Comparison between Dealloyed PtCo3 and PtCu3 Cathode Catalysts for Proton Exchange Membrane Fuel Cells

Dealloyed PtCo3 and PtCu3 catalysts supported on high surface area carbon (HSC), which were synthesized under different conditions, were tested as cathode electrodes in proton exchange membrane fuel cells. The dealloyed PtCu3/HSC gave higher initial oxygen reduction reaction (ORR) kinetic activity but much worse durability in a voltage cycling test. Detailed characterization was undertaken to develop insights toward the development of catalysts with both high activity and good durability. In situ X-ray absorption spectroscopy (XAS) analysis showed that dealloyed PtCu3/HSC exhibited stronger bulk Pt-Pt compressive strains and higher bulk cl-band vacancies (attributed in part to a greater ligand effect induced by Pt-Cu bonding) than dealloyed PtCo3/HSC, factors which can be expected to correlate with the higher initial activity of dealloyed PtCu3/HSC. Annular dark field (ADF) imaging and electron energy loss spectroscopy (EELS) mapping demonstrated that a strong majority of metal nanoparticles in both dealloyed PtCu3/HSC and PtCo3/HSC have variants of core-shell structures. However, the most prevalent structure in the dealloyed PtCo3/HSC gave multiple dark spots in ADF images, approximately half of which were due to Co-rich alloy cores and half of which arose from voids or surface divots. In contrast, the ADF and EELS data for dealloyed PtCu3/HSC suggested the predominance of Pt shells surrounding single Cu-rich cores. Further work is needed to determine whether the contrast in durability between these catalysts arises from this observed structural difference, from the differences between the corrosion chemistry of Cu and Co, or from other factors not addressed in this initial comparison between two specific catalysts.