Current inefficiency

In the present example, of the 79% carbon consumed by the above mechanism, about 12% represents excess carbon consumed due to current inefficiency, the second consumption mechanism. Present-day cells have current efficiencies in the 85-95% range. Current inefficiency reactions involve back-reactions of electrolytic CO2 with reduced metal species from the cathode. An example of this reoxidation mechanism, for dissolved aluminum, is given by the following equation ... 

In Germany, discussion on REACH has exposed the current inefficiencies in stakeholder negotiation and consultation. There is no evidence to suggest that REACH has sparked a wider debate on the sustainability of the industry. The German chemical industry will need to adopt a leading role in promoting such debate because, without its support, reforms are unlikely to succeed. Germany s administrative... 

The aluminum reacting with CO2 bubbles in this back reaction within the electrolyte may be colloidally suspended aluminum droplets, dissolved aluminum or some reduced species of aluminum present in small quantities in the electrolyte. This is not certain, but it is generally acknowledged that the reoxidation of the species by CO2 bubbles is the principal cause of current inefficiency. 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

B. Chlorate Formation. The soluble chlorine-based species in the anolyte, leading to the chlorine current inefficiency, are dissolved chlorine, HOCl, OCl, and CIOJ. 

Hydrogen current inefficiency arises from the blind current losses (BCLs) from the discharge of available chlorine at the cathode, forming NaCl and NaOH. 

A. General. Preferential transport of selected species is the primary characteristic property of membranes. In a chlor-alkali cell, for example, one equivalent of cation will pass across the cation-exchange membrane for each Faraday of electricity if the selectivity is perfect. In practice, some OH passes through the membrane in the opposite direction, resulting in current inefficiency. The membrane selectivity, therefore, directly determines the caustic current efficiency of the process. 

D. Effects of Chlorate. Chlorate ion is a Hxxluct of the current inefficiency reactions in the anolyte. Its accumulation can be controlled by purging or decomposition. 

This work showed that increased monotonically with increasing Q. The relationship between and P was more complex. When Q was laige, the current inefficiency was large and a weak function of P. In agreement with the points made above, the conclusion was that a cell should operate at high current density and therefore a high degree of conversion. 

The current efficiency for metal deposition may be around 90 %. The main reason for loss in current efficiency is the back reaction between dissolved metals and chlorine produced at the anode. Also the presence of electronic conduction will cause current inefficiency. The dissolution of metals in itself will also contribute to additional loss in current efficiency. Some metal may be lost due to the reactivity of the alkali and alkaline earth elements. Some evaporation losses of metals may also take place due to the relatively high process temperatures. 

Another OCV loss is caused by the crossover of fuel through the electrolyte. Ideally the electrolyte allows the transport of only ions. In reality, however, some fuel permeates across the membrane from the anode to the cathode. In addition, some direct transfer of electrons across the membranes can occur and cause electronic short. A fuel loss due to crossover leads to a current loss. The current loss associated with an electrical short is generally small (ca. few milli-amperes) relative to the typieal operating current of a fuel cell, and therefore is not a significant source of current inefficiency. However, these effects have a significant effect on the OCV of the cell. This is particularly true of a low-temperature cell, in which activation losses are considerable [126]. 

Electrowinning in molten salts is the only way to obtain some metals in the elementary state. The process, however is not straightforward and involves many problems. Corrosion of the cell components and electrodes is a persistent very serious problem. If the metal deposits as a solid, separation may be diflScult growth of dendrites may cause short-circuit between the anode and cathode. Solubility of the metal in the melt may be a source of current inefficiency. 

---