The fabrication of membrane/electrode assemblies (MEAs) for direct methanol fuel cells can be modified to make the cathodes hydro- phobic. These modifications improve the performances of the fuel cells, as explained below.
As in the preceding article, it is necessary to present some background information in order to give meaning to a description of the modifications. During operation of a direct methanol fuel cell, water is produced at the cathode. If not removed, the water blocks access of air to catalyst sites on the cathode, and the cell output voltage is consequently decreased. At a high rate
(a large multiple of the stoichiometric rate) of flow of air or oxygen past the cathode, the blockage is less severe because the excess flow evaporates the water. However, the equipment needed to pump the air and condense the evaporated water adds to the size and weight of the fuel-cell system and consumes a significant amount of power, thereby decreasing the efficiency of the fuel-cell system.
The size and weight of the system could be reduced and/or the efficiency of the system could be increased if it were not necessary to rely on evaporation to remove the water from the cathode and thus the system could be operated at a lower airflow rate. To reduce or eliminate reliance on evaporation, it would be desirable to exclude the water (or at least some of the water) from the cathode in the first place by rendering the cathode at least partly hydrophobic. The essence of the present modifications of the fabrication process is to implement this concept by adding a hydrophobic constituent to the cathode material.
The hydrophobic constituent in question is a poly(tetrafluoroethylene) powder with a particle size ranging from 1 to 4 μm. The powder is added to the catalytic inks used to make the electrodes. Each ink is applied to both (a) a sheet of poly(tetrafluoroethylene)-impregnated porous carbon paper and (b) a surface of a perfluoro-sulfonated ion-exchange membrane that has been roughened by use of abrasive paper to increase adhesion. Then as in the process described in the preceding article, the membrane is sandwiched between the carbon papers and the sandwich is consolidated by applying heat and pressure.
In an experiment, the electrical performance of a fuel cell containing an MEA made by the modified process was tested, along with that of a fuel cell containing an MEA made by an older process. The cell containing the MEA made by the modified process performed nearly equivalently to other cell at a third of the flow rate, and performed better at the same flow rate (see figure).
This work was done by Sekharipuram Narayanan and Thomas Valdez 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
Intellectual Property group
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
Refer to NPO-20646, volume and number of this NASA Tech Briefs issue, and the page number.