M. Vázquez

The electrochemical reduction of hydrogen peroxide on polycrystalline copper in borax buffer

Abstract The electrochemical reduction of hydrogen peroxide on copper electrodes in a borax buffer has been studied. The effects of surface oxidation and chloride ions in the electrolyte have been investigated. Hydrogen peroxide can be electrochemically reduced on Cu(I) oxidized surfaces. In the absence of chloride, the reduction is catalysed by a redox cycle involving the Cu(I)/Cu(II) couple. In chloride-containing solutions, the surface oxides are dissolved and the reduction becomes more irreversible. There appears to be a change in the reduction mechanism at potentials more negative than −0.45 V.

New evidences on the catalase mechanism of microbial corrosion

Abstract Changes on the oxygen reduction rate induced on aluminium brass by cell-free bacterial cultures of an isolate belonging to the genus Pseudomonas were studied in relation to the bacteria phase of growth and to the surface oxide layer composition after various electrochemical pre-treatments of the metal samples. Cultures isolated from the stationary phase of growth strongly influenced the oxygen reduction kinetics. Cathodic currents increased throughout the potential range tested when Cu 2 O and CuO were present simultaneously in the surface film (so-called aged surfaces). In this case, the maximum increment (35%) was observed within the oxygen reduction limiting current region. On pre-oxidised surfaces, when the oxide film was composed mainly by CuO, the effect induced by stationary phase cultures was even higher, with the limiting current density increasing by almost 60%. On pre-reduced surfaces on the other hand, when only a submonolayer of Cu 2 O was covering the surface, there was no effect as current density values were similar to those obtained in control experiments. Exponential phase cell-free cultures did not modify the limiting current values in any of the surfaces investigated. Results were in agreement with the participation of catalase as a bacterial catalyst for the oxygen reduction process. The normalised catalase activity from different stationary phase cell-free cultures ranged from 0.88 to 4.02 mg ml −1 U −1 , while there was no observable activity in exponential phase cultures. The incidence of the catalase mechanism in microbiologically influenced corrosion processes induced by aerobic biofilms is highlighted on the basis of the results obtained using metabolites from planktonic cells and their agreement with most of the experimental evidences so far reported by other authors.

The electrochemical reduction of oxygen on polycrystalline copper in borax buffer

Oxygen reduction proceeds on copper electrodes in boric + borate buffer by the four electron pathway. Difference reflectance spectroscope shows that surface oxide species are formed during oxygen reduction on pre-reduced copper. The redox couple Cu2O/CuO plays an important role in the reduction sequence and a CE mechanism is proposed, although direct electron transfer to the oxygen molecule from oxide-free surfaces appears possible. In this case, the behaviour of copper is similar to that of the isoelectronic metal gold.

Surface redox catalysis and reduction kinetics of hydrogen peroxide on copper–nickel alloys

Abstract The electrochemical reduction of hydrogen peroxide on the copper-nickel alloys Cu90Ni10 and Cu70Ni30 in 0.1 M borax has been investigated. The results have been analysed on the basis of a chemical–electrochemical mechanism involving a surface Cu(I)/Cu(II) redox couple. The electrochemical step corresponds to the reduction of Cu(II) followed by the chemical reaction of Cu(I) with hydrogen peroxide. The rate of hydrogen peroxide reduction is strongly dependent on the rate constant of the chemical step. The potential dependence of the surface coverage by Cu(I) species accounts for the large values of the Tafel slopes. The catalytic effect of Cu(I) is similar to that of AuOH noted previously for the reduction of H 2 O 2 on gold. Ellipsometric measurements also indicate the presence of an oxide/hydroxide layer at the interface at potentials very negative with respect to bulk Cu 2 O reduction.

Surface redox catalysis and reduction kinetics of oxygen on copper-nickel alloys

Abstract The reduction of oxygen on single phase Cu90Ni10 and Cu70Ni30 alloys in contact with 0.1 M borax at pH 9.2 has been investigated. The rate constant measured is independent of alloy composition for steady-state measurements. When the currents are measured in sweep experiments the current densities are larger for the alloy richer in nickel, indicating slow surface equilibrium processes. The value of n is 3.5±0.2 for pre-reduced electrodes in a wide potential range, from −1.5 to −0.8 V versus the Hgxa0∣xa0Hg 2 SO 4 xa0∣xa0K 2 SO 4 (sat) electrode. The proposed reduction mechanism involves a bifurcation between a direct and a sequential mechanism involving O 2 chemisorbed on Cu(0).

Characterization of CuInS2 thin films prepared by one-step electrodeposition

CuInS2 thin films were fabricated by one-step electrochemical deposition from a single alkaline aqueous solution and using conductive glass as the substrate. The electrolyte consisted in 0.01xa0mol L−1 CuCl2, 0.01 mol L−1 InCl3, 0.5xa0mol L−1 Na2SO3 and 0.2xa0mol L−1 Na3C3H5O(COO)3 (CitNa) at pH 8. The films were analyzed using a variety of techniques such as X-ray diffractometry, micro-Raman spectroscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy and photoelectrochemistry. After carrying out a thermal treatment in sulfur vapor, chalcopyrite CuInS2 thin films were obtained. Etching the films in KCN solution was found to be a key step, enabling a final adjustment in the stoichiometry. These thin films exhibited p-type semiconductor behavior with the bandgap of 1.43xa0eV. The results show that electrodeposition provides a cost-effective and versatile method for the preparation of thin films of CuInS2, even when acidic precursors need to be avoided.

Pulsed and potentiostatic electrodeposition of CuInSe2 on gold-coated alumina substrates

CuInSe2 thin films were deposited on gold-coated alumina electrodes using constant and pulsed potential signals. In situ electrochemical measurements were recorded with an atomic force microscope to follow CuInSe2 electrodeposition. The electrodeposited films were characterized employing scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, and Raman spectroscopy. The best stoichiometry is attained for films deposited at pulsed potentials of −0.7xa0V. These films are also the most compact, showing a homogeneous morphology. In contrast, potentiostatic films presented minor variations in their chemical composition with the deposition potential. The morphology presented porous coral-like structures. X-ray and Raman spectroscopy analysis of as-deposited films by either technique reveal the presence of secondary phases such as CuxSe and Se, especially at the most positive deposition potentials. After a thermal treatment at 500xa0°C in Se vapor, the crystallinity of the material is significantly enhanced and no secondary phases are present.

Brass corrosion in chlorinated tap water inhibited by phosphate ions

The combined effect of phosphate ions as corrosion inhibitor and sodium hypochlorite as biocide is studied on aluminum brass in contact with artificial tap water. The interaction between phosphates and hypochlorite was evaluated by electrochemical techniques for films formed between 2 and 192xa0h and by weight loss tests for immersion times of 90xa0days. Raman spectroscopy and X-ray photoelectron spectroscopy were used to study the passive layer. When the biocide agent is present, dezincification is the predominant form of localized corrosion after long time exposures. When the inhibitor is present, a nobler pitting potential was found. Electrochemical impedance spectroscopy (EIS) results suggest the development of a compact passive layer, regardless of the presence of the biocide. No localized attack was detected on samples immersed for 90xa0days at open circuit potential in the presence of phosphates, even when NaClO is present. Weight loss analysis showed inhibition efficiencies higher than 90xa0% when phosphates and the biocide are present. The oxidative nature of NaClO could favor Zn dissolution and enhance the precipitation of Zn3(PO4)2 layer, producing a more compact surface layer.

ELECTROCHEMICAL CELL DESIGN FOR IN SITU MEASUREMENTS IN CONDENSER TUBES

An electrochemical cell was specifically designed for testing in situ the quality of surface films formed on the cooling water side of condenser tubes in various situations of technical interest. Standard electrochemical techniques can be easily carried out even with the plant in operation, just requiring a half-reduction of the output. Rest potentials and polarization resistance results, together with cyclic voltammograms are presented. The results are in good agreement with those previously obtained by destructive sampling, mounting and using conventional electrochemical cells.

Cu2ZnSnS4 thin films prepared by sulfurization of co-electrodeposited metallic precursors

Cu2ZnSnS4 (CZTS) thin films were prepared by thermal sulfurization of co-electrodeposited CuZnSn (CZT) metal precursors. Electrodeposition times between 10 and 40xa0min were used to study the influence of this parameter on the composition, structure, thickness, and morphology of the sulfurized films. CZT precursors and CZTS films were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and Raman spectroscopy. The morphologies were evaluated by electronic microscopy. The electrodeposition time was found to influence the chemical composition of the metal precursor, especially the Cu and Sn content. Furthermore, XRD results showed the formation of Cu5Zn8, while Sn seemed to be present in an amorphous state. A cauliflower-like morphology was observed in the precursors, especially at long deposition times, which can be related to an electrodeposition mechanism controlled by mass transfer. A significant increase of the film thickness was observed after sulfurization. The morphology changed to round particles and presented a bi-layered structure with an internal compact layer of nanometer size particles and an external layer formed by micrometer-size particles. Raman spectroscopy, XRD, and EDS measurements confirmed the formation of crystalline CZTS after sulfurization. In addition, a disperse Cu2S secondary phase co-existed in the film. Increasing the precursor deposition time increased the amount of secondary phases in the film after sulfurization. Direct energy gap values close to 1.5xa0eV were estimated for CZTS films using transmittance spectra in the infrared region. CZTS films obtained with short electrodeposition times are promising as absorbers in kesterite thin films solar cells.Graphical Abstract