KR20030072220A - A structured body for an anode used in fuel cells - Google Patents

Abstract

The structured body intended for the anode 1 in the fuel cell includes a structure formed by the macro pores 100 and the electrode material 5. The macro pores form a communication space generated by the pore forming material. The electrode material is skeleton-like or net-like of particles 60, 70 forming two reticular systems 6, 7 that are connected and interconnected by sintering. a net-like connected structure, wherein the two reticulated systems 6, 7 comprise a first reticulated system 6 made of ceramic material and a second comprising a metal that generates electrical conductivity. Reticulated system 7. The electrode material, on the one hand, does not occur in the first reticulated system of ceramic material that is substantially altered by a multiple change between the oxidation and reduction states and on the other hand the oxidation or As a result of the reduction, the second reticulated system is formed. Moreover, the two meshed systems form a compact structure comprising micropores 110 in a oxidized state with a proportion of less than 20%, preferably less than 5%, by volume relative to the electrode material.

Description

Structure for anode used in fuel cell {A STRUCTURED BODY FOR AN ANODE USED IN FUEL CELLS}

The present invention relates to a structured body for an anode for use in a fuel cell, briefly referred to as an anode structure below. The invention also relates to a hot fuel cell having such an anode structure according to the invention and to a method of manufacturing said anode structure.

Hot fuel cells (SOFC fuel cells) are known from DE-A-19 819 453, in which the anode substrate forms a support structure. Intermediate anode layers, solid electrolytes, and layer-like cathodes, preferably in the form of very thin layers, are applied to this support structure. The anode substrate and the intermediate anode layer are made from porous cermet and nickel composed of the same material, ie ceramic material YSZ (yttrium stabilized zirconium oxide), which is also used as a solid electrolyte. The electrochemical reaction takes place at the so-called three-phase points (nickel / electrolyte / pore) at the contact region between the anode and the electrolyte, where the nickel atoms are oxidized by the oxygen ions (O ^2- ) in the electrolyte. These are reduced again by gaseous fuels (H ₂ , CO) to form H ₂ O and CO ₂ , and electrons emitted in the oxidation are passed by the anode substrate. In order to obtain a high density of dots for this three-phase point, a composition of an intermediate anode layer is provided in which the ratio by volume of nickel, YSZ and pores is close to 1: 1: 1.

However, the goal of the highest possible density of the three phases is of less importance in relation to further problems, i.e. with the requirement that the anode should have so-called redox stability. Redox reaction stability relates to the nature of the electrode material to multiple changes between oxidation and reduction states. On the other hand, such changes, referred to briefly as redox changes below, should not cause any significant change in properties for the ceramic components. On the other hand, ageing of metal components as a result of irreversible changes, i.e., redox reaction changes, should be influenced by constant ceramic components so that the electrical conductivity of the electrode material is mostly maintained. Such aging results in grain growth of nickel, where large crystals grow at the expense of small crystals, thus creating gaps in the electrically conductive connections of the anode structure.

Redox reaction stability is indeed very important. Because experiments have shown that it is not possible to hold a cell in a fuel cell in continuous operation. At every shutdown, the supply of fuel must be stopped for safety reasons. In the absence of a gaseous fuel, oxygen penetrates into the anode side of the fuel cell, and the reduced state of nickel before is changed to an oxidized state. When a fuel cell is used for home use for the purpose of simultaneous production of electrical and thermal energy, about 20 interruptions of operation can be expected per year. The fuel cell must be available for about 5 years for economic reasons. The fuel cell should not age so fast that more than 100 changes in redox reactions are possible.

However, in addition to the redox stability, good gas permeability of the anode structure is also important, and the production of the anode structure, which must be economical with respect to commercial use, is also important.

It is an object of the present invention to provide a sufficiently good anode structure with regard to redox reaction stability, gas permeability and efficiency for use in fuel cells. This object is met by the body defined in claim 1 and designated as an anode structure.

The structure provided for the anode in the fuel cell has a structure formed by macropores and electrode material. The macro pores form a communication space generated by the pore forming material. The electrode material is skeleton-like or net-like of particles forming two reticular systems that are connected by sintering and are joined together or intersected with each other. A) a two meshed system comprising a connected structure, wherein the two meshed systems comprise a first meshed system of ceramic material and a second meshed system comprising a metal for generating electrical conductivity. The electrode material, on the one hand, does not occur in the first reticulated system made of ceramic material by a substantial change between the oxidation and reduction states on the one hand and on the one hand the oxidation or reduction of the metal. As a result, the second reticulated system is formed. In addition, the two reticulated systems form a compact structure comprising micropores 110 having a ratio of less than 20%, preferably less than 5%, by volume relative to the electrode material in the oxidized state.

The term reticulated system was introduced here. This should be understood as the skeletal or net-connected structure of the particles. Due to the connection of the mesh-like system, this is given structural stability and / or electrical conductivity. In the mesh-like system, no chemical change occurs so that structural stability exists. This is the case for ceramic reticulated systems. The second reticulated system has a structure that changes due to a redox change so that only low structural stability is present. The function of the second reticulated system as an electrically conductive connection is maintained due to the structural stability of the first reticulated system. When the ratio of the volume ratios of each reticulated system reaches at least 30% and the particles are homogeneously mixed with each other, the two reticulated systems are prepared when these particles are each prepared with a narrow size spectrum for the two particle types. It naturally occurs in the form of a statistical distribution of constituent particles. (However, relatively large particles contained in an isolated manner in a matrix of fine particles can also be mixed in a ceramic reticulated system.) A system formed by pores is similar to a reticulated system. This reticulated system generates the required gas permeability of the anode structure.

In a preferred embodiment, the anode structure according to the invention has both the function of the supporting structure and the function of the intermediate anode layer. However, it only forms a support for the anode, which serves as a support structure for the intermediate anode layer. The body according to this structure must be made as strong as a supporting structure, for example withstanding a mechanical load of 20 kPa, the load of this size being present in the stack arrangement of fuel cells.

Claims 2 to 8 relate to a preferred embodiment of the anode structure according to the invention. Claims 9 and 10 are a high-temperature fuel cell having an anode structure according to the present invention, respectively, and a method of manufacturing the anode structure.

The invention will be explained below with reference to the accompanying drawings.

1 is a schematic view of a fuel cell.

2 is a cross-sectional view of a porous material that can be used as the anode structure according to the present invention.

3 and 4 are perspective views of sections taken from two reticulated systems, ie skeletal or net-connected particle structures.

5 is a perspective view of a section of the two meshed systems of FIGS. 3 and 4 coupled together;

6 is a cross-sectional view of the anode structure in which a larger grain ceramic material is incorporated into the electrode material.

In the high temperature fuel cell shown schematically in FIG. 1, the electrode reaction is carried out to produce a current I, ie the reduction reaction takes place on the anode 1 or in the anode 1, and water or carbon monoxide is a gaseous fuel. generate from the hydrogen and carbon monoxide to form, and the oxidation reaction takes place at the cathode (2), the metal conductor 20 and from the pole (24) e (e ^-), the second gas flow while taking the oxygen (e.g., air Ions (O ^2- ) are formed from oxygen molecules of O ₂ and N ₂ . Oxygen ions move through the solid electrolyte 3, which separates the two electrodes 1, 2 in a manner that is impermeable to gas and is conductive to oxygen ions at temperatures of 700 ^° C. or higher. The reducing anodic reaction takes place with oxygen ions during the release of electrons to the additional metal conductor which creates a connection to the pole 14. A consumer 4 is arranged between the poles 14, 24 that applies a load of resistor R to the fuel cell. The voltage U between the poles 14, 24 is generated 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 consisting of a homogeneously formed porous support structure 10 and a more compact extra zone 11. The pores of the support structure 10 are macropores 100 and micropores 110, see FIG. 2. The extra zone 11 in the example shown includes only micro pores 110. Preferably, the thin solid electrolyte layer 3 and the adjacent layer, which is the anode 2, can be produced, for example, by a heat injection method, which can be produced while using the screen printing method. The material for the electrodes 1 and 2 must be usable at operating temperatures up to 1000C. The macropores 100 of the anode structure 1 must form a communication space, which generates permeability to gaseous fuel suitable for the electrode reaction. This permeability exists up to the extra zone 11 which forms the boundary region under the electrolyte layer 3. Additional gas permeability is given to this boundary zone 11 by micropores 110. The pores given by the micropores 110 are larger than those shown in the reduced state of the anode structure 1.

The anode structure 1 according to the present invention has a structure formed of the macropores 100 and the electrode material 5. The macro pores 100 form a communication space created by the pore forming material. The electrode material 5 (see FIGS. 3 to 5) has a skeleton-like or net-like connection structure composed of particles 60 and 70 connected by sintering. Include. These particles 60, 70 intersect two reticular systems 6, 7 (see FIG. 5) intersected with each other, ie a first reticulated system 6 of ceramic material (see FIG. 3). And to form a second meshed system 7 (see FIG. 4) comprising a metal to generate electrical conductivity. According to the invention, the electrode material 5 has the property that the following applies to multiple changes (oxidation reduction changes) between oxidation and reduction states, while no chemical or structural changes occur in the ceramic mesh system. On the other hand, oxidation or reduction of the metal contained in the particles 70 causing aging generates a second mesh system due to the redox change. The two meshed systems 6, 7 together form a compact structure comprising micropores 110 in an oxidized state with a volume ratio of less than 20% relative to the electrode material 5. This volume ratio is preferably less than 10%, more preferably 5%. During the reduction of the metal, the size of the particles 70 in the second reticulated system 7 decreases, so that additional micropores (not shown) are generated. Upon repeated oxidation of the metal, these additional micro pores that have to be designated as "Type II micro pores" disappear. The micro pores 110 of the oxidized anode structure 1 are "Type I micro pores". "Type II micropores" have a lower impact on the structural stability of the electrode material 5 than "Type I micropores".

As could be determined empirically, the reticulated system occurs when the particles 60, 70 have a diameter of an average value (d ₅₀ ) of less than 1 μm before sintering (d ₅₀ = 50% by the volume fraction of the particle d _50). Have a smaller diameter). In this regard, micropores of less than 3 μm in diameter occur. Redox reaction stability is better for smaller diameters. Preferably, the anode structure is produced so that micropores of diameter smaller than 1 μm are generated. The volume ratio of the ceramic reticulated system to the electrode material should be at least 30%. In order that aging does not cause an unacceptable loss of electrical conductivity too quickly, the volume ratio of the second reticulated system must be greater than the volume ratio of the ceramic reticulated system, for example by a factor of at least 1.5.

The particles must be fine enough to form a mesh system. This can be done, for example, by grinding coarse particle powder. Coarse particles may be removed from the ground product by sorting (eg, by screening) as needed. The required powder quality is obtained, for example, by reprocessing the powder by spray drying. Suitable finely dispersed particles can be prepared using methods of nanotechnology, eg by reaction spray methods, spray flame pyrolysis, precipitation methods or sol / gel methods.

If zirconium oxide (YSZ) is provided as the material for the electrolyte, the same ceramic material is also preferably used for the anode structure, that is, the first mesh system. Nickel oxide particles comprising up to 10 percent by weight of non-nickel oxide material are preferably used for the second reticulated system. Such nickel oxide commercially available as a dye component is available as a low cost raw material. Such dye particles also preferably comprise a material which acts as a sintering aid in the sintering of the anode structure.

It is preferred for the production of the anode structure according to the invention that at least one kind of additive is included in the second reticulated system. Such additives may act as sintering aids upon sintering.

Additional additives can prevent damaging particle growth as inhibitors during operation of the fuel cell. Oxides or salts of Ni, Mn, Fe, Co and / or Cu may be used as sintering aids and MgO may be used as an inhibitor of particle growth.

The anode structure according to the present invention must have a communication space that allows the proper gaseous fuel to penetrate the current supply electrode reaction. This gas permeability is generated using macropores whose diameters range between 3 and 20 μm. Macro pores can be prepared using pore forming materials. For this purpose, particles or fibers of organic material, in particular cellulose, are used. These organic materials decompose during the sintering performed under the oxidized state, and the decomposition products evaporate.

In a suitable method for producing the anode structure according to the invention, the process is as follows, i.e. particles of ceramic material (e.g. YSZ) and metal oxides (e.g. green or brown nickel oxide particles) are ground and classified. By forming a fine enough shape for the formation of a mesh system. Homogeneous mixtures in the form of slurries are formed from particles, pore forming materials and liquids. The slurry is cast to form a layer. Once the slurry is cast in the absorbent mold, some of the liquid is removed from the slurry. At the same time, an extra zone occurs in which there is a lack of pore forming material. Thus, a nonhomogeneous structure occurs as shown in FIG. 2.

After the liquid is completely removed from the slurry layer by drying, sintering is performed, which is preferably performed with a solid electrolyte applied to the layer. On the one hand there is an optimum sintering temperature in which a structure with small micropores occurs and on the other hand the undesirable effects of the reticulated structure are kept to a minimum. The higher the sintering temperature, the lower the density of micropores, but the more strongly the mesh structure is damaged.

FIG. 6 shows an anode structure in which larger particles 60 'of ceramic material are incorporated into electrode material 5. Coarse particles 60 ′ are isolated and merged in a matrix of fine particles 60, 70 (see FIG. 5). The manufacturing cost can be reduced by coarse particles 60 'with diameters in the range between 2 and 10 [mu] m because less material is required to be in the required fine form. In addition, when low proportions of micropores occur in volume ratio due to the coarse particles, the greater strength of the ceramic reticulated system can be obtained by the added coarse particles.

The anode structure according to the invention is produced locally, for example in plate form or shell-shape. In plate form, it has a large range in two dimensions, and in three dimensions it has a relatively small range of thickness but is larger than 0.5 mm. The required supporting force of the anode structure is achieved at this thickness.

The ceramic mesh system 6 is in addition to stabilized zirconium oxide (YSZ), but also aluminum oxide (Al ₂ 0 ₃ ), titanium oxide (TiO ₂ ), doped ceramic oxide (CeO ₂ ), magnesium oxide ( MgO), and / or spinal compounds. Oxides of the materials Y, Ti, Gd and / or Co can be used as the doping means of CeO ₂ .

According to the manufacturing method of the present invention, unlike the high temperature fuel cells also described in DE-A 19 819 453, the anode structure corresponding to the anode substrate and the intermediate anode layer can be manufactured in the usual working steps. This is clearly a simplification of the method and has a positive effect on the manufacturing cost.

Additional methods known in principle can also be used for the production of the anode structure according to the invention, namely film casting, roll pressing, wet pressing or isostatic pressing. Can be used.

The present invention provides a sufficiently good anode structure with regard to redox reaction stability, gas permeability and efficiency for use in fuel cells.

According to the manufacturing method of the present invention, the anode structure corresponding to the anode substrate and the intermediate anode layer can be manufactured in a usual working step. This is clearly a simplification of the method and has a positive effect on the manufacturing cost.

Additional methods basically known can also be used for the production of the anode structure according to the invention, ie film casting, roll pressing, wet pressing or isostatic compression molding can be used.

Claims (10)

In a structured body for the anode (1) used for a fuel cell, A structure formed by the macropore 100 and the electrode material 5, The macro pores form a communication space generated by the pore forming material, The electrode material is connected by sintering and is a skeleton-like or net of particles 60, 70 which form two reticular systems 6, 7 which are joined together. Contains a net-like connected structure, The two reticulated systems 6, 7 are a first reticulated system 6 made of ceramic material and a second reticulated system 7 comprising a metal that generates electrical conductivity, The electrode material, on the one hand, does not occur in the first reticulated system of ceramic material that is substantially altered by a multiple change between the oxidation and reduction states and on the other hand the oxidation or As a result of the reduction, the second reticulated system is formed, The two reticulated systems form a compact structure comprising micropores 110 having a ratio of less than 20%, preferably less than 5%, by volume relative to the electrode material in the oxidized state together. Structure for anodes used in fuel cells. The method of claim 1, The ceramic reticulated system 6 may comprise relatively coarse particles 60 'having a diameter between 2 and 10 μm, the particles 60, 70 having an average value of d ₅₀ equal to 1 without being sintered. having a diameter of less than μm, the micropores 110 have a diameter of less than 3 μm and preferably less than 1 μm, and the ratio of the volume ratio of the ceramic reticulated system to the electrode material reaches at least 30%. Structure for anodes used in fuel cells. The method according to claim 1 or 2, The ceramic mesh-like system 6 is zirconium oxide (YSZ), aluminum oxide (Al ₂ O ₃ ), titanium oxide (TiO ₂ ), doped cerium oxide (CeO ₂ ), magnesium oxide stabilized with yttrium Zirconium oxide (YSZ) composed of (MgO) and / or spinal compounds, wherein the particles (60, 70) are particularly used for the electrode material (5) and stabilized for the first reticulated system ) Or used in a fuel cell comprising at least 90% nickel oxide by weight for the second reticulated system. The method according to any one of claims 1 to 3, The macro pore 100 is a structure for an anode used in a fuel cell having a diameter in the range of 3 to 20 μm. The method according to any one of claims 1 to 4, Particles or fibers of organic material, in particular cellulose, are used as a pore forming material for the particles and used in a fuel cell which is evaporated upon sintering in an oxidized state. The method according to any one of claims 1 to 5, At least one kind of additive is included in the second reticulated system 7, the first kind or the only kind of additive acting as a sintering aid during sintering and the second kind of additive during the operation of the fuel cell. A structure for anodes used in a fuel cell that acts as an inhibitor to particle growth. The method of claim 6, A structure for a positive electrode used in a fuel cell in which oxides or salts of Ni, Mn, Fe, Co, and / or Cu are used as the sintering aid and / or MgO is used as an inhibitor of grain growth. The method of claim 6, The structure is a plate-shaped or shell-like (shell-like) structure for the anode used in the fuel cell having a plate thickness or shell thickness greater than 0.5mm. A high temperature fuel cell with a structure according to any of claims 1 to 8, wherein The electrode material may be used at an operating temperature of up to 1000 ^° C., and the anode 1 forms a thin solid electrolyte layer 3 and support structures 10 and 11 for the cathode 2, wherein the anode of the anode The communication space allows gaseous fuel to sufficiently penetrate into the boundary zone 11 below the electrolyte layer for the current supply electrode reaction, and further gas permeability in the boundary zone 11 below the electrolyte layer allows the micropores of the electrode material. Given by 110 High temperature fuel cell. In the method of manufacturing a structure according to any one of claims 1 to 8, The particles 60, 70 are shaped fine enough for the formation of the reticulated systems 6, 7 by grinding and sorting, and the homogeneous mixture in slurry form is combined with the pore forming material and the liquid. Formed from particles, the slurry is poured to form a layer, the layer is sintered after removal of the liquid, and the sintering is preferably carried out with a solid electrolyte layer attached to the layer. A method of making a structure.