STEADY-STATE MULTICOMPONENT DIFFUSION AND MIGRATION TO A PLANAR ELECTRODE .2. MULTIPLE ELECTRODE-REACTIONS

A general solution has been obtained for steady-state one-dimensional diffusion and migration to a planar electrode in a multicomponent system involving any number of electrode reactions and non-reacting supporting species. The solution includes kinetic effects, making it valid over the entire potential range, and is independent of the particular form of the kinetic rate expressions. As an example, it has been applied for a comprehensive study of a system (CuSO4 + H2SO4 + Na2SO4) having two competing cathodic reactions (CU2+/CuO and H-/H-2 reduction), for which kinetic and transport data are available from the literature. Analysis of the effect of electrolyte composition on the electrode responses of both reactions show that there is a significant interaction between the two processes which depends on the presence or absence of supporting electrolyte. Of particular interest are effects that occur when the applied potential is lowered sufficiently that the more electropositive reactant (H+ in this case) begins to react and no longer behaves as a supporting ion. One of these effects - enhancement in the limiting current of the first reaction (Cu2+ reduction) - has been studied and simple formulae for this phenomenon have been derived. A second effect. which has not been reported previously, is the occurrence of a minimum in the concentration profile of the more electropositive species. Both phenomena are universal and are not restricted to the particular system under consideration.