Flexible quasi-solid state supercapacitors are widely considered to be an important clean and sustainable energy storage device. When used in an integrated photovoltaic energy storage system or grid transfer station, they are known for their light weight, portability, superior power density and long-term cycle stability^[1-7]. In theory, the design of flexible supercapacitors can be achieved by vertically superimposing on quasi-solid electrolytes or horizontally depositing electrodes next to adjacent electrodes on flexible substrates^[8-13]. Between two, the latter design is more conducive to the miniaturization of supercapacitors^[14].

The polyaniline (PANI) is considered to be the most potential conductive polymer for supercapacitors thanks to its large theoretical pseudo-capacitance, high electrical conductivity, and low-cost. However, the stability of the main chain is poor because the swollen PANI shrinks during the doping/degumming process, which may lead to mechanical degradation, electrode ion and electrochemical decay, thus its wide application in portable devices and hybrid electric vehicles is limited^[15-19]. To solve this problem, scientists have combined tantalum capacitor materials with well-stabilized carbon materials or transition metal compounds with special structures. Among them, carbon fiber (CF) as a typical electric double layer electrode material has been widely applied in flexible supercapacitors because of its flexibility and electrical conductivity. Weiet al.^[20] fabricated the asymmetric device based on MXene//MnO₂ by chemical deposition and coating on CF, which exhibits specific capacitance of 20.5 F/g at 1.5 A/g, voltage window of 1.5 V more than twice wider than that of the symmetric device and energy density of 6.4 W·h/kg at power density of 1107.7 W/kg. Zhuet al.^[21] prepared V₂O₅/CF composites with different mass ratios by a one-step hydrothermal method, whose specific capacity reaches 220.6 C/g at the current density of 1.0 A/g, and its capacitance retention is still 74.5% after 5000 incessant charge/discharge cycles. Seksaret al.^[22] proposed to generate layered double hydroxide nanosheets (LDH NSs) from CF surface by hot-water therapy (HWT) method, and the Ni–Cu–Co LDH NSs/CF electrode demonstrated maximum areal capacity of 104.2μA·h/cm² with an outstanding cycling stability of 124.5%. CF substrate exhibits excellent specific capacitance and stability performance after being loaded with tantalum capacitors such as MnO₂, V₂O₅ and LDH NSs, which can be used to make flexible supercapacitors without adhesives.

Molybdenum disulfide (MoS₂), as a typical transition metal compounds, has attracted considerable attention thanks to its special structural and chemical character, which is extensively applied in many fields including lithium-ion batteries, catalysis, and dye-sensitized solar cells. MoS₂ with nanoscale has recently been chosen for use in capacitors owing to its higher intrinsic fast ionic conductivity than oxides and higher theoretical capacity than graphite, as well as high surface area. Recently, many researchers have combined PANI with carbon materials and transition metal compound to improve the conductivity of the PANI-based electrodes to enhance the cycling stability and rate capability of the PANI-based pseudo-super-capacitors^[23-25]. Majumderet al.^[26] combined CNOs with PANI by in situ oxidative polymerization of aniline monomer, which could significantly improve the overall electrochemical performance of PANI. Xuet al.^[27] applied Co₃S₄/PANI obtained by introducing sulfur into ZIF-67/PANI to supercapacitors, where sulfur introduction can promote electron transfer with specific capacitance up to 11 times that of ZIF-67 at a current density of 1 A/g. Chenet al.^[28] proposed using highly conductive graphene as a substrate to immobilize PANI in a hydrogel-based stretchable electrode. The electrode showed a high capacitance of 500.13 mF/cm² and 100% capacitance retention after 10 000 cycles of charge and discharge.

Inspired by these ideas, we activated the CF by using of electrochemical method, and then deposited the PANI nanoparticles on the surface to further prepare the CF|PANI composite electrode materials without adhesives. The specific capacitance of the CF|PANI composite electrode is 231.63 F/g at 1 A/g, and it still has 230.77 F/g (99.6% capacity retention), even at a current density of 20 A/g. Furthermore, the capacitance retention of the CF|PANI composite electrode is 99.04% after 1000 cycles. The prepared asymmetric super capacitor (ASC) based on the CF|PANI and MoS₂|MWCNT composite electrode and symmetric super capacitor (SSC) with two CF|PANI electrodes both exhibited good specific capacitance and energy density. Besides, two such ASC can easily light a LED lamp after being connected in series. These good electrochemical performance, long cycle stability, and excellent capacitive properties indicate that the CF|PANI composite materials have potential for application in portable energy storage equipment.

The CF was first activated by using the cyclic voltammetry (CV) method for about 5 min at a scan rate of 50 mV/s between 1 and 2 V based on the previous reference^[29]. Subsequently, it was stored in absolute ethanol. Then, 3.73 g aniline monomer was added to 80 mL 1 M H₂SO₄ solution. The activated CF was in situ electrochemically deposited PANI at a cycle rate of 100 mV/s from voltage windows of –0.2 to 0.8 V by meaning of a CV cycle. The obtained CF|PANI composite materials (Fig. 1) were washed 5 times or more with deionized water and absolute ethanol, and then vacuum dried at 50 °C for 24 h. The stability of the entire system was maintained at 50 °C during electrochemical deposition. The samples with the mass loading of 4.3, 6.4, 7.8, and 8.1 mg were labeled CF|PANI2, CF|PANI3, CF|PANI4, and CF|PANI5 under cycles of 200, 300, 400, and 500 cycles, respectively.

The prepared CF|PANI electrode was first activated by 1 M KOH solution. Then, a gel electrolyte PVA-H₂SO₄ (or PVA-Na₂SO₄) was prepared, based on the previous reference^[30,31]. PVA (2 g) was dissolved in 20 mL deionized water under magnetically stirring at 90 °C until the solution became clear. Subsequently, it was uniformly mixed with 1 M H₂SO₄ (or Na₂SO₄) in a volume ratio of 1 : 1. The mixed solution was then subjected to an aging process to form the corresponding gel electrolyte. A quasi-solid SSC (CP3//CP3 SSC) was prepared by combining two CF|PANI3 electrodes and PVA-H₂SO₄ gel electrolyte. Moreover, the MoS₂|MWCNT (MM) electrodes were also prepared as in our published work^[29]. A quasi-solid ASC (MM//CP3 ASC) was also prepared by combining the MoS₂|MWCNT and CF|PANI3 electrodes with PVA-H₂SO₄ gel electrolyte.

The surface morphologies of the samples were observed using a JSM-7001F field emission scanning electron microscope (SEM). Cyclic voltammetry (CV) measurements were conducted in a three-electrode one-compartment cell, in which an as-prepared sample electrode was taken as the working electrode, a Pt sheet of 1.5 cm² as CE and an Ag/AgCl electrode as reference electrode in 6 M aqueous KOH solution. The EIS tests were carried out simulating open-circuit conditions at ambient atmosphere by using an electrochemical measurement system (CHI660E, Shanghai Chenhua Device Company, China) at a constant temperature of 20 °C with AC signal amplitude of 20 mV in the frequency range from 0.1 to 10⁵ Hz at 0 V DC bias in the dark. The galvanostatic current charge-discharge (GCD) curves were conducted by using a computer-controlled electrochemical analyzer (CHI 660E, CH Instrument). The specific capacitance (C_s), energy density (E) and power density (P) of the supercapacitor were calculated according to the following equations^[32-35]:

1Cs=IΔtmΔU=QmΔU,

2Cs=4IΔtmtΔU=4Ct,

3E=CtΔU27.2(W⋅h/kg),

4P=3600EΔt(W/kg),

whereI represents the current density (A), Δt represents the discharge time (s), ΔU represents the working potential window (V),m represents the quality of active materials (g),Q refers to the amount of charge that can be stored,C_s represents the specific capacitance of the individual electrode, andC_t andm_t represent the specific capacitance of the supercapacitor and the total mass of the active material, respectively.

Fig. 2(a) shows clearly that the color of the CF exhibited black color before pretreatment. After electrochemical pretreating, the color of the CF surface turned purple (Fig. 2(b)). Then, PANI was deposited on the pretreated CF surface, and the green CF|PANI composite was obtained as shown inFig. 2(c). After electrochemical deposition and annealing treatment, the PVA-H₂SO₄ electrolyte was loaded onto the CF|PANI3 surface, as shown inFig. 2(d).Figs. 2(e)–2(g) present the SEM images of the pretreated CF at 1000, 10 000, and 100 000 magnifications, respectively. As the amplification factor increased, the surface voids of the CF gradually appeared, which is helpful for the active substances loading on CF bundle.Figs. 2(h)–2(j) are SEM images of CF|PANI3 electrode at 1000, 10 000, and 100 000 magnifications, respectively. Among them, PANI nanoparticles are well loaded on the surface of the CF bundles. At a magnification of 100 000, it can be seen that the PANI on CF surface has more wrinkles, which greatly promotes the improvement of their specific surface area.

Fig. 3(a) shows the Raman spectrum of CF and CF|PANI3. FromFig. 3(a), the characteristic peaks of CF are close to the D and G bands of graphite material, which are near 1350 and 1580 cm^–1, respectively. The intensity ratio of D/G (about 1) and the density of defects are relatively large^[36], which is responsible for the pretreatment of CF. This is very advantageous for the active material PANI to be loaded thereon. The Raman characteristic peaks of the CF|PANI3 are all appear at 1161, 1255, 1346, 1481, and 1583 cm^–1[29]. Among them, the peak near 1583 cm^–1 is caused by the benzene ring type tensile vibration and the C–C type of the polymer type; the peak near 1481 cm^–1 is strengthened, and each band has a high/low wave number in the aniline salt spectrum^[37]. At the Raman spectrum of CF|PANI3, the characteristic peaks of CF are faintly visible. This result indicates that the nanoscale PANI has covered the CF surface very well.

Fig. 3(b) shows the XRD spectrum of the CF and CF|PANI3. Among them, the broad peak at 24° ((002) crystal plane) represents graphite carbon^[31,38]. The XRD characteristic peaks of pure PANI are about 15.1° (011), 20.5° (020), and 25.5° (200)^[39]. The appearance of the XRD peaks in the middle is due to the diffraction of the layered polymer on the alternating distance and semi-crystalline surface of polyaniline^[40].

FromFig. 4(a), it can be seen that the discharge time of the various CF|PANI composite electrodes increases at first and then decreases as the electrodeposition period increases. From Eq. (1), the corresponding specific capacitances for the above CF|PANI composite electrodes also increase at first and then decrease from. When the electrodeposition period is 300 cycles, the specific capacitance of CF|PANI3 is the largest.Fig. 4(b) shows the CVs of the various electrodes under 20 mV/s condition, in which the redox peaks of the different electrodes increase firstly and then decrease with the electrodeposition cycle increasing. Among them, the redox peak of the CF|PANI3 electrode exhibits most obviously, indicating its good pseudo-capacitance. At the same scanning speed, the area enclosed by the CV of the CF|PANI3 electrode is the largest, and thus the specific capacitance is also the largest according to Eq. (2). This result is consistent with the previous calculation of constant current charge and discharge.

Fig. 4(c) shows the EIS of the various electrodes with different deposition cycles. FromFig. 4(c), it can be seen that the internal resistance (R_s: high frequency region, the first intersection of the impedance line and the real axis) of the CF|PANI composite electrodes decreases as the electrodeposition period increases. Among them, the slope of the CF|PANI3 electrode in the low frequency region is the smallest, indicating its best ion diffusion and conductivity ability^[41]. Moreover, the lower equivalent series internal resistance and charge transfer internal resistance (R_ct: high frequency region to intermediate frequency region, the diameter of the impedance circle is semicircular) for the CF|PANI3 electrode compared to other electrodes indicates that CF|PANI3 electrode possesses better rate performance.

Table 1 shows the specific capacity (C_s) and voltage drop (U_drop) of the CP|PANI2, CP|PANI3, CP|PANI4, and CP|PANI5 electrode at 1 A/g condition. Among these electrodes, the specific capacitance of the CP|PANI3 electrode can reach 231.63 F/g, which is much larger than that of the CF|PANI2 electrode (97.98 F/g). As the scan period further increases, the corresponding specific capacitance begins to decrease (153.65 F/g for CF|PANI4 electrode and only 70.21 F/g for CF|PANI5 electrode). This result once again proves that proper deposition cycle has an important influence on the capacitance performance of the CF|PANI electrode. This is due to the moderate amount loading of PNAI, which helps to avoid stacking and gets a better layered structure. Furthermore, theU_drop of above electrodes also increases as the deposition cycle increases. This happens because the long electrodeposition cycle which can affect the conductivity of the CF|PANI electrodes.

Fig. 5(a) shows curves of the constant current charge and discharge for the CF|PANI3 composite electrode at different current densities. As the current density increases, the charge and discharge time of the CF|PANI electrodes begin to decrease, and the corresponding voltage drop also increase. The calculation of the specific capacitance shows that the CF|PANI3 composite electrode has a large specific capacitance at a lower current density. This happens because the diffusion of electrolyte ions is insufficient at higher current density, resulting in a decrease in specific capacitance^[42].Figs. 5(b) and5(c) show CV curves of the CF|PANI3 at different scan rates. It can be detected that the directions of cathodic and anodic peaks of the CF|PANI3 electrode are gradually and regularly shifting to the negative and positive orientations as the scan rate increases^[42]. Also, the directions of the cathodic and anodic peaks shift to more negative and positive orientations with the higher scan rates, which is due to the fact that ions in the electrolyte are not sufficiently diffused onto the electrode material at higher scan rates, resulting in a loss of effective specific capacitance. Moreover, the low internal resistance of the CF|PANI3 electrode contributes to rapid electron transfer and transfer, which greatly improves its rate performance^[43].Fig. 5(d) shows the little change of the specific capacitance for the CF|PANI3 electrode and 98.15% capacity retention from 0.5 A/g (235.12 F/g) to 20 A/g (230.77 F/g). This result indicates that the CF|PANI3 electrode possesses excellent rate performance. In addition, the CF|PANI3 electrode exhibits a capacity retention rate of 99.04% after 1000 cycles. This is mainly due to the excellent conductivity of the CF substrate, and the pre-treated active sites adhere well to the PANI nanomaterials electrodeposited thereon.

Fig. 6(a) shows the GCD of the CP3//CP3 SSC with different voltage windows at 3 A/g, andFig. 6(b) shows the CV diagram of the CP3//CP3 SSC at a sweep speed of 20 mV/s. FromFigs. 6(a) and6(b), the electric double layer energy storage characteristics of the CP3//CP3 SSC are exhibited with voltage windows at 0.2 and 0.4 V, but there is almost no redox peak for the CP3//CP3 SSC at the lower operating voltage window. The redox peak begins to appear and pseudo-capacitance characteristics also gradually show up for the CP3//CP3 SSC until the voltage window increases to 0.6 V^[44]. However, the capacitance of the CP3//CP3 SSC is not very large due to the limitations of the voltage window.Fig. 6(c) shows the impedance spectrum of the CP3//CP3 SSC from 0.01 to 100 000 Hz, which includes the internal resistance (R_s) and the charge transfer resistance (R_ct). In the high frequency region, the line intersects the real axis at a point and the corresponding Rs of the CP3//CP3 SSC is about 6.98 Ω. FromFig. 6(c), the as-prepared CP3//CP3 SSC displays littleR_s andR_ct due to good conductivity of the CF|PANI3 electrode. Moreover, the CP3//CP3 SSC with a very vertical line at low frequency. This happens because the CF|PANI3 electrode has good electron and ion transport channels, and the pore size distribution makes the electrolyte easily penetrate into the pores^[45]. This is attributed to the synergistic effect of the high conductivity and excellent catalytic activity of the CF and the nano PANI.

Table 2 shows the calculation results of the capacitance performance parameters of the CP3//CP3 SSC at different current densities. At a current density of 0.5 A/g, theU_drop is less than 0.01 V, while at 20 A/g, theU_drop achieves 0.35 V. The corresponding specific capacitance retention rate of the CP3//CP3 SSC is up to 98.15%. This is attributed to the small internal resistance for the CF|PANI3 electrode without adhesives. Simultaneously, the PANI with nanoscale structure covered on the CF surface provides more excellent conductivity, which contributes to the good performance of the pseudo-capacitance for the CP3//CP3 SSC. The specific capacitance C_t of the CP3//CP3 SSC with the CF|PANI3 electrodes is only 1/4 times than that of the single electrode specific capacitanceC_s (ie,C_t =C_s/4)^[46]. Moreover, the corresponding energy density of the CP3//CP3 SSC decreases as the power density increases. This means that a supercapacitor with high power density is difficult to own a large energy density.

To further improve the situation of the energy density and power density, an asymmetric hybrid supercapacitor was prepared and researched.Figs. 7(a) and7(c) show the GCD and CV plots of the MM//CP3 ASC with different voltage windows. FromFigs. 7(a) and7(c), it can be seen that the slope of the GCD curves for the MM//CP3 ASC begins to decrease as the voltage increases. In addition, the redox peak in the CV curves gradually appears as the scan voltage increases. This result indicates that a larger voltage window contributes to the performance of the tantalum capacitor. At the same time, it is known from the Eq. (3) that a larger operating voltage is also beneficial to the improvement of the energy density of the supercapacitor. Usually, the specific capacitance of the MM//CP3 ASC can reach 59.2 F/g at a current density of 0.75 A/g (current density through CF|PANI3 electrode is 1.16 A/g), which is slightly higher than that of the CP3//CP3 SSC. However, the energy density of the MM//CP3 ASC is greatly improved as the operating voltage window increases to above 1 V. Furthermore, the energy density of the MM//CP3 ASC can reach 16.12 W·h/kg, which is significantly higher than that of the CP3//CP3 SSC at a power density of 525.03 W/kg.Fig. 7(b) shows the EIS of the MM//CP3 ASC. This displays that the solution internal resistanceR_s is 15 Ω, and its charge transfer internal resistanceR_ct is about 8 Ω. In addition, the slope of the line is large in the low frequency region, indicating good capacitive response characteristics and ion-clearing channels^[45]. Therefore, the overallR_s of the quasi-solid MM//CP3 ACS is greater than that of the simple superposition of positive and negative resistances, which has a close relationship with the matching degree of positive and negative materials.Fig. 7(d) presents the relationships between the energy density and power density of the CP3//CP3 SSC and MM//CP3 ASC. The energy density of the MM//CP3 ASC decreases seriously with the increasing power density. This happens because the matching degree between positive and negative materials is distinguished under different power densities. Therefore, a power supply equipment suitable for rated power of electrical appliances can be prepared by adjusting the charge balance of positive and negative electrodes under a specific power density. In general, the contribution of asymmetric supercapacitors in voltage window and energy density is still significant.Fig. 7(e) shows that red, green and yellow LED bulbs can easily be lit by two series MM//CP3 ASCs, which shows a good potential application.

PANI nanoparticles were in situ coated onto the surface of the electrochemically activated CF by using electrochemical deposition method. In the absence of adhesive, the low internal resistance of the CF substrate can not only help to reduce the voltage drop to maintain a large voltage window but the PANI nanoparticles also have a better pseudo-capacitance to improve the specific capacity of the CF|PANI3 composite electrode. The CF|PANI3 electrode still has a 99.6% retention when the current density increases from 1 to 20 A/g, showing good rate performance and excellent cycle stability. This happens because the nanoscale PANI which can be deposited on the pretreated CF surface controllably and efficiently without binder by using the electrodeposition method. In addition, the prepared MM//CP3 ASC has higher energy density than that of the CP3//CP3 SSC. This is attributed to the expansion of operating voltage window, which helps to improve its energy density. Two MM//CP3 ASCs after being connected in series can easily light a red, green or yellow LED lamp. These results indicate that the neutral quasi-solid MM//CP3 ASC has a good application prospect.

The authors are very grateful to the joint support by NSFC (No. 61704047).