Bionics proton exchange membrane fuel cell based on snowflake structure

Abstract

The application discloses a kind of bionics proton exchange membrane fuel cell based on snowflake structure, including bipolar plate, bipolar plate includes the flow field on by 4 flow channel modules combination, each flow channel module includes serpentine channel with import and export, 4 flow channel modules on serpentine channel are sequentially connected to constitute bionic flow channel, flow channel is full in bipolar plate, and then the import of serpentine channel on the first flow channel module forms flow channel entrance, and the export of serpentine channel on the last flow channel module forms flow channel exit.The bionic flow channel of the device has good pressure drop, can make gas distribution uniform, improve the flow smoothness of flow channel, beneficial to discharge liquid water.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of proton exchange membrane fuel cells, and particularly relates to a bionic proton exchange membrane fuel cell based on a snowflake structure. BACKGROUND

[0002] At present, the flow field of the commonly used proton exchange membrane fuel cell is divided into parallel flow field and serpentine flow field, etc., wherein the significant advantage of the parallel flow field is that the pressure drop between the gas inlet and outlet is relatively small, and the pump energy loss is reduced, but due to the relatively large width of the flow field, the fluid distribution in each flow channel will appear uneven, thereby causing liquid water accumulation, increased transmission loss, insufficient supply of gas reactants in the center area of the electrode, and reduced current density, which affects the performance of the cell; and the advantage of the serpentine flow field is the drainage capacity, and the single flow path can promote the drainage of liquid water, but in a large-area flow field, the pressure drop of the serpentine flow field is large, and due to the excessively long flow channel, the pump energy loss is caused by the excessively large pressure drop, and the uniformity of the gas reactant distribution is reduced, which leads to the reduction of the performance of the fuel cell.

[0003] Therefore, how to reduce the pressure drop of the flow field on the basis of ensuring the drainage capacity of the flow field is a technical problem to be solved by those skilled in the art. SUMMARY

[0004] The purpose of the present application is to provide a bionic proton exchange membrane fuel cell based on a snowflake structure, which has a good flow channel pressure drop, can make the gas distribution uniform, and improves the flow smoothness of the flow channel, which is beneficial to the drainage of liquid water.

[0005] To achieve the above-mentioned purpose, the present application provides a bionic proton exchange membrane fuel cell based on a snowflake structure, which comprises a bipolar plate, the flow field on the bipolar plate is divided into four flow channel modules, each of the flow channel modules comprises a serpentine channel with an inlet and an outlet, and the serpentine channels on the four flow channel modules are sequentially connected to form a bionic flow channel, the flow channel is fully paved on the bipolar plate, and then the inlet of the serpentine channel on the first flow channel module forms a flow channel inlet, and the outlet of the serpentine channel on the last flow channel module forms a flow channel outlet.

[0006] Further, the cross section of the serpentine channel is square.

[0007] Further, the width of any cross section of the bionic flow channel is 2mm, and the depth is 1mm.

[0008] Further, the depth of the bionic flow channel gradually increases from the flow channel inlet to the flow channel outlet.

[0009] Further, each of the flow channel modules is a right-angled triangle area, and the right-angled sides of the four flow channel modules are spliced with each other to form a square flow field.

[0010] Further, the bipolar plate is provided with a first partition plate extending along the edge of the bipolar plate, and the middle part of the bipolar plate is provided with a cross partition plate, which is used for separating the area in the first partition plate into four right-angled triangle areas.

[0011] Further, the inner side of the first partition plate is provided with a plurality of first stop blocks uniformly spaced, and the cross partition plate is provided with a plurality of second stop blocks uniformly spaced, the first stop blocks and the second stop blocks in the same right-angled triangle area are parallel to each other, and the gap between the first stop blocks and the second stop blocks in each right-angled triangle area forms a serpentine channel.

[0012] Further, in the same right-angled triangle area, the end face of the first stop block extending from the first partition plate is parallel to the cross partition plate, and the end face of each first stop block has the same spacing distance from the cross partition plate in the extension direction.

[0013] In the same right-angled triangle area, the end face of the second stop block extending from the cross partition plate is parallel to the first partition plate, and the end face of each second stop block has the same spacing distance from the first partition plate in the extension direction.

[0014] Further, the membrane electrode is composed of a gas diffusion layer, a catalytic layer and a proton exchange membrane, and the gas diffusion layer is in contact with the bipolar plate.

[0015] The snowflake structure-based bionics proton exchange membrane fuel cell has the following beneficial effects:

[0016] 1. The design can improve the uniformity of the distribution of hydrogen and oxygen in the flow channel of the bipolar plate.

[0017] 2. The crystallization derived shape of water under natural conditions is referred to, and part of the hydrothermal characteristics is improved.

[0018] 3. The hydrogen and oxygen can be fully reacted in the bionic flow channel, the efficiency of the fuel cell is improved, and the pressure drop is good compared with the traditional structure flow channel (such as parallel, serpentine, etc.).

[0019] 4. The forced convection is formed through the structure of the bionic flow channel, the flow smoothness in the bionic flow channel is improved, and the drainage is facilitated. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0021] Figure 1 An exploded view of a snowflake structure based biomimetic proton exchange membrane fuel cell according to embodiments of the application;

[0022] Figure 2 An axial view of a bipolar plate;

[0023] Figure 3 A top view of a bipolar plate;

[0024] Figure 4 A base boundary condition line plot;

[0025] Figure 5 A mesh independence verification line plot;

[0026] Figure 6 A comparison of simulation results and experimental data line plot;

[0027] Figure 7 A comparison of three flow channel polarization curves and power density curves line plot;

[0028] Figure 8 A serpentine flow channel polarization curve and polarization curve line plot;

[0029] Figure 9 A parallel flow channel polarization curve and power density curve line plot;

[0030] Figure 10 A biomimetic flow channel polarization curve and power density curve line plot;

[0031] Figure 11 A comparison of three flow channel system pressure drops at 0.7 V line plot;

[0032] Figure 12 A flow channel pressure distribution cloud plot;

[0033] Figure 13 A reactant mass fraction distribution cloud plot at 0.7 V operating voltage;

[0034] Figure 14 An effect plot of the convection effect under the ridge;

[0035] Figure 15 A water distribution cloud plot at 0.7 V operating voltage;

[0036] Figure 16 A cathode side temperature distribution cloud plot;

[0037] Wherein: 1. anode plate, 2. membrane electrode, 3. cathode plate, 4. second stop block, 5. flow channel inlet, 6. flow channel outlet, 7. cross intersection separator, 8. first stop block, 9. first separator, 10. inclined surface, 11. right triangle area, 12. outlet, 13. inlet, 14. head end, 15. tail end, 16. serpentine channel. DETAILED DESCRIPTION

[0038] The core of the present application is to provide a snowflake structure based on bionics proton exchange membrane fuel cell, with an effective and convenient mobile water adding device, which can be operated by a single person.

[0039] In order to enable those skilled in the art to better understand the technical solutions of the present application, the present application will be further described in detail below with reference to the drawings and embodiments.

[0040] Referring to the prior art: proton exchange membrane fuel cell uses hydrogen and oxygen as fuel to directly convert chemical energy into electrical energy, which is one of the most green and environmentally friendly energy conversion devices. Hydrogen and oxygen as the reaction gas of proton exchange membrane fuel cell, after the gas flow channel is passed, the reaction gas diffuses to the center of the fuel cell into the gas diffusion layer, and then diffuses to the catalytic layer for electrochemical reaction. The gas flow channel of the proton exchange membrane fuel cell and the gas diffusion layer and the catalytic layer of the porous medium are the necessary places for the transmission of fuel reaction gas, which affects the mass transfer process of the whole fuel cell, and ultimately affects the fuel utilization efficiency of the fuel cell and the overall output performance of the fuel cell.

[0041] The gas diffusion layer is closely connected with the pole plate of the fuel cell, and constitutes a mass transfer channel, which is an important part of the proton exchange membrane fuel cell. The main function of the gas diffusion layer is to support the catalyst layer, stabilize the electrode structure, provide gas channels, electronic channels and drainage channels for electrode reaction. The main function of the pole plate is to uniformly distribute the reaction gas, realize electron conduction of the anode and cathode, and dissipate heat in time. The water in the fuel cell is generated in the cathode catalytic layer and reaches the flow field through the gas diffusion layer. The structure of the flow field is closely related to the flow state of water in the flow field. If the water in the flow field cannot be discharged in time, the "waterlogging phenomenon" will occur, which will reduce the performance of the battery. Therefore, optimizing the structure of the flow channel can make the generated water in the fuel cell be discharged faster to improve the output performance of the battery.

[0042] Please refer to Figures 1 to 4The application discloses a snowflake structure based bionics proton exchange membrane fuel cell, which comprises a bipolar plate, the bipolar plate is divided into an anode plate 1 and a cathode plate 3 according to the electrode characteristics of the cell, the flow field of the bipolar plate is divided into four flow channel modules, each flow channel module comprises a serpentine channel 16 with an inlet 13 and an outlet 12, the serpentine channels 16 on the four flow channel modules are sequentially connected to form a bionic flow channel, the flow channel is fully paved on the bipolar plate, that is, the flow channel is fully paved on the anode plate 1 or the cathode plate 3, and then the inlet 13 of the serpentine channel 16 (also referred to as an S-shaped channel) on the first flow channel module forms a flow channel inlet 5, and the outlet 12 of the serpentine channel 16 on the last flow channel module forms a flow channel outlet 6.

[0043] It should be noted that when the fuel cell starts to work, the reaction gas enters the bionic flow channel from the flow channel inlet 5, is guided to the flow channel outlet 6 after reaction and is discharged, due to the more complex structure of the bionic flow channel in the device, more corners and the pressure difference between adjacent flow channels, the flow channel with the bionic structure has a more obvious convection effect under the ridge, so that the concentration of the reactants in the area under the ridge is increased, the performance of the cell is improved, and then the snowflake-like bionic structure design in the device can make the distribution of the reactants in the effective area more uniform, that is, the reaction gas can flow into the bionic flow channel more uniformly.

[0044] When the snowflake structure based bionics proton exchange membrane fuel cell is applied, the flow field structure is improved based on the serpentine flow field, the serpentine flow field has a snowflake-like structure, the width of the bionic flow channel is the same, the internal flow of the bionic flow channel is smooth, the internal flow velocity distribution of the bionic flow channel is uniform, the gas can be uniformly distributed in the entire flow field and the electrochemical reaction rate is the same, local hot spot temperature is prevented, long-term stable operation of the cell is ensured, the uniformity of the current density distribution, water distribution and oxygen distribution of the flow field are greatly improved, and the serpentine flow field with the snowflake-like structure is not excessively long compared with the traditional structure flow channel (for example, parallel, serpentine and the like), and the pressure drop is good.

[0045] On the basis of the above embodiment, the cross section of the serpentine channel is square, and the width of any cross section of the flow channel is 2mm and the depth is 1mm, so that the width of the bionic flow channel is equal, the internal flow of the bionic flow channel is smooth, and the depth direction is perpendicular to the direction of the bipolar plate.

[0046] On the basis of the other embodiment, the depth of the bionic flow channel gradually increases from the flow channel inlet 5 to the flow channel outlet 6, the flow smoothness of the bionic flow channel is further improved, and the drainage performance is improved.

[0047] On the basis of the above embodiment, each flow channel module is a right-angled triangle area 11, and the right-angled sides of the four flow channel modules are spliced to form a square flow field.

[0048] On the basis of the above-mentioned embodiments, further disclosed is the structure of the flow field, in particular: the above-mentioned bipolar plate is provided with a first partition plate 9 extending along the edge thereof, and the middle part of the bipolar plate is provided with a cross-shaped partition plate 7, which is used for separating the area within the first partition plate 9 into four right-angled triangle areas.

[0049] The inner side of the first partition plate 9 is provided with a plurality of first stop blocks 8 uniformly spaced, and in the same right-angled triangle area 11, the end face of the first stop block 8 extending from the first partition plate 9 is parallel to the cross-shaped partition plate 7, and the spacing distance of the end face of each first stop block 8 from the cross-shaped partition plate 7 in the above-mentioned extending direction is the same.

[0050] The cross-shaped partition plate 7 is provided with a plurality of second stop blocks 4 uniformly spaced, and in the same right-angled triangle area 11, the end face of the second stop block 4 extending from the cross-shaped partition plate 7 is parallel to the first partition plate 9, and the spacing distance of the end face of each second stop block 4 from the first partition plate 9 in the above-mentioned extending direction is the same.

[0051] The gap between the first stop block 8 and the second stop block 4 in each right-angled triangle area constitutes a serpentine channel 16, and it is to be noted that the four end parts of the cross-shaped partition plate 7 are not in contact with the first partition plate 9, and the serpentine channels 16 adjacent to each other are communicated through the gap between the end part of the cross-shaped partition plate 7 and the first partition plate 9, and in the present application, the flow channel inlet 5 and the flow channel outlet 6 are close to each other, and more specifically, the gap between the head end 14 and the tail end 15 of the first partition plate 9 and the same end part of the cross-shaped partition plate 7 constitutes the flow channel inlet 5 and the flow channel outlet 6.

[0052] More specifically, the end part of the first stop block 8 is provided with an inclined surface 10, and the inclined surface 10 is parallel to the side surface of the cross-shaped partition plate 7, wherein the inclination angle of the inclined surface 10 is 45 degrees, and the cross-shaped partition plate 7 is a cross-shaped perpendicular partition plate, thereby ensuring that the width of each part of the bionic flow channel is equal.

[0053] On the basis of the above-mentioned embodiments, the first stop block 8 located in the right-angled triangle area 11 is long in the middle part and short at both ends, and the second stop block 4 located in the right-angled triangle area 11 is long in the middle part and short at both ends, thereby being adapted to the right-angled triangle area 11, so as to realize the full paving of the serpentine channel in each right-angled triangle area 11.

[0054] On the basis of the above-mentioned embodiments, further comprising a membrane electrode 2, which is composed of a gas diffusion layer, a catalytic layer and a proton exchange membrane, and the gas diffusion layer is in contact with the bipolar plate, which is a prior art, and the specific principle and structure will not be described in detail.

[0055] In addition, the inventors use the method of finite element analysis to numerically simulate the performance of the present application, thereby making an evaluation.

[0056] First step to determine the reference group of the contrast test

[0057] The research content of the present application is mainly to combine the bionic structure with the traditional serpentine flow channel, on the basis of the battery flow field design and hydrothermal management at home and abroad, a bionic flow channel based on the snowflake structure on the bipolar plate in a proton exchange membrane fuel cell is designed as shown in the figure. Figure 3 The digital modeling of the bionic flow channel and the same size of the contrast flow channel is completed by using CATIA software. The contrast flow channel is: serpentine flow channel and parallel flow channel, and the single cell is taken as the research object, the CFD simulation of the bionic flow channel and the two kinds of contrast flow channels is carried out under different working voltages by using Ansys Fluent software, the polarization curves and power density curves of the three kinds of flow channels are drawn respectively, and the performances of the three kinds of flow channels are compared.

[0058] Second step to build a PEMFC mathematical model

[0059] The research and improvement of PEMFC fuel cell (i.e. proton exchange membrane fuel cell) are usually carried out by simulation. Due to the multiphase flow and multiphysical field electrochemical reaction inside the PEMFC cell, it is difficult to accurately simulate the actual working process by experimental means, and it is not convenient to measure the changes of various parameters inside the cell. The PEM module of the commercial CFD software Fluent is used for simulation in the present application, and the multiphase reaction of the representative GDL\CL layer solution surface is described to describe the internal reaction of the cell. In order to describe the internal electrochemical working process of PEMFC and the mass and physical property changes of each component, the basic control equation, electrochemical equation and phase change equation are needed.

[0060] 2.1 Basic control equation

[0061] The mass-energy conversion of each component in the fuel cell is generally described by the conservation law of physics and chemistry. In the process of mathematical modeling of PEMFC, the control equation mainly includes mass, momentum, energy, component and current conservation equation.

[0062] 2.1.1 Mass conservation equation

[0063] Equation (2-1) is the mass control equation, which is applicable to any component and any state in the PEMFC cell:

[0064]

[0065] Wherein, ε is the porosity, i.e. the ratio of the volume of the gap of the porous medium to the total volume of the electrode layer. ρ is the fluid density, is the fluid velocity vector. Sm is the mass source term, which is usually the sum of the mass changes of each component in each layer of the cell:

[0066] Diffusion layer and flow channel:

[0067] S m = 0

[0068] Anode catalytic layer:

[0069]

[0070] Cathode catalytic layer:

[0071]

[0072] Where, MH2, MO2, MH2O are the molar mass fractions of H2, O2, H2O respectively, and F is the Faraday constant.

[0073] 2.1.2 Momentum conservation equation

[0074] Equation (2-2) is the momentum control equation:

[0075]

[0076] The left side of the equation is the convection term, and the first and second terms on the right side are diffusion terms. SM is the momentum source term. Where P is the pressure, and μ is the dynamic viscosity of the fluid. Momentum conservation in PEMFC can be derived from Stokes' viscosity law. In PEMFC, the dynamic source term of each layer region is as follows:

[0077] Flow channel:

[0078] S M = 0 (2-2.1)

[0079] Diffusion layer:

[0080]

[0081] Catalytic layer:

[0082]

[0083] 2.1.3 Energy conservation equation

[0084] The energy conservation equation in PEMFC can be expressed as:

[0085]

[0086] Where, since the physical properties of each component are related to flow velocity and temperature, temperature changes need to be considered. e S is the energy source term, mainly including electrochemical reaction heat, ohmic heat generated by current flowing through conductive medium, and exothermic heat due to changes in reaction gas state. e The calculation is shown in equation 2-3.1:

[0087]

[0088] 2.1.4 Component conservation equations

[0089] The component conservation equations in PEMFC are shown in equation (2-4):

[0090]

[0091] where S i is the additional material source term. In PEMFC, except for the catalyst layer, other parts S i are negligible. The numerical calculation of each source term in the catalyst layer is shown in equations (2-4.1), (2-4.2), (2-4.3), and (2-4.4):

[0092]

[0093]

[0094]

[0095]

[0096] 2.2 Electrochemical equations

[0097] 2.2.1 Charge conservation equations

[0098]

[0099]

[0100] where R sol and R mem are the solid-phase and membrane-phase body transfer current source terms, respectively.

[0101] Redox reactions occur during the operation of PEMFC, and the generated electric charge produces current under the action of the potential difference between the anode and cathode. The relationship between the potential and the current is represented by equations (2-5) and (2-6), in which the current source term is obtained by calculating the Butlet-Volmer equation.

[0102]

[0103]

[0104] where equations (2-7) and (2-8) are the Butlet-Volmer equation expressions on the anode and cathode, respectively, representing the anode and cathode volume exchange currents.

[0105] 2.2.3 Open-circuit voltage

[0106] The open-circuit voltage of PEMFC can be calculated by Nernst equation, and its expression is shown in equation (2-9):

[0107]

[0108] 2.3 Water transport model

[0109] During the operation of PEMFC, water will have a series of changes such as phase transition, migration and diffusion, as the reaction continuously generates and discharges water.

[0110] 2.3.1 Phase transition of water

[0111] The internal water of PEMFC will appear gas-liquid two-phase transition at the general working temperature of 80℃. The phase transition of water depends on the size of water vapor partial pressure and the corresponding temperature saturation vapor pressure. When the water vapor partial pressure is greater than the corresponding temperature saturation pressure, water vapor will be liquefied to generate liquid water: otherwise, liquid water will evaporate to generate water vapor

[10] . The phase transition model of water is represented by the liquid water conservation equation:

[0112]

[0113] To simplify the calculation, it is assumed that the capillary diffusion is dominant in the porous medium, The capillary diffusion in the flow channel can be ignored, p c =0. s is the saturation of liquid water. The capillary pressure pc is shown in equations (2-11), (2-12):

[0114]

[0115]

[0116] 2.3.2 Electro-migration of water

[0117] The electro-migration of water in PEMFC occurs in the proton exchange membrane, and the electro-migration energy is shown in equation (2-13):

[0118]

[0119] 2.3.3 Pressure migration of water

[0120] The pressure migration energy of water in PEMFC is shown in equation (2-14):

[0121]

[0122] If the pressure in the MEM changes linearly, then:

[0123] 2.3.4 Diffusion of water

[0124] The energy of water concentration diffusion in PEMFC is expressed by equation (2-15):

[0125]

[0126] Third step numerical simulation and results

[0127] 3.1 Numerical simulation of PEMFC

[0128] 3.1.1 Basic assumptions

[0129] (1) Each gas component (H2, O2, water vapor) is considered as an incompressible ideal gas or ideal gas mixture;

[0130] (2) The effect of gravity is ignored;

[0131] (3) The internal gas flow is considered as laminar flow;

[0132] (4) All porous media (proton exchange membrane, diffusion layer, catalyst layer) are considered to be isotropic and uniform.

[0133] 3.1.2 Parameter model

[0134] CFD numerical simulation needs to assign values to the parameters in the model, Table 3-1 and Table 3-2 are the relevant physical properties

[0135] Parameter expression and calculation parameter value:

[0136] Table 3-1 Calculation formula of control equation related variables

[0137]

[0138] Table 3-2 Calculation parameters

[0139]

[0140]

[0141] 3.1.3 Boundary conditions

[0142] In CFD simulation calculation, the first type of boundary condition is adopted, the inlet mass flow is set, the mass fraction of each component of the mixed gas in air is given; The outlet pressure is set at the outlet of the flow channel; All wall conditions are set to no slip. Each boundary condition is as follows Figure 4 :

[0143] (1) The surface of each boundary condition is: the electric flux and the ion flux are both 0, the solid material conductivity and the membrane ion conductivity are both constants;

[0144] (2) The surface potential of the anode plate is 0 V, i.e. φsol=0 V; the cathode potential is determined by the battery operating voltage, i.e. φsol=Vcell;

[0145] (3) In order to be as close as possible to the real fuel cell working environment, the inlet of the flow channel of the cathode plate and the anode plate adopts a mass flow inlet, and the mass fraction of each component of the reaction gas at the inlet of the flow channel is defined as Cα=Cαi; and there is:

[0146]

[0147] wherein ζ an is the anode excess coefficient, ζ ca is the cathode excess coefficient; ω H2 is the hydrogen mass fraction, ω O2

[0148] is the oxygen mass fraction;

[0149] (4) The outlet of the flow channel adopts a pressure outlet, the anode P=Pa, the cathode P=Pc, and the temperature and working pressure are assigned as 353 K and 1 atm (i.e. 101.325 kPa);

[0150] (5) All the walls are stationary walls, and the wall boundary condition is no slip.

[0151] The boundary condition parameters are shown in Table 3-3:

[0152] Table 3-3 Boundary condition parameters

[0153]

[0154]

[0155] 3.2 Calculation geometry model

[0156] 3.2.1 Solution strategy

[0157] The simulation of the proton exchange membrane fuel cell usually adopts the PEM module in the commercial CFD calculation software Ansys Fluent 19.2. Based on the SIMPLE algorithm, the discrete equation is selected as the finite volume method of differential equation, and the simulation result is obtained by iterative calculation of the software. Among them, according to the residual value of the mass and energy control equation to judge whether it is convergent, the judgment standard is: mass (1×10-6), energy (1×10-4), and the calculation cycle type and stability method are selected as F-Cycle and BCGSTAB respectively.

[0158] The operation process of the calculation of the PEM model is generally as follows:

[0159] (1) Read the grid file and check the grid quality;

[0160] (2) Define the model unit, adjust the size;

[0161] (3) Import the PEM module, define the parameters;

[0162] (4) Define the operating conditions;

[0163] (5) Define the boundary conditions;

[0164] (6) Set the calculation scheme, set the control parameters (relaxation factor, discrete method, etc.);

[0165] (7) Calculate the region initialization and iterative calculation;

[0166] (8) Post-processing.

[0167] The core of the PEM module is the multiphase flow and multi-physical field electrochemical reaction occurring in the CL layer, and the reaction is described by the complex coupling source term combined with the user-defined parameter UDS, as shown in Tables 3-4:

[0168] Table 3-4 User-defined scalar

[0169]

[0170] 3.2.2 Model geometry size

[0171] The application uses CATIA software to design and establish a 3D model of a new type of bionic flow channel, and establishes two traditional flow channels with the same geometric size: a serpentine flow channel and a parallel flow channel. The 3D model is imported into CFD simulation calculation, and the results are analyzed and compared, and the conclusion is drawn. The flow channel design is as follows Figure 1 and 3 , and the flow channel size is as shown in Table 3-5:

[0172] Table 3-5 Geometric parameter table

[0173]

[0174]

[0175] 3.2.3 Meshing and independence analysis

[0176] After the modeling of the PEMFC model, the meshing is performed using the pre-processing software hypermesh 19.1. Compared with the unstructured mesh, the truncation error of the structured mesh is smaller, so the structured mesh is generated as much as possible under the condition of allowing. Among them, due to the relatively complex corner vortex area of the bionic flow channel, the unstructured mesh is generated by selecting auxiliary lines to guide mesh generation. The fuel cell model is generated by using hexahedral mesh elements as much as possible, and each structure layer is divided into four layers of grid structure.

[0177] Based on the numerical simulation, in order to ensure the accuracy of the calculation results, the grid independence verification needs to be carried out. Because the working condition of the serpentine flow field is relatively complex, different grid division schemes with different grid cell sizes are designed, and the grid independence of the model is verified by comparing the current density under the same working voltage, so as to select the appropriate grid cell size. The grid independence analysis result is shown in Figure 5 .

[0178] According to Figure 5 It can be seen that when the grid cell size reaches 0.25mm, the calculation result is almost independent of the grid size. Considering the special structure of the biomimetic flow channel, the hexahedral grid cell of 0.2mm is selected for analysis and calculation.

[0179] 3.2.4 Model verification

[0180] As Figure 6 shown in the figure, the error between the simulation value and the experimental value is relatively small under the low current density state, and the error gradually increases with the increase of the current density. Overall, the error between the simulation result and the experimental result is not more than 5%, and the deviation between the simulation result and the experimental data is small, so it can be considered that the model established in the application has high credibility.

[0181] 3.3 Simulation results and analysis

[0182] Due to the relatively complex internal electrochemical reaction, the working performance of PEMFC fuel cell is affected by many factors. Operating conditions, physical parameters, battery materials, plate structure and other factors may have a great difference on the output performance of the battery. Therefore, in order to evaluate the influence of different geometric structure design of flow channel on the performance of PEMFC battery, under the same operating conditions, the traditional serpentine flow channel and parallel flow channel with the same material properties, physical size and flow channel section are selected as the control group. The snowflake-shaped biomimetic flow channel designed in this paper is simulated by CFD. According to the simulation results of the three kinds of flow channels, the conclusion is drawn, and the intuitive cloud chart is generated by the post-processing software CFD-Post. This paper evaluates the output performance, mass output characteristics, structure design characteristics and heat balance of the three different flow channel designs from the aspects of battery power density, current density distribution, pressure drop, reactant and product distribution, heat distribution, etc.

[0183] 3.3.1 PEMFC performance characteristics

[0184] The performance of PEMFC battery is mainly studied by simulation, recording the average current density per unit area under different working voltages, and drawing the V-i polarization curve. The output power of fuel cell is obtained by the product of current and voltage (P=iV), as Figure 7The current density polarization curves and power density curves of three kinds of structure flow channels are compared:

[0185] It can be seen that the performance of the biomimetic flow channel is significantly better than the traditional parallel flow channel, but slightly lower than the traditional serpentine flow channel. The three flow channels have relatively obvious activation loss area, ohmic loss area and concentration difference loss area. At low current density, the output power densities of the three flow channels are similar. With the increase of current density, the performance gap gradually emerges. The three different structure flow channels are analyzed as follows:

[0186] (1) Serpentine flow channel

[0187] As Figure 8 The power density curve and polarization curve of the serpentine flow channel are shown in the following figure:

[0188] The maximum current density of the serpentine flow channel is 1.4040505 A / cm2. According to the variation law of the power density curve, the power density peak value should be between 0.45-0.55 V. The serpentine flow channel has higher current density and better output power performance than the other two flow channel structures.

[0189] (2) Parallel flow channel

[0190] As Figure 9 The power density curve and polarization curve of the parallel flow channel are shown in the following figure:

[0191] The maximum current density of the parallel flow channel is 0.990215 A / cm2. According to the variation law of the power density curve, the power density peak value should be between 0.5-0.6 V. The parallel flow channel has the smallest current density and the worst performance among the three flow channels.

[0192] (3) Biomimetic flow channel

[0193] As Figure 10 The power density curve and polarization curve of the biomimetic flow channel are shown in the following figure:

[0194] The peak current density of the biomimetic flow channel is 1.286289 A / cm2. According to the variation law of the power density curve, the power density peak value should be between 0.45-0.55 V. When the working voltage is 0.5 V, the power density of the biomimetic flow channel is improved by 23.05% compared with the parallel flow channel, and the power density is reduced by 2.61% compared with the serpentine flow channel. Considering the biomimetic structure, the length of the flow channel is longer than that of the two traditional flow channels, and the structure is relatively complex. It is speculated that the insufficient supply of end reactants and the accumulation of water may lead to the decline of the performance of PEMFC. In the following part, the pressure drop and water distribution will be studied.

[0195] 3.3.2 Pressure distribution and pressure drop

[0196] The pressure drop in the PEMFC cell operation is an important factor that needs to be considered in fuel cell design

[0197] It not only affects the pump energy required in the fuel cell operation, but also affects the electrochemical reaction. When the operating voltage is 0.7 V, the three flow channels are in the stable operating area where the current density changes gently. Therefore, the simulation results at 0.7 V operating voltage are selected to compare the pressure drop and pressure distribution of the three flow channels. As shown in Figure 11 is a comparison chart of the system pressure drop of the three flow channels. The system pressure drop is the difference between the inlet and outlet pressures of the anode and cathode. It can be seen that the pressure drop of the serpentine and bionic flow channels is significantly greater than that of the parallel flow channel. The pressure loss in the flow channel is mainly related to the length of the flow channel and the viscous pressure loss affected by the change in flow rate. The parallel flow channel can be regarded as an extreme serpentine flow channel, and its flow channel length is very short, so the pressure drop is small. The bionic flow channel has a significantly increased flow channel length compared to the serpentine flow channel, but the pressure drop is similar (especially in the anode reaction area). It is speculated that the bionic flow channel has more obvious forced convection, which promotes the mass transfer of reactants between the flow channel and the diffusion layer, increases the output power of the cell, reduces the reverse diffusion phenomenon, reduces the water content in the anode flow channel, and reduces the viscous pressure loss through the flow channel, offsetting the adverse effects of flow channel length on pressure drop. Figure 12 is a pressure distribution cloud chart of the three flow channels at 0.7 V operating voltage. It can be seen that the pressure in the flow channel gradually decreases as the flow proceeds, and reaches the minimum pressure value at the outlet.

[0198] 3.3.3 Reactant distribution characteristics

[0199] The transport and distribution of reactants in PEMFC affect the quality of fuel cell mass transfer characteristics, which directly affects the performance of the cell itself. Poor reactant distribution can cause serious power loss, thereby reducing the performance of PEMFC, while relatively good reactant distribution can significantly improve the performance of the fuel cell. To facilitate comparison with the pressure distribution, analyze whether the pressure distribution has an effect on the reactant distribution, and also take 0.7 V operating voltage, respectively, through simulation, the anode hydrogen distribution and cathode oxygen distribution of the three different structure flow channels are obtained, as shown in Figure 13 .

[0200] Since the influence of reactants on the performance of the cell mainly occurs in the diffusion layer (GDL) and the catalyst layer (CL), this paper selects the GDL-CL contact surface to discuss the reactant distribution. Figure 11The mass fraction distribution of reactants on the GDL-CL contact surface of the anode and cathode is shown. As can be seen from the figure, the mass fraction of the reactants in all flow field designs shows a downward trend from the inlet to the outlet as the flow and electrochemical reaction proceed. Among them, the average hydrogen mass fraction of the anode in the biomimetic flow channel is 3.50% lower than that in the serpentine flow channel and 4.75% higher than that in the parallel flow channel; the average oxygen mass fraction of the cathode in the biomimetic flow channel is 0.86% higher than that in the serpentine flow channel and 15.14% higher than that in the parallel flow channel. Observing the image, there is a clear area of low reactant mass fraction on the right side of the parallel flow channel, because during the electrochemical reaction process, liquid water will accumulate, causing the center part of the flow channel to be blocked, and the reactants mainly pass through the peripheral flow channel, so the concentration of the reactants in this area is relatively low. The serpentine flow channel and the biomimetic flow channel both show relatively good uniformity of reactant distribution, especially the biomimetic flow channel, whose reactant mass fraction changes little between flow channels and has good uniformity of distribution. Among them, the serpentine flow channel has a relatively small amount of reactant mass fraction at the outlet, showing a trend of insufficient supply. The biomimetic flow channel is more evenly distributed. Since the serpentine flow channel and the biomimetic flow channel are single-channel through flow channels, there is a clear pressure gradient between adjacent flow channels, and there is a large pressure difference. A large enough pressure difference can drive the sub-ridge convection of the gas, and the flow of the reactants to the sub-ridge of the flow channel helps to reduce the concentration loss during the flow process. The biomimetic flow channel structure is more complex, with more corners, and a pressure difference is easily generated between adjacent flow channels, so the flow channel design of the biomimetic structure has a more obvious sub-ridge convection effect, such as Figure 14 , which makes the concentration of reactants in the sub-ridge area rise and improves the performance of the battery. In conclusion, the use of the snowflake-shaped biomimetic structure design can make the distribution of reactants in the effective area more uniform.

[0201] 3.3.4 Water management and water distribution characteristics

[0202] Water management in PEMFC is an important indicator for evaluating the performance of fuel cells, and water is a key element for maintaining the normal operation of the battery. The uniformity of water distribution in the battery is one of the main factors that determine the stability and life of a fuel cell stack. How to improve the uniformity of water distribution in the bipolar plate and avoid the "waterlogging" phenomenon is one of the main research directions in the improvement of fuel cells. To compare with the pressure distribution and reactant distribution, the simulation results at a working voltage of 0.7V are also analyzed. As Figure 15 is the water mass fraction distribution cloud diagram of the GDL-CL layer of the three flow channels.

[0203] It can be seen that the water distribution of the serpentine flow channel is highly consistent with the pressure distribution. From the inlet to the outlet, water is gradually generated as the reaction proceeds and the reactants flow, and begins to accumulate near the outlet area. The corner area of the serpentine flow channel is relatively blocked by liquid water to some extent compared to the straight area. The water distribution of the parallel flow channel has a very obvious corresponding relationship with the distribution of the reactants. Since the pressure in the center area of the parallel flow channel hardly changes, it cannot form a pressure gradient, and the water generated by the reaction is difficult to discharge in time. The water generated by the reaction accumulates in the center, which in turn affects the distribution of the reactants, thereby reducing the performance of the parallel flow channel. The bionic flow channel shows quite superior water distribution characteristics, with a low overall water mass fraction. This is because the bionic flow channel has many curves, and a pressure gradient is formed between two adjacent flow channels, forming a convection effect that accelerates the flow of reactants,

[0204] The water generated by the reaction is forced to be swept out by the tangential force from the contact surface. Therefore, this structure can improve the overall drainage capacity of the PEMFC and achieve good water distribution uniformity. However, it should be noted that due to the complex structure, the corner vortex area of the bionic flow channel will have some adverse effects on the drainage of the battery. From the inlet area, the water mass coefficient in the corner area will rise to varying degrees, and near the outlet, there will be a certain degree of liquid water accumulation.

[0205] 3.3.5 Thermal distribution characteristics

[0206] In PEMFC, only about 50% of the chemical energy can be converted into electrical energy through electrochemical reaction, and the remaining energy is released as heat in an irreversible manner as the reaction proceeds. It is necessary to analyze the thermal distribution characteristics of the fuel cell to ensure that the PEMFC operates within the appropriate temperature range and to ensure the uniformity of heat distribution, avoiding local heat accumulation and affecting the performance and durability of the fuel cell

[16] . Since the thickness of the MEM layer is very thin, the temperature change on the cathode side CL-MEM is very small, and the CL-MEM contact surface is selected for display. As Figure 16 is the temperature distribution cloud map of the CL-MEM contact surface on the cathode side of the three flow channels at a working voltage of 0.7V:

[0207] Since the fuel cell operates in a relatively closed environment, the heat exchange between the battery and the air is limited, and it is difficult to exchange heat through radiation and convection. The heat in the PEMFC is mainly carried away by the flow of reactants and generated water. From Figure 16It can be seen that, due to the liquid water accumulation in the central part caused by the parallel flow channel, the temperature distribution of the parallel flow channel is the most uneven, and there is a obvious high temperature accumulation area in the central part. The temperature distribution of the serpentine flow channel still presents obvious regularity, that is, the temperature gradually increases from the inlet to the outlet, and there is a high heat area near the outlet area. It can be preliminarily judged that the reaction material is less at the outlet end of the flow channel, and it is difficult to take away the generated water and reaction heat through the flow of reaction material. Compared with the two traditional structure flow channels, the CL-MEM contact surface of the bionic flow channel has lower heat, and the heat distribution is more uniform. Except for the vortex area at the reaction inlet corner, which has a small amount of heat accumulation due to high reaction material concentration and high electrochemical reaction rate, the other parts remain at a relatively low working temperature, and the temperature under the ridge in the non-flow channel area is lower due to the convection effect under the ridge. The structure of the flow channel significantly improves the heat dissipation capacity of the PEMFC, has good heat transfer performance, and reduces the thermal resistance requirement of the fuel cell material.

[0208] Fourth step, experimental conclusion

[0209] The present application establishes a snowflake-shaped bionic flow channel and two kinds of traditional flow channels: serpentine flow channel and parallel flow channel single cell model, and the CFD calculation results are compared and analyzed systematically, and the main conclusions are as follows:

[0210] (1) By comparing the power density and polarization curve of the three flow channels, the serpentine flow channel has the largest peak current density and higher power density, the bionic flow channel is slightly less, and the performance of the parallel flow channel is the worst.

[0211] Output power performance: serpentine flow channel > bionic flow channel > parallel flow channel.

[0212] (2) Through the analysis and research of the pressure distribution of different structure flow field, it can be found that the serpentine flow channel and the bionic flow channel with bending structure have obvious pressure drop, and the pressure distribution of the parallel flow channel is relatively uniform, and the three flow channels all show the overall trend of gradually reducing the pressure from the inlet to the outlet.

[0213] (3) Through the analysis and research of the reaction mass distribution, the reaction material distribution of the serpentine flow channel is regular, but there is a problem of low reaction material mass fraction at the outlet, which may lead to insufficient supply of reaction material at the end; the parallel flow channel has obvious low mass fraction area in the central area, and the reaction material distribution uniformity is poor; the bionic flow channel shows relatively good distribution uniformity, but there is still a certain degree of insufficient supply at the end.

[0214] (4) Through the analysis and research of water distribution, the serpentine flow channel has obvious water distribution regularity, and the overall water distribution is good, but there is some water accumulation at the outlet end; the parallel flow channel has obvious liquid water accumulation in the center, and the distribution uniformity is poor; the bionic flow channel has relatively low water mass fraction, and the water distribution is more uniform.

[0215] (5) Through the analysis of heat distribution, the serpentine flow channel presents obvious regularity, the temperature gradually increases from the inlet to the outlet, and there is obvious heat accumulation at the end; the parallel flow channel has obvious central heat accumulation area, and the heat distribution is poor; the bionic flow channel shows excellent heat distribution characteristics, except for a small amount of heat accumulation in the inlet corner area, there is no obvious large range high temperature area, the overall working temperature is low, and the heat distribution is relatively uniform.

[0216] In summary, the snowflake-shaped bionic flow channel designed in the application has similar performance to the traditional serpentine flow channel when placed horizontally, and has greater performance improvement compared with the traditional parallel flow channel; the large pressure drop is conducive to avoiding water accumulation and improving the material distribution performance. Due to the convection effect under the ridge, the bionic flow channel with large pressure drop and the bionic flow channel have better material distribution characteristics. Among them, due to the more complex structure of the bionic flow channel, the pressure gradient between adjacent flow channels is obvious, resulting in that the bionic flow channel has better material distribution uniformity and drainage performance; the bionic flow channel has a significant advantage in thermal management performance compared with the traditional flow channel, and the overall working temperature is low, and the thermal resistance requirement of the fuel cell material is low.

[0217] Finally, it should be noted that in this document, relational terms such as first and second and the like can merely be used to distinguish one entity or action from another, without necessarily requiring or implying that the entities or actions are in any way mutually exclusive or in any way arranged or ordered in succession or time. Moreover, the terms "comprises", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises a", "comprises", or "comprising" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0218] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments, and the same or similar parts between various embodiments can be referred to each other. For the device disclosed by the embodiment, since it corresponds to the method disclosed by the embodiment, the description is relatively simple, and the related parts can be referred to the method part.

[0219] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A biomimetic proton exchange membrane fuel cell based on snowflake structure comprising a bipolar plate, characterized in that, The flow field on the bipolar plate is divided into four flow channel modules, each of which includes a serpentine channel with an inlet and an outlet, and the serpentine channels of the four flow channel modules are sequentially connected to form a bionic flow channel, which is fully paved on the bipolar plate, and the inlet of the serpentine channel of the first flow channel module forms a flow channel inlet, and the outlet of the serpentine channel of the last flow channel module forms a flow channel outlet. The depth of the bionic flow channel gradually increases from the flow channel inlet to the flow channel outlet; each of the flow channel modules is a right-angled triangular area, and the right-angled sides of the four flow channel modules are spliced with each other to form a square flow field; the bipolar plate is provided with a first partition plate extending along the edge thereof, and the middle part of the bipolar plate is provided with a cross-shaped partition plate, which is used to separate the area in the first partition plate into four right-angled triangular areas.

2. The snowflake structure based biomimetic proton exchange membrane fuel cell of claim 1, wherein, The cross section of the serpentine channel is square.

3. The snowflake structure based biomimetic proton exchange membrane fuel cell as claimed in claim 2, wherein, The width of any cross section of the bionic flow channel is 2 mm, and the depth is 1 mm.

4. The snowflake structure based biomimetic proton exchange membrane fuel cell of claim 1, wherein, The inner side of the first partition plate is provided with a plurality of uniformly spaced first stop blocks, and the cross-shaped partition plate is provided with a plurality of uniformly spaced second stop blocks, the first stop blocks and the second stop blocks in the same right-angled triangular area are parallel and staggered with each other, so that the gap between the first stop blocks and the second stop blocks in each right-angled triangular area forms a serpentine channel.

5. The snowflake structure based biomimetic proton exchange membrane fuel cell as claimed in claim 4, wherein, In the same right-angled triangular area, the end face of the first stop block extending from the first partition plate is parallel to the cross-shaped partition plate, and the end face of each first stop block is spaced from the cross-shaped partition plate by the same distance in the extension direction; in the same right-angled triangular area, the end face of the second stop block extending from the cross-shaped partition plate is parallel to the first partition plate, and the end face of each second stop block is spaced from the first partition plate by the same distance in the extension direction.

6. The snowflake structure based biomimetic proton exchange membrane fuel cell as claimed in claim 1, wherein, It also includes a membrane electrode composed of a gas diffusion layer, a catalyst layer and a proton exchange membrane, and the gas diffusion layer is in contact with the bipolar plate.

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