Mg-based Metal Hydride Research Articles

Efficient hydrogen transfer carriers: hydrogenation mechanism of dibenzyltoluene catalyzed by Mg-based metal hydride

Efficient hydrogen transfer carriers: hydrogenation mechanism of dibenzyltoluene catalyzed by Mg-based metal hydride

Boosting the hydrogenation activity of dibenzyltoluene catalyzed by Mg-based metal hydrides

Hydrogenation of dibenzyltoluene (DBT) is of great significance for the application in liquid organic hydrogen carriers (LOHCs). We successfully develop Mg-based metal hydrides (Mg2NiH4, MgH2, and LaH3) reactive ball-milling for the hydrogenation of DBT. Mg-based metal hydrides milled with 500 min exhibit the best catalytic activity, the hydrogen uptake of DBT can reach 4.63 wt% at the first 4 h and finally achieve 5.70 wt% through 20 h, which is the first time to use hydrogen storage material as a catalyst for the hydrogenation of DBT. The excellent catalytic hydrogenation performance of Mg-based metal hydrides mostly originates from numerous catalytic activity centers formed at the surfaces of Mg2NiH4 nanoparticles in the MgH2 matrix. Inspired by this mechanism, more general metal hydrides can be explored for catalyzing the hydrogenation of LOHCs. The new application of Mg-based metal hydrides is beneficial to developing efficient LOHC based hydrogen storage systems and offers novel insights to hydride-based catalysts.

The effect of Na addition on the first hydrogen absorption kinetics of cast hypoeutectic Mg–La alloys

With superior properties of Mg such as high hydrogen storage capacity (7.6 wt% H/MgH2), low price, and low density, Mg has been widely studied as a promising candidate for solid-state hydrogen storage systems. However, a harsh activation procedure, slow hydrogenation/dehydrogenation process, and a high temperature for dehydrogenation prevent the use of Mg-based metal hydrides for practical applications. For these reasons, Mg-based alloys for hydrogen storage systems are generally alloyed with other elements to improve hydrogen sorption properties. In this article, we have added Na to cast Mg–La alloys and achieved a significant improvement in hydrogen absorption kinetics during the first activation cycle. The role of Na in Mg–La has been discussed based on the findings from microstructural observations, crystallography, and first principles calculations based on density functional theory. From our results in this study, we have found that the Na doped surface of Mg–La alloy systems have a lower adsorption energy for H2 compared to Na-free surfaces which facilitates adsorption and dissociation of hydrogen molecules leading to improvement of absorption kinetic. The effect of Na on the microstructure of these alloys, such as eutectic refinement and a density of twins is not highly correlated with absorption kinetics.

Vacancy-Mediated Hydrogen Spillover Improving Hydrogen Storage Properties and Air Stability of Metal Hydrides.

Hydrogen storage in metal hydrides is a promising solution for sustainable and clean energy carriers. Although Mg-based metal hydrides are considered as potential hydrogen storage media, severe surface passivation has limited their industrial application. In this study, a simple, cheap, and efficient method is proposed to produce highly reactive and air-stable bulk Mg-Ni-based hydrides by rapid treatment with water for 3 min. The nickel-decorated Mg(OH)2 nanosheets formed in situ during hydrolysis can provide a pathway for hydrogen desorption via vacancy-mediated hydrogen spillover, as revealed by density functional theory calculations, thereby significantly decreasing the peak dehydrogenation temperature by 108.2 °C. Moreover, water-activated hydrides can be stored under ambient conditions without surface decay and activity loss, exhibiting excellent air stability, which can be attributed to the chemical stability of the surface layer. The results provide alternative insights into the design of highly active, air-stable metal hydrides with low cost and promote the industrial application of hydrogenenergy.

Insight into destabilization mechanism of Mg-based hydrides interstitially co-doped with nonmetals: a DFT study

Mg-based metal hydride is one of the most promising materials for hydrogen energy storage. However, the high thermal stability due to strong bonding effects between the atoms limits its practical application. In order to reduce the thermal stability, a method of doping double nonmetals into Mg-based system was proposed in this study. The density functional theory (DFT) calculation results showed that the thermal stabilities of both the B-N co-doped Mg-based alloy and its hydride are reduced compared with pure Mg-based system. The relative formation enthalpies of the alloy and its hydride are 0.323 and 0.595 eV atom−1, respectively. The values are much higher than those for either singly B- or N-doped Mg-based system. The more significant destabilization by doping double nonmetal elements than single element is mainly attributed to a dual effect in weakening Mg–Ni/NiH4 bonds, caused by criss-cross interactions between B–Ni and N–Mg bonds.

Influences of interstitial nitrogen with high electronegativity on structure and hydrogen storage properties of Mg-based metal hydride: A theoretical study

Interstitially doping nonmetal elements has been viewed as an effective way to improve the hydrogen storage properties of Mg-based metal hydride. In this work, the influences of the concentration of N with high electronegativity on the crystal structure, hydrogen absorption/desorption kinetics and thermodynamics of Mg-based system were systematically investigated by first-principle calculations. For Mg2NiNx (x = 0, 0.25, 0.5) system and its hydride, the doping positions of interstitial N atom in the crystal structures were firstly determined. The calculation results showed that the addition of N element has the favorable effects on the improvement of both the kinetic and thermodynamic properties. N-doped Mg-based system with x = 0.5 has the best hydrogen absorption/desorption performances. Compared with pure Mg-based system, the formation enthalpy and dehydrogenation energy of Mg2NiN0.5 system are reduced respectively by up to 98% and 59% due to the strong hybridization between the orbits of N and H. Besides, Mg2NiN0.5 system is able to achieve the reversible hydrogen absorption/desorption reactions at ambient temperature, which makes it suitable for most practical applications.

First-principles insights into influencing mechanisms of metalloid B on Mg-based hydrides

First-principle calculations were carried out to provide insights into the improved dehydrogenation performance of B-doped Mg-based metal hydrides in this work. Through the analysis of the crystal structure, dehydrogenation kinetics and thermodynamics, it was found that both of the kinetic and thermodynamic properties are improved by interstitially doping metalloid B into Mg-based alloys. The relative formation enthalpies of Mg-B-based alloys and their hydrides could reach up to 0.508 and 0.303 eV·atom−1, respectively, on account of the mutual interactions between B and Mg atoms. The dehydrogenation energy and desorption temperature of Mg-B-based hydrides could be reduced respectively by up to 32.2% and 166 °C, due to the weakening of the bonding effects between Ni and H atoms caused by the hybridization of B s, B p and H s orbitals. Moreover, the Mg-B-based hydrides with the moderate desorption temperature of 75 °C are suitable for most practical applications.

Improvement in hydrogen desorption performances of magnesium based metal hydride reactor by incorporating helical coil heat exchanger

Mg-based metal hydride has been viewed as one of the most promising materials for hydrogen storage. However, large amount of reaction heat during the hydrogen absorption/desorption cycles negatively affects the heat and mass transfer, thus leading to poor performances. In this work, helical coil heat exchanger (HCHE) is proposed to be inserted into Mg-based metal hydride reactor for improving the heat and mass transfer. The models on pressure-composition-temperature properties and hydrogen desorption kinetics of Mg-based reactor are firstly established for the simulation through experiments. The comparison of hydrogen desorption performances for the reactors incorporating the conventional straight tube, finned tube and new helical coil heat exchangers is conducted based on numerical simulation. The comparison results show that the HCHE has the best effects on improving the heat and mass transfer characteristics of reactor among the three heat exchangers, due to its secondary circulation. Besides, it is suggested that the influence of hydrogen pressure gradient should be considered for the prediction of the performance of Mg-based metal hydride reactor with HCHE.

THE EFFECT OF MAGNETITE (Fe3O4)CATALYST FROM IRON SANDS ON DESORPTION TEMPERATURE OF MgH2 HYDROGEN STORAGE MATERIAL

One of the future technologies for a safe hydrogen storage media is metal hydrides. Currently, Mg-based metal hydride has a safety factor and efficient for vehicle applications. However, the thermodynamic properties of magnesium hydride (MgH2) found a relatively high temperature. High desorption temperatures caused MgH2 high thermodynamic stability resulting desorption enthalpy is also high. In this study, natural mineral (iron ore) has been extracted from iron sand into powder of magnetite (Fe3O4) and used as a catalyst in an effort to improve the desorption properties of MgH2. Magnetie has been successfully extracted from iron sand using precipitation method with a purity of 85 % , where the purity of the iron sand before extracted was 81%. Then, MgH2-Fe3O4 was milling using mechanical alloying method with a variety of catalysts and milling time. The observation by XRD showed the material was reduced to nanocrystalline scale. MgH2 phase appears as the main phase. DSC test results showed with the addition of Fe3O4, the desorption temperature can be reduced up to 366oC, compared to pure pure MgH2 reached by 409o C. Furthermore, based on gravimetric test, the hydrogen release occurs at a temperature of 388o C, weight loss of 0.66 mg during 16 minutes.

Efficiency Calculations for SOFC/SOEC Reversible System and Evaluations of Performances of Button-Size Anode-Supported Cell

The energy storage efficiency of a Solid Oxide Fuel Cell and Solid Oxide Electrolysis Cell (SOFC/SOEC) reversible system was calculated. The energy storage efficiency for the SOEC mode was estimated to reach 94%-HHV. Additionally, when the system was combined with a Mg-based metal hydride, endothermic heat for water vaporization in the SOEC mode and exhaust heat in the SOFC mode can be managed efficiently to encourage the Mgbased metal hydride to absorb and desorb hydrogen gas in the reversible system. This reveals that the SOFC/SOEC is a promising system without a significant energy loss in storing energy. In addition to the numerical simulations on the energy storage performance for the SOFC/SOEC system, the I-V characteristics of a button-size anode-supported cell were also evaluated. In contrast to the SOFC mode, a larger overvoltage was observed in the SOEC mode at a high current density over 0.6 A cm-2, which was possibly caused by insufficient supply of steam. The condition of such a large overvoltage would damage the cell and indeed we found some cracks at the 8YSZ-electrolyte part after the test.

Improvement in hydrogen storage characteristics of Mg-based metal hydrides by doping nonmetals with high electronegativity: A first-principle study

The effects of a small amount of nonmetal elements (N, F and Cl) with high electronegativity interstitially doping on improving the hydrogen storage characteristics of Mg-based metal hydrides were systematically investigated by first-principle calculations in this paper. The interstitial positions which the doping elements easily occupied were firstly determined. The calculation results showed that these elements are most likely to hold the center position of octahedral sites with two Ni and four Mg atoms. Based on this, the crystal structures, thermal stability, dehydrogenation energy and electronic structures of all the crystals, including Mg2Ni, Mg2NiN0.5, Mg2NiF0.5, Mg2NiCl0.5 and their hydrides, were further investigated. The nonmetals with high electronegativity exhibit the favorable effects on the characteristics of Mg-based metal hydrides. Doping F significantly reduces the dehydrogenation energy of Mg2NiH4 by about 25%, because of the strong hybridization between F and H atoms. When doping Cl into Mg2Ni and Mg2NiH4, the formation enthalpies decrease respectively by 0.047 and 0.024eVatom−1, due to the reduction of integral intensity of the bonding electron. Among the three elements, N has the best effects on improving both kinetics and thermodynamics. Doping N not only causes the formation enthalpies of Mg2Ni and its hydride to decrease by 0.215 and 0.141eVatom−1 respectively, but also reduces the dehydrogenation energy of Mg2NiH4.

Segregation Effect on Nanoscale Mg - Based Hydrogen Storage Materials

The nanocrystalline Mg-based metal hydrides offer a breakthrough in prospects for practical applications. In this work, we study experimentally the structure, electrochemical properties and surface segregation effect of nanocrystalline and microcrystalline Mg2M alloys and Mg2M/M’ (M=Cu, Ni; M’=C, Ni, Pd) nanocomposites. These materials were prepared by mechanical alloying (MA). In the nanocrystalline Mg2Cu powder, discharge capacity up to 30 mA h g-1 was measured. It was found that nickel substituting copper in Mg2Cu1-xNix alloy greatly improved the discharge capacity of studied material. In nanocrystalline Mg2Ni powder, discharge capacities up to 100 mA h g-1 were measured. Additionally, it was found that mechanically coated Mg-based alloys with graphite, nickel or palladium have effectively reduced the degradation rate of the studied electrode materials. Finally, the properties of nanocrystalline alloys and their nanocomposites are compared to that of microcrystalline samples. X-ray photoelectron spectroscopy studies showed that the surface segregation of Mg atoms and valence band width in the nanocrystalline Mg2M alloy are greater compared to those observed in microcrystalline Mg2M. Especially, a strong surface segregation of Mg atoms was observed for the Mg2Ni/M’ composites. In that case, Mg atoms strongly segregate to the surface and form a Mg based oxide layer under atmospheric conditions. The lower lying Ni and M’ atoms form a metallic subsurface layer and could be responsible for the observed relatively high hydrogenation rate. Furthermore, the valence band broadening observed in the nanocrystalline Mg2Ni alloys and Mg2Ni/M’ composites could also significantly influence their hydrogenation properties.

Nanostructured Mg 2Ni materials prepared by cold rolling and used as negative electrode for Ni–MH batteries

In the present work, cold rolling has been investigated as a new means of producing Mg-based metal hydrides for nickel–metal hydride (Ni–MH) batteries. Structure and electrochemical evolution of 2Mg–Ni cold-rolled samples were investigated as a function of the number of rolling passes as well as heat treatment. It was found that nanocrystalline Mg 2Ni alloy can be obtained by an appropriate three step process involving rolling, heat treatment and rolling again. It was shown that the number of primary and secondary rolling passes must be carefully optimized in order to favour the complete formation of Mg 2Ni alloy having a nanocrystalline structure (∼10 nm in crystallite size) without excessive sample oxidation. Actually, the best result was obtained by first rolling 90 times, followed by a heat treatment at 400 °C for 4 h and roll again 20 times. The resulting material displayed an initial discharge capacity of 205 mAh g −1, which is quite similar to that obtained with ball-milled Mg 2Ni alloy.

A new type of Mg-based metal hydride with promising hydrogen storage properties

Three Mg 3 RE (rare earth abbreviated as RE) compounds were prepared in Mg–La, Mg–Nd and Mg–misch metal (abbreviated as Mg–Mm) systems by induction melting. XRD patterns show that the as melted Mg 3 RE compounds have the D 0 3 structure. The lattice constants of Mg 3 La , Mg 3 Mm and Mg 3 Nd phases are 0.7498, 0.7451 and 0.7390 nm, respectively. The lattice constants of the new hydride phases formed from Mg 3 La and Mg 3 Nd during the hydrogenation process with fcc structure are 0.5607 and 0.5598 nm, respectively. The alloys have good hydrogen storage properties and could absorb hydrogen at room temperature with rapid hydriding and dehydriding kinetics. The maximum hydrogen absorption content of Mg 3 La , Mg 3 Mm and Mg 3 Nd alloys are 2.89, 2.58 and 1.95 wt%, respectively. The enthalpy ( Δ H ) and entropy ( Δ S ) of Mg 3 RE – H dehydriding reaction were also determined. Hydrogen absorption kinetic curves measured at room temperature could be well fitted by the Avrami–Erofeev equation.

Fiber optic hydrogen detectors containing Mg-based metal hydrides

We report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic hydrogen detectors. Due to the change in optical properties of Mg-based alloys on hydrogen absorption, a drop in reflectance by a factor of 10 is demonstrated at hydrogen levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg–Ni and Mg–Ti based alloys, we find that the latter has superior optical and switching properties. Using a 50 nm thick Mg 70Ti 30 film capped with a 30 nm Pd catalytic layer as the sensing layer, we report on the sensitivity of the fiber optic detector for hydrogen both in argon and oxygen, the temperature behavior and the reversibility.

The application of Mg-based metal-hydrides as heat energy storage systems

Mg-based metal hydride systems are potential high temperature heat storage media. In this paper the features and possibilities of the systems Mg/MgH 2, Mg–Ni/Mg 2NiH 4, Mg–Fe/Mg 2FeH 6 and Mg–Co–H are discussed. All the materials show cyclic stability in certain temperature ranges. The thermal energy which is released by these systems covers a temperature range from 250°C to 550°C in which high thermal energy densities of up to 2257 kJ/kg are reached.