PEM fuel cell start-up/shut-down losses vs relative humidity: The impact of water in the electrode layer on carbon corrosion

For automotive applications, one of the main challenges for proton exchange membrane fuel cells (PEMFCs) is to increase the lifetime of membrane electrode assemblies (MEAs), especially during transient operating conditions such as start-up/shut-down (SUSD) cycles. During SUSD, the carbon support in the cathode layer is known to be oxidized as a consequence of hydrogen/airanode gas fronts moving through the anode. In this work, we focus on the effect of relative humidity (RH) during SUSD events. Here we show the significant impact on PEMFC performance by both experiments with 50 cm² single-cell PEMFCs and by a simple SUSD model using the RH-dependent kinetics for the carbon oxidation reaction (COR) rate. The kinetic parameters of the COR are determined by on-line mass spectrometry, yielding a COR reaction order with respect to RH of one. Utilizing the thus determined COR kinetics in the SUSD model predicts a ≈ 3-fold lower COR during SUSD events at 80◦C for an MEA with a conventional high surface area carbon support when the RH is decreased from 100% to 25%. This agrees perfectly well with the experimentally determined factor of ≈ 3.