Cell stack and redox flow battery

Abstract

The cell stack of the present invention is formed by stacking a plurality of cell units. It includes porous electrodes and bipolar plates facing the electrodes. Each bipolar plate has an electrolyte inlet, an electrolyte outlet, and a plurality of first trenches extending from the side of the inlet to the side of the outlet. Each of the plurality of first trenches is configured such that the electrolyte flows from the first trench to the outlet. R2 / R1 is 7 × 10-11 or more and 2 × 10-4 or less. R1 is the permeation resistance, representing the ease of electrolyte flow on the electrodes, and R2 is the permeation resistance, representing the ease of electrolyte flow in the first trenches.

Description

Technical Field

[0001] This invention relates to a unit stack and a redox flow battery. This application asserts priority based on international application PCT / JP2020 / 037918 dated October 6, 2020, and incorporates all the contents of the aforementioned international application. Prior Technology

[0002] As a high-capacity storage battery, a redox flow battery is known. For example, the redox flow battery shown in Patent Documents 1 to 4 includes a cell stack formed by stacking a plurality of battery cells. Charging and discharging are performed by circulating the positive electrode electrolyte and the negative electrode electrolyte within the battery cells.

[0003] The battery cell includes a positive electrode, a negative electrode, and a separator disposed between the two electrodes. A bipolar plate is disposed between adjacent battery cells in the stacking direction. The electrodes are porous materials composed of carbon fiber aggregates or the like. In the bipolar plate with grooves, the grooves facilitate the distribution of electrolyte across the entire surface of the bipolar plate. [Previous Technical Documents] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2015-122230 [Patent Document 2] Japanese Patent Application Publication No. 2015-122231 [Patent Document 3] Japanese Patent Application Publication No. 2015-138771 [Patent Document 4] Japanese Patent Application Publication No. 2015-210849 Summary of the Invention

[0005] The unit stack of this invention, It is formed by stacking multiple battery cells, and has the following characteristics: The porous material electrode and the bipolar plate facing the aforementioned electrode, The above bipolar plate has the following features: Electrolyte inlet section; The above-mentioned electrolyte discharge section; and A plurality of first groove portions extend from the side where the inlet portion is disposed toward the side where the outlet portion is disposed; and Each of the aforementioned plurality of first trench portions is configured such that the electrolyte in the aforementioned first trench portion flows to the aforementioned discharge portion. R² / R¹ is greater than 7 × 10⁻¹¹ and less than 2 × 10⁻⁴. R1 is the permeation resistance, representing the ease with which the electrolyte flows across the electrode. R1=(L1 / K1)×μ×(1 / S1), L1 is the length of the aforementioned electrode. K1 is the transmittance of the above electrode. μ is the viscosity of the electrolyte mentioned above. S1 is the cross-sectional area of ​​the aforementioned electrode. R2 represents the permeation resistance, indicating the ease of flow of the electrolyte in the first trench. R 2=(32×μ×L 2) / (de 2×S 2), L2 is the length of the first groove portion mentioned above. de is the equivalent diameter of the first groove portion mentioned above. S2 is the cross-sectional area of ​​the first groove section mentioned above.

[0006] The redox flow battery of the present invention A unit stack possessing the present invention. Simple Explanation of the Diagram

[0007] Figure 1 is a schematic diagram illustrating the operating principle of the redox flow battery according to the embodiment. Figure 2 is a schematic diagram of the redox flow battery according to the embodiment. Figure 3 is a schematic diagram showing one example of the configuration of a unit stack in an implementation method. Figure 4 is a schematic diagram showing the positional relationship between the electrode and the bipolar plate in Embodiment 1. Figure 5 is a schematic top view of the bipolar plate shown in Embodiment 1. Figure 6A is a schematic cross-sectional view of the first groove portion of the bipolar plate shown in Embodiment 1. Figure 6B is a schematic cross-sectional view of the first groove portion having a V-shaped cross-sectional shape. Figure 7 is a schematic top view of the bipolar plate shown in Embodiment 2. Figure 8 is a schematic top view of the bipolar plate shown in Embodiment 3. Figure 9 is a schematic top view of the bipolar plate shown in Embodiment 4. Implementation

[0008] [The problem that this invention aims to solve] If the electrolyte flow at the electrodes is low, there is a risk of incomplete cell reaction at the electrodes. As a result, the electrode's reaction resistance increases. Furthermore, if the electrolyte flow in the trenches of the bipolar plates is low, the heat generated during the cell reaction tends to accumulate in the electrolyte. If the electrolyte temperature increases, the electrolyte's stability decreases. As a result, the electrode's reaction resistance increases.

[0009] One objective of this invention is to provide a cell stack where the reactive resistance of the electrodes is not easily increased. Another objective of this invention is to provide a redox flow battery with excellent battery performance.

[0010] [Description of Embodiments of the Invention] The inventors have conducted intensive research on the configuration of electrodes to reduce the reactive resistance of a unit cell. As a result, the inventors have gained the insight that the increase in electrode reactive resistance can be reduced by adjusting the flow ease of the electrolyte on the electrodes and the flow ease of the electrolyte on the bipolar plates. Based on this insight, embodiments of the present invention will be described below.

[0011] <1> Implementation method: cell stack It is made by stacking multiple battery cells. It has porous electrodes and bipolar plates facing the aforementioned electrodes. The above bipolar plate has the following features: Electrolyte inlet section; The above-mentioned electrolyte discharge section; and A plurality of first groove portions extend from the side where the inlet portion is disposed toward the side where the outlet portion is disposed; and Each of the aforementioned plurality of first trench portions is configured such that the electrolyte in the aforementioned first trench portion flows to the aforementioned discharge portion. R² / R¹ is greater than 7 × 10⁻¹¹ and less than 2 × 10⁻⁴. R1 is the permeation resistance, representing the ease with which the electrolyte flows across the electrode. R1=(L1 / K1)×μ×(1 / S1), L1 is the length of the aforementioned electrode. K1 is the transmittance of the above electrode. μ is the viscosity of the electrolyte mentioned above. S1 is the cross-sectional area of ​​the aforementioned electrode. R2 represents the permeation resistance, indicating the ease of flow of the electrolyte in the first trench. R 2=(32×μ×L 2) / (de 2×S 2), L2 is the length of the first groove portion mentioned above. de is the equivalent diameter of the first groove portion mentioned above. S2 is the cross-sectional area of ​​the first groove section mentioned above.

[0012] The inventors have defined a new index, the permeation resistance, as a measure of the ease with which an electrolyte flows. The above-described form... <1> The specification stipulates that the permeation resistance R1 of the electrode, the permeation resistance R2 of the first trench portion of the bipolar plate, and R2 / R1 are 7×10⁻¹¹ or more and 2×10⁻⁴ or less. If R2 / R1 is within the above range, the electrolyte flowing in the first trench portion of the bipolar plate can easily and appropriately flow to the electrode, and the fluidity of the electrolyte on the electrode is improved. As a result, the increase in the electrode's reactive resistance is significantly reduced. Here, the above characteristics are expressed as a percentage. <1> The range of R² / R₁ is above 7 × 10⁻⁹% and below 2 × 10⁻²%. Hereinafter, the value of R² / R₁ is sometimes expressed as a percentage.

[0013] <2> As one form of unit heap in the implementation method Examples of R2 / R1 are those with a form greater than 2×10⁻¹⁰ and less than 1×10⁻⁴.

[0014] If R2 / R1 is greater than 2×10⁻¹⁰ and less than 1×10⁻⁴, then the increase in the electrode's reactive resistance is compared to the above-described configuration. <1> This further reduces the rate. If the above form is expressed as a percentage... <2> The range of R2 / R1 is above 2×10⁻⁸% and below 1×10⁻²%.

[0015] <3> As one form of unit heap in the implementation method Examples of R2 / R1 are those with values ​​greater than 5×10⁻⁹ and less than 3×10⁻⁵.

[0016] If R2 / R1 is greater than 5 × 10⁻⁹ and less than 3 × 10⁻⁵, then the increase in the electrode's reactive resistance is compared to the above-described configuration. <1> and form <2> This further reduces the rate. If the above form is expressed as a percentage... <3> The range of R2 / R1 is above 5×10⁻⁷% and below 3×10⁻³%.

[0017] <4> As one form of unit heap in the implementation method Examples of the above-mentioned electrodes can be given of the forms constructed from multiple materials with different permeability to the electrolyte.

[0018] By constructing electrodes from multiple materials with different transmittances, the overall transmittance of the electrode can be easily adjusted. Therefore, it is easy to obtain a configuration that meets the above requirements. <1> To form <3> The cell stack within the range of R2 / R1 as specified in the code.

[0019] <5> As the above form <1> To form <3> One form of the unit heap of any one of the terms can be exemplified as follows: The above electrodes have: The first layer is constructed of carbon felt or carbon cloth; and The second layer is made of carbon paper.

[0020] Carbon paper is less prone to deformation than carbon felt and carbon cloth. Therefore, electrodes containing carbon paper are less likely to deform. Electrodes that are less prone to deformation are less likely to enter the first trench of the bipolar plate that contacts the electrode. Therefore, it is possible to prevent electrodes that have entered the first trench from obstructing the flow of electrolyte. From the viewpoint of preventing electrodes from entering the first trench, it is preferable to arrange the second layer, which is made of carbon paper, facing the bipolar plate.

[0021] <6> As the above form <5> One form of a unit heap can be exemplified as follows: The weight per unit area of ​​the first layer mentioned above is between 20 g / m² and 500 g / m². The weight per unit area of ​​the second layer mentioned above is more than 10 g / m2 and less than 100 g / m2.

[0022] By ensuring that the unit area weight of the first layer and the unit area weight of the second layer meet the aforementioned ranges, the electrolyte permeability on the electrode can easily reach an appropriate value. Therefore, it is easy to obtain a configuration that satisfies the above requirements. <1> To form <3> The cell stack within the range of R2 / R1 as specified in the code.

[0023] <7> As the above form <1> To form <3> One form of the unit heap of any one of the terms can be exemplified as follows: The aforementioned bipolar plate has a second groove portion. The aforementioned second groove portion connects two adjacent first groove portions among the aforementioned plurality of first groove portions.

[0024] The second trench facilitates the smooth flow of electrolyte in the two adjacent first trenches. Therefore, it can prevent the permeation resistance of the first trench from becoming too high.

[0025] <8> Redox flow battery of implementation method Possessing the above-mentioned form <1> To form <7> A heap of any one of the following.

[0026] The redox flow battery of this embodiment exhibits excellent battery performance. This is because the redox flow battery has a cell stack in an embodiment where the increase in electrode reaction resistance is suppressed.

[0027] [Details of the embodiments of the present invention] Specific examples of the unit stack and redox flow battery of the present invention will be described with reference to the accompanying drawings. Hereinafter, the redox flow battery will be referred to as an "RF battery". The same symbols in the figures denote the same or equivalent parts. Furthermore, the present invention is not limited to these examples, but is intended to include all modifications within the same meaning and scope as the claims, as indicated by the claims.

[0028] <Implementation Method 1> An Overview of RF Batteries Referring to Figures 1 to 3, the RF battery 1 of the embodiment will be described. The RF battery 1 shown in Figure 1 utilizes a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material for charging and discharging. Representative positive and negative electrode active materials are metal ions whose valence changes due to redox reactions. The metal ions contained in the positive and negative electrode electrolytes shown in Figure 1 are examples. Figure 1 illustrates a Ti-Mn based RF battery containing Mn ions as the positive electrode active material and Ti ions as the negative electrode active material. In Figure 1, solid arrows represent charging reactions, and dashed arrows represent discharging reactions.

[0029] Regarding the RF battery 1, it is typically connected to the power system 90 via an AC / DC converter 80 or a power transformer 81. The RF battery 1 is charged with electricity generated by the power generation unit 91, or discharges the charged electricity to the load 92. The power generation unit 91 is a power generation device utilizing natural energy such as solar power or wind power, or a general power plant. The RF battery 1 is used for applications such as load balancing, instantaneous low voltage compensation, emergency power supply, and output smoothing of natural energy power generation.

[0030] Composition of RF Batteries The RF battery 1 includes a battery cell 10, a positive electrode box 12, and a negative electrode box 13. The battery cell 10 is responsible for charging and discharging. The positive electrode box 12 stores the positive electrolyte. The negative electrode box 13 stores the negative electrolyte.

[0031] (Battery unit) The battery cell 10 is separated into a positive electrode cell 102 and a negative electrode cell 103 by a separator 101. The separator 101 is an ion exchange membrane that does not allow electrons to pass through but allows hydrogen ions to pass through, for example. A positive electrode 104 is built into the positive electrode cell 102. A negative electrode 105 is built into the negative electrode cell 103.

[0032] Positive electrolyte and negative electrolyte are supplied to the positive electrode unit 102 and negative electrode unit 103 constituting the battery unit 10, respectively. The RF battery 1 in this example includes a destination pipe 108 and a return pipe 110 connecting the battery unit 10 to the positive electrode box 12. The RF battery 1 in this example includes a destination pipe 109 and a return pipe 111 connecting the battery unit 10 to the negative electrode box 13. Pumps 112 and 113 are installed on each destination pipe 108 and 109. Positive electrolyte is supplied from the positive electrode box 12 to the positive electrode unit 102 via the destination pipe 108 by pump 112. Positive electrolyte discharged from the positive electrode unit 102 returns to the positive electrode box 12 via the return pipe 110. Negative electrolyte is supplied from the negative electrode box 13 to the negative electrode unit 103 via the destination pipe 109 by pump 113. The negative electrode electrolyte discharged from the negative electrode unit 103 returns to the negative electrode box 13 through the return pipe 111. That is, the outgoing pipes 108 and 109 and the return pipes 110 and 111 form a circulation path.

[0033] (cell stack) As shown in Figures 2 and 3, the RF battery 1 is typically used in a form called a cell stack 2, which is formed by stacking a plurality of battery cells 10. The cell stack 2 has a configuration in which the sub-stack 20 shown in Figure 3 is sandwiched between its two sides by two end plates 22. The two end plates 22 are fastened together by a fastening mechanism 23 in a direction that brings them closer to each other. Figure 3 shows a cell stack 2 having a plurality of sub-stacks 20. The sub-stack 20 has a stacked body formed by repeatedly stacking a cell frame 3, a positive electrode 104, a separator 101, and a negative electrode 105 in sequence. A feed plate 21 is arranged at both ends of the stacked body. The feed plate 21 is connected to the outgoing pipes 108 and 109 and the return pipes 110 and 111 shown in Figures 1 and 2, which constitute the above-mentioned circulation flow path. The number of battery cells 10 stacked in the cell stack 2 can be appropriately selected.

[0034] The unit frame 3 has a bipolar plate 31 disposed between the positive electrode 104 and the negative electrode 105, and a frame 32 disposed around the bipolar plate 31. The central region of the bipolar plate 31 is exposed through a through window 32o of the frame 32. The positive electrode 104 is disposed facing each other on the first surface of the bipolar plate 31. The negative electrode 105 is disposed facing each other on the second surface of the bipolar plate 31. The second surface is the opposite side of the first surface. The positive electrode 104 and the negative electrode 105 are housed inside the frame 32, separated by the bipolar plate 31. By arranging the positive electrode 104 and the negative electrode 105 between the bipolar plates 31 of adjacent unit frames 3 with a separator 101, a battery unit 10 is formed.

[0035] Within the frame 32 of the unit frame 3, supply manifolds 33 and 34, and drain manifolds 35 and 36, along with supply slits 33s and 34s and drain slits 35s and 36s, are formed. In this example, the positive electrolyte is supplied from the supply manifold 33 to the positive electrode 104 via the supply slit 33s. The positive electrolyte supplied to the positive electrode 104 is discharged through the drain slit 35s to the drain manifold 35. Similarly, the negative electrolyte is supplied from the supply manifold 34 to the negative electrode 105 via the supply slit 34s. The negative electrolyte supplied to the negative electrode 105 is discharged through the drain slit 36s to the drain manifold 36. The supply manifolds 33 and 34, and the drain manifolds 35 and 36 are arranged through the frame 32, forming the flow paths for each electrolyte by stacking the unit frame 3. These flow paths are connected via feed plates 21 to outgoing pipes 108 and 109 and return pipes 110 and 111 shown in Figures 1 and 2, respectively. The cell stack 2 is able to allow positive and negative electrolytes to flow in the battery cell 10 through the above flow paths.

[0036] As one of the features of unit cell 2 in this example, the flow of electrolyte from bipolar plate 31 of unit cell 2 toward positive electrode 104 and negative electrode 105 can be exemplified by adjusting the flow. The configuration of positive electrode 104, negative electrode 105, and bipolar plate 31 is adjusted for this purpose. In the following description, positive electrode 104 and negative electrode 105 are simply referred to as electrode 4 without distinction.

[0037] (electrode) The configuration of electrode 4 will be explained based on Figure 4. Figure 4 shows a portion of electrode 4 and a portion of bipolar plate 5 facing electrode 4. Bipolar plate 5 is the same as bipolar plate 31 shown in Figure 3. A first groove portion 51 is provided in bipolar plate 5. The first groove portion 51 will be described below.

[0038] Electrode 4 is a porous material with electrical conductivity. For example, electrode 4 comprises at least one porous material selected from the group consisting of carbon, titanium, and tungsten.

[0039] Electrode 4 can be made of a single material or a plurality of materials. In particular, electrode 4 can also be made of a plurality of materials with different electrolyte permeability. By using a plurality of materials with different permeability to construct electrode 4, the overall permeability of electrode 4 can be easily adjusted. By adjusting the permeability of electrode 4, it is easy to obtain a unit cell stack 2 that satisfies the following R 2 / R 1 within a specific range.

[0040] In this example, electrode 4 has a first layer 41 and a second layer 42 arranged in the thickness direction. Electrode 4 having the first layer 41 and the second layer 42 is an example of an electrode 4 composed of multiple materials with different transmittances. The number of layers constituting electrode 4 can also be three or more.

[0041] The first layer 41 and the second layer 42 are, for example, composed of carbon paper, carbon felt, or carbon cloth. Carbon cloth is formed by alternating weaves of carbon fibers in the longitudinal and transverse directions. Carbon felt is formed by interlacing individual carbon fibers. Carbon paper consists of a plurality of carbon fibers and an adhesive binding the carbon fibers together. Carbon paper may also contain carbon particles. The fiber diameter of the carbon fibers constituting the carbon felt and carbon paper is preferably 1 μm or more and 20 μm or less. Furthermore, the fiber length of the carbon fibers constituting the carbon felt and carbon paper is preferably 500 μm or more and 100 mm or less. Examples of adhesives for carbon paper include organic adhesives such as phenolic resin. Organic adhesives are sometimes carbonized by heat treatment. Carbon paper is less prone to deformation than carbon felt and carbon cloth. Therefore, the electrode 4 containing carbon paper is less prone to deformation. The less deformable electrode 4 is less likely to enter the first groove portion 51 of the bipolar plate 5 that contacts the electrode 4. Therefore, the flow of electrolyte in the first trench 51 can be suppressed due to the electrode 4 entering the first trench 51.

[0042] The first layer 41 can be made of carbon felt or carbon cloth, for example. The second layer 42 can be made of carbon paper, for example. From the viewpoint that it is not easy for the electrode 4 to enter the first trench portion 51, the second layer 42 made of carbon paper is preferably arranged in a manner facing the bipolar plate 5. Inevitably, the first layer 41 faces the diaphragm 101 (see Figure 3).

[0043] The unit area weight of the first layer 41 can be 20 g / m² or more and 500 g / m² or less. The unit area weight of the second layer 42 can be 10 g / m² or more and 100 g / m² or less. By ensuring that the unit area weights of the first layer 41 and the second layer 42 meet the above ranges, it is easy to achieve an appropriate electrolyte permeability for the entire electrode 4. As a result, it is easy to obtain a unit cell stack 2 with R² / R¹ satisfying the specific range described below.

[0044] (bipolar plate) The bipolar plate 5 will be described primarily with reference to FIG5. The bipolar plate 5 shown in FIG5 is the portion of the through window 32o exposed in the frame 32 in FIG3. The bipolar plate 5 includes an electrolyte inlet 5A, an electrolyte outlet 5B, and a plurality of first groove portions 51. In FIG5, the portions other than the first groove portions 51 are indicated by crosshairs.

[0045] The inlet portion 5A serves as the entrance for the electrolyte into the bipolar plate 5. The outlet portion 5B serves as the outlet for the electrolyte discharged from the bipolar plate 5. In this example, the overall flow direction of the electrolyte on the bipolar plate 5, as shown by the thick arrow on the right, is from bottom to top. Therefore, the inlet portion 5A of the bipolar plate 5 in Figure 5 is the entire lower edge of the bipolar plate 5. The outlet portion 5B of the bipolar plate 5 in Figure 5 is the entire upper edge of the bipolar plate 5.

[0046] The first trench portion 51 extends from the inlet portion 5A of the bipolar plate 5 towards the outlet portion 5B. In this example, the extension direction of the first trench portion 51 is consistent with the overall flow direction of the electrolyte. The electrolyte in the first trench portion 51 flows towards the outlet portion 5B. All the first trench portions 51 are parallel to each other. In this example, the first trench portion 51 connects both the inlet portion 5A and the outlet portion 5B. The electrolyte in the first trench portion 51 flows in a first direction from the inlet portion 5A to the outlet portion 5B. Unlike this example, in a plurality of first trench portions 51, a portion of the first trench portions 51 may also be inclined relative to the flow direction.

[0047] The width of the first groove portion 51 can be exemplified as being consistent along the extending direction of the first groove portion 51. The width of the first groove portion 51 refers to the length of the opening of the first groove portion 51 that is orthogonal to the extending direction of the first groove portion 51. The width of the first groove portion 51 may also vary at different positions along the extending direction.

[0048] The depth of the first groove portion 51 can be exemplified as being consistent along the extension direction of the first groove portion 51. The depth of the first groove portion 51 refers to the length of the first groove portion 51 from the opening to the bottom. The bottom is the deepest part of the first groove portion 51. The depth of the first groove portion 51 can also vary at different positions along the extension direction.

[0049] The cross-sectional shape of the first groove portion 51 is not particularly limited. The cross-sectional shape of the first groove portion 51 refers to the outline shape of the first groove portion 51 on a cross-section orthogonal to the extension direction of the first groove portion 51. In this example, the cross-sectional shape is rectangular. Other cross-sectional shapes include, for example, V-shaped, semi-circular, trapezoidal, etc.

[0050] (Adjustment of electrolyte flow in the unit cell) If the electrolyte flow on electrode 4 is low, there is a risk of incomplete battery reaction at electrode 4. Furthermore, if the electrolyte flow in the first trench 51 of the bipolar plate 5 is low, heat generated during the battery reaction will easily accumulate in the electrolyte. If the electrolyte flow in the first trench 51 is too high, the amount of electrolyte flowing from the first trench 51 to electrode 4 will decrease. In other words, it is important to set a proper balance between the electrolyte flow on electrode 4 and the electrolyte flow in the first trench 51 of the bipolar plate 5. In this embodiment, R1 and R2 are defined as indicators of the ease of electrolyte flow in the unit stack 2.

[0051] R1 represents the flow resistance, indicating the ease of electrolyte flow on electrode 4. R2 represents the flow resistance, indicating the ease of electrolyte flow in the first groove 51 of bipolar plate 5. Flow resistance is a new specification defined in this manual. Flow resistance will be described below.

[0052] The purpose of this example is to balance the ease of electrolyte flow on electrode 4 with the ease of electrolyte flow in the first trench portion 51 of bipolar plate 5. Therefore, in this invention, R2 / R1 is specified to be within a specific range. Specifically, the range of R2 / R1, expressed as a percentage, is 7×10⁻⁹% or more and 2×10⁻²% or less. If R2 / R1 is within the above range, the electrolyte flowing in the first trench portion 51 of bipolar plate 5 can easily and appropriately flow to electrode 4, and the flowability of electrolyte on electrode 4 is improved. As a result, the battery performance of the redox flow battery is improved. The range of R2 / R1 is more preferably 2×10⁻⁸% or more and 1×10⁻²% or less. The range of R2 / R1 is further more preferably 5×10⁻⁷% or more and 3×10⁻³% or less.

[0053] • Through resistance R1 The permeation resistance R1 of electrode 4 is as follows. R 1=(L 1 / K 1)×μ×(1 / S 1)…Equation 1-1 L1 = Length of electrode 4 (m) K1 = Transmittance of electrode 4 (m2) μ = Viscosity of the electrolyte (Pa∙s) S1 = Cross-sectional area of ​​electrode 4 (m2)

[0054] The following explains the steps to determine R1. First, the relationship between the pressure loss ΔP1 of the electrolyte on electrode 4 and the transmittance K1 satisfies Equation 1-2 based on Darcy's Law. ΔP1=(L1 / K1)×μ×(Q1 / S1)…Equation 1-2 K1: Transmittance (m2) ΔP1: Pressure loss (Pa) Q1: Electrolyte flow rate (m³ / s) μ: Viscosity of the electrolyte (Pa∙s) L1: Length of electrode 4 (m) S1: Cross-sectional area of ​​electrode 4 (m²)

[0055] The transmittance K1 is an inherent value of electrode 4, independent of the fluid type. The transmittance K1 is determined based on measured values ​​from the test unit. Specifically, the test unit is constructed using the same electrode 4 as that of the unit stack 2. The compression state of the electrode 4 in the test unit is the same as that of the electrode 4 in the unit stack 2. A box is connected to the test unit via piping, allowing a fluid of known viscosity, such as water, to flow through the test unit. A flow meter and a differential pressure gauge are pre-connected to the piping. The differential pressure gauge measures the pressure loss ΔP1 based on the electrolyte pressure at the inlet and outlet of the test unit. Since the dimensions of the electrode 4 in the test unit can be measured, values ​​in Equations 1-2 other than the transmittance K1 can be measured. Substituting the measured values ​​into Equations 1-2 yields the transmittance K1.

[0056] Secondly, the relationship between the pressure loss ΔP1 of the electrolyte on electrode 4, the flow rate Q, and the permeation resistance R1 is compared to the equation V=I×R of a circuit, and the following equations 1-3 are defined. ΔP 1=Q 1×R 1…Equation 1-3

[0057] Substituting equation 1-2 into equation 1-3, we obtain equation 1-1. The unit of R1 is Pa∙s / m3.

[0058] • Through resistance R2 The electrolyte permeation resistance R2 in the first trench 51 of the bipolar plate 5 is as follows. R 2=(32×μ×L 2) / (de 2×S 2)…Equation 2-1 L2 = Length of the first groove section 51 (m) de = equivalent diameter of the first groove section 51 (m) S2 = Cross-sectional area of ​​the first trench section 51 (m2)

[0059] The following describes the steps to calculate R². First, consider the pressure loss ΔP² of the electrolyte due to friction between the electrolyte flowing inside the circular tube and the tube wall. The pressure loss ΔP² is calculated using the Darcy-Weisbach formula shown below. ΔP 2=λ×(L 2 / d)×ρ / 2×(Q 2 / S 2) 2…Equation 2-2 λ: Pipe friction coefficient L2: Length of the circular tube (m) d: Inner diameter of the circular tube (m) ρ: Density of the electrolyte (kg / m³) Q2: Electrolyte flow rate (m³ / s) S 2: Cross-sectional area of ​​the circular tube (m 2)

[0060] Secondly, the Reynolds number Re, which represents the flow state of a fluid, is expressed by the following formula. Re = ρ × Q 2 / S 2 × d / μ…Equation 2-3 μ: viscosity of the fluid (Pa∙s)

[0061] When Re is below 2300, the flow inside the circular pipe becomes laminar. In this case, the pipe friction coefficient λ can be calculated from the following formula based on the Hagen-Poiseuille law. λ=64 / Re…Equation 2-4

[0062] If we substitute equation 2-3 into equation 2-4, we can obtain the following equation. λ=(64×S 2×μ) / (ρ×Q 2×d)…Equation 2-5

[0063] If we substitute equation 2-5 into equation 2-2, the pressure loss ΔP2 is expressed by the following equation. ΔP 2=(32×μ×L 2×Q 2) / (d 2×S 2)…Equation 2-6

[0064] The cross-sectional shape of the first groove portion 51 of the bipolar plate 5 is not circular. Therefore, the inner diameter d cannot be obtained by substituting into Equation 2-6. Therefore, the equivalent diameter de, which can be obtained from the dimensions of the first groove portion 51, is calculated. The equivalent diameter de is calculated by Equation 2-7 below. de=4×(S 2 / W)…Equation 2-7 S2: Cross-sectional area of ​​the first groove section 51 (m2) W: Length of the outer perimeter of the first groove portion 51 (m)

[0065] Here, W is the perimeter of the first groove portion 51, including the opening of the first groove portion 51. The method for calculating W is explained based on the cross-sectional views in Figures 6A and 6B. As shown in Figure 6A, when the first groove portion 51 is a rectangular groove, W = A1 + A2 + B1 + B2. A1 and A2 are the lengths of the sidewalls of the first groove portion 51. B1 is the width of the opening of the first groove portion 51. B2 is the width of the bottom of the first groove portion 51. As shown in Figure 6B, when the first groove portion 51 is a V-shaped groove, W = A1 + A2 + B1. A1 and A2 are the lengths of the sidewalls of the first groove portion 51. B1 is the width of the opening of the first groove portion 51. As shown by the cross-hatching in Figures 6A and 6B, the cross-sectional area S2 of the first groove portion 51 is the area of ​​the portion enclosed by the inner circumferential surface and the opening of the first groove portion 51.

[0066] Substituting de from equation 2-7 into d from equation 2-6, and considering the length of the circular tube as the length of the first groove 51, the pressure loss ΔP2 of the electrolyte in the first groove 51 of the bipolar plate 5 is calculated. ΔP 2=(32×μ×L 2×Q 2) / (de 2×S 2)…Equation 2-8 L2: Length of the first groove portion 51

[0067] Secondly, the relationship between the pressure loss ΔP2 of the electrolyte in the first trench portion 51 of the bipolar plate 5, the flow rate Q2, and the permeation resistance R2 is compared to the equation V=I×R of a circuit, and the following equation 2-9 is defined. ΔP² = Q² × R² … Equation 2-9

[0068] Substituting equation 2-9 into equation 2-8 yields equation 2-1. The unit of R2 is Pa∙s / m3.

[0069] • R 2 / R 1 Substituting Equation 1-1 into R1 of R2 / R1, and substituting Equation 2-1 into R2, we obtain Equation 3-1. R 2 / R 1=(32×L 2×S 1×K 1) / (de 2×S 2×L 1)…Equation 3-1

[0070] As shown in Equation 3-1, in order to calculate R2 / R1, it is necessary to obtain L1 and S1 of electrode 4 and L2, S2, and de of the first trench portion 51 of bipolar plate 5 through actual measurement. Here, we will explain which part of electrode 4 and bipolar plate 5 is the variable to be substituted into Equation 3-1.

[0071] One of the objectives of this invention is to allow an appropriate amount of electrolyte to flow from the first trench portion 51 of the bipolar plate 5 to the electrode 4, and to allow the electrolyte to flow rapidly within the electrode 4. Therefore, in this example, the ease with which the electrolyte flows from one first trench portion 51A through the electrode 4 to the other first trench portion 51B in two adjacent first trench portions 51 is used as an evaluation criterion.

[0072] Based on the aforementioned evaluation criteria, the direction of electrolyte flow on electrode 4 is a direction intersecting the extension direction of the first trench portion 51, and representatively, a direction orthogonal to the extension direction of the first trench portion 51. Therefore, the length L1 of electrode 4 is the distance between adjacent first trench portions 51A and 51B. This distance is also the width of the raised portion between two adjacent first trench portions 51A and 51B.

[0073] The direction in which the electrolyte flows from the first trench portion 51 to the electrode 4 is a direction that intersects the extending direction of the first trench portion 51, and representatively, a direction that is orthogonal to the extending direction of the first trench portion 51. Therefore, the length L2 of the first trench portion 51 is the overlap length of two adjacent first trench portions 51 in the extending direction of the first trench portion 51 (see Figure 5). In this example, the first trench portion 51 is connected to the inlet portion 5A and the outlet portion 5B. Therefore, in this example, the length L2 of the first trench portion 51 is equal to the length of the bipolar plate 5 along the overall flow direction of the electrolyte.

[0074] The cross-sectional area S1 of electrode 4 is the product of the thickness d1 (see Figure 4) and the length L2 of electrode 4. The thickness d1 is the thickness of electrode 4 compressed within unit stack 2.

[0075] The methods for determining the transmittance K1, the equivalent diameter de, and the cross-sectional area S2 of the first groove 51 are as described above.

[0076] ∙ Other Here, when electrode 4 has a first layer 41 and a second layer 42, the transmission resistance R1 of electrode 4 is calculated by the following formula. 1 / R1 = (1 / R1 - 1) + (1 / R1 - 2) ... Equation 4-1 R 1-1: Permeation resistance of the electrolyte in the first layer 41 R 1-2: Permeation resistance of the electrolyte in the second layer 42

[0077] "Effect" The unit cell 2 satisfies 7×10⁻⁹%≦R² / R¹≦2×10⁻²%. By ensuring that R² / R¹ meets the above range, the electrolyte flowing in the first trench 51 of the bipolar plate 5 of the unit cell 2 can easily and appropriately flow to the electrode 4, and the fluidity of the electrolyte on the electrode 4 is improved. As a result, the increase in the reaction resistance of the electrode 4 of the unit cell 2 is significantly reduced. Furthermore, the RF battery 1 equipped with this unit cell 2 exhibits excellent battery performance.

[0078] <Implementation Method 2> In Embodiment 2, a bipolar plate 5 having a different configuration from that in Embodiment 1 will be described based on FIG7.

[0079] As shown in Figure 7, the bipolar plate 5 in this example has a first groove portion 51A connected to the inlet portion 5A but not connected to the outlet portion 5B, and a first groove portion 51B not connected to the inlet portion 5A but connected to the outlet portion 5B. The first groove portion 51A and the first groove portion 51B are alternately arranged in a direction orthogonal to the overall flow direction of the electrolyte.

[0080] In this example, the length L2 of the first trench portion 51 used to determine R2 / R1 differs from that in Embodiment 1. The length L2 is the overlap length of adjacent first trench portions 51A and 51B in the extending direction of the first trench portion 51. Therefore, in this example, the length L2 is shorter than the length of the bipolar plate 5 in the overall flow direction of the electrolyte.

[0081] <Implementation Method 3> In Embodiment 3, a bipolar plate 5 having a different configuration from Embodiments 1 and 2 will be described based on FIG8.

[0082] In this example, the extension direction of the first trench portion 51 is inclined relative to the overall flow direction of the electrolyte. In this case, the electrolyte in the first trench portion 51 also flows in the first direction from the inlet portion 5A to the outlet portion 5B.

[0083] In this example, to determine R2 / R1, the direction of the length L2 of the first groove portion 51 of the bipolar plate 5 in Formula 3-1 is substituted, which is inclined relative to the overall flow direction of the electrolyte. The length L2 is the overlap length of two adjacent first groove portions 51 in the direction where they are parallel. The length L1 of the electrode 4 (see Figure 4) is the same as the width of the raised portion in Embodiment 1.

[0084] <Implementation Method 4> In Embodiment 4, a bipolar plate 5 having a first groove portion 51 that is different from that in Embodiments 1 to 3 will be described based on FIG9.

[0085] The bipolar plate 5 in this example has a meandering groove 50. The first end of the meandering groove 50 is connected to the inlet portion 5A, and the second end is connected to the outlet portion 5B. The flow of electrolyte in the meandering groove 50 is indicated by a thick arrow. In this example, the meandering groove 50 is constructed by connecting a first groove portion 51 and a second groove portion 52. The first groove portion 51 is the part of the meandering groove 50 that allows the electrolyte to flow along the overall flow direction of the electrolyte. The second groove portion 52 is the part of the meandering groove 50 that connects adjacent first groove portions 51A and 51B. That is, the second groove portion 52 is an S-shaped portion having a transverse groove on the outlet portion 5B side, a transverse groove on the inlet portion 5A side, and a longitudinal groove connecting the two transverse grooves. The transverse groove on the discharge section 5B side extends in a direction orthogonal to the overall flow direction of the electrolyte and connects to the end of the first groove section 51A on the discharge section 5B side. The transverse groove on the inlet section 5A side extends in a direction orthogonal to the overall flow direction of the electrolyte and connects to the end of the first groove section 51B on the inlet section 5A side. The longitudinal groove extends along the overall flow direction of the electrolyte and connects the two transverse grooves. The electrolyte flow direction in the longitudinal groove is towards the inlet section 5A. Therefore, this longitudinal groove is not for directing the electrolyte flow to the first groove section 51A on the discharge section 5B side.

[0086] In this example, the existence of the second trench portion 52 is ignored when calculating R2 / R1. Therefore, the length L1 of the electrode 4 in Equation 3-1 is the distance between the first trench portion 51A and the first trench portion 51B located at the closest positions. Also, the length L2 of the bipolar plate 5 in Equation 3-1 is the length of the overlap between the first trench portion 51A and the first trench portion 51B in the extending direction of the first trench portion 51.

[0087] <Experimental Example> In the experimental case, the effect of the ratio of R2 / R1 of the unit cell on the increase in the reactive resistance of the unit cell electrodes was investigated. The quality of the reactive resistance of the electrodes was evaluated by the reactive resistivity (Ω∙cm2).

[0088] Unit stacks of samples No. 1 to No. 12 were prepared. The difference between these unit stacks lies in the composition of the electrodes. The electrode for sample No. 1 is a single-layer electrode. The electrodes for samples No. 2 to No. 12 are double-layer electrodes with a first layer and a second layer. The electrode structures are shown in Table 1. Furthermore, the unit area weight (g / m²) of each electrode layer is also shown in Table 1.

[0089] Furthermore, the R2 / R1 ratio of each sample unit stack was calculated. The method for calculating R2 / R1 has already been explained. The values ​​of R2 / R1 are shown in Table 1. R2 / R1 in the table are expressed as percentages.

[0090] The reactive resistivity (Ω∙cm²) of the unit stacks of samples No.1 to No.12 was determined. The measurement method is as follows: First, the unit stacks of each sample were charged and discharged at a constant current density of 90 mA / cm². Multiple charge-discharge cycles were performed in this experiment. An upper and lower limit for the switching voltage was set during the experiment; when the voltage reached the upper limit during charging, the system switched to discharging, and when the voltage reached the lower limit during discharging, the system switched to charging. After each charge-discharge cycle, the unit resistivity (Ω∙cm²) of each sample was determined. The unit resistivity is calculated as: the average voltage during charging and the average voltage during discharging in any one cycle of the multiple cycles, calculated as {(difference between average voltage during charging and average voltage during discharging) / (average current / 2)} × effective area of ​​the unit. The unit resistivity is determined by the sum of the conductive resistivity and the reactive resistivity. The conductive resistivity can be determined by measuring the resistance of the unit stack using a battery tester. Therefore, the reactive resistivity is obtained by subtracting the conductive resistivity from the unit resistivity.

[0091] [Table 1] Sample No. electrode R 2 / R 1 [%] Weight per unit area [g / m²] Reactive resistivity [Ω∙cm 2] First layer Second floor First layer Second floor 1 carbon felt - 9×10⁻⁹ 428 - 0.22 2 carbon felt Carbon paper 3×10⁻⁸ 377 25 0.21 3 carbon felt Carbon paper 7×10 - 7 301 48 0.19 4 carbon felt Carbon paper 1×10⁻⁶ 322 83 0.18 5 carbon felt Carbon paper 3×10 - 3 130 30 0.19 6 carbon cloth Carbon paper 1×10⁻² 85 35 0.21 7 carbon felt Carbon paper 3×10⁻¹² 453 62 0.42 8 carbon felt Carbon paper 5×10⁻² 72 46 0.39 9 carbon felt Carbon paper 1×10⁻⁸ 555 28 0.26 10 carbon felt Carbon paper 1×10⁻² 15 37 0.31 11 carbon felt Carbon paper 1×10⁻⁷ 289 8 0.33 12 carbon felt Carbon paper 9×10⁻² 85 117 0.34

[0092] As shown in Table 1, the reactive resistivity of samples No. 1, 2, 3, 4, 5, 6, 9, 10, and 11, whose R2 / R1 ratio is above 7×10⁻⁹% and below 2×10⁻², is lower than that of other samples. This indicates that satisfying R2 / R1 of 7×10⁻⁹% to 2×10⁻² has a significant impact on reducing the increase in reactive resistivity of the unit stack electrodes.

[0093] The reaction resistivity of samples No. 2 to No. 6, with an R2 / R1 ratio of 3×10⁻⁸% or higher and 1×10⁻²% or lower, is lower than that of the other samples. Furthermore, the reaction resistivity of samples No. 3 to No. 5, with an R2 / R1 ratio of 7×10⁻⁷% or higher and 3×10⁻³% or lower, is lower than that of samples No. 2 and No. 6. Here, even when R2 / R1 is 3×10⁻⁸% or higher and 1×10⁻²% or lower, there is a tendency for samples No. 10 and No. 11, which have a layer with extremely low weight per unit area, to have higher reaction resistivity.

[0094] 1: Redox flow battery (RF battery) 2: Unit heap 3: Unit Frame 4: Electrode 5: Bipolar plate 5A: Import Section 5B: Discharge section 10: Battery Unit 12: Positive electrode box 13: Negative electrode box 20: Sub-heap 21: Arrange the boards 22: End plate 23: Fastening mechanism 31: Bipolar plate 32: Frame 32°: Through window 33: Liquid supply manifold 33s: Liquid supply slit 34: Liquid supply manifold 34s: Liquid supply slit 35:Drain manifold 35s: Drainage slit 36:Drain manifold 36s: Drainage slit 41: First Floor 42: Second layer 50: Winding ditch 51: First groove section 51A: First groove section 51B: First groove section 52: Second groove section 80: AC / DC converter 81: Transformer Equipment 90: Power System 91: Power Generation Department 92: Load 101: Diaphragm 102: Positive electrode unit 103: Negative electrode unit 104: Positive electrode 105: Negative electrode 108: Outbound piping 109: Outbound piping 110:Return piping 111:Return piping 112: Pump 113: Pump A1: Length A2: Length B1: Width B2: Width d1: Thickness L1: Length L2: Length S 1: Cross-sectional area S 2: Cross-sectional area

Claims

1. A cell stack, formed by stacking a plurality of cell units, comprising: a porous electrode and a bipolar plate facing the electrode, the bipolar plate comprising: an electrolyte inlet portion; an electrolyte outlet portion; and a plurality of first trench portions extending from the side where the inlet portion is disposed to the side where the outlet portion is disposed; and each of the plurality of first trench portions is configured such that the electrolyte in the first trench portion flows to the outlet portion, R2 / R1 is 7×10⁻¹¹ or more and 2×10⁻⁴ or less, R1 is a permeation resistance representing the ease of flow of the electrolyte on the electrode, R1=(L1 / K1)×μ×(1 / S1), L1 is the length of the electrode, K1 is the permeability of the electrode, μ is the viscosity of the electrolyte, S1 is the cross-sectional area of ​​the electrode, R 2 represents the permeation resistance, which indicates the ease of flow of the electrolyte in the first trench. R2 = (32 × μ × L2) / (de 2 × S2), where L2 is the length of the first trench, de is the equivalent diameter of the first trench, and S2 is the cross-sectional area of ​​the first trench.

2. The cell heap as requested in item 1, where R2 / R1 is greater than 2×10⁻¹⁰ and less than 1×10⁻⁴.

3. The cell heap as requested in item 1, where R2 / R1 is greater than 5×10⁻⁹ and less than 3×10⁻⁵.

4. A unit stack as claimed in any of claims 1 to 3, wherein the electrodes are constructed of a plurality of materials having different permeability to the electrolyte.

5. A cell stack as claimed in any one of claims 1 to 3, wherein the electrodes comprise: a first layer, which is formed by carbon felt or carbon cloth; and a second layer, which is formed by carbon paper.

6. The unit stack as claimed in claim 5, wherein the unit area weight of the first layer is 20 g / m² or more and 500 g / m² or less, and the unit area weight of the second layer is 10 g / m² or more and 100 g / m² or less.

7. A cell stack as claimed in any of claims 1 to 3, wherein the bipolar plate has a second trench portion, the second trench portion connecting two adjacent first trench portions among the plurality of first trench portions.

8. A redox flow battery comprising a cell stack as claimed in any one of claims 1 to 7.

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