Wang Qidong

The electrochemical charge-discharge properties of ZrCrNi hydrogen storage alloys

Abstract Zr(Cr x Ni 1− x ) 2 (0.15 ⩽ x ⩽ 0.65) alloys were prepared by arc melting in an argon atmosphere. Through X-ray diffraction, the main crystal structure of the alloys was found to change from Laves phase cubic C15 to hexagonal C14 as the Cr to Ni ratio increased, and lattice parameters and hydrogen absorption capacities were mainly determined by the Cr content. The alloy Zr(Cr 0.35 Ni 0.65 ) 2 , which was mainly of C15 Laves phase structure, was found to have maximum electrochemical discharge capacity of 305 mA h g −1 at 298 K. The activation behaviors of Zr Cr Ni-based hydride electrodes were also studied. Fine alloy powders, especially those pulverized by cycling in hydrogen gas and where the electrodes were pre-treated with HF-H 2 O 2 solution, were highly activated. As both Zr and Cr have a high resistance against alkaline solution, it is reasonable for Zr Cr Ni alloy electrodes to have durable cycle lives. The chemical valence states and composition of electrode surfaces were also analyzed with X-ray photoelectron spectroscopy before and after charge-discharge cycling. An obvious variation in the Zr 3d 5/2 peak position before and after cycling was detected. The discrepancy is ascribed to the chemical environment differences of Zr atom in primordial and activated surface layers.

Activation mechanism of hydrogen-storage electrode alloy Ml(NiCoMnTi)5 produced by gas atomization

Abstract Electrochemical activation behaviour of hydrogen-storage alloy Ml(NiCoMnTi) 5 produced by conventional induction melting (CIM) and by Ar gas atomization (AGA) has been studied. It has been found that the surface oxide layer plays a minor part, while the change of energy inside alloy before and after hydriding is the major factor affecting the behaviour of activation. The larger the change of energy inside the alloy or the strain energy caused by H-atoms entering tetrahedral and octahedral sites during hydriding, the more difficult is the activation. The effect of surface oxide film on the activation could to be considered as the oxide film causing the energy of Hatoms entering the alloy to be increased, since H-atoms must consume some energy to break the surface oxide film initially, before entering the alloy.

The effects of etching with HCl on the hydrogen sorption properties of Mg2Ni

Abstract The effects of etching with HCl on the hydrogen sorption properties of Mg 2 Ni have been investigated. On being etched with HCl, Mg 2 Ni can be activated and hydrided easily at room temperature and its low temperature desorption kinetics are improved. However, its hydriding is incomplete at room temperature and its room temperature hydrogen absorption capacity degrades rapidly on cycling. X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy analyses show that the etching treatment removes the magnesium oxide overlayer and preferentially dissolves away magnesium; this results in a new surface layer composed mainly of porous, metallic nickel microspheres. We believe that the nickel microspheres act as the active catalyst for hydrogen absorption and desorption at lower temperatures. The incomplete hydriding and the cycling degradation of the hydrogen absorption capacity at room temperature are caused, we suggest, by the cracking and disintegration of the alloy powder on each cycle, with the newly generated surfaces deactivating very quickly.

An investigation of the removal of hydrogen from gas mixtures using misch-metal-based hydrogen storage metals☆

Abstract In order to recover helium from the purge gas of synthetic ammonia production unit using Sichuan natural gas as the raw material, the present authors developed several multicomponent misch-metal hydrogen storage alloys with widely different plateau pressures. Experiments were carried out on the removal of hydrogen from simulated gas mixtures composed of H 2 -Ar in different proportions. The effect of the change in various technological parameters, involving pressure, temperature and the rate of flow, and the effect of various combinations of alloys with different plateau pressures was investigated. The results showed that, with favourable technological parameters, the concentration of remnant hydrogen could be reduced to less than 0.2%.

Hole Concentration Dependence of Tc in Bi2Sr2CaCu2Oy System

The resistance, ac susceptibility and Hall coefficient measurements were carried out for the high quality single phase Bi2Sr2CaCu2Oy samples with different oxygen contents. It is found that the number of extra oxygen atoms in the samples can be easily modulated when they are annealed in low vacuum, and that the relationship between Tc and P+ is unmonotonic with an optimum P+=0.27 holes/CuO2 plane corresponding to the highest Tc (=95 K). This unmonotonic relationship is discussed briefly.

PHOTOELECTRON SPECTROSCOPY STUDY ON THE BONDING CHARACTERISTICS OF Mg, Mg2Ni, Mg2Cu AND THEIR HYDRIDES

Magnesium-based alloys are regarded as one kind of the most promising hydrogen storage materials. But they have two evident shortages: their hydrides too stable (working only at high temperatures) and poor hydriding and dehydriding kinetics. In order to improve their hydrogen storage properties, fundamental studies are urgently required. In order to understand the bonding characteristics of magnesium-based alloys and their hydrides, the authors measured the photoelectron spectra of Mg, Mg2Ni, Mg2Cu and their hydrides. The Mg 2p electron binding energy is 49.5 eV for Mg, 53.8 eV for MgH2, 49.4 eV for Mg2Cu, 49.2 eV for Mg2Ni, 49.2 eV for MgCu2, and 50.1 eV for Mg2NiH4. The Mg 2s electron binding energy changes in the same pattern. These values show that a quite large electron transfer from Mg atoms to H atoms occurs when Mg is hydrided to MgH2, and that the Mg-H bonds are rather ionic in nature. In the formation of intermetallicses from Mg and Ni (or Cu), electron transfer occurs from Ni(or Cu) atoms to Mg atoms. In the formation of Mg2NiH4 the increase of Mg 2p (or 2s) electron binding energy is much smaller than that in forming MgH2, and there shows some increase in the Ni 2p electron binding energy, which may suggest that Ni atoms as well as Mg atoms contribute some electrons to H atoms when Mg2Ni is hydrided, and the Mg-H bonds here are not so ionic as in MgH2. From the measured results, a micro-model is proposed to explain the stability and kinetics properties of hydrides of Mg and Mg-intermetallicses Mg2Ni and Mg2Cu.

THE SEPERATION AND RECOVERY OF HYDROGEN FROM THE RECYCLING GAS IN AMMONIA PRODUCTION BY MEANS OF LANTHANUM-RICH MISCHMETAL NICKEL HYDRIDE BEDS

The seperation and recovery of hydrogen by means of a MlNi5 (Ml: La-rich mischmetal) beds were studied. The influence of the impurity gas components (O2, H2O, N2, Ar, CH4 and NH3 etc) on the hydrogen absorption capacity, hydriding and dehydriding kinetics and cycling ageing stability of the beds was investigated for both stagnent gases and continuously flowing gas streams. In small reactors, at first artificially made gas mixtures and finally the actual recycling gas from ammonia production were tested. In the presence of trace ammonia (<100ppm) in recycling gas stream, the efficiency of recovery amounted to 85 - 93% and the purity of the product hydrogen was around 99.9%. When ammonia amounted to 2.5%, the efficiency of recovery decreased to 81 -86%. The hydrogen absorption capacity of the alloy bed remained unchanged after cycling 50 times, indicating the stability of the alloy satisfactory.