Local reversed currents and degradation phenomena in long-term tested PEFC

Kurzfassung

In order to improve the long term stability of PEFC MEAs the investigation and understanding of degradation mechanisms depending on the operating conditions is of major importance. Long term tests up to 850 hours were performed in a single channel serpentine cell at constant load and in humid conditions at 80°C. The cell was equipped with a device to measure the current density distribution across the cell area with a local resolution of 0.5 cm . At regular intervals the constant operation was interrupted for a polarisation curve. The MEA was prepared at CEA using a E87-05S AQUIVION membrane with sub-gaskets, Pt/C catalyst by TKK and the GDL 25BC by SGL., In parallel flow as well as counter-flow of the gases after approximately 500 hours of operation locally reversed currents were observed at low load. The position with negative current flow was always close to the cathode inlet of the cell. At nominal load no major degradation of the current density was observed at this position in the cell. SEM, EDX, TEM, IR spectroscopy and IR microscopy measurements were performed on the MEAs after approximately 800 hours of operation investigating the GDL back side as well as cross sections and details. IR microscopy revealed very inhomogeneous PTFE distribution at the anode backing after operation. Some depositions of Si were observed that were washed out from the cell sealing but should not have major influence on the local behaviour. Different levels of Pt particle degradation were observed in the different zones of the cell. Structural damage of the membrane could be a reason of degradation