P. Hapiot

Fast sweep cyclic voltammetry at ultra-microelectrodes: Evaluation of the method for fast electron-transfer kinetic measurements

With ultra-microelectrodes (in the 10 μ m diameter range), it is possible to decrease the ohmic drop to values that allow the use of very high scan rates in cyclic voltammetry (10–1000 kV s−1. Although reduced as compared with standard size microelectrodes, the ohmic drop is, however, not negligible and thus must be appropriately corrected for. On the other hand, at these high sweep rates the double-layer charging current is a substantial portion of the total current. The correct treatment of the cyclic voltammetric data thus necessitates an ohmic drop correction that takes into account the mutual dependence of the faradaic and double-layer charging currents. Such a treatment is applied to the reduction of anthracene in dimethylformamide, a test example of very fast electron-transfer kinetics (kSap = 3.5 cm s−1), at sweep rates ranging from 20 to 250 kV s−1. The system is analysed both by direct simulation of the cyclic voltammograms and by means of convolution of the voltammograms with the diffusion function (πt)−12. The effect of band-pass limitations of the potentiostat and current measurer on the cyclic voltammetric responses is also discussed.

Oxidation of caffeic acid and related hydroxycinnamic acids

Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H a has been studied by electrochemical methods and by pulse radiolysis in aqueous and organic solvents. The results have been compared with the behaviour of 4-coumaric acid 2H 2 and ferulic acid 3H 2. The first oxidative intermediates have been characterised by their UV spectra and oxidation potentials. In the case of 2H 2 and 3H 2, the initial radicals decay by a second order process indicating a radical-radical coupling mechanism. On the contrary, for caffeic acid the oxidation leads to the formation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.

Kinetics of the charge-discharge process in conducting polymers: slow relaxation and hysteresis effects. Investigations on polyaniline by millimetric and ultramicroelectrodes

Abstract In cyclovoltammetry experiments, the oxidation wave of a conducting polymer depends on the wait-time it has been left in its insulating state. This relaxed wave differs in shape and peak position from the steady-state wave, that is, the wave obtained by repetitive cycling. This relaxation effect was studied with millielectrodes for scan rates between 2 and 200 mV/s and wait-times in the range 10−3×10 3 s, and with ultramicroelectrodes (5 μm diameter) with scan rates between 10 2 and 2×10 5 V/s for wait-times down to 0.2 ms. In both cases, it is shown that the shift in peak potentials is logarithmic as a function of both wait-time and scan rate. The steady-state wave can be thus be defined as a relaxed wave for a particular wait-time. On this basis, part of the hysteresis between oxidation and reduction comes from the relaxation effect. Ultramicroelectrodes provide a means of estimating the kinetic rate constant of polyaniline. The relaxation of polymethylthiophene is also briefly considered.

Fast electrochemical studies of the polymerization mechanisms of pyrroles and thiophenes. Identification of the first steps. Existence of π-dimers in solution

Abstract The content of several works effected on the electrochemical polymerization of several pyrroles and thiophenes is given and exemplified. It is shown that the polymerization of pyrroles proceeds via a cation-radical coupling. In the case of thiophenes, the electrochemistry of end-capped oligomers giving stable cation radicals has been first examined. The existence of associated forms of the cation radicals, the π-dimers, has been demonstrated, and the formation parameters of these π-dimers have been determined using only the electrochemical analysis of the reversible voltammograms in solution. On the other hand, the σ-polymerization of some polymerizable thiophene oligomers (quinquethiophenes and trimethylsilylterthiophenes) has been examined, and it is also shown that the preferred process in acetonitrile or dichloromethane is a cation-radical/cation-radical coupling.

Electron-transfer coupling of diffusional pathways: Theory for potential step chronoamperometry and chronocoulometry

Abstract Diffusional pathways involving couples with different diffusion coefficients can be coupled by electron-transfer reactions involving both members of each couple. This results in an enhancement or a decrease of the diffusion current at the level of the second wave. The problem is analysed in the context of potential step chronoamperometry and chronocoulometry with a view to application to homogeneous solutions and systems confined within a film deposited on the electrode surface. Procedures for calculating the enhancement, or decrease, of the diffusion current as a function of the ratio of the diffusion coefficients, the ratio of the bulk concentrations and the equilibrium constant of the coupling reaction ( K ) are described for coupling reactions that are fast compared to the diffusion processes. It was found that the limiting behaviour where K = 1 or K = ∞ is practically achieved for most experimental systems corresponding to overlapping and well-separated waves, respectively. The value of K exerts an influence within a rather narrow range. The treatment described also allows the value of the bulk concentration ratio to be predicted at which the influence of the coupling reaction is maximal for a given value of the diffusion coefficient ratio. The possible influence of the kinetics of the coupling reaction on the diffusion current is discussed for the self-exchange and irreversible cross-exchange cases. It was found that this occurs in a rather narrow range of rate constant values explaining why a number of experimental systems behave as if they were either uncoupled or completely coupled.

Electron transfer coupling of diffusional pathways. Homogeneous redox catalysis of dioxygen reduction by the methylviologen cation radical in acidic dimethylsulfoxide

It is shown that reduction of dioxygen in acidic DMSO can be catalyzed by the MV2+/MV·+ couple. This homogeneous redox catalytic process is under the kinetic control of the MV·+ + O2 → MV2+ + O·−2 electron transfer. Dioxygen has a diffusion coefficient which is much larger (6 times) than that of MV2+. This difference must be taken into account in the treatment of the kinetic data, especially for large values of the catalytic efficiency. The corresponding theory is derived in the context of cyclic voltammetry and used to analyze the variation of the catalytic efficiency with the sweep rate and the concentrations of catalyst and substrate. The rate constant of the MV·+ + O2 → MV2+ + O·−2 reaction is found to be 2.3 x 105M−1 s−1 showing that the reverse reaction is under diffusion control. This is compatible with the standard activation Gibbs energy which can be derived from the kinetics of the electrochemical reduction of dioxygen according to the Hush-Marcus theory, provided than the attraction work term for MV2+ and O·− is taken into account.

Electrochemical polymerization of several salen-type complexes. Kinetic studies in the microsecond time range

Abstract The first stages of the electrochemical polymerization of several salen-type complexes were studied by fast cyclic voltammetry on an ultramicroelectrode. In several cases, it was possible to observe the first electrogenerated intermediate and to measure its formal potential. The polymerization process was found to be very fast and to occur in the 10 microsecond time range or faster.

Instrumentation for fast voltammetry at ultramicroelectrodes: Stability and bandpass limitations

Abstract Careful design of the current-measuring amplifier circuit appears crucial for meaningful faradaic and double-layer charging responses to be obtained in cyclic voltammetry at ultramicroelectrodes for scan rates in the MV s −1 scan rate range. A step-by-step analysis of the system in the absence of a faradaic process reveals large deleterious effects of stray capacitances in addition to the intrinsic bandpass limitations of the current-measuring amplifier. Large oscillations ensue which distort the current response significantly. As a remedy, the introduction of a stabilizing capacitance in between the input and output of the amplifier is investigated. As shown by complete simulation of the current responses, proper adjustment of the stabilizing capacitance allows one to eliminate the distortion, the remaining effect being a small and correctable shift of the potential scale, up to scan rates of the order of 2 MV s −1 .

MECHANISM OF OXIDATIVE COUPLING OF CONIFERYL ALCOHOL

Abstract The dimerization mechanism of coniferyl alcohol in aqueous and non-aqueous media has been investigated by electrochemistry and by radiolysis. The coniferyl radical has been observed and shown to dimerize by a radical-radical coupling.

One-electron redox potentials for the oxidation of coniferyl alcohol and analogues

The values of one-electron redox potentials are of special interest in the knowledge of the polymerization process of coniferyl alcohol, as the primary step of this mechanism involves the monoelectronic oxidation of coniferyl alcohol into the corresponding phenoxyl radical [1,2]. However, the redox potentials at which this oxidation occurs are not known. This is due to the fact that the measurement times employed in the electrochemical studies were too long to allow the observa- tion of the phenoxyl radical by means of its reduction current, except for some hindered phenolates [3]. One-electron redox potentials for phenolate/phenoxyl radical couples can in principle be measured by pulse radiolysis; however, data can be obtained only in water and not in organic solvents like acetonitrile [4,5] which are more like the reaction site at the cell wall [6]. Using the recently developed ultramicroelectrode technique (see for example ref. 7 and references cited therein), it is possible to overcome these difficulties and to observe phenoxyl radicals through their reduction wave in fast-scan cyclic voltammetry. This permits the measurement of the formal one-electron redox potential E” of the phenolate/phenoxyl couples and also an estimation of their