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Morphology controllable nano-sheet polypyrrole–graphene composites for high-rate supercapacitor† Jianbo Zhu,a Youlong Xu,*a Jie Wang,a Jingping Wang,b Yang Baia and Xianfeng Dua Polypyrrole is a promising candidate for supercapacitor electrode materials due to its high capacitance and low cost. However, the major bottlenecks restricting its application are its poor rate capability and cycling stability. Herein, we control the morphology of polypyrrole–graphene composites by adjusting the graphene content, causing the typical ‘‘cauliflower’’ morphology of polypyrrole to gradually turn into the homogeneous nano-sheet morphology of these composites. The composites consequently exhibit good thermal stability, high protonation level (37.4%), high electronic conductivity (625.3 S m1), and fast relaxation time (0.22 s). These remarkable characteristics afford a high capacitance of 255.7 F g1 at
Received 11th May 2015, Accepted 9th June 2015
0.2 A g1, still retaining a capacitance of 199.6 F g1 at 25.6 A g1. In addition, high capacitance retention of up to 93% is observed after 1000 cycles testing at different current densities of 0.2, 1.6, 6.4,
DOI: 10.1039/c5cp02710a
12.8 and 25.6 A g1, indicating high stability. The composite’s excellent electrochemical performance is
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mainly attributed to its nano-sheet structure and high electronic conductivity, providing unobstructed pathways for the fast diffusion and exchange of ions/electrons.
1. Introduction In the wake of climate change, the deteriorating nature of the environment, and the exhaustion of the fossil resource, the development of renewable energy production and hybrid electric vehicles has attracted widespread attention in the 21st century.1,2 Over the past decade, many studies have been conducted on secondary batteries such as fuel cells and lithium ion batteries which are the forefront of energy storage technology.3 Nonetheless, with the growing requirement for portable systems and electric vehicles which need emergency high power supply, a new energy storage device in the form of the supercapacitor has become of significant interest to the public and researcher.4 Supercapacitors (SCs), also known as electrochemical capacitors or ultracapacitors, can be fully charged or discharged in seconds
a
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710021, China. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China † Electronic supplementary information (ESI) available: The file provides more detailed information regarding the fabrication of a supercapacitor by a twoelectrode cells system, the micelle particle size distribution of Py monomers and the influence of RGO addition, the calculation formulas of power and energy density, and the IR drop for each sample at different current densities. See DOI: 10.1039/c5cp02710a
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with a high power capability.5 Many materials have been investigated for use as SC electrode materials, including carbon materials possessing an electric double layer capacitance6,7 and conducting polymers (CPs)8–10 with pseudo-capacitance due to the fast and reversible redox reaction process, as well as some metal oxides (e.g. RuO2,11 MnO2,12 or MoO313). Conducting polymers (CPs) are well known as SCs electrode materials due to their high flexibility, facile synthesis, and relative high specific capacitance.14–16 Among CPs, polypyrrole (PPy) is considered as an excellent potential material due to its environmental friendliness, good electrochemical reversibility and low cost.17 However, the major problems for the application of PPy in SCs are its poor stability during charging–discharging cycles and its large capacitance loss at high current density.9,14 Thus, it is necessary to improve the long-term cycling stability and rate capability of PPy in order for it to be usefully employed in this short-term pulse application. Some composites of PPy and carbon materials, such as carbon foam,18 carbon nanotubes,19 and graphene,20–22 have been prepared to improve the electrochemical stability. Among these carbon materials, graphene, as a quasi-two-dimensional carbon with unique electronic and mechanical properties, has been received with rapidly growing research interest in recent years.23–25 Due to its high electronic conductivity, large surface area, superior chemical stability, and a broad electrochemical window, graphene and graphene-based carbon materials show
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good electrochemical performances as SC electrodes.26–29 Biswas et al. prepared multi layered architecture graphene and PPy nanowire composites with a specific capacitance of 165 F g1 at discharge current density of 1 A g1 by a simple and effective ultrasonication comixing method.30 Zhang et al.31 and Liu et al.32 reported the multi-layered structure composites of PPy fiber and PPy nanotube mixed with graphene oxide to enhance the mechanical strength and stabilize the structure during the charge–discharge process. Moreover, Ding et al. simultaneously added carbon nanotubes and reduced graphene oxide into the PPy matrix to form a 3-D highly-porous structure, bridging the defect for electron transfer between the composites.33 Currently, some literature clearly indicates that the electrochemical properties of PPy have been significantly improved by the addition of graphene,30–36 and this improvement is generally attributed to the large specific surface area and high electronic conductivity of graphene, and the synergistic effect of composites. However, further study is necessary in order to understand the more detailed reasons for why and how the addition of graphene can greatly improve the specific capacitance and rate capability, in addition to understanding its effect on charge/ion diffusion and exchange at the electrode/electrolyte interface. In this paper, we prepared the PPy–RGO composites with moderate reducing graphene as a soft template (Fig. 1), and adjusted the graphene content to control the morphology of these composites. With increasing graphene content, an obvious trend can be observed that the morphology changed from cauliflower shaped particles of PPy to graphene-like nanosheets of PPy/RGO, which can provide more accessible surface active sites for the adsorption and exchange of electrolyte ions. The as-prepared PPy–RGO composites exhibit good thermal stability, a high doping level and protonation level, and a large electronic conductivity, which endow these composites with their high capacitance and superior rate performance as supercapacitor electrodes.
graphene oxide (RGO) was prepared by reduction of GO with hydrazine hydrate according to the literature.39 In a typical experiment, 0.5 g of GO was dispersed in 500 ml DI water by ultrasonication for 1 h. Then the resulting homogeneous dispersion was mixed with 0.5 ml of hydrazine solution (N2H2, 80 wt% in water) and 2 ml of ammonia solution (NH3H2O, 26 wt% in water), and the mixture was heated at 95 1C for 10 min. The suspension was then filtered through a micropore filter, washed copiously with DI water and dried in a vacuum oven at 45 1C over night. Pyrrole (Py, Capchem, 99.8%) was distilled and stored at 10 1C in a nitrogen atmosphere prior to use. All other chemicals from Sinopharm Chemical Reagent Co. Ltd were reagent grade and used as received. Composites of reduced graphene oxide and PPy were prepared by in situ polymerization in an acidic solution. 0, 25, 50, 100, and 200 mg reduced graphene oxide was dispersed in 100 ml of 0.1 M p-toluenesulfonate acid (TOSH) solution by ultrasonication for 1 h. 10 mmol (0.677 g) of purified Py monomer was then dissolved into the solution and vigorously stirred at low temperature (below 4 1C) for 30 min, and then 3 g of potassium persulfate (PPS) dissolved in 10 ml of the precooled 0.1 M TOSH solution was added slowly to the above mixture solution. After further stirring at low temperature (below 4 1C) for 24 h, the product was collected by filtration, washed with DI water until the filtrate became a neutral solution (pH E 7), dried at 60 1C overnight under vacuum, and then named as PPy, PPy/RGO-2.5, PPy/RGO-5, PPy/RGO-10, and PPy/RGO-20, respectively. The mass percentage of RGO in these samples was estimated by weighing the mass of added RGO and the final products after polymerization. For comparison, physically mixed PPy and RGO at corresponding mass ratios were prepared to investigate the thermal stability of the PPy–RGO composites, which were named PPy97.5 + RGO2.5, PPy95 + RGO5, PPy90 + RGO10, and PPy80 + RGO20, respectively.
2. Experimental
The structure and morphology of the products were characterized by transmission electron microscopy (TEM; JEM-2100, JEOL) with an accelerating voltage of 200 kV, and field emission scanning electron microscopy (FE-SEM; JSM-6700F, JEOL; Hitachi, S-4800) coupled with energy-dispersive X-ray spectroscopy (EDS). The chemical structure of each product was ascertained by Fourier transform infrared spectroscopy (FT-IR, Bruker, VERTEX 70) and X-ray photoelectron spectroscopic (XPS) measurement with an AXIS-ULTRA DLD (Al Ka X-ray source, 1486.60 eV). The samples for FT-IR measurement were typically examined in KBr pellets after thorough mixing. All binding energies of XPS were referenced to the C 1s neutral carbon peak set at 284.6 eV to compensate for surface charging effects. Thermo-gravimetric analysis (TGA) was performed under nitrogen flow at a heating rate of 10 1C min1 with a Mettler Toledo TGA/DSC1 (STARe system) instrument. Micelle particle size distribution analysis of Py monomers and RGO in aqueous solution was studied on a Zetasizer Nano instrument (Malvern, ZEN3690). The bulk electronic conductivity measurements of the products were made on pressed
2.1
Material synthesis
Graphene oxide (GO) was synthesized from natural graphite (300 mesh) by a modified Hummers method.37,38 Reduced
Fig. 1 Illustration of the preparation process of the PPy and PPy–RGO composites.
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2.2
Material characterization
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wafers (1 cm diameter, o1 mm thickness) and measured by a four point probe resistivity meter (Guangzhou 4-probes Tect, RTS-9) at 23 1 1C. Electrochemical performances of the products were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques in a symmetrical two-electrode cell system. EIS measurements were performed at open circuit voltage in the frequency range from 100 KHz to 10 MHz. All electrochemical tests were performed on the Versatile Multichannel Potentiostat 2/Z (VMP2, Princeton applied research). Cycling stability testing was performed by GCD measurement on the Arbin battery test system. For the preparation of working electrodes, the prepared active powder was mixed with carbon black and polyvinylidene fluoride (80 : 10 : 10) in N-methyl-2-pyrrolidone. Then the slurry was coated onto titanium foil and dried at 50 1C overnight under vacuum. The mass of active material was about 1.4 0.2 mg in each electrode. To fabricate the two-electrode cell system, two electrodes with the same active material load were chosen for the symmetrical supercapacitor and measured in an airtight glass container with 3 M KCl aqueous solution as electrolyte. The distance between two electrodes was about 2 mm, and a photograph of this supercapacitor is shown in Fig. S1 (ESI†).
3. Results and discussion 3.1
Microstructure characterizations
Fig. 2 shows the FT-IR spectra of Py monomer, PPy, PPy/RGO, and RGO. Compared with the Py monomer spectrum, it can be clearly observed that the marked peaks of the PPy chain appear in both the PPy and PPy/RGO spectra, indicating that the PPy polymer has been prepared after polymerization. In the spectra of PPy and PPy/RGO, the peaks at 1540 and 1462 cm1 are due to the anti-symmetric and symmetric ring-stretching modes, respectively, which are the characteristic peaks of the PPy chains.40 The peaks near 1297 and 1174 cm1 are attributed to the stretching vibration of C–N, and the peaks at 1041 and 895 cm1 are attributed to the bending vibration of QC–H.41
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For RGO, the broad peak at 3395 cm1 is attributed to the residual hydroxyl and carboxyl groups, which are the hydrophilic groups which ensure the homogeneous dispersion of RGO in the aqueous solution. Compared with the pure PPy, PPy/RGO shows a new peak at 3395 cm1 attributed to the addition of RGO, and the peak intensity increases with the addition of the mass percentage of RGO in the composites, which indicates that the RGO was mixed with the PPy. FE-SEM and TEM images of RGO, PPy and all PPy/RGO composites are shown in Fig. 3 and 4, respectively. The PPy morphology as shown in Fig. 3b exhibits a typical ‘‘cauliflower’’ structure with a large accumulation of spherical particles of about 400 nm in diameter. During the polymerization process, PPy polymerizes into particles and naturally aggregates into the typical ‘‘cauliflower’’ structure because Py monomer disperses in aqueous solution in the form of micellar emulsions as illustrated in Fig. 1a. For PPy/RGO, with the addition of RGO (Fig. 3a), the Py monomer can stably absorb onto the surface of graphene due to the electrostatic attraction between the Py monomer and graphene.34,42 Micelle particle size distribution analysis indicates that the Py monomer easily absorbs onto RGO surface, implying that the Py monomer and RGO can form stable adsorption during the polymerization process (Fig. S2 and S3, ESI†). More nano-sheets are formed, replacing the spherical particles as the RGO content is increased from 2.11% to 19.2%. For PPy/RGO-2.5 and PPy/RGO-5, a small amount of spherical particles can be seen in Fig. 3c and d, respectively, which is due to that the amount of Py exceeding the absorption limit of the RGO present, and thus the residual free Py forms these aggregates. When the content of RGO reaches 10.2%, the morphology of PPy/RGO-10 (Fig. 3e) becomes uniform nano-sheets with a thickness of about 80 nm (Fig. S4, ESI†). Additionally, the uniform distribution of nitrogen and sulfur elements as verified by EDS elemental mapping suggests that the PPy chains are uniformly coated onto the graphene surface (Fig. S5–S10, ESI†). Moreover, TEM images of these samples (Fig. 4) also suggest that the nano-sheet morphology becomes more and more obvious with increasing RGO content. 3.2
Fig. 2 FTIR spectra of pyrrole monomer, PPy, RGO, and all of the PPy–RGO composites.
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Thermo-gravimetric and XPS analysis
The composition and thermal stability of PPy, RGO, and PPy/RGO were studied by thermo-gravimetric analysis (TGA). For comparison, physically mixed PPy and RGO samples at corresponding mass ratios were measured under the same conditions. As shown in Fig. 5a, GO shows a large mass loss (about 38.4%) between 200 and 300 1C due to the decomposition of oxygen-containing groups.43 After reduction by hydrazine, the obtained RGO shows only a 12.9% mass loss from 50 to 500 1C, indicating that most of the oxygen-containing groups have been removed during the chemical reduction process. PPy displays 56.0% mass loss from 50 to 1000 1C because of de-doping and pyrolysis. Compared with the mixed sample of PPy and PPy90 + RGO10 under the same conditions, it can be seen that PPy/RGO-10 has similar TGA curve yet shows less mass loss from 100 to 500 1C indicating its better thermal stability. A similar phenomenon can also be observed in the other samples (Fig. S11, ESI†). Furthermore, the mass
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Fig. 3
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SEM images of (a) RGO, (b) PPy, (c) PPy/RGO-2.5, (d) PPy/RGO-5, (e) PPy/RGO-10, and (f) PPy/RGO-20.
Fig. 4 TEM images of (a) RGO, (b) PPy, (c) PPy/RGO-2.5, (d) PPy/RGO-5, (e) PPy/RGO-10, and (f) PPy/RGO-20.
percentage of RGO in each PPy–RGO composite can be calculated from the mass loss of each TG curve (Fig. S11 and Table S3, ESI†), which is in good agreement with the results calculated by weighing (Table 1). On the other hand, thermal de-doping of doped ion led to significant mass loss in the temperature range of 200 to 400 1C.44,45 Fig. 5b presents the comparison of the temperature at different mass losses. The temperature increases with increasing RGO content in both the composites and physically mixed samples. Compared with the physically mixed samples, the higher degradation temperatures seen for the composites suggest that more stable ion-doping can be obtained for PPy– RGO. Two reasons may account for this thermal stability enhancement. First, the higher thermal conductivity of RGO
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may facilitate heat dissipation within the polymer composites. Second, RGO as an electron acceptor will remove electrons from the PPy chains and make the negatively charged doped-anions more stably embedded in the polymer chains.29,46,47 To investigate the protonation and doping level of PPy/RGO, X-ray photoelectron microscopy (XPS) was used for the elemental component characterization of PPy and PPy/RGO. Fig. 6 depicts the XPS survey spectra and N 1s region spectra of these samples. The N 1s and S 2p signals are due to the cationic PPy chains and their TOS dopant anions, respectively. The surface percentage doping level can be calculated from the S/N atomic ratio of PPy and PPy/RGO,48 which corresponds to the protonation level. Moreover, The N 1s spectra show relatively broad peaks, which
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Fig. 6
Fig. 5 (a) TGA curves and (b) temperatures at different mass loss of the PPy–RGO composites and the samples of PPy mixed with RGO.
Table 1
TGA and XPS analysis characteristic of PPy and all PPy/RGO
Mass percentage of RGO (%)
Doping level (%) +
Sample
Weighing
TG analysis
S/N
N /N
PPy PPy/RGO-2.5 PPy/RGO-5 PPy/RGO-10 PPy/RGO-20
0 2.18 5.01 9.62 18.18
0 2.11 5.28 10.2 19.2
26.7 28.1 29.6 31.9 32.8
22.7 28.5 31.6 33.8 37.4
can be fitted by three decomposed peaks at 399.9 0.1 eV, 401.1 0.2 eV, and 402.3 0.2 eV, and these peaks are attributed to the benzenoid amine (–NH–), protonated benzenoid amine (–N+H–), and protonated quinonoid imine (QN+H–), respectively.49,50 The shoulder on the higher energy side is considered as arising from an electrostatic interaction with the nearest doped-counterions, and the doping level can be calculated by determining the contribution of positively charged nitrogen (N+) to the total nitrogen content of PPy.50 Peak fitting of the N 1s regions for the various samples are shown in Fig. 6, and the calculated doping levels are summarized in Table 1. Both the direct S/N atomic ratios and the N+/N ratios indicate that the PPy/RGO composites have a higher doping level, which increases with increasing RGO content. For PPy/RGO, the doping
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XPS spectra and N 1s region spectra of PPy and all PPy/RGO.
level calculated form the N+/N ratios is slightly higher than that from the direct S/N ratios. The reason for this may be attributed to the RGO acting as an electron acceptor, taking away electrons from the PPy chains and leading to further protonation. Consequently, the higher doping and protonation level can improve the electronic conductivity of PPy/RGO, which is one of the key elements for its application as a high-rate supercapacitor electrode. 3.3
Electronic conductivity and electrochemical behavior
The electronic conductivities (s) of PPy/RGO, PPy and RGO were determined on pressed wafers from powder by using a conventional four point probe technique, and the average electronic conductivities are shown in Table 2. RGO has a good conductivity of 1004.2 S m1, which is lower than that reported by Li (about 7222 S m1),39 but higher than some other reports.51,52 The conductivity of PPy is only 78.7 S m1, similar to the report by Zhang et al.53 For PPy/RGO, as discussed above, the nanosheet structure composites have a higher doping and protonation level which can greatly improve their electrical properties. The electronic conductivity of the composites increases with the addition of RGO, and a high electronic conductivity of 625.3 S m1 can be obtained for PPy/RGO-10, which is about eight times of that of PPy. Fig. 7 shows the comparison of CV and GCD curves measured in a two-electrode cell system between all of the composite electrodes with different ratios of PPy and RGO. As shown in Fig. 7a, the CV curves are nearly rectangular in shape at scan rate of 50 mV s1, and the area of the quasi-rectangle corresponds to the storage capability. It can be seen that all of the electrodes have good pseudo-capacitive performance at low scan rate, and PPy/RGO-10 and PPy/RGO-20 exhibit large storage capability.
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Table 2 Electronic conductivities of PPy/RGO, PPy and RGO and their specific capacitances at different current densities in 3 M KCl (aq.)
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Samples
Electronic Specific capacitance (F g1) conductivity 0.2 A g1 1.6 A g1 6.4 A g1 25.6 A g1 (S m1)
PPy/RGO-20 633.2 PPy/RGO-10 625.3 PPy/RGO-5 561.0 PPy/RGO-2.5 385.9 PPy 78.7 RGO 1004.2
258.3 255.7 234.8 200.5 121.0 38.1
202.7 223.5 166.4 98.8 47.3 30.4
189.9 215.2 154.1 83.6 36.3 28.7
158.4 199.6 125.5 48.4 16.1 26.3
matrix, and the diameter of the arc in the high frequency region is the charge transfer resistance (Rct), which relates to interfacial processes of counter-ions through the electrode/electrolyte interface. The inset in Fig. 8a corresponds to the high frequency region of the Nyquist plots. The Rct of all PPy/RGO are significantly smaller than that of PPy. PPy/RGO-10 shows the smallest Rct which means that the counter-ions can diffuse and exchange the fastest at the PPy/RGO-10 electrode/electrolyte interface, which is attributed to the novel nano-sheet structure and the high electronic conductivity, as shown in Table 2. Moreover, the complex capacitance can be expressed as follows:55 C(o) = Cre jCim Zim ðoÞ Cre ¼ ojZðoÞj2
Fig. 7 (a and b) Cyclic voltammogram curves of the different PPy/RGO electrodes at various scan rates: 50 mV s1 and 500 mV s1, (c and d) galvanostatic charge–discharge curves of PPy/RGO at 0.2 A g1 and 1.6 A g1.
However, as the rate increases to 500 mV s1, PPy/RGO-10 still shows a quasi-rectangle with the largest area corresponding to a large specific capacitance. Moreover, it can be observed that the GCD curves are typical supercapacitor charge–discharge curves (Fig. 7c and d). The mass specific capacitance Cm (F g1) of each sample can be calculated by the following equation:54 Cm = 2 I Dt/(DV m)
(1)
where I is charge–discharge current (A), Dt is the discharge time (s), DV is the potential change during the discharge process, and m is the mass of active material of each electrode. (I/m) has been defined as charge–discharge current density. All of the specific capacitance data at different current densities are summarized in Table 2. It can be seen that the capacitance of PPy decreases rapidly as the current density increases. Nevertheless, the PPy/RGO electrodes exhibit a lower decrease corresponding to an excellent fast charging–discharging performance because of its good electronic conductivity. For PPy/RGO-10, a specific capacitance of 199.6 F g1 at 25.6 A g1 is still achieved, which is 78.1% of the initial capacitance at 0.2 A g1. Electrochemical impedance spectroscopy (EIS) is widely used to investigate the redox (charge–discharge) processes of electrode materials and to evaluate their electronic and ionic conductivities.5 Nyquist plots of all samples are shown in Fig. 8a. The nearly vertical shape in the low-frequency region indicates a good capacitive response behavior in the electrode
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Cim ¼
Zre ðoÞ ojZðoÞj2
(2) (3)
where |Z(o)| is the impedance modulus. Such expressions are very convenient to define accurately the relaxation frequency (also referred to the knee frequency) since a peak is observed on the Cim vs. f plot and Cre = C0/2 for f = fr, where C0 is the low frequency capacitance. A time constant tr is defined tr = 1/(2pfr) which is a quantitative measure of how fast the device can be charged and discharged reversibly. The evolution of the real capacitance (Cre) and imaginary capacitance (Cim) versus frequency is shown in Fig. 8b and c. With increasing RGO addition, fr increases from 0.04 Hz (PPy) to 0.72 Hz (PPy/RGO-10) and the relaxation time reduces from 3.98 s to 0.22 s (Fig. 8d). The lower relaxation time implies that PPy/RGO-10 has the faster ion/charge diffusion and exchange capability. The addition of RGO improves the frequency response and reduces the relaxation time because the nano-sheet structure provides a good electrolyte contact interface and ion diffusion-transport channels. However, the relaxation time of PPy/RGO-20 increases to 0.46 s. The reason for this can be attributed to excessive addition of RGO into the PPy matrix hindering the migration of ions due to the complete two-dimensional structure.56,57 This can be further demonstrated by the increase of the Warburg coefficient (sw), which is 1.40 O cm2 s0.5 for PPy/RGO-20, which is three times higher than that of PPy/RGO-10 (Fig. S12, ESI†). A large sw implies that excessive RGO addition will increase the resistance of ion diffusion. In order to further explore the electrochemical properties of PPy/RGO-10, we have compared the performance of the PPy and PPy/RGO-10 based electrodes. Fig. 9a and b show the CV curves of PPy/RGO-10 and PPy at different scan rates from 200 mV s1 to 2000 mV s1. For PPy/RGO-10, the curve still has a rectangular shape as the scan rate increases to 2000 mV s1. However, the curve for pure PPy exhibits an elliptical shape at the scan rate of 200 mV s1. In addition, the galvanostatic charge–discharge curves comparison of PPy/RGO-10 and PPy at 1.6 A g1 and 12.8 A g1 are shown in Fig. 9c and d, respectively. The specific capacitance of PPy/RGO-10 is nearly five times of PPy at a charge–discharge current density of 1.6 A g1. With increasing current density, the specific capacitance decay of PPy/RGO-10 is obviously slower than that of PPy. At 12.8 A g1, the specific capacitance of PPy/RGO-10 still reaches 209.2 F g1, which is
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Fig. 8 (a) Nyquist plots, (b) evolution of the real capacitance and (c) imaginary capacitance versus frequency, and (d) the relaxation frequency for PPy and all PPy/RGO.
Fig. 9 (a and b) CV curves of PPy/RGO-10 and PPy at different scan rates; (c and d) GCD curves of PPy/RGO-10 and PPy at 1.6 A g1 and 12.8 A g1, respectively; and (e) IR drop of PPy and PPy/RGO-10.
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93.6% that of the specific capacitance at 1.6 A1 (223.5 F g1), but for the PPy electrode, it is only 34% (16.1 F g1 to 47.4 F g1). The IR drop of PPy at 12.8 A g1 is up to 142 mV, and that of PPyRGO-10 is only 34 mV, about a quarter of the former (Fig. 9e). The lower IR drop is mainly dependant on the smaller equivalent resistance and can be associated with the good diffusion and exchange of electrons and counter-ions in PPy/RGO-10. A simple schematic of ion adsorption and charge exchange at the surface of the work electrodes is shown in Fig. 10. The uniform nano-sheet PPy/RGO-10 can provide more accessible surface active sites for the adsorption of electrolyte ions and exchange and reduce ion/charge transport paths, which will improve the capacitive performance of the electrodes. On the other hand, the addition of graphene with its high electronic conductivity reduces the electronic resistance and enhances the fast charge transfer of PPy/RGO-10 (Fig. 10c), and then facilitates ion exchange between the PPy chains and electrolyte. Therefore, the more accessible active sites, and the fast ion/ charge transfer and exchange are the reasons for the inherent pseudo-capacitance of PPy/RGO-10 and also its improved rate capability. To evaluate the energy/power density performance, GCD tests at different current densities were carried out using a two-electrode cell system with a cell-voltage window of 0.9 V. As shown in Fig. 11, the simplified Ragone plots of the supercapacitors based on PPy/RGO and PPy are calculated from the
Fig. 10 (a and b) Schematic of the PPy and PPy/RGO-10 electrodes, and (c) charge storage and ion transfer near the surface of the PPy/RGO-10 electrode.
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Fig. 12 (a) Rate capability of PPy/RGO-10 at the charge–discharge current density from 0.2 to 25.6 A g1, and (b) the long-term cycling stability measured at a current density of 2 A g1. The inset is the capacitance decay after each 1000 cycles at different current densities.
galvanostatic discharge curves. The excellent specific capacitance and lower IR drop (Fig. S13, ESI†) ensure that PPy/RGO-10 have a large energy density and power density. The specific energy density of PPy/RGO-10 can reach 7.02 W h kg1 at the specific power density of 89 W kg1, which is about three times of that of PPy. When the specific power density increases to 11.0 kW kg1, the specific energy density still retains 5.17 W h kg1. Meanwhile, the rate capability and cycling stability of PPy/RGO10 were measured at different current densities from 0.2 A g1 to 25.6 A g1. As shown in Fig. 12a, the initial capacitances are 255.7, 219.1, 200.4, 184.6, 172.1 F g1 and high capacitance retention ratios of 93.3%, 94.4%, 95.3%, 96.8% and 98.0% can be obtained after 1000 cycles, respectively. It can be seen that the capacitance retention ratio increases as the current density increases. This can be attributed to the fact that the transfer and exchange of ion/charge is statistically more likely to take place at the surface of the electrode material under the fast and shallow charging–discharging process at large current density. Then the insertion–deinsertion of fewer ions into the PPy matrix leads to less volume change, which ensures the better stability of the PPy matrix and it also demonstrates a relatively good cycling stability. On the other hand, the long-term cycling test at 2 A g1 shows that about 83% of the initial capacitance is retained after 4000 cycles (Fig. 12b), which reveals that PPy/RGO-10 exhibits good electrochemical cycling stability, similar to the enhancement present in polyaniline/graphene because of the introduction of RGO.58
4. Conclusions
Fig. 11 Ragone plot showing the relationship of the power density and energy density for PPy and all PPy/RGO.
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A series of PPy–RGO composites were prepared by a one-step fabrication method using RGO as a soft template during the polymerization process. By tuning the mass ratio of RGO from 2.11% to 19.2%, the morphology of the composites gradually changed from a typical ‘‘cauliflower’’ structure to the nanosheet structure. This combination of materials is advantageous as the RGO sheet can behave as an electron acceptor, taking
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away electrons from PPy chains which serve as the electron donor, thus improving the reduced degree of the RGO. This synergistic effect has been obtained, leading significant improvement in the electronic conductivity and electrochemical performance of the PPy–RGO composites. Electrochemical tests indicated that PPy/RGO-10 shows a highest specific capacitance of 255.7 F g1 at a current density of 0.2 A g1, which still reaches 199.6 F g1 when the current density is increased to 25.6 A g1. Moreover, the embedment of RGO also improves the cyclic stability of the PPy/RGO-10, which shows less than a 7% decay at charge–discharge current densities of 0.2, 1.6, 6.4, 12.8, 25.6 A g1 for 1000 cycles. In summary, PPy/RGO-10 is a potential electrode material for supercapacitors due to its high specific capacitance, excellent rate capability and long-term cycling stability.
Acknowledgements We acknowledge the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110201130005), Program for New Century Excellent Talents in University (NCET-11-0433), National Natural Science Foundation of China (Grant No. 21274115), Natural Science Foundation of Shaanxi Province (Granted No. 2014JM6231), and the 111 Project (B14040). In addition, we also thank International Center for Dielectric Research (ICDR) in Xi’an Jiaotong University and Mr Chuansheng Ma for TEM measurement.
References 1 2 3 4 5
6 7 8 9 10 11 12 13 14
P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854. J. R. Mille and P. Simon, Science, 2008, 321, 651–652. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4269. R. F. Service, Science, 2006, 313, 902. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/ Plenum Publishers, New York, 1999. E. Frackowiak and F. Beguin, Carbon, 2001, 39, 937–950. K. X. Wang, Y. G. Wang, Y. R. Wang, E. Hosono and H. S. Zhou, J. Phys. Chem. C, 2009, 113, 1093–1097. A. Malinauskas, J. Malinauskiene and A. Ramanavicius, Nanotechnology, 2005, 16, R51–R62. J. Wang, Y. L. Xu, F. Yan, J. B. Zhu and J. P. Wang, J. Power Sources, 2011, 196, 2373–2379. Y. Sun and G. Shi, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 231–253. Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H. M. Cheng, Adv. Funct. Mater., 2010, 20, 3595–3602. A. E. Fischer, K. A. Pettigrew, D. R. Rolison, R. M. Stroud and J. W. Long, Nano Lett., 2007, 7, 281–286. K. R. Prasad and N. Miura, Electrochem. Commun., 2004, 6, 849–852. L. Nyholm, G. Nystrom, A. Mihranyan and M. Stromme, Adv. Mater., 2011, 23, 3751–3769.
This journal is © the Owner Societies 2015
Paper
15 S. I. Cho and S. B. Lee, Acc. Chem. Res., 2008, 41, 699–707. 16 Q. Wu, Y. Xu, Z. Yao, A. Liu and G. Shi, ACS Nano, 2010, 4, 1963–1970. 17 C. C. Hu and X. X. Lin, J. Electrochem. Soc., 2002, 149, A1049–A1057. 18 Q. W. Zhang, X. Zhou and H. S. Yang, J. Power Sources, 2004, 125, 141–147. 19 M. Hughes, G. Z. Chen, M. S. P. Shaffer, D. J. Fray and A. H. Windle, Chem. Mater., 2002, 14, 1610–1613. 20 P. Si, S. J. Ding, X. W. Lou and D. H. Kim, RSC Adv., 2011, 1, 1271–1278. 21 A. Davies, P. Audette, B. Farrow, F. Hassan, Z. W. Chen, J. Y. Choi and A. P. Yu, J. Phys. Chem. C, 2011, 115, 17612–17620. 22 J. P. Wang, Y. L. Xu, J. B. Zhu and P. G. Ren, J. Power Sources, 2012, 208, 138–143. 23 D. Li and R. B. Kaner, Science, 2008, 320, 1170–1171. 24 C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752–7777. 25 Y. Sun, Q. Wu and G. Shi, Phys. Chem. Chem. Phys., 2011, 13, 17249–17254. 26 M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498–3502. 27 Z. Y. Lin, Y. Liu, Y. G. Yao, O. J. Hidreth, Z. Li, K. Moon and C. P. Wong, J. Phys. Chem. C, 2011, 115, 7120–7125. 28 X. W. Yang, J. W. Zhu, L. Qiu and D. Li, Adv. Mater., 2011, 23, 2833–2838. 29 D. A. C. Brownson, D. K. Kampouris and C. E. Banks, J. Power Sources, 2011, 196, 4873–4885. 30 S. Biswas and L. T. Drzal, Chem. Mater., 2010, 22, 5667–5671. 31 L. L. Zhang, S. Y. Zhao, X. N. Tian and X. S. Zhao, Langmuir, 2010, 26, 17624–17628. 32 J. H. Liu, J. W. An, Y. X. Ma, M. L. Li and R. B. Ma, J. Electrochem. Soc., 2012, 159, A828–A833. 33 B. Ding, X. J. Lu, C. Z. Yuan, S. D. Yang, Y. Q. Han, X. G. Zhang and Q. Chen, Electrochim. Acta, 2012, 62, 132–139. 34 J. Zhang and X. S. Zhao, J. Phys. Chem. C, 2012, 116, 5420–5426. 35 Y. Liu, H. Wang, J. Zhou, L. Bian, E. Zhu, J. Hai, J. Tang and W. Tang, Electrochim. Acta, 2013, 112, 44–52. 36 X. Feng, Z. Yan, R. Li, X. Liu and W. Hou, Polym. Bull., 2013, 70, 2291–2304. 37 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 38 X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang, E. Wang and H. J. Dai, Nat. Nanotechnol., 2008, 3, 538–542. 39 D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101–105. 40 B. Tian and G. Zerbi, J. Chem. Phys., 1990, 92, 3886–3891. 41 X. T. Zhang, J. zhang, Z. F. Liu and C. Robinson, Chem. Commun., 2004, 1852–1853. 42 T. Qian, C. Yu, S. Wu and J. Shen, J. Mater. Chem. A, 2013, 1, 6539–6542. 43 M. J. McAllister, J. L. Li, D. H. Adamson, H. C. Schniepp, A. A. Abdala, X. Jun Liu, O. Margarita Herrera-Alonso, D. L. Milius, R. Car, R. K. Prud’homme and I. A. Aksay, Chem. Mater., 2007, 19, 4396–4404.
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View Article Online
Published on 12 June 2015. Downloaded by University of California - San Diego on 01/07/2017 20:08:34.
Paper
´sza ´ros and M. Omastova ´, J. Therm. Anal. 44 E. Jakab, E. Me Calorim., 2007, 88, 515–521. 45 D. K. Kim, K. W. Oh, H. J. Ahn and S. H. Kim, J. Appl. Polym. Sci., 2008, 107, 3925–3932. 46 C. Xu, J. Sun and L. Gao, J. Mater. Chem., 2011, 21, 11253–11258. 47 J. An, J. Liu, Y. Zhou, H. Zhao, Y. Ma, M. Li, M. Yu and S. Li, J. Phys. Chem. C, 2012, 116, 19699–19708. 48 D. B. Cairns and S. P. Armes, Langmuir, 1999, 15, 8059–8066. 49 L. Y. Ji, E. T. Kang and K. G. Neoh, Macromolecules, 1999, 32, 8183–8188. 50 J. Joo, J. K. Lee, S. Y. Lee, K. S. Jang, E. J. Oh and A. J. Epstein, Macromolecules, 2000, 33, 5131–5136. 51 L. Mao, K. Zhang, H. S. O. Chan and J. S. Wu, J. Mater. Chem., 2012, 22, 80–85.
19894 | Phys. Chem. Chem. Phys., 2015, 17, 19885--19894
PCCP
52 J. Gao, F. Liu, Y. L. Liu, N. Ma, Z. Q. Wang and X. Zhang, Chem. Mater., 2010, 22, 2213–2218. 53 D. C. Zhang, X. Zhang, Y. Chen, P. Yu, C. H. Wang and Y. W. Ma, J. Power Sources, 2011, 196, 5990–5996. 54 Y. Korenblit, M. Rose, E. Kockrick, L. Borchardt, A. Kvit, S. Kaskel and G. Yushin, ACS Nano, 2010, 4, 1337–1344. 55 P. L. Taberna, P. Simon and J. F. Fauvarque, J. Electrochem. Soc., 2003, 150, A292–A300. 56 J. M. Yuk, J. Park, P. Ercius, K. Kim, D. J. Hellebusch, M. F. Crommie, J. Y. Lee, A. Zettl and A. P. Alivisatos, Science, 2012, 336, 61–64. 57 Y. Xu, Z. Lin, X. Zhong, X. Huang, N. O. Weiss, Y. Huang and X. Duan, Nat. Commun., 2014, 5, 4554–4562. 58 H. L. Wang, Q. L. Hao, X. J. Yang, L. D. Lu and X. Wang, Electrochem. Commun., 2009, 11, 1158–1161.
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Morphology controllable nano-sheet polypyrrole–graphene composites for high-rate supercapacitor† Jianbo Zhu,a Youlong Xu,*a Jie Wang,a Jingping Wang,b Yang Baia and Xianfeng Dua Polypyrrole is a promising candidate for supercapacitor electrode materials due to its high capacitance and low cost. However, the major bottlenecks restricting its application are its poor rate capability and cycling stability. Herein, we control the morphology of polypyrrole–graphene composites by adjusting the graphene content, causing the typical ‘‘cauliflower’’ morphology of polypyrrole to gradually turn into the homogeneous nano-sheet morphology of these composites. The composites consequently exhibit good thermal stability, high protonation level (37.4%), high electronic conductivity (625.3 S m1), and fast relaxation time (0.22 s). These remarkable characteristics afford a high capacitance of 255.7 F g1 at
Received 11th May 2015, Accepted 9th June 2015
0.2 A g1, still retaining a capacitance of 199.6 F g1 at 25.6 A g1. In addition, high capacitance retention of up to 93% is observed after 1000 cycles testing at different current densities of 0.2, 1.6, 6.4,
DOI: 10.1039/c5cp02710a
12.8 and 25.6 A g1, indicating high stability. The composite’s excellent electrochemical performance is
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mainly attributed to its nano-sheet structure and high electronic conductivity, providing unobstructed pathways for the fast diffusion and exchange of ions/electrons.
1. Introduction In the wake of climate change, the deteriorating nature of the environment, and the exhaustion of the fossil resource, the development of renewable energy production and hybrid electric vehicles has attracted widespread attention in the 21st century.1,2 Over the past decade, many studies have been conducted on secondary batteries such as fuel cells and lithium ion batteries which are the forefront of energy storage technology.3 Nonetheless, with the growing requirement for portable systems and electric vehicles which need emergency high power supply, a new energy storage device in the form of the supercapacitor has become of significant interest to the public and researcher.4 Supercapacitors (SCs), also known as electrochemical capacitors or ultracapacitors, can be fully charged or discharged in seconds
a
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710021, China. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China † Electronic supplementary information (ESI) available: The file provides more detailed information regarding the fabrication of a supercapacitor by a twoelectrode cells system, the micelle particle size distribution of Py monomers and the influence of RGO addition, the calculation formulas of power and energy density, and the IR drop for each sample at different current densities. See DOI: 10.1039/c5cp02710a
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with a high power capability.5 Many materials have been investigated for use as SC electrode materials, including carbon materials possessing an electric double layer capacitance6,7 and conducting polymers (CPs)8–10 with pseudo-capacitance due to the fast and reversible redox reaction process, as well as some metal oxides (e.g. RuO2,11 MnO2,12 or MoO313). Conducting polymers (CPs) are well known as SCs electrode materials due to their high flexibility, facile synthesis, and relative high specific capacitance.14–16 Among CPs, polypyrrole (PPy) is considered as an excellent potential material due to its environmental friendliness, good electrochemical reversibility and low cost.17 However, the major problems for the application of PPy in SCs are its poor stability during charging–discharging cycles and its large capacitance loss at high current density.9,14 Thus, it is necessary to improve the long-term cycling stability and rate capability of PPy in order for it to be usefully employed in this short-term pulse application. Some composites of PPy and carbon materials, such as carbon foam,18 carbon nanotubes,19 and graphene,20–22 have been prepared to improve the electrochemical stability. Among these carbon materials, graphene, as a quasi-two-dimensional carbon with unique electronic and mechanical properties, has been received with rapidly growing research interest in recent years.23–25 Due to its high electronic conductivity, large surface area, superior chemical stability, and a broad electrochemical window, graphene and graphene-based carbon materials show
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good electrochemical performances as SC electrodes.26–29 Biswas et al. prepared multi layered architecture graphene and PPy nanowire composites with a specific capacitance of 165 F g1 at discharge current density of 1 A g1 by a simple and effective ultrasonication comixing method.30 Zhang et al.31 and Liu et al.32 reported the multi-layered structure composites of PPy fiber and PPy nanotube mixed with graphene oxide to enhance the mechanical strength and stabilize the structure during the charge–discharge process. Moreover, Ding et al. simultaneously added carbon nanotubes and reduced graphene oxide into the PPy matrix to form a 3-D highly-porous structure, bridging the defect for electron transfer between the composites.33 Currently, some literature clearly indicates that the electrochemical properties of PPy have been significantly improved by the addition of graphene,30–36 and this improvement is generally attributed to the large specific surface area and high electronic conductivity of graphene, and the synergistic effect of composites. However, further study is necessary in order to understand the more detailed reasons for why and how the addition of graphene can greatly improve the specific capacitance and rate capability, in addition to understanding its effect on charge/ion diffusion and exchange at the electrode/electrolyte interface. In this paper, we prepared the PPy–RGO composites with moderate reducing graphene as a soft template (Fig. 1), and adjusted the graphene content to control the morphology of these composites. With increasing graphene content, an obvious trend can be observed that the morphology changed from cauliflower shaped particles of PPy to graphene-like nanosheets of PPy/RGO, which can provide more accessible surface active sites for the adsorption and exchange of electrolyte ions. The as-prepared PPy–RGO composites exhibit good thermal stability, a high doping level and protonation level, and a large electronic conductivity, which endow these composites with their high capacitance and superior rate performance as supercapacitor electrodes.
graphene oxide (RGO) was prepared by reduction of GO with hydrazine hydrate according to the literature.39 In a typical experiment, 0.5 g of GO was dispersed in 500 ml DI water by ultrasonication for 1 h. Then the resulting homogeneous dispersion was mixed with 0.5 ml of hydrazine solution (N2H2, 80 wt% in water) and 2 ml of ammonia solution (NH3H2O, 26 wt% in water), and the mixture was heated at 95 1C for 10 min. The suspension was then filtered through a micropore filter, washed copiously with DI water and dried in a vacuum oven at 45 1C over night. Pyrrole (Py, Capchem, 99.8%) was distilled and stored at 10 1C in a nitrogen atmosphere prior to use. All other chemicals from Sinopharm Chemical Reagent Co. Ltd were reagent grade and used as received. Composites of reduced graphene oxide and PPy were prepared by in situ polymerization in an acidic solution. 0, 25, 50, 100, and 200 mg reduced graphene oxide was dispersed in 100 ml of 0.1 M p-toluenesulfonate acid (TOSH) solution by ultrasonication for 1 h. 10 mmol (0.677 g) of purified Py monomer was then dissolved into the solution and vigorously stirred at low temperature (below 4 1C) for 30 min, and then 3 g of potassium persulfate (PPS) dissolved in 10 ml of the precooled 0.1 M TOSH solution was added slowly to the above mixture solution. After further stirring at low temperature (below 4 1C) for 24 h, the product was collected by filtration, washed with DI water until the filtrate became a neutral solution (pH E 7), dried at 60 1C overnight under vacuum, and then named as PPy, PPy/RGO-2.5, PPy/RGO-5, PPy/RGO-10, and PPy/RGO-20, respectively. The mass percentage of RGO in these samples was estimated by weighing the mass of added RGO and the final products after polymerization. For comparison, physically mixed PPy and RGO at corresponding mass ratios were prepared to investigate the thermal stability of the PPy–RGO composites, which were named PPy97.5 + RGO2.5, PPy95 + RGO5, PPy90 + RGO10, and PPy80 + RGO20, respectively.
2. Experimental
The structure and morphology of the products were characterized by transmission electron microscopy (TEM; JEM-2100, JEOL) with an accelerating voltage of 200 kV, and field emission scanning electron microscopy (FE-SEM; JSM-6700F, JEOL; Hitachi, S-4800) coupled with energy-dispersive X-ray spectroscopy (EDS). The chemical structure of each product was ascertained by Fourier transform infrared spectroscopy (FT-IR, Bruker, VERTEX 70) and X-ray photoelectron spectroscopic (XPS) measurement with an AXIS-ULTRA DLD (Al Ka X-ray source, 1486.60 eV). The samples for FT-IR measurement were typically examined in KBr pellets after thorough mixing. All binding energies of XPS were referenced to the C 1s neutral carbon peak set at 284.6 eV to compensate for surface charging effects. Thermo-gravimetric analysis (TGA) was performed under nitrogen flow at a heating rate of 10 1C min1 with a Mettler Toledo TGA/DSC1 (STARe system) instrument. Micelle particle size distribution analysis of Py monomers and RGO in aqueous solution was studied on a Zetasizer Nano instrument (Malvern, ZEN3690). The bulk electronic conductivity measurements of the products were made on pressed
2.1
Material synthesis
Graphene oxide (GO) was synthesized from natural graphite (300 mesh) by a modified Hummers method.37,38 Reduced
Fig. 1 Illustration of the preparation process of the PPy and PPy–RGO composites.
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2.2
Material characterization
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wafers (1 cm diameter, o1 mm thickness) and measured by a four point probe resistivity meter (Guangzhou 4-probes Tect, RTS-9) at 23 1 1C. Electrochemical performances of the products were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques in a symmetrical two-electrode cell system. EIS measurements were performed at open circuit voltage in the frequency range from 100 KHz to 10 MHz. All electrochemical tests were performed on the Versatile Multichannel Potentiostat 2/Z (VMP2, Princeton applied research). Cycling stability testing was performed by GCD measurement on the Arbin battery test system. For the preparation of working electrodes, the prepared active powder was mixed with carbon black and polyvinylidene fluoride (80 : 10 : 10) in N-methyl-2-pyrrolidone. Then the slurry was coated onto titanium foil and dried at 50 1C overnight under vacuum. The mass of active material was about 1.4 0.2 mg in each electrode. To fabricate the two-electrode cell system, two electrodes with the same active material load were chosen for the symmetrical supercapacitor and measured in an airtight glass container with 3 M KCl aqueous solution as electrolyte. The distance between two electrodes was about 2 mm, and a photograph of this supercapacitor is shown in Fig. S1 (ESI†).
3. Results and discussion 3.1
Microstructure characterizations
Fig. 2 shows the FT-IR spectra of Py monomer, PPy, PPy/RGO, and RGO. Compared with the Py monomer spectrum, it can be clearly observed that the marked peaks of the PPy chain appear in both the PPy and PPy/RGO spectra, indicating that the PPy polymer has been prepared after polymerization. In the spectra of PPy and PPy/RGO, the peaks at 1540 and 1462 cm1 are due to the anti-symmetric and symmetric ring-stretching modes, respectively, which are the characteristic peaks of the PPy chains.40 The peaks near 1297 and 1174 cm1 are attributed to the stretching vibration of C–N, and the peaks at 1041 and 895 cm1 are attributed to the bending vibration of QC–H.41
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For RGO, the broad peak at 3395 cm1 is attributed to the residual hydroxyl and carboxyl groups, which are the hydrophilic groups which ensure the homogeneous dispersion of RGO in the aqueous solution. Compared with the pure PPy, PPy/RGO shows a new peak at 3395 cm1 attributed to the addition of RGO, and the peak intensity increases with the addition of the mass percentage of RGO in the composites, which indicates that the RGO was mixed with the PPy. FE-SEM and TEM images of RGO, PPy and all PPy/RGO composites are shown in Fig. 3 and 4, respectively. The PPy morphology as shown in Fig. 3b exhibits a typical ‘‘cauliflower’’ structure with a large accumulation of spherical particles of about 400 nm in diameter. During the polymerization process, PPy polymerizes into particles and naturally aggregates into the typical ‘‘cauliflower’’ structure because Py monomer disperses in aqueous solution in the form of micellar emulsions as illustrated in Fig. 1a. For PPy/RGO, with the addition of RGO (Fig. 3a), the Py monomer can stably absorb onto the surface of graphene due to the electrostatic attraction between the Py monomer and graphene.34,42 Micelle particle size distribution analysis indicates that the Py monomer easily absorbs onto RGO surface, implying that the Py monomer and RGO can form stable adsorption during the polymerization process (Fig. S2 and S3, ESI†). More nano-sheets are formed, replacing the spherical particles as the RGO content is increased from 2.11% to 19.2%. For PPy/RGO-2.5 and PPy/RGO-5, a small amount of spherical particles can be seen in Fig. 3c and d, respectively, which is due to that the amount of Py exceeding the absorption limit of the RGO present, and thus the residual free Py forms these aggregates. When the content of RGO reaches 10.2%, the morphology of PPy/RGO-10 (Fig. 3e) becomes uniform nano-sheets with a thickness of about 80 nm (Fig. S4, ESI†). Additionally, the uniform distribution of nitrogen and sulfur elements as verified by EDS elemental mapping suggests that the PPy chains are uniformly coated onto the graphene surface (Fig. S5–S10, ESI†). Moreover, TEM images of these samples (Fig. 4) also suggest that the nano-sheet morphology becomes more and more obvious with increasing RGO content. 3.2
Fig. 2 FTIR spectra of pyrrole monomer, PPy, RGO, and all of the PPy–RGO composites.
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Thermo-gravimetric and XPS analysis
The composition and thermal stability of PPy, RGO, and PPy/RGO were studied by thermo-gravimetric analysis (TGA). For comparison, physically mixed PPy and RGO samples at corresponding mass ratios were measured under the same conditions. As shown in Fig. 5a, GO shows a large mass loss (about 38.4%) between 200 and 300 1C due to the decomposition of oxygen-containing groups.43 After reduction by hydrazine, the obtained RGO shows only a 12.9% mass loss from 50 to 500 1C, indicating that most of the oxygen-containing groups have been removed during the chemical reduction process. PPy displays 56.0% mass loss from 50 to 1000 1C because of de-doping and pyrolysis. Compared with the mixed sample of PPy and PPy90 + RGO10 under the same conditions, it can be seen that PPy/RGO-10 has similar TGA curve yet shows less mass loss from 100 to 500 1C indicating its better thermal stability. A similar phenomenon can also be observed in the other samples (Fig. S11, ESI†). Furthermore, the mass
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Fig. 3
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SEM images of (a) RGO, (b) PPy, (c) PPy/RGO-2.5, (d) PPy/RGO-5, (e) PPy/RGO-10, and (f) PPy/RGO-20.
Fig. 4 TEM images of (a) RGO, (b) PPy, (c) PPy/RGO-2.5, (d) PPy/RGO-5, (e) PPy/RGO-10, and (f) PPy/RGO-20.
percentage of RGO in each PPy–RGO composite can be calculated from the mass loss of each TG curve (Fig. S11 and Table S3, ESI†), which is in good agreement with the results calculated by weighing (Table 1). On the other hand, thermal de-doping of doped ion led to significant mass loss in the temperature range of 200 to 400 1C.44,45 Fig. 5b presents the comparison of the temperature at different mass losses. The temperature increases with increasing RGO content in both the composites and physically mixed samples. Compared with the physically mixed samples, the higher degradation temperatures seen for the composites suggest that more stable ion-doping can be obtained for PPy– RGO. Two reasons may account for this thermal stability enhancement. First, the higher thermal conductivity of RGO
19888 | Phys. Chem. Chem. Phys., 2015, 17, 19885--19894
may facilitate heat dissipation within the polymer composites. Second, RGO as an electron acceptor will remove electrons from the PPy chains and make the negatively charged doped-anions more stably embedded in the polymer chains.29,46,47 To investigate the protonation and doping level of PPy/RGO, X-ray photoelectron microscopy (XPS) was used for the elemental component characterization of PPy and PPy/RGO. Fig. 6 depicts the XPS survey spectra and N 1s region spectra of these samples. The N 1s and S 2p signals are due to the cationic PPy chains and their TOS dopant anions, respectively. The surface percentage doping level can be calculated from the S/N atomic ratio of PPy and PPy/RGO,48 which corresponds to the protonation level. Moreover, The N 1s spectra show relatively broad peaks, which
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Fig. 6
Fig. 5 (a) TGA curves and (b) temperatures at different mass loss of the PPy–RGO composites and the samples of PPy mixed with RGO.
Table 1
TGA and XPS analysis characteristic of PPy and all PPy/RGO
Mass percentage of RGO (%)
Doping level (%) +
Sample
Weighing
TG analysis
S/N
N /N
PPy PPy/RGO-2.5 PPy/RGO-5 PPy/RGO-10 PPy/RGO-20
0 2.18 5.01 9.62 18.18
0 2.11 5.28 10.2 19.2
26.7 28.1 29.6 31.9 32.8
22.7 28.5 31.6 33.8 37.4
can be fitted by three decomposed peaks at 399.9 0.1 eV, 401.1 0.2 eV, and 402.3 0.2 eV, and these peaks are attributed to the benzenoid amine (–NH–), protonated benzenoid amine (–N+H–), and protonated quinonoid imine (QN+H–), respectively.49,50 The shoulder on the higher energy side is considered as arising from an electrostatic interaction with the nearest doped-counterions, and the doping level can be calculated by determining the contribution of positively charged nitrogen (N+) to the total nitrogen content of PPy.50 Peak fitting of the N 1s regions for the various samples are shown in Fig. 6, and the calculated doping levels are summarized in Table 1. Both the direct S/N atomic ratios and the N+/N ratios indicate that the PPy/RGO composites have a higher doping level, which increases with increasing RGO content. For PPy/RGO, the doping
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XPS spectra and N 1s region spectra of PPy and all PPy/RGO.
level calculated form the N+/N ratios is slightly higher than that from the direct S/N ratios. The reason for this may be attributed to the RGO acting as an electron acceptor, taking away electrons from the PPy chains and leading to further protonation. Consequently, the higher doping and protonation level can improve the electronic conductivity of PPy/RGO, which is one of the key elements for its application as a high-rate supercapacitor electrode. 3.3
Electronic conductivity and electrochemical behavior
The electronic conductivities (s) of PPy/RGO, PPy and RGO were determined on pressed wafers from powder by using a conventional four point probe technique, and the average electronic conductivities are shown in Table 2. RGO has a good conductivity of 1004.2 S m1, which is lower than that reported by Li (about 7222 S m1),39 but higher than some other reports.51,52 The conductivity of PPy is only 78.7 S m1, similar to the report by Zhang et al.53 For PPy/RGO, as discussed above, the nanosheet structure composites have a higher doping and protonation level which can greatly improve their electrical properties. The electronic conductivity of the composites increases with the addition of RGO, and a high electronic conductivity of 625.3 S m1 can be obtained for PPy/RGO-10, which is about eight times of that of PPy. Fig. 7 shows the comparison of CV and GCD curves measured in a two-electrode cell system between all of the composite electrodes with different ratios of PPy and RGO. As shown in Fig. 7a, the CV curves are nearly rectangular in shape at scan rate of 50 mV s1, and the area of the quasi-rectangle corresponds to the storage capability. It can be seen that all of the electrodes have good pseudo-capacitive performance at low scan rate, and PPy/RGO-10 and PPy/RGO-20 exhibit large storage capability.
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Table 2 Electronic conductivities of PPy/RGO, PPy and RGO and their specific capacitances at different current densities in 3 M KCl (aq.)
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Electronic Specific capacitance (F g1) conductivity 0.2 A g1 1.6 A g1 6.4 A g1 25.6 A g1 (S m1)
PPy/RGO-20 633.2 PPy/RGO-10 625.3 PPy/RGO-5 561.0 PPy/RGO-2.5 385.9 PPy 78.7 RGO 1004.2
258.3 255.7 234.8 200.5 121.0 38.1
202.7 223.5 166.4 98.8 47.3 30.4
189.9 215.2 154.1 83.6 36.3 28.7
158.4 199.6 125.5 48.4 16.1 26.3
matrix, and the diameter of the arc in the high frequency region is the charge transfer resistance (Rct), which relates to interfacial processes of counter-ions through the electrode/electrolyte interface. The inset in Fig. 8a corresponds to the high frequency region of the Nyquist plots. The Rct of all PPy/RGO are significantly smaller than that of PPy. PPy/RGO-10 shows the smallest Rct which means that the counter-ions can diffuse and exchange the fastest at the PPy/RGO-10 electrode/electrolyte interface, which is attributed to the novel nano-sheet structure and the high electronic conductivity, as shown in Table 2. Moreover, the complex capacitance can be expressed as follows:55 C(o) = Cre jCim Zim ðoÞ Cre ¼ ojZðoÞj2
Fig. 7 (a and b) Cyclic voltammogram curves of the different PPy/RGO electrodes at various scan rates: 50 mV s1 and 500 mV s1, (c and d) galvanostatic charge–discharge curves of PPy/RGO at 0.2 A g1 and 1.6 A g1.
However, as the rate increases to 500 mV s1, PPy/RGO-10 still shows a quasi-rectangle with the largest area corresponding to a large specific capacitance. Moreover, it can be observed that the GCD curves are typical supercapacitor charge–discharge curves (Fig. 7c and d). The mass specific capacitance Cm (F g1) of each sample can be calculated by the following equation:54 Cm = 2 I Dt/(DV m)
(1)
where I is charge–discharge current (A), Dt is the discharge time (s), DV is the potential change during the discharge process, and m is the mass of active material of each electrode. (I/m) has been defined as charge–discharge current density. All of the specific capacitance data at different current densities are summarized in Table 2. It can be seen that the capacitance of PPy decreases rapidly as the current density increases. Nevertheless, the PPy/RGO electrodes exhibit a lower decrease corresponding to an excellent fast charging–discharging performance because of its good electronic conductivity. For PPy/RGO-10, a specific capacitance of 199.6 F g1 at 25.6 A g1 is still achieved, which is 78.1% of the initial capacitance at 0.2 A g1. Electrochemical impedance spectroscopy (EIS) is widely used to investigate the redox (charge–discharge) processes of electrode materials and to evaluate their electronic and ionic conductivities.5 Nyquist plots of all samples are shown in Fig. 8a. The nearly vertical shape in the low-frequency region indicates a good capacitive response behavior in the electrode
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Cim ¼
Zre ðoÞ ojZðoÞj2
(2) (3)
where |Z(o)| is the impedance modulus. Such expressions are very convenient to define accurately the relaxation frequency (also referred to the knee frequency) since a peak is observed on the Cim vs. f plot and Cre = C0/2 for f = fr, where C0 is the low frequency capacitance. A time constant tr is defined tr = 1/(2pfr) which is a quantitative measure of how fast the device can be charged and discharged reversibly. The evolution of the real capacitance (Cre) and imaginary capacitance (Cim) versus frequency is shown in Fig. 8b and c. With increasing RGO addition, fr increases from 0.04 Hz (PPy) to 0.72 Hz (PPy/RGO-10) and the relaxation time reduces from 3.98 s to 0.22 s (Fig. 8d). The lower relaxation time implies that PPy/RGO-10 has the faster ion/charge diffusion and exchange capability. The addition of RGO improves the frequency response and reduces the relaxation time because the nano-sheet structure provides a good electrolyte contact interface and ion diffusion-transport channels. However, the relaxation time of PPy/RGO-20 increases to 0.46 s. The reason for this can be attributed to excessive addition of RGO into the PPy matrix hindering the migration of ions due to the complete two-dimensional structure.56,57 This can be further demonstrated by the increase of the Warburg coefficient (sw), which is 1.40 O cm2 s0.5 for PPy/RGO-20, which is three times higher than that of PPy/RGO-10 (Fig. S12, ESI†). A large sw implies that excessive RGO addition will increase the resistance of ion diffusion. In order to further explore the electrochemical properties of PPy/RGO-10, we have compared the performance of the PPy and PPy/RGO-10 based electrodes. Fig. 9a and b show the CV curves of PPy/RGO-10 and PPy at different scan rates from 200 mV s1 to 2000 mV s1. For PPy/RGO-10, the curve still has a rectangular shape as the scan rate increases to 2000 mV s1. However, the curve for pure PPy exhibits an elliptical shape at the scan rate of 200 mV s1. In addition, the galvanostatic charge–discharge curves comparison of PPy/RGO-10 and PPy at 1.6 A g1 and 12.8 A g1 are shown in Fig. 9c and d, respectively. The specific capacitance of PPy/RGO-10 is nearly five times of PPy at a charge–discharge current density of 1.6 A g1. With increasing current density, the specific capacitance decay of PPy/RGO-10 is obviously slower than that of PPy. At 12.8 A g1, the specific capacitance of PPy/RGO-10 still reaches 209.2 F g1, which is
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Fig. 8 (a) Nyquist plots, (b) evolution of the real capacitance and (c) imaginary capacitance versus frequency, and (d) the relaxation frequency for PPy and all PPy/RGO.
Fig. 9 (a and b) CV curves of PPy/RGO-10 and PPy at different scan rates; (c and d) GCD curves of PPy/RGO-10 and PPy at 1.6 A g1 and 12.8 A g1, respectively; and (e) IR drop of PPy and PPy/RGO-10.
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93.6% that of the specific capacitance at 1.6 A1 (223.5 F g1), but for the PPy electrode, it is only 34% (16.1 F g1 to 47.4 F g1). The IR drop of PPy at 12.8 A g1 is up to 142 mV, and that of PPyRGO-10 is only 34 mV, about a quarter of the former (Fig. 9e). The lower IR drop is mainly dependant on the smaller equivalent resistance and can be associated with the good diffusion and exchange of electrons and counter-ions in PPy/RGO-10. A simple schematic of ion adsorption and charge exchange at the surface of the work electrodes is shown in Fig. 10. The uniform nano-sheet PPy/RGO-10 can provide more accessible surface active sites for the adsorption of electrolyte ions and exchange and reduce ion/charge transport paths, which will improve the capacitive performance of the electrodes. On the other hand, the addition of graphene with its high electronic conductivity reduces the electronic resistance and enhances the fast charge transfer of PPy/RGO-10 (Fig. 10c), and then facilitates ion exchange between the PPy chains and electrolyte. Therefore, the more accessible active sites, and the fast ion/ charge transfer and exchange are the reasons for the inherent pseudo-capacitance of PPy/RGO-10 and also its improved rate capability. To evaluate the energy/power density performance, GCD tests at different current densities were carried out using a two-electrode cell system with a cell-voltage window of 0.9 V. As shown in Fig. 11, the simplified Ragone plots of the supercapacitors based on PPy/RGO and PPy are calculated from the
Fig. 10 (a and b) Schematic of the PPy and PPy/RGO-10 electrodes, and (c) charge storage and ion transfer near the surface of the PPy/RGO-10 electrode.
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Fig. 12 (a) Rate capability of PPy/RGO-10 at the charge–discharge current density from 0.2 to 25.6 A g1, and (b) the long-term cycling stability measured at a current density of 2 A g1. The inset is the capacitance decay after each 1000 cycles at different current densities.
galvanostatic discharge curves. The excellent specific capacitance and lower IR drop (Fig. S13, ESI†) ensure that PPy/RGO-10 have a large energy density and power density. The specific energy density of PPy/RGO-10 can reach 7.02 W h kg1 at the specific power density of 89 W kg1, which is about three times of that of PPy. When the specific power density increases to 11.0 kW kg1, the specific energy density still retains 5.17 W h kg1. Meanwhile, the rate capability and cycling stability of PPy/RGO10 were measured at different current densities from 0.2 A g1 to 25.6 A g1. As shown in Fig. 12a, the initial capacitances are 255.7, 219.1, 200.4, 184.6, 172.1 F g1 and high capacitance retention ratios of 93.3%, 94.4%, 95.3%, 96.8% and 98.0% can be obtained after 1000 cycles, respectively. It can be seen that the capacitance retention ratio increases as the current density increases. This can be attributed to the fact that the transfer and exchange of ion/charge is statistically more likely to take place at the surface of the electrode material under the fast and shallow charging–discharging process at large current density. Then the insertion–deinsertion of fewer ions into the PPy matrix leads to less volume change, which ensures the better stability of the PPy matrix and it also demonstrates a relatively good cycling stability. On the other hand, the long-term cycling test at 2 A g1 shows that about 83% of the initial capacitance is retained after 4000 cycles (Fig. 12b), which reveals that PPy/RGO-10 exhibits good electrochemical cycling stability, similar to the enhancement present in polyaniline/graphene because of the introduction of RGO.58
4. Conclusions
Fig. 11 Ragone plot showing the relationship of the power density and energy density for PPy and all PPy/RGO.
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A series of PPy–RGO composites were prepared by a one-step fabrication method using RGO as a soft template during the polymerization process. By tuning the mass ratio of RGO from 2.11% to 19.2%, the morphology of the composites gradually changed from a typical ‘‘cauliflower’’ structure to the nanosheet structure. This combination of materials is advantageous as the RGO sheet can behave as an electron acceptor, taking
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away electrons from PPy chains which serve as the electron donor, thus improving the reduced degree of the RGO. This synergistic effect has been obtained, leading significant improvement in the electronic conductivity and electrochemical performance of the PPy–RGO composites. Electrochemical tests indicated that PPy/RGO-10 shows a highest specific capacitance of 255.7 F g1 at a current density of 0.2 A g1, which still reaches 199.6 F g1 when the current density is increased to 25.6 A g1. Moreover, the embedment of RGO also improves the cyclic stability of the PPy/RGO-10, which shows less than a 7% decay at charge–discharge current densities of 0.2, 1.6, 6.4, 12.8, 25.6 A g1 for 1000 cycles. In summary, PPy/RGO-10 is a potential electrode material for supercapacitors due to its high specific capacitance, excellent rate capability and long-term cycling stability.
Acknowledgements We acknowledge the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110201130005), Program for New Century Excellent Talents in University (NCET-11-0433), National Natural Science Foundation of China (Grant No. 21274115), Natural Science Foundation of Shaanxi Province (Granted No. 2014JM6231), and the 111 Project (B14040). In addition, we also thank International Center for Dielectric Research (ICDR) in Xi’an Jiaotong University and Mr Chuansheng Ma for TEM measurement.
References 1 2 3 4 5
6 7 8 9 10 11 12 13 14
P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854. J. R. Mille and P. Simon, Science, 2008, 321, 651–652. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4269. R. F. Service, Science, 2006, 313, 902. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/ Plenum Publishers, New York, 1999. E. Frackowiak and F. Beguin, Carbon, 2001, 39, 937–950. K. X. Wang, Y. G. Wang, Y. R. Wang, E. Hosono and H. S. Zhou, J. Phys. Chem. C, 2009, 113, 1093–1097. A. Malinauskas, J. Malinauskiene and A. Ramanavicius, Nanotechnology, 2005, 16, R51–R62. J. Wang, Y. L. Xu, F. Yan, J. B. Zhu and J. P. Wang, J. Power Sources, 2011, 196, 2373–2379. Y. Sun and G. Shi, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 231–253. Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H. M. Cheng, Adv. Funct. Mater., 2010, 20, 3595–3602. A. E. Fischer, K. A. Pettigrew, D. R. Rolison, R. M. Stroud and J. W. Long, Nano Lett., 2007, 7, 281–286. K. R. Prasad and N. Miura, Electrochem. Commun., 2004, 6, 849–852. L. Nyholm, G. Nystrom, A. Mihranyan and M. Stromme, Adv. Mater., 2011, 23, 3751–3769.
This journal is © the Owner Societies 2015
Paper
15 S. I. Cho and S. B. Lee, Acc. Chem. Res., 2008, 41, 699–707. 16 Q. Wu, Y. Xu, Z. Yao, A. Liu and G. Shi, ACS Nano, 2010, 4, 1963–1970. 17 C. C. Hu and X. X. Lin, J. Electrochem. Soc., 2002, 149, A1049–A1057. 18 Q. W. Zhang, X. Zhou and H. S. Yang, J. Power Sources, 2004, 125, 141–147. 19 M. Hughes, G. Z. Chen, M. S. P. Shaffer, D. J. Fray and A. H. Windle, Chem. Mater., 2002, 14, 1610–1613. 20 P. Si, S. J. Ding, X. W. Lou and D. H. Kim, RSC Adv., 2011, 1, 1271–1278. 21 A. Davies, P. Audette, B. Farrow, F. Hassan, Z. W. Chen, J. Y. Choi and A. P. Yu, J. Phys. Chem. C, 2011, 115, 17612–17620. 22 J. P. Wang, Y. L. Xu, J. B. Zhu and P. G. Ren, J. Power Sources, 2012, 208, 138–143. 23 D. Li and R. B. Kaner, Science, 2008, 320, 1170–1171. 24 C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752–7777. 25 Y. Sun, Q. Wu and G. Shi, Phys. Chem. Chem. Phys., 2011, 13, 17249–17254. 26 M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498–3502. 27 Z. Y. Lin, Y. Liu, Y. G. Yao, O. J. Hidreth, Z. Li, K. Moon and C. P. Wong, J. Phys. Chem. C, 2011, 115, 7120–7125. 28 X. W. Yang, J. W. Zhu, L. Qiu and D. Li, Adv. Mater., 2011, 23, 2833–2838. 29 D. A. C. Brownson, D. K. Kampouris and C. E. Banks, J. Power Sources, 2011, 196, 4873–4885. 30 S. Biswas and L. T. Drzal, Chem. Mater., 2010, 22, 5667–5671. 31 L. L. Zhang, S. Y. Zhao, X. N. Tian and X. S. Zhao, Langmuir, 2010, 26, 17624–17628. 32 J. H. Liu, J. W. An, Y. X. Ma, M. L. Li and R. B. Ma, J. Electrochem. Soc., 2012, 159, A828–A833. 33 B. Ding, X. J. Lu, C. Z. Yuan, S. D. Yang, Y. Q. Han, X. G. Zhang and Q. Chen, Electrochim. Acta, 2012, 62, 132–139. 34 J. Zhang and X. S. Zhao, J. Phys. Chem. C, 2012, 116, 5420–5426. 35 Y. Liu, H. Wang, J. Zhou, L. Bian, E. Zhu, J. Hai, J. Tang and W. Tang, Electrochim. Acta, 2013, 112, 44–52. 36 X. Feng, Z. Yan, R. Li, X. Liu and W. Hou, Polym. Bull., 2013, 70, 2291–2304. 37 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 38 X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang, E. Wang and H. J. Dai, Nat. Nanotechnol., 2008, 3, 538–542. 39 D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101–105. 40 B. Tian and G. Zerbi, J. Chem. Phys., 1990, 92, 3886–3891. 41 X. T. Zhang, J. zhang, Z. F. Liu and C. Robinson, Chem. Commun., 2004, 1852–1853. 42 T. Qian, C. Yu, S. Wu and J. Shen, J. Mater. Chem. A, 2013, 1, 6539–6542. 43 M. J. McAllister, J. L. Li, D. H. Adamson, H. C. Schniepp, A. A. Abdala, X. Jun Liu, O. Margarita Herrera-Alonso, D. L. Milius, R. Car, R. K. Prud’homme and I. A. Aksay, Chem. Mater., 2007, 19, 4396–4404.
Phys. Chem. Chem. Phys., 2015, 17, 19885--19894 | 19893
View Article Online
Published on 12 June 2015. Downloaded by University of California - San Diego on 01/07/2017 20:08:34.
Paper
´sza ´ros and M. Omastova ´, J. Therm. Anal. 44 E. Jakab, E. Me Calorim., 2007, 88, 515–521. 45 D. K. Kim, K. W. Oh, H. J. Ahn and S. H. Kim, J. Appl. Polym. Sci., 2008, 107, 3925–3932. 46 C. Xu, J. Sun and L. Gao, J. Mater. Chem., 2011, 21, 11253–11258. 47 J. An, J. Liu, Y. Zhou, H. Zhao, Y. Ma, M. Li, M. Yu and S. Li, J. Phys. Chem. C, 2012, 116, 19699–19708. 48 D. B. Cairns and S. P. Armes, Langmuir, 1999, 15, 8059–8066. 49 L. Y. Ji, E. T. Kang and K. G. Neoh, Macromolecules, 1999, 32, 8183–8188. 50 J. Joo, J. K. Lee, S. Y. Lee, K. S. Jang, E. J. Oh and A. J. Epstein, Macromolecules, 2000, 33, 5131–5136. 51 L. Mao, K. Zhang, H. S. O. Chan and J. S. Wu, J. Mater. Chem., 2012, 22, 80–85.
19894 | Phys. Chem. Chem. Phys., 2015, 17, 19885--19894
PCCP
52 J. Gao, F. Liu, Y. L. Liu, N. Ma, Z. Q. Wang and X. Zhang, Chem. Mater., 2010, 22, 2213–2218. 53 D. C. Zhang, X. Zhang, Y. Chen, P. Yu, C. H. Wang and Y. W. Ma, J. Power Sources, 2011, 196, 5990–5996. 54 Y. Korenblit, M. Rose, E. Kockrick, L. Borchardt, A. Kvit, S. Kaskel and G. Yushin, ACS Nano, 2010, 4, 1337–1344. 55 P. L. Taberna, P. Simon and J. F. Fauvarque, J. Electrochem. Soc., 2003, 150, A292–A300. 56 J. M. Yuk, J. Park, P. Ercius, K. Kim, D. J. Hellebusch, M. F. Crommie, J. Y. Lee, A. Zettl and A. P. Alivisatos, Science, 2012, 336, 61–64. 57 Y. Xu, Z. Lin, X. Zhong, X. Huang, N. O. Weiss, Y. Huang and X. Duan, Nat. Commun., 2014, 5, 4554–4562. 58 H. L. Wang, Q. L. Hao, X. J. Yang, L. D. Lu and X. Wang, Electrochem. Commun., 2009, 11, 1158–1161.
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