Solid oxide cell interconnects and methods of making thereof by casting

Abstract

A cast iron or an iron chromium alloy interconnect for an electrochemical stack includes an air side and an opposing fuel side, a fuel flow field located on the fuel side and including fuel channels separated by fuel ribs, and an air flow field located on the air side including air channels separated by air ribs.

Description

SOLID OXIDE CELL INTERCONNECTS AND METHODS OF MAKING THEREOF BY CASTING

[0001] FIELD

[0002] The embodiments of the present disclosure are generally directed to solid oxide electrochemical cell column components and more specifically to cast interconnects comprising cast iron, high chromium cast iron, high carbon and chromium cast iron, and iron chromium and molybdenum alloys.

[0003] BACKGROUND

[0004] Interconnects for a solid oxide fuel cell (SOFC) or solid oxide electrolyzer cell (SOEC) system are typically formed by powder metallurgy. A chromium powder containing about 5 weight percent iron is pressed in powder pressing apparatus to form a green interconnect, followed by sintering the green interconnect to form the chromium alloy interconnect. However, this process is relatively expensive.

[0005] SUMMARY

[0006] According to various embodiments of the present disclosure, an interconnect for an electrochemical stack comprises an air side and an opposing fuel side; a fuel flow field located on the fuel side and comprising fuel channels separated by fuel ribs; and an air flow field located on the air side comprising air channels separated by air ribs. The interconnect comprises a cast iron, comprising, in weight percent (wt.%): at least 2 wt.% carbon; at least 7 wt.% chromium; 0 to 2.9 wt.% silicon; 0 to 10 wt.%, such as 0 to 2.5 wt.% manganese; 0 to 7 wt.% nickel; 0 to 1.5 wt.% copper; optionally at least one of molybdenum, phosphorus or sulfur, and a balance of iron.

[0007] In one embodiment, the interconnect comprises a cast iron, comprising, in weight percent (wt.%): 2 wt.% to 9 wt.% carbon; 7 wt.% to 65 wt.% chromium; 0 to 10 wt.% molybdenum; 0 to 2.9 wt.% silicon; 0 to 2.5 wt.% manganese; 0 to 7 wt.% nickel; 0 to 1.5 wt.% copper; 0 to 1 wt.% phosphorus; 0 to 1 wt.% sulfur; and the balance of iron. In an alternative embodiment, the interconnect comprises an iron chromium or an iron chromium molybdenum alloy containing at least 15 wt.% iron.

[0008] According to various embodiments of the present disclosure, a method of manufacturing an interconnect for an electrochemical stack comprises pouring molten cast iron into a mold, and solidifying the molten cast iron to form the interconnect.

[0009] BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate example embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.

[0011] FIG. 1 A is a perspective view of an electrochemical cell column, FIG. IB is a perspective view of one counter-flow solid oxide electrochemical cell stack included in the column of FIG. 1 A, and FIG. 1C is a side cross-sectional view of a portion of the stack of FIG. IB.

[0012] FIG. 2A is a top view of the air side of a counter-flow interconnect of the stack of FIG. IB, and FIG. 2B is a top view of the fuel side of the interconnect.

[0013] FIG. 3 A is a perspective view of a cross-flow electrochemical cell stack, FIG. 3B is an exploded perspective view of a portion of the stack of FIG. 3A, FIG. 3C is a top view of the reactant side of an interconnect included in the stack of FIG. 3 A, and FIG. 3D is a schematic view of an electrochemical cell included in the stack of FIG. 3 A.

[0014] FIGS. 4A-4F illustrate a sand casting process, according to various embodiments of the present disclosure.

[0015] FIGS. 5A-5F illustrate an investment casting process, according to various embodiments of the present disclosure. FIG. 5G illustrates a system that can be utilized to implement a counter gravity casting process, according to various embodiments of the present disclosure.

[0016] FIG. 6 is a perspective view of an interconnect that may be formed by a casting process of embodiments of the present disclosure and finished using surface treatment process and a coating process.

[0017] FIGS. 7A-7B illustrate an open-face sand casting process, according to various embodiments of the present disclosure.

[0018] DETAILED DESCRIPTION

[0019] As set forth herein, various aspects of the disclosure are described with reference to the exemplary embodiments and / or the accompanying drawings in which exemplary embodiments of the invention are illustrated. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments shown in the drawings or described herein. It will be appreciated that the various disclosed embodiments may involve particular features, elements or steps that are described in connection with that particular embodiment. It will also be appreciated that a particular feature, element or step, although described in relation to one particular embodiment, may be interchanged or combined with alternate embodiments in various non-illustrated combinations or permutations.

[0020] The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes and are not intended to limit the scope of the invention or the claims.

[0021] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. 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 + / - 1% X. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0022] Electrochemical cell systems include fuel cell and electrolyzer cell systems. In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is directed to the cathode side of the fuel cell while a fuel (i.e., reactant) flow is directed tothe anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be hydrogen (H2) or a hydrocarbon fuel, such as methane, natural gas, ethanol, or methanol, or a hydrogen containing fuel such as ammonia. The fuel cell, operating at a typical temperature between 750°C and 950°C, enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ions combine with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and / or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ions are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.

[0023] In an electrolyzer system, such as a solid oxide electrolyzer system (SOEC), a fuel (i.e., reactant) flow comprising water (e.g., steam) is separated into hydrogen and oxygen by applying a voltage across the electrolyzer cells. In the SOEC stack, the anode is the air electrode, and the cathode is the fuel electrode. Thus, the electrode to which the fuel (e.g., hydrogen, ammonia or hydrocarbon fuel in a SOFC, and steam in a SOEC) is supplied may be referred to as the fuel electrode and the opposing electrode may be referred to as the air electrode in both SOFC and SOEC cells. As used herein, the terms fuel and reactant may be used interchangeably.

[0024] FIG. 1 A is a perspective view of an electrochemical cell column 30, FIG. IB is a perspective view of one counter-flow solid oxide electrochemical cell (e.g., SOFC or SOEC) stack 20 included in the column 30 of FIG. 1 A, and FIG. 1C is a side cross-sectional view of a portion of the stack 20 of FIG. IB.

[0025] Referring to FIGS. 1 A and IB, the column 30 may include one or more electrochemical cell stacks 20, a fuel inlet conduit 32, a fuel exhaust conduit 34, and fuel feed / return assemblies 36 (e.g., anode splitter plates (ASPs) 36). The column 30 may also include side baffles 38 and a compression assembly 40. The fuel inlet conduit 32 is fluidly connected to ASPs 36 and is configured to provide the fuel feed to each ASP 36, and fuel exhaust conduit 34 is fluidly connected to ASPs 36 and is configured to receive fuel exhaust from each ASP 36.

[0026] The ASPs 36 are disposed between the stacks 20 and are configured to provide a fuel (e.g., hydrogen or hydrocarbon fuel or steam) feed to the stacks 20 and to receive fuel exhaust from the stacks 20 (which comprises a hydrogen product stream in the case of a SOEC stack). For example, the ASPs 36 may be fluidly connected to internal fuel holes 22 formed in the stacks 20, as discussed below.

[0027] Referring to FIG. 1C, the stack 20 includes multiple electrochemical cells 1 that are separated by interconnects 10, which may also be referred to as gas flow separator plates or bipolar plates. Each electrochemical cell 1 includes an air electrode 3, a solid oxide electrolyte 5, and a fuel electrode 7.

[0028] Each interconnect 10 electrically connects adjacent electrochemical cells 1 in the stack 20. In particular, an interconnect 10 may electrically connect the fuel electrode 7 of one electrochemical cell 1 to the air electrode 3 of an adjacent electrochemical cell 1. FIG. 1C shows that the lower electrochemical cell 1 is located between two interconnects 10.

[0029] Each interconnect 10 includes fuel ribs 12A that at least partially define fuel channels 8A and air ribs 12B that at least partially define the air channels 8B. The interconnect 10 may operate as a gas-fuel separator that separates a fuel flowing to the fuel electrode 7 of one cell in the stack from oxidant, such as air, flowing to the air electrode 3 of an adjacent cell in the stack. At either end of the stack 20, there may be an air end plate or fuel end plate (not shown) for providing air or fuel, respectively, to the end electrode in the stack. Alternatively, the same interconnect 10 may be used as an air end plate or fuel end plate.

[0030] FIG. 2A is a top view of the air side of a counter-flow (or co-flow) interconnect 10, and FIG. 2B is a top view of a reactant side of the interconnect 10. Referring to FIGS. 1C and 2A, the air side includes the air channels 8B. Air flows through the air channels 8B to an air electrode 3 of an adjacent electrochemical cell 1. In particular, the air may flow across the interconnect 10 in a first direction A as indicated by the arrows.

[0031] Ring seals 23 may surround the reactant holes 22 of the interconnect 10, to prevent a reactant, such as a fuel or steam, from contacting the air electrode 3. Peripheral strip-shaped seals 24 are located on peripheral portions of the air side of the interconnect 10. The seals 23, 24 may be formed of a glass material. The peripheral portions may be in the form of anelevated plateau which does not include ribs or channels. The surface of the peripheral regions may be coplanar with tops of the air ribs 12B.

[0032] In some embodiments, the air side of the interconnect 10 may be coated with an electrically conductive protective layer 14. In particular, the protective layer 14 may be formed on at least the air ribs 12B and optionally in the air channels 8B. The protective layer 14 may be deposited by an atmospheric plasma spray (APS) process or a physical vapor deposition process. In some embodiments, the protective layer 14 may comprise a lanthanum strontium manganate and / or manganese cobalt oxide spinel material.

[0033] Referring to FIGS. 1C and 2B, the fuel side of the interconnect 10 may include the fuel channels 8A and fuel manifolds 28 (e.g., fuel plenums). A fuel such as a fuel or steam flows from one of the fuel holes 22, into the adjacent manifold 28, through the fuel channels 8 A, and to a fuel electrode 7 of an adjacent electrochemical cell 1. Excess fuel may flow into the other fuel manifold 28 and then into the adjacent fuel hole 22. In particular, the fuel may flow across the interconnect 10 in a second direction B, as indicated by the arrows. The second direction B may be opposite to the first direction A (see FIG. 2A) referred to as a counter-flow design, perpendicular to the first direction A referred to as a cross-flow design, or in the same direction as first direction A referred to as co-flow design.

[0034] A frame-shaped seal 26 is disposed on a peripheral region of the fuel side of the interconnect 10. 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 fuel ribs 12A.

[0035] FIG. 3 A is a perspective view of an internally manifolded cross-flow electrochemical cell column 300, according to various embodiments of the present disclosure, FIG. 3B is an exploded perspective view of a portion of the column 300 of FIG. 3A, FIG. 3C is a top view of the fuel side of an interconnect 200 included in the column 300, and FIG. 3D is a schematic view of an electrochemical cell included in the column 300.

[0036] Referring to FIGS. 3A-3D, the cell column 300, which may also be referred to as an electrochemical cell stack because it lacks RSPs, includes multiple electrochemical cells 310 (e.g., fuel cells or electrolyzer cells) that are separated by interconnects 200, which may alsobe referred to as gas flow separator plates or bipolar plates. One or more stacks 300 may be thermally integrated with other components of an electrochemical cell system (e.g., one or more anode tail gas oxidizers, fuel reformers, fluid conduits and manifolds, etc. of a SOFC or SOEC system) in a common enclosure or “hotbox.”

[0037] The interconnects 200 are made from an electrically conductive metal alloy. Each electrochemical cell 310 may include a solid oxide electrolyte 312, a fuel electrode 314, and an air electrode 316. In some embodiments, the fuel electrode 314 and the air electrode 316 may be printed on the electrolyte 312. In other embodiments, a conductive layer 318, such as a nickel mesh, may be disposed between the fuel electrode 314 and an adjacent interconnect 200. The electrochemical cell 310 does not include through holes, such as the fuel holes 22, described above.

[0038] An upper most interconnect 200 and a lowermost interconnect 200 of the column 300 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 electrochemical cell 310 in the stack 300. As used herein, an “interconnect” may refer to either an interconnect located between two electrochemical cells 310 or an end plate located at an end of the stack and directly adjacent to only one electrochemical cell 310. Since the column 300 does not include RSPs and the end plates associated therewith, the column 300 may include only two end plates.

[0039] The column 300 may include side baffles 302, a fuel plenum 304, and a compression assembly 306. The side baffles 302 may be formed of a ceramic material and may be disposed on opposing sides of the cell column 300 containing stacked electrochemical cells 310 and interconnects 400. The side baffles 302 may connect the fuel plenum 304 and the compression assembly 306, such that the compression assembly 306 may apply pressure to the column 300. The side baffles 302 may be curved baffle plates, such that each baffle plate covers at least portions of three sides of the cell column 300. For example, one baffle plate may fully cover the fuel inlet riser side of the column 300 and partially cover the adjacent front and back sides of the stack, while the other baffle plate fully may cover the fuel outlet riser side of the stack and partially cover 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 air to flow through the column 300. The fuel plenum 304 may be disposed below the column300 and may be configured to provide a hydrogen-containing fuel feed to the column 300 and may receive a fuel exhaust from the column 300. The fuel plenum 304 may be connected to fuel inlet and outlet conduits 308 which are located below the fuel plenum 304.

[0040] Each interconnect 200 electrically connects adjacent electrochemical cells 310 in the column 300. In particular, an interconnect 200 may electrically connect the fuel electrode 314 of one electrochemical cell 310 to the air electrode 316 of an adjacent electrochemical cell 310. As shown in FIG. 3C, each interconnect 200 may be configured to channel air in a first direction A, such that the air may be provided to the air electrode 316 of an adjacent electrochemical cell 310. Each interconnect 200 may also be configured to channel fuel in a second direction F, such that the fuel may be provided to the fuel electrode 314 of an adjacent electrochemical cell 310. Directions A and F may be perpendicular, or substantially perpendicular. As such, the interconnects 200 may be referred to as crossflow interconnects.

[0041] The interconnect 200 may include through-holes configured for fuel distribution. For example, the interconnects 200 may include one or more fuel inlets 202 and one or more fuel outlets 204, which may also be referred to as fuel exhaust outlets 204. The fuel inlets and outlets 202, 204 may be disposed outside of the perimeter of the electrochemical cells 310. As such, the electrochemical cells 310 may be formed without corresponding through holes for fuel flow. The combined length of the fuel inlets 202 and / or the combined length of the fuel outlets 204 may be at least 75% of a corresponding length of the interconnect 200 e.g., a length taken in direction A.

[0042] In one embodiment, each interconnect 200 contains two fuel inlets 202 separated by a neck portion 212 of the interconnect 200, as shown in FIG. 3B. However, more than two fuel inlets 202 may be included, such as three to five inlets separated by two to four neck portions. In one embodiment, each interconnect 200 contains two fuel outlets 204 separated by a neck portion 214 of the interconnect 200, as shown in FIG. 3B. However, more than two fuel outlets 204 may be included, such as three to five outlets separated by two to four neck portions 214. Although not illustrated, the number of fuel inlets and fuel outlets present on a single interconnect 200 can be different.

[0043] The fuel inlets 202 of adjacent interconnects 200 may be aligned in the column 300 to form one or more fuel inlet risers 303. The fuel outlets 204 of adjacent interconnects 200 may be aligned in the column 300 to form one or more fuel outlet risers 305. The fuel inlet risers 303 may be configured to distribute fuel received from the fuel plenum 304 to the electrochemical cells 310. The fuel outlet risers 305 may be configured to provide fuel exhaust received from the electrochemical cells 310 to the fuel plenum 304.

[0044] The side baffles 302 may optionally be curved around edges of the interconnects 200. In particular, the side baffles 302 may be disposed around the fuel inlets 202 and outlets 204 of the interconnects 200. Accordingly, the side baffles may more efficiently control air flow through air channels of the interconnects 200, which are exposed between the side baffles 302. In various embodiments, the column 300 may include from about 200 to 400 electrochemical cells, such as about 250 to 350 electrochemical cells, more particularly from about 275 to 325 electrochemical cells, which may be provided with fuel using only the fuel risers 303, 305.

[0045] Each interconnect 200 may be made of or may contain electrically conductive material, such as a metal alloy which has a similar coefficient of thermal expansion to that of the solid oxide electrolyte in the cells (e.g., a difference of 0-10%). The interconnects 200 may electrically connect the fuel electrode 314 of one electrochemical cell 310 to the air electrode 316 of an adjacent electrochemical cell 310. An electrically conductive contact layer, such as a nickel contact layer 318 (e.g., a nickel mesh), may be provided between fuel electrode 314 and each interconnect 200. Another optional electrically conductive contact layer may be provided between the air electrodes and each interconnect 200.

[0046] An air surface of an interconnect 200 that in operation is exposed to an oxidizing environment (e.g., air), may be coated with a protective coating layer in order to decrease the growth rate of a chromium oxide surface layer on the interconnect and to suppress evaporation of chromium vapor species which can poison the electrochemical cell air electrode. Typically, the coating layer, which can comprise a perovskite such as lanthanum strontium manganite (LSM), may be formed using a spray coating or dip coating process. Alternatively, other metal oxide coatings, such as a spinel, such as an (Mn, CojsCU spinel (MCO), can be used instead of or in addition to LSM. Any spinel having the compositionMn2-xCoi+xO4 (0 < x < 1) or written as / (MnsCU) + (I-Z CO3O4), where (1 / 3 < z < 2 / 3) or written as (Mn, Co^CL may be used. In other embodiments, a mixed layer of LSM and MCO, or a stack of LSM and MCO layers may be used as the coating layer.Cast Iron Interconnect Materials and Casting Methods

[0047] The present inventors have determined that cast iron alloys may be utilized as a less expensive substitute for chromium-based and stainless steel interconnect materials, while still providing desired mechanical, oxidation resistance, and coefficient of thermal expansion (CTE) matching properties to the ceramic material of solid oxide electrochemical cells. One such iron-based alloy is ASTM A532, which is also a known as high chromium white cast iron. Furthermore, UNS F4500X (X is an integer from 0 to 9, where UNS stands for Unified Numbering System for Metals and Alloys) alloys may also be used. For example, the high chromium white cast iron may include a balance of iron and alloying element weight percentage ranges as shown in the following Table 1.Table 1

[0048] In Table 1, each of the Si, Mn, Mo, Ni, Cu, P and / or S may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si, Mn, Mo, Ni and / or Cu may comprise greater than 0.5 weight percent.

[0049] In some embodiments, high chromium white iron may comprise alloying element weight percentages as shown in Table 2.Table 2

[0050] In Table 2, each of the Si, Mn, Mo, Ni, Cu, P and / or S may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si, Mn, Mo, Ni and / or Cu may comprise greater than 0.5 weight percent.

[0051] In some embodiments, high chromium white iron may comprise alloying element weight percentages as shown in Table 3.Table 3

[0052] In Table 3, each of the Si, Mn and / or Mo may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si, Mn and / or Mo may comprise between 0.5 and 1.5 weight percent.

[0053] In some embodiments, high chromium white iron may comprise alloying element weight percentages as shown in Table 4.Table 4

[0054] In Table 4, each of the Si and / or Mn may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si and / or Mn may comprise between 0.5 and 1 weight percent.

[0055] In some embodiments, high chromium white iron may comprise alloying element weight percentages as shown in Table 5.Table 5

[0056] In Table 5, each of the Si, Mn and / or Mo may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si, Mn and / or Mo may comprise between 0.5 and 1 weight percent.

[0057] In some embodiments, high chromium white iron may comprise alloying element weight percentages as shown in Table 6.Table 6

[0058] In Table 6, each of the Si, Mn and / or Mo may comprise zero weight percent, an unavoidable impurity weight percent (e.g., greater than zero and less than 0.05 weight percent), and / or greater than zero weight percent, such as greater than 0.05 weight percent. In one embodiment, at least one of the Si, Mn and / or Mo may comprise between 0.5 and 1 weight percent.

[0059] In some embodiments, higher carbon content cast iron compositions may be utilized to form cast iron interconnects incorporated into solid oxide fuel cell stacks. In particular, an interconnect structure may be manufactured using a cast iron comprising, in weight percent (wt. %): 2 wt.% to 9 wt.% carbon (C), 25 wt.% to 65 % chromium (Cr), 0 to 2.9 wt.% silicon (Si), 0 to 2.5 wt.% manganese (Mn); 0 to 10 wt.% molybdenum (Mo); 0 to 3.5 wt.% nickel (Ni); 0 to 1.5 wt.% copper (Cu); 0 to lwt.%, including 0 to 0.1 wt.% phosphorus (P); 0 to 1 wt.%, such as 0 to 0.06 wt.% sulfur (S); and a balance of iron (Fe). The interconnect structure may include zero weight percent or non-zero weight percent of any one or more of Si, Mn, Mo, Ni, Cu, P and / or S. For example, the interconnect structure may include 2 wt.% to 9 wt.% carbon (C), 25 wt.% to 65 % chromium (Cr), 0.1 to 2.9 wt.% silicon (Si), 0.1 to 2.5 wt.% manganese (Mn); 0.1 to 10 wt.% molybdenum (Mo); 0.1 to 3.5 wt.% nickel (Ni); 0.1 to 1.5 wt.% copper (Cu); 0.01 to 1 wt.% phosphorus (P); 0.01 to 1 wt.% sulfur (S); and a balance of iron (Fe) (e.g., at least 15 wt.% iron, such as 15 wt.% to 72 wt.% iron).

[0060] Thus, the interconnect can include at least 2 wt.% carbon and at least 7 wt.% chromium, such as 2 to 9 wt.% C and 7 to 65 wt.% Cr, including 23 to 65 wt.% Cr. The interconnect may also optionally include molybdenum and / or nickel, such as 0 to 10 wt.% molybdenum and / or 0 to 7 wt.% nickel, such as 0 to 3.5 wt.% Mo and / or 0 to 3.5 wt.% Ni.

[0061] In some embodiments, a high chromium cast iron may be utilized for forming interconnects for incorporation into solid oxide fuel cell stacks. In particular, an interconnect structure may be manufactured using a high chromium cast iron comprising, in weight percent (wt. %): 2 wt.% to 5 wt.% carbon (C), such as 2 wt.% to 4 wt.% carbon (C) or 2.8 wt.% to 5 wt.% carbon (C), 40 wt.% to wt. 75 % chromium (Cr), such as 40 wt.% to 70 wt. % chromium (Cr) or 45 wt.% to 75 wt. % chromium (Cr), 0 to 2.9 wt.% silicon (Si), 0 to 2.5 wt.% manganese (Mn), 0 to 25 wt.% molybdenum (Mo), 0 to 3.5 wt.% nickel (Ni), 0 to 1.5 wt.% copper (Cu), 0 to 1 wt.% phosphorus (P), 0 to 1 wt.% sulfur (S), and a balance of iron (Fe) (e.g., 26 to 58 wt. % iron). The interconnect structure may include zero weight percent or non-zero weight percent of any one or more of Si, Mn, Mo, Ni, Cu, P and / or S. For example, the interconnect structure may include 2 wt.% to 5 wt.% carbon (C), such as 2 wt.% to 4 wt.% carbon (C) or 2.8 wt.% to 5 wt.% carbon (C), 40 wt.% to wt. 75 % chromium (Cr), such as 40 wt.% to 70 wt. % chromium (Cr) or 45 wt.% to 75 wt. % chromium (Cr), 0.1 to 2.9 wt.% silicon (Si), 0.1 to 2.5 wt.% manganese (Mn); 0.1 to 25 wt.%, such as 1 to 25 wt.%,including 5 to 25 wt.% molybdenum (Mo); 0.1 to 3.5 wt.% nickel (Ni); 0.1 to 1.5 wt.% copper (Cu); 0.01 to 1 wt.% phosphorus (P); 0.01 to 1 wt.% sulfur (S); and a balance of iron (Fe) (e.g., at least 15 wt.% iron, 15 wt.% to 57 wt. % iron).

[0062] In further embodiments, a chromium iron alloy or chromium, iron and molybdenum alloy may be utilized for forming interconnects for incorporation into solid oxide fuel cell stacks. In particular, an interconnect structure may be manufactured using a chromium, iron and molybdenum alloy comprising, in weight percent (wt. %): 40 wt.% to 70 % chromium (Cr), 0 to 2 wt.% carbon (C), 0 to 5 wt.% boron (B), 0 to 2.9 wt.% silicon (Si), 0 to 2.5 wt.% manganese (Mn), 0 to 25 wt.% molybdenum (Mo), 0 to 3.5 wt.% nickel (Ni), 0 to 1 wt.% phosphorus (P); 0 to 1 wt.% sulfur (S), and a balance of iron (Fe) (e.g., 30 to 60 wt. % iron). This alloy may completely or substantially exclude carbon (e.g., have 0 to 0.005 wt.% carbon), or may have a relatively low amount of carbon (e.g., 0.005 to 2 wt.% carbon, such as 1 to 2 wt.% carbon). The interconnect structure may include zero weight percent or non-zero weight percent of any one or more of B, Si, Mn, Mo, Ni, P and / or S. For example, the interconnect structure may include 40 wt.% to 70 % chromium (Cr), 0.005 to 2 wt.% carbon (C), 0.1 to 5 wt.% boron (B), 0.1 to 2.9 wt.% silicon (Si), 0.1 to 2.5 wt.% manganese (Mn); 0.1 to 25 wt.%, such as 1 to 25 wt.%, including 5 to 25 wt.%, for example 10 to 25 wt.% molybdenum (Mo); 0.1 to 3.5 wt.% nickel (Ni); 0.01 to 1 wt.% phosphorus (P); 0.01 to 1 wt.% sulfur (S); and a balance of iron (Fe) (e.g., at least 15 wt.% iron, such as 15 wt.% to 59 wt. % iron). The interconnect may be formed by any suitable casting method described therein.

[0063] White iron (in addition to the other cast iron formulations disclosed herein) may beneficially have a CTE that closely matches the CTE of a solid oxide electrolyte, such as stabilized zirconia electrolyte (for electrolyte supported cells). The high chromium content may beneficially provide a suitable oxidation resistance to withstand solid oxide stack operating conditions. Unlike chromium -based interconnect materials, white cast iron and the additional cast iron formulations disclosed herein are not restricted to forming interconnects via a relatively expensive powder metallurgy process.

[0064] In particular, white cast iron and the additional cast iron formulations disclosed herein may be used to form an interconnect using a casting process. Casting is a manufacturingprocess where a melted / molten material is poured into a mold. This mold has a hollow cavity with the desired geometry / shape of the part being manufactured. After pouring, the melted / molten material solidifies, and it can be extracted or broken out of the mold. This solidified object is considered a casting or cast part, which then can be processed further with other secondary manufacturing operations to reach the desired final geometry. For example, suitable casting processes include sand casting and investment casting (also referred to as precision investment casting or precision casting).

[0065] FIGS. 4A-4F illustrate a sand casting process, according to various embodiments of the present disclosure. Referring to FIG. 4A, the first step of the sand casting process involves creating a pattern 402 that is a replica of an interconnect. The pattern 402 may be formed of any suitable material, such as plastic or metal. The pattern 402 is typically larger than the final interconnect to accommodate shrinkage during cooling. The pattern 402 may include an upper pattern 402a and a lower pattern 402b. The upper pattern 402a may correspond to the air or fuel side of the interconnect, while the lower pattern 402b may correspond to the other one of the air or fuel side of the interconnect. Casting sand is filled in each part 405a and 405b of a two part molding box 405 which is known as a casting flask or core box.

[0066] As shown in FIGS. 4B and 4C, the upper pattern 402a and the lower pattern 402b are embedded in sand to create half molds 404a, 404b, which may be referred to as a cope 404a and a drag 404b. Molding tools and techniques, such as ramming, squeezing, or jolting, may be employed to achieve proper sand compaction and mold density. Pins 406 may be inserted into the cope 404a to form openings / channels. Top molding board 407a and bottom molding board 407b may be used for embedding the upper pattern 402a and the lower pattern 402b in the cope 404a and the drag 404b, respectively.

[0067] The upper and lower patterns 402a, 402b, the pins 406 and the molding boards 407a, 407b are then removed from the casting sand (i.e., from the cope and the drag). The cope 404a and the drag 404b may then be assembled to form a complete mold 404 in the flask 405 including a cavity 409 and channels 408. As shown in FIG. 4D, the cope 404a is placed on the drag 404b by flipping the upper part 405a of the flask 405 onto the lower part 405b of the flask 405.

[0068] As shown in FIG. 4E, molten metal (e.g., molten high chromium white cast iron) is poured into one of the channels (e.g., a runner channel) 408. The molten metal flows into the cavity 409 and rises in the other one of the channels (e.g., a riser channel) 408. The metal is typically heated in a furnace and then transferred to a pouring ladle for controlled pouring into the mold 404.

[0069] After the molten metal is poured, it cools and solidifies within the mold 404. The cooling time may be controlled to produce an interconnect having desired properties and dimensional accuracy. After cooling, the mold 404 undergoes a shakeout process that involves mechanically or manually breaking the mold 404 to release an interconnect 500, as shown in FIG. 4F. This process may include removal of excess metal, which may be referred to as the casting’s “gating system (i.e., the metal portions which fill the channels 408). The interconnect 500 may undergo further processing, as discussed below.

[0070] In an alternative embodiment, a shell-mold casting method (also known as shell molding) is used instead of sand casting. Shell-mold casting is similar to sand casting. However, the shell-mold casting uses a resin covered sand instead of sand to form the half molds. The half molds in shell-mold casting comprise thin-walled shells created by applying a sand-resin mixture around the upper and lower patterns.

[0071] In another alternative embodiment that will be described in more detail below, an open mold casting method is used instead of sand casting. In the open mold casting method, the cavity in the half mold is open at the top, and the molten metal is poured into the cavity in the half mold without using a sprue or riser channels.

[0072] FIGS. 5A-5F illustrate an investment casting process (also known as a lost wax or precision investment casting process), according to various embodiments of the present disclosure. Referring to FIG. 5A, the first step of the investment casting process involves creating a wax pattern 502 that is a replica of an interconnect. The wax pattern 502 may be made of wax or another low melting point material, such as frozen mercury or plastic. The wax pattern 502 may be formed in a master mold, which is also known as a master die.

[0073] Referring to FIG. 5B, multiple wax patterns 502 may be attached to a wax sprue 504 to form a wax assembly 510.

[0074] As shown in FIG. 5C, the wax assembly 510 may be coated with a refractory slurry (i.e., the investment material, such as a glass or ceramic material). In particular, the wax assembly 510 may be dipped in the refractory slurry. Optionally, coarser particles of the investment material may be stuccoed onto the wax assembly 510 using a fluidized bed or a rainfall sander. The investment material (e.g., refractory slurry) may be allowed to harden by letting it set (i.e., by curing and drying it) at a relatively low temperature, such as a room temperature to form a mold 520. The mold 520 comprises a refractory material, such as a glass (e.g., fused silica glass) or a ceramic (e.g., zircon, alumina or aluminum silicate).

[0075] As shown in FIG. 5D, the solid mold 520 is then dewaxed by heating it to melt and remove the wax assembly 510. Cavities 509 are formed in the mold 520 in spaces that were previously occupied by the wax assembly. The mold may be subjected to a burnout preheating at between 870 and 1100 °C to remove any moisture and residual wax.

[0076] Referring to FIG. 5E, a molten metal (e.g., molten high chromium white cast iron) is poured into the cavities 509 the mold 520 and then cooled. The solidified metal forms a molding 530.

[0077] As shown in FIG. 5F, the mold 520 may be broken and removed from the molding 530. The molding 530 may then be cut to separate interconnects 500. The interconnects 500 may be further processed to remove any remaining molding material.

[0078] FIG. 5G illustrates a system that can be utilized to implement a counter gravity casting process, according to various embodiments of the present disclosure. Counter gravity casting is a metalworking technique that uses a vacuum to fill a mold as opposed to using gravity. Similar to the investment casting process described above, a counter gravity casting process can start with the creation of wax patterns attached to a central sprue to form a wax assembly, which assembly is subsequently coated by dipping into a ceramic slurry to build up a shell mold. The wax patterns are subsequently removed leaving the shell mold, which can then be fired at elevated temperatures to develop mold strength for casting purposes. For purposes of counter gravity casting, the shell mold should be gas permeable.

[0079] FIG. 5G depicts a perspective, partial cross-sectional view of a vessel 560 containing a shell mold 565. The shell mold 565 comprises a riser channel 565a, lateral channels 565b,and mold cavities 565c. The interior of the mold cavities 565c corresponds to the shape of the interconnect 500 to be cast in the counter gravity process. The number of mold cavities 565c that can be attached to riser channel 565a depends upon the height and width of the vessel 560 and the size of each mold cavity 565c. Channel openings 565d in the riser channel 565a facilitate transfer of molten metal to the mold cavities 565c.

[0080] In a reverse gravity casting process, a shell mold 565 is placed into vessel 560. The top of the riser channel 565a is covered with a gas permeable cap (not shown). The interior 560a of the vessel 560 is then filled with a support medium, typically sand, through the top of the vessel 560. A vacuum tight lid 560b is then placed on top of vessel 560 to enclose vessel 560.

[0081] As part of the casting process, vessel 560 is placed above a source of molten metal to be cast into the mold cavities 565c. The molten metal is contained in crucible 580 which is heated by induction coils 585. A fill tube 590, fluidly connected to the riser channel 565a, is lowered into the molten metal held in crucible 580. A vacuum conduit 570 is connected to the top of vessel 560. A vacuum pump (not shown) is connected to a vacuum port 575, which is fluidly connected to the vacuum conduit 570. The interior 560a of vessel 560 is evacuated to below atmospheric pressure, which also acts to reduce the gas pressure inside of shell mold 565 due to its gas permeable design. As fill tube 590 is in fluid communication with riser channel 565a, the pressure within fill tube 590 will also be reduced. As a consequence, molten metal in the crucible 580 will flow upwardly into fill tube 590, into riser channel 565a, into lateral channels 565b and into mold cavities 565c. In some embodiments, vessel 560 may be connected to machinery that causes vessel 560 to rotate about its central vertical access. In this configuration, the rotation of vessel 560 will impart a centrifugal force upon the molten metal in lateral channels 565b and mold cavities 565c, thereby facilitating filling of the mold cavities 565c.

[0082] FIG. 6 is a perspective view of an interconnect 500 that may be formed by a sand or investment casting process as described herein and finished using a surface treatment process and a coating process. Referring to FIG. 6, the interconnect 500 may be counter flow interconnect, co-flow interconnect, or cross flow interconnect as described above. After the casting, the interconnect 500 may be sand or grit blasted to remove any mold material and / ora native oxide, and to prepare one or more surfaces of the interconnect 500 for further processing.

[0083] In some embodiments, the air side of the interconnect 500 may be coated with an electrically conductive protective layer 550. In particular, the protective layer 550 may be formed on at least an air flow field 540 comprising air side ribs 542 and channels 544 of the interconnect 500, to protect the air field 540 from corrosion and / or oxidation due to exposure to high temperatures and oxygen during use in a solid oxide cell stack. The protective layer 550 may be deposited by an atmospheric plasma spray (APS) process or a physical vapor deposition process. In some embodiments, the protective layer 550 may comprise a lanthanum strontium manganate and / or manganese cobalt spinel material.

[0084] According to various embodiments, the present inventors determined that the relatively high surface area and low thickness of an interconnect may make it difficult to completely fill a mold cavity with molten metal. In particular, molten metal may prematurely solidify in the cavity during the filling process, which may result in incomplete cavity filling and / or interconnect defects. The present inventors determined that one solution to this problem is to use an open-face sand casting (also known as open-face mold casting) process to form the interconnect.

[0085] FIGS. 7 A and 7B illustrate an open-face sand casting process, according to various embodiments of the present disclosure. Referring to FIGS. 7A and 7B, the process may be performed in a sand mold 600 comprising a drag (i.e., lower mold) 610 and a cope (i.e., upper mold) 620, which may be formed of sand as discussed above with respect to FIGS. 4A-4C. The drag 610 may include a cavity 612 and at least one overflow channel 614 that may surround the cavity 612. The bottom surface of the cavity 612 may include surface features configured to form corresponding interconnect surface structures, such as ribs and channels.

[0086] The cope 620 may include a mesa structure or punch 622 and optionally at least one fill channel 624. The mesa 622 may extend from a lower surface of the main body of the cope 620 and may include surface features configured to form corresponding interconnect surface features, such as ribs and channels. The punch 622 may be configured to be inserted into the cavity 612. The opening 624 may extend through the cope 620 and may beconfigured to receive molten metal. Alternatively, the opening 624 may be omitted, and the molten metal may be poured into the cavity 612 without first passing through the cope 620.

[0087] Molten metal may be poured into the mold 600 from above. In one embodiment, the molten metal may be poured from a molten metal reservoir (e.g., ladle, etc.) 630 into cavity 612 through the fill channel(s) 624. During the filling process, the cope 620 and the drag 610 may be vertically separated by a distance D ranging from about 1 mm to about 20 mm, such as from about 5 mm to about 10 mm. This separation may allow for air to easily exit the mold 600 during the filling process, which may allow for the molten metal to rapidly fill the cavity 612. The reduction in filling time may reduce or prevent premature solidification of the molten metal.

[0088] In another embodiment, the molten metal may be poured directly into the cavity 612 without passing through the cope 620. In this embodiment, the fill channel(s) 624 may be omitted.

[0089] After the cavity 612 is completely filled, the cope 620 may be brought into contact with the drag 610, such that the punch 622 contacts the molten metal in the cavity 622. Any excess metal may be forced out of the cavity 612 and into the overflow channel(s) 614. The metal may be allowed to solidify to form an interconnect 10 or 200 that may then be removed from the mold 600.

[0090] While solid oxide electrochemical cell interconnects, end plates, and electrolytes are described above in various embodiments, embodiments can include any other fuel cell or electrolyzer interconnects or end plates, such as molten carbonate, phosphoric acid or PEM fuel cell or electrolyzer electrolytes, interconnects or end plates.

[0091] Fuel cell and electrolyzer systems of the embodiments of the present disclosure are designed to reduce greenhouse gas emissions and have a positive impact on the climate.

[0092] The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, thepresent invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

What is claimed is:

1. An interconnect for an electrochemical stack, comprising: an air side and an opposing fuel side; a fuel flow field located on the fuel side and comprising fuel channels separated by fuel ribs; and an air flow field located on the air side comprising air channels separated by air ribs, wherein the interconnect comprises a cast iron, comprising, in weight percent (wt.%): at least 2 wt.% carbon; at least 7 wt.% chromium;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper; optionally at least one of molybdenum, phosphorus or sulfur; and a balance of iron.

2. The interconnect of claim 1, wherein the cast iron comprises:2.8 wt.% to 5 wt.% carbon;45 wt.% to 75 wt.% chromium;0 to 10 wt.% molybdenum;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 1 wt.% phosphorus;0 to 1 wt.% sulfur; and the balance of iron.

3. The interconnect of claim 1, wherein the cast iron comprises:2 wt.% to 9 wt.% carbon;7 wt.% to 70 wt.% chromium;0 to 10 wt.% molybdenum;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 1 wt.% phosphorus;0 to 1 wt.% sulfur; and the balance of iron.

4. The interconnect of claim 1, wherein the cast iron comprises:2 wt.% to 3.5 wt.% carbon;7 wt.% to 40 wt.% chromium;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 3.5 wt.% molybdenum;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 0.1 wt.% phosphorus;0 to 0.06 wt.% sulfur; and a balance of iron.

5. The interconnect of claim 1, wherein the cast iron comprises:2 wt.% to 3.3 wt.% carbon;23 wt.% to 30 wt.% chromium;0 to 1.5 wt.% silicon;0 to 2.0 wt.% manganese;0 to 3.0 wt.% molybdenum;0 to 2.5 wt.% nickel;0 to 1.2 wt.% copper;0 to 0.1 wt.% phosphorus;0 to 0.06 wt.% sulfur; and the balance of iron.

6. The interconnect of claim 1, wherein the cast iron comprises:2.0 wt.% to 4.0 wt.% carbon;40 wt.% to 70 wt. % chromium;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 25 wt.% molybdenum;0 to 3.5 wt.% nickel;0 to 1.5 wt.% copper;0 to lwt.% phosphorus;0 to 1 wt.% sulfur; and the balance of iron, comprising at least 15 wt.% iron.

7. The interconnect of claim 1, wherein the interconnect is formed by a casting process comprising sand casting, open mold casting, shell-mold casting, or investment casting.

8. The interconnect of claim 1, wherein the cast iron comprises a high chromium white cast iron.

9. The interconnect of claim 1, further comprising: fuel holes that extend through the interconnect from the air side to the fuel side; and an electrically conductive protective layer located on the air flow field, wherein the electrically conductive protective layer comprises at least one of lanthanum strontium manganite or manganese cobalt spinel.

10. An electrochemical cell stack, comprising: interconnects of claim 1 stacked over one another; and electrochemical cells located between the interconnects.

11. The electrochemical cell stack of claim 10, wherein the electrochemical cells comprise solid oxide fuel cells or solid oxide electrolyzer cells.

12. A method of manufacturing an interconnect for an electrochemical stack, comprising: pouring molten cast iron into a mold; and solidifying the molten cast iron to form the interconnect.

13. The method of claim 12, wherein the molten cast iron is solidified as a molding, and further comprising: removing the molding from the mold; and cutting the molding to separate the interconnect.

14. The method of claim 13, further comprising: grit blasting the interconnect; and applying an electrically conductive protective layer to an air flow field of the interconnect.

15. The method of claim 12, further comprising placing the interconnect into an electrochemical stack comprising solid oxide fuel cells or solid oxide electrolyzer cells.

16. The method of claim 12, wherein: the mold comprises a drag and a cope disposed over the drag; the pouring comprises vertically separating the cope and the drag during the pouring; and the solidifying comprises bringing the cope into contact with the drag before the molten cast iron solidifies.

17. The method of claim 12, wherein the mold comprises a closed sand mold, an open sand mold, or a composite resin and sand shell mold, and the interconnect comprises a cast iron interconnect formed by sand casting, open mold casting or shell-mold casting.

18. The method of claim 12, wherein the mold comprises an investment casting mold, and the interconnect comprises a cast iron interconnect formed by investment casting or by counter gravity investment casting.

19. The method of claim 12, wherein the interconnect comprises a cast iron, comprising, in weight percent (wt.%): at least 2 wt.% carbon; at least 7 wt.% chromium;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 0.1 wt.% phosphorus;0 to 0.06 wt.% sulfur; optionally molybdenum; and a balance of iron.

20. The method of claim 19, wherein the cast iron comprises:2.8 wt.% to 5 wt.% carbon;45 wt.% to 75 wt.% chromium;0 to 10 wt.% molybdenum;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 1 wt.% phosphorus;0 to 1 wt.% sulfur; and the balance of iron.

21. The method of claim 19, wherein the cast iron comprises:2 wt.% to 9 wt.% carbon;7 wt.% to 65 wt.% chromium;0 to 10 wt.% molybdenum;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 7 wt.% nickel;0 to 1.5 wt.% copper;0 to 0.1 wt.% phosphorus;0 to 0.06 wt.% sulfur; and the balance of iron.

22. The method of claim 12, wherein the cast iron comprises:2.0 wt.% to 4.0 wt.% carbon;40 wt.% to 70 wt. % chromium;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 25 wt.% molybdenum;0 to 3.5 wt.% nickel;0 to 1.5 wt.% copper;0 to lwt.% phosphorus;0 to 1 wt.% sulfur; and the balance of iron, comprising at least 15 wt.% iron.

23. The method of claim 12, wherein the interconnect comprises: an air side and an opposing fuel side; a fuel flow field located on the fuel side and comprising fuel channels separated by fuel ribs; an air flow field located on the air side comprising air channels separated by air ribs; and fuel holes that extend through the interconnect from the air side to the fuel side.

24. An interconnect for an electrochemical stack, comprising: an air side and an opposing fuel side; a fuel flow field located on the fuel side and comprising fuel channels separated by fuel ribs; and an air flow field located on the air side comprising air channels separated by air ribs, wherein the interconnect comprises an iron chromium alloy comprising, in weight percent (wt.%):40 wt.% to 70 wt.% chromium;0 to 2 wt.% carbon;0 to 5 wt.% boron;0 to 2.9 wt.% silicon;0 to 2.5 wt.% manganese;0 to 25 wt.% molybdenum;0 to 3.5 wt.% nickel;0 to lwt.% phosphorous;0 to 1 wt.% sulfur; and the balance of iron, comprising at least 15 wt.% iron.

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