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.
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
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Refer to NPO-20306
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Overview
The document discusses advancements in membrane/electrode assemblies (MEAs) for methanol fuel cells, specifically focusing on those made from hydrogen form sulfonated polyether sulfone (HSPES). Traditional MEAs, often based on perfluorosulfonic acid polymers like Nafion, have been effective but costly. The new HSPES-based MEAs aim to provide similar performance at a reduced cost.
Prior to this development, HSPES-based MEAs did not perform as well as their Nafion counterparts, primarily due to poor utilization of the cathode catalyst. Analysis of polarization data revealed that performance losses were concentrated in the cathode, leading researchers to conclude that the bonding quality of the electrode was a significant factor affecting catalyst utilization.
To address these issues, the researchers modified the fabrication process of the MEAs. The new method involves a two-layer cathode structure. The first layer is sintered at a high temperature, which can reduce catalyst activity, while the second layer, which contains no polytetrafluoroethylene, is applied without sintering. This innovative approach allows for a highly active catalyst to be in direct contact with the membrane after hot pressing, improving overall performance.
In comparative tests, the improved MEA demonstrated a potential of 387 mV at a current density of 300 mA/cm², significantly outperforming the previous version, which exhibited a potential of only 212 mV. The modified process maintains the catalyst loading at 4 mg/cm², with each layer containing half of the total catalyst loading, ensuring that a greater proportion of the catalyst participates in electrochemical reactions.
The research was conducted by Andrew Kindler and Shiao-Ping Yen at Caltech for NASA's Jet Propulsion Laboratory. The findings indicate that the new HSPES-based MEAs not only enhance performance but also offer a cost-effective alternative to traditional MEAs, potentially advancing the development of more efficient methanol fuel cells.
Overall, this work represents a significant step forward in fuel cell technology, addressing previous limitations and paving the way for improved energy solutions in various applications. The document emphasizes the importance of electrode bonding and catalyst utilization in achieving optimal performance in fuel cells.
