Estimation of Mass-Transport Overpotentials During Long-Term PEMFC Operation

Abstract

A comprehensive method for separating cathode catalyst-layer and GDL mass-transport overpotentials was derived utilizing kinetic and Ohmic analysis of high-humidified H2/O2 and low-humidified H2/air polarization data. A hybridized method was applied accounting for electrochemical surface area (ECSA), hydrogen crossover, and exchange current density. This method delineates separate cathode GDL and electrode mass-transport overpotentials as a function of current density and operating time. The methodology is systematic and generalized and can be applied to polarization data from any type of durability test. The derivation was applied to periodic polarization data from a steady-state 1050-hour durability test and is shown to provide an accurate breakdown of the sources of performance losses. Increases in mass-transport overpotential for the cathode GDL and oxygen reduction reaction (ORR) overpotential were predominantly offset by improvements in the mass-transport overpotential of the cathode catalyst layer and reductions in the high-frequency resistance (HFR). Little increase in GDL mass-transport overpotential was observed during the first ~500 hour period, but a substantial increase was seen during the second ~500 hour period. The mass-transport overpotential of the cathode catalyst-layer was almost negligible at the end of ~1000 hours of operation, suggesting little O2 diffusion resistance through the ionomer and adjacent void volume.