Mechanochemically prepared polyaniline and

Mechanochemically prepared polyaniline and graphene-based nanocomposites as electrodes of supercapacitors Oleg Yu. Posudievsky, Olga A. Kozarenko, Vyacheslav S. Dyadyun, Igor E. Kotenko, Vyacheslav G. Koshechko & Vitaly D. Pokhodenko Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 J Solid State Electrochem DOI 10.1007/s10008-018-4052-6

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Author's personal copy Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-018-4052-6

ORIGINAL PAPER

Mechanochemically prepared polyaniline and graphene-based nanocomposites as electrodes of supercapacitors Oleg Yu. Posudievsky 1 & Olga A. Kozarenko 1 & Vyacheslav S. Dyadyun 1 & Igor E. Kotenko 2 & Vyacheslav G. Koshechko 1 & Vitaly D. Pokhodenko 1 Received: 4 April 2018 / Revised: 17 July 2018 / Accepted: 18 July 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Conductive nanocomposites based on polyaniline and graphene (PAni/Gr) were prepared by cheap and efficient mechanochemical method. The uniform distribution of Gr nanoparticles in the polymer matrix and the ordering of the polymer chains due to the action of mechanical shear stresses, which were established by TEM, stipulated high specific capacitance about 920 F g−1 in − 0.2–1.0 V vs. Ag/AgCl potential range. PAni/Gr-based electrodes are able to provide the specific capacitance of ~ 750 F g−1 at 2 A g−1 in symmetric supercapacitors (SSC) and stably cycle at the operating voltage V = 0.65 V for 10,000 charge-discharge cycles with 96% capacitance retention, whereas the increasing of V leads to the loss of stability as a result of the cathode degradation. PAni/Gr-based SSC possessed improved self-discharge showed high rate capability, and the specific power of such SSC could reach ~ 10 kW kg−1 at the specific energy of ~ 18 W h kg−1.

Introduction Supercapacitors (SC) have a high specific power and are able to discharge much faster than batteries that cause their application in vehicles, consumer electronics, and other devices [1, 2]. SC are divided into two main types depending on the mechanism of charge storage: (1) electric double layer capacitors (EDLCs), which operation is based on charge storage due to the nonfaradaic process of charging the double electric layer (DEL) at the electrode/electrolyte interface and (2) pseudocapacitors that function due to rapid reversible redox [1]. Usually, carbon materials with a developed surface are used in SC electrodes of the first type, whereas redox active materials, such as conducting polymers or transition metal

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-018-4052-6) contains supplementary material, which is available to authorized users. * Oleg Yu. Posudievsky [email protected] 1

L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, prospekt Nauki 31, Kyiv 03028, Ukraine

2

National Technical University of Ukraine BKyiv Polytechnic Institute^, prospekt Peremogy 37, Kyiv 03056, Ukraine

compounds, are used in SCs of the other [1–4]. EDLCs are mainly based on carbon materials. They are usually characterized by high specific energy due to quick sorption and desorption of ions, but their specific capacitance is commonly low in spite of very high specific surface area (up to 150 F g−1 in the case of organic electrolyte and less than 200 F g−1 for the aqueous media) [1–8]. Increase of the specific capacitance of carbon electrodes is possible by introducing pseudocapacitive component in the general capacitance using chemical fuctionalization of the carbon surface or inserting of redox active particles (polymers, transition metal compounds, etc.). But unfortunately, such materials appear to be unstable upon charge-discharge cycling [1–8]. At the same time, pseudocapacitors are capable to store a sufficiently greater amount of energy in comparison with EDLCs, because in this case not only the surface of the material as for EDLCs but also its volume participates in chargedischarge processes [1–8]. In this regard, one of the promising directions for creating electrodes for a new generation of SCs could be a combination of components with different types of charge storage at a nanoscale level in one material. For example, for RuO2/carbon nanocomposites, where RuO2 stores charge due to redox processes and carbon due to DEL, a significant improvement in the electrochemical characteristics of SC electrodes was demonstrated [9, 10]. Polyaniline (PAni) is one of the most attractive candidates for pseudocapacitors among conducting polymer materials

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due to its redox activity [1, 2, 11–13], sufficiently high electrical conductivity [14], and developed specific surface that can be achieved by using different synthetic approaches [15, 16]. However, the low electrochemical stability of PAni limits the use of SC on its basis, since the possibility of long-term charge-discharge cycling (> 10,000 cycles) is important for these devices [1, 2, 17, 18]. This behavior is usually unattainable for conducting polymers, although the possibility of improving stability has been demonstrated for composites of PAni with various carbon materials, as well as for the case of the presence of the active additive (quinone–hydroquinone) in the electrolyte [11, 19–25]. It should be noted that only chemical or electrochemical methods [11, 19–25] were used for the preparation of nanocomposite materials. Though the mentioned nanocomposites often possessed valuable characteristics, the complexity of the synthesis methods as well as toxicity of the required reagents (concentrated acids and aggressive reagents in Hammers synthesis of graphene oxide [26], for example) could be a considerable restriction for their practical usage. Therefore, there is a still urgent necessity of developing Bgreen^ technological approaches for preparation of efficient electrode materials for SC. A high-performance and environmentally acceptable mechanochemical method could be a promising alternative here, as it was successfully used both for the production of conducting polymers and dispersing nanosized fillers in polymer matrices [27–36]. Moreover, the mechanochemical method also allowed preparing nanocomposite materials for electrodes of lithium batteries with improved functional characteristics, as well as graphenes with different degrees of oxidation [37–48]. Taking into account the perspective of the mechanochemical approach for the creation of effective electrode materials of various electrochemical devices, in this paper, we studied the possibility of using the mechanochemical method for preparing nanocomposites based on PAni and graphene and application of such nanocomposites as active components for electrodes of symmetric supercapacitors (SSC). Much attention was paid to the study of the nanocomposite structure by electron microscopy and their functional characteristics by various electrochemical methods.

Experimental Materials Reagents Aniline, hydrochloric acid, ammonium persulfate, ammonium hydroxide, and sodium chloride of analytical grade (Aldrich) as well as graphite flakes (99.99%, Alfa Aesar) and acetylene black (MTI) were used for materials preparation. The aniline was distilled in vacuum before use.

Material preparation For chemical synthesis of PAni, chPAni, the procedure described in [29] was used. For this purpose, 10 g (0.108 mol) of aniline was dissolved in 250 mL of 1-M HCl. 19.7 g (0.086 mol) of ammonium persulfate was dissolved in 250 mL of distilled water. An aniline solution in hydrochloric acid was placed in an ice bath, and its temperature was adjusted to 0–5 °C. Then, with constant stirring, an oxidizer solution was gradually added to the solution of the monomer and the synthesis was carried out for 3 h. The resulting dark green precipitate was washed with ethanol to remove organic residues and distilled water to remove the inorganic reaction products. PAni in the state of emeraldine base was obtained by dedoping the synthesized polymer in a 2% solution of ammonium hydroxide, followed by washing with distilled water, drying in air at room temperature, removing the low molecular weight fraction by extraction in acetonitrile in a Soxhlet apparatus, and drying in vacuum at 80 °C. The preparation of nanostructured graphite (nG) was carried out accordingly to [44]. Shortly, a dry mixture of graphite flakes (50 mg) and NaCl (2 g) was mechanochemically treated in the absence of a solvent in agate 80-mL grinding bowl of planetary ball mill Pulverisette 6 (Fritsch) at a rotation rate of 500 rpm for 1 h. The product was thoroughly washed with water to remove the salt and dried in vacuum at 60 °C. The nanocomposite based on PAni and graphene (PAni/Gr) was prepared by mechanochemical treatment in the planetary ball mill of a dry mixture of 2 g of ch-PAni in the emeraldine base state and 0.3 g of nG at a rotation rate of 300 rpm for 1 h. The resulting PAni/Gr nanocomposite was used for further studies without further purification. The direct current conductivity of the nanocomposite was 0.7 S cm−1, since the indicated Gr content provided for overcoming the percolation threshold. Analogous PAni/Gr nanocomposites with 9, 23, and 31 wt% content of Gr component were prepared by the same procedure. A similar nanocomposite based on PAni and acetylene black (PAni/AB) was prepared by the mechanochemical treatment of the analogous quantities of the components (13 wt% of AB). The resulting PAni/AB nanocomposite was used for further studies without further purification. Mechanochemically treated polyaniline (mct-PAni) was prepared as a result of the mechanochemical treatment of chPAni under conditions similar to that of PAni/nG and PAni/ AB nanocomposites. Dispersion containing predominantly monolayer graphene nanoparticles in dimethyl formamide (GrDMF) was prepared by liquid exfoliation of nG in accord with [44].

Measurements Methods of atomic force and transmission electron microscopy (AFM and TEM) as well as Raman spectroscopy were used to study the structure of the samples. AFM images were


obtained using the Solver Pro M facility (NT-MDT) in a tapping mode. The samples were deposited on freshly cleaved mica (V-1 Grade, SPI Supplies) from the diluted suspension prepared by 10-s ultrasound treatment of nG in ethanol. TEM images of the particles of the prepared materials were obtained with a TEM125K (SELMI) microscope operating at a potential of 100 kV, with the samples deposited on an amorphous carbon film covering a copper mesh substrate. The selected area electron diffraction (SAED) patterns for the studied samples were also obtained on this microscope. Raman spectra were registered using an inVia Raman microscope (RENISHAW) equipped with an excitation laser of 633-nm line in an ambient air environment. Measurements of conductivity were carried out by a standard four-probe technique with an error of not more than 5%. SSC consisted of identical electrodes separated by a cellulose membrane impregnated with 1-M H2SO4 solution. Platinum plates with an area of 1 cm2 were used as current collectors. In the case of PAni/Gr and PAni/AB nanocomposites, the electrode mass consisted of 90 wt% of the active component and 10 wt% of poly(vinylidene fluoride-cohexafluoropropylene) as a binder, and in the case of ch-PAni and mct-PAni, it contained 75 wt% of the polymer, 15 wt% of acetylene black, and 10 wt% of the binder. Electrodes of SSC were formed by applying a paste prepared using acetone to platinum plates, followed by drying in air. These electrodes with the weight of 4 mg were kept in the electrolyte for an hour to dope the polymer before performing electrochemical experiments. For the studies by cyclic voltammetry (CV), chronopotentiometry, and impedance spectroscopy, μAUTOLAB III/FRA2 (ECO CHEMIE) potentiostat-galvanostat and Keithley 2400 SourceMeter were used. CV experiments were performed in 1-М aqueous H2SO4 in the three-electrode cell with Ag/AgCl reference electrode. An ability of the produced SSCs to prolonged charge-discharge cycling was measured by a computerized multichannel facility. Charge-discharge cycling was carried out at an operating voltage of 0.65, 1.0, and 1.2 V. The discharge of the charged SSCs was also performed via a resistor with resistivity of 10, 25, 50, or 100 Ohm. Equations S1–S11 (Supplementary information) were used for calculation of specific capacitance, energy, and power. Modeling of the impedance spectra by equivalent circuits was performed using ZView2 software (Scribner).

Results and discussion AFM was used to study the thickness of nG particles, and the obtained result is presented in Fig. S1. It is seen from the figure that the solvent-free mechanochemical treatment of graphite/NaCl mixture was efficient for exfoliation of the initial graphite microflakes into nG particles, which consisted

mainly of one to three layers. Using the mechanochemical approach for preparation of PAni/Gr nanocomposite, we assumed that due to the action of shear stresses, as well as π-π interaction between polymer macromolecules and nG particles, further exfoliation of nG could be achieved. The dedoped PAni in the state of the emeraldine base was used in order to exclude the possible influence of the Coulomb interaction between the dopant anions and the positively charged chains of the doped PAni on the ordering of the macromolecules packing, as well as the interaction between the components in the mixture of PAni and nG. The data of TEM about the morphology of nG particles are shown in Fig. 1. It follows from Fig. 1 which shows the brightfield TEM image of agglomerate that nG particles have 100 −250-nm length and ~ 40-nm width. SAED pattern of nG (Fig. 1b) shows a characteristic dot character, which indicates the preservation of the structure of graphite sheets in the agglomerate, clearly visible in its dark-field image (Fig. 1c). It should also be noted that a small diameter ring corresponding to the (002) reflex in graphite (indicated by arrows in Fig. 1b) is observed on the electronogram of nG. Low intensity of this reflex corresponds to the small number of layers that is in accord with the AFM presented above. Under the effect of shear stresses during the mechanochemical treatment in the planetary ball mill, the graphite microflakes are delaminated perpendicular to c axis, they became thinner, and the their packing order is disrupted by the displacement and reorientation of the graphene layers. These changes in the graphite structure led to loss of rigidity and bending of particles to minimize the surface energy, as well as the number of broken chemical bonds that are formed during the process of the particle destruction. Such bends of nG particles appear in Fig. 1a in the form of extended formations. Similar formations were observed earlier after mechanochemical treatment of graphite and coal [49, 50]. Figure 2a shows the bright-field image for the agglomerate of mct-PAni particles. In this case, SAED pattern consists of multiple periodically located point reflexes with hexagonal symmetry (Fig. 2b). The dark-field image obtained from the reflex of this electronogram is presented in Fig. 2c, which shows that the whole agglomerate is characterized by the crystalline ordering. This makes it possible to assume that the mechanochemical treatment of PAni leads to a substantial increase in the ordering degree for packing of the macromolecules. Moreover, as was shown earlier, ch-PAni basically consists of aggregates of nanoparticles that do not have a pronounced crystalline structure and are essentially amorphous [29, 51]. It should be noted that the hexagonal pattern, which is seen on the electronogram, implies the presence of the hexagonal crystalline symmetry in the polymer, which is quite unusual for PAni, which, as is known, tends to form crystals with orthorhombic lattice [51]. The interplanar distance calculated on the basis of this electronogram (Fig. 2b) is ~ 0.55 nm,


Fig. 1 Bright-field TEM image (а), SAED pattern (b), and dark-field TEM image (с) of nG

which corresponds to the distance between neighboring nitrogen atoms in the straightened PAni macromolecules [51]. Apparently, in the process of mechanochemical treatment of the polymer in the presence of nG, the crystal lattice of the polymer can be rearranged due to the alternation of the action of normal and tangential stresses on the macromolecules with formation of close-packed straightened macromolecules with hexagonal symmetry. TEM images of PAni/Gr nanocomposite are shown in Fig. 3. It should be noted that during the preparation of the sample for analysis, we encountered certain difficulties, which consisted in the fact that the nanocomposite was difficult to disperse in various solvents (water, alcohols, hexane), and rather large particles were precipitated onto the carbon substrate, through which electron beam did not pass. As a result, only the edges of such particles are clearly visible in TEM image of the nanocomposite. As could be seen from Fig. 3a, the nanocomposite components have approximately the same contrast, which makes it very difficult to clearly distinguish the components of PAni/Gr. Figure 3b shows the SAED pattern obtained for large agglomerate, whose dark-field image is shown in Fig. 3c. The presence of the ring with the largest interplanar spacing of 0.246 nm confirms the presence of the graphene component in its composition (Fig. 3b). There is no (002) reflex on the electronogram that could indicate an additional exfoliation of

nG, probably up to the graphene state (Gr), during the mechanochemical preparation of PAni/Gr nanocomposite. The ring character of the SAED pattern (Fig. 3b) testifies in favor that Gr nanoparticles in the considered agglomerate are in different spatial orientations. Using the dark-field image obtained from the diffraction of Gr, it is possible to discriminate its particles in the volume of the nanocomposite (Fig. 3c) and make a conclusion that Gr particles are uniformly distributed in the polymer matrix. An interesting feature of this material is the presence of the striped moiré pattern in the dark-field image (the absence of moiré in the bright field is due to the low contrast of the nanocomposite components) (Fig. 3c–e). As is well known, moiré patterns appear as a result of additional diffraction of the electron beam, by overlapping two crystals with a close values of the crystal lattice parameters or by two identical crystals differing in spatial orientation [52]. In our case, formation of moiré could be due to superposition of fine graphene particles and ordered PAni macromolecules (the polymer ordering follows from Fig. 2b). Since moiré patterns we did not manage to observe in both nG and mct-PAni, participation of Gr and PAni particles in the formation of the moiré pattern of PAni/ Gr nanocomposite seems to be most probable. Thus, it could be concluded based on the analysis of TEM data that during the mechanochemical treatment of the mixture of PAni and nG, the structuration of the polymer by external mechanical

Fig. 2 а Bright-field TEM image, b SAED pattern, and с dark-field TEM image of mct-PAni


Fig. 3 Bright-field TEM image (а), SAED pattern (b), and dark-field TEM image (с, d, e) of PAni/Gr nanocomposite

forces and interaction between PAni macromolecules and nG layers lead to the formation of the layered morphology due to 2D nature of carbon particles, along which the ordered straightened PAni chains are located. The structure of PAni/Gr nanocomposite as well as initial ch-PAni and GrDMF specially prepared for comparison were studied using Raman spectroscopy. It follows from Fig. 4 that the spectrum of PAni/Gr nanocomposite comprises the characteristic bands of both components. The peaks at 1585, 1475, 1406, 1216, and 1160 cm−1 are attributed to PAni, and small blue shifts compared with that of pure PAni are characteristic of them (Fig. 4) [53]. The peak at 2645 cm−1 is obviously

Fig. 4 Raman spectra of PAni and PAni/Gr powders and GrDMF film

identical to 2D mode of graphene [54]. Comparing its position with that in the spectrum of GrDMF film, one could conclude that there is a red shift. Such shift could be the consequence of charge transfer between the components in the PAni/Gr nanocomposite and consequent electron doping of Gr component [55]. This assumption is confirmed by the appearance of the new band about 1325 cm−1 in the spectrum of the nanocomposite (Fig. 4). This rather wide band is comprised of two peaks about 1318 and 1337 cm−1. The first of them is the corresponding D mode of the Gr component, red shifted relative to the spectra of GrDMF, and the second probably corresponds to the stretching of C▬N+˙ bonds in the doped PAni [53, 56, 57]. Redox activity of PAni/Gr nanocomposite was studied by CV method in − 0.2–1.0 V vs. Ag/AgCl potential range, and the obtained results are presented in Fig. S2. It was established by integrating the area under the curve in Fig. S2 in accord with Eq. S5 in Supplementary information that the nanocomposite possessed rather high specific capacitance about 920 F g−1 at the potential scan rate of 5 mV s−1. Five-fold and ten-fold increase of the scan rate led to the decrease of the specific capacitance up to ~ 910 and ~ 800 F g−1, respectively. It was concluded based on these data that PAni/Gr nanocomposites could be used as electrode material of SC. For this aim, the models of SSC based on the studied materials were produced. To determine the stability of the prepared PAni/Gr in SSC, the possibility of their prolonged charge-discharge cycling in


the galvanostatic regime was studied. The specific capacitance of PAni/Gr was determined by Eq. S1. The results are shown in Fig. 5, from which it follows that the presence of graphene particles in the bulk of the polymer results in a significant increase in not only the specific capacitance but also the stability of the SSC cycling in comparison with the similar device based on the initial PAni. For example, the ch-PAni in SSC possessed an initial specific capacitance of ~ 284 F g−1 that increased upon further cycling over 50 cycles up to ~ 363 F g−1 and subsequently continuously decreased, being after 2000 cycles not more than 240 F g−1. At the same time, the initial specific capacitance of the PAni/Gr in SSC was 750 F g−1, which decreased slightly upon further operation of the device and stabilized at the level of ~ 720 F g−1 after 1500 cycles (Fig. 5a). Control experiments simultaneously showed that the capacitance of nG as the active component did not exceed 2 F g−1. It is important to note that PAni/Gr in SSC exhibited high stability for at least 10,000 cycles (the capacitance retention was 96%), and the tendency of the change in specific capacitance was such that it is possible to suppose its further stable cycling (Fig. 5b). Both the mechanochemical preparation method and the presence of graphene nanoparticles in the bulk of the polymer could be the reason of the increased specific capacitance of PAni/Gr in SSC. To clarify the situation, the mechanochemically treated ch-PAni (mct-PAni) was also tested in SSC. Figure 6a shows the data of the prolonged cycling of such SSC, from which it follows that the mechanochemical treatment of PAni leads both to the significant increase in the specific capacity and stability of the electrochemical performance of the polymer. The observed effect is obviously related to the effect of the mechanical stresses on the structure of the polymer, which, as shown above, possesses the higher crystallinity due to the ordering of the chain packing (Fig. 2). Such increase in the polymer ordering favorably affects its electrochemical properties [27, 42]. On the other hand, since the specific capacitance of the PAni/Gr in SSC was much higher than that of mct-PAni, it

was natural to assume that this could be due to the presence of the carbon material in the bulk of the polymer. However, the question arises whether this was due solely to the presence of carbon or to the nature of the carbon component. In this connection, we also studied the electrochemical properties of the mechanochemically prepared PAni/AB nanocomposite, in which acetylene black with the same content was used instead of nG. As follows from Fig. 5a, the specific capacitance of such composite material reached the values close to those for mct-PAni, but it was significantly inferior to the characteristics of PAni/Gr. This allowed us to assume that the high value of the specific capacitance of the PAni/Gr is due not only to the mechanochemical preparation method but also to the nature of the carbon component. The presence of graphene contributed to the more efficient interaction between π-systems of the organic macromolecules and graphene layers. Such interaction could lead to an increase in access of both electrons and anions-dopants to active redox centers in the macromolecules and cause an increase in capacitance due to the faradaic process. Determination of the optimal working voltage is also important for performance of SSC. Our studies showed that an increase in the operating voltage of the PAni/Gr-based SSC to 1.0 V leads to a decrease in the stability of its cycling (27% loss of the capacitance after 2000 cycles), and an increase to 1.2 V causes sharp loss of stability (40% loss of the capacitance after 2000 cycles), whereas only 4% decrease in the capacitance is observed for the same cycling at 0.65 V operating voltage (Fig. S3). Such a significant deterioration in the electrochemical behavior of the SSC could be due to degradation of the polymer as a result of its overoxidation and hydrolysis [58]. This assumption was confirmed by the studies of CVs and charge-discharge curves of the PAni/Gr-based SSC. The CVof PAni/Gr nanocomposite contained the peaks characteristic of the main redox transitions of PAni (Fig. S2) [12, 59]. The anode peak about 0.26 V and the corresponding cathodic peak

Fig. 5 а Specific discharge capacitance of PAni-based materials during 2000 cycles. b Prolonged charge-discharge cycling of PAni/Gr nanocomposite at specific current of 2 A g−1 in 0–0.65-V voltage range


Fig. 6 Cathode and anode potentials for PAni/Gr-based SSC at V = 0.65 V (а), V = 1.0 V (b), and V = 1.2 V (c)

about 0.08 V were due to redox transitions between emeraldine (EM) and leucoemeraldine (LE). The anode wave about 0.9-V region and the backward wave about 0.7 V corresponded to transitions between EM and pernigraniline (PN) [12, 59]. Figure 6 shows the charge and discharge curves of both electrodes measured with respect to Ag/AgCl for different values of the operating voltage (V). So, in the case of V = 0.65 V (Fig. 6a), the potential of the positive electrode varied in the range 0.65–0.33 V and for negative one, it was 0.33–0.0 V. This means that the main redox reaction at the anode was LE ↔ EM transition, and the partial EM ↔ PN transition took place at the cathode. As is known, just EM ↔ PN transition leads to the degradation of the polymer in the acidic medium, whereas LE ↔ EM transition is fairly stable [58, 59]. Thus, in the case of PAni, the positive electrode determines the stability of the SSC as a whole. The presence of graphene particles under the conditions of the incomplete transition of PAni in the state of PN, as well as the mechanochemical preparation method led to the stabilization of the SSC during prolonged cycling (Fig. 6b). An increase in the operating voltage from 0.65 to 1.0 and 1.2 V caused the cathode potential range shift from 0.6–0.325 to 0.77–0.27 and 0.9–0.3 V, respectively. As could be seen from Fig. 6a, b, this led to the fact that the process of

conversion of EM into PN occurs analogously to the case when V = 0.65 V that causes the increase of the polymer degradation and decrease of the SCC stability during prolonged cycling (Fig. S3). In addition, the increase in the operating voltage could also lead to the decrease in the polymer conductivity of the anode at potentials below 0.0 V, which is manifested in a sharp change of the potential on the charge/discharge curves (Fig. 6b, c). Thus, the most effective considered operating voltage for the PAni/Gr-based SSC is V = 0.65 V. CVs of PAni/nG-based SSC were studied at the different stages of charge-discharge cycling (Fig. S4a). As could be seen from Fig. S4a, the shape of CV at the beginning of cycling and after 2000 cycles was different. First, a pair of reversible wide peaks about 0.2 V was observed on CV. However, during cycling, additional peaks appeared on CV curves, which gradually shift to the region of more positive potential and are about 0.3–0.4 V after the 50th cycle (Fig. S4a). Upon further cycling, the intensity of these peaks decreased, and the intensity of the peaks about 0.1–0.2 V increased. The reason for such changes in CV is not clear at the moment and requires further studies. CVs of PAni/Gr-based SSC become unchanged after the 1487th cycle, which was in accordance with the data shown in Fig. 5. CVs at different potential scan rates measured after


2000 charge-discharge cycles of the PAni/Gr-based SSC are shown in Fig. S4b. As follows from the figure, their form practically did not change when the potential scan rate increases. This indicates that the redox processes, as well as the accompanying diffusion of the dopant anions in the volume of the nanocomposite, proceed at a sufficiently high rate, which is probably due to the structural features of PAni/Gr nanocomposite. Figure 7a, b shows the rate characteristics of the PAni/ Gr in SSC in the galvanostatic mode at various currents, which indicate that this material can operate at the current of 30 A g−1 with capacitance of ~ 630 F g−1. According to Ragone plot, which shows how the specific energy of the SSC depends on its specific power [1, 60], the specific power of the PAni/Grbased SSC could reach 10 kW kg−1 at the specific energy of ~ 18 Wh kg−1 and the operating voltage of 0.65 V (Fig. 7c). Comparing our experimental data with those reported in literature, it should be mentioned that energy and power characteristics of SCs considerably depend on the method used for calculation of specific capacitance. Some authors consider that the usage of Eqs. S1 and S1.1 in the case of pseudocapacitive character of the charge storage mechanism could lead to substantial overestimation of this parameter. Therefore, the values

of the specific electrochemical characteristics (capacitance, energy, and power) of PAni/Gr-based SSCs were calculated by us using different equations, which are used in the framework of different electrochemical techniques (Fig. S5). The obtained results of the calculation are summarized in Table S1 (Supplementary information). As follows from Table S1, the values of specific energy and specific power for PAni/Grbased SSCs are congruent or even greater as compared with analogous devices based on composites of PAni with various carbonaceous materials considered in literature earlier [11, 19–25, 61–70]. To assess the performance of the PAni/Gr-based SSCs under conditions close to real application of these devices, their discharge through a different resistors was studied (Fig. 7d). Such approach also made possible an estimation of the specific capacitance of the SSC in accord with Eq. S6. Thus, it was established that PAni/Gr nanocomposite in the produced devices possessed the capacitance of ~ 700 F g−1, which is in accordance with the data obtained by, for instance, galvanostatic charge-discharge studies. In addition to the electrochemical characteristics presented above, the impedance spectra of the ch-PAni and PAni/Gr-

Fig. 7 a Charge-dischargecurvesforPAni/Gr-basedSSCatthedifferent currents. b Dependence of the specific capacitance of PAni/Gr on the specific current. c Ragone plot for PAni/Gr-based SSC. References are

shown in brackets. d Discharge of PAni/Gr-based SSC through different resistors


based SSC were studied during the devices cycling. As could be seen from Fig. S6a, the impedance spectra of the ch-PAnibased SSC contain an inclined line in the low-frequency region of the spectrum with an angle of inclination close to 45°, which corresponds to Warburg impedance and describes the diffusion of dopant anions in the volume of the electrode. The impedance spectra of the PAni/Gr-based SSC consist of a small semicircle corresponding to charge transfer at electrolyte/nanocomposite interface and nearly vertical line in the low-frequency region of the spectrum (Fig. S6b). These spectra could be quite perfectly fitted using two equivalent electric circuits shown in Fig. S7. The first circuit presented in Fig. S7a was earlier successfully used in [71, 72], and the second one (Fig. S7b) was utilized for modeling impedance spectra of polyaniline-based SSC in [73, 74]. The elements of the circuits have numeric values specified in Tables S2 and S3. Both circuits are quite close. Both of them contain element Re which characterizes a general Ohmic resistance of electrolyte and metallic conductors. Constant phase element ZCPEdel and Rct connected in parallel to ZCPEdel descrie capacitance of the DEL and charge transfer through electrolyte/nanocomposite interface correspondingly. The DEL capacitance is modeled in both circuits by a constant phase element due to inhomogeneous nature of the nanocomposite surface. In both circuits, constant phase element ZCPEc characterizes diffusion of anion dopants inside the polymer volume. The parallel connection of ZCPEdel and Rct reflects the presence of the semicircle in the spectra presented in Fig. S6b, while ZCPEc is due to the nearly vertical line. Comparing the fitting of the stated circuits to the experimental data (Tables S2 and S3) shows that parameter n ≈ 0.9 for ZCPEc elements that definitely shows a predominantly capacitive character of this element and its deviation from unity is due to inhomogeneity of the diffusion channels for anion dopants that is rather usual for conducting polymers [71–74]. It should be noted that the data presented in Tables S2 and S3 confirm the circuit shown in Fig. S6a as being more adequate to the studied SSC, because it is characterized by the definitely lower value of an accuracy parameter χ2. Besides, it could be seen from the spectrum (Fig. S6b) that the resistance of the SSC decreases upon cycling indicating an increase in the efficiency of the device. After 712 cycles, the impedance spectra of PAni/Gr-based SSC are practically unchanged that is the consequence of stabilization of the system operation. The impedance spectra of the ch-PAni-based SSC also show a decrease in resistance upon charge-charge cycling (Fig. S6a). Obviously, this behavior is generally characteristic of PAni. The main difference between the compared SSCs is a much larger resistance in comparison with PAni/Gr nanocomposite that could be related with the structure of the polymerbased electrodes. It was shown above that the presence of Gr particles led to the ordering of the polymer chains that facilitates the diffusion of dopant anions to the redox centers of

PAni. Macromolecules in ch-PAni are probably coiled and twisted that significantly hinders the mass transfer within the polymer. This is reflected in the form of the impedance spectra obtained for the ch-PAni-based SSC (Fig. S6a), on which an inclined line with an inclination angle of ~ 45° is observed and there is no response characteristic of SC, as is the case of PAni/Gr. The impedance spectra (Nyquist plots) for PAni/Gr-based SSC were modeled by means of an equivalent circuit (Fig. S7a), and the calculated parameters are summarized in Table S2. Extrapolating the experimental data of the impedance spectrum recorded after the 1896th cycle (Fig. S6b) to the low frequency region, it is possible to calculate the specific capacitance using Eq. S7, which appeared to be about 345 F g−1. This value is in accord with the data obtained above by other approaches. The impedance spectra were recorded in the discharged state of the device, when both electrodes have a potential of 0.325 V vs. Ag/AgCl (Fig. 6a), which according to the CV data (Fig. S2) corresponds to the polymer in the maximally conducting state of the emeraldine salt [59]. This enabled us to evaluate the DEL contribution (Eq. S11) to the total capacitance. It was established using such approach that the capacitive component of the DEL in PAni/Gr nanocomposite-based SSC is about 45 F g−1. It could be concluded therefore that the specific capacitance of the nanocomposite is predominantly determined by its redox activity. Also, the impedance spectra allowed us to determine the internal resistance of the SSC, which appeared to be equal to 0.8 Ohm for PAni/Gr-based SSC, and thus, to calculate the maximum instantaneous specific power (Eq. S10) which was found to be ~ 39 kW kg−1. An important characteristic of SC is their ability to retain an accumulated charge for a certain time after a circuit break, which is determined by the self-discharge of the device. Self-discharge is a known problem of SC, especially in the case of devices based on conducting polymers [4]. Taking this into account, we studied the self-discharge of SSC based on ch-PAni, PAni/AB, and PAni/Gr (Fig. S8a) and defined that presence of carbon particles reduces the self-discharge of SSC and leads to loss of the accumulated charge at the level of 18% for PAni/Gr and 20% for PAni/AB in comparison with 35% found for ch-PAni (Fig. S8b). Taking into account the advanced electrochemical performance of PAni/Gr nanocomposite with 13-wt% content of graphene, it was of interest to optimize its composition. We additionally considered the nanocomposites with 9-, 23-, and 31-wt% content of the carbon component, and the conducted tests allowed us to establish that the presence of Gr caused in all cases an increase of specific capacitance and its stability during charge-discharge cycling with respect to the characteristics of ch-PAni, though the highest properties were observed for the nanocomposite sample analyzed in detail above (Fig. S9).


Conclusions Thus, the nanocomposites based on PAni and Gr were prepared using the cheap and efficient mechanochemical method. The nanocomposite is electron-conducting at 13-wt% content of Gr, and its direct current conductivity is 0.7 S cm−1. The uniform distribution of nG nanoparticles in the polymer matrix was established by TEM, which in combination with the action of mechanical shear stresses leads to the structuring of the polymer and the formation of ordered straightened PAni macromolecules. It was established that the mechanochemically prepared PAni/Gr nanocomposite with the optimum composition possesses high specific capacitance about 920 F g−1 at 5 mV s−1 in − 0.2–1.0 V vs. Ag/AgCl potential range and therefore could be used for creation of efficient SSC. Such material can provide the specific capacitance of ~ 750 F g−1 at 2 A g−1 and stably cycle for at least 10,000 charge-discharge cycles with 96% capacitance retention. It was shown that the higher value of the specific capacitance of PAni/Gr-based SSC, in comparison with devices based on PAni obtained by the traditional routes, is due not only to the mechanochemical preparation but also owing to the nature of the carbon component that contributes to the formation of the specific morphology of the nanocomposite. It was established that the optimal operating voltage for the PAni/Gr-based SSC is 0.65 V, whereas the increasing of its value up to 1.0 and 1.2 V leads to loss of the operation stability of the SSC as a result of the cathode degradation. It was found that the presence of nG nanoparticles inside PAni facilitates high stability of the SCC during prolonged cycling, while the similar devices based on ch-PAni do not possess such characteristic. It was shown that the PAni/Gr show high rate capability in SSC and are able to function at currents up to 30 A g−1 and at the same time provide the capacitance of ~ 640 F g−1, and the specific power of such SSC can reach ~ 10 kW kg−1 at the specific energy of ~ 18 W h kg−1. It is also shown that the presence of graphene nanoparticles in PAni/Gr nanocomposite leads to the decrease in the selfdischarge of the corresponding SSC. Funding information This work was supported by the Targeted Research & Development Initiatives of the Science and Technology Center in Ukraine and the National Academy of Sciences of Ukraine and Targeted Comprehensive Fundamental Research Program of the National Academy of Sciences of Ukraine BFundamental problems of creating new nanomaterials and nanotechnologies.^

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