Extraordinary Oxygen Reduction Activity of Pt3Ni7

Nanostructured thin film catalysts (NSTF) with widely varying compositions of Pt x M y and Pt x M y N z (M, N = Ni, Co, Zr, Hf, Fe, Mn) have been evaluated for 0 ≤ x, y, z < 1. The catalysts' activity for oxygen reduction (ORR) was measured in 50 cm 2 fuel cell membrane electrode assemblies. Pt 1-x Ni x was found to be unique in showing an extraordinarily sharp peak in ORR activity as a function of the as-made composition around x = 0.69 ± 0.02 determined gravimetrically. This composition gave a corresponding fcc lattice parameter of 3.71 Angstroms and a grain size of 7.5 nm. Both surface area and specific activity increases contribute to the increased mass activity of the resultant dealloyed films. The ORR mass activity of the Pt 3 Ni 7 is 60% higher than for the NSTF standard Pt 68 Co 29 Mn 3 alloy. Rotating disk electrode measurements of a Pt 1-x Ni x series on NSTF coated glassy carbon disks show a similar large and sharp peak in activity. In contrast, PtCo shows a diminished but still sharply peaked mass activity in 50 cm 2 tests near x = 0.62 by electron microprobe over the 0 < x < 0.7 range examined.