Patent ID: 12255361

DETAILED DESCRIPTION

FIGS.1to3depict the basic structure of a fuel cell2as a PEM fuel cell3(polymer electrolyte fuel cell3). The principle of fuel cells2is that electric energy or electric current is produced by means of an electrochemical reaction. Hydrogen is conducted as gaseous fuel to an anode7and the anode7forms the minus pole. A gaseous oxidant, namely air with oxygen, is supplied to a cathode8, i.e. the oxygen in the air provides the required gaseous oxidant. A reduction (uptake of electrons) takes place at the cathode8. The oxidation as loss of electrons occurs at the anode7.

The redox equations of the electrochemical processes are:
O2+4H++4e−→2H2O  Cathode:
2H2→4H++4e−Anode:
2H2+O2→2H2O  Overall reaction equation of cathode and anode:

The difference between the standard potentials of the electrode pairs under standard conditions as reversible fuel cell voltage or open circuit voltage of the fuel cell2under no load is 1.23 V. This theoretical voltage of 1.23 V is not achieved in practice. In the rest state and at small currents, voltages above 1.0 V can be achieved and during operation at higher currents, voltages between 0.5 V and 1.0 V are attained. The connection series of a plurality of fuel cells2, in particular a fuel cell unit1as fuel cell stack1of a plurality of superposed fuel cells2has a higher voltage which corresponds to the number of fuel cells2multiplied by the individual voltage per fuel cell2.

The fuel cell2additionally comprises a proton exchange membrane5(PEM) which is arranged between the anode7and the cathode8. The anode7and cathode8are configured as layers or disks. The PEM5functions as electrolyte, catalyst support and separator for the reaction gases. The PEM5additionally functions as electrical insulator and prevents an electric short circuit between the anode7and cathode8. In general, proton-conducting films composed of perfluorinated and sulfonated polymers and having a thickness of from 50 μm to 150 μm are used. The PEM5conducts the proton H+and blocks ions other than protons H+to a substantial extent, so that charge transport can occur on the basis of the protons H+owing to the permeability of the PEM5. The PEM5is essentially impermeable to the reaction gases oxygen O2and hydrogen H2, i.e. it blocks the flow of oxygen O2and hydrogen H2between a gas space31at the anode7with fuel hydrogen H2and the gas space32at the cathode H with air or oxygen O2as oxidant. The proton conductivity of the PEM5increases with increasing temperature and increasing water content.

The electrodes7,8as the anode7and cathode8are present on the two sides of the PEM5, in each case facing the gas spaces31,32. A unit made up of the PEM5and the electrodes6,7is referred to as membrane electrode assembly6(MEA). The electrodes7,8are pressed together with the PEM5. The electrodes6,7are platinum-containing carbon particles which are bound to PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene-propylene copolymer), PFA (perfluoroalkoxy), PVDF (polyvinylidene fluoride) and/or PVA (polyvinyl alcohol) and are pressed hot into microporous carbon fiber, glass fiber or polymer fiber mats. A catalyst layer30is normally applied to the electrodes6,7on the side facing the gas spaces31,32. The catalyst layer30at the gas space31with fuel at the anode7comprises nanodisperse platinum-ruthenium on graphitized carbon black particles which are bound to a binder. The catalyst layer30at the gas space32with oxidant at the cathode8analogously comprises nanodisperse platinum. As binder, use is made of, for example, Nafion®, a PTFE emulsion or polyvinyl alcohol.

A gas diffusion layer9(GDL) is present on the anode7and the cathode8. The gas diffusion layer9on the anode7distributes the fuel from channels12for fuel uniformly over the catalyst layer30on the anode7. The gas diffusion layer9on the cathode8distributes the oxidant from channels13for oxidant uniformly over the catalyst layer30on the cathode8. The GDL9additionally serves to take off water of reaction in the reverse direction to the flow direction of the reaction gases, i.e. in a direction from the catalyst layer30to the channels12,13. Furthermore, the GDL9keeps the PEM5moist and conducts the current. The GDL9is, for example, made of a hydrophobicized carbon paper and a bonded carbon powder layer.

A bipolar plate10is present on the GDL9. The electrically conductive bipolar plate10serves as current collector, for conducting away water and for conducting the reaction gases through a channel structure29and/or a flow field29and for conducting away the waste heat which occurs, in particular, in the exothermic electrochemical reaction at the cathode8. To conduct away the waste heat, channels14for passage of a liquid or gaseous coolant are incorporated in the bipolar plate10. The channel structure29at the gas space31for fuel is formed by channels12. The channel structure29at the gas space32for oxidant is formed by channels13. As material for the bipolar plates10, use is made of, for example, metal, conductive polymers and composite materials or graphite.

A plurality of fuel cells2are arranged above one another in a fuel cell unit1and/or a fuel cell stack1(FIG.4).FIG.1depicts an exploded view of two superposed fuel cells1. A seal11forms a fluid-tight seal against the gas spaces31,32. Hydrogen H2as fuel is stored at a pressure of, for example, from 350 bar to 700 bar in a compressed gas store21(FIG.1). From the compressed gas store21, the fuel is conveyed through a high-pressure conduit18to a pressure reducer20in order to reduce the pressure of the fuel in an intermediate-pressure conduit17to from about 10 bar to 20 bar. From the intermediate-pressure conduit17, the fuel is conveyed to an injector19. At the injector19, the pressure of the fuel is reduced to a blowing-in pressure in the range from 1 bar to 3 bar. From the injector19, the fuel is introduced into a feed conduit16for fuel (FIG.1) and from the feed conduit16into the channels12for fuel, which form the channel structure29for fuel. The fuel thus flows through the gas space31for the fuel. The gas space31for the fuel is formed by the channels12and the GDL9at the anode7. After flowing through the channels12, the fuel which has not been consumed in the redox reaction at the anode7and possibly water from controlled moistening of the anode7are discharged from the fuel cells2through a discharge conduit15.

A gas transport device22, for example configured as a blower23or a compressor24, conveys air from the surroundings as oxidant into a feed conduit25for oxidant. From the feed conduit25, the air is introduced into the channels13for oxidant, which form a channel structure29at the bipolar plates10for oxidant, so that the oxidant flows through the gas space32for the oxidant. The gas space32for the oxidant is formed by the channels13and the GDL9at the cathode8. After flowing through the channels13or the gas space32for the oxidant32, the oxidant which has not been consumed at the cathode8and the water of reaction formed at the cathode8as a result of the electrochemical redox reaction is discharged from the fuel cells2through a discharge conduit26. A feed conduit27serves to introduce coolant into the channels14for coolant and a discharge conduit28serves to discharge the coolant which has been conveyed through the channels14. The feed conduits and discharge conduits15,16,25,26,27,28are depicted as separate conduits inFIG.1for reasons of simplicity and can actually be constructed differently, for example as holes in a frame (not shown) or as flush holes at the end region (not shown) of superposed bipolar plates10. The fuel cell stack1together with the pressurized gas store21and the gas transport device22forms a fuel cell system4. The feed conduits and discharge conduits15,16,17,18,25,26,27,28and the channels12,13,14and also the gas space31for fuel and the gas space32for oxidant each form a fluid channel37for passage of a fluid.

In the fuel cell unit1, the fuel cells2are arranged between two clamping elements33as clamping plates34. An upper clamping plate35lies against the uppermost fuel cell2and a lower clamping plate36lies against the bottommost fuel cell2. The fuel cell unit1comprises approximately 300-400 fuel cells2, which are not all depicted inFIG.4in the interests of simplicity of depiction. The clamping elements33exert a compressive force on the fuel cells2, i.e. the upper clamping plate35lies against the uppermost fuel cell2with a compressive force and the lower clamping plate36lies against the lowermost fuel cell2with a compressive force. The fuel cell stack2is thus clamped in order to ensure freedom from leaks of the fuel, the oxidant and the coolant, in particular due to the elastic seal11, and also to keep the electrical contact resistance within the fuel stack1as small as possible. To clamp the fuel cells2by means of the clamping elements33, four connecting devices39configured as pins40, which are under tensile stress, are arranged on the fuel cell unit1. The four pins40are fixed to the clamping plates34.

FIG.5depicts a section with a section plane parallel to a diffusion path38for a fluid to be sealed off through a seal11which is known from the prior art and is composed of a sealing material42. Solids, too, have a coefficient of diffusion, so that the fluid can diffuse along a straight diffusion path38as ideal diffusion direction53through the sealing material42. Owing to the straight diffusion path, the fluid diffusing through the sealing material42can cover a short minimal distance, so that a relatively large volume flow of fluid can diffuse through the sealing material42.

FIG.6shows a section with a section plane parallel to the ideal diffusion direction53for a fluid to be sealed off through a seal11in a first working example of a fuel cell unit1according to the invention. The fluid to be sealed off, for example the fuel hydrogen H2, the oxidant air or a liquid coolant, for example water containing antifreeze, is conveyed through a fluid channel37and the fluid channel37is sealed by the seal11. The seal11comprises a sealing material42, for example a polymer such as PPS (polyphenylene sulfide as heat-resistant thermoplastic polymer), EPDM (ethylene-propylene-diene rubber as synthetic rubber) or adhesive and also particles41of a particle material43. The diffusion coefficient of the particle material43is significantly lower than the diffusion coefficient of the sealing material42, so that the fluid to be sealed off by the seal11can essentially not diffuse through the particles41, so that the diffusion path38of the fluid runs virtually exclusively in the sealing material42. The particles41thus form a diffusion barrier for the fluid and the diffusion path38therefore runs around the particles41. Thus, the seal11ofFIG.6has a longer diffusion path38than in the case of the seal11shown inFIG.5. As a result of the longer diffusion path38, the seal11has a smaller total average coefficient of diffusion than the seal11of the prior art shown inFIG.5.

FIG.7shows a section with a section plane parallel to the ideal diffusion direction53for a fluid to be sealed off through a seal11in a second working example of a fuel cell unit1according to the invention. The particles41are, as inFIG.6, disk-shaped or platelet-shaped (FIG.12) and have a maximum diameter46in a first direction44and a minimum diameter47in a second direction45. The first and second directions44,45are perpendicular to one another and lie in an imaginary plane54spanned by the disk-shaped particle41(FIG.13). The maximum diameter is 500 μm and the minimum diameter is 250 μm, so that the particles41have an aspect ratio of 2:1 within the imaginary plane54. The thickness perpendicular to the imaginary plane54is 20 μm, so that the aspect ratio is 500/20=25. The first and second directions44,45and the imaginary planes54spanned by the particles41are oriented perpendicular to the ideal diffusion direction53for achieving low diffusion. The fluid is thus forced to go along a very long diffusion path38through the seal11, so that the seal11overall has a smaller average coefficient of diffusion. Only very little fluid can therefore diffuse through the seal11. The particles41are made up of an orientation layer55and a diffusion barrier layer56. The orientation layer55serves to orient the particles in an electric or magnetic field. When the particles41are arranged in a magnetic field, the orientation layer55is, for example, a ferromagnetic layer composed of γ-Fe2O3, magnesite, iron, cobalt or nickel. When the particles41are arranged in an electric field, the orientation layer55is made of a polarizable material or a material having dipole properties. These are, for example, materials composed of molecules having polar atom bonding in which the molecules have an asymmetric structure, for example aluminum chloride (AlCl3). As diffusion barrier layer56, it is possible to use, for example, glass or a metal layer, for example iron, brass or copper, having a very small coefficient of diffusion.

The steps for producing a seal11comprising the oriented particles41are shown inFIGS.8to11. Provision57of an uncured sealing material42comprising particles41is firstly carried out. A sealing material42comprising integrated particles41is applied58, e.g. by means of a dispenser, to a support layer48as first layer49to be sealed off. The first layer49to be sealed off is the bipolar plate10. A poling plate51is subsequently moved over the sealing material42comprising the particles41and an electric potential is applied between the poling plate51and the bipolar plate10, so that a static electric field having a field strength in the range from 102to 106V/m is formed between the positively charged poling plate51and the bipolar plate10as negatively charged poling plate52comprising the applied sealing material42. As an alternative, the field can also be an alternating electric field. The sealing material42as fluid medium is not yet cured inFIG.9and has such a viscosity that the particles41can perform a movement, in particular rotational movement, in the sealing material52and can, as depicted inFIG.9, become oriented59in such a way that the imaginary planes54spanned by the particles41become oriented essentially perpendicular to the ideal diffusion direction53. The sealing material42is subsequently cured60, for example thermally or by means of irradiation. After curing60of the sealing material42comprising the oriented particles41, the fuel cell unit1is assembled by a second layer50be sealed off being placed as bipolar plate10on the seal11, i.e. the seal11is arranged61between two layers49,50to be sealed off. As an alternative to the working example presented above, the sealing material42comprising the unoriented particles41can, as in the process step depicted inFIG.8, be applied to a support layer48which does not function as future layer49to be sealed off, in particular is not a bipolar plate10, so that after orientation of the particle41and curing of the sealing material42the seal11is removed from the support layer48and placed on a layer49,50to be sealed off, in particular is arranged61between two layers49,50to be sealed off.

In a further working example which is not depicted, the sealing material42comprising the particles41is, in a manner analogous to the above-described working example inFIG.9, arranged in a magnetic field in order to orient the particles41.

Overall, significant advantages are associated with the fuel cell unit1according to the invention and the process according to the invention for producing the fuel cell unit1. Owing to the required properties of the sealing material41, it is necessary to use sealing materials42having a relatively high coefficient of diffusion as sealing material42in the seals11because glass, for example, cannot be used as sealing material42. The oriented particles41have a very small coefficient of diffusion and significantly increase the length of the diffusion path38, so that the seal11has a significantly lower average coefficient of diffusion than in the case of a structure according to the prior art comprising only the sealing material42without the particles41.