Effect of Temperature on Oxygen Reduction Reaction Kinetics for Pd Core–Pt Shell Catalyst with Different Core Size

The oxygen reduction reaction (ORR) activity and the coverage of oxide species over a Pd core–Pt shell catalyst on carbon support (Pt/Pd/C) with various Pd core sizes (2.3, 4.3, and 8.0 nm) were investigated in the temperature range from 25 to 60 °C and compared with a Pt/C catalyst (TEC10V30E, Tanaka Kikinzoku Kogyo). The apparent rate constant (k app) of Pt/Pd/C increased with increasing core size at 25 °C. However, k app of Pt/Pd/C started to decrease at 50–60 °C, while that of Pt/C behaved according to the general Arrhenius equation. Eventually, the 2.3 nm core showed the highest k app at 60 °C, and the 8.0 nm core was almost the same as that of Pt/C. According to the electrochemical measurements, the coverage of oxide species on Pt/Pd/C was quite smaller than that of Pt/C. However, it increased dramatically with increasing temperature from 25 to 60 °C. Among the Pt/Pd/C, the 8.0 nm core showed the most obvious oxide coverage increase at 60 °C, which was almost identical to that of Pt/C. In contrast, the 2.3 nm core showed the lowest oxide coverage at 60 °C, which was expected to be the cause of the largest k app. Operando X-ray absorption spectroscopy indicated that the Pt–Pt bond length in Pt/Pd/C was shorter than that in the Pt/C at 25 °C due to compressive surface strain from the Pd core, which is the reason why Pt/Pd/C has higher activity than Pt/C. On the other hand, as Pd has a thermal expansion coefficient higher than that of Pt, Pt/Pd/C showed a Pt–Pt bond length larger than that of Pt/C at 60 °C. A longer Pt–Pt bond length extension was observed at 60 °C in the 8.0 nm core compared to that in the other catalysts.