Cathode oxygen depolarised

Chlorine Production with Oxygen-depolarised Cathodes on an Industrial Scale... 

Encouraging laboratory experiments since 1994 with oxygen-depolarised cathodes (ODC) in chlor-alkali as well as hydrochloric acid electrolysis motivated the development of this technique up to the industrial scale. Based on the predictions of the theory, the reduction of cell voltage could be expected up to 1 V (Fig. 4.1) for both applications. Early on, the proper choice and improvement of ODC, deriving mainly from the DeNora group, led to results with voltages as predicted in short tests as well as in endurance tests conducted over dozens of months at the Bayer endurance test facilities. 

Fig. 4.6 The method of HCI electrolysis with oxygen-depolarised cathodes (ODC).

Oxygen-depolarised cathode for aqueous hydrochloric acid electrolysis... 

Fig. 9.3 An unpurified MDI-type HCI feed test of a laboratory cell (1 dm2) with a rhodium-based oxygen-depolarised cathode.

Fig. 9.5 Cell voltage results of an HCIaq pilot electrolyser (four cells, 0.88m2 each) with rhodium-based oxygen-depolarised cathodes.

The flow-through cathode is the result of a tailored-to-the-process evolution of the GDE structure, which is available also in two additional configurations double-sided (originally developed for fuel cell servicing) and single-sided (see Fig. 9.7). The double-sided type is particularly suited for the electrochemical process where the product should not be released on to the back surface of the cathode, as in the case of oxygen-depolarised chlor-alkali electrolysis, discussed in Section 9.3. 

Oxygen-depolarised cathodes for sodium chloride electrolysis... 

The substitution of conventional hydrogen-evolving cathodes with oxygen-consuming gas-diffusion electrodes (GDE), often referred to as oxygen-depolarised cathodes (ODC), also allows a substantial reduction in the energy consumption of the chlor-alkali process. 

Fig. 9.8 Results of the electrolysis of NaCI using an original oxygen-depolarised cathode system.

Fig. 9.9 Current density distribution along the oxygen-depolarised cathode width for various types of nickel net.

Fig. 9.14 Cell voltage results during NaCI electrolysis using a latest generation oxygen-depolarised cathode.

I. Moussalem, J. Jorissen, U. Kuntz, S. Pinnow and T. Turek, Chlor-alkali electrolysis with oxygen depolarised cathodes history, present status and future prospects ,/oMwaZ of Applied Electrochemistry,(2008) 1177-1194. 

Rosenfel d" considers that SO2 can act as a depolariser of the cathodic process. However, this effect has only been demonstrated with much higher levels of SO2 (0-5%) than are found in the atmosphere (Table 2.4) and the importance of this action of SO2 has yet to be proved for practical environments. However, SO2 is 1 300 times more soluble than O2 in water" and therefore its concentration in solution may be considerably greater than would be expected from partial pressure considerations. This high solubility would make it a more effective cathode reactant than dissolved oxygen even though its concentration in the atmosphere is comparatively small. 

Oxygen Dissolved oxygen is probably the most significant constituent affecting corrosion, its importance lying in the fact that it is the most important cathodic depolariser in neutral solutions. Other depolarisers also occur, but as oxygen is an almost universal constituent of natural waters its importance will readily be understood. 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Attempts have been made to cheapen the process by producing electrolytic oxygen without the simultaneous liberation of hydrogen by the adoption of depolarising electrolytes, or cathodes also of cathodes winch absorb hydrogen and may subsequently Ik- employed as elements in gas cells.8... 

A. Mollica and A. Trevis, Correlation between the formation of a primary film and the modification of the cathodic surface of stainless steel in seawater. Proceedings of 4th Int. Cong. Marine Corrosion and Fouling, Antibe, France, 1976, pp. 351-355. A. Mollica, E. Traverso, and D. Thierry, On oxygen reduction depolarisation induced by biofilm growth on stainless steels in sea water. Aspects of Microbially Induced Corrosion (D. Thierry, ed.). University Press, Cambridge, UK, 1996. 

Oxygen dissolved in water has less influence on aluminium corrosion than on steel corrosion. It maintains corrosion by depolarisation of the cathodes, but in the case of aluminium, it also contributes to the formation of the natural oxide layer when corrosion... 

The role of oxygen in the corrosion resistance of aluminium in seawater is the same as in freshwater. It accelerates corrosion by depolarising the cathodes, but also contributes to the repair of the natural oxide layer that protects the metal [10]. The experience with desalination of seawater shows that the resistance of aluminium is the same in aerated and deaerated seawater, from room temperature up to 120 °C [11]. 

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