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

A new numerical model is developed for the scanning electrochemical microsc opy (SECM) feedback mode for reversible electron transfer (ET) processes at the interface between two immiscible electrolyte solutions (ITIES). Result s from this model were compared with data obtained using an earlier SECM fe edback model in which the back reaction was not considered, to identify whe n 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 pote ntial drop (Delta (o)(w)phi) was studied using SECM. The Delta (o)(w)phi va lue 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 an d aqueous ferrocyanide. Both the forward and back ET rate constants were fo und 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 obser ved at zero driving force for both the forward and back reactions. These ex perimental results suggest that the Butler-Volmer model applies to ET at th e ITIES, when the driving force for the reaction is low, and under conditio ns of relatively high ionic strength in both the aqueous and organic phases . (C) 2001 Elsevier Science B.V. All rights reserved.