Enhancing pseudocapacitive properties of cobalt oxide hierarchical nanostructures via iron doping

Abstract

Through co-precipitation and post-heat processing, nanostructured Fe-doped Co₃O₄ nanoparticles (NPs) were developed. Using the SEM, XRD, BET, FTIR, TGA/DTA, UV–Vis, and techniques were examined. The XRD analysis presented that Co₃O₄ and Co₃O₄ nanoparticles that had been doped with 0.25 M Fe formed single cubic phase Co₃O₄ NPs with average crystallite sizes of 19.37 nm and 14.09 nm, respectively. The as prepared NPs have porous architectures via SEM analyses. The BET surface areas of Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs were 53.06 m²/g and 351.56 m²/g, respectively. Co₃O₄ NPs have a band gap energy of 2.96 eV and an extra sub-band gap energy of 1.95 eV. Fe-doped Co₃O₄ NPs were also found to have band gap energies between 2.54 and 1.46 eV. FTIR spectroscopy was used to determine whether M–O bonds (M = Co, Fe) were present. The doping impact of iron results in the doped Co₃O₄ samples having better thermal characteristics. The highest specific capacitance was achieved using 0.25 M Fe-doped Co₃O₄ NPs at 5 mV/s, which corresponding to 588.5 F/g via CV analysis. Additionally, 0.25 M Fe-doped Co₃O₄ NPs had energy and power densities of 9.17 W h/kg and 472.1 W/kg, correspondingly.

1. Introduction

Energy storage systems for clean and renewable energy have been developed as a result of the energy crisis and environmental pollution. Global warming and severe energy difficulties are currently becoming important issues [46,77]. This predicament has sparked researchers' efforts to create energy storage systems that are clean, green, sophisticated, and effective, like fuel cells, batteries, and supercapacitors [2,21]. In that case, batteries and electrochemical capacitors known as supercapacitors are the best candidates for the impending energy crisis. Rechargeable batteries are able to store electrical energy based on electrochemical redox reactions as positive and negative electrodes [60]. Moreover, after renewable energy generation, aqueous rechargeable cells are the practical and secure option for power storage [35,65,76]. As a result extensive analysis and researches are currently underway to develop a new, environmentally friendly, high-specific capacity and energy density with improved cyclic ability for a cost-effective energy storage materials [21,23,75]. To date, supercapacitor is the most promising new energy storage technology due to its excellent charge and discharge capabilities, long cycle life, and potential to transfer more power than conventional batteries [5]. Additionally, these advantageous energy storage devices have high energy density for hybrid electric vehicles [28,42,48,57,67,84]. Due to their extraordinary qualities, supercapacitors are excellent choices for usage in a variety of industrial, hybrid cars, mobile devices, electronic devices, and memory backup's energy storage systems [15,80].There are two categories of supercapacitors: electric double-layer capacitors (EDLC) and pseudocapacitors [17,59]. These different categories differ on the basis of the charge storage mechanisms [30,43,69,86]. As a pseudocapacitors electrode materials, transition metal oxides mainly include RuO₂, MnO_2, and Co₃O₄ [13,49,79,82,85]. RuO₂ is regarded as the best pseudocapacitors electrode material due to its high theoretical capacity and rapid Faraday redox reaction [34,44,51]. However, its high price and environmental toxicity seriously limit its application in supercapacitor application [32,55,64]. Recently, among different types of transition metal oxides,Co₃O₄ has been increasing interest in the use of as an electrochemical material for pseudocapacitors due to its high theoretical capacitance value of 3650 F/g, excellent reversible redox reaction, due to its environmental friendliness, low cost and special microstructure and morphology [4,47,72,73]. However, the capacitance in real-world applications differs greatly from that in theory. One of the causes is that Co₃O₄ poor capacitance and cycle efficiency result from the material's significant volume expansion and contraction, low conductivity, and high particle aggregation [50,72,73]. Hence,Co₃O₄ nanoparticles (NPs) can be prepared via several approaches such as the hydrothermal [56], sol-gel method [61], thermal decomposition [25], co-precipitation [33,78], spray pyrolysis [11] and solvothermal methods [10].However, these methods are expensive, require complex equipment and take a long time to prepare [53,58]. Among these, co-precipitation approach, has the benefit of being quick, affordable, and simple to control throughout preparations [12,83]. Hence the nanoparticle properties and electrochemical performance may be varying with experimental parameters like synthesized method, concentration, precursor materials, reaction time, temperature and solvent. Numerous research have reported on the electrochemical activity of electrode materials based on transition metal oxides [45]. Reference [19] examined that Mn-doped Co₃O₄ microspheres using a solvothermal method and have obtained a good specific capacitance of 773 F/g in 2 M KOH aqueous solution at 1 A/g. Similarly, Ali and Khalid [7] examined that 6% Cr-doped Co₃O₄ nanoflower prepared using a hydrothermal method had a higher specific capacitance value of 1283 F/g at a 5 mV/s scan rate, which is 67% higher than pure Co₃O₄ (860 F/g). Similarly, Ali, Khalid, and Nabi et al. [8,9], reported that 5% Ce (1–7%) doped Co₃O₄ nanostructures made of nano flakes synthesized via a simple hydrothermal method showed a superior specific capacitance value of 1309.6 F/g, which is 40% higher than pure Co₃O₄ another researchers, Uma Sudharshini et al. and Khalid et al. [39,81] the successful synthesis of 5% Mo-doped Co₃O₄ porous NPs with various molybdenum concentrations using a straightforward sol-gel process, which demonstrated excellent specific capacitance of 858.09 F/g at a scan rate of 5 mV/s and good conductivity. Hence, according to the above all findings, different doping material holds ideal input for practical application of supercapacitors. Recently, research has been published on the preparation of Co₃O₄ nanomaterials with metal dopants such as Ag, Sn, Sb, Cu, Fe, Cr, Mo, Ru, Ni, Mn, and Mo for different applications. Thus, according to the literature survey, it is found that Fe-doped Co₃O₄ NPs were mostly used as photocatalytic activity/electrolytes, biosensors, gas sensors [74]. Moreover, in the literature review, there are only a few reports on Fe-doped Co₃O₄ NPs for supercapacitor applications synthesized via co-precipitation technique. Hence, in this article Fe-doped Co₃O₄ NPs for supercapacitor applications were synthesized through a co-precipitation method with post-heat treatment.

2. Materials and methods

2.1. Materials

All utilized chemicals and reagents were of analytical grade and were not further purified. From chemical markets, we received CoCl₂. 6H₂O, 99.9% (cobalt chloride hexahydrate), NH₃, 99.2% (ammonia solution), and iron (II) nitrate hexahydrate (Fe (NO₃)₂6H₂O, 99.9%).To make the solutions, distilled water was used.

2.2. Fe-doped Co₃O₄ NPs synthesis

A simple co-precipitation method was used to create iron doped-Co₃O₄ NPs. Cobalt chloride (CoCl_{2 .}6H₂O) hexahydrate (0.3 M) and the desired mole of (0.05, 0.1, 0.15, 0.2, and 0.25 M) iron (II) nitrate hexahydrate (Fe (NO₃)₂·6H₂O) were dissolved in 100 mL of distilled water for the usual production of Fe-doped Co₃O₄ NPs. The resultant mixture was then agitated for 3 h at 80 °C using a magnetic stirrer. By gradually adding 0.2 M NH₃ solution to the mixture of Fe (NO₃)_2. 6H₂O and CoCl_2. 6H₂O, the pH of the mixture was brought to 9. Following that, both items were filtered and repeatedly cleaned with distilled water and ethanol. Additionally, the samples were dried for 6 h in an oven at 100 °C to remove any remaining water and organic contaminants. The products were then calcined at 500 °C for 4 h in a muffle furnace. Un-doped Co₃O₄ was made using a similar technique. Fig. 1 provides a detailed schematic representation of the Co-precipitation developed Fe-doped Co₃O₄ NPs.

Fig. 1.

Preparation of Fe-doped Co₃O₄ NPs using co-precipitation technique.

2.3. Characterization

Using a Fourier transform infrared spectrometer (FT-IR, 6660 (JASCO MODEL)) in the wavenumber range of 4000–500 cm⁻¹, the functional groups of nanoparticles were examined. The transmittance mode was used for the functional groups analysis, and KBr pellets were used for sample analysis. The nanoparticle dry powder was mixed with KBr, milled and pressed into disk. The crystal structure and the phase purity of the nanoparticles has been analyzed by powder X-ray diffraction (MAXima-X XRD-7000, SHIMADZU) technique. The advance Cu- Kα with wavelength of 0.154 nm, Bruker's X-ray powder diffractometer was used for structural analysis. The nanoparticle powder samples were characterized in the 2Ɵ range of 10–60°. SEM (INSPECT F50) were used to analyze the morphology of the produced nanomaterials at various magnification scales. TGA/DTA analysis was used to determine the thermal characteristics. The UV–Vis, Lambda35 (PerkinElmer) spectrophotometer was used to examine the optical properties of the created nanoparticles in the wavelength range of 250–500 nm. With Quanta chrome Nova Win, the Brunner-Emmet-Teller (BET) Surface Area of nanoparticles was calculated (Quanta chrome Instruments version 11.0).

3. Results and discussion

3.1. XRD analysis

The crystal structures and phases of as-prepared pure Co₃O₄ and Fe-doped Co₃O₄ NPs were investigated using the XRD analysis (Fig. 2). The XRD technique was used to evaluate all of the as-prepared samples. The diffraction peaks at 19.68°, 31.89°, 37.47°, 39.29°, 45.5°, 55.89°, and 59.11° correspond to the cubic Co₃O₄ crystalline planes (111), (220), (311), (222), (400), (422), and (511), respectively. As a result, the XRD peaks and the JCPDS data are in good agreement (JCPDS card no. 09–0418) [41,54].

Fig. 2.

XRD pattern of (a) Pure Co₃O₄ nanoparticles, and (b) 0.25 M Fe-doped Co₃O₄ NPs.

Doping has no effect on the cubic structure of the as prepared material, however it affects the samples' crystallinity by increasing the intensities and peak positions. The average crystal size of pure Co₃O₄ NPs and 0.25 M Fe-doped Co₃O₄NPs was found using the Debye-Scherer Equation (1) [24].

$$\mathrm{D}=\frac{\mathrm{k}\lambda }{\beta \cos \theta }$$ (1)

where, D, k, ʎ, and θ represents crystal size, Debye–Scherer constant, wavelength and diffraction angle, correspondingly [6]. For 0.25 M Fe–Co₃O₄ NPs and pure Co3O4 it was discovered that the average crystal size was equal to 19.37 and 14.09 nm, correspondingly (Table 1). The outcome indicated that Fe doping reduced particle size. Because of their high surface-to-volume ratio and great capacity for charge storage, tiny crystals have a small value [8,9].

Table 1.

XRD determination of pure Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs.

Nanoparticles	2θ	FWHM	D (nm)
pure Co3O4	31.63799	0.42607	19.37945
Fe-doped Co3O4	31.63568	0.58583	14.09445

3.2. Morphology analysis

SEM morphological analysis of pure Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs at various magnification scales were illustrated in Fig. 3(a–b). At 20 μm, SEM morphologies in Fig. 3a revealed that the particles have a porous structure with tiny spherical grains. In contrast, Fig. 3b at a magnification of 10 μm, demonstrates more porous structure NPs. This implied that the presence of iron changed the surface morphology of Co₃O₄ NPs. The SEM data show a substantial difference in morphology between pure and Fe-doped Co₃O₄ NPs, demonstrating that the concentration of Fe used for doping has a significant impact on the morphology of Co₃O₄ NPs. As a result, 0.25 M Fe-doped Co₃O₄ NPs exhibit higher porosity and greater particle dispersion than Co₃O₄ NPs, which is a potential characteristic to improve the catalytic performance of the nanoparticles as generated [68].

Fig. 3.

SEM pictures of (a) Pure Co₃O₄ nanoparticles at 20 μm (b) 0.25 M Fe-doped Co₃O₄ NPs at 10 μm.

3.3. Analysis of surface area

The BET analysis was used to calculate the specific surface areas, pore volume, and pore radius of Co₃O₄ and Fe-doped Co₃O₄, as shown in Table 2 [3,31]. For Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs, the BET specific surface were calculated to be 53.066 m²/g, and 351. 560 m²/g, correspondingly. In addition, Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs showed a pore radii of 13.85 Å and 13.24 Å, respectively. The 0.25 M Fe-doped Co₃O₄ NPs greater surface area and pore volume can create more room and improve electrochemical surface reactions. The outcome is a higher concentration of electrochemically active sites, a bigger area at the electrolyte-ion interface, and a shorter diffusion path enabling rapid ion diffusion, all of which contribute to excellent performance [20]. Hence, a greater BET surface area helps the electrode to store and transport electrons and ions, creating more active sites and a higher electrochemical potential [62]. Through this study, it was discovered that the presence of iron ions caused changes in the Co₃O₄ NPs surface area, pore volume, and pore size [29].

Table 2.

BET pore-volume, pore radius and specific surface area of Co₃O₄ and Fe-doped Co₃O₄ NPs.

Samples	BET Surface area (m2/g)	Pore volume (cc/g)	Pore radius (Å)
Co3O4	53.066	0.07425	13.85
0.25 M Fe–Co3O4	351.560	0.1191	1 3.24

3.4. FT-IR analysis

Fe-doped and un-doped Co₃O₄ NPs FTIR spectra were captured in the 4000-500 cm⁻¹ region. The FTIR spectra of Co₃O₄ and Fe-doped Co₃O₄ NPs at various doping levels are shown in Fig. 4. The band at roughly 3427 cm⁻¹ corresponds to the O–H stretching vibration of water molecules, while the smaller band at approximately 1619 cm⁻¹ may be caused by the O–H stretching and bending modes of water molecules. The intensity of the OH stretching vibration absorption peak is reduced with adding iron ions, to Co₃O₄ NPs. The peaks between 3400 and 2900 cm⁻¹ were caused by carbonaceous chemicals and O–H stretching, respectively [66]. Additionally, the band roughly between 1379 and 1110 cm⁻¹ matches the O–H–Co stretching vibration. Due to the addition of iron ion, the band shifted slightly towards the higher wavenumber region. Finally, the characteristic peaks at 620 cm⁻¹ and 570 cm⁻¹ are connected to the stretching vibrations of the metal-oxygen(M–O) bond, which supports the spinel structure of Co₃O₄ NPs [26]. Moreover, the FTIR spectra investigation are well matching with others findings [63,70]. According to FTIR result the peak intensity raised with increasing iron doping levels, which also improved the crystallinity of the NPs. The existence of optical vibrational modes reveals the formation of cobalt oxide in NPs.

Fig. 4.

FTIR spectra of Co₃O₄, Fe-doped Co₃O₄ (0.05 M), Fe-doped Co₃O₄ (0.1 M), Fe-doped Co₃O₄ (0.15 M), Fe-doped Co₃O₄ (0.2 M), Fe-doped Co₃O₄ (0.25 M) NPs.

3.5. UV–Vis analysis

UV–Vis analyses were carried out to examine the optical characteristics of as-prepared nanoparticles. Fig. 5 shows the UV–visible spectra of Co3O4 and Fe-doped Co3O4 nanoparticles. The analysis displayed that Fe doping concentration affects the absorbance values in UV–Visible spectra [18]. Another notable aspect of the absorption spectrum is the presence of two pronounced absorption edges in the visible region in all observed spectra, which are ascribed to the ligand-to-metal charge transmission result of (O²⁻ → Co²⁺) and (O²⁻ → Co³⁺) in Co₃O₄ [52]. The absorption bands of Fe-doped Co₃O₄ vary with a change in the concentration of iron. The optical band gap of un-doped Co₃O₄ and Fe-doped Co₃O₄ (0.05–0.25 M) sample were calculated using Tauc relation from Eq. (2) [16,63].

$${\left(\alpha \mathrm{һ}\nu \right)}^n=\mathrm{A}\left(\mathrm{һ}\nu -E_{\mathrm{g}}\right)$$ (2)

where, hv is photon energy, A is constant, E_g is the bandgap energy α is absorption coefficient, and n is the constant [37]. Tauc plot of Co₃O₄, and Fe doped Co₃O₄ (0.05, 0.1, 0.15, 0.2 and 0.25 M) NPs were calculated by extrapolating the linear part of these plots of (αhν) ² axis to (hν) axis (Fig. 6). The bandgap energy variation is observed due to the addition Fe ions (Table 3). The maximum bandgap energy for Co₃O₄ NPs is assigned to the valence to conduction band excitation of (O²⁻ → Co²⁺ charge transfer), while the minimum band gap is assigned to the O²⁻ → Co³⁺ charge transfer at 0.25 M Fe-doped Co₃O₄ NPs where Co³⁺ ions are located below the conduction band; due to this impurity energy levels are created in the bandgap region. In contrast, Fe contributes to hole generation and increases its role with the number of charge carriers (holes) that contribute to conductivity of Co₃O₄ NPs.

Fig. 5.

UV–Visible spectrum of Co₃O₄, Fe-doped Co₃O₄ (0.05 M), Fe-doped Co₃O₄ (0.1 M), Fe-doped Co₃O₄ (0.15 M), Fe-doped Co₃O₄ (0.2 M) and Fe-doped Co₃O₄ (0.25 M) NPs.

Fig. 6.

Tauc plot of (a) pure Co₃O₄, (b) Fe-doped Co₃O₄ (0.05 M), (c) Fe-doped Co₃O₄ (0.1 M), (d) Fe-doped Co₃O₄ (0.15 M), (e) Fe-doped Co₃O₄ (0.2 M), (f) Fe -doped Co₃O₄ (0.25 M) nanoparticles.

Table 3.

Co₃O₄ and Fe-doped Co₃O₄ NPs optical band gap values.

Samples	Eg1 (eV)	Eg2 (eV)
Co3O4	1.95	2.96
Fe-doped Co3O4 (0.05 M)	1.93	2.54
Fe-doped Co3O4 (0.1 M)	1.87	2.52
Fe-doped Co3O4 (0.15 M)	1.54	2.51
Fe-doped Co3O4 (0.2 M)	1.52	2.50
Fe-doped Co3O4 (0.25 M)	1.46	2.40

3.6. Thermal analysis

Through the use of TGA and DTA, the thermal characteristics of un-doped Co₃O₄ and Fe-doped Co₃O₄ NPs were examined. The sample mass was approximately 10 mg, and it was heated in an environment of air at a rate of 20 °C per minute in a corundum crucible between 25 °C and 900 °C [36]. The TGA and DTA curves of Fe-doped Co₃O₄ and Co₃O₄ NPs are shown in Fig. 7, respectively. The TGA profiles of Co₃O₄ NPs and Fe-doped Co₃O₄ NPs show two stages of weight reduction. The first weight loss of 0.37 mg for Co₃O₄ NPs between 25 and 262 °C and the corresponding endothermic peak at 135 °C include the loss of absorbed water in the sample [83]. The degradation of the precursor materials or remaining organic ligands was associated to the second mass losses (1.54 mg) in the range of 262–599 °C, with corresponding DTA curves at 342 and 506 °C [1]. The final mass loss of 8.09 mg from the initial weight of 10 mg at 900 °C is comparable to a mass loss of 19.1% (Fig. 7a.) The endothermic peak that corresponds to this temperature may be due to the loss of physically adsorbed water as the first mass loss of 0.41 mg is observed on the 0.05 M Fe–Co₃O₄ NPs TGA curve between 25 and 279 °C (Fig. 7b). The second range of mass losses (1.15 mg) in the range of 279–598 °C were attributed to the breakdown of leftover organic ligands and the DTA curves at 339 and 509 °C. The final mass loss of 8.44 mg from the initial weight of 10 mg at 900 °C is 15.6% mass loss (Fig. 7b). Additionally, the endothermic peak that corresponds to the first mass loss of 0.89 mg between 25 and 300 °C on the 0.1 M Fe–Co₃O₄ NPs TGA curve may be produced by the loss of physically adsorbed water (Fig. 7c). The second range (0.19 mg) mass losses in the range of 300–475 °C, with corresponding DTA curves at 507 and 574 °C, were linked to the breakdown of the remaining organic ligands. The final mass loss from the starting weight of 10 mg is 8.92 mg, or 10.8%, when heated to 900 °C (Fig. 7c).The endothermic peak at 106 °C that it corresponds to the loss of physically adsorbed water, as shown by the first mass loss of 0.9 mg between 25 and 296 °C on the 0.15 M Fe-doped Co₃O₄ NPs TGA curve (Fig. 7d) [38]. The breakdown of lingering organic ligands was linked to the second range of mass losses (0.14 mg), which occurred in the range of 296–486 °C, using matching DTA curves at 415 and 458 °C. However, after 495 °C, there is no change in the TGA/DTA patterns of 0.15 M Fe–Co₃O₄ NPs. The final mass loss of 8.96 mg from the initial weight of 10 mg at 900 °C is comparable to a mass loss of 10.4% (Fig. 7d). Thermal testing demonstrates that the interaction between the Fe and Co₃O₄ NPs gives rise to the material's noticeably different thermal characteristics. As a result, Fe-doped Co₃O₄ NPs display significantly higher thermal stability as the dopant concentration increases (Table 4) [14].

Fig. 7.

TGA and DTA curve of (a) Co₃O₄ nanoparticles, (b) Fe-doped Co₃O₄ (0.05 M) nanoparticles, (c) Fe-doped Co₃O₄ (0.1 M) nanoparticles, (d) Fe-doped Co₃O₄ (0.15 M) nanoparticles.

Table 4.

Summary of TGA-DTA analysis results.

Samples	Temperature range (°C)	Endothermic peak (°C)	1st stage weight loss (TGA)		2nd stage weight loss (TGA)		Total weight loss (%)	Decomposition temperature (°C) in the DTA curve
			Temperature range °C	weight loss (mg)	Temperature range °C	Weight loss (mg)
Co3O4	25–900	135	25–262	0.37	262–599	1.54	19.1	342 and 506
Fe–Co3O4 (0.05 M)	25–900	132	25–279	0.41	279–598	1.15	15.6	339 and 509
Fe– Co3O4 (0.1 M)	25–900	95	25–300	0.89	300–475	0.19	10.8	507 and 574
Fe–Co3O4 (0.15 M)	25–900	106	25–296	0.9	296–486	0.14	10.4	415 and 458

3.7. Cyclic voltammetry (CV) analysis

Cyclic voltammetry (CV) was used for electrochemical investigations. To evaluation the capacitance of un-doped Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs used this analysis. Fig. 8a–c shows the findings of the CV curve that was recorded in the potential range of −0.1 to + 0.6 V vs Ag/Ag Cl at scan rates of 5, 10, 20, 50, and 100 mV/s in 0.1 M KOH. Wide and distinct redox peaks seen from nonlinear CV curves which support the Fe-doped Co₃O₄ NPs pseudo-capacitance property. When the scanning rate was increased from 5 to 100 mV/s, the strong redox peaks were still visible, showing good reversibility of redox reactions [22].

Fig. 8.

CV Curve of (a) Co₃O₄ and 0.25 M Fe-doped Co₃O₄ nanoparticles at 50 mV/s nanoparticles at different scan rates, (b) Co₃O₄ nanoparticles at different scan rates, and (c) 0.25 M Fe-doped Co₃O₄ nanoparticles at different scan rates.

Hence, Eq. (3) was used to estimate the specific capacitance of pure Co₃O₄ and 0.25 M Fe-doped Co₃O₄ nanoparticles determined from CV curves [8,9].

$$\mathrm{C}\mathrm{s}=\frac{\underset{v1}{\overset{V2}{\int }}IdV}{mv\Delta V}$$ (3)

where Cs, I, V₁, V₂, v, and m represents specific capacitance (F/g), oxidation/reduction current for a given voltage V (v), lower potential limit, upper potential limit, scan rate (v/s) and mass of the electrode (g), respectively. At a scan rate of 5 mV/s, the Co₃O₄ electrode only displays a high specific capacitance value of 393.6 F/g. These values are comparable with literature-reported values [40]. However, 0.25 M Fe-doped Co₃O₄ nanoparticles electrodes exhibits a high Specific Capacitance value of 588.5 F/g at a scan rate of 5 mV/s (Table 5). Notably, this particular capacitance value is higher than values previously reported in the literature.

Table 5.

The specific capacitance values of pure Co₃O₄ and Fe-doped Co₃O₄ NPs.

Scan rate (mV/s)	Specific capacitance(F/g) (Co3O4)	Specific capacitance (F/g) (0.25 M Fe-doped Co3O4)
5	393.6	588.5
10	328.7	523.1
20	230.6	395.7
50	143.1	329.4
100	109.8	261.6

3.8. Power density and energy density analysis

The power and energy density of the as-prepared materials were obtained according to Eqs. (4), (5) [27,71].

$$\mathrm{E}={\frac{1}{2}C\left(\Delta V\right)}^2/3.6$$ (4)

$$P=3600E/\Delta t$$ (5)

where C is the capacitance of the electrode (F/g), E is the energy density (Wh/kg), and ΔV (V), is the potential window of device, P is the power density (W/kg), Δt (s) is the discharge time. At a power density of 160.4 W/kg, the Co₃O₄ NPs had a maximum energy density of 6.69 W h/kg. In contrast, 9.17 W h/kg of 0.25 M Fe doped Co₃O₄ NPs had a power density of 472.1 W/kg. In this study, Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs had a greater power density than those previously reported in the literature [29].

4. Conclusion

Using a co-precipitation synthesis technique by varying the concentrations of iron, Fe-doped Co₃O₄ NPs for supercapacitor applications were investigated. Through the use of FTIR, XRD, TGA/DTA, SEM, UV-VIS, BET and CV analyses, the as-prepared materials were characterized. XRD analysis showed that an average particle size of 19.37 and 12.98 nm for Co₃O₄ and Fe-doped Co₃O₄ NPs, respectively. Morphological analysis of Co₃O₄, and 0.25 M Fe-doped Co₃O₄ showed porous structures. The BET surface areas of Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs were 53.06 m²/g and 351.56 m²/g, respectively. Co₃O₄ NPs have a band gap energy of 2.96 eV and an extra sub-band gap energy of 1.95 eV. Additionally, the band gaps of Fe-doped Co₃O₄ NPs were between 2.4 and 2.54 eV for all samples, with a second sub-band with an energy range between 1.46 and 1.93 eV. FTIR spectroscopy was used to examine the formation of M–O bonds. The doping impact of iron results in the Co₃O₄ samples having better thermal characteristics. In order to conduct the electrochemical measurements, a CV analysis in a 0.1 M KOH electrolyte solution was used. The CV test showed that, at a scan rate of 5 mV/s, the samples of Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs displayed the greatest specific capacitance values of 393.6 F/g and 588.5 F/g, respectively. Additionally, the Co₃O₄ NPs and 0.25 M Fe-doped Co₃O₄ NPs energy and power densities were 6.69 W h/kg, 160.4 W/kg, and 9.17 W h/kg, 472.1 W/kg, respective. These findings point to its possible use in energy storage technology.

Author contribution statement

Asab Fetene Alem, Ababay Ketema Worku: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Delele Worku Ayele, Nigus Gabbiye Habtu, Temesgen Atnafu Yemata: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Mehary Dagnew Ambaw: Conceived and designed the experiments; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Date will be made available on request.

Declaration of interest's statement

The authors declare no conflict of interest.