KR20200124398A - Zn-Fe Redox flow battery - Google Patents

Abstract

The present invention relates to a zinc-iron redox flow battery including a bipolar plate in which a flow path of an interlocking pattern is formed, which has excellent voltage efficiency and cycle characteristics.

Description

Zn-Fe Redox flow battery {Zn-Fe Redox flow battery}

The present invention relates to a zinc-iron redox flow battery comprising a bipolar plate having an interlocking flow path.

As the proportion of new and renewable energy has recently increased, power storage devices are attracting attention as a new alternative that can overcome the problem of fluctuations in power production and inconsistency between supply and demand, and active research on power storage devices is underway. The power storage device can efficiently reduce the gap between supply and demand by charging electricity when there is a lot of power generation and discharging electricity when there is a lot of consumption, and is the safest way to respond to fluctuations in the generation amount of renewable energy within a short time.

In addition, if the proportion of new and renewable energy increases rapidly, it is expected that the volatility of electricity production worldwide will reach a considerable scale. Accordingly, the International Energy Agency (IEA) is paying attention to power storage devices to supply new and renewable energy in the future. Therefore, from a long-term perspective, the spread of power storage devices is inevitably an indispensable factor for expanding renewable energy.

Secondary batteries for large-capacity power storage include lead-acid batteries, NaS batteries, and redox flow batteries (RFB). Lead-acid batteries are widely used commercially compared to other batteries, but have disadvantages such as low efficiency, maintenance costs due to periodic replacement, and problems in the treatment of industrial waste generated when batteries are replaced. In the case of NaS battery, it is advantageous to have high energy efficiency, but there is a disadvantage of operating at a high temperature of 300℃ or higher. In contrast, a redox flow battery has a low maintenance cost, can operate at room temperature, and can independently design capacity and output, and thus, many studies have been conducted on a large-capacity secondary battery.

The redox flow battery includes an electrolyte prepared by dissolving an active material composed of a redox couple with different oxidation numbers in a solvent. When a redox flow battery consisting of a positive electrode electrolyte and a negative electrode electrolyte containing a redox couple is charged, an oxidation reaction occurs at the positive electrode, a reduction reaction occurs at the negative electrode, and the electromotive force of the battery is redox, an active material contained in the positive electrode electrolyte and negative electrode electrolyte. It is determined by the difference in the standard electrode potential of the couple. These redox couples can obtain various combinations by the difference in oxidation/reduction, V ^(3+/2+) /V ^(4+/5+) , Fe ^(2+/3+) /Cr ^{(3 +/2+)} systems have been studied and applied. Currently, all vanadium redox flow batteries using vanadium for both anolyte and catholyte are dominant. However, the high-concentration vanadium electrolyte has low stability and high cost, so the development of a new electrolyte is required, and the development of an active material usable in a basic solution instead of a highly corrosive sulfuric acid solution is required.

The zinc-iron (Zn/Fe) redox flow battery uses [Fe(CN) ₆ ] ^3-/4- as the positive electrode active material and Zn/[Zn(OH) ₄ ] ^2- as the negative electrode active material. Due to its low toxicity, it has the advantage of being suitable for use as an active material for redox flow batteries. In addition, since it has a high discharge voltage in an aqueous strong base solution, it is attracting attention as an active material to replace vanadium. However, it is difficult to apply zinc as an active material due to a problem in that the capacity of the battery decreases due to precipitation and dissolution of zinc in the negative electrode during the charging and discharging process, and the cycle characteristics are deteriorated. Therefore, there is a need for a way to solve this problem.

Republic of Korea Patent Publication No. 10-2018-0002993 Republic of Korea Patent Publication No. 10-2017-0092040

As described above, the zinc-iron redox flow battery has low cost and toxicity, and exhibits a high discharge voltage, making it suitable for use as a redox flow battery, but due to the precipitation and dissolution of zinc in the negative electrode during the charging and discharging process, the battery There is a problem that the capacity of is reduced and the cycle characteristics are deteriorated. Accordingly, the present inventors have attempted to provide a zinc-iron redox flow battery capable of improving mass transport of an electrolyte in order to improve voltage efficiency and cycle characteristics of a zinc-iron redox flow battery.

Accordingly, an object of the present invention is to provide a zinc-iron redox flow battery having excellent voltage efficiency and cycle characteristics.

To achieve the above object,

The present invention is a separator;

Electrodes disposed on both sides of the separator;

Bipolar plates respectively disposed on one side of the electrode;

Positive electrode electrolyte; And

A zinc-iron redox flow battery comprising a cathode electrolyte,

The bipolar plate includes a flow path of an interlocking pattern,

The positive electrode electrolyte includes [Fe(CN) ₆ ] ^3-/4- and a basic aqueous solution,

The negative electrode electrolyte provides a zinc-iron redox flow battery comprising Zn/[Zn(OH) ₄ ] ^2- and a basic aqueous solution.

The zinc-iron redox flow battery of the present invention has excellent material transfer ability of an electrolyte and thus has excellent voltage efficiency and cycle characteristics.

In addition, the zinc-iron redox flow battery of the present invention has excellent charging/discharging efficiency even at a high current density, and may exhibit improved cycle characteristics.

1 is a schematic diagram showing the structure of a zinc-iron redox flow battery of the present invention.
2 is a three-dimensional perspective view showing a unit stack of the present invention.
3 is a schematic diagram showing a charging and discharging process of the zinc-iron redox flow battery of the present invention.
4 is a front view showing the bipolar plate of the present invention.
5 is a perspective view showing a bipolar plate of the present invention.
6 is a graph measuring cycle characteristics of the zinc-iron redox flow batteries of Example 1 and Comparative Example 1 at a current density of 50 mA/cm ² and a flow rate of 1 mL/cm ² ·min.
7 is a graph measuring cycle characteristics of the zinc-iron redox flow batteries of Example 1 and Comparative Example 1 at a current density of 100 mA/cm ² and a flow rate of 1 mL/cm ² ·min.
8 is a zinc-iron redox flow battery of Example 1 and Comparative Example 1, after 100 cycles at a current density of 50 mA/cm ² and a flow rate of 4 mL/cm ² ·min, a current density of 100 mA/cm ² and a flow rate of 4 mL/cm ^This is a graph of cycle characteristics obtained by running 100 cycles at ² min.
9 is a graph measuring a voltage profile according to whether or not a flow path of an interlocking pattern of a bipolar plate of the zinc-iron redox flow batteries of Example 1 and Comparative Example 1 is formed.
10 is a graph measuring cycle characteristics according to the flow rate and current density of the zinc-iron redox flow battery of Example 1. FIG.
11 is a graph measuring a voltage profile according to a flow rate at a current density of 50 mA/cm ² of the zinc-iron redox flow battery of Example 1. FIG.
12 is a graph measuring a voltage profile according to a flow rate at a current density of 100 mA/cm ² of the zinc-iron redox flow battery of Example 1. FIG.

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

In the drawings, parts not related to the description are omitted in order to clearly describe the present invention, and similar reference numerals are used for similar parts throughout the specification. In addition, the size and relative size of the components indicated in the drawings are not related to the actual scale, and may be reduced or exaggerated for clarity of description.

In the drawings, the thickness of layers and regions may be exaggerated or omitted for clarity. The same reference numbers throughout the specification denote the same elements.

The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

1 is a schematic diagram showing a zinc-iron redox flow battery according to an embodiment of the present invention, and FIG. 2 is a three-dimensional perspective view showing a unit stack.

Referring to FIG. 1, the zinc-iron redox flow battery 1000 is formed by electrically connecting unit modules 101, 102, 103, and 104 including unit stacks generating current on each side of each other. A battery module 100; A positive electrode electrolyte tank 202 for supplying a positive electrode electrolyte to the battery module 100 and storing the positive electrode electrolyte flowing out of the battery module 100; A negative electrolyte tank 204 for supplying a negative electrolyte to the battery module 100 and storing the negative electrolyte flowing out of the battery module 100; A positive electrode electrolyte pump 302 for circulating a positive electrode electrolyte between the battery module 100 and the positive electrode electrolyte tank 202; And a negative electrolyte pump 304 for circulating the negative electrolyte between the battery module 100 and the negative electrolyte tank 204.

In this case, the unit stack is formed by stacking a plurality of unit cells 130. For convenience, FIG. 2 illustrates a unit stack formed by stacking one unit cell 130.

Referring to FIG. 2, a separator 123 is disposed in the center of the unit cell 130, and electrodes 120 and 121 and bipolar plates 118 and 119 are disposed symmetrically on both sides thereof in a symmetric structure.

The unit cell 130 has a structure in which one or more units are stacked, and current collecting plates 115 and 117 and end plates 111 and 113 are stacked so as to contact the bipolar plates 118 and 119.

One side of the end plates 111 and 113 is perforated and then joined to each other using a connecting member (eg, bolt/nut) through a through hole to provide a unit cell 130, and a plurality of the unit cells 130 After arranging them in dogs, a unit stack is formed through electrical connection.

A spacer (not shown) between the separator 123, the electrodes 120 and 121, the bipolar plates 118 and 119, the current collecting plates 115 and 117, and the end plates 111 and 113 for the flow or combination of an electrolyte solution. Each may be interposed, and as an example, it is preferable to arrange it between the separator 123 and the electrodes 120 and 121.

The separator 123 is disposed between the electrodes 120 and 121, and a gasket for correcting the thickness of the electrode and preventing leakage of the electrolyte is disposed at an edge of the electrode disposed between the separator and the bipolar plate. Accordingly, the unit cell 130 is a bipolar plate 118 / a gasket located at the edge of the electrode 120 and the electrode 120 / separator 123 / a gasket located at the edge of the electrode 121 and the electrode 121 / bipolar Plates 119 may be arranged in order.

The plurality of unit cells 130 have a structure connected in series or connected in parallel as shown in FIG. 1, and are configured to generate current through circulation of an electrolyte solution. The unit stack is electrically connected to another neighboring unit stack through a bus bar (not shown). The unit modules 101, 102, 103, and 104 and the battery module 100 discharge current generated inside the unit stacks or are connected to an external power source.

3 is a schematic diagram showing a charging and discharging process of a zinc-iron redox flow battery.

The positive electrode electrolyte stored in the positive electrode electrolyte tank contains [Fe(CN) ₆ ] ^3-/4- as the positive electrode active material, and the negative electrolyte stored in the negative electrode electrolyte tank is Zn/[Zn(OH) ₄ ] ^2- It includes. In the negative electrode, charging is performed while zinc is deposited on the surface of the negative electrode, and the deposited zinc is dissolved in an electrolytic solution as ions of [Zn(OH) ₄ ] ^2- during the discharge process. In addition, in the positive electrode, [Fe(CN) ₆ ] ^4- is oxidized to form [Fe(CN) ₆ ] ^3- , and charging is performed. In the discharging process, the [Fe(CN) ₆ ] ^3- becomes [Fe( CN) ₆ ] ^4- .

On the other hand, the zinc-iron redox flow battery has the advantage of having low cost and toxicity, and having a high discharge voltage in an aqueous strong base solution. However, there is a problem in that the capacity of the battery is reduced and the cycle characteristics are deteriorated due to the precipitation and dissolution of zinc during the charging and discharging process of the negative electrode, and there is a problem that the capacity of the battery is reduced and the cycle characteristics of the battery are rapidly deteriorated, especially at high current density.

Accordingly, in the present invention, by forming a flow path of an interlocking pattern on a bipolar plate, the mass transport of the electrolyte is improved, thereby improving the precipitation and dissolution of zinc generated during the charging and discharging process, thereby providing excellent voltage efficiency and cycle characteristics. -To provide an iron redox flow battery.

The present invention is a separator;

Electrodes disposed on both sides of the separator;

Bipolar plates respectively disposed on one side of the electrode;

Positive electrode electrolyte; And

A zinc-iron redox flow battery comprising a cathode electrolyte,

The bipolar plate includes a flow path of an interlocking pattern,

The positive electrode electrolyte includes [Fe(CN) ₆ ] ^3-/4- and a basic aqueous solution,

The negative electrode electrolyte relates to a zinc-iron redox flow battery comprising Zn/[Zn(OH) ₄ ] ^2- and a basic aqueous solution.

In the present invention, the separator, electrode, and bipolar plate are included in the unit cell 130 as described above, and the separator, electrode and bipolar plate are included in the form of a unit cell in the zinc-iron redox flow battery, The unit cell has a structure in which one or more units are stacked. In addition, the unit cells 130 are included in a zinc-iron redox flow battery in the form of a unit stack formed by stacking a plurality of units.

As described above, the separator is disposed in the center of the unit cell, and electrodes and bipolar plates are disposed symmetrically on both sides thereof in a symmetric structure.

The bipolar plate includes a channel (F) of an interlocking pattern.

The bipolar plate in contact with the electrode receives a positive electrode electrolyte and a negative electrode electrolyte for an electrochemical reaction, and supplies them to the electrode at a uniform pressure and quantity.

The flow path is formed to allow the electrolyte to move, and in the present invention, the flow path preferably has an interdigitated flow field (IDFF) pattern.

Conventional bipolar plates that do not include flow paths mostly use carbon felt as an electrode material, but there is a limitation in increasing the flow rate due to the high flow resistance of the fluid, and thus it is difficult to operate a high-output battery. However, the bipolar plate including the flow path of the interlocking pattern can reduce the pressure inside the cell compared to the bipolar plate not including the flow path. Accordingly, since the electrolyte can be circulated at a high flow rate, mass transport can be improved, thereby improving the precipitation and dissolution reactions of zinc occurring in the charging and discharging process of the negative electrode, thereby improving the zinc-iron redox flow. It is possible to improve the voltage efficiency and cycle characteristics of the battery. In particular, it is possible to solve the problem of decreasing the charging/discharging efficiency and cycle characteristics of a battery at a high current density. In addition, polarization occurring in the charging process can be significantly reduced compared to the conventional bipolar plate.

The interlocking pattern refers to a structure in which flow paths in a form interlocking with each other are continuously arranged, and one surface of each of the flow paths is closed, and the entrance or exit of the flow path is alternately opened. In the case of the interlocking flow path structure, the electrolyte not only flows along the flow path, but also flows along the flow path, further increasing the chance of an electrode reaction, thereby increasing the charging/discharging reaction efficiency of the zinc-iron redox flow battery.

The passage is formed through the passage-forming partition wall 154, and the width and thickness of the partition wall 154 may be appropriately adjusted according to the size of the bipolar plate. In general, the interval between the partition walls 154 is defined as the width of the flow path, and the thickness of the partition walls 154 is defined as the depth of the flow path.

For example, 25 to 325cm when producing a bipolar plate of the ^second area, the width of the barrier rib 154 is from 2 to 8mm, the thickness is 1 to 3.5mm, and the width of the flow path of a depth of 2 to 8mm and the passage is from 1 to 3.5mm Can be.

The bipolar plate may be made of a conductive material, and the present invention is not particularly limited. In the case of a conductive material, a carbon material such as metal, stainless steel, graphite, or a conductive polymer may be used.

The bipolar plate includes an inlet 162 for introducing an electrolyte solution so as to supply an electrolyte solution to an electrode at one lower side, an outlet 161 for discharging the electrolyte solution at one upper side, and between the inlet port 162 and the flow path part F. And a supply flow path 172 for equal distribution of the electrolyte solution and a discharge flow path 171 for equal distribution of the electrolyte solution located between the discharge port 161 and the flow path part F.

In addition, the supply flow passage 172 and the discharge flow passage 171 have various shapes so that the flow rate of the electrolyte can be evenly distributed and supplied or discharged, and for example, may have a distribution flow passage shape having a plurality of branches.

Two bipolar plates are supplied with an anode electrolyte and a cathode electrolyte from the supply channel, respectively, and the electrolyte is evenly supplied to the front surface of the electrode through the interlocking pattern channel. The charging and discharging of the zinc-iron redox battery occurs through oxidation and reduction reactions of the positive electrode active material and negative electrode active material of the supplied electrolyte, and the positive electrode active material and the negative electrode active material whose oxidation number has changed from the oxidation/reduction reaction are passed through the discharge channel. It is recovered in the electrolyte tank and the cathode electrolyte tank, respectively.

In addition, since the two bipolar plates must not be in physical contact, the edge of the electrode disposed between the separator and the bipolar plate to block the physical contact of the bipolar plate, correct the thickness of the electrode, and prevent leakage of electrolyte. The gasket is located. Specifically, the gasket is located on the surface of the bipolar plate excluding a portion of the bipolar plate in contact with the electrode. The separator is positioned between the electrodes to prevent the movement of the positive active material and the negative active material, and allows only ions to be transferred.

The separator allows ions in the electrolyte to pass through, and generates electricity through electrochemical reactions of electrodes located at both sides through the electrolyte. In this case, the material, thickness, and components of the separator are not particularly limited in the present invention, and known ones may be used.

The separator may have a thickness of 20 to 100 μm, preferably 30 to 70 μm. If the thickness of the separator is 20 to 100 μm, it is a separator suitable for use in a zinc-iron redox flow battery, and the efficiency of the battery can be further increased.

The separator may be specifically, for example, a fluorine-based polymer, a partially fluorine-based polymer, or a hydrocarbon-based polymer, and more specifically, a perfluorosulfonic acid-based polymer, a hydrocarbon-based polymer, an aromatic sulfone-based polymer, an aromatic ketone-based polymer, and polybenzimida. Sol polymer, polystyrene polymer, polyester polymer, polyimide polymer, polyvinylidene fluoride polymer, polyethersulfone polymer, polyphenylene sulfide polymer, polyphenylene oxide polymer, polyphosphazen polymer , Polyethylene naphthalate polymer, polyester polymer, doped polybenzimidazole polymer, polyether ketone polymer, polyphenylquinoxaline polymer, polysulfone polymer, sulfonated polyarylene ether polymer, sulfonated Polyetherketone polymer, sulfonated polyetheretherketone polymer, sulfonated polyamide polymer, sulfonated polyimide polymer, sulfonated polyphosphagen polymer, sulfonated polystyrene polymer, and radiation polymerized sulfonated low-density polyethylene -g-a homopolymer of one or more polymers selected from the group consisting of polystyrene-based polymers (Homo copolymer), alternating copolymer, random copolymer, block copolymer, It may be selected from those that are multiblock copolymers or graft copolymers.

According to the composition of the electrolyte, one of the electrodes serves as an anode and the other serves as a cathode.

That is, the electrode receiving the positive electrolyte solution serves as an anode, and the electrode receiving the negative electrolyte solution serves as a negative electrode.

Specifically, the electrode receiving the positive electrode electrolyte containing [Fe(CN) ₆ ] ^3-/4- and a basic aqueous solution performs the function of the positive electrode, and uses Zn/[Zn(OH) ₄ ] ^2- and a basic aqueous solution. An electrode receiving the included cathode electrolyte may perform the function of a cathode.

The electrode is made of a material having conductivity, for example, carbon felt, graphite felt, carbon cloth, carbon paper, metal cloth, metal felt, metal mesh, and may include at least one selected from the group consisting of metal foam. have.

The anode electrolyte is stored in an anode electrolyte tank, and the stored anode electrolyte is supplied to a unit cell including a separator, an electrode, and a bipolar plate. The supply is made by an anode electrolyte pump for circulating the anode electrolyte between the unit cell and the anode electrolyte tank, and current is generated by the circulation of the anode electrolyte.

The positive electrode electrolyte includes a positive electrode active material and a basic aqueous solution, and specifically, a positive electrode active material is dissolved in a basic aqueous solution, and the positive electrode active material of the positive electrode electrolyte of a zinc-iron redox flow battery is an oxidation-reduction pair of [Fe(CN) ₆ ] ^3-/4- .

When [Fe(CN) ₆ ] ^3-/4- of the oxidation-reduction pair changes to a higher one of the two oxidation states, that is, when oxidation occurs, charging occurs, and when the oxidation state changes to a lower one, That is, discharge occurs when reduction occurs.

The [Fe(CN) ₆ ] ^3-/4- is very unstable in acidic solutions and cannot be used in acidic solutions because it generates toxic substances such as HCN and KCN.

The [Fe(CN) ₆ ] ^3-/4- may be included in the form of a salt including [Fe(CN) ₆ ] ^4- to exist in the ionic form in the positive electrode electrolyte. The salt may be Na ₄ Fe(CN) ₆ or K ₄ Fe(CN) ₆ , but is not limited thereto.

The basic aqueous solution is not particularly limited if it is used in the art, but preferably 1 selected from the group consisting of KOH, NaOH, LiOH, Ba(OH) ₂ , Mg(OH) ₂ and Ca(OH) ₂ It may be an aqueous solution containing more than one species, more preferably an aqueous solution containing one or more selected from the group consisting of KOH and NaOH. In the basic aqueous solution, the concentration of the base is 0.1 to 10 M, preferably 1 to 7 M.

In addition, the positive electrode active material is contained in an amount of 0.1 to 1.5M, preferably 0.2 to 1M in the positive electrode electrolyte. If the positive electrode active material is contained in less than 0.1M, the concentration is too low to be applied to a redox flow battery. If it exceeds 1.5M, the positive electrode active material may not be completely dissolved depending on the composition of the supporting electrolyte and external conditions.

The cathode electrolyte is stored in a cathode electrolyte tank, and the stored cathode electrolyte is supplied to a unit cell including a separator, an electrode, and a bipolar plate. The supply is made by a cathode electrolyte pump for circulating the cathode electrolyte between the unit cell and the anode electrolyte tank, and current is generated by the cathode electrolyte circulation.

The negative electrode electrolyte includes a negative electrode active material and a basic aqueous solution, and specifically, a negative electrode active material is dissolved in a basic aqueous solution, and the negative electrode active material of the negative electrode electrolyte of a zinc-iron redox flow battery is Zn/[Zn( OH) ₄ ] ^2- .

When Zn/[Zn(OH) ₄ ] ^2- of the oxidation-reduction pair changes to the lower of the two oxidation states, that is, when reduction occurs, charging occurs, and when the oxidation state changes to a higher level, that is, Discharge occurs when oxidation occurs. Specifically, charging is performed while zinc is precipitated on the surface of the negative electrode, and the deposited zinc is reduced to [Zn(OH) ₄ ] ^2- to cause discharge.

The Zn/[Zn(OH) ₄ ] ^2- may be included in the form of a zinc compound in the negative electrode electrolyte. The zinc compound is not particularly limited as long as it contains zinc, but preferably at least one selected from the group consisting of zinc oxide, zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc nitrate and zinc acetate. Can include.

The basic aqueous solution is not particularly limited if it is used in the art, but preferably 1 selected from the group consisting of KOH, NaOH, LiOH, Ba(OH) ₂ , Mg(OH) ₂ and Ca(OH) ₂ It may be an aqueous solution containing more than one species, more preferably an aqueous solution containing one or more selected from the group consisting of KOH and NaOH. In the basic aqueous solution, the concentration of the base is 0.1 to 10 M, preferably 1 to 7 M.

In addition, the negative active material is contained in the negative electrode electrolyte in an amount of 0.1 to 3M, preferably 0.2 to 1M. If the negative active material is contained in an amount of less than 0.1M, the concentration is too low to be applied to a redox flow battery, and if it exceeds 3M, solid zinc oxide (ZnO), which is an inactive species, may precipitate.

The zinc-iron redox flow battery of the present invention may have a flow rate of 0.5 to 6 mL/(cm ² ·min), preferably 2 to 4 mL/(cm ² ·min) of the positive electrolyte and negative electrolyte. As the zinc-iron redox flow battery of the present invention includes a bipolar plate having an interlocking pattern of flow paths, mass transport of an electrolyte may be improved, thereby improving voltage efficiency and cycle characteristics of the battery.

The higher the flow rate, the better the material transfer of the electrolyte can be, and excellent electrolyte material transfer can be achieved in the flow range, thereby improving the voltage efficiency and cycle characteristics of the battery, and reducing the overvoltage that occurs during charging and discharging. I can make it.

If the flow rate is less than 0.5 mL/(cm ² min), the amount of active material delivered to the electrode may be insufficient, resulting in a large overvoltage. If it exceeds 6 mL/(cm ² min), the pressure inside the cell increases significantly. Electrolyte may leak.

The zinc-iron redox flow battery of the present invention has a current density of 30 to 200 mA/cm ² , and preferably 50 to 150 mA/cm ² .

The zinc-iron redox flow battery is charged while zinc is deposited on the surface of the negative electrode, and the deposited zinc is reduced to [Zn(OH) ₄ ] ^2- to discharge, so that the flow path of the conventional interlocking pattern A zinc-iron redox flow battery including a bipolar plate that does not contain a battery has a problem in that charging/discharging efficiency and cycle characteristics of the battery are deteriorated at a high current density of 100 mA/cm ² or more.

However, since the zinc-iron redox flow battery of the present invention includes a bipolar plate in which a flow path of an interlocking pattern is formed, mass transport of an electrolyte can be improved, so that the voltage of the battery even at a high current density of 100 mA/cm ² or more Efficiency and cycle characteristics can be improved.

If the current density is less than 30mA/cm ^2, the output of the battery is low, and charging/discharging time is long. If it exceeds 200mA/cm ² , the voltage efficiency is greatly reduced due to a high overvoltage, resulting in very low energy efficiency. have.

In addition, in the zinc-iron redox flow battery of the present invention, the higher the current density in the above flow rate range, the better the effect of improving the voltage efficiency and cycle characteristics of the battery is. In particular, the flow rate increases because the electrochemical reaction occurs rapidly at a high current density. Due to the improvement in mass transport, the effect of reducing overvoltage of the battery is excellent.

That is, the zinc-iron redox flow battery of the present invention exhibits optimal operating conditions at a flow rate of 0.5 to 6 mL/(cm ² ·min) and a current density of 30 to 200 mA/cm ² to improve voltage efficiency and effect of cycle characteristics. And can reduce overvoltage generated during charging and discharging.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

<Manufacture of zinc-iron redox flow battery>

Example 1.

2.03 g of zinc oxide (0.025 mol) was mixed with 8 g of NaOH (0.2 mol), and then about 10 mL of distilled water was added. When the zinc oxide gradually dissolves and the color of the solution becomes transparent, distilled water was slowly added in small portions to a total volume of 50 mL to prepare a cathode electrolyte, and the concentration of zinc oxide in the cathode electrolyte was 0.5 M, and the concentration of NaOH was 4 M. Was.

After preparing an aqueous NaOH solution by adding NaOH to 25 mL of distilled water, after adding K ₄ Fe(CN) ₆ , distilled water was added so that the volume of the solution became 50 mL, and the concentration of 3M NaOH and 0.5MK ₄ Fe(CN) ₆ was added. A positive electrode electrolyte solution having a volume of 50 mL was prepared.

5×5 cm ² of carbon paper is disposed on both sides of the separator, a bipolar plate is disposed on one side of the carbon paper, and a gold current collector is disposed on one side of the bipolar plate. Gold current collector) was placed to prepare a unit cell.

Nafion 212 was used as a separator. The bipolar plate was formed with an interdigitated flow field (IDFF) pattern flow path, the width of the flow path was 4 mm, the depth of the flow path was 2.5 mm, the width of the partition wall was 5 mm, and the width of the buffer flow path was 3 mm. In addition, a gasket was placed on the edge of the carbon paper.

Comparative Example 1.

A zinc-iron redox flow battery was manufactured in the same manner as in Example 1, except that a bipolar plate having no flow path was used and carbon felt was used.

Experimental Example 1. Performance evaluation of redox flow battery

The zinc-iron redox flow batteries prepared in Example 1 and Comparative Example 1 were subjected to 100 cycles in a voltage range of 0.2 to 2.1 V, and cycle characteristics, voltage efficiency (VE), current efficiency (CE), and energy efficiency ( EE) was measured. At this time, conditions of flow rate and current density were performed differently, and conditions of flow rate and current density are shown in Table 1 below.

Current efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) were obtained by the following equations 1 to 3, respectively.

[Equation 1]

Current efficiency (CE) = discharge capacity/charge capacity

[Equation 2]

Energy efficiency (EE) = discharge energy/charge energy

[Equation 3]

Voltage efficiency (VE) = energy efficiency/current efficiency

The cycle characteristics, current efficiency, energy efficiency, and voltage efficiency of the zinc-iron redox flow batteries of Example 1 and Comparative Example 1 are shown in Table 1 below.

Current density
(mA/cm ² ) flux
(mL/(cm ² min)) Current efficiency
(%) Voltage efficiency
(%) Energy efficiency
(%) 100 ^th cycle discharge capacity retention rate (%) Example 1
(IDFF) 50 One 98 87.6 85.6 75.9 50 4 94.3 88.9 83.9 91.5 100 One 97.5 78.3 76.4 80.3 100 4 99.3 82.1 81.5 99.3 Comparative Example 1
(Flat) 50 One 98 87.5 85.8 29.9 50 4 98.1 87.8 86.1 33.1 100 One 98.8 79.3 78.4 50.0 100 4 98.9 71.9 71.1 13.9

In the results of Example 1 and Comparative Example 1 having a flow rate of 1 mL/(cm ² min) and a current density of 50 mA/cm ² , the flow of zinc-iron redox in Example 1 including a bipolar plate having an interlocking pattern of flow paths The cycle characteristics of the battery showed remarkably better results than the zinc-iron redox flow battery of Comparative Example 1 including the bipolar plate in which the flow path was not formed. More specifically, the 100th cycle discharge capacity increased by more than 150% compared to Comparative Example 1 (FIG. 6).

The results of Example 1 and Comparative Example 1 of flow rate 1 mL/(cm ² ·min) and current density 100 mA/cm ² also showed similar results as above (FIG. 7).

In Examples 1 and Comparative Example 1, the zinc and then proceeds iron LES 100 cycles the redox flow cell at a current density of 50mA / cm ² at a flow rate of ^{4mL / (cm 2 · min)} , 4mL / (cm 2 · min) Life characteristics were measured by performing 100 cycles with a current density of 100 mA/cm ² at a flow rate of. As a result, Comparative Example 1 showed very poor lifespan characteristics, whereas Example 1 showed excellent lifespan characteristics (FIG. 8). In particular, at a current density of 100mA/cm ² , Example 1 showed significantly better voltage efficiency and energy efficiency than Comparative Example 1.

In addition, in the measurement of the voltage profile, it was confirmed that in the zinc-iron redox flow battery of Comparative Example 1, the polarization phenomenon that occurs at the end of the charging process increases rapidly as the cycle progresses (FIG. 9).

From the above results, it can be seen that when the flow path of the interlocking pattern is formed on the bipolar plate, the mass transfer of the electrolyte is improved, and the electrochemical charge/discharge reaction for precipitation and dissolution of zinc is improved, thereby greatly improving the cycle characteristics. And it can be seen that the higher the current density, the better the effect of reducing the overvoltage of the battery due to the improvement of mass transport, thereby improving the voltage efficiency.

In the result of Example 1 in which conditions of flow rate and current density were different,

As the flow rate increased, the voltage efficiency and cycle characteristics were improved. It can be seen that as the flow rate increases, the electrolyte mass transport is improved, the overvoltage is reduced, and the voltage efficiency is increased.

In addition, as the current density increased, the cycle characteristics improved.

In particular, since the electrochemical reaction occurs rapidly at a high current density such as 100mA/cm ² , it was confirmed that the improvement of the electrolyte mass transfer according to the increase in flow rate had a greater effect on the reduction of the overvoltage.

Accordingly, it can be seen that the zinc-iron redox flow battery of the present invention has excellent voltage efficiency and cycle characteristics, and particularly, the higher the flow rate and current density, the more excellent the effect is.

1000: redox flow battery
100: battery module
101, 102, 103, 104: unit module
202, 204: electrolyte tank
302, 304: electrolyte pump
111, 113: end plate
115, 117: collector plate
118, 119: bipolar plate
120, 121: electrode
123: separator
130: unit cell
154: passage forming bulkhead
161: outlet
162: inlet
171: discharge flow path
172: supply euro
F: Euro

Claims (10)

Separator;
Electrodes disposed on both sides of the separator;
Bipolar plates respectively disposed on one side of the electrode;
Positive electrode electrolyte; And
A zinc-iron redox flow battery comprising a cathode electrolyte,
The bipolar plate includes a flow path of an interlocking pattern,
The positive electrode electrolyte includes [Fe(CN) ₆ ] ^3-/4- and a basic aqueous solution,
The cathode electrolyte is a zinc-iron redox flow battery comprising Zn/[Zn(OH) ₄ ] ^2- and a basic aqueous solution. The method of claim 1, wherein the interlocking pattern flow path has a structure in which flow paths interdigitated with each other are continuously arranged, each of the flow paths has a structure in which one surface is closed, and the inlet or outlet of the flow path is alternately opened. Zinc-iron redox flow cell. The zinc-iron redox flow battery according to claim 1, wherein the flow path has a width of 2 to 8 mm and a depth of the flow path of 1 to 3.5 mm. The method of claim 1, wherein the concentration of [Fe(CN) ₆ ] ^3-/4- is 0.1 to 1.5M, and the concentration of Zn/[Zn(OH) ₄ ] ^2- is 0.1 to 3M. Zinc-iron redox flow cell. The method of claim 1, wherein the basic aqueous solution is an aqueous solution comprising at least one selected from the group consisting of KOH, NaOH, LiOH, Ba(OH) ₂ , Mg(OH) ₂ and Ca(OH) ₂ Zinc-iron redox flow cell. The zinc-iron redox according to claim 1, wherein the electrode comprises at least one selected from the group consisting of carbon felt, graphite felt, carbon paper, metal cloth, metal felt, metal mesh, and foamed metal. Flow cell. The zinc-iron redox flow battery according to claim 1, wherein the flow rate of the positive electrode electrolyte and the negative electrode electrolyte is 0.5 to 6 mL/(cm ² ·min) in the zinc-iron redox flow battery. The zinc-iron redox flow battery according to claim 7, wherein the flow rate of the positive electrode electrolyte and the negative electrode electrolyte is 2 to 4 mL/(cm ² ·min) in the zinc-iron redox flow battery. The zinc-iron redox flow battery according to claim 1, wherein the zinc-iron redox flow battery has a current density of 30 to 200 mA/cm ² . The zinc-iron redox flow battery according to claim 9, wherein the zinc-iron redox flow battery has a current density of 50 to 150 mA/cm ² .

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