Integrated SOFC

Details Integrated SOFC Integrated SOFC

2000-08-18

2003-05-06

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C429S047000

Reexamination Certificate

active

06558831

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to solid oxide fuel cells (SOFCs) and, more particularly, to a multilayered, multifunctional electrolyte in solid oxide fuel cells having high mechanical strength, high ionic conductivity, stability in air and fuel, chemical compatibility to other portions of the fuel cell, and reduced temperature operation.
A solid oxide fuel cell is an energy conversion device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across an oxide electrolyte. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient power source and has been under development for a variety of power generation applications.
Under typical operating conditions, an SOFC single cell produces less than 1 V. Thus, for practical applications, single cells are stacked in electrical series to build voltage. Stacking is provided by a component, referred to as an interconnect, that electrically connects the anode of one cell to the cathode of the next cell in a stack. Conventional SOFCs are operated at about 1000° C. and ambient pressure.
An SOFC single cell is a ceramic tri-layer consisting of an oxide electrolyte sandwiched between an anode and a cathode. The conventional SOFC materials are yttria-stabilized zirconia (YSZ) for the electrolyte, strontium-doped lanthanum manganite (LSM) for the cathode, nickel/YSZ for the anode, and doped lanthanum chromite for the interconnect. Currently, there are two basic cell constructions for SOFCs: electrolyte-supported and electrode-supported.
In an electrolyte-supported cell, the electrolyte is the mechanical support structure of the cell, with a thickness typically between 150 and 250 &mgr;m. Electrolyte-supported cells are used, for example, in certain planar SOFC designs. In an electrode-supported cell, one of the electrodes (i.e., the anode or cathode) is the support structure. The electrolyte is a thin film (not greater than 50 &mgr;m) that is formed on the support electrode. Tubular, segmented-cells-in-electrical-series, and certain planar SOFC designs, employ this type of cell.
Conventional YSZ-based SOFCs typically employ electrolytes.thicker than 50 &mgr;m and require an operating temperature of 1000° C. to minimize electrolyte ohmic losses. The high-temperature operation imposes stringent material and processing requirements to the fuel cell system. Thus, the recent trend in the development of SOFCs is to reduce the operating temperature below 800° C. The advantages of reduced temperature operation for the SOFC include a wider choice of materials, longer cell life, reduced thermal stress, improved reliability, and potentially reduced fuel cell cost. Another important advantage of reduced temperature operation is the possibility of using low-cost metals for the interconnect.
Various attempts have been made to reduce the operating temperature of YSZ-based SOFCs while maintaining operating efficiency. One attempted method reduces the thickness of the electrolyte to minimize resistance losses. Various methods have been evaluated for making cells with thin films (about 5 to 25 &mgr;m thick). Electrode-supported cells (specifically, anode-supported cells) with thin electrolyte films have shown high performance at reduced temperatures. Power densities over 1 W/cm
2
at 800° C. have been reported, for example, in de Souza et al.,
YSZ
-
Thin
-
Film Electrolyte for Low
-
Temperature Solid Oxide Fuel Cell,
Proc. 2
nd
Euro. SOFC Forum, 2, 677-685 (1996); de Souza et al.,
Thin
-
film solid oxide fuel cell with high performance at low
-
temperature,
Solid State Ionics, 98, 57-61 (1997); Kim et al.,
Polarization Effects in Intermediate Temperature, Anode
-
Supported Solid Oxide Fuel Cells,
J. Electrochem. Soc., 146 (1), 69-78 (1999); Minh,
Development of Thin
-
Film Solid Oxide Fuel Cells for Power
-
Generation Applications,
Proc. 4
th
Int'l Symp. On SOFCs, 138-145 (1995); Minh et al.,
High
-
performance reduced
-
temperature SOFC technology,
Int'l Newsletter Fuel Cell Bulletin, No. 6, 9-11 (1999). An alternative attempt at reducing operating temperature has involved the use of alternate solid electrolyte materials with ionic conductivity higher than YSZ, as described in Minh,
Ceramic Fuel Cells,
J. Am. Ceram. Soc., 76 [3], 563-88 (1993). However, the work on alternate electrolyte materials is still at a very early stage.
The electrolyte and cathode have been identified as barriers to achieving efficiency at reduced operating temperatures due to their significant performance losses in current cell materials and configurations. With YSZ electrolyte-supported cells, the conductivity of YSZ requires an operating temperature of about 1000° C. for efficient operation. For example, at about 1000° C. for a YSZ electrolyte thickness of about 150 &mgr;m and about a 1 cm
2
area, the resistance of the electrolyte is about 0.15 ohm based on a conductivity of about 0.1 S/cm. The area-specific resistance (ASR) of the electrolyte is, therefore, about 0.15 ohm-cm
2
. For efficient operation, a high-performance cell with an ASR of about 0.05 ohm-cm
2
is desired. To achieve an ASR about 0.05 ohm-cm
2
at reduced temperature operation (for example, 800° C.), the required thickness (i.e., 15 &mgr;m) of YSZ can be calculated. If the desired operating temperature is less than 800° C., while the ASR remains the same, either the thickness of YSZ must be further reduced or highly conductive alternate electrolyte materials must be used.
For alternate electrolyte materials in SOFCs, the desired operating temperature determines the choice of materials to achieve high performance. The conductivity and stability of the electrolyte are two key parameters in the selection of the electrolyte material. The highest ionic conductivities are typically found in the fluorite, perovskite, and brownmillerite structures, as indicated by Boivin et al., Chem Mater., 10, p. 2870 (1998). These include doped materials of Bi
2
O
3
, CeO
2
, LaGaO
3
, and Sr—Fe—Co oxides. Of these materials, doped Bi
2
O
3
, is unstable in a fuel atmosphere and doped Sr
2
Fe
2
O
5
is a mixed ionic and electronic conductor. Therefore, these two materials are unsuitable for use as SOFC electrolytes. Doped CeO
2
in one layer and YBa
2
Cu
3
O
7
in another layer of a bilayer electrolyte have been attempted to increase the open circuit voltage (OCV) in U.S. Pat. No. 5,731,097. Another bilayer electrolyte is shown in U.S. Pat. No. 5,725,965, wherein one layer of doped Bi
2
O
3
is protected from the fuel environment by a protective layer of doped CeO
2
.
At reduced operating temperatures of about 550 to 700° C., ceria (CeO
2
) doped with Gd (CGO) and lanthanum gallate (LaGaO
3
) doped with Sr, Mg (LSGM) or Fe in addition to Sr and Mg (LSGMFe) have been considered due to their high conductivity [Feng et al., Eur. J. Solid St. Inorg. Chem., 31, p. 663 (1994); Huang et al.,
Superior Perovskite Oxide
-
Ion Conductor; Strontium
-
and Magnesium
-
Doped LaGaO
3
: I, Phase Relationships and Electrical Properties,
J. Am. Ceram. Soc., 81, [10], 2565-75 (1998); Steele,
Oxygen transport and exchange in oxide ceramics,
J. Power Sources, 49, 1-14 (1994); Ishihara et al.,
Intermediate Temperature Solid Oxide Fuel Cells Using LaGaO
3
Electrolyte Doped with Transition Metal Cations,
Proc. Electrochem. Soc. Mtg., Seattle, May 2-5 (1999)]. For example, a 15 &mgr;m LSGMFe electrolyte can be operated at about 525° C. with an ASR of 0.05 ohm-cm
2
. Of the materials above, CGO has significant electronic conductivity above 500° C. in the fuel atmosphere, leading to low open circuit voltage and decreased fuel cell efficiency.
Therefore, to be a useful electrolyte, CGO must either be used at 500° C. or lower according to Doshi et al.,
Development of Solid
-
Oxide Fuel Cells That Operate at
5

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