Nathalie Martz

Characterization of differently synthesized Pt-Ru fuel cell catalysts by cyclic voltammetry, FTIR spectroscopy, and in single cells

Carbon-supported Pt-Ru (1:1) catalysts were synthesized from aqueous solutions of Pt(IV) and Ru(IV) salts by two different reductive methods and characterized in comparison to a commercial Pt-Ru/C catalyst purchased from E-TEK. Inc. The three catalysts differ in particle size. dispersion, and degree of alloying, as determined by X-ray diffraction and transmission electron microscopy. Cyclic voltammetry in different methanol concentrations and CO-stripping experiments were conducted to check their electrocatalytic activity. The results obtained are in good agreement with single-cell measurements using H 2 /CO mixtures with concentrations of 75 and 150 ppm CO. The synthesized catalysts show improved activities for low CO concentrations at 75°C cell temperature. In addition, for the synthesized catalysts only low CO coverages were found at the electrode surface by special in situ infrared reflectance techniques in contrast to the commercial one.

Characterization of different Pt–Ru catalysts by X-ray diffraction and transmission electron microscopy

Carbon-supported Pt–Ru (1:1)-catalysts have been synthesized by two reduction methods in an aqueous phase and in a third way in an organic solution and characterized in comparison to a commercial Pt–Ru/C catalyst purchased from E-TEK. X-ray diffraction and transmission electron microscopy were carried out on the different samples in the as-synthesized state and after heat-treatment at 500°C in nitrogen and air atmospheres respectively. Powder patterns of the different catalysts in the as-synthesized state reveal a fcc pattern with d values matching or close to platinum. No evidence of metallic Ru or any oxide phases was found. After heat-treatment in nitrogen Ru reflections occur in the synthesized catalyst samples, backing the supposition of separate platinum and ruthenium particles instead of a Pt–Ru alloy.

In-situ XAFS fuel cell measurements of a carbon-supported Pt–Ru anode electrocatalyst in hydrogen and direct methanol operation

A special in-situ PEM fuel cell has been developed to make XAFS measurements during hydrogen and direct methanol operation possible. The chosen set-up in transmission allows for the on-line monitoring of changes in the catalysts oxidation states and their characteristic short-range order. In both operation modes, the catalyst is reduced during operation, as can be concluded from the XANES region of the spectra showing a decreased white-line intensity in E space.

X-ray absorption studies on alloy formation in different carbon-supported Pt–Ru electrocatalysts

Carbon-supported Pt–Ru (1 ∶ 1) catalysts were synthesized from aqueous solutions of Pt IV and Ru IV salts by two different reduction methods and in an organic solvent according to a slightly-modified Boennemann synthesis. X-ray absorption spectroscopy was applied to characterize the in-house synthesized catalysts in comparison to a commercial carbon-supported Pt–Ru/C alloy catalyst purchased from E-TEK inc. Significant geometric differences were revealed by the conventional EXAFS analysis which are attributed to differences in particle size and alloy formation. In contrast to the commercial catalyst, which is at least partially alloyed, for the in-house synthesized catalysts a much smaller number of Pt–Ru nearest neighbours has been found pointing either towards a rather inhomogeneous alloy formation or to the formation of ruthenium oxide. These findings are in excellent agreement with the results of the “Atomic” XAFS; the lower the number of Pt–Ru nearest neighbour contributions the higher the R value at which the maximum of the AXAFS feature appears. The R value, however, is supposed to be directly reflective of the d band occupancy which decreases as the degree of alloy formation increases.

Synthesis and Characterization of PtRu/C Catalysts Obtained by Colloidal and Deposition Methods for Fuel Cell Applications

The purpose of this investigation was to compare catalysts produced by the Bonnemann - colloidal method (PtRu (B1) and PtRu (B2)), and those produced by the spontaneous deposition method (PtRu (SD)). The catalysts produced by both methods had good electrochemical behavior for methanol oxidation for proton exchange membrane fuel cell applications. The structure of the catalyst was examined by transmission electron microscopy (TEM). Energy dispersive spectroscopic analysis (EDS) was used to determine the semi-quantitative composition of the catalysts, and the electrochemical behavior was determined by cyclic voltammetry (CV). The diffractograms of the binary catalysts revealed platinum and ruthenium as the only crystalline phases, as per ICDD data base. The PtRu (B1) catalyst, treated in a reducing atmosphere, has the same structure as PtRu (B2), treated in an oxidising/reducing atmosphere, except that the crystallite size was around 1.7 nm for PtRu (B1) instead of 9.9 nm for PtRu (B2). The catalysts PtRu (B2) and PtRu (SD) showed similar cyclic voltammetric behavior, which was better than that of PtRu (B1). Both methods are suitable for the production of electrocatalysts for fuel cell applications. The colloidal method is more expensive than the deposition method, but the former permits the production of ternary and quaternary catalyst systems with enhanced CO tolerance.

Synthesis and Characterization of Eletrocatalyst Powders for Application in PEM Fuel Cells

The Bonnemann method was chosen to synthesize carbon-supported Pt-Ru-Mo electrocatalyst powders, as this method leads to highly-dispersed nanoparticles with an average particle size of approximately 2 nm. Structural characterization of the resulting catalysts was done by the following techniques: X-rays fluorescence analysis (XFA) and nanosize-energy dispersive X-rays analysis (nano-EDX) to obtain information about noble metal loading and composition as well as X-rays diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) to determine structure, dispersion and crystallite size of the nanoparticles. X-rays photoelectron spectroscopy (XPS) was applied to detect amorphous phases and to determine the oxidation states of Pt, Ru and Mo. The morphology of membrane electrode assemblies, prepared by a spraying technique, was checked by scanning electron microscopy (SEM). For the determination of the electrocatalytic activity E/i curves in single cell arrangements were recorded using both H 2 /CO mixtures and methanol as feed gases.