Investigation of electrochemical reactions on copper electrode in Cu4RbCl3I2 solid electrolyte

Investigations are carried out by potentiostatic method. It is found that at potentials ϕ less than 100 mV, a reaction rate of copper deposition is limited by the formation and the three-dimensional growth of copper nuclei and the rate of copper dissolution is limited by a two-dimensional growth of holes in the metal. The rate of nucleus growth was evaluated at 10⁻⁹...10⁻⁶ μm s⁻¹ depending on the potential. At ϕ>120 mV, the reaction rate is limited by charge transport at the exchange current density of 2.7 mA cm⁻² and the anodic transfer coefficient α_a≈ 0.45. The accumulation of a divalent copper phase on Cu/Cu₄RbCl₃I₂ interface at anodic polarization is explained by a parallel course of Cu⁺ − e → Cu²⁺ reaction.     

Kinetics of electrochemical reactions on the Ag(Hg)/Ag4RbI5 interface

The electrochemical behaviour of the Ag(Hg)/Ag₄RbI₅ interface is investigated by a potentiostatic pulse method. It is found that the rate-determining step of the electrode reaction is electron transfer with an exchange current density of 68 mA cm^–2 and a transfer coefficient of approximately 0.45. The order of the electrochemical reaction for silver oxidation is estimated from polarization investigations of silver amalgam in various concentrations. From this it is deduced that the mercury is ionized and is implanted in the electrolyte together with silver under anodic polarization: 15Ag+85Hg–100e^–→15Ag⁺+85Hg⁺. From comparison of the electrochemical behaviour of the Ag(Hg)/Ag₄RbI₅ and Ag/Ag₄RbI₅ interfaces it is concluded that the rate of anodic silver dissolution on the Ag/Ag₄RbI₅ interface is limited by crystallization effects. Electronic Publication     

Electrochemical Behavior of a Copper Electrode in Solid RbCu4Cl3I2Electrolyte

The electrochemical behavior of a copper electrode in solid RbCu₄Cl₃I₂electrolyte is studied by galvanostatic and potentiostatic methods. It is found that a Cu₂O layer 1 m thick exists at the interface between the Cu electrode and RbCu₄Cl₃I₂. The layer blocks the electrochemical reaction Cu⁰– e Cu⁺, which involves with metallic copper. At low overpotentials, the Cu electrode acts as an inert redox electrode. At the Cu₂O/RbCu₄Cl₃I₂interface, the electrochemical reaction Cu⁺– e Cu²⁺occurs, which involves Cu²⁺ions. The reaction rate is limited by slow diffusion of Cu²⁺ions in RbCu₄Cl₃I₂. The initial concentration of Cu²⁺ions in the electrolyte near this interface is about 1.4 × 10¹⁷cm^–3. The exchange current density is (4 ± 2) × 10^–6A/cm². At potentials exceeding 8–10 mV, an electric breakdown of the Cu₂O layer occurs, and the reaction with metallic copper becomes unblocked. At 10 mV < < 100 mV, the rate of this reaction is limited by the nucleation of copper crystals and the nuclei growth. At > 120 mV, the reaction rate is limited by charge transfer.     

Multicycle chronoamperometry at rotating ring-disc electrode as a method of current separation into partial currents of silver ionization,Ag<Subscript>2</Subscript>O formation and its chemical dissolution in alkaline medium

A method of multicycle chronoamperometry at rotating ring-disc electrode is suggested for experimental separation of the disc polarization current into its components that correspond to the substrate metal ionization, an oxide formation, and the oxide chemical dissolution. The method was validated by the example of the Ag|Ag₂O|OH^?(H₂O) system. At moderate anodic potentials of Ag-disc (0.48–0.51 V), silver active dissolution from open areas of its surface and through film’s pores dominates; the phase-forming current, hence, the current efficiency of this process drops down rapidly. At the potentials of the maximum at voltammograms (0.52–0.53 V), when the silver active dissolution current is suppressed, the phase-forming currents dominate; they exceed the oxide chemical dissolution rate significantly. The Ag₂O film thickness increases rapidly, the current efficiency of the oxide formation process approaches 100% during the entire disc polarization period. The Ag(I)-oxide chemical dissolution rate constant practically does not depend on the anodic phase-formation potential; however, it somewhat varies depending on the oxide film thickness, thus reflecting changes in the film structure and, possibly, chemical composition (from AgOH to Ag₂O).     

Corrosion mechanism of pure aluminium in aqueous alkaline solution

The corrosion of pure aluminium in alkaline solution has been explored using an open circuit potential transient, potentiodynamic polarization experiment and a.c. impedance spectroscopy. The steady-state value of the open circuit potential (E _ocp ^ss ) of pure aluminium in alkaline solution was observed to decrease with increasing rotation rate of the specimen, which is ascribed to the enhanced anodic reaction. The extent of anodic polarization for the aluminium dissolution reaction on pure aluminium at E _ocp ^ss was found to be greater than that of cathodic polarization for the water reduction reaction. This indicates that the rate of corrosion of pure aluminium is mainly determined by the anodic reaction in alkaline solution. Based upon the experimental results, a corrosion mechanism for pure aluminium has been proposed in the presence of the native surface oxide film in alkaline solution, involving consecutive oxide film formation and dissolution, and simultaneous water reduction.     

Anodic passivation of tin in buffered phosphate electrolyte

The anodic behaviour of tin in buffered phosphate electrolyte (pH=3.1) has been studied by a variety of techniques. A number of anodic processes occur depending on potential and the conditions at the electrode/electrolyte interphase. On anodic polarisation the electrode, which is probably filmed with a phosphate layer, initially undergoes dissolution to form probably Sn(H₂PO₄·HPO₄)^? species. Impedance data indicate that this process has a corresponding Tafel slope of ～0.046 V/decade. At more positive potentials three consecutive passivating processes occur.The primary passivating process involves the blocking of the electrode by Sn₃(PO₄)₂ by a dissolution-precipitation mechanism. The formation of SnO by a slow three dimensional nucleation and growth process constitutes the second. It is formed as a result of the attainment of alkaline conditions at the electrode surface. There is also a parallel reaction path involving the formation of soluble Sn(II) species. The tertiary process consists of the oxidation of Sn to Sn(IV) species. Passivation occurs via a dissolution-precipitation mechanism when the electrode is blocked by SnO₂. The relative quantities of SnO and SnO₂ produced is a function of operating conditions.     

A Study of Nickel Electrodeposition on Paraffin‐Impregnated Graphite Electrode

The aim of the present work was to elucidate the mechanism of electrolytic deposition of Ni on paraffin‐impregnated graphite electrode (PIGE). This process is influenced by H₂ evolution, which occurs in the same potential region. On the basis of the results obtained by linear and cyclic voltammetry, elimination voltammetry with linear scan (EVLS) was used to evaluate both processes. H₂ Evolution alone was studied in sulfate supporting electrolyte, and the previously suggested mechanism for this process according to Volmer–Heyrovsky was confirmed by EVLS. It was found that both the Ni²⁺ concentration and pH affect the polarization behavior of PIGE significantly. Two separated cathodic peaks were observed at low Ni²⁺ and high H⁺ concentrations, and the separation was better at higher scan rates. EVLS confirmed the most‐probable mechanism of Ni deposition as being controlled by slow transfer of the first electron under formation of [NiOH]⁺ as an intermediate. EVLS also indicated slow reduction of H⁺ preceding the reduction of Ni²⁺. The same was confirmed by studying the anodic dissolution at different switching potentials. The results were complemented by scanning electron microscopy (SEM).     

Kinetics of silver anodic dissolution in thiosulfate electrolytes

The regularities of silver anodic dissolution are studied by using the voltammetry (at the potential scan rates from 5 to 1000 mV/s) on the electrode, which was renewed immediately in the solution by cutting-off a thin surface metal layer, and quartz microgravimetry, for various concentrations of sodium thiosulfate (0.05–0.2 M). It is shown that, in the potential range from 0 to 0.4 V (normal hydrogen electrode), the polarization curves reflect the silver dissolution, whereas the contribution of oxidation of S₂O₃²⁻ ions is insignificant. At low potential scan rates, the process kinetics is of mixed nature. The kinetics and mechanism of anodic process are studied by using the measurements at high potential scan rates (100–200 mV/s) and the calculations of equilibrium composition of near-electrode layer. It is found that the exchange current in the electrolytes studied is 5 × 10⁻⁵ A/cm², the transfer coefficient α is approximately 0.5, and both parameters are virtually independent of the concentration of S₂O₃²⁻ ions. The reaction order of silver dissolution with respect to the ligand $ \left. {\frac{{\partial logi}} {{\partial logc}}} \right|_E $ \left. {\frac{{\partial logi}} {{\partial logc}}} \right|_E is close to unity and is independent of potential. With regard for the literature data on the adsorption of thiosulfate ions on silver, this result is interpreted as the evidence for the involvement of one S₂O₃²⁻ ion from bulk solution, along with adsorbed ligands, in the elementary act of metal dissolution.     

Investigation of the anodic decomposition of the solid electrolyte RbCu4Cl3I2

The process of electrochemical decomposition of the solid electrolyte RbCu₄Cl₃I₂ at a vitreous carbon electrode has been investigated. The anodic decomposition of the electrolyte occurs in two steps. At first, the oxidizing electrode reaction of Cu⁺ ions to Cu²⁺ ions takes place at potentials higher than 0.57 V and a layer of decomposition products is formed on the electrode surface, including the divalent copper compound RbCuCl₃. Then the oxidizing reaction of I^– ions occurs at potentials higher than approximately 0.67 V, with deposition of an iodine layer onto the electrode surface. The deposition rate of the layers of decomposition products is controlled by instantaneous nucleation and two-dimensional growth of the deposit. The total thickness of the passivating layer of decomposition products on the anode is equal to ca. 1 μm. Electronic Publication     

Kinetics of the electrodissolution of metallic uranium

The kinetics of the anodic dissolution of metallic uranium in 1, 2, 3, and 4 mol^.l^-1 HNO₃ solutions at 30 °C were studied by potentiostatic polarization. The dissolved uranium was determined by polarography and the anodic dissolution rates by the initial rate method. It was observed that the dissolution rate increases with the applied potential, but is independent of the HNO₃ concentration, because it is a zero order reaction. A mechanism for the anodic reaction was proposed based on the adsorption theory.     

Application of anodic dissolution method for a study of electrocolored aluminum oxides

The anodic dissolution of color carrier of colored aluminum anodic oxide films (AOF) is studied before and after their coloring, using ac in various electrolytes containing Cu(II). The voltammetric polarization curves of anodic dissolution of colored AOF in 0.1 M H₂SO₄ depend on the amount of copper deposited in the pores and its oxidation state (Cu, Cu₂O, CuO). Analytical and X-ray diffraction examination of AOF prior to and after the anodic dissolution shows that the anodic dissolution method is inapplicable for the determination of the oxidation state of copper electrodeposited in AOF pores or the amount of copper oxides.     

Investigation of electrochemical reactions in the solid state cell Cu/RbCu4Cl3I2/C

Using experimental potential values for a vitreous carbon electrode in contact with the RbCu₄Cl₃I₂ solid electrolyte, the concentration of Cu²⁺ ions in the electrolyte was determined. At 0.5 V, the concentration of Cu²⁺ was 1.25×10¹⁸ cm^–3. The estimated values of the Cu²⁺ ion concentration in RbCu₄Cl₃I₂ (0.8%) and the potential of the vitreous carbon electrode after electrochemical decomposition of RbCu₄Cl₃I₂ (0.606 V) correspond to experimental values of 2% and 0.58 V, respectively. This demonstrates the adequacy of the model describing the electrode potential of Cu²⁺ as a function of the concentration in RbCu₄Cl₃I₂. When the C/RbCu₄Cl₃I₂ interface was polarized, the diffusion coefficient of Cu²⁺ was 1.5×10^–8 cm² s^–1. Investigations of the interface between the copper electrode and RbCu₄Cl₃I₂ were carried out by galvanostatic and potentiostatic methods. A 1-μm layer of cuprous oxide, Cu₂O, was discovered on the interface of the copper electrode with RbCu₄Cl₃I₂. This layer blocks the course of the electrochemical reaction Cu⁰–e^–⇌Cu⁺ with participation of copper metal. The copper electrode behaves as an inert redox electrode at low overvoltages. In this case, at the Cu₂O/RbCu₄Cl₃I₂ interface an electrochemical reaction with Cu²⁺ ion participation, Cu⁺–e^–⇌Cu²⁺, takes place. The results suggest that the reaction rate is limited by slowing the Cu²⁺ diffusion in RbCu₄Cl₃I₂. The initial Cu²⁺ ion concentration in the electrolyte near this interface is about 1.4×10¹⁷ cm^–3. The exchange current density is about (4±2)×10^–6 A cm^–2. At potentials ϕ>8–10 mV, an electrical breakdown of the Cu₂O layer takes place, allowing copper metal to ionize to Cu⁺. We suggest that at 10 mV<ϕ<100 mV the rate of this reaction is limited by the formation and growth of copper nuclei and at ϕ>120 mV the reaction rate is limited by charge transfer. Electronic Publication     

Kinetics of chlorine evolution on dimensionally stable anodes at high currents: Extending the concept of a self-accelerating electrode process

The chlorine evolution kinetics are studied by taking steady-state polarization measurements on rotating disc electrodes (dimensionally stable ones and those made of RuO₂) in solutions with various chloride concentration, at various acidity and temperatures, at current densities of up to 4 A/cm². At high temperatures and chloride concentrations, linear parts with an abnormally low slope of about 20 mV are observed in anodic polarization curves. Their appearance is attributed to the chlorine reaction self-acceleration due to a change in the chlorine mass transport mechanism in the anode’s porous coating.     

Linear-sweep voltammetric studies on iron in aqueous sulphate electrolytes between 20 and 200°C

Potentiostatic polarisation studies have been made on iron in 10% Na₂SO₄ at sweep rates of 0.3–30 mV s^?1 and at temperatures of 20–200°C using a platinum-platinum oxide reference electrode. The sweeps showed characteristic features of anodic dissolution and passivation. Oxide removal was evident on the cathodic sweeps. As noted by previous workers two anodic dissolution peaks were observed with clean electrode surfaces, but only one occurred when the surface was covered with oxide. The number of dissolution peaks was also found to depend on sweep rate. In contrast to other work which suggested that there was no passive region above 95°C, passivation was shown to occur even at 200°C.     

Effect of sodium vanadate on corrosion of AD31 aluminum alloy in acid media

Scanning electron microscopy, potentiodynamic polarization method, and electrochemical impedance spectroscopy were used to study the corrosion behavior of AD31 (AA6063) aluminum alloy in acid (pH 3) 0.05 M NaCl solutions containing 3 mmol dm^–3 of NaVO₃ inhibitor. It was found that the corrosion of AD31 alloy in acid sodium chloride solutions predominantly occurs locally at aluminum/intermetallic particle phase boundaries and is limited by the electrochemical stage of charge transfer. It was shown that introduction of sodium vanadate can reduce the rate of selective dissolution of magnesium from the alloy and provides a protective effect on the level of 7–10%.     

The active dissolution of tin in concentrated alkaline solution

The active dissolution of tin in alkaline solution has been studied using both rotating disc and impedance techniques. The dissolution reaction has an anodic Tafel slope of ～57 mV/decade after diffusion effects are eliminated. The order of reaction with respect to OH^? has been found to be ～2 by both techniques and is compatible with the suggested reaction mechanism involving quasi-reversible charge transfer with a following chemical reaction to form stannite ions.     

Peculiarities of anodic behavior of gold electrode in thiosulfate electrolytes

Using the methods of quartz microgravimetry and voltammetry, the anodic behavior of gold electrode in thiosulfate electrolytes is studied in the pH range of 7 to 11. It is found that, in the potential range from 0.15 to 1.0 V (NHE), the anodic current is associated predominantly with the oxidation of thiosulfate ions, and the gold dissolution rate in this electrolyte is negligibly low (< 0.02 mA/cm²). It is shown that the study of anodic processes in the neutral thiosulfate electrolytes requires stabilization of solution acidity, because the near-anode layer can be acidified to the pH values, which are sufficient for the formation of elemental sulfur. It is found that the use of Britten-Robinson buffer solution with pH 7 as the supporting electrolyte changes significantly the polarization curve of thiosulfate ion oxidation, but does not raise the gold dissolution rate. An increase in the solution pH to 11 and an exposure of electrode at various potentials (−0.5 and 0.15 V) prior to the onset of potential scanning also do not accelerate considerably the gold dissolution in the thiosulfate electrolyte. A comparison between the regularities of gold anodic behavior in the thiosulfate solutions and earlier studied gold dissolution in the cyanide and thiocarbamide electrolytes showed that they are similar. It is supposed that the specific features of anodic processes in these cases are of a similar nature: the metal dissolution proceeds with the formation of two-ligand complexes with linear structure, which is typical for all aforementioned ligands.     

Electrooxidation of the hypophosphite ion on a palladium electrode

Anodic oxidation of sodium hypophosphite on smooth Pd and Pd/Pt electrodes and a Pd membrane is studied. Thei vs.E curves for the Pd electrode exhibit two anodic current peaks. One is caused by oxidation of H₂PO ₂ ^- , and the other, by simultaneous ionization of Pd and oxidation of H₂PO ₂ ^- . The hypophosphite ion adsorbed on the Pd surface hinders the formation of the passive film. This brings about a rapid dissolution of Pd in the oxygen region and its subsequent deposition with the formation of palladium black. The oxidation probably includes a slow heterogeneous chemical reaction, specifically, a cleavage of the P-H bond of the hypophosphite ion. The change in the reaction stoichiometry following an increase in solution pH and in anodic polarization is probably due to changing conditions of the H₂PO ₂ ^- adsorption and the number of adsorption sites occupied by H₂PO ₂ ^- on the surface. Following an increase in polarization, the phosphite ion may undergo oxidation to phosphate. Deceased.     

Manipulating OH−-Mediated Anode-Cathode Cross-Communication Toward Long-Life Aqueous Zinc-Vanadium Batteries

The anode-cathode interplay is an important but rarely considered factor that initiates the degradation of aqueous zinc ion batteries (AZIBs). Herein, to address the limited cyclability issue of V-based AZIBs, Al₂(SO₄)₃ is proposed as decent electrolyte additive to manipulate OH⁻-mediated cross-communication between Zn anode and NaV₃O₈ ⋅ 1.5H₂O (NVO) cathode. The hydrolysis of Al³⁺ creates a pH≈0.9 strong acidic environment, which unexpectedly prolongs the anode lifespan from 200 to 1000 h. Such impressive improvement is assigned to the alleviation of interfacial OH⁻ accumulation by Al³⁺ adsorption and solid electrolyte interphase formation. Accordingly, the strongly acidified electrolyte, associated with the sedated crossover of anodic OH⁻ toward NVO, remarkably mitigate its undesired dissolution and phase transition. The interrupted OH⁻-mediated communication between the two electrodes endows Zn||NVO batteries with superb cycling stability, at both low and high scan rates.     

Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes

The suitability of a polymeric composite material for use as part of an anode structure in a cathodic protection system has been examined. The composite material was a conductive blend (volume resistivity typically 1.5 Ω cm) of carbon black in a polyethylene binder. A long operational lifetime for the material demands that the rate of carbon loss must be low. In the work reported here, electrochemical and in situ analytical techniques were employed to characterise the performance of the material over a wide range of anodic current densities in a variety of aqueous electrolytes. The predominant anodic electrochemical reaction on the polymeric material is CO₂ formation in acid and neutral solutions, which causes loss of carbon from the surface and the development of a non-conducting layer of polyethylene. The characteristics of the reaction suggest that it occurs via the discharge of H₂O. In alkaline pH, however, the anodic reactions are more complex. A high OH⁻ concentration (pH 12 or higher) favours the formation of oxygen rather than CO₂, particularly at low anodic potentials. The presence of CO₃ ²⁻ in the electrolyte catalyses the evolution of oxygen at pH values as low as 9. The electrochemical formation of oxygen always occurs in parallel with the generation of some humic acid in the solution. Received: 23 July 1998 / Accepted: 17 November 1998