A class of developmental membrane electrolyte materials for methanol/air and hydrogen/air fuel cells is exemplified by a composite of (1) a melt-processable polymer [in particular, poly(vinylidene fluoride) (PVDF)] and (2) a solid proton conductor (in particular, cesium hydrogen sulfate). In comparison with previously tested membrane electrolyte materials, including those described in the two preceding articles, these developmental materials offer potential advantages of improved performance, lower cost, and greater amenability to manufacturing of fuel cells.
A principal limitation on the utility of the previously tested membrane electrolyte materials is that they must be hydrated to be able to conduct protons. This requirement translates to a maximum allowable operating temperature of about 90°C, and the presence of water in the polymer matrices undesirably gives rise to high permeability by methanol. It would be desirable to reduce permeability by methanol to increase cell performance and fuel-utilization efficiency, and it would be desirable to operate fuel cells at temperatures as high as 140°C to increase their tolerance to carbon monoxide from reformate streams. Therefore, what are needed are membrane materials that conduct protons in the absence of water.
In a composite material of the type undergoing development, the polymer serves as a matrix to support the solid proton conductor. In cesium hydrogen sulfate, proton conduction occurs by a mechanism that does not depend on water. At room temperature, the protons are in a bound state and so there is little or no proton conduction. However, as the temperature rises past 130°C and toward a value between 135 and 145°C, the cesium hydrogen sulfate undergoes a phase transition to a state in which the hydrogen ions have a significant amount of mobility; that is, the material becomes a proton conductor. The conductivity can be as high as 0.1 Ω–1cm–1 — of the order of the conductivities of the previously tested membrane electrolyte materials.
Some experimental polymer/solid electrolyte membranes have been fabricated by mixing PVDF and cesium hydrogen sulfate powders and pressing the mixtures in a die at temperatures between 160 and 190°C. Other experimental membranes were prepared by forming slurries of the powder mixtures in organic solvents, casting the slurries on a plate, allowing the slurries to dry, and then hot pressing the slurries. The figure shows the logarithms of electrical conductivities of these membranes as a function of composition. At the time of reporting the information for this article, it was anticipated that these membranes would be used to fabricate membrane/electrolyte assemblies for testing in fuel cells.
This work was done by Sekharipuram Narayanan, Sossina Haile, Dane Boysen, and Calum Chisholm of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-20645, volume and number of this NASA Tech Briefs issue, and the page number.