Method for diagnosing a catalyst condition in a fuel cell

Abstract

The present invention relates to a method for diagnosing the condition of at least one catalyst (20) provided in fuel cells, in particular in high-temperature fuel cells or in solid oxide fuel cells, which are arranged in series in the form of at least one stack, comprising the following steps: - Setting a stable operating state, in particular a thermally stable operating point, of the stack; - Switching off or shutting down the load of the stack to an idle state, especially to zero; - Measuring and evaluating -- the open-circuit voltage per fuel cell and -- the concentration of the gaseous fuel supplied at the input of the stack that is still present at the output of the stack.

Description

[0001] The present invention relates to the technical field of electrochemical cells or fuel cells (SOFCs = Solid Oxide Fuel Cells), and in particular to diagnosing the condition of a catalyst provided in fuel cells, especially in high-temperature fuel cells or in solid oxide fuel cells, which are arranged serially in the form of at least one stack. State of the art

[0002] In (high-temperature) fuel cells (SOFCs = Solid Oxide Fuel Cells), which are operated with hydrocarbons and especially with natural gas or methane, the hydrocarbons in the stack, i.e. in series connection of the SOFCs, are converted into hydrogen H2 and carbon monoxide CO via a steam reforming reaction, because only H2 and CO can be converted via the reaction itself at the electrodes (cathode as oxygen electrode, anode as hydrogen electrode) of the SOFCs.

[0003] To ensure that the steam reforming process is sufficiently stable with age, the stack contains a catalyst, in particular in the form of a catalyst coating, which may also be an additional one; this catalyst is intended to ensure sufficient fuel conversion over the entire lifetime of the stack.

[0004] If this is not the case, there will be an increased emission of unused hydrocarbons at the stack's outlet, and in extreme cases – due to a lack of or insufficient conversion of the hydrocarbons into H2 and CO – a depletion of fuel (H2, CO) usable for the SOFCs will occur, leading to stack failure.

[0005] Even just one section or area with such an increased hydrocarbon emission can be critical for downstream process gas aftertreatment, for example by means of a catalytic burner.

[0006] Therefore, it is desirable to obtain a reliable statement about the condition of the catalyst, especially the catalyst coating (including any additional coatings), even during the ongoing operation of the stack, and to observe and monitor the aging behavior of the catalyst over the entire operating time of the stack in order to be able to react and remedy the situation in a timely manner if necessary.

[0007] In addition to the proposed in-situ diagnostic methods (see G. Schiller, KA Friedrich, M. Lang, P. Metzger, N. Wagner: In-Situ Diagnostic Methods for SOFC, German Aerospace Center (DLR), Institute of Technical Thermodynamics, at Germany International Symposium on Diagnostic Tools for Fuel Cell Technologies, Trondheim, Norway, June 23-24, 2009, available at https: / / elib.dlr.de / 59206 / 1 / In_situ_Diagnostics_SOFC_Schiller.pdf), conventionally proposed methods, such as gas analysis at the stack outlet, also have the disadvantage of high costs and only indicate something when the catalyst is no longer able to reach equilibrium along its entire length; in particular, such conventional analysis / monitoring methods are ineffective if the catalyst is only affected in sections or areas, especially if it is damaged or exhausted. Disclosure of the invention

[0008] Based on the disadvantages and shortcomings outlined above and taking into account the prior art described, the present invention aims to further develop a method of the type mentioned at the outset in such a way as to overcome the aforementioned disadvantages and shortcomings.

[0009] This problem is solved by the features of claim 1. Advantageous embodiments of the present invention are described in the dependent claims.

[0010] The present invention provides an in situ diagnosis for at least one, in particular additional, catalyst coating in the stack of at least one high-temperature fuel cell or solid oxide fuel cell (SOFC or Solid Oxide Fuel Cell).

[0011] Advantageously, the at least one, in particular additional, catalyst coating can be designed in the form of at least one IR catalyst, where IR stands for Internal Reforming; this means that the, in particular additional, catalyst coating is a component of an electrochemical cell or fuel cell (SOFC = Solid Oxide Fuel Cell) and is not designed as a separate catalyst.

[0012] It is also expediently not a serial arrangement, because the catalyst, in particular the, for example, additional catalyst coating, converts the gas over the entire length of the electrochemical cell or fuel cell: The catalyst, and in particular any additional catalyst coating, converts methane (CH4) into carbon monoxide (CO) and hydrogen (H2). This hydrogen (H2) is consumed in the electrochemical cell or fuel cell. The catalyst, and in particular any additional catalyst coating, then produces more hydrogen (H2) until almost all the methane (CH4) has been converted in the electrochemical cell or fuel cell. This process preferably works in parallel with the catalyst (H2 producer) and the electrochemical cell or fuel cell (H2 consumer).

[0013] The present invention is therefore based on an in situ diagnostic method, in particular diagnostic operation of the fuel cell stack, and makes it possible to continuously monitor the aging behavior of the catalyst in the stack, therefore in situ, or at certain time intervals.

[0014] In particular, it is a discontinuous process that utilizes existing components and thus offers a huge cost advantage over conventional gas analytical techniques.

[0015] Furthermore, the method according to the present invention is able to detect even early stages of catalyst aging, which would not be possible with gas analysis at the stack outlet.

[0016] Therefore, the method according to the present invention provides the possibility of measuring the entire aging range of the catalyst and thus providing early information about an impending and / or necessary replacement of the stack. Downstream components that would be damaged in the event of malfunction can thus be effectively protected.

[0017] The core of the method according to the present invention consists in entering a diagnostic mode in which the load is switched off and the electrochemical cell or fuel cell is not used to generate electricity and heat, but rather as a sensor for the hydrogen formed by steam reforming from the supplied hydrocarbon mixture or hydrocarbon, in particular natural gas and / or methane.

[0018] The electrochemical cell or fuel cell itself then gives a signal that corresponds to the average ratio of hydrogen H2 to water H2O over the entire length of the cell: In this case, an active catalyst only needs a short path to reach equilibrium, which corresponds to a high average ratio of hydrogen H2 to water H2O; a less active catalyst needs a longer path to reach equilibrium, which corresponds to a lower average ratio of hydrogen H2 to water H2O, for example due to aging.

[0019] The method proposed according to the present invention has the advantage that it is possible to average over the length of the catalyst and thus also to detect early stages of deactivation, for example an incipient deactivation, of the catalyst.

[0020] The diagnostic operation according to the method of the present invention comprises the following steps: - Setting a stable operating state, in particular a thermally stable operating point, of the stack, which is in a thermally stable state, for example by means of defined or fixed fuel gas metering and / or by means of defined or fixed cathode air flow and / or by means of defined or fixed A[node]O[ff-]G[as]R[ecirculation] rate in the case of anode gas recirculation assigned to the anodes of the fuel cells; - Switching off or shutting down the load of the stack to an idle state, especially to zero; - Measuring and evaluating -- the open-circuit voltage per fuel cell and of the stack as an evaluation signal and -- the concentration of the gaseous fuel supplied at the input of the stack that is still present at the output of the stack; optional: - (Re-)connecting or (re-)starting the load of the stack and continuing normal operation.

[0021] During the measurement, it is advantageous to ensure that the time for measuring the open-circuit voltage is sufficiently long to guarantee adequate gas exchange and thus a stable equilibrium setting.

[0022] Switching off or reducing the load, as well as switching on or increasing the load, should preferably be carried out in such a way as to avoid damage, for example from thermomechanical stress associated with switching off or on, or reducing or increasing the load.

[0023] The present invention relates in particular to the use of the method according to the above-described type in a SOFC system, also for diagnosing the thermal behavior of the stack, especially in the case of anode gas recirculation, for example with a defined or fixed A[node]O[ff-]G[as]R[ecirculation] rate.

[0024] The diagnostic operation according to the present invention is demonstrable insofar as the load is switched off, at least briefly, for in-situ diagnostics. Within the scope of the present invention, it is therefore conceivable that the thermal behavior of the SOFC stack could be diagnosed by evaluating the time-dependent temperature profiles, which could offer advantages for the control of the stack.

[0025] As discussed above, there are various ways to advantageously elaborate and further develop the teaching of the present invention. For this purpose, reference is made, on the one hand, to the claims subordinate to claim 1; on the other hand, further embodiments, features, and advantages of the present invention are described below, inter alia, with reference to the Fig. 1 to Fig. 3 illustrated examples are explained in more detail.

[0026] They show: Fig. 1 in diagrammatic representation as a function of the catalytic activity of a catalyst an embodiment corresponding to the method according to the present invention for the open-circuit voltage per fuel cell and for the concentration of the gaseous fuel supplied at the input of the stack, here methane CH4, which is still occurring at the output of a stack formed from the fuel cells; Fig. 2A in diagrammatic representation an embodiment corresponding to the method according to the present invention for the course of the concentration of the gaseous fuel supplied at the inlet of the stack, here methane CH4, along the fuel cell of the stack; Fig. 2B in diagrammatic representation an embodiment corresponding to the method according to the present invention for the course of the concentration from Fig. 2A corresponding local Nernst voltage along the fuel cell of the stack; and Fig. Figure 3 shows a schematic representation of an embodiment of the construction of the cell unit of an electrochemical cell or fuel cell (SOFC = Solid Oxide Fuel Cell) in which the method according to the present invention can be applied and / or used.

[0027] Identical, similar, or matching designs, elements, and / or features are in Fig. 1 to Fig. 3 are provided with the same reference symbols; a repeated description of these designs, elements and / or features is omitted. Fig. 1 to Fig. Figure 3 is not necessarily to scale. Any design and / or dimensioning information in Fig. 1 to Fig. 3 are purely examples.

[0028] The method according to the present invention allows the state of a catalyst 20 (cf. Fig. 3) be analyzed and diagnosed in situ, which is provided in fuel cells, in particular in high-temperature fuel cells or in solid oxide fuel cells (SOFCs = Solid Oxide Fuel Cells), which are arranged serially in the form of at least one stack.

[0029] The catalyst 20 can be formed in the form of at least one catalyst coating, wherein the material of the catalyst 20 and / or the, in particular additional, catalyst coating can be selected, for example, from the group of catalytically active metals or platinum group metals (PGMs), such as platinum (Pt), palladium (Pd), rhodium (Rh) and / or alloys thereof, such as a platinum / rhodium alloy, or nickel (Ni) (typically, nickel (Ni) is already present in the anode itself, which is why sufficient catalysis can still be provided in the new state even without an additional catalyst or catalyst coating, but this deteriorates over time and is therefore not stable over time when the electrochemical cell or fuel cell is operated with natural gas or methane).

[0030] The structure of a single cell unit of an electrochemical cell or fuel cell (SOFC = Solid Oxide Fuel Cell) can be described using Fig. 3 will be illustrated: The in Fig. The composite formed from cathode 40 (= oxygen electrode), anode 60 (= hydrogen electrode) and electrolyte 50 arranged between cathode 40 and anode 60, as shown in Figure 3 (optionally, a sealant, for example glass-based, can be provided to seal the cell layers), is equipped with two, in particular metallic, interconnectors 70, wherein these interconnectors 70 are for the electrical contacting of the Fig. The 3 cell units shown are intended to be connected to the next cell unit.

[0031] Here, the interconnector 70 is provided with a catalyst 20, in particular in the form of a catalyst coating, for example in the form of an additional catalyst coating, on its surface facing away from the free surface 42 of the cathode 40, i.e., facing away from the free surface 62 of the anode 60, which is also facing away from the electrolyte 50. Between the in Fig. 3 upper interconnector 70 and the surface 42 of the cathode 40 facing away from the electrolyte 50, a gas path 47, for example in the form of a slit-shaped air passage, in particular as a cathode space, is provided.

[0032] The in Fig. 3. The lower interconnector 70 is provided on its surface facing the surface 62 of the anode 60 in the same manner with the catalyst 20, in particular in the form of the catalyst coating, for example in the form of the additional catalyst coating. Between this on the in Fig. 3 lower interconnector 70 applied catalyst coating 20 and the surface 62 of the anode 60 facing away from the electrolyte 50, a gas path 67, for example in the form of a slit-shaped passage, in particular as an anode space, is provided for natural gas or methane (CH4), hydrogen (H2), carbon monoxide (CO), water (H2O) and carbon dioxide (CO2).

[0033] At the in Fig. 3. The chemical reaction assigned to the cathode compartment 47 is carried out by means of the cathode 40, oxygen (O2) flowing in with the air L via four electrons (e) - ) into two doubly negatively charged oxygen ions (O 2 -) converted, which are then available at the anode 60 via the electrolyte 50, where they react with the hydrogen (H2) supplied to the anode space 67 to form water (H2O) and two electrons (e - ) react. The natural gas or methane (CH4) supplied to the anode compartment 67 reacts with water (H2O) to form hydrogen (H2) and carbon monoxide (CO).

[0034] The based Fig. Figure 3 illustrates a single cell unit of an electrochemical cell or fuel cell (SOFC = Solid Oxide Fuel Cell) which can be metal-supported, wherein the assembly comprising the cathode 40, the electrolyte 50 and the anode 60 can be arranged on a metal substrate, with the anode 60 on the substrate and a gas access to the anode space 67.

[0035] First, a stable operating state, in particular a thermally stable operating point, of the stack is set according to procedure, for example by - the fuel is defined or precisely dosed and / or - a defined or fixed oxygen flow through the cathodes of the fuel cells is provided; then the load of the stack is switched off or shut down to an idle state, in particular to zero; then - the open-circuit voltage per fuel cell and - the concentration of the gaseous fuel supplied at the inlet of the stack, for example the hydrocarbon mixture or the hydrocarbon, such as natural gas and / or methane, still present at the outlet of the stack, measured and evaluated; Optionally, the load of the stack can be reactivated at the end, or be restarted and the stack can continue or be operated in normal mode.

[0036] The condition of the catalyst 20 is diagnosed from the ratio of the open-circuit voltage to the fuel concentration occurring at the output of the stack, in particular as a function of the catalytic activity of the catalyst 20.

[0037] The measurement duration is chosen so that a stable equilibrium is maintained in the stack, especially in the fuel cells, and in particular sufficient gas exchange is ensured.

[0038] In high-temperature fuel cells (SOFCs = Solid Oxide Fuel Cells) powered by hydrocarbons, and especially by natural gas or methane, the hydrocarbons are converted into hydrogen H2 and carbon monoxide CO in the stack, i.e., in series connection of the SOFCs, via a steam reforming reaction, because only H2 and CO can be converted via the reaction itself at the electrodes (cathode as oxygen electrode, anode as hydrogen electrode) of the SOFCs.

[0039] The method according to the present invention is used in situ within the framework of an anode gas recirculation assigned to the anodes of the fuel cells, in particular with a defined or fixed A[node]O[ff-]G[as]R[ecirculation] rate. In this context, the thermal behavior of the stack can also be diagnosed.

[0040] To implement the method of the present invention, the Fig. 1, Fig. 2A and Fig. 2B in the respective diagram form the measurement effect with regard to - the open-circuit voltage per fuel cell and - the concentration of the gaseous fuel supplied at the input of the stack, for example the hydrocarbon mixture or the hydrocarbon, such as natural gas and / or methane, that is still present at the output of the stack.

[0041] This is illustrated by the diagram in Fig. 1 the influence of the catalytic activity of catalyst 20 is illustrated, wherein - on the left vertical axis (= left vertical axis or left ordinate axis) the open-circuit voltage per fuel cell measured according to the procedure in volts and - on the right vertical axis (= right vertical axis or right ordinate axis) the concentration of the fuel, here methane CH4, measured according to the procedure at the output of the stack, in percent (%) each is plotted as a function of the logarithmically normalized catalytic activity of catalyst 20 on the horizontal axis (= right axis or abscissa axis), for example in the ratio R / R0.

[0042] Here, R0 represents the reaction rate of the catalyst 20, particularly any additional catalyst coating, in its new state, i.e., its catalytic activity in its new state; correspondingly, R represents the current reaction rate. The ratio R / R0 determines the average H2 / H2O ratio along the length of the fuel cell and thus the open-circuit voltage per fuel cell, i.e., the voltage per fuel cell when no current is flowing.

[0043] The curve of this open-circuit voltage per fuel cell, i.e., the open-circuit voltage across the stack divided by the number of fuel cells arranged in series in the stack, is in Fig. 1 is represented by the graph rising from left to right; the course of the methane concentration at the output of the stack is shown in Fig. 1. This is represented by the graph descending from left to right: While the open-circuit voltage per fuel cell has a continuous, i.e. increasing, profile along the horizontal axis (= right-hand axis or The gas only changes after a critical limit of catalytic activity is exceeded, as shown by the catalytic activity plotted on the abscissa axis.

[0044] Through the diagram in Fig. Figure 2A illustrates the course of the methane distribution along the fuel cell of the stack (left = input of the stack; right = output of the stack), whereby - on the vertical axis (= vertical axis or ordinate axis) the methane concentration, i.e. the percentage of methane in the gas and - on the horizontal axis (= right axis or abscissa axis) from left to right, the relative position in the fuel cell of the stack has been applied.

[0045] The course of the methane concentration in the gas along the fuel cell of the stack (left = inlet of the stack; right = outlet of the stack) is shown in Fig. 2A is represented by four graphs descending from left to right, where - the bottom graph in Fig. 2A a ratio R / R0 = 1, - the second-lowest graph in Fig. 2A a ratio R / R0 = 1 / 4, - the second-highest graph in Fig. 2A a ratio R / R0 = 1 / 8 and - the top graph in Fig. 2A corresponds to a ratio R / R0 = 1 / 16 (for the ratio R / R0: see... Fig. 1).

[0046] Through the diagram in Fig. 2B, also along the fuel cell of the stack (left = input of the stack; right = output of the stack), is the spatially resolved course of the local Nernst voltage U. n illustrated per fuel cell, whereby - on the vertical axis (= vertical axis or ordinate axis) the local Nernst stress U n in volts and - on the horizontal axis (= right axis or abscissa axis) from left to right, the relative position in the fuel cell of the stack has been applied.

[0047] The spatially resolved profile of the local Nernst voltage U plotted along the fuel cell of the stack (left = input of the stack; right = output of the stack). n per fuel cell is in Fig. 2B is represented by four graphs rising from left to right, where - the top graph in Fig. 2B a ratio R / R0 = 1, - the second-highest graph in Fig. 2B a ratio R / R0 = 1 / 4, - the second-lowest graph in Fig. 2B a ratio R / R0 = 1 / 8 and - the bottom graph in Fig. 2B a ratio R / R0 = 1 / 16 corresponds (regarding the R / R0 ratio: see below). Fig. 1).

[0048] The relationship of the diagram in Fig. 1 to the diagram in Fig. 2B is also given here by the fact that the open-circuit voltage per fuel cell of the stack corresponds to the effective mean of the local Nernst voltage per fuel cell of the stack.

[0049] From the diagrams according to Fig. 1, Fig. 2A and Fig. 2B is diagnosable, - that a still active, i.e. operational catalyst 20 (cf. Fig. 3) only a short distance is needed to reach equilibrium, which corresponds to a high average ratio of hydrogen H2 to water H2O, - whereas a less active catalyst 20, which is no longer or hardly ready, requires a greater distance for the catalyst 20 to travel, which corresponds to a reduced average ratio of hydrogen H2 to water H2O, for example due to aging.

Claims

[1] Method for diagnosing the condition of at least one catalyst (20) provided in fuel cells, in particular in high-temperature fuel cells or in solid oxide fuel cells arranged in series in the form of at least one stack, comprising the following steps: - Setting a stable operating state, in particular a thermally stable operating point, of the stack; - Switching off or shutting down the load of the stack to an idle state, especially to zero; - Measuring and evaluating -- the open-circuit voltage per fuel cell and -- the concentration of the gaseous fuel supplied at the input of the stack that is still present at the output of the stack. [2] Method according to claim 1, characterized by, that the condition of the catalyst (20) is diagnosed from the ratio of the open-circuit voltage to the fuel concentration occurring at the output of the stack, in particular as a function of the catalytic activity of the catalyst (20). [3] Method according to claim 1 or 2, characterized by that the fuel is at least a hydrocarbon mixture or at least one hydrocarbon, in particular natural gas and / or methane. [4] Method according to claim 3, characterized by Steam reforming in the stack to convert the fuel into hydrogen and carbon monoxide. [5] Method according to at least one of claims 1 to 4, characterized by the final step of reconnecting or restarting the load of the stack and continuing the stack in normal operation. [6] Method according to at least one of claims 1 to 5, characterized by, that the catalyst (20) is formed in the form of at least one, in particular additional, catalyst coating. [7] Method according to at least one of claims 1 to 6, characterized by , that to establish a stable operating state - the fuel is defined or precisely dosed and / or - a defined or fixed oxygen flow through the cathodes of the fuel cells is provided. [8] Method according to at least one of claims 1 to 7, characterized by , that the measurement duration is chosen so that a stable equilibrium, in particular sufficient gas exchange, is maintained in the stack, especially in the fuel cells. [9] Method according to at least one of claims 1 to 8, characterized by Anode gas recirculation assigned to the anodes of the fuel cells, in particular with a defined or fixed AOGR rate. [10] Use of at least one method according to at least one of claims 1 to 9 for diagnosing the thermal behavior of the stack, in particular during anode gas recirculation, for example with a defined or fixed AOGR rate.

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