SWCNT Film as an

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Facile Processing of Free-standing PANI/SWCNTs Film as Integrated Electrode for Flexible Supercapacitor Application Fuwei Liu, Shaojuan Crystal Luo, Dong Liu, Wei Chen, Yang HUANG, Lei Dong, and Lei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08382 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

Facile Processing of Free-standing PANI/SWCNTs Film as Integrated Electrode for Flexible Supercapacitor Application Fuwei Liu,†,‡ Shaojuan Luo,§ Dong Liu,† Wei Chen,|| Yang Huang,*,† Lei Dong,⊥ and Lei Wang*,† †

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials

Science and Engineering, Shenzhen University, Shenzhen, 518060, China ‡

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education

and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China §

Shenzhen

Engineering

Laboratory

of

Phosphorene

and Optoelectronics,

International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology, Shenzhen University, Shenzhen 518060, China ||

Health Science Center, Xi’an Jiaotong University, No. 28, Xianning West Road,

Xi'an 710049, China. ⊥

Department of Physics, Southern University of Science and Technology, Shenzhen

518055, China Email: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT Flexible supercapacitors (SCs) with compact configuration are ideal energy storage device for portable electronics, owing to its original advantages (e.g. fast charging/discharging). To effectively reduce the volume of SC, an integrated electrode of free-standing PANI/SWCNTs film with high-performance has been developed via a facile solution deposition method, which can be employed as current collector and active material in the meantime. Thanks to the strong π–π interactions between PANI and CNTs, an efficient conductive network with ordered PANI molecular chains is formed in this hybrid film electrode, which is beneficial for ion diffusion process and fast redox reaction resulting in a high capacitance of 446 F g-1 and outstanding cycling stability, achieving 98% retention over 13000 cycles. Predictably, solid-state SC constructed by this free-standing PANI/SWCNTs film electrode exhibited remarkable mechanical stability and flexibility in a compact configuration, let alone its excellent capacitive performance (218 F g-1). Moreover, the highest energy density of flexible solid-state SC reached 19.45 Wh kg-1 at a power density of 320.5 W kg-1, further indicating a good potential as energy storage device. This work would inspire other simple process techniques for high-performance flexible SC, catering to the demand of portable electronic device. KEYWORDS: free-standing, PANI/SWCNTs, supercapacitor, π–π interactions, continuous conductive network

2


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1. INTRODUCTION Motivated by the rising need of powerful portable electronics, flexible energy storage device with compact configuration, acting as fundamental power source to support various functions, has sprung up as the times required.1–2 Among them, supercapacitors (SCs) have gained considerable interest because of their attractive features over other contenders, such as fast charge-discharge rates, high power densities, safe operation and long life cycles.3–4 One of the biggest challenges in developing an ideal SC for portable device is to achieve outstanding energy storage performance, for example, high capacitance, together with great flexibility and compact structure. Even though a number of attempts have been made to realize these goals, some of them still require the current collector,5–6 a conventional component for constructing SC, to enhance electrical conductivity of active materials and thus improve capacitive performance, which will be a burden that results in bulky appearance and heavy weight; whereas others have developed integrated electrodes that can function as current collector and active material at the same time, effectively avoiding the redundance of SC device, which are mostly based on carbon materials (e.g. graphene) and with relatively low capacitance due to the charge storage mechanism of electrical double-layer capacitor (EDLC) materials.7 Therefore, it is still highly desirable to fabricate high-performance integrated electrode for flexible SCs with better energy storage capability, catering to the demand of portable electronic device. In order to improve energy storage performance of integrated electrode, a variety 3



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of pseudocapacitive materials, for example, metal oxides/sulfides and conductive polymers, have been introduced as additional active materials via different methods on account of their reversible redox reaction that can store more charge compared with EDLC. As one kind of conducting polymers, polyaniline (PANI) has been widely used as a potential electrode material for flexible SC, owing to its large specific capacitance, excellent flexibility, ease of synthesis and good environment stability.8–11 Recently, it was reported that with the help of m-cresol, camphorsulfonic acid doped polyaniline (PANI-CSA) film can be directly obtained via a facile solution deposition process, electrical conductivity of which exhibits two or three orders of magnitude greater than that of pristine PANI, thanks to the conformational changes of polymer chains.12–13 The strong interaction between solvent and PANI molecular chains induces PANI chains to form an expanded structure rather than a compact coil-like one which facilitates ordered molecular packing,14–15 leading to enhanced electrical conductivity as well as increased crystallinity.14,16 Thus, this modified PANI-CSA can be a promising enhancer for improving capacitive performance of flexible integrated electrode while without compromising the flexibility and conductivity alike many other metal oxides/sulfides, in consideration of its high conductivity and outstanding pseudocapacitive behavior. In this work, a flexible integrated high-performance electrode for SC was developed by a facile solution deposition method via adding a suitable amount of single-walled carbon nanotubes (SWCNTs) into PANI-CSA solution. PANI-CSA can form highly ordered interface layer on the surface of CNTs due to solution processing 4


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and strong π–π interactions between PANI and CNTs, leading to the formation of an efficient conductive network, which is beneficial for ion diffusion process and fast redox reaction at electrode/electrolyte interface resulting in better electrical conductivity and enhanced capacitive performance. The optimized free-standing PANI/SWCNTs film electrode, containing 5 wt% CNTs, exhibited a high capacitance of 446 F g-1 and outstanding cycling stability over 13000 charging/discharging cycles, retaining 98% of its original capacitance. As expected, solid-state supercapacitor fabricated by this PANI/SWCNTs integrated electrode could not only provide a relatively high capacitance of 218 F g-1, but also present perfect capacitance retention after 5000 cycles along with excellent mechanical stability and flexibility. Our approach demonstrates a facile strategy to develop flexible integrated electrodes for high-performance SCs, acting as a promising energy storage device for portable electronics. 2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used in this work were of analytical grade. Aniline (ANI), ammonium persulfate (APS), ammonia solution (28-30%), hydrochloric acid (HCl, 35-38%), sulphuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 85%), camphorsulfonic acid (CSA), and m-cresol were all received from Sun Chemical Technology Co., Ltd (Shanghai, China). Polyvinyl alcohol (PVA) was obtained from Sigma Aldrich. The aniline was distilled under reduced pressure before used, while other chemicals were used as received. SWCNTs with large specific surface area (diameter: < 3 nm, purity: >95.0 wt%) were provided by XFNANO Materials 5



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Technology Co., Ltd (Nanjing, China). 2.2. Preparation of PANI and PANI/SWCNTs free-standing films. 1 mL ANI and 2.5 g APS were added to 90 mL and 100 mL of 1 M HCl, respectively, both of which were carried out in an ice bath. Then, APS solution was added dropwise to ANI-acid mixed solution with constant stirring at 0-5 °C for 8 hours.17 After purification with water and methanol, the collected PANI powder was put into a 0.1 M ammonia solution with continuous stirring for 24 h at room temperature. Afterwards, the PANI suspended solution was filtered and washed using deionized water and methanol for several times, followed by vacuum drying at 60 °C for 24 h. In order to achieve better electrical conductivity, the PANI was re-doped using CSA with gently grinding, and a ratio of 2: 1 was set as the optimized parameter. Then the CSA-doped PANI was added into m-cresol with constant stirring for 24 h and thus forming a black-green colored solution. For PANI/SWCNTs films, CNTs with different weight fractions (0, 1, 5, 10, 30 wt%) were added into the as-obtained black-green solution, followed by 1 h stirring and 1 h ultrasonic treatment, respectively. Then, the mixed solution were cast on glass substrates and dried at 80 °C for 24 h to form flexible integrated electrodes, which can be easily peeled off from substrate through a solution immersion method. For the sake of brevity, these films were abbreviated as PANI, PANI-1, PANI-5, PANI-10, and PANI-30, respectively. It is noted that all of the PANI based samples were doped with CSA unless specified. 2.3. Preparation of solid-state supercapacitor. The electrolyte was prepared by 6


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adding 5 g PVA and 3 mL H3PO4 into 45 mL deionized water, and the mixture was subsequently heated to 95 °C with continuous stirring until a transparent solution was formed. Two pieces of film electrodes were coated with a suitable thickness of PVA/H3PO4 electrolyte and dried under vacuum at room temperature; afterwards, they were assembled into a solid-state SC by sandwiching another layer of PVA/H3PO4 electrolyte between them. 2.4. Characterization. The morphology of PANI and PANI/SWCNTs films were investigated by using a field emission scanning electron microscopy (SEM, Hitachi S-4700). Raman spectra were recorded on an inVia-Feflex confocal Raman microscope (Renishaw, UK) within the wavenumber range of 500-2000 cm-1. The molecular structures of PANI and PANI/SCNTs were investigated by a Fourier transform infrared spectrometer (FTIR, Nicolet 6700, Thermo Fisher) with a scan range from 550~4000 cm-1. The phase of PANI and PANI/CNTs were characterized via X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation in the range 2θ = 5-45°. UV-Vis absorption spectrum analysis was performed on a Thermo Fisher Evolution 220 UV-Visible spectrophotometer in a wavelength range of 200-800 nm. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher) was employed to examine changes in the electron binding energy of PANI. Cyclic voltammogram (CV) curves, galvanostatic charge-discharge (CD) curves, and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China). EIS was 7



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performed in the frequency range of 105-10-2 Hz with the amplitude of 5 mV. Cycle stability was measured using a CT2001A Land battery tester (Wuhan Landian Electronic Co., China). 2.5. Electrochemical test. The electrochemical performance of film electrodes was investigated by using a three-electrode system in 1 M H2SO4, in which PANI/SWCNTs film, saturated calomel electrode and Pt mesh were used as working electrode, reference electrode and counter electrode, respectively. The specific capacitance (Cs) of film electrodes were calculated according to the following formula: = ⁄∆

(1)

where I is constant discharge current (A), t stands for discharge time (s), ∆V and m present potential window (V) and mass of integrated film electrode (g). The specific capacitance of solid-state SC was calculated according to the following equation: = 2/∆

(2)

where, I, t, m, and ∆V represent discharge current, discharge time, mass of one single active electrode, and discharge voltage range, respectively. The energy density (Em, Wh kg-1) and power density (Pm, W kg-1) of SC device were calculated based on the capacitance values obtained from equation (2), and the formulas were shown as follows:

= ∆ ⁄3600 =

× 3600

(3) (4) 8


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where, Cc (F g-1) is the specific capacitance, ∆V (V) represents the potential window, and t (s) stands for the discharge time.18–19 3. RESULTS AND DISCUSSION 3.1. Fabrication of PANI/SWCNTs free-standing Film. The schematic illustration of fabrication process of PANI/SWCNTs free-standing film is show in Figure 1: first, PANI was synthesized via a chemical polymerization method in HCl as reported previously;17 after washing and drying, the primary dopant of PANI was removed via using ammonia solution; in order to achieve higher electric conductivity and better crystallinity, the obtained PANI was re-doped by CSA with assistant of m-cresol solvent; finally, a free-standing film electrode was formed by depositing the mixture of SWCNTs and CSA-doped PANI solution on a glass substrate, which can be readily peeled off and directly used as a flexible integrated electrode for high-performance SC. Thanks to the strong π–π interactions between PANI and CNTs, 20–21

highly ordered interface layer can be formed on the surface of CNTs, leading to

the outstanding mechanical strength of flexible film, which can be bent to arbitrary shapes without deteriorating its integrity as shown in Figure 1, making it a perfect electrode for flexible SC. 3.2. Characterization of PANI/SWCNTs free-standing films. The SEM images of surface and freeze-fractured cross sections of PANI/CNTs films presented different characteristics and morphology along with different amount of CNTs as shown in Figure 2. The pristine PANI film shows smooth and compact morphology without any pores or particle inclusions on surface and in cross section (Figure 2a and d), 9



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indicating good solubility of PANI in m-cresol. When adding a little amount of SWCNTs (1 wt%), long strips of CNTs can only be found in some section of the film with sparse dispersion, indicating effective bonding of PANI and CNTs through strong π–π interactions (Figure S1a and c). Once CNTs loading increased to 5 wt%, they were found to distribute homogeneously in the whole film, forming a highly conductive continuous network, which facilitates ions transportation and realizes fast redox reaction of SC (Figure 2b and e). With further increase of CNTs, more of them can be observed both on the surface and inside the film, despite of a slight aggregation in local area, which would hamper fast redox reaction of PANI since it might not effectively contact with electrolyte, one potential reason for the deteriorated electrochemical performance of SC as discussed later (Figure 2c, f and Figure S1b, d). As shown in Figure 3a, Raman spectroscopy was conducted to investigate structure changes of PANI/SWCNTs free-standing film with the introduction of different amount of CNTs that is in order to reveal its affection to the electrochemical performance of integrated electrode. Assignments of the main characteristic peaks are listed in Table S1, and the Raman spectrum of pristine PANI shows distinctive C–H bending vibration (quinoid/benzenoid ring, 1191 cm-1), C–N stretching vibration (1249 cm-1), C–N+ vibration (quinoid ring, 1332 cm-1), N–H stretching vibration (1512 cm-1), C=C stretching (quinoid ring, 1563 cm-1), and vibration of delocalized polarons (1623 cm-1) in extended polymeric conformation,15,20,22 suggesting the emeraldine salt form of PANI, namely, a state having sufficient charge carries, which is beneficial to fast redox reactions resulting in better capacitance.23 The peak 10


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corresponding to C–C stretching (semiquinoid ring, 1590 cm-1) appears in the films containing CNTs, the intensity of which increases with increased CNTs loading. Meanwhile, the intensity of other quinoid ring-related vibration (1332 cm-1), C–N+ vibration (1563 cm-1), and C=C stretching would also increase, which is in consistent with FTIR spectra, suggesting the formation of more quinoid ring-related bonds (Figure S2). Notably, the intensity ratio of quinoid band (I1561) and benzenoid band (I1475) is related to oxidation state and conjugation length of PANI structure.24 The increase in I1561/I1475 ratios of PANI/SWCNTs films indicates that charge transport properties such as conjugation length and degree of electron delocalization were significantly enhanced,25–26 which enhances the conductivity of polymer chains, and thus benefits the pseudocapacitive performance of PANI, as evidenced by the following UV-vis spectroscopic study (increased peak of –N=quinoid=N–), XPS analysis (increased intensity of quinoid imine (–N=) peak) and electrochemical results (improved CV, CD, and EIS). In addition, the increased density of quinoid ring-related vibration in Raman spectra could be attributed to strong π–π conjugation interaction between PANI molecular chains and CNTs.20–21 As for PANI/SWCNTs film, the conformation of PANI transforms from a “compacted coil” to an “expanded coil” because of the interaction between polymer and m-cresol solvent, creating more effective area to interact with CNTs, leading to a strengthening π–π conjugation interaction due to increasing overlap area of π–π bonds between two components, which is favorable for better capacitance since more functional groups would take part in the redox reaction during charging/discharging, besides the formation of continuous 11



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conductive network. Furthermore, the C–N+ vibration would shift to a larger wavenumber direction with increased CNTs, which resulted from the enhancement of ordering PANI chains induced by strong π–π interactions between PANI and CNTs, bringing about the enhancement of delocalization of polarons,15,20,22 and thus maintaining the fast charging/discharging process. The changes in molecular chain structure of PANI/SWCNTs free-standing films with different amount of CNTs were further characterized by XRD as shown in Figure 3b. The XRD pattern of pristine PANI presents three diffraction peaks at 15.6o, 20.7 o, and 25.6 o, which can be assigned to repeat unit of PANI chains, periodicity perpendicular and parallel of polymer backbone chain, respectively.15,20,22 After adding CNTs, the peaks of PANI at 15.6 o and 25.6 o become stronger and sharper, especially in the samples of PANI-5 and PANI-10, indicating that introduction of CNTs could enhance molecular ordering arrangement of PANI.20 When PANI molecules and CNTs co-exist in m-cresol solution, it is expected that the solution expanded PANI chains would be further stretched through strong π–π overlapping of its quinoid rings along the one-dimensional CNTs, and thus orderly attached onto their surface; meanwhile, the prior expanded PANI molecules would sequentially induce more stretched coil conformation with nearby PANI chains, resulting in their generally ordered stacking onto the surface of CNTs and consequently forming compact packing interface structures. This ordered arrangement and stacking of PANI via π–π interaction would lead to an increased crystallinity as shown in Figure 3b, bringing about good carrier mobility. UV-vis spectroscopic measurements were also 12


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performed to further illustrate the interfacial interaction between PANI and CNTs, and the results are shown in Figure 3c. The main characteristic absorption bands of PANI happen around 283 (π–π* transition centered on the quinoid ring) and 446 nm (polaron–π*),27–28 the intensity of which increases and decreases with the increasing CNTs content, respectively. Meanwhile, the characteristic band associated with π–π* transition in the composite film sample is evidently blue shifted when compared to that of the pristine PANI film, suggesting strong interactions between the two components.27 It appears that once the two phases coexist in the m-cresol solution, PANI polymer chains adsorb onto the CNTs surface through their strong π–π interaction forming an enhanced number of quinoid units oriented along the CNTs.27 Besides, dramatic increase of the designated “electronic-like absorption” peak (–N=quinoid=N–) suggests the enhancement of effective electron delocalization, and thus improved the conductivity of polymer chains.26 Moreover, with the assistance of highly conductive CNTs, an efficient charge transport network together with good mechanical properties could be successfully constructed. All of the above advantages would facilitate ion transportation and fast redox reactions of free-standing electrode and thus providing better capacitive performance. The surface structure in PANI/SWCNTs films with different content of CNTs, namely PANI and PANI-5, were also characterized via XPS, which is intended to reveal their special bonding states and the potential affection on capacitive performance. As is shown in Figure 4a, C, N, O and S elements are presented in the full spectra of PANI and PANI-5 film samples as expected. The N1s spectra of PANI 13



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can be resolved into four peaks associated with quinoid imine, benzenoid imine, positively charged imine (bipolaron state), and protonated amine (polaron state), centered at 398.5 eV, 399.5 eV, 400.8 eV, and 402.2 eV, respectively (Figure 4b).29–30 Compared to the pristine PANI, intensity of quinoid imine (–N=) peak (located at 398.5 eV) in PANI-5 increases (Figure 4c) obviously, which can be confirmed by the variation of N1s (Figure S3), suggesting the existence of strong π–π interactions between PANI molecular and CNTs, which is consistent with previous Raman analysis results. In addition, peak single area of positively charged imine in bipolaron state centered at 400.8 eV increases obviously. The increase in ratio of N+/N indicates the enriched doping level of PANI illustrating that there are more unsaturated bonds in PANI/SWCNTs films than in pristine PANI film, which causes the increase of average charge transfer number, and therefore enhancing the capacitive performance.29,31 The good wettability of PANI/SWCNTs free-standing film is another advantage for its electrochemical performance, which is crucial for faster ion diffusion process during charging/discharging (Figure S4). In general, CNTs is hydrophobic in nature, due to the lack of hydrophilic groups. However, SWCNTs as used in this work are anchored with a variety of functional groups as proved by the above characterization results. Thus, they can be easily dispersed in the solution of CSA-doped PANI. Even after two months stewing, no precipitation can be observed (Figure S4b). Besides, thanks to the strong π–π interaction between CNTs and PANI, the PANI can orderly stack onto the surface of CNTs, which possesses abundant –NH– groups and is hydrophilic in nature. Therefore, the free-standing films exhibited a good wettability 14


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(Figure S4), which brings about excellent ion diffusion properties and outstanding electrochemical performance. 3.3. Electrochemical performances of PANI/SWCNTs free-standing films. Because PANI can form highly ordered interface layer on the surface of CNTs via their strong π–π interactions, constructing a continuous conductive network that facilitate ion transportation and fast redox reaction, it is expected that this free-standing PANI/SWCNTs film can be used as flexible integrated electrode for high-performance SC, acting as current collector and active material at the same time. The CV curves of PANI/SWCNTs integrated electrodes with different CNTs content shows quasi-rectangular shape at a scan rate of 50 mV s-1 (Figure 5a). For pristine PANI film electrode, only inconspicuous redox couples are observed, due to its relatively poor electric conductivity that hampers fast reversible redox reaction during charging and discharging. Whereas, as for PANI/SWCNTs film electrodes, the redox peak couples become more obvious, which can be attributed to redox transition between

leucoemeraldine

and

protonated

emeraldine,

p-benzoquinone

and

hydroquinone, and Faradic transformation between emeraldine and pernigraniline, respectively. Even though CNTs can contribute certain capacitive performance (37 F g-1) due to the double layer capacitance (Figure S5), its significant influence to the electrochemical performance should be ascribed to the effective development of pseudo-capacitive PANI. Because PANI and CNTs would form continuous conductive network via their strong π–π interactions in the film electrode,32–33 which facilitates ion transportation and fast redox reactions to a great extent and thus improving the 15



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capacitive performance of PANI. Unsurprisingly, the CV curves of PANI/SWCNTs films show larger integrated area than that of pristine PANI film, let alone the undoped PANI (without CSA), which proves that the incorporation of CNTs can effectively enhance electrochemical activity of PANI leading to better capacitive performance. Influence of CNTs addition to the electrochemical performance is further investigated by corresponding CD measurement, and all of the curves exhibit several stages of charge-discharge durations rather than an ideal linear shape of electric double layer capacitance, implying the pseudo-capacitive performance contributed from PANI (Figure 5b). The specific capacitance of pristine PANI film is 185 F g-1, comparable with previous study,13 while it can be significantly improved by the addition of CNTs and reach a maximum value of 446 F g-1 with 5 wt% CNTs loading, namely PANI-5 sample, more than twice than that of pristine PANI electrode. This outstanding capacitive performance is comparable with or even better than other flexible PANI/CNTs hybrid electrodes (as shown in Table 1)1,5,9–10,34–39 and metal nitride/carbide based electrodes, such as VN/CNT/Inconel/CNTs (289 F g-1),40 TiN-VN fibers (247.5 F g-1),41 the TiN@C electrode (124.5 F g-1),42 Ti3C2Tx/MWCNT (150 F g-1),43 Ti3C2Tx/PPy (416 F g-1),44 and V2CTx (~100 F g-1).45 However, with further increase of CNTs, the capacitance decreases in some extent, for example, 270 F g-1 for PANI-30 film electrode, which is in accord with the variation trend of integrated area of CV curves (Figure 5a). As shown in the SEM results, after adding CNTs, PANI molecular effectively connected with them via strong π–π interactions forming the continuous conductive network, which facilitates ion transport and fast 16


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redox reaction of PANI, leading to an enhanced capacitance. Nevertheless, once addition amount is above 5%, excessive CNTs may hinder the effective interfacial contact between PANI and electrolyte, reducing redox activity sites significantly, and thus deteriorate the capacitive performance of free-standing film. The effects of added dopant (CSA) in PANI/SWCNTs free-standing film on the electrochemical performance were also kindly explored. As shown in Figure S6 and S7, the optimized mole ratio between PANI and CSA is 2:1, because excessive CSA would affect the kinetics of ion and electron transport, or insufficient acid could influence the conjugation length of PANI molecule, both of which would hamper the electrochemical performance of PANI/SWCNTs electrode.46 The CV curves of optimized PANI/SWCNTs film electrode, that is PANI-5, possess excellent electrochemical performance in a wide range of scan rates demonstrating good reversibility of redox reactions as shown in Figure 5d, and the oxidation and reduction peaks shifted positively and negatively with increased scan rate owing to the resistance of electrode.47 The quasi-triangular CD curves (Figure 5e), which is composed of double layer capacitance of CNTs and pseudo-capacitance of PANI, also illustrates good reversibility in the charging/discharging process, which is in accordance with the CV curves. In addition, PANI-5 sample exhibits outstanding rate capability thanks to the continuous conductive network between PANI and CNTs: achieving a high specific capacitance of 308 F g-1 at a high current density of 7 A g-1 (Figure S8). To get better understanding of the induced effects of CNTs on electrochemical 17



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performance of PANI, EIS was employed to investigate impedance changes in the integrated film electrodes. As shown in Figure 5c, all Nyquist curves show a semicircle in high frequency region combined with a linear part in low frequency area, which usually correlates with interfacial charge transfer resistance and a diffusion-limiting step in electrochemical process.48–49 The high-frequency intercept with X-axis is mainly associated with electrolyte solution resistance and intrinsic resistance of active materials. The expanded view of high-frequency region of Nyquist plots is shown in Figure S9. In this system, PANI/SWCNTs films exhibit much lower resistance compared with pristine PANI, indicating the enhanced electric conductivities with rapid ion responses of hybrid film, which is favorable for better capacitive performance. Diameter of semicircle mainly accounts for the resistance of electrochemical reaction on capacitor electrodes. Charge transfer resistance (Rct) of pristine PANI film is 106.6 Ω, leading to obvious voltage drop in discharging process (Figure 5b), which would significantly decrease to 50.4, 31.0, 26.8, and 19.9 Ω with increasing addition of 1, 5, 10, and 30 wt% CNTs. All of the above values achieve a satisfactory level, which are comparable with or lower than other reported PANI/CNTs electrodes.3,50 The low charge transfer resistance should be a reason for the outstanding electrochemical performance of PANI/SWCNTs film electrode. Besides, with increased CNTs, the straight line in low frequency region of EIS spectrum becomes steeper suggesting faster ion diffusion, which could be the origin of obvious redox couples in CV curves (Figure 5a), since more PANI could effectively participate in redox reaction during charging and discharging. Though 18


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better charge transfer properties and faster ion diffusion process can be achieved with more CNTs loading, the PANI redox species might decreases because of shielding effect of excessive CNTs, resulting in a relatively low capacitance as discussed before (Figure 5b). Therefore, a suitable CNTs content is necessary to achieve optimized electrochemical performance, and in our integrated film electrode, that is 5 wt% CNTs. The cycling stability of electrode materials is important for its practical application in fabricating high-performance SCs with long life span. As shown in Figure 5f, PANI-5 film electrode shows a remarkable electrochemical stability over 13000 repeated charging/discharging cycles, maintaining around 98% of its initial specific capacitance; on the contrary, pristine PANI film only exhibits a low retention rate of 74%. The obvious difference could be attributed to the formation of continuous conductive network in integrated PANI/SWCNTs film electrode, since it would effectively alleviate repeated swelling and shrinking of PANI molecular chain during long-term cycling.3 The SEM images of electrodes after cycling test (Figure S10) did confirm our inference to certain degree: no obvious defects (e.g. cracks) can be observed in the PANI-5 film electrode, whereas the surface of PANI electrode shows some obvious defects, for example, voids and cracks. This is the reason why the capacitance of PANI/SCNTs could be maintained at a satisfactory level even after long-term cycling. Obviously, the free-standing integrated PANI/SWCNTs films are perfect electrodes for flexible SCs, because of its excellent electrochemical performance, for 19



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instance, high capacitance. As shown in Figure 6a, CV curves of the solid-state SC constructed by our PANI-5 integrated electrodes exhibit quasi-rectangular shape together

with

some

weakened

redox

peaks

which

are

associated

with

pseudo-capacitive PANI, and it is noted that the potential window is intentionally set to be 0-0.8 V, since the voltage window below 0 V is ineffective in practical application and the characteristic redox couples of PANI occurred within 0.8 V (Figure 5a). As for CD curves, they appear as nearly symmetric triangular shape at various current densities (Figure 6b), suggesting good electrochemical response, and the specific capacitance is calculated to be 218 F g-1 at 1 A g-1, which is comparable or even better than other flexible SCs.10,51–53 Besides, thanks to its compact configuration design, the PANI/SWCNTs film based SC also presented a high volumetric capacitance of 0.71 F cm-3 (based on the whole device), which is comparable to other SC devices such as H-TiO2@MnO2//H-TiO2@C (0.7 F cm-3),54 TiN (0.33 F cm-3),55 graphene

(0.42

F

cm-3),56

PANI//α-Fe2O3@PANI

(0.78

F

cm-3)57

and

ZnO@ZnO-doped MnO2 core-shell nanocables (0.325 F cm-3).58 Moreover, in virtue of the continuous conductive network between PANI and CNTs, this SC is endowed with outstanding cycling stability, showing no obvious deterioration in capacitance even after 5000 charging/discharging cycles (Figure 6c). Predictably, this high-performance device exhibits good mechanical stability and flexibility thanks to the free-standing electrodes and its compact configuration (realizing a thin thickness of 145 µm as shown in Figure 6d), which can be bent to arbitrary direction while retaining over 94% of its original capacitance (Figure 6e and Figure S11), showing a 20


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great prospect to power a variety of portable electronic devices. In addition, the solid-state SC possesses a good rate capability, retaining 61.2% (134 F g-1) of its original capacitance at the current density of 10 A g-1 (Figure S12), which is attributed to the formation of a continuous conductive network between PANI and CNTs. To further demonstrate its outstanding performance as an energy storage device, energy and power densities of the solid-state SC were calculated and compared with other reported SC devices. As shown in the Ragone plots of Figure 6f, the maximum energy density of 19.45 Wh kg-1 was obtained at a power density of 320.5 W kg-1, which is comparable to or even better than a number of SCs based on different electrodes, for example, PANI/multiwalled CNT composite electrode (18 Wh kg-1, 313 W kg-1),18 porous CNTs sponge/PANI (14.87 Wh kg-1, 5340 W kg-1),59 hierarchically porous carbon/PANI composite electrode (9.6 Wh kg-1, 223 W kg-1),19 and graphene@carbon cloth electrodes (1.64 Wh kg-1, 29.5 W kg-1).60 Normally, asymmetric SCs possess higher energy and power densities due to the enlarged potential window.61–62 Surprisingly, the energy density and power density of our PANI/SWCNTs based SC, with a symmetric design, is comparable or even better than some asymmetric SCs, such as graphene//graphene-MnO2 (21.27 Wh kg-1 at 0.223 A g-1),63 activated carbon//NaMnO2 (19.5 Wh kg-1, 130 W kg-1),64 CNT//graphene-MnO2 (12.5 Wh kg-1 at 0.45 A g-1),65 activated carbon//MnO2 (17.3 Wh/kg, 605 W kg-1),66 graphene//MnO2 nanowire/graphene composite (7 Wh kg-1, 5000 W kg-1),67 activated carbon//mesoporous MnO2 (10.4 Wh kg-1 at 0.3 A g-1),68 and 90% MnO-10% nw rGO//functionalised activated carbon (2.6 Wh kg-1, around 75 W kg-1).69 In addition, 21



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when the power density was increased to 6400 W kg-1, the energy density of flexible SC device could still reach 11.9 Wh kg-1 at a current density of 10 A g-1. The above results clearly suggested that our PANI/SWCNTs based SC could provide a high energy density and a high power density concurrently, indicating a good potential as energy storage device. 4. CONCLUSIONS In conclusion, a flexible PANI/SCNTs hybrid film has been successfully developed via a facile solution deposition method that can be directly applied as current collector and active materials for high-performance flexible SC in the meantime, namely acting as integrated electrode. Structural characterization results confirmed the existence of strong π–π interaction between PANI and CNTs, which promoted the formation of ordered PANI molecular chains on the surface of CNTs resulting in a continuous conductive network, and thus facilitated ion transportation and fast redox reactions bringing about enhanced capacitive performance. By controlling CNTs content, specific capacitances of PANI/SWCNTs hybrid film (PANI-5) could achieve a high capacitance up to 446 F g-1 together with outstanding cycling stability, remaining over 98% of its original capacitance over 13000 charging/discharging cycles. As expected, this high-performance film is an ideal electrode for constructing flexible SC with compact configuration, which can deliver a specific capacitance of 218 F g-1, let alone its remarkable mechanical stability and flexibility. Moreover, the energy density of flexible SC reached 19.45 Wh kg-1 at a power density of 320.5 W kg-1, suggesting its promising application as energy storage 22


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device. Our process as demonstrated here has paved the way to develop a high-performance integrated film electrode for flexible SC that could be used as a perfect energy storage device for portable electronics. ASSOCIATED CONTENT Supporting Information SEM images, FTIR spectra, XPS spectra, water contact angle test, and electrochemical performance (CV, CD, EIS, etc.) of various PANI and/or PANI/SWCNT film electrodes. The Supporting information is available free of charge on the ACS Publication website at DOI: ********. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

*

E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge financial support from Natural Science Foundation of SZU (Grant No. 2017004), Guangdong Research Center for Interfacial Engineering of Functional Materials, National Natural Science Foundation of China (Grant Nos.51202150 and 51272161),

Shenzhen

Science

and

Technology

Research

Grant

(Nos.

JCYJ20150827155136104). The authors would also like to thank Mr. WEI Wei in the School of Materials Science and Engineering of Southwest Jiaotong University for his 23



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Figure 1. Schematic illustration of fabrication process of PANI/SWCNTs free-standing film.

Figure 2. Surface (a-c) and freeze-fractured cross sections (d-f) SEM images of PANI/CNTs films containing different contents of CNTs: (a, d) 0 wt%, (b, e) 5 wt%, (c, f) 30 wt%.

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Figure 3. (a) Raman spectra, (b) XRD patterns, and (c) UV-vis spectra of SWCNTs and PANI/SWCNTs hybrid films.

Figure 4. XPS spectra of PANI and PANI-5: (a) full spectra; (b) N1s spectra of PANI; (c) N1s spectra of PANI-5.

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Figure 5. Electrochemical properties of PANI and PANI/SWCNTs hybrid electrodes: (a) CV curves of samples with different CNTs content at a scan rate of 50 mV s-1; (b) CD curves of samples with different CNTs content at a current density of 1 A g-1; (c) Nyquist plots of PANI/SWCNTs hybrid films; (d) CV curves of PANI-5 at different scan rates; (e) CD curves of PANI-5 at different current densities; (f) Cycle stabilities of PANI and PANI-5 films during the long-term charging/discharging process at a current density of 5 A g-1.

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Figure 6. Electrochemical behaviors of the solid-state SC: (a) CV curves of different scan rates; (b) CD curves at different current densities; (c) Cycle stabilities at a current density of 5 A g-1; (d) Photographs of a flexible SC based on PANI/SWCNTs film electrode and its compact configuration, with a thickness of 145 µm; (e) Capacitance retention at different bending angles. (f) Ragone plots of the PANI/SWCNTs based solid-state SC, and comparison to other SC devices.

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Table 1. Electrochemical performance of the reported PANI/CNTs hybrid pseudocapacitor electrodes. Potential Materials

Synthesis technique

Cycle

capacitance

1 M H2SO4

-0.1 ~0.7 V at 5 mV s

In

328 F g-1

chemical

PANI/MWNTs

1 M NaNO3

-0.2 ~0.8 V

polymerization

at 5 mA cm

In situ chemical

560 F g-1

PANI/MWCNT

0.1 M H2SO4

2014/[34]

94% (1000)

2007/[35]

70.9% (2000)

2010/[9]

at 1 mV s-1

Electrochemical

403.3F g-1 1 M HClO4

PANI/VA-CNTs

90.2% (3000)

2017/[37]

81.4% (1000)

2016/[1]

92% (10000)

2016/[10]

83% (500)

2016/[38]

95% (1000)

2015/[5]

94% (1500)

2006/[39]

-2

0 ~1.0 V

polymerization

0 ~3 V -1

polymerization

at 1 A g

Electrospinning

385 F g-1 1 M H2SO4

PANI/CNT/PEO

-0.2 ~0.6 V -1

method

situ

93% (1000) -1

polymerization

In

retention

440 F g-1

In situ enzymatic

situ

life Year/Ref.

window

PANI/MWCNT

Specific

Electrolyte

at 0.5 A g

315 F g-1

chemical

CNFs/CNTs/PANI

1 M H2SO4

-0.1 ~0.8 V at 1 A g-1

polymerization

Filtration

and 146 F g-1

SWNT/PANi

electrical

synergy

6 M KOH

-0.7 ~0.3 V -1

at 0.5 A g method

In

situ

298.4 F g-1

chemical

C-CNTs/PANI

1 M H2SO4

-0.2 ~0.8 V -1

polymerization

PANI/SWCNT

Electrochemical

at 10 mV s

1 M H2SO4

0 ~0.7 V

485 F g-1

39



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at 5 mA cm-2

deposition

In

situ

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365 F g-1

chemical

PANI/HCNTs

1 M H2SO4

0 ~1.0 V

98% (5000)

2016/[36]

98% (13000)

This work

at 1 A g-1

polymerization

chemical 446 F g-1 PANI/SWCNTs

polymerization and

1 M H2SO4

-0.2 ~0.8 V -1

at 1 A g a solution process

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Table of Content (TOC) Graphic

41


SWCNT Film as an

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