Oxygen concentration polarization

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

Precipitation of inorganic compounds from seawater. The value of calcareous deposits in the effective and efficient operation of marine cathodic protection systems is generally recognized by corrosion engineers. The calcareous films are known to form on cathodic metal surfaces in seawater, thereby enhancing oxygen concentration polarization and reducing the current density needed to maintain a prescribed cathodic potential. For most cathodic surfaces in aerated waters, the principal reduction reaction is described by Eq. (2.16) ... 

In order to find the relationship between the stationary concentration of current carriers in semiconductor film and concentration of dissolved oxygen in polar liquid, it is essential to examine the expression for rate of chemisorbtion of dissolved oxygen molecules on ZnO film and its chemical desorbtion from the surface under effect of solvent... 

At room temperature, unsaturated vapours of the above specified polar and nonpolar liquids do not influence considerably the rate of adsorption and chemical activity of not only adsorbed oxygen layers, but also of acceptors of semiconductor electrons of another type, namely, of alkyl radicals [54]. This is seen from the electric conductivity of ZnO films with adsorbed alkyl radicals or oxygen being invariable in the atmosphere of the saturated vapours of the above specified solvents. In the case of oxygen, this can be also seen from the fact that the oxygen concentration features no decrease. 

This approach was coupled to a system of three NAD+-dependent enzymes comprised of alcohol dehydrogenase (EC 1.1.1.1), aldehyde dehydrogenase (EC 1.2.1.3), and formate dehydrogenase (EC 1.2.1.2) to create an electrode theoretically capable of complete oxidation of methanol to carbon dioxide, as shown in Eigure 5. The anode was, in turn, coupled to a platinum-catalyzed oxygen cathode to produce a complete fuel cell operating at pH 7.5. With no externally applied convection, the cell produced power densities of 0.67 mW/cm at 0.49 V for periods of less than 1 min, before the onset of concentration polarization. 

The oxygen concentration gradient and polarity gradient seem to be related. It is likely that the oxygen concentration gradient is created by the existence of the polarity gradient. More work is needed to establish the relationship. 

Variation of lateral mobility (fluidity) and polarity gradient have been reasonably established in certain calcium alkarylsulfonate micelles using 1-pyrene carboxylic acid probes with varying alkyl chain lengths. The existence of a polarity gradient in an inverted micelle may be responsible for the creation of an oxygen concentration gradient. 

If we Investigate the Influence of the solvent, It Is necessary to take Into account the nature of the species In the medium. In non-polar solvent we have aggregated speclesi when we add tetramethylenediamine (THEDA) we destroy the aggregates, and we have an equilibrium between Ion pairs and complexed species.In polar solvent, we have another equilibrium between the Ion pairs, the solvated Ion pairs and loose Ion pairs.11 If we compare the oxidation results (Table IV), we observe that for similar oxygen concentration, the coupling yield and the hydroperoxide functionality Increase like the lonlclty of the C-Ll linkage. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Typical curves are shown in Fig. 22 a and b for current and potential steps respectively. For the short times (trate determining step is due to a concentration polarization [78], The variation of the transient current versus ]fi is linear, described satisfactorily by the classical Cottrel-type relation (see Fig. 23). After a certain time which depends on the experimental conditions, mainly the oxygen partial pressure in the surrounding atmosphere, a depolarization effect is observed the V (t) curve shows an overshoot while the I(t) curve shows a net enhancement of current. 

Electrochemical pumps have also been successfully used to promote reactions catalytically. If oxygen is pumped through a zirconia cell into a reaction chamber which is, e.g., filled with hydrocarbons, not only the oxygen that is transferred, reacts. The anode itself can act catalytically. It seems that this so-called NEMCA effect ( Nonfaradaic Electrochemical Modification of Catalytic Activity 51) relies on a hindered surface reaction as a consequence of which the applied potential is translated into a concentration polarization, as reflected by the enrichment of a not fully oxidized oxygen species (e.g., O or O2") at the interface.51"53 For this species, redistribution equilibrium may be assumed that... 

The fluorescence lifetime and polarization of an extrinsic probe bound tightly to a spherical protein have been measured at different oxygen concentrations. The following data are obtained ... 

Although the effects of concentration polarization are not being considered in this discussion, it should be noted that the concentration of oxygen in air saturated solutions is very low and that the cathodic reaction may be subject to pronounced concentration polarization (46). 

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