KR20220146891A - PBI-based membrane doped with a sulfur-containing acid solution having improved performance, preparation method thereof and use thereof - Google Patents

Abstract

The present invention relates to a sulfur-containing acid-doped PBI-based membrane with improved performance, a manufacturing method thereof, and a use thereof. The present invention includes the steps of: pre-doping a polybenzimidazole (PBI)-based polymer membrane with a sulfur-containing first acid solution; and equilibrating the pre-doped membrane in a sulfur-containing second acid solution.

Description

PBI-based membrane doped with a sulfur-containing acid solution having improved performance, preparation method thereof and use thereof

The present invention relates to a sulfur-containing acid doped PBI-based membrane with improved performance, a method for preparing the same and a use thereof.

Efforts are being made to save fossil fuels or apply renewable energy to more fields by improving the efficiency of use in order to solve the problems of depletion of fossil fuels and environmental pollution. Renewable energy sources such as solar and wind are being used more efficiently than ever before, but these energy sources are intermittent and unpredictable. Due to these characteristics, dependence on these energy sources is limited, and the proportion of renewable energy sources among the current primary power sources is very low.

Rechargeable batteries provide a simple and efficient method of storing electricity, so efforts are being made to miniaturize them and increase their mobility to use them as intermittent auxiliary power sources or as power sources for small home appliances such as laptops, tablet PCs, and mobile phones.

A redox flow battery (RFB) is a secondary battery that can store and use energy for a long time by repeating charging and discharging by an electrochemical reversible reaction of an electrolyte. The stack and the electrolyte tank, which determine the capacity and output characteristics of the battery, are configured independently of each other, so the design of the battery is free and the installation space is small.

In addition, the redox flow battery has a load leveling function that can respond to a sudden increase in power demand by installing it in a power plant, power system, or building, and a function to compensate or suppress power failure or instantaneous low voltage. It is an energy storage technology and is a system suitable for large-scale energy storage.

These redox flow batteries include two types of electrolyte solutions. One stores the active material in the anode reaction and the other is used in the cathode reaction. In an actual redox flow battery, the electrolyte reaction is different between the cathode and the anode, and since the electrolyte solution flow phenomenon exists, a pressure difference occurs between the cathode side and the anode side. The reaction of the positive and negative electrolytes in the all-vanadium-based redox flow battery, which is a representative redox flow battery, is as follows.

Therefore, in order to overcome the pressure difference at both electrodes and to exhibit excellent battery performance even after repeated charging and discharging, a separator having excellent physical and chemical durability is required. However, when the thickness of the separator is increased in order to improve physical durability, there is a disadvantage in that resistance is increased due to this.

As one of the large-capacity energy storage technologies, the vanadium redox flow battery (VRFB) has high stability, long life cycle, high cost-effectiveness, flexible design, high reaction rate, and low environmental damage. Because of its advantages, it has been recognized by the market and is preferentially selected as a large-capacity electrochemical energy storage technology. The separator is one of the main components in the flow battery, and 1) separates the anode electrolyte solution and the cathode electrolyte solution to block crossover of the electrolyte solution; 2) It plays a role in transferring protons. The performance of these separators is directly related to conversion energy storage efficiency and battery life. Currently commercialized membranes are mainly proton exchange membranes based on perfluorosulfonic acid resins, such as Nafion, which have high proton conductivity and good chemical stability, but high crossover of vanadium ions due to high hydrophilic-hydrophobic phase separation. . Accordingly, the coulomb effect of the battery is reduced, which may adversely affect long-term stability. Compared to Nafion, sulfonated hydrocarbon membranes (e.g., sulfonated polystyrene, sulfonated polyethersulfone, sulfonated polyetherketone, sulfonated polyimine, etc.) generally have low cost, good heat resistance, Although it has advantages such as high mechanical strength and low vanadium ion crossover, most sulfonated hydrocarbon membranes contain ether groups in the main chain, so they are easily degraded by pentavalent vanadium ions (VO ₂ ⁺ ). can be Therefore, since the above-mentioned hydrocarbon-based membranes have much poorer chemical stability than Nafion, the service life of the battery is short and cannot satisfy the requirements for practical use.

Meanwhile, polybenzimidazole (PBI) is an aromatic heterocyclic polymer and has excellent chemical stability compared to most hydrocarbon polymers because it does not contain an ether group. In addition, although PBI membranes do not have ion conductivity, the protic NH group of the imidazole ring can be deprotonated by a base such as KOH, and the basic N group of the imidazole ring is formed with sulfuric acid (SA) or phosphoric acid. Since it can be protonated by an acid such as (phosphoric acid; PA), it can be immersed in an acid or a base to make it conductive. Accordingly, PBI is recently emerging as a new material for a vanadium redox flow battery (VRFB) (Journal of Electrochemical Energy Conversion and Storage, 2018, 15, article 010801). However, the conductivity is still much lower than that of Nafion (50-70 mS/cm in literature).

Overall, when the membrane is sufficiently swollen, the proton conductivity increases, while the crossover of vanadium ions also increases, so it is necessary to control it.

As a result of intensive research efforts to develop a method for manufacturing a membrane applicable to a vanadium redox flow battery, the present inventors have prepared a sulfur-containing acid with a concentration similar to that of the electrolyte solution of the battery after pretreatment with a high-concentration sulfur-containing acid solution, for example, a sulfuric acid solution. The present invention was completed by confirming that the sulfur-containing acid-doped PBI membrane prepared by equilibration with a solution exhibited excellent performance showing increased proton conductivity and improved ion selectivity.

Each description and embodiment disclosed in the present invention is also applicable to each other description and embodiment. That is, all combinations of the various elements disclosed herein fall within the scope of the present invention. In addition, it cannot be said that the scope of the present invention is limited by the specific descriptions described below.

In addition, those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Also, such equivalents are intended to be encompassed by the present invention.

In addition, throughout the specification of the present invention, when a part "includes" a certain component, it does not exclude other components unless otherwise stated, but may further include other components. it means.

Hereinafter, the present invention will be described in more detail.

In order to achieve the above object, a first aspect of the present invention is a first step of doping a polybenzimidazole (PBI)-based polymer membrane with a sulfur-containing first acid solution having a concentration of 1 M to 17 M; and a second step of equilibrating the pre-doped membrane in a sulfur-containing second acid solution having a concentration lower than that of the sulfur-containing first acid solution.

The present invention is designed to develop a PBI-based membrane with improved performance by doping PBI with an acid so that it can be used in a vanadium redox flow battery (VRFB), and a high concentration of sulfur-containing acid solution for a predetermined time. , for example, after pretreatment with a sulfuric acid solution, the area even if the concentration of the doped acid is the same or similar by equilibrating with a sulfuric acid solution having a concentration similar to the electrolyte of the battery, for example, 2 M to 3 M concentration, which is the electrolyte concentration in a redox flow battery. It is based on the discovery that it is possible to provide membranes with significantly reduced area specific resistance (ARS) and somewhat increased permeability and/or conductivity and significantly improved selectivity.

The PBI-based polymer membrane used in the present invention by itself does not exhibit ion conductivity, but contains a cationic NH group that can be deprotonated by a base such as KOH to an imidazole group contained therein, and an acid such as sulfuric acid or phosphoric acid. Since it contains a basic N group that can be protonated by , it can be modified to have ionic conductivity by immersion in an acid or a base.

For example, the polybenzimidazole (PBI)-based polymer is meta-PBI, para-PBI, ab-PBI, 2OH-PBI, O-PBI, PBI-OO, PBI-HFA (4,4'-(hexafluoroisopropylidene)) bis(benzoic acid)), PBI-OH, sulfonated para-PBI, pyridine-modified PBI, fluorinated PBI, mesityl-PBI, terphenyl-PBI, hexamethyl-p-terphenyl It may be one selected from the group consisting of PBI, or a derivative thereof, or a mixture or copolymer (co-polymer) thereof, but is not limited thereto.

For example, the sulfur-containing first acid solution and the sulfur-containing second acid solution may each independently be sulfuric acid (H ₂ SO ₄ ), methanesulfonic acid (CH ₃ SO ₃ H), or a mixture thereof. . Specifically, a sulfuric acid solution may be used as both the sulfur-containing first acid solution and the sulfur-containing second acid solution, but is not limited thereto.

For example, in the manufacturing method of the present invention, an 8 to 14 M sulfur-containing acid solution may be used as the sulfur-containing first acid solution, but is not limited thereto.

In addition, in the manufacturing method of the present invention, the first step may be repeated twice or more with a sulfuric acid solution of the same or decreasing concentration, but is not limited thereto.

For example, the manufacturing method may be carried out at -10 to 300 ℃. Specifically, it may be carried out at 25 to 60 ℃, but is not limited thereto. In a specific example of the present invention, it was confirmed that the first doping temperature with a high concentration of sulfuric acid solution in the above manufacturing method had little effect on the performance of the membrane, particularly the area resistivity (FIG. 2).

For example, the sulfur-containing second acid solution used in the second step of the manufacturing method of the present invention may further include metal ions, but is not limited thereto. Furthermore, the metal ion may be a vanadium ion, but is not limited thereto.

The second aspect of the present invention is a sulfur-containing acid-doped polybenzimidazole (PBI) equilibrated with a sulfur-containing acid solution having a concentration similar to the electrolyte concentration of the flow battery to be applied after primary doping with an 8 to 17 M sulfur-containing acid solution. Provided is a membrane for a vanadium redox flow battery (VRFB) containing a )-based polymer.

The membrane may be manufactured by the method of the first aspect, but is not limited thereto.

For example, the membrane may be supported by a porous polymer or a porous carbon layer. Specifically, the membrane may be cast on the porous support of the aforementioned material, but is not limited thereto.

For example, the membrane of the present invention exhibits increased conductivity and/or high selectivity, as well as a significantly reduced areal resistivity compared to a membrane not pretreated with a high concentration of sulfur-containing acid solution. When applied to a flow battery, it can enable fast charging/discharging.

A third aspect of the present invention provides a redox flow battery having the membrane manufactured by the method of the first aspect as a separator.

As used herein, the term “redox flow battery” means that an electrolyte containing an electroactive species flows through an electrochemical cell that reversibly converts chemical energy directly into electricity, a rechargeable, A possible type of battery is a reversible battery in which all electroactive components are dissolved in an electrolyte. A gravity feed system is also used, but mainly the additional electrolyte is stored externally, usually in a separate tank, and is supplied to the cell by a pump. Flow cells can be recharged rapidly by replacing electrolyte (in a similar way to replenishing fuel tanks in internal combustion engines) while at the same time recovering spent material for re-energization. In such a redox flow cell, energy is completely decoupled from power, as in other cells, since the cell's energy is determined by the electrolyte volume, eg tank size, and power is determined by the electrode area, eg, reactor size.

In this redox flow battery, unlike other batteries, the electroactive species exists as ions in an aqueous solution state rather than as a solid, and has a mechanism of storing energy by oxidation/reduction reactions of respective ions at the cathode and the anode. The cathode electrolyte and the anode electrolyte for causing the above-described reaction are stored in separate storage tanks, respectively, are introduced into the cell housing through the electrolyte inlet formed in the cell housing, and come into contact with the cathode electrode and the anode electrode to cause a reaction, and then each It has a circulation system that flows out through the electrolyte outlet. The battery is discharged by connecting an electric load to an external circuit including the electric load and allowing a current to flow. Conversely, charging is performed by connecting an external power source to the battery to allow a current to flow. In general, the catholyte is charged when the redox couple is oxidized to the higher of the two electron states, and discharged when reduced to the lower one. In the anode electrolyte solution, the opposite phenomenon occurs.

As mentioned above, most redox flow batteries consist of two separate electrolyte solutions. One stores the electroactive material in the anode reaction and the other is used in the cathode electrode reaction. When charging, the reverse will be applied. Fresh or used electrolyte can be circulated and stored in a single storage tank. Alternatively, the concentration of the electroactive material may be individually adjusted. An ion exchange membrane is used as a separation membrane to prevent mixing of electroactive species that can cause chemical disconnection.

As used herein, the term "separator" refers to an ion exchange membrane introduced to prevent mixing of electroactive species in the redox flow battery, and a common counter ion carrier in both electrodes separated by the separator. ) only passes through the separation membrane. For example, in a bromine-polysulfide system in which Na ₂ S ₂ is converted to Na ₂ S ₄ at the anode and Br ₂ is converted to 2Br at the cathode, excess Na ⁺ ions at the anode are removed from the cathode to maintain electroneutral conditions. is transmitted to Similarly, in a vanadium system in which V ²⁺ is oxidized to V ³⁺ at the anode and V ⁵⁺ is reduced to V ⁴⁺ at the cathode, protons and hydronium ions (H ₃ O ⁺ ) pass through a proton conducting membrane. from the anode to the cathode.

In an actual redox flow battery, the electrolyte reaction is different between the cathode and the anode, and since the electrolyte solution flow phenomenon exists, a pressure difference occurs between the cathode side and the anode side. Therefore, the separation membrane preferably has excellent physical strength so as not to be destroyed by such a pressure difference.

The redox flow battery of the present invention is characterized in that one side of the separator is provided with a cathode electrolyte and a cathode electrode, and an anode electrolyte and an anode electrode are sequentially provided on the other side of the separator.

For the cathode electrode and the anode electrode, electrode materials commonly used in the art may be used. For example, carbon felt, carbon nonwoven fabric, graphite felt, graphite plate, etc. may be used, but the present invention is not limited thereto.

When describing the redox flow battery as an example, the redox flow battery of the present invention includes a cell housing having a predetermined size; a separator installed across the center of the cell housing; a cathode electrode and an anode electrode located on both left and right sides separated by the separator inside the cell housing; a cathode electrolyte inlet and a cathode electrolyte outlet formed on the upper/lower side of the cell housing on the side where the cathode electrode is located to perform inflow and outflow of the electrolyte used in the cathode electrode; and an anode electrolyte inlet and an anode electrolyte outlet formed on the upper/lower side of the cell housing on the side where the anode electrode is located and performing inflow and outflow of the electrolyte used for the anode electrode.

For example, the redox flow battery of the present invention uses a V(IV)/V(V) redox couple as the cathode electrolyte and a V(II)/V(III) redox couple as the anode electrolyte. dog cell; Or it may be another metal-based redox battery using Fe(II)/Fe(III) redox couple as the cathode electrolyte and V(II)/V(III) redox couple as the cathode electrolyte, but is not limited thereto. .

A fourth aspect of the present invention provides an electrochemical system having a membrane produced by the method of the first aspect.

For example, the electrochemical system may be, but is not limited to, electrolyzers, fuel cells, or batteries. For example, the membranes of the present invention have improved ionic conductivity and/or selectivity, so as to separate the anode and cathode in electrochemical systems in general, fuel and oxidant in fuel cells, redox species in batteries, and/or It may be provided as a separator for blocking crossover of hydrogen and oxygen in the electrolytic cell.

In the manufacturing method of the present invention, through a simple additional process of pretreatment with a higher concentration of sulfur-containing acid solution prior to doping with a sulfur-containing acid solution of the same concentration as the electrolyte, the area resistivity is significantly reduced, as well as the permeability and/or It can be usefully applied to a vanadium redox flow battery because the conductivity is increased, and furthermore, a membrane having a very good selectivity can be provided.

1 is a diagram showing the acid doping level according to the concentration of the first doping solution of a sulfuric acid-doped PBI membrane equilibrated with a 2M sulfuric acid solution after first doping with a sulfuric acid solution of various concentrations.
2 is a diagram showing the area resistivity according to the concentration of the primary doping solution of a sulfuric acid-doped PBI membrane (10 μm thick) equilibrated with a 2 M sulfuric acid solution after primary doping with a sulfuric acid solution of various concentrations at various temperatures.

Hereinafter, the present invention will be described in more detail through examples. However, these examples are for illustrative purposes only and the scope of the present invention is not limited to these examples.

Comparative Example 1: Preparation of Sulfuric Acid Doped PBI Membrane by Standard Process

A PBI membrane doped with sulfuric acid was prepared by immersing the PBI membrane in 2M sulfuric acid solution at room temperature for 24 hours.

Example 1: Preparation of Sulfuric Acid Doped PBI Membrane by Multiple Doping Process

PBI membranes were doped by immersion in various concentrations of sulfuric acid solution (2, 4, 6, 8 or 10 M) at various temperatures (25, 40, 60 or 80 °C). The membrane was then equilibrated by immersion in a sulfuric acid solution of the concentration (eg, 2 M) of the electrolyte used for subsequent application to the cell at room temperature for 24 hours. Since PBI is known to dissolve in 18 M concentrated sulfuric acid solution, the maximum concentration of the doping solution was 12 M.

Experimental Example 1: Confirmation of Acid Doping Level of Sulfuric Acid Doped PBI Membrane

The acid doping level was measured for a series of sulfuric acid-doped PBI membranes prepared according to Comparative Example 1 and Example 1, and is shown in FIG. 1 . At low concentrations, the acid content of the PBI membrane was always in equilibrium with the surrounding solution. As shown in FIG. 1 , the membrane first doped with 8 M sulfuric acid solution absorbed twice as much acid as the membrane doped with 2 M sulfuric acid solution. However, these highly acid-doped membranes were equilibrated with 2 M sulfuric acid solution, which is the acid concentration used when applying to the battery again, and showed almost the same composition as the comparative membranes doped with 2 M sulfuric acid solution. This indicates that when treated with a high concentration of sulfuric acid solution of 10 M or more, the pretreatment with a high concentration of sulfuric acid solution does not affect the composition of the final equilibrated membrane.

Experimental Example 2: Effect of Concentration of Primary Doping Solution on Area Resistivity of PBI Membrane Doped with Sulfuric Acid

The area resistivity of a series of sulfuric acid-doped PBI membranes prepared according to Comparative Example 1 and Example 1 was calculated according to the following equation:

.

.

Furthermore, the area resistivity was calculated for the membranes prepared at different temperatures during acid treatment (25, 40, 60, or 80° C.) and is shown in FIG. 2 . As confirmed in Experimental Example 1, only a slight increase in the acid doping level was observed only when 10 M and 12 M high concentration sulfuric acid solutions were used during the first doping. In spite of the insufficient effect at the doping level as described above, as shown in FIG. 2 , the area resistivity measured for these membranes significantly decreased as the acid concentration of the primary doping solution increased, and at this time, the effect of temperature was not observed.

Experimental Example 3: Additional Physical Properties Analysis of Sulfuric Acid Doped PBI Membrane

Through additional analysis of the membranes prepared by first doping with 10 M and 12 M sulfuric acid solutions with significantly reduced area resistivity among the series of sulfuric acid-doped membranes prepared according to Example 1 and equilibrating with 2 M sulfuric acid solution, Permeability, conductivity and selectivity were measured, and the results are shown in Table 1 below. As shown in Table 1, the movement of ions was promoted in the membrane equilibrated with 2 M sulfuric acid solution after primary doping with the high concentration sulfuric acid solution prepared according to the embodiment of the present invention, and thus the permeability and conductivity through the membrane all increased. On the other hand, since protons are much smaller in size than vanadium ions, they have less electrical repulsion with imidazole groups of positively charged PBI chains, and at the same time, they are more easily compared to vanadium ions through the Grotthuss mechanism. Since it can be conducted, it exhibited a relatively high selectivity value. As a comparative example, a known literature value for Nafion 212, a standard membrane for a (pre)commercial VRFB system, was used. Compared with the literature values for Nafion 212, the PBI membrane first doped with 10 M sulfuric acid solution and equilibrated with 2 M sulfuric acid solution showed lower permeability and conductivity for vanadium ions compared to Nafion 212, whereas 29 times to It was confirmed that the selectivity was 163 times higher. That is, the membrane of the present invention exhibits significantly improved selectivity, although lower conductivity compared to the commercially available Nafion 212, and thus can be applied to the VRFB system. That is, although the membrane of the present invention has somewhat low conductivity, the permeability of vanadium ions is low and the selectivity is remarkably improved. However, in the case of Nafion, when the thickness is reduced, the crossover of vanadium ions is significantly increased, which may cause rapid self-discharge and/or low coulombic efficiency of the battery. Therefore, although the membrane of the present invention has somewhat lower conductivity compared to Nafion, which is commercially available, it can be compensated for by adjusting the thickness of the membrane based on low ion permeability and/or significantly increased selectivity, so it can be usefully used in a VRFB system. have.

Permeability (cm ² min) Conductivity (mS/cm) Selectivity (S·min·cm ^-3 ) Sulfuric Acid Solution Concentration 10M 12M 10M 12M 10M 12M 25℃ 1.99E-9 8.54E-9 9.1 11.6 4.58E6 1.36E6 40℃ 6.25E-9 1.51E-8 9.6 11.7 1.53E6 7.47E5 Nafion 212 ^[A] 3.3E-7 54 1.6E5 Nafion 212 ^[B] 1.9E-6 54 ^[A] 2.81E4 Nafion 212 ^[C] 7.8E-7 54 ^[A] 6.9E4

[A] J. Mater. Chem. A, 2017, 5: 16663-16671,

[B] ACS Appl. Mater. Interfaces, 2013, 5: 7559-7566,

[C] Int. J. Electrochem. Sci., 2014, 9: 3060-3067.

[Abbreviation]

PA: phosphoric acid

PBI: polybenzimidazole

PPA: polyphosphoric acid

VRFB: vanadium redox flow battery

SA: sulfuric acid

From the above description, those skilled in the art to which the present invention pertains will be able to understand that the present invention may be embodied in other specific forms without changing the technical spirit or essential characteristics thereof. In this regard, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of the present invention should be construed as being included in the scope of the present invention, rather than the above detailed description, all changes or modifications derived from the meaning and scope of the claims described below and their equivalents.

Claims (14)

A first step of predoping a polybenzimidazole (PBI)-based polymer membrane with a sulfur-containing first acid solution having a concentration of 1 M to 17 M; and
A second step of equilibrating the pre-doped membrane in a sulfur-containing second acid solution having a lower concentration than that of the sulfur-containing first acid solution;
The method of claim 1,
The polybenzimidazole (PBI)-based polymer is meta-PBI, para-PBI, ab-PBI, 2OH-PBI, O-PBI, PBI-OO, PBI-HFA (4,4'-(hexafluoroisopropylidene)bis( benzoic acid)), PBI-OH, sulfonated para-PBI, pyridine-modified PBI, fluorinated PBI, mesityl-PBI, terphenyl-PBI, hexamethyl-p-terphenyl PBI, Or one selected from the group consisting of derivatives thereof, or a mixture or copolymer (co-polymer) thereof, the manufacturing method.
According to claim 1,
The sulfur-containing first acid solution is an 8 to 14 M acid solution, the manufacturing method.
According to claim 1,
Which is carried out at -10 to 300 ℃, the manufacturing method.
According to claim 1,
Which is carried out at 25 to 60 ℃, the manufacturing method.
According to claim 1,
The method of claim 1, wherein the sulfur-containing second acid solution further comprises a metal ion.
7. The method of claim 6,
The metal ion is a vanadium ion, the manufacturing method.
Vanadium containing a sulfur-containing acid-doped polybenzimidazole (PBI)-based polymer equilibrated with a sulfur-containing acid solution having a concentration similar to the electrolyte concentration of the flow battery to be applied after primary doping with an 8 to 17 M sulfur-containing acid solution Membrane for vanadium redox flow battery (VRFB).
9. The method of claim 8,
A membrane prepared by the method of any one of claims 1 to 7.
A redox flow battery having a membrane manufactured by the method of any one of claims 1 to 7 as a separator.
11. The method of claim 10,
It is an all-vanadium-based redox battery using a V(IV)/V(V) redox couple as the cathode electrolyte and a V(II)/V(III) redox couple as the anode electrolyte, a redox flow battery .
11. The method of claim 10,
The redox flow battery of the other metal-based redox battery using Fe (II) / Fe (III) redox couple as the cathode electrolyte, V (II) / V (III) redox couple as the negative electrolyte.
An electrochemical system with a membrane made according to claim 1 .
14. The method of claim 13,
wherein the electrochemical system is electrolyzers, fuel cells, or batteries.

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