Environmental Engineering Reference
In-Depth Information
It is striking that j
H
k
at Pt-Ru increases with u
CO
at all temperatures, doubling
in value at u
CO
¼ 0.7 compared with u
CO
¼ 0. The dependence of j
H
k
on u
CO
at
Pt-Co is also similar to that at Pt-Ru up to 50 8C. This indicates that the HOR sites
on Pt-Ru and Pt-Co are not so rigidly blocked by CO
ad
, owing to its enhanced mobi-
lity, as indicated by EC-XPS experiments (Section 10.2.3).
However, the CO tolerance at Pt-Co degraded at 70 8C. As seen in Fig. 10.9, the
HOR activity of Pt-Co at a given u
CO
is close to that of pure Pt, although the decelera-
tion effect on the CO adsorption rate was still observed to some extent at 70 8C. Such a
“deactivated” electrode cannot recover the original CO tolerance. This can certainly be
ascribed to a severe dealloying of the nonprecious metal component (Co) in hot acid
solution. We will discuss this in Section 10.3.2.
10.3 CATHODE CATALYSTS FOR THE ORR
It is very important to develop a high performance cathode catalyst, because a sluggish
ORR causes a large overpotential at low temperatures. With respect to the total per-
formance of activity and stability, the cathode catalyst material is limited to Pt or its
alloys at present. In acidic media such as Nafion electrolyte or aqueous acid solutions,
four-electron reduction is dominant at Pt-based electrodes:
O
2
þ
4H
þ
þ
4e
!
2H
2
O
(10
:
5)
However, two-electron reduction occurs fractionally to produce H
2
O
2
, which may
cause deterioration of the polymer electrolyte membrane or gaskets:
O
2
þ
2H
þ
þ
2e
!
H
2
O
2
(10
:
6)
The ORR has been studied with a rotating ring - disk electrode (RRDE), which can
provide the j
k
and the H
2
O
2
yield P(H
2
O
2
) at around room temperature. However, for
improving the ORR activity, PEFCs should be operated at high temperature (.80 8C).
In this section, we demonstrate the temperature dependencies of ORR activity
and P(H
2
O
2
) at pure Pt (both bulk and supported catalyst) and bulk Pt alloys
(Pt-Ni, Pt-Co, and Pt-Fe).
10.3.1 Particle Size Effect on the ORR at Pt
/
CB Catalysts
The mass activity MA (in A g
21
) of the Pt catalyst is, of course, the product of the
specific activity j
s
(in A m
22
) and the specific surface area S
mass
(in m
2
g
21
): MA ¼
j
s
S
mass
. Because S
mass
is inversely proportional to the particle diameter d
Pt
, the use
of supported Pt nanoparticles is effective for increasing MA, if j
s
is a constant indepen-
dent of d
Pt
. However, even at pure Pt, conflicting results on the values of j
s
and
P(H
2
O
2
) have been reported, suggesting the presence of differences in electrochemical
properties between bulk and supported nanoparticles. For example, Bregoli [1978]
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