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MnO2 hybrids with high capacitance performance
Materials Chemistry and Physics 167 (2015) 330e337
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Fabrication of poly(p-styrenesulfonate) grafted reduced graphene oxide/polyaniline/MnO2 hybrids with high capacitance performance Yongliang Gui*, Xuteng Xing, Chunyan Song College of Metallurgy and Energy, North China University of Science and Technology, 46 Xinhua West Road, Tangshan 063009, PR China
h i g h l i g h t s Reduced graphene oxide was grafted by p-styrenesulfonate to obtain rGO-g-PSS. The rGO-g-PSS was used to induce formation of polyaniline (PANI) nanorod array. The rGO-g-PSS@PANI was assembled with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The rGO-g-PSS@PANI-MnO2 hybrids exhibit the good electrochemical properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 29 August 2015 Accepted 23 October 2015 Available online 2 November 2015
Reduced graphene oxide grafted with poly(p-styrenesulfonate) (rGO-g-PSS) was synthesized by in-situ free radical polymerization. The rGO-g-PSS was used to induce formation of polyaniline (PANI) nanorod array and then assembled with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The rGO-g-PSS@PANIMnO2 hybrids were applied to modify electrodes and the specific capacitance (SC) of the rGO-gPSS@PANI-MnO2 hybrid reached 425 F/g at the scan rate of 20 mV/s, which was much higher than those of rGO-g-PSS (190 F/g), MnO2 (203 F/g), rGO-g-PSS@PANI (305 F/g) and rGO-MnO2 (295 F/g). In addition, the rGO-g-PSS@PANI-MnO2 hybrid also showed good stability. These good electrochemical performances are due to that rGO-g-PSS@PANI-MnO2 hybrid could make full use of the high SC of PANI and MnO2, high specific surface area and conductivity of rGO, and good interfacial properties of PSS linker. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Polymers Oxides Electrochemical properties
1. Introduction Since it was reported by Geim and Novoselov in 2004 [1], graphene has received considerable attention due to its high surface area, high chemical stability, unique electronic and mechanical properties [2]. Graphene holds great promise for potential applications in nanoelectronics [3], energy resources [4], biomedical engineering [5], environmental protection [6], and hybrid materials [7], owing to its two-dimensional honeycomb lattice structure. Graphene has been widely investigated in energy devices including energy storage. It is used as electrode materials of supercapacitors and showed a specific capacitance (SC) of about 120 F/g in most reported studies [8,9]. Obviously, the SC of graphene is low compared with some metal oxides, so modification is needed to achieve a high SC for practical applications. Metal oxides such as RuO2 [10,11], V2O5
* Corresponding author. E-mail address: [email protected] (Y. Gui). http://dx.doi.org/10.1016/j.matchemphys.2015.10.052 0254-0584/© 2015 Elsevier B.V. All rights reserved.
[12,13] and MnO2 [14,15] have been often employed as electrode materials in supercapacitors. MnO2 has been a popular electrode material in supercapacitors for its low cost and a high SC (about 300 F/g) [14]. However, the electrical conductivity of MnO2 is poor. So metal oxide, like V2O5 and MnO2 were integrated with carbon nanotube or graphene to fabricate composite materials with new architectures. These composite materials usually combine advantage of the two materials and show high capacitance as electrodes in supercapacitors. The capacitive performance of the composites highly depends on the dispersion of metal oxide on supporters. For example, single or few layered graphene sheets with less agglomeration (or well dispersed carbon nanotube) provided high surface area for loading nanoparticles of metal or metal oxide [16]. One of the possible routes to obtain well dispersed graphene is the grafting modification of graphene with a hydrophilic macromolecules, for instance, polyacrylamide or poly(p-styrenesulfonate) (PSS) grafted reduced graphene oxide (rGO) [17,18]. Besides, the graphene-based materials are often incorporated with active materials like conducting polymers to obtain high
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capacitances from electrochemical through rapid and reversible redox reaction of the active materials. Conducting polymers like polyaniline (PANI) [19], polypyrrole [20] and polythiophene [21] are the common electrode materials of supercapacitors that demonstrate a high SC (about 400 F/g) [22,23]. Among these conducting polymers, PANI is even attractive as the electrode material of supercapacitors because of its simple preparation method and low cost. However, its electrochemical stability is limited because of structural conformation changes with repeated ion exchange in the electrochemical process [24]. In this paper, the reduced graphene oxide grafted with PSS (rGO-g-PSS) was prepared by in-situ free radical polymerization, and then the rGO-g-PSS was used to dope PANI (rGO-g-PSS@PANI), and finally rGO-g-PSS@PANI assembled with MnO2 nanoparticles to form rGO-g-PSS@PANI-MnO2. The rGO-g-PSS@PANI-MnO2 hybrids were applied to modify electrodes and the capacitance of these electrodes was studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (GV). 2. Experimental 2.1. Materials Graphite was obtained from Qingdao Graphite Factory. Monomer sodium p-styrenesulfonate was purchased from Aladdin Co. KMnO4, NaNO3, H2SO4, H2O2 (30%), HCl, N, N-dimethylformamide (DMF), azodiisobutyronitrile (AIBN), hydrazine monohydrate (80%), aniline, (NH4)2S2O8, KOH and all other reagents were purchased from Tianjin Chemical Reagent Co. All the chemicals were analytical grade and used as received. 2.2. Sample preparation 2.2.1. Preparation of graphene oxide (GO) GO was prepared from purified natural graphite by a modified Hummer's method [25]. 2.2.2. Preparation of the rGO-g-PSS GO (20 mg, dispersed in 50 mL of DMF) and p-styrenesulfonate monomers (2 g) were successively added into a 100 mL three-neck flask. The mixture was sonicated for 30 min to form a homogenous solution before 20 mg of AIBN was added under the protection of N2 flow and vigorous stirring. The three-neck flask was then immersed in an oil bath at 75 C to start polymerization. After reacting for a certain time (normally 48 h), the reaction was stopped and the product was filtrated, rinsed with absolute ethanol, and dried at 50 C. The product was re-dispersed in deionized water, sonicated for 15 min, and filtrated to remove any insoluble impurities. Then the filtrate was filtered through a PVDF membrane (d ¼ 0.45 mm), rinsed with deionized water to remove the un-grafted PSSNa and obtain GO-g-PSS. The GO-g-PSS dispersed in 50 mL of deionized water was added to a 100 mL three-neck flask and followed by introducing hydrazine monohydrate (80%, 200 mL). The three-neck flask was then immersed in oil bath at 85 C to reduce GO. After reducing for 24 h, the resulting product was collected by centrifugation at the speed of 9000 rpm for 5 min, and then dried at 50 C for 24 h in a vacuum oven to obtain rGO-g-PSS. 2.2.3. Preparation of rGO-g-PSS@PANI composites HCl solution (20 mL, 1 M) and rGO-g-PSS (20 mg) were added to a three-neck flask (100 mL), and then were sonicated to form a homogenous suspension before 0.466 g of aniline was added under the vigorous stirring. The three-neck flask was immersed in the ice-
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water bath for 20 min, and then 20 mL of (NH4)2S2O8 solution in a 50 mL constant pressure funnel was slowly added by dropwise to the rGO-g-PSS suspension. After the reaction had proceeded in icewater bath for 10 h, the resulting solution was filtrated, washed by absolute ethanol and deionized water, and dried at 50 C for 24 h in a vacuum oven to obtain rGO-g-PSS@PANI. 2.2.4. Preparation of the rGO-g-PSS@PANI-MnO2 hybrids A weighed amount of rGO-g-PSS@PANI was added into the KMnO4 solution and stirred for 30 min. Then MnCl2 solution was dropped into the mixture and heated in an oil bath at 80 C for 30 min. The product was filtered, rinsed with water, and dried under N2 protection at 150 C for 12 h to obtain the rGO-gPSS@PANI-MnO2 hybrid. 2.3. Characterization Fourier transform infrared (FTIR) spectroscopy analysis was recorded with a PerkineElmer Paragon-1000 Fourier transform infrared spectrometer. Scanning electronic microscope (SEM) was performed using a S4800 SEM microscope. Before observation, the sample was coated with gold using a sputter coater (Desk-II, Denton Vacuum). Transmission electron microscopy (TEM) was performed using a Philips Tecnai G2F20 microscope at 200 kV. The X-ray diffraction (XRD) patterns of the samples were measured using an X-ray diffractometer (BDX3300) with a reference target: Cu Ka radiation (l ¼ 1.54 Å), voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 80 (2q) with steps of 4 min1. Elemental analysis was conducted on an X-ray photoelectron spectrometer (XPS, PHI1600 ESCA System, PERKIN ELMER, US). The amount of MnO2 was measured by inductively coupled plasma (ICP-9000(N þ M), USA, Thermo Jarrell-Ash Corp). 2.4. Electrochemical measurement Electrochemical measurements were carried out using a threeelectrode system: the prepared electrode as the working electrode, a Hg/HgO electrode and a Pt mesh as reference and counter electrodes, respectively. The mixture containing 75 wt% active materials, 20 wt% conducting carbon black, and 5 wt% polytetrafluoroethylene (used as a binder, PTFE, 60%, Sigma Aldrich) was well mixed and then pressed onto the Ni mesh to form the working electrode. The mass loading of the active materials in the electrodes is about 8 mg. The thickness of the electrodes is about 0.54 mm. Electrochemical measurements were carried out using a threeelectrode cell with 2 M KOH as the electrolyte. The electrochemical performances of the prepared electrodes were characterized by CV and GV using a CHI660D electrochemical workstation (Shanghai Chen Hua). 3. Results and discussions 3.1. Fabrication of rGO-g-PSS@PANI-MnO2 hybrids Grafting of PSS onto rGO is crucial for the preparation of water dispersible rGO-g-PSS@PANI composite. The grafted PSS macromolecules can inhibit aggregation of the rGO sheets. The sulfonic acid groups on PSS macromolecules adsorb aniline monomers through the electrostatic interaction and therefore the in situ polymerization of aniline can occur on the surface of rGO sheets, and meanwhile PSS offers protons for acidic doping of PANI. The rGO-g-PSS was prepared through the free radical polymerization of the monomer followed by acidation. In order to determine if PSS
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Fig. 1. FTIR of rGO and rGO-g-PSS.
has been grafted onto the rGO, FTIR spectra of rGO and purified rGO-g-PSS were taken and shown in Fig. 1. The FTIR spectrum of rGO-g-PSS clearly exhibits the expected CeH stretch at 3000e2800 cm1 and the board peak at about 1680 cm1 both arising from the double bonds of aromatic carbon and sulfonic groups. The peaks near 1500 cm1 are characteristic absorption peaks for benzene and the two peaks in the fingerprint region indicate a para-substitution of sodium sulfonate; the peaks between 1000 and 1250 cm1 are characteristic CeC absorption peaks for a carbon skeleton in a macromolecular chain like PSS. So IR spectra demonstrate the grafting of PSS onto the rGO surface. XPS is a useful way to characterize carbon materials. The XPS spectra of the rGO-g-PSS are shown in Fig. 2. The S peaks indicate that PSS has been grafted onto rGO surface. The intensity of C]O bonds (about 286 eV) in Fig. 2b is much low compared with that of GO [26]. These phenomena prove that rGO is highly reduced. The above results verify that PSS has been grafted onto rGO surface successfully. The rGO-g-PSS contains negative charge eSO3 groups that can absorb aniline through electrostatic force, and therefore facilitates in situ polymerization of aniline on rGO and form rGO-g-PSS@PANI. The MnO2 were prepared in the
presence of rGO-g-PSS@PANI to form rGO-g-PSS@PANI-MnO2 hybrids. Fig. 3aed shows the SEM images of rGO-g-PSS, rGO-g-PSS@PANI and rGO-g-PSS@PANI-MnO2. The undulating morphology of rGO-gPSS sheets observed in Fig. 3a might be caused by the presence of PSS. The tethered PSS chains on the rGO surface stretched outside and repelled each other owing to the negative charge to enhance the water-solubility of the rGO when rGO-g-PSS was dispersed in water. After it was dried, PSS covered rGO and left apparent undulating morphology. The PSS made the edges of rGO sheets become smooth. In Fig. 3b, small dots distributing uniformly on the rGO-g-PSS@PANI surface are PANI. The magnified photo shows that PANI forms an array of nanorods aligned perpendicularly to the rGO sheets (Fig. 3c). The regular structure may be attributed to the grafted PSS on the rGO surface. The XPS spectrum of rGO-gPSS@PANI in Fig. 4 shows that the S and N atomic percentages are 0.8 and 4.6, respectively, so the weight percent of PSS and PANI in rGO-g-PSS@PANI is about 11.5% and 33.4%, respectively. Fig. 3d shows the SEM image of rGO-g-PSS@PANI-MnO2. The as-prepared MnO2 are nanorods. The X-ray diffraction of MnO2 (Fig. 5) clearly demonstrate that MnO2 nanorods are crystalline. The peaks at 2q values of 32.4 , 34.3 , 37.1, 41.0 , 42.6 and 56.5 are all the characteristic peaks of MnO2, correspond to the standard peaks of MnO2 (JCPDS, 53-0633; 21-0141). The TEM images of rGO-g-PSS, rGO-g-PSS@PANI and rGO-gPSS@PANI-MnO2 are shown in Fig. 6. Both PSS and PANI distributed uniformly on the rGO surface (Fig. 6aeb). Fig. 6c indicates the existence of MnO2 in rGO-g-PSS@PANI-MnO2, and the vague lattice fringes of MnO2 in Fig. 6d confirms that MnO2 is weak crystalline, which is in good accordance with the result obtained from XRD. 3.2. Electrochemical evaluation CV was employed to evaluate the electrochemical performances of the as-prepared electrodes. Fig. 7aed shows the CV curves and SCs of the rGO-g-PSS, MnO2, rGO-MnO2 (MnO2: 50 wt% determined by ICP analysis), rGO-g-PSS@PANI and rGO-g-PSS@PANI-MnO2 (with different contents of MnO2) electrodes at a scan rate of 50 mV/s, and Fig. 7eef shows the CV curves and SCs of the rGO-gPSS@PANI-MnO2 (3#, MnO2: 50 wt%) electrodes at different scan rates. Here, the total mass of these composites was used for the
Fig. 2. The XPS (a) and the C1s spectra of rGO-g-PSS.
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Fig. 3. SEM images of rGO-g-PSS (a), rGO-g-PSS@PANI (b, c) and rGO-g-PSS@PANI-MnO2 (d).
Fig. 5. XRD diagram of rGO-g-PSS@PANI-MnO2.
Fig. 4. XPS spectrum of rGO-g-PSS@PANI.
calculation of SCs. As shown in Fig. 7a, the CV curves of rGO-g-PSS, MnO2 and rGO-MnO2 are nearly rectangular, while a pair of redox peaks that come from the pseudocapacitive behavior of PANI were observed in the CV curve of rGO-g-PSS@PANI [27,28]. These peaks correspond to the transition from the emeraldine to pernigraniline oxidation state, respectively [28]. With the increasing amount of MnO2 from 25 to 83% (in Fig. 7c), the CV curves of rGO-g-PSS@PANIMnO2 were more like that of MnO2 since the redox peaks of PANI
were covered and the CV curve was dominated by MnO2. The observed peaks in Fig. 7c may be owing to the transition from the benzoquinone to phenol. The SC values calculated from CV curves in Fig. 7b show that the SC of rGO-g-PSS@PANI-MnO2 (3#) electrode is 391 F/g at a scan rate of 50 mV/s, higher than that of rGO-g-PSS (190 F/g), rGO-g-PSS@PANI (305 F/g), MnO2 (221 F/g), and rGOMnO2 (295 F/g). It is also higher than that of graphene/MnO2 electrode (251 F/g) reported by Feng [29]. The high SC of rGO-gPSS@PANI-MnO2 is attributed to its characteristic architecture that inspirits a strong synergistic effect between the components. In rGO-g-PSS@PANI-MnO2, the PANI and graphene were connected intimately through the grafted PSS, and the MnO2 dispersed in the soft mat made of PANI nanorod array to form a strong interconnecting structure that supplies a large contact surface to the electrolyte and shortens the path length for ionic transport. This also facilitates efficient electron transport to the rGO sheets and
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Fig. 6. TEM images of rGO-g-PSS (a, scale bar: 200 nm), rGO-g-PSS@PANI (b, scale bar: 200 nm) and rGO-g-PSS@PANI-MnO2 (c, scale bar: 200 nm; d, scale bar: 5 nm).
provides a continuous conductive network for fast charge collection and transfer throughout the rGO-g-PSS@PANI-MnO2 electrode [30]. In a word, the synergistic effect associated the excellent electrical conductivity of graphene and high pseudocapacitance of the MnO2 and PANI was beneficial for improving the capacitance properties. In Fig. 7d, the SCs of rGO-g-PSS@PANI-MnO2 electrodes were improved with the increasing amount of MnO2 from 25 to 50%, and a further increase in the amount of MnO2 did not result in a further increase in the SC, which may be as a result of the serious aggregation of MnO2. So the rGO-g-PSS@PANI-MnO2 (3#) has the highest SC value. As shown in Fig. 7e, the CV curves of the rGO-gPSS@PANI-MnO2 (3#) electrode at high scan rates remain the shapes at low scan rates without obvious distortion, confirming good reversibility [31]. The SCs of rGO-g-PSS@PANI-MnO2 (3#) are 425, 391, 333, and 287 F/g respectively at the scan rates of 20, 50, 100, and 200 mV/s (Fig. 7f). The capacitance retention ratio of this electrode was about 67.5% from 20 to 200 mV/s, which is superior to 60.4% from 10 to 100 mV/s for graphene/MnO2 reported by Li [32], indicating that PANI and PSSNa make contribution to the stability of rGO-g-PSS@PANI-MnO2. The decrease in SC with increasing scan rates can be attributed to the fact that the electrolyte ions are not fully accessible to the interior surfaces of the active materials for charge-storage because of the reduced diffusion time at a high scan rate [30].
Fig. 8aed shows the GV curves and SCs of the rGO-g-PSS, MnO2, rGO-MnO2 (MnO2: 50 wt%), rGO-g-PSS@PANI and rGO-gPSS@PANI-MnO2 (with different contents of MnO2) electrodes at the current density of 1 A/g and Fig. 8eef shows the GV curves and SCs of the rGO-g-PSS@PANI-MnO2 (3#) electrodes at different current densities. As shown in Fig. 8a, the GV curves of rGO-g-PSS, MnO2 and rGO-MnO2 are close to being linear, while that of rGOg-PSS@PANI deviates from linear, owing to the presence of pseudocapacitive PANI [33]. The shapes of the GV curves of rGO-gPSS@PANI-MnO2 electrodes in Fig. 8c and e are similar, while the discharge time changes with the current densities and the amount of MnO2. At the current density of 1 A/g, the SC of rGO-g-PSS@PANIMnO2 (357 F/g) is much higher than those of other four electrodes (Fig. 8b), and the SC of rGO-g-PSS@PANI-MnO2 with 50 wt% MnO2 is the largest among four rGO-g-PSS@PANI-MnO2 electrodes (Fig. 8d). Fig. 8e shows that the discharge time of rGO-g-PSS@PANIMnO2 (3#) decreased with the increasing current density, and this can be explained by slow diffusion/migration of protons in the porous electrodes at high current density. The SCs of rGO-gPSS@PANI-MnO2 (3#) at different current densities were calculated from their GV curves and shown in Fig. 8f. The SC is 357, 317, 292, 267, and 254 F/g at the current densities of 1, 3, 5, 7, and 10 A/g, respectively. The SC retained 71.1% when the current density increased from 1 A/g to 10 A/g, indicating good rate capability. This
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Fig. 7. The CV curves (a) and SCs (b) of rGO-g-PSS, rGO-g-PSS@PANI, MnO2, rGO/MnO2 (MnO2: 50 wt%) and rGO-g-PSS@PANI@MnO2 (3#) at the scan rate of 50mv/s; CV curves (c) and SCs (d) of different rGO-g-PSS@PANI-MnO2 (MnO2 wt%: 1#, 2#, 3#, 4# ¼ 25%, 33%, 50%, 83%) at the scan rate of 50mv/s; and CV curves (e) and SCs (f) of rGO-g-PSS@PANI-MnO2 (3#) at different scan rates.
phenomenon is often observed in supercapacitors [34]. For example, ɑ-MnMoO4 nanorods showed 54.5% capacitance retention as current density changed from 0.2 to 1.2 A/g. The good rate capability may be attributed to the characteristic structure of rGOg-PSS@PANI-MnO2. The nanorod array of PANI formed on the rGO surface under the directing of the grafted PSS acts as soft mat to support MnO2 nanorods and inhibits their aggregation. PANI nanorod array also acts as bridge to connect rGO sheets and MnO2. So the close contact of different components in rGO-g-PSS@PANIMnO2 hybrids ensures diffusion/migration of protons in the electrodes at high current density. The cycling stability of the rGO-g-PSS@PANI-MnO2 (3#) electrode was investigated (Fig. 9). The capacitance retentions of the rGO-g-PSS@PANI-MnO2 electrode were measured upon 2000 charge/discharge cycles at a current density of 10 A/g. After 2000-
cycle charge/discharge, the rGO-g-PSS@PANI-MnO2 electrode retained about 86% of its initial capacitance, which is similar to 86% for graphene/MnO2 after 1000-cycle charge/discharge reported by Kim [35]. The good cycle-life stability may be attributed to the presence of graphene and characteristic structure of rGO-gPSS@PANI-MnO2. The high capacitance retention indicates that the rGO-g-PSS@PANI-MnO2 electrode should be highly durable for electrochemical capacitor applications in mild aqueous electrolytes [36]. Moreover, the SCs of rGO-g-PSS@PANI-MnO2 are also higher than some reported rGO/PANI (243 F/g) [37], rGO-MnO2 (275 F/g) [38], and PANI-MnO2 (221 F/g) [39] so on. The good supercapacitive properties of rGO-g-PSS@PANI-MnO2 are owing to the fact that the PANI and graphene are connected intimately through the grafted PSS, and MnO2 disperses in the soft mat made of PANI nanorod
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Fig. 8. The GV curves (a) and SCs (b) of rGO-g-PSS, rGO-g-PSS@PANI, MnO2, rGO-MnO2 (MnO2: 50 wt%) and rGO-g-PSS@PANI-MnO2 (3#) at the current density of 1 A/g. The GV curves (c) and SCs (d) of different rGO-g-PSS@PANI-MnO2 at the current density of 1 A/g. The GV curves (e) and SCs (f) of rGO-g-PSS@PANI-MnO2 (3#) at different current densities.
Fig. 9. The GV curves (a) and capacitance retentions (b) of the rGO-g-PSS@PANI-MnO2 (3#) electrode at a constant current density of 10 A/g.
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array to form a strong interconnecting structure: (1) the strong interconnecting structure can supply a large contact surface to the electrolyte and shorten the path length for ionic transport, (2) the structure can also facilitate efficient electron transport to the rGO sheets and provide a continuous conductive network for fast charge collection and transfer throughout the electrode, (3) the close contact of different components in the hybrids ensure diffusion/ migration of protons in the electrodes at high current density. 4. Conclusions The rGO-g-PSS@PANI-MnO2 hybrids were prepared via a threestep route: the rGO-g-PSS was obtained by in-situ free radical polymerization, and then the rGO-g-PSS@PANI composite was prepared by in-situ polymerization of aniline with electrostatic adsorption between negatively-charged rGO-g-PSS and positivelycharged PANI, and finally the rGO-g-PSS@PANI was doped with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The nanorod array of PANI formed on the rGO surface under the directing of the grafted PSS acts as soft mat to support MnO2 and inhibit aggregation of MnO2 and as bridge to connect rGO sheets. The close contact of different components in rGO-g-PSS@PANI-MnO2 hybrids ensures diffusion/migration of ions in the porous electrodes, so rGO-gPSS@PANI-MnO2 hybrids demonstrated high SC and stability. The SC of the rGO-g-PSS@PANI-MnO2 hybrid reached 425 F/g at the scan rate of 20 mV/s, which is obviously improved compared to that of rGO-g-PSS (190 F/g), MnO2 (203 F/g), rGO-g-PSS@PANI (305 F/g), and rGO-MnO2 (295 F/g). The rGO-g-PSS@PANI will be a good support for other oxide electrodes in supercapacitors. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51404087) and Natural Science Foundation e Steel and Iron Foundation of Hebei Province (E2014209213). References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [2] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, Nature 448 (2007) 457. [3] Y. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Fabrication of poly(p-styrenesulfonate) grafted reduced graphene oxide/polyaniline/MnO2 hybrids with high capacitance performance Yongliang Gui*, Xuteng Xing, Chunyan Song College of Metallurgy and Energy, North China University of Science and Technology, 46 Xinhua West Road, Tangshan 063009, PR China
h i g h l i g h t s Reduced graphene oxide was grafted by p-styrenesulfonate to obtain rGO-g-PSS. The rGO-g-PSS was used to induce formation of polyaniline (PANI) nanorod array. The rGO-g-PSS@PANI was assembled with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The rGO-g-PSS@PANI-MnO2 hybrids exhibit the good electrochemical properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 29 August 2015 Accepted 23 October 2015 Available online 2 November 2015
Reduced graphene oxide grafted with poly(p-styrenesulfonate) (rGO-g-PSS) was synthesized by in-situ free radical polymerization. The rGO-g-PSS was used to induce formation of polyaniline (PANI) nanorod array and then assembled with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The rGO-g-PSS@PANIMnO2 hybrids were applied to modify electrodes and the specific capacitance (SC) of the rGO-gPSS@PANI-MnO2 hybrid reached 425 F/g at the scan rate of 20 mV/s, which was much higher than those of rGO-g-PSS (190 F/g), MnO2 (203 F/g), rGO-g-PSS@PANI (305 F/g) and rGO-MnO2 (295 F/g). In addition, the rGO-g-PSS@PANI-MnO2 hybrid also showed good stability. These good electrochemical performances are due to that rGO-g-PSS@PANI-MnO2 hybrid could make full use of the high SC of PANI and MnO2, high specific surface area and conductivity of rGO, and good interfacial properties of PSS linker. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Polymers Oxides Electrochemical properties
1. Introduction Since it was reported by Geim and Novoselov in 2004 [1], graphene has received considerable attention due to its high surface area, high chemical stability, unique electronic and mechanical properties [2]. Graphene holds great promise for potential applications in nanoelectronics [3], energy resources [4], biomedical engineering [5], environmental protection [6], and hybrid materials [7], owing to its two-dimensional honeycomb lattice structure. Graphene has been widely investigated in energy devices including energy storage. It is used as electrode materials of supercapacitors and showed a specific capacitance (SC) of about 120 F/g in most reported studies [8,9]. Obviously, the SC of graphene is low compared with some metal oxides, so modification is needed to achieve a high SC for practical applications. Metal oxides such as RuO2 [10,11], V2O5
* Corresponding author. E-mail address: [email protected] (Y. Gui). http://dx.doi.org/10.1016/j.matchemphys.2015.10.052 0254-0584/© 2015 Elsevier B.V. All rights reserved.
[12,13] and MnO2 [14,15] have been often employed as electrode materials in supercapacitors. MnO2 has been a popular electrode material in supercapacitors for its low cost and a high SC (about 300 F/g) [14]. However, the electrical conductivity of MnO2 is poor. So metal oxide, like V2O5 and MnO2 were integrated with carbon nanotube or graphene to fabricate composite materials with new architectures. These composite materials usually combine advantage of the two materials and show high capacitance as electrodes in supercapacitors. The capacitive performance of the composites highly depends on the dispersion of metal oxide on supporters. For example, single or few layered graphene sheets with less agglomeration (or well dispersed carbon nanotube) provided high surface area for loading nanoparticles of metal or metal oxide [16]. One of the possible routes to obtain well dispersed graphene is the grafting modification of graphene with a hydrophilic macromolecules, for instance, polyacrylamide or poly(p-styrenesulfonate) (PSS) grafted reduced graphene oxide (rGO) [17,18]. Besides, the graphene-based materials are often incorporated with active materials like conducting polymers to obtain high
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capacitances from electrochemical through rapid and reversible redox reaction of the active materials. Conducting polymers like polyaniline (PANI) [19], polypyrrole [20] and polythiophene [21] are the common electrode materials of supercapacitors that demonstrate a high SC (about 400 F/g) [22,23]. Among these conducting polymers, PANI is even attractive as the electrode material of supercapacitors because of its simple preparation method and low cost. However, its electrochemical stability is limited because of structural conformation changes with repeated ion exchange in the electrochemical process [24]. In this paper, the reduced graphene oxide grafted with PSS (rGO-g-PSS) was prepared by in-situ free radical polymerization, and then the rGO-g-PSS was used to dope PANI (rGO-g-PSS@PANI), and finally rGO-g-PSS@PANI assembled with MnO2 nanoparticles to form rGO-g-PSS@PANI-MnO2. The rGO-g-PSS@PANI-MnO2 hybrids were applied to modify electrodes and the capacitance of these electrodes was studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (GV). 2. Experimental 2.1. Materials Graphite was obtained from Qingdao Graphite Factory. Monomer sodium p-styrenesulfonate was purchased from Aladdin Co. KMnO4, NaNO3, H2SO4, H2O2 (30%), HCl, N, N-dimethylformamide (DMF), azodiisobutyronitrile (AIBN), hydrazine monohydrate (80%), aniline, (NH4)2S2O8, KOH and all other reagents were purchased from Tianjin Chemical Reagent Co. All the chemicals were analytical grade and used as received. 2.2. Sample preparation 2.2.1. Preparation of graphene oxide (GO) GO was prepared from purified natural graphite by a modified Hummer's method [25]. 2.2.2. Preparation of the rGO-g-PSS GO (20 mg, dispersed in 50 mL of DMF) and p-styrenesulfonate monomers (2 g) were successively added into a 100 mL three-neck flask. The mixture was sonicated for 30 min to form a homogenous solution before 20 mg of AIBN was added under the protection of N2 flow and vigorous stirring. The three-neck flask was then immersed in an oil bath at 75 C to start polymerization. After reacting for a certain time (normally 48 h), the reaction was stopped and the product was filtrated, rinsed with absolute ethanol, and dried at 50 C. The product was re-dispersed in deionized water, sonicated for 15 min, and filtrated to remove any insoluble impurities. Then the filtrate was filtered through a PVDF membrane (d ¼ 0.45 mm), rinsed with deionized water to remove the un-grafted PSSNa and obtain GO-g-PSS. The GO-g-PSS dispersed in 50 mL of deionized water was added to a 100 mL three-neck flask and followed by introducing hydrazine monohydrate (80%, 200 mL). The three-neck flask was then immersed in oil bath at 85 C to reduce GO. After reducing for 24 h, the resulting product was collected by centrifugation at the speed of 9000 rpm for 5 min, and then dried at 50 C for 24 h in a vacuum oven to obtain rGO-g-PSS. 2.2.3. Preparation of rGO-g-PSS@PANI composites HCl solution (20 mL, 1 M) and rGO-g-PSS (20 mg) were added to a three-neck flask (100 mL), and then were sonicated to form a homogenous suspension before 0.466 g of aniline was added under the vigorous stirring. The three-neck flask was immersed in the ice-
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water bath for 20 min, and then 20 mL of (NH4)2S2O8 solution in a 50 mL constant pressure funnel was slowly added by dropwise to the rGO-g-PSS suspension. After the reaction had proceeded in icewater bath for 10 h, the resulting solution was filtrated, washed by absolute ethanol and deionized water, and dried at 50 C for 24 h in a vacuum oven to obtain rGO-g-PSS@PANI. 2.2.4. Preparation of the rGO-g-PSS@PANI-MnO2 hybrids A weighed amount of rGO-g-PSS@PANI was added into the KMnO4 solution and stirred for 30 min. Then MnCl2 solution was dropped into the mixture and heated in an oil bath at 80 C for 30 min. The product was filtered, rinsed with water, and dried under N2 protection at 150 C for 12 h to obtain the rGO-gPSS@PANI-MnO2 hybrid. 2.3. Characterization Fourier transform infrared (FTIR) spectroscopy analysis was recorded with a PerkineElmer Paragon-1000 Fourier transform infrared spectrometer. Scanning electronic microscope (SEM) was performed using a S4800 SEM microscope. Before observation, the sample was coated with gold using a sputter coater (Desk-II, Denton Vacuum). Transmission electron microscopy (TEM) was performed using a Philips Tecnai G2F20 microscope at 200 kV. The X-ray diffraction (XRD) patterns of the samples were measured using an X-ray diffractometer (BDX3300) with a reference target: Cu Ka radiation (l ¼ 1.54 Å), voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 80 (2q) with steps of 4 min1. Elemental analysis was conducted on an X-ray photoelectron spectrometer (XPS, PHI1600 ESCA System, PERKIN ELMER, US). The amount of MnO2 was measured by inductively coupled plasma (ICP-9000(N þ M), USA, Thermo Jarrell-Ash Corp). 2.4. Electrochemical measurement Electrochemical measurements were carried out using a threeelectrode system: the prepared electrode as the working electrode, a Hg/HgO electrode and a Pt mesh as reference and counter electrodes, respectively. The mixture containing 75 wt% active materials, 20 wt% conducting carbon black, and 5 wt% polytetrafluoroethylene (used as a binder, PTFE, 60%, Sigma Aldrich) was well mixed and then pressed onto the Ni mesh to form the working electrode. The mass loading of the active materials in the electrodes is about 8 mg. The thickness of the electrodes is about 0.54 mm. Electrochemical measurements were carried out using a threeelectrode cell with 2 M KOH as the electrolyte. The electrochemical performances of the prepared electrodes were characterized by CV and GV using a CHI660D electrochemical workstation (Shanghai Chen Hua). 3. Results and discussions 3.1. Fabrication of rGO-g-PSS@PANI-MnO2 hybrids Grafting of PSS onto rGO is crucial for the preparation of water dispersible rGO-g-PSS@PANI composite. The grafted PSS macromolecules can inhibit aggregation of the rGO sheets. The sulfonic acid groups on PSS macromolecules adsorb aniline monomers through the electrostatic interaction and therefore the in situ polymerization of aniline can occur on the surface of rGO sheets, and meanwhile PSS offers protons for acidic doping of PANI. The rGO-g-PSS was prepared through the free radical polymerization of the monomer followed by acidation. In order to determine if PSS
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Fig. 1. FTIR of rGO and rGO-g-PSS.
has been grafted onto the rGO, FTIR spectra of rGO and purified rGO-g-PSS were taken and shown in Fig. 1. The FTIR spectrum of rGO-g-PSS clearly exhibits the expected CeH stretch at 3000e2800 cm1 and the board peak at about 1680 cm1 both arising from the double bonds of aromatic carbon and sulfonic groups. The peaks near 1500 cm1 are characteristic absorption peaks for benzene and the two peaks in the fingerprint region indicate a para-substitution of sodium sulfonate; the peaks between 1000 and 1250 cm1 are characteristic CeC absorption peaks for a carbon skeleton in a macromolecular chain like PSS. So IR spectra demonstrate the grafting of PSS onto the rGO surface. XPS is a useful way to characterize carbon materials. The XPS spectra of the rGO-g-PSS are shown in Fig. 2. The S peaks indicate that PSS has been grafted onto rGO surface. The intensity of C]O bonds (about 286 eV) in Fig. 2b is much low compared with that of GO [26]. These phenomena prove that rGO is highly reduced. The above results verify that PSS has been grafted onto rGO surface successfully. The rGO-g-PSS contains negative charge eSO3 groups that can absorb aniline through electrostatic force, and therefore facilitates in situ polymerization of aniline on rGO and form rGO-g-PSS@PANI. The MnO2 were prepared in the
presence of rGO-g-PSS@PANI to form rGO-g-PSS@PANI-MnO2 hybrids. Fig. 3aed shows the SEM images of rGO-g-PSS, rGO-g-PSS@PANI and rGO-g-PSS@PANI-MnO2. The undulating morphology of rGO-gPSS sheets observed in Fig. 3a might be caused by the presence of PSS. The tethered PSS chains on the rGO surface stretched outside and repelled each other owing to the negative charge to enhance the water-solubility of the rGO when rGO-g-PSS was dispersed in water. After it was dried, PSS covered rGO and left apparent undulating morphology. The PSS made the edges of rGO sheets become smooth. In Fig. 3b, small dots distributing uniformly on the rGO-g-PSS@PANI surface are PANI. The magnified photo shows that PANI forms an array of nanorods aligned perpendicularly to the rGO sheets (Fig. 3c). The regular structure may be attributed to the grafted PSS on the rGO surface. The XPS spectrum of rGO-gPSS@PANI in Fig. 4 shows that the S and N atomic percentages are 0.8 and 4.6, respectively, so the weight percent of PSS and PANI in rGO-g-PSS@PANI is about 11.5% and 33.4%, respectively. Fig. 3d shows the SEM image of rGO-g-PSS@PANI-MnO2. The as-prepared MnO2 are nanorods. The X-ray diffraction of MnO2 (Fig. 5) clearly demonstrate that MnO2 nanorods are crystalline. The peaks at 2q values of 32.4 , 34.3 , 37.1, 41.0 , 42.6 and 56.5 are all the characteristic peaks of MnO2, correspond to the standard peaks of MnO2 (JCPDS, 53-0633; 21-0141). The TEM images of rGO-g-PSS, rGO-g-PSS@PANI and rGO-gPSS@PANI-MnO2 are shown in Fig. 6. Both PSS and PANI distributed uniformly on the rGO surface (Fig. 6aeb). Fig. 6c indicates the existence of MnO2 in rGO-g-PSS@PANI-MnO2, and the vague lattice fringes of MnO2 in Fig. 6d confirms that MnO2 is weak crystalline, which is in good accordance with the result obtained from XRD. 3.2. Electrochemical evaluation CV was employed to evaluate the electrochemical performances of the as-prepared electrodes. Fig. 7aed shows the CV curves and SCs of the rGO-g-PSS, MnO2, rGO-MnO2 (MnO2: 50 wt% determined by ICP analysis), rGO-g-PSS@PANI and rGO-g-PSS@PANI-MnO2 (with different contents of MnO2) electrodes at a scan rate of 50 mV/s, and Fig. 7eef shows the CV curves and SCs of the rGO-gPSS@PANI-MnO2 (3#, MnO2: 50 wt%) electrodes at different scan rates. Here, the total mass of these composites was used for the
Fig. 2. The XPS (a) and the C1s spectra of rGO-g-PSS.
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Fig. 3. SEM images of rGO-g-PSS (a), rGO-g-PSS@PANI (b, c) and rGO-g-PSS@PANI-MnO2 (d).
Fig. 5. XRD diagram of rGO-g-PSS@PANI-MnO2.
Fig. 4. XPS spectrum of rGO-g-PSS@PANI.
calculation of SCs. As shown in Fig. 7a, the CV curves of rGO-g-PSS, MnO2 and rGO-MnO2 are nearly rectangular, while a pair of redox peaks that come from the pseudocapacitive behavior of PANI were observed in the CV curve of rGO-g-PSS@PANI [27,28]. These peaks correspond to the transition from the emeraldine to pernigraniline oxidation state, respectively [28]. With the increasing amount of MnO2 from 25 to 83% (in Fig. 7c), the CV curves of rGO-g-PSS@PANIMnO2 were more like that of MnO2 since the redox peaks of PANI
were covered and the CV curve was dominated by MnO2. The observed peaks in Fig. 7c may be owing to the transition from the benzoquinone to phenol. The SC values calculated from CV curves in Fig. 7b show that the SC of rGO-g-PSS@PANI-MnO2 (3#) electrode is 391 F/g at a scan rate of 50 mV/s, higher than that of rGO-g-PSS (190 F/g), rGO-g-PSS@PANI (305 F/g), MnO2 (221 F/g), and rGOMnO2 (295 F/g). It is also higher than that of graphene/MnO2 electrode (251 F/g) reported by Feng [29]. The high SC of rGO-gPSS@PANI-MnO2 is attributed to its characteristic architecture that inspirits a strong synergistic effect between the components. In rGO-g-PSS@PANI-MnO2, the PANI and graphene were connected intimately through the grafted PSS, and the MnO2 dispersed in the soft mat made of PANI nanorod array to form a strong interconnecting structure that supplies a large contact surface to the electrolyte and shortens the path length for ionic transport. This also facilitates efficient electron transport to the rGO sheets and
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Fig. 6. TEM images of rGO-g-PSS (a, scale bar: 200 nm), rGO-g-PSS@PANI (b, scale bar: 200 nm) and rGO-g-PSS@PANI-MnO2 (c, scale bar: 200 nm; d, scale bar: 5 nm).
provides a continuous conductive network for fast charge collection and transfer throughout the rGO-g-PSS@PANI-MnO2 electrode [30]. In a word, the synergistic effect associated the excellent electrical conductivity of graphene and high pseudocapacitance of the MnO2 and PANI was beneficial for improving the capacitance properties. In Fig. 7d, the SCs of rGO-g-PSS@PANI-MnO2 electrodes were improved with the increasing amount of MnO2 from 25 to 50%, and a further increase in the amount of MnO2 did not result in a further increase in the SC, which may be as a result of the serious aggregation of MnO2. So the rGO-g-PSS@PANI-MnO2 (3#) has the highest SC value. As shown in Fig. 7e, the CV curves of the rGO-gPSS@PANI-MnO2 (3#) electrode at high scan rates remain the shapes at low scan rates without obvious distortion, confirming good reversibility [31]. The SCs of rGO-g-PSS@PANI-MnO2 (3#) are 425, 391, 333, and 287 F/g respectively at the scan rates of 20, 50, 100, and 200 mV/s (Fig. 7f). The capacitance retention ratio of this electrode was about 67.5% from 20 to 200 mV/s, which is superior to 60.4% from 10 to 100 mV/s for graphene/MnO2 reported by Li [32], indicating that PANI and PSSNa make contribution to the stability of rGO-g-PSS@PANI-MnO2. The decrease in SC with increasing scan rates can be attributed to the fact that the electrolyte ions are not fully accessible to the interior surfaces of the active materials for charge-storage because of the reduced diffusion time at a high scan rate [30].
Fig. 8aed shows the GV curves and SCs of the rGO-g-PSS, MnO2, rGO-MnO2 (MnO2: 50 wt%), rGO-g-PSS@PANI and rGO-gPSS@PANI-MnO2 (with different contents of MnO2) electrodes at the current density of 1 A/g and Fig. 8eef shows the GV curves and SCs of the rGO-g-PSS@PANI-MnO2 (3#) electrodes at different current densities. As shown in Fig. 8a, the GV curves of rGO-g-PSS, MnO2 and rGO-MnO2 are close to being linear, while that of rGOg-PSS@PANI deviates from linear, owing to the presence of pseudocapacitive PANI [33]. The shapes of the GV curves of rGO-gPSS@PANI-MnO2 electrodes in Fig. 8c and e are similar, while the discharge time changes with the current densities and the amount of MnO2. At the current density of 1 A/g, the SC of rGO-g-PSS@PANIMnO2 (357 F/g) is much higher than those of other four electrodes (Fig. 8b), and the SC of rGO-g-PSS@PANI-MnO2 with 50 wt% MnO2 is the largest among four rGO-g-PSS@PANI-MnO2 electrodes (Fig. 8d). Fig. 8e shows that the discharge time of rGO-g-PSS@PANIMnO2 (3#) decreased with the increasing current density, and this can be explained by slow diffusion/migration of protons in the porous electrodes at high current density. The SCs of rGO-gPSS@PANI-MnO2 (3#) at different current densities were calculated from their GV curves and shown in Fig. 8f. The SC is 357, 317, 292, 267, and 254 F/g at the current densities of 1, 3, 5, 7, and 10 A/g, respectively. The SC retained 71.1% when the current density increased from 1 A/g to 10 A/g, indicating good rate capability. This
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Fig. 7. The CV curves (a) and SCs (b) of rGO-g-PSS, rGO-g-PSS@PANI, MnO2, rGO/MnO2 (MnO2: 50 wt%) and rGO-g-PSS@PANI@MnO2 (3#) at the scan rate of 50mv/s; CV curves (c) and SCs (d) of different rGO-g-PSS@PANI-MnO2 (MnO2 wt%: 1#, 2#, 3#, 4# ¼ 25%, 33%, 50%, 83%) at the scan rate of 50mv/s; and CV curves (e) and SCs (f) of rGO-g-PSS@PANI-MnO2 (3#) at different scan rates.
phenomenon is often observed in supercapacitors [34]. For example, ɑ-MnMoO4 nanorods showed 54.5% capacitance retention as current density changed from 0.2 to 1.2 A/g. The good rate capability may be attributed to the characteristic structure of rGOg-PSS@PANI-MnO2. The nanorod array of PANI formed on the rGO surface under the directing of the grafted PSS acts as soft mat to support MnO2 nanorods and inhibits their aggregation. PANI nanorod array also acts as bridge to connect rGO sheets and MnO2. So the close contact of different components in rGO-g-PSS@PANIMnO2 hybrids ensures diffusion/migration of protons in the electrodes at high current density. The cycling stability of the rGO-g-PSS@PANI-MnO2 (3#) electrode was investigated (Fig. 9). The capacitance retentions of the rGO-g-PSS@PANI-MnO2 electrode were measured upon 2000 charge/discharge cycles at a current density of 10 A/g. After 2000-
cycle charge/discharge, the rGO-g-PSS@PANI-MnO2 electrode retained about 86% of its initial capacitance, which is similar to 86% for graphene/MnO2 after 1000-cycle charge/discharge reported by Kim [35]. The good cycle-life stability may be attributed to the presence of graphene and characteristic structure of rGO-gPSS@PANI-MnO2. The high capacitance retention indicates that the rGO-g-PSS@PANI-MnO2 electrode should be highly durable for electrochemical capacitor applications in mild aqueous electrolytes [36]. Moreover, the SCs of rGO-g-PSS@PANI-MnO2 are also higher than some reported rGO/PANI (243 F/g) [37], rGO-MnO2 (275 F/g) [38], and PANI-MnO2 (221 F/g) [39] so on. The good supercapacitive properties of rGO-g-PSS@PANI-MnO2 are owing to the fact that the PANI and graphene are connected intimately through the grafted PSS, and MnO2 disperses in the soft mat made of PANI nanorod
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Fig. 8. The GV curves (a) and SCs (b) of rGO-g-PSS, rGO-g-PSS@PANI, MnO2, rGO-MnO2 (MnO2: 50 wt%) and rGO-g-PSS@PANI-MnO2 (3#) at the current density of 1 A/g. The GV curves (c) and SCs (d) of different rGO-g-PSS@PANI-MnO2 at the current density of 1 A/g. The GV curves (e) and SCs (f) of rGO-g-PSS@PANI-MnO2 (3#) at different current densities.
Fig. 9. The GV curves (a) and capacitance retentions (b) of the rGO-g-PSS@PANI-MnO2 (3#) electrode at a constant current density of 10 A/g.
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array to form a strong interconnecting structure: (1) the strong interconnecting structure can supply a large contact surface to the electrolyte and shorten the path length for ionic transport, (2) the structure can also facilitate efficient electron transport to the rGO sheets and provide a continuous conductive network for fast charge collection and transfer throughout the electrode, (3) the close contact of different components in the hybrids ensure diffusion/ migration of protons in the electrodes at high current density. 4. Conclusions The rGO-g-PSS@PANI-MnO2 hybrids were prepared via a threestep route: the rGO-g-PSS was obtained by in-situ free radical polymerization, and then the rGO-g-PSS@PANI composite was prepared by in-situ polymerization of aniline with electrostatic adsorption between negatively-charged rGO-g-PSS and positivelycharged PANI, and finally the rGO-g-PSS@PANI was doped with MnO2 to form rGO-g-PSS@PANI-MnO2 hybrids. The nanorod array of PANI formed on the rGO surface under the directing of the grafted PSS acts as soft mat to support MnO2 and inhibit aggregation of MnO2 and as bridge to connect rGO sheets. The close contact of different components in rGO-g-PSS@PANI-MnO2 hybrids ensures diffusion/migration of ions in the porous electrodes, so rGO-gPSS@PANI-MnO2 hybrids demonstrated high SC and stability. The SC of the rGO-g-PSS@PANI-MnO2 hybrid reached 425 F/g at the scan rate of 20 mV/s, which is obviously improved compared to that of rGO-g-PSS (190 F/g), MnO2 (203 F/g), rGO-g-PSS@PANI (305 F/g), and rGO-MnO2 (295 F/g). The rGO-g-PSS@PANI will be a good support for other oxide electrodes in supercapacitors. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51404087) and Natural Science Foundation e Steel and Iron Foundation of Hebei Province (E2014209213). References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [2] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, Nature 448 (2007) 457. [3] Y. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201.
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