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Article

Facile Synthesis of Bio-Template Tubular MCo2O4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte

by
Deepa Guragain
1,
Camila Zequine
2,
Ram K Gupta
2 and
Sanjay R Mishra
1,*
1
Department of Physics and Materials Science, The University of Memphis, Memphis, TN 38152, USA
2
Department of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA
*
Author to whom correspondence should be addressed.
Processes 2020, 8(3), 343; https://doi.org/10.3390/pr8030343
Submission received: 17 January 2020 / Revised: 10 March 2020 / Accepted: 10 March 2020 / Published: 16 March 2020
(This article belongs to the Special Issue Electrolysis Processes)
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Versions Notes

Abstract

:
In this project, we present a comparative study of the electrochemical performance for tubular MCo2O4 (M = Cr, Mn, Ni) microstructures prepared using cotton fiber as a bio-template. Crystal structure, surface properties, morphology, and electrochemical properties of MCo2O4 are characterized using X-ray diffraction (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 MCo2O4 structures was evaluated in aqueous 3M KOH electrolytes. The as-obtained templated MCo2O4 microstructures inherit the tubular morphology. The large-surface-area of tubular microstructures leads to a noticeable pseudocapacitive property with the excellent electrochemical performance of NiCo2O4 with specific capacitance value exceeding 407.2 F/g at 2 mV/s scan rate. In addition, a Coulombic efficiency ~100%, and excellent cycling stability with 100% capacitance retention for MCo2O4 was noted even after 5000 cycles. These tubular MCo2O4 microstructure display peak power density is exceeding 7000 W/Kg. The superior performance of the tubular MCo2O4 microstructure electrode is attributed to their high surface area, adequate pore volume distribution, and active carbon matrix, which allows effective redox reaction and diffusion of hydrated ions.

1. 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. A hybrid capacitor is a combination of both EDLCs and PCs; examples are carbon nanotubes, graphene, etc. [8,9]. They display hybrid charge–storage mechanisms and have the ability to deliver higher capacity [10].
The electrolyte ion transport in supercapacitor devices occurs through an ion-transport layer separated from the electrode. The charge storage mechanism follows at the electrode surface during the charging-discharging process [11].
Transition metal oxides (TMOs) with novel nano-architectures and rich in redox reactions are increasingly explored for their application in energy storage devices [12]. Among these, cobalt oxides are highly attractive because of their higher theoretical value for specific capacitance, i.e., 3560 F/g [13]. The nanoarchitecture of these metal-oxides is controlled by the synthesis route, which often requires complex technological strategies, including toxic organic reagents, which might make it difficult for their mass industrial application. Thus, it is highly sought to explore cost-effective facile synthesis strategies and environmentally benign techniques for preparing these electrodes. Furthermore, the ideal electrode should have a high specific surface area for better specific capacitance, controlled porosity for better rate capability as well as specific capacitance, and higher electronic conductivity to improve rate capability and power density of supercapacitor. Nowadays, the bio-templating technique has emerged as a convincing technique for the preparation of oxide supercapacitors [14,15,16,17]. Nature offers rich and diverse bio-templates [18,19,20] like bamboo, lotus pollen grains [21], leaf [22], sorghum straw [23], butterfly wing [24], jute fibers [25], and cotton [12]. Such bio-templates offer elaborate interior and exterior surfaces, and geometries, which make these templates attractive materials to produce multiscale hybrid and hierarchical morphologies.
Numbers of research are published on the study of transition metal oxide-based electrodes such as MnCo2O4 [5,26,27,28] and NiCo2O4 [29,30,31,32,33], Co3O4 [34], and NiFe2O4 [35] for their supercapacitive application. The benefit of transition metal oxides as electrode materials are innumerable, as their multiple oxidation states facilitate multiple redox reactions during electrochemical reaction vis-a-vise offers stable structure. The co-existence of two different cations provides abundant active sites to perform fast reversible faradic redox reaction on the electrode interface; as a result, higher specific capacity, and excellent rate capability are achieved [25,36]. Additionally, the types of bonds between transition metal ions and ligands are dictated by electronegativity and ionization energies [37]; with the former, the structure is dense, while with later the structure is more open. The valence state, ionic radius, electronegativity [38], and the local environment of the cations are affected by the change in Gibbs free energy and electrochemical potential of the electrode. An increase in the electrochemical potential of cathodes is observed with the increase in the number of electrons in d orbitals of transition metal elements. This implies a higher consumption of energy during electron transfer [39]. In mixed transition metal oxides, there is a synergetic effect between metal cations; this produces higher electrical conductivity of single metal oxide where there is low activation energy to transfer electron between metal cations and gives excellent structural stability [40,41].
Here we present a comparative study to understand the electrochemical behavior of MCo2O4 (M = Cr, Mn, and Ni) electrodes prepared via a facile bio-template method. The electronegativity differences among M ions viz. Cr (1.66), Mn (1.55), Ni (1.99), and Co (1.88) could have a substantial effect on the electrochemical performance of the said electrodes. The electronegativity difference determines the structure, covalent vs. ionic, and the electric potential of the electrode for the charge transfer, as discussed above. In the present study, MCo2O4 electrode material is prepared via the bio-template method, where the product assumes the morphology of the microstructure of bio-template and ends up with a carbon matrix. The bio-template method adapted to produce active material inherently fixed in the carbon matrix. The carbon matrix is known to enhance electrode electrochemical performance [42]. The template supported mineralization MCo2O4 at room temperature (RT) produces 3D-hierarchical and porous-MCo2O4 superstructures with tubular-like morphologies. The doped Co3O4 (MCo2O4) is explicitly explored in this study as dopant atoms or vacancies are known to affect the crystal field [43], thus modifying the electronic structure and adjusting the electrochemical potential [44].

2. Experimental

2.1. Synthesis

All the chemicals required for the synthsis, such as Cobalt nitrate hexahydrate; Co(NO3)2.6H2O, Chromium nitrate hexahydrate; Cr(NO3)2.6H2O, Manganese nitrate hexahydrate; Mn(NO3)2.6H2O, and Nickel nitrate hexahydrate; Ni(NO3)2.6H2O were purchased from Sigma-Aldrich, St. Louis, Missouri, USA. The spinel MCo2O4 (M = Cr, Mn, Ni) was synthesized by a facile bio-template method. Quantities of 1.16 g of Co(NO3)2.6H2O and 0.8 gm of Cr(NO3)2.6H2O, 0.55 g of Co(NO3)2.6H2O and 0.17 gm of Mn(NO3)2.6H2O, and 0.86 g of Co(NO3)2.6H2O and 0.43 gm of Ni(NO3)2.6H2O 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 CrCo2O4, MnCo2O4, and NiCo2O4 tubular microstructure.

2.2. Characterization

The x-ray diffraction patterns were obtained via Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Madison, WI, USA) using Cu Kα 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). MCo2O4 (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.

3. Results and Discussion

Figure 1a shows the XRD patterns of the bio-templated CrCo2O4, MnCo2O4, and NiCo2O4 microstructure. The XRD patterns match with the face-centered cubic phase of CrCo2O4, MnCo2O4, and NiCo2O4 (International Centre for Diffraction Data (ICDD) #02-0770). The main peaks at 30.9°, 36.4°, 44.3°, 58.6°, and 64.3° for CrCo2O4, 31.1°, 36.7°, 44.7°, 59.2°, and 65.9° for MnCo2O4, and 31.3°, 36.8°, 44.8°, 59.4°, and 65.2° for NiCo2O4 can be assigned to the (220), (311), (400), (511) and (440) reflections of CrCo2O4, MnCo2O4, and NiCo2O4 respectively [45,46]. The pattern of NiCo2O4 shows a peak at 43.2°, which indicates the formation of NiO cubic phase as also confirmed by TOPAS fitting. The lattice constants obtained using d-spacing for the sample are a = b = c = 0.816 nm, 0.808 nm, 0.807 nm, and for CrCo2O4, MnCo2O4, and NiCo2O4, respectively. The crystallite size of CrCo2O4, MnCo2O4, and NiCo2O4 as calculated using Scherrer’s formula [47] is around 10.57 nm, 14.65 nm, and 19.97 nm for CrCo2O4, MnCo2O4, and NiCo2O4, respectively (Table 1). FTIR spectrum, Figure 1b, further identifies the structure of the bio-templated MCo2O4. The FTIR spectrum displays two distinct bands at 515.7 (ν1) and 637 (ν2) cm−1, which arise from the stretching vibrations of the metal-oxygen bonds [48,49,50]. The ν1 band is characteristic of M-O (M = Cr, Mn, Ni) vibrations in octahedral coordination, and the ν2 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 Co3+-O2− in the octahedral sites, and at 638.6, 639.9, and 641.3 cm−1 indicate vibration of Cr3+-O−, Mn2+-O2−, and Ni2+-O− at tetrahedral sites for CrCo2O4, MnCo2O4, and NiCo2O4, respectively [52]. The presence of vibration bands confirms the development of pure phase spinal CrCo2O4, MnCo2O4, and NiCo2O4 nanostructures.
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 m2/g, 32.2 m2/g, and 18.9 m2/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. 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],
2Co(NO3)2·6H2O + Cr(NO3)2·6H2O → CrCo2O4 + 2O2 + 6NO2 + 18H2O
2Co(NO3)2·6H2O + Mn(NO3)2·6H2O → MnCo2O4 + 2O2 + 6NO2 + 18H2O
2Co(NO3)2·6H2O + Ni(NO3)2·6H2O → NiCo2O4 + 2O2 + 6NO2 + 18H2O
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 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], the anodic and cathodic peak separation are 0.121 V, 0.124 V, and 0.123 V at 2 mV/s and 0.310 V, 0.186 V, and 0.333 V at 300 mV/s for CrCo2O4, MnCo2O4 and NiCo2O4 respectively. 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].
MO + OH− ↔ MOOH + e−
CrCo2O4 + OH− + H2O ↔ CrOOH + 2CoOOH + e−
MnCo2O4 + OH− + H2O ↔ MnOOH + 2CoOOH + e−
NiCo2O4 + OH− + H2O ↔ NiOOH + 2CoOOH + e−
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 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].
C sp = ∫ V 1   V 2 i ∗ V ∗ d V m ∗ v ∗ ( V 2 − V 1 )
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 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],
i = avb
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), gives b values of ~ 0.646, 0.711, and 0.648 for CrCo2O4, MnCo2O4, and NiCo2O4, respectively. This obtained b value for our sample MCo2O4 (M = Cr, Mn, Ni) indicates the diffusive nature of the charge storage mechanism is prominent for NiCo2O4 as compared to the other two.
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):
Csp = k1 + k2 v−1/2
For which k1 and k2 can be determined from the Csp vs. v−1/2 linear plot with slope k2 and intercept k1. k1 and k2 are fractions of diffusion and capacitive contribution to the net specific capacitance at a given voltage rate. The Csp was plotted against the slow scan rate up to 20 mV/s, and a regression fit was performed using Equation (10). The obtained k1 and k2 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 CrCo2O4, MnCo2O4, and NiCo2O4, 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):
C sp = I ∗ t m ∗ Δ V
where Csp, I, ΔV, 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 OH- electrolyte 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.
Estimation of the electrochemical utilization of the active materials (CrCo2O4, MnCo2O4, and NiCo2O4 electrode), was evaluated from the fraction of cobalt sites, z. The fraction, z, can be evaluated using Faraday’s law as following [94]:
z = Csp MW ΔV/F
where Csp, MW, ΔV 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. The molecular weight of Co (84.03 g/mol) and Cr (51.996 g/mol) in CrCo2O4, Co (84.03 g/mol) and Mn (54.93 g/mol) in MnCo2O4, and Co (84.03 g/mol) and Ni (58.69 g/mol) in NiCo2O4 the specific capacitance at a current density of 1 A/g (Csp~ 231 F/g, 161 F/g, and 190 F/g) for CrCo2O4, MnCo2O4, and NiCo2O4, 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 MCo2O4 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 MCo2O4 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 (Tc) and the discharging (Td) times from GCD curves following the relation, η = Td/Tc × 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#/C1)×100
where C# and C1 are specific capacitance at various cycles and the 1st cycle, respectively. The specific capacitance of the electrode is reduced by 7.68% in CrCo2O4, reduced by 9.12% in MnCo2O4, and increased by 0.48% in NiCo2O4.
Figure 5h shows the Ragone plots of as-synthesized MCo2O4 electrodes. The energy densities (E) and power densities (P) of the electrochemical cells are calculated using the following equations [95]:
E = (1/2)CV2
P = E/t
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 CrCo2O4, MnCo2O4, and NiCo2O4 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 NiCo2O4 displays an overall better energy density of 9.3 Wh/kg and a power density of 7186.12 W/kg as compared to MnCo2O4 and CrCo2O4.
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],
Ct = 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 CrCo2O4, MnCo2O4, and NiCo2O4 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 CrCo2O4 displays higher specific capacitance. However, overall superior performance in terms of energy and power density was displayed by the NiCo2O4 electrode. NiCo2O4 is known to possess rich electroactive sites, narrow pore size, and higher electrical conductivity (at least two magnitudes higher) than that of Co3O4 and NiO, which could be the reason for the observed overall better performance of NiCo2O4 [97,98].

4. Conclusions

In conclusion, biomorphic tubular CrCo2O4, MnCo2O4, and NiCo2O4 nanostructures were prepared using cotton by a cost-effective and straightforward bio-template method. The synthesized tubular MCo2O4 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 MCo2O4 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 MCo2O4 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.

Author Contributions

Conceptualization, D.G.; methodology, D.G., and C.Z.; software, D.G. and C.Z.; validation, S.R.M., R.K.G., and D.G.; formal analysis, D.G.; investigation, S.R.M. and R.K.G.; resources S.R.M.; data curation, D.G., C.Z., and R.K.G.; writing—original draft preparation, D.G.; writing—review and editing, S.R.M.; visualization, S.R.M. and D.G.; supervision, S.R.M. and R.K.G.; project administration, S.R.M.; funding acquisition, S.R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This is supported by the grants from FIT-DRONES and Biologistics at the University of Memphis, Memphis; TN. Dr. Ram K. Gupta expresses his sincere acknowledgment of the Polymer Chemistry Initiative at Pittsburg State University for providing financial and research support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) x-ray diffraction pattern, (b) FTIR, (c) adsorption-desorption curve and inset pore volume distribution, and (d) thermogravimetric curve of tubular MCo2O4 (M = Cr, Mn, Ni) structures.
Figure 1. (a) x-ray diffraction pattern, (b) FTIR, (c) adsorption-desorption curve and inset pore volume distribution, and (d) thermogravimetric curve of tubular MCo2O4 (M = Cr, Mn, Ni) structures.
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Figure 2. SEM images of (a) cotton fiber, (b), (c), and (d) bio-templated tubular MCo2O4 (M = Cr, Mn, Ni) structures.
Figure 2. SEM images of (a) cotton fiber, (b), (c), and (d) bio-templated tubular MCo2O4 (M = Cr, Mn, Ni) structures.
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Figure 3. (a), (b), and (c) shows EDX mapping of tubular MCo2O4 (M = Cr, Mn, Ni) structures, respectively.
Figure 3. (a), (b), and (c) shows EDX mapping of tubular MCo2O4 (M = Cr, Mn, Ni) structures, respectively.
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Figure 4. (a,c,e) show cyclic voltammetry curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode obtained in the scan range of 5 mV/s to 300 mV/s measured in 3M KOH electrolyte. (b,d,f) show cyclic stability curves measured up to 1000 cycles in 3M KOH electrolyte at scan rate of 40 mV/s.
Figure 4. (a,c,e) show cyclic voltammetry curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode obtained in the scan range of 5 mV/s to 300 mV/s measured in 3M KOH electrolyte. (b,d,f) show cyclic stability curves measured up to 1000 cycles in 3M KOH electrolyte at scan rate of 40 mV/s.
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Figure 5. (a) Specific capacitance vs. scan rate, (b) and peak current vs. (scan rate)1/2, and (c) diffusion and capacitive contribution to the specific capacitance.
Figure 5. (a) Specific capacitance vs. scan rate, (b) and peak current vs. (scan rate)1/2, and (c) diffusion and capacitive contribution to the specific capacitance.
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Figure 6. (a,c,e) show charge-discharge (CD) curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode measured in the current density window of 1 to 30 A/g in 3M KOH electrolyte, where red color CD curve is for 1A/g, blue color CD curve is for 1.5A/g, orange color CD curve is for 2A/g and continuously time is decreasing with increasing current density. (b,d,f) show cyclic stability (black color) and coulombic efficiency (blue color) tested at 10 A/g current density for 5000 cycles in 3M KOH electrolytes of MCo2O4 (M = Cr, Mn, Ni).
Figure 6. (a,c,e) show charge-discharge (CD) curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode measured in the current density window of 1 to 30 A/g in 3M KOH electrolyte, where red color CD curve is for 1A/g, blue color CD curve is for 1.5A/g, orange color CD curve is for 2A/g and continuously time is decreasing with increasing current density. (b,d,f) show cyclic stability (black color) and coulombic efficiency (blue color) tested at 10 A/g current density for 5000 cycles in 3M KOH electrolytes of MCo2O4 (M = Cr, Mn, Ni).
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Figure 7. (a) Comparison of specific capacitance as a function of current density and (b) Ragone plot of power density vs. energy density.
Figure 7. (a) Comparison of specific capacitance as a function of current density and (b) Ragone plot of power density vs. energy density.
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Table
Table 1. Crystallite size and physical properties of MCo2O4 (M = Cr, Mn, Ni) determined using XRD, the Barrette–Joyner–Halenda (BJH) method, and Brunauer–Emmett–Teller (BET) surface area analyzer.
Table 1. Crystallite size and physical properties of MCo2O4 (M = Cr, Mn, Ni) determined using XRD, the Barrette–Joyner–Halenda (BJH) method, and Brunauer–Emmett–Teller (BET) surface area analyzer.
SampleBET Surface Area(m2/g)BJH Surface Area (m2/g)BJH Avg. Pore Radius (nm)BJH Avg. Pore Volume (cc/g)Crystallite Size (nm)
CrCo2O434.446.91.1350.10610.57
MnCo2O432.247.70.8390.07114.65
NiCo2O418.931.81.1290.03919.97
Table
Table 2. Elemental composition in wt % for MCo2O4 (M = Cr, Mn, Ni) obtained using energy dispersive X-Ray analysis (EDX). The elemental composition is approximately determined using EDX. Ideally, EDX can prove which elements are abundant in the particles, but not obtain the exact chemical composition.
Table 2. Elemental composition in wt % for MCo2O4 (M = Cr, Mn, Ni) obtained using energy dispersive X-Ray analysis (EDX). The elemental composition is approximately determined using EDX. Ideally, EDX can prove which elements are abundant in the particles, but not obtain the exact chemical composition.
CoCOCrMnNi
CrCo2O419.549.423.27.8
MnCo2O433.134.623.5 8.8
NiCo2O427.242.419.3 11.1
Table
Table 3. Data of specific capacitance, energy density, and power density for MCo2O4 (M = Cr, Mn, Ni) obtained from cyclic voltammetry and charge-discharge curves.
Table 3. Data of specific capacitance, energy density, and power density for MCo2O4 (M = Cr, Mn, Ni) obtained from cyclic voltammetry and charge-discharge curves.
CrCo2O4MnCo2O4NiCo2O4
Specific Capacitance at 2mv/s403.2 F/g378.1 F/g407.2 F/g
Specific Capacitance at 1A/g231 F/g161 F/g190 F/g
Energy density11.1 Wh/Kg7.8 Wh/Kg9.3 Wh/Kg
Power density7287.34 W/Kg7195.33 W/Kg7186.12 W/Kg
Table
Table 4. Comparison of electrochemical performance of MCo2O4 (M = Cr, Mn, Ni) as available from the literature.
Table 4. Comparison of electrochemical performance of MCo2O4 (M = Cr, Mn, Ni) as available from the literature.
Electrode MaterialElectrolyteSpecific CapacitanceEnergy DensityPower DensityCyclic Performance (retention)Ref.
MnCo2O4 nanofiber 108 F/g at 10 A/g54 Wh/Kg9851 W/Kg85 % after 3000 cycles[5]
Nanorods MnCo2O4 1M KOH349.8 F/g at 1 A/g35.4 Wh/Kg225 W/Kg92.7% after 50 cycles[86]
Nanoneedles MnCo2O4 6M KOH1535 F/g at 1 A/g35.4 Wh/Kg225 W/Kg94.3% after 12000 cycles[87]
Nanorods MnCo2O4 2M KOH845.6 F/g at 1 A/g35.4 Wh/Kg225 W/Kg90.2% after 2000 cycles[24]
Nanorods MnCo2O4 1M KOH308.3 F/g at 1 A/g55.5 Wh/Kg5400 W/Kg88.76% after 2000 cycles[88]
MnCo2O4 nanowires @MnO2-2262 F/g at 1 A/g85.7 Wh/Kg800 W/Kg-[89]
Nanorods MnCo2O4 718 F/g at 0.5 A/g--84 % after 1000 cycles[90]
NiCo2O4 nanorods2M KOH565 F/g at 1 A/g--77.6% after 1000 cycles[69]
RGO decorated nanorods bundle NiCo2O46M KOH1278F/g at 1 A/g--95% after 1000 cycles[84]
Nanorods assemble NiCo2O42M KOH764 F/g at 2 A/g--101.7% after 1500 cycles[91]
Nanorods NiCo2O42M KOH823 F/g at 0.823 A/g28.51 Wh/Kg-101.7% after 1500 cycles[2]
GO/ Nanorods NiCo2O41M KOH709.7 F/g at 1 A/g28 Wh/Kg8000 W/Kg94.3% after 5000 cycles[92]
Nanorods NiCo2O4 2M KOH600 F/g at 5 A/g--80% after 1500 cycles[70]
Nanorods NiCo2O4 @PANI1M H2So4901 F/g at 5 A/g--91% after 3000 cycles[93]
Templated CrCo2O4, MnCo2O4, and NiCo2O4 microstructure3M
KOH		403.4, 378, and 407.2 F/g at 2mV/s and 231, 161, and 190 F/g at 1 A/g, respectively11.1, 7.8, and 9.3 Wh/kg, respectively7287.3, 7195.3, and 7186.1 W/Kg, respectively92% after 5000 cycles
91% after 5000 cycles
and 100% after 5000 cycles,
respectively. 		[This work]

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Guragain, D.; Zequine, C.; Gupta, R.K.; Mishra, S.R. Facile Synthesis of Bio-Template Tubular MCo2O4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte. Processes 2020, 8, 343. https://doi.org/10.3390/pr8030343

AMA Style

Guragain D, Zequine C, Gupta RK, Mishra SR. Facile Synthesis of Bio-Template Tubular MCo2O4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte. Processes. 2020; 8(3):343. https://doi.org/10.3390/pr8030343

Chicago/Turabian Style

Guragain, Deepa, Camila Zequine, Ram K Gupta, and Sanjay R Mishra. 2020. "Facile Synthesis of Bio-Template Tubular MCo2O4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte" Processes 8, no. 3: 343. https://doi.org/10.3390/pr8030343

APA Style

Guragain, D., Zequine, C., Gupta, R. K., & Mishra, S. R. (2020). Facile Synthesis of Bio-Template Tubular MCo2O4 (M = Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte. Processes, 8(3), 343. https://doi.org/10.3390/pr8030343

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