Eiji Higuchi

Electrochemical properties and characteristics of a fluorinated AB2-alloy

Abstract A series of fluorination techniques have been developed in this laboratory to improve the surface condition of AB 2 alloy. The fluorination was found effective for removing oxide layer and implanting catalytic Ni to the alloy surfaces. The developed fluorination techniques improved considerably the initial activation characteristics, decreased the electrochemical reaction impedance and increased the rate capacity of AB 2 electrodes.

Improvement of the electrochemical properties of Zr-based AB2 alloys by an advanced fluorination technique

An advanced fluorination technique has been developed for improving the electrochemical performances of Zr-based AB2 alloys. During the procedure, alloys were ball milled in a complex fluoride solution. It was found that the electrode of the treated alloy exhibited excellent electrochemical performance. Its activation property and high-rate discharge capability were found to be satisfactory compared with the conventional AB5-based electrodes. The electrode demonstrated a significant decrease of the reaction resistance measured by an impedance spectrometric method. The F-treated alloy was found to give a larger specific surface area and to form a highly Ni-covered surface.

The effect of the particle pulverization on electrochemical properties of Laves phase alloys

Abstract The effect of the particle pulverization of ZrV 0.2 Mn 0.6 Ni 1.2 , ZrV 0.2 Mn 0.6 Cr 0.1 Ni 1.2 and ZrTi 0.1 V 0.2 Mn 0.6 Co 0.1 Cr 0.1 Ni 1.2 Laves phase alloys on electrochemical properties is investigated in terms of the electrochemical reaction resistance, specific surface area and element dissolution in KOH solution. It is demonstrated that electrochemical properties of the Laves phase alloy are significantly dependent on the particle pulverization during charge/discharge(C/D) cycling. In particular, the ZrTi 0.1 V 0.2 Mn 0.6 Co 0.1 Cr 0.1 Ni 1.2 alloy is examined to have a longer cycle life, but a poor high rate capability as compared to other two alloys due to the hard pulverization and good corrosion resistance. On the other hand, the particle pulverization further increases the corrosion and dissolution rate of V and Mn, which is proportional to the specific surface area due to generation of a new fresh surface during cycling. The particle pulverization is identified to be of key importance affecting electrochemical properties of the Laves phase alloy.

Effects of fluorination temperature on surface structure and electrochemical properties of AB2 electrode alloys

Abstract In order to accelerate the activation of AB 2 electrode alloys, a new fluorination treatment has been developed which removes oxides from alloy surfaces and implants metallic Ni into the alloy surface layer. The behavior of Ni during fluorination treatment depends strongly on the treatment temperature. The constituent Ni of the alloy dissolves into the treatment solution at a low temperature, but Ni complex ion in treatment solution is chemically reduced by constituent elements at a high temperature. The temperature is the key factor of the fluorination treatment. The electrochemical properties were improved through increasing the treatment temperature due to the Ni implantation into surface layer.

A duplicated fluorination technique for hydrogen storage alloys

Abstract The fluorination technique developed in 1991 has been experimentally confirmed to be effective for modifying alloy surfaces to extremely high activation characteristics and a highly protective nature against poisoning materials such as air, water vapour, and carbon oxides in gas–solid reaction. However, the dissolution of a considerable amount of Ni during the fluorination process has been left as an unsolved problem for further development. The aim of this work is to prevent the decrease of the Ni concentration on the treated surface. A fluorination technique named as the duplicated fluorination technique, presented here aims at enriching the metallic Ni onto the increased surface area of the alloy by varying the pH-value down and up repeatedly in a definite range. Experimental studies have been performed extensively on the duplicated fluorination of LaNi 4.7 Al 0.3 and LmNi 5 based alloy particles. The surface reaction characteristics were found to be greatly improved as the results of (1) the removal of oxides, (2) its high affinity to both ionic and molecular hydrogen, and (3) its high electron conductivity of the functionally-graded surface composed of the enriched Ni and fluoride layer. The microstructure, element distribution, and composition analysis were examined by using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and inductively coupled plasma spectrometry (ICPS).

Electrochemical properties and characteristics of the fluorinated Zr0.9Ti0.1V0.2Mn0.6Ni1.3La0.05 electrode

Abstract Improvement of the initial activation characteristics and electrochemical reaction activity of the Laves-phase alloy surface are of basic importance for the application of hydriding alloy to Ni/MH batteries which exhibit larger discharge capacity than those of AB 5 . The fluorination effects of the electrode made by Zr 0.9 Ti 0.1 V 0.2 Mn 0.6 Ni 1.3 La 0.05 Laves phase alloy on the electrochemical properties and characteristics, especially on the initial charge–discharge capability and electrochemical reaction activity were investigated in terms of the electrochemical impedance and surface microstructures. The fluorination procedure employed to the La-incorporated Laves phase is significantly effective for improving initial activation characteristics. The Specific surface area of the fluorinated particles were increased by about 20 times larger than those of the untreated one from 0.27 to 5.31 m 2 /g which were contributed to the formation of La-fluoride, and also by the implantation of nickel over the particle surface. Fluorination treatment employed in this study for the La-incorporated Laves phase alloy was found effective for increasing the specific surface area and the surface charge acceptance during the initial charge process.

Effects of fluorination of AB2-type alloys and of mixing with AB5-type alloys on the charge–discharge characteristics

The effects of fluorination (F-treatment) of AB2-type Laves-phase Zr0.9Ti0.1V0.2Mn0.6Co0.1Ni1.1 (AB2) alloys and of mixing with untreated AB5-type MmNi3.68Co0.78Mn0.36Al0.28 (AB5) alloys on their charge–discharge characteristics were investigated. The F-treatment of the AB2 alloy greatly increased initial activation rate and discharge capacity and slightly improved high-rate dischargeability. The mixing of the F-treated AB2 alloy with untreated AB5 (<25 μm) alloy decreased somewhat the maximum discharge capacity compared to the fluorinated AB2 alloy but it greatly improved the high-rate dischargeability. In particular, the high-rate dischargeability for the AB2(F)ue5f8AB5 (<25 μm) mixture positively deviated from the estimated values based on the maximum discharge capacity for both alloys and their content, suggesting that the AB5 alloy can work as a catalyst for dehydriding the AB2 alloy hydride besides as a hydrogen reservoir; a synergistic effect appeared by the mixing.

Deterioration of Laves phase alloy electrode during cycling

A study is made of the deterioration mechanism of a Laves phase alloy electrode without addition of Ni powder by means of electrochemical impedance spectra (EIS), X-ray diffraction (XRD), inductively coupled plasma (ICP) and X-ray photoelectron spectroscopy (XPS). The XRD analysis shows that after cycling, the alloy bulk still has the capability to store hydrogen. The deterioration mechanism is considered to be mainly a surface process. In particular, particle pulverization (increased specific surface area and reduced the particle size) is the main reason for electrode deterioration. Dissolution of V and Mn increases gradually during cycling. On the other hand, particle pulverization increases further the dissolution rates of V and Mn, which are proportional to the specific surface area due to the generation of a new, fresh surface. In addition, it is demonstrated by XPS that the depth of oxide layers of Zr and Mn increases with cycling and results in an increase in the reaction impedance and a decrease in the surface electronic conductivity. Therefore, a change in the structure of the particle surface layer, due to particle pulverization and dissolution of V and Mn, contributes to the deterioration of the Laves phase electrode.

Dynamic P-c-T relations of the La-incorporated/fluorinated AB2 hydriding alloys

Abstract Pressure–composition–temperature (P–c–T) relations of the La-incorporated and fluorinated AB 2 alloy, Zr 0.9 Ti 0.1 V 0.2 Mn 0.6 Co 0.1 Ni 1.1 , were studied under isothermal dynamic conditions of 313 K and 343 K. The La-incorporation and fluorination effectively improved both the significant deviations existing in the static and dynamic P–c–T relations of conventional metal hydrides.