The diffusion process at the electrodes in the presence of an electric current

In the previous chapter we described the changes of the concentration profiles in a liquid boundary in the presence of an electric current, in particular, the movement of the boundary was the main object of interest. When the electrodes are of solid material — or in other cases are not electrolytes and not miscible with the electrolyte — then it is more suitable to give the main emphasis to the description of the mass fluxes onto and away from the electrode material and to devote only a fairly brief discussion to the concentration variation in the electrolyte solution. It should be mentioned here that in this book the word electrode is reserved for one single metallic phase — sometimes in combination with a non-metallic solid phase — it is not understood to refer to the combined system metal-electrolyte, for instance to the “half-cell” Cu(metal)/CuSO₄ solution. Still, as has been demonstrated in Chapter 3, the boundary between two different liquid electrolyte phases in certain respects acts like an electrode although not an electrode in the sense of the definition just given. Therefore, in the last section of this chapter we shall also give an outline of the effect of the mass fluxes and the excess mass fluxes at a liquid junction between two bulk electrolytes.