Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes catalyst poisoning by membrane degradation products  which requires removal from the catalyst surface and then from the electrode layer for the performance recovery. Irreversible degradation modes include catalyst dissolution and ripening, loss of alloying agents from Pt-X catalysts, plus the effects of the various forms of carbon used in PEMFC components which include changing hydrophobicity, carbon corrosion and loss of porosity of electrode layers and Gas Diffusion Layers (GDLs). These increasing losses are primarily in the cathode catalyst layer and are attributed to both increasing transport and kinetic losses.
The FC-PAD consortium is examining these degradation mechanisms to help develop improved materials and operating strategies. Corrosion of the carbon electrocatalyst support has been measured during drive-cycle operating conditions and increases with increase potential cycling from 0.4 to 0.9 V. Carbon corrosion is one of the major contributors to degradation which leads to changes in the catalyst layer structure and reduces its activity. Reduction in catalyst layer thickness is observed during operation, exacerbated during drive cycles. This reduction can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. Membrane additives which increase membrane life-times, have been measured to migrate into the catalyst layer and appear to be associated with the carbon in the catalyst layers. Low potentials (0.2V) appear to be required to remove membrane fragment adsorbates which decrease catalyst activity. Pt alloy catalysts lose most of their alloying agents during operation; the alloying agents migrated throughout the ionomer. Durability implications of using Pt-X alloy catalysts will be discussed. Results related to the mentioned degradation mechanisms will be presented including characterization from TEM, SEM, XRF, XRD and electrochemical testing.
This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos
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