Inductive Effect on the Fuel Cell Cathode Impedance Spectrum at High Frequencies

Abstract

The high frequency electrochemical impedance measurements with positive imaginary components in the impedance complex plot of a polymer electrolyte fuel cell (PEFC) are attributable to the inductance of the electrical cables of the measurement system. This study demonstrates that the inductive effect of the electrical cables deforms the high frequency region of the cathode impedance spectrum and as such leads to an erroneous interpretation of the electrochemical mechanisms in the cathode catalyst layer (CCL). This study is divided into a theoretical analysis and an experimental analysis. In the theoretical analysis a validated model that accounts for the impedance spectrum of the CCL as reported in the authors’ previous study is applied with experimental impedance data reported in the literature. The results show that the ionic resistance of the CCL electrolyte which skews the oxygen reduction reaction (ORR) current distribution toward the membrane interface is masked in the cathode impedance spectrum by the inductive component. In the experimental analysis cathode experimental impedance spectra were obtained through a three-electrode configuration in the measurement system and with two different electrical cables connected between the electronic load and the PEFC. The results agree with the theoretical analysis and also show that the property of causality in the Kramers-Kronig mathematical relations for electrochemical impedance spectroscopy (EIS) measurements is violated by the external inductance of the measurement cables. Therefore the experimental data presenting inductance at high frequencies do not represent the physics and chemistry of the PEFC. The study demonstrates that a realistic understanding of factors governing EIS measurements can only be gained by applying fundamental modeling which accounts for underlying electrochemical phenomena and experimental observations in a complementary manner.