Improved membrane/electrode assemblies (MEAs) made partly from hydrogen form of sulfonated polyether sulfone (HSPES) have been developed for use in methanol fuel cells. In comparison with traditional fuel-cell MEAs made partly from a commercial perfluorosulfonic acid-based polymer, these MEAs perform similarly, but cost much less.
Prior to the development reported here, MEAs made partly from HSPES did not perform as well as did the traditional ones. Analysis of polarization data for an HSPES-based MEA revealed that losses in the cathode accounted for the loss in performance. Further analysis guided by previous experience led to the conclusion that the loss in performance was caused by poor utilization of the cathode catalyst. This conclusion, in turn, led to the conjecture that performance might be improved by use of a modified fabrication process that would yield a modified cathode structure, wherein the cathode catalyst would be bonded in an improved way and distributed in different structures, such that a greater proportion of the catalyst loading would participate in electrochemical reactions.
It was conjectured, further, that the best way to improve bonding and reduce migration was to immobilize some of the catalyst prior to a hot-pressing step that is part of the MEA-fabrication process. In a previous version of the process, a paint containing polytetrafluoroethylene, water, and triton was applied to carbon paper and sintered at a temperature of 350 °C under a nitrogen blanket; this immobilized the catalyst. A solution of the perfluorosulfonic acid-based polymer was then applied to the catalyst-covered electrode before hot pressing. One of the goals pursued in the development of this previous version was of the process to obtain adequate performance with a catalyst loading reduced from the value (4 mg/cm2) of the traditional MEAs. The catalyst loading achieved was 1 mg/cm2, but, as stated above, performance was below that of traditional MEAs.
In the modified process, the catalyst loading is not reduced from that of traditional MEAs. However, the catalyst is applied in two layers, each containing half (2 mg/cm2) of the total catalyst loading. The first half of the catalyst is applied as in the previous version of the process and sintered at 350 °C, but unlike in the previous version of the process, the perfluorosulfonic acid-based polymer is not applied after sintering. The second half of the catalyst loading is applied as part of a paint that also contains water and the perfluorosulfonic acid-based polymer. Unlike the first layer, the second layer is not sintered. Instead, the MEA is hot-pressed after application of the second layer.
The two-layer cathode catalytic structure offers advantages over the previous single-sintered-layer cathode catalytic structure:
- The high sintering temperature can reduce the activity of the catalyst. As a result of the placement of the unsintered layer over the sintered one, highly active catalyst is in direct contact with the membrane after hot pressing.
- Quasi-sintering of polytetrafluoroethylene can reduce catalyst activity by covering otherwise active catalytic sites. However, the unsintered layer contains no polytetrafluoroethylene. Because the unsintered layer becomes bonded to the membrane, the lower catalytic activity of the sintered layer becomes less important. For this reason, it might be possible to increase the polytetrafluoroethylene content of the sintered layer to improve the barrier to migration.
The performances of an HSPES-based MEA made by the modified process (improved MEA) and of one made by the previous version of the process were measured in a comparative test. At a current density of 300 mA/cm2, the MEA made by the previous version of the process exhibited a potential of 212 mV, whereas the improved MEA exhibited a potential of 387 mV.
This work was done by Andrew Kindler and Shiao-Ping Yen of Caltech for NASA's Jet Propulsion Laboratory.
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