Design method of large-size flow field plate of cross-medium proton exchange membrane fuel cell

Abstract

The invention discloses a method for designing a large-size flow field plate of a cross-medium proton exchange membrane fuel cell, an established model is composed of a three-dimensional meter and a one-dimensional computational domain, the three-dimensional computational domain comprises a bipolar plate, a gas channel, a gas diffusion layer and expansion layers in an anode and a cathode, and the expansion layers play a role in connecting the anode and the cathode. The microporous layer, the catalyst layer and the proton exchange membrane are simplified into a one-dimensional computational domain which is composed of internal face nodes belonging to a cathode and an anode. The method has higher calculation efficiency, takes into account the processes of mass and heat transfer, electrochemical reaction, membrane water balance and the like in the proton exchange membrane fuel cell, and can accurately simulate the large-size cross-medium working fuel cell with the activation area exceeding 300cm < 2 >. By changing the structural parameters of the flow field plate of the fuel cell, the influence of the channel ridge ratio, the flow field transverse-longitudinal ratio, the flow field fineness and the like on the cell performance, the mass transfer characteristic and the hydrothermal management can be known. Therefore, the optimal cross-medium fuel cell flow field plate structure can be designed.

Description

Technical Field

[0001] The invention belongs to the technical field of fuel cells, and in particular relates to a flow field plate design method for a proton exchange membrane fuel cell under cross-medium working conditions. Background Art

[0002] The application of hydrogen energy provides an effective solution to the current global energy problem and is also one of the important ways to achieve the goals of carbon peak and carbon neutrality. In terms of hydrogen utilization, fuel cells are currently recognized as the most suitable power generation device and have been a research hotspot in recent decades. Among them, proton exchange membrane fuel cells are the key technology for utilizing hydrogen energy, with advantages such as high power density, noiseless operation, fast dynamic response, zero emissions and low operating temperature.

[0003] In terrestrial applications, cathode air supply (hydrogen-air) fuel cells have reached maturity, especially in transportation. The advantages of fuel cells' zero emissions and noiseless operation have made them increasingly recognized for use in closed environments such as underwater and aerospace. In these airless environments, fuel cells usually use pure oxygen as the cathode supply (hydrogen and oxygen). In some application contexts, fuel cells may face the situation of cross-medium operation (cathode gas supply is converted between pure oxygen supply and air supply). For example, for fuel cells used in submarines, the cathode gas supply is more suitable for pure oxygen supply underwater, while it is more suitable for air supply when operating on the surface.

[0004] Under cross-medium operation, the influence of the flow field structure of the fuel cell on the mass transfer characteristics, performance and hydrothermal management in the cell needs to be reconsidered. However, the current designs of flow field plates for hydrogen-air and hydrogen-oxygen fuel cells are mostly carried out independently, without considering these differences in fuel cells under cross-medium operation. If a design method for large-size flow field plates of cross-medium membrane fuel cells is developed, the influence of changes in the structural parameters of the fuel cell flow field plate (including the channel groove-ridge ratio, the flow field aspect ratio and the flow field fineness on the fuel cell performance, mass transfer characteristics and hydrothermal management) can be obtained, thereby designing the optimal cross-medium fuel cell flow field plate structure. Summary of the invention

[0005] In view of the above problems, the purpose of the present invention is to propose a large-size flow field plate design method for a cross-media proton exchange membrane fuel cell, which can obtain the effects of flow channel ridge ratio, flow field aspect ratio and flow field fineness on fuel cell performance, mass transfer characteristics and water and heat management by changing the structural parameters of the fuel cell flow field plate. A comprehensive evaluation index for the cross-media fuel cell structure design is proposed, so as to design the optimal cross-media fuel cell flow field plate structure.

[0006] The technical solution of the present invention is as follows:

[0007] A full-scale local dimensionality reduction single cell model of a cross-media working fuel cell is established. The model consists of a three-dimensional computational domain and a one-dimensional computational domain. The three-dimensional computational domain includes bipolar plates, gas channels, gas diffusion layers, and extended layers in the anode and cathode. The extended layer serves as a data storage layer in the one-dimensional computational domain and plays a role in connecting the anode and cathode. The microporous layer, catalytic layer, and proton exchange membrane are simplified into a one-dimensional computational domain, which is composed of internal surface nodes belonging to the anode and cathode.

[0008] The conservation equations of mass, momentum, composition, energy, liquid water and electron potential inside the fuel cell are solved in the three-dimensional sub-model; the flux conservation equations related to the electrochemical reaction, membrane state, catalytic layer and proton exchange membrane are solved in the one-dimensional sub-model. During the calculation, the scalar value solved in the three-dimensional computational domain provides the boundary conditions for the one-dimensional computational domain. At the same time, the solution results of the one-dimensional computational domain provide the required physical parameters and source terms for the three-dimensional computational domain. Each step of the iterative process realizes data exchange between the two computational domains.

[0009] The specific implementation steps are as follows:

[0010] (1) The electron potential equation solved in the three-dimensional sub-model is solved for all regions; the mass equation, momentum equation, component equation, and energy equation are solved for all fluid regions in the three-dimensional domain; the liquid pressure is only solved in the porous electrode; and the liquid water saturation is only solved in the flow channel.

[0011] (1.1) Mass equation:

[0012]

[0013] (1.2) Momentum equation:

[0014]

[0015] (1.3) Component equation:

[0016]

[0017] (1.4) Energy equation:

[0018]

[0019] (1.5) Liquid pressure equation:

[0020]

[0021] (1.6) Liquid water saturation equation:

[0022]

[0023] (1.7)Electron potential equation:

[0024]

[0025] (2) The flux equation of the one-dimensional part includes:

[0026] (2.1) Component equation:

[0027]

[0028] (2.2) Temperature equation:

[0029]

[0030] (2.3) Hydraulic equation:

[0031]

[0032] (2.4) Membrane water content equation:

[0033]

[0034] (2.5)Electron potential equation:

[0035]

[0036] (2.6) Ionic potential equation:

[0037]

[0038] (3) The electrochemical reaction rate is calculated by the Butler-Volmer equation modified by the agglomeration model:

[0039]

[0040] Furthermore, the reversible voltage is obtained by the Nernst equation and is expressed as:

[0041]

[0042] Therefore, it is necessary to establish model boundary conditions suitable for cross-media operation of fuel cells. In the present invention, the model needs to set different inlet boundary conditions for cross-media operation conditions.

[0043] Using the full-scale local dimensionality reduction single cell model of the cross-media working fuel cell established above, by changing the structural parameters of the fuel cell flow field plate, the effects of the channel groove-ridge ratio, flow field aspect ratio and flow field fineness on the fuel cell performance, mass transfer characteristics and hydrothermal management are obtained.

[0044] The present invention proposes a comprehensive evaluation index for the structural design of a cross-media fuel cell. The evaluation of the working performance of a cross-media fuel cell cannot be limited to the output voltage, but also takes into account the pumping loss and flooding of the fuel cell. Therefore, a comprehensive evaluation index determined by the output voltage, the flow channel pressure drop and the liquid water saturation of the cathode catalyst layer is proposed. According to the proposed comprehensive evaluation index for the structural design of a cross-media fuel cell, the optimal cross-media fuel cell flow field plate structure is designed.

[0045] The characteristics and significance of the present invention are:

[0046] The proposed full-scale local dimension reduction model for PEM fuel cells working across media has higher computational efficiency and takes into account the mass transfer, heat transfer, electrochemical reaction, membrane water balance and other processes in PEM fuel cells. 2 The large-scale cross-media working fuel cell can be accurately simulated. By changing the structural parameters of the fuel cell flow field plate, the effects of the channel ridge ratio, flow field aspect ratio and flow field refinement on the performance, mass transfer characteristics and hydrothermal management of the fuel cell under cross-media operation can be obtained. A comprehensive evaluation index determined by the output voltage, channel pressure drop and liquid water saturation of the cathode catalyst layer is proposed, which can comprehensively consider the performance, pumping loss and flooding problems of the fuel cell under cross-media operation, thereby designing the optimal fuel cell flow field plate structure. This method can greatly reduce the economic and time costs of experimental testing and improve the efficiency of the design process of the flow field plate of cross-media working fuel cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Attached Figure 1 It is a schematic diagram of the principle of a full-scale local dimensionality reduction single cell model of a cross-medium working fuel cell in an example of the invention.

[0048] Attached Figure 2 The activation area is 343 cm in the invention example. 2 Single battery calculation domain.

[0049] Attached Figure 3 The output voltage of the fuel cell in the invention example varies with the channel-ridge ratio.

[0050] Attached Figure 4 The figure shows the variation of the fuel cell output voltage with the aspect ratio of the flow field in the invention example.

[0051] Attached Figure 5 The output voltage of the fuel cell in the invention example varies with the fineness of the flow field.

[0052] Attached Figure 6 The figure shows the changes of the comprehensive evaluation index of the fuel cell in the invention example along with the channel groove-ridge ratio.

[0053] Attached Figure 7 The figure shows the changes of the comprehensive evaluation index of the fuel cell in the invention example along with the aspect ratio of the flow field.

[0054] Attached Figure 8 The figure shows the changes of the comprehensive evaluation index of the fuel cell in the invention example with the refinement of the flow field. DETAILED DESCRIPTION

[0055] The method steps of the present invention are further described below through specific calculation examples. It should be noted that this embodiment is descriptive rather than restrictive, and the protection scope of the present invention is not limited thereto.

[0056] The specific implementation process of the present invention is as follows:

[0057] 1. Establish a full-scale local dimensionality reduction single cell model of a cross-media working fuel cell. Figure 1 The figure shows the schematic diagram of the model principle. The model consists of a three-dimensional computational domain and a one-dimensional computational domain. The three-dimensional computational domain includes bipolar plates, gas channels, gas diffusion layers, and extended layers in the anode and cathode. The extended layer serves as a data storage layer for the one-dimensional computational domain and plays a role in connecting the cathode and anode. The microporous layer, catalytic layer, and proton exchange membrane are simplified into a one-dimensional computational domain, which is composed of internal surface nodes belonging to the cathode and anode. Figure 2 The activation area for the construction is 343cm 2 Single battery calculation domain.

[0058] The conservation equations of mass, momentum, composition, energy, liquid water and electron potential inside the fuel cell are solved in the three-dimensional sub-model; the flux conservation equations related to the electrochemical reaction, membrane water, etc. and the catalyst layer and proton exchange membrane are solved in the one-dimensional sub-model.

[0059] Specific implementation steps:

[0060] (1) The electron potential equation solved in the three-dimensional sub-model is solved for all regions; the mass equation, momentum equation, component equation, and energy equation are solved for all fluid regions in the three-dimensional domain; the liquid pressure is only solved in the porous electrode; and the liquid water saturation is only solved in the flow channel.

[0061] (1.1) Mass equation:

[0062]

[0063] (1.2) Momentum equation:

[0064]

[0065] (1.3) Component equation:

[0066]

[0067] (1.4) Energy equation:

[0068]

[0069] (1.5) Liquid pressure equation:

[0070]

[0071] (1.6) Liquid water saturation equation:

[0072]

[0073] (1.7)Electron potential equation:

[0074]

[0075] The above conservation equations are partial differential equations, which are solved by the finite volume method. First, the conservation equations are discretized using the finite volume method, the entire computational domain is divided into several control volumes (grid units), and the corresponding conservation equations are solved in each control volume. When solving, the partial differential equations in the grid units need to be converted into linear algebraic equations, and then solved step by step using an iterative method. Each iteration updates the variables such as flow rate, pressure, temperature, and concentration in the conservation equations until the convergence criteria are met.

[0076] (2) The flux equation of the one-dimensional part includes:

[0077] (2.1) Component equation:

[0078]

[0079] (2.2) Temperature equation:

[0080]

[0081] (2.3) Hydraulic equation:

[0082]

[0083] (2.4) Membrane water content equation:

[0084]

[0085] (2.5)Electron potential equation:

[0086]

[0087] (2.6) Ionic potential equation:

[0088]

[0089] In the one-dimensional computational domain, the conservation equations such as gas component concentration, temperature, and hydraulic pressure solved in the three-dimensional computational domain are converted into flux equations for one-dimensional nodes. In addition, equations related to the catalyst layer and proton exchange membrane, such as electrochemical reactions and membrane water, are also described as flux equations. In the flux equation, only the diffusion effect along the thickness direction is considered. The flux equation in the one-dimensional computational domain is also solved step by step in each iteration step through an iterative method. During the calculation, the scalar value solved in the three-dimensional computational domain provides the boundary conditions for the one-dimensional computational domain. At the same time, the solution results of the one-dimensional computational domain provide the required physical parameters and source terms for the three-dimensional computational domain. Each iteration process realizes data exchange between the two computational domains.

[0090] (3) The electrochemical reaction rate is calculated by the Butler-Volmer equation modified by the agglomeration model:

[0091]

[0092]

[0093] In this example, R=8.314J mol -1 K -1 , F = 96487.0 C mol -1 , α a =0.5,α c =0.5, A im =4.0×10 7 m -1 , The remaining relevant parameters are obtained in each iteration step of step (2), thereby calculating the electrochemical reaction rate of the one-dimensional node.

[0094] Furthermore, the reversible voltage is obtained by the Nernst equation and is expressed as:

[0095]

[0096] In this example, T ref =298.5K, P ref =101325Pa, and the remaining parameters are obtained in each iteration step of step (2), thereby calculating the reversible voltage of the one-dimensional node of the fuel cell.

[0097] 2. Establish the model boundary conditions for the cross-medium operation of the fuel cell. The model needs to set different inlet boundary conditions for the cross-medium working conditions. The inlet boundary conditions of the gas flow channel are the mass flow inlet and the outlet is the pressure outlet. When the fuel cell is in the hydrogen-air mode, the mass flow calculation formula of the inlet is as follows:

[0098]

[0099] When the fuel cell is in hydrogen and oxygen mode, the anode inlet boundary conditions remain unchanged, and the cathode inlet mass flow rate is calculated using the following formula:

[0100]

[0101] In this example, ST a =1.5, ST c =2.5, RH a =RH c =0.4, P sat =31116.6Pa,

[0102]

[0103] I=20000A m -2 . Thus, we can calculate,

[0104] 3. Using the established full-scale local dimensionality reduction single cell model and boundary conditions of a cross-media working fuel cell, the effects of the flow field plate structural parameters on the fuel cell performance, mass transfer characteristics and hydrothermal management, including the channel groove-ridge ratio, flow field aspect ratio and flow field fineness, can be obtained.

[0105] As an example, four flow fields with flow channel groove-ridge ratios of 0.7, 1.0, 1.5 and 2.0 were built. When the flow channel groove-ridge ratio was changed, only the groove width and ridge width were changed, and other structural parameters and operating conditions remained the same.

[0106] Attached Figure 3 The output voltage of the fuel cell in the two modes is shown as a function of the groove-ridge ratio. As the groove-ridge ratio increases, the output voltage in the hydrogen-air mode increases significantly, but the increasing trend slows down at a larger groove-ridge ratio; while in the hydrogen-oxygen mode, as the groove-ridge ratio increases, the output voltage decreases slightly. The results show that increasing the groove-ridge ratio can significantly improve the performance of the fuel cell in the hydrogen-air mode, but the reduction in the performance of the fuel cell in the hydrogen-oxygen mode must be considered.

[0107] Furthermore, four flow fields with aspect ratios of 0.4, 0.6, 0.8 and 1.1 were built. In order to control the variables and exclude the influence of the change of the distribution area shape on the simulation results when changing the aspect ratio of the flow field, it is assumed that the inlet and outlet are uniformly inlet / exhaust. When changing the aspect ratio of the flow field, only the number and length of the flow channels are changed, and other structural parameters and operating conditions remain the same.

[0108] Attached Figure 4The output voltage of the fuel cell in the two modes is shown to change with the aspect ratio of the flow field. As the aspect ratio decreases, the output voltage in the hydrogen-air mode increases, and the increasing trend increases at a smaller aspect ratio; while in the hydrogen-oxygen mode, the output voltage remains almost unchanged as the aspect ratio changes. The results show that the elongated flow field structure is beneficial to improving the flow and mass transfer characteristics in the fuel cell and improving the performance of the fuel cell.

[0109] Furthermore, four flow fields with different fineness were built, with the number of flow channels being 60, 72, 90, and 120. When changing the fineness of the flow field, only the number of flow channels, the groove width, and the ridge width were changed, and the other structural parameters and operating conditions remained the same.

[0110] Attached Figure 5 The changes of fuel cell output voltage with the refinement of flow field in two modes are shown. With the increase of flow field refinement, the output voltage in hydrogen-air mode increases significantly, but the increasing trend slows down when the number of flow channels is large; while in hydrogen-oxygen mode, with the increase of flow field refinement, the output voltage does not increase significantly. The results show that the refinement of flow field is conducive to improving the flow and mass transfer characteristics in the fuel cell, and significantly improves the performance of the fuel cell in hydrogen-air mode.

[0111] 4. The comprehensive evaluation index θ of the cross-media fuel cell structure design proposed in the present invention is not limited to the output voltage, but also takes into account the pumping loss and flooding of the fuel cell. Therefore, a comprehensive evaluation index determined by the output voltage, the flow channel pressure drop and the liquid water saturation of the cathode catalyst layer is proposed, and its calculation formula is as follows:

[0112]

[0113] Take the working ratio of the fuel cell in the hydrogen-air mode as 0.5, the flow field plate structure with a groove-ridge ratio of 0.7, an aspect ratio of 0.6, and a flow channel number of 90 as an example. air =t ox =0.5, and V is calculated in step 3 1,air =0.396V, V 1,ox =0.592V, ΔP 1,air =12372.83Pa,ΔP 1,ox =2195.37Pa, In this example, V 0,air =0.454V,V 0,ox =0.590V, ΔP 0,air =11374.59Pa,ΔP 0,ox =1922.52Pa, and the values ​​of weight indexes a, b, and c are respectively 1, 1, and 2. The comprehensive evaluation index θ calculated under these working conditions and structural parameters is 0.937.

[0114] Further, according to the calculation results of step 3, the comprehensive evaluation index θ is obtained as the fuel cell channel groove-ridge ratio, flow field aspect ratio and flow field refinement change under different hydrogen-air / hydrogen-oxygen operating ratios.

[0115] Attached Figure 6 The comprehensive evaluation index θ of the fuel cell changes with the channel groove-ridge ratio under different hydrogen-air / hydrogen-oxygen operation ratios. The results show that under different hydrogen-air / hydrogen-oxygen operation ratios, the comprehensive evaluation index increases with the increase of the groove-ridge ratio, but the increase trend slows down at a larger groove-ridge ratio, which is caused by the increase of output voltage and the decrease of liquid water in the catalyst layer when the groove-ridge ratio increases. At the same time, under the same groove-ridge ratio, the comprehensive evaluation index is greater when the fuel cell operates in the hydrogen-air mode at a greater ratio, which is due to the greater liquid water content in the catalyst layer in the hydrogen-oxygen mode, resulting in a higher risk of flooding.

[0116] Attached Figure 7 The comprehensive evaluation index θ of the fuel cell changes with the aspect ratio of the flow field under different hydrogen-air / hydrogen-oxygen operating ratios. The results show that under different hydrogen-air / hydrogen-oxygen operating ratios, the comprehensive evaluation index increases with the increase of the aspect ratio, which is mainly caused by the decrease of the flow channel pressure drop when the aspect ratio increases. At the same time, under the same aspect ratio, the comprehensive evaluation index is greater when the operating ratio of the fuel cell in the hydrogen-air mode is greater.

[0117] Attached Figure 8 The comprehensive evaluation index θ of the fuel cell changes with the refinement of the flow field under different hydrogen-air / hydrogen-oxygen operating ratios. The results show that when the operating ratio of the hydrogen-air mode is larger, the comprehensive evaluation index increases first and then decreases with the increase of refinement, while when the operating ratio of the hydrogen-air mode is smaller, the comprehensive evaluation index gradually decreases with the increase of refinement. This is because the increase in the refinement of the flow field increases the output voltage in the hydrogen-air mode, but at the same time increases the flow channel pressure drop.

[0118] According to the variation law of the comprehensive evaluation index θ, the optimal cross-medium fuel cell flow field plate structure can be designed. In this example, the operation ratio t air =0.75 as an example, in the fuel cell structure design process, the number of flow channels should be selected as 72. Considering the difficulty of processing the flow field plate and the manifold, a larger flow channel groove-ridge ratio and flow field aspect ratio should be selected as much as possible.

Claims

1. A design method for a large-size flow field plate of a cross-medium proton exchange membrane fuel cell, characterized in that: A full-scale local dimensionality reduction single-cell model of a cross-media working fuel cell is established. The model consists of a three-dimensional computational domain and a one-dimensional computational domain. The three-dimensional computational domain includes bipolar plates, gas channels, gas diffusion layers, and extended layers in the anode and cathode. The extended layer serves as a data storage layer in the one-dimensional computational domain and plays a role in connecting the anode and cathode. The microporous layer, catalytic layer, and proton exchange membrane are simplified into a one-dimensional computational domain, which is composed of internal surface nodes belonging to the anode and cathode. The conservation equations of mass, momentum, composition, energy, liquid water and electron potential inside the fuel cell are solved in the three-dimensional sub-model; the flux conservation equations related to electrochemical reaction, membrane water, electroosmotic drag and catalyst layer and proton exchange membrane are solved in the one-dimensional sub-model. During the calculation, the scalar value solved in the three-dimensional computational domain provides the boundary conditions for the one-dimensional computational domain. At the same time, the solution results of the one-dimensional computational domain provide the required physical parameters and source terms for the three-dimensional computational domain. Each iterative process realizes data exchange between the two computational domains. The specific implementation steps are as follows: (1) The electron potential equation solved in the three-dimensional sub-model is solved for all regions; the mass equation, momentum equation, component equation, and energy equation are solved for all fluid regions in the three-dimensional domain; the liquid pressure is only solved in the porous electrode; Liquid water saturation is only solved in the flow channel. (1.1) Mass equation: (1.2) Momentum equation: (1.3) Component equation: (1.4) Energy equation: (1.5) Liquid pressure equation: (1.6) Liquid water saturation equation: (1.7)Electron potential equation: Where ρ is the density, is the surface velocity vector, s is the liquid saturation, P is the pressure, μ is the kinematic viscosity, Y is the mass fraction of the substance, D is the gas diffusion coefficient, C p is the specific heat capacity, T is the temperature, k is the thermal conductivity, K is the intrinsic permeability, k lw is the liquid phase relative permeability, κ ele is the electronic conductivity, φ ele is the electron potential, S m is the mass source term, S u is the momentum source term, S i is the mass source term of component i, S T is the heat source term, S lw is the hydraulic source term, S ele is the electronic potential source term, the subscript g represents gas, mix represents gas-liquid mixture, i represents gas type, including hydrogen, oxygen, and water vapor, lw represents liquid water, ele represents electricity, and the superscript eff represents effective value. (2) The flux equation of the one-dimensional part includes: (2.1) Component equation: (2.2) Temperature equation: (2.3) Hydraulic equation: (2.4) Membrane water content equation: (2.5)Electron potential equation: (2.6) Ionic potential equation: Where n=0 and 1 represent two adjacent layers respectively. When solving in the microporous layer, 0 represents the one-dimensional node at the interface between the gas diffusion layer and the microporous layer, and 1 represents the one-dimensional node at the interface between the microporous layer and the catalytic layer. When solving in the catalytic layer, 0 represents the one-dimensional node at the interface between the microporous layer and the catalytic layer, and 1 represents the one-dimensional node at the interface between the microporous layer and the proton exchange membrane. δ is the thickness, is the gas molar concentration, P1 is the liquid pressure, ρ1 is the liquid density, μ1 is the liquid kinematic viscosity, k1 is the liquid relative permeability, EW is the equivalent of dry ionomer, λ is the membrane water content, φ ion is the ion potential, S1 is the liquid pressure source term, S mw is the membrane water source term, S ion is the ion potential source term, the subscript im represents ionomer, ion represents ion, mw represents membrane water, i represents gas type, including hydrogen, oxygen, and water vapor, and eff represents effective value. (3) The electrochemical reaction rate is calculated by the Butler-Volmer equation modified by the agglomeration model: Where j represents the electrochemical reaction rate, i represents the exchange current density, A represents the specific area, θ T represents the temperature correction factor, R represents the universal gas constant, H represents the Henry coefficient, F represents the Faraday constant, α represents the transfer coefficient, η represents the overpotential gas transport resistance, and R local represents the local gas transport resistance, the superscript ref represents the reference value, eff represents the effective value, the subscript a represents the anode, c represents the cathode, pt represents the platinum loading, H2 represents hydrogen, and O2 represents oxygen. Furthermore, the reversible voltage is obtained by the Nernst equation and is expressed as: Where ΔS represents the entropy change of the electrochemical reaction, represents the inlet hydrogen pressure, and represents the oxygen pressure, T ref represents the reference temperature, P ref represents the reference pressure, the subscript a represents the anode, and c represents the cathode.

2. The method for designing a large-size flow field plate for a cross-media proton exchange membrane fuel cell according to claim 1, characterized in that: The model boundary conditions for the cross-medium operation of the fuel cell are established. The model needs to set different inlet boundary conditions for the cross-medium working conditions. The inlet boundary conditions of the gas flow channel are the mass flow inlet and the outlet is the pressure outlet. When the fuel cell is in the hydrogen-air mode, the mass flow calculation formula of the inlet is as follows: When the fuel cell is in hydrogen-oxygen mode, the anode inlet boundary conditions remain unchanged, and the cathode inlet mass flow rate is calculated using the following formula: Where ST is the stoichiometric ratio, A act is the active area, I is the average current density, A in is the inlet cross-sectional area, P g,in is the inlet gas pressure, T in is the inlet gas temperature, ρ g is the gas density, C in is the inlet volume molar concentration, RH is the relative humidity, P sat is the water saturation pressure, the superscript O2 represents oxygen, H2 represents hydrogen, air represents hydrogen-air mode, ox represents hydrogen-oxygen mode, a represents anode, and c represents cathode.

3. The method for designing a large-size flow field plate for a cross-media proton exchange membrane fuel cell according to claim 1, characterized in that: Using the established full-scale local dimensionality reduction single cell model of the cross-media working fuel cell, by changing the structural parameters of the fuel cell flow field plate, the effects of the channel groove-ridge ratio, flow field aspect ratio and flow field fineness on the fuel cell performance, mass transfer characteristics and hydrothermal management are obtained.

4. The method for designing a large-size flow field plate for a cross-media proton exchange membrane fuel cell according to claim 1, characterized in that: A comprehensive evaluation index for the structural design of a cross-media fuel cell is proposed. The performance of a cross-media fuel cell is not limited to the output voltage, but also takes into account the pumping loss and flooding of the fuel cell. Therefore, a comprehensive evaluation index determined by the output voltage, the flow channel pressure drop and the liquid water saturation of the cathode catalyst layer is proposed. The calculation formula is as follows: The time coefficient t air represents the operating time ratio of the fuel cell in hydrogen-air mode, t ox represents the operating time ratio of the fuel cell in the hydrogen-oxygen mode, and the sum of the two should be 1, V represents the output voltage of the fuel cell, ΔP represents the flow channel pressure drop of the fuel cell, Sw represents the liquid water saturation of the cathode catalyst layer of the fuel cell, a is the weight index of the output voltage, b is the weight index of the flow channel pressure drop, and c is the weight index of the liquid water saturation of the catalyst layer. The subscript 0 represents the initial parameter, and 1 represents the corresponding parameter when the structural parameter changes. For a cross-medium working fuel cell, the larger the evaluation index, the better the overall performance of the fuel cell. Furthermore, it is obtained that the comprehensive evaluation index of the fuel cell changes with the groove-ridge ratio, aspect ratio and flow field refinement of the fuel cell under different hydrogen-air / hydrogen-oxygen operating ratios. According to the change law of the obtained comprehensive evaluation index, the optimal cross-medium fuel cell flow field plate structure can be designed.

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