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Large Areal Mass, High Scalable and Flexible Cobalt Oxide/ Graphene/Bacterial Cellulose Electrode for Supercapacitors Rong Liu, Lina Ma, Shu Huang, Jia Mei, Enyuan Li, and Guohui Yuan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10475 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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The Journal of Physical Chemistry

Large Areal Mass, High Scalable and Flexible Cobalt Oxide/Graphene/Bacterial Cellulose Electrode for Supercapacitors Rong Liu, Lina Ma, Shu Huang, Jia Mei, Enyuan Li, Guohui Yuan* School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China Supporting Information ABSTRACT: Flexible energy storage devices require a simple, scalable and general strategy for fabricating high electrochemical performance and mechanically tough flexible electrodes. Herein, sustainable and biological bacterial cellulose (BC) is developed as substrate for Co3O4/Graphene (GN), which permits high flexibility (suitable for bending angle of 180o), excellent tensile strength of 63 MPa, good wettability and especially large mass loading of 9.61 mg cm−2 for flexible and free−standing supercapacitor electrode. The Co3O4/GN/BC hybrid electrode exhibits both appreciable areal capacitance of 12.25 F cm−2 and gravimetric capacitance of 1274.2 F g−1. Moreover, the remarkable cycling stability with 96.4% capacitance retention after 20000 can be achieved. This study provides a facile procedure to improve the electrochemical performance and mechanical property of flexible supercapacitor electrodes, which are promising candidates for the application of flexible power source.

INTRODUCTION Nowadays, the proliferation of portable/wearable electronics such as roll−up displays, hand−held devices, electronic textiles, distributed sensors, and large area circuits on curved objects has attracted extensive attention to flexible and lightweight energy storage devices.1−5 Among various types of energy storage devices, supercapacitors (SCs) have demonstrated their promising potential in flexible electronics because of high power density, fast charge/discharge rate and long cycle life.6−7 One of the key challenges for flexible supercapacitors is to exploit a simple, low−cost, scalable, general and environment friendly strategy for fabricating high capacitance and mechanically tough bendable electrodes. Cobalt oxide, as ideal supercapacitor electrode materials, have been intensely investigated due to the high theoretical capacitance, high redox reactivity, low cost, environmentally friendly and remarkable electrochemical performance.8−15 Moreover, Cobalt Oxide and graphene composites have been successfully synthesized, which could effectively address their limitation of the low electronic conductivity. Recent literatures have introduced cobalt oxide electrodes which are commonly based on rigid electrodes by the traditional slurry−coating technology, thus they are not suitable to be used in the flexible supercapacitors.16−18 Furthermore, the insulated and hydrophobic binder not only greatly decrease the electrical conductivity of electrode materials, but also hinder the diffusion of electrolyte ions into electrodes. These additives, current collectors and binder also will increase extra cost, needless weight, and somehow weaken the overall electrochemical properties, which is not compatible with flexible energy−storage devices. A commonly used approach to fabricate transition metal oxides/hydroxides flexible and free−standing electrodes is to incorporate the highly conductive substrate such as graphene paper and carbon nanotube films.19,20 Chemical deposition and electro−deposition are mainly two methods of directly growing the pseudo−capacitive materials onto these flexible, high electrical conductive and light−weight substrates.21,22 Despite their attractive gravimetric capacities, the limited stretchable and compressible properties, relatively elaborate procedures, low efficiency and high price would impede their further development for wide-

spread implementation and commercialization. Furthermore, the areal capacitance is still rather low due to the small mass of active materials loading through these strategies. Even more strikingly, gravimetric capacitance does not scale proportionally with the increase of mass loading, the high loading amount always results in the dramatic decrease of the specific capacitance and rate capability.23,24 Therefore, areal capacitance is performed as the standard to understand the effect of scalability for free−standing film electrode, which is a common and reasonable practice for fabricating flexible energy storage device. Besides the electrochemical performance, good mechanical properties are also key parameter for flexible electronics which mainly depend on supporting substrates. Among these substrates, bacterial cellulose (BC) is nontoxic, environmentally friendly, cheap, and can be synthesized on industrial scales using certain bacteria.25 The specific ultrafine networks structure, sufficient porosity, good mechanic properties, and high hydrophilicity make them as a perfect substrate for flexible electrode. What is more, the open space of the BC substrate reduces pulverization of metal oxides and/or hydroxides during the charging/discharging cycling, which are caused by large volume expansion and contraction, hence facilitating capacity retention for long cycle life. In this paper, we have synthesized Co3O4/GN coating on BC with a large mass loading by a simple and cost−effective “hydrothermal and filtering” method and applied them as potential flexible and freestanding electrodes for supercapacitor application. The Co3O4/GN/BC hybrid electrode exhibits appreciable areal capacitance (12.25 F cm−2), high gravimetric capacitance of 1274.2 F g−1, remarkable cycling stability (96.4% capacitance retention after 20000), high bending performance (suitable for different bending angles, even 180o), and excellent tensile strength of 63 MPa, suggesting their promising application as flexible electrodes for flexible energy−storage devices. The design has four significant advantages, namely, 1) BC can strongly bind with Co3O4/GN, and a large amount of Co3O4/GN could be uniformly loaded into BC paper due to the ultrafine nanosized 3D fibrous networks structure of BC with many hydroxyl groups on the surface. 2) The good mechanical property of BC endows the flexible electrodes with high mechanical integrity upon bending and stretching. 3) The high hydrophilicity and sufficient porosity of BC can be beneficial for the contact between electroactive

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materials and aqueous electrolytes, and provide diffusion channels for electrolyte ions. 4) This method is environmentally friendly, low−cost and can be easily scaled up to large−scale production of flexible electrodes with high areal capacitance for flexible energy storage devices. 4) This method could be applied for realizing other metal oxide/hydroxide based bendable freestanding electrodes. The Co(OH)2/GN/BC paper electrode based on the same methods is assembled with good electrochemical performance.

EXPERIMENTAL SECTION Material. An aqueous dispersion of GN sheets was synthesized using a modified Hummers’ method.26,27 The BC paper was prepared as following: the gel−like white BC pellicle (Hainan Yide Foods Co. Ltd.) was first washed thoroughly with deionized water, followed cut into small slices, and then pulped with a mechanical homogenizer at the speed of 10,000 r min−1 to obtain BC suspension (0.7 mg ml−1). The collected BC suspension (200 ml) was poured onto a nitro cellulose filter membrane (porous size of 0.22 µm) through vacuum filtration to get a uniform BC paper. Preparation of Co3O4/GN/BC paper. In a typical synthesis, three different concentration of Co(CH3COO)2·4H2O for 80, 140 and 200 mg were separately dissolved in 30 ml deionized water, and then added into 50.0 mL of a 1 mg mL−1 GN aqueous solution slowly under continuously stirring, respectively. The mixture were kept stirring and sonicating for 1 h, and then transferred into a Teflon−lined stainless steel autoclaves at 150 oC for 3 h. After cooling to room temperature, the resulting precipitate were taken out from the autoclave. Then, they were filtrated onto the as−prepared BC paper under vacuum condition to achieve Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC films

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with different amount of active materials, and dried in an oven at 60 oC overnight and then stripped away to get the freestanding membranes. The different value of mass between the Co3O4/GN/BC and pure BC is the mass loading. The loading mass of active materials (Co3O4/GN) are 6.91, 8.16 and 9.61 mg cm−2, respectively. Materials characterization. The morphology and microstructures were characterized by using scanning electron microscopy (SEM, Hitachi S−4800) and transmission electron microscopy (TEM, JEM−2100 F). The crystal structure was characterized by X−ray diffraction (XRD, Rigaku 2500) equipped with Cu Kα radiation (λ=1.5406 Å) at a scanning rate of 10° min−1 in the 2θ range from 10° to 80°. X−ray photoelectron spectroscopy (XPS, VG ESCALABMK II) was carried out to investigate the surface elemental compositions. Electrochemical measurements were performed in three−electrode system using CHI660E electrochemical workstation.

RESULTS AND DESCUSSION A schematic of preparation procedure of the Co3O4/GN/BC film is illustrated in Figure 1. In the overall synthetic process, two simple steps can be involved as follows: Co3O4 was initially grown on GN sheets via an one−pot hydrothermal process, which avoids a troublesome two−step procedure (hydrothermal deposition and subsequent annealing). Then, Co3O4/GN aqueous dispersion was poured onto BC membrane to form a Co3O4/GN/BC paper by vacuum filtration. Therefore, it is very easy to produce these flexible electrodes with a low−cost method at industrial scale.

Figure 1. Schematic illustration of the fabrication process of Co3O4/GN/BC paper. Figure 2a presents the photograph of original BC membrane, showing high water holding state and super−hydrophilicity property due to a mount of hydroxyl groups within its networks. The SEM image presents the typical micromorphology of BC, revealing a highly porous and 3D networks nanostructures with ultrafine nanofibers and cross−linked units (Figure 2b). The TEM image further shows that the diameter of these nanofibers is 20−80 nm (Figure 2c). Considering the 3D porous fibrous networks, the use of BC as supporting substrate is worth highlighting. Based on the

poor electrical conductivity of Co3O4, the sufficient contact of Co3O4 and GN is a crucial factor in determining the capacitance performance for the flexible electrode. As indicated in Figure 2d, after hydrothermal process, the GN sheets are coated by well dispersed Co3O4 nanobead, which enable good charge transfer through the highly conductive graphene channels. Furthermore, the Co3O4/GN nanocomposites directly paint into porous BC paper without binder and additives, enabling sufficient contact between the Co3O4 and the electrolyte. TEM images further reveal


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that the Co3O4 shows nanobead architecture with noticeable small edge length of about only 10−40 nm (Figure 2e). The high−resolution TEM image further reveals the lattice fringes showing an interplanar distance of 0.24 nm (Figure 2f), which corresponds to the (311) planes of the face−centered−cubic of Co3O4. The Co3O4 materials consisting of small size nanobead not only greatly facilitate the diffusion of ions but also favor excellent charge transfer to serve as the electroactive sites, which ensures the high capacitance and fast rate capability. Figure 2g displays a cross−sectional SEM image of the BC paper. It can be seen that the BC show a multi−layered structure with plenty of void pores. After filtrating the dispersion of Co3O4/GN, the active material

can immerse into the interior of BC (Figure 2h), which achieves large mass loading and hence greatly improve areal capacitance. In addition, the magnified cross−sectional SEM image (Figure 2i) reveals that the Co3O4/GN composites achieve uniformly and continuously distribution in the BC film. The good mechanical property of BC is responsible for the Co3O4/GN/BC electrode with high mechanical integrity upon high flexibility and stretching (Figure 3). Figure 3a shows the freestanding paper electrode (0.27 mm) can be bent to large angle. The stretched performance of Co3O4/GN/BC was tested with stress–strain experiment. The as−fabricated flexible electrode shows a tensile strength up to 63 MPa with a fracture strain of 5.2 % (Figure 3b).

Figure 2. (a) Photographs of original BC pellicle. (b) SEM image of BC. (c) TEM image of BC. (d) SEM image of Co3O4−H/GN/BC. (e) TEM image of Co3O4−H/GN. (f) High−resolution TEM image of Co3O4−H/GN. (g) Cross sectional SEM images of BC. (h) Cross sectional SEM image of Co3O4−H/GN/BC. (i) Enlarged cross sectional view of Co3O4−H/GN/BC (β layer).



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Figure 3. (a) Photograph of Co3O4−H/GN/BC paper. (b) Stress–strain curves of Co3O4−H/GN/BC.

Figure 4. (a) XRD patterns of GN and Co3O4/GN. (b) XPS survey spectra of Co3O4/GN. (c) High−resolution XPS spectra of the deconvoluted Co2p peak of Co3O4/GN. (d) Water contact angle measurement for the α layer, β layer and γ layer of Co3O4−H/GN/BC paper. Figure 4a shows the XRD patterns of the Co3O4/GN sample. All the peaks of the composites are in well agreement with the cubic phase Co3O4 in both position and relative intensities, except the peak at 2θ = 26o from the GN,28−30 and no other additional peaks presented suggest a high purity. More detailed state of Co3O4 is investigated by XPS test. The wide survey XPS spectrum (Figure 4b) exhibits the predominant presence of C (84.8 at. %), O (9.4 at. %), and Co (5.8 at. %). Figure 4c shows two major peaks at 779.6 and 794.8 eV, corresponding to Co 2p3/2 and Co 2p1/2 spin orbit peaks of Co3O4, respectively, in agreement with the reported data.31−33 In addition, besides two shake-up satellite peaks (sat.), both Co 2p3/2 and Co 2p1/2 are fitted into a couple

of peaks, suggesting that the Co element exists as Co2+ and Co3+ in the sample. These characterizations indicate the Co3O4/GN composites are synthesized successfully. Water contact angle measurement was conducted to further investigate the structure of Co3O4/GN/BC paper (Figure 4d). The γ layer of the flexible electrode is high hydrophilic, a water droplet spreads extensively on the surface of γ layer, with a contact angle of about 20.8o. However, the data of α layer is 91.2o, indicating the surface of flexible electrode is coated by hydrophobic Co3O4/GN nanocomposites. In contrast, the β layer of Co3O4/GN/BC is hydrophilic, with a contact angle of about 61.3o. This good wettability is mainly attributed to the well integration between BC and


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Figure 5. (a) Schematic diagram of the electrolyte ions adsorption/desorption process in the Co3O4−H/GN/BC electrode. (b) Comparison of CV plots of Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC at 3 mV s−1. (c) CV curves of Co3O4−H/GN/BC at different scan rates. (d) GCD curves of Co3O4−H/GN/BC at various current densities. (e) Areal capacitances of Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC versus different current densities. (f) Gravimetric specific capacitances of Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC versus different current densities. (g) Cycle stability of Co3O4−H/GN/BC at the current density of 50 mA cm−2. (h) Nyquist plots, the inset show the enlarged Nyquist plots in high frequency region.



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Co3O4/GN nanocomposites. The hydrophilicity of β layer can be beneficial for the contact between electroactive materials and aqueous electrolytes.

good electrochemical reversibility and capacitance of Co3O4−H/GN/BC flexible electrode. The specific capacitance derived from the discharge curves is calculated using the follow-

Figure 5a presents schematic diagram of the electrolyte ions absorbing onto the accessible surfaces of the active material. The pores and voids inside the Co3O4/GN/BC maximizes the exposed surfaces of active materials to electrolyte. Moreover, this structure can absorb electrolyte, making the Co3O4/GN directly contact with the electrolyte and decreasing the ions diffusion distance. The electrochemical measures of the as−fabricated products were performed in a three−electrode system in 2 M KOH solution. Figure 5b depicts the cyclic voltammetry (CV) curves of Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC flexible electrodes in the voltage window of 0.1−0.6 V at a scan rate of 3 mV s−1. The obvious pair of broad redox peaks are clearly observed, suggesting that the capacitance characteristics are mainly contributed by the pseudo−capacitance. These peaks correspond to the reversible oxidation state change according to reactions:

ing equation: (3).Where I is the discharge current (mA cm−2), ∆ t is the discharge time (s), ∆V is the working voltage window, S is the nominal electrode area and m is the loaded mass of the electroactive material, respectively. The calculated capacitance values as a function of current density are collected in Figure 5e and f. The current studies show that high loading amount always results in the dramatic decrease of the specific capacitance and rate capability. For example, Yang et al. have reported Co3O4 nanowire network coated on a carbon fiber paper with very high specific capacitance of 1190 F g−1 at a mass loading of 0.4 mg cm−2, When the mass loading is increased to 0.7 and 1.4 mg cm−2, the specific capacitance of the Co3O4 nanowire network electrode is reduced, respectively, to ∼1011 and ∼948 F g−1.20 However, the Co3O4−H/GN/BC flexible electrode exhibits high areal capacitance of 12.25 F cm−2 (1274.2 F g−1) with large mass loading (9.61 mg cm−2) at the current density of 3 mA cm−2 and still reaches 10.4 F cm−2 (1082.1 F g−1) even at a high current density of 50 mA cm−2. It is noticeably that such flexible paper exhibits high areal capacitance without sacrificing gravimetric capacitance, indicating prominent capacitive behavior. The Co3O4−H/GN/BC flexible electrode shows a greater than one order of magnitude larger areal capacitance than the film electrodes based on pure carbon materials, even much higher than recently reported bendable electrodes assembled by metal oxide and conducting polymer materials (Table 1). The cycling stability of the Co3O4−H/GN/BC hybrid paper electrode was examined by GCD test (Figure 5g). And a capacitance retention of about 96.4 % is achieved after 20000 cycles at the current density of 50 mA cm−2, which is higher than most of those reported for metal oxide film.

(1) (2) Figure 5c shows the representative CV curves of the Co3O4−H/GN/BC film at sweep rate between 1 and 10 mV s−1. The peak current response significantly increases with the increase of scan rate, revealing good capacitive behavior and rate capability. Galvanostatic charge−discharge (GCD) measurements were conducted at different current densities ranging from 3 to 50 mA cm−2, as shown in Figure 5d. The sloped potential plateaus can be observed in the charge and discharge curves, which is ascribed to the combination of pseudo−capacitance and electric double layer capacitance, resulting in a longer charge/discharge duration. In addition, the charge/discharge curves shows that the

Table 1. Literature on flexible electrodes for supercapacitor application Capacitance (mF cm−2)

Capacitance (F g−1)

Cycling capability

Ref.

Co3O4/RGO

781 (2 A g−1)

~96% after 3000

16

Co3O4/CNFs

556 (1 A g−1)

~100% after 2000

34

1180 (1 mV s−1)

590 (1 mV s−1)

~89.1% after 2000

35

2350 (5 mV s−1)

783.3 (5 mV s−1)

~92% after 2000

35

Flexible electrode

Co3O4/RuO2/CNFs

Mass (mg cm−2)

2

Co9S8/CNFs

840 (5 mV s−1)

Co3O4/MnO2/CNFs Co3O4/CNFs

1.4

V2O5/RGO

2.2

36

948 (50 mV s−1) 382.2 (0.05 A g−1)

178.5 (0.05 A g−1)

525 (3 mA cm−2)

110 (3 mA cm−2)

MnO2/CNFs PPY/BC

11.23

2430 (2 mA cm )

MnO2/CNT textile

8.3

~100% after 5000

23 24

216.4 (2 mA cm )

~94.5 % after 5000

37 38

2800 (0.05 mV s )

337.3 (0.05 mV s−1)

~50% after 50 000

39

MnO2/CNFs

230 (10 mV s−1)

425 (10 mV s−1)

~98.5% after 3000

40

Co3O4/GN/BC

12250 (3 mA cm )

1274.2 (3 mA cm )

~96.4 % after 20,000

This work

9.61

−2

−1

−2

−2

−2

6

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Figure 6. (a) SEM image of Co(OH)2/GN/BC paper. (b) CV curves of Co(OH)2/GN/BC at different scan rates. (c) GCD curves of Co(OH)2/GN/BC at various current densities. (d) Cycle stability of Co(OH)2/GN/BC at the current density of 50 mA cm−2. Benefiting from highly flexible BC substrate, the freestanding Co3O4/GN/BC electrode exhibits outstanding mechanical property.

The excellent electrochemical performance of the flexible electrode can be employed in depth to investigate intrinsic ohmic resistance, charge exchange resistance and ionic resistance by electrochemical impedance spectroscopy (EIS) characterization in a frequency range from 0.01 Hz to 0.1 MHz. As the Nyquist plots shown in Figure 5h, The Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC electrodes show low intrinsic resistance value acquired from the first X−intercept, revealing a faster ion response and higher electrical conductivity, which are in consistent with the conductivity values for ∼231.6 S·m−1, ∼205.02 S·m−1 and ∼189.7 S·m−1 by the four−point probe test. Meanwhile the vertical curves with a larger slope are related to the good capacitive behavior of these flexible electrodes. Via extrapolating the vertical portion with the real axis (Z’ axis) to obtain the ESR < 0.28 Ω, reflecting that all these electrodes show fast ion transport. The semicircle is a characteristics of charge transfer resistance (Rct), which are analyzed by a fitting circuit diagram (Figure S1). The results of Rct are 0.76, 0.88, and 1.12 ohm, respectively, for Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC, indicating a high electrical conductivity and fast ion response for all the flexible electrodes.

To further demonstrate this fabricating method for flexible and freestanding electrode for supercapacitor, we show the utilization of Co(OH)2/GN/BC paper using the similar approach. The typical SEM images of the Co(OH)2/GN/BC composite is shown in Figure 6a. Amount of homogeneous hexagonal platelet Co(OH)2 and corrugated GN are uniformly dispersed onto the BC substrate without agglomeration, providing a well synergistic effect and ensuring the good conductivity through the whole electrode. XRD and XPS analysis (Figure S2) further confirm that the successful synthesis of Co(OH)2/GN. The CV and GCD were performed in a three−electrode test in 2 M KOH solution, the resultant plots are shown in Figure 6b and c. The CV curves display two pairs of obvious redox couples, which is also confirmed by the GCD plots, indicating that the capacitance characteristics are mainly governed by Faradaic reactions rather than the electric double−layer capacitance. With the increase of scan rate, a well maintained curves of CV and the small hysteresis between redox reaction peaks reveal good capacitance characteristic and good rate performance. The areal capacitance of the flexible electrode based on GCD plots is about 12.91 F cm−2 (985.7 F g−1) at 5 mA cm−2, and also retain 7.82 F cm−2 even at 50 mA cm−2 (Figure S3a). Both the areal capacitance value and rate performance of the paper supercapacitor electrode are very competitive among the reported film electrodes. However, the gravimetric capacitance and rate performance of Co(OH)2/GN/BC are lower than those of Co3O4/GN/BC, which is mainly due to the resistance (Figure S3b). The close integration of Co3O4 and GN of Co3O4/GN/BC flexible electrode enable good

These excellent capacitive performances of Co3O4/GN/BC flexible electrode could be attributed to the following advantages of the design: 1) The BC supporting substrate with hydrophilic porous nanostructures not only forms large mass loading and prevents the aggregation of nanomaterials at the same time, but also shortens the diffusion pathways of electrolyte ions. 2) The extra−small size feature of Co3O4 and sufficient contact with GN provides larger ion−accessible surface area, more active surface area for the Faradaic reaction and faster electron transport. 3) 7



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charge transfer through the highly conductive graphene channels. Moreover, the operating life was verified through a GCD process at 50 mA cm−2, as shown in Figure 6d. The paper electrode shows a good stability, remaining 93.2 % of its initial capacitance after

10 000 cycles. These positive results are mainly due to the synergistic action of pseudo−capacitive behavior and the double electrode layer effect within the porous substrate.

CONCLUSIONS

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In summary, the Co3O4/GN composites have been successfully synthesized in a facile two−step for hydrothermal process and filtrating method. The Co3O4 is dispersed on the surface of GN sheets, the flexible electrode exhibits excellent performance with an appreciable areal capacitance (12.25 F cm−2), remarkable cycling stability (96.4 % capacitance retention after 20000), and high bending performance (suitable for different bending angles, even 180o). Furthermore, the facile and efficient procedure is also verified by the synthesis of the Co(OH)2/GN/BC, which indicating that this type strategy could be also compatible with various metallic oxide/hydroxide materials. In addition, the simple and low−cost design endows the electrodes with a flexible and freestanding architecture, which brings in new opportunities for advanced applications in flexible energy−storage devices.

ASSOCIATED CONTENT Supporting Information The equivalent circuit for fitting the Nyquist plots of Co3O4/GN/BC; XRD patterns of GN and Co(OH)2/GN; XPS spectra of Co(OH)2/GN; High−resolution XPS spectra of the deconvoluted Co2p peak of Co(OH)2/GN; Areal capacitance and Nyquist plots of Co(OH)2/GN/BC.

AUTHOR INFORMATION Corresponding Author

* E−mail address: [email protected] (Guohui Yuan) Notes

The authors declare no competing financial interest.

ACKNOWLEGEMENT The authors are grateful to the support of the National High Technology Research and Development Program of China (863 Program, grant number 2012AA03A212), International S&T Cooperation Program of China (2013DFR40700).

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Schematic diagram of the electrolyte ions adsorption/desorption process. (b) Areal capacitances of Co3O4−L/GN/BC, Co3O4−M/GN/BC and Co3O4−H/GN/BC versus different current densities.


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