Measurement of the forward and back rate constants for electron transfer at the interface between two immiscible electrolyte solutions using scanning electrochemical microscopy (SECM): Theory and experiment

A new numerical model is developed for the scanning electrochemical microscopy (SECM) feedback mode for reversible electron transfer (ET) processes at the interface between two immiscible electrolyte solutions (ITIES). Results from this model were compared with data obtained using an earlier SECM feedback model in which the back reaction was not considered, to identify when the latter will be important. The dependence of the ET rate constant for the oxidation of 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ(.-)) in 1,2-dichloroethane (DCE) by aqueous ferricyanide on the interfacial potential drop (Delta (o)(w)phi) was studied using SECM. The Delta (o)(w)phi value was varied by changing the concentration of NaClO4 in the aqueous phase while a fixed concentration of organic electrolyte, tetra-n-hexylammonium perchlorate, was used in the DCE phase. The results obtained were compared to earlier published studies on the forward reaction between TCNQ in DCE and aqueous ferrocyanide. Both the forward and back ET rate constants were found to depend strongly on the interfacial potential drop, with measured ET coefficients in the region of 0.5-0.6. A similar ET rate constant was observed at zero driving force for both the forward and back reactions. These experimental results suggest that the Butler-Volmer model applies to ET at the ITIES, when the driving force for the reaction is low, and under conditions of relatively high ionic strength in both the aqueous and organic phases. (C) 2001 Elsevier Science B.V. All rights reserved.