Electrode reaction - denkyokuhannou (English spelling) electrode reaction

This is a heterogeneous electrochemical reaction that takes place at the electrode-solution interface when a pair of electrodes is placed in an electrolyte solution and a voltage is applied between the electrodes. In this case, the reduction of cations (for example, reduction of hydrogen ions to hydrogen molecules, 2H ⁺ + 2e → _H2 ) occurs on the electrode (cathode) to which a negative potential is applied (this is called the cathodic reaction), and the oxidation of anions (for example, oxidation of chloride ions to chlorine molecules ^{, 2Cl-} → _Cl2 + 2e) simultaneously occurs on the electrode (anode) to which a positive potential is applied (this is called the anodic reaction), causing an electrolytic current (also called a Faraday current) to flow. If we now examine the relationship between the potential applied to one of these electrodes and the electrolytic current, we find that there is a relationship between the two, as follows:

The following relationship holds: Here, i is the magnitude of the electrolysis current, η is the electrode potential during operation measured with the electrode potential when the cathodic or anodic reaction O + n e R (O is the oxidized state, R is the reduced state) occurring on that electrode in equilibrium as the reference potential (potential 0 volts), i.e., the overvoltage, T is the absolute temperature, R is the gas constant, and α is a constant that takes different values ​​depending on the reaction. This equation is called the Tafel equation. As with other chemical reactions, when an electrode reaction is in equilibrium, the reaction rate moving to the left and the reaction rate moving to the right are considered to be equal. This reaction rate is called the exchange current. In order to investigate electrode reactions, in addition to considering the Tafel equation, impedance measurements between the electrode and solution and capacitance measurements are also performed.

[Toda Genjiro]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

1対の電極を電解質溶液中に入れ、この電極間に電圧をかけたときに、電極‐溶液の界面で進行する電気化学的な不均一系反応をいう。この場合、負の電位のかかった極（陰極）上では陽イオンの還元（たとえば、水素イオンの水素分子への還元、2H⁺＋2e→H₂）がおこり（陰極反応という）、また正の電位のかかった極（陽極）では陰イオンの酸化（たとえば、塩化物イオンの塩素分子への酸化2Cl^-→Cl₂＋2e）が同時におこり（陽極反応という）、電解電流（ファラデー電流ともいう）が流れる。いまこのなかの一つの電極について、この電極にかけた電位と電解電流との間の関係を調べると、両者の間には、

の関係が成立する。ここでiは電解電流の大きさを、ηはその電極上でおこる陰極または陽極反応O＋neR（Oは酸化状態、Rは還元状態）が平衡にある場合の電極の電位を基準（電位0ボルト）として測った動作時の電極の電位、すなわち過電圧を、Tは絶対温度、またRは気体定数、αは定数で、反応によって異なった値をとる。この式をターフェル式という。ほかの化学反応と同様に、電極反応が平衡にあるときには、この反応の左に進む反応速度と、右に進む反応速度とが等しいと考えられる。この反応速度を交換電流という。電極反応を調べるのには、ターフェル式についての考察のほか、電極‐溶液間のインピーダンス測定、電気容量の測定などが行われる。

［戸田源治郎］

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