Compact high temperature electrochemical cell stack architecture

Abstract

To provide a fuel cell assembly and a modular fuel cell system.SOLUTION: A fuel cell assembly includes a first housing base. The first housing base includes: a lower gas supply part including a first fuel inlet pipe extending from a first end of the first housing base to a second end of the first housing base and a first used fuel outlet pipe extending from the first end of the first housing base to the second end of the first housing base; an upper mounting plate including a plurality of through holes; and a gas distribution part positioned between the lower gas supply part and the upper mounting plate, the gas distribution part including a plurality of inlet gas distribution channels for fluidically coupling the first fuel inlet pipe to a first subset of the plurality of through holes and a plurality of outlet gas distribution channels for fluidically coupling the first used fuel outlet pipe to a second subset of the plurality of through holes.SELECTED DRAWING: Figure 7A

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62 / 501,633, entitled "High Power Density Compact SOFC Stack," filed May 4, 2017, the entire disclosure of which is incorporated herein by reference.

[0002] STATEMENT OF GOVERNMENT RIGHTS This invention was made with government support under Award No. DE-FE0026093 awarded by the DOE. The government has certain rights in this invention.

[0003] The present disclosure relates to high temperature fuel cell stacks and electrolysis stacks, in particular to solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) stacks, and more particularly to high power density compact SOFC stacks. [Background technology]

[0004] A solid oxide fuel cell contains an electrolyte sandwiched between a cathode and an anode. Oxygen reacts with electrons at the cathode to form oxygen ions, which are conducted through the ion-conducting ceramic electrolyte to the anode. At the anode, the oxygen ions combine with available fuel (e.g., hydrogen and carbon monoxide, methane, some other hydrocarbon, or other suitable fuel) to form products (e.g., water and carbon dioxide), thereby liberating electrons and generating electricity. Such technology also operates in reverse to perform electrolysis, forming fuel gas and oxygen when supplied with appropriate reactants (e.g., water and carbon dioxide) and electrical power. In such implementations, the technology is referred to as a solid oxide electrolysis cell. The development of SOFCs has seen numerous approaches (anode, cathode, or electrolyte supports; monolithic ceramic vs. metallic interconnects; planar vs. tubular; and variations thereof). The greatest challenge to commercializing this technology is simultaneously achieving marketable price, reasonable performance, and service life. These drivers are closely related. Summary of the Invention

[0005] Embodiments described herein relate generally to electrochemical cells, such as fuel cells or electrolysis cells, and particularly to electrochemical cell stacks including corrugated interconnects interposed between and electrically coupled to adjacent electrochemical cells, the corrugations forming a plurality of fuel channels on one side and a plurality of oxidant channels on an opposing side that are fluidly isolated via a sealing member, and the interconnects configured to provide compliance to the electrochemical cell stack.

[0006] In some embodiments, the electrochemical cell unit includes a first electrochemical cell including a first oxidant electrode and a first fuel electrode, and a second electrochemical cell including a second oxidant electrode and a second fuel electrode. The interconnect is interposed between the first electrochemical cell and the second electrochemical cell. The interconnect includes an interconnect main body defining a longitudinal channel along its longitudinal axis. The interconnect main body includes a plurality of corrugations defining a plurality of fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell. Each of the plurality of fuel channels and the plurality of oxidant channels is positioned around the longitudinal channel.

[0007] In some embodiments, the fuel channel base of each of the plurality of fuel channels is in electrical contact with the second oxidant electrode, and the oxidant channel base of each of the plurality of oxidant channels is in electrical contact with the first fuel electrode. In some embodiments, the electrochemical cell unit further includes an outer seal member positioned on the outer periphery of the interconnect on the first surface and an inner seal member positioned on the inner periphery of the interconnect on the second surface around the longitudinal channel. The outer seal member fluidically seals one of the plurality of fuel channels or the plurality of oxidant channels from a volume outside the periphery, and the inner seal member fluidically seals the other of the plurality of fuel channels or the plurality of oxidant channels from the longitudinal channel. In some embodiments, the interconnect main body defines at least one fuel inlet channel and at least one fluid outlet channel fluidically coupled to each of the plurality of fuel channels, and further defines at least one oxidant inlet channel and at least one oxidant outlet channel fluidically coupled to each of the plurality of oxidant channels.

[0008] In some embodiments, the outer seal member fluidly seals the plurality of fuel channels from a volume outside the periphery, and at least one fuel inlet channel and at least one fuel outlet channel are fluidly coupled to the longitudinal channels to receive fuel from a first portion of the longitudinal channels and discharge spent fuel into a second portion of the longitudinal channels. In some embodiments, the inner seal member may fluidly seal the plurality of oxidant channels from the longitudinal channels, and at least one oxidant inlet channel and at least one oxidant outlet channel are fluidly coupled to the periphery of the interconnect to receive oxidant from a first portion of the volume outside the periphery and discharge spent oxidant from a second portion of the volume outside the periphery. In some embodiments, the electrochemical cell unit further includes an edge seal member disposed on at least one of an outer edge of each of the first and second electrochemical cells proximate the periphery of the interconnect or an inner edge of each of the first and second electrochemical cells proximate the longitudinal channels.

[0009] In some embodiments, an electrochemical cell stack includes a stack of multiple electrochemical cell units. Each of the multiple electrochemical cell units includes a first electrochemical cell including a first oxidant electrode and a first fuel electrode, a second electrochemical cell including a second oxidant electrode and a second fuel electrode, and an interconnect interposed between the first and second electrochemical cells. The interconnect includes an interconnect main body defining a longitudinal channel along its longitudinal axis. The longitudinal channel spans the height of the electrochemical cell stack. The interconnect main body includes multiple corrugations defining multiple fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and multiple oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell, each of the multiple fuel channels and the multiple oxidant channels positioned around the longitudinal channel.

[0010] In some embodiments, each of the plurality of electrochemical cell units further includes an outer seal member positioned on an outer periphery of the interconnects on the first surface and an inner seal member positioned on an inner periphery of the interconnects on the second surface around the longitudinal channel. The outer seal member fluidically isolates one of the plurality of fuel channels or the plurality of oxidant channels from a volume outside the periphery, and the inner seal member fluidically isolates the other of the plurality of fuel channels or the plurality of oxidant channels from the longitudinal channel. In some embodiments, the interconnects included in each of the plurality of electrochemical cell units cooperate to form a bellows-like structure such that the electrochemical cell stack is compliant. In some embodiments, the electrochemical cell stack further includes posts disposed within the longitudinal channel, the posts defining at least one post inlet configured to receive one of the fuel or the oxidant and at least one post outlet configured to receive and discharge the other of the spent fuel or the spent oxidant from the electrochemical cell stack, the post inlet and the post outlet being fluidically isolated from each other.

[0011] In some embodiments, the electrochemical cell stack further includes an upper end plate positioned at an upper end of the electrochemical cell stack around the posts such that a gap is provided between the upper end plate and the posts, the gap being structured to allow movement of the posts therein to relieve thermal stress. In some embodiments, the electrochemical cell stack further includes a compliant seal member positioned within the gap, the compliant seal member providing sufficient compliance to allow movement of the posts. In some embodiments, the electrochemical cell stack further includes an upper end cap positioned on the upper end plate and a secondary seal member interposed between the upper end plate and the upper end cap. In some embodiments, the upper end plate includes a post interface tube extending axially from a surface of the upper end plate away from the posts, at least a portion of the post interface tube being positioned around a portion of the post.

[0012] In some embodiments, the electrochemical cell stack further includes a lower end plate positioned on a lower end of the electrochemical cell stack opposite the upper end. An upper compression plate is positioned on the upper end plate. A biasing member is positioned proximate to the upper end of the electrochemical cell stack and configured to exert a compressive force on the stack of multiple electrochemical cell units. At least one compression member is coupled to the upper compression plate and configured to transmit the compressive force from the upper compression plate to the lower end plate. In some embodiments, the electrochemical cell stack further includes a lower compression plate positioned at a lower end of the electrochemical cell stack, and at least one compression member is coupled to the lower compression plate. In some embodiments, the biasing member includes a stack of Belleville springs interposed between the upper compression plate and the upper end plate.

[0013] In some embodiments, the electrochemical cell stack further includes a base plate assembly positioned on a lower end of the electrochemical cell stack. The base plate assembly includes a lower end plate defining at least one fuel port and at least one oxidant port. The high-strength sealing plate is axially aligned with the lower end plate and configured to yield relative to the lower end plate to reduce transfer of mechanical stress from the high-strength sealing plate to the lower end plate. In some embodiments, the high-strength sealing plate is positioned between the stack of electrochemical cell units and the lower end plate, and the base plate assembly further includes a plurality of short tubes positioned between the high-strength sealing plate and the lower end plate. In some embodiments, the lower end plate is interposed between the stack of electrochemical cell units and the high-strength sealing plate, and the base plate assembly further includes a plurality of short tubes positioned between the high-strength sealing plate and the lower end plate. The short tubes are configured to yield in response to thermal stress such that the high-strength sealing plate is free to move laterally relative to the lower end plate to reduce transfer of stress to the lower end plate.

[0014] In some embodiments, the electrochemical cell stack includes a manifold disposed around a stack of multiple electrochemical cell units. The manifold defines a volume around an outer periphery. A first portion of the volume provides an inlet for one of fuel or oxidant into the electrochemical cell stack, and a second portion of the volume provides an outlet for spent fuel or oxidant from the electrochemical cell stack. In some embodiments, the electrochemical cell stack further includes a dielectric seal member positioned within the volume and configured to fluidically seal the first portion of the volume from the second portion of the volume.

[0015] In some embodiments, an electrochemical cell assembly includes a housing including a housing base. An array of electrochemical cell stacks is disposed on the housing base within the housing. Each of the electrochemical cell stacks included in the array includes a stack of multiple electrochemical cell units. Each of the multiple electrochemical cell units includes a first electrochemical cell including a first oxidizer electrode and a first fuel electrode, a second electrochemical cell including a second oxidizer electrode and a second fuel electrode, and an interconnect interposed between the first and second electrochemical cells. The interconnect includes an interconnect main body defining a longitudinal channel along its longitudinal axis, the longitudinal channel spanning the height of the electrochemical cell stack. The interconnect main body includes a plurality of corrugations that define a plurality of fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell, each of the plurality of fuel channels and the plurality of oxidant channels being positioned around the longitudinal channel.

[0016] In some embodiments, the electrochemical cell assembly further includes a ring separator positioned around each of the electrochemical cell stacks and a cross separator positioned between each set of four electrochemical cell stacks included in the array of electrochemical cell stacks. In some embodiments, the electrochemical cell assembly further includes an oxidant preheat tube positioned between each set of four electrochemical cell stacks via a corresponding cross separator. In some embodiments, the electrochemical cell assembly further includes a fuel inlet, a fuel outlet, an oxidant inlet, and an oxidant outlet fluidly coupled to the array of electrochemical cell stacks through the housing base. The housing base defines at least one heat exchange channel configured to provide heat exchange between fuel entering the housing base through the fuel inlet and spent fuel exiting the housing base through the fuel outlet. In some embodiments, the electrochemical cell assembly further includes a fuel bypass inlet fluidly coupled to the array of electrochemical cell stacks through the housing base, the fuel bypass inlet bypassing the at least one heat exchange channel.

[0017] The foregoing is a summary of the disclosure and, as such, contains necessarily simplifications, generalizations, and omissions of detail. Consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, features, and advantages of the devices and / or processes described herein, as defined by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0018] The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings, in which it is understood that these drawings illustrate only some implementations in accordance with the present disclosure and therefore should not be considered limiting of its scope; the present disclosure will be described with additional specificity and detail through the use of the accompanying drawings.

[0019] [Figure 1A] 1 is a cross-sectional perspective view of a portion of a fuel cell stack having hermetically sealed fuel cell units, according to one embodiment.

[0020] [Figure 1B] 1B is a schematic diagram of a fuel cell unit that can be included in the electrochemical cell stack of FIG. 1A, according to one embodiment.

[0021] [Figure 2] FIG. 2 is a front view of a manufactured fuel cell stack according to one embodiment.

[0022] [Figure 3] FIG. 1 is a top schematic view of an internal fuel manifold showing potential leak paths from fuel in to fuel out.

[0023] [Figure 4A] , [Figure 4B] , [Figure 4C] 4A and 4B are schematic top views of a fuel cell unit according to one embodiment, each showing different possible flow paths for fuel and oxidant gas based on different combinations of internal and external manifold designs. FIG. 4A shows a fuel cell unit with a single fuel inlet, a single fuel outlet, a single oxidant inlet, and a single oxidant outlet. FIG. 4B shows a fuel cell unit with two fuel inlets, two fuel profiles, two oxidant inlets, and two oxidant outlets. FIG. 4C shows a fuel cell unit with a single fuel inlet, a single fuel outlet, two oxidant inlets, and two oxidant outlets.

[0024] [Figure 5] 1 is a perspective view of an array of fuel cell stacks, according to one embodiment.

[0025] [Figure 6] FIG. 6 shows a perspective view of a portion of the array shown in FIG. 5, with some fuel cell stacks removed to show the oxidant preheat tubes.

[0026] [Figure 7A] , [Figure 7B] 1A-1C are perspective views of an array of fuel cell stacks according to two different embodiments.

[0027] [Figure 7C] Based on the progressive array of fuel cell stacks shown in Figure 7A, deployment scales of 40 kW to 350 kW are shown.

[0028] [Figure 8A] , [Figure 8B] FIG. 7C is a perspective view of the base portion of the array shown in FIGS. 7A and 7B, showing the fuel and oxidant inlets and outlets of the array.

[0029] [Figure 9A] , [Figure 9B] FIG. 7C is a top view of a portion of the array shown in FIGS. 7A and 7B, showing the oxidant preheat tubes and stack attachment points.

[0030] [Figure 10] 1 is a perspective view of a fuel cell stack having an overlapping seal design as known in the art.

[0031] [Figure 11] FIG. 1 is a cross-sectional perspective view of an interconnect, according to one embodiment.

[0032] [Figure 12A] , [Figure 12B]

[0033] Figures 12A and 12B show respective top and bottom views of the interconnect shown in Figure 11. Figure 12A shows the fuel side of the top of the interconnect. Figure 12B shows the oxidant side of the bottom of the interconnect.

[0033] [Figure 13]1 is a schematic cross-sectional view of a fuel cell stack having a bellows-like structure, according to one embodiment.

[0034] [Figure 14] 1 is a photograph showing a cross section of an electrochemical cell where the edges have been sprayed to seal the edges of the electrochemical cell.

[0035] [Figure 15A] , [Figure 15B] , [Figure 15C] 1A-1C are cross-sectional top views of a portion of a fuel cell stack according to three different embodiments, showing posts placed in the longitudinal channels of the stack.

[0036] [Figure 16A] , [Figure 16B] 15A and 15C, respectively, showing the center post along with the top plate and top cap.

[0037] [Figure 17A] , [Figure 17B] , [Figure 17C] 10A-10C are bottom perspective views of three different designs of base plate assemblies, according to embodiments.

[0038] [Figure 18] FIG. 16 is a top perspective view of the main upper plate and post interface tube of the upper compression plate assembly, according to one embodiment.

[0039] [Figure 19A] , [Figure 19B] 1A-1C are top perspective views of an upper compression plate assembly according to two different embodiments.

[0040] [Figure 20A]19B is a graph showing the spring response of a Belleville spring pack that may be used in the upper compression plate assembly of FIG. 19A, according to one embodiment. [Figure 20B] 19C is a graph illustrating the creep of a coil spring that may be used in the upper compression plate assembly of FIG. 19B, according to another embodiment.

[0041] [Figure 21A] , [Figure 21B] 1A-1C are bottom perspective views of a fuel cell stack including outer manifolds according to two different embodiments.

[0042] [Figure 22] Test data is presented from tests performed using a 225 cell (~1 kW) stack operating with a gas composition representative of a typical natural gas combustion system application.

[0043] [Figure 23] 7B shows test data obtained from tests performed using a 20-cell implementation with the fuel cell stack array of FIG. 7A operating as an electrolyzer to convert steam to hydrogen.

[0044] [Figure 24] Results are shown for a 60-cell implementation of the fuel cell stack array of FIG. 7A running a variety of hydrogen fuel cell conditions, where the total test time is over one year.

[0045] [Figure 25] Results are shown for a 45-cell implementation with the fuel cell stack array of FIG. 7B operating at a fuel cell condition (power generation) of 0.25 A / cm 2 .

[0046] [Figure 26] Results are shown for a 45-cell implementation with a fuel cell stack array operating at electrolysis (hydrogen production) conditions of -1 A / cm2.

[0047] Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, like symbols typically identify like components unless context dictates otherwise. The example implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that aspects of the present disclosure, as generally described herein and illustrated in the drawings, may be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are expressly contemplated and make part of this disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0048] Embodiments described herein relate generally to electrochemical cells, such as fuel cells and electrolysis cells, and in particular to electrochemical cell stacks including corrugated interconnects interposed between and electrically coupled to adjacent electrochemical cells, the corrugations forming a plurality of fuel channels on one side and a plurality of oxidant channels on an opposing side that are fluidly isolated via a sealing member, and the interconnects configured to provide compliance to the electrochemical cell stack.

[0049] Certain embodiments provide mechanical stack layouts that represent a design approach focused on reducing material content within the stack while maintaining (and in many cases improving) the overall feasibility of the design relative to current stack technology. Certain embodiments use relatively small, typically annular, solid oxide fuel cells and thin interconnects, the integration of which results in an order of magnitude increase in power density (W / kg) over current baselines. This can be achieved through careful thermal design to ensure maximum thermal communication between the active area of ​​the cells (locations of heat generation) and the stack environment.

[0050] Efforts to lower price tend to directly shorten lifespan, as they involve strategies to operate fuel cells harder. Efforts to improve performance (higher power output) tend to reduce lifespan and efficiency. Efforts to extend lifespan often involve expensive materials and / or operation at lower power densities, both of which increase price. For example, tubular technology has been proven for extended periods (more than five years) but at price and performance levels generally recognized as unacceptable for practical market applications. Conversely, planar SOFC technology is close to achieving its price and performance goals but faces challenges in achieving its practical service life goals. The general focus to bridge this price / performance / service life gap is to develop high-performance cells while increasing their size, primarily to reduce manufacturing costs. This is evident in almost all SOFC development efforts, whether developing large-surface-area tubular designs or planar designs. An exception to this trend exists in the form of microtubular SOFC cells, which are advocated primarily by university laboratories for applications requiring rapid thermal transients. Microtubule systems have not progressed as a viable solution for large-scale systems (typically in the range of up to several hundred watts of power).

[0051] Mobile applications generally present an additional set of constraints. Current SOFC technology demonstrates volumetric and mass-exponential power densities of around 200 W / L and 100 W / kg. Therefore, a 70 kW power unit would require the stack alone to occupy ~350 L and weigh approximately 700 kg, significantly increasing the overall power system. Small vehicles, with these weights and volumes, could not accommodate SOFC-based prime power systems. A second constraint for automotive applications is heat-up time. Current stacks require approximately one hour to reach an operating temperature of approximately 750 degrees Celsius from ambient. In practical mobile applications, start-up times of a few seconds are typically expected, although start-up times of several minutes may be acceptable due to changing expectations and / or secondary power sources such as batteries to cover the first few minutes of operation.

[0052] Finally, one of the key challenges for many fuel cell technologies is managing waste heat and temperature distribution within the stack. As stack size increases, direct heat rejection to the environment becomes increasingly impractical. Instead, large stacks rely on endothermic reactions (reforming) and / or convective cooling to the gas stream. Practical experience shows that high flow rates are required to achieve convective cooling with reasonable in-stack temperature differences.

[0053] The embodiments described herein provide a different approach to meeting price, performance, and / or lifetime targets for electrochemical cells (e.g., fuel cells or electrolysis cells) while addressing key challenges: The embodiments described herein also address the weight and volume challenges posed by mobile applications of electrochemical cells, while heat-up times are expected to be on the order of minutes.

[0054] Briefly, embodiments described herein propose to reverse the current trend of incrementally increasing cell size and performance, and instead propose to reduce cell size, reduce reliance on cell performance, and focus on tight integration of components optimized for operation in small cells. Through careful integration, certain embodiments described herein can result in stacks with the same or greater power output than current stacks, at one-seventh the volume and one-tenth the weight.

[0055] Various embodiments described herein may provide advantages including, for example, the following: (1) reduction in volume per unit of power output (e.g., 7x or greater reduction) without requiring cell performance improvements, (2) reduction in weight per unit of power output (e.g., 10x or greater reduction) without requiring cell performance improvements, (3) expected corresponding cost savings (e.g., 10x or greater reduction), (4) rapid transient response (e.g., 10x faster than current electrochemical cell stacks, providing heat-up times on the order of minutes instead of hours), (5) a level of modularity supporting a power range, e.g., from 1 kW to many MW, with the same stack for both mobile and stationary applications, and (6) anode (7) significantly reduced leakage from the stack to the cathode, increasing efficiency and expanding application possibilities; (8) higher voltage and lower current output (providing greater efficiency within power electronics); (9) inherent load sharing and redundancy at larger kW ratings (e.g., 10 kW and above); (10) enabling indirect thermal management within the stack due to the short conduction distance between the stack core and stack edge, allowing heat to be dumped into the environment; and (11) reduced balancing of plant requirements due to less airflow, easier compression requirements, higher voltage / lower current power, and / or shorter transients.

[0056] For example, the embodiments described herein describe two specific sizes and implementations of the SOFC stacks described herein for which physical hardware and test results are available. These are provided as examples of applications of the embodiments described herein, but smaller, larger, and inter-size variations of these embodiments are possible as well. An important size consideration is cell size and cell count. Some embodiments described herein are based on a 21 cm 2 or 25cm 2 The present invention includes cells with active areas of up to 234 cells per stack, and has been demonstrated with other embodiments described herein. 2 It has cells with an active area of ​​1.25 mm and is designed to operate at over 350 cells per stack, and has been demonstrated with up to 45 cells per stack.

[0057] Although various embodiments described herein refer to electrochemical cell units and electrochemical cell stacks as fuel cell units and fuel cell stacks, respectively, it should be understood that various embodiments of electrochemical cell units and electrochemical cell stacks described herein may operate in reverse flow to include electrolysis cell units and electrochemical cell stacks, or any other electrochemical cell units or stacks.

[0058] FIG. 1A is a cross-sectional perspective view of a portion of the fuel cell stack 110 shown in FIG. 2, according to one embodiment. The fuel cell stack 110, according to one embodiment, includes a plurality of fuel cell units 150, more specifically, a stack of hermetically sealed fuel cell units 110. The fuel cell stack 110 includes a plurality of solid oxide fuel cell units 150 alternating with a plurality of interconnects 152. For example, FIG. 1B shows a schematic diagram of a fuel cell unit 150 that may be included in the fuel cell stack 110. Each fuel cell unit 150 may include a first electrochemical cell 154a including a first fuel electrode 153a (e.g., an anode) and a first oxidant electrode 155a (e.g., a cathode), and may include an electrolyte interposed between the first fuel electrode 153a and the first oxidant electrode 155a. The second electrochemical cell 154b may also include a second fuel electrode 153b, a second oxidizer electrode 155b, and an electrolyte interposed between the second fuel electrode 153a and the second oxidizer electrode 155a. In some embodiments, each of the anodes optionally includes an anode support. In some embodiments, the fuel cell stack 110 can be operated in reverse flow, i.e., as an electrolysis cell stack. In such embodiments, the fuel electrode 153a / b of the electrochemical cell 154a / b can include a cathode, and the oxidizer electrode 155a / b of the electrochemical cell 154a / b can include an anode.

[0059] The interconnect 152 is interposed between the first electrochemical cell 154a and the second electrochemical cell 154b. The interconnect 152 includes an interconnect main body 152a that defines a longitudinal channel 120 along its longitudinal axis (e.g., the longitudinal axis of the electrochemical cell stack 110, so that the longitudinal channel 120 may span the fuel cell stack 110). The interconnect main body 152a includes a plurality of corrugations that define a plurality of fuel channels 157 on a first surface of the interconnect main body 152a facing the first electrochemical cell 154a and a plurality of oxidant channels 159 on a second surface of the interconnect main body facing the second electrochemical cell 154b. Each of the plurality of fuel channels 157 and the plurality of oxidant channels 159 may be positioned around the longitudinal channel 120, for example, in a symmetrical and / or annular configuration. The fuel channel base of each of the plurality of fuel channels 157 may be in electrical contact with the second oxidizer electrode 155b, and the oxidizer channel base of each of the plurality of oxidizer channels 159 may be in electrical contact with the first fuel electrode 153a.

[0060] For example, the electrochemical cells 154 a / b and interconnects 152 are molded such that, when the fuel cell stack 110 is formed, the longitudinal channel 120 extends longitudinally through the fuel cell stack 110. In the embodiment of FIG. 1A , the electrochemical cells 154 a / b and interconnects 152 have an annular shape, and the longitudinal channel 120 is a central channel located at the axial center of the fuel cell stack 110. While the fuel cell stack 110 is described as having an annular shape, it may have any other suitable shape, such as oval, hexagonal, square, or non-square, or may have other shapes so long as the longitudinal channel 120 extends longitudinally through the fuel cell stack 110. Furthermore, while the longitudinal channel 120 is described as extending along the geometric center of the fuel cell stack 110, in other embodiments, the longitudinal channel 120 may be offset from the geometric center of the fuel cell stack 110, so long as the longitudinal channel 120 does not overlap the outer edges of the fuel cell stack 110.

[0061] The electrochemical cells 154a / b are alternately sealed at the inner and outer periphery to each interconnect 152, as shown in Figures 1A and 1B. This provides a hermetically sealed yet compliant structure at the unit cell level to reduce the possibility of thermal stress buildup.

[0062] Either the fuel or the oxidant gas enters and is extracted from the fuel cell unit 150 through the longitudinal channel 120, while the other gas enters and is extracted from the fuel cell unit 150 at the periphery of the fuel cell stack 110. In certain embodiments, the fuel enters and is extracted from the longitudinal channel 120, and the oxidant is received and extracted at the periphery of the fuel cell stack 110. The closed cell and interconnect seals prevent mixing of the gases. For example, as shown in FIG. 1A , an outer seal member 158 may be positioned on the periphery of the interconnect 152 on a first surface adjacent to the first electrochemical cell 154 a, and an inner seal member 156 may be positioned on the inner periphery of the interconnect 152 on a second surface adjacent to the second electrochemical cell 154 b around the longitudinal channel 120. The outer seal member 158 can fluidly seal one of the plurality of fuel channels 157 or the plurality of oxidant channels 159 from the volume outside the periphery of the fuel cell stack 110, and the inner seal member 156 can fluidly seal the other of the plurality of fuel channels 157 or the plurality of oxidant channels 159 from the longitudinal channels 120. In particular, as shown in FIG. 1A , the outer seal member 158 fluidly seals the fuel channels 157 from the volume outside the periphery, and the inner seal member 156 fluidly seals the oxidant channels 159 from the longitudinal channels 120.

[0063] Avoiding leaks that result in fuel and oxidant mixing and combustion provides several benefits, including: (i) reduced reactant losses to the system; (ii) reduced heat load on the stack (and especially localized heating that can damage stack components); (iii) reduced steam generation on the oxidant side (which reduces chromium volatilization and transport, which can be a significant degradation mechanism for the oxidant electrode); and (iv) reduced cross-leakage during heat-up and cool-down, thereby reducing protective cover gas usage.

[0064] A fuel cell stack 110 can contain, for example, 20 to 400 fuel cell units 150, limited only by the aspect ratio (height to diameter or width) of the finished stack, which can be difficult to manufacture and package if the aspect ratio is too high. In various embodiments, the aspect ratio can range from 4:1 to 5:1, although shorter stacks may be useful for certain applications and development purposes. Multiple fuel cell units 150 may be stacked vertically into a tower with intermediate metal interconnects 152.

[0065] The power output range of each fuel cell stack is approximately 50 W to 20 kW (e.g., 0.5 kW to 20 kW, 1 kW to 15 kW, or 5 kW to 10 kW, including all ranges and values ​​therebetween), depending on operating conditions and stack size. In one embodiment, the stack has a power range of approximately 7 kW. By reducing the cell count and adjusting operating conditions, practical stacks of approximately 50 W can be produced.

[0066] The fuel cell stack 110 or any other electrochemical cell stack described herein utilizes appropriate geometric arrangements to provide a cell design that improves the ability to thermally control the electrochemical cell stack while reducing mechanical stresses induced during cell fabrication and subsequent stack operation. These two benefits allow both the interconnects 152 and the cells to be thinned without compromising their structure.

[0067] The thickness of the interconnect 152 can range from 0.05 to 0.7 mm (e.g., 0.075 to 0.4 mm, or 0.08 to 0.15 mm, including all ranges and values ​​therebetween). The thickness of the electrochemical cell unit can range from 0.2 to 0.4 mm. In certain embodiments, the thickness can range from 0.25 to 0.35 mm. A stack of this design, incorporating 0.12 mm interconnects 152 and 0.3 mm cells, has been demonstrated in operation for over a year. This is roughly 1 / 10 the thickness of the interconnect material and 1 / 2 the thickness of the cells used in typical SOFC stack designs. When including the end plates, compression system, and all other components that form the complete stack, the proposed stack weight in one embodiment has been confirmed to be approximately 1 / 10 the weight of a conventional stack per active area.

[0068] In other words, the material content of the electrochemical cell stack is reduced, and this reduction is significant. This design does not require the use of specialized materials, simplifying material requirements in many areas compared to conventional electrochemical cell stacks. The compression system may be simplified due to lower loads, as described in more detail below. The manifold may also be simplified due to lower sealing requirements, as also described in more detail below. This reduction in material content reduces the inherent cost of the electrochemical cell stack. While the number of parts per kW increases, the use of smaller parts, fewer different parts per layer, and the absence of large tolerances requiring operator intervention also improves the suitability of the parts for automation. Therefore, the cost benefits of the lower material content may outweigh the overall increase in parts count.

[0069] The fuel cell stack 110 shown in Figure 2 contains 234 cells. Each fuel cell unit 150 is annular in shape, with an outer cell diameter of 60 mm, a thickness of 300 microns, and a length of 21 cm. 2The interconnects 152 had an active area of ​​100 microns. Each interconnect 152 was a stamped metal interconnect with a material thickness of 100 microns and a flow channel height of 390 microns. The fuel cell stack 110 may also include a manifold 112 positioned around the stack of multiple fuel cell units 150 and define a volume around the periphery of the fuel cell stack 110 that may be used to introduce and exhaust one of a fuel and an oxidant around the periphery of the fuel cell stack 110. For example, a first portion of the volume may provide an inlet for one of a fuel or an oxidant into the fuel cell stack 110, and a second portion of the volume may provide an outlet for spent fuel or oxidant from the fuel cell stack 110.

[0070] The embodiments described herein can reduce material content by an order of magnitude while providing many other benefits, such as reduced cost per kW at the stack and system level. Improved thermal layout can simultaneously increase performance and reduce degradation through improved temperature control. Additionally, the embodiments described herein can improve thermal control of the electrochemical cell stack, thereby allowing for lower cooling airflow and lower inlet temperatures, both of which can improve the balance of plant efficiency.

[0071] Fuel inlet / outlet seals and oxidizer inlet / outlet seals Separation of fuel in from fuel out and oxidizer in from oxidizer out is achieved through a structurally independent manifold (e.g., manifold 112) that is separate from the stack core and can be sealed via a compliant seal that is compressible and allows relative movement between the stack core and the manifold. This allows the stack core to grow and flex under thermally induced loads independently of the manifold, which prevents or reduces thermally induced mechanical stresses on the overall structure and thereby protects the individual components. For example, ceramic cells are susceptible to brittle fracture if overstressed. The compliant seal seals between the inlet and outlet of the same gas flow. In other words, the compliant seal separates the fuel inlet from the fuel outlet and the oxidizer inlet from the oxidizer outlet. Preferably, the compliant seal does not seal between the fuel and oxidizer gases anywhere. Compliant high-temperature ceramic seals are known to leak because they typically achieve compliance by being porous, contained, and connected filled ceramic structures. In the embodiments described herein, such leakage may be acceptable because it does not result in combustion and has only a minor effect on overall efficiency as long as the leakage rate is low (e.g., less than about 5% of the total flow). This allows for the advantageous use of an external manifold design approach, offering cost, weight, and volume advantages. Figure 3 shows a fuel manifold 230, e.g., a post (e.g., a center post) placed within the longitudinal channel of an electrochemical cell stack, and a representation of the resulting leakage path from fuel in to fuel out.

[0072] Separation of inlet and outlet gases around the periphery of the stack may be achieved through a sheet metal manifold (e.g., manifold 112) structure that presses a compliant seal against the stack core ("stack core" means the assembly of repeated stack components such as cells, interconnects, seals, and end plates). The metal gas isolation components may be coated with a dielectric coating to prevent the stack from shorting to the manifold.

[0073] The annular cell design minimizes the conduction path from any portion of the heat generating area of ​​the cell to the outer surface of the stack, which helps maintain thermal control of the stack.

[0074] 4A-4C are top-down schematic views of fuel cell units 250a / b / c according to various embodiments, each showing different possible flow paths for fuel and oxidant gases based on different combinations of internal and external manifold designs. In other embodiments, fuel cell units 250a / b / c may include electrochemical cell units that operate in reverse to operate as electrolysis cell units. FIG. 4A shows fuel cell unit 250a with a single fuel inlet, a single fuel outlet, a single oxidant inlet, and a single oxidant outlet. FIG. 4B shows fuel cell unit 250b with two fuel inlets, two fuel outlets, two oxidant inlets, and two oxidant outlets. FIG. 4C shows fuel cell unit 250c with a single fuel inlet, a single fuel outlet, two oxidant inlets, and two oxidant outlets. These different flow strategies provide different thermal and pressure drop profiles for the stack and can be selected to best suit a particular application.

[0075] For example, the interconnect main body (e.g., interconnect main body 152a) of the interconnect (e.g., interconnect 152) included in each of fuel cell units 250a / b / c may define at least one fuel inlet channel and at least one fluid outlet channel fluidly coupled to each of the plurality of fuel channels (e.g., fuel channel 157). The interconnect main body may further define at least one oxidant inlet channel and at least one oxidant outlet channel fluidly coupled to each of the plurality of oxidant channels (e.g., oxidant channel 159). The at least one fuel inlet channel and the at least one fuel outlet channel may be fluidly coupled to the longitudinal channels to receive fuel from a first portion of the longitudinal channels and discharge spent fuel to a second portion of the longitudinal channels. An outer seal member (e.g., outer seal member 158) may fluidly seal the plurality of fuel channels from a volume outside the periphery. Additionally, at least one oxidant inlet channel and at least one oxidant outlet channel may be fluidly coupled to the periphery of the interconnect to receive oxidant from a first portion and discharge spent oxidant from a second portion of the volume outside the periphery. An inner seal member (e.g., inner seal member 156) may fluidly seal the plurality of oxidant channels from the longitudinal channel.

[0076] Modular Array For larger systems, stacks are arranged in modular arrays, such as array 100 in FIG. 5 or array 200 in FIG. 7A for 20 kW to 250 kW or larger, or array 300 in FIG. 7B for 40 kW to 500 kW. Larger systems may be composed of multiple arrays. The stack design is particularly suited to arrayed layouts due to its integrated compression system, direct bolted connections with integrated gas connections, short conduction paths to the environment, and high-voltage, low-current output. By simplifying or eliminating stack-to-module interfaces, stacks potentially simplify the design of larger systems. Two embodiments of stack arrays based on stack 110 in FIG. 2 are described below. Stacks can be arranged into different package sizes depending on the application. Possible sizes range from a single stack (~1.2 kW) to a 15x15 array of stacks (250 kW or larger). As an example, a 10x10, 100kW package measures approximately 0.6mx0.6mx0.3m (113L) including compression, current collection and ducting, which is competitive with an internal combustion engine.

[0077] 5 is a perspective view of an array 100 of fuel cell stacks 110, according to one embodiment. In this embodiment, fuel is supplied and extracted from the base of the fuel cell stack 110, while an oxidant gas (e.g., air) is contained in a containment volume above the stack and extracted from the base of the fuel cell stack 110. Air generally has a high convective heat capacity and may be used as the primary means of cooling the electrochemical cell stack 110. The air enters the top of the fuel cell stack 110 (or stack array) at a relatively low temperature and cools the area above the fuel cell stack 110. Spring compression and current collection may be integrated in this region, where the lower temperature allows for the use of specialized materials and / or fewer overall materials while maintaining adequate strength and current capacity.

[0078] In some embodiments, air is heated to an appropriate inlet temperature while flowing from an upper, cold region through oxidant preheat tubes 116 or inlet tubes into the volume surrounding the stacks 110, as shown in FIG. 6 . The sealing between the upper and stack zones may not be perfect, greatly simplifying the overall layout. FIG. 6 shows that the separation between the upper cooling zone and the lower stack zone is achieved by overlapping separators attached to the individual fuel cell stacks 110. In the embodiment shown in FIG. 6 , there are two separator types: ring separators 114 around each stack and cross separators 115 located between each group of four fuel cell stacks 110. For example, ring separators 114 may be positioned around each of the fuel cell stacks 110 included in the array 100, and cross separators 115 may be positioned between each set of four fuel cell stacks 110 included in the array 100. The oxidant preheat tubes 116 may be positioned through the corresponding cross separators 115.

[0079] The separators 114, 115 overlap to provide a barrier that preferentially directs gases toward the oxidant preheat tube 116. This overlapping separator geometry maintains complete freedom for the fuel cell stack 110 to sway under thermal loads without adding lateral loads to the fuel cell stack 110 or breaking down the separation between zones. The oxidant preheat tube 116 acts as a radiant heat transfer surface, using radiation from the high-temperature fuel cell stack 110 to heat the inlet air before bringing the oxidant (e.g., air) into direct contact with the stack air manifold. The air inlet to the fuel cell stack 110 can be an opening in the air manifold along the entire vertical surface, where air preheated first by the upper zone, second by the oxidant preheat tube 116, and third by direct contact with the stack manifold can finally properly enter the fuel cell stack 110. Stack cooling, a major challenge for large SOFC stacks, can be achieved by heating the oxidant stream. Unlike direct convection cooling, the multi-stage inlet approach allows for a much larger temperature rise than would be possible if the oxidant (e.g., air) were admitted directly to the fuel cell stack 110 core. With proper sizing, inlet temperatures on the order of 200 degrees Celsius (e.g., 150-250 degrees Celsius) can be achieved, compared to 600 degrees Celsius for conventional stacks. This larger temperature delta tolerance allows for less airflow, less preheat load, and simplifies and improves the efficiency of balancing components in electrochemical cell assemblies (e.g., fuel cell assemblies or electrolysis cell assemblies) that include the array 100.

[0080] Each fuel cell stack 110 may be self-contained except for the air inlet duct and exterior insulation. The packaging solutions described herein provide efficiency by sharing the air inlet and exterior insulation shell among multiple fuel cell stacks 110. In some embodiments, a fuel cell assembly (e.g., fuel cell assembly 20 of FIG. 7A ) may include the following repeating units: (1) unit cell (cells + interconnects): ∼8 W, ∼0.8 V; (2) fuel cell stack (hundreds of cells + manifolds, constrictions, etc.): ∼1200 W, 160 V to 250 V; (3) array (variable, up to 200 or more stacks, enclosures, insulation, etc.): ∼20 to 250+ kW, kV range; and (4) module (variable, road transportable array structure): 1 MW+, kV range. In other embodiments, a fuel cell assembly (e.g., fuel cell assembly 40 of FIG. 7B ) may include the following repeating units: (1) unit cell (cells + interconnects): ∼8 W, ∼0.8 V; (2) fuel cell stack (hundreds of cells + manifolds, constrictions, etc.): ∼1200 W, 160 V to 250 V; (3) array (variable, up to 200 or more stacks, enclosures, insulation, etc.): ∼20 to 250+ kW, kV range; and (4) module (variable, road transportable array structure): 1 MW+, kV range. (1) Unit cell (cells + interconnects): ~20W, ~0.8V, (2) Stack (hundreds of cells + manifold, compression, etc.): 7,000W, 160V-350V, (3) 40-350+kW, kV range, (4) Module (variable, road transportable array structure): 1MW+, kV range.

[0081] For high-power implementations (~10 kW and above), the modular approach offers additional advantages. First, stack voltages are high enough that they can be connected in parallel or series-parallel electrical configurations. This provides automatic load shedding; some stacks that underperform automatically shed their current load to electrically parallel stacks. The complete loss of a stack in a large, multi-stack array has few ill effects. Second, a failed stack can be replaced relatively inexpensively without affecting the other stacks. In conventional systems with relatively few stacks, a single vulnerability may require the removal and refurbishment of a large stack, which can be difficult to manage, but in an array of smaller stacks, localized weaknesses can be corrected by replacing only the weak stack, a smaller device, and a faster, less expensive process.

[0082] 7A and 7B are perspective views of electrochemical cell assemblies including an array of electrochemical cell stacks according to two different embodiments. The embodiments are similar to those of FIGS. 5 and 6, except that all gas services, including inlet air, are supplied from the bottom. This reduces the complexity at the top of the stack array, which may provide advantages for initial assembly and service. It also provides advantages in terms of ease of integration into a system and provides the potential for additional heat transfer between the incoming and outgoing airflows. As described herein, the electrochemical cell assemblies of FIGS. 7A and 7B include fuel cell assemblies having an array of fuel cell stacks. In other embodiments, the electrochemical cell assemblies of FIGS. 7A and 7B may be operated in reverse flow to operate as electrolysis cell assemblies including an array of electrolysis cell stacks.

[0083] FIG. 7A illustrates a fuel cell assembly 20 according to one embodiment. The fuel cell assembly 20 includes a housing 22 having a housing base 30. An array 200 of fuel cell stacks (e.g., electrochemical cell stacks 110) is positioned on the housing base 30. The array 200 includes a 6×6 array of fuel cell stacks (a 40+ kW array) with all gas services supplied from the bottom. FIG. 7B illustrates a fuel cell assembly 40 according to another embodiment. The fuel cell assembly 40 includes a housing 42 having a housing base 50 in which an array 300 of fuel cell stacks (e.g., fuel cell stacks 110) is positioned. The array 300 includes an 8×5 array (a 280+ kW array) with all gas services supplied from the bottom. In these layouts, the housing bases 30, 50 incorporate heat exchange functions to distribute and collect gases evenly to all fuel cell stacks. FIG. 8A is a perspective view of the base portion of the fuel cell assembly 20 shown in FIG. 7A, showing the fuel and oxidant inlets and outlets for the array. On the left side of FIG. 7A , two stacks are omitted so that two of the oxidant preheat tubes can be seen. As shown in FIG. 8A , the electrochemical cell assembly 20 includes a fuel inlet 22, a fuel outlet 24, an oxidant inlet 26, and an oxidant outlet 28 fluidly coupled to the array 200 of electrochemical cell stacks through a housing base 30. The housing base 30 also defines at least one heat exchange channel 34 configured to provide heat exchange between fuel entering the housing base 30 through the fuel inlet 22 and spent fuel exiting the housing base 30 through the fuel outlet 24. A plurality of stack interfaces 32 (e.g., through-holes) for communicating fuel and oxidant between the array 200 and the housing base 30, as well as one or more gas distribution channels 36, may also be provided within the housing base 30. Additionally, a fuel bypass inlet 29 is fluidly coupled to the array 200 of electrochemical cell stacks through the housing base 32 such that the fuel bypass inlet 29 bypasses the at least one heat exchange channel.Thus, fuel inlet 22 and fuel bypass inlet 29 provide dual fuel inlets, with fuel bypass inlet 29 going straight to the array 200 of fuel cell stacks and fuel inlet 22 traveling through the heat exchange and reforming section. These dual inlets provide optional, but additional, controllability of stack inlet temperature and in-stack reforming.

[0084] Figure 7C illustrates the flexibility of array size while maintaining overall modular simplicity, showing conceptual arrays from 40 kW to 350 kW based on the array of fuel cell stacks 300 shown in Figure 7B.

[0085] 8B is a perspective view of the base portion of the fuel cell assembly 40 shown in FIG. 7B, showing the fuel inlet 42, fuel outlet 44, oxidant inlet 46, and oxidant outlet 48 fluidly coupled to the array 300. In these embodiments, the upper portion 56 of the housing base 50 takes in fuel and exhausts fuel heat exchange, and may also include a fuel reforming section. Multiple stack interfaces 52 are also provided in the housing base 50. The fuel cell assembly 40 shown in FIG. 7B does not have a dual inlet.

[0086] In these embodiments, the cold air inlet enters the stack hot zone from the bottom. Figure 9A is a top view of a portion of the fuel cell stack 200 shown in Figure 7A, showing the oxidant preheat tube 216 and stack mounting interface 32. Figure 9B is a top view of a portion of the fuel cell stack 300 shown in Figure 7B, showing the oxidant preheat tube 316, fuel preheat tube 318, and stack mounting interface 52. As seen in Figures 9A and 9B, the arrays 200, 300 in these embodiments also include the oxidant preheat tube 316, which acts as a radiative surface to absorb heat from the fuel cell stack and use it to preheat the incoming gas. However, in these embodiments, the oxidant preheat tube 316 leads to the hot zone (rather than from the top down the array). In these embodiments of the array, the only connections from the top are the top current collection connections. These are relatively simple connections because the currents flowing in each stack are small (typically less than 30 A for fuel cell operation and typically less than 150 A for electrolysis operation).

[0087] Interconnect Design When designing for small cells, one of the challenges is sealing. Given a seal with some characteristic leakage that is proportional to the seal area and inversely proportional to the seal thickness in the leakage direction, larger cells are advantageous when designed to minimize leakage. First, the ratio of the cell's active area to its edge length (sealed length) roughly scales with the cell size.

number

number

[0088] These constraints mean that narrow seals with low leakage rates are desirable to support high-performance, low-leakage stacks based around small cells. This type of sealing exists in the form of glass-ceramic seals, brazed joints, or welded joints. One drawback of such seals is that they are not compliant. Compliance may be desirable in SOFC stacks because they operate over a wide temperature range and stack components move relative to each other during heat-ups or changes in operating conditions to prevent damaging stress buildup. Some stack designs build compliant features into the sheet-metal interconnects to allow for rigid joints, but the compliant features themselves are relatively bulky and suffer from the same problems as wide seals. They are space-inefficient in small cell designs.

[0089] Figure 10 is a perspective view of a fuel cell stack with an overlapping seal design known in the art, illustrating typical compliance challenges in a cross-flow stack configuration. For purposes of discussion, assume fuel flows from bottom left to top right and oxidant flows from bottom right to top left. Not shown is a manifold separating the fuel from the oxidant outside the stack.

[0090] The top visible seal is the fuel seal, which separates the fuel passing through the cell from the oxidizer against the bottom right face of the stack. Directly below the top cell is the oxidizer seal along the left edge. This separates the oxygen flowing through the cell from the fuel exposed at the left edge. The pattern is repeated throughout the stack, which may contain hundreds of cell layers.

[0091] Compliance issues arise where seals overlap, as in the foreground of Figure 10. The structure in the center foreground is made of repeated layers of seal, cell, seal, and interconnect. There is no ability to absorb strain (X, Y, or Z) in this area unless one or more of the components yields. When the goal is to use non-compliant hermetic or near-hermetic seals, the cells are often the weakest component of the structure. In this case, as stresses build up, the cells will most likely fail before yielding. This type of structure is not robust to real-world conditions.

[0092] To alleviate the inevitable thermal stresses, compliance must be built into the stack design. There are two primary approaches to incorporating compliance. In the first approach, seals are made compliant, often as filled fiber / powder ceramic composites or plate-like materials (e.g., mica) that can move between components to relieve stress. These seals inevitably leak as a result of their construction, limiting how small the cells can be before leakage begins to dominate performance. In the second approach, special interconnects or additional components are used that incorporate compliance features. This is done, for example, by rigidly sealing a thin metal sheet component to the cell, effectively stretching the perimeter of the cell, and then laser welding this cell-stretching component to the interconnect. In this strategy, there is a perimeter seal and an internal port seal that are spaced apart from both the perimeter and the cell junction. For example, the perimeter seal may contain the fuel, and the port seal may contain the oxidizer. The additional perimeter required for this compliant section means that small cell sizes are not preferred.

[0093] In contrast, in the embodiments described herein, the interconnects and overall structure directly provide the desired compliance, but without compromising the sealing properties and adding extra components or space to the design. This design allows for the use of narrow, rigid seals, achieving a compliant bellows-like structure without increasing the size of the structure. This combination of good sealing in a small area and a compliant structure allows for the effective use of small cells.

[0094] A compliant interconnect design can have many competing design constraints. First, it may be desirable for the interconnect to: (1) provide controlled flow distribution to both the cell fuel and cell oxidizer electrodes, (2) provide compliance to absorb stresses, (3) provide adequate fuel and oxidizer pressure drop, (4) provide adequate current conduction paths, and (5) isolate the fuel flow from the oxidizer flow throughout the life of the stack.

[0095] FIG. 11 is a cross-sectional perspective view of an interconnect 452, according to one embodiment. The interconnect 452 includes an interconnect main body 452a defining a plurality of fuel channels 457 and a plurality of oxidant channels 459. In FIG. 11, the fuel side is at the top. The fuel channels 457 simultaneously form an electrical contact area to the cell oxidant electrode. The oxidant channels 459 simultaneously form an electrical contact area to the cell fuel electrode. The fuel channels 457 are separated by ribs on the top surface of the interconnect 452, while the oxidant channels 459 are separated by ribs on the bottom side of the interconnect 452. That is, the fuel-side ribs form the oxidant channels 459, and vice versa. FIGS. 12A and 12B are top and bottom views, respectively, of the interconnect 452 shown in FIG. 11, depicting the longitudinal channel 420 that runs through its geometric center. FIG. 12A shows the upper fuel side of the interconnect, showing fuel inlet channels 463 (or fuel outlet channels, depending on the direction of flow) fluidly coupled to each of the fuel channels 457. FIG. 12B shows the lower oxidant side of the interconnect, showing oxidant inlet channels 465 (or oxidant outlet channels, depending on the direction of flow). FIGS. 12A and 12B show the effective active area served by each of the channels 457 on the fuel side (FIG. 12A) and the channels 459 on the oxidant side (FIG. 12B) of the interconnect 452. The flat semicircular area superimposed on the interconnect 452 represents the cell active area exposed to each interconnect channel 457, 459. The active area is a function of both the position and size of each channel 457, 459. The interconnect 452 is designed to provide flow down each channel 457, 459, proportional to the active area served by that channel 457, 459. This is achieved with respect to size and spacing constraints that provide adequate current collection from both cell electrodes. Some modifications to the geometry affect the flow and electrical properties on both sides of the interconnect 452. Optionally, a contact interlayer may be added between each cell and each interconnect 452 to aid in electrical contact.

[0096] 12A and 12B, the outer seal member 458 is a fuel seal member, which in this example is on the outer periphery (FIG. 12A). In this example, the inner seal member 456 is an oxidant seal member, which is on the inner periphery around the longitudinal channel 420 of the fuel cell stack (FIG. 12B). The separation of the fuel seal member from the oxidant seal member in space and the corrugated interconnect design provide the necessary compliance without increasing the perimeter or thickness of the stack. Of course, in embodiments in which oxidant flows through the longitudinal channel 420, the inner seal member 456 acts as a fuel seal, while the outer seal member 458 acts as an oxidant seal.

[0097] The base material of the interconnects 452 is on the order of 0.1 mm thick (e.g., 0.07 to 0.13 mm thick). This allows the stack temperature to be well controlled because the active area of ​​the cells is small and the distance from any point on the cell to its edge is relatively small. For larger stacks, or greater distances, the thickness of the interconnects must be increased to provide sufficient thermal conductivity to maintain control of the stack and cell temperatures.

[0098] Internal Seal Design The internal seals between the interconnects 452 and the cells, separating the fuel gas from the oxidant gas, may be implemented as glass-ceramic seals within the electrochemical cell stacks included in the arrays 100, 200, or 300. Their location may alternate between the inner and outer diameters to create a bellows-like structure, as can be seen in the schematic cross-section of FIG. 13, which shows a schematic diagram of an electrochemical cell stack including the interconnects 452 and the fuel cell stack 410 compressed between the top plate 440 and the bottom plate 460. In other words, the multiple interconnects 452 included in the fuel cell stack 410 cooperate to form a bellows-like structure, making the fuel cell stack 410 compliant. The interconnects 452 may be as thin as 0.1 mm. The thin material, combined with the flow-field-generating corrugations, creates interconnects 452 that easily relieve stress within their layers. This creates a robust structure that prevents layer-to-layer stress buildup. 13, preferably no additional separators or metal components are used to provide stress relief. That is, the bellows-like structure of the fuel cell stack 410 is made up of alternating cells 454, outer seals 458, interconnects 452, and inner seals 456.

[0099] In addition to the inner seal 456 and the outer seal 458, an edge seal member 461 may be disposed on at least one of the outer edges of the electrochemical cells 454 (e.g., each of the electrochemical cell units, e.g., the first electrochemical cell and the second electrochemical cell, comprising a fuel cell unit or an electrolysis cell unit) adjacent the periphery of the interconnect 452 or the inner edges of the electrochemical cells 454 adjacent the longitudinal channels 420. For example, the edges of the cell anode supports are typically porous. In the embodiment shown in FIG. 13, the edge seal member 461 is disposed on the outer edges of the electrochemical cells 454 to provide additional sealing between the fuel gas and the oxidant gas. FIG. 14 is a photograph showing a cross section of a fuel cell that may correspond to a sealed electrochemical cell (e.g., electrochemical cells 154a / b shown in FIG. 1B).

[0100] Post Design Posts can be used as manifolds for gas (either fuel or oxidant) passing through the longitudinal channels to the electrochemical cells. The posts may be placed in the longitudinal channels and configured to separate the gas inlet from the longitudinal channels into the electrochemical cells from the gas outlet from the electrochemical cells into the longitudinal channels. The posts are sealed in place with a ceramic slurry, paste, batt, or combination thereof to provide a compliant seal between the inlet and outlet flows. The posts can be machined metal, multi-piece sheet metal, brazed, or ceramic, with the ability to form vertical channels into which compliant sealing material is added.

[0101] 15A-15C are top cross-sectional views of a portion of a fuel cell stack 510 according to three different embodiments, illustrating various posts positioned in the longitudinal channels 520 of the stack 510. In these embodiments, the longitudinal channels 520 are central channels extending along the axial center of the stack 510, and thus, the posts within the channels are referred to as "central posts." However, in other embodiments, the posts may be positioned in channels offset from the center of the stack 510. This embodiment also assumes that fuel gas passes through the longitudinal channels 420. FIG. 15A illustrates a circular post 530a according to one embodiment. The post 530 defines a deep groove positioned axisymmetrically about its periphery to define a post inlet 532a configured to receive fuel and a post outlet 534a configured to receive and discharge spent fuel into the electrochemical cell stack 510. The post inlet 532a and the post outlet 534a are fluidly isolated from each other via a sealing recess 536a. Figure 15B shows another embodiment of a post 530b disposed in longitudinal channel 520. Post 530b includes two parallel plates that divide longitudinal channel 520 into a post inlet 532b, a post outlet 534b, and a seal recess 536 that fluidly isolates post inlet 532b from post outlet 534b. In the embodiment of Figures 15A and 15B, post 530a / b, which is the central post, includes one fuel inlet port and one fuel outlet port.

[0102] FIG. 15C shows a post assembly 530c including two fuel inlet plates 531c positioned opposite each other. Two fuel outlet plates 537c are positioned perpendicular to the fuel inlet plate 531c to define two post outlets 534c facing each other. In the embodiment of FIG. 15C, the post 534c includes a central channel 533c, e.g., a fuel inlet port separated from two side post channels 532c (e.g., side fuel ports) by a fuel inlet plate 531c having multiple openings 535c. Fuel flows into the central channel 533c and then into the side post channels 532c through the openings 535c. The central post assembly 530c in this embodiment has two post outlets 534c including fuel outlet ports. A seal member 539c, such as a ceramic caulking material, is inserted into a seal recess 536c to separate the inlet fuel from the outlet fuel. This seal does not need to be hermetic, as a leak path would not result in fuel-air coupling. Rather, the effect of a leak past this seal is to reduce fuel flow through the stack itself. Moderate leaks (up to a few percent of the total flow) do not significantly affect stack performance. Because solid oxide fuel cell systems typically operate with excess fuel to sweep reaction products (e.g., H2O, CO2) from the fuel electrodes, even moderate leaks may not have a noticeable effect on system performance.

[0103] The seal member 539c material used in post 530c may be designed to be somewhat compliant to allow thermal stresses to be dissipated within the structure. As a result of the compliance requirement, the seal member 539c is not tightly bonded to or sealed to the cell layer. However, it can be designed to be compliant and have sufficiently low leakage so that fuel preferentially flows around the interconnect rather than leaking through the seal.

[0104] 16A and 16B are cross-sectional perspective views of the upper portion of the fuel cell stack 510 of FIGS. 15A and 15C, respectively, showing the central posts 530a, 530c along with the top plates 540a / b and top caps 542a / b. Direct bonding of the top and bottom plates of the stack could introduce unwanted thermal stresses. To prevent this, the top (and optionally bottom) joints are structured as compliant fits. A sealant (e.g., the same sealant used on the sides of the post) is provided in the gap between the central post and the top and / or bottom plates. The allowable movement between the central post and the top plate does not need to be large, as long as it is sufficient to alleviate thermal stresses. The calculated magnitude of the desired relative movement is:

number

[0105] 16A and 16B, a top plate 540 a / c is positioned at the top of the fuel cell stack 510 around the posts 530 a / c, thereby providing a gap 541 a / c between the top end plate 540 a / c and the posts 530 a / c. The gap 541 a / c may be structured to allow movement of the posts 530 a / c therein to relieve thermal stress. In some embodiments, a compliant seal member can be positioned within the gap 541 a / c. For example, FIG. 16B shows a compliant seal member 543 c positioned in the gap 541 a / c across the post assembly 530 c. The compliant seal member 543 c can be configured to provide sufficient compliance to allow movement of the posts 530 c within the gap 541 a / c. An upper end cap 542 a / c may be positioned on the top end plate 540 a / c to close the stack, for example. Secondary seal members 544a / c may be interposed between the top end plates 540a / c and the top end caps 542a / c.

[0106] The compliant seal members 543a / c between the central posts 530a / c and the top and / or bottom plates 540a / c may leak. Secondary seal members 544a and top end caps 542a / c are added above the posts 530a / c. Compliance from the secondary seal members 544a may be undesirable, so stiffness can prevent leakage.

[0107] End Plate Design The electrochemical cell stacks (e.g., fuel cell stacks or electrolysis cell stacks) described herein may also include a lower end plate in addition to an upper end plate (e.g., upper end plates 540a / c). The lower end plate mechanically supports the stack and provides gas connections for the reactants (fuel and oxidant gas). The lower end plate provides a sealing surface for other seals in the fuel stack and / or array interface, as well as a sealing surface for the stack outer manifold (e.g., oxidant manifold) and posts. The lower end plate provides mounting clearance and isolates the stack core (cells, interconnects, and seals) from stresses generated at the sealing surfaces and bolt locations. The lower end plate also transfers compressive loads from the compression system to the stack. Additionally, the lower end plate may serve as one of the electrical connection points for the stack.

[0108] 17A-17C are bottom perspective views of three different designs of base plate assemblies 660a / b / c, according to embodiments. Each of the base plate assemblies 660a / b / c includes a lower end plate 662a / b / c defining at least one fuel port 666a / b / c and at least one oxidant port 668a / b / c, and a high-strength sealing plate 664a / b / c. The high-strength sealing plate 664a / b / c is axially aligned with the lower end plate 662a / b / c and configured to yield relative to the lower end plate 662a / b / c to reduce the transfer of mechanical stress from the high-strength sealing plate 664a / b / c to the lower end plate 662a / b / c. The bottom end plate 662a / b / c may be provided with a plurality of attachment points 665a / b / c, which allow for coupling of an electrochemical cell stack (e.g., a fuel cell or electrolysis cell stack) to the bottom end plate 662a / b / c.

[0109] In the embodiment of FIGS. 17A and 17B, high-strength sealing plates 664a / b are top-tier with high-strength sealing surfaces (e.g., made from a high-strength superalloy such as Haynes 230). This provides the necessary surface strength for interfacing with the compression sealing members. However, the thermal expansion coefficient of high-strength superalloys is typically higher than the balance of stack components allows. Therefore, separation between the high-strength sealing surfaces and the rest of the stack is provided. FIG. 17A shows a base plate assembly 660a in which high-strength sealing plate 664a is separated from the bottom end plate 662a by a short tube 663a designed to yield under imposed thermal stresses and limit the transfer of thermal stresses into the stack.

[0110] FIG. 17B illustrates a base plate assembly 660b in which the high-strength sealing plate 664b includes a yield point, resulting in the high-strength sealing plate 660b not being strong enough to transfer stresses through the lower end plate 662b and into the stack. In the embodiment shown in FIGS. 17A and 17B, one oxidizer port is routed through the lower end plates 662a / b, as are two fuel ports. In embodiments in which oxidizer, rather than fuel, is provided through the posts, the lower end plate may instead include one fuel port and two oxidizer ports. Threaded members are introduced to allow attachment of the lower end plate. The threaded members are isolated within the design to prevent stress transfer to the stack structure. The internal structure of the end plate is designed to carry loads from the compression system to the stack. This design has been demonstrated to have current collection losses of less than 0.1% at the connection between the lower end plate and the system. The upper end plate provides a sealing surface for the oxidizer manifold and posts. It also transfers compression loads from the compression system into the stack and isolates the stack core (cells, interconnects, seals) from stresses induced by the compression system. The top end plate provides a compliant sliding joint for the top of the post. The top end plate may also serve as one of the electrical connection points for the stack.

[0111] 17C illustrates a base plate assembly 660c in which a high-strength sealing plate 665c is mechanically separated but contained within a bottom end plate 662c, thereby providing the necessary strength to achieve sealing between the stack and its attached manifold without thermally stressing the stack structure due to a coefficient of thermal expansion (CTE) mismatch. In other words, the bottom end plate 662c is interposed between a stack of multiple fuel cell units and a high-strength sealing plate 664c. Multiple attachment points 665c may be provided on the high-strength sealing plate 664c. The high-strength sealing plate 664c is free to move laterally relative to the bottom end plate 662a to reduce stress transfer thereto, yet captures a portion of the bottom end plate 662c between itself and the mating manifold system when bolted to its mating manifold within the system. Thus, mechanical strength for sealing comes from the high-strength sealing plate 664c, while gas transport from the mating manifold into the stack occurs through the mechanically isolated and thermally matched bottom end plate 662c. Friction between the high-strength sealing plate 664c and the trapped, low-strength, low-CTE bottom end plate 662c may be partially mitigated by a ceramic release layer, but the trapped portion of the bottom end plate 662c is further isolated from the high-strength sealing plate 664c by a plurality of low-yield tubes (e.g., short, low-strength tubes) positioned adjacent to the high-strength sealing plate 664c and the bottom end plate 662c, thereby minimizing the possibility of transferring thermal expansion stresses to the stack structure.

[0112] 18 is a top perspective view of an upper end plate 740 including post interface tubes 748 that extend axially from a surface of the upper end plate 740, away from the posts, when the upper end plate is positioned on an electrochemical cell stack (e.g., a fuel cell stack or an electrolysis cell stack). According to some embodiments, the upper end plate 740 may be included in an upper compression plate assembly. At least a portion of the post interface tubes 748 may be positioned around a portion of the posts (e.g., the central post). The post interface tubes 748 can provide a sliding surface to allow slight relative movement between the posts and the upper end plate 740.

[0113] In some embodiments, the stack includes an integrated compression system, which allows for easy integration of the stack into an array. One advantage of the small cell area and glass-ceramic seal is that the compression load can be relatively small. For example, the stack shown in FIG. 2 used in the array 200 of FIG. 7A is designed to operate at 3.5 kgf to 9 kgf (34 N to 88 N), while another stack used in the array 300 of FIG. 7B is designed to operate at a known 121 cm 2 and 550 cm 2 The stack is designed to operate at 9 kgf to 36 kgf, compared to 360 kgf and 900 kgf, respectively, for the previous stack. This simplifies both the design of the compression elements and the design of the compression plate. First, consider the stiffness requirements of the compression plate at a high level. The maximum deflection of a uniformly loaded 2D beam with pinned end connections is given by:

number

number

[0114] Keeping in mind that this is only a comparison of orders of magnitude, the above formula can be used to compare the stiffness requirements of a compression plate to the conventional 550cm 2 The calculations can be compared to the stiffness requirements of a stack of, for example, 25 cm 2 This suggests that a fuel cell stack having an active area of ​​10,000 times less stiff while providing the same maximum deflection, while a fuel cell stack included in the array 300 of FIG. 7B, for example, can be about 440 times less stiff for the same maximum deflection. This allows for a significant simplification of the compression plate design. Note that the maximum allowable deflection is essentially independent of cell size, since deflection results in loss of electrical contact.

[0115] 19A and 19B are top perspective views of upper compression plate assemblies 870 a / b according to two different embodiments. Each of the upper compression plate assemblies 870 a / b may be positioned at an upper end of an electrochemical cell stack and base plate assembly (e.g., base plate assembly 660 a / b / c), or a lower end plate (e.g., lower end plate 662 a / b / c) may be positioned at a lower end of the electrochemical cell stack opposite the upper end. The upper compression plate assemblies 870 a / b include upper end plates 840 a / b and upper compression plates 872 a / b positioned on the upper end plates 840 a / b. The bias members 876 a / b are positioned proximate the upper end of the electrochemical cell stack and are configured to exert a compressive force on the stack of multiple electrochemical cell units. Further, at least one compression member 879a / b is configured to couple the compression plate 872a / b to a base plate assembly, e.g., a lower compression plate such as a high-strength sealing plate of the base plate assembly, and to transmit a compressive force from the upper compression plate 872a / b to the base plate assembly.

[0116] 19A is a top perspective view of an upper compression plate assembly 870 including an upper end plate 840a and post interface tube 848a (e.g., upper end plate 740 of FIG. 18 ), a base member 876a (e.g., spring pack), and a compression member 879a, along with an upper compression plate 872a, according to an embodiment deployed in a fuel cell stack included in array 200 of FIG. 7A . As previously described herein, the upper end plate 840 is positioned around the post 830a such that a gap exists therebetween. A compliant sealing member 843a is positioned in the gap above the post 830a. In this example, the bias member 876a is a stack of high-temperature Belleville springs interposed between the upper compression plate 872a and the upper end plate 840a. Other embodiments can use coil springs or wave washers of various forms. The bias member 876a generates a compressive force to compress the stack. Two compression members 879a (e.g., tension rods) carry force from the top to the bottom of the stack, where a compression plate (not shown) transfers the load to the bottom end plate. The individual Belleville springs in bias member 876a are separated by guide shims that align and guide the springs, preventing lateral drift and over-compression of the Belleville springs. The Belleville springs are designed for low stress at operating temperatures. Compression members 879a are made of a superalloy with high strength at temperatures. They have a higher coefficient of thermal expansion than the stack, which has the effect of slightly relaxing the spring pack as it heats up.

[0117] The upper compression plate assembly 870b of Figure 19B is similar to that of Figure 19A, except that the bias member 876b of the compression plate assembly 870b of Figure 19B includes multiple coil spring sets (eight in this embodiment) rather than Belleville spring packs. Additionally, the posts 830b have compliant sealing members 843b positioned thereon, which may be similar to the post assembly 530c described with respect to Figure 15C. The upper compression plate assembly 870b was implemented in the fuel cell stack included in the array 300 of Figure 7B.

[0118] FIG. 20A is a graph showing the spring response of a Belleville spring pack that may be used in the upper compression plate assembly 870a of FIG. 19A according to one embodiment. FIG. 20B is a graph showing the creep of a coil spring that may be used in the upper compression plate assembly of FIG. 19B according to another embodiment. The difference in stack growth relative to the tension rod has the effect of unloading the stack during temperature rise. This is by design, with the side benefit of providing more stack compression during shipping when temperatures are lower and material creep is not an issue. The geometry is selected so that compression is relaxed to the desired compression during operation. The circle represents the peak force point, which is also where the spring becomes unstable. If pushed to this limit, the spring risks reversing itself on the left side of the force-displacement graph. When this occurs, the spring no longer provides a useful load to the stack and cannot be recovered without disassembly.

[0119] Therefore, when the stack is cold, the design limits displacement to below the top diamond point (approximately 97 Newtons). When the stack heats up, differential thermal expansion allows the spring set to relax to the lower diamond point, where the stack is loaded to 82 Newtons and the spring stress drops to less than 50 MPa, within the creep limit of certain superalloys (e.g., Waspaloy, Haynes 282). The "+" points represent the compressive load remaining on the stack after 0.5% creep of the tension rods. After this creep, the load remains at an acceptable 18 Newtons, at which point the peak stress in the spring drops to 10 MPa. This unloading of the springs and compression members slows the net creep rate of the entire system. To maximize the spring properties, higher stresses are permitted to be experienced at room temperature, where material creep is not an issue. The figures provided illustrate a specific case. In general, design strategies that consider cold versus hot conditions, and system creep during operation, apply to all designs, but the details will depend on the goals and requirements of a particular stack.

[0120] Similarly, the coil spring in Figure 19B has a lower design load and is able to relax to their high-temperature stress target during operation. Figure 20B shows testing of a sample coil spring over 1.5 years at operating temperatures in pure creep. The desired operating range for this spring is 9 kgf to 36 kgf. The test shows that some creep occurs at 29 kgf, but when the load is reduced to 21 kgf, the spring is stable against further creep. These results validate the high-temperature spring design against the target requirements.

[0121] Manifold Design An outer manifold connects an oxidant port from each unit cell to the base plate, where oxidant can be routed to (or from) an oxidant connection adjacent to the fuel connection. The opposite face of the stack remains open to the environment, allowing oxidant to flow directly to (or from) all cells. For example, Figure 21A (fuel cell stack 110 and a fuel cell stack included in array 200) and Figure 21B (electrochemical cell stack of array 300) are bottom perspective views of electrochemical cell stacks 910a / b (e.g., fuel cell stacks or electrolysis cell stacks) including outer manifolds 912a / b.

[0122] FIG. 21A shows a configuration with two inlets (front and rear) and two outlets (left and right sides, routed to a base connection point). A lower end plate 962a is positioned at the base of the electrochemical cell stack 910a, and a lower compression plate 982a is positioned below the lower end plate 962a. A compression member 979a transfers a compression force from the upper compression plate to the lower compression plate 982a, thereby biasing the lower compression plate 982a, which presses the lower end plate 962a toward the electrochemical cell stack 910a, thereby securing the electrochemical cell stack 910a. FIG. 21B shows an alternative implementation of the structure shown in FIG. 21A. The same functional parts are present, except for a different configuration in which a mounting bolt 983b hangs from the top of the lower compression plate 982b rather than from the bottom, as shown in the embodiment of FIG. 21A. 21A and 21B, the manifolds 912a / b are made of sheet metal and are held in place by bolted clips 918a / b. Between the manifolds 912a / b and the end plates and stack core are dielectric seal members 914a / b positioned within the volume defined by the manifolds 912a / b around the electrochemical cell stack and configured to fluidly seal a first portion of the volume from a second portion of the volume. The dielectric seal members 914a / b separate the oxidant inlet from the oxidant outlet. Small leaks across the seal formed by the dielectric seal members 914a / b may be tolerated without impairing stack operation.

[0123] In the particular embodiment shown in FIG. 21B, the oxidant outlet (or inlet) port is split into two, consisting of lower left and upper right openings in a lower end plate (not shown) positioned between the lower compression plate 982b and the electrochemical cell stack 910b. These are routed within the lower end plate to volumes defined by the left and right manifolds 982b of the electrochemical cell stack 910b. In this particular embodiment, two ports are shown that provide a low parasitic pressure drop for the oxidant flow. The remaining ports (left and right openings) are fuel input and output ports, and their positions can be swapped as needed.

[0124] Depending on the application, an open or closed manifold may be preferred. The open manifold 912a shown in Figure 21A aids in thermal coupling to the environment. This is useful, for example, in electrolysis or energy storage systems where operating conditions exist that require the electrochemical cell stack 910a to absorb heat from the environment. In pure electrochemical cell systems where reforming is limited to levels where the stack is net exothermic, a full manifold may be preferred.

[0125] A complete (closed) manifold effectively isolates the stack core from its environment during operation. For exothermic operating conditions, this allows for a reduction in environmental temperature, potentially by up to 100°C. This can provide significant benefits in terms of reducing the insulation requirements around the stack or stack array, and in allowing for the use of lower quality materials around the stack. This can result in system-level cost savings. It can also mitigate other system-level challenges, such as chromium volatilization, and other material transport or material oxidation challenges.

[0126] The following sections provide examples of the performance of various electrochemical cell stacks according to the embodiments described herein. These examples are for illustrative purposes only and are not intended to limit the scope of the concepts described herein.

[0127] Experimental Examples It goes against current wisdom to suggest that overall power density (per kg and per L) can be increased by reducing component size. The accepted wisdom is that the path to increasing power density and lowering cost is to reduce the number of parts while making each part larger. This assumes that stack volume and cost are largely driven by the inactive parts of the stack (seal area, end plates, compression, etc.), and that moving to larger cells reduces the contribution of these inactive areas to the overall stack cost. It is difficult to produce large cells in planar SOFCs, where the cells are thin ceramic components. Much effort is ongoing to increase the size of SOFC cells.

[0128] In contrast, the embodiments described herein demonstrate that novel designs can result in higher power density and lower cost with smaller components. Contrary to current wisdom, moving to smaller cells has been shown to reduce the cost of inactive components by allowing them to be smaller and simpler than their larger counterparts, even when considered proportionally to overall active area or power output.

[0129] An experiment was conducted comparing three different known fuel cell stacks with an exemplary fuel cell stack according to one embodiment. All stacks were fabricated using the same basic materials. Each stack was of a planar solid oxide fuel cell design. Each stack used the same type of nickel-yttria-stabilized zirconia cell-supported anode, but with a size and thickness appropriate for the particular stack. Each stack had metal interconnects made of ferritic stainless steel. Each stack had end plates of appropriate size and strength to support the compressive forces required for stack operation. The stacks fabricated according to the embodiments described herein also included a compression system. 121 cm 2The baseline 28-cell stack with active area cells was 390mA / cm 3 It operates at 550cm and delivers a total power of 1200W. It measures 190mm x 190mm x 150mm, with a total volume of 5.4L and a weight of 17kg, achieving a power-to-weight ratio of 69W / kg and a power-to-volume ratio of 225W / L. 2 A 120-cell stack with an active area cell of 290mA / cm 2 It operates at 1000 W and delivers a total power of 16,900 W. It measures 395 mm x 395 mm x 618 mm, with a total volume of 96 L and a weight of 238 kg (including end plates), achieving a power-to-weight ratio of 71 W / kg and a power-to-volume ratio of 176 W / L. In contrast, a 25 cm 2 A stack made according to one embodiment having 225 cells, each with an active area cell of 0.39 mA / cm 2 The stack operates at 1000 W and delivers a total power of 1760 W. It measures 79 mm x 71 mm x 254 mm, with a total volume of 1.4 L and a weight of 2.4 kg, achieving a power-to-weight ratio of 733 W / kg and a power-to-volume ratio of 1257 W / L. Although relatively new compared to other stack designs, this embodiment of the stack has already achieved 10 times the power density by weight and approximately 7 times the power density by volume. This result was unexpected, especially in light of the accepted belief that the path to increasing power density and reducing cost is to reduce the number of parts while making each part larger.

[0130] Selected test data are presented in Figures 22-24. Figure 22 shows a 225-cell (~1 kW) stack included in an array 200 according to an embodiment of the fuel cell stack 110, operating with a gas composition representative of a typical natural gas combustion system application. Gas conditions include representative levels of gas utilization for the conversion of inlet natural gas to hydrogen, carbon monoxide, and carbon dioxide, representative current densities and temperatures, and representative levels of in-stack steam reforming. The stack has demonstrated stable operation for over 5000 hours, exhibiting degradation rates consistent with those expected for the cell materials used. That is, there are no degradation aspects that can be linked to the stack design. This demonstrates the stack's ability to handle flow and thermal conditions typical of a natural gas combustion system, including in-stack reforming, while maximizing the potential of the underlying cycling cell materials.

[0131] Figure 23 shows the results for a 20-cell implementation of the same stack, with a maximum of -2A / cm 2 The stack was running under very aggressive electrolysis conditions. Some test interruptions due to plant balance disturbances (not the stack's fault) occurred early in the test and can be seen as spikes in the data at approximately 25, 125, and 250 hours elapsed time. After initial degradation (electrolysis degradation manifests as an increase in voltage) likely caused by the test interruptions, the stack demonstrated over 1000 hours of operation without degradation despite the aggressive conditions. This demonstrates the stack's flexibility to operate under a wide variety of thermal and flow conditions, as well as its relative immunity to degradation.

[0132] Figure 24 shows the results of a 60-cell implementation of a similar stack running various pure hydrogen fuel cell conditions, where the total test time was over one year. While changes in conditions make determining the degradation rate difficult, the stack displayed high stability (low degradation) throughout the year of testing, including the highly exothermic test conditions. This can be seen as the relative flatness of the voltage curve, with step changes corresponding to changes in test conditions. This demonstrates the stack's relatively long-term stability and its ability to reject heat when operating in an exothermic mode without internal reforming to absorb some of that heat.

[0133] 23 and 24 show results involving thermal cycling, where no change in performance was observed before and after thermal cycling, demonstrating the thermal cycling capability of the stack regardless of the use of glass-ceramic seals. This demonstrates the success of an inherently compliant structure, which allows for the use of hermetic or near-hermetic glass-ceramic seals while preventing the buildup of thermal stresses that would otherwise cause seal or cell failure.

[0134] Figure 25 shows the current at 0.25A / cm 2 Results are presented for a 45-cell implementation of an electrochemical cell stack according to array 300, operated in electrochemical cell mode at 400 sq. m, demonstrating very low degradation after one thermal cycle. These results are comparable to those for a slightly larger cell stack (81 cm). 2 This shows a case where the active area (of the material) eliminates thermal stresses and does not hinder the ability to extract full potential from the material set.

[0135] Figure 26 shows the -1A / cm 2 The results show results for an individual 45-cell implementation of a fuel cell stack from array 300, operating in electrolysis mode at 4000 sq ft and demonstrating very low degradation, and in fact a slight improvement in overall performance after 1600 hours of testing. The test also subjected the stack to aggressive thermal cycling due to a balance of plant faults (not stack-related) that did not adversely affect stack performance, demonstrating its robustness to aggressive transients.

[0136] The integrated design described above addresses many of the major barriers between SOFC technology and the current market, in part by offering opportunities for significant cost reductions at both the stack level (due to reduced material content and easier automation of parts) and the system level, where the stack characteristics offer opportunities for system simplification (high voltage output, low current, compact packaging, low external heat exchange requirements, no external pressure load requirements, etc.).

[0137] In all cases, it will be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments representing applications of the present invention, and that numerous and varied other arrangements, including the use of different electrolytes, can be readily devised in accordance with the principles of the concepts described herein without departing from the spirit and scope of the present invention.

[0138] As used herein, the terms "approximately," "about," "substantially," and similar terms are intended to have a broad meaning consistent with commonly accepted usage by those skilled in the art to which the subject matter of this disclosure pertains. It should be understood by those skilled in the art who review this disclosure that these terms are intended to enable description of the particular features described and claimed without limiting the scope of those features to the precise numerical ranges provided. Accordingly, these terms should be interpreted to indicate that substantial or insignificant modifications or variations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.

[0139] As used herein, the terms "coupled," "connected," and the like mean that two members are joined directly or indirectly to one another. Such joining can be fixed (e.g., permanent) or movable (e.g., removable or releasable). Such joining can be achieved by the two members or the two members and an additional intermediate member being integrally formed with one another as a single, unitary body, or by the two members or the two members and an additional intermediate member being attached to one another.

[0140] References herein to the location of elements (e.g., "top," "bottom," "upper," "lower," etc.) are used merely to describe the orientation of the various elements in the figures. It should be noted that the orientation of the various elements may differ according to other exemplary embodiments, and such variations are intended to be encompassed by the present disclosure.

[0141] It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. While only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily understand that many modifications (e.g., changes in the size, dimensions, structure, shape, and proportions of various elements, parameter values, mounting arrangements, material use, color, orientation, etc.) are possible without substantially departing from the novel teachings and advantages of the subject matter described herein. For example, elements shown as integrally formed may be composed of multiple parts or elements, the position of elements may be reversed or otherwise modified, and the nature or number of individual elements or positions may be changed or modified. The order or sequence of any process or method steps may be modified or rearranged according to alternative embodiments. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the various exemplary embodiments without departing from the scope of the present invention. For example, the perforated baffles may be further optimized to achieve the intent of extending residence time without creating dead zones.

Claims

1. An electrochemical cell unit, a first electrochemical cell including a first oxidant electrode and a first fuel electrode; a second electrochemical cell including a second oxidant electrode and a second fuel electrode; an interconnect interposed between the first electrochemical cell and the second electrochemical cell, the interconnect including an interconnect main body defining a longitudinal channel along a longitudinal axis thereof, the interconnect main body including a plurality of corrugations defining a plurality of fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell, each of the plurality of fuel channels and the plurality of oxidant channels being positioned around the longitudinal channel.

2. 2. The electrochemical cell unit of claim 1, wherein a fuel channel base of each of the plurality of fuel channels is in electrical contact with the second oxidizer electrode, and an oxidizer channel base of each of the plurality of oxidizer channels is in electrical contact with the first fuel electrode.

3. an outer seal member positioned around the periphery of the interconnect on the first surface; an inner seal member positioned on the second surface around the longitudinal channel at an inner periphery of the interconnect; 2. The electrochemical cell unit of claim 1, wherein the outer seal member fluidly seals one of the plurality of fuel channels or the plurality of oxidant channels from a volume outside the periphery, and the inner seal member fluidly seals the other of the plurality of fuel channels or the plurality of oxidant channels from the longitudinal channel.

4. 4. The electrochemical cell unit of claim 3, wherein the interconnect main body defines at least one fuel inlet channel and at least one fluid outlet channel fluidly coupled to each of the plurality of fuel channels, and further defines at least one oxidant inlet channel and at least one oxidant outlet channel fluidly coupled to each of the plurality of oxidant channels.

5. 5. The electrochemical cell unit of claim 4, wherein the outer seal member fluidly seals the plurality of fuel channels from the volume outside the periphery, and the at least one fuel inlet channel and the at least one fuel outlet channel are fluidly coupled to the longitudinal channels to receive fuel from a first portion of the longitudinal channels and discharge spent fuel into a second portion of the longitudinal channels.

6. 6. The electrochemical cell unit of claim 5, wherein the inner seal member fluidly seals the plurality of oxidant channels from the longitudinal channel, and the at least one oxidant inlet channel and the at least one oxidant outlet channel are fluidly coupled to the outer periphery of the interconnect so as to receive oxidant from a first portion of the volume outside the periphery and discharge spent oxidant from a second portion of the volume outside the periphery.

7. 4. The electrochemical cell unit of claim 3, further comprising an edge seal member disposed on at least one of an outer edge of each of the first electrochemical cell and the second electrochemical cell proximate the periphery of the interconnect, or an inner edge of each of the first electrochemical cell and the second electrochemical cell proximate the longitudinal channel.

8. 1. An electrochemical cell stack comprising: a stack of a plurality of electrochemical cell units, each of the plurality of electrochemical cell units comprising: a first electrochemical cell including a first oxidant electrode and a first fuel electrode; a second electrochemical cell including a second oxidant electrode and a second fuel electrode; an interconnect interposed between the first electrochemical cell and the second electrochemical cell, the interconnect including an interconnect main body defining a longitudinal channel along a longitudinal axis thereof, the longitudinal channel spanning a height of the electrochemical cell stack, the interconnect main body including a plurality of corrugations defining a plurality of fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell, each of the plurality of fuel channels and the plurality of oxidant channels being positioned around the longitudinal channel.

9. Each of the plurality of electrochemical cell units is an outer seal member positioned around the periphery of the interconnect on the first surface; an inner seal member positioned on the second surface around the longitudinal channel at an inner periphery of the interconnect; 9. The electrochemical cell stack of claim 8, wherein the outer seal member fluidly isolates one of the plurality of fuel channels or the plurality of oxidant channels from a volume outside the periphery, and the inner seal member fluidly isolates the other of the plurality of fuel channels or the plurality of oxidant channels from the longitudinal channel.

10. The electrochemical cell stack of claim 8 , wherein the interconnects included in each of the plurality of electrochemical cell units cooperate to form a bellows-like structure such that the electrochemical cell stack is compliant.

11. 10. The electrochemical cell stack of claim 8, further comprising posts disposed within the longitudinal channels, the posts defining at least one post inlet configured to receive one of the fuel or oxidant and at least one post outlet configured to receive and discharge the other of spent fuel or spent oxidant from the electrochemical cell stack, the post inlet and the post outlet being fluidly isolated from one another.

12. 12. The electrochemical cell stack of claim 11, further comprising a top end plate positioned at a top end of the electrochemical cell stack around the posts such that a gap is provided between the top end plate and the posts, the gap being structured to allow movement of the posts therein to relieve thermal stress.

13. 13. The electrochemical cell stack of claim 12, further comprising a compliant seal member positioned within the gap, the compliant seal member providing sufficient compliance to allow the movement of the post.

14. an upper end cap positioned on the upper end plate; 14. The electrochemical cell stack of claim 13, further comprising a secondary seal member interposed between the top end plate and the top end cap.

15. 13. The electrochemical cell stack of claim 12, wherein the top end plate includes a post interface tube extending axially from a surface of the top end plate away from the post, at least a portion of the post interface tube being positioned around a portion of the post.

16. a bottom end plate positioned on a bottom end of the electrochemical cell stack opposite the top end; an upper compression plate positioned on the upper end plate; a biasing member positioned proximate the top end of the electrochemical cell stack and configured to exert a compressive force on the stack of the plurality of electrochemical cell units; 13. The electrochemical cell stack of claim 12, further comprising: at least one compression member coupled to the upper compression plate and configured to transfer the compression force from the upper compression plate to the lower end plate.

17. 17. The electrochemical cell stack of claim 16, further comprising a lower compression plate positioned at the lower end of the electrochemical cell stack, the at least one compression member being coupled to the lower compression plate.

18. 17. The electrochemical cell stack of claim 16, wherein the biasing member comprises a stack of Belleville springs interposed between the upper compression plate and the top end plate.

19. The electrochemical cell stack of claim 16 , wherein the biasing member comprises a plurality of coil springs operably coupled to the upper compression plate.

20. The electrochemical cell stack further includes a base plate assembly positioned at a lower end of the electrochemical cell stack, the base plate assembly comprising: a lower end plate defining at least one fuel port and at least one oxidant port; a high strength sealing plate axially aligned with the bottom end plate and configured to yield relative to the bottom end plate to reduce transfer of mechanical stress from the high strength sealing plate to the bottom end plate.

21. 21. The electrochemical cell stack of claim 20, wherein the high-strength sealing plate is positioned between the stack of the plurality of electrochemical cell units and the bottom end plate, and the base plate assembly further includes a plurality of short tubes positioned between the high-strength sealing plate and the bottom end plate.

22. 21. The electrochemical cell stack of claim 20, wherein the bottom end plate is interposed between the stack of the plurality of electrochemical cell units and the high-strength sealing plate, and the base plate assembly further includes a plurality of short tubes positioned between the high-strength sealing plate and the bottom end plate, the short tubes configured to yield in response to thermal stress such that the high-strength sealing plate is free to move laterally relative to the bottom end plate to reduce stress transfer to the bottom end plate.

23. 10. The electrochemical cell stack of claim 9, further comprising a manifold disposed around the stack of the plurality of electrochemical cell units, the manifold defining the volume around the periphery, a first portion of the volume providing an inlet for one of the fuel or oxidant into the electrochemical cell stack, and a second portion of the volume providing an outlet for spent fuel or oxidant from the electrochemical cell stack.

24. 24. The electrochemical cell stack of claim 23, further comprising a dielectric seal member positioned within the volume and configured to fluidly seal the first portion of the volume from the second portion of the volume.

25. 1. An electrochemical cell assembly comprising: a housing including a housing base; an array of electrochemical cell stacks disposed on the housing base within the housing, each of the electrochemical cell stacks included in the array comprising: a stack of a plurality of electrochemical cell units, each of the plurality of electrochemical cell units comprising: a first electrochemical cell including a first oxidant electrode and a first fuel electrode; a second electrochemical cell including a second oxidant electrode and a second fuel electrode; an interconnect interposed between the first electrochemical cell and the second electrochemical cell, the interconnect including an interconnect main body defining a longitudinal channel along a longitudinal axis thereof, the longitudinal channel spanning a height of the electrochemical cell stack, the interconnect main body including a plurality of corrugations defining a plurality of fuel channels on a first surface of the interconnect main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnect main body facing the second electrochemical cell, each of the plurality of fuel channels and the plurality of oxidant channels positioned around the longitudinal channel.

26. a ring separator positioned around each of the electrochemical cell stacks; 26. The electrochemical cell assembly of claim 25, further comprising: a cross separator positioned between each set of four electrochemical cell stacks included in the array of electrochemical cell stacks.

27. 26. The electrochemical cell assembly of claim 25, further comprising an oxidant preheat tube positioned between each set of four electrochemical cell stacks with a corresponding cross separator therebetween.

28. 26. The electrochemical cell assembly of claim 25, further comprising a fuel inlet, a fuel outlet, an oxidant inlet, and an oxidant outlet fluidly coupled to the array of electrochemical cell stacks through the housing base, the housing base defining at least one heat exchange channel configured to provide heat exchange between the fuel entering the housing base through the fuel inlet and spent fuel exiting the housing base through the fuel outlet.

29. 26. The electrochemical cell assembly of claim 25, further comprising a fuel bypass inlet fluidly coupled to the array of electrochemical cell stacks through the housing base, the fuel bypass inlet bypassing the at least one heat exchange channel.

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