Engineering Pore Nanostructure of Carbon Cathodes for Zinc Ion Hybrid Supercapacitors

Abstract

Zinc ion hybrid supercapacitors (ZIHCs) with both high power density and high energy density have tremendous potential for energy storage applications such as hybrid electric vehicles and renewable energy storage. However, the large radius of hydrated Zn²⁺ ions hampers their efficient storage in micropores with limited pore sizes, resulting in the limited gravimetric specific capacitance and inferior rate capability of ZIHCs. Therefore, it is critically important to understand to what extent pore size influences the storage of hydrated Zn²⁺ ions in the pores with limited sizes. Herein, porous carbon nanosheets with different pore architectures are prepared using an ammonium chloride molten salt carbonization strategy. The influence of pore size on hydrated Zn²⁺ ion storage in nanostructured carbon with divergent pore architectures is analyzed by electrochemical methods and molecular dynamic simulation. Micropores smaller than 6.0 Å obstruct the diffusion of hydrated Zn²⁺ ions, while micropores larger than 7.5 Å exhibit a low diffusion energy barrier for the hydrated Zn²⁺ ions. Mesopores improve capacitance and rate capability by exposing the electrochemically active sites and enhancing the diffusion of the hydrated Zn²⁺ ions.

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2209914 (1 of 13)
Engineering Pore Nanostructure of Carbon Cathodes for
Zinc Ion Hybrid Supercapacitors
Wenbin Jian, Wenli Zhang,* Xueer Wei, Bingchi Wu, Wanling Liang, Ying Wu, Jian Yin,
Ke Lu, Yanan Chen, Husam N. Alshareef,* and Xueqing Qiu*
Zinc ion hybrid supercapacitors (ZIHCs) with both high power density and
high energy density have tremendous potential for energy storage applica-
tions such as hybrid electric vehicles and renewable energy storage. How-
ever, the large radius of hydrated Zn2+ ions hampers their ecient storage
in micropores with limited pore sizes, resulting in the limited gravimetric
specific capacitance and inferior rate capability of ZIHCs. Therefore, it is criti-
cally important to understand to what extent pore size influences the storage
of hydrated Zn2+ ions in the pores with limited sizes. Herein, porous carbon
nanosheets with dierent pore architectures are prepared using an ammo-
nium chloride molten salt carbonization strategy. The influence of pore size
on hydrated Zn2+ ion storage in nanostructured carbon with divergent pore
architectures is analyzed by electrochemical methods and molecular dynamic
simulation. Micropores smaller than 6.0Å obstruct the diusion of hydrated
Zn2+ ions, while micropores larger than 7.5Å exhibit a low diusion energy
barrier for the hydrated Zn2+ ions. Mesopores improve capacitance and rate
capability by exposing the electrochemically active sites and enhancing the
diusion of the hydrated Zn2+ ions.
DOI: 10.1002/adfm.202209914
a new family of asymmetric superca-
pacitors, oer a voltage window of up to
1.8 V owing to their Zn anodes with low
redox potential (−0.76 V versus standard
hydrogen electrode).[1] Basically, ZIHCs
have a higher energy density than con-
ventional symmetric supercapacitors with
aqueous electrolytes.[2] Compared with
hydrated K+ and H+ ions in conventional
aqueous electrolytes, divalent Zn2+ ions
involve a two-electron charge storage pro-
cess,[3] which could eectively enhance
the gravimetric specific capacitance of
porous carbon electrodes.[4] Nevertheless,
Zn2+ ion presents as hydrated Zn(H2O)62+
ion in aqueous electrolytes. Hydrated
Zn(H2O)62+ ion has a large hydration size
of 5.5Å [5] and a large hydration enthalpy
of −2070kJmol−1,[6a] which leads to di-
culties in the desolvation of Zn(H2O)62+
ion and hinders its transportation in con-
fined nanopores. Therefore, the storage of
hydrated Zn(H2O)62+ ions is hampered in
porous carbons with micropores, which results in low specific
capacitance and inferior rate capability.[7]
To improve the specific capacitance and rate capability of
ZIHCs, researchers focus on reducing the particle size of
porous carbon[8] and building large mesopores.[9] Based on
ReseaRch aRticle
1. Introduction
The increased demand for electricity from sustainable energy
has boosted the development of supercapacitors and recharge-
able batteries. Zinc ion hybrid supercapacitors (ZIHCs), as
W. Jian, W. Zhang, X. Wei, B. Wu, W. Liang, Y. Wu, X. Qiu
Guangdong Provincial Key Laboratory of Plant Resources Biorefinery
School of Chemical Engineering and Light Industry
Guangdong University of Technology (GDUT)
100 Waihuan Xi Road, Panyu District, Guangzhou 510006, China
E-mail: wlzhang@gdut.edu.cn; cexqqiu@scut.edu.cn
W. Zhang
School of Advanced Manufacturing
Guangdong University of Technology (GDUT)
Jieyang 522000, China
J. Yin, H. N. Alshareef
Materials Science and Engineering
Physical Science and Engineering Division
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900, Saudi Arabia
E-mail: husam.alshareef@kaust.edu.sa
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202209914.
K. Lu
Institutes of Physical Science and Information Technology
Key Laboratory of Structure and Functional Regulation of Hybrid
Materials of Ministry of Education
Anhui Graphene Engineering Laboratory
Anhui University
Hefei, Anhui 230601, China
Y. Chen
School of Materials Science and Engineering
Key Laboratory of Advanced Ceramics and Machining Technology of
Ministry of Education
Tianjin Key Laboratory of Composite and Functional Materials
Tianjin University
Tianjin 300072, China
Adv. Funct. Mater. 2022, 2209914
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the relationship between the diusion path and diusion time
(l2 ∝ Dt, l is the diusion path, D is the diusion coecient,
and t is the diusion time).[10] Shortening the diusion path
could significantly reduce the diusion time, which improves
the rate capability.[11] Compared with bulk porous carbon,
porous carbon nanosheet (CNS) with limited thickness is con-
sidered an ideal porous carbon material to shorten the diu-
sion path of ions and improve the contact between electrolyte
and electrode.[12] Most importantly, the pore size in bulk porous
carbons is a critical factor that influences the diusion of
ions.[13] The diusion of Zn2+ ions ends up forming an electric
double--layer capacitor (EDLC) in the inner carbon walls of the
pores.[14] Therefore, the diusion of Zn2+ ions could be greatly
influenced by pore size.[4,15] It is of significance to investigate
the influence of pore si on the electrochemical performances of
porous carbons for storing Zn2+ ions.
Hydrated Zn(H2O)62+ ions with large sizes could only enter
the pores with limited pore size through a desolvation pro-
cess. Due to the large enthalpy of hydration of Zn(H2O)62+
ions, small pores could cause a large energy barrier for the
storage of Zn(H2O)62+ ions.[15] As a result, Zn(H2O)62+ ion dif-
fuses slowly inside small pores or is even blocked outside of
small pores (Figure1). Larger pores could eliminate the struc-
tural deformation of hydrated Zn(H2O)62+, thus enabling
the low energy barriers in adsorption and allowing fast diu-
sion of Zn(H2O)62+ ions in the pores (Figure 1).[4] Moreover,
the larger pores could maximize the exposure of active sites
and fully utilize the pseudocapacitance reaction enabled by
the specific adsorption of Zn2+ ions on the heteroatom active
sites.[16] A porous carbon with a mesoporosity of 51% (average
pore size of 4.2 nm) has 1.4 times higher capacitance than
a porous carbon with a mesoporosity of 7% (average pore
size of 2.7 nm).[17] It is revealed that large pores have a great
enhancement of both capacitance and rate capability of ZIHCs.
Liu etal.[18] reported that a porous carbon with large micropores
(0.8–2nm) and abundant mesopores could adequately expose
their surface functional groups, resulting in enhanced pseu-
docapacitance. Although porous carbons with large pore sizes
have improved rate capability and capacitance,[9b,13a,19] it is still
ambiguous how the pore size influences the storage of Zn2+
ions, and the pore size boundary that disables the storage
of Zn2+ ions.
Lignin promises to be a renewable precursor for preparing
cost-eective porous carbon materials due to its low cost and
abundant resources.[20] Sodium lignosulfonate (LS), with
abundant sulfonate groups, is dierent from alkaline lignin
and enzymatic hydrolysis lignin.[21] LS with sodium sulfonate
groups is obtained from the sulfite pulping process. The typ-
ical sulfonate group concentration is ≈3–4mmolg−1 in LS.[22]
Therefore, LS is soluble in water and could be easily mixed with
templating agents, activation agents, and dopants.[23] Moreover,
the sodium sulfonate group of LS performs the role of self-
templating for the pore formation during the high-temperature
carbonization process.[24] Since the LS-derived porous carbon
has abundant micropores,[25] we could use the obtained porous
carbons to investigate the pore size limitation for the electro-
chemical storage of Zn2+ ions. Here, we propose a molten salt
strategy of NH4Cl to tune the pore size and morphology of the
LS-derived CNSs by employing urea or thiourea as the poro-
gens and heteroatom doping agents. We studied the prepara-
tion process and determined the pore forming mechanism of
CNSs. The obtained CNSs with dierent pore architectures and
heteroatom doping chemistries were used to probe the under-
lying mechanism for the electrochemical storage of Zn2+ ions
in porous carbons. We demonstrate that the porous carbons
with more mesopores and micropores with pore size larger
than 0.75 nm could have higher capacitance and better rate
capability.
Adv. Funct. Mater. 2022, 2209914
Figure 1. Schematic illustration of the diusion of hydrated Zn(H2O)62+ in dierent pore sizes. Indigo and red spheres denote the Zn2+ ions and H2O
molecules.
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2. Results and Discussion
We prepared CNSs by carbonization of LS dispersed in NH4Cl
molten salt (Figure2). The preparation process of lignin-derived
CNSs (L-CNS), lignin-derived nitrogen-doped CNSs (L-N-CNS),
and lignin-derived nitrogen/sulfur co-doped CNSs (L-NS-CNS)
were achieved by using without urea/thiourea addition, and
urea or thiourea as nitrogen or nitrogen/sulfur doping agents
and porogens. Initially, LS was uniformly dispersed in NH4Cl
aqueous solution, followed by drying to obtain a mixture of
LS and NH4Cl. The mixture was heated up to 700 °C to pre-
pare CNSs in the molten salt of NH4Cl. Lastly, the CNSs were
obtained by washing o the inorganic salts.
The L-CNS, L-N-CNS, and L-NS-CNS are sheet-like porous
carbons observed by scanning electron microscopy (SEM)
(Figure3a–e; FigureS1a–f, Supporting Information). In con-
trast, LS-derived carbon showed bulk stone morphology
(Figure S1g–h, Supporting Information). It shows that CNSs
with sheet-like morphology could be prepared by the strategy of
high-temperature NH4Cl molten salt carbonization. We further
characterized the morphology of L-NS-CNS by transmission
electron microscopy (TEM). L-NS-CNS had irregular sheet-like
morphologies (Figure3f,g; FigureS2, Supporting Information).
The width and length of L-NS-CNS were in the micrometer
range (Figure3d,e). The wall thickness of L-NS-CNS is up to
7.5nm (Figure3g). The elemental analysis by energy-dispersive
X-ray spectroscopy shows that C, N, O, and S were uniformly
distributed in L-NS-CNS, indicating that N and S are success-
fully doped into the skeleton of L-NS-CNS (Figure3h–l, Sup-
porting Information).
X-ray diraction (XRD), Raman spectra, X-ray photoelectron
spectroscopy (XPS), and N2 adsorption/desorption measure-
ments were conducted to study the crystal structure, chemical
composition, and pore size distribution of L-CNS, L-N-CNS,
and L-NS-CNS. As confirmed by the broad XRD diraction
peaks, L-CNS, L-N-CNS, and L-NS-CNS were amorphous car-
bons (Figure4a). The ID/IG values of L-CNS, L-N-CNS, and
L-NS-CNS were 1.04, 1.02, and 1.11 calculated from Raman
spectra (Figure 4b). Therefore, it indicates that nitrogen and
sulfur co-doping could enhance the defect degree of CNS. The
N2 adsorption/desorption isotherms of L-N-CNS exhibited type
I isotherms (Figure 4c), indicating that the pore size of L-N-
CNS is dominated by micropores.[26] The N2 adsorption/des-
orption isotherms of L-NS-CNS exhibited a dramatic rise of N2
adsorption in the relative pressure ranges of 0-0.05 and 0.95-1.0
(Figure4c), indicating that L-NS-CNS contains a large number
of micropores and macropores.[27] The specific surface areas of
L-N-CNS and L-NS-CNS were 644 and 587m2g−1, respectively
(Table1). Although the specific surface areas of L-N-CNS and
L-NS-CNS were similar, their pore size and distribution dif-
fered (Figure4d). The microporous pore size of L-N-CNS was
mainly centered at 0.7nm, while the microporous pore size of
L-NS-CNS was mainly centered in the range between 0.5 and
0.65 nm (Figure4d). It could be concluded that the micropo-
rous pore size of CNS obtained by thiourea activation is
smaller than that achieved by urea activation. The mesoporous
volume and total pore volume of L-NS-CNS were 0.091 and
0.300 cm3g
−1 (Table 1), respectively, which were both larger
than those of L-N-CNS (mesoporous volume of 0.079 and total
pore volume of 0.267 cm3g
−1). The mesopore could improve
the ion diusion rate and expose the active site for electric
double layer capacitance (EDLC) and pseudocapacitance, which
could improve the rate capability of ZIHCs.[1c] Without urea or
thiourea added, the specific surface area of the obtained L-CNS
was as small as 41m2g−1 (Table1), which is in sharp contrast to
our LS-derived porous carbon under the same calcination tem-
perature of 700°C (LSC-700, 376.1m2g−1, Table1).[25] The large
specific surface area of LSC-700 results from the template role
of Na2SO4 generated in the calcination of LS. The Na2SO4 tem-
plating agent had a content of 18.4wt.%. Based on the Na2SO4
content, the content of the sulfonate group was calculated to be
2.592mmolg−1 in LS.[25] The specific surface area of L-CNS is
much lower than that of LSC-700, which indicates that the addi-
tion of NH4Cl leads to the elimination of the self-templating
Adv. Funct. Mater. 2022, 2209914
Figure 2. Schematic illustration of the preparation process of lignin-derived CNSs.
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role of Na2SO4. With this question, we need to investigate the
negative role of NH4Cl in reducing the specific surface area of
L-CNS. Furthermore, we deduced that the reason for the high
specific surface areas of L-N-CNS and L-NS-CNS is due to the
porogen role of urea and thiourea.[28] The chemical composi-
tions of L-CNS, L-N-CNS, and L-NS-CNS were characterized
by XPS. The nitrogen doping levels of L-CNS, L-N-CNS, and
L-NS-CNS were 2.3, 4.7, and 4.1at.%, respectively (Figure4e).
Even without the addition of nitrogen sources such as urea or
thiourea, NH3 produced during the decomposition of NH4Cl
could induce nitrogen doping,[29] resulting in a nitrogen doping
level of 2.3 at.% for L-CNS. L-CNS and L-N-CNS did not con-
tain sulfur, while L-NS-CNS contained trace amounts of sulfur
(0.17at.%). Sulfur elements should originate from the sulfonate
groups in LS. The sulfur doping level of LSC-700 was as high
as 3.61 at.%.[25] Therefore, it is deduced that during the calci-
nation process, the decomposition of NH4Cl produced a large
number of gaseous products, taking away the sulfur element
from LS and the CS2 gas[30] from the decomposition of thiourea,
resulting in the low sulfur doping in L-CNS, L-N-CNS, and
L-NS-CNS. The oxygen doping levels of L-CNS, L-N-CNS, and
L-NS-CNS were 13.5, 5.2, and 14.8at.%, respectively (Figure4e).
As shown by the C 1s high-resolution XPS spectra (Figure4f ),
the -COOH content of L-N-CNS was 2.62%, which was less
than that of L-NS-CNS (9.99%) (Table S3, Supporting Infor-
mation). In the O 1s high-resolution XPS spectra (FigureS3,
Supporting Information), the -COOH content of L-N-CNS and
L-NS-CNS was 1.42 and 7.35at.%, respectively, again indicating
the higher -COOH content of L-NS-CNS (TableS4, Supporting
Information). The possible reason is that urea reacts with the
carboxyl group during high calcination temperature,[31] which
causes the decreased oxygen doping of L-N-CNS.
To reveal the mechanism of the eect of NH4Cl and the
porogen role of urea and thiourea during the preparation of
L-CNS, L-N-CNS, and L-NS-CNS, XRD, and Fourier transform
infrared spectroscopy (FTIR) were performed on the sintered
products under dierent temperatures. The XRD patterns of
the LS sintered at 700°C (LSC-700-sintered products) are dis-
played in Figure5a. The LSC-700-sintered products present
Na2SO4 derived from sodium sulfonate, indicating that the
sodium sulfonate group of LS plays a porogen role as a self-
template at 700°C. As a result, LSC-700 had a specific surface
area of 376.1 m2g
−1. In sharp contrast, we could not detect
Na2SO4 in the sintering products of L-NS-CNS and L-CNS, but
NaCl inorganic salt was detected (Figure5b). It indicates that
during the high-temperature process, Cl− ions from NH4Cl
Adv. Funct. Mater. 2022, 2209914
Figure 3. Electron microscopy characterization of CNSs. SEM images of a) L-CNS, b) L-N-CNS, c–e) L-NS-CNS. f, g) TEM images, and h) high-angle
annular dark-field (scanning transmission electron microscopy) STEM image and i–l) corresponding (Energy Dispersive Spectroscopy) EDS elemental
mapping of L-NS-CNS.
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combine with Na+ ions from the sodium sulfonate group
forming NaCl crystals, while the remaining NH4+ and sulfonate
decompose, forming gaseous NH3, H2O, and SO2. Due to the
combination between Na+ and Cl− ions, the Na+ ions would not
have the role of porogens, which leads to the low specific sur-
face area of L-CNS (41m2g−1). Nevertheless, the origin of the
relatively high specific surface areas of L-N-CNS and L-NS-CNS
is still not clear. We further explored the pore-forming process
using multiple physicochemical techniques. The XRD pattern
of LS sintered at 550°C (LSC-550-sintered products) contained
crystal Na2SO4 (Figure5c), while L-NS-CNS-550-sintered prod-
ucts contained crystal NaCl, indicating that the NH4Cl has been
completely decomposed. Since there is no Na2SO4 left in the
L-NS-CNS-550-sintered products, Na2SO4 is not the porogen
for L-NS-CNS. Annealing of thiourea and urea at 550°C gener-
ates graphitic nitride (g-C3N4) which could be the porogen.[32]
The XRD pattern of L-NS-CNS-550 showed the presence of
g-C3N4 at high diraction angles (Figure5e). The appearance
of N-H (3352 cm−1),
CN
(1573 cm−1), and CN (1256 cm−1)
reveal the stretching peaks and bending peaks of the triazine
ring (808cm−1) of g-C3N4 for the calcination product of L-NS-
CNS-550 and thiourea-550,[33] whereas NH,
CN
, and CN
groups were not present in the LS-derived porous carbon under
a calcination temperature of 550 °C (LSC-550) (Figure 5f ).
Therefore, g-C3N4 plays the role of porogen in the preparation
of L-NS-CNS. The specific reasons for the porogens could be
the steric eect of g-C3N4 and the etching role of the released
gases products during pyrolysis.[34] We analyzed the decompo-
sition behavior of L-NS-CNS precursors by thermogravimetric
analysis (TGA) (Figure S5, Supporting Information). NH4Cl
and thiourea experienced intense decomposition reactions
between 210 and 370°C, yielding NH3, HCl, SO2, and H2O and
other gases. Under temperatures higher than 550 °C, g-C3N4
started to decompose forming pores.
Adv. Funct. Mater. 2022, 2209914
Figure 4. a) XRD patterns, b) Raman spectra, c) N2 adsorption/desorption isotherms, d) pore size distributions, e) XPS survey spectra for L-CNS,
L-N-CNS, and L-NS-CNS, respectively. f) The C 1s high-resolution XPS spectrum with fitting results for L-N-CNS and L-NS-CNS.
Table 1. The specific surface area and pore structure parameters of L-CNS, L-N-CNS, L-NS-CNS, LSC-700, and NLPC800, respectively.
Sample SSA Vtotal Vmeso Vmicro Vmicro/Vtotal
[m2g−1] [cm3g−1] [cm3g−1] [cm3g−1]—
L-CNS 41.22 0.0212 0.0103 0.0109 0.514
L-N-CNS 643.9 0.267 0.079 0.188 0.704
L-NS-CNS 587.1 0.300 0.091 0.209 0.697
LSC-700[25] 376.1 0.207 0.021 0.186 0.899
NLPC800[2] 548.8 0.290 0.01 0.28 0.966
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Here, we summarized the structural evolution of L-NS-CNS
(Figure 5g–i). Initially, LS, NH4Cl, and thiourea were homo-
geneously mixed into a mixed solid (Figure5g; FigureS4a–c,
Supporting Information). The thermogravimetric analysis was
used to investigate the decomposition behavior (FigureS5, Sup-
porting Information). During the mixing process, LS molecules
were uniformly dispersed in the aqueous media of NH4Cl and
thiourea. Next, NH4Cl formed a molten salt at a temperature
of 338°C,[35] and LS was carbonized to CNS. Then during the
temperature rising to 550 °C, NH4Cl, the sodium sulfonate
group in LS and thiourea decomposed to produce NH3, SO2,
H2O, HCl, and CS2 among other gases.[34a] Simultaneously, Cl−
ions from NH4Cl were combined with Na+ ions in LS, forming
NaCl crystals. The LS molecules were carbonized to CNS, while
the thiourea decomposed to yield g-C3N4 and gases products
(Figure5h; FigureS4d–f, Supporting Information). In the tem-
perature range of 550-700°C, g-C3N4 was decomposed, causing
pore formation in CNS (Figure 5i; Figure S4g–i, Supporting
Information).[28a] NH4Cl does not play the role of porogen but
plays a dispersing and sacrificial template role for the forma-
tion of nanosheet morphology. In the meantime, urea and thio-
urea enable nitrogen and sulfur doping in CNS.
The electrochemical properties of ZIHCs of L-CNS, L-N-
CNS, and L-NS-CNS were measured by cyclic voltammetry
(CV) and galvanostatic charge-discharge (GCD). Figure6a
shows the CV curves of L-CNS, L-N-CNS, and L-NS-CNS.
Their specific capacitances were 76.5, 155.6, and 179.6Fg−1 at
the scan rate of 1mVs−1. The CV curves of L-NS-CNS at dif-
ferent scan rates are shown in Figure6b. The CV curve shape
is quasi-rectangular, indicating the behavior of a quasi-ideal
capacitor. The capacitances of L-CNS, L-N-CNS, and L-NS-
CNS at 50 mV s−1 are 16.6, 74.6, and 83.8 F g−1, respectively
(Figure6c). The capacitance retention at 50mV s−1 compared
to that at 1mVs−1 are 21.7%, 47.9%, and 46.7% for L-CNS, L-N-
CNS, and L-NS-CNS, respectively. The GCD curves of L-CNS,
L-N-CNS, and L-NS-CNS showed the shape of symmetric
Adv. Funct. Mater. 2022, 2209914
Figure 5. XRD patterns of a) LSC-700-sintered products, b) L-NS-CNS-sintered products and L-CNS-sintered products, c) LSC-550-sintered products,
d) L-NS-CNS-550-sintered products, e) Thiourea-550, L-NS-CNS-550, and L-NS-CNS-700, respectively. f) FTIR of LSC-550, L-NS-CNS-550, and thio-
urea-550, respectively. g) Formation mechanism of the nanosheet morphology and pore architecture in the LS-derived CNSs.
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triangles, which are typical capacitive behavior, and the spe-
cific capacitances were 86.2, 216.5, and 233.4F g−1 at 0.1A g−1
(Figure6d). The GCD curves of L-NS-CNS at dierent current
densities are shown in figure 6e. The capacitances of L-CNS,
L-N-CNS, and L-NS-CNS were 32.5, 143.4, and 165.4 F g−1 at
1Ag−1 and are 1.6, 89.1, and 110.3Fg−1 at 10Ag−1 (Figure6f ).
The capacitance retention ratios of L-CNS, L-N-CNS, and L-NS-
CNS at 10Ag−1 compared to that at 0.1Ag−1 were 1.9%, 41.2%,
and 47.2%, respectively. From the capacitance results of CV
and GCD, L-N-CNS and L-NS-CNS showed better rate capa-
bility than L-CNS, revealing that pore-rich structure improves
capacitance and rate capability. Although L-CNS presents a
nanosheet morphology, its capacitance and rate capability are
deteriorated due to its lack of rich pore structure. The normal-
ized areal capacitances of L-CNS, L-N-CNS, and L-NS-CNS at a
current density of 0.1Ag−1 were 209.1, 33.6, and 39.8µFcm−2,
respectively (Table S5, Supporting Information), which are
higher than porous carbon obtained by supramolecular car-
bonization (20.0 µF cm−2)[18] and porous carbon prepared by
ammonia activation (13.0 µF cm−2).[16a] The ultra-high areal
capacitance of L-CNS (209.1µFcm−2) indicates that the outer
surface area of nanosheet morphology could significantly
enhance the pseudocapacitance. From the other point of view,
the areal capacitance of L-N-CNS (33.6µFcm−2) and L-NS-CNS
(39.8µFcm−2) could be limited by their rich micropores. The
normalized areal surface capacitance of the EDLC is normally
regarded in the range of 10-25 µF cm−2.[36] A small areal sur-
face capacitance of L-N-CNS and L-NS-CNS indicates that some
small pores could not contribute to the construction of EDLC
since porous carbon usually possesses pseudocapacitance. The
above results also suggest that L-CNS, L-N-CNS, and L-NS-CNS
exhibit a large portion of pseudocapacitance. This reveals that
Adv. Funct. Mater. 2022, 2209914
Figure 6. Electrochemical performances of L-CNS, L-N-CNS and L-NS-CNS as the cathodes of ZIHCs. a) CV curves at a scan rate of 1mVs−1.(b) CV
curves of L-NS-CNS at dierent scan rates. c) Specific capacitances at scan rates ranging from 1 to 200mVs−1. d) GCD curves at a current density of
0.1Ag−1. e) GCD curves of L-NS-CNS at dierent current densities. f) Specific capacitances of the porous carbon samples at current densities ranging
from 0.1 to 50Ag−1. Capacitive contribution calculated at a scan rate of 2mVs−1 for g) L-CNS, h) L-N-CNS, and i) L-NS-CNS, respectively.
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L-CNS possesses a very high pseudocapacitance at 0.1 A g−1.
We believe that the low capacitive contribution of CNS cath-
odes could result from limited EDLC constructed in small
pores and the high portion of pseudocapacitance. The capaci-
tive contribution from the fast-kinetics processes of L-CNS,
L-N-CNS, and L-NS-CNS were 31.9%, 77.7%, and 81.3%, respec-
tively (Figure 6g–i). Similarly, it is demonstrated that L-CNS
is more controlled by slow diusion due to its high portion of
pseudocapacitance. Therefore, the rate capability of L-CNS is
worse than that of L-N-CNS and L-NS-CNS. The energy den-
sity of 91.0 Wh kg−1 with a correspondingly power density of
0.094kWkg−1 could be achieved at a current density of 0.1Ag−1
for L-NS-CNS. At a current density of 10 A g−1, L-NS-CNS
exhibits an energy density of 33.8 Wh kg−1 and a power den-
sity of 9.9 kW kg−1. The energy density and power density of
L-NS-CNS have been advantageous compared to the previously
studied ZIHCs (FigureS6, Supporting Information).[1a,2,9a,16a,37]
L-NS-CNS maintained a capacitance of 94.2% after 18000 cycles
(Figure S7, Supporting Information), indicating the excellent
cycling stability of L-NS-CNS.
To compare the influence of pore size on the capacitance and
rate capability, we chose samples of porous carbons obtained
from the ammonia activation of enzymatic hydrolysis lignin at
800°C for 2h (NLPC800) in our previous work[2] as a control
sample to compare with L-N-CNS and L-NS-CNS. The specific
surface area of NLPC800 is 548.8 m2g
−1, which is similar to
that of L-N-CNS (643.9 m2g
−1) and L-NS-CNS (587.1 m2g
−1)
(Table1). The doping levels of oxygen and nitrogen of NLPC800
were 11.60 and 2.72at.%, which are also very close to L-NS-CNS
(Table S1, Supporting Information). Therefore, we could ana-
lyze the electrochemical performances of these porous carbon
materials based on the understanding of pore size distribution.
The gravimetric specific capacitances of NLPC800 was
93.4F g−1 at a current density of 0.1A g−1, and the capacitance
retention compared to the capacitance at 0.1Ag−1 was 1.7% at
10 A g−1 (Figure7a). L-CNS and NLPC800 exhibited similar
capacitance retentions and specific capacitance. The ultra-low
capacitance retentions of L-CNS and NLPC800 result from dif-
ferent origins. The low capacitance of L-CNS is due to its low
specific surface area (41m2g−1), while the low specific capaci-
tance of NLPC800 should be ascribed to the limited pore size,
which disables the construction of EDLC or pseudocapacitance.
The NLPC800, L-N-CNS, and L-NS-CNS had similar specific
surface areas, but the specific capacitance and rate capability
of NLPC800 are much inferior to those of L-N-CNS and L-NS-
CNS. Therefore, we deduce that pore size has a dramatic eect
on the electrochemical performances of ZIHCs. The rate capa-
bility of ZIHCs greatly depends on the diusion of hydrated
Zn(H2O)62+ ions in the nanopores. We analyzed the ion dif-
fusion coecient of Zn2+ ions in NLPC800, L-N-CNS, and
L-NS-CNS by electrochemical impedance spectroscopy (EIS)
technique. If the EIS was tested with a two-electrode configura-
tion, the Nyquist plots were greatly influenced by the Zn anode
(FigureS8, Supporting Information) due to the high impedance
Adv. Funct. Mater. 2022, 2209914
Figure 7. a) Specific capacitances at current densities ranging from 0.1 to 50Ag−1, b) Nyquist plots of ZIHCs by three-electrode configuration for L-CNS,
L-N-CNS, L-NS-CNS and NLPC800, respectively. c) Linear plots of real resistances (Z’) against angular frequencies (ω−1/2) in the low-frequency region
for L-N-CNS, L-NS-CNS, and NLPC800, respectively. d) Ion diusion coecients of L-N-CNS, L-NS-CNS, and NLPC800.
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of the passivation layers of Zn anode. Hence, we tested the EIS
of porous carbon electrodes using a three-electrode configura-
tion to exclude the influences of the Zn anodes. NLPC800 had a
less vertical Warburg region than L-N-CNS and L-NS-CNS, fol-
lowed by L-N-CNS (Figure 7b). It indicates that the diusion
of hydrated Zn(H2O)62+ ions in the pores of NLPC800 is slug-
gish, while the diusion in L-NS-CNS is faster. The diusion
coecient of ions in NLPC800, L-N-CNS and L-NS-CNS thus
could be calculated by the EIS technique (Figure7c, calculation
methods are shown in Supporting Information).[4] The diu-
sion coecients of L-N-CNS, L-NS-CNS, and NLPC800 were
8.4×10−9, 3.5×10−11 and 2.7×10−12m2s−1 (Figure7d; TableS6,
Supporting Information). It also suggests that the ion diu-
sion rate of NLPC800 is the slowest, while the L-NS-CNS has
the fastest ion diusion rate, which could be due to the limited
pore size in NLPC800.
NLPC800, L-N-CNS, and L-NS-CNS had similar specific sur-
face areas. However, NLPC800, L-N-CNS, and L-NS-CNS had
significant dierences in specific capacitance and rate capa-
bility. The remarkable dierence in the performance of ZIHCs
should be attributed to the dierent pore size distribution of
NLP800, L-N-CNS, and L-NS-CNS. Therefore, we revisited the
pore size distributions of NLPC800, L-N-CNS, and L-NS-CNS
and further analyzed the correlations between pore architec-
ture and electrochemical performances. The microporosity
of NLPC800 is 96.6%, and the pores were mainly centered at
0.6 nm (Table 1; Figure8a) with other pores mainly centered
below 2.0 nm. The micropores of L-N-CNS were centered at
0.75 nm with large micropores and mesopores located in the
range of 1–2.75nm (Figure8a,b). Besides, L-NS-CNS contained
mesopores with pore sizes of 7–50nm, which was not found in
NLPC800 and L-N-CNS (Figure8c). Furthermore, the mesopore
volumes of NLPC800, L-N-CNS and L-NS-CNS were 0.01, 0.079,
and 0.091cm3g−1, respectively (Table1). The inferior rate capa-
bility and lower specific capacitance of NLPC800 should be due
to the large portion of the small micropores with the size of
0.6nm. Since the hydrated Zn(H2O)62+ ions have a large size
of 5.5Å,[5] which could result in the diculty of the diusion of
hydrated Zn(H2O)62+ ions in pores with a size of 6Å. Therefore,
NLPC800 had an inferior rate capability and low specific capaci-
tance. By contrast, L-N-CNS had a large micropore pore size of
0.75nm. The diusion and storage of hydrated Zn(H2O)62+ ions
within the 0.75nm pore might not be impeded. Therefore, L-N-
CNS had high specific capacitance and decent rate capability.
The L-NS-CNS had a better rate capability and faster ion diu-
sion rate than L-N-CNS, even though it had some micropores
with small sizes of 0.55 and 0.65nm. It is due to the fact that
L-NS-CNS had a high mesopore volume of 0.091cm3g−1 and
large mesopores with sizes of 7–50nm. The large mesopores
in L-NS-CNS could enable the ecient building of interface
capacitance and diusion of ions.[18] Therefore, L-NS-CNS is
superior to NLPC800 and L-N-CNS in terms of capacitance and
rate capability. To demonstrate our assumption, here we also
chose LSC-700 as a control sample that had small micropores
with a size of 0.6nm. The mesoporous volume of LSC-700 was
0.021 cm3g
−1, which was slightly larger than NLPC800 and
was more than twice the mesoporous of NLPC800 (Table 1;
Figure S9b–d, Supporting Information). Therefore, LSC-700
had a better rate capability compared with NLPC800. At a cur-
rent density of 0.1 A g−1, LSC-700 exhibited a specific capaci-
tance of 129 F g−1 and a relatively high capacitance retention
ratio of 22.5% at 10Ag−1 (FigureS9a, Supporting Information).
It indicates that the mesopore is beneficial for improving spe-
cific capacitance and rate capability.
The specific capacitance and rate capability of NLPC800 and
L-N-CNS have significant divergence. This could be attributed
to the dierent energy required in the diusion of hydrated
Zn(H2O)62+ ions in the limited pores with sizes of 6 and 7.5Å.
We used double-layer graphene to simulate the slit pores in
porous carbon materials. We thus analyzed the energy required
for the storage of hydrated Zn(H2O)62+ ions in the slit pores
with sizes of 6 and 7.5Å by molecular dynamics simulations.
Due to the size of the SO42− molecule of 5.53 Å (FigureS10,
Supporting Information), which is comparable to the size of
hydrated Zn(H2O)62+ (5.5 Å),[5] we only calculate the interca-
lation energy of hydrated Zn(H2O)62+ in the pore when con-
ducting molecular dynamics simulations. Negative intercalation
energy suggests the process is favorable, while positive energy
suggests the process is unfavorable. From Figure9a,b, when
the hydrated Zn(H2O)62+ ions were stored in the slit pores with
sizes of 6 Å, the symmetry of hydrated Zn(H2O)62+ ions was
distorted and the adsorption energy barriers were positive for
the distance from 0 to 9Å. The adsorption energy of hydrated
Zn(H2O)62+ ions at a distance of 0 Å from the 6 Å pore was
0.56eV. It indicates that hydrated Zn(H2O)62+ ions are dicult
to diuse and be adsorbed in the micropores with a size of 6Å.
In contrast, the adsorption energy barrier was negative
throughout the adsorption process of Zn(H2O)62+ with dierent
Figure 8. a,b,c) Pore size distributions for L-N-CNS, L-NS-CNS and NLPC800, respectively.
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distances in the slit pores with a size of 7.5Å, and the adsorp-
tion energy of hydrated Zn(H2O)62+ ions at a distance of 0 Å
from the 7.5Å pore was −0.61eV, indicating that the storage
of Zn(H2O)62+ within pores with a size of 7.5 Å is favorable
(Figure9b). Therefore, we could conclude that the lower spe-
cific capacitance and inferior rate capability of the NPLC800
are due to the high energy barrier required for the diusion of
Zn(H2O)62+ in small micropores of 6Å. The micropores above
7.5Å could reduce the interaction energy barrier during adsorp-
tion and be conducive to storing hydrated Zn(H2O)62+ ions.
The capacitive contribution from the fast-kinetics processes
of NLPC800 and L-N-CNS were 33.5% and 77.7% (FigureS11,
Supporting Information), which results from the slow diu-
sion of hydrated Zn(H2O)62+ ions in pores with a size of 6Å.
Electrochemical kinetics and molecular dynamics simulations
verify that hydrated Zn(H2O)62+ ions have a slow diusion rate
in the pores with a size of 6Å, while the diusion of hydrated
Zn(H2O)62+ ions is favorable in the pore with a size of 7.5Å.
L-NS-CNS had a smaller specific surface area than L-N-CNS,
but the specific capacitance of L-NS-CNS was higher. It is attri-
buted that L-NS-CNS (14.77 at.%) contains more oxygen atoms
than L-N-CNS (5.16 at.%) (Table S1, Supporting Information),
and certain oxygen functional groups could oer pseudoca-
pacitance.[38] Therefore, to understand the pseudocapacitive
reactions during charge storage of L-NS-CNS, we further inves-
tigated the composition changes of L-NS-CNS surface during
charge and discharge (Figure10a) by employing ex situ XRD
and XPS characterizations. In Figure10b, the XRD diraction
peaks with stars are the diraction peaks of the current collector
(FigureS12, Supporting Information), and the rest of the dirac-
tion peaks are the diraction peaks of Zn4SO4(OH)6•5H2O. The
reversible generation and disappearance of Zn4SO4(OH)6•5H2O
could be observed from the ex situ XRD (Figure10b). During
discharge, H+ is adsorbed in the reaction of pseudocapacitance,
so local alkaline conditions occur, which results in the reaction
of Zn2+ with OH− forming Zn4SO4(OH)6•5H2O. During charge,
H+ is desorbed from the L-NS-CNS surface. The reaction of H+
with Zn4SO4(OH)6•5H2O occurs, leading to the disappearance
of Zn4SO4(OH)6•5H2O at 1.8 V. The variation in the amount
of CO,
CO
, and COZn could be observed from the ex
situ XPS (Figure10c–h). The relative contents of C-O-Zn and
C-O of L-NS-CNS at 0.2 V were 8.34% and 30.91%, respec-
tively, which were higher than those of C-O-Zn (4.14%) and C-O
(9.82%) when L-NS-CNS was charged to 1.8 V (Figure 10c,d
and TableS7, Supporting Information). The relative content of
CO
at 0.2 V is 6.21%, which was less than that of
CO
at
1.8V (16.24%) (Figure10c,d; TableS7, Supporting Information).
The relative contents of COZn, CO and
CO
of L-NS-
CNS were 6.71%, 22.06%, and 12.66% when discharged to 0.6V
(Figure 10e; Table S7, Supporting Information). In the high-
resolution XPS spectrum of O 1s, the
CO
content of L-NS-
CNS at 0.2, 1.8, and 0.6 V were 14.88%, 24.47%, and 17.87%,
respectively (Figure10f–h). During the discharge process, the
amount of C-O and C-O-Zn increased, while the amount of
Figure 9. a) Schematic representation of Zn(H2O)62+ diused in the slit pores with a size of 6 and 7.5Å, b) the diusion energy barriers of hydrated
Zn(H2O)62+ ions as a function of the distance from the center of the slit pores.
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CO
decreased, and the opposite was observed for the charge
process. It indicates that C-OH and C-O-Zn on the L-NS-CNS
surface electrochemically desorbed H+ and Zn2+ to
CO
during
the charge process from 0.2 V to 1.8 V, while
CO
electro-
chemically adsorbed H+ and Zn2+ forming C-OH and C-O-Zn
from 1.8 V to 0.6 V. The L-NS-CNS involves SO42−, H+, and
Zn2+ adsorption and desorption during the charge/discharge
process. From the above results, the charge storage process of
ZIHCs with L-NS-CNS//Zn could be described as follows:[38,39]
physical adsorption/desorption
C|
|SO2
eC
O
4
discharge
charge 4
2
S+→
← +
−−
(1)
C2eC
||
2discharge
charge
Zn Zn++
→
←
+− (2)
chemical adsorption/desorption
CO
He COH
discharge
charge
=+ +→
← −
+− (3)
CO eC
O
discharge
charge
Zn Zn=+ +→
← −−
+− (4)
precipitation/dissolution
4Z
nSO6OH 5H
OO
H5HO
4
2
2
discharge
charge 44 62
Zn SO
()
++ +→
← ×
+− − (5)
The
CO
and COH contents of L-NS-CNS were 2.19 and
5.26 at.%, respectively, which are more than those of L-N-CNS
(
CO
: 1.73 at.% and C-OH: 2.06 at.%) (Table S4, Supporting
Information). The
CO
and COH oxygen functional groups
could enable pseudocapacitance. Therefore, L-NS-CNS had a
Figure 10. a) GCD curve of Zn//L-NS-CNS ZIHCs at 0.1Ag−1 and b) ex situ XRD patterns of the corresponding potential of L-NS-CNS cathode. Ex situ
(c, d, e) C 1s and (f, g, h) O 1s high-resolution XPS spectrum of L-NS-CNS at c,f) 0.2V, d,g) 1.8V, and e,h) 0.6V.
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higher specific capacitance, even though the specific surface
areas of L-NS-CNS were smaller than L-N-CNS. The
CO
and
COH contents of NLPC800 were 1.17 and 3.93 at.% (TableS4,
Supporting Information), but the small pore size prevents the
oxygen functional group from adsorbing cations, thus leading
to decreased active sites. The mesopores could expose more
active sites and fully utilize the pseudocapacitance.
3. Conclusions
In summary, we prepared lignin-derived carbon nanostructures
by an NH4Cl molten salt carbonization strategy and investi-
gated the role of NH4Cl, urea, and thiourea. NH4Cl turns to the
molten state during the calcination process, which converts LS
into porous carbon nanostructure. The Cl− of NH4Cl binds with
the Na+ ions from LS, forming NaCl, resulting in the loss of
the self-templating role of Na2SO4. The urea and thiourea are
demonstrated to play the role of nitrogen doping agents and
porogens. By experimentally analyzing the electrochemical
properties of the obtained porous carbon with dierent pore
sizes and their electrochemical performances, we demon-
strate here that porous carbons having higher proportions of
mesopores and micropores with pore sizes larger than 0.75nm
can improve the capacitance and rate capability of ZIHCs.
4. Experimental Section
Materials: Sodium lignosulfonate (LS), as the carbon source of lignin-
derived porous carbon nanosheets, was purchased from Chempack Co.,
Ltd of Russia. The LS contains 2.592 mmol g−1 of sulfonate group.[25]
The sulfonate group content of LS was measured by burning method.
A certain mass of LS was burned in a mue furnace at 800 °C. The
sulfonate content was calculated by weighing the Na2SO4 product of LS
after calcination. Ammonium chloride (NH4Cl), urea and thiourea were
purchased from Aladdin. Hydrochloric acid (HCl) was purchased from
Guangzhou Chemical Reagent Factory of China. Polytetrafluoroethylene
(PTFE) emulsion was purchased from Guangdong Canrd New Energy
Technology Co., Ltd. All other chemical reagents were of analytical grade
and used without further purification.
Preparation of Carbon Nanosheets: 5g NH4Cl and 1 g LS were mixed
and dissolved in 20mL deionized water. The above solution was heated
at 80°C and stirred until all the water was evaporated to obtain a solid
mixture. Afterward, the solid mixture was calcined at 700°C in a tubular
furnace with a constant N2 flow (60 sccm) under a heat ramping rate
of 10°Cmin−1 for 1h. The sintered products were soaked in 6wt% HCl
solution to remove impurities. The obtained porous carbons were filtered
with deionized water to a pH value close to 7 and dried at 90°C for 12h.
The as-prepared porous carbon was called lignin-derived porous carbon
nanosheets (denoted as L-CNS). The recipes for lignin-derived N-doped
porous carbon nanosheets (denoted as L-N-CNS) was a mixture of 5g
NH4Cl, 1 g LS and 5 g urea, for lignin-derived N, S-co-doped porous
carbon nanosheets (denoted as L-NS-CNS) was a mixture of 5g NH4Cl,
1g LS and 5g thiourea. All the calcination protocols were the same for
L-CNS, L-N-CNS, and L-NS-CNS. The rest of the preparation process
that follows was consistent with those for L-CNS.
Material Characterization: The morphologies and microstructures
of the samples were investigated by a SU8220 scanning electron
microscope (SEM, Hitachi, Japan) and a Talos F200S transmission
electron microscopy (TEM, Thermo Fisher Scientific, USA, equipped
with an energy dispersive spectroscopy). X-ray diraction patterns were
collected on a D8 Advance X-ray diractometer (XRD, Bruker, Germany)
with a Cu Kα radiation (λ= 1.5406 Å). The degrees of graphitization
of the samples were characterized by LabRAM HR Evolution micro
confocal Raman spectrometer (HORIBA Jobin Yvon, France). The
specific surface areas and pore diameters were measured at 77 K
using ASAP 2460 N2 adsorption-desorption (Micromeritics, USA). The
elemental compositions of the samples were analyzed by Escalab 250Xi
X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientific, USA).
IS50R Fourier transform infrared spectroscopy (FTIR, Thermo Fisher
Scientific, USA) was performed using the KBr pallet method. TGA
4000 thermogravimetric analysis (TGA, PE, Holland) was employed to
analyze the pyrolytic behavior of porous carbon precursors. Ex situ XRD
was performed using a D8 Advance X-ray diractometer with a Cu Kα
radiation (λ= 1.5406 Å) and the samples were charged or discharged to
designated potentials. Ex situ XPS were collected on an Escalab 250Xi
X-ray Photoelectron Spectroscopy. Electrode samples both ex situ XRD
and XPS tests were collected at a current density of 0.1Ag−1.
Preparation of CNS Electrode: The CNS electrodes were prepared by
mixing the samples with carbon black and PTFE with a weight ratio of
8:1:1 in ethanol, respectively. The mixture was stirred and dried at 60°C.
The dried mixture was rolled into a free-standing film and dried at 90°C
for 5h. Then, the film was cut into electrode sheets with geometry sizes
of 0.8×0.8 cm2. The electrode sheet was pressed onto stainless steel
with a pressure of 10MPa to fabricate the electrode. The loading mass
of each electrode was ≈2.5 to 3.0mgcm−2.
Electrochemical Cells for Zinc Ion Hybrid Supercapacitors (ZIHCs)
Devices: ZIHCs devices: a self-made supercapacitor cells were used
for assembly ZIHCs in air with CNS electrode as a cathode, Zn foil
(1 × 1 cm−2) as the anode, 1 mol L−1 ZnSO4 aqueous as electrolyte
solution and glass microfiber (Whatman) as the separator. (Zn//CNS
ZIHCs).
Electrochemical Measurements: All cyclic voltammetry (CV),
galvanostatic charge–discharge (GCD) and electrochemical impedance
spectra (EIS) measurements were conducted on an electrochemical
workstation (VMP3E, Biologic, France). The GCD cycling test was
performed on a battery tester system (BTS4008-5V-20 mA, Neware,
China). The EIS spectra were measured in the frequency range of
1000kHz to 10mHz with a voltage amplitude of 10mV. The GCD and
CV tests were carried out between 0.2 and 1.8V.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors acknowledge the financial support from the National Natural
Science Foundation of China (22108044), the Research and Development
Program in Key Fields of Guangdong Province (2020B1111380002),
the Basic Research and Applicable Basic Research in Guangzhou City
(202201010290), the financial support from the Guangdong Provincial
Key Laboratory of Plant Resources Biorefinery (2021GDKLPRB07) and
the Special Funds for the Cultivation of Guangdong College Students’
Scientific and Technological Innovation (No. pdjh2022b0165). Research
reported in this publication was also supported by King Abdullah
University of Science & Technology (KAUST). The authors acknowledge
Jinxin Lin for her DFT simulation of sulfate ions.
Conflict of Interest
The authors declare no conflict of interest.
16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202209914 by Guangdong University Of Technology, Wiley Online Library on [21/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

www.afm-journal.dewww.advancedsciencenews.com
2209914 (13 of 13) © 2022 Wiley-VCH GmbH
Adv. Funct. Mater. 2022, 2209914
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
mesopores, micropores, porous carbons, rate capability, zinc ion hybrid
supercapacitors
Received: August 27, 2022
Revised: September 21, 2022
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