Graphene-Wrapped Polyaniline Nanowire Array Modified

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Graphene Wrapped Polyaniline Nanowire Array Modified Functionalized of Carbon Cloth for High Performance Flexible Solid-State Supercapacitor Pengcheng Du, Yuman Dong, Hongxing Kang, Xi Yang, Qi Wang, Jingye Niu, Sihan Wang, and Peng Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03278 • Publication Date (Web): 09 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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ACS Sustainable Chemistry & Engineering

Graphene Wrapped Polyaniline Nanowire Array Modified Functionalized of Carbon Cloth for High Performance Flexible Solid-State Supercapacitor Pengcheng Du,†,* Yuman Dong,† Hongxing Kang,† Xi Yang,‡ Qi Wang,† Jingye Niu,† Sihan Wang,§ Peng Liu†,* †

State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, No 222 South Tianshui Road, Chengguan District, Lanzhou 730000, People’s Republic of China ‡ Institute of Chemical Materials, China Academy of Engineering Physics, No 64 Mianzhong Road, Mianyang, 621900, China § Department of Polymer Science, The University of Akron, 170 University Ave, Akron, Ohio 44325, United States *Corresponding authors: E-mails address: [email protected] (P.C. Du), [email protected] (P. Liu)

ABSTRACT: Graphene wrapped polyaniline (PANI) nanowire array modified functionalized carbon cloth (fCC) (fCC-PANI array-rGO) is successfully fabricated and

served

as free-standing electrode for assembling flexible solid-state

supercapacitors (SCs). Carbon cloth is functionalized so as to improve both hydrophilicity and capacitance, thus the vertically aligned PANI nanowire arrays are conducive to grown on fCC. After wrapped with graphene, the fCC-PANI array-rGO electrode exhibits largest capacitance of 471 mF/cm2 at 0.5 mA/cm2. In addition, graphene layer is employed as protective layer to alleviate swelling and shrinking of PANI in order to improve the cycle stability of fCC-PANI array-rGO. As a result, the free-standing electrode can maintain 75.5% of original capacitance even up to 10 000 cycles. Furthermore, the flexible solid-state SCs based on fCC-PANI array-rGO exhibit an outstanding area capacitance of 197 mF/cm2 at current density of 0.1 mA/cm2, keeping 91.3% of its original value after 7 000 cycles at 5 mA/cm2. Remarkably, the flexible solid-state SCs exhibit excellent mechanical properties and 1

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maintain about 100% of its capacitance, when bended at 180° after 500 cycles. Moreover, the flexible solid-state SCs are further employed as energy storage device to light up a red, green, or yellow LED. Thus, the flexible solid-state SCs based on fCC-PANI array-rGO exhibit potential applications as a candidate for flexible energy-storage devices.

KEYWORDS: Flexible solid-state supercapacitor, Free-standing electrode, PANI nanowire, Graphene, Protective layer.

INTRODUCTION With the rapid development of human society, energy crisis and environmental pollution become more and more serious. Thus, sustainable and renewable clean energy (e.g. solar and wind energy) is supposed as the most reliable solution. However, the intermittent and unstable problem limit the direct application of renewable clean energy. So energy storage devices play an important role in constructing sustainable energy output systems. Supercapacitors (SCs) are recognized as one of the most promising candidates for clean energy storage devices, which have attracted intense attention due to their many advanced features, such as high energy density, fast charge-discharge time and long-term cycle stability, and so on.1-7 Recently, portable and wearable devices become popular and gradually come into our daily life. However, the traditional energy storage device cannot meet the needs of rapid development of portable and wearable devices. To meet the requirement of rapid developing of portable and wearable devices, a new generation of energy storage systems are extremely supposed to be flexible. Due to the incomparable 2


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advantages of light-weight, flexibility and safety, flexible solid-state SCs are especially important and thus attract intense attention.8-12 Moreover, these unique advantages enable them to be promising applications in many fields, such as power sources for portable and wearable electronics, and paper-like personal gadgets, etc.13 In order to satisfy the demands of the rapid developing portable and wearable devices, well-performed flexible solid-state SCs become imperative with high capacitance property and long-term cycling life. One of the best ways to meet the urgent demands is to improve the flexible solid-state SCs’ property via enlarging the capacitance of free-standing electrodes.14-16 Now, studies on flexible solid-state SCs mainly focus on fabrication of high performance free-standing electrode materials. Based on these purpose, high performance free-standing electrodes were fabricated via modifying the free-standing substrate using materials with high theoretical capacity, such as carbon materials (porous activated carbon, CNT and graphene),17-18 transition metal oxides (such as NiO, Fe2O3 and MnO2),19-21 or conductive polymers (polyaniline (PANI) and polypyrrole (PPy)).22-24 Among of wide variety of active electrode materials with pseudo-capacitive property, PANI materials have been considered as one of the most promising pseudo-capacitive materials, PANI exhibits a good deal of advantages such as environmental stability, a unique conducting mechanism, high theoretical capacity and facile chemical synthesis method. Thus, numerous efforts have been devoted to prepare composite free-standing electrodes with PANI.14, 25-27 Surprisingly, PANI can easily grow on a variety of free-standing substrates to fabricate composite free-standing electrodes. Such as print paper, filter 3



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paper, polymer film, CNT paper and 3D graphene film were choose as the free-standing substrates.14,

28-29

Among of these PANI materials with different

morphologies, the PANI nanowire can achieve higher capacitance due to high pseudo-capacitance, high specific surface area, and an optimized ion-diffusion pathway.30 PANI nanowire array was successfully synthesized and used to modify free-standing substrate in order to improve the electrochemical performance, PANI nanowire array was successfully grown on the cloth-supported single-walled carbon nanotubes (SWNT) and porous graphene composite film, and the modified free-standing electrodes exhibited immense advantage in supercapacitors.29,

31-33

However, the mechanical degradation of PANI electrode due to the swelling and shrinking extremely limits the utilization and even fade the electrochemical property. Various of methods were developed to solve the low cycle stability of PANI electrode materials. Recently, two-dimensional (2D) materials exhibited huge potential in electronic devices,34-35 which also were explored in improving the electrochemical properties of conductive polymers due to their high specific surface area and single or several atomic layers structure. 2D materials, such as graphene, MoS2 and Mxene were selected as the substrate to fabricate the composite with conductive polymers.29, 36-38

The PANI nano-needle array modified MoS2 nanosheets exhibited excellent

specific capacitance (853 F/g at 1A/g) and cycle stability (91% after 4000 cycles at current density of 10 A/g).36 However, for PANI nanomaterials that have been prepared, the protective layer has been proved to be more effective, which can prevent the

swelling

and

shrinking

problem

of

4


PANI

materials

during

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intercalating/deintercalating process, these materials (carbon, RuO2, and graphene) were detected and utilized in protecting the PANI materials.39-43 Carbon layer was used to encapsulate PANI nanowire, which could limit volumetric swelling and shrinking during charge-discharge process, and maintain remarkable 95 % capacitance up to 10 000 cycles.39 Graphene also exhibits obviously advantages in protecting the PANI so as to the swelling and shrinking due to the high specific surface area and excellent mechanical properties.33, 41 In order to further improve the capacitance and cycle stability of the free-standing electrode, the current collector was also modified to utilize as free-standing electrode.44-45 Thus, the combination of functionalized current collector, PANI electrode material and protective layer are expected to be an important strategy to achieve excellent free-standing electrode with high capacitance and long-term cycle stability. In this study, we have designed and successfully fabricated graphene wrapped PANI nanowire array modified functionalized carbon cloth (fCC) (fCC-PANI array-rGO), which are used as free-standing electrode to assemble flexible solid-state SCs. The free-standing electrode of fCC-PANI array-rGO exhibited largest capacitance of 471 mF/cm2 at current density of 0.5 mA/cm2 and maintain extremely improved cycle stability. Furthermore, the flexible solid-state SCs assembled with fCC-PANI array-rGO exhibited high capacitance, excellent cycle stability and excellent mechanical performance. Thus, it is supposed to be an excellent candidate as flexible energy storage devices. EXPERIMENTAL SECTION 5



Materials. Carbon cloth (carbon cloth plain; CCP20) was obtained from Fuel Cell Earth. Sulfuric acid (H2SO4) and nitric acid (HNO3) were purchased from Baiyin Liangyou Chemical Reagent Co., Ltd. Potassium permanganate (KMnO4) was received from Tianjin Guangfu Fine Chemical Research Institute. Aniline was purified with distillation. Hydrogen peroxide (H2O2) and perchloric acid (HClO4) were obtained from Tianjin Damao Chemistry Reagent Co., Ltd. Preparation of functionalized carbon cloth (fCC). Several pieces of CC (2×4 cm2) was fully immersed in 60 mL of concentrated H2SO4/HNO3 (2:1) solution, and stirred for 1 h at room temperature, then 6 g of KMnO4 was slowly added. After stirring in 35 °C water bath for 2 h, 100 mL of distilled water was added and stirred for another 3 h. After that, H2O2 solution (30%) was added until no gas bubbles overflowed and the solution became clear. The prepared functionalized CC was fully washed with DI water and dried in oven.44, 46 Preparation of PANI array modified functionalized CC (fCC-PANI array) Typically, 40 mL of 1 M HClO4 aqueous solution was placed in an ice bath. A piece of fCC was immersed in above solution. After the solution was fully cooled, aniline (36.5 mL, 0.4 mmol) was added and shook for 5 min to form a uniform mixture. Ammonium persulfate (APS) (60.9 mg, 0.267 mmol) was dissolved in 10 mL of 1 M HClO4. Then the precooled APS solution was added to initiate the reaction in ice bath. After 24 h, the obtained fCC-PANI array were fully washed with DI water and dried in oven for further use. 6


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Fabrication of rGO wrapped PANI array modified functionalized carbon cloth (fCC-PANI array-rGO) The fCC-PANI array sample was immersed in 0.5 mg/mL GO solution for 15 min to absorb GO, then the GO wrapped fCC-PANI array was took out and dried in oven. Next, the GO wrapped fCC-PANI array was placed in HI solution (40%) to reduce the GO in 70 °C oil bath, after reaction for 6 h, the obtained fCC-PANI array-rGO was repeatedly washed with DI water, and dried in oven. Assembly of flexible solid-state supercapacitor based on fCC-PANI array-rGO. Two free-standing electrodes of fCC-PANI array-rGO pre-coated conductive silver adhesives were fixed on PI film (70 µm) and dropped PVA-H2SO4 gel on the surface. After stored in air for several hours to evaporate the excess water, two prepared free-standing electrodes were pressed together with a separator (NKK cellulose separator) in the middle. Characterization Scanning electron microscopy (Hitachi S-4800, Japan) was used to investigate the morphologies and structure of fCC-PANI array and rGO wrapped fCC-PANI array. The Raman spectrums were carried out by the Thermo Scientific DXR Raman Microscope. The FT-IR spectroscopy analysis was used to confirm the functional group of the obtained samples by Bruker IFS 66 v/s infrared spectrometer. X-ray photoelectron spectroscopy (XPS) of as-prepared free-standing electrodes was analyzed by a PHI-5702 instrument. Electrical conductivity of free-standing electrodes was measured by a 4-probe conductivity meter, and conductivity changes 7



of free-standing electrodes were investigated using linear sweep voltammetry (LSV) by a 2-probe method. Electrochemical measurement The free-standing electrodes of fCC, CC-PANI, fCC-PANI array and fCC-PANI array-rGO were directly used as the electrode material. The load mass of PANI is about 0.35 mg. The three-electrode measurement of the free-standing electrodes were carried out at room temperature, with 1 M H2SO4 as electrolyte. Pt foil and a saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) was measured at three-electrode system with a frequency range 105-0.01Hz. Otherwise, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests and EIS tests of the flexible solid-state SCs were carried out in two-electrode configuration.

Scheme 1. Schematic diagram of the fabrication process for the free-standing electrode of fCC-PANI array-rGO and assembly of the flexible solid-state SCs based on fCC-PANI array-rGO. 8


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RESULTS AND DISCUSSION The graphene wrapped polyaniline array modified functionalized carbon cloth (fCC-PANI array-rGO) was used as the free-standing pseudo-capacitor electrode material and the schematic diagram of fabrication process is shown in Scheme 1. Firstly, the pristine CC substrate was oxidized using hummers’ method to fabricate functionalized carbon cloth (fCC), the oxidization reaction could improve both hydrophilicity and capacitance of the pristine CC. Secondly, vertically aligned PANI array was successfully grown on fCC substrate using the dilute chemical polymerization method. The PANI array modified fCC is clearly different from the pristine carbon cloth with smooth surface characterized by SEM (Figure 1a). Obviously, the as-formed PANI array on the surface of fCC exhibits ordered nanowire structure (Figure 1b-d). Unfortunately, the free-standing electrode of PANI arrays modified fCC did not show desirable long-term cycling stability due to the swelling and shrinking problem.47-48 In order to avoid the mechanical degradation of PANI, the rGO was selectively coated on the surface of PANI arrays by electrostatic adsorption, and following reduced with HI under heating status. The swelling and shrinking of PANI electrode materials was supposed to be alleviated by rGO, which also could improve mechanical strength and cycle stability of the free-standing electrode. Finally, the free-standing electrode of fCC-PANI array-rGO was successfully fabricated, and the SEM images of fCC-PANI array-rGO was shown in Figure 1f-h. It was observed that a thin layer of reduced graphene oxide was uniformly coated on the surface of PANI array and the protective graphene layer is constructed with only several 9



graphene sheets measured by SEM (Figure 1i). Additionally, the fCC-PANI array-rGO was further elucidated by the energy-dispersive X-ray spectrometry (EDS) mapping analysis of elements C, N, and O from a select area (Figure 2a), the elements mapping of C, N, and O confirmed a uniform distribution of the elements (Figure 2b-d). The element of O was mainly contributed by the oxidized carbon cloth and rGO. As shown in Figure 3a and b, the fabrication process of fCC-PANI array-rGO was further investigated by Raman and FT-IR spectra, the fCC display the same G band (~1597 cm-1) and the D band (~1335 cm-1) with the CC substrate. However, the ratios of D/G for fCC (1.43) is higher than that of the CC (1.27), the increase ratio of D/G indicates a higher degree of disorder in the fCC. After modified with PANI array, the fCC-PANI array exhibits obviously characteristics peaks of PANI at 517, 529, 813, 1165, 1221, 1326 and 1591 cm-1, respectively,25 and fCC-PANI array-rGO also owns the same characteristics peaks with fCC-PANI array after modified with rGO. As shown in FT-IR spectrum (Figure 3b), the peaks of C-C stretching of the quinoid ring and benzenoid rings existed in 1575 and 1494 cm-1 are assigned to PANI. The peaks at 1303 cm-1 is corresponding to the C-N stretching of secondary aromatic amines. In addition, the characteristics peaks at 1246 and 1127 cm-1 are attributed to the aromatic C-H bending of the benzenoid ring and the quinoid ring stretching of PANI, respectively.49 Furthermore, the fabrication process of fCC-PANI array-rGO was also analyzed by XPS as shown in Figure 4. After oxidation treatment of CC, the content of O element on surface of fCC increases from 7.15% to 22.55% (Figure 4a). Obviously, the C1s 10


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spectrum of fCC exhibits more signals than CC substrate (Figure 4b), which reveals that the oxidation treatment can introduce abundance hydrophilic groups. As shown in Figure 4c, the C1s spectrum of fCC can be fitted to three peaks at 284.7 eV, 287.4 eV, and 289.1 eV, which can be correlated to C-C, C=O, and C(O)O groups, respectively. After modified with PANI array, the high-resolution C1s spectrum of fCC-PANI array is observed to be different from fCC (Figure 4d), which consists of three peaks, locating at 284.5, 285.4, and 286.6 assigning to the groups of C=C, C=N, and C-O, respectively. Additionally, change in N element content also can proof the instruction of PANI, the value of N element content increases to 8.11% from 1.45% as shown in Figure 4a. The high-resolution N1s spectrum (Figure 4e) can be fitted into three peaks 399.6 (-N=), 401.3 (-NH-) and 403.1 eV (-N+-), respectively. Otherwise, when the adsorbed graphene oxide on the surface of fCC-PANI array is reduced by HI, the content of N element of fCC-PANI array-rGO decrease to 2.26% from 8.11 % in fCC-PANI array (Figure 4a). Thus, these results clearly confirm that the free-standing electrode of fCC-PANI array-rGO is successfully fabricated. For comparison, the electrical conductivity and conductivity changes of CC, fCC, fCC-PANI array and fCC-PANI array-rGO were investigated by a 4-probe conductivity meter and a 2-probe method using linear sweep voltammetry (LSV), the results were presented in Figure 5. As shown in Figure 5a, after oxidation treatment, the sheet resistance of fCC shots up to 107 Ω sq-1 from 1.4 Ω sq-1 of CC due to the introduction of hydrophilic groups. Furthermore, the introduced PANI array can decrease the sheet resistance of fCC-PANI array, and the sheet resistance obviously 11



decreases to 31 Ω sq-1 because of the high electrical conductivity of PANI. After reduced with HI, the sheet resistance of fCC-PANI array-rGO increases to 35 Ω sq-1, caused by change in doping status due to the treatment of HI. In addition, the change law of electrical conductivity of CC, fCC, fCC-PANI array, and fCC-PANI array-rGO is consistent with the results of conductivity changes observed by LSV (Figure 5b). Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were carried out and used to evaluate the electrochemical properties of CC, fCC, CC-PANI array, fCC-PANI array and fCC-PANI array-rGO. Figure 6a shows the CV curves of fCC-PANI array-rGO at scan rates from 5 mV/s to 100 mV/s. Obviously, two distinct redox peaks existing in the CV curves are attributed to the transition between multiple redox states. Meanwhile, the different doped transitions reveal the pseudo-capacitance behavior of PANI material and result in high capacitance. As shown in Figure 6b, the GCD curves agree well with the CV curves under different current densities from 0.5 to 50 mA/cm2. The existing plateau from 0.2 to 0.4 V is assigned to the redox reaction of PANI material, which provides better pseudo-capacitance for the free-standing electrode. The CV curves and GCD curves of CC, fCC, CC-PANI array, fCC-PANI array and fCC-PANI array-rGO was presented in Figure 6c and d for directly comparison. Obviously, the fCC-PANI array-rGO exhibits largest area in CV curves scanned at 20 mV/s, and also possesses maximum discharge time at 0.5 mA/cm2. These evidences demonstrate that the free-standing electrode of fCC-PANI array-rGO reveals excellent electrochemical performance. In addition, after modified with PANI array, the 12


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flexible electrode of fCC-PANI array possesses more excellent electrochemical performance than CC-PANI array, which is attributed to capacitance contribution by fCC. The fCC can supply partly capacitance and also be utilized as the free-standing substrate. The CC only exhibits very low capacitance (

Graphene-Wrapped Polyaniline Nanowire Array Modified

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