4 Catalyst Materials for Oxygen Reduction Reaction
127
comparisons of the low-index crystal planes of Pt and Pd at 0.9 V (vs. RHE). The
preparation of Pd nanostructures with exposed dominant crystal planes is an effective
way to improve the ORR activity of Pd.
Jiang et al. [190] used commercial Pd/C (20wt%, E-TEK) to anneal in a mixture
H 2 /N 2 (5 vol% H 2 ) at an annealing temperature of 300 °C, 400 °C, 500 °C, and 600 °C
to obtain Pd/C-300, -400, -500, and -600, and Pd nanoparticle sizes increased from 3
to 16.7 nm. When the Pd nanoparticles increased from 3 to 5 nm, their mass activity in
the 0.1 M NaOH electrolyte increased to 1.3 times of the original, and then their mass
activity decreased as the size of the Pd nanoparticles increased. Since the hydrogen
adsorption/desorption region of Pd cannot reflect the active site of the Pd surface, the
active surface area (S act ) cannot be calculated based on the hydrogen desorption peak
of Pd. The S act of Pd can be calculated from the reduction peak of a single-layer oxide
(PdO) formed on the surface of Pd particles in the cyclic voltammetry curve of Pd in
the electrolyte. The charge constant for PdO reduction is 405 μC cm
−2 . The specific
activity calculated according to S act increases with the increase of Pd particle size.
When the size of Pd nanoparticles increases from 3 to 16.7 nm, the specific activity
increases three times. Koenigsmann et al. [191] revealed the relationship between the
diameter of one-dimensional Pd nanowires and their ORR performance. They used
polycarbonate as a template to synthesize ordered self-supporting nanowire arrays of
270 and 45 nm sizes. In addition, they also used palladium nitrate as a precursor, in
a solution of octadecylamine and dodecyltrimethylammonium bromide in toluene,
and reduced with NaBH 4 as reducing agent under an inert atmosphere to obtain 2 nm
Pd nanowires, which were then loaded on carbon powder (Vulcan XC-72). In 0.1 M
HClO 4 solution, when the diameter of Pd nanowires was reduced from 270 to 2 nm,
the specific activity at 0.8 V (vs. RHE) doubled from 1.84 to 3.62 mA cm
−2 . The
ORR activity of 45 and 2 nm Pd nanowires is higher than that of commercial Pd/C.
The area specific activity of commercial Pd/C at 0.8 V (vs. RHE) is 1.8 mA cm
−2 .
Because the ORR activity of Pd (100) is higher than that of Pd (111), the prepared
Pd nanowires are analyzed based on TEM results to show that the crystal surface
exposed is (100), while the crystal surface exposed by commercial Pd/C is (111)
[198], so the ORR of the prepared Pd nanowires is higher than that of commercial
Pd/C.
ORR is a very complicated reaction process, which is related to the composition,
structure, and chemical state of the electrode material. Because Pd has a similar
crystal structure and electronic structure as Pt, Pd exhibits an ORR activity similar
to Pt, but the kinetics of single-phase Pd catalyzed ORR is still relatively slow and it
is difficult to achieve commercial application. The introduction of another transition
metal for doping modification to form an alloy can significantly change the electronic
structure of Pd and improve its kinetics of catalyzing ORR. Yang et al. [217] prepared
Pd 3 Fe (111) single crystal by sputtering under high vacuum, and then prepared five
different Pd 3 Fe (111) surfaces under ultra-high vacuum: (1) Pd 3 Fe (111) with a rough
atomic surface was prepared by sputtering with 1 keV Ar to spray clean Pd 3 Fe (111)
crystal surface, and the surface content of Fe reached 25%; (2) 0.8 single-layer Fe
atoms were precisely dispensed on the Pd 3 Fe (111) surface, and then annealed at
1000 K for 20 s to obtain Pd 3 Fe (111) with flat atomic surface; (3) Annealed at
127
comparisons of the low-index crystal planes of Pt and Pd at 0.9 V (vs. RHE). The
preparation of Pd nanostructures with exposed dominant crystal planes is an effective
way to improve the ORR activity of Pd.
Jiang et al. [190] used commercial Pd/C (20wt%, E-TEK) to anneal in a mixture
H 2 /N 2 (5 vol% H 2 ) at an annealing temperature of 300 °C, 400 °C, 500 °C, and 600 °C
to obtain Pd/C-300, -400, -500, and -600, and Pd nanoparticle sizes increased from 3
to 16.7 nm. When the Pd nanoparticles increased from 3 to 5 nm, their mass activity in
the 0.1 M NaOH electrolyte increased to 1.3 times of the original, and then their mass
activity decreased as the size of the Pd nanoparticles increased. Since the hydrogen
adsorption/desorption region of Pd cannot reflect the active site of the Pd surface, the
active surface area (S act ) cannot be calculated based on the hydrogen desorption peak
of Pd. The S act of Pd can be calculated from the reduction peak of a single-layer oxide
(PdO) formed on the surface of Pd particles in the cyclic voltammetry curve of Pd in
the electrolyte. The charge constant for PdO reduction is 405 μC cm
−2 . The specific
activity calculated according to S act increases with the increase of Pd particle size.
When the size of Pd nanoparticles increases from 3 to 16.7 nm, the specific activity
increases three times. Koenigsmann et al. [191] revealed the relationship between the
diameter of one-dimensional Pd nanowires and their ORR performance. They used
polycarbonate as a template to synthesize ordered self-supporting nanowire arrays of
270 and 45 nm sizes. In addition, they also used palladium nitrate as a precursor, in
a solution of octadecylamine and dodecyltrimethylammonium bromide in toluene,
and reduced with NaBH 4 as reducing agent under an inert atmosphere to obtain 2 nm
Pd nanowires, which were then loaded on carbon powder (Vulcan XC-72). In 0.1 M
HClO 4 solution, when the diameter of Pd nanowires was reduced from 270 to 2 nm,
the specific activity at 0.8 V (vs. RHE) doubled from 1.84 to 3.62 mA cm
−2 . The
ORR activity of 45 and 2 nm Pd nanowires is higher than that of commercial Pd/C.
The area specific activity of commercial Pd/C at 0.8 V (vs. RHE) is 1.8 mA cm
−2 .
Because the ORR activity of Pd (100) is higher than that of Pd (111), the prepared
Pd nanowires are analyzed based on TEM results to show that the crystal surface
exposed is (100), while the crystal surface exposed by commercial Pd/C is (111)
[198], so the ORR of the prepared Pd nanowires is higher than that of commercial
Pd/C.
ORR is a very complicated reaction process, which is related to the composition,
structure, and chemical state of the electrode material. Because Pd has a similar
crystal structure and electronic structure as Pt, Pd exhibits an ORR activity similar
to Pt, but the kinetics of single-phase Pd catalyzed ORR is still relatively slow and it
is difficult to achieve commercial application. The introduction of another transition
metal for doping modification to form an alloy can significantly change the electronic
structure of Pd and improve its kinetics of catalyzing ORR. Yang et al. [217] prepared
Pd 3 Fe (111) single crystal by sputtering under high vacuum, and then prepared five
different Pd 3 Fe (111) surfaces under ultra-high vacuum: (1) Pd 3 Fe (111) with a rough
atomic surface was prepared by sputtering with 1 keV Ar to spray clean Pd 3 Fe (111)
crystal surface, and the surface content of Fe reached 25%; (2) 0.8 single-layer Fe
atoms were precisely dispensed on the Pd 3 Fe (111) surface, and then annealed at
1000 K for 20 s to obtain Pd 3 Fe (111) with flat atomic surface; (3) Annealed at