Combinatorial screening of electrochemical catalysts by current-voltage methods can be unwieldy for large sample sizes. By converting the ions generated in an electrochemical half-cell reaction to a fluorescence signal, the most active compositions in a large electrode array have been identified. A fluorescent acid-base indicator was used to image high concentrations of hydrogen ions, which were generated in the electrooxidation of methanol. A 645-member electrode array containing five elements (platinum, ruthenium, osmium, iridium, and rhodium), 80 binary, 280 ternary, and 280 quaternary combinations was screened to identify the most active regions of phase space. Subsequent "zoom" screens pinpointed several very active compositions, some in ternary and quaternary regions that were bounded by rather inactive binaries. The best catalyst, platinum(44)/ruthenium(41)/osmium(10)/iridium(5) (numbers in parentheses are atomic percent), was significantly more active than platinum(50)/ruthenium(50) in a direct methanol fuel cell operating at 60 degreesC, even though the latter catalyst had about twice the surface area of the former.

The bifunctional model for methanol electro-oxidation suggests that competent catalysts should contain at least two types of surface elements: those that bind methanol and activate its C-H bonds and those that adsorb and activate water. Our previous work considered phase equilibria and relative Pt-C and M-O (M ) Ru, Os) bond strengths in predicting improved activity among single-phase Pt-Ru-Os ternary alloys. By addition of a correlation with M-C bond strengths (M ) Pt, Ir), it is possible to rationalize the recent combinatorial discovery of further improved Pt-Ru-Os-Ir quaternaries. X-ray diffraction experiments show that these quaternary catalysts are composed primarily of a nanocrystalline face-centered cubic (fcc) phase, in combination with an amorphous minor component. For catalysts of relatively high Ru content, the lattice parameter deviates positively from that of the corresponding arc-melted fcc alloy, suggesting that the nanocrystalline fcc phase is Pt-rich. Anode catalyst polarization curves in direct methanol fuel cells (DMFC's) at 60°C show that the best Pt-Ru-Os-Ir compositions are markedly superior to Pt-Ru, despite the higher specific surface area of the latter. A remarkable difference between these catalysts is revealed by the methanol concentration dependence of the current density. Although the rate of oxidation is zero order in [CH 3 OH] at potentials relevant to DMFC operation (250-325 mV vs RHE) at Pt-Ru, it is approximately first order at Pt-Ru-Os-Ir electrodes. This finding implies that the quaternary catalysts will be far superior to Pt-Ru in DMFC's constructed from electrolyte membranes that resist methanol crossover, in which higher concentrations of methanol can be used.

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