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.

Methanol oxidation was studied on arc-melted Pt-Ru-Os alloys and on fuel cell catalysts prepared by the NaBH4 reduction of metal chloride salts. Both the arc-melted alloys and the high surface area catalysts have x-ray diffraction patterns indicative of single-phase face-centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25°C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady-state voltammetry of the arc-melted alloys at 25°C confirmed that Pt-Ru-Os (65:25:10) is more active than Pt-Ru (1:1), particularly above 0.6 V. Pt-Ru-Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt-Ru (1:1) (e.g., typically at 90°C, 0.4 V; 340 mA/cm2 with Pt-Ru-Os vs. 260 mA/cm2 with Pt-Ru). InfrocluctionPractical direct methanol fuel cells require improved catalysts for the half-reaction CH3OH + H2O -* CO2 + 6W + 6e Although adsorptive dehydrogenation of methanol on ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.129.182.74 Downloaded on 2015-06-15 to IP

A family of sealant materials has been developed for use in the solid oxide fuel cell (SOFC) and in other applications in the temperature range of 800-1000 ± C. These materials are based on glasses and glass-ceramics in the SrO-La 2 O 3 -Al 2 O 3 -B 2 O 3 -SiO 2 system. The coefficients of thermal expansion (CTE) for these materials are in the range of 8-13 3 10 26 ͞ ± C, a good match with those of the SOFC components. These sealant materials bond well with the ceramics of the SOFC and, more importantly, form bonds that can be thermally cycled without failure. At the fuel cell operating temperature, the sealants have viscosities in the range of 10 4 -10 6 Pa-s, which allow them to tolerate a CTE mismatch of about 20% among the bonded substrates. The gas tightness of a sample seal was demonstrated in a simple zirconia-based oxygen concentration cell.

The concept of a proton conducting, methanol impermeable composite electrolyte system is demonstrated. A three-layered laminar electrolyte consisting of a dense, methanol impermeable protonic conductor sandwiched between proton permeable electronically insulating layers permits the selective transport of protons while eliminating methanol crossover to the cathode. We demonstrate the selectivity of the composite electrolyte using palladium foil sandwiched between two Nafion TM polymer membranes. The open-circuit voltage for an Ha/O2 fuel cell utilizing this composite electrolyte is unaffected by introduction of methanol to the H2 fuel stream, whereas conventional polymer electrolyte cells suffer severe degradation of performance due to methanol crossover.Research toward the development of low temperature (80-120~ direct methanol fuel cells (DMFCs) has primarily relied on the use of proton exchange membranes such as Nafion TM as the electrolyte. A serious drawback with these polymer electrolytebased DMFC systems is that methanol diffuses through the polymer electrolyte to the cathode, degrading cell performance. Attempts to produce polymeric electrolytes that selectively transport protons, but not small organic molecules such as methanol, have met with very limited success. 1We demonstrate here an alternative approach, a barrier concept, to the prevention of methanol crossover in DMFCs. In this design, a film of a methanol impermeable protonic conductor (MIPC), such as a metal hydride, is sandwiched between proton permeable electronic insulators, such as Nafion TM, forming a composite electrolyte. 2 Although highly permeable, inexpensive metal hydrides, such as surface modified V-15Ni-0.05Ti are known (3 • 10 -8 mol HJm s Pa ~/2, 423 K), Pd (2 • 10 9 mol H2/m s Pa ~]2, 423 K) 3 is used here as a model barrier phase; the close-packed structures of metal hydrides prevent permeation of larger molecules such as methanol and water. A hydrogen loaded palladium (palladium hydride) foil can be viewed as a proton permeable membrane: reductive adsorption of protons occurs on the surface facing the fuel anode, hydrogen diffuses through the palladium, and hydrogen atoms on the surface facing the oxygen cathode are oxidatively desorbed as protons (Fig. 1). By appropriately activating the surface of the MIPC+ the kinetics of hydrogen exchange can be markedly improved.

In situ Fourier transform infrared-diffuse reflection spectroscopy is introduced as a method for studying the catalytic surfaces of membrane electrode assemblies in working fuel cells. An annular collection electrode and a CaF2 window are used to expose a sufficient area of the electrode to the IR beam. Experimental results for methanol oxidation, CO adsorption, and (CO2 + CO) adsorption show that this method can be used to monitor the catalyst surface in a direct methanol fuel cell under load.The IR peaks associated with CO adsorption on high surface area Pt-Ru are similar to previously observed peaks on smooth electrodes reported in the literature.