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Journal of Materials Chemistry A
DOI: 10.1039/C7TA01754E
ARTICLE
Journal Name
phase diagram, it is interesting to note that the Ni-Pd alloy forms a maintained for 30 min. Other post-treatments were the same as
homogenous solid solution in all compositions and within a large those described above.
range of temperature. Such a bimetallic alloy provides a platform to
evaluate the electric-catalytic performance of core-shell structure samples onto carbon black (Vulcan XC-72, Cabot), the carbon black
alloys without the interruption of intermetallic phase. was firstly dispersed in hexane, and then a certain amount of
Preparation of carbon-supported catalysts. To immobilize the
Herein, we selected Ni-Pd alloy as a model alloy and fabricated samples in hexane was added to the solution. Finally, the mixture
o
Ni-Pd core-shell NPs with different shell compositions and contents, was stirred for 12 h and the precipitates were heated to 400 C for
2 2
and then examined their catalytic activity, selectivity and stability 2 h under an Ar/H atmosphere (5% H ) to remove capping ligands.
for ORR under both acidic and alkaline conditions. A modified The mass ratio of samples and carbon black was designed to be 1:4,
solution route was developed to fabricate the Ni-Pd core-shell NPs, and the precise loading on the carbon black was further determined
which were further tested by using various characterization by element analysis.
methods. Also, the impact of shell composition and content of the
Characterization of the prepared catalysts. The phases of the
Ni-Pd core-shell NPs on their ORR catalytic activity, selectivity and products were identified by X-ray diffraction (XRD) patterns on an
stability under acidic or alkaline conditions was investigated. X’Pert Pro Super diffractometer with graphite-monochromatized
Furthermore, the mechanisms behind the enhanced ORR Cu-Ka radiation (λ = 1.541874 Å) (Rigaku TTR-III, PHLIPS Co., the
performance of the Ni-Pd core-shell NPs (Ni/Pd=1:3) with Pd- Netherlands). The transmission electron microscopy (TEM) images
enriched surface over other prepared catalysts were elucidated. In of the samples were recorded on a TEM (H-7650, Hitachi Co., Japan)
this way, a cost-effective ORR electrocatalyst in both acidic and using an electron kinetic energy of 100 kV. The high resolution
alkaline electrolytes was prepared and an appropriate Pd-based transmission electron microscopy (HRTEM), high angle annular dark
core-shell structure to assess their influence on the ORR field scanning transmission electron microscopy (HAADF-STEM)
performance was developed.
images and corresponding energy dispersive spectroscopic (EDS)
mapping analyses were performed on a JEOL JEM0ARF200F
TEM/STEM with a spherical aberration corrector (Talos F200X, FEI
Co., USA). The surface chemical compositions and the valence
states of constituent elements were analyzed by X-ray
photoelectron spectroscopy (XPS) (ESCALAB250, Thermo Fisher Inc.,
USA). The compositions of Pd and Ni was determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES, Optima
Experimental
Materials and reagents. Palladium (II) acetylacetonate
2 2
[Pd(acac) , 98%], nickel(II) acetylacetonate [Ni(acac) , 98%],
oleylamine [OAM, 70%], 1-octadecene [ODE, 90%] and
trioctylphosphine (TOP, 90%) were purchased from Aladdin Co.,
USA. All reagents were used without any purification.
7
300 DV, Perkin Elmer Co., USA) after being digested with
chloroazotic acid. The carbon content in the supported catalysts
was determined by element analysis at pure oxygen atmosphere
Synthesis of Nix@Pdy core-shell NPs. Nix@Pdy core-shell NPs
3
4, 35
were prepared following a method reported previous.
to fabricate Ni@Pd3 core-shell NPs (Ni:Pd=1:3), Pd(acac)
mmol, 0.0915 g), Ni(acac) (0.10 mmol, 0.0258 g) and TOP (1.0 ml)
were mixed into a solution of OAM (8.0 mL) in a 100-mL three-neck
Briefly,
(
vario EL cube, Elemental Co., Germany) with a thermal conductivity
detector.
Electrochemical testing of the catalysts. All electrochemical
2
(0.30
2
o
tests were conducted with a rotating disk electrode (RDE) (Pine
Research Instrumentation Inc., USA) connected to a CHI 760E
potentiostat (Chenhua Instrument Co., China) in an acidic
2
flask. Under a N flow, the mixture was initially heated to 120 C,
which was kept for 30 min to remove low boiling point solvents,
o
and subsequently the mixture was rapidly raised to 220 C at a rate
o
-1
4
electrolyte (0.1 M HClO ) or alkaline electrolyte (0.1 M KOH). The
of 10 C min and stayed for 30 min. Then, the solution was slowly
o
o
-1
diameter of RDE was 5.0 mm and the disk geometric area was 0.196
heated to 250 C at a rate of 1.5 C min and maintained for 30 min.
Finally, the solution was cooled to room temperature by removing
the heating source. The final products were precipitated by adding
2
cm . Ag/AgCl (in 3.0 M KCl) electrode and platinum wire were
respectively used as the reference electrode and counter electrode.
To prepare the working electrode, 4.0 mg of the prepared
electrocatalysts and 10 μl Nafion solution (Sigma Aldrich Co., USA, 5
wt.%) were dispersed in 1.0 mL water-ethanol solution with a
volume ratio of 3:1 by sonicating for 1 h to form a homogeneous
ink. Then, 10 μL of the catalyst ink containing 20 μg of catalyst was
1
0 mL of ethanol and centrifuged at 8000 rpm for 5 min, washed
several times with absolute ethanol and hexane (4:1), and then re-
dispersed in hexane for further characterization. Under the similar
conditions, samples with different Pd/Ni compositions were
synthesized except the molar ratio of Pd(acac)
changed.
2 2
and Ni(acac) was
-2
loaded onto a RDE with a loading of ~0.10 mg cm . Simultaneously,
.0 mg commercial Pt/C catalyst (Sigma Aldrich Co., USA, 20 wt.%)
2
Synthesis of Pd3@Ni core-shell NPs. Pd3@Ni core-shell NPs
were prepared using a modified method mentioned above. In our
approach, a co-solvent of ODE was introduced and a bi-solvent
was also prepared for comparison following the same procedure.
All polarization curves were iR-compensated with respect to the
ohmic resistance of the electrolyte. All of the potentials were
reported with respect to the reversible hydrogen electrode (RHE)
unless otherwise mentioned, and the current densities (j) were
calculated based on the geometric area of the RDE.
synthesis method was adopted. Briefly, Pd(acac)
.0915 g), Ni(acac) (0.10 mmol, 0.0258 g) and TOP (1.0 ml) were
mixed into a solution of OAM (4.0 mL) and ODE (4.0 mL) in a 100-
mL three-neck flask. Under a N flow, the mixture was heated to
2
(0.30 mmol,
0
2
2
o
o
Line sweep voltammogram (LSV) was tested at a sweep rate of
1
20 C and degassed for 30 min, and subsequently raised to 180 C
−
1
o
-1
5
2 2
mV s and a rotating speed of 1600 rpm in a N or O -saturated
at a rate of 10 C min and stayed for 30 min. Thereafter, the
o
o
-1
electrolyte. The LSV curves were obtained after subtracting the
solution was slowly heated to 240 C at a rate of 1.5 C min and
2
| J. Name., 2012, 00, 1-3
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