Determination of Effective Surface Area and Reaction Rate Constant By Cyclic Voltammetry Considering Ohmic Resistance and CPE Effects

Abstract

One of the most commonly used electroanalytical techniques for determining the effective surface area and reaction rate constant in a redox system is linear sweep voltammetry due to its sensitive response to a reaction and easy implementation. In linear sweep voltammetry, a linear potential scan is applied to an immobile electrode submerged in an unstirred analyte. The responses to the applied potential is the total current which consists of the faradaic and non-faradaic currents. The faradaic current is the current generated by the reaction of interest, while the non-faradaic current is generated by an electrical double layer. As the faradaic processes received high attention in modeling [1-3], the non-faradaic current, which can be described by the constant phase element (CPE), received less attention. The conventional approach to evaluate the effective surface area and reaction rate constant is to assume that the non-faradaic current is linear and the ohmic drop is negligible. For a quasireversible reaction, by extrapolating the baseline, the peak current and potential are obtained and applied to the relation of Matsuda and Ayabe [4] and Nicholson’s method [5] to determine for the effective surface area and the reaction rate constant, respectively. However, this conventional approach could lead to an inaccurately estimated peak current and potential since the non-faradaic current is not always linear and the ohmic drop, which results in the peak potential shifting, occasionally cannot be negligible. In addition, the faradaic and non-faradaic currents are coupled through ohmic drop effects. Our research group has successfully developed numerical modeling of voltammetry, including the combined effects of ohmic resistance, CPE, mass transfer, and faradaic processes [6]. In this model, the CPE parameters, i.e. the pseudo-capacitance and the dispersion coefficient, are required as inputs. Generally, the CPE parameters were evaluated by electrochemical impedance spectroscopy (EIS). However, the EIS analysis is time-consuming and requires prior knowledge about the equivalent circuit of a system to obtain the CPE parameters. Furthermore, it measures only impedance of a system at one specific potential in frequency domain, while a voltammetry method is a potentiodynamic measurement, which operates under a wide potential window. Therefore, the semi-theoretical approach is introduced to evaluate the CPE parameters under voltammetry condition. This approach possesses several advantages for evaluating the CPE parameters, such as time-saving, simple, and straightforward. In this study, by considering the effects of ohmic resistance and CPE, an approach for determining the electrode effective surface area and reaction rate constant using linear sweep voltammetry was developed. By fitting the peak current and potential of the numerical simulation to those of the experimental data (see Fig. 1a), the electrode effective surface area and reaction rate constant can be determined. As the non-faradaic current becomes relatively more important at faster scan rates, it is difficult to extrapolate the baseline, as depicted in Fig. 1b. This difficulty could lead to underestimation of the effective surface area since the relation of Matsuda and Ayabe, needs the faradaic peak current as an input. Furthermore, Nicholson’s method, which evaluates the reaction rate constant directly from the peak separation, would underestimate the reaction rate constant if the ohmic drop effect was appreciable. Therefore, owing to the problem that the non-faradaic current and ohmic drop effect occasionally could not be avoided, using our approach that takes them into consideration would lead to more accurate prediction of such parameters. Acknowledgment The authors would also like to express their gratitude to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing financial support under the scholarship program for foreign students. References [1] Mahon, P.J. et al., J. Electroanal. Chem. 537 (2002) 1. [2] Aoki, K. et al., J. Electroanal. Chem. 182 (1985) 267. [3] Aoki, K. et al., J. Electroanal. Chem. 247 (1988) 17. [4] Bard, A.J. and Faulkner, L.R., John Wiley & Sons, Inc., 2001. [5] Nicholson, R.S., Anal. Chem., 31, 1351 (1965) [6] Charoen-amornkitt, P. et al., Electrochim. Acta, (In Press). Figure 1