graphene hybrid nanosheets with graphene as templates

Accepted Manuscript Enhanced Electrochemical Capacitance of Polyaniline/graphene Hybrid Nanosheets with Graphene as Templates Fei-Peng Du, Jing-Jing Wang, Chak-Yin Tang, Chi-Pong Tsui, Xiao-Lin Xie, Ka-Fu Yung PII: DOI: Reference:

S1359-8368(13)00310-7 http://dx.doi.org/10.1016/j.compositesb.2013.05.054 JCOMB 2458

To appear in:

Composites: Part B

Received Date: Accepted Date:

24 September 2012 27 May 2013

Please cite this article as: Du, F-P., Wang, J-J., Tang, C-Y., Tsui, C-P., Xie, X-L., Yung, K-F., Enhanced Electrochemical Capacitance of Polyaniline/graphene Hybrid Nanosheets with Graphene as Templates, Composites: Part B (2013), doi: http://dx.doi.org/10.1016/j.compositesb.2013.05.054

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Enhanced Electrochemical Capacitance of Polyaniline/graphene Hybrid Nanosheets with Graphene as Templates Fei-Peng Du1,2*, Jing-Jing Wang1, Chak-Yin Tang2, Chi-Pong Tsui2, Xiao-Lin Xie3, Ka-Fu Yung4

1. School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China 2. Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China 3. State Key Laboratory of Materials Processing and Die & Mould Technology, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 4. Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China

*Corresponding Author: Dr. Fei-Peng Du, [email protected] Tel: +86-27-87195661 Abstract: Polyaniline/graphene nanocomposites (PANi/GR) were prepared via PANi covalent grafting from the surface of GR. The unique structure of hybrid nanosheets was formed with uniform PANi layer coating GR without phase separation appearing when the weight ratio of aniline-to-graphene was 1:1. The unique PANi/GR hybrid nanosheets as electrode material for supercapacitors have a specific capacitance as high as 922 F/g at 10mV/s and still retain a specific capacitance of 106 F/g at a high scan rate of 1 V/s due to synergistic effect between PANi and GR. The capacitance retention was ~90% after 1000 cycles, which is much better than that of pure PANi or other PANi nanocomposites. The enhanced capacitive performance of PANi/GR hybrid nanosheets makes them have potential application in developing high performance energy storage devices. Key words: A. Nano-structures; B. Electrical properties; B. Microstructures; D. Chemical analysis

Introduction Nowadays, supercapacitors used as energy storage devices have attracted more and more attention due to their excellent performance, including fast charge-discharge, high power density, long cycle life, low cost and good environmental-friendliness.[1] Supercapacitors are of two main types: the electrical double layer capacitor (EDLC) which depends on the electrical double layer effect for the storage and release of energy, and the pseudocapacitor which depends on a redox reaction. [2, 3] The electrode materials of EDLC are mainly carbonic materials, such as activated carbon, mesoporous carbon, carbon nanotubes, and graphene.[1] In particular, graphene is considered as a more prospective electrode material over others due to its high specific surface area, excellent electronic transport properties, superior electrical conductivity and chemical stability, and it is even more easy to be obtained via the chemical reduction method. [4,5] Some investigations have shown that supercapacitors based on graphene have excellent capacitance properties, which can reach more than 200 F/g.[4] On the other hand, the electrode materials of PCs include metallic oxides (SnO2, MnO2, RuO2, V2O5, etc.), and conductive polymers.[6-10] Compared to inorganic compounds, conductive polymers process excellent electrochemistry properties due to unique doping/dedoping and redox processes, easy processability and good conductivity.[11] Among conductive polymers, polyaniline (PANi) seems to have good potential as supercapacitor electrodes with the characteristics of low cost and easy-synthesis.[12] Supercapacitors based on PANi can reach a capacitance of 503 F/g and higher.[13] Each of the EDLCs and PCs has its advantages and disadvantages. Comparatively, EDLCs have faster charge/discharge and longer cycle-life, while PCs have poor stability and rate capability due to low conductivity and a slow redox process in spite of higher specific capacitance.[11] Recently, many researches focused on the incorporation of graphene into PANi to improve its rate capability, charge/discharge stability and to further enhance the capacitance of the conductive polymer due to the conductivity and robust support for PANi. For example, Zhang et al. directly coated PANi on the surface of reduced graphene oxide (RGO) sheets via in situ

polymerization.[10] The results showed that RGO/PANi has a high rate capability at a current density of 2.0 A/g. Zhang et al achieved specific capacitance as high as 480 F/g at a current density of 0.1 A/g for PANi doped grapheme, [14] and Mao et al. prepared surfactant-stabilized graphene/polyaniline nanofiber composites for high performance supercapacitors which can reach 526 F/g at 0.2 A/g with good cycling.[15] However, more significantly, some of the particular fabricating methods for PANi/GR couldn’t control the structure of the composites where more PANi particles became free from the composite system and phase separation easily occurs. The covalently grafting method can be efficient in avoiding phase separation. Kumar et al. fabricated PANi/GR nanocomposites with the covalent grafting method for supercapacitor electrodes, however, the capacitance value is not satisfactory and the details are not sufficiently involved to control the morphology and phase separation of the nanocomposites.[16] In this work, graphene is used as a template to prepare PANi/GR nanocomposites and control the structure and morphology of the nanocomposites. To adopt the covalent grafting method and control the weight ratio of graphene and aniline, GR induces PANi to form a unique hybrid nanosheet structure due to the template effect, where PANi can be uniformly coated on the surface of graphene to form PANi/GR hybrid nanosheets, with no free PANi particles being found in the nanocomposites. This unique structure makes PANi/GR hybrid nanosheets have excellent energy storage capability with high specific capacitance. 1. Experimental Section Materials. Raw graphite powders were supplied by the Qingdao BCSM Co. (China). 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl) was purchased from the Shanghai Medpep Company (China). 4-dimethylamino-pyridine (DMAP) was purchased from the Sigma Aldrich Company (USA). Other chemical reagents were of analytical Grade and supplied by the Shanghai Reagents Co. (China). The 0.22 μm pore-diameter polycarbonate membranes and nylon membranes were purchased from the Shanghai Xingya Purification Material Factory (China).

Preparation of graphite oxide (GO). Graphite oxide was prepared from natural graphite powder using Hummer's method.[17] The procedure was as follows: 2 g of graphite powder was put into a solution of 230 mL concentrated sulfuric acid, 2 g KNO3 and 6 g KMnO4, stirring for 2 h at 0

. 180 mL H2O was then added into the

solution and the temperature was kept at (30±5)

for 30 min, followed by the

addition of 400 mL H2O into the solution keeping the temperature at 98

for 15 min.

Finally, 1000 mL H2O was added to cold the mixture solution and 30 mL H2O2 (30%) was added to react with the excess MnO2 in turn. After centrifuging and washing with dilute HCl (5%) until no sulfates were detected, the graphite oxide was dried at 50 °C in a vacuum oven.

Preparation of graphene (GR). GR was prepared from the GO by chemical reduction. [18] Firstly, 100 mg of GO was dispersed in 100 mL H2O for 1h, and 50 mL hydrazine hydrate was put into the solution, under stirring for 24 h at 98 . After vacuum-filtration, the product was washed repeatedly with H2O and ethanol. Finally, the GR was dried at 50 °C in vacuum.

Preparation of PANi/GR. PANi/GR was fabricated via covalent grafting polymerization in the presence of GR under acidic conditions. Firstly, 200 mg of GR was

dispersed

in

100

mL

NMP

mixed

with

p-Phenylenediamine,

1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl) and 4-dimethylamino-pyridine (DMAP), and stirred for 48 h at room temperature in dark conditions to obtain p-Phenylenediamine covalently grafted graphene. Secondly, p-Phenylenediamine covalently grafted graphene was dispersing in 100 mL HCl (1 M) for 30 min, and 150 mg of aniline monomer was dispersed into the above solution for 30 min. Thirdly, 150 mg (NH4)2S2O8 was added slowly into the above mixture solution, stirring in an ice bath for 8h. After vacuum-filtrating, and washing with HCl, H2O and ethanol, dark green PANi/GR were obtained through controlling the ratio of graphene and aniline and drying at 50 °C in vacuum.

Preparation of work electrode. Before preparing the work electrode, the glassy carbon electrode (GCE) was polished with 0.05 μm aluminum slurry (CH Instruments, USA), rinsed thoroughly with deionized water, and then ultrasonicated for 1 min. Finally, the GCE was coated with 2.0 mg of active materials. Characterization. FTIR spectra were examined on an Equinox 55 spectrometer. Raman spectra were obtained by Micro-Fourier transform infrared/Raman spectroscopy (Vertex 70/FRA 106/s, Bruker) using an excitation wavelength of 1064 nm. Powder X-ray diffraction (XRD) of the samples was performed on a diffractometer (X'Pert PRO) with Cu Kα radiation (λ = 1.5418 Å). The morphology was observed by field emission scanning electron microscopy (FESEM) (JEOL, JSM-6700F, Japan) and transmission electron microscopy (TEM) (FEI Co., Tecnai G220). Cyclic voltammetry (CV) was undertaken on a CHI660D electrochemical working station with a three-electrode electrochemical system equipped with a working electrode, a platinum foil counter electrode, and a standard calomel electrode (SCE) as the reference electrode. All electrochemical measurements were carried out in 1 M of H2SO4 solution as the electrolyte.

2. Results and discussion. Graphene oxide can be reduced to graphene, but some oxygen containing groups, such as carboxylic and some of epoxy groups,[19] cannot be removed completely on the surface of graphene or the edge. Hence, the covalent grafting method can be implemented via oxygen containing groups. At the same time, different morphologies of PANi/GR can be prepared via adjusting the weight ratios of the monomer and graphene. Here, PANi/GR nanocomposites were marked as PANi/GR1:9, PANi/GR1:1, PANi/GR2:1, PANi/GR9:1, corresponding to weight ratios of the monomer and graphene, respectively.

a

b

c

d

e

f

Fig. 1 SEM images: (a) PANi/GR1:9, (b) and (c) PANi/GR1:1, (d) and (e) PANi/GR2:1, and (f) PANi/GR9:1

Fig. 1 shows the structural morphologies of different PANi/GR nanocomposites. From the morphology of PANi/GR1:9 (Fig.1a), some small PANi nanoparticles are coated on the surface of the graphene. By increasing the amount of aniline, more PANi nanoparticles were coated on the surface of the graphene. As seen from Fig. 1(b), PANi/GR1:1 exhibited a hybrid nanosheet structure. In order to magnify the morphology, PANi formed a thick film and was uniformly coated on the surface of the graphene (Fig. 1c). A thicker nanosheet appeared in PANi/GR2:1 (Fig. 1d) and phase separation occurred with freer PANi particles or fibers appearing in the nanocomposites. In some areas of the sample, some nanofibers of PANi formed and spread on the surface of the hybrid nanosheets (Fig. 1d), and in other areas, free PANi nanoparticles aggregated (Fig. 1e). By further increasing the amount of aniline, a hybrid lamellar structure and free PANi nanoparticles coexisted and the agglomeration of PANi nanoparticles became more serious. Thus, the phase separation cannot be avoided with the non covalent or covalent method if the ratio of aniline and graphene is beyond a certain concentration. However, when the amount of aniline is appropriate, graphene can be used as a good template to form a uniformly hybrid nanosheet without phase separation in the covalent grafting method, such as the samples PANi/GR1:9 and PANi/GR1:1. Particularly, when the amount of

graphene is close to PANi (PANi/GR1:1), a thick layer of PANi coated graphene forms more uniform hybrid nanosheets. However, with the increasing the aniline beyond 1:1, the graphene is not sufficient as a template and more PANi nanoparticles are freed from the surface of the graphene to aggregate. Fig. 2 shows the TEM images of graphene, PANi/GR1:1, and PANi/GR9:1. Transparent nanosheets of GR are clearly seen to have folding or wrinkling silk waves, as shown in Fig.2a. Compared to GR, a thin layer of PANi nanoparticles is uniformly coated on the surface of graphene for the sample PANi/GR1:1 (Fig. 2b). In spite of the polymer covalent grafting, wrinkling of the hybrid nanosheets is still clearly visible. As seen from Fig.2c, thicker and bigger sizes of PANi nanoparticles are coated on the surface of graphene due to the PANi increasing. The SEM and TEM images indicate that graphene has a good template effect in forming PANi/GR hybrid nanosheets, with a suitable ratio of graphene and PANi. a

b

c

Fig. 2 TEM images of GR (a), PANi/GR1:1 (b), and PANi/GR9:1 (c)

Fig.3 shows the FTIR spectra of GO, GR, PANi and PANi/GR1:1. The characteristic vibrations of GO are clearly visible, such as for hydroxyl (3390 cm-1), carboxylic acid (1729cm-1), C=O (1618cm-1) and C-O (1054 cm-1, 829 cm-1). From the GR curve, only the carboxylic acid signal remains, while the other signals nearly disappear due to restoration of the most aromatic structure during chemical reduction, indicating that GR still retains some defects or active sites which can help to graft the polymer. Comparing PANi/GR1:1 with the PANi curve, some characteristic vibrations are the same, such as vN-H at 3200-3500 cm-1 for amine salt, vC-H at 2980-3050 cm-1 for aromatic, C=C tensile and deformation at 1562cm-1 and 1490cm-1

respective for quinoid rings and benzene ring, and C-C stretching and aromatic C-H in-plane bending in two aromatic amines (1128cm-1, 1299cm-1).[20] The FTIR spectra indicated that PANi has been coated on the surface of the GR.

GR

GO

Transmittance

829 1054 1729 1618 1729

3390

PANi/GR1:1 PANi

1305

3050 3500

3500 1521

1452 1191

1522

1295 1190 1455

3500

3000

2500

2000

1500

1000 3500

3000

2500

2000

1500

1000

-1

Wave number(cm

)

Fig. 3 FT-IR Spectrum of GO, GR, PANi and PANi /GR1:1

Raman spectroscopy is a powerful tool to investigate the functionalization and the structural change in graphene materials.[19] The G band arises primarily from in-plane sp2 carbon atoms, while the D band originates from sp3 carbon atoms that cause defects to appear in the graphite. As shown in Fig.4, GO has the D band at 1289 cm-1 and the G band at 1598 cm-1, while GR has the D band at 1287cm-1and the G band at 1602cm-1, respectively. The relative positions of the D and G bands are widened which indicates the aromatic restoration in the GR structure. The intensity ratio of the D band and the G band (ID/IG) can be used to indirectly assess the relative degree of functionalization or defects in the graphene. The ID/IG values are 1.67 and 1.109 for GR and GO respectively, which indicates that the average crystallite size of GR becomes smaller than GO, when directly reduced with hydrazine hydrate.[19,21] The Raman spectra of PANi show bands at the prominent peaks of 1174cm-1, 1244cm-1, 1363cm-1, and 1503cm-1, respective corresponding to C–H bending of the quinoid ring, C–H bending of the benzenoid ring, C–N+ stretching, and C–C stretching of the benzene ring, which were reported by Patil et al.[20] PANi/GR exhibit

similar bands to PANi without obvious GR bands appearing, indicating that PANi has

Raman Intensity

been uniformly coated on the surface of GR.

1 GO 2 GR 3 PANi 4 PANi/GR1:1

3

4 1 2

1000

1500

2000 -1

Wavenumber (cm )

Fig. 4 Raman Spectrum of GO, GR, PANi and PANi/GR1:1 XRD patterns (Fig.5) show that pristine graphite has a peak (002) plane at 2θ=26.4°, corresponding to the characteristic 0.34 nm basal plane spacing. Instead, a broad peak GO pattern appears at 10.5°, which corresponds to a (001) plane of GO, with an interlayer spacing of 0.84 nm. The presence of oxygen containing groups (such as hydroxyl, epoxy and carboxyl) result in GO sheets more loosely stacked, and are attributed to the increase of the interlayer spacing.[22,33 ] The GR pattern shows two weak peaks that appear at 25.3°and 26.4°, respectively. The weak or even invisible peak at 25.3° corresponds to single or a few layers of graphene sheet with the formation of a new lattice structure, as described by Zhou et al.[23]A weak peak at 26.4° indicates that the van der Waals force causes aggregate or stacking of the graphene due to oxygen removal with lower hydrophilicity.[18,24] For the PANi pattern, three characteristic peaks appear at 15.2°, 20.4° and 25.3° corresponding to (011), (020) and (200) amine salt crystals, respectively. However, for PANi/GR1:1, the peaks at 15.2° and 20.4° disappeared, with only the peak at 25.3° existing, which indicates that graphene has a strong interaction with the benzene ring of PANi. The XRD patterns suggest that PANi was successfully grafted on the surface of GR and helps to

exfoliate the GR and restrain nanosheet agglomeration. 10.5 GO

Intensity (a.u.)

26.4

20.2

25.3 PANi/GR1:1

15.1

15.2

5

10

15

20.4

GR

25.3

20 25 30 2theta (degree)

PANi

35

40

Fig. 5 X-ray diffraction patterns of GO, GR, PANi and PANI/GR

Fig. 6 shows the CV curves of PANi/GR1:9, PANi/GR1:1, and PANi/GR2:1 at different scan rates. At scan rates from 10mV/s to 200mV/s, PANi/GR1:1 displays a couple of redox peaks. The redox peaks in the range of +0.6 to 0.3V are due to the redox transition of PANi between the semiconducting-state form (leucoemeraldine) and the conductive form (emeraldine).

[25,26]

However, the redox peaks disappear

gradually at high scan rates (500mV/s or 1V/s) due to the charge transfer and diffusion mainly occurring on the surface of the hybrid nanosheets, not inside the bulk. It is also seen that the redox current increases clearly with increasing scan rate, indicating a good rate capability, and even PANi/GR1:1 still has good capacitive properties at high scan rates. All CV curves observed from PANi/GR2:1 and PANi/GR1:9 exhibit nearly rectangular shapes, but the CV loops are seriously distorted with increase of scan rate, suggesting an uncompensated resistance in the system due to the poor electrical conductivity.

250

(a)

600 PANi/GR1:1

100 50

(b) PANi/GR1:1

500

10mV/s 20mV/s 50mV/s 100mV/s 200mV/s

150

500mV/s 1V/s

400

Current density (A/g)

Current density (A/g)

200

0 -50 -100

300 200 100 0 -100 -200 -300

-150 0.0

0.2

0.4

0.6

-400

0.8

0.0

0.2

Potential (V)

(c) PANi/GR1:9

Current density (A/g)

40

60

10mV/s 20mV/s 50mV/s 100mV/s 200mV/s

30 20 10

Current density (A/g)

50

0 -10 -20 -30 -40 -50

0.4

0.6

0.8

Potential (V)

(d) PANi/GR 2:1 10mV/s 20mV/s 50mV/s 100mV/s 200mV/s

40 20 0 -20 -40

0.0

0.2

0.4

0.6

0.8

0.0

Potential (V)

0.2

0.4

0.6

0.8

Potential (V)

Fig. 6 CV curves of (a) PANi/GR1:1 with the scan rate range of 10mV/s to 200 mV/s, (b) PANi/GR1:1 with the scan rate at 500mV/s and 1V/s, (c) PANi/GR 1:9 at the scan rate from 10mV/s to 200 mV/s, and (d) PANi/GR 2:1 at the scan rate from 10mV/s to 200 mV/s. On the basis of the CV curves, the specific capacitance (Csp) of a single electrode can be calculated according to the following equation

i dV Cs = ∫ m 2 × ΔV × S

(1)

Where Cs is the specific capacitance, im is the current density, ∫im dV is the integrated area of the CV curve, ⊿V is the potential range, and s is the scan rate. Table 1 shows the specific capacitance of all samples prepared. For PANi/GR1:1, the specific capacitance is 922 F/g at a scan rate of 10 mV/s. With increase of scan rate, the specific capacitance of PANi/GR1:1 gradually decreases and has a value of 106 F/g at 1 V/s. Comparatively, the electrodes of the pure PANi nanofibers and PANi/GR9:1 also have high specific capacitance of 960 F/g and 758 F/g at 10 mV/s, respectively. PANi/GR1:9 and PANi/GR2:1, have similar patterns of change, with specific

capacitances of 419 and 355 F/g at 10mV/s. However, the specific capacitance drops sharply when the scan rate reaches 100 mV/s, except for PANi/GR1:1. All other samples have specific capacitance less than 100 F/g when the scan rate is beyond 500mV/s, especially pure PANi and PANi/GR9:1 with values of 6.1 F/g and 18 F/g at 1 V/s, respectively. At low scan rates, the charge transfer has enough time to occur in bulk so that the effective pseudocapacitive contribution of PANi makes all PANi/GR nanocomposites show high specific capacitance. At high scan rates, the electrochemical process only occurs on the surface of the bulks so that the capacitance contribution is limited. PANi/GR1:1 has an excellent rate capability and retains good capacitance properties at high scan rates due to the unique structure of the hybrid nanosheets with a synergistic effect of GR and PANi among the electrodes materials.

Table 1. The specific capacitance of various electrodes at different scan rate PANi GR PANi/GR1:9 PANi/GR1:1 PANi/GR2:1 PANi/GR9:1

10mV/s

20 mV/s

50 mV/s

100 mV/s

200 mV/s

500 mV/s

1 V/s

960 139 358 922 419 758

765 95 291 764 318 554

361 55 187 590 204 405

173 35 130 480 148 200

71 31 96 368 106 100

13 29 68 206 69 23

6.1 25 53 106 51 18

Fig. 7 shows the electrochemical stability of all electrode materials at the scan rate of 50mV/s. It can be seen that over 90% of the original capacitance is retained for PANi/GR1:1 after 1000 cycles, while pure PANi and PANi/GR9:1 both retained less than 50% of the original capacitance. The instability of PANi during the electrochemical process is a detrimental issue in supercapacitor applications.[28] Here, pure PANi fibers and PANi/GR9:1 have low dimensional stability and are prone to capacitance fading under the repeatedly charging and discharging due to continuous swelling and shrinkage of the polymer structure.[11,28] However, graphene incorporation into the conductive polymer can contribute to the improvement of cycle life due to the excellent mechanical properties.[10] The advantage of the PANi/GR1:1 over others is clearly demonstrated where graphene as a good template, and uniformly

coated PANi fibers on the surface can enhance the mechanical properties of PANi and improve the dimensional stability of PANi to support the long charge and discharge cycle ability. PANi/GR1:9 and PANi/GR2:1 both retain about 80% of the original capacitance which further indicates the enhancement effect of graphene, while the electrochemical stability is worse than that of PANi/GR1:1 due to the nonuniform distribution or phase separation, which can be seen from the SEM images (Fig. 1). Therefore, the PANi/GR hybrid nanosheets exhibit a better stability due to their unique structures. 110

Capacitive retention (%)

100 90 80 70 60 50

PANi GR PANi/GR1:9 PANi/GR1:1 PANi/GR2:1 PANi/GR9:1

40 30 20 10 0

0

100 200 300 400 500 600 700 800 900 1000

Cycle life (n)

Fig. 7 Cycle stability of various electrodes at the scan rate of 50 mV/s. 4. Conclusions In this study, unique PANi/GR hybrid nanosheets were prepared with PANi uniformly coated on the surface of the graphene, by means of controlling the ratio of PNAi and GR with the covalent grafting method. This method provides a simple and efficient way to prepare PANi nanocomposite electrodes for high performance supercapacitors. The electrochemical properties revealed a superior capacitive performance of PANi/GR nanocomposites due to the synergistic effect of GR and PANi when PANi was uniformly coated on the surface of graphene to form the unique structure. The good capacitive performances of PANi/GR hybrid nanosheets make them have potential application in developing high performance energy storage devices. ACKNOWLEDGMENT. This work was supported by the National Natural Science Foundation of China (50903034) and the Research Committee of The Hong Kong Polytechnic University (G-YK82).

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