Minoru Umeda

Kinetic Characterization of Single Particles of LiCoO2 by AC Impedance and Potential Step Methods

This is the first report of impedance technique run on single particle electrodes with the aim of clarifying its electronic and ionic transport properties. Measurements were successfully conducted on a particle of 15 μm diam resulting in impedance magnitude on the order of MΩ. The impedance spectra exhibited (i) one semicircle in the high frequency region, (ii) Warburg impedance in low frequencies, and finally, (iii) a limiting capacitance in the very low frequencies. The spectra were analyzed using a modified Randles-Ershler circuit, so that the reaction kinetics could be precisely evaluated. The charge transfer resistance decreased as the potential increased, whereas the double layer capacitance was almost invariant with the potential. Thus, the apparent chemical diffusion coefficient of lithium ions was determined to be to as function of electrode potential. These results are in agreement with those obtained by potential step chronoamperometry technique.

Porous-microelectrode study on Pt/C catalysts for methanol electrooxidation

AbstractWe havedeveloped a porous-microelectrode (PME) to investigate the electroactivity of catalyst particles for proton exchangemembrane fuel cells. The cavity at the tip of the PME was filled with Pt/C catalysts prepared by impregnation method. Cyclicvoltammograms (CVs) recorded in 1 N H 2 SO 4 aqueous solution revealed that the active area of the stacked catalysts exist not onlyat the surface but also inside of the stack. For methanol electrooxidation, 30 wt.% Pt/C exhibited the highest electroactivity, whereasthe 50 wt.% Pt/C showed extremely small current. The small current is considered as a result of a small active-surface area. Methanoloxidation peak potential shifted toward cathodic direction as Pt-loading decreased, which agrees well with the Pt-oxide formationpotential. The activation energy for methanol oxidation was assessed to be 449 / 3 kJ mol 1 for all Pt/C catalysts and Pt-discelectrode.# 2003 Elsevier Science Ltd. All rights reserved. Keywords: Porous-microelectrode; Direct methanol fuel cell; Pt-loading carbon; Methanol electrooxidation; Electroactivity

Kinetic Study of Li-Ion Extraction and Insertion at LiMn2 O 4 Single Particle Electrodes Using Potential Step and Impedance Methods

The kinetics of Li-ion extraction and insertion at LiMn 2 O 4 single particles (8-21 μm diam) were investigated by cyclic voltammetry, potential step chronoamperometry (PSCA), and electrochemical impedance spectroscopy (EIS) methods using a microelectrode technique. The EIS measurements in a frequency range from 110 kHz to 11 mHz were conducted successfully on a LiMn 2 O 4 single particle resulting in the magnitude of MΩ orders. The impedance spectra exhibited (i) a single semicircle in the high frequency region, (ii) a Warburg impedance in the low frequency region, and (iii) a limiting capacitance in the very low frequency region. The EIS spectra were fitted to a modified Randles-Ershler circuit, so that the reaction kinetics could be evaluated precisely. The dependences of the charge transfer resistance (R cl ) and the apparent diffusion coefficient of Li within the particle on the electrode potential were evaluated. Obtained values for D app were in the range of 10 -10 to 10 -6 cm2/s from EIS measurements, in fair agreement with those from PSCA results. Finally, the apparent chemical diffusion coefficients of Li-ion in the single crystal, thin-film, and single particle of LiMn 2 O 4 are compared.

ESD Fabricated Thin Films of Spinel LiMn2 O 4 for Lithium Microbatteries: I. Effects of Thickness

Electrochemical properties of thin films of LiMn 2 O 4 spinels of several thicknesses prepared by electrostatic spray deposition (ESD) were studied in 1 M LiCIO 4 /propylene carbonate solution using cyclic voltammetry and electrochemical impedance spectroscopy. The shapes of the cyclic voltammograms (CVs) is extremely affected by the scan rate. In general, at a constant film thickness, the slower the scan rates the sharper and narrower are the peaks. A progressive decrease in the charge-transfer resistance was always found with raising the film thickness and the applied potential Both the film thickness and the charge injection process effectively limit the region over which diffusion control is observed. The diffusion control domain shrinks with decreasing film thickness in agreement with the theoretical predictions. The diffusion coefficient of Li ions in the solid phase varied in a similar manner with the potential, irrespective of the film thickness. The diffusion coefficient exhibited a peak-shaped function of the electrodes potential with minima at potentials close to the peak potentials of the CV. Importantly, operating at a constant potential. the chemical diffusion coefficient decreases within the 1-0.5 μm range, and is almost constant for smaller thicknesses.

Microvoltammetric study of electrochemical properties of a single spherical nickel hydroxide particle

Abstract Electrochemical studies of spherical nickel hydroxide Ni(OH)2 with high density were carried out by using a microelectrode technique. A carbon fiber microelectrode was contacted with a single particle of nickel hydroxide in 5 M KOH aqueous solution. In this way, cyclic voltammetry and galvanostatic charge/discharge experiments were carried out. The Ni(OH)2/NiOOH redox reaction, oxygen evolution reaction and charge/discharge capacity were examined without any dilution with a binder and a conductive additive. Data were compared with those of the composite electrode consisting of the nickel hydroxide particle and a polymer binder, which is the practical form in the Ni-MH batteries. The potential-step technique was also performed to evaluate the apparent chemical diffusion coefficient of the proton (Dapp) in the nickel hydroxide. Since the nickel hydroxide particle was a dense, conductive sphere, the spherical diffusion model was employed for the analysis. The Dapp was found to be around 10 −9 cm 2 s −1 .

Photocarrier generation in a layered organic photoreceptor containing Azo pigment

Studies have been made on photocarrier generation which is caused in a highly sensitive layered organic photoreceptor for electrophotography and produces a photoinduced discharge. A series of experiments has been carried out on a layered photoreceptor which has a carrier generation layer (CGL) containing a fluorenone bisazo pigment as the main component. As a result, we have found that (1) excitons are produced in the bulk of the CGL by photon absorption, (2) the excitons dissociate into free carriers (negative electrons and positive holes) at the interface between the CGL and a carrier transport layer (CTL), and (3) the positive holes are immediately injected into the CTL, so that few free carriers recombine.

Temperature and methanol concentration dependences of direct methanol fuel cell performance measured by single cell having reference electrode

In this study, we measured the cell temperature and methanol concentration dependences of direct methanol fuel cell (DMFC) performances and its polarization characteristics using a single cell with a Ag/Ag2SO4 reference electrode, which has been developed for the DMFC research in our laboratory. The membrane electrode assembly composed of carbon-supported Pt-Ru and Pt catalysts and the Nafion membrane was used for the measurements. As a result, the power densities obtained while feeding 1 and 5 mol dm−3 methanol solutions increase in the order of 26, 40, and 60 °C. For the 10 mol dm−3 methanol, the power density at 60 °C is lower than those at 26 and 40 °C. In these cases, the polarization curves to investigate the details of the relationship between the DMFC performance and temperature indicate that for the 1 and 5 mol dm−3 methanol, the current densities measured at the same anode and cathode potentials are enhanced with the increasing temperature. As for the 10 mol dm−3 methanol, the current densities ...

In situ Raman spectroscopic study of thin-film Li1−xMn2O4 electrodes

In order to investigate structural changes in lithium manganese oxide during the insertion and extraction of lithium ions, Raman spectra of thin films of Li1−xMn2O4 produced by electrostatic spray deposition (ESD) were recorded in situ during a voltammetric scan in 1 M LiClO4 in propylene carbonate (PD)/ethylene carbonate (EC) solutions. Five Raman-active bands, as predicted by the theory, were observed for the Li1−xMn2O4 compound. The λ-MnO2 phase has a distinct band at 592 cm−1 and therefore can be clearly recognized from Raman spectra. The changes observed in Raman lines of Li1−xMn2O4 were ascribed to the occurrence of the phase transition of Li1−xMn2O4 electrodes. From the energy-dependence of Raman spectra, it is suggested that each value of x in Li1−xMn2O4 possesses its own electronic band structure.

Photocarrier generation site in a layered photoreceptor based on azo pigment: Effect of wet‐coated formations

The structures of interfaces between a carrier generation layer (CGL) and a carrier transport layer (CTL) which are considered to be a photocarrier generation site in a layered photoreceptor have been analyzed. It was found that the thickness of the CGL, which consists of the fluorenone‐based bisazo pigment (CGM) and binder resin, was expanded about 1.5 times after the CTL was deposited by a wet‐coating process. A substantial amount of the carrier transport material (CTM) had penetrated preferentially into the CGL from the CTL to the binder resin, remaining as the CTL dried out. The photocarrier generation site in such a layered photoreceptor is found not to be a planar CGL/CTL interface but a three‐dimensional CGM/CTM interface spread out over the bulk of the CGL. The CTM:CGM ratio in the CGL of the layered photoreceptor is estimated to be 0.8–0.95:1 in weight. It is also considered that the number of the photocarrier generation sites originated by the CTMs in the CGL was close to the number needed to sa...

Characterization of membrane electrode assembly for fuel cells prepared by electrostatic spray deposition

A novel electrocatalyst painting technique for realizing an extremely high yield of deposition in a limited surface area of a polymer electrolyte membrane has been developed by electrostatic spray deposition (ESD). First, a dispersion containing Pt/C powder and Nafion solution was sprayed by ESD. As a result, the dispersion was deposited on the entire surface area of a Nafion membrane placed on an Au counterelectrode, but not on the insulating poly(ethylene terephthalete) membrane. Second, for the highest-quality Pt/C catalyst layer, the solid content of the dispersion and the ejection rate were set to 3–4 wt % and 10–20 mm3min-1, respectively. The catalyst layer thus prepared demonstrated a fine, smooth structure. Third, in the case in which the sizes of the Nafion membrane and Au electrode were the same, the deposition occurred only on the Nafion/Au layered structure. This enabled an extremely high yield of deposition. The ESD of the Pt/C dispersion was conducted on a dry Nafion membrane with a water droplet on the surface. Consequently, an electrocatalyst layer was successfully formed only at the wetted point of the Nafion membrane. This technique enables the painting of the electrocatalyst layer over a limited area without the use of any surface masks. Finally, the membrane electrode assembly prepared by ESD was installed in a fuel cell and demonstrated as high a performance as that prepared by air-spraying. The cross-sectional morphology of the catalyst layer explained the coupling strength in the peel-off test well, as well as the dependence of current–voltage characteristics on catalyst layer thickness.