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 Recent Trends in Bimetallic Oxides and Their Composites as Electrode Materials for Supercapacitor Applications T. Elango Balaji, [a] Himadri Tanaya Das, [b, c] and T. Maiyalagan* [a] ChemElectroChem Reviews doi.org/10.1002/celc.202100098 1 ChemElectroChem 2021 , 8 , 1 – 25 © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 There is a growing interest in supercapacitors as energy storage systems due to their high specific power, fast charge/discharge rates, and long cycling stability. Researchers have focused recently on developing nanomaterials to enhance the capacitive performance of supercapacitors. The inclusion of electroactive components, such as transition metal oxides (TMOs), carbon- based materials, and conducting polymers (CPs), is believed to play an important role in improving the electrochemical behavior of the electrode materials. Nevertheless, supercapaci- tors containing TMOs, carbon-based materials, and CPs com- monly suffer from inferior ion-transport kinetics and poor electronic conductivity, which can affect the rate capability and cycling stability of the electrodes. Therefore, the development of TMO/CP and TMO/carbon-based electrode materials has gained widespread attention because they synergistically combine the advantages of both materials, enabling revolu- tionary applications in the electrochemical field. In general, TMOs have given good performance as electrodes for super- capacitors by further increasing the performance of the electrode when two metal cations are introduced into a single crystal structure. This Review describes and highlights recent progress in the development of bimetallic oxides regarding their design approach, configurations, and electrochemical properties for supercapacitor applications, at the same time providing new opportunities for future energy storage technol- ogies. 1. Introduction The depletion of fossil fuel and environmental pollution is projecting the research towards energy harvesting from extra- neous renewable energy resources. That is how the alternative energy resources came into play to alleviate the current energy demands. Renewable energy resources like solar and wind, became crucial support under current circumstances, as well as advanced energy storage systems with both high-power density (PD) and high energy density (ED), are key aspects to mitigate the energy crisis. It is now essential that portable light-weight conductive material with low cost, environmentally friendly energy conversion, and storage systems are the current challenges in research. [1] Electrochemical energy storage and conversion is playing a vital role in the portfolio of energy systems that includes fuel cells, supercapacitors, and batteries. Some of the most commonly used battery devices include lead- acid cells, Ni Cd batteries, Ni-Metal Hydride batteries, Lithium- ion batteries (LIBs). The emerging energy storage devices such as metal-air batteries, metal-ion batteries like Na-ion batteries, Al-ion batteries, Mg-ion batteries, Zn-ion batteries etc., are also attracting considerable attraction for researchers in recent years. On the other hand, batteries use slow faradaic reactions to store and release charge throughout the active electrode materials. Batteries have wider potential windows, high energy density which makes them run for a long time at one single charge. Unlike batteries, supercapacitors have a narrow poten- tial window and rapid charge-discharge cycling. Supercapaci- tors were introduced lately, due to their advantages of high power density, high charge-discharge (CD) capabilities, good cyclic stability, eco-friendly, cost-effective, and also long- lasting. [2] Supercapacitors and batteries are predominantly, the charge-storage devices, which have a quite similarity, further it consists of double electrodes with high conductance separated by an electrolytic medium with ionic charge carriers. Especially, supercapacitors (SCs) are being considered as striving energy storage devices, due to their high-PD, high specific capacitance, superior charge/discharge characteristics, long cycle life, and modifiable range of operating temperature [5] Supercapacitors acts as a bridge between conventional capacitors and batteries. The high power density of supercapacitors brings up its usage in high-speed electric cars as shown in Figure 1 (b). In general, a supercapacitor is mainly composed of current collectors, two active electrode materials, an electrolyte, and a separator. [6] An electrical double layer is formed at the surface of the electrode during the charging phase and the charges migrate through the electrolyte during discharge. However, within an ideal supercapacitor, only surface localized fast proceeding physical processes occur at the electrode/ electro- lyte interface. [7] The energy storage performance of the super- capacitor is massively dependent on various factors, such as the electrochemical behaviors of the electrode materials, the choice of electrolyte, and the potential window of the device. [8] Various research efforts have been going on to develop novel electrode materials for supercapacitors with appropriate structural proper- ties to facilitate effective transport and ionic diffusion. The most vital characteristics of supercapacitors are cost-efficient, eco- friendly, and flexible electrode materials with high stability, outstanding electrochemical property, and excellent mechanical performance. [9] The advantages of the supercapacitors drag attention towards its energy storage system but few shortfalls impede its practical applications. To overcome their issues, the scientist and industrialist have been investigating supercapaci- tor electrodes materials in details. The performance of supercapacitors (SCs) depends on its type of charge storage by electrode materials, on that basis it has been classified into Electrical Double Layer Capacitors (EDLC), Pseudo-capacitors (PCs), and Hybrid supercapacitors (HSCs). From Figure 2 we can see that the classification is based upon the charge storage mechanism, EDLCs store charge electrostatically; Pseudocapacitors and EDLCs are the type of [a] T. E. Balaji, Dr. T. Maiyalagan Electrochemical Energy Laboratory, Department of Chemistry SRM Institute of Science and Technology Kattankulathur, Tamil Nadu – 603 203, India E-mail: maiyalat@srmist.edu.in [b] Dr. H. Tanaya Das Department of Materials and Mineral Resources Engineering, NTUT No. 1, Sec. 3, Chung-Hsiao East Rd., Taipei 106, Taiwan, ROC [c] Dr. H. Tanaya Das Centre of Excellence for Advanced Materials and Applications Utkal university Vanivihar, Bhubaneswar-751004, Odisha, India ChemElectroChem Reviews doi.org/10.1002/celc.202100098 2 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 capacitors those are differentiated on basis of charge storage. The storage mechanism of pseudocapacitor is via rapid faradaic redox reaction happening at the surface of the electrode or pseudo-intercalation type reactions, where the EDLCs stores charge via. double layer formation. On the other hand, the performance of battery-type materials undergo purely Faradaic reactions and expressed in terms of specific capacity (mAh g 1 ) since the average capacitance is not uniform throughout the potential window. [11] The average capacitance through the potential window is known as specific capacitance. The specific capacitance can be estimated by various electrochemical techniques such as cyclic voltammetry (CV) curves or galvano- static charge/discharge curves which depicts the mechanism of electrochemical reaction undergone during a complete cycle of charge and discharge. (as shown in Figure 3). From CV and GCD specific capacity can be calculated using the Equations (1) and (2) Specific capacity ð A h g 1 Þ ¼ R i V ð Þ dV A V ð Þ m g ð Þ � # V s 1 ð Þ � 3600 (1) In this equation (1), i V ð Þ dV is the integral area of the CV curve, A V (ampere volts); ν is the scan rate and m is the mass of the active material. This specific capacity can also be calculated from GCD curves by the following equation, Specific capacity ð A h g 1 Þ ¼ R i A ð Þ dt s ð Þ 3600 � m g ð Þ (2) In the above equation, i and m denote the current density, dt denotes the discharge time. By applying the respected values in the above equation specific capacity can be calculated from GCD. [12] Hybrid supercapacitors store charge both by electrostati- cally and electrochemically combining the benefits of both EDLCs and Pseudocapacitors. In a three-electrode workstation, Thandavarayan Maiyalagan received his Ph.D in Physical Chemistry from the Indian Institute of Technology, Madras (India), and completed postdoctoral programs at Newcastle Univer- sity (UK), Nanyang Technological University (Singapore), and the University of Texas, Austin (USA). Currently, he is an Associate Professor of Chemistry at SRM Institute of Science and Technology (India). His main research interests focus on design and devel- opment of electrode nanomaterials for energy conversion and storage applications, particu- larly fuel cells, supercapacitors, and batteries. T. Elango Balaji received his Master of Science (General Chemistry) from Bishop Heber Col- lege (India). Currently, he is working under the guidance of Dr. T. Maiyalagan at the SRM Institute of Science and Technology (India). Himadri Tanaya Das joined the Centre of Advanced Materials and Applications, Utkal University (India) as a Postdoctoral Fellow in 2021. She received her Ph.D. in Physics from Pondicherry University (India) in 2019. Her Ph.D. research work was based on nano- materials in energy storage such as batteries and supercapacitors. She also holds research experience in various institutes like Nanyang Technological University (Singapore), National Tapei University of Technology (Taiwan), and National Taiwan University of Science and Technology (Taiwan). ■■ ok? ■■ Her re- search interests lay in synthesis and applica- tions of nanomaterials. Figure 1. Application of supercapacitor in domestic (a) and automobile (b) sectors. Reproduced from Ref. [3] under the terms of the Creative Commons license and with permission from Ref. [4]. Copyright (2020) The Authors and (2017) Elsevier, respectively. ■■ Please ensure and confirm that copyright permission has been obtained from [4] ■■ ChemElectroChem Reviews doi.org/10.1002/celc.202100098 3 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 the instrument consists of a working electrode, a counter electrode, and a reference electrode. Usually, the reference electrode and counter electrodes are Ag/AgCl and Pt wire electrode, respectively. The active material is coated on the current collectors like nickel foam or carbon cloth considered as working electrodes. The performance of the electrodes can be analyzed by different parameters. The electrochemical property of the material can be investigated by characteristics like Cyclic voltammetry (CV), Galvanostatic charge-discharge studies (GCD), and Electrochemical Impedance Spectroscopy (EIS). These studies reveal the applicability of the as-synthesized material to the supercapacitor application. In cyclic voltammetry studies the redox behavior, the specific capacitance of the material can be assessed using the formula in Equation (3): C s ¼ R IdV S : D V : m F g 1 (3) Where, ∫ I dV indicates the integral area of one complete cycle of CV curve, ‘S’ scan rate (mVs 1 ) ‘ ~ V’ potential window (V) and ‘m’ mass of the active material (mg), C s specific capacitance (F g 1 ). To find out the specific capacity of the material the formula used is given by Equation (4): Q ¼ I D t D U m C g 1 (4) Δ U is the width of the potential window, m is the mass of the active materials, and t is the discharging time. To find out the cyclic stability of the active electrode material charge- discharge studies can be carried out using the formula in Equation (5): C s ¼ I � t m � V F g 1 (5) where I (A), V (V), and m (g) represent the discharge current, discharge time, potential window, and mass of electrode materials, respectively. By selecting a large specific surface area, highly porous or highly electroactive electrode materials, such as amorphous carbon or nanoporous metal oxides, capacitance per gram of material is amplified. For example, activated carbon (AC) holds high specific capacitance due to the higher specific surface area of the material. In a cylindrical supercapacitor, the inner surface of the electrode is padded with activated porous carbon, resulting in a higher surface area that is about a million times Figure 2. Supercapacitors hierarchy with three main categories and their subtypes according to the possible materials like metal oxides, conducting polymers, and carbon materials. Reproduced with permission from Ref. [10]. Copyright (2020) Springer. ■■ Please ensure and confirm that all copyright permission has been obtained from [10] ■■ Figure 3. a) Two electrode device configuration. b, c) Electrochemical curves for hybrid and asymmetric supercapacitors. Reproduced with permission from Refs. [12] and [13]. Copyright (2020) Wiley-VCH and (2010) Royal Society of Chemistry, respectively. ■■ Please ensure and confirm that all copyright permissions have been obtained ■■ ChemElectroChem Reviews doi.org/10.1002/celc.202100098 4 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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. 1.1..1 Device Configuration of a Two-Electrode Setup Further, the electrodes assembled with electrolyte to form a device which energy density and power density can be calculated by using the formulae given by Equations (6) and (7): [18] E ¼ 1 2 C sp D V 2 Whkg 1 (6) P ¼ D V I m 2 Wkg 1 (7) A typical supercapacitor consists of two electrodes that are separated by a porous separator and filled with electrolyte. The active material is coated on the electrodes. Current collectors of metal foil are used to conduct electrical current from each electrode. The separator and the electrodes are immersed into an electrolyte in suitable concentration, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. A two-electrode supercapacitor module, based on the desired size and voltage, is constructed of multiple recurring unit cells. A test fixture configuration that closely mimics the unit cell configuration relatively matches the performance of a packaged cell. Two- electrode test fixtures are either available commercially or can be easily fabricated from two stainless steel. The most common organic and aqueous electrolytes are tetrafluoroborate in propylene carbonate or acetonitrile and KOH, H 2 SO 4 , respectively. According to the electrode config- uration in a supercapacitor, they are classified as symmetric, asymmetric, and hybrid supercapacitors. The symmetric super- capacitor has similar electrode material on both the electrodes. Xe et al. „ assembled symmetric solid-state supercapacitor using walnut shell derived porous carbon as both positive and negative electrodes immersed in PVA/KOH gel electrolyte. The active material showed a specific capacitance of 138 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 Co 3 O 4 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 NiMoS 4 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 physical properties like energy and power density it is mandatory to use a two-electrode setup. So, the selection of electrode material plays a vital role. Such outcomes help researchers to understand the electrode performance in supercapacitors. In general, it is seen EDLCs electrodes show high coulombic efficiency than metal oxides but with a low range of capacity relative to metal oxides. [16] The metal oxides show redox reaction so considered as battery-type electrode materials. It is seen that hybrid supercapacitors deliver high energy density with a high ability of charge storage. [17] Given below the insights on various types of supercapacitors electrode materials. 1.2.2 Factors Influencing the Energy Density and Capacitance of the Supercapacitors The governing factors for the energy density of devices are potential window, pore size distribution, surface area, electro- lytes, and device configurations. High surface area and pore size contribute to the higher capacitance, but when the pore size is very less than the charge-storage will not take place due to the minimization of interaction of nanoparticles with electrolyte ions. A study done by Gogotsi et al., , on the effect of pore size on capacitance reveals that when the size of the solvated ions is larger than the pore size then the electrolyte ions are incapable of contributing to charge storage. [18] Still, it remains a challenge to identify the optimal pore size and surface area to maximize capacitance. Fabricating electrode materials with narrow pore size distribution would increase the capacity of supercapacitors ultimately; boost the energy density without sacrificing the high power density. E ¼ 1 2 CV 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 electro- lytes aqueous electrolytes are less expensive, non-toxic, and have a good conductivity for these reasons aqueous electrolytes are mostly preferred for bimetallic oxides. ChemElectroChem Reviews doi.org/10.1002/celc.202100098 5 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 2.1. Electrical Double Layer Capacitor (EDLCs) H.I. Becker first demonstrated and patented the electrical double-layer energy storage and delivery by EDLCs with porous carbon electrodes in an aqueous electrolyte in 1957 according to the electrical double layer theory. [21] Then, NEC first brought the EDLCs-type devices into commercialization with the permission of SOHIO in1978, which was first named as super- capacitor to describe the high energy differed from conven- tional capacitors [22,23] The electrical double layer capacitor is developed from the electrical double layer model of the Helmholtz model, EDLCs store the charges in the Helmholtz interface between an electrode and electrolyte based on the electrostatic accumulation of ions in the electrolyte. Hence, the charge/ discharge process is non-faradic and reversible. The influence of thermal motion and ion absorption was not explained by Helmholtz double layer. The electrical double layer theory was later updated by Guoy et al. , [24] considered the thermal motion of ions close to the charged surface a double layer is formed during the charging phase, by introducing a diffusive layer in the electrolyte as shown in Figure 4 (a) . As per this model, the double layer is not rigid near the electrode surface but this model did not account for the ion absorption at the electrode/electrolyte interface is not taken into account according to this model double layer is not rigid at the electrode/electrolyte interface. Later Stern model combines the concept of both Helmholtz and Gouy-Chapman models. According to the stern model, electric potential varies when the distance from electrode surface varies and Grahame’s concept of inner Helmholtz plane and outer Helmholtz plane which explains the real situation of an electrical double layer. [25] The most commonly carbonaceous electrodes like gra- phene, activated carbon, carbon nanotubes, carbon aerogel etc. [26] are used for EDLCs electrode materials. Graphene is a single layer of sp 2 bonded carbon atoms tightly packed semi- conductor having zero bandgap. The calculated theoretical capacitance of graphene is 550 F g 1 with a high specific surface area of 2630 m 2 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 under- stand that each carbon material has its advantage and disadvantages. To resolve this problem, CNTs are placed in between the graphene sheets which gives rise to rapid diffusion pathways to electrolyte ions. [23] Also in this way, the structure of graphene becomes more stable as CNTs act as a binder to hold the sheets together. Figure 4. a) Mechanism of charge storage in electrical double-layer capacitors and pseudocapacitors; b) CV and GCD curves of electrical double-layer capacitors, pseudocapacitors, and battery type materials Reproduced with permission from Refs. [19] and [20]. Copyright (2020) Elsevier and (2020) The Authors, respectively. ■■ Please ensure and confirm that copyright permission has been obtained from [19] ■■ ChemElectroChem Reviews doi.org/10.1002/celc.202100098 6 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 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 NiCo 2 O 4 , [37] Carbon nanotubes (CNT) [33] and activated carbons [33] 2.2. Pseudocapacitors (PCs) The second type of supercapacitor based on charge storage mechanism is Pseudo-capacitors, which store charges by sur- face charge-transfer reaction between an electrode and electro- lyte. For storage systems, pseudo-capacitors can store very large charges by electron transfer on the surface of the electrode. In this contrast, redox reactive charge storage systems involve the volume expansion of electrodes, leading to disadvantages in terms of cyclic life and response speed. [38] Transition metal oxides (TMOs) are the electrode materials most commonly used in supercapacitors these TMOs have been widely reported a lot as an electrode material for supercapacitors due to their high thermal conductivity. B.E. Conway at first reported RuO 2 as TMOs with high pseudocapacitance. Whereas, the high cost of RuO 2 was replaced by MnO 2 followed by WO 3 , MoO 3 , V 2 O 5, etc., with cost-effective and electrochemical nature [39] Further, carbon matrix were incorporated into the metal oxides to increase the conductivity of the metal oxides since most of the transition metal oxides are semi-conductors and to enhance the stability of the metal oxides for these reasons carbon materials combined with metal oxides nanocomposites have been demonstrated good performance for the supercapacitor appli- cations. For example, Z. Fan et al. , in his work on graphene- MnO 2 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 enhan- ces the mechanical strength of metal oxides to use them in advanced flexible electronics. A hybrid supercapacitor is a fusion of an Electrical double layer capacitor and Pseudocapacitors. It has two asymmetric electrodes, one of which exhibits battery-type behavior, and the other shows a double layer capacitive behavior. The presence of faradaic behavior increases the specific capacitance and specific energy of the supercapacitor on the other hand the electrical double layer capacitance behavior provides increased cyclic stability and high specific power. Such architecture led the energy storage device to outcome with wide working voltage, better mechanical/ chemical stability, and high energy/ power density. [41] Many researchers have been tremendously working on finding a suitable electrode material for improved electrochemical performance. Often the choice of battery-type electrode materials is the metal oxides due to their high specific surface area, variable oxidation states, thermal and chemical stability. These characteristics make the metal oxides a promis- ing electrode material for supercapacitors. Among metal oxides, TMOs are a suitable material for SCs electrodes as battery-type electrodes to be pragmatic in hybrid capacitors [42] for faradaic reactions of charge storage. The metal oxides have been highly explored due to their good electrochemical performance. The metal oxides like NiO, Co 3 O 4 , Fe 2 O 3 based supercapacitors performances are constantly focused on by researchers to reach high specific capacity as their theoretical value is higher and can be experimentally tuned by altering the morphology or nanocomposites compositions. [43] To achieve theoretical ca- pacity, and tackle issues like capacity fading or low electronic conductivity of TMOs, researchers have investigated ways like; (i) doping of metal oxides, (ii) adding carbon-conductive materials or metal-organic framework, (iii) combining with a conductive polymer, and combining with other metal oxides. Das et al. , investigated the electrochemical properties of Ni/ NiO and Ni/NiO@rGO they showed a high specific capacity of 158 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, Co 2 O 3 @Fe 2 O 3 , etc., grabbed much attention due to their high electrochemical properties like variable oxidation states, syner- gistic effects, and high electrical conductivity due to these properties many research studies were conducted on mixed metal oxides. [47] Disappointingly, there is short-coming like inhomogeneity in crystal structure, these crystal structures were poorly defined when compared to those of a single phase. To overcome this problem bimetallic oxides having good crystal structures have been used as electrode material for super- ChemElectroChem Reviews doi.org/10.1002/celc.202100098 7 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 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 MnO 2 and CuMn 2 O 4 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, NiCo 2 O 4 showed a high specific capacitance of 440 Fg 1 which is higher than that of single metal oxide with doping. [48] NiCo 2 O 4 on further doping with rGO enhances the specific capacitance up to 1305 Fg 1 [49] From these, we can conclude that binary metal oxides offer higher electrochemical performance. Currently, binary metal oxides are a trending topic of research for supercapacitors electrode material. In recent years, binary metal oxides have been widely explored due to their reversible redox reactions because of their low cost, low toxicity, multiple oxidation states, and much higher electrical conductivity. There are given below different types of bimetallic oxides in supercapacitor applica- tions and their electrochemical performance of bimetallic oxides in detail with various examples. 3. Bimetallic Oxides However, bimetallic oxides are reported relatively with higher electrochemical activity than their respective single oxides. The synergistic effect of both the metals provides better electro- chemical activities. One of the famous bi-metallic metal oxides is the spinel cobaltites MCo 2 O 4 (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 (NiCo 2 O 4 ) exhibits two orders of magnitude of higher electrical conductivity than nickel oxide (NiO) or cobalt oxide (Co 3 O 4 ). [44] besides that benefits of both the oxides can be obtained in a single sample. Similarly, iron oxide and cobalt oxide both provide high electrochemical performance. Owing to low cost, some tran- sition metal oxides like Mn, Ni, Co, etc. are commonly referred to as the candidates for developing different pseudocapacitors and hybrid capacitors. Table 1, shows the specific capacitance value of various metal oxides. Similarly, many other metallic Figure 5. a, c) CV curves of activated carbon@MnO 2 and CuMn 2 O 4 at different reaction times; b, d) GCD curves of Activated carbon@MnO 2 and CuMn 2 O 4 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 Reviews doi.org/10.1002/celc.202100098 8 ChemElectroChem 2021 , 8 , 1 – 25 www.chemelectrochem.org © 2021 Wiley-VCH GmbH These are not the final page numbers! �� 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 oxides have been examined for supercapacitor studies. In this review, we have discussed various binary metal oxides as electrode materials for supercapacitors. With reported research, it has been discovered that binary metal oxide performed better than single metal oxides or mixed oxides which improved the supercapacitor‘s performance. Bimetallic metal oxides are considered as one of the best electrode materials for supercapacitors due to the properties like crystal structure, defects, spin, electronic structure, and synergetic effect. The crystal structure of bimetallic oxides has multiple lattice sites that enhance the stability and performance of the material. Defects like Schottky and Frenkel defects can help to increase the conductivity of the material since the vacancy created in the crystal lattice distorts because of that a local distortion happens which may modify the lattice vibration, which in turn determines the electrical resistivity of the material. Synergetic effect arises due to the presence of two metal cations which improves chemical functionality and charge storage capabilities by utilizing the oxidation states of two metal cations, the redox activity is improved. the single- phase crystal structure enhances the stability and performance of the material. Moreover, binary metal oxides are easy to synthesize and less harmful to the environment when com- pared to binary or single metal sulfides. A few of bimetallic oxides have been discussed below: Table 1. E lectrochemical performance of some single, mixed, and binary metal oxides. Electrode material Specific capacitance Number of cycles Retention rate Energy density [W h Kg 1 /W h Cm 2 ] Power density [W Kg 1 /W Cm 2 ] Current density [A g 1 /A cm 2 ] Electrolyte Ref. CuO 571.25 F g 1 1000 92 % NA NA 1 1 M KOH [124] RuO 2 /G 441.1 F g 1 1000 94 % NA NA 0.1 1 M Na 2 SO 4 [125] Co–MOF 450.89 F g 1 1000 95 % NA NA 0.5 6 M KOH [45] PANI/AC/Ni 1661 F g 1 2000 93 % NA NA 1 1 M KOH [126] Zr/CeO 2 448.1 C g 1 6000 96.4 % NA NA 1 2 M KOH [127] Co–MOF/PANI 504F g 1 5000 90 % NA NA 1 1 M KOH [128] MnO 2 /CuO 279.12 F g 1 10000 91.26 % NA NA 0.5 1 M KOH [129] MnO 2 /NiO 247 F g 1 1000 81.2 % NA NA 0.5 1 M KOH [130] Cu 2 O/NiO 2255.5 F g 1 5000 94.5 % NA NA 0.0011 2.0 M KOH [131] NiO/rGO 127.5 F g 1 2000 70 % NA NA 1 6 M KOH [132] TiO 2 /RuO 2 1200 F g 1 10000 95.2 % NA NA 0.5 1 M H 2 SO 4 [133] MnO 2 /FeCo 2 O 4 2.52 F cm 2 1500 94 % NA NA 2 PVA/KOH [134] FeCo 2 O 4 @NiCo 2426 F g 1 5000 91.6 % NA NA 1 PVA-KOH [135] Ni(OH) 2 @CuCo 2 O 4 295.6 mA h g 1 3000 93.7 % NA NA 1 KOH/PVA [136] FeCo 2 O 4 960 F g 1 10000 94 % NA NA 2 3 M KOH [137] RuCo 2 O 4 1469 F g 1 3000 91.3 % 36.5 3294 6 2 M KOH [138] NiCo 2 O 4 /CF 2658 F g 1 3000 80 % NA NA 2 3 M KOH [139] MnCo 2 O 4 250 F g 1 1000 NA 10.04 NA 0.25 2 M KOH [140] CuCo 2 O 4 1210 F g 1 5000 86 % 42.81 NA 2 6 M KOH [141] ZnCo 2 O 4 1841 F g 1 3000 95.8 % NA NA 1 6 M KOH [142] ZnCo 2 O 4 229 F g 1 1500 84.3 % NA NA 0.25 2 M KOH [143] NiCo 2 O 4 /AC 273.5 F g 1 3000 96 % NA NA 1 6 M KOH [144] CuCo 2 O 4 /rGO 978 F g 1 5600 1.34 times increased NA NA 3 6 M KOH [145] NiCo 2 O 4 /rGO 304 F g 1 5000 92.8 % 95 374 0.5 2 M KOH [146] LaFeO3/MOF 241.3 F g 1 5000 92.2 % 34 900 1 1 M Na 2 SO 4 [147] LiCoO 2 310.93 mF/cm 2 2000 80.26 % 5.6 X 10 5 0.0011 NA 1 M LiCl [148] MgCo 2 O 4 626.5 F g 1 5000 99.06 % 30.6 861 1 2 M KOH [149] NiV 2 O 6 565.5 Cg 1 3000 84.6 % 24.3 800 1 2 M KOH [150] NiCoO 2 778.5 C g 1 1000 97 % NA NA 0.5 6 M KOH [151] CoV 2 O 6 223 F g 1 15000 123 % NA NA 1 2 M KOH [152] CoNiO 2 184 F g 1 1000 96.8 % 15.0 14,210 1 1 M KOH [153] ZnV 2 O 4 360 F g 1 1000 89 % NA NA 1 2 M KOH [154] CoGa 2 O 4 642.4 C g 1 40000 125 % 36.71 414.1 1 3 M KOH [155] NiGa 2 O 4 1508 F g-1 10000 102 % 45.2 1600 1 6 M KOH [156] NiCr 2 O 4 187 F g 1 2000 80 % 6.5 3000 0.6 1 M KOH [157] CoFe 2 O 4 -carbon 102.5 F g 1 6000 81.5 % NA NA 0.16 5 M KOH [158] NiMoO 4 -PANI 93 F g 1 5000 98.6 % 33.07 240 0.3 PVA-KOH [121] NiCeO 2 @PANI 866 F g 1 10000 85.6 % 120.3 500.2 1 1 M Na 2 SO 4 [15