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Y. Li et al.
and reduced to form two H 2 O molecules without energy barrier. The energy barrier
of 0.49 eV is required to complete the catalytic process in the second H 2 O molecule
desorption process, so the desorption of H 2 O is a rate-determining step. On the
boundary HN-CNT site, the formation of ORR and H 2 O can be completed efficiently. For the FN-CNTs sites where both C atoms at the boundary are replaced by
N atoms, the reduction and dissociation of O 2 need to overcome the energy barrier of
0.81 eV, and the formation of H 2 O is spontaneous at the same time; ooh can adsorb
and reduce on the N–N bond in FN-CNTs, and then generate H 2 O spontaneously.
4.6.2 Boron-Doped Carbon Nanotubes
When the N atom with one more valence electron than the C atom is doped into the
π bond formed by SP
2 hybrid structure of CNTs, the extra electrons provided by the
N atom will enhance the electronic density of the system and increase the maximum
energy level of Homo of SP
2 hybrid C atom, thus promoting the ORR reaction.
When the B atom with few electrons is doped into CNTs to form boron-doped CNTs
(BCNTs), the delocalized π electrons in the conjugated system act on the 2pz space
orbit of B, resulting in electron accumulation, which can also promote ORR [362].
Ma et al. [363] prepared BCNTs with B content of 0–2.13% by chemical vapor
deposition (CVD) with benzene, triphenylborane, and ferrocene as precursors. With
the increase of B content, the morphology of BCNTs changed from hollow structure
of CNTs to bamboo-like structure, then to twisted nanotubes. The analysis of XPS
results shows that the performance of BCNTs is much better than that of CNTs, and
the performance of BCNTs increases with the increase of B content. In addition, like
ncnts, BCNTs are inert to methanol, CO, and other fuels or intermediates, and it is
also an ORR catalyst with cathode selectivity and anti-toxic performance. Different
from NCNTs, in NCNTs, the adsorption of O
2 occurs on the C atom with positive
charge distribution adjacent to the N atom, while in BCNTs, the adsorption of O
2
occurs on the B atom with doping. B in BCNTs has one less valence electron than C.
In BCNTs, the delocalized π electron in the conjugated structure of C will transfer
to the doped B atom. This charge transfer can make O
2 chemically adsorb on the
B atom as a bridge; in addition, it can weaken the O–O bond, thus promoting the
occurrence of ORR.
4.6.3 Doped Graphene
Graphene was first discovered in 2004 by Andre Geim and Konstantin Novoselov,
University of Manchester in the United Kingdom [363]. Graphene is a single-layer
SP
2 hybrid carbon structure and the basic unit of graphitic carbon. Graphene can
form many kinds of graphite allotrope. For example, it can be rolled up to form
zero-dimensional materials such as C 60 fullerenes, or it can be rolled up to form
Y. Li et al.
and reduced to form two H 2 O molecules without energy barrier. The energy barrier
of 0.49 eV is required to complete the catalytic process in the second H 2 O molecule
desorption process, so the desorption of H 2 O is a rate-determining step. On the
boundary HN-CNT site, the formation of ORR and H 2 O can be completed efficiently. For the FN-CNTs sites where both C atoms at the boundary are replaced by
N atoms, the reduction and dissociation of O 2 need to overcome the energy barrier of
0.81 eV, and the formation of H 2 O is spontaneous at the same time; ooh can adsorb
and reduce on the N–N bond in FN-CNTs, and then generate H 2 O spontaneously.
4.6.2 Boron-Doped Carbon Nanotubes
When the N atom with one more valence electron than the C atom is doped into the
π bond formed by SP
2 hybrid structure of CNTs, the extra electrons provided by the
N atom will enhance the electronic density of the system and increase the maximum
energy level of Homo of SP
2 hybrid C atom, thus promoting the ORR reaction.
When the B atom with few electrons is doped into CNTs to form boron-doped CNTs
(BCNTs), the delocalized π electrons in the conjugated system act on the 2pz space
orbit of B, resulting in electron accumulation, which can also promote ORR [362].
Ma et al. [363] prepared BCNTs with B content of 0–2.13% by chemical vapor
deposition (CVD) with benzene, triphenylborane, and ferrocene as precursors. With
the increase of B content, the morphology of BCNTs changed from hollow structure
of CNTs to bamboo-like structure, then to twisted nanotubes. The analysis of XPS
results shows that the performance of BCNTs is much better than that of CNTs, and
the performance of BCNTs increases with the increase of B content. In addition, like
ncnts, BCNTs are inert to methanol, CO, and other fuels or intermediates, and it is
also an ORR catalyst with cathode selectivity and anti-toxic performance. Different
from NCNTs, in NCNTs, the adsorption of O
2 occurs on the C atom with positive
charge distribution adjacent to the N atom, while in BCNTs, the adsorption of O
2
occurs on the B atom with doping. B in BCNTs has one less valence electron than C.
In BCNTs, the delocalized π electron in the conjugated structure of C will transfer
to the doped B atom. This charge transfer can make O
2 chemically adsorb on the
B atom as a bridge; in addition, it can weaken the O–O bond, thus promoting the
occurrence of ORR.
4.6.3 Doped Graphene
Graphene was first discovered in 2004 by Andre Geim and Konstantin Novoselov,
University of Manchester in the United Kingdom [363]. Graphene is a single-layer
SP
2 hybrid carbon structure and the basic unit of graphitic carbon. Graphene can
form many kinds of graphite allotrope. For example, it can be rolled up to form
zero-dimensional materials such as C 60 fullerenes, or it can be rolled up to form