Flexible and conductive carbonized cotton fabrics coupled with a

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Flexible and conductive carbonized cotton fabrics coupled with a nanostructured Ni(OH)2 coating for high performance aqueous symmetric supercapacitors Tian Xia, Xiaofang Zhang, Jiangqi Zhao, Qingye Li, Chenghong Ao, Rui Hu, Zhuo Zheng, Wei Zhang, Canhui Lu, and Yulin Deng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06150 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 1, 2019

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ACS Sustainable Chemistry & Engineering

Flexible and conductive carbonized cotton fabrics coupled with a nanostructured Ni(OH)2 coating for high performance aqueous symmetric supercapacitors Tian Xia,a Xiaofang Zhang,a Jiangqi Zhao,a,c Qingye Li,a Chenghong Ao,a Rui Hu,a Zhuo Zheng, a

Wei Zhang,a,b,* Canhui Lu, a,b,* Yulin Deng.d

a. State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, 24 South Section 1, Yihuan Road, Chengdu 610065, China. b. Advanced Polymer Materials Research Center of Sichuan University, 1900 Port Avenue, Shishi 362700, China. c. Department of Electrical Engineering and Computer Sciences, University of California, 550 Cory Hall, Berkeley, California 94720, United States. d. School of Chemical and Biomolecular Engineering and RBI at Georgia Tech, Georgia Institute of Technology, 500 10th Street N.W., Atlanta, Georgia 30332-0620, United States. *Authors for correspondence: E-mail: [email protected] (W. Zhang), [email protected] (C. Lu); Phone: 86-28-85460607; Fax: 86-28-85402465.

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Abstract

Flexible and wearable supercapacitor (SC) fabrics have received considerable research interests recently. However, their high hydrophobicity, poor conductivity, inferior capacitance and low energy density remain a bottleneck to be solved. Herein, a highly ﬂexible and conductive carbonized cotton fabric (CCF) covered by a unique nanostructured Ni(OH)2 layer is fabricated via a facile high-temperature carbonization process followed by an electrochemical deposition (ED) treatment. The nanostructured Ni(OH)2 greatly improves the hydrophilicity of CCF to promote electrolyte penetration and offers abundant electroactive sites, leading to dramatically increased specific capacitance and operating potential window (OPW). The resultant Ni(OH)2@CCF is then applied as the electrode for an aqueous symmetric SC device. This device has an OPW of 1.4 V and exhibits a high specific capacitance of 131.43 F g-1 at the current density of 0.25 A g-1 with a high energy density (35.78 Wh kg-1 at a power density of 0.35 kW kg-1, and it can reach 18.28 Wh kg-1 at a high power density of 14.00 kW kg-1), which outperform the performance of most aqueous symmetric SCs. In addition, the SC demonstrates excellent capacitance stability under various bending conditions, suggesting its potentials in flexible and wearable energy-storage devices. Keywords: aqueous symmetric supercapacitors, carbonized cotton fabrics, high energy density, nanostructured Ni(OH)2

Introduction


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Recently, the exhausted nonrenewable fossil fuels have accelerated the development of batteries and capacitors and the use of environmental materials. Supercapacitors (SCs) are a kind of promising energy-storage device with distinct advantages of long cycle life, high power density, low cost, and so forth. Flexible and wearable SCs with high energy densities and durability are particularly anticipated to meet the increasing demand in the modern society

1-3.

However, the inferior energy density of most SCs has greatly restricted their large-scale applications

4-7.

As is well known, the energy density of SCs is positively correlated with the

square of the operating potential window (OPW) and their specific capacitance. Hence, the energy density will be improved by boosting either the SCs’ specific capacitance or OPW

7-10.

For instance, Zhao et al. prepared flexible membrane electrodes of Ni(OH)2 for a SC which demonstrated a specific capacitance of 2198.6 F g-1 with a maximum energy density of 36.3 Wh kg-1

11.

Obviously, the ultrahigh specific capacitance did not bring about a significant

improvement on the energy density. Hybrid SCs which combine faradaic electrode material (an energy source) with electrochemical double layer capacitive (EDLC) electrode material (a power source) have been developed. They turned out to be quite effective in increasing the energy density as they could take full use of the different OPW including pseudocapacitor and EDLC 1216.

But the preparation of negative and positive electrodes of hybrid SCs tends to be quite

complicated. Consequently, symmetric SCs have increasingly gained superiority for their identical electrodes. In the meantime, most commercial supercapacitors utilize organic solvents or ionic liquids as the electrolytes because they have excellent electrochemical stability at high voltage. However, the high cost, high toxicity and flammability strongly limited their widespread applications

9,14,17.

Hitherto, exploring a facile method to widen OPW and improve energy

density of aqueous symmetric SCs remains a hot research topic.



Several flexible substrates, such as carbon cloth metal foam

35-37,

metal mesh

38-43,

6,18-22,

fabric/textile

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23-27,

fiber/yarn

19,28-34,

etc. have been explored for ﬂexible SCs. In particular, cotton

fabrics (CFs) have received considerable research interests as they can be easily converted into conductive CFs for energy storage applications through high-temperature carbonization. Nonetheless, their poor conductivity and low hydrophilicity constitute big obstacles for SCs

27.

One intelligent strategy to improve the conductivity and hydrophilicity of carbonized cotton fabric (CCF) is to combine with transitional metal oxides/hydroxides, providing both pseudocapacitance and EDLC for efficient enhancement of capacitive performance. Compared with many other transitional metal oxides/hydroxides, Ni(OH)2 has some unique attributes for SC applications, such as low impact for the environment, wide OPW, and high theoretical specific capacitance (2073 F g-1)

41,44-51.

The incorporation of Ni(OH)2 has been proved to be

very effective in improving the capacitance of SCs

45,50,52.

Particularly, nanostructured Ni(OH)2

possesses the smallest dimension and the greatest surface area, giving rise to more Faradic active sites and thereby higher pseudocapacitance 50. In this study, nanoworm/nanoflower-structured Ni(OH)2 has been synthesized at CCF’s surface through a facile two-step high-temperature carbonization process followed by an electrochemical deposition (ED) treatment. As expected, a high performance aqueous symmetric SC with a wide OPW and excellent energy storage properties could be obtained using the Ni(OH)2@CCF as the electrodes. This strategy well solved the tough problems of high hydrophobicity, narrow OPW and low specific capacitance for an ordinary CCF//CCF device. The assembled Ni(OH)2@CCF//Ni(OH)2@CCF aqueous symmetric SCs exhibited a superb capacitance of 131.43 F g-1 at 0.25 A g-1 in the two-electrode configuration. Whereas the SCs from pure CCF only showed a limited capacitance at the same testing conditions. In addition, the


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Ni(OH)2@CCF//Ni(OH)2@CCF SCs demonstrated a high energy density of 35.78 Wh kg-1 with a power density of 0.35 kW kg-1, and they could be stably operated at 1.4 V. Notably, most of the aqueous symmetric SCs demonstrated narrow OPW (less than 1.23 V) and limited energy densities (less than 10 Wh kg-1)

53.

Hence, it would be a significant breakthrough in the

capacitive performance for the present ones. Moreover, the resultant SCs were highly flexible and could retain its electrochemical capacitive performance when bended at different degrees.

Experimental Methods Materials. Nickel chloride hexahydrate (NiCl2.6H2O), KOH, acetone and ethanol were obtained from Chengdu Kelong Chemical Plant, China. Cotton fabrics (CFs) were purchased from a nearby market. Fabrication of CCF. The commercial CF was ultrasonically prewashed to remove the surface oil pollutants with sequential acetone, ethanol, and deionized water. Then, the cleaned CF was dried at 60 oC for 6 h until the water was evaporated completely. Next, the CF was heated to 240 oC

at a heating rate of 2 oC min-1 in a tube furnace (OTF-1200X, Kejing, China) under an argon

flow and was retained at this temperature for 1 h. Further heating was conducted at the same rate to 400 oC, followed by an isothermal process for another 1 h. Subsequently, the temperature was raised to 600 oC at 5 oC min-1 and kept for 1 h. Then, the sample was heated to 800 oC at the same rate and kept for another 2 h. Finally, it was cooled to 400 oC at 5 oC min-1, and then cooled down to the room temperature naturally. The obtained sample was marked as CCF. Fabrication of Ni(OH)2@CCF. The Ni(OH)2@CCF was synthesized by a one-step electrodeposition process. The CCF was first cleaned with ethanol for 2 h and then dried at 60 oC.

Next, a thin Ni(OH)2 layer was deposited onto the surface of CCF via the ED method



performed at a CHI660E electrochemical station (Chenhua, Co. Ltd., Shanghai) using the threeelectrode system (CCF, Pt sheet, and saturated calomel electrode were applied as the working electrode, the counter electrode and the reference electrode, respectively). The electrodeposition of Ni(OH)2 was conducted for different periods of 50-400s at -0.7 V in 0.1 M NiCl2.6H2O aqueous solution at room temperature. Prior to electrodeposition, the CCF was immersed in 0.1 M NiCl2.6H2O solution overnight. The obtained sample was marked as Ni(OH)2@CCF. All the samples were carefully cleaned for several times with deionized water and ethanol sequentially and then dried at 60 oC. Characterization. The scanning electron microscopy (SEM, JEOL JSM-7500F, Japan) and the transmission electron microscopy (TEM, JEOL JEM-100CX, Japan) were used to observe the surface morphologies and the nanostructures of the samples. The X-ray photoelectron spectroscopy (XPS, XASAM 800, Kratos Analysis, UK) with an Al Ka X-ray source (1486.6 eV) and an X-ray beam of about 1 mm was used to acquire the XPS spectra. The X-ray diffraction (XRD, EMPYREAN, Holland) pattern was used to identify the crystal structure of the Ni(OH)2. The drop-shape analyzer (DSA25, Kruss, Germany) was applied to measure the water contact angle of samples at room temperature. The Fourier transform infrared (FTIR) spectra were obtained on Nicolet 6700 (Thermofisher, USA) at 400-4000 cm-1. Electrochemical measurement. The electrochemical behaviors of CCF and Ni(OH)2@CCF electrodes were analyzed in the three-electrode system (CCF or Ni(OH)2@CCF, Hg/HgO and platinum sheet as work, reference and counter electrodes, respectively) in 6 mol L-1 aqueous KOH on the CHI 660E electrochemical workstation at ambient temperature. Whereas, the electrochemical performances of both CCF//CCF and Ni(OH)2@CCF//Ni(OH)2@CCF SC devices were examined with a symmetrical two-electrode configuration in 6 M aqueous KOH.


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Two slices of CCF or Ni(OH)2@CCF (2 mm  2 mm) served directly as the working electrodes without binder. They were first immersed in 6 M aqueous KOH overnight. Then, the CCF or Ni(OH)2@CCF SCs were assembled by sandwiching the electrodes with a separator of filter paper which was previously soaked in the same electrolyte overnight. Pt foils were used as the current collectors. The cyclic voltammetry (CV) and the galvanostatic charging/discharging (GCD) curves were obtained at different scan rates and current densities, respectively. The electrochemical impedance spectroscopy (EIS) was tested from 0.01 Hz to 100 kHz. Calculation. The formulae for calculating gravimetric capacitance (C, F g-1), energy density (E, Wh kg-1) and power density (P, kW kg-1) of the aqueous symmetric supercapacitor were given as follows: C = (2 × I × ∆t) / (m × ∆V)

(1)

E = (C × ∆V2) / (2 × 3.6)

(2)

P = 3.6E / ∆t

(3)

In addition, the specific capacitance for a single electrode (CS, F g-1) tested in the three-electrode configuration could be estimated from the following equation: Cs = (I × ∆t) / (m × ∆V)

(4)

Where I (A), ∆t (S), ∆V (V), and m (g) are the charge–discharge current, discharging time, OPW, and mass of one electrode material, respectively. The apparent bulk density was measured from the formula: =m/v

(5)

Where ρ (g cm-3), m (g) and v (cm3) are the density, mass and volume of one electrode material, respectively.



Results and discussion A schematic illustration of the fabrication procedure of the electrode materials was shown in Figure 1a. In a typical experiment, carbonization of CF in a tube furnace provided it with an ultralow density of 0.309 g cm-3. The resultant CCF also demonstrated excellent flexibility. It could be bent or twisted without any fracture (Figure 2b and, Video S2 in Supporting Information). Figure 2c-d clearly showed that the CCF was woven from numerous aligned single carbon filaments of ~10 µm in diameter with a uniform surface texture. Subsequently, the Ni(OH)2 nanostructures were electrodeposited on CCF. Under a constant voltage, the consumption of H+ ions owing to electrochemical reduction will result in the release/formation of hydroxyl ions (OH-), leading to rapid deposition of Ni(OH)2 at the CCF surface 27,54. A series of Ni(OH)2@CCF were prepared and designated as Ni(OH)2@CCF-50, Ni(OH)2@CCF-100, Ni(OH)2@CCF-200, Ni(OH)2@CCF-300, and Ni(OH)2@CCF-400 according to the ED time. The Ni(OH)2 layer dramatically altered the surface properties of CCF, as the equilibrium water contact angle (CA) decreased sharply from 143  2° (Figure 2a for CCF) to 0° (Figure 1b for Ni(OH)2@CCF). The Ni(OH)2@CCF was highly hydrophilic since a water droplet could be completely absorbed within 1s (Video S1, Supporting Information). By contrast, the water CA of CCF almost unchanged in 30s or even longer. It is noteworthy that the Ni(OH)2 coating could increase its density slightly (the Ni(OH)2@CCF-200 exhibited a density of 0.378 g cm-3). Moreover, the ED treatment did not affect the flexibility of CCF.


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Figure 1. a) A schematic for the preparation process of Ni(OH)2@CCF. b) a photo for the water CA measurement of Ni(OH)2@CCF.



Figure 2. a) Digital photo of the water CA of CCF. b) photograph of CCF being bended and twisted without damage. c, d) SEM images of the CCF.

XPS was adopted to disclose the composition of Ni(OH)2@CCF (Figure 3a). The XPS survey spectrum indicated the presence of Ni, C and O atoms. The dominant C and O signals appeared at the binding energies at 286.2 and 530.8 eV, respectively. And the characteristic peak at 530.8 eV for the O in Ni–O was observed in the O 1s spectrum (Figure S1, Supporting Information). Two major peaks on the Ni 2p spectrum were observed with binding energies of 855.2 eV (satellite: 860.8 eV) and 872.8 eV (satellite: 879.1 eV), which corresponded to Ni 2p3/2 and Ni


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2p1/2, respectively (Figure 3b). The spin-energy separation between Ni 2p1/2 and Ni 2p3/2 (17.6 eV) suggested the existence of a Ni(OH)2 phase 55,56. A very weak Cl 2p peak appeared at 198.0 eV in this spectrum, indicating some residual Cl- ions were settled into the space of Ni(OH)2. Additionally, XRD was also utilized to analyze the nanostructures of Ni(OH)2. According to the literature, the XRD pattern (Figure 3c) with peak positions at 2θ = 22.64, 32.88, 37.84, 54.46, 62.58, 68.92 and 75.04° corresponded to the (006), (100), (101), (102), (111), (200) and (103) planes of nanoflower-shaped Ni(OH)2

50.

And the two diffraction peaks of 25.35 and 48.09o

could be assigned to the (002) and (100) reflection of graphite structure in CCF. It is important to note that Ni(OH)2 with different nanostructures, e.g. flower-, slice-, rod- and particle- shapes, may have distinctly different capacitance properties

50.

Specifically, the nanoflower-shaped

Ni(OH)2 demonstrates superb electrochemical performance owing to its huge surface area. The FTIR spectra (Figure 3d) revealed that the oxygen-containing groups (-OH, C-O-C) of CF almost disappeared in CCF, suggesting that the thermal degradation mainly happened to those oxygencontaining species during the carbonization process. A broad band at 3350 cm-1 appeared in Ni(OH)2@CCF (Figure 3d), which could be attributed to the vibration of O–H in hydrogenbonded hydroxyls of Ni(OH)2 56.



Figure 3. a) Typical XPS spectrum of Ni(OH)2@CCF. b) XPS spectrum of Ni 2p in high resolution. c) typical XRD pattern of Ni(OH)2@CCF. d) FTIR spectra for CCF and Ni(OH)2@CCF.

The Ni(OH)2 nanostructures could be grown easily on the CCF substrates through nucleation and crystal growth processes during ED. As confirmed by SEM images in Figure 4a, uniform nanoworm-shaped Ni(OH)2 was firmly anchored at the surface of CCF when the deposition time reached 50s. As the ED time increased to 100s and 200s, the nanoworm-shaped Ni(OH)2 changed into nanoflower-shaped one. And Ni(OH)2@CCF-200 displayed more pores and larger “petal” as compared to Ni(OH)2@CCF-100 (Figure 4b-c). The TEM images (Figure 4e and


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Figure S2 in Supporting Information) clearly showed that those “nanoflowers” covered on the CCF were actually composed of numerous Ni(OH)2 “nanoworms” having an average diameter of ~250 nm. The micromorphology development of Ni(OH)2 during ED treatment was schematically presented in Figure 4d. Further increase of ED time would generate more densely packed Ni(OH)2 nanostructures. In Figure 5a, the random “nanoworms” crowded together in Ni(OH)2@CCF-300, leading to the disappearance of the “nanoflower” structure. This situation was even worse for Ni(OH)2@CCF-400 (Figure 5b) since serious cracks of bulk Ni(OH)2 were clearly visible along the carbon filaments. Consequently, the effective specific surface area would be reduced to a great extent, which resulted in capacitance fading.



Figure 4. SEM images of a) Ni(OH)2@CCF-50, b) Ni(OH)2@CCF-100, c) Ni(OH)2@CCF-200 at different magnifications. d) schematic illustration for the step-wise shape evolution of Ni(OH)2. e) TEM image for Ni(OH)2@CCF-200.


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Figure 5. SEM images for a) Ni(OH)2@CCF-300, b) Ni(OH)2@CCF-400 at different magnifications.

In order to evaluate the electrochemical performance of the flexible electrodes, CCF and Ni(OH)2@CCF were firstly tested in the three-electrode configuration. The GCD and CV measurements of CCF were conducted over a negative potential range of -1-0 V (Figure S3a-b). Whereas, the typical GCD and CV curves (Figure S3c-d) for Ni(OH)2@CCF-200 were measured in a positive potential window (0-0.4 V). The specific capacitance of a single electrode of Ni(OH)2@CCF-200 was calculated based on the discharge time (Cs = (I × ∆t) / (m × ∆V)), which decreased from 568.06 to 534.63, 502.00 and 462.50 F g-1 when the current density was increased from 0.25 to 0.5, 1.0 and 2.0 A g-1. Next,

the

electrochemical

performance

of

the

as-assembled

CCF//CCF

and

Ni(OH)2@CCF//Ni(OH)2@CCF SC devices was measured in the symmetrical two-electrode



configuration. The CV and GCD curves of the CCF//CCF SC device were shown in Figure S4ab, respectively. This device exhibited capacitive behavior with a stable OPW of 0.8 V. The GCD curves manifested good reversibility between the charge and discharge processes. Its CV curves also showed a nearly rectangular shape at different scan rates, and there was no obvious distortion. The specific capacitance of CCF//CCF was estimated from the discharge time. And it was only 11.00 F g-1 at 0.25 A g-1. The GCD curves of the Ni(OH)2@CCF-200//Ni(OH)2@CCF200 SC device in various potential windows were shown in Figure S4c. A potential limit of 1.6 V could be reached for this device. It should be noted that aqueous symmetric SCs are commonly performed under 1 or 1.2 V due to the limit of water splitting (1.23 V). Nonetheless, there are also some examples of aqueous SCs which could be operated at a higher potential 57-58. In regard to the present study, the symmetric SC (Ni(OH)2@CCF//Ni(OH)2@CCF) was actually working under an asymmetric SC mechanism (Ni(OH)2@CCF//CCF) ascribed to the fact that Ni(OH)2 is used as the active material for the positive electrode only. This is in accordance with the experimental data that redox peaks of Ni(OH)2@CCF appeared in the positive potential but were absent in the negative potential (see Figure S3d and 4d). According to numerous early studies, asymmetric SCs are capable to make use of the entire well-separated potential windows from their positive and negative electrodes, which resulted in a higher OPW 59. The GCD curves also manifested that the device could be charged to a stable potential of 1.4 V with only a 0.03 V voltage drop. Therefore, a cut-off potential of 1.4 V was selected to further investigate its electrochemical performance. The representative CV and GCD curves of the Ni(OH)2@CCF200//Ni(OH)2@CCF-200 SC device at various scan rates and current densities under an OPW of 1.4 V were depicted in Figure 6a and b, respectively. In Figure 6a, the SC exhibited typical capacitive behaviors with warped rectangle CV curves, suggesting the combination of both


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pseudocapacitance and EDLC. The clearly visible anodic and cathodic peaks at different scan rates could be assigned to the redox reaction: Ni(OH)2+OH-↔NiOOH+H2O+e-. Meanwhile, the discharge curves in Figure 6b displayed distinct nonlinearities, which were quite different from those for neat EDLC capacitors, suggesting the pseudocapacitance behaviors of Ni(OH)2 . As the current density was increased from 0.25 to 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, the specific capacitance for the Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC device decreased from 131.43 to 122.14, 113.86, 101.71, 85.00 and 67.14 F g-1, respectively (see Figure 6c). Figure 6d compared the CV curves for CCF//CCF and Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC devices. It was evident that the enclosed area of the latter was much larger, indicating that the nanostructured Ni(OH)2 could effectively promote the capacitance of the SC devices. The maximum specific capacitance of Ni(OH)2@CCF//Ni(OH)2@CCF SC device (131.43 F g-1 at 0.25 A g-1) was approximatively 12 fold higher than that of CCF//CCF ones. The GCD and CV curves for other Ni(OH)2@CCF//Ni(OH)2@CCF SC devices were supplemented in Figure S5. According to the discharging time, the specific capacitance varied from 83.57, 103.21, 131.43 and 125.36 to 111.43 F g-1 as the ED time was extended from 50 to 100, 200, 300 and 400s at 0.25 A g-1, respectively. Furthermore, the electrochemical performance was also investigated using EIS. The resultant EIS plots for CCF and Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC device were illustrated in Figure 6e. It is well accepted that the semicircle at a high frequency region reflects the charge transfer resistance (Rct) at the interfaces of electrode/electrolyte. And the ion diffusion resistance in the electrode can be estimated from the slope of the straight line. The Ni(OH)2@CCF had a distinctly larger semicircle diameter compared with CCF, which should be ascribed to the high resistance of Ni(OH)2

33.

Moreover, the higher slope of

Ni(OH)2@CCF indicated that the porous and nanostructured Ni(OH)2 could promote penetration



of electrolyte into the host materials and shorten the electron transport distance, leading to faster electrolyte ion diffusion. The Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC device demonstrated the lowest equivalent series resistance (ESR) value and the fastest ion diffusion/transport rate as compared to CCF//CCF and other Ni(OH)2@CCF//Ni(OH)2@CCF SC devices (Figure S6), in agreement with its superior electrochemical performance.


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Figure 6. a) CV curves of Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 at various scan rates in 6 M KOH. b) GCD curves of Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC devices at various current densities in 6 M KOH. c) the specific capacitance of the Ni(OH)2@CCF-200//Ni(OH)2@CCF200 SC device at various current densities. d) CV curves of the Ni(OH)2@CCF200//Ni(OH)2@CCF-200 and the CCF//CCF SC devices. e) EIS of CCF//CCF and Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC devices. Inset: an enlarged scale at high frequency. f) the Ragone plot specifying energy and power densities.

From the equation of E = 1/2CV2, the energy density (E) is positively correlated with the square of the OPW (V) and the specific capacitance (C)

10.

In this work, the SC could work

stably at a wide OPW of 1.4 V. The Ni(OH)2@CCF//Ni(OH)2@CCF aqueous symmetric SCs demonstrated a high energy density of 35.78 Wh kg-1 and a power density of 0.35 kW kg-1 at 0.25 A g-1. When the current density was increased to 10 A g-1, the energy density could still maintain 18.28 Wh kg-1 with a high power density 14.00 kW kg-1. The high specific capacitance together with the wide OPW synergistically improved the energy density. The performance of the Ni(OH)2@CCF//Ni(OH)2@CCF aqueous symmetric SCs was comparable or even outperformed many early reported aqueous symmetric supercapacitors (see Figure 6f).



Figure 7. a) Green LEDs powered by the Ni(OH)2@CCF//Ni(OH)2@CCF SCs in series. b) schematic for the assembling of aqueous symmetric SC. c) GCD profile of three Ni(OH)2@CCF//Ni(OH)2@CCF SCs in series.

Moreover, three Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 aqueous symmetric SCs were assembled in series (the area for each electrode was 2 mm  2 mm). This device could be charged to 4.5 V and power a red and green light-emitting diode (LED) for 8 and 3 min respectively, and even light up eight green LEDs simultaneously (Figure 7a-c, Figure S7 and Video S3-5 in Supporting Information), demonstrating a great potential for real-world applications. Finally, a flexible SC device was assembled by using two pieces of Ni(OH)2@CCF-200 as electrodes with soft platinum sheets as the current collectors. Figure 8 compared its CV curves at various bending angles. The CV curves looked almost the same. Remarkably, it could retain 102.61% of the initial specific capacitance even at a bending angle of 180°, suggesting its excellent capacitance stability under external forces. This strongly implied that the Ni(OH)2@CCF should be very promising in future flexible energy-storage devices.


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Figure 8. CV curves for the SC tested at various bending angles at 50 mV s-1.

Conclusion Highly flexible and conductive Ni(OH)2@CCF electrodes with nanostructured Ni(OH)2 at the surface of carbonized cotton fabric (CCF) had been successfully fabricated using a facile twostep high-temperature carbonization process followed by an ED treatment. Due to the combination of the high pseudocapacitance of nanostructured Ni(OH)2 and a wide OPW of 1.4 V, the assembled Ni(OH)2@CCF aqueous symmetric SCs demonstrated a high specific capacitance of 131.43 F g-1 at 0.25 A g-1. And its energy density could reach 35.78 Wh kg-1 with a high power density of 0.35 kW kg-1. A device of three serially connected SCs could be charged to 4.5 V and power a red and green LED for 8 and 3 min respectively, and even light up eight



green LEDs simultaneously. Besides, 102.61% of the initial specific capacitance could be retained even at a bending angle of 180°, suggesting their excellent capacitance stability under external forces. It is envisaged that this flexible and high performance SC electrode from the sustainable natural resources may find important applications in particular in wearable electronic devices.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional XPS, TEM and electrochemical performance tests.

Acknowledgment This work was supported by the National Natural Science Foundation of China (51861165203, 51473100, 51433006 and 31500789), Excellent Young Scholar Fund of Sichuan University (20822041B4205) and State Key Laboratory of Polymer Materials Engineering (sklpme2016-309). J.Q. Zhao is grateful to the funding from the China Scholarship Council (201706240140).

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TOC/Abstract Graphic

Synopsis. A flexible electrode was fabricated from the renewable and sustainable cotton fabrics for electrochemical capacitive energy storage.



Figure 1. a) A schematic for the preparation process of Ni(OH)2@CCF. b) a photo for the water CA measurement of Ni(OH)2@CCF. 216x159mm (150 x 150 DPI)


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Figure 2. a) Digital photo of the water CA of CCF. b) photograph of CCF being bended and twisted without damage. c, d) SEM images of the CCF. 157x136mm (150 x 150 DPI)



Figure 3. a) Typical XPS spectrum of Ni(OH)2@CCF. b) XPS spectrum of Ni 2p in high resolution. c) typical XRD pattern of Ni(OH)2@CCF. d) FTIR spectra for CCF and Ni(OH)2@CCF. 279x212mm (150 x 150 DPI)


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Figure 4. SEM images of a) Ni(OH)2@CCF-50, b) Ni(OH)2@CCF-100, c) Ni(OH)2@CCF-200 at different magnifications. d) schematic illustration for the step-wise shape evolution of Ni(OH)2. e) TEM image for Ni(OH)2@CCF-200. 163x180mm (150 x 150 DPI)



Figure 5. SEM images of a) Ni(OH)2@CCF-300 and b) Ni(OH)2@CCF-400 at different magnifications. 231x140mm (150 x 150 DPI)


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Figure 6. a) CV curves of Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 at various scan rates in 6 M KOH. b) GCD curves of Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC devices at various current densities in 6 M KOH. c) the specific capacitance of the Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC device at various current densities. d) CV curves of the Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 and the CCF//CCF SC devices. e) EIS of CCF//CCF and Ni(OH)2@CCF-200//Ni(OH)2@CCF-200 SC devices. Inset: an enlarged scale at high frequency. f) the Ragone plot specifying energy and power densities. 238x257mm (150 x 150 DPI)



Figure 7. a) Green LEDs powered by the Ni(OH)2@CCF//Ni(OH)2@CCF SCs in series. b) schematic for the assembling of aqueous symmetric SC. c) GCD profile of three Ni(OH)2@CCF//Ni(OH)2@CCF SCs in series. 274x95mm (150 x 150 DPI)


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Figure 8. CV curves for the SC tested at various bending angles at 50 mV s-1. 254x176mm (150 x 150 DPI)



TOC/Abstract Graphic. Synopsis. A flexible electrode was fabricated from the renewable and sustainable cotton fabrics for electrochemical capacitive energy storage. 211x184mm (150 x 150 DPI)


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Flexible and conductive carbonized cotton fabrics coupled with a

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