Current -overpotential

Solution of this model was used to determine lateral distributions of overpotential, current density and concentrations of products in the SPBER, and to simulate the effect of important operation parameters, such as gas sparing rate and applied... 

Current distribution Overpotential-current relationship E = f(i) Polarization dE/di resistance term Wagner number... 

One of the most important relations in mechanism determination is the current density-overpotential relation. At overpotentials greater than BT/aF, the rate of the reverse reaction may be neglected and the general expression for the overpotential-current density relation in the case of a cathodic reaction expressed by Eq. (41) reduces to... 

The overpotential-current density plot obtained in the electrooxidation of unsaturated hydrocarbons shows three distinct regions (i) the Tafel region which extends over two to three decades in current density with a slope of 2RTjF ii) the limiting current region and (Hi) the passivation region in which the current decreases with increase of potential (Fig. 17). 

Fig. 17. A typical overpotential-current density plot for the electrooxidation of an unsaturated hydrocarbon on a platinized Pt electrode at 80 (80).

Fio. 24. Effect of ultrasonic vibrations on the overpotential-current density relation for hydrogen evolution on a platinum electrode in 1 H28O4 at 26° 104). 

A theoretical analysis of the current distribution and overpotential-current density relations for two models of porous gas diffusion electrodes—the simple pore and thin film models—has been carried out (108,109). The results of the analysis for the simple pore model will be summarized here. The reactant gas diffuses through the pore to the gas-electrolyte interface at 2 = 0, where it dissolves in the electrolyte and the dissolved gas diffuses through the electrolyte to the various electrocatalytic sites along the pore at which the reaction occurs (Fig. 25). It is assumed that the first and second steps of diffusion of reactant gas through the electrolyte-free part of the pore (z < 0) and of dissolution of gas at the gas-electrolyte interface are fast. 

Metal oxide electrodes To obtain a thin layer, the oxide film can simply be formed by applying an anodic overpotential (current) to the surface. To form a thicker layer, a square-wave potential (current) may be helpful. Metal oxide electrodes can also be produced by chemical vapor deposition on a substrate, allowing greater control of the thickness. 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

The radius of a nucleation exclusion zone can be calculated on the basis of the following discussion, taking into account the charge transfer overpotential also. If there is a half-spherical nucleus on a flat electrode, the extent of the deviation in the shape of the equipotential surfaces which occurs around it depends on the crystallization overpotential, current density, a resistivity of the solution and on the radius of the nucleus, r. If the distance from the flat part of the substrate surface to the equipotential surface which corresponds to the critical nucleation overpotential, rj, is /n, then this changes around defect to the extent where A is a number, as is presented in Fig. 2.18. 

The relation between the current density at the electrode tip and in the homogeneous field can be estimated by considering the cathodic overpotential-current density and cell voltage-current density dependencies. According to Eq. (3.47), the tip overpotential is equal to the cell voltage for a wire electrode. 

The overpotential-current density and the cell voltage-current density plots for Cd deposition are presented in Fig. 3.14. The cathodic polarization curve obtained from potentiostatic polarization measurements has a similar shape to that found for an anode which can become passive above a certain overpotential, increasing the cathode polarization leads to a decrease in the cathodic current density followed by a range of potential in which the overpotential has little effect on the current density. Current oscillations were observed at the beginning of this plateau in some cases (see also section Organic compounds ). 

This is perhaps the singly most important technique it arises from the Stern-Geary equation which describes a linear overpotential - current relationship in the vicinity (typically + 20 mV) of the corrosion potential where the linear polarization resistance is ... 

Using this two-step model, which can be expanded to include other intermediate steps as well, a simple formulation for the electrode overpotential current relationship at a given electrode can be developed. If we assume that the reaction rate constants involved are independent of the surface coverage of reactant R, then we can derive the Langmuir kinetics model solution. With constant rate constants in Eq. (4.62), at steady state we can... 

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