Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 365

8 Limitations of Carbon Nanotubes in PEMFC
Even though the use of CNTs have shown promising results both as an electrode
support and as novel composite polymer electrolytes, it still has few limitations to
overcome before being considered for widespread use. For example, the prepa-
ration of SWCNTs yields a mixture of metallic and semiconducting SWCNTs
which may have significant variation in properties, Further, both SWCNTs and
MWCNTs contain minimal amount of impurities which may have deteriorating
effect on PEMFC performance for extended usage. It has been demonstrated that
even trace level of Fe impurities can degrade Nafion due to the presence of
sulphonic acid moieties in the backbone [ 49 ]. The use of highly purified CNTs is
more expensive which will ultimately increase the cost of MEA and PEMFC
stacks which is not desirable since cost is the single overriding concern preventing
widespread application of PEMFCs.
Another important limitation of using CNTs is lack of enough data related to
durability. Although the use of CNTs as electrode support has shown better cor-
rosion resistance over Vulcan XC-72 in short time scale, its sustained utility for
extended period of operation (such as 5,000 h or more) is yet to be evaluated.
Moreover, CNTs are normally surface oxidized for increased binding with Pt
nanoparticles. The stability of these surface groups under severe potential cycling
has not been investigated. The effect of potential induced morphology changes is
another major concern which may have severe impact on the use of CNT as a
supporting material. The details regarding such attempt will be discussed in the
following section with suitable examples. Similarly different processing conditions
of catalyst preparation and MEA fabrication are expected to create defects on the
CNT surface. For example, the use of ultrasonic homogenizer for prolonged time
is known to chop CNTs into smaller dimensions to help dispersion although for
certain applications this is harmful [ 51 , 52 ]. Further, the non-toxicity of CNTs is
yet to be completely established which necessitates cautions handling of them in
bulk [ 53 ]. Despite the favourable improvements in performance by the use of
CNTs both as electrode support and as a composite electrolyte, these limitations
require further extensive research to fulfil the enormous application potential of
CNTs and related carbonaceous materials in PEMFC.
9 Use of Graphene as Electrode Materials for PEMFC
Graphene is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms, densely
packed in a honeycomb lattice, which has attracted tremendous attention for both
fundamental researches and also possible applications in fuel cells, nanoelec-
tronics, supercapacitors, solar cells and hydrogen storage [ 93 , 94 ]. Graphene
exhibits many exciting properties, like quantum confinement resulting in finite
band gap and Coulomb blockade effects [ 95 - 98 ] which could be useful for making
many novel electronic devices.