DE102022207821A1 - Diagnostic method, diagnostic system and computer program product for diagnosing a condition of a fuel cell system - Google Patents

Abstract

The invention presented relates to a diagnostic method (100) for diagnosing a condition of a fuel cell system. The diagnostic method (100) comprises: - applying (101) a fuel cell stack of the fuel cell system to an electrical current from a power source (303), - measuring (103) the voltages on the respective fuel cells of the fuel cell stack, - determining (105) a characteristic value quantifies a state of the fuel cell system based on the measured voltage, - outputting (107) the characteristic value on an output unit, the measuring (103) of the voltage on the respective fuel cells of the fuel cell stack taking place simultaneously.

Description

The presented invention relates to a diagnostic method, a computing unit and a computer program product for diagnosing a state of a fuel cell system according to the appended claims.

State of the art

To diagnose a condition of a fuel cell system, voltage-controlled diagnostic methods, such as cyclic voltammetry, are usually used. Each individual fuel cell of a fuel cell stack is subjected to a voltage triangular curve several times in order to determine a characteristic value, in particular an active platinum surface, i.e. an electrochemically active area of the respective fuel cell.

Disclosure of the invention

As part of the presented invention, a diagnostic method, a diagnostic system and a computer program product are presented. Further features and details of the invention emerge from the respective subclaims, the description and the drawings. Features and details that are described in connection with the diagnostic method according to the invention naturally also apply in connection with the diagnostic system according to the invention or the computer program product according to the invention and vice versa, so that reference is or can always be made to each other with regard to the disclosure of the individual aspects of the invention .

The invention presented serves in particular to provide a possibility for determining a state of a fuel cell system.

According to a first aspect of the presented invention, a diagnostic method for diagnosing a state of a fuel cell system is therefore presented. The diagnostic method includes applying a constant electrical current from a power source to a fuel cell stack of the fuel cell system, measuring a voltage applied to respective fuel cells of the fuel cell stack and determining a characteristic value that quantifies a state of the fuel cell system based on the measured voltage and outputting the characteristic value on an output unit, with the measurement of the voltage on respective fuel cells of the fuel cell stack taking place simultaneously.

In the context of the invention presented, a characteristic value is to be understood as a value on a scale or a numerical value.

The diagnostic procedure presented is based on a current-controlled procedure. This means that a predetermined electrical current is applied as an independent variable to a fuel cell stack or respective fuel cells of the fuel cell stack in order to determine a change in the voltage applied to the respective fuel cells in response to the application of the electrical current as a dependent variable and evaluated, i.e. used to determine a characteristic value. Accordingly, a characteristic value is determined based on a curve of the voltage when electrical current is applied and during a subsequent self-discharge, which quantifies the state of the fuel cell or the fuel cell system.

The determined characteristic value is output, i.e. made available or stored, on an output unit, such as a display and/or a memory.

To measure the voltage on the respective fuel cells of the fuel cell system, a so-called “Cell Voltage Monitoring System” of the fuel cell system or an external voltage measuring device can be used.

Dabei steht U(t) für eine gemessene Spannung zu einem Zeitpunkt t, I_Q für einen beaufschlagten Strom, R_SC für einen Kurzschlusswiderstand, t_lb für die Zeit, bei der beim Ladevorgang U_lb erreicht wird, C_dl für eine Doppelschichtkapazität und I_H2 für einen spannungsunabhängigen Entladestrom.Based on a respective determined voltage, a respective characteristic value can be deduced using equation (1) by fitting equation (1) to the measurement data between, for example, 400mV (=U _lb ) and 500mV (=U _ub ). Only in this area do no electrochemical reactions take place on the catalyst. $$\text{U}\left(t\right)=\left[\left(I_Q-I_{H_2}\right)\cdot R_{sc}\right]-\left[\left(I_Q-I_{H_2}\right)\cdot R_{sc}-U_{tb}\right]\cdot e^{-\frac{1}{R_{sc}\cdot C_{dl}}\cdot \left(t-t_{lb}\right)}$$

U(t) stands for a measured voltage at a time t, I _Q for an applied current, R _SC for a short-circuit resistance, t _lb for the time at which U _lb is reached during the charging process, C _dl for a double-layer capacitance and I _H2 for a voltage-independent discharge current.

During self-discharge, no current flows across the short-circuit resistor, so that the double-layer capacitance in the range from U _ub to U _lb is only discharged by I _H2 : $${\frac{dU}{dt}|}_{discHarGe}=-\frac{I_{H_2}}{C_{dl}}$$

Using this discharge rate (2) of the double-layer capacity between U _ub and U _lb , the double-layer capacity can be eliminated from equation (1): $$\text{U}\left(t\right)=\left(I_Q-I_{H_2}\right)\cdot R_{sc}-\left[\left(I_Q-I_{H_2}\right)\cdot R_{sc}-U_{tb}\right]\cdot e^{{\frac{dU}{dt}|}_{discHarGe}\cdot \frac{\left(t-t_{lb}\right)}{R_{sc}\cdot I_{H_2}}}$$

This means that the equation only needs to be fitted to the measuring points between the voltages U _lb and U _ub during charging using the variables I _H2 and R _sc .

When applying the electrical current, it can be provided that a current is used that corresponds to a factor between 2 and 4 of a hydrogen crossover current of a respective fuel cell system or leads to an electrical charge of the fuel cell stack in a period of between 20 and 50 seconds.

It can be provided that a cathode of the fuel cell system is used as a working electrode and operated in a nitrogen atmosphere, and an anode of the fuel cell system is used as a counter electrode and is operated in a hydrogen atmosphere.

Alternatively, it can be provided that an anode of the fuel cell system is used as a working electrode and operated in a nitrogen atmosphere, and a cathode of the fuel cell system is used as a counter electrode and is operated in a hydrogen atmosphere.

Since nitrogen acts as an inert gas for a fuel cell, a respective electrode operated under a nitrogen atmosphere can be used as a working electrode.

It can also be provided that at least one key figure from the following list of key figures is determined as a characteristic value: electrochemically active area, hydrogen membrane leakage, double layer capacity, catalyst capacity, roughness factor and short-circuit resistance of the membrane.

The characteristic value can be output as a numerically determined value or can be assigned to a scale value, such as a color scheme and/or a value on an ordinal scale, using an assignment scheme and then output. Of course, the characteristic value can include several sub-characteristics, each of which includes key figures or is based on respective key figures.

It can also be provided that the fuel cell system is discharged by self-discharge after the electric current has been applied.

In order to be able to record the electrochemical properties of respective fuel cells, self-discharge has proven to be particularly suitable, since with this type of discharge there is a slow reduction in voltage, i.e. over a few seconds, which can only be attributed to the physical properties of the fuel cells, as is the case is given in equation (2).

The measurement of the voltage provided according to the invention can take place in particular during a discharging process of the fuel cell stack, i.e. on a descending branch of the voltage curve.

It can also be provided that the fuel cell stack is supplied with electrical current in a single charging cycle.

In contrast to voltage-controlled methods, the presented diagnostic method can be carried out with a single charging cycle. This means that the fuel cells are charged once up to a specified maximum voltage value with a specified current value and then discharged themselves. At the time the power is switched off, each fuel cell should have reached at least 500mV (U _ub ) so that the evaluation can be carried out.

It can further be provided that when determining the characteristic value, a short-circuit resistance of a respective fuel cell of the fuel cell stack is determined from an increasing range of a voltage curve when the fuel cell stack is supplied with electrical current, as is indicated, for example, in equation (3).

Using a so-called “fitting function”, the short-circuit resistance of a respective fuel cell can be determined quickly and easily.

It can further be provided that when determining the characteristic value, a voltage-independent discharge current (I _H2 ) of a respective fuel cell of the fuel cell stack is determined from an increasing range of a voltage curve when the fuel cell stack is supplied with electrical current, which corresponds to the molecular H ₂ transfer through the fuel cell membrane which describes CV/LSV methods.

A “fitting function” can also be used to quickly and easily determine the voltage-independent discharge current of a respective fuel cell. In the area of self-discharge, the discharge rate between U _ub and U _lb can be determined from the measurement data and thus also the double-layer capacity C _dl using equation (2).

Thanks to the simple relationships in equation (2), the double layer capacitance C _dl in equation (1) can be replaced by known quantities, which leads to equation (3). The fit algorithm therefore only needs to determine the two unknowns I _H2 and R _sc at the same time.

To calculate the roughness factor of the desorption, the range of charging below U _lb is now considered using the quantities determined above.

Hydrogen desorption can be viewed as a capacitor connected in parallel to the double layer, the amount of charge of which does NOT increase linearly with voltage or remain constant.

• ► Beide Kapazitäten C_dl und C_Pt werden durch den Netto-Ladestrom bis 400mV (U_lb) aufgeladen:
  • ► $I_{C_{dl}}+I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}$
    
• ► Nach I_CPt aufgelöst:
  • ► $I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}-I_{C_{dl}}$
    
• ► Aus der Kondensatorformel ergibt sich
  • ► $I_{C_{dl}}=C_{dl}\cdot \frac{\mathrm{\Delta }U}{\mathrm{\Delta }t}$
    
• ► Oben eingesetzt führt das zu
  • ► $I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}-C_{dl}\cdot \frac{\mathrm{\Delta }U}{\mathrm{\Delta }t}$
    
• ► Der Strom I_CPt wird über die Zeit von „Ladebeginn“ bis U_lb (400mV) „integriert“
  • ► $Q_{C_{Pt}}={\displaystyle {\sum }_{t=t_0}^{t\left(U_{th}\right)}{I}_{Pt}}\cdot \mathrm{\Delta }t$
    

The following relationships apply:

• ► Both capacitances C _dl and C _Pt are charged by the net charging current up to 400mV (U _lb ):
  • ► $I_{C_{dl}}+I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}$
    
• ► After I _{C Pt} dissolved:
  • ► $I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}-I_{C_{dl}}$
    
• ► The capacitor formula gives:
  • ► $I_{C_{dl}}=C_{dl}\cdot \frac{\mathrm{\Delta }U}{\mathrm{\Delta }t}$
    
• ► Inserted at the top this leads to
  • ► $I_{C_{Pt}}=I_Q-I_{H2}-\frac{U}{R_{sc}}-C_{dl}\cdot \frac{\mathrm{\Delta }U}{\mathrm{\Delta }t}$
    
• ► The current I _{C Pt} is “integrated” over the time from “start of charging” to U _lb (400mV)
  • ► $Q_{C_{Pt}}={\displaystyle {\sum }_{t=t_0}^{t\left(U_{tH}\right)}{I}_{Pt}}\cdot \mathrm{\Delta }t$
    

• ► Berechnung des Roughness-Factors für die H₂-Desorption
  • ► $r{f}_{des}=\frac{Q_{Pt}}{\mathrm{2,1}\cdot {10}^{-4}\cdot A_{MEA}}$
    
  • ► $ECS{A}_{des}=\frac{r{f}_{des}}{Platinbeladung}$
    
• ► Berechnung des Roughness-Factors für die H₂-Desorption
  • ► $r{f}_{des}=\frac{Q_{Pt}}{\mathrm{2,1}\cdot {10}^{-4}\cdot A_{MEA}}$
    
  • ► $ECS{A}_{des}=\frac{r{f}_{des}}{Platinbeladung}$
    

The time t ₀ describes the point in time from which a voltage level U ₀ is exceeded and the evaluation begins (approx. OCV or slightly above).

• ► Calculation of the roughness factor for H ₂ desorption
  • ► $r{f}_{des}=\frac{Q_{Pt}}{2.1\cdot {10}^{-4}\cdot A_{MEA}}$
    
  • ► $ECS{A}_{des}=\frac{r{f}_{des}}{PlatinbeladunG}$
    
• ► Calculation of the roughness factor for H ₂ desorption
  • ► $r{f}_{des}=\frac{Q_{Pt}}{2.1\cdot {10}^{-4}\cdot A_{MEA}}$
    
  • ► $ECS{A}_{des}=\frac{r{f}_{des}}{PlatinbeladunG}$
    

According to a second aspect, the presented invention relates to a diagnostic system. The diagnostic system presented includes a computing unit that is configured to execute a possible embodiment of the diagnostic method presented.

In the context of the invention presented, a computing unit is understood to mean a computer, a processor, a control device or any other programmable circuit. In particular, the computing unit can be a control device of a respective fuel cell system.

According to a third aspect, the presented invention relates to a computer program product with program code means which, when the computer program product is executed on a computer, configures the computer to carry out a possible embodiment of the presented diagnostic method.

The computer program product presented can be, for example, a file for downloading on a server or a data storage medium, such as a CD-ROM or a USB stick.

Further advantages, features and details of the invention emerge 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 each be essential to the invention individually or in any combination.

• 1 eine schematische Darstellung einer möglichen Ausgestaltung des vorgestellten Diagnoseverfahrens,
• 2 eine Detaildarstellung des Diagnoseverfahrens gemäß 1,
• 3 eine mögliche Ausgestaltung des vorgestellten Diagnosesystems.

Show it:

• 1 a schematic representation of a possible design of the diagnostic method presented,
• 2 a detailed description of the diagnostic procedure 1 ,
• 3 a possible design of the presented diagnostic system.

In 1 a diagnostic method 100 for diagnosing a state of a fuel cell system is shown.

The diagnostic method 100 includes an application step 101, in which a fuel cell stack of the fuel cell system is exposed to an electrical constant current from a power source, a measuring step 103, in which a voltage applied to respective fuel cells of the fuel cell stack is measured, a determination step 105, in which a characteristic value , which quantifies a state of the fuel cell system, is determined based on the measured voltage and an output step 107, in which the characteristic value is output on an output unit, with the measuring step 103 measuring the voltage on respective fuel cells of the fuel cell stack, for example on respective ones with a CVM system coupled fuel cells or all fuel cells simultaneously.

In 2 a diagram 200 is shown, which spans on its abscissa over time and on its ordinate over a voltage measured on a fuel cell.

Since it was experimentally determined that the short-circuit resistance Rsc in the descending branch of the voltage curve 201 or when discharging the fuel cell stack is greater by a factor of more than 2000 than when charging the fuel cell stack, a voltage-independent discharge current I _H2 and the measured voltage in the range can be used of the descending branch, in particular between 500mV and 400mV, a double layer capacitance C _dl can be calculated.

Using a “fitting function”, the best possible parameter combination of a voltage-independent discharge current I _H2 and a short-circuit resistance R _sc can be found based on a measured voltage curve 201. Using the fitting function, a double-layer capacitance C _dl can be determined using I _H2 in the extended voltage range (U _ub to U _lb ) between 600 mV and 300 mV, as indicated by arrow 203, while the measured voltage curve 201 is in the range between 300 mV and 0 mV is influenced by both the double layer capacitance C _dl and the catalyst layer capacitance C _H2Pt , as indicated by arrow 205.

Since the catalyst layer or its catalyst layer capacity C _H2Pt is only discharged by I _H2 , the charge of the catalyst layer Q _Pt can be determined directly, which in turn can be used to determine the roughness factor for the hydrogen desorption rf _des , which in turn indicates the electrochemical active surface for hydrogen desorption can be closed.

In 3 a diagnostic system 300 is shown. The diagnostic system 300 includes a computing unit 301 in the form of a control device that is used to carry out the diagnostic method 100 according to 1 configured and an optional constant current source 303.

The computing unit 301 includes an interface 305 for communicative and/or electrical coupling with a cell voltage monitoring system.

Claims (10)

Diagnostic method (100) for diagnosing a state of a fuel cell system, the diagnostic method (100) comprising: - applying (101) a fuel cell stack of the fuel cell system to a constant electrical current from a power source (303), - measuring (103) a voltage applied to respective fuel cells of the fuel cell stack, - Determining (105) a characteristic value that quantifies a state of the fuel cell system based on the measured voltage, - Outputting (107) the characteristic value on an output unit, with the measuring (103) of the voltage on the respective fuel cells of the fuel cell stack taking place simultaneously. Diagnostic procedure (100). Claim 1 , characterized in that a cathode of the fuel cell system is used as a working electrode and is operated in a nitrogen atmosphere, and an anode of the fuel cell system is used as a counter electrode and is operated in a hydrogen atmosphere or hydrogen/nitrogen atmosphere. Diagnostic procedure (100). Claim 1 , characterized in that an anode of the fuel cell system is used as a working electrode and is operated in a nitrogen atmosphere, and a cathode of the fuel cell system is used as a counter electrode and is operated in a hydrogen atmosphere. Diagnostic method (100) according to one of the preceding claims, characterized in that at least one key figure from the following list of key figures is determined as a characteristic value: electrochemically active area, hydrogen membrane leakage, double layer capacity, catalyst capacity, roughness factor and short-circuit resistance of the membrane. Diagnostic method (100) according to one of the preceding claims, characterized in that the fuel cell system is discharged by self-discharge after being supplied with the electrical current. Diagnostic method (100) according to one of the preceding claims, characterized in that the fuel cell stack is supplied with electrical current in a single charging cycle. Diagnostic method (100) according to one of the preceding claims, characterized in that when determining the characteristic value, a short-circuit resistance of a respective fuel cell of the fuel cell stack is determined from an ascending region of a current curve when the fuel cell stack is supplied with electrical current. Diagnostic method (100) according to one of the preceding claims, characterized in that when determining the characteristic value, a voltage-independent discharge current of a respective fuel cell of the fuel cell stack is determined from a descending region of a current curve when electrical current is applied to the fuel cell stack. Diagnostic system (300) for diagnosing a condition of a fuel cell system, wherein the diagnostic system (300) comprises a computing unit (301) which is configured to implement a diagnostic method (100) according to one of Claims 1 until 8th to carry out. Computer program product with program code means which, when the computer program product is executed on a computer, configures the computer to perform a diagnostic method (100) according to one of the Claims 1 until 8th to carry out.

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