N. Dimitrov

The Dealloying Critical Potential

This paper reexamines the concept of the dealloying critical potential by considering the critical potential to be a kinetically controlled morphological transition dependent not only on extensive system parameters such as alloy and electrolyte composition, but also on the rate of application of an intrinsic parameter such as the potential ramp rate. In the limit of a quasi-static potential sweep rate, an expression for the critical potential is derived which considers both compositional and morphological fluctuations on the alloy surface at the incipient point of either stability (passivation) or instability (bulk dealloying). In addition, we present detailed critical potential results for the entire range of Ag-Au alloy compositions in x M AgClO 4 + 1 M HClO 4 and x M AgNO 3 + 1 M HNO 3 (x = 10 -4 , 10 -3 , 10 -2 10 -1 , and 1). These results are shown to be consistent with the expression for the critical potential. Finally, we present and discuss ancillary experiments examining the effect of potential sweep rate on the determination of the critical potential.

Electrocatalytic activity of spinel related cobalties MxCo3−xO4 (M = Li, Ni, Cu) in the oxygen evolution reaction

Abstract A comparative study of the electrochemical and physicochemical behaviour of binary cobalt oxides with spinel structure M x Co 3− x O 4 (M = Li, Ni, Cu) was performed in order to elucidate the effect of the cation distribution in the crystal lattice on the electrocatalytic activity of the oxides studied in the oxygen evolution reaction in alkaline media. Teflon-bonded electrodes were studied whose true electrochemical surface area was estimated from the double layer capacitance. It was found that the activity of the spinels increases relative to that of Co 3 O 4 in the order: Co 3 O 4 x Co 3− x O 4 ⪡ Cu x Co 3− x O 4 x Co 3− x O 4 . A relation was found between the activity and the dopant content. The enhanced activity is determined by two inter-related factors: the cation distribution in the two types of spinel site (8a) and (16d) and the number and energetics of the active sites formed on the surface in the anodic peak potential range preceding the oxygen evolution potential. Tafel slopes between 2.3 RT / F and 2.3(2 RT /3 F ) at Co 3 O 4 , Cu x Co 3− x O 4 and Ni x Co 3− x O 4 electrodes and 2.3( RT /2 F ) at Li x Co 3− x O 4 electrodes were observed; the slopes decrease with increasing dopant content. The reaction order with respect to OH − concentration was nearly two. The data are consistent with two mechanisms: (i) Bockriss electrochemical oxide path with the second step, MOH + OH − → MO + H 2 O + e − , as rate determining for Co 3 O 4 , Cu x Co 3− x O 4 and Ni x Co 3− x O 4 and with the third step, 2MO → 2M + O 2 , as rate determining for Li x Co 3− x O 4 ; (ii) Krasilshchikovs path with the third step, MO − → MO + e − , as rate determining for the first three spinels and with the fourth step, 2MO → 2M + O 2 , as rate determining for the Li doped oxide (M = Co 4+ ). The decrease of the Tafel slopes with increasing dopant content is assumed to be due to the formation of active sites with larger charge transfer coefficients.

Dealloying and corrosion of Al alloy 2024-t3

We present results of a comprehensive electrochemical study aimed at determining the factors that control the corrosion evolution of the important structural alloy Al 2024-T3. An important aspect of corrosion in this alloy is copper redistribution. The redistributed copper serves to enhance the kinetics of the corrosion process and presents some major difficulties with respect to development of new families of conversion coatings. Two major sources of mobile copper are discussed, the Al 2 CuMg S-phase and copper in solid solution. In each case, a dealloying mechanism is responsible for liberating copper that becomes available to redistribution by solid state and/or liquid phase transport mechanisms. This work is concerned with identifying the relative importance of these sources of copper and the active transport mechanisms. In order to decouple the sources of copper, we designed synthetic 2024 alloys that emulate the oxygen reduction kinetics of the real alloy. Our results indicate that. in principal, both sources of copper are operative during redistribution and, for the particular alloy that we examined, both contribute about equally to the amount of redistributed copper.

Copper Redistribution during Corrosion of Aluminum Alloys

The authors describe a new quantitative electrochemical technique for measuring the copper redistribution that occurs during corrosion and pretreatment of high-strength copper-containing aluminum alloys. Underpotential deposition (upd) of Pb on Cu was used to assay the surface coverage of Cu. Model experiments using samples of known Al-Cu ratios were used to evaluate the accuracy of the technique and develop a calibration curve. The results demonstrated the accuracy of the method to within 2%. The elemental copper remaining on the surface of Al 2024-T3 samples after various pretreatment/corrosion procedures was determined using the upd technique. A simple treatment involving open-circuit etching of the alloy surface in 3 M HNO{sub 3} was found to be most efficient for the removal of Cu from the Al alloy surface. In accord with a recent observation of Bucheit et al., convection was found to enhance copper redistribution on an Al 2024-T3 sample in 0.5 M NaCl solution.

From Au to Pt via Surface Limited Redox Replacement of Pb UPD in One-Cell Configuration

This work is aimed at developing a protocol based on surface limited redox replacement (SLRR) of underpotentially deposited (UPD) Pb layers for the growth of epitaxial and continuous Pt thin films on polycrystalline and single crystalline Au surfaces. Different from previously reported papers using SLRR in multiple immersion or flow cell setups, this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. Open circuit chronopotentiometry and quartz-crystal microbalance experiments demonstrate steady displacement kinetics and a yield that is higher than the stoichiometric Pt(II)-Pb exchange ratio (1:1). This high yield is attributed to oxidative adsorption of OH(ad) taking place on Pt along with the displacement process. Also, ex situ scanning tunneling microscopy surface characterization reveals after the first replacement event the formation of a dense Pt cluster network that homogenously covers the Au surface. The Pt films grow homogenously with no significant changes in the cluster distribution and surface roughness observed up to 10 successive replacement events. X-ray diffraction analysis shows distinct (111) crystallographic orientation of thicker Pt films deposited on (111) textured Au thin films. Coarse energy dispersive spectroscopy measurements and finer X-ray photoelectron spectroscopy suggest at least 4 atom % Pb incorporating into the Pt layer compared to 13 atom % alloyed Cu when the growth is carried out by SLRR of Cu UPD.

Epitaxial Growth by Monolayer-Restricted Galvanic Displacement

The development of a new method for epitaxial growth of metals in solution is discussed and illustrated by proof-of-conceptresults. Cyclic voltammetry and scanning tunneling microscopy are employed to carry out and monitor a quasi-perfect, two-dimensional growth of up to 35 layers of Ag on Au 111 by repetitive galvanic displacement of an underpotentially deposited UPD Pb monolayer. The excellent quality of the deposit is manifested by an unchanged Pb UPD voltammetry and ascertained bya flat and uniform surface morphology maintained during the entire growth process.An X-ray photoelectron spectroscopy analysisfinds no traces of Pb in the Ag deposit.© 2005 The Electrochemical Society. DOI: 10.1149/1.2063267 All rights reserved.Manuscript submitted June 7, 2005; revised manuscript received July 18, 2005. Available electronically September 16, 2005.

Diffusion-Limited Current Density of Oxygen Reduction on Copper

There are many important processes in corrosion for which the diffusion-limited current density of oxygen reduction, i L , plays a dominant role in terms of kinetic control. The conventionally accepted value of i L (for the four-electron reduction mechanism) which can be found in many corrosion textbooks is in the range of 50-100 μA cm -2 , but the origins of this range of values are a bit mysterious. Previous research in our group aimed at ascertaining i L (under stagnant conditions) on a planar Cu electrode and a Cu microelectrode array in a naturally aerated 0.1 M Na 2 SO 4 electrolyte found that i L was in the range of 20-30 μA cm -2 . In situ scanning tunneling microscopy was used to characterize the Cu surface at relevant potentials. Rotating disk electrode studies were used to measure i L in a naturally aerated 0.1 M Na 2 SO 4 electrolyte as a function of pH for both Pt and Cu electrodes. By comparing results for Pt and Cu we conclude that oxygen reduction occurs on a Cu surface via the four-electron mechanism. The oxygen diffusion-limited current density was found to be independent of pH (in the range 1.5-14). Finally, we conclude that our previous determination of i L in a stagnant electrolyte reflects an accurate range of values of the oxygen diffusion-limited current density.

All electrochemical fabrication of a platinized nanoporous Au thin-film catalyst.

In an effort to decrease the high cost associated with the design, testing, and production of electrocatalysts, a completely electrochemical scheme has been developed to deposit and platinize a nanoporous Au (NPG) based catalyst for formic acid oxidation. The proposed route enables synthesis of an alternative to the most established, nanoparticles based catalysts and addresses issues of the latter associated with either contamination inherent from the synthetic route or poor adhesion to the supporting electrode. The synthetic protocol includes as a first step, electrochemical codeposition of a Au((1-x))Ag(x) alloy in a thiosulfate based electrolyte followed by selective electrochemical dissolution (dealloying) of Ag as the less noble metal, that generates an ultrathin and preferably continuous porous structure featuring thickness of less than 20 nm. NPG is then functionalized with Pt (no thicker than 1 nm) by surface limited redox replacement (SLRR) of underpotentially deposited Pb layer to form Pt-NPG. SLRR ensures complete coverage of the surface with Pt, believed to spread evenly over the NPG matrix. Testing of the catalyst at a proof-of-concept level demonstrates its high catalytic activity toward formic acid oxidation. Current densities of 40-50 mA cm(-2) and mass activities of 1-3 A.mg(-1) (of combined Pt-Au catalyst) have been observed and the Pt-NPG thin films have lasted over 2600 cycles in standard formic acid oxidation testing.

Surfactant Mediated Electrochemical Deposition of Ag on Au(111)

Electrochemical surfactant mediated growth of Ag on Au(111) was investigated experimentally using in situ scanning tunneling microscopy. Almost ideal layer-by-layer growth of Ag was obtained using 0.8 of a monolayer of Pb as a surfactant. Atomically flat epitaxial Ag thin films, 200 monolayer in thickness were grown on Au(111) using this technique. Energy dispersive X-ray analysis and Rutherford backscattering spectrometry showed no traces of Pb in the Ag layers grown.

Two-dimensional condensation of thymine on a basal face of a cadmium single crystal

Abstract Adsorption of thymine on a basal (0001) face of a cadmium single crystal electrode can lead to the formation of a condensed two-dimensional film.