Method of making a fuel cell device

An active cell is prepared by dispensing first electrode sub-layers, pressing in physical structures to partially embed them in an uppermost sub-layer, and dispensing more first electrode sub-layers wherein dispensing is in order of increasing porosity, then drying the sub-layers to form a first electrode layer. An electrolyte layer is then formed thereon. Further preparation includes dispensing second electrode sub-layers over the electrolyte layer, pressing in physical structures to partially embed them in an uppermost sub-layer, and dispensing more second electrode sub-layers wherein dispensing is in order of decreasing porosity, then drying the sub-layers to form a second electrode layer. A laminated stack is formed, then the physical structures are pulled out. Sintering then forms the active cell with active passages embedded in and supported by the sintered electrode layers, and with decreasing porosity in the electrode layers in a thickness direction away from the electrolyte layer.

FIELD OF THE INVENTION

This invention relates to fuel cell devices and systems, and methods of manufacturing the devices and, more particularly, to a solid oxide fuel cell device.

BACKGROUND OF THE INVENTION

Ceramic tubes have found a use in the manufacture of Solid Oxide Fuel Cells (SOFCs). There are several types of fuel cells, each offering a different mechanism of converting fuel and air to produce electricity without combustion. In SOFCs, the barrier layer (the “electrolyte”) between the fuel and the air is a ceramic layer, which allows oxygen atoms to migrate through the layer to complete a chemical reaction. Because ceramic is a poor conductor of oxygen atoms at room temperature, the fuel cell is operated at 700° C. to 1000° C., and the ceramic layer is made as thin as possible.

Early tubular SOFCs were produced by the Westinghouse Corporation using long, fairly large diameter, extruded tubes of zirconia ceramic. Typical tube lengths were several feet long, with tube diameters ranging from ¼ inch to ½ inch. A complete structure for a fuel cell typically contained roughly ten tubes. Over time, researchers and industry groups settled on a formula for the zirconia ceramic which contains 8 mol % Y2O3. This material is made by, among others, Tosoh of Japan as product TZ-8Y.

Another method of making SOFCs makes use of flat plates of zirconia, stacked together with other anodes and cathodes, to achieve the fuel cell structure. Compared to the tall, narrow devices envisioned by Westinghouse, these flat plate structures can be cube shaped, 6 to 8 inches on an edge, with a clamping mechanism to hold the entire stack together.

A still newer method envisions using larger quantities of small diameter tubes having very thin walls. The use of thin walled ceramic is important in SOFCs because the transfer rate of oxygen ions is limited by distance and temperature. If a thinner layer of zirconia is used, the final device can be operated at a lower temperature while maintaining the same efficiency. Literature describes the need to make ceramic tubes at 150 μm or less wall thickness.

An SOFC tube is useful as a gas container only. To work it must be used inside a larger air container. This is bulky. A key challenge of using tubes is that you must apply both heat and air to the outside of the tube; air to provide the O2for the reaction, and heat to accelerate the reaction. Usually, the heat would be applied by burning fuel, so instead of applying air with 20% O2(typical), the air is actually partially reduced (partially burned to provide the heat) and this lowers the driving potential of the cell.

An SOFC tube is also limited in its scalability. To achieve greater kV output, more tubes must be added. Each tube is a single electrolyte layer, such that increases are bulky. The solid electrolyte tube technology is further limited in terms of achievable electrolyte thinness. A thinner electrolyte is more efficient. Electrolyte thickness of 2 μm or even 1 μm would be optimal for high power, but is very difficult to achieve in solid electrolyte tubes. It is noted that a single fuel cell area produces about 0.5 to 1 volt (this is inherent due to the driving force of the chemical reaction, in the same way that a battery gives off 1.2 volts), but the current, and therefore the power, depend on several factors. Higher current will result from factors that make more oxygen ions migrate across the electrolyte in a given time. These factors are higher temperature, thinner electrolyte, and larger area.

Fuel utilization is a component of the overall efficiency of the fuel cell. Fuel utilization is a term that can describe the percent of fuel that is converted into electricity. For example, a fuel cell may only convert 50% of its fuel into electricity, with the other 50% exiting the cell un-used. Ideally, the fuel utilization of a fuel cell would be 100%, so that no fuel is wasted. Practically, however, total efficiency would be less than 100%, even if fuel utilization was 100%, because of various other inefficiencies and system losses. Additionally, if the gas molecules can't get into and out of the anode and cathode, then the fuel cell will not achieve its maximum power. A lack of fuel or oxygen at the anodes or cathodes essentially means that the fuel cell is starved for chemical energy. If the anode and/or cathode are starved for chemicals, less power will be generated per unit area (cm2). This lower power per unit area gives lower total system power.

In a tubular fuel cell device, such as that shown inFIG. 1where the anode lines the inside of the tube and the cathode forms the outer surface with the electrolyte therebetween, it is wishful thinking to expect high utilization of fuel. The inside diameter of the tube, which forms the fuel passage, is very large when compared to the thickness of the anode. Anode thicknesses may be on the order of 50-500 nm, whereas tube diameters may be on the order of 4-20 mm. Thus, there is a high likelihood of fuel molecules passing through the large fuel passage without ever entering the pores of the anode. An alternate geometry for the tube is to have the anode on the outside of the tube. In that case, the problem could be worse because the fuel is contained within the furnace volume, which is even larger than the volume within the tube.

Within a multilayer fuel cell device, such as the Fuel Cell Stick™ devices10depicted inFIGS. 2 and 3and developed by the present inventors, fuel utilization can be higher because the flow path for the gas can be smaller.FIG. 2is identical to FIG. 1 of U.S. Pat. No. 7,838,137, the description of which is incorporated by reference herein. Device10includes a fuel inlet12feeding a fuel passage14to a fuel outlet16, and an oxidizer inlet18feeding an oxidizer passage20to an oxidizer outlet22. An anode24is adjacent the fuel passage14and a cathode26is adjacent the oxidizer passage20, with an electrolyte28therebetween. By way of example, both the anodes24and fuel passages14can be made to a thickness of 50 nm, and this similarity in thickness, where the ratio of thickness can be near 1:1 (or a bit higher or lower, such as 2:1 or 1:2) can give a more optimal chance of molecule flow into and out of pores.

These multilayer fuel devices10are built from green materials, layer by layer, and then laminated and co-fired (sintered) to form a single monolithic device having a ceramic support structure29surrounding one or more active cells50, each active cell50having an associated anode24, cathode26and electrolyte28fed by fuel and air passages14,20. An active cell50(or active layer50) is one in which an anode24is in opposing relation to a cathode26with an electrolyte28therebetween, and the active passages are those that run along or within the active cell50.FIG. 3depicts two active cells50. Areas of the device10that lack an opposed anode24and cathode26are non-active or passive portions of the device10that form the support structure29, and passive gas passages are those that run through these passive portions of the device10. The active cells50are “within” the device10and substantially surrounded by and supported by ceramic support structure29. The device has an exterior surface and internal supporting structure, which is the ceramic support structure29, such that the active cells50are contained substantially inward of the exterior surface and are contained by the internal ceramic support structure. It should be understood that extension of all or a portion of an electrode to an edge of the device for electrical connection at the exterior surface does not compromise the support of this structure as the active cell50is still within the interior structure, and is within the scope of “substantially surrounded.” The electrolyte28in the active cell50is monolithic with the ceramic support structure29by virtue of being co-fired therewith, and may be made of the same or different material. In exemplary embodiments, the electrolyte28and ceramic support structure29are the same or similar in composition, with the primary difference between them being that the electrolyte28is that portion of the ceramic material that lies between an opposing anode24and cathode26(i.e., the middle layer in the 3-layer active cell50) and the ceramic support structure29is the remaining portion of the ceramic material (i.e., the ceramic that surrounds the 3-layer active cell50). Air and fuel are fed into the device10through the passive passages that are fluidicly coupled to the active passages that feed the active cells50. Thus, a fuel passage14and an oxidizer passage20, as referred to herein, include both the passive and active portions of the passages.

As discussed above, it is desirable to make the electrolyte28as thin as possible. However, as the electrolyte28is made thinner, the support of the structure can be compromised, and distortion of the active portion of fuel and air passages14,20that feed the anodes24and cathodes26can occur at one or more locations within the active cell50, as well as distortion of the passive portions of the passages14,20. These distortions in the passages14,20may lead to leaks that degrade the performance of the affected active cell50and of the overall device10.

One advantage of the multilayer fuel cell devices developed by the present inventors is that many active cells50can be provided within a single monolithic device, including multiple cells along a single active layer and stacks of active layers one upon another, which can be connected in various parallel and series arrangements, leading to a single device with high output. If one area of one cell distorts, there are still many other cells that produce power, such that the multilayer fuel cell devices are still superior to single cell tubular devices or stacked devices that are not monolithic, However, the more layers that are incorporated, the higher the chance for multiple distortions throughout the device.

Therefore, there is a need to provide thin electrolyte layers while still providing the needed support to prevent distortion of the gas passages within a monolithic multilayer fuel cell device.

SUMMARY OF THE INVENTION

According to an embodiment, a method of making an active cell for a monolithic fuel cell device is provided. A first plurality of first electrode sub-layers is sequentially deposited in paste form, a first plurality of elongate removable physical structures is pressed in a spaced-apart parallel orientation on the first plurality of first electrode sub-layers to partially embed the first plurality of elongate removable physical structures in an uppermost sub-layer of the first plurality of first electrode sub-layers, a second plurality of first electrode sub-layers is sequentially deposited in paste form over the first plurality of elongate removable physical structures and uppermost sub-layer of the first plurality of first electrode sub-layers, wherein sequentially dispensing the first and second plurality of first electrode sub-layers is in order of increasing porosity, and the first and second pluralities of first electrode sub-layers are dried to form a first electrode layer with the first plurality of elongate removable physical structures embedded within with at least one end of each of the first plurality of elongate removable physical structures freely protruding from the first electrode layer. One or more electrolyte sub-layers are dispensed over the first electrode layer and dried to form an electrolyte layer. A first plurality of second electrode sub-layers is sequentially deposited in paste form over the electrolyte layer, a second plurality of elongate removable physical structures is pressed in a spaced-apart parallel orientation on the first plurality of second electrode sub-layers to partially embed the second plurality of elongate removable physical structures in an uppermost sub-layer of the first plurality of second electrode sub-layers, a second plurality of second electrode sub-layers is sequentially deposited in paste form over the second plurality of elongate removable physical structures and uppermost sub-layer of the first plurality of second electrode sub-layers, wherein sequentially dispensing the first and second pluralities of second electrode sub-layers is in order of decreasing porosity, and the first and second pluralities of second electrode sub-layers are dried to form a second electrode layer with the second plurality of elongate removable physical structures embedded within with at least one end of each of the second plurality of elongate removable physical structures freely protruding from the second electrode layer. A laminated multi-layer stack is formed by pressing together the first electrode layer, the electrolyte layer, and the second electrode layer. The first and second pluralities of elongate removable physical structures are pulled out of the laminated multi-layer stack to reveal spaced-apart active passages through each of the first and second electrode layers. Thereafter, sintering the laminated multi-layer stack forms an active cell comprising the spaced-apart active passages embedded in and supported by the sintered first and second electrode layers, the active cell having decreasing porosity in the first and second electrode layers in a thickness direction away from the electrolyte layer.

DETAILED DESCRIPTION

Reference may be made to the following patents and publications by the same inventors, which describe various embodiments of a multilayer Fuel Cell Stick™ device10(et al.), the contents of which are incorporated herein by reference: U.S. Pat. Nos. 8,278,013, 8,227,128, 8,343,684, and 8,293,415, and U.S. Patent Application Publication Nos. 2010/0104910 and 2011/0117471. The inventive structures and/or concepts disclosed herein may be applied to one or more of the embodiments disclosed in the above-referenced published applications.

Various material terms will be used interchangeably, regardless of the stage of the material during manufacturing. For example, anode24, anode layer24, anode material24, etc. all refer to the anode itself or the layer in which one or more anodes are positioned, irrespective of whether the anode material is in the form of a paste, a preform layer, a sintered layer, an initial green state, or a final fired state.

In accordance with the present invention, to form the passive and active passages in multilayer fuel cell devices, removable physical structures, such as wires, are placed in the anode and cathode layers of the device as the layers are assembled in the green state. The removable physical structures travel from one end of the device, through the active area, and are spaced apart from one another with the anode or cathode material therebetween. Previous designs used removable physical structure at the ends of the device to form the passive passages, which were coupled to larger areas of organic sacrificial material that were used inside the device to form the active passages. The wires were simply placed between preformed sheets of green ceramic material with one end in contact with the sheet of sacrificial material and the other end extending outside the end of the device. After lamination, during which the preformed sheets conform to the shape of the physical structures, the removable physical structures were pulled out, and then the device was co-fired, allowing the sacrificial material to burn out and exit the end of the device through the passive passages and/or through other temporary bake-out ports in the sides. Despite embodiments that use ceramic balls in the active area to help support the active passages, the large flat active passages, as shown inFIG. 3, are still only partially supported at best and sometimes distort as the sacrificial material bakes out. As explained above, distortions have occasionally led to leaks, which have degraded the performance of the cell.

In the present method, the active area is assembled with removable physical structures, such as fine wires, for example, 0.01 inch (0.254 mm), that are spaced apart and surrounded by solid material. In other words, the removable physical structures are at least partially surrounded by solid material so as to embed them within a layer of green material, rather than placed between preformed layers. The removable physical structures will be referred to as wires, interchangeably for ease of discussion, with the understanding that the invention is not limited to wires as the only possible removable physical structures. Removable physical structures are distinguished from sacrificial materials that burn out at elevated temperatures, and refer instead to solid structures that are pulled out of the device.

FIG. 4depicts in perspective view a green layer of anode material24having a plurality of wires92extending all the way through and surrounded by the anode material24. The use of wires92to form spaced apart passages14,20in the active area50is advantageous because the active area50then has a solid support for the gas passages. The solid material is first made as a paste, filled with anode, cathode or ceramic particles, and then dispensed and dried around the wires92to fully support the wires. The solid material is necessarily porous for at least a portion of the anode24and cathode26. The wires92can be placed in parallel, if desired, and then the material is dispensed over the top of the wires. A vacuum can be pulled after dispensing the paste, in order to remove any air pockets below the wires so that the wires92are completely surrounded. Alternatively, a layer of paste can be deposited, then the wires92positioned, and then more paste applied. Thus, a layer containing the wires92may be formed separately, for example using a mold and paste materials, and then dried to form a wire-containing preformed sheet, which preformed sheets can then be stacked. Additionally or alternatively, the paste for one layer can be applied on top of a preceding layer in a manner that embeds the wires92in the paste of that layer, and that layer is then dried in place in the stack. Thus, the entire stack can be built sequentially on a surface in such a way that each layer is built and dried, and then another layer is put on top and dried, or each layer can be premade and treated as preforms, and the stack built up from many different pre-made sub components, or any combination of these two techniques can be used to assemble a complete device stack.

Once the device stack is formed, it is laminated, and then the wires92are removed. The layer-by-layer dimensions are better maintained during lamination with the present invention because the green preform layers already contain the wires92with the electrode material surrounding and conforming to the wire shape, such that the green layers need not conform around the wires92as the layers are pressed together. The result, for a single active cell50, is shown in cross-section inFIG. 5, after the wires92are pulled out, and after the porous anode and cathode materials are fired. Rather than a large flat passage14adjacent the anode24, as shown inFIG. 3, the device inFIG. 5includes a plurality of small round fuel passages14embedded within the anode24, and the same is true for the cathode26and oxidizer passages20. Round wires are not required, as other shapes can be used. Because the wires92are removed before baking the stack, binder removal from the porous materials and from any sacrificial layers will proceed more quickly because exit passages for the binder are already present.FIG. 6shows, for an anode24, how the gas in operation of the device10can flow through the fine fuel passages14, and then travel through the porous anode24to reach an electrolyte area28. A single electrode can also serve two electrolyte layers28on either side.

In the active cell50, different combinations of materials can be used in combination with the wires92.FIG. 7depicts, for an anode layer24, a combination of a porous anode24awith the fuel passages14embedded therein and a non-porous anode24badjacent thereto (and opposite the electrolyte, not shown).FIG. 8is similar toFIG. 7but with the fuel passages14formed along the interface between the porous anode24aand the non-porous anode24b. With paste materials, a layer of the porous anode paste can be laid down first, followed by laying the wires92onto the paste and pushing them halfway into the paste, and then applying the non-porous anode paste over the wires. Depending on whether and how far the wires are pushed into the porous layer will determine the position of fuel passages at the interface, e.g., off-center and residing mostly in the porous layer, off-center and residing mostly in the non-porous layer, or centered halfway in each layer.

FIG. 9shows how the paste materials can transition from one layer to the next to give a grading in the thickness direction (z direction), for example, a graded porosity with layer24abeing highly porous, layer24bhaving medium porosity, and layer24chave low porosity, for example in a direction away from the electrolyte. While not shown, rather than a multi-layer anode24surrounding the wires92, a multi-layer structure can include combinations of electrode and ceramic layers, or a grading in which ceramic material is mixed with electrode material to transition from pure electrode to pure ceramic. Each layer in a multi-layer structure can be tailored to perform different functions, such as one material being anode/cathode, and the other being ceramic or YSZ; porous versus non-porous; larger versus smaller pores; chemical composition variations; relative electrical conductivity variations; relative ionic conductivity variations; variations in the ability to bond to surrounding materials; or any other physical or chemical variation.

When coating the wires with the desired material to form a layer having the plurality of spaced-apart passages embedded therein, the material can completely cover or not completely cover the wires, and the proper choice of the coating conditions can help achieve the optimal performance. Having a majority of the wire92surrounded by the material of the layer achieves the objective of providing support for the structure.

If the surrounding material does not exceed the top and bottom of the wires having a round shape, the intervening support material is a pillar shaped structure. This is the minimum structure necessary to give a solid support structure in the active area, such that it is not required that the passages be completely encompassed within the electrode, only mostly encompassed by virtue of being essentially sandwiched between support structures. The support material can meet the wire exactly at the top and bottom surfaces or the support material can be recessed on both sides of the wires, either way forming a pillar structure. Additionally, an asymmetric structure can be formed where one side of the wires is exceeded and one side is not. By way of example, the pillar form, and in particular the recessed pillar form, can be created by using a paste that becomes much thinner as the solvent dries out of the polymer matrix or by shaving the top surface with a thin razor blade and distorting down between the wires.

As opposed to varying the material composition in the thickness direction of the wires,FIG. 10shows variation down the length of the wires92for an anode24, in particular, an anode layer24aalong a first length portion, an anode layer24balong a second length portion, and an anode layer24calong a third length portion. Any number of different material types is possible. Variations of anodes, ceramics, and cathodes down the length of the wires92are also possible, such that the structure could contribute to the formation of a series fuel cell travelling down the length direction. Another variation is to replace the anode layers24aand/or24cwith sacrificial material to form sections having large volume flow paths in combination with the supported flow sections, or to insert sacrificial segments that will form exit paths to the sides of the device10.

With respect to the wires92or other physical structures, variations are possible in terms of wire diameters, wire materials, and wire properties. The wires can be 0.02 inch, 0.01 inch, 0.005 inch, or 0.002 in, for example. The wires can be made of stainless steel, carbon steel, nickel, titanium, or any other appropriate material. The wires can be spring metal, annealed, flexible and have varying degrees of strength. The wires can be straight or curved, as discussed further below. The wires can be round, oval, semi-circular, square, rectangular, or any other shape, as desired. The plurality of wires in a single layer need not all be of the same shape or dimension, and can be different in one layer versus another layer. Additionally, the wires can change in dimension and/or shape as they travel down the length of the device. For example, a wire can have a first diameter along the length of the passive area of the device and gradually or sharply decrease to a second diameter in the active area of the device, for example, a smaller second diameter. In another example, the wire can have a first shape along the length of the passive area of the device and gradually or sharply change to a second shape in the active area of the device, such as a first round shape and a second semi-circular shape or a second oval shape. The changes in diameter and shape may be designed to achieve objectives in gas flow properties and/or to achieve less resistance to the wires being removed after lamination. It may also be advantageous to heat the device after lamination to facilitate the wire removal, for example, to about 85° C., although other temperatures are contemplated. In one embodiment, the temperature of the device is raised to above the glass transition temperature (Tg) of the organic materials of the stick to dramatically soften the material, allowing easy removal of the wires. Additionally, the wires may be coated, as necessary with a release agent. However, the use of heat may make the use of release agents unnecessary. Wires may be used to form any combination of input passages, active passages, and exhaust passages. Further, within a single layer, such as an anode layer24, the wires92may be arranged in parallel in a single layer, or multiple spaced layers. The size of the wires, and thus the size of the formed passages, may also be varied in the multiple spaced layer, for example, a row of smaller diameter passages could be formed in anode layer24bofFIG. 7, and this row could be aligned or offset with respect to the passages in anode layer24a, as desired.

Various methods are possible for connecting the gas supplies to the fuel and air passages. In an elongated device, a fuel supply can be coupled to one end, and an air supply to the opposite end, for example, by placing flexible supply tubes over the ends. In such embodiments, the fuel entering one end would have to exit the device at a point before reaching the opposite end, since the opposite end is coupled to the air supply. Thus, side exits or vertical exits have been contemplated in previous designs. When using wires92to form the passages14,20to and through the active area50, the wires for forming the fuel passages,14, for example, can extend lengthwise from a fuel input end of the device and terminate at the conclusion of the active area, or can proceed into the opposite passive area but stop short of the opposite air input end. A side exit path can then be formed using sacrificial material or additional wires in contact with the lengthwise wires, such as at the ends of the wires, and extending widthwise to the side of the device.

Alternatively, the wires can extend through the entire length of the device, such that both the fuel and oxidizer passages14,20extend from a first end11ato a second end11b, but then one of the set of passages14or20is sealed off at each end, such as by injecting a small amount of ceramic or glass paste into the passages at the ends to plug them and seal them off, or by temporarily plugging the passages to be kept with short wires and painting a paste of ceramic or glass over the passages to be sealed, drying the paste, then removing the temporary plugs. Exit passages to the sides or vertically would still need to be formed then ahead of the plugs. In yet another alternative, where the wires extend the full length of the device, supply of the gases may be made by a plurality of supply tubes, for example, ceramic tubes, that are sized to be inserted into the respective plurality of passages, in typical manifold fashion, but advantageously outside the furnace in the cold end region of the device.

In alternative embodiments, shown inFIGS. 11A-11B, 12A-12B, 13 and 14, curved wires can be used to form the passages14,20. InFIG. 11A, for forming the anode layer24, the wires92are straight at end11auntil they reach the end of the active area50, then they curve toward one side of the device10to form the fuel output16at the side. For the cathode layer26, the wires92are straight from the opposite end11b, until they reach the end of the active area50, then they curve toward one side of the device10to form the oxidizer output16at the side. With the anode layers24and cathode layers26stacked together, a device10may be formed as shown. Supply tubes (partially depicted in phantom) can be fitted over each end11a,11bfor supplying the fuel and air, respectfully, to inlets12and18, while the spent gases exit out the sides of the device10from outputs16,22before reaching the ends11a,11b.

InFIG. 11B, a two-layer electrode structure can be used to form the spaced-apart passive and active passages14,20in one layer and spaced-apart exhaust passages15,21in the other layer. In anode layer24a, straight wires92are used from the end11ato the end of the active area50to form fuel passage14. A curved wire92is used in anode layer24bto form exhaust passages15from the active area50to the side for the outlet16of spent fuel. The exhaust passages15(and21, not shown) need not extend through the entire active area50, but can begin within the active area50, such as halfway into the active area50. As shown, the exhaust passages15are offset from the active passages14, but this is not required.

InFIGS. 12A-12B, showing an anode layer24inFIG. 12Aand a resulting device10inFIG. 12B, the wires92curve the entire length of the device10from the input through the active area50and to the output, so as to form inputs12,18and outputs16,22in the sides of the device10. While this may not be a preferred embodiment in terms of connecting the fuel and air supplies, manifold-type connections are nonetheless feasible.

To better provide for separate fuel and air connections,FIG. 13uses the same curved wires92as inFIG. 12A, but splits the ends11a,11bof the device10to form a pair of first end portions11a1and11a2and a pair of second end portions11b1and11b2. This construction is made possible by the fact that the layers are assembled from the green state, and thus the green layers can be molded, cut and/or shaped to have the desired form. The pair of first end portions11a1,11a2can be coupled to fuel and air supplies to feed gases to the fuel and air inputs12,18, respectively, and the spent gases can exit from outputs16,22, respectively, in the pair of second end portions11b1,11b2.

FIG. 14depicts a combination of the various embodiments above, utilizing partially curved wires, straight wires, and a split end. A pair of first end portions11a1and11a2is formed to accommodate separate fuel and air connections to inputs12,18, respectively, with only first end portion11a1being curved. Wires92are used that curve from first end portion11a1into the active area50then straighten and exit at end11b. Straight wires92are used that enter at first end portion11a2and exit at end11b, such that both outputs16and22are at the single end11bof the device.

FIG. 15shows another embodiment for building a series fuel cell structure in the vertical (thickness) direction. This can dramatically reduce the path length for the interconnect compared to other designs because the distance from one active cell to the next becomes very short and very wide, and this gives the lowest resistance combination. In addition, the use of interconnect material, and possibly precious metal, is reduced overall.

FIG. 15depicts in schematic cross-section two active cells50stacked in series, but many active cells50can be stacked one upon another by this method. The anode layers24and cathode layers26are formed as described above, for example, as shown inFIG. 6, and stacked with intervening electrolyte layers28to form a three-layer active cell stack50of anode24/electrolyte28/cathode26. A non-conductive interconnect layer50ais positioned between each active cell stack50, with each interconnect layer50ahaving a plurality of conductive vias52extending there-through to make electrical contact with the cathode26of one active cell50and the anode24of the next active cell50. To make the interconnect layer50a, a green sheet of non-conductive ceramic material can be hole-punched to provide via holes51, and conductive paste, for example, containing precious metal, can be filled into the via holes51for making the electrical connection with the conductive vias52.

The use of filled or plugged via holes can provide a potential source of gas leaks, which negatively affect device performance, so an alternate embodiment is shown in exploded view inFIG. 16. As withFIG. 15, active cell stacks50are formed, but a variation is used to electrically connect the active cells50in series. In particular, the interconnect layer50bis made of more than one ply (in this case two) where the via holes51are offset. This offset technique can prevent or reduce the leakage of gas from one side to the other. Instead of a single sheet of non-conductive material, two non-conductive sheets are provided with via holes51, and a conductor layer53is printed on one side of each sheet and they are stacked with the printed sides facing each other, with the via holes51offset so that they do not overlap. In this embodiment, there is no need to actually fill the via holes51with interconnect material, as they can be filled instead with anode24or cathode26material, as shown. This approach then saves on the use of precious metal. In the case of the cathode side, the material (LSM, for example) can be made dense so that it contributes to leak-prevention.

In the embodiments ofFIGS. 15 and 16, the via holes51can be large or small, and can be plenty or few. By way of example, and not limitation, via holes51can be 0.1 inch (2.54 mm) in diameter, or can be 25 μm.

It was discussed above that the paste material deposited around the wires92can be varied in the length direction, for example, as shown inFIG. 10. This concept can be used to create a series design down the length of the device10, rather than vertically as shown inFIG. 15. InFIG. 17, a top view depicts an anode layer having alternating segments of ceramic support material29and anode material24embedding the wires92. A similar cathode layer may be formed with cathode material26, and these layers may be stacked with the segments of anodes24and cathodes26aligned and with an intervening electrolyte layer28to form an active cell layer having a successive series of active cells50down the length, as shown in side view inFIG. 18. The series connections may be made on the sides of the device10if all or a portion of the anodes24and cathodes26extend the full width or at least to one side of the device10, or internally using previously disclosed methods in prior applications cited above, or methods disclosed herein.

One method for forming internal series connections is to include an interconnect tab54for each electrode segment, as shown inFIG. 19(wires not shown). The exploded view depicts each of the anode layer (layer1), a combination electrolyte28/interconnect layer50acontaining both the electrolyte28and the conductive vias52(layer2), and the cathode layer (layer3), in top view and in alignment for stacking, and then in side view as stacked. The wires are not shown, but would be extending through each of layers1and3. The interconnect tab54of the anode24of one active cell50is aligned with the interconnect tab54of the cathode26of the next adjacent active cell50, and the conductive via52of interconnect layer50ais aligned with both, thereby making the series connection between the adjacent active cells50. Alternative interconnect tabs54are depicted schematically inFIGS. 20A and 20B, showing both a top view and a side view. InFIG. 20A, the tab is notched, which provides an interlock between the anode segment24, the ceramic support material29and the interconnect tab54, which may contribute to leak prevention. InFIG. 20B, the interconnect tab54is a thin extension only at the surface that interfaces with the interconnect layer50a, thereby minimizing the material used for the electrical connection.

Referring toFIGS. 21A and 21B, in side view and top view, respectively, another series structure is depicted using alternating electrode and ceramic support segments. In the stacking of the layers, the anode segments in one layer are offset with respect to the cathode segments in the other layer, with the combination electrolyte28/interconnect layer50atherebetween. The edge of the anode24of one active cell50is aligned with the edge of the cathode24of the next adjacent active cell50to provide a small overlapping region, and the conductive vias52of interconnect layer50aare then aligned with that overlapping region. This design then eliminates the need for the interconnect tabs54. Rather than the interconnect layer50a, the interconnect layer50bmay be used instead, which is formed from two sheets with the printed conductor layer53therebetween.

The various series designs enable any number of active cells, whether situated in a single active layer sequentially down the length, or vertically by stacking active cells on top of each other, or a combination of both. Thus, small devices or large devices can be provided with relatively high voltage. For example, a handheld electronic device could be provided with the design ofFIGS. 21A-21Bto give low wattage with a high voltage, for example, 0.25 W and 3.6 V.