C. Iwakura

The anodic characteristics of manganese dioxide electrodes prepared by thermal decomposition of manganese nitrate

Manganese dioxide electrodes were prepared by a thermal decomposition of manganese nitrate solution on a titanium or a platinum substrate, and their anodic characteristics were investigated mainly in 1N H2SO4 and 1N KOH. The platinum-supported manganese dioxide electrode shows the good anodic characteristics with a relatively low overvoltage for oxygen evolution while the use of the titanium-supported one as an anode needs further modification to reduce its high resistivity resulting from the thick film of titanium dioxide in spite of good adhesion of the oxide film with the titanium substrate. The primary water or hydroxide ion discharge step is rate-controlling in the anodic evolution of oxygen on the platinum-supported manganese dioxide electrode in both acidic and alkaline solutions. The oxygen overvoltage is raised and the mechanical strength of the catalytic oxide on the platinum substrate is weakened by the anodic polarization in acidic solution. These phenomena are explicable on the basis of an increase in the oxygen content in the oxide. In conclusion, the presented results suggest that the manganese dioxide film is a practical usable material for the anodic evolution of oxygen as well as chlorine, especially in alkaline solutions.

The anodic characteristics of massive manganese oxide electrode

Abstract The massive Mn oxides were synthesized and their anodic characteristics were investigated in aqueous solutions. The thermal treatment of the massive β-MnO 2 at 480°C or above leads to the formation of α-Mn 2 O 3 , and the relatively high activity was observed for the anodic evolution of oxygen on the mixed oxide of ν-MnO 2 and α-Mn 2 O 3 , compared with the pure oxide of β-MnO 2 . The active sites for the oxygen evolution reaction were regarded as the Mn(III) sites on the oxide surfaces. From the kinetic considerations for the oxygen and chlorine evolutions, it was suggested that the oxidation process of the electrode surface is a common rate-determining step in both reactions on the massive Mn oxide electrodes.

The anodic evolution of oxygen on Co3O4 film electrodes in alkaline solutions

Abstract The Co 3 O 4 film anodes for oxygen evolution were prepared by a thermal decomposition method and their anodic polarization characteristics were investigated in alkaline solutions. As a result, the polarization characteristics were found to be greatly affected by the kind of metal substrates used. Especially when Fe was used as a substrate, some Fe species were proved to be doped in the Co 3 O 4 layer during the electrode preparation and the resulting electrodes showed the attractive character for oxygen evolution.

Anodic evolution of oxygen on ruthenium in acidic solutions

Abstract Electrochemical properties of ruthenium, particularly the anodic evolution of oxygen and anodic dissolution of ruthenium have been investigated by means of polarization measurements and product analyses. The electrode surface gradually colours black during oxygen evolution. This is due to the accumulation of hydrous RuO2 resulting from decomposition of corrosion product. The black oxide film suppresses the ruthenium dissolution rate and the current efficiency for the dissolution reaction is less than 6% at a cd below 0.2 A/cm2 in 1 N H2SO4, where the anodic evolution of oxygen is predominant. The overall current for oxygen evolution is expressed by i = nFka−2H exp (2FE/RT) The probable mechanism of oxygen evolution on the ruthenium anode under the Langmuir conditions of intermediate adsorption is

Preparation of ruthenium dioxide electrodes and their anodic polarization characteristics in acidic solutions

Abstract Anhydrous ruthenium dioxide electrodes were prepared by heating hydrous ruthenium dioxide at different temperatures and their anodic polarization characteristics in acidic solutions were investigated, in connection with their physical properties. The anodic polarization characteristics are greatly affected by the preparing temperature and are closely related to their physical properties, such as the crystallinity, the crystalline size and the resistivity. Generally, the electrode composed of poorly crystalline ruthenium dioxide shows the good anodic polarization characteristics. In conclusion, it was revealed that excellent anode material is obtainable by heating hydrous ruthenium dioxide in air at about 450°C. This electrode is quite stable under the anodic polarization in acidic solutions and has strong resistance against the anodic dissolution together with very low overvoltage for the oxygen evolution.

The anodic characteristics of modified Mn oxide electrode: Ti/RuOx/MnOx☆

Abstract The Ti-supported Mn oxide electrode was modified by introducing a Ru oxide film as an intermediate layer into the Ti/Mn oxide interface, and its anodic characteristics were examined in aqueous solutions of 0.5 M H 2 SO 4 , 1 M KOH and 5 M NaCl. The intermediate thin film of the Ru oxide served effectively as a good conductor to improve the electric characteristics of the Ti-supported Mn oxide electrode and hence the modified electrode exhibited the excellent anodic characteristics similar to those of the Pt-based Mn oxide. From the kinetic considerations, it was proved that the anodic evolution of oxygen takes place on the surface of Mn oxide. The Mn oxide was found to be virtually active electrocatalyst for electrolytic chlorine evolution as well as electrolytic oxygen evolution. As a result of the evaluation of the catalytic activity, it was considered that the Mn oxide is one of the most active material of all, except for some DSA-type electrodes such as Ru and Ir oxides, for the anodic evolution of oxygen. In conclusion, the presented results suggest that the modified Mn oxide electrode has a promising character for the practical use in water electrolysis.

Some oxide catalysts for the anodic evolution of chlorine: reaction mechanism and catalytic activity

The chlorine evolution reaction was investigated on several oxide electrodes. The active site on the electrode surface is concluded to be the metal site, based on the dependence of the anodic current on pH of the electrolyte. The reaction mechanism can be determined by the d-electron configuration in the oxide; ie the reaction proceeds by the Volmer-Heyrovsky mechanism on the electrode containing the transition metal cation with partially filledt2g orbitals and empty eg orbitals, whereas the Volmer-Tafel mechanism on the electrode containing the transition metal cation with just or half filled t2g orbitals and partially filled eg orbitals.

Hydrogen evolution at LaNi5-type alloy electrodes☆

Abstract Several LaNi5-type alloy electrodes were prepared and their cathodic polarization characteristics in alkaline solutions were investigated. As a result, they were found to have a high electrocatalytic activity for the hydrogen evolution reaction compared with the constituent metals, and their activities were almost comparable with those of platinum and palladium electrodes. Such a synergism was attributed to the large heat of formation of these alloys. The surface damage of these alloy electrodes was also found to be prevented by controlling the temperature and current density.

Electrochemical behaviour of the ruthenium oxide electrode prepared by the thermal decomposition method

Abstract Basic electrochemical behaviour of the ruthenium oxide electrode was studied. Quantity of electricity measured in the potential region between oxygen evolution and RuO2 reduction is the characteristic quantity. From the potential decay in the open-circuit after charging, and the pH dependence of the quantity of electricity and the charging current, it is considered that H+ ion diffuses in the solid phase. Furthermore, the increase in oxygen content in the oxide layer could be supposed under high anodic polarization.

The anodic evolution of oxygen on platinum oxide electrode in alkaline solutions

Abstract The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasilshchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.