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polyaniline nanocomposites for flexible and transparent energy storage devices
Journal of Power Sources 348 (2017) 87e93
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Bottom-up synthesis of graphene/polyaniline nanocomposites for flexible and transparent energy storage devices Victor H.R. Souza a, Marcela M. Oliveira b, Aldo J.G. Zarbin a, * a b
(UFPR), Centro Polit Department of Chemistry, Federal University of Parana ecnico, CP 19032, CEP 81531-980, Curitiba, PR, Brazil (UTFPR), Curitiba, PR, Brazil Department of Chemistry and Biology, Federal Technological University of Parana
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First total chemical synthesis of polyaniline/graphene composites. Composites obtained as thin, transparent, free-standing films at liquidliquid interface. Improved polymer/graphene interaction results superior properties and specific capacitance. Design of a total solid supercapacitor device, ITO-free, transparent and flexible.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2016 Received in revised form 17 January 2017 Accepted 17 February 2017
An innovative, single-pot synthesis for chemically producing graphene/polyaniline nanocomposites is presented. The method, which is based on chemical reactions at liquid-liquid interfaces, begins with benzene and aniline and ultimately yields nanocomposites as thin films of polyaniline mixed with graphene. These films self-assembled at the water-benzene interface are easily transferable to any kind of ordinary substrates, plastics included. Nanocomposites prepared with different polymer/graphene ratios show differentiated structures and morphologies, resulting in excellent pseudocapacitive behaviors (specific capacitance of 267.2 F cm3). The construction of all-solid, transparent, and flexible supercapacitor device from this nanocomposite is also presented. © 2017 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis of graphene Supercapacitor Polymeric nanocomposites Liquid-liquid interfaces Thin films
1. Introduction Graphene, a single atomic layer of sp2-hybridized carbon atoms distributed in a p-conjugated two-dimensional (2D) structure, has been extensively studied in recent years due to its exceptional electrical, thermal, and mechanical properties [1]. Since its first
* Corresponding author. E-mail address: [email protected] (A.J.G. Zarbin). http://dx.doi.org/10.1016/j.jpowsour.2017.02.064 0378-7753/© 2017 Elsevier B.V. All rights reserved.
report [2], several approaches for producing graphene have been developed, such as the CVD method [3,4], micromechanical exfoliation of graphite [5], reduction of graphene oxide (GO) [6,7], and so on. Among these methods, chemical synthesis is an interesting and elegant, bottom-up way to obtain graphene. Here, polycyclic aromatic hydrocarbons (PAH's) are typically employed as building blocks to achieve graphene-like structures [8]. In light of these synthesis, we recently described a novel, bottom-up route to produce large-area graphene sheets from graphene's simplest building block, benzene, based on a heterogeneous reaction performed at a
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two immiscible liquids system [9]. This work presented both the largest graphene sheets synthesized through chemical means (average size of 800 nm2) as well as the simplest chemical route to graphene reported to date. Given its outstanding properties, graphene has great potential to be applied in many different fields. For example, graphene has been tested in transparent conductive electrodes [10], lithium ion batteries [11], sensors [12], and electrochemical capacitors [13]. The use of graphene in electrical energy storage devices (capacitor) is particularly interesting because graphene can store and release charges via non-Faradaic processes, due to the electrochemical double layer capacitance (EDLC) available at the carbon skeleton [13]. This capacitance can be further boosted by combining graphene with another material whose capacitance is based on Faradaic processes. Conducting polymers or metal oxides are the most common materials for this purpose, due to the considerably enhancement in the specific capacitance [14]. One such conducting polymer, polyaniline (PANI), shows great promise as a pseudocapacitor. PANI's electrical properties are easily tuned. Its conductivity can be modulated through the chemical doping of the polymer chains. PANI also presents fast reversibility of the Faradaic redox reactions. Furthermore, polyaniline is highly stable in many different environments, inexpensive, and easy to prepare [15,16]. Taking all of this into consideration, graphene/ polyaniline composites should perform well in electrochemical charge storage due to the synergistic effect between both graphene, which increases electron transport and acts as EDLC material, and polyaniline, which increases the device's specific capacitance due to Faradaic processes. As is usual in different types of polymeric nanocomposites, improved properties can be achieved by optimizing the interaction between components, which is strongly determined by the preparation route. Several approaches for preparing graphene/polyaniline nanocomposites have been reported, based on the methods generally employed to obtain polymer/carbon nanostructure composites: by mechanically mixing the components, or polymerizing aniline in situ over the graphene [17,18]. We recently reported the first thin, transparent and homogeneous graphene/polyaniline film [19]. For this study, our group developed a way of preparing thin films that self-assembled at the interface of two immiscible liquids and were, easily transferable to many kinds of substrates [20e23]. Recently, we demonstrated that this interfacial route was also suitable for chemically synthesizing thin films of graphene/polythiophene nanocomposites with improved electrochromic and photovoltaic properties [24]. That work was an advance in the field of polymeric nanocomposites, because it was the first report in which both polymer and graphene were synthesized together, in a one-pot reaction, starting from their simplest monomer (in this case, thiophene and benzene), directly as thin, transparent, homogeneous and transferable thin film [24].
Herein, we further investigate the liquid-liquid interfacial route to polymer/graphene nanocomposites, demonstrating the first total chemical synthesis of graphene/polyaniline, in which both graphene and polyaniline were chemically synthesized from benzene and aniline respectively, in one-pot reaction using the same reactant. Films with different polyaniline/graphene ratios were deposited over target substrates, characterized and further applied as the active layer in supercapacitors. The construction of a flexible and symmetric, all-solid supercapacitor is also demonstrated.
2. Experimental 2.1. Synthesis of nanocomposites All procedures are summarized in Scheme 1. A water/oil biphasic system consisting of 10 mL of 1 mol L1 H2SO4 aqueous solution and 10 mL of aniline (Acros) solutions in benzene (Aldrich) was prepared in a 50 mL round bottom flask. The aniline solutions were prepared by adding known volumes of twice-distilled aniline (2, 10, 20 or 100 mL) in 10.0 mL of benzene, yielding 4 different graphene/polyaniline samples, referred here as GR/PANI-2, GR/ PANI-10, GR/PANI-20 and GR/PANI-100, respectively, in which the number in the acronyms refers to the initial volume of aniline. The liquid/liquid biphasic system was kept under magnetic stirring (1000 rpm) at room temperature. Afterwards, 2 g of solid, anhydrous FeCl3 (Acros), which had been previously dried at 100 C under vacuum for 24 h, were continuously added into the system in small portions of 100 mg every 2 min, and the mixture was kept stirring for an additional 3 h. When the magnetic stirring was interrupted, a green film was observed at the interface of the immiscible liquids, while the aqueous phase was light brown. This aqueous phase was removed using a pipette and substituted with approximately 10 mL of deionized water. The procedure was repeated several times until the aqueous phase looked colorless. Afterwards, the water was removed and substituted with a NH4OH 10% (v/v) aqueous solution 3 times and 5 more times with deionized water. Then, amorphous iron oxide contamination was removed via the chemical treatment, described below. Finally, the benzene was removed and substituted with fresh toluene, and the aqueous phase with an aqueous solution of H2SO4 103 M until the solution reached a pH of 3. The thin film was transferred from the liquid-liquid interface to target substrates by keeping the substrates in the bottom of the vessel, and pulling them up towards the films. A film of raw graphene was synthesized without the presence of aniline, according to the method described in our previous report [9]. A film of neat PANI was also synthesized via a biphasic polymerization route, where 10 mL of aniline was dissolved in 10 mL of hexane instead of benzene.
Scheme 1. Experimental procedure for the graphene/polyaniline nanocomposite thin films.
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2.2. Procedure for removing amorphous iron oxide/hydroxide particles from the composites The procedure was adapted from Ref. [25], in which 850 mg of oxalic acid and 850 mg of sodium carbonate were added to a volumetric flask of 100 mL, and the volume was completed with deionized H2O. Afterwards, the aqueous phase from the round bottom flask containing the composite was exchanged with 30 mL of the previously prepared solution. This system was then maintained under magnetic stirring at 80 C, and then 250 mg of sodium dithionite was added in portions of 50 mg into the system. After all of the salt was added, the stirring was stopped and the system was cooled until room temperature. 2.3. All-solid supercapacitor The device was built following a similar procedure described previously [20]. At first, the interfacial thin film of the composite GR/PANI-20 was deposited over two PET substrates covered with a chromium/gold conductive layer. Afterwards, the two electrodes were immersed in a gel electrolyte composed of a mixture of polyvinyl alcohol (PVA) and 1 M H2SO4, removed and dried at 35 C for 30 min. Then, one of the electrodes was immersed again in the PVA mixture and joined to the other electrode. The resulting allsolid supercapacitor was dried at 35 C for 30 min. 2.4. Characterization TEM and SEM images were acquired using a TEM, JEOL JEM 120 KV and SEM-FEG/Tescan, respectively. Raman spectra were acquired with a Renishaw Raman image spectrophotometer using a He-Ne laser (1.96 eV) over silicon oxide substrates. UVeViseNIR spectra were recorded over quartz substrates using Shimadzu UV2450 spectrophotometer.
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The electrochemical measurements were performed in an Autolab potentiostat (Eco-Chimie). A three-electrode cell configuration was used with a Pt wire as the counter electrode, Ag/AgCl as the reference electrode and the samples as thin films deposited over fluorine-doped tin oxide glass substrates as the working electrode. Cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 1 M H2SO4 electrolyte. The former was measured using GPES 4.9 software, and the latter using NOVA software. The scan rate of 20 mV s1 was applied for cyclic voltammetry measurements. All the charge-discharge measurements were acquired applying a constant current of 10.8 mA. The range frequency was set at 104 to 102 Hz with an amplitude of 10 mV for all EIS measurements. The thicknesses of the films were recorded using a Dektak 150 Veeco surface profilometer. 3. Results and discussion The synthetic path to graphene/polyaniline reported here is an adaptation of the successful route recently developed by us to graphene/polythiophene, which was the first report on the total chemical synthesis of a graphene-based polymeric nanocomposite [24]. Here, one single reactant (solid FeCl3) is employed to both convert the benzene to graphene (through continuous polymerization and coupling reactions mediated by the water/benzene interface [9]), and perform the oxidative polymerization of aniline. So, both the graphene and polyaniline start to growth simultaneously, mediated and stabilized by the liquid/liquid interface, resulting in nanocomposite samples with improved contact between the components. Four nanocomposite samples at different graphene/aniline ratios were prepared, as well as two control samples of neat graphene and neat polyaniline. Fig. 1(a) shows the transmission electron microscopy (TEM) image of the neat graphene control sample. The
Fig. 1. TEM images of chemically synthesized graphene (a) and the GR/PANI-10 composite (b). SEM images of the composite films GR/PANI-2 (c), GR/PANI-10 (d), GR/PANI-20 (e) and GR/PANI-100 (f).
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presence of large, interconnected and/or stacked sheets of graphene can be observed [9]. The crystallinity of the sample can be verified by the twisted six spots array in a hexagonal lattice profile of the electron diffraction pattern (Fig. S1). The TEM image of one graphene/polyaniline film (sample GR/ PANI-10) is presented in Fig. 1(b), showing polyaniline fibers surrounding the graphene sheets. Further images that corroborate this observation are shown in Fig. S2. The presence of both graphene and polyaniline in the nanocomposite samples can be also verified by scanning electron microscopy (SEM). The images were collected directly from the films deposited over silicon substrates. The sample prepared from the smallest amount of aniline (GR/PANI-2, Fig. 1(c)) has morphology similar to the neat graphene film (Fig. S3(a)), indicating no or very low amount of polymer. The film of neat polyaniline has a fibrous morphology (Fig. S3(b)), characteristic of PANI obtained through interfacial polymerization [20,26,27]. The presence of both graphene sheets and fibrous polyaniline can be clearly seen in the SEM images of the nanocomposites GR/PANI-10, GR/PANI-20 and GR/PANI-100 (Fig. 1(d), (e) and (f), respectively), confirming the occurrence of both components in the films. Fig. 2(a) shows the UVeViseNIR spectra of all nanocomposite films. The spectrum of the GR/PANI-2 sample (Fig. 2(a)) is similar to that of neat graphene (Fig. S4(a)), characterized by a band around 260 nm assigned to the p-p* electronic transition on graphene. Three other bands, attributed to the conductive form of polyaniline (emeraldine salt), can be detected in the nanocomposite samples: one at 335 nm due to the transition related to the band gap of PANI, and others at 434 nm and 970 nm related to transitions involving the polaronic structure of the polymer [19]. The lower energy tail of the band centered at 970 nm, which is also seen in the spectra of all nanocomposite samples, is characteristic of polyaniline's secondary doping process, in which the polymer chains adopt a more aligned, expanded coil-like structure due to favorable interactions with surrounding species, known as secondary dopants [28]. As this profile is also observable in the spectrum of neat polyaniline (Fig. 2(a)), the band is attributed to the interaction of polyaniline with leftover Fe3þ from FeCl3, the oxidizing agent in our process, which is a well-known secondary dopant to polyaniline [29]. However, this band has a larger redshift in the composites than in neat PANI. It is found at 970 nm in neat PANI and 1190, 1163 and 1112 nm in the GR/PANI-10, GR/PANI-20 and GR/PANI-100 samples, respectively, suggesting that the presence of graphene contributes to the better alignment of polyaniline chains.
The inset of Fig. 2(a) shows photographs of all composite films, including neat graphene, deposited over glass substrates. The films were highly transparent, exhibiting transmittances at 550 nm of 80.0, 81.4, 84.5, 83.7, 76.1 and 85.0% for neat graphene and the samples GR/PANI-2, GR/PANI-10, GR/PANI-20, GR/PANI-100 and neat PANI, respectively. The thicknesses of the films were 81 ± 57, 88 ± 56, 252 ± 154 and 91 ± 9 for GR/PANI-10, GR/PANI-20, GR/ PANI-100 and neat PANI, respectively. Fig. 2(b) shows the Raman spectra of the samples, in which significant structural differences can be observed. The spectrum of neat graphene contains the typical D, G and 2D bands of graphene at 1340, 1584 and 2670 cm1, respectively (Fig. S4(b)) [9]. A similar profile to neat graphene was observed for the spectrum of GR/ PANI-2 sample (Fig. 2(b)), corroborating the data obtained by UVeViseNIR and SEM. The spectrum of neat PANI (Fig. 2(b)) contains characteristic bands for the conducting polymer in the emeraldine salt structure: 1622 cm1 (n CeC of benzenoid rings), 1597 cm1 (n C]C of quinoid rings), 1509 cm1 (n C]N of quinoid rings at emeraldine salt), 1475 cm1 (n C]N of quinoid rings at diimine units), 1336 cm1 (n CeNþ of polaron radical cation), 1263 cm1 (n CeN of polaronic units), 1172 cm1 (n CeH of rings with polaronic structure); 878 and 814 cm1 (out-of-plane CeH vibrations), aside from the bands at 1381 and 580 cm1 assigned to the vibrational modes of crosslinked polyaniline [30,31]. As expected, these bands were increasingly pronounced as the amount of polyaniline in each nanocomposite was increased. The bands of graphene can be seen together with the bands of polyaniline in the spectrum of the GR/PANI-10 (Fig. 2(b)). By increasing the amount of polymer in the sample even more (GR/ PANI-20 and GR/PANI-100), only bands corresponding to polyaniline can be observed. The effect of graphene on the spectra of polyaniline is clearly seen in the region containing bands associated with polarons and bipolarons in the polymer chains, suggesting a formal interaction between the components (Fig. S5). In particular, the band at 1176 cm1 (GR/PANI-10), attributed to the vibrational mode of polaronic structures of polyaniline [29,30], is redshifted to 1173 cm1 (GR/PANI-20) and 1164 cm1 (GR/PANI100). The last one is characteristic of the vibrational mode of bipolaronic structures [30,31]. The spectrum of GR/PANI-100 is similar to PANI synthesized using ammonium persulfate as oxidant, as described elsewhere [20], in which bipolarons are the majority of the charge carriers. Furthermore, the spectra of both the GR/PANI-20 nanocomposite and neat PANI contained the band related to the polaronic structure (around 1173 cm1). In
Fig. 2. (a) UVeViseNIR spectra of the composite and neat PANI films. Inset: photographs of the films deposited over glass substrates. From left to right: neat graphene, GR/PANI-2, GR/PANI-10, GR/PANI-20 and GR/PANI-100. (b) Raman spectra of the nanocomposite films.
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Fig. 3. (a) Cyclic voltammetry curves of nanocomposite and neat PANI films deposited over FTO substrates (vs Ag/AgCl, acquired in a H2SO4 1 M electrolyte, at 20 mV s1); (b) volumetric specific capacitance of the composite and neat PANI films. The inset corresponds to charge/discharge curves; (c) volumetric specific capacitance variation after 400 voltammetric cycles; (d) Nyquist plots for the different composites. The Nyquist plot for the high frequency region is presented as an inset in (d).
comparison, the vibrational mode of polaronic structure at 1176 cm1 of the GR/PANI-10 nanocomposite is assigned to polyaniline under secondary doping [32]. The presence of this band at a higher wavenumber for the GR/PANI-10 sample corroborates the contribution of the chemically synthesized graphene to stabilize the polaronic structure of the polymer, confirming the data acquired by UVeViseNIR. Therefore, the secondary doping effect observed in the polyaniline synthesized in this work should be due to the concomitant contribution of both chemically synthesized graphene and iron species remaining from the synthetic procedure. The Raman spectra of all the samples also show the presence of bands assigned to crosslinked structures. We demonstrated before that when aniline is polymerized over the previously dispersed carbon nanostructures (either carbon nanotubes or reduced graphene oxide), the crosslinked structure is suppressed because the polymer grows at the walls of the carbonaceous material [19,20,26]. The presence of the crosslinked structure in the samples presented in this work suggest that the polymer grows faster than the graphene. The electrochemical behavior of the composites was evaluated through CV, CD, and EIS, as summarized in Fig. 3. The geometric surface of active material over the ITO electrode is 0.75 cm2, and the volumetric surface loading for the samples were 6.1 106, 6.5 106, 18.9 106 and 6.8 106 cm3 for the samples GR/ PANI-10, GR/PANI-20, GR/PANI-100 and neat PANI, respectively. The cyclic voltammograms (Fig. 3(a)) of all samples except sample GR/PANI-2 show the characteristic profile of polyaniline, with the two anodic peaks (at 0.22 V and 0.82 V) attributed to the interconversion between polyaniline's leucoemeraldine/emeraldine and emeraldine/pernigraniline forms [33]. Fig. 3(b) shows the values of
volumetric specific capacitance calculated from charge-discharge curves according to Equation (1),
Cv ¼
i t
DE v
(1)
where Cv is volumetric specific capacitance (F cm3); t is the time of discharge (s); i is the current applied during the charge/discharge measurement (A); DE is the range of potential during the discharge (V) and v is volume of the material (cm3) [20]. The GR/PANI-20 composite exhibited higher specific capacitance (267.2 F cm3), while the neat PANI, the composites GR/PANI-10 and GR/PANI-100 presented specific capacitances of 79.4, 77.6 and 166.4 F cm3, respectively. The volumetric capacitance achieved here is quite reasonable compared to other graphene/polyaniline films applied to energy storage, like chemically converted graphene/PANI films prepared from vacuum filtration (160 F cm3) [17]; graphene/PANI composite paper obtained by in situ electropolymerization (135 F cm3) [34]; ternary composite films made from graphene, PANI, and carbon nanotubes (188 F cm3) [35]; chemically converted hydrogel films of graphene and PANI (572 F cm3) [36] and three-dimensional graphene/PANI composite films (581 F cm3) [37]. In addition to these values, however, the composites in the present work are transparent, which allow further application for transparent energy storage devices. The charge/discharge curves shown in the inset in Fig. 3(b) show the higher time of storage and release of the charges for the composite GR/PANI-20. The CV curves presented in Fig. 3(a) were normalized by the geometric surface of the electrode (which is the common procedure adopt for CV data), showing higher current density at sample GR/PANI-100, while the highest capacitance from the galvanostatic (Fig. 3(b)) is observed at sample GR/PANI-20. Fig. S6 shows the CV
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curves normalized by the surface loading of active material over the electrode, and it is clear that considering the amount of active material, the sample GR/PANI-20 presents the highest current density, agreeing with the charge-discharge measurements. The stability of the composites was evaluated after 400 charge/ discharge cycles (Fig. 3(c)). The composite GR/PANI-10 exhibited the highest stability of all the samples, maintaining around 88% of its initial volumetric specific capacitance after all of the charge/ discharge cycles. In contrast, the neat PANI, GR/PANI-20 and GR/ PANI-100 had a stability of 76, 76 and 72%, respectively. The stability of GR/PANI-10 is higher when compared with other graphene/PANI composite films described in the literature [17]. The higher stability of GR/PANI-10 can be explained due to the higher interaction between the graphene sheets and polyaniline, which more effectively maintain the polymer structure and hence retain the capacitance during the charge and release of the ions. Values of charge transfer resistance of the composites were calculated through electrochemical impedance spectroscopy, and their Nyquist plots are displayed in Fig. 3(d). The inset in this Figure shows the high frequency region of the curve. The equivalent circuit adopted is presented in Fig. S7. The three nanocomposites presented similar profiles and are characterized by an arc at high frequency region instead of a semicircle, which can be explained due to the low resistance in the samples. In the low frequency region, a straight vertical line can be observed almost parallel to the imaginary axis in all composites, indicating their capacitive behavior [38]. The lowest value of charge transfer resistance calculated for the composites was 1.2 U for GR/PANI-20, while the composites GR/PANI-10 and GR/PANI-100 yielded values of 5.6 and 4.1 U, respectively. These values are quite low compared to threedimensional graphene/PANI composite films, which typically
yield values higher than 35 U [37], and also when compared to neat PANI (17.1 U - Fig. S8). This result indicates that the ratio between polyaniline and graphene for the composite GR/PANI-20 is ideal for promoting electronic transport through the material. Since the composites prepared here were obtained as thin films assembled at the liquid-liquid interface, a flexible, transparent and all-solid supercapacitor from the GR/PANI-20 composite was built in order to evaluate its potential for further application in the energy field. Transparent optoelectronic and electrochemical devices are usually built employing ITO (indium tin oxide) as transparent electrode, due to its suitable transmittance (90% at 550 nm) and conductivity (sheet resistance about 10e100 U ,1) [22,39,40]. However, ITO is expensive, high brittleness and has low mechanical resistance under bending, which is a limitation for future technologies using flexible and transparent substrates. So, the preparation of ITO-free flexible devices, as described herein, is an important challenge in this field. A schematic and photographs of this solid device are shown in Fig. 4(a) and (b), respectively. The all-solid device retains its structure and voltammetric response (Fig. 4(c)) even when submitted to cycles of mechanical deformation. A high capacitive current, including an anodic peak around 0.47 V, is observed for all configurations (Fig. 4(c)). The same occurs for the charge/discharge curves presented in Fig. 4(d). The redox pairs become more defined when and after the device is twisted, an observation likely due to the diminishing of the distance between the two electrodes as previously observed for carbon nanotubes/PANI all-solid supercapacitor [20]. The value of volumetric specific capacitance for the all solid device was 95.5 F cm3. This value is slightly higher compared to a carbon nanotube/PANI all-solid supercapacitor built following a similar procedure, indicating the high potential of the
Fig. 4. (a) Scheme of the all-solid device, including all layers; (b) a photograph of the all-solid, transparent and flexible supercapacitor built from the composite GR/PANI-20 when the sample is straight or twisted; (c) cyclic voltammograms and (d) charge/discharge curves of the device in both straight and twisted states.
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interfacial method to prepare composites from the chemical synthesis of both graphene and polyaniline. 4. Conclusions In conclusion, we demonstrated the complete, one-step synthesis of nanocomposites based on graphene and polyaniline, in which both the phases were obtained starting from their simplest chemical component (benzene and aniline) and directly obtained as a thin film. This new route to synthesize graphene-based nanocomposites opens the possibility of preparing new materials without previous steps normally necessary to prepare graphene sheets. In addition, it can be deposited over different types of substrates, since the interfacial system allows for the preparation of thin and freestanding films. This is the second example reported by us of a bottom-up synthesis of an entire nanocomposite, proving the versatility, wide adaptability and general aspects of the route, which theoretically can be extended to different materials where graphene and another component may be simultaneously synthesized. The graphene/polyaniline nanocomposites show differentiated structures and morphologies, resulting in excellent pseudocapacitive behaviours and allowing for the construction of all-solid transparent and flexible supercapacitor devices. Acknowledgments Authors acknowledge the financial support of CNPq, CAPES, INCT-Nanocarbon and NENNAM (Pronex/F. Araucaria/CNPq). VHRS thanks CAPES for the fellowship. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.02.064. References [1] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906e3924. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. [3] M. Losurdo, M.M. Giangregorio, P. Capezzuto, G. Bruno, Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure, Phys. Chem. Chem. Phys. 13 (2011) 20836e20843. [4] J.W. Suk, A. Kitt, C.W. Magnuson, Y. Hao, S. Ahmed, J. An, A.K. Swan, B.B. Goldberg, R.S. Ruoff, Transfer of CVD-grown monolayer graphene onto arbitrary substrates, ACS Nano 5 (2011) 6916e6924. [5] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451e10453. mez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, [6] C. Go K. Kern, Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett. 7 (2007) 3499e3503. [7] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463e470. [8] L. Zhi, K. Mullen, A bottom-up approach from molecular nanographenes to unconventional carbon materials, J. Mater. Chem. 18 (2008) 1472e1484. [9] R.V. Salvatierra, V.H.R. Souza, C.F. Matos, M.M. Oliveira, A.J.G. Zarbin, Graphene chemically synthesized from benzene at liquideliquid interfaces, Carbon 93 (2015) 924e932. [10] X. Wang, L. Zhi, K. Müllen, Transparent, conductive graphene electrodes for dye-sensitized solar cells, Nano Lett. 8 (2007) 323e327. [11] E. Yoo, J. Kim, E. Hosono, H.-S. Zhou, T. Kudo, I. Honma, Large reversible li storage of graphene nanosheet families for use in rechargeable lithium ion batteries, Nano Lett. 8 (2008) 2277e2282. [12] J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137e3140. [13] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors,
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Genoud, I. Kulszewicz-Bajer, A. Bedel, J.L. Oddou, C. Jeandey, A. Pron, Lewis acid doped polyaniline. Part II: Spectroscopic studies of emeraldine base and emeraldine hydrochloride complexation with FeCl3, Chem. Mater. 12 (2000) 744e749. [30] R.V. Salvatierra, L.G. Moura, M.M. Oliveira, M.A. Pimenta, A.J.G. Zarbin, Resonant Raman spectroscopy and spectroelectrochemistry characterization of carbon nanotubes/polyaniline thin film obtained through interfacial polymerization, J. Raman Spectrosc. 43 (2012) 1094e1100. [31] J. da Silva, S. de Torresi, D. De Faria, M. Temperini, Raman characterization of polyaniline induced conformational changes, Synth. Met. 101 (1999) 834e835. rdoba de Torresi, M.L.A. Temperini, [32] J.E. Pereira da Silva, D.L.A. de Faria, S.I. Co Influence of thermal treatment on doped polyaniline studied by resonance raman spectroscopy, Macromolecules 33 (2000) 3077e3083. [33] Z. Gao, F. Wang, J. Chang, D. Wu, X. Wang, X. Wang, F. Xu, S. Gao, K. 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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bottom-up synthesis of graphene/polyaniline nanocomposites for flexible and transparent energy storage devices Victor H.R. Souza a, Marcela M. Oliveira b, Aldo J.G. Zarbin a, * a b
(UFPR), Centro Polit Department of Chemistry, Federal University of Parana ecnico, CP 19032, CEP 81531-980, Curitiba, PR, Brazil (UTFPR), Curitiba, PR, Brazil Department of Chemistry and Biology, Federal Technological University of Parana
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First total chemical synthesis of polyaniline/graphene composites. Composites obtained as thin, transparent, free-standing films at liquidliquid interface. Improved polymer/graphene interaction results superior properties and specific capacitance. Design of a total solid supercapacitor device, ITO-free, transparent and flexible.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2016 Received in revised form 17 January 2017 Accepted 17 February 2017
An innovative, single-pot synthesis for chemically producing graphene/polyaniline nanocomposites is presented. The method, which is based on chemical reactions at liquid-liquid interfaces, begins with benzene and aniline and ultimately yields nanocomposites as thin films of polyaniline mixed with graphene. These films self-assembled at the water-benzene interface are easily transferable to any kind of ordinary substrates, plastics included. Nanocomposites prepared with different polymer/graphene ratios show differentiated structures and morphologies, resulting in excellent pseudocapacitive behaviors (specific capacitance of 267.2 F cm3). The construction of all-solid, transparent, and flexible supercapacitor device from this nanocomposite is also presented. © 2017 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis of graphene Supercapacitor Polymeric nanocomposites Liquid-liquid interfaces Thin films
1. Introduction Graphene, a single atomic layer of sp2-hybridized carbon atoms distributed in a p-conjugated two-dimensional (2D) structure, has been extensively studied in recent years due to its exceptional electrical, thermal, and mechanical properties [1]. Since its first
* Corresponding author. E-mail address: [email protected] (A.J.G. Zarbin). http://dx.doi.org/10.1016/j.jpowsour.2017.02.064 0378-7753/© 2017 Elsevier B.V. All rights reserved.
report [2], several approaches for producing graphene have been developed, such as the CVD method [3,4], micromechanical exfoliation of graphite [5], reduction of graphene oxide (GO) [6,7], and so on. Among these methods, chemical synthesis is an interesting and elegant, bottom-up way to obtain graphene. Here, polycyclic aromatic hydrocarbons (PAH's) are typically employed as building blocks to achieve graphene-like structures [8]. In light of these synthesis, we recently described a novel, bottom-up route to produce large-area graphene sheets from graphene's simplest building block, benzene, based on a heterogeneous reaction performed at a
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two immiscible liquids system [9]. This work presented both the largest graphene sheets synthesized through chemical means (average size of 800 nm2) as well as the simplest chemical route to graphene reported to date. Given its outstanding properties, graphene has great potential to be applied in many different fields. For example, graphene has been tested in transparent conductive electrodes [10], lithium ion batteries [11], sensors [12], and electrochemical capacitors [13]. The use of graphene in electrical energy storage devices (capacitor) is particularly interesting because graphene can store and release charges via non-Faradaic processes, due to the electrochemical double layer capacitance (EDLC) available at the carbon skeleton [13]. This capacitance can be further boosted by combining graphene with another material whose capacitance is based on Faradaic processes. Conducting polymers or metal oxides are the most common materials for this purpose, due to the considerably enhancement in the specific capacitance [14]. One such conducting polymer, polyaniline (PANI), shows great promise as a pseudocapacitor. PANI's electrical properties are easily tuned. Its conductivity can be modulated through the chemical doping of the polymer chains. PANI also presents fast reversibility of the Faradaic redox reactions. Furthermore, polyaniline is highly stable in many different environments, inexpensive, and easy to prepare [15,16]. Taking all of this into consideration, graphene/ polyaniline composites should perform well in electrochemical charge storage due to the synergistic effect between both graphene, which increases electron transport and acts as EDLC material, and polyaniline, which increases the device's specific capacitance due to Faradaic processes. As is usual in different types of polymeric nanocomposites, improved properties can be achieved by optimizing the interaction between components, which is strongly determined by the preparation route. Several approaches for preparing graphene/polyaniline nanocomposites have been reported, based on the methods generally employed to obtain polymer/carbon nanostructure composites: by mechanically mixing the components, or polymerizing aniline in situ over the graphene [17,18]. We recently reported the first thin, transparent and homogeneous graphene/polyaniline film [19]. For this study, our group developed a way of preparing thin films that self-assembled at the interface of two immiscible liquids and were, easily transferable to many kinds of substrates [20e23]. Recently, we demonstrated that this interfacial route was also suitable for chemically synthesizing thin films of graphene/polythiophene nanocomposites with improved electrochromic and photovoltaic properties [24]. That work was an advance in the field of polymeric nanocomposites, because it was the first report in which both polymer and graphene were synthesized together, in a one-pot reaction, starting from their simplest monomer (in this case, thiophene and benzene), directly as thin, transparent, homogeneous and transferable thin film [24].
Herein, we further investigate the liquid-liquid interfacial route to polymer/graphene nanocomposites, demonstrating the first total chemical synthesis of graphene/polyaniline, in which both graphene and polyaniline were chemically synthesized from benzene and aniline respectively, in one-pot reaction using the same reactant. Films with different polyaniline/graphene ratios were deposited over target substrates, characterized and further applied as the active layer in supercapacitors. The construction of a flexible and symmetric, all-solid supercapacitor is also demonstrated.
2. Experimental 2.1. Synthesis of nanocomposites All procedures are summarized in Scheme 1. A water/oil biphasic system consisting of 10 mL of 1 mol L1 H2SO4 aqueous solution and 10 mL of aniline (Acros) solutions in benzene (Aldrich) was prepared in a 50 mL round bottom flask. The aniline solutions were prepared by adding known volumes of twice-distilled aniline (2, 10, 20 or 100 mL) in 10.0 mL of benzene, yielding 4 different graphene/polyaniline samples, referred here as GR/PANI-2, GR/ PANI-10, GR/PANI-20 and GR/PANI-100, respectively, in which the number in the acronyms refers to the initial volume of aniline. The liquid/liquid biphasic system was kept under magnetic stirring (1000 rpm) at room temperature. Afterwards, 2 g of solid, anhydrous FeCl3 (Acros), which had been previously dried at 100 C under vacuum for 24 h, were continuously added into the system in small portions of 100 mg every 2 min, and the mixture was kept stirring for an additional 3 h. When the magnetic stirring was interrupted, a green film was observed at the interface of the immiscible liquids, while the aqueous phase was light brown. This aqueous phase was removed using a pipette and substituted with approximately 10 mL of deionized water. The procedure was repeated several times until the aqueous phase looked colorless. Afterwards, the water was removed and substituted with a NH4OH 10% (v/v) aqueous solution 3 times and 5 more times with deionized water. Then, amorphous iron oxide contamination was removed via the chemical treatment, described below. Finally, the benzene was removed and substituted with fresh toluene, and the aqueous phase with an aqueous solution of H2SO4 103 M until the solution reached a pH of 3. The thin film was transferred from the liquid-liquid interface to target substrates by keeping the substrates in the bottom of the vessel, and pulling them up towards the films. A film of raw graphene was synthesized without the presence of aniline, according to the method described in our previous report [9]. A film of neat PANI was also synthesized via a biphasic polymerization route, where 10 mL of aniline was dissolved in 10 mL of hexane instead of benzene.
Scheme 1. Experimental procedure for the graphene/polyaniline nanocomposite thin films.
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2.2. Procedure for removing amorphous iron oxide/hydroxide particles from the composites The procedure was adapted from Ref. [25], in which 850 mg of oxalic acid and 850 mg of sodium carbonate were added to a volumetric flask of 100 mL, and the volume was completed with deionized H2O. Afterwards, the aqueous phase from the round bottom flask containing the composite was exchanged with 30 mL of the previously prepared solution. This system was then maintained under magnetic stirring at 80 C, and then 250 mg of sodium dithionite was added in portions of 50 mg into the system. After all of the salt was added, the stirring was stopped and the system was cooled until room temperature. 2.3. All-solid supercapacitor The device was built following a similar procedure described previously [20]. At first, the interfacial thin film of the composite GR/PANI-20 was deposited over two PET substrates covered with a chromium/gold conductive layer. Afterwards, the two electrodes were immersed in a gel electrolyte composed of a mixture of polyvinyl alcohol (PVA) and 1 M H2SO4, removed and dried at 35 C for 30 min. Then, one of the electrodes was immersed again in the PVA mixture and joined to the other electrode. The resulting allsolid supercapacitor was dried at 35 C for 30 min. 2.4. Characterization TEM and SEM images were acquired using a TEM, JEOL JEM 120 KV and SEM-FEG/Tescan, respectively. Raman spectra were acquired with a Renishaw Raman image spectrophotometer using a He-Ne laser (1.96 eV) over silicon oxide substrates. UVeViseNIR spectra were recorded over quartz substrates using Shimadzu UV2450 spectrophotometer.
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The electrochemical measurements were performed in an Autolab potentiostat (Eco-Chimie). A three-electrode cell configuration was used with a Pt wire as the counter electrode, Ag/AgCl as the reference electrode and the samples as thin films deposited over fluorine-doped tin oxide glass substrates as the working electrode. Cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 1 M H2SO4 electrolyte. The former was measured using GPES 4.9 software, and the latter using NOVA software. The scan rate of 20 mV s1 was applied for cyclic voltammetry measurements. All the charge-discharge measurements were acquired applying a constant current of 10.8 mA. The range frequency was set at 104 to 102 Hz with an amplitude of 10 mV for all EIS measurements. The thicknesses of the films were recorded using a Dektak 150 Veeco surface profilometer. 3. Results and discussion The synthetic path to graphene/polyaniline reported here is an adaptation of the successful route recently developed by us to graphene/polythiophene, which was the first report on the total chemical synthesis of a graphene-based polymeric nanocomposite [24]. Here, one single reactant (solid FeCl3) is employed to both convert the benzene to graphene (through continuous polymerization and coupling reactions mediated by the water/benzene interface [9]), and perform the oxidative polymerization of aniline. So, both the graphene and polyaniline start to growth simultaneously, mediated and stabilized by the liquid/liquid interface, resulting in nanocomposite samples with improved contact between the components. Four nanocomposite samples at different graphene/aniline ratios were prepared, as well as two control samples of neat graphene and neat polyaniline. Fig. 1(a) shows the transmission electron microscopy (TEM) image of the neat graphene control sample. The
Fig. 1. TEM images of chemically synthesized graphene (a) and the GR/PANI-10 composite (b). SEM images of the composite films GR/PANI-2 (c), GR/PANI-10 (d), GR/PANI-20 (e) and GR/PANI-100 (f).
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presence of large, interconnected and/or stacked sheets of graphene can be observed [9]. The crystallinity of the sample can be verified by the twisted six spots array in a hexagonal lattice profile of the electron diffraction pattern (Fig. S1). The TEM image of one graphene/polyaniline film (sample GR/ PANI-10) is presented in Fig. 1(b), showing polyaniline fibers surrounding the graphene sheets. Further images that corroborate this observation are shown in Fig. S2. The presence of both graphene and polyaniline in the nanocomposite samples can be also verified by scanning electron microscopy (SEM). The images were collected directly from the films deposited over silicon substrates. The sample prepared from the smallest amount of aniline (GR/PANI-2, Fig. 1(c)) has morphology similar to the neat graphene film (Fig. S3(a)), indicating no or very low amount of polymer. The film of neat polyaniline has a fibrous morphology (Fig. S3(b)), characteristic of PANI obtained through interfacial polymerization [20,26,27]. The presence of both graphene sheets and fibrous polyaniline can be clearly seen in the SEM images of the nanocomposites GR/PANI-10, GR/PANI-20 and GR/PANI-100 (Fig. 1(d), (e) and (f), respectively), confirming the occurrence of both components in the films. Fig. 2(a) shows the UVeViseNIR spectra of all nanocomposite films. The spectrum of the GR/PANI-2 sample (Fig. 2(a)) is similar to that of neat graphene (Fig. S4(a)), characterized by a band around 260 nm assigned to the p-p* electronic transition on graphene. Three other bands, attributed to the conductive form of polyaniline (emeraldine salt), can be detected in the nanocomposite samples: one at 335 nm due to the transition related to the band gap of PANI, and others at 434 nm and 970 nm related to transitions involving the polaronic structure of the polymer [19]. The lower energy tail of the band centered at 970 nm, which is also seen in the spectra of all nanocomposite samples, is characteristic of polyaniline's secondary doping process, in which the polymer chains adopt a more aligned, expanded coil-like structure due to favorable interactions with surrounding species, known as secondary dopants [28]. As this profile is also observable in the spectrum of neat polyaniline (Fig. 2(a)), the band is attributed to the interaction of polyaniline with leftover Fe3þ from FeCl3, the oxidizing agent in our process, which is a well-known secondary dopant to polyaniline [29]. However, this band has a larger redshift in the composites than in neat PANI. It is found at 970 nm in neat PANI and 1190, 1163 and 1112 nm in the GR/PANI-10, GR/PANI-20 and GR/PANI-100 samples, respectively, suggesting that the presence of graphene contributes to the better alignment of polyaniline chains.
The inset of Fig. 2(a) shows photographs of all composite films, including neat graphene, deposited over glass substrates. The films were highly transparent, exhibiting transmittances at 550 nm of 80.0, 81.4, 84.5, 83.7, 76.1 and 85.0% for neat graphene and the samples GR/PANI-2, GR/PANI-10, GR/PANI-20, GR/PANI-100 and neat PANI, respectively. The thicknesses of the films were 81 ± 57, 88 ± 56, 252 ± 154 and 91 ± 9 for GR/PANI-10, GR/PANI-20, GR/ PANI-100 and neat PANI, respectively. Fig. 2(b) shows the Raman spectra of the samples, in which significant structural differences can be observed. The spectrum of neat graphene contains the typical D, G and 2D bands of graphene at 1340, 1584 and 2670 cm1, respectively (Fig. S4(b)) [9]. A similar profile to neat graphene was observed for the spectrum of GR/ PANI-2 sample (Fig. 2(b)), corroborating the data obtained by UVeViseNIR and SEM. The spectrum of neat PANI (Fig. 2(b)) contains characteristic bands for the conducting polymer in the emeraldine salt structure: 1622 cm1 (n CeC of benzenoid rings), 1597 cm1 (n C]C of quinoid rings), 1509 cm1 (n C]N of quinoid rings at emeraldine salt), 1475 cm1 (n C]N of quinoid rings at diimine units), 1336 cm1 (n CeNþ of polaron radical cation), 1263 cm1 (n CeN of polaronic units), 1172 cm1 (n CeH of rings with polaronic structure); 878 and 814 cm1 (out-of-plane CeH vibrations), aside from the bands at 1381 and 580 cm1 assigned to the vibrational modes of crosslinked polyaniline [30,31]. As expected, these bands were increasingly pronounced as the amount of polyaniline in each nanocomposite was increased. The bands of graphene can be seen together with the bands of polyaniline in the spectrum of the GR/PANI-10 (Fig. 2(b)). By increasing the amount of polymer in the sample even more (GR/ PANI-20 and GR/PANI-100), only bands corresponding to polyaniline can be observed. The effect of graphene on the spectra of polyaniline is clearly seen in the region containing bands associated with polarons and bipolarons in the polymer chains, suggesting a formal interaction between the components (Fig. S5). In particular, the band at 1176 cm1 (GR/PANI-10), attributed to the vibrational mode of polaronic structures of polyaniline [29,30], is redshifted to 1173 cm1 (GR/PANI-20) and 1164 cm1 (GR/PANI100). The last one is characteristic of the vibrational mode of bipolaronic structures [30,31]. The spectrum of GR/PANI-100 is similar to PANI synthesized using ammonium persulfate as oxidant, as described elsewhere [20], in which bipolarons are the majority of the charge carriers. Furthermore, the spectra of both the GR/PANI-20 nanocomposite and neat PANI contained the band related to the polaronic structure (around 1173 cm1). In
Fig. 2. (a) UVeViseNIR spectra of the composite and neat PANI films. Inset: photographs of the films deposited over glass substrates. From left to right: neat graphene, GR/PANI-2, GR/PANI-10, GR/PANI-20 and GR/PANI-100. (b) Raman spectra of the nanocomposite films.
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Fig. 3. (a) Cyclic voltammetry curves of nanocomposite and neat PANI films deposited over FTO substrates (vs Ag/AgCl, acquired in a H2SO4 1 M electrolyte, at 20 mV s1); (b) volumetric specific capacitance of the composite and neat PANI films. The inset corresponds to charge/discharge curves; (c) volumetric specific capacitance variation after 400 voltammetric cycles; (d) Nyquist plots for the different composites. The Nyquist plot for the high frequency region is presented as an inset in (d).
comparison, the vibrational mode of polaronic structure at 1176 cm1 of the GR/PANI-10 nanocomposite is assigned to polyaniline under secondary doping [32]. The presence of this band at a higher wavenumber for the GR/PANI-10 sample corroborates the contribution of the chemically synthesized graphene to stabilize the polaronic structure of the polymer, confirming the data acquired by UVeViseNIR. Therefore, the secondary doping effect observed in the polyaniline synthesized in this work should be due to the concomitant contribution of both chemically synthesized graphene and iron species remaining from the synthetic procedure. The Raman spectra of all the samples also show the presence of bands assigned to crosslinked structures. We demonstrated before that when aniline is polymerized over the previously dispersed carbon nanostructures (either carbon nanotubes or reduced graphene oxide), the crosslinked structure is suppressed because the polymer grows at the walls of the carbonaceous material [19,20,26]. The presence of the crosslinked structure in the samples presented in this work suggest that the polymer grows faster than the graphene. The electrochemical behavior of the composites was evaluated through CV, CD, and EIS, as summarized in Fig. 3. The geometric surface of active material over the ITO electrode is 0.75 cm2, and the volumetric surface loading for the samples were 6.1 106, 6.5 106, 18.9 106 and 6.8 106 cm3 for the samples GR/ PANI-10, GR/PANI-20, GR/PANI-100 and neat PANI, respectively. The cyclic voltammograms (Fig. 3(a)) of all samples except sample GR/PANI-2 show the characteristic profile of polyaniline, with the two anodic peaks (at 0.22 V and 0.82 V) attributed to the interconversion between polyaniline's leucoemeraldine/emeraldine and emeraldine/pernigraniline forms [33]. Fig. 3(b) shows the values of
volumetric specific capacitance calculated from charge-discharge curves according to Equation (1),
Cv ¼
i t
DE v
(1)
where Cv is volumetric specific capacitance (F cm3); t is the time of discharge (s); i is the current applied during the charge/discharge measurement (A); DE is the range of potential during the discharge (V) and v is volume of the material (cm3) [20]. The GR/PANI-20 composite exhibited higher specific capacitance (267.2 F cm3), while the neat PANI, the composites GR/PANI-10 and GR/PANI-100 presented specific capacitances of 79.4, 77.6 and 166.4 F cm3, respectively. The volumetric capacitance achieved here is quite reasonable compared to other graphene/polyaniline films applied to energy storage, like chemically converted graphene/PANI films prepared from vacuum filtration (160 F cm3) [17]; graphene/PANI composite paper obtained by in situ electropolymerization (135 F cm3) [34]; ternary composite films made from graphene, PANI, and carbon nanotubes (188 F cm3) [35]; chemically converted hydrogel films of graphene and PANI (572 F cm3) [36] and three-dimensional graphene/PANI composite films (581 F cm3) [37]. In addition to these values, however, the composites in the present work are transparent, which allow further application for transparent energy storage devices. The charge/discharge curves shown in the inset in Fig. 3(b) show the higher time of storage and release of the charges for the composite GR/PANI-20. The CV curves presented in Fig. 3(a) were normalized by the geometric surface of the electrode (which is the common procedure adopt for CV data), showing higher current density at sample GR/PANI-100, while the highest capacitance from the galvanostatic (Fig. 3(b)) is observed at sample GR/PANI-20. Fig. S6 shows the CV
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curves normalized by the surface loading of active material over the electrode, and it is clear that considering the amount of active material, the sample GR/PANI-20 presents the highest current density, agreeing with the charge-discharge measurements. The stability of the composites was evaluated after 400 charge/ discharge cycles (Fig. 3(c)). The composite GR/PANI-10 exhibited the highest stability of all the samples, maintaining around 88% of its initial volumetric specific capacitance after all of the charge/ discharge cycles. In contrast, the neat PANI, GR/PANI-20 and GR/ PANI-100 had a stability of 76, 76 and 72%, respectively. The stability of GR/PANI-10 is higher when compared with other graphene/PANI composite films described in the literature [17]. The higher stability of GR/PANI-10 can be explained due to the higher interaction between the graphene sheets and polyaniline, which more effectively maintain the polymer structure and hence retain the capacitance during the charge and release of the ions. Values of charge transfer resistance of the composites were calculated through electrochemical impedance spectroscopy, and their Nyquist plots are displayed in Fig. 3(d). The inset in this Figure shows the high frequency region of the curve. The equivalent circuit adopted is presented in Fig. S7. The three nanocomposites presented similar profiles and are characterized by an arc at high frequency region instead of a semicircle, which can be explained due to the low resistance in the samples. In the low frequency region, a straight vertical line can be observed almost parallel to the imaginary axis in all composites, indicating their capacitive behavior [38]. The lowest value of charge transfer resistance calculated for the composites was 1.2 U for GR/PANI-20, while the composites GR/PANI-10 and GR/PANI-100 yielded values of 5.6 and 4.1 U, respectively. These values are quite low compared to threedimensional graphene/PANI composite films, which typically
yield values higher than 35 U [37], and also when compared to neat PANI (17.1 U - Fig. S8). This result indicates that the ratio between polyaniline and graphene for the composite GR/PANI-20 is ideal for promoting electronic transport through the material. Since the composites prepared here were obtained as thin films assembled at the liquid-liquid interface, a flexible, transparent and all-solid supercapacitor from the GR/PANI-20 composite was built in order to evaluate its potential for further application in the energy field. Transparent optoelectronic and electrochemical devices are usually built employing ITO (indium tin oxide) as transparent electrode, due to its suitable transmittance (90% at 550 nm) and conductivity (sheet resistance about 10e100 U ,1) [22,39,40]. However, ITO is expensive, high brittleness and has low mechanical resistance under bending, which is a limitation for future technologies using flexible and transparent substrates. So, the preparation of ITO-free flexible devices, as described herein, is an important challenge in this field. A schematic and photographs of this solid device are shown in Fig. 4(a) and (b), respectively. The all-solid device retains its structure and voltammetric response (Fig. 4(c)) even when submitted to cycles of mechanical deformation. A high capacitive current, including an anodic peak around 0.47 V, is observed for all configurations (Fig. 4(c)). The same occurs for the charge/discharge curves presented in Fig. 4(d). The redox pairs become more defined when and after the device is twisted, an observation likely due to the diminishing of the distance between the two electrodes as previously observed for carbon nanotubes/PANI all-solid supercapacitor [20]. The value of volumetric specific capacitance for the all solid device was 95.5 F cm3. This value is slightly higher compared to a carbon nanotube/PANI all-solid supercapacitor built following a similar procedure, indicating the high potential of the
Fig. 4. (a) Scheme of the all-solid device, including all layers; (b) a photograph of the all-solid, transparent and flexible supercapacitor built from the composite GR/PANI-20 when the sample is straight or twisted; (c) cyclic voltammograms and (d) charge/discharge curves of the device in both straight and twisted states.
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interfacial method to prepare composites from the chemical synthesis of both graphene and polyaniline. 4. Conclusions In conclusion, we demonstrated the complete, one-step synthesis of nanocomposites based on graphene and polyaniline, in which both the phases were obtained starting from their simplest chemical component (benzene and aniline) and directly obtained as a thin film. This new route to synthesize graphene-based nanocomposites opens the possibility of preparing new materials without previous steps normally necessary to prepare graphene sheets. In addition, it can be deposited over different types of substrates, since the interfacial system allows for the preparation of thin and freestanding films. This is the second example reported by us of a bottom-up synthesis of an entire nanocomposite, proving the versatility, wide adaptability and general aspects of the route, which theoretically can be extended to different materials where graphene and another component may be simultaneously synthesized. The graphene/polyaniline nanocomposites show differentiated structures and morphologies, resulting in excellent pseudocapacitive behaviours and allowing for the construction of all-solid transparent and flexible supercapacitor devices. Acknowledgments Authors acknowledge the financial support of CNPq, CAPES, INCT-Nanocarbon and NENNAM (Pronex/F. Araucaria/CNPq). VHRS thanks CAPES for the fellowship. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.02.064. References [1] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906e3924. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. [3] M. Losurdo, M.M. Giangregorio, P. Capezzuto, G. Bruno, Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure, Phys. Chem. Chem. Phys. 13 (2011) 20836e20843. [4] J.W. Suk, A. Kitt, C.W. Magnuson, Y. Hao, S. Ahmed, J. An, A.K. Swan, B.B. Goldberg, R.S. Ruoff, Transfer of CVD-grown monolayer graphene onto arbitrary substrates, ACS Nano 5 (2011) 6916e6924. [5] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451e10453. mez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, [6] C. Go K. Kern, Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett. 7 (2007) 3499e3503. [7] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463e470. [8] L. Zhi, K. Mullen, A bottom-up approach from molecular nanographenes to unconventional carbon materials, J. Mater. Chem. 18 (2008) 1472e1484. [9] R.V. Salvatierra, V.H.R. Souza, C.F. Matos, M.M. Oliveira, A.J.G. Zarbin, Graphene chemically synthesized from benzene at liquideliquid interfaces, Carbon 93 (2015) 924e932. [10] X. Wang, L. Zhi, K. Müllen, Transparent, conductive graphene electrodes for dye-sensitized solar cells, Nano Lett. 8 (2007) 323e327. [11] E. Yoo, J. Kim, E. Hosono, H.-S. Zhou, T. Kudo, I. Honma, Large reversible li storage of graphene nanosheet families for use in rechargeable lithium ion batteries, Nano Lett. 8 (2008) 2277e2282. [12] J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137e3140. [13] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors,
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