Portugaliae
Electrochimica Acta

Iron disulphide molten salt electrochemical cells are among the most promising technological options for batteries. The electrochemical behaviour of pyrite allies excellent cathodic characteristics to optimal performance and low operational costs. The cathodic iron disulphide mechanism involves many processes, encompassing from polysulphides formation to the reduction of iron to the metallic state. The use of X-ray diffraction together with scanning electronic microscopy analysis on cells of the Li/KCl-LiCl/FeS2 system made possible to identify intrinsic and extrinsic parameters to the electrochemical process involved in the establishment of the polysulphides stoichiometry. It is necessary to note that the augmentation of the cell's internal resistance and loss of electrical capacity are directly related with the formation of these same substances. Researches in the electrochemistry of these phenomena aim to elucidate the cathodic interphase processes and the effect of every reaction in the global mechanism.

On the basis of an ohmic model and a Tafel equation describing relations between current density and overpotentials in the film and at the metal/film interface, respectively, it is shown that a quantitative analysis of galvanostatic transients for the growth of passivating ultra-thin films on the so-called non-noble metals can be obtained. As an example, the growth of ZnO on Zn in a boric/borate buffer solution is considered. In this case, the values of the transfer coefficient and the exchange current density of the reaction at the metal/film interface were found to be 1.2 and 0.11 mA cm-2, respectively. It was shown that a single, first film occurred at low current densities and two films at high ones. The ionic resistivity inside the single, first film, during the transients, has an initial constant value region followed by a final increase indicating the aging process. For this variation the evolution of the point defect concentrations is taken into account. For the variation of the ionic resistivity with the galvanostatic current density two types of behaviors were found, depending on the current density. An interpretation of these results is advanced in terms of the concentrations, mobilities and recombination rate of point defects inside the film.

CoFe films were electrodeposited on platinum and copper from an acidic sulfate bath. The deposits surface morphology was analyzed using SEM and composition was determined by XPS. For deposition potentials lower than -0.80 V (SCE), the anodic stripping voltammogram showed a complex anodic current peak evidencing the complexity of the deposition process. Analysis of the deposition current transient curves showed that the nature of the substrate influenced the kinetic of the process: it was observed an instantaneous nucleation on Pt and a progressive nucleation on Cu. Pure cobalt and cobalt-iron films formed on Pt and on Cu presented binding energies for Co2p3/2 signal corresponding to cobalt oxide, while the Fe2p3/2 signal is related to metallic iron. The corresponding metallic cobalt was observed for the Co2p3/2 signal with binding energy of about 778.0 eV.

The degradation of a reactive blue 4 dye in acidic solution has been compared using electrochemical reduction and oxidation at reticulated vitreous carbon electrode (RVC) and Ti/SnO2/SbOx (3%mol)/RuO2 (30%mol) electrodes and photo-Fenton method under UV irradiation. The reduction of RB4 dye at -0.6 V on RVC electrode results in 50% of color removal and up to 64% of TOC removal. The direct oxidation of RB4 dye at +1 V on RVC promotes only the oxidation of amine group and there is no color removal. On SnO2 electrode, 58% TOC was removed and the decolorization is around 100 % after 1 hour of electrolysis at 2.4 V indicating good efficiency. Best performance was obtained by photo-Fenton method that indicates 80% elimination of TOC and 100% of color removal but the method requires oxidant addition and time of rest to remove the generated residues.

Cyclic voltammetry and controlled potential electrolysis, and other analytical techniques were used to study the reactions of mercury with iridium oxides formed by repetitive cyclic voltammetry (RCV) or by controlled potential electrolysis (CPE) on pure iridium substrate. The oxides were formed in a Hg(I)-containing solution. The SEM images, XPS spectra and CV obtained after the RCV treatment showed a more attacked surface with less mercury on the Ir. On the contrary, the SEM images, XPS spectra and the CV for the electrode prepared by CPE indicated a less attacked surface with more Hg. The oxides formed when the CPE condition was applied inhibited the UPD of mercury.

This will enable Pb(II) determination optimisation, regarding environmental aspects of the perpetuation of Quercus suber Montado (similar to Forest). It will also allow a more profound toxicological quality control of corks (barks). These two last referred aspects are very related ones, once accepted that the more advanced could the certification of the quality patterns of corks (barks) be, more will be the sustainability of Quercus suber ecosystem. Which we believe can be done, amongst other vectors, by studying such an important environmental issue as heavy metals. It is shown that Pb(II) quantification in the cork (bark) of Quercus suber tree (Cork Oak) is possible, using Differential Pulse Anodic Stripping Voltammetry (DPASV). The samples digestion was made in a mixture of H2O2 and HNO3 1:8, in a closed recipient at approximately 90 ºC. The measures were made in NaCl 0.1 M, using a Hanging Mercury Drop Electrode, a Glassy-Carbon Rod Counter Electrode, and an Ag/AgCl/KCl 3M reference electrode, after a 180 s deposition step. The results have shown the determination of a coherent stripping potential of -0.405 V (SD=0.0005 V), and a fine linear adjustment after the standard addition method (R2 = 0.997). They have also revealed the interest of further studies. The need to test other electroanalysis requisites was understood, and imposed by the proof of the complex nature of the matrix.

The behaviour of the neutral alkynyl complexes trans-[FeBr(L)(depe)2] (L = -C=C-C(=CH2)Ph 1a, -C=C-CPh2(H) 1b; depe = Et2PCH2CH2PEt2) is studied by cyclic voltammetry and the electrochemical PL and EL ligand parameters for the alkynyl ligands are estimated showing that they behave as very strong net electron-donors.

Erratum to “Electrochemical Behavior of Pt(IV) on Mercury Electrode in the Presence of Dimethylglyoxime” Volume 20, Issue: 4, Year: 2002, pp.179-189.