Flow field structure of porous metal bipolar plate fuel cell

Abstract

The invention belongs to an internal humidifying fuel cell, and particularly relates to a flow field structure of a porous metal bipolar plate fuel cell, which comprises a principle, a material, a flow field form and a whole stack structure. The auxiliary system is used for controlling the pressure and the flow speed between reaction gas and circulating water on the two sides of the porous metal bipolar plate, so that the circulating water forms bidirectional dynamic permeation in micropores of the porous metal bipolar plate, dynamic balance of gas humidification and discharge of redundant generated water is realized, the dual effects of humidification and water discharge in the battery are realized, and the service life of the battery is prolonged. The three flow field forms are suitable for different scenes, the service life of the cell is effectively prolonged, the system energy consumption is reduced, in addition, the physical property of the porous metal plate is superior to that of a porous graphite plate, and when hydrogen and oxygen are used as reaction gas, the fuel cell can realize totally-closed operation, so that the service life of the fuel cell is prolonged. The device can be applied to deep-sea diving, high-altitude low temperature, space navigation, severe impact, high-frequency vibration and other extreme environments to provide power.

Description

Technical Field

[0001] The present invention belongs to an internally humidified fuel cell, and particularly relates to a flow field structure of a porous metal bipolar plate fuel cell, including its principle, materials, flow field form, and stack structure. Background Art

[0002] Currently, the known structure and working process of a proton exchange membrane fuel cell: A fuel cell is formed by stacking a number of fuel cell units. Each fuel cell unit is composed of a membrane electrode and a bipolar plate. The membrane electrode is jointly constituted by a proton exchange membrane, a catalytic layer, and a porous gas diffusion layer. On both sides of the membrane electrode, bipolar plates processed with gas channels from graphite plates or metal plates are pressed and stacked together. As an important component of the fuel cell, the bipolar plate has main functions such as gas distribution, heat dissipation, electrical conduction, water and heat management, supporting the MEA, and structural sealing, enabling the reaction gases - hydrogen and air (or oxygen) to be respectively transported and evenly distributed on both sides of the membrane electrode. During operation, with hydrogen as the fuel gas and air (or oxygen) as the oxidant, a chemical reaction occurs under the action of the catalyst of the membrane electrode, generating water, heat, and electrical energy. During the reaction process of the fuel cell, the proton exchange membrane only has the ability to conduct hydrogen protons when it is wet, forming an electric current, and generating water on the cathode side. Under the water concentration gradient difference on both sides of the membrane electrode, water will migrate from the cathode side to the anode side. If the membrane electrode is overly dry, its proton conduction efficiency will decrease sharply, and the battery output power will decrease. At the same time, in addition to self-humidification, if there is too much water content on the surface of the membrane electrode, in the diffusion layer, and in the bipolar plate, it will block the channels of the reaction gas. If the excess water cannot be discharged in time, the battery cannot work. Therefore, to maintain the stable operation of the fuel cell, it is necessary to ensure the balance of the water volume inside the battery, neither too little nor too much. To ensure that the proton exchange membrane contains an appropriate amount of moisture, a humidification link needs to be set in the fuel cell system. There are three forms of humidification methods: self-humidification, external humidification, and internal humidification. Self-humidification refers to making the membrane electrode itself have the ability of humidification and moisture retention by reducing the thickness of the proton membrane, adjusting the hydrophilicity and hydrophobicity of the membrane electrode, or changing the gas flow direction, using the water generated by the battery. External humidification is to heat up and humidify the reaction gas before it enters the fuel cell, such as bubbling humidification, enthalpy wheel humidification, water spraying humidification, etc., which not only increases power consumption and cost, but also increases volume and weight. Internal humidification usually means that the gas is directly humidified inside the fuel cell. One common method is to add a "false cell" without a catalytic layer inside the battery as a humidification section, and directly transfer the moisture in the outlet hot and humid tail gas through the proton membrane to the dry gas at the inlet for humidification. Another method is to add a membrane humidification cavity inside the battery, that is, in the cavity, a water-permeable membrane is used to separate the reaction gas from the battery cooling water, and water evaporates into the reaction gas through the membrane under the action of the concentration difference and pressure difference. However, these two methods increase the complexity of the battery structure.

[0003] In addition to the above humidification methods, there is also a method of humidification by using a porous graphite material to make bipolar plates. For example, the method mentioned by UTC in patents US5503944 and US5700595. Utilizing the characteristics of the porosity formed during the preparation of the graphite plate, the bipolar plates of the battery made of porous graphite plates are designed with a circulating water flow channel and a gas flow channel on both sides respectively. By adjusting the pressure between the circulating water and the cathode and anode reaction gases, liquid water permeates bidirectionally from both sides of the porous graphite bipolar plate, humidifying the dry gas and discharging the excess generated water, achieving the purpose of bidirectional balance adjustment. However, this method has the following deficiencies:

[0004] 1. The density of the porous graphite bipolar plate is small, and its mechanical properties are poor. The battery volume is relatively large, restricting the power / volume density of the battery. Its fuel cell engine system cannot be widely used due to the installation space size limitation;

[0005] 2. The pores and pore diameters of the porous graphite plate are randomly formed during the preparation of the blank. Its effective porosity, pore diameter and distribution cannot be accurately controlled. Therefore, the uniformity of its humidification is inconsistent, increasing the complexity of pressure control and indirectly affecting the uniformity and stability of the battery;

[0006] 3. The material properties of the porous graphite plate are worse than those of metal materials and are not suitable for use in high-frequency vibration, severe shock, large pressure difference, extreme low temperature or vacuum environment;

[0007] 4. The flow field of the porous graphite as the bipolar plate of the battery is processed by numerical control engraving, with high cost and not suitable for mass production on a large scale.

[0008] In view of the above problems, the present invention proposes a porous metal plate as the bipolar plate of a fuel cell, including the principle, material and flow field structure to solve the above problems. Summary of the Invention

[0009] The present invention proposes to use a porous metal plate to replace the porous graphite plate to prepare the bipolar plate of the fuel cell. Among various porous metal materials, the titanium foam metal plate has the following physical characteristics:

[0010] 1. The material of the bipolar plate of the fuel cell needs to have the physical characteristics of electrical conductivity, thermal conductivity and corrosion resistance.

[0011] Foamed metal is a metal material containing foam pores. It not only retains the characteristics of metal materials such as weldability, electrical conductivity, and ductility, but also has the functional properties of porous materials such as energy absorption and vibration reduction, air and water permeability, and filtration. Foamed metals mainly include foamed aluminum, foamed magnesium, foamed copper, foamed nickel, foamed steel, foamed titanium, etc. Among them, foamed titanium is a porous material made from industrial high-purity titanium powder through sieving, static pressure forming, and then high-temperature vacuum sintering. It is suitable for the filtration of various corrosive media. It not only has excellent mechanical properties and can be machined mechanically such as cutting and welding, but also has electrical and thermal conductivity characteristics.

[0012] 2. To achieve the effects of humidification and drainage in the bipolar plate of an internal humidification fuel cell, it is necessary to have the characteristics of water passage and gas blockage:

[0013] Taking the foamed titanium metal plate as an example, its porous characteristics are formed during the production process. Using its plate to prepare the bipolar plate of a fuel cell can make the gas and water on both sides of the bipolar plate form the effects of gas blockage and water passage through flow rate, pressure gradient, and capillary action, that is: the liquid water in the circulating water chamber penetrates through the micropores of the porous titanium plate to the gas flow field on the other side, humidifying the dry gas. At the same time, the liquid water generated during the operation of the battery penetrates to the circulating water flow field on the other side of the bipolar plate through the velocity, pressure gradient, and capillary action of the fluid, achieving the drainage effect and realizing the two-way dynamic regulation of liquid water.

[0014] According to the requirements of the gas flow field and the circulating water flow field of the fuel cell, the present invention is a porous material made from metal powder through sieving, static pressure forming, and high-temperature vacuum sintering. Its parameters such as porosity, pore diameter, shape, and distribution are determined by parameters such as the diameter of raw material particles, static pressure forming process, and sintering temperature. The parameters such as the groove shape, size, and spacing of the plate are determined by the mold used in the material forming process. Through mold forming or roll forming, the grooves required for the bipolar plate flow field are obtained, and then the porous metal plate with grooves is machined mechanically to make the bipolar plate. The porous metal plate has electrical conductivity, thermal conductivity, corrosion resistance characteristics, and metal processing characteristics. Then it is stacked with the membrane electrode to form a stack, and the pressure and flow rate of the reaction gas and the circulating water on both sides of the porous metal bipolar plate are controlled respectively through auxiliary systems such as gas supply and circulating water, so that the circulating water forms two-way penetration in the micropores of the porous titanium bipolar plate to achieve the dynamic balance of humidifying the gas and discharging the excess generated water.

[0015] The present invention has the following essential differences from the methods mentioned in the patents US5503944 and US5700595 of the US UTC company:

[0016] 1. Different materials: The porous metal plate described in the present invention - the foamed titanium plate belongs to a metal material, while the graphite material used by the US UTC company in the patent is a non-metal material.

[0017] 2. The bipolar plate made of the porous metal plate - titanium foam plate of the present invention can be mass-produced through a mold, while the bipolar plate made of porous graphite can only be produced through numerical control machining, with high cost and low efficiency.

[0018] 3. Different material properties: The porous metal plate - titanium foam plate used in the present invention has excellent metal processing properties, while the porous graphite plate does not have metal processing properties.

[0019] 4. The porous metal plate - titanium foam plate of the present invention has hydrophilicity itself, while the porous graphite plate does not have hydrophilicity.

[0020] 5. The physical properties of the porous metal plate - titanium foam plate of the present invention are superior to those of the porous graphite plate, and it can be used under extreme low temperature, high pressure difference, and vacuum conditions.

[0021] Principle of the technical solution of the present invention:

[0022] According to Bernoulli's equation, an ideal fluid flowing steadily in a pipe has three forms of energy: pressure energy, potential energy, and kinetic energy. Under certain conditions, the three forms of energy in the flow field can be converted into each other, but their sum remains unchanged, that is: kinetic energy + gravitational potential energy + pressure potential energy = constant.

[0023] Based on the above law, when the reaction gas - air and circulating water flow from both sides of the porous metal bipolar plate respectively, the water permeation direction depends on the magnitude of the static pressure of the reaction gas and circulating water on both sides of the porous metal bipolar plate; the amount of water permeation is related to parameters such as the pressure difference of the fluid, capillary force, and the pore diameter, porosity, and thickness of the porous metal plate material. The specific process is described in detail as follows:

[0024] 1. Humidification process: When air and circulating water enter the flow channels of the porous metal bipolar plate respectively, through the design of the flow field and the control of the auxiliary system, when the air flow rate is greater than the circulating water flow rate and the pressure on the air side is lower than the pressure on the circulating water side, a pressure difference is generated, driving the circulating water to permeate through the micropores of the metal flow field plate to the air side, evaporating and humidifying in the air, and then permeating to the membrane electrode through capillary action and concentration difference for humidification.

[0025] 2. Drainage process: During the operation of the battery, water generated in the cathode catalyst layer permeates through the diffusion layer, gradually grows from small droplets, and converges into a liquid film attached to the surface of the porous metal bipolar plate. Its flow rate is close to zero, much lower than the flow rate of the circulating water on the other side of the bipolar plate, and the pressure of the liquid water on the cathode side is greater than the pressure of the circulating water side. Driven by the pressure difference, the liquid water on the cathode side permeates through the micropores of the porous metal plate to the circulating water side to drain excessive accumulated water on the cathode side, ensuring the smooth flow of the gas flow channel of the battery. Description of the drawings:

[0026] Appendix Figure 1It is a schematic diagram of the principle of the present invention.

[0027] Appendix Figure 2 It is a cross-sectional view of Specific Embodiment 1 of the present invention.

[0028] Appendix Figure 3 It is a three-view drawing of the flow field structure of Specific Embodiment 1 of the present invention.

[0029] Appendix Figure 4 It is a cross-sectional view of Specific Embodiment 2 of the present invention.

[0030] Appendix Figure 5 It is a three-view drawing (bottom view, front view, top view) of the flow field structure of Specific Embodiment 2 of the present invention.

[0031] Appendix Figure 6 It is a three-dimensional assembly drawing of the specific embodiment of the present invention.

[0032] Appendix Figure 1 It is a schematic diagram of the principle of the present invention. As shown in the figure, air 1 and circulating water 4 are isolated by a porous metal bipolar plate 3. When the flow rates, static pressures, concentration gradients, and capillary actions of air 1 and circulating water 4 are present, liquid water - circulating water and generated water pass through pores 2 of the porous metal bipolar plate 3 for two-way dynamic penetration between the air-side flow field and the circulating water flow field of the bipolar plate, so as to achieve gas humidification and the discharge of generated water.

[0033] Humidification process: When dry air 1 and circulating water 4 enter the flow channels on both sides of the porous metal bipolar plate respectively, the micropores 2 inside the porous metal bipolar plate 3 are wetted by the circulating water, and a bubble point pressure P_bubble is generated by the surface tension of water. When taking the atmospheric pressure as a reference, the air flow rate is greater than the circulating water flow rate, ν_air > ν_circulating water, the static pressure on the air side is less than the static pressure on the circulating water side, p_air < p_circulating water. When p_air < p_circulating water + P_bubble, driven by the pressure difference between gas and liquid, the circulating water penetrates through the micropores 2 of the porous metal bipolar plate to the air side and evaporates and humidifies in the flow channel on the air side;

[0034] Drainage process: When the fuel cell is working, the liquid water generated at the cathode catalyst layer permeates through the diffusion layer, gradually expands from small droplets, and converges into a liquid water film attached to the air-side surface of the porous metal bipolar plate. Its flow rate is close to zero, much less than the flow rate of the circulating water on the other side of the porous metal bipolar plate, ν_water on air side < ν_circulating water, the static pressure on the air side is greater than the static pressure on the circulating water side, p_water on air side > p_circulating water. Driven by the pressure difference, the liquid water film on the air side is penetrated through the micropores 2 to the circulating water side, thereby discharging the excessive generated water on the air side.

[0035] During the above process, the water permeation direction depends on the static pressures of the reaction gas side and the circulating water side on both sides of the porous metal bipolar plate; the water permeation amount is related to parameters such as the pressure difference of the fluid, the capillary force, and the pore diameter, porosity, and thickness of the porous metal plate material. The flow rates and pressures of the fluids on both sides of the porous metal bipolar plate are adjusted by methods such as the proportional valve of the auxiliary system and the speed regulation of the water pump motor; the fluids on both sides refer to the reaction gas and the circulating water, not just air and circulating water, and can also be hydrogen and circulating water. Specific embodiments:

[0036] The specific solution of the present invention: The cathode flow field, anode flow field, and circulating water flow field of the fuel cell are composed of a grooved porous metal plate, a metal wire mesh, a metal frame, and a seal. There are three structural forms: The first structural form is composed of four single-sided grooved porous metal plates; the second structural form is a combination of two single-sided grooved porous metal plates and a metal wire mesh; the third structural form is composed of one double-sided grooved porous metal plate and one single-sided grooved porous metal plate. To enable those skilled in the art to more clearly understand the technical principle, scheme structure, and technical advantages of the present invention, the present invention will be further described below in conjunction with the drawings and embodiments.

[0037] Att Figure 2 is a cross-sectional view of Specific Embodiment 1 of the present invention. As shown in the figure, a single cell is jointly composed of a cathode plate 1, a circulating water flow field 2, a seal 3, a metal frame 4, an anode plate 5, an anode diffusion layer 6, a proton membrane 7, and a cathode diffusion layer 8. Among them, the cathode plate 1 and the anode plate 5 both adopt porous metal plate materials with single-sided parallel grooves. The circulating water flow field 2 is formed by pressing a metal wire mesh into the metal frame 4 and then pressing it together with the cathode plate 1 and the anode plate 5 through the seal 3 to respectively form the cathode flow field, the anode flow field, and the circulating water flow field.

[0038] In specific implementation, the bipolar plate preferably adopts a porous metal plate with single-sided grooves. Among its two surfaces, one is a plane, and the other is processed by a forming die into a shape with grooves, ridges, alternating and parallel, which has a simple structure, is easy to form, is suitable for mass production, and has a lower cost.

[0039] In specific implementation, as the metal wire mesh used for the circulating water flow field, a mechanically pressed multi-layer metal wire mesh is preferably adopted, and a sintered metal wire mesh can also be adopted. The common point of the two is that the four sides of the metal wire mesh are cut into parallelograms.

[0040] Att Figure 3It is the three - view drawing of the circulating water flow field structure of Specific Embodiment 1 of the present invention. As shown in the figure, the diagonals of the metal frame 2 are respectively processed with the inlet 1 and outlet 5 of the circulating water. The metal wire mesh 4 is cut into a parallelogram and inlaid in the metal frame 2. The inlay method is as shown in the figure, that is, the two obtuse angles in the quadrilateral are close to the inlet and outlet of the metal frame 2, enclosing two deceleration and pressure - increasing chambers. The two sides of the parallelogram respectively form an angle A with the axis direction of the inlet and outlet water, and satisfy 0°≤A<90°, so as to facilitate the uniform distribution of the circulating water along the plane. In addition, the seal 3 is inlaid in the metal border and pressed against the cathode plate and anode plate of the bipolar plate respectively to play a sealing role. To ensure the sealing effect, the thickness L1 of the metal frame plus the seal is greater than the thickness L2 of the circulating water flow field plate 4, and the thickness L2 of the flow field plate 4 is greater than the thickness L3 of the metal frame 2, that is, the relation L1>L2>L3 is satisfied.

[0041] Appendix Figure 4 It is the cross - sectional view of Specific Embodiment 2 of the present invention. That is, it is composed of a layer of porous metal plate with grooves on both sides and a layer of porous metal plate with grooves on one side. Its characteristics are: the cathode flow field and the circulating water flow field share the porous metal plate material with grooves on both sides, and the anode flow field separately uses the porous metal plate with grooves on one side, and then is pressed against the cathode plate and anode plate through the seal to form the cathode flow field, anode flow field and circulating water flow field respectively.

[0042] Appendix Figure 4 As shown, the cathode plate 1 uses a porous metal plate with grooves on both sides. The grooves on the upper surface of the cathode plate form the flow field of air (oxygen), and the grooves on the lower surface form the flow field of circulating water. And in order to form the inlet and outlet channels of air (oxygen) and the inlet and outlet channels of circulating water, the directions of the grooves on both sides of the porous metal plate are arranged perpendicular to each other; the circulating water flow field on the lower surface of the bipolar plate is inlaid in the metal frame 3, and the seals 2 on the upper and lower surfaces of the metal frame 3 are respectively pressed against the cathode plate and anode plate to form a sealing structure, respectively forming three relatively independent cavities of air (oxygen), hydrogen and circulating water, and then are pressed against the diffusion layer and membrane electrode to form a battery unit.

[0043] Appendix Figure 5 It is the three - view drawing (bottom view, front view, top view) of the cathode flow field and the circulating water flow field structure of Specific Embodiment 2 of the present invention. As shown in the figure, the diagonals of the metal frame 2 are respectively processed with the inlet 1 and outlet 5 of the circulating water. The bottom view shows the shape of the circulating water flow field, and the side formed by the two ends of the parallel grooves forms an angle A with the axis of the inlet and outlet circulating water, and satisfies: 0°≤A<90°; the top view shows the shape of the air (oxygen) flow field 4, the cathode plate is inlaid in the metal frame 2, and seals 3 are respectively arranged on the upper and lower surfaces of the metal frame, and play a sealing role after being pressed against the cathode plate and anode plate.

[0044] Appendix Figure 6 It is the three - dimensional assembly drawing of the specific embodiment of the present invention. As shown in the appendix Figure 6As shown in the figure, multiple single-section batteries are stacked together to form a whole battery stack 5. An air intake hood 1, an air outlet hood 3, a circulating water inlet hood 8, and an outlet hood 3 are respectively arranged along the diagonals on the upper and lower surfaces of the whole battery stack 5; a hydrogen intake hood 6 and an outlet hood 10 are respectively arranged on the left and right side surfaces of the whole battery stack 5. In addition, air inlet and outlet connectors 2, 4, 7, etc. are machined on the side surfaces of each air hood.

[0045] In summary, compared with the prior art, the bipolar plate of the present invention adopts a porous metal plate, which realizes the dual functions of humidification and drainage, eliminates the humidifying device, not only has uniform humidification, timely drainage, fast dynamic response of power output, effectively prolongs the service life of the battery, but also reduces the system volume and system energy consumption. In addition, the physical properties of the porous metal plate are superior to those of the porous graphite plate. When hydrogen and oxygen are used as reaction gases, the fuel cell can operate in a fully enclosed manner and can be applied to provide power in extreme environments such as deep-sea diving, high-altitude low-temperature flight, and space aerospace.

Claims

1. Flow field structure of a porous metal bipolar plate fuel cell, comprising a fuel cell composed of porous metal bipolar plates, characterized in that: The cathode flow field, anode flow field and circulating water flow field of the fuel cell are composed of a porous metal plate with grooves, a metal wire mesh, a metal frame and a seal. There are three structural forms of the flow field: First, it is composed of four layers of porous metal plates with grooves on one side; Second, it is composed of two layers of porous metal plates with grooves on one side and a metal wire mesh; Third, it is composed of one layer of porous metal plate with grooves on both sides and one layer of porous metal plate with grooves on one side.

2. According to the second structural form described in claim 1, which is composed of a combination of two layers of single-sided grooved porous metal plates and metal wire meshes, it is characterized in that: For both the cathode and anode flow fields, a porous metal plate material with parallel grooves on one side is used. The circulating water flow field is formed by pressing a metal wire mesh into a metal frame and then pressing it with a seal against the cathode plate and anode plate to form the cathode flow field, anode flow field and circulating water flow field respectively.

3. The circulating water flow field according to claim 2 is formed by pressing a wire mesh and inlaying it into a metal frame, characterized in that: The metal wire mesh forms a parallelogram, and two of its sides form an angle A with the axial directions of the inlet and outlet water respectively, and the angle A satisfies the condition of 0° ≤ A < 90°.

4. According to the third structural form described in claim 1, it is composed of a layer of porous metal plate with grooves on both sides and a layer of porous metal plate with grooves on one side, characterized in that: The cathode flow field and the circulating water flow field share one layer of porous metal plate material with grooves on both sides. The anode flow field uses a porous metal plate with grooves on one side alone, and then is pressed with a seal against the cathode plate and anode plate to form the cathode flow field, anode flow field and circulating water flow field respectively.

5. The porous metal plate material with double-sided grooves for use in the cathode and the circulating water flow field according to claim 4, characterized in that: The directions of the grooves on both sides are perpendicular to each other.

6. The porous metal plate material with double-sided parallel grooves for the cathode and the circulating water flow field according to claim 4, characterized in that: The sides formed by the two ends of the grooves in the circulating water field form an angle A with the axis of the inlet and outlet circulating water, and the angle A satisfies the condition of 0° ≤ A < 90°.

7. The cathode flow field and anode flow field of the fuel cell according to claim 1, characterized in that: The two can be used interchangeably.

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