Recent trends in bimetallic oxides and their composites as electrode materials for supercapacitor applications

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Recent Trends in Bimetallic Oxides and Their Composites as
Electrode Materials for Supercapacitor Applications
T. Elango Balaji,[a]
Himadri Tanaya Das,[b, c]
and T. Maiyalagan*[a]
ChemElectroChem
Reviews
doi.org/10.1002/celc.202100098
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ChemElectroChem 2021, 8, 1–25 © 2021 Wiley-VCH GmbH
These are not the final page numbers! ��

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There is a growing interest in supercapacitors as energy storage
systems due to their high specific power, fast charge/discharge
rates, and long cycling stability. Researchers have focused
recently on developing nanomaterials to enhance the capacitive
performance of supercapacitors. The inclusion of electroactive
components, such as transition metal oxides (TMOs), carbon-
based materials, and conducting polymers (CPs), is believed to
play an important role in improving the electrochemical
behavior of the electrode materials. Nevertheless, supercapaci-
tors containing TMOs, carbon-based materials, and CPs com-
monly suffer from inferior ion-transport kinetics and poor
electronic conductivity, which can affect the rate capability and
cycling stability of the electrodes. Therefore, the development
of TMO/CP and TMO/carbon-based electrode materials has
gained widespread attention because they synergistically
combine the advantages of both materials, enabling revolu-
tionary applications in the electrochemical field. In general,
TMOs have given good performance as electrodes for super-
capacitors by further increasing the performance of the
electrode when two metal cations are introduced into a single
crystal structure. This Review describes and highlights recent
progress in the development of bimetallic oxides regarding
their design approach, configurations, and electrochemical
properties for supercapacitor applications, at the same time
providing new opportunities for future energy storage technol-
ogies.
1. Introduction
The depletion of fossil fuel and environmental pollution is
projecting the research towards energy harvesting from extra-
neous renewable energy resources. That is how the alternative
energy resources came into play to alleviate the current energy
demands. Renewable energy resources like solar and wind,
became crucial support under current circumstances, as well as
advanced energy storage systems with both high-power density
(PD) and high energy density (ED), are key aspects to mitigate
the energy crisis. It is now essential that portable light-weight
conductive material with low cost, environmentally friendly
energy conversion, and storage systems are the current
challenges in research.[1]
Electrochemical energy storage and
conversion is playing a vital role in the portfolio of energy
systems that includes fuel cells, supercapacitors, and batteries.
Some of the most commonly used battery devices include lead-
acid cells, Ni Cd batteries, Ni-Metal Hydride batteries, Lithium-
ion batteries (LIBs). The emerging energy storage devices such
as metal-air batteries, metal-ion batteries like Na-ion batteries,
Al-ion batteries, Mg-ion batteries, Zn-ion batteries etc., are also
attracting considerable attraction for researchers in recent
years. On the other hand, batteries use slow faradaic reactions
to store and release charge throughout the active electrode
materials. Batteries have wider potential windows, high energy
density which makes them run for a long time at one single
charge. Unlike batteries, supercapacitors have a narrow poten-
tial window and rapid charge-discharge cycling. Supercapaci-
tors were introduced lately, due to their advantages of high
power density, high charge-discharge (CD) capabilities, good
cyclic stability, eco-friendly, cost-effective, and also long-
lasting.[2]
Supercapacitors and batteries are predominantly, the
charge-storage devices, which have a quite similarity, further it
consists of double electrodes with high conductance separated
by an electrolytic medium with ionic charge carriers. Especially,
supercapacitors (SCs) are being considered as striving energy
storage devices, due to their high-PD, high specific capacitance,
superior charge/discharge characteristics, long cycle life, and
modifiable range of operating temperature[5]
Supercapacitors
acts as a bridge between conventional capacitors and batteries.
The high power density of supercapacitors brings up its usage
in high-speed electric cars as shown in Figure 1 (b).
In general, a supercapacitor is mainly composed of current
collectors, two active electrode materials, an electrolyte, and a
separator.[6]
An electrical double layer is formed at the surface
of the electrode during the charging phase and the charges
migrate through the electrolyte during discharge. However,
within an ideal supercapacitor, only surface localized fast
proceeding physical processes occur at the electrode/ electro-
lyte interface.[7]
The energy storage performance of the super-
capacitor is massively dependent on various factors, such as the
electrochemical behaviors of the electrode materials, the choice
of electrolyte, and the potential window of the device.[8]
Various
research efforts have been going on to develop novel electrode
materials for supercapacitors with appropriate structural proper-
ties to facilitate effective transport and ionic diffusion. The most
vital characteristics of supercapacitors are cost-efficient, eco-
friendly, and flexible electrode materials with high stability,
outstanding electrochemical property, and excellent mechanical
performance.[9]
The advantages of the supercapacitors drag
attention towards its energy storage system but few shortfalls
impede its practical applications. To overcome their issues, the
scientist and industrialist have been investigating supercapaci-
tor electrodes materials in details.
The performance of supercapacitors (SCs) depends on its
type of charge storage by electrode materials, on that basis it
has been classified into Electrical Double Layer Capacitors
(EDLC), Pseudo-capacitors (PCs), and Hybrid supercapacitors
(HSCs). From Figure 2 we can see that the classification is based
upon the charge storage mechanism, EDLCs store charge
electrostatically; Pseudocapacitors and EDLCs are the type of
[a] T. E. Balaji, Dr. T. Maiyalagan
Electrochemical Energy Laboratory, Department of Chemistry
SRM Institute of Science and Technology
Kattankulathur, Tamil Nadu – 603 203, India
E-mail: maiyalat@srmist.edu.in
[b] Dr. H. Tanaya Das
Department of Materials and Mineral Resources Engineering, NTUT
No. 1, Sec. 3, Chung-Hsiao East Rd., Taipei 106, Taiwan, ROC
[c] Dr. H. Tanaya Das
Centre of Excellence for Advanced Materials and Applications
Utkal university
Vanivihar, Bhubaneswar-751004, Odisha, India
ChemElectroChem
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capacitors those are differentiated on basis of charge storage.
The storage mechanism of pseudocapacitor is via rapid faradaic
redox reaction happening at the surface of the electrode or
pseudo-intercalation type reactions, where the EDLCs stores
charge via. double layer formation. On the other hand, the
performance of battery-type materials undergo purely Faradaic
reactions and expressed in terms of specific capacity (mAh g 1
)
since the average capacitance is not uniform throughout the
potential window.[11]
The average capacitance through the
potential window is known as specific capacitance. The specific
capacitance can be estimated by various electrochemical
techniques such as cyclic voltammetry (CV) curves or galvano-
static charge/discharge curves which depicts the mechanism of
electrochemical reaction undergone during a complete cycle of
charge and discharge. (as shown in Figure 3).
From CV and GCD specific capacity can be calculated using
the Equations (1) and (2)
Specific capacity ðA h g 1
Þ ¼
R
i V
ð ÞdV A V
ð Þ
m g
ð Þ � # V s 1
ð Þ � 3600
(1)
In this equation (1), i V
ð ÞdV is the integral area of the CV
curve, A V (ampere volts); ν is the scan rate and m is the mass
of the active material. This specific capacity can also be
calculated from GCD curves by the following equation,
Specific capacity ðA h g 1
Þ ¼
R
i A
ð Þdt s
ð Þ
3600 � m g
ð Þ
(2)
In the above equation, i and m denote the current density,
dt denotes the discharge time. By applying the respected values
in the above equation specific capacity can be calculated from
GCD.[12]
Hybrid supercapacitors store charge both by electrostati-
cally and electrochemically combining the benefits of both
EDLCs and Pseudocapacitors. In a three-electrode workstation,
Thandavarayan Maiyalagan received his Ph.D
in Physical Chemistry from the Indian Institute
of Technology, Madras (India), and completed
postdoctoral programs at Newcastle Univer-
sity (UK), Nanyang Technological University
(Singapore), and the University of Texas,
Austin (USA). Currently, he is an Associate
Professor of Chemistry at SRM Institute of
Science and Technology (India). His main
research interests focus on design and devel-
opment of electrode nanomaterials for energy
conversion and storage applications, particu-
larly fuel cells, supercapacitors, and batteries.
T. Elango Balaji received his Master of Science
(General Chemistry) from Bishop Heber Col-
lege (India). Currently, he is working under the
guidance of Dr. T. Maiyalagan at the SRM
Institute of Science and Technology (India).
Himadri Tanaya Das joined the Centre of
Advanced Materials and Applications, Utkal
University (India) as a Postdoctoral Fellow in
2021. She received her Ph.D. in Physics from
Pondicherry University (India) in 2019. Her
Ph.D. research work was based on nano-
materials in energy storage such as batteries
and supercapacitors. She also holds research
experience in various institutes like Nanyang
Technological University (Singapore), National
Tapei University of Technology (Taiwan), and
National Taiwan University of Science and
Technology (Taiwan).■■ok?■■ Her re-
search interests lay in synthesis and applica-
tions of nanomaterials.
Figure 1. Application of supercapacitor in domestic (a) and automobile (b) sectors. Reproduced from Ref. [3] under the terms of the Creative Commons license
and with permission from Ref. [4]. Copyright (2020) The Authors and (2017) Elsevier, respectively.■■Please ensure and confirm that copyright permission
has been obtained from [4]■■
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the instrument consists of a working electrode, a counter
electrode, and a reference electrode. Usually, the reference
electrode and counter electrodes are Ag/AgCl and Pt wire
electrode, respectively. The active material is coated on the
current collectors like nickel foam or carbon cloth considered as
working electrodes. The performance of the electrodes can be
analyzed by different parameters. The electrochemical property
of the material can be investigated by characteristics like Cyclic
voltammetry (CV), Galvanostatic charge-discharge studies
(GCD), and Electrochemical Impedance Spectroscopy (EIS).
These studies reveal the applicability of the as-synthesized
material to the supercapacitor application. In cyclic voltammetry
studies the redox behavior, the specific capacitance of the
material can be assessed using the formula in Equation (3):
Cs ¼
R
IdV
S:DV:m
F g 1
(3)
Where, ∫ I dV indicates the integral area of one complete
cycle of CV curve, ‘S’ scan rate (mVs 1
) ‘~V’ potential window (V)
and ‘m’ mass of the active material (mg), Cs specific capacitance
(F g 1
). To find out the specific capacity of the material the
formula used is given by Equation (4):
Q ¼
I Dt
DU m
C g 1
(4)
ΔU is the width of the potential window, m is the mass of
the active materials, and t is the discharging time. To find out
the cyclic stability of the active electrode material charge-
discharge studies can be carried out using the formula in
Equation (5):
Cs ¼
I � t
m � V
F g 1
(5)
where I (A), V (V), and m (g) represent the discharge current,
discharge time, potential window, and mass of electrode
materials, respectively.
By selecting a large specific surface area, highly porous or
highly electroactive electrode materials, such as amorphous
carbon or nanoporous metal oxides, capacitance per gram of
material is amplified. For example, activated carbon (AC) holds
high specific capacitance due to the higher specific surface area
of the material. In a cylindrical supercapacitor, the inner surface
of the electrode is padded with activated porous carbon,
resulting in a higher surface area that is about a million times
Figure 2. Supercapacitors hierarchy with three main categories and their
subtypes according to the possible materials like metal oxides, conducting
polymers, and carbon materials. Reproduced with permission from Ref. [10].
Copyright (2020) Springer.■■Please ensure and confirm that all copyright
permission has been obtained from [10]■■
Figure 3. a) Two electrode device configuration. b,c) Electrochemical curves for hybrid and asymmetric supercapacitors. Reproduced with permission from
Refs. [12] and [13]. Copyright (2020) Wiley-VCH and (2010) Royal Society of Chemistry, respectively. ■■Please ensure and confirm that all copyright
permissions have been obtained■■
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large as the surface area of an ordinary electrostatic capacitor
increases.
1.1..1 Device Configuration of a Two-Electrode Setup
Further, the electrodes assembled with electrolyte to form a
device which energy density and power density can be
calculated by using the formulae given by Equations (6) and
(7):[18]
E ¼
1
2
Csp DV2
Whkg 1
(6)
P ¼
DV Im
2
Wkg 1
(7)
A typical supercapacitor consists of two electrodes that are
separated by a porous separator and filled with electrolyte. The
active material is coated on the electrodes. Current collectors of
metal foil are used to conduct electrical current from each
electrode. The separator and the electrodes are immersed into
an electrolyte in suitable concentration, which allows ionic
current to flow between the electrodes while preventing
electronic current from discharging the cell. A two-electrode
supercapacitor module, based on the desired size and voltage,
is constructed of multiple recurring unit cells. A test fixture
configuration that closely mimics the unit cell configuration
relatively matches the performance of a packaged cell. Two-
electrode test fixtures are either available commercially or can
be easily fabricated from two stainless steel.
The most common organic and aqueous electrolytes are
tetrafluoroborate in propylene carbonate or acetonitrile and
KOH, H2SO4, respectively. According to the electrode config-
uration in a supercapacitor, they are classified as symmetric,
asymmetric, and hybrid supercapacitors. The symmetric super-
capacitor has similar electrode material on both the electrodes.
Xe et al.„ assembled symmetric solid-state supercapacitor using
walnut shell derived porous carbon as both positive and
negative electrodes immersed in PVA/KOH gel electrolyte. The
active material showed a specific capacitance of 138 mFcm 2
and good stability of 96% after 3000 cycles.[14]
Asymmetric
supercapacitors have two different electrode materials with two
different charge storage mechanisms. Guo et al.„ reported
Co3O4 core-shell microspheres as electrodes for asymmetric
supercapacitor using PTFE membrane as separator and 2 M
KOH which exhibited a specific capacity of 261.1 Fg 1
with
capacity retention of 90.2% after 2000 cycles. energy and
power densities were observed to be 16.6 Whkg 1
at
883 Wkg 1
.[15]
Hybrid capacitors one electrode as battery type
and capacitive electrode, Du et al.„ assembled a hybrid
capacitor with the synthesized battery type NiMoS4 as positive
electrode and activated carbon as a negative electrode which
exhibited a high specific capacity of 313 Cg 1
with high energy
and power density of 35 Whkg 1
at power density of
400 Wkg 1
. To investigate the electrode’s electrical properties
three-electrode setup can be used but when analyzing its
physical properties like energy and power density it is
mandatory to use a two-electrode setup.
So, the selection of electrode material plays a vital role.
Such outcomes help researchers to understand the electrode
performance in supercapacitors. In general, it is seen EDLCs
electrodes show high coulombic efficiency than metal oxides
but with a low range of capacity relative to metal oxides.[16]
The
metal oxides show redox reaction so considered as battery-type
electrode materials. It is seen that hybrid supercapacitors
deliver high energy density with a high ability of charge
storage.[17]
Given below the insights on various types of
supercapacitors electrode materials.
1.2.2 Factors Influencing the Energy Density and Capacitance
of the Supercapacitors
The governing factors for the energy density of devices are
potential window, pore size distribution, surface area, electro-
lytes, and device configurations. High surface area and pore size
contribute to the higher capacitance, but when the pore size is
very less than the charge-storage will not take place due to the
minimization of interaction of nanoparticles with electrolyte
ions. A study done by Gogotsi et al.,, on the effect of pore size
on capacitance reveals that when the size of the solvated ions
is larger than the pore size then the electrolyte ions are
incapable of contributing to charge storage.[18]
Still, it remains a
challenge to identify the optimal pore size and surface area to
maximize capacitance. Fabricating electrode materials with
narrow pore size distribution would increase the capacity of
supercapacitors ultimately; boost the energy density without
sacrificing the high power density.
E ¼
1
2
CV2
(8)
From Eq.8, along with capacity, increasing the potential
window will also increase the energy density as energy density
is directly proportional to the square of the potential. The
potential window for aqueous electrolytes is less than 1.0 V.
Organic electrolytes have relatively higher operating voltage
greater than 2.0 V only ionic liquids show a higher potential
window of 2.0 to 6.0 V. Thus, aqueous supercapacitors have low
energy density than non-aqueous ones. Even though organic
electrolytes and ionic liquids have advantages like wider
potential window and higher energy density, they have their
disadvantages such as organic electrolytes are expensive,
solvents used in the electrolyte like propylene carbonate and
acetonitrile are quite inflammable. Ionic liquids possess a much
higher potential window but the viscous property results in
poor ionic conductivity. When compared to these two electro-
lytes aqueous electrolytes are less expensive, non-toxic, and
have a good conductivity for these reasons aqueous electrolytes
are mostly preferred for bimetallic oxides.
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2. Types of Electrode Materials
2.1. Electrical Double Layer Capacitor (EDLCs)
H.I. Becker first demonstrated and patented the electrical
double-layer energy storage and delivery by EDLCs with porous
carbon electrodes in an aqueous electrolyte in 1957 according
to the electrical double layer theory.[21]
Then, NEC first brought
the EDLCs-type devices into commercialization with the
permission of SOHIO in1978, which was first named as super-
capacitor to describe the high energy differed from conven-
tional capacitors.
[22,23]
The electrical double layer capacitor is
developed from the electrical double layer model of the
Helmholtz model, EDLCs store the charges in the Helmholtz
interface between an electrode and electrolyte based on the
electrostatic accumulation of ions in the electrolyte. Hence, the
charge/ discharge process is non-faradic and reversible. The
influence of thermal motion and ion absorption was not
explained by Helmholtz double layer. The electrical double layer
theory was later updated by Guoy et al.,[24]
considered the
thermal motion of ions close to the charged surface a double
layer is formed during the charging phase, by introducing a
diffusive layer in the electrolyte as shown in Figure 4(a). As per
this model, the double layer is not rigid near the electrode
surface but this model did not account for the ion absorption at
the electrode/electrolyte interface is not taken into account
according to this model double layer is not rigid at the
electrode/electrolyte interface. Later Stern model combines the
concept of both Helmholtz and Gouy-Chapman models.
According to the stern model, electric potential varies when the
distance from electrode surface varies and Grahame’s concept
of inner Helmholtz plane and outer Helmholtz plane which
explains the real situation of an electrical double layer.[25]
The most commonly carbonaceous electrodes like gra-
phene, activated carbon, carbon nanotubes, carbon aerogel
etc.[26]
are used for EDLCs electrode materials. Graphene is a
single layer of sp2
bonded carbon atoms tightly packed semi-
conductor having zero bandgap. The calculated theoretical
capacitance of graphene is 550 Fg 1
with a high specific surface
area of 2630 m2
g-1.[27]
In practicality, due to restacking and
agglomeration (weak Van-der-Waals interactions) limits the
specific surface area and capacitance values experimentally.
Several synthesis strategies have been developed to solve this
issue such as heteroatom doping to graphene sheets, creating
effects in graphene sheets, or stacking of sheets by interlayer
interactions etc.[28]
, Nowadays, 3D graphene and partially
reduced graphene oxide (rGO) gives better electrical
performances.[29]
Similarly, carbon nanotubes (CNT) are an
allotrope of carbon with excellent electrical conductivity,
mechanical strength, and chemical stability.[30]
Commonly,
commercial EDLCs use activated carbon electrodes and they
exhibit a practical specific capacitance of 200 Fg 1
in aqueous
electrolytes.[31]
Specific capacitance obtained from graphene
oxide showed 306 Fg 1.[32]
The specific capacitance of CNT (1D),
rGO (2D), and mesoporous carbon (3D) showed 33 Fg 1
,
166 Fg 1
, and 202 Fg 1
obtained by chemical activation with
KOH.[33]
J. Ding et al., in their work on activated carbon, coated
CNT which exhibited a specific capacitance of 108 Fg 1
with a
retention rate reaches 95% after 10,000 cycles.[34]
Porous carbon
material such as activated carbon has a high specific surface
area and exhibit high specific capacitance.
The activated carbon has a high surface area for the
electrolyte ions to interact, yet some of the surface areas are
not accessible by electrolyte ions due to the micropores in it.
Single-walled carbon nanotubes (SWCNT) are hollow cylindrical
bundles allowing only the outermost surface accessible for
electrolyte ions.[35]
In graphene sheets, due to the van der Waals
interactions, the sheets tend to agglomerate which complicates
the flow of ions through the ultra-small pores. We can under-
stand that each carbon material has its advantage and
disadvantages. To resolve this problem, CNTs are placed in
between the graphene sheets which gives rise to rapid diffusion
pathways to electrolyte ions.[23]
Also in this way, the structure of
graphene becomes more stable as CNTs act as a binder to hold
the sheets together.
Figure 4. a) Mechanism of charge storage in electrical double-layer capacitors and pseudocapacitors; b) CV and GCD curves of electrical double-layer
capacitors, pseudocapacitors, and battery type materials Reproduced with permission from Refs. [19] and [20]. Copyright (2020) Elsevier and (2020) The
Authors, respectively. ■■Please ensure and confirm that copyright permission has been obtained from [19]■■
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The inherent properties like pores play a vital role in charge
storage of EDLCs electrodes i.e. the charge separation occurring
at an electrode-electrolyte interface of porous electrode. A
porous electrode like AC can have a very large effective surface
area, to create a large capacitor at each electrode. It indicates,
EDLC is an important class due to high specific surface area,
tunable porous electrodes providing facile pathways for easy
accessibility and transport of electrolyte ions, high electrical
conductivity, and excellent electrochemical stability.[36]
In a
given particle of the porous material, the high surface area can
be obtained by different sizes of pores or random distribution
of pores. Depending on the diameter of the pores, it is
categorized into subsets such as macropore, mesopore, and
micropore. When the pore size is below 50 nm it is macropore,
when the pore size is 2 to 50 nm it is called mesopore and
when the pore size is below 2 nm indicates microspheres. Some
of the examples of mesopore, macropore, and micropores are
NiCo2O4,[37]
Carbon nanotubes (CNT)[33]
and activated carbons[33]
2.2. Pseudocapacitors (PCs)
The second type of supercapacitor based on charge storage
mechanism is Pseudo-capacitors, which store charges by sur-
face charge-transfer reaction between an electrode and electro-
lyte. For storage systems, pseudo-capacitors can store very large
charges by electron transfer on the surface of the electrode. In
this contrast, redox reactive charge storage systems involve the
volume expansion of electrodes, leading to disadvantages in
terms of cyclic life and response speed.[38]
Transition metal
oxides (TMOs) are the electrode materials most commonly used
in supercapacitors these TMOs have been widely reported a lot
as an electrode material for supercapacitors due to their high
thermal conductivity. B.E. Conway at first reported RuO2 as
TMOs with high pseudocapacitance. Whereas, the high cost of
RuO2 was replaced by MnO2 followed by WO3, MoO3, V2O5, etc.,
with cost-effective and electrochemical nature[39]
Further,
carbon matrix were incorporated into the metal oxides to
increase the conductivity of the metal oxides since most of the
transition metal oxides are semi-conductors and to enhance the
stability of the metal oxides for these reasons carbon materials
combined with metal oxides nanocomposites have been
demonstrated good performance for the supercapacitor appli-
cations. For example, Z. Fan et al., in his work on graphene-
MnO2 composites as electrode material for supercapacitor has
achieved a specific capacitance of 310 Fg 1
at 2 mVs 1
where
pure graphene shows a specific capacitance of 104 Fg 1
at
2 mVs 1
and shows a cyclic stability as 88%.[40]
Due to the 2D
structure graphene has almost zero bandgap which makes it a
high conductive material and also due to the synergistic effect,
doping of graphene increased the charge storage capacity and
also increased the stability of the metal oxides due to its high
surface area and evenly distributed porous structure. To further
enhance the electrochemical properties of metal oxides an
electrically conductive material like graphene and also enhan-
ces the mechanical strength of metal oxides to use them in
advanced flexible electronics.
A hybrid supercapacitor is a fusion of an Electrical double
layer capacitor and Pseudocapacitors. It has two asymmetric
electrodes, one of which exhibits battery-type behavior, and the
other shows a double layer capacitive behavior. The presence
of faradaic behavior increases the specific capacitance and
specific energy of the supercapacitor on the other hand the
electrical double layer capacitance behavior provides increased
cyclic stability and high specific power. Such architecture led
the energy storage device to outcome with wide working
voltage, better mechanical/ chemical stability, and high energy/
power density.[41]
Many researchers have been tremendously
working on finding a suitable electrode material for improved
electrochemical performance. Often the choice of battery-type
electrode materials is the metal oxides due to their high specific
surface area, variable oxidation states, thermal and chemical
stability. These characteristics make the metal oxides a promis-
ing electrode material for supercapacitors. Among metal oxides,
TMOs are a suitable material for SCs electrodes as battery-type
electrodes to be pragmatic in hybrid capacitors[42]
for faradaic
reactions of charge storage. The metal oxides have been highly
explored due to their good electrochemical performance. The
metal oxides like NiO, Co3O4, Fe2O3 based supercapacitors
performances are constantly focused on by researchers to reach
high specific capacity as their theoretical value is higher and
can be experimentally tuned by altering the morphology or
nanocomposites compositions.[43]
To achieve theoretical ca-
pacity, and tackle issues like capacity fading or low electronic
conductivity of TMOs, researchers have investigated ways like;
(i) doping of metal oxides, (ii) adding carbon-conductive
materials or metal-organic framework, (iii) combining with a
conductive polymer, and combining with other metal oxides.
Das et al., investigated the electrochemical properties of Ni/
NiO and Ni/NiO@rGO they showed a high specific capacity of
158 Cg 1
and 335 Cg 1
. The as-fabricated solid-state hybrid
supercapacitor showed a high energy density of 12.8 Whkg 1
and a high power density of 2875 Wkg 1.[16]
Sivakumar et al.,
developed a controllable synthesis for cobalt oxide to enhance
the specific capacitance and the results showed a high specific
capacitance of 2751 Fg 1
with a high energy density of
31.7 Whkg 1
.[44]
Further adding a conductive network to
improve conductivity, improve redox property, and increasing
the specific surface area of the material. Co–MOF has been
reported to have a specific capacitance of 450 Fg 1
at
0.5 Ag 1[45]
and further doping of TMOs with conductivity
polymers like PANI, PEG etc. can effective way to boost
electrochemical performance.[46]
Besides that many reports were
focused on mixed oxides for the synergistic effect of both
oxides of the electrodes. Mixed metal oxides like NiO/CuO,
Co2O3@Fe2O3, etc., grabbed much attention due to their high
electrochemical properties like variable oxidation states, syner-
gistic effects, and high electrical conductivity due to these
properties many research studies were conducted on mixed
metal oxides.[47]
Disappointingly, there is short-coming like
inhomogeneity in crystal structure, these crystal structures were
poorly defined when compared to those of a single phase. To
overcome this problem bimetallic oxides having good crystal
structures have been used as electrode material for super-
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capacitors. The crystal structure of bimetallic oxides is well
oriented.
Even though single metal oxides show high electrochemical
performance, when combining two metal ions in a single crystal
the performance of the material is increased further from
Figure 5 we can see the comparison between the MnO2 and
CuMn2O4 the former shows better electrochemical curves, well
redox peaks in CV and high cyclic stability than the former.
Hence, transition metal oxides with binary metal oxides show
high electrochemical activity. When compared to bimetallic
cobaltite, NiCo2O4 showed a high specific capacitance of
440 Fg 1
which is higher than that of single metal oxide with
doping.[48]
NiCo2O4 on further doping with rGO enhances the
specific capacitance up to 1305 Fg 1
.[49]
From these, we can conclude that binary metal oxides offer
higher electrochemical performance. Currently, binary metal
oxides are a trending topic of research for supercapacitors
electrode material. In recent years, binary metal oxides have
been widely explored due to their reversible redox reactions
because of their low cost, low toxicity, multiple oxidation states,
and much higher electrical conductivity. There are given below
different types of bimetallic oxides in supercapacitor applica-
tions and their electrochemical performance of bimetallic
oxides in detail with various examples.
3. Bimetallic Oxides
However, bimetallic oxides are reported relatively with higher
electrochemical activity than their respective single oxides. The
synergistic effect of both the metals provides better electro-
chemical activities. One of the famous bi-metallic metal oxides
is the spinel cobaltites MCo2O4 (M=Mn, Ni, Cu, or Zn) attracted
much attention as it can store a large amount of charges due to
its multiple oxidation states and much higher electrical
conductivity.[52]
For example, nickel cobaltite (NiCo2O4) exhibits
two orders of magnitude of higher electrical conductivity than
nickel oxide (NiO) or cobalt oxide (Co3O4).[44]
besides that
benefits of both the oxides can be obtained in a single sample.
Similarly, iron oxide and cobalt oxide both provide high
electrochemical performance. Owing to low cost, some tran-
sition metal oxides like Mn, Ni, Co, etc. are commonly referred
to as the candidates for developing different pseudocapacitors
and hybrid capacitors. Table 1, shows the specific capacitance
value of various metal oxides. Similarly, many other metallic
Figure 5. a,c) CV curves of activated carbon@MnO2 and CuMn2O4 at different reaction times; b,d) GCD curves of Activated carbon@MnO2 and CuMn2O4 at
different reaction times. Reproduced with permission from Refs. [50] and [51]. Copyright (2017) American Chemical Society and (2017) Elsevier,
respectively.■■Please ensure and confirm that copyright permissions have been obtained■■
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oxides have been examined for supercapacitor studies. In this
review, we have discussed various binary metal oxides as
electrode materials for supercapacitors. With reported research,
it has been discovered that binary metal oxide performed
better than single metal oxides or mixed oxides which
improved the supercapacitor‘s performance.
Bimetallic metal oxides are considered as one of the best
electrode materials for supercapacitors due to the properties
like crystal structure, defects, spin, electronic structure, and
synergetic effect. The crystal structure of bimetallic oxides has
multiple lattice sites that enhance the stability and performance
of the material. Defects like Schottky and Frenkel defects can
help to increase the conductivity of the material since the
vacancy created in the crystal lattice distorts because of that a
local distortion happens which may modify the lattice vibration,
which in turn determines the electrical resistivity of the
material. Synergetic effect arises due to the presence of two
metal cations which improves chemical functionality and
charge storage capabilities by utilizing the oxidation states of
two metal cations, the redox activity is improved. the single-
phase crystal structure enhances the stability and performance
of the material. Moreover, binary metal oxides are easy to
synthesize and less harmful to the environment when com-
pared to binary or single metal sulfides. A few of bimetallic
oxides have been discussed below:
Table 1. Electrochemical performance of some single, mixed, and binary metal oxides.
Electrode
material
Specific
capacitance
Number of
cycles
Retention
rate
Energy density
[W h Kg 1
/W h Cm 2
]
Power density
[W Kg 1
/W Cm 2
]
Current density
[A g 1
/A cm 2
]
Electrolyte Ref.
CuO 571.25 Fg 1
1000 92% NA NA 1 1 M KOH [124]
RuO2/G 441.1 Fg 1
1000 94% NA NA 0.1 1 M Na2SO4
[125]
Co–MOF 450.89 Fg 1
1000 95% NA NA 0.5 6 M KOH [45]
PANI/AC/Ni 1661 Fg 1
2000 93% NA NA 1 1 M KOH [126]
Zr/CeO2 448.1 C g 1
6000 96.4% NA NA 1 2 M KOH [127]
Co–MOF/PANI 504F g 1
5000 90% NA NA 1 1 M KOH [128]
MnO2/CuO 279.12 Fg 1
10000 91.26% NA NA 0.5 1 M KOH [129]
MnO2/NiO 247 Fg 1
1000 81.2% NA NA 0.5 1 M KOH [130]
Cu2O/NiO 2255.5 Fg 1
5000 94.5% NA NA 0.0011 2.0 M KOH [131]
NiO/rGO 127.5 Fg 1
2000 70% NA NA 1 6 M KOH [132]
TiO2/RuO2 1200 Fg 1
10000 95.2% NA NA 0.5 1 M H2SO4
[133]
MnO2/FeCo2O4 2.52 F cm 2
1500 94% NA NA 2 PVA/KOH [134]
FeCo2O4@NiCo 2426 Fg 1
5000 91.6% NA NA 1 PVA-KOH [135]
Ni(OH)2@CuCo2O4 295.6 mAhg 1
3000 93.7% NA NA 1 KOH/PVA [136]
FeCo2O4 960 Fg 1
10000 94% NA NA 2 3 M KOH [137]
RuCo2O4 1469 Fg 1
3000 91.3% 36.5 3294 6 2 M KOH [138]
NiCo2O4 /CF 2658 Fg 1
3000 80% NA NA 2 3 M KOH [139]
MnCo2O4 250 Fg 1
1000 NA 10.04 NA 0.25 2 M KOH [140]
CuCo2O4 1210 Fg 1
5000 86% 42.81 NA 2 6 M KOH [141]
ZnCo2O4 1841 Fg 1
3000 95.8% NA NA 1 6 M KOH [142]
ZnCo2O4 229 Fg 1
1500 84.3% NA NA 0.25 2 M KOH [143]
NiCo2O4/AC 273.5 Fg 1
3000 96% NA NA 1 6 M KOH [144]
CuCo2O4/rGO 978 Fg 1
5600 1.34 times increased NA NA 3 6 M KOH [145]
NiCo2O4/rGO 304 Fg 1
5000 92.8% 95 374 0.5 2 M KOH [146]
LaFeO3/MOF 241.3 Fg 1
5000 92.2% 34 900 1 1 M Na2SO4
[147]
LiCoO2 310.93
mF/cm2
2000 80.26% 5.6 X 10 5
0.0011 NA 1 M LiCl [148]
MgCo2O4 626.5 Fg 1
5000 99.06% 30.6 861 1 2 M KOH [149]
NiV2O6 565.5 Cg 1
3000 84.6% 24.3 800 1 2 M KOH [150]
NiCoO2 778.5 C g 1
1000 97% NA NA 0.5 6 M KOH [151]
CoV2O6 223 Fg 1
15000 123% NA NA 1 2 M KOH [152]
CoNiO2 184 Fg 1
1000 96.8% 15.0 14,210 1 1 M KOH [153]
ZnV2O4 360 Fg 1
1000 89% NA NA 1 2 M KOH [154]
CoGa2O4 642.4 C g 1
40000 125% 36.71 414.1 1 3 M KOH [155]
NiGa2O4 1508 Fg-1 10000 102% 45.2 1600 1 6 M KOH [156]
NiCr2O4 187 Fg 1
2000 80% 6.5 3000 0.6 1 M KOH [157]
CoFe2O4-carbon 102.5 Fg 1
6000 81.5% NA NA 0.16 5 M KOH [158]
NiMoO4-PANI 93 Fg 1
5000 98.6% 33.07 240 0.3 PVA-KOH [121]
NiCeO2@PANI 866 Fg 1
10000 85.6% 120.3 500.2 1 1 M Na2SO4
[159]
MnMoO4/PANI 396 Fg 1
500 81% NA NA NA 1 M Na2SO4
[115]
PANI/MnFe2O4 371 Fg 1
2000 86.7% NA NA 1 1 M H2SO4
[160]
BiVO4/PANI 701F g 1
5000 95.4% NA NA 1 1 M KOH [161]
NiCo2O4@GO 925 Fg 1
500 98% NA NA 7 2 M KOH [162]
CuCo2O4/PPy/CNT 59.55 mFcm 2
8000 80.1% NA NA 0.008 3 M KOH [163]
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3.1. Co-Based Bimetallic Oxides
Among all other transition metal oxides, cobalt mixed with
other transition metals forms a binary metal oxide that shows
outstanding electrochemical behavior. Cobaltite has been
considered as one of the best electrode materials not only in
electrocatalysis or Li-ion batteries but also in supercapacitors.
These materials are low cost, abundant, and non-toxic. Also,
they can improve the reversible capabilities, structural stability,
electrical conductivity, and high theoretical specific
capacities.[53]
By varying the bivalent metal as M=Ni, Fe, Cu, Mn,
Zn, and keeping the trivalent metal as cobalt various studies
have been investigated and which shows excellent specific
capacitance. The cobalt as a trivalent atom can result in various
cobaltites such as NiCo2O4, ZnCo2O4, MnCo2O4, FeCo2O4,
CuCo2O4 etc.. Mostly, cobalt and nickel bimetallic combinations
are preferred for their reversible redox nature and high
theoretical specific capacity.[54]
These spinel structures have a chemical formula AB2O4 and
this belongs to the Fd3 m space group, forming an fcc lattice
cube as bimetallic oxides. The A cations occupy the 8a sites and
are tetrahedrally coordinated by X, while the B cations occupy
the octahedrally coordinated 16d sites. This occupation of
metals at octahedral and tetrahedral sites have an impact on
the properties of the spinel like color, diffusive nature, magnetic
behavior, electrical conductivity, and catalytic activity. This
spinel structure can accommodate guest cations in the 16c
octahedrally coordinated sites, this along with the tetrahedral
8a sites will form a 3-dimensional interconnected network. Kolli
et al., investigated the electrochemical properties of spinel
intercalation compounds, in their study it is proved that the
guest cation played an important role in determining the
electrochemical performance.[55]
Also, the inverse spinel showed
a higher electrochemical performance due to the occupancy of
the trivalent cation in the A site and also partially in the B site,
(Co1-xFex )Tet[CoxFe2-x ]OctO4.
When considering the normal spinels the x value will be 0,
the divalent and trivalent cations occupy the tetrahedral and
octahedral sites. In the case of inverse spinels, the value of x
will be 1. All the divalent cations occupy the octahedral sites
and trivalent cations occupy tetrahedral and octahedral sites
evenly.[56]
In this spinel and inverse spinel compounds two mechanism
follows, one is intercalation and deintercalation as like in a
battery type material and another mechanism is alloying and
dealloying both of these are very helpful to increase the charge
storage capabilities and increase the faradaic redox reaction.
For these reasons, spinel and inverse spinel compounds are
more suitable for supercapacitor electrode material.[57]
The
magnetic interaction between nickel and cobalt ions results in
spinel-type NiCo2O4. Due to the synergetic effect, Ni2+
/Ni3+
and
Co2+
/Co3+
redox couples happen when the electrochemical
process is taking place, which shows lower electron transport
activation energy when compared with single metal oxide.
NiCo2O4 showed a theoretical capacitance of higher than
3000 Fg 1
with high electrical conductivity of 2.5 Scm 1
. Gao
et al., formed a network like mesoporous NiCo2O4 arrays which
were grown on carbon cloth synthesized through hydrothermal
method the results showed a high capacitance of 1843.3 Fg 1
at 1 Ag 1
and only 10% loss after 4000 cycles with a current
density of 10 Ag 1
. The assembled device showed a high
energy density of 38.3 Whkg 1
at a power density of
396 Wkg 1
.[58]
Interestingly, CuCo2O4 has two crystal structures, normal
spinel, and inverse spinel. A normal spinel compound trans-
forms into an inverse spinel when the equation is CuxCo3-xO
with x higher than 0.2.[55]
In the inverse spinel CuCo2O4, the Co3+
cations were distributed to both A sites, and one-half of the B
sites, the Cu2+
cations occupy the B sites. CuCo2O4 showed a
high theoretical capacitance of 984 Fg 1
, with a bandgap of
0.5–0.6 eV which shows a good conductivity.[56]
The reason why
CuCo2O4 shows high electrochemical performance than single
metal oxide is due to the reason that cobalt cations can
improve the electrochemical activities on the other hand
copper cations enhance the electrical conductance. The param-
eters like morphologies play a vital role in the storage capacity
of the electrodes. Various morphologies of bimetallic oxides
synthesized via different synthetic routes have been reported
by many researchers.[59]
Das et al., reported an octahedron
morphology for CuCo2O4, synthesized via ethylenediaminetetra-
acetic acid (EDTA) assisted hydrothermal approach that is most
suited for supercapacitor application. The electrochemical
studies on the obtained single-crystalline CuCo2O4 octahedron
with a surface area of 61.97 m2
g 1
, revealed the specific
capacity of 989 Cg 1
at 5 mVs 1
. And a cyclic stability with
retention of 87% of its initial specific capacity which is achieved
after 5000 cycles at a current density of 10 Ag 1
using 6 M KOH
as electrolyte.[60]
Usually, when the binder combines with the
active material, it decreases the electrochemical performance;
binder-free electrodes have grabbed much attention. Wang
et al., was the first to report the binder-free CuCo2O4 grown on
Ni foam synthesized via electrospinning method with succes-
sive heat treatments. The as-synthesized electrode material
exhibited the nanowire network structure morphology. The
electrochemical studies showed a higher specific capacitance of
467 mFcm 2
at a current density of 1 mAcm 2
with 90% of its
initial capacitance retention after 1500 cycles. The fabricated
asymmetric supercapacitor with activated carbon as a negative
electrode showed a high ED of 0.806 mWhcm 3
and the
specific capacitance increases from 326 mFcm 2
to 467 mFcm 2
with the scan rate from 100 to 10 mVs 1
.[61]
As with Cu, Zn also has been investigated with cobalt metal
for binary metal oxides. ZnCo2O4 nanomaterials were inves-
tigated for supercapacitor application. Due to its high specific
surface area, uniform pore size distribution, improved reversible
capacities, good cyclic stability, and good environmentally
friendly nature mesoporous microspheres are considered as a
better material. Gong et al., reported ZnCo2O4 synthesized
through self-template solvothermal method followed by an-
nealing as shown in Figure 6. The electrode material showed
microsphere morphology with a high surface area of
34.60 m2
g 1
with an average pore diameter of 6.96 nm. The as-
synthesized electrode material was investigated for super-
capacitor application and it showed a maximum specific
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capacitance of 542.5 Fg 1
at a current density of 1 Ag 1
with
good cyclic stability of 95.5% capacitance retention after 2000
cycles. Further, a quasi-solid state asymmetric supercapacitor
was fabricated using the as-prepared electrode material as a
positive electrode and activated carbon as a negative electrode.
The fabricated device delivered a maximum specific capacitance
of 68.93 Fg 1
with a good cyclic stability of 76.68% after 1000
cycles at 0.5 Ag 1
and it exhibited an ED of 21.97 Whkg 1
and a
PD of 38.89 Wkg 1
.[62]
Fascinatingly, MnCo2O4 has a theoretical capacitance of
3620 Fg 1
this is higher than other materials like RuO2 (1300–
2400 Fg 1
) and MnO2 (1370 Fg 1
). MnCo2O4 has a very high
electrical conductivity of 60 Scm 1
at 800°C. MnCo2O4 has good
electrochemical properties due to the high oxidation potential
of Co and Mn can hold and transfer more electrons which leads
to high capacity.[63]
Yu et al., prepared MnCo2O4 through the
hydrothermal method followed by annealing treatment. The as-
synthesized material shows porous layered MnCo2O4 cubes. The
electrochemical studies show a higher specific capacitance of
480.5 Fg 1
with 96. 6% initial
capacitance retention after 3000 cycles, further the electrode
material shows a high-capacity retention of 75.7% even at a
current density of 25 Ag 1
. This electrode material showed a
maximum capacitance than the previously reported MnCo2O4
electrode materials which is due to the morphology-controlled
template-free hydrothermal method followed by annealing
which favors better morphology for good electron transfer.[64]
FeCo2O4 is one of the best electrode materials which was
less explored the use of iron as an electrode has many
advantages like low cost, more abundance, and environmental
benignity. It shows higher electrochemical performance due to
the variable oxidation states of Fe2+
, as Fe2+
is more active than
Ni2+
. Mohamed et al., synthesized FeCo2O4 via solvothermal
method and the morphology shows nanosheets-like structure
which is homogenously developed as shown in Figure 7 and
forming an enlarged nanosheets with a spacing of approx-
imately 100 nm. The electrochemical studies were performed
with 6 M KOH in the three-electrode system. The results show
high specific capacitance values as 853.8, 775, 716.25, and
631.25 Fg 1
at the current density of 5, 10, 15, and 25 Ag 1
,
respectively. The cyclic stability was found to be 87.5% with a
retention of 3000 cycles at a current density of 15 Ag 1
.[60]
M.
Fan et al., synthesized FeCo2O4 by a two-step hydrothermal
method which forms a core/shell hybrid structure the Co3O4
forms nanowires which can be served as core materials and the
interconnect nanosheets play the part of shell materials. It also
increases the probability of ion reaction with more efficient
charge transfer. This hybrid material gives a specific capacitance
of 1649 Fg 1
and a superior cyclic
stability of 90.6% capacitance retention after 2000 cycles using
2 M KOH as electrolyte and this much of electrochemical
performance increase is due to the core/shell structures.[66]
Binary oxides or mixed metal oxides show inhomogeneity in
their crystal structure which leads to poor stability of the
material to overcome this the single-phase bimetallic oxides
with good crystal structures are adopted, cobaltites have shown
a good crystal structure and provides an elevated stability than
other mixed metal oxides, even though cobalt shows good
electrochemical performance, it is still a semiconductor to
further enhance the conductivity by reducing the bandgap of
the material various materials like carbon, polymer, etc., have
been nanocomposite and have been discussed below.
Even though bimetallic oxides show good electrochemical
performance, the slow kinetics redox reaction and less surface
Figure 6. a) Schematic of the formation process of the ZnCo2O4 microspheres; b,c) SEM images of the ZnCo2O4 microspheres; d) CV curves of ZnCo2O4 at
various scan rates. e) The GCD curves of ZnCo2O4 at various current densities. f) The capacitance as a function of current density. Reproduced under the terms
of the Creative Commons license from Ref. [62]. Copyright (2017) The Authors.
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area metal oxides have less electrical conductivity and low
structural stability. To enhance the performance carbon materi-
als are used as support as they possess some of the properties
like highly stable, good conductive, lightweight, and high
specific surface area. Bundles of carbon nanofibers from carbon
cloth with macroporosity and high specific surface area. This
has been reckoned as a substrate for the uniform coating of
electrode materials.
3.1.1. Co-Based Bimetallic Oxides with Carbon Composites
Combining the 2D-carbon material with ZnCo2O4 could increase
the capacitance by forming a conductive carbon network with
ZnCo2O4 nanoparticles. Gao et al., reported ZnCo2O4-rGO com-
posite grown on Ni foam synthesized via. two-step process
including hydrothermal process and thermal annealing treat-
ment, this makes it a binder-free electrode material. The
morphology of the as-synthesized material shows the Zn/Co
precursor-rGO has vertically arranged nanosheets which dem-
onstrates vertically arranged nanosheets with high density free
of shedding. When looked in deep we can see the ultrathin
nanosheets intertwine together to give a porous network with
vertical channels. The SEM image of ZnCo2O4-rGO shows that
the vertically arranged ultrathin nanosheet arrays. The porous
texture composing of fused and curl sheets and vertical
macroporous channels are observable, which is mainly due to
the coating of fused ZnCo2O4 and ZnO particles onto the
vertically interconnected rGO sheets. The electrochemical
studies revealed a higher specific capacitance of 3222 Fg 1
at a
in 2 M KOH electrolyte. When it is
used in a device as a positive electrode and negative electrode
as Activated carbon, the asymmetric supercapacitor offers a
maximum device-specific capacitance of 139 Fg 1
at 0.5 Ag 1
and the ED of 49.1 Whkg 1
at PD of 400 Wkg 1
.[67]
Hu et al., reported in their work, Battery-like MnCo2O4/
Activated Carbon synthesized via hydrothermal process and
MnCo2O4 the nanofibers assembled by multi nanoparticles,
forming nanofibers with porous structure and diameter of
about 200 nm as shown in. When MnCo2O4 was combined with
activated carbon it almost wraps up MnCo2O4. The electro-
chemical investigations were made by using 1 M KOH as an
electrolyte the electrode material attained the highest specific
capacity of 443.5 Cg 1
at a current density of 0.5 Ag 1
. And it
retained only 36.85% of its initial capacitance at a scan rate of
100 mVs 1
when the mass loading of activated carbon is
400 mg the retention rate decreased gradually with the
decrease in the active loading of the activated carbon.[68]
Yuan
et al., investigated the electrochemical behavior of
MnCo2O4@Reduced Graphene Oxide this electrode material was
synthesized by hydrothermal process. The morphology of the
MnCo2O4@rGO was analyzed using TEM MnCo2O4 nanoparticles
with small diameters of around 10 nm are uniformly formed
and densely dispersed on the graphene sheets. This structure
Figure 7. a) Synthesis scheme of FeCo2O4; b) HRTEM image of FeCo2O4; c) low-magnification SEM image of FeCo2O4; d) cyclic voltammetry of FeCo2O4 at scan
rate of 5 mVs 1
; e) galvanostatic charge-discharge curves at different current densities; f) specific capacitance at different current densities. Reproduced with
permission from Ref. [65]. Copyright (2017) Elsevier.■■Please ensure and confirm that copyright permission has been obtained■■
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provides the best use of the high specific surface area of
graphene to load MnCo2O4 nanoparticles on its surface. The
electrochemical performance of the electrode material shows a
high specific capacitance of 334 Fg 1
at a current density of
1 Ag 1
and it retained its initial capacitance of 98% even after
2000 charge-discharge cycles using 2 M KOH as an
electrolyte.[69]
Shao et al., reported binder-free NiCo2O4 grown on carbon
cloth with the assistance of surfactants as electrode material for
supercapacitor the material was synthesized using a simple
hydrothermal process for the growing NiCo2O4 on Carbon Cloth
and in the second step; the precursor is thermally converted
into black network like mesoporous NiCo2O4. The material
exhibits network like mesoporous structure, the electrochemical
studies revealed the high specific capacitance of 1843.3 Fg 1
at
a current density of 1 Ag 1
the only 10% of the initial
capacitance is lost after 4000 cycles and also the supercapacitor
device showed a specific capacitance of 269 Fg 1
at a current
density of 1 Ag 1
and an energy density of 38.3 Whkg 1
at PD
of 396 Wkg 1
. The high specific capacitance of the material is
due to the porous, network-like mesoporous structure.[58]
Yedluri et al., synthesized NiCo2O4 on a highly conductive Ni
foam through a simple chemical bath deposition method as
shown in Figure 8. The as-synthesized material showed a
morphology of honeycomb nanostructure grown on Ni foam,
this morphology helps in the effective transfer of electrons. The
electrode material showed a maximum specific capacitance of
2791 Fg 1
, 99.1% retention after
3000 cycles, and an ED of 63.8 Whkg 1
and PD of
654 Whkg 1
.[70]
To boost the electrical conductivity, Gui et al.,
synthesized NiCo2O4/CNT by solvothermal method, which was
found to be NiCo2O4 nanosheets vertically grown on the CNTs
forming a honeycomb-like structure with a size of several
micrometers. Each ultrathin nanosheets are penetrated the CNT
network results in well-connected interfaces and electrical
contacts between CNTs and NiCo2O4 nanosheets. Such architec-
ture creates more active sites which lead to the direct
interaction between the electrode with electrolyte. The CNT
also reduced ion transport pathways during electrochemical
reactions. The electrochemical studies revealed a high specific
with only 7.5% loss of initial
capacitance even after 7000 cycles. The symmetrical super-
capacitor device delivers a specific capacitance of 268.4 Fg 1
at
a discharge current density of 0.1 mAcm 2
and possesses a
high energy and PD of 1.17 mWhcm 3
and 2430 mWcm 3
.[71]
On other hand, J.J. Shim et al., reported graphene/NiCo2O4
decorated on Ni foam as an electrode material for super-
capacitor. The synthesis method involves depositing the metal
precursors on Ni foam by electrodeposition method and then
by involving the thermal method for the transformation of
metal hydroxide to metal oxide. The Ni foam presents the
three-dimensional, porous, and cross-linked grid structure, and
honeycomb-like surface. The 3D grid structure with hierarchical
macroporous graphene and NiCo2O4 layers synthesized by
Figure 8. a) Schematic of the preparation process of NiCo2O4 nanoplate‘s structure. b) Low- and high-magnification FE-SEM images. c) CV curves of the
NiCo2O4 nanoplate are obtained at various scan rates of 10–100 mVs 1
. d) Galvanostatic charge-discharge curves of the NiCo2O4 at different current densities
of 5–10 Ag 1
. e) Specific capacitance of the three-electrode material at various current densities. Reproduced under the terms of the Creative Commons
license from Ref. [70]. Coypright (2019) The Authors.
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electrodeposition and thermal treatment. The growth on Ni-
foam supports the electrode materials as skeleton and results in
a high specific capacitance of 2260 Fg 1
1 Ag 1
. the charge-discharge studies revealed 92.8% retention
of initial specific capacitance even after 10000 cycles at a rate of
3 Ag 1
.[72]
Shan et al., synthesized NiCo2O4@g-C3N4 (graphite
carbon nitride) via hydrothermal method and NiCo2O4 grew on
the surface of g-C3N4 horizontally. The electrochemical studies
manifested a specific capacitance of 325.7 Fg 1
at the current
density of 1 Ag 1
with a cyclic stability of 93.6% retention of its
initial capacitance even after 2000 cycles. The hybrid asymmet-
ric capacitor was assembled and it showed a high-PD of
15.1 kW kg 1
with a retention of 83.5% from its initial
capacitance after 2000 cycles.[73]
Do-Heyoung Kim et al., synthesized FeCo2O4/graphene via a
two-step process involving the preparation of GO by a slighter
modification in Hummer’s method and the hybrid electrode is
prepared using hydrothermal process as shown in Figure 9. The
surface morphology shows the formation of layered nano-
structure of rGO nanosheets uniformly decorated with fine
FeCo2O4 nanoparticles. The electrochemical studies were per-
formed in 1 M Na2SO4 as electrolyte in a three-electrode system.
The specific capacitance of the hybrid electrode material
exhibited 1710 Fg 1
and the device
achieved a maximum specific capacitance of 260 Fg 1
with a
specific energy of 67.5 Whkg 1
and exhibited excellent cyclic
stability of 96.02% capacity retention after 5000 cycles.[74]
3.1.2. Co-Based Bimetallic Oxides with Polymer Composites
Conducting polymers like polyaniline and polypyrrole has
received more attention as they combine the electrical proper-
ties of metal and the advantages of polymers. PANI has been
identified as one of the best conductive polymers due to the
high conductivity and good electroactivity to make PANI to be
conductive it needs a proton to properly charged and dis-
charged hence protic solvent or a protic ionic liquid is required
for polyaniline to be used in supercapacitor Among other
conductive polymers, polypyrrole was considered the best
material due to the highly electronegative hetero atom present,
the electronegativity oxygen atom is higher than that of sulfur
and nitrogen also it has other properties like high conductivity,
simple preparation method, eco-friendly. Chen et al., inves-
tigated the electrochemical behavior of ZnCo2O4/Ppy grown on
Ni foam, a binder-free hybrid electrode material synthesized
through hydrothermal and annealing process. The magnified
SEM image of ZnCo2O4 shows the porous structure of ZnCo2O4
nanowires and in the SEM image of ZnCo2O4/Polypyrrole
nanowires uniformly pass through the PPy nanofilms to form
nanoarray network from the magnified SEM image PPy nano-
wires are deposited onto the surface of ZnCo2O4 nanowires. The
electrochemical studies show the specific capacitance of
1559 Fg-1 at a current density of 2 mAcm 2
and a good cyclic
stability of 90% retention of initial specific capacitance
remained even after 5000 cycles at a high current density of
10 mAcm 2
.[75]
ZnCo2O4/ PPy showed a high specific capaci-
Figure 9. a) Schematic presentation showing preparation of the rGO/FeCo2O4 hybrid electrode, b) comparison of cyclic voltammetry, c) schematic for the
MnO2/rGO/FeCo2O4 asymmetric cell with its actual demonstration, and d) CV curves for MnO2 and rGO/FeCo2O4 electrodes at scan rate of 5 mVs 1
.
Reproduced with permission from Ref. [74]. Copyright (2019) Elsevier.■■Please ensure and confirm that copyright permission has been obtained■■
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tance of 1559 Fg 1
at a current density of 2 mAcm 2
with a
cyclic stability of 90% of the initial specific capacitance
remained after 5000 cycles and a high ED of 30.9 Wh/kg at a PD
of 0.2 kW/kg.[75]
Besides, the NiCo2O4 with carbon materials, the bimetallic
composition was also investigated with the polymers. For
instance, J. Hu et al., investigated the electrochemical perform-
ance of NiCo2O4@Polypyrrole (NiCo2O4@Ppy) grown on Ni foam
synthesized via. hydrothermal and followed by an in-situ
oxidative polymerization method as shown in Figure 10.
NiCo2O4 nanowire array are grown on three dimensional and
porous backbone of Ni foil, which serves as a substrate. The
morphology of NiCo2O4@PPy shows a core/sheath arrays. The
electrochemical investigation shows a high areal capacitance of
3.49 Fcm 2
at a discharge density of 5 mAcm 2
and it retains
about 94.8% of the initial capacitance even after 5000 cycles in
6 M KOH.[76]
Along with PPy, PANI also has been reported by
researchers where NiCo2O4 was synthesized by solution com-
bustion method and NiCo2O4/PANI was synthesized by the
physical blending method. The electrode material exhibited a
specific capacitance of 887 Fg 1
at the applied current density
of 0.5 Ag 1
in 6 M KOH. From the charge-discharge studies, it
was revealed that high specific capacitance is due to spherical
morphology that provided a good insertion/exertion of electro-
lyte into electrode during the charge/discharge process.[77]
Mohamed et al., experimented with the synthesis and
electrochemical behavior of FeCo2O4 prepared via template-free
chemical growth on Ni foam followed by thermal treatment.
The material exhibited submicron tube arrays grown on Ni
foam. When the electrode material is grown on Ni foam the
tubes are separated and distributed evenly on the foam. The
electrochemical performance of the as-synthesized electrode
material was investigated using KOH as electrolyte and PVA as
separator. The electrode material achieved a specific gravimetric
capacitance (Cg) 1254 F/g with 91% of its initial capacitance
was remained after 5000 cycles with an ED of 30.9 Wh/kg and a
PD of 1551 W/kg.[78]
FeCo2O4/PPy showed a specific capacitance
of 2269 Fg 1
with cyclic stability
of 91% over 5000 cycles and an ED of 68.8 Whkg 1
and a PD of
52 Whkg 1
. It shows a core/shell nanowire structures,[79]
NiCo2O4/PANI shows a specific capacity of 720.5 Cg 1
at a
with 99.64% capacity retention after
10000 cycles, and showed a nanotube structure grown on
Carbon cloth.[80]
Among these cobaltites, NiCo2O4 showed an
excellent specific capacitance value and good cyclic stability
which makes it a better candidate for supercapacitor applica-
tion among cobaltites. It can be concluded that among all
binary cobaltites, NiCo2O4 and FeCo2O4 show a good electro-
chemical performance due to their variable oxidation states and
stable structure.
Figure 10. a) Schematic illustration of the procedure for preparing ZnFe2O4, b) CV curves of the ZnFe2O4 at different scan rates, c) the cycle life of the ZnFe2O4-
activated carbon fibers, activated carbon fibers, and ZnFe2O4 electrodes at a current density of 2 Ag-1. d) Ragone plot of ZnFe2O4-activated carbon fibers
symmetric supercapacitor device. Reproduced with permission from Ref. [85]. Copyright (2018) Elsevier.■■Please ensure and confirm that copyright
permission has been obtained■■
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3.2. 3. Co-Based Bimetallic Oxides with Other Metal Oxides
Moreover, the bimetallic oxides nanocomposites are also
investigated with different metal oxides such as NiCo2O4@MnO2
proposed by Zhang et al., proposed grown on stainless steel as
an electrode material for supercapacitor. The electrode material
was synthesized by a two-step electrodeposition method.
NiCo2O4@MnO2 hybrid nanosheet networks, where dense and
small MnO2 nanoflakes grow the surface NiCo2O4 nanosheets.
The electrochemical studies shows a specific capacitance of
913.6 Fg 1
with capacitance
retention of 87.1% after 3000 cycles in 1 M KOH,
NiCo2O4@MnO2 hybrid networks were used as the positive
electrode and assembled into an asymmetric supercapacitor
combining with AC as the negative electrode. The assembled
device gave a maximum ED of 37.5 Whkg 1
maximum PD of
7.5 kWkg 1
.[81]
Recently, the direct growth of core-shell heterostructures
had become an effective way to make adequate use of different
kinds of materials. Liu et al., explored the electrochemical
behavior of ZnCo2O4@MnO2 core-shell nanosheet arrays grown
on Ni Foam synthesized via hydrothermal method. The
morphology of the as-synthesized material shows a unique
hierarchical core-shell structure which increases its thickness
when the concentration of KMnO4 solution and the ZnCo2O4
showed a nanosheet structure. The electrochemical studies
reveal the specific capacitance of 2170 Fg 1
at a current density
of 3 mAcm 2
and it attained a retention of initial capacitance of
95.3% even after 3000 cycles in 1 M aqueous KOH. The
asymmetric supercapacitor device delivered an ED of
29.4 Whkg 1
at a PD of 628.42 Wkg 1
.[82]
Haicheng Xuan et al., reported MnO2/MnCo2O4 as an
electrode material for supercapacitor. The electrode material
was synthesized by one-step large-scale combustion at 300°C
on a large scale. Owing to the cycled electrode consists of a
binder, carbon black, and the composite, the MnO2/MnCo2O4
composite particles and the carbon black are adhered on the
surface of the binder. The electrochemical performance of
MnO2/MnCo2O4 exhibited a specific capacitance of 458 Fg 1
at a
current density of 0.5 Ag 1
with a retention rate of 60% of its
initial capacitance after 5000 cycles the electrochemical studies
were performed using 2 M KOH electrolyte.[83]
MnCo2O4@ZnO
shows a specific capacitance of 631.2 Fg 1
at a current density
of 1 Ag 1
with cyclic stability of 92.3% after 1000 cycles and an
ED of 56.10 Whkg 1
and a PD of 406 Wkg 1
and showed a
typical flower-like structure.[84]
3.3. Fe-Based Bimetallic Oxides
Ferrites show a good electrochemical performance due to the
variable oxidation states of the trivalent cation, Fe3+
which
enhances the redox behavior and improves the cyclic stability.
Moreover, iron is the most abundance metal in earth and also it
is low cost.
ZnFe2O4 has been identified as one of the best electrode
materials due to its high theoretical capacity of 1000 mAhg 1
,
cost-effective and eco-friendly. The performance of ZnFe2O4 is
hindered by low electronic conductivity, relatively low mechan-
ical stability, and severe agglomeration during the charge-
discharge process. Electrochemical studies performed by Yang
et al., revealed that the ZnFe2O4 synthesized by using active
carbon fiber as a template as shown in Figure 10 (a) which
showed a specific capacitance of 192 Fg 1
. The capacitance of
the material is enhanced due to the usage of active carbon fiber
which prevents the self-aggregation. 92.7% of capacitance
retention after 20000 cycles. Which is higher when compared to
pure ZnFe2O4 which has a capacitance retention of 81.3%.[85]
Recently, Cai et al., investigated the electrochemical properties
of NiFe2O4 synthesized via hydrothermal method the. The mass
ratios of NiFe2O4 to rGO were set as 1:9, 3:7, 5:5, 7:3, and 9:1
by adjusting the addition of precursors, and the obtained
samples were labeled as G N1, G N3, G N5, G N7, and G N9.
As the percentage of NiFe2O4 goes up, however, the trans-
parency of rGO-NiFe2O4 nanohybrids is reduced and the
arrangement density of NiFe2O4 nanoparticles is increased
(analyzed from TEM image). The electrochemical performance
was investigated using 1 M Na2SO4 electrolyte, The G-N3 (30 wt
% NiFe2O4) hybrid holds maximum specific capacitance of
210.9 Fg 1
at 0.5 Ag 1
and a good cyclic stability of no loss of
capacitance over 5000 cycles.[123]
NiFe2O4 shows a higher
specific capacitance of 240.9 Fg 1
in this interestingly the specific capacitance increased up to
128% after 2000 cycles. ED of 10.15 Whkg 1
at a PD of
140 Wkg 1
the electrode material showed a sheet-like
structure.[86]
Like cobaltite, ferrites are also investigated with
various composites like carbon, polymer, etc. which is discussed
in detail.
3.3.1. Fe-Based with Carbon Composites
CoFe2O4 has a good physiochemical property like a high
theoretical specific capacity of 228 mAhg 1
CoFe2O4 is a
partially inverted spinel structure, many studies have revealed
that inverted spinel structured compounds are the best choice
for supercapacitor applications due to the different oxidation
states of cations in both sites. CoFe2O4 / rGO shows a specific
at a scan rate of 1mVs 1
with a retention
rate of 67% after 3000 cycles, with ED of 12.14 WhKg 1
the
electrode material showed spherical structure.[88]
Gao et al.,
investigated the electrochemical properties of morphology-
controlled NiFe2O4 the specific capacitance of the electrode
has been encouragingly improved up to 240.9 Fg 1
at a current
density of 1 Ag 1
even more interestingly the specific capaci-
tance improved to 128% after 2000 cycles. The fabricated two-
electrode setup showed a higher ED of 10.15 Whkg 1
140 Wkg 1
this increased performance is due to the sheet-like
structures.[86]
NiFe2O4/graphene reaches a specific capacitance
of 464.15 Fg 1
interestingly after
5000 cycles of charging and discharging the specific capaci-
tance of the electrode material increases to 140% the material
shows an ED of 14.01 WhKg 1
at a PD of 70 WKg 1
.[28]
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Narendra Nath Ghosh reported MnFe2O4/reduced graphene
oxide synthesized via one-step hydrothermal method the as-
synthesized material has 80 wt% MnFe2O4 and 20 wt% rGO to
TGA studies have investigated to study the amount of rGO
present in the content it was reported that initially 30–100°C
approximately 3.4% of weight loss was occurred due to the
evaporation of surface H2O. After 450°C there was no weight
loss occurred. In the FESEM studies, pure MnFe2O4 shows
monodispersed spherical particles with an average diameter of
~200 nm as shown in Figure 11. Also, the spheres are broken
which indicated the hollow nature structure, FESEM images of
MnFe2O4-rGO nanocomposites show the dispersion of
MnFe2O4 spheres on rGO sheets which are nanometer-thin.
The electrochemical studies were performed using 3 M KOH
and 0.1 M K4[Fe(CN)6] with Whatman filter paper-42 as separator
and the active materials are applied in the Ni foam. A potential
window of 0–0.55 V was used for CV. electrochemical measure-
ments of pure MnFe2O4 electrode using a mixture of 3 M KOH
and 0.1 M K4[Fe(CN)6] as the electrolyte. Good redox peak has
appeared at 0.42/0.34 V at a sweep rate of 10 mVs 1
for both
3 M KOH and 3 M KOH with 0.1 M K4[Fe(CN)6]. The synthesized
MnFe2O4 was found to have 178 Fg 1
1 Ag 1
. Cs value of MnFe2O4 in 3 M KOH+0.1 M K4[Fe(CN)6] is
592 Fg 1
and this Cs value is
significantly higher than the value obtained when 3 M KOH was
used (129.6 Fg 1
at 8 Ag 1
). MnFe2O4-rGO nanocomposite
shows a large Cs (768 Fg 1
at 8 Ag 1
). The asymmetric super-
capacitor device is fabricated by keeping 80MnFe2O4-20rGO as
the positive electrode and rGO as the negative electrode. It
delivers an ED of 28.12 Whkg 1
at a PD of 750 Wkg 1
.[87]
MnFe2O4 @ Carbon showed maximum specific capacitance of
824 Fg 1
with a retention of
93.9% after 10000 cycles, ED of 27 Whkg 1,
and a PD of
290 Wkg 1
the electrode material shows nanowires like
structure.[89]
■■ dear author, please mention Figure 12 and
Figure 13 ■■
Anil V. Ghule et al., reported the synthesis and electro-
chemical properties of C@ZnFe2O4. In this article low-cost,
versatile, and efficient camphor carbon soot layer blasting
approach for the generation of nanoholes in C@ZnFe2O4 nano-
flakes morphology. The 2D holey nanoflakes have a lateral size
dimension in the range of approximately 1–5 μm and a thick-
ness of approximately 20–60 nm with holes in the size range of
approximately 10–20 nm. The synthesized material has a high
specific area of 355 m2
g 1
. The electrochemical studies were
performed in 3 M KOH electrolyte and the active material shows
a better specific capacitance of 1452 Fg 1
at 1 Ag 1
with a
cyclic stability of 98% of its initial capacitance retention over
Figure 11. a) Formation of MnFe2O4-rGO nanocomposite by one-pot hydrothermal technique, b) micrograph of 80 MnFe2O4-20 rGO, c) CV of MnFe2O4 in
different electrolytes, d) GCD of MnFe2O4 at different current densities. Reproduced with permission from Ref. [87]. Copyright (2020) American Chemical
Society.■■Please ensure and confirm that copyright permission has been obtained■■
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50,000 cycles this electrode material has an excellent applic-
ability to supercapacitor due to its high specific capacitance
and better cyclic stability also this material exhibits an ED of
81.4 Whkg 1
at a PD of 0.87 kWkg 1
.[90]
CuFe2O4/rGO showed a
specific capacitance of 797 Fg 1
with the retention of 92% after 2000 cycles and an ED of
16 WhKg 1
and a PD of 380 Wkg 1
.[91]
ZnFe2O4-active carbon
fiber shows a specific capacitance of 192 Fg 1
with 81%
retention after 20000 cycles which shows a very high cyclic
stability of this electrode material the electrode materials
Figure 12. a) Synthesis of rGO/MnFe2O4/PPy; b) SEM image of rGO/MnFe2O4/PPy; c) CV of rGO/MnFe2O4, rGO/MnFe2O4/PPy-0.25, rGO/MnFe2O4/PPy-0.50, rGO/
MnFe2O4/PPy-0.75 and PPy at scan rate of 10 mVs 1
; d) GCD of rGO/MnFe2O4, rGO/MnFe2O4/PPy-0.25, rGO/MnFe2O4/PPy-0.50, rGO/MnFe2O4/PPy-0.75 and PPy
at 0.5 Ag 1
; e) capacitance of ternary hybrids at different current densities. Adapted with permission from Ref. [94]. Copyright (2019) Elsevier.■■Please
ensure and confirm that copyright permission has been obtained■■
Figure 13. a) TEM images of ZnMn2O4; b) schematic representation of diffusion of electrolyte ions; c) variation of specific capacitance with current density; d)
CV of ZnMn2O4 at different scan rates; e) galvanostatic charge-discharge studies at different current densities; f) cyclic stability. Reproduced with permission
from Ref. [98]. Copyright (2020) Elsevier.■■Please ensure and confirm that copyright permission has been obtained■■
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showed an ED of 7.6 Whkg 1
and a PD of 523.6 Wkg 1
.[85]
ZnFe2O4/reduced graphene oxide showed a maximum specific
with a
cyclic stability of 92.3% retention after 10,000 cycles it can
reach an ED of 6.7 Whkg 1
at a PD of 300 Wkg 1
.[92]
3.3.2. Fe-Based Bimetallic Oxides with Conducting Polymer
Composites
Nagaraj et al., demonstrated the electrochemical performance
of NiFe2O4-PANI using PVA/H2SO4 electrolyte. The electrochem-
ical studies revealed a high specific capacitance of 334 Fg 1
at a
current density of 1 mAcm 2
the device also showed an
excellent cyclic stability of only 1.07*10 3
% loss of capacitance
when tested >7000 cycles. Even though the doping of the
polymer decreases the stability. But in this case, the stability is
higher than normal and also shows a subtle specific capacitance
value.[93]
Thu et al., synthesized spinel-type rGO/MnFe2O4/PPy by
varying the polymer contents (20, 33, and 42.9%) through two
steps, hydrothermal followed by oxidative polymerization of
pyrrole. The electrochemical studies revealed a high specific
. 95%
capacitance was retained after 1000 cycles. Even though rGO
support is there the stability decreased.[94]
Among ferrites,
NiFe2O4 shows a very good electrochemical performance than
other ferrites. NiFe2O4 shows a good specific capacitance value
and exhibits high cyclic stability.
3.3.3. Fe-Based Bimetallic Oxides with Metal Oxides
Hee-Je Kim et al., reported ZnO@CoFe2O4 as an electrode
material for supercapacitor. The active electrode material was
synthesized by a one-step hydrothermal approach. The material
showed a microsphere-like morphology with a diameter of
11.2 μm. The electrochemical studies were performed in 3 M
KOH electrolyte. A maximum specific capacitance of
4050.4 Fg 1
at 10 mAcm 2
was achieved it also gave a good
cyclic stability of 90.9% retention after 1000 cycles with a high
ED of 77.01 WhKg 1
at a PD of 560.54 Wkg 1
.[95]
Song et al.,
reported CuFe2O4-Fe2O3 synthesized via low temperature and
eco-friendly co-precipitation method. CuFe2O4-Fe2O3 composite
has a BET surface area of 138.18 m2
/g which is less when
compared to CuFe2O4 which is 58.69 m2
/g. The active electrode
material showed a specific capacitance of 638.24 Fg 1
and good
charge-discharge capabilities of 2000 cycles.[96]
Among ferrites,
NiFe2O4 shows a very good electrochemical performance than
other ferrites. NiFe2O4 shows a good specific capacitance value
and exhibits high cyclic stability.
3.4. Mn-Based Bimetallic Oxides
MnO2 have been reported nearly for a century, MnO2 is not a
single material it has many polymorphs like α-MnO2, β-MnO2, γ-
MnO2, etc., based on the crystal structure it has been
distinguished, these different crystal structures provide active
sites for electrochemical reactions. There are some drawbacks in
MnO2 some of them are low electrical conductivity, dissolution
of Mn in electrolytes, and an unstable volume expansion. To
resolve some of the issues in MnO2 Au was doped to MnO2 and
the results indicated that the capacity has been increased by
65% this is because Au has decreased the bandgap of the
material and made it more conductive.[97]
Similar results can be
expected from Cu and Ag as Au is expensive. As doping of
MnO2 is increasing the capacity then combining two metals
property in a single crystal phase might increase the perform-
ance even higher.
It has been widely explored as electrocatalysts, Li-ion
batteries, and sensors. In the field of supercapacitors also
manganites show superior electrochemical performance when
proper structure is formed. Saravanakumar et al., synthesized
CuMn2O4 via solvothermal method and the electrochemical
studies showed a maximum specific capacitance of 577.9 Fg 1
it retains 98% capacitance as
571.6 Fg 1
at 1 Ag 1
. The electrode material showed rice-like
morphology.[106]
Cheng et al., reported CoMn2O4 was synthesized via co-
precipitation. The electrochemical studies show a specific
at 1 Ag 1
with 67.6% retention.[99]
Fang
et al., reported ZnMn2O4 synthesized via co-precipitation meth-
od and the electrochemical studies revealed a higher specific
with
52% retention after 800 cycles which shows the poor cyclic
stability.[98]
Again, Prasankumar et al., reported LiMn2O4 synthe-
sized via glucose-mediated microwave irradiation method. the
electrochemical studies show a maximum specific capacitance
of 276 Fg 1
with a retention of 90% after 2000 cycles.[100]
Bhagwan et al., reported CdMn2O4 synthesized via electrospin-
ning technique. The electrochemical studies show a specific
after
2000 cycles the specific capacitance starts to decrease. The
electrode material showed a high ED of 25 WhKg 1
at a PD of
1.5 kWKg 1
the electrode material showed a fabric
morphology.[101]
Ray et al., reported NiMn2O4 synthesized via
sol-gel method. the electrochemical studies showed a specific
with a good cyclic stability of 91% over
10,000 cycles and an ED of 75.01 Whkg 1
at a PD of
2250.91 Wkg 1
, the electrode material exhibited porous spinel
structure.[102]
Along with magnetizes, carbon and polymer
composites have also been investigated for electrochemical
properties.
3.4.1. Mn-Based Bimetallic Oxides with Carbon Composites
Wang et al., NiMn2O4/rGO synthesized via co-precipitation
method. the electrochemical studies show a maximum specific
at 1 Ag 1
with a good cyclic stability of
91.38% retention after 2000 cycles. The electrode material
showed nanorod-like structures.[103]
S. D et al., investigated the
electrochemical properties of NiMn2O4/rGO synthesized by the
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co-precipitation method.[104]
NiMn2O4 ultrathin nanosheets are
uniformly coated on the rGO surface, leading to the formation
of NiMn2O4 Nanosheets at rGO. The closer observation of
NiMn2O4 nanosheets reveals the interconnected ultrathin
NiMn2O4 nanosheets were vertically anchored on the surface of
rGO substrate to form the extended network structures. With-
out support of the rGO sheets, the NiMn2O4 nanosheets
aggregated with each other and formed flower-like micro-
spheres of about 1 μm in size. From BET analysis the specific
surface area is reported to be 60.8 m2
g 1
. All the electro-
chemical investigations were done using 6 M KOH electrolyte at
various scan rates from 2 to 100 mVs 1
. The active electrode
material displayed a maximum specific capacitance of
1243 Fg 1
80.8% of initial
capacitance is retained after 5000 cycles.[104]
Ujjain et al.,
reported CoMn2O4/graphene as an electrode material for super-
capacitor synthesized via hydrothermal method. The graphene
nanosheets are distributed between the packed CoMn2O4
nanocubes. All the electrochemical studies were performed
using polymer [PVdF-HFP] and ionic liquid [BMIM][BF4] in a 1:4
ratio, the active electrode material delivered an ED of
44.6 Whkg 1
and a PD of 11.25 kWkg 1
it also maintained a
good cyclic stability of 95% retention even after 12000
cycles.[105]
3.5. Mo-Based Bimetallic Oxides
Binary metal oxides like NiCo2O4, ZnFe2O4, LiCoO2 gained much
attention due to their multiple oxidation states and higher
electrical conductivity than the corresponding single metal
oxide but the resources available for cobalt and iron are
decreasing due to their application in various fields. Recently
molybdates have been emerged as a good electrode material
due to their more abundant nature, low cost, and variable
oxidation states. CoMoO4 is expected to show improved
electrochemical performance because of multiple redox reac-
tions and good cyclic stability morphology also influences the
electrochemical performance. Hierarchical porous structured
morphology has been proposed for high-performance electro-
chemical energy storage. Fang et al., in his work synthesized
CoMoO4 with hierarchical morphology which demonstrated a
specific capacity of 1628.1 Cg 1
2 mAcm 2
with a good cyclic stability of 90.54% after 5000
cycles. The electrode material shows nanoneedle-like
morphology.[106]
NiMoO4 shows a high specific capacitance of
1853 Fg 1
at a current rate of 1 Ag 1
the fabricated device
retains 65% after 2500 cycles. The electrode material can
generate an ED of 117 Whkg 1
at a PD of 7527 Wkg 1.[107]
FeMoO4 shows a high specific capacity of 158.39 mAhg 1
at
2 Ag 1
with a cyclic stability of 92.37% after 4000 cycles, the
electrode material shows a honeycomb-like morphology.[108]
MnMoO4 shows a specific capacitance of 215 Fg 1
at a current
density of 1 mAcm 2
with a low internal resistance of 0.65
ohm.[109]
CaMoO4 shows a specific capacity of 118.25 mAhg 1
at
2 Ag 1
with a cyclic stability of 84% retained after 6000 cycles.
The electrode material exhibited ED of 18.68 Whkg 1
was
obtained at the PD of 362 Wkg 1.[110]
Bi2MoO6 shows a specific
at current densities of 1 Ag 1
the
fabricated device retains 95% of its initial capacitance after
3000 cycles.[111]
CeMoO4 shows a specific capacitance of
327 Fg 1
with a good cyclic stability of 96.3% retention after
4000 cycles. The electrode material displayed an ED of
24.5 Whkg 1
.[112]
Molybdates are now a trending electrode
material for supercapacitors due to their more abundance, low
cost, and high electrochemical performance not only in oxides
molybdates have become a trending topic.
3.5.1. Mo-Based Bimetallic Oxides with Metal Oxides
Shao et al., investigated the electrochemical performance of
MnO2@NiMoO4 synthesized through a two-step hydrothermal
method. The SEM image of MnO2 and NiMoO4 shows one-
dimensional morphology with a diameter ranging 24–40 nm
and ultra-thin nanoflakes, the SEM image of MnO2@NiMoO4
shows core-shell nanostructures. The electrochemical studies
were performed using 2 M KOH electrolyte, the active electrode
material showed a maximum specific capacitance of 186.8 Fg 1
at a scan rate of 10 mVs 1
interestingly the retention rate
increased after 20000 cycles.[113]
Xiaojun Zhang et al., reported
the synthesis and electrochemical behavior of Co3O4@CoMoO4
in his report the electrode material, Co3O4@CoMoO4 was
synthesized by ion exchange hydrothermal method, the as-
synthesized material shows core/ shell morphology. All the
electrochemical studies were performed using 3 M KOH, the
electrode material delivered a maximum specific capacitance of
1040 Fg 1
. The symmetric
supercapacitor had a high ED of 92.44 Whkg 1
at a PD of
6550 Wkg 1
and with a good cyclic stability of 91.22% retention
over 5000 cycling.[114]
3.6. Ni-Based Bimetallic Oxides
In past years, NiO and have been explored a lot due to their
higher theoretical capacitance of 2573 Fg 1
due to their unique
properties like high thermal conductivity, low cost, most
abundant, well-defined redox activity, and environmental
friendliness. The main drawback of Ni-based oxides and
hydroxides is due to the poor electrical conductivity. To further
enhance the electrochemical properties of NiO based materials
having various morphologies, novel synthetic routes were
examined in the previous studies which resulted in good
improvement in the electrochemical performance of NiO based
materials further to improve the electrochemical performance
Ni is combined with other metals like Co, Mn, Fe, Mo, etc.,
which are also some of the active materials for supercapacitors,
due to the synergetic effect and single-phase crystal structure
of the bimetallic oxides Ni-based bimetallic oxides have been
reported as one of the best candidate in bimetallic oxides
portfolio in the recent years.
Tuning the morphology of bimetallic oxides also increases
the specific capacitance various morphologies like nanowires,
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nanosheets, nanorods, etc.,[115,86,75]
have been investigated and
the morphology also influences the capacitance and also
enhances the stability. Recently Hao et al., reported the syn-
thesis of NiCo2O4 nanowires which showed the highest
which is one of the highest
capacitance for a bimetallic oxide recently reported.
3.6.1. Ni-Based Materials with Carbon Composites
Wu et al., prepared a self-standing NiCo2O4@CNT and achieved
a high capacitance of 1590 Fg 1
while maintaining an excellent
stability of 95% after 5000 cycles.[116]
Chang et al., developed a
flexible NiCo2O4@C/CNFs through electrospinning technique
which showed a maximum capacitance of 1586 Fg 1
with a
superior cyclic stability of 92.5% even after 5000 cycles at
10 Ag 1
.[117]
Nguyen et. prepared a binder-free NiCo2O4/
graphene grown on Ni foam which exhibited a high specific
. Wei
et al., reported N-doped carbon dots supported NiCo2O4
showed a high capacitance of 2168 Fg 1
1 Ag 1
with an outstanding retention rate of 99% even after
5000 cycles.[118]
Among all other carbon materials, N-doped
carbon dots supported NiCo2O4 displayed a very good electro-
chemical performance.
3.6.2. Ni-Based Bimetallic Oxides with Polymer Composites
Nagaraj et al., synthesized NiFe2O4 with PANI which gave a
good electrochemical performance of 668 mFcm 2
at 1 Ag 1
.[93]
Eskandari et al., reported the synthesis of coral-like NiCo2O4/
MWCNT/PANI which exhibited a high capacitance of 725 Fg 1
.[119]
Even though Ni-based
bimetallic oxides with polymer composites were unable to
reach high electrochemical performance due to the poor cyclic
stability of polymers. Ni-based materials with enhanced mor-
phology and adding carbon support show an elevated electro-
chemical performance.
3.7. Other Bimetallic Metal Oxides
H. Yang et al., investigated in their work the synthesis and
electrochemical performance of CuBi2O4 which exhibited a
higher specific capacitance of 1895 Fg 1
1 Ag 1
. The electrode material was synthesized via co-precip-
itation method the synthesized material exhibited microspheres
morphology, the electrochemical studies were reported using
2 M KOH as electrolyte.[120]
Gopi et al., reported CuNiO2 synthe-
sized via low-cost hydrothermal method and were tested for
applicability of the electrode material to supercapacitor
application. The as-synthesized material exhibited the agglom-
erated nanospheres are uniformly covered over the Ni foam
surface with a diameter range of ~400 nm to ~833 nm. The
electrochemical performance of the electrode material was
studied using 3 M KOH electrolyte; the electrode material
exhibited a maximum specific capacitance of 111.52 Fg 1
at a
with a good cyclic stability of 89.13%
retention after 1000 cycles.[111]
Add one more example: Zhang
et al., reported hierarchical ZnV2O4 as electrode material which
is synthesized by the template-free method. The electrochem-
ical studies revealed that the reported material shows a
at 1 Ag 1
after 1000 cycles 89% of its
initial specific capacitance is maintained.[120]
Xu et al., prepared
NiCr2O4 through precipitation method which showed a specific
in a three-electrode setup, the two
electrode device showed a high specific capacitance of
187 Fg 1
(the device reaches an ED of 6.5 Whcm 2
at a PD of
3000 Wkg 1
with cyclic stability of 80% under 2000 cycles.[121]
Many other bimetallic oxides have been reported as given in
Table 1. Still, many other binary metal oxides have been
explored as electrode material some of the materials from the
lanthanum series are still under research which may give good
electrochemical performance.
Even though bimetallic oxides show good electrochemical
performance but practically there is still a lot of difficulties in
synthesizing bimetallic oxides-based supercapacitors. The elec-
trical conductivity of transition metal oxides is too low to
support the fast redox reaction at higher rates. When it is
composited with carbon materials like CNT, graphene, and
mesoporous carbons the conductivity may increase but it
destroys the benefits of the pseudocapacitive nature of metal
oxide. Even though the theoretical capacitance of these
bimetallic oxides is higher, the practical capacitance has not yet
been achieved. Specific capacitance and cyclic stability needs to
be improved in the upcoming research to get the maximum
potential from it. Hence, these issues should be addressed in
the upcoming years of research. Similarly, the capacity fading is
a major shortfall for the long-term stability of bimetallic oxides-
based electrodes. The electrodes fades out by fast interaction
with electrolytes resulting in low chemical stability and
durability of the device. So researchers are trying to improve
the supercapacitors life cycle by incorporating conductive
matrix or designing specific device.[122]
In addition, a novel
design strategy directly growing bimetallic oxide on the current
collectors has been widely used to improve cyclic life.[123]
4. Future Perspective
Bimetallic oxides show good electrochemical performance
relative to single metal oxides and mixed metal oxides. Even
though the capacitance of bimetallic oxides is higher but it is
limited in the energy density of the device. To augment the
energy density some of the parameters like potential window,
device configuration and electrolytes etc., can be regulated.
Comparing to single metal oxides, bimetallic oxides show a
wider potential window due to the synergetic effect of two
metal cations. However, the energy density and potential
window is still limited and it should be improved by adopting
different morphologies like hollow spheres, mesoporous and
nanofibers in bimetallic oxides increase the electrochemical
ChemElectroChem
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Recent trends in bimetallic oxides and their composites as electrode materials for supercapacitor applications

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