Facile Synthesis of Bio-Template Tubular MCo 2 O 4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte

: In this project, we present a comparative study of the electrochemical performance for tubular MCo 2 O 4 (M = Cr, Mn, Ni) microstructures prepared using cotton ﬁber as a bio-template. Crystal structure, surface properties, morphology, and electrochemical properties of MCo 2 O 4 are characterized using X-ray di ﬀ raction (XRD), gas adsorption, scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), cyclic voltammetry (CV), and galvanostatic charge-discharge cycling (GCD). The electrochemical performance of the electrode made up of tubular MCo 2 O 4 structures was evaluated in aqueous 3M KOH electrolytes. The as-obtained templated MCo 2 O 4 microstructures inherit the tubular morphology. The large-surface-area of tubular microstructures leads to a noticeable pseudocapacitive property with the excellent electrochemical performance of NiCo 2 O 4 with speciﬁc capacitance value exceeding 407.2 F / g at 2 mV / s scan rate. In addition, a Coulombic e ﬃ ciency ~100%, and excellent cycling stability with 100% capacitance retention for MCo 2 O 4 was noted even after 5000 cycles. These tubular MCo 2 O 4 microstructure display peak power density is exceeding 7000 W / Kg. The superior performance of the tubular MCo 2 O 4 microstructure electrode is attributed to their high surface area, adequate pore volume distribution, and active carbon matrix, which allows e ﬀ ective redox reaction and di ﬀ usion of hydrated ions.


Introduction
Supercapacitors (SCs) are the energy storage device. SCs are in high demand because of their greater power density compared to batteries and higher energy density compared to that of capacitors [1]. In the capacitor, there is no time lag during the charging process; hence it can give higher power density, and in the battery, there is low self-discharge so that it can provide higher energy density [2]. Because of this, a supercapacitor is easy to charge within a short time and able to show significant performance even after prolonged use. SCs are of three types: (i) electric double-layer capacitors (EDLCs), (ii) pseudo capacitors (PCs), and (iii) hybrid capacitors. EDLCs are based on the principle that physical adsorption takes place on the interface of a solid electrode, usually carbon-based material, and liquid electrolytes [3,4]. In PCs, surface redox reaction takes place at the electrode-electrolyte interface, which is responsible for storing electronic charges [5], where metal oxides and conducting polymer-based materials are used as active electrode materials [6,7]. EDLCs have lower specific capacitance and energy density as compared to PCs, hence, practically PCs are in higher demand.

Synthesis
All the chemicals required for the synthsis, such as Cobalt nitrate hexahydrate; Co(NO 3 ) 2 2 .6H 2 O were mixed in 15 ml of distilled water separately, and the mixture was ultra-sonicated for 10 minutes to make a homogenous solution. Then, 1.0 g of cotton was soaked in the mixture solutions for 5 minutes. The resulting soaked cotton was filtered and dried at 150 • C for 30 minutes. The dried cotton was later calcined at 520 • C for 3 hours in the air to obtain bio-templated CrCo 2 O 4, MnCo 2 O 4 , and NiCo2O4 tubular microstructure.

Characterization
The x-ray diffraction patterns were obtained via Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Madison, WI, USA) using Cu K α radiation to check phase purity and determine the crystalline parameters of as-prepared samples. A scanning electron microscope (Phenom) at 10 keV analyzed the morphology of samples. The Brunauer-Emmett-Teller (BET) method was used to measure the specific surface area of the samples. The surface area measurement was carried out by adsorption-desorption isotherms at 77 K, (Quantachrome, Boynton Beach, FL 33426, model No. AS1MP) using nitrogen as adsorbing gas. Thermogravimetric analyses (TGA, Instrument Specialist, Inc., Twin Lakes, WI, USA), were performed in 24 to 550 • C temperature range. FTIR spectra were collected via Theromo-Fisher Scientific FTIR spectrometer (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA) between 450 and 1000 cm −1 .
Versastat 4-500 electrochemical workstation (Princeton Applied Research, USA) was used to perform electrochemical measurements in a standard three-electrode configuration. To prepare an electrode, slurry pastes of 80 wt % of the synthesized powder, 10 wt % of acetylene black, and 10 wt % of polyvinylidene difluoride (PVDF) were mixed in the presence of N-methyl pyrrolidinone (NMP). The thoroughly mixed paste was applied onto a nickel foam. Here, the active mass is 80% out of the total pasted mass in the electrode. The prepared electrodes were dried under vacuum at 60 • C for 10 hours. The loading mass of all samples was about 2-3 mg, measured by weighing the nickel foam before and after deposition with an analytical balance (MS105DU, Mettler Toledo, 0.01 mg of resolution). MCo 2 O 4 (M = Cr, Mn, Ni) coated nickel foam was used as a working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a platinum wire as a counter electrode. The electrochemical performance of the electrodes was evaluated at RT in 3M KOH electrolyte via cyclic voltammetry and galvanostatic charge-discharge techniques measurements. Figure 1a shows the XRD patterns of the bio-templated CrCo 2 (Table 1). FTIR spectrum, Figure 1b, further identifies the structure of the bio-templated MCo 2 O 4 . The FTIR spectrum displays two distinct bands at 515.7 (ν 1 ) and 637 (ν 2 ) cm −1 , which arise from the stretching vibrations of the metal-oxygen bonds [48][49][50]. The ν 1 band is characteristic of M-O (M = Cr, Mn, Ni) vibrations in octahedral coordination, and the ν 2 band is attributable to M-O (M -Co) bond vibration in tetrahedral coordination. These frequency bands are the signature vibrational bands for the spinel lattice [51]. Hence FTIR spectrum at 519.1, 519.02, and 519.08 cm −1 indicate stretching vibration of Co 3+ -O 2− in the octahedral sites, and at 638. 6 Figure 1c shows the BET specific surface area of tubular MCo2O4 microstructures. The specific surface area was determined from N2 adsorption-desorption isotherms obtained at 77 K between relative pressure P/Po~0.029 to 0.99, and the Barrette-Joyner-Halenda (BJH) method was used for measuring corresponding pore sized distributions. The type IV isotherm hysteresis loops [53] suggest the existence of mesopores in the samples. The BET specific surface area of biomorphic CrCo2O4, MnCo2O4, and NiCo2O4 are 34.4 m 2 /g, 32.2 m 2 /g, and 18.9 m 2 /g, respectively. Figure 1c inset shows the pore size distribution. Inset curves indicate having a more favorable condition for the fast ion transport phenomenon within the electrode surface [54][55][56][57], which is confirmed by the presence  Figure 1c shows the BET specific surface area of tubular MCo 2 O 4 microstructures. The specific surface area was determined from N 2 adsorption-desorption isotherms obtained at 77 K between relative pressure P/P o~0 .029 to 0.99, and the Barrette-Joyner-Halenda (BJH) method was used for measuring corresponding pore sized distributions. The type IV isotherm hysteresis loops [53] suggest the existence of mesopores in the samples. The BET specific surface area of biomorphic CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 are 34.4 m 2 /g, 32.2 m 2 /g, and 18.9 m 2 /g, respectively. Figure 1c inset shows the pore size distribution. Inset curves indicate having a more favorable condition for the fast ion transport phenomenon within the electrode surface [54][55][56][57], which is confirmed by the presence of a significant number of pores distribution at around 0.4 nm to 4.3 nm with the highest pore volume.

Results and Discussion
Additionally, the large BET surface area of tubular MCo 2 O 4 superstructures can provide plenty of superficial electrochemical active sites to participate in the Faradaic redox reactions.
The thermogravimetric analysis was conducted on the infiltrated samples (cotton dipped in a mixture of chemical solution and filtered it) to understand the temperature dependence mechanism of the formation of biomorphic MCo 2 O 4 . Figure 1d shows the TGA plots of MCo 2 O 4 measured in the temperature range of 24 to 550 • C. The formation of MCo 2 O 4 from the nitrate salts results in three steps. The weight loss at around 110 • C for all three MCo 2 O 4 is due to water desorption, the second weight loss up to 187 • C for CrCo 2 With the increase in calcination temperature, the removal of organic substance was achieved where the remaining few portions of the organic substance change into carbon.
FE-SEM in Figure 2a-d displays tubular morphology of the cotton fibers, samples CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 , respectively, which resembles a biomorphic structure. Figure 3a-c shows SEM images obtained using elemental mapping at chromium, manganese, and nickel energy peaks and shows that the tubular structure is well decorated with the CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 nanoparticles. Table 2 gives the elemental composition and element distribution, which is obtained via EDX (energy dispersive x-ray spectroscopy). of a significant number of pores distribution at around 0.4 nm to 4.3 nm with the highest pore volume. Additionally, the large BET surface area of tubular MCo2O4 superstructures can provide plenty of superficial electrochemical active sites to participate in the Faradaic redox reactions.
The thermogravimetric analysis was conducted on the infiltrated samples (cotton dipped in a mixture of chemical solution and filtered it) to understand the temperature dependence mechanism of the formation of biomorphic MCo2O4. Figure 1d shows the TGA plots of MCo2O4 measured in the temperature range of 24 to 550 °C . The formation of MCo2O4 from the nitrate salts results in three steps. The weight loss at around 110 °C for all three MCo2O4 is due to water desorption, the second weight loss up to 187 °C for CrCo2O4, 241 °C for MnCo2O4, and 160 °C for NiCo2O4 is due to burning of cotton and start of decomposition of Co(NO3)2·6H2O and Cr(NO3)2·6H2O, Co(NO3)2·6H2O and Mn(NO3)2·6H2O, Co(NO3)2·6H2O and Ni(NO3)2·6H2O respectively, there was no weight loss at beyond 315, 320, and 200 °C which signifies the completion of the formation of CrCo2O4, MnCo2O4, and NiCo2O4. Upon immersing fiber into the precursor solution, the water and Cr(NO3)2·6H2O, Mn(NO3)2·6H2O, Co(NO3)2·6H2O and Ni(NO3)2·6H2O molecules were absorbed onto the hydroxyl-group-rich cotton fiber substrate. With the heat treatment above 520 °C , nitrate salts decomposed in the form CrCo2O4, MnCo2O4, and NiCo2O4 as follow [58], With the increase in calcination temperature, the removal of organic substance was achieved where the remaining few portions of the organic substance change into carbon.
FE-SEM in Figure 2a-d displays tubular morphology of the cotton fibers, samples CrCo2O4, MnCo2O4, and NiCo2O4, respectively, which resembles a biomorphic structure. Figure 3a-c shows SEM images obtained using elemental mapping at chromium, manganese, and nickel energy peaks and shows that the tubular structure is well decorated with the CrCo2O4, MnCo2O4, and NiCo2O4 nanoparticles. Table 2 gives the elemental composition and element distribution, which is obtained via EDX (energy dispersive x-ray spectroscopy).   The type of electrolytes and their molar concentration play a vital role in determining the electrochemical behavior of oxide electrodes [59][60][61]. Therefore, many aqueous electrolytes such as sulfates K2SO4, H2SO4, KNO3, Na2SO4, hydroxyl KOH, NaOH, LiOH, and chlorides KCl, NaCl have been explored to be used in supercapacitors [62][63][64][65][66]. The ultimate performance of the electrode is based on the properties of the electrode material and the intercalation efficiency of the cations [51]. Since KOH electrolyte provides lower electrochemical series resistance with better conductivity as compared to other electrolytes [67], KOH is chosen as an electrolyte in this study for the electrochemical measurement.
Cyclic voltammetry and charge-discharge curves were measured to investigate the electrochemical behavior of MCo2O4 nanoparticles. Figure 4 displays the CV curves for tubular MCo2O4 electrodes measured in the 3M KOH electrolyte. Figure 4a,c, and Figure 4e shows the CV curves measured in the voltage window of 0.0 to 0.6 V and measured at different scan rate from 2 to  The type of electrolytes and their molar concentration play a vital role in determining the electrochemical behavior of oxide electrodes [59][60][61]. Therefore, many aqueous electrolytes such as sulfates K 2 SO 4 , H 2 SO 4 , KNO 3 , Na 2 SO 4 , hydroxyl KOH, NaOH, LiOH, and chlorides KCl, NaCl have been explored to be used in supercapacitors [62][63][64][65][66]. The ultimate performance of the electrode is based on the properties of the electrode material and the intercalation efficiency of the cations [51]. Since KOH electrolyte provides lower electrochemical series resistance with better conductivity as compared to other electrolytes [67], KOH is chosen as an electrolyte in this study for the electrochemical measurement.
Cyclic voltammetry and charge-discharge curves were measured to investigate the electrochemical behavior of MCo 2 O 4 nanoparticles. Figure 4 displays the CV curves for tubular MCo 2 O 4 electrodes measured in the 3M KOH electrolyte. Figure 4a,c, and Figure 4e shows the CV curves measured in the voltage window of 0.0 to 0.6 V and measured at different scan rate from 2 to 300 mV/s. A pair of redox peaks associated with the redox reactions involved in the alkaline electrolyte during the charging and discharging process was observed in all CV plots. The CV curve is asymmetric, which indicates a quasi-reversible redox reaction [68], and the anodic and cathodic peak separation is 0.123 V for 2 mV/s and 0.184 V for 200 mV/s. The presence of anodic and cathodic peaks, indicating the usefulness of the materials as a pseudocapacitor. Typical pseudo-capacitance behavior of MCo 2 O 4 nanostructures arises from the reversible surface or near-surface Faradic reactions for charge storage. The reversible redox reaction involved in the charge-discharge process for MCo 2 O 4 can be described as follows by Equations (5)-(7) [69][70][71]. MO Processes 2018, 6, x FOR PEER REVIEW 7 of 19 300 mV/s. A pair of redox peaks associated with the redox reactions involved in the alkaline electrolyte during the charging and discharging process was observed in all CV plots. The CV curve is asymmetric, which indicates a quasi-reversible redox reaction [68], and the anodic and cathodic peak separation is 0.123 V for 2 mV/s and 0.184 V for 200 mV/s. The presence of anodic and cathodic peaks, indicating the usefulness of the materials as a pseudocapacitor. Typical pseudo-capacitance behavior of MCo2O4 nanostructures arises from the reversible surface or near-surface Faradic reactions for charge storage. The reversible redox reaction involved in the charge-discharge process for MCo2O4 can be described as follows by Equations (5)-(7) [69][70][71].
CrCo2O4 + OH − + H2O  CrOOH + 2CoOOH + e − MnCo2O4 + OH − + H2O  MnOOH + 2CoOOH + e − NiCo2O4 + OH − + H2O  NiOOH + 2CoOOH + e − (7)  Pseudocapacitive characteristics of electrodes are indicated by a non-rectangular form of CV curves. Within the potential range from 0 to 0.6 V, a pair of reversible redox peaks can be observed. With the increase in the scan rate, a small positive shift of the oxidation peak potential and a negative shift of the reduction peak potential was observed, which can be primarily attributed to the influence of the increasing electrochemical polarization as the scan rate scales up. Pairs of reversible redox curve are indicative of pseudocapacitive behavior of the material with redox peaks attributed to M(II)/M(III) redox process [72]. The redox potentials and shape of the CV curves are comparable to those reported for CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 electrodes [24,[73][74][75][76], suggesting that the measured capacitance mainly arises from the redox mechanism. Figure 5a shows the specific capacitance, C sp , as a function of the voltage scan rate of the tubular MCo 2 O 4 electrode. The specific capacitance, C sp , was calculated from the CV plots using the following Equation (8) [77].
where V 1 and V 2 stand for the working potential limits, i stands for the current, m stands for the mass of the electroactive materials, and v is the scan rate in mV/s.
With the increase in the scan rate, a small positive shift of the oxidation peak potential and a negative shift of the reduction peak potential was observed, which can be primarily attributed to the influence of the increasing electrochemical polarization as the scan rate scales up. Pairs of reversible redox curve are indicative of pseudocapacitive behavior of the material with redox peaks attributed to M(II)/M(III) redox process [72]. The redox potentials and shape of the CV curves are comparable to those reported for CrCo2O4, MnCo2O4, and NiCo2O4 electrodes [24,[73][74][75][76], suggesting that the measured capacitance mainly arises from the redox mechanism. Figure 5a shows the specific capacitance, Csp, as a function of the voltage scan rate of the tubular MCo2O4 electrode. The specific capacitance, Csp, was calculated from the CV plots using the following Equation (8) [77].
Where V1 and V2 stand for the working potential limits, i stands for the current, m stands for the mass of the electroactive materials, and v is the scan rate in mV/s. It is evident for this figure that the electrode displays higher Csp up to 407.2 F/g for NiCo2O4 in 3M KOH electrolyte at 2 mV/s, which is higher than the value that is observed for either CrCo2O4 (Csp ~ 403.2 F/g) or MnCo2O4 (Csp ~ 378.1 F/g) electrode, value are given in Table 3. The specific capacitance for higher scan rates (>50 mV/s) remains practically constant because of limited ion movement only at the surface of the electrode material. Hence EDLC becomes a dominant mechanism at higher scan rates. At lower scan rates (<5 mV/s), the majority of active surface are utilized by the ions for charge storage, and hence resulting in the higher specific capacitance. The CV cyclic stability of the electrode was tested for 1000 cycles. Figure 4b,d, and Figure 4f show no significant differences in the CV curves after the 100th, 500th, and 1000th cycle of repetition. The CV It is evident for this figure that the electrode displays higher C sp up to 407.2 F/g for NiCo 2 O 4 in 3M KOH electrolyte at 2 mV/s, which is higher than the value that is observed for either CrCo 2 O 4 (C sp~4 03.2 F/g) or MnCo 2 O 4 (C sp~3 78.1 F/g) electrode, value are given in Table 3. The specific capacitance for higher scan rates (>50 mV/s) remains practically constant because of limited ion movement only at the surface of the electrode material. Hence EDLC becomes a dominant mechanism at higher scan rates. At lower scan rates (<5 mV/s), the majority of active surface are utilized by the ions for charge storage, and hence resulting in the higher specific capacitance. The CV cyclic stability of the electrode was tested for 1000 cycles. Figure 4b,d, and Figure 4f show no significant differences in the CV curves after the 100th, 500th, and 1000th cycle of repetition. The CV curves clearly show that the current response is proportionally increased with the scan rate, indicating an excellent capacitive behavior of the electrode materials. This can be ascribed to facile ion diffusion and large specific surface area of the electrode materials. Furthermore, there is almost no relation between the shape of CV curves and scan rates, which can be associated with the electron conduction and improved mass transportation of electrode material [78]. The total stored charge has a contribution from three components; first is the Faradaic contribution coming from the insertion process of electrolyte ions, second is the faradaic contribution from the charge-transfer process with surface atoms, and third is pseudocapacitance and nonfaradic contribution from the double layer effect [79]. Both pseudocapacitance and double-layer charging are substantial, due to their higher surface area of nanoparticles. The capacitive effects are characterized by analyzing the cyclic voltammetry data at various scan rates according to [80,81], where i, v, a and b, are peak current (A), voltage scan rate (mV/s), and fitting parameters, respectively. The charge storage mechanism is defined based on the value of the constant b, where b = 1 defines capacitive or b = 0.5 defines diffusion-limited charge storage mechanism. Fitting the peak current, i, vs. square root of the scan rate, SQRT (scan rate), v -1/2 , curves, Figure 5b, with Equation (6) Usually, the contribution to the current response at fixed potential comes from surface capacitive effects and diffusion-controlled insertion processes [82,83]. These contributions to the specific capacitance could be separated using the following Equation (10): For which k 1 and k 2 can be determined from the C sp vs. v −1/2 linear plot with slope k 2 and intercept k 1 . k 1 and k 2 are fractions of diffusion and capacitive contribution to the net specific capacitance at a given voltage rate. The C sp was plotted against the slow scan rate up to 20 mV/s, and a regression fit was performed using Equation (10). The obtained k 1 and k 2 values were used to determine the fractional contribution to the net specific capacitance. Figure 5c shows capacitive and diffusive fractional contributions to net specific capacitance for a slow scan rate of up to 20 mV/s. By comparing the lower green area with the total capacitance, we find that capacitive effects contribute by 48%, 54%, and 38% of the total specific capacitance for CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4, respectively. Figure 6a,c, and Figure 6e show the galvanostatic charge-discharge (GCD) plots measured in the voltage window of 0.0 to 0.6 V at different current densities between 0.75 A/g to 30 A/g in 3M KOH. From the observed non-linearity between the potential and time, it is confirmed that the capacitance of the studied materials is not constant over the studied potential ranges. The specific capacitance of electrodes was calculated using the following Equation (11): where C sp , I, ∆V, m, and t are the specific capacitance (F/g), charge-discharge current (A), the potential range (V), and the mass of the electroactive materials, and the discharging time (s), respectively. The GCD curves with a plateau, usually displayed by oxide electrodes, show pseudocapacitive behavior of electrode with respect to their discharging time for all electrolytes. This typical GCD behavior could arise from the electrochemical adsorption-desorption of OHelectrolyte and/or a redox reaction at the interface of electrode/electrolyte [84,85]. It is observed that the discharging time  Table 3. Figure 7a shows the dependence of current density on the specific capacitance of the electrode material. Usually, insufficient Faradic redox reaction is achieved at the high discharge current densities. This leads to increased potential drop due to the resistance of tubular MoCo 2 O 4 electrode resulting in an observed decrease in capacitance with the increased discharge current density. This implies ion penetration is feasible at lower current densities where ions have access to the inner structure, and thus all active area of the electrode. However, at higher current densities, the effective use of the material is limited to only the outer surface of the electrode. The specific capacitance of MCo 2 O 4 electrodes in this study is compared with the literature values at current density 1 A/g, 2 A/g, and 5 A/g and are listed in Table 4. It is evident from Table 4 that electrochemical performance of bio-templated MCo 2 O 4 comparable and, in some cases, outperformed electrodes prepared via other techniques.
where Csp, I, V, m, and t are the specific capacitance (F/g), charge-discharge current (A), the potential range (V), and the mass of the electroactive materials, and the discharging time (s), respectively. The GCD curves with a plateau, usually displayed by oxide electrodes, show pseudocapacitive behavior of electrode with respect to their discharging time for all electrolytes. This typical GCD behavior could arise from the electrochemical adsorption-desorption of OHelectrolyte and/or a redox reaction at the interface of electrode/electrolyte [84,85]. It is observed that the discharging time in biomorphic MCo2O4 is longer for CrCo2O4 in the KOH electrolyte. The specific capacitances of biomorphic CrCo2O4, MnCo2O4, and NiCo2O4 at 1 A/g are 231 F/g, 161 F/g, and 190 F/g in 3M KOH electrolytes, respectively are shown in Table 3. Figure 7a shows the dependence of current density on the specific capacitance of the electrode material. Usually, insufficient Faradic redox reaction is achieved at the high discharge current densities. This leads to increased potential drop due to the resistance of tubular MoCo2O4 electrode resulting in an observed decrease in capacitance with the increased discharge current density. This implies ion penetration is feasible at lower current densities where ions have access to the inner structure, and thus all active area of the electrode. However, at higher current densities, the effective use of the material is limited to only the outer surface of the electrode. The specific capacitance of MCo2O4 electrodes in this study is compared with the literature values at current density 1 A/g, 2 A/g, and 5 A/g and are listed in Table 4. It is evident from Table 4 that electrochemical performance of bio-templated MCo2O4 comparable and, in some cases, outperformed electrodes prepared via other techniques. (e) (f)      Estimation of the electrochemical utilization of the active materials (CrCo 2 O 4 , MnCo 2 O 4, and NiCo 2 O 4 electrode), was evaluated from the fraction of cobalt sites, z. The fraction, z, can be evaluated using Faraday's law as following [94]: where C sp , MW, ∆V and F is the specific capacitance value, the molecular weight, the applied potential window, and the Faraday's constant, respectively. The z value of 1 indicates the complete involvement of electroactive material. i.e., all active metal sites participating in the redox process.  Figure 5b, and a potential window of 0.6 V gives a z value of 0.195, 0.139, and 0.169 respectively. In other words, 20%, 14%, and 17% of the total active material Cr, Mn, and Ni atoms participate in the redox reaction for the charge storage. The observed low value of z suggests that the charge storage in tubular MCo 2 O 4 structure via a redox reaction process occurs mainly at the surface with little bulk interaction due to diffusion of OH − ions into the material. It could be concluded that the charge storage due to the redox process in MCo 2 O 4 mostly occurs only at the redox sites predominantly located on the surface of the particles [71].
Cyclic stability tests were performed to evaluate the practical performance of electrodes as a supercapacitor. The stability test of electrode materials was assessed via galvanostatic CD measurement for 5000 cycles for a current density of 10 A g −1 in 3M KOH and is shown in Figure 6b,d, and Figure 6f. The Coulombic efficiency (η) of the devices was calculated from its charging (T c ) and the discharging (T d ) times from GCD curves following the relation, η = T d /T c × 100, and is plotted in Figure 6b,d,f as a function of cyclic time. The initial η of the device was~100%, which remained practically the same even after 5000 cycles. For the practical applications, the study of cycling performance for electroactive material is very significant parameter. The percentage retention in specific capacitance was calculated using, % retention in specific capacitance = (C # /C 1 )×100 (13) where C # and C 1 are specific capacitance at various cycles and the 1st cycle, respectively. The specific capacitance of the electrode is reduced by 7.68% in CrCo 2 O 4 , reduced by 9.12% in MnCo 2 O 4 , and increased by 0.48% in NiCo 2 O 4 . Figure 5h shows the Ragone plots of as-synthesized MCo 2 O 4 electrodes. The energy densities (E) and power densities (P) of the electrochemical cells are calculated using the following equations [95]: where C, V, and t are the specific capacitance that depends on the mass of the electrodes, the operating voltage of the cell, and discharge time in seconds, respectively. The essential point for high-performance supercapacitors is to obtain a high energy density and meanwhile providing an outstanding power density. It is observed from Figure 7b that the tubular CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 electrode display superior performance over energy density up to 11.1 Wh/kg, 7.8 Wh/kg, and 9.3 Wh/kg with a peak power density up to 7287.34 W/kg, 7195.33 W/kg, and 7186.12 W/kg, respectively are given in Table 3. As supercapacitor is expected to provide higher power and energy density at the same time, hence NiCo 2 O 4 displays an overall better energy density of 9.3 Wh/kg and a power density of 7186.12 W/kg as compared to MnCo 2 O 4 and CrCo 2 O 4 .
Ideally, to develop a higher energy density battery with a given anode, a cathode with high electrochemical interaction potential is desired. This is because the energy density of the device equals the product of the working voltage, which is obtained from the electrochemical potentials different between the cathode and anode and specific capacity of the electrode materials [63]. On the other hand, the theoretical capacity of electrode materials depend on the number of reactive electrons (n) and molar weight (M) of the materials and is expressed as Equation (16) [96], C t = n * F/3.6 * M Here, F is the Faraday constant, n is the number of reactive electrons, and M is the molar weight of materials. Theoretically, the equation predicts that an electrode material having a smaller molecular weight can produce a higher capacity. The molecular weight of CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 is 233.86, 236.80, and 240.55 g/mol, respectively. Thus, in line with the Equation (11), at low current density, among three electrodes studied, the CrCo 2 O 4 displays higher specific capacitance. However, overall superior performance in terms of energy and power density was displayed by the NiCo 2 O 4 electrode. NiCo 2 O 4 is known to possess rich electroactive sites, narrow pore size, and higher electrical conductivity (at least two magnitudes higher) than that of Co 3 O 4 and NiO, which could be the reason for the observed overall better performance of NiCo 2 O 4 [97,98].

Conclusions
In conclusion, biomorphic tubular CrCo 2 O 4 , MnCo 2 O 4 , and NiCo 2 O 4 nanostructures were prepared using cotton by a cost-effective and straightforward bio-template method. The synthesized tubular MCo 2 O 4 display excellent crystallinity, phase purity and display desirable electrochemical properties, which indicate a good chance for the fabrication of high-performance supercapacitor devices. Electrodes constructed using the tubular MCo 2 O 4 demonstrate high specific capacitance, cyclic stability, power, and energy density when evaluated in 3M KOH electrolyte. The study suggests that it is imperative to account for the nature of the electroactive sites and the conductivity of materials when choosing materials from the series of transition metal oxide as the electrode for the supercapacitor application. Furthermore, the superior electrochemical performance of tubular MCo 2 O 4 microstructure owes to the presence of conducting a carbonaceous structure. The highly porous carbonaceous structure can allow electrolyte access throughout the electrode structure. Thus, it produces a large surface area for ion transfer between the electrolyte and the active materials, which leads to achieving ultrafast storage and release of energy.