6
of surface physics. The penetration of the field into the metal electrode (the Thomas–
Fermi screening distance) appears important.
1.3 Charge-Transfer Reactions
Electrochemical reactions are heterogeneous chemical reactions in which electrons
are exchanged between the electrode and the molecules or ions in the electrolyte.
The electrode is metal or other electronic conductive materials, while the electrolyte
is purely ionic conductor which includes water and nonaqueous solvents, melt or
solid electrolytes. In the course of an electrochemical reaction, the electron transfer
occurs through the electrode/electrolyte interface. Electrons can be transferred
through the interface in both directions. Particle in the electrolyte becomes either
reduced when it accepts an electron from the electrode or oxidized when it gives an
electron to the electrode. Thus, the electrochemical reaction involves the passage of
electrical current. When the electrode potential is equal to the equilibrium potential,
partial anodic and partial cathodic currents are equal, so that the total current is zero.
However, when the imposed electrode potential is more positive or more negative
than the equilibrium potential, the total current that passes through the electrode is
the anodic or cathodic current, respectively.
The simplest electrochemical reactions are those in which the electron transfer
causes only the change of the oxidation state of a reactant, and no bond formation
or splitting takes place. Much more common are cases in which the electron transfer
is followed by or occurs simultaneously with the adsorption and/or chemical
changes of a reactant, reaction intermediates, or products. Thus, the electrochemical
reactions are divided into two classes: (i) outer-sphere one-electron transfer with the
solution-phase electron donors or acceptors in the Helmholtz plane (OHP) of the
electrical double layer where the electron transfer occurs and (ii) more complex
processes where more than one electron may be transferred. Class 1 of electrochemical reactions involves a simple ionic redox process in which only the change
of oxidation state of reactants positioned in the OHP is involved. Class 2 reactions
often involve multiple steps, some can be chemical. When a reaction occurs in a
series of consecutive steps, the overall reaction rate is determined by the rate of the
slowest step, called the rate-determining step. All other preceding and following
steps can be considered to be in equilibrium. If the slowest step in the reaction
mechanism is the exchange of electrons, then the electrochemical reaction takes
place under electrochemical or activation control. Many electrochemical reactions
of organic molecules and reactions accompanied by gas evolution or dissolution are
in a class 2.
Just as for chemical reactions in general, the charge transfer is controlled by the
existence of the energy barrier between oxidized and reduced states. A unique feature of the electrode reactions is that the height of this barrier can be decreased or
increased by changing the potential across the interface.
1 Short Introduction to the Science of Electrocatalysis
of surface physics. The penetration of the field into the metal electrode (the Thomas–
Fermi screening distance) appears important.
1.3 Charge-Transfer Reactions
Electrochemical reactions are heterogeneous chemical reactions in which electrons
are exchanged between the electrode and the molecules or ions in the electrolyte.
The electrode is metal or other electronic conductive materials, while the electrolyte
is purely ionic conductor which includes water and nonaqueous solvents, melt or
solid electrolytes. In the course of an electrochemical reaction, the electron transfer
occurs through the electrode/electrolyte interface. Electrons can be transferred
through the interface in both directions. Particle in the electrolyte becomes either
reduced when it accepts an electron from the electrode or oxidized when it gives an
electron to the electrode. Thus, the electrochemical reaction involves the passage of
electrical current. When the electrode potential is equal to the equilibrium potential,
partial anodic and partial cathodic currents are equal, so that the total current is zero.
However, when the imposed electrode potential is more positive or more negative
than the equilibrium potential, the total current that passes through the electrode is
the anodic or cathodic current, respectively.
The simplest electrochemical reactions are those in which the electron transfer
causes only the change of the oxidation state of a reactant, and no bond formation
or splitting takes place. Much more common are cases in which the electron transfer
is followed by or occurs simultaneously with the adsorption and/or chemical
changes of a reactant, reaction intermediates, or products. Thus, the electrochemical
reactions are divided into two classes: (i) outer-sphere one-electron transfer with the
solution-phase electron donors or acceptors in the Helmholtz plane (OHP) of the
electrical double layer where the electron transfer occurs and (ii) more complex
processes where more than one electron may be transferred. Class 1 of electrochemical reactions involves a simple ionic redox process in which only the change
of oxidation state of reactants positioned in the OHP is involved. Class 2 reactions
often involve multiple steps, some can be chemical. When a reaction occurs in a
series of consecutive steps, the overall reaction rate is determined by the rate of the
slowest step, called the rate-determining step. All other preceding and following
steps can be considered to be in equilibrium. If the slowest step in the reaction
mechanism is the exchange of electrons, then the electrochemical reaction takes
place under electrochemical or activation control. Many electrochemical reactions
of organic molecules and reactions accompanied by gas evolution or dissolution are
in a class 2.
Just as for chemical reactions in general, the charge transfer is controlled by the
existence of the energy barrier between oxidized and reduced states. A unique feature of the electrode reactions is that the height of this barrier can be decreased or
increased by changing the potential across the interface.
1 Short Introduction to the Science of Electrocatalysis