Polarization-dependent mass transport parameters for orr in perfluorosulfonic acid ionomer membranes: an EIS study using microelectrodes

Electrochemical impedance spectroscopy (EIS), carried out with Pt microelectrodes in a solid-state electrochemical cell at 100% relative humidity, is used to study oxygen reduction reaction mass transport parameters in perfluorosulfonic acid membranes (Nafion® 1035, 117, and 1200). Complex plane plots obtained under different polarizations, temperatures, and O2 pressures are analyzed using a Randles–Ershler equivalent circuit model and complex nonlinear least-squares fitting. Under open circuit potential, oxygen mass transport contributes significantly to the impedance because of incomplete hydration of the membrane. The impedance due to O2 mass transport decreases under low to moderate polarization but increases in the moderate to high polarization regime, both due to changes in the hydration state of the ionomer membrane at the electrode interface. Mass transport parameters are compared to those extracted by chronoamperometry. The conclusion of this work is that O2 mass transport impedance changes with polarization as a result of different rates of water production and the resulting different degrees of hydration within the membrane. Mass transport parameters for three Nafion® membranes having equivalent molar masses of 1035, 1100 and 1200 g mol−1 (Nafion® 1035, 117, and 1200) are compared and the results explained in terms of their differing water content. c parameters in the low current density regime were also examined. EIS-determined parameters are larger in magnitude than those obtained by linear sweep voltammetry but the trends are the same: the exchange current density increases with temperature, oxygen pressure, and ion exchange capacity of the membrane.