Tracking Degradation Induced Structural and Compositional Changes of a Polymer Electrolyte Fuel Cell Cathode Catalyst Layer Following Voltage Cycling Using Micro-Xct
Authors: White Robin T , Eberhardt Sebastian H , Najm Marina , Orfino Francesco P , Dutta Monica , Kjeang Erik 
Published: February 2020

Degradation of cathode catalyst layers during typical automotive operation is a concern for long-term durability and performance of polymer electrolyte fuel cells (PEFCs). Although many degradation pathways exist, carbon corrosion following significant voltage fluctuations should be considered a primary degradation mechanism due to the role carbonaceous material plays in catalyst layer composition and structure. Following carbon support corrosion, catalytic activity is lost due to removal, isolation, or agglomeration of catalyst particles. Eventual collapse of the weakened carbon support structure can lead to changes in porosity and pore size distribution, thus blocking transport pathways for gas flow and water removal. Furthermore, changes in surface roughness and composition by addition of surface oxidation groups can increase hydrophilicity, increasing the possibility of flooding in the catalyst layer [ 1]. A detailed understanding of the structural and compositional changes that occur following degradation of the catalyst layer is therefore required.

To date, imaging of PEFC component degradation, in particular the cathode catalyst layer, has primarily been limited to ex situ techniques and associated qualitative morphological observations [ 2, 3]. Recent developments in lab-based X-ray computed tomography (XCT) systems have allowed for nondestructive in situ imaging of PEFCs [ 4] accomplished by using a unique device fixture design and XCT operation to obtain same location tracking with high quality three-dimensional tomographies over different stages of cathode catalyst layer (CCL) degradation [ 5]. In this work, this technique is expanded to visualize and measure quantitative morphological and compositional changes that occur with degradation of the cathode catalyst layer during voltage cycling, see Figure 1. This analysis is combined with simultaneous tracking of the liquid water distribution in the gas diffusion layer and CCL by specialized operandovisualization, not previously performed for lab-based XCT. The attenuation of X-rays is related to the elemental composition as well as the density of the material being imaged as defined by the Beer-Lambert law. By exploiting this property, novel insight into the local compositional changes of the cathode catalyst layer are investigated and uniquely correlated to the water distribution changes in an operating fuel cell, which has not been previously possible. The acquired compositional and morphological changes are further supplemented with electrochemical diagnostics measurements such as fuel cell polarization curves, electrochemical active surface area (ECSA) and double layer capacitance. This comprehensive study highlights the effect of CCL degradation on overall fuel cell performance. Significant losses in the mass transport regime of the polarization curve are correlated to possible flooding of the catalyst layer and reduced oxygen transport through observation of density increase and reduced thickness. Local material composition changes such as carbon loss, ionomer distribution and platinum loading are also calculated and discussed.

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.

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Figure 1