C.P. De Pauli

Hydrogen evolution reaction on anodic titanium oxide films

Abstract Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H 2 SO 4 . The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO 4 or H 2 SO 4 solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F . The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.

Influence of the hydrogen evolution reaction on the anodic titanium oxide film properties

Abstract The influence on the physico-chemical properties of electroformed titanium oxide film produced by the hydrogen evolution reaction (HER) was studied. The relation between the cathodic treatment in the HER potential range and the modifications produced in the oxide film was established from current density and differential capacitance measurements, as a function of potential. The products of the oxide film dissolution were identified when this was subject to a prolonged cathodic treatment. The formation of Ti(III) species was found. Depending on the kind of applied cathodic treatment, these species precipitated on the electrode surface or diffused to the bulk of the solution.

Potentiodynamic behaviour of mechanically polished titanium electrodes

Abstract The behaviour of titanium electrodes mechanically polished and/or anodically polarized at low positive potential in solutions at constant ionic strength between pH 0.3 and 11.0 is reported. The oxide electroformation potential on a mechanically polished electrode shows a complex dependence on the bulk solution pH. This dependence is similar to that obtained through acid-base titration with titanium as the indicating electrode. The formation of hydroxo-complexes on the spontaneously formed titanium oxide offers a possible explanation for the oxide electroformation potential dependence on pH. Anodic and cathodic wide current peaks are obtained between the potential of the hydrogen evolution and that of the massive oxide electroformation; the corresponding redox system becomes evident at pH 4.0 from the first potentiodynamic cycle. An interpretation of these processes involving the participation of non stoichiometric oxides and hydrogen ions is attempted.

Ellipsometric response of titanium electrodes in sulphate acid solutions under different potential perturbations

Abstract Optical characteristics and thickness of thin oxide films potentiodynamically formed on a titanium electrode in acidic sulphate solutions at pH 4.0 were investigated by dynamic and steady ellipsometric measurements. The dynamic ellipsometric response of an electrode polarized up to 1.0 V vs rhe depends on the charge involved in the massive oxide electroformation. Cycling the electrode at a constant potential sweep rate between −0.65 and 0 V produces a decrease on film thickness and the electrode reactivation. This electrode treatment becomes less effective as oxide electroformation is repeated several times. Experiments performed by holding the potential at the negative limit were not so effective in reducing film thickness and an appreciable change on optical constants was found. Results were interpreted as hydrogenated species formation within the oxide film.

A simple and novel method for preparing Ni(OH)2 Part I: Structural studies and voltammetric response

Ni/Ni(OH)2 electrodes were prepared by in situ precipitation of nickel hydroxide into porous nickel substrates. The thermal decomposition of urea in solutions containing Ni2+ was used for the synthesis of the active material. The compound prepared by this method was a poorly crystallized β(II)-type phase. The electrochemical behaviour of Ni/Ni(OH)2 electrodes thus prepared was studied by cyclic voltamperommetry and the efficiency of the in situ precipitation was followed by the measurement of the cathodic charge obtained from the j-E response. The amount of incorporated material depends on the urea concentration and the electrochemical response of the electrodes was improved by increasing the number of impregnation cycles.

Reflectometry applied to electrochemically generated phenoxy radical adsorption monitoring

Abstract This paper reports on a study by reflectometry of the adsorption process of phenolic compounds electrochemically oxidized on glassy carbon electrodes. This technique provides information about the adsorption rate and the maximum adsorbed amount. The adsorption process depends on several variables like adsorbent concentration, solution pH and working potential, while ionic strength has no significant influence. The best adsorption conditions were obtained at high phenolic concentrations (0.1 M), high pH values (>10) and working potential values higher than 0.600 V. The adsorption process on glassy carbon electrodes showed irreversible behavior, reaching the maximum adsorbed amount at relatively short times (less than 100 s).

Characterization of passive films on zinc electrodes by impedance measurements and XPS

Abstract The behaviour of the passivating layers formed by anodic polarization on zinc electrodes in borate and arsenate and in borate arsenate mixture solutions of pH 8.9 was studied by means of impedance measurements and XPS analysis. An equivalent circuit for the system is employed for the data fitting. Since an analysis based on the Mott—Schottky relationship leads to an excessively high carrier density with respect to the corresponding resistivity value, the potential drop should occur through the whole passive film. In accordance with the XPS results, a model can be proposed consisting of a layer of hydrated zinc oxide which should be responsible for the passivation. The passivating film on zinc electrodes has a more protective effect when it is formed in borate 0.1 M and arsenate 0.1 M solutions than in arsenate 0.5M solutions.

Zinc dissolution and passivation in buffered phosphate solutions: Part I. A comparative study with sodium hydroxide solutions

Abstract Zn dissolution and passivation in buffered phosphate electrolyte in the pH range between 7 and 13.5 has been studied under a wide variety of experimental conditions. The electrode rest potential shows three regions depending on pH. Between pH 7 and 10 a constant value was found while a slope of 60 mV is obtained from a plot of E r vs. pH. In the pH range 10–11.5 a mixed potential could be established due to the minimum buffer capacity of the system. The necessary charge for passivation was found to be 1.5 mC cm −2 and this was attributed to a monolayer film. From l.s.v. and RDE results a change on Zn dissolution mechanism from a dissolution precipitation to a solid phase process with pH can be postulated. Time and potential effects on product reduction would show that there are chemical transformations on the electrode surface as a second step on the electrode poassivation. From experimental kinetic parameters an empirical rate equation for Zn dissolution is found as i a =2 k a Fc OH − c (PO 4 3− ) t 0.5 exp[(3/2) EF/RT )] Accordingly a reaction mechanism is postulated where PO 4 3− ions act as dissolution promoters, while HPO 4 2− , through the formation of NaZnPO 4 ·H 2 O glass, would act as a dissolution inhibitor.

Electrochemical characterization of glassy carbon electrodes modified by resol mixtures

Abstract The present work aims to obtain more insights into the effects of the electrochemical polymerization of a 4-hydroxybenzaldehyde+formaldehyde (resol) mixture on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline), a permanent negative charge (K 3 Fe(SNC) 6 ) and either a positive or negative charge, depending on solution pH ( l -Dopa), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes is also analyzed. Electrophoretic mobilities were investigated and acid–base titrations were also carried out in order to prove that the surface is negatively charged. Electrochemical measurements showed an important electrostatic effect between the electrode surface and the couples analyzed. This effect plays an important role on the electrode response when used for analytical purposes.

Semiconducting properties of TiO2 films thermally formed at 400°C

Titanium oxide films, TiO2, were prepared on metallic titanium substrates employing a thermal treatment at 400° C under normal air atmosphere, with various annealing times (15, 25 and 45 min). Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to study the electrochemical and electrical characteristics of these films. The potential range for the voltammetric measurements was −0.80 to 8.00 V vs MSE (mercurous sulfate reference electrode). An analysis of the capacitance values of these semiconducting oxide films gave information about their electronic characteristics. From Mott-Schottky plots the donor concentration (ND) and the flat band potential (Efb) were obtained. ND values ranged from 14 × 1022 cm−3 to 3.3 × 1022 cm−3, while Efb values ranged from −0.40 to −0.98 V vs MSE, depending on the heating times for the oxide growth. The thickness of the space charge region, calculated from the minimum value of the capacitance at high band bending, varied between 1.45 and 2.13 rim.