In Situ Visualization of Cathode Catalyst Layer Degradation in Fuel Cells Using X-Ray Computed Tomography

Abstract

Polymer electrolyte fuel cells (PEFCs) have been growing in popularity as an alternative energy source for a multitude of applications, including the automotive industry. The use of fuel cells in these applications requires long-term durability with minimal degradation to be cost competitive against conventional technology sources. Current targets for automotive applications are >5,000 hours, under realistic operating conditions. One primary degradation pathway associated with these operating conditions is that of cathode catalyst support corrosion, which occurs due to the oxidation of carbon support for platinum nanoparticles, leaving the platinum unsupported and inactive. The pathway for this degradation mechanism is at elevated cathode potentials greater than 1.2 VRHE, where significant carbon corrosion, in the presence of water, occurs at rates high enough to cause significant structural degradation effects. These elevated potentials can occur during fuel starvation or gas switching during start-up and shutdown procedures [1]. A significant amount of effort has been devoted toward research regarding cathode degradation rates and mitigation, primarily using methods such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and scanning electron microscopy (SEM). These methods provide information on a global, two-dimensional perspective. That is, SEM typically provides information on catalyst layer thinning by 2-D cross-sectional images, and EIS and CV provide overall electrochemical changes affected by changes in surface area from carbon corrosion. The usefulness of the proposed X-ray computed tomography (XCT) technique is in its non-invasive nature, excellent spatial resolution, and three-dimensional imaging abilities. These advantages allow for observations to be made on a local and global level providing further insight into the cathode catalyst layer degradation process. Typically research regarding XCT is performed at a synchrotron beamline which is significantly limiting in that it is expensive, impractical and available in only short time intervals. This means that investigating in-situ degradation effects is extremely difficult. With advances in commercial X-ray sources, optics and detectors for laboratory use, many researchers are now being able to take advantage of the power of XCT scans with much lower cost and increased availability. In this work, we present an investigation toward understanding cathode catalyst layer degradation mechanisms through in-situ visualization by commercial XCT using a fully functional dual-channel, small-scale, fuel cell fixture [2]. This small-scale fixture allows imaging of the membrane electrode assembly (MEA) at multiple stages of its lifecycle during an accelerated stress test, targeting the cathode catalyst layer, in this case causing carbon corrosion. Differences under land and channel are investigated as well as water distribution, which is shown to have a significant effect on the degradation rate using a sample containing catalyst layer cracks. Image processing techniques used to obtain quantitative results are discussed which include histogram deconvolution and thresholding. Figure 1 shows the thresholding procedure results, indicating differences found under land and channel. Continued research using this tool hopes to further our understanding of the interconnectivity within a fuel cell. 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.