Contamination Effects on Improving the Hydrogenation/Dehydrogenation Kinetics of Binary Magnesium Hydride/Titanium Carbide Systems Prepared by Reactive Ball Milling

Ultrafine MgH2 nanocrystalline powders were prepared by reactive ball milling of elemental Mg powders after 200 h of high-energy ball milling under a hydrogen gas pressure of 50 bar. The as-prepared metal hydride powders were contaminated with 2.2 wt. % of FeCr-stainless steel that was introduced to the powders upon using stainless steel milling tools made of the same alloy. The as-synthesized MgH2 was doped with previously prepared TiC nanopowders, which were contaminated with 2.4 wt. % FeCr (materials of the milling media), and then ball milled under hydrogen gas atmosphere for 50 h. The results related to the morphological examinations of the fabricated nanocomposite powders beyond the micro-and nano-levels showed excellent distributions of 5.2 wt. % TiC/4.6 wt. % FeCr dispersoids embedded into the fine host matrix of MgH2 powders. The as-fabricated nanocomposite MgH2/5.2 wt. % TiC/4.6 wt. % FeCr powders possessed superior hydrogenation/dehydrogenation characteristics, suggested by the low value of the activation energy (97.74 kJ/mol), and the short time required for achieving a complete absorption (6.6 min) and desorption (8.4 min) of 5.51 wt. % H2 at a moderate temperature of 275 °C under a hydrogen gas pressure ranging from 100 mbar to 8 bar. van’t Hoff approach was used to calculate the enthalpy (∆H) and entropy (∆S) of hydrogenation for MgH2, which was found to be −72.74 kJ/mol and 112.79 J/mol H2/K, respectively. Moreover, van’t Hoff method was employed to calculate the ΔH and ΔS of dehydrogenation, which was found to be 76.76 kJ/mol and 119.15 J/mol H2/K, respectively. This new nanocomposite system possessed excellent absorption/desorption cyclability of 696 complete cycles, achieved in a cyclic-life-time of 682 h.


Introduction
Hydrogen storage is one of the key enabling technologies for realization of hydrogen energy economy [1]. Hydrogen storage materials, taking metal hydrides as a typical example, are commercially prepared by solvent-based synthesis methods or by direct gas-solid hydrogenation reactions. In contrast to the traditional gas-solid hydrogenation process, which is achieved at temperatures far above room temperature, an attractive method-so-called reactive ball milling (RBM) [2,3]-was developed in the 1990s to conduct the exothermic reactions between the gas-and metallic solid phases at almost room temperature. This relatively new process has been considered as a powerful tool for fabrication of different nanocrystalline metallic nitrides and hydrides [4]. In their room-temperature process, the starting metallic powders are subjected to dramatic shear and impact forces generated by the milling media (balls). The powders are, therefore, disintegrated into smaller particles with large surface area, and very clean or fresh oxygen-free active surfaces of the powders are created. The reactive milling atmosphere (nitrogen or hydrogen gases) was gettered and absorbed completely by the first atomically clean surfaces of the metallic ball-milled powders to react in a same manner as a typical gas-solid reaction [5]. Since then, the RBM process has become a common technique successfully used for preparing nanocrystalline metal hydrides, including magnesium hydride (MgH 2 ) and their composite powders [1,6].
High capacity hydrogen storage materials such as MgH 2 have been receiving much attention as promising solid-state hydrogen storage systems due to their high hydrogen storage capacity (7.60 wt. %), reversibility, cost effectiveness, availability and cyclability [7][8][9]. The international interest in the development of hydrogen based technologies, particularly the area of fuel cell electric vehicles, has greatly increased in recent years [9].
Unfortunately, and in contrast to the obvious advantages seen in MgH 2 binary hydrogen storage systems, the high thermal stability and the difficulty to decompose this hydride system into metal and hydrogen gas, plus the poor hydrogenation and consequence dehydrogenation kinetics, lead to restricting utilization of such a light-weight system in real automobile applications [7,9,10].
Even though and in spite of the serious drawbacks found in MgH 2 , the worldwide interest in such an attractive binary metal hydride has been increased, especially after improving its hydrogen absorption and desorption kinetics by applying a longer ball milling time that led to destabilizing the β-MgH 2 phase and increasing the volume fractions of the metastable γ-MgH 2 phase [11]. Long mechanical ball milling time always is one key approach for releasing the crystalline stored energy, leading to refining the MgH 2 grains along their grain boundaries resulting in a fine-grained structure. Such fine grains with their short-distance grain boundaries always facilitate a short diffusion path, allowing fast diffusion of the hydrogen atoms into the Mg lattice [12].
Moreover, ball milling the MgH 2 with pure metallic catalysts (e.g., Ti, Fe, Ni, Nb, V) [13], intermetallic compounds (e.g., Zr 100´x Ni x , and Ti-based alloys) [14][15][16], metal carbides such as TiC [17], metal oxides such as Nb 2 O 5 [18], metal chlorides such as MgCl 2 [19], rare earth chlorides such as LaCl 3 [20], and nanocomposite Ni/Nb 2 O 5 powders [21] led to remarkable improvement in the hydrogen absorption/desorption kinetics and lowering the thermal stability of MgH 2 . It has been shown by Ismail [20] that the improved hydrogen storage properties of MgH 2 doped with LaCl 3 were due to the catalytic effects of the La-Mg alloy and MgCl 2 . Such ultrafine micro-scaled/nano-scaled powders serving as catalysts have shown the possibility of improving the hydrogenation/dehydrogenation properties of MgH 2 to open up a new horizon for its real application.
In the present study, we have investigated the effect of FeCr contamination introduced to the MgH 2 powders upon ball milling in the long term on improving the hydrogenation/dehydrogenation properties of the metal hydride phase. Moreover, the effect of doping the as-synthesized MgH 2 nanocrystalline powders with TiC nanopowders on the hydrogen storage capacity and cyclability of MgH 2 was studied in terms of morphology and kinetics.

Experimental Procedure
Pure Mg metal powders (~80 µm, 99.8% purity provided by Alfa Aesar-Ward Hill, MA, USA), synthesized TiC nanopowders obtained upon high-energy ball milling of Ti and graphite powder (~100 nm, 2.4 wt. % FeCr), and hydrogen gas (99.999%) were used as starting materials. A certain amount of the Mg powders (5 g) was balanced inside a helium (He) gas atmosphere (99.99%)-glove box (UNILAB Pro Glove Box Workstation, mBRAUN, Garching, Germany). The powders were then sealed together with 50 FeCr-stainless steel balls into a FeCr steel vial (220 mL in volume), using a gas-temperature-monitoring system (GST; supplied by evico magnetic, Dresden, Germany). The ball-to-powder weight ratio was 40:1. The vial was then evacuated to the level of 10´3 bar before introducing H 2 gas to fill the vial with a pressure of 50 bar. The milling process was carried out at room temperature using high energy ball mill (Planetary Mono Mill PULVERISETTE 6, Fritsch, Idar-Oberstein, Germany). After 200 h of RBM, the powders were discharged from the vial inside the glove box and sealed into two Pyrex vials. The as-synthesized MgH 2 powders were then mixed in the glove box with the desired weight percentage (5%) of TiC, using an agate mortar and pestle. Five gram of the mixed powders were charged together with 50 hardened steel balls into the hardened steel vial and sealed under He gas atmosphere [21]. The vial was then filled with 50 bar of hydrogen gas atmosphere and mounted on the high-energy ball mill. The milling process was interrupted after selected time (25, and 50 h) and the powders obtained after an individual milling time were completely discharged into 8 Pyrex vials for different analyses. The average crystal structure of all samples was investigated by X-ray diffraction (XRD) with CuKα radiation, using 9 kW Intelligent X-ray diffraction system, provided by SmartLab-Rigaku, Tokyo, Japan. The local structure of the synthesized material powders at the nanoscale was studied by 200 kV-field emission high resolution transmission electron microscopy/scanning transmission electron microscopy (HRTEM/STEM) supplied by JEOL-2100F, Tokyo, Japan, equipped with Energy-dispersive X-ray spectroscopy (EDS) supplied by Oxford Instruments, Oxfordshire, UK. The morphological properties of the powders after selected ball milling times were determined by 15 kV-field emission scanning electron microscope (FE-SEM, JSM-7800F, Tokyo, Japan) equipped with EDS supplied by Oxford Instruments, UK. The concentrations of elemental Mg, Ti, Fe, and Cr in the as-ball milled powders were determined by inductively coupled plasma optical (ICP) emission spectrometry. Shimadzu Thermal Analysis System/TA-60WS, using differential scanning calorimeter (DSC), was employed to investigate the thermal stability indexed by the decomposition temperatures of MgH 2 and to estimate the activation energy, using the Arrhenius approach with different heating rates of 7, 8, 9, and 10˝C/min. The hydrogenation properties, including absorption/desorption kinetics and cycle-life-time, were investigated via Sievert's method, using PCTPro-2000, provided by Setaram Instrumentation, Caluire, France.

Results
The XRD pattern of the end-product of MgH 2 /5.2TiC/4.6FeCr nanocomposite powders obtained after 50 h of ball milling is shown in Figure 1. The powders composed of β-MgH 2 (PDF file #: 03-065-3365) and γ-MgH 2 (PDF file #: 00-035-1184) phases mixed with fcc-TiC phase (PDF file #: 00-031-1400). This end-product was significantly contaminated (~2.3 wt. %) with bcc-FeCr alloy (PDF file #: 00-054-0331) introduced to the powders upon using FeCr stainless steel as milling tool. A significant amount of bcc-FeCr was obtained as shown in Figure 1. Moreover, handling the powders outside of the glove box led to a surface oxidation of the powders and the formation of magnesium oxide layers, as indicated by the Bragg-peaks belonging to fcc-MgO phase (PDF file #: 00-004-0829) shown in Figure 1. Obviously, the as-prepared nanocomposite powders revealed broad Bragg peaks, suggesting the formation of nanocrystalline grains.

Results
The XRD pattern of the end-product of MgH2/5.2TiC/4.6FeCr nanocomposite powders obtained after 50 h of ball milling is shown in Figure 1. The powders composed of β-MgH2 (PDF file #: 03-065-3365) and γ-MgH2 (PDF file #: 00-035-1184) phases mixed with fcc-TiC phase (PDF file#:00-031-1400). This end-product was significantly contaminated (~2.3 wt. %) with bcc-FeCr alloy (PDF file#: 00-054-0331) introduced to the powders upon using FeCr stainless steel as milling tool. A significant amount of bcc-FeCr was obtained as shown in Figure 1. Moreover, handling the powders outside of the glove box led to a surface oxidation of the powders and the formation of magnesium oxide layers, as indicated by the Bragg-peaks belonging to fcc-MgO phase (PDF file#: 00-004-0829) shown in Figure 1. Obviously, the as-prepared nanocomposite powders revealed broad Bragg peaks, suggesting the formation of nanocrystalline grains.  The distribution of TiC/FeCr into the MgH2 matrix was examined by intensive EDS local analysis performed at selected points (Roman Numerals symbols shown in Figure 2a) and listed in Table 1.
The results show that the concentration of TiC/FeCr is remarkably varied from one region to another beyond the nano-level, as shown in Table 1. It is worth mentioning that significant FeCr contamination to another beyond the nano-level, as shown in Table 1. It is worth mentioning that significant FeCr contamination was evident within those TiC-rich areas (II, IV, VII, X), as shown in Table 1. This is attributed to the existence of high FeCr contamination content in the as-prepared nanocrystalline TiC powders. However, a considerable amount of FeCr contamination content existed in the as-prepared MgH 2 nanocrystalline powders, as can be seen in the rich Mg-area presented in Table 1 (I, V, VI, and VIII). In order to get more information about the TiC/FeCr distribution embedded into the host MgH 2 matrix, STEM-EDS X-ray elemental mapping was performed.  (Figure 3a,e,f) was homogeneously distributed onto the surface of MgH 2 powders. The individual TiC particle size was in the range of 10-20 nm in diameter, as shown in Figure 3e). However, some agglomerated TiC particles with apparent sizes ranging between 80 nm and 220 nm were bonded onto the MgH 2 surfaces, as shown in Figure 3e. The FeCr contamination introduced to the powders upon using steel balls was homogeneously distributed in the MgH 2 matrix, as elucidated in Figure 3g,h. We should emphasize that the concentration of FeCr contamination was higher in the regions containing TiC-particles when compared with the MgH 2 -matrix region, as shown in Figure 3e-h.
The thermal stability of nanocomposite MgH 2 /5.2TiC/4.6FeCr powders obtained after 50 h of the ball milling was investigated by DSC analysis conducted with heating rates (k) of 7, 8, 9, and 10˝C/min and presented in Figure 4. All the scans revealed single endothermic events related to the decomposition of MgH 2 phase. While the peak height increased proportionally with increasing heating rates, the peak temperatures (T p ) were significantly shifted to the higher temperature side upon increasing the heating rates from 7˝C/min to 10˝C/min, as shown in Figure 4. The peak decomposition temperature performed at a heating rate of 10˝C/min was 658 K (385˝C). When comparing this value with that (441˝C) obtained for nanocrystalline MgH 2 powders [12], one can say that doping MgH 2 with 5.2 wt. % TiC/4.6 wt. % FeCr powders led to destabilizing the metal hydride phase and decreasing the decomposition temperature by 56˝C.  The improved dehydrogenation kinetics in a helium gas atmosphere was investigated by calculating the activation energy (Ea) of the decomposition reaction. In the present work, the activation energy for dehydrogenation of MgH2 doped with TiC/FeCr was calculated according to the Arrhenius Equation: where k is a temperature-dependent reaction rate constant, R is the gas constant, and T is the absolute temperature. The value Ea of the reaction was determined by measuring the decomposition the Tp corresponded to the different heating rates (k) and then plotting ln(k) versus 1/Tp, as shown in Figure 5. A best fit for the results was calculated by the least-square method. It follows from Figure 5 that all data points lie closely on the same straight line. The Ea of 97.74 kJ/mol was obtained from the slope of line (−E/R). This value, which is far below than that one (146.53 kJ/mol) calculated for pure MgH2  The improved dehydrogenation kinetics in a helium gas atmosphere was investigated by calculating the activation energy (Ea) of the decomposition reaction. In the present work, the activation energy for dehydrogenation of MgH2 doped with TiC/FeCr was calculated according to the Arrhenius Equation: where k is a temperature-dependent reaction rate constant, R is the gas constant, and T is the absolute temperature. The value Ea of the reaction was determined by measuring the decomposition the Tp corresponded to the different heating rates (k) and then plotting ln(k) versus 1/Tp, as shown in Figure 5. A best fit for the results was calculated by the least-square method. It follows from Figure 5 that all data points lie closely on the same straight line. The Ea of 97.74 kJ/mol was obtained from the slope of line (−E/R). This value, which is far below than that one (146.53 kJ/mol) calculated for pure MgH2 powders [12], indicating a significant improvement of the dehydrogenation kinetics of the MgH2 upon The improved dehydrogenation kinetics in a helium gas atmosphere was investigated by calculating the activation energy (E a ) of the decomposition reaction. In the present work, the activation energy for dehydrogenation of MgH 2 doped with TiC/FeCr was calculated according to the Arrhenius Equation: E a "´RT lnpkq where k is a temperature-dependent reaction rate constant, R is the gas constant, and T is the absolute temperature. The value E a of the reaction was determined by measuring the decomposition the T p corresponded to the different heating rates (k) and then plotting ln(k) versus 1/T p , as shown in Figure 5. A best fit for the results was calculated by the least-square method. It follows from Figure 5 that all data points lie closely on the same straight line. The E a of 97.74 kJ/mol was obtained from the slope of line (´E/R). This value, which is far below than that one (146.53 kJ/mol) calculated for pure MgH 2 powders [12], indicating a significant improvement of the dehydrogenation kinetics of the MgH 2 upon doping with 5.2TiC/4.6FeCr. The pressure-composition temperature (PCT) relations of ball-milled MgH2/5.2TiC/4.6FeCr anocomposite powders obtained after 50 h were volumetrically investigated by Sievert's approach at ifferent temperatures of 225, 250, 275, 300, 325, and 350 °C, as elucidated in Figure 6. A single eversible hydrogenation/dehydrogenation cycle was developed for each applied temperature. he presence of clear hydrogenation plateaus can be seen in the range between 0.25 and 5.25 wt. % H2 t temperatures ranging between 275 and 350 °C, as shown in Figure 6. However, the hydrogen uptake lateau was visible only in the range of 0.25-2.5 and 0.25-3.25 wt. % H2, at temperatures of 225 °C and 50 °C, respectively. On the other hand, smooth plateaus of hydrogen release were characterized in the hole hydrogen concentrations range (0.25-5.25 wt. % H2) for all applied temperatures, as presented in igure 6. The hydrogen equilibrium pressure measurements were used in the present study to investigate he heat of hydrogen absorption, using van't Hoff equation: here is the hydrogen pressure under equilibrium at a given specific temperature, T; P0 is a reference ressure of 1 bar; R is the gas constant (0.0083145 J/K.mol); ΔH is the molar enthalpy of metal hydride ormation (MgH2); and ΔS is the entropy of absorption. Thus, ΔH can be directly calculated from lotting the natural log of each point versus the corresponding 1/T, as shown in Figure 7a. In the The pressure-composition temperature (PCT) relations of ball-milled MgH 2 /5.2TiC/4.6FeCr nanocomposite powders obtained after 50 h were volumetrically investigated by Sievert's approach at different temperatures of 225, 250, 275, 300, 325, and 350˝C, as elucidated in Figure 6. A single reversible hydrogenation/dehydrogenation cycle was developed for each applied temperature. The presence of clear hydrogenation plateaus can be seen in the range between 0.25 and 5.25 wt. % H 2 at temperatures ranging between 275 and 350˝C, as shown in Figure 6. However, the hydrogen uptake plateau was visible only in the range of 0.25-2.5 and 0.25-3.25 wt. % H 2 , at temperatures of 225˝C and 250˝C, respectively. On the other hand, smooth plateaus of hydrogen release were characterized in the whole hydrogen concentrations range (0. 25-5.25 wt. % H 2 ) for all applied temperatures, as presented in Figure 6. The hydrogen equilibrium pressure measurements were used in the present study to investigate the heat of hydrogen absorption, using van't Hoff equation: where P eq is the hydrogen pressure under equilibrium at a given specific temperature, T; P 0 is a reference pressure of 1 bar; R is the gas constant (0.0083145 J/K.mol); ∆H is the molar enthalpy of metal hydride formation (MgH 2 ); and ∆S is the entropy of absorption. Thus, ∆H can be directly calculated from plotting the natural log of each P eq point versus the corresponding 1/T, as shown in Figure 7a. In the present work, the calculated ∆H and ∆S for MgH 2 doped with 5.2TiC/4.6FeCr waś 72.74 kJ/mol and 112.79 J/mol H 2 /K, respectively. The strength of Mg-H bonds, which can be expressed by the enthalpy of decomposition can be calculated by van't Hoff approach, using the equilibrium dehydrogenation pressure in the PCT measurements. A van't Hoff plot illustrating the relationship between ln(P) and 1/T for the decomposition of MgH 2 powders doped with 5.2TiC/4.6FeCr is shown in Figure 7b. Both of ∆H and ∆S were directly calculated from the slope of the curve presented in Figure 7b and found to be 76.76 kJ/mol and 119.15 J/mol H 2 /K, respectively. Comparing these values with those reported by Reilly (77.4 kJ/mol, 138.3 J/mol H 2 /K) [22], and Klose (81.86 kJ/mol, 146.1 J/mol H 2 /K) [23], one can say that long-term ball milling led to the formation of homogeneous nanocomposite MgH 2 /5.2TiC/4.6FeCr powders, destabilizing the chemically stable phase of MgH 2 , implied by the obvious increase in the ∆H of decomposition. Until recently, it was believed that ∆S has a constant value of about 130 J/mol H 2 /K [24]. It has been suggested by Zhao-Karger et al. [24] that ∆S of the dehydrogenation process can be varied based on the MgH 2 particle size. Based on the ab initio Hartree-Fock and density functional theory calculations shown by Wagemans et al. [25], magnesium hydride becomes less stable with decreases in the cluster size to less than 20 atoms. Accordingly, and based on that study, the ∆H of hydrogen desorption decreases significantly when the grain size is smaller than 1.3 nm [25]. Figure 8 displays the STEM/BF image of the ball-milled nanocomposite sample after the PCT hydrogenation/dehydrogenation measurements under hydrogen gas pressure and temperatures ranging between 0 and 10 bar, and 225 and 350˝C, respectively. Obviously, the sample maintained its nanocrystalline structure ranging between 18 and 67 nm for MgH 2 matrix (light gray-scale particles) and 8 and 27 nm for TiC (dark particles), as shown in Figure 8. We should emphasize that the as-prepared ultrafine powders in the present study with their nanostructured grains facilitated better hydrogen desorption and shortened the diffusion distance required to accomplish a complete dehydrogenation process. In addition, TiC refractory nanoparticles acted as grain growth inhibitors maintaining the MgH 2 particles, especially when the samples were subjected to the high temperature side (300-350˝C) during the PCT analysis. and 119.15 J/mol H2/K, respectively. Comparing these values with those reported by Reilly (77.4 kJ/mol, 138.3 J/mol H2/K) [22], and Klose (81.86 kJ/mol, 146.1 J/mol H2/K) [23], one can say that long-term ball milling led to the formation of homogeneous nanocomposite MgH2/5.2TiC/4.6FeCr powders, destabilizing the chemically stable phase of MgH2, implied by the obvious increase in the ΔH of decomposition. Until recently, it was believed that ΔS has a constant value of about 130 J/mol H2/K [24]. It has been suggested by Zhao-Karger et al. [24] that ΔS of the dehydrogenation process can be varied based on the MgH2 particle size. Based on the ab initio Hartree-Fock and density functional theory calculations shown by Wagemans et al. [25], magnesium hydride becomes less stable with decreases in the cluster size to less than 20 atoms. Accordingly, and based on that study, the ΔH of hydrogen desorption decreases significantly when the grain size is smaller than 1.3 nm [25]. Figure 8 displays the STEM/BF image of the ball-milled nanocomposite sample after the PCT hydrogenation/dehydrogenation measurements under hydrogen gas pressure and temperatures ranging between 0 and 10 bar, and 225 and 350 °C, respectively. Obviously, the sample maintained its nanocrystalline structure ranging between 18 and 67 nm for MgH2 matrix (light gray-scale particles) and 8 and 27 nm for TiC (dark particles), as shown in Figure 8. We should emphasize that the as-prepared ultrafine powders in the present study with their nanostructured grains facilitated better hydrogen desorption and shortened the diffusion distance required to accomplish a complete dehydrogenation process. In addition, TiC refractory nanoparticles acted as grain growth inhibitors maintaining the MgH2 particles, especially when the samples were subjected to the high temperature side (300-350 °C) during the PCT analysis.          In general, the synthesized nanocomposite powders showed excellent potential for absorbing hydrogen gas in a short time at temperatures ranging from 250 to 275˝C under pressure ranging from 100 mbar to 8 bar, as shown in Figure 9a. After 1 min, the powders examined at 250 and 275˝C were able to uptake 3.66 and 4.55 wt. % H 2 , respectively as elucidated in Figure 9a. After 11.2 min of the absorption, the sample examined at 275˝C reached its saturated value with hydrogen storage reaching 5.51 wt. %. In contrast, 19.2 min was required for the sample examined at 250˝C to absorb 5.41 wt. % H 2 , as shown in Figure 9a.
Materials 2015, 8 11 In general, the synthesized nanocomposite powders showed excellent potential for absorbing hydrogen gas in a short time at temperatures ranging from 250 to 275 °C under pressure ranging from 100 mbar to 8 bar, as shown in Figure 9a. After 1 min, the powders examined at 250 and 275 °C were able to uptake 3.66 and 4.55 wt. % H2, respectively as elucidated in Figure 9a. After 11.2 min of the absorption, the sample examined at 275 °C reached its saturated value with hydrogen storage reaching 5.51 wt. %. In contrast, 19.2 min was required for the sample examined at 250 °C to absorb 5.41 wt. % H2, as shown in Figure 9a.  The corresponding desorption kinetics of the nanocomposite powders investigated at 250˝C and 275˝C are shown in Figure 9b,c. The powders examined at 275˝C showed excellent desorption kinetics, indexed by the relatively short time (~10 min) required to release about 5.51 wt. % of hydrogen, as shown in Figure 9b. The sample examined at this temperature desorbed 1.62 wt. % of hydrogen within a short desorption time of 2.5 min, as shown in Figure 9c. At this applied temperature, the sample released about 3.43 wt. % of its hydrogen storage capacity after 5 min of desorption, as elucidated in Figure 7c. In contrast to such fast desorption kinetics achieved at 275˝C, the sample examined at 250˝C showed a slow dehydrogenation behavior, indexed by the long time required to release its full hydrogen content (~5.5 wt. %), 79 min, as shown in Figure 9b. After 2.5 and 5 min of desorption conducted at 250˝C (Figure 9c), the sample was unable to release more than 0.32, and 0.80 H 2 wt. %, respectively, as presented in Figure 9c. Aside from the particle size effect on the ∆H and ∆S of hydrogen desorption for MgH 2 , the dehydrogenation temperature decreased from 400˝C in bulk MgH 2 to be 250-275˝C, when the crystallite size of MgH 2 was less than 10 nm in diameter (Figure 2a).
Apart from the fast kinetics of hydrogenation/dehydrogenations characterizations shown by MgH 2 /5.2TiC/4.6FeCr ternary system, the cyclic-reversibility of the fabricated nanocomposite powders examined at 275˝C under repeated hydrogenation/dehydrogenation pressure of 0/8 bar was investigated. Figure 10 shows the cycle-life-time performed at 275˝C for the nanocomposite powders obtained after 50 h of ball milling. Obviously, this new nanocomposite system exhibits excellent cyclic-reversible properties, indexed by its high cyclic stability without failure, even after about 682 h (679 cycles), as shown in Figure 10. Comparing the number of cycles achieved at 275˝C by this nanocomposite system with those performed in MgH 2 /Mn 3.6 Ti 2.4 , 1000 cycles/275˝C [16], MgH 2 /5Ni5Nb 2 O 5 , 180 cycles/250˝C [26], MgH 2 /5Fe 47 cycles/300˝C [27], and MgH 2 /10Co 350˝C [25] systems, one can consider the MgH 2 /TiC/FeCr system as one of the most stable and capable MgH 2 -based nanocomposite systems used for hydrogen storage applications.
°C showed a slow dehydrogenation behavior, indexed by the long time required to release its full hydrogen content (~5.5 wt. %), 79 min, as shown in Figure 9b. After 2.5 and 5 min of desorption conducted at 250 °C (Figure 9c), the sample was unable to release more than 0.32, and 0.80 H2 wt. %, respectively, as presented in Figure 9c. Aside from the particle size effect on the ΔH and ΔS of hydrogen desorption for MgH2, the dehydrogenation temperature decreased from 400 °C in bulk MgH2 to be 250-275 °C, when the crystallite size of MgH2 was less than 10 nm in diameter (Figure 2a).
Apart from the fast kinetics of hydrogenation/dehydrogenations characterizations shown by MgH2/5.2TiC/4.6FeCr ternary system, the cyclic-reversibility of the fabricated nanocomposite powders examined at 275 °C under repeated hydrogenation/dehydrogenation pressure of 0/8 bar was investigated. Figure 10 shows the cycle-life-time performed at 275 °C for the nanocomposite powders obtained after 50 h of ball milling. Obviously, this new nanocomposite system exhibits excellent cyclic-reversible properties, indexed by its high cyclic stability without failure, even after about 682 h (679 cycles), as shown in Figure 10. Comparing the number of cycles achieved at 275 °C by this nanocomposite system with those performed in MgH2/Mn3.6Ti2.4, 1000 cycles/275 °C [16], MgH2/5Ni5Nb2O5, 180 cycles/250 °C [26], MgH2/5Fe 47 cycles/300 °C [27], and MgH2/10Co 350 °C [25] systems, one can consider the MgH2/TiC/FeCr system as one of the most stable and capable MgH2-based nanocomposite systems used for hydrogen storage applications.

Conclusions
Nanocrystalline MgH 2 powders were synthesized by reactive ball milling of pure Mg powders, using a high-energy ball mill operated at 250 rpm under 50 bar of hydrogen atmosphere. The as-synthesized MgH 2 powders obtained after 200 h of ball milling were contaminated by about 2.2 wt. % of FeCr. The powders were doped with TiC ultrafine powders, which were already contaminated with 2.4 wt. % FeCr, and then ball milled for 50 h. Significant improvements in the hydrogenation/dehydrogenation kinetics of MgH 2 doped with 5.2TiC/4.6FeCr were achieved. Such improvements are attributed to the presence of FeCr content that played an important role in splitting the H 2 molecules and facilitating proper hydrogen diffusion into the Mg matrix. In addition, ball milling the MgH 2 powders with refractory TiC nanopowders led to further grain refining of the metal hydride phase, enabling fast hydrogen absorption/desorption processes. Moreover, the hard TiC phase inhibited grain growth, allowing to maintain the nanocrystallinity of MgH 2 powders during repeated hydrogenation/dehydrogenation cycles that extended to 697 cycles without failure or degradation.