Na-ion conducting gel polymer membrane for flexible supercapacitor application

Journal Pre-proof Na-ion conducting gel polymer membrane for flexible supercapacitor application

Jingwei Wang, Guohua Chen, Shenhua Song PII:

S0013-4686(19)32194-2

DOI:

https://doi.org/10.1016/j.electacta.2019.135322

Reference:

EA 135322

To appear in:

Electrochimica Acta

Received Date:

01 April 2019

Accepted Date:

16 November 2019

Please cite this article as: Jingwei Wang, Guohua Chen, Shenhua Song, Na-ion conducting gel polymer membrane for flexible supercapacitor application, Electrochimica Acta (2019), https://doi. org/10.1016/j.electacta.2019.135322

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Journal Pre-proof Na-ion conducting gel polymer membrane for flexible supercapacitor application Jingwei Wanga,b, Guohua Chenb,*, Shenhua Songa,* aShenzhen

Key Laboratory of Advanced Materials, School of Materials Science and

Engineering, Harbin Institute of Technology, Shenzhen, Guangdong 518055, China bDepartment

of Mechanical Engineering, The Hong Kong Polytechnic University,

Hung Hom, Kowloon, Hong Kong SAR, China

ABSTRACT A sodium-ion conducting poly(vinyl alcohol)-based gel polymer electrolyte membrane is developed by plasticizing with an ionic liquid, for the fabrication of electric double-layer capacitors. Sodium triflate is selected as the sodium salt. 1-ethyl3 methylimidazoliumtrifluoromethanesulfonate is employed as the plasticizer. The properties of the electrolyte membranes are analyzed in terms of their crystallinity, morphology, thermal stability, electrochemical stability window, ionic transference number, and ionic conductivity. The composition with poly(vinyl alcohol) - 30% sodium triflate + 10% 1-ethyl-3 methylimidazoliumtrifluoromethanesulfonate shows the optimal performances such as good thermal stability up to 150 oC and wide electrochemical stability window of 4.70 V. It is thus employed in the fabrication of electric double-layer capacitors, serving as both ion-conducting electrolyte and separator. The capacitor shows almost 100% coulombic efficiency and stable chargedischarge cyclic property with almost 100% capacity retention after 1000 cycles, when charging up to 1.6 V and 2.0 V with a specific capacitance of 103.7 and 127.8 F

*Corresponding

Chen).

author. Tel.: +852-27666647. E-mail addresse: [email protected](G.-H.

author. Tel.: +86-755-26033465. E-mail addresses: [email protected]; [email protected] (S.-H. Song). *Corresponding

1

Journal Pre-proof g-1, respectively. It compares well with that made of a liquid electrolyte. This excellent performance is attributed to the sodium triflate with large-sized anions and thus high ionic conductivity (3.8×10-3 S cm-1) of the electrolyte membrane. Keywords: Electric double-layer capacitors; Gel polymer electrolytes; Ionic conductivity; Ionic liquid plasticizer; Sodium ion supercapacitor. 1. Introduction In the current era of technological expansion, the energy crisis is one of the most challenging problems faced by humanity. The blooming market of smart-phones, electric vehicles, portable electronic devices, and other electronics products has stimulated the need for renewable energy sources. The present power supplies cannot meet the growing energy demand. Thus, to abate the burden of global energy consumption, it is highly essential to develop effective renewable and clean energy storage technologies

[1-3].

In this regard, the electric double-layer capacitor (EDLC)

has emerged as the attractive energy storage device because of its fast chargingdischarging performance, high coulombic efficiency and long service life. Since their first introduction in 2001, the lithium-ion conducting capacitors have achieved great progress. They have been fabricated by using activated carbon, carbon nanotubes, graphene, as well as Li4Ti5O12, Fe3O4 and Li3VO4 battery-type electrodes with a liquid electrolyte

[4-9].

Recently, the lithium-ion capacitors have been commercialized as the

brake energy recovery system from the subway operation in China[7]. Although lithium-ion capacitors may supplement today’s energy demand, the concern of high cost and limited availability would eventually result in the shortage of lithium metal in the near future[10]. Regardfully, the exploration of sodium metal brings hope as the next-generation sustainable energy material. The sodium metal offers a logical alternative to lithium due to its earth-abundance and closely related physical and 2

Journal Pre-proof chemical properties [11-16]. Investigation on sodium-ion capacitors started in 2012[17-19]. In the thrust of new energy materials, the paradigm is rapidly shifting towards the metal-ion conducting electrolytes. Besides the electrode material used, both the stability of electrolyte and the interface between electrode and electrolyte play a key role in improving the performances of capacitors. Accordingly, significant studies on variety of electrolytes have been done. Among them, solid-state electrolytes and gel polymer electrolytes have attracted much research interests[3, 20-25], yet rare reports are available on Na-ion conducting biodegradable gel polymer electrolytes. In contrast to liquid electrolytes, non-liquid electrolytes are regarded as safer ones because they are leakage-free and have high thermal stability. In addition, gel polymer electrolytes (GPEs) render a high ionic conductivity and excellent flexibility, making them one of the most promising candidates for the fabrication of flexible devices. It is worth noting that the demand for flexible power supply is becoming increasingly high because of the wearable and portable devices found in various fields including medical, military and outdoor sports[26]. Since Fenullade and Perche[27] found out that polymers can be plasticized with an aprotic solution, many polymer hosts such as poly(ethylene oxide) (PEO)[28-31], poly(vinylidinefluoride) (PVdF)[32], poly(methyl methacrylate)(PMMA)[33,

34],

poly(vinylidinefluoride-co-hexafluoropropylene)

(PVdF-HFP)[35-39] and poly(vinyl alcohol) (PVA)[40, 41], have been reported as GPEs with high conductivity (10-4-10-3 S cm-1 at room temperature). However, GPEs still have some disadvantages such as low mechanical stability, flammability, poor thermal and electrochemical stability[42]. In addition, it is of the prime consideration to investigate biodegradable and environmentally benign gel polymer electrolytes. PVA is especially a promising representative in terms of environmental impact, chemical stability and mechanically durability[43, 44]. PVA-based gel polymer electrolytes, such

3

Journal Pre-proof as proton-conducting electrolytes, Li-ion and Na-ion conducting electrolytes, have been investigated by several research groups[40, 44-49]. Until now, few PVA-based Naion conducting electrolytes have been studied, reporting poor electrochemical stability probably because sodium salts employed were too simple

[50].

Furthermore, the

contact between electrolyte film and the electrodes was found mechanically weak[51] when assembled into energy storage devices. Therefore, in the present work, considerable efforts have been expanded on the development of PVA-based Na-ion conducting electrolytes. In order to prepare PVA-based biodegradable gel polymer electrolyte for fabricating EDLCs with high ionic conductivity and good electrochemical stability, NaTf (sodium triflate or sodium trifluoromethanesulfonate) may serve as a better sodium salt because of the interaction between the hydroxyl group of PVA and the sulfonate group of NaTf. The trifluoro group would also be potentially beneficial for the stability of the electrolyte during operation. Moreover, NaTf can be highly dissolved in PVA, is thermally stable and non-toxic. Its interaction with PVA is a kind of weakly electrostatic coordination that renders high ionic conductivity for electrolytes[52]. Ionic liquid has some desirable performances, including high ionic conductivity, low viscosity, and good electrochemical stability. It can be used as a plasticizer for forming GPEs together with desirable electrochemical properties[53, 54]. When this NaTf containing PVA is plasticized using ionic liquid (EMITf, 1-ethyl-3 methylimidazoliumtrifluoromethanesulfonate), the electrolyte so made would have high chemical stability and ionic conductivity, as indeed seen subsequently.

4

Journal Pre-proof 2. Experimental procedures 2.1 Materials For preparing gel polymer electrolyte membranes, PVA (Sigma-Aldrich, MW =125,000 g mol-1), NaTf (Aladdin, 98%), and EMITf (IoLiTec, 99%) were used as host polymer, salt, and plasticizer, respectively. The activated carbon (AC, specific surface area = 1800-1900 m2 g-1, supercapacitor grade), poly(vinylidene fluoride) (PVdF, HSV900), and carbon black (Super P) provided by Kejingstar Technology, Shenzhen, China, were used as the active electrode material, binder, and conductive additive, respectively, in the preparation of EDLCs electrodes. 2.2 Preparation and characterization of gel polymer electrolyte (GPE) In order to identify the relationship between the ionic conductivity as well as crystallinity of the polymer electrolytes and the concentrations of the doped NaTf and EMITf, the polymer electrolyte membranes were prepared as described previously[9], using casting method after fast stirring (1000 rpm) of the mixed solution at 80 oC. In fabricating GPE membranes, different quantities of EMITf were dissolved completely in the optimal NaTf-complexed PVA system, determined through X-ray diffraction (XRD), scanning electron microscopy (SEM) and ionic conductivity analyses. The surface morphologies, structural characteristics and thermal stability of polymer electrolyte membranes were evaluated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (Hitachi S-4700 SEMEDS, Japan), X-ray diffraction (XRD,D/max 2500PC, Rigaku, Japan) with Cu-Kα radiation (λ = 1.5406 Å), and thermal analyzer (STA 449F3 Jupiter, Germany), respectively. The electrochemical stability of the membranes was evaluated using an electrochemical workstation (Model: CHI760D, CH Instruments, China). The ionic conductivity (σ) was measured using an impedance analyzer (PSM 1735, Newton,

5

Journal Pre-proof UK). The as-obtained symmetric cells were used for these three evaluations, consisting of a pair of stainless steel foils as blocking electrodes and an electrolyte membrane sandwiched between them. The measurements were carried out at room temperature (23-25 oC). In addition, the Na+ transport number of the electrolyte films was evaluated by using the Bruce-Vincent equation[55]. The Na/electrolyte film/Na cell was polarized by applying a DC potential difference of 10 mV. The cell resistance values were measured before and after polarization by using AC impedance spectroscopy. 2.3 Assembling and testing of EDLC In order to evaluate the electrochemical properties of the polymer electrolyte, it is employed in the fabrication of EDLCs. To enhance the contact between the electrode and polymer electrolyte, the EDLC was fabricated using the spinningcasting method. The PVA-NaTf + EMITf solution was spun on the prepared electrode at a high speed (1400 rpm) for 20 s to form a thin membrane (~5-6 µm) and heated to evaporate the NMP solvent at 60 oC for about 10 min to 80% of its full extent. The experimental procedure can be seen in more detail in Scheme 1. The amount of NMP evaporated was determined by weighing the sample before and after heating. The remanent solution (PVA-NaTf + EMITf) was then cast onto the surface of electrolyte membrane-electrode and dried in an oven at 70 oC for 24 h. This non-fully dried electrolyte membrane (containing ~10 wt.% solvent, determined by weighing) was joined with another piece of electrolyte membrane spun on the electrode to form a symmetrical EDLC. With this method, one could prepare numbers of EDLCs simultaneously (see Scheme 1). The liquid electrolyte, 1 mol L-1 NaTf in ethylene carbonate (EC) and propylene carbonate (PC) (1/1 v/v), was employed to fabricate CR2032-type cells with activated carbon as electrodes in an Ar-filled glove box. The

6

Journal Pre-proof measurements of cyclic voltammetry (CV) of EDLCs were performed at room temperature (23-25 oC) with different scan rates from 0 to 1.6 V and 0 to 2.0 V, respectively. Galvanostatic charge-discharge (GCD) test was carried out at a constant current density of 313 mA g-1 for 1000 cycles using Neware battery testing device. Finally, the EDLCs were employed to power a light-emitting diode (LED) to demonstrate the possibility of their practical application.

Scheme 1. Schematic illustration showing fabrication and application of assembled EDLC.

3. Results and discussion To identify the optimal salt and ionic liquid contents doped in the polymer matrix, the polymer electrolyte membranes with different contents of NaTf or EMITf were prepared using casting method. The viscosity of the solution decreases during the rapid mixing, so that the big ions, such as Tf- and EMI+, are easy to be tethered by the polymer matrix and then entangled by the polymer chains, leading to a decrease in the crystallinity of PVA[56]. The change of crystallinity is shown in Fig. 1 in terms of X-ray diffraction. The incorporation of NaTf in a concentration range of 10 to 30 wt.% enables the diffraction peak to widen and decrease in intensity, indicating the decrease of crystallinity of PVA. However, further increase in NaTf beyond 30 wt.% results in an increase in the intensity and decrease in the width of peak, indicating the increase

7

Journal Pre-proof in crystallinity. Therefore, the best composition should be P-30. In order to obtain gel electrolyte membranes, the EMITf ionic liquid was added into the P-30 mixture with continual fast stirring (1000 rpm) at 80 oC. As shown in Fig.1b, the peak intensity decreases further due to the introduction of EMITf and the best composition in terms of crystallinity is the P-30-10 mixture. The lower crystallinity can supply larger free volume, thereby leading to more segment motion and higher carrier mobility that are crucial for the electrolyte membrane to achieve high ionic conductivity. a

3500

Intensity/counts

3000 Pure PVA P-10 P-20 P-30 P-40

2500 2000 1500 1000 500 10

20

30 2/degree

40

50

b

Intensity/counts

2000 P-30-5 P-30-10 P-30-15

1500

1000

500 10

20

30 2/degree

40

50

Fig. 1. XRD patterns of (a) PVA-xNaTf(x = 0, 10, 20, 30, and 40 wt.%, are denoted as Pure PVA, P-10, P-20, P-30, and P-40) and (b) PVA-30wt.% NaTf + y EMITf (y= 5, 10, and 15 wt.%, denoted as P-30-5, P-30-10, and P-30-15) membranes.

8

Journal Pre-proof The morphologies of electrolyte membranes were observed using scanning electron microscopy. As shown in Fig. 2, the electrolyte membranes maintain a uniform and soft surface even after being plasticized by ionic liquid and exhibit a perfect structural integrity because they are self-standing. However, many particles are apparently visible on the surface of electrolyte membranes containing 20 wt.% EMITf. These particles demonstrate the existence of NaTf as confirmed by EDS (see Fig. 2f), where the peaks of carbon, oxygen, fluorine, sodium and sulphur are observed. The excess ionic liquid could lead to ion aggregates, being attributed to the combination of Na+ and Tf-, and not EMI+ and Tf- since the concentration of EMITf is low[57]. Accordingly, the optimal electrolyte composition would be the P-30-10 mixture.

9

Journal Pre-proof f

2.5

Intensity/cps

2.0

1.5

1.0

0.5

0.0

0

1

2

3

4

5

6

7

8

9

10

Energy /keV

Fig. 2. SEM images of (a-e) PVA-30 wt.% NaTf + yEMITf (y =0, 5, 10, 15 and 20 wt.%) under 5.00k magnification; (f) EDS analysis of the particle in (e).

To evaluate the thermal stability of the electrolyte membrane, the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed with the results shown in Fig. 3. Obviously, the P-30-10 membrane exhibits a ~2% weight loss at around 80oC, which should be the loss of moisture and NMP. It also shows a weight loss at around 180 oC because of the melting and the first decomposition stage of PVA[58]. A weight loss at around 240 oC due to the decomposition of NaTf is observed[50]. Furthermore, the weight loss after 300 oC may arise from volatilization of EMITf and decomposition of PVA[59]. Hence, the P-30-10 sample exhibits a relative stability before 150 oC. This indicates that this electrolyte membrane can be applied in capacitors where high temperature is a concern[60]. Besides the desirable thermal stability, the electrolyte membrane can also be completely curled without any breakage (see Fig. 4). This gives an additional advantage of mechanical stability. Consequently, the optimum PVA-based gel polymer electrolyte membrane shows a great potential for the fabrication of flexible sodium-ion capacitors as both electrolyte and separator.

10

Journal Pre-proof

Weight loss Heat flow

0.0

20 -0.5 40

-1.0

60

80

100

200 Temperature/oC

300

Heat flow/mW mg-1

Weight loss /%

0

400

Fig. 3. TGA and DSC thermograms of PVA-30 wt% NaTf + 10 wt.% EMITf membrane.

Fig. 4. The electrolyte films (a) in bending and (b) after bending

The electrochemical properties are also critical for the application of electrolyte membranes, including electrochemical stability window, ionic transference number, and ionic conductivity. The electrochemical stability window was analyzed by using cyclic voltammograms (see Fig. 5). The P-30 without the addition of EMITf shows an electrochemical stability window of 4.15 V. When 5% EMITf is added to the P-30 mixture, and the resulting P-30-5 electrolyte membrane shows a wider electrochemical stability window of ~4.70 V. This stability window is maintained even after 10% EMITf is added, implying an excellent tolerance to polarization and a great potential application for high cell-potential-difference devices. Nevertheless, further addition of EMITf to P-30 leads to some extra redox peaks, as seen from the P-30-15 CV test results, Fig. 5. These redox peaks probably arise from the 11

Journal Pre-proof precipitated sodium salt particles due to the excessive addition of ionic liquid, being

Current density/mA cm

-2

consistent with those noted from Fig. 2e. 1.2 0.6 0.0 -0.6 -1.2 0.5

P-30-15

P-30-10

0.0 -0.5 P-30-5

0.5 0.0 -0.5 0.4

P-30

0.0 -0.4 -4

-2

0

2

Applied potential difference/V

4

Fig. 5. Cyclic voltammograms of SS/PVA-30 wt.% NaTf + y EMITf/SS cell (y = 0, 5, 10, and 15 wt.% at a scan rate of 10 mV s-1, denoted as P-30, P-30-5, P-30-10, and P-30-15).

The ion transport depends on the cations (Na+ and EMI+) and anion (Tf-). The Na+ transport number of the electrolyte films (tNa+) is given by the Bruce-Vincent equation [55] 𝐼ss(𝑉 ― 𝐼0𝑅0)

(1)

𝑡Na + = 𝐼0(𝑉 ― 𝐼ss𝑅ss)

where I0 and Iss are the initial and steady-state current values, respectively; R0 and Rss are the Na/P-30-10/Na cell resistance values before and after polarization, respectively; and V is the applied potential difference. These values can be obtained from Fig. 6. The Na+ transport number for the P-30-10 electrolyte film is gained as ~0.59 at room temperature. Hence, Na+ should transfer faster than the other two ions of much larger sizes and make more contribution to ionic conduction because largesized ions can be easily pinned within the polymer matrix although the concentration of Tf- is higher[56, 61]. 12

Journal Pre-proof 400

70 Before polarization After polarization

60

350

50

Current/A

-Z''/

300 250

40 30 20 10

200

0

150

0

10

20

30 40 Z'/

50

60

70

100 50 0

0

10

20

30

40

50

60

Time/min Fig. 6. DC polarization curve of Na/P-30-10/Na cell at an applied potential difference of 10 mV and the inset shows the AC impedance curves before and after polarization of the cell.

The Nyquist impedance analyses (Fig. 7) were further conducted to confirm the effect of the addition of salt and ionic liquid on the ionic conductivity (σ/S cm-1) calculated by 𝜎 = 𝑡 (𝑅b𝐴)

(2)

where t is the thickness of electrolyte membrane, being about 0.008 cm; A is the membrane-electrode contact area; and Rb is the bulk resistance of electrolyte membrane (Rb/Ω), which can be determined from the intercept of semicircle on the real axis. As shown in Fig. 7a, when less than 40% NaTf is added to the PVA sample, the resulting electrolyte membranes (P-10, P-20 and P-30) show a decrease of Rb, which is due to the decrease in crystallinity and the increase in mobile ions. However, when the content of NaTf is up to 40%, the value of Rb increases due to the increase in ion aggregation. Hence, the P-30 sample has the highest ionic conductivity for the unplasticized systems. Similarly, the P-30-10 sample has a higher ionic conductivity (~3.8×10-3 S cm-1) than the others (<~1.8×10-3 S cm-1). When the proper amount of EMITf (less than about 10 wt.%) is added to the P-30 mixture, the amorphous region 13

Journal Pre-proof of the electrolyte membrane increases and thus enhances the ions mobility. However, further addition of EMITf would result in an increase in ion aggregation. This is consistent with the XRD results showing the lowest degree of crystallinity for this electrolyte. The transport of mobile ions within the polymer substrate depends on amorphous domains which can promote polymer segmental motions and then enhance the mobility of mobile ions[62]. Therefore, the high ionic conductivity of the P-30-10 sample should be due to its low crystallinity and large concentration of mobile ions. Furthermore, it is found that the ionic conductivities of the polymer electrolytes based on sodium, magnesium and lithium salts are of a similar order of magnitude. Overall, the present ionic conductivity compares well with reported values

[9, 40, 63].

Accordingly, the P-30-10 electrolyte membrane should be suitable for application in capacitors. 4000

a Sample

-Z''/

3000

P-10 P-20 P-30 P-40

Rb/

4170 1027 218 607

2000

1000

0

0

1000

2000

Z'/

14

3000

4000

Journal Pre-proof 15

b Rb/

Sample

5.98 2.90 5.86

-Z''/

10

P-30-5 P-30-10 P-30-15

5

0 15

20

25

30

Z'/ Fig. 7. (a) Nyquist impedance plots of PVA-x NaTf with a table of Rb as inset (x =10, 20, 30, and 40 wt.%, denoted as P-10, P-20, P-30, and P-40) and (b) PVA-30 wt.% NaTf + y EMITf with a table of Rb as inset (y= 5, 10 and 15 wt.%), recorded at room temperature.

The performance of this sodium-ion conducting gel polymer electrolyte membrane was investigated by means of acting as “electrolyte and separator” in electric double-layer capacitors with activated carbon as electrodes. Figs. 8a and 8b display the cyclic voltammograms (CVs) of capacitors with the gel polymer electrolyte membrane (P-30-10) (C//GPE//C) and with the liquid electrolyte (C//LE//C) in the potential range of 0-1.6 V at different scan rates, respectively. Obviously, both of them exhibit a nearly symmetrical and rectangular shape at low scan rates, indicating that they have a desirable capacitor performance and a low resistance, so that the mobile sodium ions can transfer and then generate electric double layers near the electrode-electrolyte interface. Nevertheless, gradually the plots lose the rectangle feature with increasing scan rates. In addition, the areas of CV curves of the C//GPE//C capacitor are slightly larger than those of the C//LE//C capacitor at low scan rates, and the difference becomes insignificant at the high scan rates of 9 and 13 mV s-1 for these two types of capacitors (see Table 1).

15

Journal Pre-proof In order to identify the performance of capacitors at a larger potential range, these two types of capacitor were also analyzed using CV measurements in the cell potential range of 0-2.0 V at different scan rates. The results are shown in Fig. 8c for the C//GPE//C) capacitor and Fig. 8d for the (C//LE//C) capacitor. Obviously, C//GPE//C shows slightly higher specific capacitances, due to its bigger areas of CV curves, than those of the C//LE//C capacitor at different scan rates (see Table 1).This means that the as-obtained gel polymer electrolyte membrane is well comparable with the liquid electrolyte. 1 mV s-1 5 mV s-1 9 mV s-1 13 mV s-1

2

1

Current/mA

Current/mA

1

0

-1

-2

a 0.0

0.2

0.4

1 mV s-1 5 mV s-1 9 mV s-1 13 mV s-1

2

0.6

0.8

1.0

1.2

1.4

0

-1

b

-2

1.6

0.0

Cell potential difference/V

1

Current/mA

Current/mA

1 0 -1

c 0.0

0.4

0.8

1.2

1.6

0.6

0.8

1.0

1.2

1.4

1.6

1 mV s-1 5 mV s-1 9 mV s-1 13 mV s-1

2

-2

0.4

Cell potential difference/V

1 mV s-1 5 mV s-1 9 mV s-1 13 mV s-1

2

0.2

0

-1

-2

d 0.0

2.0

0.4

0.8

1.2

1.6

2.0

Cell potential difference/V

Cell potential difference/V

Fig. 8. Cyclic voltammograms of EDLC fabricated with (a, c) P-30-10 electrolyte membrane and with (b, d) liquid electrolyte, determined at different scan rates in the cell potential ranges of (a, b) 0-1.6 V and (c, d) 02.0 V, respectively. Table 1. Absolute area of CV curves of C//GPE//C and C//LE//C EDLCs at different scan rates in the cell potential ranges of 0-1.6 V and 0-2.0 V.

16

Journal Pre-proof

EDLCs

Cell potential difference/V

C//GPE//C C//LE//C C//GPE//C C//LE//C

0-1.6 0-1.6 0-2.0 0-2.0

Absolute area of CV at different scan rates 1 mV s-1 5 mV s-1 9 mV s-1 13 mV s-1 0.650 2.292 3.427 4.267 0.542 1.750 3.410 4.265 1.050 3.124 4.725 5.813 0.669 2.846 4.294 5.475

The galvanostatic charge-discharge (GCD) measurements of the C//GPE//C and C//LE//C capacitors were carried out at room temperature (23-25oC) to further demonstrate the reliability of this polymer electrolyte system. Figs. 9a and 9b exhibit the GCD plots of these two kinds of EDLC for the 1st, 50th and 100th cycles recorded in the charge-discharge potential ranges of 0-1.6 V and 0-2.0 V, respectively. As can be seen from Fig. 9a, a nearly linear profile is obtained in terms of all the charging and discharging plots, further indicating a comparable capacitive property of these EDLCs, being in agreement with the CV results. Moreover, all the curves show nearly symmetrical triangles, implying that these EDLCs possess a high coulombic efficiency, which is given by



tD 100% tC

(3)

where tC and tD represent the times for charging and discharging in units of s. A sudden drop in cell potential difference, i.e. the kink at around 1.3 V, is almost identical for both C//GPE//C and C//LE//C capacitors at the initial cycling, but it decreases with increasing cycle for the capacitor with the polymer electrolyte while the drop of cell potential difference shows no change for the capacitor with the liquid electrolyte. This indicates that the C//GPE//C device has less energy loss during cycling. This cell-potential-difference drop is caused by the ohmic loss due to the internal resistance of the EDLC, i.e., the resistance induced by electrode, electrolyte, electrolyte/electrode and electrode/current collector interfaces. In addition, it can be

17

Journal Pre-proof seen that charging for the polymer electrolyte is slower than that for the liquid electrolyte as time goes because the ionic conductivity of the former is lower than that of the latter. To further identify the cycling performance of C//GPE//C and C//LE//C capacitors, the room-temperature coulombic efficiency and specific capacitance as a function of cycle are depicted in Figs. 10a and 10b under the cell potential differences of 1.6 V and 2.0 V, respectively. The value of specific capacitance (Cs/F g-1) of the single electrode is calculated from the discharge plot according to Cs  4 I /(m

dV ) dt

(4)

where I is the current in A, m is the total mass of electrodes in g, dV/dt is the slope of the linear part of the discharge curve in V/s. It can be seen from Fig. 10a that the specific capacitance of the C//GPE//C capacitor increases with increasing cycling to 170 cycles and then slightly decreases until a nearly steady value over 1000 chargedischarge cycles. By comparison, it keeps a decreasing trend up to 1000 chargedischarge cycles for the C//LE//C capacitor. This may be due to the difference of internal resistance between these two types of EDLCs. For both capacitors, the value of Cs is about 100 F g-1 in the first cycle, but it is about 103.7 F g-1 for the C//GPE//C capacitor and 63.5 F g-1 for the C//LE//C capacitor in the 1000th cycle. In addition, they both show an almost 100% coulombic efficiency after several cycles until 1000 cycles. The lower coulombic efficiencies in the initial charging-discharging cycle may be attributed to the irreversible side reactions, which is common for some capacitors[64, 65].The redox cycling performances for these EDLCs were evaluated by charging to 2.0 V, and the results are shown in Fig. 10b. Compared with the results shown in Fig. 10a (charged to 1.6 V), a similar variation trend in specific capacitance/coulombic efficiency is observed, but the C//GPE//C capacitor shows a higher coulombic efficiency than the C//LE//C capacitor. For the C//LE//C capacitor, 18

Journal Pre-proof the value of Cs is 146.1 F g-1 in the first cycle and 85.0 F g-1 in the 1000th cycle, and thus its capacity retention is only 41.8%. In contrast, for the C//GPE//C capacitor, the capacity retention is nearly 100% (125.4 F g-1 in the 1st cycle and 127.8 F g-1 in the 1000th cycle). The corresponding energy density (Ecell/W h kg-1) and power density (Pcell/W kg-1) of the EDLCs are calculated by[2, 66]:

𝐸cell =

𝐶s(∆𝑉)2 8

1000

(5)

× 3600 ∆𝑡

(6)

𝑃cell = 𝐸cell/(3600)

where ΔV is the cell potential difference excluding the cell potential drop in V and Δt is the discharge time in s. The calculated results are listed in Table 2. Clearly, the EDLC fabricated with the polymer electrolyte membrane provides a steady energy density and power density even after 1000 cycles at the charging voltages of both 1.6 V and 2.0 V, further indicating that the spinning-casting method is suitable for the fabrication of EDLCs and that the P-30-10 electrolyte membrane is a promising one for energy-storage applications. However, the EDLC fabricated with the liquid electrolyte loses about half of the energy density after 1000 charge-discharge cycles, further showing the unstable internal resistance within the C//LE//C device. It is worth noting that the relatively poor performance of the EDLC fabricated with the liquid electrolyte may be due to the sodium salt composed of large-sized anions and sodium cations. The large-sized anions are easy to migrate from the liquid electrolyte to electrode at a high cell potential difference, resulting in a jam and accumulation in electrode materials. This may lead to a larger internal resistance for EDLCs along with a smaller effective specific surface area for active carbon electrodes. By contrast, the large-sized anions in polymer electrolyte can be trapped in the polymer matrix. The as-fabricated EDLC with the present polymer electrolyte has been used to power a light-emitting diode (LED), showing that the LED emits extremely dazzling light. 19

Journal Pre-proof This further confirms the ideal electrochemical properties of the EDLC. Table 2. Energy density (Ecell/W h kg-1) and power density (Pcell/W kg-1) of C//GPE//C and C//LE//C EDLCs at different cell potential differences and redox cycles Cell potential difference/V 0-1.6 0-1.6 0-2.0 0-2.0

EDLC C//GPE//C C//LE//C C//GPE//C C//LE//C

1.8

6.5 7.3 12.4 15.9

7.2 4.4 13.8 8.1

161.4 170.0 198.4 208

167.2 166.7 207 194.4

a

polymer electrolyte liquid electrolyte

1.6

Cell potential difference/V

Ecell/1st Ecell/1000th Pcell/1st Pcell/1000th

1.4 1.2 1.0 0.8 0.6 0.4 1st

0.2 0.0

0

100 200 300 0

50th

100 200 300 0

100th

100 200 300 400

Time/s polymer electrolyte liquid electrolyte

Cell potential difference/V

2.0

b

1.5

1.0

0.5 1st

0.0

0

50th

100th

150 300 450 600 150 300 450 600 150 300 450 600

Time/s

20

Journal Pre-proof Fig. 9. The galvanostatic charge-discharge curves of EDLCs fabricated with P-30-10 electrolyte and liquid electrolyte, respectively, at 1st, 50th and100th cycles in the cell potential ranges of (a) 0-1.6 V and (b) 02.0 V. 250

a

polymer electrolyte liquid electrolyte

150

-1

Specific capacitance/F g

100 150 75 100 50 50

0

Coulombic efficiency/%

125

200

25

0

200

400

600

800

0 1000

Cycles

350

b 140

polymer electrolyte liquid electrolyte

Specific capacitance/F g

250

100 80

200

60 150 40 100 50

Coulombic efficiency/%

120

-1

300

20 0

200

400

600

800

0 1000

Cycles Fig.10. The variations of specific capacitance and coulombic efficiency with cycling for EDLCs fabricated with P-30-10 electrolyte and liquid electrolyte in the cell potential ranges of (a) 01.6 V and (b) 02.0 V.

4. Conclusions Poly(vinyl alcohol) (PVA), a biodegradable polymer, can be utilized to fabricate gel electrolyte for electric double-layer capacitors (EDLCs) using sodium ions as the 21

Journal Pre-proof main conducting charges. The proper selection of sodium salt and plasticizer is essential for its performance. Sodium triflate (NaTf) has been shown as a good choice of the sodium salt and 1-ethyl-3-methylimidazoliumtrifluoromethanesulfonate (EMITf) as the plasticizer. It is found that the composition of PVA-30% NaTf + 10% EMITf (P-30-10) exhibits an excellent room-temperature ionic conductivity of ~3.8×10-3 S cm-1, wide electrochemical stability window of ~4.70 V, and good thermal stability up to 150 oC. The as-fabricated EDLC using the P-30-10 electrolyte exhibits an excellent electrochemical performance as compared with that fabricated with the liquid electrolyte. It gives both high redox cyclic stability and high coulombic efficiency (100%) up to 1000 charge-discharge cycles in the cell potential ranges of 0-1.6 V and 0-2.0 V, respectively.

Acknowledgments: This work was supported by the Science and Technology Foundation of Shenzhen (Grant No.JCYJ20170307150808594), and the Area of Excellence Grant from HKPolyU (1-ZE30). The authors are indebted to Dr. Zeba Khanam for her help in the English language. Also, Jingwei Wang would like to thank The Hong Kong Polytechnic University for providing some financial support.

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Journal Pre-proof Author Contributions Section Prof.Song and Miss Wang conceived and designed the experiment. Miss Wang performed the experiments and wrote the paper. Prof.Song and Prof.Chen provided experimental help, and reviewed the manuscript. All authors read and approved the manuscript.

Journal Pre-proof

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: