Ac behaviour of the lead dioxide electrode: antimony effect

The impedance of PbO₂ electrodes in 8 N H₂SO₄ has been measured in a wide range of frequencies of (0.01–10,000 Hz) and applied potentials (500–1500 mV/Hg/Hg₂SO₄). Both α- and β-allotropic forms of pure and antimony doped lead dioxide have been taken into account. The effect of antimony added to the electrolyte has been considered.In the range of PbO₂ reducton potential, at least four contributions to the electrode impedance, characterized by different relaxation times, have been found. Two of them appear to be related to the electrochemical process taking place in two consecutive steps, a third one can be assigned to solid state diffusion of oxygen and/or lead towards the reaction zones and the last one refers to the resistance of a thin layer of PbSO₄.     

Application of DSA-type electrodes to a positive grid in lead-acid batteries

The possibility of the use of modified DSA-type electrodes as positive grids in lead-acid batteries was examined by anodic polarization measurements, charge-discharge tests and self-discharge tests of the Ti/RuO₂ and Ti/RuO₂/-PbO₂ electrodes. The passivation of a titanium base was retarded by using a very thin film of ruthenium dioxide. The Ti/RuO₂/-PbO₂ electrode could be used as a positive grid to a certain extent. On the other hand, the Ti/RuO₂ electrode showed the worse characteristics for the positive grid, mainly due to the low oxygen overvoltage of the ruthenium dioxide layer. Some other problems to be solved have also been pointed out.     

Corrosion behavior of a positive graphite electrode in vanadium redox flow battery

The graphite plate is easily suffered from corosion because of CO₂ evolution when it acts as the positive electrode for vanadium redox flow battery. The aim is to obtain the initial potential for gas evolution on a positive graphite electrode in 2 mol dm⁻³ H₂SO₄ + 2 mol dm⁻³ VOSO₄ solution. The effects of polarization potential, operating temperature and polarization time on extent of graphite corrosion are investigated by potentiodynamic and potentiostatic techniques. The surface characteristics of graphite electrode before and after corrosion are examined by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The results show that the gas begins to evolve on the graphite electrode when the anodic polarization potential is higher than 1.60 V vs saturated calomel electrode at 20 °C. The CO₂ evolution on the graphite electrode can lead to intergranular corrosion of the graphite when the polarization potential reaches 1.75 V. In addition, the functional groups of COOH and CO introduced on the surface of graphite electrode during corrosion can catalyze the formation of CO₂, therefore, accelerates the corrosion rate of graphite electrode.     

Corrosion of metals in molten lithium sulphate-potassium sulphate eutectic

In conjunction with the development of a new electrochemical flue gas desulphurization process, a study is made of the corrosion of several electrode materials in molten lithium sulphate-potassium sulphate eutectic at 600°C. Measurements of the open-circuit potentials are made in air, oxygen and nitrogen to determine the existence of a stable oxide layer on the electrode surface. Voltammetric measurements are also made to determine the corrosion current densities in O₂/SO₂ mixtures at various compositions. The study shows that the valve metals studies (zirconium, tantalum and titanium) develop stable oxide layers on their surfaces which are highly corrosion-resistant. The corrosion currents decrease in the following order NiFe>CuAg>Zr.     

Mechanism of the action of Ag and As on the anodic corrosion of lead and oxygen evolution at the Pb/PbO(2−x)/H2O/O2/H2SO4 electrode system

When a Pb electrode, immersed in H₂SO₄ solution, is polarized anodically in the PbO₂ potential range the Pb/PbO(_2−x)/H₂O/O₂/H₂SO₄ electrode system is established. Oxygen is evolved at the oxide—solution interface. The oxygen atoms formed as intermediates diffuse into the anodic layer and oxidize the metal. Through a solid-state reaction, the metal is oxidized first to tet-PbO and then to PbO₂. By studying the changes in the rate—potential relations of the above reactions, as well as the phase and chemical composition of the anodic layer, it was possible to elucidate the effect of Ag and As on these processes. The additives were introduced into the electrode system either by alloying with lead or by dissolving them in the H₂SO₄ solution. When added to the solution, both Ag and As lower the overvoltage of the oxygen evolution reaction. They have practically no effect on the corrosion reaction under galvanostatic polarization conditions. If alloyed in the metal, Ag reduces the oxidation rate of Pb significantly, while As enhances it. Both additives lower the stoichiometric number of the anodic oxide layer, ie they retard the oxidation of PbO to PbO₂. The results of these investigations were used to develop further the model of the mechanism of the reactions proceeding during the anodic oxidation of lead in H₂SO₄ solutions.     

Passivity and passivity breakdown of 304 stainless steel in alkaline sodium sulphate solutions

The passivity and passivity breakdown of 304 stainless steel were investigated in 0.25 M Na₂SO₄solutions of pH 10. The effect of applied potential and the presence of Cl^– ions in the electrolyte were also studied. Different electrochemical methods such as open circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results showed that the steel electrode passivates under open circuit conditions and also under potentiostatic control. The rate of passive film thickening under open circuit conditions follows a simple logarithmic law. Addition of Cl^– ion shifts the polarization curves in the active direction and above a critical chloride concentration, [Cl^– ] 0.15 M, pitting corrosion occurs and the pitting potential, E _pit, decreases linearly with the logarithm of [Cl^–]. The addition of sulphate ions to the chloride-containing solutions was found to inhibit the pitting process, and at [SO^2- ₄] 0.25 M, a complete immunity to pitting corrosion was recorded. The impedance measurements provided support for film thickening and film breakdown reactions. An equivalent circuit model which consists of a pure resistor, R , in series with a parallel combination of a pure resistor, R _p, and a constant phase element, Q, was proposed to describe the electrode/electrolyte interface. The passive film thickness was found to increase with applied potential up to a critical value of 0.3 V. At higher voltages, breakdown of the passive film occured.     

Anodic evolution of oxygen on ruthenium in acidic solutions

Electrochemical properties of ruthenium, particularly the anodic evolution of oxygen and anodic dissolution of ruthenium have been investigated by means of polarization measurements and product analyses. The electrode surface gradually colours black during oxygen evolution. This is due to the accumulation of hydrous RuO₂ resulting from decomposition of corrosion product. The black oxide film suppresses the ruthenium dissolution rate and the current efficiency for the dissolution reaction is less than 6% at a cd below 0.2 A/cm² in 1 N H₂SO₄, where the anodic evolution of oxygen is predominant. The overall current for oxygen evolution is expressed by i = nFka^?2_H exp (2FE/RT) The probable mechanism of oxygen evolution on the ruthenium anode under the Langmuir conditions of intermediate adsorption is

     

Fundamental studies on a new concept of flue gas desulphurization

A new process for removal of sulphur dioxide from waste gases is proposed consisting of both electrochemical and catalytic sulphur dioxide oxidation. In the catalytic step a part of the sulphur dioxide is oxidized by oxygen on copper producing sulphuric acid and copper sulphate. The other part is oxidized electrochemically on graphite. The cathodic reaction of this electrolysis is used for recovering the copper dissolved in the catalytic step. The basic reactions of this process have been studied experimentally in detail. It has been shown that sulphur dioxide can be electrochemically oxidized on carbon electrodes to sulphuric acid with high current efficiency. The reaction rate of the electrochemical copper deposition is increased by dissolved sulphur dioxide in the electrolyte. The catalytic oxidation of sulphur dioxide on copper has been investigated for different sulphur dioxide concentrations and temperatures. The ratio of the reaction products, sulphuric acid and copper sulphate, varies over a wide range depending on the experimental conditions.Nomenclature SO₂ concentration (gas phase) (vol % SO₂) - SO₂ concentration (electrolyte) (g l^–1) - E potential vs saturated calomel electrode (V) - E _s specific energy consumption (W g^–1 SO₂) - F Faraday constant (A s^–1 mol^–1) - i current density (mA cm^–2) - molecular weight (g mol^–1) - T temperature (° C) - U _c cell voltage (V) - v _e number of electrons being transferred - space-time yield of SO₂-oxidation (g SO₂ h^–1 dm^–3) - _cu space-time yield of Cu-corrosion (g Cu h^–1 dm^–3) - ratio - fractional conversion of SO₂ - current efficiency for SO₂ oxidation     

Corrosion of Mild Steel in low Conductive Media simulating Natural Waters

The corrosion of a mild steel was examined in two aerated neutral aqueous solutions, defined as reference solution (0.2 g L^–1NaCl) and as -solution (1.3 g L^–1NaCl + 0.63 g L^–1NaHCO₃ + 0.27 g L^–1Na₂SO₄). Their composition was chosen on the basis of the physical and chemical properties of certain natural waters. The solutions simulated the least (reference solution) and the most (-solution) aggressive waters of the Sebou river in Morocco, as determined after a four-year examination (1991–94), at 13 pump stations located along the river. Various experimental methods were used to determine the corrosion mechanism. Cathodic range voltammetry using a rotating disc allowed the kinetics of oxygen reduction process to be determined. Since the conductivity of the solutions were low, the potential was corrected for ohmic drop estimated through the high frequency limit in the Nyquist diagrams (electrochemical impedance spectroscopy) as well as the current interrupter method. After correction, the polarization curves revealed a diffusion plateau attributed to dissolved oxygen reduction. At the plateau, a two-step mechanism was derived involving oxygen diffusion through the hydrodynamic layer and through a porous inner layer formed by the corrosion products. This inner layer could not be observed by SEM, but both EIS and EHD (electrohydrodynamic impedance) confirmed the presence of a thin porous dielectric layer. At the open circuit potential, the corrosion rate was determined by the diffusion rate of dissolved oxygen in the -solution, and by charge transfer in the reference solution. This shows that the corrosion mechanism strongly depends on the electrolyte and its conductivity.     

Mechanisms of electroless metal plating. III. Mixed potential theory and the interdependence of partial reactions

Electroless plating reactions are classified according to four overall reaction schemes in which each partial reaction is either under diffusion control or electrochemical control. The theory of a technique based on the observation of the mixed potential as a function of agitation, concentration of the reducing agent and concentration of metal ions is presented. Using this technique it is shown that in electroless copper plating the copper deposition reaction is diffusion-controlled while the formaldehyde decomposition reaction is activation-controlled. Values of the kinetic and mechanistic parameters for the partial reactions obtained by this method and by other electrochemical methods indicate that the two partial reactions are not independent of each other.Nomenclature a Tafel slope intercept - A electrode area - b _M Tafel slope for cathodic partial reaction - b _R Tafel slope for anodic partial reaction - B _M diffusion parameter for CuEDTA^2– complex - diffusion parameter for dissolved oxygen - B _R diffusion parameter for HCHO - C _M bulk concentration of copper ions - bulk concentration of dissolved oxygen - C _R ^a surface concentration of HCHO - C _R bulk concentration of HCHO - D _R diffusion coefficient of HCHO - E electrode potential - E _M thermodynamic reversible potential for the metal deposition reaction - E _M ⁰ standard electrode potential for copper deposition - E _MP mixed potential - E _R thermodynamic reversible potential for reducing agent reaction - E _R ⁰ standard electrode potential for HCHO - F Faraday constant - i _M current density for metal deposition - i _M total cathodic current density - i _M ^k kinetic controlled current density for metal deposition - i _M ⁰ exchange current density for metal deposition - i _M ^D diffusion-limited current density for metal deposition - i _M ^D diffusion-limited current density for total cathodic reactions - current density for oxygen reduction - i _plat plating current density - i _R current density for HCHO oxidation - i _R ⁰ exchange current density for HCHO oxidation - i _R ^D diffusion-limited current density for HCHO oxidation - n _M number of electrons transferred in metal deposition reaction - n _R number of electrons transferred in the HCHO oxidation reaction - R gas constant - T absolute temperature - stoichiometric number - _M transfer coefficient for metal deposition - _R transfer coefficient for HCHO oxidation - _M symmetry factor - number of steps prior to rate determining step - _M overpotential for metal deposition - _R overpotential for HCHO oxidation - v kinematic viscosity - rotation rate of electrode     

The Faradaic impedance of the lithium-sulphur dioxide system. A kinetic interpretation

Impedance measurements have been made on Li/SO₂(C) cells containing an acetonitrile-based electrolyte in a range of states from newly assembled to completely discharged. The cell behaviour can be explained if it is assumed that the lithium is an irreversible electrode and that the SO₂ electrode is reversible. The nominal exchange current density on the lithium is 0.7 mA cm^–2 and 0.37 for the cell Li/LiBr(2.35 mol dm^–3), CH₃CN, S₂O ₄ ^2– ¦SO₂(6.25 mol dm^–3)C     

Behaviour of lead electrodes in sulphate electrolytes. II. Investigation of the electrode kinetics

The behaviour of lead electrodes in 0.5 M sulphuric acid has been investigated. The electrodes were of the type III variety, which were prepared by alternately cycling the lead electrodes through the Pb/PbSO₄ potential and hydrogen evolution. On these electrodes and at this concentration of electrolyte it proved possible to record highly reproducible experimental data. Various techniques were employed to investigate the Pb PbSO₄ reaction, including linear potential scans and potential steps. The electrode kinetics are discussed and, where possible, compared with theoretical models.     

Potentiometric measurements of oxygen electrodes in molten sodium sulphate

The dependence of the potential of platinum and gold electrodes in molten Na₂SO₄ on the composition of surrounding atmospheres of SO₂ + O₂, and of N₂ + O₂ has been investigated at 900°. The electrode potential dependence on the oxide ion concentration in the electrolyte in an oxygen atmosphere is also reported. It is postulated that the potential of O₂(Au) and O₂(Pt) electrodes in basic molten sulphate is controlled by the reaction: $\text{1}\text{2}$O₂ + O^2? = O^2?₂. It is thought that the sulphur dioxide electrode reaction is very complex and further studies are required to elucidate its mechanism. However, in the light of the present work it seems reasonable to consider this electrode as an oxygen electrode which also responds to SO₂ and SO₃.     

Processes in solid state at anodic oxidation of a lead electrode in H2SO4 solution and their dependence on the oxide structure and properties

During anodic oxidation of the lead electrode in H₂SO₄ solution in PbO potential region an anodic deposit is formed containing a dense layer of tet-PbO and a porous layer of PbSO₄. The rate of oxidation of Pb to tet-PbO is determined from the transport of O^2? ions through the tet-PbO layer by a vacancy mechanism.On illumination of the electrode with white light photoelectrochemical processes proceed in the anodic layer leading to the transformation of tet-PbO into PbO_n where (1 < n < 2). PbO_n is a solid electrolyte with semiconductor properties. The processes of photoelectrochemical oxidation of tet-PbO to αPbO₂ are discussed on the basis of the band energy scheme of semiconductors. These photoelectrochemical investigations show that at oxidation of tet-PbO to αPbO₂ in solid state the highest energy barrier is the band gap.In the PbO₂ potential region the PbO_n transforms into a semiconductor with electrone conductivity. The anodic corrosion of the Pb electrode in this region is discussed on the basis of semiconductor properties of the oxide. The corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO and at the second stage tet-PbO is oxidized in solid state to αPbO₂. For the second process the highest energy barrier is the band gap. Its overcoming is realized by surface states forming at the oxide/solution interface during the reactions of oxygen evolution. These are O^? radicals and O atoms. They penetrate into the oxide layer by an oxygen vacancy mechanism and oxidize the Pb to tet-PbO as well as the tet-PbO to αPbO₂. The processes in solid state in the PbO_n layer are favoured by the layer crystal structure and by the similar shape of the unit cells of tet-PbO and αPbO₂.     

Oxidation of aqueous sulfur dioxide catalyzed by poly-4-vinylpyridine–Cu(II) complex. Part 1: Homogeneous phase oxidation

The oxidation of aqueous sulfur dioxide in the presence of polymer-supported copper(II) catalyst is also accompanied by homogeneous oxidation of aqueous sulfur dioxide catalyzed by leached copper(II) ions. Aqueous phase oxidation of sulfur dioxide of low concentrations by oxygen in the presence of dissolved copper(II) has therefore been studied. The solubility of SO₂ in aqueous solutions is not affected by the concentration of copper(II) in the solution. In the oxidation reaction, only HSO is the reactive S(IV) species. Based on this observation a rate model which also incorporates the effect of sulfuric acid on the solubility of SO₂ is developed. The rate model includes a power-law type term for the rate of homogeneous phase reaction obtained from a proposed free-radical chain mechanism for the oxidation. Experiments are conducted at various levels of concentrations of SO₂ and O₂ in the gas phase and Cu(II) in the liquid phase. The observed orders are one in each of O₂, Cu(II) and HSO. This suggests a first-order termination of the free radicals of bisulfite ions.     

EQCM study of electrodeposited PbO2: Investigation of the gel formation and discharge mechanisms

Lead dioxide thin films were electrodeposited on gold substrates and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mass change occurring upon immersion in a H₂SO₄ electrolyte and during electrochemical reduction was observed in situ by electrochemical quartz crystal microbalance (EQCM). A hydrated PbO₂ gel-type layer is formed at the surface of electrodeposited PbO₂. The concentration of the H₂SO₄ electrolyte does not affect the composition of the gel nor the amount of lead dioxide involved in the hydration process. It is established that 1.3 × 10⁻⁷ mol cm⁻² of β-PbO₂ are hydrated at the surface of an electrodeposited film and that the hydration reaction occurs according to the following reaction: PbO₂^(crystal) + xH₂O ↔ (PbO(OH)₂·(x − 1)H₂O)^(gel), where x = 8.1. The mass change occurring during the first and subsequent discharge of PbO₂ was recorded. It is shown that both PbO₂^(crystal) and PbO(OH)₂·(7.1)H₂O)^(gel) are reduced to PbSO₄ during the first discharge.     

Experiences with reference electrodes in sulphate melts

The construction and the experiences with a reference electrode of the type PT/0.1 Ag₂SO₄ + 0.9 (Na, K)₂SO₄/ mullite in sulphate melts at 1173 K are reported. After an “ageing” period of approx. 200 h, during which the potential can change by up to 80 mV, the potential is stable within ± 10 mV over many hundreds of hours. Additional investigations reveal that the potential is established by formation of a silver or AgPt alloy layer, respectively, on the Pt-wire. This kind of reference electrode is suitable for long term experiments, eg corrosion tests.     

The use of alkali carbonates in carbon concentration cells

The feasibility of using alkali carbonates as electrolytes in carbon concentration cells has been investigated. The following cell was set up: The test electrode (LHS) was a carbon-permeable-iron membrane in contact with a gaseous or liquid metal environment whose carbon activity could be varied. Experiments involving argon and liquid sodium environments at 970 K showed that the potential of the-Fe test electrode was a function of its carbon activity.The potential of the electrode,-Fe, C ¦ CO ₃ ^2– , was also measured as a function of carbonate ion activity and current.It was concluded that the predominant electrode reaction at the iron electrode was reversible and involved carbon and carbonate species or species with which they were in equilibrium.     

The influence of mass transfer on the mechanism of electropolishing of nickel in aqueous sulphuric acid

The effect of mass transfer on the electropolishing of nickel in sulphuric acid solutions has been studied using a rotating disc electrode technique. Two well defined regions for different rotation rates have been observed having (a) pure diffusion control, and (b) mixed kinetic and diffusion control. The presence of a solid layer of a hydrated nickel salt covering a contaminated oxide layer is proposed. The rate of chemical attack of the underlying layer appears to depend on the SO^2?₄ ion concentration for H₂SO₄ concentrations greater than 8 M. When the rate of mass transfer is increased, changes in the morphology of the surface are observed, polishing gives way to levelling, and finally either to metallographic etching or corrosion. The disappearance of electropolishing is observed in general when the rate of mass transfer is sufficient to eliminate from the surface the solid layer of hydrated nickel sulphate, and the results indicate a coupling between diffusional rates in the solution and ionic transport through the oxide or contaminated oxide phases.     

Dependence of the phase composition of the anodic layer on oxygen evolution and anodic corrosion of lead electrode in lead dioxide potential region

At potentials more positive than 1300 than 1300 mV with respect to a Hg/Hg₂SO₄ electrode the partial currents of lead corrosion and oxygen evolution in 7_n H₂SO₄ follow the Tafel's dependence. X-ray investigations and wet analyses of the anodic layer show that it consists of tet-PbO and α PbO₂. Besides, at potentials more negative than 1530 mV, β PbO₂ forms at the oxide/solution interface.It is established that lead anodic corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO. During the second stage tet-PbP is oxidized to PbO₂. If this process is performed in solid state, α PbO₂ forms. If PbO is dissolved and then oxidized, β PbO₂ crystals are formed. The oxidation of tet-PbO to α PbO₂ proceeds at different rates in the bulk of the oxide and at its surface with the solution. At potentials more positive than 1530 mV the oxidation of tet-PbO to α PbO₂ at the surface of the oxide with the solution leads to the dropping off of part of the oxide layer.The oxidation of Pb and PbO is carried out under the action of O atoms and O^? radicals which evolve at the oxide/solution interface and penetrating the oxide reach the metal/oxide surface.