Electrochemical impedance spectroscopy (EIS) is commonly used for material characterization in the bulk. Using SECCM with micro/nanopipettes, these experiments can be also executed on the micron/nanoscale. Also sensors based on modified microelectrodes can be studied.

The EIS data are modelled using equivalent circuits to extract parameters such as solution resistance, electron transfer resistance and double layer capacitance. From these parameters, valuable information about electrode solution interfaces can be gained. Important fields of application are the study of electrode performance in corrosion or energy storage research.

The Randles Circuit is used as an easy equivalent circuit to model electrochemical reactions at the electrode solution interface (Fig. 1). Rs models the solution resistance. A parallel circuit consisting of a capacitance Cdl to model the electrochemical double layer and a resistance Rct for the electron transfer as well as a Warburg element ZW which describes the diffusion are used to model the reaction. For more complex reactions, more advanced equivalent circuits have to be used. Often the double layer capacitance Cdl is replaced by the Constant Phase Element CPE which better reflects the imperfect behavior of the double layer.

Fig.1: Randles Circuit which is used as equivalent circuit to model simple electrochemical systems.

At a microelectrode, EIS can be used to study reaction as well as interface properties. Fig. 2 shows the spectra of a 10 µm Pt microelectrode in 0.1 M KCl at OCP and 0.1 M KCl + 10 mM FcMeOH at OCP and at the have-wave potential. It can be noted that for microelectrodes large impedances are measured which require especially sensitive equipment.

Fig. 2: EIS spectra at a 10 µm Pt microelectrode in a) 10 mM FcMeOH in 0.1 M KCl at OCP (black curve) and at the half-wave potential (blue curve) and b) in 0.1 M KCl without redox medaitor.