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Effect of various aqueous electrolytes on the electrochemical performance of
α-MnO 2 nanorods as electrode materials for supercapacitor application
Article in Electrochimica Acta · November 2020
DOI: 10.1016/j.electacta.2020.137412
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Electrochimica Acta 366 (2021) 137412
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effect of various aqueous electrolytes on the electrochemical
performance of α -MnO2 nanorods as electrode materials for
supercapacitor application
M. Jayachandran a, Aleena Rose a, T. Maiyalagan b, N. Poongodi c, T. Vijayakumar a,∗
a
Futuristic Materials Research Centre for Planetary Exploration, Department of Physics and Nanotechnology, College of Engineering and Technology, Faculty
of Engineering and Technology, SRM Institute of Science and Technology, SRM Nagar, Kattankulathur-603203, Kanchipuram, Chennai, Tamil Nadu, India
Electrochemical Energy Laboratory, Department of Chemistry and Research Institute, College of Engineering and Technology, Faculty of Engineering and
Technology, SRM Institute of Science and Technology, SRM Nagar, Kattankulathur-603203, Kanchipuram, Chennai, Tamil Nadu, India
c
Department of Physics, Erode Sengunthar Engineering College, Thudupathi, Erode 638 057, India
b
a r t i c l e
i n f o
Article history:
Received 16 July 2020
Revised 27 October 2020
Accepted 31 October 2020
Available online 10 November 2020
Keywords:
Nanorods
α -MnO2
Hydrothermal
Supercapacitor
Mixture electrolyte
a b s t r a c t
We report a facile one-step hydrothermal method to prepare α -MnO2 nanorods for their application as
electrode materials for high-quality supercapacitors. The structural and morphological properties of the
prepared active materials were investigated using Powder X-ray Diffraction (XRD) and High-Resolution
Transmission Electron Microscopy (HR-TEM) analyses. Fourier Transform-Infrared Spectroscopy (FT-IR)
and Bruner-Emmert-Teller (BET) analyses were used to study the functional groups and surface area
properties of the α -MnO2 nanorods. Further the electrochemical supercapacitive performance of α -MnO2
nanorods were evaluated using Cyclic voltammetry (CV), Chronopotentiometry (CP), and Electrochemical
impedance spectroscopy (EIS) analyses in various aqueous electrolytes (1 M Na2 SO4 , 0.5 M KOH and 1 M
Na2 SO4 +0.5 M KOH). The electrochemical results show that the α -MnO2 nanorods delivered a high specific capacitance of 570F/g at 1A/g current density in the mixture electrolyte consisting of 1 M Na2 SO4
and 0.5 M KOH. In addition, a coulombic efficiency of ~80% was found at 10 A/g current density. And also,
the capacitance retention was found to be ~80% after 10,0 0 0 cycles in 1MNa2 SO4 +0.5 M KOH mixture
aqueous electrolyte solution. The present work revealed the excellent performance of α -MnO2 nanorod
electrode materials in the mixture aqueous electrolyte solution. This electrode-electrolyte combination
was found to be the prospective system for supercapacitor applications.
© 2020 Elsevier Ltd. All rights reserved.
1. Introduction
Demand for ecological and consistent energy resources and
storage systems is mounting due to fossil fuel exhaustion and environmental pollution [1,2]. In these recent years, electrochemical
supercapacitor (ultracapacitors) have attracted researchers in the
energy storage field as they possess cycle constancy, high power
density, high energy density, fast charging-discharging rate, longterm cyclability, safety and capability to function as a bridge between conventional supercapacitors and lithium-ion batteries [3,4].
Supercapacitor can be classified based on charge storage process,
namely (i) Electric double-layer capacitors (EDLC) and (ii) Pseudocapacitors (PC). EDLCs accumulate charge in an electric double layer through the electrostatic adsorption of electrolyte ions
onto the electrode surface [5–7]. In contrast, the PCs store charges
∗
Corresponding author.
E-mail address: vijayakt1@srmist.edu.in (T. Vijayakumar).
https://doi.org/10.1016/j.electacta.2020.137412
0013-4686/© 2020 Elsevier Ltd. All rights reserved.
via faradaic redox reaction, electrosorption, and intercalation. Since
the properties of electrode materials are vital in deciding the performance of supercapacitor, potential materials with excellent microstructure, high conductivity, and large surface area are essential
to be used as working electrodes [8–10]. Carbonaceous materials
(graphene, carbon nanotubes), Transition metal oxides (oxides of
ruthenium, nickel, vanadium), and conducting polymers (polypyrrole, polyaniline) are some of the commonly researched electrode
materials. Transition metal hydroxides TM (OH) such as Ni (OH)2,
Co(OH)2, Nd(OH)3 , La(OH)3 and their layered double hydroxides
have been primarily considered as pseudocapacitive electrode materials [11–14]. Previous reports reveal the potential of transition
metal oxides (TMOs), such as RuO2 [15], Co3 O4 [16], NiO [17],
V2 O5 [18], and MnO2 [19] to be used as electrode materials in supercapacitors [20–22]. In addition to this, TMOs are stable during
redox reactions, show different oxidation states, easily accessible,
and abundant [23,24]. Among TMOs, MnO2 is a prominent compound which shows superior electrochemical behavior and having
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
2.2. Synthesis of α -MnO2 nanorods
applications in catalysts, artificial oxides, dry cell components, inorganic dye for ceramics, electrochemical battery electrodes, and
supercapacitors [25–27]. MnO2 has also been found to have critical applications in a lot of energy conversion and storage processes
[28–30].
Manganese oxides can be engineered in such a way that they
can exhibit properties beneficial for energy conversion and storage. MnO2 is a cost-effective and eco-friendly electrode material
with an excellent theoretical capacitance of 1370 F/g [31]. Contrary to NiO and Co3 O4 , it gives a full working potential window (around 1 V), thereby leading to higher energy density. Besides, MnO2 crystal facilitates the diffusion of ions compared to
other spinal structures. Amongst various MnO2 polymorphs (α -,
β -, γ -, δ - and ε-MnO2 ), α -MnO2 is considered as the best because of its prominent 2 × 2 tunnel structure of about 0.46 nm
magnitude and high surface area [32,33]. Since it is challenging
to create MnO2 showing rapid and reversible surface redox reactions, a possible technique is to curtail the diffusion length amid
electrode-electrolyte interface. Nano-sized MnO2 is a key to this
problem as its enormously low dimensions can trim down the diffusion length when related to their bulk equivalents. A recent report demonstrates the synthesis of nano α -MnO2 in different morphologies for electrode applications [34–36]. Aghazadeh et al. reported the preparation of MnO2 nanorods through cathodic electrodeposition method which delivered a high specific capacitance
of 242F/g at 2A/g in 1 M Na2 SO4 electrolyte [37]. Wang et al. studied the electrochemical properties of porous nano-MnO2 prepared
through quick-redox process and achieved a specific capacitance
of 198F/g at 1A/g in 1 M Na2 SO4 +9 M KOH electrolyte [38]. Liu
et al. developed MnO2 /porous carbon microspheres which exhibited a high specific capacitance value of 459F/g at 1A/g in 6 M KOH
electrolyte [39]. Wu et al. reported the electrochemical behavior of
MnO2 in a wide working potential window, 0 to 2.2 V, with neutral
aqueous electrolytes for supercapacitor applications [40].
Among the various electrolytes, aqueous electrolytes with high
conductivity accelerate and improve the charge storage due to the
presence of ions in them. They are also recognized for the faster
carrier rates which help in achieving superior supercapacitors. It is
less expensive, nontoxic, and easy to handle in research laboratory.
Aqueous electrolytes including KOH, Na2 SO4 , NaOH, and KCl have
been extensively used in research instead of organic electrolytes
[41].
Here we present the investigation on the effect of various
aqueous electrolytes on the electrochemical properties of α -MnO2
nanorods prepared through hydrothermal method. Section 2 provides the experimental procedure and characterization techniques
used for the research. Section 3 elaborates the obtained results comprising X-ray diffractograms, transmission electron micrographs, Fourier transform infrared spectrum, and Nitrogen
adsorption-desorption isotherms. The results of electrochemical
studies such as cyclic voltammograms, charge-discharge patterns,
and Nyquist plots which focus on the performance of α -MnO2
nanorods with respect to various aqueous electrolytes (1 M
Na2 SO4 , 0.5 M KOH and 1 M Na2 SO4 +0.5 M KOH) are also included
in this section. Section 4 gives the conclusion of this research work.
The α -MnO2 nanorods were prepared using a hydrothermal
route reported earlier with some variations [42]. Briefly, 3 g
KMnO4 was dissolved in 60 ml De-ionized water (DI) water. The
solution was stirred for 30 min at a fixed temperature of 50 °C.
Then 0.2 g SiO2 dissolved in 1 ml HNO3 was added drop wise to
the mixture under constant stirring. When the solution obtained a
purple color, it was transferred into a 100 ml stainless steel autoclave (Teflon lining). After sealing, it was kept inside the air oven at
160 °C for 12 h. Once the autoclave reached room temperature naturally, the precipitate was collected by centrifugation and washed
using DI water and ethanol. Lastly, the sample was parched in a
hot air oven at 120 °C for 8 h.
2.3. Materials characterization
The crystal structure, phase composition, and sample purity
were analyzed by X-ray diffraction (XRD) (BRUKER USA D8Advance,
Davinci) method. The morphology and microstructure of the materials were observed by High-Resolution Transmission Electron Microscope (HRTEM) (JEOL Japan, JEM-2100 Plus). The Fourier Transform Infrared (FT-IR) spectrum was recorded using Perkin-Elmer
Spectrum in the range of 400 to 40 0 0 cm−1 with a resolution of
2 cm−1 using KBr pellets. The presence of different elements in
the sample was studied via Energy Dispersive Spectroscopy (EDS,
OXFORD 51-XMS). Surface area and pore size distribution analysis
was carried out using Bruner-Emmert-Teller (BET) methods.
2.4. Electrochemical measurements
The electrochemical experiments were accomplished using Biologic SP-300, with a three-electrode system, where platinum wire,
Hg/HgO, and α -MnO2 coated on nickel foam were used as the
counter electrode, reference electrode, and working electrode respectively. Active material, α -MnO2 , was mixed with the conducting additive (carbon black) and binder (PVDF) in the ratio 85:10:5
using N-Methyl-2-pyrrolidone (NMP) to form homogenous slurry.
Thus the electrode materials for testing were prepared by drop
casting the slurry onto the nickel foam (1 × 1 cm). Then the electrodes were dried at 80 °C for 12 h. The active material mass on
the nickel foam was found to be 2 mg. Electrochemical Impedance
Spectra (EIS) was recorded in the frequency range of 105 Hz –
10−1 Hz. Cyclicvoltammetry (CV) and Chronopotentiometry (CP)
techniques were performed in the potential window 0 V–0.6 V.
The above-mentioned electrochemical techniques were completed
using different aqueous electrolyte solutions such as 1MNa2 SO4 ,
0.5MKOH, and a mixture electrolyte 1 M Na2 SO4 +0.5 M KOH. The
specific capacitance of electrode material α -MnO2 was computed
from CV and CP patterns using the subsequent Eqs. (1) and (2),
(3) respectively [43,44,45].
The specific capacitance dQ/dV was calculated using the
voltammetric charge integrated from the cyclic voltammogram according to the following equation [43].
Csp (F /g) =
2. Experimental procedure
Q
(1)
E × m
In Eq. (1) Csp is the specific capacitance (in F/g), Q is the charge
(in C), ࢞E is the potential window (in V), and m is the mass of
active material (in g).
Specific capacity due to battery-Type charge storage behavior can be estimated from the CV curves using the following
Eq. (2) [44, 64]
2.1. Materials
Potassium permanganate (KMnO4 ), Nitric acid (HNO3 ), Silicon dioxide (SiO2 ), Potassium hydroxide (KOH), and Sodium sulfate (Na2 SO4 ) were procured from Sigma Aldrich. Carbon black,
Polyvinylidene fluoride (PVDF), and N-methyl-2-pyrrolidone (NMP)
was bought from Merck. For the synthesis of α -MnO2 nanorods,
substance grade chemicals were used with no extra refinement.
Specificcapacity(mAh/g ) =
2
Specificcapacitance(F/g ) × E
3.6
(2)
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Fig. 1. Hydrothermal synthesis of α -MnO2 nanorods.
In Eq. (2) E is the potential window (in V).
Csp (F /g) =
I × t
m × V
(3)
In Eq. (3) Csp is the specific capacitance (F/g), I is the applied
current (A), m denotes the mass of active material (g), V represents potential window (V), and t is the discharge time of one
cycle (s).
3. Results and discussion
3.1. X-ray diffraction analysis of α -MnO2 nanorods
Fig. 1.
X-ray diffraction (XRD) patterns of α -MnO2 nanorods are displayed in Fig. 2. The prime diffraction peaks at 2θ positions 12.88° ,
17.90° , 28.62°, 37.34° , 41.75° ,50.12°, 56.59° 60.37° ,69.82° , and 75.52°
are consistent with the characteristic diffraction peaks of MnO2
(JCPDF No: 44–014) corresponding to the lattice planes (110),
(200), (220), (310), (211), (330), (301), (411), (600), (521), (002),
(541), (730) and (332) respectively. The space group and lattice parameters for the sample were found to be I4/m, a = b = 9.78 Å,
c = 2.86 Å. Thus, the sample prepared with KMnO4 , SiO2 , and
HNO3 as precursors was identified as pure α -MnO2 with tetragonal structure [37].
The sharpness of diffraction peaks indicates high crystalline of
the synthesized α -MnO2 , and no extra peaks were observed, which
confirms its phase purity [46,47]. The average crystallite size (D)
was calculated using the following Scherer Eq. (4):
D=
kλ
β cos θ
Fig. 2. XRD pattern of α -MnO2 nanorods.
where D is the crystallite size (nm), K is the shape factor (0.9), λ
is the wavelength of X-ray used (1.5418 Å), β is the Full Width at
Half-Maximum (FWHM), and θ is the Bragg angle.
The dislocation density (δ ) was estimated from the following
Eq. (5):
δ=
1
D2
(5)
where δ is the dislocation density and D is the average crystallite
size.
(4)
3
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Table 1
Structural parameters of α -MnO2 nanorods.
Sample
α-MnO2
2θ (degree)
(hkl)
d-spacing (Å)
Crystallite size (nm)
Dislocation density (δ ) (lines/m2 ) × 1014
Lattice strain (e × 10−13 )
12.72
18.00
28.64
37.38
42.77
49.75
56.04
60.01
65.29
69.37
111
200
310
211
301
411
600
521
002
541
6.95
4.92
3.11
2.40
2.11
1.83
1.73
1.54
1.42
1.35
54.99
36.98
57.29
50.51
70.96
36.42
18.71
76.30
64.09
39.24
3.30
7.31
3.04
3.91
1.98
7.53
2.85
1.71
2.43
6.49
6.21
2.96
6.15
6.86
1.50
3.81
1.85
4.54
5.26
3.53
EDX spectrum given in Fig. 5(h) proves that the final product comprises only Mn and O, which further confirms that the nanorods
are pure α -MnO2.
3.4. Surface area analysis of α -MnO2 nanorods
Fig. 6 illustrates the nitrogen adsorption-desorption measurement of α -MnO2 nanorods coated on nickel foam (working electrode). According to IUPAC classification, the presence of hysteresis
loop signifies that the electrode is of Type IV isotherm. Initial loop
and second loop imply the adsorption and desorption respectively.
Type IV isotherm can be attributed to the mesoporous characteristic of working electrode. The specific surface area of the active site
was found to be 180 m2 .g−1 [50,51].
3.5. Electrochemical performance of α -MnO2 nanorods
The synthesized α -MnO2 nanorods can show different electrochemical behavior depending upon the properties of electrodeelectrolyte interface and ion transport rate in the charge-discharge
process. Fig. 7(a)–(c) illustrates the CV curves obtained for α -MnO2
electrodes in 1 M Na2 SO4 , 0.5 M KOH, and 1 M Na2 SO4 +0.5 M
KOH electrolytes at different scan rates (5, 10, 20, 30, 40, 50, 70,
and 100 mV/s). As shown in Fig. 7(a), the CV patterns of the material in 1 M Na2 SO4 electrolyte are not rectangular and do not
show any redox peaks resembling the surface redox pseudocapacitive behavior. This system achieved a specific capacitance of 262F/g
at 5 mV/s. Following Eqs. (7) and (8) gives the underlying mechanisms for supercapacitive charge storage in MnO2 [52–54].
Fig. 3. FTIR spectrum analysis of α -MnO2 nanorods.
The lattice strain (ε ) was determined using Eq. (6) given below:
ε=
β cos θ
4
(6)
where ε is the lattice strain, β is the Full Width at Half-Maximum
(FWHM), and θ is the Bragg angle. The structural parameters of the
material are shown in Table 1.
3.2. Fourier transform infrared analysis of α -MnO2 nanorods
Fig. 3 shows the FTIR spectrum of α -MnO2 nanorods. The two
broad, strong absorption peaks appear at 1050 and 794 cm−1 that
can be correlated to the stretching and bending vibration modes
of Mn–O–Mn band of tetrahedral and octahedral sites. The corresponding to the absorption band at 1636 cm−1 and 2081 cm−1
in this presence of carbonyl group (C–O) stretching vibration. The
weak absorption peak observed at 539 cm−1 represents moisture
absorption on the surface of sample and hence indicates the OH
bending vibration combined with Mn atoms [48].
MnO2 + M+ + e− ↔ MnOOM
(7)
(MnO2 )surface + Na+ + e− ↔ MnO−2 Na+ surface
(8)
where, M+ = H, Li, Na or K
Reaction taking place as a result of proton (alkali metal cations
(M+ )) intercalation into (or extraction from) the α -MnO2 electrode
is given in Eq. (7). This includes a reversible surface redox reaction
amid Mn4+ and Mn3+ ions. The mechanism showed in Eq. (8) is
based on the proton (alkali metal cations) adsorption to (or desorption from) the working electrode surface. Former reaction is
predominant in crystalline MnO2 while the latter is exhibited by
amorphous MnO2 [55,56].
Fig. 7(b) illustrates the cyclic voltammograms of α -MnO2 electrodes in 0.5 M aqueous KOH electrolyte. Anodic and cathodic
peaks appearing at 0.45 V and 0.35 V respectively indicates the redox reactions occurring inside the electrodes. Similar to the abovementioned system, the highest specific capacitance of 105F/g was
attained for the scan rate 5 mV/s. The presence of redox peaks in
the CV patterns suggests the faradic dominated electrochemical reactions and partial redox intercalation of α -MnO2 nanorods which
confirm the battery like behavior. The redox reactions of MnO2 as
explained in the following Eqs. (9) and (10).
3.3. Morphological analysis of α -MnO2 nanorods
Fig. 4(a)–(f) gives the HRTEM micrographs of α -MnO2 nanorods
and Fig. 4(a)–(e) reveals the nanorod like structures with a diameter of <100 nm and length up to several micrometers. The single
set of parallel planes with an interplanar spacing (d) of 0.55 nm
illustrated in Fig 4(f) is consistent with the (310) plane of α -MnO2
XRD pattern [49].
Fig. 5(g) reveals the SAED pattern of the synthesized material.
The SAED pattern displays four lattice planes of α -MnO2 -(111),
(200), (310), and (211)-which can also be observed in Fig. 2. The
4
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Fig. 4. (a)–(f) HRTEM micrographs of α -MnO2 nanorods.
Fig. 5. (g) SAED pattern and (h) EDX spectrum of α -MnO2 nanorods.
MnO2 + M+ + e− ↔ MnOOM
(MnO2 ) + K+ + e− ↔ MnO−2 K+
tions comprises the electrochemical performance of MnO2 in aqueous KOH electrolyte [57–60]. The specific capacity of the α -MnO2
nanorods at 5 mV/s in 0.5 M KOH was determined from Eq. (2) as
17.5 mAh/g.
Fig. 7(c) displays the CV patterns of α -MnO2 electrodes in
1 M Na2 SO4 +0.5 M KOH electrolyte. It is obvious that the cyclic
voltammograms obtained for both aqueous KOH and combination
of KOH–Na2 SO4 electrolytes are similar with exact anodic and ca-
(9)
(10)
Thus the non-faradic (adsorption and desorption of K+ cations
by MnO2 surface – Eq. (9)) and faradic (intercalation and removal
of electrolyte ions by interstitial sites of MnO2 – Eq. (10)) reac5
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
tained using Eq. (2) as 157 mAh/g. Reaction mechanisms of the respective electrochemical system are given in Eqs. (11) and (12) [61–
63].
(MnO2 )surface + Na+ + e− ↔ MnO−2 Na+ surface
(11)
(MnO2 ) + K+ + e− ↔ MnO−2 K+
(12)
Fig. 7(d) shows the variation of specific capacitance with scan
rate for different electrochemical systems. Specific capacitance is
lessening with the escalating scan rates. This behavior can be ascribed to the fast redox reactions or intercalation/extraction which
does not provide enough time for the ions to access the active
sites of electrode material. Hence the ability for charge storage is
very less at higher scan rates and vice versa. Among all the systems, α -MnO2 electrode in 1 M Na2 SO4 +0.5 M KOH electrolyte
achieved the highest specific capacitance of 945F/g (157mAh/g) at
5 mV/s. Table 2 shows the specific capacitance values obtained for
each electrochemical system from cyclic voltammograms at different scan rates.
The b-value which decides the mechanism of the electrode material was determined for α -MnO2 electrode in three electrolytes
based on the equation i = aυ b where i is the CV peak current
(mA/g), a and b are the adjustable parameters, and υ is the scan
rate (mV/s). The slope of log (i) vs. log (υ ) plot gives the b-value.
If the b-value is 1 it is capacitive mechanism and if the b-value is
0.5 it indicates diffusion controlled mechanism [64]. It is clear from
Fig. 8(a)−8(c) that the b-values of α -MnO2 in 1 M Na2 SO4 , 0.5 M
KOH and 1 M Na2 SO4 +0.5 M KOH electrolytes in the transitional
Fig. 6. Nitrogen adsorption-desorption isotherms of α -MnO2 nanorods.
thodic peak positions. The CV patterns reveal the faradic dominated electrochemical reactions and partial redox intercalation of
α -MnO2 nanorods which confirm the battery like behavior. The
highest specific capacitance of 945 F/g was acquired for the lowest scan rate 5 mV/s. α -MnO2 in the mixed electrolyte system acquired this highest specific capacitance as a consequence of its surface redox pseudocapacitive nature and faradic-intercalation dominated battery nature aided by Na2 SO4 and KOH, respectively. The
specific capacity of this electrochemical system at 5 mV/s was ob-
Fig. 7. CV patterns of α -MnO2 nanorods in different electrolytes (a) 1 M Na2 SO4 (b) 0.5 M KOH (c) 1 M Na2 SO4 +0.5 M KOH, and (d) Effect of scan rate on specific capacitance.
6
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Table 2
Specific capacitance calculated from CV patterns at different scan rates.
Sl. no.
Scan rate (mV/s)
1
2
3
4
5
6
7
8
5
10
20
30
40
50
70
100
Specific capacitance (F/g)
α -MnO2 in 1 M Na2 SO4
α -MnO2 in 0.5 M KOH
α -MnO2 in 1 M Na2 SO4 +0.5 M KOH
262
206
161
132
113
100
80
64
105
62
40
32
28
25
21
18
945
650
331
207
164
137
85
70
Fig. 8. log (i) vs. log (υ ) plot of α -MnO2 nanorods in different electrolytes (a) 1 M Na2 SO4 (b) 0.5 M KOH (c) 1 M Na2 SO4 +0.5 M KOH.
area (~0.5–0.7). Hence the electrochemical behavior of α -MnO2
electrode in the three electrolytes can be described as the result of
both surface redox reactions and diffusion controlled mechanisms.
Fig. 9(a)–(c) illustrates the CP curves obtained for α -MnO2 electrodes in 1 M Na2 SO4 , 0.5 M KOH, and 1 M Na2 SO4 +0.5 M KOH
electrolytes respectively for different current densities (1, 2, 3, 4,5,
7 and 10A/g). Specific capacitance from CP patterns was calculated
using Eq. (3). Fig. 9(a) illustrates the charge-discharge patterns obtained for α -MnO2 nanorods in 1 M Na2 SO4 electrolyte. The highest specific capacitance for this system was found to be 185F/g
at 1A/g current density. The result is comparable with the specific capacitance values reported in previous studies on α -MnO2 .
Zhao et al. acquired an enhanced electrochemical property for
MnO2 /HCNFs nanocomposite in 1 M Na2 SO4 with a specific capacitance of 293.6 F/g at 0.5 A/g. Tan et al reported the preparation of
MnO2 /CNT nanocomposites through chemical deposition method
for which they achieved a specific capacitance of 115F/g at 0.5A/g.
[65–67].
Fig. 9(b) shows the charge-discharge behavior of α -MnO2
nanorods in 0.5 M KOH electrolyte solution. Herein the highest
specific capacitance value, 200 F/g, was obtained for the lowest
current density, 1A/g. Tan et al. examined MnO2 /CNT nanocomposites and reported a specific capacitance of 106 F/g at 0.5 A/g
in KOH electrolyte. Fig. 9(c) reveals the CP patterns of α -MnO2
nanorods in 1 M Na2 SO4 +0.5 M KOH electrolyte. Among the
three systems, the electrochemical setup with mixture electrolyte
achieved the highest specific capacitance of 570 F/g at 1 A/g. These
values substantiate the good supercapacitive behavior displayed by
α -MnO2 nanorods in the mixture electrolyte. Hong-Qiang Wang
et al. reported the electrochemical studies on porous nano-MnO2
electrodes in the mixture electrolyte solution of 1 M Na2 SO4 and
7
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Fig. 9. CP patterns of α -MnO2 nanorods in different electrolytes (a) 1 M Na2 SO4 (b) 0.5 M KOH (c) 1 M Na2 SO4 +0.5 M KOH, and (d) Effect of current density on specific
capacitance.
Table 3
Specific capacitance calculated from CP patterns at different current densities.
Sl. no.
Current density (A/g)
1
2
3
4
5
6
7
1
2
3
4
5
7
10
Specific capacitance (F/g)
α -MnO2 in 1 M Na2 SO4
α -MnO2 in 0.5 M KOH
α -MnO2 in 1 M Na2 SO4 +0.5 M KOH
185
133
100
80
75
50
40
200
140
120
100
81
54
43
570
457
395
346
303
256
200
9 M KOH for which they obtained a specific capacitance of 120 F/g
at 5 A/g [68,69].
Fig. 9(d) gives a comparison of outcomes from the Chronopotentiometry performed on α -MnO2 nanorods in various electrolytes and variation of specific capacitance with current density.
As mentioned earlier, electrode material with mixture electrolyte
acquired higher specific capacitance when compared to the same
with single electrolyte. Hence an increase in the conductivity of
single electrolytes can be noticed when mixed with each other.
This confirms the improved electrochemical properties of α -MnO2
nanorods in mixture electrolyte system, 1 M Na2 SO4 +0.5 M KOH.
Fig. 9(d) also shows the diminution in specific capacitance with
rise in current density. At high current density the ions move very
fast which reduce their duration of intercalation or redox reaction
thereby decreasing the charge storage ability. Table 3 shows the
specific capacitance values obtained for each electrochemical sys-
tem from the charge-discharge curves recorded at different current
densities.
Comparison of material’s coulombic efficiency in different electrolytes is shown in Fig. 10. Coulombic efficiency of α -MnO2
nanorods was found to be 98% for the mixture electrolyte (1 M
Na2 SO4 +0.5 M KOH), 95% for 1 M Na2 SO4 , and 90% for 0.5 M
KOH at 1A/g current density. This result indicates the excellent reversibility of the material during charge-discharge process in 1 M
Na2 SO4+0.5 M KOH electrolyte [70-71].
Cyclic stability of α -MnO2 nanorods in different electrolytes at
10 A/g current density is illustrated in Fig. 11. The investigation
on long-term charge-discharge performance was carried out for
10,0 0 0 cycles. After this duration, the capacitance retention value
for α -MnO2 nanorods was found to be 80%, 75%, and 70% in 1 M
Na2 SO4 +0.5 M KOH, 1 M Na2 SO4 , and 0.5 M KOH electrolytes respectively. The highest capacitance retention was exhibited by the
8
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Fig. 10. Coulombic efficiency of α -MnO2 nanorods.
Fig. 11. Cycle stability analysis of α -MnO2 nanorods.
electrode material in mixture electrolyte substantiating its high
stability and conductivity. This good capacitance retention shown
by α -MnO2 nanorods throughout the long charge-discharge cycles
reveals their excellent supercapacitive behavior.
Table 4 provides the specific capacitance and capacitance retention shown by MnO2 nanorods and different MnO2 based composites in recent reports.
The electrochemical impedance spectroscopy (EIS) is carried out
for the α -MnO2 nanorods electrode materials in the frequency
range of 100 kHz to 0.01 Hz at an open circuit potential with
AC amplitude of 5 mv [80]. Fig. 12 (a–d) shows the Nyquist plots
and corresponding equivalent circuits for α -MnO2 nanorod electrodes in 1 M Na2 SO4 , 0.5 M KOH, and 1 M Na2 SO4 +0.5 M KOH
respectively. A semicircle and nearly vertical line can be found in
the high frequency range and low frequency range correspondingly
[81]. The real axis (X-axis) intercept in the high frequency region
Fig. 12. Nyquist plots of α -MnO2 in (a) 1 M Na2 SO4 (b) 0.5 M KOH (c) 1 M Na2 SO4 +0.5 M KOH, and (d) Nyquist plots of α -MnO2 in 1 M Na2 SO4 and 1 M Na2 SO4 +0.5 M
KOH.
9
M. Jayachandran, A. Rose, T. Maiyalagan et al.
Electrochimica Acta 366 (2021) 137412
Table 4
Comparison of the obtained results with other MnO2 based research works.
S. no
Electrode Materials
Electrolyte
Current density (A/g)
Specific Capacitance (F/g)
Capacitance retention (%) (Cycles)
Ref
1.
2.
3.
4.
5.
6.
7.
8.
9.
MnO2 /g-C3N4
MnO2 nanosheet
BN-MC/α -MnO2
α -Fe2 O3 /MnO2
MnO2 Nanorod
MnO2 /Graphene
MnO2 Nanorods
MnO2 Nanorods
α-MnO2 Nanorods
1
1
1
1
1
1
1
1
1
1
0.5
1
1
2
1
1
0.71
1
211
293
338
216
242
278.5
362
346
570
90% [1000]
89% [10,000]
95.6% [2000]
89.2% [1000]
76.5% [1000]
75% [10,000]
83% [5000]
86.6% [1500]
80% [10,000]
[72]
[73]
[74]
[75]
[76]
[77]
[78]
[79]
This Work
M
M
M
M
M
M
M
M
M
Na2 SO4
Na2 SO4
Na2 SO4
KOH
Na2 SO4
KOH
Na2 SO4
Na2 SO4
Na2 SO4 + 0.5 M KOH
indicates the internal resistance (Rs ), the semicircle diameter gives
the charge transfer resistance (Rct ) and the straight line implies the
Warburg diffusion resistance [82].
From Fig. 12(a), Rct values of α -MnO2 nanorods in 1 M Na2 SO4
electrolyte solution before and after 10,0 0 0 cycles were found to
be 1.5 Ω and 2.5 Ω, respectively. After stability test, the inclined
line in the low frequency region tends to be parallel to imaginary axis. This confirms the increase in supercapacitive behavior
of the material after 10,0 0 0 cycles. Similarly, from Fig. 12(b), Rct
values of α -MnO2 nanorods in 0.5 M KOH solution before and after cyclic stability were revealed as 5Ω and 10Ω respectively. This
electrochemical system shows the highest resistance values. Also
the low frequency region line becomes more inclined after stability
run indicating an increase in diffusive behavior. From Fig. 12(c), Rct
values of the electrode material in 1 M Na2 SO4 +0.5 M KOH electrolyte before and after 10,0 0 0 cycles were observed as 4 Ω and
6 Ω, respectively. Nyquist plots of two electrochemical systems are
given in Fig. 12(d) where both systems reveal comparatively good
properties.
Acknowledgments
The authors thank Micro-Raman facility of SRM Central Instrumentation Facility (SCIF), and Nanotechnology Research Center (NRC), SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India for the support in characterization studies. The author T. Vijayakumar acknowledges the financial support obtained from the Department of Space, Government of India [Grant no. B.19012/57/2016-II] through RESPOND project and
the selective excellence initiative award received from SRM Institute of Science and Technology. The author T. Maiyalagan acknowledges the financial support from the Department of Science and
Technology-Science and Engineering Research Board [DST-SERB;
No. ECR/2016/002025], through Early Career Research Award and
support of Scheme for Promotion of Academic and Research Collaboration (SPARC) of the Ministry of Human Resource Development (MHRD), Government of India, SPARC Grant No: SPARC/20182019/P1122/SL.
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Cyclic voltammograms, charge-discharge patterns, and Nyquist
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Declaration of Competing Interest
None.
Credit authorship contribution statement
M. Jayachandran: Conceptualization, Data curation, Formal
analysis, Writing - original draft, Writing - review & editing.
Aleena Rose: Data curation, Formal analysis, Writing - review &
editing. T. Maiyalagan: Conceptualization, Data curation, Formal
analysis, Writing - original draft, Writing - review & editing. N.
Poongodi: Conceptualization, Writing - original draft, Writing - review & editing. T. Vijayakumar: Conceptualization, Data curation,
Formal analysis, Writing - original draft, Writing - review & editing.
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