Perowskite for a coating of interconnectors

Details Perowskite for a coating of interconnectors Perowskite for a coating of interconnectors

1999-07-23

2001-05-08

Shaw, Clifford C. (Department: 1725)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000

Reexamination Certificate

active

06228522

ABSTRACT:

The invention relates to a Perowskite for the coating of interconnectors which are provided for use in high temperature fuel cells. It also refers to interconnectors and batteries of high temperature fuel cells.
Fuel cell batteries with cells which are arranged in the form of stacks contain connection elements which are inserted between electrochemically active plates, namely interconnectors through which the electrodes of adjacent cells are connected in an electrically conducting manner. Interconnectors of this kind must have coatings which are electrically conducting and are impermeable to chromium oxide. A coating with a known Perowskite, namely LSM, is thermally sprayed on to the surfaces of the interconnectors using a HVOF procedure (High Velocity Oxy-Fuel) up to thicknesses of 150 &mgr;m. At the high temperatures of about 900° C. which are required for the operation of high temperature fuel cells, cells with uncoated interconnectors age very rapidly so that they already no longer yield any usable electric power after 400 hours. Through the coating a chromium oxide discharge is reduced, through which the lifetime is increased up to at least 15000 hours. A degradation of the electrochemically active plates is reduced to less than 1% during an operation period of 1000 hours.
In the HVOF procedure the material to be coated, the mentioned Perowskite LSM, is sprayed on in the form of particles which are accelerated to high (supersonic) speeds, with their temperature being increased during the spraying on to a value which lies in the vicinity of the melting point of the LSM. An even higher temperature, as would be the case in a vacuum plasma spraying process, is avoided in order to be able largely to prevent a chemical segregation of the Perowskite. A segregation would worsen the properties of the coating with respect to the electrical conductivity and to the suppression of the chromium oxide transport. Layers can be manufactured with the HVOF procedure, the porosity of which is less than 25%. At the maximum value of the porosity a sufficient protection against a discharge of chromium oxide results when the layer is 150 &mgr;m thick.
In order that the Perowskite coating is largely homogeneous, the parameters of the HVOF procedure must be set in such a manner that no segregations take place. The energy which the powder particles pick up during the spraying should be kinetic energy to the greatest extent. The temperature and thus the heat energy should be as low as possible. With a large kinetic energy the length of time during which the powder particles are exposed to an increased temperature is less so that the extent of a segregation of the Perowskite is likewise less. Furthermore, the coating should be as impermeable as possible. The more impermeable it is, the greater is the electrical conductivity and the less the layer thickness can be chosen to be, both of which are advantageous with respect to an Ohmic resistance of the coating, which should be as low as possible.
Perowskites can be described by the formula ABO
3
, with A and B signifying two components which are present in addition to oxygen O. For LSM the A component is La
0.8
Sr
0.2
and the B component Mn. Lanthanides, in particular lanthanum La, are expensive materials, so that the LSM contributes in a clearly noticeable manner to the total costs of the fuel cell due to the La contained in it. In order to be able to offer fuel cells as a competitive current source, their manufacturing costs must be reduced massively. One is thus presented the problem of whether there are alternative materials which have the same functional properties as LSM with respect to an interconnector coating but which cost significantly less.
The object of the invention is to find a material for an economical coating of interconnectors which is largely equally as effective as LSM. This object is satisfied by the Perowskite ABO
3−&egr;
which is characterised in claim
1
, with in particular the A component comprising less than 50 atomic percent (w<0.5) of a lanthanide.
The Perowskite is provided for a coating in interconnectors which are used in high temperature fuel cells. Its composition can be described by the formula ABO
3−&egr;
with A=(E
1−w
Ln
w−&dgr;
) and B=(G
1−z
J
z
). In this the following hold: E is an alkaline earth metal, preferably Sr or Ca; Ln is a lanthanide, preferably La or Y; G is a transition metal, preferably Mn; J is a second transition metal, preferably Co; w is a number which is greater than 0.1 and less than 0.5, preferably equal to 0.2; &dgr; is a positive or negative number, the absolute value of which is less than about 0.02; z is a number which is greater than 0.01 and less than 0.5, preferably equal to 0.2 and &egr; is a positive or negative number, the absolute value of which is less than about 0.5.
For the corresponding formula ABO
3−&egr;
of LSM, the following holds for the comparison with the Perowskite in accordance with the invention: w=0.8,&dgr;=0, Ln=La, E=Sr, z=0, G=Mn and &egr;=0.
Whereas in the known Perowskite LSM, segregation phenomena can only be avoided by means of the HVOF procedure, other thermal spraying procedures, for example plasma spraying, can also be used in the coatings with lanthanide-poor Perowskites. Since the tendency toward the formation of segregations is also reduced through the reduction of the lanthanide component.
Claim
2
relates to particularly advantageous Perowskites, which have been selected with respect to thermal expansion and electrical conductivity.
Claims
3
to
6
relate to interconnectors which are coated with Perowskites of the composition in accordance with the invention. The subject of claim
7
is a fuel cell battery with interconnectors of this kind. The present invention also provides fuel cell batteries with interconnectors wherein the interconnectors are arranged between electrochemically active plates of a cell stack, with electrodes of electrochemically active plates of adjacent cells in each case being connected in an electrically conducting manner by an interconnector and with their interconnector coating having a specific electrical resistance which amounts at most to 20 mOHm cm.


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