1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 326

3 One-Dimensional Pd-Based Nanomaterials as Effective
Anode Electrocatalysts
3.1 Electrocatalysts for Alcohol Oxidation
Although Pt-based alloy catalysts, especially nanostructured PtRu, have been
widely used as anode electrocatalysts in DMFC, the effectiveness and wide
application of these catalysts are still largely limited due to their relatively low
electrochemical stability and low tolerance to CO poisoning [ 99 ]. To address the
disadvantages suffered from Pt-based catalysts, recent efforts have been devoted
to the development of more efficient catalysts with high stability and high
CO-tolerance. Recently, Pd-based nanomaterials have been found to exhibit high
CO tolerance and superior electrocatalytic activity toward CO oxidation [ 38 , 100 ],
methanol oxidation [ 101 , 102 ], and ethanol oxidation [ 69 ]. However, there are still
obstacles in utilizing Pd nanoparticles because they often experience irreversible
aggregation during electrocatalytic cycles, leading to a significant loss of catalytic
activity and durability. Thus, 1D Pd-based nanomaterials have emerged as a
potential electrocatalysts owing to their unique anisotropic structure. In particular,
the ordered surface structure of one-dimensional nanostructured catalysts could
affect the electrochemical and electrocatalytic properties. Among various synthetic
approaches for one-dimensional nanostructure arrays, an anodic aluminum oxide
(AAO) membrane-based method has received increasing attention because its
uniform and reproducible porous structure as an ideal template can produce highly
ordered nanotube or nanowire-type arrays.
Cheng et al. [ 71 ] prepared highly ordered Pd nanowire arrays (NWAs) using a
porous AAO template by pulse electrodeposition method. It can be clearly observed
from Fig. 8 b-d that the Pd nanowires templated by AAO are highly ordered with
uniform diameters of about 50 nm and length of 850 nm. Meanwhile, the
as-prepared Pd NWAs retain the size and near cylinder shape of the pores of the AAO
template as shown in Fig. 8 a. More interestingly, the as-synthesized Pd NWAs
exhibit good electrocatalytic activity toward isopropanol and methanol oxidation at
room temperature. By using a similar AAO template-electrodepositon method and a
subsequently magnetron sputtering techniques, Cheng's group also prepared highly
ordered Pd/Pt core-shell nanowire arrays (Pd/Pt NWAs) [ 72 ]. In the method, Pd
NWAs were first synthesized in AAO template. After removing the template in
NaOH solution, Pt film was then coated on the surface of Pd NWAs through mag-
netron sputtering. Figure 9 shows the SEM images of the as-synthesized Pd and
Pd/Pt NWAs. By using AAO as template, highly ordered Pd NWAs with smooth
surface and uniform diameter and length were produced. However, after magnetron
sputtering of Pt, the surface of Pd nanowires was covered by a thin layer (*1.7 nm)
of cotton-like aggregated Pt nanoparticles. Nevertheless, the microstructure and
morphology of Pt coated Pd NWAs is more or less the same as original Pd NWA
core. Figure 10 a shows the CVs of different electrodes in a nitrogen- saturated 0.5 M
H 2 SO 4 solution without methanol. It was found that the electrochemical active area