Overvoltage oxygen

The electrolytic processes for commercial production of hydrogen peroxide are based on (/) the oxidation of sulfuric acid or sulfates to peroxydisulfuric acid [13445-49-3] (peroxydisulfates) with the formation of hydrogen and (2) the double hydrolysis of the peroxydisulfuric acid (peroxydisulfates) to Caro s acid and then hydrogen peroxide. To avoid electrolysis of water, smooth platinum electrodes are used because of the high oxygen overvoltage. The overall reaction is... 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

The value given above takes into account the oxygen overvoltage. 

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

Hickling A, Hill S (1947) Oxygen overvoltage. 1. The influence of electrode material, current density, and time in aqueous solution. Discuss Faraday Soc 1 236-246... 

Klectrode Oxygen overvoltage in V Electrode Oxygen overvoltage in V... 

To achieve the same effect, it is sometimes recommended to increase the current density at the anode. By this a potential is obtained at which, in addition to an undesirable reoxidation process, also the oxidation of other ions, oxidizable with greater difficulty (e. g. hydroxyl ions to oxygen) can take place. At a sufficiently high current density only a small part of the applied current will be consumed for the reoxidation process, whereas, the major part will be appropriated for the oxygen formation. The discharge of hydroxyl ions can be further promoted, i. e. it can be made possible already at low current density, if for the anodes we use materials with low oxygen overvoltage, e. g. iron or nickel in alkaline solutions. 

With graphite anodes current efficiency is somewhat lower, mainly because oxygen overvoltage is lower, so that hydroxyl ions formed by the dissociation of water may also be discharged apart from chloride ions. Evolution of oxygen is also caused by the porosity of the graphite because the chloride content in... 

Chloride ions are discharged at the anode preferentially due to the high oxygen overvoltage... 

Iron is the usual material for cathodes operating at a current density between 10 to 20 A per sq. dm. Graphite or nickel is rarely used. Smooth platinum is the only suitable material for anodes as it has a sufficiently high oxygen overvoltage and so enables a high current density, of between 30 to 60 A per sq. dm, to be employed. 

Persulphuric acid or persulphates are produced by the oxidation of sulphuric acid or sulphates at an anO de of smooth pure platinum using a high current density. Only smooth platinum can be used for the anodes because no other material displays such a high oxygen overvoltage. 

Smooth platinum is the only suitable material for anodes as it has a-high oxygen overvoltage for better durability of the anode, pure platinum is not used but an alloy whioh contains 5 per cent iridium. Current efficiency decreases considerably if there is the slightest trace of any impurity deposited on the platinum such as lead dioxide, manganese dioxide, ferric oxide eto. 

The electrode material is important for several reasons. The magnitude of the hydrogen and oxygen overvoltage determines the accessible potential range special surface properties, such as adsorptive and catalytic effects, may determine the course of the reduction. In the electrocatalytic reactions the electrochemical step consists in a reduction of hydrogen ions to adsorbed hydrogen, which then reacts with the substrate as in a catalytic reaction. The study of the influence of the electrode material on the course of the reaction is an area in which further research is very much needed. 

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