Figure 1 schematically illustrates the fabrication process of the flexible PPy/Ag/GO/CF electrodes by UV-induced polymerization. The CF obtained from old T-shirts is woven by natural cellulose fibers, which has hydroxyl groups and porous microstructure (Fig. S1a). After vacuum filtration of GO suspension, GO nanosheets are coated on the CF (Fig. S1b) and has 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 AgNO3, 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 exposure 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/CF120 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 in-plane stretching vibration of sp2 hybridized carbon appear (Han et al. 2018). For PPy/Ag/3GO/CF120, 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, 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). Fig. 2b shows the XRD patterns of 3GO/CF and PPy/Ag/3GO/CF120. 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 the 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/CF120, 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.
To further confirm the chemical state of the PPy/Ag/GO/CF electrodes, 3GO/CF and PPy/Ag/3GO/CF120 are analyzed by XPS spectra. As shown in Fig. 3a, the XPS survey spectrum of 3GO/CF has two prominent peaks referring C1s and O1s, and PPy/Ag/3GO/CF120 shows four major peaks referring to C1s, O1s, N1s and Ag3d, indicating the formation of PPy and Ag via UV-induced polymerization. Fig. 3b reveals the high-resolution XPS spectrum of C1s in the PPy/Ag/3GO/CF120, in which the prominent peaks of C−C (284.0 eV), C−N (285.3 eV), =C–NH+ (286.6 eV) and –C=N+ (288.4 eV) can be distinguished (Singu and Yoon 2018). More significantly, the high-resolution XPS spectrum of N1s shown in Fig. 3c can be divided into four peaks, including –N= (397.2 eV), −NH− (399.2 eV), –N+– (400.8 eV) and NO3− (405.7 eV) (Cao et al. 2015). The existence of protonated benzenoid amine nitrogen demonstrates that the PPy has been doped. The high-resolution XPS spectrum of Ag3d shown in Fig. 3d has two peaks of Ag3d5/2 (367.7 eV) and Ag3d3/2 (373.7 eV), proving the presence of Ag nanoparticles (Zhou et al. 2020).
The effects of UV irradiation time on the morphology and electrochemical properties of the electrodes are studied. Fig. 4 shows the SEM images of PPy/Ag/1GO/CFy electrodes with different UV irradiation time. PPy/Ag/1GO/CF60 possesses plenty of holes because the PPy/Ag nanoparticles generated within 60 min are limited and cannot completely cover the surface of GO/CF. With the UV irradiation time extends, the generated PPy/Ag nanoparticles become more and more, and the surface morphology of PPy/Ag/1GO/CFy 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 properties of PPy/Ag/1GO/CFy 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/CFy 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+ (Li et al. 2018; Wei et al. 2013). GCD curves of PPy/Ag/1GO/CFy electrodes (Fig. 5b) exhibit voltage platforms, which correspond to the CV curves. The PPy/Ag/1GO/CF120 electrode exhibits the largest CV curve area and longest discharge time, suggesting it has the highest specific capacitance. By calculation, the areal specific capacitance (CA, electrode) and volumetric specific capacitance (CV, electrode) of the PPy/Ag/1GO/CFy electrodes at different current densities are shown in Fig. S4. Among them, the PPy/Ag/1GO/CF120 electrode has the highest CA, electrode of 690.3 mF cm−2 and CV, 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/CF120 electrode still shows the highest CA, electrode of 237.9 mF cm−2 and CV, electrode of 3.9 cm−3. The result demonstrates that 120 min is the optimum UV irradiation time for preparation of PPy/Ag/GO/CF electrode with high specific capacitance. Although the produced PPy/Ag nanoparticles in 150 min is more than that in 120 min, the dense and compact structure of PPy/Ag/1GO/CF150 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/CFy electrodes shown in Fig. 5c are divided into two regions, including the semicircle at the high-frequency region and the straight line at the low-frequency region. The PPy/Ag/1GO/CF120 electrode possesses the lowest charge transfer resistance (Rct) 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. Fig. 6 shows the SEM images of PPy/Ag/xGO/CF120 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. The produced PPy/Ag nanoparticles in PPy/Ag/3GO/CF120 reach the maximum value of 6.3 mg cm−2, and the Ag, C, N and O elements are uniformly distributed (Fig. 6e-h). However, when the mass loading of GO on the CF is 4 mg cm−2, the surface of PPy/Ag/4GO/CF120 shows various degrees of cracks. 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 appearance of cracks and the reduction of active sites. As a result, the produced PPy/Ag nanoparticles in PPy/Ag/4GO/CF120 (5.1 mg cm−2) is lower than that of PPy/Ag/3GO/CF120.
The electrochemical properties of PPy/Ag/1GO/CF120, PPy/Ag/2GO/CF120, PPy/Ag/3GO/CF120 and PPy/Ag/4GO/CF120 electrodes are shown in Fig. 7. The PPy/Ag/3GO/CF120 electrode exhibits the maximum value of CV curve area (Fig. 7a) and discharge time (Fig. 7b), indicating it has the best specific capacitance. As shown in Fig. S5, the CA, electrode of PPy/Ag/3GO/CF120 electrode at a current density of 0.5 mA cm−2 is 1664.0 mF cm−2 (CV, electrode: 27.0 F cm−3) and reaches 780.7 mF cm−2 when the current density is 5 mA cm−2 (CV, electrode: 12.7 F cm−3). The high specific capacitance and good rate performance of PPy/Ag/3GO/CF120 electrode is attributed to the highest content of PPy/Ag nanoparticles. In addition, the PPy/Ag/3GO/CF120 electrode has the smallest equivalent series resistance (Rs, 3.7 Ω) and Rct (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 (Yang et al. 2020). Compared with PPy/Ag/GO/CF electrodes, the specific capacitance of GO/CF is very limited. The maximum CA, electrode of GO/CF is only 14.4 F cm−2 a current density of 0.5 mA cm−2, which indicates that the PPy/Ag nanoparticles have 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/CF120 electrode are compared in Fig. S7. The CV curve of the PPy/3GO/CF electrode is an approximately rectangular shape (Fig. S7a) and the GCD curve is approximately symmetrical triangle (Fig. S7b), revealing its typical capacitive behavior (Zang et al. 2020). Significantly different from the PPy/3GO/CF electrode, the CV curve of PPy/Ag/3GO/CF120 electrode shows much larger enclosed area and the corresponding GCD curve has much longer discharge time than those of the PPy/3GO/CF electrode. The values of CA, electrode and CV, electrode of the 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, which indicate that the Ag nanoparticles can store/release charges and contribute to the capacitance (Li et al. 2018; Wei et al. 2013). Moreover, the presence of Ag nanoparticles in the electrode can increase the electrical conductivity and improve the charge transfer ability. According to the Nyquist plots shown in Fig. S7d, the Rs and Rct of the PPy/3GO/CF electrode are 8.2 Ω and 20.2 Ω, respectively, which are higher than those of the PPy/Ag/3GO/CF120 electrode.
To sum up, there are four reasons can be explained the excellent electrochemical performance of the PPy/Ag/3GO/CF120 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 beneficial to expose more active sites and achieve fast ion diffusion. (ⅲ) The Ag nanoparticles not only can effectively increase the electrical conductivity and improve the charge transfer ability of the electrode, 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. Two pieces of PPy/Ag/3GO/CF120 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 (qv) 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 (qo) of the QFSC which is a non-diffusion controlled process, is calculated to be ~0.054 C cm−2 by linear fitting. The qv 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. S8 shows the areal specific capacitance (CA, device) and volumetric specific capacitance (CV, device) of the QFSC at 0.5, 1, 2, 4 and 5 mA cm−2, which reveals its high specific capacitance and excellent rate performance. The maximum CA, device and CV, device can reach 286.6 mF cm−2 and 4.7 F cm−3 at 0.5 mA cm−2, respectively. And the values of Rs and Rct 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), rGO/1, 4, 5, 8 tetrahydroxy anthraquinone (Xu et al. 2017), PPy CNT/cotton yarn plywood (Hao et al. 2021), PPy-PVA hydrogel (Zang et al. 2017), and graphene film (Li et al. 2017). Also, the capacitance retention of the QFSC is 90.5% after 10000 GCD cycles (Fig. 9b), demonstrating its good cycling stability. 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 10000 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. Fig. 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.