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Y. Li et al.
1250 K to obtain Pd 3 Fe (111) with 11% Fe on the surface; (4) Annealed at 900 K,
Pd 3 Fe with a surface Fe content of 11% was obtained; (5) Pd 3 Fe was annealed at
1000 K, then deposit 0.5 single-layer Pd atoms on it, and then annealed at 900 K
to obtain Pd ML /Pd 3 Fe (111) with a surface Pd content of 98%. Among them, when
annealed at 1250 K, Pd 3 Fe (111) has the highest ORR performance. Its half-wave
potential (E 1/2 ) is positively shifted by 27 mV from Pd ML /Pd 3 Fe (111), and is positively shifted by 62 mV than Pd (111). In the voltage range of 0.8–0.9 V (vs. RHE),
the kinetic current density of Pd 3 Fe (111) obtained by annealing at 1250 K is 2 to
3 times that of Pd ML / Pd 3 Fe (111), and is 5 to 8 times higher than the annealed Pd
(111). In PdFe alloys, the content and state of Fe on the catalyst surface play an
important role in its ORR performance. The presence of Fe on the surface will affect
the adsorption and dissociation of O 2 , and these two steps are critical steps in the
process of ORR, even rate-determining steps. Although the mechanism of heterogeneous metal incorporation and Pd-forming alloys to enhance ORR performance
is still controversial, it can be determined that heterogeneous metal incorporation
will change the electronic structural properties of Pd, resulting in different ORR
properties. Compared with Pd (111), the d electron center of Pd ML /Pd 3 Fe (111) is
reduced by 0.25 eV, which significantly reduces the binding energy between O and
OH and the catalyst surface, thereby promoting the removal of O and OH through
protonation 145]. The synergistic effect of the ORR enhancement of Pd 3 Fe (111)
with a Fe content of 11% obtained by annealing at 1250 K can be explained by the
surface oxygen overflow effect: O 2 adsorption and dissociation at the Fe site, and then
overflow to the Pd to be reduced. The calculated structure according to the density
functional theory shows that the first O of O 2 dissociated at the Fe position can easily
diffuse to the Pd position with an energy barrier of only 0.25 eV, while the second O
has a higher diffusion energy barrier to reach with 1.25 eV. This high energy barrier
will cause irreversible adsorption on Fe and cause blocking of Fe active sites, but it
will not affect the dissociation of O 2 at the corresponding Fe position, because its
energy barrier is only 0.4 eV. Furthermore, the next O diffusion at the Fe position
is greatly promoted, and the energy barrier is reduced from 1.25 to 0.75 eV. This
characteristic keeps the dissociation of O 2 and the diffusion of O in equilibrium, so
that the reaction kinetics of ORR are maintained at a very high rate.
Liu et al. [170] prepared carbon-supported PdCo and PdNi nanoparticles by
NaBH4 reduction method, and further heat-treated under H 2 atmosphere to obtain
carbon-supported PdCo and PdNi nanoparticles with different grain sizes. In 0.1 M
HClO 4 solution, the ORR activity of carbon-supported PdCo and PdNi nanoparticles
obtained after heat treatment can exceed that of Pt/C catalyst. The ORR activity is
related to the lattice constant. As shown in Fig. 4.28, the lattice constant of the alloyed
Pd can be changed between 0.3802–0.3948 nm. The corresponding specific activity
decreases as the lattice constant increases, while the mass activity increases as the
lattice constant increases. This change in the lattice constant is thought to affect the
d-band center of Pd, and thus the ORR activity. In addition to Fe, Co, Ni, other transition metals Au, Cu, Ir, Mo, etc. can also form alloys with Pd, thereby improving the
ORR activity of the catalyst [194–196]. Figure 4.29a shows the relationship between
the d-band center ( d , related to the Fermi level) of Pd, Pd-based alloy and Pt and
Y. Li et al.
1250 K to obtain Pd 3 Fe (111) with 11% Fe on the surface; (4) Annealed at 900 K,
Pd 3 Fe with a surface Fe content of 11% was obtained; (5) Pd 3 Fe was annealed at
1000 K, then deposit 0.5 single-layer Pd atoms on it, and then annealed at 900 K
to obtain Pd ML /Pd 3 Fe (111) with a surface Pd content of 98%. Among them, when
annealed at 1250 K, Pd 3 Fe (111) has the highest ORR performance. Its half-wave
potential (E 1/2 ) is positively shifted by 27 mV from Pd ML /Pd 3 Fe (111), and is positively shifted by 62 mV than Pd (111). In the voltage range of 0.8–0.9 V (vs. RHE),
the kinetic current density of Pd 3 Fe (111) obtained by annealing at 1250 K is 2 to
3 times that of Pd ML / Pd 3 Fe (111), and is 5 to 8 times higher than the annealed Pd
(111). In PdFe alloys, the content and state of Fe on the catalyst surface play an
important role in its ORR performance. The presence of Fe on the surface will affect
the adsorption and dissociation of O 2 , and these two steps are critical steps in the
process of ORR, even rate-determining steps. Although the mechanism of heterogeneous metal incorporation and Pd-forming alloys to enhance ORR performance
is still controversial, it can be determined that heterogeneous metal incorporation
will change the electronic structural properties of Pd, resulting in different ORR
properties. Compared with Pd (111), the d electron center of Pd ML /Pd 3 Fe (111) is
reduced by 0.25 eV, which significantly reduces the binding energy between O and
OH and the catalyst surface, thereby promoting the removal of O and OH through
protonation 145]. The synergistic effect of the ORR enhancement of Pd 3 Fe (111)
with a Fe content of 11% obtained by annealing at 1250 K can be explained by the
surface oxygen overflow effect: O 2 adsorption and dissociation at the Fe site, and then
overflow to the Pd to be reduced. The calculated structure according to the density
functional theory shows that the first O of O 2 dissociated at the Fe position can easily
diffuse to the Pd position with an energy barrier of only 0.25 eV, while the second O
has a higher diffusion energy barrier to reach with 1.25 eV. This high energy barrier
will cause irreversible adsorption on Fe and cause blocking of Fe active sites, but it
will not affect the dissociation of O 2 at the corresponding Fe position, because its
energy barrier is only 0.4 eV. Furthermore, the next O diffusion at the Fe position
is greatly promoted, and the energy barrier is reduced from 1.25 to 0.75 eV. This
characteristic keeps the dissociation of O 2 and the diffusion of O in equilibrium, so
that the reaction kinetics of ORR are maintained at a very high rate.
Liu et al. [170] prepared carbon-supported PdCo and PdNi nanoparticles by
NaBH4 reduction method, and further heat-treated under H 2 atmosphere to obtain
carbon-supported PdCo and PdNi nanoparticles with different grain sizes. In 0.1 M
HClO 4 solution, the ORR activity of carbon-supported PdCo and PdNi nanoparticles
obtained after heat treatment can exceed that of Pt/C catalyst. The ORR activity is
related to the lattice constant. As shown in Fig. 4.28, the lattice constant of the alloyed
Pd can be changed between 0.3802–0.3948 nm. The corresponding specific activity
decreases as the lattice constant increases, while the mass activity increases as the
lattice constant increases. This change in the lattice constant is thought to affect the
d-band center of Pd, and thus the ORR activity. In addition to Fe, Co, Ni, other transition metals Au, Cu, Ir, Mo, etc. can also form alloys with Pd, thereby improving the
ORR activity of the catalyst [194–196]. Figure 4.29a shows the relationship between
the d-band center ( d , related to the Fermi level) of Pd, Pd-based alloy and Pt and