Su-Moon Park

Electrochemistry of Conductive Polymers VIII In Situ Spectroelectrochemical Studies of Polyaniline Growth Mechanisms

The earlier stage of the polymerization reaction of polyaniline has been studied. The results indicate that the nitrene cation appears to be a key intermediate species, which leads to all three possible dimers including a head-to-tail dimer (N-phenyl-p-phenylenediamine), a tail-to-tail dimer (benzidine), and a head-to-head dimer (hydrazobenzene). The oxidized forms of these dimers were all shown to be capable of growing polyaniline in the presence of aniline, even though aniline was not oxidized

Electrochemistry of Conductive Polymers XX. Early Stages of Aniline Polymerization Studied by Spectroelectrochemical and Rotating Ring Disk Electrode Techniques

Early stages of aniline oxidation have been studied employing spectroelectrochemical and rotating ring disk electrode experiments in solutions of low (2 mM) and high (100 mM) aniline concentrations in 1 M H 2 SO 4 . Results indicate that a large amount of the tail-to-tail dimer, benzidine, is produced in the initial stage of aniline oxidation, which is followed by the generation of the head-to-tail dimer, N-phenyl-p-phenylene diamine (PPD), as well as oligomers in solutions of high aniline concentrations. The intermediate species have been identified by spectroelectrochemical and rotating ring disk electrode (RRDE) voltammetry experiments, and the relative amount of dimeric products has been estimated from the RRDE voltammograms. Evidence for the presence of a nascent species produced upon oxidation of aniline is presented from the RRDE experiments.

Thermodynamic stabilities of semiconductor electrodes

Abstract The application of the Pourbaix diagram to the prediction and understanding of the thermodynamic stability of semiconductor materials is discussed. With the aid of flat band potential information, the thermodynamic stability of semiconductor materials under various operating conditions, types of electrodes (n- or p-) to be used, conversion efficiencies, types of redox couples to be chosen for a given semiconductor electrode material, and the proper operating pH region can be determined. It is shown from the available information that all the semiconductor materials examined, i.e., CdS, CdSe, CdTe, and GaP may be unstable with respect to anodic dissolution and that p-type electrodes may be successfully used as photocathodes for the photoassisted electrolysis of water to hydrogen. GaAs is shown to be unstable with respect to both anodic and cathodic dissolution.

Oxidation of Zinc in Alkaline Solutions Studied by Electrochemical Impedance Spectroscopy

Electrochemical dissolution and passivation reactions of zinc have been studied in 1.0 M KOH solutions by electrochemical impedance spectroscopy. Equivalent circuits have been worked out by simulating the impedance data and using the results to model the dissolution and passivation reactions. A Tafel plot constructed from the charge-transfer resistances provides an exchange current of 0.11 A/cm{sup 2} and an {alpha} value of 0.36 for zinc oxidation. The maximum rate of zinc oxidation is observed at about {minus}1.30 V vs. the Hg/HgO reference electrode as judged from the charge-transfer resistance minimum obtained from impedance measurements. A negative polarization resistance with a reverse semicircle on the Nyquist plot illustrates the transition process from an active to passive potential region at {minus}1.10 V. At high anodic over-potentials, the zinc electrode behaved as a semiconductor electrode due to a compact ZnO passive film formed on the electrode surface.

Electrochemistry of Conductive Polymers XIV . In Situ Spectroelectrochemical and Kinetic Studies on Poly(3‐Methylthiophene) Growth

The growth mechanism of poly(3-methylthiophene) has been studied employing in situ spectroelectrochemical and kinetic measurements. Results indicate that: (1) the polymer film grows only when 3-methylthiophene is oxidized, suggesting that the radical-radical coupling reaction is important for the polymer growth; (2) concentrations of intermediate species, i.e., radical cations of the trimer and tetramer, are maintained at a steady-state level during the growth; and (3) both monomer and polymer concentrations appear in the rate expression, indicating that both the monomer and polymer participate in the polymer growth. The growth is not autocatalytic, and oxidation of both the monomer and polymer participate in the polymer growth. The growth is not autocatalytic, and oxidation of both the monomer and oligomer or polymer is required for further growth of the polymer. The reaction order in the rate expression depends on experimental conditions ranging 0.5 [approximately] 1 for the monomer, while it is about 0.5 for the polymer.

In Situ Spectroelectrochemical Studies on the Reduction of Sulfur in Dimethyl Sulfoxide Solutions

The electrochemical reduction of sulfur has been studied employing in situ spectroelectrochemical techniques. Previously unreported absorption bands in the UV region observed during the electrochemical reduction are assigned to the reduced sulfur species with high electron-to-atom ratios. The spectroelectrochemical studies indicate that reduction products generated at the first reduction step are essentially identical to those formed at the second with S[sub 3][sup 2[minus]] and S[sub 4][sup 2[minus]] as ultimate reduction products. This suggests that chemical reactions following the first electron transfer to produce S[sub 8][sup 2[minus]] are important. The derivative cyclic voltabsorptometric (DCVA) results indicate that all the reduced sulfur species are generated at the first cathodic wave and three are reduced at the second reduction wave, while other species show a delayed generation. A mechanism consistent with these observations is proposed.

In situ spectro-electrochemical studies on the oxidation mechanism of brass

The oxidation mechanism of brass has been studied with in-situ spectro-electrochemical techniques. The results indicate that the brass corrosion mechanism may be interpreted as being composed of dissolution chemistries of zinc and copper, except that the oxidation potential of zinc in brass is more positive than that of pure zinc. After zinc is oxidized, copper oxidizes to Cu(I) hydroxide/oxide in two consecutive steps, followed by further oxidation to Cu(II) hydroxide/oxide. These hydroxides/oxides are reduced upon cathodic scanning of the potential to produce the brass surface in reverse sequence as for oxidation; a relatively large amount of the zincate ion and a small amount of copper (II) hydroxide are still on the electrode surface, even after a long period of reduction at −1.3 V vs the Ag/AgCl, saturated KCl reference electrode.

Electrochemistry of conductive polymers VII. Autocatalytic rate constant for polyaniline growth

Abstract We report the results of the determination of the catalytic rate constant determined employing normal pulse voltammetric (NPV) techniques. The enhancement in NPP currents due to the presence of aniline in the electrolyte solution was related to the rate of the catalytic rate in the PA growth reaction by Δ i/nFA = k c [anniline][PA] where n is the number of electrons transferred, F is the Faraday constant, A is the electrode area, and k c is the catalytic rate constant. The degree of enhancement in current was measured as a function of aniline concentration with PA films of a constant thickness on the electrode. Similar measurements were also made in a solution containing constant concentration of aniline with the PA thicknesses varied. The pseudo first order rate constant was 0.47 s −1 at a constant PA thickness of 140 nm, whereas it was 0.48 s −1 in 0.050 M aniline solution.

Spectroelectrochemical studies on dissolution and passivation of zinc electrodes in alkaline solutions

Electrochemical oxidation of zinc electrodes has been studied in 1.0 M KOH solutions employing cyclic voltammetric and in situ spectroelectrochemical techniques. The results indicate that three different processes, i.e., dissolution, prepassivation, and passivation, take place in different potential regions. Two optically different solution species absorbing at 250 and 290 nm, which are assigned to Zn(OH){sub 4}{sup 2{sup {minus}} and Zn(OH){sub 3}{sup minus}}, respectively, are produced initially during anodic oxidation of zinc at different potentials to different extents with different respective ratios. These species undergo a series of consecutive chemical reactions to eventually lead to passive films on the surface. The film compositions were identified to be ZnO{sub 1{minus}x}(OH){sub 2x}OH{sup {minus}}-doped ZnO, and Zn-doped ZnO depending on the potential regime and aging. Details of the electrochemistry and chemistry taking place during electrolysis in these three regions are discussed based on the cyclic voltammetric and spectroelectrochemical data.

Spectroelectrochemical studies of p-benzoquinone reduction in aqueous media

p-Benzoquinone reduction has been studied in aqueous solutions employing spectroelectrochemical techniques, and an intermediate species thus far believed to be unstable in aqueous media has been shown to be reasonably stable in unbuffered solutions. The detection of this intermediate species, which is produced as a one-electron transfer product, i.e. anion radical, suggests that the e−H+e−H+ mechanism, in which e− and H+ represent electron transfer and protonation steps respectively, is a major reaction pathway during the electrochemical reduction of benzoquinone to the final product, hydroquinone, in unbufferred solutions. In buffered solutions, however, p-benzoquinone reduction is a two-electron process, as has been well known. A few anomalies discovered during the reduction of p-benzoquinone in aqueous media are discussed.