Effects of Repeated Feed-Gas Exchange Processes on Degradation Processes of Pt-Based PEFC Cathode Catalysts Studied by In-situ XAFS, STEM-EDS and Electrochemical Methods

Start-up/shut-down procedures of polymer electrolyte fuel cells (PEFCs) result in temporal intermediate states of the anode Pt/C catalyst at the partial surface by exposing to both hydrogen and oxygen (air) in the feed-gas exchange processes of either air or H₂ into the anode compartment. The inhomogeneous gas mixing in the anode leads to oxygen reduction reaction (ORR) at a part of the anode surfaces, which causes the corresponding cathode potential to spike up to about 1.6 V and hence carbon corrosion and Pt dissolution to deteriorate MEA performances. Recently, we reported the behavior of Pt/C cathode catalysts in MEAs during anode gas exchange cycles (AGEX )^1,2 and cathode gas exchange cycles (CGEX)² as characterized by in-situ time-resolved XAFS and STEM-EDS. Further investigations in this study have been done with an MEA Pt₃Co/C cathode catalyst.

The Pt₃Co/C cathode catalyst gave different results from those for the Pt/C cathode catalyst under both AGEX and CGEX conditions. While both performance and ECSA of the Pt/C cathode decreased with increasing AGEX cycles, only the performance decreased with the Pt₃Co/C cathode. In the degradation process by the CGEX cycles neither performance nor ECSA decreased with the Pt₃Co/C, while only the ECSA decreased with the Pt/C. Pt L_III-edge white line peak intensities for the Pt₃Co/C showed a larger decrease under the AGEX conditions, but less under the CGEX conditions, comparable to the results obtained for the Pt/C as shown in Figure 1.

Figure1. Mass-activities (left axis) and averaged Pt L_III WL peak heights (right axis) for Pt/C (black) and Pt₃Co/C (orange and green) during AGEX (left figure) and CGEX (right figure). The in-situ XAFS were measured at top (squares, open star for Pt₃Co/C), middle (triangle), and bottom (circle) positions of the MEA.

(1)     G. Samjeské et al., ECS Transactions, 64, 113-119 (2014)

(2)     G. Samjeské et al., ChemElectroChem, DOI: 10.1002/celc.201500099