In situ analysis of oxygen partial pressure at the cathode catalyst layer/membrane interface during PEFC operation

To understand the concentration overpotential in the polymer electrolyte fuel cell (PEFC), we have performed an in situ analysis of the oxygen partial pressure (p[O-2](CL/PEM)) at the interface between the cathode catalyst layer (CL) and the polymer electrolyte membrane (PEM). Diffusion-limited oxygen reduction current was measured, with Pt probes inserted into the PEM, during cell operation by supplying H-2 to the anode and O-2 + N-2 to the cathode at 80 degrees C. It was found that the p[O-2](CL/PEM) decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm(-2) at P[O-2](gas) = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization U-O2 (from 10% to 50%). Such a change in P[O-2](CL/PEM) might result in a concentration overpoteritial of ca. 10 mV, based on the Tafel slope of 120 mV decade(-1) in the high current density region. It was also found that ohmic losses in the ionomer phase of the CL increased with decreasing humidity, from 100% to 80% RH, and became a dominant factor in the increased total overpotential, while the corresponding concentration overpotential was unchanged. The present results provide new insight into the transport of oxygen and water at the CL/PEM interface, especially at the high current densities required for the electric vehicle application. (C) 2008 Elsevier Ltd. All rights reserved.