Chemistry Reference
In-Depth Information
CNFs have conductivities almost two orders of magnitude higher: 104 and
103-104 S cm
1
, respectively,
28,29
and significantly higher specific surface
areas of 200-900 m
2
g
1
.
28
The high electric conductivity of CNTs and CNFs
promises low ohmic losses when implemented in a PEMFC as a catalyst
support, while the large surface area offers the opportunity to fabricate
highly and uniformly dispersed catalytic sites.
28,30
Many studies have shown
that the synthesis methods of Pt/CNT and Pt/CNF composites have a critical
effect on morphology and catalytic activity.
Several synthesis methods for Pt on a CNT support such as impregnation
deposition,
27,31
the sonochemical technique,
32
the microwave-heated polyol
process,
33-37
electrodeposition,
38,39
sputter-deposition,
40,41
the g-irradiation
technique,
42
the self-regulated reduction technique of surfactants,
43,44
and
the colloid method
45
have been reviewed by Lee et al.
46
Especially for impregnation deposition, the CNTs require a surface oxi-
dation treatment before the deposition of Pt. By placing the CNTs in nitric or
sulfuric acid, functional groups such as hydroxyl, carboxyl, and carbonyl
groups are created and allow for the successful integration of Pt nano-
particles on the CNT surfaces, acting as anchoring sites. Using this ap-
proach, it has been shown that smaller Pt nanoparticles with a more uniform
distribution on the CNTs achieve higher catalytic activity than Pt/CNT
without oxidation treatment.
31
A two-step sensitization-activation method
appears to create even higher catalytic activity and nanoparticle dispersion.
47
The sonochemical method further increases the number of surface
functional groups by an oxidation treatment in acidic solution, leading to
30 wt% Pt in the Pt/CNT electrode.
32
g-Irradiation can be used to establish
surface functional groups without any oxidation treatment or chemical
agents. This method can achieve Pt particles with an average size of
1.6-1.87 nm.
42
The self-regulated reduction of surfactants is another tech-
nique that avoids the danger of damaging the CNTs by acids.
43
Sputtering
can be used for thinner layers with low Pt loading.
48
The application of CNFs as a support for Pt catalysts in PEMFCs has been
investigated less than CNTs, however, there is tremendous potential for
CNFs as well. Platelet-shaped CNFs can support Pt nanoparticles of a smaller
size and more homogeneously than herringbone or tubular CNFs.
49
Like
CNTs, CNFs can decrease the required Pt loading.
50
Physical treatment by
grinding of CNFs can lead to higher numbers of Pt ion anchoring sites,
leading again to highly dispersed nanoparticles and higher catalytic
activity.
21
A first practical demonstration of a CNT-supported catalyst in a PEMFC
was conducted by NEC Corporation in 2001.
51
It was said that the micro fuel
cell containing a Pt/CNT catalyst had a fuel cell performance 20% higher
than a conventional Pt/C catalyst.
While Pt has the high catalytic activity required for ecient fuel cell
operation, it is very susceptible to CO poisoning. Small amounts of CO,
usually starting at 10 ppm, will compromise and eventually deactivate the Pt
catalyst. Since the vast majority of hydrogen for fuel cells is generated today
d
n
3
r
4
n
g
|
4
.
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