Huabin Yang

Dehydriding properties of ternary Mg2Ni1-xZrx hydrides synthesized by ball milling and annealing

Abstract The ternary hydrogen-storage alloys Mg 2 Ni 1− x Zr x (0 x ≤0.3) have been successfully synthesized by ball milling followed by annealing. Studies of the synthesis conditions of the ternary alloys and their dehydriding properties have been carried out. The results show that the ternary Mg 2 Ni 1− x Zr x (0 x ≤0.3) alloys have the same hexagonal crystal structure as that of Mg 2 Ni. Compared with a Mg 2 Ni alloy, they have a larger specific surface (∼1.20 m 2 g −1 ), more promising dehydriding kinetics, lower enthalpy of formation of hydrides than that of Mg 2 NiH 4 , and lower decomposition temperatures in an open system. An optimum desorption storage capacity of about 3.3 wt.% is observed.

The hydrogenation properties of Mg1.8Ag0.2Ni alloy

Mg-based hydrogen storage alloys are a promising choice for hydrogen energy systems. In the present work, Mg1.8Ag0.2Ni alloy has been synthesized by inter-diffusion. The structure and properties of the alloy have been characterized by XRD, SEM, XPS, PCT and measurements of discharge behavior. It is found that Mg1.8Ag0.2Ni alloy still retains its hexagonal Mg2Ni structure and an amorphous MgO layer covers the surface of the alloy. The alloy can absorb hydrogen at low temperature (150°C) without any treatment and activation due to its low absolute value of the enthalpy. The addition of Ag has no effect on the electrochemical discharge properties of alloy.

Pressure-composition isotherms of the Mg2Ni0.75Fe0.25-Mg system synthesized by replacement-diffusion method

Abstract The hydrogen-storage system Mg2Ni0.75Fe0.25-Mg (molar ratio is 2:1) has been successfully synthesized by the replacement-diffusion method (RDM). A study of the hydrogen desorption process was carried out. The results indicated that the pressure-composition isotherm (P-C-T) characteristics, plateau pressure and storage capacity change with temperature. The present system corresponds to a new hydrogen storage material with an optimum storage-capacity (3.3 wt.%).

Electrochemical studies of aluminum substituted α-Ni(OH)2 electrodes

Abstract Stabilized α -Ni(OH) 2 was used as an active material of pasted positive electrode in Ni/MH battery. Electrochemical properties such as discharge capacity and discharge potential were improved. Cyclic voltammetric studies indicated that α -Ni(OH) 2 electrode exhibited a better reversibility of Ni(OH) 2 /NiOOH redox reaction and higher oxygen evolution overpotential than β -Ni(OH) 2 electrode. This apparently would increase the charge efficiency, improve utilization of active material and minimize concurrent oxygen evolution.

Electrochemical properties of the Zr(V0.4Ni0.6)2.4 hydrogen storage alloy electrode

Abstract In order to improve the electrocatalytic activity, hydrogen adsorption performance and activation behaviour of the Zr(V 0.4 Ni 0.6 ) 2.4 alloy electrode, the alloy powder surface was modified by HF acid solution treatment. It was found that the alloy surface was transformed from a Zr-rich layer to a Ni-rich layer after the treatment and the electrocatalytic activity, hydrogen adsorption performance and activation behaviour of the alloy electrodes were significantly improved. An electrode reaction mechanism on the alloy surface (i.e. the mechanism of nickel-catalysis, hydrogen adsorption and hydrogen-transference) has been suggested. The Ni sites on the surface are not only the electrocatalytic reaction centre but also the hydrogen adsorption centre. In addition, the impedance spectra were fitted to an equivalent circuit using a non-linear, least squares fitting program.

Electrochemical impedance spectra study of the hydrogen storage electrode

The hydrogen storage electrode (MH) has been investigated by a.c. impedance measurements. The electrode impedance has been measured by superimposing an a.c. voltage of 5 mV amplitude ranging between 104 and 10−2 Hz. The impedance experiment result indicated that Cole-Cole plot for the electrode consisted of two obviously comparable semicircles and a slope related to Warburg impedance, and the semicircle in the high frequency region was very different from that in the low frequency region. The very small semicircle in the high frequency region was probably a typical double-layer capacitance; the large semicircle in the low frequency region which exhibited marked dependence on the hydrogen content was attributed to the electrochemical Heyrovsky reaction. The two semicircles were both affected by the electrode activation numbers and the depth of discharge. The impedance experiment also indicated that the semicircle in the high frequency region was almost independent of activation number, but that in the low frequency region was not. On increasing the depth of discharge, the two semicircles both reduced at first and increased later.

Calcium Zincate Synthesized by Ballmilling as a Negative Material for Secondary Alkaline Batteries

Calcium zincate has been successfully synthesized by a high-energy ballmilled method. The crystal structure, chemical composition, surface morphology, granularity distribution, and electrochemical performance have been investigated to compare with that prepared by a conventional chemical precipitation method. X-ray powder diffraction results revealed that the calcium zincate prepared by the two methods had a well-defined monoclinic structure using a Rietveld refinement method. Scanning electron microscope images and granularity analysis indicated that the ballmilled material was a finer powder with a higher specific surface area. The thermogravimetric and differential thermal analysis showed that there was a slight difference in the crystalline water content in the chemical composition of the powders prepared by the two methods. The galvanostatic charge and discharge results showed that the ballmilled powder had a promising cyclic behavior due to the better electrochemical stability as revealed from the cyclic voltammetry. It is suggested that the ballmilled calcium zincate is more suitable for applications in secondary alkaline batteries.

Characteristics of Mg2Ni0.75M0.25 (M=Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn) alloys after surface treatment

The ternary Mg2Ni0.75M0.25 (M=Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn) alloys have been successfully synthesized by the ball milling diffusion method (BDM). XRD results show that they all have a hexagonal crystal structure. We show that the alloys after fluorination treatment show good performance: the hydrogen desorption capacity reaches the highest value after only two adsorption–desorption cycles. The ratio of H/M is about 1.18 (≈3.3 wt.%) on desorption at 250°C. The hydrides have monoclinic phases. The larger the unit volume, the more unstable is the hydride. The dissociation enthalpies and temperatures of the hydrides in an open system are decreased with increasing unit cell volume. Replacement of Ni in Mg2Ni by Cr, Mn and Co has the same effect on Mg2Ni: they lower the decomposition plateau pressure; Ti and Cu have the opposite effect and Fe and Zn have little effect.

Effect of La Addition on the Electrochemical Properties of Secondary Zinc Electrodes

The electrochemical properties of secondary zinc electrodes after La addition by the solid-phase diffusion method have been investigated. The results showed that the La addition to the zinc electrode significantly improved the cycle life. After 80 cycles at 100 mA/g the zinc electrode with 0.5 atom % La addition showed the best cyclic behavior. Its discharge capacity was more than 520 mAh/g. The linear potentiodynamic polarization results showed that suitable La addition significantly suppressed the corrosion behavior of secondary zinc electrodes while little effect on the anodic behavior was found. The potentiostatic polarization results at - 100 mV overpotential showed that the growth of zinc dendrites was effectively inhibited by the La addition. Scanning electron microscopy images showed that La addition changed the formation of zinc deposition. X-ray photoelectron spectroscopy results revealed that lanthanum oxides [La 2 O 3 /La(OH) 3 ] were formed and enriched on the surface during cycling, which prevented the discharge products of zinc electrodes from dissolving in the alkaline electrolyte and suppressed the dendrite growth during charge.

Study on the multi-composition AB5 alloys including Li, made by the diffusion method, and their electrodes

The mischmetal-based hydrogen storage alloys MlNi3.8Co0.5Mn0.4Al0.3Lix (Ml=La-rich mischmetal, 0<x<0.2) have been successfully synthesized by the diffusion method (DM). The properties of these alloys have been carefully investigated by means of XRD, PAT, PCT and the measurement of electrochemical capacity. Experimental results show that the Li-containing alloy, whose defects increase greatly, has excellent discharge efficiency and its utilization ratio increases greatly at high discharging current density. The results show that the alloy can be used as the negative material in a Ni-MH battery.