58
The reaction attracts considerable attention from a fundamental point of view
focusing on theoretical considerations of four-electron reduction mechanism, its
sensitivity to the electrode surface structural and electronic properties. The fourelectron reduction can include a number of elementary reactions, involving electron
transfer steps and chemical steps, that can form various parallel-consecutive pathways [1].
Recent work on oxygen reduction has partly focused on the surface properties of
oxygen electrocatalysts aimed at an improved understanding of the structure and
composition of the electrode surface in relation to its activity for this reaction. While
Pt is the best catalyst for the reaction (see Sect. 2.1), its slow kinetics is the major
impediment. Reducing the amount of Pt while increasing activity and stability of
the catalyst has been a topic for more than two decades [2]. Most recent efforts of
several groups, besides studying Pt monolayer catalysts, are focused on nonnoble
catalysts [3], thermally treated macrocyclics [4], and nitrited non-platinum metals
in various shapes, compositions, and sizes [5].
In either case, the three surface intermediates *OOH, *O, and *OH are involved,
and thus, it is well known that the ORR activity is correlated with the adsorption
energies of O/OH on catalyst surfaces. Moreover, the linear scaling relationships
between the binding energies for *OOH, *O, and *OH are also correlated due to the
formation of metal–oxygen bonds in all cases [6].
Despite the intensive study of the fundamental problems of this reaction, many
aspects of its kinetics are not understood. Oxygen reduction is a multielectron reaction that usually includes several elementary steps in reaction mechanisms. In aqueous solutions, oxygen reduction appears to occur by two overall pathways: a “direct”
four-electron reduction and a “peroxide” pathway, which involves H 2 O 2 as the intermediate [6].
The direct four-electron pathway (in acid and alkaline solutions) is
O
H
e
H O
V vs NHE
2
2
4
4
2
1229
+
+
→
=
+
−
Ea .
.
,
(6.1)
O
H O e
OH
V
2
2
2
4
4
0401
+
+
→
=
−
−
Ea
.
.
(6.2)
The peroxide pathway is
O
H
e
H O
V
2
2 2
2
2
0 67
+
+
→
=
+
−
Ea
.
.
(6.3)
Peroxide can undergo further reduction or decomposition in acid solutions via
the reactions:
H O
H
e
H O
V
2 2
2
2
2
2
177
+
+
→
=
+
−
Ea .
,
(6.4)
2
2
2 2
2
2
H O
H O O
→
+ .
(6.5)
In alkaline solutions, the reaction:
6 Important Electrocatalytic Reactions
The reaction attracts considerable attention from a fundamental point of view
focusing on theoretical considerations of four-electron reduction mechanism, its
sensitivity to the electrode surface structural and electronic properties. The fourelectron reduction can include a number of elementary reactions, involving electron
transfer steps and chemical steps, that can form various parallel-consecutive pathways [1].
Recent work on oxygen reduction has partly focused on the surface properties of
oxygen electrocatalysts aimed at an improved understanding of the structure and
composition of the electrode surface in relation to its activity for this reaction. While
Pt is the best catalyst for the reaction (see Sect. 2.1), its slow kinetics is the major
impediment. Reducing the amount of Pt while increasing activity and stability of
the catalyst has been a topic for more than two decades [2]. Most recent efforts of
several groups, besides studying Pt monolayer catalysts, are focused on nonnoble
catalysts [3], thermally treated macrocyclics [4], and nitrited non-platinum metals
in various shapes, compositions, and sizes [5].
In either case, the three surface intermediates *OOH, *O, and *OH are involved,
and thus, it is well known that the ORR activity is correlated with the adsorption
energies of O/OH on catalyst surfaces. Moreover, the linear scaling relationships
between the binding energies for *OOH, *O, and *OH are also correlated due to the
formation of metal–oxygen bonds in all cases [6].
Despite the intensive study of the fundamental problems of this reaction, many
aspects of its kinetics are not understood. Oxygen reduction is a multielectron reaction that usually includes several elementary steps in reaction mechanisms. In aqueous solutions, oxygen reduction appears to occur by two overall pathways: a “direct”
four-electron reduction and a “peroxide” pathway, which involves H 2 O 2 as the intermediate [6].
The direct four-electron pathway (in acid and alkaline solutions) is
O
H
e
H O
V vs NHE
2
2
4
4
2
1229
+
+
→
=
+
−
Ea .
.
,
(6.1)
O
H O e
OH
V
2
2
2
4
4
0401
+
+
→
=
−
−
Ea
.
.
(6.2)
The peroxide pathway is
O
H
e
H O
V
2
2 2
2
2
0 67
+
+
→
=
+
−
Ea
.
.
(6.3)
Peroxide can undergo further reduction or decomposition in acid solutions via
the reactions:
H O
H
e
H O
V
2 2
2
2
2
2
177
+
+
→
=
+
−
Ea .
,
(6.4)
2
2
2 2
2
2
H O
H O O
→
+ .
(6.5)
In alkaline solutions, the reaction:
6 Important Electrocatalytic Reactions