Title of Invention

"AN IMPROVED METHOD OF MANUFACTURING SOLID FUEL CELLS"

Abstract

This invention relates to an improved method of manufacturing solid fuel cells, said fuel cells operating at 1000°C is leak proof, said method comprising the steps of mixing alumino-silicate based glass composition in a predetermined ratio of (SiO2 - 50%, A12O3 - 9%, CaO - 16%, MgO -9% B2O3 - 16%) by weight in powder form made from quarz, alumina, magnesium oxide, calcium carbonate and boric acid; melting the powder at 1600°C for two hours in a crucible and quenching into molds; grinding the quenched glass to a fine powder; casting the glass powder in slurry form into tape by using doctor blade and applying to form an integral seal (9, 10, 11, 12) on separators (13, 14) and yttria stabilized zirconia (YSZ) electrolyte (4) in the designated gas-sealing area (15); performing thermal cyling test and leak test in an inconel fixture having facility to pass Hydrogen/Oxygen and placed in a furnace for 300 hours at a temperature of 1000°C.

Full Text

The invention relates to an improved solid oxide fuel cell and method of manufacture thereof of the assembly in a planar design which is leak proof while operating at 1000°C. PRIOR ART A solid Oxide Fuel Cell (SOFC) consists of a cathode, an anode and a solid state electrolyte and operates in a temperature rang of 650 - 1000 °C. The anode is exposed to a reducing environment while the cathode is exposed to oxidizing environment. Seals for a fuel cell has a critical function of arresting leakage of gases. In a SOFC, the fuel and the oxidant react electrochemically to produce electricity through an oxygen-ion conducting solid state device. The basic element consists of solid oxide electrolyte layer in contact with a porous air electrode and fuel electrode on either side. The solid electrolyte is non porous, dense oxygen -ion conducting material like Yttria stabilized zirconia. Air electrode is the cathode. The cathode material is usually Strontium doped lanthanum manganite. The anode is denoted as fuel electrode. The material is usually Nickel/ yytria stabilized zirconia. The fuel and oxidant gases flow past the reverse sides of the air electrode and fuel electrode, respectively and generate electrical energy by means of electro chemical oxidation of the fuel and the electrochemical reduction oxygen. The transport rate of oxygen ions in the solid electrolyte is adequate for practical applications at high temperature of 1000° C using presently available elctrolyte materials and design. 3. Glass ceramic sealant has been effectively used in SOFC at 1000oC. A brief description based on references A,B & C is presented. A) Y Sadaki et al In glass-ceramic system. The original glass material soften and join the electrolyte and separator. At higher temperature bulk crystallization occurs and as a result, the viscosity increases. This enables the glass ceramic to seal during SOFC operation (A). B) Kueper, T.W et al In another investigation, Glass-ceramic sealing materials have been developed with mechanical and chemical properties suitable for a variety of high-temperature applications. In Glass ceramic, the authors have demonstrated the ability to tailor the thermal expansion coefficient the softening temperature adjusted such that the materials have suitable viscosities for a soft,complaint seal at temperatures ranging from 650-1000^ deg C. These materials form excellent bonds to a variety of ceramics and metals during heating to the target operation temperature. They have limited reactivity with the fuel cell materials tested and are stable both air and reducing environments (B). C) Taniguchi,et al In yet another developoment, the objective is to improve the endurance of solid oxide fuel cell (SOFC) against thremal cycles for reducing the stress caused by the difference in thermal expansion coefficients of alloy separator and electrolyte. The thermal cycle characteristics were improved by using a ceramic fiber for the sealing material. The ceramic fiber seemed to play by role of suppressing electrolyte-cracking by relaxing the stress set up during thermal cycles. The appropriate structure for the sealing material was investigated with 200 mm multiplied by 150 mm multiplied by 4combined-cell single-layer modules. The glass was arranged around the internal manifold to suppress gas leakage, and the ceramic fiber was arranged around the electrolyte to prevent the glass from contacting the electrolyte. It was confirmed that the thermal cycle characteristics can be improved and that good cell performance can be maintained by adopting this gas seal structure (C). A) Y saki et al; Glass- ceramic sealant based on CaO- A120Si02 system, Proceedings of fifth international sysmposium on Solid Oxide fuel cell SOFC5, Vol 7-40, P652 1977) B) Sealant materials for solid oxide fuel cells and other high-temperature ceramics. Kueper, T.W.Bloom, ID. Argonne National Laboratory Heat Resistant Materials II, Gatlinburg, TN, USA, 11-14 Sept. 1995 545-552. C) Improvement of thermal cycle characteristics of a planar- type solid oxide fuel cell by using ceramic fiber as sealing material, Tangiuchi, S.; Kadowaki, M, ; Yasuo, T.;Akiyama, Y.; Miyak, Y., Nisho, K., Corporate Source: SANYO, Co Ltd, Osaka, Jpn, Journal of Power Sources v 90 n 2 Oct 2000.p 163-169,2000. There are disadvantages associated with the present SOFC. One of the main disadvantages is that since it operates around 1000°C 1 the high temperature seal is not effective to prevent leakage of gas. Another disadvantage with the present SOFC is that the seal is not stable and cracks due to thermal cycling. Yet another disadvantage with the present SOFC using sealing system using sealing system based on glass ceramics is that it does not properly match with the characteristics of the solid electrolyte used in specific system. SUMMARY OF THE INVENTION Therfore the main object of th present invention is to provide a gas tight glass seal for SOFC operating at 1000°C. Another object of the present invention is to develop sealant material that forms excellent bonds to a variety of ceramics during heating to the target operation temperature and having limited reactivity with the fuel cell materials and are stable in both air and reducing environments. Yet another object of the present invention is to provide a glass seal based on Si02-Al2O3-CaO-MgO -B2O3 having a thermal expansion matching to that of solid electrolyte YSZ. Still another object of the present invention is to provide a range compositions suitable for application as a seal which are tailored to have thermal expansion coefficient between 8 and 12x10 exp-6/deg C, Further object of the present invention is to provide a range of glass seal compositions whose softening temperature is adjusted such that the materials have suitable viscosities for a soft, compliant seal at temperatures ranging from 650-1000 deg.C. According to the present invention there is provided an improved solid oxide fuel cell comprising a cell having an anode with fuel with fuel inlet and outlet and a cathode with an inlet and outlet and a YSZ solid electrolyte in between said anode and cathode is provided around which said cell is built and both end of the anode and cathode are provided with seals . . .. based on glass system having matching coefficient of thermal expansion to that of said YSZ solid electrolyte. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to the non-limiting exemplary embodiments of the invention represented in the accompanying drawings: Figure 1 of schematic diagram showing the principle of solid oxide fuel cell. Figure 2 schematics of planar SOFC stack showing gas sealing area. Figure 3 shows the diagram of the seal in relation to other components in a SOFC, Figure 4 shows the inconel fixture used for cell evaluation. Figure 5 shows the table of glass composition and thermal expansion coefficient. DESCRIPTION OF THE INVENTION The description deals with the development of a seal (9,10,11,12) based on glass system suitable for high temperature operation as in SOFC(l). The glass system has matching coefficient of thermal expansion to that of YSZ solid electrolyte around which the SOFC is built. The system based on SiO2-Al2O-CaO-MgO-B2O glass is stable against thermal cycling and verified for sealing effectiveness for prolonged operation on a cell (1). The scheme of the solid oxide fuel cell is shown in Figure 1 and figure 3 shows seal (9,10,11,12) in relation to other components in a SOFC (1). The cell (1) has two chambers having inlet (7) and out-let (8) for fuel which may be H2 or CO and iniet (5) and out-let (6) for Air which may be 02. The YSZ solid elctrolyte (4) of 250 µm is disposed centrally with an anode (3) of 60 yum and a cathode (2) of 60 µm is placed on both sides of the electrolyte (4). The anode (3) and the cathode (2) covers both inlet (5,7) and outlet (6,8) parts for fuel and air. The seals (9,10,11,12) are provided at the top and bottom of the anode (3) and the cathode (2). Figure 3 shows the schematics of planar SOFC (1) stack which shows the gas sealing area. The fuel gas flow is indicated by arrow (7) and air flow indicated by the arrow (5). Two separators (13,14) are provided on both sides of the angle cell (1) comprising air electrode (2), fuel electrode (3) and the electrolyte (4), shreaded area (5) indicates the gas sealing area. The system is based on Si02-Al2 03 -CaO-MgO -B2 03 glass which functions as an effective seal (9,10,11,12) during operation of the SOFC at 1000°C. The sealing system is chemically stable to oxidizing and reducing atmosphere, chemical stable both to electrolyte and interconnect e.g YSZ, La CrO3 and thermal expansion closely mathing to suit zirconia electrolyte (4) and thereby ensuring gas tightness. The optimized glass system was tested in fuel cell assemblies extending upto a maximum period of 750 hrs. The location of the Glass seal in a Solid Oxide Fuel Cell assembly is shown in Figure 3. The glass composition is optimized in the following ratio: SiO2 - 50%, A1203 -9%, CaO- 16%, MgO - 9% B203 16% by weight. PRINCIPLE AND METHOD OF MANUFACTURE OF GLASS SEAL FOR USE IN SOFC The glass system was optimized based on its softening behaviour during soldering and relaxation of stresses through creeping or viscous flow at operating temperature (1000°C). From a broad range of compositions, alumino-silicate based glass compositions were chosen. The original glass powder was made from commercially available quartz, alumina, magnesium oxide, calcium carbonate and boric acid using the conventional glass making technique. The raw materials were mixed in fixed ratio, then melted at 1600°C for 2 hrs. in a crucible and finally quenched onto molds. The quenched glass was ground to a fine powder for subsequent processing. Since the fuel cell (1) stack is built around solid electrolyte (4), the thermal expansion characteristics of YSZ is taken as a reference for suitability of other components. The Coefficient of thermal expansion (CTE) was measured for glass and YSZ material using a Dilatometer (make : NETZSCH-Geratebay) for a standard sample size of 6mm dia and 25 mm length. Optimized glass omposition was arrived at based on experimentation and is given in Table 1 of Figure 5. The glass powder was cast into tape by using Doctor Blade technique and applied to form an integral seal (9,10,11,12) since application in the tape form has significant advantage. Doctor blade technique is a batch casting process with a mobile doctor blade and stationary casting surface. Glass plate is used as a casting surface. Slurry is poured on one end of the glass plate and the doctor blade is moved from one end to another. Thickness control of the tape formed over the glass plate is achieved by adjusting the gap setting of the doctor blade. It also depends on various parameters such as viscosity of the slurry, temperature, casting speed etc; As a thumb rule the doctor blade gap to the final dried green tape is approximately 2:1. The CTE values are as shown in table 1 indicates composition having CTE very close to YSZ electrolyte (4) and verified for sealing effctiveness. YSZ has a typical CTE value of 10.5x x10-6 / °C. Typical the cast tape of 500 µm thickness was used as a seal with 200 yum YSZ elctrolyte plate. Prior to using in cell (1) trials, thermal cycling tests between room temperature and 1000°C as well as leak tests were carried out for tape on plate assembly. In theremal cycling test,the samples were heated to 1000°C and soaked for 60 minutes. The samples were removed from the furnace and cooled naturally for 15 min. Again the cooled samples were placed in the furnace and kept for 15 min to enable it to reach a temperature of 1000°C. The process was repeated for 20 cycles and the samples were then inspected for thermal stress induced cracks. In leak test, a special fixtures made of inconel having facility to pass Hydrogen/Oxygen gas was used. The fixture used for tape and electrolyte assembly was also used for cell evaluation and is as shown in Figure 2. The fixture was placed in the furnace and connected to a low pressure gas source. The ratio of gas flow from the fixture is monitored through bubbler. The temperature of the. furnace was raised to 1000°C at a heating rate 100°C/hr. The sample was kept at 1000°C for 300 hrs. The rate of gas flow was monitored both at inlet and outlet. Bubble count was used as a measure of leakage in the fixture . Initial tests were carried out using argon and subsequently with Hydrogen/Oxygen gases. After heat treatment of glass tape on YSZ plate, the samples showed good dimensional stability as well as good adhesion and crack free glass surface. No shrinkage was noticed. This was confirmed further by Microscopic examination. The glass system was stable and clear and no crystallization was observed. X-ray analysis confirmd the seal rings to be totally amorphous. The results were reproducible even for longer duration of operation, to an extent of 100 hrs at 1000°C. The leak test of this assembly in fixture ensured gas tightness indicating the suitability and stability of seal material as well as its compatibility. Performance of the seal in single cell experiment Number of cells (1) were assembled using seals (9,10, 11,12) made of glass composition G-2 ,G - 9 and G-13 as described in the invention. The cells were tested for a period of 100 hrs and in some cases even upto 750 hrs to evaluate the performance. The cell (1) tests confirmd the reliability of the seal with constant flow of Oxygen and Hydrogen Of the range of compositions, composition corresponding to G-13 has CTE very close to YSZ electrolyte and is most preferred. The proposed invention uses Alumino silicate glass which is a pure amorphous system. The single phase system has precise properties such as melting, glass transition temperature and can be easily processed without any micro defects. On the contrary the glass -ceramic, and glass-ceramic composites suggested in prior art requires careful processing. Yet there is a possibility of induced micro defects at the glass ceramic interface. These defects can only show up as micro leaks at a latter stage when they grow to a bigger size due to thermal stresses. It is also pertinent to refer to earlier unsuccssful attempt with pure glass seals. In earlier unsuccessful attempt (D) the Borate based system was investigated as a sealant in SOFC in the temperature range of 800 - 1000 C and was found to have many problems and discarded. D) Michael Krumplt, Ramesh Kumar, Ira Bloom, Kevinely, Sealant materials for solid oxide fuel cells, DOE/METC fuel cells, 94 contract review meeting, Morgan town, WV,Aug. 17-18 (1994) Hence amorphous alumino silicate glass system is distinctly advantageous from the point of view of processing, reliability and stability. The invention described hereinabove is in relation to the non-limiting embodiments and as defined in the accompanying claims. WE CLAIM; 1. An improved solid oxide fuel cell comprising a cell (1) having an anode (3) with fuel inlet (7) and outlet (8) and a cathode (2) with an inlet (5) and outlet (6) and a YSZ solid electrolyte (4) inbetween said anode (3) and cathode (2) is provided around which said cell (l) is built and both ends of the anode (3) and cathode (2) are provided with seals (9,10,11,12) based on glass system having matching coefficient of thermal expansion to that of said YSZ solid electrolyte (4). 2. An improved solid oxide fuel cell as claimed in claim 1 wherein the fuel (7)is H CO. 3. An improved solid oxide fuel cell as claimed in claim 1 wherein the seals (9,10,11,12) are glass based on Si02 -Al2 O3 - CaO-MgO-B2 O3 having a thermal expansion betwen 8 and 12x10 /°C. 4. The method of manufacture of an improved solid fuel cell as claimed in claim 1 wherein said fuel cell which is leak proof operating at 1000°C comprising the following steps: i) alumino-silicate based glass composition in powder form is made from quartz, alumina, magnesium oxide, calcium carbonate and boric acid which were mixed in fixed ratio, ii) the powder is then melted at 1600°C for 2 hrs in a crucible and finally quenched into molds, iii) the quenched glass was ground to a fine powder, iv) the glass powder in slurry form is then cast into tape by using doctor blade and appliedco form an integral seal (15) on separators (13,14 and the electrolyte (4) in the designatd gas-sealing area (15). vi) perform thermal cycling test and leak test is undertaken in an inconel fixture having facility to pass Hydrogen/Oxygen and put in a furnace at a temperature raised to 1000°C at a heating rate of 100°/C and kept at 1000°C for 3 hours. 5. The method as claimed in claim 4 wherein the glass composition is optimized in the following ratio: Si02- 507=,Al203~97o, CaO-16%, MgO-9%. B203- 16% by weight. 6. The method as claimec in claim wherein the composition have a CTE value close to YSZ electrolyte. 7. The method as claimed in claim 4 and 6 wherein the cast tape of 500 ^m thickness is used as a seal with 200 /urn YSZ electrolyte plate. 8. An improved solid oxide fuel cell and method of manufacture thereof as herein described and illustrated in the accompanying drawings.

Documents:

1253-del-2001-abstract.pdf

1253-del-2001-claims.pdf

1253-del-2001-correspondence-others.pdf

1253-del-2001-correspondence-po.pdf

1253-del-2001-description (complete).pdf

1253-del-2001-drawings.pdf

1253-del-2001-form-1.pdf

1253-del-2001-form-19.pdf

1253-del-2001-form-2.pdf

1253-del-2001-form-3.pdf

1253-del-2001-gpa.pdf