MoS2 composite film

Colloids and Surfaces A 581 (2019) 123815

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High performance flexible solid-state asymmetric supercapacitor composed of a polyaniline/PEDOT/polyaniline/ultralarge reduced graphene oxide tetralayer film and a PEDOT/MoS2 composite film Wai-Hwa Khoh, Boon-Hong Wee, Jong-Dal Hong

T

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Department of Chemistry, University of Incheon, 119 Academy-ro Yeonsu-gu, Incheon, 22012, Republic of Korea

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Flexible asymmetric supercapacitor Polyaniline Ultralarge reduced graphene oxide PEDOT MoS2 Capacitive performance

A flexible solid-state asymmetric supercapacitor (ASC) was assembled with a polyaniline (PANi)/PEDOT/PANi/ ultralarge reduced graphene oxide (UrGO) tetralayer film (denoted as PPPrG, used as the positive electrode) and a PEDOT/MoS2 film (denoted as PMo, used as the negative electrode) on a flexible polyethylene terephthalate (PET) substrate with polyvinyl alcohol/H2SO4 gel electrolyte. The PPPrG tetralayers were fabricated on PET via a layer-by-layer self-assembly method, whereas the PMo electrode was fabricated on PET using a drop-coating approach. The ASC exhibited a maximum energy density of 5.4 mW h/cm3 at a power density of 110 mW/cm3, demonstrating excellent electrochemical performance without performance loss despite various deformation states due to high conductivity and flexibility of the multilayer film electrodes, which act as both the active electrode and the current collector. The ASC retained an energy density of 4.0 mW h/cm3 at a power density of 265 mW/cm3 with an optimized cell voltage of 0.8 V. The excellent performance of this ASC was ascribed mainly to the conductivity of the multilayer film electrodes, which was improved by the combination of PANi, UrGO and PEDOT layers, which functioned as conducting networks in the nanostructured composite films. This flexible ASC system based on multilayer film electrodes with superior capacitive performance offers substantial promise for use in grid-scale energy storage devices integrated into portable and bio-implantable microelectronic devices.

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Corresponding author. E-mail address: [email protected] (J.-D. Hong).

https://doi.org/10.1016/j.colsurfa.2019.123815 Received 8 August 2019; Received in revised form 15 August 2019; Accepted 16 August 2019 Available online 17 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

Colloids and Surfaces A 581 (2019) 123815

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1. Introduction

graphene oxide (UrGO) tetralayers (denoted as PPPrG and used as a positive electrode), and a film composed of PEDOT and MoS2 sheets (denoted as PMo and used as a negative electrode). The PPPrG tetralayers were fabricated on a flexible PET substrate using the layer-bylayer (LBL) assembly technique, which enables not only precise control over the film thickness and morphology at the nanoscale level, but also the incorporation of various functional materials within a single film over a full range of compositions [21]. The PMo films were fabricated on PET substrates by drop-coating solutions containing PEDOT and MoS2 sheets, the composition of which was varied to optimize the conductivity of the PMo film for high electrochemical performance of the device. The electrochemical performance of the ASC assembled with PPPrG and PMo electrodes without novel metal current collectors and binders were assessed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge measurements. The results were used to determine the volumetric capacitance, electron transfer resistance, long-term cycling stability, and electromechanical durability of the devices.

Supercapacitors have been attracting increasing interest as promising energy storage devices. Because of their advantages, they are considered an intermediate system between batteries and other power sources; these advantages include high power and energy delivery, long cycle life, a wide range of operating temperatures, and environmental friendliness [1]. The rapid development of flexible and wearable electronic technology has triggered increasing demand for ultrathin, flexible, miniaturized, lightweight, highly efficient, and less toxic energy storage devices. In recent years, substantial advances have been made toward thin and/or free-standing flexible supercapacitor electrodes [2,3], including wire-shaped [4], fiber-shaped coaxial [5], carbon nanotube (CNT)-based smart textiles and highly stretchable fiber-shaped electrodes [6,7], core/shell nanowire [8–10] and petal-like nanosheets [11]. However, the performances of these supercapacitors are still limited by their inherent low specific capacitance as a result of their low charge storage, which is related to their electrochemical double layers (EDLs) [12]. The energy density (E) of a capacitor is given by the equation E = ½CV 2, where C and V represent the cell capacitance and the cell voltage, respectively. Voltage, being a squared term, plays a major role in determining the energy density of a supercapacitor. The most promising approach to increasing the working voltage window of a supercapacitor is to prepare asymmetric supercapacitors (ASCs) with dissimilar electrodes. A supercapacitor with an asymmetric electrode configuration can be fabricated with two different electrodes with different working potential windows in the same electrolyte system, leading to a greater cell voltage and thus increasing the energy density of the supercapacitor [13,14]. An ASC composed of reduced graphene oxide (rGO) sheets modified with ruthenium oxide (rGO-RuO2) (as a negative electrode) and polyaniline (rGO-PANi) (as a positive electrode) showed substantial improvement in the capacitive performance, compared with the symmetric supercapacitors fabricated with two rGO-RuO2 or two rGO-PANi electrodes [14,15]. Most flexible electrode materials, including RuO2 and CNTs on Au, still have shortcomings in practical applications, including high raw material costs and poor mechanical and chemical stability of the conducting polymers upon repeated intercalation and deintercalation of ions during the charging and discharging processes [16]. Recently, researchers have widely incorporated conducting polymers, including PANi, polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT), into carbonaceous materials in designing asymmetric supercapacitors with improved performance [17]. Conducting polymers have shown great promise in electrode materials for flexible pseudocapacitors because of their various advantageous properties, which include high redox-active capacitance, high conductivity, and high intrinsic flexibility [12]. Conventionally, slurry-based chemically polymerized conducting polymers have been mixed with a binder (or additives) to construct an electrode. Such electrodes suffer from sluggish ion transport during the redox reaction because of their high interfacial resistances and the inherent resistance of the binder materials, leading to poor device performance [12]. Thus, conducting polymers have been directly grown on current collectors for supercapacitor applications (e.g., PANi/PEDOT films electrochemically deposited onto Au-coated plastic substrates [18], multiwalled carbon nanotubes (MWCNTs)/PANi and MWCNT/PPy on polyethylene terephthalate (PET) substrates [17], PEDOT/CNTs [13], and PEDOT on Au-coated plastic substrate) [19]. Thin-film electronic technology opens a range of new applications and has various advantages for flexible and transparent small devices, which can be seamlessly attached to various objects such as bottles, patches, clothes, food, packages, machines and cars [20]. The development of flexible energy storage and conversion devices is required to satisfy the rapid increase in the demand for these flexible electronics. Thus, this article describes a flexible solid-state ASC assembled with a tetralayer film composed of PANi/PEDOT/PANi/ultralarge reduced

2. Experimental 2.1. Materials Natural graphite flake (particle size = -10 mesh) was purchased from Alfa Aesar; potassium permanganate (KMnO4) and polyaniline powder (emeraldine base, Mw-50,000) were purchased from SigmaAldrich, N,N-dimethylacetamide (DMAc, 99.5%,extra pure) was purchased from Acros organics, hydrochloric acid (HCl, 35%), concentrated sulfuric acid (H2SO4, 95%), and hydrogen peroxide (H2O2, 30%) were purchased from Samchun Pure Chemical Co., Ltd. (South Korea). Fuming nitric acid (HNO3, 93%) was purchased from Matsunoen Chemicals Ltd (Japan), and ammonium hydroxide (NH4OH, 29%) was purchased from Mallinckrodt Baker Inc (NJ, USA). PEDOT:PSS solution (1.3 wt%, CleviosTM PH1000) was purchased from Heraeus Precious Metals Gmbh & Co.KG (Germany). Polyvinyl alcohol (PVA) from DC Chemical Co. Ltd (South Korea), and silver paint was purchased from SPI supplies (USA). Transparent PET films were purchased from Saehan Industries (South Korea), silicon wafers (SiO2 thickness = 108 nm, diameter = ˜ 100 mm) was purchased from MEMC Electronic Materials Inc. (Malaysia), and fused quartz glass was purchased from Maicom Quartz GMbH (Germany). All chemicals were analytical grade, and used as received. Deionized water (18 MΩ·cm) was used for all the experiments and cleaning steps. 2.2. Synthesis 2.2.1. Preparation of substrates Quartz glass and silicon wafers used for LBL-assembly were cleaned by sonication in piranha solution (H2SO4/H2O2 = 7/3) for 1 h at 70 °C, and followed by sonication in RCA solution (H2O/H2O2/NH4OH = 5/ 1/1) for 1 h at 70 °C. The surface of PET substrates was cleaned using Digital UV-Ozone system (PSD series, Novascan) for 1 h, and then by sonication in acetone and DI water for 15 min, respectively. 2.2.2. Preparation of ultra-large graphene oxide (UGO) for LBL-assembly A UGO suspension was prepared based on the modified Hummer’s method using thermally expanded graphite [22–24]. In brief, 2 g of natural graphite flake were mixed into 60 ml of conc. H2SO4. The mixture was stirred rigorously for 24 h, and stirred further for 24 h at room temperature (RT) after the addition of 20 ml of fuming HNO3 into it. Then, 80 ml of DI water was poured slowly into the mixture and it was stirred for another 1 h. The mixture was washed with DI water (repeated for three times), followed by centrifugation at 4000 rpm for 20 min. Then, the sediment was collected and dried at 60 °C for 2 days to obtain the graphite intercalation compound (GIC). The dried GIC was placed in a ceramic boat, and then inserted into a long quartz tube. The 2

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was treated using hydroiodic acid (HI)/H2O vapor for 5 h to convert graphene oxide (GO) to its reduced form rGO in the multilayer films, resulting in a multilayer film composed of PANi/UGO bilayers. The LBL-deposition of (PANi/PEDOT)n film onto a precleaned substrate began first with immersion into PANi suspension (0.5 mM, pH 2.6) for 15 min, and rinsed three times in DI water (pH 2.6) for 1 min per each washing steps. After the substrate was dried under a gentle stream of nitrogen, the substrate (coated with a PANi monolayer) was immersed into PEDOT:PSS suspension (0.26 wt%, pH 2.6) for 15 min, and rinsed three times, and dried under a gentle stream of nitrogen. The set of the coating steps was repeated, until the desired number (n) of the PANi/ PEDOT bilayers was achieved. Then, (PANi/PEDOT)n film was immersed in aqueous HI solution (55 wt.%) for 30 min, and washed with acetone and DI water to remove residual HI prior to vacuum drying at RT for 6 h.

sealed quartz tube was purged with argon (3 cycles of purge and vacuum), and, then, was inserted slowly into a quartz tube furnace (preheated to 1000 °C), and kept there for 30 s to obtain the expanded graphite (EG) compound. The synthesis of UGO suspension proceeds as following; 1 g of EG was mixed into 200 ml of conc. H2SO4. Then, 10 g of KMnO4 was added slowly into the mixture, and stirred for 24 h. 200 ml of DI water and 50 ml of H2O2 (35%) were added slowly into the mixture, and stirred for another 30 min. As the suspension was cooled under ice bath, the color of the suspension changed from dark green to light brown. The suspension was washed three times with 10% HCl (v/v) solution by repeating the redispersion and precipitation using a centrifugation (4000 rpm, 20 min). The precipitates were washed with DI water until the suspension reached to pH 6. The as-prepared GO suspension was diluted with DI water and centrifuged at 8 000 rpm for 40 min (the small sized GO remains in the supernatant). The sediment was dispersed in DI water again, and then the UGO was collected at a centrifugation (4 000 rpm, 40 min). Finally, the UGO was redispersed in DI water, yielding the concentration of UGO suspension that was estimated to be 0.17 wt. % by gravimetry.

2.2.5. Preparation of PEDOT-MoS2 (PMo) composites film The PMo composite film was prepared according to a modified procedure reported previously [27,28]. 0.4 g of MoS2 powder was stirred 4 h under ultrasonication in 30 ml of aqueous PEDOT:PSS solution, whose concentration was adjusted to 0.4, 1.3, and 2.2 mg/ml, respectively. The composite solution (supernatant) was separated in a well dispersed state from the mixture that was centrifuged at 4000 rpm for 1 h. The composite solutions were then deposited on PET substrate (1.5 × 3.0 cm2) to form PMo films by drop-coating the aqueous PMo solution (0.3 mL). The thickness of PMo film was controlled by the volume of the drop coating solution. The deposited films were then left to dry in air at RT, and subsequently immersed in aqueous HI solution (55%) for 30 min. Finally, the PMo films were washed with acetone and DI water to remove residual HI prior to vacuum drying at RT for 6 h. Note that the composite PMo films prepared using three different PEDOT:PSS concentrations (0.4, 1.3, and 2.2 mg/ml) were denoted to PMo-1, PMo-2 and PMo-3, respectively.

2.2.3. Preparation of PANi dispersions for LBL assembly The PANi dispersion was prepared based on the method reported previously [25]. 200 mg of PANi powder was dissolved in 20 ml DMAc. The mixture was stirred and sonicated overnight, further for 10 h after lowering the sonication intensity by ˜75%. The solution was filtered through a filter paper (F1002 grade) to remove some fine particulates remained in the dispersion. A dipping solution of PANi was prepared by slowly adding one part (by volume) of the filtered PANi solution to nine parts of DI water (adjusted to pH 3.0–3.5 using 3 M HCl). The pH of the PANi solution was then lowered to be 2.5–2.6 by adding 1 M HCl into it. Note that the partially doped-PANi would be precipitated in pH < 2.5 or pH > 4.0 of the solution. The PANi solution remained stable for 2 weeks without observance in any change of the optical appearance.

2.2.6. Assembly of (PPPrG)//PMo) asymmetric supercapacitor (ASC) The ASC was assembled with PPPrG (as the positive electrode) and PMo (as the negative electrode) along with PVA/H2SO4 gel electrolyte sandwiched between the both electrodes. The gel electrolyte was prepared by mixing 3 g of PVA and 3 g of conc. H2SO4 in 30 ml of DI water. The mixture was stirred at 85 °C for 2 h to attain the homogenous and clear gel. The gel was cooled at RT, and drop-coated onto the PPPrG and PMo electrodes, respectively. These electrodes were then dried in air for 1 h, and assembled together. For comparisons with the PPPrG// PMo ASC, symmetric PPPrG and PMo supercapacitors were also assembled in same manner, respectively.

2.2.4. Preparation of (PANi/PEDOT/PANi/UGO)n tetralayer, (PANi/ UGO)n bilayer and (PANi/PEDOT)n bilayer films The layer components positively-charged PANi, negatively-charged PEDOT and UGO were LBL-deposited onto various substrates including precleaned PET (1.2 × 4 cm2), fused silica glass (1.2 × 4.5 cm2) or silicon wafer (1.2 × 4 cm2), resulting in a tetralayer film composed of PANi/PEDOT/PANi/UGO tetralayers (denoted as PPPG). According to a typical LBL-dip-coating assembly procedure [26], a precleaned substrate was first immersed into PANi suspension (0.5 mM, pH 2.6) for 15 min, and rinsed three times in DI water (pH 2.6) for 1 min per each washing steps. After the substrate was dried under a gentle stream of nitrogen, the substrate (coated with a PANi monolayer) was immersed into PEDOT suspension (0.26 wt%, pH 2.6) for 15 min, and rinsed three times, and dried under a gentle stream of nitrogen. In succession, the substrate was immersed into PANi suspension for 15 min, and followed with the washing and drying steps. The substrate was immersed into UGO suspension (0.1 mg/ml, pH 2.6) for 15 min, and followed with the washing and drying steps, leading to PPPG tetralayer films. The set of the coating steps was repeated, until the desired number (n) of the PPPG tetralayers was achieved. Finally, the PPPG were treated using hydroiodic acid (HI)/H2O vapor for 5 h to convert graphene oxide (GO) to its reduced form (rGO) in the tetralayer films, resulting in a multilayer film composed of PANi/PEDOT/PANi/UrGO tetralayers (PPPrG). The LBL-deposition of (PANi/UGO)n film onto a precleaned substrate began first with immersion into PANi suspension (0.5 mM, pH 2.6) for 15 min, and rinsed three times in DI water (pH 2.6) for 1 min per each washing steps. After the substrate was dried under a gentle stream of nitrogen, the substrate was immersed into UGO suspension (0.1 mg/ml, pH 2.6) for 15 min, and followed with the washing and drying steps. The set of the coating steps was repeated, until the desired number (n) of the PANi/UGO bilayers was achieved. Then, the (PANi/UGO)n film

2.3. Methods The UV/visible spectrometry was performed using Perkin-Elmer Lambda 40 in the wavelength range of 190–1100 nm in order to monitor the LBL-growth of PANi, PEDOT and UGO layers on fused silica, and also to investigate the effectiveness of reduction method using HI vapor for the conversion of UGO in PPPG tetralayer films to the reduced form (UrGO). The thickness of the PPPG and PPPrG multilayer films deposited on silicon wafers was measured using a real-time spectroscopic ellipsometer (Ellipso Technology, Elli-SE-F) with a Xe arc lamp (350–820 nm) equipped with a rotating polarizer, a liquid cell with optical access at an incidence angle of 60°, an analyzer, and a multichannel detection system. Employing a self-made computer program, the elliptical azimuth and phase angle were calculated for both the cleaned reference substrate and the multilayer films. Note that at least 5 sampling points were measured to obtain the average thickness. The cross-sectional thickness of the PMo film was determined using field emission scanning electron microscope (FESEM) (JEOL JSM7001 F). The Raman spectra of deposited films were obtained using Raman spectrometry (Raman-LTPT) with excitation laser 3

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Fig. 1. (a) UV/visible absorption spectra of the multilayer films composed of different number of PPPG tetralayers deposited onto a fused silica. (b) UV/visible absorption spectra of the multilayer film composed of 10 PPPG tetralayers before and after HI/H2O vapor treatment. Inset: Digital images of PPPG films before and after HI/H2O vapor treatment, n indicates the number of the PPPG tetralayers. (c) UV/visible absorption spectra of the PMo-3 films before and after HI solution treatment. (d) Cross-sectional SEM image of PMo-3 film.

confirmed on the basis of the thickness of the grown PPPG films on silicon, which tends to increase linearly with increasing numbers of tetralayers (Fig. S2). The thickness of a single PPPG tetralayer was determined (by a least-squares fit of the plot of the thickness as a function of the number of tetralayers) to be 5.04 ± 0.06 nm. The film was treated with HI/H2O vapor at 100 °C for 5 h to chemically convert the GO layer (nonconductive form) within ‘PPPG’ layers to the reduced form, rGO (conductive form) [29]. The thickness of a PPPG tetralayer in the tetralayer films was reduced by 0.84 nm—from 5.04 ± 0.06 nm to 4.20 ± 0.30 nm— after the chemical reduction of GO to rGO (Fig. S2). The significant reduction in the thickness of a tetralayer film after the HI/H2O vapor treatment was mainly attributed to the drastic lattice contraction of the GO [30], the removal of a relatively small fraction of PSS from PEDOT:PSS layers [31,32], and a reduction in the amount of H2O present within the layers. The chemical structure of the tetralayer films before and after HI/H2O vapor treatment was investigated using UV/visible (Fig. 1b) and Raman spectroscopy (Fig. S3). Typically, the conversion of GO to rGO upon reduction in a multilayer film composed of PANi/GO bilayers is indicated by the appearance of a new absorption band at 280 nm (red-shifted from the absorption band of GO at 232 nm), indicating the partial restoration of the electronic conjugation in the GO nanosheets [26]. Unfortunately, the conversion of GO to rGO upon reduction in the tetralayer film could not be clearly identified in the 230 ≤ λ ≤ 300 nm range of the UV/visible spectra because of overlap with the π-π* transition peaks of the benzenoid rings of PEDOT:PSS and PANi [29,33]. However, the conversion of GO to rGO in tetralayer film upon reduction using HI/H2O vapor was distinctively indicated by an absorption peak at 302 nm, which could be resolved from a combination of the rGO and PANi absorption bands (at 280 nm and 308 nm), respectively. The individual UV/visible absorption spectra of PEDOT:PSS, UGO, and PANi in solution are shown in Fig. S4

(λ = 532 nm). The electrochemical tests (cyclic voltammetry CV, galvanostatic charge/discharge GCD, and electrochemical impedance spectrometry EIS) of the individual electrode were performed in a three electrode cell including platinum wire and SCE electrode as the counter and reference electrodes in 1 M H2SO4, respectively. The electrochemical properties of the ASC and symmetric supercapacitors in a two electrode cell involving PVA/H2SO4 gel electrolyte were investigated at RT using a potentiostat/galvanostat (Compactstat Ivium Technology). Prior to the electrochemical measurements, the electrodes and the supercapacitors were stabilized for 50 cycles in CV test at 50 mV/s. 3. Results and discussion 3.1. Preparation and characterization of PPPG multilayer film and PMo composite film he deposition of PPPG tetralayer films onto fused silica substrate via the LBL-assembly method was monitored at the characteristic absorbance wavelength (308 nm and 880 nm) in the UV/visible spectra (Fig. 1A). These absorbances are ascribed to the π-π* transition of the benzoid ring (an interband transition) of PANi and the partially dopedPANi [29], respectively. The absorbances of PPPG (at 308 and 880 nm) increased linearly as a function of the number of PPPG tetralayers deposited onto the fused silica (Fig. 1a, inset), indicating the regular and uniform deposition of each layer component onto the substrate. Notably, the PANi, PEDOT:PSS, and UGO solutions were adjusted to pH 2.6 during the LBL-deposition of the layers to preserve the green-colored conducting emeraldine salt form of PANi [26]. The average lateral size of the UGO sheets was determined to be 47 μm by observation using FESEM (Fig. S1, suppplementary information SI). The regular and uniform deposition of PPPG tetralayers onto a substrate was also 4

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for comparison. The typical color change of the multilayer films from blue gray to dark brown (digital images in Fig. 1b, inset) provide optical evidence for the simultaneous conversion of GO to rGO in a tetralayer film during the HI/H2O vapor treatment. The structural characteristics of a (PPPG)10 film converted into a (PPPrG)10 film (where the subscript 10 indicates the number of tetralayers) by HI/H2O vapor treatment were also investigated using Raman spectrometry (λex = 533 nm), as − shown in Fig. S3. First, the two intense peaks at 1346 cm−1 and 1612 cm 1 in the Raman spectrum of PPPG film are attributed to various overlapping absorption bands of GO (including the D-band at 1350 cm−1 and the G-band at 1590 cm−1 ) [34,35], PANI (C-N+ vibration of delocalized polaronic structures at 1346 cm−1 and CC] stretching vibration of the quinonoid ring at 1584 cm−1) [29] and PEDOT:PSS (CeC stretching −1 and C = C deformation at 1365 cm−1, CC stretching at 1509 cm ]–O stretching at 1430 cm−1) [36]. The peak observed at 1179 cm−1 is ascribed to a C–H bending vibration of the semi-quinonoid rings in PANi. In addition, −1 and 3210 cm−1 three weak peaks at 2697 cm−1, 2945 cm are attributed to overtones and combination of the vibrational modes of the graphitic layers [29,37]. The intensity ratio between the D and G peaks in the spectrum of the PPPrG film was apparently greater than that of the PPPG film. However, the D/G ratio value could not be calculated from the Raman spectra because of overlap with the PANi, PEDOT, and GO peaks [38,39]. The PMo composite films were prepared by drop-coating MoS2/ PEDOT:PSS solution onto a solid substrate. The composition of the MoS2/PEDOT:PSS solution was varied, yielding three different samples: PMo-1, PMo-2, and PMo-3. The blue-colored PMo composite films darkened (digital images in Fig. S5), as the PEDOT:PSS content in the PMo solution increased. The nonconducting component PSS can be removed as PSSH from PEDOT:PSS films by an acid treatment, leading to an increase in the films’ conductivity [31,32]. In the present work, we have observed a relatively small fraction of PSS removed from PMo composite film by HI solution treatment, which led to a ˜ 40% decrease in intensity of the characteristic absorption peak at 225 nm in the spectrum of the PSS (within PEDOT:PSS) (Fig. 1c) and to an increase in conductivity of the PMo composite film (Fig. S5). The morphology of PMo-3 composite films appeared compact and featureless. The thickness of the PMo-3 composite film was determined to be 2.43 μm via the cross-sectional analysis based on its SEM image (Fig. 1d). The sheet resistances of the PMo-1, PMo-2 and PMo-3 composite films prior to HI solution treatment were measured to be 230,895 Ω/sq, 90,766 Ω/sq and 81,350 Ω/sq, respectively. The sheet resistances of the PMo composite films decreased by three orders of magnitude, i.e., 123 Ω/sq, 96 Ω/sq and 90 Ω/sq for the PMo-1, PMo-2, and PMo-3, respectively, after HI solution treatment, indicating the removal of hydrophilic and insulating component PSS of the composite films [31,32].

capacitance, which we ascribed to the synergistic effect of the tetralayer composition of PANi, PEDOT, and UrGO in the multilayer film (Fig. 2a). The capacitance of [PANi/UrGO]40 electrode was apparently greater than that of the pure conducting polymer [PANi/PEDOT]40 electrode. Collectively, the CV results indicate the UrGO sheets are an excellent electrode material contributing to both EDL capacitance and the pseudocapacitance of an electrode [42]. The equivalent series resistance (ESR) of the [PPPrG]20, [PANi/ PEDOT]40, and [PANi/UrGO]40 electrodes were determined on the basis of the Nyquist plots in the frequency range from 0.1 to 105 Hz (Fig. 2b). The intercept of the Nyquist plot with a real axis (Z′) in the high frequency region corresponds to ESR, which is associated with the intrinsic electrical resistance between the electrode and electrolyte [29]. The [PANi/UrGO]40 exhibited a much lower ESR value (28 Ω•cm2) than PPPrG (66 Ω•cm2) and [PANi/PEDOT]40 (110 Ω•cm2) (Fig. 2b), indicating a shorter ion diffusion pathway, as a result of the high conductivity of the composite UrGO/PANi film. Thus, the excellent capacitance of the PPPrG electrode could be ascribed to the conductivity of the multilayer films, which was improved by the combination of PANi, UrGO and PEDOT layers, which function as conducting networks in the nanostructured composite films. The conducting networks improved the electrochemical redox reactions of PANi by enabling its full charging or discharging [43]. The Nyquist plots for both PPPrG and [PANi/PEDOT]40 show depressed semicircles in the high frequency region as a consequence of faradaic and nonfaradaic reactions at the electrode surface (the diameter of the depressed semicircle resolves the charge transfer resistance) [29]. The semicircle diameter of PPPrG appeared much smaller than that of [PANi/PEDOT]40, indicating a higher charge transfer rate due to the presence of conductive UrGO [29]. In addition, the [PANi/UrGO]40 did not show a semicircle in the high frequency region, implying negligible charge transfer resistance across the electrode/electrolyte interface [19]. The sheet resistance of the [PANi/UrGO]40, PPPrG and [PANI/PEDOT]40 was determined to be 414 Ω/sq, 1614 Ω/sq, and 15,851 Ω/sq, respectively (Table S2). CV curves of the PPPrG electrode (Fig. S6a) were recorded at various scan rates. The shape of the CV still represented the characteristic feature of an ideal pseudocapacitor. The peak current density of the PPPrG electrode increased, as the scan rate was increased from 2 to 100 mV/s. The charge-discharge curves of the PPPrG electrode at different current densities showed good symmetric profiles, indicating the optimal capacitive performance of the electrode (Fig. S6b). The GO sheets in [PPPG]20 multilayer films were converted into rGO for PPPrG through the chemical reduction using HI/H2O vapor to improve their poor electrical conductivity by healing structural defects in the GO [29]. The thickness and surface roughness of the films substantially affect their sheet resistance [44–46]. The sheet resistance of the [PPPG]20 multilayer films was measured using a standard four-point probe technique, and the results of measurements the HI/H2O vapor treatment were compared. The sheet resistance of the PPPrG films decreased significantly after HI/H2O vapor reduction (Fig. 2c). The sheet resistance of the PPPrG films decreased exponentially to a value of 1139 ± 53 Ω/ sq as a function of the tetralayer number, as the number of tetralayers was increased to 40 and then remained constant, indicating the formation of continuous intercalation pathways for electrical conduction in the thick multilayer films. This result supports the observation that the magnitude of the CV curve of the PPPrG electrode increased without any serious distortions with increasing number of tetralayers from 10 to 50 (Fig. S7). The areal and volume capacitances of the [PPPrG]n films at a current density of 3 A/cm3 were explored as a function of the number of tetralayers (Fig. 2d). The areal capacitances of the films increased from 3 to 23 m F/cm2 in a linear relation with the number of tetralayers (n), when n was varied from 10 to 50. The thicker films were associated with a higher loading of electroactive material, hence led to a higher areal capacitance [47]. The volume capacitance of the [PPPrG]n films gradually increased to a maximum value of 1300 F/cm3, as n increased

3.2. Electrochemical properties of the PPPrG electrode in a three-electrode system The electrochemical properties of the PPPrG tetralayer film electrodes were investigated using CV, galvanostatic charge-discharge, and EIS measurements in a standard three-electrode cell configuration. Typical CV curves were recorded from [PPPrG]20 (d ˜ 84 nm), [PANi/ PEDOT]40 (d ˜ 73 nm), and [PANi/UrGO]40 (d ˜ 95 nm) films in 1 M H2SO4 electrolyte at a scan rate of 2 mV/s in a positive potential window (-0.2 to 0.8 V), as shown in Fig. 2a. Notably, the multilayer films had equivalent numbers of layers and comparable film thicknesses. The CV curves show two couples of redox peaks (a/a’ and b/b’), which are ascribed to the redox transitions of PANi [40,41]. The a/a’ peaks are attributed to the redox transition of PANi between a semiconductive state (leucoemeraldine) and a conducting state (emeraldine), and the b/b’ peaks are ascribed to the faradaic transformation of emeraldine to pernigraniline (insulating state). Compared with [PANi/PEDOT]40 and [PANi/UrGO]40 electrodes, the [PPPrG]20 electrode exhibited a larger area in its CV curve, indicating higher 5

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Fig. 2. Comparison of (a) CV curves and (b) Nyquist plot for [PANi/PEDOT]40, [PANi/UrGO]40 and [PPPrG]20 films. (c) The sheet resistance of [PPPG]20 multilayer ultrathin films before and after HI/H2O vapor reduction deposited on silicon substrate versus the number of the tetralayers using a standard four-point probe method. (d) The effect of area capacitance and volume capacitance of PPPrG (current density 3 A/cm3) on the number of tetralayers.

low frequency region, suggesting ideal capacitive behavior [19,52]. The ESR values of the PMo-1, PMo-2, and PMo-3 electrodes were measured to be 160 Ω·cm2, 80 Ω·cm2, and 70 Ω·cm2, respectively, indicating that the conductivity of the electrodes has increased gradually with increasing PEDOT content in the electrode.

to 20. However, the maximum value decreased slightly to a value of 1019 F/cm3, as n was increased from 20 to 50. The decrease of the volume capacitance was caused by an increase in resistance due to increasing thickness, along with reduced accessibility of the active sites to the electrolyte ions within the bulk of the thicker films. Notably, the volume capacitance of PPPrG electrode (1300 F/cm3) is greater than those of various thin film electrodes (composed of PANi and/or rGO) reported previously, including PANi/rGO/indium tin oxide (ITO) thin film (584 F/cm3 at current density of 3 A/cm3) [26], a poly(p-phenylenevinylene)/rGO/PET thin film (957 F/cm3 at current density of 20 A/cm3) [29] a PANi/Au/paper thin film (800 F/cm3 at current density of 1 A/cm3) [48] and a PANi/graphene/paper thin film (135 F/cm3 at scan rate of 50 mV/s) [49]. The excellent volume capacitance of the [PPPrG]20 is attributed to the synergistic combination of the layer components in the multilayer films, including UrGO and the electroactive conducting polymers (PANi and PEDOT), which led to good electrical conductivity and high pseudocapacitance.

3.4. Electrochemical properties of the flexible PPPrG//PMo ASC [PPPrG]40 and PMo-3 electrodes exhibited good electrochemical capacitive behavior, as evaluated at a scan rate of 10 mV/s in 1 M H2SO4 within the electrode potential range of -0.2 ˜ 0.8 V and 0.2 ˜ -0.4 V, respectively, as shown in Fig. 4a. An ASC was then assembled with [PPPrG]40 (as the positive electrode), PMo-3 (as the negative electrode), and a separator composed of PVA/H2SO4 gel electrolyte (Fig. 4b). The cell voltage of the ASC was expected to be as high as 1.2 V, because of the combination of the positive PPPrG electrode (with the highest cutoff potential of 0.8 V) and the negative PMo electrode (with lowest cutoff potential of -0.4 V). The cell voltage of the PPPrG// PMo ASC was measured to be 0.8 V slightly lower than the theoretical value. This slightly lower cell voltage was ascribed to over-oxidation of the PANi resulting from an excessively high cell voltage, which converted the PANi from its conducting form to either insulating nigraniline or pernigraniline [53,54]. The charge on both the positive and negative electrodes should be balanced according to the relationship; V +/V˗ = (C˗ • ΔE˗)/(C+ • ΔE+), where V˗, C˗, and ΔE˗ are the volume, volumetric capacitance and potential window of the negative electrodes, respectively, and V+, C+, and ΔE+ are the volume, volumetric capacitance and potential window of the positive electrodes, respectively [18]. The optimal volume ratio between the two electrodes in the PPPrG//PMo ASC was calculated to 0.074. The cycling stability is a critical parameter for evaluating the performance of a supercapacitor. The cycling stability of the PPPrG//PMo ASC was compared with those of symmetric supercapacitor PPPrG//PPPrG and PMo//PMo by means of CV at a scan rate of 30 mV/s (Fig. 4c). The symmetric supercapacitors PMo//PMo and PPPrG//PPPrG showed relatively poor capacitance retention. The capacitance retention of the PMo//PMo supercapacitor increased gradually to 110% of its original capacitance after the first

3.3. Electrochemical properties of the PMo negative electrode in a threeelectrode system The electrochemical properties of the PMo composite films were investigated in 1 M H2SO4 electrolyte in a negative potential window (0.2 to -0.4 V) at a scan rate of 10 mV/s. As shown in Fig. 3a, the CV curves exhibited nearly ideal rectangular shapes (with a pair of redox peaks as a result of both EDL formation and the redox reaction associated with fast doping and dedoping of electrolyte ions into the PEDOT nanostructures [50–52]. The integrated area in the CV curve of the PMo electrodes increased with increasing PEDOT:PSS content in the composite films. The volume capacitances of the PMo-1, PMo-2, and PMo-3 were calculated to be 84 F/cm3, 110 F/cm3 and 125 F/cm3, respectively. The PMo-3 electrode, which had the highest PEDOT:PSS content among the three PMo electrodes, exhibited the highest capacitance, because of the pseudocapacitance contribution was increased by the higher content of PEDOT. The Nyquist plots of the PEDOT/MoS2 composite films with different PEDOT contents (Fig. 3b) displayed vertical lines nearly parallel to the imaginary axis over the medium to 6

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Fig. 3. (a) Comparison of CV curves recorded from PMo films in three different composition of PEDOT/MoS2. (b) Nyquist plots for PMo films in three different composition of PEDOT/MoS2.

calculated to be ˜ 61 F/cm3 at a current density of 1 A/cm3 according the equation Cvol (F/cm3) = I•Δt/V•ΔE, where I is the current, Δt is the discharge time, V is the total volume of the ASC and ΔE is the voltage window. The PPPrG//PMo ASC achieved a volume capacitance of 44 F/ cm3 at a high current density of 20 A/cm3, indicating a loss of capacitance. Khomento, et.al. has reported that conducting polymer-based symmetric supercapacitors composed of PANi and PEDOT exhibit narrow operating voltage windows of 0.5 V and 0.6 V, respectively [54]. Extending the operating voltage window beyond these limits will result in the loss of capacitance during repeated charge-discharge cycling [56]. The maximum cell voltage was imposed during the chargedischarge cycling of the symmetric supercapacitors. The polarization of the negative electrode during the discharging step was not sufficiently high to enable polymer redoping. Therefore, the isolated phase is expected to accumulate in the negative electrode upon cycling of the

200 cycles because of the cycling-induced improvement in the surface wetting of the electrode, which led to a greater electroactive surface area [55]. However, the capacitance retention decreased gradually to 58% of its original capacitance as the number of cycles was extended to 1000. The capacitance retention of the PPPrG//PPPrG supercapacitor decreased gradually to 47% over 1000 cycles. The PPPrG//PMo ASC exhibited improved cycling stability compared with both of the symmetric supercapacitors. The capacitance retention of the ASC decreased rapidly to 85% relative its original capacitance during the first 200 cycles and then decreased modestly to 73% until 1000 cycles. The galvanostatic charge-discharge curves of the PPPrG//PMo ASC at different current densities (from 1 A/cm3 to 20 A/cm3) were found to be slightly nonlinear (Fig. 4d), reflecting pseudocapacitance and EDL capacitance contributions from the conducting polymer and the rGO in the ASC. The volume capacitance of the PPPrG//PMo ASC was

Fig. 4. (a) CV curves of [PPPrG]40 electrode and PMo-3 electrode recorded in a 3-electrode system in 1 M H2SO4 aqueous electrolyte at a scan rate of 10 mV/s. (b) Schematic illustration of the PPPrG//PMo ASC prototype. (c) Cyclic stability of PPPrG//PMo ASC was compared with those of PMo and PPPrG symmetric supercapacitors. (d) Galvanostatic charging-discharging curve of optimized PPPrG//PMo ASC at different current density.

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Fig. 5. (a) The Ragone plot of the PPPrG//PMo ASC at different current density. (b) The digital image for the PPPrG//PMo ASC, which were bent at different angles and CV curves of PPPrG//PMo ASC being bent at different angles. (c) The digital image for the PPPrG//PMo ASC connected in series that lights up a red LED (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

265 mW/cm3. Furthermore, the ASC showed excellent electromechanical flexibility without sacrificing electrochemical performance when deformed to various degrees. The excellent device performance of this ASC is ascribed mainly to the conductivity of the multilayer films, which was improved by the combination of PANi, UrGO and PEDOT layers, which function as conducting networks in the nanostructured composite films. A binder (or additives) (conventionally employed in the construction of the electrodes), which causes sluggish ion transport during the redox reaction due to high interfacial resistances and the inherent resistance of the binder materials, thereby leading to poor device performance, was excluded. This flexible asymmetric high-performance supercapacitor based on a facile low-cost assembly procedure offers substantial promise for applications in grid-scale energy storage devices integrated into portable and bio-implantable microelectronic devices.

supercapacitor [54]. Increasing the operating voltage would accelerate the process of capacity loss because the potential of the negative electrode would be shifted to more negative values [54]. A supercapacitor can store energy E (in Wh/cm3) and deliver the power P (in W/cm3), which are determined by equations E = ½Cvol(ΔV)2 and P = E/Δt, where Cvol is the volume capacitance of the ASC, ΔV is the voltage window, and Δt is the discharge time [57]. The energy density and power density of the PPPrG//PMo ASC at different current densities is depicted in the Ragone plot (Fig. 5a). The PPPrG// PMo ASC at a discharge current of 1 A/cm3 reached an energy density of 5.4 mW h/cm3 at a power density of 110 mW/cm3, and still exhibited an energy density of 4.0 mW h/cm3 even at 265 mW/cm3. The electrochemical performance of the PPPrG//PMo ASC yielded similar or higher energy and power densities compared with those of asymmetric supercapacitors reported previously (Table S3 in SI), such as pencildrawing graphite//PANi [57] and MnO2 nanowires//Fe2O3 nanotubes [58] and PET/Au/PANi [59]. A flexible supercapacitor must retain its electrochemical performance despite being operated under various mechanical deformations. The stability of the electrochemical performance of the PPPrG//PMo ASC under different bending conditions was examined on the basis of cyclic voltammograms recorded at 30 mV/s under various bending angles (0°, 30°, 60° and 90°) imposed using a homebuilt two-point bending device (Fig. 5b). The electrochemical performance of the ASC was almost unaffected by the various bending conditions, exhibiting remarkable flexibility and stability (Fig. 5b). As a proof-of-concept application, three as-prepared PPPrG//PMo ASCs were connected in series and charged to a cell voltage (optimized by a potentiostat) to supply power sufficient to illuminate a red light emitting diode (Fig. 5c).

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2017R1A2B4007213), and the Research Grant of Incheon National University in 2017. Appendix A. Supplementary data

4. Conclusions Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.123815.

The PPPrG and PMo electrodes were prepared on flexible PET substrates using LBL self-assembly and drop-coating methods, respectively. The tetralayer film electrode [PPPrG]20 exhibited a maximum volume capacitance of 1300 F/cm3 at a current density of 3 A/cm3, which is one of the highest volume capacitance values reported to date. The high volume capacitance of [PPPrG]20 electrode was attributed to synergistic combinations of the combined layer components, which included UrGO (good electrical conductivity) and electroactive conducting polymers PANi and PEDOT (high pseudocapacitance). The maximum volume capacitance of the PMo electrodes was determined to be 125 F/cm3 at a current density of 3 A/cm3. A flexible solid-state ASC was assembled with [PPPrG]40 (as a positive electrode) and PMo-3 (as a negative electrode) in a PVA/H2SO4 gel electrolyte at an operating voltage window of 0.8 V. The ASC showed a maximum energy density of 5.4 mW h/cm3 at a power density of 110 mW/cm3 and could still retain an energy density of 4.0 mW h/cm3 at a power density of

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