B. MacDougall

Study of the Mechanism of the Vanadium 4+/5+ Redox Reaction in Acidic Solutions

The mechanism of the vanadium VO 2+ /VO + 2 redox couple has been examined in acidic aqueous solutions. A detailed understanding of this chemistry is of interest for improving and optimizing the performance of vanadium redox-flow batteries, a promising electrochemical electricity storage technology. The vanadium 4+/5+ redox reactions were studied at a rotating disk graphite electrode and polarization curves were obtained in sulfuric acid and perchloric acid, with varying pH and vanadium concentrations. The results were compared to model predictions for different mechanisms. The data were consistent with a model with a multistep chemical-electrochemical-chemical mechanism at low overpotentials, which changes to a multistep electrochemical-chemical-chemical mechanism at higher anodic or cathodic overpotentials. Unusually high Tafel slopes (350-450 mV/decade) were observed for the reduction of VO + 2 at higher overpotentials. While this could not be directly explained by the model, insights gained through the use of the model can provide the basis for some suggestions.

Origin of Activation Effects of Acetonitrile and Mercury in Electrocatalytic Oxidation of Formic Acid

Abstract : In the presence of catalyst poisons such as acetonitrile or mercury, it is shown that the currents for formic acid oxidation, measured in the potential range +0.4 to +0.8V E(H) at Pt can be substantially increased. The effect is shown to originate from blocking of the surface at less positive potentials mainly in the H adsorption/desorption region where an inhibitor for the main formic acid oxidation reaction is normally formed in the absence of additive. Under the latter conditions, steady-state oxidation currents are normally small and decrease with time due to build-up of an inhibiting species from the HCOOH or intermediates involved in its oxidation. The competitive effects of Hg and CH3CN are different insofar as a given extent of surface blocking causes different effects on the formic acid oxidation current. Comparative experiments on the effect of CH3CN on methanol oxidation are described. (Modified author abstract)

Raman and infrared spectroscopy of α and β phases of thin nickel hydroxide films electrochemically formed on nickel.

The present work utilizes Raman and infrared (IR) spectroscopy, supported by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to re-examine the fine structural details of Ni(OH)(2), which is a key material in many energy-related applications. This work also unifies the large body of literature on the topic. Samples were prepared by the galvanostatic basification of nickel salts and by aging the deposits in hot KOH solutions. A simplified model is presented consisting of two fundamental phases (α and β) of Ni(OH)(2) and a range of possible structural disorder arising from factors such as impurities, hydration, and crystal defects. For the first time, all of the lattice modes of β-Ni(OH)(2) have been identified and assigned using factor group analysis. Ni(OH)(2) films can be rapidly identified in pure and mixed samples using Raman or IR spectroscopy by measuring their strong O-H stretching modes, which act as fingerprints. Thus, this work establishes methods to measure the phase, or phases, and disorder at a Ni(OH)(2) sample surface and to correlate desired chemical properties to their structural origins.

Electrochemisorption and reactivity of nitriles at platinum electrodes and the anodic H desorption effect

Summary It has been shown that nitriles exhibit electroactivity at Pt electrodes in aqueous acid solutions. CH 3 CN was specially studied in 0.5 M aq. H 2 SO 4 by the potentiodynamic and anodic hydrogen displacement methods. In potentiodynamic experiments, a more or less reversible, 2-electron reduction-oxidation process occurs over the “double-layer” potential region at Pt and extends into the H-region. The reactive species is a chemisorbed one. The extent of adsorption can be determined from measurements of charge passed in the double-layer and H-region in potential sweeps and in potentiostatic current transients observed following adsorption of CH 3 CN. In the H-potential region, adsorption occurs by anodic H-displacement. CH 3 CN occupies 3 Pt sites per molecule at adsorption saturation leaving 33% of the surface available for co-adsorption of H. Electroactive CH 3 CN and H-species are co-adsorbed in the H-region at Pt but the coverage of H can be quantitatively distinguished from that of the nitrile by evaluating the kinetic relaxation characteristics of the two species. The stages of reduction and oxidation of the adsorbed species are considered in terms of a consecutive 2 e process which involves a change of shape of the molecule leading to steric hindrance for completion of the reduction of the ad-layer at the least positive potentials.

Dependence of CH 3 OH Oxidation Activity for a Wide Range of PtRu Alloys Detailed Analysis and New Views

Unsupported PtRu alloy powders of a wide range of compositions were prepared at low temperatures by carefully adjusting the preparation procedure. PtRu alloys of essentially the same surface, nominal and bulk composition, were formed up to ca. 46 atom % Ru content. Adsorbed CO stripping voltammetry and CH 3 OH oxidation characteristics, namely, i-V curves and pseudo-steady-state current density values recorded at constant potentials, were the same as reported for corresponding bulk alloys, suggesting that the electrocatalytic activities of the powders can be compared to bulk alloys. CH 3 OH oxidation activities obtained for PtRu alloy, Pt, and Ru powders showed the PtRu alloy of 70:30 atom % Pt:Ru composition to exhibit the highest activity independent of the temperature and potential tested (0.3 and 0.4 V vs. a reversible hydrogen electrode). The experimental pseudo-steady-state current density values for the CH 3 OH oxidation reaction were found to show the same dependence on Ru content as theoretical values calculated assuming that an assembly of three neighboring Pt and one Ru site are involved in the oxidation of a CH 3 OH molecule.

Nature of passive films on Fe26Cr alloy

Abstract Anodic oxide films formed on Fe26Cr alloy in pH 2.0 H 2 SO 4 solution have been investigated using electrochemical techniques, and Auger electron spectroscopy (AES) combined with ion sputtering and subsequent quantitative thin film analysis. Films formed on electropolished (EP) and cathodically reduced (CR) surfaces have been examined in terms of their thickness, composition and open-circuit stability as a function of potential and time of anodization. Passive films on CR surfaces appear to be composed of either a (Fe,Cr) 2 O 3 type of oxide, or an inner layer of FeCr 2 O 4 and an outer layer of (Fe,Cr) 2 O 3 depending on the anodizing potential. The Cr/Fe ratio in the films varied with both potential and time of anodization, but all films were enriched in Cr relative to the underlying alloy. Oxide thicknesses were ≈17 A (±5−10%) irrespective of the conditions of formation. The somewhat thicker EP prior oxide contained Cl − which was removed by CR. If no CR was performed, subsequent passive films on EP surfaces contained various amounts of incorporated Cl − depending on the anodizing potential. The Cl − -containing films were also Cr rich, and their composition and thickness were potential dependent. Open-circuit decay profiles and surface reactivity measurements demonstrated that the Cl − -containing films break down faster than Cl − -free anodic films with preferential removal of Fe and Cl − occurring from the outer part of the film. The stability of both types of film increased with time of anodization.

The growth and stability of passive films

This paper will consider the growth and breakdown of passive oxide films on metals and alloys. Emphasis is placed on the use of surface-analytical techniques, particularly secondary ion mass spectrometry (SIMS) to characterize oxides formed on Fe and Fe-Cr alloys in18O-containing solutions, and to determine the oxides stability to subsequent air exposure. It is found that the stability of passive films towards air exposure decreases with increasing Cr content of the alloy. Films formed on Fe in the passive potential range in borate buffer solution are stable towardsex situ exposure, whereas films formed on Fe-26Cr alloys are not stable to air exposure after passivation at any potential in the passive region. This result has implications for examining the thickness and composition of passive films on high-Cr alloys usingex situ techniques. Passive films play a critical role in the initiation of pitting and the processes which influence film breakdown on Fe in Cl−-containing solutions are discussed. The results indicate that pitting is associated with a critical stage in the development of passive oxide films.

In Situ XANES Detection of Cr(VI) in the Passive Film on Fe‐26Cr

The passive film on sputter deposited thin film Fe-26Cr electrodes has been examined using the technique of in situ X-ray absorption near edge spectroscopy (XANES). During the X-ray spectroscopic measurements, the sample was maintained under electrochemical control in the pH 8.4 borate buffer electrolyte. The appearance of the distinctive Cr(VI) XANES pre-edge peak can be correlated with the transpassive wave in the cyclic voltammogram, and its disappearance with the corresponding reduction wave. The formation and reduction of Cr(VI) was reversible, and only small amounts were detected in the film. Cr(VI) in the passive film is not indefinitely stable, and it is completely absent after long periods of air exposure.

The electrochemical oxidation of alkaline copper cyanide solutions

Abstract A systematic investigation of the electrochemical oxidation of copper cyanide was carried out. At low pH, cyanide destruction is believed to be catalyzed by the heterogeneous reaction involving adsorbed [Cu(CN)3]2− and possibly [Cu(CN)4]3−. At high pH, rapid oxidation of cyanide was observed around 0.75 V versus Hg/HgO with the formation of a black copper oxide film. This enhanced electrocatalytic activity is believed to be related to the formation of an active copper(III) species. The transition point between low and high pH as a function of cyanide and copper concentrations is discussed. Bulk electrolysis of a copper cyanide solution at 0.90 V oxidized most of the cyanide to cyanate. Prolonged electrolysis further oxidized the cyanate to nitrate. The copper oxide film is found to be catalytic, capable of electro-oxidizing hydroxide to oxygen and cyanate to nitrate.

An aes and sims study of the influence of chloride on the passive oxide film on iron

Abstract The kinetics of passivation of Fe in pH 8.4 borate buffer solution at 0.0 V have been studied in both the presence and absence of 0.5 M Cl − . Cl − has no influence on the decay of the passive current with time over the course of several hours. Oxide films were examined by AES and SIMS for possible Cl − incorporation into the oxide lattice. Neither technique was able to detect any Cl − incorporation into the oxide film is not a precursor to pit initiation. It also appears that Cl − does not cause any film thinning.