Jae-Han Jung

The activation mechanism of Zr-based alloy electrodes

The effect of La addition on the activation behaviour of ZrCrNi based electrode alloys has been investigated. The surface morphologies of as sintered and activated samples have been studied by SEM. It has been found that La addition improves the activation behaviour, which is attributed to the initiation of fine cracks around the La-rich particles and their expansion by hydrogen absorption.

Degradation behavior of Cu-coated Ti-Zr-V-Mn-Ni metal hydride electrodes

Abstract In order to improve the durability and elucidate the degradation mechanism of Ti–Zr–V–Mn–Ni metal hydride electrodes which have a poor cycling property in KOH electrolyte, the cycling behavior of these electrodes has been investigated after Cu-coating treatment. It was observed that the cycle life of these electrodes was improved with increasing amounts of Cu-coating. However, these electrodes still degraded with further cycling. To examine the reasons for the abrupt degradation of these electrodes, surface morphologies, concentration depth profiles and polarization resistance of the degraded Cu-coated electrodes were investigated. It was found that the Ti oxide film, which has been reported as a main reason of degradation in our previous work, formed on the surface of the degraded Cu-coated electrode and the polarization resistance of these electrodes increased greatly. From these results, it is suggested that eventually the problem of degradation behavior of these electrodes is not overcome perfectly through the Cu-coating process and the degradation mechanism of these electrodes can be clarified.

Improved electrochemical performance of AB2-type metal hydride electrodes activated by the hot-charging process

Several types of AB2 metal hydride electrode have been prepared with different conductive materials and different initial particle sizes of active material. It was found that the electrochemical performance of MH electrodes is strongly influenced by the initial particle size of the alloy powder as well as by the type and amount of added conductive materials. Electrodes with large alloy particles have better properties, such as higher capacity and dischargeability rate. Electrodes with sufficient conductive additives showed low utilization efficiency of active materials and low stability, resulting in a low capacity, poor rate capability and short cycle life. The electrode with 1 wt.% carbon black had better stability than the electrode with 20 wt.% Ni powder. It was also found that electrode properties could be greatly improved by hot-changing pretreatment. Electrochemical characteristics are discussed on the basis of EIS and SEM examinations.

Self-discharge mechanism of Vanadium–Titanium metal hydride electrodes for Ni–MH rechargeable battery

In order to investigate the effect of the equilibrium hydrogen pressure (plateau pressure) of metal hydride (MH) alloys on self-discharge behavior, V0.9Ti0.1 alloys having multi-plateau pressures (low and high plateau pressures) have been used as working electrodes in a half cell. The thermal desorption experiment and open-circuit potential monitoring were conducted to observe the self-discharge behavior of the electrode. From the thermal desorption spectra of the fully charged and discharged V0.9Ti0.1 electrode (to −0.7 V versus Hg/HgO), it is found that only the higher one of the two plateau pressure regions of the V0.9Ti0.1 electrodes is electrochemically useful for battery application. But the open-circuit potential change and thermal desorption spectra of the V0.9Ti0.1 electrode after various open-circuit storage periods prove that the self-discharge behavior is attributed to the hydrogen desorption from the low plateau pressure (10−8 atm) as well as the high plateau pressure (0.1 atm). Therefore, it is suggested that the self-discharge behavior of V0.9Ti0.1 electrodes can not be effectively suppressed by reducing the plateau pressure of alloys through alloy modification. In addition, the pressure–composition–isotherms (P–C–T) of the low pressure region can be estimated by using the open-circuit potential corresponding to this region in Nernsts equation.

New activation process for Zr-Ti-Cr-Mn-V-Ni alloy electrodes: The hot-charging treatment

Abstract To improve the activation behavior of AB 2 type Zr-Ti-Cr-Mn-V-Ni multi-component alloys, a new activation process called the hot-charging treatment has been developed. Alloy electrodes were immersed in KOH solution and charged simultaneously at various electrolyte temperatures and charge current densities. It was found that the activation behavior and the rate capability of this alloy electrode were greatly improved after the hot-charging treatment, and furthermore the treated electrode was fully activated after the first cycle. The effects of this treatment have been discussed on the basis of the results of scanning electron microscopy and auger electron spectroscopy.

Effect of Cu powder as compacting material on the discharge characteristics of negative electrodes in Ni–MH batteries

Abstract In order to investigate the effect of Cu powder as a compacting material on the electrode performance, negative electrodes were prepared by mixing Ti–Zr–Cr–Mn–V–Ni alloy powder with copper powder of various sizes and contents, and were analyzed with electrochemical techniques and scanning electron microscopy (SEM) after cycling. The electrode performances such as the rate capability and the low-temperature dischargeability of the Cu-compacted electrode are improved as the size of Cu powder decreased or its content increased. These performances are better than those of the Ni-compacted electrode. The Cu-compacted electrode shows a high exchange current density for the hydrogen evolution reaction and a low contact and reaction resistance. From SEM analysis of the cycled electrode compacted with copper powder, it is also found that the surface of the MH (metal hydride) particles of the cycled Cu-compacted electrode is covered with copper grains (2∼3 μm) and whiskers. Therefore, it is suggested that the improved electrode characteristics are attributed to the copper layer deposited on the MH particles by the dissolution and precipitation (DP) process.

The effects of oxygen on diamond synthesis by hot-filament chemical vapor deposition

The effects of oxygen addition on the synthesis of diamond are extensively studied by using the hot-filament chemical vapor deposition (HFCVD) method, in which it is simple and easy to control the deposition parameters independently. Diamond films are deposited on silicon wafers under the conditions of substrate temperature 530–950 ‡C; total reaction pressure 700–8000 Pa; and methane concentration 0.4–2.4% in both CH4–H2 and CH4–H2–O2 systems.At deposition conditions of low substrate temperature, high CH4 concentration or high total pressure, soot-like carbon and/or graphite are deposited without oxygen addition. When even a small amount of oxygen (about 0.6%) is added, well-faceted diamond films are observed in scanning electron microscopy micrographs and a sharp diamond peak in the Raman spectra appears. The range of deposition parameters for high-quality diamond syntheses are extended by oxygen addition (low substrate temperature, high methane concentration and high reaction pressure).

Self-discharge mechanism of vanadium-titanium metal hydride electrodesfor Ni-MH rechargeable battery

In order to investigate the effect of the equilibrium hydrogen pressure (plateau pressure) of metal hydride (MH) alloys on self-discharge behavior, V09Ti01 alloys having multi-plateau pressures (the low and high plateau pressure) have been used as working electrodes in a half cell. A thermal desorption experiment and open-circuit potential monitoring were conducted to observe the self-discharge behavior of the electrodes. From the thermal desorption spectra of fully charged and discharged V09Ti01 electrodes (to-0.7 V vs. Hg/ HgO), it was found that only the high plateau pressure region from V09Ti01 electrodes is electrochemically useful in battery application. However, the open-circuit potential change and thermal desorption spectra of V09Ti01 electrodes after various open-circuit storage periods prove that self-discharge behavior is caused by hydrogen desorption from the low plateau pressure region (10-8 atm) as well as the high plateau pressure region (0.1 atm). Therefore, it is suggested that the self-discharge behavior of V09Ti01 electrodes cannot be suppressed effectively by reduction of the plateau pressure of alloys through alloy modification.In addition, the pressure-composition-isotherms (P-C-T) of the low pressure region can be estimated by using the open-circuit potential corresponding to this region in Nernst’s equation.