Improved capacitance of NiCo2O4/carbon composite resulted from carbon matrix with multilayered graphene

Article abstract of DOI:10.1016/j.electacta.2018.10.110

The optimized electric and ionic conductivity of metal oxides with pseudo-capacitive properties can greatly improve their electrochemical performances and can effectively amplify their applications in super-capacitors. Carbon spheres with multilayered graphene formed in their matrix are prepared by annealing them with sodium metal and they are combined with NiCo₂O₄ spheres by a simple hydro-thermal treatment. Structures and electrochemical performances of resulted samples are examined with X-ray diffraction, Raman scattering spectra, X-ray photoelectron spectroscopy, infrared spectroscopy, electron microscopy, gas physisorption, cyclic voltammetry and electrochemical impedance spectra, respectively. The largest specific capacitance of resulted composites is measured about 920 F g^?1 at 1 A g^?1, which is about 3.7 times larger than that of pure carbon spheres and 2.8 times larger than that of pure NiCo₂O₄ spheres. The formation of multilayered graphene in the matrix of carbon spheres not only increases the high specific capacitance based on electric double layers, but also makes metal oxides combined with them display greatly increased pseudo-capacitance. The electric and ionic conductivity of resulted composites with various weight ratios of NiCo₂O₄ and carbon spheres are respectively investigated and their effects on their electrochemical performances are also discussed.

Full text of DOI:10.1016/j.electacta.2018.10.110

Accepted Manuscript
Improved capacitance of NiCo O /carbon composite resulted from carbon matrix
2
4
with multilayered graphene
Zijie Xu, Liu Yang, Qiaoran Jin, Zhonghua Hu
PII:
S0013-4686(18)32350-8
10.1016/j.electacta.2018.10.110
EA 32909
DOI:
Reference:
To appear in:
Electrochimica Acta
Received Date:
Accepted Date:
06 August 2018
17 October 2018
Please cite this article as: Zijie Xu, Liu Yang, Qiaoran Jin, Zhonghua Hu, Improved capacitance of
NiCo O /carbon composite resulted from carbon matrix with multilayered graphene, Electrochimica
2
4
Acta (2018), doi: 10.1016/j.electacta.2018.10.110
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ACCEPTED MANUSCRIPT
Improved capacitance of NiCo2O4/carbon composite resulted from carbon
matrix with multilayered graphene
Zijie Xu1, Liu Yang, Qiaoran Jin, Zhonghua Hu
School of Chemical Science and Engineering, Tongji University, Shanghai,
200092, China
Abstract
The optimized electric and ionic conductivity of metal oxides with pseudo-
capacitive properties can greatly improve their electrochemical performances and can
effectively amplify their applications in super-capacitors. Carbon spheres with
multilayered graphene formed in their matrix are prepared by annealing them with
sodium metal and they are combined with NiCo2O4 spheres by a simple hydro-thermal
treatment. Structures and electrochemical performances of resulted samples are
examined with X-ray diffraction, Raman scattering spectra, X-ray photoelectron
spectroscopy, infrared spectroscopy, electron microscopy, gas physisorption, cyclic
voltammetry and electrochemical impedance spectra, respectively. The largest
specific capacitance of resulted composites is measured about 920 F g-1 at 1 A g-1,
which is about 3.7 times larger than that of pure carbon spheres and 2.8 times larger
than that of pure NiCo2O4 spheres. The formation of multilayered graphene in the
matrix of carbon spheres not only increases the high specific capacitance based on
electric double layers, but also makes metal oxides combined with them display
1 Corresponding author: xzj@tongji.edu.cn (Z. Xu)
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greatly increased pseudo-capacitance. The electric and ionic conductivity of resulted
composites with various weight ratios of NiCo2O4 and carbon spheres are respectively
investigated and their effects on their electrochemical performances are also
discussed.
Keywords: NiCo2O4; carbon sphere; multilayered graphene; super-capacitor
1. Preface
Super-capacitors including electric double layer capacitors (EDLCs), pseudo-
capacitors and hybrid capacitors are one type of energy storage devices. They usually
possess high power density and long cycle life, which are promising for their
applications in portable electronic devices, electric vehicles, backup energy systems
and user electronics [1-3]. However, their rate capabilities and energy densities need
to be improved in order to enhance the efficiency of devices [4,5]. This greatly
depends on the structural optimization of electrode materials in the devices.
Among them, one kind of redox electrochemical capacitors based on metal
oxides with pseudo-capacitive properties can generate a high load in a short period of
time and increase their final specific capacitance. Some transition metal oxides
including cupric oxide [6], nickel oxide [7], manganese oxide [8], titanium oxide [9]
or some complex metal oxides [10,11] are widely explored as electrode materials of
pseudo-capacitors. Because their electric energy storage depend on Faradaic process
generated at interfaces between metal oxides and electrolyte ions, their
electrochemical performances including specific capacitance, rate capability and cycle
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stability are mainly affected by their surface areas, electronic and ionic conductivity,
mechanical and chemical stability. However, most of pure metal oxides usually
display their poor electric and ionic conductivity because of their structural
characteristics. Although most of them possess lower production cost and can display
their prospect in super-capacitors, their electrochemical performances are greatly
affected by their structure defects. So, a lot of attentions have been paid to the
structural optimization including specific surface areas and porous structures of metal
oxides [12-17]. One of strategies is the coating of metal oxides on metal substrates in
order to improve their electric conductivity. For example, Li4Ti5O12 nanowire arrays
are grown on Ti foil [18], MnO2 nanoneedles [19], hollow NiCo2O4 nanoparticles
[20], porous NiCo2O4 nanowires [21]. Considering the fact that super-capacitors
usually need to work at large charge-discharge current densities, the iterative
oxidation and reductions of metal oxides to be carried out under large current
densities usually cause a strong concussion in their structures and make them display
their poor stability on substrates.
Under such a circumstance, another effective strategy to be carried out is the
turning some metal oxides and carbon materials into their composites. Carbon
materials not only act as supporters of metal oxides in the composites for their
improved surface characters, but also can contribute to the capacitance based on the
formation of electric double layers (EDLs) during a charge-discharge process [22-27].
They are explored as electrode materials and their electrochemical performance have
been investigated. For example, NiCo2O4/three dimensional graphene composite are
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investigated, but, their carbon substrates still display the character of typical
amorphous carbons [28-31]; Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber
hybrid composites are investigated, however, it is found that some composite display
their performances mainly based on EDLs and the others do them based on the typical
pseudocapacitance [32]; lithium ions in cotton textile are prepared and their
electrochemical performances are investigated, but it can be seen the substrate in the
composite is still composed of amorphous carbons [33, 34]. Moreover, 3D NiCo2O4
on carbon cloth [35], NiCo2O4 nanoneedles on carbon nanofibers [36], and
NiCo2O4/N-doped graphene [37] are also investigated. On the other hand, some
porous carbon materials with high specific surface areas are used to improve the
distribution of metal oxides on them. For example, NiCo2O4 nanosheets on hollow
carbon microspheres [38, 39]. In general, it can be seen that several main kinds of
carbons have been used as substrates in metal oxide/carbon composites. According to
above investigations, some of them can indeed exhibit their high electrochemical
performances, however, little attentions have been paid to the structural optimizations
of carbons, especially of amorphous carbons and their structural effects on the
performances of resulted composites. It can be seen that they are usually prepared
from metal oxides with different as-prepared carbons. The possess different structural
characters and do their different effects on the performances of resulted composites.
For example, 1D carbon materials usually possess higher electric conductivity and
they are favor to the improvement of electric conductivity of metal oxides, but their
poor porous characters make them be added little amount of metal oxides; 2D carbon
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materials usually possess marvelous electric conductivity and larger specific surface,
but, their low bulk density are not favor to their practical applications; 3D carbon
materials possess high specific surfaces and marvelous porous structures and they are
also used as supporters of metal oxides. Based on the working mechanism of super-
capacitors, no carbon materials can completely meet their requirement, so, the
structural optimization of carbons in matrix of the composites should be paid more
attentions in the field of super-capacitors.
Based on the formation of EDLs on carbons, the accumulation state of
microcrystalline graphite in matrix of carbons usually plays an important role in their
electric and ionic conductivity. It does have an important effect on electrochemical
properties both of carbons and metal oxide/carbon composites. For example, the
matrix of typical amorphous carbons are composed of disordered microcrystalline
graphite and usually make them possess large specific surfaces. They are favor to the
addition of more metal oxides on carbon substrates, but the typical amorphous
carbons usually possess poor electric conductivity and they are not favor to the
improved electric conductivity of metal oxides. On the contrary, the matrix of typical
graphite materials are composed of highly ordered micro-crystalline graphite and they
make them with high electric conductivity, but they also result in their poor porous
characters. They are not favor to the addition of more metal oxides on them. A
marvelous carbon material used as the supporter of metal oxides should possess a
high specific surface area, a proper porous structure and a high electric conductivity at
the same time. However, these as-prepared carbons used as the matrix of metal
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oxide/carbon composites are difficult to meet the requirement and further effectively
improve their electrochemical performance of the composites. In view of amorphous
carbons with the characters of easy access and low production cost, they can be used
as supporters of metal oxides and display their prospective application in super-
capacitors if their electric conductivity and porous characters are effectively
improved.
In this paper, carbon spheres with multilayered graphene to be formed in their
matrix and they are combined with NiCo2O4 spheres by a simple hydro-thermal
treatment. Various structural characterizations about carbon spheres, NiCo2O4 spheres
and resulted composites are examined and their electrochemical performances are also
investigated, respectively. The specific capacitance of the composite is obviously
larger than that of pure carbon spheres or NiCo2O4 spheres. The weight ratios of
carbon and NiCo2O4 spheres in resulted composites have an obvious effect on their
electrochemical performances. It is also found that the formation of C-O bond
between carbon and NiCo2O4 spheres is related to the improved electric performances
of resulted composites. It can be concluded that the formation of multilayered
graphene in the carbons not only makes them possess high specific capacitance based
on EDLs, but also make NiCo2O4 in the composites display their improved pseudo-
capacitive properties.
2. Experimental
2.1 Materials
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Glucose (AR), Co(NO3)2·6H2O (AR), Ni(NO3)2·6H2O (AR), NaCl (AR), isopropyl
alcohol (AR), propanetriol (AR) and sodium metal (CP) were all purchased from
Sinopharm Chemical Reagent Co., LtdS.
2.2 Preparation of carbon spheres with multilayered graphene structure
Glucose solution (aq. 0.5 mol L-1) is added in an autoclave and heated at 190 ℃
for 12 h, then, resulted mixture is filtered, washed with water and dried at 80 ℃ in a
oven, after that, it is carbonized at 400 ℃ for 4 h in nitrogen atmosphere and resulted
carbon spheres are named as GC.
GC (0.5 g) and sodium chloride (2.0 g) are mixed each other, then, the mixture
and sodium metal (1.5 g) are settled in a alumina autoclave with a cover and it is
annealed in nitrogen atmosphere at 850 ℃ for 2 h, after it being cooled at room
temperature and opened in air for several hours, resulted product is washed in water to
remove residual sodium chloride and sodium oxide, after that, it is dried at 120 °C and
the resulted sample is named as G-GC.
2.3 Preparation of NiCo2O4 spheres
Co(NO3)2·6H2O (0.5 mmol), Ni(NO3)2·6H2O (0.25 mmol) and isopropyl alcohol
(50 mL) are mixed by a strong stirring, then, propanetriol (15 mL) is dropped into the
mixture and it is continuously stirred for 10 min, after that, the mixture is added in an
autoclave with a Teflon inner liner and heated at 180 ℃ for 8 h, after it being cooled
at room temperature, resulted product is filtered and washed by water and alcohol,
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accordingly, after it being dried at 80 ℃, the product is annealed in air atmosphere at
350 ℃ for 2 h and NiCo2O4 spheres are obtained.
2.4 Preparation of G-GC/NiCo2O4 composites
G-GC and NiCo2O4 spheres are mixed at various weight ratios of 1:2, 5 and 8,
respectively, then, water (100 mL) is added to each mixture with a sonic oscillation
for 1 h, after that, each mixture is added in an autoclave and heated at 180 ℃ for 12 h,
after it being cooled to room temperature, it is filtrated and dried at 300 °C in nitrogen
atmosphere. Resulted samples are correspondingly named as G-GC/NiCo2O4-2, -5
and -8.
2.5 Structural characterizations
XRD patterns are collected on a D8 FOCUS X-ray powder diffractometer.
Acquisition conditions were CuK radiation with 40kV/40mA, a step size of 0.01/2
and a count time of 0.5s. A NIST SRM 640c silicon standard is used to correct the
line position and broadening of patterns.
Raman spectra (Raman) are obtained on a Renishaw inVia laser Raman
spectrometer, using 512 nm laser source.
X-ray photoelectron spectroscopy (XPS) measurements were carried out with a
AXIS Ultra DLD equipment with Mg K X-ray excitation source and hemispherical
electron analyzer.
Infrared spectrocopy (IR) measurements were carried out with an infrared
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spectrometer 2015 (IS10).
Scanning electron microscopy (SEM) photographs are taken on a S-4800
microscope and Energy Dispersive X-ray Spectrometric Microanalysis (EDX) with
SEM are also carried out.
Transmission electron microscopy (TEM) photographs are taken on a FEI
TECNAI G2 S-TWIN F20 microscope. Samples for TEM identification are prepared
by grinding and dispersing them in ethanol, and placing a drop on a lacey carbon grid.
These samples are observed in numerous regions.
Nitrogen adsorption and desorption isotherms of samples are measured with a
volumetric adsorption system (Micromeritics Tristar 3000). The specific surface area
and pore-width distribution are analyzed by the Brunauer-Emmett-Teller (BET) and
BJH method.
2.5 Electrochemical measurements
In order to prepare working electrodes, resulted samples of GC, G-GC, NiCo2O4
spheres and G-GC/NiCo2O4 composites are used as active materials, respectively.
Polyfluortetraethylene (PTFE) is used as a binder. The working electrode containing
2-3 mg active materials, they are mixed at a mass ratio of sample : PTFE = 8 : 1. Each
mixture is wet by a little of ethanol to form a slurry. Each working electrode is
fabricated by coating the slurry on a nickel mesh and it is dried under vacuum at 120
ºC for 5 hours and weighed. They were all dipped in KOH (6 M) for 24 hours before
electrochemical characterizations.
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Cyclic voltammetry (CV) investigations are performed by using three-electrode
system with a voltammetric analyzer (CHI600E) at various scan rates and the voltage
is swept from -1.0 to 0 V and back to -1.0 V. Resulted samples are respectively used
as working electrodes, the nickel mesh is used as the counter electrode and the
Hg/HgO electrode is used as the reference electrode. KOH (6 mole dm-3) is used as
the electrolyte.
The electrochemical impedance spectra (EIS) are also recorded on the same
electrochemical workstation. The frequency scope is from 0.01 to 1000000 Hz, the
amplitude is 0.005 V and the quite time is 2 s.
3. Result and discussion
3.1 Improved performance from structural optimization of carbon spheres
Structure characterizations about GC and G-GC are presented in Fig.1. It can be
seen the spherical morphology of GC with an average diameter about 300 nm in the
photograph as shown in Fig.1a, indicating the successful preparation of carbon
spheres from the hydrothermal treatment of glucose solution. The sphere morphology
can be also observed in the SEM photograph of G-GC as shown in Fig.1b, indicating
that the treatment of GC in sodium metal cannot destroy their sphere morphology.
TEM photograph of GC is presented in Fig.1c. These disordered graphite crystallite in
the matrix exhibit the amorphous character of GC. TEM photograph of G-GC is
presented in Fig.1d. It can be seen multilayered graphene with 4-8 layers in the matrix
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of G-GC, moreover, it can be also seen turbulent carbons around these multilayered
graphene and typical amorphous carbons near these turbulent carbons. By comparing
TEM photographs both of GC and G-GC, it can be seen that the initial disordered
graphite crystallite in the matrix of GC are rearranged and turned into multilayered
graphene, turbulent and amorphous carbons.
Fig.1 Structure characterization of GC and G-GC, SEM photograph of GC (a) and G-GC,
TEM photograph of GC (c), TEM photograph of G-GC (c), XRD (e), Raman patterns of GC
and G-GC (f), adsorption and desorption isotherm (g) and pore volume distribution curve of
G-GC (h).
The diffraction pattern for GC as shown in Fig.1e exhibits two widened
diffraction peaks at 2θ about 26 and 43°, indicating the matrix to be composed of
amorphous carbons. It can be certified by Raman spectrum as shown in Fig.1f. For
example, D- and G-band peaks with almost equal intensities indicate the laser scatting
mode on typical amorphous carbons. The amorphous carbon character of GC is
resulted from the carbonization of glucose at 400 °C. XRD pattern for G-GC as
shown in Fig.1f exhibits two sharpened diffraction peaks at 2θ about 26° and 44°,
respectively. By comparing them with those of GC, the diffract peaks for G-GC are
more sharpened than those for GC. It is surely attributed to the formation of layered
carbons in G-GC. Raman spectrum for G-GC as shown in Fig.1f displays obviously
intensified G-band peak and greatly weakened D-band peak, which is identified as the
laser scatting mode on layered carbons. Obviously, it is different from that for GC,
indicating a different accumulation state of graphite micro-crystallite to be formed in
matrix of G-GC.
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The adsorption-desorption isotherm for G-GC is presented in Fig.1g. The
isotherm with a hysteresis loop started at the relative pressure about 0.4 exhibits the
character of type IV, indicating both micro- and meso-pores to be formed in the
matrix. Moreover, a steep plateau can be clearly observed in the isotherm, indicating
uneven micro-pores to be formed in the matrix. The pore volume distribution curve as
shown in Fig.1h exhibits two pore volume distributions concentrated at the pore width
about 1.5 nm and 4.0 nm, respectively. The specific surface area of G-GC is measured
about 980 m2 g-1. It can be surely attributed to the fact that the formation of
multilayered graphene that cause a series of movements of disordered crystalline
graphite including their orientation, connection and stacking with the existence of
sodium metal at 850 °C.
Fig.2 CV curves (a) and galvanostatic charge-discharge curves (b) of G-GC
Electrochemical performances of G-GC are presented in Fig.2. The rectangle
feature can be clearly observed in CV curves at various scanning rates as shown in
Fig.2a, indicating their electrochemical performances based on the formation of EDLs
during charge-discharge process, moreover, the rectangle feature can be still observed
in the curve at 200 mV s-1. Galvanostatic charge-discharge curves for pure G-GC at
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various charge-discharge current densities are shown in Fig.2b. These symmetrical
charge-discharge branches with no obvious voltage drops can be observed in them,
indicating a marvelous charge-discharge reversibility and the sample of G-GC with
excellent electric and ionic conductivity at large charge-discharge current densities.
The specific capacitance of G-GC is measured about 250 F g-1 at 1 A g-1. It is
attributed to the improved electric conductivity from the formation of multilayer
graphens and the optimized ionic conductivity from mesopores to be formed in G-GC.
3.2 Structure and electrochemical performance of NiCo2O4 spheres
Fig.3 Structure characterization of pure NiCo2O4, SEM photograph (a) and (b), XRD
patterns (c) adsorption-desorption isotherm inserted with pore volume distribution curve (d)
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Structural characterizations for NiCo2O4 spheres are presented in Fig.3. In SEM
photograph as shown in Fig.3a, it can be seen the spheres with an average diameter
about 500 nm and the porous surface morphology. A profile of one sphere is also
observed in SEM photograph as shown in Fig.3b, exhibiting the morphology with
internal compacted core and external porous surface. The structure feature can be
ascribed to the hydro-thermal treatment of nickel and cobalt salts. At its early stage,
the crystallization of NiCo2O4 is dominated by the large concentrations both of Ni2+
and Co3+ ions and the fast crystal growth of NiCo2O4 crystals results in the compacted
core, while, at the latter stage of crystallization, the decreased concentrations both of
Ni2+ and Co3+ ions cause the crystallization of NiCo2O4 to be obviously controlled by
the interface interaction between crystalline NiCo2O4 and aqueous solution, resulting
in the crystals with a porous surface. This is why some metal oxides with a porous
morphology still possess a small specific surface area and a low specific capacitance.
XRD pattern for the sample is shown in Fig.3c. These diffraction peaks are
identified as the component of crystalline NiCo2O4, indicating that the spheres are
composed of NiCo2O4. Their adsorption-desorption isotherm is shown in Fig.3d. The
isotherm with a hysteresis loop start at the relative pressure about 0.4 is belonged to
type IV, indicating micro- and meso-pores to be formed in NiCo2O4 spheres. The
steep plateau in the isotherm indicates uneven micro-pores formed in NiCo2O4
spheres. The pore volume distribution curve is shown in the insert of Fig.3d,
exhibiting an obvious a pore volume distribution concentrated at the pore width about
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5 nm. However, their small pore volume of meso-pores is attributed to their hard
cores and porous appearances.
Fig.4 Galvanostatic charge-discharge curve (a) and capacitance retention curve (b) of
NiCo2O4 spheres
Galvanostatic charge-discharge curve of NiCo2O4 spheres is shown in Fig.4a.
The electrochemical performance displays typical pseudocapacitance character,
indicating a redox reaction of NiCo2O4 to be carried out during the charge-discharge
process. However, the charge-discharge branches display an asymmetrical style,
indicating the reaction of NiCo2O4 with an inferior reversibility during the charge-
discharge process. The specific capacitance of NiCo2O4 spheres is measured about
330 F g-1 at 1 A g-1, while, the capacitance retention curve as shown in Fig.4b exhibits
a steady specific capacitance about 290 F g-1 after 10000 charge-discharge cycles.
3.3 Improved electrochemical performances from G-GC/NiCo2O4 composite
Electrochemical performances of various G-GC/NiCo2O4 composites are
presented in Fig.5. According to widened CV curves with bulges as shown in Fig.5a,
it can be seen that their electrochemical performances are based on oxidation-
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reduction reactions from NiCo2O4 and EDLs from carbons, indicating that NiCo2O4
and carbon spheres do both contributions to electrochemical activities during the
charge-discharge process. In galvanostatic charge-discharge curves as shown in
Fig.5b, it can be seen that of G-GC/NiCo2O4-5 displays the largest charge-discharge
interval, while, that of G-GC/NiCo2O4-2 displays the smallest charge-discharge
interval. The capacitance of G-GC/NiCo2O4-2, 5 and 8 are measured about 800, 920
and 890 F g-1, respectively. In their EIS diagrams as shown in Fig.5c, semicircles in
them are obvious different each other, for example, the semicircle in that for G-
GC/NiCo2O4-5 is the smallest, indicating the small transmission resistance of ions in
the matrix, while, that for G-GC/NiCo2O4-8 displays a larger semicircle and that for
G-GC/NiCo2O4-2 exhibits the largest one. The differences in these semicircles
indicate different ionic conductivity in each composite. According to the insert as
shown in Fig.5c, Resistance values of G-GC/NiCo2O4-2, 5 and 8 are valued as 0.8, 1.0
and 1.5 Ω, respectively, indicating them with different electric conductivity. The
capacitance retention of the sample of G-GC/NiCo2O4-5 is presented in Fig.5d. The
increased specific capacitance measured before 1000 circles is attributed to its
instability during the measurement and the steady capacitance retention is found to
remain at 94% after 5000 circles.
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Fig.5 Electrochemical performances of composites , CV curves (a), galvanostatic charge-
discharge curves (b) and EIS curves (c) and capacitance retention curve for G-GC/NiCo2O4-
5 (d)
According to above investigations, it can be known that the specific capacitance
of G-GC, NiCo2O4 spheres and G-GC/NiCo2O4-5 composite are measured as 250,
330 and 927 F g-1 at 1 A g-1, respectively. So, the specific capacitance of the
composite is about 3.7 times larger than that of pure G-GC spheres and about 2.8
times larger than that of pure NiCo2O4 spheres. If they independently do their
contributions to the electrochemical performance, carbon spheres with 33 wt % in G-
GC/NiCo2O4-5 should possess the capacitance about 83 F g-1 because of pure G-GC
with the specific capacitance of 250 F g-1 at 1 A g-1, while, NiCo2O4 spheres with 67
wt% in the composite should possess the capacitance about 221 F g-1 because of pure
NiCo2O4 spheres with specific capacitance of 330 F g-1 at 1 A g-1, thus, the sum
should be about 304 F g-1 at 1 A g-1 In fact, the measured specific capacitance of the
.
composite is about 3 times larger than the sum. Obviously, it is one plus one is more
than two. The obviously improved capacitance value is resulted from the common
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contributions of carbons with NiCo2O4 spheres because of the composite with its
structure optimization.
Fig.6 Structural characterizations about G-GC/NiCo2O4 composites, SEM photograph
of G-GC/NiCo2O4-5 (a), EDS spectrum (b), XRD patterns (c), Raman spectra (d),
adsorption-desorption isotherms (e) and pore volume distribution curves (f)
Structural characterizations about G-GC/NiCo2O4 composites are presented in
Fig.6. In SEM photograph of G-GC/NiCo2O4-5 as shown in Fig.6a, it can be seen
spheres to be connected each other after the hydro-thermal treatment of G-GC and
NiCo2O4 spheres. C, Ni, Co and O elements are detected in its EDS as shown in Fig.
B, indicating the sample to be composed of G-GC and NiCo2O4. XRD patterns for
various composites are presented in Fig.6c. According to these diffraction peaks,
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crystalline NiCo2O4 in them are all detected in them. It can be also seen that their
diffraction patterns become gradually weakened with the decreased weight ratio of
NiCo2O4 in resulted composites. For example, the pattern for G-GC/NiCo2O4-8
displays clear diffraction peaks for crystalline NiCo2O4, while, that for G-
GC/NiCo2O4-2 becomes ambiguous. It is attributed to the poor sensitivity to the lower
crystalline content. Because of the annihilation from strong diffraction effect of
crystalline NiCo2O4, the diffraction patterns for layered carbons cannot be clearly
observed in them. However, they can be identified in their Raman spectra as shown in
Fig.6d, for example, intensified G-band peaks and greatly weakened D-band peaks
can be all observed in them, indicating G-GC to be added in these composites. It can
be concluded that the composites are composed of G-GC and crystalline NiCo2O4.
Their adsorption-desorption isotherms are all presented in Fig.6e. The isotherms
with hysteresis loops start at the relative pressure about 0.4 are belonged to type IV,
indicating micro- and meso-pores to be formed in them. It can be also seen that with
increasing ratios of NiCo2O4 in the composites, the plateaus in the isotherms become
more and more lower and they are all lower than that in the curve for pure GC as
shown in Fig.1f, indicating that the porous structures of the composites are obviously
affected by the weight ratio of NiCo2O4. Their pore volume distribution curves are
shown in Fig.6f. They display different pore volume distributions concentrated at the
pore width about 3.5 nm, moreover, the curve for G-GC/NiCo2O4-5 displays an
obvious pore volume distribution concentrated at the pore width about 6.5 nm.
Specific surface areas of G-GC/NiCo2O4-2, 5 and 8 are measured about 384, 205 and
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173 m2 g-1, respectively. Although their specific surface areas are all smaller than that
of pure G-GC, it can be seen that the composite of G-GC/NiCo2O4-5 with the
moderate specific surface area and crystallinity can display its marvelous
performance. It can be attributed to its obvious pore volume distribution at the pore
width range from 5 to 8 nm, which is favor to decreasing the ionic conductivity during
the charge-discharge process and does have a positive effect on their electrochemical
performances.
Fig.7 XPS general spectrum of G-GC/NiCo2O4-5 (a), fitting curves for Ni 2p (b), Co 2p (c), O
1s (d) and C 1s (e)
XPS for pristine G-GC/NiCo2O4-5 are presented in Fig. 7. Core levels of Ni 2p,
Co 2p, O 1s and C 1s can be identified from the survey of XPS. The general spectrum
as shown in Fig.6a indicates carbon, oxygen, cobalt and nickle elements detected in
the composite. It is attributed to the composite prepared from G-GC and NiCo2O4
spheres. The peaks with binding energy (BE) values of 855.7 and 873.7 eV are
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assigned to Ni2+ and these of 854.1 and 872.0 eV are assigned to Ni3+ as shown in
Fig.7b; these of 782.0 and 786.6 eV as shown in Fig.7c are assigned to Co 2p 3/2 and
its shake-up satellite for Co2p ions, respectively; these of 529.5, 531.2 and 533.1 eV
as shown in Fig.7d are assigned to the band between oxygen and metal, the oxygen
ion and the absorbed H2O, respectively; these of 284.8 and 288.2 eV as shown in
Fig.7e are assigned to C1 and C2, respectively. It can be clearly seen that there are
several active points between 284 and 874 eV, indicating that they can be used to
restore energy. Moreover, according to the C-O bond detected in XPS spectrum, it can
be seen that carbon and NiCo2O4 spheres are bonded each other after their hydro-
thermal treatment.
Fig.8 IR spectra of G-GC (1), the composite of G-GC/NiCo2O4-5 composite (2) and the
simple mixture of G-GC and NiCo2O4 with a grinding treatment (3).
IR spectra of G-GC/NiCo2O4-5 composite and G-GC are presented in Fig.8. In
the spectrum (1), the peak at 3487 cm-1 is assigned to the vibration of hydroxyl group
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in absorbed water, the peak at 1014 cm-1 can be assigned to the stretching vibration
between carbon and hydroxyl group and the peak at 801 cm-1 can be assigned to the
vibration between carbon and hydrogen atoms. In the spectrum (2), two peaks at 3413
and 3140 cm-1, respectively, are also assigned to the vibration movements of hydroxyl
groups. By comparing these peaks with that in the spectrum (2), the more vibration
movements are attributed to the hydrothermal treatment. Moreover, it can be seen one
peak at 1618 cm-1 and another at 1399 cm-1, which do not appear in the spectrum (2),
the former is assigned to the vibration between metal oxide and carbons and the latter
is assigned to the vibration between carbon and oxygen atoms [40, 41]. On the other
hand, it can be seen the small peak at 1022 cm-1, it can be attributed to the
degeneration of the peak at 1014 cm-1 in the spectrum (1). In the spectrum (3), the
peak at 3424 cm-1 is also assigned to the vibration movements of hydroxyl groups, the
weak peak at 1617 cm-1 is also assigned to the vibration between metal oxide and
carbons and the peak at 649 cm-1 is assigned to oxide absorption band. Obviously, the
spectrum for the composite is different from these of the others. It indicates the
dehydration between hydroxyl groups and the formation of C-O band in it. This
agrees with the conclusion from the XPS characterization that the formation of C-O
band is carried out after the hydrothermal treatment of G-GC with NiCo2O4 spheres.
In view of the fact that the electric performances of pure G-GC or pure
NiCo2O4 are obviously inferior to these of the composites as investigated in above
sections, it can be deduced that their obviously improved performances are related to
the following optimized structure characters.
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Firstly, the formation of multilayered graphene in the matrix of G-GC can
effectively improve its electric and ionic conductivity, which not only makes G-GC
itself display the marvelous specific capacitance of 250 F g-1, but also results in the
composite with obviously improved electric and ionic conductivity. So, it does have
an important effect on the improved electrochemical performance of resulted
composite. Secondly, the hydro-thermal treatment of G-GC and NiCo2O4 spheres
results in the formation of C-O band between them and causes more electrochemical
active spots in the composite according to XPS characterization as shown in Fig.7 and
the IR spectrum as shown in Fig.8. Thirdly, the ratio of carbon spheres in the
composite does an important effect on them.
Based on the basic working mechanism of super-capacitors, it can be known that
the comprehensive effects of the structure characters both of carbon and metal oxide
matrix. For example, their larger surfaces can provide more quantities of
electrochemical active substances, their rich porous structures can make the
electrolyte ion transport in their matrix with less resistance and their improved
material characteristics result in their marvelous electric conductivity. Obviously, the
bonding between G-GC and NiCo2O4 in resulted G-GC/NiCo2O4 results in them with
more active points to restore energy and results in a close connection between G-GC
and NiCo2O4 that does a favor to their improved electrolyte ion transmission and
electric conductivity.
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Fig.9 Sketch for carbon and NiCo2O4 phase varied in the composites
The effects of optimized structural characters on the improved performances of
the composites can be understood by the sketch as shown in Fig.9. The more content
of G-GC in C-GC/NiCo2O4-2, for example, carbon spheres with that of 33 wt%, is
favor to the formation of a continuous carbon phase in the matrix of the composite as
being symbolically described in Fig.9a and the composite possess the smallest
impedance value of 0.8 Ω, indicating the matrix with a high electric conductivity,
however, the continuous carbon phase results in a large ionic transmission resistance
as it being shown in Fig.5c; on the contrary, carbon spheres with the content of 11
wt% and NiCo2O4 spheres with that of 89 wt % in G-GC/NiCo2O4-8 is favor to the
formation of a continuous phase of NiCo2O4 in the matrix of the composite as it being
symbolically described in Fig.9b, so, it makes the composite possess a high
impedance value of 1.5 Ω as it being shown in Fig.5c because of the large electric
resistance of the oxide; however, the content of G-GC with 17 wt% and that of
NiCo2O4 with 83% in G-GC/NiCo2O4-5 may result in an optimized structure as
symbolically described in Fig.9c, which possess a smaller electric resistance and the
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largest ionic conductivity as shown in Fig.5c, so, it results in the greatly increased
specific capacitance.
4. Conclusion
Multilayered graphene formed in the matrix of amorphous carbons not only
make carbons themselves possess high specific capacitance based on electric double
layers, but also make metal oxide on them display the greatly increased pseudo-
capacitive properties. The formation of C-O bonds between the carbon spheres and
NiCo2O4 spheres can be simply carried out by a hydro-thermal treatment of the two
components. The weight ratios of NiCo2O4 and carbon spheres in the composites have
an obvious effect on the electric and ionic conductivity of resulted composites and a
moderate weight ratio in the composite can result in the greatly improved
electrochemical performances. The structural optimization of carbon substrate is an
effective route for improved electrochemical performance of metal oxides.
Acknowledgement
The work in this paper is funded by Tongji University and authors would like to
thank the Chemistry Experimental Centre at the Tongji University for providing
characterization equipments and assistances.
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