A. Katagiri

Electrodeposition of Tungsten and Related Voltammetric Study in a Basic ZnCl2­NaCl (40-60 mol %) Melt

The electrodeposition of tungsten and voltammetric experiments were performed in basic ZnCl 2 -NaCl (40-60 mol %) melts containing various tungsten compounds at 450°C. In constant potential electrolysis, metallic tungsten was obtained from WO 3 , K 2 WO4, WCl 4 , and K 2 WCl 6 , but not from K 3 W 2 Cl 9 and WCl 2 . Similar to the case of acidic melt (60-40 mol %), the oxide ion or an oxochlorotungsten species may participate in the electrodeposition process. The presence of molecular oxygen does not interfere with the process.

Preparation of a High Surface Area Nickel Electrode by Alloying and Dealloying in a ZnCl2 ­ NaCl Melt

An electrochemical method of preparing a high surface area nickel electrode by NiZn alloy formation and subsequent dealloying in ZnCl 2 -NaCl (60-40 mol %) at 450°C was studied. A nickel electrode was kept at 0.22 V vs. Zn in ZnCl 2 -NaCl(sat) where γ-NiZn alloy was formed, and then at 0.5 V where partial dealloying occurred to yield α-NiZn alloy. Scanning electron microscope observation showed that the microporous structure was formed on the surface. The roughness factor of a sample was determined to be 240 from the double layer capacitance obtained in the ac impedance measurement. Thickness, porosity, and pore size of the surface layer were estimated.

Electrochemical Formation of Porous Nickel in Zinc Chloride-Alkali Chloride Melts

An electrochemical method of preparing a porous nickel electrode by NiZn alloy formation and subsequent dealloying in zinc chloride-alkali chloride (60-40 mol %) melts at 350-450°C was studied. X-ray diffraction measurements at different stages of dealloying suggested that two step transformation from γ- to β 1 - and from β 1 - to α-NiZn alloy occurred. Dealloyed material exhibited well-developed porous structure of hundreds of nanometers in size. Electron probe microanalysis of a partially dealloyed layer showed that the dissolution of zinc occurred relatively uniformly throughout the alloy layer. Scanning electron microscopy of samples obtained at different temperatures showed that the size of porous structure (crevices and pores) was smaller at lower temperatures. The porous nickel electrode showed relatively low overvoltages in the hydrogen evolution reaction in 1 M KOH aqueous solution.

Cathodic polarization of copper electrode in CuCN–KCN solutions and the current distribution for copper deposition on grooved substrates

The cathodic polarization of a copper electrode in CuCN/KCN solution was interpreted by considering the diffusion and migration of all ionic species, chemical reactions involving cyano-copper(i) complexes, and the quasiequilibrium of charge transfer reaction. Experimental polarization curves were compared with theoretical ones, where the diffusion layer thickness was determined. The present model was applied to the prediction of thickness distribution of copper coating on grooved substrates. Two types of substrates with grooves of different size (width × depth), (a) 5mm × 5mm and (b) 1mm × 1mm, were used. Agreement between theory and experiment was satisfactory for substrate a, but not for substrate b. The effect of convective mass transfer in the groove was discussed.

Determination of Porosity of TiO2 Films from Reflection Spectra

Reflection spectra of TiO 2 films on titanium substrates were measured at different incident angles of visible light (wavelength 400-800 nm). Porosities of the TiO 2 films were estimated from the refractive indexes of the films which were determined from the dependency of the interference wavelength on the incident angle. Porous TiO 2 films formed in NaOH solutions mostly exhibit refractive indexes smaller than 2.1, indicating that the porosity is greater than 30%. As the porosity increases, the refractive index becomes less dependent on wavelength. The porosities determined from reflection spectra are in good agreement with the pore space ratios obtained by the density profile analysis of scanning electron microscopic images.

Calculation of steady-state distributions of concentrations and potential controlled by diffusion and migration of ions

A numerical method is proposed for the calculation of concentration, potential, and current distributions in electrochemical cells controlled by diffusion and migration of ions. Thus a hypothetical variablev(x, y, t) is assumed to satisfy a differential equation which is similar to that of non-steady-state heat conduction and corresponds, at steady state, to Poissons equation for the potential. The differential equation forv(x, y, t) and the diffusion-migration equations of ions are simultaneously solved by a finite difference method. Examples of calculation are given for single and mixed electrolyte solutions in one- and two-dimensional cells. The proposed method is applicable to systems in which bipolarity occurs.

A numerical method of calculating secondary current distributions in electrochemical cells

A numerical method is proposed based on the analogy between the potential distribution in an electrolytic solution and the temperature distribution in a heat-conducting medium. Thus the equation of non-steady-state heat conduction which contains a hypothetical temperaturev(x, y, t) is solved numerically with appropriate boundary conditions. In the steady state the distribution ofv(x, y, t) corresponds to the distribution of potentialφs(x,y) which satisfies Laplaces equation. The method is useful not only for conventional electrochemical cells but also for complicated systems such as a bipolar electrode for which boundary conditions provide neither the potential nor the current density at the electrode surface.

Mechanism of the Electro‐oxidation of Sulfite Catalyzed by Copper Ion

The oxidation of sulfite in aqueous solution is an important reaction in some industrial processes such as metal refining and flue gas desulfurization. Electro-oxidation of sulfite to dithionate, which is catalyzed by copper ion, has been studied by potential step chronoamperometry and potentiometry. The following mechanism has been proposed: Cu{sup I}(SO{sub 3}){sub 3}{sup 5{minus}} is oxidized to Cu{sup II}(SO{sub 3}){sub 3}{sup 4{minus}}, and the latter dimerizes to Cu{sub 2}{sup II}(SO{sub 3}){sub 6}{sup 8{minus}}, which then decomposes to S{sub 2}O{sub 6}{sup 2{minus}} and Cu{sup I}(SO{sub 3}){sub 2}{sup 3{minus}} in the rate-determining step. Formation constants of sulfito-copper(I) and -copper(II) complexes have been reported.

Simulation of a packed bipolar cell involving cathodic limiting current and anodic Tafel behaviour

v0c length of unit cell (cm) anodic Tafel b-factor (V) average electric field in solution (V cm -1) faradaic current in unit cell (A) bypass current through solution in unit cell (A) total current in unit cell (A) current density (A cmI ) , cathodic limiting current density (Acm-l ) * apparent resistance of solution in unit cell (n) radius of cylindrical electrode (cm) threshold voltage (V) characteristic voltage for anodic reaction (v) characteristic voltage for cathodic reac-

Studies on the bromine electrode in ZnBr2NaBr melts

Abstract The bromine electrode in the ZnBr 2 NaBr melts which gives a thermochemically meaningful electrode potential has been developed. Gold can be used as the electrode substrate, which is electrochemically stable in the presence of the bromine gas, and the electrocatalytic activity for the bromine electrode reaction seems to be higher than that of carbon materials.