Papers

AVS1996 Session SS+AS-FrM: Electrochemical Surface Science

Friday, October 18, 1996 8:20 AM in Room 203A

Friday Morning

Start	Invited?	Item
8:20 AM		SS+AS-FrM-1 Spatio-Temporal Pattern Formation in Electrochemical Reactions: Surface Plasmon Microscopy and Simulations K. Krischer, G. Fl\um a\tgen, N. Mazouz, P. Grauel, G. Ertl (Fritz-Haber-Institut der MPG, Germany) Dynamic instabilities, e.g. bistable or oscillatory behavior, have been commonly observed in virtually every electrochemical reaction under certain experimental conditions. Almost all investigations of these nonlinear phenomena deal with the global behavior of the systems, i.e. the temporal behavior of the current density or of the potential under potentiostatic or galvanostatic conditions, respectively. In this paper we demonstrate that the potential dependence of surface plasmon excitation can be exploited to obtain spatially resolved images of the potential drop across the double layer. We performed these measurements with a temporal resolution of ~ 1ms under oscillatory conditions. They revealed that most of the temporal instabilities are accompanied by spatial pattern formation. The observed potential waves possess velocities of cm/s to m/s and are thus several orders of magnitude faster than waves in other chemical systems. These large velocities result from spatial coupling through the electric field as opposed to the concentration field.A model describing spatio-temporal pattern formation of a large class of electro- chemical oscillators provides a deeper insight into two specific features of the spatial coupling in electrochemical systems. The range of the coupling is determined by the distance between the working and reference electrodes and can be varied between purely local coupling for short distances to global coupling for large distances. The strength of the coupling is controlled by the conductivity of the electrolyte. The characteristic length of the patterns depends on these two quantities and can thus be tuned in a wide range. .
9:00 AM		SS+AS-FrM-3 The Nature and Influence of Surface Modifiers on Electrochemical Reactions on Platinum T. Jarvi (University of Washington) Electrocatalytic reactions of C\sub 1\ species (e.g., formic acid and methanol) occur amidst coadsorbed species (carbonaceous residues and carbon monoxide) that may influence reaction selectivity and kinetics. Such species may act as spectators or participants in the reaction. To investigate their roles, we examined electrooxidation of methanol, formic acid, and carbon monoxide on Pt(111) covered with well-defined probe adlayers of carbon, carbon monoxide, and ethylidyne (CCH\sub 3\). The experiments were performed in two systems: a combined ultrahigh vacuum and electrochemical system for ex situ studies and a controlled atmosphere, multi-electrolyte cell for kinetic studies by chronoamperometry. The results show that probe adlayers may promote or poison the reaction depending on the nature of the adlayer. For example, a p(2x2) ethylidyne adlayer enhances the oxidation kinetics of formic acid beyond that of the clean surface, while at the same time removing the poisoning effect characteristic of this reaction.
9:20 AM		SS+AS-FrM-4 Reduced Electrochemical Interface Resistance in Solid Oxide Fuel Cells by Anode Modification T. Tsai, S. Barnett (Northwestern University) Electrode interfacial resistance is the major loss in thin film Solid Oxide Fuel Cells (SOFCs), especially for low operating temperatures (<800\super o\C). Ni-(Y\sub 2\O\sub 3\)\sub 0.1\(ZrO\sub 2\)\sub 0.9\ (Ni-YSZ) is the common anode and its performance is strongly dependent on the structure, primarily determined by the fabrication process. Ni-YSZ prepared by sputter deposition has shown promising results. To decrease interfacial resistance further, mixed conducting interfacial layers of 0.1 - 1.0 \mu\m thick TiO\sub 2\-doped YSZ (YZT) and Y\sub 2\O\sub 3\-doped CeO\sub 2\ (YDC) have been inserted between sputtered Ni-YSZ and YSZ electrolytes. Impedance spectroscopy measurements from 600 to 750\super o\C and at different hydrogen partial pressures showed that YDC yielded the lowest overall resistance values, e.g. about 0.1 Ohm-cm\super 2\ at 750\super o\C in 97%H\sub 2\+3%H\sub 2\O, compared with 0.2 Ohm-cm\super 2\ for YZT and 0.5 Ohm-cm\super 2\ with no layer under same deposition conditions. The YDC layers thus yield substantial improvement in SOFC performance. The impedance spectra generally showed two arcs. The interfacial layers reduced the size of the high frequency arc (HFA) but increased the low frequency arc (LFA). The HFA has been identified as the charge transfer process with an activation energy of around 1eV. The decrease in the size of HFA should result from the mixed conductivity of the interfacial layers which enables the charge transfer reaction to occur over the entire surface. The LFA decreases as the YSZ surface roughness increases, probably due to a change in Ni-YSZ structure, and is believed to be mass transport related.
9:40 AM		SS+AS-FrM-5 In Situ Characterization of the Electrochemical Oxidation/Reduction of Pt/Ru Bimetallic Alloy Particles W. O'Grady (Naval Research Laboratory); K. Pandya (North Carolina State University); E. Anderson (PSI Technology Company); D. Sayers (North Carolina State University) The alloys of platinum and ruthenium are of great interest for the application as catalysts in direct methanol fuel cells. The electrochemical properties of these alloys and especially their corrosion properties have not been investigated with x-ray absorption fine structure spectroscopy (XAFS). A series of high surface area Pt/Ru bimetallic alloy particles supported on carbon and a series of unsupported alloys were synthesized. Three compositions were prepared with atomic ratios of Pt:Ru of 8.5:1.5, 1:1 and 1.5:8.5 for the supported materials and 85:15, 50:50 and 15:85 for the unsupported materials. XAFS spectra were obtained at both the Pt L3-edge and the Ru K-edge for all the samples. The results of the analysis showed that the metals were completely alloyed with bond distances which reflected the compositions. The 1:1 Pt/Ru alloy composition gives the highest activity for methanol oxidation. Porous flooded electrodes were prepared with this material and placed in a cell where simultaneous electrochemical and XAFS studies of the oxidation and reduction of the alloy particles was carried out. The Ru-edge data showed changes in the oxidation state during the electrochemical oxidation while the Pt-edge remained largely unperturbed. The EXAFS data showed structural changes in both edges as the particles under went oxidation. The degree of oxidation determined how reversible the structural changes were. Some interesting anomalies arose in the EXAFS data analysis and interpretation which will be discussed. We acknowledge ONR for support of this research.
10:20 AM		SS+AS-FrM-7 In Situ Surface X-ray Diffraction Study on Pb UPD Structure on Cu(111) Y. Chu, I. Robinson, A. Gewirth (University of Illinois, Urbana) In-situ Surface X-ray diffraction technique was used to study the structure of the underpotentially deposited (UPD) Pb monolayer on Cu(111) single crystal. The electrolyte used was 0.1 M HClO\sub 4\ plus 5 mM PbO. The experiment was performed at beamline X16C at the National Synchrotron Light Source (NSLS). The UPD Pb monolayer was found to form an incommensurate close-packed hexagonal structure with no rotation with respect to the substrate. The in-plane Pb-Pb spacing showed a linear compression with the applied potential but with a large hysteresis. The measured two-dimensio nal compressibility of Pb monolayer was 0.96\+-\0.06\Ao\\super 2\/eV and is significantly smaller than the values for the Pb UPD layers on Ag(111) and Au(111). The substrate-dependent compressibility values suggest a considerable electronic influence from substrate to the adlayer. The structure of Pb monolayer on Cu(111) was affected by the presence of chloride in the electrolyte. Addition of a small amount of chloride to the electrolyte 1) reduced the hysteresis in the compression curve by a fact or of two, 2) increased the Pb-Pb spacing, 3) lowered the two-dimensional Pb compressibility, and 4) weakened Pb diffraction peaks. All of these changes are consistent with the interpretation that chloride may be bound to the Pb monolayer. The surface normal structure was probed with reflectivity measurements and fitting the data with a layered model. The data were described excellently by a model with a single Pb layer and the substrate with the relaxed top layer. The Pb-Cu distance turned out to be considerably contracted from the "bulk" distance estimated from the bulk metallic radii of Pb and Cu, and the topmost Cu layer spacing was found expanded.
10:40 AM		SS+AS-FrM-8 Double Layer Structure of Cu(111)/Water Interface Probed by Surface X-ray Diffraction Y. Chu, I. Robinson, A. Gewirth (University of Illinois, Urbana) The double layer structure of the Cu(111)/water interface was probed u sing surface X-ray diffraction. In situ measurements with a monolayer sensitivity was achieved using high intensity of synchrotron radiation and a specially designed electrochemical x-ray cell permitting a thin-electrolyte film geometry. Various electrolytes with a wide pH range were used to study the formation and structure of the epitaxial and/or bulk oxide. The electropolished Cu(111) single crystal sample in 0.1 M HClO\sub 4\ exhibited a well-defined voltammetric cathodic peak (around -0.5 V v s. Ag/AgCl) near the hydrogen evolution and a broader anodic peak(-0.10 V) near the Cu dissolution potential. Substantially different reflectivity and crystal truncation rods (CTR) curves were obtained at -0.60 V and -0.05 V. Fitting analysis based on layered model of the interface showed that the data at -0.60 V described a clean surface while the data at -0.05 V described a Cu(111) surface with a specifically adsorbed oxygen or hydroxide (distinction between the two is impossible due to small x-r ay cross section for hydrogen). Our data presents a proof that, at pH=1, there exist two different potential regions in which the structure of the Cu(111)/water interface is different.
11:00 AM		SS+AS-FrM-9 Reactivity and Mobility of Hydroxyl in Water and Water-Carbon Dioxide Adlayer on Silver(110) S. Lim, E. Stuve (University of Washington) To understand the roles of hydroxyl in the electrolytic adlayer we studied the interations of hydroxyl with water, and the carbon dioxide reactivity with the water-hydroxyl aqueous adlayer. From the isotopic study of the OH-H\sub 2\O interactions, we observed the migration of dueterium and labeled oxygen into the ice-like multilayer state of water in the form of HD\super 18\O. The migration of isotopes was enhanced by increasing the thickness of the water adlayer. The presence of HD\super 18\O in the multilayer water suggests the possibility that hydroxyl species desorb from the surface and are solvated by multilayer water. We also studied the reactivity of CO\sub 2\ adsorbed on a OH/H\sub 2\O adlayer. A surface reaction through either a bicarbonate (HCO\sub 3\) or formate (HCOO) intermediate was evident from the appearance of a high temperature water desorption peak at 400K. These findings suggest an ionic behavior for OH in aqueous adlayer in Ag(110).
11:20 AM		SS+AS-FrM-10 Ordering of Silicotungstate Anions on Ag, Au, and Cu Electrode Surfaces A. Gewirth, B. Niece, M. Ge, W. Klemperer, C. Wall (University of Illinois, Urbana) Adsorption of some heteropolytungstates on Ag, Au, and Cu single crystalline surfaces in redox-active solution was studied using in situ Scanning Tunneling Microscopy and electrochemical measurements. Heteropolyoxoanions spontaneously form ordered, adherent monolayers on Ag(111) surfaces, the structure of which is controlled by the shape of the anion. These monolayers exhibit domains a few tens of nanometers wide which arise as a consequence of their mechanism of adsorption to the silver surface which arises from oxophillic interactions between the polyoxometallate and the Ag. These anions also passivate the Ag electrode surface. On Au substrates, monolayer structures are also formed, but only under the influence of applied external potential. These monolayers exhibit fluxional structures due to the relatively weaker interaction with the substrate relative to Ag.