Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes

Abstract

Activated carbons were prepared by KOH-activation of argan seed shells (ASS). The activated carbon with the largest surface area and most developed porosity was superficially treated to introduce oxygen and nitrogen functionalities. Activated carbons with a surface area of around 2100 m2/g were obtained. Electrochemical measurements were carried out with a three-electrode cell using 1 M H2SO4 as electrolyte and Ag/AgCl as reference electrode. The O-rich activated carbon showed the lowest capacitance (259 F/g at 125 mA/g) and the lowest capacity retention (52% at 1 A/g), due to surface carboxyl groups hinder electrolyte diffusion into the pores. Conversely, the N-rich activated carbon showed the highest capacitance (355 F/g at 125 mA/g) with the highest retention (93% at 1 A/g), due to its well-developed micro-mesoporosity and the pseudocapacitance effects of N functionalities. This capacitance performance was among the highest reported for other activated carbons from a large variety of biomass precursors

Full text

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Activated carbons from KOH-activation of argan (Argania spinosa) seed shells
as supercapacitor electrodes
Abdelhakim Elmouwahidi1, Zulamita Zapata-Benabithe2, Francisco Carrasco-Marín,
Carlos Moreno-Castilla⇑
Departamento de Química Inorgánica, Universidad de Granada, 18071 Granada, Spain
a r t i c l ei n f o
Article history:
Received 20 December 2011
Received in revised form 1 February 2012
Accepted 2 February 2012
Available online 14 February 2012
Keywords:
Energy storage
Supercapacitors
Activated carbons
Argan seed shells
KOH-activation
a b s t r a c t
Activated carbons were prepared by KOH-activation of argan seed shells (ASS). The activated carbon with
the largest surface area and most developed porosity was superficially treated to introduce oxygen and
nitrogen functionalities. Activated carbons with a surface area of around 2100 m2/g were obtained. Elec-
trochemical measurements were carried out with a three-electrode cell using 1 M H2SO4as electrolyte
and Ag/AgCl as reference electrode. The O-rich activated carbon showed the lowest capacitance (259 F/
g at 125 mA/g) and the lowest capacity retention (52% at 1 A/g), due to surface carboxyl groups hindering
electrolyte diffusion into the pores. Conversely, the N-rich activated carbon showed the highest capaci-
tance (355 F/g at 125 mA/g) with the highest retention (93% at 1 A/g), due to its well-developed micro-
mesoporosity and the pseudocapacitance effects of N functionalities. This capacitance performance
was among the highest reported for other activated carbons from a large variety of biomass precursors.
? 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Electrochemical double-layer capacitors (EDLCs) or supercapac-
itors are energy storage devices that store the electrical energy in
an electrochemical double-layer formed at the interface between
the charged surface of the electrode and the electrolyte solution
(Qu and Shi, 1998; Conway, 1999). Porous carbon materials are
widely used as electrodes owing to their high surface area, well-
developed pore-size distribution, which can be adapted to the size
of the electrolyte ions, high conductivity, and good physicochemi-
cal stability. In addition, carbon surfaces can be decorated with
electrochemically active surface functionalities that modify the
double-layer capacitance (Pandolfo and Hollenkamp, 2006; Frac-
kowiak, 2007; Zhang and Zhao, 2009).
Activated carbons are porous carbon materials of great interest
for utilization as supercapacitor electrodes because they can be
readily prepared from cheap biomass residues and wastes. Thus,
activated carbons have been prepared from rice husk (Guo et al.,
2003), firwood and pistachio shells (Wu et al., 2004, 2005), bamboo
(Kim et al., 2006), banana fibers (Subramanian et al., 2007), waste
coffee beans (Rufford et al., 2008), corn grains (Balathanigaimani
et al., 2008), cassava peel waste (Ismanto et al., 2010), sugar cane
bagasse (Rufford et al., 2010), and sunflower seed shells (Li et al.,
2011).
The argan tree (Argania spinosa) is an endemic species in the
Southwestern region of Morocco, and its seeds are used to produce
oil of value for culinary and cosmetic uses. Argan oil processing
produces large quantities of biomass residues (ASS) that are mainly
used for heating purposes.
The objective of this study was to investigate the applicability of
activated carbons derived from ASS as supercapacitor electrodes.
Activated carbons were prepared by KOH-activation following a
method previously used to obtain activated carbons from olive-
mill waste water (Moreno-Castilla et al., 2001) and olive stones
(Ubago-Pérez et al., 2006). The activated carbon with the best sur-
face characteristics was further superficially treated to introduce
oxygen and nitrogen functionalities. The prepared activated car-
bons were characterized to determine their pore texture, surface
chemistry, and electrochemical characteristics. Results were com-
pared with published findings for activated carbons prepared from
other biomass residues and wastes.
2. Experimental
2.1. Preparation of activated carbon
Three activated carbons were prepared from ASS. Ground ASS
with a particle size between 2 and 3 mm was carbonized under
N2flow (300 cm3/min) at 500 ?C for 3 h and designated sample C.
Portions of ASS and sample C were chemically activated with
KOH. For this purpose, both samples were mixed with a
0960-8524/$ - see front matter ? 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2012.02.010
⇑Corresponding author. Tel.: +34 958 243323; fax: +34 958 248563.
E-mail address: cmoreno@ugr.es (C. Moreno-Castilla).
1Address: Departament de Chimie, Université Caddi-Ayyad, Marrakech, Morocco.
2Address: Facultad de Ingeniería Química, Universidad Pontificia Bolivariana,
050031 Medellín, Colombia.
Bioresource Technology 111 (2012) 185–190
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concentrated KOH solution to obtain two slurries with a KOH/sam-
ple weight ratio of 4. The slurries were heated at 60 ?C for 12 h and
then at 110 ?C to dryness. Next, samples were pyrolized under N2
flow (300 cm3/min) at 300 ?C for 2 h and then at 800 ?C for 3 h at
a heating rate of 10 ?C/min. The activated carbons prepared from
ASS and C were designated samples AK and CK, respectively. A
third activated carbon, AP, was prepared by physically mixing
ASS and solid KOH at a KOH/ASS weight ratio of 4. Prepared acti-
vated carbons were washed with 0.1 M HCl and then with distilled
water until chloride ions were no longer detected in the washing
water by using a silver nitrate solution.
Portions of activated carbon AK were further superficially mod-
ified by treatment with ammonium peroxydisulfate (sample AKO)
and melamine (sample AKN) to introduce surface oxygen and
nitrogen functionalities, respectively. Oxidation with ammonium
peroxydisulfate was performed as reported by Moreno-Castilla
et al. (1995). Sample AKN was prepared by mixing 1 g of AK with
70 mg of melamine dissolved in 40 mL ethanol. After stirring this
slurry, the solvent was slowly evaporated and the remaining resi-
due was heat-treated at 750 ?C for 1 h under N2flow (60 cm3/min).
This procedure was carried out according to Seredych et al. (2008)
with minor modifications. The preparation conditions and designa-
tions of these activated carbons are shown in Table 1.
2.2. Characterization of activated carbons
Activated carbons were characterized by N2and CO2adsorption
at ?196 and 0 ?C using Autosorb 1 from Quantachrome after out-
gassing the samples overnight at 110 ?C under high vacuum
(10?6mbar). The BET equation was applied to N2adsorption iso-
therms to obtain the apparent BET surface area, SBET, considering
themolecularareaofN2
at
(Rodríguez-Reinoso and Linares-Solano, 1989). The Dubinin–Rad-
ushkevich (DR) equation was applied to the N2and CO2isotherms
to obtain the micropore volume accessible to these adsorptives,
W0, and the mean micropore width, L0. The total pore volume
was obtained from the amount of N2 adsorbed at p/p0= 0.95,
V0.95, and the mesopore volume, Vmeso, from the difference between
V0.95and W0(N2). The pore size distribution (PSD) was determined
by applying Quenched Solid Density Functional Theory (QSDFT) to
the N2adsorption isotherms, assuming slit-shaped pores.
Immersion enthalpies into benzene, DHbenz, and water, DHwater,
were determined with a C80-D Setaram calorimeter after outgas-
sing the samples overnight under a dynamic vacuum of 10?6mbar
at 120 ?C. Measurements were carried out at 30 ?C and at least
twice for each sample. DHbenzwas used to determine the surface
area of the activated carbons, Sbenz, considering the immersion en-
thalpy into benzene of a non-porous graphitized carbon black to be
0.114 J/m2(Denoyel et al., 1993). The hydrophobicity of samples
was determined from Eq. (1)
?196 ?Ctobe0.162 nm2
Hydrophobicity ¼ 1 ? ðDHwater=DHbenzÞ
Temperature programmed desorption (TPD) was performed by
heating samples to 1000 ?C at 10 ?C/min and analyzing the CO
and CO2evolved by means of a model Prisma mass spectrometer
ð1Þ
from Pfeiffer (Germany). The total oxygen content, OTPD, was calcu-
lated from the amount of CO and CO2evolved. The total N content,
NCHN, was determined by elemental analysis.
X-ray photoelectron spectroscopy (XPS) was performed using
an Escalab 200R system (VG Scientific Co.) equipped with MgKa
X-ray source (hc = 1253.6 eV) and hemispherical electron analyzer.
Survey and multi-region spectra were recorded at C1s, O1s, and N1s
photoelectron peaks. Each spectral region of photoelectron interest
was scanned several times to obtain good signal-to-noise ratios.
The C1speak at 284.6 eV was used as internal standard.
2.3. Electrochemical measurements
Electrochemical measurements were carried out in a Biologic
multichannel potentiostat at room temperature using 1 M H2SO4
as electrolyte, a typical three-electrode cell with Ag/AgCl as refer-
ence electrode, and Pt wire as counter electrode. The working elec-
trode was graphite paper on which a homogeneous mixture of
finely ground activated carbon, acetylene black, and binder (poly-
tetrafluoroethylene, PTFE) at a mass ratio of 80:10:10 was pasted.
Cyclic voltammograms were obtained at different scan rates,
and the gravimetric capacitance, CCV(F/g), was calculated from
these curves by Eq. (2)
CCV¼RjIjDt
2mDV
ð2Þ
where R|I|Dt is the area of the current (A) against time (s) curve, m
the mass of active material in the electrode (g), and DV the potential
window (V). Chronopotentiograms were performed at a current
loading between 125 and 1000 mA/g in a potential interval of 0–
0.75 V. The gravimetric capacitance from these measurements, CCP
(F/g), was obtained by Eq. (3).
CCP¼IdDt
mDV
ð3Þ
where Idis the discharge current (A) in the potential range between
0.3 and 0.1 V, Dt the discharge time (s), and DV the potential inter-
val (V).
3. Results and discussion
Fig. 1 depicts N2adsorption isotherms of the activated carbons
AP, CK, and AK and of the superficially modified activated carbons
AKO and AKN. They are typical type I microporous carbons accord-
ing to IUPAC classification, but the shape of the isotherms in sam-
ple AK and its derivatives AKO and AKN showed a wide knee
indicative of a micro-mesopore structure. Fig. 2 depicts the pore
size distribution obtained by applying the QSDFT method to these
isotherms, and Table 2 exhibits the surface area and porosity
values.
All samples showed W0(N2) > W0(CO2), indicating an absence of
constrictions at micropore entrances and hence complete accessi-
bility to N2 molecules at ?196 ?C (Rodríguez-Reinoso and Lin-
ares-Solano, 1989). SBET and Sbenz surface area values were
similar, because the minimal dimensions of N2 (0.36 nm) and
Table 1
Preparation conditions of activated carbons from argan seed shell (ASS).
SampleRaw material KOH/raw material weight ratio = 4 Pyrolysis temperature (?C)–time (h)Surface treatment
C
CK
AK
AP
AKO
AKN
ASS
C
ASS
ASS
AK
AK
–
Aqueous slurry
Aqueous slurry
Physical mixture
–
–
500–2
300–2; 800-3
300–2; 800-3
300–2; 800-3
–
750–5
–
–
–
–
(NH4)2S2O8
Melamine
186
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benzene (0.37 nm) are almost identical (Villar-Rodil et al., 2002),
and the micropore width allowed the accommodation of one N2
monolayer on each micropore wall. The three activated carbons
(AP, CK, AK) showed a bimodal PSD (Fig. 2). This PSD was essen-
tially within the micropore range in the case of CK, which had
the lowest mesopore volume. However, The PSD of sample AK
was in the micro-mesopore range up to a width of a few nanome-
ters in the lower range of mesopores.
Among these activated carbons, the largest surface area and mi-
cro- and mesopore volumes were found in sample AK, prepared by
activation of ASS previously impregnated with an aqueous KOH
solution. This preparation yielded a better distribution of the acti-
vating agent within the precursor particles in comparison to the
physical mixture of solid KOH and precursor (activated carbon
AP). Use of the non-carbonized precursor to obtain AK rather than
the carbonized precursor to obtain CK created a more intensive
KOH interaction during activation, producing a larger surface area
and higher pore volumes. Thus, W0(N2) and Vmesoincreased from
0.58 and 0.10 cm3/g in CK to 0.96 and 1.22 cm3/g, respectively, in
AK. Similar observations were reported for the KOH-activation of
non-carbonized and carbonized olive stones (Ubago-Pérez et al.,
2006) and coals (Lillo-Ródenas et al., 2003).
The oxidation of AK to obtain AKO, which fixed a large amount
of oxygen on the surface (Table 3), also reduced the SBETand micro-
and mesopore volumes by partially destroying micro and meso-
pore walls (Moreno-Castilla et al., 1995, 1997). Melamine treat-
ment of AK to obtain AKN was less drastic and only slightly
reduced Vmeso, as previously reported for melamine-treated acti-
vated carbons (Seredych et al., 2008).
The microporosity of the samples with L0(N2) between 0.8 and
1.4 nm, would be fully accessible to the hydronium ions, 0.36–
0.42 nm (Eliad et al., 2001), and hydrated bisulfate ions, 0.53 nm
(Endo et al., 2001; Moreno-Castilla et al., in press), produced by
dissociation of the electrolyte (1 M H2SO4).
The surface chemistry of the activated carbons is summarized in
Tables 3 and 4. The three activated carbons AP, CP and AK showed
similar ash, oxygen, and nitrogen contents, CO/CO2 ratio, and
hydrophobicity. The oxygen content of sample AKO was very high
(20%) and was homogeneously distributed between the internal
and the external surface of the sample, as shown by the similarity
of the OTPDand OXPSvalues. The CO/CO2ratio was lower in AKO
than in AK because the oxidation mainly increased the amount of
CO2-evolving groups such as carboxyl acid groups, which are
known to increase during peroxydisulfate treatment (Moreno-
Castilla et al., 1995, 1997). The hydrophobicity (Table 3) markedly
decreased as a consequence of the fixation of oxygen functional-
ities with large polarity, e.g., carboxyl groups. As shown in Table 4,
both AK and AKO samples had a higher concentration of surface
0.0
0.5
1.0
1.5
2.0
2.5
00.5 1
Vliq(cm
3/g)
Fig. 1. N2adsorption isotherms at ?196 ?C on activated carbons:
AK;
, AKO;, AKN.
, AP; , CK; 4,
02468
d (nm)
dV/d(d) (ArbitraryUnits)
Fig. 2. Pore size distribution of activated carbons by application of QSDFT method
to N2adsorption isotherms at ?196 ?C., AP;, CK; 4, AK;, AKO; , AKN.
Table 2
Surface area and porosity of the activated carbons.
CAPCKAKAKO AKN
Surface area
SBET(m2/g)
?DHbenz(J/g)
Sbenz(m2/g)
SBET/Sbenz
21
nd
nd
nd
1279
143.7
1261
1.01
1430
154.5
1355
1.06
2132
234.3
2055
1.04
1654
186.2
1633
1.01
2062
233.9
2052
1.00
Porosity
W0(N2) (cm3/g)
W0(CO2) (cm3/g)
L0(N2) (nm)
L0(CO2) (nm)
V0.95(cm3/g)
Vmeso(cm3/g)
–
–
–
–
–
–
0.47
0.39
1.2
0.8
0.78
0.31
0.58
0.54
0.8
0.8
0.76
0.10
0.96
0.44
1.4
0.8
2.18
1.22
0.62
0.40
1.3
0.8
1.20
0.58
0.95
0.37
1.4
0.9
1.99
1.04
nd: Not determined.
Table 3
Ash content, amounts of CO and CO2evolved up to 1000 ?C, total and surface O and N
concentrations, immersion enthalpies into water and benzene, and hydrophobicity of
the activated carbons.
APCKAKAKOAKN
Ash (%)
CO (lmol/g)
CO2(lmol/g)
CO/CO2
OTPD(wt.%)
OXPS(wt.%)
NCHN(wt.%)
NXPS(wt.%)
?DHwater(J/g)
Hydrophobicity
4.6
2.92
0.53
5.51
6.4
nd
0.8
nd
84.1
0.41
4.1
2.31
0.40
5.78
5.0
nd
0.9
nd
81.4
0.43
4.4
2.97
0.54
5.50
6.5
7.6
0.8
0.9
3.1
6.48
3.01
2.15
20.0
20.3
0.5
0.6
224.4
-0.21
4.2
1.52
0.11
13.8
2.8
4.1
1.6
2.0
108.8
0.53
110.1
0.53
nd: Not determined.
Table 4
Relative surface concentrations (%) of O and N functionalities from the deconvolution
of O1sand N1sXP spectra.
Binding energy (eV)FunctionalityAKAKOAKN
O1s
531.4–531.8
533.0–533.3
534.7
C@O bonds
C–OH bonds
H2O
33
56
11
38
62
–
44
56
–
N1s
398
400
401
N-6
N-5
N-Q
20
41
39
32
48
20
43
39
18
A. Elmouwahidi et al./Bioresource Technology 111 (2012) 185–190
187

Page 4

functionalities with single C–O bonds (e.g., carboxyl and phenol
groups) than with double C@O bonds (e.g., carboxyl and quinone
groups).
Melamine treatment of AK to obtain AKN increased the N con-
tent, which was distributed across pyridinic (N-6), pyrrolic, and/or
pyridonic (N-5) and quaternary-N (N-Q) groups (Pels et al., 1995).
The treatment greatly increased the amount of N-6 functionalities
and reduced the amount of N-Q functionalities. Both OTPDand OXPS
were decreased, mainly due to the loss of CO2-evolving groups. The
hydrophobicity of the activated carbon was not changed by the
melamine treatment.
Fig. 3 depicts typical cyclic voltammograms at 0.5 mV/s. The
three activated carbons AP, CK and AK showed a symmetric and
quasi-rectangular shape profile typical of ideal EDLCs, with very
small humps attributed to pseudofaradaic redox reactions related
to the surface functionalities of the materials (Kinoshita, 1988).
CCVvalues at 0.5 mV/s are compiled in Table 5; AK had the highest
value among the activated carbons because it had the largest
microporosity and surface area.
The cyclic voltammogram of AKO (Fig. 3) shows that the capac-
itance increased with increases in the potential across the entire
range of the potential window, indicating a slow charging process
due to pore resistance. The presence of surface quinone groups in-
creases the capacitance of oxidized activated carbons by introduc-
ing pseudocapacitance effects; however, oxidation also fixes other
surface oxygen complexes such as carboxyl groups which, due to
their high polarity, bind water molecules that hinder and retard
electrolyte diffusion into the microporosity, thereby increasing
its ohmic resistance (Guo et al., 2003).
Hence, the increase in capacitance due to the presence of qui-
none groups can be offset by an increased inner resistance due to
the presence of carboxyl groups. Table 5 shows a large decrease
of CCVfrom 321 F/g in sample AK to 228 F/g in sample AKO. This
can be attributed to the large amount of CO2-evolving groups
(e.g., carboxyl acids) present in the AKO sample, producing a major
reduction in its hydrophobicity.
Sample AKN also showed a quasi-rectangular cyclic voltammo-
gram. The CCVvalue obtained at 0.5 mV/s (Table 5) was the highest
-800
-400
0
400
800
-0.4-0.200.20.40.6
C (F/g)
E (V) vs. Ag/AgCl
Fig. 3. Cyclic voltammograms of activated carbons using a three-electrode cell in
1 M H2SO4at 0.5 mV/s., AP;, CK; N, AK;, AKO;, AKN.
Table 5
Gravimetric capacitances: CCVat 0.5 mV/s and CCPat 125 mA/g and 1 A/g, and CCP
retention.
Sample
CCV(F/g)
CCP(F/g)
(at 125 mA/g)
CCP(F/g)
(at 1 A/g)
CCPretention (%)
AP
CK
AK
AKO
AKN
234
277
321
228
358
272
269
325
259
355
175
242
291
135
329
64
90
90
52
93
0
100
200
300
400
036912
s (mV/s)
CCV(F/g)
Fig. 4. Variation in CCVgravimetric capacitance with scan rate.
, AKO; , AKN.
, AP;, CK; N, AK;
0.00
0.25
0.50
0.75
0200040006000
t (s)
E (V) vs. Ag/AgCl
Fig. 5. Chronopotentiograms of activated carbons using a three-electrode cell in
1 M H2SO4at 125 mA/g., AP;, CK; N, AK;, AKO;, AKN.
0
100
200
300
400
02505007501000
I (mA/g)
CCP(F/g)
Fig. 6. Influence of current loading on CCPgravimetric capacitance of activated
carbons.
, AP;, CK; N, AK;, AKO; , AKN.
188
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obtained in any sample. It has been reported that surface N func-
tionalities (e.g., pyridinic, pyrrolic and pyridonic nitrogen) are elec-
trochemically active because they are electron-rich (Frackowiak,
2007; Rufford et al., 2008). Protons can therefore be attracted to
the electrode surface, giving rise to pseudocapacitive interactions
(Hulicova et al., 2005). The surface concentration of pyridinic nitro-
gen was higher in sample AKN than in sample AK, which may ex-
plain the higher capacitance of the former.
The gravimetric capacitance of the samples is plotted against
the scan rate in Fig. 4. CCVdecreased with increased scan rate be-
cause the formation of the electrochemical double-layer within
the micropores is slower and less complete in comparison to the
rate of variation in potential. Sample AKO showed the highest loss
of capacitance with increased scan rate, attributable to the pres-
ence of highly polar surface oxygen groups that would hinder dif-
fusion of the electrolyte into the micropores.
Typical chronopotentiograms are depicted in Fig. 5 at a current
load of 125 mA/g. CCPvalues at 125 mA/g and 1 A/g are compiled in
Table 5. CCPvalues at the lowest current load were close to CCVval-
ues at 0.5 mV/s. The variation in CCPwith increasing current load is
depicted in Fig. 6. Sample AKN showed the highest CCPvalue, 355 F/
g, at the current load of 125 mA/g, which is excellent for an acti-
vated carbon (Pandolfo and Hollenkamp, 2006; Rufford et al.,
2010). It also had the highest capacitance retention (93%) at the
higher current load of 1 A/g.
AKN and AK had similar surface areas and micropore textures,
and the higher gravimetric capacitance of the former is related to
the larger amount of pyridinic-N and therefore pseudocapacitance
effects. Both samples had the same capacitance retention at high
current load (Table 5) because of the similarity of their mesopore
textures. Small mesopores influence the capacitance retention at
high current loads, and are known to facilitate transport of ions
through the carbon porosity at fast charge–discharge rates (Rufford
et al., 2010; Zhao et al., 2010).
Sample AKO showed the lowest CCPat 125 mA/g and the lowest
capacity retention (52%) at 1 A/g, because the presence of surface
carboxyl groups hampers electrolyte diffusion into the highly polar
pores.
The performances as supercapacitor electrodes of the activated
carbons prepared from ASS by KOH-activation, especially those of
samplesAKandAKN,wasamongthebestreportedforactivatedcar-
bons prepared from a wide variety of biomass precursors (Table 6).
ASS can therefore be successfully used to prepare porous activated
carbonsbyKOH-activationforEDLCapplications.Thisisattributable
to the development of a microporous network that is accessible to
the electrolyte ions and is interconnected with small mesopores.
Their capacitance performance is further enhancedby the ease with
which N-functionalities can be introduced.
4. Conclusions
The electrochemical performance of activated carbons prepared
by KOH-activation of ASS was compared with published reports on
other activated carbons, demonstrating that ASS is an excellent
biomass precursor for the preparation of porous activated carbons
for supercapacitor applications. The highest capacitance obtained
was 355 F/g at 125 mA/g with a 93% retention capacitance at 1 A/
g, attributable to the large surface area, appropriate and well-
developed micro-mesopore texture, and N content of the activated
carbon.
Acknowledgements
The authors are grateful to AECID, Ministerio de Asuntos
Exteriores y Cooperación, Spain for financial support through
Project A/024015/09. ZZB acknowledges a pre-doctoral fellowship
from COLCIENCIAS, Colombia.
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Table 6
Maximum capacitances of activated carbons from biomass precursors. Capacitances are the three-electrode equivalents for a single electrode.
Biomass precursorActivation
method
SBET(m2/g)Maximum capacitance
(F/g)
Capacitance measurement
at
ElectrolyteReference
Rice husk
Firwood
Pistachio shell
Firwood
Bamboo
Banana fibers
Corn grains
Waste coffee beans
Sugar cane bagasse
Cassava peel waste
Sunflower seed shell
Argan seed shell
NaOH
H2O
KOH
KOH
KOH
ZnCl2
KOH
ZnCl2
ZnCl2
KOH
KOH
KOH/melamine
1886
1131
1096
1064
1251
1097
3199
1019
1788
1352
2509
2062
210
140
120
180
260a
74
257
368
300
153
311
355
0.2 mA/g
25 mV/s
10 mV/s
10 mV/s
1 mA/cm2
500 mA/g
1 mA/cm2
50 mA/g
250 mA/g
–
250 mA/g
125 mA/g
3 M KCl
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
30 wt.% H2SO4
1 M Na2SO4
6 M KOH
1 M H2SO4
1 M H2SO4
0.5 M H2SO4
30 wt.% KOH
1 M H2SO4
Guo et al. (2003)
Wu et al. (2004)
Wu et al. (2005)
Wu et al. (2005)
Kim et al. (2006)
Subramanian et al. (2007)
Balathanigaimani et al. (2008)
Rufford et al. (2008)
Rufford et al. (2010)
Ismanto et al. (2010)
Li et al. (2011)
This work
aThe two-electrode gravimetric capacitance value published was converted to the three-electrode equivalent for a single electrode.
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