Masae P. Sumino

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.

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.

Effect of heat-treatment of the oxidizability of smooth platinum anodes

Abstract The effect of heat-treatment on the electrochemical oxidizability of a smooth platinum electrode that had previously been oxidized and then reduced was investigated. The growth rate of the multilayer oxide film by the anodic oxidation at 2·18 V(nhe) decreased with increase of the pre-annealing temperature and time. On the well-annealed electrode surface, only a monolayer oxide film was formed and the multilayer oxide was not developed. The variation of the oxidizability by the heat-treatment was interpreted in term of the change in surface structure of the electrode. By the oxidation and reduction pre-treatment, the superficial platinum layer may be disordered. The activation energy for the rearrangement of platinum atoms in the disordered layer to the stable lattice position was estimated to be 11 ± 1 kcal/mol on the assumption that the oxidizability is proportional to the coverage of the disordered atoms. This result is in harmony with values calculated on the basis of both the vacancy migration mechanism and the adatom migration mechanism.

The effect of anions on the electrochemical reduction of thick oxide films on platinum electrode in acidic aqueous solutions

Abstract The cathodic galvanostatic reduction of thick oxide films on Pt has been studied in HF, HClO 4 and H 2 SO 4 solutions at 25°C as a function of the current density and the concentration of these acids. The thick oxide film was found to be completely reduced at a single potential step of 0.6 V ( vs rhe in the test solution) in HF solution and also in very dilute HClO 4 and H 2 SO 4 solutions. However, when the oxide was slowly reduced in these solutions except HF solution, the charging curve exhibited two well-defined potential arrests. That is, as the reduction current was decreased, the first arrest at 0.6 V shifted slightly towards positive potential, but the second arrest moved remarkably in negative direction. The same shift of the second arrest was observed when the solution was agitated or the acid concentration was increased. The potential shift of the second arrest is attributed to blocking the reduction of the inner-layer oxide by anions adsorbed on the bare surface which was formed by the reduction of the outermost monolayer oxide at the first arrest potential. The effect of HSO 4 − on the reduction of the inner-layer oxide was stronger than that of ClO 4 − , and the effect of F − was very weak.

Growth of oxide layers on platinum electrodes in molten alkali nitrates

Abstract The formation of oxide layers on Pt anodes and on Pt cathodes in each of the molten alkali nitrates, LiNO 3 , NaNO 3 and KNO 3 , has been investigated under galvanostatic and potentiostatic conditions at 340°C. The coverage by oxygen on the anode increased with the polarization potential and time and reached a limiting value of about 2·6 oxygen atoms per apparent surface platinum atom. No cation effect on the anodic formation of the oxide film was observed. Thick multilayer oxide films grew very rapidly on the “cathode” in the KNO 3 melt and slowly on that in the NaNO 3 melt. In the KNO 3 melt and also in the NaNO 3 melt, the growth rate of the multilayer oxide reached a maximum at −1.65 V ( vs a Ag 0.1 M Ag + in 1 kg KNO 3 reference electrode) and rapidly fell at more negative potentials because of the dissolution of the oxide into the melt. Above −2.0 V, neither the formation of the multilayer oxide nor corrosion of the electrode was observed. In the LiNO 3 melt, no multilayer oxide film was formed at any potentials.

Electrochemical activity of electrodeposited and heat-treated platinum electrodes

Abstract The electrochemical activity of the platinum electrode for hydrogen reaction in 0.5 M H2SO4 has been studied with special reference to the state of the electrode surface. The electrode, prepared by electrodepositing a small amount of platinum on a platinum substrate from H2PtCl4 solution, is characterized by extremely high and stable activity, but this is decreased remarkably by annealing in vacuum. The activity decay on annealing is associated with the settling down of superficial metastable platinum atoms to stable lattice positions. The annealed electrode may be activated by usual oxidation-reduction treatment, ie by soaking in oxidizing reagent such as chromic acid solution or by anodization, followed by cathodic reduction in each case. The platinum electrode is also activated merely by exposing the electrode to air for a short time. The activation by these treatments seems to result from the formation of a superficial active monolayer of platinum atoms, which have been forced to re-arrange to new positions by oxygen adsorbed during the oxidation or the exposure to air.