Marcos Malta

Electrochemical and chromogenic relaxation processes in polyaniline films

One critical feature for practical applications of polyaniline (PAni) in electrochromic devices is the rate of color change when an electric perturbation is applied. The electrochemical and chromogenic reactions of PAni were quantified running chronoamperometric experiments coupled to spectrophotometry in the visible region. It was verified that the process of color change does not follow the charge injection, depending on the sweep rate and also if the process is oxidation or reduction. It was shown that the reduction and oxidation processes have different kinetics, mainly related to the different polymer conformations in both redox states and it was also shown that the chromogenic process must be related to conformational changes produced by the transformation of benzenoid to quinoid character and these changes are slower than the injection charge.

Técnicas in situ de baixo custo em eletroquímica: a microbalança a cristal de quartzo

Among in situ techniques, the electrochemical quartz crystal microbalance (EQCM) is a powerful tool for the study of electrochemical reactions that produce mass changes in the electrode/solution interface. This review present some systems in which the EQCM combined with classical electrochemical techniques, gives relevant information for understanding the charge transport process at a molecular level. The aim of this review is to do a brief description of experimental arrangements, with emphasis on some special cares that must be considered by the users. Secondly, some chosen electrochemical systems where the technique was successfully applied are discussed. Finally, a brief analysis of electroacoustic impedance experiments was done in order to show when the Sauerbrey equation can be used.

Effect of Additives in the Stabilization of the α Phase of Ni ( OH ) 2 Electrodes

The present work deals with the voltammetric and electrogravimetric behavior of Ni(OH) 2 electrodes in alkaline solutions and the effect of some additives, such as Co 2+ , Cd 2+ , and Zn 2+ , in the active material. The electrochemical behavior of the α or β phases with the incorporation of the additives is analyzed by use of the electrochemical quartz crystal microbalance. Only Zn 2+ as additive leads to the complete stabilization of the a phase avoiding, since the first cycle, the insertion of alkaline cations from the electrolyte. In all other a phases, the electroneutralization of charge is achieved by a small but representative participation of cations. In the case of β electrodes, protons are the responsible for charge compensation, the cations movement being negligible.

Materiais para cátodos de baterias secundárias de lítio

In this work, cathodes employed in secondary lithium batteries are reviewed. These cathodes have great technologic and scientific importance, specifically, materials for cathodes as electronic conductor polymers (ECP), transition metal oxides (TMO) and nanocomposites of ECP/TMO. The use of a specific cathodic material is based in some intrinsic characteristics that improve the performance of the battery. Thus, some vantages and disvantages of these insertion compounds are discussed, as lithium insertion capacity, energy density, and the ciclability of these materials.

Microgravimetric study of the influence of the solvent on the redox properties of polypyrrol modified electrodes

The redox behavior of the polypyrrole films in the presence of LiClO4 salt in different solvents like propylene carbonate (PC), N,N-dimetilformamide (DMF), methanol (MetOH), ethanol (EtOH), acetonitrile (ACN) and water was investigated using simultaneous electrochemical quartz crystal microbalance and cyclic voltammetry experiments. Both charge and mass changes during redox processes were rationalized in terms of multiple regression considering some solvent parameters and ionic transport characteristics. The electroactivity of PPY modified electrodes increase in the sequence PC<DMF<EtOH<MetOH<ACN<water. Also the mass gains in the sequence PC<water<EtOH<DMF<MetOH<ACN, showing clearly the influence of the solvent physico-chemical nature on the electroactivity and electroneutralization processes.

Strong reduction of V4+ amount in vanadium oxide/hexadecylamine nanotubes by doping with Co2+ and Ni2+ ions: Electron paramagnetic resonance and magnetic studies

In this work we present a complete characterization and magnetic study of vanadium oxide/hexadecylamine nanotubes (VOx/Hexa NT’s) doped with Co2+ and Ni2+ ions. The morphology of the NT’s has been characterized by transmission electron microscopy, while the metallic elements have been quantified by the instrumental neutron activation analysis technique. The static and dynamic magnetic properties were studied by collecting data of magnetization as a function of magnetic field and temperature and by electron paramagnetic resonance. At difference of the majority reports in the literature, we do not observe magnetic dimers in vanadium oxide nanotubes. Also, we observed that the incorporation of metallic ions (Co2+, S = 3/2 and Ni2+, S = 1) decreases notably the amount of V4+ ions in the system, from 14−16% (nondoped case) to 2%−4%, with respect to the total vanadium atoms (fact corroborated by XPS experiments) anyway preserving the tubular nanostructure. The method to decrease the amount of V4+ in the nanotub...

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decreas...