Effects of Nafion® dehydration in PEM fuel cells

The current dependence of the ohmic resistance of Nafion membranes was examined with different types of humidification: cathodic (ChAd), anodic (CdAh), anodic and cathodic (ChAh) and no humidification at all (CdAd). Data show that for stacks with humidified cathodes (ChAd and ChAh), the resistance is small and relatively insensitive to the presence of the anodic humidification. On the contrary, for stacks with non-humidified cathodes (CdAh and CdAd), the membrane resistance is high and strongly dependent on current and anodic humidification. The kinetics of membrane dehydration was examined by recording the galvanostatic transients of the stack voltage and resistance, after removing the humidification. It was found that the changes in the ohmic resistance DeltaR(Omega)(t), although significant, cannot explain entirely the observed decay of the stack voltage. To account for the difference, an additional resistive term is introduced DeltaR(p)(t). Explicit equations were found for the time and current dependence of the two resistive terms DeltaR(Omega)(t) and DeltaR(p)(t) after humidification removal. A tentative explanation for the new resistive term was provided using electrochemical impedance spectroscopy (EIS). EIS data obtained at low overpotential show that dehydration of the Nafion present in the cathode catalytic layer results in an increase of the polarization resistance; the apparent deactivation of the cathode electrocatalyst appears to be due to a decrease of the electrochemically active surface area.