GB2262382A

GB2262382A - Method of producing a ceramic support for high temperature fuel cells

Info

Publication number: GB2262382A
Application number: GB9224333A
Authority: GB
Inventors: Gary Heath
Original assignee: ABB Patent GmbH
Current assignee: ABB Patent GmbH
Priority date: 1991-11-21
Filing date: 1992-11-20
Publication date: 1993-06-16
Also published as: DE4138273A1; GB9224333D0

Description

2262382 -ramic fuel cell sunDort Me invention relates to a method of

producing a ceramic, gastight support for high-temperature fuel cells which has gas-permeable wall regions. It also relates to the ceramic support produced by such a method. The support contains enclosed gas channels sealed at one end.

i--uel cell supports are needed for fuel ceh systems Such.

in which fuel cells are either applied in layers to the outside surfaces of the support or are assembled in the form of prefabricated fuel cell foils. In such application cases. the support should be gastight to prevent first reaction gases situated in the gas channels coming into direct contact with second reaction gases f!owing past outside the support. In wall regions covered with 0 fuel cells, however, the support should be gas-permeable n order to make gas passage from the gas channels to the 4 fuel Cells possible.

To produce the support, calcium-stabilised zirconium oxide is preferably used, inter alia because its coefficient of expansion is well matched to that of yttriumstabilised zirconium oxide used as electrolyte material for the fuel cells.

It is known from numerous publicationsr for example from DE 39 07 485 All DE 40 11 506 Al and DE 40 33 284 All to produce the ceramic support as a body which is porous in all its parts and which makes possible gas passage from inner gas channels to fuel cell electrodes applied on the outside. Subareas, not covered bv fuel cells, of the outer support su=ace are subsequently covered with special sealing layers in these systems. The supports known from the publLcations mentioned are f4rst produced with gas channels open at the top and bottom. in some fuel cell batteries, however, supports closed at one end are needed. Such a termination of gas channels can then be produced, for example, by applying a terminating 5 panel.

In principle, the production of a generally gastight support which is gaspermeable in selected wall regions is possible with calcium-stabilised zirconium oxide, but it requires high sintering temperatures and the provision of openings in the form of holes or slots in the selected wall regions using mechanical methods. Such a method is described in the non-prior- published patent application P 41 04 841.5. It is proposed therein to punch holes in a gastight support. The price of the starting material necessary therefor is high and the production of the support is complicated.

Proceeding therefrom one object of the invention is to provide an improved method of producing a fuel cell support. According to the invention there is provided a method of producing a ceramic support for a high-temperature fuel cell as set out in appended claim 1. Preferred features of the invention are set out in claims 2 to 8. A preferred embodiment of the invention comprises a method of producing a ceramic support for high-temperature fuel cells which has enclosed gas channels sealed at one end and gas-permeable wall regions for gas passage from the gas channels to electrodes of the fuel cells which are mounted on the outside of the finished support, which method comprises the following production steps:

a) Extrusion of a main body using an extrusion compound containing a spinel or a material mixture containing a spinel, and organic extrusion additives in each case, and sealing of the gas channels at one channel end, b) Immersion of the main body, which is still in the green state, in a solution containing Ca-Zr02 in order to form a first interlayer and subsequent heat treatment, in which process the organic additives escape and a porous structure is produced, d) e) S Masking of the support surfaces, in which 4 outer process the wall regions which are to be gaspermeable when the support is finished are covered with a wax laver or another water- resistant laver,:mmersion of the masked support in a solution -ontaining yr-trium-stabilised zirconium oxide and the gas channels in order to achieve evacuation of t the result that the zirconium oxide penetrates the mores of the support, in which process the outside surfaces covered by mask layers are not wetted by -he zirconium oxide solut4on and a sealing laver is =roduced on the support, Sintering of -:he coated support, in which process Used::.',.e spinel main body and the calcium-stabilzirconium oxide coating remain porous and the::ealina layer -'s sintered in a gastight manner.

The =rociucr--4cn -.mer-.hcd and a sucrorr- produced by::.his method have a number of advantages. Commared with a support composed c..f7 Ca-Zro., the Crice of the starting is substantially lcwer, f= example by a factor material is less than half, as a of 4. The density c he smi,,el result- of which a lcw-weight support can be produced. The sinter,. ng temperature is low. llo holes or slots have to be produced with the aid of tools. Problems due to a mlsmatch of the coefficients of exmansion or to chemical reactions can be avoided by one or more interlayers between the spinel main body and its zirconium oxide sealing layer. The layer combination on the spinel main body ensures gastightness and matching of the coefficient of expansion, and act.- as a diffusion barrier. In addition, the finished support is notable for especially good flatness of. its su=:i-.ace. This is beneficial for the assembly of the prefabricated fuel cells.

Lhe invertion also relates to a ceramic support as set out in claim 9 and 1G.

A su-.:cr-,::-,-)duce,-J a preferre-cl enibeet:LTn-en of tIne merhod accordiliz to the ;E crihed of eYarini-3 Rnd reference to the L -- _s -.le d-altr,- snolls a,'ia=,anmtJ - represent-ltion of part of a su-nlncrz 1 ihicli h=m been produced in accordance with a preferred version of the methode namely using two interlayers 4, 7.

The support 1 shown contains an extruded main body 3 which has gas channels 2. After extrusion, the channels 2 are sealed with extrusion compound at one support end.

The extrusion compound used is a spinel, preferably an MgAl204 spinel containing extrusion additives. A material mixture which contains a spinel in addition to other materials may also be used. Due to a large particle size and low sintering activity of the spinel, a support can be produced which has a good gas permeability, which is desirable for certain wall regions. However, at 9.5 x 10-6K-1, the coefficient of expansion of the MgAl204 spinel is markedly lower than the coefficient of expan- is sion of the solid electrolyte or of the air electrodes (in the event of air conduction in the gas channel) of the fuel cells, which is about 10.5 x 10-6K-1. in addition, at the high operating temperatures of the fuel cells chemical reactions might occur between the spinel and the fuel cell, so that a direct contact is to be avoided.

In the support according to the invention, these problems are solved by a f irst interlayer 4 and a second interlayer 7. The f irst porous interlayer 4 is produced by immersing the still green main body 3 in a solution containing Ca-Zr02. followed by heat treatment, in which process organic substances escape and a porous structure is produced. With said layer 4, at least a good matching of the coefficient of expansion is achieved. To improve the chemical compatibility further, this production step can be repeated using a solution which contains at least one of the riterials A1.0., ZrOV, MgO or a spinel. Very suitable, for example, is a mixture of MgO and a spinel. In this production step, the second interlayer 7, which also has a porous structure, is produced. It is obvious that the immersion in the solution is in each case carried out in such a way that the inside of the gas channels 2 is not coated.

The support 1 has a sealing layer 6 composed of yttriumstabilised zirconium oxide as outermost layer, certain wall regions 5 of the support 1 not being sealed as a 5 result of window-like openings in the sealing layer 6.

To produce the structured sealing layer 6, a mask composed of wax or another non-water- soluble material is applied to the interlayer 7, which material escapes during the subsequent sintering. The wax is applied to the wall regions 5. The support 1 is then immersed in a solution containing yttrium-stabilised zirconium oxide, in which process the wall regions 5 are not wetted. In order to achieve a well adhering joint between the layers 6 and 7, the gas channels 2 may be evacuated during the immersion operation, as a result of which the solution penetrates the layer 7. In the subsequent sintering step, sintering of the entire system is achieved, in which process the gas-permeable wall regions 5 and a flat outside surface are achieved in the region of the sealing layer 6. On the support produced in this way, prefabricated foil-type fuel cells can be assembled, for example with the aid of a ceramic adhesive, or nickel layers may be applied to it as electrical conductors or as interconnection material.

1

Claims

1. A method of producing a ceramic support for high-temperature fuel cells which has enclosed gas channels sealed at one end and gas permeable wall regions for gas passage from the gas channels to electrodes of fuel cells which are mounted on the outside of the finished support, which method comprises the step of extruding a main body using an extrusion compound containing a spinel or a material mixture containing a spinel.

2. A method according to claim 1, further comprising the step of immersing the main body in a solution containing Ca-ZrO 2 in order to form a first interlayer for separating the main body from a sealing layer subsequently applied thereto.

3. A method according to claim 3, further comprising the step of applying an yttrium-stabilised zirconium oxide sealing layer to a part of a surface of the main body.

f k

4. Method of producing a ceramic support for high-temperature fuel cells which has enclosed gas channels sealed at one end and gas-permeable wall regions for gas passage from the gas channels to electro des of the fuel cells which are mounted on the outside of the finished support characterised by the following production steps:

a) Extrusion of a main body using an extrusion compound containing a spinel or a material mixture containing a spinel and organic extrusion additives in each case, and sealing of the gas channels at one channel end, b) Immersion of the main body which is still in the green state, in a solution containing Ca-Zr02 in order to form a first interlayer and subsequent heat treatment, in which process the organic addi tives escape and a porous structure is produced, c) Masking of the outer support surfaces, in which process wall regions which are to begas- permeable when the support is finished are covered with a wax layer or another water-resistant layer, d) Immersion of the masked support in a solution containing yttrium- stabilised zirconium oxide and evacuation of the gas channels in order to achieve the result that the zirconium oxide penetrates the pores of the support, in which process the outside surfaces covered by mask layers are not wetted by the zirconium oxide solution and a sealing layer is produced on the support 11 Sintering of the coated support in which processes the spinel main body and the calcium stabilised zirconium oxide coating remain porous and the sealing layer is sintered in a gastight manner.

is z:

Method according to Claim,, characterised in that the following further production step is inserted between the production step b) and c):

bl) Immersion of the support provided with the first interlayer in a solution containing at least one of the materials A120.... ZrO, MaO and spinel, followed by heat treatment, in which process a second interlayer is produced.

6. Method according to Claim 4 or 51 characterised in that the spinel used is an MgA1,0, spinel.

7. Method according to one of 1- he preceding Claims, characterised in t. hat, in a method step c), the wall regions are covered by screen printing with a wax layer or another wazer-resistanz laver.

8. A method subszantially as herein described with reference t lo the d-awinR.

9. A ceramic Support produced by a method in accordance with any one of the precedin.g claims.

10. A ceramic support substantially as herein described with reference to the accompanying drawing.