J.X. Wang

Carbon monoxide oxidation on bare and Pt-modified and Ru(0001) single crystal electrodes

Abstract Carbon monoxide oxidation on bare and Pt-modified ruthenium surfaces with the (10 1 0) and (0001) orientations was investigated with cyclic voltammetry, scanning-tunneling microscopy and in situ Fourier transform infrared spectroscopy. Facile oxidation kinetics of CO on Ru (10 1 0) are observed, in contrast with a slow reaction on Ru(0001). Scanning tunneling microscopy (STM) measurements confirmed that spontaneous deposition of Pt produces island-like structures on both single crystal Ru surfaces. CO oxidation on a bimetallic Pt / Ru (10 1 0) surface with a Pt coverage of approximately one monolayer occurs at potentials that are 140 mV more negative than those for bare Pt. This potential is, however, more positive than the potential of the onset of the oxidation on Ru (10 1 0) . IR spectroscopy shows one peak for linearly adsorbed CO on bare and Pt-modified Ru (10 1 0) surfaces, while two peaks are visible for the Pt-modified Ru(0001) structure. A single broad peak for the bimetallic Pt / Ru (10 1 0) surface may result from addition of the red-shifted peak for Pt and the peak for the Ru (10 1 0) substrate. A large red shift of CO on the Pt / Ru (10 1 0) surface requires further work to be explained. A negative shift of CO oxidation on Pt / Ru (10 1 0) indicates a decrease of the PtCO bond strength on that surface compared with the bond with bulk Pt.

Oxygen reduction on bare and Pt monolayer-modified Ru(0001), Ru(100) and Ru nanostructured surfaces

Abstract Oxygen reduction kinetics on bare and Pt-modified Ru(10 1 0) and Ru(0001) single crystal surfaces, and on carbon-supported Ru nanoparticles have been investigated. Spontaneous deposition of Pt was used to form approximately 1.5 and 0.5 monolayers on Ru single crystals and nanoparticles, respectively. The reaction kinetics of O2 on single crystal surfaces has a small structural dependence. It is also affected by the oxidation state of the Ru surfaces. The reaction involves the exchange of approximately four electrons per O2 molecule, with the transfer of the first electron being the rate determining step. A deposit of 1.5 monolayers of Pt makes the surfaces considerably more active than bare Ru, but nevertheless still less active than bulk Pt. An electrocatalyst made by the deposition 0.5 monolayers of Pt on carbon-supported Ru nanoparticles is somewhat less active than commercial catalysts, but contains considerably lower Pt loadings.

Structure of metal adlayers during the course of electrocatalytic reactions : O2 reduction on Au(111) with Tl adlayers in acid solutions

Abstract Surface X-ray scattering has been used to determine the structure of Tl adlayers on the Au(111) electrode surface during the course of O 2 reduction in acid solution. O 2 reduction is considerably catalyzed by Tl adlayers on Au(111). The half-wave potential is shifted to more positive values in the presence of the Tl adlayer. In the potential region between −0.18 and −0.4V (vs. sce ), the reaction mechanism changes from a 2e-reduction on Au(111) to a 4e-reduction on Au(111) covered with a lowcoverage Tl phase. The close-packed rotated-hexagonal Tl phase, which exists in the potential range between −0.4V and the bulk Tl deposition at ~ −0.7V, has a lower activity for O 2 reduction than the low-coverage phase. O 2 reduction does not change the Tl coverage in this phase but causes a significant decrease of the in-plane diffracted intensity. This observation indicates that the O 2 molecules interact directly with the Tl adatoms prior to the charge transfer. It provides the most direct evidence that the outer sphere charge transfer mechanism is not operative for some surfaces. H 2 O 2 reduction is facile on the surface covered with the low-coverage Tl phase, while it is almost completely suppressed by the rotated-hexagonal phase.

Adsorption of bisulfate on the Ru(0001) single crystal electrode surface

Abstract Adsorption of anions from sulfuric acid solutions has been studied on Ru(0001) single crystal and polycrystalline surfaces by electrochemical techniques and in-situ Fourier transform infrared spectroscopy. Infrared spectroscopy shows that bisulfate is the anion adsorbed on the Ru(0001) surface. The bisulfate adsorption is detected at the H 2 evolution potential and extends into the potential region where the Ru surface is oxidized. A method for extracting unipolar bands from bipolar bands has been presented. The tuning rate of adsorbed bisulfate in the double layer potential region of Ru(0001) was found to be significantly smaller than those observed for other platinum metals. This has been ascribed to a small change in bisulfate coverage on Ru(0001) in this potential range. Bisulfate vibration frequencies are higher on this surface than at any face-centered cubic metal with the (111) orientation. Oxidation of the Ru(0001) surface is limited to one electron per Ru atom, distinctly different from the high degree of oxidation seen in polycrystalline surfaces. For oxidized polycrystalline Ru, only solution phase sulfates and bisulfates are observed in the IR spectra.

2D Cross Sectional Analysis and Associated Electrochemistry of Composite Electrodes Containing Dispersed Agglomerates of Nanocrystalline Magnetite, Fe3O4

When electroactive nanomaterials are fully incorporated into an electrode structure, characterization of the crystallite sizes, agglomerate sizes, and dispersion of the electroactive materials can lend insight into the complex electrochemistry associated with composite electrodes. In this study, composite magnetite electrodes were sectioned using ultramicrotome techniques, which facilitated the direct observation of crystallites and agglomerates of magnetite (Fe3O4) as well as their dispersal patterns in large representative sections of electrode, via 2D cross sectional analysis by Transmission Electron Microscopy (TEM). Further, the electrochemistry of these electrodes were recorded, and Transmission X-ray Microscopy (TXM) was used to determine the distribution of oxidation states of the reduced magnetite. Unexpectedly, while two crystallite sizes of magnetite were employed in the production of the composite electrodes, the magnetite agglomerate sizes and degrees of dispersion in the two composite electrodes were similar to each other. This observation illustrates the necessity for careful characterization of composite electrodes, in order to understand the effects of crystallite size, agglomerate size, and level of dispersion on electrochemistry.

Dispersion of Nanocrystalline Fe3O4 within Composite Electrodes: Insights on Battery-Related Electrochemistry

Aggregation of nanosized materials in composite lithium-ion-battery electrodes can be a significant factor influencing electrochemical behavior. In this study, aggregation was controlled in magnetite, Fe3O4, composite electrodes via oleic acid capping and subsequent dispersion in a carbon black matrix. A heat treatment process was effective in the removal of the oleic acid capping agent while preserving a high degree of Fe3O4 dispersion. Electrochemical testing showed that Fe3O4 dispersion is initially beneficial in delivering a higher functional capacity, in agreement with continuum model simulations. However, increased capacity fade upon extended cycling was observed for the dispersed Fe3O4 composites relative to the aggregated Fe3O4 composites. X-ray absorption spectroscopy measurements of electrodes post cycling indicated that the dispersed Fe3O4 electrodes are more oxidized in the discharged state, consistent with reduced reversibility compared with the aggregated sample. Higher charge-transfer resistance for the dispersed sample after cycling suggests increased surface-film formation on the dispersed, high-surface-area nanocrystalline Fe3O4 compared to the aggregated materials. This study provides insight into the specific effects of aggregation on electrochemistry through a multiscale view of mechanisms for magnetite composite electrodes.

Structure and inhibition effects of anion adlayers during the course of O2 reduction

Abstract Structures of Br adlayers were determined on the Ag(100), Au(100) and Pt(111) electrode surfaces during the course of O 2 reduction by using synchrotron surface X-ray scattering techniques and correlated with the O 2 reduction activity obtained from rotating disk electrode measurements. On Ag(100), the c(2×2) Br adlayer, precludes the O 2 adsorption in ‘bridge’ configuration, and the O 2 reduction current resulting solely from adsorption in the ‘end-on’ configuration through the four-fold symmetry holes in the c(2×2) Br lattice is observed at large overpotentials. On Au(100), O 2 reduction is completely inhibited by the c(√2×2√2)R45° phase. The reaction takes place only at potentials negative of the low potential limit of the existence of that phase. O 2 reduction is also completely blocked by the commensurate (3×3) Br adlayer on Pt(111) at high potentials. Below 0.5 V, the ordered phase vanishes and O 2 reduction takes place. Adsorbed disordered Br adions change the mechanism of O 2 reduction on Pt(111) into a 2e-reaction. O 2 affects the stability of ordered Br adlayer on Pt(111), but not on Ag(100) and Au(100).

Anomalous lattice expansion of the electrodeposited Ag bilayer on Pt(111)

Abstract A 2.2% lateral and 8.5% vertical lattice expansion, as compared to bulk Ag(111), has been found in a high resolution surface X-ray scattering study for the underpotentially deposited Ag bilayer on Pt(111). The larger-than-bulk lattice constants are unusual for an incommensurate adlayer. It presumably arises from a considerable charge transfer from Ag to Pt, which results in an additional repulsion among the adatoms.

Time response of the thin layer electrochemical cell used for in situ X-ray diffraction

As the potential applied to the bromide on Ag(001) thin layer electrochemical cell increases past a critical level, the bromide adlayer undergoes a second order phase transition from a disordered state to an ordered state. Using surface X-ray diffraction we measured the time response of this phase transition due to an applied step potential. We find that the time response of the phase transition is limited by the properties of the thin layer geometry. By modeling the electrochemical cell as a RC circuit, we argue that the observed time delay is primarily due to the slow diffusion of charge into the central region of the electrode surface.

Surface X-ray Scattering and Scanning Tunneling Microscopy Studies at the Au(111) Electrode

This chapter reviews Surface X-ray Scattering and Scanning Tunneling Microscopy results carried out at the Au(111) surface under electrochemical conditions. Results are presented for the reconstructed surface, and for bromide and thallium monolayers. These examples are used to illustrate the complementary nature of the techniques.