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Figure 1: Current-voltage curves for methanol clcctro-oxidation at pH 8, using colloidal metal catalysts supported on carbon. Electrode compositions are Pt, Pt/Ru (50/50), and Pt/Rh/Os (60/15/25). The optimum composition was found by combinatorial screening of ternary combinations of Pt, Ru, Os, Rh, and Pd, using acridine as a fluorescent indicator.

Pt(50)/Ru(50) catalyst. At lower pH, the performance of Pi(60)/C)s(25)/Rh(15) is slightly inferior to that of Pt(50)/Ru(50), but still much better than that of pure platinum. The performance of the clectrodcs at low pH is crucial, because real DMFC's s operate at low pH. The combinatorial screening was intially done at a pH of 8 bccause of the high pKa of the indicator. Nevertheless, these initial results arc encouraging bccause they vindicate the combinatorial method; that is, under conditions where the screening was done (high pH), the best catalyst was quickly identified.

In order to identify the best anode catalysts for conditions under which DMFC's normally operate, a low-pH fluorescent indicator was identified and synthesized by a modification of literature methods. PTP (pKa = 3.0) forms metal complexes with many transition metal ions. The coordinated dication withdraws electron density from PTP, so that protonation becomes more difficult. This effect lowers the pKa by another unit, to -1.8. Although the copper complex of PTP is known in the literature, Cu2+ could interfere electrochemically at potentials used in the voltammetric scans. Ni2+ was therefore used instead, and did not interfere with the electrode reaction.

With this improved indicator, the arrays were rc-evaluatcd at low pH. This investigation, which is still in progress, involves more components (Pt, Rh, Os, Ru, Pd and Ag), and a larger number of ternary compositions per array (28). Upon completion of this study, 20 arrays will have been tested, and a total of 560 individual clcctrodes assessed. A Pt(50)/Ru(50) spot is fabricated on all arrays for purposes of in-situ visual comparison. Preliminary data acquired at low pH indicate that Pt/Rh/Os electrodes and Pt/Ru/Os electrodes perform most efficiently. Quantitative comparisons of conventional binary catalysts with these new compositions will be reported at the meeting.

The authors thank Anthony Czarnik and Peter Schultz for helpful discussions. This work was supported by the Office of Naval Research (LBL Molccular Design Institute).

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