D. Tessier

Charge transfer at partially blocked surfaces: A model for the case of microscopic active and inactive sites

Abstract Non-linear diffusion effects induced by partial blocking of an electrode surface toward electron transfer are analyzed in the case where the blocking film is sprinkled with a large number of microscopic, active sites. Under such conditions, the average size of the active sites and the average distance between them are small compared to the total diffusion layer. As a result, non-linear diffusion is confined to a layer adjacent to the electrode surface which is thin compared to the total diffusion layer. It follows that, in the framework of relaxation electrochemical techniques, non-linear diffusion can be regarded as occurring under stationary conditions while linear diffusion is time-dependent. The resolution of the diffusion problem is thus greatly simplified, leading to the description of the polarization as a function of only two dimensionless parameters which are simply related to the experimental parameters: fractional coverage, distance between active sites, standard rate constant of electron transfer and time-range. The formulations thus obtained give evidence for a formal similarity with the polarization problem corresponding to the coupling of electron transfer with preceding and following chemical reactions. According to the values of these parameters, two main types of behavior are obtained: in one of them, the polarization curves show a quasi-reversible behavior, while in the other, kinetic-type waves are obtained. The effect of the various parameters on the characteristics of these waves and on the passage from one type of behavior to the other is described. Procedures for deriving estimates of the size of the active sites and the distance between them from the polarization curves are proposed.

Convolution potential sweep voltammetry V. Determination of charge transfer kinetics deviating from the Butler-Volmer behaviour☆

Abstract Convolution potential sweep voltammetry is applied to the analysis of charge transfer kinetics. The possibility of using the method without assuming a priori the validity of a given kinetic law, e.g. the Butler-Volmer law as usually done, is illustrated on the example of the reduction of tert -nitrobutane in acetonitrile and dimethylformamide. It is found that the post-factum experimental kinetics indeed deviate significantly from the Butler-Volmer behavior, involving a dependence of the transfer coefficient α upon the electrode potential. It is shown that the experimental variations of α match the orders of magnitude predicted by the Marcus theory. A procedure is proposed and applied for determining the reversible half-wave potential for redox couples in which one species is not quite stable chemically by combining the cyclic voltammetry and CPSV data.

Convolution potential sweep voltammetry: Part IV. Homogeneous follow-up chemical reactions

Abstract The application of Convolution Potential Sweep Voltammetry to mechanism analysis and rate determination in electrochemical processes involving homogeneous chemical reaction is discussed. The formal analysis of the transition between pure diffusion control and pure kinetic condition is treated in the case of a first order follow-up reaction. The practical applicability of the method is then tested on the reductive pinacolization of acetophenone in acetonitrile using as operational parameters the sweep rate, the initial concentration and the water content of the medium.

Convolution potential sweep voltammetry: III. Effect of sweep rate cyclic voltammetry

Summary The role of sweep rate in kinetic analysis by convolution potential sweep voltammetry is underlined. The method is evaluated over 3.5 orders of magnitude of sweep rate using the fluorenone-fluorenone anion couple in acetonitrile. The application of convolution procedures to cyclic voltammetry is discussed.

Kinetics of electron transfer to organic molecules at solid electrodes in organic media

Abstract Simple electron-transfer reactions to organic molecules in organic media appear to be much slower on solid metallic electrodes, such as platinum, gold and silver than on mercury or glassy carbon. An electrochemical treatment of the former electrodes is proposed, involving the repetitive cycling of the electrode potential within a negative range. A steady value of the apparent standard rate constant is thus obtained which is not very different from the values obtained on mercury and on glassy carbon. The main origin of the residual differences is likely to result from a different value of the potential at the reacting site deriving from different characteristics of the double layer. The investigations were carried out on four different compounds, tert-nitrobutane, nitrodurene, terephthalonitrile and 9-fluorenone in acetonitrile containing tetraalkylammonium salts as supporting electrolyte. The origin of the slowness of electron transfer on untreated electrodes and of the effect of the treatment is discussed in terms of a model involving the partial blocking of the electrode surface.

Variation of the electrochemical transfer coefficient with potential

The electrochemical electron-transfer rate constant has been determined as a function of the electrode potential for a series of simple electron-transfer processes to organic molecules in media containing acetonitrile or dimethylformamide and a quaternary ammonium salt as supporting electrolyte and using mercury as the electrode material. The reactions and the experimental conditions were selected so as to deal with outer-sphere processes and to minimize the magnitude of double-layer corrections. Convolution potential-sweep voltammetry and the impedance method were used for obtaining the kinetic data. Under these conditions, the electrochemical transfer coefficient was observed, in all cases, to vary, beyond experimental error, with the electrode potential. The magnitude of the variation is of the same order of magnitude as that predicted by the Marcus theory of outer-sphere electron transfer. A more complex reaction, the reduction of benzaldehyde in ethanol, involving dimerization steps following the initial electron transfer was also investigated. A definite variation of the transfer coefficient was again observed. This behaviour, observed for various solvents and functional groups, appears as a general phenomenon in the reduction of organic molecules in the case where charge transfer is fast and mainly governed by solvent reorganization.

Convolution potential sweep voltammetry: Part VI. Experimental evaluation in the kilovolt per second sweep rate range

Abstract Convolution potential sweep voltammetry is experimentally evaluated up to a sweep rate of 2278 V s−1 using a diffusion controlled system: the reduction of fluorenone in DMF. The effects of ohmic drop double layer charging and bandpass limitations of the instrument are discussed. The corresponding corrections are determined experimentally in the range 22.7–2278 V s−1. It is shown from the logarithmic analysis of the convoluted faradaic current that reliable results can be obtained up to this sweep rate after the above corrections have been carried out.

Electrodimerization: XI. Coupling mechanism of an activated olefin: p-methylbenzylidene-malononitrile as studied by convolution potential sweep voltammetry

Abstract The mechanism of the electrohydrodimerization of an activated olefin, p-methylbenzylidene-malononitrile, is studied by convolution potential sweep voltammetry aiming at confirmation of the conclusion derived from the measurements of the peak potential shifts with sweep rate and initial concentration. It is shown that CPSV is a more powerful tool than LSV for discriminating between the numerous mechanistic possibilities, based on the fact that it uses more of the information available along the current potential curves. The various convoluted experimental current-potential curves which allows the possibilities to be reduced to only three cases. The final diagnosis is made on the basis of a study of the effect of water addition. It is confirmed that the coupling is of the radical-radical type and the rate constant is determined.

A computer coupled high frequency a.c. technique for investigating fast non-Volmerian electron transfers

Abstract An a.c. technique for investigating fast electron transfer kinetics with particular emphasis on the detection of deviation from the Butler-Volmer behavior is described. Phase sensitive detection was carried out by using a lock-in amplifier and the frequency range investigated was 1–20 kHz. With such high frequencies the problem of extracting the faradaic response with a good accuracy was solved by using a shifting-amplification technique in the acquisition of data. This renders the performances comparable to those of the bridge method with in addition the advantage of a more facile automatization of the measurement procedure. Coupling of the measuring instrument with an on-line computer made possible the determination of the charge transfer kinetics for a very large number of potential values along the faradaic wave thus allowing the determination of rather subtle deviations from Butler-Volmer behavior. Instrumentation and data processing are described, performances and accuracy are discussed on the example of the reduction of nitromesitylene in DMF at a mercury electrode.