Electrochemical impedance spectroscopy

The fundamentals of electrochemical impedance spectroscopy and its application to the identification of dynamical processes in fuel cell operations are described in this chapter. The scope of the method is seen in the fact, that by applying a small signal perturbation to the cell under test conditions, the relaxation times and relaxation amplitudes of the processes governing cell performance can be identified and targeted for cell optimization. Based on the working principle and the basic measurement set-up of impedance spectroscopy, measurement arrangements for fuel cells are discussed and specific problems of the method are addressed. Various impedance data analysis techniques are described, including the well-known complex non-linear least squares curve fit to equivalent electrical circuit modes as well as the direct deconvolution of impedance data into a distribution of relaxation times. The importance of suitable physical models for the identification of microscopic cell parameters is stressed and an example for the identification procedure is given. Keywords: electrochemical impedance spectroscopy; impedance spectrum; frequency response analyzer; galvanostat; potentiostat; electrode arrangement; impedance measurement arrangements for fuel cells; impedance data analysis; complex non-linear least squares curve fit; equivalent electrical circuit model; distribution of relaxation times; impedance expression for physical processes; impedance identification of physical processes