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Hydrothermally reduced nano porous graphene–polyaniline nanofiber composites for supercapacitor
FlatChem 1 (2016) 1–5
Contents lists available at ScienceDirect
FlatChem j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl a t c
Hydrothermally reduced nano porous graphene–polyaniline nanofiber composites for supercapacitor Mrinmoy Kumar Chini ⇑, Shyambo Chatterjee Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 19 May 2016 Revised 29 July 2016 Accepted 15 August 2016
Keywords: Hydrothermal Porous graphene Supercapacitor Graphene quantum dots Power density
a b s t r a c t Hydrothermally reduced porous graphene (PGs) and polyaniline (PANI) nanofiber composites (PGs–PANI) were prepared by interfacial polymerization of aniline in the presence of reduced porous graphene (PGs) under acidic conditions. Hydrothermal reduction of graphene oxide (GO) produced graphene quantum dots (GQDs) along with porous reduced graphene (PGs). The removal of nano sized GQDs generate several round shaped edges in the graphene, which facilitated improved charge storage. In addition to preparation of porous graphene, we developed an approach to process the PGs. We envisioned preparing conjugated polymer as processing material. The capacitive behavior of PGs was characterized using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques. The specific capacitance of hydrothermally reduced GO (PGs) and PGs–PANI composites was 185 F/g and 357 F/g at 0.1 A/g in 1 M H2SO4 electrolyte. The energy density of PGs–PANI composite was obtained 49.58 W h/kg at a power density of 71.8 W/kg. Ó 2016 Elsevier BV. All rights reserved.
1. Introduction Energy conversion is as important as energy storage. Supercapacitors are one of the attractive storage devices because they bridge the gap between batteries and conventional capacitors. They possess higher energy densities than dielectric capacitors and exceptional long cyclability and higher power densities than conventional batteries [1,2]. Graphene [3–8] and conjugated polymer [9–14] based capacitors are widely explored due to the possibility of fabricating flexible and light weight supercapacitors. Variety of approaches had been developed to fabricate graphene based supercapacitors. Polyaniline grafted reduced graphene oxide, various graphene/polyaniline nanofiber composites [15–19] etc. have been employed as a supercapacitor material. The partially reduced graphene oxide nanosheet/polyaniline nanowafer (GNS/PANI) composites reported to exhibit a specific capacitance value of 329.5 F/g at 5 mV/s [20] and chemically modified graphene and polyaniline (PANI) nanofiber composites showed a specific capacitance of as high as 480 F/g at a current density of 0.1 A/g [21]. Recent reports indicate that more charges are stored at the edges of graphene [22–24]. This has not been explored as an approach to prepare materials for supercapacitors. We hypothesized
⇑ Corresponding author. E-mail address: [email protected] (M.K. Chini). http://dx.doi.org/10.1016/j.flatc.2016.08.001 2452-2627/Ó 2016 Elsevier BV. All rights reserved.
that the edges in graphene can be expanded using porous graphene (PGs). The porous graphene can be prepared by removing graphene quantum dots (GQDs). The removal of nanosized GQDs generate several round shaped edges in the graphene, which is likely to facilitate improved charge storage. In addition to preparation of porous graphene, it is essential to develop an approach to process the PGs. We envisioned preparing conjugated polymer as processing material. Polyaniline nanofibers (PANI Nano) can be easily synthesized by interfacial polymerization [25–27]. We propose to incorporate the PGs during the synthesis of PANI Nano. This material was used as electroactive component in supercapacitors, which showed capacitance of 357 F/g. This capacitance is more than double of PGs’ capacitance (185 F/g). Thus, use of combination of PANI Nano and PGs has synergistic effect in improving the performance of supercapacitors [28,29].
2. Experimental 2.1. Materials and methods Porous graphene (PGs) have been synthesized by hydrothermal treatment of GO in autoclave at 180 °C for 12 h. This simple process resulted in producing both GQDs and porous reduced grapheme (PGs) [30,31]. The reduction is carried out using a simple hydrothermal method without any reducing agent. To prepare PGs and polyaniline nanofiber composites (PGs–PANI) by in situ
2
M.K. Chini, S. Chatterjee / FlatChem 1 (2016) 1–5
interfacial polymerization method, distilled aniline (0.3 g, 3.2 mmol) was dissolved in 10 mL of 1 M HCl solution in 20 mL chloroform (CHCl3) solvent and suspension of porous reduced graphene oxide (10 mg/20 mL) in aqueous solution was prepared. Then finally ammonium peroxydisulfate (APS) (0.18 g, 0.8 mmol) in 10 mL of 1 M HCl aqueous solution was added dropwise into the aqueous layer (top layer). Polymerization was clearly observed in about 2 min with the characteristic green color of polyaniline emeraldine salt at the interface of the two layers. The bilayer solution was then allowed to keep at room temperature overnight. Then, 100 mL of D.I. water was added to the mixture followed by filtration and washing with water, ethanol and hexane successively and finally the composite was dried under vacuum at 60 °C. Here we demonstrate the preparation of PGs–PANI nanofiber composites using an in situ polymerization method to achieve a reasonably homogeneous dispersion of graphene sheets within the PANI matrix [21]. The morphology of the GO, PGs and PGs–PANI was examined by transmission electron microscopy (TEM). X-ray diffraction (XRD) measurements were carried out for structural understanding of the graphene materials. Attenuated total reflectance (ATR)infrared spectroscopic (ATR-IR) analyses were carried out to find out changes in different functionalities after hydrothermal modification. All the electrochemical measurements were performed using a multi channel autolab MAC 80038 instrument in a conventional three-electrode cell that consists of a PGs and PGs–PANI containing electrode material coated working electrode, a large platinum foil as a counter electrode, and Ag/AgCl as a reference electrode respectively. The electrolyte solution, 1.0 M H2SO4 solution, was purged with N2 prior to the each measurement.
3. Results and discussion Graphene oxide dispersion (4 mg/mL) was prepared in DMF by sonicating for 30 min. Then the dispersion was placed in Autoclave and heated at 180 °C for 12 h.
After the treatment in Autoclave, the reaction mixture was allowed to cool to room temperature. Then, the reaction mixture was dialyzed in a dialysis bag (3.5 kDa) for 3–4 days to separate out GQDs. The GQD free PGs in the dialysis bag were further washed with deionized (DI) water and ethanol and dried under vacuum. In order to identify the generation of GQDs from the graphene and formation of PGs, we collected the dialysis solution that was used for floating the dialysis bag. If GQDs were generated and released via the pores of the dialysis bag, then the solution outside of dialysis bag should be fluorescent. The fluorescence will be a function of the GQDs. We were gratified to note blue fluorescence, which indicates that the GQDs are 2–4 nm. We also dropped the solution on TEM grid. The TEM images showed the formation of GQDs with a diameter of 2–4 nm (Fig. 1). This is an indirect evidence for the formation of PGs from GOs by removal of GQDs. The ATR-IR spectra of GOs were recorded using powder sample (Fig. 2a). A broad peak corresponding to hydrogen bonding was observed around 3200 cm1. On the other hand, this peak almost disappeared in case of PGs. This indicates that the PGs preparation involves removal of GQDs and probable reduction of GO to graphene. Furthermore, evolution of a peak at 1555 cm1, indicates the restoration of sp2 carbon in PGs upon removal of GQDs from GOs. PXRD was used to understand the formation of PGs (Fig. 2b). Please note that this experiment does not provide information about porosity, however, we must get information on conversion from GOs to graphene. The PXRD of GOs was found to exhibit peak at 2h of 10.1°. After GQDs preparation, the resultant PGs showed no peak at 2h of 10.1°. Furthermore, the PGs showed a new broad peak with peak maxima at a 2h of 25.6°. The appearance of the peak at 25.6° is an indication of formation of graphene. The BET surface area for PGs was found to be 294 m2/g. Graphene oxide has been reported to show a specific surface area of 68–161 m2 g1 [32–34]. TEM experiments showed 2–4 nm diameter GQDs, which was collected from the dialysis solution (Fig. 1). The TEM of PGs did not show the pores due to very small pores formed during the PGs preparation. All these experiments indicate the formation of PGs, which is the envisioned and
Fig. 1. TEM images of graphene oxide (GO), reduced porous graphene oxide (PGs) (scale bar 200 nm for both) and graphene quantum dots (GQDs) (scale bar 10 nm) and PGs–PANI composites (scale bar 200 nm), respectively.
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Fig. 2. (a) ATR-IR spectra of GO and PGs; (b) PXRD of GO and PGs; (c) pore radius plot for porous reduced PGs, and (d) CV measurements for PGs–PANI nano composites, respectively.
Table 1 Charge storage and materials properties of PGs and PGs–PANI composite.
Scheme 1. Illustration of the process for preparation of porous graphene–PANI (PGs–PANI) composites.
desired material for supercapacitor preparation. To prepare PGs– PANI Nano composites (PGs–PANI) by in situ interfacial polymerization method, aniline was dissolved in chloroform. PGs along with ammonium persulfate (APS) were dissolved in 1 M HCl. The two immiscible solutions were brought in contact to generate interface. At the interface, the polymerization is initiated (Scheme 1). A green precipitate develops at the interface, which is PGs–PANI composite. The PGs–PANI was washed with water and ethanol and dried in vacuum at 60 °C. After hydrothermal treatment, removal of a significant amount of oxygen containing functional groups along with introduction of porosity occurred in PGs. This was evident in IR spectra and the
Sample
BET (m2/g)
Pore size (nm)
Specific capacitance (F/g) 0.1 A/g
1.0 A/g
0.2 A/g
0.5 A/g
PGs PGs–PANI PANI
294 88 –
3.9 3.4 –
185 357 128
200 290 80
150 215 45
130 160 –
presence of porosity in the PGs was confirmed by BET isotherm (Fig. 2c). The reduction of GO can be shown by ATR-IR spectrum (Fig. 2a). The TEM images of PGs–PANI nano composite showed fibrillar morphology. The morphology of this composite is not very different from that of PANI prepared by interfacial polymerization. After all the spectroscopic and morphological characterization, we proceeded to study the electrochemical and charge storage properties of the synthesized materials. The cyclic voltammograms (CVs) of PGs–PANI nano was recorded in 1 M H2SO4 using Pt counter electrode and Ag/AgCl reference electrode. The potential was swept between 0.2 V and 0.8 V. The PGs–PANI Nano did not show any sharp peak in the potential window, but a broad peak was observed around 0.5 V at higher scan rates. The absence of well define faradaic peaks indicates the pseudocapacitive nature of the PGs–PANI Nano (Fig. 2d). Then, we carried out the galvanostatic charge–discharge experiments for PGs and PGs–PANI Nano as charge storage material (Table 1). The charge–discharge experiments were carried out at various current densities and the time vs potential curves are not symmetrical (Fig. 3). This indicates that the possible IR drops in the material. This was further confirmed by non linear slope in the discharge curve. The specific capacitance calculated from the slope was found to be 357 F/g while discharging at 0.1 A/g by using the dt following equation: Csp ¼ mi dV , where, Csp is the specific capacitance of the electrode, I is the current, m is the active mass of the electrode, dt is the discharge time and dV is the voltage difference of discharge.
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Fig. 3. Galvanostatic charge–discharge (GCD) curves of (a) PGs, (b) PGs–PANI respectively at different current density, (c) Comparative GCD curves for PGs, PGs–PANI and PANI nanofibers at 0.1 °C current density, and (d) Ragone plot for PGs–PANI based supercapacitor respectively.
This decreased to 160 F/g upon an increase in discharge current density to 1 A/g. On the other hand, the PGs showed a specific capacitance of 185 F/g while discharging at 0.1 A/g. A 30% decrease in specific capacitance was observed while the discharge experiment was carried out at 1 A/g. We also calculated power (P) and energy densities (E) for the supercapacitors using the following 2
equations: E ¼ CV2 and P ¼ Et, where, C is the specific capacitance (F/g), V is the potential window of discharge (V), and t is the discharge time(s). A maximum power density of 71.8 W/kg and energy density of 49.5 W h/kg was obtained for PGs–PANI Nano composites (Fig. 3d). In case of PGs, the power density of 48.9 W/ kg and energy density of 25.7 W h/kg had been only achieved. 4. Conclusions In summary, we have synthesized porous graphene with an increased number of edges for increased charge storage. This was blended with PANI nanofibers synthesized by interfacial polymerization. The PANI nanofibers were expected to act synergistically and improve the charge storage properties. As hypothesized the specific capacitance of PGs was found to be 185 F/g. This value increased to 357 F/g in case of PGs–PANI Nano composites. Acknowledgements We thank Dr. K. Krishnamoorthy for his support. MKC thanks Council of Scientific and Industrial Research for scholarship. We thank the Department of Science and Technology for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.flatc.2016.08.001. References [1] J.R. Miller, P. Simon, Materials science. Electrochemical capacitors for energy management, Science 321 (2008) 651.
[2] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22 (2010) E28. [3] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498. [4] J.L. Xia, F. Chen, J.H. Li, N.J. Tao, Measurement of the quantum capacitance of graphene, Nat. Nanotechnol. 4 (2009) 505. [5] S.R.C. Vivekchand, C.S. Rout, K.S. Subrahmanyam, A. Govindara, C.N.R. Rao, Graphene-based electrochemical supercapacitors, J. Chem. Sci. 120 (2008) 9. [6] Z. Lin, Y. Liu, Y. Yao, O.J. Hildreth, Z. Li, K. Moon, C.-P. Wong, Superior capacitance of functionalized graphene, J. Phys. Chem. C 115 (2011) 7120. [7] B. Rajagopalan, J.S. Chung, Reduced chemically modified graphene oxide for supercapacitor electrode, Nanoscale Res. Lett. 9 (2014) 535. [8] Y. Zhu, S. Murali, M.D. Stoller, A. Velamakanni, R.D. Piner, R.S. Ruoff, Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors, Carbon 48 (2010) 2118. [9] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Conducting polymers as active materials in electrochemical capacitors, J. Power Sources 47 (1994) 89. [10] A. Laforgue, P. Simon, C. Sarrazin, J.-F. Fauvarque, Polythiophene-based supercapacitors, J. Power Sources 80 (1999) 142. [11] M. Mastragostino, C. Arbizzani, F. Soavi, Polymer-based supercapacitors, J. Power Sources 97–98 (2001) 812. [12] C. Arbizzani, M. Mastragostino, F. Soavi, New trends in electrochemical supercapacitors, J. Power Sources 100 (2001) 164. [13] V. Gupta, N. Miura, Electrochemically deposited polyaniline nanowire’s network a high-performance electrode material for redox supercapacitor, Electrochem. Solid-State Lett. 8 (2005) A630. [14] D. Belanger, X. Ren, J. Davey, F. Uribe, S. Gottesfeld, Characterization and longterm performance of polyaniline based electrochemical capacitors, J. Electrochem. Soc. 147 (2000) 2923. [15] J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance, Carbon 48 (2010) 487. [16] D.W. Wang, F. Li, J.P. Zhao, W.C. Ren, Z.G. Chen, J. Tan, Z.S. Wu, I. Gentle, G.Q. Lu, H.M. Cheng, Fabrication of graphene/polyaniline composite paper via in situ anodic electropolymerization for high-performance flexible electrode, ACS Nano 3 (2009) 1745. [17] P. Xiao, M. Xiao, P.G. Liu, K.C. Gong, Direct synthesis of a polyanilineintercalated graphite oxide nanocomposite, Carbon 38 (2000) 626. [18] H.L. Wang, Q.L. Hao, X.J. Yang, L.D. Lu, X. Wang, Graphene oxide doped polyaniline for supercapacitors, Electrochem. Commun. 11 (2009) 1158. [19] P.G. Liu, K.C. Gong, Synthesis of polyaniline-intercalated graphite oxide by an in situ oxidative polymerization reaction, Carbon 37 (1999) 706. [20] Z. Gao, W. Yang, J. Wang, B. Wang, Z. Li, Q. Liu, M. Zhang, L. Liu, A new partially reduced graphene oxide nanosheet/polyaniline nanowafer hybrid as supercapacitor electrode material, Energy Fuels 27 (2013) 568. [21] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chem. Mater. 22 (2010) 1392. [22] W. Yuan, Y. Zhou, Y. Li, C. Li, H. Peng, J. Zhang, Z. Liu, L. Dai, G. Shi, The edgeand basal-plane-specific electrochemistry of a single-layer graphene sheet, Sci. Rep. 3 (2013) 2248.
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[29] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/ polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963. [30] V. Stengl, S. Bakardjieva, J. Henych, K. Lang, M. Kormunda, Blue and green luminescence of reduced graphene oxide quantum dots, Carbon 63 (2013) 537. [31] S. Zhu, J. Zhang, X. Liu, B. Li, X. Wang, S. Tang, Q. Meng, Y. Li, C. Shi, R. Hu, B. Yang, Graphene quantum dots with controllable surface oxidation, tunable fluorescence and up-conversion emission, RSC Adv. 2 (2012) 2717. [32] L.Z. Fan, J.L. Liu, R. Ud-Din, X. Yan, X. Qu, The effect of reduction time on the surface functional groups and supercapacitive performance of graphene nanosheets, Carbon 50 (2012) 3724. [33] M. Seredych, A.V. Tamashausky, T.J. Bandosz, Graphite oxides obtained from porous graphite: the role of surface chemistry and texture in ammonia retention at ambient conditions, Adv. Funct. Mater. 20 (2010) 1670. [34] S. You, S.M. Luzan, T. Szabó, A.V. Talyzin, Effect of synthesis method on solvation and exfoliation of graphite oxide, Carbon 52 (2013) 171.
Contents lists available at ScienceDirect
FlatChem j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl a t c
Hydrothermally reduced nano porous graphene–polyaniline nanofiber composites for supercapacitor Mrinmoy Kumar Chini ⇑, Shyambo Chatterjee Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 19 May 2016 Revised 29 July 2016 Accepted 15 August 2016
Keywords: Hydrothermal Porous graphene Supercapacitor Graphene quantum dots Power density
a b s t r a c t Hydrothermally reduced porous graphene (PGs) and polyaniline (PANI) nanofiber composites (PGs–PANI) were prepared by interfacial polymerization of aniline in the presence of reduced porous graphene (PGs) under acidic conditions. Hydrothermal reduction of graphene oxide (GO) produced graphene quantum dots (GQDs) along with porous reduced graphene (PGs). The removal of nano sized GQDs generate several round shaped edges in the graphene, which facilitated improved charge storage. In addition to preparation of porous graphene, we developed an approach to process the PGs. We envisioned preparing conjugated polymer as processing material. The capacitive behavior of PGs was characterized using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques. The specific capacitance of hydrothermally reduced GO (PGs) and PGs–PANI composites was 185 F/g and 357 F/g at 0.1 A/g in 1 M H2SO4 electrolyte. The energy density of PGs–PANI composite was obtained 49.58 W h/kg at a power density of 71.8 W/kg. Ó 2016 Elsevier BV. All rights reserved.
1. Introduction Energy conversion is as important as energy storage. Supercapacitors are one of the attractive storage devices because they bridge the gap between batteries and conventional capacitors. They possess higher energy densities than dielectric capacitors and exceptional long cyclability and higher power densities than conventional batteries [1,2]. Graphene [3–8] and conjugated polymer [9–14] based capacitors are widely explored due to the possibility of fabricating flexible and light weight supercapacitors. Variety of approaches had been developed to fabricate graphene based supercapacitors. Polyaniline grafted reduced graphene oxide, various graphene/polyaniline nanofiber composites [15–19] etc. have been employed as a supercapacitor material. The partially reduced graphene oxide nanosheet/polyaniline nanowafer (GNS/PANI) composites reported to exhibit a specific capacitance value of 329.5 F/g at 5 mV/s [20] and chemically modified graphene and polyaniline (PANI) nanofiber composites showed a specific capacitance of as high as 480 F/g at a current density of 0.1 A/g [21]. Recent reports indicate that more charges are stored at the edges of graphene [22–24]. This has not been explored as an approach to prepare materials for supercapacitors. We hypothesized
⇑ Corresponding author. E-mail address: [email protected] (M.K. Chini). http://dx.doi.org/10.1016/j.flatc.2016.08.001 2452-2627/Ó 2016 Elsevier BV. All rights reserved.
that the edges in graphene can be expanded using porous graphene (PGs). The porous graphene can be prepared by removing graphene quantum dots (GQDs). The removal of nanosized GQDs generate several round shaped edges in the graphene, which is likely to facilitate improved charge storage. In addition to preparation of porous graphene, it is essential to develop an approach to process the PGs. We envisioned preparing conjugated polymer as processing material. Polyaniline nanofibers (PANI Nano) can be easily synthesized by interfacial polymerization [25–27]. We propose to incorporate the PGs during the synthesis of PANI Nano. This material was used as electroactive component in supercapacitors, which showed capacitance of 357 F/g. This capacitance is more than double of PGs’ capacitance (185 F/g). Thus, use of combination of PANI Nano and PGs has synergistic effect in improving the performance of supercapacitors [28,29].
2. Experimental 2.1. Materials and methods Porous graphene (PGs) have been synthesized by hydrothermal treatment of GO in autoclave at 180 °C for 12 h. This simple process resulted in producing both GQDs and porous reduced grapheme (PGs) [30,31]. The reduction is carried out using a simple hydrothermal method without any reducing agent. To prepare PGs and polyaniline nanofiber composites (PGs–PANI) by in situ
2
M.K. Chini, S. Chatterjee / FlatChem 1 (2016) 1–5
interfacial polymerization method, distilled aniline (0.3 g, 3.2 mmol) was dissolved in 10 mL of 1 M HCl solution in 20 mL chloroform (CHCl3) solvent and suspension of porous reduced graphene oxide (10 mg/20 mL) in aqueous solution was prepared. Then finally ammonium peroxydisulfate (APS) (0.18 g, 0.8 mmol) in 10 mL of 1 M HCl aqueous solution was added dropwise into the aqueous layer (top layer). Polymerization was clearly observed in about 2 min with the characteristic green color of polyaniline emeraldine salt at the interface of the two layers. The bilayer solution was then allowed to keep at room temperature overnight. Then, 100 mL of D.I. water was added to the mixture followed by filtration and washing with water, ethanol and hexane successively and finally the composite was dried under vacuum at 60 °C. Here we demonstrate the preparation of PGs–PANI nanofiber composites using an in situ polymerization method to achieve a reasonably homogeneous dispersion of graphene sheets within the PANI matrix [21]. The morphology of the GO, PGs and PGs–PANI was examined by transmission electron microscopy (TEM). X-ray diffraction (XRD) measurements were carried out for structural understanding of the graphene materials. Attenuated total reflectance (ATR)infrared spectroscopic (ATR-IR) analyses were carried out to find out changes in different functionalities after hydrothermal modification. All the electrochemical measurements were performed using a multi channel autolab MAC 80038 instrument in a conventional three-electrode cell that consists of a PGs and PGs–PANI containing electrode material coated working electrode, a large platinum foil as a counter electrode, and Ag/AgCl as a reference electrode respectively. The electrolyte solution, 1.0 M H2SO4 solution, was purged with N2 prior to the each measurement.
3. Results and discussion Graphene oxide dispersion (4 mg/mL) was prepared in DMF by sonicating for 30 min. Then the dispersion was placed in Autoclave and heated at 180 °C for 12 h.
After the treatment in Autoclave, the reaction mixture was allowed to cool to room temperature. Then, the reaction mixture was dialyzed in a dialysis bag (3.5 kDa) for 3–4 days to separate out GQDs. The GQD free PGs in the dialysis bag were further washed with deionized (DI) water and ethanol and dried under vacuum. In order to identify the generation of GQDs from the graphene and formation of PGs, we collected the dialysis solution that was used for floating the dialysis bag. If GQDs were generated and released via the pores of the dialysis bag, then the solution outside of dialysis bag should be fluorescent. The fluorescence will be a function of the GQDs. We were gratified to note blue fluorescence, which indicates that the GQDs are 2–4 nm. We also dropped the solution on TEM grid. The TEM images showed the formation of GQDs with a diameter of 2–4 nm (Fig. 1). This is an indirect evidence for the formation of PGs from GOs by removal of GQDs. The ATR-IR spectra of GOs were recorded using powder sample (Fig. 2a). A broad peak corresponding to hydrogen bonding was observed around 3200 cm1. On the other hand, this peak almost disappeared in case of PGs. This indicates that the PGs preparation involves removal of GQDs and probable reduction of GO to graphene. Furthermore, evolution of a peak at 1555 cm1, indicates the restoration of sp2 carbon in PGs upon removal of GQDs from GOs. PXRD was used to understand the formation of PGs (Fig. 2b). Please note that this experiment does not provide information about porosity, however, we must get information on conversion from GOs to graphene. The PXRD of GOs was found to exhibit peak at 2h of 10.1°. After GQDs preparation, the resultant PGs showed no peak at 2h of 10.1°. Furthermore, the PGs showed a new broad peak with peak maxima at a 2h of 25.6°. The appearance of the peak at 25.6° is an indication of formation of graphene. The BET surface area for PGs was found to be 294 m2/g. Graphene oxide has been reported to show a specific surface area of 68–161 m2 g1 [32–34]. TEM experiments showed 2–4 nm diameter GQDs, which was collected from the dialysis solution (Fig. 1). The TEM of PGs did not show the pores due to very small pores formed during the PGs preparation. All these experiments indicate the formation of PGs, which is the envisioned and
Fig. 1. TEM images of graphene oxide (GO), reduced porous graphene oxide (PGs) (scale bar 200 nm for both) and graphene quantum dots (GQDs) (scale bar 10 nm) and PGs–PANI composites (scale bar 200 nm), respectively.
3
M.K. Chini, S. Chatterjee / FlatChem 1 (2016) 1–5
Fig. 2. (a) ATR-IR spectra of GO and PGs; (b) PXRD of GO and PGs; (c) pore radius plot for porous reduced PGs, and (d) CV measurements for PGs–PANI nano composites, respectively.
Table 1 Charge storage and materials properties of PGs and PGs–PANI composite.
Scheme 1. Illustration of the process for preparation of porous graphene–PANI (PGs–PANI) composites.
desired material for supercapacitor preparation. To prepare PGs– PANI Nano composites (PGs–PANI) by in situ interfacial polymerization method, aniline was dissolved in chloroform. PGs along with ammonium persulfate (APS) were dissolved in 1 M HCl. The two immiscible solutions were brought in contact to generate interface. At the interface, the polymerization is initiated (Scheme 1). A green precipitate develops at the interface, which is PGs–PANI composite. The PGs–PANI was washed with water and ethanol and dried in vacuum at 60 °C. After hydrothermal treatment, removal of a significant amount of oxygen containing functional groups along with introduction of porosity occurred in PGs. This was evident in IR spectra and the
Sample
BET (m2/g)
Pore size (nm)
Specific capacitance (F/g) 0.1 A/g
1.0 A/g
0.2 A/g
0.5 A/g
PGs PGs–PANI PANI
294 88 –
3.9 3.4 –
185 357 128
200 290 80
150 215 45
130 160 –
presence of porosity in the PGs was confirmed by BET isotherm (Fig. 2c). The reduction of GO can be shown by ATR-IR spectrum (Fig. 2a). The TEM images of PGs–PANI nano composite showed fibrillar morphology. The morphology of this composite is not very different from that of PANI prepared by interfacial polymerization. After all the spectroscopic and morphological characterization, we proceeded to study the electrochemical and charge storage properties of the synthesized materials. The cyclic voltammograms (CVs) of PGs–PANI nano was recorded in 1 M H2SO4 using Pt counter electrode and Ag/AgCl reference electrode. The potential was swept between 0.2 V and 0.8 V. The PGs–PANI Nano did not show any sharp peak in the potential window, but a broad peak was observed around 0.5 V at higher scan rates. The absence of well define faradaic peaks indicates the pseudocapacitive nature of the PGs–PANI Nano (Fig. 2d). Then, we carried out the galvanostatic charge–discharge experiments for PGs and PGs–PANI Nano as charge storage material (Table 1). The charge–discharge experiments were carried out at various current densities and the time vs potential curves are not symmetrical (Fig. 3). This indicates that the possible IR drops in the material. This was further confirmed by non linear slope in the discharge curve. The specific capacitance calculated from the slope was found to be 357 F/g while discharging at 0.1 A/g by using the dt following equation: Csp ¼ mi dV , where, Csp is the specific capacitance of the electrode, I is the current, m is the active mass of the electrode, dt is the discharge time and dV is the voltage difference of discharge.
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M.K. Chini, S. Chatterjee / FlatChem 1 (2016) 1–5
Fig. 3. Galvanostatic charge–discharge (GCD) curves of (a) PGs, (b) PGs–PANI respectively at different current density, (c) Comparative GCD curves for PGs, PGs–PANI and PANI nanofibers at 0.1 °C current density, and (d) Ragone plot for PGs–PANI based supercapacitor respectively.
This decreased to 160 F/g upon an increase in discharge current density to 1 A/g. On the other hand, the PGs showed a specific capacitance of 185 F/g while discharging at 0.1 A/g. A 30% decrease in specific capacitance was observed while the discharge experiment was carried out at 1 A/g. We also calculated power (P) and energy densities (E) for the supercapacitors using the following 2
equations: E ¼ CV2 and P ¼ Et, where, C is the specific capacitance (F/g), V is the potential window of discharge (V), and t is the discharge time(s). A maximum power density of 71.8 W/kg and energy density of 49.5 W h/kg was obtained for PGs–PANI Nano composites (Fig. 3d). In case of PGs, the power density of 48.9 W/ kg and energy density of 25.7 W h/kg had been only achieved. 4. Conclusions In summary, we have synthesized porous graphene with an increased number of edges for increased charge storage. This was blended with PANI nanofibers synthesized by interfacial polymerization. The PANI nanofibers were expected to act synergistically and improve the charge storage properties. As hypothesized the specific capacitance of PGs was found to be 185 F/g. This value increased to 357 F/g in case of PGs–PANI Nano composites. Acknowledgements We thank Dr. K. Krishnamoorthy for his support. MKC thanks Council of Scientific and Industrial Research for scholarship. We thank the Department of Science and Technology for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.flatc.2016.08.001. References [1] J.R. Miller, P. Simon, Materials science. Electrochemical capacitors for energy management, Science 321 (2008) 651.
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