A method for modeling and simulation of self-discharge process of zinc-bromine flow battery

Abstract

The application provides a zinc-bromine flow battery self-discharge process modeling simulation method and relates to the zinc-bromine flow battery design and control field. The target is to solve the problem that the zinc-bromine flow battery internal self-discharge process cannot be observed and lacks an analysis method, and the core work is based on the classic zinc-bromine flow battery electrochemical model, the self-discharge reaction of the zinc-bromine flow battery is introduced and defined, the complexation reaction equation of the complex compound generation is defined, the zinc-bromine flow battery model capable of describing the self-discharge process under different working conditions is established, the coulomb efficiency, the voltage efficiency and the energy efficiency of the zinc-bromine flow battery are established, the electrochemical model of the measured data is established, and the effectiveness of the model is verified through experiments.

Description

TECHNICAL FIELD

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

[0002] Zinc-bromine flow battery is a promising long-time energy storage method due to its high energy density, long running life and high safety. However, the low cycle efficiency caused by its internal self-discharge has always been a problem that hinders its application. The analysis method of the self-discharge process of zinc-bromine flow battery is one of the technical bottlenecks in this field, which is specifically shown as follows: (1) The use of complexing agent and the existence of complexing reaction in zinc-bromine flow battery make it difficult to calculate the key variable of self-discharge process, i.e. bromine concentration gradient; (2) Different working states in the working process of zinc-bromine flow battery will affect the size of self-discharge current; (3) Self-discharge is an internal process of the battery, which cannot be observed by sensors. Therefore, it is urgent to study a modeling analysis method for the self-discharge process of zinc-bromine flow battery.

[0003] The electrochemical model of zinc-bromine flow battery is a key tool for understanding its internal reaction mechanism, optimizing design and predicting performance. For the self-discharge process of zinc-bromine flow battery which cannot be directly measured in experiments, the electrochemical model can be used as an effective means to observe its internal reaction and mechanism. However, the internal complexing reaction and different working states of the battery make it difficult to model the self-discharge process of zinc-bromine flow battery. SUMMARY

[0004] The purpose of the present application is to solve the problem that the internal self-discharge process of existing zinc-bromine flow battery cannot be observed, and there is a lack of method for theoretical analysis and control guidance of the self-discharge process of zinc-bromine flow battery. A modeling simulation method for the self-discharge process of zinc-bromine flow battery is provided, which considers the complexing reaction and different working states of the battery, and analyzes and models the reaction mechanism of the self-discharge process.

[0005] The present method is based on the two-dimensional electrochemical model of zinc-bromine flow battery, and the definition and parameter calibration of zinc-bromine flow battery self-discharge reaction and complexing reaction are introduced to realize the modeling simulation of the self-discharge process of zinc-bromine flow battery under different working states in the running process, which provides theoretical and control reference for the self-discharge problem in the design and control process of zinc-bromine flow battery.

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

[0007] A modeling simulation method for the self-discharge process of zinc-bromine flow battery, the method steps are:

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

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

[0010] Step three: The self-discharge of zinc-bromine flow battery is caused by the reaction of bromine monomer diffusing through the membrane from the positive electrode side to the zinc monomer on the negative electrode side, and the diffusion rate of bromine monomer is affected by the concentration gradient of bromine monomer, and thus affected by the complexation reaction of complexing agent;

[0011] Step four: Define the complexation reaction of complexing agent as an equilibrium reaction, and the different reaction rates of complexation reaction caused by gravity and different working states of the battery during the charge and discharge process of the battery are described by different equilibrium constants of the equilibrium reaction;

[0012] Step five: Perform parameter identification of zinc-bromine flow battery through measured data, and calibrate the electrochemical model of zinc-bromine flow battery considering the self-discharge process based on the results of parameter identification, and perform simulation of the charge and discharge process of zinc-bromine flow battery;

[0013] Step six: The self-discharge process of zinc-bromine flow battery cannot be directly measured in experiments, and the self-discharge of the battery is indirectly reflected through the calculation of voltage efficiency, coulombic efficiency and energy efficiency, and the model is verified.

[0014] Further, in step one, the model is described by the following formulas:

[0015] Formula one:

[0016] Formula two:

[0017] Formula three:

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

[0019] Formula five: j0 = z i FAk0c0 (1-α) c R α

[0020] Formula six:

[0021] Equations one to five describe the mass transport process near the electrode, the negative and positive electrode equilibrium potential, the current density distribution inside the battery, the exchange current density of the battery and the concentration of the substance in the electrolyte tank, respectively; wherein, J is the flux of substance i, D i D is the diffusion coefficient of substance i, c i c is the concentration of substance i, z i z is the number of charges carried by the substance, F is the Faraday constant, R is the gas constant, and T is the open temperature, E is the potential, E is the electrolyte flow rate, neg E is the negative electrode equilibrium potential, E is the negative electrode standard electrode potential, pos E is the positive electrode equilibrium potential, E is the positive electrode standard electrode potential, j is the local current density, j0 is the exchange current density, a is the transfer coefficient, f = F / RT, and η = E - E θ E is the overpotential, E represents the electrode equilibrium potential, E θ E represents the standard electrode potential, A is the specific surface area, k0 is the reaction rate constant, c O c is the oxide concentration, c R c is the reduction substance concentration, and Vol is the volume of the electrolyte tank, c is the inlet concentration of the substance, H is the electrolyte tank height, S is the normal component of the substance flux, and S is the cross-sectional area of the flow channel.

[0022] Further, in step two, the sampling interval is 1 s.

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

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

[0025] Equation seven: c Br2-bound = 0

[0026] Equation eight: Zn + Br2→ Zn 2+ + 2Br -

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

[0028] Further, in step four, the complex complexation reaction affecting the concentration of bromine element 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 the zinc-bromine flow battery can be defined by the following formula:

[0030] Formula nine:

[0031] The equilibrium constant K eq The equilibrium constant K is determined by the type and concentration of the complexing agent of the complexation reaction, the different working states of the zinc-bromine flow battery, and the specific design parameters of the zinc-bromine flow battery.

[0032] The zinc-bromine flow battery self-discharge process modeling and simulation method of the present application highlights four aspects of zinc-bromine flow battery electrochemical modeling, self-discharge reaction definition, complex reaction definition affecting self-discharge, and indirect observation of battery self-discharge process. Through the definition of self-discharge reaction and related reactions, the modeling and simulation of the zinc-bromine flow battery self-discharge, which is an unobservable physical process, is realized, and the connection between the efficiency calculation and the measured parameters is established. The above work can provide theoretical and control basis for solving the self-discharge current problem in the design and control process of the zinc-bromine flow battery.

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

[0034] (1) The self-discharge reaction at the negative electrode and the separator contact surface is defined, which can calculate the diffusion-induced self-discharge process;

[0035] (2) The influence of gravity on the complex and the influence of different working states of the battery on the self-discharge process of the battery are considered, and the self-discharge process modeling and simulation of the whole charging and discharging process of the battery are realized;

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

[0037] Figure 1 is a zinc-bromine flow battery self-discharge process modeling and simulation method flow chart of the present application;

[0038] Figure 2 is a negative electrode bromine element concentration change graph calculated by the model of the present application;

[0039] Figure 3is the positive bromine element concentration change diagram calculated by the model of the application;

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

[0041] Figure 5 is the battery Coulomb efficiency, voltage efficiency and energy efficiency change diagram capable of reflecting self-discharge of the application. DETAILED DESCRIPTION

[0042] The technical solutions of the application are further described below in combination with the drawings and examples, but are not limited thereto, and any modification or equivalent replacement to the technical solutions of the application without departing from the spirit and scope of the technical solutions of the application shall be covered in the protection scope of the application.

[0043] The application takes a zinc-bromine flow battery of a certain company as the research background, is based on the existing research results such as the classic zinc-bromine flow battery electrochemical model, and establishes a modeling simulation method for the self-discharge process of the zinc-bromine flow battery by applying model parameter identification and definition of battery self-discharge related reactions. 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 examples, and the results can indirectly reflect the self-discharge process of the zinc-bromine flow battery. As shown in Figure 1 The specific implementation steps are as follows:

[0044] Step one: establish a two-dimensional electrochemical model of the zinc-bromine flow battery, which is described by the following formulas:

[0045] Formula one:

[0046] Formula two:

[0047] Formula three:

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

[0049] Formula five: j0 = z i FAk0c0 (1-α) c R α

[0050] Formula six:

[0051] Formula one to formula five respectively describe the substance transfer process near the electrode, the negative electrode and the positive electrode equilibrium potential, the current density distribution in the battery, the exchange current density of the battery and the substance concentration in the electrolyte tank. Among them, Flux of species i, D i Diffusion coefficient of species i, c i Concentration of species i, z i Charge number of species, F Faraday constant, R gas constant, T open circuit temperature, Electric potential, Electrolyte flow rate, E neg Negative electrode equilibrium potential, Negative electrode standard electrode potential, E pos Positive electrode equilibrium potential, Positive electrode standard electrode potential, j local current density, j0 exchange current density, a transfer coefficient, f = F / RT, η = E - E θ Overpotential, E represents electrode equilibrium potential, E θ Standard electrode potential, A specific surface area, k0 reaction rate constant, c O Oxide concentration, c R Reduced concentration, Vol is the volume of the electrolyte tank, Inlet concentration of species, H is the height of the electrolyte tank, Normal component of species flux, S is the cross-sectional area of the flow channel.

[0052] Step two: identify 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 through the battery management system during the constant current charge and discharge operation process under different complexing agent concentrations, with a sampling interval of 1s, to obtain the curves of battery terminal voltage and current changing with time under different complexing agent concentrations constant current charge and discharge conditions;

[0053] Step three: the self-discharge process of the zinc-bromine flow battery is defined as the reaction of bromine monomer diffusing through the membrane to the negative electrode side with zinc monomer, the diffusion rate of bromine monomer is affected by the concentration gradient of bromine monomer, and the concentration of bromine monomer is affected by the complexing reaction of the complexing agent; define the fast irreversible surface reaction of zinc bromine monomer at the boundary of the negative electrode and the diaphragm to describe the process:

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

[0055] Formula seven: Zn + Br2→ Zn 2+ + 2Br -

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

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

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

[0059] Formula eight:

[0060] Wherein, C QBr-Br2 is the concentration of the complex, C QBr is the concentration of the complexing agent, C M is the concentration normalization process constant, and the equilibrium constant K eq is determined by the complexing agent species, the concentration of the complexing agent, the different working conditions of the zinc-bromine flow battery, and the specific design parameters of the zinc-bromine flow battery. In practical applications, the parameter calibration method is usually used to obtain the parameters.

[0061] Step five: parameter identification of the zinc-bromine flow battery is carried out through actual measurement data, and the zinc-bromine flow battery electrochemical model considering the self-discharge process is calibrated based on the results of parameter identification, and the simulation of the charging and discharging process of the zinc-bromine flow battery is carried out. The physical quantity parameters involved in the zinc-bromine flow battery electrochemical model are shown in Table 2, some of which have corresponding literature reference values, and some of which need to be obtained through model parameter identification:

[0062] Table 1

[0063]

[0064]

[0065] Step six: the self-discharge process of the zinc-bromine flow battery cannot be directly measured in the experiment, and the self-discharge condition of the battery is indirectly reflected through the calculation of the voltage efficiency, coulomb efficiency and energy efficiency of the battery; the voltage efficiency, coulomb efficiency and energy efficiency of the zinc-bromine flow battery can be calculated by the following formulas respectively:

[0066] Formula nine:

[0067] Formula ten:

[0068] Formula eleven:

[0069] Wherein, η v , η I , η P are the voltage efficiency, coulomb efficiency and energy efficiency of the battery respectively; U bat and I batThe end voltage and current of the battery, respectively.

[0070] Embodiment 1:

[0071] The following will be described Figures 1 to 5 The modeling simulation method for the self-discharge process of the zinc-bromine flow battery is described in this embodiment:

[0072] As Figure 1 shown, the essence of the modeling simulation method for the self-discharge process of the zinc-bromine flow battery developed by the application is to realize the modeling of the self-discharge process of the zinc-bromine flow battery by defining the related reactions of the self-discharge process of the zinc-bromine flow battery, and the specific parameters of the model are obtained by parameter identification based on measured data. The specific implementation is as follows: a classical two-dimensional electrochemical model is established, constant current charge and discharge experiments of the battery are carried out by setting different complexing agent concentrations, the model adds fast irreversible surface reactions of zinc and bromine elements to simulate the self-discharge process, the model adds equilibrium reaction equations to simulate complexation reactions and different working conditions of the battery, the model parameters are obtained by parameter identification, the battery efficiency is calculated and compared with the measured results, and the model is verified.

[0073] After the two-dimensional electrochemical model of the zinc-bromine flow battery is established according to the formulas one to six in step one, some parameters in the model need to be further calibrated, therefore, the voltage and current data of the battery during the constant current charge and discharge process under different complexing agent concentrations are measured in step two. In this embodiment, the charge and discharge current density and the complexing agent concentration are selected as shown in table 2:

[0074] Table 2

[0075] Serial number Charge current (mA / cm 2 )]]> 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 the definitions of the self-discharge reaction and the complex generation reaction in steps three and four are added, the battery model is calibrated in step five, and the advantage of this model is that it can observe the concentration distribution of the substances in the battery in real time.

[0077] As Figure 2 shown, the negative bromine element concentration calculated by the model is shown, in order to observe the amount of bromine element diffused to the negative side, the self-discharge reaction on the surface of the negative electrode and the separator is not added in the simulation process of this model, and if the reaction is added, the bromine element concentration on the negative side will remain zero. At this time, it can be seen from the bromine element concentration on the negative side that the addition of the complexing agent can effectively inhibit the diffusion of the bromine element, that is, reduce the self-discharge reaction process;

[0078] As Figure 3 shown, the positive bromine element concentration calculated by the model is shown, it is not difficult to find that the mechanism of the complexing agent inhibiting the self-discharge is that the complexation reaction generates a complex that cannot diffuse through the membrane, thereby reducing the concentration of the diffusible positive bromine element and inhibiting the diffusion of the bromine element;

[0079] As Figure 4 The battery discharge voltage curve calculated by the model is shown, and it can be concluded that the addition of complexing agent has certain side effects, that is, after reducing the positive bromine concentration, the corresponding voltage of the battery will also decrease to a certain extent according to the Nernst equation, thereby reducing the voltage efficiency of the battery;

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

[0081] As Figure 5 The coulombic efficiency, voltage efficiency and energy efficiency that can reflect the self-discharge of the battery are shown. The coulombic efficiency can most directly reflect the self-discharge process of the battery, and 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 will also reduce the voltage efficiency of the battery to a certain extent. Therefore, there is an optimal complexing agent concentration, and through the reasonable configuration of the complexing agent concentration, the zinc-bromine flow battery can achieve higher energy efficiency. The rule is consistent with the measured data results in the examples, which verifies the effectiveness of the modeling and simulation method of the self-discharge process of the zinc-bromine flow battery in the present application.

Claims

1. A method for modeling and simulating the self-discharge process of a zinc-bromine flow battery, characterized in that: The steps of the method are: Step 1: Establish a two-dimensional electrochemical model of the zinc-bromine flow battery. This model describes the relationship between the terminal voltage, current density, ion concentration, and electrolyte flow rate of the zinc-bromine flow battery, providing a model basis for the subsequent simulation of the self-discharge process. The model is described by the following formula: Formula 1: Formula 2: Formula 3: Formula 4: Formula 5: Formula 6: Formulas 1 to 5 respectively describe the material transfer process near the electrode, the equilibrium potential of the negative and positive electrodes, the current 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 equilibrium potential of the negative electrode is is the negative electrode standard electrode potential, E pos The equilibrium potential of the positive electrode is 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, 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, S is the cross-sectional area of ​​the flow channel; Step 2: Identify the parameters to be measured and identified in the two-dimensional electrochemical model of the zinc-bromine flow battery, and use the battery management system to collect the battery terminal voltage and current during the constant current charge and discharge operation of the battery at different complexing agent concentrations, and obtain curves of the battery terminal voltage and current changing with time under constant current charge and discharge conditions at different complexing agent concentrations; Step 3: Clarify that the self-discharge of zinc-bromine flow batteries is caused by the diffusion of bromine from the positive electrode through the membrane to the negative electrode and the reaction with zinc. The diffusion rate of bromine is affected by the bromine concentration gradient and thus by the complex reaction of the complex. Step 4: Define the complexation reaction of the complex as an equilibrium reaction. The different reaction rates of the complexation 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 using measured data. Based on the parameter identification results, a two-dimensional electrochemical model of the zinc-bromine flow battery is calibrated to consider the self-discharge process, and the charge and discharge process of the zinc-bromine flow battery is simulated. Step 6: It is clear that the self-discharge process of the 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, wherein: In step 2, the collection interval is 1s.

3. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, wherein: In step 3, a fast irreversible surface reaction of zinc bromine is defined at the boundary between the anode and the separator: Formula 7: Formula 8: The reaction rate is very fast, so the concentration of the reactant bromine is limited to zero.

4. The method for modeling and simulating the self-discharge process of a zinc-bromine flow battery according to claim 1, wherein: In step 4, the equilibrium reaction of the zinc-bromine flow battery can be defined by the following formula: Formula 9: Among them, C QBr-Br2 is the complex concentration, C QBr is the complexing agent concentration, 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.

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