Method for determining a transition metal cation concentration within a fuel cell system

Abstract

The method relates to a method for determining a transition metal cation concentration (H+ exch) within a fuel cell system having an unknown transition metal cation concentration (H+ exch) by means of electrochemical impedance spectroscopy, the method comprising the steps of: determining (100) an electrical high-frequency resistance (R) within the fuel cell system having an unknown transition metal cation concentration (H+ exch); normalizing (200) the determined electrical high-frequency resistance (R) with respect to a value (R0) of an electrical high-frequency resistance of a fuel cell system having a minimal transition metal cation concentration (H+ exch); comparing (300) the normalized determined high-frequency resistance (R / R0) with a profile of a calibration curve (KK) for a normalized electrical high-frequency resistance (R / R0); and determining (400) the transition metal cation concentration (H+ exch) within the fuel cell system having an unknown transition metal cation concentration (H+ exch) on the basis of the comparison of the normalized determined high-frequency resistance (R / R0) with the profile of the calibration curve (KK) for a normalized electrical high-frequency resistance (R / R0).

Description

[0001] Description

[0002] Method for determining the transition metal cation concentration within a fuel cell system

[0003] The present invention is based on a method according to the predicate of the independent method claim and a system according to the predicate of the independent system claim.

[0004] State of the art

[0005] Fuel cells are electrochemical energy converters that directly convert the chemical energy stored in a fuel (e.g., hydrogen, methanol) into electrical energy. When hydrogen is used as the energy source, the only byproduct of the conversion is water, which is why fuel cells operated in this way are used for emission-free propulsion in motor vehicles as well as for stationary power generation.

[0006] A widely used design is the polymer electrolyte membrane fuel cell (PEMFC). The PEMFC essentially consists of a membrane electrode assembly (MEA) in which a negative electrode, the anode, and a positive electrode, the cathode, are separated by a polymer electrolyte membrane. In conjunction with bipolar plates for gas supply, cooling, and electrical conductivity, several membrane electrode assemblies can be connected in series to achieve the (high) voltages required for the respective end application.

[0007] The electrodes of a MEA typically consist of a highly porous, carbon-based substrate. Nanometer-sized platinum or platinum alloy particles are deposited on this substrate; these particles, as electrochemically active species, significantly determine the cell's performance. Increased performance can be achieved either by using higher amounts of platinum or by developing more active catalyst materials. The latter allows for a reduction in the total amount of platinum required, which represents a significant cost component of the PEMFC system, as well as enabling the PEMFC system to operate at higher cell voltages, resulting in a higher efficiency in utilizing the hydrogen energy carrier.

[0008] Such more active catalyst materials are, for example, in the form of platinum alloys with the general formula Pt. xM is known, where M corresponds to another transition metal (M = Co, Ni, Fe, Cu, ...). The presence of transition metal atoms in the metallic nanoparticles results in increased catalytic activity for the oxygen reduction reaction due to electronic effects. This enables PEMFC operation at higher cell voltages and thus increases efficiency.

[0009] Unfortunately, catalyst materials are exposed to an electrochemically aggressive local environment, which includes elevated temperatures (60–120°C), voltages and voltage cycles (0–1.0 V), and, most importantly, strongly acidic conditions (pH = 0 to 2). The typically reactive transition metals of the alloy (M = Co, Ni, Fe, Cu, ...) are unstable under these conditions with respect to proton oxidation.

[0010] In real-world continuous operation, this leads to a steady dissolution of this transition metal, which remains in cationic form within the PEMFC. This has two particularly serious consequences for the performance of the PEMFC.

[0011] First, the loss of transition metal atoms from the crystal lattice of the platinum alloy leads to a decrease in the desired electronic effects, which is accompanied by a progressive loss of catalytic activity for the oxygen reduction reaction. In the limiting case, this activity is reduced to that of a pure platinum catalyst.

[0012] Secondly, the dissolved transition metal cations bind to the polymer electrolyte, which ensures the proton conductivity essential for cell operation. This is located primarily in the polymer electrolyte membrane, but also as an ionomer binder in the anode and cathode. Due to the generally higher valence of the transition metal cations (Co 2+ , Ni 2+The higher binding affinity of the dissolved transition metal cations (vs. IT) displaces protons from the polymer electrolyte, thus impairing proton conductivity. At high concentrations of dissolved transition metal cations, this leads to a substantially reduced performance, especially at high current densities.

[0013] The amount of transition metal cations dissolved from the platinum alloy catalyst over the course of operation is crucial for the performance of the PEMFC. Quantifying the transition metal cations is therefore desirable to assess the aging state of a PEMFC.

[0014] Although some approaches exist to determine the quantity of transition metal cations in a fuel cell system, these approaches are either unable to determine a transition metal cation concentration with sufficient accuracy or the systems and methods used are very complex and costly.

[0015] Disclosure of the invention

[0016] The invention relates, according to a first aspect, to a method with the features of the independent method claim and, according to a second aspect, to a system with the features of the independent system claim. Further features and details of the invention become apparent from the respective dependent claims, the description, and the drawings. Features and details described in connection with the method according to the invention naturally also apply in connection with the system according to the invention, and vice versa, so that the disclosure relating to the individual aspects of the invention always includes, or allows for, reciprocal reference.

[0017] The advantage of the inventive method for determining a transition metal cation concentration within a fuel cell system with unknown transition metal cation concentration by means of electrochemical impedance spectroscopy is particularly evident in the fact that a precise determination of the quantity of transition metal cations in a fuel cell system can be carried out in a particularly simple and cost-effective manner, so that an aging state of a fuel cell system can be reliably predicted in a simple and cost-effective manner.

[0018] The method according to the invention can be used in particular in fuel cell systems of motor vehicles, such as passenger cars or trucks. Its use in other fuel cell-powered vehicles, such as forklifts, cranes, ships, or aircraft, is also conceivable. Likewise, its use in stationary systems is imaginable.

[0019] The method according to the invention comprises the steps of determining an electrical high-frequency resistance within the fuel cell system with unknown transition metal cation concentration, normalizing the determined electrical high-frequency resistance to a value of an electrical high-frequency resistance of a fuel cell system with a minimum transition metal cation concentration, comparing the normalized determined high-frequency resistance with the course of a calibration curve for a normalized electrical high-frequency resistance, and determining the transition metal cation concentration within the fuel cell system with unknown transition metal cation concentration based on the comparison of the normalized determined high-frequency resistance with the course of the calibration curve for a normalized electrical high-frequency resistance.

[0020] According to the invention, an electrical high-frequency resistance within a fuel cell system can be understood to be, in particular, an electrical resistance that can be determined within the fuel cell system at a frequency of 100 Hz to 100 kHz, preferably 1 kHz to 10 kHz, by electrochemical impedance spectroscopy. In a plot of the imaginary part against the real part of the complex resistance (so-called Nyquist plot), the high-frequency resistance can be defined as the x-axis crossing at high frequencies or as the real part of the resistance at a specific frequency of 100 Hz to 100 kHz, preferably 1 to 10 kHz.The transition metal cation concentration within the fuel cell system can preferably be understood as the concentration of transition metal cations within a fuel cell system that are not formed in the form of platinum ions, since platinum ions can be reduced back to platinum metal at typical cell voltages. The determination of the transition metal cation concentration within the fuel cell system can advantageously be carried out within a part of the fuel cell system, e.g., within a fuel cell of a fuel cell stack of the fuel cell system, and is advantageously characteristic of the entire fuel cell system.Normalizing the determined electrical high-frequency resistance to a value of 1, as defined in the invention, can also be understood as normalizing to a value of 1, which is particularly advantageous if no value for an electrical high-frequency resistance of a fuel cell system with a minimum transition metal cation concentration is known.

[0021] Within the scope of the invention, it has been recognized that the electrical high-frequency resistance, especially under dry conditions, changes very sensitively with a change in the transition metal cation concentration.

[0022] With a view to a particularly precise determination of a transition metal cation concentration within a fuel cell system, it can advantageously be provided that the determination of the electrical high-frequency resistance within the fuel cell system with unknown transition metal cation concentration is carried out under defined system conditions, wherein preferably the defined system conditions are the same conditions that are / were also used in determining the electrical high-frequency resistance of the fuel cell system with a minimum transition metal cation concentration and / or in determining the values ​​of the electrical high-frequency resistance of the calibration curve.

[0023] For a particularly precise determination of a transition metal cation concentration within a fuel cell system, it can be provided that the defined system conditions include a relative humidity, wherein the relative humidity is preferably < 70%, more preferably < 50%, and more preferably < 30%. Within the scope of the invention, it has been found that the sensitivity of the electrical high-frequency resistance is higher at low relative humidity (at least < 70%). Furthermore, it has been found that the error arising from the uncertainty in the determination of the electrical resistance is also lowest at low relative humidity.

[0024] Similarly, the defined system conditions may include a current system voltage and / or a gas loading. To increase precision, averaging over several or as many measurements as possible can be performed, which is advantageous when an "arbitrary" or particularly frequently occurring condition is used.

[0025] Advantageously, the electrical high-frequency resistance within the fuel cell system can be determined at open-circuit voltage with hydrogen at the anode and air, oxygen, or nitrogen at the cathode. Particularly in stack applications, the system conditions can also be defined to correspond to typical operating conditions, which are advantageously characterized by low current density, high stoichiometries (gas fluxes relative to current density), and low relative humidity.

[0026] With regard to a particularly precise determination of the transition metal cation concentration within a fuel cell system, it can also be advantageous to determine the high-frequency electrical resistance at a frequency within a range of 1 kHz to 10 kHz. This frequency range has proven to be particularly informative for determining the transition metal cation concentration. It goes without saying that the high-frequency electrical resistance can also be determined at different frequencies or frequency ranges, in which case the values ​​can be averaged across the frequencies or frequency ranges to obtain a single value.

[0027] For a precise determination of the transition metal cation concentration within a fuel cell system, it can be advantageous if the determined high-frequency electrical resistance is in the form of a membrane resistance, where the membrane resistance is preferably determined by subtracting a contact resistance from the high-frequency electrical resistance. Alternatively, the normalized ionic resistance in the cathode can also be used to determine the transition metal cation concentration within the fuel cell system. Furthermore, as an alternative to electrical resistance, electrical conductivity can also be used to determine the transition metal cation concentration within the fuel cell system, since conductivity and resistance can be directly converted into each other.

[0028] With a view to quickly and easily determining a calibration curve, it is also conceivable that the calibration curve for a normalized high-frequency electrical resistance is determined at least partially by interpolation or smoothing, wherein preferably at least three values ​​for a high-frequency electrical resistance are measured to determine the calibration curve. The determination of the calibration curve can be carried out by introducing known transition metal concentrations into a fuel cell system or by releasing them in-situ from the catalyst using stress tests (e.g., voltage cycles) and subsequently determining them using known analytical methods (e.g., titration).

[0029] To determine a transition metal cation concentration even without knowledge of the electrical contact resistance, it can advantageously be further provided that a difference in the electrical high-frequency resistance within the fuel cell system is determined under two different defined system conditions, wherein the different defined system conditions relate to different relative humidity levels of the fuel cell system. Advantageously, the first defined system condition can be selected to correspond to the application of low relative humidity (e.g., 30%), while the second reference condition can be selected to correspond to the application of high relative humidity (e.g., 95%).Such a design can be particularly advantageous if an electrical contact resistance cannot be determined with sufficient accuracy, or changes in an unknown way during the service life of a fuel cell system.

[0030] In a preferred application aimed at minimizing costs and effort, the invention further provides that the process is carried out depending on the aging state of the fuel cell system with an unknown transition metal cation concentration, wherein the process is preferably carried out for the first time only after a minimum number of operating cycles of the fuel cell system. After a certain number of operating cycles, it may then be possible, for example, to increase the frequency of carrying out the process according to the invention.

[0031] It may also be advantageous to provide that an operating strategy is adjusted if the transition metal cation concentration exceeds a first threshold and / or that a maintenance procedure is initiated if the transition metal cation concentration exceeds a second threshold.

[0032] The invention also relates to a system for determining a transition metal cation concentration within a fuel cell system with an unknown transition metal cation concentration, preferably for carrying out a method described above.The system according to the invention comprises an electrical impedance spectrometer for determining an electrical high-frequency resistance within the fuel cell system, and a processing unit for normalizing the determined electrical high-frequency resistance to a value corresponding to that of a fuel cell system with a minimal transition metal cation concentration. This processing unit also serves to compare the normalized determined high-frequency resistance with a calibration curve for a normalized electrical high-frequency resistance and to determine the transition metal cation concentration within the fuel cell system based on this comparison. The system according to the invention thus offers the same advantages as those already described in detail with regard to the method according to the invention.

[0033] The invention also relates to the use of a determined value of an electrical high-frequency resistance or a determined value of a standardized determined high-frequency resistance as a diagnostic tool for maintenance decisions, preferably when a threshold value is exceeded. The use according to the invention thus offers the same advantages as those already described in detail with regard to the inventive method and the inventive system.

[0034] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description can be essential to the invention individually or in any combination.

[0035] They each show schematically:

[0036] Figure 1 shows a representation of the individual steps of a method according to the invention for determining a transition metal cation concentration within a fuel cell system with unknown transition metal cation concentration using electrochemical impedance spectroscopy.

[0037] Figure 2 shows a plot of a normalized electrical membrane high-frequency resistance against the transition metal cation concentration at different relative humidity levels. Figure 3 shows a plot of a normalized electrical membrane high-frequency resistance and a normalized ionic resistance in the cathode against the transition metal cation concentration.

[0038] Figure 4 shows a first variant (left) and a second variant (right) of plotting a calibration curve for determining a transition metal cation concentration, and

[0039] Figure 5 shows a plot of a calibration curve for determining a transition metal cation concentration based on an alternative design based on the equation shown below the plot.

[0040] Fig. 1 shows a schematic representation of the individual steps of a method according to the invention for determining a transition metal cation concentration H + exchange within a fuel cell system with unknown transition metal cation concentration H + eX ch using electrochemical impedance spectroscopy.

[0041] The inventive method comprises the steps of determining an electrical high-frequency resistance R within the fuel cell system with unknown transition metal cation concentration H. +exch, a normalization 200 of the determined electrical high-frequency resistance R to a value R° of an electrical high-frequency resistance of a fuel cell system with a minimum transition metal cation concentration H + eX ch, a comparison 300 of the normalized determined high-frequency resistance R / R° with a course of a calibration curve KK for a normalized electrical high-frequency resistance R / R° as well as a determination 400 of the transition metal cation concentration H + exchange within the fuel cell system with unknown transition metal cation concentration H +The calculation is based on a comparison of the normalized determined high-frequency resistance R / R° with the course of the calibration curve KK for a normalized electrical high-frequency resistance R / R°. Normalizing the determined electrical high-frequency resistance R to a value R° of an electrical high-frequency resistance R of a fuel cell system with a minimum transition metal cation concentration H + This can also be understood as normalization to a value of R° = 1, which is particularly advantageous when no value exists for an electrical high-frequency resistance R of a fuel cell system with a minimum transition metal cation concentration H. + eX ch is known.

[0042] Determining the electrical high-frequency resistance R within the fuel cell system with unknown transition metal cation concentration H +This process takes place under defined system conditions, which are also the same as when determining the electrical high-frequency resistance R of the fuel cell system with a minimum transition metal cation concentration H. + exch and / or were used in determining the values ​​of the electrical high-frequency resistance R of the calibration curve KK.

[0043] The defined system conditions include a relative humidity RH, wherein the relative humidity RH is preferably < 70%, more preferably < 50%, and more preferably < 30%.

[0044] Similarly, the defined system conditions can include a current system voltage and / or a gas load.

[0045] The electrical high-frequency resistance R is preferably determined at a frequency in a frequency range between 1 kHz and 10 kHz and is advantageously in the form of a membrane resistance R. membefore, which is determined by subtracting a contact resistance Rkont.

[0046] The calibration curve KK can be determined at least partially by interpolation or smoothing.

[0047] Fig. 2 shows a plot of a normalized electrical membrane high-frequency resistance Rmem / R°mem against the transition metal cation concentration H + The graph shows the increase in membrane resistance R at different relative humidity levels (RH). mem relative to the membrane resistance R o mem of a non-contaminated system (fuel cell system with a minimal transition metal cation concentration H + exC h) with increasing transition metal cation concentration H + As can be seen in Fig. 2, the sensitivity of the membrane resistance is highest at low relative humidity (e.g. RH = 30%).

[0048] Fig. 3 shows a plot of a normalized electrical membrane high-frequency resistance Rmem and a normalized ionic resistance in the cathode pH+,cath against the transition metal cation concentration H + eX ch,

[0049] As can be seen in Fig. 3, the normalized ionic resistance in the cathode pn+,cath follows the relative increase of the membrane resistance Rmem. However, its determination is significantly less accurate than that of the membrane resistance, especially at the required low relative humidity levels. Therefore, the membrane resistance is better suited for a calibration curve KK. Alternatively, the ionic resistance in the cathode PH+,cath can also be used for the calibration curve KK.

[0050] Fig. 4 shows a first variant (left) and a second variant (right) of plotting a calibration curve KK for determining a transition metal cation concentration H + exch,

[0051] As can be seen in Fig. 4, the correlation between the relative increase in membrane resistance Rmem / R°mem and the number of protons exchanged can be interpolated using common methods (e.g., with a 5th-order polynomial) and used as a calibration curve KK (left). Alternatively, the conductivity K can also be used for the determination instead of the membrane resistance Rmem / R°mem (right). Calibration based on resistance or conductivity can be considered equivalent, as they can be directly converted into one another.

[0052] Fig. 5 shows a plot of a calibration curve KK for determining a transition metal cation concentration H + exc h is based on an alternative implementation based on the equation shown below the plot. In the alternative implementation, the transition metal cation concentration H +The resistance of the electrical contact (Rcont) is determined without knowing the electrical contact resistance. Instead of the membrane resistance Rcont, the resistance of the membrane is used. mem the difference of

[0053] High-frequency resistances were measured under two reference conditions with different relative humidity levels, RSORH and R95RH, and normalized to the corresponding difference between the uncontaminated system, R°3ORH and R%5H. The first reference condition was chosen to correspond to the application of low relative humidity (30%), while the second

[0054] The reference condition is chosen to correspond to the application of high relative humidity (e.g. 95%).

[0055] Such a design is particularly advantageous when the electrical contact resistance Rkont, which is determined according to R mem = R - Rkont for determining the

[0056] Membrane resistance R memis necessary, cannot be determined with sufficient accuracy, or changes in an unknown way during the service life of a PEMFC.

Claims

Claims 1. Method for determining a transition metal cation concentration (H + exch) within a fuel cell system with unknown transition metal cation concentration (H + eX ch) using electrochemical impedance spectroscopy, comprising the steps: - Determining (100) an electrical high-frequency resistance (R) within the fuel cell system with unknown transition metal cation concentration (H) + exC h) - Normalizing (200) the determined electrical high-frequency resistance (R) to a value (R°) of an electrical high-frequency resistance of a fuel cell system with a minimum transition metal cation concentration (H) + eX ch), - Comparing (300) the normalized determined high-frequency resistance (R / R°) with a calibration curve (CC) for a normalized electrical high-frequency resistance (R / R°), - Determine (400) the transition metal cation concentration (H + exC h) within the fuel cell system with unknown transition metal cation concentration (H + exc h) based on the comparison of the normalized determined high-frequency resistance (R / R°) with the course of the calibration curve (CC) for a normalized electrical high-frequency resistance (R / R°).

2. Method according to claim 1, characterized in that the determination (100) of the electrical high-frequency resistance (R) within the fuel cell system with unknown transition metal cation concentration (H) + exch) under defined system conditions, wherein the defined system conditions are preferably the same conditions that are also used in determining the electrical high-frequency resistance (Ro) of the fuel cell system with a minimum transition metal cation concentration (H + eXch) and / or when determining the values ​​of the electrical high-frequency resistance (R) of the calibration curve (KK).

3. The method according to claim 2, characterized in that the defined system conditions include a relative humidity (RH), wherein the relative humidity (RH) is preferably < 70%, more preferably < 50%, and more preferably < 30%.

4. Method according to one of the preceding claims, characterized in that the defined system conditions include a current system voltage and / or a gas loading.

5. Method according to one of the preceding claims, characterized in that the electrical high-frequency resistance (R) is determined at a frequency in a frequency range between 100 Hz and 100 kHz, preferably 1 kHz to 10 kHz.

6. Method according to one of the preceding claims, characterized in that the determined electrical high-frequency resistance (R) is in the form of a membrane resistance (R). mem ) is formed, wherein the membrane resistance (R mem ) preferably determined by subtracting a contact resistance (Rkont).

7. Method according to one of the preceding claims, characterized in that the calibration curve (CC) for a normalized high-frequency electrical resistance (R / R°) is determined at least partially by interpolation or smoothing, wherein at least three values ​​for a high-frequency electrical resistance (R) are preferably measured to determine the calibration curve (CC).

8. Method according to one of the preceding claims, characterized in that a difference in the electrical high-frequency resistance (R) within the fuel cell system is determined under two different defined system conditions, wherein the different defined system conditions relate to a different relative humidity (RH) of the fuel cell system.

9. Method according to one of the preceding claims, characterized in that the method depends on an aging state of the fuel cell system with unknown transition metal cation concentration (H₂). + exch) is carried out, the procedure preferably being carried out for the first time only after a minimum number of operating cycles of the fuel cell system.

10. Method according to one of the preceding claims, characterized in that an operating strategy is adapted if the transition metal cation concentration (H + eX ch) exceeds a first threshold (S1).

11. Method according to one of the preceding claims, characterized in that a maintenance procedure is initiated when the transition metal cation concentration (H + exch) exceeds a second threshold (S2) 12. System for determining a transition metal cation concentration (H + exch) within a fuel cell system with unknown transition metal cation concentration (H + eX ch), preferably for carrying out a method according to one of claims 1 to 11, comprising: - an electrical impedance spectrometer for determining (100) an electrical high-frequency resistance (R) within the fuel cell system, - a processing unit for normalizing (200) the determined electrical high-frequency resistance (R) to a value (R°) of an electrical high-frequency resistance of a fuel cell system with a minimum transition metal cation concentration (H + eX ch), to compare (300) the normalized determined high-frequency resistance (R / R°) with a calibration curve (KK) for a normalized electrical high-frequency resistance (R / R°) and to determine (400) the transition metal cation concentration (H + exch) within the fuel cell system based on the comparison.

13. Use of a determined value of an electrical high-frequency resistance (R) or a determined value of a normalized determined high-frequency resistance (R / R°) as diagnostics for a maintenance decision, preferably when a threshold value (S2) is exceeded.

Images