Current efficiency oxygen

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

The oxygen contribution from these reactions is dependent on the nature of the anode material and the pH of the medium. The current efficiency for oxygen is generally 1—3% using commercial metal anodes. If graphite anodes are used, another overall reaction leading to inefficiency is the oxidation of... 

Many factors other than current influence the rate of machining. These involve electrolyte type, rate of electrolyte flow, and other process conditions. For example, nickel machines at 100% current efficiency, defined as the percentage ratio of the experimental to theoretical rates of metal removal, at low current densities, eg, 25 A/cm. If the current density is increased to 250 A/cm the efficiency is reduced typically to 85—90%, by the onset of other reactions at the anode. Oxygen gas evolution becomes increasingly preferred as the current density is increased. 

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

Selectivity of propylene oxide from propylene has been reported as high as 97% (222). Use of a gas cathode where oxygen is the gas, reduces required voltage and eliminates the formation of hydrogen (223). Addition of carbonate and bicarbonate salts to the electrolyte enhances ceU performance and product selectivity (224). Reference 225 shows that use of alternating current results in reduced current efficiencies, especiaHy as the frequency is increased. Electrochemical epoxidation of propylene is also accompHshed by using anolyte-containing silver—pyridine complexes (226) or thallium acetate complexes (227,228). 

There are other parallel electrochemical reactions that can occur at the electrodes within the cell, lowering the overall efficiency for CIO formation. Oxygen evolution accounts for about 1—3% loss in the current efficiency on noble metal-based electrodes in the pH range 5.5—6.5. 

Plating variables for this process maybe summarized as higher (87°C) operating temperatures enable the oxygen content of the metal to be reduced to 0.01% the CrO iSO ratio should be below 100 to obtain low oxygen metal current efficiencies >8% are associated with high oxygen contents and better current efficiencies are obtained at low current densities. 

Fire Refining. The impurities in bhster copper obtained from converters must be reduced before the bUster can be fabricated or cast into anodes to be electrolyticaHy refined. High sulfur and oxygen levels result in excessive gas evolution during casting and uneven anode surfaces. Such anodes result in low current efficiencies and uneven cathode deposits with excessive impurities. Fite refining is essential whether the copper is to be marketed directly or electrorefined. 

The quality of the refined metal, and the current efficiency strongly depend on the soluble vanadium in the bath and the quality of the anode feed. As the amount of vanadium in the anode decreases, the current efficiency and the purity of the refined product also decrease. A laboratory preparation of the metal with a purity of better than 99.5%, containing low levels of nitrogen (30-50 ppm) and of oxygen (400-1000 ppm) has been possible. The purity obtainable with potassium chloride-lithium chloride-vanadium dichloride and with sodium chloride-calcium chloride-vanadium dichloride mixtures is better than that obtainable with other molten salt mixtures. The major impurities are iron and chromium. Aluminum also gets dissolved in the melt due to chemical and electrochemical reactions but its concentrations in the electrolyte and in the final product have been found to be quite low. The average current efficiency of the process is about 70%, with a metal recovery of 80 to 85%. 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

The MCFC has also been tested as a true concentrator, with electric power supplied instead of H2 [32], With inlet C02 at 0.25% (in air), the outlet could be brought as low as 75 ppm, albeit with the rather low current efficiency of 40% (based on 1 mol CO2/2 F). The parasitic current is due to transport of oxyanions (Of " or 02) which are discharged as molecular oxygen at the anode. For lower C02 utilization, say below 80%, the polarization is quite acceptable, as seen in Fig. 23. Application to manned spacecraft, however, is handicapped by the high temperatures needed (> 500 °C). 

Currently, these oxygenation reactions are usually carried out in whole cells, the outcome of which is often unpredictable. The discovery of novel oxygenases and efficient hosts for protein expression remain keys to further expanding the applications of these enzymes in chemical synthesis and drug metabolism studies [34—37]. 

The intermittent operation of electrolyzers powered from a wind turbine is characterized by a dynamic power fluctuation with periods of varying overload, partial load, and shutdown. The operation of the electrolyzer below 10% of its nominal power remarkably reduced the current efficiency and the purity of the product gases, inducing the shutdown of the electrolyzer, which was programmed at a safety limit of 2 vol% hydrogen-in-oxygen [52],... 

Chemical yields from an electrochemical reaction are expressed in the usual way based on the starting material consumed. Cunent efficiency is determined from the ratio of Coulombs consumed in forming tlie product to the total number of Coulombs passed through the cell. Side reactions, particularly oxygen or hydrogen evolution, decrease the current efficiency. 

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. 

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