Flow battery polarization decoupling test method

Abstract

The invention discloses a method for testing polarization and decoupling of a flow battery. The method aims to quantify and separate three phenomena of ohmic polarization, activation polarization and concentration polarization in the flow battery. Comprising the following steps: firstly, carrying out electrochemical impedance spectroscopy testing under open-circuit voltage and different working currents respectively to obtain ohmic resistance and charge transfer resistance, and carrying out integral calculation to obtain ohmic polarization and activated polarization; then, carrying out a step type constant current charging test to obtain a total polarization curve; and finally, subtracting the ohmic polarization and the activation polarization from the total polarization to obtain concentration polarization. The method is simple and convenient to operate and suitable for various types of flow batteries, a novel and effective solution is provided for polarization analysis of the flow batteries, an accurate test basis can be provided for optimization of battery design, and improvement of performance optimization and research level of the flow batteries is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy storage, and in particular to a polarization decoupling test method for a liquid flow battery. Background Art

[0002] As an advanced energy storage technology, flow batteries play an important role in the current energy transformation and renewable energy utilization. With their long cycle life, high efficiency, fast response, scalability and safety, they have become an ideal choice for large-scale long-term energy storage systems. In the research and application of flow batteries, power density is one of the key indicators for evaluating their performance and commercial feasibility, because higher power density means higher energy output efficiency and smaller system volume, which is of great significance for improving the economy of energy storage systems. Activation polarization, ohmic polarization and concentration polarization are three key factors affecting battery power density. Activation polarization causes the reaction rate to decrease, increases voltage loss, and thus reduces power density. To reduce this effect, the reaction can be accelerated by using catalysts or improving electrode materials. Ohmic polarization originates from the resistance of current in the conductor. As the current intensity increases, the voltage loss also increases. Therefore, increasing the conductivity of the electrolyte or using highly conductive materials is an effective way to reduce ohmic polarization. Concentration polarization is caused by the change in concentration of reactants and products near the electrode. The decrease in concentration slows down the reaction rate, which in turn affects the power density. Optimizing fluid dynamics design, such as increasing flow rate, can effectively reduce concentration polarization. Comprehensive optimization of these three polarization mechanisms can significantly improve the power density of the battery, thereby improving its overall performance and application prospects.

[0003] Therefore, quantitative research on the values ​​and proportions of activation polarization, ohmic polarization and concentration polarization in voltage loss under different reaction conditions is crucial for optimizing battery design and improving overall performance. This can not only reveal the limiting mechanism of each polarization effect, but also provide theoretical guidance for the development of high-efficiency liquid flow batteries. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a liquid flow battery polarization decoupling test method, which aims to quantify and separate the three phenomena of ohmic polarization, activation polarization and concentration polarization in the liquid flow battery; using a symmetrical battery system, and combining it with electrochemical impedance spectroscopy test and step constant current charging test, to obtain voltage loss under different working conditions; including: first, electrochemical impedance spectroscopy test is performed at open circuit voltage and different working currents, ohmic resistance and charge transfer resistance are obtained, and ohmic polarization and activation polarization are obtained by integral calculation; then, step constant current charging test is performed to obtain the total polarization curve; finally, ohmic polarization and activation polarization are subtracted from the total polarization to obtain concentration polarization. The present invention is easy to operate, suitable for various types of liquid flow batteries, and provides a novel and effective solution for polarization analysis of liquid flow batteries. It can provide an accurate test basis for optimizing battery design, which is helpful to improve the performance optimization and research level of liquid flow batteries.

[0005] The design concept of the present invention is: The present invention revolves around the decoupling of three polarization phenomena (ohmic polarization, activation polarization, and concentration polarization) in liquid flow batteries. The polarization phenomenon of traditional liquid flow batteries is complex, and multiple polarization forms are coupled with each other, making it difficult to quantify them individually. In order to solve this problem, the present invention decouples through the following design ideas: 1) Use a symmetrical battery system, that is, the positive and negative electrolytes flow out from the same storage tank, thereby eliminating the influence of different electrolytes on the electrochemical process; 2) Quantify the total polarization with the help of a step constant current charging method; 3) With the help of electrochemical impedance spectroscopy testing, the ohmic resistance and charge transfer resistance are obtained respectively, and the ohmic polarization and activation polarization are quantified by integral calculation; 4) Finally, the concentration polarization is obtained by subtracting the ohmic polarization and activation polarization from the total polarization, thereby achieving effective separation and quantitative analysis of the three polarization phenomena.

[0006] In order to achieve the above technical effects, the following technical solutions are adopted: A flow battery polarization decoupling test method, comprising the following steps: Step S1: performing electrochemical impedance spectroscopy tests on a symmetrical battery with only one electrolyte storage tank at open circuit voltage and different operating currents to obtain ohmic resistance and charge transfer resistance, and obtaining ohmic polarization and activation polarization by integral calculation; Step S2: charging the symmetrical battery using a step-type constant current charging method to obtain a steady-state voltage curve, i.e., a total polarization curve; Step S3: The concentration polarization can be obtained by subtracting the ohmic polarization and the activation polarization from the total polarization.

[0007] Furthermore, the test method is used to decouple any one, any two, or all three of ohmic polarization, activation polarization, and concentration polarization in a battery; in the test method, step S1 is performed first and then step S2, or step S2 is performed first and then step S1.

[0008] Furthermore, the electrolyte is any type of electrolyte used in liquid flow batteries, and the redox couple in the electrolyte has no restrictions on concentration and ratio.

[0009] Furthermore, the total polarization is measured during step constant current charging, or during discharge; the step constant current charging is to charge at a certain working current for a period of time to obtain a steady-state voltage, then leave it for a period of time, and then charge at a different working current again for a period of time to obtain a steady-state voltage, and repeat this step. The working current can be any value within the limiting current.

[0010] Furthermore, the constant current charging only needs to obtain a steady-state voltage, and the charging time is not limited. However, in order to reduce the time cost, the constant current charging time is less than 10 minutes; the steady-state voltage refers to the voltage value measured after reaching a stable state during the charging / discharging process of the electrochemical system or battery. The steady-state voltage varies within a certain voltage range and may also show a small and stable increasing or decreasing trend.

[0011] Furthermore, the limiting current is the maximum current that can be achieved by the electrochemical system, which refers to the current value when the reaction rate on the electrode surface reaches the maximum value in the electrochemical reaction, that is, the current when the current no longer increases when the voltage continues to increase.

[0012] Furthermore, the electrolyte flowing through the positive and negative electrodes of the symmetrical battery with only one electrolyte storage tank flows out from the same electrolyte storage tank and returns to the storage tank after a redox reaction occurs in the battery; the ohmic polarization value is equal to the integral of the working current and the ohmic resistance measured at different working currents, and the ohmic resistance includes all ionic resistance and electronic resistance between the positive and negative electrodes of the battery; the activation polarization value is equal to the integral of the working current and the charge transfer resistance measured at different working currents.

[0013] Furthermore, the electrochemical impedance spectroscopy test performed at different operating currents uses a power supply to provide a constant direct current to the symmetrical battery, and simultaneously uses an electrochemical workstation to perform an electrochemical impedance spectroscopy test on the symmetrical battery at an open circuit voltage, and the power supply is the electrochemical workstation.

[0014] Furthermore, the method is applicable to, including but not limited to, all-vanadium liquid flow batteries, iron-chromium liquid flow batteries, vanadium-iron liquid flow batteries, zinc-iodine liquid flow batteries, zinc-iron liquid flow batteries, zinc-nickel liquid flow batteries, zinc-vanadium liquid flow batteries, zinc-bromine liquid flow batteries, zinc-cerium liquid flow batteries, all-iron liquid flow batteries, and organic liquid flow batteries.

[0015] Furthermore, the method is used to quantitatively decouple cell polarization when optimizing catalysts, electrodes, electrolytes, separators, and flow channels.

[0016] 1. Effective decoupling of polarization phenomena: By using a symmetrical battery system, step-type constant current charging and electrochemical impedance spectroscopy testing, ohmic polarization, activation polarization and concentration polarization can be accurately distinguished and quantified, avoiding the coupling effect between these polarization phenomena and improving the accuracy of polarization analysis.

[0017] 2. Easy to operate: It uses a step-type constant current charging method and electrochemical impedance spectroscopy technology. These methods are simple and easy to operate, do not require complex experimental equipment or procedures, and are suitable for polarization analysis of various types of flow batteries.

[0018] 3. Wide range of applications: This method is not only applicable to all-vanadium liquid flow batteries, but can also be used in various liquid flow battery systems such as iron-chromium, vanadium-iron, zinc-bromine, and all-iron, and has strong versatility.

[0019] 4. Support battery optimization: By quantitatively decoupling polarization phenomena, this method can provide accurate test basis for optimizing catalysts, electrodes, electrolytes, diaphragms and flow channels of flow batteries, helping to improve battery performance.

[0020] Overall, this invention provides a novel and effective solution for the polarization analysis of flow batteries, which helps to improve the performance optimization and research level of flow batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. The drawings in the following description are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0022] Figure 1 50% SOC Fe(Ⅱ) / Fe(Ⅲ) aqueous acidic electrolyte at 0-300mA / cm 2 The histograms of the three polarizations are decoupled in intervals; Figure 2 50% SOC Fe(Ⅱ) / Fe(Ⅲ) aqueous acidic electrolyte at 0-300mA / cm 2 Percentage plot of the three polarizations for interval decoupling. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0025] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations and / or combinations thereof.

[0026] The present invention revolves around the decoupling of three polarization phenomena (ohmic polarization, activation polarization, and concentration polarization) in liquid flow batteries. The polarization phenomenon of traditional liquid flow batteries is complex, and multiple polarization forms are coupled with each other, making it difficult to quantify them individually. In order to solve this problem, the present invention decouples through the following design ideas: 1) Use a symmetrical battery system, that is, the positive and negative electrolytes flow out from the same storage tank, thereby eliminating the influence of different electrolytes on the electrochemical process; 2) Quantify the total polarization with the help of a step constant current charging method; 3) With the help of electrochemical impedance spectroscopy testing, the ohmic resistance and charge transfer resistance are obtained respectively, and the ohmic polarization and activation polarization are quantified by integral calculation; 4) Finally, the concentration polarization is obtained by subtracting the ohmic polarization and activation polarization from the total polarization, thereby achieving effective separation and quantitative analysis of the three polarization phenomena.

[0027] Embodiment 1: The first step is to assemble the flow battery with a 10cm 2The carbon cloth electrodes were used as positive and negative electrodes respectively, the flow channels were opened on the graphite plate and used as the bipolar plate, the brass plate was used as the current collector, and the Nafion212 proton exchange membrane was used as the diaphragm. A 500ml reagent bottle was used as the electrolyte storage tank of the symmetrical battery, and the battery and the electrolyte storage tank were placed in a constant temperature oven at 60°C for testing. The electrolyte was a 400ml aqueous solution of 0.5M Fe(Ⅱ), 0.5M Fe(Ⅲ) and 2.0M hydrochloric acid, with a flow rate of 20ml / min during the test, and the liquid supply device was a peristaltic pump. In the second step, the electrochemical workstation was used to perform an electrochemical impedance spectroscopy test on the battery at an open circuit voltage, and the ohmic resistance and charge transfer resistance of the system at the open circuit voltage were measured. Subsequently, an electrochemical workstation was used to perform constant DC charging on the battery at different working currents, and another electrochemical workstation was used to perform an electrochemical impedance spectroscopy test on the battery at an open circuit voltage, and the ohmic resistance and charge transfer resistance of the system at different working currents were measured. The working current is used as the horizontal coordinate, and the ohmic resistance or charge transfer resistance is used as the vertical coordinate to integrate the two to obtain the ohmic polarization and activation polarization under different current densities. The third step is to use a charge and discharge tester to perform step-by-step constant current charging on the battery, with a starting current of 0.2A, charging for 2 minutes and then standing for 2 minutes, then increasing the current to 0.4A, charging again for 2 minutes and then standing for 2 minutes, with a step of 0.2A and a cut-off current of 3A. Repeat the above steps and measure the system at 0-300mA / cm 2 The fourth step is to use the total polarization to subtract the ohmic polarization and activation polarization from the total polarization to obtain the concentration polarization at different current densities.

[0028] The results in Example 1 are plotted as Figure 1 and Figure 2 , Figure 1 and Figure 2 The data in Figure 2 show that the total polarization increases with the increase of current density. Specifically, the current density increases from 20 mA / cm 2 Increase to 300mA / cm 2 , the total polarization of the battery increased from 8mV to 132mV. This phenomenon is in line with the basic law of electrochemistry, that is, an increase in current density will lead to an increase in polarization, because a higher current means that more electrons or ions need to pass through the electrode surface, resulting in an increase in the reaction resistance of the electrode surface, thereby increasing polarization.

[0029] Ohmic polarization also increases with increasing current density. Figure 1 and Figure 2 The data show that at lower current densities, the ohmic polarization accounts for a higher proportion of the total polarization (e.g., 20 mA / cm 2, ohmic polarization accounts for 73.2% of the total polarization), but with the increase of current density, the proportion decreases slightly (to 300mA / cm 2 Ohmic polarization accounts for 68.1% of the total polarization. Ohmic polarization is caused by the resistance of the components and electrolyte in the battery (including electronic resistance and ionic resistance). As the current density increases, the ohmic polarization in the battery components and electrolyte will also increase. However, at high current density, the growth rate of concentration polarization is faster than that of ohmic polarization, resulting in a decrease in the proportion of ohmic polarization.

[0030] The change in activation polarization is very small, and the proportion of total polarization is also very small (about 0.3% to 0.4%). This situation shows that in this system, the electrochemical reaction process contributes little to the electrode polarization, probably due to the fast reaction rate and good reversibility of the reaction. Therefore, activation polarization is not the main factor in the polarization of this system.

[0031] Concentration polarization increases significantly with increasing current density. At the same time, the proportion of concentration polarization in the total polarization also increases with increasing current density, from 26.4% to 31.6%. Concentration polarization is mainly due to the formation of a concentration gradient near the electrode surface. When the current density is high, the reactants are consumed faster on the electrode surface, resulting in a decrease in the concentration of reactants in the diffusion layer, resulting in a large concentration gradient, which ultimately leads to an increase in concentration polarization. This shows that at subsequent higher current densities, the diffusion process may become the main factor limiting the electrochemical reaction rate of flow batteries.

[0032] The results of the examples show that the flow battery polarization decoupling test method of the present invention successfully achieves effective separation and quantitative analysis of the three polarization phenomena. In a flow battery with 50% SOC Fe(Ⅱ) / Fe(Ⅲ) aqueous acidic electrolyte, at 0-300mA / cm 2 Range and ohmic polarization are the primary factors limiting the electrochemical reaction rate of liquid flow batteries. The subsequent optimization of battery performance should be primarily aimed at reducing the ohmic resistance of the system.

[0033] In summary, the present invention discloses a liquid flow battery polarization decoupling test method, which aims to quantify and separate the three phenomena of ohmic polarization, activation polarization and concentration polarization in liquid flow batteries; using a symmetrical battery system, and combining it with electrochemical impedance spectroscopy test and step constant current charging test, to obtain voltage loss under different working conditions; including: first, electrochemical impedance spectroscopy test is performed at open circuit voltage and different working currents, ohmic resistance and charge transfer resistance are obtained, and ohmic polarization and activation polarization are obtained by integral calculation; then, step constant current charging test is performed to obtain the total polarization curve; finally, the concentration polarization is obtained by subtracting ohmic polarization and activation polarization from the total polarization. The present invention is easy to operate, suitable for various types of liquid flow batteries, and provides a novel and effective solution for polarization analysis of liquid flow batteries. It can provide an accurate test basis for optimizing battery design, which is helpful to improve the performance optimization and research level of liquid flow batteries.

[0034] At this point, those skilled in the art recognize that, although the embodiments of the present invention have been shown and described in detail herein, many other variations or modifications that conform to the principles of the present invention can still be directly determined or derived based on the contents disclosed in the present invention without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be understood and recognized as covering all such other variations or modifications.

Claims

1. A liquid flow battery polarization decoupling test method, characterized in that: The method comprises the following steps: Step S1: performing electrochemical impedance spectroscopy tests on a symmetrical battery with only one electrolyte storage tank at open circuit voltage and different operating currents to obtain ohmic resistance and charge transfer resistance, and obtaining ohmic polarization and activation polarization by integral calculation; Step S2: charging the symmetrical battery using a step-type constant current charging method to obtain a steady-state voltage curve, i.e., a total polarization curve; Step S3: The concentration polarization can be obtained by subtracting the ohmic polarization and the activation polarization from the total polarization.

2. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The test method is used to decouple any one, any two, or all three of ohmic polarization, activation polarization, and concentration polarization in a battery; in the test method, step S1 is performed first and then step S2, or step S2 is performed first and then step S1.

3. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The electrolyte is any type of electrolyte used in liquid flow batteries, and the redox couple in the electrolyte has no restrictions on concentration and ratio.

4. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The total polarization is measured during step constant current charging or during discharge; the step constant current charging is to charge at a certain working current for a period of time to obtain a steady-state voltage, then leave it for a period of time, and then charge at a different working current again for a period of time to obtain a steady-state voltage, and repeat this step. The working current can be any value within the limit current.

5. A flow battery polarization decoupling test method as claimed in claim 4, characterized in that: The constant current charging only needs to obtain the steady-state voltage, and the charging time is not limited. However, in order to reduce the time cost, the constant current charging time is less than 10 minutes. The steady-state voltage refers to the voltage value measured after reaching a stable state during the charging / discharging process of the electrochemical system or battery. The steady-state voltage varies within a certain voltage range and may also show a small and stable increasing or decreasing trend.

6. A flow battery polarization decoupling test method as claimed in claim 4, characterized in that: The limiting current is the maximum current that can be achieved by the electrochemical system, and refers to the current value when the reaction rate on the electrode surface reaches the maximum value in the electrochemical reaction, that is, the current when the current no longer increases when the voltage continues to increase.

7. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The electrolyte flowing through the positive and negative electrodes of the symmetrical battery with only one electrolyte storage tank flows out from the same electrolyte storage tank and returns to the storage tank after undergoing redox reaction in the battery; the ohmic polarization value is equal to the integral of the working current and the ohmic resistance measured at different working currents, and the ohmic resistance includes all ionic resistance and electronic resistance between the positive and negative electrodes of the battery; the activation polarization value is equal to the integral of the working current and the charge transfer resistance measured at different working currents.

8. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The electrochemical impedance spectroscopy test performed at different working currents uses a power supply to provide a constant direct current to the symmetrical battery, and uses an electrochemical workstation to perform an electrochemical impedance spectroscopy test on the symmetrical battery at an open circuit voltage. The power supply is the electrochemical workstation.

9. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The method is applicable to, but not limited to, all-vanadium liquid flow batteries, iron-chromium liquid flow batteries, vanadium-iron liquid flow batteries, zinc-iodine liquid flow batteries, zinc-iron liquid flow batteries, zinc-nickel liquid flow batteries, zinc-vanadium liquid flow batteries, zinc-bromine liquid flow batteries, zinc-cerium liquid flow batteries, all-iron liquid flow batteries, and organic liquid flow batteries.

10. A flow battery polarization decoupling test method as claimed in claim 1, characterized in that: The method is used to quantitatively decouple cell polarization when optimizing catalysts, electrodes, electrolytes, separators, and flow channels.

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