Improved Anode Coatings for Direct Methanol Fuel Cells

NASA's Jet Propulsion Laboratory, Pasadena, California

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.

Polarization Levels [potentials versus a normal hydrogen electrode (NHE)] of variously coated carbon-supported Pt-Sn electrodes were measured over a range of current densities in a test cell containing a solution of 1.0 M methanol in 0.50 M sulfuric acid.

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.

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