N. Cui

Effects of yttrium additions on the electrode performance of magnesium-based hydrogen storage alloys

Abstract The effects of yttrium additions on the electrode performance of Mg 1.9 Al 0.1 Ni 1− x Y x alloys have been investigated. The addition of yttrium substantially increases the discharge capacity of the electrodes and effectively improves the electrode high-rate dischargeability. For Mg 1.9 Al 0.1 Ni 0.7 Y 0.3 alloy electrodes, a discharge capacity of 170 mAh g −1 at room temperature has been achieved. On the other hand, the addition of yttrium results in a negative effect on the cycling stability for the Mg 2 Ni-type alloy electrodes.

Characteristics of magnesium-based hydrogen-storage alloy electrodes

Abstract Various Mg 2 Ni-type hydrogen-storage alloy electrodes are prepared and characterized at room temperature. The discharge capacity is improved markedly via partial substitution of titanium for magnesium and iron for nickel in the Mg 2 Ni alloy. Composites of Mg 2 Ni with Ti 2 Ni can also increase greatly the discharge capacity of the magnesium-based electrode. As a consequence, the magnesium-based hydrogen-storage alloys are promising materials for secondary batteries and may provide further improvements in capacity and cycling performance.

Effect of cobalt addition on the performance of titanium-based hydrogen-storage electrodes

Abstract The influence of cobalt addition on the performance of a Ti 2 Ni hydrogen-storage alloy is examined. The cobalt addition is confirmed by energy dispersive spectroscopy compositional analysis. X-ray diffraction (XRD) results reveal that cobalt partially substitutes the nickel in the Ti 2 Ni alloy. Charge/discharge cycle measurements show that the specific capacity of Ti 2 Ni electrodes increases with cobalt addition, reaches a maximum at a cobalt content of 0.67 at.% (Ti 2 Ni 0.98 Co 0.02 ), and then falls with further addition. The cycle life of Ti 2 Ni electrodes increases significantly with cobalt addition. Scanning electron microscopy analysis reveals that cobalt is effective in reducing the disintegration of the Ti 2 Ni hydrogen-storage alloy powder, while XRD analysis shows that cobalt restricts the oxidation of the Ti 2 Ni hydrogen-storage alloy. By contrast, the addition of cobalt does not inhibit the formation of the irreversible Ti 2 NiH 0.5 hydride phase.

Mechanism of early capacity loss of Ti2Ni hydrogen-storage alloy electrode

Abstract The mechanism underlying the rapid, early, capacity loss of titanium-based (Ti2Ni) hydrogen-storage alloys is examined via X-ray diffraction analysis. The formation and accumulation of Ti2NiH0.5, a hydride phase that cannot be reversibly charged/discharged according to the experimental results, is proposed as a dominant cause of the early capacity loss of such electrodes.

Effects of potassium-boron addition on the performance of titanium based hydrogen storage alloy electrodes

Abstract Effects of potassium-boron addition on the performance of titanium based hydrogen storage alloy electrodes were studied for the first time in the present work. Charge/discharge cycles showed a significant increase of cycle life as well as the specific capacity of the electrodes with potassium-boron addition. Detailed studies identified that potassium is responsible for the cycle life increase and the specific capacity improvement may be attributed to the addition of boron. Based on the present studies, a novel idea was also given by the present authors for the prevention of the capacity decay of hydrogen storage electrode.

Discharge behaviour of Mg2Ni-type hydrogen-storage alloy electrodes in 6 M KOH solution by electrochemical impedance spectroscopy

Abstract The discharge behaviour of Mg 2 Ni-type hydrogen-storage alloy electrodes in 6 M KOH is investigated by a.c. impedance at room temperature. Comparative measurements are also performed on an LaNi 5 electrode. The rate-determining step of the discharge process for the magnesium-based hydrogen-storage alloy electrode is dependent on the alloy composition and depth-of-discharge. The unmodified Mg 2 Ni has a high charge-transfer and mass-transfer resistance compared with LaNi 5 . Additions of yttrium and aluminium in Mg 2 Ni reduced considerably both the resistances and, thereby, produce a remarkable improvement in discharge capacity and rate-dischargeability.

Synthesis and electrode characteristics of the new composite alloys Mg2Ni-xwt.% Ti2Ni

Abstract A new composite alloy Mg 2 Ni- x wt.% Ti 2 Ni has been successfully synthesised using a ‘particle inlaying’ method. Scanning electron microscopy and energy dispersive spectroscopy revealed that very fine Ti 2 Ni particles were inlaid onto the surface of Mg 2 Ni particles by mechanical treatment and sintering. XRD showed the composite alloys were composed of primary alloys Mg 2 Ni, Ti 2 Ni and new phases TiNi, TiMg formed in the composite procedure. The electrode characteristics of Mg 2 Ni- x wt.% Ti 2 Ni alloys in an alkaline solution have been investigated and compared with those of Mg 2 Ni. The discharge capacity of the alloy electrode was effectively improved from 8 mA h g 1 of Mg 2 Ni to 165 mA h g 1 of Mg 2 Ni-40wt.% Ti 2 Ni at ambient temperature, which is almost comparable with that of Ti 2 Ni electrode (170 mA h g 1 ). It is believed that the fine Ti 2 Ni particles inlaid on the surface of Mg 2 Ni particles play a two-fold role: firstly, they hydride-dehydride as hydrogen storage materials themselves: secondly, they provide active sites and pathways for Mg 2 Ni hydriding-dehydriding. This is supported by analysis of discharge behaviour and electrochemical impedance spectra studies.

Studies on the performance of Ti2Ni1−xAlx hydrogen storage alloy electrodes

Abstract The effects of aluminium additions on the performance of Ti 2 Ni hydrogen storage alloy electrodes have been studied in the present work. The charge/discharge cycles of the electrodes revealed that the cycle life of the Ti 2 Ni electrodes greatly increases with increasing addition of aluminium. However, the specific capacity of the electrode severely decreases with increasing aluminium content. X-Ray diffraction analysis of the alloy with aluminium addition has indicated that a new phase. Ti 2 Al, was formed during alloy melting and co-existed with Ti 2 Ni. Electrochemical measurements and Auger electron spectroscopy analysis have further shown that the passivity of the new Ti 2 Al phase is responsible for the passivity of the electrode and consequently the cycle life increase of the electrode. The passivity of the electrode is also believed to be one of the reasons responsible for the specific capacity decrease of the electrodes with added aluminium.

Surface-modified Mg/sub 2/Ni-type negative electrode materials for Ni-MH battery

In order to further improve the electrode performance of Mg/sub 1.9/Y/sub 0.1/Ni/sub 0.9/Al/sub 0.1/ alloy at ambient temperature, its surface was modified by an ultrasound pretreatment in the alkaline solution and microencapsulation with Ni-P coating. The effects of various surface modifications on the microstructure and electrochemical performance of the alloy electrodes were investigated and compared in this paper. It was found that the modification with ultrasound pretreatment significantly improved the electrocatalytic activity of the negative electrode and then reduced the overpotential of charging/discharging, resulting in a remarkable increase of electrode capacity and high-rate discharge capability but having little influence on the cycle life. However, the electrode fabricated from the microencapsulated alloy powder showed a higher discharge capacity, better high-rate discharge capability and longer cycle life as well.

Comparative study of electrochemical behaviour of single-crystalline and polycrystalline LaNi5 alloy electrodes

Abstract A.c. impedance, cyclic voltammetry and linear polarization techniques were applied to the electrochemical characterization of LaNi 5 single crystalline and polycrystalline smooth electrodes in 6 M KOH solution. The single crystalline electrode was found to have a lower charge-transfer resistance and higher electrocatalytic surface activity along the [001] direction compared with the polycrystalline electrode. The total impedance of the hydriding reaction is much less for the single crystalline alloy than for the polycrystalline alloy. The mechanism of electrode hydriding reaction is related to the crystalline structure and orientation.