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Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors
Accepted Manuscript Title: Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors Authors: Xiongwei Zhong, Jun Tang, Lujie Cao, Weiguang Kong, Zheng Sun, Hua Cheng, Zhouguang Lu, Hui Pan, Baomin Xu PII: DOI: Reference:
S0013-4686(17)31106-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.110 EA 29545
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
10-3-2017 16-5-2017 17-5-2017
Please cite this article as: Xiongwei Zhong, Jun Tang, Lujie Cao, Weiguang Kong, Zheng Sun, Hua Cheng, Zhouguang Lu, Hui Pan, Baomin Xu, Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.05.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors Xiongwei Zhonga,b, Jun Tanga, Lujie Caoa,b, Weiguang Konga, Zheng Suna, Hua Chenga, Zhouguang Lua, Hui Pan*b, Baomin Xu*a a
Department of Materials Science and Engineering, Southern University of Science and
Technology of China, Shenzhen, Guangdong Province 518055, China b
Institute of Applied Physics and Materials Engineering, University of Macau, Macao
* Corresponding authors. Tel.: +86 755 88018980; E-mail address: [email protected] (Baomin Xu); [email protected] (Hui Pan)
Highlights
A facile method to prepare gel polymer electrolyte with high conductivity by ultraviolet triggering and cross-linking between ionic liquid and poly (ethylene oxide) is proposed.
A flexible symmetric capacitor based on the prepared GPE shows ultraflexibility.
The capacitor with high voltage can power up a 3.0V LED even bended to a angle of 180o.
1
Abstract:
It is highly desirable to develop flexible solid-state electrochemical double-layer capacitors (EDLCs) with non-liquid electrolyte. However, it is still a great challenge to prepare gel polymer electrolyte (GPE) possessing high ionic conductivity and good mechanical property. In this work, a simple and novel method to improve the conductivity and mechanical properties of GPE film for their applications as electrolyte and separator in EDLC is presented. The GPE film is prepared by cross-linking ionic liquid (IL) with poly (ethylene oxide) (PEO) and benzophenone (Bp) followed by ultraviolet (UV) irradiation. Then, a non-woven cellulose separator (FPC) is used to absorb the GPE. By tuning the mass ratio (n) between IL and PEO, the flexible EDLC cooperated with low-cost active carbon and the electrolyte film with n=10 has a high capacitance of 70.84F·g-1, a wide and stable electrochemical window of 3.5V, an energy density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density of 1A∙g-1, which can drive a 3.0V light-emitting diode (LED). Importantly, the excellent performance of the flexible and low-cost EDLC can be maintained at a bending angle up to 180o, indicating the ultra-flexibility. It is expected that the IL-PEO-FPC electrolyte film is a promising candidate of GPE for flexible devices and energy storage systems.
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Keywords: Electric double-layer capacitor; gel polymer electrolyte; flexible; ionic liquid
1
Introduction
The flexible and wearable electrochemical storage device is one of the hottest research topics in wearable electronics and related multidisciplinary fields, such as flexible displays, implantable medical devices and flexible solar cells [1-9]. Among various wearable electrochemical devices, electric double-layer capacitors (EDLCs) has been considered as one of the most potential candidates due to its long cycle lifetime, high power density, excellent reliability and environmentally friendly [10-21]. Recently, it has been attracted a majority of scientists to develop non-liquid electrolytes which with high ionic conductivity, excellent flexibility, wide electrochemical window, physicochemical stability and good mechanical integrity for flexible and wearable supercapacitors [9, 22, 23]. Among the various non-liquid electrolytes, including solid [24-26] and gel electrolyte [27, 28], the gel polymer electrolyte (GPE) is considered as one of the ideal candidates for flexible devices because it exhibits high conductivity and prevents the leakage from the solution [29, 30]. The fast growth of flexible energy devices with high performance triggers increasing demands of GPE [31]. To fabricate GPE, host polymer matrix materials, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride3
co-hexafluoropropylene) (P(VDF-HFP)), poly(ethylene oxide) (PEO), and supporting electrolytes are key components [32]. Normally, the conventional electrolytes in GPE system, including aqueous and organic electrolytes, have many serious drawbacks. For example, the aqueous electrolytes (e.g., H2SO4, KOH, and Na2SO4) have a narrow electrochemical window (~1.2V) [33, 34], leading to narrow working voltage and very limited energy density. The organic electrolytes (e.g., acetonitrile and propylene carbonate) suffer from serious health and safety problems. Recently, room temperature ionic liquid (IL) has been widely pursued as an electrolyte in applications for energy storage devices [35-37], such as supercapacitors [38], lithium batteries [39] and sodium batteries [40], etc. Particularly, IL as electrolytes in GPEs has been a hot research topic because of the excellent thermal stability, high ionic conductivity, non-flammability, negligible vapour pressure, low melting point and wide electrochemical window [41]. IL composed of dissociated ions with no intervening solvent can be obtained from molten salts and is a liquid at room temperature [42]. Liu [43] et.al reported that the 1ethyl-3-methylimidazolium (EMImCl) gel formed by UV irradiating the homogeneous solution of (EMImCl), hydroxyethyl methacrylate(HEMA), Chitosan and water. This gel demonstrates good tensile property but a low electrochemical window (1.0V). Pandey and his colleagues [44] synthesized polymer electrolyte by incorporating ionic liquid, PEO, magnesium and lithium salt without UV irradiation. The EDLCs cell with the polymer electrolytes had a specific capacitance of 1.7-3.0 F/g of multi-walled carbon nanotube. Lewandowski [45] simply mixed EMImBF4, PEO and sulpholane to prepare polymer electrolyte. The operating voltage of EDCL was 1.5V. The EDLC with 4
high energy density and operating voltage by incorporated ionic liquid gel polymer electrolyte (ILGPE) has not been reported yet. In this work, we propose a novel ILGPE forming by cross-linking of IL and PEO. Then, a non-woven separator (FPC3018) adsorbs the ILGPEs to form nIL-PEO-FPC (n is the mass ratio of IL/PEO) electrolyte film for flexible EDLCs. The high ionic conductivity up to 6.7mS/cm is realised in the as-fabricated 10EMImTFSI-PEO-FPC film at room temperature. The kind of IL and the ratio of IL/PEO in a GPE are optimised by the comprehensive evaluation of the electrochemical performances and capacitance [46]. The EDLC with the optimized GPE shows high performance in electrical energy storage and high flexibility.
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2.1
Experimental
Chemicals Aqueous solutions were prepared by using deionized water with a purification of
18.3MΩ∙cm. PTFE powder and carbon black were purchased from Sigma-Aldrich. Zeolite (ZSM-5) was purchased from Alfa. Active carbon (YP-80F type, the content of carbon > 95%) and non-woven cellulose separator (FPC3018) was obtained from Kuraray Chemical Co. Ltd and SAM industrial Chemical Co. Ltd (Shenzhen, P.R China), respectively. Poly(ethylene oxide) (PEO, MW = 4,000,000), Benzophenone (Bp), LiTFSI (> 99%) and NaBF4 (> 99%) were purchased from Energy Chemical. All reagents were used without any handle.
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2.2
Preparation of materials The
procedure
to
prepare
1.3-dimethylimidazolium
bis(trifluoromethyl
sulfonyl)imide (DMImTFSI) following these steps. Firstly, the equal molar amount of iodomethane and 1-methylimidazole were added to a round-bottomed flask fitted with a reflux condenser for 24h-36h at 10oC with stirring until no more crystal produced [47, 48]. Secondly, the bottom phase, 1.3-dimethylimidazolium iodide (DMImI), was washed with ethyl acetate and dried at 70oC under vacuum. Thirdly, the DMImI was transferred to a plastic bottle; then a 1:1 molar ratio of LiTFSI was added, followed by adding appropriate deionized water. Finally, the colourless bottom phase was washed with fresh deionized water and dried under a vacuum at 100-120oC for 24-36h [49]. The preparation of 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl)imide (EMImTFSI) and 1.3-diethylimidazolium bis(trifluoromethyl sulfonyl)imide (DEImTFSI) follow the same process as for DMImTFSI described above, where iodomethane was replaced by bromoethane and 1-ethylimidazole was instead of 1methylimidazole, respectively. The 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) was synthesised by following steps. Firstly, the preparation of 1-ethyl-3-methylimidazolium bromide (EMImBr) followed the same process as DMImI, where 1-ethylimidazole and bromoethane were instead of 1-methylimidazole and iodomethane, respectively. Secondly, the EMImBr and NaBF4 (molar ratio=1:1.3) was stirred for 24h at 25-70oC in acetone solvent. Thirdly, the precipitated bromide salt (NaBr) and excess NaBF4 was removed by filtering under vacuum. The filtrate was evaporated at 40-50oC under a 6
vacuum to remove any residual acetone. Then, the filtrate was washed with dichloromethane and dried under uninterrupted vacuum (-0.1MPa) at 100-120oC for 24-36h. The synthesis of 1-propyl-3-methylimidazolium tetrafluoroborate (PrMImBF4) follows the same procedure as EMImBF4, with 1-bromopropane used instead of 1bromoethane. All prepared ionic liquids were confirmed by 1HNMR and 13CNMR and stored in an argon glovebox before use. To prepare electrolyte film, firstly, Benzophenone (Bp) was dissolved into IL by heating under vacuum. Bp was used the initiator for the cross-linking process. Secondly, PEO was dissolved to the mixture. the Bp/PEO weight ratio was kept equal to 0.05. The PEO-IL-Bp mixture was annealed under vacuum at 100°C for 2h to obtain a homogeneous semi-solid gel material. Finally, the polymer gel electrolytes were put at flat glass dish and cross-linked by UV irradiation (BZS250GF-TC UV photoirradiator equipped with a 250 W Hg lamp) for 4 min. The GPE is denoted as nIL-PEO, where n represents the mass ratio of IL/PEO. The non-woven cellulose separator (FPC) hold the nIL-PEO inside to prepare nIL-PEO-FPC electrolyte film, which is used as both of GPE and separator. The GPEs were dried under vacuum at 80oC for 4h and stored in a glove box before use. Briefly, 80wt% active carbon as an active material, 10wt% carbon black as conductive addition and 10wt% PTFE as a binder were uniformly mixed in deionized water and rolled into ~100μm thickness sheets on aluminium foil (20μm) which works as a current collector, and then the water was evaporated at 120oC under vacuum for 68h. Finally, the foil punched into Ф12mm discs as electrodes for coin cell and cut into 7
30mm*40mm rectangle as electrodes for the flexible capacitor. All electrodes were stored in the glovebox. 2.3
Materials Characterization The scanning electron microscopy (SEM, TESCAN MIR3) was used to analyse
micro-morphology. The ionic liquid structure was confirmed by nuclear magnetic resonance (NMR, Bruker Avance 400 MHz), where the (CH3)4Si and CDCl3 were used as an external standard and solvent, respectively. Nitrogen adsorption/desorption was carried out at 77K on a Micrometric ASAP 2020 apparatus. The surface area was calculated using the BET method within the relative pressure (P/Po) range of 0.05-0.45. The total pore volume was determined from the amount of nitrogen adsorbed at P/Po as close to 1, and the pore size distribution was analysed by using the density functional theory model with slit pore geometry. 2.4
Electrochemical measurements The FPC3018 dips in GPE and then hold GPE on its surface and inside as an
electrolyte film. The measurement of ionic conductivity was done by using a cell that sandwiched the electrolyte film between two stainless steel electrodes, where the electrolyte film has a size of Ф12mm and a thickness of 0.03mm. The conductivity of GPEs was measured by electrochemical impedance spectroscopy (EIS) measurements (CHI660E) at 0V under alternating current with a potential amplitude of 5mV and a frequency range of 100 kHz to 0.01Hz. The external resistance of the cell is negligible. The intercept of the measured curve on the real axis (x-axis) is the intrinsic resistance
8
of IL as the ohmic resistance [50]. The conductivity of IL is calculated according to the equation (1). σ = L/(R ∙ S)
(1)
Where 𝞼 is the ionic conductivity (mS/cm), L is the distance between the two electrodes (the thickness of electrolyte film) (cm), R is the resistance of ionic liquids (Ω), and S is the area of the electrolyte film (cm2). All coin cells (CR2025 coin-type cell) were assembled by a conventional process with two electrodes and one electrolyte film in a glovebox. The flexible EDLCs were fabricated by sandwiching one electrolytes film between two electrodes under ~1MPa pressure for 30min, then sealed with polydimethylsiloxane (PDMS). All supercapacitors were prepared in the argon glove box (O2 < 0.1 ppm and H2O < 0.1 ppm). All the electrochemical tests were carried out at room temperature (23±2oC). The electrochemical measurements were performed by using an electrochemical workstation (CHI660E, shanghai, P.R. China) in a two-electrode cell system. Cyclic voltammetry (CV) tests were conducted in the potential window, ranging from 0 to 3.5V. Galvanostatic charge and discharge (GCD) were conducted within the potential range from 0 to 3.5V at a current density of 1A/g, where the current density is the ratio of the real current value to the mass of one electrode. EIS spectra were measured within the frequency range of 100 kHz to 0.01Hz with an alternating potential amplitude of 5 mV. 9
The electrode specific capacitance (Cm, F∙g-1), energy density (E, Wh∙kg-1), equivalent series resistance (ESR, Ω), power density (P, W∙kg-1) were calculated according to the following formula (2-3): C𝑚 = 4I∆t/(M ∙ ∆V) (2) E = (Cm ∆V 2 )/8 (3) Where I (A) is the discharge current, △t (s) is the discharge time, M (g) is the total weight of two electrodes, ∆V (V) is the actual work voltage. The IR drop (iR drop, V) is defined as the electrical potential difference between the two ends of a conducting phase during the charge-discharge process.
3
Results and Discussion
The GPE can be prepared by irradiating the mixed homogeneous solution under ultraviolet (UV) light (Figure 1(a)) and the high-resolution image of electrolyte film is shown in Figure S1(b). The kind of IL and the ratio of IL/PEO in a GPE optimised by the comprehensive evaluation of the electrochemical performances and capacitance. To find the suitable IL candidate for GPE film exhibiting the best electrochemical and practical performance, we have synthesised five kinds of high conductivity ionic liquid (DMImTFSI, EMImTFSI, DEImTFSI, EMImBF4, PrMImBF4, the molecular formulas of ionic liquid are shown in Figure S2). 10
Initially, the CV and GCD curves for coin cells fabricated with 5IL-PEO-FPC and 10IL-PEO-FPC electrolyte films and activated carbon are shown in Figure 2. In 5ILPEO-FPC and 10IL-PEO-FPC electrolyte film, EMImBF4 and EMImTFSI display nearly rectangular CV responses (Figure 2(a) and 2(b)), demonstrating that the two ionic liquids have high electrochemical stability [51, 52]. In a voltage range from 0 V up to 3.5 V, the curves of EDLC with 10EMImBF4-PEO-FPC and 10EMImTFSI-PEOFPC electrolyte film are almost symmetrical, indicating an almost completely reversible ion adsorption/desorption process and no side-reaction at the surface of the porous activated carbon. In GCD measurements, the IR drop appears at the beginning of the discharge due to the internal resistance of devices. The EDLCs with 5IL-PEOFPC electrolyte film indicate high IR drop, because of low conductivity and high internal resistance induced by the high viscosity. The EDCLs with 10IL-PEO-FPC electrolyte film show low IR drop and high specific capacitances. The IR drop increase as a sequence of EMImBF4< EMImTFSI< DMImTFSI< DEImTFSI< PrMImBF4, and the specific capacitance increases as a sequence of PrMImBF4< DEImTFSI < DMImTFSI < EMImTFSI < EMImBF4 (Table S2). Moreover, the ionic conductivity of different ILs adsorbed in the 10IL-PEO-FPC electrolyte films was measured and shown in Table S1. To data, the conductivity of 10IL-PEO-FPC gel electrolyte is higher than PEO/IL/LiTFSI gel electrolyte [53]. The moderate chain of cation shows high electrochemical performance, such as EMImBF4 and EMImTFSI, the EMImBF4 exhibited high conductivity and capacitance, however, it is hydrophilic and therefore moisture was easily absorbed leading to narrow 11
electrochemical window. Whereas the EMImTFSI is hydrophobic and relatively stable in the air in short time, also the 10EMImTFSI-PEO-FPC electrolyte film revealed nearly 80% retention of ionic conductivity of neat EMImTFSI. Therefore, the EMImTFSI was found to be the best candidate among the ILs we have obtained for preparing GPE electrolyte film. Then, we fabricated coin cell type supercapacitors by using the nEMImTFSI-PEOFPC as electrolyte and active carbon (Figure S1(a)) as electrodes to optimise the ratio of IL/PEO. As a reference, an EDLC with pristine EMImTFSI and a separator is also fabricated using the same electrode. Figure 3(a) and Figure 3(b) shows that the capacitance increases with the increment of IL/PEO ratio. The specific capacitance of EMImTFSI is slightly higher than that of 15EMImTFSI-PEO-FPC electrolyte film. However, the IR drop of 15EMImTFSI-PEO-FPC electrolyte film increase dramatically because large internal resistance induced by the low viscosity of 15EMImTFSI-PEO-FPC and little amount of GPE adsorbed inside the separator (Table S3). The 2EMImTFSI-PEO-FPC electrolyte film shows poor capacitance because of high internal resistance and low ion diffusion. For the rectangular CV curves and straight GCD curves of these capacitors consisting of 10EMImTFSI-PEO-FPC exhibit standard capacitive behaviour of EDLC [54]. These devices reveal low IR drop, which is slightly higher than that of the device contains neat EMImTFSI. As a result, we have found that the optimal IL/PEO ratio for GPE was 10:1, which showed the best comprehensive performance among the GPEs we have obtained.
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To further demonstrate the flexibility and stability of the current gel polymer electrolyte, full solid symmetric supercapacitors were assembled by employing the 10EMImTFSI-PEO-FPC as an electrolyte, activated carbon as electrode and the electrochemical performance was systematically measured under different bending angles as shown in Figure 4. The CV curves and GCD curves for flexible EDLC containing the 10EMImTFSI-PEO-FPC electrolyte film with the bending angles of 0o, 50o, 100o, 150o and 180o are shown in Figure 4a and Figure 4b, respectively. The CV curves are rectangular and the GCD curves are perfectly straight and symmetry at different angles. The flexible EDCLs with 10EMImTFSI-PEO-FPC electrolyte film at different angles show similar specific capacitance with the capacitance slightly decreased in the order of 0o > 50o > 100o > 150o > 180o. The flexible EDCL with 10EMImTFSI-PEO-FPC electrolyte film at different angles also show similar IR drop, which manifests the similar internal resistance at different angles. The EIS of the EDLC with 10EMImTFSI-PEO-FPC electrolyte film at different bending angles is shown in Figure 4c. The intercept on the real axis on the Nyquist plots at high frequency (close to 100 kHz) is the intrinsic internal resistance of the electrode material, connection resistance and electrolyte of the device. The similar resistance at high frequency at different bending angles demonstrates that the bending has no impact on intrinsic internal resistance. An approximate semi-circular curve from the high to intermediate frequency region, which is relative to the interface between electrolyte and electrode material, the flexible EDLC at different bending angles show slightly different, because of a trifle interface changes between the electrolyte and electrode material, but this is 13
no effect of bending angle on electrochemical performance. The tail is an almost vertical line at low-frequency region (Warburg impedance), which is related to the diffusion of the ions into the bulk of electrodes, indicating a typical response of a perfect performance of supercapacitor with the porous electrode. The CV, GCD and EIS curves reveal that the EDLC with the 10EMImTFSI-PEO-FPC electrolyte film is completely flexible. The flexible EDLC shows a specific capacitance of 70.84F∙g-1, an energy density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density of 1A∙g-1 at a bending angle of 180o, respectively, which is better than reported flexible EDLC (0.12Wh/kg [28] and 3Wh/kg [55]). Furthermore, the flexible EDLC was fabricated with 10EMImTFSI-PEO-FPC electrolyte film, then the EDLCs was bent in between 180o and 0o for 1500 times. After 100, 500,1000 and 1500 times bending, the capacitance of this EDLC was measured at 0o. The capacitance after 1500 times bending can be retained 83% of initial specific capacitance (Figure S7), indicating that the capacitances are well maintained at a current density of 1A/g and the packing technique is effective. The maximum voltage of flexible EDCL with 10EMImTFSI-PEO-FPC electrolyte film is ~3.5V, which is comparable with about three regular AAA Ni-MH rechargeable batteries (with a height of 43.6mm, diameter of 10.1mm and an output voltage of 1.2V). As is shown in Figure 4(d) and Figure 4(e), the single flexible EDLC can drive a white or a blue light emitting diode (LDE) bulb over 2min and 5min, respectively. Therefore, it clearly shows that the as-fabricated gel polymer electrolytes derived from the crosslinking of IL and PEO, which exhibits high ionic conductivity, wide electrochemical 14
window, excellent flexibility and stability as a promising solid electrolyte for wearable energy storage devices.
4
Conclusions
In summary, a novel nIL-PEO-FPC solid electrolyte film is presented by fast cross-linking IL and PEO under UV irradiation. The EMImTFSI remains the optimal electrochemical performance after irradiation due to the moderate chain and hydrophobicity. The 10EMImTFSI-PEO-FPC electrolyte film displayed high ionic conductivity (around 6.7mS∙cm-1) and outstanding mechanical property for the flexible device. The flexible EDLC assembles 10EMImTFSI-PEO-FPC electrolyte film and two activated carbon electrodes that show a very broad electrochemical window (3.5V), a high energy density of 30.13Wh∙kg-1 and a high power density of 874.8W∙kg-1 at a current density of 1A∙g-1. This flexible EDLC shows same performance at different bending angles, indicating perfect flexibility. We firmly believe that the excellent performances of nIL-PEO-FPC shall bring new design opportunities of EDLC device configuration for clean energy storage systems for wearable and flexible electronics
Note
The authors declare no competing financial interest.
15
Acknowledgements
This work is supported by the startup funding of Southern University of Science and Technology (Grants Nos. 25/Y01256112 and 25/Y01256212), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants Nos. 2016YFA0202400 and 2016YFA0202404), and the National Natural Science Foundation of China (No. 21671096, and No. 21603094). Prof. Hui Pan thanks the support from the Science and Technology Development Fund from Macau SAR (Grants Nos. FDCT-068/2014/A2, FDCT-132/2014/A3, and FDCT-110/2014/SB) and Multi-Year Research Grants (Grants Nos. MYRG2014-00159-FST and MYRG201500017-FST) from Research & Development Office at University of Macau.
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Figure 1 Schematic representation of the preparation procedure and structure of the gel polymer electrolyte formed via the cross-linking of ionic liquid and PEO chains. a) The digital photo of a fresh GPE. b) The optimal ratio of IL/PEO in GPE formed by UV irradiation. c) The low ratio of IL/PEO in GPE formed by UV irradiation.
21
Figure 2 a), b) CV curves of the EDLC with a different kind of IL in 5IL-PEO-FPC and 10IL-PEO-FPC electrolyte films, respectively. c), d) GCD curves of the EDLC with a different kind of IL in 5IL-PEOFPC and 10IL-PEO-FPC electrolyte films, respectively.
22
Figure 3 The CV (a) and GCD (b) measurements of the nEMImTFSI-PEO-FPC electrolyte film and neat EMImTFSI.
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Figure 4 Electrochemical properties of the flexible EDLC making of the 10EMImTFSI-PEO-FPC electrolyte film under different bending angles. a) CV curves at a scan rate of 10mV∙s-1. b) GCD curves at a current density of 1A∙g-1. c) Nyquist plots in a frequency range from 10mHz to 100kHz with a potential amplitude of 5mV, the inset shows the equivalent circuit used to simulate the Nyquist plots. d) The LED (operation voltage is 3V) driven by single flexible EDLC. e) The blue LED (working voltage is 2.5V) driven by single flexible EDLC
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S0013-4686(17)31106-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.110 EA 29545
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
10-3-2017 16-5-2017 17-5-2017
Please cite this article as: Xiongwei Zhong, Jun Tang, Lujie Cao, Weiguang Kong, Zheng Sun, Hua Cheng, Zhouguang Lu, Hui Pan, Baomin Xu, Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.05.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cross-linking of polymer and ionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors Xiongwei Zhonga,b, Jun Tanga, Lujie Caoa,b, Weiguang Konga, Zheng Suna, Hua Chenga, Zhouguang Lua, Hui Pan*b, Baomin Xu*a a
Department of Materials Science and Engineering, Southern University of Science and
Technology of China, Shenzhen, Guangdong Province 518055, China b
Institute of Applied Physics and Materials Engineering, University of Macau, Macao
* Corresponding authors. Tel.: +86 755 88018980; E-mail address: [email protected] (Baomin Xu); [email protected] (Hui Pan)
Highlights
A facile method to prepare gel polymer electrolyte with high conductivity by ultraviolet triggering and cross-linking between ionic liquid and poly (ethylene oxide) is proposed.
A flexible symmetric capacitor based on the prepared GPE shows ultraflexibility.
The capacitor with high voltage can power up a 3.0V LED even bended to a angle of 180o.
1
Abstract:
It is highly desirable to develop flexible solid-state electrochemical double-layer capacitors (EDLCs) with non-liquid electrolyte. However, it is still a great challenge to prepare gel polymer electrolyte (GPE) possessing high ionic conductivity and good mechanical property. In this work, a simple and novel method to improve the conductivity and mechanical properties of GPE film for their applications as electrolyte and separator in EDLC is presented. The GPE film is prepared by cross-linking ionic liquid (IL) with poly (ethylene oxide) (PEO) and benzophenone (Bp) followed by ultraviolet (UV) irradiation. Then, a non-woven cellulose separator (FPC) is used to absorb the GPE. By tuning the mass ratio (n) between IL and PEO, the flexible EDLC cooperated with low-cost active carbon and the electrolyte film with n=10 has a high capacitance of 70.84F·g-1, a wide and stable electrochemical window of 3.5V, an energy density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density of 1A∙g-1, which can drive a 3.0V light-emitting diode (LED). Importantly, the excellent performance of the flexible and low-cost EDLC can be maintained at a bending angle up to 180o, indicating the ultra-flexibility. It is expected that the IL-PEO-FPC electrolyte film is a promising candidate of GPE for flexible devices and energy storage systems.
2
Keywords: Electric double-layer capacitor; gel polymer electrolyte; flexible; ionic liquid
1
Introduction
The flexible and wearable electrochemical storage device is one of the hottest research topics in wearable electronics and related multidisciplinary fields, such as flexible displays, implantable medical devices and flexible solar cells [1-9]. Among various wearable electrochemical devices, electric double-layer capacitors (EDLCs) has been considered as one of the most potential candidates due to its long cycle lifetime, high power density, excellent reliability and environmentally friendly [10-21]. Recently, it has been attracted a majority of scientists to develop non-liquid electrolytes which with high ionic conductivity, excellent flexibility, wide electrochemical window, physicochemical stability and good mechanical integrity for flexible and wearable supercapacitors [9, 22, 23]. Among the various non-liquid electrolytes, including solid [24-26] and gel electrolyte [27, 28], the gel polymer electrolyte (GPE) is considered as one of the ideal candidates for flexible devices because it exhibits high conductivity and prevents the leakage from the solution [29, 30]. The fast growth of flexible energy devices with high performance triggers increasing demands of GPE [31]. To fabricate GPE, host polymer matrix materials, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride3
co-hexafluoropropylene) (P(VDF-HFP)), poly(ethylene oxide) (PEO), and supporting electrolytes are key components [32]. Normally, the conventional electrolytes in GPE system, including aqueous and organic electrolytes, have many serious drawbacks. For example, the aqueous electrolytes (e.g., H2SO4, KOH, and Na2SO4) have a narrow electrochemical window (~1.2V) [33, 34], leading to narrow working voltage and very limited energy density. The organic electrolytes (e.g., acetonitrile and propylene carbonate) suffer from serious health and safety problems. Recently, room temperature ionic liquid (IL) has been widely pursued as an electrolyte in applications for energy storage devices [35-37], such as supercapacitors [38], lithium batteries [39] and sodium batteries [40], etc. Particularly, IL as electrolytes in GPEs has been a hot research topic because of the excellent thermal stability, high ionic conductivity, non-flammability, negligible vapour pressure, low melting point and wide electrochemical window [41]. IL composed of dissociated ions with no intervening solvent can be obtained from molten salts and is a liquid at room temperature [42]. Liu [43] et.al reported that the 1ethyl-3-methylimidazolium (EMImCl) gel formed by UV irradiating the homogeneous solution of (EMImCl), hydroxyethyl methacrylate(HEMA), Chitosan and water. This gel demonstrates good tensile property but a low electrochemical window (1.0V). Pandey and his colleagues [44] synthesized polymer electrolyte by incorporating ionic liquid, PEO, magnesium and lithium salt without UV irradiation. The EDLCs cell with the polymer electrolytes had a specific capacitance of 1.7-3.0 F/g of multi-walled carbon nanotube. Lewandowski [45] simply mixed EMImBF4, PEO and sulpholane to prepare polymer electrolyte. The operating voltage of EDCL was 1.5V. The EDLC with 4
high energy density and operating voltage by incorporated ionic liquid gel polymer electrolyte (ILGPE) has not been reported yet. In this work, we propose a novel ILGPE forming by cross-linking of IL and PEO. Then, a non-woven separator (FPC3018) adsorbs the ILGPEs to form nIL-PEO-FPC (n is the mass ratio of IL/PEO) electrolyte film for flexible EDLCs. The high ionic conductivity up to 6.7mS/cm is realised in the as-fabricated 10EMImTFSI-PEO-FPC film at room temperature. The kind of IL and the ratio of IL/PEO in a GPE are optimised by the comprehensive evaluation of the electrochemical performances and capacitance [46]. The EDLC with the optimized GPE shows high performance in electrical energy storage and high flexibility.
2
2.1
Experimental
Chemicals Aqueous solutions were prepared by using deionized water with a purification of
18.3MΩ∙cm. PTFE powder and carbon black were purchased from Sigma-Aldrich. Zeolite (ZSM-5) was purchased from Alfa. Active carbon (YP-80F type, the content of carbon > 95%) and non-woven cellulose separator (FPC3018) was obtained from Kuraray Chemical Co. Ltd and SAM industrial Chemical Co. Ltd (Shenzhen, P.R China), respectively. Poly(ethylene oxide) (PEO, MW = 4,000,000), Benzophenone (Bp), LiTFSI (> 99%) and NaBF4 (> 99%) were purchased from Energy Chemical. All reagents were used without any handle.
5
2.2
Preparation of materials The
procedure
to
prepare
1.3-dimethylimidazolium
bis(trifluoromethyl
sulfonyl)imide (DMImTFSI) following these steps. Firstly, the equal molar amount of iodomethane and 1-methylimidazole were added to a round-bottomed flask fitted with a reflux condenser for 24h-36h at 10oC with stirring until no more crystal produced [47, 48]. Secondly, the bottom phase, 1.3-dimethylimidazolium iodide (DMImI), was washed with ethyl acetate and dried at 70oC under vacuum. Thirdly, the DMImI was transferred to a plastic bottle; then a 1:1 molar ratio of LiTFSI was added, followed by adding appropriate deionized water. Finally, the colourless bottom phase was washed with fresh deionized water and dried under a vacuum at 100-120oC for 24-36h [49]. The preparation of 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl)imide (EMImTFSI) and 1.3-diethylimidazolium bis(trifluoromethyl sulfonyl)imide (DEImTFSI) follow the same process as for DMImTFSI described above, where iodomethane was replaced by bromoethane and 1-ethylimidazole was instead of 1methylimidazole, respectively. The 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) was synthesised by following steps. Firstly, the preparation of 1-ethyl-3-methylimidazolium bromide (EMImBr) followed the same process as DMImI, where 1-ethylimidazole and bromoethane were instead of 1-methylimidazole and iodomethane, respectively. Secondly, the EMImBr and NaBF4 (molar ratio=1:1.3) was stirred for 24h at 25-70oC in acetone solvent. Thirdly, the precipitated bromide salt (NaBr) and excess NaBF4 was removed by filtering under vacuum. The filtrate was evaporated at 40-50oC under a 6
vacuum to remove any residual acetone. Then, the filtrate was washed with dichloromethane and dried under uninterrupted vacuum (-0.1MPa) at 100-120oC for 24-36h. The synthesis of 1-propyl-3-methylimidazolium tetrafluoroborate (PrMImBF4) follows the same procedure as EMImBF4, with 1-bromopropane used instead of 1bromoethane. All prepared ionic liquids were confirmed by 1HNMR and 13CNMR and stored in an argon glovebox before use. To prepare electrolyte film, firstly, Benzophenone (Bp) was dissolved into IL by heating under vacuum. Bp was used the initiator for the cross-linking process. Secondly, PEO was dissolved to the mixture. the Bp/PEO weight ratio was kept equal to 0.05. The PEO-IL-Bp mixture was annealed under vacuum at 100°C for 2h to obtain a homogeneous semi-solid gel material. Finally, the polymer gel electrolytes were put at flat glass dish and cross-linked by UV irradiation (BZS250GF-TC UV photoirradiator equipped with a 250 W Hg lamp) for 4 min. The GPE is denoted as nIL-PEO, where n represents the mass ratio of IL/PEO. The non-woven cellulose separator (FPC) hold the nIL-PEO inside to prepare nIL-PEO-FPC electrolyte film, which is used as both of GPE and separator. The GPEs were dried under vacuum at 80oC for 4h and stored in a glove box before use. Briefly, 80wt% active carbon as an active material, 10wt% carbon black as conductive addition and 10wt% PTFE as a binder were uniformly mixed in deionized water and rolled into ~100μm thickness sheets on aluminium foil (20μm) which works as a current collector, and then the water was evaporated at 120oC under vacuum for 68h. Finally, the foil punched into Ф12mm discs as electrodes for coin cell and cut into 7
30mm*40mm rectangle as electrodes for the flexible capacitor. All electrodes were stored in the glovebox. 2.3
Materials Characterization The scanning electron microscopy (SEM, TESCAN MIR3) was used to analyse
micro-morphology. The ionic liquid structure was confirmed by nuclear magnetic resonance (NMR, Bruker Avance 400 MHz), where the (CH3)4Si and CDCl3 were used as an external standard and solvent, respectively. Nitrogen adsorption/desorption was carried out at 77K on a Micrometric ASAP 2020 apparatus. The surface area was calculated using the BET method within the relative pressure (P/Po) range of 0.05-0.45. The total pore volume was determined from the amount of nitrogen adsorbed at P/Po as close to 1, and the pore size distribution was analysed by using the density functional theory model with slit pore geometry. 2.4
Electrochemical measurements The FPC3018 dips in GPE and then hold GPE on its surface and inside as an
electrolyte film. The measurement of ionic conductivity was done by using a cell that sandwiched the electrolyte film between two stainless steel electrodes, where the electrolyte film has a size of Ф12mm and a thickness of 0.03mm. The conductivity of GPEs was measured by electrochemical impedance spectroscopy (EIS) measurements (CHI660E) at 0V under alternating current with a potential amplitude of 5mV and a frequency range of 100 kHz to 0.01Hz. The external resistance of the cell is negligible. The intercept of the measured curve on the real axis (x-axis) is the intrinsic resistance
8
of IL as the ohmic resistance [50]. The conductivity of IL is calculated according to the equation (1). σ = L/(R ∙ S)
(1)
Where 𝞼 is the ionic conductivity (mS/cm), L is the distance between the two electrodes (the thickness of electrolyte film) (cm), R is the resistance of ionic liquids (Ω), and S is the area of the electrolyte film (cm2). All coin cells (CR2025 coin-type cell) were assembled by a conventional process with two electrodes and one electrolyte film in a glovebox. The flexible EDLCs were fabricated by sandwiching one electrolytes film between two electrodes under ~1MPa pressure for 30min, then sealed with polydimethylsiloxane (PDMS). All supercapacitors were prepared in the argon glove box (O2 < 0.1 ppm and H2O < 0.1 ppm). All the electrochemical tests were carried out at room temperature (23±2oC). The electrochemical measurements were performed by using an electrochemical workstation (CHI660E, shanghai, P.R. China) in a two-electrode cell system. Cyclic voltammetry (CV) tests were conducted in the potential window, ranging from 0 to 3.5V. Galvanostatic charge and discharge (GCD) were conducted within the potential range from 0 to 3.5V at a current density of 1A/g, where the current density is the ratio of the real current value to the mass of one electrode. EIS spectra were measured within the frequency range of 100 kHz to 0.01Hz with an alternating potential amplitude of 5 mV. 9
The electrode specific capacitance (Cm, F∙g-1), energy density (E, Wh∙kg-1), equivalent series resistance (ESR, Ω), power density (P, W∙kg-1) were calculated according to the following formula (2-3): C𝑚 = 4I∆t/(M ∙ ∆V) (2) E = (Cm ∆V 2 )/8 (3) Where I (A) is the discharge current, △t (s) is the discharge time, M (g) is the total weight of two electrodes, ∆V (V) is the actual work voltage. The IR drop (iR drop, V) is defined as the electrical potential difference between the two ends of a conducting phase during the charge-discharge process.
3
Results and Discussion
The GPE can be prepared by irradiating the mixed homogeneous solution under ultraviolet (UV) light (Figure 1(a)) and the high-resolution image of electrolyte film is shown in Figure S1(b). The kind of IL and the ratio of IL/PEO in a GPE optimised by the comprehensive evaluation of the electrochemical performances and capacitance. To find the suitable IL candidate for GPE film exhibiting the best electrochemical and practical performance, we have synthesised five kinds of high conductivity ionic liquid (DMImTFSI, EMImTFSI, DEImTFSI, EMImBF4, PrMImBF4, the molecular formulas of ionic liquid are shown in Figure S2). 10
Initially, the CV and GCD curves for coin cells fabricated with 5IL-PEO-FPC and 10IL-PEO-FPC electrolyte films and activated carbon are shown in Figure 2. In 5ILPEO-FPC and 10IL-PEO-FPC electrolyte film, EMImBF4 and EMImTFSI display nearly rectangular CV responses (Figure 2(a) and 2(b)), demonstrating that the two ionic liquids have high electrochemical stability [51, 52]. In a voltage range from 0 V up to 3.5 V, the curves of EDLC with 10EMImBF4-PEO-FPC and 10EMImTFSI-PEOFPC electrolyte film are almost symmetrical, indicating an almost completely reversible ion adsorption/desorption process and no side-reaction at the surface of the porous activated carbon. In GCD measurements, the IR drop appears at the beginning of the discharge due to the internal resistance of devices. The EDLCs with 5IL-PEOFPC electrolyte film indicate high IR drop, because of low conductivity and high internal resistance induced by the high viscosity. The EDCLs with 10IL-PEO-FPC electrolyte film show low IR drop and high specific capacitances. The IR drop increase as a sequence of EMImBF4< EMImTFSI< DMImTFSI< DEImTFSI< PrMImBF4, and the specific capacitance increases as a sequence of PrMImBF4< DEImTFSI < DMImTFSI < EMImTFSI < EMImBF4 (Table S2). Moreover, the ionic conductivity of different ILs adsorbed in the 10IL-PEO-FPC electrolyte films was measured and shown in Table S1. To data, the conductivity of 10IL-PEO-FPC gel electrolyte is higher than PEO/IL/LiTFSI gel electrolyte [53]. The moderate chain of cation shows high electrochemical performance, such as EMImBF4 and EMImTFSI, the EMImBF4 exhibited high conductivity and capacitance, however, it is hydrophilic and therefore moisture was easily absorbed leading to narrow 11
electrochemical window. Whereas the EMImTFSI is hydrophobic and relatively stable in the air in short time, also the 10EMImTFSI-PEO-FPC electrolyte film revealed nearly 80% retention of ionic conductivity of neat EMImTFSI. Therefore, the EMImTFSI was found to be the best candidate among the ILs we have obtained for preparing GPE electrolyte film. Then, we fabricated coin cell type supercapacitors by using the nEMImTFSI-PEOFPC as electrolyte and active carbon (Figure S1(a)) as electrodes to optimise the ratio of IL/PEO. As a reference, an EDLC with pristine EMImTFSI and a separator is also fabricated using the same electrode. Figure 3(a) and Figure 3(b) shows that the capacitance increases with the increment of IL/PEO ratio. The specific capacitance of EMImTFSI is slightly higher than that of 15EMImTFSI-PEO-FPC electrolyte film. However, the IR drop of 15EMImTFSI-PEO-FPC electrolyte film increase dramatically because large internal resistance induced by the low viscosity of 15EMImTFSI-PEO-FPC and little amount of GPE adsorbed inside the separator (Table S3). The 2EMImTFSI-PEO-FPC electrolyte film shows poor capacitance because of high internal resistance and low ion diffusion. For the rectangular CV curves and straight GCD curves of these capacitors consisting of 10EMImTFSI-PEO-FPC exhibit standard capacitive behaviour of EDLC [54]. These devices reveal low IR drop, which is slightly higher than that of the device contains neat EMImTFSI. As a result, we have found that the optimal IL/PEO ratio for GPE was 10:1, which showed the best comprehensive performance among the GPEs we have obtained.
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To further demonstrate the flexibility and stability of the current gel polymer electrolyte, full solid symmetric supercapacitors were assembled by employing the 10EMImTFSI-PEO-FPC as an electrolyte, activated carbon as electrode and the electrochemical performance was systematically measured under different bending angles as shown in Figure 4. The CV curves and GCD curves for flexible EDLC containing the 10EMImTFSI-PEO-FPC electrolyte film with the bending angles of 0o, 50o, 100o, 150o and 180o are shown in Figure 4a and Figure 4b, respectively. The CV curves are rectangular and the GCD curves are perfectly straight and symmetry at different angles. The flexible EDCLs with 10EMImTFSI-PEO-FPC electrolyte film at different angles show similar specific capacitance with the capacitance slightly decreased in the order of 0o > 50o > 100o > 150o > 180o. The flexible EDCL with 10EMImTFSI-PEO-FPC electrolyte film at different angles also show similar IR drop, which manifests the similar internal resistance at different angles. The EIS of the EDLC with 10EMImTFSI-PEO-FPC electrolyte film at different bending angles is shown in Figure 4c. The intercept on the real axis on the Nyquist plots at high frequency (close to 100 kHz) is the intrinsic internal resistance of the electrode material, connection resistance and electrolyte of the device. The similar resistance at high frequency at different bending angles demonstrates that the bending has no impact on intrinsic internal resistance. An approximate semi-circular curve from the high to intermediate frequency region, which is relative to the interface between electrolyte and electrode material, the flexible EDLC at different bending angles show slightly different, because of a trifle interface changes between the electrolyte and electrode material, but this is 13
no effect of bending angle on electrochemical performance. The tail is an almost vertical line at low-frequency region (Warburg impedance), which is related to the diffusion of the ions into the bulk of electrodes, indicating a typical response of a perfect performance of supercapacitor with the porous electrode. The CV, GCD and EIS curves reveal that the EDLC with the 10EMImTFSI-PEO-FPC electrolyte film is completely flexible. The flexible EDLC shows a specific capacitance of 70.84F∙g-1, an energy density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density of 1A∙g-1 at a bending angle of 180o, respectively, which is better than reported flexible EDLC (0.12Wh/kg [28] and 3Wh/kg [55]). Furthermore, the flexible EDLC was fabricated with 10EMImTFSI-PEO-FPC electrolyte film, then the EDLCs was bent in between 180o and 0o for 1500 times. After 100, 500,1000 and 1500 times bending, the capacitance of this EDLC was measured at 0o. The capacitance after 1500 times bending can be retained 83% of initial specific capacitance (Figure S7), indicating that the capacitances are well maintained at a current density of 1A/g and the packing technique is effective. The maximum voltage of flexible EDCL with 10EMImTFSI-PEO-FPC electrolyte film is ~3.5V, which is comparable with about three regular AAA Ni-MH rechargeable batteries (with a height of 43.6mm, diameter of 10.1mm and an output voltage of 1.2V). As is shown in Figure 4(d) and Figure 4(e), the single flexible EDLC can drive a white or a blue light emitting diode (LDE) bulb over 2min and 5min, respectively. Therefore, it clearly shows that the as-fabricated gel polymer electrolytes derived from the crosslinking of IL and PEO, which exhibits high ionic conductivity, wide electrochemical 14
window, excellent flexibility and stability as a promising solid electrolyte for wearable energy storage devices.
4
Conclusions
In summary, a novel nIL-PEO-FPC solid electrolyte film is presented by fast cross-linking IL and PEO under UV irradiation. The EMImTFSI remains the optimal electrochemical performance after irradiation due to the moderate chain and hydrophobicity. The 10EMImTFSI-PEO-FPC electrolyte film displayed high ionic conductivity (around 6.7mS∙cm-1) and outstanding mechanical property for the flexible device. The flexible EDLC assembles 10EMImTFSI-PEO-FPC electrolyte film and two activated carbon electrodes that show a very broad electrochemical window (3.5V), a high energy density of 30.13Wh∙kg-1 and a high power density of 874.8W∙kg-1 at a current density of 1A∙g-1. This flexible EDLC shows same performance at different bending angles, indicating perfect flexibility. We firmly believe that the excellent performances of nIL-PEO-FPC shall bring new design opportunities of EDLC device configuration for clean energy storage systems for wearable and flexible electronics
Note
The authors declare no competing financial interest.
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Acknowledgements
This work is supported by the startup funding of Southern University of Science and Technology (Grants Nos. 25/Y01256112 and 25/Y01256212), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants Nos. 2016YFA0202400 and 2016YFA0202404), and the National Natural Science Foundation of China (No. 21671096, and No. 21603094). Prof. Hui Pan thanks the support from the Science and Technology Development Fund from Macau SAR (Grants Nos. FDCT-068/2014/A2, FDCT-132/2014/A3, and FDCT-110/2014/SB) and Multi-Year Research Grants (Grants Nos. MYRG2014-00159-FST and MYRG201500017-FST) from Research & Development Office at University of Macau.
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Figure 1 Schematic representation of the preparation procedure and structure of the gel polymer electrolyte formed via the cross-linking of ionic liquid and PEO chains. a) The digital photo of a fresh GPE. b) The optimal ratio of IL/PEO in GPE formed by UV irradiation. c) The low ratio of IL/PEO in GPE formed by UV irradiation.
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Figure 2 a), b) CV curves of the EDLC with a different kind of IL in 5IL-PEO-FPC and 10IL-PEO-FPC electrolyte films, respectively. c), d) GCD curves of the EDLC with a different kind of IL in 5IL-PEOFPC and 10IL-PEO-FPC electrolyte films, respectively.
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Figure 3 The CV (a) and GCD (b) measurements of the nEMImTFSI-PEO-FPC electrolyte film and neat EMImTFSI.
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Figure 4 Electrochemical properties of the flexible EDLC making of the 10EMImTFSI-PEO-FPC electrolyte film under different bending angles. a) CV curves at a scan rate of 10mV∙s-1. b) GCD curves at a current density of 1A∙g-1. c) Nyquist plots in a frequency range from 10mHz to 100kHz with a potential amplitude of 5mV, the inset shows the equivalent circuit used to simulate the Nyquist plots. d) The LED (operation voltage is 3V) driven by single flexible EDLC. e) The blue LED (working voltage is 2.5V) driven by single flexible EDLC
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