Effect of the Electrode Material

In summary, all these local measurements demonstrate the power of microelectrodes i) to determine local bulk conductivities in ionic solids and ii) to study an important phenomenon in solid state ionics, namely the occurrence of nonstoichiometry profiles in mixed conducting solids. 

A quantitative analysis of grain boundary impedances measured with macroscopic electrodes can be rather problematic if grain boundary properties vary from boundary to boundary (cf. Sec. 3.2). Hence, additional information on the distribution of grain boundary resistivities is often desired. Microelectrode measurements can yield such additional information (Sec. 4.2) and below a microcontact impedance spectroscopic study of grain boundaries in a polycrystal is exemplarily presented. The material of choice is again SrTiCE (0.2 mol % Fe-doped), which represents a model material for the technologically highly important class of perovskite-type titanates (see also above). 

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TABLE 1. Macroelectrolytes of (4-aminophenyl) phenyl sulphone (I) and (2-aminophenyl) phenyl sulphone (II) showing the effect of the electrode material and the influence of the ortho substituent ... 

The electrode potential exerts an important influence upon the course of the reduction, but there is undoubtedly a catalytic effect as well the latter is particularly evident with cathodes of copper or platinum. Lob4 and Haber8 consider that electrode potential is the determining factor, but Tafel6 emphasises the importance of the catalytic effect of the electrode material. Undoubtedly the product... 

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. 

The effect of a surface passivation layer simultaneously generated with the decomposition of the RTlLs, with respect to the effect of the electrode material on the cathodic or anodic limiting potential. 

The catalytic effect of the electrode material on the branching of an electrode reaction is most often apparent at cathodes with low overvoltage but may also be observed at materials with high hydrogen overvoltage. In the reduction of acetone in 6 N H2SO4 at a platinized platinum electrode, two independent paths are accessible One leads to propane and the other to isopropyl alcohol. The rate of formation of these two products depends on the voltage and on the history of the electrode. 

Some examples of the effect of the electrode material on the branching of reactions are the reduction of benzyltriethylammonium nitrate in dimenthylformamide (DMF) to bibenzyl or toluene at aluminum or platinum electrodes [115] and the reduction of aliphatic aldehydes [116] or alkyl arylketones [117] at different materials. 

In any event the detailed mechanism must adequately explain the 1/2 in the slope of the Tafel plot and the effect of the electrode material on the overvoltage. In strongly acidic solution, even if Reaction 13 was the primary step, hydrated electrons would be rapidly transformed to H-atoms by Reaction 3 prior to the generation of H2. 

There are basically two methods of approach to the problem of the elucidation of the effect of the electrode material on the reaction rate, one leading to the potential of zero charge (pzc), the other a constant potential, E, with respect to a fixed reference electrode 57-60) as a standard for comparison. 

In order to determine the effect of the electrode material on the degree of cleaning of a dust-covered surface, experiments were performed with a field strength of 18 kV/cm, using a continuous electrode made of aluminum, copper, brass, or sheet metal the following results were obtained for removal of dust from surfaces made of these same metals ... 

Data are listed below to illustrate the effect of the electrode material on the spot diameter, with a voltage of 2.5 kV, a frequency of 400 Hz, a needle diameter of 0.35 mm, and a treating time of 10 sec ... 

The principal theories of the mechanism of the Kolbe electrosynthesis, as discussed earlier, have been criticized. A satisfactory summary of these has been presented by Dickinson and Wynne-Jones. The main points of disagreement among these theories are, (a) whether the hydroxyl ion or the carboxylate ion is discharged at the anode, (b) the explanation of the effect of the electrode material on the reaction, and (c) the reason for the occurrence of the Kolbe synthesis in preference to oxygen evolution which could occur at lower anode potentials. Recent experimental investigations, particularly those of Dickinson and Wynne-Jones and of Conway and Dzieciuch permit a detailed analysis of the mechanism of the Kolbe reaction, in both aqueous and nonaqueous media. 

Scialdone O, Galia A, Eilardo G (2008) Electrochemical incineration of 1,2-dichloroethane effect of the electrode material. Electrochim Acta 53 7220-7225... 

The electrochemical oxidation of glucose has been reviewed (49 refs.). Special attention was given to the effect of the electrode material on the process, and to the adsorption of glucose and its oxidation products on the electrode surface. The oxidation of glucose with single-crystal platinum electrodes in different orientations has been studied. Pronounced stmctural sensitivity of the reaction was discovered, the Pt(lll) and its vicinal surfaces being the most active. ... 

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