Overvoltage - Overvoltage

When an electrode reaction is proceeding, the difference between the electrode potential and the reversible potential of that reaction is called overpotential, and is usually represented by η. Overpotential is positive for anodic reactions and negative for cathodic reactions. Overpotential is the driving force that causes current to flow, but from the other hand, it can also be said that the electrode is polarized by η when the current passes through the interface. The Tafel equation has long been known as an equation that expresses the relationship between current density and overpotential. A large overpotential corresponds to a slow reaction rate when compared at the same current density, so when the electrode reaction rate is slow (or the exchange current density is small), the expression "reaction or electrode with a large overpotential" is used. In particular, hydrogen electrode reactions are often described in terms of hydrogen overpotential instead of exchange current density. Electrode reactions consist of elementary reactions such as diffusion in the solution, charge transfer process (discharge process), and surface reactions, and the overpotentials attributable to each elementary reaction are called diffusion overpotential, charge transfer overpotential (activation overpotential), reaction overpotential, etc.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

電極反応が進行しているとき，電極電位とその反応の可逆電位の差を過電圧とよび，通常ηで表す．過電圧はアノード反応について正，カソード反応について負にとる．過電圧は電流を流す駆動力であるが，逆の見方をすれば，電流の界面通過により電極がηだけ分極されたともいえる．電流密度と過電圧の関係を表す式としてターフェルの式が古くから知られている．同じ電流密度で比較して過電圧が大きいことは反応速度が遅いことに対応するので，電極反応速度が遅い(あるいは交換電流密度が小さい)場合に，過電圧の大きい反応，あるいは電極という表現も使われる．とくに水素電極反応では，交換電流密度のかわりに水素過電圧で記述されることが多い．電極反応は，溶液内拡散，電荷移動過程(放電過程)，表面反応などの素反応から成り立っているが，その各素反応に帰因する過電圧を拡散過電圧，電荷移動過電圧(活性化過電圧)，反応過電圧などとよぶ．

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