Cathode Loading Impact on Voltage Cycling Induced PEMFC Degradation: A Voltage Loss Analysis

This study focuses on voltage cycling induced degradation of cathodes with different loading (0.4 and 0.1 mg Pt /cm 2 ) when applying square wave or triangular wave based accelerated stress tests (ASTs) between 0.6 and 1.0 V RHE . The degradation of the H 2 /O 2 and H 2 /air performance upon extended voltage cycling (up to 30000 cycles) was analyzed in terms of the voltage loss contributions from ORR kinetics, O 2 mass transport resistances and proton conduction resistances in the cathode. The extent of cathode thinning due to carbon support corrosion was determined by post mortem electrode thickness measurements. Square waves were found to cause a more rapid loss of ECSA and mass activity compared to triangular waves, which was shown to be due to the longer hold periods at high potentials rather than to the rate of the potential transient. The observed increase of the O 2 mass transport resistance with voltage cycling was found to mainly depend on the available Pt surface area, while mass transport resistances due to carbon corrosion were found to be insignificant. Finally, it was shown that by lowering the upper potential limit to 0.85 V RHE , low-loaded catalyst layers can sustain 30000 potential cycles without degradation of the H 2 /air performance. A comparison of the ECSA and the O 2 mass transport resistance evolution of MEAs aged by SW cycles leads to considerably faster degradation compared to TW cycles. To identify whether the fast po- tential transients or the potential hold phase is responsible for the faster degradation rates during SW aging, an additional procedure, consisting of a triangular potential scan followed by potential hold phase (TW-H) was tested. A comparison of the evolution of the cath- ode’s ECSA , mass activity, and mass transport resistance revealed that the potential hold rather than the rate of the potential transient is the main contributor to cathode aging in voltage cycling ASTs, so that SW and TW–H cycling profiles lead to identical aging rates with respect to the number of cycles. significant and concomitant