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Highly compressible lignin hydrogel electrolytes via double-crosslinked strategy for superior foldable supercapacitors
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Highly compressible lignin hydrogel electrolytes via double-crosslinked strategy for superior foldable supercapacitors Tao Liu, Xinle Ren, Junmei Zhang, Jing Liu, Rongxian Ou, Chuigen Guo, Xiaoyuan Yu, Qingwen Wang **, Zhenzhen Liu * College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A novel double-crosslinked lignin hydrogel electrolyte is developed. � H2SO4 induced the lignin physical hy drophobic aggregation. � Exhibiting high compressibility and ionic conductivity, excellent shape recovery. � Excellent capacitance retention after numerous bending, or strong compression.
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Double-crosslinking High compressibility Hydrogel electrolyte Supercapacitor
Employing renewable, earth-abundant, low-cost natural materials to fabricate highly compressible, foldable and high-performance energy storage devices can greatly promote the sustainable development and wide applica tions of compression-resistant electronics. Herein, a hybrid double-crosslinked (DC) lignin hydrogel electrolyte with superior compressibility is firstly developed by postformation of lignin hydrophobic aggregation via simple treatment of single chemical crosslinked (SC) lignin hydrogel using H2SO4 solution. This synthetic DC lignin hydrogel exhibits significant improvement of mechanical strength, excellent shape recovery property and high ionic conductivity (0.08 S cm 1). The compression stress at fracture is 4.74 MPa, which is 40 times compared to that of SC lignin hydrogel. Exploiting this DC lignin hydrogel as electrolyte and PANI deposited carbon cloth as electrode, a flexible supercapacitor is constructed, which possesses high specific capacitance of 190 F g 1 and excellent energy density. Remarkably, this supercapacitor retains high specific capacitance after 500 cycle numbers of 180� bending, or 80% compression strain. Thus, this presented work opens a new avenue of lignin as a prominent candidate for potential application in compression-resistant and foldable energy storage devices.
1. Introduction
application of compression-resistant electronics [1–3], which need to endure stress without losing electrochemical performance under different compressive or bended status. Among the various types of energy storage devices, supercapacitors are attractive as their fast charge-discharge rate, high power density, long term stability and
Designing highly compressible, foldable and high-performance en ergy storage device based on renewable, low-cost, earth-abundant nat ural materials is crucial for the sustainable development and wide
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wang), [email protected] (Z. Liu). https://doi.org/10.1016/j.jpowsour.2019.227532 Received 18 September 2019; Received in revised form 23 November 2019; Accepted 27 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tao Liu, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227532
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simple configuration [4–9]. Hydrogel electrolyte, as a key component of the device, has been developed to play an important role in the con struction of flexible supercapacitors, which not only avoids the harmful leakage risk of liquid electrolyte, but also owns high ionic conductivity compared to solid polymer electrolyte [10–16]. Although some tough hydrogel electrolytes have been exploited to construct supercapacitors which also display the excellent electrochemical performance [4,5,17], the main components of the reported hydrogel electrolytes are derived from the unsustainable fossil sources, which can limit the sustainable development of electronics and increase the environmental crises, or the synthetic hydrogel electrolytes can not endure compressive stress when external forces applied, which induces the degradation of electro chemical performance for the devices. Hence, a renewable, highly compressible, eco-friendly and high ionic conductivity hydrogel elec trolyte is desirable to design. Lignin is the second most abundant natural biopolymer on earth [18, 19]. It is amorphous and present in the cell wall of plants, and generated nearly 50 million tons every year as the major byproduct of paper/pulp industry [20–22]. Owning the specific physiochemical property, source-abundant and low-cost characteristics, lignin is an economically and ecologically attractive feedstock for the manufacture of energy storage device [23–28], and also an ideal alternative to minimize our reliance on non-renewable fossil fuel sources [29–32]. However, the research of lignin in electrolyte is much limited compared to electrode study. Different approaches have been reported to construct lignin-based hydrogel [33], the lignin is used either just as the additive to copolymerize with traditional polymer [34–36] or as the precursor to be chemically modified [37–39]. And the mechanical strength of resultant high lignin content hydrogel is poor, which can not satisfy the mechanical toughness requirement of flexible energy storage devices, especially the limited strength and toughness when subjected for the large stress or strain, or for the cyclic compression. Double-crosslinked strategy via two-step network formation has been demonstrated to efficiently improve the mechanical property of hydrogel [40–43]. The rigid and brittle network serves as the sacrificial bond to effectively dissipate energy, while the soft and ductile network maintains the hydrogel integrity during the deformational process [10, 44,45]. As known, lignin possesses excellent solubility in alkaline so lution because of the phenol, carboxyl, and hydroxyl groups [46], while it’s easy to induce precipitation once the acid solution adds because of the hydrophobic aggregation. And many researchers have utilized the stepwise alkali/acid treatment to purify the lignin [19,47,48]. Inspired by the different solubility of lignin in alkaline and acid so lution and hybrid chemically physically double-crosslinked approach, we synthesized a chemically crosslinked lignin hydrogel (SC lignin hydrogel) through base-catalyzed ring-opening polymerization and crosslinking reaction, and then apply a simple acid soaking strategy to convert the SC lignin hydrogel into high-mechanical hybrid doublecrosslinked lignin hydrogel (DC lignin hydrogel) via the formation of lignin hydrophobic aggregation interaction (Scheme 1). The resultant DC lignin hydrogel shows exceptional high compressive mechanical strength (4.74 MPa) which is 40-fold superior to that of SC lignin hydrogel, and excellent cyclic loading-unloading compression perfor mance. Another great benefit of this strategy is the ionic conductivity of this kind of DC lignin hydrogel electrolyte is super high (0.08 S cm 1) which is comparable to that of pure H2SO4 solution [5]. Moreover, incorporating this synthetic DC lignin hydrogel as the electrolyte to construct supercapacitor, the device exhibits remarkable specific capacitance of 190 F g 1, excellent rate capability, and high energy density of 15.24 Wh kg 1, competitive even superior to other flexible supercapacitors. Remarkably, the capacitance of the supercapacitors can be kept well under 500 cycle numbers of 180� bending or different compression strain, showing the superior foldability and high compression-resistant performance of this device.
Scheme 1. Schematic illustration of renewable double-crosslinked lignin hydrogel formed via a sequential chemical crosslinking and physical cross linking strategy, which could be as an excellent hydrogel electrolyte to be used in highly compressible and foldable supercapacitors.
2. Experimental 2.1. Materials Lignin derived from the corncob used in this study was obtained from Shandong Longlive Bio-Technology Co., LTD, China. PEGDGE (poly (ethylene glycol) diglycidyl ether, average Mn ¼ 500) was purchased from Sigma-Aldrich. All chemical reagents were used without further purification. Carbon cloth (WOS 1009 CeTech CO. Ltd., China) was obtained from Tianjin Aiweixin Chemical Engineering Technology Co., LTD. PDMS was purchased from Hangzhou Bald Advanced Materials Co., LTD. 2.2. Characterizations 31 P NMR were recorded on a Bruker Avance 500 NMR spectrometer following a method which was described in the literature. The FTIR spectra was recorded on the PerkinElmer Spectrum 100 FT-IR spec trometer and the IRTracer-100 (SHIMADZU) in ATR mode. The compressive stress-strain measurements were performed on hydrogels using a SHIMADZU AGS-X universal tensile-compressive tester (1 kN). SEM were performed in ZEISS EVO/MA15 after sputter-coated with a thin layer of gold. The cyclic voltammetry, chronopotentiometry were carried out by CHI660E electrochemical workstation (Chenhua Shanghai), and the EIS (Electrochemical Impedance Spectroscopy) was obtained by Zahner Electrochemical Workstation (Germany). The cyclic stability of the supercapacitor was tested by BTS-4008 (Shenzhen Neware CO., LTD).
2.3.
31
P NMR analysis of lignin
31 P NMR was used to analyse the hydroxyl value in lignin according to a method in cited literature [49]. The peak signal of cyclohexanol at δ ¼ 145.1 ppm was as the internal standard. The hydroxyl value for each type of hydroxyl groups was determined by comparing the corre sponding peak area to that of the internal standard.
2
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2.4. Preparation of DC lignin hydrogels
and current density. The cyclic stability of the supercapacitors was performed at room temperature with a sweep charge and discharge rate at the current density of 3 A g 1 for 10000 cycles. The specific capaci tance (C, F g 1) of the supercapacitor was calculated from the GCD curves according to Equation (2). The energy density (E, Wh kg 1) and power density (P, W kg 1) are calculated according to Equations (3) and (4).
The DC lignin hydrogel was prepared via a two-step method. Briefly, Lignin was dried thoroughly at 60 � C under vacuum for 24 h before use. 2 g of lignin was dissolved in 1 M NaOH aqueous, and different weights of PEGDGE (1.2 g, 1.6 g, 2.0 g, 2.4 g) was added into the above solution to prepare the pre-gel solution. After heating at 50 � C for about 2 h, the SC lignin hydrogel was formed. The above synthetic SC lignin hydrogel was then immersed into 1 M H2SO4 aqueous solution thoroughly for 12 h to generate the hybrid DC lignin hydrogel. In order to completely remove the Naþ inside the hydrogel and make the Hþ and SO42 as the electrolyte ions, the soaking solution of 1 M H2SO4 aqueous was changed two times.
C¼
The compressive stress-strain tests were performed on hydrogels using a universal tensile-compressive tester. The cylindrical hydrogel samples (12 mm in diameter and 12 mm in height) was placed on the lower plate and compressed by the upper plate at a speed of 2 mm min 1. The load and displacement data were collected during the ex periments. Cyclic tests were carried out by performing subsequent trials immediately after the initial loading.
E¼
CΔU 2 2
(3)
P¼
E Δt
(4)
where C is the specific capacitance calculated from Equation (2), ΔU is the voltage after IR drop (V), Δt is the discharge time (s). 3. Results and discussions As shown in Fig. 1a, the chemical-crosslinking reaction of basecatalyzed ring-opening polymerization was carried out via phenol groups in lignin and epoxy rings of PEGDGE crosslinker (poly (ethylene
2.6. Ionic conductivity measurement The ionic conductivity of hydrogel electrolyte was measured by EIS method. The hydrogel electrolyte samples were sandwiched between two Au plates (r: 0.5 cm) and firm contact was ensured. The AC amplitude was 5 mV and the frequency range of 100 kHz to 0.01 Hz at room temperature was used. The intersection of the curve at the real part is taken as the bulk resistance of the hydrogel electrolyte (Rb, Ω), and the ionic conductivity of the sample is calculated following Equation (1). L ARb
(2)
where I is the discharge current (A), Δt is the discharge time (s), m is the total mass of PANI in two electrodes (g), ΔU is the voltage after IR drop (V).
2.5. Mechanical measurement
σ¼
IΔt mΔU
(1)
where L (cm) is the thickness of the hydrogel electrolyte and A (cm2) is the electrode area. 2.7. Preparation of PANI@CC electrode Hydrophilic carbon cloths (5*5 cm) were prepared by sequentially immersing it in concentrated H2SO4, acetone and ethanol. 912.5 μL of the aniline monomer was dissolved in 20 mL of a 1 M HClO4 aqueous solution to form Solution A. Subsequently, the pre-treated carbon cloth was placed into the above-mentioned solution. After which, 1.53 g of ammonium persulfate (APS) was dissolved in another 20 mL of a 1 M HClO4 aqueous solution to form solution B. Then the Solution B was added into Solution An under magnetic stirring at room temperature. The composite solution was put in a shaker for overnight to allow for complete PANI polymerization. After the reaction, the carbon cloth was taken out from the above solution and washed thoroughly with ethanol and DI-water to remove the residual aniline, then dried at vacuum for further test. The electrochemical property of PANI activated carbon cloths was tested by cyclic voltammetry method using three electrode systems. The Pt plate as the working electrode, and the scan rate is 20 mV s 1, while 1 M H2SO4 solution as the electrolyte.
Fig. 1. Demonstration of crosslinking mechanism. a) The schematic illustration of double-crosslinking reaction of DC lignin hydrogel by sequential chemical crosslinking polymerization and physical hydrophobic aggregation cross linking. b) FTIR spectra of starting materials of PEGDGE and lignin, and the first-network lignin hydrogel (SC-1.2 lignin hydrogel) and double-network lignin hydrogel (DC-1.2 lignin hydrogel). c) The pictures of lignin in NaOH solution before and after treatment of H2SO4. The upper pictures were recorded on bright field, while the lower were irradiated at 365 nm. d) The photographs of SC-1.2 lignin hydrogel and DC-1.2 lignin hydrogel. SEM images of SC-1.2 (e) and DC-1.2 (f) hydrogel. Scale bar is 20 μm.
2.8. Fabrication and characterization of supercapacitors The flexible supercapacitors were fabricated with DC-1.2 lignin hydrogel as electrolyte (2*0.5 cm) and PANI deposited carbon cloths as electrode, and using a PDMS film to encapsulate the device. The configuration is a sandwich-like. The cyclic voltammetry and galvano static charge-discharge curves were carried out at different scan rates 3
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glycol) diglycidyl ether, Mn ¼ 500). In order to obtain highly chemicalcrosslinked network, enzymatic hydrolysis lignin (Mw ¼ 3000–5000 Da) was chosen as the starting material because of its high content of phenol groups. Before the chemical crosslinking reaction, the hydroxyl content of lignin was firstly determined by 31P NMR spectrometer (Fig. S1). Three types of hydroxyl group are observed in the spectrum, which are aromatic, aliphatic and carboxylic. And the content of hydroxyl group for each type was determined as 4.83, 3.0 and 2.65 mmol g 1 by comparing the corresponding peak area to that of internal standard. Dried unmodified lignin was dissolved in 1 M NaOH solution, then PEGDGE was subsequently added into the above solution according to different molar ratio of phenol (PO) and epoxy (EO) groups (SC-x, x ¼ 2, 1.5, 1.2, 1 (PO/EO (n/n)). After the completed chemical crosslinking reaction at 50 � C for 2 h, the SC lignin hydrogel containing a covalent network was formed. Sequentially immersing the SC lignin hydrogel into the 1 M H2SO4 solution, protonation of unreacted phenol and carboxyl groups within lignin structure induced the formation of hy drophobic interaction among lignin chains (Fig. 1a). As a result, the simple treatment of acid solutions yielded lignin physical crosslinking, and transformed the SC lignin hydrogel into the DC lignin hydrogel. FTIR was used to study the chemical crosslinking reaction between lignin and PEGDGE and the formation of second physical crosslinking interaction (Fig. 1b). For SC and DC lignin hydrogel, the occurrence of ring-opening reaction is powerfully supported by the disappearance of the bands at 907 and 753 cm 1 which is assigned to the epoxy group of PEGDGE, while the intensity of the Ph-OH band at 1212 cm 1 becomes less apparent. The stretching vibration of hydroxyl group shows a broader band at the lower wavenumber compared to that of lignin
starting material, which accounted for the formation of secondary aliphatic hydroxyl groups in SC lignin hydrogel. In addition, the appearance of very intense bands of the SC and DC lignin hydrogel at 1082, 941 and 1259 cm 1, which were attributed to the C–O–C stretching vibration and C–C stretching vibration respectively clearly indicated the successful introduction of polyoxyethylene ether. This was also supported by the increased intensity of the bands at 2915 and 2867 cm 1, which originated from the C–H stretching of methyl and methy lene in PEGDGE. For the second-step of physical hydrophobic interac – O stretching vibration at tion, the variation bands of carboxylic acid C– 1700 cm 1 and carboxylic acid O–H scissoring vibration at 1424 cm 1 in SC lignin hydrogel shifted to 1640 cm 1 and 1380 cm 1 independently comparing to the lignin starting material, then shifted back in DC lignin hydrogel, strongly suggesting the carboxyl group transformed from carboxylic acid in lignin to the corresponding sodium carboxylate in SC lignin hydrogel, then to the original carboxylic acid after the H2SO4 soaking treatment in DC lignin hydrogel. And the similar variation shifting was also observed for the unreacted phenol groups which showed in bands at 1510 (νC– – C) and 1212 (νC-O) cm 1. Moreover, in order to further demonstrate the hydrophobic aggregation of lignin in the second physical crosslinking, the unmodified lignin was as the model to test. As shown in Fig. 1c, the lignin dissolved well in NaOH solution and also had the strong fluorescence emission under 365 nm irradiation. When adding small amount of H2SO4 solution, the clear lignin solution became turbid and its fluorescence emission also disappeared, which can demonstrate the existence of lignin hydrophobic interaction in acid solution. Meanwhile, the resultant DC lignin hydrogel shows evident shrinkage from the macroscopic picture (Fig. 1d), and also smaller pore
Fig. 2. Compressive mechanical properties of SC and DC lignin hydrogels. a) The compressive stress-strain curves of SC- (2.0, 1.5, 1.2, 1.0) and DC- (2.0, 1.5, 1.2, 1.0) hydrogels. The inset is the amplification figure of the SC lignin hydrogels. Compressed speed is 2 mm min 1. b) Photographs of the compression behavior of SC1.2 and DC-1.2 hydrogel under instrumental control. c) The images to demonstrate the flexibility of DC-1.2 hydrogel. d) Compressive stress-strain curves with a varying maximum compression strain from 65% to 95%. e) Compressive stress-strain curves for the 1st, 25th, 75th, 100th cycles at 85% strain under loadingunloading cycles. 4
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unloading compressive cycles. The above results indicated the DC lignin hydrogels exhibited excellent shape-recovery property. And the improvement of mechanical property was ascribed to that introduction of the new lignin physical crosslinking formed denser network structures with increased crosslinked density and smaller pores as compared to that of SC lignin hydrogel. The increased crosslinked density made the DC lignin hydrogel stiffer than the SC lignin hydrogel, and the smaller pore sizes benefited to avoid stress concentration and crack expanding for toughening the hydrogels. The lignin physical crosslinking network could de-crosslink and dissociate to consume energy effectively during the process of deformation, and the reversible reorganization of lignin physical network endowed the hybrid DC lignin hydrogels rapid shape recovery ability and eminent fatigue resistance. As a result, DC lignin hydrogels with both high compressive strength and toughness can be obtained through a very simple fabrication process without any requirement of additives. Except possessing high mechanical property of DC lignin hydrogel, the ionic conductivity is much more important for the hydrogel elec trolyte. The DC-1.2 lignin hydrogel was chosen as the model for hydrogel electrolyte because of its high toughness and high lignin con tent. And the ionic conductivity of hydrogel electrolyte was determined by electrochemical impedance spectroscopy (EIS). Two Au plates were placed on each side of the DC lignin hydrogel to conduct the EIS mea surement. In the Nyquist plot, the high frequency intercept on the real impedance axis is regarded as the bulk resistance (Fig. 3a), and the ionic conductivity was evaluated by varying the thickness of hydrogel and the molar concentration of H2SO4 according to Equation (1). As shown in Figs. S3 and S4, the ionic conductivity of DC lignin hydrogel was essentially the same among the different gel thickness, while the ionic conductivity was lower as decreasing the molar concentration of H2SO4. And the optimized ionic conductivity is about 0.08 S cm 1 which is comparable to that of pure H2SO4 aqueous solution electrolyte (the order of 10 1 S cm 1) and even superior to other reported literatures (Table S1) [5]. H2SO4 plays the dual-role in the second-step, which not only as the soaking solution to induce the formation of lignin hydro phobic interaction, but also as the electrolyte ionic solution to provide Hþ and SO42 . Except the utilization of high ionic conductivity of H2SO4
size and denser network as compared with that of SC lignin hydrogel (Fig. 1e), which also confirm the sequential physical crosslinking induced by lignin hydrophobic aggregation after H2SO4 treatment. In order to validate the toughen effectiveness of DC lignin hydrogel, the compressive strength was systematically evaluated. Different frac ture stress was obtained by varying the hydrogel formulation. As shown in Fig. 2a, the DC-1.2 and 1.0 lignin hydrogel reached higher compressive strength, and a significant enhancement of the stress at fracture (4.74 MPa) with a fracture strain of nearly 100% occurred, which was 40-fold superior to that of SC-1.2 lignin hydrogel. As the second-step physical crosslinking was induced by lignin hydrophobic interaction because of unreacted phenol groups and carboxylic groups, DC-1.2 lignin hydrogel exhibited higher mechanical strength compared to DC-1.0 lignin hydrogel. And the lignin content is 50% wt. within the DC-1.2 hydrogel. Meanwhile, all the DC lignin hydrogels showed much higher compressive stress than that of SC lignin hydrogels, and the mechanical stress of DC lignin hydrogel can be easily tailored by adjusting the molar ratio of phenol group to epoxy group. The incor poration of physically and chemically cross-linked domains within lignin hydrogels not only increased the stress and strain at fracture, but also allows for higher deformation and energy dissipation. As shown in Fig. 2b–c, the DC lignin hydrogel was easily foldable and quickly recovered to the original state without any rupture, while the SC lignin hydrogel was easy to rupture into small pieces under the same compressive condition, demonstrating excellent flexibility and elasticity of DC lignin hydrogels. The DC lignin hydrogels also exhibited excellent mechanical dura bility which confirmed by the loading-unloading compressive tests. These curves were obtained immediately after one loading-unloading cycle. As shown in Fig. 2d, the stress is increasing with the applied strain from 65% up to 95%. Although the hysteresis exists, the compressive stress still can return to the original state after unloading for each cycle. And no breakage or even visible cracking occurred in the DC lignin hydrogel after at least 100 successive loading-unloading compressive cycles at a set strain of 85% (Fig. 2e). A similar result can be obtained from Fig. S2, the sea horse model of DC lignin hydrogel recovered to its original shape immediately after 500 successive loading-
Fig. 3. Electrochemical performance of the fabricated supercapacitor based on DC-1.2 hydrogel electrolyte. a) Nyquist plots of DC-1.2 hydrogel and PANI@CChydrogel-PANI@CC supercapacitor which were tested by EIS. b) CV curves of the device at different scan rates in the potential window of 0–0.8 V. c) GCD curves of the device at various current densities while the charge voltage is 0.8 V. d) The value of specific capacitance varies on different current densities. e) Capacitance retention and coulombic efficiency during GCD cyclic test at a current density of 3 A g 1. f) GCD profiles of the single cell and three cells in series at a current density of 1 A g 1, the inset is the photographs of the LED lamp which powered by three cells connected in series. 5
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solution, the high ionic conductivity of the DC lignin hydrogel is ascribed to the ion-conductive carboxyl functional groups in lignin, together with abundant micropores in the hydrogel network available for the filling and free movement of sufficient ion (Fig. 1f). The superior mechanical property and high ionic conductivity guarantee this kind of DC lignin hydrogel is a good candidate for flexible hydrogel electrolyte. In order to test the potential application of DC lignin hydrogel electrolyte, a symmetrical supercapacitor was fabricated. The synthetic DC lignin hydrogel can be directly utilized as the electrolyte and sepa rator without extra treatment. A polyaniline deposited carbon cloth (PANI@CC) was prepared as the electrode because of high conductivity and flexibility [5,50]. The CV curve of PANI@CC electrode shows multiple of magnitude larger current than that of pristine carbon cloth, demonstrating the successful deposition of PANI on carbon cloth (Figs. S5a–b). To fabricate the supercapacitor, the DC lignin hydrogel was sandwiched between two PANI@CC electrodes and confirming the tight contact (Fig. S6). This fabrication was facile and used fewest number of components without any separator, binder or substrate, which is quite crucial for developing high-performance flexible super capacitors and scale-up application. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and EIS methods were used to evaluate the electrochemical performance of this integrated device. Fig. 3b displays that the CV curves show the increased curves area with scan rate increasing, indicating a good capacitive behavior of this device, and the shape of CV curves are resistant with that of reported PANI-based supercapacitors [4,50]. Meanwhile, the GCD curves show a typical charge-discharge pattern for PANI-based supercapacitors (Fig. 3c) [4,50]. The specific capacitance was calculated according to Equation (2) (Fig. 3d). When the current density is increased from 0.25 to 4 A g 1, the specific capacitance re mains at a high level above 100 F g 1 upon the charge-discharge cycles, indicating an excellent rate capability. Remarkably, the maximum value of specific capacitance reaches 190 F g 1 at a current density of 0.25 A g 1 which is competitive even superior to that of previously reported
flexible supercapacitor based on hydrogel electrolytes (Table S1). Moreover, the Nyquist plot of the device shows an inconspicuous arc in the high frequency region and a steep straight line in the low frequency region (Fig. 3a), reflecting the typical capacitor behavior. The energy and power density of this flexible supercapacitor based on DC linin hydrogel electrolyte were calculated from GCD data (Equation (3) and (4)). As shown in the Ragone plot (Fig. S7), our device achieves a maximum energy density of 15.24 Wh kg 1 at a power density of 95 W kg 1 and a maximum power density of 2157.3 W kg 1 at an energy density of 9.35 Wh kg 1, which shows competitive and even greater performance comparable to that of the current flexible super capacitors based on hydrogel electrolytes (Table S1). In addition, this device provides a good cycling stability with 91% capacitance retention after 10000 GCD cycles at a current density of 3 A g 1, and the coulombic efficiency remains close to 100% during the long-term GCD cyclic test (Fig. 3e). The excellent compatibility of lignin hydrogel electrolyte and PANI@CC electrode, and the reduced interface contact resistance from the less configuration layers of SCs, are both helpful for the high electrochemical performance. Variable working window and flexibility are crucial for modern portable and wearable energy storage devices. As shown in Fig. 3f, the operating voltage of three cells in series was three times of a single cell under the same current density, and a LED lamp (2–2.2 V) was powered by three cells in series connection for a few minutes (inset of Fig. 3f and Movie S1), indicating the working potential window of this device can freely tailored by adjusting the numbers of series-connected cells. To verify the flexibility of this kind of supercapacitor, the compressive bending and folding were applied to the device and exploiting CV and GCD test to evaluate the electrochemical performances. From the results of CV (Fig. 4a) and GCD (Fig. 4b) curves, the device can still work well after different deformations, no degradation of electrochemical perfor mance. Remarkably, the capacitance retention was nearly 100% after 500 cycle numbers at 180� bending (Fig. 4c), and 85% capacitance was still displayed as the compression strain from 0% to 80% (Fig. 4d). The
Fig. 4. Mechanical durability and electrochemical stability of the fabricated supercapacitor based on DC-1.2 hydrogel electrolyte. Comparing CV (a, 75 mV s 1) and GCD (b, 2 A g 1) curves of the device under different mechanical status (flat, compressed at 500 g, folded at 180� bending). Capacitance retention of the super capacitor after different cycle numbers at 180� bending (c) and under different compression strain (d). 6
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loss of capacitance maybe ascribed to the release of H2SO4 solution contained in the hydrogel electrolyte during the strong compression strain applied to the device. Such stable electrochemical performance and mechanical durability is ascribed to the structure stability of DC lignin hydrogel electrolyte and the synergistic effect of hydrogel elec trolyte with PANI@CC electrode. These results demonstrate that this kind of foldable supercapacitor based on DC lignin hydrogel electrolyte possesses tailored working potential window, and mechanical and electrochemical robustness in mechanically extreme conditions.
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4. Conclusions In summary, we reported a facile and feasible method to construct hybrid DC lignin hydrogel electrolyte by postformation of lignin hy drophobic physical crosslinking via simple treatment of the SC lignin hydrogels using H2SO4 solutions. The hybrid DC lignin hydrogel elec trolyte displayed a significant improvement of compressive stress at fracture, higher deformation, remarkable shape recovery property and high ionic conductivity. The fabricated supercapacitor based on this DC lignin hydrogel electrolyte, exhibited superior electrochemical perfor mance of high specific capacitance, excellent rate capability and high energy density. Remarkably, the electrochemical performance of supercapacitors were well conserved under 500 cycle numbers of 180� bending or different compressive strain. This simple soaking strategy opens a new way to fabricate highly compressible lignin hydrogel electrolyte, and provides a promising and new direction for developing compressible and foldable energy storage device in a sustainable, lowcost, and eco-friendly way. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Natural Science Foundation of China [grant numbers 51903093, 31741022]; Natural Science Foundation of Guangdong Province [grant number 2018A030310146], Guangzhou Science and Technology Project [grant number 201905010005], Project of Key Disciplines of Forestry Engi neering of Bureau of Education of Guangzhou Municipality, National College Students Innovation and Entrepreneurship Training Program (201910564026), and Guangzhou Science and Technology Planning Project [grant number 201704030022]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227532. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2014) 19. [2] H. Wang, Y. Yang, L. Guo, Nature-inspired electrochemical energy-storage materials and devices, Adv. Energy Mater. 7 (2017) 1601709. [3] A. Mukhopadhyay, Y. Jiao, R. Katahira, P.N. Ciesielski, M. Himmel, H. Zhu, Heavy metal-free tannin from bark for sustainable energy storage, Nano Lett. 17 (2017) 7897–7907. [4] W. Li, F. Gao, X. Wang, N. Zhang, M. Ma, Strong and robust polyaniline-based supramolecular hydrogels for flexible supercapacitors, Angew. Chem. Int. Ed. 128 (2016) 9342–9347. [5] K. Wang, X. Zhang, C. Li, X. Sun, Q. Meng, Y. Ma, Z. Wei, Chemically crosslinked hydrogel film leads to integrated flexible supercapacitors with superior performance, Adv. Mater. 27 (2015) 7451–7457. [6] X. Liu, D. Wu, H. Wang, Q. Wang, Self-recovering tough gel electrolyte with adjustable supercapacitor performance, Adv. Mater. 26 (2014) 4370–4375.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly compressible lignin hydrogel electrolytes via double-crosslinked strategy for superior foldable supercapacitors Tao Liu, Xinle Ren, Junmei Zhang, Jing Liu, Rongxian Ou, Chuigen Guo, Xiaoyuan Yu, Qingwen Wang **, Zhenzhen Liu * College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A novel double-crosslinked lignin hydrogel electrolyte is developed. � H2SO4 induced the lignin physical hy drophobic aggregation. � Exhibiting high compressibility and ionic conductivity, excellent shape recovery. � Excellent capacitance retention after numerous bending, or strong compression.
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Double-crosslinking High compressibility Hydrogel electrolyte Supercapacitor
Employing renewable, earth-abundant, low-cost natural materials to fabricate highly compressible, foldable and high-performance energy storage devices can greatly promote the sustainable development and wide applica tions of compression-resistant electronics. Herein, a hybrid double-crosslinked (DC) lignin hydrogel electrolyte with superior compressibility is firstly developed by postformation of lignin hydrophobic aggregation via simple treatment of single chemical crosslinked (SC) lignin hydrogel using H2SO4 solution. This synthetic DC lignin hydrogel exhibits significant improvement of mechanical strength, excellent shape recovery property and high ionic conductivity (0.08 S cm 1). The compression stress at fracture is 4.74 MPa, which is 40 times compared to that of SC lignin hydrogel. Exploiting this DC lignin hydrogel as electrolyte and PANI deposited carbon cloth as electrode, a flexible supercapacitor is constructed, which possesses high specific capacitance of 190 F g 1 and excellent energy density. Remarkably, this supercapacitor retains high specific capacitance after 500 cycle numbers of 180� bending, or 80% compression strain. Thus, this presented work opens a new avenue of lignin as a prominent candidate for potential application in compression-resistant and foldable energy storage devices.
1. Introduction
application of compression-resistant electronics [1–3], which need to endure stress without losing electrochemical performance under different compressive or bended status. Among the various types of energy storage devices, supercapacitors are attractive as their fast charge-discharge rate, high power density, long term stability and
Designing highly compressible, foldable and high-performance en ergy storage device based on renewable, low-cost, earth-abundant nat ural materials is crucial for the sustainable development and wide
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wang), [email protected] (Z. Liu). https://doi.org/10.1016/j.jpowsour.2019.227532 Received 18 September 2019; Received in revised form 23 November 2019; Accepted 27 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tao Liu, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227532
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simple configuration [4–9]. Hydrogel electrolyte, as a key component of the device, has been developed to play an important role in the con struction of flexible supercapacitors, which not only avoids the harmful leakage risk of liquid electrolyte, but also owns high ionic conductivity compared to solid polymer electrolyte [10–16]. Although some tough hydrogel electrolytes have been exploited to construct supercapacitors which also display the excellent electrochemical performance [4,5,17], the main components of the reported hydrogel electrolytes are derived from the unsustainable fossil sources, which can limit the sustainable development of electronics and increase the environmental crises, or the synthetic hydrogel electrolytes can not endure compressive stress when external forces applied, which induces the degradation of electro chemical performance for the devices. Hence, a renewable, highly compressible, eco-friendly and high ionic conductivity hydrogel elec trolyte is desirable to design. Lignin is the second most abundant natural biopolymer on earth [18, 19]. It is amorphous and present in the cell wall of plants, and generated nearly 50 million tons every year as the major byproduct of paper/pulp industry [20–22]. Owning the specific physiochemical property, source-abundant and low-cost characteristics, lignin is an economically and ecologically attractive feedstock for the manufacture of energy storage device [23–28], and also an ideal alternative to minimize our reliance on non-renewable fossil fuel sources [29–32]. However, the research of lignin in electrolyte is much limited compared to electrode study. Different approaches have been reported to construct lignin-based hydrogel [33], the lignin is used either just as the additive to copolymerize with traditional polymer [34–36] or as the precursor to be chemically modified [37–39]. And the mechanical strength of resultant high lignin content hydrogel is poor, which can not satisfy the mechanical toughness requirement of flexible energy storage devices, especially the limited strength and toughness when subjected for the large stress or strain, or for the cyclic compression. Double-crosslinked strategy via two-step network formation has been demonstrated to efficiently improve the mechanical property of hydrogel [40–43]. The rigid and brittle network serves as the sacrificial bond to effectively dissipate energy, while the soft and ductile network maintains the hydrogel integrity during the deformational process [10, 44,45]. As known, lignin possesses excellent solubility in alkaline so lution because of the phenol, carboxyl, and hydroxyl groups [46], while it’s easy to induce precipitation once the acid solution adds because of the hydrophobic aggregation. And many researchers have utilized the stepwise alkali/acid treatment to purify the lignin [19,47,48]. Inspired by the different solubility of lignin in alkaline and acid so lution and hybrid chemically physically double-crosslinked approach, we synthesized a chemically crosslinked lignin hydrogel (SC lignin hydrogel) through base-catalyzed ring-opening polymerization and crosslinking reaction, and then apply a simple acid soaking strategy to convert the SC lignin hydrogel into high-mechanical hybrid doublecrosslinked lignin hydrogel (DC lignin hydrogel) via the formation of lignin hydrophobic aggregation interaction (Scheme 1). The resultant DC lignin hydrogel shows exceptional high compressive mechanical strength (4.74 MPa) which is 40-fold superior to that of SC lignin hydrogel, and excellent cyclic loading-unloading compression perfor mance. Another great benefit of this strategy is the ionic conductivity of this kind of DC lignin hydrogel electrolyte is super high (0.08 S cm 1) which is comparable to that of pure H2SO4 solution [5]. Moreover, incorporating this synthetic DC lignin hydrogel as the electrolyte to construct supercapacitor, the device exhibits remarkable specific capacitance of 190 F g 1, excellent rate capability, and high energy density of 15.24 Wh kg 1, competitive even superior to other flexible supercapacitors. Remarkably, the capacitance of the supercapacitors can be kept well under 500 cycle numbers of 180� bending or different compression strain, showing the superior foldability and high compression-resistant performance of this device.
Scheme 1. Schematic illustration of renewable double-crosslinked lignin hydrogel formed via a sequential chemical crosslinking and physical cross linking strategy, which could be as an excellent hydrogel electrolyte to be used in highly compressible and foldable supercapacitors.
2. Experimental 2.1. Materials Lignin derived from the corncob used in this study was obtained from Shandong Longlive Bio-Technology Co., LTD, China. PEGDGE (poly (ethylene glycol) diglycidyl ether, average Mn ¼ 500) was purchased from Sigma-Aldrich. All chemical reagents were used without further purification. Carbon cloth (WOS 1009 CeTech CO. Ltd., China) was obtained from Tianjin Aiweixin Chemical Engineering Technology Co., LTD. PDMS was purchased from Hangzhou Bald Advanced Materials Co., LTD. 2.2. Characterizations 31 P NMR were recorded on a Bruker Avance 500 NMR spectrometer following a method which was described in the literature. The FTIR spectra was recorded on the PerkinElmer Spectrum 100 FT-IR spec trometer and the IRTracer-100 (SHIMADZU) in ATR mode. The compressive stress-strain measurements were performed on hydrogels using a SHIMADZU AGS-X universal tensile-compressive tester (1 kN). SEM were performed in ZEISS EVO/MA15 after sputter-coated with a thin layer of gold. The cyclic voltammetry, chronopotentiometry were carried out by CHI660E electrochemical workstation (Chenhua Shanghai), and the EIS (Electrochemical Impedance Spectroscopy) was obtained by Zahner Electrochemical Workstation (Germany). The cyclic stability of the supercapacitor was tested by BTS-4008 (Shenzhen Neware CO., LTD).
2.3.
31
P NMR analysis of lignin
31 P NMR was used to analyse the hydroxyl value in lignin according to a method in cited literature [49]. The peak signal of cyclohexanol at δ ¼ 145.1 ppm was as the internal standard. The hydroxyl value for each type of hydroxyl groups was determined by comparing the corre sponding peak area to that of the internal standard.
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2.4. Preparation of DC lignin hydrogels
and current density. The cyclic stability of the supercapacitors was performed at room temperature with a sweep charge and discharge rate at the current density of 3 A g 1 for 10000 cycles. The specific capaci tance (C, F g 1) of the supercapacitor was calculated from the GCD curves according to Equation (2). The energy density (E, Wh kg 1) and power density (P, W kg 1) are calculated according to Equations (3) and (4).
The DC lignin hydrogel was prepared via a two-step method. Briefly, Lignin was dried thoroughly at 60 � C under vacuum for 24 h before use. 2 g of lignin was dissolved in 1 M NaOH aqueous, and different weights of PEGDGE (1.2 g, 1.6 g, 2.0 g, 2.4 g) was added into the above solution to prepare the pre-gel solution. After heating at 50 � C for about 2 h, the SC lignin hydrogel was formed. The above synthetic SC lignin hydrogel was then immersed into 1 M H2SO4 aqueous solution thoroughly for 12 h to generate the hybrid DC lignin hydrogel. In order to completely remove the Naþ inside the hydrogel and make the Hþ and SO42 as the electrolyte ions, the soaking solution of 1 M H2SO4 aqueous was changed two times.
C¼
The compressive stress-strain tests were performed on hydrogels using a universal tensile-compressive tester. The cylindrical hydrogel samples (12 mm in diameter and 12 mm in height) was placed on the lower plate and compressed by the upper plate at a speed of 2 mm min 1. The load and displacement data were collected during the ex periments. Cyclic tests were carried out by performing subsequent trials immediately after the initial loading.
E¼
CΔU 2 2
(3)
P¼
E Δt
(4)
where C is the specific capacitance calculated from Equation (2), ΔU is the voltage after IR drop (V), Δt is the discharge time (s). 3. Results and discussions As shown in Fig. 1a, the chemical-crosslinking reaction of basecatalyzed ring-opening polymerization was carried out via phenol groups in lignin and epoxy rings of PEGDGE crosslinker (poly (ethylene
2.6. Ionic conductivity measurement The ionic conductivity of hydrogel electrolyte was measured by EIS method. The hydrogel electrolyte samples were sandwiched between two Au plates (r: 0.5 cm) and firm contact was ensured. The AC amplitude was 5 mV and the frequency range of 100 kHz to 0.01 Hz at room temperature was used. The intersection of the curve at the real part is taken as the bulk resistance of the hydrogel electrolyte (Rb, Ω), and the ionic conductivity of the sample is calculated following Equation (1). L ARb
(2)
where I is the discharge current (A), Δt is the discharge time (s), m is the total mass of PANI in two electrodes (g), ΔU is the voltage after IR drop (V).
2.5. Mechanical measurement
σ¼
IΔt mΔU
(1)
where L (cm) is the thickness of the hydrogel electrolyte and A (cm2) is the electrode area. 2.7. Preparation of PANI@CC electrode Hydrophilic carbon cloths (5*5 cm) were prepared by sequentially immersing it in concentrated H2SO4, acetone and ethanol. 912.5 μL of the aniline monomer was dissolved in 20 mL of a 1 M HClO4 aqueous solution to form Solution A. Subsequently, the pre-treated carbon cloth was placed into the above-mentioned solution. After which, 1.53 g of ammonium persulfate (APS) was dissolved in another 20 mL of a 1 M HClO4 aqueous solution to form solution B. Then the Solution B was added into Solution An under magnetic stirring at room temperature. The composite solution was put in a shaker for overnight to allow for complete PANI polymerization. After the reaction, the carbon cloth was taken out from the above solution and washed thoroughly with ethanol and DI-water to remove the residual aniline, then dried at vacuum for further test. The electrochemical property of PANI activated carbon cloths was tested by cyclic voltammetry method using three electrode systems. The Pt plate as the working electrode, and the scan rate is 20 mV s 1, while 1 M H2SO4 solution as the electrolyte.
Fig. 1. Demonstration of crosslinking mechanism. a) The schematic illustration of double-crosslinking reaction of DC lignin hydrogel by sequential chemical crosslinking polymerization and physical hydrophobic aggregation cross linking. b) FTIR spectra of starting materials of PEGDGE and lignin, and the first-network lignin hydrogel (SC-1.2 lignin hydrogel) and double-network lignin hydrogel (DC-1.2 lignin hydrogel). c) The pictures of lignin in NaOH solution before and after treatment of H2SO4. The upper pictures were recorded on bright field, while the lower were irradiated at 365 nm. d) The photographs of SC-1.2 lignin hydrogel and DC-1.2 lignin hydrogel. SEM images of SC-1.2 (e) and DC-1.2 (f) hydrogel. Scale bar is 20 μm.
2.8. Fabrication and characterization of supercapacitors The flexible supercapacitors were fabricated with DC-1.2 lignin hydrogel as electrolyte (2*0.5 cm) and PANI deposited carbon cloths as electrode, and using a PDMS film to encapsulate the device. The configuration is a sandwich-like. The cyclic voltammetry and galvano static charge-discharge curves were carried out at different scan rates 3
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glycol) diglycidyl ether, Mn ¼ 500). In order to obtain highly chemicalcrosslinked network, enzymatic hydrolysis lignin (Mw ¼ 3000–5000 Da) was chosen as the starting material because of its high content of phenol groups. Before the chemical crosslinking reaction, the hydroxyl content of lignin was firstly determined by 31P NMR spectrometer (Fig. S1). Three types of hydroxyl group are observed in the spectrum, which are aromatic, aliphatic and carboxylic. And the content of hydroxyl group for each type was determined as 4.83, 3.0 and 2.65 mmol g 1 by comparing the corresponding peak area to that of internal standard. Dried unmodified lignin was dissolved in 1 M NaOH solution, then PEGDGE was subsequently added into the above solution according to different molar ratio of phenol (PO) and epoxy (EO) groups (SC-x, x ¼ 2, 1.5, 1.2, 1 (PO/EO (n/n)). After the completed chemical crosslinking reaction at 50 � C for 2 h, the SC lignin hydrogel containing a covalent network was formed. Sequentially immersing the SC lignin hydrogel into the 1 M H2SO4 solution, protonation of unreacted phenol and carboxyl groups within lignin structure induced the formation of hy drophobic interaction among lignin chains (Fig. 1a). As a result, the simple treatment of acid solutions yielded lignin physical crosslinking, and transformed the SC lignin hydrogel into the DC lignin hydrogel. FTIR was used to study the chemical crosslinking reaction between lignin and PEGDGE and the formation of second physical crosslinking interaction (Fig. 1b). For SC and DC lignin hydrogel, the occurrence of ring-opening reaction is powerfully supported by the disappearance of the bands at 907 and 753 cm 1 which is assigned to the epoxy group of PEGDGE, while the intensity of the Ph-OH band at 1212 cm 1 becomes less apparent. The stretching vibration of hydroxyl group shows a broader band at the lower wavenumber compared to that of lignin
starting material, which accounted for the formation of secondary aliphatic hydroxyl groups in SC lignin hydrogel. In addition, the appearance of very intense bands of the SC and DC lignin hydrogel at 1082, 941 and 1259 cm 1, which were attributed to the C–O–C stretching vibration and C–C stretching vibration respectively clearly indicated the successful introduction of polyoxyethylene ether. This was also supported by the increased intensity of the bands at 2915 and 2867 cm 1, which originated from the C–H stretching of methyl and methy lene in PEGDGE. For the second-step of physical hydrophobic interac – O stretching vibration at tion, the variation bands of carboxylic acid C– 1700 cm 1 and carboxylic acid O–H scissoring vibration at 1424 cm 1 in SC lignin hydrogel shifted to 1640 cm 1 and 1380 cm 1 independently comparing to the lignin starting material, then shifted back in DC lignin hydrogel, strongly suggesting the carboxyl group transformed from carboxylic acid in lignin to the corresponding sodium carboxylate in SC lignin hydrogel, then to the original carboxylic acid after the H2SO4 soaking treatment in DC lignin hydrogel. And the similar variation shifting was also observed for the unreacted phenol groups which showed in bands at 1510 (νC– – C) and 1212 (νC-O) cm 1. Moreover, in order to further demonstrate the hydrophobic aggregation of lignin in the second physical crosslinking, the unmodified lignin was as the model to test. As shown in Fig. 1c, the lignin dissolved well in NaOH solution and also had the strong fluorescence emission under 365 nm irradiation. When adding small amount of H2SO4 solution, the clear lignin solution became turbid and its fluorescence emission also disappeared, which can demonstrate the existence of lignin hydrophobic interaction in acid solution. Meanwhile, the resultant DC lignin hydrogel shows evident shrinkage from the macroscopic picture (Fig. 1d), and also smaller pore
Fig. 2. Compressive mechanical properties of SC and DC lignin hydrogels. a) The compressive stress-strain curves of SC- (2.0, 1.5, 1.2, 1.0) and DC- (2.0, 1.5, 1.2, 1.0) hydrogels. The inset is the amplification figure of the SC lignin hydrogels. Compressed speed is 2 mm min 1. b) Photographs of the compression behavior of SC1.2 and DC-1.2 hydrogel under instrumental control. c) The images to demonstrate the flexibility of DC-1.2 hydrogel. d) Compressive stress-strain curves with a varying maximum compression strain from 65% to 95%. e) Compressive stress-strain curves for the 1st, 25th, 75th, 100th cycles at 85% strain under loadingunloading cycles. 4
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unloading compressive cycles. The above results indicated the DC lignin hydrogels exhibited excellent shape-recovery property. And the improvement of mechanical property was ascribed to that introduction of the new lignin physical crosslinking formed denser network structures with increased crosslinked density and smaller pores as compared to that of SC lignin hydrogel. The increased crosslinked density made the DC lignin hydrogel stiffer than the SC lignin hydrogel, and the smaller pore sizes benefited to avoid stress concentration and crack expanding for toughening the hydrogels. The lignin physical crosslinking network could de-crosslink and dissociate to consume energy effectively during the process of deformation, and the reversible reorganization of lignin physical network endowed the hybrid DC lignin hydrogels rapid shape recovery ability and eminent fatigue resistance. As a result, DC lignin hydrogels with both high compressive strength and toughness can be obtained through a very simple fabrication process without any requirement of additives. Except possessing high mechanical property of DC lignin hydrogel, the ionic conductivity is much more important for the hydrogel elec trolyte. The DC-1.2 lignin hydrogel was chosen as the model for hydrogel electrolyte because of its high toughness and high lignin con tent. And the ionic conductivity of hydrogel electrolyte was determined by electrochemical impedance spectroscopy (EIS). Two Au plates were placed on each side of the DC lignin hydrogel to conduct the EIS mea surement. In the Nyquist plot, the high frequency intercept on the real impedance axis is regarded as the bulk resistance (Fig. 3a), and the ionic conductivity was evaluated by varying the thickness of hydrogel and the molar concentration of H2SO4 according to Equation (1). As shown in Figs. S3 and S4, the ionic conductivity of DC lignin hydrogel was essentially the same among the different gel thickness, while the ionic conductivity was lower as decreasing the molar concentration of H2SO4. And the optimized ionic conductivity is about 0.08 S cm 1 which is comparable to that of pure H2SO4 aqueous solution electrolyte (the order of 10 1 S cm 1) and even superior to other reported literatures (Table S1) [5]. H2SO4 plays the dual-role in the second-step, which not only as the soaking solution to induce the formation of lignin hydro phobic interaction, but also as the electrolyte ionic solution to provide Hþ and SO42 . Except the utilization of high ionic conductivity of H2SO4
size and denser network as compared with that of SC lignin hydrogel (Fig. 1e), which also confirm the sequential physical crosslinking induced by lignin hydrophobic aggregation after H2SO4 treatment. In order to validate the toughen effectiveness of DC lignin hydrogel, the compressive strength was systematically evaluated. Different frac ture stress was obtained by varying the hydrogel formulation. As shown in Fig. 2a, the DC-1.2 and 1.0 lignin hydrogel reached higher compressive strength, and a significant enhancement of the stress at fracture (4.74 MPa) with a fracture strain of nearly 100% occurred, which was 40-fold superior to that of SC-1.2 lignin hydrogel. As the second-step physical crosslinking was induced by lignin hydrophobic interaction because of unreacted phenol groups and carboxylic groups, DC-1.2 lignin hydrogel exhibited higher mechanical strength compared to DC-1.0 lignin hydrogel. And the lignin content is 50% wt. within the DC-1.2 hydrogel. Meanwhile, all the DC lignin hydrogels showed much higher compressive stress than that of SC lignin hydrogels, and the mechanical stress of DC lignin hydrogel can be easily tailored by adjusting the molar ratio of phenol group to epoxy group. The incor poration of physically and chemically cross-linked domains within lignin hydrogels not only increased the stress and strain at fracture, but also allows for higher deformation and energy dissipation. As shown in Fig. 2b–c, the DC lignin hydrogel was easily foldable and quickly recovered to the original state without any rupture, while the SC lignin hydrogel was easy to rupture into small pieces under the same compressive condition, demonstrating excellent flexibility and elasticity of DC lignin hydrogels. The DC lignin hydrogels also exhibited excellent mechanical dura bility which confirmed by the loading-unloading compressive tests. These curves were obtained immediately after one loading-unloading cycle. As shown in Fig. 2d, the stress is increasing with the applied strain from 65% up to 95%. Although the hysteresis exists, the compressive stress still can return to the original state after unloading for each cycle. And no breakage or even visible cracking occurred in the DC lignin hydrogel after at least 100 successive loading-unloading compressive cycles at a set strain of 85% (Fig. 2e). A similar result can be obtained from Fig. S2, the sea horse model of DC lignin hydrogel recovered to its original shape immediately after 500 successive loading-
Fig. 3. Electrochemical performance of the fabricated supercapacitor based on DC-1.2 hydrogel electrolyte. a) Nyquist plots of DC-1.2 hydrogel and PANI@CChydrogel-PANI@CC supercapacitor which were tested by EIS. b) CV curves of the device at different scan rates in the potential window of 0–0.8 V. c) GCD curves of the device at various current densities while the charge voltage is 0.8 V. d) The value of specific capacitance varies on different current densities. e) Capacitance retention and coulombic efficiency during GCD cyclic test at a current density of 3 A g 1. f) GCD profiles of the single cell and three cells in series at a current density of 1 A g 1, the inset is the photographs of the LED lamp which powered by three cells connected in series. 5
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solution, the high ionic conductivity of the DC lignin hydrogel is ascribed to the ion-conductive carboxyl functional groups in lignin, together with abundant micropores in the hydrogel network available for the filling and free movement of sufficient ion (Fig. 1f). The superior mechanical property and high ionic conductivity guarantee this kind of DC lignin hydrogel is a good candidate for flexible hydrogel electrolyte. In order to test the potential application of DC lignin hydrogel electrolyte, a symmetrical supercapacitor was fabricated. The synthetic DC lignin hydrogel can be directly utilized as the electrolyte and sepa rator without extra treatment. A polyaniline deposited carbon cloth (PANI@CC) was prepared as the electrode because of high conductivity and flexibility [5,50]. The CV curve of PANI@CC electrode shows multiple of magnitude larger current than that of pristine carbon cloth, demonstrating the successful deposition of PANI on carbon cloth (Figs. S5a–b). To fabricate the supercapacitor, the DC lignin hydrogel was sandwiched between two PANI@CC electrodes and confirming the tight contact (Fig. S6). This fabrication was facile and used fewest number of components without any separator, binder or substrate, which is quite crucial for developing high-performance flexible super capacitors and scale-up application. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and EIS methods were used to evaluate the electrochemical performance of this integrated device. Fig. 3b displays that the CV curves show the increased curves area with scan rate increasing, indicating a good capacitive behavior of this device, and the shape of CV curves are resistant with that of reported PANI-based supercapacitors [4,50]. Meanwhile, the GCD curves show a typical charge-discharge pattern for PANI-based supercapacitors (Fig. 3c) [4,50]. The specific capacitance was calculated according to Equation (2) (Fig. 3d). When the current density is increased from 0.25 to 4 A g 1, the specific capacitance re mains at a high level above 100 F g 1 upon the charge-discharge cycles, indicating an excellent rate capability. Remarkably, the maximum value of specific capacitance reaches 190 F g 1 at a current density of 0.25 A g 1 which is competitive even superior to that of previously reported
flexible supercapacitor based on hydrogel electrolytes (Table S1). Moreover, the Nyquist plot of the device shows an inconspicuous arc in the high frequency region and a steep straight line in the low frequency region (Fig. 3a), reflecting the typical capacitor behavior. The energy and power density of this flexible supercapacitor based on DC linin hydrogel electrolyte were calculated from GCD data (Equation (3) and (4)). As shown in the Ragone plot (Fig. S7), our device achieves a maximum energy density of 15.24 Wh kg 1 at a power density of 95 W kg 1 and a maximum power density of 2157.3 W kg 1 at an energy density of 9.35 Wh kg 1, which shows competitive and even greater performance comparable to that of the current flexible super capacitors based on hydrogel electrolytes (Table S1). In addition, this device provides a good cycling stability with 91% capacitance retention after 10000 GCD cycles at a current density of 3 A g 1, and the coulombic efficiency remains close to 100% during the long-term GCD cyclic test (Fig. 3e). The excellent compatibility of lignin hydrogel electrolyte and PANI@CC electrode, and the reduced interface contact resistance from the less configuration layers of SCs, are both helpful for the high electrochemical performance. Variable working window and flexibility are crucial for modern portable and wearable energy storage devices. As shown in Fig. 3f, the operating voltage of three cells in series was three times of a single cell under the same current density, and a LED lamp (2–2.2 V) was powered by three cells in series connection for a few minutes (inset of Fig. 3f and Movie S1), indicating the working potential window of this device can freely tailored by adjusting the numbers of series-connected cells. To verify the flexibility of this kind of supercapacitor, the compressive bending and folding were applied to the device and exploiting CV and GCD test to evaluate the electrochemical performances. From the results of CV (Fig. 4a) and GCD (Fig. 4b) curves, the device can still work well after different deformations, no degradation of electrochemical perfor mance. Remarkably, the capacitance retention was nearly 100% after 500 cycle numbers at 180� bending (Fig. 4c), and 85% capacitance was still displayed as the compression strain from 0% to 80% (Fig. 4d). The
Fig. 4. Mechanical durability and electrochemical stability of the fabricated supercapacitor based on DC-1.2 hydrogel electrolyte. Comparing CV (a, 75 mV s 1) and GCD (b, 2 A g 1) curves of the device under different mechanical status (flat, compressed at 500 g, folded at 180� bending). Capacitance retention of the super capacitor after different cycle numbers at 180� bending (c) and under different compression strain (d). 6
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loss of capacitance maybe ascribed to the release of H2SO4 solution contained in the hydrogel electrolyte during the strong compression strain applied to the device. Such stable electrochemical performance and mechanical durability is ascribed to the structure stability of DC lignin hydrogel electrolyte and the synergistic effect of hydrogel elec trolyte with PANI@CC electrode. These results demonstrate that this kind of foldable supercapacitor based on DC lignin hydrogel electrolyte possesses tailored working potential window, and mechanical and electrochemical robustness in mechanically extreme conditions.
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4. Conclusions In summary, we reported a facile and feasible method to construct hybrid DC lignin hydrogel electrolyte by postformation of lignin hy drophobic physical crosslinking via simple treatment of the SC lignin hydrogels using H2SO4 solutions. The hybrid DC lignin hydrogel elec trolyte displayed a significant improvement of compressive stress at fracture, higher deformation, remarkable shape recovery property and high ionic conductivity. The fabricated supercapacitor based on this DC lignin hydrogel electrolyte, exhibited superior electrochemical perfor mance of high specific capacitance, excellent rate capability and high energy density. Remarkably, the electrochemical performance of supercapacitors were well conserved under 500 cycle numbers of 180� bending or different compressive strain. This simple soaking strategy opens a new way to fabricate highly compressible lignin hydrogel electrolyte, and provides a promising and new direction for developing compressible and foldable energy storage device in a sustainable, lowcost, and eco-friendly way. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Natural Science Foundation of China [grant numbers 51903093, 31741022]; Natural Science Foundation of Guangdong Province [grant number 2018A030310146], Guangzhou Science and Technology Project [grant number 201905010005], Project of Key Disciplines of Forestry Engi neering of Bureau of Education of Guangzhou Municipality, National College Students Innovation and Entrepreneurship Training Program (201910564026), and Guangzhou Science and Technology Planning Project [grant number 201704030022]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227532. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2014) 19. [2] H. Wang, Y. Yang, L. Guo, Nature-inspired electrochemical energy-storage materials and devices, Adv. Energy Mater. 7 (2017) 1601709. [3] A. Mukhopadhyay, Y. Jiao, R. Katahira, P.N. Ciesielski, M. Himmel, H. Zhu, Heavy metal-free tannin from bark for sustainable energy storage, Nano Lett. 17 (2017) 7897–7907. [4] W. Li, F. Gao, X. Wang, N. Zhang, M. Ma, Strong and robust polyaniline-based supramolecular hydrogels for flexible supercapacitors, Angew. Chem. Int. Ed. 128 (2016) 9342–9347. [5] K. Wang, X. Zhang, C. Li, X. Sun, Q. Meng, Y. Ma, Z. Wei, Chemically crosslinked hydrogel film leads to integrated flexible supercapacitors with superior performance, Adv. Mater. 27 (2015) 7451–7457. [6] X. Liu, D. Wu, H. Wang, Q. Wang, Self-recovering tough gel electrolyte with adjustable supercapacitor performance, Adv. Mater. 26 (2014) 4370–4375.
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