Exchange Current Density Values Research Articles

Electrochemical properties of stabilized δ-Bi 2O 3. Oxygen pump properties of Bi 2O 3-Er 2O 3 solid solutions

The oxygen pumping characteristics of the solid electrolyte material (Bi 0.75Er 0.25) 2O 3 with gold and platinum electrocodes have been studied by three-electrode I–V measurements. These experiments show only little influence of the electrode material used on the electrode behavior. Exchange current density values are high and are confirmed by 18O 2 exchange data on the bare electrolyte. This leads to the conclusion that (Bi 0.75Er 0.25) 2O 3 has good properties for oxygen pumping applications and is an electrochemically active material in oxygen transfer processes.

Kinetics of the electrochemical doping process of polypyrrole

The kinetic parameters of the oxidation (p-doping) process of polypyrrole in electrochemical cells containing a solution of lithium perchlorate in propylene carbonate have been determined by frequency response analysis and by galvanostatic pulse transients. On the basis of the results obtained with the two techniques, values of exchange current density of the process are reported and the diffusion of the ClO 4 − dopant ions in the polymer is evaluated and discussed.

The reorganization energy of the aqueous proton

The reorganization energy of the aqueous proton was determined by analysis of experimental values of exchange current densities for the proton discharge reaction on various metals including Pt. In acid electrolytes a value of 1.95 eV was determined from the kinetic and hydrogen binding energy data for Pt electrodes. Additional values derived from similar data on the discharge reaction at other metals were 1.96 eV, Pb; 2.06 eV, Zn; 2.23 eV, Ga; 2.11 eV, In; 1.87 eV, Au; 2.03, Ag; and 1.74 eV, Cu. The average value for λH +was 2.00 ± 0.12 eV, spanning twelve orders of magnitude in discharge rate. It is concluded that the adsorbed H on Hg and Cd is mobile with equilibrium H atom coverages of 2 × 10 −7 and 3 × 10 −8 respectively.

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.

ChemInform Abstract: Gas Electrode Reactions in Molten Carbonate Media. Part l. Exchange Current Density of Oxygen Reduction in (Li + K)CO3 Eutectic at 650 ° C.

AbstractThe exchange current density values (i0), obtained on smooth Au electrodes in the title eutectic under P(O2)/P(CO2) = O.9/O.l atmosphere using three different kinetic techniques ‐ the coulostatic (CS), the potential step (PS), and the impedance (AC) method ‐ are compared to each other.

Kinetic criteria for the mechanism of the hydrogen evolution reaction

Additional criteria for distinguishing between the Volmer—Heyrovský and the Volmer—Tafel reaction mechanisms for the hydrogen evolution reaction (HER) have been derived from a theoretical treatment of polarization curves under conditions of continuous surface renewal. From data published by Tomashov and Vershinina for the HER on Pd, Ni, Fe, Sn and Pb, estimates of hydrogen coverages and rate constants have been made. Approximate values of standard exchange current densities could thus be predicted for the two reaction mechanisms; these have been compared with experimental data. In order to reconcile the various kinetic criteria with each other, it has been proposed that the Volmer—Heyrovský reaction mechanisms is operative on the majority of metals. Alternative interpretations within the Volmer—Heyrovský mechanism have been presented for Tafel slopes in the region of −30 mV dec −1 and for the relative positions of volcano curves, which might otherwise have been considered indicative of the Volmer—Tafel reaction mechanism.

Fundamental aspects of plating technology II: Morphological aspects of metal electrodeposition from complex salt solutions

It is shown that the improved quality of electrodeposits obtained from ammonium complex salt solutions compared with deposits obtained from simple salt solutions is due to the decrease in the exchange current density value for the deposition process.

Effects of radial diffusion on the efficiency of porous flow-through electrodes

A model is proposed to evaluate the effect of radial diffusion on the efficiency of porous flow-through electrodes. The model includes the electrode parameters of porosity (θ), thickness (L) and equivalent pore radius (R), the superficial flow speed of the electrolyte (v) and the molecular diffusivity of the reactant in the electrolyte (D). The effects of these variables are incorporated in a dimensionless groupφ(L) =vR2/2DLθ which controls the conversion efficiency of the electrode. Satisfactory agreement was found between the model predictions and the experimental results of several authors on different electrode-electrolyte systems over a wide range of conditions. Implications of this work for design purposes are outlined. The effect of radial diffusion limitations on the voltage efficiency of the electrode is also discussed. It is shown that radial diffusion effects predominate under conditions of large values of pore radius, polarization and exchange current density, and small values of diffusivity.

Electrochemical and morphological studies of copper electrodeposits deposited onto a copper (111) substrate in the presence of halo-compounds of acetic acid

Halo-compounds of acetic acid present in small quantities in an acid copper sulphate bath have been found to modify the growth of copper deposits on a copper (111) substrate and to increase the cathodic polarization. At low d.c. densities a transition in the type of growth from triangular pyramids to twinned pyramids and finally to polycrystalline deposit was observed. At high d.c. densities triangular and hexagonal pyramids transformed to truncated hexagonal and twinned pyramids which subsequently changed to a polycrystalline deposit. These changes are attributed to adsorption of the halo-compounds of acetic acid on the substrate. The mechanism of the deposition process is discussed. Surface coverages of addition agents determined from exchange current density values were found to fit a Langmuir adsorption isotherm. The negative free energies of adsorption of the addition agents indicate strong adsorption and inhibition of the deposition process by halo-compounds of acetic acid.

Polarization Characteristics of a Cupric Chloride Cathode in a Lewis Acid Organic Electrolyte

The polarization characteristics of copper‐cupric chloride electrodes in Lewis acid‐type organic electrolytes, using propylene carbonate as an aprotic solvent, were studied. Two experimental techniques were used: a multireference polarization cell (MRP) and a rotating disk electrode (RDE). For the latter, a range of rotational velocities from 25 to 2000 rpm was covered. Following elimination or accounting for concentration polarization, the remaining polarization is considered to be attributable not only to charge transfer steps, but also to chemical steps such as a slow dissolution of the cupric chloride crystal in this electrode of the second kind. Apparent exchange current density values in the range of have been determined for the reduction of to copper.

Effect of adsorption of organic substances on theelectroreduction of cations

Summary The exchange current density values for the zinc amalgam electrode, Zn(Hg)/Zn 2+ in 5· 10 −2 N Na 2 SO 4 in both the presence and absence of surface active organic substances, n-valeric acid, diethyl ketone and n-amyl alcohol, were determined by the galvanostatic transient method. Similarly, the exchange current density values for the cadmium amalgam electrode in 5·10 −2 N Na 2 SO 4 were determined in both the absence and presence of n-amyl alcohol. The values were corrected for the changes in the Ψ 1 potential caused by the adsorption of the organic substance. The inhibition coefficient was calculated from the corrected exchange current density values for different concentrations of the organic substance, and the values compared with the corresponding adsorption isotherm.

Investigations on the Redox Characteristics and Hydrogen Evolution Efficiencies of Pd and Mo Deposited Pd Electrodes in Alkaline Electrolyte

Redox behaviors and hydrogen evolution efficiencies of Pd and Mo deposited Pd electrodes have been investigated in 30wt.% KOH electrolyte by cyclic voltammetry. Cyclic voltammograms of Pd electrode in between the potential range - 1.05 V to + 0.75 V showed two couples of redox peaks for the transformations of Pd(0) ←→ Pd(OH)2 and Pd(OH)2 ←→ PdOOH, an anodic peak for the desorption of diffusional hydrogen (dH) and hydrogen and oxygen evolutions at the terminal potential regions. Hydrogen evolution efficiency found decreased with time and then reached to a stable condition after 35 minutes. In presence of deposited Mo, the electrode stable condition appeared after 10 minutes only. Pd found predominates over deposited Mo. Both the Pd(0) ←Pd(OH)2 and Mo(0) ← Mo(OH)2 transformations appeared at the same potential value. Mo stopped the movement of hydrogen adsorption and absorption region of Pd electrode towards negative potential direction. It increased the hydrogen evolution efficiency of Pd electrode remarkably. At the potentials - 1.1 V, - 1.2 V, - 1.25 V and - 1.3 V, currents for Pd + Mo system found 1.44, 1.25, 1.20, 1.23 times higher than those of Pd electrode. Mo also showed hydrogen migratory role to the fraction of Pd surface covered by it. Hydrogen evolution reactions (HER) over Pd and Pd + Mo surfaces seemed followed similar mechanisms. Tafel plots for the HER for both the systems showed two Tafel regions. Exchange current density values (io) for the low and high overpotential regions of Pd + Mo system showed 2.85 times and 1.29 times higher values than those of Pd electrode. 
 
 Key words: Pd, Pd-Mo, Hydrogen evolution efficiency, Hydrogen evolution reactions (HER) 
 
 DOI: 10.3329.bjsir.v43i1.861 
 
 Bangladesh J. Sci. Ind. Res. 43(1), 103-116, 2008

Effect of Ethylene Diamine Tetra Acetic Acid (EDTA) on the Surface Reaction Behavior and the Hydrogen Evolution Efficiency of Ni-30at.%Cu Electrode in Alkaline Medium

The surface reaction characteristics, hydrogen evolution efficiency and hydrogen evolution reaction kinetics of Ni-30at.%Cu electrode have been investigated in 30wt.%KOH electrolyte in presence and absence of ethylene diamine tetra acetic acid (EDTA) at room temperature by cyclic voltammetric technique. Cyclic voltammogram of the electrode showed three couples of oxidation-reduction peaks in between the potential range - 1.1 to + 0.4 V. The peak couples were originated from the electrochemical transformations of Ni(0) ←→ Ni(OH)2, Cu(0) ←→ Cu(I) and Cu(I) ←→ Cu(OH)2. An interference of the adsorped hydrogen over Cu(0) surface on the transformation of Ni(OH) 2 → Ni(0) was found. It was realized that added EDTA forms Cu-EDTA complex over the electrode surface. With increasing the amount of EDTA, the thickness of Cu(OH) 2 layer gradually decreased. Addition of 0.04g, 0.08g and 0.12g EDTAincreased the hydrogen evolution efficiency of the electrode more than 6%, 12% and 17%. Tafel plots for both the systems showed two well-defined Tafel regions. 0.12g EDTA increased the exchange current density (i0) values 1.3 times for both the Tafel regions. The observed i0 value for the high overpotential (η) region is 1.8 and 2.9 times higher than those of Ni and Cu electrodes. The Tafel slope (b) values for both the low and high η regions are smaller than those observed for without EDTA system and also for the Ni electrode. From the achieved information, it can be concluded that addition of EDTA remarkably increased the hydrogen evolution efficiency of the Ni-30at.%Cu electrode. It seems that hydrogen evolution reactions (HER) followed like mechanisms in absence and presence of EDTA. Key words: Ni-30at.%Cu electrode, EDTA, Electrochemical transformation, Surface reactions, Hydrogen evolution, Hydrogen evolution reactions (HER)DOI: 10.3329/bjsir.v43i2.955 Bangladesh J. Sci. Ind. Res. 43(2), 145-158, 2008

Oxidation-Reduction Behaviours, Electrolysis Characteristics and Efficiency of Pd-50at.%Ni Electrode in Alkaline Electrolyte

Convinced on the necessity of finding effective electrodes for electrolyser- the oxidation-reduction behaviors, surface reaction kinetics, stability, electrolysis characteristics and efficiency of a Pd-50at.%Ni electrode have been investigated in 30wt.%KOH electrolyte at room temperature using cyclic voltammetry. Cyclic voltammogram of the electrode showed three couples of oxidation-reduction peaks in between the potential range - 1.0 to + 0.65 V. These peaks were found to be originated from the transformation of zero valent metal to their higher oxides in three consecutive steps and vice versa. The peak potentials, potential differences of the couples and the peak currents were found remarkably different from those of Pd and Ni electrodes. The surface reaction kinetics at Pd-Ni electrode followed the surface reaction trends similar to those of Pd and Ni electrodes. The apparent stability of the electrode was found good. The electrode showed 1.6 and 6.8 times higher oxygen evolution efficiencies than the Ni and Pd electrodes. Addition of Ni with Pd caused hydrogen evolution potentials to move to the negative direction. Tafel plot for the hydrogen evolution reactions (HER) showed two well-defined Tafel regions. Kinetic parameters, i.e., Tafel slope and exchange current density values for the low and high overpotential regions were found 127 mV/dec and 273 mV/dec, and 1.72 x 10-2 mA/cm2 and 1.52 mA/cm2, respectively. Observed slopes indicated its better efficiency over the Pd electrode. Journal of Bangladesh Academy of Sciences, Vol. 33, No. 1, 1-13, 2009 doi: 10.3329/jbas.v33i1.2945

Sensitivity limits for the current-step method for zinc and cadmium amalgam stripping

Constant-current stripping with potential—time readout indication is applied to zinc and cadmium amalgams In order that the anodic transition time for either of these elements be a correct indication of the amount of the element present in the system, the potential of the working electrode must be controlled by an electrochemical couple involving that element, and the electrode poising must be maintained while nearly 100% of that element is stripped electrically from the electrode. The exchange current densities for zinc and cadmium amalgam electrode systems are taken as measures of the electrode poising ability for these systems. It was found that the exchange current density for an electrode system must exceed 10 −9 A/cm 2 in order that the system poise an electrode long enough to give a measurable transition time plateau on the potential—time readout- This minimum exchange current density value increases when competitive electrode systems are present. Detection limits for zinc and cadmium in solution were calculated and depend on the size of the sample.

Square wave electrolysis IV. Results and discussion

AbstractThe cyclic potential‐step method and the cyclic current‐step method are applied to two Red/Ox systems. Values of the standard exchange current density and of the transfer coefficient of both the ferro‐ferri system in 1M H2SO4 and the ferrocyanide‐ferricyanide system in 1M KCl are determined. The results obtained by the two methods are in good agreement with each other. A comparison is made with data obtained by other methods, mentioned in the literature.