FUEL CELL ASSEMBLY WITH SILICON CARBIDE SHIMS AND METHOD OF MAKING SUCH SHIMS

Abstract: A fuel cell (1) comprising first and second electrodes (11, 12), a matrix (10) for carrying electrolyte sandwiched between said electrodes and having a peripheral area, and independent integral shim means (13-16) adjacent said area of said matrix (10), the shim means (13-16) comprising silicon carbide in an amount of at least about 50 percent by weight.

Description: Fuel cell assembly with silicon carbide shims and method of making such shims Background of the Invention This invention relates to fuel cells and, in particular, to fuel cells adapted to retard leakage of electrolyte and process gases. In one type of fuel cell construction, a matrix containing the fuel cell electrolyte is sandwiched between first and second electrodes of the cell. The electrodes, in turn, are sandwiched between top and bottom plates which are provided with channels for carrying process gas to the cell. In this type of cell construction, the first and second electrodes may be made of shorter longitudinal and lateral dimension, respectively, than the matrix so that first and second longitudinal portions and first and second lateral portions of the matrix extend beyond the first and second electrodes, respectively. In such case, in order to realize proper sandwiching of the assembly, first and second shims are situated at the longitudinal ends of the first electrode overlapping the first and second longitudinal portions of the matrix and third and fourth shims are situated at the lateral ends of the second electrode overlapping the first and second lateral portions of the matrix. The shims provided for this purpose have heretofore been made of kynol or teflon. Shims made of kynol, however, have large pores and, as a result, are prone to electrolyte leakage and, therefore, process gas leakage. Moreover, shims of kynol material are likely to tear and to degrade with increased temperature. Teflon shims, on other hand, are hydrophobic in character and thus create a dry seal which due to its dry nature is also prone to electrolyte leakage and, therefore, gas leakage. Teflon furthermore is subject to creep or shrinkage over time, which also can contribute to gas and electrolyte leakage. Finally, teflon has little resiliency or give and, hence, when squeezed between adjoining cell members may not conform to their surf ace irregularities, thereby leaving gaps for the escape of gas and electrolyte. In another type of fuel cell construction, instead of utilizing shortened electrodes and independent integral members or shims adjacent the electrodes to provide sealing, the edges of the electrodes are themselves coated or impregnated with a prepared sealant material. Such a construction is disclosed in U.S. Patent 4,259,389 to Vine et al. Vine et al disclose that the sealant material used for coating or impregnating the electrode edge may be a silicon carbide powder bonded together with about a 2 percent binder of teflon or teflon like polytretrafluorethylene material to hold the powdered material in its compacted condition. Vine et al also state that carbon or graphite may be used in conjunction with or instead of silicon carbide powder or other materials inert to the electrolyte. Finally, Vine et al further teach that coating or impregnating may be realized by mixing the sealant material particles, with the binder particles therein, in a suitable slurry, then dipping the edges of the electrode substrate to the right depth in the slurry and then drying the edges. As an alternative procedure Vine et al propose application of the mixture of sealant powder and binder in a suitable vehicle such as water by the silk screen process. Vine et al therefore use the sealant material particles as a filler to the existing pores of the electrode substrate. As a result, some areas of the sealed electrode edges will be more densely packed with sealant particles than others, owing to the inherent pore distribution of the substrate. This non-uniform distribution of the sealant particles, causes a difference in resiliency and strength over the seal region and also results in a non-uniform retention of electrolyte. These effects are further aggravated by the size distribution of the sealant material particles. The net result is thus a non-uniform and less than totally effective seal. Furthermore, the dipping and screening methods described for application of the sealant material require the handling of large and often fragile substrates and, moreover, such handling must be carefully carried out to control application of sealant to a small fraction of the total area of the substrates. The overall process is thus slow and prone to substrate damage. It is, therefore, an object of the present invention to provide a fuel cell which does not suffer from the above disadvantages. Summary of the Invention In accordance with the principles of the present invention the above and other objectives are realized in a fuel cell of a type employing a shim means as a gas and electrolyte seal and wherein the shim means is an independent integral element comprised of silicon carbide in an amount of at least about 50 percent by weight. Preferably, the silicon carbide is at least 90 percent and, more preferably, at least 93 percent, by weight of the shim means and further, preferably, the shim means contains a binder which is preferably teflon. Also disclosed is a method of fabricating the shim means. Brief Description of the Drawings The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying sole drawing which shows a fuel cell having shim means in accordance with the principles of the present invention. retailed Description In the figure, the fuel cell 1 comprises an electrolyte matrix 10 which is sandwiched between first and second electrodes 11 and 12 having sides lla-lld and 12a-12d and being of smaller longitudinal and lateral dimension, respectively, than the matrix. Integral independent plates or shims 13 and 14 border the lateral edges 12a and 12b of the electrode 12 and sandwich or overlap the lateral matrix sections lOa, lOb extending therebeyond. Similarly, additional plates or shims 15 and 16 border the longitudinal edges llc and lld of the electrode 11 and sandwich extending matrix sections lOc and lOd. Bipolar plates 17 and 18, in turn, abut the electrodes 11 and 12, respectively, and the shims 13-16 and sandwich these elements and the matrix 10. The plates 17 and 18 define with the electrodes passages or channels for carrying process gases. In particular, the lower surface 17a of the plate 17 includes a plurality of grooves 17b which define with the interior region 11e of the electrode 11 a first set of process channels 17c. Similarly, the interior region 12e of the electrode 12 cooperates with grooves 18b in the upper surface 18a of the plate 18 to define a second set of process gas channels 18c, these channels being crisscross to the channels 17C. In order to prevent leakage of process gases and electrolyte from the cell 1, the shims 13-16, which are unitary structures separable from the electrodes 11 and 12, must be of appropriate construction. In accordance with the principles of the present invention, the shims are each formed from silicon carbide which is present in each shim in an amount of at least about 50 percent by weight. The shims also include a binder material for the silicon carbide which is present in an amount equal to or less than about 50 percent by weight and which, preferably, is teflon. A preferable constituent makeup for each shim is an amount of silicon carbide of at least about 90 percent by weight. A more preferable constituent makeup is at least about 93 percent silicon carbide, in which case the amount of teflon binder is equal to or less than about 7 percent by weight. With the shims 13-16 formed as aforesaid, a more uniform distribution of silicon carbide pores is found to result and this acts to more effectively retard the outflow of electrolyte and process gases from the cell. The level of electrolyte in the cell is, therefore, maintained. This, in turn, preserves cell performance and inhibits the crossover of reactant gases which otherwise could result in cell failure. Additionally, the shims now exhibit a more uniform resiliency and strength which enables them to better conform to the irregularities in the abutting surfaces of adjacent elements of the cell 1. This more effectively prevents the occurrence of possible through gaps, thereby further ensuring against leakage. Also, the shims undergo relatively little creepage with time and the seal provided is a wet seal and, therefore, possesses the usual advantages of such a seal as compared to a dry seal. The silicon carbide shims of the invention are further advantageous in that as integral independent elements they can be fabricated by a continuous rolling process. Such a process permits ready realization of desired mechnical strength and thickness and enables mass production at very low cost. The resultant shims, moreover, being much smaller than an electrode substrate, are much easier to handle. In making the shims, the mixture of silicon carbide and binder is continuously rolled to form a sheet of desired thickness. The sheet is then cut into pieces and the pieces sintered to form the desired shims. Sintering temperatures in a range of 290 degrees C to 360 degrees C are usable for mixtures containing silicon carbide and teflon binder in the ranges of about 98 to 91 percent by weight and about 2 to 9 percent by weight, respectively. A useable range of sintering times for the aforesaid range of sintering temperatures is about 1-20 minutes. A preferable range of sintering temperatures for a mixture of about 93 percent silicon carbide and 7 percent teflon is about 325-330 degrees C. Sintering time for the aforesaid sintering temperatures is about 2 minutes. In all cases, it is understood that the abovedescribed arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can readily be devised without departing from the spirit and scope of the invention. Thus, for example, the longitudinal and lateral dimensions of the electrolyte matrix 10 could also have been shortened to the same extent as the shortened dimensions of the electrodes 11 and 12. In such case, instead of overlapping the matrix edges, the shims 13-16 would be placed with their edges adjacent and abutting the matrix edges so as to provide the desired sealing effect. Moreover, the thickness of the shims would be made to accomodate the thickness of the matrix as well as the electrode.

Claims: What Is Claimed Is: 1. A fuel cell assembly comprising: first and second electrodes; a matrix for carrying electrolyte sandwiched between said electrodes, said matrix having a peripheral area, first and second process gas plates sandwiching said matrix and said electrodes; and independent integral shim means adjacent said peripheral area, said shim means comprising silicon carbide in an amount of at least about 50 percent by weight. 2. A fuel cell in accordance with claim 1 wherein: said shim means contains a binder in an amount equal to or less than about 50 percent by weight. 3. A fuel cell in accordance with claim 1 wherein: said silicon carbide is present in said shim means in an amount of about at least 90 percent by weight. 4. A fuel cell in accordance with claim 1 wherein: said peripheral area extends around the entire circumference of said matrix. 5. A fuel cell in accordance with claim 1 wherein: said plates sandwich paid peripheral area. 6. A fuel cell in accordance with claim 5 wherein: said peripheral area extends beyond said electrodes; and said shim means overlaps said peripheral area and is sandwiched by said peripheral area and said plates. 7. A fuel cell in accordance with claim 6 wherein: said first and second electrodes are each four sided; said matrix is four sided and said peripheral area of said matrix comprises first and second peri pheral portions bordering the first and second sides of said matrix and extending beyond the first and second lateral sides of said second electrode and third and fourth peripheral portions bordering the third and fourth sides of said matrix and extending beyond the first and second longitudinal sides of said first electrode; and said shim means comprises first and second shims overlapping said first and second peripheral matrix portions and third and fourth shims overlapping said third and fourth peripheral matrix portions. 8. A fuel cell in accordance with claim 7 wherein: said first and second shims are disposed between said second plate and said first and second peripheral matrix positions, respectively; and said third and fourth shims are disposed be tween said first plate and said third and fourth peripheral matrix portions, respectively. 9. A fuel cell in accordance with claim 7 wherein: said first and second shims abut the first and second lateral sides of said second electrode; and said third and fourth shims abut the first and second longitudinal sides of said first electrode. 10. A fuel cell in accordance with claim 7 wherein: said electrodes, matrix and plates are rect angular. 11. A fuel cell in accordance with claim 2 wherein: said binder is teflon. 12. A fuel cell in accordance with claim 11 wherein: said silicon carbide is present in an amount of about 93 percent by weight and said teflon in an amount of about 7 percent by weight. 13. A method of making the shim means of claim 11 and comprising; continuously rolling a mixture of said sili con carbide and binder to form a sheet; and sintering said sheet. 14. A method in accordance with claim 13 wherein said silicon carbide is present in an amount of about 98 to 91 percent by weight, said binder is teflon present in an amount of about 2 to 9 percent by weight and wherein: said sintering is at a temperature of about 290 to 360 degrees C. 15. A method in accordance with claim 14 wherein: said sintering is carried out for about 1-20 minutes. 16. A method in accordance with claim 14 wherein said silicon carbide is present in an amount of about 93 percent by weight, said binder is present in an amount of about 7 percent by weight and wherein: said sintering is at a temperature of about 325-330 degrees C. 17. A method in accordance with claim 16 wherein: said sintering is carried out for about 2 minutes.

Inventors: CHI, CHANG VUM, KUSH, ASHOK KUMAR
Applicants: ENERGY RESEARCH CORPORATION
Application year: 1984
Main classification: H01M8
IPC codes: H01M8, H01M2