Andrea F. Gulla

Electrocatalysis of reformate tolerance in proton exchange membranes fuel cells: Part I

Electrocatalysis of anode electrode tolerance resulting from the presence of both CO and CO2 in the reformer feed was investigated for Pt, Pt � /Ru (1:1) and various atomic ratios of supported Pt:Mo electrocatalysts in proton exchange membrane fuel cells (PEMFCs). In order to elucidate the effects of CO and CO2 in the reformer feed, separate systematic studies were conducted with varying levels of CO and CO2 in H2. The results were used to explain those obtained with a fixed reformate composition: 45% H2, 10 ppm CO, 15% CO2 ,1 % CH 4 balanced with N2. Results with CO in H2 showed that PtMo/C exhibits at least a threefold better CO tolerance as compared to PtRu/C and fourfold with respect to Pt/C. The variation of PtMo atomic composition has a negligible effect on CO tolerance. Additional surface poisoning was detected for all the electrocatalysts studied in the molar ratio (H2:CO2, 40:60 to 60:40). The presence of a reduced CO2 species was confirmed using cyclic voltammetry. An ensemble effect was proposed to explain the variation of tolerance to CO2 as a function of Pt: Mo atomic ratio, this is in contrast to the effect in the presence of adsorbed CO. Interestingly, the overpotential losses in the presence of H2:CO2 for PtMo/C (1:1) and PtRu/C (1:1) were very close. As the Pt content of the PtMo/C alloys was increased, the overpotential losses followed those observed for pure Pt, clearly demonstrating a relationship between overpotential loss and Pt site availability. Despite similar overpotential losses between Pt/C and PtMo/C (5:1), both of which were greater than PtRu/C (1;1) the overpotential loss observed for PtMo in a CO2� /CO reformate mix was far better than for both PtRu/C and Pt/C. # 2003 Elsevier B.V. All rights reserved.

X-Ray Absorption Spectroscopy Studies of Water Activation on an Rh x S y Electrocatalyst for Oxygen Reduction Reaction Applications

The prototype chalcogenide electrocatalyst Rh{sub x}S{sub y} was probed in situ via a synchrotron-based X-ray absorption near-edge structure (XANES) technique to elucidate specific sites and modes of water activation during oxygen reduction reaction. X-ray diffraction revealed a mixture of phases (Rh{sub 2}S{sub 3}, Rh{sub 3}S{sub 4}, and Rh{sub 17}S{sub 15}). Theoretically generated XANES on a variety of geometries of O(H) adsorption on the predominant Rh{sub 3}S{sub 4} phase were compared to the experimental data. We show for the first time that the electrocatalyst first adsorbs O(H) in a one-fold configuration at lower potentials and n-fold at potentials greater than 0.80 V. This expectedly has important consequences for oxygen reduction reaction on alternative chalcogenide materials.

Toward Improving the Performance of PEM Fuel Cell by Using Mix Metal Electrodes Prepared by Dual IBAD

Dual ion beam assisted deposition (IBAD) has been used to manufacture selected Pt-based alloy/mix (Pt-Co and Pt-Cr) electrode-catalysts by direct metallization of a standard E-TEK gas diffusion layer (GDL). Their kineticsfor oxygen reduction reaction and their performance in a proton exchange membrane fuel cells (PEMFCs) are presented. Activity enhancement, normalized to electrochemical surface area and mass activity, of 38.99 mA/cm 2 (Pt-Co) and 27.21 mA/cm 2 (Pt-Cr) are reported relative to an IBAD Pt electrode-catalyst. We report a significant new development in terms of materials and mass-manufacturability for PEMFC applications.

Dual Ion-Beam-Assisted Deposition as a Method to Obtain Low Loading-High Performance Electrodes for PEMFCs

Ultralow loading noble metal Pt electrodes, for proton exchange membrane fuel cells PEMFCs , were prepared via dual ion-beam assisted deposition of pure Pt metal particles directly onto the surface of a noncatalyzed E-TEK gas diffusion layer. Activity enhancement, based on normalization with electrochemical surface area and mass activity is reported relative to a commercial gas diffusion electrode containing carbon-supported Pt electrocatalysts. The enhanced performance was primarily dictated by the cathodic oxygen reduction reaction. Based on the morphological differences, which enable such enhanced activities and the mass manufacturability of this electrode system, we report a significant new development in terms of materials for PEMFC application.

Ab initio calculations of electric field effects on the g-tensor of a nitroxide radical

The effects of applied electric fields on the magnetic parameters of a nitroxide spin label have been calculated using an intermediate level of Rayleigh–Schrodinger perturbation theory based on unrestricted Hartree–Fock ab initio calculations. The theory is tested for selected simple model organic radicals and gives electronic g-tensors that compare well with previous calculations and experimental values. The method is then applied to calculate effects of a local electrostatic field on the larger nitroxide radical, 2,2,5,5-tetramethyl-3,4-dehydro pyrrolidine-1-oxyl (TMDP) using a finite field approach. The method slightly underestimates the gx and gy of TMDP, but the predicted shifts in these quantities with field are in excellent agreement with recent experimental observations using high-frequency (220 GHz) electron paramagnetic resonance. Results are expressed in terms of the phenomenological formalism given earlier by Mims to characterize linear electric field effects in metal centers.