The mechanism of the reactions of oxygen evolution on the lead dioxide layer has been disclosed. Lead dioxide comprises crystal and hydrated (gel) zones. The reaction of oxygen evolution proceeds in the hydrated zones their OH-groups being involved in this reaction. The latter occurs in active centres with different activation energy depending on the structure of the gel zones. The reaction of oxygen evolution has been found to proceed through the following elementary electrochemical processes:
PbO*(OH)2 + H2O —> PbO*(OH)2...(OH)o + H+ + e- (1)
PbO*(OH)2...(OH)o —> PbO*(OH)2 + O + e- + H+ (2)
2O —> O2 (3)
PbO*(OH)2 is an active centre. The OHo radicals formed remain connected to the active centres and block them.
The activation energies of the above elementary reactions have been determined through measuring the current/temperature dependence. It has been established experientially that with increase in electrode potential the activation energy of the first reaction increases, too, yielding adsorbed OH radicals. This fact has been explained through the involvement of active centres of different activation energies in reaction 1. When the electrode potential reaches a sufficiently high value to allow all active centres to take part in the first electrochemical reaction, the second electrochemical reaction starts as a result of which oxygen is formed which recombines and is evolved from the lead dioxide surface. It has been established that the activation energy of this process decreases with increase in electrode potential. These reactions exert a strong influence on the charge processes of the positive plates and on the corrosion of positive plate grids.
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Keywords: mechanism of the reactions of oxygen evolution, elementary electrochemical processes of oxygen evolution, PbO*(OH)2 active center, dependence of O2 evolution on electrode potential