Patent Application
20090317705
The invention generally relates to fuel cells. More specifically, the invention is directed to interconnect structures and materials for solid oxide fuel cells.
Solid oxide fuel cells (SOFCs) are promising devices for producing electrical energy from fuel with high efficiency and low emissions. Like most fuel cells, the SOFC devices generate electric current by the electrochemical combination of hydrogen and oxygen. In a typical SOFC, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode, and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 anions (catalytically) by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions, to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit, whereby four electrons are transferred from the anode to the cathode.
As shown in
Commercial solid oxide fuel cell structures usually consist of many of these cells stacked together—sometimes hundreds of cells, which cumulatively provide enough voltage to make the device commercially feasible. The cells are typically joined together by interconnects, such as that noted above. The interconnects are usually in the form of metallic or ceramic layers, and provide electrical contact, current distribution, and structural integrity between individual cells. In a typical cathode-electrolyte-anode stack arrangement (viewed vertically for the sake of discussion), one interconnect layer is attached to an upper surface of a cathode layer, for connection to the anode layer of an adjacent cell or “module”. Another interconnect layer is attached to the lower surface of the anode, for connection to the cathode layer of another adjacent cell.
Metallic interconnects are often used in fuel cells, based on considerations like cost and ease-of-fabrication. As in the case of any type of interconnect (e.g., ceramic), the metallic alloy composition must provide a desired level of hermeticity and electrical conductivity under fuel cell operating conditions. Moreover, the alloy material should be capable of withstanding the effects of operation at high temperatures and temperature cycling conditions.
In many cases, the cathode interconnect and the anode interconnect are formed from a ferritic stainless steel material. Ferritic stainless steels are well-known in the art, and are usually based on iron, chromium, and various other selected elements. Ferritic steels are useful for several reasons. As an example, the materials are in the form of the body-centered cubic (BCC) phase. Materials of this type have a coefficient of thermal expansion (CTE) which can be closely matched to the CTE of the electrolyte in the fuel cell. Matching of the expansion characteristics for various layers in an SOFC is critical, in terms of high-temperature electrical performance, and structural integrity.
Ferritic stainless steels are certainly very useful as interconnect materials. The chromium-containing alloys are easily formed and shaped, and are also less costly than most of the ceramic interconnect materials. However, the ferritic stainless steels have some drawbacks as well. For example, the materials are susceptible to thermally-induced oxide formation, i.e., the rapid formation of a chromium oxide (chromia) layer on the surface of the alloy component. In most cases, a thin, dense chromia layer on the surface of the component may be beneficial for protecting the metal surface, while also exhibiting relatively high electrical conductivity. However, a fast-growing, thick chromia layer can degrade the performance of the fuel cell, by increasing the overall electrical resistance of the cell in a short period of time. As a result, the useful life of the fuel cell can be decreased considerably.
With these considerations in mind, new processes for the fabrication of solid oxide fuel cells and fuel cell components would be welcome in the art. The processes should result in fuel cell structures which can provide optimum and stable electrochemical characteristics and fuel efficiency over an extended period of operation, e.g., as measured by the area specific resistance (ASR) of the fuel cell. Moreover, the fuel cell should exhibit good physical integrity and durability.
The present invention meets these and other needs by providing a method for the formation of a diffusion barrier layer on a surface of at least one fuel cell interconnect structure formed of a material comprising ferritic stainless steel. The method comprises the following steps:
(a) applying a coating of an austenite phase-stabilizer to the surface of the interconnect; and
(b) heating the coated surface to diffuse the austenite phase-stabilizer into the surface, so that a surface region of the interconnect structure is transformed from a substantially ferritic body-centered cubic (BCC) phase to a substantially austenitic face-centered cubic (FCC) phase.
In another embodiment of this invention, a solid oxide fuel cell is disclosed. The fuel cell comprises:
(i) a cathode;
(ii) an anode;
(iii) a ceramic electrolyte disposed between the anode and the cathode;
(iv) a cathode interconnect attached to an upper surface of the cathode, having an interconnect surface which faces and at least partially contacts a surface of the cathode; and
(v) an anode interconnect attached to a lower surface of the anode, having an interconnect surface which faces and at least partially contacts a surface of the anode;
wherein at least one of the cathode interconnect surface or the anode interconnect surface comprises a surface region characterized by a substantially austenitic face-centered cubic (FCC) phase.
Yet another embodiment of this invention is directed to a solid oxide fuel cell stack. The stack is formed of a plurality of interconnected fuel cells. At least one of the fuel cells comprises a cathode interconnect formed of a ferritic stainless steel material and having a cathode interconnect surface facing the surface of a cathode of the fuel cell. The cathode interconnect surface includes a surface region characterized by a substantially austenitic face-centered cubic (FCC) phase.
It should be noted that in the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” “first,” “second,” and the like are words of convenience, and are not to be construed as limiting terms. Moreover, as used throughout this disclosure, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the term “surface” may sometimes include one or more surfaces).
As alluded to above, the alloy composition for cathode interconnect 22 is formed from a ferritic stainless steel alloy (sometimes referred to herein as “ferritic steel”). Such alloys are well-known in the art. Many of those which are used in electrochemical cells comprise about 60 to about 85 weight % iron and about 15 to about 30 weight % chromium. The alloys often contain carbon (e.g., up to about 0.1 weight %) and/or manganese (e.g., up to about 1 weight %). Various other metals may also be included, such as yttrium and lanthanum, which are typically present (in total) at a level of no more than about 1 weight %. However, it should be noted that the present invention is applicable to a wide variety of iron-chromium alloys which can be characterized as “ferritic stainless steel”.
As mentioned above, a coating of an austenite-phase stabilizer is applied to the surface of the interconnect, i.e., to inner surface 23 and surface 27. The austenite-phase stabilizer comprises at least one metal selected from the group consisting of nickel, cobalt, nitrogen, carbon, and manganese. In some specific embodiments, the austenite-phase stabilizer comprises manganese, cobalt, or a combination of manganese and cobalt. However, nickel is preferred in many embodiments, while cobalt is the preferred stabilizer in other embodiments. The austenite phase-stabilizer usually must be deposited in metallic form. In some cases, though, the stabilizer can be deposited in oxide form, if it is subsequently reduced to metallic form, e.g., by a heat treatment in a reducing atmosphere, such as a hydrogen furnace.
A variety of deposition techniques could be employed to apply the austenite-phase stabilizer to the surface of the interconnect. Non-limiting examples include electroplating, electroless plating, vacuum plasma spraying, low pressure plasma spraying, vacuum arc spraying; physical vapor deposition, electron beam physical vapor deposition, sputter coating, and chemical vapor deposition. In some embodiments, electroplating is the preferred method of deposition; while in other embodiments, electroless plating would be the technique-of-choice. Those skilled in the art are familiar with these techniques, and can adapt each of them to a particular deposition situation. The amount of austenite coating material (i.e., the austenite-forming coating) which is to be applied to the interconnect surface will be determined in part by the desired depth of the austenitic surface layer or region, as discussed below. In general, the thickness of the austenite-forming coating is independent of the thickness of the substrate. Instead, the coating thickness is optimized to reduce or impede diffusion over the operating life of the fuel cell, as discussed herein.
After the austenite-phase stabilizer has been applied over the surface of the interconnect (cathode interconnect, anode interconnect, or both), a heat treatment is carried out to diffuse the material into the surface. The particular heating conditions will depend on a variety of factors, such as: the particular austenite-phase stabilizer metal employed; the manner in which the stabilizer metal was deposited; the specific composition of the underlying ferritic steel material, its microstructural characteristics; and the desired depth of the austenitic surface region. Various production factors can also be important, e.g., the amount of time required to form the desired surface region on an interconnect structure, in a typical fabrication facility.
Usually, the interconnect surface region is heated to a temperature which is at least about 40% of the melting point of the ferritic stainless steel material. In some specific embodiments, the surface region is heated to at least about 65% of the melting point of the ferritic steel material. As a non-limiting illustration in the case of a manganese or cobalt austenitic element, the diffusion temperature will be in the range of about 600° C. to about 1100° C. In some specific embodiments, the diffusion temperature will be in the range of about 800° C. to about 1000° C. Those skilled in the art understand that higher temperatures within these ranges may result in shorter heat-treatment duration, while longer heat treatments may compensate for lower diffusion temperatures. In some preferred embodiments for a commercial setting, the heat treatment time is usually in the range of about 1 hour to about 24 hours. (In the case of in-situ heat treatments, as discussed below, the heat treatment can actually be effected over the course of up to about 100 hours.) The heat treatment can be accomplished by a number of techniques. It is usually carried out in a conventional furnace, using an air or oxygen atmosphere. A reducing atmosphere (as mentioned above) or an inert atmosphere could alternatively be used.
In another embodiment of this invention, the heat treatment for the interconnect surface region can be carried out “in-situ”, i.e., while the fuel cell is in operation. As one example, the austenite phase-stabilizing material could be applied to the interconnect, and then the interconnect could be incorporated into the fuel cell structure. As the fuel cell reaches its initial operating temperature (e.g., about 700° C.-900° C.), phase transformation of the surface region of the interconnect will usually begin to occur, as discussed below. Moreover, in some embodiments, the heat treatment can be carried out by a combination of initial, conventional heating and, then, in-situ heating. (As used herein, the term “heating the coated surface” is meant to also describe partial or total in-situ treatments).
As also mentioned previously, the heat treatment transforms the surface region of the interconnect structure from a substantially ferritic body-centered cubic (BCC) phase to a substantially austenitic face-centered cubic (FCC) phase, effectively forming a diffusion barrier layer. The average depth of the surface region will depend on a number of factors, some of which were mentioned above. They include: the particular austenitic stabilizer metal employed; and the specific composition of the underlying ferritic steel material.
In general, the surface region should be thick enough (i.e., its depth) to function as a barrier layer. The barrier layer impedes the diffusion of chromium out of the interconnect surface, and thereby reduces the oxidation rate of the base metal. However, the surface region should be thin enough to ensure that the bulk interconnect structure maintains a CTE value which is similar to or substantially identical to other structural members of the fuel cell, such as the ceramic electrolyte membrane.
In some embodiments, the surface region has a depth which is in the range of about 0.1% to about 10% of the thickness of the interconnect structure. A non-limiting illustration can be provided, with reference to
The present disclosure also includes an inventive embodiment which is directed to a solid oxide fuel cell (SOFC), as described previously. With reference to
With continued reference to
In some embodiments, it may also be desirable to apply an austenite coating to the surface of the anode interconnect which faces anode 24. Thus, the coating could be applied to the walls 45 (i.e., the troughs) of the fuel flow channels 34, as well as to the surface 47 of the dividing walls of the structures. The austenite-coated surface could then be heated as described previously (for diffusion and phase transformation), either as a separate step, or as part of another heat treatment for the fuel cell. The use of the austenitic material on the anode interconnect can also provide some of the key advantages noted herein for the cathode interconnect. In the case of the anode interconnect, the austenite-phase stabilizer is usually nickel.
In operating the fuel cell, a fuel flow 40 is supplied to the fuel flow channels 34. An airflow 38, typically heated air, is supplied to the airflow channels 25. The operation of a fuel cell like that depicted in
The compositions of the various structural layers of the SOFC are known in the art. The ceramic electrolyte is typically formed of a material capable of conducting ionic species (such as oxygen ions or hydrogen ions), yet having relatively low electronic conductivity. Examples of suitable ceramic materials include, but are not limited to, various forms of zirconia, ceria, hafnia, bismuth oxide, lanthanum gallate, thoria, and various combinations of these ceramics. In certain embodiments, the ceramic electrolyte comprises a material selected from the group consisting of yttria-stabilized zirconia, rare-earth-oxide-stabilized zirconia, scandia-stabilized zirconia, rare-earth doped ceria, alkaline-earth doped ceria, rare-earth oxide stabilized bismuth oxide, and various combinations of these compounds. In an exemplary embodiment, the ceramic electrolyte comprises yttria-stabilized zirconia. Doped zirconia is attractive because it exhibits substantially pure ionic conductivity over a wide range of oxygen partial pressure levels. In one embodiment, the ceramic electrolyte comprises a thermally sprayed yttria-stabilized zirconia. One skilled in the art would know how to choose an appropriate electrolyte, based on the requirements discussed herein.
Similarly, the composition of the anode layer may depend on the end use application. In one non-limiting embodiment, the anode layer comprises a material selected from the group consisting of a noble metal, a transition metal, a cermet, a ceramic, and combinations thereof. Some examples of suitable anode materials include, but are not limited to, nickel, a nickel alloy, cobalt, nickel-yttria stabilized zirconia cermet, copper-yttria stabilized zirconia cermet, nickel-ceria cermet, nickel-samaria doped ceria cermet, nickel-gadolinium doped ceria cermet, and combinations thereof. These anode materials may be doped with many different cations. For instance, for zirconia, Y, Ca, Sc may be used as dopants. In the case of ceria, Gd and Sm may be used as dopants. In a particular embodiment, the anode layer comprises nickel. Nickel provides the advantage of easy in-situ porosity formation, and is very robust in the green state. Other advantages of nickel relate to its relatively low cost and easy availability.
The cathode layer may also be formed from conventional materials, such as a variety of electrically-conductive (and in some cases ionically-conductive) compounds. Non-limiting examples include strontium doped LaMnO3, strontium doped PrMnO3, strontium doped lanthanum ferrites, strontium doped lanthanum cobaltites, strontium doped lanthanum cobaltite ferrites, strontium ferrite, SrFeCo0.5Ox, SrCo0.8Fe0.2O3-δ; La0.8Sr0.2Co0.8Ni0.2O3-δ; La0.7Sr0.3Fe0.6Ni0.2O3-δ; and combinations thereof. Composites of these materials may also be used. In certain embodiments, the ionic conductor comprises a material selected from the group consisting of yttria-stabilized zirconia, rare-earth-oxide-stabilized zirconia, scandia-stabilized zirconia, rare-earth doped ceria, alkaline-earth doped ceria, rare-earth oxide stabilized bismuth oxide, and various combinations of these compounds.
Various other details regarding materials for fuel cell components are known in the art. Moreover, methods for the manufacture of the cells are also known. Those skilled in the art are also familiar with techniques for the fabrication of fuel cell stacks. In the exemplary embodiment shown in
The patentable scope of the invention is defined by the claims. While this invention has been described in detail, with reference to specific embodiments, it will be apparent to those of ordinary skill in this area of technology that other modifications of this invention (beyond those specifically described herein) may be made, without departing from the spirit of the invention. Accordingly, the modifications contemplated by those skilled in the art should be considered to be within the scope of this invention. Furthermore, all of the patents, patent publications, articles, texts, and other references mentioned above are incorporated herein by reference.
