High-Temperature and All-Solid-State Flexible Supercapacitors with

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Applications of Polymer, Composite, and Coating Materials

High-Temperature and All-Solid-State Flexible Supercapacitors with Excellent Long-Term Stability Based on Porous Polybenzimidazole/Functional Ionic Liquid Electrolyte Tiejun Mao, Shuang Wang, Xu Wang, Fengxiang Liu, Jinsheng Li, Hao Chen, Di Wang, Geng Liu, Jingmei Xu, and Zhe Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00452 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Materials & Interfaces

High-Temperature and All-Solid-State Flexible Supercapacitors with Excellent Long-Term Stability Based on Porous Polybenzimidazole/Functional Ionic Liquid Electrolyte

Tiejun Maoa, Shuang Wang*ab, Xu Wanga, Fengxiang Liub, Jinsheng Lia, Hao Chena, Di Wanga, Geng Liua, Jingmei Xuab, Zhe Wang*ab

aCollege

of Chemical Engineering, Changchun University of Technology, Changchun

130012, People’s Republic of China.

bAdvanced

Institute of Materials Science, Changchun University of Technology,

Changchun 130012, People’s Republic of China.

*Corresponding authors, E-mail addresses: [email protected] (S. Wang).

Keywords: Supercapacitor; Flexible; Porous polybenzimidazole; Functional ionic liquid; Polymer electrolyte; Excellent long-term stability.

Abstract A high-performance solid-state electrolyte was obtained based on porous polybenzimidazole

(pPBI)

and

ionic

liquid

(1-(3-trimethoxysilylpropyl)-3

methylimidazole). IL is hydrolyzed to form Si-O-Si networks under acidic conditions, which guarantee the enhanced mechanical properties. The porous structure and Si-OSi networks improve the acid retention capacity and conductivity simultaneously. The prepared porous composite film exhibits a proton conductivity as high as 0.103 S cm-1

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at 170 °C. Then, an all-solid-state supercapacitor (ASSC) was assembled using the obtained film electrolyte and activated carbon electrodes. The electrochemical performance was evaluated at different temperature (30 °C, 60 °C, 90 °C, 120 °C and 150 °C). The prepared ASSC displayed a specific capacitance of 85.5 F g-1 at 120 °C, which is three times higher than that at 30 °C. Meanwhile, the ASSC displayed excellent long-term cycle stability after 10,000 constant current charge and discharge tests, 91.0% capacitance retention and 95.8% coulomb efficiency. All the results indicate that the porous polymer electrolyte is promising for application in hightemperature energy storage devices.

1. Introduction The continuous development of the global culture and economy has led to two serious problems, one is the rapid consumption of fossil fuels with limited reserves, and the other is the increase of environmental pollution. Therefore, safe, environmentally friendly and energy storage technologies have received widespread attention.1–5 Among many energy storage devices, supercapacitors (SCs) have been developed rapidly and used in many fields, for instance, electric vehicles, military weapons, digital communication instruments, etc.6–8 due to high power density, fast charge and discharge capability and excellent cycle performance.8–12 SCs are mainly composed of electrodes, electrolytes and packaging materials.8,12 Electrolytes affect the performance of supercapacitors, such as maximum operating voltage and long-term stability.10,11 And electrolytes are divided into solid phase and liquid phase. Nowadays, conventional supercapacitors have been commercially produced using a liquid electrolyte.13,14 Liquid electrolytes often cause safety problems due to leakage even explosion, which greatly restrict the large-scale application of


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supercapacitors.14–16 Solid-state electrolytes will significantly simplify the fabrication process as well as improve their safety.17–21 As a core part of supercapacitors, the solidstate electrolytes acts as a separator and a conductive medium.22 Some high-performance polymers have been used as solid-state electrolyte for SCs, including Nafion,23–25 polybenzimidazole (PBI),26,27 poly(acrylonitrile) (PAN),28 polyaryletherketone (PAEK)29 and Poly (vinyl alcohol) (PVA),30 etc. Usually, the mechanical properties of polymer electrolyte will drop dramatically as the ionic conductivity increases.31 Therefore, it is still a huge challenge to develop the solid-state polymer electrolytes that not only has excellent mechanical properties but also has high ionic conductivity.32,33 In addition, SCs are often used in harsh environments, such as high temperature.7,8 Therefore, a polymer electrolyte for SCs should have characteristics of good ionic conductivity, high thermal stability and mechanical strength. Phosphoric acid-doped polybenzimidazole (PA/PBI) exhibits high ionic conductivity, good mechanical properties and thermal stability as solid-state polymer electrolytes.34–36 D. Rathod et al. used PA/PBI as a separator for supercapacitors.26 Wang et al. prepared porous PBI film with improved phosphoric acid doping level as electrolyte for fuel cells.36 However, the mechanical properties of the porous PBI films are significantly degraded due to the presence of the porous structure in the PBI films. In our previous work, we have prepared a type of composite PBI film based on PBI and ionic liquid,37 the functional IL can both improve the ionic conductivity and mechanical properties. The present study investigates a high-performance solid-state polymer electrolyte based

on

porous

PBI

and

functional

IL

(1-(3-trimethoxysilylpropyl)-3-

methylimidazolium chloride). The obtained pPBI-IL-x% (x represents the mass fraction of IL in PBI) possessed excellent mechanical properties, thermal stability and high



conductivity, which ensured it as an electrolyte for all-solid-state supercapacitors (ASSC). An ASSC is assembled by the pPBI-IL-x% composite film and two identical activated carbon electrodes. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) were used to study the electrochemical performance of all-solid-state supercapacitors at room temperature and high temperature. As far as we know, it is the first time that porous PBI has been used in high temperature solid-state supercapacitors. 2. Experimental 2.1 Materials Isophthalic acid (IPA), N-Methyl pyrrolidone (NMP), 3, 3’-diaminobenzidine (DAB) and dibutyl phthalate (DBP) were obtained from Aladdin. Phosphorus pentoxide (P2O5), Methanol and polyphosphoric acid (PPA) were purchased from Macklin. Activated carbon (AC, YP-80F) (surface area: 2100 m2 g−1), nickel foam and conductive carbon black (CC) were provided by Shanghai Huiping Energy co. LTD. China. All other chemicals like phosphoric acid (PA) and sulfuric acid (H2SO4) were provided by local suppliers. 2.2 Synthesis of polybenzimidazole Scheme 1 shows the polymerization of polybenzimidazole (PBI). Polybenzimidazole was synthesized by the condensation of isophthalic acid and 3,3'-diaminobenzidine. The specific method has been described in detail in our previous work.37 Scheme 1 Synthesis of polybenzimidazole.


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2.3 Preparation of pPBI-IL-x% films In order to obtain films with high ion conductivity, we prepared a series of porous composite

films

based

on

ionic

liquid

(1-(3-trimethoxysilylpropyl)-3-

methylimidazolium chloride) and PBI. DBP was chosen to be the porogen according to our previous work.36 A given mass of PBI powder was dissolved in DMAc with vigorous stirring. Then, a little of IL and 50 wt% DBP was added into the PBI homogeneous solution and continuously stirred until the mixture solution was uniform. Next, the mixture solution was poured onto a smooth glass plate and placed in an oven at 80 °C overnight. The obtained polymer film was immersed in methanol solution for 2 hours, and dried in an oven at 110 °C. The composite film was immersed in a 1 M H2SO4 for 24 hours. Afterwards, the sample film was washed with deionized water and placed in an oven under 100 °C for several hours. The composite films were denoted as pPBI-IL-x%, and x represents the mass percentage of IL (x=0, 5, 10, 15, 20). Finally, the pPBI-IL-x% films were immersed in the PA solution for 9 h. The reaction process is shown in Scheme 2. Scheme 2 Synthetic process of pPBI-IL-x% films.



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2.4 Fabrication of all-solid-state supercapacitors The electrode slurry consisted of AC, CC and PBI in a mass ratio of 1:0.2:0.25. The solvent consisted of DMAc and NMP (volume ratio: 7:3). The electrode slurry was sprayed onto the foamed nickel and then dried in an oven at 100 °C for 12 h. The electrode was compacted with a certain pressure (~10 MPa). The active material mass of each electrode was 0.8 mg. Nickel foam acts as a current collector for the electrodes. Then, two identical electrodes (1 × 1 cm2) and the pPBI-IL-x% composite films were assembled by a hot press at 5 MPa and 100 °C. The packaging material of the device is heat resistant tape. 2.5 Characterization 2.5.1 Porosity test The composite film was immersed in demethylation for 24 hours. The change in mass of the film before and after absorption of methanol was recorded. Calculation equation for porosity (1): P=

(W𝑎 ― W𝑏)⁄ρ 𝑉

(1)

× 100%


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where Wa is the mass of the film after absorption of methanol, Wb is the mass of the dried film sample, ρ is the density of methanol, and V is the volume of the dry composite film. 2.5.2 Mechanical properties, thermal stability and morphology of pPBI-IL-x% composite films The mechanical properties of the PA-doped pPBI-IL-x% composite films were performed on an Instron 5965 device at a test rate of 5 mm min-1. Each pPBI-IL-x% composite film sample was cut into 50 mm × 5 mm and tested at least 6 times. The thermal properties of the composite film samples were tested by using a thermogravimetric analysis (TGA) curve of Pyris1 TGA (Perkin Elmer) under a nitrogen atmosphere. All film samples were preheated in an oven at 120 °C for 48 hours to remove absorbed water and solvent before the measurement. All the samples were heated from 100 °C to 800 °C at a heating rate of 10 °C min-1. The cross section of the composite film sample was observed with a JSM-7610F field emission scanning electron microscope, and each sample was subjected to gold spray treatment. 2.5.3 Proton Conductivity The proton conductivity of the films (50 mm × 5 mm) was calculated by a fourelectrode impedance method using an electrochemical workstation (model: PGSTAT302N). The frequency range is from 0.01 Hz to 1 MHz. The temperature is between 100 and 170 °C (tested every 10 min). The equation for proton conductivity: (2)

σ = L /(R × S)

where σ is the proton conductivity in S cm-1, L is the distance between the two



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electrodes (cm), R is the resistance of the film, and S is the cross-sectional area of the film (cm2). 2.5.4 Electrochemical measurements The electrochemical measurement of the fabricated supercapacitor was measured using an electrochemical workstation (model: PGSTAT302N) from Metrohm. The electrochemical window and specific capacitance of the supercapacitor were determined by cyclic voltammetry (CV) (the scan rate is ranging from 5 to 1000 mV s-1). And analyzed the supercapacitors by electrochemical impedance spectroscopy (EIS) (0.01 Hz to 100 kHz, 10mV). Long-term stability of devices tested by galvanostatic charge-discharge (GCD) (the current density is among 0.1 and 8 A g-1). For the flexible performance evaluation, a 1× 6 cm flexible supercapacitor was prepared and tested for electrochemical performance at different degrees of bending. The cycle performance of the supercapacitor was tested by GCD at 1 A g-1 current density. The specific capacitance (C, F g-1) was calculated as follows: 𝐼𝑎

(3)

C = 𝑣×𝑚

The formulas for energy density (E, Wh kg-1) and power density (P, W kg-1) were as follows: 1

E = 2𝐶∆𝑉2

(4)

𝐸

(5)

P=

∆𝑡

where Ia (A) is the average current magnitude of the CV curve current, 𝑣 (mV s-1) is the scan rate, and m (g) is the total mass of the two electrode active species. The ∆𝑡 (s) is scan time.


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3. Results and discussion 3.1 Porosity test The pPBI composite films were prepared with different proportions of porogen and ionic liquid. As shown in Table 1, the porosity gradually decreases with the increasing content of the ionic liquid in the composite films. The reason may be that the content of the IL continuously increases, the accumulation of the silica particles generated by the hydrolysis causes the gradual decrease of the pores.37,38 Table 1 Porosity of pPBI-IL-x% composite films. Samples pPBI pPBI-IL-5% pPBI-IL-10% pPBI-IL-15% pPBI-IL-20%

IL (wt%) 0 5 10 15 20

DBP (wt%) 50 50 50 50 50

Porosity (%) 59.39 52.28 41.48 40.08 38.1

3.2 Mechanical properties The stress-strain curve of the PA-doped composite film is shown in Figure 1. The proper amount of IL improved the mechanical properties of the composite porous films, but when IL content increased to a certain extent, the mechanical properties showed a gradual decline tendency. The addition of an appropriate amount of silica particles in the composite films can improve mechanical properties, which has been concluded in our previous studies.37 However, IL increases to a certain extent, the hydrogen bonds between PBI chains may be destroyed and the chain-chain interaction is reduced. For phosphoric acid-doped composite films, there are many free acids in the films, which will not only weaken the interaction between chains, but also reduce the mechanical properties.39 However, the elongation at break of all the PA-doped composite films



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were improved relative to the pure pPBI film. Usually, the increase in elongation at break of the composite porous films is due to the plasticization of phosphoric acid. The elongation at break of pPBI-IL-15% film even reached 110% indicated that the composite film has good toughness as shown in Table 2.

Figure 1 Stress-strain curve of PA-doped pPBI-IL-x% composite films. Table 2 The mechanical properties of PA-doped pPBI-IL-x% composite films. Samples

Tensile strength

Young modulus

Elongation at break

(MPa)

(MPa)

(%)

pPBI

3.9±0.3

10.3±0.5

40.8±4.1

pPBI-IL-5%

4.6±0.6

10.2±0.3

44.9±1.5

pPBI-IL-10%

3.1±0.1

19.7±0.3

51.8±5.1

pPBI-IL-15%

2.8±0.1

27.3±0.5

110.5±5.0

pPBI-IL-20%

2.0±0.1

32.3±0.5

84.2±7.5

3.3 Thermal stability The thermal stability of solid polymer electrolytes plays a very important role in high temperature supercapacitor applications.40 The TGA curves of the pPBI-IL-x%


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composite films were shown in Figure 2. The pristine pPBI film showed a one-step degradation which began from 400 °C corresponding to the thermal decomposition of the PBI backbone. For other composite films, the thermal decomposition began at 250 °C may due to thermal decomposition of IL in the films. The thermal decomposition at 550 °C is attributed to the decomposition of the PBI backbone. The results of TGA indicated that the thermal stability of the composite films gradually decreased as the IL content in the composite films increased. Although the thermally stability of pPBI-IL-x% composite film is slightly lower than that of the pristine PBI film, the composite films can stand above 200 °C and meet the high-temperature SCs applications.

Figure 2 TGA curves of pPBI-IL-x% composite films. 3.4 Morphologies of the porous polymer electrolyte



Figure 3 Morphology characterization of pPBI (a), pPBI-IL-5% (b), pPBI-IL-10% (c), pPBI-IL-15% (d), pPBI-IL-20% (e) and surface morphology of pPBI-IL-15% (f). Surface and cross-section micrographs of porous composite films containing different IL contents were studied in detail by scanning electron microscopy. The cross-section of all composite films exhibits a honeycomb network structure as shown in Figure 3. It can also be observed that the pores in the film become denser owing to the addition of IL. Especially, the pore structure of the cross-section becomes more irregular when the IL content was increased to 20 wt%. This is mainly due to the corresponding increase of the Si-O-Si network structure in the composite films as the IL content increases. As shown in Table 1, the porosity decreases with the increase of the IL content, the porosity becomes smaller and smaller, which confirms this conclusion. The presence of a porous network structure in the composite film can absorb more PA while promoting rapid penetration of electrolyte ions into the electrodes. Figure 3f shows the surface morphology of the pBI-IL-15% film, and it is relatively smooth and uniform. 3.5 Conductivity


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Figure 4 Proton conductivity of pPBI-IL-x% composite films at different temperature. The conductivity of the film is an important parameter to evaluate the ion transport ability. Therefore, the conductivity at a high temperature (from 110 °C to 170 °C) of the pPBI-IL-x% composite films was tested as depicted in Figure 4. The proton conductivity of all pPBI-IL-x% composite films increases linearly with temperature. The conductivity increases gradually with the IL content increases at the same temperature. Among them, the pPBI-IL-15% film has the highest conductivity about 0.103 S cm-1 at 170 °C, which is nearly 50% higher than that at 110 °C. There are three main reasons for the high conductivity of pPBI-IL-x% films. First, the IL in the film can act as proton carriers, which resulting in effectively enhanced proton conductivity. Then, as the temperature increases, the proton motion becomes more intense and promotes an increase in conductivity. The last reason is that, as shown in the crosssectional electron microscope of Figure 3, the presence of porous structure in the composite film allows film it to adsorb more PA, and higher PA uptake leads to higher proton conductivity.36,41 In view of the outstanding performance, the pPBI-IL-x% film was selected as solid



electrolyte for supercapacitors. 3.6 Electrical characterization

Figure 5 Schematic representation of an ASSC consisting of the pPBI-IL-x% films and activated carbon electrodes. An all-solid-state supercapacitor (ASSC) was assembled using two identical AC electrodes and a pPBI-IL-x% porous polymer electrolyte, designated ASSC-x. Figure 5 shows a schematic of the ASSC-x.

Figure 6 (a) CV curves of the device ASSC-x at 100 mV s-1; (b) Specific capacitance


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of different devices ASSC-x. (c) CV curves of ASSC-15 for different scan rates (51000 mV s-1) at room temperature; (d) GCD curves of ASSC-15 at different current densities (0.1-8 A g-1) at room temperature. To evaluate the electrochemical properties of ASSC-x, the device ASSC-x was tested for CV curves at 100 mV s-1. The current density of the device scan window increases with increasing IL content at the same scan rate, and the CV curve area of device ASSC15 is the largest as shown in Figure 6a. Thus, ASSC-15 has the largest capacitance. In order to study the capacitance performance of the device, the specific capacitance of each device at 50 mV s-1 is calculated by formula (3) as shown in Figure 6b. The specific capacitance of the device ASSC-15 can reach 38.41 F g-1. To further evaluate the performance of the SCs, the ASSC-15 CV was tested at different scan rates (5-1000 mV s-1). CV curves of ASSC-15 exhibit a similar increase in current density with the increased CV scan rate as shown in Figure 6c. The CV curve is similar to a rectangle at a high scan rate of 1000 mV s-1, indicating that the prepared supercapacitor has good capacitive performance. At room temperature, the galvanostatic charge-discharge (GCD) test of ASSC-15 was investigated at different current densities (0.1, 0.2, 0.5, 1, 2, 5, 6 and 8 A g-1). Figure 6d shows the GCD curves at various current densities. The GCD curve appears to be relatively asymmetrical at 0.1 A g-1. This may be due to side reactions occurring during the charging process. The side reaction may be the oxidative degradation of PBI.42 The GCD curves at other current densities show showed a shape similar to an isosceles triangle, even at a high current density of 8 A g-1 with only a small IR drop. The results indicated that the prepared ASSC-15 is an alternative with



ideal capacitance behavior and small voltage drop as well as internal resistance of the device.

Figure 7 EIS curves of the supercapacitors and the inset displays the EIS for the high frequency region. Electrochemical impedance spectroscopy (EIS) is commonly used to study the charge transfer process of devices. Electrochemical impedance spectroscopy was performed on various ASSC devices using a two-electrode method at a frequency of 0.01 Hz to 100 kHz. As shown in Figure 7, the EIS curves of all devices displayed the ideal Nyquist plot of the electrochemical double layer capacitor. The intersection intercept of the EIS curves and the Z' real axis indicates the equivalent series resistance at high-frequencies which is mainly the resistance (Rs) of the electrode and the electrolyte. The intercept point of the ASSC-15 curve and the real axis is 1.52 Ω which has the lowest internal resistance among all the fabricated devices. In addition, the EIS curves of all devices is nearly parallel to the vertical axis which further proves that the device has an ideal capacitive behavior in the low frequency range and it is rooted from the diffusion of ions toward the electrolyte within the electrode. It can be attributed that


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the porous PBI composite films can absorb more PA and the addition of IL provides more charge carriers for charge transfer, which facilitates the rapid transfer of ions.

Figure 8 The CV curves of (a) ASSC-0 and (b) ASSC-15 from 30 °C to 150 °C; (c) Specific capacitance of device ASSC-15 at various temperature; (d) The relationship between the IL content and the specific capacitance at 120 °C. Commercial SCs may encounter safety problems caused by local overheating in practical application.43 We measured the electrochemical properties of ASSC at high temperature. Figure 8a and b show the CV of the SCs measured at various temperature (30 °C, 60 °C, 90 °C, 120 °C and 150 °C) for devices ASSC-0 and ASSC-15 at a scan rate of 50 mV s-1. It clearly shows that CV curves of ASSC-0 and ASSC-15 are close to a rectangle shape. The CV curve is approximately rectangular even at 150 °C, indicating that the prepared supercapacitor has good capacitance performance at high temperatures. In addition, the area of CV curves of ASSC-0 and ASSC-15 increases



with elevated temperature and current response density. The CV curve of ASSC-15 was influenced more significantly by temperature, and the area of the CV curve became the largest at 120 °C. And a redox peak appears in the electrochemical window of 0.12 V which may be caused by the oxidation-reduction reaction of IL at high temperature. As shown in Figure 8c that the specific capacitance of the device ASSC-15 at various temperature using formula (3). It can be observed that as the temperature increases, the specific capacitance is gradually enhanced. When the temperature rises to 120 °C, the specific capacitance can reach to 85.5 F g-1, which is nearly 3 times higher than that at 30 °C. The reason for the increase in capacitance is that the ion carrier PA accelerates and migrates rapidly into the pores of the activated carbon and expands the contact area between the electrode and the electrolyte at elevated temperature;27 another reason is that the pPBI-IL-15% composite porous film electrolyte assembled devices have low internal resistance and high conductivity at high-temperature, as mentioned above in Figure 4. When the temperature elevated to 150 °C, the specific capacitance will decrease, which may be caused by the formation of by-products such as pyrophosphoric acid at high temperature. In Figure 8d, the relationship between the IL content and the specific capacitance at 120 °C is compared. It is clear that when the amount of IL reaches to 15 wt%, the specific capacitance of the ASSC-15 is the highest.


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Figure 9 Ragone plot of the ASSC-15. Energy density (E) and power density (P) are two important electrochemical performance parameters for evaluating supercapacitors,44–46 calculated using equations (4) and (5). It is calculated that the maximum specific energy of the device ASSC-15 can reach 11.87 Wh kg-1 and the specific power can reach 2136 W kg-1. This has exceeded the specific energy density of commercial production.22 As shown in Figure 9, the performance of the device ASSC-15 based on the pPBI-IL-15% composite porous film electrolyte was compared with similar equipment reported in previous literature. The SC in this work possesses relatively high energy density and power density.

Figure 10 (a) CV curve of an all-solid supercapacitor measured at different bending angles (θ = 0°, 60°, 120°) and the inset displays a photo of a curved flexible device; (b)



Long-term stability of the supercapacitor. Flexibility is a key performance parameter in the practical application of energy storage devices, so the CV of the device is tested at different bending angles (θ = 0°, 60°, 120°). A supercapacitor with size of 1 cm × 6 cm was prepared for flexibility testing. Figure 10a shows the CV curves of the device ASSC-x for different bending degrees. The illustration in Figure 10a is a photo of the device being bent. It can be clearly seen that all the CV curves are substantially coincident whether bent by 60° or 120° indicating that the device has good flexibility. Long-term stability plays a crucial role in the practical application of supercapacitors.47 The device ASSC-15 was tested for galvanostatic charge-discharge at a current density of 1 A g-1. Figure 10b shows the relationship among the number of cycles, the coulombic efficiency and capacitance retention. After 3000 cycles, the capacitance retention of the device ASSC was hardly decreased. What is more remarkable is that the capacitance retention rate is 91.0% after 10,000 cycles. The coulomb efficiency is still as high as 95.8% after 10,000 cycles. Such results further indicate that the supercapacitor ASSC-15 composed of the pPBI-IL-15% composite film has practical application possibilities. 4. Conclusion Here, we successfully prepared a series of solid-state polymer electrolyte based on porous PBI and functional IL. The pPBI-IL-15% composite film can achieve not only high conductivity of 0.103 S cm-1 at 170 °C, but also has good mechanical properties and thermal stability. The specific capacitance of ASSC-15 can reach 85.5 F g-1 at


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120 °C, which is 3 times higher than that at room temperature. Further, the prepared supercapacitors can maintain stable electrochemical performance under different bending conditions. Moreover, the all-solid-state supercapacitor still has a capacitance retention rate of 91.0% after 10,000 GCD cycles, and the coulomb efficiency is still as high as 95.8% after 10,000 cycles. Therefore, the prepared high-performance porous PBI composite film is beneficial to the high-temperature application of the supercapacitor, and can be theoretically applied to other energy storage devices.

Notes The authors declare no competing financial interest.

Acknowledgments The authors gratefully acknowledge the financial support of this work by Natural Science Foundation of China (grant no.s 51603017, 51673030), Jilin Provincial Science & Technology Department (grant no.s 20180101209JC, 20160520138JH), Education Department of Jilin Province (grant no. JJKH20191286KJ) and ChangBai Mountain Scholars Program of Jilin Province.

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TOC 82x44mm (300 x 300 DPI)



Scheme 1 Synthesis of polybenzimidazole. 85x43mm (300 x 300 DPI)


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Scheme 2 Synthetic process of pPBI-IL-x% films. 124x80mm (300 x 300 DPI)



Figure 1 Stress-strain curve of PA doped pPBI-IL-x% composite films. 85x62mm (300 x 300 DPI)


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Figure 2 TGA curves of pPBI-IL-x% composite films. 85x62mm (300 x 300 DPI)



Figure 3 Morphology characterization of pPBI (a), pPBI-IL-5% (b), pPBI-IL-10% (c), pPBI-IL-15% (d), pPBIIL-20% (e) and surface morphology of pPBI-IL-15% (f). 124x62mm (300 x 300 DPI)


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Figure 4 Proton conductivity of pPBI-IL-x% composite films as a function of temperature. 85x61mm (300 x 300 DPI)



Figure 5 Schematic representation of an ASSC consisting of the pPBI-IL-x% films and activated carbon electrodes. 124x68mm (300 x 300 DPI)


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Figure 6 (a) CV curves of the device ASSC-x at 100 mV s-1; (b) Specific capacitance of different devices ASSC-x. (c) CV curves of ASSC-15 for different scan rates (5-1000 mV s-1) at room temperature; (d) GCD curves of ASSC-15 at different current densities (0.1-8 A g-1) at room temperature. 124x87mm (300 x 300 DPI)



Figure 7 EIS curves of the supercapacitors and the inset displays the EIS for the high frequency region. 85x61mm (300 x 300 DPI)


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Figure 8 The CV curves of (a) ASSC-0 and (b) ASSC-15 from 30 °C to 150 °C; (c) Specific capacitance of device ASSC-15 at various temperature; (d) The relationship between the IL content and the specific capacitance at 120 °C. 124x95mm (300 x 300 DPI)



Figure 9 Ragone plots of the ASSC-15. 85x62mm (300 x 300 DPI)


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Figure 10 (a) CV curve of an all-solid supercapacitor measured at different bending angles (θ = 0°, 60°, 120°) and the inset displays a photo of a curved flexible device.; (b) Long-term stability of the supercapacitor. 124x47mm (300 x 300 DPI)


High-Temperature and All-Solid-State Flexible Supercapacitors with

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