Sealants For Solid Oxide Fuel Cells Research Articles

Effect of alumina nanofiller on properties of heat-treated glass composite sealants.

Alkali/alkaline-earth borosilicate glass-alumina composites containing 10 vol% Al2O3 were prepared for use as solid oxide fuel cell sealants. The effect of heat treatment and Al2O3, addition on the viscosities and electrical conductivities was investigated to improve cyclic sealing performance. Upon a 48-h heat treatment, the viscosity of the glass-alumina composites at 750 degrees C was approximately four orders of magnitude higher than that of the base glass owing to the crystallization of the glass in the presence of Al2O3. Heat treatment increased the electrical conductivities of both the base glass and the glass-alumina composites. The electrical conductivities of glass-alumina composites in the range from 400 degrees C to 550 degrees C were three times higher than those of the base glass regardless of heat treatment. This increase in the conductivities and viscosities by heat treatment was attributed to the devitrification and structural densification of the sealing glass and the partial dissolution of the Al2O3 filler in alkali/alkaline-earth borosilicate sealing glass.

Effect of alumina nanofiller on the viscosity and electrical conductivity of glass-based seals for solid oxide fuel cells

Use of alumina nanofiller to strengthen the structure of the sealing glass used for solid oxide fuel cells (SOFCs) has been investigated by measurement of fracture strength, viscosity, and electrical conductivity at elevated temperatures. The electrical conductivity and viscosity of two glass-based seals for SOFCs were compared. Measurement of electrical conductivity and viscosity of the glass-based seals over the temperature range 400–600 °C showed that addition of 10 % nano-sized alumina powder to the sealing glass increased both the high-temperature electrical conductivity and viscosity of the sintered glass matrix. The higher conductivity and viscosity of glass composites containing alumina nanofiller were attributed to formation of bonds with bridging oxygen by partial dissolution of the alumina nanofiller in the sealing glass. Infrared spectroscopy and X-ray diffraction analysis were conducted to confirm this conclusion. Strengthening of the sealing glass structure to improve long-term durability may be possible by using glass composites containing alumina nanofiller.

Electrical Properties of Alkali/Alkaline-Earth Borosilicate Glass Composite Sealants for Solid Oxide Fuel Cells

Development of a reliable sealant or sealing system remains one of the top priorities in planar solid oxide fuel cells (SOFCs) technology. Recently, a compliant glass seal such as an alkali silicate glass was proposed for SOFCs. In contrast to the conventional glass–ceramic sealant which develops a rigid or semi-rigid microstructure after sealing, the compliant glass shows so called “self-healing” behavior and will remain vitreous without substantial crystallization after sealing and during operation at elevated temperatures. The objective of this study was to investigate the relationship between sealant structure and high temperature electrical conductivities of glass composite sealants for SOFCs. Alkali/alkaline-earth borosilicate glass-based sealants, both with and without Al2O3 or ZrO2 fillers, were heat treated at various temperatures for periods of up to 48 h. The effects of filler type, densification of glass and alkali/alkali-earth oxide content in sealants on the electrical conductivities of the sealants were investigated. The results showed that the electrical conductivity of a glass composite sealant containing ZrO2 filler increased ten times after 48 hours of heat treatment. The electrical conductivity of a ZrO2 filler-containing glass after heat treatment was similar to that of an Al2O3 filler-containing glass. This was attributed to the densification of sealing glass by heat treatment.

Effects of lanthanum oxide on the properties of barium-free alkaline-earth borosilicate sealant glass

We have explored the effects of lanthanum oxide (La2O3) on the properties of barium-free SrO–CaO–ZnO–B2O3–SiO2 glass system varying its content from 0 to 6mol%. The coefficient of thermal expansion and glass transition temperature of the glasses are found to be in the ranges 10.6–10.8×10−6/K (50–600°C) and 650–690°C respectively. Differential scanning calorimetric studies revealed an increase in crystallization peak temperature and thermal stability of the glasses up to 4mol% and then decrease on further addition. A monotonic increase in characteristic shrinkage temperature of glass powder compact is observed on addition of La2O3 as revealed in the heating stage microscopic study. X-ray diffraction studies revealed reduced crystallization peak intensity in La2O3 added glasses. Fourier transformed infrared spectra disclosed that the addition of La2O3 stabilized the glass structure by reducing the number of non-bridging oxygens. Vickers micro hardness of the glasses varied in the range 2.6–5.6GPa. Scanning electron microscopy revealed the formation of particulate nature of microstructures in parent glass which is not observed in the La2O3 added glasses. Activation energy of crystallization has increased from 331 to 361kJ/mol determined by Kissinger and Augis–Bennett models. Above properties of these glasses clearly advocate their applicability as solid oxide fuel cell sealants.

The Development of Sealant for Planar SOFC

Sealing is the key technique for planar solid oxide fuel cell (SOFC). Only several sealants and sealing methods could be used for SOFC due to the extreme work conditions. Several kinds of sealants and sealing methods were summarized in this paper which emphasized glass sealing, compress sealing and composite sealing. Compositions, preparations and usages were described in the article which also gave the information of development and future direction of sealing for SOFC.

Glass coated compressible solid oxide fuel cell seals

With the growing footprint of solid oxide fuel cell stacks, there is a need to extend the operating range of compressible gaskets towards lower stress levels. This article describes a method to manufacture SOFC seals by coating a compressible sealing material (Thermiculite 866) with glass to obtain good sealing performance even at compression stresses as low as 0.1 MPa. Glass layer can be coated using an organic carrier consisting of terpineol, ethanol and ethyl cellulose. The coated seals can be heat treated by simply ramping the temperature up to operating temperature at 60 Kh−1 and therefore no extra steps, which are typical to glass seals, are required. Coated seals were manufactured using this route and evaluated both ex-situ and in a real stack. Leak rates of 0.1–0.3 ml (m min)−1 were measured at 2–25 mbar overpressure using 50/50 H2/N2. A 30-cell stack was manufactured and tested using coated seals. At nominal operating conditions of 0.25 A cm−2 and 650 °C average cathode temperature, 46% fuel utilization and 20% air utilization the stack had a total hydrogen cross leak of 60 ml min−1 corresponding to 0.7% of the inlet hydrogen flow rate.

Fracture and creep of glass–ceramic solid oxide fuel cell sealant materials

The reliable operation of solid oxide fuel cell stacks depends strongly on the structural integrity of the sealing materials. In this respect, failure and deformation are aspects which need to be assessed in particular for glass–ceramic sealant materials. Bending tests and compressive tests were carried at room temperature and typical stack operation temperature for sintered bars and stack similar head-to-head joined glass–ceramic sealants specimens with different degree of crystallization and annealed temperature. The sealant material was screen printed onto the steel. Fracture stresses and creep data are reported. The results reveal a thickness dependent fracture stress and a decrease of the fracture stress for the partially crystallized sealant at operation relevant temperatures that can be associated with the residual stress state and viscous deformation of the material, respectively. A stable thickness independent fracture stress was obtained for the fully crystallized sealant. Fractographic based analysis gave insight into the failure origin.

Microstructural evolution in two alkali multicomponent silicate glasses as a result of long-term exposure to solid oxide fuel cell environments

The microstructural evolution in two potential solid oxide fuel cell (SOFC) sealing glass materials exposed to air and a gas mixture of steam + H2 + N2 at 800 °C up to 10000 h was determined. The glass exposures were performed on common SOFC substrates like alumina and zirconia. Characterization of the crystalline phases and pore size distribution was performed for the specimens with various exposure conditions. Comparison of the microstructural and chemical stability of the two glasses was performed based on known trends related to glass chemistry. It was observed that multicomponent glasses followed few rules for chemical and microstructural stability reported in the literature for glasses with fewer components. The two glasses examined in this study displayed adequate resistance to devitrification but marginal resistance to porosity changes in the SOFC environment exposure. The implications of the results for the design and long-term performance of SOFC seals are discussed.

Elevated temperature effects on the mechanical properties of solid oxide fuel cell sealing materials

A promising candidate to fulfil the solid oxide fuel cell sealant requirements with respect to gas tightness, high temperature stability and electrical insulation appear to be glass-ceramic sealing materials. However, the reliable operation of solid oxide fuel cell stacks depends strongly on the structural integrity of the sealing materials. In this respect failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. Bending tests were carried at room temperature and typical stack operation temperature for glass ceramic sealants with different degree of crystallization. Elastic moduli, fracture stresses and viscosity values are reported. In addition to sintered bars bending testing were carried out for steel specimens that were head-to-head joined with the glass ceramics similar as in a stack application, where the ceramic particle reinforced sealant material was screen printed onto the steel. The results reveal a decrease of the strength for the partially crystallized sealant at operation relevant temperatures that can be associated with the viscous deformation. Fractographic analyses based on a combination of optical, confocal and scanning electron microscopy gave insight into the failure origin.

Study of calcium–magnesium–aluminum–silicate (CMAS) glass and glass-ceramic sealant for solid oxide fuel cells

A parent glass within the CaO–MgO–Al2O3–SiO2 system and resulting glass–ceramics (GCs) have been appraised for solid oxide fuel cells (SOFCs) sealing applications. The sintering behavior was investigated by differential thermal analysis and hot stage microscopy. The glass composition exhibited single-stage shrinkage behavior with high sintering ability, and a suitable viscosity of 107.1 dPa s at the SOFCs operating temperature 900 °C. X–ray diffraction in conjunction with the Rietveld–RIR technique were employed to quantify the crystalline and amorphous phases in the GCs sintered at 900 °C for 1 h and 850 °C for 300 h. The coefficients of thermal expansion (CTE) measured were 9.7 × 10−6 K−1 (200–500 °C) and ∼10.1 × 10−6 K−1 (200–700 °C) for glass and GCs, respectively, in good agreement with those typical for SOFC components. Weibull analysis was applied on the three-point bend data of GCs in order to obtain the mechanical strength distribution, characteristic strength and Weibull modulus. Well matching CTE, flexural strength values, good sintering behavior and adhesion to the other components in air atmosphere allow proposing this glass composition as promising candidate for further experimentation as sealant for SOFCs.

Diopside – Mg orthosilicate and diopside – Ba disilicate glass–ceramics for sealing applications in SOFC: Sintering and chemical interactions studies

Diopside (CaMgSi2O6) based glasses compositions containing magnesium orthosilicate or barium aluminosilicates phases have been appraised for sealing applications in solid oxide fuel cells (SOFCs) and other solid-electrolyte devices. The sintering behavior and crystalline phase evolution of glass powders has been investigated under isothermal and non-isothermal conditions in the SOFC operating temperature range (800–900 °C). All the glass compositions exhibited two-stage shrinkage behavior resulting in well sintered and mechanically strong glass–ceramics with Augite as the primary crystalline phase. The appropriate coefficient of thermal expansion (CTE), long term thermal stability (300 h at 900 °C), high electrical resistivity, good adhesion and minimal reactivity with SOFC components makes the investigated glass–ceramics potential candidates for further experimentation as SOFC sealants.

Corrosion Resistance of SOFC and SOEC Glass-Ceramic Seal Materials in High Temperature Steam/Hydrogen

There exists concern that glass-ceramic seal materials may not tolerate the high temperature gaseous environment in solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) applications, potentially leading to the poisoning of electrodes by silica and ultimate loss of hermeticity. A mechanism for the loss of silicon from glass seals has been proposed by other workers. To address this concern, material samples of proprietary glass-ceramics were exposed to steam/hydrogen atmospheres at 800 °C for up to 1000 h. The mass losses of the samples were measured at intervals and their changes in surface chemistry were analyzed using techniques such as X-ray photoelectron spectroscopy and dynamic secondary ion mass spectroscopy. It was shown that the extent of corrosion depends on composition and surface microstructure and that some glass-ceramics are particularly resistant to steam/hydrogen corrosion.

Sol–gel synthesis and characterization of barium (magnesium) aluminosilicate glass sealants for solid oxide fuel cells

Solid oxide fuel cells (SOFC) correspond to efficient energy conversion systems coupled with low emissions of pollutants. In the aim to fabricate high temperature planar SOFC, glass and glass-ceramic sealants are developed to associate several criteria and properties : high thermal expansion (11.0 to 12.0 ⋅ 10 − 6 K − 1 ), high electrical resistance > 2 kΩ/cm 2, good thermochemical compatibility with the other active materials of the fuel cell, and stability under H 2 and H 2O atmospheres at an operation temperature of 800 °C for a long time. According to these requirements, new BAS (BaO–Al 2O 3–SiO 2) and BMAS (BaO–MgO–Al 2O 3–SiO 2) glass-ceramic sealants have been developed by sol–gel route which is a non-conventional process for such applications. By this soft chemistry process, we anticipate a decrease in the glasses processing temperature due to a better homogeneity between cationic precursors in the mixture and a more important reactivity of materials. Experimental results in terms of thermomechanical properties, thermal expansion coefficient, crystalline phase content, and microstructure were discussed. In particular, the influence of the %BaO on the thermomechanical properties of glass-ceramics was described. Changes in properties of glass-ceramics were closely related to the microstructure. The influence of MgO on glass processing temperatures, on the structure and on the microstructure is evaluated in order to confirm that these glass-ceramics are promising candidates to SOFC applications. So, after performing a systematic investigation to the various systems, the properties of suitable glass were proposed.

Thermal, structural and crystallization kinetics of SiO2–BaO–ZnO–B2O3–Al2O3 glass samples as a sealant for SOFC

The present work describes the development of a glass-ceramic in SiO2–BaO–ZnO–B2O3–Al2O3 system. The prepared glass samples were found to have good compatibility to act as a sealant in planar solid oxide fuel cell (SOFC) in terms of coefficient of thermal expansion (TEC) and glass transition temperature (Tg). The crystallization kinetics of present glass samples were investigated by various characterization techniques such as differential thermal analyzer (DTA), Dilatometery, X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization behavior of the chosen glass samples was influenced by replacing B2O3 with Al2O3. With the addition of Al2O3 there is increase in glass transition temperature (Tg) and glass crystallization temperature (Tc). Also with the addition of Al2O3 crystallization phenomenon is hindered. XRD and SEM study was done at various temperatures for different time durations. The detail of the above discussed study is done in the present paper.

Y 2O 3–BaO–SiO 2–B 2O 3–Al 2O 3 glass sealant for solid oxide fuel cells

A glass based on Y 2O 3–BaO–SiO 2–B 2O 3–Al 2O 3 (named YBA) has been investigated as sealant for planar solid oxide fuel cells (SOFCs). The YBA glass has been systematically characterized by differential thermal analysis, dilatometer, scanning electron microscopy, impedance analysis, and open circuit voltage to examine their suitability as sealant. The coefficient of thermal expansion of YBA is 11.64 × 10 −6 K −1 between 323 and 873 K. The resistivity is 9.1 × 10 4 Ω cm at 800 °C. The glass sealant is found to be well adhered with other cell components, such as electrolytes and stainless steels, at an optimum sealing temperature of 800 °C. All measured results showed that the YBA glass appears to be a promising sealant for SOFCs.

Recent developments in the preparation, characterization and applications of glass- and glass–ceramic-to-metal seals and coatings

There has, in recent years, been a major revival of interest in glass- and glass–ceramic-to-metal seals and coatings for new applications. Experience dictates that many factors need to be taken into consideration in the successful design and manufacture of high-quality seals, particularly if an adequate component lifetime is to be achieved. For example, during their preparation, undesirable reactions may occur between diffusing metal species and glass constituents, and these can lead to the formation of highly localized internal stresses, which can initiate failure of a seal either during manufacture or, more seriously, whilst in service due to the influence of static fatigue. In the case of high-temperature systems, reactions under hostile operating conditions also need to be taken into consideration. In this review, the factors learnt from past experience that influence the formation and lifetime behaviour of glass and glass–ceramic/metal systems are briefly introduced, and their relevance to the newer applications including solid oxide fuel cell sealants and coatings on titanium for biomedical applications is discussed.

Characterization of self-healing glassy composites by high-temperature environmental scanning electron microscopy (HT-ESEM)

In situ high-temperature healing of cracks in composites made of glass and vanadium boride (VB) particles was observed using an environmental scanning electron microscope equipped with a high-temperature chamber (HT-ESEM). HT-ESEM is an adequate tool for studying the self-healing property of these materials. The change in crack length as a function of redox atmospheric conditions is reported. No self-healing behaviour was observed under reducing conditions, while a complete and rapid healing of the cracks was measured under oxidizing conditions. HT-ESEM image analyses enabled the monitoring of the healing effect. The self-healing mechanism was identified as a consequence of the VB active particles oxidation and subsequent pouring of fluid oxides into the cracks. These innovative composites offer an interesting potential in the domain of solid oxide fuel cell sealants.

Development and performance of diopside based glass-ceramic sealants for solid oxide fuel cells

We report on the development and performance of diopside (CaMgSi 2O 6) based glass-ceramic (GC) sealants for solid oxide fuel cells (SOFCs). Ten glass compositions were prepared by various additions and substitutions of La 2O 3, Cr 2O 3 and B 2O 3 in diopside system. The structure of the glasses has been investigated by employing 29Si, 27Al and 11B MAS-NMR spectroscopy. The crystallization kinetics of the glasses has been investigated using differential thermal analysis (DTA) while X-ray diffraction (XRD) in conjunction with Rietveld-RIR technique has been employed to quantify the amount of crystalline and amorphous phases in the GCs. Hot-stage microscopy (HSM) has been used to investigate the influence of La 2O 3 and B 2O 3 on the sintering behavior of the glass powders. The joining and chemical interaction of the B 2O 3 and La 2O 3-containing glasses with Crofer22 APU and Sanergy HT (metallic interconnects for SOFC), has been investigated in humidified reducing atmosphere at SOFC operating temperature while the leak rate of GC sealants has been investigated using oxygen pump leak rate method. The suitable sintering and crystallization behavior of the GCs along with good adhesion to interconnect alloys and low leak rate indicate that the investigated GCs are suitable candidates for further experimentation as SOFC sealants.

Creep properties of solid oxide fuel cell glass–ceramic seal G18

This study utilizes nanoindentation to investigate and measure creep properties of a barium calcium alumino-silicate glass–ceramic used for solid oxide fuel cell seals (SOFCs). Samples of the glass–ceramic seal material were aged for 5, 50, and 100 h to obtain different degrees of crystallinity. Instrumented nanoindentation was performed on the samples with different aging times at different temperatures to investigate the strain rate sensitivity during inelastic deformation. The temperature dependent behavior is important since SOFCs operate at high temperatures (800–1000 °C). Results show that the samples with higher crystallinity were more resistant to creep, and the creep compliance tended to decrease with increasing temperature, especially with further aged samples.

Optimization of La 2O 3-containing diopside based glass-ceramic sealants for fuel cell applications

We report on the optimization of La 2O 3-containing diopside based glass-ceramics (GCs) for sealant applications in solid oxide fuel cells (SOFC). Seven glass compositions were prepared by modifying the parent glass composition, Ca 0.8Ba 0.1MgAl 0.1La 0.1Si 1.9O 6. First five glasses were prepared by the addition of different amounts of B 2O 3 in a systematic manner (i.e. 2, 5, 10, 15, 20 wt.%) to the parent glass composition while the remaining two glasses were derived by substituting SrO for BaO in the glasses containing 2 wt.% and 5 wt.% B 2O 3. Structural and thermal behavior of the glasses was investigated by infrared spectroscopy (FTIR), density measurements, dilatometry and differential thermal analysis (DTA). Liquid–liquid amorphous phase separation was observed in B 2O 3-containing glasses. Sintering and crystallization behavior, microstructure, and properties of the GCs were investigated under different heat treatment conditions (800 and 850 °C; 1–300 h). The GCs with ≥5 wt.% B 2O 3 showed an abnormal thermal expansion behavior above 600 °C. The chemical interaction behavior of the glasses with SOFC electrolyte and metallic interconnects, has been investigated in air atmosphere at SOFC operating temperature. Thermal shock resistance and gas-tightness of GC sealants in contact with 8YSZ was evaluated in air and water. The total electrical resistance of a model cell comprising Crofer 22 APU and 8YSZ plates joined by a GC sealant has been examined by the impedance spectroscopy. Good matching of thermal expansion coefficients (CTE) and strong, but not reactive, adhesion to electrolyte and interconnect, in conjunction with a low level of electrical conductivity, indicate that the investigated GCs are suitable candidates for further experimentation as SOFC sealants.