It is shown that the evidence that has at various times been advanced against the double sulphation theory of the working of the lead accumulator is invalid. From a study of the behaviour of the lead dioxide electrode, under various conditions of discharge, it is concluded that the reaction mechanism remains the same as for the reversible electrode. The consequence of this proof of the identity of mechanism is that no detailed mechanism for the electrode processes is acceptable unless it conforms with the thermodynamical requirement that it contains no spontaneous non-electrochemical stage.
A method of determining overvoltage is described in which the current density can be rapidly altered from one value to another thus enabling the determination of the overvoltage against current density relationship to be examined under constant surface conditions. The method is applied to the determination of oxygen overvoltage on lead dioxide electrodes and the results, which show the marked superiority of this method over that usually employed, conform to the Tafel equation 7) = (2.303 RT/aF) loglo i + 0, with a value of a = 0.5.
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