Synthesis and Characterization of Self-Standing and Highly Flexible Î´

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Synthesis and Characterization of Self-Standing and High Flexible #-MnO@CNTs/CNTs Composite Films for Direct Use of Supercapacitor Electrodes 2

Peng Wu, Shuang Cheng, Lufeng Yang, Zhiqiang Lin, Xuchun Gui, Xing Ou, Jun Zhou, Minghai Yao, Mengkun Wang, Yuanyuan Zhu, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07161 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Synthesis and Characterization of Self-Standing and High Flexible δ-MnO2@CNTs/CNTs Composite Films for Direct Use of Supercapacitor Electrodes Peng Wu,a Shuang Cheng,a* Lufeng Yang,a Zhiqiang Lin,b Xuchun Gui,b Xing Ou,a Jun Zhou,a Minghai Yao,a Mengkun Wang,a Yuanyuan Zhub and Meilin Liuac* a

New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006, China E-mail: [email protected]; Tel: +86-020-39380525

b

State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, P. R. China.

c

School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0245, USA. E-mail: [email protected]; Tel: +1-404-894-6114 KEYWORDS: flexible electrode, supercapacitor, self-standing, δ-MnO2@CNTs, high stability

ABSTRACT

Self-standing and flexible films worked as pseudo-capacitor electrodes have been fabricated via a simple vacuum-filtration procedure to stack δ-MnO2@carbon nanotubes (CNTs) composite layer and pure CNT layer one by one with CNT layers ended. The light-weight CNTs layers served as both current collector and supporter, while the MnO2@CNTs composite layers with

1 Environment ACS Paragon Plus


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birnessite-type MnO2 worked as active layer and made the main contribution to the capacitance. At a low discharge current of 0.2 A g-1, the layered films displayed a high areal capacitance of 0.293 F cm-2 with a mass of 1.97 mg cm-2 (specific capacitance of 149 F g-1) and thickness of only 16.5 µm, and hence an volumetric capacitance of about 177.5 F cm-3. Moreover, the films also exhibited a good rate capability (only about 15 % fading for the capacitance when the discharge current increased to 5 A g−1 from 0.2 A g−1), outstanding cycling stability (about 90 % of the initial capacitance was remained after 5,000 cycles) and high flexibility (almost no performance change when bended to different angles). In addition, the capacitance of the films increased proportionally with the stacked layers and the geometry area. E.g., when the stacked layers were three times many with a mass of 6.18 mg cm-2, the areal capacitance of the films increased to be 0.764 F cm-2 at 0.5 A g−1, indicating a high electronic conductivity. It is not overstated to say that the flexible and light-weight layered films emerged high potential for future practical applications as supercapacitor electrodes.

1. INTRODUCTTION Pseudocapacitors, one type of supercapacitors, which can store much more charges than electric double-layer supercapacitors (EDLCs) via fast and reversible surface/subsurface Faradic reactions,1 have been attracting more and more scientific and commercial interest due to their advantages of high energy density and satisfactory power density, and hence the potential to bridge lithium ion batteries and transitional capacitors.2-3 Among different type of pseudocapacitor electrodes, transition metal based compounds, including oxides,4 phosphides5-6 and nitrides,7 are always receiving specific attention due to their nature abundance and high theoretical capacitance.8-9 When the widely commercial use of RuO2 based electrodes,10 one of the most popular traditional transition metal based compounds, is limited by their high cost, other


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transition metal based compounds, such as NiO, CoOx,11 NiP,6, 12 TiN13 and etc., began to be new research hot. However, none of them is comparable with another traditional pseudocapacitor material, MnO2, due to its advantages of abundance, low cost, high environmental compatibility, wide potential window and current stability when the voltage changes linearly (close to standard capacitor’ behavior).14 Yet, the commercial use of MnO2 based pseudocapacitor electrodes is often impeded by two major drabacks15 1) low electronic conductivity, thus resulting in relative low power density16 2) low cycling life due to the dissolution of MnO2 active material attributed to the disproportionate reaction, which leads to fast performance fading during cycling.17-18 The low electronic conductivity is the main reason to restrict the practical capacitance of MnO2 based electrodes, who have a large theoretical capacitance (>1000 F g−1)17 in a potential window of 1V while behave always much less (lower than 300 F g−1) except some super thin deposition.9, 19

An effective method to solve this problem is their combination with high conductive carbon

materials, such as carbon nanofibers, CNTs and graphenes.20-24 Except this, searching of suitable current collectors with light-weight, high conductivity and high anti-corrosion ability in aqueous solution can also greatly improve their potential for commercial use. In some MnO2 based reports, specific capacitances in the range from 108 to 800 F g−1 were usually claimed.20, 25-26 However, these values would be much smaller if calculated together with the cumbersome current collectors. Thus, how to replace the cumbersome current collector, such as Ni foam, Fe foil and carbon cloth by some light-weight and high anti-corrosion materials, is becoming a new challenge.24 Therefore, CNTs seem to be one of the best choices for both current collectors and supporters in aqueous electrolyte due to their intrinsic properties of not only high conductivity and light-weight, but also high physical and chemical stability.27-29



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In the work here, to avoid the dissolution of MnO2 active material, birnessite-type δ-MnO2 was selected due to the high insertion energy of H+ in this phase structure and hence the retard of disproportionate reaction.30 In additional, light-weight and long length CNTs were chosen as both current collector and supporter. Finally, self-standing, flexible layered films for pseudocapacitor electrodes were fabricated via stacking MnO2@CNTs composite layers and pure CNTs layers alternately. The layered film displayed a high areal capacitance of 0.293 F cm-2 with a mass of 1.97 mg cm-2 and an excellent volumetric capacitance of 177.5 F cm-3 at 0.2 A g-1. It also exhibited good rate capability with only about 15 % fading for the capacitance when the discharge current increased to 5 A g−1 from 0.2 A g−1, outstanding cycling stability of about 90 % capacitance retention after 5,000 cycles and high flexible ability (almost no performance change when bended 180º). Moreover, when the layers were three times many with a mass of 6.18 mg cm-2, the areal capacitance of the films increased to be 0.764 F cm-2 at 0.5 A g−1, indicating a high electronic conductivity. This kind of high flexible and light-weight layered film emerged high potential for future practical applications as supercapacitor electrodes, not only for high electrochemical performance but also high flexibility. 2. EXPERIMENTAL 2.1. Materials CNTs sponge was used as raw material which was synthesized via a CVD process as reported by X.C. Gui’s previous work.31 Hexadecyl trimethyl ammonium Bromide (CTMAB), Potassium permanganate (KMnO4) and Manganese (II) Sulfate Monohydrate (MnSO4•H2O) were purchased from Adamas-beta. Polytetrafluoroethylene emulsion (PTFE, 60 wt.%) was taken from TCI (Shanghai) Development Co., Ltd. The cellulose filter membranes with an average


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pore size of 0.22 µm were provided by Millipore. All of the reagents were of analytical grade and used directly without further purification. 2.2. Preparation of MnO2@CNTs composite CNTs sponge was treated by ultraviolet (UV) for 1h to make it hydrophilic first, then purified by refluxing in a mix of 10 wt.% nitric acid and 50 wt.% sulfuric acid for 2 h and collected through vacuum filtration and freeze-dried at -40 ºC for 24 h in vacuum. The synthesis process of MnO2@CNTs composite was described as follows: First, CNTs (20 mg) and Hexadecyl trimethyl ammonium Bromide (CTMAB, 500 mg) were dispersed in 200 ml deionized water by ultrasonic for 2 h. Second, KMnO4 (1.2 mmol, 12.0 mmol L-1) and MnSO4•H2O (0.47 mmol, 4.7 mmol L-1) solution were added into the suspension under vigorous stirring after it was heated steadily up to 95 °C, then kept for 2 h. Precipitate of MnO2@CNTs composite were washed (several times with deionized water), collected by filtration after cooled to room temperature, dried in an oven at 80 ºC, dispersed in deionized water (0.33 mg mL-1) and ultrasonically treated for 30 min before use. 2.3. Electrode films’ fabrication First, 3.0 g of CTMAB was dissolved in distilled water (1000 mL) at 25°C. Then 33 mg of PTFE emulsion and 200 mg CNTs were subsequently added to the solution with a strong stirring for about 20 min to obtain a CTMAB, CNTs and PTFE mixture as dispersion A. Amount of the pre-obtained MnO2@CNTs composite suspension (30 mL, 10 mg) were dispersed in 20 mL of the dispersion A and stirred for 5 min by ultrasonic vibration and served as dispersion B. The final MnO2@CNTs/CNTs hybrid films were prepared by vacuum filtration of A (5 mL) layered atop B (10 mL) alternately onto a cellulose membrane along with washing by excessive deionized water. After dissolving the cellulose filter membrane by acetone and ethanol, a



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freestanding MnO2@CNTs/CNTs paper-like film was obtained. Diameter of the obtained film, ranging from 7 mm to 40 mm, was dependent on the size of the filter. A film with 11 layers was set as one unit, consisting of 5 layers’ MnO2@CNTs composite and 6 layers’ pure CNTs with CNTs layers ended on both sides to insure the good conductivity with external circuit. Areal mass of this unit film was about 2.1 mg cm-2 with acceptable deviation in each batch (e.g., 1.97 mg cm-2 in one of the batch), which was measured by a precision balance with an accuracy of 0.01 mg (Sartorius BT 25 s). Additionally, a brief flexible symmetric supercapacitor was assembled with two same pieces of MnO2@CNTs/CNTs film (0.6×1.3 cm-2) leaded by platinum wires with an electrolyte-soaked separator (filter paper with thickness of 22 µm). 2.4. Characterization techniques Surface morphologies of the samples were characterized by a field emission scanning electron microscope (FE-SEM, Hitachi LEO 1530) and Transmission electron microscopy (TEM, Tecnai G2 F20). X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance X-ray diffractometer. Raman spectra were measured on a LabRAM HR Evolution system with an Ar 514.5 nm laser. Thermogravimetric analysis (TGA) was performed at a heating rate of 10 °C/min up to 800 °C in oxygen atmosphere with Mettler LF-1100. Electrochemical properties of the electrodes, evaluated with cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD), were performed on a CHI 660E workstation (Shanghai Chenghua Instrument Co., Ltd.) using a three-electrode configuration in 1 M Na2SO4. A Pt mesh and a Ag/AgCl (in saturated KCl) were served as the counter and the reference electrodes, respectively. 3. RESULTS AND DISCUSSION


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3.1. Morphology and Structure Characterization

Figure 1. SEM images of (a) raw CNTs (b) MnO2@CNTs composite, insets are the EDS element mapping images, showing the spatial distribution of Mn and O; (c) Cross section of a unit MnO2@CNTs/CNTs stacked film with a thickness of 16.5 µm; (d) Schematics of the stacked films’ preparation process.

To explore the morphologies of the samples obtained, FESEM and energy-dispersive X-ray spectrometry (EDS) were employed first. The pristine CNTs used here possess smooth walls with a diameter of about 20−30 nm and super long length of several micrometers, as presented in Figure 1a. After treatment with acid and Mn-contain precursor via a solvothermal process, the CNTs were well covered by some cross-linked flaky materials with much increased diameters of about 60-100 nm (Figure 1b). EDS element mappings (inset of Figure 1b) corresponding to the same area of these composited CNTs indicated that Mn and O elements distribute homogeneously throughout the observing area, implying a successful and uniform deposition of target material, MnO2. After shaped to films via a simple vacuum filtration process of MnO2@CNTs composite and pure CNTs alternately with pure CNTs ended, freestanding MnO2@CNTs/CNTs hybrid films were obtained. In one unit film, which was controlled to be 11 layers with 5 layers’ MnO2@CNTs and 6 layers’ CNTs, a thickness of ~16.5 µm with an area mass density of about 1.97 mg cm-2 can be achieved, as shown in Figure 1c (a cross-section



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image of one unit multi-layer film). Additional, schematic diagrams were also drafted below the corresponding images to well expound the synthetic rout, as shown in Figure 1d.

Figure 2. (a) Raman spectra and (b) XRD patterns of pure MnO2 powder (black), CNTs (red), and MnO2@CNTs composites (blue); (c) XPS full spectrum (d) Mn 3s of the as-prepared MnO2@CNTs composites.

Before shaped to film electrodes, structural features of pure MnO2 powder, acid-treated CNTs, and MnO2@CNTs composite were investigated using Raman spectra, XRD and XPS spectrum, which are presented as Figure 2. As exhibited in Figure 2a, the acid-treated CNTs exhibited three characteristic Raman bands, D, G and G0 bands, at 1354 cm-1, 1578 cm-1 and 2690 cm-1, respectively, which represent defects (D) and graphitic (G & G0) planes.32 While, for the pure MnO2 powder of the black curve in Figure 2a, bands at 648 cm-1, 570 cm-1 and 501 cm-1 can be indexed to birnessite-type manganese dioxides (δ-MnO2).18, 33 After composition through directly


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addition of acid-treated CNTs to the solvethermal precursor, Raman spectrum revealed a simple signal superimposition of CNTs and birnessite-type MnO2 except a relative intensity change of D and G bands, which implied a change of defect content in CNTs in qualitative. The ID/IG ratios of CNTs increased to be 1.12 from 0.81 after composited with MnO2, indicating an increase of defect ratio, which should originate from the oxidation of CNTs by KMnO4 during the solvethermal process. When the samples were investigated by XRD, four sharp peaks at 12.5º, 25.3º, 35.6º, 65.7º and two broad weak peaks at 38.3º, 42.6º, which can be indexed to the (001), (002), (20 1), (312) , (003) and (112) crystal planes of birnessite-type MnO2 (JCPDS 42-1317), respectively, as shown in Figure 2b. It can be detected for the MnO2 powder sample, confirming the formation of birnessite-type MnO2; a broad peak at around 22º can be seen for the acid treated CNTs, which is the characterization peak of carbon based material;25 while a superimposed pattern of that of MnO2 and CNTs was emerged for the composited sample, indicating the additional of CNTs did not affect the phase structure of MnO2, which is coincident with the Raman results. To further elucidate the structure and chemical composition of the MnO2/CNTs electrode material, X-ray photoelectron spectroscopy (XPS) measurement was performed, and the results are shown in Figures 2c and d. The obtained XPS full spectrum (Figure 2c), showing signals from Mn, O, and C elements, suggests the presence of MnO2 on the CNTs. To reveal the valance state of Mn, high-resolution Mn 3s core level spectrum was organized and presented in Figure 2d. The energy difference of the sample is 4.77 eV, revealing a mean valance state of Ⅴ for Mn in MnO2,3 While the Mn 2p spectrum (Figure S1) exhibits two binding energies centered at 642.9 eV and 654.5 eV (with a spin energy separation of 11.6 eV), corresponding to Mn 2p3/2 and Mn 2p1/2, respectively, which is consisted with the reported results for MnO2.34-35



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Figure 3. (a)TEM image of pure CNTs and a closer view of the area indicated by the blue box (inset), (b) Normal TEM image, (c) and (d) HRTEM images with clear lattice fringe of the MnO2@CNT composites selected from different areas.

To further analyze the morphology of the MnO2@CNTs composites, transmission electron microscopy (TEM) and HRTEM observations were carried out. It can be seen clearly that the CNTs used here are multi-layer structure with a diameter of around 30 nm, as shown in Figure 3a. A clear lattice fringe of 0.35 nm corresponding to the (002) plane of CNT can be observed,3637

which was illustrated in the high magnification TEM image. After treated with Mn-contain

precursor via a solvothermal process, the CNTs was well covered (Figure 3b) by cross-linked flaky materials with much increased diameters of about 80 nm. It can be seen clearly that the coating layer is well wrapped around the CNTs like ribbons and well contacted with them, which is consistent with the SEM results. In the HRTEM iamge, as shown in Figure 3c, an interplanar spacing of 0.69 nm that corresponding to the (001) plane of birnessite-type MnO2 can be measured, which is in good agreement with other reports.38 In another HRTEM area (Figure 3d),


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the lattice planes of prior well crystallized CNTs were twisted and only blurred (002) plane can be identified, indicating a strong oxidation of KMnO4 to the CNTs during the hydrothermal process for deposition MnO2. On the surface of CNT, small nanocrystals coating with an interplanar distance of 0.23 and 0.21 nm, which can be indexed to the (003) and (112) planes of birnessite-type MnO2, respectively, was detected, which is agreed well with the above Raman and XRD results. Additional, there is no obvious gap between CNTs and MnO2 though their crystal structure did not match with each other, indicating a strong interaction and well contaction at the interface. 3.2. Electrochemical performance

Figure 4. (a) CV of the MnO2@CNTs/CNTs film electrodes collected at different scan rates; (b) GCD curves of the films collected at different current densities; (c) Dependence of specific capacitance on scan rate (corresponding to bottom axis) and on current density (top axis) determined by the CV curves and the GCD curves, respectively; (d) Cycling life test at scan rate of 50 mV s-1, inset is a photograph of one assembled flexible supercapacitor that is powering a miniature LED light.



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Before shaped to films via the vacuum filtration process, optimum weight ratio (wt. %) for MnO2:CNTs of 77:23 in the MnO2@CNTs composite was dertermined by monitoring the performance change upon their ratios (Figure S2), which is further confirmed by TGA (Figure S3). After shaped to films with pure CNTs, the electrical behavior of the MnO2@CNTs/CNTs hybrid film were evaluated in a three-electrode system with 1 M Na2SO4 solution as electrolyte. Figure 4a exhibits the CV curves of the MnO2@CNTs/CNTs hybrid film at different scan rates from 2 to 50 mV s-1 within an electrochemical window of 0 to 1.0 V vs. Ag/AgCl reference. It is clearly seen that the CV curve of the hybrid film exhibits rectangular shape with a pair of weak redox peaks resulting from treated CNTs. With increasing sweep rate, the current response increased without any obvious change in the shape of the CV curve, indicating a low resistance and good rate performance. The GCD curves of the film at various specific currents display almost symmetric and linear shapes, implying high reversibility and typical capacitive characteristics (Figure 4b). The corresponding specific capacitances at different scan rates and discharge current densities were calculated on the basis of the CV and GCD curves and presented in Figure 4c. The specific capacitance of the MnO2@CNTs/CNTs hybrid film is about 141 F g-1 at a scan rate of 2 mV s-1 and still high as 113 F g-1 at a scan rate of 50 mV s-1. Meanwhile, there is only about 15.5 % fading for the capacitance when the discharge current increased to 5 A g−1 (126 F g-1) from 0.2 A g−1 (149 F g-1), indicating a very high power density. Moreover, life stability is also an important parameter for a supercapacitor. Hence, a charge-discharge cycling test was used to evaluate the durability of the self-standing paper-like electrodes, which is shown in Figure 4d. For the pure CNTs film electrode, a specific capacitance of only about 5 F/g was obtained and there is no obvious change for the capacitance along with long-time cycling. While the initial capacitance of the MnO2@CNTs/CNTs film electrode is over 110 F g-1, much higher


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than pure CNTs film, implying MnO2 makes the main contribution to the capacitance in the films. After 5000 cycles at a scan rate of 50 mV s-1, 90% of the initial specific capacitance was retained, exhibiting high stability. Insets of Figure 4c and d are a schematic of one assembled symmetric cell in button cell and a photograph when powering a miniature LED light. It can be conclude that MnO2@CNTs/CNTs film electrode exhibits good energy storage properties: good rate capability, long cycling life and high specific capacitance (as high as 149 F g-1, even including the weight of current collector).

Figure 5. (a) CV at 20 mv s-1 and (b) GCD curves at 0.5 A g-1 of the film electrodes with various areal mass (black: 1.97 mg cm-2, red: 4.11 mg cm-2, blue: 6.18 mg cm-2); (c) Mass (left axis) and area (right axis) specific capacitance as a function of areal mass of the flexible films calculated from the GCD cureve; (d) CV at 20 mv s-1 and (e) GCD curves at 0.5 A g-1 of the films with different diameter; (f) Dependence of mass (left axis) and volumetric specific capacitance (right axis) on diameter determined by the discharge curves at 0.5 A g-1. Photograph of the flexible electrodes with different diameters (inset).

To realize commercial use as a device, the electrochemical performance change along with the extending of mass and areal is also very important. Hence, electrochemical performance of the electrodes with different areal mass densities and different areas of one individual electrode was determined and demonstrated in Figure 5. The areal mass can be controlled by the stacking of the unit film mentioned above with a mass of about 1.97 mg cm-2. Here, areal mass of 4.11 mg cm-2



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and 6.18 mg cm-2 were obtained by two and three unit films’ stacking, respectively. Figure 5a exhibits CV traces of these electrodes at a scan rate of 20 mV s-1, displaying a linear increase of the CV area along with the increase of mass. While, for the GCD curves at a same current density upon mass of 0.5 A g-1 (Figure 5b), no obvious different but a slight reduce of the discharge period along with the mass increase can be detected. Specific capacitance and areal capacitance as a function of the electrodes’ areal mass at a current density of 0.5 A g-1 were calculated and shown in Figure 5c. When the mass increased from 1.97 to 6.18 mg cm-2, the specific capacitance was slightly reduced from 130.5 F g-1 to 120 F g-1, indicating a high porous and high conductivity of these self-standing film electrodes. While, areal capacitance reveals a linear increase upon mass, leading to an areal capacitance as high as 0.764 F cm-2 at 0.5 A g−1 with a mass of 6.18 mg and thickness of ~50 µm, which is much better than a lot reports.16, 39-40 Except mass extending, performance change upon the geometry area was also characterized here at a sweep rate of 0.5 A g−1 (Figure 5d and e). With increasing diameter, the current response was increased with a broadening of CV area proportionally. The distortion of CV shape is inconspicuous even at a larger area, implying a low electronic resistance of this type electrode. While, for the GCD curves, only a slight degradation of the discharge period was detected upon area increasing of the film electrodes. Specific capacitance and volumetric capacitance as a function of these electrodes’ diameter were calculated at a measured current density of 0.5 A g-1 and shown in Figure 5f. The specific capacitance and volumetric capacitance of the self-standing film electrodes can achieve to be 125 F g-1 and 160 F cm-3, which is substantially much higher than those of the previously reported CNTs-based flexible electrodes when include current collectors.2, 6, 24


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Furthermore, a symmetric flexible cell was also assembled and the performance was measured (as shown in Figure S4). At a same discharge current density as that of a single electrode, the discharge time of the symmetric cell is half of that of the single electrode, which means the specific capacitance was remained after assembled to symmetric cells, indicating a good adaptability for practical applications.

Figure 6. (a) Photographs of the films bended to different angles; (b) CV and (c) GCD curves of the electrodes at different bending angle of 0°, 45°, 90°, 135°, and 180°; (d) Specific capacitance change upon bending angles calculated from the CV curves (black) and the GCD curves (red), inset is photographs of the three-electrode configuration.

Except the good extending ability upon area and thickness of our self-standing electrodes due to the high porous and high conductivity mentioned above, the electrochemical change along with different degrees of bending was measured and presented in Figure 6 to evaluate their flexibility for assembling flexible energy storage devices. CV and GCD results of one unit film bended to different angles of 0º, 45º, 90º, 135º and 180º (seen in Figure 6a, the corresponding schematic optical images) were exhibited in Figure 6b and c. Almost no performance fading can be found here. CV curves at 20 mV s-1 kept the near rectangle shapes, while the charge and discharge curves at 0.5 A g-1 in GCD results remained the symmetry and linear profiles. There is



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only about 4% fading for the capacitance when bended to 180º, seen from the calculated specific capacitances on the basis of the CV and GCD curves at different bending angle in Figure 6d (photographs of the three-electrode configuration), indicating a good flexibility of our electrodes. These advance performance can be explained by their unique nano architecture. Firstly, the ribbon like MnO2 was grown directly on the CNTs and made the long length CNTs tend to cross bonding. Besides, the pure CNTs layer constructed a highly conductive and porous 3-D structure which significantly increases the electronic and ionic conductivity of the electrodes. 4. CONCLUSIONS In summary, porous and well-dispersed MnO2@CNTs/CNTs hybrid self-standing films were synthesized via a simple route and were used to assemble flexible pseudocapacitors. With an areal mass of 1.97 mg cm-2, the film possesses a high areal capacitance of 0.293 F cm-2, a volume capacitance of about 177.5 F cm-3 at 0.2 A g-1, a good stability of 90 % retention for the capacitance after 5,000 charge-discharge cycles at 50 mV s-1. Besides, the electrodes also exhibited a high flexibility, almost no performance fading when bended to different angles. With a high areal mass of 6.18 mg cm-2 when stacked with more layers, an areal capacitance of as high as 0.764 F cm-2 can be achieved at a fast discharge current of 0.5 A g-1. This excellent supercapacitive performance is due to a good conductive network and abundant ion migration pathways inside the electrode based on the porous and well-distributed CNTs network. The long length CNTs that was shaped to be film via a simple vacuum filtration process are quite promising to replace the heavy metal based current collectors, which also can avoid to be corroded in aqueous-contain electrolyte. Additional, the CNTs is not only lightweight, but also high flexible, which make the design of high flexible and light supercapacitors possible. Meanwhile, birnessite-type MnO2 was chosen to be the active material and made the main contribution to the capacitance, which not only resulted in high energy storage ability but also a


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high cycling stability due to its unique crystal structure. The MnO2@CNTs/CNTs hybrid selfstanding films synthesized here are very promising electrodes for flexible supercapacitors due to the outstanding mechanical property and excellent electrochemical performance, as well as easy processing and low cost. ■ ASSOCIATED CONTENT Supporting Information Additional experimental details, including XPS about Mn 2p core level spectrum; Optimum weight percentage of MnO2 in the MnO2@CNTs composite; Thermogravimetric analysis (TGA) for pure MnO2, acid treated CNTs and MnO2@CNT composites; A brief flexible symmetric supercapacitor test. ■ ACKNOWLEDGMENTS This work was supported by the Outstanding Talent and Team Plans Program of South China University of Technology (SCUT), the Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200), Fundamental Research Funds for Central Universities of SCUT, China (no. 2015ZZ118) and National Science Foundation for Young Scientists of China (no. 21403073). ■ REFERENCES (1) Deng, W.; Ji, X.; Chen, Q.; Banks, C. E. Electrochemical Capacitors Utilising Transition Metal Oxides: an Update of Recent Developments. RSC Adv. 2011, 1, 1171-1178. (2) Peng, X.; Peng, L.; Wu, C.; Xie, Y. Two Dimensional Nanomaterials for Flexible Supercapacitors. Chem. Soc. Rev. 2014, 43, 3303-3323. (3) Toupin, M.; Brousse, T.; Bélanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184-3190.



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Synthesis and Characterization of Self-Standing and Highly Flexible Î´

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