4 Catalyst Materials for Oxygen Reduction Reaction
105
with the Pt(111) surface, the local coverage of H upd and OH ad ( Hupd and OHad ) on
the Pt 3 Ni(111) surface has decreased by 50%, which is also reflected in the decrease
in the d-band center on the Pt surface (The d-band center on the Pt 3 Ni(111) surface
is 0.34 eV lower than the Pt(111) surface). A similar situation also occurred on the
surfaces of other two crystal planes. The Hupd on the surface of Pt 3 Ni(100) was more
than 25% lower than that of Pt(100), and the d-band center decreased by 0.24 eV.
The Hupd on the surface of Pt 3 Ni(110) was more than 20% lower than Pt (110), and
the d-band center decreased by 0.16 eV. Figure 4.12 shows a comparison of the ORR
activities of different single crystal planes of Pt 3 Ni and Pt. For Pt 3 Ni, the order of
activity is Pt 3 Ni(100) < Pt 3 Ni(110) < < < Pt 3 Ni(111), where the activity of the Pt3Ni
(111) plane is orders of magnitude higher than the other two single crystal planes.
For Pt, the order of ORR activity of different crystal planes is Pt(100) Pt(111) <
Pt(110). The ORR activity on the surface of Pt 3 Ni(111) is 10 times higher than that
of Pt (111) and 90 times higher than that of commercial Pt/C surface. This is related
to its different electronic structure (d-band center) and an increase in active sites for
oxygen adsorption.
Although the theoretical and experimental research on Pt bimetallic alloy catalysts
has been very rich and in-depth, it is still difficult to predict which specific bimetallic
catalyst has the best ORR performance. The ORR performance of bimetallic catalysts
depends on two key factors: first, the formation of heteroatomic bonds will change
the electronic environment of Pt, and the change in Pt electronic structure are caused
by the change in Pt coordination; Second, the geometry structure of the bimetal
is different from Pt, for example, a change in the metal-to-metal bond length will
cause a strain effect, thereby changing the electronic structure of the metal by the
orbital overlap [156]. Strasser et al. [161] treated PtCu alloys with different Pt: Cu
components at high temperature (800 °C and 900 °C), and then dealloyed to obtain
an alloy with a thin Pt shell layer. They used K-edge X-ray emission spectra (XES)
and X-ray absorption spectra (XAS) to study the position and the specific atom
occupation state of O 2p and Pt 5d, to obtain the corresponding compressive strain,
as shown in Fig. 4.13. By testing its ORR performance, the relationship between
compressive strain and ORR activity of PtCu alloys with different components was
obtained. They used DFT to theoretically study the relationship between compressive
strain and ORR activity of PtCu alloys with different compositions, and obtained a
result similar to the volcanic map, but their experiments did not find the apex of the
volcanic map. Research in this area is still in its infancy, and a lot of research needs
to be followed up by researchers.
Although the ORR performance will increase to varying degrees when the second
metal alloy with Pt, adding elements which promote ORR performance significantly
such as Fe, Co, and Ni to the Pt lattice is not stable under acidic conditions when
running in PEMFC, and will gradually dissolve in the electrolyte during the catalysis
process, which will damage the proton exchange membrane [162, 163]. As mentioned
earlier, in the ORR reaction, the adsorption of the reactants on the Pt bimetallic surface
should not be too strong or too weak, and an appropriate amount of adsorption energy
is the most favorable for the occurrence of ORR. Figure 4.14 lists the relationship
between the adsorption energy, activity, and stability of the reactants for different
105
with the Pt(111) surface, the local coverage of H upd and OH ad ( Hupd and OHad ) on
the Pt 3 Ni(111) surface has decreased by 50%, which is also reflected in the decrease
in the d-band center on the Pt surface (The d-band center on the Pt 3 Ni(111) surface
is 0.34 eV lower than the Pt(111) surface). A similar situation also occurred on the
surfaces of other two crystal planes. The Hupd on the surface of Pt 3 Ni(100) was more
than 25% lower than that of Pt(100), and the d-band center decreased by 0.24 eV.
The Hupd on the surface of Pt 3 Ni(110) was more than 20% lower than Pt (110), and
the d-band center decreased by 0.16 eV. Figure 4.12 shows a comparison of the ORR
activities of different single crystal planes of Pt 3 Ni and Pt. For Pt 3 Ni, the order of
activity is Pt 3 Ni(100) < Pt 3 Ni(110) < < < Pt 3 Ni(111), where the activity of the Pt3Ni
(111) plane is orders of magnitude higher than the other two single crystal planes.
For Pt, the order of ORR activity of different crystal planes is Pt(100) Pt(111) <
Pt(110). The ORR activity on the surface of Pt 3 Ni(111) is 10 times higher than that
of Pt (111) and 90 times higher than that of commercial Pt/C surface. This is related
to its different electronic structure (d-band center) and an increase in active sites for
oxygen adsorption.
Although the theoretical and experimental research on Pt bimetallic alloy catalysts
has been very rich and in-depth, it is still difficult to predict which specific bimetallic
catalyst has the best ORR performance. The ORR performance of bimetallic catalysts
depends on two key factors: first, the formation of heteroatomic bonds will change
the electronic environment of Pt, and the change in Pt electronic structure are caused
by the change in Pt coordination; Second, the geometry structure of the bimetal
is different from Pt, for example, a change in the metal-to-metal bond length will
cause a strain effect, thereby changing the electronic structure of the metal by the
orbital overlap [156]. Strasser et al. [161] treated PtCu alloys with different Pt: Cu
components at high temperature (800 °C and 900 °C), and then dealloyed to obtain
an alloy with a thin Pt shell layer. They used K-edge X-ray emission spectra (XES)
and X-ray absorption spectra (XAS) to study the position and the specific atom
occupation state of O 2p and Pt 5d, to obtain the corresponding compressive strain,
as shown in Fig. 4.13. By testing its ORR performance, the relationship between
compressive strain and ORR activity of PtCu alloys with different components was
obtained. They used DFT to theoretically study the relationship between compressive
strain and ORR activity of PtCu alloys with different compositions, and obtained a
result similar to the volcanic map, but their experiments did not find the apex of the
volcanic map. Research in this area is still in its infancy, and a lot of research needs
to be followed up by researchers.
Although the ORR performance will increase to varying degrees when the second
metal alloy with Pt, adding elements which promote ORR performance significantly
such as Fe, Co, and Ni to the Pt lattice is not stable under acidic conditions when
running in PEMFC, and will gradually dissolve in the electrolyte during the catalysis
process, which will damage the proton exchange membrane [162, 163]. As mentioned
earlier, in the ORR reaction, the adsorption of the reactants on the Pt bimetallic surface
should not be too strong or too weak, and an appropriate amount of adsorption energy
is the most favorable for the occurrence of ORR. Figure 4.14 lists the relationship
between the adsorption energy, activity, and stability of the reactants for different