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[O2]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 H2 to the anode and O2 + N2 to the cathode at 80 °C. It was found that the p[O2]CL/PEM decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm−2 at p[O2]gas = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization UO2UO2 (from 10% to 50%). Such a change in p[O2]CL/PEM might result in a concentration overpotential 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.