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. 2021 Jun 21;6(25):16455–16464. doi: 10.1021/acsomega.1c01601

Electropolymerized Poly(3,4-ethylenedioxythiophene)/Screen-Printed Reduced Graphene Oxide–Chitosan Bilayer Electrodes for Flexible Supercapacitors

Chia-Hui Tseng †,⊥, Hsun-Hao Lin ‡, Cheng-Wei Hung §, I-Chung Cheng §, Shyh-Chyang Luo ‡,⊥,*, I-Chun Cheng ∥, Jian-Zhang Chen †,⊥,*
PMCID: PMC8246451  PMID: 34235317

Abstract

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An electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT)/screen-printed reduced graphene oxide (rGO)–chitosan (CS) bilayer material was coated on carbon cloth to form electrodes for gel-electrolyte flexible supercapacitors. The conductive polymer and carbon-based materials mainly contribute pseudocapacitance (PC) and electrical double-layer capacitance (EDLC), respectively. The high porosity and hydrophilicity of the PEDOT/rGO–CS bilayer material offers a large contact area and improves the contact quality for the gel electrolyte, thereby enhancing the capacitive performance. Cyclic voltammetry (CV) under a potential scan rate of 2 mV/s revealed that a maximum areal capacitance of 1073.67 mF/cm2 was achieved. The capacitance contribution ratio PC/EDLC was evaluated to be ∼67/33 by the Trasatti method. A 10,000-cycle CV test showed a capacitance retention rate of 99.3% under a potential scan rate of 200 mV/s, indicating good stability. The areal capacitance remains similar under bending with a bending curvature of up to 1.5 cm–1.

1. Introduction

Supercapacitors (SCs) with good flexibility and long lifetime have become increasingly important owing to the increasing demand for flexible and wearable electronic devices.1−4 Flexible substrates such as nickel foam, woven cloth, fiber paper, and carbon cloth (CC) have been widely used as current collectors of flexible SCs.5−7 SCs have a large specific surface area that enables storing a large amount of charges, and therefore, their energy density is much higher than that of conventional capacitors. Furthermore, SCs can be operated in a fast charging–discharging mode with a large current, thereby providing higher power density than that of a conventional battery.3,8 SCs generally store energy through two charge storage mechanisms.9 One mechanism is fast ion adsorption/desorption on the electrode/electrolyte interface, called electric double-layer capacitance (EDLC); this typically occurs in carbon-based electrode materials such as carbon nanotubes, carbon black, and graphene.10−17 The other mechanism is pseudocapacitance (PC) based on a faradaic redox reaction; in this process, ions in the electrolyte are electrochemically adsorbed to the active material to undergo a redox reaction, thus converting electrons/ions into electric charges. PC usually occurs in transition multivalence metal oxides, chalcogenides, and conductive polymers.1,3,18−21

The most frequently studied conductive polymers are polyaniline (PANI), polythiophene, polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT), and their derivatives.22−27 PEDOT is easy to process and has high conductivity, good flexibility, and good environmental stability.28−30 Owing to these attractive properties, it is considered an ideal material for various applications including fuel cells, solar cells, electrochromic device, and SCs.25,31−33 PEDOT used to fabricate SCs is usually prepared by the following three polymerization methods: (1) in situ polymerization method, in which the EDOT monomer and a catalyst are added to a water dispersion containing surfactants for polymerization; (2) vapor-phase polymerization, in which the gas-phase EDOT monomer is oxidized and polymerized on the substrate into a PEDOT film in a vacuum oven; and (3) electropolymerization, in which PEDOT is deposited on a conductive substrate in an electrochemical reactor.34−36

Previously, we have fabricated SnO2–reduced graphene oxide (rGO) and PANI–rGO–CS SCs using an arc atmospheric pressure plasma jet and dielectric barrier discharge jet, respectively.19,22 In these studies, CC was selected as the current collector for flexible SCs because of its good flexibility, good conductivity, low weight, and high porosity.37 In the present study, we use a simple and stable method in which PEDOT is electropolymerized onto CC, following which rGO–CS is screen-printed onto PEDOT. The rGO–CS coatings not only provide EDLC but also prevent PEDOT from degrading during multiple charging–discharging cycles. PEDOT/rGO–CS flexible SCs show a high areal capacitance of 1073.67 mF/cm2 with a retention rate of 99.3% after a 10,000-cycle cyclic voltammetry (CV) test. Furthermore, these SCs function well under bending with a bending curvature of 1.5 cm–1.

2. Results and Discussion

2.1. Surface Morphology of Electropolymerized PEDOT on CC

Because of the low solubility of the EDOT monomer in aqueous media, organic solvents are used for the deposition of homogeneous PEDOT.38 In this study, the electropolymerization of PEDOT on CC was carried out in acetonitrile in the presence of LiClO4 as an electrolyte to improve the stability of PEDOT growth.30Figure 1a shows a scanning electron microscopy (SEM) image of the CC. The interlacing weaving structures are clearly observed. Figure 1b shows a magnified image of the CC; the diameter of the carbon fiber is ∼10 μm. This microstructure provides a large surface area for the electropolymerization of PEDOT and ensures the accessibility of the gel electrolyte. Figure 1c–g shows SEM images of PEDOT electropolymerized on CC for various durations. PEDOT nucleated and grew on the carbon fibers as the electropolymerization proceeded. As the electropolymerization time exceeded 30 min, the fibers were well covered by the electropolymerized porous PEDOT. The porous structure of PEDOT further increased the surface area. As the electropolymerization time exceeded 30 min, the color of the electrolyte clearly changed and some materials were stripped off; in addition, the EDOT concentration and deposition rate both decreased. Figure S1 shows the final color of the EDOT solution after various electropolymerization durations. Figure 1h shows a SEM image of rGO–CS screen-printed on 30 min electropolymerized PEDOT (CC/PEDOT-30 min/rGO–CS or simply PEDOT-30 min/rGO–CS). A flake structure is observed clearly.

Figure 1.

Figure 1

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SEM images of PEDOT electropolymerized on CC for various durations. (a) Lower-magnification SEM image of CC; (b) higher-magnification SEM image of CC; PEDOT electropolymerized on CC for (c) 5, (d) 15, (e) 30, (f) 45, and (g) 60 min; and (h) screen-printed rGO–CS on 30 min electropolymerized PEDOT (CC/PEDOT 30 min/rGO–CS).

Figure 2 shows high-magnification SEM images of electropolymerized PEDOT; clustered growth and stacking of irregular particles of various sizes are observed. The areal density of electropolymerized PEDOT was measured to be 0.55, 2.03, 4.50, 6.15, and 7.18 mg/cm2 for electropolymerization durations of 5, 15, 30, 45, and 60 min, respectively. The deposition rate decreased after 30 min of electropolymerization owing to the decreased EDOT concentration in the electrolyte and decreased supporting growth area. PEDOT grows well on carbon fibers to provide a porous structure. Figure S2 shows lower-magnification SEM images of the electropolymerized PEDOT on CC.

Figure 2.

Figure 2

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High-magnification SEM images of (a) CC and PEDOT electropolymerized on CC for (b) 5, (c) 15, (d) 30, (e) 45, and (f) 60 min.

2.2. Surface Hydrophilicity Tests

Our previous study shows that the hydrophobicity of untreated CC affects the contact between the electrodes and the electrolyte to a certain extent.22 Therefore, in this study, we assessed the hydrophilicity of PEDOT electropolymerized for various durations. Figure 3a shows that the water contact angle for the untreated CC is 131°. Figure 3b–f shows the water contact angle measurement results for PEDOT electropolymerized for various durations. PEDOT electropolymerized for all durations showed a hydrophilic surface, and the testing water droplet completely penetrated the CC/PEDOT. This is because the EDOT monomer is oxidized during the electropolymerization process, and the carbon fibers at the anode may also be oxidized to form oxygen-containing functional groups such as hydroxyl groups and carboxyl groups on its surface.39 At the same time, combined with the previously observed SEM results, the granular structure formed by PEDOT electropolymerized on CC may increase the surface roughness to further improve the surface wettability. Because the EDLC of SC stores charges at the electrode/electrolyte interface, higher hydrophilicity promotes contact between the electrolyte and the electrode, in turn increasing the ion transfer rate at the electrolyte/electrode interface to improve the capacitance of the element. Furthermore, the electrode material can better access ions and electrons on the hydrophilic surface, to facilitate the PC redox reaction. Therefore, the hydrophilicity of CC/PEDOT enhances the performance of SCs.

Figure 3.

Figure 3

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Water contact angles of (a) pure CC, (b) CC/PEDOT-5 min, (c) CC/PEDOT-15 min, (d) CC/PEDOT-30 min, (e) CC/PEDOT-45 min, and (f) CC/PEDOT-60 min.

The compositions of the untreated CC and CC/PEDOT-30 min were analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Figure S3. Figure S3a shows the signals of C and O elements from carbon fiber. Figure S3b shows the C, O, and S elements corresponding to the PEDOT. The presence of Cl is possibly due to the doping agent (LiClO4) in the electropolymerization process of PEDOT. The C 1s spectra and S 2p of PEDOT-30 min electrode spectra were observed. The C 1s peak can be deconvoluted into six components (as shown in Figure S3c): C–C(284.8 eV), C–S(285.6 eV), C=C–O(286.2 eV), C–O–C(287.1 eV), −COOH(288.3 eV), and π–π*(289.6 eV).40,41 The C–S bond and C=C–O bond are contributed by the α and β positions in the thiophene ring, respectively. The C–O–C bond is derived from the ethylene bridge structure within PEDOT.40 The carboxyl group may be due to the excessive oxidation of the EDOT monomer during electropolymerization, which leads to the bonds of carbon atoms to oxygen. Finally, the asymmetric tail at the higher binding energy position is mainly caused by the π to π* transition in PEDOT.41 In the S 2p XPS spectra (Figure S3d), peaks at 163.5 and 164.6 eV correspond to S atoms in the PEDOT thiophene ring structure; these exist in the form of spin-splitting double peaks S 2p1/2 and S 2p3/2.40,42,43 Asymmetric tail peaks (S+) at higher binding energies (165–168 eV) are associated with chlorine doping. This is attributed to the positively charged sulfur in the thiophene ring that leads to a wider distribution of the binding energy of the S 2p signal, resulting in asymmetric tails on the higher binding energy side.41,42,44 These XPS characterization results indicate that the PEDOT structure has been successfully deposited on the carbon fibers by the electropolymerization process.

2.3. CV Results of PEDOT/rGO–CS Flexible SCs

Figure 4 shows the CV curves with potential scan rates of 200, 50, and 2 mV/s; Table 1 lists the corresponding areal capacitance values. The areal capacitance is calculated as follows45

2.3. 1

where I(V) is the response current; A is the area of one electrode; v is the potential scan rate; and ΔV is the applied potential window. At the high scanning rate (200 mV/s), the areal capacitance increases and then decreases as the PEDOT electropolymerization time increases, reaching a maximum value of 83.7 mF/cm2 for PEDOT-15 min/rGO–CS. At the intermediate scanning rate (50 mV/s), the optimum areal specific capacitance value is 448.61 mF/cm2 for PEDOT-45 min/rGO–CS. When the measurement was carried out at a very low scanning rate (2 mV/s), the areal capacitance increases monotonically within an electropolymerization time of 60 min, and the highest areal capacitance of 1073.67 mF/cm2 is achieved for the PEDOT-60 min/rGO–CS sample. The effect of PC becomes more apparent when the CV potential scan rate is low. In this case, the PC is mainly contributed by PEDOT. When the potential scan rate is low, the ions in the electrolytes diffuse more readily into the PEDOT structure; more PEDOT contributes PC. Therefore, the sample with a longer PEDOT electropolymerization time shows the maximum capacitance value as the potential scan rate decreases. As the deposited mass of PEDOT increases, the diffusion-limited PC mechanism begins to dominate the SC capacitance. Overfilled PEDOT may block ion transport channels and decrease the mobility of ions between the active material and the electrolyte, resulting in poorer capacitance values for samples with longer PEDOT growth times during rapid scanning. However, as the potential scan rate decreases, the ions in the electrolyte have sufficient time to diffuse completely and to complete the electrochemical reaction with PEDOT on the electrode to store charges. Therefore, the sample with a longer PEDOT electropolymerization time shows the maximum areal capacitance as the CV potential scan rate decreases.

Figure 4.

Figure 4

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CV curves of PEDOT/rGO–CS SCs under potential scan rates of (a) 200, (b) 50, and (c) 2 mV/s.

Table 1. Areal Capacitance (mF/cm2) Calculated Based on CV Curves Shown in Figure 5.

potential scan rate (mV/s) rGO–CS (mF/cm2) PEDOT-5 min/rGO–CS (mF/cm2) PEDOT-15 min/rGO–CS (mF/cm2) PEDOT-30 min/rGO–CS (mF/cm2) PEDOT-45 min/rGO–CS (mF/cm2) PEDOT-60 min/rGO–CS (mF/cm2)
200 17.30 76.42 83.70 71.72 63.24 55.25
50 19.96 123.57 272.71 389.29 448.61 415.99
2 25.39 228.60 489.43 774.32 976.05 1073.67
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The Trasatti method46,47 is used to further investigate the contribution of PC and EDLC. This method is established based on the assumption that capacitance or charge storage is related to the diffusion-limited processes. When the potential scan rate increases, ions do not diffuse easily to the inner surface of the electrode; therefore, the faradic redox reaction is limited by the ion diffusion rate. This part of the contribution to capacitance is related to the PC, and it contributes to the inner capacitance Cin in the equations below. EDLC occurs on the outer surface of the electrode and contributes to the outer capacitance Cout, which is assumed to be constant across the range of scanning rates. Therefore, the entire capacitive behavior is assumed to be divided into the contribution of the outer surface and inner surfaces of the electrode (Cout and Cin, respectively), as shown in eq 2

2.3. 2

As the potential scan rate increases, the capacitive behavior of the inner surface is assumed to be controlled by semi-infinite linear diffusion with a rate proportional to v–1/2. However, the capacitive behavior of the outer surface is independent of the potential scan rate.47

2.3. 3

where C(ν) is the capacitance measured at potential scan rate ν and k1 is a constant. Cout, which is contributed mainly by EDLC, can be obtained by the intercept on the vertical axis C(ν) by extrapolating the linear fitting line in the C(ν)–v–1/2 plot (an infinite scanning rate v → ∞, i.e., v–1/2 → 0). In contrast, at a very low potential scan rate, the entire capacitance should be contributed by both Cin and Cout because both EDLC and PC readily respond to the slowly varying applied electric field. The relation can be rewritten as47

2.3. 4

where k2 is a constant. 1/Ctotal can be obtained by the intercept on the vertical axis 1/C(ν) by extrapolating the linear fitting line on the 1/C(ν)–v1/2 plot. The PC contribution, Cin, can be obtained by subtracting Cout from Ctotal (eq 2).

Figure 5 shows the capacitance contribution ratio PC/EDLC analyzed based on the Trasatti method. Table S1 lists the calculated areal capacitances at various potential scan rates shown in Figure 5. Table S2 lists Ctotal, Cout, Cin, and the capacitance contribution ratio PC/EDLC analyzed based on the results shown in Figure 5 (CA denotes the areal capacitance). Figure 5d clearly shows that the PC contribution apparently increases after the electrodeposition of PEDOT; the PC/EDLC ratio increases from ∼24/76 to ∼67/33 with the inclusion of the electropolymerized PEDOT layer. This suggests that PC is mainly contributed by PEDOT. The PC/EDLC ratio shows no apparent change with the PEDOT (i.e., a longer electropolymerization time). However, the absolute values of Cin (contributed by PC) and Cout (contributed by EDLC) both increase with the PEDOT thickness (i.e., a longer electropolymerization time).

Figure 5.

Figure 5

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(a) 1/CA(ν)–v1/2 relation diagram, (b) magnified view of (a), and (c) CA(ν)–v–1/2 relation diagram for Trasatti analysis method. (d) Capacitance contribution ratio PC/EDLC.

2.4. GCD Analyses for PEDOT/rGO–CS Flexible SC

Figure 6a shows the galvanostatic charging–discharging (GCD) curves of PEDOT/rGO–CS SCs. The areal capacitance is calculated as45

2.4. 5

where A is the area of one electrode; Td is the discharge time; Id is the constant current; and ΔV is the voltage sweep range. The charging–discharging curves are in triangular shape, indicating fast charging and discharging behavior. At a constant current of 1 mA, the IR-drop of the PEDOT-60 min/rGO–CS flexible SC is only 0.016 V, suggesting that the internal resistance is low because of the good conductivity and high specific surface area of the electrode materials. Table S3 lists the areal capacitance values calculated based on the GCD measurements. Figure 6b shows the areal capacitance values of the PEDOT-60 min/rGO–CS SCs evaluated under various constant current densities. The results are reasonable because a longer charging time is required with smaller constant charging–discharging current (constant current). Figure 6c shows the measured areal capacitance as a function of constant current. As the constant current decreases, the areal capacitance increases slightly because the PC response becomes more apparent with the slower potential scan rate owing to the slower PC reaction than that of EDLC. The increasing amount of areal capacitance at a lower constant current becomes more apparent with a thicker PEDOT layer because more time is required to achieve the required amount of charge for the faradaic redox reaction with a smaller constant current. For the rGO–CS SC without a PEDOT layer, this increasing capacitance at a lower constant current is less obvious because the PC effect is much less dominant without the PEDOT layer. Figure 6d shows Ragone plots of the PEDOT/rGO–CS SCs analyzed with various GCD constant currents. The energy density and power density are calculated based on the areal capacitance values as follows:

2.4. 6

where E is the energy density (mW h/cm2); CA is the areal capacitance (mF/cm2); ΔV is the potential window; P is the power density (mW/cm2); and Δt is the discharging time. Furthermore, 3600 is the prefactor for unit conversion. PEDOT deposition apparently increases the energy density of the SC. The increasing energy density approaches saturation with increasing PEDOT electropolymerization time. This is because the deposition rate decreases with the electropolymerization time because of the lower EDOT concentration and fewer available growth sites. The areal capacitance value of the SC based on the PEDOT-60 min/rGO–CS electrodes at 10 mA constant charge–discharge current is 399.33 mF/cm2 with a power density of 2 mW/cm2 and a high energy density of 35.5 μW h/cm2. Compared with previously reported results, the SC fabricated in the present study exhibit good energy density and power density, as shown in Figure 6e and Table S4.13−15,23,24,48

Figure 6.

Figure 6

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GCD results of PEDOT/rGO–CS SCs: (a) areal capacitance evaluated at 1 mA constant current, (b) areal capacitance of PEDOT-60 min/rGO–CS evaluated at various constant currents, (c) areal capacitance values under various constant currents, and (d) Ragone plot of PEDOT/rGO–CS SCs for various PEDOT polymerization times. (e) Comparison of our results with those of previous studies.

In order to further evaluate the electrochemical behavior of PEDOT/rGO–CS flexible SCs, the characteristics of the SCs were analyzed by electrochemical impedance spectroscopy (EIS). The equivalent circuit used in ZView software is shown in Figure S4a to analyze the Nyquist plot. Rs is the sum of the intrinsic resistance of the current collector, the ionic resistance of the electrolyte, and contact resistance at the active material/current collector interface; Rct is the charge transfer resistance; CPEdl and CPEf are EDLC and PC, respectively; and W is the Warburg resistance.49−52Figure S4(b) shows an enlarged portion of the Nyquist plots of the PEDOT/rGO–CS flexible SC in the high frequency region. The Rs values of PEDOT-5 min/rGO–CS, PEDOT-15 min/rGO–CS, and PEDOT-60 min/rGO–CS devices are 3.19, 3.24, and 3.32 Ω, respectively. This difference can be attributed to the PEDOT continuous aggregation blocking part of the ion transport channel, thus slightly increasing the contact resistance. The Nyquist plots of PEDOT/rGO–CS flexible SCs are shown in Figure S4c. When the electropolymerization time increased from 5 to 60 min, the Rct value only increased from 0.1 to 0.2 Ω. As the electropolymerization time increased, the thicker active material layer slowed down the rate of charge transfer, thus increasing the electron transfer resistance Rct.53 The EIS results are in good agreement with the CV results at a high scanning rate (200 mV/s). It is worth noting that this resistance value is similar to the previously reported Rct value of PEDOT SC with the liquid electrolyte.50 This shows that our PEDOT/rGO–CS flexible SCs have good flexibility and portability while maintaining ideal charge transfer properties including good conductivity and reversibility.49

2.5. Long Cycle Stability Test and Bending Test of SCs

The PEDOT-60 min/rGO–CS flexible SC was evaluated through the 10,000-cycle CV test. Figure 7a and Table S5 show the experimental results. The capacitance increases slightly for the first 5000 cycles and then remains similar. We speculate that this is due to the intermolecular interaction between rGO and PEDOT that limits the change in the electrode network structure during cycling and reduces the structural damage of the PEDOT layer during the long-term charging and discharging process.54 After 10,000 cycles, the areal capacitance only decreases by 0.7%; this result demonstrates the excellent cyclic stability of the flexible SCs. The rGO layer provides some EDLC and may reduce the structural damage of the PEDOT layer during the repeated charging–discharging cycle. Figure 7b shows the results of SCs evaluated through a 1 mA constant current GCD test under bending with various bending curvatures. The capacitance remains similar for a bending curvature of up to 1.5 cm–1. Figure S5 is the schematic diagram of the tool used for bending curvature testing.

Figure 7.

Figure 7

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(a) Normalized areal capacitance of PEDOT-60 min/rGO–CS SCs for the 10,000-cycle CV stability test and (b) normalized areal capacitance of PEDOT-60 min/rGO–CS SCs for the bending test with a bending curvature of up to 1.5 cm–1.

3. Conclusions

An electropolymerized PEDOT/screen-printed rGO–CS bilayer material deposited on CC is successfully used for gel-electrolyte flexible SCs. Evaluations performed using the Trasatti method reveal that the contribution of the PC/EDLC ratio is ∼67/33 for PEDOT/rGO–CS SCs. This ratio does not change much with the PEDOT electropolymerization time. However, the absolute values of PC and EDLC both increase with the electropolymerization time (i.e., a thicker PEDOT layer). The electropolymerized PEDOT is porous and hydrophilic, and it affords high areal capacitance of 1073.67 mF/cm2, as evaluated by CV under a potential scan rate of 2 mV/s. The PEDOT-60 min/rGO–CS SC shows a power density of 2 mW/cm2 and a high energy density of 35.5 μW h/cm2, as evaluated by GCD under 10 mA constant current. The capacitance retention rate is 99.3% after the 10,000-cycle CV test under a potential scan rate of 200 mV/s. Furthermore, the areal capacitance remains similar under bending with a bending curvature of up to 1.5 cm–1.

4. Experimental Section

4.1. Preparation of rGO–CS Pastes for Screen Printing

A CS acetic acid solution was used as the thickener for screen printing pastes, making it easier and more uniform to screen-print the rGO onto the electrode. First, 0.3 g of CS powders (obtained from shrimp shells, degree of deacetylation: >75%, Sigma-Aldrich) was added to 20 mL of 0.1 M acetic acid (purity: >99.5%, AUECC) and stirred with a magnetic stirrer at 300 rpm at 50 °C for 2 h and then at room temperature for 1 h. Next, 3.6 g of CS acetic solution, 0.05 g of rGO (thickness: <5 nm, chip diameter: 0.1–5 μm, oxygen content: 5–10%, purity: >99%, Golden Innovation Business), and 1.5 g of ethanol (purity: 95%, Echo Chemical) were mixed and stirred at 300 rpm at room temperature for 1 day using a magnetic stirrer. The rGO–CS mixture was shaken for 30 min using an ultrasound shock washer, following which 5 mL of the mixture was concentrated at 55 °C for 3 min using a rotary evaporator.

4.2. Fabrication of PEDOT/rGO–CS Electrodes

Electropolymerization was performed using the trielectrode method in the potentiostatic mode with Ag/Ag+ as the reference electrode and Pt as the electrode. A solution of 10 mM EDOT (purity: >98%, TCI) and 0.1 M LiClO4 (purity: 98%, Alfa Aesar) in 20 mL of CH3CN (purity: >99.9%, Echo Chemical) was used as the electrolyte for electropolymerization. PEDOT was electropolymerized on CC (area ∼ 4 cm2) for 5, 15, 30, 45, and 60 min at a constant potential of 0.9 V. The oligomers remaining after electropolymerization were rinsed sequentially with alcohol and deionized water, following which the sample was dried in an oven at 80 °C for 10 min. The weight was measured after electropolymerization to determine the amount of PEDOT grown on CC. After PEDOT electropolymerization, rGO–CS pastes were screen-printed on the PEDOT-coated CC. Next, the sample was calcined at 80 °C for 10 min in air to complete the production of CC/PEDOT/rGO–CS (or simply PEDOT/rGO–CS) electrodes for SCs. Figure 8a–d shows the overall flowchart of the CC/PEDOT/rGO–CS electrode fabrication process.

Figure 8.

Figure 8

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Schematic diagram of the fabrication process of flexible PEDOT/rGO–CS SCs.

4.3. Assembly of Flexible SCs

The polyvinyl alcohol (PVA)–H2SO4 gel electrolyte was prepared by mixing 1.5 g of PVA (MW: 850,000–124,000, 99+% hydrolyzed, Sigma-Aldrich) into 15 mL of 1 M sulfuric acid (purity: 95–97%, AUECC). The mixture was then stirred at 800 rpm at 80 °C for 3 h until its color became clear. Figure 8e,f shows the PVA–H2SO4 gel electrolyte coating process and the assembly procedure of the flexible SC. First, 1 mL of the gel electrolyte was coated on the PEDOT/rGO–CS electrode and naturally dried at room temperature for 1 day. This gel-electrolyte coating process was repeated three times. Finally, two gel-electrolyte-coated PEDOT/rGO–CS electrodes were pressed on the gel-electrolyte side to form a symmetric gel-electrolyte flexible SC.

4.4. Characterization of PEDOT/rGO–CS and SCs

A scanning electron microscope (JSM-IT100, JEOL) was used to examine the surface morphology of the PEDOT/rGO–CS composite electrode. An optical goniometer (model 100SB, Sindetake) was used for water contact angle measurement. XPS (Sigma Probe, Thermo VG Scientific) was used for characterizing the bonding configuration. The capacitance of the PEDOT/rGO–CS SC was evaluated by CV (0–0.8 V, potential scan rate of 2–200 mV/s), GCD (potential window of 0–0.8 V, constant currents of 0.5, 0.7, 1, 3, 5, 7, and 10 mA), and EIS (0.1 Hz to 100 kHz with an amplitude of 5 mV) using an electrochemical workstation (PGSTAT204, Metrohm) in a two-electrode configuration.

Acknowledgments

This study is financially supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program of the Higher Education Sprout Project by the Ministry of Education (110L9006) and the Ministry of Science and Technology in Taiwan (MOST 110-2634-F-002-043 and MOST 108-2221-E-002-088-MY3). Part of the SEM experiments were conducted by Yuan-Tze Lee at the Instrument Center in the Department of Materials Science and Engineering, National Taiwan University. XPS experiments were conducted by Li-Fan Chen at the High-resolution Analytical Instrumentation Center, National Central University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01601.

  • Color of EDOT solution after various electropolymerization times; low-magnification SEM images of CC/PEDOT, electropolymerization time: 5 min, 15 min, 30 min, 45 min, and 60 min; XPS survey spectra of CC, XPS survey spectra of CC/PEDOT-30 min, C1s spectra of CC/PEDOT-30 min, and S 2p spectra of CC/PEDOT-30 min; equivalent circuit model, Nyquist plot of PEDOT/rGO–CS SCs (high frequency), and Nyquist plot of PEDOT/rGO–CS SCs; schematic diagram of the bending tool and schematic diagram of the bending tool with the maximum curvature (1.5 cm–1); areal capacitance (mF/cm2) (Figure 6) at various potential scan rates; capacitance contribution of PEDOT/rGO–CS as analyzed by the Trasatti method; areal capacitance (mF/cm2) calculated based on GCD measurement results; power density (mW/cm2) and energy density (μW h/cm2) calculated based on GCD measurement results; and areal capacitance (mF/cm2) at different cycle numbers (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c01601_si_001.pdf (618KB, pdf)

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