Electrochemistry, Vol. 68, No. 1, 2000


2

Li1+xV3O8 as a Lithium Insertion Host

Tomiya KISHI,* Jin KAWAKITA, and Takashi MIURA

Department of Applied Chemistry, Faculty of Science and Technology, Keio University (Hiyoshi 3-14-1, Kouhoku-ku, Yokohama 223-8522, Japan)

Received July 22, 1999 ; Accepted October 14, 1999

Lithium insertion behaviours of Li1+xV3O8 and its modified oxides were examined. Li4V3O8 phase was formed at x=1.5 in Li1+xV3O8 and had smaller insertion rate of lithium. Modification of VO5 unit in the layer and neighboring octahedral Li+ ions in the interlayer affected the formation of Li4V3O8 phase. Lattice disorder and layer stacking of the mother oxide also influenced the insertion behaviour of Li4V3O8 phase.


8

Cumulative Effect of Ether Units on Electrochemical Behaviors of Self-assembled Monolayers and Multilayers of Tetrathiafulvalenyl-tetrathiol with Oligo-ethyleneoxy Linkages

Hidetaka NAKAI, Masakuni YOSHIHARA, and Hisashi FUJIHARA*

Department of Applied Chemistry, Kinki University (Kowakae, Higashi-Osaka, 577-8502, Japan)

Received July 21, 1999 ; Accepted September 1, 1999

The electrochemical properties of the self-assembled monolayers of a tetrathiafulvalenyl-alkane monothiol (1) and the ether-substituted alkane monothiol (1-O) adsorbed onto gold electrodes showed no difference in their redox potentials and surface coverages. In contrast, the difference between the alkyl chain and the oxy-alkyl chain in the redox potentials and surface coverages was found in the self-assembled monolayers and the electropolymerized films of the tetrathiafulvalenyl-tetrathiol (2) and the ether-substituted tetrathiol (2-O) adsorbed onto gold electrodes.


11

Effect of Chemically Etched Electrolyte on Solid Oxide Fuel Cell Performance

Xinge ZHANG,* Satoshi OHARA, Kazuo MUKAI, Mitsue OGAWA, and Takehisa FUKUI

Japan Fine Ceramics Center (Mutsuo 2-4-1, Atsuta-ku, Nagoya-city 456-8587, Japan)

Received May 17, 1999 ; Accepted September 6, 1999

A chemical etching technique was used as a pretreatment of commercially available 8 mol % yittria stabilized zirconia (8YSZ) electrolyte to improve solid oxide fuel cell (SOFC) performance. It was found that the etching rate of the 8YS zincreased linearly with increases in hydrofluoric acid (HF) concentrations. Increasing the temperature of both a HF solution and a HF+HCl (hydrochloric acid) 9 solution accelerated the etching rate. Cell performance was improved not only by thinning the electrolyte but also by improved electrode performance, especially for the lanthanum strontium manganite (LSM)+YS zcomposite cathode. An electrode kinetics analysis and scanning electron microscope (SEM) results suggest that this improved electrode performance was due to increases in three phase boundary (TPB) area.


17

Charge Transport Properties of Polythionine Films Dependent on Electrode Potential and Solution pH

Wei WANG,* Teruhisa KOMURA, Takahiro YAMAGUCHI, and Kohshin TAKAHASHI

Department of Material Chemistry, Faculty of Engineering, Kanazawa University (40, Kodatsuno 2-chome, Kanazawa 920-0942, Japan)

Received April 30, 1999 ; Accepted September 21, 1999

The charge-transport properties of polythionine film were studied by cyclic voltammetry and ac impedance spectroscopy. The redox reaction of polythionine proceeded by a two-electron, two-proton transfer mechanism. Both the low-frequency resistance and the low-frequency capacitance obtained from the ac impedance response were proportional to the film thickness. The former showed a minimum, and the latter showed a maximum, at the half-wave potential. The coupled diffusion coefficient of electrons and protons also showed a maximum at the half-wave potential. This relationship between the oxidation level and the diffusion coefficient indicated that the rate of charge transport in the film may be controlled by electron transport. A decrease in the diffusion coefficient with increasing pH implied electron hopping accompanied with intermolecular proton exchange.


24

H2 Sensing Properties and Mechanism of Nb2O5-Bi2O3 Varistor-type Gas Sensors

Takeo HYODO, Eiichi KANAZAWA,a Yuji TAKAO,b Yasuhiro SHIMIZU, and Makoto EGASHIRA*

Faculty of Engineering, Nagasaki University (1-14 Bunkyo-machi, Nagasaki 852-8521, Japan)
aPresent address:Graduate School of Engineering Sciences, Kyushu University (6-1 Kasuga-koen, Kasuga-shi, Fukuoka 816-8580, Japan)
bFaculty of Environmental Studies, Nagasaki University (1-14 Bunkyo-machi, Nagasaki 852-8521, Japan)

Received July 26, 1999 ; Accepted September 29, 1999

Hydrogen sensing properties and mechanism of Nb2O5 varistors mixed with Bi2O3 (0-16.7 mol %) were investigated in the H2 concentration range of 0.2-2.0 % at 400-700 C. Pure Nb2O5 showed higher breakdown voltage and higher sensitivity of 1,200 V mm-1 to 2.0 % H2 at 400 C than the ZnO- and SnO2- based varistors reported before. The H2 sensitive properties of a Nb2O5 varistor were improved by the addition of Bi2O3 up to 5.0 mol % and the Nb2O5 varistor mixed with 1.0 mol % Bi2O3 exhibited the highest sensitivity at 400 C among the varistors tested. However, further addition of Bi2O3 resulted in significant deterioration of the sensitivity. The addition of Bi2O3 led to a slight decrease in the grain size, a change in shape of Nb2O5 particles and formation of Bi2Nb10-O28 at the surface of Nb2O5 particles. A.c. impedance measurement was performed to investigate the electric and electrochemical properties of the varistors. Resistances of the Nb2O5-Bi2O3 varistors were decomposed into four components, (i) bulk resistance (R0), (ii) grain boundary resistance (R1), (iii) resistance of oxide ion conduction (R2) and (iv)9 electrode-oxide interface resistance (R3). The R1, R2 and R3 decreased drastically with increasing H2 concentration, while R0 remained almost unchanged at 400 C. Further studies have confirmed that R1 mainly dominated the breakdown voltage of the Nb2O5-Bi2O3 varistors, and then the change in the potential barrier height per grainboundary determined the magnitude of the H2-induced shift in the breakdown voltage.


32

Charge/Discharge Performance of Electron Beam Deposited Tin Oxide Thin Film Negative Electrodes

Sang Cheol NAM,a,* Young Soo YOON,b Won Il CHO, Byung Won CHO, Kyung Suk YUN, and Hai Soo CHUNa

Battery and Fuel Cell Research Center, Korea Institute of Science and Technology, (P. O. BOX 131, Cheongryang, Seoul 136-650, Korea)
aDepartment of Chemical Engineering, Korea University (1, Anam dong 5 Ga, Seoul 136-701, Korea)
bThin Film Technology Research Center, Korea Institute of Science and Technology, (P. O. BOX 131, Cheongryang, Seoul 136-650, Korea)

Received May 19, 1999 ; Accepted September 29, 1999

Electron beam deposited tin oxide thin films were studied for use as a negative electrode for lithium rechargeable battery. Tin oxide thin films prepared at different heat treatment conditions (temperature and time) were investigated by the implementation of X-ray diffraction analysis, Auger electron spectroscopy, and atomic force microscopy. Charge/discharge performance of these thin films, typically exhibiting capacities higher than 300 mAh/g lasting beyond 100 cycles, were found to depend on the heat treatment temperatures, which influence the structure, grain sizes, and adhesion to the substrate. Capacity was decreased as film thickness increased, but capacity loss was very small in accordance with increase of charge/discharge rate. Using AC impedance analysis, it was found that capacity loss was caused by resistance increase at cut off voltage under 0.1 V. Lithiated SnO2/PAN/V2O5 type full cell showed the capacity of 200 mAh/g, with active voltage of 2.0-2.7 V.


38

Graphitic Carbon Tube Obtained by Catalytic Decomposition of CH4 for Anode of Li Ion Rechargeable Battery

Tatsumi ISIHARA,* Akira FUKUNAGA, Ranko AKIYOSHI, Masaki YOSHIO,a and Yusaku TAKITA

Department of Applied Chemistry, Faculty of Engineering Oita University (Dannoharu 700, Oita 870-1192, Japan)
aDepartment of Chemical Engineering, Faculty of Science and Engineering, Saga University, (Honjyou, Saga 840-8502, Japan)

Received June 18, 1999 ; Accepted August 20, 1999

Graphitic carbon nanotubes were prepared by the decomposition of CH4 over the Ni catalysts and the anodic properties of the obtained tubular graphite for Li-ion rechargeable battery were investigated. The Li intercalation capacity increased, however, the reversible intercalation capacity was decreased with increasing graphitic nanotube surface area. Graphitic tubes prepared at 973 K exhibited almost the theoretical Li intercalation capacity. In addition, it was found that a post annealing treatment of the graphite tube was effective for improving the coulomb efficiency.


42

Electrocatalytic Hydrogenation of Olefins Using Palladium Metal Microparticles Deposited on Viologen Film-coated Graphite Felt

Yoshitomo KASHIWAGI,* Norihiko SHIBAYAMA, Jun-ichi ANZAI, and Tetsuo OSA

Graduate School of Pharmaceutical Sciences, Tohoku University (Aramaki, Aoba-ku, Sendai 980-8578, Japan)

Received April 30, 1999 ; Accepted August 26, 1999

Palladium (Pd) metal microparticles were deposited in a viologen-containing polymer layer on the surface of a graphite felt electrode. The Pd-deposited electrode is effective for the electrocatalytic hydrogenation of the carbon-carbon double bond part of alkyl olefins (1-hexene, 1-decene, cis-2-penetene), conjugated enones (2-cyclohexene-1-one, 4-isopropyl-2-cyclohexene-1-one, 3,5,5-trimetyl-2-cyclohexene-1-one), styrene, and trans-stilbene with 100 % selectivity.



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