Composite electrode structure and zinc-bromine flow battery

Abstract

The invention belongs to the technical field of flow battery energy storage, and particularly relates to a composite electrode structure and a zinc-bromine flow battery. The electrode comprises two carbon felts which are arranged in a laminated mode and a zinc sheet located between the two carbon felts, or more than two zinc sheets which are arranged between the two carbon felts in a laminated mode and are separated by the carbon felts. And a through hole is formed in the zinc sheet. The zinc sheet is placed in the middle of the negative electrode carbon felt, zinc can be induced to preferentially deposit on the zinc sheet in the charging process of the zinc-bromine flow battery, a deposition interface of the zinc is regulated to the interior of the carbon felt and is far away from a diaphragm, and the influence of zinc dendrites can be effectively inhibited.

Description

Technical Field

[0001] This invention belongs to the field of flow battery energy storage technology, and specifically relates to a zinc-bromine flow battery structure. Background Technology

[0002] Zinc-bromine flow batteries are a low-cost, high-safety flow battery energy storage technology with high energy density. The negative electrode involves the deposition and dissolution reaction of zinc. During charging, zinc ions are converted into elemental zinc and deposited on the electrode. During discharging, elemental zinc is converted back into zinc ions and released into the electrolyte. Because zinc-bromine flow batteries are deposition-based batteries, the deposition capacity of zinc at the negative electrode limits the battery's performance and cycle stability. Excessive zinc deposition can lead to zinc dendrites, which can puncture the separator, causing a short circuit and battery failure. Therefore, suppressing zinc dendrite formation is crucial for the development of zinc-bromine flow battery technology. Summary of the Invention

[0003] To address the aforementioned technical problems, the present invention aims to provide an electrode structure for a zinc-bromine flow battery.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] The electrode comprises two stacked carbon felts and a zinc sheet located between the two carbon felts, or two or more zinc sheets stacked between two carbon felts and spaced apart by carbon felts; through holes are provided in the zinc sheets. The zinc sheets and carbon felts are stacked alternately, with the zinc sheets placed between two carbon felts, or with two or more zinc sheets stacked between two carbon felts and spaced apart by a single carbon felt. The composite electrode has 2-4 carbon felts and 1-3 zinc sheets, with the number of carbon felts minus the number of zinc sheets equal to 1. Through holes are provided in the zinc sheets, with a diameter between 5mm and 20mm, and an open area ratio between 20% and 50% on the zinc sheet surface. The carbon felt thickness is between 2mm and 5mm, and the zinc sheet thickness is between 0.5mm and 2mm. The negative electrode adopts any of the composite electrode structures described above. The composite electrode is placed in the central cavity of the annular electrode frame of the battery. Inlet and outlet channels are provided on the electrode frame. The electrolyte flows into the composite electrode in the central cavity from the inlet channel and then flows out of the electrode frame through the outlet channel. When assembling the battery, the thickness of the composite electrode is 110%-125% of the electrode frame thickness (in the embodiments and comparative examples of this invention, the thickness of the composite electrode is 115% of the electrode frame thickness). After battery assembly, the composite electrode is compressed to match the thickness of the electrode frame. Before compression, the carbon felt porosity of the composite electrode is between 0.85 and 0.9, and after compression, the carbon felt porosity of the composite electrode is between 0.4 and 0.5.

[0006] This composite electrode is used as the negative electrode in a zinc-bromine flow battery and cannot be used as the positive electrode. The positive electrode still uses the traditional flat carbon felt electrode structure. The carbon felt for the positive electrode is 3-6 mm thick.

[0007] The beneficial effects of this invention are as follows:

[0008] Placing a zinc sheet within the carbon felt of the negative electrode can induce preferential zinc deposition on the zinc sheet during charging in a zinc-bromine flow battery. By controlling the zinc deposition interface to be inside the carbon felt, away from the separator, the influence of zinc dendrites can be effectively suppressed. Furthermore, inducing zinc deposition within the carbon felt allows the elemental zinc to be protected by the compressive stress of the carbon felt during the reaction, effectively preventing zinc detachment and enhancing the reliability of the stack, thereby improving its performance. In addition, the zinc sheet must be designed with a porous structure to avoid affecting mass transfer in the battery; however, the porosity should not be too high, as this would reduce the specific surface area of ​​the zinc sheet and decrease the number of zinc deposition sites. Attached Figure Description

[0009] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0010] Figure 1 This is a schematic diagram of the zinc-bromine flow battery electrode of the present invention. Detailed Implementation

[0011] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0012] This invention provides a composite electrode structure for a zinc-bromine flow battery. The electrode comprises two stacked carbon felts and a zinc sheet located between the two carbon felts, or two or more zinc sheets stacked between two carbon felts and spaced apart by carbon felts. Through-holes perpendicular to the surface of the zinc sheet are provided. The zinc sheet and carbon felts are alternately stacked, with the zinc sheet placed between two carbon felts, or with two or more zinc sheets stacked between two carbon felts and spaced apart by one carbon felt. The composite electrode has 2-4 carbon felts and 1-3 zinc sheets, with the number of carbon felts minus the number of zinc sheets equal to 1. The zinc sheet has through-holes perpendicular to its surface, with a diameter between 5mm and 20mm, and an open area ratio between 20% and 50%. The carbon felt thickness is between 2mm and 5mm, and the zinc sheet thickness is between 0.5mm and 2mm. The negative electrode adopts any of the composite electrode structures described above. The composite electrode is placed in the central cavity of the annular electrode frame of the battery. The length and width of the electrode match the central cavity region inside the annular electrode frame. Inlet and outlet channels are provided on the electrode frame. The electrolyte flows into the composite electrode in the central cavity from the inlet channel and then flows out of the electrode frame through the outlet channel. When assembling the battery, the thickness of the composite electrode is 110%-125% of the thickness of the electrode frame. After assembling the battery, the composite electrode is compressed to match the thickness of the electrode frame. Before compression, the carbon felt porosity of the composite electrode is between 0.85 and 0.9, and after compression, the carbon felt porosity of the composite electrode is between 0.4 and 0.5.

[0013] The electrode structures described above are only used for the negative electrode of the zinc-bromine flow battery; the positive electrode still uses a traditional single-plate carbon felt electrode. The positive electrode is placed in the central cavity of the battery's annular electrode frame. The length and width of the carbon felt electrode match the central cavity region of the annular electrode frame. Inlet and outlet channels are provided on the electrode frame; the electrolyte flows into the electrode in the central cavity from the inlet channel and then flows out of the electrode frame through the outlet channel. The porosity of the carbon felt electrode is 0.9, and after compression, the porosity is 0.5. The carbon felt thickness is 5 mm.

[0014] The embodiments and comparative examples in this invention were tested under the following conditions:

[0015] The electrolyte consisted of an aqueous solution of 2 mol / L zinc bromide, 3 mol / L potassium chloride, and 0.4 mol / L MEP complexing agent. The membrane was a commercially available Daramic porous membrane; the negative and positive carbon felt electrode areas were 800 cm². 2 The zinc sheet and carbon felt electrode used in this invention are rectangular. The length and width of the zinc sheet and carbon felt electrode are the same and match the hollow area inside the annular electrode frame. That is, the surface shape and size of the zinc sheet and carbon felt electrode are the same.

[0016] Comparative Example 1

[0017] Comparative Example 1 uses a conventional electrode structure as the negative electrode to assemble a zinc-bromine flow battery. The electrode consists of a single carbon felt electrode, placed in the central cavity of the annular electrode frame. The length and width of the carbon felt electrode match the central cavity region of the annular electrode frame. Inlet and outlet channels are provided on the electrode frame. The electrolyte flows into the electrode in the central cavity from the inlet channel and then flows out of the electrode frame through the outlet channel. The porosity of the carbon felt electrode is 0.9 (before compression, the same below), and the porosity of the carbon felt electrode after compression is 0.5. The specific parameters of the stack are as follows:

[0018] Electrode area: 800 cm² 2 ;

[0019] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0020] Number of fuel cell stack sections: 20;

[0021] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0022] Battery cycle performance: Coulombic efficiency 86.2%, voltage efficiency 81.9%, energy efficiency 70.6%; Comparative Example 1 has lower performance, which is due to zinc deposited on the surface of the carbon felt, zinc dendrites gradually accumulate, and because zinc is formed on the surface, it is easier to fall off after being washed by the electrolyte, resulting in faster battery performance degradation.

[0023] Comparative Example 2

[0024] Comparative Example 2 uses a composite electrode structure as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. One carbon felt sheet is used, and one zinc sheet is placed on the bottom side of the carbon felt. The zinc sheet has through-holes with a diameter of 10 mm and a porosity (the percentage of the through-hole opening area to the zinc sheet surface area) of 30%. The zinc sheet thickness is 1 mm. The porosity of the carbon felt electrode is 0.9, and after compression, the porosity is 0.5. The specific parameters of the battery stack are as follows:

[0025] Electrode area: 800 cm² 2 ;

[0026] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0027] Number of fuel cell stack sections: 20;

[0028] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0029] Battery cycle performance: Coulombic efficiency 85.1%, voltage efficiency 77.5%, energy efficiency 65.9%;

[0030] Comparative Example 2 had even lower performance because the zinc sheet was placed on the bottom side of the carbon felt and was in direct contact with the current collector. Since the zinc sheet could not be compressed, the contact resistance between it and the current collector was extremely high, which increased the internal resistance of the battery and resulted in poor battery efficiency.

[0031] Comparative Example 3

[0032] Comparative Example 3 uses a composite electrode structure as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Two carbon felt sheets are used, with one zinc sheet placed between the two carbon felt sheets. The zinc sheet has through-holes with a diameter of 10 mm and a porosity of 80%. The zinc sheet thickness is 1 mm. The porosity of the carbon felt electrode is 0.9, and after compression, the porosity is 0.5. The specific parameters of the battery stack are as follows:

[0033] Electrode area: 800 cm² 2 ;

[0034] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0035] Number of fuel cell stack sections: 20;

[0036] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0037] Battery cycle performance: Coulombic efficiency 88.1%, voltage efficiency 79.3%, energy efficiency 69.8%;

[0038] The battery performance of Comparative Example 3 is improved compared to Comparative Examples 1 and 2, but it is still not ideal. This is because the porosity of the zinc sheet is too large, which reduces the number of zinc deposition sites on the zinc sheet. When there is not enough space for zinc deposition on the zinc sheet, it still deposits on the carbon felt surface, resulting in zinc dendrites and zinc shedding.

[0039] Comparative Example 4

[0040] Comparative Example 4 uses a composite electrode structure as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Two carbon felt sheets are used, with one zinc sheet placed between the two carbon felt sheets. The zinc sheet has through-holes with a diameter of 10 mm and a porosity of 5%. The zinc sheet thickness is 1 mm. The porosity of the carbon felt electrode is 0.9, and after compression, the porosity is 0.5. The specific parameters of the battery stack are as follows:

[0041] Electrode area: 800 cm² 2 ;

[0042] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0043] Number of fuel cell stack sections: 20;

[0044] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0045] Battery cycle performance: Coulombic efficiency 76.1%, voltage efficiency 72.3%, energy efficiency 55.0%;

[0046] Comparative Example 4 showed the worst battery performance because the porosity of the zinc sheet was too low, which affected the normal mass transfer of the battery and resulted in extremely high internal resistance, thus leading to low efficiency.

[0047] Comparative Example 5

[0048] Comparative Example 5 uses a composite electrode structure as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Five carbon felt sheets are used, and four zinc sheets are placed between two carbon felt sheets. Through-holes with a diameter of 10 mm and a porosity of 30% are formed on the zinc sheets. The zinc sheet thickness is 1 mm. The porosity of the carbon felt electrode is 0.9, and after compression, the porosity is 0.5. The specific parameters of the battery stack are as follows:

[0049] Electrode area: 800 cm² 2 ;

[0050] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0051] Number of fuel cell stack sections: 20;

[0052] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0053] Battery cycle performance: coulombic efficiency 80.5%, voltage efficiency 77.2%, energy efficiency 62.1%;

[0054] Comparative Example 5 increased the amount of carbon felt and zinc sheet in the composite electrode, resulting in increased composite electrode thickness, increased battery internal resistance, and lower battery performance.

[0055] Example 1

[0056] Example 1 uses the composite electrode structure provided by this invention as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Two carbon felt sheets are used, with a zinc sheet placed between them. The zinc sheet has through-holes with a diameter of 10 mm and a porosity of 30%. The zinc sheet thickness is 1 mm, and the porosity of the carbon felt electrode is 0.9. After compression, the porosity of the carbon felt electrode is 0.5. Specific parameters of the battery stack are as follows:

[0057] Electrode area: 800 cm² 2 ;

[0058] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0059] Number of fuel cell stack sections: 20;

[0060] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 16V.

[0061] Battery cycle performance: Coulombic efficiency 93.2%, voltage efficiency 85.3%, energy efficiency 79.5%;

[0062] Number of cycles when stack energy efficiency decreases by 5%: 402

[0063] Example 1 uses the composite electrode structure provided by the present invention. Since the zinc sheet regulates the zinc deposition site to the inside of the carbon felt, the influence of zinc dendrite formation is eliminated. Furthermore, since the zinc produced is inside the carbon felt, the problem of zinc shedding is also improved. Therefore, the battery performance is greatly improved and the cycle performance is better.

[0064] Example 2

[0065] Example 2 uses the composite electrode structure provided by this invention as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Two carbon felt sheets are used, with a zinc sheet placed between them. The zinc sheet has through-holes with a diameter of 10 mm and a porosity of 50%. The zinc sheet thickness is 1 mm, and the porosity of the carbon felt electrode is 0.9. After compression, the porosity of the carbon felt electrode is 0.5. The specific parameters of the battery stack are as follows:

[0066] Electrode area: 800 cm² 2 ;

[0067] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0068] Number of fuel cell stack sections: 20;

[0069] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 32V.

[0070] Battery cycle performance: Coulombic efficiency 94.4%, voltage efficiency 86.3%, energy efficiency 81.5%;

[0071] Number of cycles when stack energy efficiency decreases by 5%: 502

[0072] Example 2 uses the composite electrode structure provided by the present invention. Compared with Example 1, this example further improves the porosity of the zinc sheet, enhances mass transfer, and provides sufficient zinc deposition sites, thus further improving battery performance.

[0073] Example 3

[0074] Example 3 uses the composite electrode structure provided by this invention as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Four carbon felt sheets are used, with a zinc sheet placed between every two carbon felt sheets, for a total of three sheets. Through-holes with a diameter of 10 mm and a porosity of 30% are provided on the zinc sheets. The zinc sheet thickness is 1 mm, and the porosity of the carbon felt electrode is 0.9. After compression, the porosity of the carbon felt electrode is 0.5. The specific parameters of the battery stack are as follows:

[0075] Electrode area: 800 cm² 2 ;

[0076] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0077] Number of fuel cell stack sections: 20;

[0078] Current density: 40 mA / cm 2 Charging time: 3 hours; Discharge cut-off voltage: 32V.

[0079] Battery cycle performance: Coulombic efficiency 95.8%, voltage efficiency 86.6%, energy efficiency 83.0%;

[0080] Number of cycles when stack energy efficiency decreases by 5%: 651

[0081] Example 3 increased the number of carbon felt and zinc sheets. It can be seen that due to the increase in the number of zinc sheets, there are more zinc deposition sites, the deposition uniformity is improved, and the battery performance is improved. Furthermore, since zinc can be generated on multiple zinc sheets at the same time, the amount of zinc material generated at the same location is reduced, which further suppresses the influence of zinc dendrites. Moreover, due to the reduction in the amount of zinc material at the same location, the compressive stress between zinc and carbon felt electrodes is stronger, which better suppresses the problem of zinc shedding.

[0082] Example 4

[0083] Example 4 uses the composite electrode structure provided by this invention as the negative electrode to assemble a zinc-bromine flow battery. During battery assembly, the thickness of the composite electrode is 115% of the electrode frame thickness. Two carbon felt sheets are used, with a zinc sheet placed between them. The zinc sheet has through-holes with a diameter of 10 mm and a porosity of 30%. The zinc sheet thickness is 2 mm, and the porosity of the carbon felt electrode is 0.9. After compression, the porosity of the carbon felt electrode is 0.5. The specific parameters of the battery stack are as follows:

[0084] Electrode area: 800 cm² 2 ;

[0085] Before compression, the thickness of the positive and negative electrode carbon felt is 5 mm.

[0086] Number of fuel cell stack sections: 20;

[0087] Current density: 40 mA / cm 2 Charging time: 4 hours; Discharge cut-off voltage: 32V.

[0088] Battery cycle performance: Coulombic efficiency 93.9%, voltage efficiency 84.6%, energy efficiency 79.4%;

[0089] Number of cycles when stack energy efficiency decreases by 5%: 421

[0090] In Example 4, the thickness of the zinc sheet was increased to 2 mm. Due to the increased thickness of the zinc sheet, the ohmic resistance inside the battery increased, and the battery performance decreased slightly. Therefore, the thickness of the zinc sheet should not be too thick.

[0091] The comparative examples and embodiments lead to the conclusion that using the composite electrode proposed in this invention as the negative electrode of a zinc-bromine flow battery can effectively regulate the zinc deposition sites to the interior of the carbon felt, effectively suppress zinc shedding, reduce the impact of zinc dendrite formation, prevent the separator from being penetrated and causing short-circuit failure of the battery, and improve battery performance.

Claims

1. A composite electrode structure, characterized in that: The electrode comprises two stacked carbon felts and a zinc sheet located between the two carbon felts, or two or more zinc sheets separated by carbon felts stacked between the two carbon felts. Through holes are made on the zinc sheet.

2. The composite electrode structure according to claim 1, characterized in that: in, Zinc sheets and carbon felt are stacked alternately, with zinc sheets placed between two carbon felts, or two or more zinc sheets separated by one carbon felt are stacked between two carbon felts.

3. The electrode structure according to claim 1, characterized in that: The composite electrode has 2-4 carbon felts and 1-3 zinc sheets. The number of carbon felts minus the number of zinc sheets equals 1. The zinc sheets and carbon felts are stacked alternately.

4. The composite electrode structure according to any one of claims 1-3, characterized in that: Through holes perpendicular to the surface of the zinc sheet are provided on the zinc sheet. The diameter of the through holes is between 5mm and 20mm, and the opening rate of the zinc sheet surface is between 20% and 50%.

5. The composite electrode structure according to any one of claims 1-3, characterized in that: The carbon felt is 2mm-5mm thick, and the zinc sheet is 0.5mm-2mm thick. The zinc sheet and carbon felt electrodes have the same surface shape and size.

6. A zinc-bromine flow battery, characterized in that: The negative electrode adopts the composite electrode structure described in any one of claims 1-5. The composite electrode is placed in the central cavity of the battery annular electrode frame. Inlet and outlet channels are provided on the electrode frame. The electrolyte flows into the composite electrode in the central cavity from the inlet channel and then flows out of the electrode frame through the outlet channel.

7. The zinc-bromine flow battery according to claim 6, characterized in that: When assembling the battery, the thickness of the composite electrode is 110%-125% of the thickness of the electrode frame. After assembling the battery, the composite electrode is compressed to match the thickness of the electrode frame. Before compression, the carbon felt porosity of the composite electrode is between 0.85 and 0.9, and after compression, the carbon felt porosity of the composite electrode is between 0.4 and 0.

5. This composite electrode is used as the negative electrode in a zinc-bromine flow battery and cannot be used as the positive electrode. The positive electrode still adopts the traditional plate-shaped carbon felt electrode structure.

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