Christine Lefrou

Absolute determination of electron consumption in transient or steady state electrochemical techniques

Abstract A method elaborated from original work by Lingane is discussed and used to determine the absolute value of the electron stoichiometry corresponding to any electrochemical wave, under the exact conditions of its observation. The method is based on comparison of the currents measured by a pair of microscale techniques such as chronoamperometry, cyclic voltammetry, RDE or steady state methods at microelectrodes. From an evaluation of the uncertainties associated with the use of each possible pair, it is concluded that the best choice consists in chronoamperometry and a steady state method at a microelectrode. The experimental validity of the method is examined in five different cases pertaining to organic or organometallic electrochemistry, in which the determination of the absolute value of the number of electrons exchanged leads to important mechanistic or synthetic consequences.

On-line compensation of ohmic drop in submicrosecond time resolved cyclic voltammetry at ultramicroelectrodes

Abstract The feasibility of cyclic voltammetry at ultramicroelectrodes in the submicrosecond domain has been demonstrated by several groups, including ours, to be an adequate method for the direct measurements of fast rates of electron transfer or fast follow-up chemical reactions. However, even at ultramicroelectrodes and in rather conducting media, the accuracy of the method is limited by ohmic drop and variations in capacitive currents induced by faradaic currents. These sources of distortion can be removed by on-line electronic compensation of the ohmic drop. This is illustrated for the reduction of anthracene at scan rates up to 100,000 V s−1. The determination of the rate constant (ko = 3.0 ± 0.4 cm s−1) of electron transfer to anthracene obtained by this method is in rather good agreement with that (ko = 3.8 ± 0.6 cm s−1) measured previously by means of post-factum simulation of the distorted voltammograms.

New concept for a potentiostat for on-line ohmic drop compensation in cyclic voltammetry above 300 kV s−1

Abstract A new concept for a potentiostat is presented for ohmic drop compensation in submicrosecond cyclic voltammetry. Theory and simulations of the potentiostat allow the prediction of most of its characteristics, which are then confirmed experimentally on dummy cells. Reduction of anthracene in acetonitrile is examined as a test experimental case. It is thus shown that the potentiostat affords perfectly ohmic drop corrected voltammograms up to scan rates exceeding 300 kV s−1. If the data are corrected further to eliminate the slight constant potential shift due to filtering by the potentiostat (first-order filter correction), scan rates exceeding 0.5 MV s−1 are achievable without distortion.

Is cyclic voltammetry above a few hundred kilovolts per second still cyclic voltammetry

Abstract The development of ultramicroelectrodes and fast potentiostats in very recent years has allowed the investigation of electrochemical kinetics in the submicrosecond time scale by means of cyclic voltammetry at scan rates in the megavolt per second time scale. Extraction of kinetic data from such cyclic voltammograms, after their correction for distortions arising from effects due to instrumental bandpass, ohmic drop and capacitive charging currents, has been performed using classical theories for cyclic voltammetry. However, application of these classical theories supposes that the diffusion layer and the double layer can be separated theoretically. It is shown here that this classical assumption is no longer valid when scan rates in the megavolt per second range are considered, since the two layers, being then of similar sizes, are no longer physically separable. The ensuing distortions of cyclic voltanunograms, with respect to the classical theories, are examined for a single electron transfer mechanism. It is shown that the cyclic voltammograms should normally be extremely dependent on their location with respect to the point of zero charge.

Surface reactivity from electrochemical lithography: illustration in the steady-state reductive etching of perfluorinated surfaces.

The scanning electrochemical microscope (SECM) in the lithographic mode is used to assess quantitatively, from both theoretical and experimental points of view, the kinetics of irreversible transformation of electroactive molecular moieties immobilized on a surface as self-assembled monolayers (SAMs). The SECM tip allows the generation of an etchant that transforms the surface locally and irreversibly. The resulting surface patterning is detectable by different surface analyses. The quantification of the surface transformation kinetics is deduced from the evolution of the pattern dimensions with the etching time. The special case of slow etching kinetics is presented; it is predicted that the pattern evolution follows the expansion of the etchant at the substrate surface. The case of a chemically unstable etchant is considered. The model is then tested by inspecting the slow reductive patterning of a perfluorinated SAM. Good agreement is found with different independent SECM interrogation modes, depending on the insulating or conducting nature of the covered substrate. The surface transformation measurements are also compared to the reduction of solutions of perfluoroalkanes. The three-orders-of-magnitude-slower electron transfer observed at the immobilized molecules likely describes the large reorganization associated with the generation of a perfluoroalkyl-centered radical anion.

Contactless surface conductivity mapping of graphene oxide thin films deposited on glass with scanning electrochemical microscopy.

The present article introduces a rapid, very sensitive, contactless method to measure the local surface conductivity with Scanning Electrochemical Microscopy (SECM) and obtain conductivity maps of heterogeneous substrates. It is demonstrated through the study of Graphene Oxide (GO) thin films deposited on glass. The adopted substrate preparation method leads to conductivity disparities randomly distributed over approximately 100 μm large zones. Data interpretation is based on an equation system with the dimensionless conductivity as the only unknown parameter. A detailed prospection provides a consistent theoretical framework for the reliable quantification of the conductivity of GO with SECM. Finally, an analytical approximation of the conductivity as a function of the feedback current is proposed, making any further interpretation procedure straightforward, as it does not require iterative numerical simulations any more. The present work thus provides not only valuable information on the kinetics of GO reduction in mild conditions but also a general and simplified interpretation framework that can be extended to the quantitative conductivity mapping of other types of substrates.

Accurate and Simplified Consideration of the Probe Geometrical Defaults in Scanning Electrochemical Microscopy: Theoretical and Experimental Investigations

Fabrication of scanning electrochemical microscopy (SECM) tips cannot always guarantee a perfect disk geometry. In the present work, the impact of these defaults is investigated both theoretically and experimentally. The situations where these defaults can accurately be taken into account by considering that the probe behaves like a microdisk with effective geometric parameters are determined. In these situations, the quantitative analysis of the experimental results is greatly simplified. The study also proposes expressions to evaluate the apparent microdisk parameters from a picture of the probe.

Standard oxidation potentials of methylbenzenes in acetonitrile

Abstract Combined use of ultramicroelectrodes and of an ultrafast potentiostat equipped with on-line ohmic drop compensation has allowed the determination of st ( E Bz-CH 3 0 = 1.58 (hexamethylbenzene), 1.69 (pentamethylbenzene), 1.75 (durene) and 1.77 V (1,2,3,4-tetramethylbenzene) vs. a saturated rate constants for the deprotonation of the corresponding cation radicals by a series of pyridine bases. These latter values compare satisfactorily wit

A study of the scattering of valve-regulated lead acid battery characteristics

Abstract Scattering of the electrical characteristics or performances of valve-regulated lead acid (VRLA) batteries was measured under controlled conditions, using batteries from three different manufacturers. Data on open-circuit voltage, capacity, and float conditions (currents and voltages) were collected for new, fully charged batteries. The physical reasons for the observed scattering were then closely examined in terms of the parameters relating to the manufacturing process, bearing in mind the following question: Which step(s) in the manufacturing process are primarily responsible for the scattering of a particular electrical parameter?

Batteries in standby applications: comparison of alternate mode versus floating

Experimental procedures are exposed that allow quantitative conclusions about the use of alternate mode (alternation of constant current charge and open-circuit periods) instead of floating mode (constant voltage condition) in standby applications of VRLA batteries. The level of capacity and the rate of a recharge following a discharge period are particularly studied. The influence of the three parameters of an alternate mode, that is to say the charging current, the maximum and minimum voltages, is especially discussed, with regard to the previously described responses.