Preparation and electrochemical capacitive performance of polyaniline nanofiber-graphene oxide hybrids by oil–water interfacial polymerization

Synthetic Metals 189 (2014) 47–52

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Preparation and electrochemical capacitive performance of polyaniline nanofiber-graphene oxide hybrids by oil–water interfacial polymerization Yuhong Jin, Mengqiu Jia ∗ Laboratory of Electrochemical Process and Technology for Materials, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 November 2013 Received in revised form 7 December 2013 Accepted 18 December 2013 Available online 20 January 2014 Keywords: Polyaniline nanofiber Graphene oxide Interfacial polymerization Electrochemical capacitive performance

a b s t r a c t Polyaniline nanofiber-graphene oxide (PANIF-GO) hybrids were fabricated by oil–water interfacial polymerization. The structures of PANIF-GO hybrids, pure PANIF and GO were examined by high resolution transmission electron microscopy. It was found that PANIFs were homogeneously inserted between the GO layers or absorbed on the surface of the GO. Fourier transform infrared spectroscopy and X-ray diffraction showed that PANIFs effectively increased the interlayer distance of GO from 0.83 nm to 1.38 nm. Electrochemical properties for the hybrid electrode were tested by cyclic voltammetry and electrochemical impedance spectroscopy using a three-electrode system. The results indicated that a high specific capacitance of 564.7 F g−1 for the hybrids was measured at the current density of 0.5 A g−1 in a 1 M H2 SO4 aqueous solution compared to 352.8 F g−1 for pure PANIF and 30.1 F g−1 for GO. Moreover, the PANIFGO hybrids showed a very long cycle life with only 9.4% specific capacitance loss after 2000 cycles. The as-prepared hybrids are remarkable electrode materials for the supercapacitors. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, growing demands for power sources of transient high-power density have stimulated a great interest in supercapacitor with project applications in digital communications, electric vehicles, burst power generation, memory back-up devices and other related devices which require high-power pulse [1]. Compared with secondary batteries, supercapacitors, also known as electrochemical supercapacitors or ultracapacitors, exhibit faster and higher power capability, long life, wide thermal operating range, and low maintenance cost [2,3]. The main materials studied for the supercapacitor electrode are carbons, metal oxides and conducting polymers. The conducting polymers have received increasing interest as an alternative to carbons and metal oxides in supercapacitor. This is due to low fabrication cost, easy synthesis, flexibility, excellent environmental stability, high conductivity and high pseudo capacitance [4–10]. In particular, polyaniline (PANI) is one of the most promising electrode materials because of its high theoretical pseudo capacitance owing to multiple redox states [11–13]. Moreover, one dimensional nanostructure PANI such as nanotubes, nanofibers and nanowires have received great attention for electrode materials of supercapacitors, because they provide high surface area and high conductivity

∗ Corresponding author. Tel.: +86 010 64413808; fax: +86 010 64413808. E-mail address: [email protected] (M. Jia). 0379-6779/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.12.016

leading to high specific capacitance [14,15]. In particular, polyaniline nanofiber (PANIF) had been regarded as ideal supercapacitor electrode materials due to their large specific area and optimized ion diffusion path [16–19], which is necessary for the effective access of electrolyte to the electrode in both electric double layer and redox mechanism. However, PANIF is susceptible to rapid degradation in the charge/discharge process, which is attributed to the swelling and shrinkage of PANIF. In order to alleviate this limitation, the combination of PANI with carbon materials (carbon nanotubes [20,21], porous carbon [22,23], ordered mesoporous carbon [24,25], carbon nanofibers [26], active carbon [27,28] and graphene or graphene oxide [29–31]) has been proved to reinforce the stability of PANI as well as to maximize the capacitance value. Among these carbon materials, graphene and graphene oxide are predicated as an excellent support material due to high surface area, and remarkable mechanical stiffness and excellent conductivity. Due to the attractive merits of graphene or graphene oxide and the intriguing properties of PANI, composites of these materials should have novel properties as supercapacitors electrode materials. Indeed, previous researches on methods of preparation and performance of graphene or graphene oxide/PANI materials have spotlighted the interesting properties of composites obtained via various approaches. Chen and co-workers [32] studied the effect of carbon particle morphology on the electrochemical properties of nanocarbon/PANI composites. The results showed that graphene/PANI composite exhibits better performance than carbon black/PANI and carbon nanotube (CNT)/PANI. Therefore, compared

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to CNT, graphene or graphene oxide (GO) is predicated as an excellent support material due to high surface area, and remarkable mechanical stiffness and excellent conductivity. Yan [29] used in situ polymerization to synthesize a graphene nanosheet/PANI composite. Electrochemical test showed a high specific capacitance of 1046 F g−1 was obtained at a scan rate 1 mV s−1 compared to 115 F g−1 for pure PANI. Recently, Zhang [30] has reported PANIF-GO composites prepared by a rapid mixing reaction. The electrochemical tests showed that the introduction of electrical activity PANIF effectively increased the composites electrode capacitance. However, PANIF inserted into GO sheets became scaffolds for secondary growth of PANI and finally turned into irregularly shaped agglomerates containing nanofibers and particulates. It can be explained that PANIFs are formed at an early stage in the polymerization process; as more ammonium peroxydisulfate was fed into the reaction, the PANIFs became scaffolds for secondary growth of PANI, which could affect the properties of the composites. Interfacial polymerization ion is accepted as a facile approach to make bulk quantities of PANI nanofibers [33]. Wang et al. [34] reported a morphology-controlled strategy to prepare sulfonated graphene/polyaniline composites by a liquid/liquid interfacial method, using sulfonated graphene (SGE) as both a substrate and a mocromolecular acid dopant. Composites obtained with two different ratios of SGE to PANI showed higher specific capacitance of 793 F g−1 and 931 F g−1 , but lower capacity retention after 100 cycles of 77% and 76%, respectively. Herein, we report a simple method to prepare PANIF-GO hybrids by oil–water interfacial polymerization. This method has the advantage of suppressing the secondary growth of PANI. Since the monomer aniline and the initiator are separated by the boundary between the aqueous and the organic phases, polymerization occurs only at this interface where all the components needed for polymerization come together [33,35–37]. PANI is then formed into nanofibers. The twodimensional planar structure of GO is beneficial to homogeneous nucleation of a large amount of PANI on their surface, giving more active sites for the redox, and the layer-by-layer stacks of crumpled GO sheets and PANI layers prevent the peeling of PANIFs from the GO surface. Consequently, the material can tolerate considerable volume changes, swelling and shrinkage of PANI. PANIF-GO hybrid electrodes conclude two types of capacitive response. The first type is electrical double layer capacitance, which is due to graphene oxide. The second type is pseudo capacitance, which is attributed to PANIF. Interfacial polymerization method was a simple and efficient approach to prepare PANIF-GO hybrids. The electro capacitive properties of PANIF-GO hybrids in 1 M H2 SO4 electrolyte were studied and compared with that of pure PANIF and GO. 2. Experimental 2.1. Preparation of graphite oxide Graphite oxide was synthesized from natural graphite power (Crystalline, 500 meshes, provided by Qingdao Baichuan Graphite Co. Ltd., Qingdao, China) by a modified Hummers-Offeman method [38]. 1.25 g of Graphite was added into the 65 ml of concentrated sulfuric acid (98 wt%, analytical grade) within an ice bath under violent mechanical agitation and kept for 20 min. Then potassium permanganate (KMnO4 , analytical grade) was mixed gradually with the aforementioned mixture in order to control the reaction temperature at below 20 ◦ C. The reactor was kept in the ice bath for 2 h and the color changed from black to dark-green. Then the reactor was put into the 35 ◦ C water bath for 30 min. Distilled water was slowly dropped into the mixture to cause an increase in temperature of up to 95–100 ◦ C with effervescence, and the color changed from dark green to brown. The mixture was stirred at 98 ◦ C for 15 min. The water bath was removed, and 25 ml of 30 wt%

H2 O2 was added into the mixture to remove the excess KMnO4 . The mixture was rinsed and centrifugated with 5 wt% hydrochloric acid (HCl, analytical grade) and deionized water for several times. The GO dispersion was directly dispersed in the water for next experimental step. 2.2. Preparation of PANIF-GO hybrids The oil/water interfacial polymerization for PANIF-GO hybrids is illustrated in Fig. 1. The lower layer is the oil phases in which aniline (3.0 g, analytical grade) was dissolved in 100 ml chloroform (CHCl3 , analytical grade). The upper layer is the water phase in which GO aqueous suspension (6.0 g, 0.5 wt%) and Ammonium persulfate (APS, (NH4 )2 S2 O8 , 1.8 g, analytical grade) were added to 100 ml of 1 M HCl aqueous solution after ultrasonication for 2 h. The beaker was kept at room temperature for 24 h. With increasing the reaction time, the product slowly grew on the oil/water interface, and the whole upper layer became dark-green and the lower layer became yellow-brown. Finally, the product was filtered and washed carefully with the deionized water and dried in a vacuum oven at 80 ◦ C for 24 h. 3. Characterization methods of PANIF-GO hybrids The morphology was measured by high resolution transmission electron microscopy (HRTEM JEM-2010). The structure was analyzed by Fourier transform infrared spectroscopy (FT-IR, Bruker Vector 2, using KBr pellets), and X-ray diffraction (XRD, Bruker D8 with Cu K␣ radiation). 4. Electrochemical characterization of PANIF-GO hybrids The electrodes were prepared by mixing 85 wt% as-prepared composite with 10 wt% acetylene black and 5 wt% polytetrafluoroethylene dissolved in ethanol as a binder to form a slurry. The slurry was then pressed onto a stainless steel wire mesh (1 cm2 ) at 10 MPa for 1 min in order to ensure a good electrical contact and was dried in vacuum oven at 80 ◦ C for 24 h. All electrochemical experiments were carried out in 1 M H2 SO4 aqueous solution using a three-electrode system, which contained a platinum foils as a counter electrode, a saturated calomel electrode (SCE) as reference electrode, the obtained materials loaded on a stainless steel wire mesh as a working electrode. Cyclic voltammetry (CV) measurements were performed in a potential range from −0.2 to +0.8 V, and the sweep rate range from 2 to 30 mV s−1 . For the electrochemical impedance spectroscopy (EIS) measurements were also carried out in the frequency range from 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV. Galvanostatic charge/discharge (GCD) curves were measured in a potential range from −0.2 to +0.8 V, and the current density range from 0.5 to 10 A g−1 . All electrochemical data were collected by computer controlled equipment (Corrtest C350, Wuhan China). 5. Results and discussion 5.1. FT-IR characterization The FT-IR spectra of the GO, PANIF and PANIF-GO hybrids were measured as shown in Fig. 2. For the characteristic peaks of GO, the broad O H stretching peak at 3410 cm−1 , the C O stretching vibrational band at 1730 cm−1 , the C C band at 1632 cm−1 and the C O stretching vibrations at 1055 cm−1 were observed, which was similar with previous reports [39]. For PANIF, the main bands at 3441, 2926, 1575, 1492, 1300, 1138, 803, 579 cm−1 for Emeraldine PANIF may be attributed to H N H, N H, C N, C C, C N, C H

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Fig. 1. Illustration of the fabrication process for PANIF-GO hybrids. Step (1) is formation of aniline cation at the oil/water interface; step (2) is electrostatic absorption; step (3) is chemical oxide polymerization; step (4) is formation of PANIF-GO hybrids.

and C H* stretching modes for the quinoid and benzenoid rings, respectively. As was observed commonly for PANIF-GO hybrids all spectra exhibited the clear presence of the same vibrational bands as in PANIF. However, compared with the bands of PANIF, the bands of PANIF-GO hybrids corresponding to the stretching vibrations of C C in PANIF and benzenoid ring were clearly red-shift to 1568 and 1491 cm−1 . The spectra red-shift phenomena of chemically synthesized composites resulted from the – interaction and hydrogen bonding between GO sheets and the polymer (PANIF) backbone [40]. 5.2. XRD characterization Fig. 3 presented the XRD patterns of pure PANIF, GO and PANIF-GO hybrids. For PANIF, the crystalline peaks appear at

Fig. 2. FT-IR spectra of GO, PANIF and PANIF-GO hybrids.

2 = 20.7◦ and 25.2◦ , respectively, corresponding to (0 2 0) and (2 0 0) crystal planes of PANIF in its emeraldine salt form [41]. For GO, the only diffraction peak was at 2 = 10.8◦ , which can be attributed to the (0 0 2) inter-planar spacing of 0.83 nm, different from that of the graphite, which had a peak centered at 2 = 26.6◦ (d-spacing of 0.334 nm), indicating that graphite had been exfoliated forming GO, which was consistent with previous literature [42–45]. Upon intercalation with PANIFs, the (0 0 1) diffraction peaks assigned to the interlayer distance between the GO sheets gradually shifted from 10.8◦ to 6.4◦ . According to Scherrer equation [46], the layer distance (1.38 nm) of PANIF-GO hybrids became larger than that (0.83 nm) of original GO. Such an expansion of layer distance can be explained by the adsorption and intercalation of the PANIFs on the surface and between the GO sheets.

Fig. 3. XRD patterns of GO, PANIF and PANIF-GO hybrids, inset is the enlarge view of PANIF.

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Fig. 5. CV curves of PANIF-GO hybrids at different potential scan rates of 2, 5, 10, 20 and 30 mV s−1 in 1 M H2 SO4 from −0.2 to +0.8 V.

Fig. 4. HRTEM of PANIF (a and b), GO (c and d) and PANIF-GO hybrids (e and f).

5.3. Morphology characterization HRTEM was utilized to analyze the morphology of the asprepared polymeric nanofibers together with their hybrids. The HRTEM images showed typical fibrillar morphology for PANIFs (Fig. 4a and b). As to exfoliated GO (Fig. 4c and d), large sheets were observed and they resembled silk veil waves. They were transparent and entangled with each other. As reported previously, corrugation and scrolling were intrinsic to graphene nanosheets, whereas the PANIF-GO hybrids exhibited mainly irregular morphology with multishapes including both fibrillar and agglomeration (Fig. 4e and f). These changes can be explained by the absorption and intercalation of PANIF on the surface and between the GO sheets. HRTEM results were consistent with FT-IR and XRD analytical results.

density increased with increasing scan rate, suggesting a good rate ability of PANIF-GO hybrid electrode. Fig. 6 showed the CV curves of GO, pure PANIF and PANIF-GO hybrid electrodes at 2 mV s−1 . GO electrode showed a pair of peaks that originated from the transition between quinone/hydroquinone states, which was typical for carbon materials with oxygen-containing functional groups [30]. It can be seen that both pure PANIF and PANIF-GO hybrid electrodes showed two pairs of peaks in the CV curves, which corresponded to the redox transition of PANIF between a leucoemeraldine form and a polaronic emeraldine form, and between a polaronic emeraldine form and a pernigraniline form [48–50]. It was noted that the integrated area in the CV of PANIF-GO composite electrode was larger than that of other two electrode materials at the same scan rate, suggesting its higher specific capacitance. Fig. 7 demonstrated the GCD plots of the PANIF-GO hybrids at different current densities of 1–5 A g−1 . It can be noted obviously that the galvanostatic curves of PANIF-GO hybrids presented a capacitive behavior with almost symmetric charge/discharge curves. Moreover, the deviation to linearity was typical of a pseudocapacitive contribution. PANIF-GO hybrid electrode had the excellent specific capacitive ability, which was attributed to the multi-state of PANIF. The specific capacitance of the electrode can be calculated from the discharge process according to the following equation [51]: C=

I × t m × V

(1)

5.4. Electrochemical capacitive performance The CV curves obtained at 2, 5, 10, 20 and 30 mV s−1 in 1 M H2 SO4 electrolyte within potential range of −0.2 to +0.8 V for PANIF-GO hybrid electrode were shown in Fig. 5. The as-prepared hybrids showed remarkable electrochemical capacitive performance in the entire investigated potential range at the different scan rates. It can be observed that the cathodic peaks shift positively and the anodic peaks shifted negatively with increasing potential scan rates from 2 to 30 mV s−1 , which was mainly due to the resistance of the electrode [47]. Moreover, it can be noted that the peak current

Fig. 6. CV curves of GO, PANIF and PANIF-GO hybrid electrode in 1 M H2 SO4 aqueous electrolyte at 2 mV s−1 in the potential range from −0.2 to 0.8 V.

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Fig. 7. GCDD curves of PANIF-GO hybrids within the potential window −0.2 to +0.8 V at different current densities of 1, 2, 3, 4 and 5 A g−1 .

where C was the specific capacitance value (F g−1 ), I was the current density (A), t was the charge time (s), m was the mass of electro-active electrode materials (g) and V was the potential range during charge process (V). Fig. 8 showed the variation of the specific capacitance of GO, pure PANIF and PANIF-GO hybrids with the increase of current density. It can be noted that the specific capacitance of PANIF-GO hybrids was much higher than that of GO and pure PANIF at the same scan rates. The PANIF-GO hybrid electrode with a specific capacitance of 564.7 F g−1 was higher than pure GO (30.1 F g−1 ) and PANIF (352.8 F g−1 ) at the same current density of 0.5 A g−1 . Nyquist plots for GO, pure PANIF and PANIF-GO hybrid electrodes at open circuit potentials were shown in Fig. 9 and the experimental results were fitted using an equivalent circuit as shown in the set of Fig. 9. The plots consisted of a distorted semicircle in the high frequency region and an apparent straight line in the low frequency region. The diameter of the semicircle was roughly equal to the electrode resistance which rose from the charge transfer resistance (Rct ) in as-prepared material electrode and the slightly vertical line represents the capacitive behavior of the system [52]. The resistance of PANIF-GO hybrid (7.584 ) electrode was lower than that of GO (18.33 ), but higher than that of PANIF (6.165 ), which was attributed to the additive of the low conductivity of GO into PANIF. These results indicated that PANIFs inserted into GO layers not only improved conductivity but also increased the capacitance of composites.

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Fig. 9. Nyquist plots for GO, PANIF and PANIF-GO hybrid electrodes at open circuit potentials.

Fig. 10. Cycle stability of PANIF-GO hybrid electrode during the long chargedischarge process at a current density of 2 A g−1 .

The electrochemical stability was another important factor in electrochemical capacitors for the practical applications. The cycle stability tests for PANIF-GO hybrid were carried out at the same current density of 2 A g−1 , as shown in Fig. 10. The capacitance of the PANIF-GO hybrid decreased only by 9.4% (from 530.4 to 497.8 F g−1 ) after 2000 cycles. Therefore, GO can enhance the cycle stability of as-prepared hybrids. Based on the above results, it can be noted that the two dimensional planar structure for GO was beneficial to homogeneous absorption for a large amount of PANIF on the surface of GO or insertion between layers, which gave more active sites for the redox reaction, reduced the diffusion and migration length of the electrolyte ions during the fast charge/discharge process and increased the electrochemical utilization of PANIF. This means that the electrochemical capacitive performance of the obtained composites was improved due to the conductivity of PANIF and to the large surface area of GO. 6. Conclusions

Fig. 8. Specific capacitance of GO, Pure PANIF and PANIF-GO hybrids at different current densities.

An easy strategy was reported to prepare PANIF-GO hybrids by oil–water interfacial polymerization. The introduction of GO sheets into composites provided a relatively large surface area for dispersing PANIFs, which can effectively enhance the kinetics for both charge transfer and ion transport through the electrode. PANIFs with 50 nm diameters were uniformly distributed on the surface of the GO sheets or inserted between the GO sheet layers. The electrochemical measurements illustrated that the maximum

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specific capacitance was 564.7 F g−1 based on PANIF-GO hybrids compared to 30.1 and 352.8 F g−1 for GO and pure PANIF. The greatly enhanced specific capacitance was due to the synergistic effect between GO and PANIF. The reduction of PANIF-GO hybrids in the specific capacitance was only 9.4% after 2000 cycles. These intriguing features make it quite a suitable and promising electrode material for supercapacitors.

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