Überpotential

Überpotential, known in English as overpotential, is the extra electrochemical potential required beyond the equilibrium potential to drive an electrode reaction at a given rate. It is defined as the difference between the actual electrode potential under current flow and the electrode’s equilibrium potential for the redox couple at the interface (η = E − Eeq). The overpotential accounts for kinetic barriers and transport effects that prevent the reaction from proceeding exactly at its thermodynamic potential.

Overpotentials arise from several sources. Activation overpotential stems from finite electron-transfer kinetics at the electrode surface. Concentration overpotential results from mass transport limitations as reactants are consumed or products accumulate near the interface. Ohmic overpotential comes from resistance in the electrolyte, the electrode, and contacts (IR drop). In typical scenarios, activation overpotential dominates at small currents, concentration overpotential grows with current, and ohmic overpotential becomes significant at high current densities.

Quantitative descriptions often use the Butler-Volmer equation, which relates current density to overpotential by incorporating both anodic and cathodic processes. In the activation-controlled limit, the Tafel equation provides a linear relation between η and log(i). Together, these relations link kinetic parameters and transport properties to the observed overpotential. The concept is central to evaluating and improving catalytic activity, as lower overpotentials for a given reaction indicate more efficient electrodes.

Applications and relevance extend across electrolysis, fuel cells, batteries, and corrosion science. Overpotential serves as a key performance metric for catalysts and electrode materials, guiding the design of systems that minimize energy losses and enhance energy conversion efficiency.