Interpretation of Low-Frequency Inductive Loops in PEM Fuel Cell Impedance Data

Mark E. Orazem, Department of Chemical Engineering, University of Florida Gainesville, FL 32611-6005, USA.
Electrochemical Impedance Spectroscopy (EIS) is a powerful technique which, in principle, allows identification and characterization of the different processes governing electrochemical systems. Impedance spectroscopy has been applied to PEM fuel cells, but the electrical circuit analogues commonly used to interpret data do not yield unambiguous interpretations of the process. The object of this work was to explore the use of impedance measurements for fuel cells. Impedance measurements were performed for a single fuel cell with a 5 cm2 effective cell area under a variety of operating conditions and with different instrumentation. The data revealed low-frequency inductive features similar to those reported by Makharia et al. They suggested that these features could be attributed to side reactions in the fuel cell, but such low-frequency inductive loops can also be due to or affected by the non-stationary behavior. The measurement model analysis developed by Agarwal et al was applied to the data to identify the error structure of the data, including parts of the spectra that were inconsistent with the Kramers-Kronig relations. The inductive loops were found to be characteristic of the process once the fuel cell operates under stationary conditions. Impedance models were developed to account for reaction mechanisms that may be responsible for the inductive response seen at low frequencies. Models that incorporate only the hydrogen oxidation and oxygen reduction reactions cannot account for inductive features. Inductive loops can be predicted by models that account for formation of hydrogen peroxide as an intermediate in a two-step oxygen reduction reaction. Hydrogen peroxide is considered a degrading agent for materials used in the fuel cell components (membrane, electrodes) and its formation in the fuel cell operating conditions is reported by Mittal et al. Inductive loops can also be predicted by models that account for Pt dissolution and associated deactivation of catalytic activity. Experiments are under way to detect byproducts associated with these reactions. Interpretation of impedance spectra in terms of side reactions may prove useful for predicting the life-time of fuel cell performance. Mark Orazem obtained his BS and MS degrees from Kansas State University and his doctorate in 1983 from the University of California, Berkeley. In 1988 he joined the faculty of the University of Florida where, since 1992, he holds the position of Professor of Chemical Engineering. He is the director and organizer of the UF/BP Engineering Development Program for the Caspian Sea Region. Orazem is the recipient of two teaching awards and two research professorships from the University of Florida. He was recognized as an outstanding educator in 2004 by BP Azerbaijan. He was recognized as the 2005 College of Engineering Distinguished International Educator, and he received the 2006 Excellence in Teaching Award from the local student chapter of the AIChE. Orazem has over 100 refereed publications, and he has delivered 5 plenary lectures at international meetings. He is coauthoring a textbook on impedance spectroscopy.

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