111
ORR-active surface area and the significant savings of noble metals brought about
by the hollow structure accordingly enhanced up to 0.57 A/mg of the total metal
mass activity of the electrocatalyst for the ORR.
The Effects of Facets and Shapes Atoms at low-coordination sites, that is edges,
kinks, and defects, have been shown to have a stronger binding with OH than those
at 2D terrace sites and thus inhibit the oxygen reduction [24]. Those nonregistered
atoms are also prone to stronger binding with adsorbates and thus prone to
dissolution [25]. On the other hand, it has been shown that nanoparticles with a
higher ratio of (111) facets have higher activity for the oxygen reduction reaction.
Therefore, a desired nanoparticle core for Pt ML should possess a high ratio of atoms
at (111) facets and less atoms at low-coordination sites. The following paragraphs
show three examples to achieve this goal.
Reduction of Low-Coordination Sites The removal of low-coordination sites via
Br-treatment on Pd/C core engenders nanoparticles with a smooth surface and
increased content of (111) facets. As a result, 1.5-fold enhancement of the ORR
activity is achieved on Pt ML catalyst with such core [3].
Pd Nanowire Cores As it is generally observed, Pd nanowires (NW) have smooth
surfaces with fewer low-coordination sites and edges and less surface imperfections
than nanoparticles. Such surfaces are suitable for the ORR and make a good support
for a Pt ML since OH adsorption on them is shifted positively with reduced coverage.
The power density of this catalyst is close to 1 W/cm
2
with only 40 μgPt/cm
2
(0.1 mg/cm
2
PGM) content, which compares favorably with reported data for other
catalysts [26, 30].
Table 8.1 displays a comparison of activities of several Pt ML electrocatalysts having different cores from the above rational designs. A considerable difference
between some core-shell couples indicates a remarkable possibility for tuning the
activity of these catalysts and the flexibility of this approach.
8.1.3.1 Single Metal Nanoparticles Cores
Single metal nanoparticle cores can be successively used to support Pt monolayer
catalysts. As shown with the data from single-crystal extended surfaces and
nanoparticles given in the preceding text, Pd is the best support for a Pt monolayer.
Slightly contracted Pt(111) facets are among the most active surfaces for the
ORR. That was motivation to synthesize tetrahedral Pd (PdTH) nanocrystals with
cleaned surfaces. Considering the specific requirements of the ORR, their facetspecific electrochemical properties make PdTH a new, interesting support for Pt ML
electrocatalysts. The production of active facets can also be achieved by using
nanocrystals with well-defined facets as substrates. The concave Pd tetrahedron
(TH Pd) is a good example, which has a small number of low-coordination sites and
8.1 Oxygen Reduction Reaction (ORR)
ORR-active surface area and the significant savings of noble metals brought about
by the hollow structure accordingly enhanced up to 0.57 A/mg of the total metal
mass activity of the electrocatalyst for the ORR.
The Effects of Facets and Shapes Atoms at low-coordination sites, that is edges,
kinks, and defects, have been shown to have a stronger binding with OH than those
at 2D terrace sites and thus inhibit the oxygen reduction [24]. Those nonregistered
atoms are also prone to stronger binding with adsorbates and thus prone to
dissolution [25]. On the other hand, it has been shown that nanoparticles with a
higher ratio of (111) facets have higher activity for the oxygen reduction reaction.
Therefore, a desired nanoparticle core for Pt ML should possess a high ratio of atoms
at (111) facets and less atoms at low-coordination sites. The following paragraphs
show three examples to achieve this goal.
Reduction of Low-Coordination Sites The removal of low-coordination sites via
Br-treatment on Pd/C core engenders nanoparticles with a smooth surface and
increased content of (111) facets. As a result, 1.5-fold enhancement of the ORR
activity is achieved on Pt ML catalyst with such core [3].
Pd Nanowire Cores As it is generally observed, Pd nanowires (NW) have smooth
surfaces with fewer low-coordination sites and edges and less surface imperfections
than nanoparticles. Such surfaces are suitable for the ORR and make a good support
for a Pt ML since OH adsorption on them is shifted positively with reduced coverage.
The power density of this catalyst is close to 1 W/cm
2
with only 40 μgPt/cm
2
(0.1 mg/cm
2
PGM) content, which compares favorably with reported data for other
catalysts [26, 30].
Table 8.1 displays a comparison of activities of several Pt ML electrocatalysts having different cores from the above rational designs. A considerable difference
between some core-shell couples indicates a remarkable possibility for tuning the
activity of these catalysts and the flexibility of this approach.
8.1.3.1 Single Metal Nanoparticles Cores
Single metal nanoparticle cores can be successively used to support Pt monolayer
catalysts. As shown with the data from single-crystal extended surfaces and
nanoparticles given in the preceding text, Pd is the best support for a Pt monolayer.
Slightly contracted Pt(111) facets are among the most active surfaces for the
ORR. That was motivation to synthesize tetrahedral Pd (PdTH) nanocrystals with
cleaned surfaces. Considering the specific requirements of the ORR, their facetspecific electrochemical properties make PdTH a new, interesting support for Pt ML
electrocatalysts. The production of active facets can also be achieved by using
nanocrystals with well-defined facets as substrates. The concave Pd tetrahedron
(TH Pd) is a good example, which has a small number of low-coordination sites and
8.1 Oxygen Reduction Reaction (ORR)