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Oil–water interfacial synthesis of graphene–polyaniline–MnO2 hybrids using binary oxidant for high performance supercapacitor
Synthetic Metals 209 (2015) 555–560
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Oil–water interfacial synthesis of graphene–polyaniline–MnO2 hybrids using binary oxidant for high performance supercapacitor Kaijian Lia , Dengfeng Guoa , Jianmei Chena , Yong Konga,* , Huaiguo Xueb a Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China b School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 July 2015 Received in revised form 7 September 2015 Accepted 15 September 2015 Available online xxx
Graphene–polyaniline–MnO2 (G–P–Mn) hybrids were synthesized via a facile approach that includes the reduction of graphene oxide (GO) to graphene (G) by aniline and then followed by oil–water interfacial polymerization of aniline to polyaniline (PANI) using a binary oxidant of KMnO4 and (NH4)2S2O8. PANI was orderly nanofibered and MnO2-doped due to interfacial polymerization and the use of binary oxidant. The hybrids conclude electrical double layer capacitance of G and pseudo capacitance of PANI and MnO2, showing enhanced specific capacitance of 800.1 F g1 at 0.4 A g1 with good cycling stability. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Graphene MnO2 Interfacial synthesis Binary oxidant Supercapacitor
1. Introduction The past years have witnessed the development of Faradaic pseudo capacitance based supercapacitors because of the low specific capacitance of double layer capacitance [1,2]. Among the materials for pseudocapacitor electrode, polyaniline (PANI) is considered as one of the most promising material due to its facile synthesis and relatively high environmental stability [3,4]. Nanostructured PANI offers superior electrochemical properties [5–7], however, traditional chemical oxidative polymerization yields granular PANI [8]. To obtain nanostructured PANI, interfacial polymerization using (NH4)2S2O8 as oxidant was proposed as an effective approach to suppress secondary growth of PANI by removing the nanostructured PANI from the interface [9,10]. Another major problem encountered when using PANI for supercapacitor electrode is that PANI undergoes poor tolerance during continuous charge/discharge cycling [11–13]. Therefore, hybrids of PANI and other materials including graphene (G) [14–17] and MnO2 [18–20] have been prepared and applied as supercapacitor electrodes with enhanced performances. G nanosheet is predicated as an ideal support material because of its excellent electrical conductivity, remarkable mechanical strength as well as
* Corresponding author. Fax: +86 519 86330167. E-mail address: [email protected] (Y. Kong). http://dx.doi.org/10.1016/j.synthmet.2015.09.017 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
high surface area [21,22], meanwhile, MnO2 in the hybrids provides a rigid support and an ideal electrical conducting path to conducting polymers by interlinking the polymer chains thus providing enhanced charge exchange efficiency and stability during redox cycling [23]. Here, it should be pointed out that although the electrical conductivity of MnO2 is inferior to that of PANI, the nucleation of MnO2 over growing PANI chains terminates further growth of PANI chains by attachment and gives rise to a fresh nucleation of PANI. It has been reported that the structure of conducting polymer is an important parameter as the shorter and ordered polymer structure are found to be ideal structures for charge storage [24]. Moreover, the presence of MnO2 in conducting polymer scaffold can enhance the electrical conductivity of the composite by cross-linking the conducting polymer chains, which significantly reduces the chain defects responsible for hopping conduction [25]. Here, graphene–polyaniline–MnO2(G–P–Mn) hybrids were prepared via a facile approach that includes the reduction of graphene oxide (GO) to G by aniline and then followed by the interfacial polymerization performed in an oil–water biphasic system with aniline-functionalized G dispersed in an organic solvent and the binary oxidant, KMnO4 and (NH4)2S2O8, dissolved in an aqueous acid solution (Fig. 1). It should be pointed out that MnO2 could be introduced to the hybrids when only KMnO4 is used as the oxidant, however, the ordered PANI nanofibers would be destroyed seriously due to the very strong oxidation ability of
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Fig. 1. Schematic illustration of the formation process of G–P–Mn hybrids via the reduction of GO to G by aniline and successive interfacial polymerization in a chloroformwater system.
KMnO4. In order to obtain MnO2-doped hybrids and maintain the nanofibers of PANI in the hybrids, a binary oxidant composed of KMnO4 and (NH4)2S2O8 is used for the oxidative polymerization of aniline at the reactive interface. The obtained G–P–Mn hybrids not only show high specific capacitance of ordered PANI nanofibers, but also exhibit excellent rate capability and cycling stability of MnO2 and G.
reduction of GO to G can be observed by the color change of the solution from yellow to black. After being cooled to room temperature, the composite of G and aniline oligomers dispersed in aniline was obtained by liquid separation using a separatory funnel.
2. Experimental
In a typical experiment, 73 mL of the dispersion of G and aniline oligomers in aniline was dissolved in 5 mL chloroform as the oil phase, and 22.8 mg of (NH4)2S2O8 and 15.8 mg of KMnO4 were dissolved in 5 mL 0.1 M H2SO4 solution as the water phase. After the oil phase was added into a vial (2 cm in diameter), the water phase was added carefully on the top of the oil phase to avoid breaking the steady state of the interface, and the vial was remained as undisturbed at room temperature for 4 h. Next, the water phase was collected and washed thoroughly with ultrapure water for several times to remove aniline monomer and the binary oxidant. The obtained black green mixture was centrifuged at 10,000 rpm and the final product was dried in a vacuum oven at 60 C for 12 h to obtain the G–P–Mn hybrids. For comparison, PANI and MnO2-doped PANI were prepared via interfacial polymerization method without the presence of G using only (NH4)2S2O8 and KMnO4 as the oxidant, respectively. In addition, G-PANI hybrids were synthesized by the same method using only (NH4)2S2O8 as the oxidant.
2.1. Reagents and apparatus Aniline was purchased from Aladdin Chemicals Reagent Co., Ltd. (Shanghai, China) and distilled under reduced pressure before use. Natural graphite powder (99.95%, 8000 mesh), sulfuric acid (H2SO4), potassium permanganate (KMnO4) and ammonium persulfate ((NH4)2S2O8) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other reagents were of analytical grade and used as received. All solutions were prepared with ultrapure water (18.2 MV). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge were performed on a CHI 660D electrochemical workstation (Beijing, China). The morphologies of different samples were characterized on a Supra55 field-emission scanning electron microscope (FESEM, Zeiss, Germany). The FT-IR spectra were recorded on a FTIR-8400S spectrometer (Shimadzu, Japan). Chemical analysis of the graphene–polyaniline–MnO2(G–P–Mn) hybrids was carried out by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA), and the conductivity of the hybrids was measured with a model SZT-2A four-probe instrument (Suzhou, China). 2.2. Reduction of graphene oxide (GO) to graphene (G) by aniline GO was prepared from natural graphite according to method proposed by Marcano et al. [26]. 20 mg of GO was dispersed in 10 mL ultrapure water to form a yellow dispersion by sonication, and then 10 mL aniline was added into the GO dispersion. The mixture was refluxed at 95 C for 8 h under stirring, and the
2.3. Synthesis of G–P–Mn hybrids by interfacial polymerization
2.4. Electrochemical measurements All electrochemical experiments were carried out in a conventional three-electrode system consisting of the G–P–Mn hybrids loaded on a platinum disk electrode (3 mm in diameter) as the working electrode, a platinum foil and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. All electrochemical measurements were performed in 1 M H2SO4 solution at room temperature. The CV curves were recorded at varying scan rates (1, 4, 7, 10, 30, 50, 70, 90, 110, 130 and 150 mV s1) over a potential range from 0 to 0.7 V. The galvanostatic charge/discharge tests were carried out at different current densities (0.4, 0.6, 0.8, 1, 2, 3, 5 and 10 A g1) over the
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potential range from 0 to 0.7 V, and the EIS measurements were performed in the frequency range from 10 kHz to 0.1 Hz with an amplitude of the signal of 5 mV. The cycle life of the G–P–Mn hybrids and PANI was tested at a current density of 10 A g1 by repeated charge/discharge cycling. 3. Results and discussion 3.1. Interfacial synthesis of G–P–Mn hybrids Since the direct redox reaction occurred between GO and aniline or its derivatives has been reported by several groups including us [27–29], aniline is used in this work as the reducing agent to reduce GO to G. During the reduction of GO to G, aniline itself is oxidized to aniline oligomers [30], which is adsorbed on the surface of G to form aniline-functionalized G (G-aniline) via strong p–p stacking (Fig. 1). Due to the hydrophobic surface of G and the planar aromatic structure of aniline, dissolution transfer of G nanosheets from water into aniline is achieved through forming a charge-transfer complex of G and aniline [31,32]. Chloroform and water were used as the oil and water phase, respectively, for the interfacial synthesis of the hybrids. When the G-aniline complex is dispersed in chloroform, aniline oligomers adsorbed on G play a key role in preventing G from irreversible aggregation [33]. Because chloroform is heavier than water, the water layer containing the oxidant can help to seal the chloroform vapor within the reaction vial for a safety consideration. In interfacial polymerization, PANI is formed at the oil–water interface and migrates into the upper water layer immediately since protonated PANI in the emeraldine salt form is hydrophilic [9], and thus interfacial polymerization adopted here is an effective way to suppress the “secondary growth” of PANI via prompt removal of nanostructured PANI from the interface to the water layer. In addition, the initial aniline concentration is lowered when aniline is dissolved in chloroform, resulting in decreased diffusion rate of aniline from chloroform into water layer and suppressed
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“secondary growth” of primary nanostructured PANI in the water phase due to the decreased aniline concentration there [34]. FESEM was utilized to analyze the morphologies of PANI, MnO2doped PANI together with the G–P–Mn hybrids (Fig. 2A–C), in which PANI and MnO2-doped PANI were also prepared via interfacial polymerization using only (NH4)2S2O8 and KMnO4 as the oxidant, respectively. Note that ordered nanofibers of PANI are obtained when interfacial polymerization is carried out using (NH4)2S2O8 as the oxidant (Fig. 2A), agreeing well with previous report [9,10]. However, the typical fibrillar morphology of PANI is destroyed seriously when (NH4)2S2O8 is replaced totally by KMnO4 (Fig. 2B), indicating that overoxidation of PANI occurs immediately after the PANI nanofibers formed at the interface migrate into the water phase due to the strong oxidation ability of KMnO4 [35]. This assumption is also verified by the conductivity measurements of PANI (0.83 S cm1) and MnO2-doped PANI (<105 S cm1), and the significantly decreased conductivity of MnO2-doped PANI demonstrates the deteriorated nanostructure of PANI. In order to obtain MnO2-doped hybrids and maintain the ordered PANI nanofibers, a compromise is proposed here using a binary oxidant composed of KMnO4 and (NH4)2S2O8 for the interfacial synthesis of the hybrids. Although the ordered PANI nanofibers are destroyed a little compared with PANI synthesized using (NH4)2S2O8 as the oxidant, the PANI obtained from the binary oxidant is still mainly composed of nanofibers (Fig. 2C). Moreover, G sheet retains its original layerlike structure and is surrounded by the partially agglomerated PANI nanofibers (Fig. 2C). Such morphology is supposed to stabilize the three-dimensional structure of the hybrids during the charge/ discharge cycling [30]. The reduction of GO to G by aniline and introduction of MnO2 to the hybrids is confirmed by the FT-IR spectra of GO, G-PANI (using only (NH4)2S2O8 as the oxidant in interfacial polymerization) and G–P–Mn (Fig. 3). The bands at 1743, 1216 and 1054 cm1 are assigned to the stretching vibration of oxygen-containing groups in GO [14,36,37]. For G-PANI, these characteristic bands of GO disappear completely, suggesting that GO can be reduced to G
Fig. 2. FESEM images of PANI, MnO2-doped PANI and G–P–Mn hybrids made by interfacial polymerization, with the following oxidant in the aqueous phase: (A) (NH4)2S2O8, (B) KMnO4 and (C) (NH4)2S2O8 + KMnO4.
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Fig. 3. FT-IR spectra of GO, G-PANI and G–P–Mn.
successfully by aniline, and four new peaks at 1570, 1482, 1294, 1124 cm1 are associated with C¼N, C¼C, CN and C H in PANI, respectively [10]. Compared with G-PANI, a new peak corresponding to the vibration of Mn O appears at 538 cm1 [38], indicating the presence of MnO2 in the G–P–Mn hybrids synthesized using the binary oxidant of KMnO4 and (NH4)2S2O8. XPS is used to further confirm the introduction of MnO2 to the hybrids. A weak Mn 2p signal is observed at the full XPS spectra of G–P–Mn (Fig. 4A), indicating that manganese exists in the G–P–Mn
Fig. 4. Full XPS spectra (A) and Mn 2p XPS spectra (B) of G–P–Mn.
Fig. 5. Cyclic voltammograms of G–P–Mn hybrids in 1 M H2SO4 at different scan rates of 1, 4, 7, 10, 30, 50, 70, 90, 110, 130, and 150 mV s1.
hybrids after the interfacial synthesis. The two peaks of Mn 2p3/2 and Mn 2p1/2, which are centered at 642.0 and 653.7 eV, respectively (Fig. 4B), with a spin energy separation of 11.7 eV, agree well with reported data of Mn 2p3/2 and Mn 2p1/2 in MnO2 [39,40]. This result also suggests that MnO2, the reduction product of KMnO4, has been introduced into the hybrids. 3.2. Electrochemical characterization of G–P–Mn hybrids Fig. 5 shows the cyclic voltammograms of the G–P–Mn hybrids at different scan rates. Two pairs of redox peaks are observed on each curve, which are attributed to the redox transformation between leucoemeraldine based states and emeraldine salt (C1/A1), and transition between emeraldine salt and pernigraniline based states (C2/A2) of PANI, respectively [41,42]. It is noteworthy that the redox current density increases significantly with increasing scan rate from 1 to 150 mV s1 while the CV curves almost maintain the same shape, indicating that the G–P–Mn has good rate capability [43]. It is also found that the oxidation peak shifts positively and the reduction peak shifts negatively with increasing scan rate, which is mainly due to the resistance of the electrode material [1,6]. Fig. 6 shows the Nyquist plots of PANI, G-PANI and G–P–Mn recorded at the open circuit potential (0.2 V) together with an equivalent circuit used for fitting the experimental data. These plots exhibit a suppressed semicircle over the high frequency
Fig. 6. Nyquist plots of PANI (a), G-PANI (b) and G–P–Mn (c) in 1 M H2SO4. Inset is the corresponding equivalent circuit, where Rs represents the ohmic resistance of electrolyte and the internal resistance of electrode material, and CPE is the constant phase element.
K. Li et al. / Synthetic Metals 209 (2015) 555–560
region and a straight line of nearly 45 over the low frequency region, which represent the interfacial charge transfer resistance (Rct) and the Warburg resistance (Wd) resulted from ion diffusion/ transport in the electrolyte, respectively. The Rct values increase in such an order: PANI (7.4 V) > G-PANI (3.1 V) > G–P–Mn (2.1 V), suggesting that the incorporation of G and MnO2 to PANI will improve the electrochemical performance of the G–P–Mn hybrids, resulting in enhanced electrical conductivity and ion diffusion/ transport behavior.
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Meanwhile, it is found that when the same current density is applied on PANI or G-PANI, corresponding specific capacitance decreases significantly compared with that on the G–P–Mn hybrids (Fig. 7B), and the retention values are calculated to be 54.1% (G-PANI) and 48.5% (PANI), respectively. The results indicate that the incorporation of G and MnO2 to PANI will enhance the electrochemical performance and of the hybrids. 3.4. Influence of molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant
3.3. Galvanostatic charge/discharge of G–P–Mn hybrids The charge/discharge curves of the G–P–Mn hybrids recorded at different current densities of 0.4–10 A g1 exhibit almost symmetrical linear shapes with humps in the potential range from 0 to 0.7 V (Fig. 7A), suggesting that the specific capacitance of the hybrids based supercapacitor is combined with electrical double layer capacitance originated from G and pseudo capacitance originated from PANI and MnO2. The specific capacitances of the hybrids electrode in the supercapacitor can be calculated according to the following equation: C = I Dt/(m DV), where C is the specific capacitance (F g1), I is the charge/discharge current (A), Dt is the discharge time (s), m is the mass of active materials (g), and DV is the potential change during discharge process (V). The results show that the specific capacitance decreases with increasing current density from 0.4 A g1 (800.1 F g1) to 10 A g1 (504.8 F g1). The capacity retention ratio can reach as high as 63.1%, indicating that the hybrids electrode has good rate capability.
Since the purpose of this work is to introduce MnO2 into the G–P–Mn hybrids while maintain PANI in nanofibers to obtain supercapacitor electrode with enhanced electrochemical performance, a binary oxidant composed of KMnO4 and (NH4)2S2O8 was used in the interfacial polymerization of aniline. It can be concluded that the molar ratio of the two oxidants influences the capacitive properties of the hybrids based supercapacitor significantly. Fig. 8 shows the relationship between the specific capacitance and the molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant. It is found that the specific capacitance increases with increasing molar ratio from 0 (only (NH4)2S2O8 is used as the oxidant) to 0.70, suggesting that introduction of a certain amount of MnO2 to the hybrids can result in enhanced specific capacitance. However, further increase in the molar ratio leads to a significant decrease in the specific capacitance, and this can be attributed to the fact that PANI nanofibers are easily destroyed by excessive KMnO4 due to the very strong oxidation ability of KMnO4 and undergone an undesirable overoxidation. As a result, the overoxidation of PANI leads to a decreased conductivity (<105 S cm1). 3.5. Stability of G–P–Mn hybrids based supercapacitor Finally, the cycling stabilities of G–P–Mn and PANI were investigated by galvanostatic charge/discharge at a current density of 10 A g1 (Fig. 9). The capacitance of the G–P–Mn hybrids remains 71% of the initial capacitance after 800 cycles, much higher than that of pure PANI (55%). The greatly improved cycling stability can be attributed to the following reasons. The shrinkage and swelling of PANI during redox process can be effectively restrained due to the introduction of G as substrate, making the hybrids more adaptable to volumetric changes during redox process [44,45]. On the other hand, MnO2 in the hybrids provides a rigid support and an ideal conducting path to PANI by interlinking the polymer chains thus providing enhanced charge exchange efficiency and stability during redox cycling [23].
Fig. 7. (A) Galvanostatic charge/discharge curves of G–P–Mn at different current densities of 0.4, 0.6, 0.8, 1, 2, 3, 5 and 10 A g1. (B) Variations of specific capacitance of PANI, G-PANI and G–P–Mn at different current densities.
Fig. 8. Relationship between the specific capacitance and the molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant.
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Fig. 9. Cycling electrochemical stability of G–P–Mn and PANI in 1 M H2SO4 at a current density of 10 A g1.
[21]
4. Conclusions
[22] [23] [24]
In summary, G–P–Mn hybrids are synthesized via a facile oil– water interfacial polymerization method using a binary oxidant of KMnO4 and (NH4)2S2O8. MnO2 is introduced to the hybrids while PANI still maintains the structure of nanofibers. The hybrids conclude electrical double layer capacitance of G and pseudo capacitance of PANI and MnO2, showing enhanced specific capacitance of 800.1 F g1 at 0.4 A g1. The hybrids based supercapacitor also exhibits good cycling stability due to the synergistic effect of G and MnO2. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21275023, 21173183), Natural Science Foundation of Jiangsu Province (BK2012593), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Oil–water interfacial synthesis of graphene–polyaniline–MnO2 hybrids using binary oxidant for high performance supercapacitor Kaijian Lia , Dengfeng Guoa , Jianmei Chena , Yong Konga,* , Huaiguo Xueb a Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China b School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 July 2015 Received in revised form 7 September 2015 Accepted 15 September 2015 Available online xxx
Graphene–polyaniline–MnO2 (G–P–Mn) hybrids were synthesized via a facile approach that includes the reduction of graphene oxide (GO) to graphene (G) by aniline and then followed by oil–water interfacial polymerization of aniline to polyaniline (PANI) using a binary oxidant of KMnO4 and (NH4)2S2O8. PANI was orderly nanofibered and MnO2-doped due to interfacial polymerization and the use of binary oxidant. The hybrids conclude electrical double layer capacitance of G and pseudo capacitance of PANI and MnO2, showing enhanced specific capacitance of 800.1 F g1 at 0.4 A g1 with good cycling stability. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Graphene MnO2 Interfacial synthesis Binary oxidant Supercapacitor
1. Introduction The past years have witnessed the development of Faradaic pseudo capacitance based supercapacitors because of the low specific capacitance of double layer capacitance [1,2]. Among the materials for pseudocapacitor electrode, polyaniline (PANI) is considered as one of the most promising material due to its facile synthesis and relatively high environmental stability [3,4]. Nanostructured PANI offers superior electrochemical properties [5–7], however, traditional chemical oxidative polymerization yields granular PANI [8]. To obtain nanostructured PANI, interfacial polymerization using (NH4)2S2O8 as oxidant was proposed as an effective approach to suppress secondary growth of PANI by removing the nanostructured PANI from the interface [9,10]. Another major problem encountered when using PANI for supercapacitor electrode is that PANI undergoes poor tolerance during continuous charge/discharge cycling [11–13]. Therefore, hybrids of PANI and other materials including graphene (G) [14–17] and MnO2 [18–20] have been prepared and applied as supercapacitor electrodes with enhanced performances. G nanosheet is predicated as an ideal support material because of its excellent electrical conductivity, remarkable mechanical strength as well as
* Corresponding author. Fax: +86 519 86330167. E-mail address: [email protected] (Y. Kong). http://dx.doi.org/10.1016/j.synthmet.2015.09.017 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
high surface area [21,22], meanwhile, MnO2 in the hybrids provides a rigid support and an ideal electrical conducting path to conducting polymers by interlinking the polymer chains thus providing enhanced charge exchange efficiency and stability during redox cycling [23]. Here, it should be pointed out that although the electrical conductivity of MnO2 is inferior to that of PANI, the nucleation of MnO2 over growing PANI chains terminates further growth of PANI chains by attachment and gives rise to a fresh nucleation of PANI. It has been reported that the structure of conducting polymer is an important parameter as the shorter and ordered polymer structure are found to be ideal structures for charge storage [24]. Moreover, the presence of MnO2 in conducting polymer scaffold can enhance the electrical conductivity of the composite by cross-linking the conducting polymer chains, which significantly reduces the chain defects responsible for hopping conduction [25]. Here, graphene–polyaniline–MnO2(G–P–Mn) hybrids were prepared via a facile approach that includes the reduction of graphene oxide (GO) to G by aniline and then followed by the interfacial polymerization performed in an oil–water biphasic system with aniline-functionalized G dispersed in an organic solvent and the binary oxidant, KMnO4 and (NH4)2S2O8, dissolved in an aqueous acid solution (Fig. 1). It should be pointed out that MnO2 could be introduced to the hybrids when only KMnO4 is used as the oxidant, however, the ordered PANI nanofibers would be destroyed seriously due to the very strong oxidation ability of
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Fig. 1. Schematic illustration of the formation process of G–P–Mn hybrids via the reduction of GO to G by aniline and successive interfacial polymerization in a chloroformwater system.
KMnO4. In order to obtain MnO2-doped hybrids and maintain the nanofibers of PANI in the hybrids, a binary oxidant composed of KMnO4 and (NH4)2S2O8 is used for the oxidative polymerization of aniline at the reactive interface. The obtained G–P–Mn hybrids not only show high specific capacitance of ordered PANI nanofibers, but also exhibit excellent rate capability and cycling stability of MnO2 and G.
reduction of GO to G can be observed by the color change of the solution from yellow to black. After being cooled to room temperature, the composite of G and aniline oligomers dispersed in aniline was obtained by liquid separation using a separatory funnel.
2. Experimental
In a typical experiment, 73 mL of the dispersion of G and aniline oligomers in aniline was dissolved in 5 mL chloroform as the oil phase, and 22.8 mg of (NH4)2S2O8 and 15.8 mg of KMnO4 were dissolved in 5 mL 0.1 M H2SO4 solution as the water phase. After the oil phase was added into a vial (2 cm in diameter), the water phase was added carefully on the top of the oil phase to avoid breaking the steady state of the interface, and the vial was remained as undisturbed at room temperature for 4 h. Next, the water phase was collected and washed thoroughly with ultrapure water for several times to remove aniline monomer and the binary oxidant. The obtained black green mixture was centrifuged at 10,000 rpm and the final product was dried in a vacuum oven at 60 C for 12 h to obtain the G–P–Mn hybrids. For comparison, PANI and MnO2-doped PANI were prepared via interfacial polymerization method without the presence of G using only (NH4)2S2O8 and KMnO4 as the oxidant, respectively. In addition, G-PANI hybrids were synthesized by the same method using only (NH4)2S2O8 as the oxidant.
2.1. Reagents and apparatus Aniline was purchased from Aladdin Chemicals Reagent Co., Ltd. (Shanghai, China) and distilled under reduced pressure before use. Natural graphite powder (99.95%, 8000 mesh), sulfuric acid (H2SO4), potassium permanganate (KMnO4) and ammonium persulfate ((NH4)2S2O8) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other reagents were of analytical grade and used as received. All solutions were prepared with ultrapure water (18.2 MV). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge were performed on a CHI 660D electrochemical workstation (Beijing, China). The morphologies of different samples were characterized on a Supra55 field-emission scanning electron microscope (FESEM, Zeiss, Germany). The FT-IR spectra were recorded on a FTIR-8400S spectrometer (Shimadzu, Japan). Chemical analysis of the graphene–polyaniline–MnO2(G–P–Mn) hybrids was carried out by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA), and the conductivity of the hybrids was measured with a model SZT-2A four-probe instrument (Suzhou, China). 2.2. Reduction of graphene oxide (GO) to graphene (G) by aniline GO was prepared from natural graphite according to method proposed by Marcano et al. [26]. 20 mg of GO was dispersed in 10 mL ultrapure water to form a yellow dispersion by sonication, and then 10 mL aniline was added into the GO dispersion. The mixture was refluxed at 95 C for 8 h under stirring, and the
2.3. Synthesis of G–P–Mn hybrids by interfacial polymerization
2.4. Electrochemical measurements All electrochemical experiments were carried out in a conventional three-electrode system consisting of the G–P–Mn hybrids loaded on a platinum disk electrode (3 mm in diameter) as the working electrode, a platinum foil and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. All electrochemical measurements were performed in 1 M H2SO4 solution at room temperature. The CV curves were recorded at varying scan rates (1, 4, 7, 10, 30, 50, 70, 90, 110, 130 and 150 mV s1) over a potential range from 0 to 0.7 V. The galvanostatic charge/discharge tests were carried out at different current densities (0.4, 0.6, 0.8, 1, 2, 3, 5 and 10 A g1) over the
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potential range from 0 to 0.7 V, and the EIS measurements were performed in the frequency range from 10 kHz to 0.1 Hz with an amplitude of the signal of 5 mV. The cycle life of the G–P–Mn hybrids and PANI was tested at a current density of 10 A g1 by repeated charge/discharge cycling. 3. Results and discussion 3.1. Interfacial synthesis of G–P–Mn hybrids Since the direct redox reaction occurred between GO and aniline or its derivatives has been reported by several groups including us [27–29], aniline is used in this work as the reducing agent to reduce GO to G. During the reduction of GO to G, aniline itself is oxidized to aniline oligomers [30], which is adsorbed on the surface of G to form aniline-functionalized G (G-aniline) via strong p–p stacking (Fig. 1). Due to the hydrophobic surface of G and the planar aromatic structure of aniline, dissolution transfer of G nanosheets from water into aniline is achieved through forming a charge-transfer complex of G and aniline [31,32]. Chloroform and water were used as the oil and water phase, respectively, for the interfacial synthesis of the hybrids. When the G-aniline complex is dispersed in chloroform, aniline oligomers adsorbed on G play a key role in preventing G from irreversible aggregation [33]. Because chloroform is heavier than water, the water layer containing the oxidant can help to seal the chloroform vapor within the reaction vial for a safety consideration. In interfacial polymerization, PANI is formed at the oil–water interface and migrates into the upper water layer immediately since protonated PANI in the emeraldine salt form is hydrophilic [9], and thus interfacial polymerization adopted here is an effective way to suppress the “secondary growth” of PANI via prompt removal of nanostructured PANI from the interface to the water layer. In addition, the initial aniline concentration is lowered when aniline is dissolved in chloroform, resulting in decreased diffusion rate of aniline from chloroform into water layer and suppressed
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“secondary growth” of primary nanostructured PANI in the water phase due to the decreased aniline concentration there [34]. FESEM was utilized to analyze the morphologies of PANI, MnO2doped PANI together with the G–P–Mn hybrids (Fig. 2A–C), in which PANI and MnO2-doped PANI were also prepared via interfacial polymerization using only (NH4)2S2O8 and KMnO4 as the oxidant, respectively. Note that ordered nanofibers of PANI are obtained when interfacial polymerization is carried out using (NH4)2S2O8 as the oxidant (Fig. 2A), agreeing well with previous report [9,10]. However, the typical fibrillar morphology of PANI is destroyed seriously when (NH4)2S2O8 is replaced totally by KMnO4 (Fig. 2B), indicating that overoxidation of PANI occurs immediately after the PANI nanofibers formed at the interface migrate into the water phase due to the strong oxidation ability of KMnO4 [35]. This assumption is also verified by the conductivity measurements of PANI (0.83 S cm1) and MnO2-doped PANI (<105 S cm1), and the significantly decreased conductivity of MnO2-doped PANI demonstrates the deteriorated nanostructure of PANI. In order to obtain MnO2-doped hybrids and maintain the ordered PANI nanofibers, a compromise is proposed here using a binary oxidant composed of KMnO4 and (NH4)2S2O8 for the interfacial synthesis of the hybrids. Although the ordered PANI nanofibers are destroyed a little compared with PANI synthesized using (NH4)2S2O8 as the oxidant, the PANI obtained from the binary oxidant is still mainly composed of nanofibers (Fig. 2C). Moreover, G sheet retains its original layerlike structure and is surrounded by the partially agglomerated PANI nanofibers (Fig. 2C). Such morphology is supposed to stabilize the three-dimensional structure of the hybrids during the charge/ discharge cycling [30]. The reduction of GO to G by aniline and introduction of MnO2 to the hybrids is confirmed by the FT-IR spectra of GO, G-PANI (using only (NH4)2S2O8 as the oxidant in interfacial polymerization) and G–P–Mn (Fig. 3). The bands at 1743, 1216 and 1054 cm1 are assigned to the stretching vibration of oxygen-containing groups in GO [14,36,37]. For G-PANI, these characteristic bands of GO disappear completely, suggesting that GO can be reduced to G
Fig. 2. FESEM images of PANI, MnO2-doped PANI and G–P–Mn hybrids made by interfacial polymerization, with the following oxidant in the aqueous phase: (A) (NH4)2S2O8, (B) KMnO4 and (C) (NH4)2S2O8 + KMnO4.
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Fig. 3. FT-IR spectra of GO, G-PANI and G–P–Mn.
successfully by aniline, and four new peaks at 1570, 1482, 1294, 1124 cm1 are associated with C¼N, C¼C, CN and C H in PANI, respectively [10]. Compared with G-PANI, a new peak corresponding to the vibration of Mn O appears at 538 cm1 [38], indicating the presence of MnO2 in the G–P–Mn hybrids synthesized using the binary oxidant of KMnO4 and (NH4)2S2O8. XPS is used to further confirm the introduction of MnO2 to the hybrids. A weak Mn 2p signal is observed at the full XPS spectra of G–P–Mn (Fig. 4A), indicating that manganese exists in the G–P–Mn
Fig. 4. Full XPS spectra (A) and Mn 2p XPS spectra (B) of G–P–Mn.
Fig. 5. Cyclic voltammograms of G–P–Mn hybrids in 1 M H2SO4 at different scan rates of 1, 4, 7, 10, 30, 50, 70, 90, 110, 130, and 150 mV s1.
hybrids after the interfacial synthesis. The two peaks of Mn 2p3/2 and Mn 2p1/2, which are centered at 642.0 and 653.7 eV, respectively (Fig. 4B), with a spin energy separation of 11.7 eV, agree well with reported data of Mn 2p3/2 and Mn 2p1/2 in MnO2 [39,40]. This result also suggests that MnO2, the reduction product of KMnO4, has been introduced into the hybrids. 3.2. Electrochemical characterization of G–P–Mn hybrids Fig. 5 shows the cyclic voltammograms of the G–P–Mn hybrids at different scan rates. Two pairs of redox peaks are observed on each curve, which are attributed to the redox transformation between leucoemeraldine based states and emeraldine salt (C1/A1), and transition between emeraldine salt and pernigraniline based states (C2/A2) of PANI, respectively [41,42]. It is noteworthy that the redox current density increases significantly with increasing scan rate from 1 to 150 mV s1 while the CV curves almost maintain the same shape, indicating that the G–P–Mn has good rate capability [43]. It is also found that the oxidation peak shifts positively and the reduction peak shifts negatively with increasing scan rate, which is mainly due to the resistance of the electrode material [1,6]. Fig. 6 shows the Nyquist plots of PANI, G-PANI and G–P–Mn recorded at the open circuit potential (0.2 V) together with an equivalent circuit used for fitting the experimental data. These plots exhibit a suppressed semicircle over the high frequency
Fig. 6. Nyquist plots of PANI (a), G-PANI (b) and G–P–Mn (c) in 1 M H2SO4. Inset is the corresponding equivalent circuit, where Rs represents the ohmic resistance of electrolyte and the internal resistance of electrode material, and CPE is the constant phase element.
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region and a straight line of nearly 45 over the low frequency region, which represent the interfacial charge transfer resistance (Rct) and the Warburg resistance (Wd) resulted from ion diffusion/ transport in the electrolyte, respectively. The Rct values increase in such an order: PANI (7.4 V) > G-PANI (3.1 V) > G–P–Mn (2.1 V), suggesting that the incorporation of G and MnO2 to PANI will improve the electrochemical performance of the G–P–Mn hybrids, resulting in enhanced electrical conductivity and ion diffusion/ transport behavior.
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Meanwhile, it is found that when the same current density is applied on PANI or G-PANI, corresponding specific capacitance decreases significantly compared with that on the G–P–Mn hybrids (Fig. 7B), and the retention values are calculated to be 54.1% (G-PANI) and 48.5% (PANI), respectively. The results indicate that the incorporation of G and MnO2 to PANI will enhance the electrochemical performance and of the hybrids. 3.4. Influence of molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant
3.3. Galvanostatic charge/discharge of G–P–Mn hybrids The charge/discharge curves of the G–P–Mn hybrids recorded at different current densities of 0.4–10 A g1 exhibit almost symmetrical linear shapes with humps in the potential range from 0 to 0.7 V (Fig. 7A), suggesting that the specific capacitance of the hybrids based supercapacitor is combined with electrical double layer capacitance originated from G and pseudo capacitance originated from PANI and MnO2. The specific capacitances of the hybrids electrode in the supercapacitor can be calculated according to the following equation: C = I Dt/(m DV), where C is the specific capacitance (F g1), I is the charge/discharge current (A), Dt is the discharge time (s), m is the mass of active materials (g), and DV is the potential change during discharge process (V). The results show that the specific capacitance decreases with increasing current density from 0.4 A g1 (800.1 F g1) to 10 A g1 (504.8 F g1). The capacity retention ratio can reach as high as 63.1%, indicating that the hybrids electrode has good rate capability.
Since the purpose of this work is to introduce MnO2 into the G–P–Mn hybrids while maintain PANI in nanofibers to obtain supercapacitor electrode with enhanced electrochemical performance, a binary oxidant composed of KMnO4 and (NH4)2S2O8 was used in the interfacial polymerization of aniline. It can be concluded that the molar ratio of the two oxidants influences the capacitive properties of the hybrids based supercapacitor significantly. Fig. 8 shows the relationship between the specific capacitance and the molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant. It is found that the specific capacitance increases with increasing molar ratio from 0 (only (NH4)2S2O8 is used as the oxidant) to 0.70, suggesting that introduction of a certain amount of MnO2 to the hybrids can result in enhanced specific capacitance. However, further increase in the molar ratio leads to a significant decrease in the specific capacitance, and this can be attributed to the fact that PANI nanofibers are easily destroyed by excessive KMnO4 due to the very strong oxidation ability of KMnO4 and undergone an undesirable overoxidation. As a result, the overoxidation of PANI leads to a decreased conductivity (<105 S cm1). 3.5. Stability of G–P–Mn hybrids based supercapacitor Finally, the cycling stabilities of G–P–Mn and PANI were investigated by galvanostatic charge/discharge at a current density of 10 A g1 (Fig. 9). The capacitance of the G–P–Mn hybrids remains 71% of the initial capacitance after 800 cycles, much higher than that of pure PANI (55%). The greatly improved cycling stability can be attributed to the following reasons. The shrinkage and swelling of PANI during redox process can be effectively restrained due to the introduction of G as substrate, making the hybrids more adaptable to volumetric changes during redox process [44,45]. On the other hand, MnO2 in the hybrids provides a rigid support and an ideal conducting path to PANI by interlinking the polymer chains thus providing enhanced charge exchange efficiency and stability during redox cycling [23].
Fig. 7. (A) Galvanostatic charge/discharge curves of G–P–Mn at different current densities of 0.4, 0.6, 0.8, 1, 2, 3, 5 and 10 A g1. (B) Variations of specific capacitance of PANI, G-PANI and G–P–Mn at different current densities.
Fig. 8. Relationship between the specific capacitance and the molar ratio of KMnO4 to (NH4)2S2O8 in the binary oxidant.
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Fig. 9. Cycling electrochemical stability of G–P–Mn and PANI in 1 M H2SO4 at a current density of 10 A g1.
[21]
4. Conclusions
[22] [23] [24]
In summary, G–P–Mn hybrids are synthesized via a facile oil– water interfacial polymerization method using a binary oxidant of KMnO4 and (NH4)2S2O8. MnO2 is introduced to the hybrids while PANI still maintains the structure of nanofibers. The hybrids conclude electrical double layer capacitance of G and pseudo capacitance of PANI and MnO2, showing enhanced specific capacitance of 800.1 F g1 at 0.4 A g1. The hybrids based supercapacitor also exhibits good cycling stability due to the synergistic effect of G and MnO2. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21275023, 21173183), Natural Science Foundation of Jiangsu Province (BK2012593), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
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