Modelling of a spectroelectrochemical cell with radial liquid flow for in situ external reflection IR measurements

Abstract

A mathematical model is presented, which describes the current and potential distribution across the working electrode of an electrochemical thin-layer cell with controlled radial flow of electrolyte from the periphery to the centre of the electrode assuming mass transfer controlled limiting current conditions. The numerical solution of the convective diffusion equation, which characterizes the mass transfer of the electroactive species in the cylindrical coordinate system of the thin-layer cell, is obtained using the backward implicit method. The results of the calculations show that a thin-layer thickness of the order of 5 μm results in such a slow convection that the reaction takes place mainly at the outer edge of the electrode. Increasing the layer thickness to values above 7 μm leads to a situation in which the whole electrode area is utilized. The performance of the cell was evaluated using the oxidation of Fe(CN)64- to Fe(CN)63- as a model reaction. The discrepancy between the measured and calculated average limiting currents is shown to be due mainly to the deviation of the electrode surface from an ideally flat surface and/or the non-parallelism of this surface and the optical window. The fact that the intensity of the IR absorption band of Fe(CN)64-, recorded using the staircase method, increases steadily until the limiting current is reached, shows that with proper care these non-idealities do not distort spectral measurements. The difference between the potential at which the limiting current is attained and the potential of the beginning of the reaction was found to be of the same magnitude as the value obtained by calculation. © 1994.