Hydrogen Absorption Of Mg Research Articles

Catalytic mechanism of in-situ Ni/C co-incorporation for hydrogen absorption of Mg

Ni and carbon materials exhibit remarkable catalysis for the hydriding reaction of Mg. But the underlying mechanism of Ni/C hybrid catalysis is still unclear. In this work, density functional theory (DFT) calculation is applied to investigate the effect of Ni/C co-incorporation on the hydriding reaction of Mg crystal. The morphology and crystal structure of the Ni/C co-incorporated Mg sample show that the co-incorporated structure is credible. The transition state searching calculation suggests that both the incorporations of Ni and C are beneficial for the H2 dissociation. But Ni atom has a dramatic improvement for H2 dissociation and makes the H diffusion become limiting step of the hyriding reaction. The Ni dz2 orbit and H s orbit accept the electrons and combine together compactly, while the Ni dxy orbit is half-occupied. The catalytic effect of Ni on H2 dissociation can be ascribed to the bridging effect of Ni dxy orbit. The incorporation of C can weaken the over-strong interaction between Ni and H which hindered the H diffusion on Mg(0001). The Ni/C co-incorporated Mg(0001) shows the best performance during hyriding reaction compared with the clean and single incorporated Mg(0001).

Effect of Na and Cooling Rate on the Activation of Mg-Ni Alloys for Hydrogen Storage.

In spite of favourable hydrogen storage properties such as low density, high theoretical capacity (7.6 wt% H/MgH₂) and economics, commercial use of Mg-based alloys is not feasible due to long activation times, slow hydrogen sorption kinetics, and a high temperature for hydrogen release. Mg- Ni alloys have been considered promising materials for hydrogen storage systems as the Mg₂Ni intermetallic phase enhances the kinetics of hydrogen absorption in Mg-Ni alloys through a catalytic effect. It has been suggested that the refinement of eutectic in Mg-Ni alloys can further improve hydrogen absorption kinetics and that this can be achieved through trace Na additions. However, the refinement of the eutectic can also be achieved by increasing the cooling rate during solidification. In this study we investigate the effect of cooling rate and Na additions to Mg-Ni alloys on hydrogen absorption kinetics. Our results indicate that Na additions improve the hydrogen absorption kinetics independent of eutectic refinement and that the effect of the latter is relatively small.

Theoretical prediction and experimental study on catalytic mechanism of incorporated Ni for hydrogen absorption of Mg

Transition metals, including Ni, show good catalytic activity in the hydrogen storage reaction of Mg. In the present paper, first-principles calculation is performed to predict and analyze the hydriding reaction of Ni-incorporated Mg and experimental study is used to verify the accuracy of the forecast. Theoretical studies show that the hydriding reaction of Ni-incorporated Mg is a diffusion-controlled process. With Ni incorporation, the energy barrier of H2 dissociation is significantly decreased and the diffusion becomes the limiting step. Experimental studies confirm the results of theoretical studies. Besides, the material with Ni incorporation shows excellent activation performance and rapid absorption rates, leading to a high hydrogen content of 4.1 wt% in 60 s under 240 °C 3.0 MPa H2 and a low activation energy of 56.1 kJ mol−1 versus 0.4 wt% and 73.5 kJ mol−1 for the material without Ni incorporation. Atomic Ni only plays a role of catalyst.

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

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

Hydrogen sorption, kinetics, reversibility, and reaction mechanisms of MgH2-xLiBH4 doped with activated carbon nanofibers for reversible hydrogen storage based laboratory powder and tank scales

De/rehydrogenation kinetics and reversibility of MgH2 are improved by doping with activated carbon nanofibers (ACNF) and compositing with LiBH4. Via doping with 5 wt % ACNF, hydrogen absorption of Mg to MgH2 (T = 320 °C and p(H2) = 50 bar) increases from 0.3 to 4.5 wt % H2. Significant reduction of onset dehydrogenation temperature of MgH2 to 340 °C (ΔT = 70 °C as compared with pristine MgH2) together with 6.8–8.2 wt % H2 can be obtained by compositing Mg-5 wt. % ACNF with LiBH4 (LiBH4:Mg mole ratios of 0.5:1, 1:1, and 2:1). During dehydrogenation of Mg-rich composites (0.5:1 and 1:1 mol ratios), the formation of MgB2 and Mg0.816Li0.184 implying the reaction between LiBH4 and MgH2 favors kinetic properties and reversibility, while the composite with 2:1 mol ratio shows individual dehydrogenation of LiBH4 and MgH2. For up-scaling to hydrogen storage tank (∼120 times greater sample weight than laboratory scale) of the most suitable composite (1:1 mol ratio), de/rehydrogenation kinetics and hydrogen content released at all positions of the tank are comparable and approach to those from laboratory scale. Due to high purity (100%) and temperature of hydrogen gas from hydride tank, the performance of single proton exchange membrane fuel cell enhances up to 30% with respect to the results from compressed gas tank.

Experimental Challenges in Studying Hydrogen Absorption in Ultrasmall Metal Nanoparticles

Recent advances on synthesis, characterisation and hydrogen absorption properties of ultra-small metal nanoparticles (defined here as objects with average size ≤ 3 nm) are briefly reviewed in the first part of this work. The experimental challenges encountered in performing accurate measurements of hydrogen absorption in Mg- and noble metal-based ultra-small nanoparticles are addressed. The second part of this work reports original results obtained for ultra-small bulk immiscible Pd-Rh nanoparticles. Carbon supported Pd-Rh nanoalloys in the whole binary chemical composition range have been successfully prepared by liquid impregnation method followed by reduction at 300 °C. EXAFS investigations suggested that the local structure of these nanoalloys is partially segregated into Rh-rich core and Pd-rich surface coexisting within the same nanoparticles. Downsizing to ultra-small dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultra-small Pd90Rh10 nanoalloy can absorb hydrogen forming solid solutions under these conditions, as suggested by in situ XRD. Apart from this composition, common laboratory techniques such as, in situ XRD, DSC and PCI failed to clarify the hydrogen interaction mechanism : either adsorption on developed surfaces or both adsorption and absorption with formation of solid solutions. Concluding insights were brought by in situ EXAFS experiments at synchrotron: ultra-small Pd75Rh25 and Pd50Rh50 nanoalloys absorb hydrogen forming solid solutions at ambient conditions. Moreover, the hydrogen solubility in these solid solutions is higher with increasing Pd content and this trend can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS. The Rh-rich nanoalloys (Pd25Rh75 and Pd10Rh90) only adsorb hydrogen on the developed surface of ultra-small nanoparticles. In summary, in situ characterization techniques carried out at large scale facilities are unique and powerful tools for in-depth investigation of hydrogen interaction with ultra-small nanoparticles at local level.

Preparation of MgH2–Ni, MgH2–Ni–Si and MgH2–Ni–Ni2Si composites by mechanochemical method and their hydrogen absorption and desorption properties

To improve kinetics for hydrogen absorption of Mg and hydrogen desorption of MgH2, ternary composites were prepared from MgH2, Ni and Si or Ni2Si by a mechanochemical technique. The X-ray diffraction spectra of the resultant ternary composites after the initial hydrogen desorption treatment at 400 °C in vacuum suggested that in all composites the nanocrystalline Mg2Ni would be formed in the intergrain region between Mg and Ni components, while in the MgH2–Ni–Si composite the nanocrystalline Mg2Si would also be formed in the intergrain region between Mg and Si components. However, Ni2Si did not form any alloys with Mg. The hydrogen absorption rate at 250 °C for the MgH2–Ni–Ni2Si composite was comparable to that for the MgH2–Ni and MgH2–Ni–Si composites, while the hydrogen desorption rate at 250 °C decreased in the order of MgH2–Ni > MgH2–Ni–Ni2Si > MgH2–Ni–Si. In contrast, the hydrogen desorption rate at 220 °C for the MgH2–Ni–Ni2Si composite was faster than that for the MgH2–Ni composite, suggesting that Ni2Si was a key material in the improvement of hydrogen desorbability at lower temperatures. Moreover, the most plausible reaction model and the rate-determining process for hydrogen absorption and desorption at 250 °C were determined.

High-pressure and high-temperature x-ray diffraction cell for combined pressure, composition, and temperature measurements of hydrides

We present the design and construction of a high-pressure (200 bars) and high-temperature (600 °C) x-ray diffraction (XRD) cell for the in situ investigation of the hydrogen sorption of hydrides. In combination with a pressure, composition, and temperature system, simultaneous XRD and volumetric measurements become accessible. The cell consists of an x-ray semi-transparent hemispherical beryllium (Be) dome covering a heatable sample stage, which simultaneously allows sample temperatures of up to 600 °C in an applied hydrogen atmosphere of up to 200 bars. The system volume is as low as possible to maximize the precision of the volumetric measurements. Due to the high thermal conductivity of hydrogen, and in order to preserve the mechanical stability of the beryllium, the cell is water cooled. Its operability was studied on the example of the hydrogen absorption of Mg(2)Ni. The advantages and limitations of the proposed design are discussed.

Destabilization of the Mg-H System through Elastic Constraints

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

Hydrogen absorption in Mg–Y–Zn ternary compounds

The ternary magnesium alloys Mg 12YZn and Mg 3Y 2Zn 3 have been investigated from a hydrogen absorption point of view. The crystal structure, morphology and hydrogen absorption properties were investigated using X-ray diffraction, scanning electron microscopy, and differential thermal analysis. Hydrogenations were performed at temperatures between 25–400 °C and hydrogen pressures ranging from 70 kPa to 1 MPa. Both Mg 12YZn and Mg 3Y 2Zn 3 decompose into MgH 2, MgZn 2 and YH 3 at pressures above 1 MPa and 400 °C.

Hydrogen absorption and electrochemical properties of Mg 2− xNi ( x=0–0.5) alloys prepared by bulk mechanical alloying

Thermodynamic properties of hydrides of Mg 2− x Ni alloys produced by bulk mechanical alloying were determined from the pressure–composition isotherms for absorption over the temperatures from 623 to 423 K. The van’t Hoff plots for the plateau pressures of the isotherms clearly indicated the existence of high- and low-temperature hydrides with different entropy and enthalpy for hydride formation. The phase transition temperature was 525 K for Mg 2.0Ni and decreased with increasing value of x. Chemical and electrochemical behaviors of Mg 2− x Ni alloys in an alkaline solution were precisely determined. It was found that hydrogen absorption in Mg 2− x Ni alloys takes place by only immersing the alloys in 6 M KOH solution. On the other hand, during electrical charging hydrogen content increased with the quantity of charged electricity and reached the maximum value. However, the hydrogen content decreased afterwards because the corrosion of the alloys in the alkaline solution is significant.