Ellipsometry of polycrystalline iron electrodes in alkaline solutions containing chloride ions under different electrochemical conditions

The ellipsometric and voltammetric responses of iron in 0.04 M NaOH + xM NaCl (0 x 0.3) are investigated. Three main processes, barrier layer formation, outer layer formation and pitting corrosion which depend differently on the presence of NaCl can be distinguished. The first process becomes practically independent of NaCl concentration. Otherwise, under certain conditions NaCl increases the charge associated with the outer layer formation through pitting. The dependence of the latter on NaCl concentration is in agreement with data earlier reported in the literature. A relation among the three processes is discussed in terms of competitive anion adsorption on iron specimen and formation of redox couples within the hydrous oxide layer produced on iron.     

Temperature effects in the electrochemical behaviour of cycled lead electrodes in HCl solutions

The electrochemical behaviour of cycled lead electrodes has been investigated in HCl solutions in the temperature range ?6° to 30°C. Because of the low ionic strength of solution below ~0.8 M, and the associated ir drop problem, solutions stronger than 1.0 M were principally considered. In weaker solution (0.1–0.8 M) only derived current functions were analysed, whilst all potential-dependent parameters were rejected. Results are reported for both stationary and rotatiang disc electrodes.Because of the chemical dissolution of the surface layer of PbCl₂ formed on anodic scan, measurements were confined to time-scales of < 1 s. By applying these strict criteria, the overall reaction scheme was deduced. In particular, the kinetics of passivation were found to involve the three-dimensional growth of isolated PbCl₂ nuclei, which slowly dissolve into solution. At extreme anodic potentials, or at low temperature, passivation occurs by progressive, two-dimensional growth under diffusion control.A simultaneous electrochemical dissolution of Pb²⁺ at somewhat more negative potentials is noted, for which the rate-determining step is the diffusion of lead ion complexes away from the electrode surface. The nature of the reactivation of the electrode at extreme anodic potentials is also discussed.     

The potentiodynamic response of polycrystalline nickel electrodes in 0.5mK2CO3

The electrochemical behaviour of nickel in 0.5 M K₂CO₃ is investigated by applying simple and combined potentiodynamic techniques in the potential regions of the Ni(OH)₂/Ni and Ni(III)/Ni(II) redox couples. The diffusion controlled hydrated NiCO₃ precipitation interferes with the electroformation of the Ni(OH)₂ prepassive layer. Both anodic and cathodic peak multiplicities are observed in the potential range of the Ni(III)/Ni(II) electrode. The presence of CO ₃ ^2– ions is tentatively associated with a change in the hydration of the composite Ni(OH)₂/NiOOH layer and eventually with HCOc ions coming out from the CO ₃ ^2– /HCO ₃ ^– equilibrium, which depends on the local change in pH produced during the corresponding anodic and cathodic reactions.     

Discharge behaviour of plane nickel hydroxide electrodes

Plane nickel oxide electrodes were prepared by electrochemical deposition of nickel hydroxide on flexible nickel strips. Compact and adherent deposits were obtained, with thickness up to 4 × 10^?3 cm. They were cycled in 6 M KOH solution, and owing to the flexibility of the supporting nickel strips, did not show detaching. The results obtained from galvanostatic discharge curves fit the Peukert equation iⁿt = constant, giving an n value of 1.23 for the 1 × 10^?3 cm thick film and a value of 1.10 for the 4 × 10^?3 cm think film. The diffusion coefficients of moving species are in the order of magnitude of 10^?5 ?10^?6 cm²s^?1; the rate of discharge does not seem to be limited by proton diffusion. The proton diffusion in solid phase is not the only factor to be taken into account in the discharge process; diffusion and migration OH^? and H₂O species play an important role in plane thick nickel oxide film discharge.     

Aspects of electrochemical behaviour of carbon steel in different Bayer plant solutions

Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 °C, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous film. The pore resistance model describes the properties of the oxide films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe_3–x Al _x O₄. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)₃ ^– ion rather than HFeO₂ ^– ion. Moreover, the Al(OH)₄ ^– ion was considered instead of AlO₂ ^– ion in the oxidation process.     

The cycling behaviour of zinc electrodes in a nickel oxide-zinc accumulator

The behaviour of porous zinc electrodes was investigated by monitoring the potential distribution along the surface of the electrode during charging and discharging in a nickel oxide-zinc battery. Impedance measurements of the zinc electrode were taken as a function of state-of-charge and of the cycle number. The composition of the electrode was varied with HgO and PbO additives. Shape change was observed for all electrodes. The observed potential distribution has led to the tentative explanation that the shape change phenomenon is caused by diffusion of zincate along the surface.     

The electrochemical behaviour of silver sulphide in sulphuric acid solutions

The electrochemical behaviour of synthetic silver sulphide (acanthite) electrodes in sulphuric acid solutions has been investigated using several techniques including cyclic voltammetry, anodic polarization and constant potential experiments. Under anodic polarization the dissolution has been attributed to the reaction Ag₂S=2Ag⁺+S+2e which occurs in two sequential, single electron transfer steps. A kinetic model for this stepwise anodic dissolution process at lower overpotential, where the current is a function of potential, is provided. At high dissolution rates (i.e. high currents) the slightly soluble silver sulphate salt is formed on the surface due to the saturation of the electrolyte near the Ag₂S interface. This observation is supported by the influence of electrolyte composition on the cyclic voltammetry and the polarization curve. A paralinear film growth model has been found to describe the formation and growth of the silver sulphate product layer indicating an initial region of parabolic kinetics which gradually changes to linear kinetics as the rate of film dissolution approaches that of film formation.     

The electrochemical behaviour of copper in sodium salicylate aqueous solutions

Cyclic voltammetry has been used to investigate the behaviour of copper surfaces in sodium salicylate aqueous solutions. The observed copper anodic passivation dependence on the presence of salicylate ion in solution is explained. The analysis of the experimental data supports the formation of a complex passivating film formed by a Cu₂O inner layer and a mixed cupric oxide/cupric salicylate outer layer; this film provides a partial passivation of the copper surface and can be completely removed upon excursion to negative potentials values; the composition of the passivating layer depends on the electrolyte nature, i.e. sodium salicylate ion and solution pH, and on the potential programme the copper electrode is subjected to.     

Photocurrents at irradiated mercury electrodes in solutions of nickel ions

The influence of concentration and potential on photocurrents produced at mercury electrodes in solutions of nickel ions have been investigated using a pulsed xenon lamp. It is concluded that the mechanism involves scavenging of hydrated electrons by the nickel ions followed by an oxidative regeneration, rather than the reduction of excited nickel ions.     

The electrochemical behaviour of copper in alkaline solutions containing sodium sulphide

The electrochemical behaviour of copper in NaHCO₃ solution (pH 9) and NaOH solutions (pH 14) in the presence of sodium sulphide (10^–3 to 5×10^–2m) was investigated by using rotating disc electrode and rotating ring-disc techniques, triangular potential scanning voltammetry and potentiostatic steps. When the potential increases from –1.2V upwards, copper sulphide layers are firstly formed at potentials close to the equilibrium potentials of the Cu/Cu₂S and Cu/CuS reversible electrodes. When the potential exceeds 0.0 V (NHE), the copper oxide layer is electroformed. Pitting corrosion of copper is observed at potentials greater than –0.3 V. The charateristics of copper pitting are also determined through SEM optical microscopy and EDAX analysis. There are two main effects in the presence of sodium sulphide, namely, the delay in the cuprous oxide formation by the presence of the previously formed copper sulphide layer and the remarkable increase of copper electrodissolution when the potential exceeds the cuprous oxide electroformation threshold potential. These results are interpreted on the basis of a complex structured anodic sulphide layer and on a weakening of the metal-metal bond by the presence of adsorbed sulphur on the copper surface.     

The electrochemical behaviour of stainless steel AISI 304 in alkaline solutions with different pH in the presence of chlorides

Nowadays, stainless steel reinforcements appear as an effective solution to increase the durability of reinforced concrete structures exposed to very aggressive environments. AISI 304 is widely used for this purpose. Although the improved durability of reinforcing AISI 304, when compared to carbon steel, there is a high probability of pitting susceptibility in the presence of chlorides. Thus, the present work aims at studying the passivation and passivation breakdown of AISI 304 in alkaline solutions of different pH (pH from 13 to 9), simulating the interstitial concrete electrolyte. These solutions were contaminated with different concentrations of chloride ions (3% and 10%, as NaCl). The electrochemical behaviour was evaluated by d.c. potentiodynamic polarization and by electrochemical impedance spectroscopy (EIS).The morphological features and the changes observed in the surface composition were evaluated by Scanning Electron Microscopy (SEM) together with EDS chemical analysis.The results evidence that pH plays an important role in the evolution of the film resistance and charge transfer processes. Moreover, the effect is highly dependent upon the chloride content and immersion time.     

Structure and electrochemical properties of nickel hydroxide electrodes with cobalt additives

In this article,cobalt additives are introduced into nickel hydroxide electrodes by two incorporation methods—co-precipitated cobalt hydroxide during the nickel hydroxide synthesis or post-added CoO with nickel hydroxide. The results of X-ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests indicate that (i) the diffraction peaks show a decrease in intensity and increase in the half peak breadths for Ni(OH)₂ with co-precipitated cobalt hydroxide; (ii) the electrochemical activity of nickel hydroxide can be improved by both incorporated cobalt and the effects of post-added CoO are more notable; (iii) CoOOH derived from post-added CoO is not stable in the KOH electrolyte when the potential of the Ni(OH)₂ electrode is lowered and its reduction product may be inactive, thus results in an irreversible capacity loss of nickel-metal-hydride battery after over-discharge-state storage.     

The effect of the surface state on the electrochemical behaviour of copper electrodes

All investigated treatments of copper electrodes, which include mechanical (grinding and polishing with abrasives of different grain size: 210, 85, 75 and 42 μm), chemical (polishing, etching and shine-etching) and electrochemical (polishing), lead to an increase both of the anodic (copper dissolution) and of the cathodic (hydrogen evolution and oxygen reduction) polarization. This effect may be explained by the combined action of several types of factors.(1) Change in the real surface area of the examined electrode. This is the only factor in the case of mechanical treatment and an important factor for all other cases.(2) Formation of oxygen layers on the surface, an important factor in all types of chemical and electrochemical treatment.(3) Removal of the deformed layer composed upon mechanical polishing. This factor fully affects the electrochemically polished samples, partially the chemically polished one and does not affect the other types of chemical treatment.(4) These data correlate well with, and confirm, the concept about the higher electrochemical homogeneity of the electrochemically polished surface in comparison to the mechanically polished one.Our investigations show that a correct choice of the nature and sequence of the preliminary treatment of copper electrode surface makes it possible to impart on it the desired surface and electrochemical properties.     

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions has been investigated. The results of hydrogen collection experiments showed that aluminium has a lower corrosion rate in alkaline methanol solutions compared to water based solutions and that the corrosion rate increases with increasing water content of the solution. Polarization and galvanostatic discharge experiments showed that there is a wide potential window of electrochemical activity and a better discharge performance in the alkaline methanol solutions with a certain amount of water. Scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) showed that the passivation in the later stages of discharge in alkaline methanol solutions at relatively high current densities is due to the formation of a dense Al(OH)₃ layer on the surface of the anode.     

Studies concerning charged nickel hydroxide electrodes. VI. Voltammetric behaviour for pre-cycled electrodes

In this study cyclic voltammetry/coulometry has been combined with oxygen volume measurements to identify the species giving the multiple anodic and cathodic peaks for a pre-cycled -Ni(OH)₂ starting material. A complex sequence of 1 and 2 electron transfer reactions involving several U/V and U/V coexisting phase pairs has been identified. This system is complicated by overlap of the various processes in some cases and also the chemical transformation of unstable -phases. As many as six anodic and four cathodic processes can be encountered depending on the charging history.     

A study of the electrochemical behaviour of electrodes in operating solid-state supercapacitors

The electrochemical behaviour of electrodes and of complete solid-state supercapacitors has been studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (CD) measurements using two independent electrochemical equipments. The first one controlled the execution of the test and recorded the voltage and current values of the complete supercapacitor while the other one recorded the potential changes of the single electrodes. In this work, two different types of capacitors were studied: (a) a symmetric supercapacitor using carbon electrodes, and (b) a hybrid (asymmetric) supercapacitor with ruthenium oxide/carbon in the positive electrode and carbon in the negative electrode. The studies evidenced that in the symmetric capacitors the positive electrode controlled the capacitive performance and an optimal mass ratio from 1.2:1 to 1.3:1 between the positive and the negative electrodes was found in the investigated conditions. For the hybrid supercapacitor it was observed that the ruthenium-based positive electrode influenced the capacitive performance of carbon-based negative electrode and that an accurate balance of carbon loading in the negative electrode was necessary.     

Investigation of electrochemical behaviour of storage device prototypes with carbon electrodes

The electrochemical behavior of storage device prototypes with carbon electrodes and ionic liquids as electrolyte was investigated. The current-voltage dependences using different working electrodes and the dependences of specific capacitance, charge and discharge dynamics of supercapacitor cells on time were obtained. The optimal values of specific capacitance and specific energy of film-like supercapacitor samples at different voltages were obtained.     

The electrochemical behaviour of cobalt in alkaline nitrate melts at different temperatures

The electrochemical behaviour of cobalt in molten alkali nitrates in the temperature range from 141 to 321°C has been investigated. At lower temperatures the metal dissolves as Co(II); the oxidation product at higher temperatures is Co₃O₄. Nitrogen oxides are also formed. Passivation and localized corrosion occur under definite anode potential, Co(II) concentration and temperature conditions. These effects have been studied by non-stationary measurements. Co(II) dissolved in the melt can be electrodeposited either on cobalt or platinum cathodes.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.