Modeling and evaluation of three-terminal impedance measurement configurations for solid oxide fuel cell electrodes

Abstract

Solid oxide fuel cells (SOFCs) are a promising solution to various technological challenges, as they convert chemical energy directly into electrical energy. The electrochemical properties of the cell's electrodes and electrolyte, can be investigated by means of impedance spectroscopy (IS). To measure the separate impedance response of only one electrode of the electrochemical cell, a three-terminal configuration is required. However, implementing this method in solid state electrochemistry goes along with non-trivialities and error sources that do not exist to the same extent in liquid electrochemistry. This work considers potential error sources and evaluates each of them quantitatively with special emphasis on their impact on thin film electrode measurements by means of finite elements analysis (FEA), electric circuit simulations and conducted measurements. Three potential error sources were identified as crucial factors. Asymmetric electrode geometries go along with a shift of the reference equipotential line with frequency. This can lead to measurement errors and additional features like inductive loops in the impedance spectrum. Second, particular sample geometries go along with an electric potential spread across the reference electrode (RE). This causes a current flow through the reference electrode and a (frequency dependent) bypass of a part of the sample. Third, especially for high ohmic systems, coupling capacitances between the three electrodes are crucial, as they influence the impedance transfer function which can provoke additional features in the spectrum. With the results of this work, it was possible to introduce an optimized sample and electrode geometry. This setup minimizes the error extent significantly in a wide range of electrode properties.