Influence of convection through gas-diffusion layers on limiting current in PEM FCs using a serpentine flow field

Three gas-diffusion layers (GDL) with distinctively different gas permeability were used to study the influence of convection through the GDL on the cathode limiting current in proton exchange membrane (PEM) fuel cells. Several flow rates between 50 and 1000 cm(3)/min were used in constant flow rate operation with oxidant gases being air, 4% oxygen/nitrogen, and 21% oxygen/helium. The study was conducted at three cell temperature/relative humidity conditions: 80degreesC/75% inlet cathode R.H., 100degreesC/70% inlet cathode R.H., and 120degreesC/35% inlet cathode R.H., with the objective to evaluate the influence of cell temperature, oxygen mole fraction, relative humidity, and cathode flow rate on the limiting current due to reactant gas transport under conditions where there is no significant flooding. A conventional single-serpentine graphite flow field was used. Cell relative humidity significantly affected the limiting current by reducing oxygen transport through the ionomer thin film of the cathode catalyst layer as the relative humidity decreased. At all three conditions, increasing the cathode dry flow rate increased the limiting current mainly due to more convection. A GDL with higher gas permeability in the microporous layer had a higher limiting current due to more enhanced convection. This accentuates the significance of high gas permeability as a criterion for optimization of GDL. Convection contributes to the limiting current of hydrogen/air PEM fuel cells even when using a conventional flow field pattern (i.e., not interdigitated). (C) 2004 The Electrochemical Society.