ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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 2021, 8, 1 – 25 1 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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), carbonbased materials, and conducting polymers (CPs), is believed to play an important role in improving the electrochemical behavior of the electrode materials. Nevertheless, supercapacitors containing TMOs, carbon-based materials, and CPs commonly 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 revolutionary applications in the electrochemical field. In general, TMOs have given good performance as electrodes for supercapacitors 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 technologies. 1. Introduction cyclic stability, eco-friendly, cost-effective, and also longlasting.[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/ electrolyte interface.[7] The energy storage performance of the supercapacitor 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 properties to facilitate effective transport and ionic diffusion. The most vital characteristics of supercapacitors are cost-efficient, ecofriendly, 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 supercapacitor 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 The depletion of fossil fuel and environmental pollution is projecting the research towards energy harvesting from extraneous 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 leadacid cells, Ni Cd batteries, Ni-Metal Hydride batteries, Lithiumion 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 potential window and rapid charge-discharge cycling. Supercapacitors were introduced lately, due to their advantages of high power density, high charge-discharge (CD) capabilities, good [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 2021, 8, 1 – 25 www.chemelectrochem.org 2 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 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]■■ R Specific capacity ðA h g 1 Þ ¼ 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 galvanostatic 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) R Specific capacity ðA h g 1 Þ ¼ i ðAÞdt ðsÞ 3600 � mðgÞ (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 electrostatically and electrochemically combining the benefits of both EDLCs and Pseudocapacitors. In a three-electrode workstation, 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 nanomaterials 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 research interests lay in synthesis and applications of nanomaterials. T. Elango Balaji received his Master of Science (General Chemistry) from Bishop Heber College (India). Currently, he is working under the guidance of Dr. T. Maiyalagan at the SRM Institute of Science and Technology (India). www.chemelectrochem.org (1) In this equation (1), iðVÞdV 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, Thandavarayan Maiyalagan received his Ph.D in Physical Chemistry from the Indian Institute of Technology, Madras (India), and completed postdoctoral programs at Newcastle University (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 development of electrode nanomaterials for energy conversion and storage applications, particularly fuel cells, supercapacitors, and batteries. ChemElectroChem 2021, 8, 1 – 25 iðVÞdV ðA VÞ mðgÞ � #ðV s 1 Þ � 3600 3 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 studies the redox behavior, the specific capacitance of the material can be assessed using the formula in Equation (3): R Cs ¼ IdV Fg S:DV:m 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): Q¼ I Dt Cg DU m 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 chargedischarge studies can be carried out using the formula in Equation (5): 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]■■ Cs ¼ 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 I �t Fg m �V 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 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■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 4 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 large as the surface area of an ordinary electrostatic capacitor increases. 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. 1.1..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] E¼ 1 C DV2 Whkg 2 sp P¼ DV Im Wkg 2 1 (6) 1 1.2.2 Factors Influencing the Energy Density and Capacitance of the Supercapacitors (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. Twoelectrode 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 configuration in a supercapacitor, they are classified as symmetric, asymmetric, and hybrid supercapacitors. The symmetric supercapacitor 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 mF cm 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 F g 1 with capacity retention of 90.2 % after 2000 cycles. energy and power densities were observed to be 16.6 W h kg 1 at 883 W kg 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 C g 1 with high energy and power density of 35 W h kg 1 at power density of 400 W kg 1. To investigate the electrode’s electrical properties three-electrode setup can be used but when analyzing its ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org The governing factors for the energy density of devices are potential window, pore size distribution, surface area, electrolytes, 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. E¼ 1 CV2 2 (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 electrolytes aqueous electrolytes are less expensive, non-toxic, and have a good conductivity for these reasons aqueous electrolytes are mostly preferred for bimetallic oxides. 5 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 2. Types of Electrode Materials capacitance of graphene is 550 F g 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 F g 1 in aqueous electrolytes.[31] Specific capacitance obtained from graphene oxide showed 306 F g 1.[32] The specific capacitance of CNT (1D), rGO (2D), and mesoporous carbon (3D) showed 33 F g 1, 166 F g 1, and 202 F g 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 F g 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 understand 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. 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 supercapacitor to describe the high energy differed from conventional 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 graphene, 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 semiconductor having zero bandgap. The calculated theoretical 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]■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 6 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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] 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 promising 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 capacity, 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 C g 1 and 335 C g 1. The as-fabricated solid-state hybrid supercapacitor showed a high energy density of 12.8 W h kg 1 and a high power density of 2875 W kg 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 F g 1 with a high energy density of 31.7 W h kg 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, synergistic 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- 2.2. Pseudocapacitors (PCs) The second type of supercapacitor based on charge storage mechanism is Pseudo-capacitors, which store charges by surface charge-transfer reaction between an electrode and electrolyte. 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 applications. For example, Z. Fan et al., in his work on grapheneMnO2 composites as electrode material for supercapacitor has achieved a specific capacitance of 310 F g 1 at 2 mV s 1 where pure graphene shows a specific capacitance of 104 F g 1 at 2 mV s 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 enhances the mechanical strength of metal oxides to use them in advanced flexible electronics. ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 7 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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 electrochemical 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 transition 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■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 8 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 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 RuO2/G Co–MOF PANI/AC/Ni Zr/CeO2 Co–MOF/PANI MnO2/CuO MnO2/NiO Cu2O/NiO NiO/rGO TiO2/RuO2 MnO2/FeCo2O4 FeCo2O4@NiCo Ni(OH)2@CuCo2O4 FeCo2O4 RuCo2O4 NiCo2O4 /CF MnCo2O4 CuCo2O4 ZnCo2O4 ZnCo2O4 NiCo2O4/AC CuCo2O4/rGO NiCo2O4/rGO LaFeO3/MOF LiCoO2 571.25 F g 1 441.1 F g 1 450.89 F g 1 1661 F g 1 448.1 C g 1 504F g 1 279.12 F g 1 247 F g 1 2255.5 F g 1 127.5 F g 1 1200 F g 1 2.52 F cm 2 2426 F g 1 295.6 mA h g 960 F g 1 1469 F g 1 2658 F g 1 250 F g 1 1210 F g 1 1841 F g 1 229 F g 1 273.5 F g 1 978 F g 1 304 F g 1 241.3 F g 1 310.93 mF/cm2 626.5 F g 1 565.5 Cg 1 778.5 C g 1 223 F g 1 184 F g 1 360 F g 1 642.4 C g 1 1508 F g-1 187 F g 1 102.5 F g 1 93 F g 1 866 F g 1 396 F g 1 371 F g 1 701F g 1 925 F g 1 59.55 mF cm 1000 1000 1000 2000 6000 5000 10000 1000 5000 2000 10000 1500 5000 3000 10000 3000 3000 1000 5000 3000 1500 3000 5600 5000 5000 2000 92 % 94 % 95 % 93 % 96.4 % 90 % 91.26 % 81.2 % 94.5 % 70 % 95.2 % 94 % 91.6 % 93.7 % 94 % 91.3 % 80 % NA 86 % 95.8 % 84.3 % 96 % 1.34 times increased 92.8 % 92.2 % 80.26 % NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 36.5 NA 10.04 42.81 NA NA NA NA 95 34 5.6 X 10 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 3294 NA NA NA NA NA NA NA 374 900 0.0011 1 0.1 0.5 1 1 1 0.5 0.5 0.0011 1 0.5 2 1 1 2 6 2 0.25 2 1 0.25 1 3 0.5 1 NA 1 M KOH 1 M Na2SO4 6 M KOH 1 M KOH 2 M KOH 1 M KOH 1 M KOH 1 M KOH 2.0 M KOH 6 M KOH 1 M H2SO4 PVA/KOH PVA-KOH KOH/PVA 3 M KOH 2 M KOH 3 M KOH 2 M KOH 6 M KOH 6 M KOH 2 M KOH 6 M KOH 6 M KOH 2 M KOH 1 M Na2SO4 1 M LiCl [124] 5000 3000 1000 15000 1000 1000 40000 10000 2000 6000 5000 10000 500 2000 5000 500 8000 99.06 % 84.6 % 97 % 123 % 96.8 % 89 % 125 % 102 % 80 % 81.5 % 98.6 % 85.6 % 81 % 86.7 % 95.4 % 98 % 80.1 % 30.6 24.3 NA NA 15.0 NA 36.71 45.2 6.5 NA 33.07 120.3 NA NA NA NA NA 861 800 NA NA 14,210 NA 414.1 1600 3000 NA 240 500.2 NA NA NA NA NA 1 1 0.5 1 1 1 1 1 0.6 0.16 0.3 1 NA 1 1 7 0.008 2 M KOH 2 M KOH 6 M KOH 2 M KOH 1 M KOH 2 M KOH 3 M KOH 6 M KOH 1 M KOH 5 M KOH PVA-KOH 1 M Na2SO4 1 M Na2SO4 1 M H2SO4 1 M KOH 2 M KOH 3 M KOH [149] MgCo2O4 NiV2O6 NiCoO2 CoV2O6 CoNiO2 ZnV2O4 CoGa2O4 NiGa2O4 NiCr2O4 CoFe2O4-carbon NiMoO4-PANI NiCeO2@PANI MnMoO4/PANI PANI/MnFe2O4 BiVO4/PANI NiCo2O4@GO CuCo2O4/PPy/CNT 1 2 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, ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 5 [125] [45] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [150] [151] [152] [153] [154] [155] [156] [157] [158] [121] [159] [115] [160] [161] [162] [163] 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 singlephase 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 compared to binary or single metal sulfides. A few of bimetallic oxides have been discussed below: 9 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 3.1. Co-Based Bimetallic Oxides were grown on carbon cloth synthesized through hydrothermal method the results showed a high capacitance of 1843.3 F g 1 at 1 A g 1 and only 10 % loss after 4000 cycles with a current density of 10 A g 1. The assembled device showed a high energy density of 38.3 W h kg 1 at a power density of 396 W kg 1.[58] Interestingly, CuCo2O4 has two crystal structures, normal spinel, and inverse spinel. A normal spinel compound transforms 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 parameters 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 ethylenediaminetetraacetic 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 C g 1 at 5 mV s 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 A g 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 successive heat treatments. The as-synthesized electrode material exhibited the nanowire network structure morphology. The electrochemical studies showed a higher specific capacitance of 467 mF cm 2 at a current density of 1 mA cm 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 mW h cm 3 and the specific capacitance increases from 326 mF cm 2 to 467 mF cm 2 with the scan rate from 100 to 10 mV s 1.[61] As with Cu, Zn also has been investigated with cobalt metal for binary metal oxides. ZnCo2O4 nanomaterials were investigated 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 annealing 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 assynthesized electrode material was investigated for supercapacitor application and it showed a maximum specific 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 F g 1 with high electrical conductivity of 2.5 S cm 1. Gao et al., formed a network like mesoporous NiCo2O4 arrays which ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 10 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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. capacitance of 542.5 F g 1 at a current density of 1 A g 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 F g 1 with a good cyclic stability of 76.68 % after 1000 cycles at 0.5 A g 1 and it exhibited an ED of 21.97 W h kg 1 and a PD of 38.89 W kg 1.[62] Fascinatingly, MnCo2O4 has a theoretical capacitance of 3620 F g 1 this is higher than other materials like RuO2 (1300– 2400 F g 1) and MnO2 (1370 F g 1). MnCo2O4 has a very high electrical conductivity of 60 S cm 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 assynthesized material shows porous layered MnCo2O4 cubes. The electrochemical studies show a higher specific capacitance of 480.5 F g 1 at a current density of 1 A g 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 A g 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 ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 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 approximately 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 F g 1 at the current density of 5, 10, 15, and 25 A g 1, respectively. The cyclic stability was found to be 87.5 % with a retention of 3000 cycles at a current density of 15 A g 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 F g 1 at a current density of 1 A g 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 11 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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 mV s 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■■ area metal oxides have less electrical conductivity and low structural stability. To enhance the performance carbon materials 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. the coating of fused ZnCo2O4 and ZnO particles onto the vertically interconnected rGO sheets. The electrochemical studies revealed a higher specific capacitance of 3222 F g 1 at a current density of 1 A g 1 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 F g 1 at 0.5 A g 1 and the ED of 49.1 W h kg 1 at PD of 400 W kg 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 electrochemical investigations were made by using 1 M KOH as an electrolyte the electrode material attained the highest specific capacity of 443.5 C g 1 at a current density of 0.5 A g 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 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 composite grown on Ni foam synthesized via. two-step process including hydrothermal process and thermal annealing treatment, 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 demonstrates 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 ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 12 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 2791 F g 1 at a current density of 5 A g 1, 99.1 % retention after 3000 cycles, and an ED of 63.8 W h kg 1 and PD of 654 W h kg 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 architecture 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 capacitance of 1752.3 F g 1 with only 7.5 % loss of initial capacitance even after 7000 cycles. The symmetrical supercapacitor device delivers a specific capacitance of 268.4 F g 1 at a discharge current density of 0.1 mA cm 2 and possesses a high energy and PD of 1.17 mW h cm 3 and 2430 mW cm 3.[71] On other hand, J.J. Shim et al., reported graphene/NiCo2O4 decorated on Ni foam as an electrode material for supercapacitor. 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 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 F g 1 at a current density of 1 A g 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 F g 1 at a current density of 1 A g 1 the only 10 % of the initial capacitance is lost after 4000 cycles and also the supercapacitor device showed a specific capacitance of 269 F g 1 at a current density of 1 A g 1 and an energy density of 38.3 W h kg 1 at PD of 396 W kg 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 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 mV s 1. d) Galvanostatic charge-discharge curves of the NiCo2O4 at different current densities of 5–10 A g 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. ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 13 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 3.1.2. Co-Based Bimetallic Oxides with Polymer Composites electrodeposition and thermal treatment. The growth on Nifoam supports the electrode materials as skeleton and results in a high specific capacitance of 2260 F g 1 at a current density of 1 A g 1. the charge-discharge studies revealed 92.8 % retention of initial specific capacitance even after 10000 cycles at a rate of 3 A g 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 F g 1 at the current density of 1 A g 1 with a cyclic stability of 93.6 % retention of its initial capacitance even after 2000 cycles. The hybrid asymmetric 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 nanostructure of rGO nanosheets uniformly decorated with fine FeCo2O4 nanoparticles. The electrochemical studies were performed in 1 M Na2SO4 as electrolyte in a three-electrode system. The specific capacitance of the hybrid electrode material exhibited 1710 Fg 1 at a current density of 3 Ag 1and the device achieved a maximum specific capacitance of 260 Fg 1with a specific energy of 67.5 Whkg 1 and exhibited excellent cyclic stability of 96.02 % capacity retention after 5000 cycles.[74] Conducting polymers like polyaniline and polypyrrole has received more attention as they combine the electrical properties 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 discharged 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., investigated 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 nanowires are deposited onto the surface of ZnCo2O4 nanowires. The electrochemical studies show the specific capacitance of 1559 F g-1 at a current density of 2 mA cm 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 mA cm 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 mV s 1. Reproduced with permission from Ref. [74]. Copyright (2019) Elsevier.■■Please ensure and confirm that copyright permission has been obtained■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 14 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 tance of 1559 Fg 1 at a current density of 2 mA cm 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 performance 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 F cm 2 at a discharge density of 5 mA cm 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 combustion method and NiCo2O4/PANI was synthesized by the physical blending method. The electrode material exhibited a specific capacitance of 887 F g 1 at the applied current density of 0.5 A g 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 electrolyte 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 at a current density of 1 Ag 1 with cyclic stability of 91 % over 5000 cycles and an ED of 68.8 Wh kg 1 and a PD of 52 Wh kg 1. It shows a core/shell nanowire structures,[79] NiCo2O4/PANI shows a specific capacity of 720.5 Cg 1 at a current density of 1 Ag 1 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 application among cobaltites. It can be concluded that among all binary cobaltites, NiCo2O4 and FeCo2O4 show a good electrochemical 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 ZnFe2O4activated carbon fibers, activated carbon fibers, and ZnFe2O4 electrodes at a current density of 2 A g-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■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 15 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 3.2. 3. Co-Based Bimetallic Oxides with Other Metal Oxides cost-effective and eco-friendly. The performance of ZnFe2O4 is hindered by low electronic conductivity, relatively low mechanical stability, and severe agglomeration during the chargedischarge 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 F g 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 transparency 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 F g 1 at 0.5 A g 1 and a good cyclic stability of no loss of capacitance over 5000 cycles.[123] NiFe2O4 shows a higher specific capacitance of 240.9 F g 1 at a current density of 1 A g 1 in this interestingly the specific capacitance increased up to 128 % after 2000 cycles. ED of 10.15 W h kg 1 at a PD of 140 W kg 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. 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 F g 1 at a current density of 0.5 A g 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 W h kg 1 maximum PD of 7.5 kW kg 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 F g 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 W h kg 1 and a PD of 406 Wkg 1 and showed a typical flower-like structure.[84] 3.3.1. Fe-Based with Carbon Composites CoFe2O4 has a good physiochemical property like a high theoretical specific capacity of 228 mAh g 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 capacitance of 195 Fg 1 at a scan rate of 1mVs 1with a retention rate of 67 % after 3000 cycles, with ED of 12.14 W h Kg 1 the electrode material showed spherical structure.[88] Gao et al., investigated the electrochemical properties of morphologycontrolled NiFe2O4 the specific capacitance of the electrode has been encouragingly improved up to 240.9 F g 1 at a current density of 1 A g 1 even more interestingly the specific capacitance improved to 128 % after 2000 cycles. The fabricated twoelectrode setup showed a higher ED of 10.15 W h kg 1 140 W kg 1 this increased performance is due to the sheet-like structures.[86] NiFe2O4/graphene reaches a specific capacitance of 464.15 F g 1 at a current density of 1 A g 1 interestingly after 5000 cycles of charging and discharging the specific capacitance of the electrode material increases to 140 % the material shows an ED of 14.01 W h Kg 1 at a PD of 70 W Kg 1.[28] 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 mA h g 1, ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 16 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 Narendra Nath Ghosh reported MnFe2O4/reduced graphene oxide synthesized via one-step hydrothermal method the assynthesized 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 measurements 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 mV s 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 at a current density of 1 Ag 1. Cs value of MnFe2O4 in 3 M KOH + 0.1 M K4[Fe(CN)6] is 592 F g 1 at a current density of 8 Ag 1 and this Cs value is significantly higher than the value obtained when 3 M KOH was used (129.6 F g 1 at 8 A g 1). MnFe2O4-rGO nanocomposite shows a large Cs (768 F g 1 at 8 A g 1). The asymmetric supercapacitor device is fabricated by keeping 80MnFe2O4-20rGO as the positive electrode and rGO as the negative electrode. It delivers an ED of 28.12 Wh kg 1 at a PD of 750 W kg 1.[87] MnFe2O4 @ Carbon showed maximum specific capacitance of 824 Fg 1 at a current density of 0.1 Ag 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 electrochemical 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 nanoflakes morphology. The 2D holey nanoflakes have a lateral size dimension in the range of approximately 1–5 μm and a thickness 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 F g 1 at 1 A g 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■■ ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 17 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 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 mV s 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 A g 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■■ 50,000 cycles this electrode material has an excellent applicability to supercapacitor due to its high specific capacitance and better cyclic stability also this material exhibits an ED of 81.4 W h kg 1 at a PD of 0.87 kW kg 1.[90] CuFe2O4/rGO showed a specific capacitance of 797 F g 1 at a current density of 2 Ag 1 ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org with the retention of 92 % after 2000 cycles and an ED of 16 W h Kg 1 and a PD of 380 W kg 1.[91] ZnFe2O4-active carbon fiber shows a specific capacitance of 192 F g 1 with 81 % retention after 20000 cycles which shows a very high cyclic stability of this electrode material the electrode materials 18 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 showed an ED of 7.6 W h kg 1 and a PD of 523.6 W kg 1.[85] ZnFe2O4/reduced graphene oxide showed a maximum specific capacitance of 352.9 F g 1 at a current density of 1 A g 1 with a cyclic stability of 92.3 % retention after 10,000 cycles it can reach an ED of 6.7 W h kg 1 at a PD of 300 W kg 1.[92] 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 performance 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 F g 1 at a current density of 0.5 A g 1 it retains 98 % capacitance as 571.6 F g 1 at 1 A g 1. The electrode material showed rice-like morphology.[106] Cheng et al., reported CoMn2O4 was synthesized via coprecipitation. The electrochemical studies show a specific capacitance of 788 F g 1 at 1 A g 1 with 67.6 % retention.[99] Fang et al., reported ZnMn2O4 synthesized via co-precipitation method and the electrochemical studies revealed a higher specific capacitance of 447 F g 1 at a current density of 1 A g 1 with 52 % retention after 800 cycles which shows the poor cyclic stability.[98] Again, Prasankumar et al., reported LiMn2O4 synthesized via glucose-mediated microwave irradiation method. the electrochemical studies show a maximum specific capacitance of 276 F g 1 with a retention of 90 % after 2000 cycles.[100] Bhagwan et al., reported CdMn2O4 synthesized via electrospinning technique. The electrochemical studies show a specific capacitance of 210 F g 1 at a current density of 1 A g 1 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 kW Kg 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 capacitance of 875 F g 1 with a good cyclic stability of 91 % over 10,000 cycles and an ED of 75.01 W h kg 1 at a PD of 2250.91 W kg 1, the electrode material exhibited porous spinel structure.[102] Along with magnetizes, carbon and polymer composites have also been investigated for electrochemical properties. 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 electrochemical studies revealed a high specific capacitance of 334 F g 1 at a current density of 1 mA cm 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 capacitance of 66.1 F g 1 at a current density of 0.5 A g 1. 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 F g 1 at 10 mA cm 2 was achieved it also gave a good cyclic stability of 90.9 % retention after 1000 cycles with a high ED of 77.01 W h Kg 1 at a PD of 560.54 W kg 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. 3.4.1. Mn-Based Bimetallic Oxides with Carbon Composites Wang et al., NiMn2O4/rGO synthesized via co-precipitation method. the electrochemical studies show a maximum specific capacitance of 693 F g 1 at 1 A g 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 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, γChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 19 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 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. Without support of the rGO sheets, the NiMn2O4 nanosheets aggregated with each other and formed flower-like microspheres of about 1 μm in size. From BET analysis the specific surface area is reported to be 60.8 m2 g 1. All the electrochemical investigations were done using 6 M KOH electrolyte at various scan rates from 2 to 100 mV s 1. The active electrode material displayed a maximum specific capacitance of 1243 F g 1 at a current density of 3 Ag 1 80.8 % of initial capacitance is retained after 5000 cycles.[104] Ujjain et al., reported CoMn2O4/graphene as an electrode material for supercapacitor 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 W h kg 1 and a PD of 11.25 kW kg 1 it also maintained a good cyclic stability of 95 % retention even after 12000 cycles.[105] obtained at the PD of 362 W kg 1.[110] Bi2MoO6 shows a specific capacitance of 182 F g 1 at current densities of 1 A g 1 the fabricated device retains 95 % of its initial capacitance after 3000 cycles.[111] CeMoO4 shows a specific capacitance of 327 F g 1with a good cyclic stability of 96.3 % retention after 4000 cycles. The electrode material displayed an ED of 24.5 W h kg 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 onedimensional 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 F g 1 at a scan rate of 10 mV s 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 assynthesized 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 F g 1 at a current density of 1 A g 1. The symmetric supercapacitor had a high ED of 92.44 W h kg 1 at a PD of 6550 W kg 1 and with a good cyclic stability of 91.22 % retention over 5000 cycling.[114] 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 reactions and good cyclic stability morphology also influences the electrochemical performance. Hierarchical porous structured morphology has been proposed for high-performance electrochemical energy storage. Fang et al., in his work synthesized CoMoO4 with hierarchical morphology which demonstrated a specific capacity of 1628.1 C g 1 at a current density of 2 mA cm 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 F g 1 at a current rate of 1 A g 1 the fabricated device retains 65 % after 2500 cycles. The electrode material can generate an ED of 117 W h kg 1 at a PD of 7527 W kg 1.[107] FeMoO4 shows a high specific capacity of 158.39 mA h g 1 at 2 A g 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 F g 1 at a current density of 1 mA cm 2 with a low internal resistance of 0.65 ohm.[109] CaMoO4 shows a specific capacity of 118.25 mA h g 1 at 2 A g 1 with a cyclic stability of 84 % retained after 6000 cycles. The electrode material exhibited ED of 18.68 W h kg 1 was ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 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, 20 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH Reviews doi.org/10.1002/celc.202100098 ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 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 synthesis of NiCo2O4 nanowires which showed the highest capacitance of 2876 F g 1 which is one of the highest capacitance for a bimetallic oxide recently reported. exhibited a maximum specific capacitance of 111.52 F g 1 at a current density of 2 A g 1 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 electrochemical studies revealed that the reported material shows a capacitance of 360 F g 1 at 1 A g 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 capacitance of 422 F g 1 in a three-electrode setup, the two electrode device showed a high specific capacitance of 187 F g 1 (the device reaches an ED of 6.5 W h cm 2 at a PD of 3000 W kg 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 electrical 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 oxidesbased 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] 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 F g 1 with a superior cyclic stability of 92.5 % even after 5000 cycles at 10 A g 1.[117] Nguyen et. prepared a binder-free NiCo2O4/ graphene grown on Ni foam which exhibited a high specific capacitance of 1950 F g 1 at a current density of 7.5 A g 1. Wei et al., reported N-doped carbon dots supported NiCo2O4 showed a high capacitance of 2168 Fg 1 at a current density of 1 A g 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 electrochemical 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 mF cm 2 at 1 A g 1.[93] Eskandari et al., reported the synthesis of coral-like NiCo2O4/ MWCNT/PANI which exhibited a high capacitance of 725 F g 1 at a current density of 1 A g 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 morphology and adding carbon support show an elevated electrochemical performance. 3.7. Other Bimetallic Metal Oxides 4. Future Perspective H. Yang et al., investigated in their work the synthesis and electrochemical performance of CuBi2O4 which exhibited a higher specific capacitance of 1895 F g 1 at a current density of 1 A g 1. The electrode material was synthesized via co-precipitation method the synthesized material exhibited microspheres morphology, the electrochemical studies were reported using 2 M KOH as electrolyte.[120] Gopi et al., reported CuNiO2 synthesized via low-cost hydrothermal method and were tested for applicability of the electrode material to supercapacitor application. The as-synthesized material exhibited the agglomerated 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 ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 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 21 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 higher specific capacitance. In general, it is always encouraged to widen the potential window to get more capacitance to achieve this a hybrid electrode material should be considered which also increases the ED of the supercapacitor. On looking into the future aspects of supercapacitors, it is advised to develop fiber shape binary metal oxides which are flexible so that it can be used in flexible electronics also. performance of supercapacitors due to its high surface area, more active sites, and good conductivity. Device configuration also plays an important role in enhancing the energy density, such as the hybrid supercapacitor (having one battery-like electrode and another capacitive electrode) exhibit higher energy density rather than symmetric or asymmetric configurations. In the future, the electrochemical performance of the bimetallic oxides should be evaluated using the two-electrode device setup to get a deeper understanding of the material for practical usage. New combinations or design of the device can lead to a higher energy density that can fulfil demand of consumer. Electrolytes also key factor to determine the energy density viz. aqueous electrolytes (potential window 1.0 V) exhibit low energy density than ionic liquids/ organic electrolytes (potential window of up to 6 V). To enhance the electrochemical performance further, suitable electrolytes with long-term stability, non-toxic and cost-effective should be selected. Device configuration should be further investigated in such a manner that the potential window can be amplified. Furthermore, conductive materials should be composited to enhance the electrical conductivity of metal oxides. Metal oxides-based are not much flexible making them unfit for application in flexible supercapacitors, to overcome this drawback bimetallic oxides are composited with 2D materials like MXenes, graphenes as these have good electrical conductivity and flexible. To get the maximum performance from bimetallic oxides, the synthetic parameters should be well optimized to get the perfect crystal structure that gives out the full potential of the material. Acknowledgements The authors acknowledge the financial support from the Scheme for Promotion of Academic and Research Collaboration (SPARC) of the Ministry of Human Resource Development (MHRD), Government of India, SPARC Grant No. SPARC/2018-2019/P1122/SL. H.T.D. acknowledges RUSA, Utkal university, Vanivihar, Bhubaneswar-751004, Odisha, India for financial support. Conflict of Interest The authors declare no conflict of interest. Keywords: energy storage · supercapacitors · bimetallic oxides · electrodes · coulombic efficiency [1] A. Afif, S. M. Rahman, A. Tasfiah Azad, J. Zaini, M. A. Islan, A. K. Azad, J. Energy Storage 2019, 25, 100852. [2] H. Liu, X. Liu, S. Wang, H. K. Liu, L. Li, Energy Storage Mater. 2020, 28, 122–145. [3] M. Jaszczur, Q. Hassan, Appl. Energy 2020, 279,115776. [4] T. Wilberforce, Z. El-Hassan, F. N. Khatib, A. Al Makky, A. Baroutaji, J. G. Carton, A. G. Olabi, Int. J. Hydrogen Energy 2017, 42, 25695–25734. [5] D. P. Dubal, O. Ayyad, V. Ruiz, P. Gómez-Romero, Chem. Soc. Rev. 2015, 44, 1777–1790. [6] L. L. Zhang, R. Zhou, X. S. Zhao, J. Mater. Chem. 2010, 20, 5983–5992. [7] M. Horn, B. Gupta, J. MacLeod, J. Liu, N. Motta, Curr. Opin. Green Sustain. Chem. 2019, 17, 42–48. [8] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 2014, 7, 867–884. [9] C. V. V. Muralee Gopi, R. Vinodh, S. Sambasivam, I. M. Obaidat, H. J. Kim, J. Energy Storage 2020, 27, 101035. [10] V. V. Jadhav, R. S. Mane, P. V. Shinde, Bismuth-Ferrite-Based Electrochemical Supercapacitors, Springer, Cham, 2020, pp. 11–36. [11] T. Brousse, D. Bélanger, J. W. Long, J. Electrochem. Soc. 2015, 162, A5185–A5189. [12] N. R. Chodankar, H. D. Pham, A. K. Nanjundan, J. F. S. Fernando, K. Jayaramulu, D. Golberg, Y. K. Han, D. P. Dubal, Small 2020, 16, 2002806. [13] M. D. Stoller, R. S. Ruoff, Energy Environ. Sci. 2010, 3, 1294–1301. [14] X. Xu, J. Gao, Q. Tian, X. Zhai, Y. Liu, Appl. Surf. Sci. 2017, 411, 170–176. [15] D. Guo, X. Song, F. Li, L. Tan, H. Ma, L. Zhang, Y. Zhao, Colloids Surf. A 2018, 546, 1–8. [16] H. T. Das, K. Mahendraprabhu, T. Maiyalagan, P. %J S. reports Elumalai 2017, 7, 1–14. [17] E. Duraisamy, H. T. Das, A. Selva Sharma, P. Elumalai, New J. Chem. 2018, 42, 6114–6124. [18] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P. L. Taberna, Science 2006, 313, 1760–1763. [19] Z. Wang, M. Zhu, Z. Pei, Q. Xue, H. Li, Y. Huang, C. Zhi, Mater. Sci. Eng. R 2020, 139, 100520. [20] P. Forouzandeh, V. Kumaravel, S. C. Pillai, Catalysts 2020, 10, 1–73. [21] H. I. Becker, ■■ missing journal title ■■ 1954. [22] H. Choi, H. Yoon, Nanomaterials 2015, 5, 906–936. [23] X. Chen, R. Paul, L. Dai, Natl. Sci. Rev. 2017, 4, 453–489. [24] O. Stern, Zeitschrift fur Elektrochemie 1924, 30, 508–516. 5. Conclusions To meet the energy needs of renewable energy storage for various applications like electronic devices, electric vehicles, and some large industrial scale machineries, supercapacitors have become clean and outstanding energy storage devices. The development of electrode material with high specific surface area has been proven to improve the ion/electron transport through the SSA which increases the specific capacitance. But still synthesizing proper structured binary metal oxides remain a challenge since the specific capacitance is greatly influenced by factors like morphology and pore size. In this review, recent progress in binary metal oxides has been reviewed and pointed out. Even though binary metal oxides deliver a good electrochemical performance, they suffer from some limitations such as poor conductivity of metal oxide which results in low specific capacitance. These problems can be solved by introducing EDLC based carbon materials that have high electrical conductivity and porous structure. To date the best electrode material among carbon is graphene but still, there are many issues in graphene which needed to be solved to get the full performance from it. Also conducting polymers gives excellent electrochemical performance only the stability of the conducting polymer remains unsolved. By modifying the synthesis procedure and morphology a highly stable conductive polymer can be obtained which will give a higher stability as well as ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org 22 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 [25] M. S. Uddin, H. Tanaya Das, T. Maiyalagan, P. Elumalai, Appl. Surf. Sci. 2018, 449, 445–453. [26] Z. S. Iro, C. Subramani, S. S. Dash, Int. J. Electrochem. Sci. 2016, 11, 10628–10643. [27] A. Goljanian Tabrizi, N. Arsalani, A. Mohammadi, H. Namazi, L. Saleh Ghadimi, I. Ahadzadeh, New J. Chem. 2017, 41, 4974–4984. [28] X. Gao, J. Bi, W. Wang, H. Liu, Y. Chen, X. Hao, X. Sun, R. Liu, J. Alloys Compd. 2020, 826, 154088. [29] A. Yu, I. Roes, A. Davies, Z. Chen, Appl. Phys. Lett. 2010, 96, 253105. [30] B. You, L. Wang, L. Yao, J. Yang, Chem. Commun. 2013, 49, 5016–5018. [31] S. Ahmed, A. Ahmed, M. Rafat, Surf. Coat. Technol. 2018, 349, 242–250. [32] H. Yang, S. Kannappan, A. S. Pandian, J. H. Jang, Y. S. Lee, W. Lu, Nanotechnology 2017, 28, 445401. [33] R. Xiong, Y. Zhang, W. Zhou, K. Xia, Q. Sun, G. Chen, B. Han, Q. Gao, C. Zhou, Colloids Surf. A 2020, 597, 124762. [34] J. Xu, X. Wang, X. Zhou, N. Yuan, S. Ge, J. Ding, Electrochim. Acta 2019, 301, 478–486. [35] Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, L. C. Qin, Phys. Chem. Chem. Phys. 2011, 13, 17615–17624. [36] M. Y. Bhat, N. Yadav, S. A. Hashmi, Electrochim. Acta 2019, 304, 94–108. [37] J. Bhagwan, G. Nagaraju, B. Ramulu, S. C. Sekhar, J. S. Yu, Electrochim. Acta 2019, 299, 509–517. [38] J. H. Shin, H. J. Park, Y. il Song, Y. S. Choi, S. J. Suh, Electrochim. Acta 2020, 348, 136210. [39] O. Ghodbane, J. L. Pascal, F. Favier, ACS Appl. Mater. Interfaces 2009, 1, 1130–1139. [40] J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, F. Wei, Carbon N. Y. 2010, 48, 3825–3833. [41] R. B. Choudhary, S. Ansari, B. Purty, J. Energy Storage 2020, 29, 101302. [42] B. E. Conway, Proc. Int. Power Sources Symp. 1991, 138, 319–327. [43] C. An, Y. Zhang, H. Guo, Y. Wang, Nanoscale Adv. 2019, 1, 4644–4658. [44] P. Sivakumar, M. Jana, M. Kota, M. G. Jung, A. Gedanken, H. S. Park, J. Power Sources 2018, 402, 147–156. [45] Z. Wang, J. Chen, R. Bi, W. Dou, K. Wang, F. Mao, H. Wu, S. Wang, J. Solid State Chem. 2020, 283, 121128. [46] R. Srinivasan, E. Elaiyappillai, E. J. Nixon, I. Sharmila Lydia, P. M. Johnson, Inorg. Chim. Acta 2020, 502, 119393. [47] S. Tajik, D. P. Dubal, P. Gomez-Romero, A. Yadegari, A. Rashidi, B. Nasernejad, Inamuddin, A. M. Asiri, Int. J. Hydrogen Energy 2017, 42, 12384–12395. [48] M. Sethi, D. K. Bhat, J. Alloys Compd. 2019, 781, 1013–1020. [49] P. Osaimany, A. S. Samuel, Y. Johnbosco, Y. P. Kharwar, V. Chakravarthy, Composites Part B 2019, 176, 107327. [50] X. Wang, S. Chen, D. Li, S. Sun, Z. Peng, S. Komarneni, D. Yang, ACS Sustainable Chem. Eng. 2018, 6, 633–641. [51] B. Saravanakumar, S. Muthu Lakshmi, G. Ravi, V. Ganesh, A. Sakunthala, R. Yuvakkumar, J. Alloys Compd. 2017, 723, 115–122. [52] S. K. Chang, Z. Zainal, K. B. Tan, N. A. Yusof, W. M. D. W. Yusoff, S. R. S. Prabaharan, Ceram. Int. 2015, 41, 1–14. [53] S. Lalwani, M. Munjal, G. Singh, R. K. Sharma, Appl. Surf. Sci. 2019, 476, 1025–1034. [54] Y. Liu, S. Wen, W. Shi, Mater. Lett. 2018, 214, 194–197. [55] S. K. Kolli, A. Van Der Ven, ACS Appl. Mater. Interfaces 2018, 1, 6833– 6839. [56] Y. H. Hou, Y. J. Zhao, Z. W. Liu, H. Y. Yu, X. C. Zhong, W. Q. Qiu, D. C. Zeng, L. S. Wen, J. Phys. D 2010, 43, 445003. [57] S. Yuvaraj, R. K. Selvan, Y. S. Lee, RSC Adv. 2016, 6, 21448–21474. [58] S. Gao, F. Liao, S. Ma, L. Zhu, M. Shao, J. Mater. Chem. A 2015, 3, 16520–16527. [59] S. Fu, L. Li, Y. Jing, Y. Zhang, X. Wang, S. Fang, J. Wang, G. Li, Cryst. Growth Des. 2018, 18, 6107–6116. [60] A. K. Das, N. H. Kim, S. H. Lee, Y. Sohn, J. H. Lee, Composites Part B 2018, 150, 269–276. [61] Q. Wang, D. Chen, D. Zhang, RSC Adv. 2015, 5, 96448–96454. [62] Y. Gai, Y. Shang, L. Gong, L. Su, L. Hao, F. Dong, J. Li, RSC Adv. 2017, 7, 1038–1044. [63] T. Pettong, P. Iamprasertkun, A. Krittayavathananon, P. Sukha, P. Sirisinudomkit, A. Seubsai, M. Chareonpanich, P. Kongkachuichay, J. Limtrakul, M. Sawangphruk, ACS Appl. Mater. Interfaces 2016, 8, 34045– 34053. [64] M. Li, W. Yang, J. Li, M. Feng, W. Li, H. Li, Y. Yu, Nanoscale 2018, 10, 2218–2225. [65] S. G. Mohamed, S. Y. Attia, H. H. Hassan, Microporous Mesoporous Mater. 2017, 251, 26–33. ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org [66] Q. Wu, Y. Zhao, J. Yu, D. Song, R. Chen, Q. Liu, R. Li, M. Fan, Int. J. Hydrogen Energy 2019, 44, 31780–31789. [67] Z. Gao, L. Zhang, J. Chang, Z. Wang, D. Wu, F. Xu, Y. Guo, K. Jiang, Appl. Surf. Sci. 2018, 442, 138–147. [68] X. Hu, H. Nan, M. Liu, S. Liu, T. An, H. Tian, Electrochim. Acta 2019, 306, 599–609. [69] Y. Yuan, H. Bi, G. He, J. Zhu, H. Chen, Chem. Lett. 2014, 43, 83–85. [70] A. K. Yedluri, H. J. Kim, RSC Adv. 2019, 9, 1115–1122. [71] Y. Zheng, Z. Lin, W. Chen, B. Liang, H. Du, R. Yang, X. He, Z. Tang, X. Gui, J. Mater. Chem. A 2017, 5, 5886–5894. [72] V. H. Nguyen, J. J. Shim, J. Power Sources 2015, 273, 110–117. [73] Q. Y. Shan, B. Guan, J. M. Zhang, F. Dong, X. Y. Liu, Y. X. Zhang, J. Nanosci. Nanotechnol. 2018, 19, 73–80. [74] N. R. Chodankar, D. P. Dubal, S. H. Ji, D. H. Kim, Electrochim. Acta 2019, 295, 195–203. [75] T. Chen, Y. Fan, G. Wang, Q. Yang, R. Yang, RSC Adv. 2015, 5, 74523– 74530. [76] J. Hu, M. Li, F. Lv, M. Yang, P. Tao, Y. Tang, H. Liu, Z. Lu, J. Power Sources 2015, 294, 120–127. [77] V. Shanmugavalli, K. Vishista, Mater. Res. Express 2019, 6, 045021. [78] B. Zhu, S. Tang, S. Vongehr, H. Xie, J. Zhu, X. Meng, Chem. Commun. 2016, 52, 2624–2627. [79] X. He, Y. Zhao, R. Chen, H. Zhang, J. Liu, Q. Liu, D. Song, R. Li, J. Wang, ACS Sustainable Chem. Eng. 2018, 6, 14945–14954. [80] C. Pan, Z. Liu, W. Li, Y. Zhuang, Q. Wang, S. Chen, J. Phys. Chem. C 2019, 123, 25549–25558. [81] Y. Zhang, B. Wang, F. Liu, J. Cheng, X. wen Zhang, L. Zhang, Nano Energy 2016, 27, 627–637. [82] D. Yu, Z. Zhang, Y. Meng, Y. Teng, Y. Wu, X. Zhang, Q. Sun, W. Tong, X. Zhao, X. Liu, Inorg. Chem. Front. 2018, 5, 597–604. [83] Y. Zhang, H. Xuan, Y. Xu, B. Guo, H. Li, L. Kang, P. Han, D. Wang, Y. Du, Electrochim. Acta 2016, 206, 278–290. [84] C. V. V. M. Gopi, M. Venkata-Haritha, S. K. Kim, K. Prabakar, H. J. Kim, RSC Adv. 2016, 6, 102961–102967. [85] S. Yang, Z. Han, F. Zheng, J. Sun, Z. Qiao, X. Yang, L. Li, C. Li, X. Song, B. Cao, Carbon 2018, 134, 15–21. [86] X. Gao, W. Wang, J. Bi, Y. Chen, X. Hao, X. Sun, J. Zhang, Electrochim. Acta 2019, 296, 181–189. [87] P. Makkar, N. N. Ghosh, ACS Appl. Mater. Interfaces 2020, 3, 2653–2664. [88] K. V. Sankar, R. K. Selvan, D. Meyrick, RSC Adv. 2015, 5, 99959–99967. [89] L. Geng, F. Yan, C. Dong, C. An, Nanomaterials 2019, 9, 777. [90] M. M. Vadiyar, S. B. Bandgar, S. S. Kolekar, J. Y. Chang, Y. C. Ling, Z. Ye, A. V. Ghule, ACS Appl. Mater. Interfaces 2019, 2, 6693–6704. [91] M. Chandel, D. Moitra, P. Makkar, H. Sinha, H. S. Hora, N. N. Ghosh, RSC Adv. 2018, 8, 27725–27739. [92] S. Yang, Z. Han, J. Sun, X. Yang, X. Hu, C. Li, B. Cao, Electrochim. Acta 2018, 268, 20–26. [93] R. Nagaraj, K. Aruchamy, M. Halanur, N. Maalige, R. D. Mondal, S. K. Nataraj, D. Ghosh, J. Electroanal. Chem. 2019, 851, 113482. [94] T. V. Thu, T. Van Nguyen, X. D. Le, T. S. Le, V. Van Thuy, T. Q. Huy, Q. D. Truong, Electrochim. Acta 2019, 314, 151–160. [95] A. E. Reddy, T. Anitha, C. V. V. Muralee Gopi, S. S. Rao, B. Naresh, H. J. Kim, Anal. Methods 2018, 10, 223–229. [96] R. Khan, M. Habib, M. A. Gondal, A. Khalil, Z. U. Rehman, Z. Muhammad, Y. A. Haleem, C. Wang, C. Q. Wu, L. Song, Mater. Res. Express 2017, 4, 0–15. [97] J. Shin, J. K. Seo, R. Yaylian, A. Huang, Y. S. Meng, Int. Mater. Rev. 2020, 65, 356–387. [98] Q. Fang, C. Chen, Z. Yang, X. Chen, X. Chen, T. Liu, J. Alloys Compd. 2020, 826, 154084. [99] J. Cheng, X. Liu, Y. Lu, X. Hou, L. Han, H. Yan, J. Xu, Y. Luo, Mater. Lett. 2016, 165, 231–234. [100] T. Prasankumar, J. Vigneshwaran, M. Bagavathi, S. Jose, J. Alloys Compd. 2020, 834, 155060. [101] J. Bhagwan, A. Sahoo, K. L. Yadav, Y. Sharma, J. Alloys Compd. 2017, 703, 86–95. [102] A. Ray, A. Roy, M. Ghosh, J. Alberto Ramos-Ramón, S. Saha, U. Pal, S. K. Bhattacharya, S. Das, Appl. Surf. Sci. 2019, 463, 513–525. [103] Z. Wang, Z. Zhu, C. Zhang, C. Xu, C. Chen, Electrochim. Acta 2017, 230, 438–444. [104] L. Li, H. Hu, S. Ding, Inorg. Chem. Front. 2018, 5, 1714–1720. [105] S. K. Ujjain, P. Ahuja, R. K. Sharma, J. Mater. Chem. A 2015, 3, 9925– 9931. [106] L. Fang, F. Wang, T. Zhai, Y. Qiu, M. Lan, K. Huang, Q. Jing, Electrochim. Acta 2018, 259, 552–558. 23 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH ChemElectroChem 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Reviews doi.org/10.1002/celc.202100098 [136] C. Wang, K. Guo, W. He, X. Deng, P. Hou, F. Zhuge, X. Xu, T. Zhai, Sci. Bull. 2017, 62, 1122–1131. [137] S. Lalwani, M. Munjal, G. Singh, R. K. Sharma, Appl. Surf. Sci. 2019, 476, 1025–1034. [138] D. P. Dubal, N. R. Chodankar, R. Holze, D. H. Kim, P. Gomez-Romero, ChemSusChem 2017, 10, 1771–1782. [139] J. Du, G. Zhou, H. Zhang, C. Cheng, J. Ma, W. Wei, L. Chen, T. Wang, ACS Appl. Mater. Interfaces 2013, 5, 7405–7409. [140] S. Sahoo, K. K. Naik, C. S. Rout, Nanotechnology 2015, 26, 455401. [141] A. Pendashteh, S. E. Moosavifard, M. S. Rahmanifar, Y. Wang, M. F. ElKady, R. B. Kaner, M. F. Mousavi, Chem. Mater. 2015, 27, 3919–3926. [142] C. Wu, J. Cai, Q. Zhang, X. Zhou, Y. Zhu, L. Li, P. Shen, K. Zhang, Electrochim. Acta 2015, 169, 202–209. [143] L. Xu, Y. Zhao, J. Lian, Y. Xu, J. Bao, J. Qiu, L. Xu, H. Xu, M. Hua, H. Li, Energy 2017, 123, 296–304. [144] J. Xu, F. Liu, X. Peng, J. Li, Y. Yang, D. Jin, H. Jin, X. Wang, B. Hong, ChemistrySelect 2017, 2, 5189–5195. [145] P. Liang, F. Wang, Z. A. Hu, Chem. Eng. J. 2018, 350, 627–636. [146] M. Isacfranklin, G. Ravi, R. Yuvakkumar, P. Kumar, D. Velauthapillai, B. Saravanakumar, M. Thambidurai, C. Dang, Ceram. Int. 2020, 46, 16291– 16297. [147] Y. Zhang, J. Ding, W. Xu, M. Wang, R. Shao, Y. Sun, B. Lin, Chem. Eng. J. 2020, 386, 124030. [148] J. Lu, H. Ran, J. Li, J. Wan, C. Wang, P. Ji, X. Wang, G. Liu, C. Hu, Electrochim. Acta 2020, 331, 135426. [149] Y. Wang, X. Ma, S. Li, J. Sun, Y. Zhang, H. Chen, C. Xu, J. Alloys Compd. 2020, 818, 152905. [150] Y. Li, H. Sun, Y. Yang, Y. Cao, W. Zhou, H. Chai, J. Colloid Interface Sci. 2020, 580, 298–307. [151] H. Huang, H. Zhang, Y. Fan, X. Deng, G. Li, X. Liang, W. Zhou, J. Guo, S. Tang, Appl. Surf. Sci. 2019, 481, 1220–1227. [152] Y. Wang, H. Chai, H. Dong, J. Xu, D. Jia, W. Zhou, ACS Appl. Mater. Interfaces 2016, 8, 27291–27297. [153] W. Du, Y. Gao, Q. Tian, D. Li, Z. Zhang, J. Guo, X. Qian, J. Nanopart. Res. 2015, 17, 368. [154] F. K. Butt, M. Tahir, C. Cao, F. Idrees, R. Ahmed, W. S. Khan, Z. Ali, N. Mahmood, M. Tanveer, A. Mahmood, I. Aslam, ACS Appl. Mater. Interfaces 2014, 6, 13635–13641. [155] X. Chen, H. Chai, Y. Cao, D. Jia, A. Liu, W. Zhou, Chem. Eng. J. 2018, 354, 932–940. [156] S. Liu, K. S. Hui, K. N. Hui, H. F. Li, K. W. Ng, J. Xu, Z. Tang, S. C. Jun, J. Mater. Chem. A 2017, 5, 19046–19053. [157] X. Xu, J. Gao, W. Hong, RSC Adv. 2016, 6, 29646–29653. [158] N. Heydari, M. Kheirmand, H. Heli, Int. J. Green Energy 2019, 16, 476– 482. [159] Q. Gong, Y. Li, H. Huang, J. Zhang, T. Gao, G. Zhou, Chem. Eng. J. 2018, 344, 290–298. [160] N. Arsalani, A. G. Tabrizi, L. S. Ghadimi, J. Mater. Sci. Mater. Electron. 2018, 29, 6077–6085. [161] R. Srinivasan, E. Elaiyappillai, S. Anandaraj, B. Kumar Duvaragan, P. M. Johnson, J. Electroanal. Chem. 2020, 861, 113972. [162] D. Carriazo, J. Patiño, M. C. Gutiérrez, M. L. Ferrer, F. Del Monte, RSC Adv. 2013, 3, 13690–13695. [163] X. Liang, Q. Wang, Y. Ma, D. Zhang, Dalton Trans. 2018, 47, 17146– 17152. [107] H. S. Chavan, B. Hou, A. T. A. Ahmed, Y. Jo, S. Cho, J. Kim, S. M. Pawar, S. N. Cha, A. I. Inamdar, H. Im, H. Kim, Sol. Energy Mater. Sol. Cells 2018, 185, 166–173. [108] H. W. Nam, C. V. V. M. Gopi, S. Sambasivam, R. Vinodh, K. V. G. Raghavendra, H. J. Kim, I. M. Obaidat, S. Kim, J. Energy Storage 2020, 27, 101055. [109] F. Nti, D. A. Anang, J. I. Han, Mater. Lett. 2018, 217, 146–150. [110] J. Bhagwan, S. K. Hussain, J. S. Yu, ACS Sustainable Chem. Eng. 2019, 7, 12340–12350. [111] H. H. Joo, C. V. V. M. Gopi, R. Vinodh, H. J. Kim, S. Sambasivam, I. M. Obaidat, J. Energy Storage 2019, 26, 100914. [112] H. R. Naderi, A. Ghaderi, Z. S. Seyedi, M. Eghbali-Arani, Anal. Bioanal. Chem. 2019, 11, 679–690. [113] X. Wang, H. Xia, J. Gao, B. Shi, Y. Fang, M. Shao, J. Mater. Chem. A 2016, 4, 18181–18187. [114] Z. Gu, R. Wang, H. Nan, B. Geng, X. Zhang, J. Mater. Chem. A 2015, 3, 14578–14584. [115] Y. Johnbosco, V. Elumalai, M. Bhagavathiachari, A. S. Samuel, E. Elaiyappillai, P. M. Johnson, J. Electroanal. Chem. 2017, 797, 78–88. [116] P. Wu, S. Cheng, M. Yao, L. Yang, Y. Zhu, P. Liu, O. Xing, J. Zhou, M. Wang, H. Luo, M. Liu, Adv. Funct. Mater. 2017, 27, 1702160. [117] L. Chang, C. Li, H. Ouyang, J. Huang, Q. Huang, Z. Xu, Mater. Lett. 2019, 240, 21–24. [118] J. S. Wei, H. Ding, P. Zhang, Y. F. Song, J. Chen, Y. G. Wang, H. M. Xiong, Small 2016, 12, 5927–5934. [119] M. Eskandari, C. A. García, D. Buceta, R. Malekfar, P. Taboada, J. Electroanal. Chem. 2019, 851, 113481. [120] Y. C. Zhang, H. Yang, W. P. Wang, H. M. Zhang, R. S. Li, X. X. Wang, R. C. Yu, J. Alloys Compd. 2016, 684, 707–713. [121] H. Gao, F. Wu, X. Wang, C. Hao, C. Ge, Int. J. Hydrogen Energy 2018, 43, 18349–18362. [122] W. Raza, F. Ali, N. Raza, Y. Luo, K. H. Kim, J. Yang, S. Kumar, A. Mehmood, E. E. Kwon, Nano Energy 2018, 52, 441–473. [123] L. Lin, L. Li, S. Hussain, S. Zhao, L. Wu, X. Peng, N. Hu, Appl. Surf. Sci. 2018, 452, 113–122. [124] P. K. Singh, A. K. Das, G. Hatui, G. C. Nayak, Mater. Chem. Phys. 2017, 198, 16–34. [125] R. Thangappan, M. Arivanandhan, R. Dhinesh Kumar, R. Jayavel, J. Phys. Chem. Solids 2018, 121, 339–349. [126] E. Elanthamilan, A. Sathiyan, S. Rajkumar, E. J. Sheryl, J. P. Merlin, Sustain. Energy Fuels 2018, 2, 811–819. [127] M. Sun, Z. Li, H. Li, Z. Wu, W. Shen, Y. Q. Fu, Electrochim. Acta 2020, 331, 1–10. [128] S. Arunpandiyan, S. Bharathi, A. Pandikumar, S. Ezhil Arasi, A. Arivarasan, Mater. Sci. Semicond. Process. 2020, 106, 104765. [129] K. Mohamed Racik, A. Manikandan, M. Mahendiran, P. Prabakaran, J. Madhavan, M. Victor Antony Raj, Phys. E 2020, 119, 114033. [130] K. M. Racik, K. Guruprasad, M. Mahendiran, J. Madhavan, T. Maiyalagan, M. V. A. Raj, J. Mater. Sci. Mater. Electron. 2019, 30, 5222–5232. [131] Y. Miao, X. Zhang, Y. Sui, E. Hu, J. Qi, F. Wei, Q. Meng, Y. He, Z. Sun, Y. Ren, Z. Zhan, Mater. Lett. 2020, 265, 127300. [132] J. Xu, L. Wu, Y. Liu, J. Zhang, J. Liu, S. Shu, X. Kang, Q. Song, D. Liu, F. Huang, Y. Hu, Surfaces and Interfaces 2020, 18, 100420. [133] S. Park, D. Shin, T. Yeo, B. Seo, H. Hwang, J. Lee, W. Choi, Chem. Eng. J. 2020, 384, 123269. [134] B. Zhu, S. Tang, S. Vongehr, H. Xie, X. Meng, ACS Appl. Mater. Interfaces 2016, 8, 4762–4770. [135] X. He, R. Li, J. Liu, Q. Liu, R. R. chen, D. Song, J. Wang, Chem. Eng. J. 2018, 334, 1573–1583. ChemElectroChem 2021, 8, 1 – 25 www.chemelectrochem.org Manuscript received: January 22, 2021 Revised manuscript received: February 26, 2021 Accepted manuscript online: March 11, 2021 24 These are not the final page numbers! �� © 2021 Wiley-VCH GmbH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 REVIEWS Bi the way: 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 technologies. T. E. Balaji, Dr. H. Tanaya Das, Dr. T. Maiyalagan* 1 – 25 Recent Trends in Bimetallic Oxides and Their Composites as Electrode Materials for Supercapacitor Applications ## SPACE RESERVED FOR IMAGE AND LINK Share your work on social media! 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