14
Z. Zhao and P. K. Shen
For an electrode reaction O + ne R, the current (or electrode reaction rate)
is determined by the rate in the following process: ➀ substance transfer (transfer of
oxide O from the bulk solution to the electrode surface or reduced species R generated from electrode surface transfer to the bulk solution); ➁ adsorption, dissociation,
desorption, protonation process of the oxide O on the surface of the electrode; and
adsorption and desorption of the reducing material R on the electrode surface;➂
Electron transfer on the surface of the electrode is a chemical reaction kinetics
related to the potential applied to the electrode surface, and is also a core part of
our electrochemical concerns.
In general, the magnitude of the current during an electrochemical reaction is
generally limited by one or more slow reactions, which are referred to as ratedetermining steps, where any step in the reaction given by Fig. 2.2 may become
a speed-stop step. Below we will discuss in detail the three main processes that
affect the reaction rate of the electrode, namely the mass transfer, the adsorption and
desorption of molecules on the electrode surface and the kinetics of the electrode
reaction.
During the oxygen reduction reaction, oxygen molecules and intermediates
dissolved in the solvent participate in the adsorption process, so the mechanism
of the adsorption is indistinguishable from the mechanism of the heterogeneous
catalytic reaction. According to the adsorption principle, adsorption can be divided
into physical adsorption and chemical adsorption, and the adsorption force is also
different. The physical adsorption is caused by the inter-force, which is relatively
weak and does not affect the molecular structure. The chemical adsorption force is
a force formed by a chemical bond between an oxygen atom and a platinum atom,
and belongs to a chemical bond force. This force is relatively strong, involving electron rearrangement between the adsorbate molecules and solids, and breaking or
formation of chemical bonds. Therefore, the chemical adsorption force has a great
influence on the structure of the adsorbed molecules.
The change of the potential energy of the adsorption system during the adsorption
process is shown in Fig. 2.3: The energy change of the A molecule near the solid
surface is divided into physical adsorption and chemical adsorption [6]. The physical adsorption process is represented by the AYX curve and the chemical adsorption
process is represented by the BXZ line. The B position in the figure is the energy state
when the molecule dissociates into an atom. D is the dissociation energy. When the
molecules are close to the metal surface, the potential energy decreases. When physical adsorption occurs (at the Y point), the heat Qp is released. When the molecule
absorbs more energy and reaches the X point, this position is the chemical adsorption state of the dissociated atom (Z point in the figure), and the absorbed energy is
called the adsorption activation energy E a . The intersection X is a transition state that
is physically adsorbed to chemical adsorption. From the chemical adsorption state,
desorption, and to molecular state, an E d energy barrier is to be overcome. This part
of the energy is called desorption activation energy.
The change in energy from molecular adsorption and desorption is described by
the potential energy curve of adsorption. In addition to this, it is necessary to understand the interaction between the molecule and the catalytic surface to understand
Z. Zhao and P. K. Shen
For an electrode reaction O + ne R, the current (or electrode reaction rate)
is determined by the rate in the following process: ➀ substance transfer (transfer of
oxide O from the bulk solution to the electrode surface or reduced species R generated from electrode surface transfer to the bulk solution); ➁ adsorption, dissociation,
desorption, protonation process of the oxide O on the surface of the electrode; and
adsorption and desorption of the reducing material R on the electrode surface;➂
Electron transfer on the surface of the electrode is a chemical reaction kinetics
related to the potential applied to the electrode surface, and is also a core part of
our electrochemical concerns.
In general, the magnitude of the current during an electrochemical reaction is
generally limited by one or more slow reactions, which are referred to as ratedetermining steps, where any step in the reaction given by Fig. 2.2 may become
a speed-stop step. Below we will discuss in detail the three main processes that
affect the reaction rate of the electrode, namely the mass transfer, the adsorption and
desorption of molecules on the electrode surface and the kinetics of the electrode
reaction.
During the oxygen reduction reaction, oxygen molecules and intermediates
dissolved in the solvent participate in the adsorption process, so the mechanism
of the adsorption is indistinguishable from the mechanism of the heterogeneous
catalytic reaction. According to the adsorption principle, adsorption can be divided
into physical adsorption and chemical adsorption, and the adsorption force is also
different. The physical adsorption is caused by the inter-force, which is relatively
weak and does not affect the molecular structure. The chemical adsorption force is
a force formed by a chemical bond between an oxygen atom and a platinum atom,
and belongs to a chemical bond force. This force is relatively strong, involving electron rearrangement between the adsorbate molecules and solids, and breaking or
formation of chemical bonds. Therefore, the chemical adsorption force has a great
influence on the structure of the adsorbed molecules.
The change of the potential energy of the adsorption system during the adsorption
process is shown in Fig. 2.3: The energy change of the A molecule near the solid
surface is divided into physical adsorption and chemical adsorption [6]. The physical adsorption process is represented by the AYX curve and the chemical adsorption
process is represented by the BXZ line. The B position in the figure is the energy state
when the molecule dissociates into an atom. D is the dissociation energy. When the
molecules are close to the metal surface, the potential energy decreases. When physical adsorption occurs (at the Y point), the heat Qp is released. When the molecule
absorbs more energy and reaches the X point, this position is the chemical adsorption state of the dissociated atom (Z point in the figure), and the absorbed energy is
called the adsorption activation energy E a . The intersection X is a transition state that
is physically adsorbed to chemical adsorption. From the chemical adsorption state,
desorption, and to molecular state, an E d energy barrier is to be overcome. This part
of the energy is called desorption activation energy.
The change in energy from molecular adsorption and desorption is described by
the potential energy curve of adsorption. In addition to this, it is necessary to understand the interaction between the molecule and the catalytic surface to understand