Effect of ZrC Nanopowders on Enhancing the Hydro/Dehydrogenation Kinetics of MgH2 Powders

Hydrogen has been receiving great attention as an energy carrier for potential green energy applications. Hydrogen storage is one of the most crucial factors controlling the hydrogen economy and its future applications. Amongst the several options of hydrogen storage, light metal hydrides, particularly nanocrystalline magnesium hydride (MgH2), possess attractive properties, making them desired hydrogen storage materials. The present study aimed to improve the hydrogen storage properties of MgH2 upon doping with different concentrations of zirconium carbide (ZrC) nanopowders. Both MgH2 and ZrC were prepared using reactive ball milling and high-energy ball milling techniques, respectively. The as-prepared MgH2 powder was doped with ZrC (2, 5, and 7 wt%) and then high-energy-ball-milled for 25 h. During the ball milling process, ZrC powders acted as micro-milling media to reduce the MgH2 particle size to a minimal value that could not be obtained without ZrC. The as-milled nanocomposite MgH2/ZrC powders consisted of fine particles (~0.25 μm) with a nanosized grain structure of less than 7 nm. Besides, the ZrC agent led to the lowering of the decomposition temperature of MgH2 to 287 °C and the reduction in its apparent activation energy of desorption to 69 kJ/mol. Moreover, the hydrogenation/dehydrogenation kinetics of the nanocomposite MgH2/ZrC system revealed a significant improvement, as indicated by the low temperature and short time required to achieve successful uptake and release processes. This system possessed a high capability to tackle a long continuous cycle lifetime (1400 h) at low temperatures (225 °C) without showing serious degradation in its storage capacity.


Introduction
The global use of fossil fuels has increased drastically as people's standard of living has improved. Climate change, driven by increasing carbon dioxide emissions, will wreak havoc on society [1]. The threat of pollution will be exacerbated by the forced expansion of using low-grade fossil fuels. Improved living standards will raise the demand for chemicals derived from fossil fuels, which will be used to make higher-value end-use commodities such as plastics and pharmaceuticals. Due to the severe rivalry over dwindling supplies, the price of energy and chemicals derived from fossil fuels will increase. Due to environmental and health points of view, fossil fuels emit carbon dioxide and other harmful air pollutants such as SO 2 and NO x when burned [2].
The depletion of fossil fuels is a major motivator for finding clean alternative sources to power energy systems. As a result, using renewable and sustainable energy sources such as solar, wind, and geothermal energy to produce clean and efficient energy systems has become one of the world's hottest research subjects, drawing a large number of academics and researchers [3].
Among the different green-energy options, hydrogen, which possesses a high energy density, has been considered the most suitable carbon-free energy carrier that can be used as an alternative to fossil fuel [4]. The convenience, safety, and versatility of hydrogen as an energy carrier are linked to its unique features of being easy to manufacture from a renewable energy system and convert to a desired form of energy [5]. One of the key advantages of hydrogen is that when burned, CO 2 is not produced. Fuel cells (FCs) that run on hydrogen offer a wide range of potential uses, ranging from a few watts to gigawatts. Furthermore, compared to an internal combustion engine, hydrogen has the ability to drive a fuel-cell engine more efficiently [6]. The FCs could supply auxiliary power for electrical appliances such as air conditioning and refrigerators in mobile applications [7]. For automobile [8] and vehicle powertrain applications [9], hydrogen-polymer electrolyte membrane (PEM) FCs have become a well-known option [10].
The hydrogen economy and its potential applications are influenced by three variables. Apart from hydrogen generation and transportation, hydrogen storage is seen to be the most important aspect influencing the possible usage of hydrogen in real-world applications. Metal hydrides, notably MgH 2 , have very appealing features as compared to typical hydrogen storage systems. This system has a higher hydrogen storage density of 6.5 H atoms/cm 3 for MgH 2 when compared with pure hydrogen gas (0.99 H atoms/cm 3 ) or liquid hydrogen (4.2 H atoms/cm 3 ). Mg metal has various advantages for hydrogen storage, including low cost, light weight, and high gravimetric (7.60 wt%) and volumetric (110 g/L) hydrogen storage capacities [11][12][13][14][15][16][17][18]. In spite of the attractive and useful properties of Mg/MgH 2 , there are some serious drawbacks that should be solved before using this system for FC-hydrogen storage applications. The tetragonal-MgH 2 (β-phase) is thermodynamically very stable (H for = −75 kJ/mol. H 2 ) and decomposes at a high temperature of 350 • C [19]. Furthermore, it has a high apparent activation energy (above 130 kJ/mol) and very slow hydrogenation/dehydrogenation kinetics under 325 • C [20].
Within the last three decades, great efforts have been dedicated in order to improve the hydrogen storage behavior of MgH 2 , using mechanical treatment and catalyzation approaches. Long-term high-energy ball milling [21], cold-rolling [22,23], equal channel angular pressing [24], and high-pressure torsion [25,26] are some techniques used to introduce severe plastic deformation to the Mg lattice, leading to the destabilization of the β-MgH 2 phase that tends to transform into a less stable phase (γ-MgH 2 ) with desired kinetics of hydrogen uptake/release.
In the present study, we attempted to improve the kinetics of the uptake/release of MgH 2 powders, using a new catalytic agent of ZrC. This compound was selected according to its great hardness value. It can then be expected that upon ball milling with Mg/MgH 2 powders, ZrC can play a great role as micro-milling media for reducing the particle size of the base material, leading to the enhancement of the kinetic characteristics of the hydride phase. Besides, ZrC with its high thermal stability would not decompose during the hydrogenation process of Mg metal. Moreover, ZrC does not react with Mg and/or MgH 2 to form any undesired reactive or intermediate compounds. Thus, it can be safely used as a heterogenous catalytic agent without the drastic decrease in the hydrogen storage capacity of Mg. The present work was undertaken in part to introduce a new nanocomposite system of MgH 2 -based materials that has an excellent performance of cycle lifetime with fast hydrogenation/dehydrogenation kinetics. An X-ray technique was employed to monitor the progress of the gas-solid reaction undertaken between Mg powders and H 2 , using the reactive ball milling (RBM) method. The general crystal structural changes in hcp Mg and the formation of new phases were investigated after the early (1-3 h), intermediate (3-6 h), and final stages of the RBM time. The X-ray diffraction (XRD) pattern of the starting feedstock powder, which is displayed in Figure 1a, displayed sharp Bragg-diffraction peaks related to hcp Mg (PDF # 00-004-0770). After a few minutes (30 min) of RBM, the powders were agglomerated due to the effect of cold welding created by the ball-powder-ball collisions, as displayed in Figure 2a. These agglomerated powders tended to disintegrate into small aggregates (~20-180 µm in diameter) upon the increase in the RBM time to 1 h, as shown in Figure 2b. The corresponding diffractogram of the disintegrated powders at this stage of RBM time is shown in Figure 1b. An X-ray technique was employed to monitor the progress of the gas-solid reaction undertaken between Mg powders and H2, using the reactive ball milling (RBM) method. The general crystal structural changes in hcp Mg and the formation of new phases were investigated after the early (1-3 h), intermediate (3-6 h), and final stages of the RBM time. The X-ray diffraction (XRD) pattern of the starting feedstock powder, which is displayed in Figure 1a, displayed sharp Bragg-diffraction peaks related to hcp Mg (PDF # 00-004-0770). After a few minutes (30 min) of RBM, the powders were agglomerated due to the effect of cold welding created by the ball-powder-ball collisions, as displayed in Figure  2a. These agglomerated powders tended to disintegrate into small aggregates (~20-180 μm in diameter) upon the increase in the RBM time to 1 h, as shown in Figure 2b. The corresponding diffractogram of the disintegrated powders at this stage of RBM time is shown in Figure 1b. Besides the Bragg peaks of unreacted hcp peaks, low-intensity diffracted lines were detected, suggesting the formation of the reacted β-MgH2 phase (PDF # 00-012-0697), as increasing the RBM time to 18 h, as indexed by the staking faults appearing in β-MgH2 (002), as shown in Figure 3a,b. The generation of these lattice imperfections led to further disintegration of the MgH2 powders and the formation of nanocluster powder particles after 25 h (Figure 2d). The XRD pattern of this end-product revealed significant broadening, obtained as a result of the grain refinement and lattice strain, as displayed in Figure  1e. The existence of metastable γ-MgH2 (PDF# 00-035-1184) was produced due to distortion of the most stable phase of β-MgH2 (Figure 1e).

ZrC Catalytic Agent Nanopowders
For the present work, equiatomic nanocrystalline fcc-ZrC powders were synthesized from elemental Zr and graphite powders, using a high-energy ball mill (HEBM). Figure Besides the Bragg peaks of unreacted hcp peaks, low-intensity diffracted lines were detected, suggesting the formation of the reacted β-MgH 2 phase (PDF # 00-012-0697), as presented in Figure 1b. At the intermediate stage of RBM (3-6 h), the large Mg aggregates were disintegrated along their grain boundaries to form smaller grains with new active surfaces, as displayed in Figure 2c. Initiation of these new surfaces promoted a forward gas-solid reaction and the formation of a reactive MgH 2 phase. This is implied by the formation of pronounced high-intensity Bragg lines, which corresponded to the β-MgH 2 phase (Figure 1c). The formation of larger volume fractions of fresh surface Mg powders upon increasing the RBM time (6 h) enhanced the reaction between the diffusion couples (Mg and H 2 ), leading to increases in the volume fraction of the reacted phase against pure Mg metal. Accordingly, the Bragg peaks related to hcp Mg were hardly detected in the diffractogram related to the powders obtained after 6 h of RBM ( Figure 1d).
Severe lattice imperfections were continuously developed in the MgH 2 lattice upon increasing the RBM time to 18 h, as indexed by the staking faults appearing in β-MgH 2 (002), as shown in Figure 3a,b. The generation of these lattice imperfections led to further disintegration of the MgH 2 powders and the formation of nanocluster powder particles after 25 h (Figure 2d). The XRD pattern of this end-product revealed significant broadening, obtained as a result of the grain refinement and lattice strain, as displayed in Figure 1e. The existence of metastable γ-MgH 2 (PDF# 00-035-1184) was produced due to distortion of the most stable phase of β-MgH 2 ( Figure 1e).  Figure 2c. Initiation of these new surfaces promoted a forward gas-solid reaction and the formation of a reactive MgH2 phase. This is implied by the formation of pronounced high-intensity Bragg lines, which corresponded to the β-MgH2 phase (Figure 1c). The formation of larger volume fractions of fresh surface Mg powders upon increasing the RBM time (6 h) enhanced the reaction between the diffusion couples (Mg and H2), leading to increases in the volume fraction of the reacted phase against pure Mg metal. Accordingly, the Bragg peaks related to hcp Mg were hardly detected in the diffractogram related to the powders obtained after 6 h of RBM ( Figure 1d). Severe lattice imperfections were continuously developed in the MgH2 lattice upon increasing the RBM time to 18 h, as indexed by the staking faults appearing in β-MgH2 (002), as shown in Figure 3a,b. The generation of these lattice imperfections led to further disintegration of the MgH2 powders and the formation of nanocluster powder particles after 25 h (Figure 2d). The XRD pattern of this end-product revealed significant broadening, obtained as a result of the grain refinement and lattice strain, as displayed in Figure  1e. The existence of metastable γ-MgH2 (PDF# 00-035-1184) was produced due to distortion of the most stable phase of β-MgH2 (Figure 1e).

ZrC Catalytic Agent Nanopowders
For the present work, equiatomic nanocrystalline fcc-ZrC powders were synthesized from elemental Zr and graphite powders, using a high-energy ball mill (HEBM). Figure

ZrC Catalytic Agent Nanopowders
For the present work, equiatomic nanocrystalline fcc-ZrC powders were synthesized from elemental Zr and graphite powders, using a high-energy ball mill (HEBM). Figure 4a,b display the XRD patterns for the starting feedstock powders of hcp Zr (PDF File# 00-005-065) and hcp-C (PDF File# 00-056-0159), respectively. A new phase was detected after 25 h of HEBM, as indexed in the XRD pattern shown in Figure 4c. Analysis of the diffracted lines that matched well with PDF File# 00-19-1487 indicated the formation of a single phase of fcc-ZrC. The HEBM ZrC powders had a cluster-like morphology composed of spherical nanoparticles with a diameter of less than 50 nm (Figure 4d). The field emission high-resolution transmission electron microscope (FE-HRTEM) image indicated the formation of uniform nanospheres in the range between 2 and 7 nm in diameter, as shown in Figure 4e. Moreover, the corresponding nanobeam diffraction pattern (NBDP), implied the formation of fcc ZrC, characterized by (111), (200), (220) and (311), as displayed in Figure 4f. The absence of sharp spots from the Debye-Scherrer rings implied the existence of nanocrystalline ZrC powders.  The ZrC nanoparticles obtained after 25 h of milling were used to improve the hydrogen storage characteristics of MgH2 powders upon mechanically induced mixing for 50 h with different mass fractions (2, 5, and 7 wt.%), using HEBM. The XRD pattern of nanocomposite MgH2/5 wt.% ZrC powders obtained after 50 h of HEBM is displayed in Figure 5a. The diffractogram possessed broad Bragg peaks of the base MgH2 powders and the modifier agent of ZrC, as presented in Figure 5a. Both phases kept their existence after milling for 50 h with no evidence for the formation of any reacted phase.
The atomic-resolution TEM image of the nanocomposite powders obtained after 50 h of milling is presented in Figure 6. The figure shows two individual MgH2 particles, indexed by β-MgH2 (110) and (101). Meanwhile, two ZrC nanoparticles, characterized by fcc -ZrC (111) and (200), were adhered to the MgH2 particles, as displayed in Figure 6. The ZrC nanoparticles obtained after 25 h of milling were used to improve the hydrogen storage characteristics of MgH 2 powders upon mechanically induced mixing for 50 h with different mass fractions (2, 5, and 7 wt.%), using HEBM. The XRD pattern of nanocomposite MgH 2 /5 wt.% ZrC powders obtained after 50 h of HEBM is displayed in Figure 5a. The diffractogram possessed broad Bragg peaks of the base MgH 2 powders and the modifier agent of ZrC, as presented in Figure 5a. Both phases kept their existence after milling for 50 h with no evidence for the formation of any reacted phase.
Examining the local structure of the interfaces between these two different materials beyond the atomic level suggested the absence of any reacted phase(s). This implies that ZrC was natural during milling with MgH2 powders and played the role of a heterogeneous catalyst. It is worth mentioning that the positions of the Bragg peak for MgH2 shifted to the low-angle side, suggesting the expansion of the lattice parameter without altering the crystal.   The atomic-resolution TEM image of the nanocomposite powders obtained after 50 h of milling is presented in Figure 6. The figure shows two individual MgH 2 particles, indexed by β-MgH 2 (110) and (101). Meanwhile, two ZrC nanoparticles, characterized by fcc -ZrC (111) and (200), were adhered to the MgH 2 particles, as displayed in Figure 6. Examining the local structure of the interfaces between these two different materials beyond the atomic level suggested the absence of any reacted phase(s). This implies that ZrC was natural during milling with MgH 2 powders and played the role of a heterogeneous catalyst. It is worth mentioning that the positions of the Bragg peak for MgH 2 shifted to the lowangle side, suggesting the expansion of the lattice parameter without altering the crystal. Examining the local structure of the interfaces between these two different materials beyond the atomic level suggested the absence of any reacted phase(s). This implies that ZrC was natural during milling with MgH2 powders and played the role of a heterogeneous catalyst. It is worth mentioning that the positions of the Bragg peak for MgH2 shifted to the low-angle side, suggesting the expansion of the lattice parameter without altering the crystal.   The powders were composed of MgH 2 matrix, where spherical ZrC nanoparticles (<10 nm) were embedded uniformly into the matrix.
The distribution of ZrC nanoparticles in MgH 2 powders after processing for 25 h of HEBM was investigated by intensive EDS analysis, using FE-SEM. Figure 7a displays a FE-SEM micrograph of nanocomposite MgH 2 /5 wt.% ZrC powder obtained after 25 h of milling. The indexed rectangular zone (5 µm × 4 µm), shown in the middle part of the micrograph, refers to the selected area chosen for EDS analysis. The area was classified into subrectangular zones (0.5 µm × 0.4 µm), where the analysis was conducted in the middle of each subzone (Figure 7a). In general, the powder obtained after this stage of HEBM possessed a narrow distribution particle size range (100-300 nm) with a spherical-like morphology, as shown in Figure 7a. We should emphasize that refractory ZrC nanopowders played the role of micro-milling media and led to drastic grain refining of the MgH 2 powders.
The distribution of ZrC nanoparticles in MgH2 powders after processing for 25 h of HEBM was investigated by intensive EDS analysis, using FE-SEM. Figure 7a displays a FE-SEM micrograph of nanocomposite MgH2/5 wt.% ZrC powder obtained after 25 h of milling. The indexed rectangular zone (5 μm × 4 μm), shown in the middle part of the micrograph, refers to the selected area chosen for EDS analysis. The area was classified into subrectangular zones (0.5 μm × 0.4 μm), where the analysis was conducted in the middle of each subzone (Figure 7a). In general, the powder obtained after this stage of HEBM possessed a narrow distribution particle size range (100-300 nm) with a sphericallike morphology, as shown in Figure 7a. We should emphasize that refractory ZrC nanopowders played the role of micro-milling media and led to drastic grain refining of the MgH2 powders.
In addition, further ball milling time (50 h) promoted a homogeneous distribution of nanograined ZrC powders into the metal hydride matrix. The compositional analysis obtained from EDS measurements was employed to design isochemical contour maps for Mg (Figure 7b

Thermal Stability
Atmospheric helium-pressure DSC was employed to investigate the thermal stability, indexed by decomposition temperature (T p ) and activation energy (E a ), for synthesized nanocrystalline MgH 2 before and after doping with ZrC nanoparticles. The DSC thermogram of MgH 2 nanopowders, obtained after 50 h of RBM, are displayed in Figure 8a with different heating rates (k) of 5, 10, 20, 30, and 40 • C/min. All the scans revealed single endothermic events related to the decomposition of the MgH 2 phase. This is implied by the XRD pattern of the sample heated up to 500 • C, which revealed sharp Bragg peaks related to pure Mg and fcc ZrC overlapped with an undecomposed minor volume fraction of MgH 2 , as displayed in Figure 9. While the peak height increased proportionally with the increase in k, T p significantly shifted to the higher-temperature side upon the increase in the heating rates from 5 to 40 • C/min, as shown in Figure 8a. The as-synthesized MgH 2 nanopowders revealed a high decomposition temperature, exemplified by the T p measured at 10 • C/min (360 • C), as shown in Figure 8a.

Thermal Stability
Atmospheric helium-pressure DSC was employed to investigate the thermal stability, indexed by decomposition temperature (Tp) and activation energy (Ea), for synthesized nanocrystalline MgH2 before and after doping with ZrC nanoparticles. The DSC thermogram of MgH2 nanopowders, obtained after 50 h of RBM, are displayed in Figure  8a with different heating rates (k) of 5, 10, 20, 30, and 40 °C/min. All the scans revealed single endothermic events related to the decomposition of the MgH2 phase. This is implied by the XRD pattern of the sample heated up to 500 °C, which revealed sharp Bragg peaks related to pure Mg and fcc ZrC overlapped with an undecomposed minor volume fraction of MgH2, as displayed in Figure 9. While the peak height increased proportionally with the increase in k, Tp significantly shifted to the higher-temperature side upon the increase in the heating rates from 5 to 40 °C/min, as shown in Figure 8a. The as-synthesized MgH2 nanopowders revealed a high decomposition temperature, exemplified by the Tp measured at 10 °C/min (360 °C), as shown in Figure 8a.  The improved dehydrogenation kinetics, measured under a helium gas atmosphere, was evaluated by calculating the Ea of the decomposition reaction for both pure MgH2 and corresponding MgH2/5 wt.% ZrC samples. In the present work, the activation energy for dehydrogenation was investigated according to the Arrhenius Equation (1):

Thermal Stability
Atmospheric helium-pressure DSC was employed to investigate the thermal stability, indexed by decomposition temperature (Tp) and activation energy (Ea), for synthesized nanocrystalline MgH2 before and after doping with ZrC nanoparticles. The DSC thermogram of MgH2 nanopowders, obtained after 50 h of RBM, are displayed in Figure  8a with different heating rates (k) of 5, 10, 20, 30, and 40 °C/min. All the scans revealed single endothermic events related to the decomposition of the MgH2 phase. This is implied by the XRD pattern of the sample heated up to 500 °C, which revealed sharp Bragg peaks related to pure Mg and fcc ZrC overlapped with an undecomposed minor volume fraction of MgH2, as displayed in Figure 9. While the peak height increased proportionally with the increase in k, Tp significantly shifted to the higher-temperature side upon the increase in the heating rates from 5 to 40 °C/min, as shown in Figure 8a. The as-synthesized MgH2 nanopowders revealed a high decomposition temperature, exemplified by the Tp measured at 10 °C/min (360 °C), as shown in Figure 8a.  The improved dehydrogenation kinetics, measured under a helium gas atmosphere, was evaluated by calculating the Ea of the decomposition reaction for both pure MgH2 and corresponding MgH2/5 wt.% ZrC samples. In the present work, the activation energy for dehydrogenation was investigated according to the Arrhenius Equation (1): The improved dehydrogenation kinetics, measured under a helium gas atmosphere, was evaluated by calculating the E a of the decomposition reaction for both pure MgH 2 and corresponding MgH 2 /5 wt.% ZrC samples. In the present work, the activation energy for dehydrogenation was investigated according to the Arrhenius Equation (1): where k is a temperature-dependent reaction rate constant, R is the gas constant, and Tp is the absolute temperature. The value E a of the reaction was determined by measuring the decomposition of T p corresponding to the different heating rates (k) and then plotting ln(k) versus 1/T p , where the E a of synthesized MgH 2 nanopowders was 123 kJ/mol, and the E a obtained for nanocomposite MgH 2 /5 wt.% ZrC was 69 kJ/mol. This indicates a significant destabilization of the MgH 2 upon doping with 5 wt.% ZrC.

Pressure-Composition-Temperature
Pressure-composition-temperature (PCT), also known as pressure-composition isotherms (PCI), is the most basic measurement, used to investigate the thermodynamic properties of a hydrogen storage system. The PCT correlations of nanocomposite MgH 2 /5 wt.% ZrC powders, obtained after 25 h of HEBM, were volumetrically investigated by Sievert's approach at different temperatures of 225, 250, 275, 300 and 325 • C, as displayed in Figure 10a. The PCT measurements were started by introducing H 2 to the system at the desired temperature, and then the system was closed while the sample and gas reacted under isochoric conditions. All PCT experiments were achieved, using the run-of-RBM (raw-nanocomposite powders), before conducting powder activation. This was necessary in order to realize the original shape of the PCT curve, and its slope. In all applied temperatures, single reversible hydrogenation/dehydrogenation cycles were characterized, with the absence of wide pressure gaps (∆P) between the hydrogenation (P abs ) and dehydrogenation (P des ) plateaus, as shown in Figure 10b. Accordingly, the near values of ∆P (∆P = P abs − P des ) indicate the absence of the hysteresis phenomenon for this hydrogen storage nanocomposite system, as can be realized in Figure 10a,b.

Pressure-Composition-Temperature
Pressure-composition-temperature (PCT), also known as pressure-composition isotherms (PCI), is the most basic measurement, used to investigate the thermodynamic properties of a hydrogen storage system. The PCT correlations of nanocomposite MgH2/5 wt.% ZrC powders, obtained after 25 h of HEBM, were volumetrically investigated by Sievert's approach at different temperatures of 225, 250, 275, 300 and 325 °C, as displayed in Figure 10a. The PCT measurements were started by introducing H2 to the system at the desired temperature, and then the system was closed while the sample and gas reacted under isochoric conditions. All PCT experiments were achieved, using the run-of-RBM (raw-nanocomposite powders), before conducting powder activation. This was necessary in order to realize the original shape of the PCT curve, and its slope. In all applied temperatures, single reversible hydrogenation/dehydrogenation cycles were characterized, with the absence of wide pressure gaps (ΔP) between the hydrogenation (P abs ) and dehydrogenation (P des ) plateaus, as shown in Figure 10b. Accordingly, the near values of ΔP (ΔP = P abs − P des ) indicate the absence of the hysteresis phenomenon for this hydrogen storage nanocomposite system, as can be realized in Figure 10a,b.   Figure 10b. The hydrogen equilibrium pressure measurements were used in the present study to investigate the heat of hydrogen absorption, using the van't Hoff Equation (2): where P eq is the hydrogen pressure under equilibrium at a given specific temperature, T; P o 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. The ∆H of hydrogenation can be directly calculated from plotting the natural log of each P eq point versus the corresponding 1/T, as shown in Figure 11a.
where Peq is the hydrogen pressure under equilibrium at a given specific temperature, T; Po 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 (MgH2); and ∆S is the entropy of absorption. The ∆H of hydrogenation can be directly calculated from plotting the natural log of each Peq point versus the corresponding 1/T, as shown in Figure 11a.
In the present work, the calculated ∆H and ∆S for MgH2 doped with 5 wt.% ZrC were −72.74 kJ/mol and 112.79 J/mol H2/K, respectively. On the other hand, the strength of Mg-H bonds, which is indicated by the enthalpy of decomposition, can be calculated via the van't Hoff approach, using the equilibrium dehydrogenation pressure in the PCT measurements. The ΔH of decomposition and the corresponding ΔS of the nanocomposite MgH2/5 wt.% ZrC were calculated for the slope of the line displayed in Figure 11b, and they were found to be 75.97 kJ/mol and 119.15 J/mol H2/K, respectively.   Figure 12a,b display the synergetic effect of RBM time and applied temperature on the hydrogenation and dehydrogenation kinetics of RBM MgH2 powders obtained after 50 h of milling, respectively. As expected, the synthesized pure MgH2 powders exhibited poor hydrogenation/dehydrogenation kinetics, particularly at applied temperatures of less than 325 °C, as shown in Figure 12. For example, the times required to uptake and discharge ~6.5 wt.% H2 at 325 °C were 2500 s and 7800 s, as presented in Figure 12a,b, respectively. The absorption and desorption kinetics dropped significantly upon decreasing the temperature to 275 °C. This is indicated by the time necessary to absorb (~2500 s) and desorb (~40000 s) 6.28 wt.% H2, as presented in Figure 12a,b, respectively. In the present work, the calculated ∆H and ∆S for MgH 2 doped with 5 wt.% ZrC were −72.74 kJ/mol and 112.79 J/mol H 2 /K, respectively. On the other hand, the strength of Mg-H bonds, which is indicated by the enthalpy of decomposition, can be calculated via the van't Hoff approach, using the equilibrium dehydrogenation pressure in the PCT measurements. The ∆H of decomposition and the corresponding ∆S of the nanocomposite MgH 2 /5 wt.% ZrC were calculated for the slope of the line displayed in Figure 11b, and they were found to be 75.97 kJ/mol and 119.15 J/mol H 2 /K, respectively. Figure 12a,b display the synergetic effect of RBM time and applied temperature on the hydrogenation and dehydrogenation kinetics of RBM MgH 2 powders obtained after 50 h of milling, respectively. As expected, the synthesized pure MgH 2 powders exhibited poor hydrogenation/dehydrogenation kinetics, particularly at applied temperatures of less than 325 • C, as shown in Figure 12. For example, the times required to uptake and discharge~6.5 wt.% H 2 at 325 • C were 2500 s and 7800 s, as presented in Figure 12a,b, respectively. The absorption and desorption kinetics dropped significantly upon decreasing the temperature to 275 • C. This is indicated by the time necessary to absorb (~2500 s) and desorb (~40,000 s) 6.28 wt.% H 2 , as presented in Figure 12a      Under an applied pressure of 10 bar, this system showed a good ability of absorbing 2.7 and 2.3 wt.% H 2 within 32.4 and 22.7 min at low temperatures of 100 and 150 • C, respectively, as displayed in Figure 13b. It is worth mentioning that the system was able to absorb 0.9 wt.% H 2 within 25.9 min at 50 • C under 10 bar, as shown in Figure 13b.  (Figure 13d).

Cycle Lifetime
To obtain more information of the capability of the nanocomposite MgH 2 /5 wt.% ZrC system to perform a large number of uptake/release cycles without degradation, the powders were first activated at a higher temperature of 275 • C under hydrogenation/dehydrogenation hydrogen pressures of 35 and 0.2 bar, respectively. This activation step was required to improve the hydrogen storage capacity, which reached 6.9 wt.%, as shown in Figure 14a. During the thermal treatment (surface cleaning) under such high pressure and temperature, the bonding between Mg and O 2 tended to break down. Accordingly, the yielded metallic Mg reacted with hydrogen to form MgH 2 , where the dissociated oxygen atoms combined with hydrogen to form a vapor of water molecules that were continuously evacuated outside of the system. As a result, the volume fraction of MgH 2 was increased against MgO, leading to the enhancement of the hydrogenation/dehydrogenation kinetics (Figure 14a). More importantly, surface cleaning of MgH 2 powders enhanced the kinetics of hydrogenation/dehydrogenation processes, as implied by the powder capability of achieving 1400 h (~60 days) of charging/discharging processes with the absence of failure, as displayed in Figure 14a. However, this long testing time led to obvious degradation in both the hydrogenation/dehydrogenation kinetics of the last cycle ( Figure 14c) when compared with the first cycle (Figure 14b).
13d displays the effect of ZrC nanopowder on improving the dehydrogenation kinetics of MgH2 powder. As shown in the figure, MgH2 doped with 2, 5 and 7 wt.% ZrC possessed excellent releasing kinetics, indicated by the relatively short time (200 s) needed to desorb about −4.5, −5.5, and 5.3 wt.%, respectively. After 400 s, the three systems approached saturated values in the range between −5.5 and −5.8 wt.% H2 (Figure 13d).

Cycle Lifetime
To obtain more information of the capability of the nanocomposite MgH2/5 wt.% ZrC system to perform a large number of uptake/release cycles without degradation, the powders were first activated at a higher temperature of 275 °C under hydrogenation/dehydrogenation hydrogen pressures of 35 and 0.2 bar, respectively. This activation step was required to improve the hydrogen storage capacity, which reached 6.9 wt.%, as shown in Figure 14a. During the thermal treatment (surface cleaning) under such high pressure and temperature, the bonding between Mg and O2 tended to break down. Accordingly, the yielded metallic Mg reacted with hydrogen to form MgH2, where the dissociated oxygen atoms combined with hydrogen to form a vapor of water molecules that were continuously evacuated outside of the system. As a result, the volume fraction of MgH2 was increased against MgO, leading to the enhancement of the hydrogenation/dehydrogenation kinetics (Figure 14a). More importantly, surface cleaning of MgH2 powders enhanced the kinetics of hydrogenation/dehydrogenation processes, as implied by the powder capability of achieving 1400 h (~60 days) of charging/discharging processes with the absence of failure, as displayed in Figure 14a. However, this long testing time led to obvious degradation in both the hydrogenation/dehydrogenation kinetics of the last cycle ( Figure 14c) when compared with the first cycle ( Figure 14b).
However, a marginal decrease (~less than 0.5 wt.% H2) in the storage capacity was detected after a compilation of 1400 h, as displayed in Figure 14a. This may be attributed to a moderate grain growth that occurred in MgH2 powders due to the long processing cycle lifetime. It was necessary to examine the morphological characteristics and local structure of powders after such a long cycle lifetime.  However, a marginal decrease (~less than 0.5 wt.% H 2 ) in the storage capacity was detected after a compilation of 1400 h, as displayed in Figure 14a. This may be attributed to a moderate grain growth that occurred in MgH 2 powders due to the long processing cycle lifetime. It was necessary to examine the morphological characteristics and local structure of powders after such a long cycle lifetime.
The FE-SEM image of the cycled powders is presented in Figure 15. The powders were composed of dark-grey Mg aggregates with a rough surface, containing numerous numbers of pores with a crater-like morphology, as displayed in Figure 15. The presence of such pores facilitated successful hydrogen absorption/desorption. Besides, the ZrC nanopowders were embedded into the surface and subsurface of Mg metal through the pores to act as a grain growth inhibitor, as shown in Figure 15.
uptake/release kinetics. The advantages of using ZrC particles as a hydrogen storage modifier for the MgH2 binary system may be attributed to the abrasion effect of ZrC nanopowders, which led to a severe reduction in MgH2 powders upon HEBM. Moreover, the ZrC nanopowders played a critical role as a grain growth inhibitor that prevented MgH2 powders from severely increasing in size, thus overcoming any undesired kinetics degradation during the long cycle lifetime.  The local structure of the nanocomposite MgH 2 /5 wt% ZrC system obtained after a cycle lifetime for 1400 h was examined with atomic-resolution FE-TEM. The HRTEM image displayed fringe nanograins related to β-MgH 2 (zone I in Figure 16a) oriented to the axis zone of (110), as displayed in Figure 16b. Besides, a ZrC nanograin (zone II in Figure 16a) was embedded into the MgH 2 matrix, as indicated by the interplanar spacing (0.236 nm), which corresponds to ZrC (200), as presented in Figure 16c. It is worth mentioning that we could not detect any reacted phase such as pure hcp-Zr, fcc-ZrH 2 , and/or MgZr phases. This may suggest that ZrC nanopowder is a typical heterogeneous catalyst, where it does not react with MgH 2 to form any intermediate phase.
Comparison of the present results indicates the capability of the MgH 2 /ZrC binary system to achieve a long cycle lifetime with excellent performance. For example, MgH 2 /Mn 2 Ti [33] and MgH 2 /10 wt% big-cube Zr 2 Ni [35] systems showed excellent performances for achieving about 1400 h; however, the process was undertaken at higher temperatures (250 • C to 275 • C) under a hydrogen pressure of 10 bar. These systems, however, had a lower hydrogen storage capacity of less than 5 wt%H 2 and suffered from slower uptake/release kinetics. The advantages of using ZrC particles as a hydrogen storage modifier for the MgH 2 binary system may be attributed to the abrasion effect of ZrC nanopowders, which led to a severe reduction in MgH 2 powders upon HEBM. Moreover, the ZrC nanopowders played a critical role as a grain growth inhibitor that prevented MgH 2 powders from severely increasing in size, thus overcoming any undesired kinetics degradation during the long cycle lifetime.

Preparations of Nanocrystalline MgH2 Powders
Elemental powders of Mg (99.9 wt%, 80 μm) were provided by Alfa Aesar (CAS Number 7439-95-4), Kandel, Germany, and high-purity (99.999 wt.%) hydrogen gas was used as starting materials. A small amount (~5 g) of the powder was sealed into a tool steel vial (150 mL), using a GST (gas-temperature-monitoring system) inside a helium (He) gas atmosphere-glove box (UNILAB Pro Glove Box Workstation, mBRAUN, Daimlerstraße 29-31, D-76316 Malsch, Germany). A quantity of 50 tool steel balls (11 mm in diameter) were used as milling media. The ball-to-powder weight ratio was 42 to 1. The vial was then pressurized with 50 bar of hydrogen. The reactive ball milling was carried out at room temperature for 25 to 50 h with a rotation speed of 250 rpm, using a planetarytype ball mill (PM400) provided by RETSCH GmbH, Berlin, Germany.

Preparations of ZrC Nanopowders
Elemental Zr (99.5 wt%, 20 μm, CAS Number7440-67-7) and graphite (99 wt.%, 20 μm, CAS Number7782-42-5) powders, provided by Alfa Aesar, Kandel, Germany, were employed as feedstock materials. The two powder species were balanced and mixed inside the glove box to obtain 5 g of equiatomic composition. The powders were then sealed in a tool steel vial (150 mL) together with 50 tool steel balls (11 mm in diameter), using a ball-to-powder weight ratio of 40 to 1. The system was mounted on the planetary-type ball mill (PM400), where the milling process started at room temperature for 25 h with a rotation speed of 250 rpm.

Preparations of Nanocrystalline MgH 2 Powders
Elemental powders of Mg (99.9 wt%, 80 µm) were provided by Alfa Aesar (CAS Number 7439-95-4), Kandel, Germany, and high-purity (99.999 wt.%) hydrogen gas was used as starting materials. A small amount (~5 g) of the powder was sealed into a tool steel vial (150 mL), using a GST (gas-temperature-monitoring system) inside a helium (He) gas atmosphere-glove box (UNILAB Pro Glove Box Workstation, mBRAUN, Daimlerstraße 29-31, D-76316 Malsch, Germany). A quantity of 50 tool steel balls (11 mm in diameter) were used as milling media. The ball-to-powder weight ratio was 42 to 1. The vial was then pressurized with 50 bar of hydrogen. The reactive ball milling was carried out at room temperature for 25 to 50 h with a rotation speed of 250 rpm, using a planetary-type ball mill (PM400) provided by RETSCH GmbH, Berlin, Germany.

Preparations of ZrC Nanopowders
Elemental Zr (99.5 wt%, 20 µm, CAS Number7440-67-7) and graphite (99 wt.%, 20 µm, CAS Number7782-42-5) powders, provided by Alfa Aesar, Kandel, Germany, were employed as feedstock materials. The two powder species were balanced and mixed inside the glove box to obtain 5 g of equiatomic composition. The powders were then sealed in a tool steel vial (150 mL) together with 50 tool steel balls (11 mm in diameter), using a ball-to-powder weight ratio of 40 to 1. The system was mounted on the planetary-type ball mill (PM400), where the milling process started at room temperature for 25 h with a rotation speed of 250 rpm.

Preparations of Nanocomposite MgH 2 /ZrC Nanopowders
The as-milled MgH 2 powders were doped with the desired mass of as-prepared ZrC to obtain three nominal compositions of MgH 2 /x wt% ZrC (x; 2, 5 and 7) inside the glove box. The three composite systems were individually charged into tool steel vials (150 mL) and sealed together with 50 tool steel balls (11 mm), using 45:1 as the ball-to-powder weight ratio. The vial was then pressurized with 50 bar of hydrogen. The mechanically induced solid-state mixing was conducted at a milling speed of 250 rpm for 25 h, using a planetary-type ball mill (PM400).

Crystal Structure and Morphology
X-ray diffraction (XRD) with CuKα radiation was employed to investigate the average crystal structure of all samples, using the 9 kW Intelligent X-ray diffraction system, provided by SmartLab-Rigaku, Kawasaki, Japan. Then, a 200 kV-field-emission high-resolution transmission electron microscope (FE-HRTEM, JEOL-2100 F), supplied by JEOL, Chiba, Japan, was used to investigate the local structure of the synthesized powders. The morphological properties of the powders were studied by a 15 kV-field-emission scanning electron microscope (FE-SEM, JSM-7800 F/EDS, Chiba, Japan). The local composition of the as-prepared samples was investigated via FE-SEM/energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, Andor, 277.3 mi, Belfast, UK).

Thermal Stability
The thermal stability of the samples was investigated by the Shimadzu Thermal Analysis System (TA-60 WS, Tokyo, Japan), using a differential scanning calorimeter (DSC) with different heating rates of 5, 10, 20, 30, and 40 • C/min. All DSC measurements were conducted under a flow (75 mi/min) of He.

Hydrogenation/Dehydrogenation Kinetics
The absorption/desorption kinetics behavior of the samples were investigated via Sievert's method, using PCTPro-2000, provided by Setaram Instrumentation, 12 Rue de Verdun, 69,300 Caluire-et-Cuire, France. An amount of 250-280 mg of the powders was handled and balanced to obtain the desired mass inside the He-glove box. The powders were then sealed into a vial made of chromium-resistance (Swagelok ® ) alloy inside the glove box. The vial was then inserted inside a portable standard autoclave Cu-clad holder. Prior to the kinetics measurements, the powders were activated at 350 • C under 35 bar of hydrogen overnight. Then, six volume calibrations were individually conducted at room temperature and at the desired measurement temperature.

Conclusions
Reactive ball milling was employed to fabricate nanocrystalline MgH 2 powders in a gas-solid reaction fashion performed under high hydrogen pressure (~50 bar) for 25 h to 50 h. The hydrogenation/dehydrogenation kinetics of the as-prepared MgH 2 powder was very slow and required the application of high temperature (above 325 • C) to absorb/desorb 6.5 wt.% H 2 within 2500 s and 7800 s, respectively. Besides, the as-prepared MgH 2 system obtained after 50 h of reactive ball milling possessed a high decomposition temperature of 376 • C at a heating rate of 20 • C/min with an apparent activation energy of 123 kJ/mol. To improve the hydrogen storage behavior of the as-prepared MgH 2 binary system, ultrafine fcc-ZrC nanopowder was prepared via the carbonization reaction between metallic Zr and graphite powders, using a room-temperature high-energy ball mill. The as-prepared ZrC powders obtained after 25 h of milling possessed good morphological properties of being an ultrafine powder (less than 50 nm in diameter) with spherical-like nanograins, ranging between 2 and 7 nm in diameter. In the present work, MgH 2 powders were doped with three individual concentrations of ZrC (2, 5 and 7 wt.%) and high-energy ball-milled for 25 h. The results showed that ZrC powders acted as micro-milling media to reduce the MgH 2 particle size to a minimal value that could not be obtained without ZrC.
After 25 h of milling, the hard ZrC nanopowders were embedded into the MgH 2 matrix to form nanocomposite MgH 2 /ZrC powders that had an homogeneous composition and fair distribution of ZrC particles beyond the nano-level. The ZrC agent led to the minimization of the decomposition temperature (287 • C) of MgH 2 and lowering in the apparent activation energy of desorption for MgH 2 to 69 kJ/mol. The hydrogenation/dehydrogenation kinetics of the nanocomposite MgH 2 /ZrC system revealed significant improvement, as indicated by the low temperature and short time required to achieve successful uptake and release processes. This system possessed a high capability of tackling a long continuous cycle lifetime (1400 h) at low temperature (225 • C) without showing serious degradation in its storage capacity.