Environmental Engineering Reference
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stated that j
s
for the ORR decreased with increasing S
mass
from 20 to 80 m
2
g
21
(corresponding to a decrease in d
Pt
from about 14 nm to 3.5 nm). Ross and co-workers
observed a similar but more pronounced effect in the region of S
mass
greater than
80 m
2
g
21
with d
Pt
, 3 nm [Ross, 1986; Sattler and Ross, 1986]. This so-called
“particle size effect” might be attributed to an electronic effect [Mukerjee and
McBreen, 1989; Takasu et al., 1996] or to a morphological effect (exposed facet
dependent on d
Pt
) [Kinoshita, 1990]. In contrast, Watanabe and co-workers found
that j
s
was independent of d
Pt
, but did depend on the interparticle distance
[Watanabe et al., 1988, 1989]. We consider that this inconsistency is partly due to
inappropriate evaluation techniques for j
s
or MA. We have therefore made a great
effort to establish standard methods of evaluating the real activities [Higuchi et al.,
2005; Wakabayashi et al., 2005a, b; Yano et al., 2006a].
In this section, we demonstrate the real ORR activities (apparent rate constant
per real active surface area, k
app
) and P(H
2
O
2
) at bulk Pt and nanosized Pt catalysts dis-
persed on carbon black (Pt/CB) with d
Pt
¼ 1.6+0.4, 2.6+0.7, and 4.8+1.0 nm in
the practical temperature range 30 - 110 8C [Yano et al., 2006b]. The use of a channel
flow double-electrode (CFDE) cell allowed us to evaluate k
app
and P(H
2
O
2
) precisely.
Details of the experimental set-up for the ORR measurements using the CFDE cell
and a flow circuit of electrolyte solution are described in the literature [Wakabayashi
et al., 2005a, b; Yano et al., 2006a]. The working electrode consisted of Pt/CB catalysts
perfectly dispersed on a gold substrate electrode (1 mm
4 mm) at a constant loading of
the carbon support, 5.45 mgcm
22
, which corresponds to approximately a monolayer of
CB particles. Nafion solution 0.2 wt% was put on top of the catalyst layer to yield an
average film thickness of 0.1 mm, which is thinner than the critical value determined
for the ORR [Higuchi et al., 2005]. Finally, Nafion2Pt/CB on gold was heated at
130 8C for 30 minutes in air. Hydrodynamic voltammograms at the working electrode
under a flow of O
2
-saturated 0.1 M HClO
4
solution (U
m
¼ 10 - 50 cm s
21
)were
recorded by scanning its potential from 0.3 to 1.0 V at 0.5 mV s
21
.
10.3.1.1 H
2
O
2
Production Rate
Based on the collection efficiency N at the Pt
collecting electrode located downstream from the working electrode, we calculated the
ratio of the H
2
O
2
production rate to the overall ORR rate, P(H
2
O
2
):
2I
C
NI
W
þ
I
C
100%
P(H
2
O
2
)
¼
(10
:
7)
where I
W
and I
C
are the currents at the working and collecting electrodes, respectively.
Because we found that Nafion-coated carbon black (without Pt) and Au substrate
produced H
2
O
2
with P(H
2
O
2
) ¼ 98% only at potentials lower than 0.65 V, the
H
2
O
2
production at E . 0.7 V was ascribed completely to the Nafion - Pt/CB inter-
face. As shown in Fig. 10.10, the values of P(H
2
O
2
) at all Nafion - Pt/CB are
almost the same as those at Nafion2Pt(bulk) within experimental error, irrespective
of d
Pt
and Pt loading. P(H
2
O
2
) increases slightly on lowering the potential, i.e.,
0.6% and 1.0% at 0.80 and 0.70 V, respectively, and is almost independent of the
temperature from 50 to 110 8C. Considering P(H
2
O
2
) ¼ 0 at Pt(bulk) (without
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