Method and apparatus for diagnosing a condition of a fuel cell stack

Abstract

A method for diagnosing a condition of a fuel cell stack (10) in that it is contaminated, comprising: Measuring an impedance of the fuel cell stack (10) at a predetermined low frequency and a predetermined high frequency; Increasing an amount of fuel or oxidant supplied to the fuel cell stack (10) when the impedance at the predetermined high frequency is less than a first critical point (Z1) and the impedance at the predetermined low frequency is greater than a second critical point (Z2); re-measuring the impedance of the fuel cell stack (10) at the predetermined low frequency; and Detecting that the fuel cell stack (10) is contaminated when the impedance at the predetermined low frequency measured during the re-measurement is greater than the second critical point (Z2).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a technology for detecting fouling of a fuel cell, and more particularly to a method and apparatus for diagnosing a fouled state of a fuel cell stack by measuring the impedance of the fuel cell stack at a high frequency and a low frequency. BACKGROUND

[0002] A fuel cell device is a power-generating device that directly converts the chemical energy of a fuel into electricity. A fuel cell device includes a fuel cell stack that generates electricity, a fuel supply unit that supplies fuel (e.g., hydrogen) to the fuel cell stack, an air supply unit that supplies air (specifically, oxygen) to the fuel cell stack as an oxidizer needed to initiate an electrochemical reaction, and a heat and water management unit that removes heat from the fuel cell stack and controls the operating temperature of the fuel cell stack. The fuel cell stack produces electricity through an electrochemical reaction between hydrogen (fuel) and oxygen (air) and also generates byproducts (heat and water) that must be removed from the fuel cell stack.

[0003] A fuel cell stack, suitable for use in a fuel cell vehicle, comprises many individual cells arranged in series. Each individual cell has a membrane-electrode assembly (MEA) located at the center. An MEA has an electrolyte membrane that allows protons to pass through. Catalyst layers serving as the cathode and anode, where hydrogen and oxygen react, are arranged on respective surfaces of the electrolyte membrane. Gas diffusion layers (GDLs) are arranged on the surfaces of the catalyst layers. Separators with corresponding flow fields (channels), through which fuel and air are supplied to the anode and cathode, are arranged on the surfaces of the GDLs. End plates are arranged at respective ends of each individual cell to firmly connect all these elements.

[0004] In a fuel cell stack, hydrogen and oxygen are ionized through chemical reactions via the catalyst layers. Then, an oxidation reaction occurs, producing protons (hydrogen ions) and electrodes at a fuel electrode, to which hydrogen is supplied, and a reduction reaction involving hydrogen ions and oxygen ions, producing water at an air electrode, to which air is supplied. A typical electrode catalyst used in a fuel cell consists of a catalyst support made of a carbon material and a co-catalyst such as Ru, Co, Cu, or the like. Hydrogen is supplied to an anode (also referred to as an "oxidation electrode"), and oxygen (air) is supplied to a cathode (also referred to as a "reduction electrode"). The hydrogen supplied to the anode is converted into H+ Protons and e - Electrons are split by the catalyst on the electrode layers arranged on the corresponding surfaces of the electrolyte membrane. Of the protons and electrons, only the protons can selectively pass through the electrolyte membrane, which is called the proton exchange membrane, and reach the cathode. The electrons move through the GDLs (conductive layers) and the separators to reach the cathode.

[0005] Hydrogen ions reaching the cathode through the electrolyte membrane and electrons reaching the cathode through the separators react with the oxygen contained in the air supplied to the cathode via the air supply unit, producing water. At the same time, the movement of the hydrogen ions causes an electric current to flow through an external line, and in addition to the water, heat is produced as a byproduct.

[0006] In a fuel cell device, the performance of a fuel cell stack deteriorates due to various causes. Therefore, technology is needed to detect the causes of fuel cell stack deterioration. Potential causes of fuel cell stack deterioration include flooding (specifically, flooding, overflow, and / or over-wetness), drying out (specifically, over-dryness), and fouling of the fuel cell stack.

[0007] For example, if the cause of a fuel cell stack's performance deterioration is mistakenly identified as flooding when the actual cause is fuel cell stack contamination, operation control may be performed by increasing the air flow rate to solve the flooding problem, which may exacerbate the dryness of the fuel cell stack. This means that no appropriate action is taken, which does not solve the fuel cell stack's problem and may potentially generate new problems. Therefore, it is important to accurately and precisely diagnose the condition of a fuel cell stack to identify the causes of fuel cell stack performance deterioration so that appropriate measures can be taken to solve the fuel cell stack problem when a problem occurs in the fuel cell device.

[0008] The foregoing is merely intended to aid in an understanding of the background of the present disclosure and does not imply that the present disclosure is within the scope of related art already known to those skilled in the art. SHORT DESCRIPTION

[0009] Accordingly, the present disclosure has been made in consideration of the aforementioned problems encountered in the related art, and the present disclosure is intended to provide a method and an apparatus for diagnosing a condition of a fuel cell stack to detect a cause of deterioration of performance of the fuel cell stack by measuring an impedance of the fuel cell stack at a high frequency and a low frequency.

[0010] To achieve the above object, according to one aspect of the present disclosure, a method for diagnosing a state of a fuel cell stack as being contaminated, in other words, a method for detecting contamination of a fuel cell stack, is provided.The method comprises: measuring the impedance of the fuel cell stack at a predetermined low frequency and a predetermined high frequency; increasing an amount of a fuel or an oxidizer supplied to the fuel cell stack if the impedance at the predetermined high frequency is less than a first critical point, for example a first threshold, and the impedance at the predetermined low frequency is greater than a second critical point, for example a second threshold; re-measuring the impedance of the fuel cell stack at the predetermined low frequency; and detecting that the fuel cell stack is contaminated if the impedance at the predetermined low frequency measured in the re-measurement is higher than the second critical point.

[0011] The method may further comprise detecting that the fuel cell stack is dried out, in particular too dry, if the impedance at the predetermined high frequency measured in the measurement, in particular the first measurement, is higher than the first critical point.

[0012] The method may further comprise detecting that the fuel cell stack is normal when the impedance at the predetermined high frequency measured during the measurement, in particular the first measurement, is lower than the first critical point and the impedance at the predetermined low frequency measured during the measurement, in particular the first measurement, is lower than the second critical point.

[0013] The method may further comprise detecting that the fuel cell stack is flooded when the impedance at the predetermined low frequency measured in the re-measurement is less than the second critical point.

[0014] To achieve the object of the disclosure, according to another aspect, a method for diagnosing a fouled condition of a fuel cell stack is provided, the method comprising: measuring an impedance of the fuel cell stack at a predetermined low frequency and a predetermined high frequency; increasing an amount of fuel or oxidant supplied to the fuel cell stack when the impedance at the predetermined high frequency measured during the impedance measurement is less than a first critical point, for example, a first threshold, and the impedance at the predetermined low frequency measured during the impedance measurement is greater than a second critical point, for example, a second threshold; measuring an output voltage of the fuel cell stack;and detecting that the fuel cell stack is contaminated when the output voltage measured in the output voltage measurement is not increased by a predetermined critical voltage level compared to an output voltage of the fuel cell stack output immediately before the impedance measurement is performed;

[0015] The method may further comprise detecting that the fuel cell stack is dried out, in particular too dry, when the impedance at the predetermined high frequency measured when measuring the impedance is greater than the first critical point.

[0016] The method may further comprise detecting that the fuel cell stack is normal when the impedance at the predetermined high frequency measured in measuring the impedance is less than the first critical point and the impedance at the predetermined low frequency is lower than the second critical point.

[0017] The method may further comprise detecting that the fuel cell stack is flooded when the output voltage measured when measuring the output voltage is increased by the predetermined critical voltage level compared to the output voltage of the fuel cell stack output immediately before performing the impedance measurement.

[0018] To achieve the object of the disclosure, according to another aspect, there is provided an apparatus for diagnosing a state of a fuel cell stack as being contaminated, the apparatus comprising: an impedance meter that measures the impedance of the fuel cell stack at a predetermined high frequency and a predetermined low frequency; and a control unit that diagnoses the state of the fuel cell stack in accordance with the impedance of the fuel cell stack at the predetermined high frequency and the impedance of the fuel cell stack at the predetermined low frequency, the impedance being measured by the impedance meter, the control unit receiving the impedance at the predetermined high frequency and the impedance at the predetermined low frequency, the impedance being measured by the impedance meter;wherein the control unit performs an operation control in the form of increasing an amount of fuel or oxidant supplied to the fuel cell stack when the impedance at the predetermined high frequency is less than a first critical point and the impedance at the predetermined low frequency is greater than a second critical point; wherein the control unit receives the impedance of the fuel cell stack at the predetermined low frequency after performing the operation control to increase the amount of fuel or oxidant supplied to the fuel cell stack, and the control unit detects that the fuel cell stack is contaminated when the newly measured impedance at the predetermined low frequency is greater than the second critical point.

[0019] To achieve the object of the disclosure, according to another aspect, there is provided an apparatus for diagnosing a state of a fuel cell stack as being contaminated, the apparatus comprising: an impedance meter that measures an impedance of the fuel cell stack at a predetermined high frequency and a predetermined low frequency; and a control unit that diagnoses a state of the fuel cell stack in accordance with the impedance at the predetermined high frequency and the predetermined low frequency; wherein the control unit receives the impedance at the predetermined high frequency and the impedance at the predetermined low frequency, the impedance being measured by the impedance meter;wherein the control unit performs an operation control of increasing an amount of fuel or oxidant supplied to the fuel cell stack when the impedance at the predetermined high frequency is less than a first critical point and the impedance at the predetermined low frequency is greater than a second critical point; wherein the control unit receives an output voltage of the fuel cell stack measured by a voltmeter after performing the operation control of increasing the amount of fuel or oxidant supplied to the fuel cell stack;and wherein the control unit detects that the fuel cell stack is contaminated if the measured output voltage is not increased by a predetermined voltage level compared to an output voltage of the fuel cell stack measured by the voltmeter immediately before the impedance at the predetermined high frequency and the impedance at the predetermined low frequency are measured.;

[0020] The method and apparatus for diagnosing a fuel cell stack for being contaminated in accordance with the present disclosure have the following advantages.

[0021] When a performance of a fuel cell stack is deteriorated, it is possible to precisely and accurately identify a cause of the deterioration of the performance of the fuel cell stack, especially when the deterioration is attributable to contamination of the fuel cell stack, allowing a user to immediately take appropriate measures to solve the problem.

[0022] Additionally, it is possible to prevent the fuel cell stack from being exposed to adverse conditions attributable to inappropriate measures that might be taken if an inaccurate diagnosis of the fuel cell stack problem is made. For example, it may be the case that a fuel cell stack is subject to flooding, but upon diagnosis, the fuel cell stack problem is incorrectly identified as dryout. In this case, operation control is performed by reducing the flow rate of air supplied to the fuel cell stack. This aggravates the flooding of the fuel cell stack, thereby deteriorating the stability and durability of the fuel cell stack. On the other hand, it may be the case that a fuel cell stack is subject to dryout, but the fuel cell stack is incorrectly diagnosed as flooded.In this case, operational control is performed by increasing the flow rate of air supplied to the fuel cell stack. This intensifies the drying out of the fuel cell stack, thereby continuously deteriorating the performance of the fuel cell stack. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating an apparatus for diagnosing a fouled condition of a fuel cell stack in accordance with an embodiment of the present disclosure; Fig. 2 and Fig. 3 are flowcharts each illustrating a method for diagnosing a contaminated condition of a fuel cell stack in accordance with embodiments of the present disclosure; Fig. 4A and Fig. 4B are graphs showing measurements of the impedance of a fuel cell stack having a catalyst layer that is fouled; and Fig. Figure 5 is a graph showing measurements of the impedance of a fuel cell stack in accordance with humidity in a fuel cell stack. DETAILED DESCRIPTION

[0023] Hereinafter, with reference to the attached drawings, a method and apparatus for diagnosing a contaminated condition of a fuel cell stack will be described in accordance with preferred embodiments of the present disclosure.

[0024] Fig. 1 is a block diagram illustrating an apparatus for diagnosing a fouled condition of a fuel cell stack according to an embodiment of the present disclosure.

[0025] Referring to Fig. 1, the device for diagnosing may include a fuel cell stack 10, a voltmeter 11 that detects an output voltage of the fuel cell stack 10, an impedance meter 12 that measures an impedance of the fuel cell stack 10, and a control unit 13 that receives information about the impedance and the voltage of the fuel cell stack 10 and detects a state of the fuel cell stack 10.

[0026] The fuel cell stack 10 may be an element that produces electricity by means of an oxidation-reduction reaction of oxygen and hydrogen.

[0027] The fuel cell stack 10 may include a plurality of individual cells stacked in series. Each individual cell may include a membrane-electrode assembly and separators. The membrane-electrode assembly may be formed from an electrolyte membrane, catalyst layers, and gas diffusion layers. The separators may be disposed on the surfaces of the respective gas diffusion layers and may include flow regions through which fuel and air are supplied to a cathode and an anode, and through which water (produced as a byproduct) may be drained from the fuel cell stack.

[0028] Hydrogen and oxygen are ionized by chemical reactions on the catalyst layers. Then, oxidation reactions occur, producing protons (hydrogen ions) and electrons at an electrode to which hydrogen is supplied, and reduction reactions involving the hydrogen ions and oxygen ions occur, producing water at an electrode to which air is supplied. A typical electrode catalyst used in a fuel cell consists of a catalyst support made of a carbon material and a co-catalyst such as Ru, Co, Cu, or the like. Hydrogen is supplied to the anode, and oxygen (air) is supplied to the cathode. Hydrogen supplied to the anode is converted into protons (H) + and electrons e -split by catalysts at the electrode layers arranged on the corresponding surfaces of the electrolyte membrane. Of the protons and electrons, only the protons can selectively pass through the electrolyte membrane, which is called the proton exchange membrane, and reach the cathode. While the protons move through the electrolyte membrane, the electrons move through the gas diffusion layers (conductive layers) and separators to reach the cathode.

[0029] Hydrogen ions reaching the cathode through the electrolyte membrane and electrons reaching the cathode through the separators combine with the oxygen contained in the air supplied to the cathode via an air supply unit, producing water. Simultaneously, the movement of hydrogen ions causes an electric current to flow along an external wire, and in addition to the water, heat is produced as a byproduct.

[0030] The diagnostic device according to a present embodiment may be a device for detecting contamination of the catalyst layers of a single cell constituting the fuel cell stack 10.

[0031] The impedance meter 12 may be an element that measures the impedance of the fuel cell stack 10. The impedance meter 12 may be configured in various ways. For example, the impedance meter 12 may apply a voltage signal or a current signal at a predetermined frequency to a single cell, to a single group of cells comprising a predetermined number of individual cells, or to all cells of the fuel cell stack 10, detect a current or voltage of the fuel cell stack 10 at the predetermined frequency, and determine an impedance for each frequency component.

[0032] According to a present embodiment, the impedance measured by the impedance meter 12 may be a high-frequency impedance, which is the impedance of the fuel cell stack when a voltage or current signal of a predetermined high frequency is applied, or a low-frequency impedance, which is the impedance of the fuel cell stack when a voltage or current signal of a predetermined low frequency is applied. The frequencies of the voltage signal used for measuring the high-frequency impedance and the low-frequency impedance may vary depending on the specifications of the fuel cell stack. For example, the high frequency may be 1000 Hz or more, and the low frequency may be 10 Hz or less.

[0033] The impedance meter 12 can be operated in dependence on a control of the control unit 13.

[0034] The control unit 13 can control the overall operation of the fuel cell device. According to a present embodiment, the control unit 13 can process information needed to determine the contamination of the fuel cell stack 10 using the information provided by the voltmeter 11 and the impedance meter 12, and can control other elements of the fuel cell device as necessary.

[0035] The control unit 13 can detect a plurality of states of the fuel cell stack 10 depending on the high-frequency impedance and the low-frequency impedance measured by the impedance meter 12, the output voltage of the fuel cell stack 10 measured by the voltmeter 11, and changes thereof.

[0036] The following describes a method for diagnosing a fouled condition of a fuel cell stack, which is performed in the diagnostic device described above. The operation of the diagnostic device will be better understood with reference to the diagnostic method described below.

[0037] Fig. 2 and Fig. 3 are flowcharts illustrating a method for diagnosing a fouled condition of a fuel cell stack according to embodiments of the present invention.

[0038] First, in a step S11, the control unit 13 may receive an output voltage of the fuel cell stack 10 from the voltmeter 11 and, in a step S12, may determine whether it is necessary to diagnose the condition of the fuel cell stack 10 based on the received output voltage of the fuel cell stack 10. The control unit 13 may also be supplied with a current value measured by an ammeter 14 that measures the current supplied from an output terminal of the fuel cell stack 10 to a load. That is, the control unit 13 may receive the output voltage and output current of the fuel cell stack 10 and, in step S12, may determine whether the output voltage corresponding to the output current of the fuel cell stack 10 is lower than a predetermined reference voltage V1.

[0039] Next, in a step S13, the control unit 13 may instruct the impedance meter 12 to measure the impedance (high-frequency impedance and low-frequency impedance) of the fuel cell stack 10 when the output voltage is lower than the predetermined reference voltage V1 and receives the high-frequency impedance and the low-frequency impedance measured by the impedance meter 12.

[0040] Next, in a step S14 and a step S15, the control unit 13 may compare the high-frequency impedance and the low-frequency impedance measured by the impedance meter 12 with a predetermined first critical point Z1 and a predetermined second critical point Z2, respectively. In a step S15, the control unit 13 may increase an amount of fuel or oxidant supplied to the fuel cell stack 10 to increase the output voltage of the fuel cell stack 10 when the high-frequency impedance is greater than the first critical point Z1 and when the low-frequency impedance is greater than the second critical point Z2.For this purpose, the control unit 13 may open a purge valve 15 on a residual hydrogen return line to increase the amount of hydrogen, which is the fuel of the fuel cell device, or may enhance an operation of a circulation pump 16 that returns residual hydrogen to the fuel cell stack 10. In addition, the control unit 13 may enhance an operation of a blower 17 (an air supply unit) to increase an amount of air (oxidizer) supplied to the fuel cell stack 10.

[0041] The control unit 13 may, in a step S17, again instruct the impedance meter 12 to measure the low-frequency impedance of the fuel cell stack after a predetermined period of time has elapsed and, in a step S18, may compare the low-frequency impedance measured again by the impedance meter 12 with the second critical point Z2.

[0042] If the comparison result in step S18 shows that the low-frequency impedance of the fuel cell stack 10 has not decreased but is still higher than the second critical point Z2, the control unit 13 can recognize that a reduction in the output voltage of the fuel cell stack is due to the contamination of the fuel cell stack 10.

[0043] This detection of the control unit 13 may be based on a connection between characteristics of the high-frequency impedance and the low-frequency impedance of a fuel cell stack and the contamination of the fuel cell stack.

[0044] Fig. 4A and Fig. 4B show measurements of the impedance of a fuel cell stack when a catalyst layer of the fuel cell stack is fouled, and Fig. Figure 5 shows measurements of the impedance of a fuel cell stack in accordance with humidity in the fuel cell stack.

[0045] In particular, the Fig. 4A and Fig. 4B Changes in the impedance of the fuel cell stack depending on the degree of CO contamination of a catalyst layer of the fuel cell stack. As shown in the Fig. 4A and Fig. As shown in Figure 4B, when a catalyst layer of a fuel cell stack is heavily contaminated, the low-frequency impedance may decrease. However, the high-frequency impedance may not change depending on the degree of contaminated catalyst layer.

[0046] With reference to Fig. 5, it is found that the low frequency impedance increases dramatically as a degree of flooding in the fuel cell stack is intensified.

[0047] Referring to the Fig. 4A, Fig. 4B and Fig. 5, the low-frequency impedance may increase as fouling or flooding increases. The fouling problem of a fuel cell stack cannot be solved by a measure of supplying fuel or oxidant, but the flooding problem can be solved by the measure of increasing the amount of fuel or oxidant supplied to the fuel cell stack because the supplied fuel or oxidant removes water or moisture. Therefore, in accordance with an embodiment of the present invention, when the low-frequency impedance of the fuel cell stack 10 is higher than a predetermined level, the control unit 13 may perform operation control to supply more fuel or oxidant in a step S16 to solve the flooding problem. That is, the control unit 13 may increase the supply of hydrogen or air.If the low-frequency impedance of the fuel cell stack 10 decreases as a result of this operation control, the condition of the fuel cell stack 10 can be diagnosed as not being contaminated (or having previously been flooded) in a step S181. However, if the low-frequency impedance of the fuel cell stack 10 is still high even after the operation control by the control unit 13, the condition of the fuel cell stack 10 can be diagnosed as being contaminated in a step S19.

[0048] Fig. 3 shows a method for diagnosing a contaminated condition of a fuel cell stack according to another embodiment.

[0049] The embodiment according to Fig. 3 is essentially the same as the embodiment according to steps S11 to S16 Fig. 2, but is different in that the control unit 13 can diagnose a state of the fuel cell stack 10 using a result of detecting an output voltage of the fuel cell stack 10 after performing operation control to increase an output voltage of the fuel cell stack 10 in step S16.

[0050] In accordance with the embodiment according to Fig. 3, when a predetermined period of time has passed after the operation control to increase the output voltage of the fuel cell stack 10 (step S16), the control unit 13 may compare the output voltage of the fuel cell stack 10, which was sent from the voltmeter 11, with a predetermined critical level in steps S21 and S22, and may diagnose the condition of the fuel cell stack 10 as being contaminated in step S23 if the output voltage of the fuel cell stack 10 is not increased to the extent that it exceeds the critical level. This is based on the idea that, if the reduction in the output voltage of the fuel cell stack 10 is due to the flooding of the fuel cell stack 10, the degree of flooding can be reduced and the output voltage of the fuel cell stack 10 can be increased by means of the operation control in step S16.Therefore, when the output voltage of the fuel cell stack 10 is increased to exceed the predetermined critical level in step S18, the control unit 13 can recognize that the reduction in the output voltage of the fuel cell stack 10 is due to the flooding.

[0051] In the embodiments according to the Fig. 2 and Fig. 3, if it is detected in step S14 that the high-frequency impedance is greater than the first critical point Z1, the control unit 13 can detect that the fuel cell stack 10 is dry (step S141). If it is detected that the high-frequency impedance is less than the first critical point Z1 (S14) and it is detected that the low-frequency impedance is less than the second critical point Z2 (S15), the control unit 13 can detect that the fuel cell stack 10 is normal in step S151.

[0052] The method and apparatus for diagnosing according to the embodiments of the present disclosure can precisely and accurately detect the cause of deterioration in the performance of a fuel cell stack and therefore enable appropriate measures to be taken to correct the problems.

[0053] The present disclosure prevents a fuel cell stack from suffering from problems resulting from inappropriate measures that may be taken when an inaccurate diagnosis of a condition of the fuel cell stack is made. For example, when a misdiagnosis is made that the deterioration in the performance of a fuel cell stack is due to drying out of the fuel cell stack, although the actual cause of the deterioration is flooding, operation control is performed to reduce an air flow rate, thereby deteriorating the stability and durability of the fuel cell stack. The present disclosure can prevent such a misdiagnosis, thereby preventing the stability and durability of the fuel cell stack from deteriorating.Conversely, if a false diagnosis is made that the deterioration in the performance of a fuel cell stack is due to flooding in the fuel cell stack, although the actual cause of the deterioration is dryout, operation control may be performed to increase an air flow rate, and the performance of the fuel cell stack will continuously deteriorate. The present disclosure prevents such false diagnoses, thereby preventing the performance of the fuel cell stack from continuously deteriorating.

[0054] Although the embodiments of the present disclosure have been described for purposes of illustration, those skilled in the art will recognize that various modifications, additions, and substitutions are possible without departing from the scope and spirit of the disclosure as disclosed in the appended claims.

Claims

[1] A method for diagnosing a condition of a fuel cell stack (10) in that it is contaminated, comprising: Measuring an impedance of the fuel cell stack (10) at a predetermined low frequency and a predetermined high frequency; Increasing an amount of fuel or oxidant supplied to the fuel cell stack (10) when the impedance at the predetermined high frequency is less than a first critical point (Z1) and the impedance at the predetermined low frequency is greater than a second critical point (Z2); re-measuring the impedance of the fuel cell stack (10) at the predetermined low frequency; and Detecting that the fuel cell stack (10) is contaminated when the impedance at the predetermined low frequency measured during the re-measurement is greater than the second critical point (Z2). [2] The method of claim 1, further comprising: Detecting that the fuel cell stack (10) is dried out when the impedance at the predetermined high frequency measured during the measurement is greater than the first critical point (Z1). [3] Method according to one of claims 1 or 2, further comprising: Detecting that the fuel cell stack (10) is normal when the impedance at the predetermined high frequency measured in the measurement is smaller than the first critical point (Z1) and the impedance at the predetermined low frequency measured in the measurement is smaller than the second critical point (Z2). [4] Method according to one of the preceding claims, further comprising: Detecting that the fuel cell stack (10) is flooded when the impedance at the predetermined low frequency measured during the re-measurement is less than the second critical point (Z2). [5] A method for diagnosing a condition of a fuel cell stack (10) in that it is contaminated, comprising: Measuring an impedance of the fuel cell stack (10) at a predetermined low frequency and a predetermined high frequency; Increasing an amount of fuel or oxidant supplied to the fuel cell stack (10) when the impedance at the predetermined high frequency measured during the impedance measurement is less than a first critical point (Z1) and the impedance at the predetermined low frequency measured during the impedance measurement is greater than a second critical point (Z2); Measuring an output voltage of the fuel cell stack (14); and Detecting that the fuel cell stack (10) is contaminated when the output voltage measured in the output voltage measurement is not increased by a predetermined critical voltage level compared to an output voltage of the fuel cell stack (10) output immediately before the impedance measurement is performed. [6] The method of claim 5, further comprising: Detecting that the fuel cell stack (10) is dried out when the impedance measured in the impedance measurement is greater than the first critical point (Z1) at the predetermined high frequency. [7] The method of claim 6, further comprising: Detecting that the fuel cell stack (10) is normal when the impedance measured in the impedance measurement is smaller than the first critical point (Z1) at the predetermined high frequency and the impedance measured in the impedance measurement is smaller than the second critical point (Z2) at the predetermined low frequency. [8] Method according to one of claims 5 to 7, further comprising: Detecting that the fuel cell stack (10) is flooded when the output voltage measured during the output voltage measurement is increased by the predetermined critical voltage level compared to the output voltage of the fuel cell stack (10) output immediately before the impedance measurement is performed. [9] Device for diagnosing a condition of a fuel cell stack (10) in that it is contaminated, the device comprising: an impedance meter (12) that measures an impedance of the fuel cell stack (10) at a predetermined high frequency and a predetermined low frequency; and a control unit (13) which diagnoses the state of the fuel cell stack (10) depending on the impedance of the fuel cell stack (10) at the predetermined high frequency and the impedance of the fuel cell stack (10) at the predetermined low frequency, the impedance being measured by means of the impedance meter (12), wherein the control unit (13) receives the impedance at the predetermined high frequency and the impedance at the predetermined low frequency, the impedance being measured by means of the impedance meter 12, wherein the control unit (13) performs an operation control such that an amount of fuel or oxidant supplied to the fuel cell stack (10) is increased when the impedance at the predetermined high frequency is smaller than a first critical point (Z1) and the impedance at the predetermined low frequency is larger than a second critical point (Z2), wherein the control unit (13) receives the impedance of the fuel cell stack (10) at the predetermined low frequency after the operation control has been performed to adjust the amount of fuel or oxidant supplied to the fuel cell stack (10), and wherein the control unit (13) detects that the fuel cell stack (10) is contaminated if the re-measured impedance at the predetermined low frequency is greater than the second critical point (Z2). [10] Device for diagnosing a condition of a fuel cell stack (10) in that it is contaminated, the device comprising: an impedance meter (12) for measuring an impedance of the fuel cell stack (10) at a predetermined high frequency and a predetermined low frequency; and a control unit (13) for diagnosing a state of the fuel cell stack (10) depending on the impedance at the predetermined high frequency and the impedance at the predetermined low frequency, wherein the control unit (13) receives the impedance at the predetermined high frequency and the impedance at the predetermined low frequency, the impedance being measured by means of the impedance meter (12), wherein the control unit (13) performs an operation control such that an amount of fuel or oxidant supplied to the fuel cell stack (10) is increased when the impedance at the predetermined high frequency is smaller than a first critical point (Z1) and the impedance at the predetermined low frequency is larger than a second critical point (Z2), wherein the control unit (13) receives an output voltage of the fuel cell stack (10) measured by a voltmeter (11) after the operation control is performed to increase the amount of fuel or oxidant supplied to the fuel cell stack (10), and wherein the control unit (13) detects that the fuel cell stack (10) is contaminated if the measured output voltage is not increased by a predetermined critical voltage level compared to an output voltage of the fuel cell stack (10) measured by the voltmeter (11) immediately before the impedance at the predetermined high frequency and the impedance at the predetermined low frequency are measured.

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