Fuel-cell element stack with stress relief and methods

A fuel-cell assembly has a plurality of unit cells, each of the unit cells comprising an anode, an electrolyte, a cathode, and a current collector. The fuel-cell assembly has a plurality of electrical interconnection elements, at least one electrical interconnection element being connected respectively to each anode, to each cathode, and to each current collector of the unit cells. The unit cells are arranged in a stack and are mechanically supported by electrical interconnection elements such that each of the unit cells of the stack has at least one edge free to move relative to the electrical interconnection elements.

TECHNICAL FIELD

This invention relates to fuel cells and more particularly to stress relief in MEMS-based fuel-cell structures and related methods.

BACKGROUND

Various portable devices, such as laptop computers, personal digital assistants (PDA's), portable digital and video cameras, portable music players, portable electronic games, and cellular phones or other wireless devices, require portable power sources. The weight and inconveniences of single-use batteries and rechargeable batteries have motivated efforts to replace those power sources for portable use. Thus, there is an increasing demand for light-weight, re-usable, efficient, and reliable power sources in such applications and in many other applications as well. In attempts to meet these needs, various portable fuel cells have been developed, such as ceramic-based solid-oxide fuel cells, direct methanol fuel-cell (DMFC) systems, reformed-methanol-to-hydrogen fuel-cell (RMHFC) systems, and other proton-exchange-membrane (PEM) fuel-cell systems. Microscale design principles have been applied to the design of portable fuel cells to provide improved power density and efficiency and to provide lower cost. However, microscale designs can be susceptible to thermally-induced mechanical stress. There is a continuing need and a large anticipated market for improved practical compact portable fuel cells with rapid startup times and improved efficiency. There is a particular need for compact portable fuel cells with improved relief of thermally-induced mechanical stress.

DETAILED DESCRIPTION OF EMBODIMENTS

Throughout this specification and the appended claims, the term “fuel cell” means a fuel cell in its usual meaning or a battery cell having at least one each of an anode, a cathode, and an electrolyte. A “unit cell” is one cell comprising an anode, a cathode, and an electrolyte. The term “MEMS” has its conventional meaning of a micro-electro-mechanical system. The term “lateral” is used to mean generally parallel to the principal plane of a generally planar unit cell. For clarity of the description, the drawings are not drawn to a uniform scale. In particular, vertical and horizontal scales may differ from each other and may vary from one drawing to another.

One aspect of the invention is a fuel-cell stack10comprising a number of unit cells20connected together. A first embodiment of such a fuel-cell stack10is shown inFIGS. 1,2A, and2B. A second embodiment of such a fuel-cell stack10is shown inFIG. 3. In the embodiment ofFIGS. 1,2A, and2B, the unit cells20are vertically aligned with each other. In the embodiment ofFIG. 3, unit cells20are staggered, i.e., alternate unit cells extend in opposite directions.

Each unit cell has an anode30, a cathode50, and an electrolyte40in contact with the anode and cathode. As shown inFIG. 2B, each unit cell may also have a current collector55. The unit cell may be a MEMS-based unit cell in which MEMS techniques are used to make the anode30, cathode50, electrolyte40and current collector55. As described in the parent applications incorporated by reference, any of the three elements, anode30, electrolyte40, or cathode50, may support the other two elements of the unit cell. While the unit cell embodiments shown in1,2A,2B, and3have the anode30at the top, the cathode50at the bottom and the electrolyte40in the middle of each unit cell, that configuration is shown for illustrative purposes only and is not intended to be limiting of the invention. A person skilled in the art will readily recognize from the detailed descriptions of the applications incorporated herein by reference that various other configurations of each unit cell may be made.

Each unit cell20of stack10has electrical interconnection elements60, at least one electrical interconnection element60being connected to each anode30and to each cathode50respectively. The unit cells20are also mechanically supported by electrical interconnection elements60. Each unit cell20has at least one edge25free to move relative to electrical interconnection elements60. In the embodiment of1,2A, and2B, the electrical interconnection elements60of unit cells20are vertically aligned with each other. In the embodiment ofFIG. 3, the electrical interconnection elements60are vertically aligned with each other, although unit cells20are staggered, extending alternately in opposite directions from the common alignment axis of the electrical interconnection elements60by which unit cells20are mechanically supported. However, it is not generally necessary for all the electrical interconnection elements60to be vertically aligned along a single common axis. Various arrangements of the electrical interconnection elements60may be used, with various degrees of alignment.

Each of the electrical interconnection elements60may be a conductive pin, for example, mounted in a conventional pin opening (not shown) in each unit cell20. Many suitable interconnection pin materials and configurations are known in the art. The material should have good electrical conductivity and a thermal expansion coefficient that is similar to the overall thermal expansion coefficient of the unit cells20. The material should also be catalytically inert. In the embodiments illustrated inFIGS. 1,2A,2B, and3, each of the electrical interconnection elements60is a ball bond, e.g., a gold ball. Such an interconnection element has advantages, including the inertness and relative softness of gold and a relatively small contact area with unit cell20. However, it is not required that the spherical shape be retained. Square, rectangular, triangular, or other shapes may be used for electrical interconnection elements60if convenient. As shown inFIG. 3, the interior electrical interconnection elements60may be flattened when the stack10is compressed.

As shown inFIGS. 1,2A,2B, and3, unit cells20have four edges25free to move relative to electrical interconnection elements60. It will be readily understood that a mechanical constraint of any of the edges25could reduce the number of edges free to move, but it is desirable to leave at least one edge25of each unit cell unconstrained and free to move relative to electrical interconnection elements60. Thus, various embodiments like those ofFIGS. 1,2A,2B, and3may have one, two, three or four edges25free to move relative to electrical interconnection elements60. An example of an embodiment with only one edge25free to move is one with circular or elliptical unit cells, in which the entire periphery may be considered one edge.

In the embodiments illustrated inFIGS. 1,2A,2B, and3, each of the unit cells20is cantilevered from the electrical interconnection elements60by which it is mechanically supported. The electrical interconnection elements60are disposed between the unit cells of each pair of adjacent unit cells, and each electrical interconnection element60is shared by the two adjacent unit cells.

A third embodiment is shown inFIGS. 4A–4C. As inFIG. 3, the embodiment ofFIG. 4Aalso has unit cells20staggered, i.e., alternate unit cells extend in opposite directions. In this embodiment, the unit cells are supported by bond-pad interconnection elements65, which connect both electrically and mechanically to current collectors55. In this embodiment, the unit cells20are cantilevered from the bond-pad interconnection elements65. The particular dual-comb-shaped configurations of current collectors55shown inFIGS. 2B,4B, and4C are illustrative of a particular design choice for a current collector and should not be considered limiting of the invention. A person skilled in the art will recognize that many other useful design configurations may be used besides the layout shown in the drawings.

It will also be recognized that many types of electrical interconnection elements60or65are suitable, depending on the application, the temperature ranges occurring during startup and operation of the fuel-cell stack, etc. Types of electrical interconnections that are suitable in various applications include ball bonds, bond pads, pins, clips, nail-head bonds, wire bonds, ultrasonic bonds, solder bonds, controlled-collapse bonds, surface-mount bonds, brazed bonds, compression bonds, and welded bonds, for example.

FIG. 5illustrates schematically an exploded perspective view of major components of a fuel-cell assembly, in which the stack10of unit cells is held in a housing or fixture70that has openings or slots for supplying fuel and air or other source of oxidant. Not shown inFIG. 5are leads for current collection from the fuel cell, fasteners, and mounting hardware, for example.

As shown inFIG. 6, a fixture75may be provided, having slots80for gas flow.

In operation each unit cell20has an operable range of thermal expansion determined by the temperature range reached and the thermal expansion coefficients of the various constituents. The fuel-cell assembly is configured so that the edges25that are free to move laterally relative to electrical interconnection elements60remain free to move laterally throughout the entire operable range of thermal expansion. For example, as shown inFIG. 7, a housing or fixture70may be provided, having trays90adapted to receive portion(s) of each unit cell20including the edge(s)25that are free to move laterally relative to interconnection elements60. Thus, each of the movable edges25is loosely (i.e., movably) positioned in one of the trays90of fixture70. Fixture70has vent openings to allow adequate flow of fuel and an adequate oxidant source such as air flow across the unit cells. Not shown inFIGS. 5–7are conventional thermocouples that may be used for monitoring stack temperatures and heaters that may be used for heating the unit cells.

As temperatures vary during start-up, operation, and shut-down of the fuel cell assembly, the unit cells may expand and contract within fixture70in accordance with the composite thermal expansion coefficients. Throughout the operable temperature range, the freely movable edges25remain laterally unconstrained, thus preventing stresses that would otherwise accompany the thermal expansion.

FIG. 7illustrates another embodiment of a fuel cell assembly. In the embodiment ofFIG. 7, stacking trays90also provide alignment of the unit cells, during bonding of the stack. As shown inFIG. 7, each stacking tray90has top and bottom openings large enough to allow interconnection elements60to extend through the openings throughout the stack. The openings are symmetric in shape and size. Thus, while only the top opening is visible inFIG. 7, the bottom opening is identical to the top opening in this embodiment. Stacking trays90also have internal slots (not visible inFIG. 7). As shown inFIG. 7, stacking trays90may be left in place when stack10is assembled into housing70, maintaining alignment of the unit cells20while allowing lateral expansion of the edges25within their individual internal slots. As will be readily understood by those skilled in the art, the height of the internal slot of each stacking tray90may be chosen to limit out-of-plane bending of the unit cell and to limit motion of the unit cell more or less perpendicular to its principal plane due to non-uniform thermal expansion or contraction, while still allowing lateral expansion or contraction substantially parallel to the unit cell's principal plane.

Thus, one aspect of the invention may be embodied in a fuel-cell assembly that has elements for generating electric current. Each of these current-generating elements includes an anode, an electrolyte, and a cathode, and may include a current collector. The fuel-cell assembly also has elements for electrically interconnecting the current-generating elements. At least one of those interconnecting elements is connected to each anode, and at least one of the interconnecting elements is connected to each cathode. Electrical interconnecting elements are also connected to the current collector if one is present. The current-generating elements are mechanically supported by the electrical interconnecting elements, such that each of the current-generating elements has at least one edge free to move relative to the electrical interconnecting elements. In this fuel-cell assembly, each of the current-generating elements has an operable range of thermal expansion. Each edge that is free to move relative to the electrical interconnecting elements remains free to move throughout the operable range of thermal expansion.

Various embodiments illustrate two useful features: the support of each unit cell of a stack by fixed electrical interconnections which also serve as mechanical supports, and the freedom of lateral expansion/contraction of the unit cells, while limiting the range of bending or motion of the unit cells in directions more or less perpendicular to the surface of each unit cell. The latter feature is illustrated by stacking trays90, for example.

Fixture75has a tray90for each unit cell20. Each unit cell is positioned with a portion of the unit cell (including edge25) in an internal slot of tray90, loosely enough positioned so that the edge25of unit cell20may move freely in lateral directions in response to thermal expansion, but the internal slot limits out-of-plane bending and motion of the unit cell. It will be understood that each of the unit cells has an operable range of thermal expansion and that the portion of unit cell20in tray90remains within tray90throughout the operable range of thermal expansion. Thus, each tray90is adapted to receive a unit cell of the stack, and tray90may be used to limit out-of-plane bending and motion while allowing lateral motion of unit cell edges25due to thermal expansion.

Fabrication

Another aspect of the invention is a method for fabricating fuel-cell assemblies. An embodiment for such a method for fabricating the fuel-cell assembly is illustrated by the flowchart ofFIG. 8. The method includes the steps of (S10) providing a multiplicity of fuel-cell unit cells20(each fuel-cell unit cell comprising an anode30, an electrolyte40, and a cathode50); (S20) providing interconnection elements60adapted to connect to the anode30and cathode50of each of the unit cells; (S30) assembling a stack10by stacking the multiplicity of unit cells with at least one of the interconnection elements60disposed between each pair of adjacent unit cells of the stack, while leaving at least one edge25of each of the unit cells free to move; and (S50) affixing each unit cell20in the stack by connecting interconnection elements60to each unit cell20. Stack10is placed (S60) in a fixture75. Fixture75may be housed in a housing70. The assembling step (S30) may performed by the step (S40) of disposing the unit cells20in a staggered configuration. One way of assembling the stack of unit cells20is by disposing each interconnection element60near one edge of each unit cell; then at least one distal edge25of each unit cell is free to move. If alternate free ends25extend in opposite directions, the unit cells20will be staggered. This is one configuration in which unit cells20are cantilevered from interconnection elements60. Affixing step (S50) may be performed by compressing the stack of unit cells20(compression bonding). Alternatively, various other methods of bonding known in the art may be employed, such as brazing, welding, wire- bonding, ultrasonic bonding, or soldering.

Industrial Applicability

Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes can be made thereto by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims. For example, if the thermal expansion of each unit cell is anisotropic, the interconnection elements60and internal slots of the trays may be disposed to direct maximum expansion into the direction of the trays' internal slots. Similarly, several interconnection elements60may be disposed so that they share the load of supporting an individual unit cell. The interconnection elements60may be disposed proximate to one edge of each unit cell, for example.