Determination of oxygen reduction on platinic and non-platinic electrodes and sulfur oxidation reaction mechanisms on platinum and platinum-cobalt electrodes using in situ X-ray Absorption Spectroscopy.

摘要

X-ray Absorption Spectroscopy (XAS) has been used to examine several active fuel cell catalysts with the objective of extending the applicability of the novel Deltamu XANES technique to more complex systems, and to develop a catalyst that is more stable, less prone to poisoning, and less expensive than the current standard Pt nanoparticles supported on carbon. In this work we examined Au nanoparticles supported on SnOx, ligand-stabilized Pt nanoparticles, sulfur poisoned Pt cathodes, and Rh-based sulfide, Rh xSy, in situ in an electrochemical cell. Both Extended X-ray Absorption Fine Structure (EXAFS) analysis, as well as the novel difference technique utilizing the X-ray Absorption Near Edge Structure (Deltamu-XANES) were utilized.;The XAS data, taken at the National Synchrotron Light Source at Brookhaven National Laboratory, were measured in situ to acquire information on the oxygen reduction reaction (ORR) mechanism on the indicated electrocatalyts. Particle modeling and EXAFS data were used to estimate catalysts nanoparticle size and morphologies. Full multiple scattering calculations were performed on small atomic models representing the catalysts to interpret the XANES Deltamu signatures. The geometric binding sites and relative coverage of simple H, OH, and O species to more complicated organic ligands and various sulfur species adsorbed on the electrocatalysts were determined at different applied electrochemical potentials. The results aided in improving our understanding of how the ORR reaction occurs on different catalysts such as RhxSy, Au-SnO2, ligand stabilized Pt, and S poisoned Pt.;Au nanoparticles supported on tin-oxide (Au-SnOx) and mixed with Vulcan carbon (VC) are active electrocatalysts for the ORR, while Au/VC by itself is inactive. These results indicate that the SnOx support either enhances the ORR activity of the Au, or participates directly in the reaction. Results obtained from the Deltamu XANES and EXAFS show that a bifunctional mechanism plays the dominant role in the ORR reaction. O 2 adsorbs and dissociates on the nearby SnOx surface with apparent simultaneous electron transfer from the Au, and then ultimate reduction to water.;XAS data were also acquired on Pt nanoparticles stabilized by triphenyl phosphine triphosphonate (TPPTP) ligands attached to the Pt particles. Deltamu analysis shows the Pt particles in the Pt/TPPTP catalyst are complexed via the P (i.e., Pt-Ptp) with about 0.3 ML of TPPTP (where the tp indicates the triphenyl groups); these species exist on the surface along with a dramatically reduced OH coverage. The reduction in OH coverage enhances the surface specific ORR rate relative to the same sized Pt particles on carbon, but is not sufficient to increase the mass specific ORR rate.;A broad range of experimental tools were used to study RhxS y/C. These tools include X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), micro-analysis and electrochemical investigations, along with the XAS. The adsorption of water, OH, and O as a function of overpotential is reported. Heating of the RhxSy catalysts causes Rh segregation and the formation of Rh metal particles, and immersion in TFMSA causes S dissolution and the formation of a Rh skin on the RhxS y samples. It is shown that some Rh-Rh interactions are needed to carry out the ORR.;Finally XAS data were collected and analyzed using a modified Deltamu XANES technique to distinguish the adsorption of O and SOx species on Pt and Pt3Co electrocatalysts. This modified Deltamu technique enabled the direct observation of O coverage and SOx oxidation state, and the results were correlated with previous electrochemical data and theory to assign oxidation and reduction potential regions. The data reveal why S oxidation and ultimate removal from Pt3Co are much more efficient than that which occurs when S interacts with Pt. SO2 to SO3/4 oxidation occurs concurrently with OH and O adsorption on Pt3Co, while on Pt the potential apparently has to be cycled to higher potentials to oxidize the SO2 in the presence of O/Pt, and then back down to lower potentials to remove the SO3/4 in the presence of OH/Pt.