Highly active and durable chemically ordered Pt-Fe-Co intermetallics as cathode catalysts of membrane-electrode assemblies in polymer electrolyte fuel cells

摘要

A cathode catalyst with enhanced activity for the oxygen reduction reaction and high durability is greatly required in polymer electrolyte fuel cells (PEFCs). We report that chemically ordered intermetallics, Pt_2FeCo/C with L1_0 ordered structure and Pt_6FeCo/C with L1_2 ordered structure, achieve both higher activity in solution and higher durability as the cathode catalyst of a membrane-electrode assembly (MEA) under actual PEFC operating conditions, compared with a commercial Ft/C. The mass activity of Pt_2FeCo/C is about twice as high as that of a commercial Pt/C, and Pt_6FeCo/C has a slightly higher mass activity than a commercial Pt/C in solution. These intermetallics were evaluated for durability as the cathodes of MEAs. During the load cycle durability test, the electrochemical surface area (ECSA) of Pt_2FeCo/C is retained, while that of Pt_6FeCo/C decreases, although less than a commercial Pt/C. The retention of ECSA is consistent with smaller changes in the particle size, observed with transmission electron microscopy, of the intermetallics than of a commercial Pt/C. X-ray diffraction patterns after the durability test show that while some part of ordered intermetallics is transformed into pure Pt, the chemically ordered structure of Pt_2FeCo/C is largely retained. Highly active and durable ordered intermetallics are promising as cathode catalysts of PEFCs.