 165.91 kJ/mol (Figure 6a).Further milling time lead to a drastic decrease of Ea value that reached 92.95 kJ/mol after 50 h, as illustrated in Figure 6a. Figure 6b summarizes the concentration effect of (ZrNi5/Nb2O5) on the Ea and decomposition temperature (Temp.decom ) for the nanocomposite powders obtained after 50 h of milling.Both of the parameters tended to monotonically decrease with increasing the milling time.For example, pure MgH2 that was obtained upon RBM for 200 h had Ea and Temp.decom of 133.64 kJ/mol and 721 K, respectivelyy (Figure 6b).
Doping the hydride phase with 5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) led to a significant decrease on Ea and Temp.decom to reach a lower values of 92.98 kJ/mol and 783 K, respectively.These values were continuously decreased with increasing the dose of cataysts to 7 wt% (88.27 kJ/mol and 475 K), 10 wt% (85.07 kJ/mol and 469 K), and 15 wt% (81.69 kJ/mol and 458 K), as eludicated in Figure 6b.A further increase of the catalytic dose had a beneficial effect on decreasing the Ea and Temp.decom to 77.25 kJ.mol -1 /446 K (20 wt%) and 75.66 kJ.mol −1 /441 K (30 wt%), as displayed in Figure 6b.The Ea of decomposition of nanocomposite MgH2/y wt%(x ZrNi5 + x Nb2O5) systems had lower Ea when compared with different MgH2 catalyzed systems, as shown in Table 2.In contrast, the Ea of the present system shows higher values when compared with MgH2 doped with Cr2O3, as presented in Table 2.

Pure MgH2 Nanocrystalline Powders
The hydrogenation/dehydrogenation kinetics of pure MgH2 nanocrystalline powders that were MgH 2 /5 wt%(2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders that were obtained after 12.5 h of milling had Ea of 165.91 kJ/mol (Figure 6a).Further milling time lead to a drastic decrease of Ea value that reached 92.95 kJ/mol after 50 h, as illustrated in Figure 6a. Figure 6b summarizes the concentration effect of (ZrNi 5 /Nb 2 O 5 ) on the E a and decomposition temperature (Temp.decom ) for the nanocomposite powders obtained after 50 h of milling.Both of the parameters tended to monotonically decrease with increasing the milling time.For example, pure MgH 2 that was obtained upon RBM for 200 h had E a and Temp.decom of 133.64 kJ/mol and 721 K, respectivelyy (Figure 6b).
Doping the hydride phase with 5 wt% (2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) led to a significant decrease on E a and Temp.decom to reach a lower values of 92.98 kJ/mol and 783 K, respectively.These values were continuously decreased with increasing the dose of cataysts to 7 wt% (88.27 kJ/mol and 475 K), 10 wt% (85.07 kJ/mol and 469 K), and 15 wt% (81.69 kJ/mol and 458 K), as eludicated in Figure 6b.A further increase of the catalytic dose had a beneficial effect on decreasing the E a and Temp.decom to 77.25 kJ.mol -1 /446 K (20 wt%) and 75.66 kJ.mol −1 /441 K (30 wt%), as displayed in Figure 6b.The Ea of decomposition of nanocomposite MgH 2 /y wt%(x ZrNi 5 + x Nb 2 O 5 ) systems had lower E a when compared with different MgH 2 catalyzed systems, as shown in Table 2.In contrast, the E a of the present system shows higher values when compared with MgH 2 doped with Cr 2 O 3 , as presented in Table 2.

Pure MgH 2 Nanocrystalline Powders
The hydrogenation/dehydrogenation kinetics of pure MgH 2 nanocrystalline powders that were obtained after 200 h of RBM are displayed in Figure 7a,b, respectively.The hydrogenation kinetic was measured under 10 bar of H 2 , where the kinetic of dehydrogenation was investigated under 200 mbar of hydrogen.Both measurements were obtained at applied temperatures of 200 • C and 300 • C.   The fine MgH2 powders (Figure 3b) that were obtained after such long-term of milling (200 h) were capable of absorbing 2.57 wt% H2 in 300 s at rather low temperature (200 °C), as presented in Figure 7a.The absorption kinetic was increased with increasing the temperature to 300 °C, as indicated by the larger molar fraction of H2 (4.72 wt%) that absorbed at the same length of time (300 s).Increasing the applied time 2,000 s) increased the hydrogen amount that was absorbed by the system to reach to 6.1 wt% and 6.69 wt% at 200 °C and 300 o C, respectively (Figure 7a).Further improving on the hydrogen storage capacity (6.28 wt%) was attained with increasing the absorption time to 2,500 s for the sample that was measured at 200 °C, as displayed in Figure 7a.Comparing the hydrogenation kinetics of as-prepared MgH2 nanocrystalline powders of this study with several of the reported values in the litrature (Table 3) led us to realize that long term milling always leads us to deducing the large aggregated powders into finer particles (Figure 3b).
Applying long milling time was essential in increasing the lattice imperfections and mechanical deformation that were intoduced to the milled powders, and hence deduced the kinetics barrier (inset of Figure 7a).It is worth mentioing that longer milling time is responsible for modifying the thermodynamic barrier of the system upon the formation of significant volume fraction of -MgH2 metastable phase (inset of Figure 7a), as suggested by many authors [27][28][29][30][31][32][33][34].From Table 3, it can be summarized that the starting MgH2 powders that were prepared in the present work had better The fine MgH 2 powders (Figure 3b) that were obtained after such long-term of milling (200 h) were capable of absorbing 2.57 wt% H 2 in 300 s at rather low temperature (200 • C), as presented in Figure 7a.The absorption kinetic was increased with increasing the temperature to 300 • C, as indicated by the larger molar fraction of H 2 (4.72 wt%) that absorbed at the same length of time (300 s).Increasing the applied time 2000 s) increased the hydrogen amount that was absorbed by the system to reach to 6.1 wt% and 6.69 wt% at 200 • C and 300 • C, respectively (Figure 7a).Further improving on the hydrogen storage capacity (6.28 wt%) was attained with increasing the absorption time to 2500 s for the sample that was measured at 200 • C, as displayed in Figure 7a.Comparing the hydrogenation kinetics of as-prepared MgH 2 nanocrystalline powders of this study with several of the reported values in the litrature (Table 3) led us to realize that long term milling always leads us to deducing the large aggregated powders into finer particles (Figure 3b).
Applying long milling time was essential in increasing the lattice imperfections and mechanical deformation that were intoduced to the milled powders, and hence deduced the kinetics barrier (inset of Figure 7a).It is worth mentioing that longer milling time is responsible for modifying the thermodynamic barrier of the system upon the formation of significant volume fraction of γ-MgH 2 metastable phase (inset of Figure 7a), as suggested by many authors [27][28][29][30][31][32][33][34].From Table 3, it can be summarized that the starting MgH 2 powders that were prepared in the present work had better hydrogenation kinetics and higher storage capacity when compared to the other reported ones that were prepared with shorter ball milling time.Ball milling under Ar gas for 20 h 6.0 Absorption: 300 • C/120 min [28] Ultrafine Mg nanoparticles Hydrogen-plasms-metal reaction under a mixture of 70% Ar + 30% H 2 at 25 V/300A.

7.5
Absorption: 300 • C/40 bar/30 min [35] In contrast to the rather fast hydrogenation kinetic, the MgH 2 powders obtained after 200 h of RBM revealed very slow dehydrogenation kinetic, as shown in Figure 7b.At 200 • C, the powders released a very small amount of its hydrogen storage capacity (−0.18 wt%), even after 12,000 s (200 min), as displayed in Figure 7b.Increasing the applied temperature led to enhancing the desorption kinetics, as indicated by the rather short time that is required to release −6.36 wt% H 2 within 4000 s (66.67 min).At this temperature, the hydrogen released tended to be saturated at −6.52 wt%, as indexed in Figure 7b.The catalyzation step was necessary to improve both the hydrogenation and dehydrogenation kinetics of as-prepared MgH 2 nanocrystalline powders that were obtained after 200 h of RBM time.Binary (x-ZrNi 5 /x-Nb 2 O 5 ) catalytic agent was selected with different concentrations to study their synergetic effect on enhancing the storage properties of MgH 2 powders.For all compositions, the MgH 2 powders were doped with the proper dose of the catalytic binary system and experienced 50 h of high energy ball milling to ensure the formation of homogeneous powders that were closed to the starting nominal composition.In the present study, six batches of nanocomposite powders were prepared under the same conditions.
The samples can be classified into four batches, composited of low, medium, and high concentrations of catalysts.Table 4 lists the code number of each sample and the corresponding catalytic concentrations.The kinetics of hydrogenation and dehydrogenation for four samples corresponding to Batch #I (Table 4) are displayed in Figure 8a-c, respectively.In general, the hydrogenation kinetics that were measured under 10 bar at 175 • C of all the catalyzed samples with different concentrations were very fast absorption kinetics (Figure 8a) when compared with the undoped MgH 2 powders (Figure 7a and Table 3).Kinetically, they behave almost same, however they significantly differed in their hydrogen storage capacity, as shown in Figure 8a,b.The hydrogen storage degradations shown in samples 2, 3, and 4 were attributed to the large molar fraction of heavy-molecular mass of the catalytic agent that was used for doping MgH 2 powders (Table 4).Based on these results, it can be concluded that MgH 2 /5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) nanocomposite system combined excellent hydrogen storage properties, as indicated by the fast absorption of 5 wt% within 50 s, as presented in Figure 8a,b.This nanocomposite system saturated at 5.6 wt% H 2 , after 300 s (5 min), is shown in Figure 8a,b.

Hydrogenation/dehydrogenation cycle-life-time
The cyclability of MgH2 that was doped with 5 wt% (2.5 ZrNi5/2.5Nb2O5)powders and milled for 50 h was examined by cycle-life-test for 100 h (Figure 10a).This test was conducted under 10 bar H2 (hydrogenation)/200 mbar (dehydrogenation) at 200 °C.The sample was first activated under 40 bar of hydrogen at 370 °C for 54 h.This activation process was necessary to breakdown the thin MgO In contrast to samples #1 and #2, the dehydrogenation kinetics that are related to samples #3 and #4 showed unexpected slow kinetics behaviors with a serious decrease of their storage capacity, as can be seen in Figure 8c.Based on several morphological examinations of these two samples, we believe that using large molar fractions (10 wt% and 15 wt%) of catalysts led to sealing/plugging the "gateway pores" existing on outer surface of MgH 2 powders and hence prevented a fast release of hydrogen.This assumption was based on FE-SEM observations (Figure 9c) of sample #4-powders that were obtained after 300 s of desorption time (Figure 8b).The overall Mg/MgH 2 powder matrix hosted a large volume fraction of ZrNi 5 /Nb 2 O 5 nanoparticles that tended to adhere on the outermost shell of the matrix powders in heterogeneous distribution fashion (Figure 9a).Most of the pores that formed on the powder surface during the hydrogenation process were "sealed" by these fine ZrNi 5 /Nb 2 O 5 nanoparticles to create a hydrogen releasing barrier.Accordingly, the gas was trapped inside the core of Mg powders.More study is required to investigate the drawback of using "overdose" catalytic agents on the kinetics behavior and storage capacity of metal hydride powders.Figure 9b

Hydrogenation/dehydrogenation cycle-life-time
The cyclability of MgH2 that was doped with 5 wt% (2.5 ZrNi5/2.5Nb2O5)powders and milled for 50 h was examined by cycle-life-test for 100 h (Figure 10a).This test was conducted under 10 bar H2 (hydrogenation)/200 mbar (dehydrogenation) at 200 °C.The sample was first activated under 40 bar of hydrogen at 370 °C for 54 h.This activation process was necessary to breakdown the thin MgO

Hydrogenation/Dehydrogenation Cycle-Life-Time
The cyclability of MgH 2 that was doped with 5 wt% (2.5 ZrNi 5 /2.5Nb 2 O 5 ) powders and milled for 50 h was examined by cycle-life-test for 100 h (Figure 10a).This test was conducted under 10 bar H 2 (hydrogenation)/200 mbar (dehydrogenation) at 200 • C. The sample was first activated under 40 bar of hydrogen at 370 • C for 54 h.This activation process was necessary to breakdown the thin MgO layer that coated the powders to increase the mass molar fraction of fresh Mg metal surfaces against the oxide phase.Accordingly, the hydrogen storage capacity was significantly increased to ~6.5 wt%.In addition, this activation process led to creating large volume fractions of pores that were desired for facilitating fast charging/discharging processes.
This new system possessed good performance, as indicated by the large number of achieved hydrogenation/dehydrogenations cycles, reaching to 228, as displayed in Figure 10a.The last 9.5 cycles achieved between a time interval of 95 h to 100 h, as displayed in Figure 10b.Obviously, the sample maintained its fast hydrogen uptake/released kinetics with almost constant storage capacity value in the range between 6.48 wt% to 6.49 wt%, as displayed in Figure 10b.
layer that coated the powders to increase the mass molar fraction of fresh Mg metal surfaces against the oxide phase.Accordingly, the hydrogen storage capacity was significantly increased to ~ 6.5 wt%.In addition, this activation process led to creating large volume fractions of pores that were desired for facilitating fast charging/discharging processes.This new system possessed good performance, as indicated by the large number of achieved hydrogenation/dehydrogenations cycles, reaching to 228, as displayed in Figure 10a.The last 9.5 cycles achieved between a time interval of 95 h to 100 h, as displayed in Figure 10b.Obviously, the sample maintained its fast hydrogen uptake/released kinetics with almost constant storage capacity value in the range between 6.48 wt% to 6.49 wt%, as displayed in Figure 10b.
A low magnification secondary SEM micrograph of the powders that were obtained after completion of the last desorption cycle is presented in Figure 11.The sample consisted of numerous ultrafine clusters of Mg and (ZrNi5/Nb2O5) that were agglomerated together due to Van der Waals attraction to form large aggregates with a porous structure, as shown in Figure 11.
In order to realize the possibility of the formation of an intermediate phase during the cycle-lifetest, the XRD of the powder that was taken after compilation of the last dehydrogenation cycle was conducted (Figure 12a).The powders contained large Mg crystals, as indicated by the sharp Bragg peaks shown in Figure 11a.Both of ZrNi5 and Nb2O5 phases revealed their corresponding Bragg lines, as presented in Figure 12a.Neither the MgH2 phase nor reacted product, such as ZrH2, NbH, ZrO2, etc. could be detected.The FE-HRTEM micrograph of the powders taken after the cycle-life-test is shown in Figure 11b.The image displays an aggregated Mg powder, indexed by a Miller fringe image of (101).This particle was adhered by to fine crystals corresponding to ZrNi5 (311) and Nb2O5 (002), as elucidated in Figure 12b.A low magnification secondary SEM micrograph of the powders that were obtained after completion of the last desorption cycle is presented in Figure 11.The sample consisted of numerous ultrafine clusters of Mg and (ZrNi 5 /Nb 2 O 5 ) that were agglomerated together due to Van der Waals attraction to form large aggregates with a porous structure, as shown in Figure 11.In order to realize the possibility of the formation of an intermediate phase during the cycle-life-test, the XRD of the powder that was taken after compilation of the last dehydrogenation cycle was conducted (Figure 12a).

MgH2 Nanocrystalline Powders
In the present study, Mg powders (99.8 wt%, 80 m in diameter) supplied by Sigma-Aldrich, St. Louis, MI, USA.were used as starting reactant materials with hydrogen gas (99.99 wt%) provided by local company in Kuwait.Seven gram of the powders were sealed into a tool steel vial with 60 tool steel balls of 12 mm in diameter inside the He glove box.The ball-to-powder weight ratio was 50:1.The vial had been charged with H2 gas under 55 bar before it mountained on planetary type ball mill.The reactive ball milling (RBM) process was started for 200 h to obtain a fully reacted MgH2 nanocrystalline powders.

 ZrNi5 Intermetallic Compound
Pure bulk Zr pellets (99 wt%) and Ni foil (99.99 wt%) supplied by Sigma-Aldrich, St. Louis, Missouri, USA were snipped into small pieces and balanced to give the nominal atomic composition

MgH 2 Nanocrystalline Powders
In the present study, Mg powders (99.8 wt%, 80 µm in diameter) supplied by Sigma-Aldrich, St. Louis, MI, USA.were used as starting reactant materials with hydrogen gas (99.99 wt%) provided by local company in Kuwait.Seven gram of the powders were sealed into a tool steel vial with 60 tool steel balls of 12 mm in diameter inside the He glove box.The ball-to-powder weight ratio was 50:1.The vial had been charged with H 2 gas under 55 bar before it mountained on planetary type ball mill.The reactive ball milling (RBM) process was started for 200 h to obtain a fully reacted MgH 2 nanocrystalline powders.

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ZrNi 5 Intermetallic Compound Pure bulk Zr pellets (99 wt%) and Ni foil (99.99 wt%) supplied by Sigma-Aldrich, St. Louis, Missouri, USA were snipped into small pieces and balanced to give the nominal atomic composition of ZrNi 5 .The starting materials were etched with 70% H 2 O + 30% HCl, rinsed by ethanol, and then dried at 250 • C for 3 h.Arc melter equipment was used in Ti-gettered He atmosphere to fabricate the desired ZrNi 5 alloy, while using water-cooled Cu crucible.The melting process was started by melting Ti and then melting the metallic bulk mixture of Zr and Ni.The metallic button that was obtained after melting was turned over and re-melted six times to ensure the homogeneity.The master alloy was then removed out from the arc melter, washed by acetone and ethanol, and then crushed down into small shots (~18 mm), while using 100 ton cold press.The shots were then crushed down into finer particles (less than 100 µm in diameter) while using a disk mill.

Figure 1 .
Figure 1.X-ray diffraction (XRD) patterns of (a) MgH2 powders obtained after 200 h of reactive ball milling, (b) ZrNi5 powders obtained after 10 h of high energy ball milling and (c) nanocomposite MgH2 doped with 5 wt% (2.5 ZrNi5/2.5 Nb2O5) powders after milling for 50 h.The powders were contaminated with 1.36 wt% Fe, which came from using stainless steel milling tools, as indicated by Bragg peak of Fe(110) shown in (c).The FE-HRTEM micrograph of nanocomposite powders obtained after 50 h of milling is displayed in (d).

Figure 2 .
Figure 2. Dark field image (DFI) of nanocomposite MgH2/5 wt% (2.5 ZrNi5/2.5 Nb2O5) powders obtained after milling for 50 h.The results of EDS analysis for the zones indexed in the figure are presented in Table1.

Figure 1 .
Figure 1.X-ray diffraction (XRD) patterns of (a) MgH 2 powders obtained after 200 h of reactive ball milling, (b) ZrNi 5 powders obtained after 10 h of high energy ball milling and (c) nanocomposite MgH 2 doped with 5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) powders after milling for 50 h.The powders were contaminated with 1.36 wt% Fe, which came from using stainless steel milling tools, as indicated by Bragg peak of Fe(110) shown in (c).The FE-HRTEM micrograph of nanocomposite powders obtained after 50 h of milling is displayed in (d).

Figure 1 .
Figure 1.X-ray diffraction (XRD) patterns of (a) MgH2 powders obtained after 200 h of reactive ball milling, (b) ZrNi5 powders obtained after 10 h of high energy ball milling and (c) nanocomposite MgH2 doped with 5 wt% (2.5 ZrNi5/2.5 Nb2O5) powders after milling for 50 h.The powders were contaminated with 1.36 wt% Fe, which came from using stainless steel milling tools, as indicated by Bragg peak of Fe(110) shown in (c).The FE-HRTEM micrograph of nanocomposite powders obtained after 50 h of milling is displayed in (d).

Figure 2 .
Figure 2. Dark field image (DFI) of nanocomposite MgH2/5 wt% (2.5 ZrNi5/2.5 Nb2O5) powders obtained after milling for 50 h.The results of EDS analysis for the zones indexed in the figure are presented in Table1.

Figure 2 .
Figure 2. Dark field image (DFI) of nanocomposite MgH 2 /5 wt% (2.5 ZrNi 5 /2.5 Nb 2 O 5 ) powders obtained after milling for 50 h.The results of EDS analysis for the zones indexed in the figure are presented in Table1.

Figure 4 .
Figure 4. Hydrogenation/dehydrogenation behaviors and thermal stability of as-synthesized nanocomposite MgH2/5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) powders.The high-pressure differential scanning calorimetry (HP-DSC) traces achieved under 30 bar hydrogen pressure for nanocomposite powders obtained after 50 h of milling time are shown in (a) at different heating rates.The hydrogenation peaks (low-temperature peaks) are presented in (b) with different temperature range.The hydrogenation peak temperature obtained at different heating rates were used to calculate the activation energy of hydrogenation, while using Arrhenius plot (c).

Figure 5 .
Figure 5. Decomposition behavior of as-synthesized nanocomposite MgH2/5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) powders.Normal pressure-DSC curves achieved under 1 bar of He gas at different heating rates are displayed for the powders obtained after (a) 12.5 h and (b) 50 of milling time. .

Figure 4 .
Figure 4. Hydrogenation/dehydrogenation behaviors and thermal stability of as-synthesized nanocomposite MgH 2 /5 wt% (2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders.The high-pressure differential scanning calorimetry (HP-DSC) traces achieved under 30 bar hydrogen pressure for nanocomposite powders obtained after 50 h of milling time are shown in (a) at different heating rates.The hydrogenation peaks (low-temperature peaks) are presented in (b) with different temperature range.The hydrogenation peak temperature obtained at different heating rates were used to calculate the activation energy of hydrogenation, while using Arrhenius plot (c).

Figure 4 .
Figure 4. Hydrogenation/dehydrogenation behaviors and thermal stability of as-synthesized nanocomposite MgH2/5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) powders.The high-pressure differential scanning calorimetry (HP-DSC) traces achieved under 30 bar hydrogen pressure for nanocomposite powders obtained after 50 h of milling time are shown in (a) at different heating rates.The hydrogenation peaks (low-temperature peaks) are presented in (b) with different temperature range.The hydrogenation peak temperature obtained at different heating rates were used to calculate the activation energy of hydrogenation, while using Arrhenius plot (c).

Figure 5 .
Figure 5. Decomposition behavior of as-synthesized nanocomposite MgH2/5 wt% (2.5 ZrNi5 + 2.5 Nb2O5) powders.Normal pressure-DSC curves achieved under 1 bar of He gas at different heating rates are displayed for the powders obtained after (a) 12.5 h and (b) 50 of milling time. .

Figure 5 .
Figure 5. Decomposition behavior of as-synthesized nanocomposite MgH 2 /5 wt% (2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders.Normal pressure-DSC curves achieved under 1 bar of He gas at different heating rates are displayed for the powders obtained after (a) 12.5 h and (b) 50 of milling time.

Figure 6 .
Figure 6.(a) Influence of milling time on the apparent activation energy of decomposition for nanocomposite MgH 2 /5 wt%(2.5 ZrNi 5 + 2.5 Nb 2 O 5 ) powders and (b) concentration effect of (x ZrNi 5 /x Nb 2 O 5 ) catalytic agent on apparent activation energy of decomposition (red sybmoles) and decomposition temperatures (blue symboles) for nanocomposite powders obtained after 50 h of milling.

Figure 7 .
Figure 7. Kinetics of (a) hydrogenation and (b) dehydrogenation for pure MgH2 nanocrystalline powders obtained after 200 h of RBM time.The measurements were conducted under hydrogen pressure of 10bar and 200 mbar, respectively.

Figure 7 .
Figure 7. Kinetics of (a) hydrogenation and (b) dehydrogenation for pure MgH 2 nanocrystalline powders obtained after 200 h of RBM time.The measurements were conducted under hydrogen pressure of 10bar and 200 mbar, respectively.

Figure 8 .
Figure 8. Kinetics of (a,b) hydrogenation and (c) dehydrogenation for nanocomposite MgH 2 doped with low, medium, and high concentrations of binary ZrNi 5 /Nb 2 O 5 catalysts after milling for 50 h.The dehydrogenation kinetics of samples #1, #2, #3, and #4 are displayed in Figure8c.All of the measurements were conducted at 200 • C under 200 mbar of H 2 pressure.The samples showed significant differences in their basic hydrogen storage properties, depending on the concentrations of catalytic agent that was added to each of them.For example, samples #1 and #2 showed excellent kinetics, implied by desorbing −2.26 and −2.15 wt% H 2 within very short time (50 s), as shown in Figure8c.The hydrogen that was released for these two samples was saturated after 200 s at −5.49 and −5.27 wt% (Figure8c).The low-magnification SEM-secondary electron detector (BED) micrograph that was taken for sample 1 after compilation of dehydrogenation test is presented in Figure9a.Obviously presented, the fine catalytic (ZrNi 5 /Nb 2 O 5 ) nanoparticles that are indicated by the yellow arrows shown in Figure9aare homogeneously adhered on each individual Mg particle to form uniform nanocomposite powders.This homogeneous distribution was confirmed by scanning transmission electron microscope (STEM)/dark field image (DFI) beyond the nanoscale level.ZrNi 5 /Nb 2 O 5 nanoparticles occupied their catalytic sites on the surface of Mg powder matrix in a homogeneous fashion without agglomeration, as indexed in Figure9b.Such homogeneous distribution of ZrNi 5 /Nb 2 O 5 nanoparticles among the powder surfaces of Mg/MgH 2 that are shown in the present work is believed to be the major factor leading to superior improvement on hydrogen uptake/release kinetics.In contrast to samples #1 and #2, the dehydrogenation kinetics that are related to samples #3 and #4 showed unexpected slow kinetics behaviors with a serious decrease of their storage capacity, as can be seen in Figure8c.Based on several morphological examinations of these two samples, we believe that using large molar fractions (10 wt% and 15 wt%) of catalysts led to sealing/plugging the "gateway pores" existing on outer surface of MgH 2 powders and hence prevented a fast release of hydrogen.This assumption was based on FE-SEM observations (Figure9c) of sample #4-powders that were obtained after 300 s of desorption time (Figure8b).The overall Mg/MgH 2 powder matrix hosted a large volume fraction of ZrNi 5 /Nb 2 O 5 nanoparticles that tended to adhere on the outermost shell of the matrix powders in heterogeneous distribution fashion (Figure9a).Most of the pores that formed on the powder surface during the hydrogenation process were "sealed" by these fine ZrNi 5 /Nb 2 O 5 nanoparticles to create a hydrogen releasing barrier.Accordingly, the gas was trapped inside the core of Mg powders.More study is required to investigate the drawback of using "overdose" catalytic agents on the kinetics behavior and storage capacity of metal hydride powders.Figure9b presents presents the STEM/DFI of this sample.In the figure, a huge amount of nano-spherical ZrNi 5 /Nb 2 O 5 particles were adhered on Mg/MgH 2 powders, handicapping hydrogen release.

Figure 8 .
Figure 8. Kinetics of (a, b) hydrogenation and (c) dehydrogenation for nanocomposite MgH2 doped with low, medium, and high concentrations of binary ZrNi5/Nb2O5 catalysts after milling for 50 h.
The powders contained large Mg crystals, as indicated by the sharp Bragg peaks shown in Figure 11a.Both of ZrNi 5 and Nb 2 O 5 phases revealed their corresponding Bragg lines, as presented in Figure 12a.Neither the MgH 2 phase nor reacted product, such as ZrH 2 , NbH, ZrO 2 , etc. could be detected.The FE-HRTEM micrograph of the powders taken after the cycle-life-test is shown in Figure 11b.The image displays an aggregated Mg powder, indexed by a Miller fringe image of (101).This particle was adhered by to fine crystals corresponding to ZrNi 5 (311) and Nb 2 O 5 (002), as elucidated in Figure 12b.

Table 1 .
Elemental Analysis Corresponding to the Indexed Zones Shown in Figure2.

Table 2 .
Apparent Activation energy (Ea) of Decomposition for MgH 2 Doped with Selected Catalysts.

Table 2 .
Apparent Activation energy (Ea) of Decomposition for MgH2 Doped with Selected Catalysts.

Table 3 .
Hydrogen storage properties for different types of MgH 2 powder.

Table 4 .
Batches of as-prepared nanocomposite powders with different catalytic concentrations.