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 (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 [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.