Robert A.W. Dryfe

Diffusional mass transport to microband electrodes of practical geometries : a simulation study using the strongly implicit procedure

Abstract The strongly implicit procedure is used to model the diffusion-only chronoamperometric responses of microband electrode geometries which deviate from the ideal. Specifically, attention is focused on four electrode shapes: elevated microband electrodes (with conducting supporting sides), recessed microband electrodes (with insulating pit walls), platform electrodes (with insulating supporting sides) and, for the purposes of comparison, a hypothetical line electrode without any support which permits diffusional mass transport to both sides of the infinitesimally thin electrode. In all cases the short time transient is found to be described by simple analytical expressions in which the contributions from linear diffusion over the electrode area(s) and non-linear diffusion to the electrode perimeters are clearly distinguishable.

Modelling electrode reactions using the strongly implicit procedure

Abstract The strongly implicit procedure is shown to be an easy-to-use stable, rapid and computationally efficient method of solving the coupled mass transport equations which describe complex electrode reactions involving diffusion, convection and homogeneous chemical steps. The method is illustrated with a diversity of problems pertaining to both microelectrodes and electrodes of conventional dimensions located in a rectangular channel through which electrolyte is pumped under laminar conditions. In all cases excellent agreement with experiment and/or existing analytical or numerical theory is noted.

In situ electrochemical ESR studies of reactive radicals: the reductions of bromo-anthraquinone and methyl viologen

The application of channel electrodes for quantitative in situ electrochemical ESR studies of unstable radicals is discussed with reference to two systems. First, the reduction of 1-bromo-anthraquinone in acetonitrile solution in the presence of laser light of wavelength 488 nm is shown to lead to the loss of bromide and the formation of the anthraquinone radical anion via a photo-ECE process. The kinetics of this reaction as determined by channel electrode ESR studies are found to be in good agreement with those measured by independent methods. Second, the one-electron reduction of the methyl viologen dication in aqueous solution is shown to lead to the formation of viologen radicals which are known to dimerise. However, it is shown that since this reaction is rapid and reversible, measurements of the ESR signal as a function of electrolyte flow rate and current are essentially blind to this reaction. Lower estimates for the dimerisation rate constant are deduced.

Excimer laser-induced electrochemical activity in carbon ink films

The effects on the microscopic and electrochemical properties of composite ink films after exposure to UV radiation from a 193 nm excimer laser source were investigated. The work considered two carbon ink types, a commercial screen ink material and a preparation consisting of micro-spherical glassy carbon with polystyrene. A scanning electron microscopy study of these ink film surfaces has concluded that laser treatments selectively etch the organic binding polymer from the composite surface thereby exposing sub-layers of carbon particulates. The photoablative processes by which the UV radiation interacts with materials not only remove the ink polymer, thereby increasing the surface area, but also chemically modify the carbon surface resulting in an increased rate of electron transfer. The cyclic voltammetric characteristics of potassium hexacyanoferrate(III) and ferrocene carboxylic acid at these modified ink films are analysed using the methodology proposed by Amatore et al. (J. Electroanal. Chem, 147 (1983) 39).

The photoelectrochemical reduction of p-bromo-nitrobenzene: An ECEE, ECE or DISP process? Mechanistic resolution via channel electrode voltammetry

Abstract The photoelectrochemical reduction of p -bromo-nitrobenzene in acetonitrile solution, containing tetrabutylammonium perchlorate as supporting electrolyte, was studied using a platinum channel electrode. It is shown that p -bromo-nitrobenzene is reduced at a half-wave potential of −0.98 V (vs. Ag pseudo-reference electrode) to form a radical anion which is stable in the absence of light. On irradiation with light of wavelength 330 nm, corresponding to an absorption band in the radical anion, appreciable photocurrents flow and in situ electrochemical electron spin resonance (ESR) shows the formation of the nitrobenzene radical onion. The mechanism of formation is shown to occur via a “photo-ECE” mechanism rather than either “photo-ECEE” or “photo-DISP” processes by means of (i) the variation in photocurrent with flow rate, and (ii) novel phototransient experiments in which the photocurrent is followed after the step-wise application of light to the system after steady-state transport l imited currents have been established in the dark at various flow rates. Experiments conducted in the presence of different concentrations of added Br − show steady-state photocurrents that are diminished relative to those flowing under corresponding conditions in the absence of added Br − . The kinetic modelling of this effect is consistent with a light-induced breakage of the carbon-bromine bond in the C-step of the ECE process followed by reaction of the ·φNO 2 radical thus formed with both Br − and the solvent system.

Micro-glassy carbon inks for thick-film electrodes

Reference LEPA-ARTICLE-1997-016View record in Web of Science Record created on 2005-11-07, modified on 2017-05-12

Voltammetry under high mass transport conditions. The application of the high speed channel electrode to the reduction of pentafluoronitrobenzene

Abstract The electroreduction of pentafluoronitrobenzene in dimethylformamide solution resulting in the formation of the dimer, octafluoro- 4, 4′-dinitro-biphenyl, is studied. The dimeric species is reduced further at the potentials applied to the corresponding di-anion. Microband electrodes are used in conjunction with a high speed channel flow cell to explore the mechanism of this process and to quantify the associated very fast homogeneous kinetics. In particular, the formation of the dimer is seen to proceed via the rate determining loss of fluoride anion from the initially formed radical anion of pentafluoronitrobenzene with a first order rate constant of 4.2 × 10 5 s −1 . The reduction of the dimer to its di-anion is seen to occur via a DISP2 mechanism in which the rate determining step is the disproportionation of the mono-anions of octafluoro,4,4′-dinitro-biphenyl with a rate constant of 2 × 10 8 M −1 s −1 . The merits of high speed channel electrodes for the measurement of fast homogeneous kinetics are examined and compared with those of microdisc voltammetry in stationary solution.

Photoelectrochemical dehalogenation of p-halo-nitrobenzenes

The efficient dehalogenation of aromatic substrates by electrochemical means is of potential synthetic value given the possibility of controlled and selective halide loss at low temperatures. Mechanistically the general picture [l-111 which has emerged is that whereas alkyl halides undergo one-electron reduction with simultaneous carbon-halogen cleavage, aryl halides follow a sequential route in which bond breaking follows electron transfer and radical anions of varying stability are formed as intermediates. The purpose of the work described in this preliminary note is to study whether photochemical activation of the electrogenerated radical anion intermediates can usefully influence the course of reaction. In particular, we examine the influence of light of wavelengths between 300 and 600 nm on the electroreduction of the p-halo-nitrobenzenes (p-X-N no evidence for nitrite loss is observed.

TIME-RESOLVED LASER-INDUCED FLUORESCENCE STUDY OF PHOTOINDUCED ELECTRON TRANSFER AT THE WATER/1,2-DICHLOROETHANE INTERFACE

Reference LEPA-ARTICLE-1997-005View record in Web of Science Record created on 2005-11-07, modified on 2017-05-12

Theory of ECE processes at hemicylinder and band microelectrodes

Numerical simulations are used to predict the chronoamperometric current transients resulting from an ECE process at both hemicylinder and band microelectrodes. Working curves are presented to permit the analysis of experimental data. It is demonstrated that the long-time behaviour at a band of width w and length xe is identical with the current at a hemicylinder of width w and radius a = xc4, as has previously been shown to be the case for electrode processes involving simple electron transfer in the absence of coupled homogeneous kinetics.