V.A. Macagno

Kinetics and mechanisms of electrochemical reactions on platinum with solutions of iodine-sodium iodide in acetonitrile☆

The kinetics of the iodine-tri-iodide-iodide electrode in acetonitrile has been investigated in the temperature range from 0 to 30°C. Current/voltage curves were determined using a platinum rotating disk electrode working at 200 to 3000 rpm. The anodic and cathodic current/voltage curves are characterized by two well defined waves. The total reactions related to the first and second anodic waves are respectively 3I− = I3− + 2 e and 2I3− = 3I2 + 2 e, while the reverse reactions represent the second and first cathodic waves respectively. The diffusion coefficients of the diffusing species have been evaluated. Current/voltage curves in the region preceding the limiting cd exhibit a measurable irreversibility. The kinetic parameters related to the activated process were determined, and the kinetics of the reactions interpreted in terms of reaction mechanisms that involve an ion plus atom reaction as rate determining step for the anodic process and the reverse reaction for the cathodic process.

The electrochemical response of binary mixtures of hydrous transition metal hydroxides co-precipitated on conducting substrates with reference to the oxygen evolution reaction

Abstract The potentiodynamic behaviour of colloidal electrodes obtained by co-precipitation of binary mixtures of hydrous transition metal hydroxides on conducting substrates in alkaline solutions is described as well as some aspects about single precipitated hydroxides. The mixed interphases investigated can be classified into three groups according to the interaction between the components. The catalytic activity of the mixtures towards the oxygen evolution reaction depends also on that interaction. The interphases involving the co-precipitation of hydrous Ni(OH) 2 with either Mn(OH) 2 or Fe(OH) 2 give the best performance as electrocatalysts for O 2 -evolution; in both mixed electrodes the rate of this reaction is higher than that of the corresponding individual components.

Influence of the forming electrolyte on the electrical properties of tantalum and niobium oxide films: an EIS comparative study

The electrochemical oxidation of Ta and Nb and the dielectric behaviour of the oxide films thus formed were investigated in the following electrolytes: H2SO4, HNO3, H3PO4 and NaOH. Characterization of the films was carried out by means of potentiodynamic current–potential profiles (in the range 0–8V) and electrochemical impedance spectra (in the range 0.1Hz–100kHz). The a.c. response of the oxide films was modelled as a single layer structure on the basis of an equivalent circuit with constant phase elements (CPE). The dependence of the oxide resistance and oxide capacitance with potential is also reported.

Characterization of hafnium anodic oxide films: An AC impedance investigation

Abstract The electrochemical behaviour of hafnium oxide was investigated in the potential range 0–150V, in the following electrolytes: H 2 SO 4 , H 3 PO 4 , NaOH and HNO 3 . After film growth, AC impedance measurements were performed in the frequency range 0.1 Hz–100 KHz. The impedance spectra of the oxides showed a multilayer structure as the oxide thickness increased, due to anion incorporation from the electrolyte and the corrosion process enhanced by anodic breakdown at high final formation potentials. Anodic breakdown was also investigated. An electric field strength increase with thickness was observed, which could be associated to changes from amorphous to crystalline oxide structure. This fact could produce internal stresses during the oxide growth resulting in an anodic fracture which can be correlated, in turn, with dielectric constant changes.

Silver oxide particles/silver nanoparticles interconversion: susceptibility of forward/backward reactions to the chemical environment at room temperature{

The thermal stability of the silver oxide particles (Ag2O)/metallic silver nanoparticles (AgNPs) system in aqueous and gaseous environments is investigated with UV-Visible spectroscopy, TEM, SEM and DLS as characterisation techniques, and with calculations using electromagnetic theory. Thermal decomposition of aqueous Ag2O colloids to produce AgNPs is conclusively demonstrated and used as a base reaction to produce clean AgNPs without any external reducing agent. Such a spontaneous character of Ag2O decomposition in alkaline aqueous/water-enriched environments at room temperature makes the formation of silver oxide films on silver nanoparticles/nanostructures unlikely, keeping the silver surface oxide-free, a crucial feature in determining the silver catalytic and Raman enhancing properties. The synthetic suitability of this reaction to develop new routes to produce AgNPs is explored by analyzing the effect of temperature, complexing agents, and environment polarity on the AgNPs size/shape control. Thermal decomposition of Ag2O colloids in aqueous/water-enriched environments offers the possibility to produce AgNPs at low cost, with easy, clean, safe and green chemistry procedures.

Influence of thiourea on silver deposition: Spectroscopic investigation

Abstract The interfacial behaviour of thiourea (TU) in perchloric acid solutions on glassy carbon electrodes during silver electrodeposition has been studied using surface enhanced Raman spectroscopy (SERS). The spectroscopic results are compared with those obtained on electrochemically activated silver electrodes. Potential-dependent thiourea adsorption (occurring via sulphur) and ClO4− coadsorption are observed. No evidence for the adsorption of solution silver–thiourea complexes regardless of TU concentration was found. The time evolution of the TU SERS signals during the electroreduction of silver is a function of the crystallite formation (nucleation) and stabilization.

The oxygen and chlorine evolution reactions at titanium oxide electrodes modified with platinum

Abstract The oxygen (OER) and chlorine (CLER) evolution reactions were used as probes to test the electronic properties of modified electrodes (Ti/TiO 2 -Pt). Both reactions were studied in acidic solutions by means of current-potential (j-E) and impedance measurements under steady state conditions. The effect of dopant concentration, TiO 2 thickness, and the concentration of the electroactive species in the solution was analyzed. A diminution of the electron transfer current with titanium oxide thickness was obtained whereas an increase of the current was found on augmenting Pt amount. At low titanium oxide thickness both inner-sphere reactions behave in the same way as in the case of pure Pt, indicating that platinum sites are the active ones. However, at higher titanium oxide thickness, the platinum sites become less active and the valve metal oxide (TiO 2 ) begins to block OER and CLER reactions. The electrochemical impedance spectroscopy results are compared with those of the current-potential measurements and a good correlation is found between them.

Influence of the forming electrolyte on the electrical properties of anodic zirconium oxide films: Part II. Ac impedance investigation

Abstract The potentiodynamic oxidation of Zr at ν = 0.05 V s−1 was investigated in the potential range 0–90 V in the following electrolytes: H2SO4, HNO3, H3PO4 and NaOH. After film growth, the oxides were characterized by ac impedance measurements in the frequency range 0.1 Hz ≤ f ≤ 10 kHz. The impedance responses of the films formed in H2SO4 and NaOH were modelled with a transfer function corresponding to a single-layer model. The impedance spectra of the oxides formed in H3PO4 and HNO3 showed two time constants which were adequately simulated by a transfer function corresponding to a two-layer oxide structure. The nature of the outer layer of the films formed in H3PO4 was attributed to phosphate incorporation from the electrolyte, which was confirmed by X-ray photoelectron spectroscopy measurements. The variation of the oxide electrical parameters (resistance, capacitance etc.) with final formation potential is reported and the reasons leading to the complex film structure are discussed.

Electrochemical behaviour of the chloride/chlorine system at platinum electrodes in acetonitrile solutions☆

Abstract The electrochemical behaviour of chlorine and chloride solutions in acetonitrile at platinum electrodes has been studied. The rotating disk electrode technique was applied in a temperature range of −6 to 30 ± 0·1°C. The over-all reaction Cl 2 + 2 e − ⇄ 2 Cl − undergoes a two-electron exchange per mol of chlorine and only one anodic and one cathodic wave are found. The analysis of the polarization curves shows that the reaction is under joint control at the rde. From the diffusion limiting currents the diffusion coefficients of the reacting species are calculated, and from the dependence of the activation overpotential on the maximum current the kinetic parameters are obtained. The most likely mechanism and rate-determining step are postulated.

The effect of iron hydroxide on nickelous hydroxide electrodes with emphasis on the oxygen evolution reaction

The electrochemical response of mixed Ni(OH)2 and Fe(OH)2 electrodes prepared either by chemical coprecipitation or layer by layer precipitation in alkaline solutions at 25°C was studied. Ni(OH)2 coprecipitated with Fe(OH)2 forms a mixed hydrated oxide with specially high catalytic activity towards the oxygen evolution reaction (OER). In layer by layer precipitated electrodes, the catalytic activity for the OER depends on the sequence of precipitation of the hydroxides and there is a loss of efficiency when the inner layer is made of Fe(OH)2 because of a poisoning of the electrode. Kinetic and mechanistic aspects are discussed. Some long term experiments at controlled potential were also carried out.