# Ultralong-Cycling and Free-Standing Carboxylated Graphene/PEDOT:PSS Films as Electrode for Flexible Supercapacitors

Authors: Rongfang Wu; Xinjin Xu; Na Li; Congcong Liu; Xiao Chen; Zhihong Chen; Xiaoqi Lan; Qinglin Jiang; Jingkun Xu; Fengxing Jiang; Peipei Liu
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The rapid development of current wearable electronics motivates the promotion of energy storage devices. Supercapacitor with flexible, lightweight, highly efficient, and long cycling life is one of the potential and glittering candidates. Here, the composite film of carboxylated graphene (CG) and commercially available poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) are prepared as electrodes for flexible electrochemical capacitors by ultrasonication and vacuum filtration. The CG/PEDOT:PSS films were roundly characterized by SEM, XPS, FTIR, XRD and Raman spectrometer. The as-prepared free-standing films exhibited high specific capacitance (192.7 F g-1 at 0.5 A g-1), excellent rate capability (up to 80.4% from 0.5 to 20 A g-1), and good electrochemical stability (95.5% of specific capacitance retention after 50000 cycles). Surprisingly, the post treated free-standing films by Dimethyl Formamide (DMF) shows better capacitive properties than those of the pristine electrode, such as higher specific capacitance (204.3 F g-1 at 0.5 A g-1), and ultralong-cycling stability (104.6% retention at a high current density of 20 A g-1 after 50000 cycles). It is expected that the free-standing CG/PEDOT:PSS films can be used as flexible electrodes for various wearable energy storage devices.


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## Body

## 1. Introduction

Conducting polymers have attracted extensive attention in flexible electronic device applications due to their unique conjugated chain structures, excellent physical, and chemical properties. [1–4] The typical conducting polymers include polyacetylene, polypyrrole, polythiophene, polyaniline, and their derivatives. Among them, as a kind of the derivatives of polythiophene, PEDOT:PSS-based materials have been widely studied, owing to its advantage of conspicuous thermal stability, solution processability, fast charge/discharge kinetics, environmental friendliness, high transparency, and adjustable conductivity (10-4 ~ 103 S cm-1). [5–12] However, one of the challenges for the PEDOT:PSS-based materials is their poor cycling stability, which is caused by volume changes during repeated charge-discharge processes (insertion/deintercalation of counter ions). The expansion and contraction of PEDOT:PSS chains may result in structural instability and mechanical degradation, sequentially affecting their capacitive performance. [13, 14] In addition, PEDOT:PSS as an electrode usually suffers from compact morphology, because of its dense growth, limiting its sufficient contact with electrolytes [15, 16].Graphene, as a popular material in which carbon atoms are connected by sp2 hybridization, has attracted increasing attention due to its high theoretical surface area and fast electron transportation. [13, 17, 18] But, the near-perfect graphene surface has few active sites. Therefore, a series of methods are applied to change the chemical structure of graphene, such as surface functionalization, chemical doping, and chemical modification, etc. [19] Carboxylated graphene (CG) is to carboxylate the oxygen-containing groups (epoxy, carbonyl, and hydroxyl) between graphene oxide sheets, increasing the conjugated chain and the active site. The CG with carboxyl groups can easily disperse in water and interact with polymers strongly to form more structurally stable composites. [20] The composites of carboxylated graphene and pseudocapacitive materials possess both physical and mechanical properties. For example, Zhou et al. [21] prepared CG/PPy composite electrodes by the one-pot electrochemical polymerization method. The composite electrode showed a specific capacitance of 170.9 mF cm-2 at 0.5 mA cm-2, acceptable cycle stability (96.9% of initial specific capacitance maintained after 5000 cycles), and excellent rate capability. Nonetheless, the reported cycling stability of CG-based composites has not yet met the requirements for supercapacitors in practical application.In this work, CG/PEDOT:PSS films with high specific capacitance and long-term cycling stability as free-standing electrodes were prepared using a simple sonication and vacuum filtration method. The CG/PEDOT:PSS exhibit better electrochemical properties compared to previously reported electrodes, the main reasons are as follows: (1) PEDOT:PSS enters the CG sheet, which can prevent its stacking, increase the interlayer spacing, and provide a large number of active sites; [22, 23] (2) the interaction between carboxyl groups and PSS chain similar to hydrogen bond, forming cross-linked pores and increasing ion transport channels; and (3) the electrostatic attraction between the PEDOT+ cation and the −COO- anion makes the composite more stable and spontaneously forms an ordered structure, which is beneficial to the stability during long-term cycling. Besides, the treated electrode by DMF showed a high specific capacitance (204.3 F g-1 at 0.5 A g-1), a satisfactory rate capability (80.8% from 0.5 to 20 A g-1), and outstanding cycling stability (the initial specific capacitance retained 104.6% after 50000 cycles).


## 2. Experimental

### 2.1. Materials

PEDOT:PSS (Clevios PH1000, PEDOT:PSS=1:2.55, C=13.6mgmL−1) was purchased from Heraeus. Carboxylated graphene (CG, C=2mgmL−1) was obtained from Nanjing XFNANO Material Tech Co. Ltd. Ethanol (C2H5OH,AR,≥99.7%) and dimethylformamide (DMF,AR,≥99.5%) were bought from Shanghai Titan Scientific Co., Ltd. Dimethyl sulfoxide (DMSO,AR,≥99.0%) was acquired from Shanghai Lingfeng Chemical Reagent Co., Ltd. Ethylene Glycol (EG, AR, 99.0%) was achieved from Beijing J&K Scientific Co., Ltd. The water mentioned in the experiment was deionized water (Millipore Co., Ltd.).


### 2.2. Preparation of CG/PEDOT:PSS

Firstly, a certain amount of PEDOT:PSS was diluted in ethanol with a total volume of 5 mL. Then, mixed the CG and PEDOT:PSS by sonication for 15 mins. For comparison, various ratios of CG and PEDOT:PSS (0 : 2000, 1 : 2000, 2 : 2000, 4 : 2000, 10 : 2000, and 20 : 2000) mixture with a total mass of 5 mg were prepared. The CG/PEDOT:PSS films were obtained by vacuum filtration using PVDF membrane (diameter of 5.0 cm, pore size of 0.45μm). After stripping in DI water, the films were dried at room temperature and then placed in a 60°C blast oven for 3 h. Finally, a rectangular film of 1×1.2 cm was cut with a scalpel for electrochemical testing. The prepared films with different mass ratios of CG and PEDOT:PSS (0 : 2000, 1 : 2000, 2 : 2000, 4 : 2000, 10 : 2000, and 20 : 2000) were denoted as PG0, PG1, PG2, PG4, PG10, and PG20, respectively.


### 2.3. Polar Solvent Posttreatment of CG/PEDOT:PSS

To remove excess PSS, PG1 films were immersed in DMF, DMSO, and EG for 30 mins at room temperature, respectively. The solvent-treated PG1 films were sequentially rinsed with DI water and C2H5OH. After drying in vacuum for 1 h at 80°C, the post-treatment CG/PEDOT:PSS films were acquired. The PG1 films treated by DMF, DMSO, and EG are denoted as PG1-M, PG1-D, and PG1-E, respectively.


### 2.4. Materials Characterization

The morphology of the prepared CG/PEDOT:PSS was studied using scanning electron microscopy (SEM, JEOL, JSM-7500F, Japan). Structural characterization was performed by Nicolet Avatar 320 FT-IR spectrometer (Nicolet Instrument Corporation, Madison, WI, USA), UV-visible spectroscopy (SPECORD 200 PLUS), X-ray diffraction (XRD, Rigaku SmartLab SE) and Raman spectroscopy (Horiba Jobin-Yvon Lab-RAM H80). In addition, the elemental and chemical state composition of the CG/PEDOT:PSS films were analyzed by XPS (Thermo Fisher Scientific, USA).


### 2.5. Electrochemical Measurements

The electrochemical tests in this work were all performed using a CHI660E electrochemical workstation (Shanghai CH Instrument Co., Ltd., China) in 1 M H2SO4 electrolyte. Ag/AgCl, CG/PEDOT:PSS films and platinum mesh were used as reference electrodes, working electrodes, and counter electrodes, respectively. The electrochemical properties of CG/PEDOT:PSS films were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements.The specific capacitance (Cs) of the film electrode was evaluated according to the GCD curve, which is calculated by the following formula [24]:
(1)Cs=ItmVwhere I (A), t (s), m (g), and V (V) are the charge-discharge current, discharge time, the effective mass of the electrode material, and potential window, respectively.


## 2.1. Materials

PEDOT:PSS (Clevios PH1000, PEDOT:PSS=1:2.55, C=13.6mgmL−1) was purchased from Heraeus. Carboxylated graphene (CG, C=2mgmL−1) was obtained from Nanjing XFNANO Material Tech Co. Ltd. Ethanol (C2H5OH,AR,≥99.7%) and dimethylformamide (DMF,AR,≥99.5%) were bought from Shanghai Titan Scientific Co., Ltd. Dimethyl sulfoxide (DMSO,AR,≥99.0%) was acquired from Shanghai Lingfeng Chemical Reagent Co., Ltd. Ethylene Glycol (EG, AR, 99.0%) was achieved from Beijing J&K Scientific Co., Ltd. The water mentioned in the experiment was deionized water (Millipore Co., Ltd.).


## 2.2. Preparation of CG/PEDOT:PSS

Firstly, a certain amount of PEDOT:PSS was diluted in ethanol with a total volume of 5 mL. Then, mixed the CG and PEDOT:PSS by sonication for 15 mins. For comparison, various ratios of CG and PEDOT:PSS (0 : 2000, 1 : 2000, 2 : 2000, 4 : 2000, 10 : 2000, and 20 : 2000) mixture with a total mass of 5 mg were prepared. The CG/PEDOT:PSS films were obtained by vacuum filtration using PVDF membrane (diameter of 5.0 cm, pore size of 0.45μm). After stripping in DI water, the films were dried at room temperature and then placed in a 60°C blast oven for 3 h. Finally, a rectangular film of 1×1.2 cm was cut with a scalpel for electrochemical testing. The prepared films with different mass ratios of CG and PEDOT:PSS (0 : 2000, 1 : 2000, 2 : 2000, 4 : 2000, 10 : 2000, and 20 : 2000) were denoted as PG0, PG1, PG2, PG4, PG10, and PG20, respectively.


## 2.3. Polar Solvent Posttreatment of CG/PEDOT:PSS

To remove excess PSS, PG1 films were immersed in DMF, DMSO, and EG for 30 mins at room temperature, respectively. The solvent-treated PG1 films were sequentially rinsed with DI water and C2H5OH. After drying in vacuum for 1 h at 80°C, the post-treatment CG/PEDOT:PSS films were acquired. The PG1 films treated by DMF, DMSO, and EG are denoted as PG1-M, PG1-D, and PG1-E, respectively.


## 2.4. Materials Characterization

The morphology of the prepared CG/PEDOT:PSS was studied using scanning electron microscopy (SEM, JEOL, JSM-7500F, Japan). Structural characterization was performed by Nicolet Avatar 320 FT-IR spectrometer (Nicolet Instrument Corporation, Madison, WI, USA), UV-visible spectroscopy (SPECORD 200 PLUS), X-ray diffraction (XRD, Rigaku SmartLab SE) and Raman spectroscopy (Horiba Jobin-Yvon Lab-RAM H80). In addition, the elemental and chemical state composition of the CG/PEDOT:PSS films were analyzed by XPS (Thermo Fisher Scientific, USA).


## 2.5. Electrochemical Measurements

The electrochemical tests in this work were all performed using a CHI660E electrochemical workstation (Shanghai CH Instrument Co., Ltd., China) in 1 M H2SO4 electrolyte. Ag/AgCl, CG/PEDOT:PSS films and platinum mesh were used as reference electrodes, working electrodes, and counter electrodes, respectively. The electrochemical properties of CG/PEDOT:PSS films were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements.The specific capacitance (Cs) of the film electrode was evaluated according to the GCD curve, which is calculated by the following formula [24]:
(1)Cs=ItmVwhere I (A), t (s), m (g), and V (V) are the charge-discharge current, discharge time, the effective mass of the electrode material, and potential window, respectively.


## 3. Results and Discussion

### 3.1. Materials Characterization

Figure1(a) schematically illustrates the preparation process of the CG/PEDOT:PSS composite films. The free-standing films were obtained by two simple steps sonication and vacuum filtration. Then, the composite films were soaked in polar solvent to remove part of the PSS chain. As shown in Figure 1(b), the as-prepared films can be folded, revealing their high flexibility. In addition, it is clear that the pristine PEDOT:PSS film exhibits dense smooth surface, while the PG1 film possesses numerous spherical particles with uneven size and macropores in the polymer matrix, as exhibited in Figures 1(c) and (d), implying that the addition of CG may affect the chemical properties of the prepared composite film [25].Figure 1
(a) Schematic illustration of the preparation of CG/PEDOT:PSS films. (b) CG/PEDOT:PSS film lay flat, folding once, and folding twice. (c) SEM images of PEDOT:PSS and (d) PG1 composite film.
(a)(b)(c)(d)Figure2(a) demonstrates the UV-vis spectra of CG, PEDOT:PSS, and PG1 films. Obviously, CG possesses a characteristic peak at 228 nm and an acromion at 294 nm, which are credited to π-π∗ transition of aromatic C−C bonds and n−π∗ transition of C=O bonds, respectively [26–28]. The two absorption bands of PEDOT:PSS originate from the aromatic rings of PSS at 195 and 225 nm, and the broad peaks at 400~1100 nm are neutral states of PEDOT monopolarons and bipolarons. [28–32] The absorption spectrum of the PG1 exhibits the characteristic peaks of both the CG and PEDOT:PSS, confirming the successful preparation of the composite film.Figure 2
UV-vis absorption spectra (a) and FTIR absorption spectra (b) of CG, PEDOT:PSS, and PG1 film. (c) Raman spectra of CG, PEDOT:PSS, and PG1 without and with DMF treatment. (d) The survey XPS spectrum of the PEDOT:PSS and PG1. (e) XPS spectrum of S 2p of PG1 before and after DMF treatment.
(a)(b)(c)(d)(e)Figure2(b) presents the FT-IR spectra of CG, PEDOT:PSS, and PG1 film. The characteristic peaks at 1734, 1632, and 1387 cm-1 of CG can be seen, which correspond to the carboxyl group C=O, stretching vibrations corresponding to C=C and − COO− bending vibration, respectively, confirming that CG contained carboxylic and carbon-carbon double bond, respectively. [26, 33] For the PEDOT:PSS film, there are two obvious characteristic peaks at 1632 cm-1 (C=C alkane stretching vibration peak) and 1502 cm-1 (C=C aromatic stretching vibration peak), respectively. Moreover, the peaks at 1174 and 683 cm-1 are ascribed to the C − S stretching vibrations on the PSS ring. [34, 35] The vibrational bands can be observed for PG1, the characteristic peak of −COO− shifted from 1387 to 1384 cm-1, which may result from the π-π stacking of CG and PEDOT:PSS [36].The structural changes of CG, PEDOT:PSS, and PG1 composite films were characterized by Raman spectroscopy. The Raman spectrum of the CG in Figure 2(c) demonstrates the presence of D (1350 cm-1) and G band (1591 cm-1). The D band originates from sp3 vibrations of defective and disordered carbon atoms, and the G band can be attributed to the sp2 vibration of carbon atoms in a 2D hexagonal lattice. [37–39] In the spectrum of PEDOT:PSS, the main peaks at 437, 676, 575 and 988, 1108, 1256, 1424, 1503, and 1553 cm-1 can be ascribed to SO2 bending, symmetric C − S − C deformation, oxyethylene ring deformation, C − O − C deformation, C − C interring stretching, C=C symmetrical stretching, C=C asymmetrical stretching, and C=C antisymmetrical stretching, respectively. [40, 41] However, the intensity of the peak of PSS is weak, because a part of PSS is carried away by the solvent during the preparation process. In the Raman spectrum of PG1, the peak related to C=C symmetrical stretching transformed from 1424 to 1421 cm-1, further indicating the successful preparation of the CG/PEDOT:PSS composites. The X-ray diffraction patterns of the PEDOT:PSS, CG, and PG1 films are presented in Figure S1. The PEDOT:PSS peak in the XRD of PG1 is shifted and the diffraction peak of CG almost disappears, demonstrating that CG may be wrapped, further suggesting π-π superposition and electrostatic interactions between PEDOT:PSS and CG [42].The surface chemistry of PG1 film was further investigated by XPS. There are four elements in the full spectrum, i.e. C, N, O, and S, as shown in Figure 2(d). Element N comes from the carboxyl graphene branch chain. The S element has two different types of peaks: the S2p3/2 and S2p1/2 peaks at 168.1 eV and 169.4 eV correspond to the element S in PSS sulfonic group, and the doublet S2p3/2 and S2p1/2 peaks at 163.7 eV and 165.1 eV represent the PEDOT ring of the element S in the thiophene, respectively. [35, 43–45] As displayed in Figure 2(e), the ratios of PEDOT and PSS in PG1 and PG1-M films were 0.71 and 0.62, respectively, which were calculated by the peak area of S 2p spectra, confirming partial removal of PSS during solvent treatment process [46, 47].


### 3.2. Electrochemical Measurements

To investigate the electrochemical properties of PG films, the electrochemical properties of CG were tested, and the composite with different weight ratios of CG and PEDOT:PSS was prepared. As shown in FigureS2, the specific capacitance of pure CG at 1 A g-1 is only 10.4 F g-1. The CV curves of PG film with different CG contents (PG0, PG1, PG2, PG4, PG10, and PG20) at 5 mV s-1 are shown in Figure 3(a). All CV curves exhibit regular rectangle characteristics, implying their remarkable capacitive properties. Figure 3(b) displays the GCD plots of the composites at 1 A g-1. Obviously, PG1 shows a longer discharge time, indicating that the interaction between CG and PEDOT:PSS contributes to the improvement of capacitive performance. The GCD curves of PG1 are exhibited in Figure 3(c) under potential window of 0 ~ 0.8 V. It is can be seen that the GCD curve still maintains a regular isosceles triangle even at a high current density of 20 A g-1, which further proves the outstanding capacitive performance of PG1 electrode. Figure 3(d) shows the calculated specific capacitances of PG1 based on the GCD curves, revealing remarkable rate capability of 80.4% from 0.5 to 20 A g-1. In addition, the PG1 film shows similar discharge time under 0, 90, and 180° bends, suggesting the good flexibility of the prepared electrode, as displayed in Figure S3.Figure 3
Electrochemical performance of the prepared PG electrodes: (a) CV curves at 5 mV s-1, (b) GCD profiles at 1 A g-1. Capacitive properties of PG1 electrode: (c) GCD profiles at current densities of 0.5, 1, 2, 5, 10, 15, 20 A g-1, (d) Specific capacitance, (e) Cycling stability test (The inset demonstrates the GCD plots before and after 3000, 15000, 30000 and 50000 charge-discharge cycles at 5 A g-1), (f) The Nyquist plots of PG0 and PG1 (The inset shows a magnified high-frequency region).
(a)(b)(c)(d)(e)(f)The long-term cycling stability of electrode materials is an essential parameter for electrochemical evaluation. As demonstrated in Figure3(e), the PG1 composite electrode exhibits splendid cycling stability after 50000 charge-discharge cycles at 5 A g-1 with specific capacitance retention of 95.5%, which is better than those of the PEDOT:PSS, as reported in our previous work. [48] The superior capacitive property can be attributed to the electrostatic attraction of ions and the covalent connections between CG and PEDOT:PSS. The inset of Figure 3(e) shows the GCD curves before and after 3000, 15000, 30000, and 50000 charge-discharge cycles. Apparently, the discharge time under the GCD curve decreases first, then increases, and finally decreases slowly. In the first 3000 cycles, the capacitance decreased, probably because the ion/electron transport channel was not fully open. With the continuous insertion and extraction of ions, the transport channel was opened during the cycling process, facilitating adequate contact between the electrolyte and the electrode material. Meanwhile, the addition of CG gave enough space for the expansion and contraction of PEDOT:PSS. [49, 50] Finally, as the number of cycles increases, the active material is continuously depleted, resulting in a decrease in electrode capacitance. Figure 3(f) exhibits the Nyquist plots of PG0 and PG1 in the frequency range from 105 to 10-2 Hz. The PG1 electrode shows a solution resistance value (Rs) of 1.7 Ω, which consist of the electronic resistance of the electrode and the ionic resistance of the electrolyte, suggesting that the PG1 electrode has a high conductivity. [51] The near-vertical line in the low-frequency region manifests remarkable capacitive behavior. Furthermore, the relatively small semicircle of the PG1 electrode seen in the high-frequency region indicates a lower charge transfer resistance (Rct), as shown in the inset of Figure 3(f). The pure PEDOT:PSS film shows very similar Nyquist plots to PG1, but it possesses a lower slope in the low-frequency region and a larger semicircle in the high-frequency region, which could account for their poor capacitive performance. In short, the free-standing PG1 electrode shows better capacitive properties compared to those of other prepared materials. Therefore, PG1 was selected for further electrochemical investigations.To achieve better electrochemical performance, the PG1 films were treated with various polar solvents (DMF, DMSO, EG). The CV curves of the PG1 films show regular rectangles, indicating that they have good capacitive performance, as displayed in Figure 4(a). Figure 4(b) shows the GCD behaviors of all films at 0.5 A g-1. And the corresponding specific capacitance of PG1, PG1-M, PG1-D, and PG1-E calculated by the formula (1) are 192.7, 204.3, 189.8, and 183.1 F g-1, respectively, as can be seen from Figure 4(c). The results show that PG1-M has the highest Cs, which may be because the treated PEDOT turns into the quinoid structure, which is more favorable for charge transfer [52].Figure 4
Comparisons of electrochemical performance of PG1, PG1-M, PG1-D, and PG1-E: (a) CV curves at 5 mV s-1, (b) GCD curves at 0.5 A g-1, (c) Specific capacitance at 0.5 A g-1, and (d) Nyquist plot (the inset shows the magnified view of the high-frequency region of the impedance spectra and fitting electrical equivalent circuit).
(a)(b)(c)(d)Although the films treated with DMSO and EG have the same effect as above, theirCs was not improved. Probably because the films treated by DMSO and EG become more compact, which is not conducive to the transport of charges, resulting in a slight decrease of Cs. [48] The EIS curves and fitted equivalent circuit diagrams for the PG1 electrodes treated by various solvents are shown in Figure 4(d). Rs, CPE, Rct, and Wo represent the solution resistance, double-layer capacitance, charge transfer resistance, and diffusion-controlled Warburg impedance, respectively. As listed in Table S1, the PG1 treated by DMF in the high-frequency region has the lowest Rct, implying the highest capacitive properties of the PG1-M electrode compared to those of PG1, PG1-D, and PG1-E.The electrochemical properties of PG1-M were further systematically investigated. The CV curves retain their rectangular shape even at a high scan rate of 100 mV s-1, signifying favorable fast charge-discharge kinetics of the PG1-M electrode, as exhibited in Figure 5(a). The GCD profiles with negligible voltage drop also demonstrate fast charge transfer and ion transport of the electrode (Figure 5(b)). As shown in Figure 5(c), the rate capability of PG1-M is as high as 80.8% from the current density of 0.5 to 20 A g-1.Figure 5
Electrochemical performance of the PG1-M film in 1 M H2SO4 electrolyte: (a) CV curves at 5, 10, 20, 50, and 100 mV s-1 scan rates, (b) GCD profiles at current densities of 0.5, 1, 2, 5, 10, 15, and 20 A g-1, (c) specific capacitance at different current densities, (d) Nyquist plot (the inset shows a magnified high-frequency region), and (e) cycling stability study during 50000 charge-discharge cycles (the inset shows SEM images of the PG1-M electrode before and after cycling.).
(a)(b)(c)(d)(e)The Nyquist plot in Figure5(d) shows a semicircle with a small radius in the high-frequency region, indicating the low charge transfer impedance of the PG1-M electrode. The almost perpendicular line in the low-frequency region verifies the fast transport of ions during the charging-discharging of the treated electrode. The cycling stability of the PG1 composite treated by DMF is evaluated, as displayed in Figure 5(e), manifesting that the initial specific capacitance retention rate reaches 104.6% after 50000 cycles even at a high current density of 20 A g-1. Probably because with the continuous charging and discharging, more and more active sites appear, enhancing the specific capacitance. [48] Exhilaratingly, the capacitive properties of the treated PG1-M electrode are better than those of reported conducting polymer based-materials, as listed in Table 1. The possible reasons for the satisfactory capacitive performance of the treated composite are as followings: (1) the DMF solvent can remove parts of the PSS, accommodating large volumetric expansion and contraction during charging-discharging and alleviating degradation of PEDOT:PSS [47, 51] ; (2) the exposed active sites of the treated free-standing films increased with the continuous cycles, improving the specific capacitance of the electrode; and (3) there is still a part of PSS that may be removed during the charging and discharging process, which makes the film surface rougher, increasing the specific capacitance [48, 53].Table 1
Capacitive performance comparisons of PG1-M with previously reported conducting polymer based-materials electrode materials.
Electrode materialsCs (F g-1)Current density (A g-1)Retention (cycles)Ref.PPy/GO144291.2% at 0.5 A g-1 (4000)42PEDOT:PSS591082.1% at 10 A g-1 (5000)54PEDOT:PSS:EG/rGO1740.590% at 0.5 A g-1 (5000)55PEDOT:PSS/rGO1601091% at 5 A g-1 (5000)56G-COOZn2380.05>95% at 0.05 A g-1 (500)57PTh/MWCNT110190% at 1 A g-1 (1000)58PEDOT:PSS/rGO62195% at 1 A g-1 (10000)59PG1-M204.3 1650.5 20104.6% at 20 A g-1 (50000)This workNote: GO: graphene oxide; rGO: reduced graphene oxide; G-COOZn: carboxylated graphene/ZnO; MWCNT: multi-walled carbon nanotubes.


## 3.1. Materials Characterization

Figure1(a) schematically illustrates the preparation process of the CG/PEDOT:PSS composite films. The free-standing films were obtained by two simple steps sonication and vacuum filtration. Then, the composite films were soaked in polar solvent to remove part of the PSS chain. As shown in Figure 1(b), the as-prepared films can be folded, revealing their high flexibility. In addition, it is clear that the pristine PEDOT:PSS film exhibits dense smooth surface, while the PG1 film possesses numerous spherical particles with uneven size and macropores in the polymer matrix, as exhibited in Figures 1(c) and (d), implying that the addition of CG may affect the chemical properties of the prepared composite film [25].Figure 1
(a) Schematic illustration of the preparation of CG/PEDOT:PSS films. (b) CG/PEDOT:PSS film lay flat, folding once, and folding twice. (c) SEM images of PEDOT:PSS and (d) PG1 composite film.
(a)(b)(c)(d)Figure2(a) demonstrates the UV-vis spectra of CG, PEDOT:PSS, and PG1 films. Obviously, CG possesses a characteristic peak at 228 nm and an acromion at 294 nm, which are credited to π-π∗ transition of aromatic C−C bonds and n−π∗ transition of C=O bonds, respectively [26–28]. The two absorption bands of PEDOT:PSS originate from the aromatic rings of PSS at 195 and 225 nm, and the broad peaks at 400~1100 nm are neutral states of PEDOT monopolarons and bipolarons. [28–32] The absorption spectrum of the PG1 exhibits the characteristic peaks of both the CG and PEDOT:PSS, confirming the successful preparation of the composite film.Figure 2
UV-vis absorption spectra (a) and FTIR absorption spectra (b) of CG, PEDOT:PSS, and PG1 film. (c) Raman spectra of CG, PEDOT:PSS, and PG1 without and with DMF treatment. (d) The survey XPS spectrum of the PEDOT:PSS and PG1. (e) XPS spectrum of S 2p of PG1 before and after DMF treatment.
(a)(b)(c)(d)(e)Figure2(b) presents the FT-IR spectra of CG, PEDOT:PSS, and PG1 film. The characteristic peaks at 1734, 1632, and 1387 cm-1 of CG can be seen, which correspond to the carboxyl group C=O, stretching vibrations corresponding to C=C and − COO− bending vibration, respectively, confirming that CG contained carboxylic and carbon-carbon double bond, respectively. [26, 33] For the PEDOT:PSS film, there are two obvious characteristic peaks at 1632 cm-1 (C=C alkane stretching vibration peak) and 1502 cm-1 (C=C aromatic stretching vibration peak), respectively. Moreover, the peaks at 1174 and 683 cm-1 are ascribed to the C − S stretching vibrations on the PSS ring. [34, 35] The vibrational bands can be observed for PG1, the characteristic peak of −COO− shifted from 1387 to 1384 cm-1, which may result from the π-π stacking of CG and PEDOT:PSS [36].The structural changes of CG, PEDOT:PSS, and PG1 composite films were characterized by Raman spectroscopy. The Raman spectrum of the CG in Figure 2(c) demonstrates the presence of D (1350 cm-1) and G band (1591 cm-1). The D band originates from sp3 vibrations of defective and disordered carbon atoms, and the G band can be attributed to the sp2 vibration of carbon atoms in a 2D hexagonal lattice. [37–39] In the spectrum of PEDOT:PSS, the main peaks at 437, 676, 575 and 988, 1108, 1256, 1424, 1503, and 1553 cm-1 can be ascribed to SO2 bending, symmetric C − S − C deformation, oxyethylene ring deformation, C − O − C deformation, C − C interring stretching, C=C symmetrical stretching, C=C asymmetrical stretching, and C=C antisymmetrical stretching, respectively. [40, 41] However, the intensity of the peak of PSS is weak, because a part of PSS is carried away by the solvent during the preparation process. In the Raman spectrum of PG1, the peak related to C=C symmetrical stretching transformed from 1424 to 1421 cm-1, further indicating the successful preparation of the CG/PEDOT:PSS composites. The X-ray diffraction patterns of the PEDOT:PSS, CG, and PG1 films are presented in Figure S1. The PEDOT:PSS peak in the XRD of PG1 is shifted and the diffraction peak of CG almost disappears, demonstrating that CG may be wrapped, further suggesting π-π superposition and electrostatic interactions between PEDOT:PSS and CG [42].The surface chemistry of PG1 film was further investigated by XPS. There are four elements in the full spectrum, i.e. C, N, O, and S, as shown in Figure 2(d). Element N comes from the carboxyl graphene branch chain. The S element has two different types of peaks: the S2p3/2 and S2p1/2 peaks at 168.1 eV and 169.4 eV correspond to the element S in PSS sulfonic group, and the doublet S2p3/2 and S2p1/2 peaks at 163.7 eV and 165.1 eV represent the PEDOT ring of the element S in the thiophene, respectively. [35, 43–45] As displayed in Figure 2(e), the ratios of PEDOT and PSS in PG1 and PG1-M films were 0.71 and 0.62, respectively, which were calculated by the peak area of S 2p spectra, confirming partial removal of PSS during solvent treatment process [46, 47].


## 3.2. Electrochemical Measurements

To investigate the electrochemical properties of PG films, the electrochemical properties of CG were tested, and the composite with different weight ratios of CG and PEDOT:PSS was prepared. As shown in FigureS2, the specific capacitance of pure CG at 1 A g-1 is only 10.4 F g-1. The CV curves of PG film with different CG contents (PG0, PG1, PG2, PG4, PG10, and PG20) at 5 mV s-1 are shown in Figure 3(a). All CV curves exhibit regular rectangle characteristics, implying their remarkable capacitive properties. Figure 3(b) displays the GCD plots of the composites at 1 A g-1. Obviously, PG1 shows a longer discharge time, indicating that the interaction between CG and PEDOT:PSS contributes to the improvement of capacitive performance. The GCD curves of PG1 are exhibited in Figure 3(c) under potential window of 0 ~ 0.8 V. It is can be seen that the GCD curve still maintains a regular isosceles triangle even at a high current density of 20 A g-1, which further proves the outstanding capacitive performance of PG1 electrode. Figure 3(d) shows the calculated specific capacitances of PG1 based on the GCD curves, revealing remarkable rate capability of 80.4% from 0.5 to 20 A g-1. In addition, the PG1 film shows similar discharge time under 0, 90, and 180° bends, suggesting the good flexibility of the prepared electrode, as displayed in Figure S3.Figure 3
Electrochemical performance of the prepared PG electrodes: (a) CV curves at 5 mV s-1, (b) GCD profiles at 1 A g-1. Capacitive properties of PG1 electrode: (c) GCD profiles at current densities of 0.5, 1, 2, 5, 10, 15, 20 A g-1, (d) Specific capacitance, (e) Cycling stability test (The inset demonstrates the GCD plots before and after 3000, 15000, 30000 and 50000 charge-discharge cycles at 5 A g-1), (f) The Nyquist plots of PG0 and PG1 (The inset shows a magnified high-frequency region).
(a)(b)(c)(d)(e)(f)The long-term cycling stability of electrode materials is an essential parameter for electrochemical evaluation. As demonstrated in Figure3(e), the PG1 composite electrode exhibits splendid cycling stability after 50000 charge-discharge cycles at 5 A g-1 with specific capacitance retention of 95.5%, which is better than those of the PEDOT:PSS, as reported in our previous work. [48] The superior capacitive property can be attributed to the electrostatic attraction of ions and the covalent connections between CG and PEDOT:PSS. The inset of Figure 3(e) shows the GCD curves before and after 3000, 15000, 30000, and 50000 charge-discharge cycles. Apparently, the discharge time under the GCD curve decreases first, then increases, and finally decreases slowly. In the first 3000 cycles, the capacitance decreased, probably because the ion/electron transport channel was not fully open. With the continuous insertion and extraction of ions, the transport channel was opened during the cycling process, facilitating adequate contact between the electrolyte and the electrode material. Meanwhile, the addition of CG gave enough space for the expansion and contraction of PEDOT:PSS. [49, 50] Finally, as the number of cycles increases, the active material is continuously depleted, resulting in a decrease in electrode capacitance. Figure 3(f) exhibits the Nyquist plots of PG0 and PG1 in the frequency range from 105 to 10-2 Hz. The PG1 electrode shows a solution resistance value (Rs) of 1.7 Ω, which consist of the electronic resistance of the electrode and the ionic resistance of the electrolyte, suggesting that the PG1 electrode has a high conductivity. [51] The near-vertical line in the low-frequency region manifests remarkable capacitive behavior. Furthermore, the relatively small semicircle of the PG1 electrode seen in the high-frequency region indicates a lower charge transfer resistance (Rct), as shown in the inset of Figure 3(f). The pure PEDOT:PSS film shows very similar Nyquist plots to PG1, but it possesses a lower slope in the low-frequency region and a larger semicircle in the high-frequency region, which could account for their poor capacitive performance. In short, the free-standing PG1 electrode shows better capacitive properties compared to those of other prepared materials. Therefore, PG1 was selected for further electrochemical investigations.To achieve better electrochemical performance, the PG1 films were treated with various polar solvents (DMF, DMSO, EG). The CV curves of the PG1 films show regular rectangles, indicating that they have good capacitive performance, as displayed in Figure 4(a). Figure 4(b) shows the GCD behaviors of all films at 0.5 A g-1. And the corresponding specific capacitance of PG1, PG1-M, PG1-D, and PG1-E calculated by the formula (1) are 192.7, 204.3, 189.8, and 183.1 F g-1, respectively, as can be seen from Figure 4(c). The results show that PG1-M has the highest Cs, which may be because the treated PEDOT turns into the quinoid structure, which is more favorable for charge transfer [52].Figure 4
Comparisons of electrochemical performance of PG1, PG1-M, PG1-D, and PG1-E: (a) CV curves at 5 mV s-1, (b) GCD curves at 0.5 A g-1, (c) Specific capacitance at 0.5 A g-1, and (d) Nyquist plot (the inset shows the magnified view of the high-frequency region of the impedance spectra and fitting electrical equivalent circuit).
(a)(b)(c)(d)Although the films treated with DMSO and EG have the same effect as above, theirCs was not improved. Probably because the films treated by DMSO and EG become more compact, which is not conducive to the transport of charges, resulting in a slight decrease of Cs. [48] The EIS curves and fitted equivalent circuit diagrams for the PG1 electrodes treated by various solvents are shown in Figure 4(d). Rs, CPE, Rct, and Wo represent the solution resistance, double-layer capacitance, charge transfer resistance, and diffusion-controlled Warburg impedance, respectively. As listed in Table S1, the PG1 treated by DMF in the high-frequency region has the lowest Rct, implying the highest capacitive properties of the PG1-M electrode compared to those of PG1, PG1-D, and PG1-E.The electrochemical properties of PG1-M were further systematically investigated. The CV curves retain their rectangular shape even at a high scan rate of 100 mV s-1, signifying favorable fast charge-discharge kinetics of the PG1-M electrode, as exhibited in Figure 5(a). The GCD profiles with negligible voltage drop also demonstrate fast charge transfer and ion transport of the electrode (Figure 5(b)). As shown in Figure 5(c), the rate capability of PG1-M is as high as 80.8% from the current density of 0.5 to 20 A g-1.Figure 5
Electrochemical performance of the PG1-M film in 1 M H2SO4 electrolyte: (a) CV curves at 5, 10, 20, 50, and 100 mV s-1 scan rates, (b) GCD profiles at current densities of 0.5, 1, 2, 5, 10, 15, and 20 A g-1, (c) specific capacitance at different current densities, (d) Nyquist plot (the inset shows a magnified high-frequency region), and (e) cycling stability study during 50000 charge-discharge cycles (the inset shows SEM images of the PG1-M electrode before and after cycling.).
(a)(b)(c)(d)(e)The Nyquist plot in Figure5(d) shows a semicircle with a small radius in the high-frequency region, indicating the low charge transfer impedance of the PG1-M electrode. The almost perpendicular line in the low-frequency region verifies the fast transport of ions during the charging-discharging of the treated electrode. The cycling stability of the PG1 composite treated by DMF is evaluated, as displayed in Figure 5(e), manifesting that the initial specific capacitance retention rate reaches 104.6% after 50000 cycles even at a high current density of 20 A g-1. Probably because with the continuous charging and discharging, more and more active sites appear, enhancing the specific capacitance. [48] Exhilaratingly, the capacitive properties of the treated PG1-M electrode are better than those of reported conducting polymer based-materials, as listed in Table 1. The possible reasons for the satisfactory capacitive performance of the treated composite are as followings: (1) the DMF solvent can remove parts of the PSS, accommodating large volumetric expansion and contraction during charging-discharging and alleviating degradation of PEDOT:PSS [47, 51] ; (2) the exposed active sites of the treated free-standing films increased with the continuous cycles, improving the specific capacitance of the electrode; and (3) there is still a part of PSS that may be removed during the charging and discharging process, which makes the film surface rougher, increasing the specific capacitance [48, 53].Table 1
Capacitive performance comparisons of PG1-M with previously reported conducting polymer based-materials electrode materials.
Electrode materialsCs (F g-1)Current density (A g-1)Retention (cycles)Ref.PPy/GO144291.2% at 0.5 A g-1 (4000)42PEDOT:PSS591082.1% at 10 A g-1 (5000)54PEDOT:PSS:EG/rGO1740.590% at 0.5 A g-1 (5000)55PEDOT:PSS/rGO1601091% at 5 A g-1 (5000)56G-COOZn2380.05>95% at 0.05 A g-1 (500)57PTh/MWCNT110190% at 1 A g-1 (1000)58PEDOT:PSS/rGO62195% at 1 A g-1 (10000)59PG1-M204.3 1650.5 20104.6% at 20 A g-1 (50000)This workNote: GO: graphene oxide; rGO: reduced graphene oxide; G-COOZn: carboxylated graphene/ZnO; MWCNT: multi-walled carbon nanotubes.


## 4. Conclusions

The free-standing CG/PEDOT:PSS films were obtained by simple sonication and vacuum filtration steps. The prepared free-standing electrode shows an acceptable specific capacitance of 192.7 F g-1 at 0.5 A g-1 and superior cycling stability with 95.5% capacitance retention after 50000 charge-discharge cycles at 5 A g-1. The remarkable electrochemical performance can be ascribed to the interaction between the CG and PEDOT:PSS. More excitingly, the treated CG/PEDOT:PSS electrode by DMF displays a better capacitive performance compared to the pristine film. In particular, the treated film demonstrates superior cycling stability (104.6% retention after 50000 cycles at 20 A g-1) than previously reported conductive polymer-based electrode materials. In sum, this investigation provides the possibility to achieve capacitive electrodes with ultralong cycle life and satisfactory specific capacitance for energy storage devices.



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*Source: 5695694-2023-02-09.xml*