A method of fabricating membrane electrode structures for methanol fuel cells involves, among other things, the use of improved sprayers to deposit inks containing catalytic metals on membranes of Nafion™ (or equivalent) perfluorosulfonic acid-based hydrophilic, proton-conducting ion-exchange polymer. In comparison with older methods, the present method provides for faster production with more efficient utilization of the expensive catalytic metals. The present method is thus better suited to mass production.
The older methods do not provide for deposition of uniformly thin catalytic layers on membranes, nor do they afford the flexibility of depositing well-defined multiple thin catalytic layers of different compositions. The present method does provide these capabilities, thus making it possible to tailor catalytic layers more precisely to achieve comparable cell performances with reduced catalyst loadings. The improved sprayers used in the present method contribute to the attainment of these objectives in that they produce slow, fine sprays that can be aimed more precisely on the surface areas to be coated, without wasting sprays on adjacent areas. Moreover, whereas the sprayers used in the older methods frequently became clogged, the improved sprayers are designed to prevent clogging.
The catalytic ink usually comprises the catalytic metal (Pt for the cathode or a mixture of Pt and Ru for the anode), a Nafion™ (or equivalent) ionomer solution, water, and isopropanol, with perhaps a small amount of a polytetrafluoroethylene-based additive. The ingredients of the ink are mixed well by use of ultrasound, and the viscosity of the ink is adjusted, by addition of small amounts of water and isopropanol, to enhance sprayability. The ink is then transferred to the sprayer.
In fabricating a membrane electrode according to the present method, the membrane is first conditioned in water, then soaked in an aqueous solution of isopropanol or methanol; this soaking is necessary to swell the membrane to prevent wrinkling that would otherwise occur when the sprayed catalytic ink subsequently comes into contact with the membrane. The membrane is then mounted in a frame (see figure), which must be nonmetallic to prevent corrosion. Then before the membrane dries out, it is sprayed on one side with catalytic ink. The position and settings of the spray head and the rate of flow of compressed air that drives the sprayer are adjusted to regulate the characteristics of the spray to ensure that the spray does not dry out on its way to the membrane and so that the deposited material bonds to the membrane and forms an electrochemically active surface. The spray-coated membrane is then dried with air at ambient temperature or, optionally, air heated to a temperature between 40 and 60 °C to accelerate drying.
Additional coats of catalytic ink on the same side or on opposite sides can be applied by repeating the spraying and drying steps. Preferably, the layers are applied alternately on the anode and cathode sides to minimize any stresses remaining after the coating process. The areal mass density of a single coat can be as small as 0.1 mg/cm².
Upon completion of coating, the membrane is released from the frame and hot-pressed between anode- and cathode-side carbon-paper supports. The resulting sandwich electrode structure is then stored in water until use.
In experiments, the performances of fuel cells containing electrode structures made by the present method were found to be comparable to those of fuel cells containing electrode structures made by older methods. However, the amounts of catalysts used in the present method ranged from 1 to 2 mg/cm², whereas the amounts used in the older methods were typically about 4 mg/cm². The achievement of comparable performance with less catalytic material by use of techniques suitable for mass production is a significant step toward commercialization and lowering the costs of producing fuel cells.
This work was done by William Chun, Sekharipuram Narayanan, Barbara Jeffries-Nakamura, Thomas I. Valdez, and Juergen Linke 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
Technology Reporting Office JPL Mail Stop 122-116 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240
Refer to NPO-19941, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Improved fabrication of electrodes for methanol fuel cells
(reference NPO19941) is currently available for download from the TSP library.
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