Discrepancies in the Measurement of Ionic Conductivity of PEMs Using Two- and Four-Probe AC Impedance Spectroscopy

Two- and four-probe cells were designed for comparative studies of ionic conductivity of proton conducting membranes (PEMs) using electrochemical impedance spectroscopy. Nafion 115 membrane was employed to examine the influence of cell configuration, probe geometry, and arrangement of probe. Whereas a single arc and linear responses, were observed in Nyquist plots using the 2-probe cell, multiple arcs were observed using the 4-probe cell. The linear response observed in the 2-probe configuration and the low frequency arc observed in the 4-probe configuration are due to transmission line behavior that results from a distributed interfacial capacitance coupled with the membrane's ionic resistance at the Pt/PEM interfacial region. Increasing the distance between the probes and/or reducing the electrode contact area reduces or eliminates these low-frequency artifacts so that accurate data for ionic resistance can be obtained. Equivalent circuits for 2-probe and 4-probe cell geometries are constructed and used to extract conductivity data.