Undoped MgH2 Research Articles

Inclusion of CoTiO3 to ameliorate the re/dehydrogenation properties of the Mg–Na–Al system

For the first time, the MgH2–NaAlH4 (ratio 4:1) destabilized system with CoTiO3 addition has been explored. The CoTiO3-doped MgH2–NaAlH4 sample begins to dehydrogenate at 130 °C, which is declined by 40 °C compared to the undoped MgH2–NaAlH4. Moreover, the de/rehydrogenation kinetics characteristics of the CoTiO3-doped MgH2–NaAlH4 were greatly ameliorated. With the inclusion of CoTiO3, the MgH2–NaAlH4 composite absorbed 5.2 wt.% H2, higher than undoped MgH2–NaAlH4. In the context of dehydrogenation, the CoTiO3-doped MgH2–NaAlH4 sample desorbed 2.6 wt.% H2, almost doubled compared to the amount of hydrogen desorbed from the undoped MgH2–NaAlH4 sample. The activation energy obtained by the Kissinger analysis for MgH2 decomposition was significantly lower by 35.9 kJ/mol than the undoped MgH2–NaAlH4 sample. The reaction mechanism demonstrated that new phases of MgCo and AlTi3 were generated in situ during the heating process and are likely to play a substantial catalytic function and be useful in ameliorating the de/rehydrogenation properties of the destabilized MgH2–NaAlH4 system with the inclusion of CoTiO3.

Exceptional catalytic effect of novel rGO-supported Ni-Nb nanocomposite on the hydrogen storage properties of MgH2

The design of an excellent active catalyst to improve the sluggish kinetic and thermodynamic properties of magnesium hydride (MgH2) remains a great challenge to achieve its practical application. In this study, a novel Ni-Nb/rGO nanocomposite catalyst was successfully prepared by one-spot hydrothermal and subsequent calcination methods. The novel Ni-Nb/rGO nanocomposite exhibits an exceptional catalytic effect on improving MgH2 sorption properties. Specifically, the onset desorption temperature of MgH2 + 10 wt% Ni-Nb/rGO composite is reduced to 198 °C, much lower than that of undoped MgH2 (330 °C). Interestingly, the composite can release 5.0, 5.9, and 6.0 wt% H2 within 10 min at 245, 260, and 275 °C, respectively. Furthermore, the dehydrogenated MgH2 + 10 wt% Ni-Nb/rGO composite starts to absorb hydrogen even at room temperature with approximate 2.75 wt% H2 uptake at 75 °C under 3 MPa H2 pressure within 30 min and exhibits excellent stability by maintaining 6.0 wt% hydrogen content after 20 cycles at 300 °C. Chou's model suggests that the de/hydrogenation kinetics of Ni-Nb/rGO-modified MgH2 switches from surface penetration model to diffusion model at lower temperatures. Additionally, the activation energies (Ea) for the de/hydrogenation of MgH2 + 10 wt% Ni-Nb/rGO are reduced to 57.8 kJ/mol and 33.9 kJ/mol, which are significantly lower than those of undoped MgH2. The work demonstrates that the addition of a novel ternary Ni-Nb/rGO catalyst is an effective strategy to not only boost the sorption kinetics of MgH2 but also maintain its cycling property.

Carbon composite support improving catalytic effect of NbC nanoparticles on the low-temperature hydrogen storage performance of MgH2

Ultrafine carbon-based transition metal compounds have been widely investigated as efficient catalysts for enhancing the hydrogen storage performance of magnesium hydride. In this work, the carbon thermal shock method is applied to synthesize the ultrafine carbon-encapsulated NbC nanoparticles with an average grain size of 17.3 nm. The MgH2–10 wt% NbC/C composites show excellent low-temperature hydrogen storage performance with the onset dehydrogenation temperature of 196.1 °C, which is 92.2 °C and 98 °C lower than that of MgH2–10 wt% NbC and undoped MgH2, respectively. Specifically, MgH2–10 wt% NbC/C can absorb 6.71 wt% H2 at 100 °C within 30 min around and retain almost 100% reversible hydrogen desorption capacity after 10 cycles. For the catalytic mechanism, the electron transfer process between multi-valence Nb cations of in-situ formed NbHx and Mg, H atoms can greatly improve the cyclic de/rehydrogenation kinetics of MgH2-NbC/C. Besides, the enhancement of dehydrogenation kinetics can also be ascribed to MgH2 particle refinement by NbC nanoparticles, and destabilization of the Mg-H bond caused by carbon substrate. This investigation not only proves that carbon-encapsulated NbC nanoparticles can greatly enhance the hydrogen storage performance of MgH2 but provides an idea of preparing carbon-based transition metal carbides as effective catalysts for magnesium-based hydrogen storage materials.

Boosting the hydrogen storage performance of magnesium hydride with metal organic framework-derived Cobalt@Nickel oxide bimetallic catalyst

In this study, a MOF-derived bimetallic [email protected] catalyst was synthesized and doped into MgH2 to improve the hydrogen desorption and resorption kinetics. The [email protected] catalyst decreased the onset dehydrogenation temperature of MgH2 by 160 °C, compared with un-doped MgH2. The MgH2 + 9% (mass) [email protected] composite released 6.6% (mass) hydrogen in 350 s at 315 °C and uptook 5.4% (mass) hydrogen in 500 s at 165 °C, showing greatly accelerated de/rehydrogenation rates. Besides, the desorption activation energy of MgH2 + 9% (mass) [email protected] was decreased to (93.8 ± 8.4) kJ·mol−1. Noteworthy, symbiotic Mg2NiH4/Mg2CoH5 clusters were in-situ formed from bimetallic precursors and inlaid on MgH2 surface, which are considered as “multi-step hydrogen pumps”, and provides surface pathways for hydrogen absorption. Meanwhile, the introduced Mg2NiH4/Mg2CoH5 interfaces could provide numerous low energy barrier H diffusion channels, therefore accelerating the hydrogen release and uptake. This research proposes new insights to design high-efficiency bimetallic catalyst for MgH2 hydrogen storage.

Density Functional Theory studies of structural, electronic and optical properties of Fluorine Doped Magnesium Hydride

In order to overcome the disadvantages of magnesium hydride (MgH2) towards its applications in solar cell science and technology, doping with non-metals such as Fluorine (F) doping is a promising approach to tune its large band gap. In order to expose the hidden potential in F doped MgH2, details analysis of electronic and optical absorptions is needed. Theoretical calculations of structural, electronic and optical properties of F doped MgH2 are studied using first-principles approach within density functional theory (DFT) framework. The calculated lattice constants with PBE-GGA are in better agreement with experimental result. The bandgap value of 3.34 eV for the undoped MgH2 is close to experimental value. When one atom of F is introduced into MgH2 at Mg site, the doping effects modified the band gap from 3.34 to 2.72 eV. Also, by introducing one atom of F to H site, the band gap value reduced to 1.59 eV. Our findings confirmed that non-metal doping narrow the energy band gap of semiconductor materials. The results of optical absorptions indicate that F doped MgH2 at H has strong absorption behavior in the visible light frequency, which depicts its suitability for solar cell applications.

Hydrogen release mechanisms of MgH2 over NiN4-embedded graphene nanosheet: First-principles calculations

In this study, first-principles calculations were performed to investigate the catalytic effect of NiN4-G on the dehydrogenation of MgH2. Side-on MgH2 can be adsorbed stably on the NiN4-G monolayer and is preferentially adsorbed on the NiN4 site compared with the graphene site. The hydrogen desorption process, in which MgH2 dissociated to the Mg atom on the NiN4 site or graphene site and an H2 molecule in the vacuum, should overcome lower barriers than pure MgH2. This is because the corresponding Mg–H bond is weakened owing to the electron transfer between the Mg atom and the substrate. The hydrogen desorption enthalpies of the (MgH2)5 cluster on the NiN4 active and graphene sites are significantly smaller (0.11 eV and 1.50 eV, respectively) when H2+H2 is released from the cluster compared with those of the undoped MgH2 cluster (2.48 eV). Therefore, the NiN4-G monolayer can provide the double effect of the NiN4 active and graphene sites on improving the dehydrogenation performance of MgH2.

Influence of Nanosized CoTiO3 Synthesized via a Solid-State Method on the Hydrogen Storage Behavior of MgH2.

Magnesium hydride (MgH2) has received outstanding attention as a safe and efficient material to store hydrogen because of its 7.6 wt.% hydrogen content and excellent reversibility. Nevertheless, the application of MgH2 is obstructed by its unfavorable thermodynamic stability and sluggish sorption kinetic. To overcome these drawbacks, ball milling MgH2 is vital in reducing the particle size that contribute to the reduction of the decomposition temperature. However, the milling process would become inefficient in reducing particle sizes when equilibrium between cold-welding and fracturing is achieved. Therefore, to further ameliorate the performance of MgH2, nanosized cobalt titanate (CoTiO3) has been synthesized using a solid-state method and was introduced to the MgH2 system. The different weight percentages of CoTiO3 were doped to the MgH2 system, and their catalytic function on the performance of MgH2 was scrutinized in this study. The MgH2 + 10 wt.% CoTiO3 composite presents the most outstanding performance, where the initial decomposition temperature of MgH2 can be downshifted to 275 °C. Moreover, the MgH2 + 10 wt.% CoTiO3 absorbed 6.4 wt.% H2 at low temperature (200 °C) in only 10 min and rapidly releases 2.3 wt.% H2 in the first 10 min, demonstrating a 23-times-faster desorption rate than as-milled MgH2 at 300 °C. The desorption activation energy of the 10 wt.% CoTiO3-doped MgH2 sample was dramatically lowered by 30.4 kJ/mol compared to undoped MgH2. The enhanced performance of the MgH2–CoTiO3 system is believed to be due to the in situ formation of MgTiO3, CoMg2, CoTi2, and MgO during the heating process, which offer a notable impact on the behavior of MgH2.

Enhanced hydrogen sorption kinetics of MgH2 catalyzed by a novel layered Ni/Al2O3 hybrid

Metal-containing catalysts have been successfully applied in the Mg-based hydrogen storage materials. Here, a layered Ni/Al2O3 obtained by calcination of NiAl-layered double hydroxide (NiAl-LDH) is firstly reported for enhancing hydrogen sorption kinetics of MgH2. The hybrid catalyst of Ni/Al2O3 with high stability shows better catalytic performance than the corresponding complex oxides: the onset dehydrogenation temperature of MgH2-Ni/Al2O3 (190 °C) is much lower than that of MgH2-NiO/Al2O3 (240 °C) and the as-milled MgH2 (298 °C), and the MgH2-5 wt% Ni/Al2O3 desorbs about 5.6 wt% hydrogen within 3500 s at 275 °C, while only 4.6 wt% hydrogen is released for MgH2-5 wt% NiO/Al2O3 and the undoped MgH2 can hardly dehydrogenate at the same conditions. The catalytic mechanism of Ni/Al2O3 was investigated in detail by combining the experiment and density functional theory (DFT) calculations. The enhanced hydrogen desorption kinetics is mainly due to the destabilizing effect of MgH2 caused by Ni/Al2O3 acting as electron acceptors to capture the electrons in the Mg−H bond, resulting in a weakening of the bond strength. This study might be of interest for designing high-efficiency metal-metal oxide hybrid catalysts applied in the field of hydrogen storage.

Study of the Hydrogen Storage Properties and Catalytic Mechanism of a MgH2–Na3AlH6 System Incorporating FeCl3

In this work, the catalytic effects of FeCl3 toward the hydrogen storage properties of the MgH2–Na3AlH6 composite were investigated for the first time. The temperature-programed desorption results indicated that the onset temperature of the hydrogen release of a 10 wt % FeCl3-doped MgH2–Na3AlH6 composite was ∼30 °C lower than that of the undoped MgH2–Na3AlH6 composite. The addition of FeCl3 into the MgH2–Na3AlH6 composite resulted in improved absorption and desorption kinetics performance. The absorption/desorption kinetics measurements at 320 °C (under 33 and 1 atm hydrogen pressure, respectively) indicated that within 10 min, the doped sample absorbed ∼4.0 wt % and desorbed ∼1.5 wt % hydrogen. By comparison, the undoped sample absorbed only ∼2.1 wt % and desorbed only ∼0.6 wt % hydrogen under the same conditions and time. Comparably, the apparent activation energy value of the doped composite is 128 kJ/mol, which is 12 kJ/mol lower than that of the undoped composite (140 kJ/mol). The formation of the new species of MgCl2 and Fe in the doped composite was detected from X-ray diffraction analysis after heating and absorption processes. These two components were believed to play a vital role in reducing the decomposition temperature and kinetics enhancement of the MgH2–Na3AlH6 composite.

Effect of adding different percentages of HfCl4 on the hydrogen storage properties of MgH2

Magnesium hydride (MgH2) is the best candidate material to store hydrogen in the solid-state form owing to its advantages such as good reversibility, high hydrogen storage capacity (7.6 wt%), low raw material cost and abundance in the earth. Nevertheless, slow desorption/absorption kinetics and high thermodynamic stability are two issues that have constrained the commercialization of MgH2 as a solid-state hydrogen storage material. So, to boost the desorption/absorption kinetics and to alter the thermodynamics of MgH2, hafnium tetrachloride (HfCl4) was used as a catalyst in this study. Different percentages of HfCl4 (5, 10, 15 and 20 wt%) were added to MgH2 and their catalytic influences on the hydrogen storage properties of MgH2 were investigated. Results showed that the 15 wt% HfCl4-doped MgH2 sample was the best composite to enhance the hydrogen storage performance of MgH2. The onset decomposition temperature of the 15 wt% HfCl4-doped MgH2 composite was decreased by ~75 °C compared to as-milled MgH2. Meanwhile, the desorption/absorption kinetic measurements showed an improvement compared to the undoped MgH2. From the Kissinger analysis, the apparent dehydrogenation activation energy was 167.0 kJ/mol for undoped MgH2 and 102.0 kJ/mol for 15 wt% HfCl4-doped MgH2. This shows that the HfCl4 addition reduced the activation energy of the hydrogen decomposition of MgH2. The desorption enthalpy change calculated by the van't Hoff equation showed that the addition of HfCl4 to MgH2 did not affect the thermodynamic properties. Scanning electron microscopy showed that the size of the MgH2 particles decreased and there was less agglomeration after the addition of HfCl4. It is believed that the decrease in the particle size and in-situ generated MgCl2 and Hf-containing species had synergistic catalytic effects on enhancing the hydrogen storage properties of the HfCl4-doped MgH2 composite.

Highly efficient ZrH2 nanocatalyst for the superior hydrogenation kinetics of magnesium hydride under moderate conditions: Investigation and mechanistic insights

Magnesium hydride (MgH2) has attracted extensive attention over the past decades due to its highly reversible hydrogen storage density (7.6 wt%). But intrinsic high activation energy barriers for de/rehydrogenation of MgH2/Mg restrict its potential application as an on-board hydrogen carrier. Herein, ZrH2 nanocatalyst homogeneously distributed on the surfaces of MgH2 exhibits pronounced low temperature de/rehydrogenation kinetics, resulting from the vast lattice distortions between ZrH2 and MgH2/Mg phases. The ZrH2 doped MgH2 composite (MgH2-ZrH2) starts to release H2 at 203 °C, 102 °C lower than that of undoped MgH2. Most importantly, MgH2-ZrH2 quickly absorbs 5.90 wt% H2 at the temperature as low as 65 °C. Further deep investigations found that H atoms preferentially accumulates around ZrH2, since the lower hydrogen dissociation energy on the surfaces of ZrH2 (1.421 eV) than that of pure Mg (2.301 eV). Then, the large lattice disordered zone between ZrH2 and Mg phases enables H atoms diffuse easily into the lattice of Mg nearby, thus triggering the formation of MgH2. These findings are critically important to understand the mechanism of other metal hydride catalysts, as well as offer an opportunity for the practical on-board fuel cell applications in the future.

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.

Self-templated carbon enhancing catalytic effect of ZrO2 nanoparticles on the excellent dehydrogenation kinetics of MgH2

Development of highly effective carbon-based multifunctional catalysts for improvement of light metal hydrogen storage materials is of great significance. Herein, a novel self-template method is proposed to synthesize carbon-encapsulated ZrO2 (5–10 nm) nanoparticles (ZrO2/C). It is found that carbon could highly improve the catalysis of ZrO2 towards MgH2 than that of ZrO2 without C. Specifically, MgH2–ZrO2/C composite remarkably reduces the onset operating temperature to 208 °C, much lower than that of MgH2–ZrO2 (248 °C) and undoped MgH2 (309 °C). Furthermore, DFT calculations were applied to elucidate the detailed catalysis mechanism of C in ZrO2/C. The results demonstrated that for the MgH2–ZrO2/C sample, Zr atoms can weaken the Mg–H bonding strength. When Mg–H bond is broken, C substrates easily “pull” the electrons from H− atoms and eliminate the bound effect of Zr on H−, thus leading to the lower hydrogen desorption energy. This work provides clear evidence to understand the synergistic catalysis of ZrO2 and C, and offers a new strategy to synthesize carbon-encapsulated metal oxides with efficient catalysis towards light metal hydrogen storage materials.

MoSe2 hollow nanospheres decorated with FeNi3 nanoparticles for enhancing the hydrogen storage properties of MgH2

The FeNi3, MoSe2 and MoSe2@FeNi3 hollow nano-spheres synthesized by wet chemical method are used for the first time to improve the hydrogen storage properties of the MgH2. The results show that they all possess excellent catalytic activity in the hydrogen desorption/sorption reactions of MgH2. Because of the synergistic catalytic effect of MoSe2 and FeNi3, the MoSe2@FeNi3 has the most outstanding catalytic effect. In detail, the onset dehydrogenation temperature of MgH2 is reduced from 310 °C to 194 °C, and its maximum dehydrogenation rate is increased by 19.8 times by the introduction of 10 wt% MoSe2@FeNi3. Furthermore, the 10 wt% MoSe2@FeNi3-doped MgH2 sample, 5.8 wt% H2 can be absorbed in 0.5 min at 150 °C. However, the un-doped MgH2 sample hardly absorbs hydrogen under the same conditions. The onset hydrogen absorption temperature of the composite is only 73 °C. More importantly, it has an excellent cycling stability. These improvements are because the in situ formed MgSe, Mo, Mg2Ni and Fe can serve as active species, and they as well as their homogeneous distribution on MgH2 matrix can remain stable during dehydrogenation/hydrogenation. In addition, the presence of Mg2Ni can convert MgH2 to an easier pathway of hydrogen absorption/desorption. These help to facilitate the hydrogen storage performance of MgH2.

Effect of nano-structured Ta2C on hydrogen absorption−desorption kinetics of Mg−H2 system

High purity Ta2C was successfully prepared and the hydrogen absorption−desorption kinetic properties of MgH2−10 wt% Ta2C composites were investigated systematically. It was found that the hydrogen absorption of Mg−10 wt% Ta2C (20 nm) composite takes about 5 min to reach saturation at 573 K, and its hydride fully desorbs hydrogen within 15 min at 623 K. These kinetic properties are much better than those of the undoped Mg and MgH2 prepared under the same condition, respectively. The improvement in the hydrogen storage kinetics is ascribed to the catalytic effect of Ta2C and its inhibition role in crystallite growth.

The hydrogen storage properties and catalytic mechanism of the CuFe2O4-doped MgH2 composite system

The influence of CuFe2O4 addition on the sorption performances of MgH2 prepared by ball milling was studied for the first time. The MgH2 + 10 wt% CuFe2O4 sample exhibited an enhancement in hydrogen storage performance compared to that of as-milled MgH2, with the onset decomposition temperature decreased from 340 °C to 250 °C. Dehydrogenation kinetic result revealed that CuFe2O4-added MgH2 released around 5.3 wt% H2 within 10 min at 320 °C, while the as-milled MgH2 released below 1.0 wt% H2 under the same condition. Furthermore, about 5.0 wt% H2 was absorbed at 250 °C in 30 min for the 10 wt% CuFe2O4-doped MgH2 sample. In contrast, the un-doped MgH2 only absorbed 4.0 wt% H2 at 250 °C in 30 min. From the Kissinger analysis, the apparent activation energy of as-milled MgH2 was 166.0 kJ/mol and this value decreased to 113.0 kJ/mol for 10 wt% CuFe2O4-added MgH2. The enhanced sorption performance of MgH2 in the presence of CuFe2O4 is believed to be due to the role of in situ formed Fe, Mg-Cu alloy, and MgO phases as an active species to catalyse the hydrogen storage properties of MgH2.

Understanding the dehydrogenation properties of MgH2 catalysed by Na3AlF6

The catalytic effect of Na3AlF6 on the dehydrogenation properties of the MgH2 with X wt% (X = 5, 10, 20 and 50) have been investigated by ball milling technique. Based on the temperature-programme-desorption result, the addition of 10 wt% Na3AlF6 to the MgH2 has demonstrated the best dehydrogenation properties performance. The dehydrogenation temperature of the un-doped MgH2 has experienced a reduction for about 60 °C after doped with 10 wt% Na3AlF6. The dehydrogenation kinetics also has been improved with the addition of 10 wt% Na3AlF6. Based on the Kissinger analysis, it was observed that the apparent activation energy of MgH2 desorption is remarkably decreased from 158 kJ/mol to 129 kJ/mol with the addition of 10 wt% Na3AlF6. Meanwhile, the formations of new species, the NaMgF3, the NaF and the AlF3 in the doped composite after the de/rehydrogenation processes are found in the X-ray diffraction analysis. These new species are expected to act as the active species that probably contributes to enhance the dehydrogenation properties of MgH2.

Nanolayer-like-shaped MgFe2O4 synthesised via a simple hydrothermal method and its catalytic effect on the hydrogen storage properties of MgH2.

In this study, the effect of nanolayer-like-shaped MgFe2O4 that is synthesised via a simple hydrothermal method on the performance of MgH2 for hydrogen storage is studied. MgH2 + 10 wt% MgFe2O4 is prepared by using the ball milling method. The MgFe2O4-doped MgH2 sample started to release H2 at approximately 250 °C, 90 °C and 170 °C lower than the milled and pure MgH2 respectively. At 320 °C, the isothermal desorption kinetic study has shown that the doped sample has desorbed approximately 4.8 wt% H2 in 10 min while the milled MgH2 desorbed less than 1.0 wt% H2. For isothermal absorption kinetics, the doped sample can absorb approximately 5.5 wt% H2 in 10 min at 200 °C. Meanwhile, the undoped sample absorbs only 4.0 wt% H2 in the same condition. The activation energy of 10 wt% MgFe2O4-doped MgH2 composite is 99.9 kJ mol−1, which shows a reduction of 33.1 kJ mol−1 compared to the milled MgH2 (133.0 kJ mol−1). X-ray diffraction spectra display the formation of new species which are Fe and MgO after dehydrogenation, and these new species are believed to act as the real catalyst that plays a crucial role in improving the sorption performance of the MgFe2O4-doped MgH2 system by providing a synergetic catalytic effect.

MnFe2O4 nanopowder synthesised via a simple hydrothermal method for promoting hydrogen sorption from MgH2

The effects of MnFe2O4 nanopowder synthesised via a simple ‘hydrothermal’ method on the hydrogen storage properties of MgH2 are investigated for the first time. The particle size of the as-synthesised MnFe2O4 nanoparticles is determined to be about 10 nm. We observe that MnFe2O4 catalyst decreases the decomposition temperature of MgH2 and enhances the sorption kinetics. Interestingly, the onset hydrogen desorption temperature of 10 wt% MnFe2O4-doped MgH2 sample gets lowered from 350 °C to 240 °C with faster kinetics, and the sample shows an average dehydrogenation rate 8–9 times faster than that of the as-milled MgH2 sample. By adding 10 wt% of as-prepared MnFe2O4 to MgH2, approximately 5.5 wt% hydrogen can be absorbed in 10 min at 200 °C. In contrast, the un-doped MgH2 sample absorbed only 4.0 wt% hydrogen in the same period of time. From the Kissinger analysis, the apparent activation energy for hydrogen released in the MnFe2O4-added MgH2 composite is found to be 108.42 kJ/mol, which is much lower than the activation energy for hydrogen released in the as-milled MgH2 (146.57 kJ/mol). It is believed that the in situ formed Fe particle and Mn-containing phases together play a synergistic role in remarkably improving MgH2 storage properties.

Controllable fabrication of Ni-based catalysts and their enhancement on desorption properties of MgH2

Searching for effective catalysts is one of the most attractive subjects to enhance the hydrogen storage properties, especially the sorption kinetics of the metal-based hydrides. The effect of the Ni morphology (including size and shape) on the desorption performance of MgH2 was studied in the paper, in which a modified wet chemical route was used to control the morphology of Ni. By introducing the as-synthesized Ni-based catalysts into MgH2, a well-distinguished desorption behavior was found, evidencing the existence of the morphology-dependent effect of the catalytic phase. Among them, graphene sheets (GS) supported Ni (Ni6GS4) exhibited the best catalytic performance with a lowered MgH2 onset desorption temperature of 225 °C (the undoped MgH2 initiated to desorb hydrogen at 308 °C) and a saturated hydrogen desorption capacity of 6.74 wt%. The significant enhancement of the hydrogen storage kinetics was ascribed to not only the relative small particle size of the initial Ni active phase but also its well-dispersed distribution state and further refinement during ball milling. In addition, we believe that the good dispersive ability of GS was of particular importance in separating the Ni nanoparticles from each other and preventing aggregating of the MgH2 particles, leading to a desirable cyclic performance. Our results have experimentally shown the influence of shape and size of metal-based catalysts on the MgH2 system, providing guideline or strategy designing nanostructured catalysts with light weight, ultra-fine particle size, well-formed distribution and high activity.