Multistage Activation of Anthracite Coal-Based Activated Carbon for High-Performance Supercapacitor Applications

Multistage Activation of Anthracite Coal-Based Activated Carbon for
High-Performance Supercapacitor Applications
Guanrong Song, Carlos Romero, Tom Lowe, Greg Driscoll, Boyd Kreglow, Harold Schobert,
Jonas Baltrusaitis,* and Zheng Yao*
Cite This: https://doi.org/10.1021/acs.energyfuels.2c03487 Read Online
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ABSTRACT: An anthracitic coal-derived activated porous carbon is proposed as a promising carbon electrode material for
supercapacitor (SC) applications. The specific capacitance of this activated carbon SC electrode is related to the characteristics, such
as specific surface area, pore size distribution, wettability, and conductivity. In the present work, a series of anthracite-based activated
carbons (ABAC) were prepared via a multistage activation process and used as electrode materials for SCs. The multistage activation
experiment was developed by exploring different activation temperatures, precursor/activating agent mass ratios, and process treating
environments. The electrochemical performance of ABACs was evaluated in a three-electrode testing system. Multiple electrolytes
were utilized, such as 1 M sulfuric acid (H2SO4) and 1 and 6 M potassium hydroxide (KOH) solutions. An optimum ABAC
electrode was obtained, characterized by its largest wettability and superior conductivity, and achieved excellent electrochemical
performance. The three-electrode system exhibited a specific capacitance of 288.52 and 260.30 F/g at 0.5 A/g in the 1 M H2SO4 and
6 M KOH electrolytes, respectively. It was found that moderate multistage activation temperatures are beneficial for the electrolyte
uptake which enhances the specific capacitance. The high content of the oxygen functional groups on the activated carbon surface
greatly improved its specific capacitance due to the increase in wettability. In the 1 M H2SO4 electrolyte, the working electrode
exhibited better performance than in 1 M KOH because the ion diameter in the acidic electrolyte was more suitable for pore
diffusion. The concentrated KOH electrolyte leads to an increase in specific capacitance due to increased ions being adsorbed by a
certain number of the hydrophilic pores. Moreover, the specific capacitance of the optimum ABAC sample remained at 95.4% of the
initial value after 1000 galvanostatic charge−discharge tests at 0.5 A/g, which is superior to the performance of SC grade commercial
carbon.
1. INTRODUCTION
Supercapacitors (SCs) are considered promising energy
storage devices due to their ability to store and supply energy
quickly and efficiently, as well as their long life cycles.1
SCs are
widely used in applications such as automobiles, memory
backup systems, and wind turbines that require rapid power
supply.2,3
However, although the energy density of the SCs is
much higher than conventional capacitors, it is still not large
enough to be used in certain applications. Characteristics, such
as quick recharge or prolonged life cycle, need to be enhanced
for the further development of SCs.4
The improvement in
capacitance and maximum operating cell potential of SCs, with
high energy density, has become the focus of recent studies. It
was demonstrated that enhancement of the capacitance and
voltage can both occur when focusing on the development of
the connection among different parameters, such as chemical
Received: October 14, 2022
Revised: December 9, 2022
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composition, pore structure, and interaction with the electro-
lyte on the electrode. Great efforts to improve the capacitance
mainly rely on the modification of electrode surface
functionalization, electrode composite, electrolyte, and device
configuration.5−7
Electrode material plays an important role in the enhance-
ment of electrochemical performance in SCs. The most
common porous carbon material for manufacturing commer-
cial electrodes is activated carbon (AC), due to its merits of
high specific surface area (SSA), well-developed pore structure,
easy synthesis, moderate cost of sources, and acceptable
electrical conductivity in both aqueous and organic electro-
lytes.8,9
A large SSA is one of the most important parameters
leading to a large gravimetric capacitance and hence electric
energy storage capacity. However, it has been demonstrated
that the specific capacitance remains lower than expected even
given a very high SSA, which can be explained by the pore size
distribution (PSD) of the electrode.10
Micropores can provide
surface area for charge storage and mesopores provide the ion-
transport pathway and facilitate ion diffusion whereas macro-
pores serve the purpose of acting as an electrolyte ion-buffering
reservoir for shortening diffusion time.11
Furthermore, it has
been reported that the electrochemical performance of AC-
based electrodes can be greatly enhanced by the size and
volumes of micropores and mesopores, which was attributed to
the pore structure matching the radius of bare electrolyte
ions.12,13
The wettability of the carbon material is also
important for good performance of SCs, as the wettability of
micropores improves stronger adsorption ability of the
electrolyte, thereby enhancing the electrochemical perform-
ance of the ACs material.14
Based on this, it is necessary to select a suitable carbon
precursor to prepare porous carbon materials with excellent
capacitance properties. An accessible and high SSA and
hierarchical pore structure are requirements for the utilization
of new material in electrochemical devices. Anthracite, as the
highest rank coal, is of interest for the type of affination, due to
its high carbon content and already existing micropores. The
majority of carbon atoms in anthracite are sp2
hybridized
whereas its high crystallite structural order can contribute to
lower charge-transfer resistance at the electrode−electrolyte
interface.15,16
Anthracitic coal is abundant and has good
mechanical properties, high carbon content, and low heteroele-
ment (mainly S, O, and N), which are conducive to obtaining
porous carbon materials with high yield and purity17
content.
Anthracite is a macromolecule composed of countless similar
units including the mainly ordered structures of various
aromatic rings and the few disordered structures of alkyl side
chains/functional groups. The abundant ordered structures can
greatly improve conductivity.18
It has been reported that the
higher rank coal is more easily wetted than lower rank coals.19
Additionally, the micropore structure of the anthracitic coal
can be developed during the carbonization process.20
Optimization of PSD can be pursued for the formation of
micropores and mesopores of anthracite, leading to enhancing
the accessibility of the porosity. Generally, coal-based ACs
have been synthesized by chemical or physical activation
methods. Physical activation has been done by carbonization of
the carbon precursor in an inert atmosphere to remove non-
carbon elements, followed by activation in the presence of a
suitable oxidized gasifying agent, such as carbon dioxide (CO2)
and steam, which contributes to the formation of micropores.21
Chemical activation is generally made by mixing carbonaceous
materials with a chemical activating agent [usually potassium
hydroxide (KOH)], which gives very high SSA, considerable
micropores, and small mesopores.8
A multistage activation
method may potentially combine the merit of both physical
and chemical activations methods, resulting in a well-
developed pore structure with a high SSA that is easily
accessible to the electrolyte. Therefore, multistage activation
carbons can be specifically tailored to determine the optimal
relationship of pore size and resultant microporous electrode
structure because accessible micropores to the electrolyte can
be beneficial in electrochemical performance. Besides, aqueous
electrolytes also contribute to the electrochemical perform-
ance. The difference in the specific capacitance is related to the
hydrated ionic radius, ionic mobility, and molar ionic
conductivity of the electrolyte solutions.
A multistage activation method for anthracite activation and
enhancement of electrochemical performance of anthracite-
based porous carbon (ABAC) electrodes was investigated in
the present study. The preparation methods for anthracitic coal
include a combination of physical and chemical activation. The
carbonized anthracite was physically activated in CO2 and
steam under two different temperatures (800 and 900 °C) and
selected AC after physical activation was mixed with KOH
powder and chemically activated under different conditions
(i.e., temperature, chemical agent/carbon ratio), followed by a
sampling washing process for discarding any remaining ions.
Electrochemical measurements with ABAC electrodes were
performed in a three-electrode system to test the specific
capacitance and resistance in different aqueous electrolytes
[sulfuric acid (H2SO4) and KOH]. The three-electrode system
was used to measure the performance of electrodes in different
analytes at the working electrode (ABAC electrode). Overall,
this study investigated the effect of parameters in the anthracite
multistage activation process, including the physical and
chemical activation temperature and the chemical agent/AC
mass ratio, as well as the effect of different aqueous electrolytes
on the electrochemical performance of the ABACs.
2. EXPERIMENTAL MATERIALS AND METHODS
2.1. Preparation and Activation of Anthracite Samples. The
anthracitic coal, provided by Blaschak Anthracite Corporation from
their no. 5 Lattimer mining site, was used in this study. The
physicochemical analysis of the anthracite sample is summarized in
Table 1. It can be seen that this anthracitic coal has a high carbon
content of 84.79% and ash content is 9.66%, on a dry basis.
Anthracitic coal is classified as a high carbon and low impurity coal
rank, which is prosperous for the AC preparation and further
application of SCs.
The anthracitic coal was initially crushed to 0.3 mm size and then
U.S. standard sieve no. 50 was used to sieve the anthracitic coal to
obtain <0.3 mm size samples. Collected, crushed, and sieved
Table 1. Analysis Data of the Raw Anthracite Sample
component analysis results
composition as determined (%) dry basis (%)
carbon 83.43 84.79
hydrogen 1.9 1.75
nitrogen 0.86 0.87
sulfur 0.67 0.68
ash 9.51 9.66
oxygen (calculation) 3.63 2.24
moisture 1.6 N/A
Energy & Fuels pubs.acs.org/EF Article
B

anthracitic coal was dried overnight in a stainless-steel electric oven at
110 °C. After cooling, the dehydrated anthracite was modified by
carbonization, physical activation, and chemical activation process in a
vertical tube furnace (Sentro Tech). The prepared sample was first
carbonized under 750 °C in a nitrogen (N2) atmosphere for 2 h.
During the process of pyrolysis, the carbonized anthracite was
physically activated under two different physical activation environ-
ments, namely CO2 and steam at two different activation temper-
atures of 800 and 900 °C in a tube furnace system with a gas flow rate
of 0.3 L/min. The heating rate was set at 5 °C/min and the
temperature was increased to the designated temperature and kept at
this temperature for 2 h. The selected physically activated anthracite
samples were then mixed with KOH powder (Sigma-Aldrich, Inc.)
while stirring to obtain an AC/KOH ratio of 1:1 and 1:2 by weight.
The mixtures were then arranged in alumina trays and activated at a 5
°C/min heating rate in the furnace system under different target
temperatures of 650, 750, and 850 °C for 1 h in a 0.3 L/min flow rate
of N2 environment. The selection of the chemical activation
temperatures was based on the physical activation conditions. A
programable temperature controller was used to regulate the
temperature and heating rate in the furnace system. The scheme of
the ABACs’ multistage activation process can be found in Figure 1a
and the tube furnace system used in the activation process can be
found in Figure 1b. Aalborg GFC mass flow controller was designed
to set the flow rate of N2/CO2 (Airgas Inc., USA). A boiler was used
for the steam generator, and the flow rate was controlled. After the
activation process, aqueous HCl (Fisher Scientific Inc., USA) was
then utilized to get rid of any remaining ions in the anthracitic coal-
based porous AC samples. Deionized water was used for washing the
samples over 12 times until the carbon products were clear of K+
and
Cl−
. In the end, each washed carbon sample was dried under 110 °C
over at least 12 h.
One commercial AC, BPL manufactured by Calgon Carbon
Corporation, USA, was also used in the experiments. BPL is a granular
commercial bituminous coal-based AC with a high surface area of
1100.39 m2
/g and a high pore volume of 0.58 cm3
/g which is
designed for use in gas-phase applications.22
BPL granular AC was
crushed with a coffee grinder to obtain a powder AC with a size of
<355 μm. BPL was used to compare electrochemical performance in
comparison to the produced ABAC samples.
2.2. Characterization of Synthesized Anthracite-Based
Porous Activated Carbon. The contact angle measurement was
used to explain wetting phenomena, which was conducted using a
contact angle goniometer (Ramé-Hart Instrument Co.). A scanning
electron microscope was utilized to evaluate the morphology features
of the anthracite-based porous carbon samples. X-ray diffraction
(XRD) was implemented on a diffractometer (Bragg Brentano) with
Cu Kφ radiation to characterize modified ABACs. The voltage and
current of the X-ray were 40 kV/40 mA with a scanning angle from 10
to 80° and the scanning increment was set at 0.04°. Carbon surface
chemistry measurements were performed using a custom-built SPECS
X-ray photoelectron spectroscopy (XPS) instrument using a photon
energy of 1486.6 eV Al Kα to determine the surface chemistry of
samples. The pass energy for all core level scans was 10 eV and the
pass energy for survey scans was 70 eV. An Accelerated Surface Area
and Porosimetry 2020 system (Micromeritics, USA) was used to
compute the porous structure of ABACs. The N2 isotherms were
employed to calculate the SSA and total pore volume according to the
Brunauer−Emmett−Teller model at relative pressures ranging from
0.001 to 1. The density functional theory method was used to
calculate PSD. The average pore diameter (D) of the sample was
calculated from eq 1
Figure 1. (a) Scheme of the multistage activation process. (b) ABACs’ activation experimental setup.
C

D
V
4
SSA
total
=
(1)
SSA results, average pore size results, Barrett−Joyner−Halenda
adsorption cumulative volume of mesopore results, were obtained for
four physical activated anthracite samples, six ABAC samples, and
BPL by the physical adsorption of gas (N2 adsorption at −196 °C).
2.3. Preparation and Electrochemical Performance of the
Electrode. For the preparation of the carbon electrode, the ABACs,
conductive carbon black, and polytetrafluoroethylene binder were
mixed with a mass ratio of 8:1:1 into ethanol. The prepared carbon
electrode wet slurry was uniformly applied to the current collector and
dried in the oven at 110 °C over 12 h. Subsequently, the above
current collector and electrode were compacted by a press machine
under a pressure of 10 MPa. The prepared working electrode was
then soaked in the electrolyte solution at room temperature for over
12 h. The electrochemical experiments were carried out by a three-
electrode system. The three-electrode configuration is useful in the
investigation of the chemistry of the active electrode surface as well as
the redox behavior of the active electrode material.23
Two different
aqueous electrolytes were used in the electrochemical experiments.
One is 1 M H2SO4, the other is KOH solutions with two
concentrations (1 and 6 M). In the acid electrolyte experiments, a
1 cm × 1 cm stainless-steel mesh was employed as the current
collector. A Ag/AgCl electrode was used as the reference electrode
and a platinum wire ring acted as the counter electrode. The mass
loading of active material on the electrode was about 5 mg. In the
alkaline electrolyte experiments, a 1 cm × 1 cm nickel foam was used
as the current collector. A Hg/HgO electrode was utilized as the
reference electrode and a platinum wire ring was acted as the counter
electrode. The active material’ mass loading on the electrode was 5
mg. The electrochemical performance of galvanostatic charge−
discharge (GCD), cyclic voltammetry (CV), and electrochemical
impedance spectroscopy (EIS) was measured by an electrochemical
workstation (Gamry Instruments Inc.). The GCD tests were
performed at a room temperature of 20 °C, at a current density of
0.5−10 A/g. The CV scanning voltage ranges were 0−1 V in acid
electrolytes and −1 to 0 V in alkaline electrolytes and the scanning
rates were 20, 40, 60, 80, and 100 mV/s for both electrolytes. The
amplitude of the alternating signal that was applied by the EIS was 10
mV and the frequency range was 10−2
to 105
Hz.
The specific capacitance (C, F/g) in the three-electrode system of
the prepared electrodes was determined using eq 2
C
I t
m V
=
·
(2)
where I is the charge or discharge current in A, m represents the mass
of AC in g, Δt stands for the total charge or discharge time interval
during the experiment in seconds, and ΔV is the voltage difference
during the charging or discharging process in volts.
3. RESULTS AND DISCUSSION
The produced carbon samples’ nomenclature includes carbon-
ization temperature, physical activation temperature, physical
activation environment, chemical activation temperature, and
the physically activated anthracite sample/KOH mass ratio.
For example, 750-800-steam-650-1:2 shows the anthracitic
coal activated at a 750 °C-carbonization temperature, followed
by 800 °C physical activations with steam, after which the
physically activated sample was then chemically activated with
an AC/chemical agent mass ratio of 1:2 at 650 °C. The
multistage activation conditions are summarized in Table 2.
3.1. Microstructure and Composition of ABACs.
3.1.1. SSA, Vtotal, and Dave Analyses. Surface property results
of the raw anthracitic coal and physical activated anthracite
samples are shown in Table 3. After physical activation, the
porosity of the sample was developed. It is expected that larger
SSA results would be beneficial for the specific capacitance. A
higher physical activation temperature intensifies the reaction
between steam and AC and consumes more ACs, further
leading to the decomposition of carbon around some
heteroatoms (mainly composed of oxygen), which in turn
further increases the SSA and Vtotal of the ABACs (Table 3).
Therefore, the 750-800-steam and 750-900-steam samples with
the higher SSA and total pore volume results were selected to
proceed with chemical activations.
The SSA and pore structure properties of six ABACs and
one BPL are shown in Table 4. The porosity features of ABAC
samples were analyzed via the N2 adsorption−desorption
methods.
It can be seen in Tables 3 and 4 that the SSA, Vtotal, and Dave
of the ABAC samples show a trend of gradual increase with
physical activation temperature. The increase in Vtotal is
ascribed to the combination of mesopores and micropores
formed when the temperature increases. As the physical
Table 2. Description of Multistage Activation Conditions of Samples
description
carbonization
temperature (°C)
physical activation
temperature (°C)
physical activation
environment (°C)
chemical activation
temperature (°C)
AC/chemical agent
impregnation ratio
750-800-CO2 750 800 CO2 N/A N/A
750-800-steam 750 800 steam N/A N/A
750-900-CO2 750 900 CO2 N/A N/A
750-900-steam 750 900 steam N/A N/A
750-800-steam-650-1:1 750 800 steam 650 1:1
750-800-steam-650-1:2 750 800 steam 650 1:2
750-800-steam-750-1:1 750 800 steam 750 1:1
750-800-steam-750-1:2 750 800 steam 750 1:2
750-900-steam-750-1:2 750 900 steam 750 1:2
750-900-steam-850-1:2 750 900 steam 850 1:2
Table 3. Pore Structure Analysis of Physically Activated
Anthracite Samples
sample
number
sample
description
SSA
(m2
/g)
total
pore
volume
Vtotal
(cm3
/g)
micropore
volume
Vmic
(cm3
/g)
mesopore
volume
Vmes
(cm3
/g)
0 anthracitic coal 3.08 0.004 0.001 0.003
1 750-800-CO2 187.39 0.088 0.062 0.02
2 750-800-steam 540.98 0.247 0.013 0.053
4 750-900-CO2 294.47 0.128 0.099 0.029
5 750-900-steam 752.81 0.342 0.217 0.125
D

activation temperature increases, more energy is supplied to
the activation reaction leading to more carbon atoms at the
active sites that react with the activator and generate pore
structure, eventually resulting in the widening of the spaces
into the carbon matrix, thereby increasing the SSA, Vtotal, and
Dave of ABACs. Furthermore, the continuous increase of
chemical activation temperature would lead to the damage of
pore wall and development of a larger pore diameter and thus
cause the SSA ablation of ABACs. Many small holes were
gradually destroyed with this increasing temperature. The
decrease in micropore volumes and the increase in the average
pore diameter led to a decrease in the SSA and Vtotal and an
Table 4. Summary of Pore Parameters of ABACs and BPL
sample
number description SSA (m2
/g)
Vtotal
(cm3
/g)
Vmic
(cm3
/g)
Vmes
(cm3
/g) average pore diameter Dave (nm) yield of ABAC (Y) (%)
1 750-800-steam-650-1:1 709.92 0.32 0.23 0.09 1.81 62.6
2 750-800-steam-650-1:2 784.88 0.34 0.26 0.08 1.73 60.7
3 750-800-steam-750-1:1 581.05 0.28 0.18 0.10 1.95 58.8
4 750-800-steam-750-1:2 627.77 0.29 0.20 0.09 1.89 56.9
5 750-900-steam-750-1:2 1384.46 0.68 0.34 0.34 1.98 39.2
6 750-900-steam-850-1:2 990.92 0.55 0.17 0.38 2.20 34.3
7 BPL 1100.39 0.58 0.22 0.36 2.12 N/A
Figure 2. (a) N2 adsorption and desorption isotherms of ABACs and BPL. (b) PSD of the 750-800-steam-650-1:2 sample. (c) PSD of BPL. (d)
XRD patterns of three particular ABACs and BPL.
E

increase in the Dave of the ABACs.24,25
With a chemical
activation temperature increase near to the physical activation
temperature, pore walls become thinner, which can negatively
influence electric double layer formation. The potential
distribution has the greatest impact on the SC-specific
capacitance of porous carbon electrodes.26
It was confirmed
that the largest contribution to SSA is from the micropores. As
the number of micropores in the sample is increased, the larger
the SSA becomes, and the more mesopores of the samples are
included, the larger the Dave, at the expense of a smaller SSA.27
It can be seen from Table 4 that the SSA and Vtotal of the
ABACs increase as the impregnation ratio increases. A higher
impregnation ratio would provide more available KOH reagent
for the activation reaction, and it would be beneficial in the
activation of carbon atoms and thus increasing pore structure.
An increase in the impregnation ratio can accelerate the
formation of microporosity, which is favorable for more
electrolytes being absorbed, thereby improving the electro-
chemical properties of the electrode material.28
However, it
can be seen from Table 4 that the Dave of the ABACs decreases
with the increase in the impregnation ratio, which results from
the fact that an increase in the KOH/AC ratio at a moderate
activation temperature (650 °C/750 °C) involves an increase
in wide microporosity volume, which was confirmed by a
previous study, implying that an increase in the impregnation
ratio entails a decrease in the narrower microporosity (<0.7
nm) at any activation temperature (650−900 °C).29
According to the results of pore structure analysis, it can be
concluded that both impregnation ratio and multistage
activation temperatures are significant for the SSA and Vtotal
development of the ABACs. Therefore, the combination of
multistage activation temperature and a suitable impregnation
ratio is important in the optimization of pore textural
properties of the ABACs.27
3.1.2. Pore Structure Properties of ABACs. N2 adsorption
isotherm curves of six ABACs are shown in Figure 2a,
including the BPL. All samples show a type IV adsorption
isotherm curve. It can be observed that the isotherm increases
drastically at low relative pressure, then this tendency gradually
flattens at P/Po between 0.2 and 1.0. N2 adsorption and
appears to take place mostly in the microporous structure of
AC samples during the low relative pressure level. It is noted
that ABACs’ N2 adsorption capacity is higher when it is in the
low relative pressure stage, demonstrating the abundant
microporous structures presented in the ABACs. The N2
adsorption shifts from monolayer adsorption to multilayer
adsorption with an increase in the relative pressure, indicating
the existence of a mesoporous structure in the ABACs.
A hysteresis loop is also observed in Figure 2a in the range
from 0.4 to 0.96, representing a potential capillary con-
densation in mesopores or macropores of the samples.30
The
isothermal adsorption capacity increases as physical activation
temperature increases, suggesting that the SSA of samples
gradually increases. A similar effect is noticed with the AC/
KOH ratio. As the impregnation ratio increases, the isothermal
capacity for adsorption increases.
The PSD of the 750-800-steam-650-1:2 sample and BPL is
presented in Figure 2b,c. The result indicates that the samples
are mainly composed of micropores but also contain certain
mesopores. Table 3 indicates that the average pore diameter of
five of the ABACs is relatively small and less than 2 nm in size
whereas the Dave of 750-900-steam-850-1:2 and BPL samples is
larger than 2 nm. As shown in Figure 2b, 750-800-steam-650-
1:2 presents narrow PSD. The micropores are mainly
distributed between 0.5 and 2 nm and the mesopores are in
a range between 2 and 7 nm. However, for the BPL (Figure
2c), most of the width of the pores is concentrated at around 2
nm and only a few pore diameters are below 1 nm, indicating
that BPL is a high mesoporous carbon. It has been reported
that pores with a pore diameter of 0.7 nm can effectively
increase specific capacitance.31
The specific capacitance can be
improved by increasing the number of micropores with
suitable pore diameters. Therefore, the ABACs, especially the
750-800-steam-650-1:2 prepared sample has a greater potential
of demonstrating outstanding electrochemical performance
when employed in SC applications.
In electrode applications for SCs, as the numbers of
macropores increased, electrolyte ion diffusion to the micro-
pore process would be enhanced. Higher utilization of
micropores is favorable for the electrode material to adsorb
more electrolyte ions, thereby generating more electric double-
layer capacitance.32
Electrode material with both high SSA and
reasonable PSD has a greater potential to provide excellent
electrochemical performance. It has been reported that at low
current density (<1 A/g), micropores are dominant and
enhance the value of greater capacitance than the meso-
pores.33,34
In this study, the 750-800-steam-650-1:2 sample
provided the highest Vmic/Vtotal of 76.47% at a low current
density of 0.5 A/g with a specific capacitance of 258.36 and
288.52 F/g in 6 M KOH and 1 M H2SO4, respectively. Hence,
SCs can be strongly motivated in carbon electrodes with
tailored micropores for capacitance and sufficient number of
mesopores for high-rate charge−discharge performance.
3.2. XRD Analysis. Figure 2d shows the XRD patterns of
selected ABAC samples and BPL. All the samples displayed
two broad diffraction peaks at 2θ centered at around 26 and
43°, which is due to the diffuse (002) and (100) diffraction,
representing an amorphous carbon structure for these types of
carbon samples. It can be concluded that all samples possess a
porous carbon (002) microcrystal face as well as a (100)
amorphous structure, embedded with a partially graphitic
structure.35
A microcrystalline graphite structure is favorable
for increasing the conductivity of ACs and improving
electrochemical performance. This suggests that the crystallites
in the ACs have an intermediate structure between graphite
and the amorphous state, known as a random layer lattice
structure. The sharp peak at around 26° is related to the (002)
graphitic plane and it indirectly ensures the presence of in-
plane conductivity required for electrochemical applications.
The peak at 26° of the 750-800-steam-650-1:2 sample was
found to be the largest. This is due to the abundant
microcrystalline graphite structure present in ACs and
electrolyte ions can more readily enter the material. In
addition, it also reveals that a sharp peak at 26° will be
gradually decreased with a multistage temperature increase and
it confirms that the ABAC samples are more graphitic. A
decrease was observed in the intensity of (002) peak with the
increase in activation temperatures. This could reflect an
increment of crystallite disorder, with more defects in the
structure of carbon because the structure of graphitized
samples was severely altered at high temperatures during
activation.36
Higher chemical ratios result in a higher value of
(002). This can be interpreted as the ablative effect of KOH
and the impact of microcrystals resulting from the reactions
between KOH and carbon. Generally, due to their numerous
pore structures, a high SSA AC usually exhibits a very low
F

electric conductivity. However, the presence of graphite
microcrystalline can significantly improve this, thereby
enhancing their electrochemical performance.37
3.3. Morphological Features of Modified ABACs. The
scanning electron microscopy (SEM) images of ACs obtained
in the multistage activation process, the raw anthracitic coal,
and commercial AC (BPL) are shown in Figure 3a−d. A
significant difference between raw anthracite material and the
other ACs can be observed. As shown in Figure 3a, the
examination of the raw anthracitic coal reveals a dense
structure and smooth surface without obvious pores, which is
attributed to the high degree of coalification of anthracite with
relatively low porosity. In Figure 3b, physical activation
treatment with steam creates a gradual porosity development.
The 750-800-steam sample exhibited an irregular and
heterogeneous topography, whose external surface has cracks
and connected cavities. The smooth surface turned uneven.
Following steam activation, the surface of the sample shows
some discernible pores, and there are some erratic pieces on
the surface and in the crack of the sample. The same
observation can be found in Figure 3d showing the SEM image
of the commercial AC-BPL. This appearance of the 750-800-
steam sample is mainly due to the release of the coal volatiles
and the reaction between the steam and carbon. In addition to
the present of substantial number of amorphous carbons, a
microcrystalline stripe structure resembling graphite started to
form. The reactions (eqs 3−6) summarize the mechanism of
H2O activation31,38
C H O CO H
2 2
+ = + (3)
C 2H O CO 2H
2 2 2
+ = + (4)
CO H O CO H
2 2 2
+ = + (5)
C CO 2CO
2
+ = (6)
A comparable amount of CO and CO2 was likely formed by
the primary reaction of the steam with the carbon and by the
water−gas reaction (eqs 3−6), which can be attributed to the
widening of the pore size of the ACs. In addition, the
development of the pore structure of the ACs also improved as
the CO2 produced by the steam−carbon reaction further
reacted with carbon to make CO and further etch the carbon.
After KOH activation, high porosity was further developed
and different sizes of pores appeared on the surface of the
sample (Figure 3c). The 750-800-steam-650-1:2 sample has a
large number of pores with uniform size after the chemical
activation. It presents an amorphous structure with slits and
collapsed pores, which are a result of thermal stress due to the
double heat treatment. The development of the larger SSA and
high porosity in KOH/AC is the result of the synergistic
comprehensive actions. Several simultaneous reactions in
KOH activation below 700 °C have been proposed, as
indicated by eqs 7−1124,25,39
2KOH K O H O
2 2
= + (7)
C H O CO H
2 2
+ = + (8)
C 2H O CO 2H
2 2 2
+ = + (9)
CO H O CO H
2 2 2
+ = + (10)
K O CO K CO
2 2 2 3
+ = (11)
It has been indicated that K2CO3 forms at about 400 °C, and
at 600 °C KOH is completely consumed.40
The surface carbon
matrix further reacts with the breakdown of KOH to K2O and
H2O to release CO, CO2, and the C−O−K complex to etch
the carbon, which enhances the formation of the mesoporous
and microporous structures of the ACs. Additionally, the active
CO2 constantly oxidizes the K−C complex to create a new C−
O−K structure, sustaining the catalytic processes and creating
new pores.39
At the micro-scale, the prepared ABACs show
amorphous porous structures with different extremely irregular
particle morphological architecture related to the applied
activation pathway. Furthermore, the graphite-like streaks of
750-800-steam-650-1:2 significantly thrived and amorphous
carbon started to change into an ordered graphite structure.
The surface of sample 750-800-steam-650-1:2 exhibits loose
and porous features and majority of the pores that extend to
Figure 3. SEM images of (a) anthracite, (b) 750-800-steam, (c) 750-800-steam-650-12, and (d) BPL.
G

the surface have a circular or elliptical shape. The circular and
elliptical pore structures have superior ion-transport efficiency
than irregular pore structures such as mesh and crack, which
can lower resistance in the electrolyte ion-transport process.
3.4. XPS Analysis. Surface elemental content and the
chemical bond information of the samples were obtained using
XPS. Two peaks of C and O were observed on the surface of
the different modified samples (Figure 4a). The strength of
these peaks varied with the different activation processes,
indicating that the content of the C and O atoms changed with
the activation processes. With the proceeding of the activation
process, the C content in samples increased, and the O content
decreased gradually, which could be interpreted as the
formation of a corresponding gas at reasonably high temper-
atures as a result of chemical bonds rupturing into functional
groups that contain O. The C content of the sample dropped,
whereas the O content increased as the chemical activation
temperature reached 650 °C. This is primarily caused by the
activation reaction between C and KOH consuming the C
element in the anthracite and producing an element that
Figure 4. (a) XPS survey spectra of samples, (b) high-resolution XPS spectra C 1s region of the 750-800-steam 650-1:2 sample, (c) high-resolution
XPS spectra O 1s region of the 750-800-steam 650-1:2 sample, (d) high-resolution XPS spectra in C 1s region of BPL, and (e) high-resolution XPS
spectra in O 1s region of BPL.
Table 5. C and O Elemental Percentage of Samples
element chemical state anthracite (%) 750-800-steam (%) 750-800-steam-650-1:2 (%) BPL (%)
C C−C sp2
65.97 71.64 84.70
C−C sp3
5.69 1.56 44.58 3.68
C−O,C−OH 2.26 4.90 25.97 1.65
C�O 6.71
O−C�O 6.67
O C−OH 15.97 9.00 4.82 5.05
C�O 5.27 5.60 14.74 2.81
H

contains O. In the XPS spectrum of the 750-800-steam-650-
1:2 sample (Table 5), the contents of C and O were 83.93 and
19.56%, respectively. A very small amount of Si was found in
the sample with a peak at 102 eV as shown in Figure 4a. The
appearance of Si on the XPS spectra is related to the use of
anthracite as a precursor for the carbon electrode. Meanwhile,
the data show the presence of oxygen in the carbon which was
introduced on the modified sample surface after the physical
activation, the O content of the 750-800-steam-650-1:2 sample
was significantly higher than in the other samples, indicating
that the 750-800-steam-650-1:2 sample is a highly graphitic
oxidized carbon.
The high-resolution XPS spectrum of C 1s and O 1s of the
750-800-steam-650-1:2 sample is shown in Figure 4b,c. The C
1s spectrum can be deconvoluted into C−C (284.8 eV), C−
O/C−OH (∼286 eV), C�O (286.5 eV), and O−C�O
(289.1 eV) of the graphite oxide structure.41
The 750-800-
steam-650-1:2 sample displayed broad symmetrical peaks
corresponding to C−O, C�O, and O−C�O, implying its
high graphitization degree.42
The O 1s spectrum fitted peak
positions at C�O (531.5 eV) and C−O/C−OH (532.7 eV).
C−O/C−OH, C�O, and O−C�O are hydrophilic groups,
those rich oxygen-containing functional groups would improve
the wettability of the electrode, allowing electrolyte ions to
diffuse and move through the micropores.43
These functional
groups can enhance wettability in aqueous electrolytes, which
is conducive to promoting affinity between material surface
and electrolyte ions. Additionally, the quinone group (C�O)
relative content of the 750-800-steam-650-1:2 sample was
significantly increased to 6.71%. The C�O functional group
would increase the specific capacitance of the SCs by
increasing its pseudocapacitance.44
Therefore, with increased
content of the C�O functional group on the surface of the
sample, the charge storage ability of pseudocapacitance in the
SCs will be greatly improved. The high-resolution XPS
spectrum of C 1s and O 1s of BPL is also shown in Figure
4d,e. As compared with the 750-800-steam-650-1:2 sample,
BPL is a highly amorphous carbon with a high C�C content.
Additionally, BPL lacks hydrophilic groups such as C�O, C−
OH, and O−C�O, leading to low wettability which would
result in less favorable electrochemical performance.
3.5. Contact Angle Measurement Results. The
wettability of carbon with the electrolyte is essential for its
use as an electrode material in SCs. In order to describe
electrode−electrolyte interactions and clarify the intrinsic
electrolyte wettability of the electrode surface, contact angle
measurements were carried out as a major characterization
tool. The level of wettability of a solid with a probing liquid is
determined by the contact angle (α), which immediately
reveals information on the interaction of energy between the
surface and the liquid.45
A high-resolution camera with
software to capture and analyze the angle created between
the solid/liquid interface is used to measure the contact angle
of the flat interface from a drop of suitable liquid resting on a
surface. According to the Yang equation (eq 12),46
the
wettability of the AC electrode to the KOH electrolyte can be
expressed by cos α. The larger the α, the stronger the surface
hydrophobicity of the AC electrode and the smaller the α, the
better the surface wettability.
cos
sv sl lv
= + (12)
where α is the contact angle; γsl
is the solid/liquid interfacial
free energy; γsv
is the solid surface free energy; and γlv
is the
liquid surface free energy.
In this study, the wettability investigation of the electrode
was conducted by using the ABACs, and 1 mol/L KOH
electrolyte was used as the probing liquid because the KOH
solution was used as the electrolyte in the electrochemical cell
during the electrochemical measurements. The electrolyte was
added dropwise to the surface of the AC electrode. The droplet
eventually spreads and the contact angle shrinks as the amount
of time the droplet spends in contact with the electrode surface
increases. The angle of the electrodes was recorded during 120
s stabilization, and the wettability of four ABAC electrodes and
BPL is compared in Figure 5. The wettability of the different
modified ABAC electrodes was significantly different than the
BPL material. For the 750-800-steam-650-1:2 sample, the
droplet will be completely and immediately soaked when it
comes in contact with the surface within 60 s, characteristics of
a super-hydrophilic. BPL shows poor wettability, reaching
34.5° after the 120 s. It was determined that the wettability of
the electrode is related to the utilization of micropores and
mesopores and functional groups on the surface. The
utilization of micropores and mesopores is key to improving
wettability. Mesopores, act as a transport channel for
electrolyte ions to enter the porous carbon structure, which
could increase the rate of contact with the micropores,
encourage the electrode materials to adsorb the electrolyte
Figure 5. Contact angle measurement of four ABACs and BPL.
I

ions, which can greatly enhance the wettability of the
electrodes, and result in a higher specific capacitance. The
oxygen-containing functional groups is vital to AC surface
properties including hydrophilicity and hydrophobicity. The
investigation of the relationship between AC wettability and its
oxygen-containing functional groups is helpful to uncover the
characteristics of wettability from a microscopic view.47
The
presence of carboxyl group (C�O) and a hydroxyl group (C−
OH), indicated by XPS results as shown in Figure 4b,c, are the
greatest promoters to surface wettability.48
In comparing the
Vmic/Vmes values of the samples, there is a higher micropore
utilization but less mesopore utilization in the 750-800-steam-
750-1:2 sample, however, there are more mesopores in the
750-900-steam-750-1:2 sample and the increase in mesopores
allows more electrolyte ions to enter the inside of the 750-900-
steam-750-1:2 sample, which provides sufficient electrolyte
ions for the micropores and increases the capillary force of the
adsorbent. For BPL, the low utilization of the micropore
reduces its wettability. The 750-800-steam-650-1:2 sample has
the greatest utilization rate of micropores, which would
translate into a great improvement in specific capacitance in
SCs. Although it has relatively low volumes of micropores and
mesopores, due to its low activation temperature, it was
embedded with oxygen-rich functional groups on the surface,
which is greatly beneficial in improving its wettability. Thus, it
can be concluded that the ABACs ended up with an optimized
contact with the electrolyte.
3.6. Electrochemical Performance of the ABACs. The
SC behavior of all the ABAC samples prepared in this study,
also BPL, was evaluated by GCD, CV, and EIS in a three-
electrode system with two different aqueous electrolytes.
The GCD curves of the ABACs at a current density of 0.5
A/g in 1 M KOH are shown in Figure 6a. The GCD curves are
roughly isosceles triangles, indicating that all samples were
mainly based on the electric double-layer capacitor character-
istics and good electrochemical reversibility.49
Clearly, the 750-
800-steam-650-1:2 sample shows the longest charging time,
indicating it has the largest specific capacitance among all
ABACs. In addition, compared with other ABACs, the specific
capacitance of the 750-800-steam-650-1:2 sample at 244.25 F/
g is more than 1.5 times higher than the corresponding specific
capacitance of the 750-900-steam-850-1:2 sample of 160.44 F/
g, proving the importance of activation temperature and the
impregnation ratio in the multistage activation processes. The
Figure 6. Electrochemical behavior of six cases of ABACs in a three-electrode system of 1 M KOH: (a) GCD curves of six cases of ABACs at 0.5
A/g, (b) CV curves of six cases of ABACs at 100 mV/s, (c) CV curves of 750-800-steam-650-1:2 from 20 to 100 mV/s, (d) GCD curves of 750-
800-steam-650-1:2 and BPL at a current density of 0.5 A/g, and (e) CV curves of 750-800-steam-650-1:2 and BPL at 100 mV/s.
J

CV curves for each ABAC are displayed in Figure 6b at a
scanning rate of 100 mV/s. All curves display an almost
rectangular shape and excellent capacitive behavior, demon-
strating that the electrical double-layer capacitance behavior is
the primary source of capacitance in all samples.50
Moreover, it
was noted that the CV curve is somewhat distorted, which is a
result of the pseudocapacitance effect caused by the fast redox
reaction of oxygen-containing functional groups on the carbon
surface.51
Furthermore, the 750-800-steam-650-1:2 sample
displayed the largest CV area, which is due to the co-
promotion of pseudocapacitance induced by the oxygen-
containing functional groups and the double-layer capacitance
provided by the utilization of SSA. Combined with XPS results,
it can be confirmed that a large amount of C−O/C−OH in the
750-800-steam-650-1:2 sample modified the electrode’s
wettability, effectively reducing the diffusion resistance of
electrolyte ions in the pores and improving the accessibility of
electrolyte ions. Meanwhile, the carbonyl (C�O) functional
groups increase the pseudocapacitance.52
The CV curves at
different rates for the 750-800-steam-650-1:2 sample are
shown in Figure 6c, as the scan rate increased, the distortion
range of the CV curves also increased. As a result, the
micropores in the electrode material could not be used to their
full potential. This is due to the limited diffusion of electrolyte
ions in the electrode and the inability of the inner electrode
pores to be suitably saturated. Besides, as the scan rate
increases, there is an increase in the redox reactions between
electrolyte ions and the outer surface of the electrode. The
GCD and CV curves of the 750-800-steam-650-1:2 and BPL
samples in 1 M KOH are shown in Figure 6d,e. As compared
with commercial AC-BPL, the CV and GCD curves of the 750-
800-steam-650-1:2 sample exhibited good capacitive behavior.
As shown in Figure 6d,e, the capacitance of the 750-800-steam-
650-1:2 sample is more than 1.7 times higher than that of BPL
in 1 M KOH at a current density of 0.5 A/g. The charge
storage capacities of the 750-800-steam-650-1:2 sample were
superior to the capacitance of BPL due to its reasonable PSD,
high conductivity, and rich oxygen surface functional groups,
contributing to the improvement in wettability of the electrode
by changing the polarity of the carbon surface.
To investigate the resistance and capacitance behavior of the
ABACs, EIS analyses were performed in the frequency range
from 100 to 0.01 kHz. Nyquist plots of the ABACs in 1 M
KOH are shown in Figure 7. In the high-frequency region, all
ABACs show a semicircle feature. The curves at the x-axis
intersection reflect the intrinsic resistance (Rs), including
contact resistance between the electrode and current collector,
electrode resistance, and electrolyte resistance. The 750-800-
steam-650-1:2 sample has the smallest intrinsic resistance. The
diameter of the semicircle represents the charge-transfer
resistance (Rct), which is related to the pore structure in
carbon materials.53
The 750-800-steam-650-1:2 sample has the
smallest Rct, indicating that the charge-transfer resistance of
electrons in the electrode is the smallest. As compared with the
Rct of the 750-800-steam-750-1:2 sample, the Rct increased
sequentially, which is due to the decrease in the degree of
graphitization of the ABACs during different activation
conditions, which was confirmed by XRD results. It is
favorable to enhance the SSA utilization to increase the
efficiency of electrolyte ions entering the internal pores of the
carbon material.54
The slope of the linear portion of the
Nyquist plots in the mid-frequency region represents the ionic
diffusion resistance (Rd) between electrolyte ions and electrode
materials.55
The larger the slope, the closer to the ideal double-
layer capacitor behavior. Generally, a high SSA utilization ratio
and moderate pore width are beneficial in promoting fast ion
diffusion and thereby enhancing the specific capacitance. The
Rd of the 750-900-steam-850-1:2 sample was the largest,
demonstrating that the ion diffusion inside the electrode was
severely constrained. The reported resistance parameters are
summarized in Table 6. The resistance increase at high
multistage activation temperatures is probably due to a volume
increase of the large pores. The decrease in capacitance can be
explained by the content decrease of C�O groups and
graphitization degree with increasing activation temperature,
which influences pseudocapacitance. As the chemical activation
temperature becomes higher, the pore wall becomes thinner,
which can negatively influence the double layer formation by
leading to an increase in Ohmic loss due to the loss of contact
between C particles in the electrode. The smaller resistance of
the 750-800-steam-650-1:2 sample promotes its high specific
capacitance in SCs.
The GCD curves for the 750-800-steam-650-1:2 sample at a
current density of 0.5 A/g and different concentrations (1 and
6 M) of KOH in the voltage range of 0−1 V are shown in
Figure 8a. The specific capacitance values increase from 244.25
to 260.30 F/g as the KOH concentration increases from 1 to 6
M. These values show that capacitance is affected by the
concentration of KOH. The increase in the concentration of
the electrolyte leads to more adsorbed K+
and OH−
. As more
electrolyte ions is adsorbed, larger capacitances can be
achieved. Figure 8b shows the CV curves of the 750-800-
steam-650-1:2 sample for different electrolyte concentrations
in the voltage range of 0−1 V at a scan rate of 20 mV/s. The
Figure 7. Nyquist plots of six ABACs.
Table 6. Summary of Resistance Parameter for ABACs in 1
M KOH
sample Rsum (Ω) Rs (Ω) Rct (Ω) Rd (Ω)
750-800-steam-650-1:1 1.49 1.09 0.16 0.24
750-800-steam-650-1:2 1.47 0.99 0.23 0.25
750-800-steam-750-1:1 1.52 1.04 0.17 0.31
750-800-steam-750-1:2 1.50 1.09 0.19 0.22
750-900-steam-750-1:2 1.48 1.09 0.19 0.20
750-900-steam-850-1:2 1.49 1.05 0.19 0.25
K

shape of the CV curves is distorted at lower electrolyte
concentrations. The near rectangular CVs suggest a capacitive
type of charge storage mechanism including surface or near
surface charge transfer.56
The largest area in 6 M KOH is
enclosed by the CV curves. One important factor that affects
the performance of electrochemical reactions is the electro-
lyte’s specific conductivity. It was found that the specific
conductivity increases as the concentrations of KOH increase
to the maximum at 6 M.57
Figure 9a,b shows the CV behavior of the 750-800-steam-
650-1:2 sample evaluated at a voltage window of 1 V in two
different aqueous electrolytes at 20 mV/s. As it is shown in
Figure 9a,b, both curves exhibit a malformed curvilinear curve,
representing good conductivity of the 750-800-steam-650-1:2
sample. However, the H2SO4 electrolyte displays a relatively
large hysteresis area on the curve and better electrode
performance. The differences observed from the CV curves
of the 750-800-steam-650-1:2 sample in different electrolytes
come from the different physical properties of the electrolyte
ions. Some of these properties include the ionic radius, radius
of ionic hydration sphere, molar conductivity, and ionic
mobility.58
The specific capacitance of the H2SO4 electrolyte is
higher than the KOH electrolyte. The main factors in
determining capacitive performance using different electrolytes
are related to mobility and conductivity. 1 M H2SO4 has an
extremely high conductivity of 0.8 cm−1
at 25 °C.59
Compared
to the basic electrolyte KOH, the acidic electrolyte H2SO4 is
thought to have a more suitable ion diameter for ion diffusion
in the ABACs.60
Out of the largest mobility and conductivity of
ions, H+
cations is determined to have the highest ion
conductivity, which is almost fivefold as compared with K+
cations at 73.5 whereas the radius of hydration sphere H+
is
smaller by 2.8 Å than that of K+
at 3.31 Å. Thus, the H+
cation
has the highest mobility and conductivity of the ions and has
the smallest radius of the hydration spheres. As a result, when
charging and discharging, H+
cations could easily migrate into
the electrolyte bulk and onto the electrode−electrolyte surface.
However, the bigger SO4
2−
anion at 3.74 Å (as compared to
the OH−
anion of 3 Å) would slightly reduce the mobility of
H+
cation; thus, making the electrochemical behavior of the
capacitor in the H2SO4 electrolyte less favorable. The
difference in specific capacitance is attributable to the hydrated
Figure 8. (a) GCD and (b) CV curves of 750-800-steam-650-1:2 for 1 and 6 M KOH at 0.5 A/g.
Figure 9. CV curves of 650-800-steam-650-1:2 in (a) 1 M H2SO4 and (b) 1 M KOH at 20 mV/s. (c) EIS curves of 750-800-steam-1:2 in 1 M
KOH and 1 M H2SO4.
L

ionic radius, ionic mobility, and molar ionic conductivity,
whereas H2SO4 has a smaller cationic radius, greater ionic
mobility, and greater conductivity. The EIS data provide
additional information on the ion restriction in the various
electrolytes. Figure 9c represents the Nyquist plot of the
electrolyte systems with the inset in the high-frequency region
and the degree of the deviation from the vertical curve showing
the inner ion diffusion. The H2SO4 system exhibits a nearly
vertical Nyquist plot with the lowest impedance and the
smallest intrinsic resistance, with good capacitive behavior, as
compared to the KOH system, demonstrating the higher
capacitive behavior of the H2SO4. As mentioned before, the
semicircle in the high- frequency region is considered the
charge-transfer resistance. The H2SO4 system has a larger
semicircle, capable of more intense redox reactions that
occurred on the electrode’s surface in H2SO4 as compared to
the KOH. The redox reactions on the surface of the electrode
would be contributed to pseudocapacitance. All of those results
point to the best electrochemical capacitive performance of the
H2SO4 system, exhibiting good CV and GCD results.
Figure 10a shows the GCD curves of the 750-800-steam-
650-1:2 sample as a function of current density in a 1 M
H2SO4 electrolyte system. The 750-800-steam-650-1:2 sample
has a highest capacitance of 288.52 F/g at a current density of
0.5 A/g. When the current density is increased to a high value
of 20 A/g, the capacitance decreases to 201.23 F/g, at which
the electrolyte diffusional limitations became evident. With an
increased scanning rate, the 750-800-steam-650-1:2 sample still
maintained a good rectangular characteristic. It was shown that
the 750-800-steam-650-1:2 sample has good reversibility and
fast ion response. The pore structure of this material could
satisfy the rapid diffusion of electrolytes, thus making the
material capable of good SC performance. Although micro-
pores are less stable than mesopores, the capacitance of
micropores decreases with the increase in current density. At a
high current density, rapid movement of electrolyte ions are
capable only in mesopores, hence, mesopores could form high
capacitance.61
The other reason for this behavior could be
ascribed to the interfacial electrolytes adsorbing electrode ions
and resulting in the concentration of the electrolyte ion at the
interface rapidly decreasing and increasing polarization.62
This
not only greatly shortens the charging time of super-
capacitances, but also exhibits excellent electrochemical
performance in high-power applications, making SCs useful
for a wider range of applications.
For SCs, long-term cycling stability is also one important
aspect. The cyclic stability of the 750-800-steam-650-1:2 and
BPL samples was studied via GCD at a current density of 0.5
A/g (Figure 10b). The specific capacitance of the 750-800-
steam-650-1:2 sample was kept steady at first and slightly
decreased after 300 cycles. After 1000 cycles, the specific
capacitance of the 750-800-steam-650-1:2 sample retention
rate was up to 95.4% of the first cycle, which shows that this
material possesses superior cyclic stability and excellent rate
performance. BPL maintained capacitance retention of 88.7%
after 1000 cycles, which is lower than the 750-800-steam-650-
1:2 sample. The enhanced cycling stability for the activated
anthracite material is due to the synergistic effect between the
active material and the electrolyte interfaces. The moderate
microporous and mesoporous nature of the ABACs prevented
continuous degradation during the charge and discharge
processes by withstanding strain/stress relaxation during the
electrochemical reaction while also providing a stable channel
for electron transportation.63
The specific capacitances of ABACs in 1 M KOH, 6 M
KOH, and 1 M H2SO4 are listed in Table 7. The capacitive
behavior of the ACs is affected by many factors, such as PSD,
surface functional groups, wettability, and graphitization
degree. To improve the specific capacitance of the ABACs
by adjusting their pore structure, the relationship between
specific capacitance and pore textural properties was studied,
and the overall results are shown in Table 4. As shown in Table
4, the activation temperatures and the chemical mass ratio
corresponds to different yields of ABACs. The 750-800-steam-
650-1:2 sample activated under moderate activation temper-
atures with a relatively higher chemical mass ratio presenting a
relatively higher yield of 60.7%. In addition, the content of
micropores and mesopores increases with an increase in the
physical activation temperature of the ABACs and the
impregnation ratio, whereas more ultra-micropores are evident
in the 750-800-steam-650-1:2 sample. However, the pore
depth of micropores and mesopores also affects the rate
capability of active materials.64
An increase in porosity leads to
an increase in adsorbed electrolytes.65
For mesopores, Vmes/
Vtotal increase gradually with an increase in physical temper-
ature. However, the results show that Vmic/Vmes became smaller
with the increase of physical activation temperature. The
presence of micropores can significantly improve the material
capacity, as the hydrated ions strip off the hydrated shells and
deforms into micropores.63
The smaller space in the
micropores effectively reduces the distance between the lattice
layers, which abnormally increases the capacitance. Compared
with all ABACs and BPL, the 750-800-steam-650-1:2 sample
has the largest Vmic/Vmes, thus, the maximum utilization rate of
micropores can be achieved. However, there may be a
congestion in the process of ions entering the micropores
Figure 10. (a) GCD curves of 750-800-steam-650-1:2 in 1 M H2SO4 at different current densities and (b) cycling performance of 750-800-steam-
650-1:2 and BPL at 0.5 A/g after 1000 cycles.
M

because Vmic/Vmes is small in the 750-900-steam-850-1:2
sample. Thus, the micropores are not fully utilized, making
more inner pores inaccessible. In the 750-900-steam-750-1:2,
750-800-steam-750-1:1, 750-800-steam-750-1:2, and 750-800-
steam-650-1:1 samples, the congestion is alleviated when Vmic/
Vmes increases, and the utilization of micropores is improved.
Although the Vmic/Vmes value of the 750-900-steam-750-1:2
sample is not higher than that for the 750-800-steam-750-1:1,
750-800-steam-750-1:2, and 750-800-steam-650-1:1 samples,
it has a relatively higher Vmes/Vtoal, which would be beneficial
in increasing the electron/ion transfer rate and afford a double
layer to achieve a higher specific capacitance. A decrease in the
inner resistance and an increase in specific capacitance are not
associated with the increase in adsorbed electrolyte volume
with a higher activation temperature. Three things can account
for these phenomena. Pore depth, the first factor, might make
it difficult for ion penetration, gating, and uneven charge
distribution in the micropores. Due to ion solvation shell
distortion within carbon nanostructures, the second effect,
micropore narrowing, may positively affect specific capacitance.
As a result, the distance between the ion center and electrode
surface are closer, which is beneficial in improving specific
capacitance. The capacitive behavior is also influenced by the
third factor, surface oxygen-containing functional groups.
These functional groups can contribute to SCs’ capacitance
by redox reactions of C�O and also increase the hydro-
philicity of the pore surface by changing the polarity of the
carbon group, where the electric double layer forms. Thus,
ABACs have the greatest potential for SC applications.
4. CONCLUSIONS
Anthracite-based porous carbons were synthesized by multi-
stage activation and successfully used as electrode materials for
SCs. The impact of multistage activation temperature and the
impregnation ratio of chemical agents on the pore structure of
ABACs and the effect of different aqueous electrolytes on the
electrochemical performance were studied. An optimal 750-
800-steam-650-1:2 sample was obtained after preparation at a
carbonization temperature of 750 °C, a physical activation
temperature of 800 °C with steam, and chemical activation at
650 °C with 1:2 impregnation ratios of AC/KOH. This
engineered sample achieved a reasonable PSD with great
utilization of SSA and narrow micropore structure, rich oxygen
functional group, and high graphitization degree. The electro-
chemical performance of this sample also reveals that in a 1 M
H2SO4 electrolyte system, the 750-800-steam-650-1:2 sample
obtained the highest specific capacitance (288.52 F/g at 0.5 A/
g), which corresponds to an outstanding rate performance
(69.74% at 20 A/g). The optimal sample also exhibited
remarkable cycling stability (95.4% capacitance retention after
1000 cycles), which is superior to a commercial grade SC AC
(BPL). The excellent electrochemical performance of the
anthracite-derived material in 1 M H2SO4 is attributed to
reasonable PSD, high utilization of micropore structure, high
surface wettability, and high conductivity of the ABACs as well
as higher ion mobility and conductivity of the 1 M H2SO4
system. This study confirms the merit of the synthesis of
ABACs for utilization in SCs and provides useful guidance for
the future commercial exploration of anthracitic coal as SC
electrode material. This study also shows that not only the
electrode material is very important for the design of a SC but
a good choice of electrolytes also plays a vital role in the
development of SCs.
Table
7.
Specific
Capacitance
of
Six
Cases
of
ABACs
and
BPL
in
1
M
KOH,
6
M
KOH,
and
1
M
H
2
SO
4
sample
electrolytes
750-800-steam-650-1:1(F/g)
750-800-steam-650-1:2(F/g)
750-800-steam-750-1:1(F/g)
750-800-steam-750-1:2(F/g)
750-900-steam-750-1:2(F/g)
750-900-steam-750-1:2(F/g)
BPL
(F/g)
V
mic
/V
mes
2.55
3.25
1.80
2.22
1
0.44
0.61
V
mes
/V
total
0.09
0.08
0.1
0.09
0.34
0.38
0.36
1
M
KOH
194.54
244.25
168.81
208.59
203.57
160.77
142.84
6
M
KOH
202.24
260.30
187.78
220.72
230.81
166.74
148.82
1
M
H
2
SO
4
202.98
288.52
183.09
222.62
249.41
181.74
163.13
N

■ AUTHOR INFORMATION
Corresponding Authors
Jonas Baltrusaitis − Department of Chemical and
Biomolecular Engineering, Lehigh University, Bethlehem,
Pennsylvania 18015, United States; orcid.org/0000-
0001-5634-955X; Email: job314@lehigh.edu
Zheng Yao − Energy Research Center, Lehigh University,
Bethlehem, Pennsylvania 18015, United States;
Email: zhy4@lehigh.edu
Authors
Guanrong Song − Energy Research Center, Lehigh University,
Bethlehem, Pennsylvania 18015, United States
Carlos Romero − Energy Research Center, Lehigh University,
Bethlehem, Pennsylvania 18015, United States
Tom Lowe − Blaschak Anthracite Corporation, Mahanoy
City, Pennsylvania 17948, United States
Greg Driscoll − Blaschak Anthracite Corporation, Mahanoy
Boyd Kreglow − Blaschak Anthracite Corporation, Mahanoy
Harold Schobert − Blaschak Anthracite Corporation,
Mahanoy City, Pennsylvania 17948, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.energyfuels.2c03487
Notes
The authors declare the following competing financial
interest(s): Tom Lowe, Greg Driscoll, Boyd Kreglow are
employees of Blaschak Anthracite Corporation while Harold
Schobert is a paid consultant.
■ ACKNOWLEDGMENTS
This study was funded by the Pennsylvania Infrastructure
Technology Alliance (PITA) and Blaschak Anthracite Corp.,
with headquarters in Mahanoy City, Pennsylvania, USA.
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