Hydrogen Storage Properties Of MgH2 Research Articles

Improved hydrogen storage properties of MgH2 using transition metal sulfides as catalyst

In this study, some transition metal sulfides (TiS2, NbS2, MoS2, MnS, CoS2 and CuS) are used as catalyst to enhance the hydrogen storage behaviors of MgH2. The MgH2-sulfide composites with different sulfides addition are prepared by ball-milling. The phase composition and hydrogen storage properties are studied in detail. The results confirm that all these sulfides can significantly increase the hydrogen desorption and absorption kinetics of MgH2. The MgH2–TiS2 has the best hydrogenation and dehydrogenation kinetics, followed by the MgH2–NbS2, MgH2–MnS, MgH2–MoS2, MgH2–CoS2, MgH2–CuS and MgH2. Also, the onset dehydrogenation temperature of the MgH2–TiS2 is about 204 °C, which is lower about 126 °C than that of the MgH2. The dehydrogenation activation energy can be reduced to 50.8 kJ mol−1 when doping TiS2 in MgH2. The beneficial catalytic effects of the sulfides can be ascribed to the in-situ formation of MgS, TiH2, NbH, Mo, Mn, Mg2CoH5 and MgCu2 phases.

Facile synthesis of a Ni3S2@C composite using cation exchange resin as an efficient catalyst to improve the kinetic properties of MgH2

Carbon materials have excellent catalytic effects on the hydrogen storage performance of MgH2. Here, carbon-supported Ni3S2 (denoted as Ni3S2@C) was synthesized by a facile chemical route using ion exchange resin and nickel acetate tetrahydrate as raw materials and then introduced to improve the hydrogen storage properties of MgH2. The results indicated the addition of 10 wt.% Ni3S2@C prepared by macroporous ion exchange resin can effectively improve the hydrogenation/dehydrogenation kinetic properties of MgH2. At 100 °C, the dehydrogenated MgH2-Ni3S2@C-4 composite could absorb 5.68 wt.% H2. Additionally, the rehydrogenated MgH2-Ni3S2@C-4 sample could release 6.35 wt.% H2 at 275 °C. The dehydrogenation/hydrogenation enthalpy changes of MgH2-Ni3S2@C-4 were calculated to be 78.5 kJ mol−1/−74.7 kJ mol−1, i.e., 11.0 kJ mol−1/7.3 kJ mol−1 lower than those of MgH2. The improvement in the kinetic properties of MgH2 was ascribed to the multi-phase catalytic action of C, Mg2Ni, and MgS, which were formed by the reaction between Ni3S2 contained in the Ni3S2@C catalyst and Mg during the first hydrogen absorption–desorption process.

Achieving superior hydrogen storage properties of MgH2 by the effect of TiFe and carbon nanotubes

Multiple catalysts have exhibited high activity on improving the hydrogen storage performance of magnesium hydride. Herein, TiFe as a superior catalyst was successfully prepared, and then doped into MgH2 via ball milling to improve the de/rehydrogenation properties of MgH2 at low temperatures. Compared with as-prepared MgH2, the onset desorption temperature could be reduced to 175 °C after adding 15 wt%-TiFe to MgH2. Furthermore, the 10 wt%-TiFe doped MgH2 released approximately 6.5 wt% H2 within 10 min at 300 °C, and 5.3 wt% H2 could be rapidly absorbed at low temperature of 125 °C under hydrogen pressure of 3 MPa. Moreover, the additional doping of carbon nanotubes (CNTs) evenly distributed on the surface of MgH2 particles, enabling the MgH2 + 10 wt%-TiFe + 5 wt%-CNTs composite with outstanding cycling performance. The activation energy of hydrogenation for MgH2 was reduced from 72.5 ± 2.7 kJ/mol to 60.7 ± 8.0 kJ/mol after doping with TiFe and CNTs. However, the addition of TiFe and CNTs had limited influence on the thermodynamic properties of MgH2. The analysis of XRD, TEM, SEM and EDS indicated that uniformly distributed TiFe and CNTs served as active catalytic unit and aggregation preventer on enhancing the hydrogen storage properties of MgH2, respectively.

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH2-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO2, and Ti4Fe2Ox suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH2 and hydrogenation of nano-Ti and Ti4Fe2Ox. The possible reduction of TiO2 during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO2 compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH2 by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH2 down to 74 and 67 kJ/mol for MgH2 modified with nano-TiO2 and Ti4Fe2O0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH2. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti4Fe2O0.3 nanocomposite, ΔHdes = 76 kJ/mol H2 and ΔSdes = 138 J/K·mol H2, correspond to the reported literature values for pure polycrystalline MgH2. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO2 and Ti4Fe2Ox additives. Cycling stability of Mg/Ti4Fe2Ox was substantially improved by introduction of 3 wt% graphite into the composite.

Catalytic effect of TiO2 on hydrogen storage properties of MgH2

MgH2is a promising material among metal hydrides due to its high gravimetric hydrogen storage, good reversibility and low cost.However, it has some problems, including high decomposition temperature and slow ab/desorption kinetics. In this work, different wt % concentration of TiO2(x = 0, 15) is added to MgH2 to synthesize Nanocomposites using ball milling for 2 h with 300 rpm. The de/hydriding properties of the prepared nanocomposites have been studied by DTA and TGA. The onset temperature 130 °C for the MgH2- 15 wt% TiO2 nanocomposite and maximum dehydrogenation achieved ~ 5.98 wt% from the TGA analysis. The DTA result showed a wide signal range from temperature 392 – 435 °C for the absorption and desorptionprocess. The XRD and SEM data also supports the above data in these Nanocomposites.

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.

Enhancing hydrogen storage properties of MgH2 through addition of Ni/CoMoO4 nanorods

In this work, one-dimensional NiMoO4 and CoMoO4 nanorods are prepared and introduced into MgH2 to improve its hydrogen storage properties. It is observed that the MgH2–NiMoO4 system exhibits lower dehydrogenation temperature and faster kinetics than the MgH2–CoMoO4 system and pure MgH2, indicating the superior promoting effect of NiMoO4 over CoMoO4. Concretely, NiMoO4 reacts with MgH2 during the first dehydrogenation to in situ form Mg2Ni and Mo0, both of which are crucial factors for the following hydrogen cycling process. The ‘Mg2Ni/Mg2NiH4’ mutual transformation upon hydrogen release/uptake is the well-known ‘hydrogen pump’ effect which boosts the hydrogen storage performance of MgH2. Moreover, theoretical calculations reveal the dual roles of Mo0 played for the hydrogen storage in MgH2: i) it accelerates the hydrogen de/absorption of MgH2 through weakening the Mg–H bonding; ii) it facilitates the mutual ‘Mg2Ni/Mg2NiH4’ transformation as the formation energy of Mg2NiH4 decreases under the influence of Mo0, which we call ‘facilitated hydrogen pump’ effect in this work. Ascribing to the collaborative action between Ni- and Mo- containing species, the hydrogen storage kinetics of MgH2 has been accelerated.

Enhancing hydrogen storage properties of MgH2 by core-shell CoNi@C

Introducing multicomponent nanoparticles can improve remarkably de/hydrogenation kinetic properties of MgH2. In this work, the original core-shell CoNi@C bimetallic nanoparticles were obtained via hydrothermal and calcination reduction technology. MgH2-8 wt% CoNi@C starts to dehydrogenate at 173 °C with the dehydrogenation capacity of 5.83 wt% within 1800 s at 275 °C. Moreover, the composite absorbs 4.83 wt% H2 within 1800 s at 100 °C with prominent advantage at low temperature. The notable improvement of hydrogen storage properties of MgH2 is attributed to the reversible phase transitions of Mg2Co/Mg2CoH5 and Mg2Ni/Mg2NiH4, and the excellent thermal conductivity and confinement effect of the carbon shell. As a result, the apparent activation energy of dehydrogenation reaction reduces to 78.5 kJ/mol. This work extends the horizon of bimetallic nanoparticles with especial microstructure for enhancing the hydrogen storage properties of metal hydrides.

Using a Self-Assembled Two-Dimensional MXene-Based Catalyst (2D-Ni@Ti3C2) to Enhance Hydrogen Storage Properties of MgH2.

In this work, we report the remarkable catalytic effects of a novel Ti3C2 MXene-based catalyst (Ni@Ti-MX), which was prepared via self-assembling of Ni nanoparticles onto the surface of exfoliated Ti3C2 nanosheets. The resultant Ni@Ti-MX catalyst, characterized by ultradispersed Ni nanoparticles being anchored on the monolayer Ti3C2 flakes, was introduced into MgH2 through ball milling. In situ transmission electron microscopy (TEM) analysis revealed that a synergetic catalytic effect of multiphase components (Mg2Ni, TiO2, metallic Ti, etc.) derived in the MgH2 + Ni@Ti-MX composite exhibits remarkable improvements in the hydrogen sorption kinetics of MgH2. In particular, the MgH2 + Ni@Ti-MX composite can absorb 5.4 wt % H2 in 25 s at 125 °C and release 5.2 wt % H2 in 15 min at 250 °C. Interestingly, it can uptake 4 wt % H2 in 5 h even at room temperature. Furthermore, the dehydrogenation peak temperature of the MgH2 + Ni@Ti-MX composite is about 221 °C, which is 50 and 122 °C lower than that of MgH2 + Ti-MX and MgH2, respectively. The excellent hydrogen sorption properties of the MgH2 + Ni@Ti-MX composite are primarily attributed to the peculiar core-shell nanostructured MgH2@Mg2NiH4 hybrid materials and the interfacial coupling effects from different catalyst-matrix interfaces. The results obtained in this study demonstrate that using self-assembling of transition-metal elements on two-dimensional (2D) materials as a catalyst is a promising approach to enhance the hydrogen storage properties of MgH2.

Graphene-induced growth of N-doped niobium pentaoxide nanorods with high catalytic activity for hydrogen storage in MgH2

High operation temperatures and slow kinetics remain big challenges for using magnesium (Mg) as a practical hydrogen storage medium. In this work, a novel graphene-guided nucleation and growth process was developed for the preparation of N-doped Nb2O5 nanorods that enable remarkably improved hydrogen storage properties of MgH2. The nanorods were measured to be 10–20 nm in diameter. MgH2 doped with 10 wt% of the nanorods released 6.2 wt% H2 from 170 °C, which is 130 °C lower than additive-free MgH2, thanks to a 40% reduction in the kinetic barriers. About 5.5 wt% of H2 was desorbed in isothermal dehydrogenation test at 175 °C. Reloading of hydrogen was notably completed at 25 °C under 50 atm of hydrogen pressure, which has not been reported before. Density functional theory (DFT) calculations demonstrate the extended bond lengths and weakened bond strengths of Mg-H or H-H when MgH2/H2 adsorbs on the Nb-N-O/graphene model, consequently favouring lower operating temperatures and improved kinetics for hydrogen storage in MgH2 catalyzed by the graphene-guided N-Nb2O5 nanorods. Our findings provide useful insights in the design and preparation of high-performance catalysts of transition metals and rare metals for on-board hydrogen storage.

Electrospun carbon nanofibers with in-situ encapsulated Ni nanoparticles as catalyst for enhanced hydrogen storage of MgH2

Transition-metals have emerged as promising catalyst candidates for improving the hydrogen storage properties of MgH2. However, the preparation of uniformly dispersed and extra-fine transition-metals catalysts with high catalytic activity still remains a challenge. In this paper, an electrospinning-based reduction approach is presented to generate nanostructured nickel catalyst, which is protected from irreversible fusion and aggregation in subsequent high-temperature pyrolysis, in carbon nanofibers (Ni@C) in situ. The obtained Ni@C reveals remarkable catalytic effect on improving the hydrogen storage properties of MgH2. For example, the MgH2-10 wt%Ni@C composite delivers dehydrogenation capacities of 5.79 wt% and 6.12 wt% at 280 °C and 300 °C, respectively, whereas the as-milled MgH2 hardly decomposes at the same temperature. By Arrhenius plots, the calculated Ea of the dehydrogenation of MgH2-10 wt%Ni@C is 93.08 kJ mol−1, which is 94.33 kJ mol−1 lower than that of the as-milled MgH2. Furthermore, the microstructure of Ni@C is remained during the re/dehydrogenation process and the Ni nanoparticles are still distributed homogeneously in the composite, accounting for the excellent cycling performance. This study could render combinations of ultrafine metal nanoparticles with carbon accessible, thereby, extending opportunities in catalytic applications for hydrogen storage.

Enhancing (de)hydrogenation kinetics properties of the Mg/MgH2 system by adding ANi5 (A = Ce, Nd, Pr, Sm, and Y) alloys via ball milling

Magnesium (Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity, low cost, light weight, etc. However, the poor de/hydrogenation kinetic properties dramatically hinder the practical applications. In this work, the MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites were prepared by a high-energy ball milling method, which can effectively refine the particle size thus improving the kinetic properties. Experimental results reveal that the MgH2-ANi5 composites mainly consist of Mg2NiH4, MgH2 and rare earth (RE) hydride, which will be dehydrogenated to form Mg2Ni, Mg and stable RE hydride reversibly. Accordingly, the as-milled MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%, 5.7 wt%, 6.21 wt%, 6.38 wt%, and 6.5 wt% at a temperature of 300 °C, and show much better kinetic properties in comparison to the pure MgH2 without any additive. In-situ formed Mg2Ni and stable RE hydride (such as CeH2.73 and YH2) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH2. The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.

Enhancing Hydrogen Storage Properties of MgH2 by Transition Metals and Carbon Materials: A Brief Review.

Magnesium hydride (MgH2) has attracted intense attention worldwide as solid state hydrogen storage materials due to its advantages of high hydrogen capacity, good reversibility, and low cost. However, high thermodynamic stability and slow kinetics of MgH2 has limited its practical application. We reviewed the recent development in improving the sorption kinetics of MgH2 and discovered that transition metals and their alloys have been extensively researched to enhance the de/hydrogenation performance of MgH2. In addition, to maintain the cycling property during the de/hydrogenation process, carbon materials (graphene, carbon nanotubes, and other materials) have been proved to possess excellent effect. In this work, we introduce various categories of transition metals and their alloys to MgH2, focusing on their catalytic effect on improving the hydrogen de/absorption performance of MgH2. Besides, carbon materials together with transition metals and their alloys are also summarized in this study, which show better hydrogen storage performance. Finally, the existing problems and challenges of MgH2 as practical hydrogen storage materials are analyzed and possible solutions are also proposed.

Superior catalytic effect of facile synthesized LaNi4.5Mn0.5 submicro-particles on the hydrogen storage properties of MgH2

Catalytic doping is an important strategy for enhancing the hydrogen storage performance of magnesium hydride. Hence, LaNi4.5Mn0.5 submicro-particles were successfully prepared via a facile wet chemical ball milling method. To further study the catalytic performance, various amount of as-received LaNi4.5Mn0.5 were introduced to MgH2 and the submicro-LaNi4.5Mn0.5 showed best catalytic effect on MgH2. Specifically, experimental results showed that LaNi4.5Mn0.5 submicro-particles modified MgH2 started to release hydrogen at 175 °C, about 150 °C lower than that of additive-free MgH2. At 300 °C, the composite could desorb 6.6 wt% H2 within 6 min. Moreover, the fully dehydrogenated sample could take up hydrogen at room temperature and 4.1 wt% H2 was charged within 10 min at 150 °C. The apparent activation energy of MgH2 for hydrogenation after doping submicro-LaNi4.5Mn0.5 was greatly reduced from 72.50 ± 2.70 to 16.63 ± 1.40 kJ/mol. Furthermore, a hydrogen capacity of 6.1 wt% was maintained after 10 cycles. The synergetic effects between the in-situ formed Mg2Ni/Mg2NiH4, Mn and LaH3 may contribute to the promoted hydrogen dissociation and diffusion along the Mg/MgH2 interface, thus reducing the energy barrier and enhancing the hydrogen storage performance of MgH2.

Catalytic effect of Ni@rGO on the hydrogen storage properties of MgH2

Abstract Uniform-dispersed Ni nanoparticles (NPs) anchored on reduced graphene oxide (Ni@rGO) catalyzed MgH2 (MH-Ni@rGO) has been fabricated by mechanical milling. The effects of milling time and Ni loading amount on the hydrogen storage properties of MgH2 have been investigated. The initial hydrogen desorption temperature of MgH2 catalyzed by 10 wt.% Ni4@rGO6 for milling 5 h is significantly decreased from 251 °C to 190 °C. The composite can absorb 5.0 wt.% hydrogen in 20 min at 100 °C, while it can desorb 6.1 wt.% within 15 min at 300 °C. Through the investigation of the phase transformation and dehydrogenation kinetics during hydrogen ab/desorption cycles, we found that the in-situ formed Mg2Ni/Mg2NiH4 exhibited better catalytic effect than Ni. When Ni loading amount is 45 wt.%, the rGO in Ni@rGO catalysts can prevent the reaction of Ni and Mg due to the strong interaction between rGO and Ni NPs.

Synergistic effect of rGO supported Ni3Fe on hydrogen storage performance of MgH2

Bimetallic catalysts possess unique physical and chemical properties that distinct from the individual, which offer the opportunity to ameliorate the hydrogen storage properties of MgH2. Herein, a Ni3Fe catalyst homogeneously loaded on the surface of reduced graphene oxide (Ni3Fe/rGO) was prepared based on layered double hydroxide (LDH) precursor. The novel Ni3Fe/rGO nano-catalyst was subsequently doped into MgH2 to improve its hydrogen storage performance. The MgH2-5 wt.% Ni3Fe/rGO composite requires only 100 s to reach 6 wt% hydrogen capacity at 100 °C, while for MgH2 doped with 5 wt% Ni3Fe, Ni/rGO and Fe/rGO all require more than 500 s to uptake 3 wt% hydrogen under the same condition. The onset dehydrogenation temperature of the MgH2-5 wt.% Ni3Fe/rGO composite is about 185 °C, much lower than that of the MgH2 doped with 5 wt% Ni3Fe (205 °C), Ni/rGO (210 °C) and Fe/rGO (250 °C), and it can release H2 completely even in 1000 s at 275 °C. Besides, the MgH2-5 wt% Ni3Fe/rGO displays the lowest dehydrogenation apparent activation energy of 59.3 kJ/mol calculated by Kissinger equation. The synergetic effect attributing to rGO, in-situ formed active species of Mg2Ni and Fe is in charge of the excellent catalytic effect on hydrogen storage behavior of MgH2. Meanwhile, this study supplies innovative insights to design high efficiency catalysts based on the LDH precursor.

Insights into 2D graphene-like TiO2 (B) nanosheets as highly efficient catalyst for improved low-temperature hydrogen storage properties of MgH2

The development of low temperature hydrogen storage system is crucial for the wide application of renewable energy. A key obstacle for this system is the lack of efficient catalysts, in which two-dimensional nanosheets have attracted considerable attention because of their unique properties. Herein, we synthesized graphene-like TiO2 (B), and applied them as a highly efficient catalyst to dramatically enhance the low temperature hydrogen storage performances of MgH2. The MgH2s–TiO2 (B) quickly absorbs 5.32 and 5.50 wt% H2 at low temperatures of 50 and 60 °C, respectively, which is superior to most of the previously catalyzed MgH2 composites. Moreover, MgH2–TiO2 (B) begins to desorb hydrogen at ~200 °C and release ~6.29 wt% H2 below 288 °C. Careful microstructure analyses reveal that TiO2 (B) nanosheets are reduced to metallic Ti nanoparticles and wrinkled Ti2O3 upon ball milling and (de)hydriding processes, which creates lots of boundary interfaces between MgH2 and Ti-based catalysts, thus facilitating the hydrogen diffusion. Besides, the in-situ formed Ti has the intermediate electronegativity between Mg and H, which could weaken the Mg–H bonds and decrease the dehydrogenation kinetic barriers. While in rehydrogenation, Ti nanoparticles act as effective heterogeneous nucleation agents of MgH2 nuclei, further promoting hydrogen absorption properties of MgH2–TiO2 (B). The present investigation provides clear evidence for remarkable catalytic effect of graphene-like TiO2 (B) on hydrogen absorption/desorption properties of MgH2, which is not only important for deeply understanding the mechanism, but also sheds lights for catalysis design towards practical low temperature hydrogen storage.

Extreme high reversible capacity with over 8.0 wt% and excellent hydrogen storage properties of MgH2 combined with LiBH4 and Li3AlH6

Abstract Magnesium hydride has attracted great attention because of its high theoretical capacity and outstanding reversibility, nevertheless, its practical applications have been restricted by the disadvantages of the sluggish kinetics and high thermodynamic stability. In this work, an unexpected high reversible hydrogen capacity over 8.0 wt% has been achieved from MgH2 metal hydride composited with small amounts of LiBH4 and Li3AlH6 complex hydrides, which begins to release hydrogen at 276 °C and then completely dehydrogenates at 360 °C. The dehydrogenated MgH2+LiBH4/Li3AlH6 composite can fully reabsorb hydrogen below 300 °C with an excellent cycling stability. The composite exhibits a significant reduction of dehydrogenation activation energy from 279.7 kJ/mol (primitive MgH2) to 139.3 kJ/mol (MgH2+LiBH4/Li3AlH6), as well as a remarkable reduction of dehydrogenation enthalpy change from 75.1 kJ/mol H2 (primitive MgH2) to 62.8 kJ/mol H2 (MgH2+LiBH4/Li3AlH6). The additives of LiBH4 and Li3AlH6 not only enhance the cycling hydrogen capacity, but also simultaneously improve the reversible de/rehydrogenation kinetics, as well as the dehydrogenation thermodynamics. This notable improvement on the hydrogen absorption/desorption behaviors of the MgH2+LiBH4/Li3AlH6 composite could be attributed to the dehydrogenated products including Li3Mg7, Mg17Al12 and MgAlB4, which play a key role on reducing the dehydrogenation activation energy and increasing diffusion rate of hydrogen. Meanwhile, the LiBH4 and Li3AlH6 effectively destabilize MgH2 with a remarkable reduction on dehydrogenation enthalpy change in terms of thermodynamics. In particular, the Li3Mg7, Mg17Al12 and MgAlB4 phases can reversibly transform into MgH2, Li3AlH6 and LiBH4 after rehydrogenation, which contribute to maintain a high cycling capacity. This constructing strategy can further promote the development of high reversible capacity Mg-based materials with suitable de/rehydrogenation properties.

Improved hydrogen storage properties of MgH2 by the addition of KOH and graphene

Mg hydride is a competitive candidates for hydrogen storage based on its high gravimetric hydrogen capacity and accessibility. In this study, a small amount of KOH and graphene were added into MgH2 by high energy ball milling. MgH2 doped with both KOH and graphene has a greatly improved hydrogen storage performance. The existence of graphene and the in-situ formed KMgH3 and MgO decreased activation energy of MgH2 to 109.89 ± 6.03 kJ/mol. The both KOH and graphene doped sample has a reversibly capacity of 5.43 wt % H2 and can released H2 as much as 6.36 times and 1.84 times faster than those of undoped sample and only KOH doped sample at 300 °C, respectively. The addition of graphene not only can provide more “H diffusion channels”, but also can disperse the catalyst.

Enhanced Hydrogen Storage Properties of MgH2 Using a Ni and TiO2 Co-Doped Reduced Graphene Oxide Nanocomposite as a Catalyst.

To improve the hydrogen storage properties of Mg/MgH2, a Ni and TiO2 co-doped reduced graphene oxide [(Ni-TiO2)@rGO] nanocomposite is synthesized by a facile impregnation method and introduced into Mg via ball milling. The results demonstrated that the dispersive distribution of Ni and TiO2 with a particle size of 20–200 nm in the reduced graphene oxide matrix led to superior catalytic effects on the hydrogen storage properties of Mg-(Ni-TiO2)@rGO. The initial hydrogenation/dehydrogenation temperature for Mg-(Ni-TiO2)@rGO decreased to 323/479 K, 75/84 K lower than that of the additive-free sample. The hydrogen desorption capacity of the Mg-(Ni-TiO2)@rGO composite released 1.47 wt.% within 120 min at 498 K. When the temperature was increased to 523 K, the hydrogen desorption capacity increased to 4.30 wt.% within 30 min. A hydrogenation/dehydrogenation apparent activation energy of 47.0/99.3 kJ·mol−1 was obtained for the Mg-(Ni-TiO2)@rGO composite. The improvement in hydrogenation and dehydrogenation for the Mg-(Ni-TiO2)@rGO composite was due to the reduction of the apparent activation energy by the catalytic action of (Ni-TiO2)@rGO.