Shigeo Shibata

Kinetics and mechanism of electrochemical reduction of multilayer oxides on a smooth platinum electrode surface in acidic electrolyte

Abstract The electrochemical reduction of multilayer oxides which were formed on a smooth Pt electrode surface under severe anodic conditions was investigated using a galvanostatic transient, a linear potential sweep and a potential step technique. Four regions of the surface oxide reduction were distinguished in the galvanostatic E−t curve and four corresponding cathodic current peaks were observed in the potentiodynamic i−E profile. These four regions or peaks are attributed to the reduction of four O-containing layers: an oxygen monolayer adsorbed on the oxide surface, two oxide layers in a first and a second lattice and a multilayer oxide in the deeper lattices having a phase property. The reduction rate of the first lattice oxide layer is determined by a second electron transfer. Under a rapid stripping condition, the reduction of the second oxide layer is considered to be controlled by the place exchange reaction. The extremely large reduction rate of the multilayer oxide compared with the formation rate is explained in terms of the proton-electron model.

Supersaturation of oxygen in acidic solution in the vicinity of an oxygen-evolving platinum anode

The concentration of O2 in 1 M H2SO4 solution in the vicinity of the O2-evolving smooth Pt anode was measured as a function of anodic cdia using the galvanostatic potential—transient method. The solution near the O2-evolving anode was supersaturated with O2. When the anodic current was interrupted, supersaturated concentration C* decreased at a rate proportional to C*  C0, where C0 is the solubility of O2 in the electrolyte at 1 atm. The rate constant of the decay of the supersaturation under the open circuit condition was measured to be 0.069 sec−1 at 25°C. At ia 200 mA/cm2, however, the supersaturation exhibited a limiting value of 9.0 × 10−2 mol/l.

Electron-diffraction study of electrochemically and thermally treated platinum electrode surfaces☆

Abstract Surface states of a platinum electrode that has been anodized, or anodized then cathodized, in 0·5 M H 2 SO 4 , have been studied by means of a reflexion electron-diffraction technique. It has been shown that the multilayer oxide film which is formed on the electrode surface under the heavy anodization is composed of a poorly crystalline PtO 2 and that by reducing its oxide film, the electrode surface is platinized. The platinization occurs also by repeating the formation and reduction of monolayer oxide films. A electrodeposited platinum layer becomes amorphous when it is heated in a gas flame then cooled in air. This amorphous layer is slightly recrystallized by the successive anodic oxidation and cathodic reduction.

The electrochemical Peltier heat for the adsorption and desorption of hydrogen on a platinized platinum electrode in sulfuric acid solution

Abstract The electrochemical Peltier heat for the surface hydrogen process at a pt-Pt electrode in 0.5 M H2SO4 solution was measured under controlled-potential and controlled-current polarizations using a thick film thermistor electrode. The observed Peltier heat is related to the entropy change of the reversible hydrogen process. In the hydrogen potential region, four stepwise heat changes were observed. These heat changes corresponds to the adsorption of four hydrogen species with different adsorption strenghts, respectively. The most weakly bonded hydrogen species Hw exhibited the largest Peltier heat. This is possibly due to the strong interaction of Hw with the water molecules of the solvent. Peltier effects for the other three adsorption species are explained in terms of the nature of the adsorption sites where hydrogen atoms adsorb with a different mobility or vibrational movement, resulting in a different entropy.

Conductance measurement of thin oxide films on a platinum anode

Abstract Conductance of two types of oxides, which were galvanostaticaly formed on a smooth platinum anode with 200 mA/cm 2 at 45°C in 0.5 M H 2 SO 4 , were measured with a new conductance cell. It was found that two types of oxide films differ in conductance property. The conductivity of the one, a outermost monolayer oxide film, was equal to that of metalic Pt. The other, a multilayer oxide which grows under the monolayer oxide, showed a definite conductivity lower than that of the metal. The conductivity per monolayer of this oxide in the vertical direction of the film plane was 0.67 × 10 5 Ω −1 per unit real surface area. The specific conductance value of the multilayer oxide was estimated to be about 1–2 × 10 −3 Ω −1 cm −1 , assuming that the multilayer oxide is composed of PtO 2 and that this monolayer is twice the thickness of a PtO monolayer.

Growth of multilayer oxide films on platinum electrodes by potentiostatic anodization in sulphuric acid solution

Abstract The growth of multilayer oxide films on smooth platinum anodes in 0.5 M H 2 SO 4 has been investigated under the potentiostatic conditions where oxygen was evolved vigorously. The range of anodizing condition covered was: electrode potential, 2.07–2.22 V(nhe); 20–35°C; duration up to 60 min. The growth rate of the multilayer oxide, although it was very small as compared with that of a superficial monolayer oxide, increased with increasing anodization potential, but decreased inversely at potentials higher than 2.13 V. The growth rate of the multilayer oxide film was increased, as a whole, by increasing temperature, but always showed the maximum value at 2.13 V at any temperatures from 20° to 35°C. At potentials higher than 2.17 V, no multilayer oxide film was formed. This could be caused by the formation of a passive, monolayer oxide film preventing further growth of oxide.

Electrochemical reduction of thick oxide film on platinum electrode in alkaline solutions

Abstract The electrochemical reduction of the thick oxide film formed on Pt electrode by severe preanodization has been studied in LiOH, NaOH and KOH solutions of different concentrations (0.001 ∼ 1.0 M) using a galvanostatic technique. An outermost monolayer oxide and an inner multilayer oxide of the thick oxide film exhibit different potential behaviors in the cathodic reduction. In dilute solution, both the oxides are completely reduced in a potential range of 0.6-0.4 V ( vs rhe in the test solution) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide only shifts rapidly into a H-electrosorption potential region and the amount of the oxide reduced at this potential decreases. The remaining oxide is slowly reduced at H 2 -evolution potential. The retardation of the reduction of the inner oxide is related to cations adsorbed on Pt electrode. This retardation effect increases in the order, K + + + .

Kinetics and mechanism of reduction of oxide film on smooth platinum electrodes with hydrogen

The kinetics and mechanism of the reaction of chemisorbed oxide (Pt-O) layer on a smooth Pt electrode with H2 dissolved in 1 M H2SO4 solution were investigated under the open circuit condition. It was found that a monolayer of Pt-O on the electrode surface is reduced first at a slow rate which is of second order in chemisorbed oxygen in the range 1 ≧ θ ≥ 0·65 and then at a rapid rate which is proportional to (1 – nθ)2 in the range 0·6 ≥ θ > 0, where θ is the fraction of the surface covered by oxygen. The factor n, which was constant for the electrode oxidized under a given condition, was assumed to be the number of Pt sites deactivated by each one of the chemisorbed oxygen atoms. For the transiently formed oxide, n was estimated to be 1·64. It was also observed that all over the coverage range the reaction rate was proportional to the partial pressure of H2. The variation in reduction rate with the decrease of the coverage was interpreted in terms of change in reduction mechanism from the chemical reaction to the electrochemical reaction.

Specific adsorption of hydrogen on polycrystalline platinum electrode

Abstract When a polycrystalline Pt electrode was activated by repeating anodic cycles, peak III appeared between peak I and peak II in the anodic-going voltammogram. However, in the cathodic-going voltammogram peak III did not appear at the same potential as anodic peak III. Peak III was observed to increase in height and the potential of peak III was shifted towards the noble potential as the potential sweep rate was increased. It was considered that the adsorption potential of H III was in the region of the weakly adsorbed hydrogen, H w , and it took longer for H III to be adsorbed in comparison with H w . The site which would be adsorbed by a H w atom was considered to be adsorbed by two H III atoms.

An improved heat-responsive electrode for the measurement of electrochemical peltier heat: The peltier heat for electrosorption and electrodesorption of oxygen on a platinized platinum electrode in sulfuric acid solution

Abstract A new plate-type electrode with high sensitivity and responsive to temperature change was made using a thick film thermistor. The electrochemical Peltier heat for the oxygen surface process at a platinized platinum electrode in 0.5 M H 2 SO 4 solution was measured with this electrode by potentiodynamic and galvanostatic transient techniques. It was demonstrated that most of the Peltier heat is caused by the overpotential due to the irreversible oxygen electrode reaction. That is, a Tafel-type relations between the Peltier heat and the current was confirmed. A step-wise heat change corresponding to consecutive stages of platinum lattice occupation by OH was observed. The amount of heat evolved on PtO formation was apparently larger than that on PtOH formation. The results were compared with those obtained by the voltammetric measurement.