Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 355

5.1 Novel Carbon Supports and Their Functionalized
Surfaces

One of the major modes of failures of a PEMFC is the breakdown of membrane
and the loss in active surface area of the Pt electrocatalyst with time in addition to
the corrosion of the support. The lifetime and cost as a function of performance is
intimately linked with design, materials and operation strategies and various tar-
gets are often used by fuel cell researchers and funding agencies for comparison
and also for measuring progress. The cost targets of the US DOE for PEM fuel cell
stack is an unrealistic $30/kW by 2015 which is a way down from the current
value of $110/kW [ 33 ]. Most of this reduction has to be from the Pt catalyst,
bipolar plate and PEM although total elimination of Pt has been recently indicated
as a tangible possibility [ 34 ]. Development of different types of inexpensive car-
bon plays a critical role in accomplishing some of these objectives as it is well
known that carbon with varying properties could be prepared to meet these
technology-specific requirements. For example, the surface area of carbon can be
varied from few metres to few thousand metres per gram although other useful
properties such as pore size distribution, mechanical strength and electrical con-
ductivity vary dramatically some times in an adverse manner. Consequently,
carbon has been extensively engineered using diverse methods by a huge number
of groups as a support material in PEMFC [ 35 - 40 ].
Earlier attempts to overcome these challenges were restricted mainly by
selecting three different forms of carbon, i.e. activated carbon, carbon black and
graphite or graphitized materials, as the primary choice of support for catalyst
materials in different types of fuel cells [ 35 ]. The preferred form of carbon in fuel
cell electrode including that in GDL is Vulcan XC-72, a kind of activated carbon,
with moderate surface area (250 m 2 /g) and good electrical conductivity, which is
in stark contrast to the preference of activated carbon with a surface are of more
than 3,000 m 2 /g for certain other applications, perhaps due to poor electrical
conductivity and different pore size distribution. However, the mesopores in
Vulcan XC-72 result in part, of the Pt nanoparticles getting buried deeply inside
the pores (especially if they are few nm) and hence becoming inaccessible for the
TPB formation, which is essential for sustaining the electrode reactions in fuel
cells. Further, Vulcan XC-72 undergoes corrosion (more important under peroxide
intermediate formation conditions of fuel cell cathodes) resulting in the aggrega-
tion as well as dissolution of Pt nanoparticles [ 36 - 42 ].
Attempts to improve the carbon support by different strategies have generated
mixed results. Coin like hollow carbon (Fig. 2 ) prepared by a simple solvothermal
method has been used to support Pd electrocatalyst in methanol oxidation with an
improved mass activity of 2,930 A g -1 against 870 A g -1 of Pd supported on
Vulcan XC-72 carbon [ 37 ]. Similarly, carbon nanofibres and even scrolls are also
attempted as a support due to its ease of fabrication [ 38 - 41 ]. Although all these
improvements on carbon alone cannot solve most of the above challenges