High-Performance Asymmetric Supercapacitors Based on Monodisperse CuO@C Polyhedron Nanocomposites

Article abstract of DOI:10.1002/bkcs.12263

Herein, CuO nanocrystals spatially embedded inside carbon polyhedron (CuO@C) derived via morphology-preserved transformation of metal–organic frameworks (MOFs) are utilized for high-performance asymmetric supercapacitors (SCs). Using a conventional MOF (several micrometers in size), pore-filling with polymer inside MOF (polymer@MOF) via vapor-phase polymerization (VPP) process was achieved that amount of polymer used for VPP can be readily adjusted to control the carbon content of CuO@C after thermolysis and subsequent oxidation processes. When monodisperse and nano-sized MOF is used for CuO@C (denoted as nCuO@C_1), it presents superior electrochemical performance because monodispersity and smaller size reduce interfacial resistance and promote mass-transport property, respectively. Asymmetric SC of nCuO@C_1 with carbon sphere (CS) as a counter electrode presents excellent energy density of 55.47 Wh/kg and long-term stability of 88.7% at 5000 cycles, comparable to the best MO-based asymmetric SCs derived from MOFs.

Full text of DOI:10.1002/bkcs.12263

Article
DOI: 10.1002/bkcs.12263
BULLETIN OF THE
H. S. Kim et al.
KOREAN CHEMICAL SOCIETY
High-Performance Asymmetric Supercapacitors Based on
Monodisperse CuO@C Polyhedron Nanocomposites
Hee Soo Kim,†,† Min Seok Kang,†,† Incheol Heo,‡,† and Won Cheol Yoo‡,§,
*
†Department of Applied Chemistry, Hanyang University, Ansan 15588, Republic of Korea
‡Department of Applied Chemistry, Center for Bionano Intelligence Education and Research, Hanyang
University, Ansan 15588, Republic of Korea
§Department of Chemical and Molecular Engineering, Hanyang University, Ansan 15588, Republic of
Korea. *E-mail: wcyoo@hanyang.ac.kr
Received February 5, 2021, Accepted February 25, 2021, Published online March 11, 2021
Herein, CuO nanocrystals spatially embedded inside carbon polyhedron (CuO@C) derived via
morphology-preserved transformation of metal–organic frameworks (MOFs) are utilized for high-
performance asymmetric supercapacitors (SCs). Using a conventional MOF (several micrometers in size),
pore-filling with polymer inside MOF (polymer@MOF) via vapor-phase polymerization (VPP) process
was achieved that amount of polymer used for VPP can be readily adjusted to control the carbon content
of CuO@C after thermolysis and subsequent oxidation processes. When monodisperse and nano-sized
MOF is used for CuO@C (denoted as nCuO@C_1), it presents superior electrochemical performance
because monodispersity and smaller size reduce interfacial resistance and promote mass-transport prop-
erty, respectively. Asymmetric SC of nCuO@C_1 with carbon sphere (CS) as a counter electrode presents
excellent energy density of 55.47 Wh/kg and long-term stability of 88.7% at 5000 cycles, comparable to
the best MO-based asymmetric SCs derived from MOFs.
Keywords: Asymmetric supercapacitor, Pseudocapacitor, MOF-derived metal oxide@carbon, Reduced
interfacial resistance, Promoted mass-transport property
Introduction
drawbacks of MOs in psuedocapacitors, however, of which
is intrinsically low electrical conductivity resulting in poor
rate performance and long-term stability.5 Thus hybrid
pseudocapacitors along with carboncaceous materials
such as graphene/graphene oxide6,7 and carbon nanotube
(CNT)8,9 has widely been employed for boosting-up elec-
trochemical properties of pseudocapacitors.
The rapid development of energy consumption due to fast
industrialization and population growth has made high-
performance sustainable energy storage and conversion
devices more important. One of the most well-known
energy storage systems, SCs have attracted significant
attention owing to outstanding power density (~ 10 K W/
kg), very short charging time (on the order of tens of sec-
onds), and ultrahigh long-term stability (~10 K cycles) as
compared to lithium-ion batteries (LIBs).1–4 However, infe-
rior energy density (~10 Wh/kg) of SCs compared to LIBs
has hampered widespread utilization.
There are two different mechanisms to store energy in
SCs: one is electrical double layer capacitor (EDLC) that
stores ions on the oppositely charged surface and the other
is pseudocapacitor that stores energy via Faradaic reactions
on the surface.3 Carbonaceous materials possessing high
specific surface area (SSA) and electrical conductivity have
widely been employed as EDLC-type electrode. On the
other hands, various metal oxides (MOs) and conducting
polymers have shown promising platforms for surface
redox chemistry, which typically delivery high capacitance
compared to EDLC-type SCs. There are important
Among various precursors for MOs, metal–organic
framework (MOF) that is a crystalline porous material com-
posed of metal node and organic ligand strut has been con-
sidered as an ideal platform to prepare MOF-derived
carbons10,11 and MOs due to diverse choice of metal ions,
controllable porosity and content of MO, and unique poly-
hedron morphology.12–15 Yamauchi and co-workers pre-
pared asymmetric psuedocapacitors consisting in 3D
nanoporous carbon and Co3O4 nanocrystal derived from a
single MOF, presenting a high energy density of 36 Wh/kg
with excellent long-term stability up to 2000 cycles with
ca. 89% retention.2 Guan et al. reported hollow NiCo2O4
array derived from 2D Co-MOF with subsequent ion-
exchange/etching process, which showed high energy den-
sity of 31.9 Wh/kg with 86.7% retention at 20 000 cycles.16
There are numerous means to make MOs with controllable
porosity and unique morphology using MOFs; however,
control of porosity and content of carbonaceous materials
of hybrid psuedocapacitors have rarely been achieved.
†These authors contributed equally to this work.
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Meanwhile, a facile synthesis of MOF-derived metal/
metal oxide@carbon composites (M/MO@Cs) via
morphology-preserved transformation has been recently
reported.17,18 The site-specific polymerization, called VPP,
at metal node inside MOF (polymer@MOF) has been suc-
cessfully achieved that carbon content of M/MO@C after
thermolysis and subsequent oxidation processes, respec-
tively, can be readily controlled by adjusting amount of
polymer used for VPP process.
PVP, several washing preocess using MeCN was carried
out.19
Synthesis of Polymer@MOF. Polymer@MOF compos-
ites were synthesized via VPP process that phenol
(360 mg) and formaldehyde (260 mg, PF) and activated
MOF (220 mg) were placed inside reaction vessel for 12 h
at 80 ꢀC where a static vacuum was already applied, den-
oted as polymer@MOF_2. Polymer@MOF_0 and 1 were
synthesized by using no PF and half amount of PF, respec-
tively. In order to remove less polymerized PF on the sur-
face during VPP process, polymer@MOF composites were
washed several times with THF and dried at 80 ꢀC for 12 h
under a static vacuum prior to the carbonization and oxida-
tion processes.17,18
Synthesis of CuO@C Composites. As-synthesized poly-
mer@MOF composites and pristine MOF (100 mg) were
placed in a crucible and then placed in a tube furnace and
heatꢀed to 800 ꢀC under N2 (99.9%) with a heating rate of
4.4 C/min. After reaching the target temperature, the sam-
ples were maintained for 3 h and then naturally allowed to
cool down to room temperature. The final black powder of
Cu@C samples was obtained and then Cu@C composites
were heated to 200 ꢀC for 16 h under air to synthesize
CuO@C samples. These samples were denoted as CuO,
CuO@C_0, 1, and 2, and nCuO@C_1.
Synthesis of CS. First resorcinol and formaldehyde were
mixed with a solution containing (200 mL) deionized water
(H2O) and aqueous ammonia (28% NH4OH). After stirring
for 24 h at room temperature, the reaction mixture was
moved to an oven and was subsequently heated for 24 h at
90 ꢀC without stirring. Solid product was recovered by cen-
trifugation and was washed several times with deionized
water. And then, CO2 activation was performed using
500 mg of carbon samples. The samples were placed in an
alumina boat in a quartz tube in the isothermal zone of a
tubular furnace that was purged with flowing nitrogen. The
samples were heated to 900 ꢀC (heating rate 30 ꢀC/min)
under inert conditions (N2), followed by changing the gas
flux to CO2 (1000 cc/min) for 720 min.20,21
Electrochemical Measurements (Three-Electrode Sys-
tem). Electrochemical measurements were carried out on a
computer-controlled potentiostat (CHI 660H CH Instru-
ment, Austin, TX) with a three-electrode electrochemical
cell. The standard three-electrode electrochemical cell was
fabricated using glassy carbon with the deposited sample as
the working electrode, platinum wire as the counter elec-
trode, and Ag/AgCl as the reference electrode. The working
electrodes were fabricated as follows: first, 5 mg of the car-
bon sample was added to 0.25 mL of DI water/isopropanol
(1:1) and then subjected to sonication for an hour. The sus-
pension was dropped onto a glassy carbon electrode. After
drying, a Nafion solution (5 wt % in methanol) was coated
on the sample as the binder. All the electrochemical mea-
surements were carried out at room temperature using 6 M
KOH electrolyte. The potentials for electrochemical mea-
surements are reported relative to an Ag/AgCl (saturated
Herein, monodisperse and nano-sized CuO@C polyhe-
dron derived from the iconic HKUST-1 (Cu-BTC,
benzenetricarboxylate) with controllable carbon content is
synthesized for high-performance asymmetric SCs along
with activated CS as a counter electrode. Using a MOF
with size of several micrometers produced by a conven-
tional method, polydisperse polymer@MOF_0, 1, 2 where
the numbers stand for the amount of polymer used for VPP
process are used for preparation of CuO@C_0, 1, 2, respec-
tively. Even the carbon content CuO@C_1 is medium compare
to the others, electrochemical performance of CuO@C-1 is
superior owing to well-balanced carbon and MO contents. Fur-
thermore, when monodisperse and nano-sized (ca. 500 nm)
MOF (nMOF) produced using a modulator is used for CuO@C
(denoted as nCuO@C_1) with the same carbon content of
CuO@C_1, electrochemical performance is even better than
CuO@_1 because monodispersity and smaller size of
nCuO@C_1 reduce interfacial resistance and ameliorate mass-
transport property, respectively. Asymmetric SC composed of
nCuO@C_1//CS presents excellent energy density of
55.47 Wh/kg and long-term stability of 88.7% at 5000 cycles,
comparable to the best MO-based asymmetric SCs derived
from MOFs.
Experimental Section
Reagents. All the reagents and solvents were used as
received. Cu(NO3)2Á2.5H2O (98%), PVP (99%, MW
~55 000) and paraformaldehyde (95%) were obtained from
Aldrich. 1,3,5-benzenetricarboxylic acid (H3btc, 98%), phenol
(98%) were purchased from TCI. Acetonitrile (MeCN,
99.5%) and tetrahydrofuran (THF, 99.5%) were supplied by
JUNSEI. N, N-dimethylformamide (DMF, 99.5%) was pur-
chased from DAEJUNG, nitric acid (60%) were purchased
from JUNSEI, potassium hydroxide (85%) were purchased
from DEAJUNG.
Synthesis of HKUST-1 [Cu3(btc2)]. First, Cu(NO3)2Á
2.5H2O (500.0 mg, 2.15 mmol) was dissolved in DMF
(15 mL) and btc (301.2 mg, 1.43 mmol) was added. After
stirring for 30 min, the solution was kept in a glass vial at
80 ꢀC for 16 h and then cooled to room temperature. Blue
crystals were obtained. After washing with DMF and
solvent-exchange with MeCN for 1 D, the product was
heated to 80 ꢀC under a static vacuum for 12 h. Then, the
activated HKUST-1 was used for VPP. For nano-sized
HKUST-1, 3.46 g of polyvinylpyrrolidione (PVP) was
added into aforementioned solution. To completely remove
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3 M NaCl) reference electrode and the potential window
In this equation, C is specific capacitance of assembled
asymmetric SC (based on the total mass of nCuO@C_1
and CS). V and t represent the potential window and dis-
charge time, respectively.
for cycling was confined between −0.8 and 0 V. The gravi-
metric specific capacitance
C (F/g) was calculated
according to the following equation using the discharge
portion of the charge–discharge curves:
Characterizations.
Transmission electron microscopy
(TEM) images and energy dispersive X-ray (EDX) were
obtained with JEOL TEM-2100F microscope. EDX analy-
sis and elemental mapping were performed using the EDX
spectroscopy attached to the OXFORD INCA. Scanning
electron microscopy (SEM) images were obtained with a
HITACHI S-4800 microscope with an accelerating voltage
of 15.0 kV and an applied current of 10 mA, the thermo-
gravimetric analysis (TGA) was performed using an SDT
Q600 (TA Instruments Inc.) X-ray diffraction (XRD) data
IΔt
C =
mΔV
where I is the constant current (A), m is the total mass of
the samples, ΔV is the potential rage (V), and Δt is the dis-
charge times (s).
Electrochemical Measurements (Two-Electrode System).
The electrodes were prepared by mixing 80 wt % of active
material, 10 wt % of Super P and 10 wt % of polyvinylidene
fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone
(NMP) to form a slurry and then pasted onto the carbon paper
current collector by dip coating process. Then, 2032 coin
cells were assembled by using nCuO@C-1 as anode and CS
as a cathode inside an Ar-filled glovebox using 6 M KOH as
the electrolyte and cellulose paper (Wellcose) as the separa-
tor. The anode to cathode mass ratio was 1:2. All measure-
ments were carried out at room temperature. The operating
voltage window for GCD and CV was to 1.5 V, which were
measured using potentiostat (CHI 660E). The current densi-
ties of asymmetric SC were calculated based on the mass of
their positive and negative electrodes was calculated based
on charge balance principles as follows:
was recorded on
a Rigaku, Miniflex600 instrument
equipped with Cu Kα radiation (λ = 1.5406 Å) at 40 kV
and 100 mA. The crystallite size of CuO nanoparticles was
calculated by applying the Scherrer equation. The nitrogen
sorption isotherms were measured at 77 K on a Belsorp
Mini-II. All samples were degassed at 150 ꢀC under a
dynamic vacuum for overnight. The Brunauer–Emmett–
Teller (BET) method was applied to estimate specific sur-
face areas (SSA) and total pore volumes were obtained at
P/P0 (= 0.99). Pore-size distributions (PSDs) were obtained
from the corresponding isotherms using the Barrett–
Joyner–Halenda (BJH) method. Micropore volumes were
calculated from the corresponding isotherms using the t-
plot method. The PSD and accumulated pore volume of the
CS sample was calculated from the adsorption branches of
the nitrogen-sorption isotherms using the BJH method. X-
ray photoelectron spectroscopy (XPS) spectra were mea-
sured using a PHI Versa Probe system with a 100-W ALK
Alpha X-ray source. Electrochemical impedance spectros-
copy (EIS) was obtained from the frequency range from
0.01 Hz to 100 kHz at 5 mV amplitude voltage in
6 M KOH.
m+ C− × ΔV −
=
m− C+ × ΔV −
In this equation, C− and V− are specific capacitance and
working potential window of AC. C+, and V+ are the spe-
cific capacitance and potential range of nCuO@C_1 and
CS, respectively. The followed equations were used to cal-
culated the energy density (E) and Power density (P):
Results and Discussion
0:5 × C × V2
E =
3:6
First, schematic illustration of CuO@C polyhedron that is
derived from MOF via VPP and subsequent carbonization
and oxidation processes is depicted (Scheme 1). HKUST-1
prepared by a conventional hydrothermal reaction with size
3600 × E
P =
t
Scheme 1. Schematic illustration of CuO@C that are pseudomorphically transformed from polymer@MOF via subsequent carbonization
and oxidation processes.
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of several micrometers was utilized for VPP process.
According to SEM images (Supporting Information
Figure S1), the morphology and the surface of the poly-
mer@MOF_1 and 2 are the same as the pristine MOF, and
characteristic XRD peaks corresponding to HKUST-1 of
for nCuO@C_1. According to SEM image of nCuO@C_1
(Figure 1(c)), monodisperse and nano-sized polyhedral con-
taining nanoparticles inside are clearly identified and its
crystallinity corresponds to CuO with size of ca. 8 nm simi-
lar to the crystal size of CuO@C_1 (Figure 1(a)).
According to further TEM study (Figure 1(d)), hollow par-
ticles with size of 50–80 nm are distributed inside carbon
matrix and each hollow particle consists in sub-10 nm
nanoparticles (Figure 1(c) inset). Formation of hollow
architecture of aggregated CuO nanoparticles could be
ascribed to the Kirkendal effect that causes transformation
of solid interior of Cu nanoparticle after thermolysis to hol-
low architecture of CuO nanoparticles after oxidation.18,23
In attempts to confirm this transformation, we prepared
nCu@C_1 that is a product right after thermolysis process.
According to XRD, characteristic XRD peaks
corresponding to Cu are clearly identified and TEM image
for nCu@C_1 shows solid interior of Cu nanoparticles with
size less than 58 nm (Supporting Information Figure S7).
Therefore, it could be concluded that hollow-like aggre-
gated CuO nanoparticles with sub-10 nm are formed via
the Kirkendal effect from solid interior Cu nanoparticles.
In addition, high-resolution TEM investigation was fur-
ther carried out that sub-10 nm crystal domains are identi-
fied and characteristic (111) and (002) crystal planes for
CuO are confirmed, which is also proved by the electron
diffraction patterns in Figure 1(e) and inset. Elementary
mapping of Cu, C, and O for nCuO@C_1 performed by
STEM-HAADF images definitively present spatially dis-
tributed CuO nanocrystals embedded inside carbon matrix
(Figure 1(f)). In addition, according to high-resolution XPS
analysis (Figure 2(a)), oxidation state of Cu2+ is only iden-
tified, confirming successful transformation of Cu to CuO
nanoparticles.
polymer@MOF_1 and
2
are well-preserved intact
(Supporting Information Figure S2). This results reveal that
the site-specific polymerization gradually occurs only inside
MOF pore that does not affect MOF morphology and crys-
tallinity.17,18 Then nanoparticle embedded polyhedron mor-
phology of CuO@C_0, 1, and 2 is prepared via thermolysis
and subsequent oxidation processes using polymer@MOF
samples that morphology-preserved transformation is
clearly determined by SEM images (Supporting Informa-
tion Figure S3). According to XRD analysis, only charac-
teristic CuO peaks are observed22 and the size of
nanocrystals measured by the Scherrer equation are
ca. 13, 8.2, and 6.2 nm for CuO@C_0, 1, and 2, respec-
tively (Supporting Information Figure S4). In addition,
CuO contents obtained from TGA for CuO@C_0, 1, and
2 are 93, 64, 48%, respectively (Supporting Information
Figure S5 and Table 1). Meanwhile, CuO derived from
MOF by direct oxidation was also prepared for comparison
that the crystal size measured from the Scherrer equation is
ca. 22 nm (Supporting Information Figure S4) and the CuO
content obtained by TGA is ca. 99% (Supporting Informa-
tion Figure S5 and Table 1).
Meanwhile, monodisperse nano-sized MOF (nMOF) was
prepared by using a PVP modulator, a synthetic procedure
of which is described at experimental section in detail.19 As
observed under SEM investigation, monodisperse polyhe-
dron with size of ca. 500 nm is clearly identified and crys-
tallinity of nMOF is confirmed to correspond to HKUST-1
(Supporting Information Figure S6 and Figure 1(a)). The
same VPP process as for polymer@MOF_1 was then
applied for nMOF to form polymer@nMOF_1 that the sur-
face, morphology, and crystallinity of polymer@nMOF_1
are same as the pristine nMOF (Figure 1(b)), which
undergoes thermolysis and subsequent oxidation processes
Lastly, the content of CuO for nCuO@C_1 obtained
from TGA is ca. 62% similar to CuO@C_1 (Supporting
Information Figure S5 and Table 1).
The textural features of CuO@C samples were analyzed
by nitrogen sorption isotherms. Isotherm for CuO sample
Table 1. Textural properties, specific capacitance, rate retention, and TGA results for CuO, CuO@C_0, 1, and 2, nCuO@C_1, CS, and
nCuO@C-1//CS.
SBET
Vmicro
Vtot
Carbon content
(wt %)
Capacitance (F/g @
0.5 A/g)
Retention
(%)
TGA
(w%)
Sample
(m2/g)
(cm3/g)
(cm3/g)a
CuO
6
97
0.001
0.01
0.08
0.12
0.08
1.08
-
0.04
0.07
0.18
0.31
0.14
1.24
-
1
7
36
52
38
-
307
473
482
459
494
326
177.5b
15
20
52
43
58
99
93
64
48
62
-
CuO@C_0
CuO@C_1
CuO@C_2
nCuO@C_1
CS
233
398
215
3891
-
63
63b
nCuO@C_1//
CS
-
-
a Measured at P0/P = 0.99.
b Obtained from two-electrode system.
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Figure 1. XRD results (a) for nMOF, polymer@nMOF, and nCuO@C_1, SEM images of polymer@nMOF (b) and nCuO@C_1 (c),
TEM image of nCuO@C_1 with enlarged image in the inset (d), high-resolution TEM image with selected area electron diffraction as an
inset (e), and HAADF STEM elementary mapping of Cu, C, and O for nCuO@C_1 (f).
exhibits almost nonporous feature that the specific surface
area (SSA) determined by the BET model is 15 m2/g and
PSD plotted by the BJH method presents no PSD (Figure 2
(b) and (c)). On the other hands, abrupt increases at very
low relative pressure indicative of microporosity occur for
CuO@C_1 and 2 compared to CuO@C_0 that shows
almost no uptake (Figure 2(b) and (c)). Because different
carbon contents for CuO@C_0, 1, and 2 are of 7, 36, and
52%, respectively, are the reason for different micropore
volumes, those of which are 0.01, 0.08, and 0.12 cm3/g,
respectively (Table 1). The SSAs for CuO@C_0, 1, and
2 are of 97, 233, and 398 m2/g, respectively, strongly
related to extent of carbon content (Table 1). In addition,
PSD plots after 10 nm for CuO@C samples are similar
each other, indicative of well development of large
mesoporosity derived from spatially distributed CuO nano-
crystals embedded inside carbon matrix (Figure 2(c))
Meanwhile, nCuO@C_1 shows SSA (218 m2/g), micropore
volume (0.08 cm3/g), and PSD that are almost identical to
CuO@C_1 (Figure 2(b) and (c) and Table 1). Because the
textural feature, CuO content, and crystal size for
nCuO@C_1 and CuO@C_1 are almost identical to each
Figure 2. High-resolution of Cu 2p XPS spectra (a), nitrogen sorption isotherms (b), and BJH plots (c) CuO, CuO@C_0, 1, and 2, and
nCuO@C_1 samples.
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other; therefore, we anticipate that the difference in electro-
chemical performance for both samples can be attributed
entirely to the influence of monodispersity and size.
better supercapacitive due to existence of conductive carbona-
ceous material. The specific capacities obtained from GCD
profiles at current density of 0.5 A/g for CuO and CuO@C_0,
1, 2 are 307, 473, 482, and 459 F/g, respectively. CuO@C_1
with medium carbon content presents slightly higher capacity
than the others and the specific capacity of CuO@C_2 is even
lower than CuO@C_0, strongly referring the importance of
well-balanced carbon and MO content for high-performance
hybrid SCs. In addition, nCuO@C_1 shows a higher specific
capacity of 494 F/g than CuO@C_1 probably owing to
monodispersity and smaller size of nCuO@C_1.
Specific capacitances at various current densities (0.5–50
A/g) for all samples are plotted as in Figure 3(e). Rate
retentions that are derived from the capacitance ratio
obtained at 1 and 50 A/g for CuO and CuO@_0 are of
15 and 20%, respectively, owing to poor electrical conduc-
tivity of MOs. On the other hands, CuO@C_1 and 2 show
much better rate retentions of 52 and 43%, clearly con-
firming advantage of using conductive carbon for hybrid
SCs. Higher rate retention of CuO@C_1 with smaller car-
bon content than CuO@C_2 definitely clarifies importance
of well-balanced carbon and MO content. Furthermore, rate
retention (58%) of nCuO@C_1 is even higher than
CuO@C_1, distinctly demonstrating beneficial effects
using monodisperse and smaller size electrodes for high-
performance SCs.
Electrochemical performances of CuO@C samples are
measured under a three-electrode system and cyclic
voltammetry (CV) with different scan rate (10–200 mV/s)
is first carried out in 6 M KOH solution. CuO sample pre-
sents characteristic pseudocapacitive peaks corresponding
to redox peaks of CuO (Supporting Information Figure S8
(a)). On the other hands, CV curves of CuO@C_0, 1, and
2 show characteristic redox peaks with rectangular shape
curves associated with pseudocapacitive and EDLC-type
peaks, respectively (Supporting Information Figure S8
(b) and (d) and Figure 3(a)).3 In addition, nCuO@C_1 pre-
sents similar CV profiles with CuO@C_1 (Figure 3(a) and
(b)). It is noted that more rectangular shape is getting devel-
oped for higher carbon content samples such as CuO@C_1
and 2 and nCiO@C_1 due to larger contribution of EDLC
nature (Figure 3 and Supporting Information Figure S8).
Galvanostatic charge–discharge (GCD) profiles for CuO
sample and CuO@C samples were obtained by
chronopotentiometry with current density range of 0.5–50 A/g
(Supporting Information Figure S8(e–h) and Figure 3(c)).
Significant IR-drop at initial potential range and capacitive
development after around −0.4 V at current density of 1 A/g
are clearly observed for all samples, corresponding to charac-
teristic pseudocpaptive feature (Figure 3(d)). CuO sample pre-
sents a short discharge time; on the other hands, CuO@Cs
and nCuO@C_1 show much longer discharge time referring a
The origin of electrochemical performances for samples
with well-balanced carbon and MO content (CuO@C_1 vs.
CuO@_0 and 2) and monodisperse and small size of
Figure 3. CV profiles obtained at scan rate of 50 mV/s for CuO@C samples and nCuO@C_1 (a), CV profiles with various scan rates
(b) and GCD profiles (c) with various current densities of nCuO@C_1, GCD profile comparison (d) obtained at current density of 1A/g
and rate retention (e) with various current densities, and Nyquist plots for CuO, CuO@C_0, 1, and 2, and nCuO@C_1.
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Figure 4. Schematic illustration (a), CV curves (b) with various scan rates, GCD profiles (c) with various current densities, rate retention
(d), long-term stability (e) at current density of 1 A/g for 5000 cycles, and Ragone plot (f) of nCuO@C_1//CS with references obtained
from MOF-derived MO-based asymmetric SCs.
electrode (nCuO@C_1 vs. CuO@C_1) is uncovered by
electrochemical impedance spectroscopy (EIS) study.
Impedance spectra fitted into an equivalent circuit model
for all samples are plotted that Nyquist plots for all samples
show similar shapes that semi-circles appear at high fre-
quency region with rapidly increase of long tails at low-
frequency region (Figure 3(f)). The intercept with x-axis is
the equivalent series resistance (Rs) that is the total electri-
cal conductivity including electrode and electrolyte, CuO,
CuO@C_0, 1, and 2 of which are 10, 8, 3, and 4 Ω, respec-
tively (Supporting Information Table S1).24 As expected,
samples with lower carbon contents (CuO and CuO@C_0)
have higher Rs values than the other samples with higher
carbon contents. The charge transfer resistance (Rct) values
at the interface between electrode and electrolyte for CuO,
CuO@C_0, 1, and 2 are 98, 51, 24, and 38 Ω, respectively
(Supporting Information Table S1).24 In particular, CuO
sample presents very high Rct value than the other samples
probably due to large crystal size of ca. 22 nm and no car-
bon content. In addition, Rs and Rct values of CuO@C_1
are slightly lower than those of CuO@C_2, which could be
possibly ascribed to synergetic effect of well-balanced car-
bon and MO content.
Nyquist plot in the mid-frequency region for nCuO@C_1
is lower (97 Ωs−1/2) than that (110 Ωs−1/2) of CuO@C_1,
clearly confirming the advantages of using nano-sized poly-
hedron composite (Figure 3(f) and Supporting Information
Table S1).24,25 Therefore, the better electrochemical perfor-
mance of monodisperse and nano-sized nCuO@C_1 than
CuO@C-1 is clearly demonstrated.
Meanwhile, CS that is
a
counter electrode of
nCuO@C_1 for asymmetric SC (nCuO@C_1//CS) is pre-
pared and the detail synthetic procedure is described in the
experimental section. Monodisperse CS with size of
390 nm shows very high SSA of 3891 m2/g obtained from
the BET model using nitrogen sorption isotherm and PSD
derived from the BJH method up to 4 nm (Supporting
Information Figure S9 and Table 1).20,21 CV profiles of CS
with various scan rates (10–200 mV/s) are displayed in the
Supporting Information Figure S10 that quasi-rectangular
shape indicative of characteristic EDLC-type capacitive
property is clearly confirmed.24 GCD profiles of CS with
various current densities (1–50 A/g) present isosceles trian-
gular shape, which is also characteristic EDLC feature and
rate retention of CS is ca. 63% (Supporting Information
Figure S10). Therefore, asymmetric SC composed of
nCuO@_1 with CS as a counter electrode (nCuO@C-1//
CS) is further investigated (Figure 4(a)).
In addition, nCuO@C_1 presents lower Rs (2 Ω) and Rct
(17 Ω) values than those of CuO@C_1 probably due to
more effective contact between electrode materials and cur-
rent collector. In particular, the Warburg diffusion resis-
tance (Aw) that is related to efficiency of mass-transport of
electrolyte toward electrode surface obtained from the
Electrochemical performance of asymmetric SC of
nCuO@C-1//CS is evaluated that CV profiles show revers-
ible and rectangular shapes with working voltage up to
1.5 V, confirming effective capacitive energy storage
Bull. Korean Chem. Soc. 2021, Vol. 42, 649–657
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KOREAN CHEMICAL SOCIETY
(Figure 4b). The specific capacitance of nCuO@C_1//CS
obtained from GCD at current density of 0.5 A/g is
177.5 F/g with excellent rate retention of 63% (Figure 4
(c) and (d) and Table 1). In addition, nCuO@C-1//CS pre-
sents outstanding long-term stability (88.7%) at 5000 cycles
tested at current density of 2 A/g (Figure 4(e)). Further-
more, nCuO@C-1//CS deliveries prominent energy density
of 55.47 Wh/kg at power density of 375 W/kg and remark-
able maximum power density of 37 498 W/kg with excel-
lent energy density of 26.04 Wh/kg (Figure 4(f)). The
outstanding specific capacitance, rate retention, long-term
stability, energy, and power densities of nCuO@C-1//CS
are attributed to monodisperse and nano-sized polyhedron
with well-balanced carbon and MO content of nCuO@C-1,
which is comparable with the best values obtained from
MOF-derived asymmetric SCs (Supporting Information
Table S2).2,16,26–38
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