Patent Publication Number: US-8535848-B2

Title: Structured body for an anode used in fuel cells

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/355,624, filed Jan. 31, 2003, which claims the benefit of European Patent Application No. 02 405 163.3, filed Mar. 4, 2002; each preceding disclosure is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a structured body for an anode used in fuel cells which is briefly termed an anode structure in the following. The invention also relates to a high temperature fuel cell having such an anode structure and to a method of manufacturing the anode structure in accordance with the invention. 
     A high temperature fuel cell (SOFC fuel cell) is known from DE-A-19 819 453 in which an anode substrate forms a support structure. An intermediate anode layer, a solid electrolyte, preferably in the form of a very thin layer, and a layer-like cathode are applied to this support structure. The anode substrate and the intermediate anode layer are manufactured from the same material, namely from a porous cermet which consists of the ceramic material YSZ (yttrium stabilized zirconium oxide), which is also used as the solid electrolyte, and nickel. Electro-chemical reactions take place at so-called three-phase points (nickel/electrolyte/pore) in the contact region between the anode and the electrolyte in which nickel atoms are oxidized by oxygen ions (O 2− ) of the electrolyte and these are reduced again by a gaseous fuel (H 2 , CO), with H 2 O and CO 2  being formed and the electrons released in the oxidization being passed on by the anode substrate. To obtain a large density of points for these three-phase points, a composition of the intermediate anode layer is provided for which the ratio of the proportions by volume of nickel, YSZ and pores lies close to 1:1:1. 
     The aim of the largest possible density of the three-phase points is, however, of lesser importance with respect to a further problem, namely with respect to the requirement that the anode should have a so-called redox stability. The redox stability relates to the properties of the electrode material with respect to a multiple change between oxidizing and reducing conditions. On the one hand, this change, which is briefly termed a redox change in the following, should not result in any major changes in properties for the ceramic components. On the other hand, an irreversible change, i.e. an ageing of the metallic components as a result of the redox change, should be influenced by means of the constant ceramic components such that the electrical conductivity of the electrode material is largely maintained. With such an ageing, a grain growth of the nickel takes place in which large crystallites grow at the cost of small ones and so allow gaps to arise in electrically conductive connections of the anode structure. 
     The redox stability is very important in practice because, according to experience, it is not possible to keep a battery with fuel cells in continuous operation. At each operation stop, the supply of the fuel must be stopped for safety reasons. When the gaseous fuel is absent, oxygen penetrates onto the anode sides of the fuel cells and the previously reduced state of the nickel changes into the oxidized state. When fuel cells are used in domestic technology for the purpose of the simultaneous production of electrical and thermal energy, around 20 interruptions to operation can be expected per year. A fuel cell must be usable for around five years for economical reasons. The fuel cell must thus only age so fast that up to 100 redox changes are possible. 
     However, in addition to the redox stability, good gas permeability of the anode structure is also important, as is—with respect to commercial use—a production of the anode structure which should be economical. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an anode structure which is sufficiently good with respect to redox stability, gas permeability and efficiency for use in fuel cells. This object is satisfied by the anode structure in accordance with the embodiments of the present invention. 
     The structured body, which is provided for use for an anode in fuel cells, has a structure formed by macro-pores and an electrode material. The macro-pores form communicating spaces which are produced by pore forming materials. The electrode material includes skeleton-like or net-like connected structures of particles connected by sintering which form two reticular systems which interengage or cross into one another: a first reticular system of ceramic material and a second reticular system which includes metals to bring about electrical conductivity. The electrode material has the properties so that, with multiple changes between oxidizing and reducing conditions, no major changes in properties occur in the ceramic reticular system, on the one hand, and an oxidization or reduction of the metals results in the second reticular system, on the other hand. In addition, the two reticular systems together form a compact structure which includes micro-pores in the oxidized state whose proportion by volume is less than 20%, preferably less than 5%, with respect to the electrode material. 
     The term “reticular system” has been introduced here. This is to be understood as a skeleton-like or net-like connected structure of particles. Due to the connection of the reticular system, this is given a structural stability and/or electrical conductivity. No chemical changes can take place in the reticular system so that structural stability is present. This is the case for the ceramic reticular system. The second reticular system has a structure which changes due to the redox change such that only a low structural stability is present. The function of the second reticular system as an electrically conductive connection is maintained due to the structural stability of the first reticular system. The two reticular systems result in a natural manner in the form of a statistical distribution of constituting particles if these particles are prepared such that they each have a narrow size spectrum for the two particle kinds when the proportion by volume for each reticular system amounts to at least 30% and when the particles are mixed homogeneously with one another. (However, even relatively large particles, which are included in insular manner in a matrix of fine particles, may be mixed in the ceramic reticular systems.) The system formed by the pores is likewise a reticular system. This reticular system results in the required gas permeability of the anode structure. 
     In a preferred embodiment, the anode structure in accordance with the invention has both the function of a support structure and the function of the above-mentioned intermediate anode layer. It can, however, also only form the supporting part of the anode which serves as a support structure for an intermediate anode layer. The body in accordance with the structure must be made so strong as the support structure that it withstands a mechanical load of, for example, 20 kPa; a load of this order of magnitude is typically present on installation in a stack-like arrangement of fuel cells. 
     The present invention enables various advantageous anode structures. Furthermore, the present invention enables the formation of a high temperature fuel cell having these advantageous anode structures. 
     The invention will be explained in the following with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a fuel cell; 
         FIG. 2  is a cross-section through a porous material which can be used as an anode structure in accordance with the invention; 
         FIGS. 3 ,  4  are sections from two reticular systems, namely skeleton-like or net-like connected structures of particles; 
         FIG. 5  shows the two reticular systems of  FIGS. 3 and 4  which are interengaged; and 
         FIG. 6  shows an anode structure in whose electrode material larger particles of ceramic material are incorporated. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a high temperature fuel cell such as is shown schematically in  FIG. 1 , electrode reactions are carried out to produce an electric current I; that is, reducing reactions arise on or in an anode  1 , wherein water or carbon dioxide arise from hydrogen and carbon monoxide, which form the gaseous fuel; and oxidizing reactions arise on a cathode  2  in which ionic oxygen O 2−  is formed—while taking up electrons e −  from a metallic conductor  20  and a pole  24 —from molecular oxygen of a second gas flow (e.g. air: O 2  and N 2 ). The oxygen ions move through a solid electrolyte  3 , which separates the two electrodes  1 ,  2  in a gas impermeable manner and which is conductive for the oxygen ions at temperatures above 700° C. The reducing anode reaction takes place with the oxygen ions while emitting electrons to a further metallic conductor which produces a connection to a pole  14 . A consumer  4  is arranged between the poles  14 ,  24  which load the fuel cell with a resistance R. The voltage U between the poles  14 ,  24  is produced by a stack of cells connected in series in the practical application of the fuel cell. 
     In a preferred embodiment, the anode structure  1  has a heterogeneous design made up of a homogeneously structured, porous support structure  10  and a more compact marginal zone  11 . The pores of the support structure  10  are macro-pores  100  and micro-pores  110 : see  FIG. 2 . The marginal zone  11  in the example shown includes only micro-pores  110 . The adjoining layers, that is, the preferably thin solid electrolyte layer  3  and the cathode  2 , can be manufactured, for example, by means of a thermal injection method; they can also be manufactured while using a screen printing method. The material for the electrodes  1  and  2  must be usable at an operating temperature of up to 1000° C. The macro-pores  100  of the anode structure  1  form communicating spaces which result in a permeability for the gaseous fuel which is adequate with respect to the electrode reactions. This permeability exists up to the marginal zone  11  which forms a boundary zone beneath the electrolyte layer  3 . A further gas permeability is given in this border zone  11  by the micro-pores  110 . The porosity given by micro-pores  110  is larger than shown in the reduced state of the anode structure  1 . 
     The anode structure  1  in accordance with the invention has a structure formed by the macro-pores  100  and an electrode material  5 . The macro-pores  100  form communicating spaces which are produced by pore forming materials. The electrode material  5 —see FIGS.  3  to  5 —includes skeleton-like or net-like connected structures made up of particles  60  and  70  connected by sintering. These particles  60 ,  70  form two reticular systems  6  and  7  ( FIG. 5 ) which are crossed into one another, namely a first reticular system  6  ( FIG. 3 ) of ceramic material and a second reticular system  7  ( FIG. 4 ) which includes metals to effect electrical conductivity. In accordance with the invention, the electrode material  5  has the properties that the following applies with a multiple change between oxidizing and reducing conditions (redox change): on the one hand, no chemical or structural changes occur in the ceramic reticular system. On the other hand, an oxidization or reduction of the metals contained in the particles  70 , where ageing occurs, results in the second reticular system due to the redox change. The two reticular systems  6 ,  7  together form a compact structure which includes micro-pores  110  in the oxidized state whose proportion by volume is less than 20% with respect to the electrode material  5 . This proportion by volume is preferably less than 10% or, even better, 5%. During the reduction of the metals, the dimensions of the particles  70  reduce in the second reticular system  7 ; further micro-pores (not shown) therefore occur. On a repeated oxidization of the metals, these further micro-pores, which should be designated by “type II micro-pores”, disappear. The micro-pores  110  of the oxidized anode structure  1  are “type I micro-pores”. The “type II micro-pores” have a lower influence on the structural stability of the electrode material  5  than the “type 1 micro-pores”. 
     As was determined empirically, reticular systems result when the particles  60 ,  70  have diameters whose mean value d 50  is less than 1 μm prior to sintering (d 50 =50% by volume of the particles have a diameter lower than d 50 ). In this connection, micro-pores arise whose diameters are smaller than 3 μm. The redox stability becomes even better for smaller diameters. An anode structure is preferably manufactured such that micro-pores arise with diameters lower than 1 μm. The proportion by volume of the ceramic reticular system—with respect to the electrode material—should amount to at least 30%. So that the ageing does not too quickly result in an intolerable loss in electrical conductivity, the proportion by volume of the second reticular system should be larger than that of the ceramic reticular system, for example larger by at least a factor of 1.5. 
     The particles must be put into a sufficiently fine form for the formation of the reticular systems. This can be done, for example, by grinding a coarse grain powder. Coarser particles can—if necessary—be removed from the ground product by classification (e.g. by screening). The required powder quality is obtained, for example, by reprocessing the powder by means of spray drying. Suitable finely dispersed particles can also be prepared using methods of nanotechnology: for example by means of reaction spray methods, spray flame pyrolysis, deposition methods or sol/gel methods. 
     If zirconium oxide, YSZ, is provided as the material for the electrolyte, then the same ceramic material is advantageously also used for the anode structure, i.e. for its first reticular system. Nickel oxide particles, which contain up to 10% by weight of materials other than nickel oxide, are preferably used for the second reticular system. Such nickel oxide particles, which are available on the market as dye components, are available as low-cost raw material. These dye particles advantageously also contain materials which act as sintering aids in the sintering of the anode structure. 
     It is of advantage in the manufacture of the anode structure in accordance with the invention for additives of at least one kind to be contained in the second reticular system. Such additives can serve as a sintering aid on sintering. Further additives can impede damaging grain growth as inhibitors during the operation of the fuel cell. Oxides or salts of Ni, Mn, Fe, Co and/or Cu can be used as sintering aids and MgO as the inhibitor of grain growth. 
     The anode structure in accordance with the invention must have communicating spaces which enable permeability for the gaseous fuel which is adequate with respect to the current supplying electrode reactions. This gas permeability is brought about using macro-pores whose diameters lie in the region between 3 and 20 μm. The macro-pores can be manufactured using pore forming materials. For this purpose, particles or fibers of an organic material, in particular cellulose, are used. This organic material is decomposed during sintering which is carried out under oxidizing conditions; the decomposition products evaporate. 
     In a suitable method of manufacturing the anode structure in accordance with the invention, the procedure is as follows: particles of ceramic material (e.g. YSZ) and of a metal oxide (e.g. green or brown nickel oxide particles) are put into a sufficiently fine form for the formation of the reticular systems by grinding and classification. A homogeneous mixture in the form of a slurry is formed from the particles, the pore forming materials and a liquid. The slurry is cast to form a layer. If the slurry is cast in an absorbent mold, some of the liquid is removed from it. At the same time, a marginal zone arises in which a lack of pore forming materials is present. An inhomogeneous structure thus results such as is shown in  FIG. 2 . 
     After a complete removal of the liquid from the slurry layer by drying, sintering is carried out, with the sintering preferably being carried out together with a solid electrolyte applied to the layer. There is an optimum sintering temperature at which a structure with small micro-pores arises, on the one hand, and an unfavorable influencing of the reticular structure remains minimal, on the other hand. The larger the sintering temperature is, the lower is the density of the micro-pores, but the more strongly the reticular structure is impaired. 
       FIG. 6  shows an anode structure in whose electrode material  5  larger particles  60 ′ of ceramic material are incorporated. The coarse particles  60 ′ are incorporated in an insular manner in a matrix of the fine particles  60 ,  70  (see  FIG. 5 ). The manufacturing costs can be reduced with the coarse particles  60 ′, whose diameters lie in the range between 2 and 10 μm, because less material has to be put into the required fine form. A greater strength of the ceramic reticular system can also be obtained with added coarse particles if a lower proportion by volume of micro-pores results due to the coarse particles. 
     The anode structure in accordance with the invention is made areally, for example in plate-shape or shell-shape. In plate-shape, it has a large extent in two dimensions; in a third dimension, it has a thickness of relatively small extent which is, however, larger than 0.5 mm. The required supporting force of the anode structure is achieved with this thickness. 
     The ceramic reticular system ( 6 ) can consist, in addition to the stabilized zirconium oxide, YSZ, also of aluminum oxide Al 2 O 3 , of titanium oxide TiO 2 , of doped cerium oxide CeO 2 , of magnesium oxide MgO, and/or of a spinel compound. Oxides of the substances Y, Ti, Gd and/or Co can be used as doping means for the CeO 2 . 
     With the manufacturing method in accordance with the invention and unlike the high temperature fuel cell described in DE-A 19 819 453, an anode structure corresponding to the anode substrate and the intermediate anode layer can be manufactured in common working steps. This is obviously a simplification of the method which has a positive effect on the manufacturing costs. 
     Further methods known per se can also be used for the manufacture of the anode structure in accordance with the invention: film casting, roll pressing, wet pressing or isostatic pressing.