Zinc-bromine flow battery self-discharge process modeling simulation method

Abstract

The invention provides a zinc-bromine flow battery self-discharge process modeling simulation method, and relates to the field of zinc-bromine flow battery design and control. The objective of the invention is to solve the problems that the self-discharge process in the zinc-bromine flow battery cannot be observed and an analysis method is lacked, and the core work is based on a classical zinc-bromine flow battery electrochemical model, and a self-discharge reaction of the zinc-bromine flow battery and a complex reaction equation generated by a complex are introduced and defined. A zinc-bromine flow battery model capable of describing the self-discharge process in different working states is established, and the method is characterized in that an electrochemical model of coulombic efficiency, voltage efficiency, energy efficiency and actually measured data of the zinc-bromine flow battery is established, and the effectiveness of the model is verified through experiments.

Description

Technical Field

[0001] The invention relates to the field of zinc-bromine liquid flow battery design and control, in particular to a method for simulating the internal self-discharge process of a zinc-bromine liquid flow battery by adopting modeling simulation. Background Art

[0002] Zinc-bromine flow batteries are a promising long-term energy storage method due to their high energy density, long operating life and high safety. However, the low cycle efficiency caused by internal self-discharge has always been a pain point that restricts its application. The analysis method of the self-discharge process of zinc-bromine flow batteries is one of the technical bottlenecks in this field. The specific manifestations are as follows: (1) The use of complexing agents and the existence of complexing reactions inside zinc-bromine flow batteries make it difficult to calculate the concentration gradient of bromine, a key variable in the self-discharge process; (2) Different working states during the operation of zinc-bromine flow batteries will affect the size of the self-discharge current; (3) Self-discharge is a process inside the battery, and the self-discharge current cannot be observed by sensors. Therefore, it is urgent to study a modeling and analysis method for the self-discharge process of zinc-bromine flow batteries.

[0003] The electrochemical model of zinc-bromine flow batteries is a key tool for understanding their internal reaction mechanisms, optimizing their designs, and predicting their performance. For the self-discharge process of zinc-bromine flow batteries, which cannot be directly measured in experiments, electrochemical models can serve as an effective means of observing their internal reactions and mechanisms. However, the internal complexation reactions and different working states of the battery make it difficult to model the self-discharge process of zinc-bromine flow batteries. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that the internal self-discharge process of the existing zinc-bromine liquid flow battery cannot be observed and there is a lack of methods to theoretically analyze and control the self-discharge process of the zinc-bromine liquid flow battery. A modeling and simulation method for the self-discharge process of a zinc-bromine liquid flow battery is provided. The method takes into account the complex reaction and different working states of the battery, and analyzes and models the reaction mechanism of the self-discharge process.

[0005] This method is based on the two-dimensional electrochemical model of zinc-bromine flow battery. By introducing the definition of self-discharge reaction and complex reaction of zinc-bromine flow battery and parameter calibration, it realizes the modeling and simulation of self-discharge process of zinc-bromine flow battery under different working conditions during operation, providing theoretical and control reference for self-discharge problem in the design and control process of zinc-bromine flow battery.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for modeling and simulating the self-discharge process of a zinc-bromine flow battery, the method steps comprising:

[0008] Step 1: Establish a two-dimensional electrochemical model of zinc-bromine flow battery. This model describes the relationship between the terminal voltage, current density, ion concentration and electrolyte flow rate of zinc-bromine flow battery, providing a model basis for the subsequent simulation of self-discharge process;

[0009] Step 2: clarify the parameters to be measured and identified in the parameters of the two-dimensional electrochemical model of the zinc-bromine flow battery, and collect the battery terminal voltage and current during the constant current charge and discharge operation of the battery under different complexing agent concentrations through the battery management system, and obtain the curves of the battery terminal voltage and current changing with time under the constant current charge and discharge operation of different complexing agent concentrations;

[0010] Step 3: It is clear that the self-discharge of zinc-bromine flow battery is caused by the diffusion of bromine element from the positive electrode side through the membrane to the negative electrode side and the reaction with zinc element. The diffusion rate of bromine element is affected by the concentration gradient of bromine element, and thus affected by the complex reaction of the complex.

[0011] Step 4: Define the complex reaction of the complex as an equilibrium reaction. The different reaction rates of the complex reaction caused by gravity and different working states of the battery during the battery charging and discharging process are described by different equilibrium constants of the equilibrium reaction.

[0012] Step 5: Parameter identification of the zinc-bromine flow battery is performed through measured data. Based on the results of parameter identification, the electrochemical model of the zinc-bromine flow battery considering the self-discharge process is calibrated to simulate the charge and discharge process of the zinc-bromine flow battery;

[0013] Step 6: It is clear that the self-discharge process of zinc-bromine flow battery cannot be directly measured in the experiment. The self-discharge of the battery is indirectly reflected by calculating the battery's voltage efficiency, coulomb efficiency and energy efficiency to verify the model.

[0014] Furthermore, in step 1, the model is described by the following formula:

[0015] Formula 1:

[0016] Formula 2:

[0017] Formula 3:

[0018] Formula 4: j=j0[e -αfη -e (1-α)fη ]

[0019] Formula 5: j0=z i F A k0c (1-α) c R α

[0020] Formula 6:

[0021] Formulas 1 to 5 respectively describe the material transfer process near the electrode, the negative and positive equilibrium potentials of the electrode, the current fluid density distribution inside the battery, the exchange current density of the battery, and the material concentration in the electrolyte tank; where, is the flux of substance i, D i is the diffusion coefficient of substance i, c i is the concentration of substance i, z i is the charge number of the substance, F is the Faraday constant, R is the gas constant, and T is the Kelvin temperature. is the electric potential, is the electrolyte flow rate, E neg The negative electrode balance potential, is the standard electrode potential of the negative electrode, E pos is the positive electrode equilibrium potential, is the standard electrode potential of the positive electrode, j is the local current density, j0 is the exchange current density, α is the transfer coefficient, f = F / RT, η = EE θ is the overpotential, E represents the electrode equilibrium potential, and E θ represents the standard electrode potential, A is the specific surface area, k0 is the reaction rate constant, c O is the oxide concentration, c R is the concentration of the reducing substance, Vol is the volume of the electrolyte tank, is the inlet concentration of the substance, H is the height of the electrolyte tank, represents the normal component of the material flux and S is the cross-sectional area of ​​the flow channel.

[0022] Furthermore, in step 2, the sampling interval is 1 s.

[0023] Furthermore, in step 3, the self-discharge process of the zinc-bromine flow battery is described by defining the reaction of zinc and bromine on the negative electrode side;

[0024] Specifically, the fast irreversible surface reaction of zinc bromine is defined at the boundary between the negative electrode and the separator:

[0025] Formula 7: c Br2-bound =0

[0026] Formula 8: Zn+Br2→Zn 2+ +2Br -

[0027] The reaction rate is very fast, so the concentration of the reactant bromine is limited to zero.

[0028] Furthermore, in step 4, the complex reaction affecting the concentration of bromine is defined as an equilibrium reaction, and the uneven distribution of the complex caused by gravity and different working states of the zinc-bromine flow battery is described by different equilibrium constants of the equilibrium reaction;

[0029] The equilibrium reaction of zinc-bromine flow battery can be defined by the following formula:

[0030] Formula 9:

[0031] Equilibrium constant K eq It is determined by the type and concentration of the complexing agent in the complexing reaction, the different working conditions of the zinc-bromine liquid flow battery, and the specific design parameters of the zinc-bromine liquid flow battery.

[0032] The self-discharge process modeling and simulation method of zinc-bromine flow battery constructed by the present invention highlights four aspects: modeling of zinc-bromine flow battery electrochemical model, definition of self-discharge reaction, definition of complex reaction affecting self-discharge, and indirect observation of battery self-discharge process. By defining the self-discharge reaction and related reactions, the modeling and simulation of the unobservable physical process of zinc-bromine flow battery self-discharge is realized, and the connection with the measured parameters is established through efficiency calculation. The above work can provide a theoretical and control basis for solving the self-discharge current problem in the design and control process of zinc-bromine flow battery.

[0033] The present invention introduces the definition of self-discharge and complex reaction into the electrochemical model of zinc-bromine flow battery, and further realizes the modeling and simulation of the self-discharge process of zinc-bromine flow battery through parameter identification, and achieves the following effects:

[0034] (1) Define the self-discharge reaction at the interface between the negative electrode and the separator, and be able to calculate the self-discharge process caused by diffusion;

[0035] (2) Taking into account the influence of gravity on the complex and the influence of different working conditions of the battery on the self-discharge process of the battery, the self-discharge process modeling and simulation of the entire battery charging and discharging process is realized;

[0036] (3) The self-discharge process of the battery can be indirectly reflected by calculating the coulombic efficiency, voltage efficiency and energy efficiency of the battery, and the effectiveness of the model can be confirmed through experimental test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of a method for modeling and simulating the self-discharge process of a zinc-bromine flow battery of the present invention;

[0038] Figure 2 This is a graph showing changes in the concentration of bromine in the negative electrode calculated by the model of the present invention;

[0039] Figure 3This is a graph showing changes in the concentration of bromine in the positive electrode calculated by the model of the present invention;

[0040] Figure 4 is a battery discharge voltage curve diagram calculated by the model of the present invention;

[0041] Figure 5 The present invention is a graph showing changes in the battery coulomb efficiency, voltage efficiency and energy efficiency that can reflect self-discharge. DETAILED DESCRIPTION

[0042] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0043] This invention takes a zinc-bromine flow battery of a certain company as the research background, and is based on the existing research results such as the classic zinc-bromine flow battery electrochemical model. It applies model parameter identification and battery self-discharge related reaction definition to establish a modeling and simulation method for the self-discharge process of zinc-bromine flow batteries. In order to highlight the effectiveness of the method, the coulomb efficiency, voltage efficiency and energy efficiency calculated by the model are compared with the results in the embodiment. The results can indirectly reflect the self-discharge process of zinc-bromine flow batteries. Figure 1 As shown, the specific implementation steps are as follows:

[0044] Step 1: Establish a two-dimensional electrochemical model of zinc-bromine flow battery, which is described by the following formula:

[0045] Formula 1:

[0046] Formula 2:

[0047] Formula 3:

[0048] Formula 4: j=j0[e -αfη -e (1-α)fη ]

[0049] Formula 5: j0=z i F A k0c (1-α) c R α

[0050] Formula 6:

[0051] Formulas 1 to 5 respectively describe the material transfer process near the electrode, the negative and positive equilibrium potentials of the electrode, the current density distribution inside the battery, the exchange current density of the battery, and the material concentration in the electrolyte tank. is the flux of substance i, D i is the diffusion coefficient of substance i, c i is the concentration of substance i, z i is the charge number of the substance, F is the Faraday constant, R is the gas constant, and T is the Kelvin temperature. is the electric potential, is the electrolyte flow rate, E neg The negative electrode equilibrium potential, is the standard electrode potential of the negative electrode, E pos is the positive electrode equilibrium potential, is the standard electrode potential of the positive electrode, j is the local current density, j0 is the exchange current density, α is the transfer coefficient, f = F / RT, η = EE θ is the overpotential, E represents the electrode equilibrium potential, and E θ represents the standard electrode potential, A is the specific surface area, k0 is the reaction rate constant, c O is the oxide concentration, c R is the concentration of the reducing substance, Vol is the volume of the electrolyte tank, is the inlet concentration of the substance, H is the height of the electrolyte tank, represents the normal component of the material flux and S is the cross-sectional area of ​​the flow channel.

[0052] Step 2: Clarify the theoretical parameters and measured identification parameters of the two-dimensional electrochemical model of the zinc-bromine flow battery, and collect the battery terminal voltage and current during the constant current charge and discharge operation of the battery under different complexing agent concentrations through the battery management system. The sampling interval is 1s, and the curves of the battery terminal voltage and current changing with time under constant current charge and discharge conditions with different complexing agent concentrations are obtained;

[0053] Step 3: It is clear that the self-discharge process of zinc-bromine flow battery is caused by the diffusion of bromine element from the positive electrode through the membrane to the negative electrode side and the reaction with zinc element. The diffusion rate of bromine element is affected by the concentration gradient of bromine element, and the concentration of bromine element is affected by the complexation reaction of the complexing agent. Define the rapid irreversible surface reaction of zinc-bromine element at the boundary between the negative electrode and the diaphragm to describe the process:

[0054] Formula 6: c Br2-bound =0

[0055] Formula 7: Zn+Br2→Zn 2+ +2Br -

[0056] The reaction rate is very fast, so the concentration of the reactant bromine is limited to zero.

[0057] Step 4: Define the complexing reaction of the complexing agent as an equilibrium reaction. The uneven distribution of the complex due to different flow modes of the electrolyte under different working conditions during the battery charging and discharging process is described by different equilibrium constants of the equilibrium reaction.

[0058] In the present invention, the equilibrium reaction of the zinc-bromine flow battery can be defined by the following formula:

[0059] Formula 8:

[0060] Among them, C QBr-Br2 is the complex concentration, C QBr is the concentration of complexing agent, C M is the concentration normalized process constant, the equilibrium constant K eq It is determined by the type and concentration of the complexing agent in the complexing reaction, the different working conditions of the zinc-bromine flow battery, and the specific design parameters of the zinc-bromine flow battery. In practical applications, it is mostly obtained through parameter calibration methods.

[0061] Step 5: Parameter identification of zinc-bromine flow battery is performed through measured data. Based on the results of parameter identification, the electrochemical model of zinc-bromine flow battery considering the self-discharge process is calibrated, and the charge and discharge process of zinc-bromine flow battery is simulated. The physical quantity parameters involved in the electrochemical model of zinc-bromine flow battery are shown in Table 2 below. Some parameters have corresponding literature reference values, and some parameters need to be obtained through model parameter identification:

[0062] Table 1

[0063]

[0064]

[0065] Step 6: It is clear that the self-discharge process of zinc-bromine flow battery cannot be directly measured in the experiment. The self-discharge of the battery can be indirectly reflected by calculating the voltage efficiency, coulomb efficiency and energy efficiency of the battery. The voltage efficiency, coulomb efficiency and energy efficiency of zinc-bromine flow battery can be calculated by the following formulas:

[0066] Formula 9:

[0067] Formula 10:

[0068] Formula 11:

[0069] Among them, η v , η I , η P are the voltage efficiency, coulomb efficiency and energy efficiency of the battery respectively; U bat and I batare the terminal voltage and current of the battery respectively.

[0070] Embodiment 1:

[0071] Combine the following Figures 1 to 5 The following describes the modeling and simulation method for the self-discharge process of zinc-bromine flow battery in this embodiment:

[0072] like Figure 1 As shown, the essence of the method for modeling and simulating the self-discharge process of zinc-bromine liquid flow battery developed by the present invention is to realize the modeling of the self-discharge process of zinc-bromine liquid flow battery by defining the related reactions of the self-discharge process of zinc-bromine liquid flow battery, and the specific parameters of the model are obtained by parameter identification through measured data. The specific implementation method is: establish a classic two-dimensional electrochemical model, perform constant current charge and discharge experiments of the battery by setting different complexing agent concentrations, add the fast irreversible surface reaction of zinc-bromine single substance to the model to simulate the self-discharge process, add the equilibrium reaction equation to the model to simulate the complexing reaction and different working conditions of the battery, obtain the model parameters through parameter identification, calculate the battery efficiency and compare it with the measured results, and verify the model.

[0073] After the two-dimensional electrochemical model of the zinc-bromine flow battery is built by formulas 1 to 6 in step 1, some parameters in the model need to be further calibrated. Therefore, the voltage and current data of the battery during constant current charge and discharge under different complexing agent concentrations are measured and obtained through step 2. In this embodiment, the selected charge and discharge current density and complexing agent concentration are shown in Table 2:

[0074] Table 2

[0075] Serial number <![CDATA[Charging current (mA / cm 2 )]]> <![CDATA[Discharge current (mA / cm 2 )]]> Complexing agent concentration (mol / L) 1 40 40 0 2 40 40 0.2 3 40 40 0.4 4 40 40 0.6 5 40 40 0.8

[0076] After adding the definitions of the battery self-discharge reaction and the complex formation reaction in step three and step four, the battery model is calibrated through step five. The advantage of this model is that it can observe the material concentration distribution inside the battery in real time.

[0077] like Figure 2 The figure shows the concentration of bromine at the negative electrode calculated by the model. In order to facilitate the observation of the amount of bromine diffused to the negative electrode, the self-discharge reaction on the surface of the negative electrode and the diaphragm was not added to the simulation process of the model. If this reaction was added, the concentration of bromine at the negative electrode side would remain zero. At this time, it can be obtained from the concentration of bromine at the negative electrode side that the addition of the complexing agent can effectively inhibit the diffusion of bromine, that is, reduce the self-discharge reaction process;

[0078] like Figure 3 The figure shows the positive electrode bromine concentration calculated by the model. It is not difficult to find that the mechanism of the complexing agent inhibiting self-discharge is that the complexing reaction will generate a complex that cannot diffuse through the membrane, thereby reducing the diffusible positive electrode bromine concentration and inhibiting the diffusion of bromine.

[0079] like Figure 4 The figure shows the battery discharge voltage curve calculated by the model. It can be concluded that the addition of the complexing agent has certain side effects, that is, after reducing the concentration of bromine in the positive electrode, according to the Nernst equation, the corresponding voltage of the battery will also decrease to a certain extent, thereby reducing the voltage efficiency of the battery.

[0080] The above laws are consistent with the laws obtained through experiments.

[0081] like Figure 5 The figures show the coulombic efficiency, voltage efficiency and energy efficiency that can reflect the self-discharge of the battery. Among them, the coulombic efficiency can most intuitively reflect the self-discharge process of the battery. It can be seen that the addition of the complexing agent effectively improves the coulombic efficiency of the battery and inhibits the generation of self-discharge, but at the same time it also reduces the voltage efficiency of the battery to a certain extent. Therefore, there is an optimal concentration of the complexing agent. Through the reasonable configuration of the concentration of the complexing agent, the zinc-bromine liquid flow battery can achieve higher energy efficiency. This law is consistent with the measured data results in the embodiment, which verifies the effectiveness of the modeling and simulation method of the self-discharge process of the zinc-bromine liquid flow battery in the present invention.

Claims

1. A method for modeling and simulating the self-discharge process of a zinc-bromine flow battery, characterized in that: The method steps are: Step 1: Establish a two-dimensional electrochemical model of zinc-bromine flow battery. This model describes the relationship between the terminal voltage, current density, ion concentration and electrolyte flow rate of zinc-bromine flow battery, providing a model basis for the subsequent simulation of self-discharge process; Step 2: clarify the parameters to be measured and identified in the parameters of the two-dimensional electrochemical model of the zinc-bromine flow battery, and collect the battery terminal voltage and current during the constant current charge and discharge operation of the battery under different complexing agent concentrations through the battery management system, and obtain the curves of the battery terminal voltage and current changing with time under the constant current charge and discharge operation of different complexing agent concentrations; Step 3: It is clear that the self-discharge of zinc-bromine flow battery is caused by the diffusion of bromine element from the positive electrode side through the membrane to the negative electrode side and the reaction with zinc element. The diffusion rate of bromine element is affected by the concentration gradient of bromine element, and thus affected by the complex reaction of the complex. Step 4: Define the complex reaction of the complex as an equilibrium reaction. The different reaction rates of the complex reaction caused by gravity and different working states of the battery during the battery charging and discharging process are described by different equilibrium constants of the equilibrium reaction. Step 5: Parameter identification of the zinc-bromine flow battery is performed through measured data. Based on the results of parameter identification, the electrochemical model of the zinc-bromine flow battery considering the self-discharge process is calibrated to simulate the charge and discharge process of the zinc-bromine flow battery; Step 6: It is clear that the self-discharge process of zinc-bromine flow battery cannot be directly measured in the experiment. The self-discharge of the battery is indirectly reflected by calculating the battery's voltage efficiency, coulomb efficiency and energy efficiency to verify the model.

2. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, characterized in that: In step 1, the model is described by the following formula: Formula 1: Formula 2: Formula 3: Formula 4: j = j0[e -αfη -e (1-α)fη ] Formula 5: Formula 6: Formulas 1 to 5 respectively describe the material transfer process near the electrode, the negative and positive equilibrium potentials of the electrode, the current flow density distribution inside the battery, the exchange current density of the battery, and the material concentration in the electrolyte tank; where, is the flux of substance i, D i is the diffusion coefficient of substance i, c i is the concentration of substance i, z i is the charge number of the substance, F is the Faraday constant, R is the gas constant, and T is the Kelvin temperature. is the electric potential, is the electrolyte flow rate, E neg The negative electrode equilibrium potential, is the standard electrode potential of the negative electrode, E pos is the positive electrode equilibrium potential, is the standard electrode potential of the positive electrode, j is the local current density, j0 is the exchange current density, α is the transfer coefficient, f = F / RT, η = EE θ is the overpotential, E represents the electrode equilibrium potential, and E θ represents the standard electrode potential, A is the specific surface area, k0 is the reaction rate constant, c O is the oxide concentration, c R is the concentration of the reducing substance, Vol is the volume of the electrolyte tank, is the inlet concentration of the substance, H is the height of the electrolyte tank, represents the normal component of the material flux and S is the cross-sectional area of ​​the flow channel.

3. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, characterized in that: In step 2, the sampling interval is 1s.

4. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, characterized in that: In step 3, a fast irreversible surface reaction of zinc bromine is defined at the boundary between the anode and the separator: Formula 7: c Br2-bound =0 Formula 8: Zn+Br2→Zn 2+ +2Br - The reaction rate is very fast, so the concentration of the reactant bromine is limited to zero.

5. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, characterized in that: In step 4, the equilibrium reaction of the zinc-bromine flow battery can be defined by the following formula: Formula 9: Equilibrium constant K eq It is determined by the type and concentration of the complexing agent in the complexing reaction, the different working conditions of the zinc-bromine liquid flow battery, and the specific design parameters of the zinc-bromine liquid flow battery.

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