Two perfluoroalkanesulfonic acids and perfluoroalkanesulfonimides have shown promise as anodecoating materials for improving the performances of direct methanol fuel cells (DMFCs). Heretofore, the state-of-the-art material commonly used for coating anodes in DMFCs has been Nafion™-H — a perfluorosulfonic acid-based hydrophilic, proton-conducting ion-exchange polymer that exhibits relatively high thermal and electrochemical stability. Relative to Nafion™-H, the present coating materials afford greater rates of electro-oxidation of methanol, smaller polarization losses and, hence, greater energy-conversion efficiencies.
Perfluorinated solid polymer electrolytes — in particular, Nafion™-H — have been used as anode coatings in DMFCs to (1) ensure contact between electrolyte membranes and electrocatalytic anode materials (typically, alloys containing Pt) and (2) help prevent catalysts from being poisoned by adsorption of anions. However, the performances of electrodes coated by perfluorinated solid polymer electrolytes have not been ideal, especially at room temperature. Consequently, there has been continued interest in developing means of reducing polarization losses and increasing rates of oxidation and efficiencies of utilization of methanol in order to improve the performances and increase the energy-conversion efficiencies of DMFCs.
In preparation for experiments, DMFC anodes made of carbon-supported Pt, Pt-Ru, and Pt-Sn were prepared and coated, variously, with Nafion™-H or six different perfluoroalkanesulfonic materials: perfluorooctanesulfonic acid (C8 acid), perfluorododecanesulfonic acid (C12 acid), perfluoroheptadecanesulfonic acid (C17 acid), bis-perfluoro-n-butyl sulfonyl acid imide, bis-perfluoro-noctylsulfonic acid imide (C8-C8 imide), or perfluoro-n-butyl-perfluoro-n-octylsulfonic acid imide. The experiments involved electro-oxidation of methanol on each anode installed as one of the electrodes in a three-electrode electrochemical test cells. The performances of the electrodes were characterized by galvanostatic polarization measurements and cyclic voltammetry. Of the compounds investigated, C12 acid and C8-C8 imide were found to afford the greatest increases in rates of oxidation of methanol and the greatest reductions in levels of polarization (see figure), relative to those of Nafion™-H.
This work was done by G. K. Surya Prakash, Qun-Jie Wang, and George A. Olah of the University of Southern California and Marshall C. Smart, Sekharipuram Narayanan, and Subbarao Surampudi, of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online 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:
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Refer to NPO-40503, volume and number of this NASA Tech Briefs issue, and the page number
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Improved Anode Coatings for Direct Methanol Fuel Cells
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Overview
The document titled "Improved Anode Coatings for Direct Methanol Fuel Cells" is a Technical Support Package from NASA's Jet Propulsion Laboratory, aimed at disseminating research findings related to advancements in fuel cell technology. It focuses on the electro-oxidation of methanol on platinum (Pt) electrodes, particularly those coated with perfluoroalkane sulfonic acids or perfluoroalkanesulfonimides.
Key findings from the research include the performance characteristics of various ionomeric coatings on Pt-Sn electrodes when oxidizing methanol in a sulfuric acid solution. The study presents data on current density and potential (NHE) for different coatings, such as Nafion, C12-acid, and C8-C8 imide, illustrating their effectiveness in enhancing the electrochemical reactions involved in methanol oxidation.
Figures included in the document depict the current density versus potential curves, showcasing the minimum polarization for each type of coated electrode. The performance of these electrodes is influenced by the thickness of the coatings, as noted in the findings attributed to Watanabe regarding Nafion-coated Pt electrodes. The stability of the coated electrodes over time is also examined, with results indicating the performance of bare electrodes compared to those coated with C8-C8 imide and Nafion H.
The document emphasizes the importance of these advancements in the context of direct methanol fuel cells, which are of significant interest for their potential applications in portable power sources and automotive technologies. The research highlights the ongoing efforts to improve the efficiency and stability of fuel cell systems, which are critical for the development of sustainable energy solutions.
In addition to the experimental results, the document provides references to previous studies and publications that contribute to the understanding of fuel cell technology. It serves as a resource for researchers and engineers in the field, offering insights into the latest developments and potential future directions for research in anode coatings and fuel cell performance.
Overall, this Technical Support Package encapsulates NASA's commitment to advancing fuel cell technology, showcasing innovative research that has implications beyond aerospace applications, potentially benefiting various sectors seeking cleaner energy solutions.
