Artefacts in Electrochemical Impedance Measurements Due to Stray Capacitances

Electrochemical impedance spectroscopy (EIS) is a powerful and widely used technique for the investigation of electrochemical systems. Measurements in two-electrode setup are a fast and easy way to study a variety of processes in electrochemistry. However, in many cases it is difficult to distinguish between the impedance contributions of both electrodes. A solution to this problem is the utilization of a three-electrode setup. Under ideal conditions, this setup allows for measuring exclusively the impedance of the working electrode. However unfortunately, there are many sources that can lead to severe artefacts in three-electrode impedance spectra. One possible origin for artefacts is the so called voltage divider effect. If the impedance of the reference electrode is not negligible compared to the analyzer input impedance, this can lead to artefacts in the impedance spectra caused by a voltage drop over the reference electrode. Since the impedance of the reference electrode is a complex quantity, not only the measured magnitude of the impedance but also the phase angle can be wrong.[1] Another problem arises from the requirement that the reference electrode should probe an equipotential line in the cell. Since the position of equipotential lines is frequency dependent, artefacts can be caused by a non-ideal positioning of the reference electrode.[2] Even in the case of ideal reference electrode positioning and a very high analyzer input impedance, stray capacitances between the three electrodes can lead to impedance artefacts. Stray capacitances can be minimized, but they cannot be eliminated completely. By means of a three-terminal equivalent network, Fletcher showed that stray capacitances can cause artefacts over the entire frequency range. However, Fletcher considered the electrodes to be purely resistive.[3] Sadkowski et al. extended this model and considered the working electrode to have a complex impedance consisting of a parallel RC-circuit in series with a resistor, representing the charge transfer resistance, the double-layer capacitance and the electrolyte resistance, respectively. However, the counter electrode and the reference electrode are still considered to be purely resistive.[4] In this work we present results for an extended model in which all three electrodes exhibit a complex impedance consisting of a parallel RC element in series with a resistor (see figure 1). Using this model, it is e.g. possible to calculate the three-electrode impedance spectra of an electrochemical cell with blocking and non-blocking electrodes. By comparing model spectra with experimental spectra, the stray capacitances in electrochemical cells can be estimated. Figure 1: Three-terminal equivalent network used for the simulations. [1] G. Hsieh, S. J. Ford, T. O. Mason, L. R. Pederson, Solid State Ionics, 91 , 191 (1996). [2] S. Klink, E. Madej, E. Ventosa, A. Lindner, W. Schuhmann, F. La Mantia, Elechtrochemistry Communications, 22 , 120 (2012). [3] S. Fletcher, Elechtrochemistry Communications, 3 , 692 (2001). [4] A. Sadkowski, J.-P. Diard, Electrochimica Acta, 55 , 1907 (2010). Figure 1