graphene paper for flexible supercapacitor: large areal mass exhibits excellent areal capacitance

Accepted Manuscript Title: Freestanding conductive film based on polypyrrole/bacterial cellulose/graphene paper for flexible supercapacitor: large areal mass exhibits excellent areal capacitance Author: Lina Ma Rong Liu Haijun Niu Fang Wang Li Liu Yudong Huang PII: DOI: Reference:

S0013-4686(16)32312-X http://dx.doi.org/doi:10.1016/j.electacta.2016.10.195 EA 28283

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

1-9-2016 13-10-2016 29-10-2016

Please cite this article as: Lina Ma, Rong Liu, Haijun Niu, Fang Wang, Li Liu, Yudong Huang, Freestanding conductive film based on polypyrrole/bacterial cellulose/graphene paper for flexible supercapacitor: large areal mass exhibits excellent areal capacitance, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.10.195 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Freestanding conductive film based on polypyrrole/bacterial cellulose/graphene paper for flexible supercapacitor: large areal mass exhibits excellent areal capacitance Lina Maa, Rong Liua, Haijun Niub, Fang Wanga, Li Liua, Yudong Huanga* a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State

Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China b

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Department of

Macromolecular Materials and Engineering, School of Chemical and Chemical Engineering Heilongjiang University, Harbin 150080, P. R. China *Corresponding author: Tel: +86-451-86413711; E-mail: [email protected] (Yudong Huang)

Abstract The development of polypyrrole (PPY) flexible supercapacitors has been recognized as one of the most effective strategies for preparing advanced flexible energy storage devices. However, they are still limited by low mass loading and poor areal capacitance during charge-discharge process. Here, freestanding conductive film is designed and prepared by PPY/bacterial cellulose (BC) nanofibers in combination with graphene (RGO) through a simple in situ polymerization and filtering method. The porous and flexible BC nanofiber is used as a substrate and template for successive polymerization of PPY, which is responsible for such a large areal mass of 13.5 mg cm-2. Thus, the high areal capacitance of 3.66 F cm-2 at 1 mA cm-2 and 2.59 F cm-2 at 50 mA cm-2 are achieved. The assembled symmetric supercapacitor by coupling of two freestanding electrodes delivers high areal capacitance of 1.67 F cm-2, high areal energy density of 0.23 mWh cm-2 and a maximum power density of 23.5 mW cm-2. Therefore, it is believed that this strategy holds great promise for design of freestanding conductive polymer electrodes on high performance flexible supercapacitors. Keywords: Polypyrrole, Bacterial cellulose, Flexible electrode, Supercapacitor

1. Introduction Rapid development of roll-up displays, bendable mobile electronic products, wearable and stretchable electronics requires the use of flexible devices for energy storage [1-4]. Supercapacitor, as a class of state-of-the-art energy storage devices by combining the advantages of rechargeable battery and conventional capacitor, exhibits high power density, fast charge discharge rate and good cyclic stability [5-11]. The key issue for fabricating well-performance flexible supercapacitors is to design flexible electrodes for the aim of guaranteeing electrochemical characteristic and mechanical stability. Furthermore, the flexible supercapacitors are expected to own high areal capacitance that store enough energy in a limited space [12-15]; therefore, development of the foldable electrode with high mass is a critical technological breakthrough. Freestanding conductive films are considered to be ideal flexible electrodes for the high flexibility, lightweight property and the left-off of binder, which not only render fundamental physical property, but also facilitate the continuous electron transfer. Currently, carbon-based paper-like materials, such as RGO paper and carbon nanotube (CNT) paper emerge to garner major interest because of high conductivity and large specific surface area. Nevertheless, the small mass loading greatly limits their potentials. An attractive approach for preparing carbon freestanding film with high mass is coating or growing carbon materials on flexible, porous and light-weight substrates. However, the limited improvement of areal capacitance for them which store energy in their electrochemical double-layers prove that it is unideal strategy for only pure carbon materials. Recently, numerous researchers have transferred attention to integrate carbon materials with pseudocapacitive materials (conductive polymers, metal oxides and/or hydroxides). For example, Hu et al. electrodeposited MnO2 nanoflowers onto a CNT-enabled conductive textile fibers to form a freestanding electrode, which achieved a high areal capacitance of 2.8 F cm-2 (8.3 mg cm-2) [16]. Liu et al. fabricated the PANI/carbon cloth and PPY/carbon cloth flexible electrode by electrochemically deposition with good areal capacitance of 787.40 mF cm-2 and 136.99 mF cm-2 [17]. Zeng et al. developed a CNT/PANI composite electrode through electrochemical polymerization method to show a large specific capacitance of 680 mF cm-2 in the three-electrode system [18]. Despite that they showed excellent electrochemical

performance, these novel approach for fabricating bendable freestanding conductive films could be limited by the weak mechanical performance, elaborate and complex procedures, relatively high cost and time-consuming fabrication procedures. Further practical applications of bendable freestanding electrodes require a simple, scalable and low-cost strategy for flexible supercapacitors. PPY, as a kind of conducting polymer, has been demonstrated to be a promising pseudocapacitive electrode material for flexible supercapacitors. Competitive advantages of the PPY include high specific capacitance, high electrical conductivity, facile synthesis, low cost and good environmental stability [19]. The ability to store charges is mainly attributed to the fast and reversible Faradic redox reactions from a doping–dedoping process [20]. Besides, the operating voltage of the PPY can be extended by introducing permanent doping anions into the polymer, which is contributed from cations extraction/ anion insertion in the oxidation process and anion extraction/ cation insertion during the reduction process of the polymer [21]. However, direct use of PPY as the flexible electrode material will seriously impede their utilization for its poor mechanical properties. Moreover, the swelling-shrinking during the ion intercalating/deintercalating process causes the fading of electrochemical performance during cycling [22]. Several works have attempted to directly electro-polymerization or chemical polymerization PPY onto the well-integrated mechanical flexibility, fibrous and light-weight foldable substrates [23-28]. Given the mechanical properties and fabrication cost, the selection of substrate is key factor for the construction of PPY flexible electrode. To date, various supporting substrates have been tested to construct flexible electrodes, including porous cotton textile, synthetic sponge, official paper, and carbon based paper. BC, as a special type of cellulose, composes of an ultrafine nanosized three-dimensional (3D) fibrous network structure with the diameters between 10 and 100 nm to provide a large surface for combining with PPY [29-31]. Also, many hydroxyl groups on the surface of BC could interact with pyrrole (PY) monomers due to the hydrogen bands which can be helpful for in situ polymerization of PPY. The entire structure of the BC makes it great superiorities as nano-templates for PPY to prepare bendable freestanding electrode. Besides, the remarkable tensile strength (at least 2 GPa) ensures the mechanical property of flexible electrode. However, the conductivity of this electrode composites is still much lower than that of the RGO paper and CNT paper, thus, rational amount of RGO is incorporated.

RGO, as the fundamental 2D carbon structure, features high electrical conductivity, large specific surface area and excellent electrochemical stability [32-35]. The synergistic effect of ultrafine network BC, pseudocapacitive PPY and high conductive RGO have contribution on the areal performance of this freestanding film. Here, a freestanding conductive PPY/BC/RGO film is fabricated by a simple and low-cost “polymerization and vacuum filtration” method. Combining PPY and BC to produce high mass amount flexible electrode has been proven effectively to improve areal capacitance with the mass loading in the range of 7-14 mg cm-2. The incorporating of RGO effectively resolve the poor conductivity of PPY/BC to ensure a high electron and ion transfer. Thus, the flexible supercapacitor electrode with large mass-loading (13.5 mg cm-2) shows high areal capacitance of 3.66 F cm-2, good rate performance of 70.7 % retention from 1 mA cm-2 to 50 mA cm-2, and good capacitance retentions of 73.5 after 8 000 cycles. Meanwhile, the flexible electrode exhibits well flexibility, which can be bent to a large degree. The symmetric supercapacitor, which is fabricated with two PPY/BC/RGO paper electrodes, can offer a large areal capacitance (1.67 F cm-2), high energy density of 0.23 mWh cm-2 and a maximum power density of 23.5 mW cm-2. The high mass loading, mechanically tough, flexible and freestanding electrode in a facile method may represents an alternative promising candidate for flexible energy storage devices. 2. Experimental 2.1 Preparation of PPY/BC/RGO: RGO suspension (1 mg mL-1) was obtained by a modified Hummers method according to the references [36,37]. The PPY/BC composites were performed by in situ chemical oxidative polymerization of PY onto the surface of BC for different polymerization time in the presence of FeCl3 as an oxidant and C7H8O3S•H2O (P-TSA) as an dopant. Firstly, the BC membranes (Hainan Yide Industry Co. Ltd.) were cut into small pieces and washed by deionized water for several times, then pulped with a mechanical homogenizer at the speed of 10,000 rpm, subsequently diluted into 500 mL deionized water to obtain the slurry of BC (2.8 mg mL-1). Secondly, PY monomer (0.25 mL) and P-TSA (665 mg) were then added into the as-prepared BC suspension (50 mL) slowly under ultrasonication to form a dispersed solution, the homogeneous suspension was then transferred into an ice-water with vigorous stirring.

Afterward, 30 mL of ferric chloride (973 mg) aqueous solution was added dropwise into the PY/BC suspension with constant stirring for different times (2h, 4h, 6h, 8h and 10h) at 0-5 oC to obtain PPY/BC suspension. After every polymerization, 18 mL of RGO suspension was poured into the above suspension slowly to form PPY/BC/ RGO homogeneous suspension. The products were filtered via vacuum filtration using a 0.22 µm porous nitro cellulose membrane to form a serials of PPY/BC/RGO composites, namely PPY2/BC/RGO, PPY4/BC/RGO, PPY6/BC/RGO, PPY8/BC/RGO and PPY10/BC/RGO hybrid paper. Furthermore, the sample PPY8/BC was fabricated without adding RGO. Finally, these films were dried at 55 oC for 8 h and automatically peeled off to get a free standing membrane. The pure BC papers were used as the blank sample and conducted in the similar method. The different value of mass between the PPY/RGO/BC and pure BC paper is the mass loading contents. The loading mass of active materials (PPY and RGO) were 7.2, 10.6, 12.9, 13.5, 13.6 mg cm-2 for PPY/BC/RGO and 11.7 mg cm-2 for PPY8/BC. 2.2 Characterization and Electrochemical Measurements The morphology and microstructure of the samples were characterized through scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2100 F). The compounds and composites were investigated by Fourier transform infrared spectroscopy analyzer (FT-IR, PerkinElmer Spectrum 100 Model) and X-ray diffraction (XRD, Rigaku 2500) equipped with Cu Kα radiation (λ=1.5406 Å). Electrochemical studies were performed with a CHI660E electrochemical workstation. The single electrode was tested in a three-electrode system in 1M NaNO3 aqueous solution with an active carbon and a saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. Symmetric supercapacitor was measured in two electrode configuration using both flexible hybrid PPY8/BC/RGO paper as electrodes, which separated with diaphragm.

3. Results and discussion

Fig. 1. Schematic illustration of the fabrication process of PPY/BC/RGO paper electrode. Photograph of (a) original BC pellicle, (b) PPY8/BC/RGO paper.

The strategy for producing PPY/BC/RGO flexible and freestanding electrode is illustrated in Fig. 1. The PPY/BC composite was conducted by in situ oxidative polymerization of PY with P-TSA as a dopant and BC (Fig. 1a) as the template, and the mass of PPY in the substrate can be easily controlled by reaction time of chemical polymerization. Subsequently, amount of RGO was mixed with the synthesized PPY-coated BC nanofibers to form a uniform and conductive flexible membrane. Benefiting from the utilization of BC, the freestanding electrode can be bent to large angle (Fig. 1b). The BC pellicle (Fig. 1a) as substrate exhibits a highly hydrophilic property due to numerous hydroxyl groups on the surface. Further characterization by SEM shows that the BC consists of three dimensional interconnected nanofibrous networks with the diameter in the range from 20 to 60 nm (Fig. 2a), which can be identified by the TEM test in Fig. 2b. The porous structure of BC assists dispersion of the PY molecules along the continuous nanofibers. Also, the hydrogen bands between hydroxyl groups of BC and PY could be acted as a traction force, which is necessary for an efficient growing of PPY onto BC surface. As shown in Fig. 2c and d, the bead-like PPY is uniformly grown onto BC nanofibers without large-scale aggregate formation after in situ polymerization. Fig. 2e presents SEM image of the PPY8/BC/RGO paper, in which RGO can be clearly observed. Based on the low electrical conductivity of PPY8/BC, the firm connection of PPY8/BC and RGO is an important factor in

determining the capacitance performance for the flexible electrode. As shown in Fig. 2f, it is found that the PPY8/BC nanofibers deposit on the surface of RGO to form a close contact for a highly conductive film. The excellent contact between the PPY8/BC nanofibers and RGO can significantly improve the electrical conductivity of the paper electrode. Meanwhile, the cross-linked porous networks are also retained in the PPY8/BC/RGO paper. The channels in the film can be able to provide short ion diffusion paths and much accessible active-sites. Consequently, the fabricated PPY8/BC/RGO paper can directly serve as flexible electrode in the absent of current collector, binder and other additives. Also, this approach for flexible supercapacitor electrodes is simple and low-cost, which is promising for flexible energy storage devices.

Fig. 2. SEM and TEM image of (a,b) BC. (c,d) PPY8/BC. (e,f) PPY8/BC/RGO.

Additionally, IR and XRD analysis were also used to identified the structures and changes from BC to PPY8/BC, then to the final product PPY8/BC/RGO. Fig. 3a presents the FTIR spectra of BC, PPY8/BC and PPY8/BC/RGO films. In the spectrum of BC, the characteristic broad peaks around 3346 and 2892 cm-1 are assigned to the O-H group and the asymmetrically stretching vibration of C-H in the paranoid ring [38,39]. For PPY8/BC, the obvious peaks at 3316, 1557 and 1476 cm-1 are attributed to the N-H stretching vibration,

asymmetric (C-N) and symmetric (C=C) ring-stretching vibration of PPY, respectively. What is more, the strong peaks at 1153 and 955 cm-1 are related to the stretching vibration of the sulfonic group from p-TSA and out-of-plane bending vibration of the C-C, implying the successful polymerization of PY onto the BC [40,41]. While for PPY8/BC/RGO composite, the peaks belonged to PPY8/BC still remain, but they have a blue-shift in comparison with that of PPY8/BC, confirming that the RGO is incorporated in to the PPY8/BC network, and distributes uniformly [42].

Fig. 3. (a) FTIR spectra of BC, PPY8/BC and PPY8/BC/RGO. (b) XRD patterns of BC, PPY, PPY8/BC and PPY8/BC/RGO.

The XRD patterns were shown in Fig. 3b, the characteristic broad peak of PPY ascribes to the amorphous nature appears at 25o [43], and the pattern of original BC displays three sharp peaks at 14.3, 16.7 and 22.6 o, assigning to the typical (110), (110), and (020) planes of cellulose I, respectively [44,45]. After polymerization of the PPY onto the BC, the XRD pattern of PPY8/BC has the comprehensive peaks of PPY and BC. While in the curve of PPY8/BC/RGO, besides the typical peaks of BC and PPY, other two distinct peaks located at 26.4 and 54.4o own to the RGO, indicating the uniform mixture of the PPY8/BC/RGO hybrid. The above characterization results verify the PPY8/BC/RGO paper is achieved with the feature of 3D pours architecture. To learn the wettability of the integrate film, the dynamic contact angles test was performed as shown in Fig. 4. The PPY8/BC/RGO film has a good hydrophilicity with the initial contact angle of 59.3° and the droplet is completely sucked up within 80 s, which benefit to the transport and diffusion of electrolyte.

Fig. 4. Dynamic water contact angle measurement for the PPY8/BC/RGO. The photograph at 1 s was taken immediately after resting the water droplet on the surface.

To estimate electrochemical performance of PPY/BC/RGO, the freestanding conductive films were directly used as working electrode in a three-electrode system in 1.0 M NaNO3 electrolyte. Fig. 5a illustrates the CV curves of PPY8/BC/RGO in a potential window between -0.4 and 0.6 V at the scan rate of 1-50 mV s-1. Rectangular shapes of the CV curves correlates to better capacitive performance, specifically, a successive and reversible faradaic reaction of PPY. The CV response is close to quasi-rectangular shape when the scan rate is lower than 20 mV s-1, demonstrating that the electrode has a good capacitive behavior. When the scan rate increases to 50 mV s-1, the shape deviates from that of a rectangular shape, possibly due to the polarization of the electrode as well as the increasing overpotential from ion transport between the electrolyte and electrode materials [46]. At higher scan rates, the diffusion of electrolyte ions from solid/liquid interface to the interior of electrode materials is not fast enough to satisfy the electrochemical reactions, and the concentration of ions on the solid/liquid interface increase rapidly [47,48]. Fig. 5b and c compare the GCD curves of the PPY2/BC/RGO, PPY4/BC/RGO, PPY6/BC/RGO, PPY8/BC/RGO, PPY10/BC/RGO and PPY8/BC paper electrodes collected at 1 mA cm-2 and 10 mA cm-2. The GCD curves of PPY8/BC/RGO show longest discharge time, reflecting an improvement of the areal capacitance that the amount of mass loading has a great influence on the electrochemical

performance. All these GCD curves are in quasi-triangular shapes at the scan rate of 1 mA cm-2. However, with increasing the current density to 10 mA cm-2, the PPY8/BC/RGO keeps a rather better triangular-like GCD curve as compared to the case for others, especially for PPY8/BC, suggesting more efficient charge carrier transfer/storage. The results mainly come from the high conductive RGO composites uniformly distributing on the surface of porous PPY8/BC, forming fast electronic and ionic conducting channels, and hence enhance the electrochemical performance of PPY8/BC/RGO paper. These results can also be verified by electrochemical impedance spectroscopy (EIS). The detailed GCD profiles of PPY8/BC/RGO flexible electrode are shown in Fig. 5d, which display a typical triangular shape with a small iR drop, indicating that this flexible electrode possesses good electrical conductivity, high coulombic efficiency and reversibility. The results could be attributed to the hydrophilic characteristic and porous structure of this freestanding film, making the internal electroactive materials directly contact with the aqueous electrolyte, and shortening the ions diffusion and migration pathways without the use of additives and binder.

Fig. 5. Electrochemical performance of the PPY/BC/RGO hybrid electrodes. (a) CV curves of PPY8/BC/RGO at different scan rates. (b) Comparison of GCD plots of each samples at 1 mA cm-2. (c) Comparison of GCD plots of each samples at 10 mA cm-2. (d) GCD curves of PPY8/BC/RGO versus different current densities. (e) Areal capacitance versus different current densities. (f) Gravimetric capacitance versus different current densities. (g) Cycle stability of PPY8/BC/RGO electrode at the current density of 40 mA cm-2 at room temperature. (h) Nyquist plots.

On the basis of the discharge curves, the specific capacitance can be calculated by the formula, C = 𝐼 ∗ 𝑡 /𝑚(𝑠) ∗ 𝑉, where I is the discharge current, t is the discharge time, s is the nominal electrode area, m is the mass of the active materials, and V is the voltage range. The calculated specific capacitance as a function of the current densities is shown in Fig. 5e and f. The PPY8/BC/RGO electrode exhibits the highest areal capacitance of 3.66 F cm-2 at 1 mA cm-2, which corresponds to a gravimetric capacitance of 271 F g-1. With increasing the polymerization time from 2 to 8 h, the areal capacitance has obvious enhancement, which contributes to the increasing pesudocapacitance. However, the areal capacitance decreases at the polymerization time of 10 h. This result is possibly attributed to the aggregate, which blocks the diffusion of ions into the interior of the electrode materials and increase the resistance. It should be noted that the areal capacitance of 3.66 for PPY8/BC/RGO is slightly higher than that of 3.57 F cm–2 for PPY8/BC at low current density of 1mA cm–2. Therefore, the difference between the two capacitance values is the electric double-layer capacitance contribution from RGO, which is about 0.09 F cm–2 (50 F g–1), and the specific capacitance of PPY phase within this composite is as high as 306 F g–1. Moreover, an areal capacitance of 2.59 F cm-2 is still remained at a relatively high current density of 50 mA cm-2, demonstrating the good rate capacity. Such high areal capacitance is remarkable compared with that of other reported flexible electrodes (Table 1). In addition, the PPY8/BC/RGO displays a high cyclic

stability. After 8000 cycles the capacitance retention remains 73.5 % of the initial capacitance at the current density of 40 mA cm-2 (Fig. 5g). To better clarify the electrochemical properties of various hybrid electrodes, EIS was conducted, and the Nyquist plots are shown in Fig. 5h. All these plots feature almost vertical slopes in a low-frequency region, indicating a nearly ideal capacitive behavior. The semicircle is considered to be related to an interfacial charge-transfer resistance. The smallest semicircle of PPY8/BC/RGO clearly reveals that the conductivity of this flexible electrode can be greatly improved by RGO introduction and the content of coating PPY. The outstanding electrochemical performance mainly results from the integrity effect of 3D porous hierarchical BC substrate, high capacitive PPY and high conductive RGO. First, the PPY uniformly deposited on the surface of porous BC provide a large mass amount, leading to high areal capacitance. Secondly, The PPY/BC nanofibers in combination with RGO ensure effective electron transport among the film. Thirdly, highly hydrophilic and porous electrode can provide good diffusion channels for electrolyte solution and permit more electroactive sites exposed to the electrolyte.

Table 1. Literature on flexible electrodes for supercapacitor application Mass

Capacitance

Capacitance

(mg cm-2)

(mF cm-2)

(F g-1)

0.45

94.5

215

[49]

PPY/carbon cloth

136.99

114.08

[17]

PANI/carbon cloth

787.4

189.7

[17]

RGO/cellulose

81

120

[50]

165

[51]

Materials

Multilayered RGO films

Ref.

Carbon nanoparticle/MnO2

0.562

109

MnO2/CNT textile fibers

8.3

2800

[16]

CNT/PANI

680

[18]

Graphite nanosheets/PANI

355.6

[52]

BC/PPY

11.2

2430

216.9

[39]

MnO2 /carbon cloth

230

425

[53]

PPY/gold

1.8

270

[54]

NiO//RGO

2.7

1054

392

[55]

PPY/BC/RGO electrode

13.5

3660

271

This work

PPY/BC/RGO supercapacitor

27

1670

This work

Fig. 6. Electrochemical performance of the PPY8/BC/RGO symmetrical supercapacitor. (a) CV curves at different scan rates. (b) GCD curves under different current densities. (c) Areal capacitance versus different current densities. (d) Cycling performance at a current density of 30 mA cm-2. (e) Ragone plot.

Symmetric supercapacitor was assembled by two PPY8/BC/RGO flexible films, as shown in Fig. 6. Fig. 6a shows the CV curves collected at different current densities in the voltage window of 1 V, which exhibit a quasi-rectangular shape corresponding to a capacitive behavior. The CV current respond increases with the scan rate, revealing a good electrochemical reversibility. The GCD curves at different current densities display an almost

symmetric triangular shape with a small voltage drop (Fig. 6b), demonstrate that the device has good capacitive property and a little overall resistance. High areal capacitance of 1.67 F cm-2 at a discharge current of 0.5 mA cm-2 can be achieved from GCD (Fig. 6c). As the current density varies from 0.5 to 50 mA cm-2, the areal capacitance decreases to 0.67 F cm-2. These values are higher than those reported carbon-based flexible symmetric supercapacitors and the most of conductive polymer flexible devices in the literature. In addition, the cycling performance of PPY8/BC/RGO device was tested at 30 mA cm-2 for 8000 cycles, which shows high capacitance retention of 65.4% (Fig. 6d). Fig. 6e shows the areal power and energy density of PPY8/BC/RGO symmetric supercapacitor at different current densities. This device exhibits an energy density of 0.23 mWh cm-2 at a power density of 0.25 mW cm-2 for a 1 V window voltage. The maximum power density as high as 23.5 mW cm-2 can be achieved, with energy density of 0.093 mWh cm-2 is retained. Taken together, the outstanding electrochemical performance of the PPY8/BC/RGO symmetric device can be reasonably attributed to large mass loading, porous structure and large accessible surface areas for electrolyte penetration. 4. Conclusion In summary, this work provides a simple and low-cost synthesis strategy to fabricate freestanding conductive film for flexible electrode. Benefiting from the continuous nanofibers and porous network of the BC substrate, large areal mass (13.5 mg cm-2) of active materials could be grown on the surface without compromising its electrochemical performances. As a result, this electrode exhibits an excellent areal capacitance of 3.66 F cm-2 at 1 mA cm-2, good rate capacity of 2.59 F cm-2 at 50 mA cm-2 and a stable capacitance retention of 73.5 % after 8000 cycles. Moreover, the assembled PPY/BC/RGO symmetric supercapacitor achieves high areal capacitance of 1.67 F cm-2, high areal energy density of 0.23 mWh cm-2 and a maximum power density of 23.5 mW cm-2. These promising results demonstrate that the low-cost, easy scale-up freestanding electrode could serve as a good candidate for the development of flexible energy storage devices. Acknowledgement The authors gratefully acknowledge financial supports from the Chang Jiang Scholars Program and National Natural Science Foundation of China (51073047).

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