Yibo Mo

In Situ Quartz Crystal Microbalance Studies of Nickel Hydrous Oxide Films in Alkaline Electrolytes

The microgravimetric characteristics of electrodeposited nickel and composite Ni/Co hydrous oxide films on Au electrodes in KOH electrolytes were examined in situ with a combination of quartz crystal microbalance (QCM) and voltammetric techniques. In the case of freshly prepared {alpha}-Ni(OH){sub 2} hydrous oxide films, denoted as {alpha}-Ni(OH){sub 2}(hyd), the mass increased during oxidation and then decreased upon subsequent reduction. As the redox cycling was continued, however, this behavior gradually reversed, i.e., the mass decreased upon oxidation and then increased following further reduction. This unique effect could be correlated with a shift in the position of the oxidation peak maximum (E{sub p}{sup ox}) in the voltammogram toward more positive potentials, which has been ascribed to the transformation of {alpha}-Ni(OH){sub 2}(hyd) into {beta}-Ni(OH){sub 2}(hyd). Based on these findings, it can be concluded that the mechanism of ionic charge transport in these two types of lattices is markedly different. In contrast, composite Ni/Co (9:1) hydrous oxide films displayed qualitatively the same behavior as pure freshly prepared Ni(OH){sub 2}(hyd) in the same alkaline media regardless of the extent of cycling. This observation provides evidence that the incorporation of cobalt into Ni(OH){sub 2}(hyd) modifies the structural properties of the lattice during the oxidation/reduction process.

The electrochemical oxidation of carbon monoxide adsorbed on Pt(111) in aqueous electrolytes as monitored by in situ potential step-second harmonic generation

The dynamics of electrooxidation of adsorbed CO on Pt(111) microfacets was examined in CO-saturated 0.1 M HClO4 aqueous solutions by in situ time resolved second harmonic generation (SHG). Analysis of the temporal dependence of the intensity of the SHG signals recorded for experiments in which the potential was stepped to values high enough for adsorbed CO oxidation to ensue, was found to be consistent with the mean field theory model, yielding rate constants somewhat higher than those reported by Lebedeva et al. (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. v. Santen, J. Electroanal. Chem., 2002, 524-525, 242-251) in sulfuric acid solutions. The smaller rates observed by these authors may be ascribed to the presence of anions, ie. in all likelihood bisulfate, which are capable of competing effectively for Pt sites thereby blocking formation of oxygenated species on the surface. Also discussed in this work are the virtues and limitations of in situ SHG for monitoring fast surface processes.

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.

Electrochemical characterization of unsupported high area platinum dispersed on the surface of a glassy carbon rotating disk electrode in the absence of Nafion † or other additives

Abstract A method is described herein to immobilize dispersed unsupported high area Pt particles (ca. 30 m 2 g −1 ) on the surface of a glassy carbon (GC) disk of a rotating Pt(ring)/GC(disk) electrode (RRDE) assembly, without the use of Nafion® or other additives, allowing electrochemical measurements to be performed under forced convection with only minimal losses of Pt from the surface. Diffusion limiting currents, I lim , and half-wave potentials for O 2 reduction recorded with dispersed Pt/GC electrodes at the highest attainable loadings (ca. 8.3 equivalent Pt monolayers and rotation rates 400≤ ω ≤2500 rpm, were found to be differ only slightly from those observed with a bulk Pt RDE of the same cross sectional area, e.g. I lim (Pt) ca. 1.06 I lim (Pt/GC), under otherwise identical conditions. In addition, this novel strategy enables in situ surface modification of dispersed Pt particles on the GC disk of the RRDE by ad-atoms and subsequent voltammetric and electrocatalytic characterization for a wide spectrum of redox reactions.

Platinum-based electrocatalysts for generation of hydrogen peroxide in aqueous acidic electrolytes: Rotating ring-disk studies

The electrocatalytic activity of selenium-, and sulfur-modified polycrystalline Pt electrodes for O 2 reduction in acid media has been examined using rotating ring-disk electrode techniques. The results obtained indicated that, within a rather narrow range of coverages, both Se-, and S-modified Pt surfaces promote O 2 reduction via a two-electron pathway to yield hydrogen peroxide at close to 100% faradaic efficiency over a wide potential region. Also presented in this work is an experimental procedure for Se-modification of high area, unsupported Pt particles based on fluidized packed bed reactor principles. Such a strategy could be readily scaled up opening new prospects for the development of large scale hydrogen peroxide generation in acid media, including electrochemically based, room temperature, Nafion-based oxygen concentrators.

In Situ X‐Ray Absorption Fine Structure Studies of a Manganese Dioxide Electrode in a Rechargeable MnO2 / Zn Alkaline Battery Environment

Electronic and structural aspects of a MnO{sub 2} electrode in a rechargeable MnO{sub 2}/Zn battery environment have been investigated by in situ Mn K-edge x-ray absorption fine structure (XAFS). The relative amplitudes of the three major Fourier transform shells of the extended XAFS function of the rechargeable MnO{sub 2} electrode in the undischarged state were similar to those found for ramsdellite, a MnO{sub 2} polymorph with substantial corner-sharing linkages among the basic MnO{sub 6} octahedral units. The analyses of the background-subtracted pre-edge peaks and absorption edge regions for the nominally 1-e{sup {minus}} discharged electrode were consistent with Mn{sup 3+} as being the predominant constituent species, rather than a mixture of Mn{sup 4+} and Mn{sup 2+} sites. Furthermore, careful inspection of both the x-ray absorption near edge structure and EXAFS indicated that the full recharge of MnO{sub 2}, which had been previously discharged either by one or two equivalent electrons, generates a material with decreased corner-sharing linkages compared to the original undischarged MnO{sub 2}.

Topographical and Electrochemical Characterization of Optically Smooth Zn Films Prepared by Physical Vapor Deposition

Optically smooth Zn films supported on Cu-coated glass and quartz substrates have been obtained by physical vapor deposition of Zn in metallic form. The method employed involves resistive heating of a Mo boat filled with high purity Zn shot in an Ar atmosphere at pressures of about 3-5 m Torr. Atomic force mieroscopy images revealed that the resulting Zn deposits consist of smooth features (rms roughness ca. 0.3 nm) with dimensions on the order of 200 nm. Preliminary results indicate that the electrochemical behavior of these films in strongly alkaline solutions is somewhat different than that observed for Zn in hulk commercial form.