Eighteen Pt-M binary (M = Sn, Ta, W, Mo, Ru, Fe, In, Pd, Hf, Zn, Zr, Nb, Sc, Ni, Ti, V, Cr, Rh) thin film composition spreads were deposited at low M concentrations using magnetron sputtering and screened for methanol and ethanol electrooxidation activity using a fluorescence assay. Characterization of these thin films was performed using high energy X-ray diffraction and X-ray fluorescence. The electrochemical fluorescence assay revealed highest activity in the films with M = Sn, Zn, In, Fe, and Ru. Pt-M (M = Sn, Zn, In) showed highest activity at concentrations below 5 atom-% with a high fraction of Pt fcc(111) texturing and Pt-Fe showing the best activity at 10 atom-% Fe. On the other hand, the best (most negative) fluorescence onset potential in the Pt-Ru system was observed at a concentration of 35 atom-% Ru with only slight texturing of the film. To explore the potential origins of the observed catalytic activity, preliminary calculations on the d-band center shift with alloying were performed for bulk concentrations of up to 30 atom-% for Fe and 16 atom-% for M = Sn, Zn.Fuel cells present a promising energy conversion technology that is not limited by the Carnot cycle efficiency. Fuel cell efficiencies can reach up to 80-90% whereas a typical Carnot engine can only reach a maximum efficiency of 45-50%. 1 Fuel cells, in principle, could provide higher efficiency, an environmentally friendly form of energy conversion and high power/energy density. While the ideal system is a hydrogen-oxygen proton exchange membrane fuel cell (PEMFC), transportation, storage, and widespread availability of hydrogen is challenging due to the required high pressures and low volumetric energy density when compared to other fuels. Methanol, ethanol, and other small organic molecules (SOMs) have the advantage of higher volumetric energy densities, easier transportation, and diversity of availability from biosources. 2 Present limitations for widespread deployment of fuel cells are the cathode and anode materials which are expensive, easily poisoned, and show degradation over time. For methanol and ethanol oxidation, the process is more complicated than in the case of hydrogen, since the carbon in the fuel must be oxidized to CO 2 for maximal efficiency. The catalyst must also exhibit high catalytic activity for fuel oxidation with minimal poisoning. Platinum is the most commonly used catalyst, but it is readily poisoned with even low (ppm) levels of CO. As CO stands as a general and ubiquitous intermediate in the oxidation of SOMs, different catalysts must be used/found. Many materials have been investigated as anode catalysts for the oxidation of methanol, ethanol, and ethylene glycol, including alloys, 3 intermetallics, 4 nonPt containing materials (carbides, nitrides, oxides), 5 and core-shell structures. 6 For the cathode case, Strasser et al. reported improved activity for oxygen reduction with a Pt shell and a Pt-Cu alloyed core when compared to Pt alone. 6a They also considered the improved activity to be a result of the...
Materials with long-term durability and electrical conductivity at low pH (<2) and high potentials (∼1.4 V vs RHE) are of great interest as catalyst supports in proton exchange membrane (PEM) fuel cells. We have evaluated Ta−Ti−Al nitrides for this purpose. Combinatorial sputter-deposition of Ta−Ti−Al nitride thin films allowed the composition of these films to be varied spatially over a substrate at ∼1 atomic %/mm, enabling the investigation of the conductivity and microstructure of these materials over a wide range of compositions. Conductive probe atomic force microscopy (cp-AFM) is shown to facilitate high-throughput screening of electrical conductivity as a function of composition. Local, tip-induced oxidation of the film indicated that films annealed in the presence of oxygen were most resistant to oxidation-induced losses of conductivity. Ti-rich compositions exhibited conductivities similar to carbon black and best retained their conductivity after tipinduced oxidation. Small amounts of Ti (∼20 atomic %) were sufficient to impart desired conductivities to compositions rich in Ta and Al, which without Ti exhibited insulating behavior. Electron energy-loss spectroscopy (EELS) imaging revealed the formation of a <2 nm oxide layer at the surface of the nitride films, which is expected to make these materials more durable. Remarkably, high conductivities were observed in the presence of this oxide layer. Segregation of elements was observed at sub-10-nm length scales, yet mapping the lattice constant of the film with X-ray diffraction showed that the majority phase is a wellmixed alloy with a lattice constant that varies smoothly over the entire range of compositions. The rock-salt structure was observed at all compositions except those with high levels of Al.
A combined scanning differential electrochemical mass spectrometer (SDEMS)-scanning electrochemical microscope (SECM) apparatus is described. The SDEMS is used to detect and spatially resolve volatile electrochemically generated species at the surface of a substrate electrode. The SECM can electrochemically probe the reactivity of the surface and also offers a convenient means of leveling the sample. It is possible to switch between these two different scanning tips and techniques without moving the sample and while maintaining potential control of the substrate electrode. A procedure for calibration of the SDEMS tip-substrate separation, based upon the transit time of electrogenerated species from the substrate to the tip is also described. This instrument can be used in the characterization of combinatorial libraries of direct alcohol fuel cell anode catalysts. The apparatus was used to analyze the products of methanol oxidation at a Pt substrate, with the SDEMS detecting carbon dioxide and methyl formate, and a PtPb-modified Pt SECM tip used for the selective detection of formic acid. As an example system, the electrocatalytic methanol oxidation activity of a sputter-deposited binary PtRu composition spread in acidic media was analyzed using the SDEMS. These results are compared with those obtained from a pH-sensitive fluorescence assay.
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