Synthesis, characterization and electrochemical properties of activated coconut fiber carbon (ACFC) and CuO/ACFC nanocomposites for applying as electrodes of supercapacitor devices

Surfaces and Interfaces 25 (2021) 101174

Contents lists available at ScienceDirect

Surfaces and Interfaces


journal homepage: www.sciencedirect.com/journal/surfaces-and-interfaces




Synthesis, characterization and electrochemical properties of activated
coconut fiber carbon (ACFC) and CuO/ACFC nanocomposites for applying
as electrodes of supercapacitor devices

c
Sasiphimol Sawadsitang a,b , Thanawut Duangchuen a,b , Attaphol Karaphun a,b , Thanin Putjuso ,
d
Pisist Kumnorkaew , Ekaphan Swatsitang a,b,*
a
Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), NANOTEC -KKU, RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen
University, Khon Kaen 40002, Thailand
b
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
c
School of General Science, Faculty of Liberal Arts, Rajamangala University of Technology Rattanakosin, Wang Klai Kangwon Campus, Hua Hin, Prachaubkerekhan
77110, Thailand
d
National Nanotechnology Center (NANOTEC), National Science and Technology, Development Agency (NSTDA), Pathumthani 12120, Thailand
ARTICLE INFO ABSTRACT
Keywords: CuO nanoparticles (CuO NPs) and CuO/Activated Coconut Fiber Carbon nanocomposites (CuO/ACFC Ncps) with
CuO nanoparticles different ACFC loading of 0.1, 0.2, 0.3, and 0.4 g (designated as CuO/ACFC-1, 2, 3, 4 Ncps, respectively) were
CuO/Activated Coconut Fiber Carbon
prepared by a one-pot hydrothermal method. CuO NPs and all CuO/ACFC Ncps displayed a monoclinic phase of
nanocomposites
CuO with space group C2/c as confirmed by X-ray diffraction. Field emission scanning electron microscopy
hydrothermal method
electrochemical properties. revealed a lotus root-like shape with grooves on a rough surface of ACFC and an oval-like shape for CuO NPs.
Transmission electron microscopy revealed a bar-let shape of CuO NPs with average diagonal of 167.57 ± 2.32
nm. Cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy were
performed to probe the electrochemical properties of all samples. Interestingly, the specific capacitance (C s ) of
60.14 F g 1 for CuO NPs electrode was improved to 145.33 F g 1 at 1 A g 1 in CuO/ACFC-0.3 Ncps electrode
with capacity retention of 85.24%. Moreover, an assembled asymmetric supercapacitor (ASC) device with ACFC
as a negative electrode and CuO/ACFC-3 Ncps as a positive electrode (ACFC//CuO/ACFC-3) could exhibit the C s
value of 24.94 F g 1 at 0.5 A g 1 with the maximum energy density (E sp ) and power density (P sp ) of 7.78 Wh kg 1
1
and 3,000 W kg , respectively. Additionally, good capacitance retention of 75.2% was achieved in this device
after 1500 continuous GCD cycling test.

1. Introduction charge storage mechanisms, SC can be categorized into three types [11].
Electrochemical double layer capacitor (EDLC) is the first one of which
Presently, supercapacitor (SC) is proposed to be one of the most charge is collected at the surface of electrode, and the commonly active
effective and reliable electrochemical energy storage device, because it electrode materials are the carbonized materials [5,6,10–16]. The sec-
can provide reasonably high power density (P sp ) over 15 kW kg 1 and ond type is a pseudocapacitor (PC) of which the electrode materials are
1
specific energy density (E sp ) of approximately 10 Wh kg , including the mainly metal/transition metal oxides, hydroxides, sulfides and con-
high stability and long cycle lifetime of fast charge/discharge process ducting polymers [4,5,13], and it displays a faradaic redox reaction. The
[1–9]. Moreover, the fabrication process of the device is facile, clean, last one is derived from the combination of EDLC and PC types and it is
and environmentally benign, which can help to solve not only the known as hybrid capacitor [4,13,14] or asymmetric supercapacitor
problem of global warming but also the environmental deterioration. (ASC). According to the ASC type, the positive electrode displays fara-
Additionally, SC is a source of sustainable energy storage system that daic redox reaction in active materials, while the negative one is mostly
meets the ever-increasing demands in a novel technology and made of carbonized materials that significantly show the EDLC behavior
fast-growing electronics industry [6–10]. Normally, according to the [1]. Generally, it is well known that the specific capacitance (C s ) of SC

* Corresponding author at: Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand.
E-mail address: [email protected] (E. Swatsitang).
https://doi.org/10.1016/j.surfin.2021.101174
Received 17 December 2020; Received in revised form 20 April 2021; Accepted 30 April 2021
Available online 16 May 2021
2468-0230/© 2021 Elsevier B.V. All rights reserved.

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
depends mainly on the electrode materials. Thus, in order to improve the Loba Chemie, Sigma Aldrich and Alfa Aesar, respectively. Potassium
C s value together with increasing the conductivity and cycle stability, hydroxide (KOH, 85%), N-methyl-2-pyrrolidone (NMP, 99.5%) and
one of the most attractive solutions is to explore for novel active mate- ethanol (C 2 H 5 OH, 99.9%) were supplied by RCI Labscan. Activated
rials to be used as negative and positive electrodes [1,3,6,10,17]. In the coconut fiber carbon (ACFC) was derived by pyrolysis the coconut fibers
investigation of active materials for negative electrode, carbonized (CFs) selectively separated from dry mature coconut shell (brown color)
◦
materials such as carbon black, activated carbon, carbon nanotubes and collected from the local area, at 500 C for 6 h in argon atmosphere. This
graphene have been reported to be the appropriate ones for EDLC due to pyrolysis temperature ascertains the existence of graphitic layers (for
their high power density and cycle stability [1,2,12,15,16,18]. Other good conductivity) in a strengthen ACFC structure (the details for this
than these, some carbonized materials derived from different parts of were omitted and described elsewhere) that can support the growth of
particular plants such as activated coconut fiber carbon (ACFC) obtained CuO NPs on their surface during the hydrothermal process.
by the pyrolysis of coconut fiber (coconut fiber is a natural filamentous
material selectively separated from dry coconut shell) have been 2.2. Synthesis of CuO NPs and CuO/ACFC Ncps
considered to be a prospective one that can be applied as electrode
materials of ASC with high performance. Moreover, it was reported that Fig. 1 illustrates the schematic diagram for the preparation of CuO
ACFC has a good electrical properties, good mechanical strength, NPs and CuO/ACFC Ncps with different ACFC loading of 0.1, 0.2, 0.3
non-toxic material, high surface area with unique fiber pore structure, and 0.4 g (designated as CuO/ACFC-1, 2, 3, 4 Ncps, respectively) by a
cost-effective, world-wide available, industry-viable technology and one-pot hydrothermal method. In the preparation of pure CuO NPs,
offer high production rate [18–20]. Furthermore, as referred to research stoichiometric amount of Cu(NO 3 ) 2 .3H 2 O was dissolved in 40 ml
articles, the raw materials for positive electrodes that prepared from the deionized (DI) water. After that, appropriate amount of KOH pellets
composite of some metal oxide nanomaterials and ACFC are emerged as were added into this solution to yield a 0.5 M concentration with
prominent materials, which can induce a high energy density in ASC [1, vigorous stirring using a magnetic bar at room temperature (RT) for 6 h.
21]. So far, various kinds of oxide materials that have been explored for Then, the solution was transferred into a cylindrical teflon container
SC electrodes are such as transition metal pyrophosphates (e.g., with a tight cover lid and was placed in a fit stainless steel autoclave of
Co 2 P 2 O 7 , Mn 2 P 2 O 7 , Fe 2 P 2 O 7 , Cu 2 P 2 O 7 , Ni 2 P 2 O 7 and Zn 2 P 2 O 7 ), metal which a cover lid was tighly closed with 6 screws. This close system was
◦
hydroxides (e.g., Ni(OH) 2 , Cu(OH) 2 , and Co(OH) 2 ), metal oxides (e.g., placed in an electric oven (Memmert, Model UF 55) and heated at 180 C
CuO, Cu 2 O, CuO 2 , TiO 2 , MnO 2 , and NiCo 3 O 4 ), and metal chlorides (e.g., for 6 h. Likewise, CuO/ACFC-1, 2, 3, 4 Ncps were prepared by the same
CuCl 2 ). All of these materials have been suggested to be the appropriate process under the same conditions, however, each of the mixture was
ones for high performance electrodes with the achievement of different stirred for 12 h before transferring to hydrothermally treat
C s values and retentions. Among these, copper oxide (CuO), which is Finally, the obtained powders of all products were subsequently
◦
inexpensive, reliably good conductivity and easy to prepare, has washed with DI water, dried in an electric oven at 85 C for 8 h and alls
attracted extra interest for applying as a positive electrode of SC. were ground using agate mortar to obtain fine powders for further
Moreover, according to the charge/discharge process, the oxidation characterization and study.
state of CuO can be dynamically transformed between Cu(II) and Cu (I)
species during the faradaic redox reactions [2,3,6,10,11,22,23]. Addi- 2.3. Characterization
tionally, CuO nanostructures can have different morphologies such as
nanoparticles, nanoflowers, nanosheets, nanorods, depending on the X-ray diffraction (XRD, EMPYREAN, PANalytical with Cu K α radia-
method and condition of preparation. Besides, it is well known that tion, λ = 0.15406 nm) was used to examine the crytalline structure of all
morphology, nanocomposites and conductivity are the important fac- the products. Morphology of all samples were studied using field emis-
tors, which can influence the electrochemical performance of electrode sion scanning electron microscope (FE-SEM, Helios Nanolab G3 CX, FEI)
2
materials [24]. For instance, Zhang et al. [25] synthesized the flower and transmission electron microscope (TEM, Tecnai G 20 twin, FEI).
1
like CuO nanostructures and reported the C s value of 133.6 F g . Li Element mapping of Ca, C, O and Si in ACFC sample and those of Cu, C
et al. [26] expanded CuO nanostructures on a copper foam substrate and with O in CuO NPs and all CuO/ACFC Ncps samples were investigated by
1
N
obtained the C s value of 212 F g . Additionally, a high C s of 569 F g 1 energy-dispersive X-ray spectroscopy (EDS-mapping, X-Max 80, Ox-
could be achieved in CuO nanosheets, as reported by Wang et al. [27]. ford). Thermogravimetric analysis (TGA, STA7200, HITACHI) was used
Moreover, it was reported that the mesoporous CuO of nanoribbons to study the phase formation and determined the amount of CuO NPs in
◦
shape could be successfully synthesized in tetraethylammonium bro- all CuO/ACFC Ncps samples from RT to 800 C, using a heating rate of
◦
mide (TEAB) with the C s value of 137 F g 1 [28]. In addition, a 3D 10 C min 1 under nitrogen flow. Functional groups in all samples were
porous CuO of gear like morphology had been prepared on a Cu sheet verified by Fourier transform infrared spectroscopy (FT-IR, Tensor 27,
with the achieved C s value of 348 F g 1 [29]. BRUKER), using the KBr pellet technique. Specific surface area and pore-
In this work, it is our intention to investigate the electrochemical size distribution of all the products were analyzed by the nitrogen
properties of CuO NPs and CuO/ACFC Ncps for positive electrode adsorption-desorption isotherm (BELPREP VAC II, BELSORP MINI X)
application of the ASC with ACFC as negative electrode and fiber glass as employing the Bruanauer-Emmett-Teller (BET) and Barrett-Joyner-
a separator in 3 M KOH electrolyte. In the experiment, a one-pot hy- Halenda (BJH) technique, respectively. The chemical surface of CuO
drothermal method was employed to prepare CuO NPs and CuO/ACFC NPs and all CuO/ACFC Ncps samples was investigated by X-ray photo-
Ncps with different ACFC loading of 0.1, 0.2, 0.3 and 0.4 g. Up to now, electron spectroscopy (XPS, AXIS Ultra DLD, Kratos Analytical Ltd.).
the study of ACFC loading on the electrochemical properties of CuO/
ACFC Ncps to be used as positive electrode of ASC has not been reported 2.4. Electrochemical study
yet.
Each electrode was fabricated by coating the slurry on a 1 × 2 cm 2
2
2. Experimentals nickel foam sheet (coating area of 1 × 1 cm ). The slurry for each
electrode was prepared by mixing 80 wt% of each active material, 10 wt
2.1. Materials % polyvinylidenefluoride and 10 wt% acetylene black in the dissolved
0.25 ml of N-methyl pyrrolidone using a ball milling process. After
◦
The chemicals used for the preparation of CuO NPs were purchased coating, all of them were dried at 80 C for 24 h in an electric oven and
from various companies. Cu(NO 3 ) 2 •3H 2 O (99.50%), polyvinylidene uniaxially compressed at 3 ton for 2 min. Electrochemical properties of
fluoride (PVDF) and acetylene black (99.9+%) were purchased from all electrodes were studied at RT by cyclic voltammetry (CV),

2

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.














Fig. 1. Schematic diagram for the preparation of CuO NPs and CuO/ACFC Ncps by a one-pot hydrothermal method.

galvanostatic charge/discharge (GCD) and electrochemical impedance 3. Results and discussion
spectroscopy (EIS) techniques via electrochemical workstation (CS350
Potentiostat/Galvanostat, Wuhan Corrtest Instruments Corp. Ltd.), 3.1. Structure analysis
using a three-electrode cell system in 3.0 M KOH aqueous solution with
Ag/AgCl and a platinum plate as reference and counter electrodes, The XRD results of all samples are presented in Fig. 2(a). As displayed
◦
◦
respectively. The C s value of each electrode was calculated from the GCD in Fig. 2(a) two large broaden peaks at 2θ = 24.44 and 43.87 of ACFC
data using Eq. (1), belongs to the amorphous hexagonal graphite predominantly formed in
the natural coconut fibers [18,19,21, 30], corresponding to the JCPDS
IΔt
C s = (1) card no. 75-1621 [21]. This result confirms the presence of a significant
mΔV
amount of graphitic carbon in ACFC sample. The observed weak
◦
1
where C s is a specific capacitance (F g ), I is a constant discharge diffraction peak at 2θ = 32.77 relates to the presence of a specific silica
current (A), Δt is a discharge time (s), ΔV is a potential window (V) and (JCPDS card no. 39-0973) that naturally occurs in coconut fibers [19,
m is a mass of active material in electrode (g). 21]. In cases of CuO NPs and all of CuO/ACFC Ncps, the observed
◦
◦
diffraction peaks in a 2θ range of 15 - 70 compose of two major (111)
2.5. Fabrication of asymmetric supercapacitor (ASC) device and (111) planes, corresponding to a monoclinic phase of CuO with
space group C2/c as established in a standard XRD data (JCPDS card no.
With CuO/ACFC-3 Ncps as a positive electrode and ACFC as a 05-0661) [4,11,22,24,31–34]. Moreover, Fig. 2(b) shows the magnifi-
negative electrode, the ASC device of CR-2032-coin cell was assembled cation of (111) peak in a 2θ range of 34.8–37.0 for all samples, pre-
◦
2
◦
within a geometric surface area of 1.5 cm , using a few drops of 3 M KOH senting an infinitesimal shift of this plane at 34.96 to a higher 2θ value
solution as electrolyte and fiber glass as a separator. In order to obtain with increasing of full width at half maximum (FWHM, β value), indi-
the widest range of working potential window, the mass ratio of positive cating the decrease of crystallite size of particles. Furthermore, the
electrode to negative electrode was determined according to the charge average crystallite sizes (D XRD , nm) of CuO NPs and those of CuO in all
+
balance theory (q = q ), given by Eq. (2) [1], CuO/ACFC Ncps were determined from the X-ray line broadening of
(111), (111),(202), (202) and (113) planes, using Scherrer equation
m + /m = (C ×ΔE )/(C + ×ΔE + ) (2)
[35] given in Eq. (5),
+
where m + /m - , q /q , C + /C - , and ΔE + /ΔE - , are the ratios of mass, charge, 0.89λ
specific capacitance and potential window of positive and negative D XRD = βcosθ (5)
electrodes, respectively. According to the specific capacitance value of
47.75 F g 1 at current density 1 A g 1 for ACFC and 145.33 F g 1 for where λ is the wavelength of an incident X–ray beam (1.5406 Å), θ is the
CuO/ACFC-3 Ncps, with the appropriate potential window experimen- Bragg diffraction angle, β is the full width at half maximum (FWHM) and
tally obtained for ACFC (1.0 V) and CuO/ACFC-3 Ncps (0.5 V), the 0.89 is a value of a shape factor (k). Accordingly, the D XRD value of CuO
optimal mass ratio (m + /m - ) was evaluated to be 0.657 for the ASC de- NPs was found to be 34.1 ± 3.3 nm, whereas those in CuO/ACFC Ncps
vice. Consequently, the mass loading of negative and positive electrodes decrease with increasing ACFC content as can be seen in Table 1. Fig. 2
are 12.00 mg and 7.88 mg, respectively. Therefore, the total mass of (c) shows the Rietveld refinement pattern of CuO NPs, employed for the
both electrodes materials in the ASC device is 19.88 mg cm 2 . determination of unit cell volume (V), lattice constant (a, b, and c), β
1
The E sp (Wh kg ) value of ASC device was determined from the angle, and other related parameters (e.g. weight fraction (%), reliability
specific cell capacitance (C s, cell ), corresponding to 1/4 of C s value in the factors of weighted profile residual (R wp ), profile residual (R p ), expected
GCD curve using Eq. (3), residual (R ex ), and goodness of fit (GOF)) using a space group C2/c [9,
(( ) ) 35]. All of these determined parameters for all samples were listed in
) C s × (ΔV) 2 ) Table 1 and found to correspond with the standard values of a mono-
1 C s,cell × (ΔV) 2 1 4 1 C s × (ΔV) 2
E sp = = = (3) clinic structure CuO [4,9,11,36,37]. Remarkably, the results of XRD
2 3.6 2 3.6 8 3.6
certify a pure CuO phase in all samples.
1
Moreover, the P sp (W kg ) value of ASC device can be determined
from the E sp value divided by the discharge time t as given in Eq. (4),
3.2. Morphology analysis
E sp × 3600
P sp = (4)
t
As shown in Fig. 3(a) is the FE-SEM image of ACFC, showing the
filamentous morphology with roughly longitudinal grooves on surface
and several tubes inside the filament similar to that of lotus root
[18–20]. Fig. 3(b) displays the FE-SEM image of CuO NPs, revealing the
very fine oval-like shape. For CuO/ACFC-1 Ncps sample, as shown in
Fig. 3(c), the very fine CuO NPs of icing like are densely agglomerated
3

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.





























































◦
Fig. 2. (a) XRD patterns of ACFC, CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps, (b) Enlargement of the XRD results for (111) plane in 2θ range 34.8–37.0 of these samples
and (c) Rietveld refinement of CuO NPs sample.
and spreadable on the surface of ACFC due to the high CuO and low pattern of ACFC (a lower inset of Fig. 4(a)) displays two faint halo
ACFC loading. For others CuO/ACFC-2, 3, 4 Ncps, the SEM images in diffraction rings, indicating the amorphous nature of ACFC corresponds
Fig. 3(d), (e) and (f) show less dense agglomeration and distribution of with the two broaden peaks of XRD result for ACFC shown in Fig. 2. TEM
CuO NPs on ACFC surface correspond with high ACFC and low CuO image of CuO NPs, as shown in Fig. 4(b), displays the obviously irregular
loading in these samples. The distribution of CuO NPs on the surface of bar-let of monoclinic structure with average diagonal of 167.57 ± 2.32
ACFC in all CuO/ACFC Ncps samples are suggested to affect their spe- nm. The SAED pattern of CuO NPs (inset of Fig. 4(b)) displays the
cific surface area as will be further discussed by BET analysis. Most irregularly concentric diffraction rings with bright spots on the cir-
importantly, such a specific morphology of ACFC and the optimum cumferences, demonstrating a polycrystalline nature of CuO NPs with
composition of CuO/ACFC Ncps might benefit not only to support the the indexed planes of monoclinic CuO phase, corresponding to the XRD
CuO NPs but also improve the contact and interaction between active results for CuO NPs shown in Fig. 2. In all cases of CuO/ACFC Ncps, the
materials and electrolyte, including the assistance to prevent the decay TEM images as shown in Fig. 4(c) – (f) demonstrate the agglomeration of
of charge during the charge/discharge process, which could result in irregular bar-let CuO NPs on the ACFC surface. The variation in size and
high cycle life and electrochemical performance of electrode [36]. shape of CuO NPs in all CuO/ACFC Ncps might be due to the increasing
Fig. 4 (a) illustrates the TEM image of ACFC, revealing the presence loading of ACFC and low content of CuO in CuO/ACFC Ncps, and this
of extensive mesoporous character of ACFC [21]. Moreover, the SAED would possibly alter the growth process of CuO NPs in all of these

4

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
Table 1
Lattice parameters (a, b and c), angles (α, β and γ), cell volume (V), Rietveld refinement parameters (R ex , R p , R wp and GOF) and average crystallite sizes (D XRD ) of CuO
NPs and CuO/ACFC-1, 2, 3, 4 Ncps.
Parameter CuO NPs CuO/ACFC-1 Ncps CuO/ACFC-2 Ncps CuO/ACFC-3 Ncps CuO/ACFC-4 Ncps
Space group C2/c C2/c C2/c C2/c C2/c
Crystal structure Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic
a 4.684(3) 4.687(3) 4.691(4) 4.687(4) 4.693(2)
Lattice parameter (Å) b 3.423(2) 3.422(2) 3.422(4) 3.421(3) 3.423(2)
c 5.125(3) 5.126(2) 5.125(4) 5.126(3) 5.128(2)
◦
α = γ ( ) 90 90 90 90 90
◦
β ( ) 99.510(1) 99.530(1) 99.550(1) 99.490(1) 99.574(9)
6
3
Cell volume (10 pm ) 81.03 81.08 81.12 81.07 81.25
R ex (%) 4.7331 5.3895 4.9934 5.2199 5.2424
R p (%) 4.5838 3.9797 4.0386 4.2830 4.6028
R wp (%) 6.0787 5.1291 5.2259 5.5440 6.0595
GOF 6.0973 4.1996 3.1963 4.6255 4.1498
D XRD (nm) 34.1 ± 3.3 29.3 ± 3.3 23.1 ± 2.8 25.8 ± 7.3 23.8 ±8.2





















































Fig. 3. FE-SEM images of (a) ACFC, (b) CuO NPs and (c)–(f) for CuO/ACFC-1, 2, 3, 4 Ncps, respectively.
samples. Furthermore, the SAED patterns of all CuO/ACFC Ncps, as 3.3. EDS analysis
depicted in the insets of Fig. 4(c)–(f), are similar to those observed in
Fig. 4(b), suggestion the polycrystalline nature of these samples, as well. The EDS spectra and element distribution mapping of all samples
were used to confirm the presence of elemental components on their
surfaces. Fig. 5(a) shows the EDS spectrum of ACFC, indicating the

5

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.






















































Fig. 4. TEM images and selected area electron diffraction (SAED) patterns of (a) ACFC, (b) CuO NPs and (c) – (f) for CuO/ACFC-1, 2, 3, 4 Ncps, respectively.

◦
presence of Ca, C, O and Si elements with the proper dispersion of them corresponding to the dehydration process (up to 200 C with estimated
on ACFC surface (inset of Fig. 5(a)). These elements are the typical ones weight loss of 10%) [19], the degradation of some residual organic
◦
that generally found in coconut fibers [20]. For CuO NPs, the EDS materials in natural coconut fibers (from 200–573.81 C with approxi-
spectrum in Fig. 5(b) reveals the presence of primary spectra of both Cu mate weight loss of 90%) [20] and the completeness of the activated
◦
and O elements, signifying the high purity of CuO NPs. The observed carbonization process (from 573.81 to 800 C with 0.64% of weight
spectra for C element in this sample are due to the carbon tape substrate. loss), respectively. In case of CuO NPs, a single step weight loss of
◦
As shown in Fig. 5(c)–(f) and their insets are the EDS spectra with approximately 0.46% is observed from RT to 800 C due to the loss of
element mapping images of CuO/ACFC-1, 2, 3, 4 Ncps, displaying the some organic materials adsorbed on CuO NPs surface [4]. For all cases of
main elements of Cu, O and C properly existed and dispersed in these CuO/ACFC Ncps, the curves reveal three main regions of weight loss
samples with no sign of any impurity. Obviously, the calculated content similar to that of ACFC. In addition, the residual weight of each
◦
of C element (wt%) increases with increasing ACFC loading, as depicted CuO/ACFC Ncps were determined from a clear plateau at 800 C, and
in the insets of Fig. 5(c) – (f) and vice versa for the Cu content, corre- found to be 91.27, 83.20, 76.00 and 67.12% for CuO/ACFC-1, 2, 3, 4
sponding to the different composition ratios of Cu to ACFC in Ncps, respectively. These TGA results of all CuO/ACFC Ncps confirm the
CuO/ACFC-1, 2, 3, 4 Ncps samples. greater decrease of weight loss in sample of CuO/ACFC Ncps with high
ACFC loadings, corresponding to the composition ratio of CuO:ACFC in
each sample by mass.
3.4. TGA, FT-IR and BET analysis Fig. 6 (b) displays the FT-IR spectra of all samples. The appeared
vibrational peak at ~3385 cm 1 in all samples was assigned to the O–H
◦
The TGA curves of all samples obtained from RT to 800 C, using a stretching mode of hydroxyl group owing to the H 2 O molecule formed
1
◦
heating rate of 10 C min , as shown in Fig. 6(a), were employed to on their surfaces [4,11,19]. In addition, the C=C stretching mode of the
verify the composition ratio in wt% of CuO:ACFC in CuO/ACFC Ncps. aromatic ring resulted in an absorption band at 1642 cm . The
1
For ACFC sample, the TGA curve decomposes into three main regions,
6

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.





























































Fig. 5. EDS spectra with mapping images of elements (Ca, C, O and Si) for (a) ACFC, and (Cu, C and O) for (b) CuO NPs and (c) – (f) CuO/ACFC-1, 2, 3, 4 Ncps,
respectively.

absorption band at 1384 cm 1 was attributed to the ether stretching nm) and mesopores (diameter 2–50 nm) structures that can assist to
mode of –OCH 3 group, which is absent in the ACFC spectrum. The promote the double-layer capacitance during the charge/discharge
vibrational peak of C–H stretching mode can be observed in all samples process, resulting in a high specific capacitance of ACFC [2, 8, 20]. In
at ~755 cm 1 due to the carbonization of these samples formed as the case of CuO NPs, the BET curve in Fig. 7(b) shows a type- IV isotherm
aromatic ring after the activation by KOH during the hydrothermal corresponding to the mesopores which are usually functioned as ion
1
process [19,20]. Moreover, those of vibrational peaks at ~601 cm , buffers, resulting in high pseudo-capacitive [14,24]. However, those
~525 cm 1 and ~448 cm 1 were assigned to Cu-O stretching modes [4, BET curves of CuO/ACFC-1, 2, 3, 4 Ncps shown in Fig. 7(c) – (f),
11] which can be seen in samples of CuO NPs and all CuO/ACFC Ncps. respectively, provide the adsorption that changes significantly at low
FT-IR results confirm the presence of CuO and ACFC in all CuO/ACFC-1, and high partial pressures, whereas an intermediate value remaining
2, 3, 4 Ncps. relatively constant, implying the isotherms of type IV as well. The per-
Fig. 7 (a) – (f) display the BET results of ACFC, CuO NPs and CuO/ fect pore structure and appropriate BET surface modification of the
ACFC-1, 2, 3, 4 Ncps, respectively. The BET curve of ACFC in Fig. 7(a) materials are suggested to result in a fast ion transfer and increase the
displays a hysteresis loop of type IV for N 2 adsorption–desorption efficiency of ion diffusion at the electrode/electrolyte surface. The
isotherm, which are a typical one of porous carbon materials at a high specific surface area and pore-size distribution of all samples were
partial pressure [12], corresponding to the micropores (diameter < 2 calculated from these BET curves using BJH technique and the obtained


7

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.

































































Fig. 6. (a) TGA results, (b) FT-IR spectra of ACFC, CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps.

results were listed in Table 2. Undeniably, the appeared high surface the peak positions of Cu 2p state show the Cu 2p 3/2 lines at approxi-
0
area and properly small pore size of CuO/ACFC-3 Ncps over others mately 933.2, 934.4 and 935.7 eV, illustrating the coexistence of Cu or
+
CuO/ACFC Ncps, as seen in Table 2, might be good enough to supply the Cu and Cu 2+ states which correspond to CuO phase in their structure
sufficient path ways for ion transportation and electrolyte diffusion, and and Cu(OH) 2 on samples surface. These results confirm the presence of
could result in the enhancement of pseudo-capacitive performance. Cu 2+ ions in all of these samples [1,4,6,11,22]. Furthermore, the XPS
spectra of C 1 s state in all of these samples, as shown in Fig. 8(c), display
the binding energy positions at 285.0, 286.2, 286.8 and 288.6 eV, cor-
3.5. XPS analysis
responding to the specific peaks of C=C, C–C, C–O and O–C=O func-
tional groups of carbon materials, respectively [1,4,16,18,20].
The surface atomic composition and chemical state of elements in Moreover, it is obviously seen in samples of CuO/ACFC-3 and 4 Ncps
CuO NPs and all CuO/ACFC Ncps were investigated by XPS technique.
that there is a peak of C–OH at 284.0 eV, confirming the existence of
As seen in Fig. 8(a), the complete XPS survey spectra of all samples hydroxyl group in both samples [18]. In Fig. 8(d), the XPS spectra of O 1
reveal the existence of Cu 2p, O 1 s and C 1 s states at the binding energy
s state in all of these samples display the peak positions at binding
position of 934.2, 284.9 and 530.1 eV, respectively [1,4,11]. In Fig. 8(b),
8

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.


































































Fig. 7. Nitrogen sorption isotherms with inset a pore-size distribution curves of (a) ACFC, (b) CuO NPs and (c) – (f) for CuO/ACFC-1, 2, 3, 4 NCPs, respectively.

energy of 530.0, 531.5, 532.9 and 534.0 eV, corresponding to Cu–O were studied at RT using a three-electrode cell system in 3.0 M KOH
bonding, OH , C–O and H–O–H (H 2 O), respectively [12,16,18,20]. In aqueous electrolyte within a potential window range of 1.0 V to 0.0 V
addition, a peak of C=O is observed at 529.2 eV in all CuO/ACFC Ncps. against an Ag/AgCl reference and a platinum plate counter electrode, of
Clearly, the XPS results evidently indicate that the surface of CuO NPs which the obtained results are displayed in Fig. 9(a)–(c). The CV plots in
0
+
and CuO/ACFC-1, 2, 3, 4 Ncps consist of Cu ions with Cu or Cu and Fig. 9(a) of ACFC electrode at various scan rates (5, 10, 20, 50 and 100
1
Cu 2+ states, O elements and C derived from ACFC, suggesting to benefit mV s ) offer precisely a typical behavior of EDLC, which is identical at a
for improving the surface area, conductivity of electrodes and the higher scan rate i.e. the curve shape remains the same [2,12,16,18],
Faradaic reaction [1]. representing the excellent stability and the well prepared electrode.
Furthermore, the GCD curves of ACFC electrode at various current
1
densities (1, 2, 3, 5 and 10 A g ) display the EDLC behavior as depicted
3.6. Electrochemical properties of ACFC
in Fig. 9(b), illustrating a practically symmetrical shape of triangle [8,
12]. In Fig 9(c), the GCD results display the highest electrochemical
The electrochemical properties of ACFC, as a working electrode,
9

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
Table 2 3.7. Electrochemical properties of CuO NPs and CuO/ACFC-1, 2, 3, 4
wt% of CuO in CuO/ACFC Ncps calculated from TGA spectra with specific Ncps
surface area and pore-size distribution of ACFC, CuO NPs and CuO/ACFC-1, 2, 3,
4 Ncps. The CV curves at scan rate 20 mV s 1 within a potential window 0 -
a
Samples CuO (wt Specific surface area b Pore-size distribution c 0.5 V of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes are shown in
2
%) (m g 1 ) (nm) Fig. 10(a). Obviously seen in Fig. 10(a), all curves display a redox re-
ACFC – 188.11 2.32 action of which the peak for oxidation appears around 0.39 – 0.40 V and
CuO NPs 99.54 7.00 63.67 that of reduction around 0.19–0.20 V, suggesting the pseudocapacitive
CuO/ACFC-1 91.27 45.71 3.05 behavior for all of these electrodes, mainly originates from the Faradaic
NCPs
CuO/ACFC-2 83.20 76.35 2.99 redox reaction between electrode surface and electrolyte [7,9,10,17,
22–24, 29,36–40]. Moreover, the Faradaic redox peak was found to
NCPs
CuO/ACFC-3 76.00 93.67 3.01 intensify with the increase of scan rate by a quasi-reversible redox
+
NCPs transition between Cu 2+ and Cu , corresponding to the plausible
CuO/ACFC-4 67.12 45.93 3.42 mechanism for the reaction proposed by Vidhyadharan et al. [9] as
NCPs
given in the following reaction (6).
a
Calculated from TGA spectra.
b 2CuO + H2O + 2e ↔ Cu2O + 2OH (6)
Determined by the nitrogen adsorption-desorption isotherm using BET
technique. 2+
c According to the reaction given in (6), Cu ions in CuO NPs of all
Determined by the nitrogen adsorption-desorption isotherm using BJH +
technique. electrodes are exchanged with Cu ions, as confirmed by the XPS results.
In addition, charges can be stored on the surface of each CuO/ACFC-1, 2,
3, 4 Ncps electrodes by the adsorption/desorption of electrolyte ions
reaction of ACFC electrode during charge/discharge reversibility with
the calculated specific capacitance (C s ) of 47.75, 36.94, 30.11, 20.60 (OH ) with CuO NPs on ACFCs surface [9] as illustrated in the following
and 9.37 F g 1 at current densities of 1, 2, 3, 5 and 10 A g 1 , respec- reaction (7)
tively. Furthermore, the inset of Fig 9(c) displays the cycling stability of CuO + OH ↔ CuOOH (7)
ACFC electrode at a current density of 5 A g 1 tested for 1000 cycles,
illustrating the ultimate capacitance of 104.41% with a good life-time In addition, the area enclosed by CV curves of CuO/ACFC-3 Ncps
and excellent stability [12,16,18]. electrode, as clearly seen in Fig. 10(a), is larger than those of other
electrodes, indicating the higher C s value and excellent electrochemical
performance of this electrode. Fig. 10(b)–(f) display the CV curves of
CuO NPs with all of those CuO/ACFC NCPs electrodes at scan rate of 5,
10, 20, 50, 80 and 100 mV s 1 in the same potential window range 0 -









































Fig. 8. XPS spectra of (a) survey spectra, (b) Cu 2p spectra, (c) C 1 s spectra and (d) O 1 s spectra of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps.

10

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
results.
In addition, the calculated C s values of these electrodes at different
current densities by GCD test are shown in Fig. 12(a) and listed in
Table 3. As depicted in Fig. 12(a), it is obvious that all CuO/ACFC Ncps
electrodes reveal the higher C s values than that of CuO NPs electrode.
Remarkably, the C s value of 145.33 F g 1 at current density 1 A g 1 of
CuO/ACFC-3 Ncps electrode is higher than that of CuO NPs (60.14 F
1
g ) and other CuO/ACFC-1, 2, 4 Ncps electrodes (103.35, 122.81, and
1
115.48 F g , respectively), indicating the significant role of appropriate
amount ACFC on the improvement of electrochemical performance of
1
CuO/ACFC Ncps. However, at higher current densities (2–10 A g ), the
C s value of all samples decreases mostly owing to the increasing
distortion in the unapproachable sites within their electrodes and the
less effective diffusion of electrolyte ions at a high current density [9,10,
13,22,37–40].
In order to verify the cycling stability of CuO NPs and CuO/ACFC-1,
2, 3, 4 Ncps electrodes, the GCD test at 5 A g 1 was carried out using
continuous measurement for 1000 cycles with the obtained results
depicted in Fig. 12(b) and listed in Table 3. Consequently, the capacity
retention after 1000 cycle test of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps
electrodes were found to be 84.68, 86.05, 86.10, 85.24 and 85.43%,
respectively. According to these results, the cycling stability of all CuO/
ACFC Ncps electrodes can be obviously improved, as compared to that of
CuO NPs electrode, owing to the excellent properties of ACFC with the
exceptional function as a good supporting material for CuO NPs to bind
on their surfaces [14,17]. In addition, the properly dispersed CuO NPs
on the surface of ACFC can prevent the agglomeration of CuO NPs and
could probably strengthen the structure of CuO/ACFC Ncps throughout
a long period of the GCD performance [17]. Furthermore, Fig. 12(c)
displays the Nyquist plots with the fitting plots of ACFC, CuO NPs and
CuO/ACFC-1, 2, 3, 4 Ncps electrodes. As displayed in Fig. 12(c), the
plots exhibit semicircle and straight line in a high frequency and a low
frequency region, respectively [7,14,22]. To analyze the EIS spectra of
all electrodes, all the experimental data were fitted with the equivalent
circuit model, as depicted in the inset of Fig. 12(c), using ZView soft-
ware, and the obtained results were listed in Table 3. The main com-
ponents R s , CPE, R ct , C p and DE diff in the equivalent circuit are referred
to the solution resistance, a constant phase element to account for the
double-layer capacitance, the charge transfer resistance, a pseudoca-
pacitive element from redox process of active materials, and the diffu-
sion resistance, respectively. Generally, the series resistance (R s ) of a
solution at the interface of electrode/electrolyte was determined from
′
the intercept on Z axis in a high frequency region of the plots [7,9,14,
22]. Principally, a semicircular curve is ascribed to the charge transfer
resistance (R ct ), consistence with the transportation of charge at the
interface of electrolyte/electrode [15,22,36]. As listed in Table 3, the R s
values of ACFC, CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes are
1.2020, 1.0530, 0.9968, 1.0630, 1.0100 and 0.9973 Ω, respectively.
Fig. 9. (a) CV curves at different scan rates, (b) GCD curves at different current
densities, and (C) specific capacitance at different current densities of ACFC Moreover, the R ct values of 1.9940, 0.5904, 3.8940, 1.8350, 0.7141 and
electrodes determined by GCD test with inset showing the cycling stability at a 1.0120 Ω were obtained for ACFC, CuO NPs and CuO/ACFC-1, 2, 3, 4
current density of 5 A g 1 . Ncps electrodes, respectively. As seen, the CuO/ACFC-3 Ncps displays
the low values of R s and R ct , implying a faster charge transfer mechanism
0.5 V, respectively. At a high scan rate, as shown in Fig. 10(b) – (f), the at the electrolyte/electrode surface which confirms the high C s value of
CV curves of all electrodes reveal the gradually increase of intensity and this electrode [9,22]. In addition, the slight decrease of DE diff values of
area with the increase of scan rate, suggesting the existence of a properly electrolyte ions, as seen in Table 3, implies that more electrolyte ions can
reversible redox reaction of a capacitive performance at the interface of easily move to electrodes surface. Additionally, the slope for the plot of
electrodes and KOH electrolyte [9,16,22,23]. CuO/ACFC-3 Ncps electrode is higher than that of other electrodes,
To further investigate the electrochemical properties of these elec- representing a good electrical conductivity owing to the influence of
trodes, the GCD test of these electrodes were performed at a current ACFC [14,22,36]. Thus, it is suggested that the improvement of elec-
density of 1 A g 1 in a potential window range 0–0.5 V with the obtained trochemical performance of CuO/ACFC-3 Ncps electrode is strongly
results displayed in Fig. 11(a). Moreover, the same tests for these elec- regulated by the ion transport/diffusion process of pseudocapacitive
trodes at various current densities within the same potential window are behavior, according to the proper presence of ACFC in CuO/ACFC-3
depicted in Fig. 11(b)–(f), respectively. Clearly seen in Fig. 11(b)–(f), the Ncps electrode.
GCD curves of all electrodes disclose the reversible redox reaction of the
pseudocapacitive type [9,11,22,23,36], correlating with their CV


11

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.


































































Fig. 10. (a) CV curves at a scan rate 20 mV s 1 of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes, (b) – (f) CV curves at different scan rates of CuO NPs and CuO/
ACFC-1, 2, 3, 4 Ncps electrodes, respectively.

3.8. ACFC//CuO/ACFC-3 device assembly presents the CV curves of the optimized ACFC//CuO/ACFC-3 device
scanned under operating voltage of 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5 V at a
To explore the real application of fabricated electrode, CuO/ACFC-3 scan rate 100 mV s 1 . According to the results in Fig. 13(a), the CV curve
Ncps of the highest C s value was selected to be used as a positive elec- of ACFC//CuO/ACFC-3 device performed at a different range of oper-
trode and ACFC as a negative electrode for the ASC assembling using a ating voltage has a similar shape of the pseudo-capacitive response [1, 6,
CR-2032-coin cell. Accordingly, both electrodes were pasted with 3 M 22]. Furthermore, it is obvious that the enclose area of these CV curves
KOH electrolyte and separated by a fiber glass sheet. According to the increase with the increase of operating voltage, corresponding to the
difference of potential range employed for the electrochemical analysis increase in capacitive response and consistent with their GCD curves
of a negative ACFC electrode ( 1.0 V to 0.0 V) and a positive CuO/ shown in Fig. 13(b). In addition, the GCD curves of ACFC//CuO/ACFC-3
ACFC-3 Ncps electrode (0.0 V to 0.5 V), it is thus suggested to extend device at current density 0.5, 1, 1.5, 2, 3 and 5 A g 1 under a constant
the working voltage for ACFC//CuO/ACFC-3 device to 1.5 V. Fig. 13(a) working voltage between 0 and 1.4 V (as seen in Fig. 13(c)) retains the


12

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.






























































Fig. 11. (a) GCD curves at a current density 1 A g 1 of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes, (b) – (f) GCD curves at different current densities of CuO
NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes, respectively.

similar shape, indicating their good capacitive performance [3, 22]. electrochemical performance characteristic before and after 1000
Moreover, the calculated C s values of ACFC//CuO/ACFC-3 device at continuous GCD cycles test as shown by the Nyquist plots in Fig. 13(f),
different current densities as shown in Fig. 13(d) present an outstanding and the results show that both Nyquist plots are almost the same, indi-
value of 24.94, 17.86, 13.25, 9.35, 6.44 and 4.53 F g 1 (within a wide cating the outstanding cycling stability of the device [1, 22, 36].
voltage window and a potential from 0.0 to 1.5 V) at current densities The CV results of ACFC//CuO/ACFC-3 device with a single cell and
1
0.5, 1, 1.5, 2, 3 and 5 A g , respectively. In addition, as shown in the two cells connected in series, obtained in different potential ranges of
inset of Fig. 13(d), the ACFC//CuO/ACFC-3 device shows the E sp of 7.78, 0–3.0 V at scan rate 100 mV s 1 as displayed in Fig. 14(a), represent the
5.58, 4.12, 2.91, 2.00 and 1.41 Wh kg 1 and the P sp of 375, 750, 1125, stability of assembly as a result of pseudocapacitive behavior of the
1
1500, 2250 and 3000 W kg at current density 0.5, 1, 1.5, 2, 3 and 5 A hybrid device [22]. Furthermore, the GCD curves at different current
g 1 , respectively. Furthermore, the capacity retention of densities in a potential range 0 – 2.6 V of two cells connected in series of
ACFC//CuO/ACFC-3 device as investigated by GCD at 2 A g 1 after ACFC//CuO/ACFC-3 device (as displayed in Fig. 14(b)) reveals a curve
1500 cycles test can maintain at about 75.2% to the initial value, as of similar shape at different current densities, confirming the stability of
shown in Fig. 13(e). Additionally, the EIS measurements of capacitive performance [5,22,24]. Additionally, a simple application to
ACFC//CuO/ACFC-3 device were performed to illustrate the load of a green-light emitting diode has been demonstrated to validate


13

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.




















































Fig. 12. (a) Specific capacitance of CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes at different current densities, (b) cycling stability at a current density 5 A g 1
of all electrodes, (c) Nyquist plots with fitting lines of all electrodes. The upper and lower insets in (c) are the equivalent circuit used for fitting and the enlargement of
the plots near origin, respectively.


Table 3
Specific capacitance at different current densities calculated by GCD test, cycling stability after 1000 cycles GCD test at a current density of 5 A g 1 and EIS analysis of
ACFC, CuO NPs and CuO/ACFC-1, 2, 3, 4 Ncps electrodes.
Parameter ACFC CuO NPs CuO/ACFC-1 Ncps CuO/ACFC-2 Ncps CuO/ACFC-3 Ncps CuO/ACFC-4 Ncps
1 A g 1 47.75 60.14 103.35 122.81 145.33 115.48
C s (F g 1 ) 2 A g 1 36.99 54.48 81.47 98.30 117.55 91.91
3 A g 1 30.11 50.87 73.03 88.66 107.08 83.47
4 A g 1 - 48.10 67.60 82.57 100.78 78.28
5 A g 1 20.60 45.62 62.91 77.95 96.02 73.80
8 A g 1 - 40.20 54.66 68.58 86.87 65.93
1
10 A g 9.37 37.26 50.52 64.18 81.87 61.48
Capacity retention (%) 104.41 84.68 86.05 86.10 85.24 85.43
R s (Ω) 1.2020 1.0530 0.9968 1.0630 1.0100 0.9973
EIS analysis CPE-T 0.0463 0.0249 0.0423 0.0388 0.0730 0.3561
CPE-P 0.6238 0.6593 0.5393 0.5467 0.5181 0.3773
R ct (Ω) 1.9940 0.5904 3.8940 1.8350 0.7141 1.0120
C p 0.0050 0.0108 0.0197 0.0148 0.0721 0.0570
DE diff -R 0.0019 0.1112 0.0103 0.0057 0.0058 0.0059
DE diff -T 0.3488 0.6224 0.6189 0.6381 0.7853 0.5875
DE diff -P 0.9609 1.9100 1.0260 0.9308 0.9069 0.9499
DE diff -U 0.0297 0.0588 0.0723 0.0322 0.0289 0.0429




14

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.






























































1
Fig. 13. (a) CV curves at scan rate 100 mV s , (b) GCD curves at current density 0.5 A g 1 in different potential ranges from 0 to 1.5 V, (c) GCD curves at different
current densities in a potential range 0 1.5 V, (d) Specific capacitance with inset the Ragone plots at different current densities, (e) the cycling stability at a current
density of 2 A g 1 and (f) Nyquist plots with fitting lines measuring before and after stability of ACFC//CuO/ACFC-3 device, respectively. Upper and lower insets of
(f) are the equivalent circuit for fitting and the enlargement of plots near origin, respectively.

the potential usefulness of three cells connected in series of 4. Conclusion
ACFC//CuO/ACFC-3 device, as shown in Fig. 14(c). Amazingly, as seen
in Fig. 14(c), three cells connected in series of ACFC//CuO/ACFC-3 In summary, pure phase of CuO NPs and CuO/ACFC Ncps with
device with green LED loading can light up for more than 120 s. From different ACFC loading of 0.1, 0.2, 0.3 and 0.4 g were successfully
◦
these attractive results, CuO/ACFC-3 Ncps is suggested to be a promising prepared by a one-pot hydrothermal method at 180 C for 6 h. XRD
electrode material for a high performance supercapacitor in a wide results revealed a monoclinic phase of CuO in these samples, belonging
range application of interest [24]. In addition, the maximum value of C s , to a space group C2/c with average crystallite size of 34.1 ± 3.3 nm for
E sp and P sp obtained in this work were compared for the performance CuO NPs. FE-SEM revealed a characteristic morphology of each products
evaluation with others work, as listed in Table 4. i.e. a filamentous shape with grooves on surface and several tubes inside
similar to that of lotus root for ACFC, a very fine oval-like shape for CuO
NPs and an icing-like CuO NPs that grown on the surface of ACFC for all
CuO/ACFC Ncps. All positive CuO/ACFC-1, 2, 3, 4 Ncps electrodes


15

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.














































Fig. 14. (a) CV curves at scan rate 100 mV s 1 of single cell and two cells connected in series of ACFC//CuO/ACFC-3 device, showing at different potential range of
0–3.0 V, (b) GCD curves at different current densities in a potential range of 0–2.6 V for two cells connected in series of the same device, and (c) a photographic of
three cells connected in series of the same device in loading of a green LED.

Table 4
Comparison of the maximum C s , E sp and P sp values obtained in this work with others work.
Electrode materials [ref.] 3 Electrode Devices
Electrolyte C s (F g ¡1 ) Capacity retention(%) E sp (Wh P sp (W Voltage range Capacity retention
kg ¡1 ) kg ¡1 ) (V) (%)
1
CuO-rGO [41] 2 M KOH 296 (1 A g ) 96.1%, 2000 cycle(2 A - - - -
1
g )
C/CuO@g-C 3 N 4 [42] 0.5 M NaOH 247.2(1 A g 1 ) 92.1%, 6000 cycle(1 A - - - -
1
g )
CuO/Co 2 O 4 @ N-MWCNT [43] 5 M KOH 278(0.5 A g 1 ) - ~7.544 ~251 - 89%,
5000 cycle
(0.5 A g 1 )
1
CuO/MPC [44] 1 M KOH 616(1 A g ) 57%, 5000 cycle(10 A 26.6 438 0–1.4 69%,
1
(3electrode) g ) 5000 cycle
PVA/KOH (5 A g 1 )
(devices)
CuO/CNS-20 [45] 1 M KOH 371.1(1 A g 1 ) 87%, 2000 cycle(1 A 19.36 355.6 0–1.4 52%,
g 1 ) 2000 cycle
CuO/CNS-20 [45] 1 M Na 2 SO 4 183.9(1 A g 1 ) 51.3%, 2000 cycle(1 A 12.46 355.6 0–1.2 94.4%,
g 1 ) 2000 cycle
Anisotropic carbon /polylaminate 3 M KOH 694.8(0.5 A - 13.6 350.3 0–1.5 91.2%,
CuO [46] g 1 ) 5000 cycle
1
(10 A g )
CuO/ACFC-0.3 g Ncps [This work] 3 M KOH 145.33(1 A 85.24%, 1000 cycle(5 A 7.78 3000 0–1.5 75.2%, 1500 cycle
g 1 ) g 1 ) (2 A g 1 )


16

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
exhibited pseudocapacitive behavior, mainly originate from the Fara- [13] S. Min, C. Zhao, Z. Zhang, K. Wang, G. Chen, X. Qian, Z. Guo, Hydrothermal
daic redox reaction by quasi-reversible redox transition between Cu 2+ growth of MnO 2 /RGO/Ni(OH) 2 on nickel foam with superior supercapacitor
performance, RSC Adv 5 (2015) 62571–62576.
+
and Cu in electrodes and the adsorption/desorption of electrolyte ions [14] M. Sham Lal, T. Lavanya, S. Ramaprabhu, An efficient electrode material for high
(OH ) at ACFC surface. The maximum C s value of 145.33 F g 1 at a performance solid-state hybrid supercapacitors based on a Cu/CuO/porous carbon
current density 1 A g 1 was achieved in CuO/ACFC-3 Ncps electrode nanofiber/TiO 2 hybrid composite, Beilstein J. Nanotechnol. 10 (2019) 781–793.
[15] Y. Li, L. Xu, J. Gao, X. Jin, Hydrothermal fabrication of reduced graphene oxide/
with capacity retention of 85.24% after 1000 cycles of charge/discharge activated carbon/MnO 2 hybrids with excellent electrochemical performance for
1
test at 5 A g . Remarkably, the ASC using a CR-2032-coin cell of supercapacitors, RSC Adv 7 (2017) 39024–39033.
ACFC//CuO/ACFC-3 device could provide an expansive potential [16] J. Han, Q. Li, J. Wang, J. Ye, G. Fu, L. Zhai, Y. Zhu, Heteroatoms (O, N)-doped
porous carbon derived from bamboo shoots shells for high performance
voltage window in a range 0.0–1.5 V and displayed the C s value of 24.94 supercapacitors, J. Mater. Sci. Mater. Electron. 29 (2018) 20991–21001.
A g 1 at 0.5 A g 1 . In addition, the device illustrated the maximum E sp of [17] Y. Li, X. Wang, Q. Yang, M.S. Javed, Q. Liu, W. Xu, C. Hu, D. Wei, Ultra-fine CuO
7.78 Wh kg 1 and P sp of 375 W kg 1 with cycle stability 75.2% at 2 A g 1 nanoparticles embedded in three-dimensional graphene network nano-structure
for high-performance flexible supercapacitors, Electrochim. Acta 234 (2017)
after 1500 continuous GCD cycling test. Compared with other expensive
63–70.
carbon electrode materials, CuO/ACFC-0.3 Ncps is recommended to be a [18] A. Jayakumar, J. Zhao, J.-.M. Lee, A coconut leaf sheath derived graphitized N-
beneficial one for the efficient and high-performance electrode material doped carbon network for high-performance supercapacitors, ChemElectroChem 5
of supercapacitor due to its low cost, environmental benign and simple (2018) 284–291.
[19] K. Thakur, S. Kalia, B.S. Kaith, D. Pathania, A. Kumar, Surface functionalization of
fabrication process. coconut fibers by enzymatic biografting of syringaldehyde for the development of
biocomposites, RSC Adv. 5 (2015) 76844–76851.
[20] L. Zhang, L.-.Y. Tu, Y. Liang, Q. Chen, Z.-.S. Li, C.-.H. Li, Z.-.H. Wang, W. Li,
Coconut-based activated carbon fibers for efficient adsorption of various organic
Declaration of Competing Interest
dyes, RSC Adv. 8 (2018) 42280–42291.
[21] D. Li, Y. Zhu, E. Xu, H. Wang, T. Chen, J. Quan, Y. Zhang, L. Wang, Y. Jiang, Ion-
The authors declare that they have no known competing financial and air-tailored micro-honeycomb structures for superior Na-ion storage in coir-
interests or personal relationships that could have appeared to influence derived hard carbon, New J. Chem. 43 (2019) 10449–10457.
[22] A.K. Mishra, A.K. Nayak, A.K. Das, D. Pradhan, Microwave-assisted solvothermal
the work reported in this paper.
synthesis of cupric oxide nanostructures for high-performance supercapacitor,
J. Phys. Chem. C 122 (2018) 11249–11261.
[23] S.M. Pawar, J. Kim, A.I. Inamdar, H. Woo, Y. Jo, B.S. Pawar, S. Cho, H. Kim, H. Im,
Acknowledgments
Multi-functional reactively-sputtered copper oxide electrodes for supercapacitor
and electro-catalyst in direct methanol fuel cell applications, Sci. Rep. 6 (2016)
This work has received scholarship under the Post - Doctoral 21310.
[24] S.E. Moosavifard, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi,
Training Program from Khon Kaen University, Thailand and it has been
Designing 3D highly ordered nanoporous CuO electrodes for high-performance
partially supported by the Research Network NANOTEC (RNN) program asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 4851–4860.
of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry [25] H. Zhang, J. Feng, M. Zhang, Preparation of flower-like CuO by a simple chemical
of Higher Education, Science, Research and Innovation (MHESI) and precipitation method and their application as electrode materials for capacitor,
Mater. Res. Bull. 43 (2008) 3221–3226.
Khon Kaen University, Thailand. Moreover, this research was also sup-
[26] Y. Li, S. Chang, X. Liu, J. Huang, J. Yin, G. Wang, D. Cao, Nanostructured CuO
ported by the Basic Research Fund of Khon Kaen University (Grant no. directly grown on copper foam and their supercapacitance performance,
1500147). Electrochim. Acta 85 (2012) 393–398.
[27] G. Wang, J. Huang, S. Chen, Y. Gao, D. Cao, Preparation and supercapacitance of
CuO nanosheet arrays grown on nickel foam, J. Power Sources 196 (2011)
References 5756–5760.
[28] L.J. Yu, Y. Li, L. Ma, J. Wang, G. Geng, B. Zhang, X., Facile synthesis of mesoporous
CuO nanoribbons for electrochemical capacitors applications, Int. J. Electrochem.
[1] L. Xu, J. Li, H. Sun, X. Guo, J. Xu, H. Zhang, X. Zhang, In situ growth of Cu 2 O/CuO Sci. 8 (2013) 1366–1381.
nanosheets on Cu coating carbon cloths as a binder-free electrode for asymmetric [29] L. Yu, Y. Jin, L. Li, J. Ma, G. Wang, B. Geng, X. Zhang, 3D porous gear-like copper
supercapacitors, Front. Chem. 7 (2019) 420.
oxide and their high electrochemical performance as supercapacitors,
[2] K.P.S. Prasad, D.S. Dhawale, T. Sivakumar, S.S. Aldeyab, J.S.M. Zaidi, K. Ariga,
CrystEngComm 15 (2013) 7657–7662.
A. Vinu, Fabrication and textural characterization of nanoporous carbon electrodes
[30] D. Das, D.P. Samal, B. Meikap, Preparation of activated carbon from green coconut
embedded with CuO nanoparticles for supercapacitors, Sci. Technol. Adv. Mater.
shell and its characterization, J. Chem. Eng. Process Technol. 6 (2015) 5.
12 (2011), 044602.
[31] S. Meghana, P. Kabra, S. Chakraborty, N. Padmavathy, Understanding the pathway
[3] M. Huang, Y. Zhang, F. Li, Z. Wang, N.Hu Alamusi, Z. Wen, Q. Liu, Merging of
of antibacterial activity of copper oxide nanoparticles, RSC Adv. 5 (2015)
Kirkendall growth and ostwald ripening: CuO@MnO 2 core-shell architectures for
12293–12299.
asymmetric supercapacitors, Sci. Rep. 4 (2014) 4518.
[32] A.S. Ethiraj, D.J. Kang, Synthesis and characterization of CuO nanowires by a
[4] M.G.T Nathan, J.M. Boby, P. Basu, R. Mahesh, S. Harish, S. Joseph, P. Sagayaraj,
simple wet chemical method, Nanoscale Res. Lett. 7 (2012) 70.
One-pot hydrothermal preparation of Cu 2 O-CuO/rGO nanocomposites with [33] S. Sundar, G. Venkatachalam, S.J. Kwon, Biosynthesis of Copper Oxide (CuO)
enhanced electrochemical performance for supercapacitor applications, Appl. Surf. nanowires and their use for the electrochemical sensing of dopamine,
Sci. 449 (2018) 474–484.
Nanomaterials 8 (2018) 823.
[5] S. Kamila, B. Mohanty, A.K. Samantara, P. Guha, A. Ghosh, B. Jena, P.V. Satyam, B. [34] A. Umar, J.-.H. Lee, R. Kumar, O. Al-Dossary, A.A. Ibrahim, S. Baskoutas,
K. Mishra, B.K. Jena, Highly active 2D layered MoS 2 -rGO hybrids for energy Development of highly sensitive and selective ethanol sensor based on lance-
conversion and storage applications, Sci. Rep. 7 (2017), 8378–8378.
shaped CuO nanostructures, Mater. Des. 105 (2016) 16–24.
[6] A.M. Zardkhoshoui, S.S.H. Davarani, Flexible asymmetric supercapacitors based on [35] E. Swatsitang, A. Karaphun, T. Putjuso, Influence of Fe:Co co–doping on the
CuO@MnO 2 -rGO and MoS 2 -rGO with ultrahigh energy density, J. Electroanal. morphology, optical and magnetic properties of Cu 1-(x+y) Fe x Co y O nanostructures
Chem. 827 (2018) 221–229.
[7] K. Wang, X. Dong, C. Zhao, X. Qian, Y. Xu, Facile synthesis of Cu 2 O/CuO/RGO prepared by a hydrothermal method, Physica B Condens. Matter 583 (2020),
412044.
nanocomposite and its superior cyclability in supercapacitor, Electrochim. Acta [36] N. Budhiraja, S.V. Kumar, M. Tomar, V. Gupta, S.K. Singh, Multifunctional CuO
152 (2015) 433–442.
[8] M. Kim, Y. Kim, K.M. Lee, S.Y. Jeong, E. Lee, S.H. Baeck, S.E. Shim, nanosheets for high-performance supercapacitor electrodes with enhanced
photocatalytic activity, J. Inorg. Organomet. Polym. Mater. 29 (2019) 1067–1075.
Electrochemical improvement due to alignment of carbon nanofibers fabricated by [37] K.G. Chandrappa, T.V. Venkatesha, Electrochemical bulk synthesis and
electrospinning as an electrode for supercapacitor, Carbon 99 (2016) 607–618.
[9] B. Vidhyadharan, I.I. Misnon, R.A. Aziz, K.P. Padmasree, M.M. Yusoff, R. Jose, characterisation of hexagonal-shaped CuO nanoparticles, J. Exp. Nanosci. 8 (2013)
516–532.
Superior supercapacitive performance in electrospun copper oxide nanowire [38] Y.N. Sudhakar, H. Hemant, S.S. Nitinkumar, P. Poornesh, M. Selvakumar, Green
electrodes, J. Mater. Chem. A 2 (2014) 6578–6588.
[10] A. Aljaafari, N. Parveen, F. Ahmad, M.W. Alam, S.A. Ansari, Self-assembled cube- synthesis and electrochemical characterization of rGO–CuO nanocomposites for
supercapacitor applications, Ionics 23 (2017) 1267–1276.
like copper oxide derived from a metal-organic framework as a high-performance [39] C.J. Jafta, F. Nkosi, L. le Roux, M.K. Mathe, M. Kebede, K. Makgopa, Y. Song,
electrochemical supercapacitive electrode material, Sci. Rep. 9 (2019) 9140.
[11] S. Rai, R. Bhujel, J. Biswas, B.P. Swain, Effect of electrolyte on the supercapacitive D. Tong, M. Oyama, N. Manyala, S. Chen, K.I. Ozoemena, Manganese oxide/
graphene oxide composites for high-energy aqueous asymmetric electrochemical
behaviour of copper oxide/reduced graphene oxide nanocomposite, Ceram. Int. 45 capacitors, Electrochim. Acta 10 (2013) 228–233.
(2019) 14136–14145.
[12] B. Hou, T. Zhang, R. Yan, D. Li, Y. Mo, L. Yin, Y. Chen, High specific surface area [40] S.-.J. Li, Y. Xing, L.-.L. Hou, Z.-.Q. Feng, Y. Tian, J.-.M. Du, Facile synthesis of NiO/
CuO/reduced graphene oxide nanocomposites for use in enzyme-free glucose
activated carbon with well-balanced micro/mesoporosity for ultrahigh
sensing, Int. J. Electrochem. Sci. 11 (2016) 6747–6760.
supercapacitive performance, Int. J. Electrochem. Sci. 11 (2016) 9007–9018.
17

Surfaces and Interfaces 25 (2021) 101174
S. Sawadsitang et al.
[41] A. Liu, Y. Bai, Y. Liu, M. Zhao, J. Mu, C. Wu, X. Zhang, G. Wang, H. Che, [44] B. Saravanakumar, C. Radhakrishnan, M. Ramasamy, R. Kaliaperumal, A.J. Britten,
Hierarchical dandelion-like copper oxide wrapped by reduced graphene oxide: M. Mkandawire, Copper oxide/mesoporous carbon nanocomposite synthesis,
hydrothermal synthesis and their application in supercapacitors, Mater. Res. Bull. morphology and electrochemical properties for gel polymer-based asymmetric
84 (2016) 85–92. supercapacitors, J. Electroanal. Chem. 852 (2019), 113504.
[42] S.V.P. Vattikuti, B.P. Reddy, C. Byon, J. Shim, Carbon/CuO nanosphere-anchored [45] X. Yuan, X. Yan, C. Zhou, J. Wang, D. Wang, H. Jiang, Y. Zhu, X. Tao, X. Cheng,
g-C3N4 nanosheets as ternary electrode material for supercapacitors, J. Solid State Decorating carbon nanosheets with copper oxide nanoparticles for boosting the
Chem. 262 (2018) 106–111. electrochemical performance of symmetric coin cell supercapacitor with different
[43] S. Ramesh, A. Kathalingam, K. Karuppasamy, H.-.S. Kim, H.S. Kim, Nanostructured electrolytes, Ceram. Int. 46 (2020) 435–443.
CuO/Co 2 O 4 @ nitrogen doped MWCNT hybrid composite electrode for high- [46] C. Wan, W. Tian, J. Zhou, Y. Qing, Q. Huang, X. Li, S. Wei, L. Zhang, X. Liu, Y. Wu,
performance supercapacitors, Compos. B. Eng. 166 (2019) 74–85. Green anisotropic carbon-stabilized polylaminate copper oxide as a novel cathode
for high-performance hybrid supercapacitors, Mater. Des. 198 (2021), 109309.












































































18