Patent Grant
7858261
Fuel cells produce electricity from chemical reactions. The chemical reactions typically react a fuel, such as hydrogen, and air/oxygen as reactants, and produce water vapor as a primary by-product. The hydrogen can be provided directly, in the form of hydrogen gas, or can be produced from other materials, such as hydrocarbon liquids or gasses, which are reformed to isolate hydrogen gas. Fuel cell assemblies may include one or more fuel cells in a fuel cell housing that is coupled with a fuel canister containing the hydrogen and/or hydrocarbons. Fuel cell housings that are portable coupled with fuel canisters that are portable, replaceable, and/or refillable, compete with batteries as a preferred electricity source to power a wide array of portable consumer electronics products, such as cell phones and personal digital assistants. The competitiveness of these fuel cell assemblies when compared to batteries depends on a number of factors, including their size, efficiency, and reliability.
However, these factors are constrained by limitations in the art. For example, existing fabrication methods limit the number of fuel cell units that can be housed within a fuel cell housing of a given size. Additionally, each fuel cell in a housing includes a limited region in which it produces electricity, known as the active area of the fuel cell, and existing fabrication methods limit the active areas of each of the fuel cell units. Moreover, existing fabrication methods do not balance the design tradeoffs inherent to portable fuel cells. Certain fuel cells operate at extremely high temperatures, which thermally stresses fuel cell components and may disable the fuel cell. Existing devices do not adequately support the fuel cell components to withstand the stresses.
Thus, a need exists for fuel cell assemblies and fabrication methods that provide fuel cells which overcome limitations in the art.
This invention, in various embodiments, addresses deficiencies in the prior art by arranging a plurality of fuel cell units in one or more planar stacks within a fuel cell housing, and electrically coupling the fuel cell units to provide increased voltages, currents, and/or power. In certain embodiments, a manufacturer adjusts design parameters, including physical dimensions of fuel cell components, to balance fundamental tradeoffs inherent to portable planar fuel cell stacks. More particularly, in one aspect, the fuel cell units are arranged efficiently within the planar stacks such that each fuel cell unit has an active area of more than about 70%. In one aspect, the planar stacks are contained in a fuel cell housing that is less than about 30 cubic centimeters and/or produces more than about 0.1 volts per cubic centimeter of the housing. In certain configurations, the fuel cell assemblies disclosed herein produce more than about 0.5 volts per cubic centimeter, and in others more than about 2 volts per cubic centimeter. In another aspect, the planar stacks are provided in a housing having a volume of between about 0.1 cubic centimeters and about 30 cubic centimeters, and have active areas of between about 0.5 square centimeters and about 100 square centimeters. In certain configurations, the active areas are between about 0.5 square centimeters and about 20 square centimeters, or between about 2 square centimeters and about 10 square centimeters. In certain embodiments, the invention includes structural supports disposed on the fuel cell that reduce thermal stress from, for example, high temperatures and thermal cycles. In one aspect, the invention includes methods of fabricating the fuel cell units, the structural supports, and other structures using micron-level fabrication techniques such as etching. In another aspect, groups of fuel cell units are arranged in respective planar stacks. By varying the number of fuel cell units in the groups and/or varying the size of individual fuel cell units, a manufacturer improves certain performance metrics, such as power density and/or voltage production.
In one aspect, the devices include a planar fuel cell stack of a plurality of fuel cells, comprising an anode layer including a first anode and a second anode, an electrolyte layer, a cathode layer including a first cathode and a second cathode, at least one interconnect at least partially disposed within the electrolyte layer, and electrically and mechanically coupling the first anode and the second cathode, and an elongate structural support oriented perpendicular to a plane extending through the planar stack and at least partially disposed laterally between two adjacent electrodes.
In one configuration, the stack further includes a second structural support in contact with the stack and spaced apart from the elongate structural support. The stack may include an insulating material. For example, the structural support includes an electrically insulating coating. In one feature, the structural support includes an oxidized surface. The structural support may include one or more of a silicon material, a yttria stabilized zirconia (YSZ) material, a magnesium oxide material, a ferro-chromium material, and a ceramic material.
In certain configurations, the structural support has a width of between about 30 microns and about 200 microns, and a height of greater than about 100 microns. In one configuration, the structural support mechanically couples with the electrolyte later.
According to one feature, the first anode and the second anode are laterally separated by a first distance, the first cathode and the second cathode are laterally separated by a second distance, and at least one of the first distance and the second distance is between about 5 microns and about 500 microns. For example, at least one of the first distance and the second distance can be between about 5 microns and about 200 microns.
In one feature, the stack of claim 1, wherein the electrolyte layer includes a first electrolyte region disposed between the first anode and the first cathode and providing a first voltage differential between the first anode and the first cathode, a second electrolyte region disposed between the second anode and the second cathode and providing a second voltage differential between the second anode and the second cathode and the first electrolyte region and the second electrolyte region comprise a monolithic electrolyte structure.
According to certain configurations, the stack comprises one or more of a solid-oxide fuel cell and a proton exchange membrane fuel cell.
According to one feature, the stack includes active regions of the electrolyte layer wherein respective anodes and cathodes have overlapping projections on a surface of the electrolyte layer, and the active regions comprise at least 70% of an area of the surface of the electrolyte layer. The stack may be contained within a housing having a volume of less than about 30 cubic centimeters. The stack may produce a voltage differential of more than about 5 volts, and in some configurations more than about 10 volts or more than about 15 volts. The at least one interconnect may have a cross-section parallel to a plane extending through the electrolyte layer of less than about (100 microns)2. The stack may further include a plurality of interconnects at least partially disposed within the electrolyte layer, and electrically and mechanically coupling the first anode and the second cathode. The plurality of interconnects may comprise a linear array of interconnects, wherein the center-to-center spacings of adjacent ones of the plurality of interconnects are substantially equal.
In one feature, the stack includes three or more fuel cells electrically connected in series, wherein the three or more fuel cells comprise a non-linear array of fuel cells. The devices may include two or more series electrical connections of the fuel cells coupled in a parallel electrical connection.
In one aspect, the devices include a planar fuel cell stack of a plurality of fuel cells, comprising an anode layer including a first anode and a second anode, an electrolyte layer, a cathode layer including a first cathode and a second cathode, at least one interconnect at least partially disposed within the electrolyte layer, and electrically and mechanically coupling the first anode and the second cathode, and an elongate structural support oriented perpendicular to a plane extending through the planar stack and having at least a portion of the elongate structural support aligned with the at least one interconnect on an axis perpendicular to a plane extending through the fuel cell stack
The plurality of fuel cells may comprise one or more of solid oxide fuel cells and/or proton exchange membrane (PEM) fuel cells. The structural support may have a width of between about 30 microns and about 200 microns, and a height of greater than about 100 microns. In one configuration, the first anode and the second anode are laterally separated by a first distance, the first cathode and the second cathode are laterally separated by a second distance, and at least one of the first distance and the second distance is between about 5 microns and about 500 microns.
In one feature, the stack is contained within a housing having a volume of less than about 30 cubic centimeters. The stack may produce a voltage differential of more than about 10 volts.
In another aspect, the devices include a fuel cell assembly, comprising a housing having a volume of less than about 30 cubic centimeters, and a planar fuel cell stack contained in the housing and having an anode layer including a plurality of anodes arranged in a first plane, a cathode layer including a plurality of cathodes arranged in a second plane, and an electrolyte layer disposed between the anode layer and the cathode layer, wherein the stack includes active regions of the electrolyte layer wherein respective anodes and cathodes have overlapping projections on a surface of the electrolyte layer, and the active regions comprise at least about 50% of an area of the surface of the electrolyte layer. In certain configurations, the active regions comprise at least about 70% or at least about 85% of an area of the surface of the electrolyte layer.
In one aspect, the devices include a fuel cell assembly, comprising a housing, and a planar fuel cell stack contained in the housing and having respective anodes arranged in an anode layer, respective cathodes arranged in a cathode layer, and an electrolyte layer, wherein the fuel cell assembly produces more than about 0.1 volts per cubic centimeter of the housing.
According to one feature, the housing has a volume of less than about 30 cubic centimeters.
In another aspect, the devices include a fuel cell assembly, comprising a housing having a volume of between about 0.1 cubic centimeters and about 30 cubic centimeters, and a planar fuel cell stack having an anode layer including a plurality of anodes arranged in a first plane, a cathode layer including a plurality of cathodes arranged in a second plane, and an electrolyte layer disposed between the anode layer and the cathode layer, wherein the stack includes active regions of the electrolyte layer wherein respective anodes and cathodes have overlapping projections on a surface of the electrolyte layer, and the active regions have an area on the surface of the electrolyte layer of between about 0.5 square centimeters and about 100 square centimeters.
In one aspect, the methods include a method of fabricating a planar fuel cell stack with at least one structural support, comprising providing a substrate, disposing an electrolyte layer and one or more electrode layers above the substrate, and forming the at least one structural support from the substrate by selectively removing portions of the substrate.
According to one configuration, the at least one structural support is formed to be elongate and extend perpendicular to a plane extending through the electrolyte. In one feature, selectively removing portions of the substrate comprises etching the substrate. In one feature, the methods include coating the at least one structural support with an insulating material. For example, the methods may include oxidizing the at least one structural support. The method may include disposing the electrolyte directly on the substrate.
In one feature, the methods include providing an anode layer including a first anode and a second anode, providing a cathode layer including a first cathode and a second cathode, and electrically and mechanically interconnecting the first anode and the second cathode through the electrolyte layer. At least one of providing an anode layer and providing a cathode layer may include providing a dam structure on the electrolyte to define a first electrode region and a second electrode region, and disposing electrode material on the first electrode region and the second electrode region. The dam structure may comprise the structural support. The methods may further include removing the dam structure to define at least two electrodes.
The methods may include electrically and mechanically interconnecting the first anode and the second cathode includes etching microfeatures within the electrolyte layer and disposing conductive material within the microfeatures. Etching microfeatures may include etching a linear array of microfeatures with center-to-center spacings of adjacent ones of the microfeatures being substantially equal. Etching a plurality of microfeatures may include etching microfeatures having respective cross-sections with respective areas of less than about (100 microns)2. In one feature, the plurality of microfeatures are etched within a monolithic electrolyte structure.
Thus, the systems and methods described herein arrange many fuel cell units in a single fuel cell housing sized for use in portable electronics devices; provide fuel cells with high active areas; produce high power densities while being sized and shaped for portable electronics applications; provide fabrication methods that balance design tradeoffs inherent in portable fuel cell assemblies, and withstand high thermal stress.
These and other features and advantages will be more fully understood by the following illustrative description with reference to the appended drawings, in which like elements are labeled with like reference designations and which may not be drawn to scale.
The invention, in various embodiments, provides devices and methods for portable fuel cell assemblies.
The fuel cell units 7 produce electricity when a fuel contacts the anodes 4 and oxygen contacts the cathodes 6. Exemplary types of fuel include hydrogen, carbon monoxide, hydrocarbon based fuels such as methane, ethane, methanol, butane, pentane, methanol, formic acid, ethanol, and/or propane, and/or non-hydrocarbon based fuels such as ammonia or hydrazine. The anodes 4 and cathodes 6 may be porous, and catalyst (e.g., a platinum based catalyst) is optionally disposed at the interfaces of the anodes 4 and the electrolyte layers 14 and 16 and/or at the interfaces of the cathodes 6 and the electrolyte layers 14 and 16. The hydrogen and oxygen electrochemically react with the anodes 4, the cathodes 6, the electrolyte membranes 14 and 16, and catalysts (not shown) to produce voltage differentials between respective anodes and cathodes. The respective voltage differentials created by the fuel cells 7 combine through the series connection of fuel cells 7, and can be used to drive electrical current and power a load (not shown).
In operation, a fuel stream 20 enters through the fuel inlet 32. The fuel stream 20 can include liquid or gas, and can comprise the exemplary fuel types described above. Providing the fuel cell units 7 with pure hydrogen may improve the efficiency of the assembly 5, and thus in certain embodiments, such as when a hydrocarbon based fuel is used, the fuel stream 20 may pass through a reformer region 10 and chemically react with catalysts in the reformer region 10 to isolate/produce hydrogen and/or carbon monoxide from the fuel stream 20. However, if the fuel stream 20 comprises pure hydrogen or if the fuel cell units 7 are configured to operate directly with the other fuel types mentioned above, the reformer region 10 is not necessary. The reformer region 10 can comprise one or more of a steam reformer, a partial oxidation reformer, a preferential oxidation reformer, an aqueous shift reformer, and/or a thermal cracking reformer. The actual reformer used will depend upon the Application, and any suitable reformer may be employed.
The fuel stream 20, including hydrogen gas produced by the reformer region 10, then flows along path 21 and contacts the anodes 4 before flowing through tail gas burner 12, where the unexhausted fuel combusts with oxygen (not shown). Additionally, an air stream 26, which can comprise air and/or oxygen gas, flows through internal routing channels (not shown) and contacts the cathodes 6 through the air routing layer 15. As mentioned above, the cathodes 6 and anodes 4 include pores (not shown), and as a result the hydrogen and oxygen/air flow through the pores and contact the electrolyte membranes 14 and 16. When hydrogen and oxygen contact the cathodes 6, anodes 4, and electrolyte membranes 14 and 16, they electrochemically react to produce electricity.
More particularly, the fuel cell assembly 5 includes a plurality of anodes 4 disposed on the outer surface 16b of the electrolyte membrane layer 16 and the outer surface 14b of the electrolyte membrane layer 14, a plurality of cathodes 6 disposed on the inner surface 16a of the electrolyte membrane layer 16 and the inner surface 14a of the electrolyte membrane layer 14, and structural supports 44 disposed on electrolyte membranes 14 and 16. The cathodes 6 disposed on surface 14a directly face the cathodes 6 disposed on surface 16a of membrane 16. This is beneficial in part because both cathode layers share the same flow of air 26 and air routing channel 15, and thus do not require separate air routing channels. In certain embodiments, the air routing channel 15 can be as thin as 1.0 mm or thinner than about 0.5 mm.
The anodes 4 can be constructed of a wide variety of materials, including cermet composites such as nickel and YSZ cermets, platinum, silver, palladium, iron, cobalt, ceria, other oxide matrix materials, or combinations thereof. The cathodes can be constructed from lanthanum (strontium) manganate (LSM), lanthanaum (strontium) cobaltite (LSC), and lanthanum (strontium) cobalt-ferrite (LSCF). The electrolyte layers 14 and 16 can comprise yttria-stabilized zirconia (YSZ) and/or doped ceria materials. Other materials, configurations, and fabrication methods for the electrolyte layers 14 and 16 are described in PCT application WO 2005/030376, incorporated herein by reference in its entirety.
The electrolyte membranes 14 and 16 are vulnerable to thermal stresses due to thermal cycling of the fuel cell (for example, when the fuel cell is turned on and off repeatedly), and structural supports 44 add structural integrity to the electrolyte membranes 14 and 16. The structural supports 44 will be discussed in more detail below.
The anodes 4, cathodes 6, and electrolyte membranes 14 and 16 are contained within a first/inner housing 18. The inner housing 18 can have a volume of between about 0.1 cubic centimeters and about 100 cubic centimeters. In certain embodiments, the inner housing 30 has a volume of between about 0.1 cubic centimeters and about 30 cubic centimeters, between about 0.1 cubic centimeters and about 10 cubic centimeters, or between about 1 cubic centimeter and about 5 cubic centimeters.
The depicted housing 18 comprises a thermally conductive material, and thus the housing comprises a substantially isothermal zone. This is beneficial so that all of the components within the housing operate at substantially the same temperature.
In certain embodiments, the operating temperature within the inner housing 18 is more than 400 degrees C., and in other embodiments can be greater than 750 degrees C. Thus, to maintain this temperature and in order to protect an end-user from these high temperatures, the inner housing 18 is housed in an outer housing 30 that is thermally isolated from the inner housing 18 by an insulating volume 28. The insulating volume 28 may comprise a variety of insulating materials with low thermal conductance, but in certain embodiments, the insulating volume comprises a low pressure region and in some cases comprises a vacuum. In order to maintain a vacuum, a getter material 42 absorbs background gases and maintains vacuum-like conditions. In certain embodiments, the getter material 42 comprises non-evaporable getter, which can be activated through electrical heating.
A vacuum, however, may not prevent/eliminate the heat loss due to radiation. Thus the outer housing 30 includes radiation reflectors 40 on its inner surfaces. The reflectors 40 can comprise a metallic coating which is deposited on the inner surfaces of the wall 30, or by means of a metallic or infrared reflective material which is mechanically attached to the inner surfaces of the vacuum wall. In addition, a series of parallel infrared reflectors can be provided between the inner and outer surfaces of the outer housing 30. As mentioned above, the fuel cell assembly 5 may be portable, and in certain embodiments the outer housing 30 can have a volume of between about 0.1 cubic centimeters and about 100 cubic centimeters. In certain embodiments, the outer housing 30 has a volume of between about 0.1 cubic centimeters and about 30 cubic centimeters, between about 0.1 cubic centimeters and about 10 cubic centimeters, or between about 1 cubic centimeter and about 5 cubic centimeters.
In operation, the interconnects 51-53 connect the fuel cell units 54-57 in series and provide additive voltage gains. In particular, during operation the electrochemical reactions of fuel cell 54 produces a voltage differential between anode 60 and cathode 65, and electrochemical reactions of fuel cell 55 produce a voltage differential between anode 61 and cathode 66. The interconnect 51 comprises a low resistance material, such as platinum and/or lanthanum chromite, and thus forms a low resistance electrical connection between cathode 65 and anode 61, and thus the cathode 66 is maintained at a voltage that is about equal to the voltage of cathode 65 plus the voltage differential between anode 61 and cathode 66. Connecting additional fuel cell units with series connections can add voltage gains. In certain embodiments, planar stacks including 15-20 fuel cell units can produce total voltage differentials of greater than 15 volts. In others, the planar stacks produce more than about 3 volts, more than about 5 volts, or more than about 10 volts . These exemplary voltages apply to the other fuel cell stacks described herein. Thus, planar stacking as described herein can provide relatively high voltages in small volumes. The higher voltages within portable housings are beneficial at least in part because many commercial applications for the portable fuel cell assemblies described herein, such as cell phones or laptop computers, require the higher voltages.
As mentioned, during operation the electrochemical reactions of fuel cell units 54-57 produce a voltage differential between anode 60 and cathode 65, and electrochemical reactions of fuel cell 55 produce a voltage differential between anode 61 and cathode 66. However, the voltage differentials are generally produced within active areas of the fuel cell units 54-57 wherein cathodes 65-68 and corresponding anodes 60-63 directly overlie each other. By way of example, the active area 54a of fuel cell unit 54 is shown. The active area 54a is generally constrained to be the region 54a wherein cathode 65 and anode 60 have overlapping projections on the upper surface of the electrolyte layer 48. Fuel cell units 55-57 similarly have active areas 55a, 56a, and 57a. As shown, gaps between adjacent electrodes (i.e., gaps between adjacent cathodes 65-58 and between adjacent anodes 60-63), and the interconnects 51-53 reduce the active areas 54a, 55a, 56a, and 57a. In one feature, the devices and methods discussed below increase the active areas of fuel cell assemblies by reducing these gaps and by reducing the area of the interconnects 51-53.
However, in some cases the voltage maintained at electrical connector 72 may be less than the sum of the two voltage differentials, in part because of internal resistances of the stack 50 and in part because of leakage and/or parasitic currents that oppose the current flow along path 74. These resistances, leakage currents, and parasitic currents can be reduced by adjusting and/or optimizing the dimensions, configurations, and fabrication materials of the various components of stack 50. However, these adjustments may reduce the active area of the fuel cell stack 50, resulting in a set of design tradeoffs that can be balanced in order to increase the efficiency of the fuel cell stack.
For example, one source of internal resistance is the anodes 60-61 and cathodes 65-66. This internal resistance can be lowered by reducing the lateral widths 65a and 61a of the cathodes and anodes. However, reducing the lateral widths 65a and 61a lowers the active area of the fuel cell stack 50. Alternatively, the internal resistance of the stack 50 can be lowered by increasing the thicknesses 65b and 61b of the cathode 65 and the anode 61, respectively. In certain embodiments, this invention reduces internal resistances using a combination of the above-mentioned approaches. In particular, Table 1 shows electrode thicknesses 65b and 61b that correspond to different lateral widths 65a and 61a chosen to achieve relatively low internal resistances. Units and materials are noted in parentheses.
In certain embodiments, and in accordance with the results of Table 1, stack 50 is fabricated with lateral widths 65a and 61a of less than about 2 mm and with electrode thicknesses 65b and 61b of less than about 50 microns. However, in other embodiments, the lateral widths 65a and 61a can be between about 5 microns and about 500 microns. In certain configurations, the lateral widths 65a and 61a are between about 500 microns and about 2 mm.
Another configuration that reduces the resistance of the anodes 60-61 and the cathodes 65-66 includes current collecting films. In this configurations, a material (not shown) with a high conductivity, such as platinum, is disposed as one or more films under the anodes 60-61 (i.e., on the surface of the anodes 60-61 facing away from electrolyte 48) and over the cathodes 65-66 (i.e., on the surface of the cathodes 65-66 facing away from the electrolyte 48). In certain embodiments, the electrode thicknesses 65b and 61b are relatively thin (i.e., less than about 25 microns) and the platinum films are also relatively thin (i.e., less than about 25 microns) so that the laminates of the cathodes 65-66 and platinum films and the laminates of the anodes 60-61 and the platinum films have respective thicknesses of less than about 50 microns. The addition of the high conductivity material reduces the internal resistance. However, in other embodiments, thicker electrode and platinum film layers (i.e., less than about 50 microns each) are used. The platinum films are porous, and thus allow fuel to flow through the platinum films to contact the anodes 60-61 and allow oxygen/air to flow through the platinum films to contact the cathodes 65-66.
As mentioned, in addition to internal resistances, leakage currents can reduce the voltage maintained at electrical connector 72. Leakage currents, depicted by current paths 80 and 82, develop in part from voltage differentials between adjacent cathodes 66 and 65, and adjacent anodes 61 and 60. The leakage currents 80 and 82 must bridge respective electrode gaps 86 and 88. By increasing the lengths 86a and 88a of the gaps 86 and 86 between adjacent cathodes 65 and 66 and adjacent anodes 60 and 61, these leakage currents can be reduced and/or eliminated. In certain embodiments, the lengths 86a and 88a are more than about five times greater than the thickness 48a of the electrolyte membrane 48, and in other embodiments the lengths 86a and 88a are about fifty times greater than the thickness 48a of the electrolyte membrane 48 in order to reduce and/or eliminate the leakage currents 80 and 82.
However, as noted above, increasing the lengths 86a and 88a of the gaps 86 and 88 will reduce the active area of the fuel cell stack 50. Thus, in one aspect, the thickness 48a of the electrolyte membrane 48 is chosen to be relatively small. This allows the lengths 86a and 88a of the gaps to be several times the thickness 48a of the electrolyte membrane layer 48, while still being small enough to preserve the active area of the stack 50. More particularly, in certain embodiments, the thickness 48a of the electrolyte membrane 48 is less than about 40 microns, and in others the thickness is less than about 10 microns or less than about 5 microns. Accordingly, the lengths 86a and 88a of the gaps 86 and 88 are, in certain embodiments, less than about 500 microns, less than about 100 microns, or less than about 25 microns. With these dimensions, the active area of the stack 50 can be greater than about 50%, and in certain configurations is greater than about 70%, or greater than about 85%. With these relatively small thicknesses, the electrolyte membrane layer 48 may not provide adequate structural support to electrodes and/or may unable to withstand thermal stresses of thermal cycling. Therefore, in certain embodiments, the electrolyte membrane layer 48 is provided with structural support members that will be discussed in more detail below.
In addition to leakage currents 80 and 82, parasitic currents can reduce the efficiency of the stack 50. Generally, fuel cell units operate by transferring ions between the anode and cathode of that fuel cell unit through the electrolyte 48 (i.e., negatively charged oxygen ions are transferred through the electrolyte 48 from cathode 65 to anode 60, and from cathode 66 to anode 61) in regions where the anode and the cathode overlap. However, in the depicted stack 50, in certain regions anodes and cathodes of adjacent fuel cells overlap as well. For example, cathode 65 overlaps with anode 61 generally near the region of the interconnect 51 and ions may transfer through the electrolyte in this region as well (i.e., negatively charged oxygen ions may transfer from cathode 65 to anode 61). Since the cathode 65 is electrically connected to anode 61 by interconnect 51, negatively charged electrons can conduct back through the interconnect 51 from anode 61 to cathode 65. The result is a parasitic current loop that consumes fuel but does not produce useful electricity for the load powered by the fuel cell stack 50.
In certain embodiments, this parasitic current is reduced/eliminated by placing a barrier material that does not conduct ions between the electrolyte 48 and one or both of the electrodes 65 and 61 in the region where the electrodes 65 and 61 overlap. The barrier material can comprise one or more of platinum, metal, and a ceramic material. The barrier can be deposited as a film using any one or more of the deposition techniques discussed elsewhere herein.
Alternatively, or in addition, to the above-described technique, the parasitic currents may be reduced/eliminated by altering or replacing the materials of the stack 50 in the region where the cathode 65 and the anode 61 overlap with material that yields little or no voltage differential when in contact with the fuel or oxygen. For example, a manufacturer may alter or replace the material of the cathode 65 and/or the anode 61 so that they comprise less catalytic materials. Additionally or alternatively, the manufacturer may avoid disposing catalyst material between the cathode 65 and the electrolyte 48, or between the anode 61 and the electrolyte 48. In certain embodiments, the materials are chosen such that the stack 50 has a power density in the region where cathode 65 and anode 60 overlap of more than about 10 times the power density in the region where cathode 65 and anode 61 overlap.
In other embodiments, the parasitic current is reduced/eliminated by configuring the stack 50 such that electrodes in adjacent fuel cell units do not overlap.
As illustrated in
In certain embodiments, the interconnect material seals the cathode side 116 of the stack 100 from the anode side 114 of the stack 100. The seal prevents hydrogen from diffusing from region 114 near the anode 106 to region 116 near the cathode 104, and similarly prevents air and/or oxygen from diffusing from region 116 to region 114. When gas diffuses through membrane layer 112, the stack 100 may lose efficiency. To form a tighter seal, the manufacturer chooses an interconnect material that has a higher coefficient of thermal expansion than that of the electrolyte layer 112. As a result, the interconnect 102 expands relative to the electrolyte layer 112 when the stack 100 heats to its operating temperature, and thereby forms a tight seal at the interface of the electrolyte layer 112 and the interconnect 102.
In addition, the interconnects 102 include a top cap 102a and a bottom cap 102c. which improves the seal between regions 114 and 116. In certain embodiments, the shaft 102b has a circular cross-section with diameter of about 40 microns, while the top cap 102a and the bottom cap 102b have circular cross-sections at their respective interfaces with the shaft 102b having diameters of about 80 microns.
The distance 120 between the center of the interconnect 102 and the center of the gap between adjacent cathodes 104 and 105 is known as the cathode standoff 120, and the distance 122 between adjacent anodes 106 and 107 is known as the anode standoff 122. In certain embodiments, both the anode standoff 122 and the cathode standoff 120 are less than about 100 microns, less than about 95 microns, or less than about 70 microns.
Table 2 describes the active area obtained in one illustrative embodiment of the invention. In this illustrative embodiment, the distance between corresponding components in adjacent fuel cell units, known as the repeat distance 126, is 2.25 mm, although in other embodiments the repeat distance 126 is between about 1 mm and about 4 mm. The active area in this illustrative embodiment is about 89%, though higher active areas are possible using techniques described above. For example, a manufacturer can increase the active area without significantly compromising voltage gain by using a thinner electrolyte layer 112 and reducing the length of the gap 129 between adjacent cathodes and/or adjacent anodes.
As mentioned above, fuel cell assemblies in certain embodiments operate at extremely high temperatures, which stresses fuel cell components due to excessive heat and due to thermal cycling. In particular, certain solid oxide fuel cell embodiments of this invention operate at temperatures exceeding 400 degrees, 500 degrees C., 600 degrees C., 700 degrees C., or 800 degrees C. In some cases, the fuel cells operate at temperatures that exceed 1100 degrees C. Fuel cell assemblies that power portable consumer electronics products may turn on and off repeatedly over the lifetime of the product. As a result, the fuel cell thermally cycles repeatedly between ambient temperatures and the high operating temperatures mentioned above. Since the components of fuel cell assemblies can comprise several materials, and since the materials may have varying coefficients of thermal expansion, the thermal cycling can stress fuel cell components at their interfaces with other components. Thus, in certain embodiments, the fuel cell assembly includes additional structural supports.
The positioning is also beneficial to reduce/eliminate parasitic current. As discussed above, parasitic currents occur generally in regions where electrodes of adjacent fuel cell units overlap and react with, for example, fuel and oxygen to transport ions through the electrolyte 170. In the depicted stack 150, the structural support 154 blocks access of fuel to the anode 163 in the region where the anode 163 overlaps with the cathode 166 of the adjacent fuel cell. Thus, fuel does not react with anode 163 in the region that would otherwise give rise to parasitic currents. Additionally, or alternatively, the parasitic currents can be reduced/eliminated by a structural support disposed over cathode 166 over the region where cathode 166 overlaps with anode 163 by preventing oxygen from contacting the cathode 166 in this region.
In certain embodiments, the structural supports 154-155 comprise a material with a similar coefficient of thermal expansion as other devices in the planar stack 150. This reduces stresses from thermal cycling at the interfaces of structural support members 154-155 and other components of the stack 150. For example, if the electrolyte membrane 170 comprises YSZ, then the support structure can comprise zirconia, magnesium oxide, ferritic stainless steel (Fe—Cr), and/or combinations thereof. In other embodiments, the coefficient of thermal expansion of the structural supports is similar to that of anodes 162-163 and/or cathodes 166-168.
In certain embodiments, the structural supports 154-155 comprise electrical insulators to, in part, insulate electrodes that are mechanically coupled by one of the structural support members 154-155, such as cathodes 166 and 167, or anodes 162 and 163. For example, the structural supports 154-155 can comprise a ceramic material including oxides, non-oxides, and/or composites. Exemplary ceramic materials include titanates, oxides, and nitrides. In certain embodiments, the structural supports 154-155 comprise plastic materials.
Certain materials that insulate at room temperature, however, do not insulate at the high operating temperatures mentioned above (e.g., silicon). Thus, the structural supports 154-155 include coatings 154a and 155a. In certain embodiments, the coating comprises a separate material applied to the structural supports 154-155. However, in other embodiments, the coatings 154a and 155a are formed via chemical reactions to surfaces of the structural supports 154. In certain embodiments, the surfaces of the structural supports 154-155 are oxidized to form oxidized coatings 154a and 155a.
In certain embodiments, the structural support 154 has a width 154b of between about 30 microns and about 200 microns, and a height of greater than about 100 microns. In some implementations, the width is between about 10 microns and about 400 microns. In some implementations, the height is between about 300 microns and 500 microns, but can be as large as 1 mm, 2 mm, or 3 mm. The height can also be between about 50 microns and 100 microns.
In certain embodiments, the structural supports 154-155 support the stack 150 so that electrolyte layers 170 can have a thickness of less than about 2 microns or less than about 1 micron while still withstg thermal stress.
Turning to
The manufacturer then patterns via holes 504 in the electrolyte layer. In exemplary techniques, the manufacturer flows and/or spreads a positive photoactive photoresist material 506 on the electrolyte 502 and the photoresist 506 cures. The thickness 506c of the photoresist 506 can be between about 1 micron and about 50 microns. The manufacturer exposes the region of the electrolyte overlying via 504 to visible light using a photomask (not shown). The manufacturer then applies a developer solution which removes the photoresist in areas 508 exposed to the light through the photomask. The remaining portions of the photoresist 506a and 506b protect underlying regions of the electrolyte 502 when the manufacturer fabricates via 504.
The manufacturer then etches the via 504. In certain embodiments, the etch comprises ion milling, wherein a stream of argon ions bombard the electrolyte 502. The argon ion stream can comprise a flux greater than about 10 mA/cm2 and an acceleration voltage in excess of 500 volts. The via 504 is then etched at a rate of more than about 40 angstroms per minute. However, in other embodiments, the etch comprises wet etching using a solution of hydrofluoric acid, using a combination of hydrofluoric and hydrochloric acids and plasma etching from a plasma of CFCl3 gas, and/or laser ablation.
Optionally, the manufacturer then forms the cavity 512 that will shape a cap shape of an interconnect. In certain embodiments, the manufacturer exposes the structure 501 to a plasma comprising sulfur hexaflouride for more than about 10 minutes.
Next, the manufacturer electroplates the via 504 and cavity 512 to form an interconnect. More particularly, the manufacturer applies a seed layer of interconnect material. In certain embodiments, the seed layer is applied by exposing the structure 501 to a flux of interconnect material atoms in a vacuum or low pressure environment.
Turning to
The manufacturer then electroplates the cavity 512 and the via 504. In particular, the manufacturer exposes the structure 501 to a plating solution of chloroplatinic acid or platinum sulfate. The manufacturer provides a wafer chuck which electrically couples to the back side 514 of the silicon wafer. Electroplating currents flow through the silicon substrate 500. The electrolyte 502 insulates the electroplating current from the plating solution except in region 512. Thus, the interconnect material fills the cavity 512 and then fills the via 504. The electroplating currents continue to flow through the substrate 500 until a cap 516 forms, completing the interconnect 518.
The manufacturer then forms the structural supports from substrate 500 and in particular selectively removes portions of substrate 500 to form structural supports. Turning to
As mentioned above, the active area of fuel cell stacks is impacted by the length of gaps between adjacent electrodes and standoff distances. The electrode gaps and standoff distances discussed above, which were in certain embodiments less than 100 microns, have relatively small dimensions, and it is desirable that the various deposition, patterning, and etching steps described herein align with high precisions. By way of example, if, in
Thus, exemplary fabrication techniques align the photomasks with the photoresist 506 and/or the substrate 500 with precisions of less than about 10 microns, less than about 5 microns, or less than about 1 micron. In certain embodiments, the substrate 500 is marked with alignment marks etched or otherwise formed on the surfaces of the substrate 500. By way of example, box-shaped or circular alignment marks can be disposed on the substrate 500 to indicate a target region for the interconnect 518. Photomasks discussed above can also include matching marks which align with corresponding marks on the substrate 500. The substrate 500 can be moved laterally by a photo-alignment assembly with the above-mentioned precisions in order to precisely align with the photomasks. In certain embodiments, the photo-alignment assembly locates and images the alignment markings. The imaging can include infrared optical detection so that the photo-alignment assembly can detect markings on the surface 532 of substrate 500 from the back side 514 of the substrate 500.
Next, the manufacturer dispenses cathode ink 540 and 542 on the electrolyte layer 502. The ink may comprise the exemplary cathode materials described above. In certain embodiments, the manufacturer needle dispenses the cathode ink. The needle may be stationary, and an alignment assembly, such as the photo-alignment assembly described above, laterally moves the electrolyte 502 and structural support 515 with respect to the stationary needle. The needle may dispense less than about 100 nanoliters of ink per drop. The drops spread over the electrolyte membrane 502 until the ink abuts against the dam 544. Next, the manufacturer fires the cathode ink 540 and 542 in, for example, oxygen at 500 degrees C. and thus burns off binders or plasticizers in the ink system. The manufacturer may optionally planarize the cathodes 540 and 542 by, for example, spinning or pressing. Subsequent to being fired, the cathodes 540 and 542 may be in a porous state to allow gas (such as oxygen or air) to diffuse through the cathodes 540 and 542. In other embodiments, the ink can be deposited using, for example, screen-printing, immersion, or dip coating, sputtering, plasma or vapor spray, and/or electrophoretic deposition.
Anodes can be patterned similarly. In the depicted embodiment, the structural support 515 itself serves as the dam to separate and define the adjacent anodes 546 and 548. After disposing anode ink 546 and 548 and cathode ink 540 and 542, the manufacturer fires the structure 501 in oxygen at 800 degrees C. to sinter particles in the anode ink 546 and 548 and cathode ink 540 and 542. This also coats the support structure 515 with an oxide coating 515a. As mentioned above, the coating 515a insulates the support structure 515 from the rest of the structure 501.
More particularly,
Groups of planar fuel cell stacks can be arranged to comprise arrays of fuel cell units.
The connectors 624 and 626 can comprise thin films of platinum that are deposited onto portions of the device by means of vacuum evaporation. A manufacturer can pattern the connectors 624 and 626 using photoresist and a photomask as described above in connection to patterning electrolyte membrane layers. In certain embodiments, a manufacturer deposits the connectors 624 and 626 before he deposits electrodes and/or before he patterns support structures. The platinum of the connectors 624 and 626 then cannot contaminate the electrodes after the electrodes are fired into a porous state. The platinum can either be formed as a dense thin film using techniques such as sputtering or vacuum evaporation, or the platinum can be formed by firing from an ink or paste formulation, whereby ink or paste is deposited by means of needle dispense, screen-printing, and immersion or dip coating.
In certain embodiments, fuel cell units are grouped and electrically coupled in accordance with the positions of the respective groups of fuel cell units in relation to a fuel source in order to, for example, produce more power, voltage, current, and/or provide more efficiency. More particularly,
In one implementation, different groups of fuel cells include respective different configurations of fuel cell units in order to drive the groups with different current densities to in part compensate for the varying power densities described above, and correspondingly improve overall power density.
A manufacturer can adjust the current densities for groups 702 and 704 by adding or removing fuel cell units from groups 702 and 704. In certain embodiments, the manufacturer may include more fuel cell units in group 702 than in group 704.
In addition to configuring groups of fuel cell units differently based on their respective locations with respect to a fuel source in order to adjust current densities that drive the fuel cell units, individual fuel cell units can be sized and shaped differently based their respective locations with respect to a fuel source. More particularly,
Since downstream fuel cell units such as fuel cell units 812 and 814 have smaller areas, they will have larger current densities than upstream fuel cell units such as fuel cell units 802 and 804. However, in alternate embodiments, downstream fuel cell units have larger active areas, which may be advantageous in certain situations.
The configurations, devices, and methods described above can be used in any operative combination with other configurations, devices, and methods, including those described in U.S. Patent and Publication Nos. 2005/0069737; 2005/0132648; 6,939,632; 2004/0241061; 2004/0072039; 2005/0249993; 6,680,139; 2004/0028975; 6,852,436; 6,623,881; 2003/0096147; 2005/0221131; 5,925,477; 5,190,834; 5,479,178; 6,183,897; and 5,595,833, all of which are incorporated herein by reference in their entireties.
The fuel cell assemblies discussed above may be any type of fuel cell, such as solid-oxide fuel cells and/or proton exchange membrane fuel cells (PEM). They may be provided in a housing which integrates one or more of the functions of a fuel reformer, a set of fuel cell membranes, a tail gas burner, and all internal fluid manifolds in one thermal zone, can be fabricated through any number of fabrication techniques. In particular, embodiments of the invention can be fabricated using MEMS techniques (micro-electro-mechanical systems) or micromachining techniques. Such techniques make it possible to integrate thin film materials (for instance thin film electrolytes, anodes, cathodes and/or electrical connections) along with etched microchannels for control of fluid flow onto a common substrate that is thermally conductive and mechanically robust.
For example, an integrated housing can be assembled from a group of substantially planar or non-planar semiconductor structures. Specifically, five silicon substrates can be bonded together to form the “box” that various fuel cell apparatus components are integrated within. Bonding together the five silicon substrates, results in a stacked configuration. In one embodiment, the substrates can be stacked as follows: (1) fuel reformer substrate including fluidic interconnects; (2) a membrane electrode assembly, (3) a fluid routing layer, (4) another membrane electrode assembly, and (5) a top fluid routing layer including tail gas burner. Thus, a stack of layers can form some or all of the integrated fuel cell apparatus.
In certain embodiments, silicon is chosen as the substrate for building the fuel cell membranes and other manifold structures. However, micromachining techniques also exist for building fluid flow channels in rigid wafers of glass and ceramic, all materials which possess the high temperature strength required for solid oxide fuel cells. In order to prevent electrical shorting between different points of the membrane assembly, a silicon substrate can be coated with layers of silicon oxide or silicon nitride to render it electrically insulating.
Etched fluidic microchannels are formed in the above substrates by a variety of techniques, including wet and dry chemical etching, laser ablation, diamond milling, tape casting, or injection molding. A variety of substrate or wafer bonding techniques are available including fusion bonding, anodic bonding, sealing by means of eutectic solder materials or thin films, or sealing by means of glass frits.
Fuel cell assemblies, including the anode, cathode, and electrolyte can be deposited by a variety of thin and thick film deposition techniques including sputtering, evaporation, chemical vapor deposition, laser ablation, screen-printing, dip coating, or vapor spray techniques.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
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