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Polymer-Plastics Technology and Engineering
ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20
Synthesis of Polystyrene Microspheres-Supported
Ag-Ni Alloyed Catalysts with Core-Shell Structures
for Electrocatalytic
Yue Yu, Dongxue Luan, Changlong Bi, Yu Ma, Yongheng Chen & Dongyu Zhao
To cite this article: Yue Yu, Dongxue Luan, Changlong Bi, Yu Ma, Yongheng Chen & Dongyu
Zhao (2017): Synthesis of Polystyrene Microspheres-Supported Ag-Ni Alloyed Catalysts with
Core-Shell Structures for Electrocatalytic, Polymer-Plastics Technology and Engineering, DOI:
10.1080/03602559.2017.1354250
To link to this article: http://dx.doi.org/10.1080/03602559.2017.1354250
Accepted author version posted online: 24
Oct 2017.
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Download by: [UAE University]
Date: 25 October 2017, At: 10:06
Synthesis of polystyrene microspheres-supported Ag-Ni
alloyed catalysts with core-shell structures for
electrocatalytic
Downloaded by [UAE University] at 10:06 25 October 2017
Yue Yu
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
Dongxue Luan
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
Changlong Bi
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
Yu Ma
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
Yongheng Chen
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
Dongyu Zhao
School of Chemistry and Materials Science, Heilongjiang University, Harbin, China
1
Key Laboratory of Chemical Engineering Process and Technology for High-efficiency
Conversion, University of Heilongjiang, Harbin, China
Address correspondence to Dongyu Zhao. E-mail: hacar1201@aliyun.com
ABSTRACT
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Rational designed synthesis strategy for well-defined morphology which can endow the
catalysts with unexpectedly enhanced catalytic properties remains a significant challenge in
heterogeneous catalytic reactions. Hence, here we report a facile and controllable synthesis of
polystyrene microspheres-supported Ag-Ni alloyed catalysts (PS@Ag-Ni) with uniform
core-shell structures via sulfonated treatment coupled with the subsequent liquid phase reduction
strategy. In this typical synthesis, sulfuric acid acts as the bifunctional roles in directing the
core-shell morphology and the linker between the polystyrene microspheres and Ag-Ni alloy.
The as-obtained PS@Ag-Ni optimized by tuning in a mass ratio of 1:1 shows superior oxygen
reduction reaction activity and electrocatalytic performance toward the degradation of
p-nitrophenol in comparison with other range of polystyrene microspheres and Ag-Ni alloy
feeding ratios. The superior electrocatalytic and ORR activity are attributed to its highly uniform
core-shell morphology and exposure of much more active sites. Moreover, our as-prepared
core-shell electrocatalysts will enable further investigation in other catalytic reactions.
Graphical Abstract
2
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KEYWORDS: core-shell structure, electrocatalytic, oxygen reduction reaction, p-nitrophenol,
surface functionalized
1. Introduction
Nanocomposites with peculiar geometry and novel morphologies have been extensively
studied in various fields such as photocatalysis, refractory and pH-sensitive materials.[1–4] In
particular, core-shell structure attracted much more attention because of high surface-to-volume
ratios and controllable dielectric properties. Synthesis methods of diverse materials with
core-shell structure have been widely reported. The ingredient of core was usually prepared by
polymer, gelatin, and inverse miniemulsion[5–9] with a large variety of shapes of sphericity,
raspberry-type, dumbbell-shaped, and petaling[10–13]. In comparison with above structures,
however, the spherical-shaped nanocomposites have the high reproducibility and excellent
performance which can be prepared by emulsion polymerization, sol-gel and self-assembly
3
method.[14–16] A well-prepared core which possesses large specific surface area, high stability and
the possibility should have been easily modified. As the most popular and functional core
material, polystyrene (PS) microspheres was extensively used as biocarriers, templates and
catalyst supports[17–19] owning to the controllable and low cost. Core-shell materials with PS as
matrix can be prepared via various methods including one-step reactions such as self-assembly
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process and seed-mediated growth method[20],[21], which is usually a way of preparing polymers
coating PS matrix. Nevertheless, Surface functionalization treatment is a simple and effective
way to connect different groups onto the surface of target catalysts.
After surface functionalization treatment, PS microspheres will connect with plentiful
active groups such as carbon nanotubes, polyaniline, metals[22–26] and so on. Wang and
co-workers[27] synthesized polystyrene/polyaniline core/shell structured catalysts and improved
the stability of the epoxy-based conductive composites. Zhang[28] reported PS/Fe3O4 multihollow
microspheres with porous walls, which indicates PS supports are well to corrosion-resistant and
easily magnetically recoverable. Inspired by those ideas, the synthesis for corrosion-resistant and
stable PS-supported catalyst should be highly valuable.
Metallic nanoparticles are of importance due to their excellent catalytic activities and
optical properties[29–32]. In recent years, noble metal nanoparticles, in particular Au and Pd et al,
have been used in applications for electrochemical catalysis and oxygen reduction reaction
(ORR)[33],[34]. However, the high cost and shortage of reserve abundance restrict the further
4
development of the noble metal. By using the promising substitutes like Ag metals or Ag alloy,
the disadvantage metioned above can be overcomed without reducing the performance of the
catalysts. It has been reported silver/PS composite microspheres exhibits improved catalytic
performance in the oxidation-reduction reaction of methylene blue by NaBH4[35]. While much
promising alloy nanoparticles possess higher property of physics and chemistry than single
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component[36–38], and catalytic activity is intensely sensitive to the composition and well-defined
chemometry so on. Introducing a certain amount of Ni into Ag is a good choice for tailoring its
properties and the cost. The preparations of composites with Ag-Ni alloy have been widely
reported, but the process is all complicated rather than one step as far as I concerned.[39],[40]
Hence, how to synthesize core-shell composites structure combined with the PS supports and
Ag-Ni alloy via simple reaction and to improve the monodispersity of the composites are worth
to be investigated.
Herein, we provided a facile and controllable approach to synthesize PS@Ag-Ni
nanocomposites with core-shell structure via sulfonated treatment coupled with the subsequent
liquid phase reduction process. To the best of our knowledge, PS@Ag-Ni is a novel example to
assemble Ag-Ni nanoalloys onto the surface of PS microspheres and used for bifunctional
electrocatalysis. PS microspheres were regarded as a support in order to prevent aggregation of
Ag-Ni alloy to enhance the monodispersity of the core-shell structure, Ag and Ni ions can be
reduced simultaneously forming Ag-Ni alloy in-situ encircling PS microsphere as the shell for
5
uniform core-shell structures. SEM images showed the core-shell structure of PS@Ag-Ni
nanocomposites uniform distribution and the particle size was about 300-320 nm. This
nanocomposites we prepared can be used as multifunctional catalysts in the ORR and
degradation of p-nitrophenol in water with rotating disk electrode (RDE) and carbon electrode,
respectively. PS@Ag-Ni nanocomposites showed the highest catalytic performance towards the
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degradation of p-nitrophenol and the oxygen reduction reaction in the mass ratio of 1:1. By
comparing with the commercial Pt/C electrode, PS@Ag-Ni nanocomposites got better current
density in methanol tolerance. As a result, the PS@Ag-Ni nanocomposites synthesized via liquid
phase reduction method could be a promising material in application of catalytic chemistry.
2. Experimental Section
Materials: Styrene (St, 99%) and divinyl benzene (DVB, 80%) was disposed via reduced
pressure distillation in order to remove the polymerization inhibitor and deposited in refrigerator.
Initiator potassium persulfate (KPS), dispersant sodium dodecyl benzene sulfonate (SDBS),
nickel sulfate (NiSO4), silver nitrate (AgNO3), sodium hydroxide (NaOH) and concentrated
sulfuric acid need no purification in this work. Hydrazine hydrate (80%) stored at temperature
about 4 ℃ was used as reductive.
2.1 Preparation of surface sulfonate polystyrene microspheres
Cross-linked polystyrene microspheres were synthesized by emulsion polymerization,
using SDBS as the emulgator. The reaction was carried out in a 1 L three-necked flask equipped
6
with a mechanical stirrer for 300 rpm, a thermometer, and a condenser. The reactor was charged
with 0.5 g of SDBS, 100 mL ethanol and 400 mL deionized water under nitrogen atmosphere.
After dispersing SDBS adequately in ethanol-water mixture system, the polymerization was
started by adding 25 mL St and 5 mL DVB. Not until the reactor was heating to 80 ℃, 10 mL
KPS solution with a concentration of 0.185 mol/L was introduced into the reactor and allowed to
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continue reaction for 12 h under nitrogen atmosphere. Ultimately, the resultant cross-linked PS
microspheres were separated from the emulsion by centrifuge, and washed with deionized water
to obtain cross-linked polystyrene microspheres with particle size at about 220 nm. Then the
microspheres were dispersed by a small amount of deionized water under ultrasound. The
reaction of surface sulfonation was accomplished in concentrated sulfuric acid medium in a
three-necked flask equipped with a mechanical stirrer, in which the dosage of concentrated
sulfuric acid was approximately 50 mL. After heating the mixture of PS emulsion and
concentrated sulfuric acid at 65 ℃ for more than 24 h, the sulfonated polystyrene microspheres
(SPS) was washed by deionized water until the system turn neutral.
2.2 Synthesis of polystyrene microspheres-supported Ag-Ni alloyed catalysts with
core-shell structures
A small amount of resultant SPS particles were dispersed into deionized water with the
mixture of aqueous solution of NiSO4 and AgNO3 for more than 30 min under ultrasonic.
Subsequently, the mixture was added into a three-necked flask, and then the flask was putted into
7
a water bath with the stirring at a speed of 300 rpm. When the temperature reaches 80 ℃, 10 mL
NaOH aqueous solution (0.2 M) was added into the system, after a while, 10 mL of hydrazine
hydrate was dropped into the mixture. The reaction was stirred for 2 h. After centrifuging and
washed with alcohol and water, the functionalized polystyrene supporting nano-silver-nickel
alloy (PS@Ag-Ni) nanocomposites were synthesized successfully.
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2.3 Characterization
The morphologies of the prepared PS, SPS and SPS@Ag-Ni nanocomposites were tested
via a scanning electron microscope (SEM, S4800, JEOL, Japan). For characterization, all
products were attenuated with ethanol, and disposed by ultrasonic for less than 0.5 h. Whereafter,
the dispersion liquid was dropped onto conductive adhesive on the Al pan and the Al pan with
the simples was putted into a stove at 50 ℃ for more than 24 h. The powder X-ray diffraction
(XRD) was operated on a Rigaku Electric Company diffractometer with Cu-Kα radiation in a
scanning range of 10° to 80° (2θ). Sample for FT-IR characterization were disposed at 60 ℃
under vacuum for 24 h, dispersed in KBr matrices, and measured in the range of 4000-300 cm-1
by PerkinElmer instruments. X-ray photoelectron spectroscopy (XPS) analysis was measured on
a ULTR AXIS DLD with an Al Kα (1235.6 eV) achromatic X-ray source. The cyclic
voltammograms (CVs) were conducted on a CH Instruments I604 electrochemical analyzer with
a three-electrode system for testing the performance of PS@Ag-Ni nanocomposites for the
catalytic property towards degradation of p-nitrophenol in water and performance of the oxygen
8
reduction reaction. The former used Hg/HgO as the reference electrode and the carbon electrode
modified with 5 mg as-prepared PS@Ag-Ni nanocomposites dispersing in 0.1 mL Nafion
solution was used as the working electrode. The electrolyte was composed of 1.5 M NaOH
solution and 0.01M p-nitrophenol solution. The scan rate was 100 mV with the voltage range of
0-1.0 V. The latter used rotating disk electrode (RDE) modified by 10 μL 0.06 g/mL simple
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dispersed with Nafion as working electrode which was disposed by O2-saturated 0.1 M KOH
solution and Ag/AgCl as reference electrode with the potential range form -0.8 to 0 V.
3 Results and Discussion
3.1 Characterization of PS@Ag-Ni Nanocomposites
The PS@Ag-Ni nanocomposites were synthesized by sulfonated treatment coupled with
the subsequent liquid phase reduction strategy, in this typical synthesis, cross-linked polystyrene
microspheres were synthesized by emulsion polymerization by modifying a previously reported
method. The formation process of PS@Ag-Ni nanocomposites had been illustrated in Scheme 1.
Firstly, we make sulfonic group encircling the surface of cross-linked polystyrene microspheres
as precursor. Thereinto, sulfonic group plays vital role on directing the core-shell morphology
and the linker between the polystyrene microspheres and Ag-Ni alloy. Secondly, the target
catalyst is achieved by using hydrazine reduction and ultraphonic assembly strategy combine
in-situ formation of Ag-Ni alloy on sulfonate polystyrene microspheres into an entity. The
core-shell morphology enhanced uniform dispersion of Ag-Ni alloy and exposed much more
9
active sites, which directly leading to the excellent electrocatalytic and oxygen reduction reaction
activity.
The structural state of PS@Ag-Ni nanocomposites was measured by X-Ray Diffraction
(XRD). Five distinct diffraction peaks observed in Figure 1, the 2θ at round 38.08˚ and 64.42˚
are the characteristic peak of Ag (111), Ag (220), also 51.80˚ is the characteristic peak of Ni
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(200). The slightly wider hybrid diffraction maximum at 44.38˚ and 77.40˚ corresponding to Ag
(220), Ni (111) and Ag (311), Ni (220), which indicates that the layer of nanocomposites has a
structure of face-centered cubic arrangement.
There is a broad diffraction peak present at 2θ between 10˚ and 30˚, which is ascribed to
the semicrystalline nature of PS microspheres. On the basis of Scherrer equation ‘ D  kγ / Bcosθ
’, we can deduce that the particle size has changed after the PS was sulfonated via the variation
of the displacement of the peak between 10˚ and 30˚. Nevertheless, after loading, Ag-Ni alloy
covered up the surface of SPS, which influenced the crystallization of SPS microspheres. Also
seen XRD pattern of PS@Ag-Ni nanocomposites with different ratio was disposed in Figure S1,
from which we can discern that with the increase of the content of Ag-Ni alloy, the peak appears
more prominent indicating that crystal particle size has increased.
Fourier transform infrared (FT-IR) is regard as forceful instrument for analyzing the
structure and chemical interactions of the nanocomposites we prepared. Figure 2 exhibited the
FT-IR spectra of PS, SPS, and SPS@Ag-Ni nanoparticles. Peaks in the ranges of 500-1018 cm-1,
10
1602 cm-1, 2900-3100 cm-1 corresponding to the absorption of stretching vibration of C-H in the
benzene ring, stretching vibration of C = C, and bending vibration of C-C out-of-plane in the
benzene ring, and the peak belonging to stretching vibrations of C-C of the benzene ring
appeared at 1445-1700 cm-1. All these absorption peaks appear in the curve of SPS and
SPS@Ag-Ni composites. However, a significant peak at 1181 cm-1 in curve SPS and PS@Ag-Ni
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is corresponding to –SO3H, which indicates the surface of PS microspheres was successfully
loaded with sulfonyl after sulfonation for 24 h[41].
As shown in Figure 2, SPS@Ag-Ni, peak at 3434cm-1 corresponding to the N-H
stretching vibration band[42] introduced by hydrazine hydrate via the process of reduction.
The cross-linked PS microspheres were synthesized via emulsion polymerization with an
average diameter of 220 nm. SEM images of the PS microspheres is shown in Figure 3a,b which
reveals that the PS microspheres as-prepared have uniform distribution with smooth surfaces. PS
particles formed a structure of spherical as a result of the polymerization occurred in the micelle
which was generated by adding surfactant SDBS.
Sulfonated PS microspheres from Figure 3c,d demonstrate there is little variation in
morphology and size of these microspheres, which is because the sulfonyl on PS microspheres
are atomic groups which cannot be observed via SEM and meanwhile it can prove the stability of
cross-linked PS microspheres.
11
For comparing, Figure 3e,f show the morphologies of the representative Ag-Ni alloys
coated SPS as-prepared by liquid phase reduction method. The Ag-Ni alloy particles were
distributed on the surface of PS microspheres uniformly and the arrangement were completely
pyknotic, which was attribute to the –SO3H connected on the surface of PS microspheres via
covalent bond after sulfonation. We can precisely observe that the particles size increased to
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300-320 nm, and after the reaction the morphologies keep well (seen in Figure S1), which
confirms that the SPS microspheres have been covered with a layer of Ag-Ni alloy nanoparticles.
The morphologies of different mass ratio between PS and Ag-Ni alloy were shown in Figure S1,
the ratios of PS and Ag-Ni alloy nanoparticles were in 2:1, 1:1 and 1:2 with homogeneous
distribution and particles size were all approximate 300-320 nm. Compared with (A) and (C), (B)
had shown a complete and uniform coating layer of Ag-Ni alloy on the surface of PS
microspheres with little agglomeration.
Not until the supplementation less than 100%, Ag-Ni alloy coating could not cover the
whole surface of the PS microspheres completely. However when the recruitment was excessive,
the alloy nanoparticles would agglutinate on the surface of PS microspheres as observed in
picture Figure S1.
For analyzing the interaction and chemical composition between Ag-Ni alloy
nanoparticles and PS microspheres, X-ray photoelectron spectroscopy (XPS) was introduced into
the measurements with a wide range of 0-1200eV. Figure 4a is the full spectrum of
12
PS@Ag-Ni nanocomposites, from which we can discern that electron binding energy at
284.4, 369.0, 375.0, 528.1, 855.4, 873.5 eV corresponding to C 1s, Ag 3d5/2, Ag 3d3/2, O 1s, Ni
2p3/2 and Ni 2p1/2, respectively. The other five images are the detailed spectrogram of major
elements. Spectrum of Ag 3d and Ni 2p are shown in Figure 4b,c. The binding energy of the C
1s narrow interval fitting spectra in Figure 4d of 284.8, 268.8 and 288.7 eV, which correspond
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to C = C, C-O, and C = O coming from benzene ring and the oxygen-containing groups on the
surface of benzene ring, respectively. The presence of S = O in Figure 4e and + 6-valent sulfur
at the binding energy of 168.5 eV in Figure 4f has further confirmed the existence of sulfonic
acid groups on the surface of PS microspheres after disposing by concentrated sulfuric acid.
3.2 Performance analysis of PS@Ag-Ni nanocomposites
Figure 5 is the CV graph which demonstrates the catalytic property of as-prepared
nanocomposites. Redox peaks can be observed definitely at all the modified carbon electrode.
When the ratio of PS and Ag-Ni alloy in 1:1, the nanocomposites show the maximum difference
of current, which means the PS@Ag-Ni nanocomposites have the best catalytic performance in
this ratio. And most nanocomposites reveal better catalytic performance than the pure Ag-Ni
alloy, which attributes to the uniform distribution of Ag-Ni alloy nanoparticles loading onto the
surface of PS enhancing the specific surface area and effective contact area of the
nanocomposites. In other words, pure Ag-Ni alloy was more likely to be agglomeration than
nanocomposites, which could decrease the effective contact area with the reaction system.
13
The reduction peak at about 0.303 V is identified as the reduction of nitryl to amidogen
and the oxidation peak at about 0.578 V is identified as the reversible reaction. The redox
performance attributes to the microstructure and large specific surface area of the PS@Ag-Ni
nanocomposites. With the decrease of the ratio of PS and Ag-Ni alloy, the redox performance
shows an increasing trend, which is because the coverage of Ag-Ni alloy become more
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integrated on the surface of PS. However when the coverage reaches a critical value, due to the
agglomeration of Ag-Ni alloy on the surface of PS, the specific surface area and subsequently the
active site recedes, which has decreased the catalytic property of the nanocomposites.
The performance of the catalysts for the oxygen reduction reaction was tested by rotating
disk electrode (RDE) which was manipulated in O2-saturated 0.1 M KOH electrolyte solution
with the potential range from -0.8 to 0 (vs. Ag/AgCl). Figure 6a shows the influence of different
ratio of PS and Ag-Ni alloy towards the ORR activity, from which we can observe in the ratio of
1:1 reveals the most positive potential. The identical phenomenon was that most nanocomposites
showed better performance towards oxygen reduction reaction than pure Ag-Ni alloy occurred
during this reaction compared with tendency of the catalytic process of p-nitrophenol’s
degradation. Figure 6b reveals the characteristic of the nanocomposites in the ratio of 1:1 for the
oxygen reduction reaction tested by RDE with various rotations from 400 rpm to 1600 rpm, from
which we can calculate the transferred electron number towards ORR was 3.9 via
Koutecky-Levich equation (1) and (2).
14
1 1
1
 
j jk jlim (1)
2
3
1
6
1
2
jlim  0.20nFD υ C0ω  Bω
1
2
(2)
The stability of as-prepared nanocomposites were assessed by a measurement named
chronoamperometric which was taken in O2-saturated 0.1 M KOH solution at a rotation rate of
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1600 rpm. From Figure 6c we can observe that PS@Ag-Ni only get lost approximately 15.2% in
the oxygen reduction reaction, which is less than the uniform conditions of commercial Pt/C.
Moreover, methanol tolerance measurement is another method for evaluating the stabilities of
catalysts. Figure 6d shows the characteristic discrepancy between as-prepared PS@Ag-Ni
nanocomposites and commercial Pt/C. 3 M methanol was injected at the time of 400 s after the
beginning of methanol tolerance testing. There is less significant change can be viewed form the
ORR current of PS@Ag-Ni sample than that of commercial Pt/C in the same condition.
4 Conclusions
In summary, we have demonstrated a facile and reproducible strategy for fabricating
highly dispersive PS@Ag-Ni core-shell nanocomposites via liquid phase reduction and assembly
method combined with concentrated sulfuric acid served as structure direct-agent and linkers.
Varying the sulfonated polystyrene microspheres and Ag-Ni alloy feeding ratio are vital for
generating suitable nanostructures as well as a significant improvement in ORR and
electrocatalysis. This can ascribe to sufficient catalytic active sites at both their interior
polystyrene microspheres and exterior Ag-Ni alloy shell surfaces. Therefore, we anticipate this
15
method can be widely applicable to produce other indeed promising and novel nanostructures for
extendible catalytic applications.
Acknowledgments
This work was supported by Harbin Scientific and Technological Special Fund for
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Innovative Talents (Grant No. 2012RFXXG093).
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19
Figure 1. X-Ray diffraction pattern of PS@Ag-Ni nanocomposites, Ag-Ni alloy, SPS, and PS.
The inset image is detailed spectrogram of the displacement of different materials in XRD
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pattern.
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Figure 2. Fourier transform infrared spectra of the SPS, PS and PS@Ag-Ni particles.
21
Figure 3. SEM images of PS (A) and (B), SPS (C) and (D) and PS@Ag-Ni nanocomposites (E)
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and (F).
22
Figure 4. X-ray photoelectron spectra of PS@Ag-Ni nanocomposites. (A) the whole spectrum of
PS@Ag-Ni nanocomposites; (B)-(F) the detailed spectrogram of the elements of Ag, Ni, C, O, S
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involved in the nanocomposites.
23
Figure 5. CV curves for carbon electrode modified by different ratios of PS@Ag-Ni (0:1, b 2:1,
c 1:1 and d 1:2) tested in 1.5 M NaOH solution adding 0.01 M p-nitrophenol at the voltage range
Downloaded by [UAE University] at 10:06 25 October 2017
of 0 V-1.0 V with a scan rate of 0.1V.
24
Figure 6. (A) RDE voltammograms response in O2-saturated 0.1 M KOH electrolyte solution at
a scan rate of 5 mV/s with different ratios of PS and Ag-Ni alloy nanoparticles and the electrode
rotation rate was 1600 rpm. The inset image in (A) is the CV curves for ORR on different ratios
of PS@Ag-Ni, and (B) PS@Ag-Ni LSV curves measured in an O2-saturated 0.1 M KOH
electrolyte with various rotating speeds at scan rates of 5 mV/s and the inset image of CV curves
for ORR on PS@Ag-Ni in O2-saturated and N2-saturated 0.1 M KOH electrolyte. (C) and (D) is
the chronoamperometric response of PS@Ag-Ni and Pt/C electrodes in O2-saturated 0.1 M KOH
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electrolyte without and with adding 1.0 M CH3OH at 500s.
25
Scheme 1. Schematic illustrating the formation process of PS@Ag-Ni nanocomposites via liquid
phase reduction method. a PS microspheres prepared by emulsion polymerization. b Sulfonated
PS microspheres (SPS) treated by concentrated sulfuric acid for more than 24 h. c PS@Ag-Ni
nanocomposites with core-shell structures were synthesized via liquid phase reduction method,
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which assembled Ag-Ni onto the surface of SPS microspheres forming compact shell.
26
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