Carbon for energy storage derived from granulated white sugar by hydrothermal carbonization and subsequent zinc chloride activation

Article abstract of DOI:10.1149/2.0681709jes

Various electrochemical methods have been applied to establish the electrochemical characteristics of the electrical double layer capacitor consisting of the activated carbon material based electrodes and 1 M triethylmethylammonium tetrafluoroborate solution in acetonitrile and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid as the electrolytes. Activated carbon material used for the preparation of electrodes has been synthesized from hydrochar prepared via hydrothermal carbonization process of granulated white sugar solution in H₂O, followed by activation with ZnCl₂ with a mass ratio of 1:4 at the temperature 700°C. High porosity and Brunauer-Emmett-Teller specific surface area (SBET = 2100 m² g^-1), micropore surface area (S_micro = 2080 m² g^-1) and total pore volume (Vtot = 1.05 cm³ g^-1) have been achieved for the granulated white sugar derived carbon (GWS carbon) material. Wide region of ideal polarizability (ΔE ≤ 3.0 V), short characteristic relaxation time (0.5 s and 4.0 s), high specific series capacitance (125 F g^-1 and 140 F g^-1) and high energy density (39 W h kg^-1 and 48 W h kg^-1) have been calculated for the GWS carbon material in 1 M triethylmethylammonium tetrafluoroborate solution in acetonitrile and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, respectively, demonstrating that these systems are very promising for energy storage devices.

Full text of DOI:10.1149/2.0681709jes

A1866
Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
Carbon for Energy Storage Derived from Granulated White Sugar
by Hydrothermal Carbonization and Subsequent Zinc Chloride
Activation
∗
,z
∗
M. H a¨ rmas, T. Thomberg, T. Romann, A. J a¨ nes, and E. Lust
Institute of Chemistry, University of Tartu, 50411 Tartu, Estonia
Various electrochemical methods have been applied to establish the electrochemical characteristics of the electrical double layer
capacitor consisting of the activated carbon material based electrodes and 1 M triethylmethylammonium tetrafluoroborate solution
in acetonitrile and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid as the electrolytes. Activated carbon material used for
the preparation of electrodes has been synthesized from hydrochar prepared via hydrothermal carbonization process of granulated
◦
white sugar solution in H2O, followed by activation with ZnCl2 with a mass ratio of 1:4 at the temperature 700 C. High porosity
2
−1
2
−1
and Brunauer-Emmett-Teller specific surface area (SBET = 2100 m g ), micropore surface area (Smicro = 2080 m g ) and total
3
−1
pore volume (Vtot = 1.05 cm g ) have been achieved for the granulated white sugar derived carbon (GWS carbon) material. Wide
region of ideal polarizability (ꢀE ≤ 3.0 V), short characteristic relaxation time (0.5 s and 4.0 s), high specific series capacitance (125
−
1
−1
−1
−1
F g and 140 F g ) and high energy density (39 W h kg and 48 W h kg ) have been calculated for the GWS carbon material
in 1 M triethylmethylammonium tetrafluoroborate solution in acetonitrile and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic
liquid, respectively, demonstrating that these systems are very promising for energy storage devices.
©
The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0681709jes] All rights reserved.
Manuscript submitted May 11, 2017; revised manuscript received June 21, 2017. Published July 5, 2017.
In recent years, environmentally friendly energy storage devices
solvents or catalysts and requires only application of the low process-
◦
15–20
have gained increasing attention due to the growing requirement of
energy storage for sustainable energy applications. Among the vari-
ous existing energy storage devices, supercapacitors (electrical dou-
ble layer capacitors (EDLC) and hybrid capacitors (HC)) are con-
sidered as the most promising short-term energy storage devices due
to their high power density, short characteristic time constant, excel-
ing temperature (normally not higher than 300 C).
The process
consists of the heat-treatment of an aqueous solution of organic ma-
terials at desired autogenous pressure and temperature. The resulting
solid carbon product (termed hydrochar) generally possesses very
low porosity and additional pyrolysis leads to the moderate increase
in porosity of porous carbon materials. Therefore, the carbon material
porosity (the pore size distribution, the average pore width and the
specific surface area) must be increased by using additional activation
lent coulombic reversibility (98% or higher), high energetic efficiency
6
(
92–94%), long cycle life (over 10 cycles) and wide operation tem-
1
–3
perature range filling the gap between dielectric capacitors and
methods based on the application of air, carbon dioxide, steam as well
4
–7
21,22
traditional batteries. EDLCs store energy in the electrical double
layer, where the adsorption of ions is based mainly on the electrostatic
interactions. The unique characteristics of EDLCs allow them to re-
place or combine with batteries and fuel cells in applications where
the high power pulses are important, such as the different energy
recuperation systems and peak power sources, hybrid electric vehi-
cles, wind turbines, digital communication devices, cameras, mobile
3 4
as KOH, NaOH, H PO , etc. as an activation agents.
The objective of this study was to investigate the applicability
limits of carbon material derived from granulated white sugar (GWS
carbon) by HTC method combined with subsequent zinc chloride ac-
tivation step of hydrochar, for supercapacitor electrodes. Synthesized
GWS carbon material was used as an electrode material in the two-
electrode single cells of EDLC filled with 1 M triethylmethylammo-
8
–10
phones, laptops, etc.
nium tetrafluoroborate (TEMABF
ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (EMImBF
as the electrolytes. The mixture of TEMABF and AN was selected
4
) solution in acetonitrile (AN) or 1-
Porous carbon materials are considered to be the most promising
electrode materials for portable supercapacitors due to the high surface
area, good electrical conductivity, good chemical stability, low gravi-
4
)
4
due to the low viscosity (0.41 mPa s), high electrical conductivity (κ
1
–8
−1
metric density and low cost. The electrical charge accumulated in
EDLC depends on the electrochemically active surface area and, thus,
on the porosity and hierarchical porous structure of a carbon material.
In addition, the presence of mesopores in porous carbon materials
determines the power density of EDLC due to a strong effect on the
rate of mass transfer (diffusion and migration) and adsorption rate of
charge carriers inside the hierarchical porous matrix. Therefore, the
characteristics of micro- and mesoporous carbon materials (especially
the ratio of micropore and mesopore surface areas and pore volumes)
have to be optimized to improve further the specific energy and power
= 50.2 mS cm ) and good electrochemical stability within a very
20,23–27
wide region of cell potentials.
4
EMImBF was chosen due to
−
1
high bulk conductivity (κ = 13.6 mS cm ) and wide potential region
28–31
of electrochemical stability.
However, compared to TEMABF
4
solution in AN, EMImBF has almost two order of magnitude higher
4
2
3
viscosity (38.2 mPa s). Cyclic voltammetry (CV), constant cur-
rent charge/discharge (CC), electrochemical impedance spectroscopy
(EIS) and constant power discharge (CP) methods were used to study
the electrochemical performance of EDLCs.
1
,11–14
density of EDLCs.
The classical porous carbon synthesis route uses pyrolysis of or-
ganic compounds followed by activation. However, control over pore
size distribution by this route is poor. Thus, development of novel
methods for the synthesis of porous carbons with controlled pore
size distribution is important for further development of supercapac-
itors technology. Hydrothermal carbonization (HTC) is an attractive
process to prepare spherical carbon materials from natural precur-
sors being cheap and “green” since it does not use expensive organic
Experimental
Chemicals and reagents used.—1-ethyl-3-methylimidazolium
tetrafluoroborate (Sigma-Aldrich, for electrochemistry, ≥ 99.0%, H
2
O
<
200 ppm), zinc chloride (≥ 97% purity, anhydrous, Sigma-Aldrich)
and hydrochloric acid (50% v/v aqueous solution, Alfa Aesar) were
used as received. Granulated white sugar (GWS, 17000 kJ/kg, price
∼
0.45 €/kg) was used for the preparation of sugar solution. 1 M tri-
ethylmethylammonium tetrafluoroborate solution in acetonitrile was
prepared from pure acetonitrile (H
O < 0.003%, Riedel-de Ha e¨ n,
2
∗
stored over molecular sieves before use) and dry triethylmethylammo-
nium tetrafluoroborate (Stella Chemifa Corporation). Ultrapure water
Electrochemical Society Member.
E-mail: alar.janes@ut.ee
z
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
Milli-Q , 18.2 Mꢁ cm, Millipore) was used for the preparation of the
A1867
+
(
sugar solution, for cleaning the resulting solid product formed during
HTC process, and for cleaning the GWS carbon from activating agent
and by-products.
Synthesis and physical characterization of GWS carbon
material.—The HTC of sugar aqueous solution (17 g of GWS was
dissolved in water to prepare 100 ml of solution) was carried out in a
◦
high-pressure reactor (B u¨ chi limbo, vessel volume 285 ml) at 200 C
for 24 hours. After HTC step, the hydrochar was collected and washed
+
with Milli-Q water for several times and dried overnight in a vacuum
◦
oven (Vaciotem-TV, J.P. Selecta) at 120 C and 50 mbar. The yield of
HTC process was 38%.
GWS carbon material was synthesized by activation of the hy-
◦
drochar with ZnCl
2
with mass ratio 1:4 at 700 C using heating up
◦
−1
rate of 5 C min . Activation procedure has been carried out accord-
ing to the method described in Ref. and the yield of this process
33
was 36%. Additional treatment of GWS carbon material with H
rity 99.9999%) at 800 C for 2 hours was performed to reduce the
2
(pu-
◦
surface functional groups because the H
demonstrate wider potential region of ideal polarizability.
2
reduced carbon materials
2
7,32,33
X-ray diffraction (XRD) patterns were collected by a Bruker D8
diffractometer (Bruker Corporation) with position sensitive LynxEye
◦
detector using CuKα radiation with an angular step 0.01 and counting
time 2 s for each angle measured. Diffraction spectra were recorded
◦
◦
at 25 C ± 1 C temperature. Raman spectra were recorded using a
Renishaw inVia micro-Raman spectrometer with Ar laser excitation
beam (λ = 514 nm). Obtained spectra were fitted with the combination
of two Lorentzian and two Gaussian functions to model the D- and
34
G-bands observed.
The porous structure of the synthesized GWS carbon material was
characterized by the nitrogen sorption method.35–37
sorption ex-
N
2
Figure 1. SEM images for hydrochar (a) and GWS carbon material (b).
periment was performed at the boiling temperature of liquid nitrogen
◦
(
−195.8 C) using an ASAP 2020 system (Micromeritics, USA). The
specific surface area (SBET) was obtained applying Brunauer-Emmett-
arate mechanically the two working electrodes. All electrochemical
36,38
◦
Teller (BET)
.05 to 0.1. The total pore volume (Vtot) was calculated from the
amount of adsorbed nitrogen at relative pressure p/p
= 0.995. The
t-plot method was used to obtain the surface area (Smicro) and volume
micro) of micropores in carbon material studied. The pore size distri-
bution was calculated using a fitting of N isotherm by non-local den-
sity functional theory (NLDFT) model “Carbon-N2-77, 2D-NLDFT
theory to multiple points in the pressure range from
experiments were carried out at temperature 20 ± 1 C.
0
The electrochemical characteristics of EDLCs were established by
CV, CC, EIS and CP methods. Impedance spectra (over ac frequency
range from 1 mHz to 300 kHz with modulation 5 mV), CV and CC
curves were recorded using a frequency response analyzer 1252A and
potentiostat SI1287 (Solartron, UK). The CP tests were performed on
a BT2000 testing system (Arbin Instruments, USA).
0
(
V
2
39–43
Heterogeneous Surface” (SAIEUS v2.02, Micromeritics).
The surface structure and morphology of the synthesized hy-
drochar and carbon material was examined by scanning electron mi-
croscopy (SEM), using the ZEISS EVO MA15 SEM system. The
surfaces of the samples were sputter-coated with gold before SEM
measurements.
Results and Discussion
Physical characterization of GWS carbon material.—The SEM
images of the hydrochar and the GWS carbon material are given in
Fig. 1. It can be seen that the hydrochar and GWS carbon material
exhibit interconnected spheres with various sizes in the range from 1
to 20 μm, which is characteristic of hydrochar and carbon materials
Electrode preparation, cell assembly and electrochemical char-
acterization of material.—Carbon electrodes were prepared from syn-
thesized GWS carbon material (94 wt%) adding a 6 wt% suspension of
binder (prepared from polytetrafluoroethylene, PTFE, 60% dispersion
15,27,33
synthesized by HTC process.
The XRD patterns for synthesized carbon material, given in
Fig. 2 inset, show very weak reflections corresponding to the graphite
◦
in H
2
O, Sigma-Aldrich). The mixture of synthesized carbon material
(hexagonal symmetry, space group P6
3
/mmc) 002 peak at 2θ ∼ 26
◦
and binder was laminated and roll-pressed (HS-160N, Hohsen Corpo-
ration, Japan) to form a flexible layer of the carbon electrode material
with a thickness of 105 ± 5 μm. After drying under vacuum, for
better electrochemical (homogeneous) polarization of carbon layers,
the pure Al layer (3 μm) was deposited onto one side of the carbon
electrode using the plasma activated physical deposition method. The
and 100/101 peak at 2θ ∼ 43 , characteristic for mainly amorphous
6
,8,11,16,33
carbon materials.
The first-order Raman spectra, given in Fig. 2, show two character-
−
1
istic peaks for amorphous carbon materials: one peak at ∼1338 cm
−
1
8,27,28,33,44–47
(D-peak) and second peak at ∼1577 cm (G-peak).
For
hexagonal graphite, G-peak is assigned to E2g vibrational mode, which
−
2
2
electrode material loading into one electrode was about 7 mg cm
mass of binder and current collector are included what were about
0% of the total mass). A standard two–electrode test cell (aluminum,
HS Test Cell, Hohsen Corporation) with two identical carbon elec-
is attributed to in-plane stretching motion of sp carbon atom pairs.
(
1
D-peak originates due to the existence of disorder in a graphitic struc-
ture which is assigned to A1g symmetry and this peak reflects the pres-
ence of amorphous areas in carbon particles. The ratio of intensities of
2
trodes (geometric area of 2.0 cm ) and 1 M TEMABF
EMImBF was completed inside a glove box (Labmaster sp, MBraun,
and H O concentrations lower than 0.1 ppm). The vacuum dried 25
μm thick TF4425 (Nippon Kodoshi) separator sheet was used to sep-
4
in AN or
D-peak and G-peak (I
a lower value means the higher degree of graphitization.
on D-peak and G-peak fitting results the relatively high value of I
(about 1.31) was obtained demonstrating low graphitization degree of
D G
/I ) represent the degree of graphitization and
33,44–47
4
Based
/I
O
2
2
D
G
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A1868
Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
Table I. Results of sorption measurements of carbon materials.
Carbon
material
SBET
−1
Smicro
−1
Vmicro
(cm g
Vtot
(cm g
2
2
3
−1
3
−1
)
(m g
)
(m g
)
)
GWS carbon
2100
1544
1236
2080
1503
1232
1.01
0.71
0.550
1.05
0.87
0.557
11
TiC-CDC-900
27
GDAC-12h
the GWS carbon material synthesized is highly microporous. How-
ever, this carbon material contains also reasonable amount of small
mesopores (2.0 nm < d < 3.2 nm). Compared with carbon dioxide
27
2
activated D-glucose derived carbon, ZnCl , KOH or their mixture
3
3
8,25,26
activated carbon, different carbide-derived carbons
and other
5
,15,16,19,20
porous carbon materials,
the GWS carbon material demon-
strates high specific surface area and micropore surface area as well as
total pore volume values, being essential for energy storage materials.
Figure 2. Raman spectra and characteristic XRD patterns (inset) for GWS
carbon material.
Cyclic voltammetry and capacitance vs. cell potential data.—
The specific capacitance (CCV), calculated from CV curves, vs. cell
potential (ꢀE) curves for assembled EDLCs are presented in Fig.
4. The CCV values of a test-cell were calculated from CV curves
according to Eq. 1 and Eq. 2:
the GWS carbon, which is in agreement with the XRD results. Thus,
Raman spectroscopy and the XRD data reveal that the synthesized
carbon material is mainly amorphous.
The N
2
sorption isotherms for the porous GWS carbon material
C = jν−1
,
[1]
(
Fig. 3a) can be approximated to the isotherm of type I according to the
IUPAC classification,
37,40
characteristic of microporous carbon mate-
rial with an average pore width less than 2 nm. According to the poros-
ity measurements results, given in Table I, activation of hydrochar by
2
C ,
C
m
=
[2]
m
2
-1
ZnCl
2
produces carbon material with high SBET = 2100 m g , Smicro
2 -1 3 −1 3 −1
where j is the current density (per geometrical surface area of elec-
trode), ν is the potential scan rate, C is the cell capacitance and m is
the weight of one carbon electrode.
=
2080 m g , Vmicro = 1.01 cm g and Vtot = 1.05 cm g values.
The pore size distribution plot demonstrated in the Fig. 3b shows that
Eq. 1 is correct if the capacitance C does not depend on the potential
ꢀ
s
→
Eq. 2 can be implemented if the positively and
negatively charged electrodes have the same capacitance at the cell
potential applied.
The specific capacitance vs. cell potential curves for EDLC based
on the GWS carbon material in 1 M TEMABF
4
solution in AN and
EMImBF ionic liquid have nearly symmetrical shape in respect to
4
−
1
the zero-current line at low potential scan rates (v ≤ 50 mV s
,
not shown for clearness of figure) and cell potentials (ꢀE ≤ 3.0 V)
demonstrating nearly ideal capacitive behavior (Fig. 4). EDLC based
4
on 1 M TEMABF solution in AN demonstrate nearly ideal capacitive
−
1
behavior even at very high potential scan rates (v ≤ 500 mV s ).
Figure 4. Cyclic voltammetry curves expressed as specific capacitance vs.
cell potential dependencies for the EDLCs based on the GWS carbon material
in 1 M TEMABF4 in AN (dotted lines) and EMImBF4 ionic liquid (solid lines)
Figure 3. N2 sorption isotherms (a) and differential pore size distribution
vs. pore width plots (b) obtained using NLDFT method in conjunction with
SAIEUS pore surface model for GWS carbon material.
−
1
−1
at cell potential scan rates ν = 5 mV s and ν = 200 mV s (noted in figure)
within the cell potential range from 0 to 3.0 V.
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
A1869
Table II. Electrochemical parameters for EDLCs based on
different carbon materials and electrolytes.
CCV
CCC
−1
Cs
−1
τR
(F g 1) (F g ) (F g
−
)
(s)
Carbon material and electrolyte
GWS carbon in EMImBF4
GWS carbon in 1 M TEMABF4 in AN
TiC-CDC-900 in 1 M TEMABF4 in AN
GDAC-12h in 1 M TEMABF4 in AN
134
110
136
125
136
107
N/A
N/A
144 4.0
125 0.5
135 0.5
126 0.34
1
1
2
7
CCV – specific capacitance calculated from CV curves within the cell
potential range from 0 to 3.0 V using the integrated charge q obtained
according to Eq. 3, CCC – average specific capacitance calculated from
CC curves within the cell potential range from 1.5 to 3.0 V, according to
Eq. 4, Cs – specific series capacitance calculated from electrochemical
impedance data at cell potential ꢀE = 3.0 V and ac frequency f =
1
mHz, τR – characteristic time constant.
−
1
However, the highest specific capacitance value CCV = 134 F g (at
−1
ꢀ
E ≤ 3.0 V and v = 1 mV s ) has been calculated for EDLC based
on the ionic liquid electrolyte (Table II).
The capacitance values have been calculated over the cell potential
range ꢀE from 0 to 3.0 V, using the integrated total charge q values
obtained according to Eq. 3:
ꢀ
E2
ꢀE2
ꢀ
ꢀ
q =
i(ꢀE)dt ≡
i(ꢀE)
d(ꢀE).
[3]
ν
ꢀ
E1
ꢀE1
EDLC based on 1 M TEMABF
4
solution in AN demonstrate high
capacitance retention and maintain ∼70% of the initial capacitance
−
1
at potential scan rate v = 1000 mV s showing specific integral
−1
capacitance CCV = 76 F g . Ionic liquid based EDLC system shows
somewhat faster capacitance decrease due to the higher viscosity of
the ionic liquid. However, this system demonstrated high capacitance
Figure 5. Constant current charge/discharge cycles at current density j = 0.1
−1
−1
A g (a) and j = 1 A g (b) for the EDLCs based on the GWS carbon
material in 1 M TEMABF in AN and EMImBF ionic liquid.
4
4
−1
retention up to potential scan rate v = 100 mV s showing specific
−1
capacitance CCV = 94 F g (∼70% of the initial capacitance). At
−1
potential scan rate v = 1000 mV s the specific capacitance decreased
=
−
1
to CCV = 27 F g
.
−1
Calculated values of capacitance for GWS carbon material derived
from granulated white sugar by HTC and subsequent zinc chloride ac-
tion in AN demonstrated somewhat lower specific capacitance CCC
tivation method in 1 M TEMABF
liquid are comparable with the specific capacitance values established
4
solution in AN and EMImBF
4
ionic
−1
=
109 F g . The calculated CCC values for EDLCs in both elec-
trolytes are in a good agreement with cyclic voltammetry (Table II)
and electrochemical impedance spectroscopy data (discussed later).
27,28,33
for carbon materials derived from D-glucose
expensive raw material ∼40 €/kg). In comparison with binary and
ternary carbides (Mo C, Ta HfC , WTiC ) derived carbons, the GWS
(i.e. from the more
At higher current densities EDLC based on 1 M TEMABF
in AN demonstrated high capacitance retention even at current density
4
solution
2
4
5
2
carbon offers much cheaper supercapacitor electrodes with compa-
rable or even higher capacitance. In addition, the high-temperature
chlorination (or HCl treatment) step can be avoided for the prepara-
tion of carbon powders.
−1
−1
j = 5 A g with the specific capacitance of CCC = 102 F g . The
ionic liquid based EDLC system shows a somewhat faster decrease of
−1
capacitance (specific capacitance CCC = 71 F g at current density j
−1
=
5 A g ) with increased discharge current density.
The cycling efficiency, i.e. the coulombic efficiency (Qeff) values
Constant current charge/discharge data.—EDLCs were tested
have been calculated as a ratio of integrated charges, measured during
52
at different fixed constant current (CC) regimes (from 0.1 to 5 A
discharging and charging steps of EDLCs. The Qeff values are within
−
1
g
) applying charging/discharging steps within cell potential region
the range from 98 to 99%, indicating to a very good reversibility of
from 1.5 to 3.0 V. The discharge and charge capacitances have been
calculated from the data of the third charge/discharge cycle according
to Eq. 4:
EDLCs. However, the energy efficiencies (E ) for studied EDLCs
were slightly lower staying within the range from 92 to 97% for
eff
TEMABF solution in AN at charging current density range j = 0.1
4
−
1
−1
ꢀ
E2
E1
A g to j = 5 A g , respectively. EMImBF
4
system demonstrated
∫ꢀ
jdt
similar (93 to 95%) Eeff values for charging current density range j =
C =
[4]
−
1
−1
d (ꢀE)
0.1 A g to j = 1 A g . However, higher current densities resulted
−1
in significantly lower Eeff values (77% at j = 5 A g ).
where j is the current density, dt is the change in time and ꢀE is
the cell potential. The charge/discharge curves (Figs. 5a and 5b) are
nearly linear and symmetrical for EDLCs in 1 M TEMABF
4
solution
Nyquist plots.—The shape of Nyquist plots (imaginary part Z(Im)
vs. real part Z(Re) of the electrochemical impedance) (Fig. 6a)
1
,53,54
in AN and EMImBF ionic liquid with carbon electrodes prepared
4
from GWS carbon material at different current densities, demonstrat-
ing very good electrochemical reversibility, thus coulombic and en-
ergy efficiency. The longest charge/discharge cycle has been observed
depends noticeably on the electrolyte conductivity and viscosity used,
but is nearly independent of ꢀE applied, if ꢀE ≤ 3.0 V. This in-
dicates that there are no quick faradaic charge transfer processes at
GWS carbon | electrolyte interface. The Nyquist plot (Fig. 6a) con-
4
for EDLC in EMImBF ionic liquid, corresponding to the highest
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A1870
Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
Figure 6. Impedance complex plane plots (a), phase angle vs. ac frequency plots (b), specific series capacitance vs. ac frequency dependencies (c) and imaginary
part of capacitance vs ac frequency plots (d) for the EDLCs based on the GWS carbon material in 1 M TEMABF4 in AN and EMImBF4 ionic liquid (noted in
figure) at different cell potentials (noted in figure).
sists mainly of three parts: Refs. 11, 28, 33 (1) the characteristic very
small semicircle at ac frequencies higher than 300 Hz; (2) the very
frequencies which demonstrate a nearly ideal capacitive behavior of
these systems. The phase angle starts to increase at higher cell poten-
◦
◦
well expressed so-called “porous” region with a slope of nearly −45
tials ꢀE ≥ 3.0 V (Fig. 6b) (|φ | ≤ 80 at 3.2 V), especially for ionic
within the ac frequency range from 0.1 to 300 Hz; and (3) the double
liquid based system, due to the initiation of faradaic reactions occur-
◦
1,5,14,27,55
layer capacitance region with a slope of α ≤ −85 at very low ac
ring at highly negatively and positively charged electrodes.
frequency f < 0.1 Hz. The semicircle at high frequencies depends on
the adsorption kinetics of ions at the porous carbon electrode and on
the series resistance of a material as well as on the mass transfer resis-
tance of electrolyte inside of the porous carbon structure, both being
high for ionic liquid based systems. The nearly linear region at inter-
mediate frequencies characterizes the mass transfer limited processes
in the porous carbon electrode matrix. The nearly vertical region of
the Nyquist plots for the carbons with micro- and mesoporous struc-
ture is assigned to the finite length adsorption effects determined by
the microporosity of the carbon material under study. Data in Fig. 6a
s
The values of specific series capacitance, C , (Fig. 6c and Table II),
overlapping with C values at f < 0.1 Hz, and demonstrating long
p
plateaus, are in a good agreement with the values obtained from the
CV and CC data. Slightly higher specific capacitance value (140 F
−
1
g
) has been achieved for ionic liquid based EDLC system because
in CV measurement conditions adsorption equilibrium has not estab-
lished jet. Only for EMImBF based EDLC noticeable increase of C
4
s
at ꢀE > 3.2 V has been observed explained by the very slow faradaic
processes.
For further characterization of EDLCs, the values of the imaginary
ꢁꢁ
inset show that high frequency series resistance R
higher for EMImBF based EDLC than for TEMABF
EDLC. Therefore, the high-frequency semicircle is more pronounced
for EMImBF ionic liquid electrolyte system due to the higher viscos-
s
is nearly 3 times
part C (ω) of capacitance have been calculated according to the
4
4
+ AN based
Eq. 5:
ꢁ
Z (ω)
4
ꢁꢁ
C (ω) =
[5]
2
ity and lower conductivity of ionic liquid which have a considerable
influence on the mass transfer resistance of the electrolyte ions and ad-
sorption capacitance values of porous carbon material.11,25,28,33 Data in
ω|Z(ω)|
where |Z(ω)| is the impedance modulus.53,54 C (ω) corresponds to en-
ergy dissipation of the capacitor by an irreversible faradaic charge
transfer process, which can lead to the hysteresis of the electro-
ꢁꢁ
Fig. 6a inset indicate that the so-called “porous” region with a slope
◦
of nearly −45 is well expressed for EDLCs in EMImBF
4
, which
ꢁꢁ
could be explained by the increased mass-transfer resistance of ions
in the porous carbon material for EMImBF ionic liquid electrolyte
This observation is in a good accordance with CV
chemical processes. According to the results in Fig. 6d, the C vs.
f dependencies have a well-expressed maximum at the so-called re-
laxation frequency f , determining the characteristic time constant τ
4
1
1,25,28,32
system.
and CC data.
The phase angle value for ionic liquid based system (Fig. 6b) ap-
R
R
−
1
= (2πf ) values. Comparison of the data for the EDLC systems
R
completed with different electrolytes indicates a very strong influence
◦
◦
proaches −87 and for acetonitrile based electrolyte −89 at low ac
of electrolyte properties on τ values (Table II). The value of τ in-
R
R
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
A1871
◦
60
rial synthesized at chlorination temperature 950 C (TiC-CDC-950)
and for carbon dioxide activated D-glucose derived carbon material
(
28
4
GDAC-12h) based systems in EMImBF ionic liquid have been
introduced into Fig. 7. These results show that GWS carbon material
synthesized in this work exhibit similar energy density values at low
−
1
constant power values (39 W h kg ) and slightly lower energy density
values at higher constant power values in 1 M TEMABF solution in
AN compared with TiC-CDC-900 and GDAC-ZnCl carbons based
EDLCs. However, in EMImBF ionic liquid electrolyte GWS carbon
material shows the highest energy density at low constant power values
4
2
4
−
1
(
48 W h kg ) compared with GDAC-12h and TiC-CDC-950 carbon
materials and slightly lower energy density values at higher constant
power values compared with GDAC-12h carbon based EDLC.
Conclusions
Granulated white sugar derived carbon (GWS carbon) material was
synthesized from hyrdochar, obtained from hydrothermal carboniza-
tion of granulated white sugar (GWS) aqueous solution, applying
Figure 7. Specific energy vs. specific power plots for the EDLCs based on
different carbon materials in 1 M TEMABF4 in AN and EMImBF4 ionic liquid
(noted in figure), obtained from constant power tests within the cell potential
additional activation with ZnCl
2
in mass ratio 1:4 at the temperature
range from 3.0 V to 1.5 V.
◦
700 C.
Based on the XRD and Raman spectroscopy data, the synthesized
GWS carbon material is mainly amorphous. N sorption measure-
2
creases with the decrease of electrolyte conductivity and increase of
electrolyte viscosity, but the value of τ is only slightly dependent of
R
ments results show that GWS carbon material is mainly microporous,
however, this carbon contains also reasonable amount of small meso-
pores and shows high porosity, i.e. high Brunauer-Emmett-Teller spe-
ꢀ
E applied, at ꢀE ≤ 3.0 V. The characteristic time constant values
for EDLCs in 1 M TEMABF
4
solution in AN and EMImBF
4
ionic
2
−1
cific surface area SBET = 2100 m g , micropore surface area Smicro
liquid are τ
The τ value established for EDLCs in 1 M TEMABF
in AN (τ
material synthesized from natural organic precursor (cellulose, potato
starch, eucalyptus wood sawdust) using HTC process followed by the
R
∼ 0.5 s and τ
R
∼ 4.0 s, respectively.
2
−1
3
−1
=
2080 m g , micropore pore volume Vmicro = 1.01 cm g and
3 −1
R
4
solution
total pore volume Vtot = 1.05 cm g
.
R
∼ 0.5 s) is substantially shorter than that of the carbon
The electrochemical characteristics of the EDLCs consisting of
the GWS carbon material based electrodes in 1 M triethylmethy-
lammonium tetrafluoroborate (TEMABF
4
) solution in acetonitrile
2
0
subsequent KOH activation (τ
comparable with τ for EDLCs based on D-glucose derived carbon
electrodes (carbon material activated for 12 h with carbon dioxide
R
∼ 10 s). The calculated τ
R
values are
(
AN) and in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liq-
R
4
uid (EMImBF ) were tested applying the electrochemical impedance
spectroscopy, cyclic voltammetry, constant current charge/discharge
and the constant power discharge methods.
The electrochemical impedance spectroscopy, cyclic voltammetry
and constant current charge/discharge measurement results show that
27
(
τ
R
= 0.34 s)), for activated sucrose-derived carbon electrodes (τ
R
4
8
∼
2
0.5 s) and for KOH and ZnCl activated carbon electrodes (τ
R
3
3
∼
0.8 s) based EDLCs.
The τ value obtained for EDLC based on the GWS carbon mate-
ionic liquid (τ
obtained for sucrose derived carbon (τ
R
the values of specific capacitance are somewhat higher for EMImBF
4
rial in EMImBF
4
R
= 4.2 s) is substantially shorter than
−1
(
±
135 ± 5 F g ) electrolyte compared to the TEMABF
4
in AN (110
5
6
τ
R
R
= 20 s), coconut shell ac-
−1
5 F g ). The specific energy vs. specific power dependencies are
51
29
tivated carbon (τ
R
∼ 10 s) and for carbon cloth (τ
R
= 8 s) and only
similar with the previous results and at low constant power values the
slightly longer compared with characteristic time constant obtained
stored energy is higher for EDLC based on EMImBF
4
ionic liquid
in AN
57
for carbon nanosheets (τ
R
∼ 2.5 s) and titanium carbide-derived
−1
(
48 W h kg ) compared with EDLC based on TEMABF
4
58
carbon based EDLC (τ
R
= 1.76 s).
−1
electrolyte (39 W h kg ). However, the best capacitance retention
and shortest relaxation time constant τ
∼ 0.5 s were established for
EDLCs in 1 M TEMABF solution in AN due to the lower viscosity
and higher electrical conductivity compared to the ionic liquid based
electrolyte. Therefore, EDLC based on 1 M TEMABF solution in
R
Ragone plots.—The Ragone plots, i.e. the energy vs. power
4
1
,2,27,32,59
relationships
AN and EMImBF
power discharge tests within the cell potential range from 3.0 V to
.5 V (Fig. 7) (taking into account the total weight of electrodes, i.e.
for the EDLCs in 1 M TEMABF
4
solution in
4
ionic liquid have been calculated from the constant
4
AN electrolyte delivers substantially higher energy at higher constant
power values.
1
mass of carbon material, binder and current collectors). The specific
energy vs. specific power plots for the EDLCs based on GWS carbon
Thus, the GWS carbon material synthesized using cheap and abun-
dant GWS as the starting material, show good electrochemical per-
material and 1 M TEMABF
4
solution in AN or EMImBF
4
ionic liquid
4 4
formance in TEMABF in AN as well in EMImBF ionic liquid and
show a noticeable influence of the electrolyte properties, i.e. viscosity
and electrical conductivity on the stored specific energy and delivered
specific power values. At low constant power values, the stored energy
is promising carbon material for the high energy and power density
supercapacitor application. It is important to mention that synthesis of
carbon nanospheres is simpler and environmentally more friendly than
the preparation of molybdenum carbide or titanium carbide derived
carbon powders. As a next step, the carbon powders will be synthe-
sized from the raw syrup or from the molasses of different plants (thus
less expensive than GWS) and tested as electrode materials for EDLC
and other devices.
4
is somewhat higher for EDLC based EMImBF ionic liquid (48 W h
−
1
kg ) in a good agreement with the specific capacitance values ob-
tained from the CV, CC and EIS data. At higher constant power values
4
the EDLC based on 1 M TEMABF solution in AN electrolyte de-
liver substantially higher energy due to the lower viscosity and higher
electrical conductivity compared to the ionic liquid based EDLC.
For comparison of the achieved energy- and power density val-
ues established in this work, similar dependencies for the titanium
carbide-derived carbon material synthesized at chlorination tempera-
Acknowledgments
This research was supported by the EU through the Euro-
pean Regional Development Fund (Centers of Excellence, 2014–
2020.4.01.15-0011 and 3.2.0101–0030, TeRa project SLOKT12026T.
Higher education specialization stipends in smart specialization
◦
11
ture 900 C (TiC-CDC-900) and for D-glucose derived zinc chloride
33
activated carbon material (GDAC-ZnCl
2
)
based systems in 1 M
TEMABF solution in AN, the titanium carbide-derived carbon mate-
4
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A1872
Journal of The Electrochemical Society, 164 (9) A1866-A1872 (2017)
growth areas 2014–2020.4.02.16-0026) and Institutional Research
grant IUT20–13. This work was partially supported by Estonian Re-
search Council grants PUT1033 and PUT55.
29. H. Kurig, M. Vestli, K. T o˜ nurist, A. J a¨ nes, and E. Lust, J. Electrochem. Soc., 159,
A944 (2012).
3
0. S. Zhang, N. Sun, X. He, X. Lu, and X. Zhang, J. Phys. Chem. Ref. Data, 35, 1475
2006).
(
3
1. M. Gali n´ ski, A. Lewandowski, and I. Stepniak, Electrochim. Acta, 51, 5567 (2006).
ꢂ
3
2. T. Tooming, T. Thomberg, L. Siinor, K. T o˜ nurist, A. J a¨ nes, and E. Lust, J. Elec-
trochem. Soc., 161, A222 (2014).
References
1
. B. E. Conway, Electrochemical supercapacitors: scientific fundamentals and tech-
nological applications, Springer, New York (1999).
33. M. H a¨ rmas, T. Thomberg, H. Kurig, T. Romann, A. J a¨ nes, and E. Lust, J. Power
Sources, 326, 624 (2016).
2
3
4
. R. K o¨ tz and M. Carlen, Electrochim. Acta, 45, 2483 (2000).
. A. Burke, J. Power Sources, 91, 37 (2000).
. G. Salitra, A. Soffer, L. Eliad, Y. Cohen, and D. Aurbach, J. Electrochem. Soc., 147,
34. J. Ribeiro-Soares, M. E. Oliveros, C. Garin, M. V. David, L. G. P. Martins,
C. A. Almeida, E. H. Martins-Ferreira, K. Takai, T. Enoki, R. Magalh a˜ es-Paniago,
A. Malachias, A. Jorio, B. S. Archanjo, C. A. Achete, and L. G. Can c¸ ado, Carbon,
95, 646 (2015).
35. P. I. Ravikovitch and A. V. Neimark, Colloids Surf. Physicochem. Eng. Asp., 187–188,
11 (2001).
2486 (2000).
5. E. Frackowiak and F. Beguin, Carbon, 39, 937 (2001).
6
. T. Thomberg, H. Kurig, A. J a¨ nes, and E. Lust, Micropor. Mesopor. Mater., 141, 88
(
2011).
36. S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc., 60, 309 (1938).
37. S. J. Gregg and K. S. W. Sing, Adsorption, surface area, and porosity, p. 328, Aca-
demic Press, (1991).
38. S. Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 62, 1723
(1940).
7
8
. A. Burke and M. Miller, J. Power Sources, 196, 514 (2011).
. I. Tallo, T. Thomberg, K. Kontturi, A. J a¨ nes, and E. Lust, Carbon, 49, 4427
(2011).
9
. L. Eliad, G. Salitra, A. Soffer, and D. Aurbach, Langmuir, 21, 3198 (2005).
0. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J. E. Fischer, B. Yi, H. C. Foley, and
M. W. Barsoum, Nat. Mater., 2, 591 (2003).
1
39. P. I. Ravikovitch, A. Vishnyakov, and A. V. Neimark, Phys. Rev. E, 64, 0116021
(2001).
1
1. I. Tallo, T. Thomberg, H. Kurig, A. J a¨ nes, K. Kontturi, and E. Lust, J. Solid State
40. J. Rouquerol, F. Rouquerol, and K. S. W. Sing, Adsorption by Powders and Porous
Solids: Principles, Methodology and Applications, 1 edition., p. 467, Academic Press,
San Diego (1998).
41. W. D. Harkins and G. Jura, J. Am. Chem. Soc., 66, 1366 (1944).
42. J. Jagiello, Langmuir, 10, 2778 (1994).
43. J. Jagiello and J. P. Olivier, Adsorption, 19, 777 (2013).
44. F. Tuinstra and J. L. Koenig, J. Chem. Phys., 53, 1126 (1970).
45. A. C. Ferrari, Solid State Commun., 143, 47 (2007).
46. M. S. Dresselhaus, G. Dresselhaus, and M. Hofmann, Vib. Spectrosc., 45, 71 (2007).
47. H. Kurig, M. Russina, I. Tallo, M. Siebenb u¨ rger, T. Romann, and E. Lust, Carbon,
100, 617 (2016).
Electrochem., 17, 19 (2013).
1
1
2. J. Chmiola, G. Yushin, R. Dash, and Y. Gogotsi, J. Power Sources, 158, 765 (2006).
3. M. Oschatz, E. Kockrick, M. Rose, L. Borchardt, N. Klein, I. Senkovska,
T. Freudenberg, Y. Korenblit, G. Yushin, and S. Kaskel, Carbon, 48, 3987 (2010).
4. I. Tallo, T. Thomberg, A. J a¨ nes, and E. Lust, J. Electrochem. Soc., 159, A208
1
1
1
1
(
2012).
5. M.-M. Titirici, Ed., Sustainable carbon materials from hydrothermal processes, p.
57, Wiley, Chichester (2013).
6. Q.-Y. Li, H.-Q. Wang, Q.-F. Dai, J.-H. Yang, and Y.-L. Zhong, Solid State Ion, 179,
69 (2008).
7. R. Demir-Cakan, N. Baccile, M. Antonietti, and M.-M. Titirici, Chem. Mater., 21,
84 (2009).
3
2
48. L. Wei and G. Yushin, Carbon, 49, 4830 (2011).
4
49. A. Lewandowski and M. Gali n´ ski, J. Phys. Chem. Solids, 65, 281 (2004).
50. G. Sun, K. Li, L. Xie, J. Wang, and Y. Li, Micropor. Mesopor. Mater., 151, 282
(2012).
51. M. Galinski, K. Babeł, and K. Jurewicz, J. Power Sources, 228, 83 (2013).
52. A. Lahe a¨ a¨ r, P. Przygocki, Q. Abbas, and F. B e´ guin, Electrochem. Commun., 60, 21
(2015).
53. E. Barsoukov and J. R. Macdonald, Impedance spectroscopy: theory, experiment, and
applications, Wiley, John & Sons, Inc., (2005).
54. P. L. Taberna, P. Simon, and J. F. Fauvarque, J. Electrochem. Soc., 150, A292 (2003).
55. T. Romann, O. Oll, P. Pikma, H. Tamme, and E. Lust, Electrochim. Acta, 125, 183
(2014).
56. L. Wei and G. Yushin, J. Power Sources, 196, 4072 (2011).
57. D. Zhou, H. Wang, N. Mao, Y. Chen, Y. Zhou, T. Yin, H. Xie, W. Liu, S. Chen, and
X. Wang, Micropor. Mesopor. Mater., 241, 202 (2017).
58. H. Kurig, A. J a¨ nes, and E. Lust, J. Electrochem. Soc., 157, A272 (2010).
59. K. T o˜ nurist, T. Thomberg, A. J a¨ nes, and E. Lust, J. Electrochem. Soc., 160, A449
(2013).
1
1
2
8. W. Zhang, H. Tao, B. Zhang, J. Ren, G. Lu, and Y. Wang, Carbon, 49, 1811 (2011).
9. H. Zhu, X. Wang, F. Yang, and X. Yang, Adv. Mater., 23, 2745 (2011).
0. L. Wei, M. Sevilla, A. B. Fuertes, R. Mokaya, and G. Yushin, Adv. Energy Mater., 1,
356 (2011).
21. K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, 1 edition.,
p. 560, Wiley, New York (1988).
2
2
2. H. Marsh and F. R. Reinoso, Activated Carbon, p. 555, Elsevier, London (2006).
3. M. Arulepp, L. Permann, J. Leis, A. Perkson, K. Rumma, A. J a¨ nes, and E. Lust, J.
Power Sources, 133, 320 (2004).
2
2
4. A. J a¨ nes and E. Lust, J. Electroanal. Chem., 588, 285 (2006).
5. E. Tee, I. Tallo, H. Kurig, T. Thomberg, A. J a¨ nes, and E. Lust, Electrochim. Acta,
1
61, 364 (2015).
6. I. Tallo, T. Thomberg, H. Kurig, K. Kontturi, A. J a¨ nes, and E. Lust, Carbon, 67, 607
2014).
2
2
2
(
7. T. Thomberg, T. Tooming, T. Romann, R. Palm, A. J a¨ nes, and E. Lust, J. Electrochem.
Soc., 160, A1834 (2013).
8. T. Tooming, T. Thomberg, H. Kurig, A. J a¨ nes, and E. Lust, J. Power Sources, 280,
60. K. T o˜ nurist, T. Thomberg, A. J a¨ nes, I. Kink, and E. Lust, Electrochem. Commun.,
22, 77 (2012).
667 (2015).
Downloaded on 2017-08-01 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).