Realizing Hydrogen De/Absorption Under Low Temperature for MgH2 by Doping Mn-Based Catalysts

Magnesium hydride (MgH2) has been considered as a potential material for storing hydrogen, but its practical application is still hindered by the kinetic and thermodynamic obstacles. Herein, Mn-based catalysts (MnCl2 and Mn) are adopted and doped into MgH2 to improve its hydrogen storage performance. The onset dehydrogenation temperatures of MnCl2 and submicron-Mn-doped MgH2 are reduced to 225 °C and 183 °C, while the un-doped MgH2 starts to release hydrogen at 315 °C. Further study reveals that 10 wt% of Mn is the better doping amount and the MgH2 + 10 wt% submicron-Mn composite can quickly release 6.6 wt% hydrogen in 8 min at 300 °C. For hydrogenation, the completely dehydrogenated composite starts to absorb hydrogen even at room temperature and almost 3.0 wt% H2 can be rehydrogenated in 30 min under 3 MPa hydrogen at 100 °C. Additionally, the activation energy of hydrogenation reaction for the modified MgH2 composite significantly decreases to 17.3 ± 0.4 kJ/mol, which is much lower than that of the primitive MgH2. Furthermore, the submicron-Mn-doped sample presents favorable cycling stability in 20 cycles, providing a good reference for designing and constructing efficient solid-state hydrogen storage systems for future application.


Sample Preparation
Powders of manganese chloride (MnCl 2 ) was purchased from Sinopharm Chemical Reagent and Mn powders was commercially purchased from Aladdin Industrial Corporation. Submicron-Mn particles were prepared by a wet-chemical ball milling method. At first, 4 g Mn powders (99.95%, Aladdin Industrial Corporation, Shanghai, China), 12 mL heptane (98.5%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), 0.6 mL oleic acid (90%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), 0.2 mL oleylamine (98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and 240 g balls were mingled in a home-made stainless steel jar under 0.1 MPa of Ar. The mixture was milled at a speed of 400 rpm for 60 h in the planetary ball mill (QM-3SP4, Nanjing, China). The treated slurry mixed with another 15 mL heptane, 1 mL oleic acid, and 1 mL oleic acid was then placed in a centrifuge tub. In addition, ethanol was used to centrifuge and wash the mixed solution eight times to remove larger particles and residual organic solvent. Finally, Mn submicron particles (submicron-Mn) can be acquired after vacuum-drying at room temperature for 12 h.
The MgH 2 used was synthesized in our laboratory. First, Mg powder (99%, 100-200 mesh, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was placed at 380 • C under the hydrogen pressure of 6.5 MPa to absorb hydrogen for 2 h. The second step was to ball-mill the processed samples at 450 rpm for 5 h. After repeating the above hydrogenation heat treatment, MgH 2 can be finally acquired.
The MgH 2 + x wt% submicron-Mn (x = 5, 10 and 15) and MgH 2 -MnCl 2 composites were prepared by mechanical ball milling at 450 rpm for 2 h under 0.1 MPa of Ar (the ball to material ratio is 40:1). In order to avoid oxidation and contamination, all samples were handled and transferred in an Ar-filled glove box (Mikrouna, Shanghai, China) where the oxygen /water concentration was kept less than 0.1 ppm.

Sample Characterization
X-ray diffraction (XRD) analyses of all samples were performed on an X'Pert Pro X-ray diffractometer (PAN alytical, Royal Dutch Philips Electronics Ltd, Amsterdam, Netherlands) with Nanomaterials 2020, 10, 1745 3 of 11 Cu K alpha radiation at 40 KV, 40 mA to detect the phase compositions. To avoid air and water contamination, a special container was adopted for transferring and scanning samples. A scanning electron microscopy (SEM, Hitachi SU-70, Tokyo, Japan) with an energy dispersive spectroscopy (EDS) was performed to further characterize morphologies and element distribution of the samples. The hydrogen absorption and desorption properties were tested in a Sieverts-type apparatus. During testing non-isothermal hydrogen desorption properties, about 75 mg sample was heated to 450 • C at a heating rate of 2 • C min −1 in a sealed stainless steel reactor. For hydrogenation, the samples were gradually heated from room temperature to 400 • C at an average rate of 1 • C min −1 under 3 MPa H 2 . For isothermal measurements, the samples were first heated up to the desired temperature and then keeping the temperature constant in the whole test. In order to get the exact values of hydrogenation capacity, the second dehydrogenation measurements were also conducted to verify the accuracy of the values. Moreover, controlling the hydrogen pressure for de/hydrogenation tests well is also important, the isothermal absorption tests were performed at various temperatures under 3 MPa while the isothermal desorption performance was tested at different temperatures under hydrogen pressure below 0.001 MPa.

Results and Discussion
To investigate the catalytic effect of MnCl 2 on the hydrogen storage properties of MgH 2 , 5 wt% MnCl 2 was ball milled with MgH 2 to prepare the MgH 2 + 5wt% MnCl 2 composite, and temperature-programmed desorption (TPD) tests were conducted from room temperature to 450 • C, the results are shown in Figure 1a. The un-doped MgH 2 began to release hydrogen from 315 • C and about 7.45 wt% hydrogen could be desorbed after the non-isothermal test. It is clear that the dehydrogenation temperature shifts to lower temperature after doping MnCl 2 powders, which started to release hydrogen at 230 • C and about 6.8 wt% hydrogen could be attained when heating up to 350 • C. XRD measurements were also used to shed light on the microstructure evolution in the desorption process. Figure 1b shows that MgH 2 still dominates the diffraction peaks for MgH 2 -MnCl 2 sample and no apparent new phases appeared during the ball-milling process, indicating that it was only a physical mixture after the ball milling process. After dehydrogenation, it is interesting that the signal of Mn phase emerged at 43 • besides Mg phase. It is much likely that MnCl 2 reacted with MgH 2 during the dehydrogenation process, just as reported in other transition metal halides-modified MgH 2 systems [42,44]. Therefore, Mn may be the key to enhance the dehydrogenation performance of MgH 2 . In order to confirm this conjecture, submicron-Mn particles were further synthesized and a series of tests were performed. Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 11 X-ray diffraction (XRD) analyses of all samples were performed on an X'Pert Pro X-ray diffractometer (PAN alytical, Royal Dutch Philips Electronics Ltd, Amsterdam, Netherlands) with Cu K alpha radiation at 40 KV, 40 mA to detect the phase compositions. To avoid air and water contamination, a special container was adopted for transferring and scanning samples. A scanning electron microscopy (SEM, Hitachi SU-70, Tokyo, Japan) with an energy dispersive spectroscopy (EDS) was performed to further characterize morphologies and element distribution of the samples. The hydrogen absorption and desorption properties were tested in a Sieverts-type apparatus. During testing non-isothermal hydrogen desorption properties, about 75 mg sample was heated to 450 °C at a heating rate of 2 °C min −1 in a sealed stainless steel reactor. For hydrogenation, the samples were gradually heated from room temperature to 400 °C at an average rate of 1 °C min −1 under 3 MPa H2. For isothermal measurements, the samples were first heated up to the desired temperature and then keeping the temperature constant in the whole test. In order to get the exact values of hydrogenation capacity, the second dehydrogenation measurements were also conducted to verify the accuracy of the values. Moreover, controlling the hydrogen pressure for de/hydrogenation tests well is also important, the isothermal absorption tests were performed at various temperatures under 3 MPa while the isothermal desorption performance was tested at different temperatures under hydrogen pressure below 0.001 MPa.

Results and Discussions
To investigate the catalytic effect of MnCl2 on the hydrogen storage properties of MgH2, 5 wt% MnCl2 was ball milled with MgH2 to prepare the MgH2 + 5wt% MnCl2 composite, and temperatureprogrammed desorption (TPD) tests were conducted from room temperature to 450 °C, the results are shown in Figure 1a. The un-doped MgH2 began to release hydrogen from 315 °C and about 7.45 wt% hydrogen could be desorbed after the non-isothermal test. It is clear that the dehydrogenation temperature shifts to lower temperature after doping MnCl2 powders, which started to release hydrogen at 230 °C and about 6.8 wt% hydrogen could be attained when heating up to 350 °C. XRD measurements were also used to shed light on the microstructure evolution in the desorption process. Figure 1b shows that MgH2 still dominates the diffraction peaks for MgH2-MnCl2 sample and no apparent new phases appeared during the ball-milling process, indicating that it was only a physical mixture after the ball milling process. After dehydrogenation, it is interesting that the signal of Mn phase emerged at 43° besides Mg phase. It is much likely that MnCl2 reacted with MgH2 during the dehydrogenation process, just as reported in other transition metal halides-modified MgH2 systems [42,44]. Therefore, Mn may be the key to enhance the dehydrogenation performance of MgH2. In order to confirm this conjecture, submicron-Mn particles were further synthesized and a series of tests were performed.   The as-prepared Mn particles were analyzed by XRD and SEM measurements, as shown in Figure 2. It can be seen from Figure 2a that the diffraction peaks in the XRD patterns of as-prepared Mn correspond well to that of the standard peaks of Mn phase (PDF#0637). Moreover, the diffraction peaks for as-prepared Mn become wider and weaker, suggesting a decrease in crystallite size under the effect of ball-milling. The SEM images from Figure 2b-d shows that the purchased Mn powders present the appearance of micrometer range particles while the particle size of as-prepared submicron-Mn ranged from 500 nm to 800 nm, which could be defined as submicron particles. Combing XRD results with SEM pictures, it can be concluded that submicron-Mn particles were successfully synthesized and a great catalytic effect was expected on the hydrogen storage properties of MgH 2 .
Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 11 The as-prepared Mn particles were analyzed by XRD and SEM measurements, as shown in Figure 2. It can be seen from Figure 2a that the diffraction peaks in the XRD patterns of as-prepared Mn correspond well to that of the standard peaks of Mn phase (PDF#0637). Moreover, the diffraction peaks for as-prepared Mn become wider and weaker, suggesting a decrease in crystallite size under the effect of ball-milling. The SEM images from Figure 2b-d shows that the purchased Mn powders present the appearance of micrometer range particles while the particle size of as-prepared submicron-Mn ranged from 500 nm to 800 nm, which could be defined as submicron particles. Combing XRD results with SEM pictures, it can be concluded that submicron-Mn particles were successfully synthesized and a great catalytic effect was expected on the hydrogen storage properties of MgH2. To witness the modification impact of the as-synthesized Mn submicron particles on MgH2, isothermal and non-isothermal dehydrogenation tests were operated. As a comparison, the purchased Mn was also doped into MgH2. The non-isothermal dehydrogenation curves in Figure 3a depicted that the onset temperatures of MgH2 + 5wt% Mn composite was 225 °C, 5 °C, and 90 °C lower than that of MgH2 + 5wt% MnCl2 composite and additive-free MgH2, respectively. Just as expected, after submicron-Mn was doped to MgH2, MgH2 + 5wt% submicron-Mn composite began to release hydrogen at 183 °C, superior to purchased Mn and MnCl2. In order to figure out the best doping amount, different amounts of submicron-Mn were ball-milled with MgH2 and further isothermal and non-isothermal measurements were conducted on the MgH2 + submicron-Mn composites. It could be clearly seen from Figure 3b that the volumetric release curves of submicron-Mn modified samples shifted toward lower temperatures with the increasing doping amount. The MgH2 + 5 wt% submicron-Mn composite possessed onset dehydrogenation temperatures of 183 °C, about 132 °C lower than that of prepared MgH2. As for the MgH2 + 10 wt% submicron-Mn and MgH2 + 15 wt% submicron-Mn composites, the onset desorption temperatures further decreased to 175 °C and 165 °C, respectively. When the temperature rose to 350 °C, about 6.8 wt%, 6.5 wt%, and 6.1 wt% H2 could be obtained for the MgH2 + 5 wt% submicron-Mn, MgH2 + 10 wt% submicron-Mn, and MgH2 To witness the modification impact of the as-synthesized Mn submicron particles on MgH 2 , isothermal and non-isothermal dehydrogenation tests were operated. As a comparison, the purchased Mn was also doped into MgH 2 . The non-isothermal dehydrogenation curves in Figure 3a depicted that the onset temperatures of MgH 2 + 5wt% Mn composite was 225 • C, 5 • C, and 90 • C lower than that of MgH 2 + 5wt% MnCl 2 composite and additive-free MgH 2 , respectively. Just as expected, after submicron-Mn was doped to MgH 2 , MgH 2 + 5wt% submicron-Mn composite began to release hydrogen at 183 • C, superior to purchased Mn and MnCl 2 . In order to figure out the best doping amount, different amounts of submicron-Mn were ball-milled with MgH 2 and further isothermal and non-isothermal measurements were conducted on the MgH 2 + submicron-Mn composites. It could be clearly seen from Figure 3b that the volumetric release curves of submicron-Mn modified samples shifted toward lower temperatures with the increasing doping amount. The MgH 2 + 5 wt% submicron-Mn composite possessed onset dehydrogenation temperatures of 183 • C, about 132 • C lower than that of prepared MgH 2 . As for the MgH 2 + 10 wt% submicron-Mn and MgH 2 + 15 wt% submicron-Mn composites, the onset desorption temperatures further decreased to 175 • C and 165 • C, respectively. When the temperature rose to 350 • C, about 6.8 wt%, 6.5 wt%, and 6.1 wt% H 2 could be obtained for the MgH 2 + 5 wt% submicron-Mn, MgH 2 + 10 wt% submicron-Mn, and MgH 2 + 15 wt% submicron-Mn samples, respectively. Further isothermal dehydrogenation measurements of the above three samples are performed at 275 • C. Figure 3c shows that only 4.7 wt% of H 2 was desorbed in the first 10 min for the MgH 2 + 5 wt% submicron-Mn composite. For the MgH 2 + 10 wt% submicron-Mn and the MgH 2 + 15 wt% submicron-Mn samples, the values increased to 6.1 wt% and 6.0 wt%. On the contrary, the pristine MgH 2 sample could hardly release hydrogen under the same condition.
submicron-Mn and the MgH2 + 15 wt% submicron-Mn samples, the values increased to 6.1 wt% and 6.0 wt%. On the contrary, the pristine MgH2 sample could hardly release hydrogen under the same condition.
According to the results of TPD curves, it could be concluded that the addition of submicron-Mn could remarkably improve the hydrogen desorption kinetics of MgH2. Moreover, the initial dehydrogenation temperature did not decrease obviously after increasing the doping amount of catalyst. From a comprehensive perspective of the dehydrogenation temperature and capacity, the MgH2 + 10 wt% submicron-Mn was chosen for further study. Figure 3d presented the isothermal dehydrogenation curves of MgH2 + 10 wt% submicron-Mn composite at 250 °C, 275 °C, and 300 °C, respectively. The MgH2 + 10 wt% submicron-Mn composite could quickly release 6.6 wt% hydrogen in 8 min at 300 °C (almost 96.5% of theoretical hydrogen storage capacity). At 275 °C, this sample could desorb the same amount hydrogen with 20 min. Furthermore, about 6.0 wt% H2 could be acquired at 250 °C. Besides the significantly improved dehydrogenation properties, hydrogen absorption kinetics of MgH2+ submicron-Mn composites were also investigated. As shown in Figure 4, the isothermal and non-isothermal hydrogenation tests were performed. From Figure 4a, it can be seen that the dehydrogenated MgH2 + 10 wt% submicron-Mn sample could absorb H2 even at room temperature and about 5.5 wt% hydrogen could be re-absorbed when heating up to 250 °C. However, the dehydrogenated MgH2 sample sluggishly took up hydrogen from 186 °C. The hydrogen absorption curves of MgH2 + 10 wt% submicron-Mn at relatively low temperature were also performed, shown in Figure 4b. Even at a low temperature of 50 °C, the dehydrogenated MgH2 + 10 wt% submicron-Mn sample still absorbed 1.8 wt% hydrogen within 40 min. When the temperature went up to 75 °C, the According to the results of TPD curves, it could be concluded that the addition of submicron-Mn could remarkably improve the hydrogen desorption kinetics of MgH 2 . Moreover, the initial dehydrogenation temperature did not decrease obviously after increasing the doping amount of catalyst. From a comprehensive perspective of the dehydrogenation temperature and capacity, the MgH 2 + 10 wt% submicron-Mn was chosen for further study. Figure 3d presented the isothermal dehydrogenation curves of MgH 2 + 10 wt% submicron-Mn composite at 250 • C, 275 • C, and 300 • C, respectively. The MgH 2 + 10 wt% submicron-Mn composite could quickly release 6.6 wt% hydrogen in 8 min at 300 • C (almost 96.5% of theoretical hydrogen storage capacity). At 275 • C, this sample could desorb the same amount hydrogen with 20 min. Furthermore, about 6.0 wt% H 2 could be acquired at 250 • C.
Besides the significantly improved dehydrogenation properties, hydrogen absorption kinetics of MgH 2 + submicron-Mn composites were also investigated. As shown in Figure 4, the isothermal and non-isothermal hydrogenation tests were performed. From Figure 4a, it can be seen that the dehydrogenated MgH 2 + 10 wt% submicron-Mn sample could absorb H 2 even at room temperature and about 5.5 wt% hydrogen could be re-absorbed when heating up to 250 • C. However, the dehydrogenated MgH 2 sample sluggishly took up hydrogen from 186 • C. The hydrogen absorption curves of MgH 2 + 10 wt% submicron-Mn at relatively low temperature were also performed, shown in Figure 4b. Even at a low temperature of 50 • C, the dehydrogenated MgH 2 + 10 wt% submicron-Mn sample still absorbed 1.8 wt% hydrogen within 40 min. When the temperature went up to 75 • C, the hydrogen uptake of the submicron-Mn-doped sample amounted to 2.3 wt% under the same condition. After hydrogenation at 100 • C for 40 min, the fully dehydrogenated composite could absorb 3.2 wt% hydrogen. As a comparison, non-isothermal hydrogenation measurement of MgH 2 sample were also conducted (Figure 4c). For the dehydrogenated MgH 2 sample, only 2.6 wt% H 2 was absorbed even at 210 • C within 30 min.
where α is the fraction of Mg converted to MgH2 with time, k is an effective kinetic parameter, and n is the Avrami exponent. The numerical values of n and nlnk carried out by fitting the JMAK plots are shown in Figure S1. In accordance with the curves shown in Figure 4d, the calculated Ea value of the hydrogenation process for the dehydrogenated MgH2 was calculated to be 72.5 ± 2.7 kJ/mol, while the value was reduced to 17.3 ± 0.4 kJ/mol for the dehydrogenated MgH2 + 10 wt% submicron-Mn sample. The greatly reduced activation energy also indicates that the energy barrier for hydrogenation was distinctly decreased after the addition of submicron-Mn, which well explains the evidently improved hydrogenation kinetics of the MgH2 + 10 wt% submicron-Mn sample.  In addition, the Ea values of the hydrogenation reaction were calculated to further explore the improved kinetics of hydrogenation for MgH 2 + 10 wt% submicron-Mn sample. Some kinetic models such as Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for the gas-solid reaction were adopted to simulate the evolution of kinetics [46,47]. Figure 4d depicts the JMAK model through fitting the absorption curves and the Ea values for the hydrogenation reactions were finally calculated according to Arrhenius equation [48].
where α is the fraction of Mg converted to MgH 2 with time, k is an effective kinetic parameter, and n is the Avrami exponent. The numerical values of n and nlnk carried out by fitting the JMAK plots are shown in Figure S1. In accordance with the curves shown in Figure 4d, the calculated Ea value of the hydrogenation process for the dehydrogenated MgH 2 was calculated to be 72.5 ± 2.7 kJ/mol, while the value was reduced to 17.3 ± 0.4 kJ/mol for the dehydrogenated MgH 2 + 10 wt% submicron-Mn sample. The greatly reduced activation energy also indicates that the energy barrier for hydrogenation was distinctly decreased after the addition of submicron-Mn, which well explains the evidently improved hydrogenation kinetics of the MgH 2 + 10 wt% submicron-Mn sample.
Although doping submicron-Mn into MgH 2 has shown great improvement in the reversible hydrogen storage properties, the catalytic mechanism of submicron-Mn in modifying MgH 2 remained unknown. To further elucidate the hydrogen de/absorption mechanism, XRD tests of the MgH 2 + 10 wt% submicron-Mn sample in ball-milled, dehydrogenated, and rehydrogenated state were performed. In the ball-milled state (Figure 5a), MgH 2 phases still dominated the XRD pattern while the diffraction peaks of doped submicron-Mn were also found at around 43 • . Interestingly, the XRD results demonstrated that the submicron-Mn was stable and persistently acted as an active substance during the process of de/hydrogenation. After dehydrogenation (Figure 5b) and 20th rehydrogenation (Figure 5c), the primary phase transformation during cycling is the transformation between Mg and MgH 2 . However, the diffraction peak of Mn which could be found at 43 • was stable after 20 cycles and no other phases of Mn-related composites occurred, illuminating that Mn was the active catalyst to enhance the hydrogen storage properties. Although doping submicron-Mn into MgH2 has shown great improvement in the reversible hydrogen storage properties, the catalytic mechanism of submicron-Mn in modifying MgH2 remained unknown. To further elucidate the hydrogen de/absorption mechanism, XRD tests of the MgH2 + 10 wt% submicron-Mn sample in ball-milled, dehydrogenated, and rehydrogenated state were performed. In the ball-milled state (Figure 5a), MgH2 phases still dominated the XRD pattern while the diffraction peaks of doped submicron-Mn were also found at around 43°. Interestingly, the XRD results demonstrated that the submicron-Mn was stable and persistently acted as an active substance during the process of de/hydrogenation. After dehydrogenation (Figure 5b) and 20th rehydrogenation (Figure 5c), the primary phase transformation during cycling is the transformation between Mg and MgH2. However, the diffraction peak of Mn which could be found at 43 ° was stable after 20 cycles and no other phases of Mn-related composites occurred, illuminating that Mn was the active catalyst to enhance the hydrogen storage properties. In order to realize the practical application of hydrogen energy, preserving long-term kinetics is considered as one of the important indexes to evaluate the practicability for hydrogen storage materials. In this case, as shown in Figure 6, cycling tests of the MgH2 + 10 wt% submicron-Mn composite were further operated under the conditions of isothermal dehydrogenation and hydrogenation at 275 °C. As revealed in this pattern, the MgH2 + 10 wt% submicron-Mn sample could acquire a hydrogen capacity of 6.46 wt% in the first desorption. When exposed to hydrogen atmosphere of 3 MPa, the dehydrogenated sample quickly absorbed 5.94 wt% hydrogen. After 20 cycles, this composite could also release 5.72 wt% H2 (almost 89% of the original capacity). Compared with our previous study [29], the cycling property was better than that of MgH2 + nano-Fe samples, which had an evident decrease in the first 20 cycles. In general, the degenerating cycling properties caused by that MgH2 particles tend to grow and aggregate during the process of thermolysis [9,49]. Thus, although the addition of submicron-Mn can significantly enhance the de/hydrogenation performance of MgH2; other technics should still be explored to improve the cycling properties. In order to realize the practical application of hydrogen energy, preserving long-term kinetics is considered as one of the important indexes to evaluate the practicability for hydrogen storage materials. In this case, as shown in Figure 6, cycling tests of the MgH 2 + 10 wt% submicron-Mn composite were further operated under the conditions of isothermal dehydrogenation and hydrogenation at 275 • C. As revealed in this pattern, the MgH 2 + 10 wt% submicron-Mn sample could acquire a hydrogen capacity of 6.46 wt% in the first desorption. When exposed to hydrogen atmosphere of 3 MPa, the dehydrogenated sample quickly absorbed 5.94 wt% hydrogen. After 20 cycles, this composite could also release 5.72 wt% H 2 (almost 89% of the original capacity). Compared with our previous study [29], the cycling property was better than that of MgH 2 + nano-Fe samples, which had an evident decrease in the first 20 cycles. In general, the degenerating cycling properties caused by that MgH 2 particles tend to grow and aggregate during the process of thermolysis [9,49]. Thus, although the addition of submicron-Mn can significantly enhance the de/hydrogenation performance of MgH 2 ; other technics should still be explored to improve the cycling properties. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 11 Figure 6. Isothermal dehydrogenation/hydrogenation cyclic kinetics curves of the MgH2 + 10 wt% submicron-Mn sample.

Conclusions
In summary, MnCl2 and Mn particles were doped as catalysts into MgH2 to improve its hydrogen storage properties and the submicron-Mn particles exhibited superior catalytic effect. The MgH2 + 10 wt% submicron-Mn composite started to release hydrogen at 175 °C and about 6.6 wt% hydrogen could be obtained within 8 min at 300 °C. For absorption performance, the dehydrogenated MgH2 + 10 wt% submicron-Mn sample began to absorb H2 at room temperature the completely dehydrogenated sample could assimilate 3.0 wt% H2 within 30 min under 100 °C while the dehydrogenated MgH2 needed a high temperature of 210 °C to absorb the same amount of H2. Besides, the Ea of hydrogen absorption of MgH2 was reduced to 17.3 ± 0.4 kJ/mol because of the addition of submicron-Mn. Moreover, the MgH2 + 10 wt% nano-Mn composite exhibited good cycling performance that 89% of initial hydrogen could still be maintained after 20 cycles.

Conclusions
In summary, MnCl 2 and Mn particles were doped as catalysts into MgH 2 to improve its hydrogen storage properties and the submicron-Mn particles exhibited superior catalytic effect. The MgH 2 + 10 wt% submicron-Mn composite started to release hydrogen at 175 • C and about 6.6 wt% hydrogen could be obtained within 8 min at 300 • C. For absorption performance, the dehydrogenated MgH 2 + 10 wt% submicron-Mn sample began to absorb H 2 at room temperature the completely dehydrogenated sample could assimilate 3.0 wt% H 2 within 30 min under 100 • C while the dehydrogenated MgH 2 needed a high temperature of 210 • C to absorb the same amount of H 2 . Besides, the E a of hydrogen absorption of MgH 2 was reduced to 17.3 ± 0.4 kJ/mol because of the addition of submicron-Mn. Moreover, the MgH 2 + 10 wt% nano-Mn composite exhibited good cycling performance that 89% of initial hydrogen could still be maintained after 20 cycles.