In current efficiency

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

Electrolysis in molten salts obeys Faraday s laws, although the demonstration of their validity is sometimes very difficult, as mentioned earlier. In fact, often during the electrolysis of molten electrolytes there are considerable and not readily avoidable losses in the current efficiency. Some of the causes of such losses are (i) evaporation or distillation of metal separated in the molten state (ii) secondary reactions between the separated molten metal and the materials with which it comes into contact and (iii) the solubility of the metal in the electrolyte. The latter cause appears to be the main one leading to a loss in current efficiency. 

The limiting current density may be defined as the current density at which the depression of the Na+ concentration at the interface of the membrane s sulphonic and carboxylic layers results in an abrupt rise in cell voltage and drop in current efficiency. 

The circulation conditions obtained in the small laboratory cell cannot be attained in a full-scale cell. The effects of Na+ diffusion to the membrane and non-uniformity in its intracell concentration cannot be entirely eliminated, and a greater decrease in current efficiency will tend to occur at high current densities. 

AGC has modified the carboxylic polymer and prepared one experimental membrane. This membrane is similar to F-8934 in the arrangement of the sub-structure, and it is almost the same as the F-8934 in both mechanical strength and ohmic resistance. Figure 19.13 illustrates the current efficiency trend of this experimental membrane in a laboratory test run at 8 kA irf2, compared with the F-8934 tested under the same conditions. The absolute value of the membrane s current efficiency is approximately 97.5%. No decline in current efficiency has been observed. AGC is now evaluating the stability and is optimising the carboxylic polymer feature and fabrication process for commercial production of this type of membrane. 

Atobe and Nonaka [67] have used a 20 kHz (titanium-alloy) sonic horn as the electrode (called sonoelectrode) for electroreductions of various benzaldehyde derivatives. This they did after insulating the submerged metal part of the horn-barrel with heat-shrink plastic. They found an improvement in current efficiency with insonation, but in addition noted some change in product selectivity towards one-electron-per-mole-cule products. Although the authors quote enhanced mass transfer across the electrode interface as the origin of the sonoelectrochemical trend towards products from the lesser amount of electrons per substrate molecule, the involvement of surface species on the reactive electrode provides a complication. 

On a large scale, it is more difficult to maintain constant electrode potential and conditions of constant current are employed. Under these conditions, as the concentration of the substrate falls, the voltage across the cell rises in order to maintain the imposed reaction rate at the electrode surface. This causes a drop in current efficiency towards the end of the reaction, since as the working electrode potential rises, either oxygen or hydrogen evolution becomes significant. 

The electrochemical reduction of CO2 to form hydrocarbons has been studied on several electrodes. The production of methane at both ruthenium and Cu electrodes has been reported. For ruthenium, electrolysis of CO2 in aqueous solutions containing 0.1 M H2SO4 resulted in current efficiencies as high as 20% for methane production at 65 [12,... 

The complete reduction of coal proceeds from aromatic rings through rings containing olefinic double bonds to saturated compounds. While reduction of the benzene ring takes place at a current efficiency of about 80% (8), current efficiency for reducing an olefin (1-decene) is only 27% as shown in this paper. The slow step in the coal reduction may very well be reduction of olefinic double bonds. An increase in the rate of olefinic double bond reduction may therefore lead to a considerable increase in current efficiency. 

Effect of Proton Donor. The increase in current efficiency on adding a proton donor is consistent with the work reported by Krapcho and Bothner-By (3) and Krapcho and Nadel (4). These authors showed that the rate of reduction of the benzene ring and of olefins by alkali metals in liquid ammonia and amines is proportional to the concentration of alkali metal, substrate, and proton donor. 

The 41% increase in current efficiency on reducing 1-decene achieved by adding an equimolar amount of butyl alcohol may prove significant in connection with the reduction of coal. It has been shown previously (6) that current efficiency for coal reduction is about 15%. An increase of 41% in current efficiency would represent a substantial saving in time and current. 

Based on results of electrochemical reductions of tetralin in ethylenediamine, current efficiency is highest with aluminum as cathode material and with lithium chloride as electrolyte. A substantial increase in current efficiency was obtained in reducing 1-decene by adding a proton donor. 

The principal reason for the loss in current efficiency (CE) in aluminum electrolysis is the metal reoxidation by the anode gas, according to reaction (87). 

The current efficiency would be 100% if the recombination reaction of sodium with chlorine did not occur. Although the diaphragm prevents sodium droplets formed at the cathodes to react with the chlorine bubbles formed at the anodes, another phenomenon causes the decrease in current efficiency. Thus, the slight solubility of sodium in the melt causes the melt to become a partial electronic conductor (see Section I.B.4 Electrical Conductivity of Metal-Molten Salt Mixtures ). This electronic conductivity and the recombination reaction of sodium and chlorine dissolved in the electrolyte decrease the current efficiency. 

These values show that lithium and sodium are at the negative potential limit of the electrochemical window (-2 V) (see Figure 49), close to the reduction potential of the imidazolium cation to neutral radical. Therefore, there is a competition between these processes with a resulting decrease in current efficiency. But Reichel and Wilkes [454], Campbell and Johnson [468], Scordilis-Kelley and Carlin [467,469] and Gray et al. [470] showed that an extension of the electrochemical window to -2.4 V is obtained by the addition of HC1 to the AICI3-MEIC neutral melt buffered with NaCl or LiCl. Under these conditions, plating and stripping of sodium and lithium occurs at inert electrodes in room temperature chloroaluminate molten salts. The effect of HC1 addition disappears quickly because of evaporation. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Fig. 13.12 Variations in current efficiency and energy consumption for the electrochemical HDH of 20 mM DCP in paraffin oil at different current densities. Other conditions are as in Fig. 13.7...

Of course, losses in current efficiency can originate from the cathodic reduction of CIO- (see Sect. 5.2.5.1) and of ClOjT already generated in the electrode compartment... 

The current efficiency in modern cells of aluminum electrolysis may exceed 95%. It is generally accepted that the major part of loss in current efficiency is due to the reaction between dissolved metal and electrolyte. Model studies by 0degard et al. (1988) indicates that sodium dissolves in the electrolyte in the form of free Na, while dissolved Al is predominantly present as the monovalent species ALF. Any electronic conductivity is most likely associated with the Na species, which may form trapped electrons and electrons in the conduction band. Morris (1975) ascribed the loss in current efficiency during Al production to electronic conduction. In a theoretical and experimental study. Dewing and Yoshida (1976) subsequently maintained that the electronic conductivity was too low to account for the loss in current efficiency in industrial aluminum cells. However, the existence of electronic conduction in NaF-AlF3 melts was demonstrated later by Borisoglebskii et al. (1978) also. 

Electrocatalysis at metal electrodes in aqueous (1.2) and non-aqueous ( ) solvents, phthalocyanine ( ) and ruthenium ( ) coated carbon, n-type semiconductors (6.7.8),and photocathodes (9,10) have been explored in an effort to develop effective catalysts for the synthesis of reduced products from carbon dioxide. The electrocatalytic and photocatalytic approaches have high faradaic efficiency of carbon dioxide reduction (1,6). but very low current densities. Hence the rate of product formation is low. Increasing current densities to provide meaningful amounts of product, substantially reduces carbon dioxide reduction in favor of hydrogen evolution. This reduction in current efficiency is a difficult problem to surmount in light of the probable electrostatic repulsion of carbon dioxide, or the aqueous bicarbonate ion, from a negatively charged cathode (11,12). 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Influence of Electrolysis Conditions. Among the various electrolysis conditions, brine purity has the most significant effect on the life of the membranes. The presence of a small amount of multivalent cations leads to formation of metal hydroxide deposits in the membrane, and thus causes a decrease in current efficiency, an increase in cell voltage, and damage to the polymer structure of the membrane. With perfluorocarboxylic acid membrane, the presence of more than 1 ppm of calcium ion will begin to cause these problems in a very short period (1 - 8). To obtain stable current efficiency and cell voltage, it is therefore essential to establish effective brine purification methods. 

The same experiment was carried out several times with different pieces of Nafion 390 cut from a large sheet all data showed similar trends in current efficiency and electro-osmotic water with increasing current density. However, results from each membrane were nonidentical, presumably because of macroscopic inhomogeneities. 

Figure 2.1 Change in current efficiency (transport number) with concentration of sodium hydroxide in catholyte. (1) Perfluorocarbon membrane NEOSEPTA-F C-1000 (2) Perfluorocarbon membrane NEOSEPTA-F C-2000. Electrolysis at 20 A dm 2 at 80 °C using 3.5 N sodium chloride solution as anolyte and sodium hydroxide solution of various concentrations as catholyte.

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