115
Importantly, as-prepared Pd 9 Au NWs maintain significantly enhanced oxygen
reduction reaction activity (0.40 mA/cm
2
), as compared with elemental Pd NW/C
(0.12 mA/cm
2
) and Pt nanoparticles (NP)/C (0.20 mA/cm
2
), respectively. A volcanotype composition dependence is observed in the ORR activity of the Pt ∼ Pd 1 − x Au x
NWs as the Au content is increased from 0% to 30% with the activity of the
Pt ∼ Pd 9 Au NWs (0.98 mA/cm
2
, 2.54 A/mg Pt ), representing the optimum performance. The platinum group metal (PGM) activity of the ultrathin 2 nm NWs (0.64
A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs
(0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance. These shortfalls in the
catalytic performance of Pt NP/C are often accompanied by corresponding deficits
in the stability and durability of these catalysts. Thus, a significant amount of attention has been paid to manipulate the morphology, chemical composition, and structural motif (e.g., core−shell hierarchies) of platinum-based catalysts.
The high-resolution transmission electron microscopy (HRTEM) images of the
samples are presented as insets in Fig. 8.9. It is seen that all deposited Pd
nanostructures are single crystalline. Lattice fringes with interplanar spacing of
0.22 nm, ascribed to (111) planes of the face-centered cubic (fcc) structure of Pd,
are clearly seen in the HRTEM images. The fast Fourier transform pattern of a
selected area from the single nanowire indicated that most of the Pd atoms are
ordered in (111) facets. It also confirms that the surface fraction of Pd atoms on the
(111) facets is predominant in the sample of interest.
An optimum performance has been observed in the case of the ultrathin
Pt ∼ Pd 9 Au NWs, which display outstanding specific, platinum, and PGM mass
activities amounting 0.98 mA/cm
2
, 2.54 A/mgPt, and 0.64 A/mg, respectively
(Fig. 8.10). The observed enhancement can most likely be attributed to the unique
composition-dependent structural and electronic properties of the Pd 1-x Au x alloytype nanostructures but also to their ultrathin diameters.
8.1.3.3 Hollow Cores Supported Pt Monolayer Catalysts
In order to further reduce the Pt content, Pt ML hollow Pd or Pd 20 Au nanoparticle
electrocatalysts were synthesized. A hollow architecture of the Pd 20 Au particles was
achieved by a delicate balance between galvanic displacement of Ni nanoparticles
and the Kirkendall effect in controlling reaction kinetics. The enhanced Pt mass and
total metal mass activities of Pt ML catalysts supported on hollow cores to the smooth
surface morphology of the Pt ML catalyst supported on hollow Pd-based cores is evident in the TEM images.
High-activity electrocatalysts for the oxygen reduction reaction comprising a Pt
monolayer shell were formed on compact hollow nanoparticles. Pulseelectrodeposited Ni nanoparticles were replaced galvanically by Pd and Pd–Au ions
to obtain corresponding hollow nanoparticles. Pt monolayer catalysts supported on
such hollow cores exhibited total metal mass activities ranging from 0.41 to 0.57 A
mg
−1
, doubling that of 0.25 A mg
−1
for Pt monolayer catalysts on solid Pd cores.
8.1 Oxygen Reduction Reaction (ORR)
Importantly, as-prepared Pd 9 Au NWs maintain significantly enhanced oxygen
reduction reaction activity (0.40 mA/cm
2
), as compared with elemental Pd NW/C
(0.12 mA/cm
2
) and Pt nanoparticles (NP)/C (0.20 mA/cm
2
), respectively. A volcanotype composition dependence is observed in the ORR activity of the Pt ∼ Pd 1 − x Au x
NWs as the Au content is increased from 0% to 30% with the activity of the
Pt ∼ Pd 9 Au NWs (0.98 mA/cm
2
, 2.54 A/mg Pt ), representing the optimum performance. The platinum group metal (PGM) activity of the ultrathin 2 nm NWs (0.64
A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs
(0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance. These shortfalls in the
catalytic performance of Pt NP/C are often accompanied by corresponding deficits
in the stability and durability of these catalysts. Thus, a significant amount of attention has been paid to manipulate the morphology, chemical composition, and structural motif (e.g., core−shell hierarchies) of platinum-based catalysts.
The high-resolution transmission electron microscopy (HRTEM) images of the
samples are presented as insets in Fig. 8.9. It is seen that all deposited Pd
nanostructures are single crystalline. Lattice fringes with interplanar spacing of
0.22 nm, ascribed to (111) planes of the face-centered cubic (fcc) structure of Pd,
are clearly seen in the HRTEM images. The fast Fourier transform pattern of a
selected area from the single nanowire indicated that most of the Pd atoms are
ordered in (111) facets. It also confirms that the surface fraction of Pd atoms on the
(111) facets is predominant in the sample of interest.
An optimum performance has been observed in the case of the ultrathin
Pt ∼ Pd 9 Au NWs, which display outstanding specific, platinum, and PGM mass
activities amounting 0.98 mA/cm
2
, 2.54 A/mgPt, and 0.64 A/mg, respectively
(Fig. 8.10). The observed enhancement can most likely be attributed to the unique
composition-dependent structural and electronic properties of the Pd 1-x Au x alloytype nanostructures but also to their ultrathin diameters.
8.1.3.3 Hollow Cores Supported Pt Monolayer Catalysts
In order to further reduce the Pt content, Pt ML hollow Pd or Pd 20 Au nanoparticle
electrocatalysts were synthesized. A hollow architecture of the Pd 20 Au particles was
achieved by a delicate balance between galvanic displacement of Ni nanoparticles
and the Kirkendall effect in controlling reaction kinetics. The enhanced Pt mass and
total metal mass activities of Pt ML catalysts supported on hollow cores to the smooth
surface morphology of the Pt ML catalyst supported on hollow Pd-based cores is evident in the TEM images.
High-activity electrocatalysts for the oxygen reduction reaction comprising a Pt
monolayer shell were formed on compact hollow nanoparticles. Pulseelectrodeposited Ni nanoparticles were replaced galvanically by Pd and Pd–Au ions
to obtain corresponding hollow nanoparticles. Pt monolayer catalysts supported on
such hollow cores exhibited total metal mass activities ranging from 0.41 to 0.57 A
mg
−1
, doubling that of 0.25 A mg
−1
for Pt monolayer catalysts on solid Pd cores.
8.1 Oxygen Reduction Reaction (ORR)