Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt(-1) at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt(-1)). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra.

We demonstrate the synthesis of a core-shell catalyst consisting of a Pt monolayer as the shell and porous/hollow Pd-Cu alloy nanoparticles as the core. The porous/hollow Pd-Cu nanoparticles were fabricated by selectively dissolving a less noble metal, Cu, using an electrochemical dealloying process. The Pt mass activity for the oxygen reduction reaction of a Pt monolayer deposited on such a porous core is 3.5 times higher than that of a Pt monolayer deposited on bulk Pd nanoparticles and 14 times higher than that of state-of-the-art Pt/C electrocatalysts.

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Hydrogen peroxide ͑H 2 O 2 ͒ formation rates in a proton exchange membrane fuel cell ͑PEMFC͒ anode and cathode were estimated as a function of humidity and temperature by studying the oxygen reduction reaction ͑ORR͒ on a rotating ring disk electrode. Fuel cell conditions were replicated by depositing a film of Pt/Vulcan XC-72 catalyst onto the disk and by varying the temperature, dissolved O 2 concentration, and the acidity levels in hydrochloric acid ͑HClO 4 ͒. The HClO 4 acidity was correlated to ionomer water activity and hence fuel cell humidity. The H 2 O 2 formation rates showed a linear dependence on oxygen concentration and square dependence on water activity. The H 2 O 2 selectivity in ORR was independent of oxygen concentration but increased with the decrease in water activity ͑i.e., decreased humidity͒. Potential dependent activation energy for the H 2 O 2 formation reaction was estimated from data obtained at different temperatures.Proton exchange membrane fuel cell ͑PEMFC͒ technology, owing to its high efficiency, operational flexibility, and superior modularity, has the capability to be the structural and fundamental unit of an impending hydrogen economy. Two main issues that impede its progress toward commercialization are cost and durability. The U.S. Department of Energy's ͑DOE͒ projected performance requirements 1 for the year 2010 are 5000 h ͑with 20,000 start/stops͒ at $45/kW for automotive stacks and upwards of 40,000 h at $400-$750/kW for stationary power plants. In addition, current engineering requirements demand stack operation at higher temperatures ͑Ͼ100°C͒ and low relative humidities ͑Ͻ75% RH͒. Elevated temperature operation offers better tolerance to CO, faster oxygen reduction reaction ͑ORR͒ kinetics, and better water and thermal management enabling easier system integration. However, elevated temperatures and the desire to operate at ambient pressures means the fuel cell needs to be operated at lower relative humidities. Since much of ionomer and membrane technologies have evolved around the water dependent perfluorinated systems such as Nafion, both high temperature and low humidity conditions cause severe performance degradation 2-8 and remain an impediment toward achieving DOE's performance and durability targets.One of the mechanisms for catalyst/ionomer chemical degradation in PEMFCs involves the formation of hydroxyl and hydroperoxyl ͑OH· and OOH·͒ radicals 9,10 caused by hydrogen peroxide ͑H 2 O 2 ͒ formation on the catalyst surface via Reaction 1vs SHE ͓1͔ and subsequent decomposition via Reactions 2 and 3. Using a novel in situ spin trap electron paramagnetic resonance study, 11 Panchenko et al. 12 reported no evidence of OH· and OOH· radicals in the anode. They observed the presence of radicals in the cathode and near the membrane-cathode interface. Therefore, the H 2 O 2 diffuses into the membrane and chemically breaks down to hydroxyl radicals and ions on metal ions present 13 in the membrane H 2 O 2 + M 2+ → M 3+ + OH· + OH − ͓2͔ OH· + H 2 O 2 → OOH· + H 2 O ͓3͔These ra...

The ability to control the nanoscale size, composition, phase, and facet of multimetallic catalysts is important for advancing the design and preparation of advanced catalysts. This report describes the results of an investigation of the thermal treatment temperature on nanoengineered platinum−nickel−cobalt catalysts for oxygen reduction reaction, focusing on understanding the effects of lattice strain and surface properties on activity and stability. The thermal treatment temperatures ranged from 400 to 926 °C. The catalysts were characterized by microscopic, spectroscopic, and electrochemical techniques for establishing the correlation between the electrocatalytic properties and the catalyst structures. The composition, size, and phase properties of the trimetallic nanoparticles were controllable by our synthesis and processing approach. The increase in the thermal treatment temperature of the carbon-supported catalysts was shown to lead to a gradual shrinkage of the lattice constants of the alloys and an enhanced population of facets on the nanoparticle catalysts. A combination of the lattice shrinkage and the surface enrichment of nanocrystal facets on the nanoparticle catalysts as a result of the increased temperature was shown to play a major role in enhancing the electrocatalytic activity for catalysts. Detailed analyses of the oxidation states, atomic distributions, and interatomic distances revealed a certain degree of changes in Co enrichment and surface Co oxides as a function of the thermal treatment temperature. These findings provided important insights into the correlation between the electrocatalytic activity/stability and the nanostructural parameters (lattice strain, surface oxidation state, and distribution) of the nanoengineered trimetallic catalysts.

The effect of humidity on the chemical stability of two types of membranes ͓i.e., perfluorosulfonic acid type ͑PFSA, Nafion 112͒ and biphenyl sulfone hydrocarbon type, ͑BPSH-35͔͒ was studied by subjecting the membrane electrode assemblies ͑MEAs͒ to open-circuit voltage ͑OCV͒ decay and potential cycling tests at elevated temperatures and low inlet-gas relative humidities. The BPSH-35 membranes showed poor chemical stability in ex situ Fenton tests compared to that of Nafion membranes. However, under fuel cell conditions, BPSH-35 MEAs outperformed Nafion 112 MEAs in both the OCV decay and potential cycling tests. For both membranes, ͑i͒ at a given temperature, membrane degradation was more pronounced at lower humidities and ͑ii͒ at a given relative humidity operation, increasing the cell temperature accelerated membrane degradation. Mechanical stability of these two types of membranes was also studied using relative humidity ͑RH͒ cycling. Due to decreased swelling and contraction during wet-up and dry-out cycles, Nafion 112 lasted longer than BPSH-35 membranes in the RH cycling test.

Electrochemical impedance spectroscopy was successfully employed to investigate ionic permeability into self-assembled monolayers (SAMs) deposited on a Au(111) single crystal in propylene carbonate electrolyte solutions. The ionic permeability strongly depends on the applied dc potential, implying the existence of different potential dependent phases in monolayers exposed to propylene carbonate solutions. Increasing the temperature changes the ionic permeability of the SAM irreversibly. This is suggested to be due to changes in film structure due to Ostwald ripening. An appropriate equivalent circuit for characterization of the ionic conductivity of these self-assembled monolayers was developed.

The formation and behavior of self-assembled monolayers made of n-dodecaneselenol (DDSe) on Au(111) as a blocking dielectric medium toward heterogeneous electron-transfer were assessed by electrochemical impedance spectroscopy. Microelectrode array theory was applied to study the growth kinetics of the self-assembled monolayer. The results show that adsorption from a n-dodecaneselenol solution in ethanol can be described using two different time constants. The monolayer is formed within the first minute after the electrode has been brought into contact with the deposition solution. The second step involves film reorganization and self-ordering, which can last for several hours. LA0203483