Overpotential

Overpotential is the difference between an electrode’s actual potential and its equilibrium potential under current flow. It measures the extra driving force required to sustain an electrochemical reaction at a given rate.

Butler–Volmer equation: An equation relating electrode current density to overpotential through anodic and cathodic reaction rates. It quantifies how activation-controlled current changes as overpotential increases.
Electrode potential: The electrical potential of an electrode relative to a specified reference electrode. Overpotential is defined by comparing an electrode potential with its equilibrium value.
Water electrolysis: The electrochemical splitting of water into hydrogen and oxygen using electrical energy. Electrode overpotentials make the required cell voltage exceed water’s thermodynamic decomposition voltage.
Electromotive force: The energy supplied per unit charge by a source, equal to its open-circuit voltage under specified conditions. It describes a source’s driving voltage, whereas overpotential is an electrode’s departure from equilibrium.
Activation overpotential: The portion of overpotential associated with the energy barrier for charge transfer at an electrode. It captures the extra drive needed to overcome the interfacial reaction barrier.
Nernst equation: An equation relating an electrochemical reaction’s equilibrium potential to temperature and the activities of its chemical species. It determines the equilibrium potential from composition and temperature.
Oxygen evolution reaction: The multistep anodic reaction that produces molecular oxygen from water or hydroxide. Its sluggish kinetics often require substantial anodic overpotential in electrolyzers.
Polarization (electrochemistry): The change in an electrode’s potential from its equilibrium value when current flows. Polarization is the broader phenomenon quantified by overpotential.
Concentration overpotential: The potential shift caused by differences between reactant or product concentrations at an electrode surface and in the bulk electrolyte. It arises when mass transport cannot replenish reactants at the electrode fast enough.
Equilibrium potential: The electrode potential at which the forward and reverse rates of an electrochemical reaction balance. It is the baseline subtracted from the actual potential to obtain overpotential.

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