A method for calculating concentration polarization suitable for carbon felt / bipolar plate structure of flow battery

Abstract

The application belongs to the field of liquid flow battery, and particularly relates to a concentration polarization measurement method suitable for carbon felt / bipolar plate structure of a liquid flow battery. First, the limiting current density under different electrolyte flow rates is measured by using a symmetrical battery structure, the local mass transfer coefficient under the corresponding flow rate is obtained from the relationship between the limiting current density and the local mass transfer coefficient, then the relationship between the local mass transfer coefficient and the flow rate is fitted, and finally the relationship is substituted into the concentration polarization formula, and the concentration polarization size and spatial distribution of the battery under specific conditions are calculated by using a finite element method. The application has the advantages of simple method, low cost, easy implementation, and the like, can be widely applied to various forms of electrode / bipolar electrode structures, various types of redox flow batteries, and has strong practicability.

Description

Technical Field

[0001] This invention belongs to the field of flow batteries, specifically a method for calculating concentration polarization applicable to carbon felt / bipolar plate structures in flow batteries. Background Technology

[0002] The widespread application of renewable energy is a crucial support for achieving my country's carbon peaking and carbon neutrality goals. Renewable energy sources such as wind and solar power are characterized by discontinuity and uncontrollability, severely restricting their grid integration. Energy storage technology, due to its ability to effectively address the instability caused by large-scale renewable energy integration into the grid and improve the safety, economy, and flexibility of grid operation, has become a promising technological development direction.

[0003] Flow batteries, with their advantages of high safety, large energy storage capacity, and individually adjustable power and capacity, have promising applications in large-scale energy storage. One unique advantage of flow batteries compared to other batteries is that the electrolyte is in a flowing state during operation, resulting in faster ion transport rates. The electrolyte flow process is closely related to the flow field structure. To fully utilize the high mass transfer rate of flow batteries, minimize concentration polarization, and improve battery performance, optimizing the flow field structure of the flow battery, i.e., the carbon felt / bipolar plate structure, and calculating the concentration polarization before and after optimization is crucial. Summary of the Invention

[0004] The purpose of this invention is to provide a concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries. By assembling a symmetrical battery with or without flow channels using carbon felt / bipolar plate structures, the mass transfer coefficient is calculated, and then the concentration polarization magnitude and spatial distribution of the battery under specific conditions are calculated using the finite element method. This method can simply and quickly calculate the concentration polarization and its spatial distribution.

[0005] The technical solution of this invention is:

[0006] A method for calculating concentration polarization in carbon felt / bipolar plate structures for flow batteries is proposed. The carbon felt / bipolar plate structures include those with flow channels, those with flow channels in the bipolar plate, and those without flow channels in either the carbon felt or bipolar plate. A symmetrical cell is used, and the local mass transfer coefficient k is measured. m Then, based on the concentration polarization formula, the magnitude and spatial distribution of concentration polarization on the carbon felt electrode are calculated by finite element simulation.

[0007] The concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries includes, but is not limited to, serpentine, forked, or parallel flow channels in the carbon felt / bipolar plate structure.

[0008] The concentration polarization calculation method applicable to carbon felt / bipolar plate structure of flow battery is described above. The positive and negative electrodes of the symmetrical battery use the same electrolyte, that is, the positive and negative electrodes of the battery have the same redox pair, and the electrolyte of the positive and negative electrodes flows through the same storage tank.

[0009] The concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries, wherein the concentration polarization of the positive / negative electrodes is calculated by the following formula:

[0010]

[0011] η c Concentration polarization, unit: V; R: ideal gas constant, unit: J / (mol·K); T: electrolyte temperature, unit: K; n: number of transferred electrons; F: Faraday constant, unit: C / mol; I: local current density, unit: A / m³ 2 ;k m c is the local mass transfer coefficient, in m / s. r The concentration of reactants is expressed in mol / m³. 3 .

[0012] The concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries, with local mass transfer coefficient k m It is closely related to the electrolyte flow rate v. By measuring the limiting current density at different flow rates, the following formula can be used:

[0013] I lim =n·F·k m ·c r

[0014] The local mass transfer coefficient at different flow velocities was calculated, where: I lim The limiting current density is expressed in A / m. 2 n is the number of electrons transferred, F is the Faraday constant, and the unit is C / mol; k m c is the local mass transfer coefficient, in m / s. r The concentration of reactants is expressed in mol / m³. 3 ;

[0015] The final fit is the local mass transfer coefficient expression in the following form:

[0016] k m =av b

[0017] k m is the local mass transfer coefficient, in m / s; v is the electrolyte flow rate, in m / s; a and b are fitting coefficients.

[0018] The concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries is described above. A simplified 3D finite element cell model is constructed to simulate and calculate the local current density I and reactant concentration c at various points on the carbon felt electrode. r .

[0019] The concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries uses concentration polarization formulas in a 3D finite element battery model to simulate and calculate the magnitude and spatial distribution of concentration polarization on the carbon felt electrode.

[0020] The design concept of this invention is:

[0021] This invention first uses a symmetrical battery structure to measure the limiting current density at different electrolyte flow rates. The mass transfer coefficient at the corresponding flow rate is obtained from the relationship between the limiting current density and the mass transfer coefficient. Then, the relationship between the mass transfer coefficient and the flow rate is fitted. Finally, this relationship is substituted into the concentration polarization formula to calculate the concentration polarization of the battery under specific conditions.

[0022] The advantages and beneficial effects of this invention are:

[0023] 1. This invention provides a concentration polarization calculation method applicable to carbon felt / bipolar plate structures in flow batteries, which has the advantages of being simple, low-cost, and easy to implement.

[0024] 2. The concentration polarization calculation method for carbon felt / bipolar plate structure of flow battery described in this invention can be widely applied to various forms of electrode / bipolar structure and various redox flow batteries, and has strong practicality. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of an electrode / bipolar plate structure applicable to this invention. In the figure, 1 is the bipolar plate, 2 is the carbon felt electrode, and 3 is the flow channel.

[0026] Figure 2 This is a schematic diagram of another electrode / bipolar plate structure to which this invention applies. In the figure, 1 is the bipolar plate, 2 is the carbon felt electrode, and 3 is the flow channel.

[0027] Figure 3 This is a schematic diagram of another electrode / bipolar plate structure to which this invention applies. In the figure, 1 is a bipolar plate and 2 is a carbon felt electrode.

[0028] Figure 4 This is a schematic diagram of the symmetrical battery structure used in this invention. In the figure, 4 is the liquid storage tank, 5 is the positive bipolar plate, 6 is the positive carbon felt electrode, 7 is the separator, 8 is the negative carbon felt electrode, 9 is the negative bipolar plate, and 10 is the peristaltic pump.

[0029] Figure 5 This is a flowchart illustrating the simulation calculation using a 3D finite element battery model in this invention. Detailed Implementation

[0030] In the specific implementation process, the present invention uses a symmetrical battery structure to measure the limiting current density under different electrolyte flow rates. The mass transfer coefficient at the corresponding flow rate is obtained from the relationship between the limiting current density and the mass transfer coefficient. Then, the relationship between the mass transfer coefficient and the flow rate is fitted. Finally, this relationship is substituted into the concentration polarization formula, and the concentration polarization magnitude and spatial distribution of the battery under specific conditions are calculated using the finite element method.

[0031] like Figure 4 As shown, the key components of the symmetrical battery structure used in this invention include: an electrolyte storage tank 4, a positive bipolar plate 5, a positive carbon felt electrode 6, a separator 7, a negative carbon felt electrode 8, a negative bipolar plate 9, and a peristaltic pump 10. The positive and negative electrodes of the symmetrical battery use the same electrolyte, meaning the positive and negative electrodes have the same redox pair, and the electrolytes for both electrodes flow through the same storage tank. Its specific structure is as follows:

[0032] A positive carbon felt electrode 6 and a positive bipolar plate 5 are sequentially and tightly contacted on one side of the diaphragm 7, and a negative carbon felt electrode 8 and a negative bipolar plate 9 are sequentially and tightly contacted on the other side of the diaphragm 7. The positive carbon felt electrode 6 and the positive bipolar plate 5 are symmetrical to the negative carbon felt electrode 8 and the negative bipolar plate 9, respectively. The upper and lower ends of the positive carbon felt electrode 6 and the negative carbon felt electrode 8 are connected in parallel to the inlet and outlet of the electrolyte storage tank 4 through pipelines, respectively. A peristaltic pump 10 is installed on the pipeline connected to the outlet of the electrolyte storage tank 4. The electrolyte in the electrolyte storage tank 4 enters the positive carbon felt electrode 6 and the negative carbon felt electrode 8 through the peristaltic pump 10, and then flows back to the electrolyte storage tank 4 through the pipeline.

[0033] like Figure 1 As shown, the present invention provides an electrode / bipolar plate structure, including a bipolar plate 1 and a carbon felt electrode 2 that are in relatively close contact. The bipolar plate 1 has flow channels 3 corresponding to the carbon felt electrode 2. The flow channels 3 are distributed as follows: all flow channels 3 are inlet flow channels, with flow channels of 3.5cm and 2.6cm lengths distributed alternately, and the cross-sectional area of ​​the flow channels is 0.1cm×0.1cm.

[0034] To better understand the technical solutions and advantages of the present invention, the present invention will be further described in detail below with reference to specific embodiments.

[0035] Example

[0036] In this embodiment, a carbon felt with flow channels is used. Figure 2 Neither carbon felt bipolar plates nor carbon felt bipolar plates have flow channels. Figure 3 Taking the carbon felt / bipolar plate structure as an example, the carbon felt electrode dimensions are 4cm × 7cm × 0.4cm. Wherein:

[0037] like Figure 2As shown, the present invention provides an electrode / bipolar plate structure, including a bipolar plate 1 and a carbon felt electrode 2 that are in close contact. The carbon felt electrode 2 has a flow channel 3 corresponding to the bipolar plate 1. The flow channel 3 is distributed as follows: all flow channels 3 are inlet flow channels, with flow channels of 3.5cm and 2.6cm lengths distributed alternately, and the cross-sectional area of ​​the flow channel is 0.1cm×0.1cm.

[0038] like Figure 3 As shown, another electrode / bipolar plate structure applicable to this invention includes a bipolar plate 1 and a carbon felt electrode 2 in relatively close contact, neither of which has a flow channel.

[0039] In this embodiment, the electrolyte in the electrolyte storage tank of the symmetrical battery is 0.075 mol / L V (Vo (molar ratio)). 2+ :VO2 + =1:2) + 3mol / L H2SO4, flowing through the positive and negative electrodes, and then returning to the electrolyte storage tank. The limiting current density at each flow rate was obtained using a stepped constant potential charging method at flow rates of 0.00132m / s, 0.00198m / s, 0.00265m / s, and 0.00317m / s, respectively, and then calculated using the following formula:

[0040] I lim =n·F·k m ·c r

[0041] The local mass transfer coefficient, I, was calculated at different flow velocities. lim The limiting current density is expressed in A / m. 2 n is the number of electrons transferred (1), F is the Faraday constant (96485 C / mol); k m c is the local mass transfer coefficient, in m / s. r The reactant concentration is 75 mol / m³. 3 .

[0042] The final fit is the local mass transfer coefficient expression in the following form:

[0043] k m =av b

[0044] In this embodiment, k m is the local mass transfer coefficient, in m / s; v is the electrolyte flow rate, in m / s; a and b are fitting coefficients. The fitting yields a flow channel on the carbon felt ( Figure 2 Neither carbon felt bipolar plates nor carbon felt bipolar plates have flow channels. Figure 3 The local mass transfer coefficient of the carbon felt / bipolar plate structure and the flow velocity are respectively k m =0.45373v 1.4946 and k m=0.02705v 1.1963 This relationship can be used to obtain the local mass transfer coefficient at any flow rate.

[0045] Substituting the fitted expression for the local mass transfer coefficient into the following concentration polarization formula:

[0046]

[0047] In this embodiment, η c Concentration polarization, unit V; R is the ideal gas constant, 8.314 J / (mol·K); T is the electrolyte temperature, 298.15 K; n is the number of transferred electrons (l); F is the Faraday constant, 96485 C / mol; I is the local current density (the specific distribution is obtained by simulation calculation using a 3D finite element cell model), unit A / m. 2 ;k m c is the local mass transfer coefficient, in m / s. r The reactant concentration (the specific distribution is calculated using a 3D finite element cell model), in mol / m³. 3 .

[0048] like Figure 5 As shown, the process of constructing a 3D finite element battery model is as follows: First, draw a simplified 3D battery geometry. Then, set material properties for each solution domain. Next, set boundary conditions in the corresponding physics interface. Then, perform mesh generation and evaluate the mesh quality. If the quality does not meet the requirements, re-mesh. If the requirements are met, solve the model. Post-process the solution results and evaluate the results. If they are unreasonable, start from the first step to re-check and optimize. If they are reasonable and the error is within the allowable range, output the results.

[0049] The applied current density of 2000 A / m is calculated in this 3D finite element battery model. 2 At a flow rate of 0.00265 m / s and 80% SOC charging, the local current density I and reactant concentration c at various points on the carbon felt electrode are as follows: r The concentration polarization distribution was then simulated and calculated using the concentration polarization formula described above, determining the magnitude and spatial distribution of concentration polarization across the entire electrode domain.

[0050] Comparing the concentration polarizations of the two carbon felt / bipolar plate structures calculated using the above method, it was found that the maximum concentration polarization (approximately 0.05 V) at the electrode-bipolar plate contact interface of the carbon felt structure with flow channels was significantly smaller than that of the structure without flow channels (approximately 0.1 V). Furthermore, the average concentration polarization across the entire electrode of the structure with carbon felt flow channels (approximately 0.05 V) was also significantly smaller than that of the structure without flow channels (approximately 0.02 V). This is consistent with existing literature reports that introducing flow channels into the carbon felt can uniformly reduce concentration polarization. In addition, full-cell experiments were conducted on both carbon felt / bipolar plate structures. The discharge curves were IR-corrected, and the active area of ​​the carbon felt was measured to eliminate the influence of ohmic and activation polarization. The results showed that the carbon felt / bipolar plate structure with flow channels had even smaller concentration polarization, consistent with the relative magnitude of the concentration polarization calculated using the method described in this invention.

[0051] The results of the embodiments show that the present invention is applicable to the concentration polarization calculation method of carbon felt / bipolar plate structure of flow battery. It only requires measuring the limiting current density at different flow rates using symmetrical cells, and the subsequent calculation method is simple. Therefore, the present invention is low in cost and easy to implement.

[0052] Furthermore, the carbon felt / bipolar plate structure and flow channel form are not limited to the few mentioned in this invention. This method can be used for various redox flow batteries and various carbon felt bipolar plate structures, and has a wide range of applications and strong practicality.

Claims

1. A method for measuring the concentration polarization suitable for carbon felt / bipolar plate structure of flow battery, characterized in that, Carbon felt / bipolar plate structure includes carbon felt with flow channel, bipolar plate with flow channel, carbon felt / bipolar plate without flow channel; symmetric battery is adopted to measure local mass transfer coefficient k m According to the concentration polarization formula, the size and spatial distribution of the concentration polarization on the carbon felt electrode are calculated through finite element simulation. The same electrolyte is used for the positive and negative electrodes of the symmetric battery, i.e., the positive and negative electrodes have the same redox pair, and the electrolyte of the positive and negative electrodes flows through the same storage tank; The concentration polarization of the positive / negative electrode of the battery is calculated by the following formula: η c for concentration polarization, unit V; R is the ideal gas constant, unit J / (mol K); T is the electrolyte temperature, unit K; n is the number of transferred electrons, F is the Faraday constant, unit C / mol; I is the local current density, unit A / m 2 ; k m is the local mass transfer coefficient, unit m / s; c r is the reactant concentration, unit mol / m 3 ; Local mass transfer coefficient k m Closely related to the electrolyte flow rate v, by measuring the limiting current density at different flow rates, then by the following formula: I lim = n · F · k m · c r The local mass transfer coefficient at different flow rates is calculated, where: I lim is the limiting current density, in A / m 2 ; n is the number of transferred electrons, F is the Faraday constant, in C / mol; k m is the local mass transfer coefficient, in m / s; c r is the reactant concentration, in mol / m 3 ; Finally, the local mass transfer coefficient expression is fitted in the following form: k m = av b k m k is the local mass transfer coefficient, in m / s; v is the electrolyte flow rate, in m / s; a, b are fitting coefficients; A simplified 3D finite element battery model was constructed to simulate the local current density I and reactant concentration c at each point on the carbon felt electrode r .

2. The method for measuring the concentration polarization suitable for the carbon felt / bipolar plate structure of the flow battery according to claim 1, characterized in that, The flow channel form in the carbon felt / bipolar plate structure includes but is not limited to a serpentine, a finger, or parallel.

3. The method for measuring the concentration polarization of the carbon felt / bipolar plate structure of the liquid flow battery according to claim 1, characterized in that, The concentration polarization size and spatial distribution on the carbon felt electrode are simulated and calculated by using the concentration polarization formula in the 3D finite element battery model.

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