DE102022134409A1 - Method and device for determining the state of an energy converter - Google Patents

Abstract

The technology disclosed here relates according to the invention to a device (103) for determining the state of an electrochemical energy converter (102). The device (103) is designed to cause an AC excitation signal (131) at the energy converter (102), wherein a frequency (302) of the AC excitation signal (131) is changed according to a frequency sweep (311, 312, 313). The device (103) is further designed to detect an AC measurement signal (132) caused by the AC excitation signal (131) at the energy converter (102). In addition, the device (103) is configured to determine an impedance-frequency curve for the energy converter (102) on the basis of the AC excitation signal (131) and on the basis of the AC measurement signal (132), and to determine the state of the energy converter (102) on the basis of the impedance-frequency curve.

Description

The technology disclosed here relates to a method and a corresponding device for determining the state of an electrochemical energy converter, in particular a fuel cell stack.

An electrically powered vehicle can have a fuel cell system with a fuel cell stack with one or more fuel cells, wherein the fuel cell stack is designed to generate electrical energy for operating the electric drive motor of the vehicle based on a fuel, in particular based on hydrogen (H ₂ ). The fuel for the fuel cell stack is typically fed from a pressure vessel to the fuel cell stack.

During operation of the fuel cell system, the fuel cell stack may assume conditions that may lead to impairment, in particular damage, of the fuel cell stack.

It is a preferred object of the technology disclosed here to reduce or eliminate at least one disadvantage of a previously known solution or to propose an alternative solution. It is a preferred object of the technology disclosed here to determine the state of a fuel cell stack, generally an electrochemical energy converter, in an efficient, reliable, fast and precise manner, in particular to avoid deterioration of the fuel cell stack.

The problem(s) is/are solved by the subject matter of the independent patent claims. The dependent claims represent preferred embodiments.

• • ein Normalzustand, bei dem der Energiewandler z.B. eine ausreichende Menge an Brennstoff, Oxidationsmittel und/oder Reaktionsprodukten aufweist;
• • ein Brennstoff-Mangelzustand, bei dem der Energiewandler z.B. eine unzureichende Menge an Brennstoff aufweist;
• • ein Oxidationsmittel-Mangelzustand, bei dem der Energiewandler z.B. eine unzureichende Menge an Oxidationsmittel aufweist;
• • ein Überflutungs-Zustand, bei dem in dem Energiewandler z.B. eine überhöhte Menge an Reaktionsprodukten (insbesondere Wasser) angeordnet ist; und/oder
• • ein Austrocknungs-Zustand, bei dem in dem Energiewandler z.B. eine unzureichende Menge an Reaktionsprodukten angeordnet ist.

According to one aspect, a device for determining the state of an electrochemical energy converter (in particular a fuel cell stack having one or more fuel cells) is described. Example possible states are,

• • a normal state in which the energy converter has, for example, a sufficient amount of fuel, oxidant and/or reaction products;
• • a fuel starvation condition in which the energy converter, for example, has an insufficient amount of fuel;
• • an oxidant deficiency condition, where the energy converter, for example, has an insufficient amount of oxidant;
• • a flooding condition in which an excessive amount of reaction products (in particular water) is present in the energy converter; and/or
• • a drying-out condition in which, for example, an insufficient amount of reaction products is present in the energy converter.

The device is designed to cause an AC (alternating current) excitation signal at the energy converter. The frequency of the AC excitation signal can be changed according to a frequency sweep. The frequency sweep can, for example, have a total duration of one second or less (e.g. 500 ms). Alternatively or additionally, the frequency sweep can have N different frequencies, e.g. with N equal to 100 or more, or N equal to 500 or more. A frequency sweep can also be referred to as a frequency sweep. Furthermore, a frequency sweep can extend from a first frequency (e.g. 5 Hz or less) to a second frequency (e.g. 1 kHz or more). The AC excitation signal can optionally be caused by a DC voltage converter.

Furthermore, the device is configured to detect an AC measurement signal caused by the AC excitation signal at the energy converter (wherein the AC measurement signal typically has the frequencies from the frequency sweep). The AC excitation signal can comprise an AC measurement current (also referred to as alternating measurement current) and the AC measurement signal can comprise an AC measurement voltage (also referred to as alternating measurement voltage). Alternatively, the AC excitation signal can comprise an AC measurement voltage and the AC measurement signal can comprise an AC measurement current. The AC measurement signal can be detected using a high-pass filter.

The device is further configured to determine an impedance frequency curve for the energy converter based on the AC excitation signal and on the AC measurement signal (in particular based on the amplitudes and phases of the respective signals). The impedance frequency curve has an impedance value for each of the different frequencies from the frequency sweep.

In addition, the device is designed to determine the state of the energy converter on the basis of the impedance-frequency curve. The energy converter can then be operated depending on the determined state, in particular with the aim of returning the energy converter to the normal state.

A device is thus described which makes it possible to determine the state of an energy converter in an efficient, rapid and precise manner, in particular in order to avoid impairment of the energy converter (by means of a correspondingly adapted operation of the energy converter). For this purpose, the impedance frequency curve and the state of the energy converter can be determined repeatedly, in particular periodically (eg with a frequency of 0.1 Hz or more).

The device can be configured to change the frequency sweep of the AC excitation signal, in particular with respect to how the frequency of the AC excitation signal changes over time. For example, a linear, a logarithmic or an exponential change in the frequency can be brought about within the frequency sweep.

A modified impedance-frequency curve for the energy converter can then be determined using the modified frequency sweep. Furthermore, the state of the energy converter can be determined, in particular adjusted, on the basis of the modified impedance-frequency curve.

For example, using a first frequency sweep, a first impedance frequency curve can be determined and, based on this, the (provisional) state of the energy converter can be determined. The frequency sweep can then be changed (taking the determined state into account), and a changed impedance frequency curve can be determined using the changed frequency sweep in order to determine the state of the energy converter with increased accuracy based on this.

• • einen linearen Frequenzdurchlauf, bei dem die Frequenz linear entlang der Zeit von der ersten Frequenz bis zu der zweiten Frequenz (monoton steigend oder sinkend) verändert, insbesondere erhöht oder reduziert, wird;
• • einen exponentiellen Frequenzdurchlauf, bei dem die Frequenz exponentiell entlang der Zeit von der ersten Frequenz bis zu der zweiten Frequenz (monoton steigend oder sinkend) verändert, insbesondere erhöht oder reduziert, wird; und/oder
• • einen logarithmischen Frequenzdurchlauf, bei dem die Frequenz logarithmisch entlang der Zeit von der ersten Frequenz bis zu der zweiten Frequenz (monoton steigend oder sinkend) verändert, insbesondere erhöht oder reduziert, wird.

The device can be set up to select a frequency sweep from a set of different frequency sweeps (eg on the basis of a previously determined (provisional) state of the energy converter). The different frequency sweeps can differ with respect to the temporal change of the frequency within the time period of the respective frequency sweep. The set of different frequency sweeps can include, for example,

• • a linear frequency sweep in which the frequency is changed, in particular increased or reduced, linearly over time from the first frequency to the second frequency (monotonically increasing or decreasing);
• • an exponential frequency sweep in which the frequency is changed, in particular increased or reduced, exponentially over time from the first frequency to the second frequency (monotonically increasing or decreasing); and/or
• • a logarithmic frequency sweep in which the frequency is changed, in particular increased or reduced, logarithmically over time from the first frequency to the second frequency (monotonically increasing or decreasing).

The impedance-frequency curve for the energy converter (which is used to determine the state of the energy converter) can be determined using the selected frequency sweep. This can further increase the accuracy of the state determination.

The device can be set up to determine a frequency-pass-specific impedance frequency curve from the set of frequency passes (which has, for example, two or more, or three or more different frequency passes) using the individual frequency passes. A set of frequency-pass-specific impedance frequency curves can thus be determined for the corresponding set of different frequency passes.

The state of the energy converter can then be determined in a particularly precise manner based on the set of frequency sweep-specific impedance-frequency curves.

The device can be set up to select the state of the energy converter from a set of different possible states of the energy converter based on at least one determined impedance-frequency curve. The set of possible states can include, for example, two or more, or three or more, or four or more states. By selecting a possible state, the accuracy of the state determination can be further increased.

The device can be set up to compare the determined impedance frequency curve with a set of (pre-determined) reference frequency curves for the corresponding set of different possible states of the energy converter. The state of the energy converter can then be selected from the set of different possible states of the energy converter based on the comparison.

In particular, for the individual reference frequency curves from the set of reference frequency curves, the value of a distance measure (e.g. the mean square distance) for the distance of the determined impedance frequency curve with the respective reference frequency curve can be determined. The state The energy converter can then be selected in a particularly precise and robust manner based on the determined values of the distance measure from the set of different possible states of the energy converter (e.g. as the possible state with the lowest value of the distance measure).

The device can be set up to compare the individual frequency sweep-specific impedance frequency curves with a set of frequency sweep-specific reference frequency curves for the corresponding set of different possible states of the energy converter. A set of frequency sweep-specific reference frequency curves can thus be provided for the different frequency sweeps.

The state of the energy converter can then be selected in a particularly precise manner based on the comparisons from the set of different possible states of the energy converter. For this purpose, as described above, values of the distance measure can be determined and compared with each other.

According to another aspect, a fuel cell system is described which includes the device described in this document.

According to a further aspect, a (road) motor vehicle (in particular a passenger car or a truck or a bus or a motorcycle) is described which comprises the device described in this document and/or the fuel cell system described in this document.

According to a further aspect, a method for determining the state of an electrochemical energy converter is described. The method comprises causing an AC excitation signal to the energy converter, wherein the frequency of the AC excitation signal is changed according to a frequency sweep. The method further comprises detecting an AC measurement signal caused by the AC excitation signal to the energy converter. The method further comprises determining, on the basis of the AC excitation signal and on the basis of the AC measurement signal, an impedance frequency curve for the energy converter, and determining the state of the energy converter on the basis of the impedance frequency curve.

According to a further aspect, a software (SW) program is described. The SW program can be configured to be executed on a processor (e.g. on a control unit of a vehicle) and thereby to carry out the method described in this document.

According to a further aspect, a storage medium is described. The storage medium can comprise a software program which is designed to be executed on a processor and thereby to carry out the method described in this document.

It should be noted that the methods, devices and systems described in this document can be used alone or in combination with other methods, devices and systems described in this document. Furthermore, any aspects of the methods, devices and systems described in this document can be combined with one another in a variety of ways. In particular, the features of the claims can be combined with one another in a variety of ways. Furthermore, features listed in brackets are to be understood as optional features.

• 1a ein beispielhaftes Brennstoffzellensystem mit einem Brennstoffzellenstapel;
• 1b eine beispielhafte Messanordnung zur Ermittlung des Impedanz-Frequenzverlaufs der Impedanz eines Brennstoffzellenstapels;
• 2 eine beispielhafte Brennstoffzelle;
• 3a beispielhafte Frequenzdurchläufe (auf Englisch, frequency sweeps) zur Ermittlung des Impedanz-Frequenzverlaufs für einen Brennstoffzellenstapel;
• 3b beispielhafte Referenz-Frequenzverläufe; und
• 4 ein Ablaufdiagramm eines beispielhaften Verfahrens zur Ermittlung des Zustands eines Brennstoffzellenstapels.

The invention is described in more detail below using exemplary embodiments.

• 1a an exemplary fuel cell system with a fuel cell stack;
• 1b an exemplary measuring arrangement for determining the impedance-frequency curve of the impedance of a fuel cell stack;
• 2 an exemplary fuel cell;
• 3a Example frequency sweeps to determine the impedance-frequency response for a fuel cell stack;
• 3b exemplary reference frequency curves; and
• 4 a flowchart of an exemplary method for determining the condition of a fuel cell stack.

As stated at the beginning, this document deals with the efficient, fast and reliable determination of the state of an energy converter, in particular a fuel cell stack. The following will specifically address a fuel cell stack (as an example of an energy converter). It should be noted that the aspects described are generally applicable to an (electrochemical) energy converter.

1a shows a fuel cell system 100 with a fuel cell stack 102 with at least one fuel cell 101. The fuel cell system The fuel cell 100 is intended, for example, for mobile applications such as motor vehicles, in particular for providing electrical energy for at least one electric drive motor for moving a motor vehicle. A fuel cell 101 is an electrochemical energy converter that converts fuel and oxidizing agent into reaction products and thereby produces electricity and heat. A fuel cell 101 comprises (as in 2 shown) an anode 201 and a cathode 202, which are separated by an ion-selective or ion-permeable separator 203. The anode 201 is supplied with fuel 211. Preferred fuels 211 are: hydrogen (H ₂ ), low molecular weight alcohol, biofuels, or liquefied natural gas. The cathode 202 is supplied with oxidizing agent 212. Preferred oxidizing agents 212 are: air, oxygen and peroxides. The ion-selective separator 203 can be designed, for example, as a proton exchange membrane (PEM). A cation-selective polymer electrolyte membrane is preferably used. Materials for such a membrane are, for example: Nafion®, Flemion® and Aciplex®.

In addition to the at least one fuel cell 101, a fuel cell system 100 comprises peripheral system components (BOP (balance of plant) components) that can be used in the operation of the at least one fuel cell 101. As a rule, several fuel cells 101 are combined to form a fuel cell stack 102. Furthermore, the fuel cell system 100 typically comprises at least one pressure vessel, in particular a pressure tank, 110, which can be used to provide the fuel 211 for the one or more fuel cells 101. The pressure vessel 110 is connected to the one or more fuel cells 101 via one or more lines 112. The electrical power provided by the fuel cell stack 102 can be controlled and/or regulated by a (control) device 103 of the fuel cell system 100. In this context, the mass flow of fuel 211 and/or the mass flow of oxidant 212 in the fuel cell stack 102 can be controlled and/or regulated. The mass flow of oxidant 212 can be adjusted and/or changed by an oxidant conveyor 205, in particular by a compressor.

The anode 201 and the cathode 202 of a fuel cell 101 or a fuel cell stack 102 can be connected to contact parts 204. An operating voltage is typically present between the contact parts 204 (e.g., approximately 1V for a fuel cell 101) and a current can be provided. By connecting several fuel cells 101 in series (i.e., by providing a stack or fuel cell stack 102), the operating voltage of the fuel cell stack 102 can be increased.

The fuel cells 101 of the fuel cell stack 102 generally each comprise two separator plates (not shown). The ion-selective separator 203 of a fuel cell 101 is generally arranged between two separator plates. One separator plate forms the anode 201 together with the ion-selective separator 203. The further separator plate arranged on the opposite side of the ion-selective separator 203 forms the cathode 202 together with the ion-selective separator 203. Gas channels for fuel 211 or for oxidizing agent 212 are preferably provided in the separator plates.

The separator plates can be designed as monopolar plates and/or as bipolar plates. In other words, a separator plate expediently has two sides, one side forming the anode 201 of a first fuel cell 101 together with an ion-selective separator 203 and the second side forming the cathode 202 of the second fuel cell 101 together with a further ion-selective separator 203 of an adjacent second fuel cell 101.

Between the ion-selective separators 203 and the separator plates, so-called gas diffusion layers or gas diffusion layers (GDL) are usually provided.

The electrical current generated by the fuel cell stack 102 can be provided via a DC-DC converter 120 to a current sink 121, e.g. an electric drive motor and/or an electrical energy storage device of a vehicle.

• • ein Normalzustand, bei dem der Brennstoffzellenstapel 102 mit einer passenden Menge an Brennstoff 211 und Oxidationsmittel 212 versorgt ist, und bei dem eine passende Menge an Reaktionsprodukten (insbesondere Wasser) in dem Brennstoffzellenstapel 102 vorliegt;
• • ein Brennstoff-Mangelzustand, bei dem der Brennstoffzellenstapel 102 mit einer unzureichenden Menge an Brennstoff 211 versorgt wird;
• • ein Oxidationsmittel-Mangelzustand, bei dem der Brennstoffzellenstapel 102 mit einer unzureichenden Menge an Oxidationsmittel 212 versorgt wird;
• • ein Überflutungs-Zustand, bei dem der Brennstoffzellenstapel 102 eine zu hohe Menge an Reaktionsprodukten (insbesondere Wasser) aufweist; und/oder
• • ein Austrocknungs-Zustand, bei dem der Brennstoffzellenstapel 102 eine unzureichende Menge an Reaktionsprodukten (insbesondere Wasser) aufweist.

During operation of the fuel cell system 100, the fuel cell stack 102 can assume different states, which may lead to impairment of the fuel cell stack 102. Example states are,

• • a normal state in which the fuel cell stack 102 is supplied with an appropriate amount of fuel 211 and oxidant 212, and in which an appropriate amount of reaction products (in particular water) is present in the fuel cell stack 102;
• • a fuel starvation condition in which the fuel cell stack 102 is supplied with an insufficient amount of fuel 211;
• • an oxidant deficiency condition in which the fuel cell stack 102 is operating at an insufficient sufficient amount of oxidizing agent 212;
• • a flooding condition in which the fuel cell stack 102 has an excessive amount of reaction products (particularly water); and/or
• • a dry-out condition in which the fuel cell stack 102 has an insufficient amount of reaction products (particularly water).

The measures described in this document make it possible to determine the state of the fuel cell stack 102 (from a predefined set of different possible states) in an efficient, fast and reliable manner.

• • einen ersten Referenz-Frequenzverlauf 331 für den Normalzustand;
• • einen zweiten Referenz-Frequenzverlauf 332 für den Überflutungs-Zustand;
• • einen dritten Referenz-Frequenzverlauf 333 für den Brennstoff-Mangelzustand;
• • einen vierten Referenz-Frequenzverlauf 334 für den Oxidationsmittel-Mangelzustand; und
• • einen fünften Referenz-Frequenzverlauf 335 für den Austrocknungs-Zustand.

For this purpose, the impedance-frequency curve of the fuel cell stack 102 can be determined. The impedance-frequency curve can show (eg in a Nyqvist diagram) the imaginary part 322 and the real part 321 of the impedance of the fuel cell stack 102 as a function of frequency (see 3b) . The impedance frequency curve of the fuel cell stack 102 can be compared with one or more reference frequency curves 331, 332, 333, 334, 335 in order to determine the state of the fuel cell stack 102. 3b shows

• • a first reference frequency curve 331 for the normal state;
• • a second reference frequency curve 332 for the flooding condition;
• • a third reference frequency curve 333 for the fuel shortage condition;
• • a fourth reference frequency curve 334 for the oxidant deficiency state; and
• • a fifth reference frequency curve 335 for the drying state.

The device 103 can be set up to determine the value of a distance measure (e.g. the mean square deviation) for the distance and/or for the deviation of the impedance frequency curve from the individual reference frequency curves 331-335. The state of the fuel cell stack 102 can then be determined on the basis of the determined values of the distance measure. In particular, the state for which the lowest value of the distance measure was determined can be selected. In this way, the state of the fuel cell stack 102 can be determined in a reliable and precise manner.

The device 103 can also be configured to operate the fuel cell system 100 depending on the determined state of the fuel cell stack 102. If necessary, one or more measures can be taken that are aimed at bringing the fuel cell stack 102 into the normal state.

1b shows an exemplary measuring arrangement for determining the impedance frequency curve of the fuel cell stack 102. The DC-DC converter 120 can be used to cause an AC (alternating current) measuring current 131 in the fuel cell stack 102. The AC measuring current 131 can be superimposed with the DC (direct current) current of the fuel cell stack 102. The AC measuring current 131 can have a specific frequency 302.

Thus, the DC-DC converter 120 can be used to generate the excitation signal (i.e., the AC measurement current 131) for exciting the fuel cell stack 102. Alternatively, a dedicated excitation unit can be provided to generate the excitation signal.

Furthermore, the AC measurement voltage 132 resulting from the AC measurement current 131 at the poles of the fuel cell stack 102 can be recorded (e.g. using a voltage measuring unit). A high-pass filter can be used to record the AC measurement voltage 132 in a particularly precise manner. The impedance of the fuel cell 102 for the specific frequency 302 can be determined on the basis of the AC measurement current 131 and on the basis of the AC measurement voltage 132 (in particular on the basis of the amplitude and the phase of the AC measurement current 131 and the AC measurement voltage 132).

It should be noted that alternatively, an AC measurement voltage 132 may be applied to the poles of the fuel cell stack 102 and the resulting AC measurement current 131 may be sensed to determine the impedance of the fuel cell stack 102. In general, an AC excitation signal (e.g., an AC measurement current 131 or an AC measurement voltage 132) may be generated and the AC measurement signal (e.g., the AC measurement voltage 132 or the AC measurement current 131) caused by the AC excitation signal may be sensed. The impedance of the fuel cell stack 102 may then be determined based on the AC excitation signal and based on the AC measurement signal.

To determine the impedance frequency curve, the frequency 302 of the AC measuring current 131 or the AC measuring voltage 132 can be changed along the time 301, as shown for example in 3a The frequency 302 can be set between a first (lower) frequency 303 (e.g. at 1Hz) and a second (upper) frequency 304 (e.g. at 2 kHz) (in a monotonically increasing or decreasing manner). A frequency sweep 311, 312, 313 from the first frequency 303 to the second frequency 304 can have a specific total duration 305 (e.g. of 1 second or less). The frequency 302 can be changed linearly with time 301, in particular increased or reduced (see frequency sweep 311). Alternatively, the frequency 302 can be changed logarithmically with time 301 (see frequency sweep 313) or exponentially with time 301 (see frequency sweep 312). The individual frequency sweeps 311, 312, 313 can each have N different frequencies 302, e.g. with N=100 or more, or N=500 or more. The resulting impedance-frequency curve can have N impedance values for the corresponding N frequencies 302.

The frequency 302 of the AC measurement current 131 (or alternatively the AC measurement voltage 132) can thus be changed according to a predefined frequency sweep 311, 312, 313. The resulting AC measurement voltage 132 (or alternatively the resulting AC measurement current 131) can be recorded. Furthermore, on the basis of the AC measurement current 131 and on the basis of the AC measurement voltage 132 for the predefined frequency sweep 311, 312, 313, the impedance frequency curve of the fuel cell stack 102 (for the N frequencies 302 from the predefined frequency sweep 311, 312, 313) can be determined. The impedance frequency curve of the fuel cell stack 102 can thus be determined in a particularly efficient and quick manner.

The device 103 can be set up to determine a set of different impedance frequency curves for a corresponding set of different frequency sweeps 311, 312, 313. In particular, an impedance frequency curve can be determined for a linear frequency sweep 311, for a logarithmic frequency sweep 312 and/or for an exponential frequency sweep 313. The state of the fuel cell stack 102 can then be determined in a particularly precise and reliable manner on the basis of the set of different impedance frequency curves for the corresponding set of different frequency sweeps 311, 312, 313.

For example, a set of frequency sweep-specific reference frequency curves 331-335 can be determined for each of the different frequency sweeps 311, 312, 313. The individual frequency sweep-specific impedance frequency curves can then be compared with the respective set of frequency sweep-specific reference frequency curves 331-335. In particular, values of the distance measure can be determined in this process. The state of the fuel cell stack 102 can then be determined in a particularly precise and robust manner on the basis of the comparisons, in particular on the basis of the determined values of the distance measure.

The individual frequency sweeps 311, 312, 313 can each have a sequence of N different frequencies 302, e.g. with N equal to 100 or more, or N equal to 500 or more. Impedance frequency curves with a relatively high frequency resolution (with impedance values for N different frequencies 302 between the first frequency 303 and the second frequency 304) can thus be determined.

4 shows a flow chart of an exemplary (possibly computer-implemented) method 400 for determining the state of an electrochemical energy converter 102 (in particular a fuel cell stack). The method 400 includes causing 401 an AC excitation signal 131 (eg an alternating current or an alternating voltage) at the energy converter 102. The frequency 302 of the AC excitation signal 131 changes according to a frequency sweep 311, 312, 313 (ie according to a frequency sweep). The AC excitation signal 131 can be generated eg using a voltage converter 120.

The method 400 further comprises detecting 402 an AC measurement signal 132 (e.g. an alternating voltage or an alternating current) caused by the AC excitation signal 131 at the energy converter 102. The AC measurement signal 132 can be detected by a measuring unit.

Furthermore, the method 400 comprises determining 403, on the basis of the AC excitation signal 131 and on the basis of the AC measurement signal 132 (in particular on the basis of the respective amplitude and/or phase), an impedance frequency curve for the energy converter 102. The impedance frequency curve indicates the impedance of the energy converter 102 for the N frequencies 302 of the frequency sweep 311, 312, 313.

In addition, the method 400 includes determining 404 the state of the energy converter 102 based on the impedance-frequency curve.

By means of the measures described in this document, the state of a fuel cell stack 102 can be determined in a particularly efficient, rapid and reliable manner, in particular in order to avoid any impairment of the fuel cell stack 102.

The present invention is not limited to the embodiments shown. In particular, it should be noted that the description and the figures are intended only to illustrate the principle of the proposed methods, devices and systems by way of example.

List of reference symbols

100

Fuel cell system

101

Fuel cell

102

Fuel cell stack

103

(Control) device

110

pressure vessel

112

Fuel line

120

DC-DC converter

121

electrical energy storage

131

AC excitation signal (e.g. AC measuring current or AC measuring voltage)

132

AC measuring signal (e.g. AC measuring voltage or AC measuring current)

201

anode

202

cathode

203

separator

204

Contact part (electrode)

205

Oxidant conveyor (compressor)

211

Fuel (especially hydrogen)

212

Oxidizing agents (especially air)

301

Time

302

frequency

303

first (lower) frequency

304

second (upper) frequency

305

Duration (of a frequency sweep)

311-313

Frequency sweep

321

Real part (impedance)

322

Imaginary part (impedance)

331-335

Reference frequency response

400

Method for determining the condition of a fuel cell stack

401-404

Process steps

Claims (12)

Device (103) for determining a state of an electrochemical energy converter (102); wherein the device (103) is configured to - cause an AC excitation signal (131) at the energy converter (102); wherein a frequency (302) of the AC excitation signal (131) is changed according to a frequency sweep (311, 312, 313); - to detect an AC measurement signal (132) caused by the AC excitation signal (131) at the energy converter (102); - to determine an impedance frequency curve for the energy converter (102) based on the AC excitation signal (131) and on the basis of the AC measurement signal (132); and - to determine a state of the energy converter (102) based on the impedance frequency curve. Device (103) according to Claim 1 , wherein the device (103) is configured to - change the frequency sweep (311, 312, 313) of the AC excitation signal (131), in particular with respect to how the frequency (302) of the AC excitation signal (131) changes over time (301); - determine a changed impedance frequency curve for the energy converter (102) using the changed frequency sweep (311, 312, 313); and - determine the state of the energy converter (102) on the basis of the changed impedance frequency curve. Device (103) according to one of the preceding claims, wherein the device (103) is configured to - select a frequency sweep (311, 312, 313) from a set of different frequency sweeps (311, 312, 313); and - determine the impedance-frequency curve for the energy converter (102) using the selected frequency sweep (311, 312, 313). Device (103) according to Claim 3 , wherein - the different frequency sweeps (311, 312, 313) differ in relation to a temporal change of the frequency (302) within a time period (305) of the respective frequency sweep (311, 312, 313); and/or - the set of different frequency sweeps (311, 312, 313) comprises in particular - a linear frequency sweep (311), in which the frequency (302) is changed linearly over time (301) from a first frequency (303) to a second frequency (304), in particular increased or reduced; - an exponential frequency sweep (312), in which the frequency (302) is exponentially increased over time (301) from a first frequency (303) to a second frequency (304). Time (301) from the first frequency (303) to the second frequency (304) is changed, in particular increased or reduced; and/or - a logarithmic frequency sweep (313) in which the frequency (302) is changed, in particular increased or reduced, logarithmically over time (301) from the first frequency (303) to the second frequency (304). Device (103) according to one of the Claims 3 until 4 , wherein the device (103) is set up - to determine a frequency-pass-specific impedance frequency curve from the set of frequency passes (311, 312, 313) using the individual frequency passes (311, 312, 313) in order to provide a set of frequency-pass-specific impedance frequency curves for the corresponding set of frequency passes (311, 312, 313); and - to determine the state of the energy converter (102) on the basis of the set of frequency-pass-specific impedance frequency curves. Device (103) according to one of the preceding claims, wherein - the device (103) is set up to select the state of the energy converter (102) from a set of different possible states of the energy converter (102) on the basis of the determined impedance frequency curve; and - the set of different possible states comprises in particular, - a normal state in which the energy converter (102) has a sufficient amount of fuel (211), oxidizing agent (212) and/or reaction products; - a fuel deficiency state in which the energy converter (102) has an insufficient amount of fuel (211); - an oxidizing agent deficiency state in which the energy converter (102) has an insufficient amount of oxidizing agent (212); - a flooding state in which an excessive amount of reaction products is arranged in the energy converter (102); and/or - a dry-out state in which an insufficient amount of reaction products is arranged in the energy converter (102). Device (103) according to Claim 6 , wherein the device (103) is configured to - compare the determined impedance frequency curve with a set of reference frequency curves (311-315) for the corresponding set of different possible states of the energy converter (102); and - select the state of the energy converter (102) from the set of different possible states of the energy converter (102) based on the comparison. Device (103) according to Claim 7 , wherein the device (103) is set up - to determine a value of a distance measure for a distance of the determined impedance frequency curve with the respective reference frequency curve (311-315) for the individual reference frequency curves (311-315) from the set of reference frequency curves (311-315); and - to select the state of the energy converter (102) based on the determined values of the distance measure from the set of different possible states of the energy converter (102). Device (103) according to one of the Claims 7 until 8th with reference to Claim 5 , wherein the device (103) is configured to - compare the individual frequency sweep-specific impedance frequency curves with a set of frequency sweep-specific reference frequency curves (311-315) for the corresponding set of different possible states of the energy converter (102); and - select the state of the energy converter (102) from the set of different possible states of the energy converter (102) based on the comparisons. Device (103) according to one of the preceding claims, wherein the device (103) is configured to operate the energy converter (103) depending on the determined state, in particular with the aim of transferring the energy converter (103) to a normal state. Device (103) according to one of the preceding claims, wherein - the energy converter (102) comprises a fuel cell stack with one or more fuel cells (101); and/or - the AC excitation signal (131) comprises an AC measurement current and the AC measurement signal (132) comprises an AC measurement voltage; or - the AC excitation signal (131) comprises an AC measurement voltage and the AC measurement signal (132) comprises an AC measurement current; and/or - the frequency sweep (311, 312, 313) has a total duration (305) of one second or less; and/or - the frequency sweep (311, 312, 313) has N different frequencies (302), with N equal to 100 or more, or N equal to 500 or more. Method (400) for determining a state of an electrochemical energy converter (102); the method (400) comprising - causing (401) an AC excitation signal (131) at the energy converter (102); wherein a frequency (302) of the AC excitation signal (131) is changed according to a frequency sweep (311, 312, 313); - detecting (402) an AC measurement signal (132) caused by the AC excitation signal (131) at the energy converter (102); - determining (403), on the basis of the AC excitation signal (131) and on the basis of the AC measurement signal (132), an impedance frequency curve for the energy converter (102); and - determining (404) a state of the energy converter (102) on the basis of the impedance frequency curve.

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