100
Y. Li et al.
In order to achieve a reduction in the amount of Pt, there are currently several
ways to (1) increase the utilization of catalyst and the number of active sites. The
method generally improves the utilization of the catalyst by reducing the particle size
of the Pt catalysts and increasing the Pt dispersion, thereby greatly increasing the
ratio of surface atoms to bulk atoms [129–131]; (2) increasing the intrinsic activity
of the catalyst, which is usually alloyed by introducing the second element or more
elements (PtM/N (M/N = Pd, Fe, Co, Ni, Cu, Cr, Y, etc.) to form alloys [132–
137]; (3) synergistic effect, introduce a second substance or load Pt on other carriers
that can synergize with Pt, such as loading Pt on some nano-carbides, transition
metal oxides, graphenes, etc.; there is a certain electronic effect between Pt and its
carrier, which produces a synergistic effect, thereby increasing the ORR activity
of Pt [138–140]; (4) constructing special nanostructures, due to nano-scale effects,
special nano-structures will change in their electronic structure and properties, which
will be beneficial to the improvement of ORR catalytic activity, such as nanowires,
nanotubes, nanoframes, nanocages, nanodenrites, and polyhedrons with high crystal
face index [141–143]. In the actual synthesis of high-performance Pt-based ORR
catalysts, one or more of the above-mentioned pathways are usually included. For
example, the prepared PtFe nanowires belong to both the alloying of pathway 2 and
the special nanostructure of pathway 4.
4.2.1 Pt Atom Cluster
For spherical particles, the smaller the particle size, the larger the specific surface area
and the more atoms are exposed to the outside. For example, the size of Pt particles
is reduced from 15 nm to about 3 nm, and the electrochemical active area (ECSA) is
increased from 5 to 80 m
2 g
−1
Pt , and the ratio of the number of surface atoms to the
total number of atoms is increased from 3 to 13%. This greatly increases the effective
utilization of precious metal Pt, which is called the size effect of Pt nanoparticles.
JOO et al. [144] prepared Pt nanoparticles with sizes of 2.7 to 6.7 nm and loading
of 60wt% on ordered mesoporous carbon (OMC) by continuous impregnation reduction. The size of the particles was reduced by temperature and frequency to control.
Their experimental results show that the ORR performance of 3.3 nm Pt is best
in 0.1 M HClO 4 medium, as shown in Fig. 4.8. This is consistent with the results
reported in some other literatures [145, 146]. The case where the ORR activity of
Pt with size of below 3 nm decreases as the Pt particle becomes smaller is called
a “negative particle size effect”. This effect can be attributed to the following three
possible reasons: (1) The orientation of the crystal plane orientation of the surface
of Pt nanoparticles below 3 nm varies with the decrease of the particle size, while
the ORR activity of each crystal plane of Pt is different: Pt (100) < Pt(111) < Pt(110)
[147, 148]; (2) The crystallinity of Pt particles decreases with the decrease of crystal
grains, and the coordination of Pt atoms in the surface layer changes greatly; (3)
Inhibition of anions adsorbed on different crystal plane orientations and the crystal
plane orientation of the surface of the Pt nanoparticle vary with the particle size.
Y. Li et al.
In order to achieve a reduction in the amount of Pt, there are currently several
ways to (1) increase the utilization of catalyst and the number of active sites. The
method generally improves the utilization of the catalyst by reducing the particle size
of the Pt catalysts and increasing the Pt dispersion, thereby greatly increasing the
ratio of surface atoms to bulk atoms [129–131]; (2) increasing the intrinsic activity
of the catalyst, which is usually alloyed by introducing the second element or more
elements (PtM/N (M/N = Pd, Fe, Co, Ni, Cu, Cr, Y, etc.) to form alloys [132–
137]; (3) synergistic effect, introduce a second substance or load Pt on other carriers
that can synergize with Pt, such as loading Pt on some nano-carbides, transition
metal oxides, graphenes, etc.; there is a certain electronic effect between Pt and its
carrier, which produces a synergistic effect, thereby increasing the ORR activity
of Pt [138–140]; (4) constructing special nanostructures, due to nano-scale effects,
special nano-structures will change in their electronic structure and properties, which
will be beneficial to the improvement of ORR catalytic activity, such as nanowires,
nanotubes, nanoframes, nanocages, nanodenrites, and polyhedrons with high crystal
face index [141–143]. In the actual synthesis of high-performance Pt-based ORR
catalysts, one or more of the above-mentioned pathways are usually included. For
example, the prepared PtFe nanowires belong to both the alloying of pathway 2 and
the special nanostructure of pathway 4.
4.2.1 Pt Atom Cluster
For spherical particles, the smaller the particle size, the larger the specific surface area
and the more atoms are exposed to the outside. For example, the size of Pt particles
is reduced from 15 nm to about 3 nm, and the electrochemical active area (ECSA) is
increased from 5 to 80 m
2 g
−1
Pt , and the ratio of the number of surface atoms to the
total number of atoms is increased from 3 to 13%. This greatly increases the effective
utilization of precious metal Pt, which is called the size effect of Pt nanoparticles.
JOO et al. [144] prepared Pt nanoparticles with sizes of 2.7 to 6.7 nm and loading
of 60wt% on ordered mesoporous carbon (OMC) by continuous impregnation reduction. The size of the particles was reduced by temperature and frequency to control.
Their experimental results show that the ORR performance of 3.3 nm Pt is best
in 0.1 M HClO 4 medium, as shown in Fig. 4.8. This is consistent with the results
reported in some other literatures [145, 146]. The case where the ORR activity of
Pt with size of below 3 nm decreases as the Pt particle becomes smaller is called
a “negative particle size effect”. This effect can be attributed to the following three
possible reasons: (1) The orientation of the crystal plane orientation of the surface
of Pt nanoparticles below 3 nm varies with the decrease of the particle size, while
the ORR activity of each crystal plane of Pt is different: Pt (100) < Pt(111) < Pt(110)
[147, 148]; (2) The crystallinity of Pt particles decreases with the decrease of crystal
grains, and the coordination of Pt atoms in the surface layer changes greatly; (3)
Inhibition of anions adsorbed on different crystal plane orientations and the crystal
plane orientation of the surface of the Pt nanoparticle vary with the particle size.