DE10317976A1 - Solid oxide fuel cell, especially for electrolysis, has graded alteration of electrochemical activity of anode and cathode layers along the gas flow path - Google Patents

Abstract

A solid oxide fuel cell comprises anode and cathode layers separated by an electrolyte layer, gas inlet and outlet, and a gas flow path. The electrochemical activity or characteristics of at least one layer is so graded as to alter the activity along the flow path between gas inlet and outlet. An independent claim is also included for a production process for the cell above.

Description

The Invention relates to a solid electrolyte fuel cell according to the The preamble of claim 1 and a method for producing the solid electrolyte fuel cell according to the preamble of claim 10.

The Solid electrolyte fuel cells belong to the so-called high-temperature fuel cells. The Solid electrolyte fuel cell, hereinafter abbreviated with SOFC) takes its name from its solid as opposed to "liquid" oxide ceramic electrolytes. This electrolyte layer separates the anode from the cathode as a separator. The operating temperature of a SOFC is between 650 and 1000 ° C. The anode and the cathode has many open pores for good flow-through, to the inside of the reaction gases (e.g. hydrogen and oxygen) a big Surface for the chemical Offer response. A typical SOFC cathode usually consists of a ceramic mixed oxide with a perovskite structure (e.g. ceramic Lanthanum strontium manganese oxide (LSM), which is also made with electrolyte material can be mixed. This cathode is in contact with the thin electrolytic ceramics, - that from for example made of yttrium-stabilized zirconium oxide (YSZ). Such a porous one Using a partial pressure difference, cathode can be used like a sponge through the air. On the big one surface inside the cathode is in an electrochemical reaction gaseous Oxygen is broken down into its atoms and these by absorption of each 2 electrons converted into negatively charged ions. The oxygen ions migrate through the electrolyte to the anode side of the cell while the Hydrogen protons remain bound to the anode. See to that the electrolyte, which takes over the function of the separator in the SOFC. It is only for the oxygen ions permeable. They flow through it to the anode and react there with the protons to water. In the area where the particles of the porous cathode material are in contact with the electrolyte, the oxygen ions in get to the electrolyte. To get into the electrolyte needs each oxygen atom additional Electrons. The cathode provides this. They come from the anode, where they split off from the hydrogen atoms. From there they pour as a transporter of electrical energy from the fuel cell via electrical Lines to a consumer. They change you in the consumer huge Part of the transported energy in useful work. Then they flow back to the fuel cell in the cathode, where it is now the oxygen its reduction available stand.

High temperature fuel cell assemblies and methods for applying layers are known in the art DE-OS 3907485 described. One of the usual methods for applying layers on a base, in particular on yttrium-stabilized zirconium oxide, is plasma gasoline. In plasma spraying, a powder is melted in a plasma generated by an arc and sprayed onto a substrate at high speed, where the material solidifies as a layer.

From the DE 4122942 C1 is a process for producing a powder which is provided as a plasma spray powder for producing a fuel cell anode on a ceramic substrate, in particular an yttrium-stabilized zirconium oxide electrolyte disk. In this process, a wettable powder in the grain size range of approximately 30-90 micrometers is produced from certain starting substances by wet grinding and subsequent drying and pre-calcining as well as subsequent coarse and fine grinding, which can then be sprayed on.

From "Fabrication of the Electrolyte-Electrodes Assembly by applying the Vacuum Plasma Spraying Technology "; G. Schiller, R. Henne and M. Lang; 3. European Solid Oxide Fuel Cell Forum, Nantes, 2–5.06.1998 it is known that plasma spraying under vacuum is an option opened, the complete multi-layer electrode structure in a fast multi-stage spraying process manufacture. The production by plasma spraying requires a modified one planar cell design that matches the properties of the spray is adjusted. The development is based on a porous carrier substrate and thin Electrodes and electrolyte layers, which are consecutive be applied. The plasma spraying process enables both Deposition of very thin ones but dense layers of electrolyte as well as of porous electrode layers with over the Thick graded composition and porosity.

From WO 02/084774 A2 a multi-layer and multi-functional cathode for SOFC is known, which should have a high conductivity, a high catalytic activity, an excellent compatibility with other areas of a fuel cell and a suitability for a reduced working temperature. The cathode comprises a conductive layer, a catalytic layer and a graded composite layer. The conductive layer has a first density, the catalytic layer has a second density which is less than the first density, and the layer with a graded composition is characterized in that the electron conductivity and the ion conductivity are graded over the layer thickness. The electron conductivity is said to go from a lower side of the layer to an upper side the layer of the composite graded layer, the underside being the area immediately adjacent to the catalytic layer. In contrast, the ion conductivity should preferably decrease from the lower layer to the surface. On the surface, the layer should be 70-100% electron conductor, while in the bottom area 50-70% ion conductivity should be present. The grading should be achieved in that several layers are brought together by spraying, pouring or laminating or deposition.

Out WO 97/41612 discloses layers with different layers functionality matched to the oxygen partial pressure in a cell to layer, with a functional grading accordingly the chemical properties should be achieved. As in the rest of the Technique too, the ceramic reaction layers are anode and Thickness cathode with reaction-promoting rare earth metals graded and stabilized.

at The known SOFC is disadvantageous in that Efficiency are not optimal and destruction due to thermally induced voltages may occur.

task the invention is to provide a solid electrolyte fuel cell which is a higher one Efficiency than known solid electrolyte fuel cells and greatly reduced in the case of thermally induced mechanical stresses become.

This Task is with a solid electrolyte fuel cell with the features of claim 1 solved.

advantageous Further developments are characterized in the dependent claims dependent thereon.

It is another object using a method of manufacturing a solid electrolyte fuel cell reduced thermally induced mechanical stresses and higher efficiency, in particular according to claim 1. The task comes with solved a method with the features of claim 10.

Further advantageous embodiments are dependent in the dependent claims characterized.

by virtue of the series connection of many fuel cells to a fuel cell block must Electrodes of the individual cells parallel to their surface that needed to generate electricity Flowing with fuel gas become. As fuel gases can Hydrogen, methanol, natural gas or reformates of higher quality hydrocarbons such as B. gasoline, diesel or kerosene. On the anode side it occurs between the fuel gas inlet and outlet along the anode surface to deplete the combustible components (reactants) in the Fuel gas. The reactants diffuse out of the fuel gas flow in the anode layer are implemented there by electrochemical reaction and released back into the gas stream as exhaust gases. The concentration of those contained in the fuel gas and usable by the fuel cells So reactants decrease along the cell surface. Since the local Electric power production proportional to the fuel cell Number of electrochemical reactions behaves, has a depletion of the combustible components of the fuel gas cause electrical Electricity production of the fuel cell is not the same over its entire area but in the gas flow direction decreases from the fuel gas inlet to the outlet.

According to the invention recognized that the electricity generation potential those fuel cell areas that are closer to the gas outlet, cannot be used optimally for energy conversion if one do not electrically overload the cell areas at the gas inlet want.

specially electricity is generated at SOFC only takes place above a certain response temperature and increases with increasing Temperature. According to the invention recognized that there is a temperature gradient during the heating process and train in the company. warmer Cell areas thus reach the response temperature earlier and generate earlier electricity than colder Cell regions. Since there is also energy loss in the generation of electricity in the form of heat is produced, the warm area continues to heat up, which is a even higher electricity generation favored. According to the invention, it was recognized that these temperature gradients resulting from the heating process to damage the SOFC can lead through thermally induced mechanical stresses.

According to the invention, the problems recognized by the inventors are eliminated in that the electrochemical properties of the fuel cells or the layers of the fuel cells are influenced locally in a targeted manner along the flow direction of the reactants. According to the invention, in the areas of SOFC in which later a high supply of fuel gas and high temperatures prevail during the heating process, a smaller amount of ion-conductive material is applied or less nickel is present in the anode. Along the direction of the fuel gas flow, the proportion of the ionic is better capable material in relation to the temperature gradient.

in this connection is beneficial that increased ion conductivity in the area of lower fuel gas supply (for fuel gas outlet hin) and lower cell temperature to an equalization of the electricity generation over the cell area and to an over the cell area simultaneous use of electricity generation during the SOFC starts. This makes it possible maximum electricity yield with a minimum of cell area and in particular to achieve the use of materials. From a decrease the cell area there is also a reduction in the volume and mass of the fuel cell stack with the resulting savings in terms of housing.

The inventive method provides for different ion-conducting or different thicknesses SOFC materials about To apply plasma spraying, the during the spraying Total amount and / or the composition of the material to be sprayed-on, in particular of a powder - continuously or changed in stages. A grading of the material composition and / or Material thickness and therefore also the properties along the flow direction and / or the direction of application of the plasma torch can be achieved.

The The invention is explained by way of example with reference to a drawing. in this connection demonstrate:

1 schematically shows a plan view of a functional layer of an SOFC from the fuel gas inlet to the fuel gas outlet;

2 schematically shows a plan view of a functional layer of an SOFC with a registered movement of a plasma torch during the manufacturing process;

3 a functional layer after 2 , wherein a further direction of movement of the plasma torch is shown during the manufacturing process.

1 shows simplified a plan view of an anode surface of a solid electrolyte fuel cell 1 , Along a flow direction 2 a suitable fuel gas flows from an inlet side of the combustion gas 3 to a fuel gas outlet side 4 , As fuel gases can be hydrogen, methanol, natural gas or reformates of higher quality hydrocarbons, such as. B. gasoline, diesel or kerosene. About the flow direction 2 depletes the fuel gas of reactants and is enriched with reaction products. There is therefore an area richer in reactants 5 and a poorer area of reactants 6 ,

In order to equalize the electrical current density and the fuel cell temperature along the flow direction according to the invention 2 the local ionic conductivity of the electrolyte or the nickel concentration in the anode of the solid electrolyte fuel cell along the flow direction 2 changed. For example, yttrium-stabilized zirconium oxide (YSC) has a lower ionic conductivity and a higher response temperature than scandium-stabilized zinconium oxide. The same also applies to different stoichiometric compositions of yttrium and zirconium oxide. However, these materials are not only the components of the electrolyte of the SOFC, they are also present in the anode and ideally also in the cathode of the SOFC. The composition of the ion conductor, the relative content of the ion conductor in the anode and cathode and the thickness of the ion-conducting layers thus influence all electrochemically active layers of the SOFC. According to the invention, the ion conductivity is from the fuel gas inlet 3 to the exhaust outlet 4 and / or increased from the warmer to the colder area of the fuel cell. The increase can take place continuously or in increments, whereby the increments can exist with the same or different compositional differences. The concentration gradient between the input side 3 and exit side 4 can be linear according to a straight line or non-linear according to a parabola or hyperbola, for example. With a non-linear grading, for example, the flow behavior of the fuel gas, such as. B. the formation of boundary layers and thus both the diffusion and heat transfer behavior to the fuel gas are taken into account.

In general, a less ion-conductive material is applied in the areas of the SOFC in which a high fuel gas supply and / or high temperatures prevail later during operation. In the course of the fuel gas flow direction 2 the proportion of the more ion-conductive material then increases according to the above specification.

The method according to the invention provides for the graded functional layer to be applied by means of plasma spraying. In this process, the powdery starting materials of the solid electrolyte fuel cell are fed to a burner, melted in an ionized gas stream (plasma), accelerated strongly and applied to the carrier material (substrate). Here is a Plasma torch, similar to a paint gun, meandered over the substrate. In this way, the anode, then the electrolyte and finally the cathode are realized layer by layer. According to the invention, two materials with the same function but different properties, in particular properties with regard to ion conductivity, are deposited in suitable areas of the SOFC. According to the method according to the invention, this takes place in a particularly simple but highly effective manner in that the type and quantity of the material transported to the plasma torch and then sprayed on is determined by regulating the material supply. According to the invention, a powder feed device with at least two separate powder conveying systems, each with a differently ion-conductive powder, is connected to the plasma torch. During the burner process, a control device controls the powder feed device and in particular the powder conveying systems in such a way that the material composition within the individual SOFC layers, in particular the electrode layers, is controlled in a targeted manner via the later direction of flow 2 of a fuel gas is influenced.

Will, as in 2 shown, the burner move so that the meanders 7 run perpendicular to the fuel gas flow direction, the composition becomes transverse to the later fuel gas flow direction during a movement of the burner 2 kept constant and when moving the burner with or against the flow direction - depending on the movement of the burner - influences the powder conveying systems so that the amount (feed rate) of one powder is increased at the expense of the amount (feed rate) of the other powder or vice versa. As a result, the less ion-conductive material can be metered in and applied more strongly in the areas of the SOFC, in which a high fuel gas supply and / or high temperatures prevail later during operation.

In a further advantageous embodiment of the method, the burner is operated in parallel with or against the later direction of flow 2 process, in the conveying of the powder to be sprayed on, the amount of powder of one material is increased continuously or in stages at the expense of the other material during the movement of the burner over the substrate. After the burner has run over the substrate, it can carry out a lateral offset movement and can perform a backward movement counter to the direction of movement first carried out, in which the process sprayed first runs backwards, that is to say that the material which was depleted during the first spraying process is now during the Spray movement in the opposite direction is enriched accordingly. This process control is carried out until the complete substrate is coated according to the requirements.

at Another advantageous embodiment is the burner movement with simultaneous spraying only in one direction, for example only in the direction of flow instead, so that the control is carried out so that the more ion-conductive material first is metered in less and the metering or the proportion during the run over of the substrate rises continuously or in steps. After being run over of the substrate leads the Plasma torch backward movement by, at the same time the dosage is controlled such that after completing the return movement again with a high concentration of the less ion-conductive material is started.

at a further advantageous embodiment of the method according to the invention takes place additionally grading along the direction of flow in the thickness direction, so that once the substrate is completely covered with a Layer covered was another thin layer the same function, for example an anode function, applied becomes, in which already the starting composition to the starting composition the underlying layer is different, for example higher or lower ionic conductivity. This can be done, for example, by one of the two powdery Substances are enriched at the expense of others or a third Substance if desired is fed. Here, of course the third substance can also be a mixture of several substances, which is either fed to the burner or in the same way like the first two substances only in the area of the burner head through a powder conveying system is mixed.

According to the invention emphasized that plasma spraying is particularly suitable is the grading according to the invention along the flow direction to reach. The grading according to the invention can be carried out with corresponding effort along the flow direction but can also be realized with other order processes, for example with appropriately trained thin film (film) casting processes.

In the case of a SOFC designed according to the invention with a grading of the ionic conductivity along the flow direction of the fuel gases, it is advantageous that this leads to an equalization of the electricity generation over the cell surface and to a simultaneous use of the electricity generation during the start program of the SOFC. This makes it possible to achieve a maximum of electricity yield with a minimum of cell area or use of material chen. As a result, the total costs of the SOFC are also achieved, in particular also by reducing the volume and the mass of the fuel cell stack and thus also reducing the size of the housing and the insulation.

at the inventive method is advantageous that the grading according to the invention is particularly simple and effective both in the longitudinal direction, d. H. in the direction of gas flow, as well as in the thickness direction.

The The invention is in no way limited to just the anode surfaces or grading the anode layer accordingly along the direction of flow, grading can also be achieved with other functional layers become.

The In addition, the invention is not limited to a fuel cell. she is with the same success and advantage with an electrolyzer who chemical reversal of a fuel cell, can be used.

Claims (25)

Solid electrolyte fuel cell with a functional layer serving as anode and a functional layer serving as cathode, which are separated from one another by a separator or electrolyte layer, the functional layers being electrochemically active and a gas inlet and a gas outlet being present, with the gases to be converted from the gas inlet to the Gas outlet flow over the respective functional layer along a predetermined flow direction, characterized in that the electrochemical activity or the properties of at least one functional layer is designed in such a way that the electrochemical activity or the electrochemical properties of the gas inlet ( 3 ) to the gas outlet ( 4 ) along the flow direction ( 2 ) change. Solid electrolyte fuel cell according to claim 1, characterized characterized in that the electrochemical activity or electrochemical properties of the functional layer over the flow direction constantly changing. Solid electrolyte fuel cell according to claim 1 and / or 2, characterized in that the electrochemical activity or electrochemical properties of the functional layer over the direction of flow linear, non-linear or graded change. Solid electrolyte fuel cell according to one or more of the preceding claims, characterized in that the electrochemical activity or electrochemical properties over the direction of flow changed according to a curve function such as a parabola or a hyperbola. Solid electrolyte fuel cell according to one or more of the preceding claims, characterized in that at least one of the functional layers also about the thickness is a grade of the electrochemical activity or has electrochemical properties. Solid electrolyte fuel cell according to claim 5, characterized characterized that the grading across the thickness of the functional layer continuous or linear or graded or non-linear or corresponding to one Curve function is designed like a parabola or hyperbola. Solid electrolyte fuel cell according to one or more of the preceding claims, characterized in that the electrochemical activity or the electrochemical properties of the gas inlet ( 2 ) to the gas outlet ( 3 ) increases corresponding to the depletion of reactants in the gas to be converted. Solid electrolyte fuel cell according to one or more of the preceding claims, characterized in that as an electrochemical activity or property the electron and / or ion conductivity is graded is. Solid electrolyte fuel cell according to one or more the previous one. Expectations, characterized in that the functional layer essentially is formed from zirconium oxide, the zirconium oxide with doping elements is doped, with the grading of the electrochemical properties the type and amount of the doping elements contained is changed. Method for producing a solid electrolyte fuel cell with a functional layer serving as anode and a cathode, which are separated from one another by a separator layer, the functional layers being electrochemically active and a gas inlet and a gas outlet being present, the gases to be converted from the gas inlet to the gas outlet Flow over the respective functional layer along a flow direction, in particular for producing a solid electrolyte fuel cell according to one or more of Claims 1 to 9, characterized in that at least one of the functional layers is formed from a base material and at least one doping element which influences the electrochemical properties, the Functional layer the electrochemical properties of the operational functional layer over a gas flow direction ( 2 ) is changed in that a concentration gradient of the at least a doping element over the direction of flow ( 2 ) is generated so that the at least one doping element is enriched or depleted during the application. A method according to claim 10, characterized in that while depleted and at least one doping element another enriched at the expense of the depleted doping element becomes. A method according to claim 10 and / or 11, characterized characterized in that for the enrichment and depletion of the respective doping element the mixing ratio at least two components with essentially the same order volume changed with a first component of the mixture being the base material and contains at least a first doping element and a second component the mixture, the base material and at least one second doping element contains. Method according to one or more of claims 10 to 12, characterized in that the functional layer by plasma spraying, Thin Film or film casting or others to produce thinner Layers suitable process is generated. Method according to one or more of claims 10 to 13, characterized in that for the production of at least one Functional layer in plasma spraying at least two powdered components fed to a plasma spraying device and according to the desired mixing ratio be splashed. A method according to claim 14, characterized in that the powdery Components of the mixture fed to a plasma torch, in one ionized gas stream (plasma) melted, accelerated and up a carrier material (Substrate) can be applied. A method according to claim 15, characterized in that the plasma torch is similar a paint gun meandering over the Led substrate becomes. Method according to one or more of claims 14 to 16, characterized in that layer by layer the first Anode, then the separator and finally the cathode are sprayed on. Method according to one or more of claims 10 to 17, characterized in that a powder feed device with at least two separate powder feed systems for powders, each with different electrochemically active components, is connected to the plasma torch, with a control device the powder feed device and in particular the powder feed systems during the burner process is controlled in such a way that the composition of the sprayed material within the individual solid electrolyte fuel cell layers is targeted via the later direction of flow ( 2 ) of a fuel gas is changed. Method according to one or more of claims 10 to 18, characterized in that the burner is moved so that the meanders ( 7 ) perpendicular to the direction of fuel gas flow ( 2 ) run, the composition of the mixture during a movement of the burner transversely to the later direction of fuel gas flow ( 2 ) kept constant over the substrate and when moving the burner with or against the flow direction ( 2 ) - depending on the movement of the burner - the powder delivery systems are influenced so that the amount (delivery rate) of one powder or one component is increased at the expense of the amount (delivery rate) of the other powder or other component or vice versa. Method according to one or more of claims 10 to 18, characterized in that the burner in parallel with or against the later direction of flow ( 2 ) is moved over the substrate, the amount of one material or one component being increased or decreased at the expense of the other material or the other component during the movement of the burner over the substrate during the conveyance of the powder to be sprayed on. A method according to claim 20, characterized in that after the burner has run over the substrate, a side Offset movement of the burner is carried out and against the first conducted Direction of movement a return movement is driven in which the first sprayed process runs backwards, d. H. the material or component, which during the first spraying process has been depleted, now while corresponding to the spray movement in the opposite direction is enriched. A method according to claim 20, characterized in that the burner is moved with simultaneous spraying only in one direction, ie in the direction of flow or counter to the direction of flow, the control being carried out in such a way that the one component is initially metered in less and the metering or The proportion of the component increases continuously or in stages during the passage of the substrate, while the proportion of the other component decreases accordingly, the plasma being passed over the substrate burner carries out a return movement, the metering being controlled at the same time such that after the return movement has ended, a high concentration of the first component is started again and no spraying takes place in the return movement. Method according to one or more of claims 11 to 22, characterized in that also a grading of the electrochemical Properties in the thickness direction of the layers is carried out so once the substrate is completely covered with a layer was another thin layer the same function, for example an anode function, applied where the starting composition becomes the starting composition the underlying layer with respect to the electrochemical Activity is changed. A method according to claim 23, characterized in that one of the components is enriched at the expense of the other or is depleted or a third component is added. Use of a fuel cell according to one or several of the claims 1 to 9 and in particular a fuel cell using a method was produced according to one or more of claims 10 to 24, as an electrolyzer.