Evolution of Structure and Activity of Alloy Electrocatalysts during Electrochemical Cycles: Combined Activity, Stability, and Modeling Analysis of PtIrCo(7:1:7) and Comparison with PtCo(1:1)
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This study explores the changes in bulk composition/structure and oxygen reduction activity of two alloys, Pt7IrCo7 and PtCo, caused by Co leaching during electrochemical cycles and as a result of membrane electrode assembly (MEA) fabrication procedures. Exposure to liquid electrolyte and electrochemical cycles in a rotating disc electrode (RDE) environment resulted in substantial Co loss and no stabilization from the low levels of Ir used in the ternary material. The true composition of the ternary material was determined as Pt8IrCo3 following initial exposure to 0.1 M HClO4 (before cycling) and Pt11IrCo4 after 5000 cycles. Density functional theory (DFT) modeling of the cycled catalyst compositions indicated that structures with Pt-rich upper layers would show the highest stability; however, addition of 0.25 ML oxygen adsorption favored Co segregation from second and third atomic layers. The high initial activities (>0.44A/mgPt) achieved in the RDE environment decreased with cycles and were not reproduced in MEAs. X-ray diffraction (XRD) analysis revealed a measurable increase in lattice parameter caused by the MEA preparation procedure, consistent with Co (and some Ir) leaching into the ionomer phase and relaxation of the lattice. MEA fabrication procedures and cycling in 1 M H 2SO4 at 80-C showed greater changes to catalyst structure and increased Ir and Co loss compared to exposing the catalyst to RDE like conditions (0.1 M HClO4, RT) explaining the observed discrepancy in activity between RDE and MEA. 2013 American Chemical Society.