CPE Analysis from Cyclic Voltammetry and Electrochemical Impedance Spectroscopy

Abstract

The electric properties of an electrode can be evaluated by the determination of the capacitance of the interface. However, the literature reports a large panel of experimental protocols for capacitance determination, which involve either cyclic voltammetry (CV) or electrochemical impedance spectroscopy (EIS). On the one hand, CV is a routine electrochemical technique widely used for the determination of electric properties of super capacitors or energy storage devices. The capacitive charge is determined through the variation of the scan rate in a non-faradaic current domain and is often used to estimate the performances of these electrodes. On the other hand, EIS is used for the determination of interfacial processes and also provides precise information on the double layer capacitance and the oxide capacitance in the case of more complex systems. However, substantial discrepancies in the assumptions are made between both techniques as in CV, the system is usually considered as ideal (and the current-scan rate relationship is assumed to remain linear at all frequencies), whereas in EIS, the system is usually considered as non-ideal and a CPE element is introduced. Although both techniques are used to provide similar information on the capacitive characteristics of electrochemical interfaces, a clear correlation between the results obtained with each technique is not straightforward. In fact, recent work from Allagui et al. [1]revealed that the time-voltage relationship cannot be used when the system is considered as ideal.In this work, electrochemical impedance spectroscopy and cyclic voltammetry were performed on well-known systems showing capacitive behaviours on a wide frequency domain and over a wide potential range: a blocking electrode (Au) and an electrode with a thin oxide film (Al). The capacitive charge was determined using cyclic voltammetry from the analysis of the conventional current to scan rate linear relationship and EIS measurement. In CV measurements, the calculated capacitance predominantly stems from the double layer capacitance for the considered scan rate ranges (low frequency range). Additionally, it was shown that the value of the determined capacitance is scan-rate dependent. Such experimental results highlight the arbitrary character of the scan rate ranges used in the literature as they usually remain in low frequency range. In fact, careful selection of scan rate is critical and should be adapted to the electrode considered, particularly for systems involving a thin oxide film as the Cdl or the Cox response depends on the frequency range. Similarly, the EIS response shows that the capacitance determination cannot be performed without a large error when a non-ideal behaviour is experimentally obtained. From a physical point of view, such outcome in this frequency domain is the results of observations made in the selected time domain [2].