Patent Description:
A typical solid oxide fuel cell stack includes multiple fuel cells separated by metallic interconnects (IC) which provide both electrical connection between adjacent cells in the stack and channels for delivery and removal of fuel and oxidant. The metallic interconnects are commonly composed of a Cr based alloy such as an alloy which has a composition of 95wt% Cr-5wt% Fe, or Cr-Fe-Y having a 94wt% Cr-5wt% Fe- 1wt%Y composition. The CrFe and CrFeY alloys retain their strength and are dimensionally stable at typical solid oxide fuel cell (SOFC) operating conditions, e.g. <NUM>-900C in both air and wet fuel atmospheres. However, fabrication of the interconnects is relatively complex and expensive. It is also known in the prior art to further protect Cr alloy interconnects by coating them with a protective coating, as described in documents <CIT> Al and <CIT>.

According to the invention, a method of forming a protective coating on a chromium alloy interconnect for an electrochemical device stack includes using a wet coating process to deposit a powder on the chromium alloy interconnect, the powder comprising perovskite metal oxide powder; and laser sintering the powder to form the protective coating on the chromium alloy interconnect.

The drawings are not necessarily to scale, and are intended to illustrate various features of the invention.

When such a range is expressed, examples include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about" or "substantially" it will be understood that the particular value forms another aspect. In some embodiments, a value of "about X" may include values of +/- <NUM>% X.

<FIG> is a perspective view of a conventional fuel cell column <NUM>, <FIG> is a perspective view of one counter-flow solid oxide fuel cell (SOFC) stack <NUM> included in the column <NUM> of <FIG> is a side cross-sectional view of a portion of the stack <NUM> of <FIG>.

Referring to <FIG>, the column <NUM> may include one or more stacks <NUM>, a fuel inlet conduit <NUM>, an anode exhaust conduit <NUM>, and anode feed/return assemblies <NUM> (e.g., anode splitter plates (ASPs) <NUM>). The column <NUM> may also include side baffles <NUM> and a compression assembly <NUM>. The fuel inlet conduit <NUM> is fluidly connected to ASPs <NUM> and is configured to provide the fuel to each ASP <NUM>, and anode exhaust conduit <NUM> is fluidly connected to ASPs <NUM> and is configured to receive anode fuel exhaust from each ASP <NUM>.

The ASPs <NUM> are disposed between the stacks <NUM> and are configured to provide a hydrocarbon fuel to the stacks <NUM> and to receive anode fuel exhaust from the stacks <NUM>. For example, the ASPs <NUM> may be fluidly connected to internal fuel riser channels <NUM> formed in the stacks <NUM>, as discussed below.

Referring to <FIG>, the stack <NUM> includes multiple fuel cells <NUM> that are separated by interconnects <NUM>, which may also be referred to as gas flow separator plates or bipolar plates. Each fuel cell <NUM> includes a cathode electrode <NUM>, a solid oxide electrolyte <NUM>, and an anode electrode <NUM>.

Each interconnect <NUM> electrically connects adjacent fuel cells <NUM> in the stack <NUM>. In particular, an interconnect <NUM> may electrically connect the anode electrode <NUM> of one fuel cell <NUM> to the cathode electrode <NUM> of an adjacent fuel cell <NUM>. <FIG> shows that the lower fuel cell <NUM> is located between two interconnects <NUM>.

Each interconnect <NUM> includes ribs <NUM> that at least partially define fuel channels 8A and air channels 8B. The interconnect <NUM> may operate as a gas-fuel separator that separates a fuel, such as a hydrocarbon fuel, flowing to the fuel electrode (i.e. anode <NUM>) of one cell in the stack from oxidant, such as air, flowing to the air electrode (i.e. cathode <NUM>) of an adjacent cell in the stack. At either end of the stack <NUM>, there may be an air end plate or fuel end plate (not shown) for providing air or fuel, respectively, to the end electrode.

<FIG> is a top view of the air side of a conventional interconnect <NUM>, and <FIG> is a top view of a fuel side of an interconnect <NUM>. Referring to <FIG> and <FIG>, the air side includes the air channels 8B. Air flows through the air channels 8B to a cathode electrode <NUM> of an adjacent fuel cell <NUM>. In particular, the air may flow across the interconnect <NUM> in a first direction A as indicated by the arrows.

Ring seals <NUM> may surround fuel holes 22A of the interconnect <NUM>, to prevent fuel from contacting the cathode electrode. Peripheral strip-shaped seals <NUM> are located on peripheral portions of the air side of the interconnect <NUM>. The seals <NUM>, <NUM> may be formed of a glass material. The peripheral portions may be in the form of an elevated plateau which does not include ribs or channels. The surface of the peripheral regions may be coplanar with tops of the ribs <NUM>.

Referring to <FIG> and <FIG>, the fuel side of the interconnect <NUM> may include the fuel channels 8A and fuel manifolds <NUM> (e.g., fuel plenums). Fuel flows from one of the fuel holes 22A, into the adjacent manifold <NUM>, through the fuel channels 8A, and to an anode <NUM> of an adjacent fuel cell <NUM>. Excess fuel and exhaust byproducts may flow into the other fuel manifold <NUM> and then into the adjacent fuel hole 22A. In particular, the fuel may flow across the interconnect <NUM> in a second direction B, as indicated by the arrows. The second direction B may be the same, perpendicular to, or opposite to the first direction A (see <FIG>).

A frame-shaped seal <NUM> is disposed on a peripheral region of the fuel side of the interconnect <NUM>. The peripheral region may be an elevated plateau which does not include ribs or channels. The surface of the peripheral region may be coplanar with tops of the ribs <NUM>.

Accordingly, a conventional counter-flow fuel cell column, as shown in <FIG>, <FIG>, may include complex fuel distribution systems (fuel rails and ASPs). In addition, the use of an internal fuel riser may require holes in fuel cells and corresponding seals, which may reduce an active area thereof and may cause cracks in the ceramic electrolytes of the fuel cells <NUM>.

The fuel manifolds <NUM> may occupy a relatively large region of the interconnect <NUM>, which may reduce the contact area between the interconnect <NUM> and an adjacent fuel cell by approximately <NUM>%. The fuel manifolds <NUM> are also relatively deep, such that the fuel manifolds <NUM> represent relatively thin regions of the interconnect <NUM>. Since the interconnect <NUM> is generally formed by a powder metallurgy compaction process, the density of fuel manifold regions may approach the theoretical density limit of the interconnect material. As such, the length of stroke of a compaction press used in the compaction process may be limited due to the high-density fuel manifold regions being incapable of being compacted further. As a result, the density achieved elsewhere in the interconnect <NUM> may be limited to a lower level by the limitation to the compaction stroke. The resultant density variation may lead to topographical variations, which may reduce the amount of contact between the interconnect <NUM> and fuel cell <NUM> and may result in lower stack yield and/or performance.

Another important consideration in fuel cell system design concerns operational efficiency. Maximizing fuel utilization is a key factor to achieving operational efficiency. Fuel utilization is the ratio of how much fuel is consumed during operation, relative to how much is delivered to a fuel cell. An important factor in preserving fuel cell cycle life may be avoiding fuel starvation in fuel cell active areas, by appropriately distributing fuel to the active areas. If there is a maldistribution of fuel such that some flow field channels receive insufficient fuel to support the electrochemical reaction that would occur in the region of that channel, it may result in fuel starvation in fuel cell areas adjacent that channel. In order to distribute fuel more uniformly, conventional interconnect designs include channel depth variations across the flow field. This may create complications not only in the manufacturing process, but may also require complex metrology to measure these dimensions accurately. The varying channel geometry may be constrained by the way fuel is distributed through fuel holes and distribution manifolds.

One possible solution to eliminate this complicated geometry and the fuel manifold is to have a wider fuel opening to ensure much more uniform fuel distribution across the fuel flow field. Since fuel manifold formation is a factor in density variation, elimination of fuel manifolds should enable more uniform interconnect density and permeability. Accordingly, there is a need for improved interconnects that provide for uniform contact with fuel cells, while also uniformly distributing fuel to the fuel cells without the use of conventional fuel manifolds.

Owing to the overall restrictions in expanding the size of a hotbox of a fuel cell system, there is also a need for improved interconnects designed to maximize fuel utilization and fuel cell active area, without increasing the footprint of a hotbox.

<FIG> is a perspective view of a fuel cell stack <NUM>, according to various embodiments of the present disclosure, <FIG> is an exploded perspective view of a portion of the stack <NUM> of <FIG> is a top view of the fuel side of an interconnect <NUM> included in the stack <NUM>, and <FIG> is a schematic view of a fuel cell included in the stack <NUM>.

Referring to <FIG>, the fuel cell stack <NUM>, which may also be referred to as a fuel cell column because it lacks ASPs, includes multiple fuel cells <NUM> that are separated by interconnects <NUM>, which may also be referred to as gas flow separator plates or bipolar plates. One or more stacks <NUM> may be thermally integrated with other components of a fuel cell power generating system (e.g., one or more anode tail gas oxidizers, fuel reformers, fluid conduits and manifolds, etc.) in a common enclosure or "hotbox.

The interconnects <NUM> are made from an electrically conductive metal material. The interconnects <NUM> comprises a chromium alloy, such as a Cr-Fe alloy. A chromium-alloy interconnect <NUM> may comprise more than about <NUM>% chromium by weight, such as about <NUM>-<NUM>% (e.g., <NUM>%) chromium by weight. An interconnect <NUM> may also contain less than about <NUM>% iron by weight, such as about <NUM>-<NUM>% (e.g., <NUM>%) iron by weight, may contain less than about <NUM>% by weight, such as about zero to <NUM>% by weight, of other materials, such as yttrium or yttria, as well as residual or unavoidable impurities. Alternatively, any other suitable conductive interconnect material, such as stainless steel (e.g., ferritic stainless steel, SS446, SS430, etc.) or iron-chromium alloy (e.g., Crofertm <NUM> APU alloy which contains <NUM> to <NUM> wt% Cr, less than <NUM> wt% Mn, Ti and La, and balance Fe, or ZMGtm <NUM> alloy which contains <NUM> to <NUM> wt% Cr, <NUM> wt% Mn and less than <NUM> wt% Si, C, Ni, Al, Zr and La, and balance Fe), may be used to form the interconnect <NUM>.

Each fuel cell <NUM> may include a solid oxide electrolyte <NUM>, an anode <NUM>, and a cathode <NUM>. In some embodiments, the anode <NUM> and the cathode <NUM> may be printed on the electrolyte <NUM>. In other embodiments, a conductive layer <NUM>, such as a nickel mesh, may be disposed between the anode <NUM> and an adjacent interconnect <NUM>.

Various materials may be used for the cathode electrode <NUM>, electrolyte <NUM>, and anode electrode <NUM>. For example, the anode electrode <NUM> may comprise a cermet comprising a nickel containing phase and a ceramic phase. The nickel containing phase may consist entirely of nickel in a reduced state. This phase may form nickel oxide when it is in an oxidized state. Thus, the anode electrode <NUM> is preferably annealed in a reducing atmosphere prior to operation to reduce the nickel oxide to nickel. The nickel containing phase may include other metals in additional to nickel and/or nickel alloys. The ceramic phase may comprise a stabilized zirconia, such as yttria and/or scandia stabilized zirconia and/or a doped ceria, such as gadolinia, yttria and/or samaria doped ceria.

The electrolyte <NUM> may comprise a stabilized zirconia, such as scandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ). Alternatively, the electrolyte may comprise another ionically conductive material, such as a doped ceria.

The cathode <NUM> may comprise an electrically conductive material, such as an electrically conductive perovskite material, such as lanthanum strontium manganite (LSM). Other conductive perovskites, such as LSCo, etc., or metals, such as Pt, may also be used. The cathode electrode <NUM> may also contain a ceramic phase similar to the anode electrode <NUM>. The electrodes and the electrolyte may each comprise one or more sublayers of one or more of the above described materials.

The fuel cell <NUM> does not include through holes, such as the fuel holes of conventional fuel cells. Therefore, the fuel cell <NUM> avoids cracks that may be generated due to the presence of such through holes.

An upper most interconnect <NUM> and a lowermost interconnect <NUM> of the stack <NUM> may be different ones of an air end plate or fuel end plate including features for providing air or fuel, respectively, to an adjacent end fuel cell <NUM>. As used herein, an "interconnect" may refer to either an interconnect located between two fuel cells <NUM> or an end plate located at an end of the stack and directly adjacent to only one fuel cell <NUM>. Since the stack <NUM> does not include anode splitter plates (ASPs) (i.e., fuel manifolds) and the end plates associated therewith, the stack <NUM> may include only two end plates. As a result, stack dimensional variations associated with the use of intra-column ASPs may be avoided.

The stack <NUM> may include side baffles <NUM>, a fuel plenum <NUM>, and a compression assembly <NUM>. The side baffles <NUM> may be formed of a ceramic material and may be disposed on opposing sides of the fuel cell stack <NUM> containing stacked fuel cells <NUM> and interconnects <NUM>. The side baffles <NUM> may connect the fuel plenum <NUM> and the compression assembly <NUM>, such that the compression assembly <NUM> may apply pressure to the stack <NUM>. The side baffles <NUM> may be curved baffle plates, such each baffle plate covers at least portions of three sides of the fuel cell stack <NUM>. For example, one baffle plate may fully cover the fuel inlet riser side of the stack <NUM> and partially covers the adjacent front and back sides of the stack, while the other baffle plate fully covers the fuel outlet riser side of the stack and partially covers the adjacent portions of the front and back sides of the stack. The remaining uncovered portions for the front and back sides of the stack allow the air to flow through the stack <NUM>. The curved baffle plates provide an improved air flow control through the stack compared to the conventional baffle plates <NUM> which cover only one side of the stack. The fuel plenum <NUM> may be disposed below the stack <NUM> and may be configured to provide a hydrogen-containing fuel to the stack <NUM>, and may receive an anode fuel exhaust from the stack <NUM>. The fuel plenum <NUM> may be connected to fuel inlet and outlet conduits <NUM> which are located below the fuel plenum <NUM>.

Each interconnect <NUM> electrically connects adjacent fuel cells <NUM> in the stack <NUM>. In particular, an interconnect <NUM> may electrically connect the anode electrode of one fuel cell <NUM> to the cathode electrode of an adjacent fuel cell <NUM>. As shown in <FIG>, each interconnect <NUM> may be configured to channel air in a first direction A on one side of the interconnect <NUM>, such that the air may be provided to the cathode of an adjacent fuel cell <NUM>. Each interconnect <NUM> may also be configured to channel fuel in a second direction F on one side of the interconnect <NUM>, such that the fuel may be provided to the anode of an adjacent fuel cell <NUM>. Directions A and F may be perpendicular, or substantially perpendicular to each other. As such, the interconnects <NUM> may be referred to as cross-flow interconnects.

The interconnect <NUM> may include fuel holes that extend through the interconnect <NUM>. The fuel holes are configured for fuel distribution. For example, the fuel holes may include one or more fuel inlets <NUM> and one or more fuel outlets <NUM>, which may also be referred to as anode exhaust outlets <NUM>. The fuel inlets and outlets <NUM>, <NUM> may be disposed outside of the perimeter of the fuel cells <NUM>. As such, the fuel cells <NUM> may be formed without corresponding through holes for fuel flow. The combined length of the fuel inlets <NUM> and/or the combined length of the fuel outlets <NUM> may be at least <NUM>% of a corresponding length of the interconnect <NUM> e.g., a length taken in direction A.

In one embodiment, each interconnect <NUM> contains two fuel inlets <NUM> separated by a neck portion <NUM> of the interconnect <NUM>, as shown in <FIG>. However, more than two fuel inlets <NUM> may be included, such as three to five inlets separated by two to four neck portions <NUM>. In one embodiment, each interconnect <NUM> contains two fuel outlets <NUM> separated by a neck portion <NUM> of the interconnect <NUM>, as shown in <FIG>. However, more than two fuel outlets <NUM> may be included, such as three to five outlets separated by two to four neck portions <NUM>.

The fuel inlets <NUM> of adjacent interconnects <NUM> may be aligned in the stack <NUM> to form one or more fuel inlet risers <NUM>. The fuel outlets <NUM> of adjacent interconnects <NUM> may be aligned in the stack <NUM> to form one or more fuel outlet risers <NUM>. The fuel inlet riser <NUM> may be configured to distribute fuel received from the fuel plenum <NUM> to the fuel cells <NUM>. The fuel outlet riser <NUM> may be configured to provide anode exhaust received from the fuel cells <NUM> to the fuel plenum <NUM>.

Unlike the flat related art side baffles <NUM> of <FIG>, the side baffles <NUM> may be curved around edges of the interconnects <NUM>. In particular, the side baffles <NUM> may be disposed around the fuel inlets <NUM> and outlets <NUM> of the interconnects <NUM>. Accordingly, the side baffles may more efficiently control air flow through air channels of the interconnects <NUM>, which are exposed between the side baffles <NUM> and are described in detail with regard to <FIG>.

In various embodiments, the stack <NUM> may include at least <NUM>, at least <NUM>, at least <NUM>, or at least <NUM> fuel cells, which may be provided with fuel using only the fuel risers <NUM>. In other words, as compared to a conventional fuel cell system, the cross-flow configuration allows for a large number of fuel cells to be provided with fuel, without the need for ASPs or external stack fuel manifolds, such as external conduits <NUM>, <NUM> shown in <FIG>.

Each interconnect <NUM> is made of or contains electrically conductive material, namely a chromium alloy (e.g., chromium-iron alloy) which has a similar coefficient of thermal expansion to that of the solid oxide electrolyte in the cells (e.g., a difference of <NUM>-<NUM>%). For example, the interconnects <NUM> comprises a chromium-iron alloy, such as <NUM>-<NUM> weight percent iron, optionally <NUM> or less weight percent yttrium and balance chromium alloy), and may electrically connect the anode or fuel-side of one fuel cell <NUM> to the cathode or air-side of an adjacent fuel cell <NUM>. An electrically conductive contact layer, such as a nickel contact layer (e.g., a nickel mesh), may be provided between anode and each interconnect <NUM>. Another optional electrically conductive contact layer may be provided between the cathode electrodes and each interconnect <NUM>.

<FIG> are plan views showing, respectively, an air side and a fuel side of the cross-flow interconnect <NUM>, according to various embodiments of the present disclosure. Referring to <FIG>, the air side of the interconnect <NUM> may include ribs <NUM> configured to at least partially define air channels <NUM> configured to provide air to the cathode of a fuel cell <NUM> disposed thereon. The air side of the interconnect <NUM> may be divided into an air flow field <NUM> including the air channels <NUM>, and riser seal surfaces <NUM> disposed on two opposing sides of the air flow field <NUM>. One of the riser seal surfaces <NUM> may surround the fuel inlets <NUM> and the other riser seal surface <NUM> may surround the fuel outlets <NUM>. The air channels <NUM> and ribs <NUM> may extend completely across the air side of the interconnect <NUM>, such that the air channels <NUM> and ribs <NUM> terminate at opposing peripheral edges of the interconnect <NUM>. In other words, when assembled into a stack <NUM>, opposing ends of the air channels <NUM> and ribs <NUM> are disposed on opposing (e.g., front and back) outer surfaces of the stack, to allow air to flow through the stack. Therefore, the stack may be externally manifolded for air.

Riser seals <NUM> may be disposed on the riser seal surfaces <NUM>. For example, one riser seal <NUM> may surround the fuel inlets <NUM>, and one riser seal <NUM> may surround the fuel outlets <NUM>. The riser seals <NUM> may prevent fuel and/or anode exhaust from entering the air flow field <NUM> and contacting the cathode of the fuel cell <NUM>. The riser seals <NUM> may also operate to prevent fuel from leaking out of the fuel cell stack <NUM> (see <FIG>).

Referring to <FIG>, the fuel side of the interconnect <NUM> may include ribs <NUM> that at least partially define fuel channels <NUM> configured to provide fuel to the anode of a fuel cell <NUM> disposed thereon. The fuel side of the interconnect <NUM> may be divided into a fuel flow field <NUM> including the fuel channels <NUM>, and a perimeter seal surface <NUM> surrounding the fuel flow field <NUM> and the fuel inlets and outlets <NUM>, <NUM>. The ribs <NUM> and fuel channels <NUM> may extend in a direction that is perpendicular or substantially perpendicular to the direction in which the air-side channels <NUM> and ribs <NUM> extend.

A frame-shaped perimeter seal <NUM> may be disposed on the perimeter seal surface <NUM>. The perimeter seal <NUM> may be configured to prevent air entering the fuel flow field <NUM> and contacting the anode on an adjacent fuel cell <NUM>. The perimeter seal <NUM> may also operate to prevent fuel from exiting the fuel risers <NUM>, <NUM> and leaking out of the fuel cell stack <NUM> (see <FIG>).

The seals <NUM>, <NUM> may comprise a glass or ceramic seal material. The seal material may have a low electrical conductivity. In some embodiments, the seals <NUM>, <NUM> may be formed by printing one or more layers of seal material on the interconnect <NUM>, followed by sintering.

<FIG> is a sectional view illustrating one of the fuel-side ribs <NUM> of the interconnect <NUM> and the anode <NUM> of an adjacent fuel cell <NUM>, according to various embodiments of the present disclosure. Referring to <FIG>, the interconnect <NUM> is electrically connected to the anode <NUM> by a Ni mesh <NUM> covering the fuel side of the interconnect <NUM>. The Ni mesh <NUM> may operate as a current collector with respect to the anode <NUM>. Although only one fuel-side rib <NUM> is shown, each fuel-side rib <NUM> of the interconnect <NUM> may be connected to the anode <NUM> in a similar manner.

Conventionally, oxides or other contaminants may form over time on the surface of an interconnect, and in particular, at an interface between a Ni mesh and corresponding ribs of the interconnect. As such, the contact resistance between the Ni mesh and an interconnect may increase, which may reduce the useful life of a fuel cell stack.

Referring again to <FIG>, the interconnect <NUM> may include a contact layer <NUM> configured to prevent or reduce such an increase in contact resistance. In particular, the contact layer <NUM> may be formed on the tips of the ribs <NUM>. For example, the contact layer <NUM> may be disposed at an interface between the interconnect <NUM> and the Ni mesh <NUM> (e.g., between the rib <NUM> and the Ni mesh <NUM>). In some embodiments, the contact layer <NUM> may cover substantially all of the upper (e.g., distal) surface of the rib <NUM>. However, in other embodiments, the contact layer <NUM> may cover only a portion of the tip of the rib <NUM>.

The contact layer <NUM> may have a higher iron content than the interconnect <NUM>. For example, the contact layer <NUM> may have greater than <NUM> wt% iron, such as from about <NUM> to about <NUM> wt% iron, or about <NUM> to about <NUM> wt% iron, while the interconnect <NUM> may include an alloy having from about <NUM> to about <NUM> wt% iron. Accordingly, the contact layer <NUM> may operate to prevent oxide growth at the interface between the rib <NUM> and the Ni mesh <NUM>. The contact layer <NUM> may also improve the metallurgical joining of the Ni mesh <NUM> and the interconnect <NUM>.

In some embodiments, the thickness (e.g., depth) and/or width of the contact layer <NUM> may be controlled to reduce mechanical distortion of the interconnect <NUM>. For example, the thickness of the contact layer <NUM> may range from about <NUM> to about <NUM>. The contact layer <NUM> may have an iron to chromium ratio that varies in the thickness direction thereof. For example, the iron to chromium ratio may increase as a distance from the tip of the rib <NUM> to the top surface of the contact layer <NUM> increases.

According to various embodiments, the contact layer <NUM> may be formed by depositing an iron-based material, such as metallic iron or iron oxide, on the tips of the ribs <NUM>. The interconnect <NUM> can then be sintered, such that the iron and chromium in the interconnect at least partially inter-diffuse, thereby creating the contact layer <NUM>. In particular, iron of the contact layer <NUM> may partially diffuse into the tip of the rib <NUM>, and chromium of the interconnect <NUM> may partially diffuse into the contact layer <NUM>.

In some embodiments, the contact layer <NUM> may be formed by disposing iron wire on the tips of the ribs <NUM>. The interconnect <NUM> may then be sintered to facilitate iron and chromium inter-diffusion, as described above. The iron wire may have a thickness (e.g., diameter) ranging from about <NUM> to <NUM> microns, such as from about <NUM> to about <NUM> microns, or from about <NUM> to about <NUM> microns.

In some embodiments, the iron wire may be deposited on an interconnect powder, and the resultant structure may be compressed into the shape of an interconnect, followed by sintering. In other embodiments, the iron wire may be deposited on a compressed interconnect, and the resultant structure may then be sintered.

In some embodiments, the contact layer <NUM> may be formed by contacting an iron powder to the tip of the rib <NUM>. The iron powder may have an average particle size ranging from about <NUM> to about <NUM> microns, such as from about <NUM> to about <NUM> microns, or from about <NUM> to about <NUM> microns. In some embodiments, the iron powder may be deposited on an interconnect powder, and the resultant structure may be compressed into the shape of an interconnect, followed by sintering. For example, the interconnect powder may be deposited into a die cavity using a first shoe, and then the iron powder may then be deposited onto the interconnect powder using a second shoe or by a spraying process.

In other embodiments, the iron powder may be deposited on a compressed interconnect, or the compressed interconnect may be placed onto the iron powder ribs down, and the resultant structure may then be sintered. Methods of forming interconnects will be discussed in more detail below.

<FIG> is a sectional view of a fuel cell <NUM> connected to the air side of an interconnect <NUM>, according to various embodiments of the present disclosure. Referring to <FIG>, a protective coating or layer <NUM> may be disposed on the air side of the interconnect <NUM>, and a cathode contact layer (CCL) <NUM> may be disposed on the coating <NUM>. In some embodiments, the protective coating <NUM> and/or the CCL <NUM> may be deposited only on the tops of the cathode side ribs <NUM>, without depositing the protective coating <NUM> and/or the CCL <NUM> in the oxidant (e.g., air) channels <NUM> (i.e., without depositing the coating <NUM> and/or the CCL <NUM> on or over the sidewalls of the ribs <NUM> and/or without depositing the coating <NUM> and/or the CCL on or over the bottom of the oxidant channels <NUM>).

The coating <NUM> may be configured to limit the diffusion of chromium ions (e.g., Cr<NUM>+) from the interconnect <NUM> into cathode <NUM> and into seals <NUM>. The coating <NUM> may also be configured to suppress the formation of native oxide on the surface of the interconnect <NUM>. The native oxide is formed when oxygen reacts with chromium in the interconnect alloy to form a relatively high resistance layer of Cr<NUM>O<NUM>. If the interconnect coating <NUM> can suppress the transport of oxygen and water vapor from the air to the surface of the interconnect <NUM>, then the kinetics of oxide growth can be reduced.

According to the invention, the coating <NUM> includes a metal oxide perovskite material, such as lanthanum strontium manganite (LSM). It may further include a spinel material, such as a manganese cobalt oxide (MCO) spinel material. In an embodiment, the MCO spinel material encompasses the compositional range from Mn<NUM>CoO<NUM> to Co<NUM>MnO<NUM>. That is, any spinel material having the composition Mn<NUM>-xCo<NUM>+xO<NUM> (<NUM> ≤ x ≤ <NUM>) or written as z(Mn<NUM>O<NUM>) + (<NUM>-z)(Co<NUM>O<NUM>), where (<NUM>/<NUM> ≤ z ≤ <NUM>/<NUM>) or written as (Mn, Co)<NUM>O<NUM> may be used, such as Mn<NUM>Co<NUM>O<NUM>, MnCo<NUM>O<NUM> or Mn<NUM>CoO<NUM>. Many of the spinels that contain transition metals exhibit good electronic conductivities and reasonably low anion and cation diffusivities and are therefore suitable coating materials. Examples of such materials may be found in <CIT> and <CIT>.

The CCL <NUM> may be an electrically conductive metal oxide layer configured to improve an electrical connection between the interconnect <NUM> and the cathode <NUM>. In some embodiments, the CCL <NUM> may include metal oxide materials that have a low cation diffusivity in the perovskite family, such as a lanthanum strontium oxide, e.g., La<NUM>-xSrxMnO<NUM> (LSM), where <NUM>≤x≤<NUM>, such as <NUM>≤x≤<NUM>. In the case of LSM, the material has high electronic conductivity yet low anion and cation diffusion. Other perovskites such as La<NUM>-xSrxFeO<NUM>-d, La<NUM>-xSrxCoO<NUM>-d, and La<NUM>-xSrxCo<NUM>-yFeyO<NUM>-d all exhibit high electronic conduction and low cation conduction (low chromium diffusion rates) and may be used as the CCL <NUM>.

Such materials generally have sintering temperatures of more than <NUM>. However, such temperatures may result in the oxidation of metal alloys included in the interconnect <NUM>. As such, it may be difficult to properly sinter a CCL <NUM>.

In view of the above and/or other problems, the CCL <NUM> may include a sintering aid configured to increase the density of the CCL <NUM> and to improve interfacial strength, and increase layer bonding. In some embodiments, the CCL <NUM> may include a glass material as a sintering aid. The glass material may be included at an amount less than about <NUM> wt%, such as from about <NUM> to about <NUM> wt%, such as from about <NUM> to about <NUM> wt%, with the remainder of the CCL <NUM> being a conductive perovskite metal oxide, such as LSM. In particular, glass amounts of greater that about <NUM> wt% may unnecessarily reduce the conductivity of the CCL <NUM>.

In addition to better sintering, the addition of the glass material may make the CCL <NUM> more compliant and tolerant to mechanical stresses induced by thermal cycles during SOFC operation. As such, the glass material may be selected from glass materials that have a relatively low glass transition (Tg) or softening temperature, to avoid crystallization. In particular, the glass material may have a Tg or softening temperature that is low enough to allow the glass material to remain viscous at temperatures below <NUM>, such as temperatures of from about <NUM> to about <NUM>. In some embodiments, selected glass materials may have a glass transition temperature of <NUM> or less, such as <NUM> or less, such as ranging from about <NUM> to about <NUM>. For example, the glass material may remain viscous at fuel cell operating temperatures ranging from about <NUM> to about <NUM>, such as from about <NUM> to about <NUM>. Remaining viscous at such temperatures allows the CCL <NUM> to conform to the tops of the air-side ribs of the interconnect <NUM>. Accordingly, the CCL <NUM> allows for improved coverage of rib tops, an increased effective contact surface area, reduced degradation, and the ability to self-heal cracks formed during thermal cycling.

According to various embodiments, the glass material may be selected from various compositions, such as alumino-silicate, boro-silicate, boro-aluminate, and alkali-free compositions, and may include Al, Si, Ca, Ba, B, La, Sr, Mg, or mixtures thereof.

Referring to <FIG> and <FIG>, in one embodiment, the protective coating <NUM> is formed on the air side of the interconnect <NUM> using a laser sintering method. The laser sintering method forms a dense adhesive protective coating <NUM> which suppresses chromium evaporation from the interconnect and reduces interconnect <NUM> oxidation. As described above, the protective coating material includes perovskite materials, such as lanthanum strontium manganite (LSM) or other perovskites described above, and their mixtures with spinel materials, such as manganese cobalt oxide (MCO).

The protective coating powder is deposited on the air side of the interconnect <NUM> via a wet process (e.g., from a suspension containing a solvent), optionally dried, and then sintered with a laser. The suspension may also be referred to as an ink. The wet process may include an aqueous (i.e., water based) solvent and/or an organic solvent. The suspension (e.g., the ink) may also include a binder and/or plasticizer in addition to the protective coating powder and the solvent. Non-limiting examples of the wet process include but are not limited to spray coating, dip coating, screen printing, electrochemical plating, electrophoresis, electrostatic attraction, pad printing, roller coating or tape casting (also known as doctor blading or knife coating). In pad printing, a suspension of a powder in a solvent is deposited onto a sponge having a negative surface profile of the ribs and channels on the air side of the interconnect. The sponge is then contacted against the air side of the interconnect to transfer the suspension to the air side of the interconnect. In roller coating, a roller containing the suspension of a powder in a solvent is rolled against the air side of the interconnect, similar to a paint roller. The powder may have the same composition as the protective coating <NUM> or may comprise an intermediate powder which is converted into the protective coating during or after laser sintering.

In one embodiment, the deposited wet suspension (e.g., the ink coated interconnect) is subject to laser sintering without first drying the suspension. The solvent and the optional binder and/or plasticizer are removed from the ink during the laser sintering. In another embodiment, the suspension (e.g., ink) is dried at a relatively low temperature (e.g., between <NUM> and <NUM>) to remove (e.g., evaporate) the solvent without removing the binder and/or plasticizer prior to the laser sintering. The remaining binder and/or plasticizer are then removed from the dried powder during the laser sintering. In another embodiment, the suspension (e.g., ink) is dried at a relatively high temperature (e.g., between <NUM> and <NUM>) to remove (e.g., evaporate) the solvent as well as to remove (i.e., burn out) the binder and/or plasticizer prior to the laser sintering. The dry powder is then laser sintered.

In one embodiment, the starting powders which are applied to the air side of the interconnect comprise metal oxide powders, such as perovskite powders, with or without spinel powders. Examples of perovskite oxide phases include LSM, which may have a formula (La<NUM>-xSrx)yMnO<NUM>-d where <NUM> < x ≤ <NUM>, for example <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y ≤ <NUM>, and <NUM> ≤ d ≤ <NUM>, such as (La<NUM>Sr<NUM>)<NUM>MnO<NUM>-d or (La<NUM>Sr<NUM>)<NUM>MnO<NUM>-d. Examples of spinel phases include MCO, which may have a formula Mn<NUM>-xCo<NUM>+xO<NUM> where <NUM> < x < <NUM>, such as Mn<NUM>Co<NUM>O<NUM>, Mn<NUM>Co<NUM>O<NUM>, or Mn<NUM>Co<NUM>O<NUM>. Other spinel phases include manganese copper oxide, manganese copper cobalt oxide, manganese iron oxide, cobalt iron oxide, manganese copper nickel oxide, or nickel cobalt iron oxide, for example Mn<NUM>Cu<NUM>O<NUM>, Mn<NUM>Co<NUM>Cu<NUM>O<NUM>, Mn<NUM>Co<NUM>Cu<NUM>O<NUM>, Mn<NUM>Co<NUM>Cu<NUM>O<NUM>, Mn<NUM>Ni<NUM>Cu<NUM>O<NUM>, Mn<NUM>Co<NUM>Fe<NUM>O<NUM>, Ni<NUM>Co<NUM>Fe<NUM>O<NUM>, or Co<NUM>Fe<NUM>O<NUM>.

In this embodiment, a single powder such as LSM is deposited on the air side of the interconnect <NUM> via a wet process (e.g., from a suspension containing a solvent), optionally dried, and then sintered with a laser. Alternatively, a powder blend of perovskite and spinel powders, such as a blend of LSM and MCO powders may be deposited, optionally dried and then laser sintered. The blend may contain <NUM> to <NUM> weight percent perovskite and <NUM> to <NUM> weight percent spinel, such as <NUM> wt. % LSM and <NUM> wt. % MCO for example.

As shown in <FIG>, a laser beam <NUM> is scanned across the wet or dried powder coating to sinter the dried powder coating to form the protective coating <NUM>. The powder may be optionally melted during the laser sintering. Any suitable laser may be used which is capable of sintering the powder.

Depending on the atmosphere during laser sintering, and specifically partial pressure of oxygen in the sintering atmosphere, the final phases of the coating materials may be non-stoichiometric. However, upon heating in a fuel cell stack, the oxide phases will reassemble and equilibrate (e.g., become stoichiometric) with local oxidizing atmosphere, such as air for example.

In another embodiment not according to the present invention, the starting powders comprise metal powders rather than oxide powders. For example, metal powders can include two or more of Mn, Co, Ni, Cu and/or Fe metal powders with average particle sizes between <NUM> and <NUM> microns. The metal powders may be blended at various ratios, deposited by a wet process on the air side of the interconnect <NUM>, optionally dried, and then sintered with the laser beam <NUM>. The metal powders may optionally be melted during the laser sintering.

The laser sintered metal coating may be oxidized to form the protective coating <NUM>. The oxidation may be conducted during the laser sintering, after the laser sintering but before placing the interconnect into a fuel cell or electrolyzer stack, or during use in the fuel cell or electrolyzer stack. Thus, the laser sintering atmosphere may comprise vacuum, air, oxygen or an inert atmosphere, such as nitrogen or noble gas (e.g., argon) atmosphere.

The laser sintered coating may be metallic in nature, a mixture of metals and oxides, or an oxide. For example, to form the MCO spinel phase composition Mn<NUM>Co<NUM>O<NUM>, a <NUM>:<NUM> ratio of Co and Mn metal powders is deposited on the interconnect and laser sintered. After laser sintering, and depending on atmospheric conditions, a resulting coating can be metallic and comprise Co and Mn metals (e.g., if laser sintered in a non-oxidizing ambient), or a mixture of oxides and metals, for example Co metal and Mn oxide or Co oxide and Mn metal (e.g., if laser sintered in a mixed oxygen containing atmosphere), or a mixture of binary oxides such as MnOx and CoOx, or a final oxide phase such as MCO (e.g., if laser sintered in an oxidizing ambient). Regardless of the makeup of the intermediate coating phase, after subsequent heating of the interconnect coating in a fuel cell or electrolyzer stack, the phase of the coating will assemble and equilibrate based on temperature and atmosphere. To form conductive spinel phases, powder blends can be comprised of various blends and ratios of Ni, Fe, Mn, Co, and/or Cu metal powders. Some examples include but are not limited to Mn-Fe, Mn-Co, Ni-Fe, Ni-Co, Cu-Mn, Mn-Co-Fe, Mn-Co-Cu, Ni-Mn-Co, Ni-Mn-Fe, or Mn-Co-Fe-Cu.

In another embodiment not according to the present invention, the starting powders are intermediate metal oxide powders such as binary oxides which then form a ternary metal oxide protective coating <NUM>. Examples of binary oxides include, but are not limited to MnO, Mn<NUM>O<NUM>, Mn<NUM>O<NUM>, FeO, Fe<NUM>O<NUM>, Fe<NUM>O<NUM>, NiO, CuO, and/or Co<NUM>O<NUM>. The oxide powers are mixed at specific ratios to form spinel phases. For example, a <NUM>% molar mixture of MnO and CoO may be used to form a final protective coating having the Mn<NUM>Co<NUM>O<NUM> composition. The oxide powder blends are deposited on the interconnect <NUM> by a wet process, optionally dried, and laser sintered. The oxide powders may optionally be melted during the laser sintering. Upon thermal treatment outside or within the fuel cell or electrolyzer stack, the phase of the coating will assemble and equilibrate based on temperature and atmosphere.

In another embodiment not according to the present invention, starting powders are a mixture of one or more metal powders and one or more metal oxide powders such as binary oxides or ternary oxides (e.g., perovskite and/or spinel powders) described above. For example, a single phase powder, such as LSM or MCO, is blended with either a metal powder, such as Mn, Co, Ni, Cu, and/or Fe, and/or with another oxide powder, such as a binary oxide powder. Examples of binary oxides include, but are not limited to MnO, Mn<NUM>O<NUM>, Mn<NUM>O<NUM>, FeO, Fe<NUM>O<NUM>, Fe<NUM>O<NUM>, NiO, CuO, and/or Co<NUM>O<NUM>.

A non-limiting example not according to the present invention, includes a mixture of MCO powder and Fe<NUM>O<NUM> powder, with a weight percentage between <NUM>% and <NUM>% of each constituent, such as <NUM> wt. % MCO and <NUM> wt. % Fe<NUM>O<NUM> powders or <NUM> wt. % MCO and <NUM> wt. % Fe<NUM>O<NUM> powders. Another example includes a mixture of MCO powder with Fe metal powder, with a weight percentage between <NUM>% and <NUM>% of each constituent, such as <NUM> wt. % MCO and <NUM> wt. % Fe or <NUM> wt. % MCO and <NUM> wt. The powders are deposited onto the air side of the interconnect, optionally dried, and sintered with a laser. Upon heat treatment in an oxidizing atmosphere, such as an air atmosphere, for example in a fuel cell or electrolyzer stack or prior to being placed into the stack, the resulting coating comprises two or more oxide phases. For example, MCO and Fe starting powder may result in a composite coating comprised of MCO, iron oxide and one or more optional intermediate interfacial oxides.

The laser sintering of the protective coating <NUM> reduces fabrication cost, improves coating performance, and increased coating thickness uniformity.

Claim 1:
A method of forming a protective coating on a chromium alloy interconnect for an electrochemical device stack, comprising:
using a wet coating process to deposit a powder on the chromium alloy interconnect, the powder comprising perovskite metal oxide powder; and
laser sintering the powder to form the protective coating on the chromium alloy interconnect.