Flexible and Freestanding Supercapacitor Electrodes Based on

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Flexible and Freestanding Supercapacitor Electrodes Based on Nitrogen-Doped Carbon Networks/Graphene/ Bacterial Cellulose with Ultrahigh Areal Capacitance Lina Ma, Rong Liu, Haijun Niu, Lixin Xing, Li Liu, and Yudong Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11034 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

Flexible and Freestanding Supercapacitor Electrodes Based on NitrogenDoped Carbon Networks/Graphene/Bacterial Cellulose with Ultrahigh Areal Capacitance †

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Lina Ma , Rong Liu , Haijun Niu , Lixin Xing , Li Liu , Yudong Huang*,

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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 ‡

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 KEYWORDS: flexible electrode, N-doped, bacterial cellulose, carbon nanofibers, supercapacitor

ABSTRACT: Flexible energy storage devices based on supercapacitors rely largely on scrupulous design of flexible electrodes with both good electrochemical performance and high mechanical properties. Here, nitrogen-doped carbon nanofiber networks/graphene/bacterial cellulose freestanding paper (N-CNFs/RGO/BC) is first designed as a high-performance, mechanically tough and bendable electrode for supercapacitor. The BC is exploited as both supporting substrate for a large mass loading of 8 mg cm-2 and biomass precursor for N-CNFs by pyrolysis. The one-step carbonization treatment not only fabricates the N-doped threedimensional (3D) nanostructured carbon composite materials, but also forms the reduction of the GO sheets at the same time. The fabricated paper electrode exhibits an ultrahigh areal capacitance of 2106 mF cm-2 (263 F g-1) in KOH electrolyte and 2544 mF cm-2 (318 F g-1) in H2SO4 electrolyte, exceptional cycling stability (∼100 % retentions after 20,000 cycles), and excellent tensile strength (40.7 MPa). The symmetric supercapacitor shows a high areal capacitance (810 mF cm-2 in KOH and 920 mF cm-2 in H2SO4), thus, delivers a high energy density (0.11 mWh cm-2 in KOH and 0.29 mWh cm-2 in H2SO4) and a maximum power density (27 mW cm-2 in KOH and 37.5 mW cm-2 in H2SO4). This work shows that the new procedure is powerful and promising way to design flexible and freestanding supercapacitor electrodes.

INTRODUCTION The current development trend of wearable and portable electronics such as collapsible displays, on-body sensors, bendable mobile electronic products and electronic papers has promoted the demand for flexible, small, thin, lightweight, and highly efficient energy storage devices.1-3 Supercapacitors are widely considered as a class of state-of-the-art energy storage devices due to rapid charge and discharge rates,4 moderate energy densities, long operating lifetimes5 and power densities.6-10 However, most of the conventional supercapacitors in the market are made up of rigid electrodes based on slurrycoating technology which cannot withstand bending, folding or rolling. Moreover, the use of metallic current collectors and other additives will increase the manufacture cost and the total weight which is not compatible with flexible electronics. Furthermore, the insulated and hydrophobic binder will greatly increase the contact resistance, hindering their potential application in high-performance aqueous supercapacitors.11-13 Therefore, one of the key challenges is to explore a facile, low-cost, scalable and renewable method for fabricating flexible and freestanding electrodes with both high electrochemical performance and good mechanical properties for flexible and lightweight supercapacitors.14-16 Carbon materials usually have low cost, ease of processability, good conductivity, controllable porosity, low weight, very fast charging/discharging kinetics, and bipolar operational

flexibility, thus are the main option to exploit the advanced flexible supercapacitors.17-20 Recently, significant studies have been focused on carbon material films which are considered as one of the promising candidates for producing flexible supercapacitor devices.3,21-23 RGO or carbon nanotubes (CNTs) and their composites film prepared by vacuum filtration have been widely investigated in recent literatures.24-27 Despite their attractive electronic conductivity, the disadvantages of small mass loading and irreversible agglomeration,28 which cause low areal capacitance and poor cycling life. Furthermore, the relatively high production cost of single RGO and CNTs for fabricating flexible supercapacitors have limited their widespread commercialization. Currently, carbon materials derived from renewable biomass precursors have attracted much attention given their low cost, easy fabrication and environmental friendly.29 BC, which can be produced on industrial scales via fermentation of bacterium, consisting of ultrafine nanosized 3D fibrous networks.30-32 Thus, there is growing interest for the interconnected 3D carbon nanofibers networks (CNFs) from pyrolyzed BC owing to its low-cost, high conductivity, good electromechanical stability, specific surface area and sufficient porosity. Furthermore, the engineering of incorporation of N-heteroatom into the carbon lattice is an effective way to enhance special capacitance, improve rate capability and cyclingperformance.31,33-35 In addition, the rigid electrodes have to be replaced by bendable ones to fulfill the flexible electronics. An attractive

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approach is coating CNFs on flexible, porous and light-weight supporting substrates, and the CNFs can bind strongly with substrates. Office copy paper is the most widely investigated as the flexible substrate due to its low-cost, easier fabrication, and high bending or folding property, nevertheless, the lower mass loading of the active materials hinders its potential applications. Cellulose is the most abundant and renewable natural material which can bind strongly with carbon materials. BC, as a special type of cellulose, can largely absorb carbon materials ink based on the specific ultrafine networks and sufficient porosity structure.36 The remarkable tensile strength (at least 2 GPa) can be help for the mechanical property of flexible electrodes. The high hydrophilicity of BC can increase the contact between electroactive materials and aqueous electrolytes. Thus, BC has shown great potential as an ideal substrate for flexible electrodes.37-39 Herein, an N-CNFs/RGO/BC paper electrode with superior performance is prepared in a highly efficient and novel strategy. Besides the direct use as supporting substrate for flexible electrode, BC can also serve as porous carbon precursor to introduce heteroatoms into carbon framework. The porous and highly flexible BC substrate is responsible for a large areal mass of 8 mg cm-2. The precursor is fabricated through PPY coating on BC/GO mixture via in-situ polymerization. The one-step pyrolysis treatment not only fabricates the N-doped 3D nanostructured carbon composite materials, but also forms the reduction of the GO sheets at the same time. Furthermore, GO is easy assembled into porous BC that reduce irreversible agglomeration and restacking after carbonization. This results in excellent contact between the N-CNFs and RGO, which leads to fast electrical conductivity of the flexible electrode. As a consequence, the freestanding N-CNFs/RGO/BC paper electrode displays a good areal specific capacitance of 2106 mF cm-2 (263 F g-1) in KOH electrolyte and 2544 mF cm-2 (318 F g-1) in H2SO4 electrolyte, meanwhile exhibiting a very stable capacitance retention (~100 % after 20,000 cycles). When directly coupled with two paper electrodes, the symmetric supercapacitor is assembled, and it reveals excellent electrochemical performance: a high areal capacitance of 810 mF cm2 in KOH and 920 mF cm-2 in H2SO4, remarkable cycle life (~99.6 % retention after 10,000 cycles) and distinguished areal maximum energy density/maximum power density (0.11 mWh cm-2/27 mW cm-2 in KOH and 0.29 mWh cm-2/37.5 mW cm-2 in H2SO4) being achieved. And this kind of paper electrodes represent an alternative promising candidate for flexible supercapacitors.

EXPERIMENTAL SECTION Synthesis of N-CNFs/RGO Nanocomposites. GO suspension was prepared according to a modified Hummers’ method.40 In order to obtain a uniform BC nanofibers suspension, the BC pellicles (Hainan Yide Industry Co. Ltd.) were first washed by deionized water until neutral, cut into small pieces, pulped with a mechanical homogenizer at the speed of 10,000 rpm, and then diluted into 300 ml deionized water to obtain the slurry of BC (2 mg ml-1). 30 ml GO suspension (2 mg ml-1) was slowly poured into BC dispersion and stirred for 30 min to obtain a uniform suspension. Subsequently, 0.3 mL pyrrole (PY) monomer was added into the composite suspension, followed by 60 min of stir and sonication, and cooled to 4 oC in an ice water bath. 30 ml of ammonium persulfate (APS, 5.26 mmol) solution was added into the mixture suspension dropwise under continuous stirring and kept at 0-4 °C. After fur-

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ther stirring for 5 h, a black precipitate was obtained and separated by filtration, washed with deionized water, freeze-dried to preserve the tenuous network, and named as PPY/BC/GO. Then, the PPY/BC/GO composite was pyrolyzed in a nitrogen atmosphere at 900 °C for 2.0 h to form N-CNFs/RGO. Finally, the N-CNFs/RGO ink was fabricated by dispersing 80 mg NCNFs/RGO with 80 mg sodium dodecyl sulfate (SDS) surfactant in 150 mL deionized water, which permits a homogeneous dispersion for N-CNFs/RGO in deionized water. Synthesis of N-CNFs/RGO/BC Flexible Paper. 200 ml BC suspension (0.7 mg ml-1) was then filtered via vacuum filtration using a 0.22 µm porous nitro cellulose membrane to form a BC film. Then, the as-fabricated N-CNFs/RGO ink was poured onto the prepared BC paper to form a hybrid membrane. Finally, the precipitate was dried at 60 oC for 8 h and automatically peeled off to get the free standing paper. The loading mass of active materials was 8 mg cm-2. For comparison, the various concentration of PPY of 0.1, 0.2, 0.3, 0.5 and 0.8 ml were used by in situ polymerization while other synthesis conditions were kept the same, and the obtained sample was named as N1-CNFs/RGO/BC, N2-CNFs/RGO/BC N3CNFs/RGO/BC N5-CNFs/RGO/BC and N8-CNFs/RGO/BC. Materials Characterization. The crystallographic structure of the samples was characterized by X-ray diffraction (XRD, Rigaku 2500) equipped with Cu Kα radiation (λ=1.5406 Å). The surface chemical species were measured using X-ray photoelectron spectroscopy (XPS, VG ESCALABMK II) with Mg Kα radiation. Fourier transform infrared spectroscopy (FTIR) was recorded on a PerkinElmer Spectrum 100 Model FT-IR spectrometer to analyze chemical structure. The morphology and microstructures were observed by Scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2100 F). Specific surface area and pore structure of the samples were characterized by nitrogen adsorption–desorption isotherm using NOVA 2000 at 77 K. Electrochemical Measurements. Electrochemical studies were performed with a CHI660E electrochemical workstation. The prepared flexible N-CNFs/RGO/BC paper was directly used as the working electrode and no binders and other additives were added. Electrochemical measurements of the individual electrode were carried out using a three-electrode cell with active carbon as counter electrode and Hg/HgO as reference electrode in a 6.0 M KOH aqueous electrolyte. The symmetric supercapacitor was built using two pieces of NCNFs/RGO/BC paper electrodes with a cellulose acetate membrane separator, and performed in two-electrode configuration in 6.0 M KOH solution. The areal specific capacitance can be calculated based on the following equation: (1) Where I is charge/discharge current at a discharge time ∆ t, ∆V is the working voltage window and S is the nominal electrode area, respectively. The energy density (E, mWh·cm-2) and the power density (P, mW·cm-2) were calculated from GCD testing using following equation: (2) (3) Where C is the areal specific capacitance and t is the discharge time.

RESULTS AND DESCUSSION


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Figure 1. Schematic illustration of the fabrication process of N-CNFs/RGO/BC paper electrode. (a) Photograph of BC pellicle. (b) Photograph of N-CNFs/RGO/BC paper.

The specific strategy for fabricating lightweight, flexible and freestanding N-CNFs/RGO/BC paper electrode is illustrated in Figure 1. The procedure started by a BC slurry mechanically mixed with the prepared GO dispersion to get a uniform suspension. The PPY/BC/GO precursor was fabricated via in situ polymerization using PY and APS as the monomer and oxidant, respectively. N-CNFs/RGO composite was obtained through direct pyrolysis of the carbon precursor under flowing N2. The one-step carbonization treatment applied here not only fabricates the 3D N-CNFs, but also forms the reduction of the GO sheets at the same time. Subsequently, the resultant N-CNFs/RGO was deposited onto the prepared BC paper via vacuum filtration technique to obtain NCNFs/RGO/BC paper electrode. The high mass loading of NCNFs/RGO is readily incorporated into BC networks through a simple vacuum filtration and the composite can strongly bind with the BC by hydrogen bonding and electrostatic interaction with no need for any binders. It is worth noting that the paper can be rolled up, twisted and bent to arbitrary angle even 180° (Figure 1b). The BC pellicle (Figure 1a) is employed as precursor and supporting substrate to fabricate flexible and freestanding electrodes. It exhibits high water holding morphology due to the many hydroxyl groups along its networks. The SEM and TEM images (Figure 2a and b) further show that the BC consists of 3D interconnected nanofibrous networks for about 2060 nm, where the void volume allows GO to diffuse into the networks and strongly binds with the BC through hydrogen bonding and electrostatic interaction, which can be clearly identified in Figure 2c. The porous structure as well as hydrogen bands between BC and PY monomers can serve as a driving force to ensure the uniform distribution of PY and assist the growing of the continuous PPY onto the surface of cellulose and GO. It can be seen that the core-shell structure is formed with a little PPY aggregates and the PPY coating is assembled along the BC/GO template in random orientation (Figure 2d), which is observed more obviously in TEM image (Figure 2e), indicating that PPY/BC/GO can act as a good precursor for carbonization. After pyrolysis of PPY/BC/GO, the N-CNFs/RGO sample still retains numerous intertwined ultrathin nanofibers of original BC pellicles (Figure 2f). Moreover, the N-CNFs/RGO nanocomposites directly paint

into porous BC without binder and other additives to form porous networks structure, ensuring sufficient contact between the N-CNFs/RGO and the electrolyte. Figure 2g shows that the numerous carbon nanofibers and energy storage unit grown on RGO to form a conformal coating on its surface. The highresolution TEM (Figure 2h, i) images further confirm that the random carbon nanofibers and energy storage unit are interconnected and bridged with short-range ordered RGO to form high conductive 3D structures, which could be conducive to fast electron transport during the chage/dischage process and hence enhance the rate capability. For practical applications of flexible electrode, good mechanical properties is currently required. The BC substrate with ultrahigh tensile strength and flexibility acts as mechanical support for N-CNFs/RGO composites, resulting in the excellent mechanical properties of this paper electrode. As shown in Figure 3, the paper electrode for 0.32 mm thickness and 2 mm width can hold a weight of 1 kg steadily (Figure 3b), indicating a remarkable tensile strength, which is further demonstrated by stress–strain experiment. The high tensile strength of 40.7 MPa at dry state and 37.8 MPa at wet state can be achieved (Figure 3a), which is much larger than those of reported flexible RGO films, CNTs films, RGO-cellulose paper and others with a similar structure, suggesting that the BC is perfect candidate as substrate for carbon based flexible electrodes. Figure 4a presents the FTIR of BC, GO, PPY, PPY/BC/GO and N-CNFs/RGO samples. 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.13,41,42 For GO, the peak at 3436 cm-1 is attributed to hydroxyl group (-OH) on the surface of GO, the stronger C=O stretching vibration peak at 1731 cm-1, and the C–O stretching vibration peak at 1210 cm-1, revealing the exist of oxygen-containing functional groups on the surface of the GO. For PPY, the broad peaks at 3418, 1557 and 1476 cm-1 are attributed to the N–H stretching vibration, asymmetric and symmetric ring-stretching vibration of PPY. Compared to individuals, their respective characteristic bands



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Figure 2. (a) SEM image of BC. (b) TEM image of BC. (c) SEM image of BC/GO. (d) SEM image of PPY/BC/GO. (e) TEM image of PPY/BC/GO. (f) SEM image of N3-CNFs/RGO/BC. (g) TEM image of N3-CNFs/RGO. HRTEM image of N3-CNFs/RGO for (h) energy storage unit and (i) N-CNFs.

Figure 3. (a) Stress–strain curve of N-CNFs/RGO/BC paper at the dry and wet state. (b) Photograph of N-CNFs/RGO/BC paper for mechanical property test with a weight (1kg).

could be indexed in the PPY/BC/GO, suggesting a successful polymerization along the surface of BC/GO. In the spectrum of N-CNFs/RGO, the C=O characteristic peak (1731cm-1) is disappeared after carbonization, which conforms that the GO is successful reduced to the RGO. Additionally, XRD analysis of samples was used to identify the structures and changes (Figure 4b). It could be seen that

the characteristic broad peak of PPY ascribes to the amorphous nature appears at 25 o. GO displays an obvious sharp peak at 11.2 o. The original BC shows a crystalline structure owning to the sharp peaks at 22.45 16.77 and 14.36 o, which can be assigned to the typical (110), (110), and (020) planes of cellulose I, respectively.30 For PPY/BC/GO, PPY and GO peaks cannot be observed obviously after polymerization due to the lower contents of GO and weaker peaks of PPY which


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Figure 4. (a) FTIR spectra and (b) XRD patterns of BC, GO, PPY, PPY/BC/GO and N-CNFs/RGO. (c) XPS survey spectra of NCNFs/RGO. (d) High-resolution XPS spectra of the deconvoluted N1s peak of N3-CNFs/RGO.

are covered by the crystalline structure of BC. After carbonization, the peak at 2θ = 10.8 ° disappears for N-CNFs/RGO, indicating that the reduction of the GO has been successfully performed.41 Meanwhile, the broad peak is attributed to the poorly ordered doped carbon composites along their packing arrangement. XPS further ascertain the elemental composition and bonding configurations in order to understand the relationship between capacitive performance and nitrogen functionalities. The characteristic peaks of N1s in the survey spectra are observed (Figure 4c), suggesting that the as-obtained samples are indeed introduced of nitrogen into carbon matrices, the amount of nitrogen is increasing with the concentration of PY, and the percentage of N in N1-CNFs/RGO, N2-CNFs/RGO, N3-CNFs/RGO, N5-CNFs/RGO and N8-CNFs/RGO are about 2.66, 3.35, 3.58, 4.58 and 4.82 atom %, respectively. The high resolution spectrum of the N1s peak (Figure 4d) further confirms the four forms of nitrogen functional groups in the asprepared sample including pyrrolic N (N-5, 400.7eV), pyridinic N (N-6, 398.5 eV), quaternary N (N-Q, 401.2 eV) and oxidized N (N-oxide 402.3 eV). Nitrogen doping is profitable to form electrochemically active sites and promotes the electric conductivity and surface polarity of the carbon materials,31,35 and therefore improves the electrochemical perfor-

mance of electrodes. Notably, the high ratio of nitrogen functionality of N-5 and N-6 is expected to be present in carbon framework, which can provide additional pseudo-capacitance through the redox reactions, resulting in the improvement of special capacitance. Moreover, the presence of quaternary-N can be help for conductivity of carbon materials to enhance fast charge/discharge property. This results demonstrate the carbonization of PPY may be an effective strategy to introduce nitrogen functionalities into the carbon materials. Additionally, Nitrogen adsorption/desorption analyses based on Brunauer–Emmett–Teller (BET) and the BJH (BarrettJoyner-Halenda) methods were utilized to obtain and evaluate specific surface areas and pore size distributions of the samples prepared of different monomer concentration, and the results are shown in Figure 5. It can be seen that all isotherms exhibit type IV with steep uptakes below P/P0 = 0.01 and small hysteresis loops ranging from 0.4 to 1.0, which imply the coexistence of micropores (50 nm) in these samples.36 The specific surface area decreases from 720.88 to 284.5 m2 g-1 with the increase of PPY content from N1-CNFs/RGO to N8CNFs/RGO. The N3-CNFs/RGO has the biggest average pore size of 18.9 nm with a specific surface area of 614 m2 g-1 and total pore volume of 0.55 cm3 g-1. The results imply that the



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Figure 5. Nitrogen adsorption-desorption isotherm and Pore size distribution (inset) of (a) N1-CNFs/RGO, (b) N2-CNFs/RGO, (c) N3CNFs/RGO, (d) N5-CNFs/RGO, (e) N8-CNFs/RGO and (f) detail results.

resultant N-CNFs/RGO composites maintain the 3D porous architecture after carbonization with high surface-to-volume ratio, which can enhance the double layer charge storage and enable high rate capacity.42-45 To further identify the wettability of flexible electrode for aqueous supercapacitor, the contact angle measurement was conducted using the N3-CNFs/RGO/BC film wetting a 6 M KOH droplet (Figure 6). The hybrid film shows a good hydrophilic property with an initial contact angles of 60.7 o and the droplet is taken up within 40 s. As expected, the N-

CNFs/RGO/BC paper electrode displays a superior wettability owing to the introduced polar C-N bonds after N-doping and the high hydrophilic BC substrate, which supply fast transport and diffusion of electrolyte. In order to demonstrate the advantages of the high mass, flexible and freestanding N-CNFs/RGO/BC paper electrodes for supercapacitors, the electrochemical measurements were carried out in three-electrode system in different electrolyte of 6 M KOH and 1 M H2SO4 aqueous solution, as shown in Figure 7. It is well known that a perfect rectangular shape of


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Figure 6. Dynamic water contact angle measurement for the N3-CNFs/RGO/BC. The photograph at 1 s was taken immediately after resting the water droplet on the surface

cyclic voltammetry (CV) correlates to better capacitive behavior. The CV curves of N3-CNFs/RGO/BC paper electrode show quasi-rectangular response (Figure 7a), suggesting good capacitive performance based on a fast, reversible, successive pseudo-capacitive contribution of nitrogen-containing groups and electric double-layer capacitance (EDLC). The nearly rectangular shapes are retained even at a high scan rate of 500 mV s-1, indicating excellent rate capability. The nearly triangular galvanostatic charge/discharge (GCD) curves in both KOH and H2SO4 electrolyte at different current densities ranging from 1 to 50 mA cm-2 (Figure 7b and c) imply remarkable rate capability and quick Faradaic reactions occur on the surfaces of N3-CNFs/RGO/BC paper electrodes, which are in agreement with the results of the CV analysis. Additionally, small IR drop is observed for the N3-CNFs/RGO/BC paper at a high current density of 10 mA cm-2, demonstrating small overall resistance and good electrochemical reversibility. That can be attributed to remarkable electrical conductivity, porous structure and excellent wettability of the N-CNFs/RGO/BC flexible electrodes. Areal specific capacitance is a more common and reasonable factor than the gravimetric one to evaluate the real application of film electrode for flexible energy-storage devices. As shown in Figure 7d, sample N3-CNFs/RGO/BC shows a higher areal capacitance of 2106 mF cm-2 in KOH and 2544 mF cm-2 in H2SO4 at 1 mA cm-2, higher than N1-CNFs/GN/BC (1776 mA cm-2), N2-CNFs/GN/BC (2040 mA cm-2), N5CNFs/GN/BC (1898 mA cm-2) and N8-CNFs/GN/BC (2010 mA cm-2). The results may be ascribed to the combined effect of the nitrogen doping level and specific surface area, which is in consistent with the XPS and BET measurements. Significantly, benefiting from the large mass (8 mg cm-2) of our paper electrode, such an areal capacitance is much higher than the reported carbon flexible electrodes, and even can be comparable to most of metallic oxide and/or conducting polymer flexible electrodes (Table 1). Rate capability is a critical factor of supercapacitors used for power applications. It is found that the areal capacitance can retain 76 % in KOH and 67 % in H2SO4 electrolyte at current densities from 1 to 50 mA cm-2. As shown, the areal capacitance decreases gradually with the increase of current density, which is mainly related to the ions diffusion resistance. The diffusing and migrating of electrolytic ions into the interior of active materials is easy at low current density. While the higher current density requires a faster electrolytic ion diffusion to access all the active materials, block diffusion effect likely limits the movement of ions due to the time constraint, leading to some active surface areas of the hybrid paper electrodes to become inaccessible for charge storage.32,46-48 In addition, the long-term cycling performance

of the flexible electrode is tested at the current density of 50 mA cm-2 (Figure 7e), and there is almost no degradation of capacitance until the end of 20,000 cycles through the testing cycles. Electrochemical impedance spectroscopy (EIS) was conducted to understand the internal resistance, charge exchange resistance, and ion diffusion process of the N-CNFs/RGO/BC paper electrodes. In the Nyquist plots, all these plots are characterized by a semicircle in the high frequency region related to interfacial charge-transfer resistance, and a linear part in the low frequency region corresponds to the capacitive behavior. Moreover, the first intercept along the real axis for the semicircle in high frequency represents the intrinsic ohmic resistance.49,50 Notably, all the impedance profiles exhibit the most vertical line in low frequency region, indicating a nearly ideal capacitive performance. The low first X-intercept (< 0.34 Ω) reveal that all the N-CNFs/RGO/BC paper electrodes show the low intrinsic ohmic resistance. It is observed that the sample N3-CNFs/RGO/BC has a slightly smaller charge transfer resistance than those of others, which can be further analyzed by a fitting circuit diagram (inset in Figure 7f). The charge transfer resistance (Rct) of the equivalent circuit of N1CNFs/RGO/BC, N2-CNFs/RGO/BC, N3-CNFs/RGO/BC, N5CNFs/RGO/BC and N8-CNFs/RGO/BC are 0.345, 0.307, 0.231, 0.285, 0.437 Ω, respectively. Apparently, the N3CNFs/RGO/BC exhibits the smallest value, suggesting higher electrical conductivity and faster ion response originated from its synergy of high N-doped and large pore structure, which are in agreement with the results of the XPS and XRD analysis. These results demonstrate the well design for this flexible electrode with high loading mass, which does not impede other essential properties, including fast charge/discharge and excellent cycle life. The excellent electrochemical performance can be ascribed to its unique N-CNFs/RGO conductive networks and compatibility with flexible BC substrates. To further clarify actual device behavior of the as-prepared flexible N-CNFs/RGO/BC paper electrode, we employed a two-electrode test to investigate the electrochemical performance of our paper electrode in a symmetric system, where the freestanding N3-CNFs/RGO/BC films were directly used as electrodes, and the results are showed in Figure 8. CV of the flexible symmetrical device is measured at different scan rates ranging from 10 to 800 mV s-1 in voltage window of 0-1 V (Figure 8a). The CV profiles display nearly rectangular-like shapes even at a high scan of 800 mV s-1, suggesting a prominent capacitive behavior with excellent rate performance. Meanwhile, the almost symmetric with a slightly nonlinear



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Figure 7. (a) CV curves of N3-CNFs/RGO/BC at different scan rates in 6 M KOH. (b) GCD curves at various current densities in 6 M KOH. (c) GCD curves at various current densities in 1 M H2SO4. (d) Comparison of areal capacitances of N1-CNFs/RGO, N2-CNFs/RGO, N3-CNFs/RGO, N5-CNFs/RGO and N8-CNFs/RGO versus different current densities. (e) Cycle stability of N3-CNFs/RGO/BC at the current density of 50 mA cm-2. (f) Comparison of Nyquist plots of N1-CNFs/RGO, N2-CNFs/RGO, N3-CNFs/RGO, N5-CNFs/RGO and N8CNFs/RGO, the insets show the corresponding equivalent circuit and the enlarged Nyquist plots in high frequency region. Table 1. Literature on Flexible Carbon-Based Electrodes for Supercapacitor Application Mass Flexible materials RGO film electrode

Ref.

(F g-1)

3.6

234

67

52

87.9

39.05

53

Activated carbon cloths electrode RGO films electrode

Cycling capability

(mF cm )

1

0.45

-2

Capacitance

(mg cm )

RGO film electrode RGO//RGO supercapacitor

Capacitance -2

71

96.7 % after 5000

54

76

~97 % after 20,000

55

94.5

215

97 % after 10,000

56

81

120

~99 % after 50

57

2.3

12

90 % after 15,000

58

0.98

200.4

54 % after 1000

59

545

51

RGO/MnO2

94.5

315

PANI/carbon cloth

787.4

189.7

PPY/gold supercapacitor

1.8

270

78 % after 900

63

2106 in KOH;

263 in KOH;

2544 in H2SO4

318 in H2SO4

~100 % after 20,000 cycles

Our work

~99.6 % after 10,000 cycles

Our work

RGO/Cellulose electrode Graphite/cellulose supercapacitor

0.1

CNT/RGO fibers supercapacitor MnO2/CNF

N-CNFs/RGO/BC electrode

N-CNFs/RGO/BC symmetric supercapacitor

9

8 810 in KOH; 16 920 in H2SO4


60 95 % after 5000

61 62

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Figure 8. (a) CV curves of N3-CNFs/RGO/BC symmetric supercapacitor at different scan rates in 6 M KOH. (b) GCD curves of the symmetric supercapacitor at different current densities in 6 M KOH. (c) GCD curves of the symmetric supercapacitor at different current densities in 1 M H2SO4. (d) Areal capacitance calculated from GCD curves under different current densities. (e) Cycling stability performance at a current density of 20 mA cm-2. (f) Ragone plots.

characteristics of GCD curves of N3-CNFs/RGO/BC symmetric supercapacitor (Figure 8b) indicate the coexistence of EDLC and pseudo-capacitance. As known, to improve the energy density of the supercapacitors, higher operating voltage is an effective design by altering different electrolytes while maintaining special capacitance based on the equation: . H2SO4 aqueous solution is often employed as electrolytes for N-doping carbon material which can provide high special capacitance and increase the operating voltage due to strong solvation effect of ions inhibiting the decomposition of water.51 Hence, the performance of NCNFs/RGO/BC symmetric supercapacitor is further confirmed using 1M H2SO4 electrolyte (Figure 8c). The GCD curves display no obvious distortion in a voltage window of 1.5 V, indicating that the device allows a higher operating voltage up to 1.5 V. The areal capacitance of the symmetric supercapacitor is calculated from the GCD curves (Figure 8d). Benefiting from the high mass of our electrodes, the N3-CNFs/RGO/BC symmetric supercapacitor achieves a maximum areal capacitance of 810 mF cm-2 at 2 mA cm-2, and retains 755 mF cm-2 at 50 mA cm-2 in KOH. In the meantime, a high areal capacitance of 920 mF cm-2 at 2 mA cm-2 and 826 mF cm-2 at 50 mA cm-2 can be obtained in H2SO4. From this results, high rate capacity can be obviously observed in both KOH and H2SO4 electrolyte. Cycle life is another important factor for the practical application of supercapacitors, which is evaluated at a high current density of 20 mA cm-2 (Figure 8e). The flexible electrode exhibits impressive cycling stability of ~99.6 % capacitance retention after 10,000 cycles. Ragone plot of as-assembled symmetric supercapacitor (Figure 8f) in H2SO4 electrolyte exhibits a maximum energy density of 0.29 mWh cm-2 at a power density of 1.5 mW cm-2, and still remains 0.26 mWh cm-2 at a high power density of

Figure 9. (a) CV curves under different bending condition at 100 mV s-1, inset shows the optical image of the supercapacitor device. (b) schematic illustration of the flexible supercapacitor device. 37.5 mW cm-2. The high energy density/power density of 0.11 mWh cm-2/1 mW cm-2 and 0.1 mWh cm-2/25 mW cm-2 also can be achieved in KOH electrolyte. Further study is carried out to demonstrate the toughnessenabled flexibility of our symmetric supercapacitor, and the device was tested in flat and bent state with a bending angle of approximately 45o, 90o, 120o at a high sweep rate of 100 mV s1 , as shown in Figure 9. CV curves of the symmetric supercapacitor show almost the same quasi-rectangular shapes with slight effect at both flat and bent states, demonstrating the remarkable stability and good capacitive behavior even at the bent state. Owing to the high mass of as-prepared flexible electrodes, the energy density for N3-CNFs/RGO/BC symmetric supercapacitor is worth highlighting and much higher than those of carbon flexible symmetric supercapacitors,23,52-55,57,60,64 including large-area hierarchical porous RGO//RGO film supercapacitor (0.0098 mWh cm-2),54 interfacial gelation based RGO//RGO film supercapacitor (0.00466 mWh cm-2),23 RGO–



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cellulose paper supercapacitor (0.015 mWh cm-2). 57 Such values are also comparable to those of flexible asymmetric MnO2/CNF//Bi2O3/CNF supercapacitor (0.0434 mWh cm-2),60 V2O5/rGO//rGO supercapacitor (0.089 mWh cm-2),53 RGO/MnO2//RGO supercapacitor (0.0351 mWh cm-2).52 The high performance of N3-CNFs/RGO/BC symmetric supercapacitor could be attributed to the unique flexible electrode architecture, which integrates the advantages of excellent mechanical property, hierarchical porous structure, good conductivity and remarkable wettability.

CONCLUSIONS In summary, we have developed a facile but efficient procedure to synthesize flexible and freestanding N-CNFs/RGO/BC paper electrode for supercapacitors, where BC is used as both substrate and precursor. High conductive N-CNFs/RGO uniformly coated on porous BC substrate, providing fast electron transfer and ion transport throughout the electrode. Benefiting from its unique hierarchical microstructure and numerous pseudo-capacitive functional groups, the flexible electrode exhibits robust mechanical performance, large mass loading, ultrahigh areal capacitance, long cycle life, excellent rate performance. Therefore, the facile, low-cost, flexible and freestanding electrode may represent an alternative promising candidate for flexible energy storage devices.

AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] (Yudong Huang) Notes

The authors declare no competing financial interest.

ACKNOWLEGEMENT The authors gratefully acknowledge financial supports from the Chang Jiang Scholars Program (51073047) and National Natural Science Foundation of China (91016015).

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Nitrogen-Doped Carbon Networks/Graphene/Bacterial Cellulose paper was demonstrated to be a large mass loading, mechanical, ultrahigh areal capacitance flexible and freestanding supercapacitor electrodes. And this work represents an alternative promising candidate for flexible supercapacitors.


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The N-CNFs/RGO/BC paper was demonstrated to be a mechanical, high-performance flexible electrode. And represents an alternative promising candidate for flexible supercapacitors. Graphic abstract 208x190mm (96 x 96 DPI)


Flexible and Freestanding Supercapacitor Electrodes Based on

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