The effects of relative humidity on the performances of PEMFC MEAs with various Nafion^® ionomer contents

For PEMFC operation, water management is very important to provide both sufficient proton conductivity and mass transport. Therefore, in this study, the effect of the relative humidity (38-87%) on cell performance is examined for PEMFC MEAs with various Nafion^® ionomer contents. The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method. As the relative humidity of the cathodes (RHC) increases, the cell voltages at 0.4 and 1.2 A/cm² increased for a MEA with 20 wt% ionomer. This can be explained in terms of the expansion of active sites with enhanced ionic conductivity (activation overpotential). In contrast, with a higher RHC value, the cell voltages for 35 wt% ionomer (more hydrophilic) gradually decreased as a result of slower gas transport (concentration overpotential). For MEAs with intermediate ionomer contents, 25 and 30 wt%, the cell voltages at 0.4 A/cm ² showed maximum values at a RHC of 67%, at which point the mass transport begins to be the more dominant factor. The highest unit cell performance was observed in a MEA with an ionomer content of 25 wt% at a RHC of 59% and in a MEA at 20 wt% ionomer content at a higher humidity of 87%.