Jeon Choi

Effect of V and Zr on the electrochemical properties of La-based AB5-type metal hydride electrodes

In this paper, investigation has been carried out to find the effects of V- or Zr-addition and fluorination on the discharge capacity of AB5-type alloy with a multicomponent La–Ni–Al–Co–Mn–My (M=V, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3) system. Spectral studies have been made by EDS and SEM, and the crystal structure was also determined by XRD. In order to calculate the hydrogen storage capacity, the pressure–composition isotherms (PCT curves) were utilized. The metal hydride electrode was also used as a galvanostatic cycling test. As the amount of added V or Zr is increased, the portion of the second phase is also increased gradually, but the crystal structure, CaCu5-type Laves phase, is not changed. But, in the matrix, the main component is Ni, in one of the major phases the main component is La and in another minor phase the main component is V or Zr. It has been confirmed that the addition of V or Zr causes improvement in the discharge capacity (Zr, y≤0.05; V, y≤0.1) and the cyclability (Zr, y≤0.05; V, y≤0.1) up to 200 cycles. However, adding more than y=0.2 has some significant effect on these properties. Generally, V-containing alloys may have many second phases of a round shape appearing in dispersed form in the matrix, while Zr-containing alloys’ second phases appear with a plate or needle-like shape. La-rich particles precipitated along boundaries of the second phases which can act as sites of preferred hydrogen absorption, to increase the discharge capacity. In addition, many micro-cracks are formed around the La-rich particles at the surface of the La-rich particle itself. Therefore, such second phases are considered as catalytic sites for hydrogen penetration, surface reaction, initial activation and change transfer properties.

Effect of Ti and Zr additions on the characteristics of AB5-type hydride electrode for Ni–MH secondary battery

Abstract AB 5 -type intermetallic compounds were prepared by arc-melting in an argon atmosphere. The composition of a stoichiometric compound ( LMNi 3.6 Al 0.4 Co 0.7 Mn 0.3 ) M y ( LM = La - rich mischmetal , M=Ti, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3 ) with a hexagonal CaCu 5 structure was varied by stoichiometric and nonstoichiometric addition of Ti or Zr. The alloy surface was analyzed by energy-dispersive spectroscopy and scanning electron microscopy and the crystal structure was characterized by X-ray diffraction. To determine hydrogen storage capacity, the pressure composition isotherms (PCT curves) were utilized. Metal hydride electrode was characterized by galvanostatic cycling test. It was found that the addition of Ti ( y =0.05) or Zr ( y =0.02) improves the activation, discharge capacity and cycle life property. But, the alloy with Ti addition ( y ⩾0.2), containing a Zr addition ( y ⩾0.2), is found to have bad discharge capacity and cycle life. The kinetic properties are additionally increased by the formation of a Ti- or Zr-rich second phase. The improvement in discharge capacity caused by increasing the Ti or Zr content (0.1⩾ y ) in the alloy is attributed to active sites for the electrochemical reaction. Also, they seem to be strongly related with the shapes and amounts of the second phases.

Effect of vanadium content on electrochemical properties of la-based AB5-type metal hydride electrodes

Abstract Commercial La–Ni–Al–Co–Mn–V hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure and electrochemical properties. It is found that for the stoichiometric ‘B’-rich compound, single phase with CaCu 5 -type exists. However, for B-poor compounds, there is principally a CaCu 5 -type phase with a small amount of V-rich type phase and the amount of V-rich phase reduces with vanadium. With the increase of V y ⩽0.1 content, hydrogen storage capacity is enhanced, whereas when y =0.2–0.3 it is decreased. The discharge capacity and cyclability are increased considerably by addition of vanadium in the range 0.02–0.1 with a maximum value at about 0.02%. The decrease of capacity for high V content was also correlated with the amount of V-rich phase. The V-rich phase is consisted of La 0.1 Ni 2.6 Al 0.2 Co 2.0 Mn 0.6 V 1.3 . The improvement of kinetics is due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This can be explained because the improvement of capacity for alloys with low V content is due to better kinetics. These alloys have been subjected to analysis by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (PCT curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.

The influence of titanium on the electrochemical properties of the AB5-type metal hydrides

Abstract AB 5 -type intermetallic compounds were prepared by arc-melting in argon atmosphere. The composition of a stoichiometric compound LaNi 3.6 Al 0.4 Co 0.7 Mn 0.3 with a hexagonal CaCu 5 structure was varied by stoichiometric and nonstoichiometric addition of Ti. With the increase of the Ti y ⩽0.05 content in LaNi 3.6 Al 0.4 Co 0.7 Mn 0.3 Ti y , the hydrogen storage capacity is enhanced, whereas when y =0.1–0.3, it is decreased. The discharge capacity and cyclability are increased considerably by addition of titanium in the range of 0.02–0.1 with a maximum value at about 0.1%. The highest maximum capacity is achieved for a nonstoichiometric addition of 0.05% Ti. The kinetic properties are also additionally improved by the formation of a titanium-rich second phase. This can explain the improvement of the capacity for alloys with low Ti content. The decrease in capacity for high Ti content was also correlated with the amount of the Ti-rich phase. Therefore, the improvement of kinetics are due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This study has been aimed to improve the electrode properties of a series of multicomponent LaNi 3.6 Al 0.4 Co 0.7 Mn 0.3 Ti y ( y =0.0, 0.02, 0.05, 0.1, 0.2, 0.3) alloys which have mutual complementary properties. All the prepared alloys have been subjected to analyses by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms ( P – C – T curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.

High Temperature Charging Efficiency and Degradation Behavior of High Capacity Ni - MH Batteries

Recently the Ni/MH secondary battery has been studied extensively to achieve higher energy density, longer cycle life and faster charging-discharging rate for electric vehicles and portable computers, and etc. In this work, the charging efficiency of the Ni−MH battery which uses Ni electrode with addition of various compounds and the degradation behavior of the 90Ah battery were studied. The battery using the Ni electrode with Ca(OH)2 addition showed the charging efficiency and the utilization ratio significantly better than electrodes without added compounds. After 418 cycles, the residual capacities at the Ni electrode showed nearly the same values in the upper, middle and lower regions. In the case of the MH electrode, the residual capacity in the upper region appeared lower than that in other regions. As a result of ICP analysis, the amount of dissolved elements in the three regions appeared almost the same. The faster degradation in the upper region of the MH electrode was caused by the TiO2 oxide film formed at the electrode surface because of overcharging. The thickness of the oxide film increases with cycling, so it will form a layer that is not able to allow hydrogen to penetrate into the MH electrode.