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Y. Li et al.
cracking, ball milling, and hydrothermal method. These methods can be summarized in two ways: one is to dope n simultaneously in the process of synthesis of
CNTs, which is called in situ method; the other is to carry out post-processing on
the synthesized CNTs to achieve N-doping, which is called post-processing [343]. In
the process of in-situ doping, N atoms from the precursors are incorporated into the
framework of CNTs in the process of CNTs growth. N atoms can come from different
precursors, such as transition metal macrocycles [344], ammonia [356], acetonitrile
[357], pyridine [358], melamine [236] and polymers. This in-situ doping method
usually requires the use of transition metals such as Fe, Co, Ni, Mn as catalysts
for NCNTs growth. After heat treatment, simple acid treatment can not completely
remove these transition metals, which have a great impact on the ORR performance
of the samples [344, 359]. In order to avoid the influence of these transition metals on
ORR performance, acid treatment and electrochemical treatment should be carried
out before electrochemical test, which may damage the structure of NCNTs and the
N in NCNTs. The preparation of NCNTs by heat treatment of purified CNTs and
N-source precursors without transition metals can effectively avoid the influence
of transition metals in the synthesis process. The precursors of this post-treatment
method include ammonia [342], urea [351], dicyandiamide, polyaniline (PANI) [343,
350], polypyrrole [360], etc.
Different methods of preparation will affect the morphology, structure, specific
surface area, nitrogen content, nitrogen doping form of NCNTs, etc. These characteristics will affect the ORR performance of NCNTs. Table 4.3 lists the NCNTs
prepared by different methods. These NCNTs prepared by different methods show
different ORR properties. Finding a method for preparing NCNTs with the best ORR
performance has become a research hotspot in this field.
4.6.1.4 ORR Mechanism of NCNTs
Until now, there is no perfect explanation for the enhancement mechanism of
nitrogen-doped carbon materials on oxygen reduction performance, but it is certain
that the nitrogen atoms and their bonding states play a decisive role in oxygen reduction performance. Dai et al. [334] thought that nitrogen of pyridine type and pyrrole
type played an important role in the process of oxygen reduction, and through DFT
(density functional theory) calculation shows that the substitution of nitrogen atom
for carbon atom will lead to the change of asymmetric spin density and atomic charge
density, and make the carbon atom near it have net positive charge, thus the adsorption mode of oxygen molecule changes from Pauling mode on carbon nanotube to
bridge mode on nitrogen-doped carbon nanotube, weakening the O = O bond, which
is conducive to the dissociation adsorption of oxygen. This kind of charge transfer
can promote oxygen reduction reaction. Gao et al. [361] used the spin polarization
density function theory of the first principle to study the process of ORR catalyzed
by NCNTs open boundary. O 2 and OOH can be chemically adsorbed on the C-N
bond at the NCNTs boundary without energy barrier, and can be partially reduced;
the adsorbed O 2 , OOH, and H
+ in the surrounding environment can be combined
Y. Li et al.
cracking, ball milling, and hydrothermal method. These methods can be summarized in two ways: one is to dope n simultaneously in the process of synthesis of
CNTs, which is called in situ method; the other is to carry out post-processing on
the synthesized CNTs to achieve N-doping, which is called post-processing [343]. In
the process of in-situ doping, N atoms from the precursors are incorporated into the
framework of CNTs in the process of CNTs growth. N atoms can come from different
precursors, such as transition metal macrocycles [344], ammonia [356], acetonitrile
[357], pyridine [358], melamine [236] and polymers. This in-situ doping method
usually requires the use of transition metals such as Fe, Co, Ni, Mn as catalysts
for NCNTs growth. After heat treatment, simple acid treatment can not completely
remove these transition metals, which have a great impact on the ORR performance
of the samples [344, 359]. In order to avoid the influence of these transition metals on
ORR performance, acid treatment and electrochemical treatment should be carried
out before electrochemical test, which may damage the structure of NCNTs and the
N in NCNTs. The preparation of NCNTs by heat treatment of purified CNTs and
N-source precursors without transition metals can effectively avoid the influence
of transition metals in the synthesis process. The precursors of this post-treatment
method include ammonia [342], urea [351], dicyandiamide, polyaniline (PANI) [343,
350], polypyrrole [360], etc.
Different methods of preparation will affect the morphology, structure, specific
surface area, nitrogen content, nitrogen doping form of NCNTs, etc. These characteristics will affect the ORR performance of NCNTs. Table 4.3 lists the NCNTs
prepared by different methods. These NCNTs prepared by different methods show
different ORR properties. Finding a method for preparing NCNTs with the best ORR
performance has become a research hotspot in this field.
4.6.1.4 ORR Mechanism of NCNTs
Until now, there is no perfect explanation for the enhancement mechanism of
nitrogen-doped carbon materials on oxygen reduction performance, but it is certain
that the nitrogen atoms and their bonding states play a decisive role in oxygen reduction performance. Dai et al. [334] thought that nitrogen of pyridine type and pyrrole
type played an important role in the process of oxygen reduction, and through DFT
(density functional theory) calculation shows that the substitution of nitrogen atom
for carbon atom will lead to the change of asymmetric spin density and atomic charge
density, and make the carbon atom near it have net positive charge, thus the adsorption mode of oxygen molecule changes from Pauling mode on carbon nanotube to
bridge mode on nitrogen-doped carbon nanotube, weakening the O = O bond, which
is conducive to the dissociation adsorption of oxygen. This kind of charge transfer
can promote oxygen reduction reaction. Gao et al. [361] used the spin polarization
density function theory of the first principle to study the process of ORR catalyzed
by NCNTs open boundary. O 2 and OOH can be chemically adsorbed on the C-N
bond at the NCNTs boundary without energy barrier, and can be partially reduced;
the adsorbed O 2 , OOH, and H
+ in the surrounding environment can be combined