Guoxiang Liang

Hydrogen desorption kinetics of a mechanically milled MgH2+5at.%V nanocomposite

Abstract The hydrogen desorption kinetics of mechanically milled MgH 2 +5at.%V nanocomposite were determined under various desorption pressures and temperatures. The reaction rate constant was extracted from the time-dependent desorption curves. The relationships of rate constant with pressure and temperature were established. It was found that the hydrogen desorption at high temperature and under high driving force, is controlled by the interface (Mg/MgH 2 ) motion. When the driving force is small, the early stage of hydrogen desorption is controlled by nucleation and growth and the later stage is controlled by long range hydrogen diffusion. At temperatures below 523 K, the nucleation and growth process dominates the hydrogen desorption. High temperature annealing (673 K) of the nanocomposite results in slower desorption kinetics and increased activation energy of desorption. At high temperatures, the rate-limiting step changes from interface control (before annealing) to surface control (after annealing), while at low temperatures, the rate-limiting step of desorption does not change after annealing.

Recent developments in the applications of nanocrystalline materials to hydrogen technologies

The paper discusses the application of nanocrystalline alloys as hydrogen storage materials and as electrocatalysts for solid polymer electrolyte fuel cells. After reviewing some of the requirements of metal hydrides for hydrogen fueled vehicles, the paper presents new results on the structure and hydrogen sorption properties of high storage capacity ball-milled magnesium hydride. The great advantages of milling the hydride instead of the pure metals for producing novel nanostructures with high surface area and for improving hydrogen sorption kinetics are presented. In the second part of the paper, the same technique has been extended to the milling of carbides and chlorides and coupled to a lixiviation process to produce new electrocatalysts for polymer electrolyte fuel cells. This new technology offers the possibility of producing nanoparticles with metastable structures whose specific surface area is much larger than that of any nanocrystalline powders made by conventional ball milling techniques. Pt-based nanoparticles were fabricated and tested as anode in fuel cells under pure and CO-contaminated hydrogen feedstreams.

Hydrogen storage properties of the mechanically alloyed LaNi5-based materials

Abstract Mechanical alloying has been used to synthesize LaNi 5 -based hydrogen storage alloys. Mechanical milling of the La and Ni powder blend results in the direct formation of nanocrystalline AB 5 phase. Hydrogen storage measurements show that this as-milled LaNi 5 compound does not absorb much hydrogen reversibly. Annealing leads to grain growth, release of microstrain, and to an increase of storage capacity. Substitution of La or Ni by a third element can easily be achieved by mechanical alloying. The structure and hydrogen storage properties of these LaNi 5 -based alloys prepared by mechanical alloying and annealing show no big difference with those of melt casting alloys.

Structure of nanocomposite metal hydrides

Abstract It has recently been shown that MgH2–(V, Nb) nanocomposite has very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium which eases hydrogen penetration into the material and by the particular microstructure of this nanocomposite. In order to have a better understanding of the nature and hydrogen desorption mechanism, a systematic structural study has been undertaken. The powder morphology and chemical phase distribution were observed by SEM. X-ray diffraction under hydrogen pressure was performed at different temperatures in order to see the effect of absorption/desorption process on the crystal structure. Crystallite size was evaluated by X-ray powder diffraction peak broadening. Real time X-ray investigations of hydrogen desorption in MgH2–Nb nanocomposite were performed using synchrotron radiation. For the first time, we were able to get direct evidence of the dehydrogenation mechanism. A metastable new niobium hydride phase associated with the desorption process was detected. This metastable hydride is probably the result of long-range ordering of hydrogen in the niobium hydride lattice in order to facilitate the hydrogen flow.

Magnesium-based nanocomposites chemical hydrides

Abstract The hydrolysis reaction of nanostructured MgH 2 and nanocomposites MgH 2 –X (X=Ca, Li, LiAlH 4 ) prepared by ball-milling was studied as a function of milling time and component proportion. It was found that nanocrystallinity greatly enhances the hydrolysis kinetics. Moreover, in this new class of chemical hydrides, the reaction also proceeds to full completion, contrary to some conventional chemical hydrides where the reaction stops before total completion due to the formation of passivation layers. The effect of addition of acidic solutions was also investigated.

Mechanical alloying and hydrogen storage properties of CaNi5-based alloys

Abstract CaNi 5 -based hydrogen storage materials have been synthesized by mechanical alloying of the elemental Ca and Ni powder blend, followed by an isothermal annealing. It was found that high energy ball milling of a Ca and Ni blend does not result in the formation of the CaNi 5 intermetallic compound. However, DSC measurements show that the CaNi 5 phase forms upon heating the mechanically milled powders to 395°C. Replacement of Ca by Ce or Mm (Mischmetal), and of Ni by Zn leads to direct formation of nanocrystalline alloy phases with CaCu 5 structure. The as-milled nanocrystalline powder does not absorb much hydrogen. A post-milling isothermal annealing at 640°C leads to grain growth and release of microstrain, and to an improvement of the hydrogen storage properties. Replacement of Ca by Ce or Mm increases the plateau pressure, while replacement of Ni by Zn reduces the desorption plateau pressure.