A micro-tubular fuel cell contains multiple membrane/electrode assemblies, each comprising a tubular protonexchange membrane with the anode on the inner surface and the cathode on the outer surface (see figure). Targeted dimensions include an inner membrane diameter of 600 µm, membrane thickness of 50 µm, anode thickness of 25 µm, and cathode thickness of 125 µm. One end of each micro-tubular membrane/ electrode assembly (µT-MEA) is closed, while the other end is open and connected to a current-collection manifold. At the open end of each µT-MEA, a conical anode current collector and iffuser is inserted in the tube, and a cathode current-collector/crimping ring is placed around the outside of the tube. Hydrogen gas diffuses into the interiors of the tubes, while air or oxygen is blown across the outside of the tubes in a crossflow configuration.
The anode and cathode current collectors are connected by an end-plate assembly (not shown in the figure) in the hydrogen- gas manifold that defines the parallel and serial electrical connections of the µT-MEAs. Because each µT-MEA produces a relatively small current, parallel and serial connections can be made at their ends without incurring an unacceptably large amount of ohmic heating. Although the cylindrical geometry causes the current density at the anode in each µT-MEA to exceed that at the cathode, this feature detracts only slightly from cell performance because it is a fundamental property of any PEMFC that the anode polarization loss is much less than the cathode polarization loss at a given current density.
The elimination of the bipolar plates in favor of the much less bulky and massive manifold and current-collector assembly is the single greatest contribution to more efficient utilization of available volume and thus to increased power density. It has been estimated that after further optimization of dimensions, materials, and fabrication processes, it should be possible to make micro-tubular fuel cells with power densities as great as 6.4 W/g and 6.9 kW/L.
This work was done by Michael C. Kimble, Everett B. Anderson, Karen D. Jayne, and Alan S. Woodman of Physical Sciences Inc. for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809.
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
Physical Sciences Inc.
20 New England Business Center
Andover, MA 01810-1077
Telephone No.: (978) 689-0003
Fax No.: (978) 689-3232
Refer to MSC-23012, volume and number of this NASA Tech Briefs issue, and the page number.