Y.-M Sun

SURFACE PROPERTIES OF THE F-TREATED ZRTIVNI ALLOY

Abstract The effects of the chemical treatment by an aqueous fluorine-containing solution (F-treatment) on the surface properties of the dendrite-free AB 2 alloy, Zr 0.5 Ti 0.5 V 0.75 Ni 1.5 , were investigated by electron probe microanalysis, electron spectroscopy for chemical analysis and inductively coupled plasma spectrometry. The F-treatment significantly improves the electrochemical activation and kinetic properties of the alloy because of the formation of microcaves on the Ni-rich alloy surface after F-treatment.

Studies on the fluorination method for improving surface properties and characteristics of AB5-types of hydrides

A fluorination technique aiming at increasing the specific surface area has been studied in this work. The fluorination process as a surface improvement procedure has been studied with regards to its fluoride formation mechanisms. In this paper, the proposed fluorination process is based on the control of pH values in a certain range by adding a buffer solution. Buffer solutions suitable for controlling pH change were considered and were selected. Surface structures and composition changes have been studied by ICPS, SEM, and EPMA.

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.

Studies on the properties and characteristics of the fluorinated AB5 hydrogen-absorbing electrode alloys

Abstract Fluoride layers have been known to contribute to the selective permeability of hydrogen molecules in the gas–solid reaction and provide a protective barrier against impurities such as water vapor, carbon monoxide, air, and others. One of the important roles of the fluorination processes is the removal of the oxide layer from the surface which considerably improves the initial charge/discharge characteristics. However, it has also been found that during fluorination, metallic Ni on the surface dissolves rapidly into the aqueous F − and HF 2 − ion containing solution (here after, denotes F-solution) and as the result, the electric conductivity is significantly reduced in the fluoride layer. This paper is aimed at investigating the effects of fluorination conditions on the electrochemical properties and characteristics of the AB 5 electrodes for Ni–metal hydride batteries. Special care has been taken to control fluoride formation and its thickness on the particle surface. Ni 2+ ions were added to the F-solution in order to protect Ni from being dissolved and lost in the vicinity of the particle surface. The thickness of the fluoride layer depends largely upon the specific surface area to be treated as a function of time. The thickness of the fluoride layer and the state of metallic Ni near the surface region was investigated by EPMA and ICPS and they were correlated with the initial discharge capacity, charge/discharge cycle life, and durability, against a 6 M KOH electrolyte solution.

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.

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.

A recovery of carbon oxides by methanation reactionthrough a pressure-temperature swing process by applyingactive protium in the fluorinated metal hydride

Abstract A method has been developed which uses hydriding alloys to formmethane through the reaction of absorbed carbon dioxide on the hydride surfaces with thedesorbed hydrogen from the hydride. As we know, hydriding alloys can easily absorb and desorbthrough changing temperature at low temperatures. It is possible to convert carbon dioxide tomethane using the Pressure–Temperature-swing process. It has been confirmed that fluorinated hydriding alloys with large specific surface area canbe used for methanation. Unlike untreated hydriding alloys, the water that is produced duringmethanation reaction shows less effect on the methanation reaction if using fluorinated hydridingalloys as the catalyst. The methane conversion rate can be calculated from mass conservation.

EFFECTS OF ZIRCONIUM ON THE SURFACE STRUCTURES OF THE FLUORINATED HYDROGEN-ABSORBING ALLOYS

Abstract The aim of this work was to utilize the F-treatment method to obtain a Ni-rich surface which has catalytic effects on the electrochemical characteristics of hydrogen-absorbing alloys. During the F-treatment of the hydrogen-absorbing alloys (AB5-type alloys), e.g. in the case of LaNi5, the Ni was partially dissolved into the F-solution, during the formation of LaF3 on the surface. By analysing the treatment results for different kinds of alloys, it was found that different ions have different dissolving speeds. By utilizing this property, if some appropriate elements (e.g. zirconium) are selected to be added to the LaNi5 alloys, the surface composition might be changed through F-treatment.