Environmental Engineering Reference
In-Depth Information
Hydrophobic carbon layer
Integrated
pulse generator
e -
Cathode
Anode
Pt-Co deposits
Plating bath
Catalyst
particle
Carbon cloth
Hydrophilic carbon layer
Figure 28.4
Schematic diagram of pulse electrodeposition method. reprinted with permission from ref. [52]. © Elsevier.
nonprecious-metal catalysts with high activity and practical durability for the methanol oxidation and oxygen reduction
reactions in PEFCs. Attention was particularly given to the oxygen reduction reaction since this appears to be responsible for
major voltage losses within the cell. For this reaction, some promising Pt-free catalsysts were studied intensively: nonprecious-
metal chalcogenides [55], transition metal carbides [56], nitrides [57], and organometallic components [58]. The review by
Othman and Dicks [59] outlines classes of nonprecious-metal systems that have been investigated over the past 10 years.
Although CNTs are used as supports for a nanoparticle metal catalyst, it was recently demonstrated that surface-modified
CNTs and graphene show electrocatalytic activity for the oxygen reduction reaction in alkaline and acidic media [5]. Particularly,
nitrogen-doped graphene that was synthesized by Qu et al. [60] by chemical vapor deposition of methane in the presence of
ammonia acts as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance
to crossover effect than platinum catalysts. As demonstrated by Wang et al. [61], some polyelectrolytes have a strong electron-
accepting ability to withdraw electrons from carbon atoms in the CNT plane to induce the net positive charge, facilitating the
catalytic activity of the nitrogen-free CNTs.
28.4
NaNomaterials as Catalyst suPPort
The performance and durability of fuel cells seriously depend on catalyst support materials [5]. The nature of the support is
most important, since it determines the dispersion and stability of the metal crystallites, the electronic properties of the metal,
and mass transfer and ohmic resistances of the catalyst layer [62]. Current catalysts for PEFCs are Pt or Pt alloys supported
on carbon black in the form of highly dispersed nanoparticles. recent studies, however, show that Pt-based nanoparticles sup-
ported on other forms of carbon, such as carbon nanofibers, CNTs, and graphene, display higher electrocatalytic activities [63].
Although the underlying mechanisms for activity enhancement are still not well understood, it was suspected that the surface
structures and electronic properties of CNTs may well be responsible. Figure 28.5 shows the transmission electron microscopy
(TEm) images of highly dispersed Pt nanoparticles with very small and uniform size distribution (2-3 nm) that were obtained
on carbon nanofibers and CNT supports [64].
CNTs have high specific surface area, corrosion resistance, good electronic conductivity, and high stability [65]. The main methods
for the preparation of Pt-based catalysts supported on CNTs involve the reduction of Pt(ii) salts by microemulsion, chemical vapor
deposition, and electrochemical deposition, among others. A series of green techniques for synthesizing CNT-supported platinum
nanoparticles for methanol fuel cell applications have been reported by Shimizu et al. [66]. These techniques utilize either the super-
critical fluid carbon dioxide or water as a medium for depositing platinum nanoparticles on surfaces of multiwalled or single-walled
CNTs. An improved catalyst support for DmFCs on the base of polyphosphazene-coated CNTs was prepared by Qian et al. [67].
Graphene has a unique advantage as a catalyst support material compared to other carbon nanomaterials such as carbon
black and CNTs owing to much larger surface-to-mass ratio. An overview of graphene nanosheets used as supports for fuel cell
catalysts is presented by Antolini [68]. A systematic and comparative study of the effects of structure, composition, and carbon
support properties on the electrocatalytic activity and stability of graphene-supported Pt-Ni alloy nanocatalysts for methanol
Search WWH ::




Custom Search