Temperature dependence of oxygen reduction activity at carbon-supported PtXCo (X=1, 2, and 3) alloy catalysts prepared by the nanocapsule method

Monodispersed PtXCo (atomic ratio, X = 1, 2, and 3) alloy nanoparticles supported on carbon black (CB, 50 wt % metal loading) were prepared by the nanocapsule method. The average particle diameters (d) measured by scanning transmission electron microscopy ranged from 1.9 (X = 3) to 2.4 nm (X = 1). The alloy composition was found to be well-controlled to the projected value among the supported particles. Oxygen reduction reaction (ORR) activity and H2O2 yield at Nafion-coated PtXCo/CB catalysts were examined in O-2-saturated 0.1 M HClO4 solution at 30 to 100 degrees C by using the multichannel flow double electrode (M-CFDE) method. In the temperature range from 30 to 80 degrees C, the value of apparent rate constant k(app) (per real active surface area) for the ORR at X = 3 (d = 1.9 nm) was the largest, by a factor of 2.2 compared with that of Pt/CB (50 wt % Pt, d = 2.6 nm), i.e., the mass activity of Pt was expected to be enhanced by 3.3 times due to an advantage of higher specific surface area. The Pt3Co/CB also exhibited the highest stability for an immersion test in air-saturated H2SO4 at 70 degrees C.