Key aspects of the microenvironment surrounding the Fe center in the nitrosyl adduct of iron phthalocyanine, [Fe(Pc)(NO)], have been elucidated from the analysis of the Fe K-edge extended X-ray absorption fine structure (EXAFS) of the material adsorbed on the surface of a high area carbon electrode recorded in situ, in 0.5 M H(2)SO(4). Statistical best fits to the EXAFS data place the Fe center in a five-coordinated square pyramidal configuration shifted away from the Pc plane toward the axially bound NO bent at an angle of ca. 40 degrees with respect to the normal to the Pc plane. This environment is analogous to that of Fe in the nitrosyl adduct of crystalline [Fe(TPP)], where TPP = meso-tetraphenylporphyrinato(2-), determined from X-ray diffraction.

Changes in the state of charge of a high-area RuOx electrode in an operating RuOx|Nafion|IrOx supercapacitor were monitored in situ by time-resolved, transmission Ru K-edge X-ray absorption spectroscopy (XAS). Linear and reversible variations in the intensity of the transmitted X-ray beam as a function of time were found by fixing the energy of the incident X-ray beam, E-i, at judiciously selected values within the Ru K-edge X-ray near edge structure (XANES), while the supercapacitor was charged and discharged at constant current. The sign of the slope of these temporal signals was found to vary, depending on the value of E-i. This behavior could be rationalized based on the spectral differences between the Ru K-edge XANES of RuOx in the fully oxidized and fully reduced states, recorded in situ from films of the material electrodeposited on a gold substrate in the fluorescence mode.

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. (C) 2002 The Electrochemical Society.

Structural and electronic aspects of IrO2 films prepared by electrodeposition on Au substrates were investigated by in situ LIII-edge X-ray absorption and surface enhanced Raman spectroscopies in both acid and alkaline aqueous solutions. Linear correlations were found between the extent of oxidation of Ir3+ in the films determined from a statistical fit of the white line, which includes contributions from each of the sites differing by a single electron, and from coulometric analysis of the voltammetric curves. Analysis of the extended X-ray absorption fine structure (EXAFS) yielded Ir−O bond lengths decreasing in the sequence 2.02, 1.97, and 1.93 A for Ir3+, Ir4+, and Ir5+ sites, respectively. Whereas SERS provided evidence for the presence of crystalline IrO2 in the highly hydrated films, the lack of intense shells in the Fourier transform of the EXAFS function beyond the nearest oxygen neighbors indicates that the films do not display long-range order.

The electronic properties of electrodeposited Ru oxide films supported on Au have been examined in situ by Ru LIII and LII edge X-ray absorption near edge structure (XANES) in acid media. The results obtained are consistent with the main voltammetric peak centered at 0.7 V vs RHE as being attributed to a redox couple involving formally Ru3+ and Ru4+ sites in the lattice. A linear correlation was found between the extent of oxidation of the film as derived from a deconvolution of the spectral features and the charge as determined from coulomteric analysis of the voltammetric curves. This observation suggests that only two Ru-based redox states are involved in this electrochemical process.

A method is described herein to immobilize dispersed unsupported high area Pt particles (ca. 30 m2 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, Ilim, and half-wave potentials for O2 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. Ilim(Pt) ca. 1.06Ilim(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.

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, i.e. 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.

Vibrational aspects of thin RuO2 films (ca. 7-8 X 10(-8) mol Ru sites/cm(2)) electrodeposited on preroughened Au electrodes were investigated in situ in 0.50 M H2SO4 aqueous solutions using surface enhanced Raman scattering (SERS). Prominent peaks could be observed for the material in the predominantly fully oxidized state (+0.80 V vs. SCE) at 456 and 710 cm(-1), and predominantly fully reduced state (+0.10 V vs. SCE) at 614 and 750 cm(-1). The first set of features is in good agreement with in situ SERS measurements reported by Chan et al. for oxidized metallic Ru films in acid electrolytes polarized at high potentials and ascribed by those authors to a hydrated form of Ru oxide in the 4+ formal oxidation state. Based on our recent in situ Ru K-edge X-ray absorption fine structure of thicker RuO2 films prepared by the same procedure employed in this work, the peaks observed for the first time for the predominantly reduced film are attributed to a highly disordered oxide incorporating Ru sites in the 3+ oxidation state. (C) 2001 The Electrochemical Society.

Limiting currents for the reduction of hexacyanoferrate-(III), i(lim), in aqueous solutions have been recorded in the presence of convective flow generated by a focused acoustic source with its main axis placed normal to the surface of a circular Au electrode embedded in a coplanar Teflon shroud. The results obtained could be fitted to a formula of the type -i(lim) = a(Uz(ss))b, where Uz(ss) is the axial velocity of the fluid along the center line of the lens evaluated at the focal point using computer simulation routines developed by Kamakura and co-workers (Kamakura, T.; Matsuda, K; Kumamoto, Y.; Breazeale, M. A. J. Acoust. Soc. Am. 1995, 97, 2740-2746). The fit yielded a value of b approximately 0.5 in agreement with that of rotating disk and impinging jet electrodes.

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 tilled with high purity Zn shot in an Ar atmosphere at pressures of about 3-5 mTorr. Atomic force microscopy 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 bulk commercial form. (C) 2001 The Electrochemical Society. [DOI: 10.1149/1.1376120] All rights reserved.

Certain aspects of the electrochemical deposition of Al on Au electrodes from ambient-temperature acidic AlCl3/1-ethyl-3-methyl imidazolium chloride (EMIC) melts have been examined in situ using a quartz crystal microbalance (QCM). These experiments were performed using benzene as a cosolvent to decrease the very high viscosity of the neat molten salt. A linear correlation was found between the change in the frequency of the QCM and the amount of electrodeposited Al for amounts of Al ranging from 0.05 to 0.27 mug/cm(2). This observation suggests that within this mass range the films are relatively smooth and rigidly bound to the underlying substrate. For lower masses, the plot was also linear, except that the slope was significantly smaller than that expected based on the weight of Al, a phenomenon attributed to mass compensation. Large deviations from linearity were observed when the amount of deposited Al increased beyond 0.27 mug/cm(2). This behavior is consistent with the growth of discrete Al clusters leading to increased surface roughness and to liquid entrapment within the cluster array, which contributes to the frequency changes measured by the QCM. Long times were required for the mass of the Au electrode to return to its original value following deposition and stripping, which is consistent with the formation of bulk Al/Au alloys as has been earlier proposed. (C) 2001 The Electrochemical Society.

In situ time-resolved second-harmonic generation (SHG) signals have been obtained from well-defined Pt(1 1 1) facets of micrometer dimensions formed spontaneously on the surface of Pt single-crystal microspheres grown by melting/cooling techniques in the ambient atmosphere. Plots of the intensity of the SHG signal, I-pp(2 omega), versus the potential, E, where pp refers to p-input and p-output polarization, recorded while E was scanned linearly between two prescribed values at a rate of 50 mV/s in 0.1 M HClO4 aqueous solutions were found to be linear in the range 0.05-0.6 versus RHE and, thus, in excellent agreement with the behavior believed to be characteristic of larger Pt(1 1 1) surfaces prepared by more conventional techniques, Measurements were also performed in which the input voltage to the potentiostat was repeatedly stepped (> 500 000 times) between 0.1 and 0.7 V versus RHE at frequencies of 250 Hz, while accumulating I-pp(2 omega) signals from a single Pt(1 1 1) microfacet. In situ I-pp(2 omega) transients acquired with submicrosecond resolution revealed a damped oscillatory behavior both in the actual potential of the Pt single-crystal microspherical electrode. as measured with a high-impedance oscilloscope, and also in I-pp(2 omega). These oscillations lasted for ca. 100 ps before the potential and I-pp(2 omega) settled at constant values. Extensions of this overall strategy to the acquisition of time-resolved SHG measurements of well-defined surfaces in even shorter time domains are discussed.

The First in situ Fe K-edge x-ray absorption fine structure study is reported on the electronic and structural properties of a nitrosyl [meso-tetrakis(p-methoxphenyl)porphyrinato]iron(II) adduct, [Fe(NO)-(TMPP)], adsorbed onto a high-area carbon electrode. This adsorbed complex displayed a remarkable stability over a wide potential range, and multiple-scattering XAFS analysis yielded bond lengths of Fe-N-p = 1.99, Fe-N-NO = 1.74, and Fe-OH2 = 2.02 angstrom, and an Fe-N-O bond angle of 15 degrres, characteristic of (FeNO)-N-II porphyrin adducts.

Modifications in electronic and structural aspects of RuO2 films electroprecipitated onto Au electrodes induced by changes in the applied potential have been examined in situ in aqueous 0.50 M H2SO4 by Ru K-edge X-ray absorption spectroscopy (XAS). The Fourier transform of the k(3)-weighted extended X-ray absorption fine structure (EXAFS), k(3)chi>(*) over bar * (k), for the film polarized at +1.20V vs RHE is characterized by two shells attributed to Ru-O and Ru-Ru interactions with average distances of 1.94(1) and 3.12(2) Angstrom, respectively, in agreement with results obtained ex situ for RU4+ in hydrous RuO2 by other groups. In contrast, films in the reduced state, i.e., +0.40 V vs RHE, yielded only a single shell ascribed to a Ru-O interaction at 2.02(1) Angstrom with no evidence for a distant Ru-Ru shell. The long Ru-O distance is in agreement with that reported earlier for the hydrous Ru3+ ion [Ru-(OH2)(6)](3+) in the solid state. Moreover, the difference between the average Ru-O bond lengths for the reduced and oxidized films is consistent with the difference in the ionic radii of Ru3+ and Ru4+. On, this basis it has been suggested that films in the reduced state contain Ru3+ sites, consistent with the electrochemical results, in a phase with apparently less order beyond the Ru-O coordination sphere than for hydrous RuO2.

Abstract : An experimental set up has been designed and built for the acquisition of ultrafast second harmonic generation (SHG) measurements to monitor surface dynamics at electrified interfaces with special emphasis on processes induced by variations in the applied potential across single crystal metal-electrolyte solution interfaces. Much of the effort has been devoted to the study of reactions of interest to fuel cells, including hydrogen and CO adsorption, as well as methanol oxidation on Pt surfaces. As evidenced by the experimental results obtained, the feasibility of measuring transient effects is limited by the geometry of the cell and particularly by the electrode size. Attempts to overcome these problems are being made by using single crystal ultramicroelectrodes taking advantage of spontaneous facetting to allow single crystalline microsurfaces to be examined optically without interference from other areas of the electrode.

Structural and electronic properties of electrodeposited Cu(OH)(2) films on Au electrodes in alkaline solutions have been characterized in situ by Co K-edge X-ray absorption spectroscopy (XAS). The results obtained have shown that freshly prepared films undergo spontaneous partial and irreversible oxidation in 1 M KOH solution even when polarized at -0.58 V vs. Hg/HgO,OH-, as evidenced by the presence of contributions (ca. 20%) from Co-O and Co-Co shells in the extended X-ray absorption fine structure (EXAFS) attributed to CoOOH. Subsequent polarization of the electrode at + 0.6 V oxidizes a significant fraction, but not all of the Co(OH)(2) into CoOOH, as judged by the occurrence of Co-O and Co-Co shells in the EXAFS ascribed to Co(OH)(2). Further reduction of the same film at -0.58 V vs. Hg/HgO,OH- increased the amount of Co(OH)(2) from 28 to 43%, indicating that the fraction of electrochemically active sites does not exceed ca. 15%. On this basis, the redox activity of Co hydroxide is probably associated with surface rather than bulk Co2+/Co3+ sites in the lattice. These results suggest that Co(OH)(2) present as a distinct pure phase in technical Ni oxide electrodes does not contribute significantly to charge storage during cycling, nor to changes in the redox properties of the electrochemically active material. (C) 2000 The Electrochemical Society. All rights reserved.

X-ray absorption fine structure (XAFS) and optical reflectance spectroscopy (RS) have been used to examine in situ electronic and structural aspects of nickel hydrous oxide, α-Ni(OH)2(hyd), electrodes supported on gold in alkaline electrolytes as a function of their state of charge. The extended X-ray absorption fine structure (EXAFS) of α-Ni(OH)2(hyd) electrodes in the uncharged (UC, or discharged) and overcharged (OC, orfully charged) states yielded, in each case, a single set of two distinct nearest-neighbor shells, with distances, d(Ni—O)1 = 2.05 ± 0.02 Å and d(Ni—Ni)1 = 3.11 ± 0.02 Å for UC, and d(Ni—O)1 = 1.87 ± 0.02 Å and d(Ni—Ni)1 = 2.83 ± 0.02 Å for OC. The in situ EXAFS of films allowed to self-discharge following overcharge could be fit with contributions from both sets of shells, suggesting that only two types of nickel sites are sufficient to account for the redox chemistry of this material. These data, in addition to information derived both from quantitative X-ray absorption near-edge structure (XANES) and optical RS in the visible range, indicate that the excess anodic charge, i.e., beyond the one-electron oxidation of Ni2+ sites, observed during the first oxidation of freshly prepared α-Ni(OH)2(hyd) electrodes may not be related to oxidation state changes involving nickel sites in the lattice, and, therefore, do not support the existence of nickel sites with a formal oxidation state higher than three for charged or overcharged electrodes in this media. Keywords: nickel oxide, reflectance spectroscopy, EXAFS, in situ measurements.

Electronic and structural aspects of a MnO2 electrode In a rechargeable MnO2/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 EXAFS (extended XAFS) function of the rechargeable MnO2 electrode In the undischarged state were found to be similar to those found for ramsdellite, a MnO2 polymorph with substantial corner-sharing linkages among the basic MnO6 octahedral units. The analyses of the background-subtracted pre-edge peaks and absorption edge regions for the nominally 1-e discharged electrode were consistent with Mn3+ as being the predominant constituent species, rather than a mixture of Mn4+ and Mn2+ sites. Furthermore, careful Inspection of both the XANES (X-ray absorption nera edge structure) and EXAFS indicated that the full recharge of Mn-O2, which had been previously discharged either by (1) under bar or (2) under bar-equivalent electrons, generates a material with decreased corner-sharing linkages compared to the original undischarged MnO2.

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.