Solid oxide electrolyte fuel cell

Details Solid oxide electrolyte fuel cell Solid oxide electrolyte fuel cell

1999-07-13

2001-10-02

Brouillette, Gabrielle (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06296963

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a compact and economical solid oxide electrolyte fuel cell which comprises a simple structure to provide a large power generation area, and the thermal stress generated therein is low to ensure reliable fuel cell characteristics.
2. Background Technology
Conventionally, as a planar type solid oxide electrolyte fuel cell (hereinafter SOFC) in which power generation films comprise a dimpled structure (hereinafter “dimples”), the configuration shown in
FIGS. 15 and 16
is known.
FIG. 15
is an exploded perspective view of the SOFC and
FIG. 16
is a sectional view taken along line X—X in FIG.
15
. In these figures, reference numeral
1
denotes an interconnector (also called a gas separator);
2
, a power generation film, which together with the interconnector
1
constitute a unified fuel cell structure
10
(hereinafter “stack”) in such a manner that from top to bottom an interconnector
1
, a power generation film
2
, an interconnector
1
, and so on, are alternately superposed.
The power generation film
2
is the smallest unit cell (also called a cell) that constitutes a SDFC, and is mainly composed of a solid electrolyte film
20
comprising convex dimples
21
and concave dimples
22
substantially all over its surfaces, an oxygen electrode
23
on one side of the solid electrolyte film
20
, and a fuel electrode
24
on the other side thereof.
In the SOFC shown in
FIGS. 15 and 16
, the side having convex dimples
21
is the oxygen electrode
23
and the side having concave dimples serves as the fuel electrode
24
. The above power generation film
2
is surrounded by a seal material
3
in the circumference thereof, except for gas inlet and outlet apertures through which oxidant gases (e.g. air, hereinafter air) and fuel gases pass, and each film is sandwiched between two interconnectors
1
to form air flow passages
41
and fuel gas flow passages
42
.
The above interconnectors
1
, on the other hand, are connected with the seal material
3
to provide space between themselves and the adjacent power generation film
2
and constitute gas flow passages, while providing electrical functions for series connection by contacting or connecting with the dimple protrusions of the adjacent power generation films.
The SOFC stack
10
thus constituted is then kept in a high temperature range of 800° C.~1000° C., and power is generated when air flows through the air passages
41
and a fuel gas through the gas passages
42
, respectively, as illustrated in
FIGS. 15 and 16
.
In the example in
FIG. 15
, the air flow and the fuel gas flow are perpendicular to each other on the top and the bottom side of each power generation film
2
. For this reason, a SOFC having this kind of gas flow layout is generally called a cross flow type.
In the cross flow type shown in
FIG. 15
, an air inlet
43
is provided on one of the four sides of each planar type cell and an air outlet
44
is constituted on the opposite side thereof, whereas a fuel gas inlet
45
is given on one of the remaining sides and a fuel outlet (not shown) is prepared on the facing side thereof.
On the other hand, there is a planar cross flow type SOFC, in which each power generation film
2
is a flat plate without any dimple and grooves are provided in the interconectors
1
to constitute gas flow passages. A typical example of this configuration is shown in
FIGS. 17 and 18
.
FIG. 17
is an exploded perspective view of the said SOFC; FIGS.
18
(
a
) and
18
(
b
) are sectional views taken along line X—X and line Y—Y in
FIG. 17
, respectively
Each power generation film
2
is a flat plate without any dimple, and is composed of a solid electrolyte film
20
, an oxygen electrode
23
on one side of the power generation film
20
, and a fuel electrode
24
on the other side thereof in FIG.
18
.
In
FIG. 17
, reference numeral
1
denotes an intermediate interconnector on both sides of which multiple rows of grooves
33
for gas flow are provided along the direction of gas flow. Reference numerals
1
c
and
1
d
in the same figure indicate an upper interconnector and a lower interconnector of the stack
11
, respectively. On the surface facing a power generation film
2
of each of these interconnectors, multiple grooves
33
are provided along the direction of gas flow, and the opposite surface thereof is usually flat to fit power collecting parts for taking out electric current. The interconnectors
1
, the upper interconnector
1
c
, and the lower interconnector
1
d
alternately isolate the power generation films
2
, thereby forming air flow passages
41
and fuel gas flow passages
42
between themselves and the adjacent power generation films
2
, and having at the same time functions for electrical series connection by contacting or connecting with the protrusions
32
of the interconnectors and the oxygen electrodes
23
as well as the fuel electrodes
24
of the adjacent power generation films.
In
FIG. 17
, the SOFC stack
11
is composed of an upper interconnctor
1
c
, a power generation film
2
, an intermediate interconnctor
1
, a power generation film
2
, . . . , a lower interconnector
1
d
, which are superposed alternately from top to bottom, thus constituting a unified structure. The stack
11
is then kept in a temperature range of 800° C.~1000° C., and power is generated, as shown in
FIG. 18
, by letting air flow through the air flow passages
41
and a fuel gas through the fuel gas flow passages
42
.
In a SOFC, the operation temperature is as high as 800° C.~1000° C., and the reaction in the fuel cell generates heat. As a result, the temperature distribution in the fuel cell is such that the area near the gas inlet is in a low temperature range and the area near the gas outlet is in a high temperature range. In the SOFC's as shown in FIGS.
15
~
18
in which the gas flow occurs according to the cross flow method, a temperature distribution as shown in FIG.
20
(
a
) is observed. The % values in FIG.
20
(
a
) indicate approximate ratios when the temperature difference between the gas inlet and the gas outlet is regarded as 100%. Once such a temperature distribution occurs, thermal stress is generated in each part of the fuel cell. If the thermal stress becomes too high, the heat build-up associated with the cell reaction increases by taking out much output, for example, and an excessive temperature difference between the gas inlet and the gas outlet results in a higher thermal stress, thus causing the electrical connection between the stacked cells to deteriorate partially, or damaging the surrounding gas seal parts to cause a decrease in power generation capability, which in some cases could lead to fractures of the interconnectors or the power generation films. In such cases, the expected power output cannot be obtained and the function as a fuel cell itself may be lost.
One of the means to avoid such trouble is to decrease the temperature difference between the gas inlet and the gas outlet of the fuel cell by providing much air to remove the reaction heat of the cell, thus maintaining reliable characteristics of the fuel cell.
However, such a method requires high ventilating power to send a large volume of air, as well as a large-sized heat exchanger or heater in order to preheat the large volume of air up to a temperature close to the operating temperature of the SOFC. As a result, the fuel cell becomes uneconomical as a power generation unit.
On the other hand, as a structural means to solve the above problem, there is the so-called co-flow method by which the air and the fuel gas flow parallel to each other in the same direction.
A typical example which employs this method is shown in
FIGS. 21 and 22
. In these figures, reference numeral
5
denotes a header;
6
, a gas pre-flow rectifying section. Both
5
and
6
are provided as rectifying sections for the air or the fuel gas to flow uniformly in one direction. The other reference numerals are the same as those explained in
FIGS. 15
,
16
and
17

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