Current Distribution on a Rotating Disk Electrode

The radial variation of deposit thickness on a rotating disk electrode was measured for deposition of copper from a solution 0.1M in CuSO4 and O.IM in H2SO4. The results confirm the theory of nonuniform current distribution on such electrodes below the limiting current. The basic theory of the rotating disk electrode was developed by Levich in 1942. Since then, considerable work has been done to refine the theory of this elec-trode system. It has been widely used for the study of electrode processes and mass transfer in electrochem-ical systems. Measurement of diffusion coefficients and bulk concentrations and the study of moderately fast electrode reactions are some of the common applica-tions. Theoretically predictable velocity and concen-tration profiles and elimination of natural convection effects due to strong forced convection make it suitable for the above-mentioned and other applications. Fur-thermore, the current density is uniform at the limit-ing current. Recently, Newman (6) has shown that below the limiting current he current density is nonuniform and can be obtained by solving for the concentration and potential distribution simultaneously ( ubject o elec-trode kinetic boundary conditions). The convective diffusion equation for the diffusion layer can be re-placed by an integral equation (7, 8, 13) relating the reactant concentration and the normal derivative of the reactant concentration at the electrode surface nFD Oc nFD /av \ I /3 In-i =---- l y=0- l-t-+ / V---Sb-J 7 r f dCo} r(4/3) o dr [r = x (r 3-- x3) 1/3 [1] where i is the current density on the disk and Co and c ~ are the surface and bulk concentrations of the re-actant. The potential distribution outside the diffusion layer satisfies Laplace's equation and can be expressed as a series solution, from which one obtains r16