is that it is difficult to determine how much of a measured overpotential is derived from a specific source. Overpotentials can be grouped into three categories: activation, concentration, and resistance. Activation overpotential The activation overpotential is the potential difference above the equilibrium value required to produce a current that 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 anolyte. This sort of overpotential can also be called "electron transfer overpotential" and is a component of "polarization overpotential", a phenomenon observed in cyclic voltammetry and partially described by the Cottrell equation. Reaction overpotential Reaction overpotential is an activation overpotential that specifically relates to chemical reactions that precede electron transfer. Reaction overpotential can be reduced or eliminated with the use of electrocatalysts. The electrochemical reaction rate and related current density is dictated by the kinetics of the electrocatalyst and substrate concentration. The platinum electrode common to much of electrochemistry is electrocatalytically involved in many reactions. For example, hydrogen is oxidized and protons are reduced readily at the platinum surface of a standard hydrogen electrode in aqueous solution. Substituting an electrocatalytically inert glassy carbon electrode for the platinum electrode produces irreversible reduction and oxidation peaks with large overpotentials. Concentration overpotential Concentration overpotential spans a variety of phenomena that involve the depletion of charge-carriers at the electrode surface. Bubble overpotential is a specific form of concentration overpotential in which the concentration of charge-carriers is depleted by the formation of a physical bubble. The "diffusion overpotential" can refer to a concentration overpotential created by slow diffusion rates as well as "polarization overpotential", whose overpotential is derived mostly from activation overpotential but whose peak current is limited by diffusion of analyte. The potential difference is caused by differences in the concentration of charge-carriers between bulk solution and the electrode surface. It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution. The rate of reaction is then dependent on the ability of the charge-carriers to reach the electrode surface. Bubble overpotential Bubble overpotential is a specific form of concentration overpotential and is due to the evolution of gas at either the anode or cathode. This reduces the effective area for current and increases the local current density. An example is the electrolysis of an aqueous