High-Performance Biomass-Based Flexible Solid-State

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High Performance Biomass-Based Flexible Solid-State Supercapacitor Constructed of Pressure-Sensitive Lignin-Based and Cellulose Hydrogels Zhiyuan Peng, Yubo Zou, Shiqi Xu, Wenbin Zhong, and Wantai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05171 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

High

Performance

Biomass-Based

Flexible

Solid-State

Supercapacitor Constructed of Pressure-Sensitive Lignin-Based and Cellulose Hydrogels Zhiyuan Peng,† Yubo Zou,† Shiqi Xu,† Wenbin Zhong,†,* Wantai Yang‡

† College of Materials Science and Engineering, Hunan University, Changsha, 410082, P. R. China. ‡ Department of Polymer Science, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. *E-mail address: [email protected] (W. Zhong) Keywords: biopolymer, lignosulfonate, cellulose, pressure-sensitive hydrogel, flexible supercapacitor

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Abstract Employing renewable, earth-abundant, environmentally friendly, low cost nature materials to design flexible supercapacitors (FSCs) as energy storage device in wearable/portable electronics represents the global perspective to build sustainable and green society. Chemical stable and flexible cellulose, and electroactive lignin have been employed to construct biomass-based FSC for the first time. The FSC was assembled using lignosulfonate/SWCNTHNO3 (Lig/SWCNTHNO3) pressure-sensitive hydrogel as electrodes and cellulose hydrogel as electrolyte separator. The assembled biomass-based FSC shows high specific capacitance (292 F g-1 at current density of 0.5 A g-1), excellent rate capability and outstanding energy density of 17.1 Wh kg-1 at a power density of 324 W kg-1. Remarkably, the FSC presents outstanding electrochemical stability even suffering 1000 bending cycles. Such excellent flexibility,

stability

and

electrochemical performance

enable

the

designed

biomass-based FSCs as prominent candidates in applications of wearable electronic devices.

1. Introduction Flexible supercapacitors (FSCs) possess high power density, ultra-long cycle life, extraordinary mechanical flexibility to endure bending, twisting or folding, making them competitive candidates as energy storage devices in the flexible/wearable electronics.1-4 Tremendous efforts have been dedicated to developing high performance flexible electrode materials. Various carbon materials (such as

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graphene,5 SWCNT,6 nitrogen-doped carbon4), conductive polymers (such as polyaniline (PANI)7), carbon-based composites with metal oxides (such as MWCNT/MnO28) or conducting polymers (such as CNT/PANI,9 reduced graphene oxide (rGO)/hydroquinone10) and other electroactive materials are employed to construct three-dimensional porous,4,7,10 fiber-like8,9,11 or paper-like5,6 electrodes. Choi et al. fabricated fiber-like flexible electrodes through electrodeposition of MnO2 onto aligned carbon nanotube sheets.8 Paper-like electrodes were obtained via filtration assembly of partially reduced graphene oxide with subsequent freeze-casting process.5 Functionalized graphene hydrogels were casted onto a gold-coated polyimide substrate to prepare three-dimensional (3D) flexible electrodes.10 Wang et al. produced textile electrodes by coating PANI hydrogel onto carbon fibers cloth.7 Recently, PANI/rGO or activated carbon deposited on cellulose substrates also were created to obtain paper-like electrodes.11,30 However, the present electrode materials still suffer from inevitable practical problems. For example, metal oxides are non-renewable and exhibit low cost-effect, which limits its wide applications.12,13 Even though conductive polymers are easily synthesized, the synthesis of conductive polymers uses toxic monomers and produces needless by-products during the synthesis process, and they exhibit unstable electrochemical performance. In addition, polyvinyl alcohol based gels (such as PVA/H2SO4,10 PVA/KOH,8 PVA/H3PO49), as widely used electrolyte for FSCs is corrosive or hardly degradable.14,15 Hence, the fabrication of FSCs with aforementioned electrodes and polymer gel electrolyte separator arises environmental concerns, huge resource consumption as well as

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tremendous electronic garbage after their service lifetime.13 Thus, developing alternative renewable, earth-abundant, low-cost, biodegradable, environmentally friendly biomass-based FSCs and searching for facile and green fabrication methods are of great challenges.16 Lignin is the second abundant natural biopolymer.17 Lignosulfonate (Lig), as the main derivative of lignin, is a waste product in the sulfonation process of papermaking.18 Although a small amount of Lig is used as anionic surfactant due to the intrinsically good wettability, dispersibility, absorptivity and other colloidal properties,19 most of the Lig is discarded. However, the electroactive, renewable and source-abundant characteristics make it prominent in energy storage applications. Lig possesses phenol groups that can induce reversible redox reaction, such as sinapyl alcohol (S), coniferyl alcohol (G) and p-coumaryl alcohol (H).20 Thus Lig possesses charge storage capacity. But the inherent electronical insulativity makes it impossible to be directly applied as the energy storage electrode material.18 To overcome the drawback, it is feasible to prepare the composites of Lig and conductive materials to fulfill its charge storage capacity. The construction of electroactive interpenetrating networks of Lig/polypyrrole composite exhibits high specific capacitance (up to 1000 F g-1).18,20 In our previous work, Lig/rGO hydrogel was successfully made into self-supported hydrogel electrode. The well-defined 3D porous network structure facilitates the fast ion and charge transport while Lig provides superior reversible faradaic reaction. This excellent composite leads to the largely improved specific capacitance (549.5 F g-1), excellent rate capability and cycling stability.21

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Subsequently, Li et al. designed a flexible solid-state supercapacitor based on Lig/rGO hydrogel electrodes and polymer gel (PVA/H2SO4) electrolyte, which exhibits an energy density of 13.8 Wh kg-1.22 However, the mechanical properties, especially pressure-sensitive performance, as important evaluation aspect for wearable electronic devices and pressure sensors, in which large deformation is generated and recovered with the load and release of a small stress, respectively,23-25 has not been investigated for Lig/rGO hydrogels. SWCNTs, present excellent conductivity, outstanding mechanical tensile strength and flexibility,26,27 making it promise to serve as pressure-sensitive and conducting component in Lig-based electrodes. In addition, as the most abundant biopolymer, cellulose possesses many advantages such as good biocompatibility, high thermal/chemical stability, robust mechanical strength and flexibility, thus indicating its accessibility to be environmentally friendly electrolyte separator and the possibility to replace the synthetic polymer gel electrolyte separator.28-30 It is suggested that assembling Lig-based flexible electrodes with cellulose electrolyte separator as biomass-based supercapacitors could be a promising approach towards the environmental friendly and low cost FSCs. Herein, a novel renewable and environmentally friendly, low cost biomass-based flexible solid-state supercapacitor is designed and fabricated. The flexible high performance Lig/SWCNTHNO3 hydrogel is adopted as electrodes and biocompatible cellulose/Li2SO4 gel is used as electrolyte. Lig/SWCNTHNO3 pressure-sensitive hydrogel was produced in a simple hydrothermal process and cellulose hydrogel

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prepared via a facile phase-inversion method. The Lig/SWCNTHNO3 hydrogel presents ultrahigh water content (99.6%) and pressure-sensitive property. The assembled biomass-based symmetric FSC possesses notable electrochemical performance and flexibility, which paves the new way towards the development of energy storage devices for wearable electronics. 2. Experimental Section 2.1 Materials Single-walled carbon nanotube (SWCNT, 5-30 µm in length, purity > 95 wt %) was purchased from Chengdu Organic Chemicals Co., Ltd. (Chengdu, China). Sodium Ligninsulfonate (Lig, molecular weight ca. 20000) was purchased from Borregaard LignoTech (Norway). Cellulose (100-200 mesh) was purchased from Meryer Chemical Technology Co., Ltd. (Shanghai, China). Nitric acid (AR) was purchased from Sinopharm Chem. Reagent Co.,Ltd. (Beijing, China). All material and agent were used as received. 2.2 Preparation of Lig/SWCNT and Lig/SWCNTHNO3 hydrogels SWCNTs (70 mg) were mixed with nitric acid (40 mL) with stirring and under refluxing at 90 °C for 6 h. The resulting dispersion was then diluted in water and filtered. The obtained products were washed up to pH = 7. These acid treated SWCNTs were used for next step directly. As-prepared acid treated SWCNTs were dispersed in deionized water (10 mL). The Lig (280 mg) solution (dissolved in 10 mL deionized water) was subsequently added. The SWCNTs were ultimately completely dispersed in Lig solution via stirring

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and ultrasonic vibration to obtain Lig/SWCNT dispersion. Then 6.3 mL Lig/SWCNT dispersion mixed with 0.7 mL HNO3 (0.1 mol L-1), after stirring for 15 min, the mixture was sealed in a Teflon-lined autoclave hydrothermal at 180 °C for 12 h and subsequent natural cooling to obtain Lig/SWCNTHNO3 hydrogel. For comparasion, the Lig/SWCNT was prepared as that for Lig/SWCNTHNO3 but without HNO3. 2.3 Preparation of cellulose hydrogel The cellulose hydrogel was prepared by phase-inversion process (the formation process diagram as shown in Figure S11).55 Firstly, NaOH-urea aqueous system was established by mixing NaOH (1.4 g), urea (2.4 g) and deionized water (15 mL). Secondly, the resultant mixture was pre-cooled to -12.0 °C, then 1.2 g cellulose was added immediately under vigorous stirring for 5 min to obtain a cellulose solution. Thirdly, the resulting solution was transferred into a 25 mL beaker, afterwards add 10 mL H2SO4 aqueous solution (0.5 mol L-1). After standing 24 h at room temperature, a white cellulose hydrogel was obtained by washing with excess deionized water to remove the residual chemical reagents. 2.4 Fabrication of the symmetric supercapacitor in two-electrode system, compressible supercapacitor device, flexible all-solid-state supercapacitor and electrochemical measurements The working electrodes of two-electrode system were prepared as follows: as-prepared Lig/SWCNTHNO3 hydrogel was cut into self-supported slices (1 mm in thickness and 1 × 1 cm2 in area), the hydrogel slice was clamped by two pieces of stainless steels under a pressure of 15 MPa for 5 min. The electrodes were immersed

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in 1 M Li2SO4 aqueous electrolyte for 12 h. The separator was prepared by cutting cellulose hydrogel into slices (0.3 mm in thickness and 1.1 × 1.1 cm2 in area). The two-electrode system was constructed with two pieces of Lig/SWCNTHNO3 hydrogel slices used as electrodes, cellulose hydrogel slice used as separator and 1 M Li2SO4 used as electrolyte. The two-electrode system based on Lig/SWCNT electrodes was fabricated identically. The acid treated SWCNT based electrodes were fabricated by coating onto stainless steel meshes and assembled at the same procedure. The compressible supercapacitor device was prepared by two slices of Lig/SWCNTHNO3 (thickness of about 3 mm and area of 1 × 1 cm2) separated by a cellulose hydrogel separator (before use all the hydrogels were soaked into 1 M Li2SO4 aqueous electrolyte), and carbon cloth as the current collectors. The compression process was enforced by various stationary weight. The

flexible

all-solid-state

supercapacitor

was

prepared

as

follows:

Lig/SWCNTHNO3 hydrogel was cut into rectangular pieces (area of 1 × 1 cm2), and soaked in 1 M Li2SO4 for 12 h, then pressed on a substrate of stainless steel (SS) under 0.2 MPa to act as electrodes. Two pieces of electrodes with a cellulose hydrogel separator (soaking in 1 M Li2SO4 for 12 h) were pressed with 2 MPa into a flexible all-solid-state supercapacitor. The mass percentage of Lig (WLig) in Lig/SWCNTHNO3 hydrogel electrode is about 38.3 wt %, based on the equation: WLig = (MLig/SWCNTHNO3-MSWCNT) /MLig/SWCNTHNO3 × 100%,21 where MLig/SWCNTHNO3 is the mass of freeze-dried Lig/SWCNTHNO3 hydrogel and MSWCNT is the mass of freeze-dried Lig/SWCNTHNO3

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hydrogel removing Lig by soaking in 1 M HNO3 and ethyl alcohol. Relatively, the mass percentage of Lig in Lig/SWCNT hydrogel is just 21.7 wt %. The electrochemical performances were measured on an electrochemical working station (CHI660C, Shanghai, China). Cyclic voltammetry (CV) tests were carried out in a wide scan rates range of 5 to 2000 mV s-1 with a potential window of 1.3 V. Galvanostatic charge/discharge (GCD) tests were measured under various current densities in the same potential window. Electrochemical impedance spectroscopy (EIS) tests were conducted in an amplitude of 10 mV and frequency range of 100 kHz to 0.1 Hz at open circuit potential. The specific capacitance (Cm, F g-1) of two-electrode system, compressible supercapacitor devices and flexible supercapacitors were calculated from the GCD curve by Eq. (1): Cm =

4×I×∆t

(1)

m×∆V

The energy density (E, Wh kg-1) and power density (P, W kg-1) were obtained based on Eq. (2) and Eq. (3), respectively: E= P=

Cm×∆V2

(2)

8×3.6 E×3600

(3)

∆t

Where I, ∆t, ∆V, m were the discharge current (A), the discharge time (s), the potential window (V), and the mass of active materials on both two symmetric electrodes (g), respectively. 2.5 Characterization and measurement

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The structure and morphology of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Titan G2 60-300). Nitrogen adsorption-desorption isotherms were obtained by using an ASAP-2020 surface analyzer (Micromeritics Instruments) to determine the Brunauer-Emmett-Teller (BET) specific surface area and density functional theory (DFT) pore size distribution of samples. X-ray diffraction (XRD) measurements were performed using X-ray power diffraction (D8 Advance, Germany) with nickel-filtered Cu Ka radiation of l = 0.154 nm. The X-ray photoelectron spectroscopy (XPS) was obtained using a Thermo ESCALAB 250Xi instrument. Fourier transform infrared (FT-IR) spectra were recorded on an IRAffinity-1 spectrometer (shimadzu, Japan). 3. Results and Discussions The fabrication process of Lig/SWCNTHNO3 pressure-sensitive hydrogel is illustrated in Figure 1a,b. The acid treated SWCNTs with hydroxyl (-OH) and carboxyl (-COOH) groups were first dispersed in Lig solution.31 During hydrothermal process a Lig/SWCNTHNO3 hydrogel with 3D network structure was obtained within a trace of nitric acid (Figure 1b), attributed to hydrogen bonding and π-π stacking interaction between Lig and SWCNTs.32 The optimized reaction conditions of Lig/SWCNTHNO3 hydrogel were determined by evaluating the electrochemical performance of related hydrogel electrode (Figure S1-5). The hydrogel prepared under optimal conditions possesses high content of water (99.6%), and an ultra-light Lig/SWCNTHNO3 aerogel (5.7 mg mL-1) was obtained after freeze-drying (Figure 1c).

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Figure 1. (a) Schematic diagram of Lig and acid treated SWNCT. (b) Lig/SWCNTHNO3 hydrogel. (c) Lig/SWCNTHNO3 aerogel. (d) Schematic diagram of Lig/SWCNTHNO3 aerogel microstructure. The

interior

morphologies

and

microstructure

of

Lig/SWCNT

and

Lig/SWCNTHNO3 were characterized by SEM and TEM (Figure 2). As shown in Figure 2a, an isotropic porous network structure linked by long and straight nanotubes is observed for Lig/SWCNT. The surface sheath and crosslinking structure (the thickness of Lig sheath is around 1.35 nm) is clearly observed in the TEM image of Lig/SWCNT (Figure 2b). In comparison, hydrogel cannot be found in pure acid treated SWCNTs after hydrothermal process (Figure S6). The formation of Lig/SWCNT may be attributed to a series of interactions between Lig and SWNCTs during hydrothermal process: (1) the hydrogen bonding and π-π stacking interaction between Lig and SWCNTs make it form a surface sheath structure. (2) The physical crosslinking between Lig sheaths facilitates the formation of 3D network by hydrogen bonds and van der Waals force. (3) The free Lig can bind the crosslinking sheaths through hydrogen bonds and π-π stacking interaction. From the Figure 2c, the SEM image of Lig/SWCNTHNO3 shows a porous network structure linked by relative flexural long nanotubes and the sheath structures are assembled into bundles. 11



Moreover, a crosslinking surface sheath structure with much more Lig (the thickness of Lig sheath is around 5.2 nm) is observed in the TEM image of Lig/SWCNTHNO3 (Figure 2d). The different structure of Lig/SWCNT and Lig/SWCNTHNO3 due to the introduced trace of nitric acid enables SWCNTs to be further oxidized and etched31 as well as Lig is nitro-functionalized during the hydrothermal process.33 Thus it facilitates Lig can be adsorbed on the surface of the SWCNTs via hydrogen bonding. Moreover, the introduction of nitro also increases the number of hydrogen bonds,34 thereby the structure of the hydrogel is more stable (Figure 1d). According to the comparison of the mechanical tests (Figure S7-8), Lig/SWCNTHNO3 has much stable mechanical properties, suggesting the existence of a much stable network structure. The content of Lig is calculated to be around 38.3 wt % (Experimental Section).

Figure 2. SEM and TEM images of (a, b) Lig/SWCNT and (c, d) Lig/SWCNTHNO3. The XRD patterns of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3

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are shown in Figure 3a. It is found that for acid treated SWCNT, two characteristic diffraction peaks locate at ca. 25.9° and 44.7°, corresponding to (002) and (100) planes of the graphic structure, respectively.35 The Lig/SWCNT and Lig/SWCNTHNO3 exhibit a certain degree graphic structure as well. Moreover, peaks located at ca. 24.8° and 24.6° are observed in the patterns of Lig/SWCNT and Lig/SWCNTHNO3, respectively, which may be due to the amorphous Lig attached on the surface of the SWCNTs.21 The broadening of full width at half maximum for Lig/SWCNTHNO3 compared to that for Lig/SWCNT indicates a lower ordering along the sheet stacking direction.12 The relative low crystallization of Lig/SWCNTHNO3 may be related to the further oxidation of SWCNTs and increased amount of Lig adsorbed on the surface of SWCNTs.

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Figure 3. (a) XRD patterns, (b) FT-IR spectra, (c) N2 absorption and desorption curves, (d) pore distribution and (e) XPS spectra of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3. C 1s spectrum of (f) acid treated SWCNT, (g) Lig/SWCNT and (h) Lig/SWCNTHNO3. Fourier transform infrared (FT-IR) spectra of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3 are presented in Figure 3b. The characteristic peaks at 3750, 1748 and 1565 cm-1 are observed for acid treated SWCNT, representing hydroxyl group

(O-H),

carbonyl

group

(C=O)

and

C=C

stretching

vibrations,

respectively.21,31,36 For Lig/SWCNT and Lig/SWCNTHNO3, the bands at 2923, 2816, 1457 and 1406 cm-1 are correlated to the vibration of methoxy group.21,37 The peak at 1044 cm-1 is associated to the S=O stretching vibration, further demonstrating the existence of Lig.21,37 Additional, a peak at 1366 cm-1 is found for Lig/SWCNTHNO3,

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coupled with red shift of C=C stretching band to 1544 cm-1, indicating the presence N-O stretching.33 Furthermore, the characteristic peaks of C-N stretching vibration (1643-1632, 1273-1196 and 1044 cm-1) can also be found in Lig/SWCNTHNO3, implying that Lig is nitro-functionalized.31,33 N2 adsorption/desorption measurements of the acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3 are presented in Figure 3c. All of the three samples display a type IV isotherm characteristic and a small H3-type hysteresis loop, implying the presence of large number of mesopores. For acid treated SWCNT, an obvious sharp increase of adsorption appears at very low relative pressure (P/P0 < 0.1) and increase tendency at high relative pressure (P/P0 > 0.9) indicating the presence of large number of micropores and macropores, the existence of macropores may be due to the accumulation of SWCNTs during the test. An abrupt increase in the high relative pressure region (P/P0 > 0.9) is also observed for Lig/SWCNT and Lig/SWCNTHNO3, implying the presence of large number of macropores within hydrogels,41 which can be seen clearly in the SEM images (Figure 2a,c). The specific surface areas of Lig/SWCNT and Lig/SWCNTHNO3 are calculated to be 150.5 m2 g-1 and 155.7 m2 g-1, respectively, which are lower than that of acid treated SWCNT (713 m2 g-1), meanwhile there are almost no micropores for the hydrogels (Table S1), these results may be due to the blocking of pore structure by Lig adhesion on the surface of the SWCNTs during hydrothermal process.40 The pore-size distribution curves of Lig/SWCNT, Lig/SWCNTHNO3 and acid treated SWCNT as shown in Figure 3d. With the introduction of Lig, the Lig/SWCNT and Lig/SWCNTHNO3 samples display a

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broad pore size distribution of 5-50 nm, confirming the presence of mesopores. In addition, the content of pore width at ca. 2, 3 and 6 nm of Lig/SWCNTHNO3 is obviously lower than that of acid treated SWCNT and Lig/SWCNT, while the content of pore width at ca. 15, 22 and 28 nm of Lig/SWCNTHNO3 is obviously higher than that of acid treated SWCNT and Lig/SWCNT, which is attributed to more Lig blocking of pore structure at lower pore width and the structure bundling by abundant active functional groups in Lig/SWCNTHNO3. With larger number of mesopores and macropores as ionic and electronic transport channels, hydrogels may show outstanding electrochemical performance.41 Figure 3e shows the X-ray photoelectron spectroscopy (XPS) characterization of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3 with the related chemical compositions presented in Table S2. The acid treated SWCNT exhibits high O content (9.62 at.%), attributed to that the hydroxyl and carboxyl groups are introduced into SWCNTs during acid treatment.31 Extra S 2p peak is observed at 168 eV for Lig/SWCNT and Lig/SWCNTHNO3, in comparison with acid treated SWCNT, further proving the successful coupling of Lig and SWCNT.21 In particular, the intensities of S 2p and O 1s (~ 533 eV) peaks are obviously higher in Lig/SWCNTHNO3 than that in Lig/SWCNT. Meanwhile, a N 1s peak (~ 400 eV) is found in Lig/SWCNTHNO3, due to the introduction of nitro into Lig.12,38,39 Figure 3f-h present the C 1s peak of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3, respectively. The C 1s peak is fitted into four peaks for each sample, with the peaks at 284.8, 285.5 ~ 285.8, 287.3 and 289.2 eV corresponding to C=C/C-C, C-O/C-N/C-S, C=O and O-C=O,

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respectively.21,38,39 The content of C-O/C-S (285.6 eV) is 28.2 at.% for Lig/SWCNTHNO3, 21.2 at.% for Lig/SWCNT and 13.6 at.% for acid treated SWCNT. The larger content of C-O bond obtained for Lig/SWCNT and Lig/SWCNTHNO3 is associated to the additional contribution of oxygen-containing groups from Lig. Two different N bonds are obtained for Lig/SWCNTHNO3 from the high-resolution N 1s spectrum (Figure S9), namely -NH- (400.2 eV) and -NO2 (406.1 eV). The appearance of -NH- bond reflects the chemical reduction of -NO2 to NH2 under X-ray irradiation during XPS measurements.49 Thus, the XPS results demonstrate that Lig structures are nitro-functionalized, which promotes the increase of Lig content on the surface of SWCNTs, as consistent with the FT-IR characterizations. Additionally, the content of C=O bond is slightly higher in Lig/SWCNTHNO3 (5.4 at.%) than that in Lig/SWCNT (3.8 at.%) and acid treated SWCNT (4.4 at.%), indicating a larger amount of quinoid functional groups existed in Lig/SWCNTHNO3.18,20 Moreover, the content of O-C=O (290.2 eV) bond in Lig/SWCNT and Lig/SWCNTHNO3 have decreased dramatically compared to that of acid treated SWCNT, which further proves the Lig is absorbed onto SWCNTs to form a sheath structure. Compression experiments were carried out to test the mechanical properties of the samples as shown in Figure 4a, in which Lig/SWCNTHNO3 shows a recoverable deformation under a light weight of 10 g with the strain about 40%. Figure 4b presents the compressive stress-strain curves of Lig/SWCNTHNO3 under different strains (20%, 40% and 60%). There are two strikingly different stages during the whole loading process, with the linear elastic deformation under low compressive

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strain followed by inelastic hardening and densification under high compressive strain.27,43 Densification region with a rapidly increased slope is observed with strain up to 40%, due to the impinging among the nanotubes.42 The hydrogel can even withstand compression up to 60% and almost fully recover after releasing the compression force. Impressively, the Lig/SWCNTHNO3 shows no significant strength degradation after compression at the strain of ε = 40% with a light weight for 1000 cycles

(Figure

4c).

These

results

suggest

Lig/SWCNTHNO3

possesses

pressure-sensitive property and mechanical stability. The morphology changes during compression were characterized by SEM images. Before compression, the hydrogel has a planar isotropic 3D network structure (Figure S10). During the compression process (preparation of the SEM sample: freeze drying of hydrogel under a fixed weight to keep a 40% strain), the zigzag buckles without cracks are formed along the Lig/SWCNTHNO3 vertical surface (Figure 4d). From the microstructure a relatively loose porous network can be seen before compression (Figure 2c), which is conformably densified with some network overlapping under 40% compression strain (Figure 4e). Even though the original straight SWCNTs have a tendency to bend, the porous network is well maintained without any fracture under 40% strain. Interestingly, there is no changes of the 3D network structure and pore size in Lig/SWCNTHNO3 even after 1000 compression cycles, which responds to its highly recoverability and mechanical stability (Figure 4f-g). These results further prove the Lig/SWCNTHNO3 possesses stable pressure-sensitive properties.

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Figure 4. (a) The digital photograph of compression-recovery processes for Lig/SWCNTHNO3 hydrogel. The compressive cyclic stress-strain curves of the Lig/SWCNTHNO3 hydrogel with (b) different strains and (c) different cycles. SEM images with (d) low and (e) high magnifications of the surface of Lig/SWCNTHNO3

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with 40% strain. SEM images with (f) low and (g) high magnifications of the surface of Lig/SWCNTHNO3 after 1000 compression cycles. The electrochemical performance of acid treated SWCNT, Lig/SWCNT and Lig/SWCNTHNO3 were characterized in a two-electrode system with a cellulose hydrogel as electrolyte separator. The CV curve of acid treated SWCNT presents a typical quasi-rectangular-like shape, corresponding to the ideal double-layer capacitive behavior (Figure 5a).39,43 A couple of noticeable faradaic peaks (0.1 ~ 0.4 V) are observed in the CV curves of Lig/SWCNT and Lig/SWCNTHNO3, attributed to the reversible redox reaction of Q/QH2 groups of Lig.21,22 These redox peaks can be more clearly identified in a three-electrode system (Figure S5). A quasi-rectangular-like shape is well maintained for Lig/SWCNTHNO3 even at an extremely high 2000 mV s-1 scan rate, indicating a good reversibility and quick charge transfer capability (Figure 5b).39 The Q/QH2 structure in Lig experiences a reversible redox change (QH2 ⇌ Q + 2e- + 2H+) during the charging/discharging process, which contributes to a large number of pseudocapacitance.18,20,21 As a result, the Lig/SWCNTHNO3 shows a specific capacitance of 288 F g-1 at a current density of 1 A g-1 (Figure 5c), which is higher than that of Lig/SWCNT (216 F g-1) and acid treated SWCNT (51 F g-1). The Lig/SWCNTHNO3 still retains a specific capacitance of 153 F g-1, as current density increased to 20 A g-1 (Figure 5d). The excellent rate capability of Lig/SWCNTHNO3 is attributed to the 3D porous networks structure and good electrical conductivity of the hydrogel electrodes, which facilitates the rapid ion diffusion and charge transport at high current density.21 It is noticeable that a steep oblique line is observed in the low

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frequency region in EIS curves, showing the ideal capacitive response (Figure 5e).40,44 Lig/SWCNTHNO3 presents a smaller semi-circle in the high frequency region, compared to Lig/SWCNT, indicating a lower charge transfer resistance,44,45 which may be attributed to the stronger forces between Lig and SWCNTs that make the network structure more compact and homogeneous to facilitate the transfer of electrons. The intercept of the curve at high frequency, which represents the equivalent series resistance (ESR), is lower in Lig/SWCNTHNO3 than that in Lig/SWCNT, owing to the easy electron and ion transmission in the robust porous network of Lig/SWCNTHNO3. The electrochemical stability of the hydrogel electrodes were tested at a current density of 10 A g-1 for 3000 cycles (Figure 5f). Lig/SWCNTHNO3 remains 78.3% specific capacitance after 3000 charge/discharge cycles, which is higher than that of Lig/SWCNT (76.2%). The excellent cycling stability of Lig/SWCNTHNO3 electrodes can be attributed to the synergistic effect of Lig and SWCNTs, as well as the superior 3D porous network structure.

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Figure 5. Electrochemical properties in two-electrode system. (a) CV curves at a scan rate of 5 mV s-1. (b) CV curves of Lig/SWCNTHNO3 at scan rates ranging from 100 to 2000 mV s-1. (c) GCD curves at a current density of 1 A g-1. (d) The specific capacitance at different current densities. (e) Nyquist plots of the samples over the frequency rang from 100 kHz to 0.1 Hz. (f) The cycling stability of Lig/SWCNT and Lig/SWCNTHNO3. To test the compressibility of the supercapacitors, a biomass-based compressible supercapacitor

with

an

integrated

configuration

is

assembled

based

on

Lig/SWCNTHNO3 hydrogel electrodes, cellulose/Li2SO4 gel electrolyte and carbon cloth current collectors. The electrodes and separator were soaked in 1 M Li2SO4 solution for 12 h before the electrochemical test. The biomass-based compressible supercapacitor exhibits excellent pressure-sensitive feature, with the shape of

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supercapacitors fully recoverable in no plastic deformation after a compression cycle (Figure 6a). Moreover, the supercapacitor can maintain the thickness after a repeated compression. The electrochemical performance of the compressible supercapacitor was evaluated by both CV and GCD tests. From the Figure 6b, only a slight deviation emerges under 40% strain in the CV curves for compressed supercapacitor, which further proves the pressure-sensitive Lig/SWCNTHNO3 possesses a stable 3D network structure. The GCD curves of the supercapacitor show no changes under different strains, indicating the excellent recoverability. The specific capacitance of Lig/SWCNTHNO3 hydrogel electrode in compressible supercapacitor is around 238 F g-1 at a current density of 1 A g-1 (Figure 6c). Remarkably, the specific capacitance of the supercapacitor retains 97% of the initial value post 1000 compressing-releasing cycles (Figure 6d,e), identifying the outstanding electrochemical stability of the biomass-based compressible supercapacitor.

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Figure 6. (a) Real-time optical images of the compressible supercapacitor showing a compressing and recovering process. (b) CV curves at 5 mV s-1 and (c) GCD curves at 1 A g-1 of the compressible supercapacitor at different strains. (d) The specific capacitances at different compression (ε = 40%) states for 1000 compression cycles. (e) GCD curves of the compressible supercapacitor during 1000 compression cycles. The prepared hydrogels were ultimately assembled into flexible supercapacitor (FSC) to investigate the electrochemical performance and their potential applications in wearable devices and other light-weight, flexible portable devices.1-2 Figure 7a shows the photos of the assemble FSC. The flexible electrode was prepared by cutting the Lig/SWCNTHNO3 hydrogel into self-supported slices with a thickness of ca. 2 mm (1 × 1 cm2), soaking in 1 M Li2SO4 electrolyte and then pressing onto stainless steel (SS). The Lig/SWCNTHNO3 hydrogel films maintain 3D-network structure (Figure

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S15a,b). The electrolyte separator was prepared by soaking cellulose hydrogel in 1 M Li2SO4 for 12 h. The FSC possesses a sandwiched structure with cellulose/Li2SO4 electrolyte in-between two pieces of Lig/SWCNTHNO3 thin-film electrodes (Figure 7b). The CV curves of the FSC present a rectangular-like shape, as that in the two-electrode system, with the potential window shifted stepwise from 0 ~ 0.8 V to 0 ~ 1.3 V (Figure 7c). There is no obvious deviation of the rectangular-like shape and the low potential curve is completely included within the curve at potential window of 0 ~ 1.3 V, indicating a good capacitive characteristics.46 Figure 7d displays the GCD curves in different working windows from 0 ~ 0.8 to 0 ~ 1.3 V at the current density of 1 A g-1. The striangular-like shape is well maintained at a working windows up to 0 ~ 1.3V, which further certifies the excellent capacitive behavior, as consistent with the CV results. The FSC also shows excellent capacitance at different current densities (292 F g-1 at a current density of 0.5 A g-1), according to the related GCD curves (Figure 7e), which can be attributed to the redox reactions provided by Q/QH2 structure in Lig that contribute to high pseudocapacitance and the excellent conductivity of SWCNTs. Moreover, the FSC exhibits good rate capability, with 156 F g-1 retention at 20 A g-1 and 131 F g-1 retention at 50 A g-1 (Figure 7f). This excellent rate capability can be attributed to the stable 3D continuous conductive network with larger number of mesopores and macropores in Lig/SWCNTHNO3 and close contact between electrodes with separator.47 In addition, the FSC shows excellent electrochemical stability with 83.6% and 80.1% retention of the initial capacitance after 3000 and 10000 cycles, respectively (Figure 7g), ascribed to the

25



superior structure stability and reversible Faradaic reaction in Lig/SWCNTHNO3.48 The Ragone plot of the FSC is illustrated in Figure 7h. The FSC exhibits a highest energy density of 17.1 Wh kg-1 at a power density of 324 W kg-1, and maintains 7.69 Wh kg-1 even at an ultrahigh power density of 32512 W kg-1. The energy storage capability of the as-prepared FSC is superior to those of the FSCs based on hydrothermal graphene/Lig hydrogels (13.8 Wh kg-1, 500 W kg-1),22 graphene/graphite-paper (8.87 Wh kg-1, 178.5 W kg-1),45 hydrazine reduced graphene hydrogels (7.2 Wh kg-1, 500 W kg-1),50 hydrothermal reduced graphene hydrogels (6.1 Wh kg-1, 670 W kg-1),51 mCel-membrane based activated carbon micro-supercapacitor (4.37 Wh kg-1, 2490 W kg-1),30 nitrogen and boron co-doped graphene aerogels (8.7 Wh kg-1, 1650 W kg-1)52 and mesoporous carbon/graphene aerogels (6.2 Wh kg-1, 3545 W kg-1)53 (more details are listed in Table S3. Figure 7i shows the CV curves of FSC under various bend angles, in which the electrochemical performance is almost identical even under a large bend angle of 150°. Such excellent flexibility is mainly attributed to the outstanding mechanical stability of Lig/SWCNTHNO3 and cellulose gel separator (Figure S16). Moreover, the FSC retains 98% specific capacitance after 1000 bending cycles for 90° (Figure 7j). These encouraging results presented here further make the biomass-based FSCs a promising energy storage device for flexible wearable electronics.

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Figure 7. (a) Digital photograph of single Lig/SWCNTHNO3 flexible electrode and biomass-based FSC. (b) The schematic diagram of FSC. (c) CV at 5 mV s-1 and (d) GCD curves at 1 A g-1 of FSC under the potential window from 0~0.8 V to 0~1.4 V. (e) GCD curves at different current density and (f) the specific capacitance from 0.5 A g-1 to 50 A g-1. (g) Cycling tests of FSC at a current density of 5 A g-1 for 10000 cycles. (h) Ragone plot of FSC. (i) CV curves at 5 mV s-1 for different bending angles. (j) The specific capacitance changes within 1000 bending cycles for 90°. 4. Conclusion In summary, the construction of sustainable, environmentally friendly, low-cost biomass-based FSCs derive from renewable and earth-abundant Lig-based composite hydrogel electrodes and cellulose/Li2SO4 hydrogel electrolyte has been proposed and fulfilled for the first time. Facile fabrication processes have been achieved, in which pressure-sensitive

Lig/SWCNTHNO3

hydrogel

is

produced

through

simple

hydrothermal treatment and the cellulose hydrogel is prepared via a phase-inversion methods. The assembled symmetric FSC shows high specific capacitance (292 F g-1 at current density of 0.5 A g-1), excellent rate capability and outstanding energy density of 17.1 Wh kg-1. Furthermore, the FSC shows outstanding electrochemical stability

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with 98% capacitance retention in 1000 bending cycles for 90°. The idea of employing biomass-based materials to fabricate environmentally friendly and biocompatible FSCs, which provides prominent opportunities for the development of sustainable energy storage system. Supporting Information Three-electrode system tests, TEM of SWCNT, compression contrast of hydrogels, XPS N1s of Lig/SWCNTHNO3, different cellulose separator effects on the electrochemical properties, SEM of Lig/SWCNTHNO3 hydrogel film and cellulose hydrogel, the table of chemical composition of samples and comparison for flexible electrode materials. Notes The authors declare no competing financial interest. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51673062 and 51473049). References (1) Dong, L.; Xu, C.; Li, Y.; Pan, Z.; Liang, G.; Zhou, E.; Kang, F.; Yang, Q. Breathable and Wearable Energy Storage Based on Highly Flexible Paper Electrodes. Adv. Mater. 2016, 28, 9313-9319. (2) Wu, H; Huang, Y.; Xu, F.; Duan, Y.; Yin, Z. Energy Harvesters for Wearable and Stretchable Electronics: From Flexibility to Stretchability. Adv. Mater. 2016, 28, 9881-9919.

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Table of Contents

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High-Performance Biomass-Based Flexible Solid-State

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