Patent Application
20150171459
In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates.
A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”.
To facilitate formation of a stack of fuel cell stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost. In forming the stack, the cell-picture frame assembly of each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack.
The separator plate provides for fluid flow separation between the anode and cathode of adjacent cells in the fuel cell stack, and also provides part of an electrically conductive path connecting the anode from one cell in series with the cathode of an adjacent cell. In some fuel cell stack designs, the separator plate itself is configured on one or both sides to provide a three-dimensional structure that provides contact with the electrode of an adjacent fuel cell at a number of locations so that electrical connectivity, with spaces between the points of contact so that fluid (air or fuel) can flow along the surface of the electrode. In other designs, a separate interconnect structure is disposed in the stack between separator plate and the adjacent fuel cell(s).
The cells in a fuel cell stack are electrically connected in series from the anode of one cell through the electrically conductive separator plate to the cathode of an adjacent cell. Electrical contact between the separator plate and the cathode and anode of adjacent cells is typically provided at discrete points of contact the adjacent electrodes with spaces between the points of contact to allow for fluid flow. The points of contact can be provided in various ways, such as by the physical configuration of the separator plate itself (e.g., dimples or ridges) or by interconnect elements disposed between the separator plate and each of the adjacent electrodes. The fuel cell stack is typically sealed along the periphery to contain the fuel and air flows within the stack. However, in order to preclude short circuits around the series connection of the cells through the separator plates, the peripheral seal between adjacent cassettes is typically formed from an electrically insulating seal material such as a glass ceramic.
Typical seals utilized for SOFC stack sealing applications are formed from an alkaline earth aluminosilicate glass, such as a barium-calcium-aluminosilicate based glass, also known as G-18 glass, developed by Pacific Northwest National Laboratory (PNNL). G-18 glass provides a seal material that offers high electrical resistively, high coefficient of thermal expansion, high glass transition temperature, and good chemical stability. Another known type of seals for SOFC stack sealing applications are composite glass seals, which are formed from glass materials mixed with fibers to increase the structural integrity of the glass matrix. Viscous glasses, defined as any glass that remains in a fully or partially amorphous phase within the standard operating temperature of an SOFC stack of about 500° C. to 1000° C., and retains its ability to flow. Examples of viscous glass include B—Ge—Si—O glasses, which retains approximately 70% amorphous phase after 1500 hours at 850 [deg.]C; barium alkali silicate glass; and SCN-1 glass, commercially available from SEM-COM Company, Inc.
Glass ceramic seals are typically sandwiched between two planar surfaces parallel to the plane of the mounted fuel cell. The stack assembly is restrained and/or loaded in the direction perpendicular to the planar fuel cells and the seals to reduce tensile stresses in the seal joint, which ideally results in compressive stress perpendicular to seal/fuel cell plane. This can be beneficial because seal materials such as glass ceramic are often lowest in strength to tensile stress, but highest in strength to compressive stress. Although compressive loading of the fuel cell stack can reduce tensile stresses to which the seal joints are subjected, such loading has no effect on shear stresses within the plane of the seal joint. Although the shear strength of seal materials such as glass ceramic is stronger than tensile strength, it is often not strong enough to meet operational requirements, particularly those experienced during thermal cycling.
Based on the foregoing and other factors, there remains a need for different alternatives for seal joints in fuel cell stacks.
The present invention provides a fuel cell stack assembly comprising first and second fuel cell cassettes joined together by an electrically insulating seal material wherein the seal material is disposed in a first seal retaining area between a recessed portion of the first cassette and a protruding portion of the second cassette.
In another aspect of the invention, the first fuel cell cassette comprises a first fuel cell retainer plate having a first fuel cell subassembly mounted in a central opening of the first cell retainer plate, and a first separator plate. The first separator plate and the first cell retainer plate are joined along mutual edge portions thereof and configured to enclose a first captive space having inlet and outlet openings thereto for fluid flow along a surface of the first fuel cell subassembly. The second fuel cell cassette comprises a second fuel cell retainer plate having a second fuel cell subassembly mounted in a central opening of the second cell retainer plate, and a second cell retainer plate joined along mutual edges thereof and configured to enclose a second captive space comprising having inlet and outlet openings thereto for fluid flow along a first surface of the second fuel cell subassembly. The first cassette and the second cassette are joined together along mutual edge portions of the first separator plate and the second cell retainer plate by an electrically insulating seal material, and are configured to enclose a third captive space having inlet and outlet openings thereto for fluid flow along a second surface of the second fuel cell subassembly.
In yet another aspect of the invention, a method of assembling a fuel cell stack comprises disposing an electrically insulating seal material in a first seal retaining area between a recessed portion of a first fuel cell cassette and a protruding portion of a second fuel cell second cassette, and curing the seal material.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
Referring now to the Figures, the invention will be described with reference to specific embodiments, without limiting same. Where practical, reference numbers for like components are commonly used among multiple figures.
The invention is not limited to a particular cassette design or configuration, as it is directed to the electrically insulating seal between the cassettes, and the design and manufacture of the mating components on adjacent cassettes and the stack assembly. Referring to
Referring now to
The invention provides a robust configuration that is resistant to the deleterious effects of stress on the fuel cell stack structure. The sealing material is retained within a nested configuration between adjacent cassettes where at least a portion of the seal material is captured between opposing cassette surfaces that are perpendicular to or at a substantial angle to the plane of the fuel cell. This geometry can be easily produced inexpensively and reproducibly by stamping sheet metal parts (e.g., having thicknesses of from 0.10 mm to 0.75 mm) Additionally, this geometry can provide retention of viscous glasses, which have shown promise because of their ability to self-heal from cracks, but can be subject to flow-out at SOFC operating temperatures.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.
This invention was made with Government support under DE-NT003894 or DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention.
