Studies of the double-layer and exchange reactions at copper and cadmium electrodes

The structures of the electrical double layers at some metal/aqueous solution interphases have been studied. The kinetics of the exchange reactions at copper and cadmium electrodes have been investigated in selected electrolytes using a.c. impedance and galvanostatic techniques. The contributions by various possible steps to the overall exchange process have been investigated. In nitrate electrolyte (NO-3 not adsorbed) the component process controlling the rate of exchange at copper electrodes is shown (at ≈ 40°C) to change from crystallisation (and dissolution of the lattice) to charge transfer. In sulphate electrolytes (SO2-4 adsorbed at the electrode) the control is solely by the processes of crystallisation (and dissolution of the lattice). The exchange process at a cadmium electrode system was found to be difficult to study in aqueous solution. The system is very susceptible to interference from impurities in the electrolyte. In NaClO4 electrolyte the exchange process is controlled by crystallisation (and dissolution) effects. In alkaline electrolytes the exchange reaction is a very rapid process. In this case however, it is likely that the exchange proceeds via a surface film and there is no evidence for any crystallisation effects. For some metals, the mechanism of the exchange process (i.e. whether the reaction proceeds by surface diffusion or direct transfer to a kink site across the double layer) has been discussed in connection with the crystal structure.