Compact high-temperature electrochemical cell stack architecture

Abstract

To provide solid oxide (SOFC) and solid oxide electrolysis cell stacks, and more particularly, high power density compact SOFC stacks.SOLUTION: An electrochemical cell unit 150 comprises a first electrochemical cell comprising a first oxidant electrode and a first fuel electrode and a second electrochemical cell comprising a second oxidant electrode and a second fuel electrode. An interconnect 152 is interposed between the first electrochemical cell and the second electrochemical cell. The interconnect comprises an interconnect main body 152a defining a longitudinal channel along a longitudinal axis thereof. 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.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the priority and benefit of U.S. Provisional Patent Application No. 62 / 501,633, entitled "High Power Density Compact SOFC Stack," filed on 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 number DE - FE0026093 awarded by the DOE. The government has certain rights in this invention.

[0003] This disclosure relates to high - temperature fuel cell stacks and electrolysis stacks, and more particularly to solid oxide (SOFC) and solid oxide electrolysis cell (SOEC) stacks, and even more particularly to high - power - density compact SOFC stacks.

Background Art

[0004] A solid oxide fuel cell includes an electrolyte sandwiched between a cathode and an anode. Oxygen reacts with electrons at the cathode to form oxygen ions, and the oxygen ions are conducted to the anode through an ion-conductive ceramic electrolyte. At the anode, the oxygen ions combine with available fuel (e.g., hydrogen and carbon monoxide, methane, some other hydrocarbons, or other suitable fuels) to form products (e.g., water and carbon dioxide), thereby liberating electrons and generating electricity. Such a technology can be operated in reverse to perform electrolysis and form fuel gas and oxygen when appropriate reactants (e.g., water and carbon dioxide) and electricity are supplied. In such an implementation, this technology is called a solid oxide electrolysis cell. In the development of SOFCs, numerous approaches (anode, cathode, or electrolyte support, monolithic ceramic versus metal interconnect, planar versus tubular, and their variations) have been seen. The greatest challenge in commercializing this technology is the simultaneous achievement of a marketable price, reasonable performance, and service life. These drivers are closely related.

Summary of the Invention

[0005] Embodiments described herein generally relate to an electrochemical cell such as a fuel cell or an electrolysis cell, and particularly to an electrochemical cell stack including a corrugated interconnect inserted between and electrically coupled to adjacent electrochemical cells, the corrugation 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 seal member, the interconnect being 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 that defines a longitudinal channel along its longitudinal axis. 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 each of the plurality of oxidant channels are 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 fluidly seals one of the plurality of fuel channels or the plurality of oxidant channels from the volume outside the outer 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. In some embodiments, 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.

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

[0009] In some embodiments, the electrochemical cell stack includes a stack of a plurality of electrochemical cell units. Each of the plurality of 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 interconnecting portion interposed between the first electrochemical cell and the second electrochemical cell. The interconnecting portion includes an interconnecting portion main body that defines a longitudinal channel along its longitudinal axis. The longitudinal channel spans the height of the electrochemical cell stack. The interconnecting portion main body includes a plurality of corrugations that define a plurality of fuel channels on a first surface of the interconnecting portion main body facing the first electrochemical cell and a plurality of oxidant channels on a second surface of the interconnecting portion main body facing the second electrochemical cell, and each of the plurality of fuel channels and the plurality of oxidant channels is positioned around the longitudinal channel.

[0010] In some embodiments, each of the plurality of electrochemical cell units further includes an outer seal member positioned at the outer periphery of the interconnect on the first surface and an inner seal member positioned at the inner periphery of the interconnect on the second surface around the longitudinal channel. The outer seal member fluidly isolates one of the plurality of fuel channels or the plurality of oxidant channels from the volume outside the outer 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. 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 has compliance. In some embodiments, the electrochemical cell stack further includes a post disposed within the longitudinal channel, the post defining at least one post inlet configured to receive one of fuel or oxidant and at least one post outlet configured to receive and discharge the other of the spent fuel or spent oxidant from the electrochemical cell stack, the post inlet and the post outlet being fluidly isolated from each other.

[0011] In some embodiments, the electrochemical cell stack further includes an upper end plate positioned at the upper end of the electrochemical cell stack around the post such that a gap is provided between the upper end plate and the post, the gap being structured to allow movement of the post 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 post. In some embodiments, the electrochemical cell stack further includes an upper end cap positioned on the upper end plate and a secondary seal member inserted between the upper end plate and the upper end cap. In some embodiments, the upper end plate includes a post interface tube that extends axially from the surface of the upper end plate away from the post, 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 the lower end of the electrochemical cell stack opposite the upper end. The upper compression plate is positioned on the upper end plate. The bias member is positioned proximate to the upper end of the electrochemical cell stack and is configured to exert a compressive force on the stack of a plurality of electrochemical cell units. At least one compression member is coupled to the upper compression plate and is configured to transmit a 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 the lower end of the electrochemical cell stack, and at least one compression member is coupled to the lower compression plate. In some embodiments, the bias member includes a stack of Belleville springs inserted 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 the lower end portion of the electrochemical cell stack. The base plate assembly includes a lower end plate that defines 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 with respect to the lower end plate to reduce the transmission 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 a plurality 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 inserted between the stack of a 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 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 with respect to the lower end plate to reduce stress transmission to the lower end plate.

[0014] In some embodiments, the electrochemical cell stack includes a manifold disposed around the stack of a plurality of electrochemical cell units. The manifold defines a volume around the 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, the electrochemical cell assembly includes a housing that includes 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 a plurality of electrochemical cell units. Each of the plurality of electrochemical cell units includes a first electrochemical cell that includes a first oxidant electrode and a first fuel electrode, a second electrochemical cell that includes a second oxidant electrode and a second fuel electrode, and an interconnect disposed between the first electrochemical cell and the second electrochemical cell. The interconnect includes an interconnect main body that defines a longitudinal channel along its longitudinal axis, and the longitudinal channel spans 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, and each of the plurality of fuel channels and the plurality of oxidant channels is 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 within 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 a 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 at least one heat exchange channel.

[0017] The foregoing is a summary of the disclosure, and as such, necessarily contains some simplification, generalization, and omission 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 to be interpreted in conjunction with the accompanying drawings.

Brief Description of the Drawings

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

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[0047] Throughout the following detailed description, reference is made to the accompanying drawings. In the drawings, like reference numerals generally identify like components unless the context indicates otherwise. The illustrative implementations described in the detailed description, the drawings, and the 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. Aspects of the present disclosure may be arranged, substituted, combined, and designed in a variety of different configurations, as generally described herein and shown in the drawings, all of which are explicitly contemplated and will be readily understood to be part of the present disclosure.

Best Mode for Carrying Out the Invention

[0048] The embodiments described herein generally relate to electrochemical cells such as fuel cells and electrolytic cells, and in particular to an electrochemical cell stack including a corrugated interconnect that is inserted between and electrically coupled to adjacent electrochemical cells, the corrugation forming a plurality of fuel channels on one side and a plurality of oxidant channels on the opposite side that are fluidically isolated via a seal member, and the interconnect being configured to provide compliance to the electrochemical cell stack.

[0049] According to certain embodiments, a mechanical stack layout is provided that represents a design approach focused on reducing the material content within the stack while maintaining (and in many cases improving) the feasibility of the overall design relative to current stack technology. Certain embodiments use relatively small generally circular solid oxide fuel cells and thin interconnects, and their integration results in a one-order-of-magnitude increase in power density (W / kg) compared to the current baseline. This can be achieved by careful thermal design to ensure maximum thermal communication between the active area (heat generation location) of the cell and the stack environment.

[0050] Efforts to reduce cost tend to directly shorten the lifespan. This is because it involves strategies to operate the fuel cell more intensively. Efforts to improve performance (high power output) tend to reduce lifespan and efficiency. Efforts to extend lifespan often involve expensive materials and / or operation at low power density, both of which increase cost. For example, tubular technology has been proven over a long period (more than 5 years), but is generally recognized as having price and performance levels that are not acceptable for actual market applications. Conversely, planar SOFC technology is close to achieving price and performance goals, but faces the challenge of achieving practical durability goals. A common focus for bridging this price / performance / durability gap is to develop high-performance cells while increasing size mainly to reduce manufacturing costs. This is evident in almost all SOFC development activities, whether it is the development of a tubular design with a large surface area or the development of a planar design. An exception to this trend exists in the form of microtubular SOFC cells, which are mainly proposed by university research institutes for applications that require rapid thermal transients. Microtubular systems have not advanced as a viable solution for large-scale systems (usually in the output range of up to several hundred watts).

[0051] Generally, there is an additional set of constraints for mobile applications. Current SOFC technology shows that the output density of volume and mass index is about 200 W / L and 100 W / kg. Therefore, a 70 kW power unit occupies ~350 L and weighs about 700 kg with just the stack, and increases significantly for the entire power system. Small cars cannot accommodate an SOFC-based prime power system with these weights and volumes. The second constraint for automotive applications is the warm-up time. Current stacks require about 1 hour to reach an operating temperature of about 750 degrees Celsius from ambient temperature. In actual mobile applications, startup times of usually a few seconds are expected, but startup times of a few minutes may be acceptable with a secondary power source such as a battery to cover changes in expected values and / or the operation during the first few minutes.

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

[0053] The embodiments described herein provide different approaches for meeting the price, performance, and / or lifetime goals of an electrochemical cell (e.g., a fuel cell or an electrolysis cell) while addressing the major challenges. The embodiments described herein also address the weight and volume challenges posed by mobile applications of electrochemical cells, while the warm-up time is expected to be on the order of a few minutes.

[0054] Briefly, the embodiments described herein propose to reverse the current trend of gradually increasing the size and performance of the cell, and instead propose to reduce the size of the cell, reduce the dependence on the performance of the cell, and focus on the tight integration of components optimized for operation with small cells. Through careful integration, certain embodiments described herein can result in a stack with an output that is the same as or greater than that of current stacks at 1 / 7 of the volume and 1 / 10 of the weight.

[0055] The various embodiments described herein may provide advantages including, for example, the following: (1) reduction in volume per unit power output without the need for improved cell performance (e.g., a reduction of seven times or more), (2) reduction in weight per unit power output without the need for improved cell performance (e.g., a reduction of ten times or more), (3) expectation of corresponding approximate cost reduction (e.g., a reduction of ten times or more), (4) rapid transient response (e.g., ten times faster than current electrochemical cell stacks, providing a heating time of about minutes instead of hours), (5) modularity level that supports power ranges from, for example, 1 kW to many MW with the same stack for both mobile and fixed applications, (6) significantly reducing leakage from anode to cathode to enhance efficiency and expand application possibilities, (7) higher voltage and lower current output (providing higher efficiency within power electronics), (8) inherent load distribution and redundancy at larger kW ratings (e.g., 10 kW or more), (9) short conduction distance between the stack core and the stack edge, enabling indirect thermal management inside the stack as heat can be released to the environment, and (10) reduction in the balance of plant requirements due to less air flow, simpler 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 specific examples of the use of the embodiments described herein, although smaller, larger, and variations between sizes of these embodiments are equally possible. Important size considerations are cell size and cell count. Some embodiments described herein include cells having an active area of 21 cm 2 or 25 cm 2 and have been demonstrated with up to 234 cells per stack. Other embodiments described herein have cells with an active area of 81 cm 2 and are designed to operate with 350 cells or more per stack and have been demonstrated with up to 45 cells per stack.

[0057] While various embodiments described herein refer to an electrochemical cell unit and an electrochemical cell stack as a fuel cell unit and a fuel cell stack, it should be understood that the various embodiments of the electrochemical cell units and electrochemical cell stacks described herein can operate in reverse flow, such as to include an electrolytic cell unit and an electrochemical cell stack, or to include any other electrochemical cell unit or stack.

[0058] FIG. 1A is a cross-sectional perspective view of a portion of a fuel cell stack 110 shown in FIG. 2, according to one embodiment. The fuel cell stack 110 includes, according to one embodiment, a plurality of fuel cell units 150, and more particularly, a stack of 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 view of a fuel cell unit 150 that may be included in the fuel cell stack 110. Each fuel cell unit 150 includes a first electrochemical cell 154a that includes 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 also includes a second fuel electrode 153b, a second oxidant electrode 155b, and may also include an electrolyte interposed between the second fuel electrode 153a and the second oxidant electrode 155a. In some embodiments, each of the anodes optionally includes an anode support. In some embodiments, the fuel cell stack 110 can operate in reverse flow, i.e., as an electrolytic cell stack. In such embodiments, the fuel electrodes 153a / b of the electrochemical cells 154a / b can include cathodes, and the oxidant electrodes 155a / b of the electrochemical cells 154a / b can include anodes.

[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, such that the longitudinal channel 120 can 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 can be positioned around the longitudinal channel 120, for example, in a symmetric and / or circular configuration. The fuel channel base of each of the plurality of fuel channels 157 can be in electrical contact with the second oxidant electrode 155b, and the oxidant channel base of each of the plurality of oxidant channels 159 can be in electrical contact with the first fuel electrode 153a.

[0060] For example, the electrochemical cells 154a / b and the interconnect 152 are shaped 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 154a / b and the interconnect 152 have an annular shape, and the longitudinal channel 120 is a central channel positioned at the axis center of the fuel cell stack 110. Although the fuel cell stack 110 is described as having an annular shape, it may have any other suitable shape, such as an oval, hexagonal, square or non-square shape, or any other shape as long as the longitudinal channel 120 extends longitudinally through the fuel cell stack 110. Further, although 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 as long as the longitudinal channel 120 does not partially overlap with the outer edge of the fuel cell stack 110.

[0061] As shown in FIGS. 1A and 1B, the electrochemical cells 154a / b are sealed to each interconnect 152 alternately at the inner and outer circumferences. Thereby, although hermetically sealed, a compliant structure is obtained at the unit cell level to reduce the possibility of thermal stress accumulation.

[0062] Either the fuel or the oxidant gas flows into the fuel cell unit 150 through the longitudinal channel 120 and is extracted from the fuel cell unit 150, while the other gas flows into the fuel cell unit 150 at the outer periphery of the fuel cell stack 110 and is extracted from the fuel cell unit 150. In certain embodiments, the fuel flows into and is extracted from the longitudinal channel 120, and the oxidant is received and extracted at the outer periphery of the fuel cell stack 110. The seal between the sealed cell and the interconnect prevents gas mixing. For example, as shown in FIG. 1A, an outer seal member 158 may be positioned at the outer circumference of the interconnect 152 on the first surface proximate the first electrochemical cell 154a, and an inner seal member 156 may be positioned at the inner circumference of the interconnect 152 on the second surface proximate the second electrochemical cell 154b 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 outer 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 channel 120. In particular, as shown in FIG. 1A, the outer seal member 158 fluidly seals the fuel channel 157 from the volume outside the outer periphery, and the inner seal member 156 fluidly seals the oxidant channel 159 from the longitudinal channel 120.

[0063] By avoiding leaks that would result in the mixing and combustion of fuel and oxidant, several advantages are obtained, including (i) a reduction in the loss of reactants to the system, (ii) a reduction in the heat load on the stack (and in particular, local heating that could damage stack components), (iii) a reduction in vapor generation on the oxidant side (which reduces the volatilization and transport of chromium, which can be a significant degradation mechanism for the oxidant electrode), and (iv) a reduction in the use of protective cover gas due to reduced cross-leakage during warm-up and cooling.

[0064] The fuel cell stack 110 can include, for example, 20 to 400 fuel cell units 150, and is limited only by the aspect ratio (height to diameter or width) of the completed stack, which can make manufacturing and packaging difficult if the aspect ratio is too high. In various embodiments, the aspect ratio can be in the range of 4:1 to 5:1, although shorter stacks can be useful for certain applications and development purposes. The plurality of fuel cell units 150 may be stacked vertically with intermediate metal interconnects 152.

[0065] The output range of each fuel cell stack is from about 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 the operating conditions and stack size. In one embodiment, the stack has a power range of about 7 kW. By reducing the cell count and adjusting the operating conditions, a practical stack of about 50 W can be manufactured.

[0066] The fuel cell stack 110 or any other electrochemical cell stack described herein provides a cell design that utilizes an appropriate geometric arrangement to improve the ability to thermally control the electrochemical cell stack while reducing mechanical stresses induced during cell manufacturing and subsequent stack operation. These two advantages enable both the interconnects 152 and the cells to be made thinner without compromising their structures.

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

[0068] In other words, the material content of the electrochemical cell stack is reduced, and the reduction is significant. In this design, there is no need to use special materials, and in many fields, the material requirements are simplified compared to conventional electrochemical cell stacks. The compression system may be simplified due to low loads, as will be described in more detail below. As will also be described in more detail below, the manifold is also simplified due to low sealing requirements. This reduction in material content reduces the essential cost of the electrochemical cell stack. The number of parts per kW increases, but the use of small parts, the reduction of different parts per layer, and the lack of large tolerances that require operator intervention also improve the compatibility of the parts for automation. Therefore, the cost advantages of having less material content may outweigh the overall increase in the number of parts.

[0069] The fuel cell stack 110 shown in FIG. 2 includes 234 cells. Each fuel cell unit 150 has an annular shape, an outer cell diameter of 60 mm, a thickness of 300 microns, and 21 cm 2It had an active area. 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 also included a manifold 112 positioned around the stack of a plurality of fuel cell units 150, which could define a volume around the outer periphery of the fuel cell stack 110 for use in introducing and discharging one of fuel and oxidant around the outer periphery of the fuel cell stack 110. For example, a first portion of the volume could provide an inlet for one of fuel or oxidant into the fuel cell stack 110, and a second portion of the volume could provide an outlet for spent fuel or oxidant from the fuel cell stack 110.

[0070] The embodiments described herein can reduce the material content by one digit while providing many other advantages, such as reducing the cost per kW at the stack and system levels. The improved thermal layout, with improved temperature control, can simultaneously improve performance and reduce degradation. Further, the embodiments described herein can improve the thermal control of the electrochemical cell stack, thereby allowing for a lower cooling air flow and a lower inlet temperature, both of which can improve the balance of plant efficiency.

[0071] Fuel inlet / outlet seal and oxidant inlet / outlet seal Separation of fuel in from fuel out and oxidant in from oxidant out is achieved through a structurally independent manifold (e.g., manifold 112) that is separate from the stack core and is compressible and sealable via a compliant seal that allows relative movement between the stack core and the manifold. Thereby, the stack core grows and bends due to thermally induced loads independent of the manifold, which prevents or reduces thermally induced mechanical stresses throughout the structure, thereby protecting the individual components. For example, ceramic cells are susceptible to brittle fracture when overly stressed. 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 oxidant inlet from the oxidant outlet. Preferably, the compliant seal does not seal between the fuel and the oxidant gas anywhere. Compliant high-temperature ceramic seals are known to have leaks because they typically achieve compliance by being porous, interconnected packed ceramic structures. In the embodiments described herein, such leaks are acceptable because they do not cause combustion and have little impact on overall efficiency as long as the leak rate is low (e.g., less than about 5% of the total flow rate). This enables the advantageous use of external manifold design techniques, providing cost, weight, and volume advantages. FIG. 3 shows a fuel manifold 230, e.g., a post (e.g., a central post) placed within the longitudinal channels of an electrochemical cell stack, and the resulting leak path from fuel in to fuel out.

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

[0073] With the annular cell design, the conduction path from any part of the cell's heat - generating area to the outer surface of the stack is minimized, which supports maintaining the thermal control of the stack.

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

[0075] For example, the interconnect main body (e.g., interconnect main body 152a) of the interconnect (e.g., interconnect 152) included in each of the 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 a 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 a 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 a longitudinal channel to receive fuel from a first portion of the longitudinal channel and discharge spent fuel to a second portion of the longitudinal channel. An outer seal member (e.g., outer seal member 158) may fluidly seal the plurality of fuel channels from an outer volume of the outer perimeter. Further, the at least one oxidant inlet channel and the at least one oxidant outlet channel may be fluidly coupled to the outer perimeter of the interconnect to receive oxidant from a first portion and discharge spent oxidant from a second portion of the outer volume of the outer perimeter. 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 large-scale systems, the stack is deployed in a modular array, for example, in an array of 20 kW to 250 kW or more as shown in array 100 of FIG. 5 or array 200 shown in FIG. 7A, or in an array of 40 kW to 500 kW as shown in array 300 of FIG. 7B. A large-scale system may be composed of multiple arrays. The stack design is particularly suitable for an arranged layout due to an integrated compression system, direct bolt connection with an integrated gas connection, a short conduction path to the environment, and high voltage - low current output. By simplifying or eliminating the interface from the stack to the module, the stack may simplify the design of large-scale systems. Two embodiments of a stack array based on stack 110 of FIG. 2 are described below. The stack can be arranged in different package sizes depending on the application. The possible size range is from a single stack (~1.2 kW) to a stack of a 15x15 array (250 kW) or more. As an example, a 10x10, 100 kW package measures approximately 0.6 m x 0.6 m x 0.3 m (113 L) including compression, current collection, and ducts, which is competitive with internal combustion engines.

[0077] FIG. 5 is a perspective view of an array 100 of a fuel cell stack 110 according to one embodiment. In this embodiment, fuel is supplied and extracted from the base of the fuel cell stack 110, while the oxidant gas (e.g., air) is introduced into the containment volume above the stack and extracted from the base of the fuel cell stack 110. Air can generally be used as the main means of cooling the electrochemical cell stack 110 because of its high convective heat capacity. Air enters above 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 can be integrated in this area, where at low temperatures, less use of special materials and / or overall materials is required while maintaining appropriate strength and current capacity.

[0078] In some embodiments, air is heated to an appropriate inlet temperature as it flows from the upper low temperature region, through the oxidizer preheat tube 116 or the inlet tube, into the volume surrounding the stack 110, as shown in FIG. 6. The sealing between the upper zone and the stack zone may not be complete, and the overall layout is significantly simplified. FIG. 6 shows that the separation between the upper cooling zone and the lower stack zone is done by overlapping the separators mounted on the individual fuel cell stacks 110. In the embodiment shown in FIG. 6, there are two separator types, a ring separator 114 around each stack, and a cross separator 115 located between each group of four fuel cell stacks 110. For example, the ring separator 114 may be positioned around each of the fuel cell stacks 110 included in the array 100, and the cross separator 115 may be positioned between each set of four fuel cell stacks 110 included in the array 100. The oxidizer preheat tube 116 may be positioned through the corresponding cross separator 115.

[0079] The separators 114, 115 overlap to provide a barrier that preferentially directs gas to the oxidant preheating tube 116. This overlapping geometric arrangement of the separators maintains the 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 the separation between zones. The oxidant preheating tube 116 acts as a radiative heat transfer surface and uses the radiation from the high-temperature fuel cell stack 110 to heat the inlet air before the oxidant (e.g., air) comes 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 plane, where the air is first preheated in the upper zone, then by the oxidant preheating tube 116, and finally by direct contact with the stack manifold, and can ultimately enter the fuel cell stack 110 properly. Stack cooling, which is a major challenge for large SOFC stacks, can be achieved by heating the oxidant flow. Unlike direct convective cooling, a multi-stage inlet approach allows for a much larger temperature rise than when the oxidant (e.g., air) is directly introduced into the core of the fuel cell stack 110. With proper sizing, an inlet temperature of about 200 degrees Celsius (e.g., 150 - 250 degrees Celsius) can be achieved compared to 600 degrees Celsius for conventional stacks. This large temperature delta tolerance reduces the air flow and the preheating load, simplifying and improving the efficiency of the balance of components in the electrochemical cell assembly (e.g., fuel cell assembly or electrolytic cell assembly) including the array 100.

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

[0081] In high - power implementations (~10kW and above), a modular approach offers additional advantages. First, since the stack voltages are high enough, they can be connected in parallel or series - parallel electrical configurations. This provides automatic load limiting. Some stacks with degraded performance will automatically divert the current load to stacks in electrical parallel. Even if a stack is completely lost in a large multi - stack array, there is little adverse impact. Second, a failed stack can be replaced at relatively low cost without affecting other stacks. In conventional systems with relatively few stacks, when a single vulnerability occurs, it may be necessary to remove and repair a large stack, which is difficult to manage. In an array of small stacks, however, local weaknesses can be corrected by replacing only the weak stack, smaller devices, and a more rapid and low - cost process.

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

[0083] FIG. 7A shows 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., an electrochemical cell stack 110) is disposed on the housing base 30. The array 200 includes a 6×6 array (40+kW array) of fuel cell stacks, and all gas services are supplied from below. FIG. 7B shows a fuel cell assembly 40 according to another embodiment. The fuel cell assembly 40 includes a housing 42 having a housing base 50 on which an array 300 of fuel cell stacks (e.g., a fuel cell stack 110) is positioned. The array 300 includes an 8×5 array (280+kw array), and all gas services are supplied from below. In these layouts, the housing bases 30, 50 incorporate a heat exchange function and evenly distribute and collect gas 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 inlet and outlet of fuel and oxidant of the array. Since two stacks are omitted on the left side of FIG. 7A, two of the oxidant preheating 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 that are fluidly coupled to an array 200 of electrochemical cell stacks through the housing base 30. The housing base 30 also defines at least one heat exchange channel 34 configured to provide heat exchange between the fuel entering the housing base 30 through the fuel inlet 22 and the 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. Further, 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 at least one heat exchange channel.Accordingly, the fuel inlet 22 and the fuel bypass inlet 29 provide a dual fuel inlet, with the fuel bypass inlet 29 leading straight to the fuel cell stack array 200 and the fuel inlet 22 moving through the heat exchange and reforming section. These dual inlets are optional but provide additional controllability of the stack inlet temperature and in-stack reforming.

[0084] Figure 7C shows the flexibility of the array size while maintaining the overall module simplicity. Based on the fuel cell stack array 300 shown in Figure 7B, a conceptual array from 40 kW to 350 kW is shown.

[0085] Figure 8B is a perspective view of the base portion of the fuel cell assembly 40 shown in Figure 7B, showing the fuel inlet 42, the fuel outlet 44, the oxidant inlet 46, and the oxidant outlet 48 fluidly coupled to the array 300. In these embodiments, the upper portion 56 of the housing base 50 can take in fuel and discharge heat-exchanged fuel and can also include a fuel reforming section. A plurality of stack interfaces 52 are also provided on the housing base 50. The fuel cell assembly 40 shown in Figure 7B does not have a dual inlet.

[0086] In these embodiments, the cold air inlet enters the stack hot zone from below. FIG. 9A is a top view of a portion of the fuel cell stack 200 shown in FIG. 7A, showing the oxidant preheat tube 216 and the stack mounting interface 32. FIG. 9B is a top view of a portion of the fuel cell stack 300 shown in FIG. 7B, showing the oxidant preheat tube 316, the fuel preheat tube 318, and the stack mounting interface 52. As seen in FIGS. 9A and 9B, the arrays 200, 300 of these embodiments also include an oxidant preheat tube 316 that functions as a radiating 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 (not from the top of the array downward). In these embodiments of the array, the only connection from the top is the upper current collection connection. These are relatively simple connections because the current flowing through each stack is small (usually less than 30 A in fuel cell operation and usually less than 150 A in electrolysis operation).

[0087] Interconnection design When designing for small cells, one of the challenges is sealing. Given a seal with some characteristic leaks that are proportional to the seal area and inversely proportional to the seal thickness in the leakage direction, designing to minimize leakage favors larger cells. First, the ratio of the active area of the cell to the length of the edge (the sealed length) is approximately based on the cell size.

Number

Number

[0088] These constraints imply that a narrow seal with a low leakage rate is desirable to support a high-performance, low-leakage stack 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 non-compliant. Compliance may be desirable in an SOFC stack because the SOFC stack operates over a wide temperature range and components of the stack move relative to each other during warm-up or changes in operating conditions, preventing damage from stress buildup. Some stack designs incorporate a compliant function into the sheet metal interconnects to allow for rigid joints, but the compliant function itself is relatively bulky and suffers from the same problems as wide seals. They are not space-efficient in small cell designs.

[0089] FIG. 10 is a perspective view of a fuel cell stack having an overlapping seal design known in the art, showing typical compliance challenges in a cross-flow stack configuration. For purposes of discussion, assume that fuel flows from the lower left to the upper right and oxidant flows from the lower right to the upper left. The manifolds that separate the fuel from the oxidant outside the stack are not shown.

[0090] The top visible seal is a fuel seal that separates the fuel passing through the cell from the oxidant to the right lower face of the stack. Directly below the upper cell is an oxidant 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 and can include hundreds of cell layers.

[0091] As shown in the foreground of Figure 10, compliance issues occur where the seals overlap. The central foreground structure is made of repeated layers of seals, cells, seals, and interconnects. There is no ability to absorb strain (X, Y, or Z) in this region unless one or more of the components yield. If the goal is the use of non-compliant or nearly sealed gaskets, the cells are often the weakest components of the structure. In this case, most often, when stress builds up, the cells will break before they yield. This type of structure is not robust against real-world situations.

[0092] To mitigate unavoidable thermal stresses, compliance needs to be incorporated into the stack design. There are mainly two approaches to incorporate compliance. In the first approach, the 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 will inevitably leak as a result of their structure, limiting the extent to which small cells can be used before leakage begins to dominate performance. In the second approach, special interconnects or additional components with compliance functionality are used. This is done, for example, by rigidly sealing a thin metal sheet component to the cell, effectively stretching around the cell, and then laser welding this cell-stretched component to the interconnect. In this strategy, there are perimeter seals and internal port seals placed away from both the perimeter and the cell junctions. For example, the perimeter seals can contain fuel and the port seals can contain oxidizer. The additional perimeter required for this compliant portion means that a small cell size is not preferred.

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

[0094] There can be many competing design constraints in a compliant interconnect design. First, the interconnect may desirably: (1) provide a controlled flow distribution to both the cell fuel electrode and the cell oxidant electrode, (2) provide compliance to absorb stress, (3) provide an appropriate fuel and oxidant pressure drop, (4) provide an appropriate current conduction path, and (5) isolate the fuel flow from the oxidant 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 that defines a plurality of fuel channels 457 and a plurality of oxidant channels 459. In FIG. 11, the fuel side is on 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 upper surface of the interconnect 452, while the oxidant channels 459 are separated by ribs on the lower 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 a longitudinal channel 420 that passes through their geometric centers. FIG. 12A shows the fuel side of the top 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 oxidant side of the bottom 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 supplied by each of the channels 457 on the fuel side of the interconnect 452 (FIG. 12A) and the channels 459 on the oxidant side (FIG. 12B). The flat semi-circular area overlaid on the interconnect 452 represents the cell active area exposed to each interconnect channel 457, 459. The active areas are each a function of both position and size, as well as each channel 457, 459. The interconnect 452 is designed to provide flow downward through each channel 457, 459, and each channel 457, 459 is proportional to the active area supplied by that channel 457, 459. This is achieved with respect to size and spacing constraints that provide proper current collection from both cell electrodes. Some changes in the geometric arrangement affect the flow and electrical characteristics on both sides of the interconnect 452. Optionally, a contact intermediate layer may be added between each cell and each interconnect 452 to assist with electrical contact.

[0096] In the embodiment shown in FIGS. 12A and 12B, the outer seal member 458 is, in this embodiment, a fuel seal member on the outer periphery (FIG. 12A). In this embodiment, the inner seal member 456 is an oxidant seal member and 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 the space and the corrugated interconnect design provide the required compliance without increasing the stack perimeter or thickness. Of course, in embodiments where the 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 interconnect 452 is about 0.1 mm thick (for example, 0.07 to 0.13 mm thick). This enables the stack temperature to be appropriately controlled because the active area of the cell is small and the distance from any point on the cell to the edge is relatively small. For large-scale stacks or large distances, it is necessary to increase the thickness of the interconnect to obtain sufficient thermal conductivity to maintain control of the stack and cell temperatures.

[0098] Internal seal design An internal seal that is between the interconnect 452 and the cell and separates the fuel gas from the oxidant gas may be implemented as a glass-ceramic seal within the electrochemical cell stack included in the array 100, 200, or 300. Their positions can be seen in the schematic cross-sectional view of FIG. 13, which shows a schematic of an electrochemical cell stack including the interconnect 452 and the fuel cell stack 410 that is compressed between the upper plate 440 and the lower plate 460, and may be staggered between the inner diameter and the outer diameter in means for generating a bellows-like structure. In other words, the plurality of interconnects 452 included in the fuel cell stack 410 cooperate to form a bellows-like structure so that the fuel cell stack 410 has compliance. The interconnect 452 can be about 0.1 mm thick. A thin material combined with the corrugation that generates the flow field creates the interconnect 452 that easily relieves the stress within that layer. Thereby, a robust structure is generated that prevents the accumulation of stress from layer to layer. As shown in FIG. 13, it is preferable that no additional separator or metal component is used to provide the stress relief function. That is, the bellows-like structure of the fuel cell stack 410 is made of an alternating arrangement of cells 454, outer seals 458, interconnects 452, and inner seals 456.

[0099] In addition to the inner seal 456 and the outer seal 458, the edge seal member 461 may be disposed at at least one of the outer edge of the electrochemical cell 454 (e.g., each of the first and second electrochemical cells, including an electrochemical cell unit, e.g., a fuel cell unit or an electrolytic cell unit) proximate to the outer periphery of the interconnect 452, or the inner edge of the electrochemical cell 454 proximate to the longitudinal channel 420. For example, the edge of the cell anode support is typically porous. In the embodiment shown in FIG. 13, the edge seal member 461 is disposed at the outer edge of the electrochemical cell 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., the electrochemical cell 154a / b shown in FIG. 1B).

[0100] Post design The post can be used as a manifold for a gas (either fuel or oxidant) that passes through the longitudinal channel and reaches the electrochemical cell. The post may be positioned in the longitudinal channel and configured to separate the gas inlet from the longitudinal channel into the electrochemical cell from the gas outlet from the electrochemical cell into the longitudinal channel. The post is sealed in place with a ceramic slurry, paste, bat, or combinations thereof to provide a compliant seal between the inlet flow and the outlet flow. The post can be a machined metal, a sheet metal consisting of multiple parts, brazed, or ceramic with a function of forming a vertical channel to which a compliant seal material is added.

[0101] Figures 15A through 15C are top cross-sectional views of a portion of a fuel cell stack 510 according to three different embodiments, showing various posts disposed in the longitudinal channels 520 of the stack 510. In these embodiments, the longitudinal channels 520 are central channels that extend 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 disposed in channels that are offset from the center of the stack 510. In this embodiment, it is also assumed that fuel gas passes through the longitudinal channels 420. FIG. 15A shows a circular post 530a according to one embodiment. The post 530 defines a deep groove that is axially symmetrically disposed around it so as to define a post inlet 532a configured to receive fuel and a post outlet 534a configured to receive spent fuel and discharge it to the electrochemical cell stack 510. The post inlet 532a and the post outlet 534a are fluidly isolated from each other via a seal recess 536a. FIG. 15B shows a post 530b according to another embodiment disposed in the longitudinal channel 520. The post 530b includes two parallel plates that divide the longitudinal channel 520 into a post inlet 532b, a post outlet 534b, and a seal recess 536 that fluidly isolates the post inlet 532b from the post outlet 534b. In the embodiments of FIGS. 15A and 15B, the posts 530a / b, which are central posts, include 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 to each other. Two fuel outlet plates 537c are positioned perpendicular to the fuel inlet plates 531c so as to define two post outlets 534c facing each other. In the embodiment of FIG. 15C, the post 534c includes a fuel inlet port separated from two side post channels 532c (e.g., side fuel ports) by a central channel 533c, e.g., a fuel inlet plate 531c having a plurality of 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 of this embodiment has two post outlets 534c including fuel outlet ports. A seal member 539c, such as a ceramic coking material, is inserted into the seal recess 536c to separate the inlet fuel from the outlet fuel. This seal does not need to be airtight because the leakage path does not result in a combination of fuel and air. Rather, the effect of leakage through this seal is to reduce the flow of fuel through the stack itself. Moderate leakage (a few percent of the total flow) does not significantly affect the performance of the stack. Since a solid oxide fuel cell system typically operates with excess fuel to sweep reaction products (such as H2O, CO2, etc.) from the fuel electrode, even moderate leakage may not have a significant impact on system characteristics.

[0103] The material of the seal member 539c used in the post 530c may be designed to be somewhat compliant in order to allow thermal stresses to dissipate within the structure. As a result of the compliance requirement, the seal member 539c is not firmly adhered to the cell layer and is not airtight. However, it can be designed to be compliant at the same time and have a low enough leakage such that fuel preferentially flows around the interconnect rather than through the seal.

[0104] Figures 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 together with the upper plates 540a / b and the upper caps 542a / b. Direct adhesion to the upper and lower plates of the stack can induce unwanted thermal stresses. To prevent this, the upper (and optionally lower) joints are structured as compliant fit parts. A sealing material (e.g., the same sealing material used on the sides of the posts) is provided in the gap between the central post and the upper plate and / or the lower plate. There is no need to increase the allowed movement between the central post and the upper plate as long as it is sufficient to relieve the thermal stress. The magnitude of the desired relative movement is calculated as follows.

Number

[0105] When further enlarged, as shown in FIGS. 16A and 16B, the upper plates 540a / c are positioned at the upper end of the fuel cell stack 510 around the posts 530a / c, and thus a gap 541a / c is provided between the upper end plates 540a / c and the posts 530a / c. The gap 541a / c may be structured to allow movement of the posts 530a / c therein to relieve thermal stress. In some embodiments, a compliant seal member can be positioned within the gap 541a / c. For example, FIG. 16B shows a compliant seal member 543c positioned in the gap 541a / c across the post assembly 530c. The compliant seal member 543c can be configured to provide sufficient compliance to allow movement of the post 530c within the gap 541a / c. The upper end caps 542a / c may be positioned, for example, on the upper end plates 540a / c to close the stack. A secondary seal member 544a / c may be inserted between the upper end plate 540a / c and the upper end cap 542a / c.

[0106] The compliant seal member 543a / c between the central post 530a / c and the upper plate 540a / c and / or the lower plate may leak. The secondary seal member 544a and the upper end cap 542a / c are added above the post 530a / c. Since the compliance from the secondary seal member 544a may not be desirable, it can be made rigid and leak-free.

[0107] Design of the end plate The electrochemical cell stacks described herein (e.g., fuel cell stacks or electrolytic cell stacks) may also include a bottom end plate in addition to the top end plate (e.g., top end plates 540a / c). The bottom end plate mechanically supports the stack and provides gas connections for reactants (fuel and oxidant gases). The bottom end plate provides a seal surface for the fuel stack and / or other seals of the array interface, and further provides a seal surface for the stack outer manifold (e.g., oxidant manifold) and the post. The bottom end plate allows for some tolerance in mounting and isolates the stack core (cells, interconnects, and seals) from stresses occurring at the seal surfaces and bolt locations. Also, the bottom end plate transfers a compressive load from the compression system to the stack. Further, the bottom end plate may function as one of the electrical connection points of the stack.

[0108] Figures 17A - 17C are bottom perspective views of three different designs of a base plate assembly 660a / b / c according to an embodiment. Each of the base plate assemblies 660a / b / c includes a bottom end plate 662a / b / c that defines 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 bottom end plate 662a / b / c and is configured to yield with respect to the bottom end plate 662a / b / c to reduce the transfer of mechanical stress from the high-strength sealing plate 664a / b / c to the bottom end plate 662a / b / c. A plurality of attachment points 665a / b / c can be provided on the bottom end plate 662a / b / c, thereby enabling attachment of the bottom end plate 662a / b / c of an electrochemical cell stack (e.g., a fuel cell or an electrolytic cell stack).

[0109] In the embodiments of FIGS. 17A and 17B, the high-strength sealing plate 664a / b is the topmost having a high-strength sealing surface (made of, for example, a high-strength superalloy such as Haynes 230). This provides the surface strength required for the interface with the compression sealing member. However, the coefficient of thermal expansion of high-strength superalloys is typically higher than that allowed by the balance of the stack components. Thus, separation is provided between the high-strength seal surface and the rest of the stack. FIG. 17A shows a base plate assembly 660a in which the high-strength sealing plate 664a is separated from the lower-end plate 662a by a short tube 663a designed to yield under the imposed thermal stress and limit the transmission of the thermal stress into the stack.

[0110] FIG. 17B shows a base plate assembly 660b in which the high-strength sealing plate 664b includes a yield point, and as a result, the high-strength sealing plate 660b does not have sufficient strength to transmit stress into the stack through the lower-end plate 662b. In the embodiments shown in FIGS. 17A and 17B, one oxidant port, like the two fuel ports, is sent through the lower-end plate 662a / b. In embodiments where oxidant rather than fuel is provided through the post, the lower-end plate may instead include one fuel port and two oxidant ports. A threaded member is introduced to enable attachment of the lower-end plate. The threaded member is isolated within the design to prevent stress from being transmitted to the stack structure. The internal structure of the end plate is designed to carry the load from the compression system to the stack. In this design, it has been demonstrated that the current collection loss at the connection between the lower-end plate and the system is less than 0.1%. The upper-end plate provides a sealing surface for the oxidant manifold and post. It also transmits a compressive load from the compression system into the stack and isolates the stack core (cells, interconnects, seals) from the stress induced by the compression system. The upper-end plate provides a compliant sliding joint above the post. The upper-end plate may also function as one of the electrical connection points of the stack.

[0111] Figure 17C shows a base plate assembly 660c where the high-strength sealing plate 665c is mechanically separated but contained within the lower end plate 662c, thereby providing the strength necessary to achieve a seal between the stack and the manifold to which it is attached without applying thermal stress to the stack structure due to a mismatch in the coefficient of thermal expansion (CTE). In other words, the lower end plate 662c is inserted between the stack of multiple fuel cell units and the high-strength sealing plate 664c. A plurality of 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 lower end plate 662a to reduce stress transfer to it, but when bolted to its mating manifold within the system, it captures a portion of the lower end plate 662c between itself and the mating manifold system. Thus, the mechanical strength for sealing is provided by the high-strength sealing plate 664c, while the transport of gas from the mating manifold into the stack occurs through the lower end plate 662c which is mechanically separated and has a matching thermal expansion. The friction between the high-strength sealing plate 664c and the captured low-strength and low-CTE lower end plate 662c can be partially mitigated by a ceramic release layer, but the captured portion of the lower 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 lower end plate 662c, thereby minimizing the possibility of transferring thermal expansion stress to the stack structure.

[0112] FIG. 18 is a top perspective view of an upper end plate 740 including a post interface tube 748 that extends axially from the surface of the upper end plate 740 away from a post when the upper end plate is positioned on an electrochemical cell stack (e.g., a fuel cell stack or an electrolytic 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 tube 748 may be positioned around a portion of a post (e.g., a central post). The post interface tube 748 can provide a sliding surface to allow for a small amount of relative movement between the post and the upper end plate 740.

[0113] In some embodiments, the stack includes an integrated compression system. This allows the stack to be easily integrated into an array. One advantage of a small cell area and a glass-ceramic seal is that the compression load may 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), and another stack used in the array 300 of FIG. 7B), for example, is designed to operate at 9 kgf to 36 kgf compared to 360 kgf and 900 kgf of the stacks of 121 cm 2 and 550 cm 2 respectively. This simplifies both the design of the compression element 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 the following.

Equation

number

[0114] Keeping in mind that this is only a comparison of orders of magnitude, the above formula can be used to reduce the stiffness requirement of a compression plate to the traditional 550 cm 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, may be about 440 times less stiff for the same maximum deflection. This allows the design of the compression plate to be greatly simplified. Note that the maximum allowable deflection is essentially independent of cell size, since deflection results in loss of electrical contact.

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

[0116] When further enlarged, FIG. 19A is a top perspective view of an upper compression plate assembly 870 including an upper end plate 840a and a post interface tube 848a (e.g., the upper end plate 740 of FIG. 18), a base member 876a (e.g., a spring pack), and a compression member 879a, together with an upper compression plate 872a, according to an embodiment provided in the fuel cell stack included in the 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 inserted between the upper compression plate 872a and the upper end plate 840a. Other embodiments can use coil springs, or various forms of wave washers. The bias member 876a generates a compressive force for compressing the stack. Two compression members 879a (e.g., tension rods) carry the force from the top to the bottom of the stack where the compression plate (not shown) transmits the load to the lower end plate. The individual Belleville springs of the bias member 876a are separated by guide shims that align and guide the springs, such that the Belleville springs do not drift laterally or become overly compressed. The Belleville springs are designed to have low stress at the operating temperature. The compression members 879a are made of a superalloy having high strength at temperature. They have a higher coefficient of thermal expansion than the stack and have the effect of slightly releasing the spring pack when the temperature rises.

[0117] The upper compression plate assembly 870b of FIG. 19B is similar to that of FIG. 19A, except that the bias member 876b of the compression plate assembly 870b of FIG. 19B includes a plurality of coil spring sets (eight in this embodiment) rather than a Belleville spring pack. Further, the post 830b has a compliant sealing member 843b positioned thereon, which can be similar to the post assembly 530c described with respect to FIG. 15C. The upper compression plate assembly 870b is mounted to the fuel cell stack included in the array 300 of FIG. 7B.

[0118] FIG. 20A is a graph showing the spring response of a Belleville spring pack that can 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 can be used in the upper compression plate assembly of FIG. 19B, according to another embodiment. The difference in stack growth with respect to the tension rod has the effect of unloading the stack during a temperature rise. This is by design and has the secondary advantage of providing more stack compression during transportation when the temperature is low and creep of the materials is not a problem. The geometric arrangement is selected such that the compression is relaxed to the desired compression during operation. The circles represent the peak force points and are also the locations where the spring becomes unstable. Pushed to this limit, the spring runs the risk of inverting itself on the left side of the force-displacement graph. If this occurs, the spring will no longer provide an effective load to the stack and will not be able to recover without disassembling.

[0119] Therefore, in the design when the stack is cold, the displacement is limited to below the top diamond point (about 97 Newtons). When the stack is heated, the thermal expansion difference allows the spring set to relax to the lower diamond point, where the stack is loaded to 82 Newtons and the spring stress drops below 50 MPa, which is within the creep limit of certain superalloys (e.g., Waspaloy, Haynes 282). The "+" point represents the compressive load remaining on the stack after 0.5% creep of the tension rod. After this creep, the load remains at an acceptable 18 Newtons, at which point the peak stress of the spring drops to 10 MPa. This unloading of the spring and the compressive member slows down the net creep rate of the entire system. To make the most of the spring characteristics, it is permitted to experience higher stresses at room temperature where creep of the material is not a problem. The numbers provided illustrate a specific case. Generally, design strategies considering 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 specific stack.

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

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

[0122] Figure 21A shows a configuration with two inlets (front and rear) and two outlets (left and right sides, with a path defined to a base connection point). The lower end plate 962a is positioned at the base of the electrochemical cell stack 910a, and the lower compression plate 982a is positioned below the lower end plate 962a. The compression member 979a transmits a compressive force from the upper compression plate to the lower compression plate 982a, thereby biasing the lower compression plate 982a, causing the lower end plate 962a to be pushed towards the electrochemical cell stack 910a, thereby fixing the electrochemical cell stack 910a. Figure 21B shows an alternative implementation of the structure shown in Figure 21A. As shown in the embodiment of Figure 21A, there are the same functional parts except for a different configuration where the mounting bolts 983b descend from above rather than from below the lower compression plate 982b. Referring to Figures 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, there are dielectric seal members 914a / b that are positioned within the volume defined by the manifolds 912a / b around the electrochemical cell stack and are configured to fluidly seal a first part of the volume from a second part of the volume. The dielectric seal members 914a / b separate the oxidant inlet from the oxidant outlet. Small leaks across the seals formed by the dielectric seal members 914ab may be tolerated without impairing stack operation.

[0123] In a particular embodiment shown in FIG. 21B, the oxidant outlet (or inlet) port is split into two and consists 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 in 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 to the flow of oxidant. The remaining ports (left and right openings) are the fuel input and fuel output ports and can be interchanged in position as needed.

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

[0125] A complete (closed) manifold effectively isolates the operating stack core from its environment. In the case of exothermic operating conditions, this potentially allows the ambient temperature to be reduced by up to 100 °C. This can result in significant benefits in terms of reducing the insulation requirements around the stack or stack array and in terms of being able to use lower quality materials around the stack. This can result in system-level cost savings. It can also reduce other system-level issues such as chromium volatilization and issues with the transport or oxidation of other materials.

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

[0127] Experimental Example It is contrary to current wisdom that reducing the size of components can increase the overall power density (per kg and per L). The accepted wisdom is that the process for increasing power density and reducing cost is to increase the size of each component while reducing the number of components. This is based on the fact that the volume and cost of the stack are greatly influenced by the non-active parts of the stack (sealing area, end plates, compression, etc.), and that by moving to larger cells, the overall contribution of these non-active areas to the stack cost decreases. In planar SOFCs where the cells are thin ceramic components, it is difficult to produce large cells. Considerable efforts have continued to increase the size of SOFC cells.

[0128] In contrast, the embodiments described herein demonstrate, with a novel design, that the smaller the components, the higher the power density can be and the more likely it is to be of low cost. Contrary to current wisdom, moving to smaller cells shows that the cost of non-active components can be reduced by making them smaller and simpler than their larger counterparts, even when considered in proportion to the total active area or power output.

[0129] Experiments were conducted comparing three different known fuel cell stacks with an exemplary fuel cell stack according to one embodiment. All stacks were manufactured using the same basic materials. Each stack was of a planar solid oxide fuel cell design. Each stack used an anode supported on the same type of nickel-yttria stabilized zirconia cell, but the size and thickness were appropriate for a particular stack. Each stack had a metallic interconnect 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 stack manufactured according to the embodiments described herein also included a compression system. 121 cm 2The cell stack of baseline 28 with an active area cell operated at 390 mA / cm 3 and supplied a total power of 1200 W. It was measured to achieve a power-to-weight ratio of 69 W / kg and a power-to-volume ratio of 225 W / L at a size of 190 mm × 190 mm × 150 mm with a total volume of 5.4 L and a weight of 17 kg. A stack of 120 cells with an active area cell of 550 cm 2 operated at 290 mA / cm 2 and supplied a total power of 16,900 W. It was measured to achieve a power-to-weight ratio of 71 W / kg and a power-to-volume ratio of 176 W / L at a size of 395 mm × 395 mm × 618 mm with a total volume of 96 L and a weight of 238 kg (including end plates). In contrast, a stack fabricated according to one embodiment having 225 cells each with an active area cell of 25 cm 2 operated at 0.39 mA / cm 2 and supplied a total power of 1760 W. It was measured to achieve a power-to-weight ratio of 733 W / kg and a power-to-volume ratio of 1257 W / L at a size of 79 mm × 71 mm × 254 mm with a total volume of 1.4 L and a weight of 2.4 kg. Although relatively young compared to other stack designs, the stack of this embodiment has already achieved a power density 10 times in weight and approximately 7 times in volume. This result was unexpected from the perspective of the accepted belief that the process for increasing power density and reducing cost is to reduce the number of components while increasing the size of each component

[0130] The selection of test data is presented in FIGS. 22-24. FIG. 22 shows a 225-cell (~1 kW) stack included in array 200, according to an embodiment of fuel cell stack 110, operating with a gas composition representative of typical natural gas combustion system applications. Gas conditions include representative levels of gas utilization to convert inlet natural gas to hydrogen, carbon monoxide, and carbon dioxide, representative current density and temperature, and representative levels of in-stack steam reforming. The stack has demonstrated stable operation for over 5000 hours and shows a degradation rate consistent with that expected from the cell materials used. That is, there are no aspects of degradation that can be linked to stack design. This results in demonstrating the stack's ability to handle thermal conditions including typical flows and in-stack reforming of a natural gas combustion system, while maximizing the potential of the underlying repeating cell materials.

[0131] FIG. 23 shows the results of a 20-cell implementation of the same stack, running under very aggressive electrolysis conditions of -2 A / cm 2 . Some test interruptions, due to plant balance impairments (not the stack's fault), occurred early in the test and can be seen as data spikes at elapsed times of approximately 25, 125, and 250 hours. After possible initial degradation caused by the test interruptions (degradation from electrolysis appears as an increase in voltage), the stack demonstrated operation longer than 1000 hours without degradation despite the aggressive conditions. This shows the flexibility of the stack to operate under a 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 exceeded one year. While the changing conditions made it difficult to determine the degradation rate, the stack exhibited high stability (low degradation) throughout the year of testing, including very 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 relatively long-term stability of the stack and its ability to reject heat when operating in the exothermic mode without internal reforming to absorb some of that heat.

[0133] Figures 23 and 24 show results including thermal cycles, where no change in performance was observed before and after the thermal cycles, demonstrating the stack's thermal cycling ability regardless of the use of glass-ceramic seals. This demonstrates the success of an essentially compliant structure, which allows the use of sealed or nearly sealed glass-ceramic seals while preventing the accumulation of thermal stresses that could cause seal or cell failure in other respects.

[0134] Figure 25 shows the results of a 45-cell implementation of an electrochemical cell stack by Array 300 operating in electrochemical cell mode at 0.25 A / cm 2 demonstrating very low degradation after one thermal cycle. These results show a case where a somewhat larger cell stack (81 cm 2 active area) does not prevent the ability to eliminate thermal stresses and draw out all possibilities from the material set.

[0135] Figure 26 shows the results of an individual 45-cell implementation of a fuel cell stack by Array 300 operating in electrolysis mode at -1 A / cm 2 demonstrating very low degradation and actually a slight improvement in overall performance after 1600 hours of testing. This test also demonstrated its robustness against aggressive thermal cycles and aggressive transients due to a balance of plant failures (not stack-related) that did not affect the stack's performance.

[0136] The above integrated design addresses many of the major barriers between SOFC technology and the current market, and in part provides significant cost reduction opportunities at both the stack level (by reducing material content and facilitating component automation) and the system level, and its stack characteristics provide opportunities for system simplification (such as high voltage output, low current, compact packaging, low external heat exchange requirements, no external compressive load requirements, etc.).

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

[0138] As used herein, the terms "about," "approximately," "substantially," and similar terms are intended to have a broad meaning in harmony with the usage generally accepted 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 of this disclosure that these terms are provided to enable the description and claim of specific features without limiting the scope of these features to the exact numerical ranges provided. Accordingly, these terms should be construed to indicate that minor or non-essential modifications or variations of the subject matter being described and claimed are considered to be within the scope of the invention recited in the appended claims.

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

[0140] References to the positions of elements in this specification (e.g., "above," "below," "upper," "lower," etc.) are used solely to describe the orientation of various elements in the figures. Note that the orientation of the various elements may vary 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 merely illustrative. Although only some embodiments have been described in detail in this disclosure, those skilled in the art considering this disclosure will readily understand that many modifications (e.g., the sizes, dimensions, structures, shapes, and ratios of various elements, the values of parameters, the mounting arrangements, the use of materials, the colors, the changes in orientation, etc.) can be made 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 positions of the elements may be reversed or otherwise changed, the nature or number of individual elements or positions may be varied or changed, or the order or sequence of any process or method steps may be changed or rearranged according to alternative embodiments. Without departing from the scope of the present invention, other substitutions, modifications, changes, and omissions can also be made in the design, operating conditions, and arrangement of the various exemplary embodiments. For example, the perforated baffle can be further optimized to achieve the intention of extending the residence time without creating a dead zone.

Claims

**Claim 1** A base plate assembly for an electrochemical cell stack, comprising: A lower end plate defining a fuel inlet port, a fuel outlet port, and an oxidant port; A high-strength sealing plate having a plurality of openings axially aligned with each of the fuel inlet port, the fuel outlet port, and the oxidant port, and configured to be located on the side opposite to the electrochemical cell stack with respect to the lower end plate; A plurality of tubes located between the lower end plate and the high-strength sealing plate; The tubes are configured to yield so as to reduce the transmission of mechanical stress from the high-strength sealing plate to the lower end plate. The base plate assembly. **Claim 2** The base plate assembly according to claim 1, wherein the high-strength sealing plate further comprises a plurality of threaded members, each of which is configured to receive a fastener for attaching the electrochemical cell stack to the housing base of the electrochemical cell assembly. **Claim 3** The base plate assembly according to claim 2, wherein the high-strength sealing plate has a plurality of additional openings, each of which is configured to allow one of the fasteners to extend therethrough. **Claim 4** The base plate assembly according to claim 2, wherein the high-strength sealing plate has a plurality of additional openings, and each threaded member extends through a corresponding one of the openings in the high-strength sealing plate. **Claim 5** The base plate assembly according to claim 1, wherein the high-strength sealing plate has additional openings configured to receive fasteners for attaching the electrochemical cell stack to the housing base of the electrochemical cell assembly, the additional openings are located at the peripheral edge of the high-strength sealing plate, and the high-strength sealing plate is configured to move laterally with respect to the lower end plate so as to further reduce the transmission of the mechanical stress from the high-strength sealing plate to the lower end plate due to thermal expansion. **Claim 6** The base plate assembly according to claim 5, further comprising a ceramic layer disposed between the high-strength sealing plate and the lower end plate. **Claim 7** The base plate assembly according to claim 1, wherein the high-strength sealing plate has a higher coefficient of thermal expansion than the lower end plate.

8. The base plate assembly according to claim 7, wherein the high-strength sealing plate is formed of Haynes 230 alloy.

9. An electrochemical cell stack assembly, A plurality of electrochemical cells configured in a stack, the stack comprising a plurality of electrochemical cells having a longitudinal channel extending therethrough, An outer gas manifold disposed around a peripheral portion of the stack, A lower end plate configured to interface with the outer gas manifold to seal a lower end portion of the stack, the lower end plate including a fuel inlet port, a fuel outlet port, and an oxidant port fluidly coupled to the longitudinal channel, A high-strength sealing plate having a plurality of openings axially aligned with each of the fuel inlet port, the fuel outlet port, and the oxidant port, the high-strength sealing plate being located on a side opposite the plurality of electrochemical cells with respect to the lower end plate, A plurality of tubes positioned between the lower end plate and the high-strength sealing plate, the tubes being configured to yield to reduce transmission of mechanical stress from the high-strength sealing plate to the lower end plate, an electrochemical cell stack assembly.

10. The electrochemical cell stack assembly according to claim 9, wherein the high-strength sealing plate further comprises a plurality of through holes located at a peripheral portion of the high-strength sealing plate and configured to receive fasteners from above to couple the electrochemical cell stack assembly to a housing of an electrochemical cell module assembly.

11. The electrochemical cell stack assembly according to claim 9, wherein the high-strength sealing plate further comprises a plurality of attachment points for coupling the electrochemical cell stack assembly to a housing of an electrochemical cell module assembly.

12. The electrochemical cell stack assembly according to claim 11, wherein the attachment points comprise threaded members configured to receive fasteners.

13. The high-strength sealing plate is configured to move laterally with respect to the lower-end plate so as to further reduce the transmission of mechanical stress from the high-strength sealing plate to the lower-end plate due to thermal expansion, the electrochemical cell stack assembly according to claim 10.

14. The electrochemical cell stack assembly according to claim 13, further comprising a ceramic layer disposed between the high-strength sealing plate and the lower-end plate.

15. The electrochemical cell stack assembly according to claim 9, wherein the high-strength sealing plate has a higher coefficient of thermal expansion than the lower-end plate.

16. The electrochemical cell stack assembly according to claim 15, wherein the high-strength sealing plate is formed of Haynes 230 alloy.

17. A base plate assembly for an electrochemical cell stack, a lower-end plate defining a fuel inlet port, a fuel outlet port, and an oxidant port, a high-strength sealing plate having a plurality of openings axially aligned with each of the fuel inlet port, the fuel outlet port, and the oxidant port, configured to be located on a side opposite to the electrochemical cell stack with respect to the lower-end plate, and further comprising a yield point configured to reduce the transmission of mechanical stress to the lower-end plate.

18. The base plate assembly according to claim 17, wherein the high-strength sealing plate has a higher coefficient of thermal expansion than the lower-end plate.

19. The base plate assembly according to claim 18, wherein the high-strength sealing plate is formed of Haynes 230 alloy.

20. The base plate assembly according to claim 17, further comprising a threaded member configured to receive a fastener for coupling the base plate assembly to a fuel cell assembly housing, wherein the high-strength sealing plate is configured to fluidly seal a connection between the base plate assembly and the fuel cell assembly housing.

21. An upper compression plate assembly for an electrochemical cell stack, an upper-end plate configured to interface with an upper end of a stack of electrochemical cells, an upper compression plate located on the upper-end plate, An upper compression plate assembly comprising a plurality of springs coupled to a peripheral portion of the upper compression plate and configured to exert a compressive force on the upper compression plate against the upper end plate.

22. The upper compression plate assembly according to claim 21, wherein the spring comprises a coil spring configured to exert a tensile force on the upper compression plate.

23. The upper end plate comprises a central opening configured to allow a post to extend therethrough, and the upper compression plate assembly further comprises a post interface tube coupled to the upper end plate and positioned around the central opening, and a compliant seal member positioned in the post interface tube and configured to fluidly isolate a first portion of the post from a second portion of the post, the compliant seal member providing sufficient compliance to allow the post to move within the post interface tube. The upper compression plate assembly according to claim 21.

24. The upper compression plate assembly according to claim 23, wherein the upper compression plate comprises an opening and the post interface tube extends through the opening.

25. an upper end cap positioned at an end of the post interface tube, and a secondary seal member positioned between the upper end cap and the post interface tube. The upper compression plate assembly according to claim 23.

26. An electrochemical cell stack assembly comprising a plurality of electrochemical cells configured in a stack, the stack comprising a plurality of electrochemical cells having a longitudinal channel extending therethrough, an outer gas manifold disposed around a peripheral portion of the stack, an upper end plate configured to interface with an upper end portion of the outer gas manifold, an upper compression plate positioned on the upper end plate, and a plurality of springs coupled to a peripheral portion of the upper compression plate and configured to exert a compressive force on the upper compression plate against the upper end plate.

27. The electrochemical cell stack assembly according to claim 26, wherein the spring comprises a coil spring configured to exert a tensile force on the upper compression plate.

28. The electrochemical cell stack assembly according to claim 26, wherein the spring is further coupled to a lower compression plate located at a lower end portion of the outer gas manifold.

29. The upper end plate includes a central opening configured to allow a post to extend therethrough, and the electrochemical cell stack assembly further includes a post interface tube coupled to the upper end plate and located around the central opening, and a compliant seal member located in the post interface tube and configured to fluidly isolate a first portion of the post from a second portion of the post, the compliant seal member providing sufficient compliance to allow the post to move within the post interface tube. The electrochemical cell stack assembly according to claim 26.

30. The electrochemical cell stack assembly according to claim 29, wherein the upper compression plate includes an opening, and the post interface tube extends through the opening.

31. an upper end cap located at an end of the post interface tube, and a secondary seal member located between the upper end cap and the post interface tube. The electrochemical cell stack assembly according to claim 29.

32. An upper compression plate assembly for an electrochemical cell stack, comprising an upper end plate configured to interface with an upper end portion of a stack of electrochemical cells, an upper compression plate located on the upper end plate, and a plurality of Belleville springs located between the upper compression plate and the upper end plate and configured to exert a compressive force of the upper compression plate on the upper end plate. An upper compression plate assembly.

33. The upper compression plate assembly according to claim 32, wherein the upper compression plate includes a plurality of openings configured to receive tension rods.

34. The upper end plate includes a central opening configured to allow a post to extend therethrough, and the upper compression plate assembly further includes a post interface tube coupled to the upper end plate and located around the central opening, A compliant seal member located in the post interface tube and configured to fluidly isolate the first portion of the post from the second portion of the post, the compliant seal member providing sufficient compliance to allow the post to move within the post interface tube, the upper compression plate assembly according to claim 32.

35. The upper compression plate according to claim 34, wherein the upper compression plate has an opening, and the post interface tube extends through the opening.

36. An upper end cap located at an end of the post interface tube; The upper compression plate assembly according to claim 34, further comprising a secondary seal member located between the upper end cap and the post interface tube.

37. An electrochemical cell stack assembly, A plurality of electrochemical cells configured in a stack, the stack comprising a plurality of electrochemical cells having a longitudinal channel extending therethrough; An outer gas manifold disposed around a peripheral portion of the stack; An upper end plate configured to interface with an upper end portion of the outer gas manifold; An upper compression plate located on the upper end plate; An electrochemical cell stack assembly comprising a plurality of Belleville springs located between the upper compression plate and the upper end plate and configured to exert a compressive force on the upper compression plate against the upper end plate.

38. The electrochemical cell stack assembly according to claim 37, wherein the upper compression plate is further coupled to a lower compression plate located at a lower end portion of the outer gas manifold by a plurality of tension rods.

39. The electrochemical cell stack assembly according to claim 38, wherein the tension rod has a higher coefficient of thermal expansion than the stack.

40. The electrochemical cell stack assembly according to claim 39, wherein the tension rod is formed of a superalloy.

41. The upper end plate has a central opening configured to allow a post to extend therethrough, and the electrochemical cell stack assembly further comprises: A post interface tube coupled to the upper end plate and located around the central opening; A compliant seal member positioned in the post interface tube and configured to fluidly isolate the first portion of the post from the second portion of the post, the compliant seal member providing sufficient compliance to allow the post to move within the post interface tube, the electrochemical cell stack assembly of claim 37.

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