The role of the potential distribution at the electrode interface in determining the electron transfer rate constant

Simultaneous electrochemical impedance (EI) and electroreflectance (ER) measurements were performed on 'bare' gold, N-acetyl-L-cysteine(NAC) modified gold, and cytochrome c (cyt c) adsorbed on a NAC modified gold electrode. The electrical impedance of the electrode interface was modeled by a series connection of a constant phase element (CPE), a capacitor, and a resistor. The analysis of the combined EI and ER data yielded the heterogeneous electron transfer (ET) rate constant at each frequency of the modulating potential. This is in contrast to previous techniques which used the frequency dependence of the EI or ER response to obtain a value of the rate constant. Assuming that the Faradaic potential was equal to the potential at the nodes of the three element impedance model yielded frequency dependent rate constants. A small modification of the three element impedance model was needed to obtain a frequency independent rate constant of 850 +/- 80 s(-1). This suggests that the distribution of the potential at the electrode interface (reflected in the choice of the electrical impedance model) is an important factor in the determination of the ET rate constant. (C) 1999 Published by Elsevier Science S.A. All rights reserved.