Equivalent circuit for electrochemical impedance spectroscopy analysis of direct ammonia fuel cell

Abstract

The invention discloses an equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell, and provides a standard equivalent circuit of the direct ammonia fuel cell or an equivalent circuit of the direct ammonia fuel cell considering an anode solution aggregation phenomenon. By adopting the two equivalent circuits, the electrochemical impedance spectrum of the tested battery can be fitted, so that polarization impedance values including mass transfer impedance, activation impedance and ohmic impedance of the tested battery can be obtained. The two equivalent circuits provided by the invention can accurately and reasonably explain the internal electrochemical phenomenon of the low-temperature direct ammonia fuel cell under the operation condition and completely explain the internal electrochemical characteristics of the low-temperature direct ammonia fuel cell. According to the analysis method provided by the invention, the electrochemical performance of the low-temperature direct ammonia fuel cell can be represented from the angle of polarization loss, quantitative analysis of impedance of each part in the cell under the operation condition is realized, and the test technology of the low-temperature direct ammonia fuel cell is enriched to a certain extent.

Description

Technical Field

[0001] The invention belongs to the field of fuel cells, and in particular relates to an equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell. Background Art

[0002] As a carbon-neutral fuel rich in hydrogen, ammonia has the advantages of high energy density, perfect preparation, storage and transportation technology, and high degree of commercialization. It is expected to solve the hydrogen energy industry chain problems faced by fuel cells from the perspective of fuel storage and transportation. At present, low-temperature direct ammonia fuel cell (DAFC) technology is in a rapid development stage after early exploration, and is receiving more and more attention in the energy field. In a low-temperature direct ammonia fuel cell, humidified oxygen is passed into the cathode of the battery and an oxygen reduction reaction occurs to generate hydroxide ions. Ammonia is used as an anode fuel to combine with the hydroxide ions generated at the cathode and undergo an ammonia oxidation reaction to generate nitrogen and water. Among them, the anode is often supplied with ammonia fuel in the form of a liquid ammonia / potassium hydroxide mixed solution. The reaction at the two poles of the battery usually takes place in a temperature range of 60 to 100°C.

[0003] Low-temperature direct ammonia fuel cell technology has made rapid breakthroughs in the past five years, but the testing technology used by related research institutes is relatively simple, and most of them only use polarization testing methods to characterize the overall performance of the battery. With the rapid improvement of DAFC performance, quantitative analysis of the impedance size of each part of the battery during operation can help explain the internal heat and mass transfer characteristics of the battery, optimize the battery structure, and further improve the electrochemical performance of the battery. Electrochemical impedance spectroscopy (EIS) is a very important research method in electrochemical measurement technology, and is widely used in the study of electrode process dynamics and various electrochemical systems. EIS is a transient electrochemical technology. The disturbance signal is a small-amplitude sinusoidal potential or current. With the help of an impedance spectrum measuring instrument, the impedance spectrum of the electrode system at different frequencies can be obtained. Using EIS testing in DAFC research and analyzing the battery electrochemical impedance spectrum under operating conditions can understand the electrochemical process contained in the battery and achieve quantitative characterization of battery polarization impedance including mass transfer impedance, electrochemical activation impedance, and ohmic impedance.

[0004] Based on this, the present invention proposes an equivalent circuit for electrochemical impedance spectroscopy analysis of direct ammonia fuel cells. The equivalent circuit can effectively explain the internal electrochemical behavior of DAFC, characterize the electrochemical performance of the battery from the perspective of polarization loss, and realize quantitative analysis of the impedance of each part of the battery under operating conditions. Using this equivalent circuit to perform electrochemical impedance spectroscopy analysis of direct ammonia fuel cells can, to a certain extent, make up for the limitations of battery testing technology and enrich the means of characterizing battery electrochemical performance, thereby effectively promoting the research progress of DAFC in the directions of hydrothermal management and fault diagnosis. Summary of the invention

[0005] The object of the present invention is to provide an equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell.

[0006] The technical solution of the present invention is: the standard equivalent circuit of the direct ammonia fuel cell includes: an inductor, an ohmic impedance element, a constant phase element representing the capacitance characteristics of the electrolyte membrane, an impedance element representing the two-pole Faraday process, a capacitor, an impedance element representing the mass transfer loss inside the battery, and a Faraday process impedance element at the anode solution gathering place, etc. The circuit composition structure is: the constant phase element representing the capacitance characteristics of the electrolyte membrane is connected in parallel with the impedance element representing the two-pole Faraday process and the impedance element representing the mass transfer loss inside the battery, and then connected in series with the inductor element and the ohmic impedance element to form the standard equivalent circuit of the direct ammonia fuel cell.

[0007] A constant phase element characterizing the capacitance characteristics of the electrolyte membrane is connected in parallel with an impedance element characterizing the Faraday process at both electrodes and an impedance element characterizing the mass transfer loss inside the battery; a capacitance element is connected in parallel with the Faraday process impedance element at the anode solution accumulation point; the two parallel circuits are connected in series and then connected in series with an inductance element and an ohmic impedance element to form an equivalent circuit of a direct ammonia fuel cell taking into account the anode solution accumulation phenomenon.

[0008] The electrochemical impedance spectroscopy test results of the direct ammonia fuel cell are fitted using the standard equivalent circuit of the direct ammonia fuel cell, or using the equivalent circuit of the direct ammonia fuel cell taking into account the anode solution aggregation phenomenon, so as to obtain the full battery impedance information of the battery ohmic impedance, activation impedance and finite layer diffusion coefficient.

[0009] By fitting the measured DAFC electrochemical impedance spectrum with the battery equivalent circuit model, the parameters of each component of the equivalent circuit are obtained, thereby achieving quantitative analysis of the battery polarization impedance including mass transfer impedance, activation impedance and ohmic impedance.

[0010] By fitting the measured DAFC electrochemical impedance spectrum with the battery equivalent circuit model, the key parameters of the equivalent circuit are obtained, thereby achieving quantitative analysis of the battery polarization impedance including mass transfer impedance, activation impedance and ohmic impedance.

[0011] The direct ammonia fuel cell under this technical solution generally uses an anion exchange membrane as a solid electrolyte membrane, and the anode reactant is ammonia gas or liquid ammonia water. Among them, the DAFC under test using liquid ammonia water as the anode supply needs to select an equivalent circuit according to the specific operation method.

[0012] In addition, combined with the actual operating characteristics of the direct ammonia fuel cell, the present invention also proposes an equivalent circuit of the direct ammonia fuel cell taking into account the anode solution aggregation phenomenon.

[0013] For the DAFC under test that uses ammonia gas as the anode supply, the standard equivalent circuit of the direct ammonia fuel cell should be used for parameter fitting. For the DAFC under test that uses ammonia water as the anode supply, it is necessary to determine whether to use the standard equivalent circuit of the direct ammonia fuel cell or the direct ammonia fuel cell equivalent circuit that takes into account the anode solution aggregation phenomenon based on the characteristics of the actual electrochemical impedance spectrum.

[0014] The characteristics and beneficial effects of the present invention are:

[0015] (1) Using electrochemical impedance spectroscopy as a test method for direct ammonia fuel cells, the equivalent circuit is fitted according to the test results to obtain the parameters of each component of the equivalent circuit. This can achieve quantitative analysis of the internal mass transfer loss, activation loss and ohmic loss of DAFC, evaluate the performance of the battery from the perspective of polarization loss, and enrich the research methods of DAFC.

[0016] (2) The proposed electrochemical impedance spectroscopy equivalent circuit model of direct ammonia fuel cells can be used for DAFCs whose anode feed is gaseous or liquid. The equivalent circuit model can accurately and reasonably explain the electrochemical phenomena occurring inside DAFCs under operating conditions.

[0017] (3) For direct ammonia fuel cells that use ammonia solution as the anode supply, an equivalent circuit model of the direct ammonia fuel cell that takes into account the anode solution aggregation phenomenon is proposed. As a modification of the standard model, this model can more completely explain the electrochemical characteristics inside the DAFC. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1( a ) is a schematic diagram of a standard equivalent circuit of a direct ammonia fuel cell according to the present invention.

[0019] FIG. 1( b ) is a schematic diagram of an equivalent circuit of a direct ammonia fuel cell according to the present invention taking into account the anode solution aggregation phenomenon.

[0020] FIG. 2( a ) is a Nyquist diagram drawn from the EIS test results according to an embodiment of the present invention.

[0021] FIG. 2( b ) is a Nyquist diagram drawn according to the EIS test results of an embodiment of the present invention (anode solution aggregation phenomenon occurs).

[0022] FIG. 3( a ) is a comparison between the measured results and the fitting results of an embodiment of the present invention (DAFC standard equivalent circuit).

[0023] FIG. 3( b ) is a comparison between the measured results and the fitting results of an embodiment of the present invention (DAFC equivalent circuit considering the anode solution aggregation phenomenon).

[0024] Table 1 is the quantitative analysis result of polarization impedance of the embodiment of the present invention (DAFC standard equivalent circuit).

[0025] Table 2 is the quantitative analysis result of polarization impedance of the embodiment of the present invention (DAFC equivalent circuit considering the anode solution aggregation phenomenon). DETAILED DESCRIPTION

[0026] The content of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments are descriptive rather than restrictive, and the protection scope of the present invention is not limited thereto.

[0027] The equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell includes: an inductor element L1, an ohmic impedance element R0, a constant phase element CPE characterizing the capacitance characteristics of an electrolyte membrane, an impedance element R1 characterizing the bipolar Faraday process, a capacitor element C, an impedance element Z characterizing the internal mass transfer loss of the battery, and a Faraday process impedance element R1' at the anode solution aggregation point, and is characterized in that: the constant phase element CPE characterizing the capacitance characteristics of an electrolyte membrane is connected in parallel with the impedance element R1 characterizing the bipolar Faraday process and the impedance element Z characterizing the internal mass transfer loss of the battery, and then connected in series with the inductor element L1 and the ohmic impedance element R0 to form a standard equivalent circuit of a direct ammonia fuel cell.

[0028] The constant phase element CPE representing the capacitance characteristics of the electrolyte membrane is connected in parallel with the impedance element R1 representing the two-electrode Faraday process and the impedance element Z representing the mass transfer loss inside the battery, and the capacitance element C is connected in parallel with the Faraday process impedance element R1' where the anode solution is gathered. The two parallel circuits are connected in series and then connected in series with the inductance element L1 and the ohmic impedance element R0 to form an equivalent circuit of a direct ammonia fuel cell considering the anode solution accumulation phenomenon.

[0029] The standard equivalent circuit of the direct ammonia fuel cell or the equivalent circuit of the direct ammonia fuel cell considering the anode solution aggregation phenomenon is used to fit the electrochemical impedance spectrum test results of the direct ammonia fuel cell, and the parameters of each component of the equivalent circuit are calculated to obtain the full battery impedance information of the battery ohmic impedance, activation impedance, and finite layer diffusion coefficient.

[0030] The present invention adopts liquid ammonia water as the anode supply solution of the direct ammonia fuel cell and adopts an anion exchange membrane as the electrolyte membrane.

[0031] The battery under test was subjected to electrochemical impedance spectroscopy analysis using the standard equivalent circuit of a direct ammonia fuel cell. The water contact angle of the anode diffusion layer of the battery was greater than 130°. The anode diffusion layer did not show solution aggregation under the battery operating conditions. The impedance real part-impedance imaginary part change curve of the battery during the EIS test showed two arc shapes. The impedance real part-impedance imaginary part change curve is customarily called the Nyquist curve.

[0032] The battery under test was subjected to electrochemical impedance spectroscopy analysis using an equivalent circuit of a direct ammonia fuel cell taking into account the anode solution aggregation phenomenon. The water contact angle of the anode diffusion layer of the battery was less than 130°, and solution aggregation occurred in the anode diffusion layer under the battery operating conditions. The Nyquist curve of the battery during the EIS test showed a three-segment arc shape.

[0033] L1 is an inductance element representing the inductance characteristics of the wire, R0 is an impedance element representing the ohmic impedance, CPE is a constant phase element representing the capacitance characteristics of the electrolyte membrane, R1 is an impedance element representing the impedance characteristics of the two-pole Faraday process, Z is an impedance element representing the mass transfer impedance of hydroxide ions inside the battery, C is a capacitance element representing the capacitance effect generated by the anode solution aggregation phenomenon, and R1' is an impedance element representing the impedance characteristics of the Faraday process occurring at the anode solution aggregation point.

[0034] The specific operation method is as follows:

[0035] (1) Determine the test parameters of the EIS test (frequency range, AC disturbance amplitude, sampling period, etc.) according to the actual operating parameters of the DAFC under test (output voltage, output current).

[0036] (2) The battery under test is tested using an electrochemical workstation with EIS test function, and key data such as frequency, real impedance, and imaginary impedance are recorded.

[0037] (3) The Nyquist curve of the battery is drawn based on the EIS test data, and the DAFC equivalent circuit is selected based on the Nyquist image characteristics.

[0038] (4) The mathematical model of the equivalent circuit is fitted with the measured data of the battery through the multivariate regression method, and the key parameters of the equivalent circuit obtained are the polarization impedance of each part of the battery.

[0039] The standard equivalent circuit of the direct ammonia fuel cell proposed in the present invention is shown in FIG. 1( a ); the equivalent circuit of the DAFC considering the anode solution aggregation phenomenon is shown in FIG. 1( b ). Specific embodiment:

[0041] Prepare anion exchange membrane electrode and assemble the direct ammonia fuel cell prototype to be tested. The membrane electrode of the DAFC prototype to be tested is prepared by pressing an anion exchange membrane with a bipolar diffusion electrode. The effective active area of ​​the membrane electrode is 2×2cm 2 The active component of the anode catalyst is PtIr / C, and the platinum group metal loading of the catalyst layer is 1.5 mg cm -1 The active component of the cathode catalyst is Pt / C, and the platinum group metal loading of the catalyst layer is 0.5 mg cm -1 .

[0042] The test conditions of DAFC were as follows: the battery operating temperature was 80°C, the anode reactant was a mixed solution of 3M ammonia and 1M potassium hydroxide, and the supply rate was 4ml min -1 The cathode reactant is humidified air with a humidity of 100% at the corresponding operating temperature, and the supply rate is 0.2 SLPM. The operating conditions are maintained until the open circuit voltage of the DAFC prototype under test is stable to activate the battery.

[0043] Electrochemical impedance spectroscopy test: The anode of the DAFC prototype to be tested is connected to the auxiliary electrode and reference electrode of the electrochemical workstation through a wire, and the cathode is connected to the working electrode of the electrochemical workstation. The EIS test is performed on the prototype to be tested by the alternating current method. The EIS test parameters of the electrochemical workstation are set as follows: the constant current is 200mA, 100mA, and 80mA respectively, the AC disturbance value is 10% of the battery constant current, the frequency scanning direction is high frequency to low frequency, and the upper frequency limit is 10 5 Hz, and the lower frequency limit is 0.1Hz. Record the real and imaginary impedance values ​​at each AC frequency, and plot the Nyquist diagram of DAFC with the real impedance as the X-axis and the imaginary impedance as the Y-axis.

[0044] Analysis of electrochemical impedance spectrum: The Nyquist curve obtained by testing the DAFC prototype under the condition of constant current of 200mA is shown in Figure 2(a), and the curve shape is two arcs of different sizes. The horizontal axis and vertical axis of the Nyquist diagram represent the real and imaginary values ​​of the overall impedance of the battery under test during the EIS test. The EIS test direction of the fuel cell is usually from high frequency to low frequency. The electrochemical process of DAFC under high-frequency disturbance is only related to ohmic loss. At this time, there is no phase difference between the response voltage and the change of the disturbance current, and the imaginary part of the impedance is almost zero. The electrochemical process of DAFC under medium-frequency AC disturbance is generally related to activation loss. The electrode activation loss causes a phase difference between the input disturbance current and the output voltage. At this time, the overall impedance of the battery is no longer a real number and needs to be expressed in a complex form with real and imaginary parts. This makes the Nyquist curve appear in the positive direction of the X-axis. The size of this arc can qualitatively characterize the size of the battery activation loss. As the frequency decreases further, the mass transfer loss of the reactants inside the battery gradually manifests itself during the transfer process, which also causes a phase difference between the voltage and current. Therefore, a second arc appears in the positive direction of the X-axis in the Nyquist diagram, and the size of the arc is related to the mass transfer impedance.

[0045] By adjusting the hydrophilicity of the anode diffusion layer of the prototype, the Nyquist curve of the prototype under the anode solution aggregation phenomenon is shown in Figure 2(b), and the curve shape is three arcs of different sizes. Compared with the Nyquist curve of the DAFC without anode solution aggregation, the extra third arc is located between the activation loss arc and the mass transfer loss arc in the X-axis direction. This is because the anode electrolyte solution exhibits a capacitive effect under the action of the potential difference between the two electrodes of the battery after aggregation, resulting in a phase difference between the response voltage and the disturbance current.

[0046] For the Nyquist diagram composed of two arcs, the standard equivalent circuit of the direct ammonia fuel cell is used for fitting; for the Nyquist diagram composed of three arcs, it can be judged that solution aggregation occurs at the battery anode, and the direct ammonia fuel cell equivalent circuit that takes into account the anode solution aggregation phenomenon is used for fitting.

[0047] Equivalent circuit fitting and quantitative analysis of polarization loss: Based on the above judgment criteria, the corresponding equivalent circuit is used to fit the electrochemical impedance curve of the DAFC under test. Based on the two equivalent circuits, the mathematical expressions of the change of the overall impedance of the equivalent circuit corresponding to the AC frequency under the above test conditions are calculated respectively. Through the multivariate regression method, the expressions are fitted with the measured data to obtain the mass transfer impedance, activation impedance and ohmic impedance of the DAFC under test respectively.

[0048] Figure 3(a) shows the comparison between the fitting curve of the standard equivalent circuit of the direct ammonia fuel cell and the measured data; Figure 3(b) shows the comparison between the fitting curve of the DAFC equivalent circuit considering the anode solution aggregation and the measured data when the anode solution aggregation phenomenon occurs.

[0049] It can be seen from the above Figures 2 and 3 that the fitting curve is very close to the measured data, indicating that the standard equivalent circuit can numerically simulate the impedance change characteristics of DAFC during the EIS test, and the equivalent circuit considering the anode solution aggregation phenomenon can also numerically simulate the impedance change characteristics of DAFC during the EIS test when the anode solution is aggregated.

[0050] Table 1 shows the numerical calculation results of the polarization impedance of each part obtained by fitting the standard equivalent circuit of the direct ammonia fuel cell. The ohmic impedance value is the impedance of the battery as a whole generated by the ohmic effect, and the activation impedance value is the impedance of the battery corresponding to the activation loss of the electrochemical reaction under the operating conditions. In addition, the mass transfer loss in the ion transfer process in the battery is represented by the finite layer diffusion coefficient, and the size of this value can directly reflect the difficulty of ion transfer.

[0051] Table 1. Quantitative analysis results of polarization impedance of the embodiment (DAFC standard equivalent circuit)

[0052]

[0053] Table 2 shows the numerical calculation results of the polarization impedance of each part of DAFC under the condition of anode solution accumulation, obtained by fitting the DAFC equivalent circuit taking into account the anode solution accumulation phenomenon.

[0054] Table 2. Quantitative analysis results of polarization impedance of the embodiment (DAFC equivalent circuit considering the anode solution aggregation phenomenon)

[0055]

Claims

1. Equivalent circuit for electrochemical impedance spectroscopy analysis of direct ammonia fuel cells. The standard equivalent circuit of direct ammonia fuel cells includes: An inductor element, an ohmic impedance element, a constant phase element characterizing the capacitance characteristics of the electrolyte membrane, an impedance element characterizing the bipolar Faraday process, a capacitor element, an impedance element characterizing the mass transfer loss inside the battery, and a Faraday process impedance element at the anode solution gathering place, characterized in that: the constant phase element characterizing the capacitance characteristics of the electrolyte membrane is connected in parallel with the impedance element characterizing the bipolar Faraday process and the impedance element characterizing the mass transfer loss inside the battery, and then connected in series with the inductor element and the ohmic impedance element to form a standard equivalent circuit of a direct ammonia fuel cell; The constant phase element representing the capacitance characteristic of the electrolyte membrane is connected in parallel with the impedance element representing the Faraday process at both electrodes and the impedance element representing the mass transfer loss inside the battery, and the capacitance element is connected in parallel with the Faraday process impedance element at the anode solution accumulation point. The two parallel circuits are connected in series and then connected in series with the inductance element and the ohmic impedance element, thus forming an equivalent circuit of a direct ammonia fuel cell considering the anode solution accumulation phenomenon. The electrochemical impedance spectroscopy test results of the direct ammonia fuel cell are fitted using the standard equivalent circuit of the direct ammonia fuel cell, or using the equivalent circuit of the direct ammonia fuel cell taking into account the anode solution aggregation phenomenon, so as to obtain the full battery impedance information of the battery ohmic impedance, activation impedance and finite layer diffusion coefficient.

2. The equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell according to claim 1, characterized in that: Liquid ammonia water is used as the anode supply solution of the direct ammonia fuel cell, and an anion exchange membrane is used as the electrolyte membrane.

3. The equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell according to claim 1, characterized in that: The battery under test that uses the standard equivalent circuit of the direct ammonia fuel cell for electrochemical impedance spectroscopy analysis has a water contact angle of anode diffusion layer greater than 130°, no solution aggregation occurs in the anode diffusion layer under battery operating conditions, and a curve of real impedance part-imaginary impedance change of the battery during electrochemical impedance spectroscopy testing presents two arc shapes.

4. The equivalent circuit for electrochemical impedance spectroscopy analysis of a direct ammonia fuel cell according to claim 1, characterized in that: The battery under test that uses the equivalent circuit of the direct ammonia fuel cell taking into account the anode solution aggregation phenomenon for electrochemical impedance spectroscopy analysis has a water contact angle of the anode diffusion layer of the battery that is less than 130°, and solution aggregation occurs in the anode diffusion layer under the battery operating conditions. The impedance real part-impedance imaginary part change curve of the battery during the electrochemical impedance spectroscopy test presents a three-segment arc shape.

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