Takuro Kodera

Electrochemical behavior of H2O2 at Ag in HClO4 aqueous solution

Concentration dependences of the steady rest potential (φr) and polarization characteristics in the anodic and cathodic directions were observed at an Ag electrode in acidic solutions of various concentrations of H2O2 and HClO4. Results lead to the conclusioin that (i) the cathodic reaction is the reduction of H2O2 with the slowest step being the first electron transfer, (ii) the anodic reaction is the dissolution of Ag where the subsequent diffusion of the dissolved Ag+ is rate-controlling and (iii) φr is the mixed potential determined by the above two reactions. The open circuit potential (φ′r after the cathodization showed abnormal behavior and potential oscillations were observed at higher cathodic current densities. A mechanism for the φ′r behavior is proposed in connection with the potential oscillation.

Electrooxidation of methanol on platinum bonded to the solid polymer electrolyte, Nafion

The electrooxidation of methanol was enhanced on PtSn-SPE, PtRu-SPE and PtIr-SPE in sulfuric acid solution, when compared with the activity of Pt-SPE, which has already been shown to have a higher activity than a Pt electrode. SPE is an abbreviation for Nafion, a solid polymer electrolyte. It is suggested that this dual enhancement of the oxidation rate for PtSn-SPE and PtRu-SPE catalysts is due to the modification of the oxidation state of Pt by Sn and Ru and to the presence of H2O and CH3OH, both modified by the SPE matrix. This modification appears to weaken their hydrogen bonds in solution. Both Pt and Ir have catalytic properties for methanol oxidation, but a PtIr-SPE catalyst showed a more enhanced catalytic activity than either of them. This will be discussed in terms of Ir, oxidized at relatively low positive potentials, assisting the redox process of Pt0/Pt2+ or Pt2+/Pt4+ in the SPE matrix, where CH3OH and H2O are present in modified forms. For comparison, IrPd-SPE was also used as an electrode and showed a higher activity than Ir alone, although Pd did not have any activity toward methanol oxidation in sulfuric acid solution. Irrespective of the kind of Pt-SPEs, the Tafel slope was approximately 120 mV; the CH3OH concentration dependence was of the order of 0.2–0.6. The pH dependence was nearly 0.5 against NHE. The activation energy of the Pt-SPEs for the reaction ranged between 20 and 33 kJ mol−1.

Electrochemical Behaviour of hydrazine on platinum in alkaline solution

Abstract Concentration dependences of the rest potential and polarization characteristics in the anodic and catholic directions were observed on a Pt electrode in alkaline solutions at various concentrations of N2H4 and OH−. On the basis of these observations, it is concluded that the oxidation of N2H4 proceeds through the electrochemical oxidation N2H4 + 2OH− → N2H2(a) + 2H2O + 2e−, followed by the chemical decomposition N2H2(a) → N2H(a) + H(a) which is rate determining.

On the electrochemical behavior of H2O2 AT Ag in alkaline solution

Electrochemical oxidation and reduction of H2O2 on Ag were studied in alkaline solution of 10−3−0.3 M H2O2 and 2 × 10−3 −1.0 M KOH under N2 bubbling. Steady i-φ curves obtained by a cyclic potential sweep method in a potential range where no electrode oxidation takes place, lead to the following results: (1) icd (A cm−2) (cathodic limiting current density) = 1.0 × [H2O2]1.0T (M), (2) i1d (A cm−2 (anodic limiting one) = icd ([KOH] ⪖ [H2O2]T) or 1.0 × [KOH] < [H2O2]T), (3) φm (V) (mixed potential) = 0.126-0.060 log [KOH]1.0 and (4) (∂φ/∂i)φ=φm (Ωcm2) (reaction resistance at φ = φm) = 0.057 × [H2O2]−1.0T (M−1), where [H2O2]T designates a total H2O2 concentration and the others have their usual meanings. The above results are explained by the following mechanism; HO−2 formed by the reversible chemical reaction, H2O2 + OH ⇌ HO−2 + H2O, is oxidised in anodic reaction by two steps: HO−2 HO2 (a) + e− and HO2(a) + OH− → O2 + H2O + e−, whereas in cathodic reaction, H2O2 is reduced by H2O2 + e− OH(a) + OH−, OH(a) + e− → OH−. Here, designates a rate determining step, Catalytic decomposition of H2O2 on the electrode is also discussed.

Limit cycle in electrochemical oscillation—potential oscillation during anodic oxidation of H2

Abstract A limit cycle was obtained by simulating kinetic equations describing the potential oscillation due to coupling of the anodic oxidation of H2 with Ag deposition and dissolution. Attractive interaction between Ag adatoms on Pt test electrodes was found to be essential for producing the limit cycle.

Potential oscillation during anodic oxidation of hydrogen at a platinum electrode—II. Kinetic analysis

A kinetic model was constructed representing the coupling of the anodic oxidation of H2 with Ag deposition and dissolution. The calculated potential oscillation curve reproduced well the observed one. Furthermore, the model was compatible with the following experimental results reported in the preceding paper. (1) The potential oscillation is observed in a certain range of the applied current, I, and a plot of log I against the maximum potential, φMAX, in oscillation gives a straight line. (2) On the other hand, in the region where I is lower than a threshold value, potential does not oscillate and a plot of log I vs the steady potential, φ, is found to be linear. (3) A smaller bubbling rate of H2 results in a longer period, τ, and a higher φMAX.

Potential oscillation during anodic oxidation of hydrogen at a platinum electrode—I. experimental

Sustained potential oscillations were observed in a certain range of anodic current, during the oxidation of H2 in various acidic solutions with a small amount of a metal ion, such as Ag+, Cu2+ and Pb2+, and the following results were obtained. The maximum potentials in oscillations are linear to the logarithm of the anodic current but are scarcely dependent on the concentration of metal ion. A smaller bubbling rate of H2 results in a higher maximum potential and a longer periodic time. The mechanism of the oscillation is discussed qualitatively.