4 Catalyst Materials for Oxygen Reduction Reaction
117
reach 7.8 mA/cm
2 @ 0.9 V and 4.3 A/mg Pt @0.9 V, respectively. Different from
the traditional view, the density functional theory analysis thinks that the moderate
expansion stress on the Pt(110) crystal plane is helpful to reduce the interaction
between Pt-O and achieve a better value.
Compared with pure Pt nanoparticle catalysts, the M@Pt core–shell catalysts
have improved Pt utilization obviously. However, the enhancement of ORR performance of the M@Pt core–shell electrocatalyst in structural design is not limited to the
improvement of the utilization of Pt atoms. This core–shell structure also makes an
important contribution to the improvement of ORR activity in the following aspects:
(1) The formation of the core–shell double-layer structure can affect and change the
electronic structure and properties of the outer Pt layer, and it can even adjust the
electronic structure and properties of the Pt atoms on the surface by adjusting the
change of the thickness of inner transition metal and Pt layer; (2) Surface modification through surface functionalization can increase the stability and dispersibility of
catalyst particles; (3) Further assembly of core–shell nanoparticles will also generate
new physical and chemical properties.
The M@Pt core–shell electrocatalyst can achieve higher ORR activity and
stability with less Pt, which greatly reduces the cost of PEMFC. The optimized
design of the M@Pt core–shell ORR electrocatalyst can furtherly accelerate the
commercialization process of PEMFC [202].
4.2.5 Pt-Based Nanoframes and Nanocages ORR
Electrocatalyst
ORR is a surface-catalyzed electrochemical process, so only Pt atoms on the surface
can participate in the catalytic reaction, while Pt atoms inside the particles cannot
directly participate in the ORR catalytic process [188]. Although the utilization of
Pt nanoparticles can be increased through the core–shell structure, it is still the goal
of researchers to further improve the utilization of internal Pt atoms. Therefore,
researchers have proposed a method to etch the surface and core of nanomaterials
to obtain a framework-only nanoframe structure or nanocage structure material. By
increasing the specific surface area of the catalyst, the exposure and utilization of
internal Pt atoms can be increased, and increase the catalytic performance of oxygen
reduction of nanomaterials.
Peidong Yang et al. [204] reported for the first time the synthesis of Pt-Ni nanocrystals with a fully transparent framework structure. First, PtNi 3 nanocrystals with a
rhombic dodecahedron structure were synthesized. Dispersed in a non-polar solvent
for two weeks, such as n-hexane and chloroform, these nanocrystals are transformed
into Pt 3 Ni nanoframework, and its symmetry and size have not been changed. The
open frame structure of Pt 3 Ni nanoframes and Pt(111) surface skin make the active
site fully exposed. Therefore, in the oxygen reduction reaction catalysis, the mass
activity of the Pt 3 Ni nanoframe is as high as 5.7 A/mg Pt @0.9 V, which is 36 times
117
reach 7.8 mA/cm
2 @ 0.9 V and 4.3 A/mg Pt @0.9 V, respectively. Different from
the traditional view, the density functional theory analysis thinks that the moderate
expansion stress on the Pt(110) crystal plane is helpful to reduce the interaction
between Pt-O and achieve a better value.
Compared with pure Pt nanoparticle catalysts, the M@Pt core–shell catalysts
have improved Pt utilization obviously. However, the enhancement of ORR performance of the M@Pt core–shell electrocatalyst in structural design is not limited to the
improvement of the utilization of Pt atoms. This core–shell structure also makes an
important contribution to the improvement of ORR activity in the following aspects:
(1) The formation of the core–shell double-layer structure can affect and change the
electronic structure and properties of the outer Pt layer, and it can even adjust the
electronic structure and properties of the Pt atoms on the surface by adjusting the
change of the thickness of inner transition metal and Pt layer; (2) Surface modification through surface functionalization can increase the stability and dispersibility of
catalyst particles; (3) Further assembly of core–shell nanoparticles will also generate
new physical and chemical properties.
The M@Pt core–shell electrocatalyst can achieve higher ORR activity and
stability with less Pt, which greatly reduces the cost of PEMFC. The optimized
design of the M@Pt core–shell ORR electrocatalyst can furtherly accelerate the
commercialization process of PEMFC [202].
4.2.5 Pt-Based Nanoframes and Nanocages ORR
Electrocatalyst
ORR is a surface-catalyzed electrochemical process, so only Pt atoms on the surface
can participate in the catalytic reaction, while Pt atoms inside the particles cannot
directly participate in the ORR catalytic process [188]. Although the utilization of
Pt nanoparticles can be increased through the core–shell structure, it is still the goal
of researchers to further improve the utilization of internal Pt atoms. Therefore,
researchers have proposed a method to etch the surface and core of nanomaterials
to obtain a framework-only nanoframe structure or nanocage structure material. By
increasing the specific surface area of the catalyst, the exposure and utilization of
internal Pt atoms can be increased, and increase the catalytic performance of oxygen
reduction of nanomaterials.
Peidong Yang et al. [204] reported for the first time the synthesis of Pt-Ni nanocrystals with a fully transparent framework structure. First, PtNi 3 nanocrystals with a
rhombic dodecahedron structure were synthesized. Dispersed in a non-polar solvent
for two weeks, such as n-hexane and chloroform, these nanocrystals are transformed
into Pt 3 Ni nanoframework, and its symmetry and size have not been changed. The
open frame structure of Pt 3 Ni nanoframes and Pt(111) surface skin make the active
site fully exposed. Therefore, in the oxygen reduction reaction catalysis, the mass
activity of the Pt 3 Ni nanoframe is as high as 5.7 A/mg Pt @0.9 V, which is 36 times