Zhenghao Wang

Simultaneous microgravimetric and optical reflectivity studies of lithium underpotential deposition on Au(111) from propylene carbonate electrolytes

The underpotential deposition (upd) of lithium on highly oriented Au(111) electrodes was examined in propylene carbonate (PC)-LiAsF6 solutions by simultaneous in situ quartz crystal microgravimetry (QCM), UV-visible reflectance spectroscopy (Δ R/R), and cyclic voltammetry (CV). An analysis of the QCM and Δ R/ R data was performed in the potential range 0.4 < E < 2.5 V vs Li | Li+(PC) after the voltammetric response had reached steady state, i.e., following the irreversible formation of a film on the electrode surface. The results obtained indicated that both the (background corrected) charge under the Li upd stripping peak and the loss in the effective mass of the QCM (Δ meff) are consistent with the oxidation of one to two monolayers equivalent of Li on a smooth Au(111) substrate. The optical reflectivity recorded during these experiments revealed large, on the order a few tenths of a percent, and wavelength dependent changes in Δ R/R as a function of the applied potential. A combination of these data with those derived from the coulometric/micro-gravimetric analysis of the Li upd stripping peak yielded, for Li coverages of about 0.1, values of Δ R/R as large as 0.3% at wavelengths of about 650 nm. A comparison between the experimental Δ R/R spectra at fixed Li coverages, and that predicted theoretically based on an ideal, non-interacting, three-layer model, indicates that the features observed are mostly due to 1 /R contributions due to the bare Au substrate.

In Situ UV‐Visible Spectroscopic Imaging of Laminar Flow in a Channel‐Type Electrochemical Cell

Transmission ultraviolet-visible spectroscopy was employed to image concentration profiles of ferricyanide, generated in a channel-type cell by the oxidation of ferrocyanide on a gold electrode, by monitoring the absorbance as functions of distances either parallel to the flow or transverse to the flow. Steady-state measurements performed under diffusion-limited conditions yielded excellent agreement with an earlier model of convection and diffusion, in the absence of any adjustable parameters. Transient measurements following the application of a current pulse showed that it was possible to detect substantially all of the produced ferricyanide, in agreement with Faradays law. With further development, spectroscopic methods should be useful for on-line chemical analysis under forced convection

Theoretical Aspects of Laminar Flow in a Channel‐Type Electrochemical Cell as Applied to In Situ Attenuated Total Reflection‐Infrared Spectroscopy

Analytic expressions are derived, within the Leveque approximation, for the steady-state concentration profile of a reactant or a stable product generated via first-order kinetics at an electrode in a channel-type electrochemical cell under fully developed laminar flow. It is shown that for cell geometries and other experimental conditions easily realizable in the laboratory, the concentration of species directly above the insulating flat surface adjacent to the downstream edge of the electrode c(x,y), where x is the direction of fluid flow and y is the distance normal to the electrode surface, differs only slightly (less than 5%) from c(x,0), the concentration on the surface at the prescribed x, for values of y on the order of microns. Implications of these results to the quantitative analysis of in situ attenuated total reflection-infrared spectroscopy in a channel-type spectroelectrochemical cell (Barbour et al. 6 ) are discussed.

In Situ Spectroscopy in the Presence of Convective Flow under Steady‐State Conditions: A Unified Mathematical Formalism

A generalized mathematical treatment is presented that enables a quantitative analysis of various in situ spectroscopi.c experiments involving detection of solution-phase species generated at the surface of rotating disk and ring-disk, channel, and tube-type electrodes under steady state. This theory is valid for experimental conditions easily realizable in the laboratory and is only applicable to first-order irreversible heterogeneous electron-transfer processes without complications derived from homogeneous phase reactions. Illustrations of the advantages of this formalism are provided, including a rederivation of equations obtained by other authors, an analytic route to the solution of the rotating disktransparent ring electrode, and various aspects of normal incidence reflection absorption spectroscopy at rotating ring and ring-disk electrodes. The results were in excellent agreement with those of other workers (when available), and in some cases, yielded better accuracy than solutions generated by digital simulation techniques reported in the literature.

Beam Probe Deflection Analysis of Redox Active Species Irreversibly Adsorbed on Electrode Surfaces

Theoretical aspects of probe beam deflection (PBD) as applied to voltammetric studies of redox active species irreversibly adsorbed on a flat electrode surface have been examined using Weeks numerical inverse Laplace transform algorithm. Excellent agreement was found between the time-resolved profiles calculated based on this approach and those obtained via conventional space-time discretization techniques over the interval of relevance to actual experimental measurements. In agreement with the behavior reported elsewhere for related systems, the shape of the PBD response is highly sensitive to the distance between the probing beam and the electrode surface. In particular, plots of the dimensionless derivative of the concentration of the electrolyte with respect to the dimensionless distance normal to the electrode surface, X (which is proportional to the deflection), ∂θ/∂Χ vs dimensionless time, T (or, equivalently, potential, for voltammetric measurements) for small Χ, yielded curves similar to the voltammetric behavior of a redox active solution phase species in a thin layer cell configuration (which closely resemble the voltammetry of the actual adsorbed redox couple). As Χ was increased, however, the ∂θ/∂Χ vs T curves acquired characteristics reminiscent of solution-phase voltammetry recorded with a microelectrode and farther away with a larger electrode. Further evidence of the accuracy of Weeks method was obtained from the analysis of square-wave periodic boundary conditions at the interface, which yielded time-resolved profiles away from the interface, in harmony with the analytical solutions published in the literature.