An improved method of fabricating membrane electrode structures for methanol fuel cells is similar to the method described in the preceding article, except that in this method, the catalytic ink is poured and spread onto the membrane instead of sprayed. The development of both improved methods was prompted by a need to utilize expensive catalytic material more efficiently
Older methods of fabricating such electrode structures involve the deposition of catalysts on porous carbon paper substrates, which are then hot-pressed onto solid-electrolyte membranes. In a typical case, at least half the catalyst becomes immobilized deep within the pores of the substrate. This portion of the catalyst is not accessible for electrochemical reactions, and is not available for the formation of the desired large-area, electrochemically active interface between the catalyst-coated substrate and the membrane
Both improved processes include pretreatment of membranes by soaking in aqueous isopropanol solutions. The benefits of this pretreatment are twofold: (1) After pretreatment, the membrane and the catalytic ink have similar solvent compositions, so that the membrane will not become wrinkled when the ink is subsequently applied; and (2) The membrane becomes very soft, so that the catalytic layer can integrate with the membrane more readily during the subsequent drying and hot-pressing steps
As in the method described in the preceding article, the pretreated membrane is mounted in a frame. The membrane in the frame is laid down horizontally on a fine, absorbent, lint-free tissue to help control the rate of evaporation (as explained below) of volatile constituents (water and alcohols) of the ink to be applied next. The horizontal orientation is necessary to help ensure uniformity of the ink coating. The ink is then poured on the part of the membrane exposed in the aperture of the frame, and the ink is spread over the membrane by use of a glass rod.
Slow evaporation of the volatile constituents of the ink is necessary to obtain a crack-free catalytic coat on the membrane. Therefore, immediately after spreading of the ink, the membrane is placed in a polyethylene bag with a very small orifice for escape of the volatile constituents. After 24 to 48 hours, the coating is dry and the membrane can be taken through the hot-pressing process as in the method of the preceding article.
The performances of electrode structure made by a previous method and by the present improved method were evaluated in a laboratory fuel-cell apparatus at a temperature of 90 °C. In each case, a 1 molar aqueous methanol solution was circulated past the anode side, while air at 20 psig (gauge pressure of 14 kPa) was circulated past the anode. The measured performance of the improved version was superior; in particular, the peak power density of the improved version was 210 mW/cm², as compared to 160 mW/cm² for the previous version. Because of the greater power density, the weight of a fuel-cell stack could be reduced by about 25 percent without loss in performance.
This work was done by William Chun, Sekharipuram Narayanan, Barbara Jeffries-Nakamura, and Thomas I. Valdez of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-19940, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Another improved method for making fuel-cell electrodes
(reference NPO19940) is currently available for download from the TSP library.
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