Apparent Exchange Current Density Research Articles

Kinetics of the surface reaction H ad ↔ OH ad on n-Ge(1 1 1)

Different surface states (GeOH and GeH) were found on n-Germanium. These two surface states can be controlled by potential (GeH at U<−0.2 V, GeOH at U>0.1 V in 0.5 M H 2SO 4). The surface reaction H ad↔OH ad is reversible and can be described by: (1) Ge H+ p ++ H 2 O↔ Ge OH+2 H ++ e − Measurements were carried out on n-Ge(1 1 1). Starting from GeH, anodic potentiostatic pulse experiments yield the kinetics of the surface reaction (1). Fast surface states, e.g. radicals were formed at relatively short times ( t<10 ms) due to the breaking of GeH-bonds. Then, OH-groups were generated and a GeOH monolayer was formed at polarization potentials U P>0.1 V and longer times ( t>10 ms). The shift of flat band potentials yields a contribution Δ φ Dip of OH dipoles to the total potential drop Δ φ at the interface. Reaction (1) was also studied by cathodic pulse experiments starting from GeOH. As a result, the kinetics of the surface reaction were obtained in dependence on U and θ. Reactions B and E are diffusion controlled, while surface reactions C and F are transfer controlled at low current densities. For θ H= θ OH=0.5, i.e. Δφ Dip=const., Tafel lines with b(C)=83 and b(F)=87 mV are obtained. The correspondent transfer coefficients α(C)=0.7 and α(F)=0.3 indicate that they refer to different elementary reactions. Hence, an apparent equilibrium potential U 0=0.01 V, and apparent exchange current density i 0=0.1 μA cm −2 can be estimated only from Tafel-plots.

A new approach to metal electrodeposition at a periodically changing rate

Abstract A method of increasing the apparent deposition exchange current density value in reversing overpotential (RO) copper electrodeposition is described. In pulsating overpotential (PO) metal electrodeposition, the current density during the ‘off’ period is anodic, and proportional to the exchange current density value. The current during the ‘off’ period in PO copper electrodeposition can be increased by appropriate anodic polarization. This method is referred to as the RO method to distinguish it from the pulsating method. In order to maintain the average current density in this case, it is necessary to increase the cathodic current density during the ‘on’ period. Hence, the current density during the anodic period can be considered as proportional to the exchange current density for some other deposition process, characterized by a larger exchange current density value. The apparent deposition process exchange current density can be increased easily by using different current densities during the anodic period, and different morphologies for the same average current density can be expected. In this way, previously unknown morphologies of copper electrodeposits in the overpotential region used were obtained as spongy and needle-like forms.

Structure investigations of SOFC anode cermets. Part I: Porosity investigations

The number of electrolyte/metal/pore three-phase boundaries in the SOFC anode cermet was determined in a cross-section photo according to the intercepted-segment method and given as the volume concentration of the electrochemically active phase boundary. The apparent geometrical exchange current density of hydrogen oxidation served as a relativizing measure of electrochemical activity. A direct correlation was found between the number of optically visible three-phase boundaries of ‘electrolyte/nickel agglomerates/gas pores’ in the unit volume of the electrode and the apparent electrochemical exchange current density. The electrode was also analysed porosimetrically by a standard porosimetry method (MSP) and the porosimetric curves were compared with the optical/electrochemical measurements of the same electrode. This comparison revealed that the sum of the integrated volume of nickel agglomerates and the single nickel particles in contact with the electrolyte give the total amount of the active nickel volume whose surface is directly proportional to the amount of optically visible three-phase boundaries in the unit volume.

Mathematical Modeling of Oxygen Exchange and Transport in Air‐Perovskite‐Yttria‐Stabilized Zirconia Interface Regions: II. Direct Exchange of Oxygen Vacancies

The transport of oxygen in a porous perovskite solid oxide fuel cell cathode with a relatively high oxygen ion conductivity is modeled by taking into account exchange kinetics at the gas/electrode interface, bulk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interface. The electrochemical mechanism is assumed to be controlled by direct exchange of oxygen vacancies between the cathode and electrolyte phases. Simulated polarization curves typically exhibit Tafel‐like behavior in the cathodic direction, which, however, is caused by concentration rather than activation polarization. In the anodic direction, a limiting current behavior is predicted, due to occupation of oxygen lattice sites on the cathode side of the interface. The effective polarization resistance either decreases or remains constant upon reduction of the oxygen partial pressures depending on prevailing kinetic and material parameters. Analytical expressions valid for the asymptotic case of a fast oxygen adsorption process at the gas/electrode interface are derived for the apparent Tafel slope, apparent exchange current density, anodic limiting current, and the effective polarization resistance. The theoretical results are consistent with experimental data in the literature for dense perovskite electrodes and for porous electrode materials with high oxygen nonstoichiometries. An overall assessment of the two parts of this study indicates that the catalytic properties of the perovskite surface, which enhances adsorption and surface diffusion of oxygen, is more significant than processes involving the bulk material, such as fast oxygen exchange with the bulk and vacancy diffusion, in determining cathode performance.

Electrocatalytic activity of nickel black electrodes for the hydrogen evolution reaction in alkaline solutions

The electrocatalytic activity of nickel black electrodes for the hydrogen evolution reaction (HER) in alkaline solutions was studied. Three types of electrode were obtained through electrodeposition from different plating baths and their electrocatalytic properties were evaluated through slow potentiodynamic sweeps. The apparent and real exchange current densities and the Tafel slopes were calculated and compared to those of Raney nickel electrodes. The importance of the superficial topography on the apparent electrocatalytic characteristics was emphasized and some evidence was given to demonstrate that, when the electrode is previously aged in alkaline solution, the HER is produced on a thin nickel hydroxide overlayer.

The Faradaic—reactive impedance for a sinusoidal electrode

The faradaic—reactive impedance for a sinusoidal-shaped electrode is determined from a perturbation solution to the Laplace equation in which the ratio of the amplitude to period length ( A) is the perturbation parameter. Terms in the expansion up to and including fourth order in A are included, but A must be less than 1/2π for convergence. A formula for the impedance minus its infinite frequency limit is presented for the reference electrode far removed from the electrode surface. The Nyquist diagram of the imaginary versus the real component of the impedance for the sinusoidally rough surface is shown to be always perpendicular to real axis at both low and high frequency and is nearly semicircular with a diameter which, at a given amplitude, decreases asymptotically for low exchange current density but increases asymptotically for high exchange current density. At a given exchange current density, the diameter may decrease or increase with amplitude depending upon whether the exchange current density is low or high, respectively. If the Nyquist diagram for the faradaic—reactive impedance of the sinusoidal-shaped electrode is (incorrectly) interpreted as that from a planar electrode, the resulting apparent exchange current density and double-layer capacitance both differ from the actual values, and a means to correct the measurements to obtained the intrinsic values is given. The results show that for an irreversible reaction the apparent transfer coefficient determined from the slope of a plot of the inverse of the low frequency limit of the impedance versus the dc bias voltage may be nonconstant and vary with A. The impedance for an ideally blocking sinusoidal-shaped electrode is also presented. At low and high frequencies the phase lag is −π/2 and the modulus is inversely proportional to frequency, with a proportionality constant which is smaller at low frequency and is A dependent for all frequency; however, over a frequency range which is also A dependent, a deviation from the pseudo-planar behavior is found with the phase lag less than −π/2.

Gas Electrode Reactions in Molten Carbonate Media: IV . Electrode Kinetics and Mechanism of Hydrogen Oxidation in Eutectic

The hydrogen oxidation reaction in mole percent at 650°C has been studied at Au, Ag, Cu, Pt, Ir, Pd, Ni, Co, and Fe electrodes by using cyclic voltammetry, ac impedance, potential step, and chronocoulometric methods. Impedance data were analyzed by computer fitting with an equivalent circuit, taking hydrogen adsorption into account. The exchange current density decreased, under the gas composition of , in the following series: . The chronocoulometric method was employed to determine the stoichiometric number ν using a new technique described in this paper, and the value obtained was . The apparent transfer coefficient, α, and exchange current density were also determined from an Allen‐Hickling plot, and found to be for Au, Ir, and Ag, and for Ni and Pt. The concentrations and diffusion coefficients of the reactant and the product species, including the diffusion process of hydrogen dissolved in the Ni lattice, were estimated, based on an estimated value for from the Wilke‐Chang equation. Based on these results, a detailed reaction mechanism is proposed in this paper.

Electrical response of oxygen sensing TiO 2 surfaces and fractal Pt/YSZ interfaces

Titania and YSZ single crystals with 9.4 mol% Y 2 O 3 have been investigated by impedance, voltage relaxation and current-voltage studies. In the case of TiO 2 separation of the surface boundary layer conductivity from the bulk conductivity yields a first indication of rather different P O2 dependencies in the two domains. For high current densities and CO 2/O 2 gas mixtures the apparent exchange current density of YSZ is enhance by up to a factor of 40 in the low temperature regime. At T⩾ 750° C the CO 2 effect vanishes and the exchange gets independent of gas composition and temperature. A similar continuous change is observed in the impedance of Pt/O 2/YSZ under blocking conditions. The fractal description of the systems proved to be unsatisfactory Especially, there is a disagreement with the Nyikos and Pajkossy model of the impedance of fractal blocking electrodes. It appears that the morphology of the platinum is not the unique parameter determining the interface properties.

Apparent ion-exchange current densities at valinomycin-based potassium ion-selective PVC membranes obtained with an AC-impedance method

The complex ac-impedance spectra of thin (< 70 μm) valinomycin-based potassium-selective PVC membranes symmetrically bathed in KCl, NaCl and LiCl solutions of different concentrations, have been measured with high resolution from 0.01 Hz to 10 kHz. With the exception of those KCl solutions > 10 −4 M, all impedance spectra obtained showed a more or less pronounced and stable second semicircle at medium frequencies, which could be identified as the double-layer capacitance in parallel to a charge-transfer resistance of the corresponding ion equilibrating across the interface PVC membrane/aqueous solution. Using the formalism originally derived for redox reactions at metallic electrodes, apparent standard ion-exchange current densities and consistent charge-transfer coefficients could be obtained with high reproducibility. The apparent standard exchange current density of K + at a valinomycin-based PVC membrane with tetraphenylborate as an additive, lies in the range of about 25 mA cm −2 which is the highest value reported so far for these membranes. A strong correlation was found between the potentiometricall y observed ion selectivity expressed by the selectivity coefficient used in an extended Nernst equation and the apparent exchange current densities of the corresponding ions.

On electrochemical behaviour of zinc in organic solvents

The electrochemical behaviour of Zn has been investigated in DMSO, DMF and AN by potentiostatic and voltammetric methods. The values of α n and β n equal to 0.5 and 1 show that the charge-discharge of Zn 2+ in these solvents proceeds as a two-step reaction. The values of apparent exchange current densities depend on the employed measurement techniques. The assumption about the passivation of Zn electrode by the products of secondary chemical reaction has been formulated.

Experimental Studies of Flow‐Through Porous‐Graphite Chlorine Electrode

The kinetics and mass transfer behavior of the chlorine flow‐through graphite electrode have been studied in solution at 25°C. Discharge cathodic polarizations of the chlorine electrode indicate that the limiting current density depends on the flow rate of the electrolyte, in agreement with the analytical solution for flow‐through under limiting current. Furthermore, the limiting current increases significantly after the activation process, due to the increase in the specific surface area of the graphite. Analysis of limiting current data shows that the macro area exposed to flow increases typically from 20 to 35 cm−1 and is accompanied by an increase in permeability. Attempts to measure the exchange current density for chlorine on graphite indicates that different results are obtained for different porous graphites. The apparent exchange current density is 10−3 A/cm2 and on porous graphite and dense graphite, respectively. The true exchange current density on pyrolytic graphite is measured to be . The electrochemical specific area for porous graphite is estimated from the ratio of the exchange current densities of porous and pyrolytic graphite to be 22 cm−1, in good agreement with the macro area exposed to flow.

Electrochemical studies on ion-selective polymer membrane electrodes

The selectivity and response of neutral carrier based polymer membrane electrodes are investigated via exchange current measurements for systems containing valinomycin, dibenzo-18-crown-6, and plasticizer alone in a polyvinyl chloride matrix. Using a transient galvanostatic step method, apparent exchange current densities of 1.3 X 10 −3 A/cm 2, 5.4 X 10 −6 A/cm 2, and 2.2 X 10 −9 A/cm 2 were obtained with K + as the primary ion for the three types of membranes, respectively. Preliminary results indicate that the exchange current data obtained with this technique are complementary to the potentiometric response observed for the membranes studied.

Sinusoidal voltage modulation of a copper rotating disk electrode

An experimental and theoretical study has been made of the sinusoidal potential modulation of a copper rotating disk electrode in acidic copper sulfate solution. The mathematical model takes into account the fluctuations of the surface concentration, the effect of the double capacitance, the surface overpotential, and the ohmic resistance in the electrolyte. Numerical computations were performed to show the effect of alternating voltage (av) on the shape of direct current polarization curves, the changes in the apparent rest potential, the apparent exchange current density and the apparent Tafel slopes. The agreement between the theory and the experiment was within ±15%. At sufficiently high av frequencies, the model can be further used to predict the av modulation of a rotating hemispherical electrode.

The Kinetic Parameters of the Fe ( CN ) 63 − / 4 − Redox System: New Results with the Ring Electrode in Turbulent Pipe Flow

A theoretical analysis of a ring electrode in turbulent pipe flow is given in order to estimate the limits of resolution of a given experimental setup. Special attention is paid to error analysis, which is necessary for the correct evaluation of the data. To demonstrate the potentialities of this method for kinetic studies, the apparent exchange current density of the ferroferricyanide system was measured. The values obtained are higher than those reported in the literature.

An Admittance Study of the Tin Electrode

Voltammetric and admittance measurements for tin deposition at a rotating disk electrode from sulfuric and fluoboric acid Sn++ solutions have been analyzed. Apparent exchange current densities ca. 10 A/cm2 are reduced 100‐fold upon addition of polyethylene glycol. With or without the additive, the conductance representing the exchange process varies with potential directly as the surface Sn++ concentration. Localized three‐dimensional concentration polarization zones in the electrolyte phase arising from discrete growth sites form the preferred interpretation, although an alternative kinetic process occurring prior to charge transfer cannot be excluded as the rate‐limiting step.

ChemInform Abstract: MODIFICATION OF APPARENT ELECTROCATALYSIS FOR ANODIC CHLORINE EVOLUTION ON ELECTROCHEMICALLY CONDITIONED OXIDE FILMS AT IRIDIUM ANODES

AbstractKinetische Untersuchungen der Cl2‐Entwicklung an Ir‐Anoden, auf denen cyclisch voltammetrisch Oxidfilme mit Dicken von einer Monoschicht aufwärts gebildet wurden, zeigen, daß die scheinbaren Austauschstromdichten an solchen Filmen mit zunehmender Oxiddichte fast linear zunehmen, obwohl der Mechanismus der Cl2‐Entwicklung unabhängig von der Dicke des gebildeten Oxidfilms ist.

Modification of Apparent Electrocatalysis for Anodic Chlorine Evolution on Electrochemically Conditioned Oxide Films at Iridium Anodes

Studies of evolution kinetics on iridium anodes on which oxide films of various thicknesses from a monolayer upwards were formed by a potential cycling program, are reported. The multilayer oxide films were of the kind that exhibit reversible reduction/oxidation behavior. The kinetic experiments conducted in aq. at pH 2 give log [current] vs. overpotential relations having a constant Tafel slope of for all anode oxide surfaces prepared, indicating that the evolution mechanism is independent of the thickness of the oxide film produced. However, the apparent exchange current densities increase in an almost linear way with thickness. The latter behavior is attributed to an increase in the real area of the electrode available for Cl−discharge, coupled possibly with improved electrocatalysis, e.g., on account of changes that can arise in the band structure of the oxide. Increases of evolution rates of up to ca. 180× are found. Effects of Cl− adsorption on the development of the initial monolayer of oxide of Ir were also studied and compared with the behavior at Pt. Cl− adsorption is found to inhibit the formation of the thick oxide films at Ir.

Influence of crystal surface orientation on redox reactions at semiconducting MoS 2

The redox system Cu(NH 3) 2+/1+ x , Fe(CN) 3− 4− 6 and Fe 3+/2+ and oxygen reduction have been studied on MoS 2 surfaces normal to the C-axis and parallel to the C-axis, using rotating disc electrodes under potentiostatic conditions. Current—voltage curves were obtained and apparent exchange current densities ( i 0) and rate constants ( k 0 app) have been evaluated. The i 0 and k o app values are one order of magnitude larger for the |- C surfaces than for the ⊥- C surfaces of MoS 2. Both forward and reverse reactions occurred readily in the case of Cu(NH 3) 2+/1+ x whereas the reverse reaction was partly or totally blocked in the case of Fe(CN) 3− 4− 6 and Fe 3+ 2+ , respectively. The results indicate that surface states and crystal imperfections play a very important role for all these redox reactions. These surface states seem to be connected with Mo-atoms at edges and steps on the surface where they are directly exposed to the electrolyte. The differences in the electron transfer rate between the two surfaces is attributed to the greater overlapping of the electron orbitals of the conduction band at the |- C surface of MoS 2 with the electron orbitals of the redox systems, than of those at the ⊥- C surface.

Interfacial Phenomena at Solid Aluminum Electrodes in Solutions of AlBr3 / KBr in Aromatic Hydrocarbons

Solutions of and in toluene, mesitylene, or their mixture have been prepared under high purity conditions on a vacuum line. Anodic dissolution and cathodic deposition at a pure aluminum electrode were studied. Special attention was focused on the mechanism of rupture of the oxide film initially present on the surface and the formation of pits under the surface. The apparent exchange current density was found to depend on the anodic current density employed to activate the surface. The current density was found to be a linear function of potential over ca. 0.2V on both sides of the reversible potential. This was interpreted as being due to the solution resistance inside small flaws (pinholes) in the oxide layer on the surface.

トリフルオロ酢酸ニッケル‐塩化アンモニウム‐エタノール浴からのニッケル電析

This paper describes the electrodeposition of nickel from Ni(CF3COO)2-NH4Cl-MeOH solutions for studying the electrodeposition mechanism of nickel. Bright and smooth nickel deposits were obtained from Ni(CF3COO)2 500g/l-NH4Cl20.0g/l-MeOH bath under current density of 2.0-22.0Amp/dm2 at 30-50°C. The current efficiencies in this case were about 100% for both of cathode and anode. The rate controlling step of electrodeposition of nickel from Ni(CF3COO)2-NH4Cl 20.0g/l-MeOH bath under current density of 1.0-5.8Amp/dm2 was a charge transfer reaction. The main values in these experiments were as follows: Apparent transfer coefficient: αc=0.75, Apparent valence: n=0.78, Apparent exchange current density: ioc=0.23Amp/dm2, Tafel slope: bc=-0.12V, Apparent activation energy: Ec=5.7kcal/mol.