Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 331

Figure 10.12 Arrhenius plots of k app for the ORR at Pt 54 Fe 46 ( † ), Pt 68 Co 32 (V), Pt 63 Ni 37 (O),
and Pt ( .... ) electrodes in 0.1 M HClO 4 at - 0.525 V vs. E o . (From Wakabayashi et al. [2005b],
reproduced by permission of the American Chemical Society.)
(Pt 68 Co 32 ), and 2.4 (Pt 63 Ni 37 ), respectively. However, the k app values on these alloys
decrease with increasing temperature above 60 8C, and settle at almost the same value
as for the pure Pt electrode. The degraded ORR activity after heating at high tempera-
ture in 0.1 M HClO 4 solution was never recovered, even in the low temperature region.
The values of P(H 2 O 2 ) were found to be almost independent of potential between
E ¼ 0.8 and 0.3 V, namely, 1.2% at Pt 54 Fe 46 (20 - 50 8C) and 0.16% at Pt 68 Co 32 and
Pt 63 Ni 37 (20 - 60 8C). However, further elevation of temperature decreases H 2 O 2
production below the limit of detection (,0.05%). After these alloy electrodes
experienced high temperatures (.60 8C) in the solution, H 2 O 2 production was
never detected over the whole temperature range. It should be noted that the critical
temperature of about 60 8C for loss of the high ORR activity and H 2 O 2 production
activity corresponds well to that of a sharp decrease in the S Pt . This deactivation in
the ORR as well as the HOR is certainly attributable to a dealloying of the nonprecious
metal component in hot acid solution, resulting in a thick Pt layer, the electronic state
of which is no longer affected by the underlying alloy.
Finally, we briefly discuss the mechanism of enhanced specific ORR activities
(k app ) at the Pt skin layer on the alloys. The Tafel slope in the high current density
region at the Pt skin layer was found to be 2120 mV/decade [Toda et al., 1999;
Wakabayashi et al., 2005b], indicating that the enhancement of ORR activities at