Gas Cross-over and Durability of Membrane Electrode Assemblies

Durability of the membrane electrolyte is a key factor for the longevity of polymer electrolyte fuel cells. A loss of gas separation is the most detrimental failure mode of perfluorosulfonic acid membranes. Mechanical formation of micron sized membrane defects is the main degradation process during fuel cell operation. This process is accelerated by variations in the operating conditions, such as humidity and temperature, and by imperfections and local defects of the gas diffusion electrodes.

The impact of these local defects, termed pinholes is studied by implementing artificial micron sized defects in the membrane by focused ion beam (FIB) milling. Such membranes were then operated in different accelerated stress tests to study the development of chemical, mechanical and thermal degradation processes. Degradation at membrane defects was investigated post mortem using synchrotron based X-ray tomographic microscopy (XTM) and FTIR spectro-microscopy (see Figure 1) [1]. Gas cross-over, a key function for degradation, is controlled by operating conditions and material characteristics and was measured in-situ by mass spectrometry using a tracer gas concept [2, 3].

From the results an overall degradation mechanism at membrane defects can be thus formulated [4].

[1]    S. Kreitmeier, M. Michiardi, A. Wokaun, F. N. Büchi, Factors determining the gas crossover through pinholes in polymer electrolyte fuel cell membranes, Electrochim. Acta, 80, 240-247 (2012)

[2]    G. A. Schuler, A. Wokaun, F. N. Büchi, Local online gas analysis in PEFC using tracer gas concepts, J. Power Sources, 195, 1647–1656 (2010)

[3]    S. Kreitmeier, G. A. Schuler, A. Wokaun, F. N. Büchi, Investigation of membrane degradation in polymer electrolyte fuel cells using local gas permeation analysis, J. Power Sources, 212, 139-147(2012)

[4]    S. Kreitmeier, P. Lerch, A. Wokaun, F.N. Büchi, Local degradation at membrane defects in polymer electrolyte fuel cells, J. Electrochem. Soc. 160, F456-F463 (2013)