2 Mechanism of Oxygen Reduction Reaction
23
material increases, which indicates that oxygen reduction is more likely to generate
H 2 O 2 than H 2 O.
Therefore, through the results of thermodynamic calculations, both ways are
achievable. In an electrochemical environment, it takes 498 kJ/mol to break the O–O
bond, which requires that the bond energy M–O between the metal and the oxygen
atom must be greater than 250 kJ/mol. But by transferring an electron to generate
O 2
− , then separating O 2
− only require 98.7 kJ/mol, and the associative mechanism
is the main pathway under the action of electric field. By conclusion, Neurock et al.
[11] believe that the initial step of the oxygen reduction reaction should be that the
oxygen molecule adsorbed on the surface of the electrode first gets an electron and
then combines with the hydrogen proton diffused on the surface of the electrode to
form HO 2 *.
On the other hand, when
18 O 2 ,
16 O
18 O and
16 O 2 are used as reactants, no isotope
exchange effect is observed in the product H 2 O 2 /HO 2
− , so oxygen molecules are
more likely to form O 2
− as an intermediate product. This result also supports the
path of oxygen reduction through the combination of oxygen molecule.
2.1.2 Control Factor of Oxygen Reduction Reaction
Although the rate-determining step of the oxygen reduction reaction has been in
dispute. Currently, the control reaction process in which O 2 first acquires electrons
is widely accepted. However, there are many other theories that hold different views.
The control reaction is to destroy the O–O double bond by the double-active site
mechanism, or the process of O 2 initial adsorption to the surface of the electrocatalyst,
and the desorbing process of O or OH from the surface of the electrocatalyst. Among
them, Wang et al. [13] consider that the OH ads desorption in the potential interval
of OH ads adsorption is a control step, while in the interval where Pt has no product
adsorption, O 2 acquiring electron is a control step. These theories all indicate that the
oxygen reduction multielectron reaction is a complex multistep reaction, and there
are many reaction paths depending on the morphology of electrode and the electrode
potential.
A large number of studies have shown that oxygen reduction reaction conditions are the key factors affecting the mechanism of oxygen reduction reaction.
They mainly include different catalyst metals or metal compounds, different carrier
materials, catalyst surface state, electrode structure and oxygen concentration.
This book will analyze the effects of catalysts and carrier materials in Chap. 4.
For details, please refer to Chap. 4. Currently, for commercial catalyst systems, Pt
metal particles (Pt/C) supported on high surface area carbon materials as catalysts still
exhibit the best activity and stability in both experimental and industrial applications.
Therefore, in this section, the Pt/C catalyst is taken as an example to discuss the
surface state and the electrode structure of the Pt metal for study of ORR mechanism.
The important influencing factors on the Pt/C electrocatalyst are first the oxygen
concentration Po 2 adsorbed on the surface, and the rate of the ORR reaction is linear
23
material increases, which indicates that oxygen reduction is more likely to generate
H 2 O 2 than H 2 O.
Therefore, through the results of thermodynamic calculations, both ways are
achievable. In an electrochemical environment, it takes 498 kJ/mol to break the O–O
bond, which requires that the bond energy M–O between the metal and the oxygen
atom must be greater than 250 kJ/mol. But by transferring an electron to generate
O 2
− , then separating O 2
− only require 98.7 kJ/mol, and the associative mechanism
is the main pathway under the action of electric field. By conclusion, Neurock et al.
[11] believe that the initial step of the oxygen reduction reaction should be that the
oxygen molecule adsorbed on the surface of the electrode first gets an electron and
then combines with the hydrogen proton diffused on the surface of the electrode to
form HO 2 *.
On the other hand, when
18 O 2 ,
16 O
18 O and
16 O 2 are used as reactants, no isotope
exchange effect is observed in the product H 2 O 2 /HO 2
− , so oxygen molecules are
more likely to form O 2
− as an intermediate product. This result also supports the
path of oxygen reduction through the combination of oxygen molecule.
2.1.2 Control Factor of Oxygen Reduction Reaction
Although the rate-determining step of the oxygen reduction reaction has been in
dispute. Currently, the control reaction process in which O 2 first acquires electrons
is widely accepted. However, there are many other theories that hold different views.
The control reaction is to destroy the O–O double bond by the double-active site
mechanism, or the process of O 2 initial adsorption to the surface of the electrocatalyst,
and the desorbing process of O or OH from the surface of the electrocatalyst. Among
them, Wang et al. [13] consider that the OH ads desorption in the potential interval
of OH ads adsorption is a control step, while in the interval where Pt has no product
adsorption, O 2 acquiring electron is a control step. These theories all indicate that the
oxygen reduction multielectron reaction is a complex multistep reaction, and there
are many reaction paths depending on the morphology of electrode and the electrode
potential.
A large number of studies have shown that oxygen reduction reaction conditions are the key factors affecting the mechanism of oxygen reduction reaction.
They mainly include different catalyst metals or metal compounds, different carrier
materials, catalyst surface state, electrode structure and oxygen concentration.
This book will analyze the effects of catalysts and carrier materials in Chap. 4.
For details, please refer to Chap. 4. Currently, for commercial catalyst systems, Pt
metal particles (Pt/C) supported on high surface area carbon materials as catalysts still
exhibit the best activity and stability in both experimental and industrial applications.
Therefore, in this section, the Pt/C catalyst is taken as an example to discuss the
surface state and the electrode structure of the Pt metal for study of ORR mechanism.
The important influencing factors on the Pt/C electrocatalyst are first the oxygen
concentration Po 2 adsorbed on the surface, and the rate of the ORR reaction is linear