Miniature fuel cells in a “flat-pack” configuration are being developed as alternatives to recharge- able batteries in cellular telephones, laptop computers, and other small, portable electronic devices. These fuel cells exploit the electrochemical oxidation of organic fuel (usually methanol) in air. Whereas power sources based on state-of-the-art lithium-ion batteries have specific energies of no more than ≈150 W⋅h/kg, power sources based on the present developmental fuel cells are expected to have specific energies between 500 and 1,000 W⋅h/kg. Moreover, whereas one must often wait for batteries to be recharged before using them, a fuel cell can be refueled and used immediately.
Conventional fuel-cell assemblies include bipolar plate stacks, pumps, blowers, and other ancillary items that not only contribute to cost but also add bulk and complexity, thereby posing considerable impediments to miniatur- ization. In the present developmental fuel cells, the flat-pack configuration is part of an overall improved design that eliminates much of the bulk and complexity.
A flat-pack fuel-cell assembly can include one or more fuel cells electrically connected in series and/or parallel to obtain the required current and/or voltage rating. A typical basic flat-pack fuelcell assembly (see figure) contains a single polymer electrolyte membrane that serves multiple cells. The cathodes of all the cells are located side by side in the same plane on one side of the membrane, while the anodes of all the cells are similarly located on the other side of the membrane. A fuel-feed manifold and a wick deliver fuel in regulated amounts to the anodes. The cathodes are exposed to air in a manner similar to that of metal/air batteries.
Series electrical connections between adjacent cells are made in the form of posts that extend through the membrane. These posts are made from such corrosion-resistant, electronically conductive materials as graphite, platinum, and/or gold, along with (if needed) an appropriate stable polymeric binder. Alternatively or in addition, parallel and/or series electrical connections among cells can be made in the form of thin edge connector plates that include segmented strip conductors made of gold or graphite.
Fabrication of a multiple-cell membrane/ electrode assembly like that shown in the figure involves the application, to the membrane, of catalyst layers and backing structures for the anodes and cathodes. The techniques of fabrication include the use of catalyst inks and either the use of pre-coated electrodes or else sputter deposition of the electrodes. Gas-diffusion backing layers are preferably bonded to the membrane by hot pressing. Optionally, nonbonded backing layers can be used instead of bonded ones.
The preferred anode catalyst is Pt-Ru; the preferred cathode catalyst is Pt. The anode structure is made hydrophilic so that an aqueous solution of liquid organic fuel can readily flow to the catalyst layer and the carbon dioxide product can readily leave the anode surface. The cathode is made hydrophobic to exclude water and thereby facilitate the flow of air.
This work was done by S. R. Narayanan, T. I. Valdez, Filiberto Clara, and Frank Harvey of Caltech for NASA’s Jet Propulsion Laboratory.
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
Refer to NPO-21066, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Miniature Fuel Cells for Small, Portable Electronic Devices
(reference NPO-21066) is currently available for download from the TSP library.
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