Electrochimica Acta 285 (2018) 221e229

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Electrochimica Acta

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Inesitu hybridization of polyaniline nanofibers on functionalized reduced graphene oxide films for high-performance supercapacitor

* Kai Jin a, b, Weijie Zhang a, Yixuan Wang a, Xinli Guo a, b, , Zhongtao Chen a, Long Li c, ** Yao Zhang a, Zengmei Wang a, Jian Chen a, Litao Sun b, d, Tong Zhang d, a Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China b Center for Advanced Carbon Materials, Southeast University, Jiangnan Graphene Research Institute, Changzhou 213100, China c Yinbang Clad Material Co., Ltd, Wuxi 214145, China d School of Electronic Science & Engineering, Southeast University, Nanjing 210096, China article info abstract

Article history: High-performance hybrid supercapacitor is still hampered by lacking of proper electrode materials of Received 11 June 2018 desired nanostructures. However, the hybridization of polyaniline (PANI) on functionalized reduced Received in revised form graphene oxide (FrGO) could form such desired nanostructure with uniform graphene distribution, high 28 July 2018 carriers density and significantly improved cycling stability and rate capability of PANI. Here we report a Accepted 30 July 2018 facile process for the hybridization of polyaniline nanofibers (PANI NFs) on functionalized reduced Available online 2 August 2018 graphene oxide (FrGO) films by filtering the hybrid suspension of graphene oxide (GO) and in-situ polymerized PANI NFs followed by hydrothermal treatment to reduce GO and functionalization Keywords: fi fl Polyaniline nanofiber/functionalized of rGO. The as-prepared PANI NFs/FrGO composite lms were uniform, exible and stable with a high fi 1 1 reduced graphene oxide (PANI NFs/FrGO) speci c capacitance of 692.0 F/g at 1 A g and an excellent capacitance retention of 53.5% at 40 A g . composite films Furthermore, the assembled all-solid-state supercapacitor using the as-prepared composite films as 1 1 In-situ hybridization electrodes exhibited a high capacitance of 324.4 F/g at 1 A g and an energy density ~16.3 Wh kg at a Supercapacitor power density of 300 W kg 1, showing a great promising in the applications for portable power and Sulfur functionalization flexible supercapacitor. Flexible © 2018 Elsevier Ltd. All rights reserved.

1. Introduction the graphene films' layer-like structure has a spontaneous ten- dency of stacking and agglomeration, resulting in a relatively low Supercapacitors are considered as one of environmentally- surface area and specific capacitance. In contrary, polyaniline friendly energy-storage devices due to their advanced character- (PANI), a kind of conducting materials with a controllable istics including high power density, long cycling life and low morphology by varying polymerization conditions exhibits high maintenance cost. In particular, flexible all-solid-state super- pseudocapacitive performances [8e11]. He et al. [12] synthesized capacitors have attracted increasing attention due to its application various PANI nanostructures with spherical, tubular, belt-like, in portable, wearable and flexible electronics [1e5]. Two- polyhedral and dendritic morphologies by tuning the preparation dimensional graphene films possess notable features including conditions. Pan et al. [13] synthesized PANI with the morphology flexibility, large surface area, high electrical conductivity, great varying from fiber to dendrite fibers, plates and spheres by tuning double-layer capacitance property and excellent rate capability. the concentration of HCl. The difference in the morphologies of the Therefore, the graphene films are considered as a candidate of products is due to the difference in the relative rate of chain electrodes in flexible all-solid-state supercapacitors [6,7]. However, propagation to spontaneous nucleation, which is influenced by various parameters including the concentration of , the mole ratio of and initiator and the kind of solvent, etc. Compared with particles and sheets, PANI nanoneedles, nanorods and nano- * Corresponding author. Jiangsu Key Laboratory of Advanced Metallic Materials, fibers have been demonstrated a significant enhancement on the School of Materials Science and Engineering, Southeast University, Nanjing 211189, China. energy storage properties due to their higher surface area and ** Corresponding author. better ion-diffusion pathway [14]. However, PANI has poor cycling E-mail addresses: [email protected] (X. Guo), [email protected] (T. Zhang). https://doi.org/10.1016/j.electacta.2018.07.220 0013-4686/© 2018 Elsevier Ltd. All rights reserved. 222 K. Jin et al. / Electrochimica Acta 285 (2018) 221e229 stability and rate capability as the result of the swelling and and PANI/GO composites films were transferred to a Teflon-lined shrinking during the charge/discharge process. Thus, to combine autoclave (100 mL) and heated at 180 C for 10 h. After cooled to the advantages of graphene and PANI and overcome their draw- room temperature, the PANI/FrGO composites films were backs, hydride films of them have been developed. Zhang Kai [15] immersed in 10 mL of 1 M HCl solution containing 0.06 g of APS for reported that the PANI NFs/GO composite films exhibited a specific 10 h to re-oxidize the PANI. The as-prepared hybrid films were capacitance of 480 F/g at a current density of 0.1 A/g. Meng et al. washed with distilled water and then freeze-dried to get PANI NFs/ [16] prepared a graphene film that was composed of PANI nanowire FrGO composites films. For comparison, the PANI NFs/rGO com- arrays; the modified film had a specific capacitance of 385 F/g at 0.5 posites films were also prepared without adding thiourea to func- A/g. Zhang et al. [17] prepared a 3D structure graphene/PANI tionalize rGO during above mentioned process. composite films by hydrothermal treatment, which exhibited a specific capacitance of 1182 F/g at 1 A/g. 2.3. Construction of supercapacitor electrodes and properties In addition, recent reports have indicated that functionalizing measurement graphene with chemical moieties such as nitrogen, sulfur and bo- ron has an enhanced electrical conductivity and capacity due to the For the three-electrode testing system, the as-prepared films introduction of additional n-type or p-type carriers in carbon sys- were cut into 1 1cm2 rectangular slice and then pressed on tems or groups which provide short ion diffusion paths and exhibit stainless steel mesh under 20 MPa for 5 min and used as electrodes pseudocapacitive performances [18e20]. Hydrothermal treatment for electrochemical performance tests. The loadings percentage of is an effective method to reduce GO and functionalize graphene PANI of PANI NFs/FrGO-100, 300, 500 and 700 is calculated as because it's high pressure and high temperature. Moreover, unique 28.3%, 32.4%, 49.5% and 61.3%, respectively and their mass areal porous structure can be constructed during the hydrothermal density is calculated as 0.64, 0.71, 0.91 and 1.12 mg/cm2, respec- treatment because of the pathway of water steam and O2 released tively. Platinum wire was adopted as the counter electrode and from reduction of GO [21]. Hao et al. [22] prepared boron-doped C(K2SO4) electrode served as the reference electrode. 1 M H2SO4 rGO/PANI which achieved a specific capacitance about 241 F/g at solution was used as the electrolyte for the electrochemical 0.5 A/g. Liu et al. [17] prepared nitrogen-doped 3D reduced gra- measurements. phene oxide/polyaniline composite, which exhibited a specific For the two-electrode testing system, a flexible all-solid-sate capacitance of 282 F/g at 1 A/g. supercapacitor device was assembled. All composite films were In this work, we report a novel PANI NFs/FrGO composite film cut into 1 4cm2 rectangular strips and pressed on carbon cloth to performed by a facile process of filtering the hybrid suspension of form flexible electrodes. The solid electrolyte was prepared by GO and in-situ polymerized PANI NFs followed by hydrothermal heating the mixture of 6 g H2SO4, 6 g PVA and 60 mL H2Oto80 Cin treatment to reduce GO and sulfur functionalization of rGO. The as- water bath with stirring. The heated sol electrolyte was poured on prepared composite films were uniform, flexible, stable and the electrodes. A piece of cellulosic paper, which served as sepa- exhibited high performance for the potential applications using as rator, was immersed into the heated electrolyte for 0.5 h. After the electrodes in portable power and flexible supercapacitor. electrodes and separator were cooled down, pressing them together like a sandwich structure. 2. Experimental The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) of the 2.1. Preparation of PANI NFs/GO composite films as-prepared composites films were measured by using electro- chemical workstation system. CV tests were measured at scan rates PANI NFs/GO composite films were prepared by filtering hybrid ranging from 10 to 100 mV/s within a voltage range of 0.6 Ve0.4 V suspension of GO and PANI which were synthesized by in-situ (vs. SCE) and galvanostatic charge/discharge curves were recorded polymerization of aniline in a suspension of graphene oxide in at current density range of 1e50 A/g. EIS was conducted by acidic solution. Typically, 10 mg GO obtained from modified Hum- applying an AC voltage with a 5 mV amplitude in a frequency range mers methods was dissolved in 40 mL 1 M HCl solution and then from 0.02 Hz to 100 kHz under open circuit potential conditions. the solution was treated with ultrasonic dispersion for 1 h to ensure The specific capacitance from CV plots can be calculated by the uniformity of the dispersion. The aniline was dissolved in 1 M using the following equation: HCl at varying concentration. Then the aniline solution was added Z in the well dispersed GO solution slowly while maintaining ImdV vigorous magnetic stirring. Another solution of ammonium per- Csp ¼ oxydisulfate (APS), with a mole ratio to aniline of 1:4 in 1 M HCl was Vv prepared. Then, these solution were kept in refrigerator for 1 h. The where C is the specific capacitance in F/g, I is the current density APS solution was rapidly poured to the hybrid suspension of GO and sp m (A/g), !IdV is the area under the backward CV curve, V is the po- aniline followed by vigorous shaking for 30 s. The resulting solution tential range and v is the scan rate (mV/s). was kept at 0 C in refrigerator for 12 h. The circular PANI/GO The specific capacitances of the electrodes in three-electrode composite films with a diameter of ~4 cm and a thickness of ~7 mm testing system from GCD measurements can be calculated by us- were prepared by filtering the green resultant hybrid suspension ing the following equation: and then immersed into ethanol and distilled water for 1 h, fi respectively. Four kinds of composite lms PANI NFs/GO-100, PANI I t C ¼ m NFs/GO-300, PANI NFs/GO-500 and PANI NFs/GO-700 were pre- sp DV pared at the weight ratio of 100%, 300%, 500% and 700% of aniline to graphene oxide, respectively. where DV is the potential sweep in V, t is the discharge time. As for symmetrical two-electrode configuration, the specific capacitance 2.2. Preparation of PANI NFs/FrGO composite films of the device can be calculated by the following equation:

The as-prepared PANI/GO composites films were immersed in 2Imt C v ¼ the 80 mL solution containing 1 g thiourea for 1 h. Then the solution sp de ice DV K. Jin et al. / Electrochimica Acta 285 (2018) 221e229 223

The efficiency of materials is very often presented as coulombic composite films. Typically, HCl was used as doping proton , efficiency (h), determined by the ratio of Qdischarge to Qcharge, which which caused the inter-molecular and inner-molecular delocaliza- can be calculated from: tion in PANI and improved the conductivity [23]. Aniline monomers were adsorbed to GO substrate during the long time stirring by It p p Q ¼ hydrogen bonds, - stacking, Van der Waals' forces and dehy- m dration. While the aniline monomers adsorbed to GO substrate played a role of anchor sites and enabled the subsequent in-situ Q h ¼ discharge ¼ td polymerization of PANI NFs attaching tightly on the surface of GO Q charge tc [24]. In addition, during the ultrasonic dispersion, GO was activated in high centration HCl solution and increased active sites were The power density and the energy density can be calculated by formed. APS was added to induce aniline polymerization. After the following equations: hydrothermal treatment, APS solution immersion was necessary because parts of PANI NFs were reduced from highly conductive CDV2 E ¼ oxidized emeraldine base (EB) state to the reduced neutral leu- 2 coemeraldine (LB) state during the hydrothermal process for the reducibility of thiourea [25]. Thus, reoxidation and reprotonation of E P ¼ proton acid happened during the process of immersion in APS so- t lution and resulting in EB state PANI NFs. The thiocarboxylic acid ester was formed by thiourea reducing and doping due to the thiol- carboxylic acid esterification between carboxyl in GO and thione 2.4. Microstructure characterization (C]S) in thiourea [16]. Raman spectra of the PANI NFs/GO, PANI NFs/rGO, PANI NFs/ Raman spectra (Thermo Fish) and Fourier transform infrared FrGO samples were showed in Fig. 2 (a). To distinguish the peaks spectroscopy (FT-IR, Nicolet 5700 Fourier-IR spectrometer) are used clearly, a spectrum of PANI NFs/FrGO-500 was separately showed in for qualitative analyses on the components composition. The Fig. 2 (b). We found that there were two dominant D bands and G morphologies, microstructures and chemical constitution of sam- bands. The D band is associated with the sp3 domain structure of ples were characterized by scanning electron microscopy (SEM). disorder carbon atoms, while the G band comes from the in-plane The distribution of elements was characterized by energy disper- vibration of the graphitic structure [26]. The intensity ratio of the D sive x-ray spectroscopy (EDS). and G bands (ID/G) can be used to evaluate the degree of disorder which represents the reduction of GO. The ID/G values of PANI NFs/ 3. Results and discussion GO-500, PANI NFs/rGO-500 and PANI NFs/FrGO-500 were calcu- lated as 0.89, 1.15 and 1.01, respectively. The higher ID/G value rep- Fig. 1 illustrates the preparation process of PANI NFs/FrGO resented more defects and disorders in rGO generated in the

Fig. 1. Schematic showing the preparation process of PANI NFs/FrGO composite films. 224 K. Jin et al. / Electrochimica Acta 285 (2018) 221e229

Fig. 2. (a) Raman spectra of PANI NFs/GO, PANI NFs/rGO and PANI NFs/FrGO, (b) Raman spectra of PANI NFs/FrGO-500, (c) FTIR spectra of PANI, rGO and PANI NFs/FrGO-500. reduction process of GO due to the removal of oxygen functional bonding of PANI with rGO. groups [14,27]. The reason that ID/G of PANI NFs/FrGO-500 was The morphology and structure of rGO and PANI, PANI NFs/GO, smaller than PANI NFs/rGO-500 is probably attributed to the de- PANI NFs/rGO and PANI NFs/FrGO samples were shown in Fig. S1 (a- fects generated by replacement of oxygen functional groups with d), respectively. The typical porous structure for the rGO, PANI NFs/ sulfur functional groups. It was noted that there is a slight blueshift GO, PANI NFs/rGO and PANI NFs/FrGO samples were formed by the 1 of G band from 1580 to 1591 cm for the samples of PANI NFs/rGO- releasing of water steam and O2 during the hydrothermal treat- 500 and PANI NFs/FrGO-500. This was caused by the increase of ment process. Such kind of porous structure are believed to be sulfur functional groups linked with benzene ring of rGO [28]. The helpful in improving the electrolyte infiltration and the electrical PANI bands at 415, 521, 804, 1341 and 1471 cm 1 are correspond to contact of PANI on rGO substrate for the electrode application in the out-of-plane CeH wag of quinoid ring, out-of-plane CeNeC double-layer capacitance. The PANI NFs can not be observed on the þ torsion, CeH out-of-plane bending of quinoid ring, CeN stretching surface of composite films, probably due to the vigorous water of semiquinone radical, C]N stretching vibration of quinoid ring, steam generated during the hydrothermal treatment destroyed the respectively [23]. It was observed that the intensity of PANI Raman PANI NFs outside. spectra increases with the increase of aniline content, which indi- The chemical constitution of PANI NFs/FrGO composite films cated that aniline was successfully polymerized. The bands of PANI were typically identified by EDS mapping, as shown in Fig. S2(a). in spectra of the PANI NFs/rGO-500 and PANI NFs/FrGO-500 are Except of carbon, oxygen and nitrogen elements (see Figs. S2(bed)), weaker than those in spectra of PANI NFs/GO-500, which may be the sulfur element was also clearly observed (see Fig. S2(e)), which due to the loss of parts of PANI NFs caused by its' weak bonding is consistent with the results of Raman spectra of Fig. 2(aeb) that with graphene substrate. sulfur functional groups were homogeneously doped in graphene. The FT-IR spectra were showed in Fig. 2(c). Peaks at 3400, 1569 The morphologies of PANI NFs/FrGO-500 composite films were and 1176 cm 1 can are assigned to O-H in -COOH, C]C in benzoid characterized by SEM, as shown in Fig. 3. Short PANI NFs arrays on units and C-H bending in benzenoid ring, respectively for all the the surface of rGO nanosheets and long PANI NFs on edges of rGO spectra [29]. The new peaks at 1569, 1485, 1297, 1117 and 801 cm 1 nanosheets can be observed in one image (see Fig. 3(a)). While were observed for both PANI and PANI NFs/FrGO-500 spectra. magnified images of long PANI NFs and short PANI NFs arrays were While the peaks at 1569 and 1485 cm 1 are attributed to the C]C shown in Fig. 3(b) and (c), respectively. The long PANI NFs grew and C]N stretching modes of vibration for the quinoid and benzoid disorderly and interlinked with the edge of graphene sheets. The units, the peak at 1297 cm 1 is due to C]N stretching, the peak at PANI NFs' length and diameter were about 500 nm and 30 nm, 1120 cm 1 is assigned to the characteristic mode of the quinoid unit respectively. The growth of long PANI NFs was attributed to the and the peak at 800 cm 1 is due to the C-H out-of-plane bending of massive broken bonds and oxygen functional groups on the edge of 1,4 disubstituted benzene [30], which demonstrated the successful graphene sheets, which are beneficial for the adsorption of aniline K. Jin et al. / Electrochimica Acta 285 (2018) 221e229 225

Fig. 3. SEM image of PANI NFs/FrGO-500 (a), arrays of long PANI NFs (b) and short PANI NFs (b). monomer [31]. In addition, it was supposed that aniline monomers agglomerated PANI coating at an excessive aniline contents of 700 were difficult to enter the space between GO nanosheets during the (see Fig. 4(c) and (d)). Because a narrow space between GO nano- standing polymerization process. Because GO nanosheets in acid sheets and a high concentration of aniline tended to allow the solution always tends to stack layer by layer. When polymerization lateral-directional polymerization rather than directional fiber- began, a balance was achieved between aniline concentration growth, resulting in agglomerated PANI coating, which covered gradient and repulsion force of p-p stacking, resulting in an accu- the surface of graphene sheets and destroyed the porous structure. mulation of aniline monomers on the edge of graphene sheets. Fig. 5(a) shows the CV curves of PANI NFs/GO, PANI NFs/rGO and PANI NFs were attached with rGO nanosheets because aniline PANI NFs/FrGO samples. Two couples of redox peaks were monomers on rGO nanosheets served as nucleation centers, which observed, which can be ascribed to the leucoemeraldine/emer- reduced interfacial energy between nucleation centers and aniline aldine and emeraldine/pernigraniline transition of PANI, indicating solution and promoted fiber-growth process [32]. In summary, the the main pseudocapacitance behavior of the hybrid films [33]. PANI preparation mechanism of PANI NFs/FrGO composite films was was stable during the hydrothermal treatment due to the high schematically shown in Fig. S3. bonding between in-situ polymerized PANI and graphene sheets. Fig. 4 shows the morphologies of PANI with different aniline The other peaks in the curve of PANI NFs/FrGO are attributed to the contents. Comparing with Fig. 3, it can be seen that with the in- functionalization of sulfur, indicating the improvement in capaci- crease of aniline content, the morphologies of short PANI NFs arrays tance due to the redox reaction between thiocarboxylic acid ester on rGO sheets became denser and longer, finally resulting to and sulfone. In addition, the CV curve of PANI NFs/FrGO exhibits a

Fig. 4. SEM image of PANI NFs/FrGO-100 (a) and (b), and PANI NFs/FrGO-700 (c), and (d). 226 K. Jin et al. / Electrochimica Acta 285 (2018) 221e229

Fig. 5. (a) CV curves and (b) Nyquist plots of different samples with the same weight ratio of aniline to GO of 500, and direct observation of surface wettability for (c) PANI NFs/rGO- 500 and (d) PANI NFs/FrGO-500 composite films. more rectangular shape than unfunctionalized PANI NFs/rGO due to with different aniline mass ratio at 10 mV/s and 1 A/g, respectively. its larger double-layer capacitance. The electrolyte ion transport The specific capacitance of PANI NFs/FrGO-500 was calculated by and capacitance behaviors of functionalized and unfunctionalized GCD curve and gave the highest value 692.0 F/g and 629.7 mF/cm2. samples were further investigated by EIS. As shown in Fig. 5(b), the Because agglomerated PANI led to a lower specific surface area with obtained Nyquist plots include two parts of high-frequency region higher aniline mass ratio. and low-frequency region. The high-frequency region of Nyquist The GCD curves of PANI NFs/FrGO-500 at different current plots showing small incomplete semicircles is attributed to the density are shown in Fig. 7(a) and Fig. 7(b). All the curves show a impact of charge-transfer resistance. It's obvious that unfunction- deviation from the ideal triangular shape due to the contribution of alized PANI NFs/rGO exhibits a larger solution resistance of 1.0 U faradic reaction between PANI and electrolyte. Furthermore, the and a charge-transfer resistance of 3.6 U according to the contact good symmetry of the GCD curves indicate a large double-layer point between curves and x axis and diameter of semicircles, capacitance. This is attributed to the large specific surface area respectively. The low-frequency region of Nyquist plots involves and the efficient ions diffusion caused by hydrothermal treatment Warburg impedance which is about mass-transfer resistance. The and sulfur functionalization, respectively. The specific capacitance curves slope of PANI NFs/FrGO composite films is larger than that of and coulombic efficiency at different current densities were shown PANI NFs/rGO composite films, indicating that the capacitive per- in Fig. 7(c). The specific capacitance and coulombic efficiency show formance of PANI NFs/FrGO composite films is much closer to an a plateau varying from 15 A g 1 to 40 A g 1, owing to the combi- ideal supercapacitor. The active sites provided from sulfur func- nation of several capacitive components. When the current density tional groups might enhance the diffusion rate of active ions. The was as high as 40 A/g, the specific capacitance was 370.1 F/g, big difference of double-layer capacitance and diffusion rate of exhibiting an excellent capacitance retention of 53.5%, while the active ions between PANI NFs/FrGO and PANI NFs/rGO can be coulombic efficiency stayed about 94% at high current density. explained by Fig. 5(c) and (d) as well. The surface modification These results indicate that the combination of functionalized rGO induced by the introduction of sulfur functional groups made the and PANI can effectively enhance the rate performance compared surface hydrophilic and therefore facilitated efficient ion diffusions with bare PANI, which generally only maintains a capacitance of aqueous electrolytes throughout the film electrodes, which retention lower than 50% at high current density [17,35]. A high enlarged the actual contact area between electrolytes and elec- coulombic efficiency at high current density represents an increase trodes [34]. in kinetic quasi-reversibility, which is attributed to remarkable Fig. 6 demonstrates the CV and GCD curves of PANI NFs/FrGO double-layer capacitance and decreased redox reaction of PANI. K. Jin et al. / Electrochimica Acta 285 (2018) 221e229 227

Fig. 6. (a) CV curves and (b) GCD curves of PANI NFs/FrGO samples with different weight ratio of aniline to GO.

Fig. 7. (a) and (b) GCD curves of PANI NFs/FrGO-500 in different current density, (c) Specific capacitance and coulombic efficiency of PANI NFs/FrGO-500 in different current density, and (d) Capacitance retention after 1000 cycles of PANI NFs/FrGO-500 at constant current density of 10 A g 1.

Cycling stability of PANI NFs/FrGO-500 was conducted at 10 A/g hybrid film electrodes were assembled to evaluate their electro- for 1000 cycles (see Fig. 7(d)). The capacitance and coulombic ef- chemical performances. Fig. 8(a) demonstrates the CV curves ficiency of electrode remains 83.3% and 92.9% respectively at the recorded in different voltage windows at 10 mV/s, which exhibits a end of the cycling test, indicating a high electrochemical stability. pair of relatively obvious redox peaks due to the intercalation/de- Because the in-situ polymerization led to the tight contact between intercalation of PANI and sulfur functional groups. Fig. 8(b) shows rGO nanosheets and short PANI NFs arrays. The rGO nanosheets the GCD curves. The device exhibited a good specific capacitance, substrate not only constructed a porous structure but also main- calculated as 324.4 F/g at 1 A/g when it was charged to 1.2 V. The tained the stability of PANI during insertion/de-insertion process of coulombic efficiency under the different electrochemical window the faradic reaction. It is worth mentioned that there are no obvious of 0.4 V, 0.8 V and 1.2 V can be calculated as 62.0%, 72.8% and 55.1%. morphology changes for the electrode materials after the cycle test In the typical supercapacitor devices, the main reason for efficiency compared with those before the cycle test, which gives another loss would be dissipated heat and irreversibility of faradaic pro- identification for its good cycling property. cesses [36]. The coulombic efficiency reached a highest point when All-solid-state supercapacitor based on PANI NFs/FrGO-500 the electrochemical window was 0.8 V, because the main efficiency 228 K. Jin et al. / Electrochimica Acta 285 (2018) 221e229

Fig. 8. (a) CV curves of device in different voltage windows at 10 mV/s. (b) GCD curves in different voltage windows at 1 A g 1.

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