Activation Overpotential

Definition - What does Activation Overpotential mean?

Activation overpotential is the energy lost due to the slowness of electrochemical reactions at the anode and the cathode electrodes.

Activation overpotential is a measure of the activity of electrodes. It represents the overpotential required for electrochemical reaction—the current density (typically small) at which the overpotential is measured.

Activation potential is the potential difference above the equilibrium value required to produce a current which depends on the activation energy of the redox event. While ambiguous, "activation overpotential" often refers exclusively to the activation energy necessary to transfer an electron from an electrode to an analyte.

Corrosionpedia explains Activation Overpotential

Activation overpotential is a condition occurring when the reaction rate is controlled by the slowest "step" in a series of reaction steps. For example, the steps in a hydrogen reduction reaction are:

    1. Hydrogen ions are absorbed from the solution onto the anode surface.
    2. Electron transfer occurs from anode to the hydrogen ions to form hydrogen.
    3. The hydrogen elements form hydrogen gas molecules.
    4. Hydrogen gas bubbles form.

The slowest of these four steps dictates how fast the overall reduction reaction happens.

In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency.

There are three categories of overpotentials:

  • Activation
  • Concentration
  • Resistance

Factors affecting activation overpotential:

  • Current density
  • Materials
  • Surface roughness
  • Temperature
  • Pressure
  • pH
  • Agitation
  • Adsorption of ions

At low current density, the shape of the polarization curve is governed by the activation overpotential, which arises from the kinetics of charge transfer reactions across interfaces and represents the magnitude of activation energies when reactions propagate at the rate required by the current. This depends on:

  • Type of reactions and catalyst materials
  • Electrode microstructure
  • Reactant activities
  • Electrolyte material (acidic/alkaline)
  • Temperature
  • Current density

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