Florentina Golgovici

Electrochemical Activity and Corrosion Protection Properties of Doped Polypyrrole Electrodeposited at Pure Aluminium Electrode

ABSTRACT Polypyrrole films were electrodeposited at pure aluminium from aqueous solution of sodium sulphate containing pyrrole and organic compounds as dopants. Substrate-adherent polypyrrole films were obtained by electrochemical oxidation of pyrrole using the potentiodynamic method and cycling the electrode potential from 50 up to 100 cycles on the potential range of 0 ÷ 800 mV. Since the conducting polymers, such as polypyrrole, are electrosynthesized easily at inert electrodes such as gold and platinum, but much more difficult at aluminium electrodes, the surface protective oxide, Al2O3, was removed, as it acts as a barrier inhibiting electron transfer and the polymerization process. The electrochemical properties of polypyrrole films electrosynthesized from aqueous solutions containing sodium dodecylsulphate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as doping electrolytes were studied by cyclic voltammetry and electrochemical impedance spectroscopy. The results indicate that the mechanism of the redox process is complex and may be governed by the diffusion of the electrolyte. The cyclic voltammograms of polypyrrole film synthesized in solutions having different concentrations of SDS and AOT indicate that the dopant concentration plays a very relevant role in the electrochemical response of the doped polypyrrole films like as: PPY/SDS/aluminium oxide and PPY/AOT/aluminium oxide. The results indicate that the SDS and AOT anions favor redox processes which are faster and more reversible than those associated to usual polypyrrole electrodes. However, the aluminium substrate had a considerable effect on the electrochemical activity of the polypyrrole films and that, because in the presence of pyrrole, anodization of these electrolytes resulted in formation of Al2O3 and PPY layers simultaneously. This is consistent with a galvanic interaction between the polymer and the aluminium substrate, giving rise to oxidation of the aluminium and reduction of the polymer. The corrosion performance of polypyrrole coated aluminium was evaluated by DC polarization and Electrochemical Impedance Spectroscopy. Our results show that the presence of polypyrrole coatings significantly increases the corrosion potential and drastically reduces the corrosion current and corrosion rate of pure aluminium. The corrosion resistance of polypyrrole coated aluminium was higher than of uncoated aluminium.

The influence of different aggressive anions on the electrochemical behaviour of aluminium in sodium nitrate aqueous solutions

The effects of different experimental parameters influencing the determination of critical pitting and protection potentials of aluminium and its alloys have been studied by potentiostatic and potentiodynamic methods. The resistance of aluminium against corrosion in aqueous media can be attributed to a rapidly formed surface oxide film. The addition of the aggressive anions like: chloride, thiocyanide, hydroxyl, sulphide, formate, and acetate (Cl - , SCN - , OH - , S 2- , HCOO - and CH 3 -COO - ) lead to extensive lo-calized attack in all of the cases. The breakdown of the passive film takes into account the migration of aggressive anions through the film. Breakdown occurs when aggressive anions reach the metal-film interface. E π is the critical pitting potential, E p ist the protection potential and the pitting can be formed only in the E π -E p polarization range as it was proved in many experiments [1-3]. The most likely action mechanism of aggressive anions is not a complete dissolution of the film, nor penetration of aggressive anions through solid oxide as suggested for nickel and iron [4]. It is more likely to be somewhere in between the two i.e. action of aggressive anions is that of complexing aluminium ions and pulling in water to hydrate the layer in a way similar to that occurring at cathodic hydrogen evolution, where such dramatic increase of hydrogen evolution rate is observed after a certain cathodic potential is reached. Localized corrosion can be prevented by the action of adsorptive inhibitors which prevent the adsorption of the aggressive anions or by the formation of a more resistant oxide film on the metal surface. The corrosion mechanism is not modified by the addition of ammonium rhodanide but only slowed down.

Ni–Mo alloy nanostructures as cathodic materials for hydrogen evolution reaction during seawater electrolysis

In the case of hydrogen production involving seawater electrolysis, one of the main targets is to develop more active cathodic materials, to optimize the efficiency of hydrogen evolution reaction (HER) and, by doing so, enhance the overall energy efficiency of electrolysis. Thus, to develop suitable HER electrocatalysts either an increase of the electrode active surface area or a design of a material having high intrinsic catalytic activity should be taken into consideration, both of them decreasing the HER overpotential. In the present work, various Ni–Mo alloy nanostructures (10–40 wt% Mo) have been prepared involving electrochemical deposition from aqueous and deep eutectic solvent (DES)-based electrolytes as potential cathodic materials suitable for hydrogen evolution reaction during water electrolysis. The electrocatalytic activity of the obtained layers has been investigated using real seawater electrolyte. The determined Tafel slopes suggested that the electrodeposited Ni–Mo alloy coatings follow an HER mechanism controlled by the Volmer reaction step. The EIS results indicated that the use of choline chloride-based ionic liquids as electrolytes facilitated Ni–Mo alloy coatings showing a significant increase in surface roughness. Studies of the intrinsic activity showed that the main contribution towards the apparent activity comes from the increase of the real surface area, although a slight increase of the intrinsic electrocatalytic activity in the case of Ni–Mo alloy coatings electrodeposited on Ni foam was also noticed. These results showed that Ni–Mo alloy coatings electrodeposited from the novel electrolytes based on choline chloride–urea–citric acid ternary mixtures associated with a porous substrate may represent a promising technological approach to build cathodic materials suitable for seawater electrolysis.

Biomimetic calcium phosphate coating of CoCr alloys

CoCr aloys are most common material used as implantable material because of it reduced cost and good mechanical properties. This work presents our results of obtaining biomimetic calcium phosphate coating of CoCr and to characterize the properties of such coatings. The coating was prepared by immersing the CoCr substrates into the simulated body fluid (SBF) containing Ca ions in sealed plastic bottles, kept at room temperature for one, fourteen and twenty one days. Detailed characterization including chemical, structural and morphological characterization (SEM, EDS, X-ray diffraction) were perfomed. ICP/MS (inductively coupled plasma mass spectrometer) determinations sustain chemical results put in evidence by Fourier Transformed Infrared Spectroscopy (FTIR), the ions release being much smaller for phosphate coatings formed after longer immersion time in SBF. The hydrophilic/hydrophobic character of the coatings was put in evidence by contact angle measurements (CA).

Cathodic deposition of BiTe as thermoelectric films using choline chloride based ionic liquids

This paper reports the electrodeposition of BiTe by potential control electrolysis using a ionic liquid based on choline chloride and malonic acid mixture (1:1 moles) in the 25-60°C temperature range. From cyclic voltammetry and impedance experiments carried out in order to characterize the cathodic process on Pt electrode it was found that the deposition of BiTe from electrolytes occurs on a Te-covered Pt substrate at less negative potentials than for deposition of singular Bi or Te films. Nyquist and Bode impedance spectra showed differences in Pt behavior due to its polarization at various cathodic potentials. The morphology and chemical composition of BiTe films deposited on Cu were determined by AFM, SEM and TEM microscopy.