Apparent current efficiency

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

Paired electrooxidation is feasible, allowing apparent current efficiencies of 200% and higher. 

The same relative oxidation power was found for the above methods in the degradation of all aromatics, as deduced from the percentages of TOC removal after 3 h of electrolysis of solutions with lOOmgdm-3 TOC of 4-CPA, MCPA, 2,4-D, and 2,4,5-T at pH 3.0 and 100 mA given in Table 19.2. All initial chlorine of these compounds was released as Cl-, which remained stable in solution. The apparent current efficiency (ACE) for these trials was then calculated from the following equation ... 

Table 19.2 Percentage of TOC removal and apparent current efficiency (ACE) determined after 3 h of treatment of 100 cm3 of solutions of chlorophenoxyacetic acid herbicides with a concentration equivalent to 100 mg dm 3 TOC in 0.05 M Na2S04 + H2SO4 of pH 3.0 by different indirect electro-oxidation methods under comparable conditions at 100 mA ...

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

There are several reasons for apparent deviations from Faraday s laws, and why current efficiencies of less than 100% are encountered. Among these, mention may be made of the following ... 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

The behavior along CPE varies according to the metal. The current density along CD remains fairly constant for metals such as iron, chromium and stainless steel as the potential increases the metal continues to dissolve at 100 percent efficiency. Along DE the metals dissolve in their higher valence state but at low current efficiency, since evolution of oxygen, or other anodic reaction where possible, now occurs. The film which forms on these metals is apparently very thin and readily allows the transitions of electrons from the anion to the metal by conduction or the tunnel effect. 

During the electrolytic preparation of composite cathodes from solutions of Ni or Co salts with molybdate or tungstate, the current efficiency for deposition of the two metals is far from 1(X)%, so cathodic Hj evolution, with codeposition (sorption) of the H intermediate, is unavoidable. Hence it is virtually certain that these composite cathode materials are formed as hydride materials. It was suggested in Ref. (75) that this may be one of the reasons for their excellent electrocatalytic behavior in the HER, in contrast to that of bulk, thermally prepared alloys of the same metals, Ni and Mo. In this respect, hydrided metals may behave like Pt cathodes where the HER proceeds with good electrocatalysis on a full monolayer of UPD H and, under appreciable applied current densities, on a Pt surface region containing apparently some significant quantity of three-dimensionally sorbed H (136). 

Fig. 4. Faradaic efficiencies of CH4 and C2H4 vs. time for CO2 reduction at an activated Cu electrode (0.28 cm ) in 0.5 M KHCO3 at 22°C E = -2V. Dotted line shows evolution of the apparent current density during 8 days-long electrolysis nm.

Simultaneous electrode reactions. Current efficiency If, as is frequently the case, several electrochemical reactions occur simultaneously at an electrode, Faraday s law will be found to hold only if the total number of equivalents which have entered into reaction arc used in the computation. Failure to include all the reactions at the electrode will thus result in an apparent deviation from the law. The ratio of the number of equivalents of a single electrode product to the total possible number computed by Faraday s law is called the current efficiency with respect to the electrochemical reaction in question. For instance, in the electrodeposition of zinc from an aqueous solution of one of its salts, hydrogen is always evolved. The ratio of the number of equivalents of zinc deposited to the total number of chemical equivalents (zinc and hydrogen) is the current efficiency of the deposition of zinc. Thus, as we have seen, a current efficiency of less than 100 per cent does not indicate a failure in the application of the law, but only that all the electrochemical reactions have not been included in the computation. 

Permselectivity of counter-ions through the ion exchange membrane depends on the fixed ion concentration of the membrane (Chapter 2.3). Many attempts have been made to increase the fixed ion concentration of the membrane to increase the ion exchange capacity and to decrease the water content of the membrane, namely, to increase the fixed ion concentration without increasing the electrical resistance of the membrane. Figure 4.8 shows an example of the relationship between current efficiency to produce sodium hydroxide and the fixed ion concentration of the membrane for the electrolysis of sodium chloride solution.17 It is apparent that the current efficiency increases with increasing fixed ion concentration of the membrane. 

After 2 hours or less, solid potassium permanganate should start to crystallize out. This effect will be manifest by an apparent sharp drop in the current efficiency as found by titration, After 2 hours, shut off the current temporarily, remove the anode and clean it (it will be found to be coated with manganese dioxide), and restore the alkalinity of the solution by adding 1 mole of solid potassium hydroxide for each faraday of electricity passed. If this is not done, potassium bicarbonate will crystallize along with the permanganate. 

Seventeen of the reporting plants operated electrolytic cells, and data for the electrolysis process are given in Tables XIV and XV. Figure 6 illustrates the relationship between cell power consumption and current density. The current efficiency for electrolysis is compared with the average current density in Figure 7. No correlation between the two variables is apparent. The electrolyte concentrations of the plants are reported in Table XVI, and the electrolyte additives are shown in Table XVII. 

The prevailing type of charge carriers in the film should affect considerably the apparent character of the inner electrochemical process. Should these carriers be alkali metal cations, their discharge at the inner boundary would be preferable. Thus, the formation of alkali metal is expected to be the side process in such systems resulting in the low current efficiency of the targeted process of the reduction of a polyvalent metal species. Evidently, that accounts for higher current efficiency of elementary silicon electrodepositiOTi from potassium halide baths relatively to the electrolytes based on sodium halides. 

The current efficiency for this scheme is about 85% at an apparent current density of about 2 mA cm in laboratory cells. This is a clean process that has lowered the chlorate level from 3.6 to 0.01 gL . The process seems to have economic potential but has not been optimized for commercial application. 

Figure 6.39 Galvanostatic polarization of Ni in the transpassive potential region in sodium nitrate solution (a) anodic polarization curve in the transpassive potential region (b) the current efficiency for metal dissolution and (c) the apparent thickness of the film as measured by coulometry [36].

The apparent valence of Mg in anodic dissolution at values less than two can also be reflected by the low anodic dissolution current efficiency. Theoretically, each mole of Mg atoms after dissolving into Mg ions will... 

A number of factors will determine whether or not an electrolysis will proceed with 100 per cent current efficiency. Certainly the presence of any electro-active impurities will cause the apparent electrolysis current to be in error so that the total number of coulombs passed will exceed the theoretical quantity of electricity required for the actual oxidation or reduction of the material to be determined. In most cases it should be possible to improve the purity of solvents, reagents, and other constituents sufficiently by pre-electrolysis or chemical means. Alternatively, the applied potential might be shifted in such a way as to minimize the effect of impurities at the expense of some increase in electrolysis time. Naturally, oxygen and other atmospheric contaminants should be eliminated by flushing with an inert gas. 

Note 1. In some runs the current dropped to a low level soon after the requisite number of coulomibs had passed for about a 3-electron reduction of the uranium present. In other oases, the current did not decrease, but discontinuance of the run beyond any point where twice the theoretical current had pabsed gave satisfactory uranium recovery. In the latter oases, a gray ether-insoluble, but alcohol-soluble precipitate (apparently a mercury cupferrate), was usually evident in the aiiueous phase. The current efficiency for the desired process appeared to be good In most runs. 

Typical polarization curves for alkaline fuel cells are shown in Fig, 27-63, It is apparent that the all aline fuel cell can operate at about 0,9 and 5()() rnA/cnr current density. This corresponds to an energy conversion efficiency of about 60 percent IIII, The space shuttle orbiter powder module consists of three separate units, each measuring 0,35 by 0,38 by I rn (14 by 15 by 40 in), weighing 119 kg (262 lb), and generating 15 kW of powder. The powder density is about 100 W/L and the specific powder, 100 W/kg,... 

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