{Cubic like CoMn2O4 nanostructures as advanced high-performance pseudocapacitive electrode:} http://dx.doi.org/10.5599/jese.1353 777 J. Electrochem. Sci. Eng. 12(4) (2022) 777-786; http://dx.doi.org/10.5599/jese.1353 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Cubic like CoMn2O4 nanostructures as advanced high-performance pseudocapacitive electrode Puratchimani Mani1, Venkatachalam Vellaikasi2,, Xavier Thankappan Suryabai3, Abraham Rajasekar Simon2 and Thamizharasan Kattaiyan2 1Department of Physics, As-Salam College of Engineering and Technology, Thirumangalakudi, Aduthurai- 612 012. India 2Department of Physics, Sir Theagaraya College, Chennai -600 025, India 3Department of Physics, Centre for Advanced Material Research, Govt. College for Women, Thiruvananthapuram, Kerala- 695014, India Corresponding author: venkatcnst12@gmail.com Received: April 24, 2022; Accepted: June 21, 2022; Published: July 18, 2022 Abstract In this work, a synthesis of cubic-like CoMn2O4 uniform nanostructures with KOH-NaOH involved in the hydrothermal method has been reported. The crystal structure phase purity, functional groups, and morphology of the CoMn2O4 have been investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-reso- lution transmission electron microscopy (HR-TEM) analyses. The electrochemical beha- viour of CoMn2O4 electroactive material has been examined for supercapacitors. The elec- trode displays excellent capacitive behaviour with superior electrochemical properties. The cubic-like morphology structure with enough free space is beneficial for improving electrochemical performance. The CoMn2O4 electrode exhibits a faradaic capacitance with the highest specific capacitance value of 762.4 F g-1 at a scan rate of 5 mV s-1. The coulombic efficiency of the CoMn2O4 electrode was found to be 91.2 % after 2000 charging-discharging cycles. The nanostructures of CoMn2O4 make a prominent contrib0- ution to the excellent electrochemical performance of the prepared electrode. Keywords Hydrothermal method; faradaic capacitance; long-term stability; energy storage Introduction In recent years, supercapacitors (SCs) have been focused on energy storage applications due to their distinctive advantages such as low cost, high-energy density, rapid charge-discharge rates, and environmental benignity when compared to batteries. The choice of electrode materials plays an important role in determining SC behaviour. These factors have triggered a significant research interest in the development and design of new and superior electrode materials for supercapacitors. http://dx.doi.org/10.5599/jese.1353 http://dx.doi.org/10.5599/jese.1353 http://www.jese-online.org/ mailto:venkatcnst12@gmail.com J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 CoMn2O4 AS PSEUDOCAPACITIVE ELECTRODE 778 Among numerous available electrode materials, binary metal oxides with excellent electro- chemical performance were found to be promising, effective, and scalable alternatives. In addition, binary metal oxides possess achievable oxidation states, high electrical conductivities, excellent electrochemical properties, and moreover, they are affordable and environmentally friendly when compared to single-component oxides. In general, transition metal oxides with multiple oxidation states undergoing some redox re- actions were already being used as pseudocapacitive or faradaic materials [1-3]. A huge interest has been put towards the synthesis and development of metal-oxide nanostructures with excellent elec- trochemical properties. Simple binary metal oxide electrode materials such as MnO2 [4], Co3O4 [5] and NiO [6] have been widely studied as electrode materials for supercapacitor with excellent electrochemical behaviour. Meanwhile, ternary metal oxides with different metal cations like CoFe2O4, FeMn2O4, MnCo2O4 and NiFe2O4 have also attracted attention because of their promising applications in the energy storage field [7-11]. The coupling of two metal species might increase oxidation-state-rich redox reactions crucial for pseudocapacitors and different combinations of cations during the charging-discharging process could provide great opportunities to manipulate the physical and chemical properties of electrodes. CoMn2O4 has already been considered a promising material for supercapacitors owing to its outstanding reversible capacity and widespread availability. The Co2MnO4 material is found to exhibit an excellent capacitive behavior because cobalt exhibits higher oxidation potential while manganese can transport more electrons [12]. For example, Jiang et al. [13] reported the growth of cobalt-manganese composite oxide nanostructures on Ni foam by hydrothermal method, displaying specific capacitance of even 840.2 F g-1 at a current density of 10 A g-1 and showing the excellent cycling performance. Ren and his co-workers [14] have reported the solvothermal synthesized uniform and decentralized flower-like CoMn2O4 of hierarchical nanostructures that exhibited a specific capacitance of 188 F g-1 in Na2SO4 solution. Metal oxide nanostructures as supercapacitor electrodes are fascinating owing to their large surface area and reduced particle size, which generates more interfacial active sites. Various synthesis methods for the preparation of metal oxide nanostructures have been widely explored, including the co-precipitation technique, hydrothermal method, template-assisted synthesis, and electrochemical deposition. Among these methods, hydrothermal is a powerful method to synthesize nanostructures, where the size, crystal structure, and morphology of the product materials can easily be controlled. Herein, we have developed a facile method to prepare CoMn2O4 nanostructured material by the hydrothermal method, involving potassium and/or sodium hydroxide solution treatment. The prepared material has been characterized by some surface techniques, while its pseudocapacitive and stability properties are determined by CV and galvanostatic charging-discharging experiments. Experimental Material synthesis CoMn2O4 nanostructures were prepared using double-hydroxide treatment involved in the hydrothermal method. The material was synthesized as follows: initially, 9 g of mixed hydroxides solution was prepared by adding an equal molar ratio of potassium hydroxide (KOH) and Sodium hydroxide (NaOH) molten pellets. To this solution, 1 M cobalt nitrate (Co(NO3)26H2O) and 2 M man- ganese nitrate (Mn(NO3)26H2O) solutions were added and then mixed thoroughly by stirring the solution for a few minutes to ensure the homogeneity. The solution was then transferred into an P. Mani et al. J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 779 autoclave containing a Teflon vessel and kept in the furnace for 24 hours at a constant temperature of 180 °C. After completion of the reaction, the autoclave containing solution was cooled down naturally. Finally, the precipitate was washed several times by using water and ethanol to remove the impurities, dried, and annealed at 500 °C for 5 hours and collected for characterization. Possible chemical reactions resulting in the formation of pure CoMn2O4 nanostructured material can be expressed by Eqs. (1) - (3): Mn2+ + 2OH− → Mn(OH)2 (1) Co2++ 2OH−→ Co(OH)2 (2) 2Mn(OH)2 + Co(OH)2 + 1/2O2 → CoMn2O4 + 3H2 (3) Surface characterization The crystal structure of the CoMn2O4 material was studied by powder XRD (XRD, Rigaku miniflux (II)-c). The Debye-Scherer formula was used to evaluate the average crystallite size of the synthesized material. 0.9 cos d    = (4) where  is the x-ray wavelength,  is the full-width half-maximum,  is the Bragg diffraction angle, and d is the crystallite size. Fourier-transform infrared (FT-IR) study was performed using a Perkin Elmer spectrometer with KBr pellet-based samples in the frequency range between 4000 and 400 cm-1. Morphology of the synthesized material was viewed by field emission scanning electron microscopy (FE-SEM) and high- resolution electron microscopy (HR-TEM) using a Hitachi H7650. Electrochemical measurements and preparation of the electrode The electrochemical behaviour of CoMn2O4 nanostructures was characterized by cyclic voltam- metry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) analyses. These measurements were performed using a conventional three-electrode system with platinum wire as the counter electrode and Ag/AgCl/ sat. KCl as the reference electrode. For the preparation of working electrodes, the CoMn2O4 material was mixed with polyvinylidene difluoride and activated carbon added to the above mixture in order to achieve homogeneity. The prepared material was brush coated onto nickel foil (NF). The mass of the loaded active material ranged from 0.4 and 0.5 mg. All electrochemical experiments were conducted at room temperature. The specific capacitance value was calculated from cyclic voltammetry and Galvanostatic charging/discharging curves. Cyclic voltammetry was recorded to determine the charge-storage capacity within a potential window of 0.0 to 0.5 V (vs. Ag/AgCl sat. KCl) at various scan rates from 5 to 100 mV s-1 using CHI (7081C) electrochemical workstation (USA). For testing charging and dis- charging properties, chronopotentiometry measurements at various constant currents (2-4 A g-1) were applied between 0.0 and 0.5 V (vs. Ag/AgCl sat. KCl). Electrochemical impedance spectrum was also carried out at 0.5 V vs. Ag/AgCl sat. KCl, in the frequency region between 1 MHz and 1 Hz. Results and discussion XRD analysis The structure and phase purity of the prepared CoMn2O4 material after calcining were studied by X-ray powder diffraction, as shown in Figure 1. All reflections could be indexed as body-centred- http://dx.doi.org/10.5599/jese.1353 J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 CoMn2O4 AS PSEUDOCAPACITIVE ELECTRODE 780 tetragonal (bct) phase (JCPDS no. 77-0471) with a distorted spinel structure. The sharpening of the peaks is observed, which indicates the improvement in crystalline quality. No residues or contaminants have been detected in the XRD pattern, which indicates the complete transformation from Co–Mn-DH (double hydroxide) precursors to CoMn2O4 and the high phase purity of the sample. The mean crystallite size was calculated from the (211) peak with the Scherrer formula given by Eq. (4), and the size is found to be 34 nm for the CoMn2O4 sample [15,16] 2 / o Figure 1. XRD patterns of CoMn2O4 nanostructured material FT-IR analysis FT-IR spectrum of the CoMn2O4 nanostructured material prepared by DH (double hydroxide) treatment is shown in Figure 2. The peaks observed around 3415 (broad) and 2921 (small) cm-1 correspond to the stretching vibration of hydrogen-bonded hydroxyl groups. The shoulder peak at 1626 cm-1 corresponds to the bending mode of absorbed water molecules. The characteristic absorption small bands at around 621 cm-1 are attributed to the vibration of the tetrahedral oxygen environment (mainly cobalt oxide), while the peak observed at 493 cm-1 can be correlated to the vibration of the octahedral oxygen environment (mainly manganese oxide), which further demonstrate the formation of pure CoMn2O4 by annealing at 500 oC [17]. Figure 2. FT-IR spectrum of CoMn2O4 nanostructured material T / % P. Mani et al. J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 781 Morphology analysis The morphology of the prepared CoMn2O4 material was investigated by FE-SEM and HR-TEM with different magnifications. Figure 3(a) displays FE-SEM images of CoMn2O4 prepared by double hydroxide solution treatment, while the corresponding HR-TEM images are shown in Figure 3 (b and c). Clearly, the prepared CoMn2O4 is composed of disordered cubic-like nanostructure with small agglomerates and rough surfaces. In Figure 3(d), the selected area diffraction (SAED) pattern displays well-defined rings, representing the characteristic polycrystalline nature of CoMn2O4 and not amorphous. It is anticipated that this material can absorb and strongly retain electrolyte ions, ensuring sufficiently fast faradic reactions, specifically at high current densities. In addition, this kind of morphology can afford higher active sites and a specific surface area leading to high performance in the electrochemical energy storage system. Figure 3. (a) FE-SEM, (b, c) HR-TEM images and (d) SAED pattern of CoMn2O4 nanostructured material Electrochemical properties Cyclic voltammetry analysis Cyclic voltammetry (CV) was used to study the charge storage capacity and charge storage mechanism of the CoMn2O4 electrode. The electrochemical performance of the CoMn2O4 electrode was tested using a three-electrode working system in 2 M KOH electrolyte. The CV curves in the potential region from 0 to 0.5 V at various scan rates are shown in Figure 4(a). For the as-obtained electrode, the shape of CV curves indicates a predominant pseudocapacitive behaviour. The current values were found to increase upon increasing the scan rate due to the fast electronic and ionic transport rates between nanostructured electrode and electrolyte. At low scan rates, CV curves exhibit nearly rectangular shapes, indicating no kinetic limitations and well stability of the http://dx.doi.org/10.5599/jese.1353 J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 CoMn2O4 AS PSEUDOCAPACITIVE ELECTRODE 782 supercapacitor [18]. At higher scan rates, CV curves are slightly deviated from a rectangular-like response but still demonstrate capacitive behaviour at scan rates up to 100 mVs-1. This indicates the rapid current response to voltage reversal (small contact resistance) and reveals pseudocapacitive properties of the CoMn2O4 electrode. The probable charge storage mechanism may be described ba eq. (5): (CoMn2O4)surface + K+ + e− = [KCoMn2O4]surface (5) According to the reaction described by Eq. (5), energy storage is based on the adsorption/absorp- tion of electroactive ions (K+) at the surface (or near-surface) electrode region, which is undergoing charge transfer and without bulk phase transformation. The specific capacitance values were calculated using eq. (6): ( ) c a s a c 1 d V V C IV V vm V V = −  (6) where  c a d V V IV V is the integral value during the cathodic scan, m is the mass of the active electrode material loaded on the substrate, 𝑣 is the scan rate, and Va – Vc is the working potential window range. Based on Figure 4(a) and eq. (6), specific capacitance values of CoMn2O4 electrode material were calculated to be 762.4, 601.35, 454.08, 292.39 and 256.91 F g-1, for 5 , 10, 25, 50 and 100 mV s-1, respectively. From the CV results, the modified electrode reveals the pseudocapacitive behaviour. The specific capacitance value of the electrode material decreased as the scan rate increased, which may be owing to the availability of cations near the surface of the electrode, which cannot be effectively utilized on the surface of the electroactive material. Anyhow, the CV testing of the electrode under scan rate of 5 mV s-1 and obtained Cs of 762.4 F g-1 suggests a potential use of such material in SCs used in high power applications [19]. Table 1 shows that here obtained specific capacitance of CoMn2O4 is higher than found in earlier reported studies and other ternary/binary transition metal oxides. Table 1. Comparison of the specific capacitance value of CoMn2O4 with earlier studies. Material Synthesis method Specific capacitance, F g-1 Electrolyte Reference NiCo2O4 Solvothermal 758 at 1 A g-1 2 M KOH [20] V2O5/carbon Electrospinning 150 at 1 mA 6 M KOH [21] Carbon/Nickel Electrospinning 103.8 at 0.5 A g-1 6 M KOH [22] CoMn2O4 Hydrothermal 140.6 mAh g-1 at 1 mA cm-2 3 M KOH [23] CoMn2O4 Electrospinning 121 at 5 mV s-1 3 M NaOH [24] CoMn2O4 Hydrothermal 762.4 at 5 mV s-1 2 M KOH Present work Charging-discharging analysis The charge/discharge analysis confirms the excellent reversibility characteristics with a nearly symmetric charging/discharging profile. Galvanostatic charge-discharge analysis was carried out at different current densities of 1, 2, 3 and 4 A g-1 in the potential window range between 0 to 0.5 V, as shown in Figure 4(b). At high current densities, the discharge curves are nearly symmetrical to the corresponding charge curves, which indicates a highly capacitive nature of the CoMn2O4 electrode and electrochemical reversibility of the underlying faradaic reaction. The nearly symmetrical charge/discharge profiles are in good agreement with almost rectangular CV curves. P. Mani et al. J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 783 The apparent deviation from straight lines in the charge-discharge plots is obtained at low currents, indicating some inhibition of the faradic pseudocapacitive reaction of as-synthesized material [25]. Also, the curves are highly symmetric at peak values, showing that the electrode has low internal resistance. Furthermore, the specific capacitance values were calculated using Eq. (7), Z’ /  Scan rate, mV s-1 Figure 4. (a) CV curves of CoMn2O4 nanostructured material in 2 M KOH at different scan rates; (b) GCD curves at different current densities; (c) Nyquist plot at 0.5V; (d) variation of specific capacitance with scan rate; insert: variation of specific capacitance with current density applied; (e) coulombic efficiency during 2000 charge-discharge cycles recorded at a current density of 2 A g-1; insert: few long term cycles -Z '' /  http://dx.doi.org/10.5599/jese.1353 J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 CoMn2O4 AS PSEUDOCAPACITIVE ELECTRODE 784 d s It C m V =  (7) where, I is the constant current applied, td is discharge time, m is the mass of the electroactive material loaded, and V is the potential difference (0.5 V) [24]. Specific capacitance values calculated from charging/discharging curves in Figure 4(b) and eq. (7) are presented in the inset of Figure 4(d). The specific capacitance values of the CoMn2O4 material calculated from GCD results concord with CV results. Long-term cycle charge/discharge analysis was also carried out to evaluate the cyclic stability of the prepared CoMn2O4 electrode material. A constant current density of 2 A g−1 was applied to examine the CoMn2O4 electrode in 2 M KOH electrolyte, and 2000 charging-discharging cycles were performed. The results are in the form of coulombic efficiency vs. cycle number presented in Figure 4(e). The coulombic efficiency of the CoMn2O4 electrode was calculated using the relation,  = (td / tc) 100 (8) where,  is coulombic efficiency, td is the discharge time and tc is charging time derived from charge- discharge curves. Figure 4(e) shows a gradual increase of the coulombic efficiency during the first few cycles, which indicates a certain electro-activation process of the electrode under given testing con- ditions (voltage range of 0 to 0.5 V and current density of 2 A g-1). After that, the coulombic efficiency slightly decreased with the cycling but retained 91.2 % of the initial efficiency after 2000 cycles. Electrochemical impedance spectroscopy analysis The electrochemical impedance spectroscopy analysis is exploited to evaluate the electro- chemical properties of CoMn2O4 material. Figure 4(c) shows the Nyquist plot of the CoMn2O4 elec- trode in 2 M KOH, measured 0.5 V within the frequency region of 1 MHz to 1 Hz. The plot reveals a distinct semicircle at high frequencies and a sloping straight line in the lower frequency region. The semicircle impedance response in the high-frequency region corresponds to the charge-transfer resistance at the electrode/electrolyte interface, and the straight line observed at low frequencies corresponds to the diffusion process of electroactive species (Warburg impedance) [26]. The equi- valent circuit diagram proposed to fit the impedance spectrum is shown in the inset of Figure 4(c). The resistance (Rs), which involves ohmic resistance of the active material and electrolyte, as well as the contact resistance at the interface between the active material and current collector, is obtained as the high-frequency intercept of the semicircle at the real impedance axis. Faradaic interfacial charge-transfer resistance (Rct) is associated with the diameter of the semicircle in the high-frequency range. The Warburg impedance, which indicates the frequency dependence of ion diffusion [27], is probably related to the diffusion of K+ within the surface and near-surface layer according to the charge storage mechanism described by eq. (5). Conclusion In summary, spinel CoMn2O4 nanostructure was successfully synthesized using facile hydrothermal route at 180 °C followed by annealing at 500 °C. The prepared material was charac- terized by XRD, FT-IR, FE-SEM and HR-TEM analyses, revealing a nanostructured material of polycrystalline nature. The electrochemical properties were investigated in 2 M KOH aqueous electrolyte using cyclic voltammetry, charge-discharge, long-term cycle stability performance and electrochemical impedance spectroscopy measurements. The maximum specific capacitance of 762.4 F g-1 was obtained from CV measurements at 5 mV s-1, while about 600 F g-1 was obtained by galvanostatic charging-discharging at 1 A g-1. The material showed sufficient cycling stability over P. Mani et al. J. Electrochem. Sci. Eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 785 2000 cycles. The results demonstrate that the spinel-CoMn2O4 nanostructure should be a promising electrode material for supercapacitor applications. References [1] L. Li. H. Hu, S. Ding, Inorganic Chemistry Frontiers 5(7) (2018) 1714-1720. https://doi.org/10.1039/C8QI00121A [2] C. Zhu, R.-G. Lu, L. Tian, Q. Wang, IEEE Vehicle Power and Propulsion Conference (2006) 24172345. https://doi.org/10.1109/VPPC.2006.364372 [3] Y. Wang, Y. Song, Y. 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This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1177%2F155892501300800405 https://doi.org/10.1016/j.matlet.2018.11.100 https://doi.org/10.1016/j.jallcom.2016.09.005 https://doi.org/10.1016/j.est.2020.101483 https://doi.org/10.1016/j.electacta.2012.01.095 https://doi.org/10.1007/s11696-020-01448-z https://creativecommons.org/licenses/by/4.0/) @Article{Mani2022, author = {Mani, Puratchimani and Vellaikasi, Venkatachalam and Suryabai, Xavier Thankappan and Simon, Abraham Rajasekar and Kattaiyan, Thamizharasan}, journal = {Journal of Electrochemical Science and Engineering}, title = {{Cubic like CoMn2O4 nanostructures as advanced high-performance pseudocapacitive electrode:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {777--786}, volume = {12}, abstract = {In this work, a synthesis of cubic-like CoMn2O4 uniform nanostructures with KOH-NaOH involved in the hydrothermal method has been reported. The crystal structure phase purity, functional groups, and morphology of the CoMn2O4 have been investigated by X‑ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-reso­lution transmission electron microscopy (HR-TEM) analyses. The electrochemical beha­viour of CoMn2O4 electroactive material has been examined for supercapacitors. The elec­trode displays excellent capacitive behaviour with superior electrochemical properties. The cubic-like morphology structure with enough free space is beneficial for improving electrochemical performance. The CoMn2O4 electrode exhibits a faradaic capacitance with the highest specific capacitance value of 762.4 F g-1 at a scan rate of 5 mV s-1. The coulombic efficiency of the CoMn2O4 electrode was found to be 91.2 % after 2000 charging-discharging cycles. The nanostructures of CoMn2O4 make a prominent contr­ib0­ution to the excellent electrochemical performance of the prepared electrode.}, doi = {10.5599/JESE.1353}, file = {:D\:/OneDrive/Mendeley Desktop/Mani et al. - 2022 - Cubic like CoMn2O4 nanostructures as advanced high-performance pseudocapacitive electrode.pdf:pdf;:13_jESE_1353.pdf:PDF}, keywords = {Hydrothermal method, energy storage, faradaic capacitance, long, term stability}, publisher = {International Association of Physical Chemists (IAPC)}, url = {https://pub.iapchem.org/ojs/index.php/JESE/article/view/1353}, }