Environmental Engineering Reference
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catalysts for PEMFC, but these are very expensive, and Pt-based electatalysts
and their associated catalyst layers contribute over 55% of the total cost of
fuel cells. Thus, exploring new catalysts withl platinum or are platinum-free
is in high demands [46, 47].
9.3.1 Pt-Based Catalysts
Pt-based catalysts are the mainly catalysts used in current fuel cells. It has
a face-centered cubic (fcc) crystal structure with a lattice parameter of
3.93 Ả, and it surface energies ( γ ) of the low-index crystallographic planes
are in the order of γ (111) <  γ (100) <  γ (110). Usually, nano-sized Pt-based
catalysts have high specific surface area, and are high active than bulk, while
the exposed facets, morphologies as well as composites affect the catalytic
activities obviously [48]. Thus, synthesis of new nanostructured materials is
a topic of intense research and development to optimize the existing Pt NP
catalysts [49].
In 2007, Xia et al. at Washington University described the synthesis of
platinum nanoparticles of an unusual shape (Fig. 9.4). Remarkably, the crys-
tals were concave nanocubes enclosed by high index facets, including {510},
{720}, and/or {830} surfaces. Their as-prepared Pt concave nanocubes
exhibited substantially enhanced specific activity compared with those of Pt
nanocubes, cuboctahedra, and commercial Pt/C catalysts that are bounded
by low index facets, such as {100} and {111} toward the oxygen reduction
reaction, and have significant potential in fuel cell application [50].
Meanwhile, the Pt alloys have also been widely explored. Yang et al.
synthesized Pt-Pd bimetallic heteronanostructures by using Pd as a metallic
support for Pt nanoparticles (Fig. 9.5). They found that the alloys exhibited
high reactivities and improved stabilities, which could be to the favorable
interfacial structures between Pt and Pd supports, as well as the larger than
usual overall particle size of Pt-on-Pd nanostructures, which prevented the
small Pt from dissolution in the oxygen reduction reaction.
9.3.2 Nonnoble Metal Catalysts
Though Pt-based materials have long been investigated as active catalysts
for oxygen reduction reaction, the large-scale application of fuel cells has
been hampered by its high cost and the inadequacy of this metal. Recently,
platinum-free catalysts for the oxygen reduction reaction have attracted enor-
mous interest as an alternative to platinum-based catalysts. These mainly
include three kinds of materials: organometallic complexes, nitrogen-doped
carbon-supported metal ions, and transition metal oxides and chalcogenides
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