State Polarization Curves Research Articles

Electrode reactions on the aluminum electrode in 1-butyl-3-methylimidazolium chloride chloroaluminate ionic liquid: A comprehensive study

Mechanisms of Al electrochemical deposition/dissolution in acidic AlCl3–1-butyl-3-methylimidazolium chloride (BMImCl) ionic liquids are studied. Stationary polarization curves (SPCs) are obtained in the anodic range of potentials. The anodic SPCs are characteristic of electrode processes, which are complicated by passivation phenomena. The peaks intensity of AlCl4−, Al2Cl7− ions were obtained by in-situ Raman spectroscopy during the aluminum electrode polarization and correlated with the behavior of the SPCs. Based on the results obtained, a mechanism of electrochemical oxidation of aluminum in AlCl3–BMImCl IL is proposed. According to the mechanism in question, the anodic process is complicated by the presence of a following chemical reaction involving AlCl3. Chronopotentiometric studies were carried out to determine diffusion coefficients of Al2Cl7− anion, which are unaffected by the change in the molar ratio of AlCl3 to BMImCl (N). The activation energy (Ea) for the Al2Cl7− diffusion coefficient is independent of N and equals 14.5 ± 0.4 kJ mol−1. Exchange current density (i0) at the Al | AlCl3–BMImCl interface was determined using impedance spectroscopy for the compositions with 1.1 ≤ N ≤ 2.0 at temperatures from 303 to 386 K. The i0 increases from 0.54 ± 0.13 to 2.73 ± 0.22 mA cm−2 with the concentration of the electrochemically active Al2Cl7− anion in the concentration range indicated above at 303 K. The data on i0 is used to calculate the transfer coefficient of cathodic reaction (α = 0.18 ± 0.02), the electrochemical reaction rate constant (ks= (1.1 ± 0.3)∙10−6 cm s−1 (at T = 303 K)) and the Ea for the ks (Eaks = 36.5 ± 0.9 kJ mol−1), which do not depend on the IL composition.

Electrochemical behavior of aluminum in 1-ethyl-3-methylimidazolium chloroaluminate ionic liquids

The mechanism of electrochemical oxidation and reduction of aluminum in 1-ethyl-3-methylimidazolium-based (EMImCl) chloroaluminate ionic liquids is studied across the concentration range of N = 1.1–2.0 (N the molar ratio of AlCl3 to EMImCl) at temperatures between 303 K and 418 К. Anodic stationary polarization curves (SPCs) indicate that the process of anodic dissolution of aluminum is complicated by passivation. The current density in the passive region and the passivation potential decrease with a higher concentration of Al2Cl7– ion in the melt. To elucidate the processes taking place at the aluminum electrode, the relationship between the concentration of the ions in the vicinity of the electrode and the potential applied in the cathodic and anodic region was analyzed by in-situ Raman spectroscopy for the concentrations of N = 1.1 and 2.0. The resulting dependences of relative peak intensity vs. potential correlate with the cathodic and anodic polarization curves. In the anodic region the content of Al2Cl7– grows with the potential, while the concentration of AlCl4– ion decreases. In the cathodic region the opposite process is observed: the concentration of Al2Cl7– decreases and that of AlCl4–increases with growing potential. It is shown that a precipitate of AlCl3 forms on the electrode surface in the region of passive aluminum dissolution, which is responsible for the electrode passivation. Based on the results obtained, we propose a model for the process of aluminum oxidation/reduction preceded and followed by a chemical reaction of aluminum chloride formation for the cathodic and anodic process, respectively. Temperature dependences of diffusion coefficients are studied by chronopotentiometry. The results indicate that diffusion coefficients are not affected by the concentration across the temperature range studied. The activation energy for diffusion was found to be 13.65 kJ∙mol−1. Impedance spectroscopy was used to investigate the concentration and temperature dependences of exchange current density. The exchange currents increase with increasing aluminum chloride content in the melt. The transfer coefficient and electrochemical reaction rate constant were found to be α = 0.17 ± 0.04, ks = (1.7 ± 0.4)∙10−6 cm∙s−1, respectively. The activation energy for the electrochemical reaction rate constant does not depend on the concentration and was found to be 33.5 kJ∙mol−1.

EFFECT OF POLYVINYLPYRROLIDONE ON THE SYNTHESIS AND CATALYTIC PROPERTIES OF PLATINUM-CONTAINING OXYGEN ELECTROREDUCTION CATALYSTS

Results of the work on the synthesis of highly effective nanocomposite platinum-containing oxygen reduction electrocatalysts for hydrogen-oxygen fuel cells are presented and ways of increasing their activity are considered. Fuel cells (FCs) represent the most promising renewable and environmentally friendly energy sources that can operate at low temperatures with high efficiency and with low or zero levels of hazardous waste. The main problem of fuel cells is that the oxygen reduction reaction is slowed down in relation to the hydrogen oxidation reaction, which leads to an increase in the overvoltage at the cathode and, as a result, to a decrease in the productivity of FCs. To solve this problem, the new methods are being developed for the synthesis of highly efficient platinum electrocatalysts, which currently have the best performance, since platinum has the highest specific catalytic activity among other materials used in low temperature FCs. The purpose of this work is to determine the optimal composition of reaction mixture and conditions for the preparation of a platinum based catalyst for oxygen electroreduction where Vulcan XC-72 nanodispersed carbon black is used as a carrier. We have worked out 6 methods for preparing Pt (40 %)/XC-72 catalysts. To evaluate the activity of the prepared catalysts and to determine the kinetic parameters of the oxygen reduction on them, cathodic stationary polarization curves were recorded in a 0.5 M H2SO4 solution on a floating gas diffusion electrode superficially modified by the catalysts. For preparation of all electrocatalysts we are used 3.6 ml of a hexachloroplatinic acid (H2PtCl6) solution, containing 18.8 mg Pt/ml, 100 ml of ethylene glycol or its mixture with deionized water (3 : 1) and 100 mg of Vulcan XC-72 carbon black, previously oxidized in HNO3 solution. 1 M NaOH or KOH solutions were used to create the necessary medium (pH = 11). Some Pt(40 %)/XC-72 electrocatalysts were prepared when additions of polyvinylpyrrolidone (PVP) (100 mg or 300 mg) and/or 60 ml of formaldehyde were introduced into reaction mixture. Temperature during syntheses was in the range 75 - 160 °C. It was shown that reducing the synthesis temperature from 160 to 75 °C and introduction of PVP together with formaldehyde into reaction mixture had a positive effect on the activity of obtained nanocomposite platinum-containing catalysts Pt(40 %)/XC-72 for oxygen electroreduction.The sizes of the obtained platinum clusters on the surface of carbon black nanoparticles were in the range of 1.7 - 5.7 nm, and under optimal synthesis conditions they were evenly and densely distributed on the surface of the carbon carrier. The obtained kinetic characteristics of oxygen electroreduction on the studied catalysts indicate that the mechanism of this process depended on both the current density and the method of catalyst synthesis.

OXYGEN REDUCTION ON THE SURFACE OF Fe3O4&Au NANOCOMPOSITES

Functional materials based on ferromagnetics and noble metals are attractive due to their unique optical, catalytic and magnetic properties. Nowadays these materials find their application in various fields of science and technology: medicine, biotechnology, chemistry, physics and energy sector. The aim of the present work is to study the electrochemical characteristics of the oxygen reduction on the surface of Fe3O4&Au nanocomposites. The formation of the composite particles took place on the surface of a rotating steel disk which was in contact with HAuCl4 aqueous solution and air. Initial gold concentration was varied from 0.2 to 10.0 mg/l. Oxygen reduction was studied on a floating gas diffusion electrode (pressed carbon black P803 + 30 % PTFE) with surface, modified by prepared Fe3O4&Au nanocomposites, in 1 M KOH solution at 20 ºC. Electrochemical measurements were carried out using a PI-50-1.1 potentiostat.
 Tafel slopes of the stationary polarization curves ∂E/∂lgj of oxygen reduction on the electrode modified by composites formed at с(Au3+) from 0.2 to 5.0 mg/l lay in the ranges: b1 = 0.048 – 0.060 V, b2 = 0.119 – 0.131 V. When nanocomposites were obtained at с(Au3+) from 7.0 to 10 mg/l these values were: b1 = 0.042 –0.061 V, b2 = 0.079 – 0.105 V. It was concluded that oxygen reduction mechanism is multistep and includes formation of the hydrogen peroxide as intermediate. Obtained b1 and b2 values indicate that electrode process me-chanism changes when current density grows.
 The largest exchange current (8.51∙10–3 A/g) was achieved when electrode was modified by composite formed at с(Au3+) = 1.0 mg/l, whereas when the composite formed at с(Au3+) = 10.0 mg/l was used, the j0 value was smallest. Perhaps, the increase in the initial Au3+ concentration during nanocomposite formation led to an increase in the number of separate gold clusters on the surface of Fe3O4 nanoparticles and to raise of composite catalytic activity. But when с(Au3+) exceeded 1.0 mg/l the core-shell Fe3O4&Au composites with compact gold shell were formed. Such coreshall composites had lower catalytic activity in the oxygen reduction than composites with separate gold clusters on the surface of Fe3O4 particles.

Electrochemical characterization of the liquid aluminium bipolar electrode for extraction of noble metals from spent catalysts

Regularities of the cathodic and the anodic behaviour of aluminium on tungsten in halide melts are investigated. The study of such systems is relevant in connection with the prospects for their use as bipolar electrodes for efficient extraction of noble metals from spent petrochemical Al2O3-based catalysts by the electrometallurgical method with associated production of high-purity aluminium and oxygen. Stationary and non-stationary polarization curves were obtained for the reduction and the oxidation of aluminium in molten NaCl – KCl + 5 wt.%AlF3 and 1.3KF – AlF3 systems at 700–850 oC (973–1123 K) in the range of current densities from 0.01 to 1.5 A·cm–2 and in the range of potential sweep rates from 0.01 to 5 V·s–1. Electrochemical characterization of the electrode showed that electroactive particles in chloride-based melts have higher diffusion coefficients (in comparison with fluoride melt) due to the high concentration of Na(I) and K(I) cations, low density and viscosity of the melt. Electrochemical dissolution and reduction of aluminium in both chloride-based and fluoride melts are quasi-reversible diffusion-controlled processes complicated by several parallel phenomena. Cathodic reduction of Al(III) in both studied melts occurs at high overvoltages (more than 200 mV at 0.1 A·cm–2) with the limiting current densities of 0.3–0.4 A·cm–2 at 800 oC. The cathodic reduction of Na(I) starts in chloride-based melts at the potential of –0.5 – (–0.7) V relative to Al reference electrode; the reduction of K(I) in both studied melts starts at –1.0 – (–1.2) V. In general, the kinetic parameters of the processes show that the extraction in two-sectioned cell can be beneficially performed at 800 oC with high current densities, and the liquid aluminium bipolar electrode seems to be the promising solution.

Corrosion Inhibition of Mild Steel in 0.5 M H2SO4 Solution by Artemisia herba-alba Oil

Artemisia herba-alba oil (AHAO) was obtained by hydrodistillation of the aerial parts of the plant and analyzed by GC and GC–MS. AHAO was tested as corrosion inhibitor of mild steel in 0.5 M H2SO4 solution using weight loss measurement and the stationary polarization curves. Results obtained indicate that the corrosion rate is reduced and AHAO adsorbs on the metal surface and then inhibits corrosion process. The highest inhibiting efficacy reached 88% at 298 K at 2.76 g L−1. The influence of the temperature (303–343 K) on the inhibition efficacy at different concentrations of AHAO was investigated. Thermodynamic data for AHAO inhibitor adsorption and mild steel corrosion led to suggest the occurrence of comprehensive adsorption physical adsorption for the inhibitor species on mild steel.

Galvanic displacement and electrochemical leaching for synthesizing Pd-Ag catalysts highly active in FAOR

The galvanic displacement (GD) of a silver electrodeposit on gold by palladium (displacing agent – Pd2+) is carried out. The resulting composite Pd0(Ag)in is subjected to potential cycling in the supporting electrolyte (0.5М Н2SO4) solution (1–3cycles) in the Е interval 0.09–1.4V (RHE). All samples are characterized by SEM, XPS, AES-ICP, and CVA methods. The GD fails to produce a Pd shell, which is explained by the fact that Ag adatoms are incorporated into the surface layer on the top of Pd0 and cannot be substituted by Pd2+. Potential cycling of the initial deposit Pd0(Ag)in results in preferential dissolution of Ag; however, no sufficiently dense Pd shell is formed. Stationary polarization curves at Е<250mV (vs RHE) in 0.5М Н2SO4+0.5М НСOОН solution reveal the very high specific activity for all Pd0(Ag) composites (per cm2 of EASA) in FAOR, 5–10 times exceeding the activity of electrodeposited (e.d.) Pd. The promotion of Pd by Ag additions was explained by both the inhibition of formation of chemisorbed species and the development of new active sites at the Pd/Ag boundary.

Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC).

The performance of a molten carbonate electrolysis cell (MCEC) is to a great extent determined by the anode, i.e. the oxygen production reaction at the porous NiO electrode. In this study, stationary polarization curves for the NiO electrode were measured under varying gas compositions and temperatures. The exchange current densities were calculated numerically from the slopes at low overpotential. Positive dependency on the exchange current density was found for the partial pressure of oxygen. When the temperature was increased in the range 600-650 °C, the reaction order of oxygen decreased from 0.97 to 0.80. However, there are two different cases for the partial pressure dependency of carbon dioxide within this temperature range: positive values, 0.09-0.30, for the reaction order at lower CO2 concentration, and negative values, -0.26-0.01, with increasing CO2 content. A comparison of theoretically obtained data indicates that the oxygen-producing reaction in MCEC could be reasonably satisfied by the reverse of oxygen reduction by the oxygen mechanism I, an n = 4 electron reaction, assuming a low coverage of oxide ions at high CO2 content and an intermediate coverage for a low CO2 concentration.

Nanostructured catalysts for oxygen electroreduction based on bimetallic monoethanolamine complexes of Co(III) and Ni(II)

Oxygen reduction electrocatalysts based on the monoethanolmine complexes {(CoEtm)2(l-Etm)4Ni(NO3)2} and {(CoEtm)2(l-Etm)4Ni(NO3)2} ? activated carbon AG-3 have been obtained by high-temperature synthesis. The nature of active centers on the synthesized electro- catalysts was described. Using potentiostatic and cyclic potentiodynamic voltammetry, the kinetic characteristics of catalysts in the oxygen electroreduction reaction were determined. Thermal decomposition of the thermally unstable complexes was described and character of the active centers formed was discussed. The optimal synthesis temperature of electrocatalysts is 600 C in an inert atmosphere. The calculated exchange current densities for the oxygen electroreduction reaction at the catalysts in 1 M KOH at 20 C was j0 = 1.01 9 10 -3 Ag -1 -3.3 9 10 -3 Ag -1 . The Tafel slopes of stationary polarization curves are 0.054-0.063 V for b1 and 0.106-0.125 V for b2. The prepared electrocatalysts can be recommended only for electrochemical systems with alkaline electrolyte.

The Electrodeposition of Lead in LiCl-KCl-PbCl2 and LiCl-KCl-PbCl2-PbO Melts

The mechanism for the electrode process that occurs on a molybdenum substrate in LiCl-KCl-PbCl2 and LiCl-KCl-PbCl2-PbO melts depending on the concentration of PbO was investigated over the temperature range of 723–823 K using stationary polarization curves and voltammetry techniques. An increase in the concentration of PbO was found to decrease the diffusion-limiting current density for the recovery of lead, which was identified by the appearance of an additional peak in the voltammograms; it can be explained by a decrease in the concentration of Pb2+ ions decrease due to the formation of Pb2O2+ complex ions in the electrolyte. The evaluation of the equilibrium constant KE and the rate constant of the reaction suggests that this reaction is strongly shifted toward the formation of the Pb2O2+ complex ions. A theoretical polarization curve for the diffusion kinetics was calculated.

Optimization of platinum/iridium ratio in thin sputtered films for PEMFC cathodes

The paper presents an investigation on the electrocatalytic activity of co-sputtered PtIr thin films towards the ORR. The catalyst composition was controlled by varying the sputtering power of the Ir target in the range 20–100W at constant power of 100W for the Pt target. The films were analyzed applying SEM, XRD, EDX, XPS (physical characterization); stationary and quasi-stationary polarization curves, cyclic voltammetry and ac impedance (“in situ” tests performed in single PEMFC). The influence of the Pt/Ir ratio on the atomic interaction, morphology, and the resulting electrocatalytic activity was studied in detail. The contribution of the film content, morphology and the Pt/Ir electrocatalytic synergism to the electrode performance was evaluated. The optimal balance between the favourable effect of Ir addition and the lost of active surface sites due to substitution of Pt for Ir was established.

Influence of tartaric acid on zinc electrodeposition from sulphate bath

The effect of tartaric acid on zinc electrodeposition from sulphate plating bath was investigated by electrochemical impedance spectroscopy (EIS), stationary polarization curves, X-ray diffraction and SEM imagery. The study shows that it is possible to obtain homogeneous, compact and dendrites-free zinc deposits from sulphate solutions containing tartaric acid. For various hydrodynamic methods (rotating disc electrode and static vertical or horizontal electrode), the results indicate that, the presence of only small quantity of tartaric acid, may induce significant changes on deposition rates and deposit quality.

Electrodeposition of Zn in acid sulphate solutions: pH effects

The kinetics of zinc electrodeposition from acid sulphate solution on a platinum electrode was investigated by means of stationary polarization curves, interfacial pH measurement and electrochemical impedance spectroscopy. The effect of pH, namely pH 2, 3 and 4, was analyzed. A significant dependence of Zn electrodeposition with solution pH was verified. The results obtained cannot be predicted by the available models for Zn electrodeposition. A reaction model is then proposed based on the predominant steps as a function of the potential and the electrode surface nature.

ChemInform Abstract: The Role of Molybdenum in the Active-Passive Transition of Iron- Chromium Alloys

Abstract In order to clarify the role of Mo on the passivation of FeCrMo alloys, the critical current density in stationary polarization curves of several alloys, with and without Mo, are compared with the maximum current density in potential step measurements. The results showed that Mo increases the dissolution rate of the alloys, but the increase is compensated by the faster formation of the passive film, thus increasing the resistance against corrosion of the FeCr alloys.

Experimental determination of effective surface area and conductivities in the porous anode of molten carbonate fuel cell

Stationary polarization curves and electrochemical impedance spectroscopy of a porous nickel anode in a molten carbonate fuel cell were obtained in order to determine the active surface area and conductivities with varying degree of electrolyte filling for two anode feed-gas compositions, one simulating operation with steam reformed natural gas and the other one gasified coal. The active surface area for coal gas is reduced by around 70–80% compared to the standard gas composition in the case of Li/Na carbonate. Moreover, an optimal degree of electrolyte filling was shifted toward higher filling degree in the case of operation with coal gas. In order to evaluate the experimental data a one-dimensional model was used. The reaction rate at the matrix/electrode interface is about five times higher than the average reaction rate in the whole electrode in case of 10% electrolyte filling. This result suggests that the lower limit of the filling degree of the anode should be around 15% in order to avoid non-uniform distribution of the reaction in the electrode. Therefore, in the case of applying Li/Na carbonate in the MCFC, an electrolyte distribution model taking into account the wetting properties of the electrode is required in order to set an optimal electrolyte filling degree in the electrode.

Electrochemical characterization of porous nickel–cobalt oxide electrodes

The electrochemical characterization of mixed oxides containing cobalt and nickel prepared by cathodic electrodeposition is presented. Their catalytic properties are discussed according to the results obtained employing stationary polarization curves, voltammetry, electrochemical impedance spectroscopy, and scanning electron microscopy. Impedance results were analyzed considering a porous electrode and approximated in terms of a finite transmission line model of conical pores linked in parallel. For sake of comparison, results obtained with Co–Ni cobaltites prepared by thermal decomposition are also presented.

Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell

The electrochemical performance of the porous nickel anode in a molten carbonate fuel cell was experimentally investigated in this study. The electrode structure was studied by scanning electron microscope and Hg porosimetry. The effect of electrolyte filling, on overpotential at a constant current density, was also examined. Kinetics for the hydrogen reaction was studied by obtaining stationary polarization curves, for porous nickel anodes at varying temperatures and anode gas compositions. The slopes of these polarization curves were analyzed at low overpotentials, with the assumption that the porous anode was under kinetic control, and the corresponding exchange current densities were determined using a simplified porous electrode model. The obtained partial pressure dependencies of the exchange current density were high, and therefore, difficult to explain by the generally assumed mechanisms. © 2001 The Electrochemical Society. All rights reserved.

Chromic acid regeneration process with fuel cell electrode assistance. Part II: Electrochemical characterization, material compatibility and energy consumption

Electrochemical characteristics, energy consumption and material compatibility aspects of a novel process for regeneration of spent hard chromium plating baths using a Nafion-117 separator and a fuel cell cathode are discussed. Electrochemical impedance spectroscopy (EIS) and stationary polarization curves were used to characterize the performance of a fuel cell cathode in the regeneration cell. The configuration of the MEA, current collector, and flow distributing backing plate may, during long-term operation, lead to excessive ohmic resistance, which necessitates a special design of the cathode assembly. X-ray diffraction indicated that Cu, Fe, Ni, and Cr were deposited on the electrode matrix, leading to deactivation of the Pt-catalyst. The deactivation causes a rising cell voltage during electrolysis. Nevertheless, the energy consumption of the regeneration cell is at least 1 V less than that of a comparable cell with hydrogen evolving cathode.

Corrosion resistance and electrocatalytic activity of some refractory compounds of transition metals

The corrosion resistance of some metal-like compounds in H2SO4 of concentration 9.7 M is studied. It is established that the composition TaC is the most corrosion-resistant. The method of stationary polarization curves is used to study the reaction of electric oxidation of hydrogen on sintered and thin-film TaC electrodes in H2SO4. It is shown that under the given conditions TaC is a catalyst of the reaction of oxidation of hydrogen.

Morphological and electrochemical characterization of Ni–polyvinylchloride composites

Morphological characteristics and electrochemical behaviour of nickel– Polyvinylchloride (Ni–PVC) composite were studied in alkaline solutions. It was found that prolonged cathodic polarization and continue potential cycling on samples seriously alters the surface morphology, surface composition and adsorption properties of the composite samples. This was confirmed by SEM, EDS, IR spectroscopy and osmometry. Oxygen evolution reaction (oer) was also studied on fresh and pre-activated samples by stationary polarization curves.