Anal. Chem. 1996, 68, 1711-1716
A Fast-Response, UV-Visible Optically Transparent Thin-Layer Cell for Potential Scan Spectroelectrochemistry Scott C. Paulson† and C. Michael Elliott*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
We report the design of a thin-layer spectroelectrochemical cell that has been optimized for obtaining spectral data under non-steady-state electrochemical conditions. A variety of applications involving the simultaneous use of cyclic voltammetry and UV-visible spectroscopy is possible. For example, scan rates as high as 25 mV/s have been used while still maintaining peak separations of 165 mV or less for millimolar electroactive species in acetonitrile. More importantly, under the same conditions, the spectral and coulometric responses correlate temporally. Because of flexibility in the design, the cell can be tailored for specific solution applications. Its use can be further extended to electropolymerization and the study of electroactive polymers. Most electrochemical cells employing optically transparent thin-layer electrodes (OTTLEs) are designed to optimize coulometric and/or spectral responses. Optimization for coulometry (usually employing potential steps) generally requires severe restriction of solution species exchange between the thin layer and bulk solution in order to minimize so-called “edge effects”.1 Consequently, standard OTTLE designs have two distinct but related problems in applications where non-steady-state measurements are attempted. The thin-layer design, by necessity, introduces considerable uncompensated resistance into the cell, which is particularly exacerbated in low-dielectric-constant organic solvents. Also, the auxiliary electrode is usually positioned near an opening at one edge of an otherwise enclosed thin-layer compartment. These factors lead to a very poor distribution of current between the working and auxiliary electrodes. When the large uncompensated resistance is convoluted with the poor current distribution, a very nonuniform, temporally varying concentration of redox species develops across the OTTLE surface (i.e., perpendicular to the light path). The practical result of this situation is that usually there is little or no meaningful temporal correlation between the current and spectral responses. It is only when the cell approaches equilibrium (i.e., i f 0) that a uniform distribution of redox species is established across the spectral window. As a consequence, only very slow potential scans (
