EP3152794A1

EP3152794A1 - Fuel cell having flat-tubular anode

Info

Publication number: EP3152794A1
Application number: EP15803627.7A
Authority: EP
Inventors: Yeshwanth Narendar; Aravind Mohanram
Original assignee: Saint Gobain Ceramics and Plastics Inc
Current assignee: Saint Gobain Ceramics and Plastics Inc
Priority date: 2014-06-05
Filing date: 2015-05-27
Publication date: 2017-04-12
Also published as: JP2017522691A; WO2015187416A1; EP3152794A4

Abstract

A solid oxide fuel cell (SOFC) including a flat-tubular anode and a cathode. In an embodiment, the flat-tubular anode and the cathode can be substantially thick. In an embodiment, the cathode can have a coefficient of thermal expansion (CTE) that is substantially the same as a CTE of the flattubular anode. In an embodiment, the cathode can have channels formed within the cathode for the intake of air/oxygen from an outside source.

Description

FUEL CELL HAVING FLAT-TUBULAR ANODE

FIELD OF THE INVENTION

The present invention relates to a solid oxide fuel cell and processes of forming the same.

A fuel cell is an energy conversion device that transforms chemical energy into electric power through the electrochemical oxidation of fuel. A typical fuel cell includes a cathode, an anode, and an electrolyte between the cathode and the anode. Among various fuel cells, solid oxide fuel cells (SOFCs) use a hard, ceramic compound metal oxide as an electrolyte.

Typically, in a solid oxide fuel cell (SOFC), oxygen gas (O₂) is reduced to oxygen ions (O ²) at the cathode, and a fuel gas such as hydrogen (H₂ ) or a hydrocarbon such as methane (CH₄) is oxidized with the oxygen ions to form water and carbon dioxide (from hydrocarbon) at the anode.

The design of a fuel cell is typically based on its support structure and shape. For example, US Patent Publication No. 2004/0219411 describes an anode- supported fuel cell, which is also known as a flat-tubular fuel cell because of the flat-tubular shape of the anode. In an anode- supported cell, the anode is typically the thickest layer of the fuel cell, and therefore, provides the support for forming other layers of the fuel cell. However, there remains a need in the art for improvements in fuel cell design.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 illustrates a solid oxide fuel cell (SOFC) according to an embodiment; and

FIG. 2 illustrates an SOFC stack according to another embodiment. DETAILED DESCRIPTION

The following description in combination with the figures is provided to assist in understanding the teachings disclosed herein. The following discussion will focus on specific implementations and embodiments of the teachings. This focus is provided to assist in describing the teachings and should not be interpreted as a limitation on the scope or applicability of the teachings.

As used herein, the terms "comprises," "comprising," "includes,"

"including," "has," "having," or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, "or" refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

The use of "a" or "an" is employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural, or vice versa, unless it is clear that it is meant otherwise.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples are illustrative only and not intended to be limiting. To the extent not described herein, many details regarding specific materials and processing acts are conventional and may be found in textbooks and other sources within the solid oxide fuel cell arts. In accordance with an embodiment, an SOFC can include a cathode having a thickness capable of providing structural support for other layers of the SOFC. In an embodiment where the control of oxygen or air delivery is desired, the SOFC can include a cathode having channels formed therein. Such aforementioned features are not capable in a conventional anode- supported cell having a relatively thin cathode.

FIG. 1 illustrates a solid oxide fuel cell (SOFC) 100 in accordance with an embodiment. As illustrated, the SOFC 100 can include a cathode 109. In an embodiment, the cathode 109 can be planar. For example, the cathode 109 can include an upper surface and a lower surface opposite the upper surface, wherein the upper surface and lower surface are substantially parallel to each other.

The cathode 109 can have a particular thickness. A cathode 109 having a thickness of less than 0.6 mm may not provide enough structural support for the entire SOFC 100 and may not allow for the inclusion of adequately- sized channels to formed in the cathode 109. On the other hand, a thickness of greater than 5.0 mm may increase production cost and ionic/electrical resistance without any significant increase in power density of the SOFC 100. Therefore, in accordance with an embodiment, the cathode 109 can have a thickness of at least 0.6 mm, such as at least 1.0 mm, at least 1.5 mm, or even at least 2.0 mm. In a non-limiting embodiment, the cathode 109 may include a thickness of not greater than 5.0 mm, such as not greater than 4.0 mm, not greater than 3.0 mm, or even not greater than 2.0 mm. The cathode 109 can have a thickness that is within a range of any minimum or maximum value indicated above, such as, for example, within a range of 0.6 mm to 5 mm, 1.0 mm to 2.0 mm, or 0.6 mm to 1.5 mm.

In an embodiment, the cathode 109 can include channels 111 to help provide more uniform delivery or distribution of a fluid (e.g., oxygen or air) across the cathode 109. More uniform delivery or distribution of a fluid may provide for greater power density of the SOFC 100 that capable in an SOFC that does not have channels formed therein. In a particular embodiment, the channels 111 can be formed in the cathode bulk layer 110 of the cathode 109. The cathode bulk layer 110 can include a thickness capable of having channels 111 formed therein, such as a thickness of at least 0.6 mm, at least at least 1.0 mm, at least 1.5 mm, or at least 2.0 mm. In a non-limiting embodiment, the cathode bulk layer 110 may include a thickness of not greater than 5.0 mm, such as not greater than 4.0 mm, or not greater than 3.0 mm, or not greater than 2.0 mm. The cathode bulk layer 110 can have a thickness that is within a range of any minimum or maximum value indicated above, such as, for example, within a range of 0.6 mm to 5 mm, 1.0 mm to 2.0 mm, or 0.6 mm to 1.5 mm.

The channels 111 can be of any desired shape, such as, for example, circular, rectangular, or oblong. In the case in which the channels 111 are circular, they can have a particular diameter. The diameter may be chosen for particular flow and pressure characteristics. For example, a diameter of less than 0.5 mm may not be large enough to provide adequate fluid flow through the channels. In an embodiment, the diameter of the channels 111 can be at least 0.5 mm, such as at least 1.0 mm, or even at least 1.5 mm. In a non- limiting embodiment, the diameter of the channels 111 may be not greater than 2.0 mm, such as not greater than 1.5 mm, or even not greater than 1.0 mm. The cathode 109 may have channels 111 of a diameter within a range of any minimum or maximum value indicated above, such as, for example, within a range of from 0.5 mm to 2.0 mm, 0.5 to 1.5mm, or 1.5 to 1.0 mm.

In another embodiment, the channels can have a rectangular shape, or even an oblong shape, as viewed from a cross section of FIG. 1, and can have a minimum dimension (e.g. height or width) and a maximum dimension (e.g., other of the height or width of the channels 111) separately within a range of any of the aforementioned diameters indicated with respect to circular channels. For example, the channels 111 can have a rectangular or oblong shape having a height of from 0.5 mm to 2.0 mm and a width of from 0.5 mm to 2.0 mm. In a particular embodiment, the rectangular or oblong channels 111 can have a height of 0.5 mm and a width of 1.5mm.

In accordance with an embodiment, the channels 111 can have a height (or diameter) of at least 50% of the thickness of the cathode bulk layer 110. In a non- limiting embodiment, the channels 11 can have a height of not greater than 90% of the thickness of the cathode bulk layer 110, such as not greater than 75% or nor greater than 60% of the thickness of the cathode bulk layer 110. The channels 111 can have a height that is within a range of any minimum of maximum value indicated above. For example, the channels 111 can have a height that is within a range of 50-60% of the thickness of the cathode bulk layer 110. In a particular embodiment, a cathode bulk layer 110 can have a channel of having a thickness of 1 mm and can have a channel height of 0.50 mm to 0.60 mm.

A minimum thickness of the cathode 109 may be at least 0.05 mm thicker than the dimension of the channels 111 in the thickness direction (e.g., diameter or height of the channels 111 as viewed from the cross section illustrated in FIG. 1). For example, the minimum thickness of the cathode 109 may be at least 0.1 mm thicker, such as 0.5 mm thicker, or even at least 1.0 mm thicker, than the dimension of the channels 111 in the thickness direction. Such a minimum thickness can help ensure that structural integrity is maintained in a cathode 109 having channels therein. As will be appreciated by those of ordinary skill in the art, the minimum thickness of the cathode 109 having channels 111 therein may also be dependent on the strength or porosity of the cathode 109 in that a cathode having greater porosity or less strength may require a greater minimum thickness to provide adequate structural integrity.

As discussed in more detail further herein, the SOFC 100 can include a flat tubular anode 101 having a particular thickness. The thickness of the cathode 109 and the flat- tubular anode 101 can be defined with respect to each other. For example, the cathode 109 can have a thickness (Th_c) and the flat- tubular anode 101 can have a thickness (Th_a). In accordance with an

embodiment, a ratio (Th_c)/(Th_a) can be at least at least at least 0.50, such as at least 0.75, or even at least 1.0. In a non-limiting embodiment, the ratio

(Th_c)/(Th_a) may be not greater than 1.50, such as not greater thanl.25, or even not greater than 1.0. The ratio (Th_c)/(Th_a) can be within a range of any minimum or maximum ratio described above, such as, for example, within a range of 0.50 to 1.5, or 0.75 to 1.25. In a certain embodiment, the ratio

(Th_c)/(Th_a) can be about 1.0, indicating that the cathode 109 and the flat- tubular anode 101 have the same thicknesses. The use of electrodes having a ratio (Th_c)/(Th_a) within a range of 0.50 to 1.5, or more particularly 0.75 to 1.25 may reduce geometric issues such as curvature of one or more components of the SOFC 100 resulting during formation or use. Moreover, because embodiments herein can include a ratio (Th_c)/(Th_a) within a range of 0.50 to 1.5, or more particularly 0.75 to 1.25, the cathode 109 can be thick enough to provide structural support to the SOFC similarly to a relatively thick anode 101 of an embodiment. Thus, the SOFC 101 in accordance with an embodiment may be categorized as an electrode- supported fuel cell.

In addition to fluid flow and pressure, the size and shape (e.g., diameter, width, height) of the cathode channels 111 or anode channels 108 can affect pressure drop within the cathode 109 or anode 101, respectively. For channels of a given diameter, as the length increases the pressure drop increases, parasitic power consumption increases and there may be insufficient pressure for the fuel or oxidant to flow through the entire length of the channels as intended. For example, if channel size is relatively small, pressure drop will be relatively high. Thus, the pressure drop across the channels may be regulated to obtain needed or desired flow characteristics. In an embodiment, the channels 111 or channels 108 may have a pressure drop that is not greater than 1.5 kPa, such as not greater than 1 kPa, or even not greater than 0.5 kPa. In an embodiment, the channels 111 or channels 108 may have a pressure drop that is at least O.OlkPa.

In an embodiment, the cathode 109 can include a cathode bulk layer 110 and a cathode functional layer 104. The cathode bulk layer 110 can include a greater percent porosity than the cathode functional layer 104. The porosity of the cathode bulk layer 110 can aid in the transport of fluid (e.g., air or oxygen) through the cathode 109 and toward the electrolyte 103 where the electrochemical reaction of the cell takes place. The porosity of cathode functional layer 104 can help to provide a greater density of triple phase boundaries (TPBs) at the interface of the cathode 109 and the electrolyte layer 103. TPBs are the confined spatial sites where electrolyte, gas, and electrically connected catalyst regions contact and where the oxygen reduction reaction and the hydrogen oxidation reaction of the fuel cell take place. To that end, and in accordance with an embodiment, the cathode functional layer 104 is disposed between, and can be in direct contact with, the cathode bulk layer 110 and the electrolyte 103.

In an embodiment, the cathode bulk layer 110 can have a particular total porosity, including both open porosity and closed porosity. Porosity can be calculated based on Archimedes Principle. In a particular embodiment, the majority of the total porosity is open porosity that creates an interconnected network of pores to allow fuel to traverse through the cathode bulk layer 110 toward the electrolyte 103. A total porosity of less than 25 vol% may provide insufficient interconnected porosity, while a total porosity of greater than 50% may provide insufficient structural integrity of the cathode bulk layer 110. In an embodiment, the cathode bulk layer 110 can have a total porosity of at least 25 vol%, such as at least 30 vol%, or even at least 40 vol%. In a non-limiting embodiment, the cathode bulk layer 110 may have a total porosity of not greater than 50 vol%, such as not greater than 40 vol%. A skilled artisan will appreciate from the description herein that the cathode bulk layer 110 can have a total porosity that is within a range of any minimum or maximum value indicated above, such as, for example, within a range from 25 vol% to 40 vol%.

In an embodiment, the cathode functional layer 104 can have a total porosity of at least 10 vol%, such as at least 15 vol%, or even at least 20 vol%. In a non-limiting embodiment, the cathode functional layer 104 may have a total porosity that is not greater than 35 vol%, such as not greater than 25 vol%, or even not greater than 20 vol%. The cathode functional layer 104 can have a total porosity that is within a range of any minimum or maximum value indicated above, such as, for example, within a range from 15 vol% to 25 vol%.

In an embodiment, the cathode bulk layer 110 can include a majority of the thickness of the cathode 109, and the cathode functional layer 104 can include a minority of the thickness. In a particular embodiment, the cathode bulk layer 110 can include at least 90% of the total thickness of the cathode 109, such as at least 95%, or even at least 99% of the total thickness of the cathode 109. The cathode functional layer 104 may include a thickness that is not greater than 10% of the thickness of the cathode 109, such as not greater than 5%, or even not greater than 1% of the total thickness of the cathode 109. In a particular embodiment, the cathode functional layer 104 can have a thickness of at least 15 microns and not greater than 50 microns. If the cathode functional layer 104 is too thin (i.e., less than 15 microns thick), the cathode functional layer 104 will not have enough volume for electro-chemical activity. However, if the cathode functional layer 104 is too thick (i.e., greater than 50 microns), the thickness could prevent gas diffusion through the cathode functional layer 104.

In one embodiment, the cathode bulk layer 110 can be directly bonded to the cathode functional layer 104. In another embodiment, the cathode bulk layer 110 can be bonded to the cathode functional layer 104 by a cathode bond layer 107. The cathode bond layer 107 can include characteristics such as electrical connectivity, gas permeability, mechanically strength, and thermal stability throughout the operational temperature range of the SOFC 100. For example, the cathode bond layer 107 can improve electrical contact and conductivity between the cathode bulk layer 110 and the cathode functional layer 104 by bonding the cathode bulk layer 110 directly to the cathode functional layer 104. The cathode bond layer 107 can thereby improve the dimensional tolerance between the cathode bulk layer 110 and the cathode functional layer 104 to enhance the physical integrity (i.e., mechanical strength) of the SOFC 100. The cathode bond layer 107 can include a thickness of at least 15 microns, such as at least 20 microns, or at least 25 microns. In a non- limiting embodiment, the cathode bond layer 107 may have a thickness that is not greater than 300 microns, such as not greater than 200 microns, no greater than 100 microns, not greater than 50 microns, or no greater than 30 microns. A skilled artisan will appreciate from the description herein that the cathode bond layer 107 can have a thickness that is within a range of any minimum or maximum value indicated above, such as, for example, within a range from 15 microns to 30 microns. In a particular embodiment, the cathode bond layer 107 can have the same thickness as the cathode functional layer 104. The cathode bond layer 107 can include a variety of materials, which may or may not include materials in common with the layers to which it is bonded (e.g., cathode layer and interconnect layer). However, by including common materials with the cathode bulk layer 110 or the cathode functional layer 104, the cathode bond layer 107 can have a similar CTE to the adjacent layer, and thus provide thermal stability of the entire cathode 109. For example, the cathode bond layer 107 can include a lanthanum strontium manganite (LSM) material, , a lanthanum chromite material, a lanthanum strontium cobaltite (LSC) material, or a combination thereof.

A variety of cathode materials can be used in the embodiments described herein. For example, the cathode 109 can be made of a doped lanthanum manganite material. The doped lanthanum manganite material can have a general composition represented by the formula, (La₁__xA_x)_yMnO₃, where the dopant material is designated by "A" and is substituted within the material for lanthanum (La), on the A-sites of a perovskite crystal structure. The dopant material can include an element, such as Mg, Ba, Sr, Ca, Co, Ga, Pb, Z, or any mixture thereof. According to a particular embodiment, the dopant is Sr, and the cathode 109 may include a lanthanum strontium manganite material, known generally as LSM. In a particular embodiment, the cathode 109 can consist essentially of LSM. As used herein, the term "consist essentially of," "consists essentially of," or "consisting essentially of means that the described material(s) is (are) limited to that (those) specified and to any other material(s) that do not materially affect the basic characteristics of the specified material(s). The value of x within the doped lanthanum manganite composition (La^ _xA_x)_yMnO₃ represents the amount of dopant "A" substituted for La within the structure. In accordance with an embodiment, x is not greater than about 0.5, such as not greater than about 0.4, not greater than about 0.3, not greater than about 0.2, or even not greater than about 0.1. In an embodiment, the value of x can be greater than about 0.05. In a particular embodiment, the value of x can be within a range of from about 0.4 to 0.05. In an embodiment, y is not greater than about 1.0, and the ratio of La/Mn is not greater than about 1.0. The cathode bulk layer 110 and the cathode functional layer 104 of the cathode 109 can include the same or different materials.

Components of an SOFC may be susceptible to damage caused by fluctuations in temperature during their formation or use. Specifically, materials employed to form the various components, including ceramics of differing compositions, exhibit distinct material, chemical, and electrical properties that can result in breakdown and failure of the SOFC article. For example, an anode material including equal parts nickel oxide (NiO) and yttria stabilized zirconia (YSZ) can have an average CTE of about of 12.5xl0^{"6 0}C\ and a cathode material including LSM can have an average CTE of from about 12.2xl0^"6 °C^_1 to about 12.4xl0^"6 °C^_1. Moreover, a CTE of an electrolyte material including YSZ is generally in a range of from about 10.5xl0^~6 °C^_1 to about 1 lxlO^"6 °C^_1. The difference between the CTEs of the various materials of different layers of the SOFC would generate a large thermal mismatch stress in the SOFC. Using CTE matched layers between the cathode and anode layers may reduce the occurrence of curvature of SOFC layers that could otherwise result with mismatched CTEs. A fuel cell in which both the anode and the cathode are relatively thick are particularly susceptible to coefficient of thermal expansion (CTE) mismatches between adjacent components to the anode or cathode, which can lead to failure of the fuel cell, particularly at operating temperatures of 780 °C to 950 °C.

Stress generation of SOFC components can also occur when cooling down from a sintering temperature (e.g., 1300-1400°C) to room temperature, particularly when two or more SOFC components (i.e., layers) are sintered together if there is any mismatch in the CTE among the two or more

components. Where there is sufficient mismatch of CTE among the

components, any temperature change (especially a temperature change that is too rapid) can cause fracture and consequent failure of the SOFC. Because of the much larger thickness of the cathode 109 of the embodiments, a majority of the stress can generated by the mismatch between the CTEs of the flat-tubular anode 101 and the cathode 109, particularly the cathode bulk layer 110. In contrast, the selection of a relatively thin (e.g. 30 microns to 100 microns) cathode, such as that of a conventional anode- supported fuel cell, is typically not based on the CTE characteristics of the cathode material because such a consideration is not imperative at such small cathode thicknesses. For example, relatively thin cathodes will not typically expand or contract with a large enough degree to negatively affect (e.g., crack or distort) other layers of the SOFC. According to an embodiment, the cathode 109 can be chosen to have a CTE that is well-matched to one or more other layers of the SOFC 100, particularly to that of the flat-tubular anode 101. CTE mismatch between the flat-tubular anode 101 and the cathode 109 may be determined based on the following formula: [(CTE_a - CTE_d)/CTE_a] x 100%, wherein CTE_a is the CTE of the anode, and CTE_C is the CTE of the cathode. In a particular embodiment, the CTE mismatch may be not greater than about 5%, such as not greater than about 4%, not greater than about 3%, not greater than about 2%, or even not greater than about 1%. In a particular embodiment, the CTE mismatch may be at least about 0.001%.

In an embodiment, a cathode bond layer 106 can be used to bond a cathode from another subcell to the interconnect 105. In general, a bond layer should be porous and thin to allow for a high oxygen partial pressure at an interface between the bond layer 106 and the interconnect 105. A cathode bond layer can improve electrical contact between the interconnect 105 and an adjacent cathode that is bonded thereto. The cathode bond layer 106 can also improve the dimensional tolerance between the interconnect 105 and the adjacent cathode to enhance the physical integrity of the SOFC 100. The cathode bond layer 106 can include a variety of materials, which may or may not include materials in common with the layers to which it is bonded (e.g., cathode layer and interconnect layer). For example, the cathode bond layer 106 can include a lanthanum strontium manganite (LSM) material, , a lanthanum chromite material, a lanthanum strontium cobaltite (LSC) material, or a combination thereof. The cathode bond layer 106 can include a thickness of at least 15 microns, such as at least 25 microns, at least 50 microns, at least 100 microns 200 microns, at least 300 microns. In a non-limiting embodiment, the cathode bond layer 106 may have a thickness that is not greater than 300 microns. A skilled artisan will appreciate from the description herein that the cathode bond layer 106 can have a thickness that is within a range of any minimum or maximum value indicated above, such as, for example, within a range from 15 microns to 300 microns, or 100 microns to 300 microns.

As also illustrated in FIG. 1, SOFC 100 can include a flat- tubular anode 101. In an embodiment, the flat-tubular anode 101 can include a thickness of at least 0.6 mm. In embodiments described herein in which the anode 101 can include channels for the delivery of fluid, it is may be desirable for a minimum thickness of the anode 101 to be at least 0.1 mm thicker than the dimension of the channels in the thickness direction (e.g., diameter or height of the channels as viewed from the cross section illustrated in FIG. 1). For example, the minimum thickness of the anode 101 may be at least 0.5 mm thicker, or even at least 1.0 mm thicker, than the dimension of the channels in the thickness direction. Such a minimum thickness can help ensure that structural integrity is maintained in a flat-tubular anode 101 having channels therein. Still, it may be desirable for the anode 101 to not have a thickness in excess of 5.0 mm in order to not unnecessarily increase ionic/electronic resistance. Thus, in a non-limiting embodiment, the flat- tubular anode 101 may include a thickness of not greater than 5.0 mm, such as not greater than 4 mm, not greater than 3 mm, not greater than 2 mm, or even not greater than 1.5 mm. A skilled artisan will appreciate from the description herein that the flat-tubular anode 101 can have a thickness that is within a range of any minimum or maximum value indicated above. For example, in a particular embodiment, the flat-tubular anode 101 can have a thickness within a range from 0.6 mm to 1.5 mm.

In a certain aspect, the flat-tubular anode 101 can have a shape that is defined at least in part by an exterior surface 102. For example, the exterior surface 102 can be substantially flat at one portion of the flat-tubular anode 101, and substantially rounded at another portion of the flat- tubular anode 101. In an embodiment the exterior surface 102 can be substantially flat at a portion of the flat-tubular anode 101 that contacts an interconnect 105. In an embodiment, the exterior surface 102 can be substantially rounded at opposite ends of the flat- tubular anode 101, as generally illustrated in FIG. 1. The rounded ends may help the electrolyte 103 to have a more uniform thickness along the ends of the flat-tubular anode 101.

In an embodiment, the flat-tubular anode 101 can include channels 108 for the delivery of a fluid (e.g., fuel) to the SOFC 100 from an external source. The channel 108 can include the shapes and dimensions as discussed herein with respect to cathode channels 111.

The flat-tubular anode 101 can include a particular porosity. The total porosity can include both open porosity and closed porosity. In a particular embodiment, the majority of porosity of the flat-tubular anode 101 is open porosity that creates an interconnected network of pores to allow for fluid (e.g., fuel) to traverse through the anode 101 to the electrolyte 103. A total porosity less than 25 vol% may provide inadequate interconnected porosity, while a total porosity of greater than 60% may provide insufficient structural integrity of the flat-tubular anode 101. Therefore, in an embodiment, the flat-tubular anode 101 can have a total porosity of at least 25 vol%, such as at least 30 vol%, or even at least 40 vol%. In a non-limiting embodiment, the flat-tubular anode 101 may have a total porosity of not greater than 50 vol%, such as not greater than 40 vol%. A skilled artisan will appreciate from the description herein that the flat- tubular anode 101 can have a total porosity that is within a range of any minimum or maximum value indicated above, such as, for example, within a range from 25 vol% to 40 vol%.

A variety of anode materials can be used in embodiments described herein. A specific example of an anode material is one that includes a Ni cermet. "Ni cermet" generally refers to a ceramic metal composite that includes Ni, such as about 20 wt %-70 wt % of Ni. An example of a Ni cermet is a material that includes Ni (or NiO) and yttria- stabilized zirconia (YSZ). In an embodiment, YSZ can include ZrO₂ containing anywhere from 3 mol% to 15 mol% of Y₂O₃. In a particular embodiment, YSZ can include 8 mol% Y₂O₃ (8YSZ). In a particular embodiment, the flat-tubular anode 101 can include Ni, NiO, YSZ, or any combination thereof.

In an embodiment, the SOFC 100 can include an electrolyte layer 103. The electrolyte layer 103 can be disposed over a portion of the exterior surface 102 of the flat-tubular anode 101. In particular embodiments, the electrolyte layer 103 can be disposed over at least 50% of the exterior surface 102 of the flat-tubular anode 103. For example, the electrolyte layer 103 can be disposed over at least 60% of the exterior surface 102 of the flat-tubular anode 103, such as at least 65%, at least 75%, at least 80%, at least 90%, or even at least 95%. In particular embodiments, the electrolyte layer 103 may be disposed over not greater than 95% of the exterior surface 102 of the flat-tubular anode 101. For example, the electrolyte layer 103 may be disposed over not greater than 90% of the exterior surface 102 of the flat-tubular anode 103, such as not greater than 85%, not greater than 80%, not greater than 75%, not greater than 70%, not greater than 65%, not greater than 60%, or even not greater than 50%. As skilled artisan will appreciate from the disclosure herein that the electrolyte layer 103 can be disposed over the exterior surface 102 of the flat-tubular anode 101 within any range of minimum or maximum values indicated above. In particular embodiments, the electrolyte layer 103 can be disposed over at least one of the rounded portions of the flat-tubular anode 101, as shown in FIG. 1. In other particular embodiments, the electrolyte layer 103 can be disposed only on one or more of the flat portions of the flat- tubular anode 101. In a certain embodiment, the electrolyte layer 103 can be disposed only between the flat- tubular anode 101 and the cathode 109.

A variety of electrolyte materials can be used for electrolyte layer 103. For example, an electrolyte material that can be used may include a ZrO2 based material, such as a Sc₂O₃-doped ZrO₂, Y₂O₃-doped ZrO₂, or a Yb₂O₃-doped ZrO₂; a CeO₂ based material, such as a Sm₂O₃-doped CeO₂, Gd₂O₃-doped CeO₂, Y₂O₃-doped CeO₂ or a CaO-doped CeO₂; a Ln-gallate based material (Ln=a lanthanide, such as La, Pr, Nd or Sm), such as a LaGaO₃ doped with Ca, Sr, Ba, Mg, Co, Ni, Fe or a mixture thereof (e.g., Lao_.8Sro_.2Gao_.8 go_.2O₅,

Lao_.8Sro_.2Gao_.8Mgo_.i₅COo_.05O₅, Lao_.9Sro_.iGao_.8Mgo.2O5, LaSrGaO₄, LaSrGa₃O₇ or La₀.9A_0.iGa₃ where A=Sr, Ca or Ba); or any mixture thereof. Another example includes doped yttrium- zirconate (e.g., YZr₂O₇), doped gadolinium-titanate (e.g., Gd₂Ti₂O₇), brownmillerites (e.g., Ba₂In₂O₆ or Ba₂In₂O₅), or the like.

Any suitable thickness can be used for electrolyte layer 103. For example, the thickness of the electrolyte layer 103 can be at least 5 microns, such as at least 10 microns, at least 30 microns, at least 50 microns, at least 80 microns, or even at least 100 microns. In a non- limiting embodiment, the thickness of the electrolyte layer 103 may be not greater than 120 microns, such as not greater than 100 microns, not greater than 80 microns, not greater than 50 microns, not greater than 30 microns, or even not greater than 10 microns. In one specific embodiment, the thickness of electrolyte layer 103 can be in a range of about 5 microns to about 20 microns, such as of about 5 microns to about 10 microns. In another specific embodiment, the thickness of electrolyte layer 103 is thicker than about 100 microns.

The separation of the fuel gas in flat-tubular anode 101 and oxidizer gas in cathode 109 by the electrolyte layer 103 creates an oxygen partial pressure gradient. This gradient causes oxygen ions to be transported across the electrolyte layer 103 and to react with the fuel in the flat- tubular anode 101. The anode 101, cathode 109, and electrolyte 103 together can form a subcell. Similarly, another flat-tubular anode, cathode, and electrolyte can form another subcell. Two or more subcells according to embodiments described herein can be stacked one upon another. For example, as illustrated in FIG. 2, SOFC 201 and SOFC 203 are subcells which can form a stack 200 having an interconnect 202 disposed between SOFC 201 and SOFC 203. An electrically conductive interconnect layer can be formed between an anode layer of one subcell and a cathode layer of an adjacent subcell to connect the subcells in series so that the electricity each subcell generates can be combined.In an embodiment, a cathode bond layer 204 can be disposed between the interconnect 202 of SOFC 201 and the cathode 205 of SOFC 203. The pattern may be repeated multiple times to form a stack with a large number of individual subcells.

In accordance with an embodiment, the SOFC 100 can include an interconnect layer 105. The interconnect layer 105 can be disposed on the flat- tubular anode 101. In a particular embodiment, the interconnect layer 105 can be in direct contact with the flat- tubular anode 101. The interconnect layer 105 can be planar along its entire length. In a particular embodiment, the

interconnect layer 105 can be disposed over at least 20% of the exterior surface 102 of the flat-tubular anode 101, such as at least 30%. In a non-limiting embodiment, the interconnect layer 105 may be disposed over not greater than 50% of the exterior surface 102 of the flat-tubular anode 101, such as not greater than 40%, or not greater than 30%. A skill artisan will appreciate from the description herein that the interconnect layer 105 can be disposed over a portion of the exterior surface 102 of the flat-tubular anode 101 that is within a range of any of the minimum or maximum values indicated above.

A variety of interconnect materials can be used to form the interconnect layer 105. Such materials can include a ceramic material, such as an inorganic material. In particular, the interconnect layer 105 can include an oxide material, such as a chromite or a titanate material. More particularly, the interconnect layer 105 can include lanthanum (La), manganese (Mn), strontium (Sr), titanium (Ti), niobium (Nb), calcium (Ca), gallium (Ga), cobalt (Co), yttria (Y), or any combination thereof. In certain instances, the interconnect layer 105 can include a chromium oxide-based material, nickel oxide-based materials, cobalt oxide-based materials, and titanium oxide-based materials (e.g., lanthanium strontium titanate). In particular, the interconnect layer 105 can be made of a material, such as LaSrCrO₃, LaMnCrO₃, LaCaCrO₃, YCrO₃, LaCrO₃, LaCoO₃, CaCrO₃, CaCoO₃, LaNiO₃, LaCrO₃, CaNiO₃, CaCrO₃, or any combination thereof.

LaCrO₃- based materials have been widely used as an interconnect material for SOFCs. However, LaCrO₃- based materials are difficult to sinter densely and typically require high sintering temperatures to obtain a sufficiently dense material useful as an interconnect. On the other hand, LST-based materials have superior sintering characteristics in that they can typically sinter to greater density at lower sintering temperatures than LaCrO₃- based materials. Thus, in particular, the interconnect layer 105 can include an LST-based material (e.g., alone or in combination with YST). In another particular embodiment, the interconnect layer 105 may consist essentially of LST, such as, La_0.2Sr_0.8TiO₃. In yet another particular embodiment, the interconnect layer 105 can consist essentially of LST having one or more dopants, such as Nb. The interconnect material may include an A- site deficient material, wherein for example, the lattice sites typically occupied by lanthanum or strontium cations are vacant, and thus the material has a non- stoichiometric composition.

The interconnect layer 105 can be a particularly thin, planar layer of material. For example, the interconnect layer 105 may have an average thickness of not greater than about 100 microns, not greater than about 80 microns, not greater than about 50 microns, or even not greater than about 25 microns. Still, the interconnect layer 105 can have an average thickness of at least about 1 micron, such as at least about 2 microns, at least about 5 microns, at least about 8 microns, or at least about 10 microns. The average thickness of the interconnect layer 105 can be within a range of from any of the minimum and maximum values noted above. In a particular embodiment using an interconnect including a titanate material, an interconnect having a relatively thin titanate interconnect layer has been discovered by the inventors to perform better than a relatively thick titanate interconnect layer. Although not wishing to be bound to any particular theory, it is believed that the low conductivity of titanates in air causes a relatively thick titanate interconnect layer to perform more poorly, or even not at all, than a relatively thin titanate interconnect layer. Therefore, in a particular embodiment, the interconnect layer 105 including a titanate material may have a maximum average thickness of not greater than about 100 microns.

In order to form a solid oxide fuel cell according to the embodiments herein, each of the layers can be formed individually before assembling the layers in the SOFC subcell or jointly with other layers of the subcell through, for example, a co-sintering process. That is, the layers can be formed separately as green layers and assembled together into a subcell or stack. Alternatively, the layers may be formed in green state in succession on each other, such that a first green electrolyte layer is formed, and thereafter, a green electrode layer can be formed overlying the green electrolyte layer, and thereafter, a green interconnect layer can be formed overlying the green electrode layer. The method further including sintering the green SOFC cell in a single sintering process to form a sintered SOFC cell.

Reference herein to "green" articles is reference to materials that have not undergone sintering to affect densification or grain growth. A green article is an unfinished article that may be dried and have low water content, but is unfired. A green article can have suitable strength to support itself and other green layers formed thereon.

The layers described according to the embodiments herein can be formed through techniques including casting, deposition, printing, extruding, lamination, die-pressing, gel casting, spray coating, screen printing, roll compaction, injection molding, or any combination thereof.

Many embodiments described herein can include a cathode having a thickness capable of providing structural support for other layers of the SOFC. In an embodiment where the control of air or O₂ delivery is desired, the SOFC can include a cathode having channels formed therein. Such aforementioned features are not capable in a conventional anode- supported cell having a relatively thin cathode (e.g., in a range of from 30 microns to 100 microns). Such a thin cathode is generally not thick enough to allow channels to be formed therein for actively supplying air or oxygen to the SOFC. Instead, a cathode of a conventional anode- supported fuel cell receives air or oxygen passively from the environment by the entire SOFC structure being placed within an enclosure that provides air or oxygen generally around the entire SOFC structure. Diffusion alone is used to distribute the air or oxygen across the thin cathode of the conventional anode- supported fuel cell, which may limit the power density of the cell. Moreover, such a thin cathode is generally not thick enough to provide for structural support of the SOFC and must rely on a relatively thick anode structure for support. Also, the cathode of an

embodiment can serve to replace the Cr-felt current collector used for cell-cell connection in some convention anode- supported cells, which also tend to exhibit high degradation and low temperature constraint issues.

Items

Item 1. A solid oxide fuel cell, comprising: a flat- tubular anode; a cathode including a cathode bulk layer; and an electrolyte disposed between the flat-tubular anode and the cathode.

Item 2. A solid oxide fuel cell, comprising: a flat-tubular anode; a cathode including channels; and an electrolyte disposed between the flat-tubular anode and the cathode.

Item 3. A solid oxide fuel cell, comprising: a flat-tubular anode; a cathode including a thickness of at least 0.6 mm; and an electrolyte disposed between the flat-tubular anode and the cathode.

Item 4. The solid oxide fuel cell of any of the above items, wherein the flat-tubular anode has a coefficient of thermal expansion (CTE_a), and the cathode has a coefficient of thermal expansion (CTE_C); wherein a CTE Mismatch is defined by the formula: [(CTE_a - CTE_c/CTE_a] x 100%, and wherein the CTE Mismatch is not greater than 5%.

Item 5. The solid oxide fuel cell of any of the above items, wherein the cathode includes a coefficient of thermal expansion in a range of lO.OxlO^"6 °C to 12.5xl0^"6 °C^_1.

Item 6. The solid oxide fuel cell of any of the above items, wherein the cathode has a thickness (Th_c) and the flat-tubular anode has a thickness (Th_a), and wherein a ratio (Th_c)/(Th_a) is in a range of 0.50 to 1.50.

Item 7. The solid oxide fuel cell of any of the above items, wherein the cathode includes a thickness in a range of 0.6 mm to 5 mm.

Item 8. The solid oxide fuel cell of any of the above items, wherein the cathode includes channels.

Item 9. The solid oxide fuel cell of any of the above items, wherein the channels of the cathode have a diameter of at least 0.5 mm.

Item 10. The solid oxide fuel cell of any of the above items, wherein the channels of the cathode have a diameter of not greater than 2 mm.

Item 11. The solid oxide fuel cell of any of the above items, wherein the cathode is substantially planar.

Item 12. The solid oxide fuel cell of any of the above items, wherein the cathode includes a cathode bulk layer and a cathode functional layer.

Item 13. The solid oxide fuel cell of any of the above items, wherein the cathode includes LSM.

Item 14. The solid oxide fuel cell of any of the above items, wherein the flat-tubular anode includes Ni, NiO, YSZ, or a combination thereof.

Item 15. The solid oxide fuel cell of any of the above items, further comprising an interconnect disposed on the flat-tubular anode.

Item 16. The solid oxide fuel cell of any of the above items, wherein the electrolyte is in contact with a rounded portion of the anode. Item 17. A solid oxide fuel cell stack comprising two or more of the solid oxide fuel cells according to any of the above items, wherein the two or more solid oxide fuel cells are stacked one upon the other.

Item 18. The solid oxide fuel cell of item 17, further comprising a cathode bond layer disposed between the interconnect of a first solid oxide fuel cell and a cathode of a second solid oxide fuel cell.

Item 19. The solid oxide fuel cell of item 18, wherein the cathode bond layer includes LSM.

Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed is not necessarily the order in which they are performed.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.

The specification and illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The specification and illustrations are not intended to serve as an exhaustive and comprehensive description of all of the elements and features of apparatus and systems that use the structures or methods described herein. Certain features, that are for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in a subcombination. Further, reference to values stated in ranges includes each and every value within that range. Many other embodiments may be apparent to skilled artisans only after reading this specification. Other embodiments may be used and derived from the disclosure, such that a structural substitution, logical substitution, or another change may be made without departing from the scope of the disclosure. Accordingly, the disclosure is to be regarded as illustrative rather than restrictive.

Although the present invention has been described with reference to particular embodiments, as will occur to those skilled in the art, changes and additions to such embodiment can be made without departing from the scope of the present invention as set forth in the appended claims.

Claims

What is claimed is:

1. A solid oxide fuel cell, comprising:

a flat-tubular anode;

a cathode including a cathode bulk layer; and

an electrolyte disposed between the flat-tubular anode and the cathode.

2. A solid oxide fuel cell, comprising:

a flat-tubular anode;

a cathode including channels; and

an electrolyte disposed between the flat-tubular anode and the cathode.

3. A solid oxide fuel cell, comprising:

a flat-tubular anode;

a cathode including a thickness of at least 0.6 mm; and

an electrolyte disposed between the flat-tubular anode and the cathode.

4. The solid oxide fuel cell of any of the above claims, wherein the flat-tubular anode has a coefficient of thermal expansion (CTE_a), and the cathode has a coefficient of thermal expansion (CTE_C); wherein a CTE Mismatch is defined by the formula: [(CTE_a - CTE_c/CTE_a] x 100%, and wherein the CTE Mismatch is not greater than 5%.

5. The solid oxide fuel cell of any of the above claims, wherein the cathode includes a coefficient of thermal expansion in a range of lO.OxlO^"6 °C^_1 to 12.5xl0^"6 °C^_1.

6. The solid oxide fuel cell of any of the above claims, wherein the cathode has a thickness (Th_c) and the flat-tubular anode has a thickness (Th_a), and wherein a ratio (Th_c)/(Th_a) is in a range of 0.50 to 1.50.

7. The solid oxide fuel cell of any of the above claims, wherein the cathode includes channels.

8. The solid oxide fuel cell of any of the above claims, wherein the cathode is substantially planar.

9. The solid oxide fuel cell of any of the above claims, wherein the cathode includes a cathode bulk layer and a cathode functional layer.

10. The solid oxide fuel cell of any of the above claims, wherein the cathode includes LSM.

11. The solid oxide fuel cell of any of the above claims, wherein the flat-tubular anode includes Ni, NiO, YSZ, or a combination thereof.

12. The solid oxide fuel cell of any of the above claims, further comprising an interconnect disposed on the flat-tubular anode.

13. The solid oxide fuel cell of any of the above claims, wherein the electrolyte is in contact with a rounded portion of the anode.

14. A solid oxide fuel cell stack comprising two or more of the solid oxide fuel cells according to any of the above claims, wherein the two or more solid oxide fuel cells are stacked one upon the other.

15. The solid oxide fuel cell of claim 14, further comprising a cathode bond layer disposed between the interconnect of a first solid oxide fuel cell and a cathode of a second solid oxide fuel cell.