Journal of Solid State Electrochemistry

https://doi.org/10.1007/s10008-023-05676-7

ORIGINAL PAPER

Hierarchical 3D flowers of 1T@2H‑MoS2 assembled with an array

of ultrathin nano‑petals for high‑performance supercapacitor electrodes
Mahesh R. Charapale1 · Tukaram D. Dongale2 · Omkar. A. Patil2 · Aviraj M. Teli3 · Sonali A. Beknalkar3 ·
Sajid B. Mullani4 · Sagar M. Mane5 · Jaewoong Lee5 · Shivanand A. Masti1

Received: 23 June 2023 / Revised: 18 August 2023 / Accepted: 8 September 2023

© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023

Abstract
The use of transition metal dichalcogenides (TMDs) for energy storage and/or conversion applications has become quite popular.
Molybdenum sulfide ­(MoS2), one of many TMDs, has become a promising option for energy storage devices such as recharge-
able batteries and supercapacitors due to its peculiar chemical and structural characteristics. Assembled with incredibly thin
nano-petals, hierarchical 3D flowers of the 1T@2H-MoS2 were created in this study using a straightforward one-pot hydrother-
mal method. The physiological and chemical features of the hierarchical 3D ­MoS2 were examined using a variety of approaches.
The formation of hexagonal crystallinity was revealed by investigating X-ray diffraction. The presence of only two bands (­ E2g
and ­A1g) in Raman spectroscopy confirms phase formation. Scanning electron microscope (SEM) images reflect bunched 3D
flowers of M­ oS2 assembled with a large number of ultrathin nano-petals. The average thickness of nano-petals remains below
40 nm. Elemental presence was rectified through energy-dispersive X-ray spectroscopy (EDS) and their states were examined
using X-ray photoelectron spectroscopy (XPS). The electrode of such a 3D hierarchical architecture flaunts a higher specific
capacitance of 207.14 F/g at a current density of 1A/g and exceptional stability of 93.6% across 1000 charge–discharge cycles.
This study elaborates on the easiest path to develop the 3D hierarchical architecture of ­MoS2 for a variety of applications.

Keywords MoS2 · Hydrothermal · 3D sub-micron flowers · Ultrathin nano-petals · Supercapcitor · Charge-storage kinetics

Introduction

Generation and storage of green energy are in greater

demand with exhausting fossil fuels to avoid the coming
* Sagar M. Mane
manesagar99@gmail.com severe energy crisis. Direct storage of energy in electrical
form is challenging another approach to conversion and stor-
* Jaewoong Lee
jaewlee@yu.ac.kr age in other forms and again conversion in its original form
is suggested. The only ecologically benign technologies that
* Shivanand A. Masti
shivamasti111@gmail.com can store electrical energy in converted form and supply it in
its original form when needed are solar cells, fuel cells, bat-
1
Department of Physics, Dr. Ghali College Gadhinglaj, teries, capacitors, and supercapacitors [1–3]. Among these
Shivaji University, Kolhapur Maharashtra 416502, India
supercapacitors which are known to store electrical energy
2
Computational Electronics and Nanoscience Research in chemical form are gaining a huge interest as a reliable
Laboratory, School of Nanoscience and Biotechnology, and effective energy storage and/or energy conversion sys-
Shivaji University, Kolhapur, Maharashtra 416004, India
3
tem [4]. The supercapacitor has various benefits over bat-
Division of Electronics and Electrical Engineering, Dongguk teries and ordinary capacitors, including a longer lifespan,
University, 30, Pildong‑ro, Jung‑gu, Seoul 04620, Korea
4
a high power density, a quick charge/discharge cycle rate,
Department of Chemistry, Shivaji University, Kolhapur, and pollution-free, lightweight, quick fabrication process,
Maharashtra 416004, India
5
low fabrication cost, and secure inherent operation [5–10].
Department of Fiber System Engineering, Yeungnam Supercapacitors can be divided into three types, includ-
University, 280 Daehak‑ro, Gyeongsan 712‑749,
Republic of Korea ing (i) electrical double-layer capacitors (EDLCs), (ii)

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 Journal of Solid State Electrochemistry

pseudocapacitors (PCs), and (iii) hybrid capacitors (HCs), Several researchers have examined the supercapacitive
based on their methods for storing energy [11, 12]. Carbon- performance of electrodes made of M ­ oS2 to date. Huang
based materials fall under the EDLC category and in this type et al. [30] created a ­MoS2 nanosheet and reached a maximum
of material, the capacitance value is lesser as it is derived capacitance of 129.2 F/g at 1 A/g. ­MoS2 nanosheets on car-
from the non-faradic charge transfer mechanism. In another bon fabric were created by Zhou et al. [31] and can display
type of supercapacitor, the capacitance was achieved by the a capacitance of 151 F/g at a current density of 10 mA/cm2.
reversible surface redox reactions which makes it easy to With a maximum possible specific capacitance value of 255
achieve the higher value of specific capacitance. Various F/g at 0.25 A/g, 2D nanostructures of ­MoS2 were created by
metal ions in their hydroxide/oxide/sulfide form and con- Gupta et al. [32]. Spherical nanorods of the ­MoS2 after an
ducting polymers show pseudocapacitive features. On the expansion of the interlayer spacing reported by Pan et al.
other hand, because of their widespread use in many appli- [33] have able to deliver a maximum specific capacitance
cations, together with energy storage and/or conversion of 165.5 F/g at 0.5 A/g and give ultralong cycling stabil-
devices, transition metal dichalcogenides (TMDs), such as ity with 93.8% capacitance retention at 5 A/g over 30,000
­MoS2, ­WS2, ­MoSe2, and W ­ Se2 [13–15], have drawn atten- cycles. Flower-like morphology of ­MoS2 assembled with
tion. Molybdenum sulfide ­(MoS2) is a more likely option nanosheets fabricated and reported by Fan et al. [34] can
from a range of TMD families for incredible applications deliver a specific capacitance of 19.1 F/g at a very low cur-
because of its chemical and structural characteristics. The rent density of 0.2 A/g while with a decoration of carbon
tremendous applications include batteries [16], electrochemi- on the sheets, the specific capacitance enhances to 201.4
cal capacitors (ECs) [17], solar cells [18], memristors [19], F/g at the same current density. Flower-like M ­ oS2 micro-
biosensors [20], solid lubricants [21], gas sensors [22], and spheres reported by Ma et al. [35] give a maximum spe-
photocatalysts [23] among others. This chalcogenide has sev- cific capacitance of 223 F/g at a lower current density of 0.6
eral layers created by covalent interaction between ­Mo4+ and A/g. Nevertheless, the weak conductivity of ­MoS2 makes it
­S2− atoms, creating an S-Mo-S framework with a minuscule challenging to use active materials to boost the capacitance
van der Waals gap between each layer [24]. The Mo atom and overall electrochemical performance. Designing and
at the center has several redox states such as 2­ +, ­3+, ­4+, ­5+, creating the various hierarchical nano-architectures of this
and ­6+, which makes this TMD more advantageous for the chalcogenide material is one technique to improve overall
charge storage application [25]. Six sulfur atoms harmonize supercapacitive performance. Because redox processes in
the Mo atom in each layer. Here, active sulfur sites offer a pseudocapacitors might potentially occur on surfaces as
higher diffusion rate and faster movement of the electrolytic small as a few nanometers thick, shrinking the material will
ions over and in the gap between various layers through the ultimately increase the amount of active material used [36].
reversible redox reaction process [16, 26]. In general, 1 T Therefore, achieving a hierarchical morphology with diverse
(trigonal), 2H (hexagonal), and 3R (rhombohedral) are the dimensions particles scaled to a few nanometers is a crucial
three crystal prototypes found in ­MoS2 depending on the element for the creation of electrodes with excellent electro-
coordination between Mo and S atom and the sequence of the chemical performance.
all stacked layers. The numbers associated with each crystal This paper outlines the simple hydrothermal technique
phase, i.e., 1, 2, 3 point out the number of stacked layers asso- used to create the 3D flower-like hierarchical architecture
ciated with each crystal phase in the unit cell of the M ­ oS2. of ­MoS2 with combined polymorphs of 1 T and 2H phases
Out of these three two polymorphs, the most common (2H) constructed from the 2D ultrathin mesoporous nano-petals.
and metastable (3R) are semiconducting while 1 T has pure The average thickness of these nano-petals remains below
metallic characteristics. Conversion between the most com- 40 nm and the length is up to a few hundred nanometers.
mon semiconducting 2H and metallic 1 T phase is possible Such a massive, multi-dimensionally constructed morphol-
through electron doping which gives an easy way to tune ogy is eager to offer better paths for charge transfer dur-
the electronic features of the M ­ oS2 [27, 28]. However, due ing the reversible faradaic redox reaction process, improv-
to the larger technical issues in the fabrication of a single ing the material’s overall electrochemical performance.
polymorph, ­MoS2 with the mixture of 1 T and 2H phases was The maximum specific capacitance of 207.14 F/g can be
most preferred for the various advanced applications. Fur- attained by the as-fabricated electrode of a 3D flower-like
thermore, the coexistence of 1 T and 2H phases gives rise to ­MoS2 material at a high applied current density of 1 A/g,
an expansion of interlayer spacing which leads to the ampli- and the electrode maintains 93.6% of its initial capacitance
fication of the electrical conductivity and boosts the diffu- after 1000 charge–discharge cycles. Additionally, a power
sion of electrolytic ions (specifically sodium ion), whereas law analysis was used to thoroughly examine the charge-
the drawback of low surface hydrophilicity associated with storage mechanism of this pseudocapacitive electrode. An
2H-MoS2 confines the diffusion rate of electrolytic ions on increase in specific capacitance was demonstrated for the
the surface [16, 28, 29]. ­MoS2 with combined 1 T and 2H phases when a comparison

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Journal of Solid State Electrochemistry

was made with the previous studies. These improvements are was reached following the reaction. The as-obtained solu-
mostly related to more reactive sites created by an increase tion was centrifuged to recover the final product of 3D
in specific surface area, demonstrating this hierarchical flower-like ­MoS2 which was further washed thoroughly
architecture of ­MoS2 is predominantly useful for electrodes using DI water and ethanol and further placed for drying
in energy storage devices. at 80 °C for overnight. Investigations were conducted on
the physicochemical characteristics and electrochemical
performance of this dried powder made of 3D flower-like
Materials and techniques ­MoS2 combined with extremely thin nano-petals. Sche-
matically representation for the synthesis procedure of the
Chemicals 3D flower-like ­MoS2 assembled with 2D ultrathin petals
was provided in Scheme 1.
Analytical-grade chemicals with higher (99%) purity were
utilized without any further treatment or purification for the Material characterization
development of M ­ oS2. Thiourea (CS(NH2)2), ammonium
molybdate tetrahydrate ((NH4)6Mo7O24.4H2O), and ethanol X-ray diffraction (XRD) patterns were recorded using
­(C2H6O) were obtained from Loba Chemie Pvt. Ltd. (Mum- a diffractogram with Cu-Kα radiation of 0.15418 nm
bai, Maharashtra, India) India. Polyvinylidene difluoride (D8-Bruker, Billerica, MA, USA). Raman spectroscopy
(PVDF) and carbon black (CB) were obtained from, Alfa (Xplora micro-Raman spectrometer, Horiba, Kyoto, Japan)
Aesar (Ward Hill, MA, USA). was used to investigate the purity and phase formation of
the as-prepared M­ oS2 sample. Chemical bondings were ana-
Synthesis of 3D flower‑like ­MoS2 lyzed in the frequency range of 4000 and 400 ­cm−1 through
Fourier transform infrared spectroscopy (FTIR-Bruker
MoS2 was constructed in a 3D flower-like structure with Alpha (100508) Billerica, MA, USA). X-ray photoelectron
ultrathin nano-petals using a straightforward hydrothermal spectroscopy (XPS, JEOL JPS-9030, Akishima, Tokyo,
synthesis process that was slightly modified than reported Japan) was performed to get insight into the chemical states
earlier [37]. Here, 8 mM ammonium molybdate and 0.8 M of composed elements. Morphological features and elemen-
thiourea were combined with deionized water as the sol- tal analysis were investigated using field-scanning electron
vent to create a 40 ml homogeneous solution. This mixture microscopy (FE-SEM, TESCAN MIRA3, Brno, Czech
was continuously stirred for 60 min. This homogeneous Republic) and energy-dispersive X-ray spectroscopy (EDS)
solution was then transferred into a 50 ml Teflon vessel and using SEM (JEOL JSM-IT200, Akishima, Tokyo, Japan).
placed in an autoclave, which was further shifted to an oven Surface wettability was investigated through a contact angle
and heated for 16 h at 200 °C. The resulting solution was measurement (Surface Electro-Optics, Model-Phoenix 150
taken out of the autoclave as soon as the room temperature CA analyzer, Selangor, D.E. Malaysia).

Scheme 1  Synthesis mecha-

nism of 3D flower-like M
­ oS2
assembled with an array of 2D
ultrathin petals

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 Journal of Solid State Electrochemistry

Electrochemical characterization flower-like ­MoS2. The gel layer of the PVA-Na2SO4 elec-
trolyte served as a membrane between these two electrodes.
Utilizing a cell setup of three electrodes on a biological elec- Here, a similar process as given in the above section was fol-
trochemical workstation (VSP-3e potentiostat, Vaucanson, lowed to make the two separate electrodes. The active sub-
Seyssinet-Pariset, France) and a 1 molar aqueous solution stance ­(MoS2) was consistently placed onto the pre-cleaned
of ­Na2SO4, the electrochemical properties of the ­MoS2 elec- current collector (nickel foam) at about 10 mg for each elec-
trode were elaborated. For a three-electrode system work- trode. The apparatus was sealed tightly with paraffin layers
ing electrode of 3D flower-like, M ­ oS2 was fabricated on a to prevent out air. A fabricated symmetric supercapacitor
conducting 3D current collector, i.e., Ni foam. The reference was subjected to various electrochemical tests (CV, GCD,
electrode (Ag/AgCl) and counter electrode (platinum wire) EIS) to realize the real application of the electrode material.
were employed as another two electrodes. To fabricate the
working electrode slurries of the M ­ oS2 (active material):
Polyvinylidene difluoride (PVDF) + N-methyl pyrrolidone Results and discussion
(NMP): carbon black (CB) were prepared in 80:10:10 pro-
portion and pasted on the Ni-foam. Ni-foam (1 × 1 c­ m2) was To get insight into the crystallinity of the as-prepared 3D
thoroughly cleaned using an ultrasonic cleaner in a 3 molar flowers of 1T@2H-MoS 2 diffraction patterns has been
HCl solution before being subjected to acetone, water, and evaluated at room temperature. The creation of a hexago-
ethanol for 15 min each. A piece of Ni-foam with painted nal crystal structure with the space group P63/mmc is con-
slurries was vacuum dried at 80 °C for 6 h which was further firmed by the M­ oS2 diffraction patterns displayed in Fig. 1.
used for electrochemical measurement. Before the electro- All indexed crystal planes are associated with the JCPDS
chemical measurement mass loading of the active material card number 00–037-1492 and as reported by Wang et al.
on the electrode was evaluated by measuring the weight [40] and Tian et al. [41]. It was discovered that the 1T@2H-
of the clean electrode and the weight of the M ­ oS2 loaded MoS2’s most conspicuous crystal plane (002) was separated
electrode which was found to be 10 mg. Electrochemical into two planes, one sharp plane centered at 2θ = 9.5° and
impedance spectroscopy (EIS), galvanostatic charge/dis- another broad plane at 2θ = 14.4°, respectively. Besides this,
charge (GCD), and cyclic voltammetry (CV) were made a new crystal plane (004) centered at 2θ = 18.4° was noticed.
to assess the electrochemical characteristics of this M ­ oS2 The separation of the prominent plane (002) and the exist-
electrode. These observations were used to study the charge ence of a new plane reflect an increase in d-spacing and
storage mechanism using cyclic voltammetry, while the elec- interlayer expansion due to the intercalation of ammonium
trode’s specific capacitance (Cs), energy density (E.D.), and ions [35, 40, 42].
power density (P.D.) were calculated using charge–discharge Raman spectroscopy was explored to analyze the phase
curves and formulae [38, 39]: purity and atomic structure defects. Figure 2 reflects a
Raman spectrum for 1T@2H-MoS2 that includes several
I dt distinctive strong peaks along with the three prominent
Cs = (1)
mdV

0.5Cs (dV)2
E.D. = (2)
3.6

(E) 3600
P.D. = (3)
dt
where I is the discharge current (A), dt is the discharging
duration (s), m is the mass of the active material on the Ni
foam, and dV is the potential window (V).

Fabrication of solid‑state symmetric supercapacitor (SC)

Assessing the real-world viability of an electrode material

involves constructing a device capable of functioning within
a two-electrode system. The design of a symmetric super-
capacitor (SSC) cell includes both cathode and anode of 3D Fig. 1  X-ray diffraction pattern of 3D flower-like ­MoS2

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Journal of Solid State Electrochemistry

Fig. 2  Raman spectra of ­MoS2 Fig. 3  Fourier transform infrared (FTIR) spectra of ­MoS2

Raman active modes E ­ 1g, E2g

1
, and A
­ 1g. These longitudinal
Using X-ray photoelectron spectroscopy (XPS), the sur-
acoustic phonon modes, which have respective centers at face chemical states of the included elements Mo and S
282, 378, and 402 ­cm−1, have been linked to the ­MoS2’s 2H were examined. The survey spectrum for ­MoS2 is shown
phase [40, 41]. Out of these characteristic phonon modes, in Fig. 4(a), which solely shows the existence of the rela-
­E1g is prohibited, while E2g
1
denotes the displacement of the tive elements Mo, S, and C with the differentiating spec-
Mo and S elements within the layer, and the third ­A1g is in tra of Mo 3d and S 2p. The peak with a binding energy of
charge of the displacement of the sulfur atoms symmetri- 225.95 eV accommodates the presence of the 2 s state of the
cally outside the layer [43]. The high intensity of those sulfur atom. With a high-resolution XPS spectrum, insight
conspicuous peaks indicates well-crystallization of the as- analysis is enabled. According to Fig. 4(b), which depicts
prepared ­MoS2 in the hexagonal phase as noticed through the high-resolution spectra of Mo 3d, the two states Mo ­3d5/2
investigation of diffraction patterns. Furthermore, the and Mo ­3d3/2 were respectively centered at 228.5 eV and
peaks centered at lower wave numbers, i.e., 147 ­c m −1, 231.7 eV. Here, shifting of the binding energy was noted
235 ­cm−1, and one centered at 336 ­cm−1 in between E ­ 1g, towards the lower side by 1 eV as compared with XPS of
and E2g 1
modes are assigned as ­J1, ­J2, and ­J3 respectively. commercial ­MoS2 which reflects the inclusion of metallic
These three Raman vibrations are associated with the pure 1 T phase [46]. Furthermore, each state of the Mo 3d spec-
metallic 1 T phase of the ­MoS2 [36]. trum has composed of two components as revealed from the
Fourier transform infrared spectroscopy (FTIR) deconvolution indicating the exitance of Mo with two redox
determined whether M ­ oS 2 had vibrational groups capa- species. The 1 T and 2H phases of the ­MoS2 are responsi-
ble of bending and stretching. The stretching vibration ble for emerging of those two components in each state,
between the Mo and S atoms is responsible for the mod- respectively. The stronger peaks inside the 3d envelope of
est absorption peak shown in Fig. 3 at 611.8 ­c m −1. The Mo with binding energies of 228.5 eV and 231.7 eV repre-
asymmetric stretching of the sulfide oxide (S–O) is senting ­3d5/2 and ­3d3/2 states respectively owe to the ­Mo4+
attributed to the bond with a center at 1101 ­c m −1 [44]. oxidation state and reflect the presence of metallic 1 T phase
The intercalation of the ammonium group is confirmed in the ­MoS2, whereas peaks representing the 2H phase were
by the presence of the absorption peak at 1398 ­c m −1 , centered at 229.4 eV (­ 3d5/2 state) and 232.5 eV ­(3d3/2 state)
while the two broad absorption bands with centers at respectively. These two small intense 3d doublets are attrib-
1631 ­c m −1 and 3420 ­c m −1 are a reflection of the bend- uted to the ­Mo5+ oxidation state which may come due to the
ing and stretching vibrations of the O–H bonds of the air exposure of the ­MoS2 sample [11, 47–49]. Figure 4(c)
intercalated water molecules [39, 44]. Additionally, the represents the high-resolution spectrum for another con-
symmetric and antisymmetric stretching vibrations of stituted element, i.e., sulfur (S 2p). These spectra’s decon-
the -CH 2 (C-H bond) are reflected in the two absorp- volution reveals the presence of two states with centers at
tion peaks with centers at 2852 ­c m −1 and 2921 ­c m −1 , 163.9 eV and 162.7 eV, respectively, known as S 2­ p3/2 and
respectively [39, 45]. S ­2p1/2. This confirms sulfur exists in dominating oxidation

13
 Journal of Solid State Electrochemistry

­ oS2, b high-resolution deconvoluted spectrum of Mo 3d and c S 2p

Fig. 4  a XPS survey spectra of M

state of ­S2− [50]. The higher binding energy peak centered with an average thickness below 40 nm while it ranges up
at 169.2 eV was assigned to the oxidized sulfur bond (S–O to a few hundred nanometers in length. The formation of
bond) adsorbed at the surface [51]. The atomic percent- hierarchical 3D architecture contained with 2D ultrathin
age estimated from the XPS analysis reflects 32.8% of Mo nano-petals results in the enhancement of the charge stor-
while 67.2% of S atoms were constituted in the ­MoS2 with a age process through an improvement in the reversible sur-
ratio of 1:2. Overall XPS analysis confirms the formation of face redox reactions. The EDS spectra of the hydrothermally
­MoS2 while metallic Mo (­ Mo4+/Mo5+) and abundant inter- synthesized ­MoS2 (Fig. 5(e)) elaborates only two elements,
layer space linking succeeding layers at the sulfur edges play i.e., Mo and S in their respective energy region. EDS con-
the role of redox centers in the charge storage process of the firms the formation of ­MoS2 as no traces of the additional
supercapacitor electrode. elements were detected. The estimated atomic ratio between
Morphological images reflect the formation of hierarchi- the two constituted elements (Mo and S) closely remains
cal 3D flowers with submicron size assembled with ultrathin 1:2 with an atomic weight percentage of 29.35 and 70.65
nano-petals. The SEM images of 3D hierarchical ­MoS2 are respectively which well agree with the XPS analysis. An
shown in Fig. 5(a–d) when examined at different magnifica- investigation of the surface wettability of as-prepared M ­ oS2
tions. From Fig. 5(a) and (b), it could be seen that ultrathin confirms that the angle between the solid (­ MoS2) and liquid
nano-petals are tightly contained at one point like a recep- (water) is an acute one, i.e., less than 90°. The solid–liquid
tacle seen in flowers. Moreover, a high-resolution image contact angle in our case is 39.5° (left angle) and 45.8° (right
in Fig. 5(c) and (d) confirms that those petals are very thin angle) as reflected in Fig. 5(f). This confirms the hydrophilic

13
Journal of Solid State Electrochemistry

­ oS2 micro flowers with different magnifications, e EDS spectrum of M

Fig. 5  a–d FE-SEM images of M ­ oS2, f solid–liquid angle of contact of
­MoS2 on the substrate

formation of the 3D flower-like M­ oS2 making it predomi- adsorption–desorption isotherm used to evaluate the spe-
nant for electrochemical energy storage purposes. cific surface area while the pore diameter was estimated
Specific surface area and pore size distribution of the based on Fig. 6(b) Barrett-Joyner-Halenda (BJH) plot. The
as-prepared ­MoS2 was accumulated through Brunauer- BET hysteresis loop shown in Fig. 6(a) is a type IV iso-
Emmet-Teller (BET). Figure 6(a) ref lects the ­ N2 therm confirming the sample has a higher surface area and

13
 Journal of Solid State Electrochemistry

Fig. 6  Surface area and pore size distribution analysis. a Nitrogen absorption–desorption, and b Barrett-Joyner-Halenda (BJH) plots of M
­ oS2

is constituted with a large number of mesoporous structures where a and b can have random values. The value of “b”
[29, 34, 48, 52]. Our M ­ oS2 sample demonstrates a large elaborates whether the charge storage kinetics result from
specific surface area of about 17.36 m ­ 2/g while the peak the surface-controlled (non-faradaic redox reactions) or
point of the BJH plot majority pores has a diameter of about diffusion-controlled (reversible faradaic redox reactions)
4.05 nm. Both of these observations confirm the formation mechanism. Diffusion-controlled processes are indicated
of 3D hierarchical architecture with copious mesoporosity by b = 0.5, whereas surface-controlled processes are indi-
which favors larger ion diffusion at the surface and an easy cated by b = 1.
channel for transmission. Figure 7(b) demonstrates the log (peak current) vs log
With the use of three-electrode cell CV, GCD, and EIS (scan rate), the slope of these points gives the b-value of the
properties of the ­MoS2 were measured with 1 molar ­Na2SO4 ­MoS2 electrode. Here, in our case, the slope value is found to
aqueous electrolyte. As shown in Fig. 7(a), CV curves with be 0.56 indicating reversible faradaic redox reactions domi-
a voltage window of − 0.9 to − 0.2 V vs SCE potential and a nate the charge storage kinetics of ­MoS2. Furthermore, the
plethora of applied scan rates (5 to 100 mV/s) are depicted. contribution of surface and diffusion was estimated for each
Analyzing closely, the rectangular-shaped behavior was applied scan rate using the equation:
noted for the CV curves indicating ­MoS2 electrode exhib-
its the pseudocapacitive type of charge storage behavior i = k1 𝜗 + k2 𝜗1∕2 (7)
[11]. The estimated under-curve area increases as the scan
where the current (i) was separated into capacitive (k1ϑ;
rate increases; however, no significant change in the shape
Qc) and diffusive (k2ϑ; Qd)-controlled processes. A column
of these CV curves was noticed this sign that M ­ oS2 has a
graph as shown in Fig. 7(c) demonstrates the contribution
good rate performance [37]. The possible reversible fara-
arising from capacitive and diffusive mechanisms for ­MoS2
daic reaction mechanism between the electrolytic ions ­(Na+)
electrode in ­Na2SO4 electrolyte with a variation in scan
and active material ­(MoS2) can be represented through two
rate. This makes it obvious that when the scan rate rises, the
equations as follows [53]:
non-faradic process accelerates. This mechanism contributes
MoS2 + Na+ + e− ↔ MoS − SNa (4) 20.5% at 10 mV/s and increases to 54.20% at 100 mV/S. For
the contribution originating from the reversible faradaic pro-
cess, an exact reverse condition can be observed, where the
MoS2 surface + xNa+ + xe− ↔ (Na+ − MoS−2 )surface (5)
( )
contribution from this process dominates at a lower range of
scan rate and declines as the scan rate increases. The contri-
Furthermore, power law was used to scrutinize the charge
bution of this mechanism, in this case, is 79.5% at 10 mV/s
storage kinetics of ­MoS2 with an aqueous ­Na2SO4. The
and drops to 45.8% at 100 mV/s.
power is expressed in terms of the following equation:
Charge–discharge (GCD) measurements were made to
i = a𝜗b ; log(i) = log(a) + b log (𝜗) (6) determine the electrochemical characteristics of the 3D
flower-like ­MoS2, including specific capacitance and other

13
Journal of Solid State Electrochemistry

Fig. 7  Electrochemical performance of M ­ oS2. a Cyclic voltammetry and diffusion-controlled contribution of ­MoS2 electrode measured at
at different scan rates (5–100 mV/s), b current response versus scan different scan rates
­ oS2, c capacitive
rate (log i vs. log ϑ) plots at each redox peak of M

important aspects. Figure 8(a) demonstrates GCD curves specific capacitance is illustrated in Table 1. A hierarchical
measured in 1 M N ­ a2SO4 electrolyte within − 0.9 to − 0.2 V architecture of M
­ oS2 fabricated in our case is mainly related
vs SCE potential for ­MoS2 electrode at various current den- to dominance in the specific capacitance when compared
sities (1 to 5 A/g). Pseudocapacitive performance can be with the 3D and 2D systems found in previous reports. Here,
noted from the shape of the GCD curves of the ­MoS2 elec- the synergistic effect of the 3D flowers constituted with 2D
trode. Based on the discharging time recorded from the GCD ultrathin mesoporous petals has a prominent role in boost-
curves, the specific capacitance was assessed using Eq. 1. ing the capacitance. The amplification of the Faradaic redox
The estimated specific capacitance with respective current reactions and consequent capacitance is caused by the com-
density was represented in Fig. 8(b). This value declines bination of hybrid structure, which provides essential space
while going from lower to higher current density, i.e., 207.14 for ion diffusion and creates a convenient channel for the
F/g to 1 A/g and reaches 50 F/g at 5 A/g. Additionally, at 1.5 migration of charges over an electrode–electrolyte inter-
A/g, the M­ oS2 electrode’s rate capability is roughly 70%. It face. The fluctuation in this electrode’s estimated energy
is noticed that the value of specific capacitance estimated in and power densities at various current densities is shown
our case overrides some of the previously reported hierarchi- in Fig. 8(c). When the current density is 1 A/g, the ­MoS2
cal architectures of the M
­ oS2. The comparative part focus- electrode displays a 14.2 Wh/kg energy density with a power
ing on different architectures of the M­ oS2 and its estimated density of 0.34 kW/kg. Energy density further decreases and

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 Journal of Solid State Electrochemistry

Fig. 8  a Galvanostatic charge–discharge at different current densi- trode, d Ragone plot of ­MoS2 based electrodes, e cyclic stability and
ties (1–5 A/g), b specific capacitance from GCD vs current density, c coulombic efficiency of M­ oS2 over 1000 charge–discharge cycles at
estimated values of current density and power density for ­MoS2 elec- 2 A/g

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Journal of Solid State Electrochemistry

Table 1  Comparative study of M

­ oS2-based supercapacitor electrodes
Electrode Morphology Electrolyte Specific capacitance @ Energy and power density Reference
current density

MoS2 Micrograins 2 M KOH 87 F/g @ 0.5 A/g - 3

MoS2 3D semispherical nanograins 1 M ­Na2SO4 137 F/g @ 1.1 mA/ ­cm2 23 Wh/kg and 2.1 kW/kg 4
MoS2/Mo 2D nanosheets 1 M ­Na2SO4 192.7 F/g @ 1 mA/ ­cm2 38.54 Wh/kg and 0.75 kW/kg 11
MoS2 Flower-like microspheres 1 M ­Na2SO4 44.1 F/g @ 1A/g 14.5 Wh/kg and 0.13 kW/kg 25
MoS2 2D nanosheets 1 M ­Na2SO4 129.2 F/g @ 1 A/g - 30
MoS2 2D vertical nanosheets 1 M ­Na2SO4 151.1 F/g @ 10 mA/cm2 - 31
MoS2 Spherical-flower 3 M KOH 255 F/g @ 0.25 A/g 35.5 Wh/kg and 0.25 kW/kg 32
MoS2 Spherical naorods 1 M ­MgSO4 79 F/g @ 0.5 A/g - 33
E-MoS2 Spherical naorods 1 M ­MgSO4 165.6 F/g @ 0.5 A/g - 33
MoS2 Intercalated nanosheets 1 M ­Na2SO4 19.1 @ 0.2 A/g - 34
MoS2/C Flower-like 1 M ­Na2SO4 201.4 F/g @ 0.2 A/g - 34
MoS2 Flower-like microspheres 3 M KOH 223 F/g @ 0.6 A/g - 35
MoS2 Nanoparticles 6 M KOH 30.66 F/g @ 0.5 A/g - 49
MoS2/carbon dots Flower-like 6 M KOH 149.21 F/g @ 0.5 A/g - 49
MoS2 3D flowers 1 M ­Na2SO4 207.14 F/g @ 1 A/g 14.2 Wh/kg and 0.34 kW/kg Present work

power density increases concerning the increase in current performance of ­MoS2 was shown by a straight line that was
density up to 5 A/g as seen through the Ragone plot repre- almost precisely parallel to the imaginary axis. The low R ­ s
sented in Fig. 8(d). A slight decrease in the energy density and ­Rct value is due to good contact established between the
and power density than a few previous reports attributes ­MoS2-Ni-foam electrode and the 3D flower-like structure
to the higher specific capacitance and enhanced potential which provide highly active sites.
window [4]. The cyclic test, which was carried out at 2 A/g Figure 10(a) displays the cyclic voltammetry (CV) of
over 1000 GCD cycles, shows that 3D flower-like ­MoS2 the SSC device, recorded over various potential ranges at
has a loss in original capacitance of only 6.4%. Moreover, a scanning rate of 60 mV/s. This exploration aimed to opti-
this electrode reflects symmetric charge–discharge curves mize the suitable operational potential window. The results
as there is no obvious change in the charging and discharg- indicate that the potential window of − 1 to + 1 V (2 V)
ing time noticed from the first cycle to the 1000th cycle as is well-suited for the SSC device’s functioning. Further-
reflected from the coulombic efficiency curve. This elec- more, the CV profiles of the SSC device were examined
trode of M­ oS2 reflects great capacitance retention of 93.6% across different scan rates, ranging from 10 to 100 mV/s
with 99% of coulombic efficiency as shown in Fig. 8(e). The
capacitance gradually decreases over 1000 cycles; this loss
of capacitance is because over time continuous reversible
electrochemical processes taken at higher scan rates hinder
the easy diffusion of the electrolytic ions on the surface of
the material. Furthermore, detachment of the material from
the substrate due to chemical reactions and mechanical stress
also impacts the specific capacitance over a large number of
cycles [54].
The series resistance of the electrochemical cell was
measured by measuring electrochemical impedance in a
1 M ­Na2SO4 electrolyte at 10 mV potential with frequency
ranges from 100 kHz to 0.1 Hz. The hydrothermally syn-
thesized ­MoS2 electrode’s Nyquist plot with an equivalent
circuit (inset) is shown in Fig. 9. This figure revealed a semi-
circle in the high frequency region and a straight line in
the lower-frequency region. The estimated value of series
resistance ­(Rs) is 2.52 Ω that of the charge transfer resist-
ance ­(Rct) is 4.14 Ω, respectively. The excellent capacitive Fig. 9  Nyquist plot and fitting circuit (inset) of ­MoS2 electrode

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 Journal of Solid State Electrochemistry

Fig. 10  Electrochemical properties of ­MoS2//MoS2 SSC. a CV curve at different potential windows at 60 mV/s, b CV curves at different scan
rates, c GCD profiles at different current density, d Ragone plot of SSC device, e Nyquist plot and fitting circuit (inset) of ­MoS2//MoS2

13
Journal of Solid State Electrochemistry

as represented in Fig. 10(b). Interestingly, the area beneath fabricate the hierarchical architectures of other prominent
the curve exhibited an increase without distorting the shape materials for a variety of applications.
of the CV curve as the scan rate was elevated. This behav-
Acknowledgements The author thanks the CSMNRF-2021 for the
ior underscores the robust rate capability of the SSC device funds provided by the Chhatrapati Shahu Maharaj Research Training
[55]. To analyze the energy storage properties of the SSC and Human Development Institute (SARATHI), Pune. The authors are
device, galvanostatic charge–discharge (GCD) curves were thankful to the Principal and Management of Dr. Ghali College Gadh-
captured under varying current densities (1, 2, 3, 4, and 5 inglaj. This work was supported by the Korea Institute for Advance-
ment of Technology(KIAT) grant funded by the Korea Government
A/g) within a 2 V potential range, as depicted in Fig. 10(c). (Ministry of Trade, Industry and Energy-MOTIE) (P0012770).
Using the estimated discharging time from GCD profiles
of the SSC device at the respective applied current density
device performance factors such as specific capacitance,
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formance of self-assembled nanoflowers of ­MoS2 nanosheets as
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doi.​org/​10.​1021/​acsom​ega.​9b010​58 exclusive rights to this article under a publishing agreement with the
50. Sharkawy HME, Dhmees AS, Tamman AR, Sabagh SME, author(s) or other rightsholder(s); author self-archiving of the accepted
Aboushahba RM, Allam NK (2020) N-doped carbon quantum manuscript version of this article is solely governed by the terms of
dots boost the electrochemical supercapacitive performance and such publishing agreement and applicable law.

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