Fuel cell system and method for operating a fuel cell system

Abstract

A fuel cell system is disclosed. The fuel cell system comprises a main fuel cell and a recirculation path. The main fuel cell comprises an anode inlet through which fuel can flow into the main fuel cell, and an anode outlet from which an anode exhaust gas, including unconsumed fuel, can flow out. The recirculation path, through which the anode exhaust gas can flow as recirculate, connects the anode inlet to the anode outlet. The recirculation path comprises an electrochemical filter and a bypass path. The electrochemical filter is configured to extract the unconsumed fuel from the recirculate. The extracted fuel can be recirculated to the anode inlet. The recirculate can be recirculated from the anode outlet to the anode inlet via the bypass path, bypassing the electrochemical filter.The fuel cell system is designed for operation with a bypass ratio of at least 85 percent. The bypass ratio corresponds to the ratio of the portion of recirculate flowing through the bypass path to the total amount of recirculate. A corresponding method for operating a fuel cell system is also disclosed.

Description

[0001] The present disclosure relates to a fuel cell system and a method for operating a fuel cell system.

[0002] A fundamental development goal for fuel cells in general and especially for proton exchange membrane fuel cells (Proton Exchange Membrane Fuel Cells) or polymer electrolyte membrane fuel cells (Polymer Electrolyte Membrane Fuel Cells), hereinafter referred to as PEM fuel cells, is to achieve the highest possible system efficiency of the fuel cell system.

[0003] A good lever for this is the most efficient use of fuel on the anode side of the fuel cell.

[0004] In this case, all common fuels for fuel cells are suitable, especially hydrogen and methanol. However, hydrogen-containing gases, such as reformate gas, which is formed from the reforming of methanol, ethanol, natural gas, or LPG (liquefied petroleum gas), can also be used to operate fuel cells.

[0005] To ensure a good supply of fuel to the catalyst layers in all areas of the fuel cell (and thus increase system efficiency), the anode can be operated at a superstoichiometric level. During normal operation of the fuel cell, more fuel is supplied to the anode than would (theoretically) be required to maintain normal operation. This superstoichiometric operation ensures that even the areas of the anode located downstream of the fuel flow, such as the areas near the anode outlet, are still sufficiently supplied with fuel. However, the release of the resulting anode exhaust gas causes higher losses, as unused fuel is released into the environment. This reduces system efficiency.

[0006] Furthermore, the release of unused fuel is also undesirable for environmental reasons. Hydrocarbon-based fuels and (pure) hydrogen can contribute to atmospheric warming and damage the ozone layer. For example, in April 2022, the British government published a study showing that hydrogen released into the atmosphere causes global warming eleven times faster than an equal amount of CO2.

[0007] To minimize the escape of unused fuel, the anode exhaust gas can be recirculated. This can be done passively (using a jet pump) or actively (using a blower). During recirculation, the anode exhaust gas is returned via the anode outlet of the anode chamber of a fuel cell to the anode inlet of the anode chamber via a flow path, the recirculation path. This allows the previously unused fuel to reenter the anode chamber and thus be available for further reaction.

[0008] By means of the high volume flow of fuel in the fuel cell generated by the superstoichiometric operation of the fuel cell including anode exhaust gas recirculation, it is fundamentally possible to achieve a high stoichiometry at the catalyst layers and thus a uniform fuel distribution without significant fuel losses. However, recirculation leads to an enrichment of reaction products, such as nitrogen and water, and thus to fuel depletion in the anode circuit. This undesirable enrichment occurs even without superstoichiometric operation in systems with recirculation on the anode side, since the reaction products diffuse from the cathode side to the anode side via the fuel cell's membrane system.

[0009] This can have undesirable effects on the function of the fuel cell stack, such as a voltage drop during operation. To minimize or compensate for these effects and also maintain the longevity of the fuel cell system, an anode circuit purge can be used. The reaction products enriched in the recirculate can thus be removed from the system by removing the mixture of fuel and reaction products contained in the recirculate (preferably intermittently, but also continuously) from the recirculation path via a purge device, such as a purge valve, and replacing it with pure fuel.

[0010] On the one hand, this represents a significant loss of efficiency because unused fuel is released into the environment during this purging process. The fuel expelled from the system is no longer available for power generation in the fuel cell. On the other hand, this release of fuel into the environment is also undesirable due to its undesirable impact on the greenhouse effect.

[0011] An object of the present invention is therefore to prevent or minimize the accumulation of reaction products on the anode side, while avoiding or reducing harmful emissions to the environment. More generally, the object of the invention is to provide a fuel cell system that can be operated efficiently and in an environmentally friendly manner.

[0012] This object is achieved by the fuel cell system and method according to the independent claims. Advantageous developments of the present invention are the subject of the dependent claims.

[0013] In general, the accumulation of reaction products in the anode recirculation or the anode side can be counteracted by separating liquid components of the reaction products in the anode recirculation using a liquid separator. For example, liquid water can be removed from the anode recirculation in this way.

[0014] On the other hand, enrichment with reaction products can be reduced or even completely prevented by separating gaseous components of reaction products (inert gas) from the gaseous fuel and flushing them out of the recirculation path. In the case of a fuel cell powered by hydrogen and atmospheric oxygen, such as a PEM fuel cell, it is therefore desirable to separate a gaseous hydrogen-nitrogen mixture into its components hydrogen and nitrogen. This would allow the hydrogen to be recirculated into the anode compartment and the nitrogen to be flushed out of the system.

[0015] This would increase system efficiency and reduce or even eliminate unwanted emissions. Firstly, purging processes would be less frequent and, in the best case, unnecessary at all. Secondly, the compositions of substances purged from the system would contain less fuel.

[0016] One technical approach to minimize both liquid and gaseous reaction products in the recirculated product fed back to the anode is mechanical or physical fuel separation. However, this technology has room for improvement, particularly in terms of efficiency.

[0017] Electrochemical fuel separation from the recirculate is therefore preferred.

[0018] The present invention is based on an electrochemical filter for fuel (electrochemical fuel filter, which can be, for example, an electrochemical hydrogen filter), such as hydrogen, or for depleting reaction products, such as nitrogen and liquid water. Compared to mechanical alternatives, such as centrifuges, an electrochemical filter requires less power and is more efficient with regard to the overall efficiency of the anode side and thus of the fuel cell system as a whole. The electrochemical filter continuously depletes reaction products from the anode recirculation path or the anode side of the fuel cell system, so that high levels of reaction products do not reach the anode recirculation path, which increases the overall system efficiency and the service life of the fuel cell system.In comparison, the intermittent action of a purge valve without an upstream electrochemical filter is used. The intermittent action of a purge valve operating alone allows for the accumulation of larger amounts of reaction products in the anode recirculation path, which leads to reduced fuel cell performance and other disadvantages.

[0019] In order to minimize the proportion of liquid reaction products, such as water, as well as the proportion of gaseous reaction products, such as nitrogen, in the recirculated product fed back to the fuel cell anode, media separation or filtration is advantageous. During operation of the fuel cell system, the exhaust stream from the fuel cell anode outlet is fed to the anode of the electrochemical filter. In the electrochemical filter, the fuel, such as molecular hydrogen, is oxidized and releases electrons. The fuel ions are transported through the proton-conducting membrane system of the electrochemical filter, which may comprise a polymer membrane.The electrons flow via the electrically conductive components and the external circuit to the cathode of the electrochemical filter, where the fuel ions, such as protons, are reduced again and recombine to form fuel, such as molecular hydrogen. Mainly fuel ions can be transported through the membrane system of the electrochemical filter. Other components of the fuel cell's anode exhaust gas remain essentially entirely on the anode side of the electrochemical filter and are removed from the recirculation path or the fuel cell system as anode exhaust gas of the electrochemical filter (and the fuel cell). Through permeation processes, a minor exchange of the reaction product molecules, such as nitrogen and water molecules, occurs between the anode and cathode sides of the electrochemical filter.

[0020] The electrochemical filter operates very similarly to a fuel cell. Therefore, it can also be advantageous to integrate it into the fuel cell or fuel cell system. To distinguish it from the electrochemical filter, the fuel cell of the fuel cell system, in whose anode recirculation path the electrochemical filter is located, is also referred to below as the main fuel cell.

[0021] To achieve the above-described object, the present invention comprises a fuel cell system having a main fuel cell and a recirculation path. The main fuel cell comprises an anode inlet and an anode outlet. Fuel can flow into the main fuel cell through the anode inlet (during operation of the fuel cell system). In other words, the anode inlet is configured to supply fuel to the main fuel cell or to an anode compartment of its anode. An anode exhaust gas, including unused fuel, can flow out of the anode outlet (during operation of the fuel cell system). In other words, the anode outlet is configured to discharge or expel the anode exhaust gas from the anode compartment. The recirculation path connects the anode inlet to the anode outlet of the main fuel cell, so that the anode exhaust gas can flow as recirculate from the anode outlet to the anode inlet.In other words, the recirculation path is configured to (partially) return or recirculate an anode exhaust gas from the anode compartment of the main fuel cell to the anode compartment. The recirculation path comprises an electrochemical filter and a bypass path. In other words, the electrochemical filter and the bypass path are arranged in the recirculation path. The electrochemical filter is configured to extract the unused fuel from the recirculate. The fuel extracted or recovered by the electrochemical filter can then be directed to the anode inlet or returned (recirculated) to the anode compartment. The recirculate, as a mixture of the fuel and the reaction products, can be directed to the anode inlet or returned (recirculated) to the anode compartment through the bypass path.In other words, the bypass path allows the recirculate to be recirculated from the anode outlet to the anode inlet of the main fuel cell, bypassing the electrochemical filter.

[0022] When reference is made here to the anode exhaust gas of an anode, this refers to the composition of substances flowing out of the anode outlet. This composition of substances can contain not only gaseous components but also liquid components.

[0023] When reference is made here to a flow direction, this generally means the flow direction of the recirculate from the anode outlet to the anode inlet, unless a different flow direction is defined in the individual case.

[0024] According to the invention, a bypass ratio is defined. The bypass ratio is a quotient of a (volume) portion of the recirculate flowing through the bypass path to a total amount (a total volume) of the recirculate (more precisely, the exhaust gas discharged from the anode outlet). In other words, the bypass ratio is the ratio of the exhaust gas volume unfiltered by the electrochemical filter to the total exhaust gas volume discharged from the anode of the main fuel cell. The sum of the quotients of the bypass ratio and the exhaust gas flow exiting the anode outlet of the main fuel cell, as well as the volume flow flowing through the electrochemical filter and the exhaust gas flow exiting the anode outlet of the main fuel cell, is one.In other words, the sum of the bypass flow rate, which is the flow rate through the bypass path, the fuel flow rate extracted from the recirculate, which is the flow rate discharged from the cathode outlet of the electrochemical filter, and the fuel flow rate purged from the fuel cell system or the recirculation path, which is the flow rate discharged from the anode outlet of the electrochemical filter, equals the flow rate discharged from the anode outlet of the main fuel cell. The bypass flow rate does not need to be completely recirculated to the anode inlet of the main fuel cell.

[0025] According to the invention, the bypass ratio is at least 85 percent. This means that during normal operation of the fuel cell system, only a maximum of 15 percent of the anode exhaust gas from the main fuel cell flows through the electrochemical filter. The fuel cell system is designed to operate with a bypass ratio of at least 85 percent during normal (continuous) operation.

[0026] The relatively high bypass ratio results in a relatively low flow rate through the electrochemical filter. This, in turn, leads to low consumption associated with the operation of the electrochemical filter. In other words, the high bypass ratio ensures that little power needs to be supplied to the electrochemical filter, which has a positive effect on the overall system efficiency of the fuel cell system.

[0027] Preferably, the bypass ratio is in a range of 85 to 99 percent. Even more preferably, the bypass ratio is in a range of 90 to 98 percent. Even more preferably, the bypass ratio is in a range of 92.5 to 97.5 percent. Even more preferably, the bypass ratio is in a range of 95 to 97 percent. As described, the highest possible bypass ratio is desirable with regard to overall system efficiency.

[0028] On the other hand, a certain volume flow through the electrochemical filter is desirable in order to extract unused fuel or to remove reaction products from the recirculation.

[0029] The recirculation path preferably comprises a purge path. The purge path is configured to discharge the recirculate flowing out of the electrochemical filter, depleted of the extracted fuel, from the recirculation path or from the fuel cell system. In other words, the purge path is configured to purge the anode exhaust gas of the electrochemical filter. Accordingly, the purge path is fluidly connected to the anode outlet of the anode chamber of the electrochemical filter.

[0030] Optionally, the purge path includes a purge valve. The purge valve can be operated intermittently or continuously. The purge valve can be used to control the volume flow purged from the fuel cell system.

[0031] Preferably, the main fuel cell comprises a main anode, a main cathode, and a main membrane system. The main membrane system is arranged between the main anode and the main cathode.

[0032] The main membrane system can provide a (first) inert gas permeation (ṅ IMain ). In this context, inert gas permeation generally refers to a molar mass flow (mmol / s) of an inert gas from one side of a cathode to one side of an anode via a membrane system. In the case of the main membrane system, the (first) inert gas permeation refers to a molar mass flow of an inert gas, such as nitrogen, from one side of the main cathode (main cathode side) to one side of the main anode (main anode side) of the main fuel cell via the main membrane system.

[0033] Preferably, the electrochemical filter comprises a filter anode, a filter cathode, and a filter membrane system. The filter membrane system is arranged between the filter anode and the filter cathode.

[0034] The filter membrane system may have a (second) inert gas permeation.

[0035] Preferably, the second inert gas permeation is significantly or so much lower than the first inert gas permeation that the amount of inert gas passing through the filter membrane system (during normal operation of the fuel cell system) can be neglected.

[0036] Even more preferably, the second inert gas permeation is set up to be lower than the first inert gas permeation such that a ratio (SR) of a molar mass flow of the inert gas (ṅ EFPurge ), which (during normal operation of the fuel cell system) flows out of an anode outlet of the filter anode instead of via the filter membrane system, and the first inert gas permeation (ṅ IMain ) is at least 1.5. Therefore, SR=n˙EFPurgen˙IMain>1,5.

[0037] More preferably, SR > 1.75, and even more preferably, SR > 2.0.

[0038] In other words, during normal operation of the fuel cell system, significantly more inert gas flows from the anode outlet of the electrochemical filter (and thus from the recirculation path or the fuel cell system) than inert gas reaches the main anode side via the main membrane system. This reduces or prevents enrichment of the main anode side with inert gas, such as nitrogen, or fuel depletion of the main anode side.

[0039] The recirculation path can have a valve, such as a directional control valve, for controlling the bypass ratio. The valve can then be used to directly control the composition of the recirculated stream with regard to the fuel content and the proportions of reaction products, as well as the power consumption of the electrochemical filter. The operation or control of the valve can thus have a direct impact on the overall efficiency and service life of the fuel cell system. The valve can be designed simply and thus easily and relatively inexpensively to integrate into the fuel cell system.

[0040] Alternatively, the fuel cell system can be configured to control the bypass ratio (exclusively) by energizing the electrochemical filter. In other words, the bypass ratio can be controlled by varying, for example, the power of the electrochemical filter. Preferably, no valve, such as a directional control valve, is provided or required to control the bypass ratio. This reduces the complexity, costs, and susceptibility to mechanical failures of the fuel cell system, but can still offer similar advantages to the provision of a valve (controllability of the bypass ratio and thus the overall efficiency and service life of the fuel cell system).

[0041] The recirculation path may include a recirculation drive. The recirculation drive is preferably a blower or a jet pump.

[0042] The electrochemical filter can be arranged parallel to the recirculation drive in the recirculation path. Alternatively, the electrochemical filter can be arranged in series with the recirculation drive in the recirculation path. This provides great flexibility in the design of the recirculation path.

[0043] By reducing the inert gas content or maintaining a low level of inert gas by means of the electrochemical filter during operation of the fuel cell system, a reduction in the power of the recirculation drive can be achieved compared to a system without an electrochemical filter.

[0044] In general, the present invention is not limited to any particular type of fuel cells, but is preferably directed to PEM fuel cells.

[0045] The fuel is preferably hydrogen. Alternatively or additionally, the inert gas is nitrogen.

[0046] The main fuel cell is preferably a main fuel cell capable of operating at a superstoichiometric rate. The operation of the fuel cell system according to the invention or of the main fuel cell comprised therein is not limited to a superstoichiometric rate, as long as the main fuel cell is capable of expelling unused fuel from the anode compartment or as long as the main fuel cell expels unused or unconsumed fuel during operation.

[0047] Optionally, the fuel cell system or recirculation path for purging (the recirculate or main anode exhaust gas depleted of the fuel extracted in the electrochemical filter) does not have a purge valve. The fuel cell system or recirculation path is configured to control purging via the bypass ratio (e.g., via the directional control valve described above) and / or the energization of the electrochemical filter. This reduces the complexity (e.g., the number of components and / or the control system) of the fuel cell system or recirculation path, as well as the associated costs and efficiency losses, and increases the robustness of the system with respect to mechanical failures.

[0048] Preferably, the fuel cell system or the recirculation path does not have a water separator (liquid separator). The fuel cell system or the recirculation path is configured to separate any water (liquid) contained in the recirculate via the electrochemical filter. This reduces the complexity (e.g., the number of components) of the fuel cell system or the recirculation path, as well as the associated costs and efficiency losses, and increases the robustness of the system with respect to mechanical failures.

[0049] Preferably, the fuel cell system does not have a humidification device in a main fuel train. The main fuel train is understood to be the flow path from the fuel (pressure) reservoir via a metering device toward the anode chamber, to which the fuel extracted in the electrochemical filter is added. Since the electrochemical filter already introduces more moisture into the extracted fuel than other separation devices for fuel and reaction products, a dedicated humidification device in the main fuel train can preferably be dispensed with. This reduces the complexity (e.g., the number of components and / or the control system) of the fuel cell system as well as the associated costs and efficiency losses, and increases the robustness of the system with regard to mechanical failures.

[0050] In order to achieve the above-described object, the present invention also includes a method for operating a fuel cell system.

[0051] The fuel cell system can be one of the fuel cell systems described above. The fuel cell system according to the invention comprises a main fuel cell and a recirculation path. The recirculation path connects an anode outlet of an anode compartment of an anode of the main fuel cell to an anode inlet of the anode compartment of the anode. The recirculation path includes an electrochemical filter.

[0052] The method according to the invention comprises discharging and splitting an exhaust gas as well as extracting and supplying a fuel.

[0053] The exhaust gas discharges from the anode compartment through the anode outlet into the recirculation path. The exhaust gas comprises unconsumed fuel. The exhaust gas discharged from the anode outlet flows into the recirculation path as recirculate toward the anode inlet.

[0054] Splitting involves dividing a total recirculate flow into at least two substreams. A (first) substream flows through the electrochemical filter. In other words, the first substream is recirculated (partially, with depleted exhaust gas not being recirculated) through the electrochemical filter (to the anode compartment of the main fuel cell). A (second) substream flows to the anode inlet, bypassing the electrochemical filter. This means that the second substream of the recirculate is recirculated (to the anode compartment of the main fuel cell) without flowing through or past the electrochemical filter.

[0055] Extraction includes extracting the unused fuel contained in the first partial stream of the recirculate through the electrochemical filter.

[0056] The feeding process involves feeding the fuel extracted from the first partial stream of the recirculate through the electrochemical filter to the anode inlet. The extracted fuel also contains reaction products, such as nitrogen, but in negligible amounts. The extracted fuel may also contain significant amounts of water. This can be advantageously used to humidify the fuel flowing in the main fuel train. The feeding process further involves feeding the second partial stream, which contains unused fuel but also reaction products, to the anode inlet.

[0057] The first partial flow, which flows through the electrochemical filter and from which the fuel is extracted, amounts to a maximum of 15 percent of the total flow of recirculated or exhaust gas leaving or being expelled from the anode chamber. The advantages of the comparatively low volume flow through the electrochemical filter and the associated comparatively high bypass ratio with regard to the function and efficiency of the fuel cell system have been explained above.

[0058] Preferably, the first partial flow flowing through the electrochemical filter and from which the fuel is extracted is 1 to 15 percent of the total flow of the recirculate or exhaust gas leaving the anode compartment, more preferably 2 to 10 percent, more preferably 2.5 to 7.5 percent, and even more preferably 3 to 5 percent.

[0059] Preferably, the process also includes a continuous depletion of an inert gas from the recirculate. The advantage achieved is that the proportion of inert gas on the anode side of the fuel cell system can always be kept at a low level.

[0060] Preferably, the method also includes purging the portion of the recirculate or exhaust gas flowing out of the electrochemical filter, which has been depleted of the extracted fuel, from the recirculation path. Even more preferably, the purging takes place without using a purge valve. The advantages of being able to dispense with a purge valve have been explained above.

[0061] Preferably, the (second) inert gas permeation at the membrane system of the electrochemical filter (filter membrane system) is lower than the (first) inert gas permeation at the membrane system of the main fuel cell (main membrane system).

[0062] Even more preferably, the second inert gas permeation is so lower than the first inert gas permeation that the molar mass flow of the inert gas (during normal operation of the fuel cell system) through the filter membrane system is negligible.

[0063] Even more preferably (during normal operation of the fuel cell system), due to the low second inert gas permeation, a ratio of a molar mass flow of an inert gas flowing out of the anode outlet of the anode of the electrochemical filter (filter anode) instead of via the membrane system of the electrochemical filter, to the inert gas permeation at the membrane system of the main fuel cell (main membrane system) is at least 1.5, preferably at least 1.75, and even more preferably at least 2.0. The advantages of a comparatively high molar mass flow from the filter anode relative to the inert gas permeation at the main membrane system have been explained above.

[0064] Preferably, the total flow of recirculated material is divided by energizing or controlling the line of the electrochemical filter, more preferably exclusively by energizing or controlling the power. In the latter case, a valve, such as a directional control valve, for dividing the total flow, which is arranged upstream of the electrochemical filter in the recirculation path, can be omitted. This brings with it the advantages described above.

[0065] Preferably, water contained in the recirculated material is separated via the electrochemical filter. Even more preferably, the water is separated exclusively via the electrochemical filter. This eliminates the need for a separate water separation device or one specifically designed for water separation. The associated advantages have been described above.

[0066] Preferably, the fuel supplied to the main anode is humidified via the electrochemical filter. Even more preferably, the fuel is humidified exclusively via the electrochemical filter. This eliminates the need for a separate fuel humidification device or one specifically designed for fuel humidification. The associated advantages have been described above.

[0067] In the method according to the invention, the main fuel cell is preferably a PEM fuel cell.

[0068] In the process according to the invention, the fuel is preferably hydrogen. Additionally or alternatively, the inert gas is preferably nitrogen.

[0069] In the process according to the invention, the main fuel cell is preferably operated at a superstoichiometric rate. This has the previously described advantage that all catalyst layers can be well supplied with fuel and the fuel cell can be operated efficiently.

[0070] Preferred embodiments of the present invention are described in detail below, but are not intended to be limiting. All features described generally above and described below as part of the embodiments can be combined with one another as desired, as long as this appears technically or economically expedient. Fig. 1 shows a schematic view of a fuel cell system. Fig. Figure 2 shows a schematic view of an electrochemical filter for the fuel cell system. Fig. Figure 3 shows a schematic view of a main fuel cell and an example of an anode side including recirculation path and main fuel train of the fuel cell system. Fig. Figure 4 shows a schematic view of a main fuel cell and another example of an anode side including recirculation path and main fuel train of the fuel cell system. Fig. Figure 5 shows a schematic view of a main fuel cell and yet another example of an anode side including recirculation path and main fuel train of the fuel cell system. Fig. Figure 6 shows a schematic view similar to that of Fig. 3, for example, material flows in the main fuel cell and on the side of the main anode. Fig. Figure 7 shows a graph illustrating the relationship between the volume flow through the electrochemical filter and the power consumption of the electrochemical filter.

[0071] The present invention generally relates to a fuel cell system. The fuel cell system comprises a main fuel cell and a recirculation path. The main fuel cell comprises an anode inlet through which fuel can flow into the main fuel cell, and an anode outlet from which an anode exhaust gas, including unconsumed fuel, can flow out. The recirculation path, in which the anode exhaust gas can flow as recirculate, connects the anode inlet to the anode outlet. The recirculation path comprises an electrochemical filter and a bypass path. The electrochemical filter is configured to extract the unconsumed fuel from the recirculate. The extracted fuel can be recirculated to the anode inlet. The recirculate can be recirculated from the anode outlet to the anode inlet via the bypass path, bypassing the electrochemical filter.The fuel cell system is designed for operation with a bypass ratio of at least 85 percent. The bypass ratio corresponds to the ratio of the portion of recirculated material flowing through the bypass path to the total amount of recirculated material.

[0072] The present invention also generally relates to a method for operating a fuel cell system having a main fuel cell and a recirculation path. The recirculation path connects an anode outlet of an anode compartment of an anode of the main fuel cell to an anode inlet of the anode compartment. The recirculation path includes an electrochemical filter. The method comprises discharging, splitting, extracting, and feeding. Discharging is discharging an exhaust gas comprising unconsumed fuel from the anode compartment via the anode outlet into the recirculation path as recirculate. Splitting is splitting a total flow of the recirculate into at least two partial flows: a first partial flow flowing through the electrochemical filter and a second partial flow flowing to the anode inlet, bypassing the electrochemical filter.Extraction involves extracting the unconsumed fuel in the first partial flow through the electrochemical filter. Feeding involves feeding the fuel extracted from the first partial flow and the second partial flow to the anode inlet. The first partial flow constitutes a maximum of 15 percent of the total flow.

[0073] In the following, identical parts are designated by the same reference numerals.

[0074] As described above, the fuel cell is not limited to a specific type. Likewise, it has been described that the fuel is not limited to a specific fuel. In the following embodiments, a PEM fuel cell system with hydrogen (H2) as the fuel is described by way of example. Atmospheric oxygen (O2) serves as the oxidant. Accordingly, the reaction products are primarily water (H2O) and nitrogen (N2).

[0075] In Fig. 1 shows a fuel cell system 1. The fuel cell system 1 comprises a fuel cell 3, hereinafter referred to as the main fuel cell 3, and a recirculation path 2 for the main fuel cell 3. The main fuel cell 3 has an anode chamber 4 on the anode side (left of the main fuel cell 3 in the illustration of Fig. 1) and a cathode chamber 5 on the cathode side (right of the main fuel cell 3 in the illustration of the Fig. 1).

[0076] The cathode side of the main fuel cell 3 comprises a turbine-driven compressor, a cooling device, a humidification device, and a liquid separator (water separator). The cathode side of the main fuel cell 3 serves to supply and preferably recirculate the oxidizing agent (oxygen) to and from the main fuel cell 3. The cathode side of the main fuel cell 3 shown here represents a conventional cathode side of a known main fuel cell.

[0077] The anode side of the main fuel cell 3 comprises a recirculation path 2, which recirculates fuel from an anode outlet 12 to an anode inlet 13. A separation device 6, such as a water separator, is arranged in the recirculation path 2 downstream of the outlet 12 relative to a flow direction of the recirculate, indicated by arrows, which initially contains the fuel (hydrogen) and the reaction products (water and nitrogen). This separation device separates liquid (water) from the recirculate.

[0078] Downstream of the water separator 6, a branch 10 of the recirculation path 2 is arranged. The branch 10 is configured to direct the flow of the now gaseous recirculate toward a conveying device 7 and a flushing device 8. The branch 10 can direct the flow either toward the conveying device 7 or toward the flushing device 8, or toward both simultaneously.

[0079] The purging device 8 is arranged downstream of the branch 10 in the recirculation path 2. The purging device 8 can be a purge valve. The purge valve 8 is designed to continuously or intermittently purge recirculated fluid from the recirculation path 2 or the fuel cell system 1. The purge valve 8 can have an intermittent mode of operation. In the Fig. In the fuel cell system 1 shown in Figure 1, the recirculate contains inert gas (nitrogen) and gaseous fuel (hydrogen). Purging accordingly reduces the system efficiency due to the loss of hydrogen and, due to the discharge of hydrogen into the environment, is associated with environmental disadvantages, as described above. Purging is nevertheless required or accepted in the fuel cell system 1 shown, since otherwise the recirculate becomes increasingly enriched with nitrogen, which, during operation of the main fuel cell 3, diffuses from the air in the cathode compartment 5 into the anode compartment 4 through the membrane system of the main fuel cell 3 arranged therebetween, hereinafter referred to as the main membrane system. Accordingly, due to the recirculation, the gas supplied to the anode compartment 4 becomes increasingly enriched with nitrogen, which impairs the operation of the main fuel cell 3, as described above.

[0080] Also downstream of the branch 10, the conveying device 7, such as a recirculation fan, is arranged in the recirculation path 2. The recirculation fan 7 sucks in the gaseous recirculate and expels it toward the anode chamber 4.

[0081] Any two of the three or all three components comprising the recirculation fan 7, the flushing valve 8 and the water separator 6 can be designed as an integrated unit in the recirculation path 2.

[0082] Downstream of the recirculation fan 7, a further branch 11 is arranged in the recirculation path 2. The branch 11 directs the recirculate expelled by the upstream recirculation fan 7 towards the inlet 13 of the anode chamber 4. Also upstream of the branch 11 (in the flow direction of the hydrogen), a valve 9, such as a metering valve, is shown in a main fuel line 14 of the anode chamber 4, which serves as a supply valve and directs the hydrogen from a fuel storage 15 (for example, a pressure vessel; see Fig. 3-6) and / or shuts off the hydrogen supplied to the anode chamber 4 or allows it to pass through to the anode chamber 4. The branch 11 is designed to direct the flow of recirculate and / or hydrogen from the main fuel line 14 towards the anode chamber 4. The branch 11 can be designed to interrupt the gas supply to the anode chamber 4. The branch 11 can, for example, be a jet pump 16 (see Fig. 3 and Fig. 6). The branch 11 connects the main fuel line 14 to the recirculation path 2 upstream of the inlet 13 of the anode of the main fuel cell 3 (hereinafter referred to as the main anode).

[0083] Fig. Figure 2 shows an electrochemical filter which, according to the invention, is arranged in the recirculation path 2 of the Fig. 1 and extracts unused fuel from a recirculation of the main anode, here also referred to as main anode recirculation.

[0084] The electrochemical filter 20 is very similar in its operating principle to a fuel cell, such as the main fuel cell 3.

[0085] In order to keep the proportion of liquid reaction products, such as the water content H2O, as low as possible as well as the proportion of gaseous reaction products, such as the nitrogen content N2, in the recirculate fed back to the main anode, media separation or filtration is advantageous.

[0086] As in Fig. As shown schematically in Figure 2, during operation of the fuel cell system 1, the anode 21 of the electrochemical filter 20, also referred to herein as the filter anode 21, is supplied with the exhaust gas stream from the anode outlet 12 of the main fuel cell 3 via the anode inlet 24 of the electrochemical filter 20, also referred to herein as the filter anode inlet 24. In the electrochemical filter 20, the molecular hydrogen H2 is oxidized and releases electrons e-. The hydrogen ions H+ are transported through the proton-conducting membrane system 23 of the electrochemical filter 20 (also referred to herein as the filter membrane system 23), which comprises, for example, a polymer membrane.The electrons e- flow via the electrically conductive components and the external circuit to the cathode 22 of the electrochemical filter 20 (here also referred to as the filter cathode 22), where the hydrogen ions H+ (protons) are reduced again and recombine to form molecular hydrogen H2. The molecular hydrogen H2 then flows via the outlet 26 of the filter cathode 22, here also referred to as the filter cathode outlet 26, toward the main fuel line 14 or the inlet 13 of the anode chamber 4 of the main anode.

[0087] Mainly hydrogen ions H+ can be transported through the filter membrane system 23. Other components of the anode exhaust gas of the main fuel cell 3 preferably remain essentially completely on the side of the filter anode 21 and are removed from the recirculation path 2 or the fuel cell system 1 as anode exhaust gas of the electrochemical filter 20 (filter anode exhaust gas) via the outlet 25 of the filter anode 21, here also referred to as the filter anode outlet 25. However, through permeation processes, a slight exchange of the nitrogen molecules N2 and water molecules H2O takes place between the side of the filter anode 21 and the side of the filter cathode 22, as shown in Fig. 2. It may be advantageous, particularly with regard to the humidification of the fuel, to provide a filter membrane system 23 that allows a large amount of water to pass through to the filter cathode 22.

[0088] As described above, the electrochemical filter 20 consumes comparatively little power and is efficient with respect to the overall efficiency of the main anode side or the fuel cell system 1. The electrochemical filter 20 can continuously deplete the nitrogen N2 from the main anode side or from the recirculation path 2, so that low nitrogen levels can be maintained on the main anode side, which contributes to improving the overall efficiency of the fuel cell system 1, the performance of the main fuel cell 3, and its service life.

[0089] Fig. 3-6 show the part of the fuel cell system 1 which is Fig. 1 is surrounded by a dashed line and which essentially comprises the main fuel cell 3 of the fuel cell system 1 as well as the anode side of the main fuel cell 3 including the recirculation path 2 and the main fuel line 14.

[0090] Fig. Figure 3 shows an example of a side of the main anode in which the electrochemical filter 20 is used to extract unused fuel from the main anode recirculate.

[0091] The anode outlet 12 of the main fuel cell 3 is connected to a line 30. During operation of the fuel cell system 1, the exhaust gas from the anode chamber 4 of the main anode flows into the line 30 as recirculation.

[0092] At branch 33, line 30 branches into lines 31 and 32. Line 31 leads to the electrochemical filter 20, which is arranged in the recirculation path 2, and more precisely to the filter anode inlet 24. Via line 31, the recirculate, including the fuel it contains, enters the electrochemical filter 20 so that the fuel can be extracted from the recirculate. Line 32 leads past or bypasses the electrochemical filter 20 (directly) toward the main fuel line 14 or the anode inlet 13 of the main fuel cell 3. Via line 32, the recirculate, including the unextracted fuel it contains, returns to the anode chamber 4 or toward the main fuel line 14.

[0093] The (total) volume flow of the exhaust gas from the main anode, divided at branch 33 (where 100 percent of the exhaust gas volume is present at the anode outlet 12 or in line 30), is divided into two volume flows: a (first) partial volume flow (partial flow) that flows through line 31 toward the electrochemical filter 20, and a second partial volume flow (partial flow) that flows through line 32 and bypasses the electrochemical filter 20. Relative to the total volume flow (i.e., to the 100 percent at the anode outlet 12 or in line 30), a maximum of 15 percent of the volume flow flows into line 31 and at least 75 percent into line 32 during operation of the fuel cell system 1.

[0094] It is conceivable to provide a valve, such as a directional control valve, in the recirculation path 2 instead of the branch 33.

[0095] In the electrochemical filter 20, the processes described above with reference to Fig. 2 described processes.

[0096] The filter cathode outlet 26 is connected to line 34. This leads to a branch 35, which connects line 34 to a line 38, which is described below. During operation, the extracted fuel (hydrogen) returns via line 34 to the anode chamber 4 of the main fuel cell 3 or can mix with the (pure) fuel (hydrogen) supplied via the main fuel line 14 from the fuel reservoir 15 (the fuel source) and flow into the anode chamber 4 of the main fuel cell 3.

[0097] The portion of the recirculate (or the exhaust gas from the main anode) depleted by the electrochemical filter 20 is discharged from the electrochemical filter 20 via the filter anode outlet 25 during operation. The filter anode outlet 25 is connected to a line 36. In the example, the line 36 is part of a purge path. The purge path includes an optional purge valve 8 provided in the line 36. During operation, the depleted portion of the recirculate (which is not recirculated) or (more precisely) the depleted portion of the exhaust gas from the main anode is discharged or purged from the recirculation path 2 or the fuel cell system 1 via the line 36.

[0098] The main fuel line 14 comprises a fuel source, which in the present example is a fuel reservoir (pressure accumulator) 15, a line 37 connected to the fuel reservoir 15, and an optional metering valve 9 (metering device). During operation, (fresh) fuel flows from the fuel reservoir 15 toward the anode chamber 4 of the main fuel cell 3 to operate the main fuel cell 3, preferably in an amount that enables superstoichiometric operation of the main fuel cell 3.

[0099] The main fuel train 14 leads in the example of Fig. 3 to a or comprises a jet pump 16, which also serves as a recirculation drive during operation. The jet pump 16 serves as a branch point where the two lines 32 and 37 meet. In other words, in the jet pump, during operation, the fresh fuel from the fuel reservoir 15 and the recirculate containing the unused fuel are combined or mixed with each other. The jet pump 16 can contain a convergent-divergent passage to promote the mixing of the flows and minimize the total pressure loss across the jet pump 16 or to increase the pressure of the fuel at the outlet of the jet pump 16.

[0100] The jet pump 16 is connected at its output to line 38. During operation, the mixture of fresh fuel and non-depleted recirculate flows via line 38 to the anode chamber 4 of the main fuel cell.

[0101] In the example shown, the electrochemical filter 20 and the jet pump 16 are arranged in parallel in the recirculation path 2.

[0102] In the example shown, the bypass path corresponds to lines 32 and 38 with the components possibly arranged therein, such as the jet pump 16.

[0103] The two lines 34 and 38 meet at the junction 35. During operation, the mixture emerging from the jet pump 16 and the essentially pure fuel extracted by the electrochemical filter 20 are combined or mixed at the junction 35 and flow further in the direction of the anode inlet 13 or the anode chamber 4 of the main fuel cell 3. It is conceivable to provide a mixing device, such as a further jet pump, instead of the junction 35 at which the two lines 34 and 38 are combined.

[0104] Due to the use of the electrochemical filter 20, a dedicated humidification device on the main anode side of the fuel cell system 1 is not required. However, such a humidification device may be provided if necessary.

[0105] The Fig. The embodiment shown in Figure 3 enables a simple and cost-effective implementation of the inventive concept, particularly if no directional control valve is provided instead of the branch 33, no purge valve 8, and no humidification device. The provision of a passive recirculation drive in the form of the jet pump 16 also enables a simple design and simple control of the volume flow through the recirculation path 2. Because the jet pump 16 has no active drive, i.e., it consumes no power during operation, the overall efficiency of the fuel cell system 1 can be improved.

[0106] Fig. Figure 4 shows another example of a side of the main anode in which the electrochemical filter 20 is used to extract unused fuel from the main anode recirculate.

[0107] The Fig. The part of the fuel cell system 1 shown in Figure 4 corresponds to the one shown in Fig. 3, except that instead of the jet pump 16, the branch 11 is provided and the recirculation fan 7 is provided as a recirculation drive in the recirculation path 2 or is arranged on the line 32.

[0108] This means that, in contrast to the case with reference to Fig. 3 described operation, in which fuel cell system 1 according to Fig. 4 The recirculate branched off at branch 33 (i.e., at least 75 percent of the total volume of the exhaust gas from the anode chamber 4 of the main fuel cell 3) flows during operation through the recirculation fan 7 to branch 11. Furthermore, the fresh fuel from the fuel reservoir 15 flows via the optional metering valve 9 to branch 11, where it mixes with the non-depleted portion of the recirculate. The mixture then flows further toward branch 35, where it mixes with the essentially pure fuel extracted by the electrochemical filter. The mixture then flows further toward the anode inlet 13 or the anode chamber 4 of the main fuel cell 3.

[0109] In the example shown, the electrochemical filter 20 and the recirculation fan 7 are arranged in parallel in the recirculation path 2.

[0110] In the example shown, the bypass path corresponds to lines 32 and 38 with any components arranged therein, such as the recirculation fan 7.

[0111] The Fig. The embodiment shown in Figure 4 enables a simple and cost-effective implementation of the inventive concept and also offers the advantage of an active recirculation drive in the form of the recirculation fan 7. This allows the volume flow through the recirculation path 2 to be actively controlled and adapted or optimized to the operation of the main fuel cell 3 and / or the electrochemical filter 20. This can improve the overall efficiency of the fuel cell system 1 under certain operating conditions or at certain operating points of the fuel cell system 1.

[0112] Fig. Figure 5 shows another example of a side of the main anode in which the electrochemical filter 20 is used to extract unused fuel from the main anode recirculate.

[0113] The Fig. The part of the fuel cell system 1 shown in Figure 5 corresponds to the one shown in Fig. 4, except that the recirculation fan 7 is arranged upstream of the branch 33 or on the line 30 and the bypass path 32 and the line 34 connected to the filter cathode outlet 26 are brought together at a branch 39.

[0114] This means that, in contrast to the case with reference to Fig. 4 described operation, in which fuel cell system 1 according to Fig. 5 During operation, the total volume flow of the exhaust gas from the main anode flows through the recirculation fan 7. This can be advantageous, for example, if the recirculation fan 7 has an integrated water separator and it is desired to separate as much water as possible from the recirculation. The recirculation then flows downstream to the branch 33, where it is then divided into the two partial flows, which flow towards the electrochemical filter (first partial flow) and into the bypass path, which corresponds to line 32 with the components possibly arranged therein (second partial flow). The division ratio is, as described above, that a maximum of 15 percent of the total volume flow forms the first partial flow, which flows through the electrochemical filter 20.

[0115] During operation of the fuel cell system 1, the fuel separated in the electrochemical filter 20 as described above flows into the line 34 via the filter cathode outlet 26, is combined or mixed with the bypass volume flow from the line 32 at the branch 39 and then flows in a common line 41 to the branch 40.

[0116] At branch 40, the pure fuel from line 37 or the main fuel line 14 is combined or mixed with the mixture from the bypass path 32 and the filter cathode 22. The mixture then flows via line 42 to the anode inlet 13 of the main anode.

[0117] It is also conceivable that the three volume flows from line 37 or the main fuel line 14 (pure fuel), from the bypass path or line 32 (recirculate containing unused fuel), and from line 34 or the filter cathode outlet 26 (extracted pure fuel) converge or mix at the same branch 39 or 40. Then, one of the branches 39 and 40, as well as the line 41 arranged between them, can be omitted.

[0118] In the example shown, the electrochemical filter 20 and the recirculation fan 7 are arranged serially in the recirculation path 2.

[0119] The Fig. The embodiment shown in Figure 5 enables a simple and cost-effective implementation of the inventive concept and also offers the advantage of an active recirculation drive in the form of the recirculation blower 7, through which the entire volume flow in the recirculation path 2 flows. Thus, the volume flow through the recirculation path can be actively controlled and adapted or optimized to the operation of the main fuel cell 3 and / or the electrochemical filter 20. This can improve the overall efficiency of the fuel cell system 1 under certain operating conditions or at certain operating points of the fuel cell system 1. Furthermore, water can be separated from the entire recirculated material if the recirculation blower 7 has an integrated water separator.

[0120] Fig. 6 corresponds to the Fig. 3 and schematically illustrates the material and volume flows that occur during operation of the fuel cell system 1. Similar representations based on the Fig. 4 and Fig. 5 are apparent to the expert.

[0121] In the main fuel cell 3 shows Fig. 6 the mass flow of the fuel (ṅ FMain ) via the main membrane system from the main anode chamber 4 to the main cathode chamber 5. Furthermore, the mass flow of the inert gas (ṅ IMain in mmol / s) across the main membrane system from the main cathode chamber 5 to the main anode chamber 4. The main membrane system is configured such that the desired mass flow of the fuel is significantly greater than the undesired mass flow of the inert gas, as indicated by the respective arrow lengths.

[0122] The total volume flow (V̇ Ges ) of the exhaust gas of the main anode is in Fig. 6 is indicated by a relatively wide line, from which a relatively thin line branches off at the junction 33, which symbolizes the small first partial volume flow through the electrochemical filter 20.

[0123] The mass flow of the fuel passing through the filter membrane system essentially corresponds to the volume flow flowing out of the filter cathode outlet 26, since the mass flow of the reaction products passing through the filter membrane system is negligible (and therefore not included in the representation of the Fig. 6 is also not shown).

[0124] The exhaust gas of the main anode, depleted of the extracted fuel, flows during operation essentially as a mass flow of the inert gas (ṅ EFPurge ) from the filter anode outlet 25 into the line 36 or into the flushing path.

[0125] As described above, SR=n˙EFPurgen˙IMain>1,5.

[0126] The ratio SR expresses that the main anode side (always) has a low level of inert gas during operation of fuel cell system 1. This is even more true for SR > 1.75 and SR > 2.0.

[0127] For the sake of completeness, it should be mentioned that Fig. 6 also the second partial flow (bypass flow) of the total volume flow (V̇ Ges ) of the exhaust gas of the main anode shows a significantly broader (because significantly larger) flow compared to the first partial flow.

[0128] In addition, Fig. 6 the fuel flow of the pure fuel in the main fuel line 14 upstream of the jet pump 16 in the flow direction of the fuel.

[0129] Fig. 7 illustrates the disproportionate (approximately exponential) increase in the power consumption of the electrochemical filter 20 with increasing volume flow of the recirculate through the electrochemical filter 20. In Fig.7 shows that for small volume flows (of the first partial volume flow through the electric filter) up to approximately 15 percent, the relationship between the volume flow and the power consumed is, to a very good approximation, approximately linear. Therefore, in this range of volume flows of the first partial volume flow, there is a good or satisfactory balance between the amount of extracted fuel or filtered-out inert gas, such as the nitrogen content (at 15 percent of the total volume share of the main anode exhaust gas, the nitrogen content can be reduced to almost zero) and the power consumption, thereby improving or optimizing the system efficiency. From approximately 15 percent of the volume flow, the exponential curve becomes noticeable, and a further increase in extracted fuel must be paid for with an undesirably high power consumption of the electrochemical filter.This has a negative impact on the overall efficiency of the fuel cell system 1.

[0130] According to the invention, the range of the first partial volume flow through the electrochemical filter 20 above 15 percent of the total volume flow of the main anode exhaust gas is therefore avoided.

Claims

[1] Fuel cell system (1) with a main fuel cell (3) comprising an anode inlet (13) through which fuel can flow into the main fuel cell, and an anode outlet (12) from which an anode exhaust gas including unused fuel can flow out; and a recirculation path (2) in which the anode exhaust gas can flow as recirculate, which connects the anode inlet to the anode outlet and which includes the following: an electrochemical filter (20) for extracting the unused fuel from the recirculated fuel, wherein the extracted fuel is recirculable to the anode inlet, and a bypass path (32, 38) through which the recirculated fluid can be recirculated from the anode outlet to the anode inlet, bypassing the electrochemical filter; wherein the fuel cell system is set up for operation with a bypass ratio, which is a quotient of a proportion of the recirculated fluid flowing through the bypass path to a total amount of the recirculated fluid, of at least 85 percent. [2] Fuel cell system according to claim 1, wherein the bypass ratio is in a range of 85 to 99 percent, preferably 90 to 98 percent, more preferably 92.5 to 97.5 percent and more preferably 95 to 97 percent. [3] Fuel cell system according to claim 1 or 2, wherein the recirculation path includes a purge path (36) which is configured to discharge the recirculated fuel, depleted of the extracted fuel, from the electrochemical filter into the recirculation path, the flushing path optionally includes a flushing valve (8). [4] Fuel cell system according to one of the preceding claims, wherein the main fuel cell comprises a main anode, a main cathode and an intermediate main membrane system, which has a first inert gas permeation, which is a molar mass flow of an inert gas from one side of the main cathode to one side of the main anode via the main membrane system; and the electrochemical filter comprises a filter anode (21), a filter cathode (22) and an intermediately arranged filter membrane system (23) which has a second inert gas permeation, which is a molar mass flow of the inert gas from one side of the filter cathode to one side of the filter anode via the filter membrane system; wherein The second inert gas permeation is set up to be so lower than the first inert gas permeation that the ratio of the molar mass flow rate of the inert gas, which flows out of an anode outlet (25) of the filter anode instead of via the filter membrane system, to the first inert gas permeation is at least 1.5, preferably at least 1.75 and even more preferably at least 2.

0. [5] Fuel cell system according to one of the preceding claims, wherein The recirculation path also includes a valve for controlling the bypass ratio; or the fuel cell system is set up to control the bypass ratio by energizing the electrochemical filter, and The recirculation path for controlling the bypass ratio preferably has no valve and the fuel cell system is set up to control the bypass ratio exclusively by energizing the electrochemical filter. [6] Fuel cell system according to one of the preceding claims, wherein the recirculation path comprises a recirculation drive (7, 16), preferably a blower (7) or a jet pump (16); and the electrochemical filter is arranged parallel to the recirculation drive in the recirculation path; or The electrochemical filter is arranged in series with the recirculation drive in the recirculation path. [7] Fuel cell system according to any of the preceding claims, wherein the main fuel cell is a proton exchange membrane fuel cell; and / or the fuel is hydrogen and / or the inert gas is nitrogen; and / or the main fuel cell is a superstoichiometric main fuel cell. [8] Fuel cell system according to one of the preceding claims, wherein the recirculation path for flushing does not have a flushing valve and is set up to control flushing via the bypass ratio and / or the energizing of the electrochemical filter; the recirculation path does not have a water separator and is set up to separate any water contained in the recirculated fluid via the electrochemical filter; and / or The fuel cell system does not have a humidification device in a main fuel line leading to the anode inlet. [9] Method for operating a fuel cell system (1) comprising a main fuel cell (3) and a recirculation path (2) connecting an anode outlet (12) of an anode compartment (4) of an anode of the main fuel cell to an anode inlet (13) of the anode compartment and comprising an electrochemical filter (20), the method comprising a release of exhaust gas containing unused fuel from the anode chamber via the anode outlet into the recirculation path as recirculated gas, a division of a total recirculated current into at least two partial currents, a first partial current flowing through the electrochemical filter and a second partial current flowing to the anode inlet bypassing the electrochemical filter, an extraction of the unused fuel in the first partial stream through the electrochemical filter, and a supply of the fuel extracted from the first partial stream and the second partial stream to the anode inlet, where the first partial flow is at most 15 percent of the total flow. [10] Method for operating a fuel cell system according to claim 9, wherein The process further comprises a continuous removal of an inert gas from the recirculated gas in order to keep the proportion of inert gas on one anode side of the fuel cell system at a low level; and / or the first partial flow is between 1 and 15 percent of the total flow, preferably 2 to 10 percent, more preferably 2.5 to 7.5 percent, and even more preferably 3 to 5 percent; and / or The method further comprises purging the exhaust gas flowing from the electrochemical filter, depleted of the extracted fuel, from the recirculation path, preferably without using a purge valve; and / or an inert gas permeation at a membrane system (23) of the electrochemical filter is so lower than an inert gas permeation at a membrane system of the main fuel cell that the ratio of a molar mass flow rate of an inert gas, which flows out of an anode outlet (25) of an anode of the electrochemical filter instead of via the membrane system of the electrochemical filter, to the inert gas permeation at the membrane system of the main fuel cell is at least 1.5, preferably at least 1.75 and even more preferably at least 2.0; and / or The division is carried out by energizing the electrochemical filter, preferably exclusively by energizing it; and / or Water contained in the recirculated fluid is separated via the electrochemical filter, preferably exclusively via the electrochemical filter; and / or the fuel is moistened via the electrochemical filter, preferably exclusively via the electrochemical filter; and / or the main fuel cell is a proton exchange membrane fuel cell; and / or the fuel is hydrogen and / or the inert gas is nitrogen; and / or the main fuel cell is operated superstoichiometrically.

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