Solid polymer electrolyte membranes based on quaternized polysulfone and solvent-free fluid as separators for electrical double-layer capacitors

Accepted Manuscript Solid polymer electrolyte membranes based on quaternized polysulfone and solventfree fluid as separators for electrical double-layer capacitors Yongsheng Ji, Na Liang, Jing Xu, Rong Qu, Dongzhi Chen, Hongwei Zhang PII:

S0013-4686(18)31444-0

DOI:

10.1016/j.electacta.2018.06.156

Reference:

EA 32149

To appear in:

Electrochimica Acta

Received Date: 23 November 2017 Revised Date:

13 June 2018

Accepted Date: 24 June 2018

Please cite this article as: Y. Ji, N. Liang, J. Xu, R. Qu, D. Chen, H. Zhang, Solid polymer electrolyte membranes based on quaternized polysulfone and solvent-free fluid as separators for electrical doublelayer capacitors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.06.156. 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.

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An electrical double-layer capacitor using new solid polymer electrolyte shows a specific capacitance of 114.0 F g-1 at a current density of 1 A g-1 and outstanding cycling stability, which is comparable to that of a device using porous polypropylene (PP) membrane.

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Solid polymer electrolyte membranes based on quaternized polysulfone and solvent-free fluid as separators for electrical double-layer capacitors Yongsheng Ji1, Na Liang1, Jing Xu, Rong Qu, Dongzhi Chen, Hongwei Zhang∗

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College of Materials Science and Engineering, Wuhan Textile University, WuHan, 430073, PR China

Abstract

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A room temperature ionic liquid is immobilized on the surfaces of layered double hydroxides to form a solvent-free fluid via a silylation reaction. A series of solid polymer electrolyte membranes based on

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quaternized polysulfone and the solvent-free fluid are fabricated by solution casting method, which are tough and flexible and display moderate KOH aqueous solution uptakes. A symmetric electrical double-layer capacitor is assembled by using an optimized quaternized polysulfone-based solid polymer electrolyte membrane with KOH solution and two activated carbon electrodes. The single

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carbon electrode delivers a specific capacitance of 114.0 F g-1 at a current density of 1 A g-1 and outstanding cycling stability, indicating that this novel solid polymer electrolyte membrane may be

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appropriate for electrical double-layer capacitors.

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Keywords: quaternized polysulfone, solvent-free fluid, solid polymer electrolyte membrane, EDLC

1

The two authors contributed equally to this work.

*Corresponding author: Tel.: +86-27-59367850; fax: +86-27-59367578. E-mail: [email protected]. 1

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1. Introduction Supercapacitors are considered as promising energy conversion and storage devices due to high power density and long lifecycle, which are commonly classed into electrical double-layer capacitors (EDLCs)

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and pseudo-capacitors depending on the charge-storage mechanism. [1] It is believed that supercapacitors could bridge the energy/power gap between batteries/fuel cells and conventional

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dielectric capacitors. [2] As an emerging branch of supercapacitors, flexible supercapacitors are drawing great attention with the rapid advancements of portable electronic devices in recent years.

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Typical flexible supercapacitors generally consist of flexible electrodes, electrolytes, separators, flexible packaging materials and current collectors in some cases. [3] It is obvious that self-supported, flexible solid-state electrolytes are desired for flexible supercapacitors compared to liquid electrolytes because of easy encapsulation and leakage-free electrolytes.

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Solid-state electrolytes can be divided into two classes, namely gel polymer electrolytes (GPEs) and solid polymer electrolytes (SPEs). GPEs with gel-network structures containing solvents have high

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ionic conductivity and are widely used in flexible supercapacitors. [4-7] However, mechanical properties of GPEs are often deteriorated during practical operation. To deal with the problem, Na et al.

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prepared micro-porous polymer membranes filled with gel electrolyte for solid EDLCs, which showed low internal resistance and excellent cycling performance. [8] Liu et al. reported cross-linked nanocomposite hydrogels supported on eggshell membranes, which resulted in a specific capacitance of 161 F g-1 at a current density of 1 A g-1 after assembled in EDLCs. [9] SPEs often have lower ionic conductivity even under fully hydrated conditions than GPEs. As a consequence, it is necessary to absorb liquid electrolytes in SPEs for high performance of EDLCs. Sulfonated poly(fluorenyl ether

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nitrile oxynaphthalate) doped with H2SO4 aqueous solution, quaternized poly(aryl ether sulfone) doped with KOH aqueous solution and poly [2,5-benzimidazole] doped with H3PO4 aqueous solution have been employed for EDLCs based on activated carbon (AC) electrodes.[10-12] All of them

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demonstrated a promising prospect as potential materials for flexible EDLCs. But large-scale applications of them will be limited by their expensive starting monomers and tedious synthesis steps.

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Consequently, we adopted polysulfone (PSF), a commercial polymer with good mechanical property, as starting materials to synthesize chloromethylated polysulfone (CMPSF). Layered double

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hydroxides (LDHs) are a typical class of two-dimensional “anionic” clay, which consists of positively charged brucite-like layers and exchangeable interlayer anions. [15] LDHs are favorable for the anionic transference in quaternized polysulfone (QPSF)-based membranes because their conductivity can reach the order of 10-3 S cm-1. [16] The plenty hydroxyl groups on LDH sheets endow them with

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hydrophilicity, which results in incompatibility between LDH sheets and matrix polymer matrix. To address the issue, we employed a room temperature ionic liquid (RTIL) containing trimethoxysilyl

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groups to modify LDHs. Schematic illustration of the synthetic routes for RTIL and LDH-RTIL were briefly illustrated in Figure 1.

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The self-supported composite membranes based on QPSF and modified LDHs for EDLCs were fabricated and evaluated. The EDLC, which consisted of QPSF-based composite membrane doped with KOH aqueous solution and AC electrodes, exhibited a specific capacitance of 114 F g-1 (based on AC mass of an electrode) at a current density of 1 A g-1, which was comparable to that of the EDLC assembled with two AC electrodes and a porous polypropylene (PP) membrane in a 6 M KOH aqueous solution.

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2. Experimental 2.1 Chloromethylation of polysulfone Chloromethylated polysulfone (CMPSF) was prepared via the same method in our published paper [13].

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In a typical process, PSF (11 g) was firstly dissolved in dichloromethane (300 mL). Secondly, chloromethyl ether (16 mL) and anhydrous tin chloride (3 mL) were added into the solution, and

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followed by keeping 40 min at 30 oC to synthesize CMPSF. Thirdly, the solution was poured into 3 L ethanol to obtain precipitated CMPSF. Lastly, the CMPSF was obtained by filtration, washing with

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water and dehydration. 2.2 Synthesis of RTIL

RTIL was synthesized by an ion-exchange reaction followed by extracting of by-product. At first, the ion-exchange reaction between 50 g of dimethyloctadecyl[3-(trimethoxysilyl) propyl] ammonium

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chloride (DMAOP, 60% in methanol) and 150 g of poly(ethylene glycol) 4-nonylphenyl 3-sulfopropyl ether sodium salt (PEG-SO3Na, 60% in water) occurred in 200 mL dichloromethane (CH2Cl2) for 24 h

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at room temperature. Thereafter 20 mL of water was added into the mixture to extract the generated NaCl. After stirring for 4 h, the aqueous layer was removed. This step was repeated at least 5 times.

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And then the remainder CH2Cl2 layer was evaporated in oven at 60 oC for 24 h, followed by further treatment in a vacuum oven for 24 h at 80 oC to obtain RTIL (Figure 1a). 2.3 Modification of LDHs

LDHs were synthesized by the same method in our published paper [14]. To anchor RTIL onto LDHs, the as-synthesized LDHs (1 g) and RTIL (10 g) were mixed in 20 mL of methanol for 2 h and then refluxed for 24 h at 75 oC. Subsequently, the reactant was re-dispersed by adding 20 mL of CH2Cl2 and

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stood for 24 h to remove sedimentation. The as-obtained CH2Cl2 layer was evaporated at 60 oC for 24 h and in a vacuum oven at 80 oC for 24 h to obtain LDHs anchored by RTIL (LDH-RTIL) (Figure 1b). 2.4 Fabrication of QPSF-based membranes

LDH-RTIL

of

preassigned

content

(if

necessary)

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At first, the 15% (w/v) solution of CMPSF in dimethylacetamide (DMAc) was blended with for

casting

films.

After

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N,N,N‫׳‬,N‫׳‬-tetramethylethylenediamine (TMEDA) as crosslinking agent (0.3 mL / 10 g CMPSF casting solution) was added into the casting solution, the casting solution was spread on a clean glass plate with

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a glass knife. The as-prepared film was dried at 80°C for 24h under vacuum condition. To induct quaternary groups, the resultant membrane was treated in 30 wt% trimethylamine (TMA) solution for 24h. Subsequently, the membrane was immersed into 1M KOH solution for another 24 h. Thereafter, the membrane was washed with distilled water until pH 7-8 and kept in water for further testing. The

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plain membrane without LDH-RTIL was designated as QPSF membrane. The QPSF-based composite membranes were designated as QPSF/x% LDH-RTIL membrane, where x (=10 and 20) meant the

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contents of LDH-RTIL.

2.5 Materials Characterization

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LDH-RTIL samples were characterized by VERTEX70 spectrometer and rheometer (AR2000EX). The membrane samples were characterized by transmission electron microscope (TEM, Tecnai G2 20) and INSTRON WN5566 Mechanical Testing Machine. 2.6 Hydroxide ion conductivity The ionic conductivity (σ) of PP in 6 M KOH and LDH-RTIL samples with 6 M KOH was measured using two-probe AC method on an Autolab work station. The measurements were performed in a 5

ACCEPTED MANUSCRIPT frequency range from 1Hz to 500 KHz with an excitation signal of 10 mV. AEMs with size of 15 mm × 15 mm were immersed in 6 M KOH for 48 h prior to measurement. After a sample was clamped between two platinum electrodes, it was put into an open, temperature controlled cell. Repeated

perpendicular direction was calculated by the equation (1):

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(1)

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impedance measurements were performed under 100% RH at a 25 oC. The σ of them in the

where d and A are the distance between the two electrodes and face area of the electrode, respectively,

2.7 Electrochemical Measurement

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and R is the membrane resistance value obtained from the AC impedance data.

The capacitive performance was investigated in a two-electrode configuration. In order to prepare the electrode, commercial AC (80 wt%) was mixed with polytetrafluoroethylene (5 wt%) as binder and

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carbon black (15 wt%) as conductive agent. A piece of as-formed paste with an area of 1.0 cm-2 was roll-pressed onto a foamed nickel by a rolling machine and the as-prepared electrode was dried at 80 °C

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for 24 h. Before a sandwich-type cell was assembled by placing a separator between two electrodes, the separator and two electrodes were immersed in 6 M KOH aqueous solution for 12 h. The

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electrochemical performance of the as-assembled EDLC cell was evaluated with an electrochemical workstation (CHI 660E, Shanghai Chen Hua Co., Ltd). The specific capacitance of the single electrode was calculated from the galvanostatic charge/discharge data by using the equation of C= 4I∆t/(m∆V), where I is the constant current (A), m is the total mass of AC for both electrodes (g), ∆t is the discharge time (s), and ∆V is the voltage range after the IR drop during the discharge process (V).

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3. Results and Discussion 3.1 Synthesis of LDH-RTIL

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Fig.2 Fig.3 From Figure 2a, it could be found that the RTIL exhibited a light yellow liquid state, indicating a typical liquid feature at room temperature. After RTIL was anchored onto LDH sheets, LDH-RTIL

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appeared a viscous solvent-free fluid (Figure 2b), which was made up of LDH sheets as cores and RTIL as shells. The facile condensation reactions between hydroxyl groups of LDHs and alkoxy group of

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silane coupling agents had been well proved. [17] We investigated fourier transform infrared (FTIR) spectra to confirm the successful immobilization of RTIL on LDH sheets (Figure 2c). The C-H stretching modes at 2860, 2928 and 2966 cm-1 were observed, which could be attributed to the –CH2and CH3 groups in RTIL. The moderately intensive peak at 472 cm-1 was still observed despite the low

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content of LDH in LDH-RTIL, which was assigned to the newly-generated Si-O-M (M=Mg or Al) bonds.[18-20] These results manifested that RTIL have successfully reacted with LDH sheets. The

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rheological behavior of LDH-RTIL was also investigated (Figure 3). In the whole testing temperature range, the shear-loss modulus (Gʹʹ) was remarkably higher than the storage modulus (Gʹ), which meant

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a liquid-like behavior, namely solvent-free fluid. [21] 3.2 Properties and morphology of QPSF-based membranes Table 1

The properties of QPSF and QPSF-based membranes were listed in table 1. It could be observed that three membranes had similar thickness and were tough and flexible. They could be used as self-supported membranes because of their enough tensile strength and acceptable elongation at break. 7

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Furthermore, the tensile strength decreased and the elongation at break increased with the increase of LDH-RTIL content in membranes probably owing to the plasticization effect of LDH-RTIL. The PP membrane had the highest KOH aqueous solution uptake due to the porous feature. The introduction of

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LDH-RTIL brought two opposite effects. On the one hand, the LDH-RTIL could loosen the packing structures of polymer chains and resulted in more spaces to accommodate KOH aqueous solution. On

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the other hand, the hydrophobic LDH-RTIL could suppress the absorption of KOH aqueous solution. Based on the balance of two effects, QPSF/10% LDH-RTIL membrane showed the highest KOH

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aqueous solution uptake among the QPSF and QPSF-based membranes. Furthermore, all the samples displayed similar values of hydroxide ion conductivity (0.41 – 0.52 S/cm) because the KOH provided dominant contribution for anionic transfer.

The distribution of LDH-RTIL in QPSF-based membranes was showed in Figure 4. It could be found

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that the agglomerates, which consisted of several LDH-RTIL sheets, had a size of less than 100 nm and were unevenly dispersed in QPSF-based membranes. The size of LDH-RTIL sheets was only 10-20 nm,

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which suggested a good compatibility between LDH-RTIL sheets and QPSF chains because of the immobilization of RTIL onto LDHs.

Moreover, the agglomerates in the QPSF/20% LDH-RTIL

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membrane demonstrated larger size than those in the QPSF/10% LDH-RTIL membrane due to higher contents of LDH-RTIL in QPSF-based membranes. Fig. 4

3.3 Electrochemical Behavior of EDLCs Fig. 5 Cyclic voltammograms (CVs) of EDLCs using PP, QPSF, QPSF/10% LDH-RTIL and QPSF/20% 8

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LDH-RTIL membranes at different scan rates were presented in Figure 5. Obviously, all CV curves displayed a shape approximately close to an ideal rectangle at low scan rates from 5 to 20 mV s-1, which indicated that all EDLCs had excellent capacitive behaviors generated from electric double

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layers formed at the electrode/electrolyte interfaces. Although these CV curves maintained a nearly rectangular shape at a scan rate of 100 mV s-1, they still deviated from rectangles at different degrees. It

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could be attributed to the fact that OH- ions had less time to migrate between the KOH aqueous solution and the surface of AC during the charging/discharging process at high scan rates, which

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consequently led to insufficient utilization of the surface area of AC materials. [22] The degree deviated from rectangles of these CV curves at high scan rates was determined by the membrane used in EDLCs since the same AC electrodes were used in EDLCs. It could be seen that the CV curve of the EDLC using QPSF/20% LDH-RTIL membrane exhibited the highest deviation degree at a scan rate of

application in EDLCs.

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100 mV s-1. The result meant that the QPSF/20% LDH-RTIL membrane was unsuitable for the

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Fig. 6

The galvanostatic charge/discharge measurements of four EDLCs were carried out at a current

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density of 1 A g-1. Just as shown in Figure 6a, charge/discharge curves of these EDLCs were nearly isosceles triangular shapes, which suggested good Coulombic efficiency and excellent capacitive behaviors. [23] The IR drops during the discharging measurement of EDLCs were related to the internal resistances of corresponding EDLCs. As a consequence, the internal resistances of four membranes could be listed in decreasing order: QPSF/20% LDH-RTIL membrane > QPSF/10% LDH-RTIL membrane, QPSF membrane > PP membrane. From the linear portion of discharge curves,

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the specific capacitances of the single electrode in four EDLCs were calculated to be 114.2, 112.3, 114.0 and 88.7 F g-1 at a current density of 1 A g-1, respectively. The value of 114.0 F g-1 was slightly lower than 118.63 F g-1 (@ 1 A g-1) of an EDLC using micro-porous polymer membranes filled with

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gel electrolyte [8], but higher than 92.79 F g-1 (@ 0.1 A g-1) of an EDLC using quaternary ammonium functionalized poly(aryl ether sulfone) membrane absorbed 6 M KOH aqueous solution [11].

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The impedance analysis of EDLCs was also performed by electrochemical impedance spectroscopy. Figure 6b displayed the Nyquist plots of four EDLCs, which included three parts: intrinsic internal

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resistance (Rs) in the high-frequency region, interfacial charge-transfer resistance (Rct) in the medium-high frequencies region and Warburg impedance (Rw) in the medium-low frequencies region. [24] Rs was obtained from the intercept with the real axis (Zʹ), which stood for the ohmic resistance of the electrolyte, membrane, electrode materials, and the contact resistance. Rct was determined from the

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diameter of semicircle region, which represented interface nature of membrane, electrolyte and electrode materials. Rw was caused by the diffusion of the ions into the bulk of electrodes, which was

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corresponded to the straight 45o sloped line. Furthermore, a straight Warburg line larger than 45o in the low-frequency region indicated good capacitive behaviors. [23-25] As shown in Figure 6b, porous PP

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membrane had the least Rs and Rct values. Among three membranes based on QPSF, the QPSF/20% LDH-RTIL membrane presented the least Rs value due to abounding ionic bonds on LDH-RTIL, but the largest Rct value owing to the hindrance of the LDH-RTIL in the ionic channels. As a result, the EDLC using QPSF/20% LDH-RTIL membrane delivered the lowest specific capacitance, which was consistent with the low KOH aqueous solution uptake, big size of agglomerates and high internal resistance of the QPSF/20% LDH-RTIL membrane. On the contrary, QPSF and QPSF/10% LDH-RTIL

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membranes had Rs and Rct values similar to that of the PP membrane, which induced that the EDLCs using QPSF and QPSF/10% LDH-RTIL membranes delivered specific capacitances comparable to that of the EDLC using the PP membrane.

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Long cycling durability is very important for determining the practical application of EDLCs. The cycling life test was measured by continuous galvanostatic charging/discharging at a current density of

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1 A g-1. Fig. 6c plotted curves between capacitance retentions of four EDLCs versus the number of charge/discharge cycles. With increasing cycle numbers, the curves of four EDLCs exhibited tiny

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fluctuation, suggesting good stability and outstanding electrochemical reversibility under short-term charge/discharge cycles. In order to further explore the long term stability of the QPSF/10% LDH-RTIL membrane as a separator under practical conditions, the EDLC was subjected to 5000 charge/discharge cycles at a current density of 1 A g-1 (Fig. 6d) . In the first 200 cycles, the specific capacitance

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increased to 107% of the original value because of the activation of the electrode. Then it steadily reduced to 98% of the original value after 5000 cycles. The inset photo in Fig. 6d demonstrated that the

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EDLC could drive a mini-fan and maintained for at least 12.4 s even after 5000 charge/discharge cycles, indicating an excellent stability of the QPSF/10% LDH-RTIL membrane.

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4. Conclusions

In summary, a solvent-free fluid prepared via LDHs anchored by RTIL was incorporated in QPSF to fabricate SPE membranes. The optimum QPSF-based SPE membrane showed moderate KOH aqueous solution uptake, enough tensile strength and acceptable elongation. An EDLCs assembled by optimum SPE membranes doped with KOH aqueous solution and electrodes based on AC materials delivered a high specific capacitance of 114.0 F g-1 and an excellent cycling durability, which were comparable to

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those of an EDLC using commercial PP membrane in 6 M KOH aqueous solution. The results suggested that QPSF/LDH-RTIL membranes could serve as a class of promising SPEs for EDLCs.

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC) (51503161) and Hubei Provincial Natural Science Foundation of China (2018CFB267)

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Table Caption

1.

Thickness, KOH aqueous solution uptake, mechanical properties and specific capacitance of

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EDLCs using various membranes. Fig. Captions

Schematic diagram of (a) RTIL synthesis and (b) LDH anchored by RTIL.

2.

Digital photo of (a) RTIL and (b) LDH-RTIL, (c) FTIR spectra of LDH and LDH-RTIL.

3.

The modulus-temperature curve of LDH-RTIL.

4.

TEM image of (a) QPSF/10% LDH-RTIL membrane, (b) QPSF/20% LDH-RTIL membrane.

5.

Cyclic voltammetry curves of EDLCs using (a) PP, (b) QPSF, (c) QPSF/10% LDH-RTIL or (d)

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1.

QPSF/20% LDH-RTIL membrane at different scan rates.

(a) galvanostatic charge/discharge curves of EDLCs using PP, QPSF, QPSF/10% LDH-RTIL or

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QPSF/20% LDH-RTIL membrane at a current density of 1 A g-1, (b) Nyquist plots of EDLCs using PP, QPSF, QPSF/10% LDH-RTIL or QPSF/20 % LDH-RTIL membrane, (c) The cyclic properties of EDLCs using PP, QPSF, QPSF/10% LDH-RTIL or QPSF/20% LDH-RTIL membrane at a

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current density of 1 A g-1 (d) The cycling stability test of the EDLC using QPSF/10%LDH-RTIL membrane at a current density of 1 A g-1 for 5000 cycles (Inset: Digital photo of a mini-fan

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powered by the EDLC using QPSF/10% LDH-RTIL membrane).

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Ionic conductivity (S/cm) at 25 oC

Tensile strength (MPa)

Maximum elongation (%)

Specific capacitance (F/g)

85 84 87 88

21.16 18.24 19.02 15.17

0.52 0.46 0.49 0.41

/ 18.16 14.03 11.91

/ 6.78 9.82 15.23

114.2 (@ 1 A g-1) 112.3 (@ 1 A g-1) 114.0 (@ 1 A g-1) 88.7 (@ 1 A g-1)

TE D

M AN U

SC

Table 1

RI PT

KOH aqueous solution uptake (%)

EP

PP QPSF QPSF/10%LDH-RTIL QPSF/20%LDH-RTIL

Thickness (µm)

AC C

Membranes

16

EP AC C

Fig.1

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

17

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

(c)

Transmittance / a.u.

TE D

LDH-RTIL

2860

472

AC C

EP

2928

4000

3500

3000

LDH

2500

2000

1500

1000

500

-1

Wavenumber / cm

Fig.2

18

ACCEPTED MANUSCRIPT

80

G' G"

RI PT

Modulus / Pa

60

40

SC

20

30

40

50

M AN U

0

60

70

80

90

o

Temperature / C

AC C

EP

TE D

Fig.3

19

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

Fig.4

20

ACCEPTED MANUSCRIPT (a)

4

50 mV/s 5 mV/s

100 mV/s 10 mV/s

3

20 mV/s

1 0 -1 -2 -3 0.0

0.2

0.4

0.6

0.8

4

100 mV/s 10 mV/s

3

Current/A g-1

2 1 0

-3

TE D

-1 -2

20 mV/s

50 mV/s 5 mV/s

M AN U

(b)

1.0

SC

Cell Potential/ V

RI PT

Current/A g-1

2

0.0

0.2

0.4

0.6

0.8

1.0

EP

Cell Potential/ V

4

50 mV/s 5 mV/s

100 mV/s 10 mV/s

AC C

(c)

3

20 mV/s

Current/A g-1

2 1 0

-1 -2 -3 0.0

0.2

0.4

0.6

0.8

1.0

Cell Potential/ V

21

ACCEPTED MANUSCRIPT (d)

3

50 mV/s 5 mV/s

100 mV/s 10 mV/s

20 mV/s

1

0

-1

-2 0.0

0.2

0.4

0.6

1.0

SC

Cell Potential/ V

0.8

RI PT

Current/A g-1

2

AC C

EP

TE D

M AN U

Fig.5

22

ACCEPTED MANUSCRIPT (a)

PP QPSF QPSF/10%LDH-RTIL QPSF/20%LDH-RTIL

1.0

Cell Potential/ V

0.8

0.4

0.2

0.0 0

10

20

30

40

50

7

0.5

M AN U

(b)

0.4

-Z''/ohm cm2

6 5

-Z''/ ohm cm2

60

SC

Time/ s

RI PT

0.6

PP QPSF QPSF/10%LDH-RTIL QPSF/20%LDH-RTIL

4 3

1 0 0

TE D

2

1

2

3

0.3 0.2 0.1

0.0 0.0

0.1

0.2

0.3

0.4

0.5

Z'/ohm cm2

4

5

6

7

2

EP

Z'/ ohm cm

AC C

(c)

Retention/%

100

80

PP QPSF QPSF/10%LDH-RTIL QPSF/20%LDH-RTIL

60

40 0

200

400

600

800

1000

Cycle number

23

ACCEPTED MANUSCRIPT (d) 110 100

80 70 60 50 40 0

1000

2000

3000

5000

SC

Cycle number

4000

RI PT

Retention/%

90

AC C

EP

TE D

M AN U

Fig.6

24

ACCEPTED MANUSCRIPT
A solvent-free fluid based on layered double hydroxide has been prepared.
The SPE containing 10% solvent-free fluid shows the best performance.

AC C

EP

TE D

M AN U

SC

RI PT


Two EDLCs using a SPE or PP membrane display similar specific capacitances.