Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 329

where [H þ ] is the bulk concentration of H þ (0.1 M) and S Pt is the active area of Pt,
electrochemically determined with CV in N 2 -purged 0.1 M HClO 4 at 30 8C. It was
found that the values of S Pt of d Pt ¼ 4.6 and 2.6 nm catalysts were unchanged even
after measurement at 110 8C. Although S Pt at the 1.6 nm catalyst was almost constant
after measurement at 50 8C, it decreased significantly once heated to a temperature
higher than 60 8C. It was also observed by transmission electron microscopy (TEM)
that the Pt particles had grown to about 1.9 nm from the original size of 1.6 nm.
Hence, we will consider the ORR properties of the 1.6 nm catalyst at 30 - 50 8C,
because the effect of particle size on stability is not within the scope of this chapter.
Figure 10.11 shows Arrhenius plots for k app at various electrodes. Linear relation-
ships between log k app and 1/T are observed at all the electrodes. Each solid line is the
least squares fit of all the data at the constant applied potential. From the slope,
we obtained an apparent activation energy E a ¼ 37 kJ mol 21 at - 0.525 V vs. E o ,
which is in very good agreement with that for bulk Pt electrodes [Wakabayashi
et al., 2005a]. It is very striking that the values of k app ( per real surface area) at
Nafion - Pt/CB with different d Pt agreed beautifully with those at Nafion - Pt(bulk)
in the whole temperature range and in the practical potential range. Hence, the ORR
rate constant and its activation energy are independent of Pt particle size.
Figure 10.11 Arrhenius plots of the ORR rate constants k app obtained at various electrodes.
The symbols are the same as those in Fig. 10.10. Each solid line is the least squares fit of all
the data at the constant applied potential. Since the standard potential E o and E[RHE(t)] shift
to less positive values in a different manner, the corrected potential E is applied so as to keep
a constant overpotential for the ORR at each temperature. The applied potentials of - 0.485,
- 0.525, and - 0.585 V vs. E o correspond to 0.80, 0.76, and 0.70 V vs. RHE, respectively, at
30 8C. (From Yano et al. [2006b], reproduced by permission of the PCCP Owner Societies.)