A cross-linked violet polymer and its preparation method

Abstract

This invention relates to the field of organic optoelectronic functional materials, specifically to a cross-linked violet polymer and its preparation method. The cross-linked violet polymer film prepared by cyclic voltammetry exhibits extremely strong color-changing stability. Furthermore, through rational molecular structure design during the preparation process, the molecular structure of the synthesized cross-linked violet polymer can be controlled. In addition, by continuously optimizing the monomer structure and electrochemical polymerization reaction conditions during the preparation process, the polymer structure and film thickness can be controlled. The prepared cross-linked violet polymer can achieve reversible color changes under a step voltage of 0 to -1.2 V, exhibiting high contrast and good electrochromic properties, showing potential application prospects in the field of electrochromism.

Description

Technical Field

[0001] This invention relates to the field of organic optoelectronic functional materials, specifically to a cross-linked violet polymer and its preparation method. Background Technology

[0002] Violet is a class of organic compounds with a bipyridine molecular structure, often referred to as 1,1'-disubstituted-4,4'-bipyridine cationic salts. Because its molecules can undergo significant photoelectron transfer under external stimuli such as light, electricity, heat or pressure, it is a widely used organic optoelectronic functional material, mainly used in photochromism, thermochromism or electrochromism.

[0003] Violet compounds, as a class of electron-deficient small organic molecules, exhibit color changes due to intramolecular electron transfer. The divalent cation state is the most stable structure of violet, exhibiting a colorless state due to the absence of photocharge transfer caused by anions. With the application of a certain voltage, it gains an electron and transforms into a monovalent cation state, at which point the molar absorptivity is very high, resulting in strong and deep color. Further increasing the voltage causes the monovalent cation to gain an electron and transform into a neutral state, at which point the color is lighter. However, the difficulty in fabricating small molecules into films severely limits the application of violet in the field of organic electrochromism.

[0004] Kamata et al. (KAMATA K, SUZUKI T, KAWAI T, et al. Voltammetric anion recognition by a highly cross-linked polyviologen film [J]. Journal of Electroanalytical Chemistry, 1999, 473(1-2): 145-55.) successfully synthesized a cross-linked polyviologen polymer film using cyclic voltammetry. However, the literature did not provide more information on different molecular structures, the synthesized material had a single structure, and the electrochromic properties of the synthesized film needed further verification.

[0005] The applicant of this invention previously disclosed an electrochromic polymer with phenanthroline groups in the main chain in publication number CN101293961A. In the test of the synthesized polymer film, it was found that as the test progressed, the film partially dissolved and thinned; although the color change still existed, it was weakened to a certain extent.

[0006] Therefore, when using violet small molecules as electrochromic films, the color-changing stability of the films still needs to be considered. Summary of the Invention

[0007] The present invention aims to overcome the shortcomings of existing violet polymers, such as poor color-changing stability and uncontrollable structure, by providing a cross-linked violet polymer and its preparation method to overcome the above-mentioned defects.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] The first object of the present invention is to provide an active monomer represented by the following formula (Ⅰ):

[0010] Formula I

[0011] Where X2 - For Cl - ,Br - Or I - R is H, CH3 or C2H5.

[0012] This invention regulates the synthesis of active monomers with different structures, such as C0TDI and C2TDI, by controlling the number of cyano groups introduced. Through electrochemical polymerization, cross-linked violet polymers with different molecular structures can be prepared from active monomers with different structures within a relatively low voltage range (0 to -1.2 V). Furthermore, the cross-linked violet polymers prepared using the method described in this invention exist in the form of a thin film. The prepared film does not easily detach when washed with deionized water, and it does not dissolve or melt during testing. Therefore, the loss of polymer contained in the film is minimal, and the color change is more stable.

[0013] The color change of the small molecule violet is achieved through redox reactions, involving the gain and loss of electrons and changes in valence state. As the voltage increases, the color changes from yellow to blue and then to pale yellow; conversely, as the voltage decreases, the color changes back from pale yellow to blue and then back to yellow, repeating this cycle. The UV-Vis absorption spectra of the polymer film at different potentials show that the film is yellow at -1.2 V, blue at -0.8 V, and pale yellow at 0 V. The kinetic properties of the polymer film were tested using an electrochemical workstation coupled with a UV-Vis spectrophotometer. The results indicate that the prepared film exhibits high contrast and good electrochromic properties, suggesting that this material has potential applications in the field of electrochromism.

[0014] The preparation method of the above-mentioned active monomer is as follows:

[0015] Under an inert atmosphere, 4-cyanopyridine as shown in formula (II) and substance a are added to an acetonitrile solution, stirred and reacted, then filtered, washed and dried to obtain active monomer powder.

[0016] Formula II

[0017] Where X2 is Cl, Br or I; R is H, CH3 or C2H5.

[0018] Preferably, the molar ratio of 4-cyanopyridine to substance a is (3.0~3.5):1.

[0019] Preferably, the inert atmosphere is an N2 atmosphere or an Ar atmosphere.

[0020] Preferably, the reaction temperature is 80~90 ℃ and the reaction time is 18~24 h.

[0021] A second object of the present invention is to provide a crosslinked violet polymer represented by the following formula (III):

[0022] Formula III

[0023] Among them, X1 2- SO4 2- or CO3 2- X2 - For Cl - ,Br - Or I - R represents H, CH3, or C2H5; n represents the degree of polymerization.

[0024] The above-mentioned cross-linked violet polymer is obtained by polymerizing the active monomers as described above through an electrochemical oxidation method, which is as follows:

[0025] S1. Add the active monomer and supporting electrolyte to a three-electrode electrolytic cell to form an electrolyte;

[0026] S2. A cross-linked violet polymer film was obtained by electrodeposition on the working electrode using cyclic voltammetry.

[0027] Chemical polymerization is a common method for preparing polymers from small molecule monomers. Generally, the small molecules are first prepared into a precursor solution, and then the polymer film is obtained by electrodeposition on the working electrode in a three-electrode electrolytic cell using cyclic voltammetry, potentiostatic method, or galvanostatic method.

[0028] The commonly used three-electrode system uses three electrodes: the working electrode can be a gold, platinum, lead, titanium, graphite, glassy carbon, or ITO electrode; the counter electrode can be a gold, platinum, lead, titanium, graphite, or ITO electrode; and the reference electrode can be a silver-silver ion electrode, a silver-silver chloride electrode, or a saturated calomel electrode.

[0029] In the three-electrode system used in this invention, a platinum electrode is used as the counter electrode, an Ag / AgCl electrode is used as the reference electrode, and an ITO conductive glass is used as the working electrode. Kamata et al. (KAMATA K, SUZUKI T, KAWAI T, et al. Voltammetric anion recognition by a highly cross-linked polyviologen film[J]. Journal of Electroanalytical Chemistry, 1999, 473(1-2): 145-55.) successfully synthesized violet polymer films using cyclic voltammetry. Therefore, the inventors of this invention followed the cyclic voltammetry method used in the literature for the next step of the experiment.

[0030] Preferably, in step S1, the supporting electrolyte can be any one of K2CO3, Na2CO3, Na2SO4, KCl, NaCl, KBr, NaBr or KI, and the electrolyte concentration is 0.1~0.3 mol / L.

[0031] K2CO3, Na2CO3, Na2SO4, KCl, NaCl, KBr, NaBr, or KI solutions can all be used as supporting electrolyte solutions, and they all have the same function: to achieve the final cross-linked violet polymerization film formation.

[0032] Preferably, in step S1, the concentration of the active monomer is 1~5 mol / L.

[0033] Preferably, in step S2, the voltage range during cyclic voltammetric polymerization is 0 to -1.2 V, the scan rate is 50 mV / s, and the number of cycles is 20 to 40.

[0034] As an n-type polymer, violet polymers exhibit redox properties only under negative voltages; therefore, the voltage range for electrochemical polymerization is set under negative voltages. Furthermore, the redox peaks of violet are between -0.6 and -0.8 V, and no water peaks are observed. Therefore, in this invention, a test voltage of 0 to -1.2 V is selected, which sufficiently ensures the redox properties of the polymerization without exceeding the voltage window for water.

[0035] For ease of experimentation, the scanning rate used in this invention is set to 50 mV / s, referring to the literature.

[0036] The range of scanning cycles depends primarily on whether a cross-linked violet polymer film of suitable thickness is synthesized. Too few scan cycles hinder polymer film formation and result in an excessively thin film, making subsequent performance testing difficult. Conversely, too many scan cycles result in an excessively thick polymer film, which is also detrimental to subsequent testing and practical applications. A scan cycle count of 20–40 cycles produces a polymer film with a thickness of approximately 200–300 nm, which is considered a suitable film thickness.

[0037] The cross-linked violet polymer prepared by this invention can be used in electrochromic applications, energy storage materials, and supercapacitor applications. The cross-linked violet polymer prepared according to the method described in this invention exists in the form of a thin film, which does not easily detach even after washing with deionized water. It does not dissolve or melt during testing, thus minimizing polymer loss and resulting in more stable color changes. Besides its application in electrochromic fields, violet can also be used in energy storage materials, such as COFs or POPs materials, or in supercapacitor applications.

[0038] Therefore, the present invention has the following beneficial effects:

[0039] (1) The cross-linked violet polymer film prepared by the cyclic voltammetry method of the present invention has extremely strong color change stability; at the same time, the molecular structure of the synthesized cross-linked violet polymer can be controlled by reasonable molecular structure design during the preparation process.

[0040] (2) The present invention uses cyclic voltammetry to cycle 20 to 40 times on the working electrode. By controlling different number of cycles, cross-linked violet polymer films of different thicknesses can be obtained, thereby achieving adjustable polymer film thickness.

[0041] (3) The cross-linked violet polymer designed and prepared by the present invention can achieve reversible color change under a voltage of 0~-1.2 V, exhibiting high contrast, and has a broad application prospect in the field of electrochromism;

[0042] (4) The present invention uses electrochemical polymerization to prepare cross-linked violet polymer films in a low voltage range, which can solve the problem that violet small molecules are difficult to form films. Attached Figure Description

[0043] Figure 1 This is a flowchart illustrating the electrochemical polymerization of cross-linked violet polymers.

[0044] Figure 2 The cyclic voltammetric polymerization curves of the monomer in Example 5 are shown.

[0045] Figure 3 The cyclic voltammetry curves of the cross-linked violet polymer in Example 5 at different scan rates are shown below.

[0046] Figure 4 The UV-Vis absorption curve and color change under different voltages of the cross-linked violet polymer in Example 5 are shown.

[0047] Figure 5 The kinetic curve of the crosslinked violet polymer in Example 5 at 392 nm;

[0048] Figure 6 The kinetic curve of the cross-linked violet polymer of Example 5 at 600 nm is shown. Detailed Implementation

[0049] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0050] Example 1

[0051] Preparation of cross-linked violet polymer

[0052] 1. Preparation of active monomers

[0053] This embodiment provides a method for preparing the active monomer COTDI in a crosslinked violet polymer, the structure of which is as follows:

[0054]

[0055] The reaction formulas involved in the preparation of the active monomer COTDI are shown below:

[0056]

[0057] The specific preparation method is as follows: 4-cyanopyridine (200 mg, 1.96 mmol) and 1,3,5-tris(bromomethyl)benzene (233 mg, 0.65 mmol) were placed in a 100 mL two-necked round-bottom flask. Under Ar protection, 20 mL of acetonitrile was added to the reaction flask, stirred, and heated under reflux at 85 °C for 24 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with 100 mL of acetonitrile. Finally, the filter cake was placed in an oven and dried at 50 °C for two hours to obtain a yellow solid powder.

[0058] 2. Preparation of cross-linked violet polymer

[0059] 13.3 mg of the obtained active monomer C0TDI was added to a 10 mL volumetric flask, followed by 284 mg of Na2SO4 as the supporting electrolyte. The mixture was brought to volume with deionized water and sonicated for 5 min. After complete dissolution of the solid, cyclic voltammetric electrochemical polymerization was performed to obtain a cross-linked violetin polymer film. During polymerization, an ITO glass electrode was used as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode, forming a three-electrode system. Polymerization parameters were: voltage range 0 to -1.2 V, scan rate 50 mV / s, and 30 cycles. After electrochemical polymerization, the film was rinsed with deionized water.

[0060] Example 2

[0061] Preparation of cross-linked violet polymer

[0062] 1. Preparation of active monomers

[0063] This embodiment provides a method for preparing the active monomer C2TDI in a crosslinked violet polymer, the structure of which is as follows:

[0064]

[0065] The reaction formulas involved in the preparation of the active monomer C2TDI are shown below:

[0066]

[0067] The specific preparation method is as follows: 4-cyanopyridine (163 mg, 1.58 mmol) and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (200 mg, 0.45 mmol) were placed in a 100 mL two-necked round-bottom flask. Under N2 protection, 20 mL of acetonitrile was added to the reaction flask, stirred, and heated under reflux at 90 °C for 20 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with 100 mL of acetonitrile. Finally, the filter cake was placed in an oven and dried at 50 °C for two hours to obtain a yellow solid powder.

[0068] 2. Preparation of cross-linked violet polymer

[0069] 15.0 mg of the obtained active monomer C2TDI was added to a 10 mL volumetric flask, followed by 142 mg of Na2SO4 as the supporting electrolyte. The mixture was brought to volume with deionized water and sonicated for 5 min. After complete dissolution of the solid, cyclic voltammetric electrochemical polymerization was performed to obtain a cross-linked violetin polymer film. During polymerization, an ITO glass electrode was used as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode, forming a three-electrode system. Polymerization parameters were: voltage range 0 to -1.2 V, scan rate 50 mV / s, and 40 cycles. After electrochemical polymerization, the film was washed with deionized water.

[0070] Example 3

[0071] Preparation of cross-linked violet polymer

[0072] 1. Preparation of active monomers

[0073] This embodiment provides a method for preparing the active monomer C1TDI in a crosslinked violet polymer, the structure of which is as follows:

[0074]

[0075] The reaction formulas involved in the preparation of the active monomer C1TDI are shown below:

[0076]

[0077] The specific preparation method is as follows: 4-cyanopyridine (182 mg, 1.75 mmol) and 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene (200 mg, 0.50 mmol) were placed in a 100 mL two-necked round-bottom flask. Under N2 protection, 20 mL of acetonitrile was added to the reaction flask, stirred, and heated under reflux at 80 °C for 18 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with 100 mL of acetonitrile. Finally, the filter cake was placed in an oven and dried at 50 °C for two hours to obtain a yellow solid powder.

[0078] 2. Preparation of cross-linked violet polymer

[0079] 4.7 mg of the obtained active monomer C1TDI was added to a 10 mL volumetric flask, followed by 142 mg of Na2SO4 as the supporting electrolyte. The mixture was brought to volume with deionized water and sonicated for 5 min. After complete dissolution of the solid, cyclic voltammetric electrochemical polymerization was performed to obtain a cross-linked violetin polymer film. During polymerization, an ITO glass electrode was used as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode, forming a three-electrode system. Polymerization parameters were: voltage range 0 to -1.2 V, scan rate 50 mV / s, and 20 cycles. After electrochemical polymerization, the film was rinsed with deionized water.

[0080] Example 4

[0081] Preparation of cross-linked violet polymer

[0082] 1. Preparation of active monomers

[0083] This embodiment provides a method for preparing the active monomer C1TDI in a crosslinked violet polymer, the structure of which is as follows:

[0084]

[0085] The reaction formulas involved in the preparation of the active monomer C1TDI are shown below:

[0086]

[0087] The specific preparation method is as follows: 4-cyanopyridine (163 mg, 1.58 mmol) and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (213 mg, 0.48 mmol) were placed in a 100 mL two-necked round-bottom flask. Under N2 protection, 20 mL of acetonitrile was added to the reaction flask, stirred, and heated under reflux at 80 °C for 20 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with 100 mL of acetonitrile. Finally, the filter cake was placed in an oven and dried at 50 °C for two hours to obtain a yellow solid powder.

[0088] 2. Preparation of cross-linked violet polymer

[0089] 23.5 mg of the obtained active monomer C1TDI was added to a 10 mL volumetric flask, followed by 142 mg of Na2SO4 as the supporting electrolyte. The mixture was brought to volume with deionized water, sonicated for 5 min, and after complete dissolution of the solid, cyclic voltammetric electrochemical polymerization was performed to obtain a cross-linked violetin polymer film. During polymerization, an ITO glass electrode was used as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode, forming a three-electrode system. Polymerization parameters were: voltage range 0 to -1.2 V, scan rate 50 mV / s, and 20 cycles. After electrochemical polymerization, the film was rinsed with deionized water.

[0090] Example 5

[0091] Preparation of cross-linked violet polymer

[0092] 1. Preparation of active monomers

[0093] This embodiment provides a method for preparing the active monomer C1TDI in a crosslinked violet polymer, the structure of which is as follows:

[0094]

[0095] The reaction formulas involved in the preparation of the active monomer C1TDI are shown below:

[0096]

[0097] The specific preparation method is as follows: 4-cyanopyridine (182 mg, 1.75 mmol) and 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene (212 mg, 0.53 mmol) were placed in a 100 mL two-necked round-bottom flask. Under N2 protection, 20 mL of acetonitrile was added to the reaction flask, stirred, and heated under reflux at 80 °C for 20 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with 100 mL of acetonitrile. Finally, the filter cake was placed in an oven and dried at 50 °C for two hours to obtain a yellow solid powder.

[0098] 2. Preparation of cross-linked violet polymer

[0099] 14.2 mg of the obtained active monomer C1TDI was added to a 10 mL volumetric flask, followed by 142 mg of Na2SO4 as the supporting electrolyte. The mixture was brought to volume with deionized water and sonicated for 5 min. After complete dissolution of the solid, cyclic voltammetric electrochemical polymerization was performed to obtain a cross-linked violetin polymer film. During polymerization, an ITO glass electrode was used as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode, forming a three-electrode system. Polymerization parameters were: voltage range 0 to -1.2 V, scan rate 50 mV / s, and 20 cycles. After electrochemical polymerization, the film was rinsed with deionized water.

[0100] The cyclic voltammetric polymerization curve of this embodiment is as follows: Figure 2 As shown. Observation Figure 2 As the number of scan cycles gradually increases, the peak current intensity of the reduction injection gradually increases. Simultaneously, a visible polymer film gradually forms on the ITO surface; testing shows that the thickness of this polymer film is approximately 300 nm. Notably, the film does not dissolve in deionized water during rinsing, thus confirming the successful preparation of the polymer film.

[0101] [Performance Testing]

[0102] 1. Electrochemical performance testing

[0103] Cyclic voltammetry curves of the polymer films obtained above were tested using an electrochemical workstation at different scan rates. The test method was as follows: 142 mg of Na₂SO₄ was added to a 10 mL volumetric flask and diluted to volume with deionized water to serve as the supporting electrolyte solution for performance testing. A blank supporting electrolyte solution was used as the test solution. A three-electrode system was constructed using ITO coated with the polymer film as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode. The test voltage window was 0 to -1.2 V, and the scan rates were 10 mV / s, 20 mV / s, 30 mV / s, 50 mV / s, 100 mV / s, 200 mV / s, 300 mV / s, 400 mV / s, and 500 mV / s. The test structure is shown below. Figure 3 As shown.

[0104] Depend on Figure 3 As can be seen, the scan rate and peak current intensity exhibit a cross-linking relationship, indicating that the precursor (i.e., the active monomer) can polymerize well on the ITO glass surface during the electrochemical polymerization process. This also demonstrates that the electrochemical process is reversible and non-diffusion-controlled. Furthermore, it can be observed that the film changes color from yellow to blue and then to light yellow as the voltage increases, and when the voltage decreases, the color changes from light yellow to blue and then back to yellow, repeating this cycle.

[0105] 2. Ultraviolet-Visible Absorption Spectroscopy Test

[0106] The UV-Vis absorption spectra of the polymer film at different potentials were tested using an electrochemical workstation coupled with a UV-Vis spectrophotometer. The test method was as follows: 142 mg of Na₂SO₄ was added to a 10 mL volumetric flask and diluted to volume with deionized water to serve as the supporting electrolyte solution for performance testing. A blank supporting electrolyte solution was used as the test solution. A three-electrode system was constructed using ITO coated with the polymer film as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode. The test voltage range was 0 to -1.2 V, with a step size of -0.1 V. The test results are shown below. Figure 4 As shown. Records are from Figure 4 The relationship between the potential and the color of the polymer film obtained from the results is shown in Table 1:

[0107] Table 1: Relationship between potential and color of cross-linked violet polymer film

[0108]

[0109] 3. Dynamic performance testing

[0110] The kinetic curves of the polymer film were tested using an electrochemical workstation coupled with a UV-Vis spectrophotometer. The test method was as follows: 142 mg of Na₂SO₄ was added to a 10 mL volumetric flask and diluted to volume with deionized water to serve as the supporting electrolyte solution for performance testing. A blank supporting electrolyte solution was used as the test solution. A three-electrode system was constructed using ITO coated with the polymer film as the working electrode, a platinum sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode. Figure 4 The kinetic properties of the polymer film were tested at peak values ​​of 392 nm and 600 nm. The test results are as follows: Figure 5 , Figure 6 As shown.

[0111] Depend on Figure 5 , Figure 6 As can be seen, the contrast ratio of this polymer film at wavelengths of 392 nm and 600 nm is 30% and 33%, respectively. Contrast ratio is an important parameter for electrochromic characterization; the higher the contrast ratio, the more obvious the surface color change and the better the material performance.

Claims

1. An active monomer represented by the following formula (Ⅰ): Formula I Where X2 - For Cl - ,Br - Or I - R is C2H5.

2. A method for preparing the active monomer as described in claim 1, characterized in that, The process includes the following steps: Under an inert atmosphere, 4-cyanopyridine and substance a are added to an acetonitrile solution, stirred and reacted, then filtered, washed and dried to obtain the active monomer powder. Formula II Where X2 is Cl, Br or I; R is C2H5.

3. The method for preparing the active monomer according to claim 2, characterized in that, The molar ratio of 4-cyanopyridine to substance a is (3.0~3.5):

1.

4. The method for preparing the active monomer according to claim 2, characterized in that, The inert atmosphere is either N2 or Ar.

5. The method for preparing the active monomer according to claim 2, characterized in that, The reaction temperature is 80~90 ℃, and the reaction time is 18~24 h.

6. A cross-linked violet polymer of formula (Ⅲ), the structural formula of which is as follows: Formula III in, X1 2- SO4 2- or CO3 2- X2 - For Cl - ,Br - Or I - R is C2H5; The cross-linked violet polymer is obtained by electrochemical oxidation polymerization of the active monomer described in claim 1, wherein the electrochemical oxidation method is as follows: S1. Add the active monomer and supporting electrolyte to a three-electrode electrolytic cell to form an electrolyte; S2. A cross-linked violet polymer film was obtained by electrodeposition on the working electrode using cyclic voltammetry. The voltage range for the cyclic voltammetric polymerization is 0 to -1.2 V, the scan rate is 50 mV / s, and the number of cycles is 20 to 40.

7. The cross-linked violet polymer according to claim 6, characterized in that: In step S1, the supporting electrolyte can be any one of K2CO3, Na2CO3, and Na2SO4, and the electrolyte concentration is 0.1~0.3 mol / L.

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