Temperature Dependence of Oxygen Reduction Activity at Carbon-Supported PtXCo (X = 1, 2, and 3) Alloy Catalysts Prepared by the Nanocapsule Method

Monodispersed Pt X Co (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 H 2 O 2 yield at Nafion-coated Pt X Co/CB catalysts were examined in O 2 -saturated 0.1 M HClO 4 solution at 30 to 100 °C by using the multichannel flow double electrode (M-CFDE) method. In the temperature range from 30 to 80 °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 Pt 3 Co/CB also exhibited the highest stability for an immersion test in air-saturated H 2 SO 4 at 70 °C.