Faradaic yield

The reaction products often compromise a mixture of various substances. Moreover, the reduction of carbon dioxide in aqueous solutions in the cathodic potential region is always accompanied by hydrogen evolution. Hence, an important criterion that describes the reaction selectivity is the faradaic yield ri for each individual Mh organic reaction product. 

Faradaic yields and the composition of reaction products are influenced mainly by the nature of the cathode material. Metallic electrodes used in aqueous solutions can be classified into four groups according to the nature of the principal reaction product ... 

The reaction selechvity and faradaic yields are sensitive to the reaction conditions (e.g., the electrode potenhal, the solution composition, the presence of accidental impurities). This is the reason for discrepancies between the data on the composition of the reachon products reported by various authors. 

From a prachcal standpoint, formic acid or its salts are the least valuable reaction products. The energy content of formic acid upon its reverse oxidation to CO2 is insignificant, and its separation from the solutions is a labor-consuming process. At present, maximum effort goes into the search for conditions that would ensure purposeful (with high faradaic yields) synthesis of methanol, hydrocarbons, oxalic acid, and other valuable products. 

The faradaic yield of CO formation on group II metals strongly depends on the value of the electrode potential. On silver and gold at definite potentials, yields up to 90 to 100% can be achieved. On zinc also, high yields (80%) were reported. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Of great interest and importance are studies on carbon dioxide reduction on copper electrodes, performed primarily by Japanese scientists. Under certain conditions, formation of methane and ethylene with high faradaic yields (up to 90%) was observed. The efficiency and selectivity of this reaction depends very much on the purity and the state of the surface of the copper electrode. For this reason, many of the published results are contradictory. 

The monomer and lower oligomers are soluble in the electrolyte, but with increasing polymerization degree the solubility decreases. After attaining some critical value, an insoluble film is formed on the anode. Lower (soluble) oligomers can also diffuse from the electrode into the bulk of the electrolyte, hence the faradaic yield of electrochemical polymerization is, at least in the primary stages, substantially lower than 100 per cent. 

SWNTs, the stability of the (C, PF ) ionic compound should be lower than in flat graphite layers. Therefore, during the electrochemical intercalation a chemical de-intercalation (decomposition) may take place, which explains the low faradaic yield of the anodic intercalation. 

The anodic reaction used is an indirect oxidation of benzene by Ag(I)/Ag(II) as redox mediator, because of its high faradaic yield. The high yield of BQ of 84% (of the theoretical yield) compared to the yield of the direct oxidation on the Pb02 anode of 62% indicates that some mechanism to minimize side reactions such as formation of o-BQ is operative. The highest yields are achieved with AgC104, (cf. Table 2, Ref. [66]). The use of AgC104 excludes its application in large scale synthesis. 

U(III) species and a second three-electron reduction to give U(0) metal. The first reduction, U(IV)/U(III) couple, is elec-trochemically and chemically irreversible except in hexamethylphosphoramide at 298 K where the authors report full chemical reversibility on the voltammetric timescale. The second reduction process is electrochemically irreversible in all solvents and only in dimethylsulfone at 400 K was an anodic return wave associated with uranium metal stripping noted. Electrodeposition of uranium metal as small dendrites from CS2UCI6 starting material was achieved from molten dimethylsulfone at 400 K with 0.1 M LiCl as supporting electrolyte at a platinum cathode. The deposits of uranium and the absence of U CI3, UCI4, UO2, and UO3 were determined by X-ray diffraction. Faradaic yield was low at 17.8%, but the yield can be increased (55.7%) through use of a mercury pool cathode. 

When the electrochemical reaction is carried out in DMF solution19, in the presence of a zinc anode and at a low current intensity, the organozinc compounds CF3ZnBr and (CF3)2Zn are produced with faradaic yields higher than 100%. Interestingly, the lower the current density, the higher the faradaic yield. 

First, C02 reduction at metal electrodes in both aqueous and nonaqueous media, as well as in systems coupled with electron-mediating complexes are detailed. The faradaic efficiency of such a system can be used as a measure of efficiency and selectivity. For a specific, electrochemically generated product, the faradaic efficiency is the ratio of the actual and theoretical amounts of product formed within the same time interval, based on charge passed. An efficient and selective system will lead to a 100% faradaic yield for a single product in other words, all of the charge passed in the system has gone into the production of that product. 

In an aqueous C02-saturated Na2S04 electrolyte, using electroplated Ru electrodes, Frese and Leach observed faradaic efficiencies of up to 42% for methanol production at a temperature of 333 K at a potential of only -0.55 V (versus SCE) [54]. Faradaic yields of up to 30% were likewise obtained for methane. When Popic et al. examined Ru02 electrodes, either alone or with Cu and Cd adatoms [64], in 0.5 M NaHC03 at a potential of-0.8 V (versus SCE), they were able to reduce C02 to methanol with faradaic efficiencies of 17%, 41%, and 38% after 480 min of electrolysis for Ru02, Ru02/Cu, and Ru02/Cd electrodes, respectively. 

In general, few electrochemical systems have been shown to reduce C02 to higher-order alcohols, as most operate at fairly large overpotentials and produce low faradaic yields and selectivity. Details of these systems are provided elsewhere [42]. 

While the overpotentials are high for methane production, rather high current densities have also been achieved at Cu electrodes. For example, Cook et al. were able to reduce C02 to methane in aqueous 0.5 M KHC03 solutions at a current density of 38mAcm-2, with 33% faradaic efficiency [71], although the potential was -2.29V (versus SCE). Subsequently, it proved possible to increase the faradaic yields to 79% for methane and ethene together on Cu-coated glassy carbon electrodes from in situ Cu deposition, but in this case the potential was —2.0 V (versus SCE), in the same electrolyte with current densities of up to 25 mA cm 2. 

Daube et al. also reported the reduction of C02 at redox polymer-coated p-Si electrode containing a Pd catalyst [118], In an aqueous bicarbonate solution, formic acid was produced with 70% faradaic yield and a small overpotential. 

Both, Beley et al. and Petit et al. reported the reduction of C02 to CO using Ni(cyclam)2+ catalysts in aqueous solution at p-GaAs and p-GaP photoelectrodes [121-123]. Here, the faradaic yields for CO approached 100%, at potentials of —1.0V and -0.44V (versus SCE), respectively, although carbon deposits on the surfaces of the electrodes led eventually to a degradation of the system. 

Trifluoromethylzinc compounds can be prepared by electrolysis of CF3Br in DMF between a zinc anode and a stainless steel cathode.7 Faradaic yields are higher than 100%, thus indicating the occurrence of a chemical route (Equation 8.10) at the surface of the anode along with the electrochemical process (Equation 8.9). 

It was found, in keeping with this, that the lower the current density, the greater the chemical attack at the anode. For example, at the low 5 mA cm"2 current density the faradaic yield of the formation of trifluoromethylzinc compounds is higher than 300%. This chemical process only takes place during electrolysis. 

Spasojevic, M., Krstajic, N. and Jaksic, M. (1984), Electrocatalytic optimization of faradaic yields in the chlorate cell process. Surf. Technol., 21(1) 19-26. 

This results in high turbulence and good faradaic yield, up to 65% have been reported, a typical run reduced the copper content of a wastewater by almost two orders of magnitude. 

Fig. 5. Faradaic yields for C2H4 and CH4 recorded together with cathodic current during electrolysis of CO2 at a Zr02-modified, periodically activated, Cu electrode (0.28 cm ) in 0.5MK2SO4at5°C E = -1.8 V.

Because of a relatively small CO2 solubility in water under atmospheric pressure (0.033 mol/dm at 15°C), the reduction process meets limitations due to the slow transport of the reactant to the electrode. Such limitations appear clearly for an activated Cu electrode where a continuous increase in the cathodic current during prolonged electrolysis runs (cf Fig. 4) leads finally to a decrease of the faradaic yield for hydrocarbons. To overcome this problem, several authors conducted electrolysis experiments at elevated CO2 pressures. 

Fig. 7 Faradaic yields of hydrocarbons (CH4 and C2H4) obtained during electrolyses of CO2 (20 atm) in 0.5 M KHCO3 at an activated, respectively, non-activated copper electrode 22°C, E = -2.85 V.

Fig. 8. Effect of the Cu electrode potential upon faradaic yields of hydrocarbons, formed during electrolyses of CO2 under increased, 20 atm pressure in 0.5 M KHCO3 at 22°C.

The general electrochemical procedure for the carbon dioxide incorporation was based on the use of one-compartment cells fitted with consumable anodes of magnesium or zinc [12]. Electrocarboxylations were carried out in DMF at constant current density, using tetrabutylammonium tetrafluoroborate (10 2 m) as supporting electrolyte. The catalyst was introduced in a 10% molar ratio with respect to the substrate and carbon dioxide was bubbled through the solution at atmospheric pressure. Electrolyses were generally run at room temperature and reactions were stopped when starting material was consumed or when the faradaic yield attained 30%. 

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