Enhancing hydrogen storage properties of the Mg/MgH2 system by the addition of bis(tricyclohexylphosphine)nickel(II) dichloride

This is a first report on the use of the bis(tricyclohexylphosphine)nickel (II) dichloride complex (abbreviated as NiPCy3) into MgH₂ based hydrogen storage systems. Different composites were prepared by planetary ball-milling by doping MgH₂ with (i) free tricyclohexylphosphine (PCy3) without or with nickel nanoparticles, (ii) different NiPCy3 contents (5–20 wt%) and (iii) nickel and iron nanoparticles with/without NiPCy3. The microstructural characterization of these composites before/after dehydrogenation was performed by TGA, XRD, NMR and SEM-EDX. Their hydrogen absorption/desorption kinetics were measured by TPD, DSC and PCT. All MgH₂ composites showed much better dehydrogenation properties than the pure ball-milled MgH₂. The hydrogen absorption/release kinetics of the Mg/MgH₂ system were significantly enhanced by doping with only 5 wt% of NiPCy3 (0.42 wt% Ni); the mixture desorbed H₂ starting at 220 °C and absorbed 6.2 wt% of H₂ in 5 min at 200 °C under 30 bars of hydrogen. This remarkable storage performance was not preserved upon cycling due to the complex decomposition during the dehydrogenation process. The hydrogen storage properties of NiPCy3-MgH₂ were improved and stabilized by the addition of Ni and Fe nanoparticles. The formed system released hydrogen at temperatures below 200 °C, absorbed 4 wt% of H₂ in less than 5 min at 100 °C, and presented good reversible hydriding/dehydriding cycles. A study of the different storage systems leads to the conclusion that the NiPCy3 complex acts by restricting the crystal size growth of Mg/MgH₂, catalyzing the H₂ release, and homogeneously dispersing nickel over the Mg/MgH₂ surface.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Effects of Ti-based catalysts on hydrogen desorption kinetics of nanostructured magnesium hydride

In the present work, the synergetic effect of Ti-based catalysts (TiH₂ and TiO₂ particles) on hydrogen desorption kinetics of nanostructured magnesium hydride was investigated. Nanostructured 84 mol% MgH₂–10%mol TiH₂–6%mol TiO₂ nanocomposite powder was prepared by high-energy ball milling and subjected to thermal analyses. Evaluation of the absorption/desorption properties revealed that the addition of the Ti-based catalysts significantly improved the hydrogen storage performance of MgH₂. A decrease in the decomposition temperature (as high as 100 °C) was attained after co-milling of MgH₂ with the Ti-based catalysts. Meanwhile, solid-state chemical reactions between MgH₂ and TiO₂ nanoparticles during co-milling slightly decreased the maximum hydrogen capacity. It was also found that formation of micro-cracks at the particle surfaces during thermal cycling enhanced the H-kinetics. Isothermal and non-isothermal thermal analysis revealed that the addition of Ti-catalysts reduced the decomposition activation energy of MgH₂ by 20–30 kJ/mol.     

Facile synthesized Fe nanosheets as superior active catalyst for hydrogen storage in MgH2

Reversible hydrogen storage in MgH₂ under mild conditions is a promising way for the realization of “Hydrogen Economy”, in which the development of cheap and highly efficient catalysts is the major challenge. Herein, A two-dimensional layered Fe is prepared via a facile wet-chemical ball milling method and has been confirmed to greatly enhance the hydrogen storage performance of MgH₂. Minor addition of 5 wt% Fe nanosheets to MgH₂ decreases the onset desorption temperature to 182.1 °C and enables a quick release of 5.44 wt% H₂ within 10 min at 300 °C. Besides, the dehydrogenated sample takes up 6 wt% H₂ in 10 min under a hydrogen pressure of 3.2 MPa at 200 °C. With the doping of Fe nanosheets, the apparent activation energy of the dehydrogenation reaction for MgH₂ is reduced to 40.7 ± 1.0 kJ mol⁻¹. Further ab initio calculations reveal that the presence of Fe extends the Mg–H bond length and reduces its bond strength. We believe that this work would shed light on designing plain metal for catalysis in the area of hydrogen storage and other energy-related issues.     

Phase-structural and morphological features,dehydrogenation/re-hydrogenation performance and hydrolysis of nanocomposites prepared by ball milling of MgH2 with germanium

Ball milling of magnesium hydride with germanium additives in argon for up to 10 h was shown to result in the formation of nanocomposites of α- and γ-modifications of MgH₂ together with individual Ge phase. When the as milled samples are heated-up, Ge interacts with MgH₂ to form Mg₂Ge at T = 350–400 °C; the phase transformation overlaps with MgH₂ decomposition and Mg₂Ge phase remains in the material after its re-hydrogenation at T = 300 °C and P(H₂)∼15 bar.The ball milled Ge-MgH₂ nanocomposites were found to be characterised by an improved hydrogen release via hydrolysis in organic acid solutions as compared to pure MgH₂ prepared and tested at the same conditions. It was found that the composite containing 5 wt% of Ge and ball milled for 5 h showed the best hydrogen generation performance, with total hydrogen release exceeding 1.55 NL/g (yield close to 100%) in one minute.     

ZrO2@Nb2CTx composite as the efficient catalyst for Mg/MgH2 based reversible hydrogen storage material

While Mg/MgH₂ system has a high hydrogen storage capacity, its sluggish hydrogen desorption rate has hindered practical applications. Herein, we report that the hydrogen absorption and desorption kinetics of Mg/MgH₂ system can be significantly improved by using the synergetic effect between Nb₂CT_x MXene and ZrO₂. The catalyst of Nb₂CT_x MXene loading with ZrO₂ (ZrO₂@Nb₂CT_x) is successfully synthesized, and the dehydrogenation activation energy of MgH₂ becomes as low as 60.0 kJ/mol H₂ when ZrO₂@Nb₂CT_x is used as the catalyst, which is far smaller than the case of ZrO₂ (94.8 kJ/mol H₂) and Nb₂CT_x MXene (125.6 kJ/mol H₂). With the addition of ZrO₂@Nb₂CT_x catalyst, MgH₂ can release about 6.24 wt.% and 5.69 wt.% of hydrogen within 150 s at 300 °C and within 900 s even at 240 °C, respectively. Moreover, it realizes hydrogen absorption at room temperature, which can uptake 2.98 wt.% of hydrogen within 1800 s. The catalytic mechanism analysis demonstrates that the in-situ formed nanocomposites can weaken the Mg–H bonding and provide more hydrogen diffusion channels, enabling the dissociation and recombination of hydrogen under milder reaction conditions.     

Synthesis of low-cost biomass charcoal-based Ni nanocatalyst and evaluation of their kinetic enhancement of MgH2

In this study, a low-cost biomass charcoal (BC)-based nickel catalyst (Ni/BC) was introduced into the MgH₂ system by ball-milling. The study demonstrated that the Ni/BC catalyst significantly improved the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂ + 10 wt% Ni/BC-3 composite starts to release hydrogen at 187.8 °C, which is 162.2 °C lower than the initial dehydrogenation temperature of pure MgH₂. Besides, 6.04 wt% dehydrogenation can be achieved within 3.5 min at 300 °C. After the dehydrogenation is completed, MgH₂ + 10 wt% Ni/BC-3 can start to absorb hydrogen even at 30 °C, which achieved the absorption of 5 wt% H₂ in 60 min under the condition of 3 MPa hydrogen pressure and 125 °C. The apparent activation energies of dehydrogenation and hydrogen absorption of MgH₂ + 10 wt% Ni/BC-3 composites were 82.49 kJ/mol and 23.87 kJ/mol lower than those of pure MgH₂, respectively, which indicated that the carbon layer wrapped around MgH₂ effectively improved the cycle stability of hydrogen storage materials. Moreover, MgH₂ + 10 wt% Ni/BC-3 can still maintain 99% hydrogen storage capacity after 20 cycles. XRD, EDS, SEM and TEM revealed that the Ni/BC catalyst evenly distributed around MgH₂ formed Mg₂Ni/Mg₂NiH₄ in situ, which act as a “hydrogen pump” to boost the diffusion of hydrogen along with the Mg/MgH₂ interface. Meanwhile, the carbon layer with fantastic conductivity enormously accelerated the electron transfer. Consequently, there is no denying that the synergistic effect extremely facilitated the hydrogen absorption and desorption kinetic performance of MgH₂.     

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.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C，which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

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 Ti₄Fe₂O_x 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 Ti₄Fe₂O_x. 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 Ti₄Fe₂O_0.3 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–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 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 Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

CNTs decorated with CoFeB as a dopant to remarkably improve the dehydrogenation/rehydrogenation performance and cyclic stability of MgH2

The chain-like carbon nanotubes (CNTs) decorated with CoFeB (CoFeB/CNTs) prepared by oxidation-reduction method is introduced into MgH₂ to facilitate its hydrogen storage performance. The addition of CoFeB/CNTs enables MgH₂ to start desorbing hydrogen at only 177 °C. Whereas pure MgH₂ starts hydrogen desorption at 310 °C. The dehydrogenation apparent activation energy of MgH₂ in CoFeB/CNTs doped-MgH₂ composite is only 83.2 kJ/mol, and this is about 59.5 kJ/mol lower than that of pure MgH₂. In addition, the completely dehydrogenated MgH₂−10 wt% CoFeB/CNTs sample can start to absorb hydrogen at only 30 °C. At 150 °C and 5 MPa H₂, the MgH₂ in CoFeB/CNTs doped-MgH₂ composite can absorb 6.2 wt% H₂ in 10 min. The cycling kinetics can remain rather stable up to 20 cycles, and the hydrogen storage capacity retention rate is 98.5%. The in situ formation of Co₃MgC, Fe, CoFe and B caused by the introduction of CoFeB/CNTs can provide active and nucleation sites for the dehydrogenation/rehydrogenation reactions of MgH₂. Moreover, CNTs can provide hydrogen diffusion pathways while also enhancing the thermal conductivity of the sample. All of these can facilitate the dehydrogenation/rehydrogenation performance and cyclic stability of MgH₂.     

Catalytic mechanism of TiO2 quantum dots on the de/re-hydrogenation characteristics of magnesium hydride

In the present study, the catalyst anatase titanium dioxide (TiO₂) quantum dots (QDs) of size ∼ (2.50–4.00)nm was successfully synthesized by the hydrothermal method. The formation of TiO₂: QDs has been established by UV–Vis spectroscopy and confirmed by transmission electron microscopy. Here, we report the catalytic action of TiO₂:QDs on de/re-hydrogenation properties of magnesium hydride (MgH₂/Mg). By catalyzing MgH₂ through this catalyst, the onset desorption temperature of MgH₂ gets reduced significantly from ∼360 °C (for ball-milled MgH₂) to ∼260 °C. Moreover, the Mg-TiO₂: QDs sample absorbed a significant amount of hydrogen up to ∼6.10 wt% in just 77sec at 280 °C. Improved rehydrogenation kinetics has been found even at lower temperatures by absorbing ∼5.30 wt% in 74 s at 225 °C and ∼5.0 wt% of hydrogen in 30 min at 100 °C. Based on structural,.microstructural, and XPS investigations, a feasible mechanism for improved hydrogen sorption and cyclic stability in MgH₂ catalyzed with TiO₂:QDs has been explained and discussed. To our knowledge, no studies have been carried out on the sorption of hydrogen in MgH₂ catalyzed by TiO₂:QDs.     

Superior catalysis of NbN nanoparticles with intrinsic multiple valence on reversible hydrogen storage properties of magnesium hydride

Magnesium hydride, as a potential solid state hydrogen carrier has attracted great attention around the world especially in the energy storage domain due to the high hydrogen storage capacity and the good cycling stability. But kinetic and thermodynamic barriers also impede the practical application and development of MgH₂. Nanoscale catalysts are deemed to be the most effective measure to overcome the kinetic barrier and lower the temperature required for hydrogen release in MgH₂. NbN nanoparticles (~20 nm) with intrinsic Nb³⁺-N and Nb⁵⁺-N were prepared using the molten salt method and used as catalysts in the MgH₂ system. It is found that the NbN nanoparticles exhibit a superior catalytic effect on de/rehydrogenation kinetics for the MgH₂/Mg system. About 6.0 wt% hydrogen can be liberated for the MgH₂+5NbN sample within 5 min at 300 °C, and it takes 12 min to desorb the same amount of hydrogen at 275 °C. Meanwhile, the MgH₂+5NbN sample can absorb 6.0 wt% hydrogen within 16 min at 150 °C, and absorb 5.0 wt% hydrogen within 24 min even at 100 °C. Particularly, the catalyzed samples exhibit excellent hydrogen absorption/desorption kinetic stability. After multiple cycles, there is no kinetic attenuation and the hydrogen capacity remains at about 6.0 wt%. It is demonstrated that the NbN nanoparticles with intrinsic multiple valence can be the critical effect in improving the hydrogen storage kinetics of MgH₂. The stability of Nb₄N₃ phase and Nb³⁺-N and Nb⁵⁺-N valence states can ensure a stable catalytic effect in the system.     

Synergistic catalytic effect of SrTiO3 and Ni on the hydrogen storage properties of MgH2

Study on the synergistic catalytic effect of the SrTiO₃ and Ni on the improvement of the hydrogen storage properties of the MgH₂ system has been carried out. The composites have been prepared using ball milling method and comparisons on the hydrogen storage properties of the MgH₂ – Ni and MgH₂ – SrTiO₃ composites have been presented. The MgH₂ – 10 wt% SrTiO₃ – 5 wt% Ni composite is found to has a decomposition temperature of 260 °C with a total decomposition capacity of 6 wt% of hydrogen. The composite is able to absorb 6.1 wt% of hydrogen in 1.3 min (320 °C, 27 atm of hydrogen). At 150 °C, the composite is able to absorb 2.9 wt% of hydrogen in 10 min under the pressure of 27 atm of hydrogen. The composite has successfully released 6.1 wt% of hydrogen in 13.1 min with a total dehydrogenation of 6.6 wt% of hydrogen (320 °C). The apparent activation energy, E_a, for decomposition of SrTiO₃-doped MgH₂ reduced from 109.0 kJ/mol to 98.6 kJ/mol after the addition of 5 wt% Ni. The formation of Mg₂Ni and Mg₂NiH₄ as the active species help to boost the performance of the hydrogen storage properties of the MgH₂ system. Observation of the scanning electron microscopy images suggested the catalytic role of the SrTiO₃ additive is based on the modification of composite microstructure.     

Graphene-anchored Ni6MnO8 nanoparticles with steady catalytic action to accelerate the hydrogen storage kinetics of MgH2

Transition metal-based oxides have been proven to have a substantial catalytic influence on boosting the hydrogen sorption performance of MgH₂. Herein, the catalytic action of Ni₆MnO₈@rGO nanocomposite in accelerating the hydrogen sorption properties of MgH₂ was investigated. The MgH₂ + 5 wt% Ni₆MnO₈@rGO composites began delivering H₂ at 218 °C, with about 2.7 wt%, 5.4 wt%, and 6.6 wt% H₂ released within 10 min at 265 °C, 275 °C, and 300 °C, respectively. For isothermal hydrogenation at 75 °C and 100 °C, the dehydrogenated MgH₂ + 5 wt% Ni₆MnO₈@rGO sample could absorb 1.0 wt% and 3.3 wt% H₂ in 30 min, respectively. Moreover, as compared to addition-free MgH₂, the de/rehydrogenation activation energies for doped MgH₂ composites were lowered to 115 ± 11 kJ/mol and 38 ± 7 kJ/mol, and remarkable cyclic stability was reported after 20 cycles. Microstructure analysis revealed that the in-situ formed Mg₂Ni/Mg₂NiH₄, Mn, MnO₂, and reduced graphene oxide synergically enhanced the hydrogen de/absorption properties of the Mg/MgH₂ system.     

Improved hydrogen storage characteristics of magnesium hydride using dual auto catalysts (MgF2+CsH)

This study discusses the improvement in the hydrogen sorption properties of MgH₂ with dual auto-catalysts, MgF₂ and CsH. The auto-catalysts are formed due to the reaction between MgH₂ and CsF during the dehydrogenation reaction of MgH₂. It has been observed that MgF₂ and CsH not only improve MgH₂'s hydrogen sorption properties, also aids its positive thermodynamic tuning, which is favourable for hydrogen storage. The on-set desorption temperature of MgH₂ catalysed by MgF₂+CsH is 249 °C, which is 106 °C lower than that of ball-milled MgH₂ without any additives measured under identical measurement conditions. The catalysts helped in improving both the de/rehydrogenation kinetics of MgH₂. The MgH₂ catalysed by MgF₂+CsH released 4.73 wt % H₂ in 15 min at 300 °C. Furthermore, its initial re-hydrogenation rate under isothermal conditon at 300 °C is 4.62 wt % H₂ in 5 min. The catalysed sample exhibits negligible hydrogen storage degradation of 0.39 wt % H₂ after 25 de/re-hydrogenation cycles. Using the Kissinger method, the activation energy of MgH₂ catalysed by MgF₂+CsH was estimated to be 98.1 ± 0.5 kJmol^-1. From the Van't Hoff plot, the decomposition and formation enthalpies of MgH₂ were determined to be 66.6 ± 1.1 kJmol^-1 and 63.1 ± 1.2 kJmol^-1, respectively. From the experimental observation, a feasible mechanism for the de/re-hydrogenation behaviour of MgH₂ with MgF₂+CsH is proposed.     

Enhanced hydrogen absorption and desorption properties of MgH2 with graphene and vanadium disulfide

Magnesium hydride (MgH₂) is the most prominent carrier for storing hydrogen in solid-state mode. However, their slow kinetics and high thermodynamics become an obstacle in hydrogen storage. The present study elaborates on the catalytic effect of graphene (Gr) and vanadium disulfide (VS₂) on MgH₂ to enhance its hydrogen sorption kinetic. The temperature-programmed desorption study shows that the onset desorption temperature of MgH₂ catalyzed by VS₂ and MgH₂ catalyzed by Gr is 289 °C and 300 °C, respectively. These desorption temperatures are 87 °C and 76 °C lower than the desorption temperature of pristine MgH₂. The rapid rehydrogenation kinetics for the MgH₂ catalyzed by VS₂ have been found at a temperature of 300 °C under 15 atm H₂ pressure by absorbing ∼4.04 wt% of hydrogen within 1 min, whereas the MgH₂ catalyzed by Gr takes ∼3 min for absorbing the same amount of hydrogen under the similar temperature and pressure conditions. The faster release of hydrogen was also observed in MgH₂ catalyzed by VS₂ than MgH₂ catalyzed by Gr and pristine MgH₂. MgH₂ catalyzed by VS₂ releases ∼2.54 wt% of hydrogen within 10 min, while MgH₂ catalyzed by Gr takes ∼30 min to release the same amount of hydrogen. Furthermore, MgH₂ catalyzed by VS₂ also persists in the excellent cyclic stability and reversibility up to 25 cycles.     

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 performance of MgH2 through the addition of BaMnO3 synthesized via solid-state method

Currently, magnesium hydride (MgH₂) as a solid-state hydrogen storage material has become the subject of major research owing to its good reversibility, large hydrogen storage capacity (7.6 wt%) and affordability. However, MgH₂ has a high decomposition temperature (>400 °C) and slow desorption and absorption kinetics. In this work, BaMnO₃ was synthesized using the solid-state method and was used as an additive to overcome the drawbacks of MgH₂. Interestingly, after adding 10 wt% of BaMnO₃, the initial desorption temperature of MgH₂ decreased to 282 °C, which was 138 °C lower than that of pure MgH₂ and 61 °C lower than that of milled MgH₂. For absorption kinetics, at 250 °C in 2 min, 10 wt% of BaMnO₃-doped MgH₂ absorbed 5.22 wt% of H₂ compared to milled MgH₂ (3.48 wt%). Conversely, the desorption kinetics also demonstrated that 10 wt% of BaMnO₃-doped MgH₂ samples desorbed 5.36 wt% of H₂ at 300 °C within 1 h whereas milled MgH₂ only released less than 0.32 wt% of H₂. The activation energy was lowered by 45 kJ/mol compared to that of MgH₂ after the addition of 10 wt% of BaMnO₃. Further analyzed by using XRD revealed that the formation of Mg_0·9Mn_0·1O, Mn₃O₄ and Ba or Ba-containing enhanced the performance of MgH₂.     

Activation effects during hydrogen release and uptake of MgH2

Scandium(II)hydride, ScH₂, and scandium(III)chloride, ScCl₃, are explored as additives to facilitate hydrogen release and uptake for magnesium hydride. These additives are expected to form more homogeneous composites with Mg/MgH₂ as compared to metallic scandium. However, scandium(III)chloride, reacts with MgH₂ during mechano-chemical treatment and form ScH₂ and MgCl₂ (that later crystallise during heat treatment). The composite MgH₂−ScH₂ was investigated using in-situ synchrotron radiation powder X-ray diffraction during up to five cycles of continuous release and uptake of hydrogen at isothermal conditions at 320, 400 and 450 °C and p(H₂) = 100–150 or 10⁻² bar. The data were analysed by Rietveld refinement and no reaction is observed between either MgH₂/ScH₂ or Mg/ScH₂ during cycling. The extracted sigmoidal shaped curves for formation or decomposition of Mg/MgH₂ suggest that a nucleation process is preceding the crystal growth. The reaction rate increases with increasing number of cycles of hydrogen release and uptake at isothermal conditions possibly due to activation effects. This kinetic enhancement is strongest between the first cycles and may be denoted an activation effect.