An Improved Technique for Measuring Cathode Overpotential During Electrodeposition

Abstract

The determination of the activation overpotential/current density relationship (η_A—i) during the electrodeposition of metals is subject to errors owing to the changing surface topography of the substrate. Under these circumstances, the true current density varies continuously as electrodeposition proceeds. To overcome this difficulty, a technique has been developed which involves the use of a platinum cathode which is coated with a very thin deposit of copper prior to each determination of η_A at constant i. Removal of the deposit is effected anodically and automatically terminated before oxygen gas can be evolved. This avoids the possibility of gas bubbles adhering to the platinum and causing errors in the subsequent plating stage. Although the studies have been confined to copper, it is considered that the method is of general application. The preparation of copper sulphate electrolytes has been investigated and, using highly purified electrolytes in conjunction with the techniques described, it has been found possible to obtain linear Tafel lines during the electrocrystallisation of copper.