Arezoo Avid1, Robin Theiss1, Jonathan Braaten1, Lei Cheng1, Nathan Craig1, Iryna V. Zenyuk2,3, Christina M. Johnston1, Sarah Stewart1
1 Bosch Research and Technology Center North America, Sunnyvale, CA 94085, USA
2 National Fuel Cell Research Center, University of California, Irvine, CA 92697, USA
3 Department of Chemical and Biomolecular Engineering, University of California, Irvine, CA 92697, USA
One of the attempts to improve catalytic activity of Pt is by alloying it with the base-metal Co. These bimetallic surfaces have reported 2-4 times larger oxygen reduction reaction specific activity compared to Pt. However, the main drawback of the Pt-Co alloy catalysts is their lack of long-term operational durability1. It has been shown that Co leaches out of the alloy nanoparticles poisoning ionomer in the catalyst layer (CL) and membrane by ion exchanging with H+ in sulfonic acid groups2. Among lifetime limiting factors, transient operation of start-up (SU) and shut-down (SD) is detrimental to the cathode catalyst layer due to the reverse-current decay mechanism where the H2/airanode gas front pass through the anode flow-field causing an oxidative current on the cathode and a rise in the local potential as high as 1.5 V. This will result in severe carbon corrosion, substantial increase in the oxygen mass transport resistance and performance decay as a result3,4. Better understanding of these degradation fundamentals during real-world operations like SUSD and their correlation with morphological transformations of the catalyst layer will help optimize durability of fuel cells. This is the key to the commercialization and mass production of affordable fuel cell electric vehicles (FCEVs) for the transportation sector.
In this work, to elucidate the evolution of cathode catalyst layer during SUSD events for Pt and Pt-Co electrodes with high-surface area (HSAC) carbon supports, we studied double-layer capacitance, changes of electrochemical surface area, and the evolution of ionomer and sulfonic groups (SO3-) coverage utilizing CO displacement/stripping and electrochemical impedance spectroscopy (EIS) in dry and wet conditions. These in-situ electrochemical characterization experiments revealed the distribution of Pt nanoparticles within the pores, and extent of ionomer redistribution and degradation during and after SUSD events due to the collapse of carbon support framework5. We also investigated the changes of Pt-Co|ionomer and Pt|ionomer interfaces and their role in the total performance of the fuel cell. The loading of Pt and Pt-Co catalyst on carbon supports were either 30 or 50%. We adapted unmitigated SUSD protocol by applying a well-defined residence time during the SUSD events under relevant fuel cell operating conditions. The protocol involved initial current hold in H2/Air with the purpose of MEA rehydration followed by a hold at open circuit voltage. H2/Airanode gas fronts for SU and SD were created by switching between H2 and air gas streams on the anode side in Greenlight G100 fuel cell test station that provides separate humidifiers for H2 and air in the anode compartment. The duration of each SU and SD event was 120 s3.
CL thickness was assessed along the channels assessed using scanning electron microscopy (SEM) cross-sections. X-ray fluorescence spectroscopy (XRF) mapping studies were used to understand Pt and Co distributions at the beginning of life and at the end of life. The study provides design considerations for Pt-Co electrodes with HSACs to enable durable electrodes that can withstand SUSD events.
References
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