Oxygen depolarized electrolysis

Ziegelbauer JM, Guild AF, O Laoire C, Urgeghe C, Allen RJ, Mukerjee S (2007) Chalcogenide electrocatalysts for oxygen-depolarized aqueous hydrochloric acid electrolysis. Electrochim Acta 52 6282-6294... 

Fig. 14 Schematic representation of the chlor-alkali electrolysis using an oxygen depolarized cathode.

Oxygen depolarized cathodes can be used in hydrochloric acid electrolysis, too. 

Recent developments in cell technology include the use of ion-exchange membranes instead of diaphragms [29] and the use of oxygen-depolarized cathodes [30]. Section 11.2.2.2C discusses the latter as an alternative to the standard cathode that reduces the reversible potential by about 1.23 V. Section 17.2.2.2 identifies these cathodes as an important emerging technology. A commercial plant for production of about 24,000 tons of chlorine by electrolysis of HCl with depolarized cathodes was recently announced [31]. 

Hine and coworkers studied this electrolysis with oxygen-depolarized graphite cathodes [41]. This combination is known as the Kyoto cell, and it operates with either air or oxygen supplied to the cathode compartment. The reactions are the same as in the Westvaco process, but the oxidation of cuprous chloride takes place in the cell rather than in an external reactor. 

Moussallem I, Jorissen J, Kunz U, Pinnow S, Tutek T (2008) Chlor-alkali electrolysis with oxygen depolarized cathodes history, present status and future prospects. J Appl Electrochem 38 1177-1194. doi 10.1007/s10800-008-9556-9... 

Jorissen J, Turek T, Weber R (2011) Chlorine production with oxygen depolarized cathode. Energy saving in electrolysis (language German). Chem Unserer Zeit 45 172-183. doi 10.1002/ ciuz.201100545... 

Hydrochloric acid electrolysis was developed in 1942. Since 1964, it is operated in industry and continuously optimized until today [4—6] (See section Hydrochloric Acid Electrolysis). A far-reaching modification starting in 1999 was the implementation of Oxygen Depolarized Cathodes (ODCs) which decrease substantially the energy consumption (See section Hydrochloric acid electrolysis using Oxygen Depolarized Cathode (ODC)). An overview about hydrochloric acid electrolysis is given in [1,2], detailed information about both processes is available in [4]. 

Hydrochloric Acid Electrolysis Using "Oxygen Depolarized Cathode" (ODC)... 

The functional principle of an ODC/GDE is elucidated for chlor-alkah electrolysis in the entry Chlorine and Caustic Technology, Using Oxygen Depolarized Cathode. There are problems discussed which result from handling of the liquid product caustic soda solution and the pressure difference between gas phase and electrolyte (hydrostatic pressure). Such problems are irrelevant in case of hydrochloric acid electrolysis which is in consequence easier to operate. 

Hydrochloric Acid Electrolysis, Fig. 2 Scheme of hydrochloric acid electrolysis rising Oxygen Depolarized Cathode... 

Maljusch A, Nagaiah TC, Schwambom S, Bron M, Schuhmann W (2010) Pt-Ag catalysts as cathode material for oxygen-depolarized electrodes in hydrochloric acid electrolysis. Anal Chem 82(5) 1890-1896... 

This process causes depolarization of the anode as it proceeds more easily than reaction (XX-14). Caro s acid is. therefore, harmful not only for causing a loss in active oxygen but also for its depolarizing effect, which lowers the necessary high anode potential whereby current efficiency is decreased. For this reason electrolysis should be carried out under conditions which suppress as much as possible the hydrolysis of persulphuric acid according to equations (XX-15) and (XX-16) and, therefore, the secondary reactions (XX-17) and (XX-18). 

The electrolysis of aromatic acids by no means offers results which are comparable to those obtained by the electrolysis of aliphatic acids. In so far as the aromatic acids, or their salts, act as electrolytes, a regeneration of the acid from the anion RCOO and water, with evolution of oxygen, occurs almost exclusively. A splitting off of carbonic acid, which makes possible the manifold reactions of aliphatic acids, almost never occurs here. The results obtained with aromatic acids are, therefore, only of a more general interest so far as the acids, by substitutions in the benzene nucleus, can act as cathodic or anodic depolarizers, and can in this way exert reduction and oxidation effects. 

There are several problems associated with hydrogen production however, some of them are really a side product associated inconvenience, such as oxygen formation in the course of electrolysis. The use of new anode depolarizers seems to be one of the solutions to avoid the large overpotentials associated with the anodic process. All these anodes are patent protected, but some of them are not new material, but only a product of a side process with lower activation energy. The Westinghouse thermo(electro)chemical hybrid process for hydrogen production is an example [25] with 45% of... 

The high temperature electrolysis can be performed in two different modes cells with oxygen anode and cells with depolarized anodes. Both are possible with current SOFC cell arrangements. 

By the mid 1970s it was clear that the hydrodimcrizalion of acrylonitrile could be run very effectively with only a low concentration ortctraalkylammoniurn ion and that a saturated solution of acrylonitrile in an aqueous buffer was an appropriate medium. In such circumstances, it seemed likely that the electrolysis could be run jn an undivided cell without the additional complication of an anode depolarizer. Such a system was first reported by Phillips Petroleum. They ran a pilot plant with an undivided cell consisting of a lead cathode and a steel anode a very simple electrolyte, 6% acrylonitrile and 0,03% tetrabutylaminon-ium salt in aqueous dipotassium hydrogen phosphate was employed and the anode reaction was oxygen evolution. The yield of adiponitrile remained above 90% and the chief by-products were propioniirile and trimer. No base-initiated chemistry was observ due to the use of an effective buffer. 

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