3D Failure Analysis of PEM Fuel Cell Catalyst Layers Using Multi-Length Scale X-Ray Computed Tomography

Catalyst layers are a critical component in polymer electrolyte membrane (PEM) fuel cells and are where the electrochemical reactions take place. These layers play a major role in both performance and durability of fuel cells. Catalyst layer degradation is a result of carbon corrosion, detachment of platinum particles from their carbon support, formation of carbon surface oxides, electrolyte expansion and contraction, etc. [1]. Hence, understanding CL degradation mechanisms in fuel cells is an area of intense, but very challenging study. One challenge lies in the structural characterization of the catalyst layers of the bonded membrane electrode assembly (MEA). A challenge which significantly increases as the MEA degrades, and the catalyst layers are intermingled with other layers in the MEA and difficult to separate. Even if the layers were to separate, the samples might be altered chemically and structurally. A non-destructive technique which allows one to study the catalyst layer properties is X-ray computed tomography (XCT). Such scanners use X-rays to scan the MEA in a completely non-invasive process. XCT scanners operate at ambient conditions which are ideal since fewer morphological artifacts due to dehydration of the ionomer would be observed.

The objective of the present work is to qualify the XCT technique for 3D failure analysis of degraded catalyst layers. Previous research studies have shown that catalyst layers with higher catalyst crack area show higher carbon corrosion degradation [2] after 4700 voltage cycles with an upper potential limit of 1.3 V. The EOL MEA sample along with a conditioned beginning of life (BOL) sample with the high catalyst crack area are analyzed and compared upon failure using ZEISS Xradia 520 Versa and ZEISS Xradia 810 Ultra XCT instruments having a maximum image resolution of 700 and 50 nm, respectively. A 1cm x 1cm sample is scanned in the Versa whereas a 350 µm diameter sample is punched out of the 1cm x 1cm sample and scanned in the Ultra.

A significantly increased crack size in the cathode catalyst layer (CCL) is observed due to degradation. As illustrated in Figure 1, the average crack diameter at BOL is 6 µm and grows to about 18 µm at EOL.

The brighter surface observed in the EOL CCL is interpreted as being due to carbon corrosion; hence, a cross sectional study is performed to validate this assumption. Thickness measurements show catalyst layer thinning on both anode and cathode catalyst layers. Loss of surface area in the catalyst layers is believed to occur via Pt dissolution, coalescence of Pt nanoparticles, loss of the carbon support due to carbon corrosion, loss of Pt surface area due to agglomeration, and carbon corrosion [3]. This is believed to be one of the main reasons for performance degradation during fuel cell operation. Here, substantial catalyst layer thinning indicates severe carbon corrosion.

The absorption contrast mode of the Ultra XCT instrument is used to perform high-resolution scans on the individual island structures in the BOL CCL. The data are post-processed in Avizo and the BOL CCL porosity is determined to be 52.6%. This result is compared with the mercury intrusion porosimetry (MIP) technique, which results in 53.0% porosity. A similar study on the EOL CCL sample is in progress. The comparison between the BOL and EOL porosity data is expected to confirm structural collapse of the CCL due to carbon corrosion.

Acknowledgements

Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.

References

1. S. Zhang, X.Yuan, J Hin, H Wang, K Friedrich, M. Schulze, A review of platinum-based catalyst layer degradation in proton exchange membrane fuel cells. J Power Sources 2009; 194(2): 588–600
2. Silvia Wessel, David Harvey, “Development of Micro-Structural Mitigation Strategies for PEM Fuel Cells: Morphological Simulations and Experimental Approaches”, 2013 Annual Merit Review Proceedings: Fuel Cells, 16 May 2013, Project ID# FC049.
3. J. Zhang, PEM fuel cell electro catalysts and catalysts layer, London: Springer, May 2008: p. 1063-1087