Membrane dehydration with increasing current density at high inlet gas relative humidity in polymer electrolyte membrane fuel cells

The membrane hydration state of a polymer electrolyte membrane fuel cell is investigated in operando at high current densities (> 1.5 A cm(-2)) for various inlet gas relative humidities (RH). The ohmic resistance, an indicator for membrane hydration, is found to have a non-monotonic dependence on current densities between 0 and 3.0 A cm(-2), and the current density corresponding to the local minimum ohmic resistance is defined as the optimal hydration current density. While the relative thickness of the membrane increases with increasing inlet RH, an interesting observation was made whereby elevating the catalyst-coated membrane (CCM) temperature at current densities above the optimal hydration current density leads to membrane dehydration. Surprisingly, at high inlet gas RH values (70% and 100%) and high current densities, membrane dehydration results in membrane shrinkage (relative to its thickness at 0 A cm(-2)). We attribute the membrane dehydration and associated shrinkage to the reduced local RH at the CCM, even though the net water flux from the cathode to anode increases. The relative membrane thicknesses as a function of inlet RH conditions are determined from the low frequency arc time constants obtained via electrochemical impedance spectroscopy measurements.