Nanosheet floral clusters of Fe-doped Co₃O₄ for high-performance supercapacitors

Abstract

Because of their superior power and energy densities, supercapacitors have garnered considerable amounts of attention in energy storage technologies. Co₃O₄ is considered as a regularly used electrode material for supercapacitors because of its high theoretical capacitance and outstanding electrochemical activity. However, owing to the Co₃O₄ electrode materials’ rapid capacity decay and low intrinsic conductivity, it is challenging to achieve high theoretical capacitance. As a result, doping Co₃O₄ with metal components can enhance its electrical and surface characteristics while also raising its conductivity. Through a straightforward hydrothermal reaction and subsequent thermal decomposition, a series of Fe doping into Co₃O₄ has been successfully realized in this paper. This generates a nanosheet floral cluster structure and exposes more active sites, and the two metal elements work in concert to improve the capacitance performance. The electrochemical performance of the Co-0.2Fe-450 electrode material appears satisfactory when the doping amount of Fe is 0.2 mmol and its thermal decomposition temperature is 450 °C. The specific capacitance at 1 A g⁻¹ is 680 F g⁻¹. The capacitance retention rate at 10 A g⁻¹ current density after 5000 charge–discharge cycles is 84.67%. Notably, the assembled asymmetric supercapacitor retains 105.5% of its capacitance after 5000 cycles and has a power density of 803.13 W kg⁻¹ and an energy density of 17.78 W h kg⁻¹. Fe-doped Co₃O₄ has a strong future in energy storage, as evidenced by its exceptional electrochemical characteristics.

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1. Introduction

Excessive use of traditional fossil fuels increases the cumulative emission of greenhouse gases, leading to serious disturbances in the ecological balance, and it is necessary to use new clean and renewable sources of energy and to take a green development path in order to achieve the major strategic goal of “dual carbon” for sustainable development.^1–3 Research is now focused on finding appropriate energy storage devices so that power produced by sustainable energy can be easily stored and used whenever and wherever it is needed.^4,5 The two most common kinds of energy storage devices are supercapacitors and batteries. Supercapacitors typically have a higher power density than batteries. They also offer the benefits of a long cycle life, excellent safety, and quick charging and discharging speeds.^6–8 In particular, electrode materials are a crucial component and essential component of supercapacitors. Supercapacitor electrode materials can be broadly classified into three categories: carbon-based materials,⁹ conductive polymers,¹⁰ and compounds based on transition metals.¹¹ Each category has pros and cons of its own. Therefore, the development of electrode materials with a distinctive structure and form as well as excellent performance is extremely crucial to improve the electrochemical performance of supercapacitors.^12,13

The most popular electrode materials for supercapacitors are transition metal oxides (TMOs),^14–17 which include NiO,¹⁸ CuO,¹⁹ MnO₂,²⁰ Fe₃O₄,²¹ Co₃O₄,²² and others. These materials offer several benefits, including rich oxidation states, great mechanical stability, and outstanding electrochemical activity.^23,24 Among them, Co₃O₄ is a significant metal oxide that has found widespread application as an electrode material for supercapacitors due to its high theoretical capacitance, outstanding reversible REDOX behavior, low cost, and excellent environmental friendliness.^25,26 However, its actual applicability in supercapacitors is restricted due to its drawbacks, including weak conductivity, structural stacking, and poor REDOX reversibility; also, its capacitance is substantially lower than its theoretical value of 4292 F g⁻¹.²⁷ A variety of tactics have been used by researchers to overcome these problems. To increase the conductivity of Co₃O₄ electrode materials and buffer volume expansion during charge and discharge, for instance, cover carbon materials or conductive polymers. This coating method's primary drawbacks include its more complicated electrode material manufacturing, easily broken coating, and challenging large-scale production.²⁸ Another effective strategy is to prepare composite electrode materials, which have better properties due to their synergistic effects. In an effort to enhance the electrochemical behavior of metal oxide electrode materials, researchers have recently combined metal oxides with carbon substrates. This combination increases the specific surface area and reactive site of the material.²⁹ For example, Madhu et al.³⁰ prepared cellular porous carbon–Co₃O₄ nanocomposites (PSAC/Co₃O₄) using seed shells as carbon sources (PSAC), which have significant specific capacitance and excellent long-term cyclic stability. Zou et al.³¹ fabricated a layered electrode structure (Co₃O₄/NGF) composed of Co₃O₄ and nitrogen-doped graphene foam (NGF). In electrochemical measurements, the capacitive performance of the Co₃O₄/NGF electrode was increased from 320 F g⁻¹ of the original Co₃O₄ to 451 F g⁻¹ with excellent rate capability. Although the composite electrode material can make up for the defects of a single material and improve the electrochemical performance, the contact resistance between the composite components may increase the total resistance of the material, and the preparation process is complex, which is not conducive to large-scale production in a short time. Doping TMOs with additional elements has shown to be a successful method for resolving these issues, as it enhances the intrinsic conductivity and electrical characteristics of TMOs while also increasing their stability.^32,33 Wang et al.³⁴ synthesized Fe-doped MnO₂ nanostructures, which showed significantly improved specific capacitance, magnanimity, and cycle life compared to undoped MnO₂. Co₃O₄ has a spinel structure, and the introduction of Fe doping can exert the synergistic effect of the two substances to modify the electronic structure of Co₃O₄, expose more reactive sites, and thus improve the conductivity and electrochemical performance. Wang et al.³⁵ prepared Fe-doped Co₃O₄/graphene nanocomposites by a hydrothermal method. The specific capacitance reached 671 F g⁻¹ at a current density of 5 A g⁻¹, and the capacitance remained at 98.7% after 1000 cycles, which proved to be a significant candidate for doping.

Therefore, in this study, Fe-doped Co₃O₄ nanosheet floral cluster structures were prepared by a simple hydrothermal method and heat treatment reaction. Due to the doping of Fe elements and the excellent microstructure, the electrode material prepared has excellent conductivity and large specific surface area, and shows an excellent electrochemical performance in supercapacitors. The specific capacitance at 1 A g⁻¹ in the three-electrode system can reach 680 F g⁻¹, and after 5000 cycles of charge and discharge at 10 A g⁻¹, the capacitance retention rate is 84.67%. To build asymmetric supercapacitors, activated carbon (AC) serves as the negative electrode and iron-doped Co₃O₄ as the positive electrode. Even after 5000 cycles, the capacitance retention may reach 105.5% when the power density is 803.1 W kg⁻¹ and the energy density is 17.78 W h kg⁻¹. The remarkable cycle stability and electrochemical performance of iron-doped Co₃O₄ electrode materials validate their extensive potential for use in supercapacitor energy storage devices.

2. Experimental section

2.1. Materials

Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, urea and ammonium fluoride are all purchased from Shanghai Aladdin Biochemical Materials Co., Ltd. All chemicals are not purified and belong to the analytical grade. Deionized water is prepared in the laboratory.

2.2. Method of synthesis

The Fe-doped Co₃O₄ was made via a straightforward hydrothermal process. A homogeneous solution was formed by dissolving 2 mmol of Co(NO₃)₂·6H₂O (0.582 g), 0.2 mmol of Fe(NO₃)₃·9H₂O (0.081 g), 10 mmol of urea (0.601 g) and 6 mmol of NH₄F (0.222 g) in 65 mL of deionized water and stirring for 30 minutes. The resulting solution was then transferred to a 100 mL Teflon lined reactor and heated in an oven to 120 °C for 8 h. At the end of the reaction, the solution was allowed to cool naturally to room temperature. It was then washed two to three times with ethanol and deionized water and dried under vacuum at 60 °C for 12 h. The dried powder samples were annealed under nitrogen at 450 °C for 2 h. The effects of different Fe (NO₃)₃·9H₂O contents (0, 0.2, 0.4, and 0.6 mmol) and annealing temperatures (T = 350, 450, and 600 °C) on the electrochemical performance of supercapacitors were also investigated.

2.3. Materials characterization

(1) X-ray diffraction (XRD) was performed using an XRD diffractometer (D8 ADVANCE), using JADE to characterize the crystal structure composition of the active substance. In the experimental test, the test angle is 10–80° and the scan rate is 10° min⁻¹. (2) Using field emission scanning electron microscopy (SEM), a small quantity of the active ingredient (powder) was dissolved and ultrasonically dispersed in ethanol, bound to a conductive adhesive, sprayed with gold, and its shape was examined. (3) Transmission electron microscopy (TEM), which involves dissolving a little quantity of an active material in ethanol for ultrasonic dispersion, adding two to three drops to a copper mesh, letting it dry naturally, and using a transmission electron microscope to view the samples' internal microstructure. (4) X-ray photoelectron spectroscopy (XPS), which typically uses X-ray excitation to determine the chemical state of an active material by examining the inner or valence electrons of its atoms or molecules. (5) Specific surface area analysis and determination (BET), through the specific surface area analysis and measurement instrument to test the specific surface area and aperture distribution of active substances. (6) Fourier transform infrared absorption spectroscopy (FTIR), analytical reagent for KBr, analysis of functional group changes of active substances.

2.4. Electrochemical measurements

Three electrode test: First, the active substance, acetylene black, and polyvinylidene fluoride (PVDF), were mixed uniformly in N-methylpyrrolidone (0.015, 0.003, and 0.002 g) and coated on nickel foam with a coating quantity of 3–4 mg cm⁻². After pressing and drying, a working electrode was produced. An electrochemical test of the three-electrode system was performed using a 1 M KOH alkaline aqueous solution as the electrolyte, a saturated calomel electrode as the reference electrode, and a platinum foil electrode as the electrical level.

Dual electrode test: commercial activated carbon (AC) is employed as the negative electrode material, and the active material working electrode that was previously created serves as the positive electrode. A series of electrochemical experiments are conducted in a 1 M KOH electrolyte, and the fabrication procedure for the negative electrode material is comparable to that for the active electrode material.

The specific capacity (C, F g⁻¹), energy density (E, W h kg⁻¹) and power density (P, W K g⁻¹) are calculated as follows:^36,37

where C, I, Δt, m and ΔV represent the specific capacitance, current density, discharge time, active substance mass and voltage window, respectively.

3. Results and discussion

The preparation process of Fe-doped Co₃O₄ layered nanosheet floral cluster structure is shown in Fig. 1. First, a straightforward hydrothermal process was used to dissolve Co²⁺, Fe³⁺, urea, and ammonium fluoride in deionized water to create the CoFe precursor. Thermal reduction of Fe³⁺ produced the Fe-doped Co₃O₄ flake-like floral cluster structure after the precursor was annealed at high temperature under N₂ protection. Then, without adding Fe³⁺, Co₃O₄/CoO composite sea urchin-like structures were produced. It is important that the electrochemical characteristics are greatly impacted by annealing temperature changes and the addition of various concentrations of Fe³⁺.

Fig. 1 Structure diagram of Fe doped Co₃O₄.

The sample's crystal structure and phase composition were examined using XRD. The impact of varying doping concentrations on the crystal structure of Co₃O₄ at 450 °C is seen in Fig. 2a. The sample is mostly Co₃O₄ (PDF No. 42-1467) when the iron source is not doped, with a minor quantity of CoO (PDF No. 48-1719). The manufactured Co-0.2Fe-450 and Co-0.4Fe-450 demonstrate the presence of metal Fe in 44.8° and 44.6°. The angle moves to the left with increasing Fe doping concentration. Moreover, the Co-0.6Fe-450 sample displayed Fe_xC at 42.5°, which can be the result of the iron and carbon atoms’ strong binding force at high temperatures, which makes it simple to create interstitial compounds. A local enlargement of the material is shown in Fig. 2b. The lattice spacing rises and the diffraction peak changes to the left as a result of the varied Fe doping replacing Co²⁺, indicating the successful doping of Fe elements.

Fig. 2 (a) and (b) XRD spectra of samples prepared at different ratios.

The FT-IR absorption spectra of several materials following heat treatment at 450 °C are displayed in Fig. S1 (ESI†). The broad peaks centered at 3440 cm⁻¹ in all samples represent the –OH absorbed by the water molecules in the sample, while the peaks at 1633 cm⁻¹ represent the C–O bond's stretching vibration. Furthermore, in the Co-0Fe-450 samples, the stretching vibration absorption peaks of Co₃O₄ were seen at 665 cm⁻¹ and 658 cm⁻¹, suggesting that Co₃O₄ was the predominant product at this point in time.³⁸ The Fe doped sample's characteristic peak is nearly identical to Co₃O₄'s and does not alter Co₃O₄'s functional group structure, suggesting that the two materials interact non-covalently and providing preliminary evidence of the material's successful production.

The chemical state and elemental composition of the samples were examined using XPS. The full XPS spectra of the Co-0Fe-450 and Co-0.2Fe-450 samples are shown in Fig. 3a. There is no signal peak of Fe in the full XPS spectrum of the Co-0Fe-450 sample, while the XPS spectrum of the Co-0.2Fe-450 sample shows the simultaneous presence of the elements Co, Fe, C, and O, which confirms that the element Fe is successfully doped into Co₃O₄. C1s spectra are shown in Fig. S2 (ESI†), and the peaks located at 284.8, 286.3, and 288.6 eV correspond to C=C, C–N and C–O bonds, and high-temperature thermal decomposition is the main source of the presence of C elements. The O 1s spectra of the two groups of samples are shown in Fig. 3b. The three signal peaks in the figure represent surface adsorbed water molecular bonds (H–O–H), oxygen defect bonds (M–O–H), and metal–oxygen–metal bonds (M–O–M),³⁹ respectively. The Co 2p spectra, which have two prominent peaks with binding energies of 780.2 eV and 795.4 eV for Co 2p^3/2 and Co 2p^1/2, can be seen in Fig. 3c. The two main peaks can be simulated to synthesize four peaks, among which 780.1 eV and 795.2 eV correspond to Co³⁺, and the peaks at 781.6 eV and 796.8 eV correspond to Co²⁺, indicating the successful formation of Co₃O₄.⁴⁰ Interestingly, the signal peaks of Co-0Fe-450 and Co-0.2Fe-450 samples did not shift significantly indicating that the chemical environment of the atoms inside the material had not changed. Fig. 3d is the Fe 2p spectrum, and the combined energy corresponds to two main peaks of Fe 2p^3/2 and Fe 2p^1/2 at 712.8 eV and 724.3 eV, where 710.7 eV and 722.0 eV correspond to Fe⁰, and 712.9 eV and 724.4 eV correspond to Fe²⁺. 715.2 eV and 727.8 eV correspond to Fe³⁺, and 718.2 eV and 732.7 eV are a set of satellite peaks.⁴¹ The test results of XPS further indicate that Fe is successfully doped in Co₃O₄.

Fig. 3 XPS spectra of (a) survey, (b) O 1s, (c) Co 2p and (d) Fe 2p of Co-0.2Fe-450 and Co-0Fe-450.

Using the N₂ adsorption–desorption method, the specific surface area and mesoporous characteristics of several samples were examined. The outcome is displayed in Fig. 4. A type IV hysteresis loop with clear mesoporous features was seen in every sample. Co-0Fe-450, Co-0.2Fe-450, Co-0.4Fe-450, and Co-0.6Fe-450 have specific surface areas of 13.12, 43.00, 42.10, and 37.97 m² g⁻¹, in that order. The findings demonstrated that following Fe doping, the electrode material's specific surface area greatly increased. Dynamic ion/electron rapid transport channels are generated, and more exposed active sites are revealed. The Co-0.2Fe-450 sample has the most specific surface area of any of them. The average pore size of the four sets of samples is 13.01 nm, 9.58 nm, 8.40 nm, and 10.41 nm, respectively, as shown by the pore size distribution diagram in Fig. 4, which illustrates a mesoporous structure. The aforementioned study's findings demonstrate that high temperatures promote the formation of pore structures. Interestingly the surface area of Co-0.2Fe decreased significantly (23.87 m² g⁻¹) after high temperature thermal decomposition at 600 °C, which can be attributed to the fact that with the increase of the thermal decomposition temperature, the morphological structure of the Co-0.2Fe-600 samples was disrupted, shrinkage occurred and larger particles were produced (Fig. S3, ESI†). The above findings suggest that the formation of mesoporous pore structure can be induced by thermal decomposition reaction at the appropriate temperature.^42,43 In addition, the nanosheet flower cluster structure formed by Fe doping can effectively increase the contact area between the electrolyte and the electrode material, which is favorable for the electrochemical performance.

Fig. 4 N₂ adsorption desorption curves for (a) Co-0Fe-450, (b) Co-0.2Fe-450, (c) Co-0.4Fe-450, (d) Co-0.6Fe-450. (Inset: pore size distribution map).

SEM was used to examine the morphology and structure of the material. As shown in Fig. 5a, in the absence of Fe doping, the Co-0Fe samples exhibit anisotropic radial nano-needle-like structures similar to those of sea urchins. With the addition of Fe, a lamellar structure was formed and the nanosheets were interspersed to form floral clusters accompanied by a small number of rod-like structures. It is noteworthy that the morphology and structure of the samples did not change significantly with the increase of iron content (Fig. 5b and Fig. S4, ESI†). As shown in Fig. S5 (ESI†), after thermal decomposition at 450 °C, the morphology of the Co-0Fe samples without Fe-doped elements changed from the original nano-needle structure to a rod-like structure, and the surface became rough. The morphology and structure of the Co-0.2Fe-450 and Co-0.4Fe-450 samples have good stability. However, the Co-0.6Fe-450 sample experienced a collapse of the structure, which may be due to the fact that the addition of more Fe elements drastically affects the morphology and structure of the product. Elemental mapping analysis of the Co-0.2Fe-450 sample (Fig. 5c) demonstrated that the Co, Fe and O elements were uniformly distributed in the Co-0.2Fe-450 sample, which further indicated that Fe was successfully doped into the product (Fig. 5d–f). In addition, we investigated the effect of different temperatures on the appearance and morphology of Co-0.2Fe samples. As can be seen from Fig. S6 (ESI†), at 350 °C, the surface of the sample became rough and a large number of particles were generated; at 600 °C, the structure of the sample collapsed, which could be attributed to the high thermal decomposition temperature, which made the structure of the sample unstable and possibly fractured. Meanwhile, due to the addition of urea and NH₄F during the sample preparation, the nucleation and crystal growth processes of the samples were well regulated by the alkaline solution.

Fig. 5 SEM images of (a) Co-0Fe, (b) Co-0.2Fe and (c) Co-0.2Fe-450. (d)–(f) Elemental mapping of Co-0.2Fe-450.

Transmission electron microscopy (TEM) was used to further investigate the Co-0.2Fe-450 sample's structural properties. The floral-cluster Co-0.2Fe-450, as shown in Fig. 6a and b, was made up of ultra-thin nanosheets. The homogenous nanosheets helped to preserve the sample structure's regularity, which was in line with the SEM results. The lattice fringe of Co-0.2Fe-450 is depicted in Fig. 6c. Lattice fringes with crystal face spacing of 0.286 nm correspond to the (220) plane of Co-0.2Fe-450. The rapid Fourier transform is used to extract the selected electron diffraction pattern (SAED) in Fig. 6d from the selected region in the HRTEM. The diffraction rings in the image correspond to the (222), (440), and (422) crystal faces of Co-0.2Fe-450, respectively, which correspond to the standard XRD cards. The figure shows that polycrystal features are seen.

Fig. 6 (a) and (b) TEM images, (c) high-resolution TEM images, (d) selected electron diffraction patterns of Co-0.2Fe-450.

Three electrode electrochemical tests were conducted on the samples to verify that Fe doping is a successful method of improving the electrochemical characteristics of Co₃O₄ materials. Fig. S7 (ESI†) displays the cyclic voltammetry (CV) and constant current charge and discharge (GCD) curves for Co-0Fe, Co-0.2Fe, Co-0.4Fe, and Co-0.6Fe. The fact that each set of CV curves contains a pair of clearly visible cathode and anode peaks as well as a clearly visible charging and discharging platform in the GCD curve suggests that the materials can have some of the properties of batteries. Co-0.2Fe is selected as the precursor material due to the fact that the figure shows that it has the highest specific capacitance. The CV and GCD curves of products produced at various temperatures during the thermal decomposition process are displayed in Fig. 7a and b. It is assumed that the CV curve of Co-0.2Fe-450 has the highest specific capacitance due to its biggest closed area. The specific capacitance of Co-0.2Fe-450 is up to 680 F g⁻¹ at 1 A g⁻¹, as shown in the GCD curve, which is much superior to that of Co-0Fe (288 F g⁻¹) and Co-0.2Fe-350 (590 F g⁻¹). Co-0.2Fe (400 F g⁻¹) and Co-0.2Fe-600 (450 F g⁻¹) demonstrated the accuracy of the CV curve's derivation. The Nyquist diagram, which is depicted in Fig. 7c, exhibits charge transfer resistance in the high frequency zone and diffusion resistance in the low frequency region. With a steeper slope at low frequencies, the produced Co-0.2Fe-450 material displays diffusion-controlled capacitive behavior, suggesting quicker charge transfer and diffusion between the electrode material and the electrolyte. The CV curve of Co-0.2Fe-450 is shown in Fig. 7d. When the sweep speed increases from 10 mV s⁻¹ to 50 mV s⁻¹, the anode peak moves to high potential and the cathode peak moves to low potential, and the area of the CV curve remains basically unchanged, indicating that the Co-0.2Fe-450 electrode material has excellent magnification performance. The specific capacitance of Co-0.2Fe-450, as illustrated in Fig. 7e, is 680, 540, 424, 372, 328, and 302 F g⁻¹, respectively, under current densities of 1, 2, 4, 6, 8, and 10 A g⁻¹. In comparison to other comparable electrode materials indicated in Table 1, this suggests exceptional capacitive performance that is noticeably superior. As shown in Fig. S8 (ESI†), when the current density was increased from 1 A g⁻¹ to 10 A g⁻¹, the specific capacitance decreased from 680 F g⁻¹ to 295.5 F g⁻¹ with a capacitance retention of 43.5%. The Co-0.2Fe-450 sample's specific capacitance retention after 5000 cycles at a current density of 10 A g⁻¹ is 84.67% of the starting value (Fig. 7f). Table 2 lists the stability properties of other similar materials reported in recent years, and it can be seen that Co-0.2Fe-450 has strong cycling stability. Fe doping is a simple and effective way to enhance the electrochemical performance of Co₃O₄ electrode materials.

Fig. 7 Co-0Fe, Co-0.2Fe, Co-0.2Fe-350, Co-0.2Fe-450, and Co-0.2Fe-600 with (a) CV curves at 10 mV s⁻¹, (b) GCD curves at 1 A g⁻¹ current density, and (c) Nyquist plots. (d) CV curve of Co-0.2Fe-450, (e) GCD curve of Co-0.2Fe-450, and (f) cycling stability performance of Co-0.2Fe-450.

Table 1 Comparison of specific capacitance of the Co-0.2Fe-450 electrode material with similar material

Electrode material	Electrolyte	Current density	Specific capacitance	Ref.
Co-0.2Fe-450	1 M KOH	1 A g^−1	680 F g^−1	This work
Mn0.05Co2.95O4	4 M KOH	1 A g^−1	80.8 F g^−1	44
Co3O4	6 M KOH	1 A g^−1	523 F g^−1	45
Co3O4-T	3 M KOH	1 A g^−1	216.4 F g^−1	46
Mn–Co3O4	1 M KOH	1.5 A g^−1	537 F g^−1	47
Co3O4-NLIG	1 M KOH	0.5 mA cm^−2	216.3 F g^−1	48
Co3O4/N-CP	6 M KOH	—	316.2 F g^−1	49
Fe–Co3O4	3 M KOH	1 A g^−1	153 F g^−1	50
Co3O4@CSCN	6 M KOH	1 A g^−1	508 F g^−1	51

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Table 2 Comparison of cycle stability of Co-0.2Fe-450 electrode materials with similar materials

Electrode material	Electrolyte	Current density	Cycle number	Capacitance retention	Ref.
Co-0.2Fe-450	1 M KOH	10 A g^−1	5000	84.67%	This work
Co3O4@N/C	1 M KOH	1 A g^−1	5000	83%	52
N-Co3O4	6 M KOH	10 A g^−1	3000	80.1%	53
O67/C67/CC	3 M KOH		6000	70%	54
Ni–Co3O4	1 M KOH	5 A g^−1	1000	89.3%	54
Mn–Co3O4	2 M KOH	5 A g^−1	5000	73.9%	55
Mn–Co3O4	2 M KOH	5 A g^−1	2000	71.2%	56
S-ZnCo2O4	2 M KOH	5 A g^−1	5000	78%	57
S-NixCo3−xO4	2 M KOH	4 A g^−1	2000	80.8%	58

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The two-electrode test is used to confirm the electrode material's practical applicability. Due to its high voltage window, commercial activated carbon (AC) is typically employed as a negative material for supercapacitors. In order to determine the mass ratio of Co-0.2Fe-450 to AC, an electrochemical test of AC was first performed in a three-electrode system. The CV curve of AC, as illustrated in Fig. S9 (ESI†), exhibits an essentially rectangular shape that is typical of carbon-based materials. There is no discernible degradation in the curve shape as sweep speed increases. The GCD curve has double electric layer characteristics and is triangular in shape. It lacks an evident charging and discharging platform. The specific capacitance under a current density of 1 A g⁻¹ is 120 F g⁻¹, and eqn (4) may be used to derive the mass ratio of CO-0.2Fe-450 : AC = 0.18. Fig. 8a shows an asymmetric supercapacitor assembled with Co-0.2Fe-450 and AC (Co-0.2Fe-450//AC ASC). Fig. 8b shows the operating voltage window of Co-0.2Fe-450 and AC. Among them, the voltage window of Co-0.2Fe-450 electrode ranges from 0 to 0.7 V, and the potential window of AC electrode ranges from −1 to 0 V. The two have the characteristics of battery capacitor and double layer capacitor respectively when the sweep speed is 20 mV s⁻¹, further confirming the operating voltage window of ASC, as shown in Fig. S10 (ESI†). When the voltage window increases to 1.7 V, the curve becomes more polarized, so it is determined that the ASC's operating voltage window is 0–1.6 V. CV curves of ASC at different sweep speeds are shown in Fig. 8c. With the increase of sweep speed, CV curves basically remain unchanged, which proves that ASC has excellent magnification performance. In addition, according to the shape of CV curve, it can be observed that the double layer and Faraday reaction exist simultaneously. Fig. 8d displays the ASC GCD curve at various current densities. Under current densities of 1, 2, 4, 6, 8, and 10 A g⁻¹, the specific capacitance of ASC is 50, 44, 35, 30, 27.5, and 25 F g⁻¹, in that order. Fig. S11 (ESI†) shows the relationship between the current density and the specific capacitance. When the current density increases from 1 A g⁻¹ to 10 A g⁻¹, the capacitance retention rate is 50%, which has a good magnification performance. After 5000 cycles of charging and discharging, the material activation of the ASC electrode material causes the capacitance retention rate to reach 105.5% at a current density of 10 A g⁻¹. It has satisfactory cycle stability. Eqn (2) and (3) can be used to obtain the energy density and power density of ASC, as seen in Fig. 8f. The ASC can achieve a maximum energy density of 17.78 W h kg⁻¹ and a power density of 803.13 W kg⁻¹. Furthermore, the energy density can reach 3.35 W h kg⁻¹ at a high power density of 8014.28 W kg⁻¹. These outcomes significantly surpass those of other analogous electrode materials that have been previously documented, including Co₃O₄//AC,⁵⁹ Fe-N-CHPs,⁶⁰ Co₃O₄//AC,⁶¹ CoNW//CF,⁶² and Co₃O₄//AC.⁶³The practical application of ASC is further proved.

Fig. 8 (a) Schematic diagram of Co-0.2Fe-450//AC asymmetric supercapacitor, (b) CV curves of Co-0.2Fe-450 and AC at 20 mV s⁻¹, (c) and (d) CV and GCD curves of Co-0.2Fe-450//AC, (e) cycling stability performance of Co-0.2Fe-450//AC, (f) energy density and power density of Co-0.2Fe-450//AC versus other similar materials.

4. Conclusions

To summarize, Fe-doped Co₃O₄ with a nanosheet floral cluster structure was effectively created using a straightforward hydrothermal technique and a subsequent thermal decomposition reaction. By optimizing the influence of different doped iron contents and thermal decomposition temperature on the electrochemical performance, the electrode material of the Co-0.2Fe-450 supercapacitor prepared at the best ratio and the most suitable temperature has excellent electrochemical performance. Fe doping can enhance an electrode material's electrical characteristics while exposing more reactive sites to create diffusion channels that will facilitate the movement of ions and electrons. In the three-electrode system, the specific capacitance was 680 F g⁻¹ at a current density of 1 A g⁻¹, and the capacitance retention rate was 84.67% after 5000 cycles. The assembled asymmetric supercapacitor (Co-0.2Fe-450//AC) has an excellent energy storage performance and cycling stability with a power density of 803.13 W kg⁻¹ at an energy density of 17.78 W h kg⁻¹ and a capacitance retention of 105.5% after 5000 cycles. Thus, these excellent results indicate that the synthesis strategy of Fe-doped Co₃O₄ can provide a feasible pathway for other doped materials.

Author contributions

Design of the study and supervision of the overall project: Songjun Li, Maiyong Zhu. Performing of the experiments, collection of data, and drafting of the manuscript: Congcong Lu. Contribution to the result discussion: Congcong Lu, Yu Yang.

Conflicts of interest

The authors declare no competing financial interest.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00170b

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