4 Catalyst Materials for Oxygen Reduction Reaction
141
chemical environment around C, O, and Co are the reasons for the synergy between
Co 3 O 4 and N-rmGO.
Similar to Co 3 O 4 /N-rmGO, Liang et al. [144] also prepared spinel MnCo 2 O 4
and nitrogen-doped graphene composites(MnCo 2 O 4 /N-rmGO) by a similar method.
Compared with Co 3 O 4 /N-rmGO, the ORR performance of MnCo 2 O 4 /N-rmGO in
0.1 M KOH solution has been further improved, and it is closer to Pt/C catalyst.
They used the same X-ray absorption near edge structure analysis to reveal the
synergy between MnCo 2 O 4 and N-rmGO, and reached similar conclusions. Because
of the synergistic effect with nitrogen-doped graphene, the ORR performance
of its composites Fe 3 O 4 nanoparticles supported on three-dimensional nitrogendoped graphene aerogel (Fe 3 O 4 /N-GAs) [184], Mn 3 O 4 nanoparticles supported on
nitrogen-doped grapheme [227] and so on, has been improved. As an ORR catalyst,
this type of transition metal oxide has an ORR activity that is very close to that
of a Pt/C catalyst. However, this type of catalyst has a big limitation that its application as an ORR catalyst can only be used in alkaline media, and such materials
are unstable in acidic media. However, with the development of alkaline fuel cells,
more in-depth research on such materials and exploring such new catalysts with
ORR performance closer to or even exceeding Pt/C catalysis have great research and
application significance and requirements.
4.4.3.4 Transition Metal Sulfur/Selenide ORR Catalyst
Transition metal sulfides and selenides have very high ORR catalytic activity in acidic
media [228, 229], but such catalysts are very unstable under acidic conditions. When
the catalyst is run for a period of time, its catalytic activity gradually decreases. The
main reason is that S/Se atoms are gradually replaced partially or completely by O.
For example, Co 3 S 4 /C nanoparticles have an ORR open-circuit potential of about
0.67 V in an acidic medium, and have good thermodynamic stability in an air atmosphere at 300 oC. However, due to the poor chemical stability of Co 3 S 4 /C, Co 3 S 4 /C
will be converted into CoSO 4 during operation, which will lead to performance
degradation [230].
The process of the ORR reaction that occurs on the transition metal sulfur/selenide
surface is as follows:
1. O 2 + H
+
+ e
−
+
*
→ HOO
∗
2. HOO
∗
+ H
+
+ e
−
→ H 2 O + O
∗
3. O
∗
+ H
+
+ e
−
→ OH
∗
4. OH
∗
+ H
+
+ e
−
→ H 2 O
+*
Where ∗ is the active position of the reaction, e
− is the electron in the electrode,
and H
+ is the proton in the electrolyte. The reaction rate-determining step of the ORR
process of the material on the left of the volcano is the reaction of electron/proton
conversion to OH
∗ , that is, reaction step 4. The reaction rate-determining step of the
ORR process of the material on the right of the volcano is the step of converting
protons and electrons to adsorbed O 2 , that is, reaction step 1. Metal Pt is at the top
141
chemical environment around C, O, and Co are the reasons for the synergy between
Co 3 O 4 and N-rmGO.
Similar to Co 3 O 4 /N-rmGO, Liang et al. [144] also prepared spinel MnCo 2 O 4
and nitrogen-doped graphene composites(MnCo 2 O 4 /N-rmGO) by a similar method.
Compared with Co 3 O 4 /N-rmGO, the ORR performance of MnCo 2 O 4 /N-rmGO in
0.1 M KOH solution has been further improved, and it is closer to Pt/C catalyst.
They used the same X-ray absorption near edge structure analysis to reveal the
synergy between MnCo 2 O 4 and N-rmGO, and reached similar conclusions. Because
of the synergistic effect with nitrogen-doped graphene, the ORR performance
of its composites Fe 3 O 4 nanoparticles supported on three-dimensional nitrogendoped graphene aerogel (Fe 3 O 4 /N-GAs) [184], Mn 3 O 4 nanoparticles supported on
nitrogen-doped grapheme [227] and so on, has been improved. As an ORR catalyst,
this type of transition metal oxide has an ORR activity that is very close to that
of a Pt/C catalyst. However, this type of catalyst has a big limitation that its application as an ORR catalyst can only be used in alkaline media, and such materials
are unstable in acidic media. However, with the development of alkaline fuel cells,
more in-depth research on such materials and exploring such new catalysts with
ORR performance closer to or even exceeding Pt/C catalysis have great research and
application significance and requirements.
4.4.3.4 Transition Metal Sulfur/Selenide ORR Catalyst
Transition metal sulfides and selenides have very high ORR catalytic activity in acidic
media [228, 229], but such catalysts are very unstable under acidic conditions. When
the catalyst is run for a period of time, its catalytic activity gradually decreases. The
main reason is that S/Se atoms are gradually replaced partially or completely by O.
For example, Co 3 S 4 /C nanoparticles have an ORR open-circuit potential of about
0.67 V in an acidic medium, and have good thermodynamic stability in an air atmosphere at 300 oC. However, due to the poor chemical stability of Co 3 S 4 /C, Co 3 S 4 /C
will be converted into CoSO 4 during operation, which will lead to performance
degradation [230].
The process of the ORR reaction that occurs on the transition metal sulfur/selenide
surface is as follows:
1. O 2 + H
+
+ e
−
+
*
→ HOO
∗
2. HOO
∗
+ H
+
+ e
−
→ H 2 O + O
∗
3. O
∗
+ H
+
+ e
−
→ OH
∗
4. OH
∗
+ H
+
+ e
−
→ H 2 O
+*
Where ∗ is the active position of the reaction, e
− is the electron in the electrode,
and H
+ is the proton in the electrolyte. The reaction rate-determining step of the ORR
process of the material on the left of the volcano is the reaction of electron/proton
conversion to OH
∗ , that is, reaction step 4. The reaction rate-determining step of the
ORR process of the material on the right of the volcano is the step of converting
protons and electrons to adsorbed O 2 , that is, reaction step 1. Metal Pt is at the top