superior supercapacitive performance in porous...
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Superior supercapacitive performance in porous nanocarbons
Gomaa A.M. Ali , Shoriya Aruni Abdul Manaf , Divyashree A ,Kwok Feng Chong , Gurumurthy Hegde
PII: S2095-4956(16)30043-2DOI: 10.1016/j.jechem.2016.04.007Reference: JECHEM 152
To appear in: Journal of Energy Chemistry
Received date: 12 January 2016Revised date: 8 March 2016Accepted date: 15 March 2016
Please cite this article as: Gomaa A.M. Ali , Shoriya Aruni Abdul Manaf , Divyashree A ,Kwok Feng Chong , Gurumurthy Hegde , Superior supercapacitive performance in porous nanocar-bons, Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.04.007
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Superior supercapacitive performance in porous nanocarbons
Gomaa A. M. Alia,b,
, Shoriya Aruni Abdul Manafb, Divyashree A
c, Kwok Feng
Chong
b,
Gurumurthy Hegdec,
a Chemistry Department, Faculty of Science, Al–Azhar University, Assiut, 71524, Egypt
b Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300, Gambang, Kuantan,
Malaysia
c BMS R and D Centre, BMS College of Engineering, Basavanagudi, Bangalore, 560019, India
Article history:
Received 12 January 2016
Revised 8 March 2016
Accepted 15 March 2016
Available online
Abstract
Porous nanocarbons with average particle size 20–40 nm were developed using biowaste oil palm
leaves as a precursor. Simple pyrolysis was carried out at 700 oC under nitrogen atmosphere. Obtained
porous nanocarbons showed excellent porous nature along with spherical shape. Symmetric
supercapacitor fabricated from porous nanocarbons showed superior supercapacitance performance
where high specific capacitance of 368 F/g at 0.06 A/g in 5 M KOH were reported. It also exhibited
high stability (96% over 1700 cycles) and energy density of 13 Wh/kg. Low resistance values were
obtained by fitting the impedance spectra, thus indicating the availability of these materials as
supercapacitors electrode. The presented method is cost effective and also in line with waste to wealth
approach.
Keywords: Porous carbon nanoparticles; Supercapacitor; Catalyst free; Biowaste
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* Corresponding author. Tel: +91 8762520397; E-mail address: [email protected]
(Prof. Gurumurthy Hegde). **
Corresponding author. Tel: +60 95492403; E-mail address: [email protected];
[email protected] (Gomaa A. M. Ali).
1. Introduction
Carbon is conventionally preferred over metal oxide material in supercapacitor application
for several good reasons such as its abundance, high surface area, excellent electrical
conductivity and low production cost [1]. Owing to the enhanced pore volume distribution of
carbon, it has high stability and conductivity.
Although there are various super capacitors made up of several materials available in
the literature, biowaste approach is highly influential due its capacity to bulk produce. These
materials are highly efficient for energy storage device [2]. In the recent past, carbon is
focused upon as the precursor for the electrodes in supercapacitor application because of its
extensive electrochemical storage property [3]. In addition to this, carbon electrodes can be
easily polarized [4]. Electrodes synthesized from carbon are stable both in acidic and basic
solutions [5]. Upon implication of several physical activation methods, electrodes with huge
surface area can be obtained. These chemical and physical attributes of carbon contribute to
the application of carbon in storage of energy [6].
Developing countries have rapid growth in the past few decades which is powered by
coal and fossil fuels [7,8]. This growth which is accelerated by the consumption of fossil
fuels is hazardous and also results in the greenhouse effect [9]. A healthy development needs
source of abundant supply of clean energy. The statistics show that the biowaste materials
make up major solid waste and a wise way to address this issue is to utilize biowaste
materials for the commercial purpose [10,11]. The major advantage of the biowaste materials
is that they possess cellulose, lignin and hemicellulose which are the potential material for
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energy storage devices [12,13]. In comparison with fossil fuels, biowaste materials have
negligible negative impact to the environment [14].
Various carbon materials like single-walled carbon nanotubes, multi-walled carbon
nanotubes, activated carbon, carbon nanospheres, carbon nano-onion, graphene have been
tested as apt materials for the fabrication of EDL (Electrical Double Layer) electrodes [15–
21]. The selection of the biowaste precursor and its activation process emphasizes the pore
size distribution, surface area, specific capacitance and electrochemical performance of the
supercapacitor.
In this paper we describe our continued investigation on Oil Palm Leaves (OPL)
which are the lignocellulosic biowaste as the precursor for the electrode material. This study
showed superior super capacitance performance in comparison with our earlier reports.
2. Experimental
2.1. Sample preparation
Oil palm leaves which are the biowaste material were used as a precursor for the production
of porous carbon nanoparticles (PCNs). OPLs were dried in an oven at 110 oC for 48 h to
eliminate all the moisture content in the sample. The dried sample was crushed and grinded at
a speed of 12,000 rpm using a grinder. The ground sample was furthered sieved to the
particle size of 63 µm. The sieved sample was synthesized by one step catalyst free pyrolysis
technique in a tube furnace at 700 oC under nitrogen atmosphere (with continuous flow of
150 mL/cm3) for two hours at a heating rate of 10
oC/min followed by cooling to room
temperature. The aqueous NaOH, 2.5 M is employed to remove silica from the obtained
product which converts the product in to carbon nanosphere (template approach). Detailed
experimental method is discussed in our earlier papers [22–25].
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2.2. Porous carbon nanoparticles characterization
PCNs were characterized using X-ray diffraction (XRD, Rigaku Mineflex II),
field emission scanning electron microscope-energy dispersive X-rays (FESEM-EDX, JEOL,
JSM-7800F), transmission electron microscopy (TEM, JEOL, JSM 1230) and N2 adsorption-
desorption (Micromeritics ASAP 2020) techniques. Full details about the PCNs were given in
Ref. [22].
2.3. Electrochemical studies
The following procedure is adopted to prepare samples for electrochemical
studies. PCNs with 5 wt% polytetrafluoroethylene (PTFE) and 15 wt% carbon black were
mixed, followed by pressing the mixture onto a nickel foam to prepare the electrode. Coin
cell design is adopted in this experiment to measure specific capacitance. The total mass of
the both electrode is around 9.13 mg and the electrode dimension is around 1 cm × 0.8 cm.
The electrochemical tests were performed using a two-electrode type system, in which the
electrodes were electrically isolated from each other by porous membrane in 5 M KOH
electrolyte. The data were collected using an electrochemical workstation (Autolab/PGSTAT
M101) equipped with a frequency response analyzer. Cyclic voltammetry tests were
performed between 0 and 1 V with scan rates range from 5 to 100 mV/s. Charge-discharge
galvanostatic tests were performed at current densities up to 1 A/g. Impedance data were
collected from 500 kHz to 0.01 Hz, with 10 mV in ac amplitude signal at open circuit
potential (OCP).
3. Results and discussion
3.1. Structural and morphological characterizations
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Figure 1(a) shows XRD pattern for the PCNs synthesized at 077 oC from OPL. The peaks at
2θ = 26.85o, 44.55
o, 50.45
o and 60.10
o are referring to graphite carbon according to ICDD
card 96-901-2231. In addition, the peak at 2θ = 21.10o is close to the reflection for carbon
(ICDD card 96-901-4005) [26]. The peak at 26.85o is because of the high crystalline cellulose
fibres which are formed due to the hemicelluloses and celluloses of OPL. The peaks at
44.55o, 50.45
o and 60.10
o show graphitic nature of the PCNs. From these peaks, it is clear
that the PCNs obtained from the synthesis of OPL show graphitic structure. In addition, we
evidence another peak at 68o, and this peak corresponds to the (2 2 0) plane. The reduction in
the size of this peak in comparison to other peaks shows the reduction in the crystallinity
facilitating the formation of the smaller particle size.
The lattice vibration of carbon materials was investigated by Raman spectroscopy. The D
band at 1365 cm-1
is known as disorder-induced character of graphite (see Fig. 1(b)). The G
band appears at 1610 cm-1
[26]. The Raman band between 2700 and 2900 cm-1
which
corresponds to the overtone of D band is known as 2D. The ID/IG ratio is 0.903.
Figure 1. (a) XRD pattern and (b) Raman spectrum for PCNs.
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To investigate the morphology of the obtained PCNs, FESEM and TEM were
performed. Figure 2(a) shows the spherical shape without any irregularity in the PCNs. TEM
analysis showed the average size of the PCNs to be 20–40 nm which is shown in Figure 2(b).
The particle size distribution was obtained from the TEM image and shown in Figure 2(c).
The histogram shows the average particle size of the PCNs in the sample. This fine particle
size of the obtained PCNs would be ideal for electrochemical measurements as it facilitates
the ion diffusion between the fine particles.
Figure 2. (a) FESEM, (b) TEM images and (c) particle size distribution for PCNs.
The surface area and pore width were measured using N2 adsorption-desorption
technique (BET method) (Micromeritics ASAP 2020) with degassing at 200 °C for 12 h. The
results showed that, PCNs having a surface area of 37.3 m2/g and 22 m
2/g of t-plot micropore
values (see Figure 3, left). In addition, PCNs showed a micropore percentage of 56.4% and
pore diameter of 1.98 nm (Figure 3, right).
Although the reason for decreasing surface area increasing specific capacitance with
respect to high temperature is not completely sure, this phenomenon happened due to the
aggregation of PCNs where it intact and exhibit smooth surfaces thus the surface area was
drastically decreased compared to the earlier reports on carbon nanospheres [24,25].
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Figure 3. N2 adsorption-desorption isotherms for PCNs (left) and pore volume with respect to
pore diameter (right).
3.2. Electrochemical studies
3.2.1. Cyclic voltammetry and galvanostatic charge–discharge
Cyclic voltammetry curves measured at different scan rates in 5 M KOH are shown in Figure
4. CV curves exhibit almost rectangular–like shape with no obvious redox peaks, which
imprints the EDLC behavior. Higher current densities have been obtained, indicating high
specific capacitance. Different annealing conditions play dominant role in increasing super
capacitance values [23]. The CV curves are a bit deviated from the ideal shape which is still
acceptable, even at the 100 mV/s.
The charge‒discharge test was performed at different current densities in 5 M KOH aqueous
solution and is shown in Figure 5(a). The data display nearly a straight line with neglected IR
drop, indicating a good current–voltage response. The specific capacitance was calculated
from charge‒discharge data using the pre-formulated equation [27,28]. The calculated
specific capacitance decreased with increase in discharge current and the highest specific
capacitance of 368 F/g was obtained at 0.70 A/g.
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Figure 4. Cyclic voltammetry curves at different scan rates for PCNs.
These values are much higher than those reported for novel corn grains-based
activated carbons (257 F/g in 6 M KOH) [29], activated carbon from waste Camellia oleifera
shell (266 F/g in 6 M KOH) [30] and melamine-based carbon (204.8 F/g in 1 M H2SO4) [31].
The specific capacitance decreased with increasing the current density, but it still shows high
values even at higher currents (225 F/g at 2 A/g). It is evident that the specific capacitance of
PCNs synthesized at 700 oC is much greater than that of reported PCNs which were
synthesized at 600 oC (309 F/g in 0.06 A/g) [23]. This increased value in the specific
capacitance is attributed to the fine particle distributions as evident in FESEM data. These
well distributed PCNs are more easily accessible to the electrolyte ions and also possess low
impurities at higher pyrolysis temperatures. These findings support that these materials are
interesting candidates for energy storage devices.
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Figure 5. (a) Galvanostatic charge-discharge curves at different current densities, (b) specific
capacitance as functions of current density.
3.2.2. Life stability
The cycling stability of PCNs was performed using galvanostatic charge‒discharge at
3 A/g for 1700 cycles and the result is presented in Figure 6. PCNs showed very good cycling
stability where it still maintained more than 96% of its original capacitance in 5 M KOH after
1700 cycles, which is higher than the capacitance retention obtained for C60 (91%) after only
1000 cycles [32] and for graphene (86%) after 1100 cycles [20]. Moreover, it shows high
Coulombic efficiency of 98%. Coulombic efficiency was calculated from the ratio of
discharging to charging time. The inset of Figure 6 shows the first and the last cycles in the
charge-discharge cycling test. From Figure 6 it is evident that there is no much change in the
linearity of the charging or discharging curves indicating the stability of PCNs.
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Figure 6. Cycle life stability curve (left vs. bottom) and Coulombic efficiency (right vs.
bottom) at 1 A/g current density, the inset shows the charge-discharge curves for different
cycles.
3.2.3. Electrochemical impedance spectroscopy (EIS)
EIS is performed to investigate the charge kinetics on the electrode surface. Nyquist plot for
PCNs at OCP is shown in Figure 7(a). The insets represent the high frequency region of the
recorded full impedance plot and the equivalent circuit used to fit the experimental data. A
small semicircle in the high frequency region and a vertically straight line in the low
frequency region can be seen. Rs and Rct (0.39 and 0.57 , respectively) were found to be
very small, indicating high electrical conductivity of PCNs. The vertical-linear section in the
low frequency region demonstrates a pure capacitive behavior and represents an ideal
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supercapacitor. The result of EIS measurement indicated that the PCNs have good capacitive
performance. All fitting parameters are summarized in Table 1.
Figure 7. (a) Nyquist plot, the insets are zoomed view of Nyquist plots at high-frequency
region and the equivalent circuit, (b) Bode plot, (c) and (d) real and imaginary parts of the
capacitance as functions of the frequency.
Bode plot shown in the Figure 7(b) explains the relation between the phase angle and
frequency. It is seen that, the phase angle is -70o which is in some extent close to the angle
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for ideal capacitor (-90o) [33]. Plots of real (C′) and imaginary (C″) parts of capacitance as a
function of frequency are plotted from the equations formulated by other authors [34,35] and
shown in Figure 7(c) and (d) respectively. The relaxation time (τ) value was found to be 2.68
s, indicating good electrochemical supercapacitance properties and fast charge–discharge
characteristic response of this composite. The relaxation time (τ) values distinguished the
transition between resistive and capacitive behaviors, as it defines predominantly resistive
behavior at frequencies above 1/τ and capacitive behavior below 1/τ. All the above mentioned
electrochemical parameters are listed in Table 1.
EIS was performed again after 1700 charge-discharge cycles to investigate the
electrochemical stability of the electrode. Figure 8 shows Nyquist plots before and after
cycling stability. The Nyquist plot after the completion of 1700 charge-discharge cycles is
almost similar to the one performed earlier. The minor increment in the resistance and the
relaxation time is due to some changes in the electrode porosity. The similarity in the plots
implies close parameters values before and after charge-discharge cycling, indicating the
electrochemical stability of these materials.
Table 1. Fitting parameters of the experimental impedance data for PCNs before and after cycling
stability at OCP.
Rs
(Ω)
Rct
(Ω)
C
(mF)
CPE
(F)
W
(Ω)
τ
(s)
Before 1700 cycle 0.43 0.43 0.27 0.41 0.20 2.68
After 1700 cycle 0.61 0.35 0.26 0.54 0.28 3.85
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The energy (E) and power (P) densities can be calculated from charge–discharge data
using the equations reported elsewhere [1,36]. The Ragone plot for PCNs is as seen in Figure
9, which shows high energy density of 13 Wh/kg at power density of 41 Wh/kg. This energy
density is about three times higher than the reported activated carbon electrodes from fibres
of oil palm empty fruit bunches (4.297 Wh/kg) [34]
and higher than the energy density of
ZnCl2 activated carbon prepared from sugar cane bagasse (10 Wh/kg) [37].
Figure 8. Nyquist plots of PCNs before and after cycling stability; the insets are zoomed view
of Nyquist plots at high-frequency region.
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Figure 9. Ragone plot for PCNs.
4. Conclusions
Very fine porous carbon nanoparticles (20–40 nm) prepared at 700 oC showed superior
supercapacitance properties where it showed high specific capacitance value of 368 F/g at
0.06 A/g in 5 M KOH with high stability (96% over 1700 cycles) and energy density of 13
Wh/kg. Low resistance values were obtained by fitting the impedance spectra indicating the
availability of these materials as precursor in the fabrication of supercapacitors electrode.
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Graphical Abstract
Superior supercapacitance behaviour was achieved using catalyst free, simple one step pyrolysis
based porous nano carbons. Obtained materials are from waste materials and also having high
porous nature with uniform shape showing excellent capacitance behaviour.