30
C. Juhong et al.
Research on the electrochemical behavior of oxygen has been around for a long
time, and there are many research teams and review articles. Although the reduction
of molecular oxygen is a crucial step, the oxygen reduction reaction is an extremely
complex four-electron reaction, and unstable intermediates are easily formed in the
reaction, such as hydrogen peroxide generation by two-electron reaction (H 2 O 2 ),
thereby reducing energy conversion efficiency. The intermediate, H 2 O 2 , will damage
the membrane in fuel cells by combining transition metal cations, like iron ion, to
proceed with a Fenton reaction as follows.
M
2+
+ H 2 O 2 → M
3+
+
• OH + OH
−
(3.1)
M
3+
+ H 2 O 2 → M
2+
+
• OOH + H
+
(3.2)
where M
2+ is the transition metal cation. These metal ions will react with hydrogen
peroxide and produce-free radicals to attack the weak sites in a membrane. In a
perfluorinated sulfonic acid membrane electrode, the weak polymer end groups or
the side chain cleavage are the potential sources of carboxylic acid end groups. Once
the carboxylic acid end groups are formed, it will be attacked by free radicals in an
unzipping reaction:
∼ CF 2 COOH + 2OH
−
→ COOH + CO 2 + 2HF
(3.3)
If the structure of the membrane is damaged, the pinhole formation, fragmentation
and short circuit of the system become possible. The following is a brief introduction
to the common reaction mechanism of oxygen reduction in fuel cell systems.
3.1.1 Oxygen Reduction Reaction Mechanism
In the acidic solution, the total reaction formula of the four-electron reaction in which
oxygen is reduced to water is expressed as follows.
O 2 + 4H
+
+ 4e
−
→ 2H 2 OE = 1.229 V vs. SHE25
◦ C
( 3 . 4 )
The overall reaction equation in an alkaline solution is,
O 2 + 2H 2 O + 4e
−
→ 4OH
− E = 0.40 V vs. SHE25
◦ C
(3.5)
1.229 and 0.40 V are theoretical equilibrium potentials, calculated from thermodynamic data.
C. Juhong et al.
Research on the electrochemical behavior of oxygen has been around for a long
time, and there are many research teams and review articles. Although the reduction
of molecular oxygen is a crucial step, the oxygen reduction reaction is an extremely
complex four-electron reaction, and unstable intermediates are easily formed in the
reaction, such as hydrogen peroxide generation by two-electron reaction (H 2 O 2 ),
thereby reducing energy conversion efficiency. The intermediate, H 2 O 2 , will damage
the membrane in fuel cells by combining transition metal cations, like iron ion, to
proceed with a Fenton reaction as follows.
M
2+
+ H 2 O 2 → M
3+
+
• OH + OH
−
(3.1)
M
3+
+ H 2 O 2 → M
2+
+
• OOH + H
+
(3.2)
where M
2+ is the transition metal cation. These metal ions will react with hydrogen
peroxide and produce-free radicals to attack the weak sites in a membrane. In a
perfluorinated sulfonic acid membrane electrode, the weak polymer end groups or
the side chain cleavage are the potential sources of carboxylic acid end groups. Once
the carboxylic acid end groups are formed, it will be attacked by free radicals in an
unzipping reaction:
∼ CF 2 COOH + 2OH
−
→ COOH + CO 2 + 2HF
(3.3)
If the structure of the membrane is damaged, the pinhole formation, fragmentation
and short circuit of the system become possible. The following is a brief introduction
to the common reaction mechanism of oxygen reduction in fuel cell systems.
3.1.1 Oxygen Reduction Reaction Mechanism
In the acidic solution, the total reaction formula of the four-electron reaction in which
oxygen is reduced to water is expressed as follows.
O 2 + 4H
+
+ 4e
−
→ 2H 2 OE = 1.229 V vs. SHE25
◦ C
( 3 . 4 )
The overall reaction equation in an alkaline solution is,
O 2 + 2H 2 O + 4e
−
→ 4OH
− E = 0.40 V vs. SHE25
◦ C
(3.5)
1.229 and 0.40 V are theoretical equilibrium potentials, calculated from thermodynamic data.