High performance cathodes for solid oxide fuel cells

Details High performance cathodes for solid oxide fuel cells High performance cathodes for solid oxide fuel cells

2001-04-10

2003-10-14

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06632554

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to cathodes for solid oxide fuel cells (SOFCs) and, more particularly, to a multi-layered, multifunctional cathode having high conductivity, high catalytic activity, minimized coefficient of thermal expansion (CTE) mismatch, excellent 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 1V. 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.
A 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 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.
Data and information in the literature indicate that SOFC cells can be further developed and optimized to achieve high power densities and high performance at reduced temperature. 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.
Various attempts have been made to reduce the operating temperature of YSZ-based SOFCs while maintaining operating efficiency. 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 an 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 of 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.
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 been 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 other barrier to achieve efficiency at reduced temperature is the cathode
13
. LSM-based cathodes have been used in high-temperature (>900° C.) SOFCs as either a porous structure of sintered LSM particles or as LSM/YSZ mixtures. For operation at reduced temperatures (e.g., 700 to 900° C.), optimization of the mixtures of LSM and YSZ in the cathode has resulted in a cathode ASR of 0.2 to 0.3 ohm-cm
2
at 800° C. For thin-film electrode-supported cells, the total cell ASR is typically less than 0.4 ohm-cm
2
. The performance and losses from each of the cell components of a typical thin-film YSZ electrolyte with an Ni/YSZ anode-support electrode and with an optimized LSM/YSZ cathode are showed in FIG.
1
. As seen in the figure, the loss from the cathode contributes to the majority of the total cell performance loss. When the cell operating temperature is decreased, the cell ASR increases significantly due to an increase in both electrolyte resistance and cathode polarization.
Recently, there have been attempts to increase performance at reduced operating temperatures by developing new cathode materials in combination with new and higher-conductivity electrolytes. These cathode materials are typically designed to overcome the limitations from LSM's oxide ion conductivity described in Steele,
Survey of Materials Selection for Ceramic Fuel Cells II. Cathode and Anode,
Solid State Ionics, 86-88, p. 1223 (1996), the rate of oxygen exchange reaction on the LSM surface, and the moderate electronic conductivity of LSM. One approach involves the development of Ag/yttria-doped bismuth oxide (YDB) cermet cathodes for doped ceria (CeO
2
) electrolytes. The combination of high oxide ion conductivity of YDB and high electronic conductivity of Ag yielded some enhancement in performance between 500 and 700° C. as discussed in Doshi et. al.,
Development of Solid
-
Oxide Fuel Cells That Operate at
500° C., J. Electrochem. Soc., 146 (4), 1273-1278 (1999), and Wang et. al.,
Lowering the Air
-
Electrode Interfacial Resistance in Medium
-
Temperature Solid Oxide Fuel Cells,
J. Electrochem. Soc., 139 (10), L8 (1992). However, YDB is not suitable for use above 700° C. as it reacts with ceria. In addition, Ag tends to d

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