Synthesizing a Gel Polymer Electrolyte for Supercapacitors, Assembling a Supercapacitor Using a Coin Cell, and Measuring Gel Electrolyte Performance

Ohhyun Kwon; Jihyeon Kang; Seohyeon Jang; Seyoung Choi; Hojong Eom; Junhyeop Shin; Jong-Kwon Park; Soomin Park; Inho Nam

doi:10.3791/64057

JoVE Journal > Engineering

Ingénierie

Synthesizing a Gel Polymer Electrolyte for Supercapacitors, Assembling a Supercapacitor Using a Coin Cell, and Measuring Gel Electrolyte Performance

Published: November 30, 2022

doi:

10.3791/64057

Ohhyun Kwon¹, Jihyeon Kang¹, Seohyeon Jang¹, Seyoung Choi¹, Hojong Eom¹, Junhyeop Shin¹, Jong-Kwon Park¹, Soomin Park², Inho Nam¹

¹School of Chemical Engineering and Materials Science, Department of Intelligent Energy and Industry, Department of Advanced Materials Engineering,Chung-Ang University, ²School of Energy, Materials and Chemical Engineering,Korea University of Technology and Education

Summary

We present a protocol to test the electrochemical and physical properties of a supercapacitor gel polymer electrolyte using a coin cell.

Abstract

Supercapacitors (SC) have attracted attention as energy storage devices due to their high density and long cycle performance. SCs used in devices operating in stretchable systems require stretchable electrolytes. Gel polymer electrolytes (GPEs) are an ideal replacement for liquid electrolytes. Polyvinyl alcohol (PVA) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) have been widely applied as a polymer-matrix-based electrolytes for supercapacitors because of their low cost, chemically stable, wide operating temperature range, and high ionic conductivities. Herein, we describe the procedures for (1) synthesizing a gel polymer electrolyte with PVA and PVDF-HFP, (2) measuring the electrochemical stability of the gel polymer electrolytes by cyclic voltammetry (CV), (3) measuring the ionic conductivity of the gel polymer electrolytes by electrochemical impedance spectroscopy (EIS), (4) assembling symmetric coin cells using activated carbon (AC) electrodes with the PVA- and PVDF-HFP-based gel polymer electrolytes, and (5) evaluating the electrochemical performance using galvanostatic charge-discharge analysis (GCD) and CV at 25 °C. Additionally, we describe the challenges and insights gained from these experiments.

Introduction

Flexible SCs have grown rapidly in recent years for the fabrication of electronics with stretchable displays and wearable energy devices. Flexible SCs typically consist of flexible electrodes¹, separators², and the electrolyte³ in a flexible assembly. Therefore, GPEs are the most effective structure owing to their flexibility⁴, separator-free nature, relatively high ionic conductivity⁵, and thin-film forming ability⁶.

To prepare the polymer matrices of GPEs, materials such as polymethylmethacrylate (PMMA), PVDF-HFP, and PVA have been developed in recent years. PVA and PVDF-HFP have especially been widely applied as polymer-matrix-based electrolytes for SCs due to their low cost, chemically stable, wide operating temperature range, and high ionic conductivities at room temperature (RT).

Herein, we describe a synthetic method for two representative polymer-matrix materials-PVA⁷ and PVDF-HFP-and the electrochemical characterization of the polymer-matrix material-based gel electrolyte. In summary, we illustrate the general synthesis, material processing methods, and performance evaluation methods employed to fabricate stretchable SCs.

For application in flexible SCs, polymer electrolytes should exhibit the following properties: (1) high ionic conductivity at ambient temperature, (2) high chemical and electrochemical stability, (3) good mechanical properties of dimensional stability, and (4) sufficient thin film processability. These features were confirmed using EIS, CV, and tensile tests. The EIS and CV measurements were conducted using a coin cell. First, the ionic conductivity of the polymer-matrix-based electrolyte was estimated according to the equation using impedance. Second, the chemical and electrochemical stabilities of the polymer-matrix-based electrolyte were estimated by the CV and GCD tests. The stabilities of the polymer-matrix-based electrolytes were demonstrated by controlling the voltage range tested by the CV. Third, the mechanical properties of the polymer-matrix-based electrolytes were evaluated by conducting tensile tests.

A coin cell was fabricated using PVA- and PVDF-HFP polymer-matrix-based electrolytes with AC symmetric cells. The supercapacitor performances of the two different coin cell supercapacitors were evaluated at 25 °C. Because this work mainly involves PVA- and PVDF-HFP polymer-matrix-based electrolytes, the remainder of this paper focuses on these electrolytes. The detailed procedures of these experiments, difficulties in execution, and insights gained from these experiments are described as below.

Protocol

1. Synthesis of PVA- and PVDF-HFP polymer-matrix-based electrolytes

NOTE: When handling methanol, it is best to avoid direct exposure as much as possible.

PVA polymer-matrix-based electrolyte synthesis
1. Dissolve PVA (1 g) (Mw 146,000-186,000)in double-distilled water (10 mL) in a water bath at 90 °C and stir at 500 rpm until a clear solution is obtained. Then, add H₃PO₄ (1 mL) to the hot solution with constant stirring at RT for 24 h.
2. Pour the stretchable polymer electrolyte into a glass Petri dish and dry it overnight in a vacuum oven at 40 °C.
  NOTE: The thickness of the as-formed GPE should be approximately 1 mm.
3. Peel off the dried films from the molds and cut them into 19 mm specimens for further testing.
PVDF-HFP polymer-matrix-based electrolyte synthesis
1. Prepare a gel polymer electrolyte using the solution-casting method. First, dissolve PVDF-HFP (MW 400,000) (3 g) in dimethylformamide (DMF, 15 mL) with a lidded container at RT and stir at 500 rpm for 3 h until a homogeneous, low-viscosity solution is formed.
2. Add diglycidyl ether of bisphenol-A (DEBA; 1 g), poly (ethylene glycol) diglycidyl ether (PEGDE; 3 g), and diamino-poly (propylene oxide) (DPPO; 8 g) to the solution prepared in step 1.2.1 and stir at 500 rpm constantly at ambient temperature for 6 h.
3. Pour the resultant mixture into a round polytetrafluoroethylene plate or plastic Petri dish and heat it in a vacuum oven at 80 °C for 24 h to evaporate the DMF solution and to afford the desired GPE.
4. Cool the resultant GPE to RT, wash it thrice with methanol using a centrifuge at 12,329 × g for 5 min to remove the unreacted monomer, and dry under a vacuum for 12 h at 60 °C.
  NOTE: The thickness of the as-formed gel polymer electrolyte is approximately 100 µm. The resultant gel polymer electrolyte showed excellent mechanical properties when the weight ratio of PEGDE:DEBA:DPPO was optimized to 3:1:8 and the content of PVDF-HFP was optimized to 20 wt%.
5. Prepare the synthesized GPEs by immersing the porous membranes in a liquid electrolyte (1 M LiPF₆ in EC/DMC = 1/1, v/v) for 24 h in an argon-filled glove box.
  NOTE: The liquid electrolyte uptake after soaking for 24 h was approximately 350 wt%.

2. Characterization of the GPEs

Fourier transform infrared (FTIR) spectroscopy
NOTE: We recommend using a slid-on attenuated total reflection (ATR) accessory with the FTIR spectrometer for collecting spatially resolved FTIR spectra (high spatial resolution of ~10 µm²) of the interaction between the polymer-matrix-based electrolytes.
1. Select a sample with appropriate dimensions for the FTIR microscope to ensure high-quality spectra using the ATR-FTIR accessory.
2. Calibrate the FTIR and take the same sample measurements in the range of 500-4500 cm^-1 at a 5 cm^-1 resolution. This process includes cooling the detector and allowing sufficient time for stabilization.
3. Collect an appropriate background spectrum to subtract from the sample spectrum.
4. Depending on the appropriate objective, select the area of interest and focus on the same area for analysis.
5. After determining the area of interest, attach the ATR accessory to the FTIR microscope objective. Lower the ATR accessory until it contacts the sample closely, and then collect the sample spectrum.
6. Perform data processing after collecting the FTIR spectra.
X-ray diffraction (XRD)
1. Mill the sample powder using an agate mortar. Then, deposit the powder on the sample holder of the X-ray diffractometer to fill the hole until it overflows, and press to form a uniform, smooth surface. The instrumental parameters of the XRD analysis are described in references⁸^,⁹.
2. Before measuring the film-type XRD patterns of the sample, keep the polymer-matrix-based electrolyte as flat as possible in the holder. The instrumental parameters of the XRD analysis were the same as those described in step 2.2.1.

3. Preparation of the composite AC electrode

Prepare a powdered composite electrode by mixing AC, conductive carbon, and PTFE binder in a mass ratio of 8:1:1 using a mortar until it becomes a dough. Add a drop of isopropanol (IPA; 0.1-0.2 mL) to the dough, and spread the mixture repeatedly to thoroughly mix it.
Roll the dough using a roller to achieve the desired thickness (~100 mm), and construct AC electrodes with radius of 14 mm.
Dry the AC electrode in an oven at 80 °C for 24 h to completely evaporate the IPA.

4. Coin cell preparation and testing

Heat 15 mL of H₃PO₄-PVA at 80 °C and immerse the AC electrodes in this solution for 10 min. After the process, dry the electrodes in a hood for 4 h to evaporate the water.
Press the two AC electrodes face-to-face with the polymer electrolyte placed in between to form a sandwich structure.
Similarly, to prepare the coin cell containing the PVDF-HFP gel, assemble the AC symmetric cell using the electrolyte soaked in step 4.1
NOTE: Figure 3 shows a schematic of the coin-cell assembly.
To prepare the coin cells for testing, close the 2032 coin cell with a cell cap and crimp two or three times using a manual crimping machine.

5. EIS, CV, and GCD testing methods for the PVA and PVDF-HFP GPEs

NOTE: The potentiostats consist of a working sensor (WS), a working electrode (WE), a reference electrode (RE), and a counter electrode (CE).

Before testing the two-electrode system, combine the WS line with the WE line, which is working as WE, and the RE line with the CE line, which is working as CE.
Then, insert the coin cell into the holder used for the electrochemical test and connect the WE line and CE line on both sides.
NOTE: All tests were conducted using the coin cells that were prepared.
EIS test
NOTE: The 'Rest Time' step is necessary to stabilize the cell before the EIS test. Smart Management 6 program is used for setting the sequence and measuring the electrochemical result.
1. Run the program and set the EIS measurement experiment sequence file.
2. Click on the Experiment option to generate a new file, and then click on the Add button to generate the first step.
3. Then set the Rest Time parameters of the sequence file. Set the Control tab as Constant. Set the Type, Mode, and Range in the Configuration tab as PSTAT, Timer Stop, and Auto, respectively.
4. Conduct complex impedance measurements using an EIS system in the frequency range 100 kHz-0.01 Hz.
5. Click on the Add button to generate the next step.
6. Click on the Control button and set it as EIS; for the Configuration, set the Type, Mode, and Range as PSTAT, LOG, and AUTO, respectively.
7. Conduct the EIS at 100 kHz-0.01 Hz. For this, set the Initial (Hz) and Middle (Hz) as the same value, 100 x 10³, and the Final (Hz) value as 1 x 10^-2. Set the Bias (V) values as 400 x 10^-3. Then, click on the Ref button and set it as Eref.
8. The resulting signal must exhibit a linear response to the applied signal. Therefore, set the amplitude (Vrms) to 10 x 10^-3.
9. Set the Density and Iteration as 10 and 1, respectively, for this experiment.
10. Click on the Save As button to save the file for EIS testing.
11. Click on Apply to CH, and run the file for EIS testing to obtain results.
CV test
NOTE: In this case, the operating voltage depends on the solvent used to prepare the GPE.
1. Run the program to generate the sequence file.
2. Click on Experiment to generate a new file, and then click on the Add button to generate the first step.
3. Set the parameters of the sequence file: Control as SWEEP, Type, Mode, and Range de Configuration as PSTAT, CYCLIC, and AUTO, respectively, Ref as Eref, Initial (V) and Middle (V) as 0.0, and the Final (V) as 800 x 10^-3.
4. Conduct CV at scan rates of 5, 10, 20, 50, and 100 mV/s. For this, create five identical steps, and set the scan rate (V/s) to 5 x 10^-3, 10 x 10^-3, 20 x 10^-3, 50 x 10^-3, and 100 x 10^-3 for the aforementioned scan rates taken in order. Set the other parameter values as the same as those in step 5.4.3.
  1. In each scan rate, set the Quiet Time(s) value as 0 and Segments as 21. The formula "2n+1" (n is the number of desired cycles) was used to determine the value of the Segments. For the Cut Off condition, Item was set as Step End and Go Next as Next.
  2. In the Misc. setting, set the Item Value as Time(s) and OP as >=. The Delta Value expresses the conditions for data collection. To collect nearly 300 data points at each scan rate, set the Delta Value as 0.9375, 0.5, 0.25, 0.125, and 0.0625.
  3. For the cycling stability test, set the scan rate (V/s) to 100 x 10^-3 and set Segments as 2001 for the 1000 cycle test. Set the other parameter values as the same as those in step 5.4.4.2.
5. To save the sequence file for the CV test, click on the Save As button.
6. Click on Apply to CH and run the sequence file of the CV test to obtain the results.
GCD test
1. Run the program as mentioned in step 5.3.1 and create a new file for the GCD test.
2. Click on Experiment to generate a new file, and then click on the Add button to generate the first step.
3. Set the parameters of the sequence file. Set Control as Constant. Set the Type, Mode, and Range in Configuration as GSTAT, Normal, and Auto, respectively. The GCD test starts with a charge.
4. Set Ref. as Zero. The current(A) value depends on the current density and electrode weight. A current density of 1 mA/g was selected for the GCD test.
  1. For the Cut Off condition, click on Item and set it as Voltage. Set OP as >=, Delta Value as 800 x 10^-3, and Go Next as Next. For the Misc. setting, set the Item as Time(s), OP as >=, and Delta Value as 1.
5. Click on the Add button to create the next step (Discharge step).
  NOTE: This step is set the same as the charge step, but the current direction is different.
  1. For the discharge, the value of the current is the same as the charging flow, but the current direction is the opposite. Set the value of Current(A) to be the same as step 5.5.4, which is a charing step.
  2. For the Cut Off condition, set the Item as Voltage, OP as <=, Delta Value as 0, and Go Next as Next. For the Misc. setting, set the Item as Time(s), OP as >=, and Delta Value as 1.
6. Click on the Add button to create the next step (Loop step).
  1. Set the Control as Loop, and for Configuration, set the Type as Cycle and Iteration as 21.
  2. For Condition-1 of the Cut Off condition, set the Item at List 1 as Loop Next. For each current density, set Go Next as SETP-2 for 1 mA/g.
7. Click on the Save As button to save the sequence file of the GCD test.
8. Click on Apply to CH, and run the sequence file of the GCD test to obtain the results.

6. Stretchable gel testing

Prepare rectangular shape gel films with dimensions of 1 cm × 10 cm, and repeat the following steps at least twice with different samples to obtain a reasonable value.
Fix the prepared sample between two grips of the tensile testing machine. In this study the gap was set as 5 cm.
Set the desired gap value by adjusting the button to lower the grip on the top.
Run the program to generate the sequence file.
1. Choose the test method. Here, the stress-strain test was selected.
2. Next, choose the number of trials, and apply the selected number. Then, check the test conditions, and set the stretching rate to 50 mm/min.
3. Save the file and apply it to the program. Then, click on the Start button.

7. Stretchable gel deformation test

Prepare rectangular shaped gel films with dimensions of 1 cm × 10 cm, and repeat the test twice.
Fix the prepared sample between two grips of the tensile testing machine with a gap of 5 cm.
Run the program to generate the sequence file.
1. Choose the test method. Here, select the stress-strain test.
2. Select the number of trials and apply the selected number. Then, check the test conditions, set the stretching rate to 50mm/min, and the displacement to 10 mm. Repeat this procedure 10 times.
3. Save the file and apply it to the program. Then, click on the Start button.

Representative Results

PVA was widely applied as a polymer-matrix-based electrolyte for SCs because it is biodegradable, inexpensive, chemically stable and non-toxic, has a wide operating temperature range, and has a transparent-film forming capability¹⁰^,¹¹. PVA enhances ionic conductivity due to its hydroxyl groups which absorb water¹². In this study, we prepared the PVA-based gel electrolyte by mixing H₃PO₄/H₂O, which served as a proton source. In gel electrolytes, H₃PO₄ generally provides free ions that act as charge carriers, and the solvent acts as a conductive medium that helps salt dissolution¹³. As shown in Figure 1A, there are three proton migration mechanisms: (1) Grotthuss or hopping mechanism, where the proton diffuses by forming and cleaving a hydrogen bond network¹⁴. (2) Diffusion mechanism, where the proton forms a complex with water molecules and its driving force is the proton concentration¹⁵. (3) Direct transfer mechanism, where protons can move freely through the polymer chains¹⁶.

PVDF-HFP-based GPEs have also been used to achieve high ionic conductivities at RT while maintaining long-cycle stability¹⁷. Furthermore, PVDF-HFP has a linear structure and high dielectric constant, which provides membrane flexibility and numerous charge carriers. Three additional polymers were used in this synthetic strategy (Figure 1B): poly(ethylene glycol) diglycidyl ether (PEGDE), diamine poly(propylene oxide) (DPPO), and diglycidyl ether of bisphenol-A (DEBA). DEBA was used as a supporting polymer to enhance the strength of the gel polymer network. DPPO and PEGDE were connected to the DEBA network¹⁸.

FTIR and XRD analyses were performed to determine the structural properties of the GPEs. The FTIR spectra of the pure PVA and PVA-based gel electrolyte films are shown in Figure 2A, which indicates that phosphoric acid and PVA interact with each other. The O-H and C-O stretching vibrations of the pure PVA and PVA-based gel electrolyte are significantly different. The peaks at 3293 and 1088 cm^-1 correspond to the O-H and C-O stretching vibrations, respectively. The decrease in the peak intensity and red shift in the spectra indicate that PVA forms a network. These peaks confirmed the successful interaction between H₃PO₄ and PVA through the O-H and C-O groups. The FTIR spectrum of the PVDF-HFP-based gel electrolyte is shown in Figure 2B. The peaks at 1250, 380, 1610, and 1510 cm^-1 are attributed to C-C stretching. The peak at 1110 cm^-1 is ascribed to the C-O-C stretching bond, whereas the peak at 930 cm^-1 corresponds to C-N bonding, which confirms that the reaction occurred.

XRD is a useful technique for studying the structural characteristics of gel electrolytes. Figure 2C shows the XRD patterns of the pure PVA and PVA-based gel electrolyte. These patterns confirm that the acid interacts with the polymer phases and changes the properties of the polymer. In the XRD pattern of pure PVA, the reflection plane (101) indicates a semicrystalline nature. The peak corresponding to the (110) phase is high because of the hydroxyl groups in the PVA side chains, which result in a high degree of hydrogen bonding. This peak broadened with the addition of H₃PO_4, indicating that the gel had an amorphous nature compared to the pure PVA film. The XRD patterns confirmed the formation of the H₃PO₄ and PVA complexes.

Figure 2D shows the XRD pattern of the PVDF-HFP-based GPE. The peaks at 2θ = 18.2°, 19.9°, and 38.9° correspond to the (100), (020), and (020) crystalline peaks, respectively¹⁹. Compared to the reference crystallinity data²⁰, the crystallinity of the polymer is considerably lower. The intensities of the crystalline peaks are low and broad.

Next, we performed electrochemical tests to confirm the ionic conductivity, chemical stability, and electrochemical stability of the PVA- and PVDF-HFP polymer-matrix-based electrolytes. A coin cell was used to evaluate the electrochemical properties of the PVA- and PVDF-HFP-based GPEs. The supercapacitor design, components, and assembled structure are shown in Figure 3, where two stainless steel current collectors or AC electrodes were positioned at the ends to create sandwich-like batteries. Symmetric coin cells were used to measure the ionic conductivity and chemical stability. An ideal GPE has high ionic conductivity and chemical and electrochemical stabilities. The chemical stability of the gel polymer electrolyte was determined at 25 °C by CV, using a coin-cell setup (Figure 3). CV was performed to measure the electrochemical stability using the following coin cells: "SUS|| PVA-based GPE||SUS" and "SUS|| PVDF-HFP-based GPE||SUS". The CV of the PVA-based GPE was performed at 25 °C in the range 0-0.8 V, and that of the PVDF-HFP-based GPE was performed in the range 0-2.5 V at a scan rate of 1 mV/s. The operating voltage of the aqueous electrolyte was limited to 1.23 V, which is the electrolysis voltage of water²¹. Organic electrolytes have a higher electrochemical potential window (2.5-2.8 V)²² than aqueous electrolytes. Figure 4A,B demonstrates that the PVA and PVDF electrolytes were stable in the tested voltage range.

The ionic conductivities of the electrolytes were measured using EIS. The electrolyte membrane was cut into round disks and sandwiched between two stainless steel current collectors.

where L is the gel electrolyte film thickness or separating distance between the two electrodes, A is the contact area between the GPE and the AC electrodes or SUS plate, and R_e is the GPE resistance, which can be estimated from the real part of the impedance at high frequency. As shown in Figure 5, the intercept of the real axis (Z' axis) indicates that R_e is associated with the intrinsic resistance of the electrode material, ionic resistance of the GPE, and contact resistance at the interface. Figure 5A,B shows the Nyquist plots of the PVA- and PVDF-HFP-based electrolytes. Their ionic conductivities were calculated using ohmic resistance. The R values are listed in Table 1.

To evaluate the electrochemical stability of the gel polymer electrolytes, CV and GCD tests were performed using an AC symmetric coin cell. Figure 6A shows the CV curves of the PVA-based GPE SC at different scan rates ranging from 5 to 100 mV/s. The PVA-based GPE SC exhibits a potential window of 0.8 V and remains rectangular with increasing scanning rates of the CV loops. The CV curves of PVDF-HFP GPE are shown in Figure 6B. The test was conducted under the same conditions as those used for the PVA-based GPE. Next, we performed a GCD test. The GCD graphs (Figure 6C) demonstrate that both the PVA- and PVDF-HFP-based gel electrolytes exhibited symmetrical curves, which are characteristic of an electric double layer capacitor. Cycling stability is an important parameter that reflects the electrochemical performance of the supercapacitor. To evaluate the cycling stability of the supercapacitor with the GPE, CV tests were conducted at a scan rate of 100 mV/s_.The electrochemical stability of the AC electrode with the PVA-based gel electrolyte at a scan rate of 100 mV/s is shown in Figure 7A. It was observed that 98.6% of the original capacitance was retained after 1000 cycles, which is an acceptable cycle ability for the PVA-based gel electrolyte. As shown in Figure 7B, the AC electrode with the PVDF-HFP GPE retained 101% of its original capacitance after 1000 cycles. The performance of the supercapacitor improved as the electrodes were gradually wetted by the electrolyte²³^,²⁴.

The mechanical properties of the PVA- and PVDF-HFP-based gel electrolytes are shown in Figure 8. As shown in Figure 8A, the PVA-based gel electrolyte exhibited an elongation at break because of the phosphoric acid, which entangles the polymer chains. The tensile strength of the PVA gel electrolyte was 20.3 MPa with a maximum strain of 291.53%. As shown in Figure 8B, the tensile strength of the PVDF-HFP GPE was 2.2 MPa with a maximum strain of 32.07%. The deformation properties were investigated using loading-unloading tests. Figure 9A,B shows the loading-unloading curves of the PVA-and PVDF-HFP-based gel electrolytes, which exhibited a significant hysteresis loop in the first cycles. However, this effect became negligible as the loading-unloading processes were repeated.

Figure 1: Mechanism and synthesis of PVA- and PVDF-HFP polymer-matrix-based electrolyte. (A) A schematic illustration showing the proton conduction mechanism in PVA: 1) Grotthuss or hopping mechanism, 2) diffusion of the vehicle mechanism, and 3) decoupling of the proton from the segmental motion in the polymer electrolyte. (B) Schematic of the synthesis of the PVDF-HFP-based gel polymer electrolyte. Please click here to view a larger version of this figure.

Figure 2: Structural characterizations of PVA- PVDF-HFP-based gel polymer electrolytes. FTIR spectra of (A) the PVA-based gel electrolyte and (B) the PVDF-HFP-based gel electrolyte. XRD patterns of (C) the pure PVA- and PVA-based gel electrolyte and (D) the PVDF-HFP-based gel electrolyte. Please click here to view a larger version of this figure.

Figure 3: Schematic of the coin-cell assembly. Please click here to view a larger version of this figure.

Figure 4: Chemical stability characterization data for PVA- and PVDF-HFP polymer-matrix-based electrolyte. CV curve of the optimized gel polymer electrolyte system at 0.1 mV/s scan rate for each cell, (A) SUS||PVA-based gel polymer electrolyte||SS and (B) SS||PVDF-HFP-based gel polymer electrolyte||SS. Please click here to view a larger version of this figure.

Figure 5: Ionic conductivity measurements. Nyquist plots of (A) the PVA-based electrolyte and (B) the PVDF-HFP-based gel polymer electrolyte. Please click here to view a larger version of this figure.

Figure 6: Electrochemical test. (A) CV curves of the PVA-based gel electrolyte based on AC symmetric electrodes at various scan rates. (B) CV curves of the PVDF-HFP-based gel electrolyte based on AC symmetric electrodes at various scan rates. (C) GCD curves of the PVA- and PVDF-HFP-based gel electrolytes at 0.1 mA/cm² Please click here to view a larger version of this figure.

Figure 7: Cycling stability test. (A) AC symmetric electrode with the PVA-based gel polymer electrolyte cycling test at 100 mV s^-1 for 1000 cycles. (B) AC symmetric electrode with the PVDF-HFP electrolyte cycling test at 100 mV·s^-1 for 1000 cycles. Please click here to view a larger version of this figure.

Figure 8: Stress-strain curves. (A) PVA- and (B) PVDF-HFP-based gel electrolytes. Please click here to view a larger version of this figure.

Figure 9: Loading-unloading cycle test (A) The PVA-based gel electrolyte. (B) The PVDF-HFP-based gel electrolyte. Please click here to view a larger version of this figure.

Electrolyte type	The thickness of the electrolyte film (mm)	Bulk resistance of the electrolyte (ohm)	Ionic conductivity (mS/cm)
PVA	0.213	603.749	2.29 x 10^-5
PVDF-HFP	0.23	52.987	2.82 x 10^-4

Table 1: Comparison of the ionic conductivities of PVA- and PVDF-HFP-based gel polymer electrolytes.

Discussion

Our approach for developing stretchable SCs involved the synthesis of GPEs and their subsequent evaluation in prototypical coin cells. In particular, the PVA- and PVDF-HFP-based GPEs were tested in coin cells with symmetric AC electrodes or SUS plates. The critical steps in this approach include 1) preventing bubble generation during the preparation of GPEs, 2) developing a cell assembly procedure that accords with a working supercapacitor, and 3) setting an appropriate experimental range.

Polymer electrolytes should exhibit high ionic conductivity at ambient temperature²⁵, high chemical and electrochemical stabilities²⁶, good mechanical properties of dimensional stability²⁷, and sufficient thin-film processability to be eligible for use in flexible supercapacitors. Achieving these characteristics in a gel electrolyte is challenging. To date, PVA- and PVDF-HFP-based electrolytes have been widely used to best implement these characteristics in supercapacitor electrolytes. In this protocol, the properties primarily determined in the gel electrolyte and a facile methodology to verify the same have been described.

Most polymerization processes are accompanied by the generation of bubbles. However, bubbles deteriorate the gel electrolyte performance, shorten the cycle life, and impact the physical properties of the materials²⁸. These bubbles act as resistive elements inside the cell. Thus, the temperature should be increased in steps, and a vacuum must be maintained to create a uniform GPE without bubbles.

Several difficulties were encountered during the GPE SC assembly because of the high interfacial resistance between the GPE and electrode²⁹. When testing a gel polymer electrolyte with an AC electrode, it was challenging to wet all the AC electrode pores. Therefore, the AC electrode was placed in the GPE solution and heated to 60 °C for 10 min to ensure complete wetting of the AC electrode. After this step, the interfacial resistance was minimized when the SC performance was tested.

Furthermore, the potential range for electrochemical tests was determined based on the electrochemical kinetics of the electrolytes. Aqueous electrolytes have a narrow potential range that is restricted by the decomposition of water. For example, with respect to 0 V ,the standard hydrogen electrode is the negative electrode, in which the hydrogen evolution reaction occurs; at 1.23 V, the positive electrode, the oxygen evolution reaction occurs³⁰. To avoid gas evolution, the cell voltage of an aqueous electrolyte is usually restricted to approximately 1.0 V. In contrast, the organic electrolytes have a wide potential window, typically in the range of 2.5-2.8 V³¹.

The performance of the cell was accurately evaluated by first measuring the amount of current by CV and then performing GCD with that current. If the GCD is tested with a current higher than that obtained by CV, data validation becomes difficult owing to the large resistance.

Even though the safety and electrochemical tests on the PVA- and PVDF-HFP-based GPEs were easily tested in a coin cell with an AC electrode, determining the simultaneous performance changes in the physical properties of gel electrolytes and thereby predicting the performance of the flexible system were difficult. To solve this problem, tests must be conducted using electrode materials with a modulus similar to that of the GPE. Using this coin cell testing method, the electrochemical performance of the gel polymer was determined without any deformation.

In conclusion, we presented the preparation of PVA- and PVDF-HFP-based GPEs and assembled an AC symmetric coin cell SC. XRD and FTIR analyses were performed to confirm the structures of the GPEs. Ionic conductivity, chemical stability, electrochemical stability, and physical properties were measured. We expect that these validated experimental procedures will facilitate the study of GPEs for application in supercapacitors.

Divulgations

The authors have nothing to disclose.

Acknowledgements

The research was supported by the Competency Development Program for Industry Specialists of the Korean MOTIE operated by KIAT (No. P0012453, Next-generation Display Expert Training Project for Innovation Process and Equipment, Materials Engineers), and the Chung-Ang University Research Scholarship Grants in 2021.

Materials

1 M LiPF6 in EC/DMC=1/1, v/v	Sigma aldrich	746738	Electrolyte for pvdf-hfp polymer based gel electrolyte
Activated carbon	Sigma aldrich	902470	Active material
Ag/AgCl electrode	BASi	RE-5B	Reference electrode
Carbon black	Sigma aldrich	699632	Conductive material
Diamino-poly (propylene oxide) (DPPO)	Sigma aldrich	80506-64-5	corss linking material for pvdf-hfp polymer based gel electrolyte
Diglycidyl ether of bisphenol-A (DEBA)	Sigma aldrich	106100-55-4	corss linking material for pvdf-hfp polymer based gel electrolyte
Dimethylformamide (DMF)	Samchun	D0551
Electrode pressing machine	Rotech	MP200
Extractor	WonA Tech		Convert program (raw data to Excel )
Isopropanol(IPA)	Samchun	I0346	Solvent to melt the binder
Phosphoric acid	Samchun	00P4277
poly (ethylene glycol) diglycidyl ether (PEGDE)	Sigma aldrich	475696	corss linking material for pvdf-hfp polymer based gel electrolyte
Polytetrafluoroethylene(PTFE)	Sigma aldrich	430935	Binder
polyvinyl alcohol (PVA)	Sigma aldrich	9002-89-5
Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP)	Sigma aldrich	427160
Potentiostat	WonA Tech	Zive SP1
Pt electrode	BASi	MW-018122017	Counter electrode
Smart management 6(SM6)	WonA Tech		Program of setting sequence and measuring electrochemical result
Sulfuric acid	Samshun	S1423	Electrolyte
Tensile testing machine	Nanotech	NA-50K	tensile testing machine
Zman	WonA Tech		EIS program

References

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