Electrode Kinetic Data: Geometric vs. Real Surface Area

Kinetic data reporting the rate of electron transfer across an electrified interface are of fundamental as well as practical importance. They report the electric current caused by coupling the flow of electrons inside the electronically conducting electrode and the flow of ions in the adjacent ionically conducting phase. At equilibrium or rest, potential currents in both directions at the established dynamic equilibrium have the same absolute value: the net current is zero. This current describes the electrocatalytic activity of an electrode and is called the exchange current; with respect to the surface area, it is called the exchange current density. This study inspected the actually used surface areas because the reported activities may depend critically on the selection of this area. Charge transfer resistances corresponding to exchange currents I0 were determined for a simple redox system using a platinum disc electrode with constant geometric surface area but variable roughness. At all studied degrees of roughness, changes in I0 were found. With an electrochemically active surface area, exchange current densities j0 could be calculated, but the obtained values showed a dependency on roughness that could not be accounted for by using this surface area instead of the geometric one. It is suggested that j0 may be reported with respect to geometric surface area, but at least roughness data of the studied electrode should be provided.