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In contrast, Paulus and co-workers reported that j
k
( per real surface area) for the
ORR at Nafion - Pt/CB were lower than those at bulk Pt at 60 8CinO
2
-saturated
0.5 M H
2
SO
4
, namely, 190 mAcm
22
(20 wt% Pt/Vulcan XC72) compared with
400 mAcm
22
[Pt(bulk)] at 0.85 V vs. RHE [Paulus et al., 2001]. Such a noticeable
reduction in activity should be ascribed to the stacked multilayer (about 10 mono-
layers) of Pt/CB catalyst coated on the glassy carbon disk substrate. Hence, it is
essential for the evaluation of the true activity of electrocatalysts supported on CB
to disperse them uniformly as in our experimental procedure. The present results
have been supported by
195
Pt electrochemical nuclear magnetic resonance spec-
troscopy measurements (EC-NMR) on the same set of Pt/CB catalysts, which
showed that the surface peak position and the spin - lattice relaxation time of surface
Pt atoms exhibited practically no change with d
Pt
[Yano et al., 2006b].
Because both the ORR activity per real active surface area and P(H
2
O
2
) are the
same, we can reduce the mass of Pt in the catalyst layer by using smaller particles,
as long as their durability can be ascertained. It is also found that, at 20.525 V vs.
E
o
, the k
app
values for both bulk Pt and Nafion2Pt/CB at 60 and 110 8C are 5 and
35 times higher, respectively, than at 30 8C. This clearly shows the possibility of sig-
nificant reduction of Pt loading in PEFCs by such factors by elevating the operating
temperature, if the catalyst layer could properly be designed to supply sufficient O
2
gas to Pt active sites.
10.3.2 Enhanced ORR Activity at Pt Alloy Electrodes
Enhanced ORR activities of Pt alloyed with nonprecious metals such as Fe, Co, Ni,
Mn, Cr, and V have been reported in acidic electrolyte solutions [Paffett et al.,
1988; Mukerjee and Srinivasan, 1993; Mukerjee et al., 1995; Thamizhmani and
Capuano, 1994; Toda et al., 1999; Min et al., 2000; Neergat et al., 2001;
Stamenkovic et al., 2002, 2003; Antolini et al., 2002; Drillet et al., 2002; Yang
et al., 2004]. In this section, we investigate the temperature dependencies of the real
ORR activities k
app
and P(H
2
O
2
) at Pt-Fe, Pt-Co, and Pt-Ni sputtered film electrodes
at 20 - 90 8C by the CFDE method [Wakabayashi et al., 2005b].
As described in the previous sections, a stable Pt skin of a few nanometers is formed
on the Pt-Fe, Pt-Co, and Pt-Ni alloy surfaces after electrochemical stabilization.
Figure 10.12 shows Arrhenius plots of k
app
on the alloy electrodes at 20.525 V vs.
E
o
in comparison with that of a pure Pt electrode. In the low temperature region
(20 - 50 8C for Pt
54
Fe
46
,20-608C for Pt
68
Co
32
and Pt
63
Ni
37
), linear relationships
between log k
app
and 1/T are observed at all the electrodes, corresponding to the follow-
ing Arrhenius equation:
k
app
ΒΌ
Z exp
1
a
RT
(10
:
9)
The apparent activation energy 1
a
on each alloy was found to be 41 kJ mol
21
,
which is almost comparable to that on the pure Pt electrode. The values of k
app
on
the alloys are larger than that on the Pt electrode by factors of 4.0 (Pt
54
Fe
46
), 3.1
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