High-performance cotton fabric-based supercapacitors consisting of polypyrrole/Ag/graphene oxide nanocomposite prepared via UV-induced polymerization

Cotton fabric (CF) fabricated by natural cellulose fibers has porous structure, abundant hydrophilic hydroxyl groups and good mechanical flexibility, which is a promising substrate for flexible fabric-based supercapacitors. Graphene oxide (GO) nanosheets were fixed on the CF by vacuum filtration, and then the pyrrole monomers and silver ions (Ag+) were adsorbed to the surface of GO/CF by π-π and electrostatic interactions, respectively. Polypyrrole/silver (PPy/Ag) nanoparticles were generated on the surface of GO/CF via in situ UV-induced polymerization, forming the flexible PPy/Ag/GO/CF electrodes. The electrodes combine three active materials (PPy, Ag and GO), which show good electrochemical performance. The electrode prepared under optimum conditions (UV irradiation time: 120 min; mass loading of GO on the CF: 3 mg cm−2) exhibits a high specific capacitance of 1664.0 mF cm−2. The flexible symmetric quasi-solid-state fabric-based supercapacitor (QFSC) based on the optimum electrode also keeps superior electrochemical performance and excellent mechanical flexibility, which has a great application prospect for wearable energy storage devices.

supercapacitors are particularly noticeable due to their environmental friendliness, fast charge/discharge rate, high power density, long cycle life and superior mechanical flexibility Jia et al. 2020;Keum et al. 2020;Xiong et al. 2021;Zhang et al. 2021). Especially, flexible fabric-based supercapacitors are regarded as the suitable energy storage device for wearable electronic devices because they are easy to integrate into cloths. In addition, the 3D porous structure and excellent mechanical flexibility of the fabrics are conducive to loading more active materials on the fabrics and improving the electrochemical and mechanical performance of the fabric-based supercapacitors (Barakzehi et al. 2019;Liu et al. 2020a). Compared with the synthetic fabrics consisting of man-made fibers, cotton fabric (CF) fabricated by natural cellulose fibers has good hydrophilicity, breathability and renewability, which is a promising candidate for flexible fabric-based supercapacitors (Li et al. 2019;Lv et al. 2019a;Wang et al. 2021;Zhang et al. 2019).
To fabricate flexible supercapacitors, graphene is often applied as the active material on account of its large specific surface area, good electrical conductivity, high mechanical robustness and outstanding cycling stability (Huang et al. 2018;Purkait et al. 2018;Wu and Shao 2021). However, the poor hydrophilicity, strong van der Waals forces and π-π interactions of graphene can have negative effects on its electrochemical performance. After oxidation, the obtained graphene oxide (GO) shows improved hydrophilicity and the abundant oxygen-containing functional groups of GO can have strong interaction with the hydroxyl groups of CF, which are advantageous to obtaining the flexible fabric-based supercapacitor with good electrochemical and mechanical performance by loading GO on the CF (GO/CF). Even so, limited by the electric double layer capacitance (EDLC) mechanism of GO, the specific capacitance of GO/CF needs to be further improved by introducing other active materials.
Polypyrrole (PPy), as a representative conducting polymer, is one of the most concerned pseudocapacitive materials for flexible supercapacitors in light of its good environmental stability, outstanding electrochemical properties, easy preparation, high conductivity and low cost (Kim and Shin 2021;Liu et al. 2019;Wang et al. 2020a;Zang et al. 2020). The composite electrodes containing pseudocapacitive PPy and electric double layer capacitive GO possess improved electrochemical performance due to the synergistic effect (Wang et al. 2018a(Wang et al. , b, 2020bZhang et al. 2017). To obtain PPy, UV-induced polymerization of pyrrole in the presence of silver ions (Ag + ) has been proven to be a simple and effective method Zang et al. 2016). This strategy can not only obtain PPy nanoparticles in a relatively short time, but also generate Ag nanoparticles simultaneously. The incorporation of Ag nanoparticles can improve the electrical conductivity and store energy via reversible redox reaction Liang et al. 2021), which can further enhance the electrochemical performance of the electrode materials.
In this study, GO nanosheets were anchored on the CF by vacuum filtration to obtain the flexible GO/ CF. Pyrrole monomers and Ag + ions were gathered on the surface of GO/CF due to the π-π interaction between GO and pyrrole monomers and electrostatic interaction between GO and Ag + ions. Then PPy/ Ag nanoparticles were generated on the GO/CF via in situ UV-induced polymerization, forming the flexible PPy/Ag/GO/CF electrodes. The as-prepared PPy/ Ag/GO/CF electrodes show superior electrochemical performance owing to the synergistic effect of the three active materials (PPy, Ag and GO). In addition, the strong interactions among components and high mechanical flexibility of CF endow the PPy/Ag/GO/ CF electrodes with outstanding mechanical flexibility and robustness. The effects of each component, UV irradiation time and mass loading of GO on the CF on the electrochemical properties were studied. The PPy/Ag/GO/CF electrode with the best electrochemical performance was further used to fabricate a flexible symmetric supercapacitor, exhibiting excellent mechanical flexibility and electrochemical performance.
Fabrication of graphene oxide/cotton fabric (GO/CF) CF (5 × 5 cm 2 ) was cut from old T-shirts and washed with ethanol to remove impurities before use. GO was synthesized according to the modified Hummers method , and then was dispersed in distilled water to obtain a GO suspension (2 mg mL −1 ). GO/CF was prepared by vacuum filtration and then dried at 80 °C for 12 h. The as-prepared GO/CF with 1, 2, 3 and 4 mg of GO on one square centimeter of CF are labeled as 1GO/CF, 2GO/CF, 3GO/CF and 4GO/ CF, respectively.

Fabrication of flexible PPy/Ag/GO/CF electrodes
The PPy/Ag/GO/CF electrodes were prepared by UVinduced polymerization. Typically, 0.25 mL of pyrrole was stirred in 25 mL of distilled water for 30 min in an ice-water bath, and then 10 mL of AgNO 3 solution (containing 0.612 g of AgNO 3 ) was added with stirring for 5 min. Then the mixture and GO/CF were transferred to a petri dish to perform the UV-induced polymerization (wavelength of the UV light: 365 nm). After UV irradiation for a certain time, the samples were washed with distilled water and ethanol, and dried at 40 °C in a vacuum oven for 12 h to obtain the PPy/ Ag/GO/CF electrodes. The final PPy/Ag/GO/CF electrodes are named as PPy/Ag/xGO/CF y , where x is the mass loading of GO on one square centimeter of CF (1, 2, 3 and 4 mg cm −2 ) and y means the UV irradiation time (60, 90, 120 and 150 min).
For comparison, traditional chemical polymerization of pyrrole was also performed to prepare the fabric-based electrode. In brief, 0.25 mL of pyrrole and 0.33 g of HNO 3 were stirred in 25 mL of distilled water for 30 min in an ice-water bath, and then 3GO/ CF was added into the mixture. Afterwards, 10 mL of APS solution (containing 0.83 g of APS) was slowly added into the mixture within 20 min, and the mixture was stirred for 8 h in an ice-water bath to obtain the PPy/3GO/CF electrode.
Assembly of flexible quasi-solid-state fabric-based supercapacitor (QFSC) Firstly, PVA/H 2 SO 4 gel electrolyte was prepared by dissolving 6 g of PVA in 60 mL of 1 M H 2 SO 4 solution at 85 °C and then cooled to room temperature. Two pieces of PPy/Ag3GO/CF 120 electrodes were immersed in the PVA/H 2 SO 4 electrolyte for 5 min and dried for 60 min at room temperature. The QFSC with a sandwich structure was assembled by pressing the above electrodes together (effective area: 1 × 1 cm 2 ).

Characterization
The morphology and structure of the samples were characterized by field emission scanning electron microscope (SEM, ZEISS Sigma 300), Raman spectroscopy (Thermo Scientific DXR), X-ray diffractometer (XRD, PANalytical X' Pert Pro) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The electrochemical properties of the electrodes and QFSC were tested by CS2350H electrochemical workstation (Wuhan Corrtest Instruments Co. Ltd.). The electrochemical calculations are given in the Supporting Information.

Results and discussion
Figure 1 schematically illustrates the fabrication process of the flexible PPy/Ag/GO/CF electrodes by UVinduced polymerization. The CF obtained from old T-shirts is woven by natural cellulose fibers, showing a porous microstructure (Fig. S1a). After vacuum filtration of GO suspension, GO nanosheets are coated on the CF (Fig. S1b) and have strong interactions with CF owing to the hydrogen bonds formed between oxygen-containing functional groups of GO and hydroxyl groups of CF. As confirmed in Fig. S2, no GO nanosheets fall off the CF even after ultrasonic treatment for 120 min. When GO/CF is submerged in the aqueous solution containing pyrrole and AgNO 3 , the pyrrole monomers are gathered on the surface of GO/CF due to π-π interaction between pyrrole and GO. Meanwhile, Ag + ions are also adsorbed onto the surface of GO/CF due to electrostatic interaction between Ag + and GO. The formation mechanism of PPy/Ag composite by UV-induced polymerization is shown in Fig. S3 (Zang et al. 2016). Pyrrole monomers are excited and undergo polymerization to form PPy when exposed to UV light, and Ag + ions are reduced to the metallic Ag nanoparticles at the same time, leading to the formation of flexible PPy/Ag/GO/ CF electrodes.
Raman spectra of 3GO/CF and PPy/Ag/3GO/CF 120 are displayed in Fig. 2a. For 3GO/CF, two marked peaks at 1346 cm −1 (D band) attributed to the defects and/or edges of carbon atom lattice and 1592 cm −1 (G band) assigned to the in-plane stretching vibration of sp 2 hybridized carbon appear . For PPy/Ag/3GO/CF 120 , the main peaks of PPy are confirmed, including ring in-plane deformation associated with radical cation at 974 cm −1 , symmetrical C-H in-plane bending at 1041 cm −1 , C-C in-ring stretching at 1340 cm −1 and C=C stretching of the PPy backbone at 1581 cm −1 (Lv et al. 2019b;Zuo et al. 2020). Additionally, the D band and G band of GO are shifted and overlapped with the peaks of PPy, which is resulted from π-π interaction between PPy and GO (Singu and Yoon 2018). Figure 2b shows the XRD patterns of 3GO/CF and PPy/Ag/3GO/CF 120 . The strong diffraction peak of 3GO/CF at 2θ = 10.6°  is ascribed to the (001) diffraction of GO. The interlayer distance between graphene layers of the GO is 0.86 nm according to Bragg's equation, which is wider than that of the corresponding graphite due to the presence of oxygen-containing functional groups (Fan et al. 2017;Saleem et al. 2018). For the XRD pattern of PPy/Ag/3GO/CF 120 , five peaks loaded at 38.0°, 44.2°, 64.3°, 77.3° and 81.5° are corresponded to the (111), (200), (220), (311) and (222) planes of Ag with face-centered cubic structure (JCPDS 87-0597) (Cai et al. 2017). It is indicated that Ag + ions have been reduced to metallic Ag nanoparticles during UV-induced polymerization process. Moreover, the diffraction peak of GO decreases because the 3GO/CF is covered by PPy/Ag nanoparticles.
The effects of UV irradiation time on the morphology and electrochemical properties of the electrodes are studied. Figure 4 shows the SEM images of PPy/ Ag/1GO/CF y electrodes with different UV irradiation time. PPy/Ag/1GO/CF 60 possesses plenty of holes because the PPy/Ag nanoparticles generated within 60 min are limited and cannot completely cover the surface of GO/CF. As the UV irradiation time extends, the generated PPy/Ag nanoparticles become more and more, and the surface morphology of PPy/ Ag/1GO/CF y electrode gets denser and denser. Specifically, the produced PPy/Ag nanoparticles on the 1GO/CF in 60, 90, 120 and 150 min are 0.9, 2.9, 3.5 and 3.9 mg cm −2 , respectively. The electrochemical Fig. 3 a XPS survey spectra of 3GO/CF and PPy/Ag/3GO/CF 120 and corresponding XPS spectra of b C1s, c N1s and d Ag3d in the PPy/Ag/3GO/CF 120 properties of PPy/Ag/1GO/CF y electrodes are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). As shown in Fig. 5a, the CV curves of PPy/Ag/1GO/CF y electrodes have a pair of redox peaks situated in ~ 0.56 V and ~ 0.48 V, which are associated with faradic reactions of Ag/ Ag + Wei et al. 2013). GCD curves of PPy/Ag/1GO/CF y electrodes (Fig. 5b) exhibit voltage platforms, which correspond to the CV curves. PPy/Ag/1GO/CF 120 electrode exhibits the largest CV curve area and longest discharge time, suggesting it has the highest specific capacitance. By calculation, the areal specific capacitance (C A, electrode ) and volumetric specific capacitance (C V, electrode ) of the PPy/ Ag/1GO/CF y electrodes at different current densities are shown in Fig. S4. Among them, the PPy/ Ag/1GO/CF 120 electrode has the highest C A, electrode of 690.3 mF cm −2 and C V, electrode of 11.4 F cm −3 at a current density of 0.5 mA cm −2 . When the current density increases to 5 mA cm −2 , the PPy/Ag/1GO/ CF 120 electrode still shows the highest C A, electrode of 237.9 mF cm −2 and C V, electrode of 3.9 cm −3 , but the capacitance retention is only 34.5%. The result demonstrates that 120 min is the optimum UV irradiation time for the preparation of PPy/Ag/GO/CF electrode with high specific capacitance. Although the produced PPy/Ag nanoparticles in 150 min are more than that in 120 min, the dense and compact structure of PPy/Ag/1GO/CF 150 will cause that less active sites can be exposed and the diffusion of electrolyte ions becomes more difficult. Nyquist plots of the PPy/ Ag/1GO/CF y electrodes shown in Fig. 5c are divided into two regions, including the semicircle at the highfrequency region and the straight line at the lowfrequency region. The PPy/Ag/1GO/CF 120 electrode possesses the lowest charge transfer resistance (R ct ) of 12.4 Ω, indicating that it has easy charge transfer at the electrode/electrolyte interface.
With more profound research, it is found that the mass loading of GO on the CF significantly affects the morphology and electrochemical properties of the electrodes. Figure 6 shows the SEM images of PPy/ Ag/xGO/CF 120 electrodes with different mass loading of GO on the CF. The produced PPy/Ag nanoparticles gradually increase and then decrease as the mass loading of GO on the CF increases. To be specific, the produced PPy/Ag nanoparticles on the 1GO/CF, 2GO/CF, 3GO/CF and 4GO/CF in 120 min are 3.5, 4.3, 6.3 and 5.1 mg cm −2 , respectively. The produced PPy/Ag nanoparticles in PPy/Ag/3GO/CF 120 reach the maximum value of 6.3 mg cm −2 , and the Ag, C, N and O elements are uniformly distributed (Fig. 6e-h). The GO has abundant oxygen-containing functional groups on its surface. The higher mass loading of GO on the CF means that more active sites are exposed, so more Ag + ions and pyrrole monomers are adsorbed on the surface of GO/CF to realize the high yield of PPy/Ag nanoparticles. But when the mass loading of GO on the CF is superabundant (4 mg cm −2 in this work), agglomeration happens due to the strong interactions between GO nanosheets, which leads to the Fig. 6 SEM images of a PPy/Ag/1GO/CF 120 , b PPy/ Ag/2GO/CF 120 , c PPy/ Ag/3GO/CF 120 and d PPy/ Ag/4GO/CF 120 ; EDS mappings of e Ag, f C, g N and h O of PPy/Ag/3GO/CF 120 appearance of cracks and the reduction of active sites. As a result, the produced PPy/Ag nanoparticles in PPy/Ag/4GO/CF 120 (5.1 mg cm −2 ) is lower than that of PPy/Ag/3GO/CF 120 .
The electrochemical properties of PPy/Ag/1GO/ CF 120 , PPy/Ag/2GO/CF 120 , PPy/Ag/3GO/CF 120 and PPy/Ag/4GO/CF 120 electrodes are shown in Fig. 7. PPy/Ag/3GO/CF 120 electrode exhibits the maximum value of CV curve area (Fig. 7a) and discharge time (Fig. 7b), indicating it has the best specific capacitance. The rate performance of PPy/Ag/xGO/CF 120 electrodes is shown in Fig. S5. The C A, electrode of PPy/Ag/3GO/CF 120 electrode at a current density of 0.5 mA cm −2 is 1664.0 mF cm −2 (C V, electrode : 27.0 F cm −3 ) and reaches 780.7 mF cm −2 when the current density is 5 mA cm −2 (C V, electrode : 12.7 F cm −3 ), with a capacitance retention of 46.9%. The high specific capacitance of PPy/Ag/3GO/CF 120 electrode is attributed to the highest content of PPy/Ag nanoparticles. In addition, PPy/Ag/3GO/CF 120 electrode has the smallest equivalent series resistance (R s , 3.7 Ω) and R ct (1.9 Ω) according to the Nyquist plots shown in Fig. 7c, which is also conducive to the electrochemical performance. The CV curves of 1GO/CF, 2GO/ CF, 3GO/CF and 4GO/CF shown in Fig. S6a present nearly rectangle shapes and the corresponding GCD curves shown in Fig. S6b display nearly symmetrical triangles, demonstrating the EDLC behavior of GO . Compared with PPy/ Ag/GO/CF electrodes, the specific capacitance of GO/CF is very limited. The maximum C A, electrode of GO/CF is only 14.4 F cm −2 at a current density of 0.5 mA cm −2 , which indicates that PPy/Ag nanoparticles have a major contribution to the capacitance. To better illustrate the effect of Ag nanoparticles, the electrochemical properties of PPy/3GO/CF electrode and PPy/Ag/3GO/CF 120 electrode are compared in Fig. S7. The CV curve of PPy/3GO/CF electrode is an approximately rectangular shape (Fig. S7a) and the GCD curve is an approximately symmetrical triangle (Fig. S7b), revealing its typical capacitive behavior . Significantly different from PPy/3GO/CF electrode, the CV curve of PPy/Ag/3GO/CF 120 electrode shows much larger enclosed area and the corresponding GCD curve has much longer discharge time than those of PPy/3GO/ CF electrode. The values of C A, electrode and C V, electrode of PPy/3GO/CF electrode are only 393.7 mF cm −2 and 6.5 F cm −3 at a current density of 0.5 mA cm −2 , respectively (Fig. S7c). The pair of strong peaks in the CV curve and the voltage platforms in the GCD curve are attributed to the reversible Ag/Ag + redox, indicating that the Ag nanoparticles can store/release charges and contribute to the capacitance Wei et al. 2013). Moreover, the presence of Ag nanoparticles in the electrode can increase the electrical conductivity and improve the ability of charge transfer. According to the Nyquist plots shown in Fig.  S7d, the R s and R ct of PPy/3GO/CF electrode are 8.2 Ω and 20.2 Ω, respectively, which are higher than those of PPy/Ag/3GO/CF 120 electrode.
To sum up, there are four reasons can be explained the excellent electrochemical performance of PPy/Ag/3GO/CF 120 electrode. (i) The 3GO/CF can provide more active sites to effectively adsorb more pyrrole monomers and Ag + ions on its surface to form more PPy/Ag nanoparticles. (ii) To perform the UV-induced polymerization for 120 min can achieve the high yield of PPy/Ag nanoparticles with appropriate compactness, which is Fig. 7 Electrochemical properties of PPy/Ag/xGO/CF 120 electrodes. a CV curves at 1 mV s −1 ; b GCD curves at 0.5 mA cm −2 ; c Nyquist plots beneficial to provide more active substances, expose more active sites and achieve fast ion diffusion. (iii) The Ag nanoparticles not only can effectively increase the electrical conductivity and improve the ability of charge transfer, but also can contribute to the capacitance through the reversible Ag/ Ag + redox. (iv) The synergistic effect of the three active materials (PPy, Ag and GO) is advantageous to improve the electrochemical performance of the electrode. Moreover, PPy/Ag/3GO/CF 120 electrode shows outstanding mechanical flexibility owing to the strong interactions among components and the high mechanical flexibility of CF. As shown in Fig.  S8, the tensile strength of PPy/Ag/3GO/CF 120 electrode is 5.42 MPa with a break elongation of 6.34%, which is even a little higher than that of CF (tensile strength: 5.26 MPa, break elongation: 5.84%). Two pieces of PPy/Ag/3GO/CF 120 electrode are further used to prepare the flexible QFSC (Fig. 8a). The CV curves of the QFSC show a pair of redox peaks at low scan rates, and the peaks gradually shift and become weak and wide with the increase of scan rate (Fig. 8b). To better understand the charge storage mechanism of the QFSC, the relationship between the total measured voltammetry charge (q v ) and inverse of the square root of the scan rate ( v −1∕2 ) is plotted in Fig. 8c. According to the electrochemical calculations given in the Supporting Information (Ardizzone et al. 1990), the outer surface charges (q o ) of the QFSC which is a non-diffusion controlled process, is calculated to be ~ 0.054 C cm −2 by linear fitting. The q v of the QFSC at 1, 2, 4, 5, 10 and 20 mV s −1 are 0.19, 0.16, 0.14, 0.13, 0.10 and 0.07 C cm −2 , respectively. The corresponding ratios of non-diffusion contribution at different scan rates are shown in Fig. 8d, which increases from 28.5% at 1 mV s −1 to 77.9% at 20 mV s −1 because the electrolyte ions don't have enough time to diffuse into and diffuse out of the inner part of the active materials at high scan rates. The GCD curves with good symmetry demonstrate that the QFSC has superior electrochemical reversibility and high Coulombic efficiency (Fig. 8e). Fig.  S9 shows the areal specific capacitance (C A, device ) and volumetric specific capacitance (C V, device ) of the QFSC at 0.5, 1, 2, 4 and 5 mA cm −2 . The maximum C A, device and C V, device can reach 286.6 mF cm −2 and 4.7 F cm −3 at 0.5 mA cm −2 , respectively, and C A, device and C V, device still can reach 187.2 mF cm −2 and 3.0 F cm −3 at 5 mA cm −2 , respectively, Fig. 8 Electrochemical properties of the QFSC based on the PPy/Ag/3GO/CF 120 electrode. a Structure schematic illustration; b CV curves at different scan rates; c The dependence of q v on v −1∕2 ; d The ratios of non-diffusion contribution at different scan rates; e GCD curves at various current densities; f Nyquist plots revealing its high specific capacitance and excellent rate performance. And the values of R s and R ct obtained from the Nyquist plot in Fig. 8f are 5.3 Ω and 2.4 Ω, respectively.
The Ragone plots of the QFSC are shown in Fig. 9a. The maximum energy density is 25.5 μWh cm −2 at a power density of 101.6 μW cm −2 , and the maximum power density is 1149.5 μW cm −2 at an energy density of 16.6 μWh cm −2 . It is superior compared with other reported similar flexible supercapacitors, such as rGO/PPy (Barakzehi et al. 2019), graphene/nanotube/PANI (Liu et al. 2020b), PPy CNT/cotton yarn plywood (Hao et al. 2021), PPy/graphite/newspaper , PANI/ PPy/graphite/gold-coated sandpaper (Alcaraz-Espinoza and de Oliveira 2018), and PPy/gold-coated surgical mask (Zuo et al. 2022). Also, the capacitance retention of the QFSC is 90.5% after 10,000 GCD cycles (Fig. 9b). The good cycling stability may be attributed to two reasons: (1) No active materials peel off during GCD cycling process due to the strong interactions among components; (2) The flexible substrate (CF) can ease the structural breakdown of PPy caused by repetitive insertion and de-insertion of counter ions. In addition, the QFSC exhibits outstanding flexibility. As shown in Fig. 9c, the CV curves are negligibly changed at different bending curvatures of 60°, 120° and 180° compared with its normal state. Surprisingly, after 10,000 bending cycles at a bending angle of 180°, the capacitance retention is still as high as 89.7%, which is ascribed to that the 3GO/CF can provide robust and flexible support for PPy/Ag nanoparticles to maintain the electrode integrity. At the same time, the working voltage and capacitance can be adjusted by connecting multiple QFSCs in series and in parallel to meet the practical requirements. Figure 9e shows the GCD curves of multiple QFSCs connected in series and in parallel. The voltage window can extend to 2.4 V by connecting three QFSCs in series, and the discharge time increases three folds while the working voltage is unchanged by connecting three QFSCs in parallel. For example, as shown in Fig. 9f, three QFSCs connected in series can power the electronic clock effortlessly, indicating the as-prepared QFSC is a good candidate for wearable electronics. Fig. 9 Electrochemical performance of the QFSC based on the PPy/Ag/3GO/CF 120 electrode. a Ragone plots; b Cycling stability at 8 mA cm −2 ; c CV curves at 10 mV s −1 under different bending angles; d Capacitance retention during 10,000 bending cycles at a bending angle of 180°; e GCD curves of QFSCs connected in series and in parallel; f Digital image of an electronic clock powered by three QFSCs connected in series

Conclusion
In summary, the flexible PPy/Ag/GO/CF electrodes can be conveniently obtained by efficient UV-induced polymerization. The incorporation of Ag nanoparticles can effectively enhance the electrical conductivity and contribute to the capacitance through the reversible Ag/Ag + redox. The optimum UV irradiation time and the mass loading of GO on the CF are determined to be 120 min and 3 mg cm −2 , respectively. The corresponding PPy/Ag/3GO/CF 120 electrode exhibits a high specific capacitance of 1664.0 mF cm −2 (27.0 F cm −3 ) owing to the high yield of PPy/Ag nanoparticles with appropriate compactness and the synergistic effect of PPy, Ag and GO. The asassembled QFSC based on the PPy/Ag/3GO/CF 120 electrode possesses maximum specific capacitance of 286.6 mF cm −2 (4.7 F cm −3 ), energy density of 25.5 μWh cm −2 and power density of 1149.5 μW cm −2 . In addition, the device retains 89.7% of its original capacitance after 10,000 bending cycles, exhibiting its excellent mechanical flexibility and electrochemical stability. In light of the outstanding electrochemical performance and feasible preparation process, this study offers an effective method to obtain flexible and wearable energy storage device with high performance.