Method for establishing performance prediction simulation model of direct ammonia fuel cell

Abstract

The application discloses a kind of performance prediction simulation model establishment methods of direct ammonia fuel cell, steps include: establishing direct ammonia fuel cell three-dimensional two-phase geometric model, setting the property parameter in model, carry out three-dimensional model grid division, respectively set for boundary condition inlet mass flow rate, working temperature and establish direct ammonia fuel cell solution model.The modeling method proposed accurately describes the electrochemical process and mass transfer characteristics involved in the fuel cell, accurately analyzes the electrochemical reaction kinetics through multi-physical field coupling, clarifies the two-phase transport characteristics in the anode and the ammonia transmembrane transport mechanism, quantifies the ammonia molecule diffusion flux caused by concentration difference and the electroosmotic flux caused by electroosmotic drag, thereby obtaining the regulation of current density on ammonia permeation, which helps to analyze the distribution characteristics of multi-physical field and its influence law on cell performance, and has guiding significance for optimizing water and heat management, designing low-permeation composite membrane and reducing ammonia permeation loss.

Description

Technical Field

[0001] This invention belongs to the field of fuel cell technology, specifically relating to a three-dimensional simulation modeling method that directly utilizes the two-phase flow of an ammonia fuel cell. Background Technology

[0002] Ammonia fuel cells, using ammonia as fuel, offer a new approach to addressing bottlenecks such as high costs and an incomplete industrial chain in hydrogen energy storage and transportation due to their high energy density, mature storage and transportation system, and carbon neutrality. Direct ammonia fuel cells (DAFCs), as an innovative technology for solving hydrogen energy storage and transportation challenges, have demonstrated significant technological advantages and industrialization potential in recent years, driven by the in-depth development of polymer electrolyte membrane materials and extensive research on AOR (ammonia oxidation reaction) catalysts. However, DAFCs are complex nonlinear systems involving multi-component reactions, multidimensional transport, and multiple physical fields. Experiments alone are insufficient to characterize their internal transport phenomena and multi-physical field distributions. Therefore, research on the transmembrane transport mechanism and characteristics of ammonia within DAFCs, focusing on internal transport and reaction processes, is a forward-looking scientific endeavor.

[0003] Establishing a simulation model coupling internal transport and electrochemical reaction within a DAFC full cell is an efficient research method. This model couples multiple physics fields, including electrochemical reactions, fluid flow within porous media, and concentrated substance diffusion, to simulate the flow field and ammonia-water-nitrogen transport process in porous electrodes. This helps elucidate the two-phase transport characteristics within the cell anode and the ammonia transmembrane transport mechanism, quantify the ammonia transmembrane permeation flux, and the resulting parasitic current density. The ammonia fuel cell performance prediction simulation model proposed in this invention can reveal the ammonia / water / gas transport kinetics mechanism under multiphysics coupling, analyze the distribution characteristics of multiple physics fields and their impact on cell performance, and provide methodological guidance for further proposing hydrothermal management and control strategies to improve cell performance. Summary of the Invention

[0004] The purpose of this invention is to propose a method for establishing a performance prediction simulation model for direct ammonia fuel cells, enabling performance prediction under different operating conditions and obtaining polarization curves and internal multiphysics field distributions under multiple combinations of operating conditions.

[0005] The steps of constructing the model in this invention are as follows:

[0006] (1) Establish a three-dimensional two-phase geometric model of a direct ammonia fuel cell, including: finite element model of cathode and anode flow channels, cathode and anode diffusion layer, cathode and anode catalyst layer and anion exchange membrane.

[0007] (2) Set the physical property parameters of the seven parts of the finite element model: anion and anode channels, anion and anode diffusion layers, anion and anode catalyst layers, and anion exchange membrane. Specifically, these parameters include: anion exchange membrane permeability and inherent conductivity, anion and anode catalyst layers, diffusion layers permeability, conductivity and porosity, gas phase and liquid phase density and dynamic viscosity.

[0008] (3) Use mesh generation software to perform three-dimensional model mesh generation, and perform additional mesh refinement on the membrane and catalyst layer; set velocity inlet boundary conditions and pressure outlet boundary conditions at the corresponding inlet and outlet positions respectively.

[0009] (4) Set the intake mass flow rate and operating temperature for the boundary conditions in step (3).

[0010] (5) Establish a three-dimensional two-phase geometric mathematical model of a direct ammonia fuel cell. The fluid domains of the anode and cathode in the model include: flow channel, diffusion layer and catalyst layer; the porous electrodes of the anode and cathode include: diffusion layer and catalyst layer; the electron transport part includes: catalyst layer and diffusion layer.

[0011] Furthermore, the governing equations of the three-dimensional model include: the continuity equation, the momentum conservation equation, the momentum conservation equation for porous media, the mass transport equation, the liquid water transport equation (under steady-state operating conditions), the liquid water volume fraction solved using the Leverett-J equation, the charge conservation equation, and the electrochemical reaction rate solved using the modified Butler-Volmer equation.

[0012] The established simulation model is used to simulate its performance, and the polarization curves and internal multiphysics fields under different operating temperature and speed conditions are output. The ammonia permeation flux driven by diffusion and electroosmosis and the resulting parasitic current density are calculated according to formula (13).

[0013] Increased current density reduces the net ammonia permeation flux through a dual mechanism: on the one hand, the current consumes ammonia at the anode, reducing the concentration gradient and inhibiting diffusion flux; on the other hand, enhanced ion migration strengthens the counteracting effect of electroosmosis on diffusion. By calculating the variation of ammonia permeation flux with current density, the regulation law of ammonia permeation by current density can be derived, providing a quantitative basis for optimizing hydrothermal management and inhibiting ammonia leakage.

[0014] This simulation model realizes a fully coupled dynamic simulation of electrochemistry, two-phase flow, and transmembrane transport. It can analyze the multi-physics distribution characteristics inside the DAFC and their impact mechanism on battery performance. Furthermore, it can effectively predict battery performance under different combinations of operating conditions, thereby reducing the large amount of time and economic costs consumed in experiments. It is helpful to explore hydrothermal management control methods to improve heat and mass transfer, such as operating conditions and battery design, which can improve DAFC performance.

[0015] The features and advantages of this invention are as follows: (1) The proposed modeling method comprehensively considers the electrochemical processes and mass transfer characteristics involved in the direct ammonia fuel cell, including processes such as gas-liquid two-phase transport in porous media. The electrochemical reaction kinetics are accurately analyzed through multi-physics coupling, and the two-phase transport characteristics in the anode and the ammonia transmembrane permeation mechanism are clarified. (2) The synergistic effect of ammonia oxidation at the anode and oxygen reduction at the cathode is simulated by combining the modified Butler-Volmer equation. (3) The diffusion flux of ammonia molecules caused by concentration difference and the electroosmotic flux caused by electroosmotic drag are quantified, which has guiding significance for the design of low-permeability composite membranes and reducing ammonia permeation loss. Attached Figure Description

[0016] Figure 1 This is a geometric schematic diagram of the three-dimensional model established in the example of the present invention.

[0017] Figure 2 This is an example of the ammonia permeation flux diagram of the present invention.

[0018] Figure 3 This is a parasitic current density diagram of an example of the present invention.

[0019] Figure 4 This is an example of an ammonia concentration cloud map output by the present invention.

[0020] Figure 5 This is a nitrogen concentration cloud map output as an example of the present invention. Detailed Implementation

[0021] The method steps of the present invention will be further described below with reference to the accompanying drawings and specific calculation examples. It should be noted that this embodiment is descriptive and not limiting, and is not intended to limit the scope of protection of the present invention.

[0022] 1. The steps for establishing a performance prediction simulation model for a direct ammonia fuel cell are as follows:

[0023] (1) Establish a three-dimensional two-phase geometric model of a direct ammonia fuel cell, including: cathode flow channel, anode flow channel, cathode diffusion layer, anode diffusion layer, cathode catalyst layer, anode catalyst layer and anion exchange membrane of the finite element model.

[0024] (2) Set the physical property parameters of the seven parts of the finite element model: cathode flow channel, anode flow channel, cathode diffusion layer, anode diffusion layer, cathode catalyst layer, anode catalyst layer and anion exchange membrane. Specifically, these parameters include: anion exchange membrane permeability, inherent conductivity, permeability, conductivity and porosity of cathode and anode catalyst layers and diffusion layers, gas phase density, liquid phase density and dynamic viscosity.

[0025] (3) Use mesh generation software to perform three-dimensional model mesh generation, and perform additional mesh refinement on the membrane and catalyst layer; set velocity inlet boundary conditions and pressure outlet boundary conditions at the corresponding inlet and outlet positions respectively.

[0026] (4) Set the intake mass flow rate and operating temperature for the boundary conditions in step (3).

[0027] (5) Establish a three-dimensional two-phase geometric mathematical model of a direct ammonia fuel cell. The fluid domains of the anode and cathode in the model include: flow channel, diffusion layer and catalyst layer; the porous electrodes of the anode and cathode include: diffusion layer and catalyst layer; the electron transport part includes: catalyst layer and diffusion layer.

[0028] The governing equations of the specific three-dimensional two-dimensional geometric mathematical model include:

[0029] (1) Continuity equation:

[0030] (1)

[0031] in -Density kg m -3 , - Velocity vector (ms) -1 .

[0032] (2) Momentum conservation equation

[0033] (2)

[0034] in - Pressure (Pa), -Kinetic viscosity of mixture (kg m) -1 s -1 , -Unit tensor, - Velocity vector (ms) -1 , - Velocity vector gradient.

[0035] (3) Momentum conservation equation for porous media (Brinkman equation):

[0036] (3)

[0037] in -Porosity of porous media -Permeability of porous media (m 2 ), -Ergun parameter.

[0038] (4) Equation of mass transport:

[0039] (4)

[0040] in - Quality score, - mole fraction, -mole fraction gradient, -Reaction source term, The effective diffusion coefficient m of substance i relative to substance j 2 s -1 The effective diffusion coefficient Using the reference diffusion coefficient Based on m 2 s -1 Simultaneously, porosity and liquid water are introduced to correct the integral:

[0041] (5)

[0042] (6)

[0043] in -Liquid water volume fraction, - Porosity of porous media.

[0044] (5) The liquid water transport equation under steady-state operating conditions is:

[0045] (7)

[0046] in -Permeability m of porous media 2 , - Relative permeability of liquid water m 2 , -Liquid water dynamic viscosity (kg m) -1 s -1 , -Liquid pressure (Pa), -Source item kg m -3 s -1 .

[0047] (6) Solve for the volume fraction of liquid water using the Leverett-J equation:

[0048] (8)

[0049] (9)

[0050] in -Capillary pressure Pa -Porous media contact angle °, - Surface tension of liquid water N .

[0051] (7) Charge conservation equation:

[0052] (10)

[0053] (11)

[0054] in -Effective ionic conductivity S m -1 , -Effective electronic conductivity S m -1 .

[0055] (8) Solve for the electrochemical reaction rate using the modified Butler-Volmer equation:

[0056] (12)

[0057]

[0058] in -Anode reference exchange current density, - Cathode reference exchange current density -Ammonia concentration (mol / m) -3 , -Oxygen concentration (mol / m) -3 , -water concentration (mol / m) -3 , -Ammonia reference concentration (mol / m) -3 , -Oxygen reference concentration (mol / m) -3 , -Water reference concentration (mol / m) -3 a, b, and c are the reaction orders. -Anodic charge transfer coefficient, - Charge transfer coefficient of the cathode, R - Universal gas constant 8.314 J mol -1 K -1 , -Activation overpotential V of the anode, -Activation overpotential V of the cathode.

[0059] Using the simulation models established above, simulation calculations are performed to simulate its performance. The polarization curves and internal multiphysics fields under different operating conditions (operating temperature and speed) are output, and the ammonia permeation flux driven by diffusion and electroosmosis drag and the resulting parasitic current density are calculated according to the following formula (13):

[0060] (13)

[0061] (14)

[0062] in -Diffusion coefficient of liquid ammonia in the membrane m 2 s -1 , -Electroosmotic drag coefficient, - Mole fraction of ammonia - Permeability m of anion exchange membrane 2 , - Pressure difference across the membrane (Pa) - Membrane thickness m, - Ammonia concentration (mol / m) -3 The ammonia permeation flux was calculated from the diffusion term caused by the concentration gradient, the electroosmosis term caused by electric drag, and the convection term caused by the pressure gradient. and parasitic current density .

[0063] The model parameters involved in this embodiment are given during the specific calculation process.

[0064] (1) Establish a three-dimensional two-phase geometric model of a direct ammonia fuel cell (as shown in the attached figure). Figure 1 (As shown). The model includes a finite element model of the anion and anolyte channels, anion and anolyte diffusion layers, anion and anolyte catalyst layers, and anion exchange membrane.

[0065] In this step, the geometric parameters of each component are determined, such as... Figure 2 As shown in the cross-sectional view, the inlet and outlet cross-sections of the flow channel are 0.4 meters wide. 0.4 meters high The cathode / anode diffusion layer thickness is 0.33 mm. The thickness of the anode / cathode catalyst layer is 0.04 mm. The film thickness is 0.015. .

[0066] (2) Set the material properties of each component. Anion exchange membrane permeability. for The static contact angle θ of both the anode diffusion layer and the catalyst layer is 110°. The inherent permeability of the anode and cathode diffusion layers... for The inherent permeability of the catalyst layer for The inherent conductivity of the anode and cathode diffusion layers, catalyst layer, and membrane. , , They are respectively , and Porosity of the anode and cathode diffusion layers and catalyst layer , The values ​​are 0.7 and 0.4 respectively. The surface tension coefficient σ is... transfer coefficient , 0.46 and 1 respectively, constant voltage enter.

[0067] (3) Use mesh generation software to perform mesh generation of the three-dimensional model, and perform additional mesh refinement on the membrane and catalyst layer.

[0068] (4) Set boundary conditions and operating conditions, setting velocity inlet boundary conditions and pressure outlet boundary conditions at the corresponding inlet and outlet positions. The inlet velocity is... The outlet pressures of both the cathode and anode are Set temperature boundary conditions, battery operating temperature The temperature is 80℃.

[0069] As can be seen from the formula, the ammonia permeation flux is mainly caused by three mechanisms: first, the diffusion term caused by the concentration gradient; second, the electroosmotic term caused by electric drag; and third, the convection term caused by the pressure gradient. Thus, the ammonia permeation flux diagram and parasitic current density diagram have been obtained through calculation, as shown in the appendix. Figure 2 , 3 As shown.

[0070] Figure 2 The figure presents the relationship between ammonia permeation flux and current density in a direct ammonia fuel cell: the diffusion flux is driven by the ammonia concentration gradient between the anode and cathode. As the current density increases, ammonia in the anode is consumed by electrochemical reactions, reducing the concentration gradient and weakening the diffusion driving force; therefore, the calculated ammonia diffusion flux decreases monotonically with increasing current density. Meanwhile, the electroosmotic flux is determined by the direction in which ion migration "drags" ammonia molecules; the direction of ion migration is opposite to the direction of ammonia diffusion, thus inhibiting diffusion. Its intensity initially increases with current density, then tends to reach equilibrium due to the decrease in intramembrane ammonia concentration, as shown in the figure (300-400). The flux reaches its peak in the range, then decreases slightly and tends to stabilize. The "initially rising and then stabilizing" trend of electroosmotic flux reflects the dynamic balance between ammonia transport and ion migration within the membrane: initially, the ion migration rate dominates the electroosmotic intensity, while later, the ammonia concentration within the membrane becomes the limiting factor.

[0071] Figure 3 The figure shows the variation of parasitic current generated by the electrochemical reaction after ammonia permeates into the cathode with the working current density. The intensity of the parasitic current is directly related to the ammonia permeation flux, and it decreases monotonically as the battery working current density increases. The decrease in the curve in the figure indicates that increasing the battery working current density can suppress ammonia permeation and reduce side reaction losses.

[0072] In addition, the simulation model can also provide a cloud map of ammonia concentration distribution, as shown in the attached image. Figure 4 As shown, this is a cloud map using pseudo-color to represent the concentration gradient. The map shows that the flow channel is a high-concentration ammonia "transport zone," while the area near the catalyst layer is a low-concentration ammonia "consumption-equilibrium zone." Along the flow channel direction, the ammonia concentration near the catalyst layer exhibits a gradual change from "lower on the left and higher on the right" due to "strong consumption at the inlet and reduced consumption along the flow path."

[0073] The simulation model also provides a cloud map of nitrogen concentration distribution, as shown in the attached figure. Figure 5 As shown in the figure, the coupled process of "reaction generation-diffusion-transport" is clearly presented. The spatial gradation of color intuitively reflects the chain logic of "ammonia consumption → reaction rate decrease → nitrogen generation decrease → mass transfer and transport attenuation". It is a visual representation of the "reactant consumption-product evolution" of the anode in a direct ammonia fuel cell.

Claims

1. A method for establishing a performance prediction simulation model for direct ammonia fuel cells, characterized by: The steps for building the model are as follows: (1) Establish a three-dimensional two-phase geometric model of a direct ammonia fuel cell, including: the cathode flow channel, anode flow channel, cathode diffusion layer, anode diffusion layer, cathode catalyst layer, anode catalyst layer and anion exchange membrane of the finite element model; (2) Set the physical property parameters of the seven parts of the finite element model: cathode flow channel, anode flow channel, cathode diffusion layer, anode diffusion layer, cathode catalyst layer, anode catalyst layer and anion exchange membrane. Specifically, these parameters include: anion exchange membrane permeability, inherent conductivity, permeability, conductivity and porosity of cathode and anode catalyst layers and diffusion layers, gas phase density, liquid phase density and dynamic viscosity. (3) The three-dimensional model was meshed using mesh generation software, and additional mesh refinement was performed on the membrane and catalyst layer; velocity inlet boundary conditions were set at the inlet and outlet positions, and pressure outlet boundary conditions were set at the outlet position. (4) Set the intake mass flow rate and operating temperature for the boundary conditions in step (3); (5) Then, a three-dimensional two-phase geometric mathematical model of the direct ammonia fuel cell is established. The fluid domains of the anode and cathode in the model include: flow channels, diffusion layers and catalyst layers; the porous electrodes of the anode and cathode include: diffusion layers and catalyst layers; the electron transport part includes: catalyst layers and diffusion layers. The governing equations of the three-dimensional two-phase geometric mathematical model include: (1) Continuity equation: （1） in It's density. It is a velocity vector; (2) Momentum conservation equation （2） in It's pressure. It is the dynamic viscosity of the mixture. It is a unit tensor. It is a velocity vector. It is the velocity vector gradient. (3) Momentum conservation equation for porous media: （3） in It is the porosity of porous media. It is the permeability of porous media. These are Ergun parameters; (4) Equation of mass transport: （4） in It is the quality score. It is the mole fraction. It is a mole fraction gradient. It is the reaction source term. It is the effective diffusion coefficient of substance i relative to substance j. Using the reference diffusion coefficient As a baseline, porosity and liquid water volume fraction are also introduced for correction: （5） （6） in It is the volume fraction of liquid water. It is the porosity of porous media; (5) The liquid water transport equation under steady-state operating conditions is: （7） in It is the permeability of porous media. It is the relative permeability of liquid water. It is the dynamic viscosity of liquid water. It is liquid pressure. It is the source term; (6) Solve for the volume fraction of liquid water using the Leverett-J equation: （8） （9） in It is capillary pressure. It is the contact angle of porous media. It is the surface tension of liquid water; (7) Charge conservation equation: （10） （11） in It is the effective ionic conductivity, It is the effective electronic conductivity; (8) Solve for the electrochemical reaction rate using the modified Butler-Volmer equation: （12） ； in It is the anode reference exchange current density. It is the cathode reference exchange current density. It is the ammonia concentration. It is oxygen concentration. It's the water concentration. This is the ammonia reference concentration. This is the oxygen reference concentration. Here is the reference concentration of water, and a, b, and c are the reaction orders. It is the anodic charge transfer coefficient. R is the charge transfer coefficient of the cathode, and R is the universal gas constant. It is the activation overpotential of the anode. It is the activation overpotential of the cathode.

2. The method for establishing a performance prediction simulation model for a direct ammonia fuel cell according to claim 1, characterized in that: The established simulation model was used to simulate its performance, and the polarization curves and internal multiphysics fields under different operating temperature and speed conditions were output. The ammonia permeation flux driven by diffusion and electroosmosis and the resulting parasitic current density were calculated according to the following formula (13): （13） （14） in It is the diffusion coefficient of liquid ammonia in the membrane. It is the electroosmotic drag coefficient. It is the mole fraction of ammonia. It refers to the permeability of the anion exchange membrane. It is the pressure difference across the membrane. The ammonia permeation flux is calculated from the membrane thickness, the diffusion term caused by the concentration gradient, the electroosmosis term caused by electro-draft, and the convection term caused by the pressure gradient. and parasitic current density .

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