polyaniline for supercapacitor

Synthetic Metals 162 (2012) 2107–2111

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Bridge effect of silver nanoparticles on electrochemical performance of graphite nanofiber/polyaniline for supercapacitor Ki-Seok Kim a , Soo-Jin Park b,∗ a b

Korea CCS R&D Center, Korea Institute of Energy Research, 152 Gajeongro, Yuseoung-gu, Daejeon 305-343, South Korea Department of Chemistry, Inha University, 100 Inharo, Nam-gu, Incheon 402-751, South Korea

a r t i c l e

i n f o

Article history: Received 6 July 2012 Received in revised form 24 September 2012 Accepted 26 September 2012 Available online 16 November 2012 Keywords: GNFs Polyaniline Silver nanoparticles Electrochemical properties

a b s t r a c t In this work, silver (Ag) nanoparticles were deposited onto graphite nanofibers (GNFs) by chemical reduction while polyaniline-coated Ag-GNFs (Ag-GNFs/PANI) were prepared by in situ polymerization. The effect of the Ag nanoparticles intercalated in composite interface on the electrochemical performances, such as CV curve, charge–discharge behaviors, and specific capacitance of the GNFs/PANI was investigated. It was found that nano-sized Ag particles could be uniformly deposited onto the GNFs and that Ag-GNFs were successfully coated by PANI via in situ polymerization. According to the charge–discharge curves, the highest specific capacitance (212 F/g) of the Ag-GNFs/PANI was obtained at a scan rate of 0.1 A/g, as compared to 153 F/g for GNFs/PANI and 80 F/g for PANI. This indicated that the Ag nanoparticles that were deposited onto the GNFs led to a bridge effect between GNFs and PANI to improve the charge transfer, which resulted in the enhanced electrochemical performances of the composites due to a synergistic effect. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The various carbon materials such as graphite nanofibers (GNF), carbon nanotubes (CNT), carbon black, and graphites provide alternate candidates for carbon support for electrodes and fillers for polymer composites due to their superior chemical stability, electrical and mechanical properties, and large surface area [1–3]. Among the carbon materials, vapor-grown graphite nanofibers (GNFs) have been studied for use as adsorbents, hydrogen storage devices, and electrode materials because of their good graphitic structure, high electric conductivity, and high specific surface area [4–6]. Moreover, GNFs are interesting candidate materials to support metal nanoparticles by enhancing the electrochemical properties and electrocatalytic activity of the GNF-based electrode. Actually, the high electrical conductivity of GNF and the orientation of the metal nanoparticles on the GNF are believed to be the reasons for the observed enhanced electrocatalytic activity [7–9]. Carbon materials reinforcing conductive polymer (CP) nanocomposites have attracted much attention for their enhancement of electrical and mechanical properties by a synergistic effect through the interaction of the two components, compared with the performance of monolithic polymers [10–12]. For example, carbon nanotubes (CNTs) or graphene/CP nanocomposites have been studied as sensors, supercapacitors, batteries, etc. [13–15].

Among numerous conductive polymers, polyaniline (PANI) have been studied as a promising candidate for carbon/CP nanocomposites due to their easy synthesis, controllable conductivity, and possibility of use in various applications including electrode materials [16,17]. Recently, carbon/PANI and metal doped PANI composites have a considerable attraction as electrode materials due to the combination effect of both PANI and carbon (or metal). However, there have been a limited number of studies on the effect of metal incorporation between carbon materials and CP on the electrochemical properties of carbon materials/CP nanocomposites [18–20]. Therefore, in the present work, graphite nanofibers (GNFs) as support of metal are firstly doped by silver (Ag) nanoparticles and Ag deposited GNFs (Ag-GNFs) are coated by polyaniline (PANI) using in situ oxidation polymerization of the corresponding aniline monomer. The morphologies and structural features of Ag-GNFs and PANI-coated Ag-GNFs (Ag-GNFs/PANI) are investigated. The effect of the Ag nanoparticles on electrochemical performance such as current density, charge–discharge, and specific capacitance of GNFs/PANI electrode is also discussed.

2. Materials and methods 2.1. Materials

∗ Corresponding author. Tel.: +82 32 860 8438; fax: +82 32 860 8438. E-mail address: [email protected] (S.-J. Park). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.09.021

Graphite nanofibers (GNFs) were obtained from Nanomirea (Korea). Aniline, silver nitrate, polyvinyl pyrrolidone (PVP), and ammonium persulfate (APS) were supplied by Aldrich.

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O HOC HO HO O HOC

O COH OH O COH OH O COH

HO



Ag

Silver Nitrate

Ag

Hydrazine

Ag

Ag

Ag

Ag

Aniline

Ag

APS

Ag

Ag

Ag Ag

Ag Ag

Ag





Fig. 1. Schematic diagram showing the preparation of Ag-GNFs and Ag-GNFs/PANI.

3-aminepropyltrimethoxysilane (APTMS) and hydrazine monohydrate were supplied by TCI and Sam Chum Chem. (Korea), respectively. The sulfuric acid, nitric acid, and all other organic solvents used in this study were of analytical grade and used without further purification.

coated onto the Ni foam, which was dried at 100 ◦ C for 12 h. Cyclic voltammetry measurements were carried out on an IviumStat instrument in 6.0 M KOH solution at different scan rates of 10–100 mV/s in the voltage range 0.3 to −0.7 V. Galvanostatic charge/discharge curves were measured at different current densities from 0.1 to 1 A/g.

2.2. Preparation of PANI-coated Ag-GNFs Before silver doping, GNFs were acid-treated by a sulfuric acid and nitric acid mixture (3:1, v/v) for 6 h at 80 ◦ C with stirring. Acid treated GNFs (A-GNFs) were washed using water until pH 7.0 and the washed samples were filtered and dried at 100 ◦ C. Ag nanoparticle-deposited GNFs (Ag-GNFs) were prepared by the reduction of silver ions using hydrazine. A-GNFs were added in 0.5 wt% PVP solution containing 0.5 M APTMS with constant stirring for 1 h. 0.1 M HNO3 was used to adjust the pH of the solution before the reaction. 10 ml of silver nitrate solution (0.1 mol/L) was slowly dropped to the A-GNF solution within 30 min at 60 ◦ C and then hydrazine was added to the mixture solution. This reaction was remained for 24 h mixture with stirring at room temperature. Finally, Ag-GNFs were obtained by filtration and washed with deionized water, and then dried in a vacuum oven for 24 h. Ag-GNFs were dispersed in 1 M HCl solution with ultrasonication for 1 h. The aniline monomer was added to the Ag-GNFs solution with stirring. The APS solution as initiator was dropped into the Ag-GNFs/aniline solution for 30 min. The polymerization of aniline was continued for 12 h at 0–5 ◦ C. Then, the PANI-coated Ag-GNFs (Ag-GNFs/PANI) was washed using water and methanol. The washed product was dried in a vacuum oven at 80 ◦ C for 24 h. The preparation procedure of the Ag-GNFs and Ag-GNFs/PANI is presented in Fig. 1.

3. Results and discussion 3.1. Characterization of Ag-GNFs The morphologies of pristine GNFs and Ag-GNFs were observed using TEM analysis. Fig. 2(a) and (b) displays the TEM images of GNFs and Ag-GNFs. GNFs have a diameter of about 100 nm and show a clean surface without any Ag nanoparticles. After chemical reduction, Ag nanoparticles are uniformly deposited onto GNFs without agglomeration, and particle sizes are from about 2 nm to 4 nm. These results suggest that the oxygen-containing surface groups, i.e. carboxyl and hydroxyl groups, on A-GNFs can play as anchoring sites for binding Ag precursor to prevent the aggregation of the Ag nanoparticles, resulting in high dispersion of Ag nanoparticles onto GNFs [21]. Also, the Ag deposition onto the GNFs is further confirmed using EDX spectrum. The EDX spectrum reveals the presence of Ag in the Ag-GNF samples, as shown in Fig. 2(c). X-ray diffraction patterns for pristine GNFs and Ag-GNFs are shown in Fig. 2(d). As expected, the GNFs revealed the weak d(0 0 2) and d(1 0 0) peaks at 2 = 26◦ and 43◦ corresponding to crystalline graphite characteristics [22]. In Ag-GNFs, three peaks are detected at 2 = 38◦ , 44◦ , and 64◦ , which correspond to the crystal faces of d(1 1 1), d(2 0 0), and d(2 2 0) of silver nanoparticles, consistent with the formation of face centered-cubic silver crystallite in the GNFs after chemical reduction.

2.3. Measurements Ag-GNFs were confirmed using transmission electron microscopy (TEM, JEOL FE-TEM 2006) and X-ray diffraction (XRD, Rigaku D/Max 2200V) at 40 kv and 40 mA using Cu K␣ radiation. The XRD patterns were obtained in 2 ranges between 2◦ and 70◦ at a scanning rate of 2◦ /min. Infrared spectra of A-GNFs, Ag-GNFs, and Ag-GNFs/PANI were confirmed with a Fourier transform infrared spectrophotometer (FT-IR 4200, Jasco). The morphologies of Ag-GNFs and Ag-GNFs/PANI were observed by scanning electron microscopy (SEM, S-4200, Hitachi). Electrochemical performances of PANI, GNFs/PANI, and AgGNFs/PANI were characterized using a three electrode electrochemical cell. The three-electrode cell consisted of a Pt wire as a counter electrode, an Ag/AgCl reference electrode, and Ni foam coated with samples as the working electrode. For working electrodes, the Ag-CNFs/PANI, carbon black, and PVDF (70:20:10, w/w) were mixed and dispersed in NMP. Then, the mixture slurry was

3.2. Characterization of PANI-coated Ag-GNFs composites FT-IR spectra of A-GNFs, Ag-GNFs, and Ag-GNFs/PANI are shown in Fig. 3. Acid-treated GNFs exhibit a strong and broad O H stretching peak at 3270 cm−1 ; absorbance at 1730 cm−1 and 1635 cm−1 , which are attributed to carbonyl groups and to O H bending vibration due to the functionalization of pristine GNFs consisted of carbon. However, in the Ag-GNFs, the intensity of the hydroxyl and carbonyl peaks is lower compared to that of the A-GNFs. It suggests that electron density of the carboxyl oxygen atom was decreased due to the interaction between silver nitrate and the functional groups of the A-GNFs [23]. The bands around 1570 cm−1 and 1490 cm−1 in Ag-GNFs/PANI are characteristic stretching bands of nitrogen quinoid (N Q N) and benzenoid (N B N) in PANI, indicating the conducting state of the PANI. The 1290 cm−1 and 1230 cm−1 peaks are assigned to the bending vibrations of N-H and the asymmetric C N stretching modes of the polaron structure of

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Fig. 2. TEM images of (a) pristine GNFs and (b) Ag-GNFs, (c) EDX image of Ag-GNFs, and (d) XRD pattern of pristine GNFs and Ag-GNFs.

PANI, respectively. The absorption band around 1040 cm−1 indicates C N stretching [17]. Fig. 4 shows the XRD patterns of PANI and Ag-GNFs/PANI. Pure PANI have two broad peaks at 2 = 20◦ and 25◦ , indicating the periodicity parallel and perpendicular characteristics of the polymer chain [24]. Ag-GNFs/PANI displays the features of both the Ag-GNFs and PANI with weak Ag peaks, presenting successful hybridization of Ag-GNFs and PANI. The Ag-GNFs and the core/shell Ag-GNFs/PANI are observed using SEM analysis, as shown in Fig. 5. The diameter of the Ag-GNFs is about 100 nm with the length of a few ␮m (Fig. 5a). Compared to the Ag-GNFs, the surface of the Ag-GNF/PANI composites becomes rough and the diameter increases to the range of about 120–200 nm (Fig. 5b and c), resulting from the coating of Ag-GNFs by PANI. It is known that the thickness of the PANI layers on the Ag-GNFs is over 20 nm. Also, Ag-GNFs/PANI is further confirmed using TEM image (Fig. 5d).

3.3. Electrochemical performance of PANI-coated Ag-GNFs composites Fig. 6 shows the cyclic voltammogram of pure PANI, GNFs/PANI, and Ag-GNFs/PANI, which were measured at 10 mV/s in a 6.0 M KOH solution as electrolyte. In Fig. 6a, the current density of Ag-GNFs/PANI is larger than that of pure PANI and GNFs/PANI, indicating the higher specific capacitance of Ag-GNFs/PANI. It indicates that the incorporation of Ag nanoparticles between GNFs and PANI increases the electrochemical utilization of GNFs/PANI electrodes. Fig. 6b shows CV curves of Ag-GNFs/PANI as a function of different scan rates (10–100 mV/s). It can be found that the current density of Ag-GNFs/PANI is proportionally increased as increasing scan rate without peak deformation, indicating a good rate capability of Ag-GNFs/PANI electrodes. The cathodic peaks and anodic peaks,

(a) GNF/PANI (b) Ag-GNFs/PANI

Ag

Intensity (a. u.)

Transmittance (%)

A-GNFs Ag-GNFs Ag-GNFs /PA NI

Ag

(b)

(a)

4000

3500

3000

2500

2000

1500

1000

-1

Wav eleng th (cm ) Fig. 3. FT-IR spectra of A-GNFs, Ag-GNFs, and Ag-GNFs/PANI.

0

10

20

30

40

50

60

2 Fig. 4. XRD patterns of PANI and Ag-GNFs/PANI.

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2

Current (mA/cm )

20

(a)

10

0

-10 PANI GNFs/PANI Ag-GNFs/PANI

-20 -0.8

-0.6

-0.4

-0.2

0.0

0.2

(a)

0.4

Potential (mA vs. Ag/AgCl)

Fig. 5. SEM images of (a) Ag-GNFs, (b and c) Ag-GNFs/PANI, and (d) TEM image of the Ag-GNFs/PANI.

0.2

PANI GNFs/PANI Ag-GNFs/PANI

0.0 -0.2 -0.4 -0.6 -0.8 0

0.4

20

40

60

(b) (b)

Potential (mA vs. Ag/AgCl)

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10 mV/s 20 mV/s 30 mV/s 50 mV/s 100 mV/s

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100 120 140 160 180

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Voltage (V vs Ag/AgCl) 0.4

1 A/g 0.5 A/g 0.2 A/g 0.1 A/g

0.2 0.0 -0.2 -0.4 -0.6 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Voltage (V vs Ag/AgCl) Fig. 6. Cyclic voltammetry of (a) pure PANI, GNFs/PANI, and Ag-GNFs/PANI at 10 mV/s scan rate and (b) Ag-GNFs/PANI with different scan rates from 10 to100 mV/s.

0

100

200

300

400

500

Time (s) Fig. 7. Charge–discharge curves of (a) pure PANI, GNFs/PANI, and Ag-GNFs/PANI at 0.2 A/g current density and (b) Ag-GNFs/PANI with different current densities from 0.1 to 1 A/g.

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Ag nanoparticles on electrochemical performances of GNFs/PANI was investigated. It was found that Ag nanoparticles having 2–4 nm diameter were easily deposited onto the GNFs by chemical reduction of silver ions. From cyclic voltammetry, the Ag-GNFs/PANI showed superior electrochemical performance compared to pure PANI and GNFs/PANI. The specific capacitance of PANI, GNFs/PANI, and Ag-GNFs/PANI is 80 F/g, 153 F/g, and 212 F/g, respectively. Higher electrochemical performance of AgGNFs/PANI was attributed to the bridge effect by Ag nanoparticles incorporated between GNFs and PANI.

250 PANI GNFs/PANI Ag-GNFs/PANI

Capacitance (F/g)

200

2111

150

100

50

Acknowledgements 0 0.0

0.2

0.4

0.6

0.8

This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (0031985).

1.0

Current density (A/g) Fig. 8. Specific capacitance of pure PANI, GNFs/PANI, and Ag-GNFs/PANI with different current density from 0.1 to 1 A/g.

however, shift to positive and negative directions, respectively, due to the resistance of the electrodes [25]. Fig. 7 shows the galvanostatic charge/discharge curves of pure PANI, GNFs/PANI, and Ag-GNFs/PANI. It is clear that the charge–discharge duration of pure PANI increase by the incorporation of GNFs. Especially, the incorporation of Ag-GNFs in PANI provide more improved charge/discharge behaviors to PANI, indicating the increase of energy storage capacity. Fig. 7b shows the charge/discharge of the Ag-GNFs/PANI with different current densities from 0.1 to 1 A/g. The charge/discharge duration of the AgGNFs/PANI is depended on the current density. The discharge time is longer than the charge time, which can be probably attributed to the interaction between electrode materials and electrolyte ions [15]. From the charge–discharge curve, the specific capacitance (Cspec ) for the composite electrodes is shown in Fig. 8. It can be calculated as: Cspec =

I×t V × m

(1)

where I is the discharge current, m is the electrode mass, t is the discharge time, and V is the voltage range. As shown in Fig. 8, the Cspec of Ag-GNFs/PANI is higher than that of pure PANI and GNFs/PANI. The highest Cspec value (212 F/g) of Ag-GNFs/PANI is obtained at 0.1 A/g current density compared to 80 F/g for pure PANI and 153 F/g for GNFs/PANI. The enhanced electrochemical performances of Ag-GNFs/PANI are due to a synergistic effect of Ag nanoparticles and GNFs/PANI. This indicated that the presence of Ag nanoparticles in interface of core/shell structure can provide a bridge effect between GNFs and PANI, leading to decrease of contact resistance between GNFs and PANI and improved charge transfer of Ag-GNFs/PANI [26]. 4. Conclusions In this study, we prepared core/shell Ag-GNFs/PANI composites as electrode material for supercapacitor. The effect of

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