1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 315

mechanism, ruthenium provides oxygen-containing species, which could then
oxidize CO on the adjacent platinum sites at more negative potentials than pure
platinum. Due to the large surface area, various Pt-based bimetallic nanostructures
have been synthesized and applied to both anode and cathode catalysts. For
example, Xia and co-workers [ 25 ] synthesized Pd-Pt bimetallic nanodendrites,
which exhibited enhanced catalytic activity for oxygen electro-reduction. Recently,
Stamenkovic et al. [ 12 ] demonstrated that extended single crystal surfaces of
Pt 3 Ni(111) exhibit an enhanced ORR activity that is 10-fold higher than the cor-
responding Pt(111) surface and 90-fold more active than the current state-of-the-art
Pt/C catalysts for PEMFCs. Such a remarkable activity was attributed to the unusual
electronic structure (d-band center position) and arrangement of surface atoms in
near-surface region. For the anode reaction, Yang et al. [ 10 ] have found that the
(100) facet-terminated Pt 3 Co nanocubes are much more active than the Pt nano-
cubes due to the weaker and slower CO adsorption.
In recent years, to further reduce the loading of Pt, intensive studies have
focused on low-Pt or non-Pt electrocatalysts [ 26 - 37 ]. Among the studied non-
platinum electrocatalysts, Pd-based nanomaterials have become hot research
topics as electrocatalysts in fuel cells because of their similar intrinsic properties to
platinum, such as the valence band structure and lattice parameters [ 38 , 39 ]. More
significantly, compared to Pt, Pd is cheaper and exhibits higher electrochemical
stability. Recent research progress in Pd-based catalysts has revealed that Pd can
catalyze the oxidation of formic acid and alcohols at the anode of polymer elec-
trolyte membrane fuel cells (PEMFCs) with greater tolerance to CO than Pt cat-
alysts and comparable activity toward the cathode oxygen reduction [ 40 - 50 ].
For catalysts on nanoscale, their catalytic efficiency, selectivity and reaction
durability are highly dependent on the size, shape, composition, and surface
structure [ 8 , 26 , 41 , 51 - 54 ]. Similarly to Pt-based nanoalloys, Pd-based alloy
nanomaterials have also shown enhanced electrocatalytic activities compared to
pure Pd catalyst due to the synergistic effect or the modulation effect [ 55 , 56 ].
Recent investigations have found that alloying Pd with some transition metals,
such as Au [ 57 ], Ag [ 58 ], Fe [ 59 ], Co [ 60 ], Ti [ 55 , 61 ], Ni [ 62 ], Sn [ 63 ], etc., is an
effective way to improve the catalytic activity of Pd metal. At the same time, some
of Pd alloys exhibited comparable activity and stability to those of Pt catalysts.
Among various Pd or Pd-based nanomaterials, one-dimensional (1D) nano-
structured electrocatalysts, such as nanowires (NWs) [ 64 - 72 ], nanorods (NRs)
[ 44 , 73 ], nanoleaves (NLs) [ 74 ], and nanotubes (NTs) [ 75 - 80 ], have attracted more
and more attention in recent years due to their unique structures and high surface
area. From a structural perspective, 1D Pd nanomaterials possess largely pristine
surfaces with long segments of crystalline planes, as compared with their corre-
sponding 0D morphologies. Additionally, the anisotropic growth of 1D structured
materials typically results in the preferential surface display of low energy crystal
facets in order to minimize the surface energy of the systems [ 81 ]. In the previous
studies on 1D Pd nanostructures, high resolution transmission electron microscopy
(HRTEM) and selected-area-electron diffraction patterns (SAED) showed that the
surfaces of the Pd nanowires are enclosed by {111} and {200} planes, suggesting