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Y. Li et al.
4.6.4.2 Reparation of N-Graphene
At present, the main methods of N-doping for graphene are plasma treatment [383,
384], arc discharge [385], chemical vapor deposition in NH3 atmosphere [377, 386],
carbon nitrogen segregation [387]. Guo et al. [388] annealed graphene treated by N
+
ion irradiation in NH 3 atmosphere, and prepared N-graphene with nitrogen content
of 1.1%. P-graphene and N-graphene can be selectively prepared by controlling
doping experimental parameters. Wang et al. [383] first prepared graphene by chemical method, and then treated graphene by nitrogen plasma to obtain N-graphene
with nitrogen content of 1.26%. Qu et al. [377] successfully prepared N-graphene
with nitrogen content of 4% on Ni-modified SiO 2 /Si substrate by CVD in a mixed
gas (NH 3 : CH 4 : H 2 : Ar = 10:50:65:200), and the N-graphene showed good ORR
performance.
Geng et al. [380] first prepared graphene by heat treatment of graphene oxide,
and then heat treatment in NH 3 atmosphere, successfully prepared N-graphene with
nitrogen content of 2.8%. Li et al. [389] used NH 3 as reducing agent and nitrogen
source to prepare n-graphene by one-step heat treatment of graphene oxide. When
the heat treatment temperature was 500 °C, the nitrogen content could reach 5%.Li
et al. [390] prepared N-graphene with thickness of 2–6 by DC arc method using
NH 3 as discharge medium and pure graphite as electrode. Similar to the preparation
methods of NCNTs, the preparation methods of N-graphene can be divided into two
categories: one is to prepare n-graphene by one-step method [377, 389]; the other is
to prepare graphene first, and then, by plasma containing N or chemical treatment to
obtain N-graphene [383, 388].
4.6.4.3 ORR Mechanism of N-Graphene
In explaining the enhancement of ORR activity of CNTs by nitrogen doping, it
is generally believed that the charge transfer caused by N-doping can promote
the adsorption and dissociation of O 2 , thus promoting ORR, while different Ndoping forms play different roles in promoting ORR. For N-graphene, the theoretical research results of density functional theory show that the pyridine nitrogen
and pyrrole nitrogen formed by N atom replacing C atom in graphene will lead
to asymmetric distribution of spin density and atomic charge density, and the ORR
performance of N-graphene may come from this [391]. Sheng et al. [392] prepared Ngraphene mainly doped with pyridine type nitrogen by heat treatment of the mixture
of GO and melamine, and confirmed the promoting effect of pyridine type nitrogen
on ORR by experiments. An important reason for the slow kinetics of ORR reaction is that ORR is a complex multi-step reaction process. Boukhvalov et al. [393]
compared the energy barriers of graphene, N-graphene, and Pt surfaces in different
ORR reaction steps. The results show that N-graphene has the lowest energy barrier
in each ORR reaction step. Their research also shows that the ORR performance of
N-graphene with about 4% N-doping content is the best. In many ORR reaction steps,
the energy barrier of the first electron transfer step is often the highest, which makes
Y. Li et al.
4.6.4.2 Reparation of N-Graphene
At present, the main methods of N-doping for graphene are plasma treatment [383,
384], arc discharge [385], chemical vapor deposition in NH3 atmosphere [377, 386],
carbon nitrogen segregation [387]. Guo et al. [388] annealed graphene treated by N
+
ion irradiation in NH 3 atmosphere, and prepared N-graphene with nitrogen content
of 1.1%. P-graphene and N-graphene can be selectively prepared by controlling
doping experimental parameters. Wang et al. [383] first prepared graphene by chemical method, and then treated graphene by nitrogen plasma to obtain N-graphene
with nitrogen content of 1.26%. Qu et al. [377] successfully prepared N-graphene
with nitrogen content of 4% on Ni-modified SiO 2 /Si substrate by CVD in a mixed
gas (NH 3 : CH 4 : H 2 : Ar = 10:50:65:200), and the N-graphene showed good ORR
performance.
Geng et al. [380] first prepared graphene by heat treatment of graphene oxide,
and then heat treatment in NH 3 atmosphere, successfully prepared N-graphene with
nitrogen content of 2.8%. Li et al. [389] used NH 3 as reducing agent and nitrogen
source to prepare n-graphene by one-step heat treatment of graphene oxide. When
the heat treatment temperature was 500 °C, the nitrogen content could reach 5%.Li
et al. [390] prepared N-graphene with thickness of 2–6 by DC arc method using
NH 3 as discharge medium and pure graphite as electrode. Similar to the preparation
methods of NCNTs, the preparation methods of N-graphene can be divided into two
categories: one is to prepare n-graphene by one-step method [377, 389]; the other is
to prepare graphene first, and then, by plasma containing N or chemical treatment to
obtain N-graphene [383, 388].
4.6.4.3 ORR Mechanism of N-Graphene
In explaining the enhancement of ORR activity of CNTs by nitrogen doping, it
is generally believed that the charge transfer caused by N-doping can promote
the adsorption and dissociation of O 2 , thus promoting ORR, while different Ndoping forms play different roles in promoting ORR. For N-graphene, the theoretical research results of density functional theory show that the pyridine nitrogen
and pyrrole nitrogen formed by N atom replacing C atom in graphene will lead
to asymmetric distribution of spin density and atomic charge density, and the ORR
performance of N-graphene may come from this [391]. Sheng et al. [392] prepared Ngraphene mainly doped with pyridine type nitrogen by heat treatment of the mixture
of GO and melamine, and confirmed the promoting effect of pyridine type nitrogen
on ORR by experiments. An important reason for the slow kinetics of ORR reaction is that ORR is a complex multi-step reaction process. Boukhvalov et al. [393]
compared the energy barriers of graphene, N-graphene, and Pt surfaces in different
ORR reaction steps. The results show that N-graphene has the lowest energy barrier
in each ORR reaction step. Their research also shows that the ORR performance of
N-graphene with about 4% N-doping content is the best. In many ORR reaction steps,
the energy barrier of the first electron transfer step is often the highest, which makes