Water-Free Proton-Conducting Membranes for Fuel Cells

NASA’s Jet Propulsion Laboratory, Pasadena, California

Sunday, July 01 2007

Each of these preparations was brushed onto an open mat of glass fibers. The coated mats were dried in flowing air at a temperature of 60 °C for about an hour. The coated mats were further dried in a vacuum oven at 60 °C to remove traces of water.

The thermal stability of P4VPBS was evaluated by differential scanning calorimetry. The results showed that P4VPBS undergoes a glass transition at a temperature of about 182 °C and that it melts at about 298.7 °C, with no evidence of decomposition. These thermal properties are consistent with the requirements for stability under operating conditions in fuel cells.

The coated mats were tested to determine their ionic conductivities and to quantify their performances as membranes in hydrogen/oxygen fuel cells. The ionic conductivity of the mat coated with the P4VPBS/silica composite was slightly greater than that of the mat coated with P4VPBS: this was expected because the composite contains additional molecular groups that are presumably available for forming hydrogen bonds. On the basis of the observed temperature dependence of the conductivity, the activation energy for conduction was estimated to be about 0.1 eV, suggesting hopping-type conduction through hydrogen bonds. While the measured conductivity values were two orders of magnitude lower than desirable for fuelcell applications, the degree of solid-state proton conduction was the highest observed thus far in polymeric salts. It is anticipated that the polymer backbone could be modified to facilitate formation of hydrogen bonds to obtain more sites for proton hopping and, hence, greater proton conductivity.

In preparation for the tests of fuel-cell performance, the coated mats were further coated with catalytic anode and cathode layers to form membrane/electrode assemblies. No attempt was made to optimize the catalytic layers. In the fuel-cell tests, stable maximum cell potentials of 0.85 V were attained. The anticipated maximum cell voltage was 1.0 V. The decrease from the expected maximum value was attributed to some crossover of hydrogen and oxygen through the membranes. The figure shows some of the data from the fuel-cell test of the mat coated with the P4VPBS/silica composite. The power density indicated by these data is low for a fuel cell operating at the indicated temperature — presumably because of the lack of optimization of the catalyst layers. Nevertheless, the data suggest that optimization of catalysts and enhancement of conductivity should make it possible to realize high-temperature fuel cells.

This work was done by Sekharipuram Narayanan and Shiao-Pin Yen of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp 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:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Refer to NPO-30873, volume and number of this NASA Tech Briefs issue, and the page number.