Martin C. Henstridge

Mass transport to micro- and nanoelectrodes and their arrays: a review.

The use of micro- and nanoelectrodes and their arrays has become commonplace in modern electrochemistry. Numerical simulation is often required for detailed analysis of voltammetric data and this relies upon an understanding of the prevailing mass transport operating under the experimental conditions. The theoretical basis of our understanding of mass transport, particularly diffusion and migration, has developed greatly in recent years. We review both theoretical and experimental studies which have probed the mass transport at micro- and nanoelectrodes and their arrays.

Investigation of the optimal transient times for chronoamperometric analysis of diffusion coefficients and concentrations in non-aqueous solvents and ionic liquids

We report the optimal transient times for chronoamperometric experiments in order to simultaneously determine accurate values of concentration (c) and diffusion coefficient (D), or alternatively the number of electrons passed (n) providing c is known. This is achieved by the analysis of the current-time transients resulting from potential steps for the oxidation of ferrocene in acetonitrile and the reduction of cobaltocenium in 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide. The analysis is based upon Shoup and Szabo approximation, which has been reported to describe the current response over all time values to within an error of 0.5% [D. Shoup and A. Szabo, Journal of Electroanalytical Chemistry, 1982, 140, 237-245]. The error is quantified through comparing the resulting chronoamperometric data with simulation at all transient times. In addition, an alternative simple approach to the simultaneous determination of nc and D values is proposed by independently investigating the short and long time regimes of chronoamperometric transients. The chronoamperometry of hydrazine is investigated as a multielectron example process.

Mass transport to and within porous electrodes. Linear sweep voltammetry and the effects of pore size: The prediction of double peaks for a single electrode process

We simulate an electrode modified with a conducting porous film, where the electrolysis occurs both at the surface of the film and within it, in order to study the effect of pore size on the peak current in linear sweep voltammetry. For redox systems with reversible electrode kinetics we find that for both very large and very small pores the peak current is given by the Randles-Ševčik equation. For intermediate pore size, however, we observe a greatly enhanced peak current. When considering systems with irreversible electrode kinetics a very similar pattern is observed, except for the case of very small pores. In this case the peak current is actually smaller than expected from the Randles-Ševčik equation because the peak splits into two distinct peaks; one due to “thin layer” diffusion within the film and another caused by planar diffusion from bulk solution. The experimental implications of this observation are significant given the widespread use of modified electrodes for analysis.

Ammonium-Directed Olefinic Epoxidation: Kinetic and Mechanistic Insights

The ammonium-directed olefinic epoxidations of a range of differentially N-substituted cyclic allylic and homoallylic amines (derived from cyclopentene, cyclohexene, and cycloheptene) have been investigated, and the reaction kinetics have been analyzed. The results of these studies suggest that both the ring size and the identity of the substituents on nitrogen are important in determining both the overall rate and the stereochemical outcome of the epoxidation reaction. In general, secondary amines or tertiary amines with nonsterically demanding substituents on nitrogen are superior to tertiary amines with sterically demanding substituents on nitrogen in their ability to promote the oxidation reaction. Furthermore, in all cases examined, the ability of the (in situ formed) ammonium substituent to direct the stereochemical course of the epoxidation reaction is either comparable or superior to that of the analogous hydroxyl substituent. Much slower rates of ring-opening of the intermediate epoxides are observed in cyclopentene-derived and cycloheptene-derived allylic amines as compared with their cyclohexene-derived allylic and homoallylic amine counterparts, allowing for isolation of these intermediates in both of the former cases.

Generator−Collector Experiments at a Single Electrode: Exploring the General Applicability of This Approach by Comparing the Performance of Surface Immobilized versus Solution Phase Sensing Molecules

We demonstrate proof-of-concept that generator-collector experiments can be performed at a single macroelectrode and used to determine mechanistic information. The practical advantages of such a system over conventional generator-collector techniques are also outlined. The single-electrode generator-collector technique is applied to study the known mechanism of oxygen reduction in aqueous conditions as a model system. We seek to demonstrate that the single-electrode generator-collector approach is capable of detecting local pH changes, immediately adjacent to the electrode surface during a redox reaction. Experiments are performed using a molecular pH probe attached to the electrode surface. Comparison of experimental data with numerical simulations verifies that the reduction of oxygen at pH 6.8 proceeds via a two-electron, two-proton mechanism. Experiments were also performed with a molecular pH probe dissolved in the electrolyte solution in order to explore the feasibility of this approach, which is potentially applicable to a much wider range of electrochemical systems.

Switching transition states with changing electrode potential: Zn(II)/Zn electrodeposition on glassy carbon in the N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid

The mechanisms for the electrodeposition and stripping of Zn2+|Zn in the N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid are investigated via cyclic voltammetry. Analysis showed that the deposition of Zn onto a bulk Zn surface occurred via a two-electron process, with the first electron transfer being rate determining. The electrodissolution was found to occur via a potential-dependent mechanism with the first electron transfer being rate determining near the formal potential, while an intermediate chemical step became rate determining at more positive potentials.