Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Site density effect of Ni particles on hydrogen desorption of MgH2

Fine Ni particles are effective in catalyzing hydrogen sorption of MgH₂, but there is confusion about the extent of this effect in relation to Ni particle size and content. Here, effects of Ni particles of different sizes on hydrogen desorption of MgH₂ were comparatively investigated. MgH₂ mixed with only 2 at% of fine Ni particles rapidly desorbs hydrogen up to 6.5 wt% around 200–340 °C, but there is no significant difference in the desorption temperature of the mixture when Ni particles vary from 90 to 200 nm. Increasing the content of Ni to 4 at%, or a combined (2 at% Ni + 2 at% Fe), leads to hydrogen desorption starting from 160 °C. Further analyses of the literature suggest that the effectiveness of Ni catalysis largely depends on its site density over MgH₂ surface, i.e., an optimal site density of catalytic particles is important in balancing the sorption properties of MgH₂. The projected trend suggests that MgH₂ can desorb hydrogen from 100 °C, the targeted temperature for fuel cells, if the number of catalyst sites is around 4 × 10¹⁴ per m² of MgH₂, or the number ratio of Ni to MgH₂ particles is about a million to one.     

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.     

Synergetic effect of Ni and Nb2O5 on dehydrogenation properties of nanostructured MgH2 synthesized by high-energy mechanical alloying

Nanostructured MgH₂-Ni/Nb₂O₅ nanocomposite was synthesized by high-energy mechanical alloying. The effect of MgH₂ structure, i.e. crystallite size and lattice strain, and the presence of 0.5 mol% Ni and Nb₂O₅ on the hydrogen-desorption kinetics was investigated. It is shown that the dehydrogenation temperature of MgH₂ decreases from 426 °C to 327 °C after 4 h mechanical alloying. Here, the average crystallite size and accumulated lattice strain are 20 nm and 0.9%, respectively. Further improvement in the hydrogen desorption is attained in the presence of Ni and Nb₂O₅, i.e. the dehydrogenation temperature of MgH₂/Ni and MgH₂/Nb₂O₅ is measured to be 230 °C and 220 °C, respectively. Meanwhile, the dehydrogenation starts at 200 °C in MgH₂–Ni/Nb₂O₅ system, revealing synergetic effect of Ni and Nb₂O₅. The mechanism of the catalytic effect is presented.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Interfacial engineering of nickel/vanadium based two-dimensional layered double hydroxide for solid-state hydrogen storage in MgH2

As a high-density solid-state hydrogen storage material, magnesium hydride (MgH₂) is promising for hydrogen transportation and storage. Yet, its stable thermodynamics and sluggish kinetics are unfavorable for that required for commercial application. Herein, nickel/vanadium trioxide (Ni/V₂O₃) nanoparticles with heterostructures were successfully prepared via hydrogenating the NiV-based two-dimensional layered double hydroxide (NiV-LDH). MgH₂ + 7 wt% Ni/V₂O₃ presented more superior hydrogen absorption and desorption performances than pure MgH₂ and MgH₂ + 7 wt% NiV-LDH. The initial discharging temperature of MgH₂ was significantly reduced to 190 °C after adding 7 wt% Ni/V₂O₃, which was 22 and 128 °C lower than that of 7 wt% NiV-LDH modified MgH₂ and additive-free MgH₂, respectively. The completely dehydrogenated MgH₂ + 7 wt% Ni/V₂O₃ charged 5.25 wt% H₂ in 20 min at 125 °C, while the hydrogen absorption capacity of pure MgH₂ only amounted to 4.82 wt% H₂ at a higher temperature of 200 °C for a longer time of 60 min. Moreover, compared with MgH₂ + 7 wt% NiV-LDH, MgH₂ + 7 wt% Ni/V₂O₃ shows better cycling performance. The microstructure analysis indicated the heterostructural Ni/V₂O₃ nanoparticles were uniformly distributed. Mg₂Ni/Mg₂NiH₄ and metallic V were formed in-situ during cycling, which synergistically tuned the hydrogen storage process in MgH₂. Our work presents a facile interfacial engineering method to enhance the catalytic activity by constructing a heterostructure, which may provide the mentality of designing efficient catalysts for hydrogen storage.     

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.     

Synthesis by reactive ball milling and cycling properties of MgH2–TiH2 nanocomposites: Kinetics and isotopic effects

MgH₂, MgH₂–TiH₂ nanocomposites and their deuterated analogues have been obtained by reactive ball milling and their kinetic and cycling hydrogenation properties have been analysed by isotope measurements and high-pressure differential scanning calorimetry (HP-DSC). Kinetics of material synthesis depends on both Ti-content and the isotopic nature of the gas. For pure Mg, the synthesis is controlled by isotope diffusion in Mg and therefore MgH₂ forms faster than MgD₂. For the MgH₂–TiH₂ nanocomposites, the synthesis is controlled by the efficiency of milling. Kinetics of reversible hydrogen/deuterium sorption in nanocomposites have been studied at 548 K. The rate limiting step is isotope diffusion for absorption and Mg/MgH₂ interface displacement for desorption. HP-DSC measurements demonstrate that the TiH₂ phase acts as a gateway for hydrogen sorption even in presence of MgO and provides abundant nucleation sites for Mg and MgH₂ phases. The 0.7MgH₂–0.3TiH₂ nanocomposite exhibits steady hydrogen storage capacity after 100 cycles of absorption–desorption.     

Remarkable catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen sorption properties of MgH2

In the present study, the catalytic effect of Ni and ZrO₂ nanoparticles on the hydrogen absorption and desorption properties of MgH₂ has been investigated. The MgH₂ nanocomposites were prepared by high-energy ball-milling. The morphology, phase structure, thermal behavior, and hydrogen storage properties of the materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), and the pressure-composition temperature (PCT) methods. ZrO₂ and Ni nanoparticles were homogenously dispersed into the MgH₂ matrix. The calculated apparent activation energy for dehydrogenation was 63.4 kJ/mol, which was decreased by 80.1 kJ/mol compared to that of as-milled MgH₂. As a result, MgH₂+5 wt.%Ni+5 wt.%ZrO₂ demonstrated improved dehydrogenation and hydrogenation kinetics at 310 °C. The MgH₂+5 wt.%Ni+5 wt.%ZrO₂ sample released about 6.83 wt.% and absorbed about 6.10 wt.% in less than 30 min. Therefore, the co-catalysis of Ni and ZrO₂ significantly enhances the hydrogenation and dehydrogenation properties of MgH₂.     

Effects of KNbO3 catalyst on hydrogen sorption kinetics of MgH2

The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ternary compound catalyst such as K₂NiF₆, the influence of KNbO₃ on hydrogen storage properties of MgH₂ has been investigated for the first time. The MgH₂ - KNbO₃ composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH₂ and MgH₂-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH₂ with other Nb-contained catalysts such as Nb, NbO and Nb₂O₃, the desorption rate was almost the same as the one registered for as-milled MgH₂. The analysis of differential scanning calorimetry (DSC) showed that MgH₂-KNbO₃ composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH₂ without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol⁻¹ for this material, while for the as-milled MgH₂ was about 165 ± 2.0 kJ mol⁻¹. It is believed that Nb-ternary oxide catalyst (KNbO₃) showed a good catalytic effect and enhance the sorption kinetics of MgH₂.     

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.     

Hydrogen desorption properties of MgH2/LiAlH4 composites

The hydrogen desorption properties of MgH₂–LiAlH₄ composites obtained by mechanical milling for different milling times have been investigated by Thermal Desorption Spectroscopy (TDS) and correlated to the sample microstructure and morphology analysed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The MgH₂–LiAlH₄ composites show improved hydrogen desorption properties in comparison with both as-received and ball-milled MgH₂. Mixing of MgH₂ with small amount of LiAlH₄ (5 wt.%) using short mechanical milling (15 min) shifts, in fact, the hydrogen desorption peak to lower temperature than those observed with both as-received and milled MgH₂ samples. Longer mixing times of the MgH₂–LiAlH₄ composites (30 and 60 min) reduce the catalytic activity of the LiAlH₄ additive as revealed by the shift of the hydrogen desorption peak to higher temperatures.     

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.     

Improved de/re-hydrogenation properties of MgH2 catalyzed by graphene templated Ti–Ni–Fe nanoparticles

The present investigation deals with the excellent catalytic effect of graphene templated Ti–Ni–Fe nanoparticles (Ti–Ni–Fe@Gr) on de/re-hydrogenation characteristics of MgH₂. The catalytic effect of Ti–Ni–Fe@Gr on MgH2 has also been compared with Ti@Gr, Ni@Gr, and Fe@Gr. It has been found that Ti–Ni–Fe@Gr lowers the onset desorption temperature up to 252 °C with improved kinetics and cyclability for the hydrogen release and absorption from MgH₂. The presence of a multivalence environment around Mg/MgH₂ has been analyzed by XPS analysis which gives the evidence of possible electronic exchange between the catalyst and Mg/MgH₂ during de-/rehydrogenation. Since Mg/MgH₂ and Ti–Ni–Fe are both anchored on graphene template, agglomeration detrimental to cycling is not possible. Thus negligible degradation of 0.22 wt% has been observed even after 24 cycles of de/re-hydrogenation.     

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.     

Integrated Ni/Nb2O5 nanocatalytic agent dose for improving the hydrogenation/dehydrogenation kinetics of reacted ball milled MgH2 powders

Reactive ball milling (RBM) technique was employed to synthesize ultrafine powders of MgH₂, using high energy ball mill operated at room temperature under 50 bar of a hydrogen gas atmosphere. The MgH₂ powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250 °C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics of as synthesized MgH₂ powders, three different types of nanocatalysts (metallic Ni, Nb₂O₅ and (Ni)_x/(Nb₂O₅)_y) were utilized with different weight percentages and independently ball milled with the MgH₂ powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results showed that the prepared nanocomposite MgH₂/5Ni/5Nb₂O₅ powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). This nanocomposite system showed excellent hydrogenation/dehydrogenization behavior, indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt% H₂ at 250 °C. At this temperature the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 180 complete cycles. No serious degradation on the hydrogen storage capacity could be detected and the system exhibited nearly constant absorption and desorption values of +5.46 and −5.46 wt% H₂, respectively.     

Hydrogen sorption in MgH2-based composites: The role of Ni and LiBH4 additives

In the present work we investigate the hydrogen sorption properties of composites in the MgH₂–Ni, MgH₂–Ni–LiH and MgH₂–Ni–LiBH₄ systems and analyze why Ni addition improve hydrogen sorption rates while LiBH₄ enhance the hydrogen storage capacity. Although all composites with Ni addition showed significantly improved hydrogen storage kinetics compared with the pure MgH₂, the fastest hydrogen sorption kinetics is obtained for Ni-doped MgH₂. The formation of Mg₂Ni/Mg₂NiH₄ in Ni-doped MgH₂ composite and its microstructure allows to uptake 5.0 wt% of hydrogen in 25 s and to release it in 8 min at 275 °C. In the MgH₂–Ni–LiBH₄ composite, decomposition of LiBH₄ occurs during the first dehydriding leading to the formation of diborane, which has a Ni catalyst poison effect via the formation of a passivating boron layer. A combination of FTIR, XRD and volumetric measurements demonstrate that the formation of MgNi₃B₂ in the MgH₂–Ni–LiBH₄ composite happens in the subsequent hydriding cycle from the reaction between Mg₂Ni/Mg₂NiH₄ and B. Activation energy analysis demonstrates that the presence of Ni particles has a catalytic effect in MgH₂–Ni and MgH₂–Ni–LiH systems, but it is practically nullified by the addition of LiBH₄. The beneficial role of LiBH₄ on the hydrogen storage capacity of the MgH₂–Ni–LiBH₄ composite is discussed.     

Effect of Ni and SAPO-34 co-additive on enhancing hydrogen storage performance of MgH2

The hydrogen storage performances of MgH₂ improved by the addition of Ni and SAPO-34 were studied in detail. The mixture of MgH₂ with Ni and SAPO-34 was a physical reaction as shown by the X-ray diffraction (XRD) results. The SAPO-34 and Ni were uniformly distributed on the surface of MgH₂. The thermodynamic and kinetic properties of 90MgH₂/5Ni/5SAPO-34 were investigated by differential scanning calorimetry (DSC) and pressure-composition-isothermal (PCI) methods. The results showed that the dehydrogenation activation energy of 90MgH₂/5Ni/5SAPO-34 decreased by 64.3 kJ/mol compared with that of MgH₂. In addition, the relationship between the value of dehydrogenation heat and hydrogen content was also investigated by in-situ calorimetry. The enthalpy value of each sample in the dehydrogenation processes were calculated by in-situ calorimetry measurement. The dehydrogenation enthalpies of as-milled MgH₂ and 90MgH₂/5Ni/5SAPO-34 were 63.2 kJ/mol H₂ and 53.6 kJ/mol H₂, respectively. Thus, the co-doping of Ni and SAPO-34 contributed significantly to decrease the thermodynamic stability and improve the hydrogen sorption kinetic properties of MgH₂.     

Room temperature conversion of Mg to MgH2 assisted by low fractions of additives

In recent works, it was noticed that Mg/MgH₂ mixed with additives by high energy ball milling allows temperature reductions of H₂ absorption/desorption without necessarily changing thermodynamic properties. Thus, the objective of this work was to investigate which additives, mixed in low fractions with MgH₂ powder would act as efficient hydrogen absorption/desorption catalysts at low temperatures, mainly at room temperature (RT). MgH₂ mixtures with 2 mol% additives (Fe, Nb₂O₅, TiAl and TiFe) were prepared by high energy reactive ball milling (RM). MgH₂–TiFe mixture showed the best results, both during desorption at 330 °C and absorption at RT. The hydrogen absorption was ≈ 2.67 wt% H₂ in 1 h and ≈ 4.44 wt% H₂ in 16 h (40% and 67% of maximum theoretical capacity, respectively). The MgH₂–TiFe superior performance was attributed to the hydrogen attraction by the created high energy interfaces and strong TiFe catalytic action facilitating the H₂ flow during Mg/MgH₂ reactions.     

Excellent synergistic catalytic mechanism of in-situ formed nanosized Mg2Ni and multiple valence titanium for improved hydrogen desorption properties of magnesium hydride

Nowadays, catalytic doping has been regarded as one of the most promising and effective methods to improve the sluggish kinetics of magnesium hydride (MgH₂). Herein, we synthesized Ni/TiO₂ nanocomposite with the particle sizes about 20 nm by an extremely facile solvothermal method. Then, the Ni/TiO₂ nanocomposite was doped into MgH₂ to enhance its reversible hydrogen storage properties. A remarkably enhancement of de/rehydrogenation kinetics of MgH₂ can be achieved by doped with Ni/TiO₂ nanocomposite, compared to that solely doped with Ni or TiO₂ nanoparticles. The hydrogen desorption peak temperature of MgH₂Ni/TiO₂ is 232 °C, which is 135.4 °C lower than that of ball-milled MgH₂ (367.4 °C). Moreover, the MgH₂Ni/TiO₂ can desorb 6.5 wt% H₂ within 7 min at 265 °C and absorb ∼5 wt% H₂ within 10 min at 100 °C. In particular, the apparent activation energy of MgH₂Ni/TiO₂ is obviously decreased from 160.5 kJ/mol (ball-milled MgH₂) to 43.7 ± 1.5 kJ/mol. Based on the analyses of microstructure evolution, it is proved that metallic Ni particles can react with Mg easily to form fine Mg₂Ni particles after dehydrogenation, and the in-situ formed Mg₂Ni will transform into Mg₂NiH₄ in the subsequent rehydrogenation process. The significantly improved hydrogen absorption/desorption properties of MgH₂Ni/TiO₂ can be ascribed to the synergistic catalytic effect of reversible transformation of Mg₂Ni/Mg₂NiH₄ which act as “hydrogen pump”, and the multiple valence titanium compounds (Ti^4+/3+/2+) which promote the electrons transfer of MgH₂/Mg.