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 in the literature. It has been 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 our group 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). Inductive loops can also be predicted by models that account for Pt dissolution and associated deactivation of catalytic activity. Preliminary experiments have detected 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.