Quantitative theory, scanning electrochemical

Scanning electrochemical microscopy (SECM the same abbreviation is also used for the device, i.e., the microscope) is often compared (and sometimes confused) with scanning tunneling microscopy (STM), which was pioneered by Binning and Rohrer in the early 1980s [1]. While both techniques make use of a mobile conductive microprobe, their principles and capabilities are totally different. The most widely used SECM probes are micrometer-sized ampero-metric ultramicroelectrodes (UMEs), which were introduced by Wightman and co-workers 1980 [2]. They are suitable for quantitative electrochemical experiments, and the well-developed theory is available for data analysis. Several groups employed small and mobile electrochemical probes to make measurements within the diffusion layer [3], to examine and modify electrode surfaces [4, 5], However, the SECM technique, as we know it, only became possible after the introduction of the feedback concept [6, 7],... 

Snowden, M.E., Giiell, A.G., Lai, S.C.S. et al. (2012) Scanning electrochemical cell microscopy theory and experiment for quantitative high resolution spatially-resolved voltammetry and simultaneous ion-conductance measurements. Analytical Chemistry, 84, 2483-2491. 

The quantitative scanning electrochemical microscopic (SECM) theory has been developed for various regimes of measurements and electrochemical mechanisms. Different operating modes of the SECM, for example, feedback and generation/collection (G/C) modes, steady-state and transient... 

McGeouch, C. A., Edwards, M. A., Mbogoro, M., Parkinson, C., Unwin, P. R. Scanning electrochemical microscopy as a quantitative probe of acid-induced dissolution Theory and application to dental enamel. Anal. Chem. 2010, 82, 9233. 

The kinetics of the fac lmer isomerization step can be determined quantitatively from the scan-rate dependence of the oxidation process. Both theory and experiment show that the peak potential corresponding to the oxidation of the /ac° species ( p ) shifts to less positive potentials as the scan rate is increased. This occurs because the oxidation charge-transfer process is electrochemically reversible. Under these circumstances, the isomerization step following the charge transfer removes the product and causes the equilibrium position to move to the right in (41), which effectively facilitates the oxidation step. Consequently, at low scan rates, when the isomerization step is important, the oxidation process requires a lower thermodynamic driving force in order to occur and hence a less positive potential is observed. If the electron-transfer (E) step had been irreversible, the isomerization reaction would have no effect on the voltammetric response since the C step would not be rate determining and no kinetic data could be obtained. 

Electrochemical simulations of the concentration and scan-rate dependence of the voltammetry potentially provide the composition of the intermediates formed during the reaction cycle together with estimates of the rate and equilibrium constants. As shown in the preceding section spectroscopic information can greatly assist the elucidation of the molecular details of these reactions, however, reliable deduction of the structure is greatly enhanced by the incorporation of structural and computational information (Section 1.6). The rapid advance in computer power and implementation of density-functional theory allows a more quantitative approach for evaluation of proposed structures based on spectroscopic information and estimation of the relative energies of the proposed spe-cies. The recent computational study of the electrocatalytic reaction cycle proposed for illustrates the opportunities presented by the approach. 

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