Tubular solid oxide fuel cell stack

Details Tubular solid oxide fuel cell stack Tubular solid oxide fuel cell stack

2002-02-14

2004-11-30

Kalafut, Stephen J. (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C264S618000, C264S619000, C096S010000

Reexamination Certificate

active

06824907

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to fluid separators and in particular to a stack of tubular solid oxide fuel cells.
BACKGROUND OF THE INVENTION
In general, a solid oxide fuel cell (SOFC) comprises a pair of electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of about 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO
3
), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO
2
and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:
H
2
+O
=
→H
2
O+2e
−
CO+O
=
→CO
2
+2e
−
CH
4
+4O
=
→2H
2
O+CO
2
+8e
−
Cathode reaction:
O
2
+4e
−
→2O
=
Known SOFC designs include planar and tubular fuel cells. Applicant's own PCT application no. PCT/CA01/00634 discloses a method of producing a tubular fuel cell by electrophoretic deposition (EPD). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube.
It is also known to arrange a plurality of tubular fuel cells in an array or “stack” to increase electrical output. Designs have been proposed for stacking together relatively large-diameter (≧2 mm) thick-walled tubular fuel cells that are essentially self-supporting; for example it is known to stack large diameter tubular fuels cells in a grid-like pattern and interconnect the fuel cells with nickel felt spacers. This and other known designs for large diameter self-supporting tubular fuel cells are not particularly well suited for small diameter fuel cells (≦2 mm), especially if such small diameter fuel cells are arranged into a tightly-packed array. It is therefore desirable to provide an improved stack design that enables the close-packing of a plurality of small-diameter tubular fuel cells.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a fuel cell stack comprising a plurality of tubular fuel cells embedded in a solid-state electronic or mixed (electronic and ionic) conductive porous matrix. Each fuel cell comprises an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers. A first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof. The matrix may have a foam-like microstructure, and may have a porosity of between 40 and 95%.
The fuel cells may be of the solid-oxide type and in such case the matrix composition may include an electronic or mixed conductive material. In particular, the matrix material may be lanthanum strontium manganate. The diameter of at least one of the fuel cells may be in the range of about 10 &mgr;m to 2000 &mgr;m. The inner electrode layer may be an anode and the outer electrode layer a cathode, and in such case, the first reactant is oxidant and the second reactant is fuel. The inner electrode layer of at least one the fuel cells may be produced by one of electrophoretic deposition, metal electrodeposition, or composite electrodeposition.
According to another aspect of the invention, there is provided a method of producing a fuel cell stack that comprises:
(a) producing a plurality of tubular fuel cells, each fuel cell having an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers;
(b) coating the fuel cells with a slurry having a composition that includes a matrix material that upon sintering, becomes a solid-state electronic or mixed (electronic and ionic) conductive porous matrix;
(c) stacking the fuel cells such that the slurry coating of each fuel cell is in contact with the slurry coating of adjacent fuel cells; and
(d) sintering the coated and stacked fuel cells to solidify the matrix and embed the fuel cells therein,
thereby producing a stack wherein a first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells, and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof.
The step of producing the fuel cell may comprise first forming an inner electrode layer on a combustible deposition cathode by one of electrophoretic deposition, metal electrodeposition, or composite electrodeposition, then forming an electrolyte layer on the inner electrode layer by electrophoretic deposition, then forming an outer electrode layer onto the electrolyte layer, and then applying a sintering step that combusts the deposition cathode, thereby leaving a hollow tubular fuel cell.
The matrix material in the slurry may be one in the group of lanthanum strontium manganate, doped LaCrO
3
(La
1−x
Sr
x
Cr0
3
, La
1−x
Ca
x
Cr0
3
, La
1−x
Mg
x
Cr0
3
, LaCr(Mg)0
3
, LaCa
1−x
Cr
y
0
3
, LaCr(Mg)O
3
, stainless steel (316, 316L), cermet (such as Ni-Yittria stabilized zirconia or any Ni and doped zirconia cermet, Ni doped—Ce0
2
cermet, Cu doped-ceria cermet), silver and its alloys, Inconel steel or any super alloy, or ferritic steel, SiC, MoSi
2
. The slurry may further include a foaming agent, such that upon a selected heat treatment, a solid-state porous matrix is formed having a foam-like microstructure. The slurry may also or instead include combustible particles, such that upon a selected heat treatment, a solid-state porous matrix is formed having a porous microstructure.
The steps of coating the fuel cells with slurry and stacking the fuel cells may comprise stacking the fuel cells in a container, then adding the slurry into the container such that the fuel cells in the container are immersed in the slurry. Alternatively, the steps of coating the fuel cells with slurry and stacking the fuel cells comprise coating each fuel cell then placing combustible spacers between the fuel cells before stacking. Yet another alternative approach comprises coating the fuel cells then placing the coated fuel cells on a flexible sheet, then manipulating the sheet such that the fuel cells are arranged into a desired stack configuration.
According to yet another aspect of the invention, there is provided a method of producing a fuel cell stack that comprises:
(a) producing a plurality of tubular fuel cells, each fuel cell having an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers;
(b) providing a container of combustible members in a desired stack configuration and immersed in a slurry having a composition that includes a matrix material that upon sintering, becomes a solid-state electronic or mixed (ionic and electronic) conductive porous matrix;
(c) sintering the slurry and combustible members such

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