Reduced Graphene Oxide/Polyaniline Electrochemical Supercapacitors Fabricated by Laser PDF

Applied Surface Science 467–468 (2019) 691–697 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Reduced graphene oxide/polyaniline electrochemical supercapacitors fabricated by laser T A. Ladrón-de-Guevaraa,b, A. B...

Applied Surface Science 467–468 (2019) 691–697 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Reduced graphene oxide/polyaniline electrochemical supercapacitors fabricated by laser T A. Ladrón-de-Guevaraa,b, A. Boscáa,b, J. Pedrósa,b, E. Climent-Pascualc, A. de Andrésc, F. Callea,b, ⁎ J. Martíneza,d, a Instituto de Sistemas Optoelectrónicos y Microtecnología, Universidad Politécnica de Madrid, Av. Complutense 30, Madrid 28040, Spain Departamento de Ingeniería Electrónica, E.T.S.I de Telecomunicación, Universidad Politécnica de Madrid, Av. Complutense 30, Madrid 28040, Spain c Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco, Madrid 28049, Spain d Departamento de Ciencia de Materiales, E.T.S.I de Caminos, Canales y Puertos, Universidad Politécnica de Madrid, C/ Professor Aranguren s/n, Madrid 28040, Spain b ARTICLE INFO ABSTRACT Keywords: Graphene Graphene oxide Polyaniline Supercapacitor Specific capacitance Energy storage We report on the precise fabrication of low-cost high-performance electrochemical supercapacitors using reduced graphene oxide/polyaniline nanofiber composite electrodes. An infrared laser has been used to reduce the graphene oxide, converting the initial graphene oxide compact layer into a three dimensional open network of exfoliated graphene flakes. This highly conducting porous structure is very well suited for electrodepositing pseudocapacitive materials owing to its large surface area. Polyaniline nanofibers have been controllably electrodeposited on the graphene flake network, not only extending further the electrode surface area and providing it with a strong pseudocapacitance but also preventing the restacking of the graphene sheets during the subsequent device processing and charge-discharge cycling. The composite electrode presents a specific capacitance of 442 F g−1, as compared to 81 F g−1 for the bare reduced graphene oxide counterpart, and a capacitance retention of 84% over 2000 cycles. 1. Introduction Graphene has attracted increasing attention in recent years due to its excellent mechanical, optical and electrical properties. Its high theoretical specific surface area (SSA = 2630 m2 g−1) and high electrical conductivity make it an attractive material for many industrial applications. Also, it is a flexible transparent material that can be used for solar cells, light emitting diodes (LEDs, OLEDs), touchscreens and LCD displays [3,4], and in the near future, its flexibility will let to create foldable and wearable devices. In particular, as a consequence of the increasing demand for more efficient, longer-lasting and more compact portable electronic devices, the use of graphene in energy storage devices is one of the most promising applications for this material. Unlike conventional batteries and capacitors, the most common type of electrochemical capacitors (ECs) stores the charge in an electric double layer (EDL) formed at the interface between the electrode and the electrolyte, that is why they are called electric double layer capacitors (EDLCs). These energy storage devices offer larger energy densities than capacitors and higher power densities than batteries [6–8]. Since the charge accumulates at the electrode surface, EDLC electrodes should have high surface area and high porosity. Carbon materials are often used as electrode materials for EDLCs owing to their properties. Specifically, graphene has proven to be an outstanding electrode material for EDLCs due to its high surface area, high electrical conductivity and electrochemical stability [10,11]. In the case of pseudocapacitors, another class of ECs, the capacitance is driven by faradaic reactions (redox reactions, capacitive deionization, etc.) at the electrode surface. Pseudocapacitors can achieve higher specific capacitances than EDLCs but they offer lower power densities and poor electrochemical stability. Metal oxides and conductive polymers are typically used as pseudocapacitive electrode materials [12–15]. Among conducting polymers, polyaniline (PANI) has been widely studied as a pseudocapacitive material owing to its relatively high conductivity, low cost and easy synthesis which allows to produce structures at the nanometric scale [16,17]. The combination of an EDL formed at the electrode surface together with fast and reversible faradaic processes offer both high energy and power densities together with good cycling stability. Therefore, the synthesis of composite electrodes leading to a hybrid system between EDLCs and pseudocapacitors is being widely ⁎ Corresponding author at: Instituto de Sistemas Optoelectrónicos y Microtecnología, Universidad Politécnica de Madrid, Av. Complutense 30, Madrid 28040, Spain. E-mail address: [email protected] (J. Martínez). https://doi.org/10.1016/j.apsusc.2018.10.194 Received 20 June 2018; Received in revised form 11 October 2018; Accepted 22 October 2018 Available online 24 October 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved. Applied Surface Science 467–468 (2019) 691–697 A. Ladrón-de-Guevara et al. studied [18–21]. In this work, we report a 3-dimensional composite electrode based on laser-reduced graphene oxide (LrGO) on which an entangled mesh of PANI nanofibers is electrodeposited. The reduction of the graphene oxide (GO) is achieved by using an infrared laser. This is a single-step and scalable procedure which allows to make devices, circuits and even complex designs on different substrates without the need for chemicals, masks, transfer techniques, catalysts or other expensive equipment [22–26]. The PANI nanofibers are then selectively electrodeposited only on the areas previously irradiated by the laser, thus allowing to reproduce the predesigned pattern. The PANI-LrGO composite electrodes combines both the lightweight and high conductivity of the LrGO and the SSA and high pseudocapacitance of the PANI nanofibers. While the LrGO serves as an extended collector, the PANI nanofibers provide a large electrochemically active surface area. As a result, we show a significant increase in the specific or gravimetric capacitance (Cwt) of the LrGO-based ECs from 81 F g−1 up to 442 F g−1 when PANI nanofibers are electrodeposited on the LrGO electrodes. Furthermore, the PANI-LrGO composite electrodes present a capacitive retention of 84% after 2000 cycles. This work demonstrates that graphene-PANI nanofiber composite electrodes are promising candidates for the fabrication of high-performance ECs. (Sartorius SE2) with a capacity of 2.1 g and 0.1 μg resolution was employed to weigh all the samples. 2.4. Electrochemical measurements To evaluate the performance of the developed supercapacitors, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were performed with a potentiostat/galvanostat system (Autolab PGSTAT204) and Nova software. Electrochemical impedance spectroscopy (EIS) was also measured using the FRA32M frequency response analyzer module of the PGSTAT204 system, applying a sinusoidal signal with amplitude of 10 mV at open circuit over a frequency ranging from 100 kHz to 10 mHz. LrGO electrodes were made by cutting rectangular pieces of LrGO. To ensure a reliable electrical connection, copper foil was attached to the backside of the electrodes with conductive silver paint and covered with Kapton tape, defining an active area of ∼1 cm2 which is in contact with the electrolyte. A symmetric supercapacitor was formed by placing an ion porous separator (VWR Qualitative 413 filter paper) between two LrGO electrodes and an aqueous electrolyte of 1 M H2SO4 (Fig. 1c). A three-electrode configuration was used to evaluate the electrochemical response of the PANI-LrGO composite as the working electrode, a Pt mesh as counter electrode and an Ag/AgCl electrode (KCl 1 M) as reference. In the PANI-LrGO composite, the LrGO acts as an extended collector. No binder was needed, as the electrodeposited PANI ensured an intimate electrical contact with the LrGO. The electrochemical measurements were carried out in a 1 M H2SO4 aqueous solution. The capacitance of the devices was calculated from the GCD curves using the formula Ccell = I/(dV/dt), where I is the constant current applied and dV/dt is the slope of the discharge curve. In the twoelectrode configuration, the gravimetric capacitance Cwt was calculated according to the equation Cwt = 2 Ccell/m, where m is the mass of one electrode. In the three-electrode configuration, the factor 2 is removed in the above mentioned equation and m is the mass of the LrGO/PANI composite. The equivalent series resistance (ESR) is estimated by using the expression ESR = Vdrop/(2I), where Vdrop is the voltage drop produced after reversing the current flow in the GCD curves. 2. Experimental section 2.1. GO reduction A plastic foil of polyethylene terephthalate (PET) was coated using an aqueous dispersion of GO with the appropriate concentration (4 mg ml−1) which was obtained by the modified Hummer’s method. A density of 0.13 ml cm−2 of the solution was drop-casted onto the substrate, and it was dried overnight under ambient conditions after applying spin-coating in order to obtain a homogeneous coating layer. Apart from PET, other substrates such as aluminium foil or polydimethylsiloxane (PDMS) were used. The laser of a LightScribe recorder drive (λ = 788 nm, Pout = 5 mW) was used to effectively reduce the GO on top of the PET to LrGO at specific locations according to a desired pattern. This method has been shown to be useful not only for printing computer designed patterns directly on the GO, but also to produce large areas of reduced graphene by a single step, taking an average time of 25 min per cycle. The process is schematically shown in Fig. 1a. 3. Results and discussion As a result of the laser reduction process of GO, oxygen groups are removed causing the reestablishment of carbon networks so that LrGO was finally obtained as indicated by a color change from brown to black. The color is not the only feature that changes after laser irradiation, but the electrical properties also vary from a near insulating behavior of the GO film to highly conductive in the case of LrGO. As observed from I to V curves in Fig. 2a, the GO shows a nonlinear asymmetric behavior while the LrGO exhibits a linear I-V curve which is associated with a significant increase in the conductivity. In addition, the extent of those changes depends on the number of cycles performed and the greyscale used for each pattern causing the reduction process to be controlled in two ways, as illustrated in Fig. 2b. On the one hand, the more times the process is repeated, the lower the sheet resistance. On the other hand, the darker the pattern (i.e. lower R-G-B values), the higher the conductivity. Raman spectroscopy was used to understand the effects of the low energy infrared laser on the GO chemical structure. The Raman spectra for both GO and LrGO are shown in Fig. 2c. The three characteristic peaks D, G, and 2D, in order from left to right, can be observed in both spectra. The D band at ∼1360 cm−1 indicates the presence of sp3carbon bonds. In the LrGO spectrum, the D band suffers from a significant narrowing and decrease in intensity. With regard to the G band, there is a clear increase in intensity and narrowing in the LrGO Raman spectrum. It is also observed a peak shift from 1589 to 1581 cm−1. The decrease in the intensity ratio ID/IG from ID/IG ∼ 1 to 0.3 is a 2.2. Electrodeposition of PANI nanofibers PANI nanofibers were electrodeposited on top of the LrGO pattern using a three electrode system with a Pt mesh as the counter electrode and an Ag/AgCl reference electrode, following the method described by Pedrós et al.. The LrGO obtained through the laser reduction of GO was used as the working electrode, and the whole three electrode system was immersed in an electrolyte made of a mixture of HCl:CH3OH:aniline (1:0.5:0.2 M). First, the LrGO electrode was immersed in the mixture of HCl:aniline and the air trapped within the LrGO layers was removed using a membrane vacuum pump. The CH3OH was then added to the solution in order to avoid its evaporation during the previous air removal process. Finally, a DC potential of 0.8 V was applied during 9 min while stirring softly. The setup used for the three electrode system is schematically depicted in Fig. 1b. 2.3. Material characterization Morphological changes between the initial GO and the LrGO were assessed using a scanning electron microscope (SEM) (FEI Inspect F50). A confocal Raman spectrometer (λ = 488 nm) with a spot size of 1 μm was used to obtain chemical and structural information of the samples before and after the laser treatment. A high accuracy microbalance 692 Applied Surface Science 467–468 (2019) 691–697 A. Ladrón-de-Guevara et al. Fig. 1. (a) Scheme of the reduction process. Initially, GO is deposited on the flexible substrate by drop casting. Spin coating at low spin speeds is applied in order to obtain a uniform thin GO film. After allowing GO to dry during the night under ambient conditions, an IR laser is used to effectively reduce and the GO film producing LrGO patterns at specific locations. (b) Electrodeposition of the PANI nanofibers. The PANI-LrGO composites are prepared using a three-electrode system where the LrGO acts as the working electrode, an Ag/AgCl electrode (KCl 1 M) is the reference electrode (+0.235 V vs. a standard hydrogen electrode), and a Pt mesh is used as the counter electrode. The electrolyte used is a mixture of HCl:CH3OH:aniline (1:0.5:0.2 M) and a potential of 0.8 V was applied at room temperature. After the electrodeposition process, a homogeneous network of entangled PANI nanofibers coats all the surface of the LrGO filling the space among the sheets that form it. consequence of the removal of oxygen species causing the reestablishment of sp2-carbon bonds. This suggests that the amount of defects in LrGO has decreased and there is a lower concentration of structural edge defects, resulting in an increase in size of the graphene domains [28,29]. On the other hand, the appearance of a well-defined 2D band at ∼2710 cm−1 is the most significant change between both spectra. This is a consequence of the GO reduction and the value of the intensity ratio I2D/IG of about 0.6 indicates the presence of few-layer graphene [30,31]. Therefore, the analysis of the Raman spectra fully proves that treating GO with a low energy infrared laser results in an efficient, costeffective and controllable reduction process for producing few layer graphene. Morphological differences were also observed between GO and LrGO after the laser treatment. Fig. 2d shows a large-scale SEM image in which it is noticed that LrGO is a well expanded and exfoliated material while GO remains compact and stacked. This can be a consequence of a rapid release of gases generated during laser irradiation which caused the reduction, expansion and exfoliation of GO forming a three dimensional network with a high specific surface area (in the order of 1500 m2/g ). A higher magnification SEM image of LrGO is presented in Fig. 2e showing how the original compact and stacked material becomes expanded and exfoliated. In turn, this is also a fundamental aspect related to a high performance EC, since LrGO shows an open network which is more accessible for the electrolyte, thus improving the ion mobility. Therefore, after the reduction of GO, the obtained LrGO has shown excellent structural and electrical properties. Specifically, its high electrical conductivity (1.28 kΩ/sq) and its high specific surface area (∼1500 m2/g ) suggest that LrGO can be used as both the active electrode and the current collector of the EC. Two-electrode GCD as well as CV assessments were performed to study the electrochemical characteristics of the LrGO EC in 1 M H2SO4 aqueous electrolyte. Compared to the linear CV curve of the GO, the CV profiles of the LrGO are nearly rectangular in shape (Fig. 3a) which points to the nearly ideal capacitive behavior of the LrGO-based ECs. Moreover, a linear relationship is observed between the capacitive current, extracted from the CV profiles at a fixed voltage of 0.5 V, and the scan rate (Fig. 3b). This linear trend indicates the formation of an efficient EDL and, therefore, a fast ion transport within the EC. Fig. 3c shows the GCD curves obtained at different current densities. The nearly triangular shape of the GCD curves further confirms that an EDL is successfully formed. The voltage drop observed when the current flow is reverted indicates an ESR of ∼260 Ω. Fig. 3d shows the gravimetric capacitance calculated from the GCD curves as a function of the current density for both LrGO and GO. As can be observed, the electrochemical performance significantly increases after the laser treatment of the GO, and a Cwt of 81 F g−1 is obtained at a current density of 0.1 A/g. This result is comparable to those of other rGO-based ECs (Supplementary Table S2). EIS was also used to assess the higher electrochemical performance of the LrGO. The Nyquist plot in Fig. 3e shows that the impedance spectra of both LrGO and GO present a linear region at low frequencies. This trend in which the imaginary part of the impedance increases rapidly is associated with a capacitive behavior. The interception of the impedance with the real axis indicates the equivalent series resistance. Fig. 3f shows the Bode plot of the ECs. At a phase angle of 45°, where the resistive and capacitive impedances are equal, the characteristic frequency f0 of the LrGO-EC is 0.52 Hz. The corresponding time constant τ0 = 1/f0 is 1.92 s, as compared to the resistive behavior shown by 693 Applied Surface Science 467–468 (2019) 691–697 A. Ladrón-de-Guevara et al. Fig. 2. (a) I-V curves of GO and LrGO. The inset shows a magnified view of the I-V curve of GO. (b) Correlation between the sheet resistance of the LrGO and the two variables controlling the reduction process: the number of times that the sample is irradiated with the IR laser and the grayscale color determined by the Red-GreenBlue (RGB) values. (c) Raman Spectra of GO and LrGO. (d) SEM image of a top-view of GO before laser treatment (lower right corner) and LrGO. (e) Higher magnification SEM image of LrGO within the marked area in (d) showring the details of the expanded and exfoliated structure which results in an open network with a high specific surface area. Fig. 3. Electrochemical performance of symmetric GO and LrGO ECs in 1 M H2SO4 aqueous electrolyte: (a) Cyclic voltammograms of LrGO at scan rates of 5, 10, 20, 50 and 100 mV/s compared to that of GO electrode at scan rate of 100 mV/s. (b) Capacitive current extracted from the CV curves at 0.5 V as a function of the scan rate. A linear relationship is observed for the charge and discharge curves. (c) Galvanostatic charge-discharge curves of the LrGO EC at various current densities. The shortest processes are magnified in the inset. (d) Gravimetric capacitance of the LrGO and GO ECs as a function of the current density. (e) Nyquist and (f) Bode plots of the GO and LrGO ECs over a frequency range from 100 kHz to 0.01 Hz. The inset provides a magnified view of the high-frequency region. 694 Applied Surface Science 467–468 (2019) 691–697 A. Ladrón-de-Guevara et al. the GO. This improvement in the frequency response is due to the expansion and exfoliation of graphene sheets which enhances the ion transport within the electrodes. The inset of Fig. 3f shows the corresponding equivalent circuit (EC) used to fit and analyze the measured impedance spectra [32–35]: ESR is the equivalent series resistance which is the sum of the contact resistance, the intrinsic resistance of the electrode material, and the electrolyte resistance and CPE is a constant phase element associated with the double layer capacitance. The ESR values extracted for the LrGO and GO electrodes are 296 Ω and 6.8 kΩ, respectively, showing how the conductivity of the LrGO is improved after the laser irradiation. Moreover, it indicates that the expansion and exfoliation of graphene sheets enhances the ion access and transport within the electrodes (Supplementary Table S1). Therefore, the EIS measurements confirm that the laser treatment of GO plays an important role not only in increasing the electrical conductivity of the material, but also in making its surfaces accessible and in facilitating the electrolyte ion diffusion across its structure, thus improving the overall electrochemical performance of the ECs. However, considering these outstanding properties of LrGO as an electrode material for EC, the gravimetric capacitance obtained is not as promising as expected, especially at high current densities, where Cwt degrades considerably. This is a common setback in supercapacitors based on graphene-derived composites where the performance seems to be hindered by the aggregation and restacking of graphene sheets during processing and charge-discharge cycling most likely due to the strong π-π interaction among their basal planes [36,37]. As a consequence of this compacting process, the active surface area lowers and the pore size decreases hindering access to electrolytes. One of the methods to avoid this problem is the addition of pseudocapacitive materials such as conductive polymers. Moreover, the high surface area of the open and accessible structure of the graphene sheets make LrGO remarkably suitable for adding pseudocapacitive materials. The deposition of a pseudocapacitive material will take place all over the LrGO structure, thus considerable increasing the electroactive area and the specific capacitance of the final device. Here, LrGO was functionalized with electrodeposited polyaniline nanofibers coating all the surface of the LrGO homogeneously and filling the space between the LrGO flakes so that their restacking can be prevented and a large electrochemically active surface area is provided. A SEM image of the LrGO coated with a homogeneous network of PANI nanofibers is shown in Fig. 4a. The coating process is governed by the electrodeposition conditions which allow for controlling the extension of the PANI nanofibers as well as the density of nanofibers. As a result of the electrodeposition process, a PANI-LrGO composite electrode is assembled, which combines the remarkable electrical conductivity of the underlying LrGO acting as an extended current collector, with the rapid diffusion of the electrolyte ions into the PANI nanofiber/LrGO network due to its high specific surface area. The electrochemical performance of the assembled PANI-LrGO composite electrode was studied by three-electrode CV and GCD experiments in a potential window from −0.2 to 0.5 V with respect to the Ag/AgCl reference electrode while immersed in a 1 M H2SO4 aqueous solution. Fig. 4b shows the CV curves at different scan rates. When comparing with the CV curves obtained for LrGO electrodes, the much larger CV curve area indicates a stronger electroactivity and, thus, a larger specific capacitance of the PANI-LrGO composite electrode. In addition, this also demonstrates that the contribution of the pseudocapacitive material to the capacitance is larger than that of the LrGO, known to only contribute by means of the EDL capacitance principle. The pair of anodic (a1) and cathodic (c1) current peaks which appear at 0.35 and 0.01 V, respectively, at a scan rate of 5 mV/s are associated with the oxidation/reduction of leuco-emeraldine and emeraldine stages. The shift of these two peaks when increasing the scan rate can be attributed to an increase of the internal diffusion resistance within the pseudocapacitive material. However, the linear relationship between the peak current density and the scan rate (as shown in Fig. 4c) suggests that this redox process is not limited by the ion diffusion into the bulk owing to the high surface area to volume ratio of the PANI nanofibers. GCD curves at different current densities (Fig. 4d) were used to calculate the Cwt of the PANI-LrGO composite electrode, which are plotted in Fig. 4e. As a result of the electrodeposition of PANI nanofibers, a much higher capacitance than that of the bare LrGO electrode is obtained, as evidenced by a Cwt value of 442 F g−1 at a current density of 0.18 A/g. As the current density increases, the Cwt of the PANI-LrGO composite electrode decreases slowly. After increasing the current density up to 7.1 A/g, a Cwt value of 245 F g−1 was obtained demonstrating a capacitance retention of 55%. The cycling stability of the fabricated PANI-LrGO composite electrode was also tested. After 2000 charge-discharge cycles at a current density of 3.7 A/g, the electrode retains 84% of the initial capacitance maintaining its initial coulombic efficiency of around 80% (Fig. 4f). These results show the excellent stability of the PANI-LrGO composite electrode. The inset in Fig. 4f shows the CV curves of the PANI/LrGO composite electrode measured at a scan rate of 10 mV s−1 before and after the 2000 cycles. As can be observed, the a1/c1 peaks are smoothed after the cycling test. Moreover, there is also a shift of the a1 and c1 redox peaks to more positive and negative values, respectively. The flattening and shifting of the redox peaks is typically caused by an increment in the resistance of the PANI. Therefore, the electrodeposition of polyaniline is not only useful to avoid the restacking and compacting processes of the graphene sheets during processing and charge-discharge cycling, but it also enhances the electrochemical performance and makes the PANI-LrGO composite electrode suitable for high-performance supercapacitors due to the high pseudocapacitance, high conductivity, and large specific surface area of the PANI nanofibers which leads to high values of Cwt. In particular, the Cwt value obtained in this work is almost twice as high as those obtained when rGO sheets are directly coated by conducting polymers via in situ polymerization [41,42]. This demonstrates the importance of the controllable electro-polymerization process reported through which a homogeneous layer of PANI nanofibers is deposited to prevent the agglomeration of the graphene sheets. As reviewed by Moussa et al. , PANI-graphene composites have received much attention to develop supercapacitor electrodes. Although high-performance electrodes using 3D graphene nanostructures have been demonstrated, most of them involved complicated growth or processing techniques such as chemical, thermal and hydrothermal processes , etching of the substrates , transfer processes , alignment of PANI fibers , or the use of a third material in the electrode composite (ternary electrodes). Despite some of these processes could be automated , their complexity limits their suitability for mass production. The simple laser-driven reduction process of the GO and the controllable electrodeposition of the PANI nanofibers on the LrGO reported here therefore strongly simplifies the fabrication of composite electrodes while obtaining similar results (see Table S3 for a detailed comparison of figures of merit), paving the way for a cost effective large-scale fabrication route. In addition, the high cycling stability demonstrated by the PANI-LrGO composite electrode, better than most of the electrodes reviewed by Moussa et al. , indicates the efficient ionic and electronic transport within this hybrid composite. 4. Conclusions We have developed a simple, cost effective, and scalable method to manufacture graphene-based material from the reduction of GO by using an infrared laser. The reduction of GO is not only effectively made, but the graphene-based material produced by this technique, called LrGO, shows both high conductivity and SSA values which makes it promising for electrochemical electrodes. However, the restacking and aggregation of LrGO sheets during the charge-discharge cycling 695 Applied Surface Science 467–468 (2019) 691–697 A. Ladrón-de-Guevara et al. Fig. 4. (a) SEM image of the PANI-LrGO composite electrode. Three-electrode electrochemical characterization of the PANI-LrGO composite electrode immersed in 1 M H2SO4 aqueous electrolyte: (b) Cyclic voltammetry of PANI-LrGO electrode at scan rates of 5, 10, 50 mV/s compared to that of LrGO electrode at scan rates of 5 mV/s; (c) anodic peak potential (left axis) and current density (right axis) as a function of the scan rate; (d) Galvanostatic charge-discharge curves at different current densities. The shortest processes are magnified in the inset; (e) gravimetric capacitance as a function of the current density for the GO, LrGO, and PANI-LrGO electrodes. (f) Capacitance retention and coulombic efficiency of the PANI-LrGO electrode during cycling test. Capacitance was calculated from GCD curves at a current density of 3.7 A/g. The inset shows the CV curves of the PANI-LrGO composite electrode measured at 10 mV s−1 before and after the 2000 cycles. seems to hinder its electrochemical performance as indicated by the significant decrease of Cwt values at high current densities in the case of bare LrGO-based ECs. The functionalization of the LrGO flakes with electrodeposited PANI nanofibers not only avoids this problem but provides an enhanced electrochemical performance. The assembled PANI-LrGO composite electrode combines both the light weight and high conductivity of the LrGO, which acts as an extended current collector, and the high pseudocapacitance and large SSA of the PANI nanofibers leading to high Cwt values. Specifically, a Cwt value of 442 F g−1 at a current density of 0.18 A/g has been reported for the PANI-LrGO composite electrode, 4.5 times higher than the value obtained for the bare LrGO electrode measured at the same conditions. In addition, the PANI-LrGO composite electrode shows good electrochemical stability with a capacitance retention of 84% after 2000 charge-discharge cycles at a current density of 3.7 A/g. In summary, the combination of high Cwt along with its high cycling stability makes the PANI-LrGO composite an outstanding material for high performance ECs. Competing interests Declarations of interest: none. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.10.194. References B. Luo, S. Liu, L. Zhi, Chemical approaches toward graphene-based nanomaterials and their applications in energy-related areas, Small 8 (2012) 630–646. M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498–3502. X. Cao, Y. Shi, W. Shi, G. Lu, X. Huang, Q. Yan, Q. Zhang, H. Zhang, Preparation of novel 3D graphene networks for supercapacitor applications, Small 7 (2011) 3163–3168. Y. Huang, J. 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