DE102022202521A1 - Method for examining an electrochemical cell by electrochemical impedance spectroscopy - Google Patents

Abstract

The invention relates to a method for examining an electrochemical cell (100) by electrochemical impedance spectroscopy (EIS), comprising: imposing a frequency (fi) on a load signal (I, U) on at least one segment (101) of the electrochemical cell (100 ),- Detecting a signal response (li, Ui) from the electrochemical cell (100),- Changing the frequency (fi) in order to impose a further frequency (fi+1) on the load signal (I, U) and produce a further signal response ( li + 1, Ui + 1) from the segment (101) of the electrochemical cell (100), the above-mentioned steps being carried out for a certain frequency range (df) in order to obtain a series of signal responses (I, U) for different To obtain frequencies (fi) in the specific frequency range (df), - Assigning signal responses (li, Ui) in the series of signal responses (I, U) to corresponding frequencies (fi), - Determining impedances (Z) for different frequencies (f) in the specific frequency range (df) depending on corresponding signal responses (li, Ui).

Description

The invention relates to a method for examining a, in particular segmented, electrochemical cell, such as. B. a fuel cell, for example a PEM fuel cell, by electrochemical impedance spectroscopy or using electrochemical impedance spectroscopy. The invention further relates to a corresponding computer program product. The invention further relates to a corresponding control unit for carrying out a corresponding method. The invention also relates to a corresponding fuel cell system with a corresponding control unit.

State of the art

Electrochemical cells, e.g. B. used in fuel cell systems are basically known. The electrochemical cells can, for example, be used as fuel cells, for example PEM fuel cells. “Proton Exchange Membrane Fuel Cells” (PEMFC). In fuel cell systems, the electrochemical cells are usually assembled in several layers to form a stack. Electrochemical cells are playing an increasingly important role in the transformation of the mobility sector towards greater sustainability. Automotive operation of electrochemical cells is very transient and involves significant spatial inhomogeneities along the flow direction of a single cell and through the layers of the stack. Optimizing cell design and operations toward improved cost, increased lifetime, and improved performance requires a profound understanding of the spatiotemporal distribution of internal states of a cell. An important internal condition that must be controlled is membrane humidification. This cannot be measured directly in-situ, but can be approximately derived from the membrane resistance. The membrane resistance can be determined, for example, using electrochemical impedance spectroscopy (EIS). Local investigations of an electrochemical cell using electrochemical impedance spectroscopy can reveal strong inhomogeneities in current density and membrane resistance along a flow channel of the reactants, even during stationary operation of the cell, which can only be revealed using electrochemical impedance spectroscopy carried out in segments. There are also approaches that look at the temporally but not spatially resolved membrane resistance at a constant examination frequency of, for example, 1 kHz. These approaches showed that cell-averaged membrane resistance can only provide unreliable information about membrane moisture. Therefore, both temporally and spatially resolved membrane resistance acquisitions are required to correctly observe membrane moisture under automotive-relevant operation.

Disclosure of the invention

According to the first aspect, the present invention provides: A method for examining an, in particular segmented, electrochemical cell, such as. B. a fuel cell, for example a PEM fuel cell, by electrochemical impedance spectroscopy or using electrochemical impedance spectroscopy with the features of the independent method claim. According to the second aspect, the present invention provides: A computer program product with the features of the independent product claim. According to the third aspect, the present invention provides: A control unit with the features of the independent device claim. According to the fourth aspect, the present invention provides: A fuel cell system with the features of the independent device claim. Features and details that are described in connection with different embodiments and/or aspects of the invention naturally also apply in connection with the other embodiments and/or aspects and vice versa, so that with regard to the disclosure of the individual embodiments and/or aspects always is or can be mutually referenced.

The invention provides: a method for examining an, in particular segmented, electrochemical cell, such as. B. a fuel cell, for example a PEM fuel cell, or a corresponding cell stack by electrochemical impedance spectroscopy or using electrochemical impedance spectroscopy (EIS for short).

The electrochemical cell (hereinafter referred to as a cell for short) can be, for example, a PEM fuel cell (PEMFC) or another electrochemical cell (e.g. PEMEC, SOFC, SOEC, AEMFC, AEMEC, etc.).

• - Aufprägen einer Frequenz auf ein Lastsignal auf mindestens einem Segment der elektrochemischen Zelle,
• - Erfassen einer Signalantwort von der elektrochemischen Zelle, insbesondere von dem Segment der elektrochemischen Zelle, insbesondere in Form von Informationen, die zur Erfassung der Impedanz notwendig oder nützlich sind (z.B. Strom- und/oder Spannungsantwort),

wobei vorzugsweise das Aufprägen und Erfassen innerhalb einer bestimmten Messzeit (bspw. 2 Perioden der aufgeprägten Frequenz) erfolgt,

• - Verändern der Frequenz, um eine weitere Frequenz auf das Lastsignal aufzuprägen und eine weitere Signalantwort von dem Segment der elektrochemischen Zelle zu erhalten,

wobei die oben genannten Schritte für einen bestimmten Frequenzbereich durchgeführt werden,
um eine Reihe an Signalantworten für unterschiedliche Frequenzen in dem bestimmten Frequenzbereich (z. B. mit einem Frequenzbereich von 10 kHz bis 50 Hz) zu erhalten,
insbesondere bevor die Auswertung von Signalantworten beginnt,

• - Zuordnen von Signalantworten in der Reihe an Signalantworten zu korrespondierenden Frequenzen,
• - Bestimmen von Impedanzen für unterschiedliche Frequenzen in dem bestimmten Frequenzbereich in Abhängigkeit von korrespondierenden Signalantworten.

The procedure has the following steps/actions:

• - imposing a frequency on a load signal on at least one segment of the electrochemical cell,
• - Detecting a signal response from the electrochemical cell, in particular from the segment of the electrochemical cell, in particular in the form of information that is necessary or useful for detecting the impedance (e.g. current and/or voltage response),

wherein the imprinting and recording preferably takes place within a certain measuring time (e.g. 2 periods of the imprinted frequency),

• - changing the frequency to impose a further frequency on the load signal and to obtain a further signal response from the segment of the electrochemical cell,

where the above steps are carried out for a specific frequency range,
to obtain a range of signal responses for different frequencies in the specific frequency range (e.g. with a frequency range of 10 kHz to 50 Hz),
especially before the evaluation of signal responses begins,

• - Assigning signal responses in the series of signal responses to corresponding frequencies,
• - Determining impedances for different frequencies in the specific frequency range depending on corresponding signal responses.

The measurement of different frequencies in the specific frequency range (e.g. with a frequency range of 10 kHz to 50 Hz) can preferably take place in less than one second. Only then can the evaluation of the signal responses begin, which in particular includes assigning signal responses in the series of signal responses to corresponding frequencies and determining impedances for different frequencies in the specific frequency range depending on corresponding signal responses.

The frequency range that is measured can be set variably, e.g. B. from 10 kHz to 50 Hz. The frequency range that is measured can be shifted, shortened or extended as required.

The idea behind the invention is that all frequencies are first impressed and measured before the evaluation of signal responses begins. In this way, a quick sampling of all frequencies can be made possible before the comparatively complex post-processing begins. Overall, the procedure can be carried out in less than a second.

With the help of the invention, a diagnostic method, so-called rapid and local EIS method (RaLo EIS), is provided using electrochemical impedance spectroscopy (EIS), which is locally and temporally resolved and can be used quickly, easily and reliably for transient conditions can be used to examine the electrochemical cell.

Transient conditions can in particular be understood to mean events in the operation of the electrochemical cell which lead to a change in the internal state of the electrochemical cell, for example in the event of a load change, a temperature change, in particular a coolant temperature change, a coolant flow change, a change in stoichiometry, a change the chemical composition in the operation of the electrochemical cell, etc. Transient events last a comparatively longer time, for example several seconds, than a run of the method according to the invention, which can take place under one second.

Different parameters can then be determined from the impedance spectra that can describe the internal state of the electrochemical cell, such as: B. the membrane resistance.

With the EIS measurement, current and voltage can be measured continuously and with high temporal resolution without interrupting the measurement for evaluation (ending and restarting the measuring process takes around 30 ms). During the ongoing measurement, all desired frequencies can be impressed serially on the cell and, for example, held for at least 2 periods. After all the desired frequencies have been applied, the measurement can be ended. The measurement time is therefore only the sum of the number of respective periods divided by the respective frequencies. In a downstream evaluation procedure that is therefore temporally decoupled from the measurement, the time series for current and voltage can finally be converted into impedance data (amplitude and phase shift). Each time point in the time series is assigned the associated frequency, so that separate, completed voltage and current time series are provided for each frequency. These time series can then be fitted to the sine curves of the respective frequencies. The impedance is calculated from the two fitted sine curves for the current and voltage time series.

Furthermore, it can be provided that the method is carried out in all segments of the, in particular segmented, electrochemical cell, preferably in parallel and/or simultaneously, in order to obtain corresponding impedances for different frequencies for all segments of the segmented electrochemical cell. Through the By parallelizing the process and reducing the measurement time, internal changes in the membrane resistance and current distribution can be observed and analyzed locally and in a temporally resolved manner.

In principle, it is also conceivable that the method can only be used globally to examine the transient behavior of a cell or a cell stack as a whole.

• - eine Signalantwort für Strom durch das Segment der elektrochemischen Zelle, und
• - eine Signalantwort für Spannung an dem Segment der elektrochemischen Zelle

erfasst werden.Furthermore, it can be provided that when detecting a signal response from the electrochemical cell, in particular from the at least one segment of the electrochemical cell:

• - a signal response for current through the segment of the electrochemical cell, and
• - a signal response for voltage on the segment of the electrochemical cell

be recorded.

At the electrochemical cell, the current can be recorded locally for each segment using current sensors (e.g. Hall sensors, shunt resistors, etc.). The method can be used with all known current and voltage sensors or methods that are temporally resolved to the required extent. The procedure can be carried out with a global voltage measurement. In addition, the cell voltage can be recorded individually for each segment. The local voltage measurement can be advantageous in order to be able to compensate for impedances that do not belong to the cell (e.g. cable-related impedances) during the measurement. By directly measuring current and voltage on the segments, the quality of the measured impedances can be significantly increased. The current and voltage time series can be measured using measurement channels with sufficiently high sampling rates and/or through parallelized measurement channels in order to enable the shortest possible measurement time for the largest possible frequency range in electrochemical impedance spectroscopy.

Furthermore, it can be provided that the detection of signal responses for current on segments of the electrochemical cell takes place via individual current outputs. Segmented scanning of the cell can thus advantageously be made possible.

In addition, it can be provided that the detection of signal responses for voltage on segments of the electrochemical cell takes place via individual voltage outputs or via a common voltage output. Using individual voltage outputs, improved results when determining impedances can be achieved. With the help of a common voltage output, the construction of a diagnostic tool for carrying out the procedure, in particular Rapid and Local EIS procedures (RaLo EIS), can be simplified.

It can also be provided that different frequencies are impressed on a load signal on segments of the electrochemical cell via a common input or via individual inputs. With the help of a common input, the construction of a diagnostic tool for carrying out the procedure, in particular Rapid and Local EIS procedures (RaLo EIS), can be simplified. However, a structure with individual inputs is also conceivable.

• - Bestimmen von mindestens einem Parameter der elektrochemischen Zelle in Abhängigkeit von den bestimmten Impedanzen für unterschiedliche Frequenzen.

Furthermore, it can be provided that the method has at least one further step:

• - Determining at least one parameter of the electrochemical cell depending on the specific impedances for different frequencies.

The at least one parameter can in particular include a membrane resistance (Rmem), a proton resistance (Rp) in the catalyst layer, a charge transfer resistance (Rct) and a so-called double layer capacitance (Cdl). These parameters can be used to examine the internal state of the cell. But other parameters that are suitable for describing the internal state of the cell can also be included in the at least one parameter.

Advantageously, the method, which can proceed as described above, can be carried out for at least one representative frequency range. Advantageously, depending on the determined impedances for different frequencies in the representative frequency range, a starting value and/or at least one fit limit for determining at least one parameter of the electrochemical cell can be determined.

With the diagnostic method described above, the so-called Rapid and Local EIS method (RaLo EIS), it is now possible to measure impedance spectra with temporal and spatial resolution within a specified frequency range. The resulting impedance data is interpreted at the end of the procedure to determine parameters of the cell that can indicate its internal state, such as: B. the membrane resistance. A specific frequency range, e.g. B. between 10 kHz and 50 Hz, which can be sampled for transient events within a fast turnaround time of less than a second, unsampled frequencies in the lower frequency range have zen. A full frequency range, e.g. B. between 10 kHz and 0.1 Hz, however, can be advantageous in order to determine the parameters that describe the internal state of the cell with increased accuracy.

Using a representative sequence of the diagnostic procedure for an extended, preferably complete frequency range, e.g. B. between 10 kHz and 0.1 Hz, insights can be gained that can be used to improve the diagnostic process for narrower frequency ranges, e.g. B. between 10 kHz and 50 Hz. The findings can be provided, for example, in the form of starting values and/or fit limits for corresponding impedance models. With the help of specifically determined starting values and/or fit limits, a high level of accuracy in determining cell parameters can be achieved even with a smaller frequency range, so that the measurement time can be reduced but the high level of accuracy is maintained. This makes it possible to fit and interpret RaLo-EIS data reliably and quickly. In this way, even incomplete impedance spectra that were obtained depending on location and time using the diagnostic method, the so-called Rapid and Local EIS method (RaLo EIS), can be evaluated reliably and quickly.

Such a representative sequence of the diagnostic method can advantageously be carried out at the moments when the cell is operated in a stationary state, for example when no load fluctuations are to be expected and/or when the computing power and time are available. Representative processes of the diagnostic procedure can, for example, be carried out before and after the RaLo-EIS measurement and provide full impedance spectra (EIS1 and EIS2, with the full frequency range from 10 kHz to 0.1 Hz). The same impedance model that is used for the RaLo-EIS data can be fitted to these full spectra. Fit limits and starting values for all fit procedures can also be obtained from the resulting results. These fit limits and starting values can then be used to enable reliable and faster fitting of the RaLo-EIS data.

Such representative processes of the diagnostic method can also be carried out periodically, event-controlled, in particular during a start, during stationary operation, when switching off the electrochemical cell, etc., and/or in a forward-looking manner, in particular depending on weather data, navigation data, calendar data, user settings, etc become. In this way, additional insights can be gained that can be used to further refine the diagnostic process.

Furthermore, it can be provided that the starting value is defined as a function, in particular as an average, of at least one (or more) specific parameter(s). In this way, a suitable starting point for fitting the RaLo-EIS data can be determined quickly and easily, which makes evaluating data even faster.

In addition, it can be provided that the at least one fit limit is determined depending on at least one mean value for determining at least one parameter. In this way, suitable fit limits for evaluating RaLo-EIS data can be determined quickly and easily, which means that the data can be analyzed quickly and reliably.

With the fit limits and starting values obtained, the fit for the RaLo-EIS data can then be carried out according to the selected fit model. Various impedance models, cost functions and/or various minimization algorithms can be used for this step. It is also conceivable to divide the measuring section so that the measuring time is divided into shorter sections, between which a full EIS data set is always recorded and is included in the calculation of fit limits and starting values. The starting values and fit limits could also be calculated according to different rules. The starting values and the fit limits can be used to ensure that each parameter remains within realistic limits, that the search space for each parameter is reduced and thus the calculation time is shortened and that the optimum fit is found quickly.

The method, which can proceed as described above, can be carried out in a preliminary manner, upon a transient event in the operation of the electrochemical cell, in order to examine at least one parameter of the electrochemical cell during the transient event. A transient event can be defined as an event that leads to a change in the internal state of the electrochemical cell, for example during a load change, a temperature change, a change in stoichiometry, a change in the chemical composition during operation of the electrochemical cell. Such a transient event usually lasts for a longer time, for example a multiple time, than a run of a diagnostic procedure, so-called Rapid and Local EIS procedure (RaLo EIS).

According to the second aspect, the present invention provides: a computer program product comprising instructions which, when the computer program product is executed by a computer, cause the computer to carry out a method, which can proceed as described above. With the help of the computer program product according to the invention, the same advantages are achieved that were described above in connection with the method according to the invention. These advantages are fully referenced here.

According to the third aspect, the present invention provides: a control unit, comprising a computing unit and a storage unit in which a code is stored which, when at least partially executed by the computing unit, carries out a method which can run as described above. With the help of the control unit according to the invention, the same advantages are achieved that were described above in connection with the method according to the invention. These advantages are fully referenced here.

The present invention provides according to the fourth aspect: a fuel cell system comprising a control unit which can be implemented as described above. With the help of the fuel cell system according to the invention, the same advantages are achieved that were described above in connection with the method according to the invention. These advantages are fully referenced here.

Preferred embodiments:

• 1 einen beispielhaften Aufbau für eine Strom und Spannungsmessung bei einer Impedanzmessung,
• 2 einen beispielhaften Ablauf eines Verfahrens im Rahmen der vorliegenden Offenbarung (b) im Vergleich zu einem herkömmlichen Ablauf einer Impedanzmessung (a), und
• 3 einen beispielhaften Ablauf eines Verfahrens für verlässliches Fitten von RaLo-EIS-Daten.

The invention and its further developments as well as their advantages are explained in more detail below with reference to drawings. They show schematically:

• 1 an exemplary setup for a current and voltage measurement during an impedance measurement,
• 2 an exemplary sequence of a method within the scope of the present disclosure (b) in comparison to a conventional sequence of an impedance measurement (a), and
• 3 an exemplary sequence of a procedure for reliable fitting of RaLo-EIS data.

In the different figures, the same parts of the invention are always provided with the same reference numerals, which is why they are generally. d. R. can only be described once.

The 1 until 3 serve to explain a method which was developed for examining a, in particular segmented, electrochemical cell 100 using electrochemical impedance spectroscopy or using electrochemical impedance spectroscopy (EIS for short).

The electrochemical cell 100 or cell for short may be a PEM fuel cell (PEMFC) or another electrochemical cell (e.g. PEMEC, SOFC, SOEC, AEMFC, AEMEC, etc.). The electrochemical cell 100 may have at least one or more segments 101.

• 301 Aufprägen einer Frequenz fi auf ein Lastsignal I, U auf mindestens einem Segment 101 der elektrochemischen Zelle 100,
• 302 Erfassen einer Signalantwort Ii, Ui von der elektrochemischen Zelle 100, insbesondere von dem Segment 101 der elektrochemischen Zelle 100, insbesondere in Form von Informationen, die zur Erfassung der Impedanz notwendig oder nützlich sind (z.B. Strom- und/oder Spannungsantwort),

wobei vorzugsweise das Aufprägen und Erfassen innerhalb einer bestimmten Messzeit (bspw. zwei Perioden) erfolgt,

• 303 Verändern der Frequenz fi, um eine weitere Frequenz fi+1 auf das Lastsignal I, U aufzuprägen und eine weitere Signalantwort li+1, Ui+1 von dem Segment 101 der elektrochemischen Zelle 100 zu erhalten,

wobei die oben genannten Schritte 301 bis 303 für einen bestimmten Frequenzbereich df durchgeführt werden,
um eine Reihe 304 an Signalantworten I, U für unterschiedliche Frequenzen fi in dem bestimmten Frequenzbereich df z. B. mit einem Frequenzbereich von 10 kHz bis 50 Hz zu erhalten.The method 300 has the following steps/actions:

• 301 imposing a frequency fi on a load signal I, U on at least one segment 101 of the electrochemical cell 100,
• 302 Detecting a signal response Ii, Ui from the electrochemical cell 100, in particular from the segment 101 of the electrochemical cell 100, in particular in the form of information that is necessary or useful for detecting the impedance (e.g. current and/or voltage response),

wherein the imprinting and recording preferably takes place within a certain measuring time (e.g. two periods),

• 303 changing the frequency fi in order to impose a further frequency fi+1 on the load signal I, U and to obtain a further signal response li+1, Ui+1 from the segment 101 of the electrochemical cell 100,

wherein the above-mentioned steps 301 to 303 are carried out for a specific frequency range df,
around a series 304 of signal responses I, U for different frequencies fi in the specific frequency range df z. B. with a frequency range of 10 kHz to 50 Hz.

The impressed frequencies fi can, for example, be held for at least 2 periods.

The aforementioned steps, which serve to measure 301 to 303 different frequencies fi in the specific frequency range df, can preferably be carried out before the evaluation 305, 306 of signal responses I, U.

• 305 Zuordnen von Signalantworten Ii, Ui in der Reihe an Signalantworten I, U zu korrespondierenden Frequenzen fi,
• 306 Bestimmen von Impedanzen Z für unterschiedliche Frequenzen f in dem bestimmten Frequenzbereich df in Abhängigkeit von korrespondierenden Signalantworten Ii, Ui.

The evaluation of signal responses I, U has the following steps/actions:

• 305 Assigning signal responses Ii, Ui in the series of signal responses I, U to corresponding frequencies fi,
• 306 Determination of impedances Z for different frequencies f in the specific frequency range df depending on corresponding signal responses Ii, Ui.

The impedances Z for different frequencies f can also be referred to as impedance spectra.

The measurement 301 to 303 can be carried out, for example, for a frequency range of 10 kHz to 50 Hz. The measurement 301 to 303 can therefore take place in less than a second. The steps for surveying 301 to 303 are in 2 B ) outlined with a dashed arrow.

Only then can the evaluation 305, 306 of the signal responses I, U from the series 304 of signal responses I, U begin. The steps for evaluation 305, 306 are in 2 B ) outlined with a solid arrow.

The specific frequency range df that is measured can be set variably, for example from 10 kHz to 50 Hz, and shifted, shortened or lengthened as required. The specific frequency range df is in the 2 B ) indicated using a series of frequencies f1, f2, f3, ....

How it is 2 B ) clarifies, all frequencies fi = f1, f2, f3, ... are first impressed and measured before the evaluation of signal responses I, U begins.

The difference between the method according to the invention 300 (cf. 2 B )) and the conventional method 200 (cf. 2 , a)) visible.

How it is 2, a ) indicates, in the conventional method 200 the steps of evaluation 205, 306 will be carried out at each frequency f. The measurement must be interrupted for evaluation 205, 206. The conventional method 200 includes N = Nf repetitions, where Nf is the number of measured frequencies f. Since ending and restarting the measuring process takes around 30 ms, a lot of time is lost.

In contrast, the method 300 according to the invention only has one pass N = 1 for measuring 301 to 303 of all frequencies fi = f1, f2, f3, .... In this way, a quick measurement can be made on all frequencies fi = f1, f2, f3, ... before the comparatively complex evaluation 305, 306 begins.

The measurement time is therefore only the sum of the number of the respective periods divided by the respective frequencies fi. In a downstream evaluation procedure 305, 306 that is therefore temporally decoupled from the measurement, the time series for current I and voltage U can finally be converted into impedance data (amplitude and phase shift).

With the help of the invention, a diagnostic method, so-called rapid and local EIS method (RaLo EIS), can be provided, which is locally and temporally resolved and which is quick, simple and reliable for transient conditions in the operation of the electrochemical cell 100 can be applied to examine the internal parameters P of the cell 100.

Under transient conditions, any changes in the internal state of the electrochemical cell 100 may be subsumed, for example load changes, temperature changes, in particular coolant temperature changes, coolant flow changes, stoichiometry changes, chemical composition changes, etc. Transient events last for a longer period of time, for example several times a time of several seconds, e.g. B. a 10-fold time, for example 10 seconds, as one run of the method 300 according to the invention, which can take place under one second.

• - Membranwiderstand Rmem,
• - Protonenwiderstand Rp in der Katalysatorschicht,
• - Ladungstransferwiderstand Rct, und/oder
• - Doppelschichtkapazität Cdl usw.

From the impedance spectra from step 306, different parameters P can then be determined in step 307, which can describe the internal state of the electrochemical cell 100, for example:

• - membrane resistance Rmem,
• - proton resistance Rp in the catalyst layer,
• - Charge transfer resistance Rct, and/or
• - Double layer capacity Cdl etc.

How it is 1 indicates, the method 300 can be carried out in all segments 101 of the, in particular segmented, electrochemical cell 100, preferably in parallel and/or simultaneously, in order to obtain corresponding impedances Z for different frequencies f for all segments 101 of the segmented electrochemical cell 100.

• - eine Signalantwort Ii für Strom I durch das Segment 101 der elektrochemischen Zelle 100, und
• - eine Signalantwort Ui für Spannung U an dem Segment 101 der elektrochemischen Zelle 100

erfasst werden.Furthermore, the 1 assume that when detecting a signal response Ii, Ui from the electrochemical cell 100, in particular from the at least one segment 101 of the electrochemical cell 100:

• - a signal response Ii for current I through the segment 101 of the electrochemical cell 100, and
• - a signal response Ui for voltage U on the segment 101 of the electrochemical cell 100

be recorded.

Furthermore, the 1 indicates that the detection of signal responses Ii for current I on segments 101 of the electrochemical cell 100 can take place via individual current outputs.

In addition, it indicates 1 indicates that the detection of signal responses Ui for voltage U on segments 101 of the electrochemical cell 100 can take place via a common voltage output. At the same time, individual voltage outputs can be provided.

In addition, it indicates 1 indicates that the imprinting of different frequencies fi on a load signal I, U on segments 101 of the electrochemical cell 100 takes place via a common input. However, a structure with individual inputs is also conceivable.

Furthermore, the 3 indicates that the method 300, which can run as described above, is carried out for at least one representative frequency range df1, df2. Advantageously, depending on the determined impedances Z for different frequencies f in the representative frequency range df1, d2, a starting value Pstart and / or at least one fit limit P1, P2 for determining at least one parameter Rmem, Rp, Rct, Cdl of the electrochemical cell 100 can be determined.

Developed using the process 300 described above, so-called Rapid-and-Local-EIS RaLo EIS. RaLo EIS, it is now possible to measure impedance spectra with temporal and spatial resolution within a specific frequency range df. The resulting impedance data Z is interpreted at the end of the method 300 to determine parameters P of the cell 100, which may be specific to the internal state of the cell 100, such as. B. the membrane resistance. A specific frequency range, e.g. B. between 10 kHz and 50 Hz, which can be sampled in transient events within a fast turnaround time of less than a second, can have unsampled frequencies in the lower frequency range. A full frequency range, e.g. B. between 10 kHz and 0.1 Hz, however, can be advantageous in order to determine the parameters that describe the internal state of the cell with increased accuracy.

Using a representative sequence of the diagnostic method 300 (df1, df2) for an extended, preferably complete frequency range df1, df2, e.g. B. between 10 kHz and 0.1 Hz, insights can be gained that can be used to develop the diagnostic method 300 for narrower frequency ranges df, e.g. B. between 10 kHz and 50 Hz., to refine. The findings can, for example, be provided in the form of starting values Pstart and/or fit limits P1, P2 for corresponding impedance models in order to enable quick and efficient determination of cell parameters in a smaller frequency range. of starting values Pstart and/or fit limits P1, P2, it is possible to fit and interpret RaLo-EIS data reliably and quickly. In this way, even incomplete impedance spectra, which were obtained depending on location and time using the diagnostic method, so-called Rapid-and-Local-EIS RaLo EIS, can be analyzed reliably and quickly.

Such a representative sequence of the diagnostic method 300 (df1, df2) can advantageously be carried out at the moments when the cell 100 is operated in a stationary state, for example when no load fluctuations are to be expected and/or when the computing power and time are available stand.

Representative processes of the diagnostic procedure can, for example, be carried out before and after the RaLo-EIS measurement and provide full impedance spectra (EIS1 and EIS2, with the full frequency range from 10 kHz to 0.1 Hz).

Such representative sequences of the diagnostic method can also be periodic, event-controlled, in particular during a start, during stationary operation, when switching off the electrochemical cell 100, etc., and / or predictively, in particular depending on weather data, navigation data, calendar data, user settings, etc. be performed.

Furthermore, the shows 3 that the starting value Pstart is defined as a function, for example as an average, of at least one (or more) specific parameter (s) (P = Rmem, Rp, Rct, Cdl): $$\left(\text{P1means}(\text{ICE1}\right)+\text{P2means}\left(\text{ICE2}\right))\text{/2}\text{.}$$

In addition, the at least one fit limit P1, P2 can be determined depending on at least one mean value (P1mean(EIS1) + P2mean(EIS2))/2 for determining at least one parameter P.

• 1) Der Mittelwert aus P1mittel(EIS1) und P2mittel(EIS2) kann als Startwert Pstart bestimmt werden,
• 2) Ferner kann ein ΔP berechnet werden, bspw. gleich einem Teil, z. B.10% oder 5%, der Differenz zwischen P1mittel(EIS1) und P2mittel(EIS2),
• 3) Ein Minimum aus den beiden Parametern P1mittel(EIS1) und P2mittel(EIS2) minus ΔP kann bspw. als eine untere Fitgrenze P1 für den Parameter P festgelegt werden.
• 4) Ein Maximum aus den beiden Parametern P1mittel(EIS1) und P2mittel(EIS2) plus ΔP kann als eine obere Fitgrenze P2 für den Parameter P festgelegt werden.

In an example of two representative sequences of diagnostic methods 300 (df1, df2) or EIS1 and EIS2, steps can be carried out for each of the parameters (P = Rmem, Rp, Rct, Cdl) of the impedance model:

• 1) The mean value of P1mean(EIS1) and P2mean(EIS2) can be determined as the starting value Pstart,
• 2) Furthermore, a ΔP can be calculated, for example equal to a part, e.g. B.10% or 5%, the difference between P1average(EIS1) and P2average(EIS2),
• 3) A minimum of the two parameters P1mean(EIS1) and P2mean(EIS2) minus ΔP can, for example, be set as a lower fit limit P1 for the parameter P.
• 4) A maximum of the two parameters P1mean(EIS1) and P2mean(EIS2) plus ΔP can be set as an upper fit limit P2 for the parameter P.

With the obtained fit limits P1, P2 and starting values Pstart, the fit for the RaLo-EIS data can then be carried out according to the selected fit model. Various impedance models, cost functions and/or various minimization algorithms can be used for this step.

It is also conceivable to divide the measuring section so that the measuring time is divided into shorter sections, between which a full EIS data set is always recorded and is included in the calculation of fit limits and starting values.

The starting values Pstart and fit limits P1, P2 could also be calculated according to other rules.

The starting values Pstart and the fit limits P1, P2 can be used to ensure that each parameter P remains within realistic limits, that the search space for each parameter is reduced and thus the calculation time is shortened and that the optimum fit is found quickly.

The transmission line model, for example, can be used as an impedance model.

The above description of the figures describes the present invention exclusively in the context of examples. Of course, individual features of the embodiments can, if it makes technical sense, be freely combined with one another without departing from the scope of the invention.

Claims (16)

Method for examining an, in particular segmented, electrochemical cell (100) by electrochemical impedance spectroscopy (EIS), comprising: - imposing a frequency (fi) on a load signal (I, U) on at least one segment (101) of the electrochemical cell (100), - Detecting a signal response (li, Ui) from the electrochemical cell (100), - Changing the frequency (fi) in order to impose a further frequency (fi+1) on the load signal (I, U) and a further signal response (li+1, Ui+1) from the segment (101) of the electrochemical cell (100 ), wherein the above steps are carried out for a specific frequency range (df) in order to obtain a series of signal responses (I, U) for different frequencies (fi) in the specific frequency range (df), - Assigning signal responses (li, Ui) in the series of signal responses (I, U) to corresponding frequencies (fi), - Determining impedances (Z) for different frequencies (f) in the specific frequency range (df) depending on corresponding signal responses (li, Ui). Procedure according to Claim 1 , characterized in that the method is carried out in all segments (101) of the, in particular segmented, electrochemical cell (100), preferably in parallel and / or simultaneously, in order to obtain corresponding impedances for all segments (101) of the segmented electrochemical cell (100). (Z) for different frequencies (f). Procedure according to Claim 1 or 2 , characterized in that when detecting a signal response (li, Ui) from the electrochemical cell (100): - a signal response (li) for current (I) through the segment (101) of the electrochemical cell (100), and - a signal response (Ui) for voltage (U) on the segment (101) of the electrochemical cell (100) can be detected. Method according to one of the preceding claims, characterized in that the detection of signal responses (li) for current (I) on segments (101) of the electrochemical cell (100) takes place via individual current outputs. Method according to one of the preceding claims, characterized in that the detection of signal responses (Ui) for voltage (U) on segments (101) of the electrochemical cell (100) takes place via individual voltage outputs or via a common voltage output. Method according to one of the preceding claims, characterized in that different frequencies (fi) are impressed on a load signal (I, U) on segments (101) of the electrochemical cell (100) via a common input or via individual inputs. Method according to one of the preceding claims, characterized in that the method has at least one further step: - determining at least one parameter (Rmem, Rp, Rct, Cdl) of the electrochemical cell (100) as a function of the determined impedances (Z) for different frequencies (f). Method according to one of the preceding claims, characterized in that the method according to one of the preceding claims is carried out for at least one representative frequency range (df1, df2), in particular depending on the specific impedances (Z) for different frequencies (f) in which representative frequency range (df1, df2), a starting value (Pstart) and / or at least one fit limit (P1, P2) for determining at least one parameter (Rmem, Rp, Rct, Cdl) of the electrochemical cell (100) are/will be determined. Method according to the preceding claim, characterized in that the method according to the preceding claim is periodic, event-controlled, in particular at a start, during stationary operation, when switching off the electrochemical cell (100), etc., and / or in a forward-looking manner, in particular depending on of weather data, navigation data, calendar data, user settings, etc. Method according to one of the preceding Claims 8 or 9 , characterized in that the starting value (Pstart) is defined as a function, in particular as an average, of at least one specific parameter (Rmem, Rp, Rct, Cdl). Method according to one of the preceding Claims 8 until 10 , characterized in that the at least one fit limit (P1, P2) is determined depending on at least one mean value for determining at least one parameter (Rmem, Rp, Rct, Cdl). Method according to one of the preceding Claims 8 until 11 , characterized in that determining at least one parameter (Rmem, Rp, Rct, Cdl) of the electrochemical cell (100), in particular according to Claim 7 , using the starting value (Pstart) and/or the at least one fit limit (P1, P2). Method according to the preceding claim, characterized in that the method according to the preceding claim is carried out during a transient event (E) during operation of the electrochemical cell (100) in order to adjust at least one parameter (Rmem, Rp, Rct, Cdl) of the electrochemical cell (100) to examine the transient event, in particular a transient event (E) being determined as an event which leads to a change in the internal state of the electrochemical cell (100), for example during a load change, a temperature change, a change the stoichiometry, a change in the chemical composition during operation of the electrochemical cell (100), preferably a transient event (E) lasting a longer time, for example a multiple time, than a run of a method according to one of the preceding ones Claims 1 until 7 . Computer program product, comprising instructions which, when the computer program product is executed by a computer, cause the computer to carry out a method according to one of the preceding claims. Control unit, comprising a computing unit and a storage unit in which a code is stored which, when at least partially executed by the computing unit, carries out a method according to one of the preceding claims. Fuel cell system comprising a control unit according to the preceding claim.

Images