Fuel cell water-gas transmission model based on breakthrough pressure and blockage degree and application of fuel cell water-gas transmission model

Abstract

The invention relates to a fuel cell water vapor transmission model based on breakthrough pressure and clogging degree and application thereof. According to the model, the condition of invalid holes is considered, the concept of water clogging degree is introduced and used for replacing traditional water saturation, the concept of breakthrough pressure is defined and combined with the water clogging degree to form a new PEMFC model, and finally, the water-gas transmission process of a gas diffusion layer in the fuel cell is deeply clarified. The model is suitable for water-gas transmission and gas-liquid transmission processes in porous media in micro-reactor devices such as hydrogen fuel cells, electrolyzed water, methanol fuel cells and reversible fuel cells, and has the advantages of accurate and reliable results and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and particularly relates to a fuel cell water-gas transport model based on breakthrough pressure and blockage degree and its application. Background Art

[0002] As one of the main hydrogen energy conversion devices, proton exchange membrane fuel cells (PEMFCs) can directly convert chemical energy into electrical energy, and have the advantages of high efficiency, high volume energy ratio, light weight, zero carbon emissions, etc., and have great application potential in fields such as transportation and low altitude. The power generation principle of fuel cells is to use hydrogen and oxygen in the air to undergo a chemical reaction to convert into water and output energy. With long-term and high-power electrical energy output, a large amount of liquid water will be generated inside the fuel cell. If these excess liquid waters cannot be discharged in time, it will hinder the reaction gas from reaching the reaction site, resulting in a decline in the performance of the fuel cell.

[0003] The coupling of the internal drainage and gas supply processes of fuel cells makes water-gas management very complex. The cathode gas diffusion layer (GDL), as a gradient porous hydrophobic component, can greatly improve the water management ability of fuel cells. How to accurately analyze the influencing factors of water-gas transport at the GDL and its interface with the catalyst layer is the key to the performance iteration of fuel cells. For the internal water-gas transport process of fuel cells, the traditional capillary force drainage mechanism for gas-liquid two-phase flow is a commonly used calculation method. However, the capillary force drainage model in this method only considers the drainage of water from the pores of the GDL, while ignoring the capillary force that needs to be overcome when water is generated from the catalyst layer and enters the pores of the GDL. In addition, the water saturation concept in the traditional capillary force drainage model has defects for microporous structures, resulting in the calculated theoretical value of water-gas transport deviating from the actual value. Therefore, it is necessary to establish a more accurate theoretical model to analyze and evaluate the internal water-gas transport process of fuel cells. Summary of the Invention

[0004] The main purpose of the present invention is to solve the problems of inaccurate results and deviation from the actual situation existing in the existing models when calculating the internal water-gas transport process of fuel cells, construct a new fuel cell water-gas transport model based on breakthrough pressure and blockage degree, and use this model to evaluate the water-gas transport process of fuel cells. The results are more accurate and reliable, which is helpful for the research and development improvement of fuel cells. To achieve the above object, the technical solutions provided by the present invention are as follows:

[0005] A fuel cell water-gas transport model based on breakthrough pressure and blockage degree, and the establishment process of this model includes: establishing the concepts of breakthrough pressure and water blockage degree, and therefrom deriving the gas pressure P of the fuel cell gas and the relationship with the blockage degree and the concentration overpotential η conc and the relationship with the blockage degree, and using this relationship to clarify the water-gas transport process in the gas diffusion layer inside the fuel cell.

[0006] Furthermore, the liquid water pressure P at the inlet of the MPL pores water exceeds the critical value, enabling liquid water to enter and be transported within the pores. This critical value is defined as the breakthrough pressure P b .

[0007] Even further, the calculation formula for the breakthrough pressure P b is as follows:

[0008] P b = P c + P gas

[0009]

[0010] where r is the pore diameter of the MPL pores, σ is the surface tension of the liquid water, θ is the contact angle of the MPL surface, P c represents the capillary force at the inlet of the MPL pores, and P gas represents the gas inlet pressure within the MPL pores.

[0011] Furthermore, the water blockage degree is defined as the ratio of the volume of the blocked pores in the MPL to the total pore volume when the liquid water pressure generated by the CL inside the fuel cell reaches the breakthrough pressure P b .

[0012] Furthermore, during the modeling process, the cracks or pores of the MPL at the CL interface are divided into large pores r1 and small pores r2, and the corresponding breakthrough pressures are denoted as P b1 and P b2 respectively; the liquid water generated in the CL accumulates at the CL-MPL interface, increasing the liquid water pressure. The liquid water preferentially enters the large pores r1 of the MPL at the CL interface. As the liquid water pressure increases to the breakthrough pressure P b2 of the small pores r2, the liquid water will enter and block the small pores r2, affecting the water-vapor transport.

[0013] Even further, when the liquid water pressure P water < the capillary force P c2 of the small pores r2, the liquid water cannot enter the small pores r2, and the gas can be transported within the small pores r2. At this time, the small pores r2 are effective pores; when the liquid water pressure P water is between the capillary force P c2 of the small pores r2 and the breakthrough pressure P b2 , the liquid water enters and blocks the small pores r2, and both the liquid water and the gas cannot be transported; when the liquid water pressure P water > the breakthrough pressure P b2 of the small pores r2, the liquid water will block the small pores r2 and be transported within them, and at this time the gas cannot be transported.

[0014] Furthermore, let P water = P b2 An equation relating the gas pressure P gas to the small hole r2 was established, and using this equation, the relationship between P gas and P b2 was obtained. Finally, a function relationship between the gas pressure P gas and the degree of blockage was established as follows:

[0015]

[0016] where r1 and r2 are the radii of the large and small holes respectively, i represents the current density, F represents the Faraday constant, represents the molecular weight of water, μ lq is the dynamic viscosity of liquid water, ΔL is the distance between the two seepage cross-sections, ρ lq represents the density of liquid water, K is the permeability of the MPL porous medium, and ε is the porosity of the seepage through the large holes in the porous medium.

[0017] Furthermore, K is proportional to the square of the pore diameter d. The solution process for the proportionality coefficient ξ between the two is as follows: First, the true permeability of the GDL sample is measured using a full-functional membrane pore size analyzer, then the pore size distribution curve is obtained by fitting using a neural network algorithm based on the pore size distribution of the GDL, and finally, the coefficient ξ is obtained by the segmentation method.

[0018] Furthermore, using the gas pressure P gas as a bridge, the relationship between the fuel cell gas pressure P gas and the concentration overpotential η conc was established through a zero-dimensional steady-state isothermal two-phase flow model:

[0019]

[0020] where R is the gas constant, T is the thermodynamic temperature during fuel cell operation, n is the reaction electron transfer coefficient, α is the transport coefficient, J L represents the limiting current, D MPL represents the diffusion coefficient of the MPL, is the 1.5th power of the GDB porosity ε GDB , Cd L-pore is the degree of blockage of the large pore r1, Cd S-pore is the degree of blockage of the small pore r2, C1 is the concentration when the gas just reaches the MPL, and L MPL is the thickness of the MPL.

[0021] The second object of the present invention is to provide the application of the above fuel cell water and gas transport model based on breakthrough pressure and blockage degree in hydrogen fuel cells, water electrolysis, methanol fuel cells, and reversible fuel cells.

[0022] Compared with the prior art or models, the progressive effects of the present invention are mainly reflected in the following aspects:

[0023] (1) The concept of water blockage degree is creatively introduced, and it is used to replace the traditional water saturation. At the same time, the concept of breakthrough pressure is defined, and a PEMFC model based on breakthrough pressure and blockage degree is proposed accordingly. The new model more accurately clarifies the water and gas transport process in the gas diffusion layer inside the fuel cell.

[0024] (2) The model proposed by the present invention is widely applicable to micro reaction devices such as hydrogen fuel cells, water electrolysis, methanol fuel cells, and reversible fuel cells, and can accurately analyze and evaluate the water and gas transport and gas-liquid transport processes in different porous media, and the results are highly credible.

[0025] (3) Experimental results show that the calculation results of the fuel cell water and gas transport model based on breakthrough pressure and blockage degree provided by the present invention are more accurate, very close to the actual situation, and are beneficial to the development of the fuel cell industry. Description of the Drawings

[0026] Figure 1 It is a schematic diagram of the transport of liquid water in a hydrophobic porous medium.

[0027] Figure 2 It is a schematic diagram of the comparison between the water blockage degree proposed by the present invention and the traditional water saturation.

[0028] Figure 3 It is a schematic diagram of the transport of liquid water generated in the fuel cell catalyst layer to the MPL.

[0029] Figure 4 It is a comparison diagram of the pore size distribution curve simulated by the neural network algorithm and the pore size distribution curve of SGL28BC.

[0030] Figure 5 It is a curve graph of the simulated calculation results and measured values of SGL28BC and SGL22BB.

[0031] Figure 6 It is a curve graph of the simulated calculation results and measured values of JNTG21-A6L and JNTG21-A6L.

[0032] Figure 7 It is a curve graph of the simulated calculation results and measured values of the model of the present invention and the traditional capillary force model. Detailed Embodiments

[0033] To enable those of ordinary skill in the art to fully understand the technical solutions and beneficial effects of the present invention, the following further detailed description is provided in conjunction with specific embodiments and the accompanying drawings. It should be emphasized that the embodiments listed below are all preferred embodiments of the present invention rather than all of them, and any simple improvements made on this basis should fall within the protection scope of the present invention.

[0034] A proton exchange membrane fuel cell mainly includes a proton exchange membrane, cathode and anode catalyst layers (CL), cathode and anode gas diffusion layers (GDL), flow channels, and seals, etc. Among them, the GDL generally includes a substrate layer (GDB) and a microporous layer (MPL) in two parts, and the MPL is in contact with the CL. The thickness of the GDL is 100 - 300 microns, the pore size distribution is 20 - 200 microns, the thickness of the MPL is 10 - 60 microns, and the pore size distribution is 10 - 100 nm. The micropores in the MPL are interconnected in the thickness direction and have relatively low connectivity with each other in the direction perpendicular to the thickness direction. Considering that the MPL has a relatively large weight in the influence of water vapor transport at the GDL and its interface with the CL, the present invention focuses on introducing the water vapor transport mechanism of the MPL.

[0035] To improve the output performance of the fuel cell, the usual measure is to increase the inlet pressure of the cathode and anode. During long - term and high - power operation, a large amount of liquid water will be generated inside the fuel cell. If this liquid water cannot be discharged in time, it will hinder the reaction gas from reaching the reaction site, resulting in a decline in battery performance. Both the CL and MPL of the fuel cell have a microporous structure, and their drainage behavior conforms to the capillary force drainage mechanism.

[0036] The liquid water generated by the CL must overcome the capillary pressure in the micropores of the MPL and the gas inlet pressure to successfully enter the MPL and be effectively discharged. The liquid water discharged from inside the fuel cell generally goes through the following two steps: In the first step, the liquid water is generated in the CL and enters the hydrophobic MPL from the hydrophilic CL. In this process, the liquid water needs to overcome the capillary pressure in the micropores of the MPL and the gas inlet pressure. In the second step, the liquid water entering the MPL enters the large pores from the small pores under the action of capillary force, realizing the discharge of the liquid water.

[0037] The breakthrough pressure proposed by the present invention, which is the first step of the liquid water discharged from inside the fuel cell, is defined as when the water pressure P water at the pore entrance of the MPL exceeds the critical value, the liquid water can enter the pores and be transported therein, and this critical value is called the breakthrough pressure. The capillary force P c and the breakthrough pressure P b The calculation formula is:

[0038]

[0039] P b = P c + Pgas (2)

[0040] where r is the pore diameter of the MPL pores, σ is the surface tension of liquid water, θ is the contact angle on the MPL surface, and P c represents the capillary force at the entrance of the MPL pores, and P b represents the breakthrough pressure, and P gas represents the gas intake pressure inside the MPL pores.

[0041] When liquid water is generated in the CL and is ready to flow into the MPL, the pressure at the entrance of the MPL pores must be greater than the sum of the capillary force and the gas intake pressure at this pore diameter, so that the liquid water in the CL can smoothly enter the pores of the MPL, and under capillary action, it flows from the small pores of the MPL into the large pores of the MPL until it is completely discharged.

[0042] Due to the porous structure of the GDL being interconnected in the thickness direction but having low connectivity with each other perpendicular to the thickness direction, once liquid water enters the pore channels of the GDL, the back end of this pore diameter will be regarded as an ineffective pore regardless of the presence or absence of liquid water, and the reactive gas cannot cross the liquid water in the pore diameter to enter the reaction site for reaction. Specifically, as Figure 2 shown, the pore channels filled or half-filled with liquid water cannot conduct gas transmission and are regarded as ineffective pores; while the pore channels without liquid water and capable of conducting gas transmission are regarded as effective pores.

[0043] The traditional water saturation is defined as the ratio of the volume of liquid water occupied in the porous medium to the total pore volume. This definition obviously cannot distinguish ineffective pores, resulting in a deviation between the theoretical calculation and the actual situation. Therefore, the present invention proposes the concept of water blockage degree, which is defined as the pressure P of the liquid water generated in the CL inside the fuel cell water reaching the breakthrough pressure P b when the ratio of the volume of blocked pores in the MPL to the total pore volume. The water blockage degree takes into account the situation of ineffective pores and can replace the traditional water saturation definition in the calculation, thereby improving the accuracy of the theoretical calculation.

[0044] The difference between the water blockage degree described in the present invention and the traditional water saturation is as Figure 2 shown. Figure 2The figure below is a schematic diagram of the cross section of the MPL, where the black vertical bars represent the MPL solid phase, and the white vertical bars represent the large pores R1 and small pores R2, respectively. In this diagram, the pore volume of the large pores is four times the pore volume of the small pores (V) (i.e., 4V). In case 1, six small pores on the left are blocked by liquid water. Calculations show that the traditional water saturation and water blockage are both 0.3 (i.e., V*6 / (V*8+4V*3)). In case 2, four small pores are completely blocked by liquid water, and one large pore is half blocked by liquid water. The traditional water saturation is 0.3 (i.e., (V*4+4V*0.5) / (V*8+4V*3)), while the blockage is 0.4 (i.e., (V*4+4V*1) / (V*8+4V*3)). This is because although the large pores are half blocked by liquid water, they have actually lost their gas transmission capacity, resulting in a large deviation between the two calculated results. Similarly, in case 3, the traditional water saturation is still 0.3 (i.e., (V*2+4V*2*0.5) / (V*8+4V*3)), while the blockage is 0.5 (i.e., (V*2+4V*2) / (V*8+4V*3)). In summary, for the cases 2 and 3 where liquid water occupies part of the pore but not the entire pore, this unfilled state is when the liquid water pressure is between the capillary pressure and the breakthrough pressure. At this point, water can enter the pore but cannot be transported within it. The pore cannot transport gas and should be considered an ineffective pore. However, the traditional definition of water saturation cannot distinguish between these two pores and still assumes that they have the ability to transport gas. This is clearly inconsistent with the facts and leads to deviations in the calculation results. By comparison, it can be seen that our proposed concept of water blockage can indeed distinguish between ineffective and effective pores, while the traditional concept of water saturation cannot.

[0045] After establishing the concepts of water blockage and breakthrough pressure, we attempted to improve and optimize the traditional capillary force drainage mechanism model and establish a fuel cell water vapor transport model based on breakthrough pressure and blockage. The specific reasoning process is as follows:

[0046] Figure 3 Schematic diagram of the transfer of liquid water generated in the CL to the MPL. As the reaction proceeds, the liquid water generated in the CL accumulates at the interface between the CL and MPL, causing an increase in the liquid water pressure. This liquid water preferentially enters the cracks at the MPL interface and the larger pores r1. As the liquid water pressure increases to the breakthrough pressure P of the small pores r2, b2 Afterwards, the liquid water will enter and block the small pores r2, affecting the water vapor transmission.

[0047] The transmission of liquid water in different pore sizes faces the following situations: ① When the liquid water pressure P water <Capillary force P of small hole r2 c２When the liquid water cannot enter the small pore r2 while the gas can be transported in the small pore r2, the small pore r2 is an effective pore at this time; when the liquid water pressure P water is between the capillary force P c2 of the small pore r2 and the breakthrough pressure P b2 when the liquid water enters and blocks the small pore r2, and at this time neither the liquid water nor the gas can be transported; when the liquid water pressure P water > the breakthrough pressure P b2 of the small pore r2, the liquid water will block the small pore r2 and be transported therein, and at this time the gas cannot be transported. Among them, P c1 , P c2 , P b1 , P b2 are the capillary pressures and breakthrough pressures of the large and small pore channels respectively, and their calculation formulas can be seen in the above formulas (1)-(2).

[0048] The transport of liquid water satisfies Darcy's law:

[0049] <gh_stars>0

[0050] A = πr 2 (4)

[0051]

[0052] where Q is the flow rate through the porous medium, K is the permeability of the porous medium, ΔP is the pressure difference between the two seepage cross-sections, μ lq is the dynamic viscosity of the liquid water, ΔL is the distance between the two seepage cross-sections, A is the seepage pore cross-sectional area, n is the number of pores, r is the radius of the pores, B is the effective seepage area of the porous medium, and ε is the porosity of the pores that seep through the large pores in the porous medium.

[0053] The water flux J generated by the fuel cell CL is:

[0054]

[0055] where i represents the current density, F represents the Faraday constant, represents the water molecular weight, ρ l represents the liquid water density.

[0056] Combining formulas (3)-(6) gives:

[0057]

[0058] Therefore, the liquid water pressure P water at the interface between the CL and the MPL is:

[0059]

[0060] According to the above formula, the following two inferences can be obtained:

[0061] Inference 1: If P water > P c2 , then the small hole is blocked and the gas cannot be transmitted;

[0062] Inference 2: If P water < P c2 , then the small hole is not blocked and the gas can be transmitted.

[0063] Let P water = P b2 , an equation of the relationship between the gas pressure P gas and the small hole r2 is established. Using this equation, the relationship curve between P gas and P b2 is obtained. Finally, the functional relationship between the gas pressure P gas and the degree of blockage as shown in formula (9) can be established.

[0064]

[0065] After the functional relationship between the gas pressure P gas and the degree of blockage is established, it is necessary to study more deeply the influence of the degree of blockage on the performance of the fuel cell. The gas pressure P gas can be used as a bridge. The relationship between the gas pressure P gas and the concentration overpotential η conc requires the establishment of a zero-dimensional steady-state isothermal two-phase flow model. The following assumptions are made for simplified calculation:

[0066] Assumption 1: There is no gas loss in the flow channel and the gas follows Fick's law in the GDB;

[0067] Assumption 2: The pore distribution in the MPL is uniform, the gas diffusion follows Knudsen diffusion and molecular diffusion, and the degree of blockage of liquid water is also uniform;

[0068] Assumption 3: The CL is infinitely thin;

[0069] Assumption 4: The pores in the MPL are divided into large pores r1 and small pores r2, and the breakthrough pressure P b determines the water-vapor transmission inside the pores.

[0070] The inlet concentration C0 of the reaction gas can be determined by the inlet pressure P gas (i.e., the gas inlet pressure inside the pores of the aforementioned MPL, approximately the fuel cell inlet pressure or the external pressure) and the humidity RH:

[0071]

[0072] where RH is the relative humidity of the cathode, P sat-r$P_{v0}$ is the saturated vapor pressure of water in MPL, $R$ is the gas constant, and $T$ is the thermodynamic temperature when the fuel cell is operating.

[0073] The transport of gas in the porous medium follows Fick's law, so the diffusion flux $J$ of the reactive gas is:

[0074]

[0075] where $D_{eff}$ is the effective diffusion coefficient of GDB, $C_1$ is the concentration of the gas when it first reaches the MPL, and $L$ GDB is the thickness of GDB.

[0076] The effective diffusion coefficient of GDB can be calculated by equations (12) and (13):

[0077]

[0078]

[0079] where $D$ GDB is the intrinsic diffusion coefficient of GDB, $P$ represents $P$ gas , $V$ N2 , $V$ O2 are the molecular diffusion volumes of $N_2$ and $O_2$ respectively, $M$ N2 , $M$ O2 are the molecular molar masses of $N_2$ and $O_2$ respectively, $\epsilon$ GDB is the porosity of GDB, is the 1.5th power of $\epsilon$ GDB .

[0080] Based on equations (11)-(13), $C_1$ is obtained as:

[0081]

[0082] Both molecular diffusion and Knudsen diffusion exist in the MPL. The diffusion coefficient equation of the MPL is as follows:

[0083]

[0084] where $D$ MPL represents the diffusion coefficient of the MPL, $D$ MPL，Fick is the molecular diffusion coefficient in the MPL, $D$ MPL，Knudsen is the Knudsen diffusion coefficient in the MPL, and $r$ Knudsen is the Knudsen diffusion radius.

[0085] According to the water vapor diffusion related parameters and structural parameters in the MPL, the concentration overpotential $\eta$ conc caused by the water vapor transport through the MPL can be calculated:

[0086]

[0087] Among them, n is the reaction electron transfer coefficient, α is the transmission coefficient, and J L represents the limiting current, and Cd L-pore is the clogging degree of the large pore r1, and Cd S-pore is the clogging degree of the small pore r2, and L MPL is the thickness of the MPL.

[0088] The concentration overpotential is the main factor affecting the high current density performance of the fuel cell. By defining the concepts of water clogging degree and water breakthrough pressure, the present invention successfully establishes the relationship between the concentration overpotential and the clogging degree, and clarifies the water vapor transport process in the gas diffusion layer inside the fuel cell.

[0089] Previously, when theoretically calculating the water vapor transport process in the MPL inside the fuel cell, the parameter K, as the permeability of the MPL porous medium, was usually given by empirical values or empirical formulas. This treatment method led to large errors in the subsequent calculation results. The present invention derives the K-value equation based on the actual water permeability test of the GDL and the neural network algorithm, and obtains the permeability parameters at specific pore diameters.

[0090] The seepage law of liquid in a single capillary follows the following Poisenille's law:

[0091]

[0092] Among them, Q represents the flow rate through the porous medium; ΔP represents the pressure difference between the two ends of the capillary; μlq represents the dynamic viscosity of water; and ΔL represents the distance between two seepage cross-sections.

[0093] Combining formula (20) with formula (3) can obtain:

[0094]

[0095] Among them, A represents the cross-sectional area of the seepage pores; n represents the number of pores.

[0096]

[0097] Among them, K represents the water permeability of the porous medium.

[0098]

[0099] Among them, ε represents the porosity; Vp represents the volume of the pores; V represents the total volume; d is the pore diameter, and r is the pore radius.

[0100]

[0101] As can be seen from Equation (24), K is proportional to the square of the pore diameter d. However, for a porous medium with a complex structure, its coefficient cannot be simply determined by the porosity. Equation (24) can be transformed into the following form:

[0102] K = ξd 2 (25)

[0103] For the solution of the coefficient ξ, the present invention first uses a full-functional membrane pore size analyzer to measure the true permeability of the GDL sample Then, according to the pore size distribution of the GDL, the pore size distribution curve is fitted using a neural network algorithm, and the coefficient ξ is obtained by the segmentation method. The specific method is as follows:

[0104] Figure 4 Figure 14 is a comparison diagram of the pore size distribution curve simulated by the neural network algorithm and the pore size distribution curve of SGL28BC (GDL carbon paper model). Among them, the black solid line is the measured pore size distribution map of SGL28BC, and the gray solid line is the pore size distribution curve simulated by the neural network algorithm. It can be seen from the figure that the simulated curve in the figure coincides with the measured curve. Assuming that the pore size distribution curve is segmented into n segments, then can be expressed as:

[0105]

[0106] where d1, d2, d3... d n are the micro-element pore diameters corresponding to the curve segmentation, and di represents the i-th micro-element pore diameter. Therefore, the coefficient ξ can be expressed as:

[0107]

[0108] According to Equation (27) and the pore size distribution obtained by the neural network algorithm, the coefficient ξ is calculated by MATLAB, and then Equation (25) is transformed into:

[0109] K = 6.8059×10 -6 d 2 (28)

[0110] If d = 10 μm, then K = 6.8059×10 -16 m 2 .

[0111] Example 1

[0112] In this embodiment, two types of carbon papers, SGL28BC and SGL22BB, are selected for verification. Among them, the thickness of SGL28BC is 235 microns, and the corresponding MPL thickness is 45 microns; the thickness of SGL22BB is 215 microns, and the corresponding MPL thickness is 40 microns. The pore size distributions of SGL28BC and SGL22BB are similar, but the pore sizes of the corresponding MPLs of SGL28BC or SGL22BB are smaller, mainly distributed between 0.01 and 20 μm. In this pore size region, the pore volume of SGL22BB is greater than that of SGL28BC. The porosity of MPL in SGL28BC is 0.5, and the porosity of MPL in SGL22BB is 0.6.

[0113] The effective area of the membrane electrode MEA is 25 cm 2 (with dimensions of 5×5 cm 2 ), and this membrane electrode is composed of a proton exchange membrane, cathode and anode catalyst layers (CL), and cathode and anode gas diffusion layers (GDL). The cathode and anode gas diffusion layers (GDL) are assembled using the same specifications of SGL28BC and SGL22BB each time a test is conducted. The assembled fuel cell is tested on an automated HEPHAS (HTS-125) fuel cell test station, and the polarization curve of the single cell test is recorded to evaluate the performance of the MEA. During the test, a single serpentine flow channel is used, the test temperature is set at 80 °C, hydrogen and air are introduced into the anode and cathode as fuel and oxidant respectively, the outlet back pressures of the cathode and anode are set at 0 kPa, 50 kPa, 100 kPa, and 150 kPa respectively, and the initial gas flow rates are 210 cc / min (anode) and 500 cc / min (cathode). Throughout the test process, the stoichiometric ratio of hydrogen to air is maintained at 2.0. The test is conducted under the condition that the relative humidity of the cathode and anode is simultaneously 100%, 70%, and 50% (tested at three humidity levels respectively). Before testing the polarization curve, the battery also needs to be activated for 8 h to reach its optimal performance.

[0114] Substitute the above parameters of SGL28BC and SGL22BB into the fuel cell water and gas transport model based on breakthrough pressure and blockage degree (i.e., in Equations 9 and 19) for theoretical calculation, and compare with the experimental values. The results are as Figure 5 shown.

[0115]

[0116] Figure 5The left figure in the middle is the polarization curve of two types of GDLs, SGL28BC and SGL22BB, under atmospheric pressure, and the right figure is the polarization curve of SGL28BC and SGL22BB under atmospheric pressure calculated based on the breakthrough pressure and blockage degree fuel cell water-vapor transport model. It can be seen from the figure that the performance of SGL22BB is higher than that of SGL28BC under atmospheric pressure conditions, especially the performance gap is more obvious at high current densities, and the concentration polarization of SGL22BB is significantly lower than that of SGL28BC. This trend is consistent with the concentration overpotential and simulated polarization curve calculated by the MPL water-vapor transport model, thus proving the correctness and feasibility of the model. In summary Figure 5 The experimental results show that the theoretical values calculated by using the fuel cell water-vapor transport model based on breakthrough pressure and blockage degree of the present invention have a high degree of agreement with the experimental results, and the accuracy of this model is relatively good.

[0117] Example 2

[0118] In this example, two types of carbon papers, JNTG18B-A6L and JNTG21-A6L, were selected for verification. Among them, the thickness of JNTG18B-A6L is 180 microns, and the corresponding MPL thickness is 40 microns; the thickness of JNTG21-A6L is 210 microns, and the corresponding MPL thickness is 40 microns. The porosity of MPL in SJNTG18B-A6L is 0.5, and the porosity of MPL in JNTG21-A6L is 0.6.

[0119] The effective area and composition of the membrane electrode MEA are the same as those in Example 1. Among them, the gas diffusion layers (GDLs) of the anode and cathode are assembled with the same specifications of JNTG18B-A6L and JNTG21-A6L each time during the test. The assembled fuel cell is tested on an automated HEPHAS (HTS-125) fuel cell test station, and the polarization curve of the single cell test is recorded to evaluate the performance of the MEA. The single serpentine flow channel is used for the test, the test temperature is set at 80 °C, hydrogen and air are respectively introduced into the anode and cathode as fuel and oxidant, the outlet back pressures of the anode and cathode are respectively set at 0 kPa, 50 kPa, 100 kPa and 150 kPa, and the initial gas flow rates are 210 cc / min (anode) and 500 cc / min (cathode). During the whole test process, the stoichiometric ratio of hydrogen to air is maintained at 2.0. Tests were carried out under the conditions that the relative humidity of the anode and cathode is simultaneously 100%, 70% and 50% (tested under three humidity conditions respectively). Before testing the polarization curve, the battery also needs to be activated for 8 h to make it reach the best performance.

[0120] Referring to the method in Embodiment 1, the above parameters of JNTG21 - A6L and JNTG21 - A6L are brought into the fuel cell water - gas transport model based on breakthrough pressure and blockage degree for theoretical calculation, and compared with the experimental values. The results are as Figure 6 shown.

[0121] Figure 6 The left figure in the following is the polarization curve of JNTG18B - A6L and JNTG21 - A6L under atmospheric pressure, and the right figure is the polarization curve of two GDLs of JNTG18B - A6L and JNTG21 - A6L calculated based on the fuel cell water - gas transport model of breakthrough pressure and blockage degree under atmospheric pressure. It can be seen from the figure that the performance of JNTG21 - A6L is higher than that of JNTG18B - A6L under the same back pressure, and the performance gap will be more obvious at high current densities. The concentration polarization of JNTG21 - A6L is significantly lower than that of JNTG18B - A6L. This trend is consistent with the concentration overpotential and simulated polarization curve calculated by the MPL water - gas transport model, which also proves the correctness and feasibility of the above - mentioned model of the present invention.

[0122] Embodiment 3

[0123] In order to verify the practicability of the fuel cell water - gas transport model based on breakthrough pressure and blockage degree, the results are compared with the simulation results of the traditional capillary force model and the actual measured polarization curve data.

[0124] For different diffusion media, considering different gas transport mechanisms and water saturations, the formula of the traditional capillary force model is as follows:

[0125]

[0126] where x represents GDB, MPL or CL; S lq is the water saturation.

[0127] In this embodiment, SGL28BC carbon paper was selected for verification, and its parameters were the same as those in Embodiment 1. The effective area and composition of the membrane electrode MEA were the same as those in Embodiment 1. Tests were carried out on an automated HEPHAS (HTS-125) fuel cell test station, and the polarization curves of the single cell test were recorded to evaluate the performance of the MEA. A single serpentine flow channel was used in the experiment. The test temperature was set at 80 °C. Hydrogen and air were introduced into the anode and cathode as fuel and oxidant respectively. The outlet back pressures of the anode and cathode were set at 0 kPa, 50 kPa, 100 kPa, and 150 kPa respectively, and the initial gas flow rates were 210 cc / min (anode) and 500 cc / min (cathode) respectively. During the whole test process, the stoichiometric ratio of hydrogen to air was maintained at 2.0, and the test was carried out under the condition that the relative humidity of the anode and cathode was 100% at the same time. Before testing the polarization curve, the battery also needed to be activated for 8 h to reach its optimal performance.

[0128] Figure 7 The left middle figure is the measured polarization curve under different back pressures. It can be seen from the figure that as the current density further increases, in the concentration polarization region, the performance of the battery gradually decreases with the increase of the back pressure, and the higher the current density and the back pressure, the more obvious the performance degradation. The polarization curves under different back pressures intersect at high current densities.

[0129] However, according to the polarization curves under different back pressures calculated by the traditional capillary force model ( Figure 7 the middle figure), it can be seen that there is no so-called intersection at high current densities. This is because the concept of water saturation in the traditional capillary force drainage model has defects for the microporous structure and cannot distinguish effective pores from ineffective pores, that is, it is considered that the pores not filled with liquid water can all conduct gas transmission, thus deviating from the actual value.

[0130] Figure 7 The right figure is the polarization performance curve under different back pressures calculated according to the model of the present invention. It can be seen from the figure that at high current densities, when the back pressure increases, the liquid water will break through the capillary force of the pore diameter and enter the pores. During the actual blocking process, the blocking of one pore may affect the gas transmission of multiple pores, resulting in a doubling of the concentration polarization and thus affecting the performance under high back pressure. This result shows that the model of the present invention starts from the actual state of the microporous structure, and the proposed concepts of water blockage degree and breakthrough pressure make the calculation results closer to the actual situation.

Claims

1. A fuel cell water-vapor transport model based on breakthrough pressure and blockage degree, characterized in that The establishment process of this model includes: establishing the concepts of breakthrough pressure and water blockage degree, and based on this, deriving the gas pressure P of the fuel cell gas The relationships between the blockage degree, the concentration overpotential and the blockage degree are used to clarify the water vapor transport process in the gas diffusion layer inside the fuel cell.

2. The model according to claim 1, characterized in that: The liquid water pressure P at the entrance of the MPL pores water If it exceeds the critical value to enable water to enter and transport in the pores, then this critical value is defined as the breakthrough pressure P b .

3. The model according to claim 2, characterized in that: The breakthrough pressure P b is calculated by the following formula: P b = P c + P gas where r is the pore diameter of the MPL, σ is the surface tension of liquid water, θ is the contact angle on the MPL surface, P c is the capillary force at the pore entrance of the MPL, and P gas is the gas intake pressure inside the pores of the MPL.

4. The model according to claim 3, wherein: The water blockage degree is defined as the ratio of the volume of the blocked pores in the MPL to the total pore volume when the pressure of the liquid water generated inside the fuel cell in the CL reaches the breakthrough pressure P b .

5. The model according to claim 3, characterized in that: During the modeling process, the cracks or pores of MPL in the CL interface are divided into large pores r1 and small pores r2, and the corresponding breakthrough pressures are denoted as P b1 , P b2 ; The liquid water generated in the CL preferentially enters the large pores r1. As the pressure of the liquid water increases to the breakthrough pressure P b2 of the small pores r2, the liquid water enters and blocks the small pores r2, affecting the water-vapor transmission.

6. The model according to claim 5, wherein: When the pressure P of liquid water water <the capillary force P of the small hole r2 c2 , the liquid water cannot enter the small hole r2, while the gas can be transmitted in the small hole r2. At this time, the small hole r2 is an effective hole; when the pressure P of liquid water water is between the capillary force P of the small hole r2 c2 and the breakthrough pressure P b2 , the liquid water enters and blocks the small hole r2, and neither the liquid water nor the gas can be transmitted; when the pressure P of liquid water water >the breakthrough pressure P of the small hole r2 b2 , the liquid water will block the small hole r2 and be transmitted in it. At this time, the gas cannot be transmitted.

7. The model according to claim 3, characterized in that: Let P water = P b2 An equation for the relationship between the gas pressure P gas and the small hole r2 was established, and using this equation, the relationship between P gas and P b2 was obtained. Finally, a relationship curve between the gas pressure P gas and the degree of blockage was established as follows: where r1 and r2 are the radii of the large and small pores respectively, i represents the current density, F represents the Faraday constant, represents the molecular weight of water, μ lq is the dynamic viscosity of liquid water, ΔL is the distance between two seepage cross-sections, ρ lq represents the density of liquid water, K is the permeability of the MPL porous medium, and ε is the porosity of seepage through the large pores in the porous medium.

8. The model according to claim 7, characterized in that: K is proportional to the square of the pore diameter d, and the solution process of the proportionality coefficient ξ between the two is as follows: First, test the true permeability of the GDL sample, then use the neural network algorithm to fit the pore size distribution curve according to the pore size distribution of the GDL, and finally obtain the coefficient ξ by the segmentation method.

9. The model according to claim 1, characterized in that: Utilize the gas pressure P gas As a bridge, establish the relationship between the fuel cell gas pressure P gas and the concentration overpotential η conc by means of a zero-dimensional steady-state isothermal two-phase flow model: where R is the gas constant, T is the thermodynamic temperature during fuel cell operation, n is the reaction electron transfer coefficient, α is the transport coefficient, J L represents the limiting current, D MPL represents the diffusion coefficient of the MPL, is the 1.5th power of the GDB porosity ε GDB Cd L-pore is the degree of blockage of the large pore r1, Cd S-pore is the degree of blockage of the small pore r2, C1 is the concentration when the gas just reaches the MPL, L MPL is the thickness of the MPL.

10. Application of the fuel cell water-gas transmission model based on breakthrough pressure and blockage degree according to any one of claims 1-9 in hydrogen fuel cells, water electrolysis, methanol fuel cells, and reversible fuel cells.

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