CN116264304A - Method for detecting cathode fluid distribution of proton exchange membrane fuel cell stack - Google Patents
Method for detecting cathode fluid distribution of proton exchange membrane fuel cell stack Download PDFInfo
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Abstract
The present application discloses a method for detecting cathode fluid distribution of proton exchange membrane fuel cell stack, which is based on the principle that the relation (f) between characteristic frequency of flow path impedance and stoichiometric ratio of cathode fluid ch And 1/ln (1-1/lambda) to determine the cathode fluid distribution of each cell in the stack. The detection method is simple, practicable, practical and effective. All parameters are extracted from the electrochemical impedance spectrum measured in real time, and the method does not need to be corrected in advance, so that the method has great significance for high-consistency electric pile design and health state monitoring of each cell of the electric pile.
Description
Technical Field
The invention belongs to the technical field of fuel cells, and particularly relates to a method for detecting cathode fluid distribution of a proton exchange membrane fuel cell stack.
Background
The proton exchange membrane fuel cell is a power generation device for directly converting fuel chemical energy into electric energy, has the advantages of high energy conversion efficiency, high specific energy, low pollution and the like, and has wide application prospects in the aspects of power sources of transportation equipment (including automobiles, ships, unmanned aerial vehicles and the like), portable mobile power sources, fixed power stations and the like.
A typical proton exchange membrane fuel cell consists of a polar plate with a flow channel, an anode, an electrolyte, a cathode and a polar plate with a flow channel in sequence. Since the output power of a single fuel cell is limited, a high-power stack is formed by stacking several tens to hundreds of identical fuel cells in series. Achieving high performance and long life stacks remains an important goal for large-scale commercial applications. Improving consistency of stack internals, thermal, electrical, force distribution is critical to ensure optimal stack performance and durability.
Ideally, the stack would need to provide the same flow of reactant gas to each cell through the manifold. However, even fluid distribution within the stack is practically difficult to achieve due to branching flow problems, especially for commercial stacks with multiple cell counts and limited manifold space. Cathode fluid maldistribution can affect the operating characteristics of a single cell, such as electrochemical reactions, ionic/electronic conductivity, etc. Cells with less flow supply experience under-gassing and flooding, resulting in voltage drops and damage to the electrolyte membrane and catalyst. And the excessive flow supply of the battery can dehydrate the electrolyte membrane, reducing the conductivity. Therefore, online measurement of cathode fluid distribution in an online measurement cell stack is important for improving cell stack performance and enhancing durability, and can provide powerful support for high-consistency cell stack design and cell stack real-time health monitoring. However, a simple and effective method for detecting the consistency of the fluid distribution of the proton exchange membrane fuel cell stack is still lacking at present.
In the prior art, the method for detecting the fluid distribution consistency of the proton exchange membrane fuel cell stack mainly comprises the following steps: 1) An embedded microsensor: a thermal or optical micro flow sensor is embedded in the flow channel; the pressure drop of each cell is measured by inserting a pressure measuring rod into the main pipe of the electric pile. Because of the millimeter-sized flow channel, the sensor is difficult to embed, and interference to the operation of the galvanic pile is possible. 2) Limiting current method: and introducing hydrogen diluted by inert gas into the cathode side of the electric pile, and testing and comparing the limiting current of each single cell. Because of the many factors that affect the uniformity of the limiting current, it is not possible to determine whether this is caused by uniformity of fluid distribution and is not suitable for on-line monitoring. 3) Voltage decay method: according to the fact that ohmic loss is mainly affected by the water content in the proton exchange membrane, the flow is judged by monitoring the attenuation value of the voltage of each battery. The method is only suitable for low-temperature proton exchange membrane fuel cells with ohmic resistance easily affected by the water content of the electrolyte. 4) Electrochemical impedance spectroscopy: the flow rate is judged based on the relation between the flow channel impedance and the cathode reaction gas flow, and the flow rate is usually related to fixed frequency impedance, flow channel impedance arc size or a physical model at present. These processing methods require parameter correction in advance in an off-line state or are costly to calculate.
Disclosure of Invention
Electrochemical impedance is currently the most commonly used non-invasive non-destructive monitoring technique, and therefore, flow measurement methods based on this principle are easier to implement. Aiming at the problems, the invention provides a rapid, simple and convenient in-situ on-line method for detecting the consistency of cathode fluid distribution of a proton exchange membrane fuel cell stack.
The invention is mainly based on electrochemical impedance measurement of each cell of the electric pile, and the cathode reaction gas flow of each cell of the electric pile is calculated through simple correlation of the constructed cathode reaction gas flow (or stoichiometric ratio) and the characteristic frequency related to the oxygen mass transfer process.
Aiming at the problems of large embedding difficulty, interference, off-line measurement, limited applicable system, high calculation cost and the like, the invention provides a rapid, simple and in-situ on-line method for detecting the consistency of cathode fluid distribution of a fuel cell stack.
In one aspect of the present application, a method for detecting cathode fluid distribution of a proton exchange membrane fuel cell stack is provided, the method comprising: characteristic frequency f using flow path impedance ch The volumetric cathode gas flow rate Q (T, p) of each cell in the stack is determined in linear relation to the cathode fluid stoichiometry 1/ln (1-1/lambda).
The detection principle of the invention is based on the correlation between the characteristic frequency of the flow path impedance and the cathode reactant gas flow (or stoichiometric ratio). As shown in FIG. 1, the local impedance nature of the low frequency band is that of local sinusoidal oxygen concentration oscillations within the cathode catalyst layer
Is a local response of (c). />
Comprising sinusoidal oxygen concentration fluctuations of both processes. The oxygen fluctuation corresponding to the process I occursDisturbance at the cathode electrode by alternating current>
Excited local oxygen concentration oscillation->
The oxygen fluctuation corresponding to process II is oxygen concentration oscillation transmitted along the flow channel>
It originates from upstream +.>
The oxygen consumption along the flow channel causes the oxygen concentration to decrease along the flow channel, thereby causing the oxygen concentration of the cathode electrode to fluctuate +.>
Extending into the flow channel. These upstream oxygen concentration fluctuations (++) also due to the forced air convection effect within the flow channel>
I.e. < ->
) Carried downstream along the flow channel and with +.>
The coupling, in turn, affects the oxygen concentration in the downstream catalytic layer. Process ii contributes significantly to the impedance response, thereby creating a second low frequency arc, the so-called "runner impedance". Lower air stoichiometry means lower air velocity, therefore +.>
Is carried more slowly along the flow path (corresponding to a smaller characteristic frequency of the flow path impedance), which is why the characteristic frequency of the flow path impedance can be correlated with the air stoichiometry.
In order to realize the detection of the consistency of the fluid distribution of the proton exchange membrane fuel cell stack, the invention adopts the following technical scheme: under the premise of keeping the normal and stable operation of the electric pile, the electrochemical impedance spectrum (the frequency range from 10kHz to 0.1 Hz) of each single cell of the electric pile is tested by using a multi-channel electrochemical workstation, the characteristic frequency of the flow channel impedance (the second low-frequency arc) related to the oxygen transmission process in the flow channel is extracted through physical model fitting, equivalent circuit fitting (ECM) or relaxation time Distribution (DRT) analysis, and the fluid flow rate or the stoichiometric ratio of each cell is calculated according to the correlation between the characteristic frequency and the fluid flow rate or the stoichiometric ratio (as shown in figure 2).
Optionally, the detection method specifically includes the following steps:
s001, extracting the characteristic frequency f of the oxygen transmission process in the cathode flow channel of each battery according to the electrochemical impedance spectrum analysis of each battery ch The method comprises the steps of carrying out a first treatment on the surface of the Extracting the effective depth h of the cathode flow channel of each battery according to the electrochemical impedance model considering the flow channel impedance and the flow channel impedance arc fitting of the actually measured electrochemical impedance;
s002, calculating the effective depth of an average cathode runner of the galvanic pile and the relative deviation of each section; if the obtained relative deviation is within 0-10%, calculating the air metering ratio lambda of each battery according to the formula 1 n,cal ；
H is the effective depth of the average cathode flow channel of the electric pile which is arbitrarily smaller than or equal to the effective depth of the anode flow channel of the electric pile;
wherein c ref Inlet oxygen concentration for each cell; f is Faraday constant; j is the current density of the pile in steady state operation;
the error analysis by numerical analysis shows that if the relative deviation of h extracted by each battery of the electric pile is less than 10%, the average value of all the batteries h is uniformly used as a calculated value, and the resulting relative error is small (6.95%). Further, a value smaller than the average value is uniformly used as the calculated value, and the relative error thereof is smaller. Therefore, in this specific example, any effective depth of the cathode flow channels smaller than or equal to the whole stack average can be used as the calculated value of h.
S003, the lambda obtained in the step S002 is matched with the formula 2 n,cal Correcting to obtain accurate stoichiometric ratio lambda n,cor ；
Where N is the total number of cells, lambda stack Is the stoichiometric ratio of air in the whole stack;
optionally, for the case where the model extracted h is low overall, λ obtained in step S002 is calculated according to equation 2 n,cal Correction is performed.
S004, calculating the volume flow of each section of cathode gas according to the method 3;
wherein Q (T, p) is the volume flow rate (Lmin) of each cathode gas -1 ) A is the active surface area (cm) of the cell 2 ) N is the electron transfer number, lambda is the accurate stoichiometric ratio lambda of the cathode gas of each battery obtained in step S003 n,cor 。
Optionally, f as described in step S001 ch Obtained by the following method:
(1) During the stable operation of the proton exchange membrane fuel cell stack, collecting the electrochemical impedance spectrum of each cell of the stack;
(2) Analyzing the electrochemical impedance spectrum obtained in the step (1) to obtain f ch 。
Optionally, in the step (1), the electrochemical impedance spectrum of each cell of the collecting pile is collected by an electrochemical workstation, and the collecting frequency range is equal to the collecting frequency range.
Optionally, in step (2), the resolving comprises physical model fitting, equivalent circuit fitting or relaxation time distribution analysis.
Alternatively, step S002In said c ref Calculated according to the method of 4,
wherein p is the average pressure of the gas in the manifold, T is the average temperature of the gas in the manifold,
is the molar ratio of oxygen, R is the gas constant and is 8.314J K -1 mol -1 。
Inlet oxygen concentration c of each battery ref Depending on the humidity, temperature and pressure within the manifold. The experimental results show that the changes in humidity and temperature have little effect on the characteristic frequency, as shown in fig. 3. Also, the manifold humidity change itself is not obvious due to the short residence time of the gas in the stack manifold and the lack of a humidification source. In addition, the pressure difference within commercial stack manifolds is small (about 1.3%). Thus, the oxygen concentration at the manifold inlet can be taken as the oxygen concentration c at each cell inlet ref 。
Alternatively, when the proton exchange membrane fuel cell stack is fed with dry non-humidified air,
0.21.
Optionally, the electrochemical impedance model and the measured electrochemical impedance spectrum data in the step S001 are converted and fitted according to the formulas 5 and 6 respectively;
wherein Z is the complex impedance, re (Z) is the real part of the complex impedance, and Im (Z) is the imaginary part of the complex impedance; r is R ∞ Is the internal resistance of the film; phi is the impedance dataPhase angle.
Wherein tan phi exp Obtained from experimental data.
Optionally, Z exp The complex impedance is obtained by testing, and only the flow channel impedance arc data of the electrochemical impedance is selected for calculation. The real part Re (Z) exp ) Subtracting the internal resistance R of the film ∞ Is to match the assumption of neglected ohmic resistance in the model, R ∞ The extraction can also be resolved by physical model fitting, equivalent circuit fitting (ECM) or relaxation time Distribution (DRT).
Optionally, the Z exp Obtained by testing electrochemical impedance spectra;
the R is ∞ Obtained by the following method: during the stable operation of the proton exchange membrane fuel cell stack, collecting the electrochemical impedance spectrum of each cell of the stack; analyzing the obtained electrochemical impedance spectrum to obtain R ∞ Wherein the parsing includes any one of physical model fitting, equivalent circuit fitting (ECM), or relaxation time distribution analysis (DRT);
optionally, the electrochemical impedance spectrum of each cell of the collecting pile is collected through an electrochemical workstation, and the collecting frequency interval is 0.1 Hz-10 kHz.
The application describes an electrochemical impedance model, as shown in formula 7, according to formula 7, fitting and extracting the effective depth h of each battery cathode runner, and in the actual detection process, the electrochemical impedance model is not limited to the electrochemical impedance model mentioned in the application;
wherein, the liquid crystal display device comprises a liquid crystal display device,
m＝4Fhc ref (8)
n=λ qJ (formula 9)
Im(D)＝-mβω-nδm 2 ω 3 +mnγωcosα+(n 2 -mδω 2 ) γsin α (formula 11)
β＝γn-Jm 2 ω 2 (13)
Gamma= (λ -1) Jn (formula 14)
the effective depth h (cm) of each cell cathode flow channel will be less than the pre-assembly measurement due to the embedded flow channels of the assembled gas diffusion layers, etc. Thus, the effective depth h needs to be fitted and extracted according to the electrochemical impedance model of the formula 1;
according to formulas 7 to 17, tan can be obtained
With respect to ω, λ, h, J, f ct Function of equal parameters. In order to improve accuracy, all parameters except h are calculated from experimental data.
Where ω is the impedance data of the test (ω=2pi f) scaled. J is the current density of the stack operation during the test.
Optionally, said f ct Obtained by the following method:
(1) During the stable operation of the proton exchange membrane fuel cell stack, collecting the electrochemical impedance spectrum of each cell of the stack;
(2) Analyzing the electrochemical impedance spectrum obtained in the step (1) to obtain f ct 、。
Optionally, the detection method is suitable for the relative deviation of the effective depth of the cathode flow channel of each battery to be within 0-10%.
Because the method is established based on the relation between the flow passage impedance characteristic and the gas flow, the method is used on the premise that each cell of the electric pile can measure the flow passage impedance. From the above-described test principle, it is known that the flow channel impedance is mainly caused by the consumption of oxygen concentration in the flow channel. The range of use of the method is therefore dependent on the flow channel geometry and stack operating conditions (gas flow, current density, gas oxygen concentration, etc.).
The beneficial effects that this application can produce include:
compared with the prior art, the fuel cell stack fluid distribution consistency detection method is simple, feasible, practical and effective. All parameters are extracted from the electrochemical impedance spectrum measured in real time, and the method does not need to be corrected in advance, so that the method has great significance for high-consistency electric pile design and health state monitoring of each cell of the electric pile.
Drawings
FIG. 1 is a schematic diagram of the flow path impedance generation principle.
FIG. 2 is a graph of characteristic frequency of flow channel impedance versus air stoichiometry associated with oxygen transport in a flow channel.
FIG. 3 is the effect of operating conditions on relaxation time distribution, wherein the operating condition of plot a is humidity; the operating condition for the change in b plot is temperature.
FIG. 4 is a schematic view of model fitting for extracting effective flow channel depth, experimental data tan φ exp And model tan phi c Is a comparison of (c).
The maximum relative deviation of the effective flow channel depth at different stoichiometries and the depths in the three cells for each cell extracted from case 1 in the example of fig. 5.
FIG. 6 shows the results of the fluid dispensing test of example 1.
Wherein 1 is a cathode flow channel, 2 is a gas diffusion layer, and 3 is a cathode catalytic layer.
Detailed Description
Example 1
In order to verify the accuracy of the method in the embodiment, the electric pile used in the embodiment is a specially designed electric pile with independent air paths, wherein three high-temperature proton exchange membrane fuel cells are stacked in series, and each cell is provided with a mass flowmeter for supplying air, so that a reference value is conveniently provided for the measured value. Introducing non-humidified gas into anode inlet or cathode inlet of fuel cell stack, and setting voltage acquisition point at cathode and anode of single cell by using Solartron
Electrochemical workstations (including 1470E potentiostat and 1455 frequency response analyzer) record electrochemical impedance spectra in constant current mode. And simultaneously recording the impedance spectrum of each cell in the electric pile through an auxiliary voltage division channel of the workstation, and calculating the inlet air metering ratio of each single cell, thereby judging the consistency of fluid distribution of the fuel cell electric pile. The test galvanic pile has the working temperature of 160 ℃ and the current density of 100mA cm -2 And running in a lower steady state. Because each gas path is independent, three different gas distribution conditions (the condition 1 is uniformly distributed corresponding to each battery, the relative standard deviation of the distribution is 5% corresponding to the condition 2, and the relative standard deviation of the distribution is 10% corresponding to the condition 3) are simulated altogether, and each distribution condition also simulates the condition of different total flow.
The method for detecting the consistency of the fluid distribution of the fuel cell stack comprises the following steps:
1) During the stable operation of the electric pile, the electrochemical impedance spectrum of each cell of the electric pile is collected through an electrochemical workstation, and the collection frequency interval is 10 kHz-0.1 Hz;
2) Electrochemical impedance spectroscopy by relaxation time Distribution (DRT) analysis, and extraction of internal resistance R of membrane therefrom ∞ Characteristic frequency f related to oxygen reduction reaction process ct And a characteristic frequency f related to the oxygen mass transfer process ch And the current density J is read to calculate the oxygen concentration c ref ；
3) Combining the parameters obtained in the step 2) and the data of the flow passage impedance interval measured through experiments, fitting with the model to obtain the effective flow passage depth h of each cell, wherein partial fitting results are shown in fig. 4, and calculating the average cathode flow passage effective depth of the galvanic pile and the relative deviation of each cell;
4) Judging whether the calculated relative deviation is smaller than 10%, if yes, adopting the effective depth of an average cathode flow channel of a galvanic pile and the characteristic frequency f related to the oxygen mass transfer process of each battery ch Substituting formula (18) to calculate air metering ratio lambda of each battery n,cal . Otherwise, the method has larger error and is not applicable; as shown in FIG. 5, the relative deviations of the extracted effective runner depths are all within 0-10%.
5) Correction is performed according to formula (19) to obtain an accurate stoichiometric ratio, and further, the stoichiometric ratio can be converted into a volume flow according to formula (20).
The test results are compared with the air metering ratio calculated by the mass flowmeter as shown in fig. 6. As can be seen from the graph, the relative error of all test results in this example is less than 3.11% compared with the reference value calculated by the mass flowmeter, and the accuracy is good.
Example 2
According to the following steps, an active area of 45cm 2 Is provided with a snake-shaped runner, and the high temperature proton exchange membrane fuel cell with the snake-shaped runner is at 160 ℃ and 200mA cm -2 Running under the condition of testing electrochemical impedance spectrums by testing different cathode air stoichiometric ratios; then the impedance spectrum is resolved to extract f ch . Fig. 2 is obtained, and it can be seen from fig. 2 that there is a linear relationship between the characteristic frequency of the flow path impedance associated with the oxygen transport process in the flow path and the stoichiometric air ratio.
Example 3 influence of operating conditions on relaxation time distribution.
The test was performed as follows: an active area of 45cm 2 Is provided with a snake-shaped runner, and the high temperature proton exchange membrane fuel cell with the snake-shaped runner is at 160 ℃ and 200mA cm -2 Running under the condition of changing the humidity of cathode air of the battery, and testing electrochemical impedance spectrum; then the impedance spectrum is resolved to extract f ch The method of fig. 3a is obtained,
an active area of 45cm 2 The high-temperature proton exchange membrane fuel cell with the snake-shaped runner runs under 200mA cm < -2 >, the running temperature of the cell is changed, and the electrochemical impedance spectrum is tested; then the impedance spectrum is resolved to extract f ch . Obtaining FIG. 3b
The test results are shown in FIG. 3, wherein the operating condition for the change in panel a is humidity; b changing the operating conditions of the graph to temperature; wherein the P1 peak is related to the process of oxygen transmission in the flow channel, and the P2A peak is related to the oxygen reduction reaction process. The peak top corresponding frequency is the characteristic frequency of the corresponding process. As can be seen from FIG. 3, the characteristic frequency f of the temperature and humidity versus the impedance of the flow path ch The influence is small.
Example 4 experimental data tan
And model tan->
Is a comparison of (2)
The data is obtained from a single active area of 160cm 2 Three-section electric pile of high temperature proton exchange membrane fuel cell with parallel flow passage at 160 deg.C and 100mA cm -2 The results were tested below. The test results are shown in FIG. 4; wherein the solid marking line is a model tan
A predicted value; the hollow mark line is experimental data tan +.>
The foregoing description is only a few examples of the present application and is not intended to limit the present application in any way, and although the present application is disclosed in the preferred examples, it is not intended to limit the present application, and any person skilled in the art may make some changes or modifications to the disclosed technology without departing from the scope of the technical solution of the present application, and the technical solution is equivalent to the equivalent embodiments.
Claims (10)
1. A method for detecting cathode fluid distribution of a proton exchange membrane fuel cell stack, the method comprising: characteristic frequency f using flow path impedance ch The volumetric cathode gas flow rate Q (T, p) of each cell in the stack is determined in linear relation to the cathode fluid stoichiometry 1/ln (1-1/lambda).
2. The method of detection according to claim 1, characterized in that the method of detection comprises the steps of:
s001, extracting the characteristic frequency f of the oxygen transmission process in the cathode flow channel of each battery according to the electrochemical impedance spectrum analysis of each battery ch The method comprises the steps of carrying out a first treatment on the surface of the Extracting the effective depth h of the cathode flow channel of each battery according to the electrochemical impedance model considering the flow channel impedance and the flow channel impedance arc fitting of the actually measured electrochemical impedance;
s002, calculating the effective depth of an average cathode runner of the galvanic pile and the relative deviation of each section; if the relative deviation is obtainedWithin 0-10%, the air metering ratio lambda of each battery is calculated according to the formula 1 n，cal ；
H is the effective depth of the average cathode flow channel of the electric pile which is arbitrarily smaller than or equal to the effective depth of the anode flow channel of the electric pile;
wherein c ref Inlet oxygen concentration for each cell; f is Faraday constant; j is the current density of the pile in steady state operation;
s003, the lambda obtained in the step S002 is matched with the formula 2 n，cal Correcting to obtain accurate stoichiometric ratio lambda n，cor ；
Where N is the total number of cells, lambda stack Is the stoichiometric ratio of air in the whole stack;
s004, calculating the volume flow of each section of cathode gas according to the method 3;
wherein Q (T, p) is the volume flow of each cathode gas, A is the active surface area of the battery, n is the electron transfer number, and lambda is the accurate stoichiometric ratio lambda of each battery cathode gas obtained in step S003 n,cor 。
3. The method of claim 2, wherein,
f described in step S001 ch Obtained by the following method:
(1) During the stable operation of the proton exchange membrane fuel cell stack, collecting the electrochemical impedance spectrum of each cell of the stack;
(2) For step (1) obtainIs analyzed by electrochemical impedance spectrum to obtain f ch 。
4. The method according to claim 3, wherein,
in the step (1), the electrochemical impedance spectrum of each cell of the collecting pile is collected through an electrochemical workstation, and the collecting frequency interval is 0.1 Hz-10 kHz.
5. The method according to claim 3, wherein,
in the step (2), the analysis is selected from at least one of physical model fitting, equivalent circuit fitting or relaxation time distribution analysis.
6. The method of claim 2, wherein,
in step S002, the c ref Calculated according to the method of 4,
wherein p is the average pressure of the gas in the manifold, T is the average temperature of the gas in the manifold,
r is the gas constant, which is the molar ratio of oxygen.
7. The method according to claim 6, wherein,
when the proton exchange membrane fuel cell stack is fed with dry non-humidified air,
0.21.
8. The method of claim 2, wherein,
the electrochemical impedance model and the measured electrochemical impedance spectrum data in the step S001 are converted and fitted according to the formulas 5 and 6 respectively;
wherein Z is the complex impedance, re (Z) is the real part of the complex impedance, and Im (Z) is the imaginary part of the complex impedance; r is R ∞ Is the internal resistance of the film; phi is the phase angle of the impedance data.
9. The method according to claim 8, wherein,
the Z is exp Obtained by testing electrochemical impedance spectra;
the R is ∞ Obtained by the following method: during the stable operation of the proton exchange membrane fuel cell stack, collecting the electrochemical impedance spectrum of each cell of the stack; analyzing the obtained electrochemical impedance spectrum to obtain R ∞ Wherein resolving is selected from at least one of a physical model fit, an equivalent circuit fit, or a relaxation time distribution analysis;
preferably, the electrochemical impedance spectrum of each cell of the collecting pile is collected by an electrochemical workstation, and the collecting frequency range is 0.1 Hz-10 kHz.
10. The method according to claim 1, wherein,
the detection method is suitable for the fact that the relative deviation of the effective depth of the cathode flow channel of each battery is within 0-10%.
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