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 alloys such as CrFe alloys, which have 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>-<NUM> in both air and wet fuel atmospheres. However, during operation of the SOFCs, chromium in the CrFe or CrFeY alloys react with oxygen and form chromia, resulting in degradation of the SOFC stack. <CIT> discloses methods for fabricating an interconnect for a fuel cell stack that include the steps of providing a metal powder, and rapidly compressing the metal powder, such as with a combustion-driven compaction apparatus, in a lubricant-free and/or subatmospheric environment to form the interconnect. <CIT> discloses methods of fabricating an interconnect for a fuel cell stack include providing a powder in a die cavity of a powder press apparatus, where the powder includes at least one of a pre-alloyed powder and a pre-sintered powder, compressing the powder in the die cavity of the powder press apparatus using high velocity compaction to form a pressed powder interconnect, and incorporating the pressed powder interconnect into a fuel cell stack, wherein the pressed powder interconnect is incorporated into the fuel cell stack without first sintering the pressed powder interconnect. <CIT> discloses methods for fabricating an interconnect for a fuel cell stack that include providing a protective layer over at least one surface of an interconnect formed by powder pressing pre-alloyed particles containing two or more metal elements and annealing the interconnect and the protective layer at elevated temperature to bond the protective layer to the at least one surface of the interconnect. <CIT> discloses a component, such as a SOFC interconnect, and methods of making the component are provided using various chromium powders, including powder particles with a chromium core covered with an iron shell, a pre-alloyed Cr-Fe powder or a chromium powder produced by hydrogen reduction with hydrogen.

Various embodiments provide a method of making an interconnect for fuel cell stack, the method including compressing an interconnect powder to form an interconnect, the interconnect power comprising Cr, Fe and at least one transition metal selected from Co, Cu, Mn, Ni, or V, wherein the at least one transition metal is pre-alloyed with particles of at least one of the Cr or the Fe, and sintering the interconnect as defined in the claims.

Various embodiments provide a method of forming an interconnect for a fuel cell stack, comprising compressing an interconnect powder to form an interconnect, the interconnect power comprising Cr powder particles, Fe powder particles and at least one transition metal shell selected from Co, Cu, Mn, Ni, or V, wherein the at least one transition metal forms a shell on particles of the Cr or the Fe particles, and sintering the interconnect as defined in the claims.

Various embodiments provide an interconnect for a fuel cell stack, comprising an interconnect body comprising a chromium based alloy containing iron and at least one transition metal selected from Cu, Ni, or V, the interconnect body comprising fuel flow channels separated by fuel ribs on a fuel side and air flow channels separated by air ribs on an air side, wherein a concentration of the at least one transition metal selected from Cu, Ni, or V is higher on the air side of the interconnect than on the fuel side of the interconnect, a protective coating comprising an oxide of at least one of Mn or Co located over the air side of the interconnect body, and a chromium-transition metal oxide spinel interfacial layer located between the air side of the interconnect body and the protective coating as defined in the claims.

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 sectional view of a portion of a solid oxide fuel cell (SOFC) stack <NUM>, according to various embodiments of the present disclosure, <FIG> schematic side view of an interconnect <NUM> of <FIG>, <FIG> is a top view of the air side of an interconnect <NUM> of <FIG>, and <FIG> is a top view of a fuel side of the interconnect <NUM>.

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>.

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 or hydrogen 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.

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 of <FIG>.

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 or glass-ceramic material. The peripheral portions of the interconnect <NUM> 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>.

As shown in <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 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.

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>.

The interconnect <NUM> shown in <FIG> is externally manifolded for air and internally manifolded for fuel. The air and fuel flow in counter-flow (i.e., opposite directions) across the air and fuel sides of the interconnect <NUM>. Other interconnect configurations may also be used. In alternative embodiments, co-flow or cross-flow interconnect configurations may be used, such as for example, a cross-flow interconnect described in <CIT>. Furthermore, the interconnects may be internally manifolded for air and/or externally manifolded for fuel.

Interconnects are typically made from a Cr-based alloys such as CrFe alloys, which have a composition of <NUM>-<NUM> wt. % Cr and <NUM>-<NUM> wt. % Fe (e.g., <NUM> wt. % Cr and <NUM> wt. % Fe) or CrFeY alloys having a <NUM> wt. % Cr, <NUM> wt. % Fe and <NUM> wt. % 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>-<NUM> in both air and wet fuel atmospheres.

However, during operation in a stack, the chromium in interconnects formed of CrFe and CrFeY alloys may react with oxygen in air and form a chromium oxide (chromia, Cr<NUM>O<NUM>) layer on air-sides of the interconnects. Water in the air may also react with the chromia layer to form gaseous phases such as CrO<NUM>(OH)<NUM> that diffuse into fuel cell cathodes and decrease the electrochemical activity of the cathodes. In addition, the high resistivity of the chromia layer on the surface of the interconnects increases the ohmic resistance of the interconnects, which increases the area specific resistance (ASR) of the stack over time, resulting in performance degradation.

For example, chromia formation may result in higher stack ohmic resistance due to the formation of native chromia on the interconnect. The chromia layer may grow in thickness on the surfaces of the interconnects over time. Thus, the ohmic resistance of the interconnects increases over time due to this chromia layer. Furthermore, chromium containing gaseous phase evaporation may result in chromium poisoning of the cathodes <NUM>. Typical SOFC cathode materials, such as perovskite materials, (e.g., LSM, LSC, LSCF, and LSF) are particularly vulnerable to chromium poisoning.

In some embodiments, the air-side surface of the interconnects <NUM> may be coated with a protective coating <NUM> (see <FIG>), in order to decrease the growth rate of the chromium oxide on the surface of the interconnects <NUM> and to suppress evaporation of chromium vapor species which can poison the fuel cell cathodes <NUM>. However, in some embodiments the protective coating <NUM> may be formed on both the air-side and the fuel side of the interconnects <NUM>. The protective coating <NUM> may be formed using a spray coating process, such as plasma spraying, or a dip coating process.

Typically, the protective coating <NUM> may comprise a perovskite material such as lanthanum strontium manganite (LSM). Alternatively, other metal oxide spinel materials, such as a (Mn, Co)<NUM>O<NUM> spinel materials, can be used instead of or in addition to LSM. Any spinel 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. Many of the spinels that contain transition metals exhibit good electrical conductivities and reasonably low anion and cation diffusivities and are therefore suitable coating materials.

However, the present inventors determined that the protective coating <NUM> alone may not be sufficient to prevent the ASR degradation due to chromia accumulation on the interconnects <NUM> and/or cathode chromium poisoning.

Accordingly, in various embodiments, a transition metal alloying element is added to the interconnects <NUM> prior to forming the protective coatings <NUM>, in order to facilitate the formation of a chromium - transition metal oxide spinel phase interfacial layer <NUM> between the protective coating <NUM> and the interconnect <NUM>, as shown in <FIG>. Alloying elements may be transition metal elements that form a binary or ternary spinel material with Cr and that have a higher octahederal site occupancy preference as compared to Cr. For example, transition metal alloying elements in the interconnect <NUM>, such as Mn, Co, Ni, Cu, and/or V, may promote the formation of the chromium - transition metal oxide spinel phase interfacial layer <NUM>, in place of or in addition to the native chromia phase interfacial layer formed on the surface of the interconnect <NUM>, during operation in a fuel cell stack. Such chromium - transition metal oxide spinel phases may provide reduced Cr evaporation and increased electrical conductivity, as compared to native chromia phases.

Chromium containing spinel materials have the general formula CrxM<NUM>-xO<NUM>, where <NUM> ≤ x ≤ <NUM> and M comprises at least one of Mn, Co, Ni, Cu, or V. In general, the electrical conductivity of the spinel phases is higher if a transition metal atom, such as Mn, Co, Ni, Cu, and/or V, occupies the octahedral position in the resulting spinel phase, rather than Cr.

For example, Mn reacts with Cr<NUM>O<NUM> to form (Mn, Cr)<NUM>O<NUM> type spinel phases having compositions ranging between MnCr<NUM>O<NUM> and Mn<NUM>CrO<NUM>, with typical compositions of Mn<NUM>Cr<NUM>O<NUM>, Mn<NUM>Cr<NUM>O<NUM> and Mn<NUM>Cr<NUM>O<NUM>. These spinel oxide phases have substantially higher electrical conductivity and lower chromium evaporation, as compared to Cr<NUM>O<NUM>. For example, these spinel phases may have electrical conductivities that are more than <NUM> times greater than the electrical conductivity of chromia. Furthermore, these spinel phases may have reduced Cr diffusion rates, as compared to chromia, which may significantly lower the rate of Cr evaporation.

For example, addition of Mn to a Cr-Fe alloy interconnect material may promote the formation of a (Fe, Mn, Cr)<NUM>O<NUM> spinel phase interfacial layer <NUM>, wherein the elemental site preference may be determined by composition, temperature, and oxygen partial pressure (pO<NUM>) of an atmosphere during the formation thereof. Example spinel materials may include NiCr<NUM>O<NUM> of the (Ni,Cr)<NUM>O<NUM> family, CoCr<NUM>O<NUM> of the (Co,Cr)<NUM>O<NUM> family, and CuCr<NUM>O<NUM> of the (Cu, Cr)<NUM>O<NUM> family. One or more additional alloying elements may be included to promote the formation of other spinel phases, such as (Mn, Co, Cr)<NUM>O<NUM>, (Cu, Mn, Cr)<NUM>O<NUM>, (Ni, Cu, Cr)<NUM>O<NUM>, and (Mn, Ni, Co, Cr)<NUM>O<NUM>, wherein some of the additional alloying elements partially occupy the octahedral positions of the spinel phase.

Various embodiments provide an interconnect <NUM> for a fuel cell stack <NUM>, comprising an interconnect body 10B comprising a chromium based alloy containing iron and at least one transition metal selected from Cu, Ni, or V, the interconnect body 10B comprising fuel flow channels 8A separated by fuel ribs <NUM> on a fuel side, and air flow channels 8B separated by air ribs <NUM> on an air side, wherein a concentration of the at least one transition metal selected from Cu, Ni, or V is higher on the air side of the interconnect than on the fuel side of the interconnect, a protective coating <NUM> comprising an oxide of at least one of Mn or Co located over the air side of the interconnect body 10B, and a chromium-transition metal oxide spinel interfacial layer <NUM> located between the air side of the interconnect body 10B and the protective coating <NUM> as defined in the claims.

In one embodiment, the chromium-transition metal oxide spinel interfacial layer <NUM> comprises an oxide of chromium and at least one of Co, Cu, Mn, Ni, or V. The protective coating <NUM> comprises at least one of lanthanum strontium manganite or a (Mn, Co)<NUM>O<NUM> spinel coating.

According to various embodiments, interconnects may be formed by powder metallurgy which includes pressing an interconnect powder including at least one transition metal powder, Cr and Fe, in a hydraulic or mechanical press to produce a part having the desired interconnect shape. The interconnect powder may also include an organic lubricant / binder and when pressed using a powder metallurgy technique, forms so-called "green parts". The "green parts" have substantially the same size and shape as the finished interconnect (i.e., "near net shape"). The lubricant / binder in the green parts may be removed before the parts are sintered. For example, the lubricant / binder may be removed in a debinding process in a furnace at a temperature of <NUM> to <NUM>. After debinding, the compressed powder interconnects may be sintered at high-temperature (e.g., <NUM>-<NUM>), in a reducing atmosphere, to promote metal interdiffusion. The interconnects may undergo a separate controlled oxidation treatment, such as by exposing the interconnects to an oxidizing ambient, such as air, at a high temperature after sintering and prior to use of the interconnects in the stack. A protective layer may be applied after the sintering of an interconnect.

The incorporation of transition metals, such as Co, Mn, V, Ni, and/or Cu, into a powder metallurgy process used to form the interconnect may suffer from various challenges. For example, if a transition metal elemental powder is mixed with Cr and Fe elemental powders, and the resultant interconnect powder is pressed to a desired shape, the high vapor pressure of the transition elements may result in significant loss of material during high-temperature sintering of the interconnect. Secondly, the low melting temperature of some transition metal elements can result in loss of material and contamination of furnace furniture. Herein, "elemental" powders refer to powders that do not comprise alloyed elements (i.e., pure Co, Cu, Mn, Ni, or V powders).

The at least one transition metal is pre-alloyed with particles of at least one of the Cr or the Fe; or the at least one transition metal forms a shell on particles of at least one of the Cr or Fe as defined in the claims. According to various embodiments, interconnects may be formed by powder metallurgy using an interconnect powder that includes a powder comprising Fe and/or Cr pre-alloyed with one or more transition metals, such as Co, Cu, Mn, Ni, and/or V. For example, the interconnect powder may be formed by mixing a Cr elemental powder and a pre-alloyed transition metal-Fe powder. In some embodiments, the interconnect powder may include, by weight, from about from about <NUM>% to about <NUM>% pre-alloyed transition metal-Fe powder and from about <NUM>% to about <NUM>% of an elemental Cr powder. In the alternative, an interconnect powder may include an elemental Fe powder mixed with a pre-alloyed transition metal-Cr alloy powder.

The ratio of the metals in the pre-alloyed powders may be tailored to promote the formation of the desired spinel phase (e.g., during SOFC operation), while lowering the evaporation rate of the materials during powder metallurgy processing. Suitable pre-alloyed Fe powders may include Co-Fe, Mn-Fe, V-Fe, and Ni-Fe. Suitable pre-alloyed Cr powders including Co-Cr, Mn-Cr, V-Cr, and Ni-Cr. In some embodiments, pre-alloyed powders may include ternary alloys, such as Co-Mn-Fe and Cu-Ni-Fe, for example.

In various embodiments, a Mn-Fe alloy powder may include, by weight, from about <NUM>% to about <NUM>% Mn and from about <NUM>% to about <NUM>% Fe, such as from about <NUM>% to about <NUM>% Mn and from about <NUM>% to about <NUM>% Fe. A Cu-Fe alloy powder may include, by weight, from about <NUM>% to about <NUM>% Cu and from about <NUM>% to about <NUM>% Fe, such as about <NUM>% Cu and about <NUM>% Fe. A Co-Fe alloy powder may include, by weight, from about <NUM>% to about <NUM>% Co and from about <NUM>% to about <NUM>% Fe, such as about <NUM>% Co and <NUM>% Fe.

The pre-alloying of transition metals may unexpectedly lower the activity of the transition metals, which thereby lowers the vapor pressure and/or increases the melting temperature of the alloyed transition metal. For example, the alloying of Mn with Fe lowers the activity of the Mn, and thus, lowers the vapor pressure of the Mn and reduces evaporation during subsequent high-temperature sintering. Likewise, alloying Cu with Fe significantly increases the melting temperature of the Cu, thereby reducing problems associated with liquid phases.

<FIG> is a flow diagram illustrating a method of forming an interconnect, according to various embodiments of the present disclosure. Referring to <FIG>, in step <NUM>, an interconnect powder may be loaded into a die between an upper punch and a lower punch of a hydraulic or mechanical press and compacted under high pressure to form a green interconnect.

The interconnect powder may include Cr, Fe, and one or more pre-alloyed transition metals, such as Co, Mn, V, Ni, and/or Cu as described above. For example, a pre-alloyed transition metal-Fe powder may be mixed with a Cr elemental powder to form the interconnect powder. In the alternative, a pre-alloyed transition metal-Cr powder may be mixed with a Fe elemental powder to form the interconnect powder. In some embodiments, the interconnect powder may also include an organic lubricant / binder.

In various embodiments, the transition metal-Fe powder and/or the transition metal-Cr powder may be formed by coating a Fe elemental powder, a Cr elemental powder, or a mixture thereof, with at least one transition metal. In particular, one or both of the elemental powders may be coated using an electroless plating process, a solution deposition process, or a sol-gel process. For example, an elemental powder may be wetted by a solution comprising transition metal precursors, dried, and reacted at an appropriate temperature and atmosphere, to form a transition metal coated Cr or Fe powder. The coated powders may have a core-shell structure, including a Cr or Fe core covered by a transition metal shell. The coated powder (e.g., Fe core with a transition metal shell) may be mixed with an elemental powder (e.g., Cr powder) to form the interconnect powder.

In the above embodiments in which the powder containing the transition metal element is evenly distributed throughout the die cavity, a concentration of the at least one transition metal selected from Co, Cu, Mn, Ni, or V is substantially from the air side of the interconnect <NUM> to the fuel side of the interconnect <NUM> after the pressing and sintering.

In an alternative embodiment, step <NUM> may alternatively include loading a Cr elemental powder, a Fe elemental powder, and a transition metal alloy powder in the die cavity using a graded filling process. In particular, a layer of the transition metal alloy powder may be loaded at the top or bottom of the die cavity, and a remainder of the die may be filled with a mixture of elemental Fe and Cr powders. As such, the transition metal alloy powder may be localized on one side of the resultant pressed interconnect, such as on an air-side of the interconnect. In this embodiment, a concentration of the at least one transition metal selected from Co, Cu, Mn, Ni, or V is higher on the air side of the interconnect than on the fuel side of the interconnect.

In step <NUM>, the green interconnect may be removed from the press, loaded into a furnace, and heated at a temperature ranging from about <NUM> to about <NUM>, to remove organic component such as the lubricant / binder.

In step <NUM>, the interconnect may be sintered at a high-temperature, such as a temperature ranging from about <NUM> to about <NUM>, in a reducing atmosphere (e.g., forming gas or H<NUM> atmosphere), to promote metal interdiffusion.

In step <NUM>, the interconnect may be oxidized by heating the interconnect at a controlled oxidation treatment, such as by exposing the interconnect to an oxidizing ambient, such as air, at a high temperature. The oxidation may operate to reduce the porosity of the interconnect. A surface oxide may be removed by grit blasting or other methods.

In step <NUM>, the protective coating <NUM> may be formed on at least one side of the interconnect <NUM>. For example, the protective coating may be formed by applying a perovskite and/or spinel material, such as LSM and/or MCO on the sintered interconnect using an air plasma spray (APS) spray process or a dip coating process, for example. The air plasma spray process is a thermal spray process in which powdered coating materials are fed into the coating apparatus. The coating particles are introduced into a plasma jet in which they are melted and then accelerated toward the substrate. On reaching the substrate, the molten droplets flatten and cool, forming the coating. The plasma may be generated by either direct current (DC plasma) or by induction (RF plasma). Further, unlike controlled atmosphere plasma spraying (CAPS) which requires an inert gas or vacuum, air plasma spraying is performed in ambient air.

In step <NUM>, interconnects may be assembled with fuel cells <NUM> and other stack components to form the fuel cell stack <NUM>. Operation of the stack may result in the formation of chromium-transition metal oxide spinel layers <NUM> between the interconnect <NUM> and the protective coating <NUM> on air sides of the interconnects.

In other embodiments, step <NUM> may alternatively include pressing an interconnect powder that comprises Fe and Cr elemental powders and that does not comprise a pre-alloyed powder. The method may additionally include enriching the surface of the interconnect with a transition metal alloy, before or after the sintering of step <NUM>. For example, a Fe or Cr-transition metal alloy may be applied to the surface of the interconnect by a deposition technique, such as electroplating, plasma vapor deposition (PVD), chemical vapor deposition (CVD), or spray coating.

Claim 1:
A method of forming an interconnect for a fuel cell stack, comprising:
compressing an interconnect powder to form an interconnect, the interconnect powder comprising Cr, Fe and at least one transition metal selected from Co, Cu, Mn, Ni, or V, wherein:
the at least one transition metal is pre-alloyed with particles of at least one of the Cr or the Fe; or
the at least one transition metal forms a shell on particles of at least one of the Cr or Fe; and
sintering the interconnect.