Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 358

Fig. 3 a Scheme for the preparation of oriented Pt/CNT film-based MEAs. b TEM image of Pt
deposited CNT. c SEM image of the oriented CNTs on Nafion membrane (adapted from Ref. [ 60 ]
with permission from American Chemical Society). [In the above figure EG refers to ethylene
glycol which acts as both reducing agent and solvent in the reaction]
Carbon nanotubes are generally hydrophobic in nature which helps in con-
trolling gas diffusion properties. Oriented CNTs are shown to have increased
hydrophobicity than that of disordered CNTs. Further, the electronic conductivity
is higher along the tubes than across the tube along with increased gas perme-
ability which would help in better mass transport conditions [ 63 - 65 ]. Considering
all these benefits associated with oriented CNTs Yan et al. have developed a
unique method to orient the CNTs by a filtration method followed by transfer to
the membrane (Fig. 3 ) to prepare the GDL. In this method, the surface of CNTs is
endowed with functional groups such as -COOH by refluxing with Con.HNO 3 /
Con.H 2 SO 4 mixture followed by in situ chemical reduction of Pt precursor solu-
tions on them for the proper anchoring of Pt nanoparticles on the CNT surface.
Subsequent to the chemical reactions, the functionalized CNTs are filtered through
a hydrophobic nylon membrane with precisely controlled pore size/distribution in
CNTs standing up with the preferred orientation and length.
The use of this type of oriented CNTs results in better fuel cell performance
than that of randomly aligned CNT-based electrodes and Pt/C-based electrodes.
Table 1 illustrates this use of CNTs to enhance the fuel cell performance compared
to that of commercial Pt/C catalysts. Durability of a fuel stack is mainly restricted
by the corrosion of carbon support under the operating conditions of the cathode
especially due to the production of hydrogen peroxide (H 2 O 2 ) as an intermediate.
Several studies have established beyond doubt that Pt nanoparticles are expected to
double in size with an operation time of around 200 h [ 62 ]. In this regard, the use
of CNTs which is known for their chemical inertness and remarkable mechanical
strength can increase the endurance of a fuel cell MEA. More specifically, an
attempt to prove the durability of CNTs carried out by potentiostatic treatment for
168 h for Pt supported on CNTs and Vulcan XC-72 reveals that CNTs have lesser
surface oxides than that of Vulcan XC-72, concomitantly demonstrating 30 %
lower corrosion rates [ 62 ].
A more recent work from Lin et al. has attempted to reduce the cost of the fuel
cell by decreasing the particle size as well as increasing the Pt distribution by a wet