Nonlinear Impedance Spectroscopy and Its Application to Solid Oxide Fuel Cells

Abstract

Electrochemical impedance spectroscopy (EIS) is recognized as a powerful tool for characterizing the charge transfer reaction on heterogeneous interfaces, thus has been widely applied in various fields of research and in many applications, such as corrosion, development of various types of batteries and fuel cells. Due to the limitation of linearization inherent to the EIS technique the nonlinear information of an investigated system is automatically abandoned. This work focuses on extending the traditional linear EIS technique into the nonlinear domain, nonlinear EIS (NLEIS), and has demonstrated that the nonlinear information can be utilized for gaining a more comprehensive understanding of a system.

In this work, the NLEIS technique is discussed from a basic theoretical point of view, including the fundamental definition of nonlinear “impedance” and experimental issues on which a consensus has yet to be reached within the community. A LabView programmed experimental setup was developed during the project to perform NLEIS measurements and preliminary data analysis, which appeared superior to commercial systems (higher frequency range) and systems developed at other institutes (lower noise level). A qualitative approach for data analysis in NLEIS is discussed and prompted in this work, which is more intuitive and suitable for fast first stage analysis compared to an in-depth quantitative modelling approach.

To test the NLEIS methodology developed, a classic redox couple- ferri-ferrocyanide- was examined. Information on nonlinearity and symmetry of the system was directly investigated by measuring harmonic responses up to 5th order and it was demonstrated that in contrast to EIS a full set of kinetic parameters could be directly calculated from a single measurement. In addition, features generic to the methodology were discovered, which are independent of systems under investigation.

Finally, the NLEIS technique was used in investigating the oxygen reduction reaction on strontium and iron doped lanthanum cobaltite (LSCF) cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The decrease of the level of nonlinearity and the increase of asymmetry indicates a potential mechanism change, with increased operating temperature.