Tadashi Sotomura

Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Electrochemical reduction of oxygen (O2) in air electrode with manganese oxides (MnOx) as electrocatalysts was studied with MnOx/Nafion-modified gold (Au) electrodes using cyclic voltammetry, potential-controlled amperometry and rotating ring–disk electrode (RRDE) voltammetry in alkaline aqueous solution. At Nafion-modified (MnOx free) Au electrode, O2 reduction undergoes two successive two-electron processes with HO2− as intermediate. The presence of MnOx, including Mn2O3, Mn3O4, Mn5O8 and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O2 to hydrogen peroxide (HO2− in this case) and decreases the second one of HO2− to OH−, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO2− to O2 and OH− and thus, the O2 reduction in air electrode was considered to include an initial two-electron reduction of O2 to HO2− followed by a disproportionation reaction of HO2− into O2 and OH− catalyzed by MnOx. The excellent activity of MnOx for the follow-up disproportionation reaction substantially results in an overall four-electron reduction of O2 at MnOx/Nafion-modified Au electrodes in the first reduction step, depending on potential scan rate and the kind of MnOx. The present work provides a scientific significance of the mechanism of O2 reduction in air electrode using MnOx as electrocatalysts to effect a four-electron reduction of O2 to OH−.

Electrochemical Characterization of Catalytic Activities of Manganese Oxides to Oxygen Reduction in Alkaline Aqueous Solution

The catalytic function and activity of manganese oxides (MnOx: Mn 2 O 3 , Mn 3 O 4 , Mn 5 O 8 , and MnOOH) to the electrochemical reduction of O 2 in 0.10 M KOH aqueous solution have been investigated by cyclic voltammetry at MnOx/Nafion-modified gold electrodes. Two successive reduction current peaks were observed at Nafion-modified electrodes in the cyclic voltammograms, i p,1 for a two-electron reduction of O 2 to hydrogen peroxide (HO 2 ) and i p,2 for a two-electron reduction of HO 2 to OH - . The peak current heights of i p,1 and i p,2 changed greatly depending on the kind of MnOx species incorporated into the MnOx/Nafion-modified gold electrodes; i p,1 increased and i p,2 decreased. On the assumption that HO 2 produced in the first reduction step is chemically decomposed into O 2 and OH with a catalytic action of MnOx and that this regenerated O 2 is reduced again in the same first reduction step, we evaluated the catalytic activity of MnOx using the values of i p,1 and i p,2 . MnOOH provided the highest catalytic activity to the electrochemical reduction of O 2 . This result was supported by another experiment by using a chemical method where catalytic decomposition of HO 2 with MnOx was estimated by measuring the O 2 concentration directly with a commercial oxygen sensor.

New organodisulfide—polyaniline composite cathode for secondary lithium battery

A new composite cathode used for high-energy density secondary lithium batteries is developed, which consists of 2,5-dimercapto-1,3,4-thiadiazole (organodisulfide), polyaniline, and gel electrolyte. Polyaniline works to promote the redox reaction of the organodisulfide in the composite electrode, providing the cyclic voltammogram showing a narrower peak-potential separation of 0.5 V between cathode and anode peaks. The lithium cell using the composite cathode and gel electrolyte based on acrylonitrile—methylacrylate copolymer has over 3.0 V cell voltage with 303 Wh kg−1 cathode energy density at a current density of 0.1 mA cm−2 at room temperature. In practice, a model cell with 80 μm thick lithium anode, 50 μm thick gel electrolyte, and 170 μm thick composite cathode is expected to deliver 220 Wh kg−1 cell energy density at 0.1 mA cm−2.

Size and Crystallographic Orientation Controls of Gold Nanoparticles Electrodeposited on GC Electrodes

Gold nanoparticles were electrodeposited onto glassy carbon-electrodes (nano-Au/GC) in the presence of two different additives, namely, cysteine and iodide ions. The electrochemical characterization of the electrodeposited nano-Au/GC electrodes was performed via the measurements of the reductive desorption patterns of a thiol (e.g., cysteine) self-assembled monolayer as well as the cyclic voltammetric response toward the oxygen reduction reaction in alkaline medium. The structural characterization of the electrodeposited Au nanoparticles was performed by the scanning electron microscope. The nano-Au/GC electrodes prepared in the presence of 100 μM cysteine were surprisingly found to be enriched in the Au(100) and Au(110) facets and are characterized by a relatively big particle size up to 300 nm as well as low particle density (number of particles per unit area). The Au nanoparticles prepared in the presence of 100 μM I - ions were found to be much enriched in the Au(111) facets and are characterized by a relatively narrow particle size distribution range (10-40 nm) as well as a high particle density. Analysis of the X-ray diffraction data revealed a significantly decreased ratio of Au(111) domains of the Au nanoparticles electrochemically deposited in the presence of cysteine. These preliminary results suggest a simple way to control the size as well as the preferential crystallographic orientations of gold nanoparticles.

An organosulfur polymer cathode with a high current capability for rechargeable batteries

The charge-discharge capability of a polymer composite cathode prepared from 2,5-dimercapto-1,3,4-thiadiazole (DMcT), polyaniline, 3-alkylcarboxylate-4-methylpyrrole, and acetylene black has been investigated on different kinds of current collectors including copper, nickel, aluminum, and titanium foil, gold-plated titanium foil, and a porous carbon film in a lithium cell system with a gel-like polymer electrolyte. The polymer composite cathode with a copper current collector provides a relatively flat discharge potential difference (3.4 to 2.8 V) and high current capability (137 mA/g-cathode) without undue deterioration of the energy density. The battery can be charged up to 550 mWh/g-cathode within 1.25 h, and it can be reversible discharged within 1.25 h. This unique charge-discharge performance might be attributed to the redox reaction of a copper (I or II)-DMcT complex which is formed in the first several cycles as a result of oxidative dissolution of copper. The use of a thin copper current collector in place of a rather thick porous carbon film enables one to fabricate polymer lithium rechargeable batteries with a thin-film configuration.

Morphological Selection of Gold Nanoparticles Electrodeposited on Various Substrates

Gold nanopanicles with different morphologies (nanocrystallites, perfect nanospheres, plumbs, and nanoaggregates) have been electrodeposited on different substrates, namely, glassy carbon (GC), highly oriented pyrolytic graphite (HOPG), and Au(111) single-crystalline substrates. Au particles with particle size ranging from a few nanometers to a few micrometers have been prepared. The morphology of the electrodeposited Au particles was largely dependent on the nature of the substrate as well as the composition of the electrodeposition bath. For instance, the inclusion of iodide ions during electrodeposition was found to enhance two-dimensional (2D) growth of the Au nanoparticles, and particles with a relatively small particle size down to 10 nm were obtained. The inclusion of L-cysteine (as an additive) during the electrodeposition of the Au nanoparticles resulted in a significant influence on the morphology (and the particle size of the Au particles), which strongly depends on the nature of the substrate. Au nanoparticles with crystalline geometry were prepared on the Au(lll) substrates in the presence of L-cysteine, while under the same experimental conditions Au aggregates of size up to 300 nm were electrodeposited on the GC substrates. Au particles with a perfect spherical shape were electrodeposited on the HOPG electrodes. X-ray diffraction measurements of the electrodeposited Au particles revealed significantly different crystallinity of the Au particles and in turn different ratios of the single-crystalline domains constituting the Au particles. The cyclic voltammetric response toward the oxygen reduction reaction at the different Au nanoparticles showed a versatile behavior ranging from a quasi-reversible two-electron reaction to an irreversible overall four-electron reaction in O 2 -saturated 0.5 M KOH solution, demonstrating the entirely different electrocatalytic activity of the thusprepared Au nanoparticles on different substrates.

Effects of Adding Copper(II) Salt to Organosulfur Cathodes for Rechargeable Lithium Batteries

The authors report the preliminary analysis of a new cathode material and the performance of secondary lithium cells which contain it. The material is comprised of an organosulfur compound, 2,5-dimercapto-1,3,4-thiadiazole (DMcT), doped with a copper(II) salt in a poly(aniline) matrix. Secondary lithium cells which use this material as cathodes can be discharged at 300 Ah per kg-cathode for ca. 10 cycles 260 Ah per kg-cathode for at least 80 cycles, or 170 Ah per kg-cathode for ca. 130 cycles. Further, after failure of these cells, replacement of the lithium anode restores the original capacity. Vibrational and electronic spectroscopy of the copper(II)/DMcT system agree with previous reports on the redox chemistry which occurs between Cu(II) and thioamide groups. A preliminary cyclic voltammetric study of this system indicates that the electroactive compound is probably a Cu(I)DMcT complex which does not exhibit traditional electrocatalytic behavior, most likely due to the effects of proton-coupling in the redox processes of DMcT.

Dimercaptan‐Polyaniline Cathodes for Lithium Batteries: Addition of a Polypyrrole Derivative for Rapid Charging

A polymer composite cathode prepared from polyaniline and 2,5-dimercapto-1, 3,4-thiadiazole shows high gravimetric energy density when it is coupled with a lithium anode. However, charging and discharging currents should be 0.05 mA/sq cm or less, otherwise the cycle life is shortened. Addition of a polypyrrole derivative, poly(3-butylcarboxylate- 4-methylpyrrole), to the composite cathode enabled rapid charging at 0.2 mA/sq cm without undue deterioration of the energy density, but it was not effective for rapid discharging. This effect of the polypyrrole derivative may be ascribale to the electrical conductivity in its oxidized state.

Electrocatalytic Reduction of Oxygen in a Novel Catalytic System with Cobalt Phthalocyanines and Manganese Oxide

This article concerns the efficient dioxygen (O 2 ) reduction by a novel catalytic system that is comprised of two catalysts: one for the electroreduction of O 2 through the two-electron process and the other for the subsequent chemical decomposition of hydrogen peroxide generated. Here we represent the combined catalytic system of cobalt phthalocyanines and manganese oxide (MnOOH). Each catalyst performs well independently, leading to the totally four-electron reduction of O 2 in alkaline media. This was confirmed from almost twofold increases of the cathodic current in cyclic voltammograms and the steady-state current in rotating ring disk electrode voltammograms, the collection efficiency and the number of electrons calculated from the Koutecky-Levich plot. As a consequence, it is concluded that this combined catalytic system works efficiently for the O 2 reduction.

Spectroscopic identification of 2,5-dimercapto-1,3,4-thiadiazole and its lithium salt and dimer forms

Abstract In this contribution we report initial results of our solid phase vibrational spectroscopic study of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) and derivatives representing its protonation and oxidation states. We have succeeded in assigning observed bands to modes which are diagnostically useful for studies concerning the electrochemical character of DMcT when used with polyaniline (PAn) as a composite cathode material in a secondary lithium-ion cell. We discuss the implications of the current study for our further investigations of the DMcT/PAn cathode.