Interpreting Concentration and Activation Overpotentials in Electrochemical Systems: A Critical Discussion

When partitioning the cell potential, concentration (mass transfer) and activation (reaction kinetics) overpotentials are frequently defined in different ways. In this work, we compare two potential partitions resulting from the use of either equilibrium-based or activation-based concentration overpotential. We emphasize that the different definitions for activation and concentration overpotential in the two potential partitions (i) cannot be chosen independently, (ii) can lead to considerably different overpotential values, and (iii) result in different meanings as they assume the respective other resistance to be either zero or at the current value. If not applied carefully, they can thus easily result in misinterpretations.