Minoru Inaba

Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device

Polycrystalline n-ZnO/p-Cu2O heterojunctions have been fabricated by low-temperature eletrodepositions of ZnO and Cu2O layers in aqueous solutions. The condition for forming the Cu2O layer significantly reflected the electrical rectification characteristic and the photovoltaic performance, and the heterojunction fabricated under optimized conditions showed an excellent electrical rectification characteristic and a photovoltaic performance of 1.28% in conversion efficiency under an AM 1.5 illumination.

Polymer electrolyte fuel cell durability

Stack Components.- Dissolution and Stabilization of Platinum in Oxygen Cathodes.- Carbon-Support Requirements for Highly Durable Fuel Cell Operation.- Chemical Degradation of Perfluorinated Sulfonic Acid Membranes.- Chemical Degradation: Correlations Between Electrolyzer and Fuel Cell Findings.- Improvement of Membrane and Membrane Electrode Assembly Durability.- Durability of Radiation-Grafted Fuel Cell Membranes.- Durability Aspects of Gas-Diffusion and Microporous Layers.- High-Temperature Polymer Electrolyte Fuel Cells: Durability Insights.- Direct Methanol Fuel Cell Durability.- Influence of Metallic Bipolar Plates on the Durability of Polymer Electrolyte Fuel Cells.- Durability of Graphite Composite Bipolar Plates.- Gaskets: Important Durability Issues.- Cells and Stack Operation.- Air Impurities.- Impurity Effects on Electrode Reactions in Fuel Cells.- Performance and Durability of a Polymer Electrolyte Fuel Cell Operating with Reformate: Effects of CO, CO2, and Other Trace Impurities.- Subfreezing Phenomena in Polymer Electrolyte Fuel Cells.- Application of Accelerated Testing and Statistical Lifetime Modeling to Membrane Electrode Assembly Development.- Operating Requirements for Durable Polymer-Electrolyte Fuel Cell Stacks.- Design Requirements for Bipolar Plates and Stack Hardware for Durable Operation.- Heterogeneous Cell Ageing in Polymer Electrolyte Fuel Cell Stacks.- System Perspectives.- Degradation Factors of Polymer Electrolyte Fuel Cells in Residential Cogeneration Systems.- Fuel Cell Stack Durability for Vehicle Application.- R&D Status.- Durability Targets for Stationary and Automotive Applications in Japan.

Impedance Study on the Electrochemical Lithium Intercalation into Natural Graphite Powder

Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.

Effects of Some Organic Additives on Lithium Deposition in Propylene Carbonate

The effects of some film-forming organic additives, fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfite (ES), on lithium deposition and dissolution were investigated in 1 M LiClO 4 dissolved in propylene carbonate (PC) as a base solution. When 5 wt % FEC was added, the cycling efficiency was improved. On the contrary, addition of 5 wt % VC or ES significantly lowered the cycling efficiency. The surface morphology of lithium deposited in each electrolyte solution was observed by in situ atomic force microscopy (AFM). In PC + FEC, the surface was covered with a uniform and closely paced layer of particle-like deposits of about 100-150 nm dram. The surface film seemed to be more solid in PC + VC, and inhomogeneous in PC + ES. From ac impedance measurements, it was revealed that the surface film formed in PC + FEC has a lower resistance than that in the additive-free solution, whereas that formed in PC + VC or PC + ES has a higher resistance. Large volume changes during lithium deposition and dissolution require that the surface film should be elastic (or soft) and be self-repairable when being damaged. In addition, a nonuniform current distribution is liable to cause dendrite formation, which requires that the surface film should be uniform and its resistance should be as low as possible. PC + FEC gave a surface film that satisfies all these requirements, and therefore only FEC was effective as an additive for deposition and dissolution of lithium metal.

In Situ Raman Study on Electrochemical Li Intercalation into Graphite

Electrochemical lithium intercalation into graphite materials has been extensively studied for use in negative electrodes of secondary lithium batteries. Electrochemical lithium intercalation into highly oriented pyrolytic graphite and natural graphite powder was investigated using in situ Raman spectroscopy. Three plateaus were observed on the charging curve for both samples. From the Raman spectral changes the first plateau was assigned to a phase transition from dilute stage 1 to stage 4, the second from a stage 2 phase to another stage 2 phase, and the third from stage 2 to stage 1, which are in good agreement with Dahns results by in situ X-ray diffraction. The spectral changes associated with the phase transitions occurred reversibly during a charge and discharge cycle. It was shown from the Raman spectral changes of the HOPG electrode that the electrode potential during the electrochemical intercalation is determined by the surface stage of graphite intercalation compounds.

Effect of Agglomeration of Pt/C Catalyst on Hydrogen Peroxide Formation

Various amounts of 20 wt % Pt/C catalysts (56.7-5.7 μg c a r b o n cm - 2 ) were loaded on glassy carbon (GC) disk electrode, and the effect of agglomeration on hydrogen peroxide formation in oxygen reduction was investigated by the rotating ring-disk electrode technique. The formation of H 2 O 2 was enhanced with a decrease in agglomeration of Pt/C. Even in the operating potential range of polymer electrolyte fuel cell cathodes (0.6-0.8 V), 10% hydrogen peroxide was formed at 5.7 μg c a r b o n cm - 2 Pt/C loaded on GC. These results revealed that series two-electron reduction pathway, which is negligible on clean bulk Pt surface, does exist on Pt particles supported on carbon.

Raman study of layered rock-salt LiCoO2 and its electrochemical lithium deintercalation

Unpolarized and polarized Raman spectra (200–800 cm-1) of LiCoO2 with a layered rock-salt structure were measured. The Raman-active lattice modes of LiCoO2 were assigned by polarized Raman measurements of a c-axis oriented thin film. The variation of the Raman spectra of Li1-xCoO2 powder prepared by electrochemical lithium deintercalation was investigated, and the spectral changes were well correlated with the structural changes determined by x-ray diffraction except that peak splitting by the distortion in the monoclinic phase was not observed. The observed line broadening of the second hexagonal phase and the monoclinic phase indicated that the lithium ions remaining in the lattice after deintercalation randomly occupy the available sites on the lithium planes in the lattice the layered rock-salt structure.

Surface Film Formation on Graphite Negative Electrode in Lithium-Ion Batteries: AFM Study in an Ethylene Carbonate-Based Solution

In situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV in 1 mol dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle, the thickness of the precipitate layer was 40 nm, and increased to 70 nm after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane.

A.c. impedance analysis of electrochemical lithium intercalation into highly oriented pyrolytic graphite

Abstract Electrochemical lithium intercalation into graphite was studied by cyclic voltammetry and a.c. impedance spectroscopy. Highly oriented pyrolytic graphite was used as a model graphite material to distinguish the difference in electrochemical behavior between the basal and the edge planes at graphite. A comparison between cyclic voltammograms of the basal plane and the whole surface of highly oriented pyrolytic graphite revealed that electrochemical lithium intercalation proceeds predominantly at the edge plane/electrolyte interface. The charge-transfer resistance changed continuously with electrode potential, and no significant change was observed at stage transition potentials (210, 120, and 90 mV versus Li/Li + ). From the variations of the Warburg impedance of samples of different sizes, it was concluded that lithium diffuses from the edge plane to the interior in the direction parralel to the basal plane and that its diffusivity changes with the stage structure of the bulk lithium—graphite intercalation compound.

Structural and Electrical Characterizations of Electrodeposited p-Type Semiconductor Cu2O Films

The p-type semiconductor cuprous oxide (Cu 2 O) film has been of considerable interest as a component of solar cells and photodiodes due to its bandgap energy of 2.1 eV and high optical absorption coefficient. We prepared Cu 2 O films on a conductive substrate by electrodeposition at 318 K from an aqueous solution containing copper sulfate and lactic acid. The structural and electrical characterizations of the resulting films were examined by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption measurements, and the Hall effect measurement, respectively. The resistivity varied from 2.7 × 10 4 to 3.3 X 10 6 Ω cm, while the carrier density was from 10 1 2 to 10 1 4 cm - 3 and the mobility from 0.4 to 1.8 cm 2 V - 1 s - 1 , depending on the preparation conditions, i.e., solution pH and deposition potential. The carrier density was sensitive to the atomic ratio of Cu to O in the films and the mobility to the grain size.