Impedance Model of Oxygen Reduction in Water-Flooded Pores of Ionomer-Free PEFC Catalyst Layers

We present an impedance model of oxygen reduction in water-flooded pores of ionomer-free, ultrathin catalyst layers in polymer electrolyte fuel cells. This work expands on the continuum, single pore approach of a previously developed steady state model (JES 158, B18, 2011), which postulated that protons are transported into water-flooded pores through their electrostatic interaction with the metal surface charge density. We derive approximate analytical expressions for the impedance response in various limiting cases. In contrast to the response of electrolyte-filled pores, the resistive and capacitive elements of the water-flooded pores are highly dependent on the surface charge density of the metal, which is determined by the applied bias potential and the potential of zero charge of the metal. We discuss the capabilities of the model to determine the kinetic, electrostatic, and transport contributions to the overall Faradaic current density, and to extract metal vertical bar solution interfacial parameters of electrocatalytic materials in ionomer-free catalyst layers from impedance data. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.045202jes] All rights reserved.