Solid oxide fuel cells and interconnectors

Details Solid oxide fuel cells and interconnectors Solid oxide fuel cells and interconnectors

2002-05-30

2003-11-25

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000, C429S006000, C429S006000, C429S047000, C429S047000, C429S047000, C427S115000, C427S126300, C427S126600, C427S372200, C029S623100, C029S623500, C029S623300

Reexamination Certificate

active

06653009

ABSTRACT:

This invention relates to novel designs for solid oxide fuel cells (hereinafter SOFC) that can convert the chemical energy of fuel and oxidant (air) directly to electrical and heat energy. More particularly, this invention relates to planar SOFCs, novel interconnectors, SOFC stacks, methods for fabricating them, and apparatus including them.
BACKGROUND OF THE INVENTION
Fuel cells are electrochemical devices that convert the chemical energy of fuel and oxidant (air) directly to electrical energy and heat energy. A fuel cell consists of two electrodes, an anode and a cathode, with an electrolyte layer between them. Fuel, such as hydrogen, hydrocarbons or carbon monoxide, is continually fed to the anode and oxidized there to release electrons to an external circuit. An oxidant, such as air, is continually fed to the cathode and reduced there, accepting electrons from the anode through the external circuit. The electrolyte is a gas-tight, pure ionic conductive membrane through which only reactive ions can be transmitted
Such fuel cells have high energy conversion efficiency, since the fuel cell generates electrical energy from chemical energy directly, without any intermediate thermal and/or mechanical energy conversions. Generally a series of such cells are operated together in a stack to provide higher voltage, wherein an interconnector connects the anode of one cell to the cathode of the next cell in the stack.
Current thin film solid oxide fuel cells comprise an anode supporting electrode, a cathode electrode, and a thin film electrolyte between them.
As shown in
FIG. 1
, a dense, thin electrolyte layer
14
about 10 microns thick is deposited between a porous anode support layer
12
and a porous cathode layer
10
. The cathode layer
10
can also be used as the supporting substrate for the thin film electrolyte layer
14
.
These fuel cells have traditionally been made by depositing the electrolyte layer
14
on the anode layer
12
; sintering the bi-layer at high temperatures of about 1400° C.; screen printing the cathode layer
10
on the other side of the electrolyte layer
14
; and sintering the resulting tri-layer at about 1250° C. The need for two firing steps adds to the cost of manufacture, and, because of the difficulty of obtaining a For continuous electrode reactions. Electrons flow through the external circuit from anode to cathode, producing direct current electricity. strong bond between the cathode
10
and the sintered electrolyte layer
14
, good cathode/electrolyte interface properties are not obtained.
One type of ceramic fuel cell has been made by casting a plurality of green tapes comprising an oxide powder and an organic vehicle, including plasticizers, binders, dispersants and the like, for each green tape layer, and stacking them together. The layers are laminated at a predetermined temperature and pressure to obtain a monolithic multilayer green tape stack. The stack is then sintered at high temperatures to remove the organic materials and to form a single solid body.
In order to maintain good bonding between the different layers, it is necessary to keep a high plasticity in the green tape slurry, using sufficient organic additives, typically about 10% by weight of the slurry, of binders and plasticizers. When sub-micron size ceramic powders are to be added however, more organic additives are needed, on the order of about 20-30% by weight of the slurry, in order to obtain good strength and flexibility in the green tape. However, these organic additives decrease the concentration of the ceramic particles, and increase the sintering temperature needed to obtain a fully densified layer. If a thin film layer, i.e., the electrolyte layer, is included in the multilayer structure, this traditional process has to face the challenges of handling the thin film green tape, and ensuring uniformity of the thin film layer thickness after lamination.
Different types of fuel cells are known, typically named for the electrolyte it uses. Solid oxide fuel cells use a solid ceramic as the electrolyte. Planar fuel cells use a solid, thin, flat plate ceramic as the electrolyte, which can be an oxygen ion conductor or a proton conductor. The operating temperature is above 400° C. and generally is about 600-800° C. with high output power density. This high temperature promotes rapid kinetics with non-precious catalyst materials, allows use of hydrocarbon fuels directly, and generates heat as a by-product. However, due to these elevated temperatures, the materials and stack design must adhere to rigorous requirements, both for the materials used and the stack design.
For example, high temperature seals are required at the edges of the electrolyte layers, which are difficult to make. Compressive seals, cement seals and glass seals have been proposed. Compressive seals, using metal rings, can lead to non-uniform stress distribution on the ceramic, causing cell cracks and unstable bonds of the cells to succeeding layers. Gas-tight cement seals are difficult to form. Glass seals are difficult to maintain and they make stack assembly difficult.
A typical thin film electrolyte can be 8 mol % yttria stabilized zirconia, hereinafter YSZ or Y8SZ. Various known methods can be used to make the dense thin film electrolyte, including tape calendering, colloid spray coating, plasma spray coating, sol-gel deposition, sputtering, dip coating, tape cast-laminating and screen printing. However, these various methods have problems of high cost, high processing temperatures and limitations on the materials from which the support anode is made.
A suitable anode can be a porous Ni-YSZ cermet about 500-2000 microns thick, to provide mechanical strength; and the cathode can be a Sr doped lanthanum manganite-(LSM)-YSZ composite about 50 microns thick. Another advantage of using Ni-Y8SZ as the supporting anode is that NiO-YSZ composite, the starting material for the Ni-Y8SZ anode, is stable with YSZ electrolyte at the high sintering temperature of 1400° C. The Ni-Y8SZ, for the most part, is a good anode material with excellent electrical and catalytic properties.
However, the above porous support anode cannot be used with dry hydrocarbon fuels because carbon deposits rapidly in the anode at SOFC working temperatures. Further, this layer, e.g., of Ni-YSZ, must be quite thick, over 500 microns, to provide adequate mechanical support, although the effective reaction zone is a surface layer only about 10-50 microns thick. The thickness of the anode layer will slow down mass transport of fuel gases in the porous anode, and will decrease fuel utilization of the cell.
The cathode suitably can be made of a porous layer about 50 microns thick of a composite of strontium doped lanthanum manganite (LaMnSrO
3
) or LSM and Y8SZ.
In order to increase the voltage generated by a SOFC. a stack or series of such fuel cells is made, connected together by means of an interconnector that connects the cathode of one cell to the anode of the adjacent cell.
The interconnector materials must be electrically conductive, strong and tough at operating temperatures of 650-800° C.; must be chemically and physically stable and non-reactive to other components of the SOFC in both oxidizing and reducing atmospheres at high operating temperatures of 600-800° C.; must have low surface/interface electrical or ohmic resistance; and must have a TCE (thermal coefficient of expansion) that is closely matched to the ceramic components, about 10-11×10
−6
/° C. Further, air and fuel channels must be machined or otherwise formed into the interconnector. These requirements limit the choices of suitable materials from which to make the interconnectors. Conductive ceramics, such as doped lanthanum chromites, are expensive and difficult to machine. Metal interconnects corrode in the presence of reactive gases at high temperatures, weakening them structurally.
High temperature metal alloys have been tried to make interconnectors, such as nickel-based high temperature alloys. However, they have a higher TCE than other components of the

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