Theodore Kuwana

Activation and deactivation of glassy carbon electrodes

Abstract A procedure is described for treating glassy carbon to create an active electrode. For the first time, a level of activity comparable to that of active Pt has been attained. This was determined by the heterogeneous electron transfer rate (0.14±0.01 cm/s) of ferri-/ferro-cyanide. The possible factors in the activation of the glassy carbon electrode (GCE), such as electrode surface cleanliness, roughness and functional groups were studied. However, the GCE was found to deactivate with time; the rate of this deactivation appeared to correlate with the loss rate of hydrogen adsorption/desorption sites at Pt. These results suggest that impurity adsorption played an important role in the deactivation. In addition to assessing the level of activity via the ferri-/ferro-cyanide redox probe, the monitoring of electroactive surface functionalities by differential pulse voltammetry is proposed as a diagnostic for the validation of electrode preparation steps.

Spectroelectrochemical study of indirect reduction of triphosphopyridine nucleotide: I. Methyl viologen, ferredoxin-TPN-reductase and TPN

Summary The indirect electrochemical reduction of triphosphopyridine nucleotide was investigated spectroelectrochemically using electrogenerated methyl viologen radical cation in the presence of the electron carrier, spinach ferredoxin-TPN-reductase. The experimental results using a tin oxide optically transparent electrode clearly showed that the free radical cation could reduce TPN+ to TPNH at an appreciable rate only in the presence of the reductase. For these particular experiments the kinetic rate was found to be first order in reductase. Further confirmation of the reduction sequence and rate was afforded by the technique of chronocoulometry where the charge-time response function was evaluated for a current limited condition.

Electrochemical dispersion of Pt microparticles on glassy carbon electrodes

The effect of various electrode pretreatments on the properties of Pt microparticles (dia. ca. 0.1 μm) electrodeposited on glassy carbon (GC) is evaluated. On freshly polished GC surfaces, Pt particles are not observed by scanning electron microscopy (SEM) after the electrodeposition process. In addition, these electrodes exhibit very low catalytic activity toward hydrogen generation. Heat treatment of GC, prior to deposition at 0.185 V, markedly improves the catalytic activity of the electrode, and Pt particles are observed on the GC surface by SEM. It is believed that a thin layer of finely divided carbon is formed by the polishing procedure. Reduction of Pt onto this layer either did not produce Pt particles or produced particles which are removed when the electrode is washed with water. Heat treatment of the GC prior to deposition removes the carbon layer, allowing Pt reduction to occur on the intrinsic carbon surface with concomitant particle formation. Cyclic voltammograms show multiple reduction waves for the deposition of Pt on GC. In the potential range of 0.2 to 0.5 V, the first reduction exhibits a double peak which is assigned to the reduction of Pt(IV) to Pt(II). However, Pt microparticles are observed by SEM on the GC surface when the deposition potential is held within the limits of the first wave. A slow chemical step (probably disproportionation of Pt(II) to Pt0 and Pt(IV)) is involved in the nucleation and growth of the Pt particles at ⪢- 50 mV. For electrodes exhibiting high particle stability, hydrogen evolution, with exchange current densities comparable to bulk platinum, is achieved at loading levels of 10 to 20 μg cm−2 Pt.

Electroanalysis of oxygen reduction: Part III. Selective reduction to hydrogen peroxide or water using polymeric attachment of metalloporphyrins

Abstract Fe(III) tetra-( o -aminophenyl)porphyrin and Fe(III) tetra-( N -(2-hydroxyethyl)pyridyl)-porphyrin were attached to glassy carbon electrodes through amidization or esterifification of the methyl acryl chloride polymer. Long-term stability as well as high coverage (up to 10 −9 mol cm −2 ) enabled these electrodes to be useful with respect to catalysis of oxygen reduction. The reduction of O 2 to H 2 O 2 or H 2 O can be controlled by changing the surface coverage of the porphyrin. Based on results obtained for dissolved, adsorbed and polymercoated porphyrin, a general mechanism, for O 2 electrocatalysis is proposed.

Non-aqueous electrochemistry using optically transparent electrodes

Abstract Tin oxide, gold and platinum film optically transparent electrodes are evaluated for use in spectro-electrochemical studies of organic oxidations and reductions in non-aqueous solvents. Acetonitrile, dimethylformamide, nitromethane, propylene carbonate, dichloroethane, monoglyme and diglyme were used, usually with tetraalkylammonium perchlorate or hexafluorophosphate as supporting electrolyte. Care was taken in purifications and all experiments were run in a controlled atmosphere dry box. The background potential limits for these electrodes depend greatly on the type and purity of solvent and supporting electrolyte, and in well-purified systems are sufficiently large to allow the study of oxidation of benzene or the reduction of phenanthrene or napthalene. Data are reported for the reduction of methyl viologen, a6nthraquinone and three nitroaromatics, and for the oxidation and reduction of several aromatic hydrocarbons. The optical windows of these electrodes have made possible the in situ observation of the intermediates and products of electrode reactions; some examples, including work using rapid scanning spectrophotometry, are presented.

Characteristics of the electrode-solution interface under faradaic and non-faradaic conditions as observed by internal reflection spectroscopy

Summary Changes in the optical absorbance in the visible spectral region as observed by internal reflection spectroscopy with transparent electrodes have been studied as a function of potential. For non-faradaic conditions this change is related to the electric charge and can be partially explained, both in wavelength and amplitude-dependence, by the variation of the optical constants as related to the free electron model. The absorbance change as a function of time during potential step has been theoretically and experimentally examined. The effect of a homogeneous chemical reaction involving the optically monitored species formed electrochemically on the absorbance versus time curve is discussed quantitatively for the catalytic case.

Kinetic and mechanism studies of o-tolidine electro-oxidation using optically transparent electrodes

The kinetics and mechanism for the oxidation of o-tolidine have been studied at both platinum and optically transparent electrodes. Spectral data show that the intermediate is a dimeric species. Evaluation of fast kinetic rates for homogeneous reactions following charge transfer is demonstrated using transparent electrode.

Indirect electrochemical titration of beef heart cytochrome c oxidase

Titration of beef heart cytochrome c oxidase with electrochemically generated reductant, methyl viologen cation radical MV+, and oxidant, molecular oxygen, has enabled cycling of the oxidase repeatedly from its totally oxidized to its totally reduced forms. Spectrocoulometric results clearly show that cytochrome oxidase accepts four electrons during both reduction with MV+ and oxidation with O2. Oxidation with coulometrically generated O2 produces oxidized oxidase, with no evidence of the “oxygenated” form.

Charge distribution in electron transport components: Cytochrome c and cytochrome c oxidase mixtures

Abstract Mixtures of cytochrome c oxidase and cytochrome c have been titrated by coulometrically generated reductant, methyl viologen radical cation, and physiological oxidant, O2. Charge distribution among the heme components in mixtures of these two redox enzymes has been evaluated by monitoring the absorbance changes at 605 and 550 nm. Differences in the pathway of the electron transfer process during a reduction cycle as compared to an oxidation cycle are indicated by variations found in the absorbance behavior of the heme components during successive reductive and oxidative titrations. It is apparent that the potential of the cytochrome a heme is dependent upon whether oxidation or reduction is occurring.

Electrochemical and spectroscopic studies of metal hexacyanometalate films—III. Equilibrium and kinetics studies of cupric hexacyanoferrate

Abstract The potential response of cupric hexacyanoferrate (CuHCF) thin films on glassy carbon substrate was studied by cyclic voltammetry in potassium and ammonium ions. The shift in potential with ammonium ion concentration is Nernstian with a difference of +0.70 V in 1 M NH + 4 with respect to 1 M K + . Equilibrium spectroelectrochemical studies have shown that the mole fraction of CuHCF is a reflection of film activity. Kinetic studies have indicated that the redox reaction is faster in ammonium ion than in potassium ion. Similarities and differences in the equilibrium and kinetic response of the film in both electrolytes are discussed.