DE102007061061A1 - Fuel cell stack for traction system of motor vehicle, has hydrogen supply line for supplying hydrogen to anodes and dummy cell, and hydrogen discharge line for removing residual hydrogen from anode and dummy cell - Google Patents

Abstract

The stack (10) has a set of solitary cells that are arranged in stacks, where each cell has a membrane-electrode-unit (12) with one proton-conductive membrane (14), a cathode (16) and an anode (18). Bipolar plates are arranged at the unit (12). A dummy cell (22) is arranged between the solitary cells and exhibits a hydrogenate material that is suitable to form hydrogen under formation of a hydride. A hydrogen supply line (24) supplies hydrogen to the anodes and dummy cell, and a hydrogen discharge line removes residual hydrogen from the anode and dummy cell.

Description

The The invention relates to a fuel cell stack having a plurality Stacked arranged single cells consisting of membrane-electrode units and bipolar plates and a hydrogen transport system for delivery from hydrogen to the bipolar plates and dissipation of Residual hydrogen from these.

Fuel cells use the chemical transformation of hydrogen and oxygen into water to generate electrical energy. For this purpose, fuel cells contain as core component the so-called membrane electrode assembly (MEA for membrane electrode assembly), which is a composite of a proton-conducting membrane and in each case one on both sides of the membrane arranged electrode (anode and cathode). In general, a fuel cell stack (stack) is formed by a plurality of stacked arranged individual cells, each having an MEA and on both sides of these subsequent bipolar plates and their electrical services add up. During operation of the fuel cell, hydrogen H ₂ or a hydrogen-containing gas mixture is fed to the anode, where an electrochemical oxidation of H ₂ to H ⁺ takes place with release of electrons. Via the membrane, which separates the reaction spaces gas-tight from each other and electrically isolated, takes place (water-bound or anhydrous) transport of protons H ⁺ from the anode compartment in the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line. The cathode is further supplied with oxygen or an oxygen-containing gas mixture, so that a reduction of O ₂ to O ² takes place with absorption of the electrons. At the same time, these oxygen anions in the cathode compartment react with the protons transported via the membrane to form water. The direct conversion of chemical into electrical energy fuel cells achieve over heat engines due to the circumvention of the Carnot factor improved efficiency.

The Each electrode has a catalyst layer facing the membrane often on a gas-permeable substrate, the so-called gas diffusion layer (GDL for gas diffusion layer), for the homogeneous supply of the reaction gases is applied. The catalyst layer contains reactive centers which are in usually consists of platinum as a catalytically active component, which is on an electrically conductive porous carrier material, For example, carbon particles, supported.

Between The MEAs are arranged so-called bipolar plates, the supply the reaction gases and the removal of the residual gases as well serve the product water and also the electrical contact of the MEAs. The structure of MEA and the two sides Bipolar plates adjoining the MEA represents the single cell which, in a stacked arrangement, the fuel cell stack forms.

The currently the most advanced fuel cell technology based on polymer electrolyte membranes (PEM) in which the membrane itself consists of an ionic conductive polymer. The most common PEM based on a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion; Copolymer of tetrafluoroethylene and a sulfonyl fluoride derivative a perfluoroalkyl vinyl ether) or on Nafion-analogous plastics. The electrolytic line takes place via hydrated Protons take place, which is why for the proton conductivity the presence of liquid water is a condition from which a number of disadvantages, especially when using the fuel cell in traction systems of motor vehicles. That's how it works the PEM fuel cell requires moistening of the operating gases, which means a high system overhead. Does it come to a failure of the humidification system, are power losses and irreversible Damage to the membrane-electrode assembly the result. Furthermore, the maximum operating temperature of these fuel cells - too due to the lack of thermal stability of the Membranes - limited to below 100 ° C at standard pressure (Which is why this type of fuel cell in the present case as a low-temperature PEM fuel cell (NT-PEM fuel cell) is called). For the mobile as well as the stationary use are operating temperatures desirable above 100 ° C.

To overcome these problems, high temperature polymer electrolyte membrane fuel cells (HT-PEM or HTM fuel cells) have been developed which operate at operating temperatures of 120 to 180 ° C and which require little or no humidification. The electrolytic conductivity of the membranes used in these second-generation fuel cells is based on liquid electrolytes bound to the polymer backbone by electrostatic complex bonding, in particular acids or bases which ensure proton conductivity even when the membrane is completely dry above the boiling point of water. For example, poly (benzimidazole) high-temperature membranes (PBI) complexed with acids such as phosphoric acid, sulfuric acid or others are present in US Pat US 5,525,436 . US 5,716,727 . US 5,599,639 . WO 01/18894 A . WO 99/04445 A . EP 0 983 134 B and EP 0 954 544 B described.

For a use as a vehicle drive is the cold start capability desirable for fuel cells at low temperatures, which ideally starts at temperatures around -40 ° C. At temperatures below freezing come from water in addition to the reduced conductivity of the membranes problems due to icing. While at Nafion-based Fuel cells cold start at temperatures around -20 ° C this has been possible with HTM fuel cells so far not yet realized. There is also a quick heating up to the respective optimum operating temperature of the fuel cell desirable in which there is optimum efficiency and at all fuel cell types well above room temperature lies. This is the necessary operating temperature of (nafion-based) NT-PEM fuel cells at 80 to 90 ° C and in so-called solid oxide fuel cells (SOFC) even at 800 to 1000 ° C. Because at low temperatures The power density of the fuel cell is very low, also finds no appreciable self-heating due to the exothermic fuel cell reactions instead of. Therefore, an additional energy supply for heating required.

On On the other hand, the fuel cell stack must be exceeded the optimum operating temperature to be cooled To prevent damage. For this purpose is in fuel cell systems a cooling circuit with cooler provided.

Out EP 1 351 330 A2 a fuel cell is known whose bipolar plates, in particular the flow channels for the process gases thereof, consist at least partially of a material or are coated with a material which forms a hydride in an exothermic hydrogenation reaction, as a result of which the reaction gases are heated as a result of the heat release, which leads to heating of the MEA. In a preferred embodiment, this internal heating measure is combined with further, external or internal measures, such as electrical heating. However, the coating of the bipolar plates with a corresponding material for manufacturing reasons has proven to be costly.

Of the Invention is therefore based on the object, a fuel cell stack to provide that over known systems a improved cold start capability and a fast heating behavior to the operating temperature, and at the same time easy to manufacture is.

• (a) eine Mehrzahl gestapelt angeordneter Einzelzellen, die jeweils eine Membran-Elektroden-Einheit (12) mit einer protonenleitfähigen Membran (14) und zwei beidseitig an die Membran (14) anschließenden Elektroden (16, 18), von denen eine als Kathode (16) und die andere als Anode (18) geschaltet ist, aufweisen sowie seitlich an die Membran-Elektroden-Einheit (12) anschließende Bipolarplatten (20),
• (b) zumindest eine (membran- und elektrodenfreie) Blindzelle, die zwischen den (aus MEA und Bipolarplatten bestehenden) Einzelzellen und/oder endständig an dem Stapel der Einzelzellen angeordnet ist und die zumindest ein hydrierbares Material aufweist, das geeignet ist, temperatur- und/oder druckabhängig in einer reversiblen exothermen Hydrierungsreaktion Wasserstoff unter Bildung eines Hydrides zu binden, und
• (c) ein Wasserstofftransportsystem zur Zuführung von Wasserstoff zu den Anoden der Membran-Elektroden-Einheiten und zu der zumindest einen Blindzelle und zur Abführung von Restwasserstoff aus diesen, das als gemeinsames Versorgungssystem ausgestaltet ist oder als ein unabhängig für die Membran-Elektroden-Einheiten und die zumindest eine Blindzelle betreibbares Versorgungssystem.

This object is achieved by a fuel cell stack with the features of claim 1. The fuel cell stack according to the invention comprises

• (a) a plurality of stacked single cells each comprising a membrane-electrode assembly ( 12 ) with a proton-conductive membrane ( 14 ) and two on both sides of the membrane ( 14 ) subsequent electrodes ( 16 . 18 ), one of which as a cathode ( 16 ) and the other as anode ( 18 ), and laterally to the membrane-electrode unit ( 12 ) subsequent bipolar plates ( 20 )
• (B) at least one (membrane and electrode-free) dummy cell, which is arranged between the (consisting of MEA and bipolar plates) single cells and / or terminal to the stack of single cells and which has at least one hydrogenatable material which is suitable, temperature and to bind hydrogen to form a hydride in a reversible exothermic hydrogenation reaction; and / or pressure dependent
• (c) a hydrogen transport system for supplying hydrogen to the anodes of the membrane-electrode assemblies and to the at least one dummy cell and discharging residual hydrogen therefrom, configured as a common supply system or as independent for the membrane-electrode assemblies and the at least one dummy cell operable supply system.

The Hydrogenatable material produced by the hydrogen transport system Hydrogen is contacted with this reacts to a Hydride, due to the exothermic nature of the reaction heat is released, causing a rapid warming of the Systems leads. Above a substance-dependent Desorption temperature of the material is again an endothermic Release of hydrogen and cooling in the Operation of the fuel cell used for cooling the system can be.

According to the invention, an additional heat source is thus introduced into the stack by a further exothermic chemical reaction, which does not correspond to the actual, electricity-generating fuel cell reaction, which provides for additional heating. The dummy cells according to the invention have a simple structure and in particular dispense with the installation of the expensive membrane and the electrodes, that is to say they do not comprise MEA. You only need to have a hydrogen supply that can be realized on the general supply system of the anodes or separately seen from this before. In addition, they have the advantage that they can be installed with conventional components of fuel cells and no modifications to the standard components must be made. By their arrangement adjacent to the bipolar plates, a rapid heating of the bipolar plates and thus the preheating of the reaction gases flowing through them are also achieved. This way will very quickly, the anode and / or cathode reaction of individual MEAs set in motion, which in turn gives off heat and thus causes further heating of the entire fuel cell stack to its operating temperature. As a result, a rapid start of the fuel cell according to the invention at low temperatures is achieved, wherein the lower starting temperature of the hydrogen absorption region of the hydrogenation material used or the combination of several hydrogenatable materials depends. It is also advantageous that the heating takes place "fuel-neutral", since the absorbed hydrogen can be released again when it reaches a substance-specific desorption and used as the operating gas of the fuel cell.

Regarding their arrangement allows the dummy cells regularly or irregular over the entire stack distributed. Preference is given to a regular Distribution within the stack, for example after every fifth an MEA-containing single cell or the like. Alternatively or In addition, the dummy cells can also be terminal be arranged on the stack adjacent to there existing end plates.

The Bipolar plates placed between the membrane-electrode units and / or the at least one dummy cell are arranged, serve the uniform Distribution of hydrogen over the area of the Anodes or of interposed gas diffusion layers of the MEAs and the dummy cells and the removal of residual hydrogen. (At the same time usually takes over the cathode side the bipolar plates supply the cathodes with oxygen (air) and the removal of residual oxygen and product water.) According to a preferred embodiment comprises the hydrogen transport system hydrogen supply lines to the supply line from hydrogen to the bipolar plates and hydrogen discharges for the discharge of residual hydrogen from these. In an advantageous addition The invention may also include the hydrogen transport system, i. H. the Hydrogen supply lines and / or the hydrogen derivatives, and / or the bipolar plates have at least one hydrogenatable material.

The Type of incorporation of the hydrogenatable material can be varied Way done. In particular, this can be at least one hydrogenatable Material in the form of a coating on the corresponding hydrogen leading elements of the dummy cell (s) and / or the hydrogen transport system, For example, as an inner coating of the conduit system or be formed by gas channels or webs in the bipolar plate. Alternatively, the material may also be in the form of material inclusions the at least one dummy cell and / or the hydrogen transport system available. Likewise, these parts can at least partially or completely from the at least one hydrogenatable material be prepared. Also, combinations of the aforementioned embodiments conceivable. Within the dummy cell (s), the hydrogenatable material also in powder form or in the form of coatings more porous Materials, such as sintered materials, fabrics or the like present, with the largest possible surface is sought.

Of the Fuel cell stack according to the invention is with Advantage of the initially described HT-PEM type, on a membrane based on an electrolyte-impregnated polymer material consists. However, it can also be an NT PEM fuel cell whose membrane is deposited on an ionically conductive (Nafion or Nafion-analogous) polymeric material, or a solid oxide fuel cell (Solid oxide fuel cells, SOFC) or other fuel cell types. In any case, the advantageous ability of the invention comes Fuel cell stack used to start the fuel cell and / or to heat up to their operating temperature, which also clearly above room temperature (80-90 ° C for NT-PEM fuel cells and 800-1000 ° C at SOFC).

The Hydride formation is - at a given pressure and temperature - a Spontaneous exothermic process. The hydride formation is temperature dependent, with increasing temperature, the reaction rate increases. Once the reaction has started, it accelerates by the energy released autocatalytically until it reaches the reverse temperature. The substance-specific inversion temperature is the Temperature at which the hydride formation stops and the (endothermic) desorption of hydrogen begins. The way this way released hydrogen is thus either the energy by the fuel cell reaction or re-absorption through another hydrogenatable material available. In the total balance over the operating time runs the cold start is fuel neutral. The endothermic desorption of Although used materials causes a certain amount of cooling, however, this is due to the onset of the fuel cell reaction, which also leads to a warming of the system, compensated.

The reversibility of the hydrogenation of the material is an important prerequisite, since only repeatable hydrogenation (loading) and dehydration (discharge) allows the repeated application of the fuel cell cold start material. In order to have a sufficient amount of hydrogenatable material available for the next cold start, the most complete possible dehydration of the material is therefore prior to stopping required for the system. The absorption and desorption does not proceed without hysteresis, that is, at the same temperature, the absorption pressure is usually higher than the desorption pressure. Therefore, the desorption is preferably carried out at a lower pressure than the absorption. Another parameter for shifting the equilibrium in favor of the desorption reaction when shutting down the system is given by an increased temperature, which is ensured by the increased operating temperature of the MEA.

According to one preferred embodiment of the invention is in the at least one hydrogenatable material around a metal or a metal alloy, which are capable of cryogenic hydride formation. Especially For this metals come from the group Ti, Fe, Cr and / or Zr in question, optionally as further constituents Ca, Mg, Cu, Ni and / or Mn. Concrete examples are in called the embodiments.

The most materials have no absorption temperature range, from very low temperatures to the operating temperature the fuel cell is enough. According to a further advantageous embodiment Therefore, according to the invention, the hydrogenatable material comprises two or more Hydrogenatable materials whose absorption temperature ranges themselves cascade overlapping, so they together one preferably complete total absorption temperature range form. In this way, a cold start of the fuel cell can be made done almost any temperature. At the same time can through appropriate selection and combination of materials due to the over the overall temperature range continuous hydrogenation a heating up of the system up to a certain minimum operating temperature respectively. Thus, a combination with other heating methods, For example, in the form of electrical heating, be waived, although Such combinations are not excluded within the scope of the invention are.

It is preferably provided that the at least one hydrogenatable material or in case of several hydrogenatable materials at least one the materials chosen so that its absorption temperature range at -10 ° C, especially at at most -20 ° C, preferably at -30 ° C at most starts. In particular for applications in motor vehicles lends itself to the use of a material that already has temperatures at -40 ° C or even below hydrogen below Heat dissipation absorbs, leaving a cold start already off this temperature is possible. Furthermore, the should Hydrogen at the latest when the operating temperature is reached, which depends on the fuel cell type between 60 and 1000 ° C can be released again and then the fuel cell are available again - either for the actual fuel cell reaction or for a renewed hydrogenation cycle of the material for heating purposes.

Around a complete heating of the fuel cell stack from the initial temperature to the temperature at which sufficient Performance of the MEA is guaranteed, to achieve the hydrogenatable materials are preferably so chosen that their absorption temperature ranges together one Cover total absorption temperature range, at least approximately to a lower operating temperature of the MEA is sufficient. Especially It is preferable that the upper limit temperature of the total absorption temperature range up to 10 K below the lower operating temperature of the MEA, preferably to 5 K below the lower operating temperature. It is in this case, the lower operating temperature than that temperature the MEA defines at least 50% of its maximum electrical Performance available. In this way, when reaching the top Limit temperature of the total absorption temperature range already some performance of the MEA, which in turn is another Warming leads. In particular, the materials chosen so that the total absorption temperature range in the case of an HT-PEM fuel cell from -40 to 180 ° C is enough for an NT-PEM fuel cell of -40 ° C up to 90 ° C and at a SOFC of -40 ° C up to 1000 ° C.

It it is further preferred to select the hydrogenatable material so that its absorption or desorption pressure substantially within the operating pressure range of the fuel cell stack is because operation of the fuel cell under too high pressure this can permanently damage. In this context can be beneficial be provided a pressure-controlled hydrogen transport system. In this way it can be applied to the desorption of hydrogen be reacted by the hydrogenated material resulting pressure, for example, by not supplying hydrogen to the MEAs for so long until the released hydrogen is consumed.

So that in a subsequent cold start of the fuel cell stack enough hydrogen-free material for a new hydrogenation is available, it must be ensured during operation of the fuel cell, that the material is as completely as possible dehydrated. If this is not automatically the case, for example because the system has not yet reached the desorption temperature of the material, it is preferably provided that Allow the system to run on until the required temperature is reached. Only then will the system be shut down. Alternatively, through the use of a sufficiently large amount of the hydrogenatable material, a corresponding buffer can be set up, which ensures repeated charging with hydrogen, even if a preliminary discharge does not take place.

By Enabling the start of the fuel cell stack very low temperatures, that is for vehicle-relevant Temperatures, this can be particularly beneficial in mobile applications, as in traction systems of motor vehicles or for additional energy supply as so-called APU (for auxiliary power unit) in motor vehicles be used. Of course he is the same can be used for stationary applications, in particular for small power plants or domestic energy supply facilities or as a battery replacement for electronic devices.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

The Invention will be described below in embodiments explained the accompanying drawing. The only FIG. 1 shows a schematic representation of a fuel cell stack according to an advantageous embodiment of the invention with dummy cells installed in the stack.

In the figure is a fuel cell stack 10 shown comprising a plurality of series-connected single cells. Each single cell has a membrane-electrode assembly 12 (MEA), each having a proton-conducting membrane 14 which is in particular a polymer electrolyte membrane formed from a suitable polymer material. In the case of an HT-PEM fuel cell is an impregnated with an electrolyte membrane, for example from the group of polyazoles and polyphosphazenes, in particular PBI, which is impregnated with phosphoric acid or other. However, the present invention is not limited to HT-PEM fuel cells. Every MEA 12 includes fer ner two adjoining the two outer membrane surfaces electrodes 16 . 18 namely, an electrode connected as a cathode 16 on the cathode side of the membrane 14 and an electrode connected as an anode 18 on anode side. These each comprise a non-illustrated microporous catalyst layer, which is the polymer electrolyte membrane 14 contacted on both sides. The catalyst layers contain as actual reactive centers of the electrodes, a catalytic material, which is usually a noble metal as a catalytically active substance, such as platinum, iridium or ruthenium or transition metals, such as chromium, cobalt, nickel, iron, vanadium or Tin or mixtures or alloys of these. The catalytic substance is preferably fixed on a porous, electrically conductive carrier material. About the support material of the catalyst layers is an electrically conductive connection of the reaction centers of the electrodes 16 . 18 realized with an external circuit (not shown). Unless the electrodes 16 . 18 are designed as gas diffusion electrodes, they also each comprise a gas diffusion layer (GDL for gas diffusion layer, also not shown), which at the respective outer, of the polymer membrane 14 connect opposite surfaces of the catalyst layer. Function of the GDL is to ensure a uniform flow of the catalyst layers with the reaction gases oxygen or air on the cathode side and hydrogen on the anode side.

The single cells further comprise between the MEAs 12 arranged bipolar plates 20 , which provide both sides of the MEA compound electrically contacting for the supply of process gases and the discharge of the product water and also the individual MEAs 12 in the fuel cell stack 10 separate gas-tight from each other. The bipolar plates 20 have a plurality of inner transport channels, which serve to supply the reaction gases (in the case of the anode hydrogen and in the case of the cathode oxygen or air) and the cathode side also the removal of the product water. Materials for sealing and stabilizing the MEAs 12 are not shown.

According to the invention is in the fuel cell stack 10 at least one single cell through a so-called dummy cell 22 replaced. In the example shown, a total of two dummy cells 22 available. These are components with comparable dimensions as the single cells, but they have neither a membrane nor electrodes. In material terms, the dummy cells can 22 consist of the same material as the other stack components, in particular stainless steel or graphite. In a preferred embodiment, the dummy cells are 22 hollow bodies which have in their interior at least one rever sibel and exothermic hydrogenatable material whose hydrogen absorption for heating the MEAs 12 or the fuel cell 10 is being used. The material may, for example, be present as a solid porous body or as a coating of such. The dummy cells 22 be just like the MEAs 12 containing cells of two bipolar plates 20 enclosed on the side, being one of the two Be supplied with hydrogen. Details of the hydrogenatable material will be described below.

The fuel cell stack 10 also has hydrogen supply lines 24 on which the bipolar plates 20 Feed hydrogen gas. An inner anode-side channel system of the bipolar plates 20 directs the supplied hydrogen to the dummy cells 22 and the anodes 18 the membrane electrode units 12 to where it is oxidized to protons. About hydrogen discharges 26 connected to another anode-side internal channel system of the bipolar plates 20 In connection, the unused residual hydrogen (and through the membrane 14 diffused product water) and recycled. Furthermore, air supply lines 28 provided with which air and thus oxygen in the bipolar plates 20 and from there via a cathode-side channel system of the same the cathode 16 is forwarded. Via another cathode-side channel system of the bipolar plates 20 and connected air discharges 30 the discharge of the remaining air and the product water takes place.

The stack of MEAs 12 , Dummy cells 22 and bipolar plates 20 becomes laterally from end plates 32 limited.

In the illustrated embodiment, the hydrogen supply of the dummy cells takes place 22 via the same hydrogen supply lines 24 and the hydrogen discharges 26 that includes the MEAs 12 supply. That means the dummy cells 22 be over the appropriate bipolar plates 20 supplied with hydrogen, which is supplied to the hydrogenatable material and binds to this. Unconsumed hydrogen or hydrogen released from the hydrogenatable material is passed over the corresponding bipolar plates 20 and about the hydrogen discharges 26 derived and recycled.

In an alternative embodiment it can be provided that the dummy cells 22 supplying hydrogen supply lines are equipped with controllable valves, whereby one of the MEAs 12 independent supply of the dummy cells 22 is possible. In a further variant, the hydrogen supply system for the dummy cells can also be completely derived from the hydrogen supply system of the MEAs 12 be separated.

In a further modification of the invention, the dummy cells 22 and the associated bipolar plates 20 also present as an integrated component. Essential for the dummy cells according to the invention is only the presence of hydrogenatable material and the hydrogen supply and removal. The type of hydrogen transport within the dummy cell 22 or the associated bipolar plate 20 may be configured as desired, for example, parallel-guided or meandering hydrogen channels may be present or the hydrogen supply via porous materials, in particular sintered materials.

According to a further embodiment, hydrogen-carrying elements of the bipolar plates can also be used 20 and / or the hydrogen supply lines 24 and / or the hydrogen derivatives 26 have a hydrogenatable material, such as in the form of coatings or arbitrarily large material inclusions. Alternatively, some of these elements may also be made directly from such a material.

at the hydrogenation material (s) used is preferably to metals or metal alloys that are at temperatures below of freezing in contact with hydrogen cryogenic hydrides form in an exothermic reaction. There are in particular such Materials used as the main alloying components titanium (Ti), iron (Fe), zirconium (Zr), chromium (Cr) or a mixture of these included. Suitable materials are summarized in Table 1 compiled with some of their physical parameters.

The amount of hydrogenatable material - or their total amount in the case of several hydrogenatable materials - is determined by the amount of heat used to heat the MEAs 12 is required to their minimum operating temperature or a few Kelvin below, it is preferably assumed that the lowest possible starting temperature at -40 ° C, for example. In particular, the amount of heat provided by the hydrogenation must provide the heat of fusion for the frozen water / electrolyte phase and be sufficient to heat surrounding layers to the target temperature. The amount of heat that can be used for these purposes can be determined using the substance-specific reaction enthalpy of the hydrogenation reaction and the amount of hydrogen absorbed. Examples of suitable hydrides and their enthalpies of reaction are listed in Table 1 and typically range from 15 to 75 kJ / mol _H2 (per mol of H ₂ absorbed). About the mass uptake of hydrogen, which is between 2 to 5% (Table 1, absorption capacity), the necessary mass of the / of the hydrogenatable materials can be determined.

In preferred embodiment is a mixture of several hydrogenatable Materials used, allowing as possible the entire temperature range starting from the lowest possible starting temperatures up to Reaching a target temperature at or just below the minimum Operating temperature of the MEA through the absorption areas of the materials is covered.

This allows a cold start from different starting temperatures and the system can be heated to the target temperature, without the need for a combination with other heating methods. The selection of materials is from the viewpoint of staggered cascade-like temperature behavior with respect to absorption and desorption, with an overlap of the absorption temperature ranges of at least 10 K being desirable (Table 1). For example, a mixture of Ti-Zr-Cr-Fe (absorption range -60 ... -20 ° C), Mn-Ca-Ni-Al (-40 ... + 40 ° C), Ti-Cr-V- Fe (-10 ... + 80 ° C) and Ti-Zr-Mn-V-Fe (+40 ... + 160 ° C) are used. At a start at -40 ° C, hydrogenation of Ti-Zr-Cr-Fe initially takes place, which stops when a temperature of -20 ° C is reached. There is a desorption of hydrogen from Ti-Zr-Cr-Fe, which, however, re-absorbed by Mn-Ca-Ni-Al. This cascade is continued until a ΔT of preferably ≥ 5 K below the minimum operating temperature is reached in the reaction zone and the electrical load is switched on. As the operation of the fuel cell 10 For example, if it now runs at 80% of its maximum capacity and also causes warming as a result of internal losses, further heating will take place automatically. In this way, a continuous rise of the temperature starting from -40 ° C or lower up to the operating temperature of 160 ° C or even 180 ° C is possible. Table: Properties of commercially available metal hydrides (manufacturer information) alloy Typical composition Absorptive capacity [wt%] Equilibrium pressure [MPa] Reaction heat [kcal / mol _H2 ] Temperature range [° C] Ti-Fe-C TiFe _0.8 C _0.1 1.9 0.15 (20 ° C) -6.1 10 ... 50 Ti-Fe-Ta TiFe _0.96 Ta _0.01 1.7 0.20 (25 ° C) -6.2 10 ... 50 Ti-Fe-Ca TiFe _0.93 Ca _0.03 1.7 0.15 (50 ° C) -6.7 20 ... 60 Ti-Fe-Ca-Mn TiFe _{0 . 8} Mn _{0 . 15} Ca _0.01 1.7 0.50 (60 ° C) -6.8 20 ... 60 Ti-Cr-Fe TiCr _1.3 Fe _0.5 1.8 0,50 (-60 ° C) -5.1 <-40 Ti-Zr-Cr-Fe Ti _0.7 Zr _0.3 Cr _1.3 Fe _0.5 1.9 0.15 (-60 ° C) -5.9 -60 ... -20 Ti-Zr-Cr-Fe-Mn-Cu Ti _0.6 Zr _0.4 Cr _1.0 Fe _0.6 Mn _0.4 Cu _0.03 1.7 0.40 (20 ° C) -6.9 -20 ... 60 Zr-Ti-Cr-Fe-Mn-Cu Zr _0.7 Ti _0.3 Cr _1.0 Fe _0.6 Mn _0.4 Cu _0.03 1.6 0.35 (110 ° C) -8.6 50 ... 150 Ti-Mn-V TiMn _1.43 V _0.43 2.0 0.61 (40 ° C) -6.9 -20 ... 60 Ti-Zr-Mn-V-Fe Ti _0.8 Zr _0.2 Nn _1.42 V _0.52 Fe _0.09 1.8 0.20 (45 ° C) -7.5 40 ... 160 Ca-Mn-Ni-Al Ca _0.85 Mn _0.15 Ni _4.85 Al _0.15 1.5 0.18 (50 ° C) -7.8 20 ... 60 Mn-Ca-Ni-Al Mn _0.7 Ca _{0 . 3} Ni _4.7 Al _{0 . 3} 1.5 0.40 (-40 ° C) -6.5 -40 ... 40 Ti-Cr-V Ti ₂₀ Cr ₃₀ V ₅₀ 2.4 0.04 (20 ° C) -10.0 0 ... 80 Ti-Cr-V-Fe Ti _27.8 Cr _4.22 V ₂₅ Fe ₅ 2.2 0.29 (-10 ° C) -9.6 -10 ... 80

10

fuel cell stack

12

Membrane-electrode assembly (MEA)

14

membrane

16

Electrode (cathode)

18

electrode (Anode)

20

bipolar

22

dummy cell

24

Hydrogen supply lines

26

Hydrogen derivatives

28

air lines

30

air derivatives

32

endplate

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 5525436 [0006]
• US 5716727 [0006]
• US 5599639 [0006]
• WO 01/18894 A [0006]
• WO 99/04445 A [0006]
• EP 0983134 B [0006]
• EP 0954544 B [0006]
• - EP 1351330 A2 [0009]

Claims (9)

Fuel cell stack ( 10 ) comprising (a) a plurality of stacked single cells, each comprising a membrane-electrode assembly ( 12 ) with a proton-conductive membrane ( 14 ) and two on both sides of the membrane ( 14 ) subsequent electrodes ( 16 . 18 ), one of which as a cathode ( 16 ) and the other as anode ( 18 ), and to the membrane-electrode unit ( 12 ) subsequent bipolar plates ( 20 ), (b) at least one dummy cell ( 22 ), which is arranged between the individual cells and / or terminally on the stack of the individual cells and which has at least one hydrogenatable material which is suitable for binding temperature and / or pressure in a reversible exothermic hydrogenation reaction hydrogen to form a hydride, and ( c) a hydrogen transport system ( 24 . 26 ) for supplying hydrogen to the anodes ( 18 ) of the membrane-electrode assemblies ( 14 ) and to the at least one dummy cell ( 22 ) and for removal of residual hydrogen therefrom, which is designed as a common supply system or as an independent for the membrane-electrode units ( 14 ) and the at least one dummy cell ( 22 ) operable supply system. Fuel cell stack ( 12 ) according to claim 1, characterized in that the hydrogen transport system ( 24 . 26 ) Hydrogen feed lines ( 24 ) for supplying hydrogen to the bipolar plates ( 20 ) and hydrogen derivatives ( 26 ) for the removal of residual hydrogen from these. Fuel cell stack ( 12 ) according to claim 1 or 2, characterized in that the bipolar plates ( 20 ) and / or the hydrogen supply lines ( 24 ) and / or the hydrogen derivatives ( 26 ) also have at least one hydrogenatable material. Fuel cell stack ( 12 ) according to claim 1 or 3, characterized in that the at least one hydrogenatable material in the form of a coating and / or in the form of material inclusions of the at least one dummy cell ( 22 ) and / or the hydrogen transport system or these parts consist at least in sections of the at least one hydrogenatable material. Fuel cell stack ( 10 ) according to one of the preceding claims, characterized in that the dummy cells ( 22 ) at regular intervals or irregularly over the fuel cell stack ( 10 ). Fuel cell stack ( 10 ) according to one of the preceding claims, characterized in that the at least one hydrogenatable material is a metal capable of forming low-temperature hydride or a metal alloy which contains in particular a metal selected from the group consisting of Ti, Fe, Cr and Zr and optionally Ca, Mg, Cu, Ni and / or Mn. Fuel cell stack ( 10 ) according to one of the preceding claims, characterized in that the hydrogenatable material comprises two or more hydrogenatable materials whose absorption temperature ranges overlap in a cascade, so that together they form a Gesamtabsorptionstemperaturbereich. Fuel cell stack ( 10 ) according to one of the preceding claims, characterized in that the at least one hydrogenatable material or in the case of several hydrogenatable materials at least one of the materials is chosen so that its absorption temperature range at -10 ° C, especially at -20 ° C, preferably at at most -30 ° C begins. Fuel cell stack ( 10 ) according to claim 6 or 7, characterized in that the absorption temperature ranges of the hydrogenatable materials together cover a Gesamtabsorptionstemperaturbereich which at least approximately up to a lower operating temperature of the membrane-electrode units ( 12 ) ranges, insbesonde re to 10 K below the lower operating temperature, preferably to 5 K below the lower operating temperature.