Hydrogen and oxygen overvoltage

The evolution of hydrogen and oxygen are well-known phenomena during electrolysis of dilute aqueous solutions of acids and bases between inert metal electrodes. If bright platinum electrodes are used for the electrolysis, it is found that in most cases the minimum potential difference which must be applied between them before gas bubbles appear is close to 1 -7 V. That a similar value s)iould usually be observed is hardly surprising since the same overall chemical process is occurring, viz. the decomposition of water. 

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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. 

This type of battery further supports this funclion by the fact that it develops a very high hydrogen and oxygen overvoltage which enhances the efficiency of conversion and ensures a sharp and reproducible rise in on-charge voltage to regulate current acceptance best. 

Standard lead-acid batteries lose a small amount of water by evaporation, but the major mechanism for water loss is by electrolysis to form hydrogen and oxygen, as described by eqs (5.5) and (5.6). The presence of small quantities of foreign elements lowers the overvoltages for these processes and leads to an accelerated water loss it has been shown that the elements antimony, arsenic, cobalt, manganese, nickel, platinum and tellurium all have a deleterious effect, even at low levels. Two general approaches have been taken in the development of MF batteries ... 

Molten salts are favored electrolytes in cases where aqueous solutions cannot be used because the decomposition voltage of water is lower than that of the salt in question. Although high overvoltages for hydrogen and oxygen evolution allow us to extend the use of aqueous electrolytes somewhat beyond the limits set by thermodynamics, there are many substances which cannot be electrowon or plated from aqueous solutions. 

In appl5nng Jprgensen s approach, it should also be remembered that there are usually kinetic factors (so-called overvoltage effects ) which results in hydrogen and oxygen being evolved only at potentials beyond the range of the thermodynamic ones [81). There is also often the question of the effect of complex ion formation. 

Although the decomposition potential of an aqueous solution of sulphuric acid to produce hydrogen and oxygen is constant at about 1.7 V with smooth platinum electrodes, due to the overvoltage phenomenon, the value is different if other materials are employed as electrode materials. If the cathode is lead and the anode is platinum, for example, the decomposition potential increases to about 2.2 V. 

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... 

Metallurgical palladium-rathenium alloys are of interest insofar as at a component ratio of 40% Ru and 60% Pd, hydrogen is adsorbed as on pure Pd, and oxygen is adsorbed as on pure Ru. The overvoltage for hydrogen evolution at this alloy is just as low as it is at Pd, that for oxygen evolution is as low as it is at Ru. 

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