Electrodes passivation

Passivation of an electrode with respect to a certain electrochemical reaction is the term used for the strong hindrance experienced under certain conditions by a reaction which under other conditions (in the electrode s active state) will occur without hindrance at this electrode. Passivation of metals imphes the hindrance frequently observed with respect to anodic metal dissolution. 

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

J.E. Malinsky, G.E. Jabbour, S.E. Shaheen, J.D. Anderson, A.G. Richter, T.J. Marks, N.R. Armstrong, B.K. Pulak Dutta, and N. Peyghambarian, Self-assembly processes for organic LED electrode passivation and charge injection balance, Adv. Mater., 11 227-231, 1999. 

Other Models. In addition to Besenhard s model, the other models were mainly modifications developed from the original Peled s concept for lithium electrode passivation, with surface reaction as the major process, and emphasis was placed upon the composition and structure of the precipitated film or the interaction between the precipitated products and the bulk electrolyte components. 

On the basis of the results from XPS studies by Kanamura and co-workers that the SEI has a multilayered structure,Peled and co-workers modified their lithium electrode passivation model to include carbonaceous anodes and proposed a so-called mosaic model to describe the SEI structure on the anode, as Figure 15a shows.According to this model, multiple reductive decompositions occur between the negatively charged anode surface and the various electrolyte components simultaneously, depositing a mixture of insoluble products on the anode. This heteropolymicrophase SEI consists of many microregions that are of entirely different chemical... 

An important feature of the positive electrode discharge concerns the nature of the PbS04 deposit since the formation of dense, coherent layers can lead to rapid electrode passivation. Lead dioxide exists in two crystalline forms, rhombic (a-) and tetragonal (/3-), both of which are present in freshly formed electrode structures. Since PbS04 and a-Pb02 are iso morphic, crystals of lead dioxide of this modification tend to become rapidly covered and isolated by lead sulphate, and their utilization is less... 

If (23) is selected as the dihalosilane, a convenient way of modifying the nickel surface is available.64 The electrochemical properties of the treated nickel electrode are very similar to those of a similarly derivatized platinum electrode for example, both are equally effective in the elec-trocatalytic oxidation-reduction of solution ferrocene. Normally oxidation of the nickel surface would be a competing process ultimately rendering the electrode passive. The surface modification clearly eliminates this problem and opens up the possibility of using surface modified inexpensive metals as electrodes. 

Gap between electrodes (passive mixing) 800 pm Hydrophobic layer thickness 50 nm... 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

LijPOyFz, and Li BO F types. The latter two species result from partial hydrolysis of the BF3 or PF5 species (which may also be present in these salt solutions) with trace water, followed by electrochemical reduction in the presence of Li+. d. It should be emphasized that a critical parameter for the nature of the surface films formed on nonactive electrodes and the properties of the electrode passivation due to these surface films is the ratio between the electrode surface and the solution volume. The lower this ratio, the more pronounced is the rate of the above secondary reactions between the surface species initially formed and contaminants such as H20 and HF. 

Apart from a technical focus on quantifying interactions between the foundation technologies and an economic focus on the relative cost of electrocoagulation, future research also needs to examine reliable means of reducing electrode passivation. 

Direct electrochemical reduction of oxidized nicotinamide cofactors is not useful because of the formation of dimers via intermediate radicals. On the other hand, direct electrochemical oxidation of NAD(P)H to NAD(P)+ can be performed successfully [90]. However, it requires relatively high oxidation potentials and may result in electrode passivation. 

Electrode surface must be resistant to erosion by sonication. Electrode passivation is reduced by cavitational cleaning. 

Avoidance of electrode passivation (nonconducting polymer formation on the electrode)... 

Despite its beneficial effects on electroanalytical techniques, which include avoiding electrode passivation, enhancing mass transport and current intensity, and the ability to modify process kinetics, US assistance has not yet gained widespread acceptance in routine analytical laboratories [132,133]. 

The large three-dimensional structure of Hb, the resulting inaccessibihty of the heme centre, and the subsequent electrode passivation due to protein adsorption make it difficult to obtain DET between Hb and electrodes. Numerous efforts have been made to improve the electron transfer characteristics by using mediators or promoters [163]. The most efficient way is to modify preferentially carboneous electrodes with polymeric and membrane forming films, for example, surfactants [38], clay [57], and composite films of surfactant and bentonite. The redox potential of immobilized haemoglobin was determined between - 100 and - 380 mV vs. SCE. 

Oxidation of arenes under nonaqueous conditions often results in polymerization [93], which has been observed for most of the simple arenes, such as benzene [94-97], naphthalene [98], pyrene [99], biphenyl [96], triphenylene [99], fluoranthene [99], and fluor-ene [99-101]. (See Chapter 32 for details.) Polymerization is often accompanied by severe electrode passivation [100]. 

X 10 to 1.4 X 10 cm s , see in all cases the calculated value is always greater by 10-20 times than the experimental one. This is perhaps due to the electrode passivation that cuts off part of the electrode surface from the process. The effect of passivation on the electrode reactions of solvated electrons is discussed in detail in Section 7. Here we shall only mention that passivation does not alter the limiting diffusion current. The different effect of passivation on diffusion and kinetic currents is related to the fact that at the employed rotation rates the thickness of diffusion layer (that determines ij) far exceeds that of the reaction layer (that determines iJ... 

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