Environmental Engineering Reference
In-Depth Information
to fuel production, which is typically located in rural areas. in this connection, coal-based hydrogen would likely lead to
increased emissions compared to conventional vehicles using petroleum-based fuels [43].
28.3
NaNomaterials as Catalysts
One of the most important challenges for the ultimate commercialization of fuel cells is the preparation of active, robust, and
low-cost catalysts [44]. The catalyst most frequently used for the electrochemical oxidation of hydrogen and methanol in the
PEFC system consists of carbon-supported Pt or multimetallic nanoparticles (Pt, Co, Ni, V, Fe, Cu, Pd, W, Ag, Au, etc.) [5].
Figure 28.3 illustrates possible processes of surface adsorption and reactivity for the methanol oxidation reaction in the case of
a gold-platinum alloy catalyst in DmFCs [45, 46]. The catalytic reaction of Pt in the alloy occurs via a combination of reaction
steps, such as the adsorption of Ch
3
Oh on Pt followed by dehydrogenation, the formation of intermediate CO
ad
/Pt, the transfer
of CO
ad
/Pt to neighboring Au-atop sites forming CO
ad
/Au, the formation of Oh
ad
/Au or surface oxides on gold, and the reactions
of Pt-CO
ad
+ Au-Oh
ad
and Au-CO
ad
+ Au-Oh
ad
toward the final product (CO
−3
) [45].
Several techniques are used to depose uniform catalyst spherical nanoparticles on the high-surface carbon support. These
techniques can be subdivided into chemical, electrochemical, and physical ones. Chemical methods use colloidal, impregnation-
reduction, chemical vapor deposition, and reverse micelles processes. A simple, environment-friendly colloidal-precipitation
method was successfully employed by yao et al. [47] to prepare carbon-supported platinum nanoparticles with a narrow size
distribution. in this method, (Nh
4
)
2
WO
4
was used to react with h
2
PtCl
6
, which forms (Nh
4
)
2
PtCl
6
and h
2
WO
4
simultaneously.
The precipitation of (Nh
4
)
2
PtCl
6
and the colloidal of h
2
WO
4
can protect the formation of the Pt nanoparticles. in the impregna-
tion method, the metal precursors impregnate the carbon support in aqueous and/or organic media followed by reduction using
a reducing agent such as organic alcohols [48]. Fuel cell electrode catalysts with improved electrochemical properties were
prepared by Kim and moon [49] by dispersing Pt nanoparticles onto CNTs using a chemical vapor deposition method. Pt
particles synthesized by this method have a relatively uniform size of approximately 1 nm, which is substantially smaller than
in the case of a commercial Pt/carbon black catalyst (≤4.5 nm) prepared by wet impregnation. A reverse micelle synthesis was
used by Cheney et al. [50] to improve the nanoparticle size uniformity of bimetallic Pt/Ni nanoparticles supported on γ-Al
2
O
3
.
The electrochemical method for Pt nanoparticle deposition on porous and high-surface carbon substrates such as carbon
black and CNTs is an alternative way to prepare gas diffusion electrodes for PEFCs that has the ability to localize the metal
particles on the surface of the electrode and reduce the thickness of the catalysis layer [51]. The deposition procedure can be
carried out by using either a potentiostatic or a galvanostatic technique, which involves direct and pulse techniques.
Electrodeposition via the galvanostatic pulse technique is considered convenient to improve the current distribution. Therefore,
it is easy to control the particle size and composition of the alloy simply by varying experimental parameters such as on/off time
and peak current density [52]. Figure 28.4 illustrates the schematic diagram of the pulse electrodeposition method.
The physical sputter deposition technique allows obtaining ultralow levels of catalyst loading (on the order of 5 μgPt cm
−2
contrary to a commercial catalyst having a Pt loading of 0.3 mgPt cm
−2
), localization of the catalyst on the uppermost surface
of the carbon support, and increasing the activity for methanol oxidation compared to commercial catalysts [53]. Advantages
of the sputter deposition compared with chemical deposition are easy preparation of the catalyst (absence of reducing and
deflocculating agents, no heat treatment in hydrogen) and low Pt loading, allowing a cost reduction in the product together with
easy industrial transfer of the process. it should be noted that the sputter deposition method is already a commercial technique
for thin film deposition for other industrial applications [54].
recently, numerous efforts were made to replace Pt-based catalysts in PEFCs due to the high cost of the metal [5]. The
progress in nanotechnology and material science has raised high expectations in the design and synthesis of alternative new
OO
C
C
H
2
O
-e
-e
CO
=
CH
3
-OH
(CH
3
-OH)
ads
/Pt
(OH)
ads
/Au
Pt
Au
AuPt/C
Figure 28.3
Schematic illustration of the electrocatalytic oxidation of methanol on an AuPt/C catalyst in alkaline electrolyte. reprinted
with permission from ref. [45]. © Elsevier.
