Comprehensive characterization and electrochemical performance of Fe-doped Co3O4 nanoparticles for energy storage applications

This study investigates the structural, spectroscopic, and electrochemical properties of Fe-doped cobalt oxide nanoparticles (Fe-doped Co3O4 Nps). X-ray diffraction (XRD) analysis reveals that the Fe-doped samples have a spinel cubic Co3O4 structure with peaks corresponding to (220), (311), (400), (511), and (400) reflection planes. Fourier-transform infrared (FTIR) analysis shows that the major peaks correspond to Co2+ and Co3+ vibrations in the spinel Co3O4 crystal structure, and their positions shift with the increase in Fe doping concentration. X-ray photoelectron spectroscopy (XPS) studies confirm the presence of Co2+ and Co3+ in the Co 2p spectrum and identify Fe3+ and Fe2+ in the Fe 2p spectrum. Scanning electron microscopy (SEM) reveals the surface morphology of the Fe-doped Co3O4 Nps, showing hexagonal/granular structures with varying pore sizes. High-resolution transmission electron microscopy (HR-TEM) analysis confirms the nanocrystalline nature of the Fe-doped Co3O4 Nps. Energy-dispersive X-ray spectroscopy (EDAX) elemental analysis confirms the presence of Co, O, and Fe in the doped samples. Fe-doped Co3O4 nanoparticles were characterized through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopic analysis (EIS). The CV curves displayed consistent electrochemical behavior with observed redox peaks, confirming pseudocapacitive nature. Increasing scan rates led to higher current responses and minor peak position shifts. The Fe-doped Co3O4 nanoparticles exhibit a specific capacity of 253 C/g at a current density of 1.5 A/g, and they maintain 95% of their initial specific capacity after undergoing 1500 cycles. Specific capacity increased with higher Fe doping concentrations, attributed to electron injection and ion diffusion enhancement. GCD profiles showed nonlinear plateau regions, indicating pseudocapacitive behavior, with longer discharge curves for Fe-doped samples, demonstrating superior specific capacity. Electrochemical impedance spectroscopy revealed disrupted ion transport paths. Cyclic stability tests showed good capacity retention for all samples, with the 7 wt% Fe-doped Co3O4 sample exhibiting the highest capacity and cyclic stability, making it suitable for pseudo-capacitor applications.


Introduction
The increasing popularity of portable electronic devices such as mobile phones, tablets, power banks, earphones, and digital cameras, as well as electric cars, has attracted significant attention from researchers aiming to enhance the performance of long-lasting energy storage devices.Previous studies primarily focused on batteries and capacitors, which are the most common types of energy storage used in these gadgets.Batteries are more suitable for applications that require moderate power over an extended period, while capacitors are utilized for high-power demands in a short time frame.However, the majority of applications necessitate both high energy and high power for prolonged operation, which neither batteries nor capacitors can adequately fulfill simultaneously.As a result, researchers have turned their attention to a new type of energy storage device called supercapacitors or ultracapacitors.Supercapacitors are renowned for their high power density, rapid charging and discharging capabilities, and extended lifespan with large capacities.They can be broadly classified into pseudocapacitors (faradaic) and electric double-layer capacitors (non-faradaic) [1].Electric double-layer capacitors (EDLC) possess low energy density, excellent cyclic stability, and considerable specific capacitance.On the other hand, pseudocapacitors exhibit high energy density, high specific capacitance, and limited rate capabilities [2].The higher energy density of pseudocapacitors compared to EDLC is attributed to Faradaic reactions responsible for the charge transfer between the electrode and electrolyte [3].Pseudocapacitors commonly employ transition metal oxides, conducting polymers, and metaldoped carbon as electrode materials.Among various metal oxides such as RuO 2 , Co 3 O 4 , TiO 2 , and MnO 2 , cobalt oxide has attracted significant attention due to its high theoretical specific capacity of approximately 3560 C/g, abundance in nature, and eco-friendliness [4].Extensive research has been conducted on cobalt oxide electrode materials using various techniques to achieve its high theoretical specific capacitance, including nanostructure engineering, 2D and 3D templates, nanorod and nanosphere structures, morphological modifications, and doping.For instance, the hydrothermal synthesis of Co 3 O 4 nanorods exhibited a specific capacitance of 1081 F/g at 0.1 A/g [5], while solvothermal synthesis of porous Co 3 O 4 nano needle-like structures showed a specific capacitance of 111 F/g at a scan rate of 15 mV/s [6].Furthermore, hydrothermal fabrication of Co 3 O 4 nano sheets-RGO resulted in a specific capacitance of 187 F/g at a current density of 1.2 Ag −1 [7], and green synthesis of mesoporous Co 3 O 4 cubes demonstrated a specific capacity of 160 C/g at 2.0 A/g [8].The electrochemical performance of cobalt oxide materials is strongly influenced by factors such as electrical conductivity of the electrode, surface area of the electrode, active sites on the particles, and crystal defects.Therefore, in order to enhance the electrochemical performance of Co 3 O 4 , we attempted to improve its electrical conductivity through doping.Given the similarity in ionic radius between Fe (0.64 Å) and Co (0.61 Å), as well as their comparable oxidation states [9], Fe was selected as the dopant to incorporate into the lattice structure of Co 3 O 4 nanoparticles.
In this study, we employed a cost-effective and straightforward coprecipitation method to synthesize Fe-doped Co 3 O 4 nanoparticles (NPs).All the prepared samples were characterized by structural, morphological, optical properties and impedance spectroscopy, respectively.The electrochemical properties of Fe-doped Co 3 O 4 NPs with concentrations of 3wt%, 5wt%, and 7wt% were investigated using cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy.The obtained results were then discussed.

Materials used
The precursor materials, including cobalt nitrate hexahydrate [Co(NO)    times with deionized water and ethanol.The doped sample was collected, placed in a hot air oven, and dried at 80 °C for 2 h.The obtained samples were subsequently calcined at 600 °C for 3 h using a muffle furnace.The resulting samples were collected and ground in a mortar.The 3 wt% Fe-doped Co 3 O 4 sample obtained from this procedure was labeled as Co 3 O 4 :3%Fe.The same procedure was followed to prepare 5 wt% and 7 wt% Fe-doped Co 3 O 4 nanoparticles, which were named Co 3 O 4 :5%Fe and Co 3 O 4 :7%Fe, respectively.

Characterization technique
The X-ray diffraction analysis of Fe-doped Co 3 O 4 nanoparticles (NPs) was performed using a Bruker Axs D8 Advance diffractometer with CuKα radiation as the X-ray source.Fourier-transform infrared (FTIR) transmission studies were conducted using a Nicolet 5DX FTIR spectrometer.The structural, morphological, and elemental analysis of the samples were carried out using field emission-scanning electron microscopy (FESEM) with a GEMINI SUPRA 55VP Zeiss instrument.The elemental composition and oxidation states of the elements were investigated using an X-ray photoelectron spectrometer, specifically the PHI-VERSA PROBE III (k-alpha) from Thermo Fischer Scientific, operating at 15 kV with an Al target under ultra-high vacuum conditions.Transmission electron microscopy (TEM) studies of the Fe-doped Co 3 O 4 samples were performed using HRTEM on a JOEL/JEM 2100 instrument.Electrochemical studies were conducted using a three-electrode cell connected to an electrochemical workstation, specifically a potentiostat-PGSTAT 204-Metrohm Autolab.

Preparation and fabrication of working electrode
A few pieces of Ni foam (sized 1 cm × 2 cm) were taken and washed with deionized water.They were then rinsed with 1 M nitric acid (HNO 3 ) followed by deionized water to remove oxides.Subsequently, the Ni foam pieces were soaked in 2 M hydrochloric acid to further eliminate any

Electrochemical cell setup
The electrochemical analysis of Fe-doped Co 3 O 4 was conducted using a three-electrode cell configuration.The cell consisted of an Ag/AgCl electrode as the reference electrode, the Fe-doped Co 3 O 4 -coated foam as the working electrode, and a platinum coil as the counter electrode.All the electrodes were immersed in a cell containing a 1.0 M KOH solution.

X-ray diffraction analysis
The X-ray diffraction analysis (XRD) was performed on the powder samples to determine the structure, phase purity, and crystallite size of the synthesized Fe-doped cobalt oxide nanoparticles (Fe-doped Co 3 O 4 NPs).The XRD patterns of the Fe-doped Co 3 O 4 samples before cycling were obtained by scanning angles (2θ) ranging from 20° to 80°, as shown in Fig. 1.All the Fe-doped samples exhibited similar peaks, corresponding to the (220), (311), (400), (511), and (400) reflection planes.These diffraction peaks matched well with the standard crystallographic XRD pattern of the spinel cubic Co 3 O 4 structure [JCPDS no: 042:1467].Notably, for the preferential peak corresponding to the (311) reflection plane, it was observed that the peaks shifted towards lower angles with increasing Fe doping concentration.This shift may be attributed to a slight lattice distortion caused by the presence of Fe atoms, which have a slightly larger radius than Co [10].As Fig. 1 illustrates, after cycling the sample, it becomes evident that the intensity of the peak decreases as the concentration of Fe doping increases, while there is no shift in the peak's position.This suggests that Fe atoms are successfully incorporated into the cobalt oxide crystal structure.The average crystallite size of the Fe-doped Co 3 O 4 NPs was calculated using Scherrer's formula, as given in Eq. 1.
(1) The crystal size of the Fe-doped samples, Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe, corresponding to the well-diffracted plane (311), was calculated using the Scherrer's formula.In the formula, K represents the Scherrer's constant (0.9), λ is the wavelength of the X-ray source (λ = 1.54 Å), and β denotes the full width at half maximum (FWHM).Applying Eq. ( 1), the crystal sizes of the Fe-doped samples were determined as follows: Co 3 O 4 :3%Fe-27 nm, Co 3 O 4 :5%Fe-38 nm, and Co 3 O 4 :7%Fe-38 nm.It was observed that the crystal size slightly increased with the higher Fe doping concentration, possibly due to the larger Fe 3+ ions (with an ionic radius of 6.4 nm) replacing Co 3+ ions (with an ionic radius of 6.2 nm) in the lattice [10].

FTIR spectral analysis
The FTIR analysis of Fe-doped Co 3 O 4 Nps was conducted within the wavenumber range of 400 to 4000 cm −1 , as shown in Fig. 2. All the Fe-doped Co 3 O 4 Nps exhibited two prominent peaks at 565 cm −1 and 661 cm −1 .These peaks correspond to the vibrations of Co 2+ ions in the octahedral sites and Co 3+ ions in the tetrahedral sites of the spinel Co 3 O 4 crystal structure [11].Upon closer examination, it was observed that these major peaks shifted to lower wavenumbers as the Fe doping concentration in the Co 3 O 4 nanostructure increased.This shift could be attributed to the alteration of surface defects caused by Fe doping.Additionally, the peak at 1036 cm −1 was associated with the C-O band [12], while the broad absorption band at 3435 cm −1 was attributed to the O-H mode of vibrations.This wide absorption band arises from the presence of water molecules that are either surface-adsorbed or trapped within the samples [13].

X-ray photoelectron spectroscopy studies (XPS)
The XPS analyses were conducted on 5 wt% Fe-doped Co 3 O 4 Nps to determine the valence states of Co, O, and Fe. Figure 3  3+ and Co 2+ which is consistent with the previous reports [9,14].Co 2p 3/2 and Co 2p 1/2 with 15.20 eV energy separation corresponds to Co3p [10].In addition, two satellite peaks are centered at 787.9 eV and 804 eV, indicating the presence of Co2p [15,16].Figure 3(c) displays the O1s XPS spectrum of Fe-doped Co 3 O 4 Nps, revealing the deconvolution of the major peak into two components.The peak at 531.73 eV corresponds to oxygen from hydroxyl groups or water, while the peak at 530.37 eV corresponds to O 2− in the Co 3 O 4 lattice [15].Figure 3(d) depicts the Fe2p XPS spectrum of Fe-doped Co 3 O 4 Nps, which consists of spin-orbit doublets and a satellite peak.The peaks observed at 713.69 and 717.09 eV are attributed to Fe 3+ , while the peak at 724.57eV is assigned to Fe 2+ [16].Specifically, the peaks at 711.2 and 724 eV are assigned to Fe2p [17,18], while the peaks at 713.5 and 725.9 eV are ascribed to Fe3p [19,20].Fe 2p 3/2 and Fe2p 1/2 with the energy separation of 12.9 eV corresponds to Fe3p [21][22][23].These XPS findings align with the EDAX mapping results, confirming the successful incorporation of Fe into the Co 3 O 4 lattices.

Surface morphological analysis
The surface morphology of Fe-doped Co 3 O 4 Nps before cycling was examined using scanning electron microscopy (SEM), and the corresponding SEM images are presented in Fig. 4. In Fig. 4(a), the surface morphology of the 3 wt% Fe-doped Co 3 O 4 Nps reveals the formation of uniformly distributed hexagonal/granular structures with numerous pores throughout the surface.The average particle size, determined through image analysis using ImageJ software, was found to

EDAX-elemental analysis
The elemental composition of Fe-doped Co 3 O 4 Nps was determined using energy-dispersive X-ray spectroscopy (EDAX), and the results are presented in Fig. 6.The EDAX analysis revealed the presence of characteristic peaks corresponding to Co, O, and Fe elements in all three Fe-doped Co 3 O 4 samples, without any additional peaks.This confirms the purity of the Fe-doped samples.The atomic weight percentages of Co, O, and Fe are indicated in the insert of each EDAX profile, providing quantitative information about the elemental composition of the samples.

Electrochemical studies
The electrochemical activities of Fe-doped Co 3 O 4 Nps with varying concentrations of Fe (3 wt%, 5 wt%, and 7 wt%) were assessed in a 1.0 M KOH solution.The experiments were conducted using a three-electrode setup, consisting of an Ag/AgCl reference electrode, the prepared samples coated on Ni foam as the working electrode, and a platinum wire coil as the counter electrode.To comprehensively evaluate the electrochemical performance of the as-prepared Fe-doped Co 3 O 4 Nps, cyclic voltammetry (CV) studies, galvanostatic charge-discharge (GCD) studies, and electrochemical impedance spectroscopy (EIS) studies were performed.

Cyclic-voltammetry analysis
The Fig. 7 presents the cyclic voltammetry (CV) curves of Fe-doped Co 3 O 4 Nps with varying Fe concentrations (3 wt%, 5 wt%, and 7 wt%) at different scan rates (100 mV/s, 50 mV/s, 20 mV/s, 10 mV/s, and 5 mV/s).The shape of the curves for all Fe-doped Co 3 O 4 samples is similar, indicating consistent electrochemical behavior.Notably, redox peaks are observed in all Fe-doped cobalt oxide samples, confirming the pseudocapacitive nature of the electrode material.The anodic peaks are observed in the voltage range of 0.35 V to 0.48 V, while the cathodic peaks appear in the range of 0.24 V to 0.16 V.As the scan rate increases, the current response also increases, accompanied by a slight shift in the positions of the cathodic and anodic peaks.This phenomenon can be attributed to structural changes in the material or voltage drop resulting from variations in the electrode-electrolyte resistance [24,25].The increase in electrode peak current with increasing scan rate indicates that the doped Co 3 O 4 samples exhibit excellent rate capabilities, likely due to rapid quasi-reversible faradic mechanisms [26].Furthermore, the curves demonstrate consistent behavior at different scan rates, suggesting good reversibility of the electrode material.The specific capacitance of the Fe-doped Co 3 O 4 Nps can be calculated using Eq. 2, providing a measure of their electrochemical performance.
where, SC denotes the specific capacity of the prepared active material, ϑ denotes the applied scan rate, m represents the active mass of the electrode material, V 1 − V 2 is the range of the potential window and ∫ IVdV denotes the area under the CV curve.The specific capacity of Fe-doped Co 3 O 4 samples measured at different scan rates are listed in the following Table 1.

Effect of Fe doping concentration on specific capacity
The specific capacity values of Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe were determined at a scan rate of 5 mV/s, IV dV yielding values of 145 C/g, 161 C/g, and 168 C/g, respectively.These results indicate a clear trend: as the concentration of Fe doping increases, the specific capacity of the Co 3 O 4 nanoparticles also increases.This trend suggests that the introduction of Fe into the Co 3 O 4 nanostructures has a positive impact on their capacitive behavior.
Mechanism behind the increase in specific capacity increase in specific capacity with higher Fe doping concentration is attributed to the injection of electrons and charge compensating ions (cations) into the nanostructures.This injection of charge carriers is likely a result of the Fe doping process, which introduces additional electron carriers into the material.Furthermore, the larger ionic radius of Fe 3+ compared to Co 3+ may lead to a local lattice distortion in the Co 3 O 4 structure.This distortion can facilitate the diffusion of ions within the material, potentially increasing its capacitive performance [27][28][29].

Scan rate dependence of specific capacity
The variation of specific capacity with different scan rates is also discussed.It is observed that in all three samples (Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe), the specific capacity decreases linearly as the current density increases at higher scan rates.This behavior is a common characteristic in electrochemical energy storage systems, where higher scan rates may limit the diffusion of ions within the electrode material, resulting in reduced capacity.
Optimal specific capacity conditions Figure 8 graphically depicts the fluctuation in specific capacitance across distinct scan rates for Fe-doped samples.The maximum specific capacity of 168 C/g is achieved in the Co 3 O 4 :7%Fe doped sample at a scan rate of 5 mV/s.This result suggests that the 7% Fe doping concentration, in combination with the chosen scan rate, provides the optimal conditions for achieving the highest specific capacity.The longer diffusion time for ions at this scan rate allows for more efficient utilization of the extensive surface area of the electrode, resulting in higher capacity.

Galvanostatic charge-discharge test (GCD)
To further investigate the electrochemical behavior of Fedoped Co 3 O 4 Nps, galvanostatic charge-discharge (GCD) studies were conducted at various current densities of 9 A/g, 6 A/g, 3 A/g, and 1.5 A/g. Figure 9

Electrochemical Impedance spectroscopic Analysis (EIS)
Electrochemical impedance spectroscopy (EIS) was used to study the impedance and capacitive behavior of the prepared electrode materials.The Nyquist plots of Fe-doped Co 3 O 4 Nps are presented in Fig. 10a-b, which exhibit a depressed semi-circle in the high-frequency region and a straight sloped line (with an inclination of almost 45°) in the low-frequency region, corresponding to the Warburg element.The Warburg element is due to disruptions in the ionic diffusion path and ion transport phenomena [40][41][42].Figure 10(b) shows the Nyquist plot in the high-frequency region, and the value of R s (solution resistance) for 3 wt%, 5 wt%, and 7 wt% Fedoped Co 3 O 4 Nps is determined to be 1.02 Ω, 1.19 Ω, and 1.5 Ω, respectively, indicating that the 3 wt% Fe-doped Co 3 O 4 electrode has the lowest resistance compared to the others.

Cyclic stability
The capacity retention and cyclic stability of Fe-doped Co 3 O 4 samples were evaluated in a 1 M KOH solution by performing 1500 continuous CV cycles at a scan rate of 100 mV/s, as shown in Fig. 11.All the samples exhibited good capacity retention throughout the 1500 cycles.In the case of the 3 wt% Fe-doped Co 3 O 4 sample, the specific capacity initially dropped rapidly but then increased.The capacity retention remained at around 90% between cycles 200 and 1500, indicating the activation of the electrode after a certain number of cycles [43].From the figure, it can be observed that the capacity retention of Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe samples after 1500 cycles was 95%, 59%, and 67%, respectively.Among the three Fedoped Co 3 O 4 Nps, the 3 wt% Fe-doped Co 3 O 4 exhibited the highest capacity, indicating its suitability for long-term cyclic stability in pseudo capacitor applications.

Conclusion
The facile coprecipitation method was employed to synthesize iron-doped cobalt oxide nanoparticles.X-ray diffraction (XRD) analysis revealed that the Fe-doped samples exhibited peaks corresponding to the spinel cubic Co 3 O 4 structure, indicating the successful incorporation of Fe atoms within the crystal structure.The average crystallite size of the Fe-doped Co 3 O 4 Nps was found to increase slightly with the increase in Fe doping concentration, likely due to the larger radius of Fe atoms replacing Co atoms in the lattice.Fourier-transform infrared spectroscopy (FTIR) analysis showed major peaks corresponding to Co 2+ and Co 3+ vibrations in the octahedral and tetrahedral sites of the Co 3 O 4 crystal structure, respectively.
The peak positions shifted to lower wavenumbers with increasing Fe doping concentration, suggesting changes in surface defects caused by Fe doping.The presence of C-O and O-H bands indicated the presence of surfaceadsorbed or trapped water molecules.X-ray photoelectron spectroscopy (XPS) analysis provided information about the valence states of Co, O, and Fe.The Co2p XPS profile showed peaks corresponding to Co 2+ and Co 3+ states, while the O1s spectrum exhibited peaks attributed to oxygen in hydroxyl groups or water and O 2− in the Co 3 O 4 lattice.The Fe2p spectrum showed peaks corresponding to Fe 3+ and Fe 2+ states, indicating the successful incorporation of Fe into the Co 3 O 4 lattice.Scanning electron microscopy (SEM) images revealed the surface morphology of the Fe-doped Co 3 O 4 Nps, showing hexagonal/granular and nanoflakes structures with varying degrees of agglomeration and porosity.High-resolution transmission electron microscopy (HR-TEM) analysis confirmed the nanocrystalline nature of the Fe-doped Nps, with well-separated particles and defined boundaries.Energy-dispersive X-ray spectroscopy (EDAX) elemental analysis confirmed the purity of the Fe-doped samples, showing peaks corresponding to Co, O, and Fe elements without any additional peaks.Electrochemical studies were conducted to evaluate the electrochemical performance of the Fe-doped Co 3 O 4 Nps.Cyclic voltammetry (CV) analysis showed redox peaks indicative of pseudo-capacitive behavior.The specific capacity of the Fe-doped samples increased with the increase in Fe doping concentration, suggesting enhanced charge storage capabilities.Galvanostatic charge-discharge (GCD) tests further confirmed the pseudo-capacitive behavior, and the Fe-doped samples exhibited longer discharge curves, indicating better specific capacity behavior.The specific capacity values increased with increasing Fe doping concentration and current density.
Overall, the Fe-doped Co 3 O 4 nanoparticles demonstrated promising structural, morphological, and electrochemical properties, making them potential candidates for energy storage applications.Further optimization and characterization can be done to explore their full potential and improve their performance.

Fig. 5 a
Fig. 5 a TEM images (b) SAED pattern of of 7 wt% Fe doped Co 3 O 4 sample

Fig. 7
Fig. 7 CV profile of (a) 3 wt% Fe doped Co 3 O 4 sample, (b) 5 wt% Fe doped Co 3 O 4 sample and (c) 7 wt% Fe doped Co 3 O 4 sample (a) illustrates the wide XPS survey spectrum of 7% Fe-doped Co 3 O 4 Nps, while Fig. 3(b) presents the Co2p XPS profile of the Fe-doped sample.The peaks observed at 780.18 and 795.34 eV correspond to Co 2p 3/2 , whereas the peaks at 781.35 and 796.77eV correspond to Co 2p 1/2 .The energy difference between the prominent peaks at 780.20 eV and 795.60 eV is 15.40 eV, indicating the presence of Co

Fig. 8
Fig. 8 Variation of SC with respect to different scan rate of (a) 3 wt% Fe doped Co 3 O 4 sample, (b) 5 wt% Fe doped Co 3 O 4 sample and (c) 7 wt% Fe doped Co 3 O 4 sample

Fig. 9
Fig. 9 GCD profile of (a) 3 wt% Fe doped Co 3 O 4 sample, (b) 5 wt% Fe doped Co 3 O 4 sample and (c) 7 wt% Fe doped Co 3 O 4 sample illustrates the GCD profiles of Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe samples.Notably, all the GCD profiles of the doped samples exhibit a nonlinear plateau region, which confirms the pseudo-capacitive behavior of the electrode material.It is worth mentioning that the GCD profiles of Co 3 O 4 :3%Fe, Co 3 O 4 :5%Fe, and Co 3 O 4 :7%Fe samples display a longer

Fig. 10
Fig. 10 Nyquist plot of (a) Fe doped Co 3 O 4 samples.b Fe doped Co 3 O 4 samples at low frequencies

Table 1
Specific capacitance of Fe doped Co 3 O 4 samples at various scan rates

Table 2
Specific capacitance of Fe doped Co 3 O 4 samples at various current densities

Table 3
Comparison study of Specific capacitance of pure and Fe doped Co 3 O 4 nanoparticles

Table 3
provides a detailed comparison of specific capacities for both Co 3 O 4 and Fe: Co 3 O 4 -based electrode materials [14, 30-39].