Micro-tubular fuel cells that would operate at power levels on the order of hundreds of watts or less are under development as alternatives to batteries in numerous products — portable power tools, cellular telephones, laptop computers, portable television receivers, and small robotic vehicles, to name a few examples. Micro-tubular fuel cells exploit advances in the art of proton-exchange- membrane fuel cells. The main advantage of the micro-tubular fuel cells over the plate-and-frame fuel cells would be higher power densities: Whereas the mass and volume power densities of low-pressure hydrogen-andoxygen- fuel plate-and-frame fuel cells designed to operate in the targeted power range are typically less than 0.1 W/g and 0.1 kW/L, micro-tubular fuel cells are expected to reach power densities much greater than 1 W/g and 1 kW/L. Because of their higher power densities, micro-tubular fuel cells would be better for powering portable equipment, and would be better suited to applications in which there are requirements for modularity to simplify maintenance or to facilitate scaling to higher power levels. The development of PEMFCs has conventionally focused on producing large stacks of cells that operate at typical power levels >5 kW. The usual approach taken to developing lower-power PEMFCs for applications like those listed above has been to simply shrink the basic plate-and-frame configuration to smaller dimensions. A conventional plate-and-frame fuel cell contains a membrane/electrode assembly in the form of a flat membrane with electrodes of the same active area bonded to both faces. In order to provide reactants to both electrodes, bipolar plates that contain flow passages are placed on both electrodes. The mass and volume overhead of the bipolar plates amounts to about 75 percent of the total mass and volume of a fuel-cell stack. Removing these bipolar plates in the micro-tubular fuel cell significantly increases the power density.

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.