Glass composition for fuel cell stack sealing

Abstract

This invention relates to glass compositions and sealing materials comprising glass compositions, which are suitable for electrochemical devices requiring hermetically sealed environments, such as solid oxide fuel cells (SOFCs) and solid oxide electrolyzer (SOEC) stacks.

Description

Technical Field

[0001] This invention relates to glass compositions and sealing materials comprising glass compositions, which are suitable for electrochemical devices requiring hermetically sealed environments, including solid oxide fuel cell stacks and similar devices, such as solid oxide electrolyzer cell stacks.

[0002] Related Applications

[0003] This application claims priority to Australian provisional patent application AU AU 2021900273 and Australian patent application AU2021218224, the entire contents of which are incorporated herein by reference. Background of the Invention

[0005] An electrochemical device, or electrochemical cell, is a device capable of generating electrical energy from a chemical reaction or using electrical energy to induce a chemical reaction. An example of an electrochemical device is a solid oxide fuel cell (SOFC) unit, which converts the chemical energy of a gaseous fuel (such as hydrogen) into electrical energy through electrochemical oxidation. A typical SOFC stack consists of multiple interconnected cells, each with a porous ceramic cathode and a porous ceramic anode, separated by a dense, ion-conducting solid oxide electrolyte. The stack typically includes a support structure consisting of one or more supports made of a suitable material, such as a suitable metal. During operation of the SOFC stack, fuel, such as natural gas, is supplied to the anode of each cell, and an oxidant, such as air, is supplied to the cathode of each cell. The cell assembly is assembled in such a way that fuel and oxidant can be supplied to the anode and cathode of each cell, respectively. Another example of an electrochemical device is a solid oxide electrolyzer (SOEC) unit, which is essentially a SOFC operating in a regenerative (reverse) mode and performs the electrolysis of water to produce hydrogen and oxygen.

[0006] SOFC and SOEC stacks require hermetically sealed batteries to prevent fuel and oxidant mixing, making them crucial for the performance, durability, and safe operation of the SOFC or SOEC stack. Depending on the stack design requirements, seals are typically used to separate the anode and cathode cavities of the SOFC or SOEC stack from each other and from the surrounding environment. These seals also enable the mechanical bonding of the SOFC or SOEC stack components and the electrical insulation between the bonded components.

[0007] During operation, SOFC and SOEC fuel cell stacks reach elevated temperatures, typically ranging from approximately 500°C to approximately 1000°C, and undergo intentional and unintentional temperature fluctuations (thermal cycling), from low temperatures to ambient temperatures to operating temperatures with varying heating and cooling rates. To ensure the commercial viability of SOFC and SOEC fuel cell stacks, the seals must maintain their integrity and meet all the aforementioned requirements under thermal cycling conditions and thousands of hours of isothermal operation. For example, the mismatch between each seal and other components of the SOFC or SOEC fuel cell stack due to thermal expansion and contraction should be sufficiently low to prevent seal or any other component from failing under the thermal stress generated during thermal cycling. Furthermore, the seals should not adversely interact with other components of the SOFC or SOEC fuel cell stack, either by releasing unwanted volatile substances that alter the chemical or physical properties of other components, or by reacting with other components in contact with the seals.

[0008] Several types of glass have been developed for use as seals in SOFC and SOEC fuel cell stacks. One type of glass is designed to retain most of its liquid glass phase. This provides the glass with the ability to flow under generated thermal stress (exhibiting viscous relaxation), serving as a primary means of reducing the magnitude of stresses applied to other components and at interfaces with other components at temperatures above the glass transition temperature (Tg). This type of glass has several drawbacks. For example, it is generally prone to cracking at temperatures below Tg, where viscous relaxation is not present. Furthermore, the glass typically contains significant amounts of components such as alkali metal oxides and B₂O₃, which can (a) make the seal a poor electrical insulator, (b) volatilize or leach in the humid gas environment within the fuel cell stack, leading to continuous changes in the chemical and physical properties of the seal, and (c) cause adverse reactions with other components.

[0009] Another type of glass is designed to become a highly crystalline rigid glass-ceramic at SOFC and SOEC operating temperatures. While this type of highly crystalline glass mitigates the drawbacks associated with the reactivity of the aforementioned low-crystallinity glass seals, densifying seals made from this type of glass and eliminating large inherent defects can be extremely difficult. The presence of large inherent defects, along with the lack of sufficient glass phase to reduce stress concentration at the tips of existing defects, makes this type of glass prone to cracking under harsh thermal cycling due to the propagation of existing inherent defects.

[0010] The aforementioned defects may impair the performance of glass seals currently used in commercial SOFC and SOEC fuel cell stacks. Therefore, there is a need for alternative glass seals suitable for electrochemical devices requiring hermetically sealed environments (such as SOFC and SOEC fuel cell stacks).

[0011] References to any prior art in this specification are not intended to acknowledge or imply that such prior art constitutes part of common general knowledge in any jurisdiction, or that such prior art can be reasonably understood, regarded as relevant, and / or combined with other prior art by a person skilled in the art. Summary of the Invention

[0012] The inventors have developed a glass composition capable of forming glass seals suitable for SOFC devices. The formed glass seals advantageously comprise one or more crystalline phases and a glass phase.

[0013] In one aspect, the present invention provides a glass composition comprising, by weight (mol%):

[0014] - Approximately 50 to 60 mol% SiO2;

[0015] - Approximately 2 to approximately 10 mol% of B2O3;

[0016] - Approximately 0.5% to approximately 3 mol% of Al2O3;

[0017] - Approximately 4 to 6 mol% TiO2;

[0018] - Approximately 1 to approximately 4 mol% of CeO2;

[0019] - Approximately 2 to 30 mol% of SrO; and

[0020] - Approximately 2 to approximately 25 mol% BaO.

[0021] In some embodiments of the glass composition, condition (a) and one or both of conditions (b) and (c) are satisfied:

[0022] (a)mol%BaO>(2x mol%TiO2+mol%B2O3);

[0023] (b) (mol% BaO + mol% SrO-2x mol% TiO2-mol% B2O3) ≤ 0.5x (mol% SiO2 – 2x mol% TiO2-2 / 3x mol% B2O3);

[0024] (c) (mol% BaO+mol% SrO-2x mol% TiO2) / (mol% SiO2-2 x mol% TiO2)<0.5.

[0025] In some embodiments, the glass composition is substantially free of alkali metal oxides.

[0026] On the other hand, the present invention provides a sealing material for an electrochemical device comprising the glass composition described herein. The electrochemical device can be any electrochemical device requiring a hermetically sealed seal. In a preferred embodiment, the electrochemical device is an SOFC or SOEC fuel cell stack.

[0027] On the other hand, the present invention provides an electrochemical device comprising one or more batteries, each battery including a cathode, an anode, and a solid electrolyte; a support structure including one or more supports; and a sealing material described herein. The electrochemical device can be any electrochemical device requiring a hermetically sealed environment. In a preferred embodiment, the electrochemical device is an SOFC or SOEC stack.

[0028] On the other hand, the present invention provides the use of the glass compositions or sealing materials described herein to form seals in electrochemical devices. The electrochemical device can be any electrochemical device requiring a hermetically tight seal. In a preferred embodiment, the electrochemical device is an SOFC or SOEC fuel cell stack.

[0029] On the other hand, the present invention provides a method for forming a seal in an electrochemical device, which is an SOFC or SOEC stack, the method comprising:

[0030] - Apply the sealing material described herein to either or both of the cells and support structures of the SOFC or SOEC stack;

[0031] - The sealing material is subjected to a sintering thermal cycle, wherein the glass composition of the sealing material softens to provide a sintered glass, and subsequently undergoes controlled crystallization to provide a glass ceramic comprising one or more crystalline phases and a glass phase;

[0032] - This forms a seal in the SOFC or SOEC fuel cell stack.

[0033] Further aspects of the invention, and other embodiments of the aspects described in the preceding paragraphs, will become apparent from the following description given by way of example and with reference to the accompanying drawings.

[0034] Brief description of the attached figures

[0035] Figure 1 This is a schematic diagram of a portion of a solid oxide fuel cell stack, with the battery assembly shown in an exploded view.

[0036] Figure 2 This is a typical particle size distribution diagram of glass powder prepared from the glass composition of the present invention.

[0037] Figure 3 The images show scanning electron microscope images of sintered glass samples prepared from the glass composition of the present invention at two different magnifications.

[0038] Figure 4 This is a graph showing the difference in expansion between the metal used in the support structure of the SOFC stack and the sintered glass rod prepared from the glass composition of the present invention.

[0039] Figure 5 This is a graph showing the difference in expansion between the metal used in the support structure of the SOFC stack and a sintered glass rod prepared from the glass composition of the present invention, which has been subjected to 0, 1000, 2000, 4000 and 6000 hours in an atmospheric environment at 850°C.

[0040] Figure 6 Scanning electron microscope images of sintered glass rods prepared from the glass composition of the present invention are shown, which have been aged in air at 850°C for 0, 1000, 2000 and 6000 hours.

[0041] Figure 7 This is a graph showing the difference in expansion between the metal used in the support structure of the SOFC stack and a sintered glass rod prepared from the glass composition of the present invention, which has been subjected to 0, 1000, 2000, 4000 and 6000 hours in a fuel environment at 850°C.

[0042] Figure 8 Scanning electron microscope images of sintered glass rods prepared from the glass composition of the present invention are shown, which have been fuel aged at 850°C for 0 hours (top left), 500 hours (top right), 1000 hours (bottom left), and 2000 hours (bottom right).

[0043] Figure 9 Scanning electron microscope images of the glass composition of the present invention before and after fuel aging are shown.

[0044] Figure 10 This is a graph showing the percentage of voltage degradation relative to the number of thermal cycles when an SOFC stack having the glass composition of the present invention undergoes approximately 100 thermal cycles over approximately 9000 hours.

[0045] Figure 11 An optical microscope image of a glass seal made from the glass composition of the present invention is shown after SOFC stack testing. Detailed Implementation

[0046] definition

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While any methods and materials similar to or equivalent to those described herein may be used in the practice or testing of this invention, only preferred methods and materials are described. For the purposes of this invention, the following terms are defined as follows.

[0048] As used herein, the term “about” means that the quantity, value, size, or amount varies by no more than 30%, 25%, 20%, 15%, or 10% compared to a reference quantity, value, size, or amount.

[0049] As used herein, unless the context otherwise requires, the term “comprise” and variations thereof, such as “including,” “containing,” and “having,” are not intended to exclude additional additives, components, integers, or steps.

[0050] Glass composition

[0051] This invention provides a glass composition comprising, in mol% of the glass composition:

[0052] - Approximately 50 to 60 mol% SiO2;

[0053] - Approximately 2 to approximately 10 mol% of B2O3;

[0054] - Approximately 0.5% to approximately 3 mol% of Al2O3;

[0055] - Approximately 4 to 6 mol% TiO2;

[0056] - Approximately 1 to approximately 4 mol% of CeO2;

[0057] - Approximately 2 to 30 mol% of SrO; and

[0058] - Approximately 2 to approximately 25 mol% BaO.

[0059] In a preferred embodiment of the glass composition, condition (a) and one or both of conditions (b) and (c) are satisfied:

[0060] (a)mol%BaO>(2 x mol%TiO2+mol%B2O3);

[0061] (b) (mol% BaO+mol% SrO-2 x mol% TiO2-mol% B2O3) ≤ 0.5x (mol% SiO2–2 x mol% TiO2-2 / 3 x mol% B2O3);

[0062] (c) (mol% BaO + mol% SrO-2 x mol% TiO2) / (mol% SiO2-2 x mol% TiO2) < 0.5.

[0063] Advantageously, satisfying condition (a) and one or both of conditions (b) and (c) allows the glass composition to form a glass seal having a glass phase, substantially free of BaO and B2O3, respectively. As used herein, in the context of a glass phase, "substantially free of" means that the glass phase does not contain a specific metal oxide, or contains only a specific metal oxide in an amount that has no measurable effect on the properties and / or performance of the glass seal formed from the glass composition. Thus, the term "substantially free of BaO and B2O3" will be understood to mean that the glass phase does not contain BaO and B2O3 or contains an amount of BaO and B2O3 that has no measurable effect on the properties and / or performance of the glass seal formed from the glass composition. Thus, the glass phase may contain small amounts of BaO and / or B2O3, as long as these amounts have no measurable effect on the properties and / or performance of the glass seal formed from the composition. Not wishing to be bound by theory, the inventors assume that condition (a) and one or more of conditions (b) and (c) respectively allow substantially all of BaO and Ba2O3 to be in crystalline form in the glass seal.

[0064] In a preferred embodiment, the glass composition is substantially free of alkali metal oxides. Glass seals containing alkali metal oxides may become contaminated, electrochemically unstable, and lack robustness, which could lead to performance degradation in SOFC or SOEC stacks or other electrochemical devices requiring hermetically sealed environments.

[0065] The glass composition may optionally not contain other metal oxides, that is, it does not contain any metal oxides other than SiO2, B2O3, Al2O3, TiO2, CeO2, SrO, and BaO. In some embodiments, the glass composition does not contain CaO. In some embodiments, the glass composition does not contain ZrO2.

[0066] In some embodiments, the glass composition, based on mol% of the composition, is substantially composed of the following or consists of the following:

[0067] - Approximately 50 to 60 mol% SiO2;

[0068] - Approximately 2 to approximately 10 mol% of B2O3;

[0069] - Approximately 0.5% to approximately 3 mol% of Al2O3;

[0070] - Approximately 4 to 6 mol% TiO2;

[0071] - Approximately 1 to approximately 4 mol% of CeO2;

[0072] - Approximately 2 to 30 mol% of SrO; and

[0073] - Approximately 2 to approximately 25 mol% BaO.

[0074] In this document, the terms “consistently composed of” and “composed of” will be understood to imply that the composition does not contain any additional metal oxides, that is, the composition contains only the metal oxides specified in the composition.

[0075] In these embodiments of the glass composition, it is preferred to satisfy condition (a) and one or both of conditions (b) and (c):

[0076] (a)mol%BaO>(2 x mol%TiO2+mol%B2O3);

[0077] (b) (mol% BaO+mol% SrO-2 x mol% TiO2-mol% B2O3) ≤ 0.5x (mol% SiO2-2 x mol% TiO2-2 / 3 x mol% B2O3);

[0078] (c) (mol% BaO + mol% SrO-2 x mol% TiO2) / (mol% SiO2-2 x mol% TiO2) < 0.5.

[0079] The glass composition may contain any suitable range of metal oxide components within the widest range specified for each metal oxide. The amount of each metal oxide in the composition may be appropriately selected based on the desired performance of the glass seal formed from the glass composition.

[0080] In some embodiments, the glass composition, based on mol% of the glass composition, comprises one or more of the following, or is composed of one or more of the following:

[0081] - Approximately 52 to approximately 59 mol% SiO2, especially approximately 54 to approximately 58 mol% SiO2;

[0082] - Approximately 3 to approximately 10 mol% B2O3, especially approximately 5 to approximately 7 mol% B2O3;

[0083] - about 0.5 to about 2 mol% Al2O3, especially about 1 to about 2 mol% Al2O3;

[0084] - Approximately 4 to approximately 5.5 mol% TiO2;

[0085] - Approximately 2 to 3 mol% CeO2, especially approximately 2 to 2.5 mol% CeO2;

[0086] - about 9 to about 20 mol% SrO, especially about 9 to about 12 mol% SrO, even more especially about 10 to about 12 mol% SrO, and even more especially about 10 to about 11 mol% SrO;

[0087] - about 15 to about 25 mol% BaO, especially about 16 to about 21 mol% BaO, even more especially about 17 to about 20 mol% BaO, and even more especially about 17 to about 19 mol% BaO.

[0088] The glass compositions of the present invention can be prepared by methods known in the art. The glass compositions are typically provided in the form of glass powder. Glass can also be provided in the form of frit, wherein the frit is ground into a powder having a desired particle size distribution for use as a sealing material. In short, the metal oxide component of the glass composition or its precursor is weighed in the correct proportions to obtain the desired glass composition. The weighed powders are mixed to produce a homogeneous mixture, which is then melted. The melt is poured onto a suitable surface, such as a marver or mold, and then rapidly cooled to provide molten glass frit. The molten glass frit can be ground, for example using a ball mill, to produce glass powder. The ground glass powder can be suitably sieved to provide glass powder having a desired particle size or particle size distribution (PSD). The desired PSD can be suitably selected according to, for example, the technique used to apply glass sealant to components.

[0089] The glass compositions of the present invention can be used to provide seals in electrochemical devices requiring hermetic sealing. Therefore, the present invention also provides the use of the glass compositions of the present invention in forming seals in electrochemical devices, particularly SOFC or SOEC fuel cell stacks. Advantageously, as shown in the examples and described in more detail below, the glass compositions of the present invention are capable of forming glass seals having properties that make them suitable for use in SOFC (and SOEC) fuel cell stacks.

[0090] Sealing material

[0091] The glass composition of the present invention can be used as a sealing material in electrochemical devices (including SOFC or SOEC stacks) requiring hermetic sealing. Therefore, the present invention provides a sealing material comprising the glass composition described herein. The present invention also provides the use of the sealing material in forming seals in electrochemical devices, particularly SOFC or SOEC stacks.

[0092] The sealing material may contain one or more fillers. Preferably, the filler is substantially chemically inert to the seal formed from the glass composition, which allows the filler to be used to not affect the performance of the seal. The filler may also preferably have a CTE similar to that of glass and / or have high strength. Examples of suitable fillers include, but are not limited to, ZrO2, cerium dioxide, and barium silicate in powder or fibrous form.

[0093] In some embodiments, the sealing material comprises about 80 to about 100 vol% of a glass composition and about 0 to about 20 vol% of one or more fillers, based on the total amount of the sealing material.

[0094] Glass compositions of sealing materials can undergo suitable sintering thermal cycling to provide glass seals for electrochemical devices, particularly SOFC or SOEC stacks. Suitable thermal cycling may include a first step that allows the glass powder particles of the glass composition to soften and sinter together to provide a sintered glass with relatively low viscosity, and a second step that allows the sintered glass to transform into a stable glass-ceramic with relatively high viscosity by forming crystals of many different compositions. Advantageously, glass seals formed from the glass compositions of the present invention can provide the beneficial properties of high vitreousness and high crystallinity seals currently used in SOFC and SOEC stacks.

[0095] Therefore, in some embodiments, the glass composition of the sealing material of the present invention is softened after undergoing a sintering thermal cycle to provide a sintered glass and subsequently undergoes controlled partial crystallization to provide a glass ceramic comprising one or more crystalline phases and a glass phase.

[0096] In some implementations, suitable sintering thermal cycles include:

[0097] - The first step is carried out at a temperature above the glass transition temperature and about 10 to about 30°C lower than the glass crystallization initiation temperature for about 30 to about 120 minutes, particularly for a period of about 30 to about 60 minutes; and

[0098] - The second step is carried out for a period of about 2 to 5 hours at a temperature at least 50°C higher than the expected operating temperature of the electrochemical device (especially SOFC or SOEC stack) and at least 50°C higher than the glass crystallization initiation temperature.

[0099] During the first step, the glass powder particles of the composition soften and sinter together to eliminate interconnecting pores and facilitate flow into the gaps between the components of the electrochemical device to be sealed (e.g., either or both of the cells and interconnecting support structures of an SOFC / SOEC stack). Advantageously, the sintered glass can establish a hermetically tight seal between the components of the electrochemical device. The sintered glass can also advantageously provide a strong mechanical bond between the components on either side of the seal. Furthermore, the presence of a specific amount of B2O3 in the glass phase prior to crystallization can improve the wetting of the glass to the electrochemical device components during the first step, which can advantageously lead to a strong bond between the seal and the components. The first step may take a longer time, although this increases production costs. However, performing the first step in a shorter time may result in poor sealing, for example, poor adhesion of the seal to other stack components, or poor sintering of the seal, leaving a high level of porosity, resulting in weak mechanical properties and partial permeation. The temperature at which glass crystallization begins can be determined by methods known in the art, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC).

[0100] During the second step, the sintered glass seal partially crystallizes to form a stable glass-ceramic comprising one or more crystalline and glassy phases. The crystals of each crystalline phase can advantageously enhance the mechanical strength of the glass-ceramic. The crystalline phases can also advantageously impart thermal expansion and contraction properties to the glass-ceramic that closely match the thermal expansion and contraction properties of other components in SOFC or SOEC stacks or other electrochemical devices requiring hermetically sealed environments. The duration of the second step can be appropriately selected based on one or more factors. One factor may be the temperature of the second step; generally, higher temperatures result in shorter required times. For example, if the temperature of the second step is about 50°C higher than the crystallization initiation temperature of the SOFC or SOEC stack, a time period of 2 hours may be sufficient to stabilize the glass through crystallization. It should be understood that longer time periods at temperatures significantly higher than the expected operating temperature of the SOFC or SOEC stack may lead to undesirable and irreversible changes in other components of the stack. It should also be understood that longer time periods will increase production costs. The expected operating temperature of an SOFC or SOEC stack (or other suitable electrochemical device requiring a hermetically sealed environment) can be appropriately selected based on the design of the SOFC or SOEC stack and the characteristics of other functional components in the stack, such as the anode, cathode, electrolyte, and metal support. In some embodiments, the expected operating temperature of the SOFC or SOEC stack (or other suitable electrochemical device requiring a hermetically sealed environment) is from about 500°C to about 1000°C, particularly from about 500°C to about 900°C. The temperature at which glass crystallization begins can be determined by methods known in the art, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC).

[0101] The sintering cycle may optionally include a binder burn-out step prior to the first and second steps of the sintering thermal cycle. The binder burn-out step may suitably be performed to burn off organic materials present in the sealant and / or battery coating. Examples of suitable binder burn-out steps include heating to a temperature of approximately 445°C to approximately 455°C, particularly approximately 450°C, over a period of approximately 0.5 hours.

[0102] In some embodiments, sintered glasses that can be formed from the glass compositions of the present invention form a hermetically sealed seal within an SOFC or SOEC stack or other electrochemical device requiring hermetically sealed seals upon undergoing suitable sintering thermal cycling.

[0103] In some embodiments, the glass-ceramic subsequently formed from the sintered glass comprises one or more crystalline phases and a glassy phase. In some embodiments, based on the total amount of the glass-ceramic, the glass-ceramic comprises about 45 to about 80 vol%, particularly about 50 to about 70 vol%, of one or more crystalline phases and about 20 to about 55 vol%, particularly about 30 to about 50% glassy phase.

[0104] In some embodiments, one or more crystalline phases of the glass-ceramic comprise crystals having a structure selected from: 2BaO.TiO2.2SiO2, 2SrO.TiO2.2SiO2, 3BaO.3B2O3.2SiO2, BaO.2SiO2, BaO.B2O3, and combinations thereof.

[0105] BaO in the glass composition can be consumed during the crystallization of the sintered glass into a glass ceramic, such that substantially all of the BaO is in crystalline form within the glass ceramic. Similarly, B2O3 in the glass composition can be consumed during crystallization, such that substantially all of the B2O3 is in crystalline form within the glass ceramic. Therefore, in some embodiments, the glass phase of the glass ceramic is substantially free of BaO. In some embodiments, the glass phase of the glass ceramic is substantially free of B2O3. Advantageously, this can provide a low-reactivity, high-viscosity silicate glass matrix. This is because BaO and B2O3 in the glass phase may interact adversely with other components of SOFCs or SOECs or other electrochemical devices requiring hermetically sealed environments, but may become substantially inert during crystallization. Hereinafter, "substantially inert" means that BaO and / or B2O3 do not react with other components of the electrochemical device during crystallization or react only in a manner that has no measurable effect on the characteristics and / or performance of the glass seal formed from the glass composition.

[0106] Within the temperature range where glass is rigid (i.e., below the glass transition temperature), glass-ceramics preferably have thermal expansion and contraction characteristics that closely match those of other components of the electrochemical device (especially SOFC or SOEC stacks). This advantageously allows the thermal stress generated during operation of the electrochemical device to not exceed the mechanical strength of the device's components. Therefore, in some embodiments, at any temperature up to the glass transition temperature of the glass phase, the glass-ceramic has a thermal expansion and contraction mismatch of about -0.04 (negative 0.04) to about 0.10 (positive 0.10) with any other stack component it is bonded to, where the thermal expansion and contraction mismatch is defined as:

[0107]

[0108] Wherein, "glass" refers to glass-ceramics, and "others" refers to other electrochemical device components bonded to the glass (e.g., in the case of SOFC or SOEC stacks, one or both of the battery and interconnect support structure). The glass transition temperature of the glass phase depends on its composition and can be determined by methods known in the art, such as by performing an expansion measurement test. Advantageously, as shown in the examples, glass samples prepared from the glass compositions of the present invention exhibit stable expansion mismatch when exposed to air or fuel environments at high temperatures for extended periods.

[0109] Glass-ceramics can have a coefficient of thermal expansion (CTE), which allows them (and therefore sealing materials) to be used in electrochemical devices requiring hermetically sealed environments, particularly SOFC or SOEC stacks. The CTE can be substantially the same as that of any other component in an SOFC or SOEC stack or other electrochemical device requiring a hermetically sealed environment. In some embodiments, the glass-ceramic (or sealing material) has a CTE of approximately 10 x 10⁻⁶. -6 / ℃ to approximately 13x 10 -6 CTE at / ℃.

[0110] The sealing material of the present invention can be used to form glass seals in electrochemical devices requiring hermetically sealed environments, particularly SOFC or SOEC stacks. Therefore, the present invention provides an electrochemical device, preferably an SOFC or SOEC stack, comprising one or more cells, each cell including a cathode, an anode, and a solid electrolyte; a support structure including one or more supports; and the sealing material described herein. The present invention also provides an electrochemical device, preferably an SOFC or SOEC stack, comprising one or more cells, each cell including a cathode, an anode, and a solid electrolyte; a support structure including one or more supports; and a glass seal formed of the sealing material described herein. The glass seal can be formed using a suitable sintering thermal cycle as described herein. The support structure is an interconnected support structure comprising one or more supports made of a suitable material, such as a suitable metal like steel. In some embodiments, the support structure is a set of interconnected plates. It should be understood that each plate can be interpreted as a support of the support structure, and each cell may include one or more plates.

[0111] The present invention also provides a method for forming a seal in an electrochemical device serving as an SOFC or SOEC stack, the method comprising:

[0112] - Apply the sealing material described herein to either or both of the cells and support structures of an SOFC or SOEC stack; and

[0113] - The sealing material is subjected to a sintering thermal cycle, wherein the glass composition of the sealing material softens to provide a sintered glass, and subsequently undergoes controlled crystallization to provide a glass ceramic comprising one or more crystalline phases and a glass phase;

[0114] - This forms a seal in the SOFC or SOEC fuel cell stack.

[0115] The characteristics and properties of suitable sintering thermal cycles and sealing materials (or sintered glass, glass ceramics or glass seals formed therefrom) that can be used in the methods of the present invention are as described herein.

[0116] Figure 1 An example of an SOFC stack is shown. The figure is a schematic diagram of a portion of an SOFC stack (1) which includes battery components, namely cathode (2), anode (3) and electrolyte (4); support structure (5) and glass seal (6), as shown in the exploded view.

[0117] Advantageously, as illustrated in the embodiments, SOFC stacks sealed with the glass composition of the present invention exhibit less degradation during prolonged operation at standard operating temperatures compared to those sealed with comparative glass seals currently used in the production of SOFC stacks. Therefore, in some embodiments, after approximately 10,000 hours of operation and approximately 100 thermal cycles from room temperature (approximately 20°C to approximately 25°C) to the intended operating temperature of the SOFC (or SOEC) stack, the SOFC (or SOEC) stack of the present invention experiences less than 10%, particularly less than 6%, more particularly less than about 3%, and even more particularly less than about 2% of total performance degradation.

[0118] It should be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more individual features mentioned or obvious from the text or drawings. All these different combinations constitute various alternative aspects of the invention.

[0119] Example

[0120] The invention will be further described by way of non-limiting embodiments. Those skilled in the art will understand that many modifications can be made without departing from the spirit and scope of the invention.

[0121] Example 1. Glass composition and powder

[0122] To determine suitable glass seals for electrochemical devices such as SOFC and SOEC stacks, 19 different glass compositions were evaluated. Table 1 provides the glass compositions.

[0123] Table 1. Glass Compositions

[0124]

[0125]

[0126] Condition (a): (BaO-2*TiO2-B2O3). Glass compositions with a value > 0 satisfy this requirement. For glass compositions that satisfy this requirement, it is expected that in the glass ceramics formed from these glass compositions, essentially all of the B2O3 will be in crystalline form.

[0127] Condition (b): (BaO + SrO - 2*TiO2 - B2O3) / (SiO2 - 2*TiO2 - 2*B2O3 / 3). Glass compositions with a value ≤ 0.5 satisfy this requirement. For glass compositions that satisfy this requirement, it is expected that substantially all BaO in the glass ceramics formed from these glass compositions will be in crystalline form.

[0128] Condition (c): (BaO + SrO - 2*TiO2) / (SiO2 - 2*TiO2). Glass compositions with a value < 0.5 satisfy this requirement. For glass compositions that satisfy this requirement, it is expected that substantially all BaO in the glass ceramics formed from these glass compositions will be in crystalline form.

[0129] Glass powders corresponding to glass compositions 1-19 are prepared by the following method: Oxides or precursors of each metal component are weighed in the correct proportions to produce the desired glass composition. The weighed powders are thoroughly mixed to produce a homogeneous mixture and melted at 1450°C for 2 hours. After the raw material is converted into a melt, it is poured onto an amarver and then rapidly cooled in water to produce glass frit.

[0130] The molten glass feed was dried, ball-milled, and sieved to provide glass powder with the desired particle size distribution (PSD). The particle size fell within the range shown in Table 2. Typical particle size distributions of the glass powder were measured using laser diffraction. Figure 2 As shown.

[0131] Table 2. Particle size distribution of glass powder

[0132] Particle size (μm) <![CDATA[d 10 ]]> 58-68 <![CDATA[d 50 ]]> 110-120 <![CDATA[d 90 ]]> 200-220

[0133] Chemical analysis of glass powders was performed using inductively coupled plasma (ICP) spectroscopy, according to ASTM International Standard No. C1463-13, to identify the composition of each glass powder. A slightly modified procedure was used to prepare the sample solution for ICP analysis, following Section 22.2 of C1463-13, except that nitric acid was used instead of the mixture of hydrochloric acid and oxalic acid in Section 22.2.8.

[0134] Example 2. Characterization of sintered glass by SEM and XRD

[0135] Scanning electron microscopy (SEM) was used to determine the microstructure of the sintered glass samples. To prepare the sintered glass samples, glass powders corresponding to compositions 1-19 in Table 1, prepared according to the procedure described in Example 1, were sintered via a two-step sintering thermal cycle to produce sintered rods. The same temperature profile as that used for SOFC stack sintering was used to ensure that the microstructure of the sintered samples was similar to that of the seals in the sintered stack.

[0136] Figure 3 The microstructures of the sintered glass sample corresponding to glass composition 11 are shown at two different magnifications. The microstructures indicate that the sintered glass possesses a variety of different crystalline and glassy phases. Some of the crystalline phases identified by X-ray diffraction include 2BaO.TiO2.2SiO2, 2SrO.TiO2.2SiO2, and BaO.2SiO2.

[0137] Example 3. Characterization of sintered glass by dilatation measurement

[0138] The expansion mismatch between the sintered glass rod and the stainless steel interconnecting the fuel cell stack was determined using an expansion method. Sintered glass rod samples corresponding to glass compositions 1-19 in Table 1 were prepared using the procedures described in Examples 1 and 2.

[0139] Figure 4 The expansion mismatch of sintered glass rod samples 9, 10, 11, 14, and 19 relative to the stainless steel metal support material is shown, given by the following formula:

[0140]

[0141] The glass sample is a sintered glass rod, and the others are made of stainless steel.

[0142] Figure 4 The two dashed lines in the diagram enclose a preferred region of expansion mismatch between room temperature and the glass transition temperature to minimize stress within the fuel cell stack, thereby ensuring safe operation and thermal cycling. Results show that glass compositions 9, 10, 11, 14, and 19 exhibit expansion differences relative to the metal support, which fall within the two dashed lines and thus within the defined range. Therefore, the results indicate that the glasses provided by glass compositions 9, 10, 11, 14, and 19 have thermal expansion and contraction characteristics that closely match those of other components of an SOFC (or SOEC) fuel cell stack within the temperature range where the glass is rigid (i.e., below the glass transition temperature).

[0143] Example 4. Characterization of air-aged sintered glass

[0144] The suitability of glass powder for sealing SOFC stacks was evaluated by aging sintered glass rods in a high-temperature air environment. Sintered glass rod samples corresponding to glass compositions 1-19 in Table 1 were prepared using the procedures described in Examples 1 and 2. The sintered glass rods were aged in an atmospheric environment at 850°C for 0, 1000, 2000, 4000, and 6000 hours, and then the microstructure and expansion mismatch were characterized.

[0145] Figure 5 The expansion mismatch of sintered rods prepared from glass composition 11 was shown after being placed in an atmospheric environment at 850°C for 0, 1000, 2000, 4000, and 6000 hours. The air-aged samples of glass composition 11 exhibited relatively stable expansion mismatch with the metal over the extended time periods.

[0146] Figure 6SEM micrographs of an air-aged sample of glass composition 11 are shown. SEM analysis indicates that the initially formed crystalline phase crystals have coarsened, while a small number of new crystal types have grown with exposure time, but the glass generally remains non-porous, where the formation of pores could lead to seal failure.

[0147] Example 5. Characterization of fuel-aged sintered glass

[0148] The suitability of glass powder for sealing SOFC stacks was evaluated by aging sintered glass rods in a high-temperature fuel environment. Sintered glass rod samples corresponding to glass compositions 1-19 in Table 1 were prepared using the procedures described in Examples 1 and 2. The sintered glass rods were aged in a 60% H2 + 40% steam environment at 850°C for a time similar to that of the air aging test.

[0149] Figure 7 The expansion mismatch of sintered rods prepared from glass composition 11 was shown after 0, 1000, 2000, 4000, and 6000 hours in a fuel environment at 850°C. Although the fuel environment is more reactive to glass than air, the expansion mismatch of the fuel-aged samples of glass composition 11 remained relatively stable over the extended time periods.

[0150] Figure 8 SEM micrographs of a fuel-aged sample of glass composition 11 are shown. SEM analysis indicates some crystal growth, but not as much as in the air-aged sample. A degree of porosity increase was observed in the glass, although it was minimal. Figure 9 As shown.

[0151] Example 6. Verifying the effectiveness of glass as a sealing element for SOFC fuel cell stacks

[0152] Glass compositions 11, 14, and 18 from Table 1 were selected as sealants for SOFC stacks for validation. Glass powder from each composition was converted into a paste using a suitable binder / solvent system, applied to the stack assembly requiring sealing (on the battery and / or interconnect support structure), stacked to construct the stack, and then sintered using a suitable sintering temperature procedure as described in paragraph

[0048] above to provide an SOFC stack with a hermetically sealed seal. Notably, in addition to the two steps required for the glass sealant, the stack sintering cycle also includes a binder burn-off step as described in paragraph

[0051] above to burn off organic materials present in the sealant paste and battery coating.

[0153] The fuel cell stack operated at a standard fuel cell operating temperature of 750°C and underwent approximately 100 thermal cycles over approximately 9000 hours. Table 3 provides a summary of the percentage voltage degradation results for the fuel cell stacks, and the percentage voltage degradation of the fuel cell stack with glass composition 18 during each thermal cycle is shown in Table 3. Figure 10 The degradation percentage for each thermal cycle includes both inherent degradation of the fuel cell stack and degradation purely due to thermal cycling; that is, degradation of the stack under normal operation combined with degradation due to thermal cycling. These degradations are combined and normalized to the number of thermal cycles experienced by the fuel cell stack. It is worth noting that the degradation percentage is expected to be smaller if no thermal cycling occurs.

[0154] Table 3. Summary of Degradation Percentage Results for Test Stacks

[0155] Glass composition for forming a seal Test duration (h) Average degradation per thermal cycle % 11 9800 0.044％ 14 9400 0.041％ 18 9200 0.047％

[0156] The porosity growth of the glass seals in the test fuel cell stack was examined. Glass seals near the fuel vents in the stack were chosen for analysis because they are in the most reactive environment. Porosity levels were determined through image analysis. Acceptable porosity levels depend on many factors, including seal strength, the level of generated thermal stress, and the CTE mismatch between the glass and other components. Continued porosity growth in the glass eventually leads to seal failure. Therefore, fuel cell stack life typically increases as the rate of pore formation and growth decreases.

[0157] Figure 11 An optical microscope image of the glass seal is shown, taken from a fuel cell stack of glass composition 18 after 9200 hours of testing. The image indicates that the glass seal exhibits minimal porosity growth.

Claims

1. A glass composition, comprising, by mol% of the glass composition: - 50 to 60 mol% SiO2; - 2 to 10 mol% B2O3; - 0.5 to 3 mol% Al2O3; - 4 to 6 mol% TiO2; - 1 to 4 mol% CeO2; - 2 to 30 mol% SrO; and - 2 to 25 mol% BaO, and The glass composition thereon satisfies condition (a) and one or both of conditions (b) and (c): (a)mol% BaO > (2 x mol% TiO2 + mol% B2O3); (b)(mol% BaO + mol% SrO - 2 x mol% TiO2 - mol% B2O3) ≤ 0.5 x (mol% SiO2 –2 x mol% TiO2 - 2 / 3 x mol% B2O3); (c)(mol% BaO + mol% SrO - 2 x mol% TiO2) / (mol% SiO2 - 2 x mol% TiO2) <0.

5.

2. The glass composition according to claim 1, wherein the glass composition is substantially free of alkali metal oxides.

3. The glass composition according to any one of claims 1 to 2, wherein the glass composition does not contain CaO.

4. The glass composition according to any one of claims 1 to 2, wherein the glass composition does not contain ZrO2.

5. The glass composition according to any one of claims 1 to 2, wherein, based on mol% of the glass composition, the glass composition comprises one or more of the following: - 52 to 59 mol% SiO2; - 3 to 10 mol% B2O3; - 0.5 to 2 mol% Al2O3; - 4 to 5.5 mol% TiO2; - 2 to 3 mol% CeO2; - 9 to 20 mol% SrO; - 16 to 21 mol% BaO.

6. The glass composition according to any one of claims 1 to 2, wherein, based on mol% of the glass composition, the glass composition comprises one or more of the following: - 54 to 58 mol% SiO2; - 5 to 7 mol% B2O3; - 1 to 2 mol% Al2O3; - 4 to 5.5 mol% TiO2; - 2 to 3 mol% CeO2; - 10 to 12 mol% SrO; - 17 to 19 mol% BaO.

7. A glass composition, comprising, in mol% of the glass composition, the following: - 50 to 60 mol% SiO2; - 2 to 10 mol% B2O3; - 0.5 to 3 mol% Al2O3; - 4 to 6 mol% TiO2; - 1 to 4 mol% CeO2; - 2 to 30 mol% SrO; and - 2 to 25 mol% BaO.

8. The glass composition according to claim 7, wherein condition (a) and one or both of conditions (b) and (c) are satisfied: (a)mol% BaO > (2 x mol% TiO2 + mol% B2O3); (b) (mol% BaO + mol% SrO - 2 x mol% TiO2 - mol% B2O3) ≤ 0.5 x (mol% SiO2 -2 x mol% TiO2 - 2 / 3 x mol% B2O3); (c)(mol% BaO + mol% SrO - 2 x mol% TiO2) / (mol% SiO2 - 2 x mol% TiO2) <0.

5.

9. A sealing material for an electrochemical device comprising the glass composition according to any one of claims 1 to 8.

10. The sealing material according to claim 9, wherein the sealing material further comprises one or more fillers.

11. The sealing material of claim 10, wherein, based on the total amount of the sealing material, the sealing material comprises 80 to 100 vol% of a glass composition and 0 to 20 vol% of one or more fillers.

12. The sealing material according to any one of claims 9 to 11, wherein the glass composition is softened after undergoing a sintering thermal cycle to provide a sintered glass, and subsequently undergoes controlled crystallization to provide a glass ceramic comprising one or more crystalline phases and a glass phase.

13. The sealing material according to claim 12, wherein the sintering thermal cycle comprises: - The first step is carried out at a temperature above the glass transition temperature and 10 to 30°C lower than the glass crystallization initiation temperature for a period of 30 to 120 minutes; and - The second step is carried out for a period of 2 to 5 hours at a temperature at least 50°C higher than the expected operating temperature of the electrochemical device and at least 50°C higher than the glass crystallization initiation temperature.

14. The sealing material according to claim 12, wherein the sintered glass forms an hermetically sealed seal with the electrochemical device.

15. The sealing material of claim 12, wherein, based on the total amount of the glass-ceramic, the glass-ceramic comprises 45 to 80 vol% of one or more crystalline phases and 20 to 55 vol% of a glass phase.

16. The sealing material according to claim 15, wherein each of one or more crystalline phases of the glass-ceramic comprises crystals having a structure selected from: 2BaO.TiO2.2SiO2, 2SrO.TiO2.2SiO2, 3BaO.3B2O3.2SiO2, BaO.2SiO2, BaO.B2O3, and combinations thereof.

17. The sealing material of claim 15, wherein at any temperature up to the glass transition temperature of the glass phase, the glass-ceramic has a thermal expansion and contraction mismatch of -0.04 to 0.10 with any other fuel cell assembly to which it is bonded, the thermal expansion and contraction mismatch being defined as: 。 18. The sealing material according to claim 15, wherein the glass phase of the glass ceramic is substantially free of BaO.

19. The sealing material according to claim 15, wherein the glass phase of the glass ceramic is substantially free of B2O3.

20. The sealing material of claim 15, wherein the glass ceramic has a 10 x 10 -6 / °C to 13 x 10 -6 Coefficient of thermal expansion (CTE) at / °C.

21. An electrochemical device comprising one or more batteries, each battery comprising a cathode, an anode, and a solid electrolyte; a support structure comprising one or more supports; and a sealing material according to any one of claims 9 to 20.

22. The electrochemical device according to claim 21, wherein the electrochemical device is an SOFC or SOEC stack.

23. Use of the glass composition according to any one of claims 1 to 8 or the sealing material according to any one of claims 9 to 20 in forming a seal in an electrochemical device.

24. A method for forming a seal in an electrochemical device, said electrochemical device being an SOFC or SOEC stack, the method comprising: - Apply the sealing material as described in any one of claims 9 to 20 to any one or both of the cells and support structures of the SOFC or SOEC stack; - The sealing material is subjected to a sintering thermal cycle, wherein the glass composition of the sealing material softens to provide a sintered glass, and subsequently undergoes controlled crystallization to provide a glass ceramic comprising one or more crystalline phases and a glass phase; This forms a seal in the SOFC or SOEC fuel cell stack.

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