Structural and electrochemical characterization of binary, ternary, and quaternary platinum alloy catalysts for methanol electro-oxidation

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 (RI 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-lr 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 are-melted fee alloy, suggesting that the nanocrystalline fee phase is Pt-rich. Anode catalyst polarization curves in direct methanol fuel cells (DMFC's) at 60 degrees 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 [CH3OH] 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.