Electrode element

Abstract

Fuel cell (110) with at least two electrode elements (10, 10') and a membrane element (30), wherein the membrane element (30) is arranged between the two electrode elements (10, 10'), in an electrochemical reaction at least one fuel gas flows along at least one flow path (42, 42') over at least one electrode element (10, 10'), at least one of the electrode elements (10, 10') is provided with a catalyst (60), wherein a density (61) of the catalyst (60) increases along the flow path (42, 42') in order to prevent poisoning of the catalyst (60) with inactivating substances, wherein the fuel gas can be guided to the electrode element by a feed element (130, 131), characterized in that a water (H2O) adsorbing adsorbent is arranged in and / or on the feed element (130, 131).

Description

[0001] The present invention relates to an electrode element for the electrochemical conversion of a fuel gas in a fuel cell, comprising the features of the preamble of claim 1, wherein, in an installed state, the fuel gas flows along a flow path over the electrode element, and the electrode element is provided with a catalyst. The invention also relates to a fuel cell according to the preamble of claim 5, comprising at least two electrode elements and a membrane element, wherein the membrane element is arranged between the two electrode elements, and, during an electrochemical conversion, at least one fuel gas flows along at least one flow path over at least one electrode element, and at least one of the electrode elements is provided with a catalyst. State of the art

[0002] Well-known fuel cells are used to convert hydrogen into usable electrical energy. To carry out this electrochemical reaction, the fuel cell has two electrodes to which two reactants, such as hydrogen and oxygen, are supplied. The hydrogen supplied to one electrode – the anode – splits into hydrogen, releasing electrons in the process. +Electrons are released. While the electrons can be used to generate energy via an external circuit, the protons diffuse through a membrane element to the second electrode – the cathode. At the cathode, the protons can then react with oxygen, producing water. The two electrodes, together with the membrane element, form a membrane electrode assembly (MEA). For high-temperature fuel cells, it is known from DE 195 19 847 C1 that the catalyst loading of the electrode increases along a flow path. For a PEM fuel cell, it is known from DE 199 14 680 C2 that the catalyst loading of the electrode varies in the thickness direction.

[0003] A flow field plate serves to distribute the reactants across an active area of ​​the electrode while ensuring mechanical stability. To achieve this, known flow field plates incorporate flow channels. A gas diffusion layer is positioned between the membrane electrode assembly and the flow field plate. This gas diffusion layer serves to distribute the fuels—also called reactants—hydrogen or oxygen evenly across the entire electrode surface and to dissipate the reaction products (electricity, heat, and water) from the electrodes.

[0004] In conventional fuel cells, particularly PEM fuel cells, precious metal catalysts such as platinum are used to catalyze the electrochemical reaction of hydrogen and oxygen. The use of precious metal catalysts represents a significant cost factor in fuel cell manufacturing. A disadvantage has been identified: these catalysts can be poisoned by the carbon monoxide (CO) contained in the fuel gas. The carbon monoxide is preferentially absorbed at the platinum reaction centers, irreversibly damaging the catalyst at typical operating temperatures of T < 120°C and anode potentials of approximately 0 V relative to the reference hydrogen electrode (RHE). This catalyst poisoning significantly reduces the efficiency of the electrochemical reaction. Furthermore, severe poisoning can impair the long-term stability of the fuel cell.For currently operating PEM fuel cells, it has been found that the carbon monoxide concentration in the fuel must be less than 100 ppm if the fuel cell is operated at a temperature below 150°C. Ensuring such low carbon monoxide concentrations within the hydrogen used as fuel requires extensive production steps—steam reforming, water shift reaction, and / or selective CO oxidation—to enable the use of hydrogen produced from fossil fuels in fuel cells. Furthermore, US Patent 4,910,099 A discloses the possibility of introducing oxygen into the fuel stream to oxidize the carbon monoxide present in the fuel flow. However, this approach has proven to be technically complex and costly.Additionally, the amount of hydrogen available for the electrochemical reaction is reduced.

[0005] Further fuel cells with varying catalyst loading can be found in the documents US 4 851 377 A, EP 0 654 837 A1, US 5 843 195 A and DE 11 2006 002 453 T5. Purpose and advantages of the invention

[0006] The object of the present invention is to provide a fuel cell which overcomes the above-mentioned disadvantages, in particular one which can also be operated at high carbon monoxide concentrations without poisoning of the catalyst.

[0007] This problem is advantageously solved by the fuel cell with the features of claim 1. Furthermore, the problem is advantageously solved by the fuel cell with the features of claim 5. Further advantageous embodiments of the present invention are described in the respective dependent claims.

[0008] The fuel cell according to the invention is characterized in that the density of the catalyst increases along the flow path in order to prevent poisoning of the catalyst with inactivating substances.

[0009] The invention is based on the surprising finding that inactivating substances introduced into the fuel cell with the fuel gas reach the catalyst at locally varying concentrations. Within the scope of the invention, inactivating substances are defined as all those components of the fuel gas that damage the catalyst to such an extent that it can no longer be used for the catalysis of the reactants entering the fuel cell. It is known, for example, that carbon monoxide (CO) and / or sulfur compounds can lead to lasting damage to the platinum catalyst. Since the fuel gas is to be converted electrochemically within the fuel cell as efficiently as possible, it is introduced into the fuel cell at a low flow rate. It has been found that the amount of inactivating substances decreases along the flow path at low flow rates.This can result from the fact that at low flow velocities, the inactivating substance diffuses into the gas diffusion layer. Thus, the concentration of the inactivating substance along the gas flow path across the active area of ​​the electrode element decreases. In contrast, higher volume flows and therefore higher flow velocities lead to a reduction in the diffusion of the inactivating substance into the gas diffusion layer. Simultaneously, the potential contact time of the inactivating substance with the catalyst decreases, so that lower absorption along the flow path is to be expected. Consequently, the reduction of the inactivating substance along the flow path decreases at high flow velocities. However, to achieve high fuel cell efficiency, the lowest possible flow velocity of the fuel gases is desirable.Based on this finding, the invention provides that the density of the catalyst increases along the flow path. Thus, the density of the catalyst is adapted to and inversely proportional to the amount of the inactivating substance in the fuel gas. This ensures that poisoning of the catalyst by the inactivating substance is significantly reduced and, in some cases, even eliminated. Within the scope of the invention, the statement that poisoning of the catalyst with inactivating substances is prevented means that, compared to the prior art, significantly less catalyst is poisoned by inactivating substances. The catalyst arrangement according to the invention thus reduces and / or avoids and / or minimizes poisoning of large portions of the catalyst. Therefore, poisoning of the catalyst is significantly reduced and / or even completely avoided compared to the prior art.

[0010] An advantageous embodiment of the fuel cell according to the invention is characterized in that the electrode element includes an oxidation element for oxidizing the inactivating substances. In contrast to the catalyst, which serves for the electrochemical conversion of the fuel gas, the function of the oxidation element is to convert the inactivating substances in such a way that they cannot permanently damage the catalyst. If the fuel gas is hydrogen, the predominant inactivating substance is carbon monoxide. The following reaction equation is suitable for catalyzing carbon monoxide: CO + H2O => CO2 + H2 2 CO + O2 => 2 CO2

[0011] The oxidizing element serves to catalyze the conversion of carbon monoxide to carbon dioxide according to the reaction equations mentioned above. This can be produced, in particular, by the absorption of carbon monoxide and water and / or oxygen by the oxidizing element.

[0012] It has proven advantageous for the density of the oxidizing element to decrease along the flow path. Consequently, the oxidizing element can have a density that decreases in proportion to the increase in the density of the catalyst along the flow path. The oxidizing element thus converts the inactivating substances along the flow path, preventing any risk of catalyst poisoning and allowing its density to increase along the flow path.

[0013] Gas diffusion layers (GDLs) can be applied to both sides of the membrane electrode assembly. These GDLs are typically made of carbon fiber paper or fabric and allow for good access of the fuels to the reaction layers and efficient drainage of the cell current and the water produced. The fuels—also called reactants—such as hydrogen and oxygen, and the reaction product, water, flow through the flow channel—also called the flow path—of the flow field plate. The reactants primarily serve the electrochemical generation of electrical energy. Since this electrochemical reaction is exothermic, the fluids are simultaneously used to dissipate excess heat of reaction.It has therefore proven advantageous for the flow paths on the outer surface of the flow field plate to have a meandering structure. In this arrangement, the flow paths cover a large proportion of the flow field plate's surface. The electrode element and the flow field plate are essentially parallel to each other, meaning that the planes spanned by the electrode element and the flow field plate, respectively, do not intersect or only intersect at an angle of less than 10°. Within the scope of this patent, the term "density" refers to the quantity of catalyst relative to the volume it occupies.

[0014] According to the invention, the density of the catalyst increases along the flow path. If a flow field plate is used, the fuel flows in a meandering pattern over the electrode element. In this case, the density of the catalyst can increase along the flow path of the flow field plate. It is also conceivable that a constant catalyst density is provided for each meander loop, but that the catalyst density increases from the top surface of an outer surface of the flow field plate to the bottom surface.

[0015] To enable efficient conversion of the fuel and / or the inactivating substances, it has proven advantageous for the catalyst and / or the oxidizing element to contain at least one of the following: an element of the platinum group, such as platinum, palladium, ruthenium, or indium, or an element of the copper group, such as copper or gold. Platinum catalysts primarily serve to convert the fuel gas in the fuel cell. However, they can also be used to oxidize the inactivating substances. For the oxidizing element, however, the use of copper group elements has proven to be particularly advantageous and cost-effective.

[0016] In a further advantageous embodiment, the catalyst comprises two catalyst elements. The first catalyst element is primarily located in a region of high concentration of inactivating substances, while the second catalyst element is positioned offset towards the flow path and primarily serves for the electrochemical conversion of the fuel gas. It has proven advantageous if the catalyst and / or the first catalyst element are platinum-free in an inlet region of the flow path. A platinum-free catalyst or first catalyst element has the advantage of not being affected by the problem of irreversible adsorption of the inactivating substances within the operating temperature range.It has proven particularly advantageous if the catalyst and / or the first catalyst element and / or the oxidation element contains at least one of the following components: metalloporphyrins, in particular co-porphyrins.

[0017] It has proven advantageous for the electrode element to consist of carbon-supported platinum (Pt / C). Platinum is applied to carbon particles, which are then deposited onto the membrane via screen printing or sputtering. The catalyst concentration is independent of the electrode element's thickness or depth. This ensures that sufficient catalyst is present even in deeper layers of the electrode element to carry out the electrochemical reaction between the two reactants within the fuel cell.

[0018] The problem according to the invention is also solved by a fuel cell with at least two electrode elements and a membrane element, wherein the membrane element is arranged between the two electrode elements, and in an electrochemical reaction, at least one fuel gas flows along at least one flow path over at least one electrode element, and at least one of the electrode elements is provided with a catalyst. According to the invention, the density of the catalyst increases along the flow path in order to prevent poisoning of the catalyst with inactivating substances. The fuel cell according to the invention is characterized by the use of the electrode element according to the invention. This ensures that inactivating substances that enter the fuel cell via the fuel flow are not absorbed by the catalyst and poisoned.Features and details listed in connection with the electrode element naturally also apply to the electrode element and vice versa.

[0019] For the fuel cell according to the invention, it has also proven advantageous for it to include an oxidation element to oxidize the inactivating substances. It is advantageously provided that the density of the oxidation element decreases along the fuel flow path. In a first embodiment, the oxidation element is arranged on the electrode element, with the amount of the oxidation element being adapted to the concentration of the inactivating substance. Since this decreases – as described above – during the fuel flow, the density of the oxidation element can also decrease along the flow path.

[0020] In another advantageous embodiment, the oxidizing element is arranged on a gas diffusion layer. The gas diffusion layer is positioned between the electrode element and the flow field plate and serves to distribute the fuel gas evenly across the entire surface of the electrode element and to remove the reaction products from the electrode element. As described above, some of the inactivating substances penetrate the gas diffusion layer. Therefore, it is advantageous if the oxidizing element is at least partially located within the gas diffusion layer. This allows for a rapid and effective conversion of the inactivating substances.

[0021] In a further advantageous embodiment, the fuel cell has a feed element. The fuel gas can be supplied to the electrode element by means of the feed element. The fuel for operating the fuel cell is generally taken from a storage container and flows into the fuel cell through the feed element. The feed element can be arranged such that the fuel flows from the feed element into an inlet area of ​​the flow field plate. The feed element thus ensures that the fuel is introduced into the fuel cell so that it can be electrochemically converted there.

[0022] In an advantageous embodiment, the oxidation element for reacting the inactivating substances is arranged in and / or on the feed element. Consequently, the oxidation element is used to oxidize the inactivating substances even during the feed and before the electrochemical reaction. This reduces the amount of inactivating substances in the fuel flow, thus reducing the probability of catalyst poisoning.

[0023] It has proven particularly advantageous to arrange a water (H₂O) adsorbent in and / or on the inlet element. This adsorbent serves as a catalyst for the conversion of carbon monoxide. The electrooxidation of carbon monoxide with water according to one of the reactions described above is preferred, as this can be carried out without the supply of oxygen-containing gas such as air, thus reducing the complexity of the fuel cell design.

[0024] Another advantageous embodiment is characterized by the arrangement of a trapping layer in and / or on the inlet element, the trapping layer serving to oxidize the inactivating substances. According to the invention, the oxidizing element interacts with the trapping layer to further reduce the probability of carbon monoxide contamination of the catalyst. The reaction described above, in which carbon monoxide is converted to carbon dioxide with the aid of water, is preferably used. Since the fuel gas is usually humidified, water is available as an oxidizing agent. It has proven particularly advantageous if the trapping layer is constructed from a membrane, the membrane being provided with the oxidizing element.The trapping layer can consist, in particular, of an ionomer coated with a suitable oxidizing element, whereby techniques such as impregnation, screen printing, etc., can be used to apply the oxidizing element. Besides an arrangement in the feed element, it has also proven advantageous to arrange the trapping layer in the region of the electrode element.

[0025] The fuel cell described according to the invention can also be arranged in a fuel cell system. Within the fuel cell system, at least one of the described fuel cells is then integrated. Furthermore, it has proven advantageous if the fuel cell has an electrode element possessing the features described above. Examples of implementation

[0026] Further advantages, features or details of the invention are described in the following description, in which exemplary embodiments of the invention are explained in detail with reference to the drawings.

[0027] They show: Fig. 1 a fuel cell system according to the invention, Fig. 2 a flow field plate, Fig. 3 two distributions of inactivating substances within an inlet element of a fuel cell according to the invention, Fig. 4 a section through an electrode element and the flow field plate made of Fig. 2, Fig. 5 a distribution of a catalyst on an electrode element according to the invention, Fig. 6 a further embodiment of the catalyst on the electrode element and Fig. 7 another embodiment of the electrode element according to the invention.

[0028] In Fig. Figure 1 shows a fuel cell system 100, which here comprises two fuel cells 110. These fuel cells 110 are arranged adjacent to each other in a housing 120. Each of the fuel cells 110 has two electrode elements 10, 10' arranged on a membrane element 30. By applying two different fuels – also referred to as reactants – to the electrode elements 10, 10', an electric current is generated by an electrochemical reaction. The two reactants are often provided in the form of different fluids. An example of the two corresponding electrode reactions is as follows: H2 => 2H + + 2e - (Anode reaction) 2H + + 2e - + ½ O2 => H2O (cathode reaction).

[0029] The generated electrical current can be used in a load element. The reactant oxygen can be supplied to the fuel cell 110 in the form of ambient air. By connecting the various fuel cells 110 in series via a conductor element, it is possible to achieve a high voltage, which can be supplied to the load element, such as an electric motor. To achieve a uniform distribution of the reactants on the electrode elements 10, 10', the fuel cell 110 has a flow field plate 40. The two reactants flow into the interior of the fuel cell via the inlet elements 130, 131. A fuel such as hydrogen is supplied to the fuel cell 110 via the inlet element 130. The oxidant flows into the fuel cell via the inlet element 131.To achieve a uniform distribution of the fuel over the entire surface of the electrode element 10,10', two flow field plates 40 are arranged in the fuel cell 110. A space is created between the electrode element 10,10' and the respective flow field plate 40, in which a gas diffusion layer 70 is arranged. This gas diffusion layer 70 generally consists of a pressed carbon fleece.

[0030] In Fig. Figure 2 shows a flow field plate 40. The flow field plate 40 serves to distribute the reactants over the active area of ​​the electrode element 10, 10' and thereby ensure mechanical stability. To enable this, the flow field plate 40, in the illustrated embodiment, has two flow paths 42, 42'. The two flow paths 42, 42' run approximately in a meandering pattern over an outer surface 41 of the flow field plate 40. The reactant is introduced into the flow paths 42, 42', as also illustrated by the motion arrows 44. The reactant flowing through the flow paths 42, 42' is then intended to react electrochemically with a catalyst 60, thus generating electrons and ions so that the fuel cell 110 can generate an electric current. The amount of reactant introduced into the flow paths 42,42' is generally greater than the electrochemically reacted portion.Thus, some of the reactant still flows out of the flow paths 42, 42', as illustrated by the two arrows 45. The outflowing reactant simultaneously serves to cool the flow field plate 40, which is heated by the exothermic reaction.

[0031] In the Fig. Figure 3 shows two adsorption diagrams. The x-axis (abscissa) represents position 210 within the inflow element 130,131. The y-axis (ordinate) represents the orthogonal distance 220 from the center of the inflow element 130,131. During the production of fuels such as hydrogen, a certain amount of carbon monoxide is generated, which can poison the catalyst. After contact with carbon monoxide, the catalyst can no longer be used for the catalytic conversion of the fuel gases. As the Fig. Figure 3 illustrates that the concentration of carbon monoxide depends on the position in the inlet element 130 and on a flow velocity 211,211'. To achieve the most uniform distribution of the fuel on the flow field plate 40, the fuel is guided through the inlet element 130,131 at a low flow velocity 211. In the upper half of the figure of the Fig. 3 the flow velocity is approximately 1 cm 211 3 / min. The fuel flows in a direction indicated by the flow velocity arrow 211. As can be seen, a particularly large amount of carbon monoxide is adsorbed in the initial section 200. This reduces the amount of carbon monoxide present in the fuel gas along position 210 within the inlet element 130,131. If, in contrast, a higher flow velocity 211' is selected - here 3 cm 3 / min - the adsorption is 200' lower, like the lower half of the Fig. Figure 3 illustrates this. Consequently, significantly more carbon monoxide remains within the fuel gas.

[0032] The inactivating substance – such as carbon monoxide, H₂S, or other sulfur compounds – leads to the poisoning of the catalyst. After poisoning, the catalyst can no longer be used to electrochemically generate an electric current from the reactants supplied to the fuel cell. Based on this knowledge and the above-described characteristics of the distribution of the inactivating substances, an electrode element 10, 10' designed according to the invention is proposed. The purpose of this electrode element 10 is to prevent the poisoning of the catalyst. Fig. Figure 4 illustrates the structure of the electrode element 10 according to the invention. The electrode element 10 serves for the electrochemical conversion of a fuel gas in the fuel cell 110, wherein, in an installed state, the fuel gas is introduced into the fuel cell 110 through an inlet element 130, 131. The fuel gas flows along a flow path 42 over the flow field plate 40. The flow paths 42 are delimited by webs 43. As the Fig. As illustrated in Figure 4, the gas diffusion layer 70 is arranged between the flow rock plate 40 and the electrode element 10 designed according to the invention. To achieve an electrochemical conversion of the fuel gas, the electrode element 10 is provided with a catalyst 60. As shown in Figure 4, the gas diffusion layer 70 is arranged between the flow rock plate 40 and the electrode element 10 designed according to the invention. Fig. As shown in Figure 3, the concentration of the catalyst-inactivating carbon monoxide decreases along the flow path 42. Therefore, according to the invention, the density 61 of the catalyst 60 increases along the flow path 42 in order to prevent poisoning of the catalyst 60 with the inactivating substance. Fig. 4 The amount of catalyst 60 introduced into the electrode element increases from the inlet element 130 towards the outlet 132. As in Fig. As illustrated in Figure 2, the flow path 42 can be arranged in a meandering pattern. In this case, there are two possibilities for the distribution of the catalyst on the electrode element 10. Firstly, the density 61 of the catalyst – as shown in Figure 2 – can be... Fig. As indicated in Figure 4, the density decreases uniformly vertically. In this case, the density 61 is constant for parts of the flow path 42. Only when the flow path 42 enters another meandering loop does the density 61 of the catalyst 60 decrease. It is also conceivable that the catalyst 60 is positioned in such a way that it is arranged in accordance with the meandering structure of the flow path 42. Consequently, the density 61 of the catalyst 60 on the electrode element 10 also exhibits a meandering arrangement.

[0033] In Fig. 5 is the one from Fig. Figure 4 shows the electrode element 10 as part of a membrane electrode assembly (MEA). The MEA comprises the membrane 30, which is provided on both sides with an electrode element 10, 10'. The electrode element 10' serves to convert the fuel, which acts as an oxidizing agent. Since the oxidizing agent immediately oxidizes any inactivating substances, the density 61 of the catalyst 60 does not need to be adjusted. In contrast, the increase in the density 61 of the catalyst 60 according to the invention can be seen in the electrode element 10. The flow arrow 140 is intended to illustrate the flow direction of the fuel.

[0034] In Fig. Figure 6 shows a further embodiment of the electrode element 10 according to the invention. This electrode element 10 is provided with an oxidation element 80. The oxidation element 80 serves to oxidize the inactivating substances. It is therefore a type of catalyst that serves exclusively to convert the inactivating substances into substances that do not poison the catalyst 60. Since the amount of the inactivating substance decreases along the flow path 42, the invention provides that the density 81 of the oxidation element 80 is reduced along the flow path.

[0035] In Fig. Figure 7 shows a further embodiment of the fuel cell 110 according to the invention. In contrast to the embodiment shown in Fig.As shown in Figure 6, the oxidation element 80 is integrated into the gas diffusion layer 70. The fuel containing the inactivating substances flows from the inlet element 130 into the fuel cell 110 in the direction of the flow arrow 140. A portion of the inactivating substances also flows into the gas diffusion layer 70. The oxidation element 80 ensures that the inactivating substances are oxidized before they can reach the catalyst 60. This ensures that the catalyst 60 is not poisoned by the inactivating substances.

Claims

[1] Fuel cell (110) with at least two electrode elements (10, 10') and a membrane element (30), wherein the membrane element (30) is arranged between the two electrode elements (10, 10'), in an electrochemical reaction at least one fuel gas flows along at least one flow path (42, 42') over at least one electrode element (10, 10'), at least one of the electrode elements (10, 10') is provided with a catalyst (60), wherein a density (61) of the catalyst (60) increases along the flow path (42, 42') in order to prevent poisoning of the catalyst (60) with inactivating substances, wherein the fuel gas can be guided to the electrode element by a feed element (130, 131), characterized by , that a water (H2O) adsorbing adsorbent is arranged in and / or on the inflow element (130,131). [2] Fuel cell (110) according to claim 1, characterized by, that the fuel cell (110) has an oxidizing element (80) to oxidize the inactivating substances, wherein in particular the density (81) of the oxidizing element (80) decreases along the flow path (42,42'). [3] Fuel cell (110) according to claim 2, characterized by , that the oxidation element (80) is arranged on the electrode element (10,10') and / or the membrane element (30) and / or a gas diffusion layer (70). [4] Fuel cell (110) according to claim 2 or 3, characterized by , that the oxidation element (80) is arranged in and / or on the inflow element (130,131). [5] Fuel cell (110) with at least two electrode elements (10, 10') and a membrane element (30), wherein the membrane element (30) is arranged between the two electrode elements (10, 10'), in an electrochemical reaction at least one fuel gas flows along at least one flow path (42, 42') over at least one electrode element (10, 10'), at least one of the electrode elements (10, 10') is provided with a catalyst (60), wherein a density (61) of the catalyst (60) increases along the flow path (42, 42') in order to prevent poisoning of the catalyst (60) with inactivating substances, wherein the fuel gas can be guided to the electrode element by a feed element (130, 131), characterized by , that a trapping layer is arranged in and / or on the inflow element (130,131), wherein the trapping layer serves to oxidize the inactivating substances, in particular that the trapping layer is arranged in the region of the electrode element (10,10'). [6] Fuel cell (110) according to claim 5, characterized by , that the trapping layer is composed of a membrane, wherein the fuel cell (110) has an oxidizing element (80) to oxidize the inactivating substances, wherein in particular the density (81) of the oxidizing element (80) decreases along the flow path (42,42'), wherein the membrane is provided with the oxidizing element (80), in particular that the trapping layer is composed of an ionomer. [7] Fuel cell system (100), characterized by , that the fuel cell system (100) comprises at least one fuel cell (110) according to any one of claims 1 to 6.

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