1) A facile route is presented for synthesizing nitrogen-doped hollow graphitic carbon spheres (NHGCSs) through the direct pyrolysis of solid melamine–formaldehyde (MF) resin spheres. 2) The MF resin spheres are prepared via a hydrothermal method without templates or catalysts, and pyrolyzed at temperatures above 400°C to form hollow carbon spheres with graphitic carbon shells. 3) The NHGCSs exhibit excellent capacitive performance as electrode materials for supercapacitors, achieving a high specific capacitance of 306 Fg-1 at 0.1 A g-1.

2. GC University Faisalabad (Layyah Campus)
Seminar On
A facile route for
nitrogen doped
hollow graphitic
carbon spheres
with superior
performance in
supercapacitors.
Presented To:
Dr Rehan H Shah
Presented By:
Muhammad Mohsan
Roll No: 359
BS-Chemistry
Session (2014-2018)
Analytical Chemistry
8th Semester.
Department of Chemistry

3.  Approach to prepare nitrogen-doped hollow carbon
microspheres with graphitic carbon shells
(NHGCSs).
 The obtained NHGCSs by the direct pyrolysis of
solid melamine–formaldehyde (MF) resin spheres
exhibit excellent capacitance performance.
 The specific capacitance reaches 306 Fg⁻1 at a current
density of 0.1 A g⁻1 (in 2M H₂SO₄), and remains as
high as 230 Fg⁻1 even at a high current density of 3 A
g⁻1, which indicates the optimistic prospects for
using NHGCSs as supercapacitor electrode
materials.
Abstract

4. 
 Carbon materials with various microtextures and wide
availabilities represent very attractive materials for
energy storage.
 Among them, hollow carbon spheres (HCSs) exhibit
potential applications in lithium batteries, fuel cells,
catalyst supports, gas storage media and adsorbents,
hollow spheres composites, or as templates for the
synthesis of other useful hollow spheres.
 HCSs have been used as supercapacitor electrode
materials due to their advantages, such as high surface
areas, short mass diffusion and transport resistance.
Introduction

5. Microtexture
Another type of profilometer
is for measuring the surface
texture of a road and how it
relates to the coefficient of
friction and thus to skid
resistance. Pavement texture
is divided into three
categories: megatexture,
macrotexture,
and microtexture.
Lithium Batteries

6. 
 An important strategy to improve the carbon
capacitance is doping (N,O,P) which can have
reversible pseudo-capacitance and enhance the
electronic conductivity.
 HCS materials with graphitic walls are improved
electrical conductivity and outstanding stability.
 It is great significance to find a direct, low-cost and
effective approach to prepare nitrogen-doped hollow
graphitic carbon spheres (NHGCSs).
Pseudo-Capacitance

7. A pseudo capacitor is part of an electrochemical capacitor, and forms together
with an electric double-layer capacitor (EDLC) to create a
supercapacitor. Pseudo capacitance and double-layer capacitance add up to a
common inseparable capacitance value of a supercapacitor.

8.  Up to now, the fabrication of HCSs is widely based
on template methods.
 These methods are commonly carried out with
multiple procedures and time consuming synthetic
steps. In addition, these HCSs are mostly composed
of amorphous carbon shells.
 There are only a few reports about the preparation of
HGCSs.

9. Raymond E. Schaak et al. demonstrated a
multistep template-based strategy for
generating graphitic carbon shells using Ni3C
nanoparticle precursors.
Hui Lu et al. prepared HGCSs by the pyrolysis
of hollow polymers with the aid of transition-
metal species.

10.  In some other reports, crystalline carbon hollow
spheres and/or Nano graphene-based hollow
carbon spheres have been fabricated using silica
spheres as the hard templates.

11. 
Composites-A need
 Although there are several reports on
nitrogen-doped hollow carbon
spheres fabricated by the pyrolysis of
a self-polymerized dopamine shell
wrapped on SiO2 microspheres, the
carbon shell does not have a graphitic
structure.
 To date, reports about NHGCSs are
rather scarce.
 The only report in 2006 concerned
NHGCSs that were prepared using
silica spheres as hard templates via
chemical vapor deposition of
acetonitrile,and the preparation
process is quite complicated.

12.  In this paper, we develop a novel template-free
approach for the one-pot preparation of nitrogen-
doped hollow carbon microspheres with graphitic
carbon shells by direct pyrolysis of solid melamine–
formaldehyde (MF) resin spheres.
 Micrometer sized solid MF spheres were produced by
condensation of melamine with formaldehyde in
aqueous solution through hydrothermal treatment,
which can be easily converted into NHGCSs through
simple heat-treatment at certain temperatures under
high-purity nitrogen stream.

13.  Melamine resin or melamine formaldehyde (also
shortened to melamine) is a hard, thermosetting plastic
material made from melamine and formaldehyde by
polymerization.

14.  The process of forming hollow structures is investigated in detail
and a preliminary formation mechanism for the NHGCSs is also
proposed.
 It is worth noting that the NHGCSs show remarkable
performance as the electrode materials of supercapacitors with a
high specific capacitance of 306 Fg⁻1 at a current density of 0.1 A
g⁻1, which is superior to most cases of reported nitrogen-doped
microporous activated carbon (230 F g⁻1).
 Melamine-based carbon (204.8 F g⁻1), and nitrogen enriched
mesoporous carbon spheres (211 F g⁻1), and is comparable to the
later reported nitrogen-containing hydrothermal carbon material
(300 F g1).

15.  The MF spheres were prepared by the polymerization of
melamine and formaldehyde through hydrothermal treatment
without the addition of any catalysts or surfactants.
 In a typical SEM image of the obtained polymer spheres.
 The spheres have diameters in the range of 4.8–5.5 mm and are
monodisperse.
 The inset shows an image of a ruptured sphere, which indicates
that the MF spheres are solid.

16. The chemical structure of the MF resin is studied
by FT-IR.
Raymond E. Schaak et al. demonstrated a multistep
template-based strategy for generating graphitic
carbon shells using Ni3C nanoparticle precursors.
These results prove that the MF-4 spheres are
composed of triazine rings that are almost
primarily connected through methylene linkages.

17. Basic Principle of FT-IR

18. 
 There are two principle linkages during the condensation
of methylol melamine.
i. Ether linkages (–N–CH2–O–CH2–N–)
ii. Methylene linkages (–N–CH2–N–)
 Many reported melamine resins that are prepared in
solution at ambient pressure are mainly cross-linked
through ether linkages (with a small part of methylene
linkages in the condensed system).
 Compared to these results, it seems that both the
synthesis methods and the molar ratio of formaldehyde to
melamine exert important influences on the type of
bridges between the triazine rings.
Formation of MF resin
spheres

19. Schematic illustration of the
formation process of the MF
resin spheres.
 The reaction of melamine with
formaldehyde leads at first to
hydroxymethylation, whereas the
hydrogen atoms in the NH2 groups
of the melamine are substituted by
methylol groups (–CH2OH).
 Under hydrothermal conditions,
these methylolmelamines cross-link
through methylene linkages to
form sheet-like colloidal
nanoparticles.
 This phenomenon can be observed
when the reaction solution is
hydrothermal-treated within one
hour.
 These nanoparticles further cross-
link through the remaining
methylol groups during
condensation, resulting in uniform
solid MF spheres.

20. 
 In order to obtain carbon products, the spherical MF-4
resins were pyrolyzed at certain temperatures under
nitrogen flow.
 A typical SEM image of the sample after heating from
room temperature to 800 C at a ramp rate of 1⁰C min1,
and then keeping at 800 C for 5 hours.
 The diameter of the obtained carbon spheres is about 1.8
mm with a total shrinkage of about 64% compared to the
MF-4 spheres (5 mm).
 If we take a close look at the ruptured particle that is
obtained by grinding the carbon spheres, a hollow core
and shell structure can be observed, and the shell
thickness is about 200 nm.
Morphological Evolution

21. Typical SEM images of the MF spheres after heating at different temperatures for 5
hours: (a) 150 C, (b) 300 C, (c) 400 C and (d) 700 C. Inset is the enlarged image of the
ruptured spheres to show the interior morphology.

22.  This result reveals the formation of hollow carbon
spheres instead of solid ones.
o It is worth noting that this hollow structure can always
been obtained, nomatter the ramp rate from 1 °C /min1
up to 10 °C/min1.
 It is difficult to reveal the hollow structure from the
TEM image (Fig. 3b).
 The quite thick carbon shell prevents the penetration of
electron beams through the shell.
 To the best of our knowledge, this is the first report on
the formation of such hollow carbon particles through a
simple pyrolysis process of MF spheres.

23.  High-resolution transmission electron microscope
(HRTEM) images are used to identify the formed
carbon phases and the crystallinity of the carbon shell.
 The results clearly illustrate that the carbon shell
features graphitic order with weak edge terminations
and small crystalline domains.
 These small crystalline regions are formed by 4–8
parallel fringes with an interlayer d-spacing of 0.348
nm.
 The graphitic structures of the carbon shells are
further confirmed by XRD and Raman analyses.
 A typical XRD pattern of the carbonized sample.
 There are two diffraction peaks centered at 25.4 and
43.7,which can be indexed to the (002) and (101)
diffractions from the graphitic phase.

24.  The (002) diffraction peak is relatively low in intensity and broad in
shape, which is often related to the turbostratic carbon structure
having randomly oriented graphene layers.
 A representative Raman spectrum of the carbonized sample shows
two bands centered at 1575 cm1 (G band) and 1357 cm1 (D band),
respectively.
 The G band is closely related to the graphitic carbon phase with sp2
electronic configuration, such as graphene layers.
 The D band is a common feature of all disordered graphite carbon.
 The relative intensity of these two lines depends on the type of
graphitic materials and reflects the degree of graphitization.

25.  These results clearly demonstrate that the shells of the
hollow carbon spheres are composed of turbostratic
carbon with a weakly ordered graphitic microstructure.
 Nitrogen absorption/desorption isotherms of the
NHGCSs are presented.
 It is obvious that the isotherm is type I with
microporous feature, which means that the shell of the
NHGCSs has a microporous structure (pores less than 2
nm).
 The Brunauer–Emmett–Teller (BET) specific surface
area is 753 cm3 g⁻1, indicating that the NHGCSs have
good porosity.

26.  CHN elemental analysis results prove that an
appreciable amount of nitrogen has been introduced
into the carbon matrix by the pyrolysis ofMF-4 resin
in inert gas.
 The nitrogen content remains at 6.02 wt.% even after
heat-treatment at 800 C for 5 hours (Table S2†).
 The chemical state of nitrogen presented in the
carbonized samples is further studied by X-ray
photoelectron spectroscopy (XPS).
 XPS results reveal that nitrogen is indeed embedded
into the graphitic structure in the form of pyridinic
nitrogen (397.9 eV),pyrrolic/pyridone (399.9 eV), and
quaternary nitrogen (400.6 eV) in the graphitic
layers.41,42

27. 
 The morphological evolution of the MF spheres under the
pyrolysis process is monitored by SEM to deduce the
formation mechanism of the hollow carbon spheres.
 The MF-4 microspheres are annealed in N2 atmosphere at
different temperatures for 5 hours.
 It is found that the MF particles are solid spheres when
heat-treated below 300 C and the particle diameter
decreases from 5.0 mm to 4.0 mm, due to continued loss
of water and formaldehyde that are released during
further cross-linking of the residual methylol groups.
Morphological Evolution By
Pyrolysis Process

28.  The MF sphere morphology changes dramatically
after heating at 400 ⁰C.
 Some cavities appear inside the spheres, and the
sphere diameter decreases further by about 14% (from
4.2 mm to 3.6 mm) When the heating temperature is
up to 700 ⁰C, hollow spheres with a diameter of 1.9
mm are clearly formed.
 It should be pointed out that some hollow spheres,
after heat-treating at 400 C, exhibit a collapsed shape
like deflated balloons.
 This is likely caused by the softening (or melting) of
the spheres and, at the same time, the rapid discharge
of volatile compounds from the interior of the
spheres.

29.  The TGA test of the MF-4 resin shows a sharp weight
loss at about 400 ⁰C.
 The overall weight change is about 22% (from 80% to
58%), which presumably corresponds to the thermal
decomposition of the MF resin.
 Further weight loss can be observed up to 800 ⁰C, due
to the evolution of ammonia and hydrogen cyanide
from the spheres.
 The sharp weight loss at 400 ⁰C may play an
important role in producing the hollow structure.

30. Fig. (a) Typical SEM image. (b) TEM, (c) HRTEM images of ruptured carbon shell. (d) XRD
pattern of the hollow carbon spheres.

31. 
 The pyrolysis reaction of MF-4 resin is further monitored
by FT-IR spectra.
 The characteristic peaks almost remain unchanged after
heat-treatment at 300 ⁰C, suggesting that the chemical
structure of the treated samples do not undertake great
changes.
 At the same time, the C/N molar ratio decreases
gradually with increases of the treatment temperature,
due to the condensation of remaining methylol groups.
 When the MF resin is heat-treated up to 400 ⁰C, the FT-IR
spectrum is completely different.
FT-IR Spectra

32. Fourier transform infrared (FT-IR) spectra of MF-4 spheres heat-treated at different
temperatures for 5 hours: (a) as-synthesized, (b) 300 C, (c) 400 C, (d) 500 C,
(e) 600 C, (f) 700 C, (g) 800 C.

33.  The characteristic peaks for MF resin completely
disappear, indicating that the material is no longer MF
resin.
 Some broad new peaks located at 1220–1620 cm1
appear, corresponding to C–N and C=N stretching
vibrations.
 This result is quite different from that reported by
Bettina Friedel et al., when they heat-treated their MF
resin (mainly ether linkages), and found a sharp
absorption peak appeared at around 1700 cm⁻1.38
 Solid-state NMR data showed an intense peak at 163
ppm with a shoulder at 156 ppm, which can be
attributed to external carbons and internal carbons of
C–N and C=N ring structures, respectively.

34. 
 The FT-IR results for these white powers show the
characteristic peaks of melamine, which indicates that the MF
resin is decomposed and melamine is sublimated.
 The decomposition and carbonization reactions continue with
the further increase of the pyrolysis temperature, and the broad
infrared peaks at 1220–1620 cm1 becomes weaker.
 At 800⁰C, all of the infrared signals of the organic residues
completely disappear, indicating the formation of a carbon
product.
Peaks of Melamine

35.  According to the above measurement and analysis results, a
formation procedure for the nitrogen-doped hollow
graphitized carbon spheres can be proposed: the MF resin
sphere remains stable below 400 C in inert atmosphere, due
to the strong methylene linkages formed between the
triazine rings.
 At about 400 C, deep-seated rupture of the polycondensate
system takes place.
 A number of independent reactions involving both side-
chains and ring degradation give rise to gaseous products
among which ammonia, hydrogen cyanide, carbon
monoxide and melamine have been observed.38

36. 
 Cyclic voltammetry (CV) and cyclic
chronopotentiometric curves were used to
characterize the capacitive properties of the nitrogen
doped hollow graphitic carbon spheres.
 The typical cyclic voltammogram plots of the carbon
electrode at different scan rates.
 The rectangular-like shape and the appearance of
humps in the CV curves indicate that the capacitive
response comes from the combination of EDLC and
redox reactions, which relate to the heteroatom
functionalities of the materials.
Cyclic voltammetry

37.  The cyclic chronopotentiometric curve is given in
Fig. a. The transition periods can be easily noticed
between 0.6 V and 0.4 V, which also indicate the
redox reactions performed in the charge–discharge
process.
 The NHGCSs material demonstrates excellent
capacitance performance in acid electrolyte.
 The specific capacitance is 306 F g1 at a current
 density of 0.1 A g1.

38.  Fig. 6b presents the relationships between the specific
capacitance and the charge–discharge current density.
 The specific capacitance at a high charging–discharging
rate remains at a very high level, such as 260, 243, 215 F g⁻1
at 1, 2, and 4 A g⁻1.
 Cycling performance is another key factor in determining
the supercapacitor electrodes for many practical
applications.
 The cycling stability of the hollow carbon sphere electrode
was examined using galvanostatic charge–discharge
cycling at a current density of 1 A g⁻1.
 During the 1000 cycles, the specific capacitances are almost
constant (the capacitance retention nearly remaining 100%),
which demonstrates good cycling performance.
 Therefore, the NHGCSs are much more suitable for direct
application.

39. Fig. Electrochemical performance of NHGCSs using a three-electrode cell in 2 M
H2SO4 solution: (a) cyclic chronopotentiometric curves at different charge–discharge
current densities, (b) relationship of the specific capacitance with respect to the
charge–discharge current densities. (c) Cycling stability at a current density of 1 A g⁻1.

40. 
 In conclusion, the formation of methylene linkages
between triazine rings in the hydrothermally synthesized
MF resins is very critical for the production of hollow
carbon spheres.
 Our results proved, for the first time, the possibility to
obtain NHGCSs by simply heat-treating solid MF resin
spheres.
 This method is thought to be extendable to the
preparation of other hollow structures, by simple
pyrolysis of methylene bridged nitrogen-contained
heterocyclic polymers.
Conclusion

41.  The obtained hollow carbon spheres exhibit both
nitrogen-doped and graphitic structure properties,
which lead to superior capacitance performance in
acid electrolyte.
 By combining these unique properties with the
inherent properties of hollow carbon spheres, such as
low density, high surface area, thermal insulation, and
electronic properties, these nitrogen-doped hollow
graphitic spheres have great potential for a variety of
applications, such as supports for catalysts,
 Fuel cell electrode materials
 Dye-sensitized
 Solar cell electrodes
 Supercapacitor electrodes

42. 
This work was supported by the
National Natural Science Foundation of
China (51072048, 51102083),
Natural Science Foundation of
Heilongjiang Province (B201107),
Heilongjiang Educational Department
(2011CJB006), and the Innovation Team
of Heilongjiang University (Hdtd2010-
04).
Acknowledgement

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