Ageing of PEMFC’s due to operation at low current density: Investigation of oxidative degradation

The paper was aimed at investigating PEMFC ageing upon current cycling: periods of operation under nominal conditions (540 mA cm−2) were followed by periods at low current density (120 or 20 mA cm−2), the two phases of the cycles being hour-long periods. The progressive ageing of the MEA in the single 25 cm2 FC was followed by monitoring the cell voltage, impedance spectroscopy at the end of the various test periods, and by analysis of the water fractions collected. Ageing at 120 mA cm−2 was shown to be of reduced intensity as confirmed by the weak increase in the various resistances of the equivalent cell circuit, whereas repeated operation at 20 mA cm−2 resulted in far more severe degradation. This was confirmed by the significance of Pt dissolution from the cathode and recrystallisation in the bulk of the perfluorinated sulfonate polymer, as revealed by TEM photographs of the MEA after the long-term tests. Besides, the loss of sulfur from the membrane was shown to depend little on the ageing procedure, whereas the loss of fluorine was four times larger at 20 mA cm−2 than at 120 mA cm−2: the high cathode potential allowed by the lower current density might change the decomposition mechanism of the polymer chain, as discussed in the paper.