Porous electrode detection method for flow battery

Abstract

The invention belongs to the technical field of flow battery detection, and discloses a porous electrode detection method for a flow battery. Comprising the steps of assembly preparation, accelerated circulation of electrodes in electrolyte, in-situ XCT scanning, high-frequency EIS testing and result analysis. According to the test method, electrode optimization efficiency is improved, life prediction precision is improved, and time is shortened.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid flow battery detection, and in particular relates to a method for detecting porous electrodes for liquid flow batteries. Background Art

[0002] Existing electrode characterization technologies are mostly static tests, where physical and electrochemical data are isolated to characterize the characteristics of electrode materials. They are unable to dynamically reveal the characteristics of electrode materials under the multi-factor coupling mechanism during electrode operation. More specifically, the electrical performance of the electrode is closely related to factors such as current density, operating temperature, electrolyte concentration and flow rate, and collector channel structure. Therefore, it is extremely important to reveal the characteristics of electrode materials during service. Currently, most existing characterization methods are static and lack in-situ observation of the dynamic process of the electrode-electrolyte interface under multi-factor changes and the changes in the electrode pore structure during service.

[0003] High-power density fuel cell stacks place higher demands on electrode materials, requiring them to maintain extremely low performance degradation at high current densities. Traditional single-cell cycle stability testing is subject to numerous interference factors and stringent conditions, making it difficult to accurately assess cycle stability. Retesting individual cells in the event of test anomalies consumes significant manpower, material resources, and testing efficiency is low, with numerous factors influencing the results.

[0004] Conventional X-rays have insufficient resolution (>5μm) and cannot analyze micron-scale pore connectivity. Electrochemical testing equipment is separated from physical / chemical characterization equipment and cannot dynamically display the characteristics of the electrode reaction process. This has certain equipment limitations.

[0005] The commonly used technical means today are:

[0006] Physical characterization: Scanning electron microscopy (SEM) is routinely used to statically observe the electrode surface morphology, or mercury intrusion porosimetry is used to measure porosity.

[0007] Chemical characterization: Surface elemental composition analysis (without bulk / surface differentiation) or electrochemical activity assessment by cyclic voltammetry (CV)

[0008] Electrochemical testing: Single parameter measurements, such as polarization curves or EIS impedance spectroscopy, lack multi-parameter simultaneous correlation analysis.

[0009] The life of carbon felt electrodes was evaluated by SEM+electrochemical cycling test, but the structural evolution under dynamic service conditions was not involved; XPS was used to analyze the oxygen-containing functional groups on the electrode surface, but quantitative depth analysis was not achieved; an electrode state monitoring method based on EIS was proposed, but it was not correlated with physical structural parameters. Summary of the Invention

[0010] In order to overcome the shortcomings of the existing technology, the present invention provides a porous electrode detection method for liquid flow batteries, which performs online in-situ detection, tests the dynamic electrode characteristics through multi-factor coupling, and improves the electrode optimization rate.

[0011] The above object of the present invention is achieved through the following technical solution: A method for detecting porous electrodes for flow batteries, comprising the following steps:

[0012] 1. Component preparation: Seal and clamp the electrode material to be tested to form a CT unit. Prepare a single battery cell with a pair of positive and negative electrodes to be tested as a CD unit.

[0013] 2. Place the electrodes in the electrolyte to accelerate the cycle: Use a blue electric charge and discharge instrument to charge and discharge the CD unit; initially activate the 3.5-valent electrolyte, charge and discharge in constant current mode for three cycles, then discharge and stop for standby use. Charge the CD unit to a certain state of charge and stop charging. Connect the SC and CT units to share the electrolyte at that state of charge.

[0014] 3. In-situ XCT scanning: XCT is used to quickly scan the CT unit and analyze the electrode pore connectivity;

[0015] 4. High-frequency EIS test: Perform EIS test on the CD unit to obtain the conductivity of the electrolyte in the electrode. The tortuosity is calculated and characterized. The CD unit is charged and discharged cycled, and EIS test is performed on the electrode at irregular intervals to see whether Rion increases as the test progresses and what the growth pattern is.

[0016] 5. Result analysis: Perform corresponding analysis on the parameters obtained in steps 2-4.

[0017] Furthermore, the step 1 is specifically as follows: the electrode material to be tested is sealed and clamped with a transparent plastic end plate, and the electrode area is 1cm 2 This unit is called a CT unit. The electrode compression ratio of this unit is adjustable. A 48cm 2 The battery cell is a single battery cell with a pair of positive and negative electrodes to be tested, with an electrode area of ​​48cm 2 The energy unit consists of 50mL of electrolyte for each positive and negative electrode. An external blue-electric charge and discharge instrument is used to charge and discharge the single battery. This unit is called a CD unit.

[0018] Furthermore, the step 2 is specifically as follows: the CD unit is charged and discharged using a blue electric charge and discharge instrument, the state of charge SOC is between 20-80%, the electrolyte temperature is between 37-55 ° C, and the compression ratio is adjusted synchronously with the CT unit. The 3.5-valent electrolyte is initially activated, and the constant current mode charge and discharge current density is 160mA·cm -2After three cycles, the battery is discharged and then used for standby. The CD unit is charged to a certain state of charge and the SC and CT units are connected to share the electrolyte at that state of charge.

[0019] Furthermore, the parameters of the in situ XCT scan in step 3 are as follows:

[0020] Voltage: 60-80kV

[0021] Voxel resolution: 0.5-5 μm;

[0022] Number of projections: 500-1000;

[0023] Reconstruction algorithm: FBP / SIRT iterative algorithm;

[0024] Segmentation threshold method: Otsu / manual thresholding.

[0025] Furthermore, the parameters and calculation formula of the high-frequency EIS test in step 4 are as follows:

[0026] Frequency range: 50000-0.01Hz;

[0027] Amplitude: 5mV;

[0028] Typical circuit: RΩ+Rion∥Cd;

[0029] Fitting software: ZView, EC-Lab;

[0030] Calculation formula: Where σbulk is the bulk ionic conductivity of the electrolyte, and σeff is the ionic conductivity of the electrolyte in the electrode, which is calculated from Rion obtained from the AC impedance. The formula is: Where A is the electrode area and L is the electrode thickness.

[0031] Furthermore, the step 5 is specifically as follows: analyzing the contribution of the pore structure in the electrode to particle diffusion and the double-layer capacitance charge storage capacity by the pore connectivity retention rate; and predicting the service life of the electrode by the change in Rion impedance.

[0032] The beneficial effects of the present invention compared with the prior art are:

[0033] Improved electrode optimization efficiency: Online in-situ detection, through multi-factor coupling, tests the dynamic electrode characteristics and improves the electrode optimization rate;

[0034] Improved accuracy of life prediction and shortened time: By eliminating the interference of operating conditions as much as possible, the accuracy of one-time life judgment is improved, and trend judgment can be used to shorten the test judgment time. DETAILED DESCRIPTION

[0035] The present invention is described in detail below by specific examples, but the scope of protection of the present invention is not limited. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can be obtained from commercial channels.

[0036] Example 1

[0037] The mainstream electrodes used in all-vanadium liquid flow batteries are porous carbon-based materials, which have good electron conductivity, ion diffusion channels and acid corrosion resistance.

[0038] 1. High power density conditions place higher demands on the electrode's ion diffusion capacity, namely, pore connectivity γ and tortuosity τ. Rapid ion diffusion accelerates redox reactions. Pore connectivity and tortuosity vary with changes in electrode compressibility and electrolyte state of charge.

[0039] Definition of pore connectivity γ: the ratio of the volume of interconnected pores to the total pore volume.

[0040] Definition of tortuosity τ: A dimensionless parameter that describes the complexity of the transmission path in porous media. The tortuosity τ can be inferred from the ionic conductivity.

[0041] High power density operating conditions place higher demands on electrode surface activity. During use, this activity decreases, leading to accelerated degradation of electrical performance. This requires characterization of the degree of failure. Considering the multi-factor coupling of failure and the sustainability of failure monitoring, it is necessary to employ in-situ non-destructive methods to characterize degradation in another dimension. Specifically, this involves measuring the ionic conductivity parameter Rion at a fixed state of charge and observing its increase over cycling.

[0042] Develop an acid corrosion-resistant in-situ test characterization electrolytic cell, and use high-precision X-ray computer tomography (XCT) and an electrochemical workstation to jointly characterize the characteristics of the electrode during operation.

[0043] XCT operating parameters: ZEISS Xradia 620Versa was used to characterize the pore connectivity of the electrode during changes in compression ratio and temperature.

[0044] Voltage: 60-80kV

[0045] Voxel resolution: 0.5-5μm (multi-area scanning and stitching required)

[0046] Number of projections: 500-1000 (fast scan)

[0047] Reconstruction algorithm: FBP / SIRT iterative algorithm (such as SIRT) is suitable for low signal-to-noise ratio data

[0048] Segmentation threshold method: Otsu / manual threshold, which needs to be determined in combination with histogram analysis

[0049] Electrochemical workstation: Autolab was used to perform EIS analysis on the CD unit and characterize the electrode tortuosity.

[0050] Frequency range: 50000-0.01Hz

[0051] Amplitude: 5mV

[0052] Typical circuit: RΩ+Rion∥Cd, where: RΩ: ohmic resistance (electrolyte body + contact resistance), Rion: ionic resistance (transmission resistance in the porous electrode), Cdl: double-layer capacitance.

[0053] Fitting software: ZView, EC-Lab.

[0054] Data processing:

[0055]

[0056] Where σbulk is the bulk ionic conductivity of the electrolyte (measured by a Mettler Toledo conductivity meter), and σeff is the ionic conductivity of the electrolyte in the electrode, which is calculated from Rion (the conductivity of ions in the electrolyte in the electrode) obtained from the AC impedance. The formula is:

[0057]

[0058] Where A is the electrode area and L is the electrode thickness.

[0059] Auxiliary equipment: Blue Electric charging and discharging equipment 5V20A.

[0060] Component preparation: Use transparent plastic end plates to seal and clamp the electrode material to be tested. The electrode area is 1cm 2 This unit is called CT unit. The compression ratio of the electrode of this unit is adjustable. XCT scanning is performed on this unit later. 2 The battery cell is a single battery cell with a pair of positive and negative electrodes to be tested, with an electrode area of ​​48cm 2 The energy unit contains 50mL of electrolyte for each positive and negative electrodes. An external Blue Electric charger and discharger is used to charge and discharge the single battery. This unit, called a CD unit, has an adjustable electrode compression ratio. Subsequent AC impedance testing of this unit requires an external electrochemical workstation. The CT and CD units share electrolytes at different states of charge and temperatures. The distance between the CT and SC units is minimized, and the piping is insulated with thermal insulation to minimize temperature loss.

[0061] The specific operation process is to charge and discharge the CD unit using a blue electric charge and discharge instrument, with the state of charge (SOC) between 20-80%, the electrolyte temperature between 37-55°C, and the compression ratio adjusted synchronously with the CT unit. The parity 3.5-valent electrolyte is initially activated, and the constant current mode charge and discharge current density is 160mA·cm -2 After three cycles, the unit was discharged and ready for use. The CD unit was charged to a certain state of charge and the SC and CT units were connected to share the electrolyte at that state of charge. The CT unit was scanned quickly by XCT to obtain the electrode pore connectivity. The CD unit was tested by EIS (electrochemical impedance spectroscopy) to obtain the conductivity of the electrolyte in the electrode. The tortuosity was calculated and characterized, as shown in Table 1. The CD unit was charged and discharged in a constant current mode of 300 mA cm -2 The EIS test was carried out on the electrode from time to time to see whether Rion increased as the test progressed and what the growth pattern was, as shown in Table 2.

[0062] Table 1 Table 2 Rion / Ω·cm 2

[0063] Number of cycles S1 S2 S3 40 0.03 0.04 0.025 200 0.032 0.08 0.045 300 0.031 0.09 0.074

[0064] Note: S1-3 are three electrodes with different cycle life performances

[0065] Example 2

[0066] Rapid screening of electrode materials:

[0067] The test method is the same as in Example 1

[0068] Application scenarios: 3 types of modified carbon felt electrodes (A: ozone treatment; B: nitrogen doping; C: graphene composite);

[0069] 1. Testing process:

[0070] In 3M H2SO4+1.6MV 4+ In electrolyte, at 300mA / cm 2 Perform an accelerated loop (synchronous execution after every 50 loops):

[0071] a. In-situ XCT scanning (0.5 μm resolution, scan time <10 min) - In-situ XCT uses X-ray tomography technology to scan and analyze the sample using CT units, thereby visualizing the pore structure inside the material in three dimensions.

[0072] b. High-frequency EIS test (frequency range 50kHz-10mHz) to perform in-situ testing of the reaction impedance of the electrode material;

[0073] 2. Result analysis (analyze those parameters accordingly):

[0074] The electrode shows the best comprehensive performance after a certain number of cycles at high current density, which is characterized by multi-dimensional parameters such as pore connectivity and lifespan:

[0075] a. The retention rate of pore connectivity is crucial, as it determines the pore structure in the electrode's ability to assist ion diffusion and its double-layer capacitance storage capacity.

[0076] b. Predict the service life of the electrode based on the change of Rion impedance

[0077] Example 2: Fault diagnosis of service electrodes

[0078] Application scenario: Abnormal capacity attenuation of the all-vanadium liquid flow battery system in a certain energy storage power station

[0079] Diagnostic process:

[0080] Disassemble and obtain the electrode sheet for multi-dimensional characterization:

[0081] XCT shows whether there is pore blockage in local areas;

[0082] EIS detected the increase in charge transfer resistance Rct.

[0083] The above-described embodiments are only preferred embodiments of the present invention, and are not intended to be all feasible embodiments of the present invention. Any obvious modifications made by a person skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for detecting porous electrodes for flow batteries, characterized in that the steps include: S1. Component Preparation: Seal and clamp the electrode material to be tested to form a CT unit. Seal a single battery cell with a pair of positive and negative electrodes to be tested to form a CD unit. S2. Placing the electrodes in the electrolyte to accelerate cycling: Charge and discharge the CD unit using a blue-powered charge-discharge instrument. Initially activate the parity 3.5-valent electrolyte, perform three cycles of constant current charge and discharge, and then discharge the unit for standby use. Charge the CD unit to a specific state of charge and then connect the SC and CT units to share the electrolyte at that state of charge. S3. In-situ XCT scanning: XCT rapid scanning of the CT unit is used to analyze the electrode pore connectivity. S4. High-frequency EIS testing: EIS testing is performed on the CD unit to determine the conductivity of the electrolyte in the electrode. The tortuosity is calculated and characterized. The CD unit is charged and discharged for cycling, and EIS testing is performed on the electrode at irregular intervals to determine whether Rion increases as the test progresses and what the growth pattern is. S5. Result analysis: perform corresponding analysis on the parameters obtained in steps S2-S4.

2. The method for detecting porous electrodes for flow batteries according to claim 1, wherein: The step S1 specifically includes: using transparent plastic end plates to seal and clamp the electrode material to be tested, which is called a CT unit, and the electrode compression ratio of this unit is adjustable; preparing another battery unit, which is a single battery unit with a pair of positive and negative electrodes to be tested, and connecting the energy unit to an external blue electric charge and discharge instrument to charge and discharge the single battery, which is called a CD unit.

3. The method for detecting porous electrodes for flow batteries according to claim 2, wherein: The CT unit electrode area is 1 cm 2 , the battery cell electrode area is 48cm 2 .

4. The method for detecting porous electrodes for flow batteries according to claim 2, wherein: The energy unit is 50mL of electrolyte for each positive and negative electrode.

5. The method for detecting porous electrodes for flow batteries according to claim 1, wherein: The step S2 specifically includes: using a blue electric charge and discharge instrument to charge and discharge the CD unit; initially activating the parity 3.5 valence electrolyte, and charging and discharging in a constant current mode with a current density of 160 mA·cm -2 , discharge after cycling and stop standby, charge the CD unit to a certain state of charge and stop charging, connect the SC and CT units to share the electrolyte at the state of charge.

6. The method for detecting porous electrodes for flow batteries according to claim 5, wherein: When the blue-electric charge-discharge instrument performs charge-discharge operations on the CD unit, the state of charge (SOC) is between 20-80%, the electrolyte temperature is between 37-55° C., and the compression ratio is adjusted synchronously with the CT unit.

7. The method for detecting porous electrodes for flow batteries according to claim 5, wherein: The device is discharged after the cycle and is ready for use, and the cycle is three cycles.

8. The method for detecting porous electrodes for flow batteries according to claim 1, wherein: The parameters of the in-situ XCT scan in step S3 are: Voltage: 60-80kV Voxel resolution: 0.5-5 μm; Number of projections: 500-1000; Reconstruction algorithm: FBP / SIRT iterative algorithm; Segmentation threshold method: Otsu / manual thresholding.

9. The method for detecting porous electrodes for flow batteries according to claim 1, wherein: The parameters and calculation formula of the high-frequency EIS test in step S4 are as follows: Frequency range: 50000-0.01Hz; Amplitude: 5mV; Typical circuit: RΩ+Rion∥Cd; Fitting software: ZView, EC-Lab; Calculation formula: Where σbulk is the bulk ionic conductivity of the electrolyte, and σeff is the ionic conductivity of the electrolyte in the electrode, which is calculated from Rion obtained from the AC impedance. The formula is: Where A is the electrode area and L is the electrode thickness.

10. The method for detecting porous electrodes for flow batteries according to claim 1, wherein: The step S5 specifically includes: analyzing the contribution of the pore structure in the electrode to particle diffusion and the double-layer capacitance charge storage capacity by the pore connectivity retention rate; and predicting the service life of the electrode by the change in Rion impedance.

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