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Abstract
This study delves into the synthesis and electrochemical performance of Co-, Ni-, and Cu-based ferrite/g-C3N4 composites for supercapacitor applications. The investigation covers the synthesis procedures for NiFe2O4, CuFe2O4, and CoFe2O4, as well as their composites with graphitic carbon nitride (g-C3N4). The electrochemical characterization includes cyclic voltammetry, charge-discharge tests, electrochemical impedance spectroscopy, and stability measurements. The results demonstrate that the incorporation of g-C3N4 significantly enhances the specific capacitance, rate capability, and cycling stability of the ferrite materials. The study concludes that the CoFe2O4/g-C3N4 composite exhibits the highest specific capacitance and superior performance across various current densities, making it a promising candidate for high-performance supercapacitor electrodes. The detailed analysis of the structural and electrochemical properties provides a comprehensive understanding of the synergistic effects between the ferrite and g-C3N4 components, highlighting their potential for advanced energy storage applications.
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Abstract
This study focus on the synthesis and electrochemical characterization of spinel ferrite and C3N4 composite materials for energy storage applications. Spinel ferrites such as NiFe2O4, CuFe2O4, and CoFe2O4 were synthesized and combined with C3N4 to improve their performance in supercapacitor applications. The synthesis process involved coating 1 mg of electrode slurry onto nickel foam to exploit its porous structure, which enhances active material retention. Electrochemical measurements revealed that at a current density of 1 A/g, NiFe2O4 exhibited a capacitance value of 238.09 F/g, while the NiFe2O4/C3N4 composite reached 847.61 F/g, demonstrating superior capacitance retention and good cycling stability. Similarly, CuFe2O4 and CuFe2O4/C3N4 composites were characterized, achieving a capacitance of 424.7 F/g and maintaining 92.8% efficiency over 10,000 cycles. CoFe2O4 and CoFe2O4/C3N4 composites displayed even higher performance, with a capacitance of 2291.03 F/g at 1 A/g. The combination of high surface area C3N4 with ferrites enhances active material loading, ion–electron interactions, and charge transfer, leading to improved electrochemical performance. These materials, derived from abundant, non-toxic elements, offer an environmentally friendly, cost-effective solution for energy storage, reducing environmental impact while enhancing performance.
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1 Introduction
Spinel ferrites are a group of magnetic compounds characterized by the general formula MFe2O4, where M stands for a metal ion, such as Fe2+, Co2+, Ni2+, and others, and the ferrite structure consists of these metal ions being arranged in both octahedral and tetrahedral sites within the spinel lattice [1‐4]. This class of materials has garnered considerable attention due to their exceptional electronic, magnetic, and electrochemical properties. These unique characteristics, combined with their high natural abundance, low cost, and biocompatibility, have enabled a wide range of applications in diverse fields, from biomedicine and sensing to water purification and energy storage Technologies [5].
One of the most promising applications of spinel ferrites is in energy storage, particularly in supercapacitors. Supercapacitors, also known as ultracapacitors, are energy storage devices that are gaining widespread recognition due to their ability to deliver high power densities, rapid charge/discharge cycles, and long cycle lives. This makes them suitable for a variety of applications, including renewable energy storage, portable electronic devices, and electric vehicles. Unlike conventional batteries, which rely on chemical reactions for energy storage, supercapacitors store energy through the physical process of ion adsorption and desorption at the electrode/electrolyte interface, allowing for faster charge and discharge rates [6‐8].
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Supercapacitors can generally be categorized into two types based on their charge storage mechanism: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store charge through electrostatic interactions between the electrode surface and ions in the electrolyte, whereas pseudocapacitors rely on fast Faradaic reactions that involve electron transfer between the electrode material and the electrolyte ions. Pseudocapacitors combine the high energy density of batteries with the fast charge/discharge capabilities of EDLCs, positioning them as highly desirable for applications that require both high power and high energy storage. Aqueous supercapacitors, which typically operate at lower voltages (1.5–2 V), offer significant advantages over other energy storage systems, including higher power density, better cycling stability, lower cost, and enhanced safety. These features make aqueous supercapacitors an attractive, more sustainable alternative to traditional lithium-ion and sodium-ion batteries [9].
Spinel ferrites, particularly those based on iron oxide, such as magnetite (Fe3O4), are emerging as ideal candidates for use as electrode materials in supercapacitors due to their high electrical conductivity, electrochemical stability, and abundant availability. Magnetite's structure, where Fe2+ and Fe3+ ions coexist in octahedral sites, allows for high electronic conductivity (on the order of 100 S/cm), making it an attractive material for electrochemical applications. Moreover, the high specific surface area of nanostructured ferrites facilitates better interaction with the electrolyte ions, improving the electrochemical performance of supercapacitors. By doping magnetite with other metal ions (e.g., Co2+, Ni2+, or Cu2+), it is possible to further tune the properties of the ferrite, enhancing its energy storage capacity and cycling performance [10‐14].
In addition to their use in supercapacitors, spinel ferrites have a wide range of other applications. Their magnetic properties, including superparamagnetism in nanoparticles with sizes smaller than ~ 30 nm, have made them valuable in biomedicine for applications such as magnetic resonance imaging (MRI), targeted drug delivery, and thermal therapy. The combination of their magnetic and electrochemical properties also makes them promising candidates for use as electrocatalysts in energy conversion devices, such as in the hydrogen generation process for water splitting, which is crucial for renewable energy Technologies [15].
Graphitic carbon nitride (g-C3N4) has recently emerged as an attractive partner material for ferrite-based composites due to its unique structural and electronic attributes. Its high nitrogen content provides abundant chemically active sites, facilitating strong interfacial interaction with metal ions in ferrite lattices and promoting efficient charge transfer across the heterointerface. Moreover, the 2D layered architecture of g-C3N4 offers a large surface area and short diffusion pathways, which support the dispersion of ferrite nanoparticles and enhance electrolyte accessibility. In addition, its favorable electronic properties such as moderate bandgap, good electron mobility, and intrinsic stability enable improved conductivity and synergistic electron transport when integrated with spinel ferrites. These characteristics collectively make g-C3N4 a suitable component for designing high-performance ferrite composites, as demonstrated in several prior studies. Incorporating g-C3N4 therefore provides a solid foundation for achieving improved electrochemical behavior and underscores the novelty of the present work [16, 17].
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While significant progress has been made in utilizing spinel ferrites for energy storage and other applications, their full potential has yet to be realized. One of the main challenges lies in the development of simple, low-cost, and environmentally friendly synthesis methods that can yield high-performance ferrite nanomaterials. Recent research efforts have focused on green chemical synthesis routes, such as hydrothermal and sol–gel methods, which offer the advantages of scalability, simplicity, and low energy requirements [18].
In summary, spinel ferrites are highly promising materials for energy storage and conversion due to their unique crystal structure, excellent electrochemical properties, and broad application potential. Their ability to deliver both high power and energy densities, along with their cost-effectiveness and environmental sustainability, positions them as a key material in the development of next-generation supercapacitors and other electrochemical devices. Despite challenges in their synthesis and performance optimization, ongoing research in this field continues to explore new avenues for improving the properties of spinel ferrites and expanding their applications across various domains.
In this study, we aimed to enhance the performance of spinel ferrite and C3N4 composite materials for energy storage applications. Spinel ferrites such as NiFe2O4, CuFe2O4, and CoFe2O4 were synthesized and combined with C3N4 to improve their electrochemical properties in supercapacitor applications. The synthesis process involved coating the electrode slurry onto nickel foam to take advantage of its porous structure, which aids in retaining active materials. Electrochemical characterization showed that the composite materials exhibited superior capacitance retention and excellent cycling stability compared to the individual ferrites. The incorporation of C3N4 with spinel ferrites enhanced the overall electrochemical performance by improving active material loading, ion–electron interactions, and charge transfer. These composites, derived from abundant, non-toxic elements, present a sustainable and cost-effective approach for energy storage applications, offering an environmentally friendly alternative to traditional materials.
2 Experimental
2.1 Materials
Citric acid, Nitric acid, Urea, Ni(NO3)20.6H2O ve Fe(NO3)30.9H2O, ethylene glycol, CuCl2·2H2O, FeCl3·6H2O2 NaAca Co(NO3)2 and polyvinylpyrrolidone (PVP) was purchased from Merck.
2.2 Synthesis of g-C3N4
Following a modified protocol, bulk graphitic carbon nitride (g-C3N4) was synthesized via a thermal polycondensation process. In a representative procedure, 10 g of urea (CO(NH2)2) was calcined in a covered muffle furnace at 550 °C for 4 h under ambient atmosphere, with a controlled heating rate of 30 °C min−1. The resulting bulk g-C3N4 was chemically exfoliated by dispersing it in 50 mL of nitric acid solution (pH 1) under continuous magnetic stirring and refluxing at 90 °C for 8 h. The treated material was then washed sequentially with deionized water and absolute ethanol to remove residual acid and neutralize the pH to 7. The purified sample was dried under vacuum at 85 °C for 15 h. The resulting yellowish g-C3N4 powder, exhibiting enhanced structural properties, was stored in a desiccator under anhydrous conditions to prevent moisture absorption prior to further characterization and application.
2.3 Synthesis of Fe2NiO4
Nickel ferrite (NiFe2O4) was synthesized using nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and iron nitrate nonahydrate (Fe(NO3)3·9H2O) as precursors in a stoichiometric molar ratio of 1:2 (Ni:Fe). The precursors were dissolved in 50 mL of a solvent mixture of deionized water and ethylene glycol under continuous stirring, followed by ultrasonication for 30 min to ensure complete dissolution and homogenization. Subsequently, 9 mmol of urea was added, and the solution was further ultrasonicated to achieve uniform dispersion.
The homogeneous solution was then transferred into a Teflon-lined stainless steel autoclave for hydrothermal treatment at 160 °C for 18 h under autogenous pressure. After the reaction, the product was collected by centrifugation, washed sequentially with deionized water and ethanol to remove residual impurities, and dried at 70 °C in a convection oven to obtain the final nickel ferrite powder.
2.4 Synthesis of CuFe2O4
Copper ferrite (CuFe2O4) was synthesized using copper(II) chloride dihydrate (CuCl2·2H2O, 2 mmol) and iron(III) chloride hexahydrate (FeCl3·6H2O, 4 mmol) as metal precursors, in combination with sodium acetate (NaAc, CH3COONa, 6 mmol) and polyvinylpyrrolidone (PVP, 0.5 g). These components were dissolved in 30 mL of ethylene glycol under vigorous magnetic stirring to form a homogeneous solution.
The solution was then transferred into a Teflon-lined stainless steel hydrothermal reactor and subjected to hydrothermal treatment at 160 °C for 24 h under autogenous pressure. After naturally cooling to room temperature, the resulting precipitate was collected by centrifugation, washed several times with deionized water and ethanol to remove unreacted residues, and dried in a vacuum oven at 90 °C to obtain the final copper ferrite product.
2.5 Synthesis of CoFe2O4 and CoFe2O4/g-C3N4
The synthesis of cobalt ferrite (CoFe2O4) was performed following procedures inspired by Yao et al. [19]. Cobalt nitrate (Co(NO3)2·6H2O, 10 mmol), iron(III) nitrate (Fe(NO3)3·9H2O, 20 mmol), and polyvinylpyrrolidone (PVP, 1 g) were stoichiometrically combined and dissolved in ethylene glycol. Under continuous stirring, NaOH (0.2 mol) was added dropwise (1 drop/s) to obtain a homogeneous precursor solution. The solution was then transferred into a 100 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 160 °C for 24 h. The resulting product was collected by centrifugation and washed sequentially with ethanol and distilled water.
For the preparation of the CoFe2O4/g-C3N4 composite, the acid-modified g-C3N4 powder and CoFe2O4 nanoparticles were dispersed in 1,3-butylene glycol using ultrasonic agitation combined with vigorous mechanical stirring to form a uniform slurry. This slurry was transferred to a Teflon-lined autoclave and thermally treated at 160 °C for 24 h. After the heat treatment, the resulting material was dried under vacuum at 60 °C for 12 h and designated as CoFe2O4/g-C3N4.
2.6 Synthesis of C3N4/CoFe2O4
A mixture was prepared by combining 100 mg of CoFe2O4 with 25 mg of C3N4 and adding 40 mL of water. The suspension was stirred on a magnetic stirrer until a homogeneous mixture was achieved. Subsequently, the mixture was transferred to a hydrothermal reactor and heated at 160 °C for 18 h. After the hydrothermal treatment, the resulting product was washed several times with water and dried in a vacuum oven.
2.7 Synthesis of C3N4/Fe2NiO4
Nickel ferrite (NiFe2O4) was synthesized via a hydrothermal route using nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and iron nitrate nonahydrate (Fe(NO3)3·9H2O) as precursors in a stoichiometric molar ratio of 1:2 (Ni: Fe). The precursors were dissolved in a 50 mL binary solvent system comprising deionized water and ethylene glycol (1:1 v/v) under vigorous stirring. The solution was ultrasonicated for 30 min to ensure complete dissolution and homogenization, followed by adding 9 mmol urea as a precipitating agent. The mixture was further agitated using an ultrasonic stirrer until a colloidal suspension was achieved.
The homogeneous suspension was transferred into a Teflon-lined stainless steel autoclave and subjected to hydrothermal treatment at 160 °C for 18 h under autogenous pressure. Post-synthesis, the product was isolated via centrifugation, sequentially washed with deionized water and ethanol to remove unreacted species, and dried at 70 °C in a convection oven.
To fabricate the NiFe2O4/g-C3N4 composite, 100 mg of the synthesized nickel ferrite was uniformly blended with 40 mg of graphitic carbon nitride (g-C3N4) via mechanical grinding. The powder mixture was dispersed in 40 mg of deionized water under continuous magnetic stirring to form a slurry. This slurry was hydrothermally treated under identical conditions (160 °C, 18 h) to promote interfacial interaction. The final composite was centrifuged, rinsed thoroughly with deionized water to eliminate residual impurities, and dried in a vacuum oven at 60 °C.
2.8 Electrochemical characterization
The electrochemical performance of the supercapacitor was initially evaluated in a three-electrode configuration to characterize the anode, cathode, and electrolyte independently. Following the optimization of electrode parameters, a full-cell supercapacitor was assembled using the tailored electrodes. The electrodes were fabricated by homogenizing a slurry composed of 85 wt% electroactive material (anode/cathode), 10 wt% activated carbon (AC) as a conductive additive, and 5 wt% polyvinylidene fluoride (PVDF) as a binder. N-methyl-2-pyrrolidone (NMP) was employed as a solvent-based binder to achieve uniform slurry dispersion. The slurry was cast onto a current collector and dried in a vacuum oven at 80 °C to ensure complete solvent evaporation, thereby minimizing residual impurities.
During the study, a Gamry Reference 3000 potentiostat was used to set up the electrochemical system. For the working electrode (WE), a 1 cm × 1 cm nickel foam substrate was employed, onto which 1–1.5 mg of active material was deposited. A platinum (Pt) wire served as the counter electrode (CE), while a silver/silver chloride (Ag/AgCl) electrode was used as the reference electrode (RE). The operating potential of the electrode was first determined using cyclic voltammetry (CV) within the three-electrode system. Subsequently, galvanostatic charge/discharge (GCD) tests were performed under a constant current to assess the charge/discharge capacity, and efficiency calculations were made after extended cycling. In addition, electrochemical impedance spectroscopy (EIS) measurements were conducted to calculate the equivalent series resistance (Rs) and the charge transfer resistance (Rct).
The capacitance of the electrode material was calculated from the CV curves using the following equation:
where ∫I(V) dV is the integrated area of the CV curve, m is the mass of the synthesized active material, s is the scan rate (mV/s), and ΔV is the applied potential window.
2.9 Structural Characterization
The substrate morphology was analyzed by scanning electron microscopy (SEM) imaging performed at 3 kV (FESEM, Zeiss Gemini 500). X-ray diffraction patterns of silver decorated ZnO thin film hybrid nanostructures (Ag@ZnO) were analyzed by X-ray Diffraction (XRD) technique in the range of 10°−95° 2θ on a Panalytical X-ray diffractometer. The specific surface area (SSA) was determined via the Brunauer–Emmett–Teller (BET) method. At the same time, pore size distribution and total pore volume were derived from nitrogen adsorption–desorption isotherms using the Barrett-Joyner-Halenda (BJH) model. Fourier-transform infrared (FT-IR) spectroscopy was conducted with a PerkinElmer 400 spectrometer (Waltham, MA, USA) equipped with a MIRacle attenuated total reflectance (ATR) accessory. Spectra were acquired in the mid-infrared region (500–4000 cm−1) under ambient conditions (25 °C), with a spectral resolution of 4 cm−1.
3 Results and discussion
In this study, the focus was on the synthesis of CoFe2O4, NiFe2O4 and CuFe2O4 spinel ferrite structures and their g-C3N4 doped variants, with a subsequent comparison of their supercapacitor performances. A particular focus of the study was the effect of gC3N4 doping on the energy storage properties of these materials in supercapacitors.
As shown in Fig. 1, the synthesis procedure of spinel ferrite materials and the associated structural transformations are illustrated. The preparation of various metal ferrites and their g-C3N4-modified derivatives was performed using the hydrothermal synthesis method. The preparation steps involving metal salts and iron salts for Ni, Cu, and Co ferrites are provided in Fig. 1a, b, and c, respectively. The resulting crystal structures obtained after synthesis are presented at the end of each column. The impact of g-C3N4 modification on structural integrity and its influence on supercapacitor performance will be discussed based on structural characterisation and electrochemical analysis.
Fig. 1
Synthesis procedure of spinel Ferrit materials by hydrothermal reactor. a synthesis of spinel NiFe2O4, b synthesis of spinel CuFe2O4, c synthesis of spinel CoFe2O4
Figure 2 presents SEM images of synthesised spinel ferrite materials along with their g-C3N4-modified counterparts. SEM images of NiFe2O4 and g-C3N4-modified NiFe2O4 nanomaterials are provided in Fig. 2a and b, respectively, revealing no distinct topological differences upon inspection. Furthermore, the SEM images of CoFe2O4 and its g-C3N4-modified counterpart (shown in Fig. 2c and d, respectively) do not demonstrate any notable alterations in the nanostructures. This observation suggests that g-C3N4 incorporation does not induce significant deformation in the crystal structure. In order to provide a comprehensive elucidation of this aspect, supplementary structural analyses such as XRD and FTIR are required. Conversely, the SEM images of CuFe2O4 and g-C3N4-modified CuFe2O4 nanomaterials (Fig. 2e and f, respectively) exhibit discernible structural variations. Consequently, further characterisation techniques should be employed to thoroughly investigate the structural changes induced by g-C3N4 modification in spinel ferrite materials.
Fig. 2
SEM images of spinel ferrite materials a NiFe2O4b NiFe2O4/g-C3N4c CoFe2O4d CoFe2O4/g-C3N4e CuFe2O4f CuFe2O4/g-C3N4
As seen in Fig. 3a, the crystalline phases of pure NiFe2O4 and NiFe2O4/g-C3N4 composites are shown. As demonstrated in Fig. 4a (red line), the characteristic diffraction peak of g-C3N4 at 2θ = 27.3°, corresponding to the (002) crystal plane associated with interlayer graphitic stacking (JCPDF 87-1526) [20], is not clearly observed. In addition, the XRD spectrum of pure NiFe2O4 (Fig. 3a) exhibits distinct diffraction peaks at 30.3°, 35.7°, 43.4°, 53.8°, 57.3°, and 63.0°, attributed respectively to the (220), (311), (222), (400), (511), and (440) crystallographic planes [21]. The observed diffraction peaks confirm the formation of spinel-type NiFe2O4, which closely matches the reference diffraction data (JCPDF 10–0325).
Fig. 3
Structural characterization of Spinel ferrit NiFe2O4 & NiFe2O4/g-C3N4. aXRD analysis of NiFe2O4 (bottom)and NiFe2O4/g-C3N4 (top), b FTIR analysis of NiFe2O4 & NiFe2O4/g-C3N4, c N2 adsorption–desorption analysis of NiFe2O4, d N2 adsorption–desorption analysis of NiFe2O4/g-C3N4
Structural characterization of Spinel ferrit CoFe2O4 & CoFe2O4/g-C3N4. a XRD analysis of CoFe2O4 (bottom) and CoFe2O4/g-C3N4 (top), b FTIR analysis of CoFe2O4 & CoFe2O4/g-C3N4, c N2 adsorption–desorption analysis of CoFe2O4, d N2 adsorption–desorption analysis of CoFe2O4/g-C3N4, e Raman analysis of CoFe2O4 & CoFe2O4/g-C3N4, f UV–Vis spectrometer results of CoFe2O4 & CoFe2O4/g-C3N4
The FTIR spectra of NiFe2O4 and NiFe2O4/g-C3N4 reveal characteristic absorption bands corresponding to metal–oxygen vibrations in the spinel ferrite structure, typically observed around 400–600 cm−1. In the NiFe2O4/g-C3N4 composite, additional peaks corresponding to the stretching vibrations of C–N and C=N groups are evident, confirming the successful incorporation of graphitic carbon nitride into the composite matrix. These structural features suggest the presence of strong interfacial interactions between NiFe2O4 nanoparticles and the g-C3N4 sheets, which can enhance charge transfer and facilitate electron mobility.
N2 adsorption–desorption analysis analysis further supports the structural observations. The specific surface area of pure NiFe2O4 is measured to be approximately 63 m2/g, while the NiFe2O4/g-C3N4 composite exhibits a slightly lower surface area of 62 m2/g. Despite this marginal decrease, the composite material demonstrates superior electrochemical performance. This apparent contradiction can be attributed to the synergistic effect between NiFe2O4 and g-C3N4, where the latter not only provides a conductive network but also helps maintain a porous structure that promotes ion diffusion and enhances the accessibility of electroactive sites.
Therefore, the improved electrochemical behavior of the NiFe2O4/g-C3N4 composite is not solely governed by surface area but also significantly influenced by the structural and chemical characteristics imparted by the g-C3N4 component. These factors collectively contribute to faster charge–discharge kinetics and better overall capacitance performance, as observed in the electrochemical measurements.
As illustrated in Fig. 4a, the crystalline phases of pure CoFe2O4 and CoFe2O4/g-C3N4 composites are evident. As demonstrated in Fig. 4a (black line), pure g-C3N4 displays a discernible diffraction 2θ peak at 27.3°, corresponding to the (002) crystal plane. This peak is indicative of interlayer graphitic stacking (JCPDF 87–1526) [20]. In the same figure, the XRD spectrum of pure CoFe2O4 reveals characteristic diffraction peaks at 30.2°, 35.6°, 43.3°, 53.7°, 57.3°, and 62.8°, which correspond respectively to the crystal planes (111), (220), (311), (222), (400), (511), and (440). The presence of these peaks confirms the formation of spinel-type CoFe2O4 [25], a finding which is in close agreement with the standard diffraction data (JCPDF 22–1086).
The FTIR spectra of CoFe2O4 and CoFe2O4/g-C3N4 provide clear insight into the structural features of the materials. In both samples, characteristic bands related to metal–oxygen stretching vibrations within the spinel ferrite structure appear in the region of 400–600 cm−1, confirming the formation of CoFe2O4. In the CoFe2O4/g-C3N4 composite, additional absorption bands associated with C–N and C = N bonds are observed, indicative of the successful integration of graphitic carbon nitride (g-C3N4) into the composite system. These spectral changes suggest strong interfacial interactions between the ferrite particles and the g-C3N4 matrix, which can play a crucial role in improving charge transport properties.
N2 adsorption–desorption analysis surface area analysis further supports these structural findings. The specific surface area of pure CoFe2O4 is measured at approximately 38 m2/g, whereas the CoFe2O4/g-C3N4 composite shows a slightly increased surface area of 40 m2/g. Although the increase in surface area is relatively modest, it implies that the g-C3N4 component contributes to maintaining a porous structure that can facilitate electrolyte ion diffusion and increase the number of accessible active sites.
This slight enhancement in surface area, combined with the conductive and chemically interactive nature of g-C3N4, explains the observed improvement in the electrochemical performance of the CoFe2O4/g-C3N4 composite. The presence of g-C3N4 not only aids in electron transfer but also stabilizes the electrode architecture during charge–discharge processes.
As demonstrated in Fig. 5a, the crystalline phases of pure CuFe2O4 and CuFe2O4/g-C3N4 composites are evident. As demonstrated in Fig. 4a (black line), pure g-C3N4 displays a discernible diffraction 2θ peak at 27.3°, corresponding to the (002) crystal plane. This peak is indicative of interlayer graphitic stacking (JCPDF 87-1526) [20]. In the exact figure, the XRD spectrum of pure CuFe2O4 reveals characteristic diffraction peaks at 24.2°, 33.2°, 35.7°, 40.9°, 49.5°, 54.1°, 57.6°, 62.5°, and 64.1°, which correspond respectively to the crystal planes (111), (220), (311), (222), (400), (511), and (440). The presence of these peaks confirms the formation of spinel-type CuFe2O4 [26], a finding which is in close agreement with the standard diffraction data (JCPDF 06–0545).
Fig. 5
Structural characterization of Spinel ferrit CuFe2O4 & CuFe2O4/g-C3N4. a XRD analysis of CuFe2O4 (bottom) and CuFe2O4/g-C3N4 (top), b FTIR analysis of CuFe2O4 & CuFe2O4/g-C3N4, c N2 adsorption–desorption analysis of CuFe2O4, d N2 adsorption–desorption analysis of CuFe2O4/g-C3N4
Here, the wavelength of X-ray radiation is denoted by λ, the Bragg's angle of the peaks by θ, and the angular width of the peaks at the full width at half maximum (FWHM) by β. The mean crystal size was determined to be 17.10, 17.79, 41.25, 18.17, 18.74 and 32.32 nm for NiFe2O4, CoFe2O4, CuFe2O4, NiFe2O4 with g-C3N4, CoFe2O4 with g-C3N4, and CuFe2O4 with g-C3N4, respectively.
FTIR spectra of CuFe2O4 show prominent absorption bands around 500–600 cm−1, typical of metal–oxygen vibrations within the spinel lattice. In the CuFe2O4/g-C3N4 composite, additional peaks associated with C–N and C = N stretching vibrations are observed, confirming the successful integration of g-C3N4. The presence of both ferrite and g-C3N4 signatures supports the formation of a hybrid structure with potential for improved electrochemical interfaces.
N2 adsorption–desorption measurements indicate that pure CuFe2O4 possesses a surface area of approximately 18 m2/g, while the CuFe2O4/g-C3N4 composite shows a slightly lower surface area of 17 m2/g.
Despite the minor decrease, the composite exhibits superior electrochemical performance. This counterintuitive behavior is explained by the synergistic effects between CuFe2O4 and g-C3N4: the g-C3N4 matrix provides a more conductive framework and facilitates better dispersion of ferrite nanoparticles, enhancing charge transport and preserving structural integrity during cycling.
Overall, the combination of spinel CuFe2O4 with g-C3N4 results in a composite material with improved structural and interfacial properties that directly contribute to enhanced electrochemical activity. These findings highlight that surface area alone does not dictate performance; rather, the nature of the interfaces and the structural compatibility between components play a critical role in determining electrochemical behavior [28].
In Fig. 6; the electrochemical performance of NiFe2O4 and its composite with graphitic carbon nitride (g-C3N4) was assessed using cyclic voltammetry (CV), charge–discharge curves, electrochemical impedance spectroscopy (EIS), and stability tests. The obtained results provide valuable insights into the impact of g-C3N4 integration on the capacitive behavior and rate capability of the materials. CV Measurement of NiFe2O4 at 20, 50, 100 mV/s (Fig. 6a).
Fig. 6
Electrochemical characterization of NiFe2O4 & NiFe2O4/g-C3N4. a CV measurement of NiFe2O4 at 20, 50, 100 mV/s. b CV measurement of NiFe2O4/g-C3N4 at 20, 50, 100 mV/s. c charge–discharge curves of NiFe2O4 at different current load per 1 g. d charge–discharge curves of NiFe2O4/g-C3N4 at different current load per 1 g. e impedance measurements of NiFe2O4 & NiFe2O4/g-C3N4. f stability measurement of NiFe2O4 & NiFe2O4/g-C3N4 at same cycles
The cyclic voltammograms (CV) of NiFe2O4 at various scan rates (20, 50, and 100 mV/s) show a typical redox behavior, indicating good electrochemical reversibility. However, the peak currents slightly decrease as the scan rate increases, which is typical of materials with relatively slower ion diffusion kinetics. CV Measurement of NiFe2O4/g-C3N4 at 20, 50, 100 mV/s (Fig. 6b).
In contrast, the CV of NiFe2O4/g-C3N4 composite demonstrates higher peak currents and more defined redox peaks at the same scan rates, indicating improved electrochemical kinetics. This is attributed to the enhanced conductivity and improved ion diffusion facilitated by the g-C3N4 matrix. The composite material exhibits better charge storage properties, which directly correlates with the improved electrochemical performance observed in charge–discharge tests. Charge–Discharge Curves of NiFe2O4 at Different Current Load per 1 g (Fig. 6c).
The charge–discharge curves for pure NiFe2O4 at various current densities show a typical triangular shape, characteristic of a capacitive process. However, the material exhibits a noticeable drop in specific capacitance as the current density increases, with a value of 238.09 F/g at 1 A/g. This decrease suggests limited ion diffusion at higher current loads, reducing the overall performance of the material at higher rates.Charge–Discharge Curves of NiFe2O4/g-C3N4 at Different Current Load per 1 g (Fig. 6d) In contrast, the charge–discharge curves of NiFe2O4/g-C3N4 composite at the same current densities exhibit better rate capability and higher capacitance retention. At 1 A/g, the composite reaches 847.61 F/g, demonstrating a significant improvement over pure NiFe2O4. This suggests that the integration of g-C3N4 enhances the material’s ability to maintain high capacitance values even under higher current densities, owing to the improved charge transport and enhanced ionic conductivity provided by the g-C3N4 framework.
At 2.5 A/g, pure NiFe2O4 shows a capacitance of 214.6 F/g, while the composite retains 657.14 F/g. At 5 A/g, the capacitance of pure NiFe2O4 is further reduced to 224.86 F/g, whereas the composite material shows 519.04 F/g, indicating that the g-C3N4 matrix significantly mitigates the decrease in capacitance at higher current densities.Impedance Measurements of NiFe2O4 and NiFe2O4/g-C3N4 (Fig. 6e). Electrochemical impedance spectroscopy (EIS) measurements further support the enhanced electrochemical performance of the composite. The Nyquist plots of NiFe2O4 show a larger semicircular arc at high frequencies, indicating higher charge transfer resistance and slower electron transport. In contrast, the Nyquist plot of NiFe2O4/g-C3N4 exhibits a smaller arc, suggesting lower charge transfer resistance and enhanced conductivity. This is consistent with the improved rate capability and higher capacitance retention observed in the charge–discharge tests.Stability Measurement of NiFe2O4 and NiFe2O4/g-C3N4 at Same Cycles (Fig. 6f). The stability tests reveal that the NiFe2O4/g-C3N4 composite maintains a higher percentage of its initial capacitance after multiple charge–discharge cycles, demonstrating superior cycling stability. This is attributed to the structural stability provided by the g-C3N4 matrix, which helps to prevent the aggregation or degradation of the ferrite nanoparticles during prolonged electrochemical cycling.The integration of g-C3N4 into the NiFe2O4 matrix significantly enhances the electrochemical performance of the composite. The increased capacitance values, especially at higher current densities (1 A/g: 847.61 F/g vs. 238.09 F/g for pure NiFe2O4, 2.5 A/g: 657.14 F/g vs. 214.6 F/g, and 5 A/g: 519.04 F/g vs. 224.86 F/g), demonstrate the superior charge storage ability of the composite material. The impedance measurements indicate that the g-C3N4 matrix reduces charge transfer resistance, leading to improved rate performance. Additionally, the composite's stability over multiple cycles further suggests that g-C3N4 contributes to the long-term structural integrity of the material.
In Fig. 7,the electrochemical behavior of CoFe2O4 and its composite with graphitic carbon nitride (g-C3N4) was evaluated using cyclic voltammetry (CV), charge–discharge tests, electrochemical impedance spectroscopy (EIS), and stability measurements. The results indicate a significant enhancement in the capacitive performance and rate capability of the composite material.CV Measurement of CoFe2O4 at 20, 50, 100 mV/s (Fig. 7a).
Fig. 7
Electrochemical characterization of CoFe2O4 & CoFe2O4/g-C3N4. a CV measurement of CoFe2O4 at 20, 50, 100 mV/s. b CV measurement of CoFe2O4/g-C3N4 at 20, 50, 100 mV/s. c charge–discharge curves of CoFe2O4 at different current load per 1 g. d charge–discharge curves of CoFe2O4/g-C3N4 at different current load per 1 g. e Impedance measurements of CoFe2O4 & CoFe2O4/g-C3N4. f Stability measurement of CoFe2O4 & CoFe2O4/g-C3N4 at same cycles
The cyclic voltammetry (CV) curves of CoFe2O4 at various scan rates (20, 50, and 100 mV/s) reveal typical redox behavior, indicating that the material is capable of fast charge and discharge processes. However, as the scan rate increases, the peak current decreases, suggesting a relatively slower ion diffusion process and limited conductivity at higher scan rates. CV Measurement of CoFe2O4/g-C3N4 at 20, 50, 100 mV/s (Fig. 7b). In contrast, the CV curves of CoFe2O4/g-C3N4 at the same scan rates exhibit more prominent redox peaks and higher peak currents, highlighting the enhanced electrochemical kinetics of the composite material. This suggests that the incorporation of g-C3N4 improves the conductivity and provides more efficient ion transport, resulting in a better electrochemical response. Charge–Discharge Curves of CoFe2O4 at Different Current Loads per 1 g (Fig. 7c). The charge–discharge curves of pure CoFe2O4 at different current densities show typical capacitive behavior, with a noticeable decrease in the specific capacitance as the current load increases. At 1 A/g, the capacitance of CoFe2O4 is 228 F/g. As the current density increases to 2.5 A/g, the specific capacitance decreases to 512 F/g, and further decreases to 333 F/g at 5 A/g. These results suggest that the pure CoFe2O4 material exhibits a moderate rate capability, with a significant reduction in capacitance at higher current densities due to limited ion diffusion and slower charge transfer processes.Charge–Discharge Curves of CoFe2O4/g-C3N4 at Different Current Loads per 1 g (Fig. 7d). On the other hand, the charge–discharge curves of CoFe2O4/g-C3N4 composite show much better rate capability and higher capacitance retention at the same current densities. At 1 A/g, the composite achieves 2191.03 F/g, which is a substantial improvement over pure CoFe2O4. At 2.5 A/g, the composite material retains 1653 F/g, and at 5 A/g, the capacitance remains at 1387 F/g. The high specific capacitance and better retention at higher current densities can be attributed to the synergistic effects between CoFe2O4 and g-C3N4, where g-C3N4 not only enhances electron conductivity but also maintains an efficient ion diffusion pathway. Impedance Measurements of CoFe2O4 and CoFe2O4/g-C3N4 (Fig. 7e). Electrochemical impedance spectroscopy (EIS) measurements further clarify the enhanced performance of the composite material. The Nyquist plots of pure CoFe2O4 show a larger semicircular arc at high frequencies, indicating higher charge transfer resistance, which limits the overall conductivity and ion diffusion. In contrast, the Nyquist plot for the CoFe2O4/g-C3N4 composite exhibits a smaller semicircular arc, suggesting lower charge transfer resistance and enhanced conductivity. This enhanced conductivity likely contributes to the better electrochemical performance observed in the CV and charge–discharge tests, particularly at higher current densities.Stability Measurement of CoFe2O4 and CoFe2O4/g-C3N4 at Same Cycles (Fig. 7f). The stability tests indicate that the CoFe2O4/g-C3N4 composite material maintains a higher percentage of its initial capacitance after multiple cycles, highlighting its superior cycling stability. This stability is attributed to the structural reinforcement provided by g-C3N4, which prevents the aggregation of CoFe2O4 particles and maintains the integrity of the electrode during repeated charge–discharge cycles.
The incorporation of g-C3N4 into CoFe2O4 significantly enhances the electrochemical performance of the composite. Specifically, the 1 A/g capacitance of the composite material (2191.03 F/g) is substantially higher than the pure CoFe2O4 (228 F/g), showcasing the superior charge storage capacity. At higher current densities, the composite material also demonstrates much better capacitance retention: 1653 F/g at 2.5 A/g and 1387 F/g at 5 A/g, compared to 512 F/g and 333 F/g for pure CoFe2O4 at the same currents, respectively [29].
The impedance measurements reveal that the composite material has lower charge transfer resistance, which allows for faster charge/discharge kinetics and higher rate capability. This improvement in performance is likely due to the enhanced conductivity of g-C3N4, which forms a conductive network that aids in efficient electron transport and facilitates ion diffusion throughout the electrode material.
In addition, the stability tests confirm that the CoFe2O4/g-C3N4 composite retains a higher percentage of its initial capacitance after multiple cycles, demonstrating excellent cycling stability. This makes the composite an ideal candidate for high-performance supercapacitor applications, where high capacitance retention and long-term stability are critical.
In Fig. 8a, b; the CV curves for both CuFe2O4 and the CuFe2O4/g-C3N4 composite display quasi-rectangular shapes with redox peaks, indicating pseudocapacitive behavior. With increasing scan rates from 20 to 100 mV/s, the current response increases proportionally, demonstrating good rate capability for both materials. However, the CV curves of the composite (b) exhibit higher current density than those of pure CuFe2O4 (a), suggesting enhanced electrochemical activity due to the presence of g-C3N4, which likely improves conductivity and surface area.The GCD curves further confirm the superior performance of the composite. At all current densities, the CuFe2O4/g-C3N4 composite (d) shows longer discharge times compared to the pure CuFe2O4 (c), indicating higher specific capacitance (Fig. 8c, d). Pure CuFe2O4 delivers a specific capacitance of 107.6 F/g, while the CuFe2O4/g-C3N4 composite achieves a significantly higher value of 424.7 F/g.At 2.5 A/g pure CuFe2O4 shows 369.81 F/g, the composite reaches an impressive 3511.90 F/g. At 5 A/g, pure CuFe2O4 maintains 292.17 F/g, while the composite still achieves 2678.57 F/g.
Fig. 8
Electrochemical characterization of CuFe2O4 & CuFe2O4/g-C3N4. a CV measurement of CuFe2O4 at 20, 50, 100 mV/s. b CV measurement of CuFe2O4/g-C3N4 at 20, 50, 100 mV/s. c Charge–discharge curves of CuFe2O4 at different current load per 1 g. d Charge–discharge curves of CuFe2O4/g-C3N4 at different current load per 1 g. e Impedance measurements of CuFe2O4 & CuFe2O4/g-C3N4. f Stability measurement of CuFe2O4 & CuFe2O4/g-C3N4 at same cycle
This remarkable improvement in capacitance, especially at higher current densities, demonstrates that the composite not only stores more charge but also maintains performance under increased load—indicating excellent rate capability and conductivity enhancements due to g-C3N4.
The EIS plots (Nyquist plots) show that the composite exhibits a smaller semicircle in the high-frequency region and a steeper slope in the low-frequency region compared to pure CuFe2O4. This indicates lower charge-transfer resistance and better ion diffusion kinetics in the CuFe2O4/g-C3N4 material, further confirming the synergistic effect of g-C3N4 on improving conductivity and electrochemical behavior (Fig. 8e) [30].
The cycling stability curves demonstrate that the CuFe2O4/g-C3N4 composite retains a higher percentage of its initial capacitance over repeated cycles compared to pure CuFe2O4. This improved stability is critical for long-term applications and is attributed to the structural stability and enhanced charge transport of the composite (Fig. 8f).
The CuFe2O4/g-C3N4 composite significantly outperforms pure CuFe2O4 across all electrochemical tests. The incorporation of g-C3N4 improves conductivity, enhances capacitance (especially at high current densities), reduces internal resistance, and increases cycling stability. These features make it a promising material for high-performance energy storage applications.
The electrochemical performance data of the synthesized materials CuFe2O4, NiFe2O4, CoFe2O4, and their respective g-C3N4 composites are clearly summarized in Table 1. This table presents the specific capacitance values at different current densities, allowing for a direct and detailed comparison between the pure ferrite materials and their composite counterparts.
Table 1
The capacity values calculated at different current densities for MFe2O4(Cu, Ni, Co) and MFe2O4(Cu, Ni, Co)/C3N4 composites
Current density (A)
Samples
NiFe2O4
NiFe2O4- C3N4
CuFe2O4
CuFe2O4- C3N4
CoFe2O4
CoFe2O4- C3N4
1
238.09
847.61
107.6
424.7
228
2191.03
2.5
214.6
657.14
369.81
3511.90
512
1653.54
5
–
519.04
292.17
2678.57
333
1387.09
10
200.7
257.14
766.7
2309.52
309
1274.19
20
47.61
184.5
186.8
1761.90
179
1229.03
50
32.4
126.7
110.7
1190.47
–
1230.32
100
16.6
89.4
–
142.85
–
483.87
From the data in Table 1, it is evident that the CuFe2O4/g-C3N4 composite exhibits a significantly higher specific capacitance compared to pure CuFe2O4. Similar enhancements are observed in the NiFe2O4/g-C3N4 and CoFe2O4/g-C3N4 composites, emphasizing the positive impact of g-C3N4 on the overall electrochemical performance across different transition metal ferrites.
The observed differences in electrochemical performance among the CoFe2O4/g-C3N4, NiFe2O4/g-C3N4, and CuFe2O4/g-C3N4 composites can likely be attributed to the intrinsic properties of the respective metal cations. Cobalt, nickel, and copper ions differ in their electronic conductivity, redox potentials, and ability to facilitate electron transfer within the ferrite lattice. These variations affect the overall charge-transfer efficiency and the kinetics of the redox reactions at the electrode/electrolyte interface. For example, cobalt-based ferrites are known to exhibit higher redox activity and better electronic conductivity compared to their nickel and copper counterparts, which can explain the superior performance observed in the CoFe2O4/g-C3N4 composite [31, 32]. This discussion provides mechanistic insight into the metal-dependent behavior of the composites.
The results in Table 1 also highlight that the materials were tested under various current densities, confirming the excellent rate capability of the composite structures. In all cases, the composite materials outperformed their pure ferrite counterparts, especially at higher current loads—demonstrating improved ion diffusion, lower internal resistance, and enhanced charge storage ability due to the synergistic effect with g-C3N4 [33].
This data not only validates the role of g-C3N4 in enhancing electrochemical behavior but also reinforces the superiority of the CuFe2O4/g-C3N4 composite, making it a promising candidate for high-performance supercapacitor applications.
4 Conclusion
The electrochemical performance analysis conducted at varying current densities clearly demonstrates the superior capacitive behavior of the MFe2O4(Cu, Ni, Co)/C3N4 composites in comparison to the pristine MFe2O4(Cu, Ni, Co) materials. At a current density of 1 A g−1, the composite exhibited a specific capacitance of 2191 F g−1, significantly higher than the 228 F g−1 recorded for the unmodified ferrite. This performance gap persisted across all tested current densities, indicating the robustness of the composite structure.
The enhanced capacitive performance is attributed to the synergistic interaction between the spinel ferrite phase and the graphitic carbon nitride (C3N4) matrix, which contributes to improved electrical conductivity, higher electroactive surface area, and more efficient ion diffusion pathways. Moreover, the composite material retained over 85% of its initial capacitance at higher current densities (e.g., 5 A g−1), reflecting excellent rate capability and stability.
These results highlight the significant role of C3N4 in boosting the charge storage characteristics of ferrite-based materials, establishing the MFe2O4(Cu, Ni, Co)/C3N4 composite as a highly promising candidate for high-performance supercapacitor electrodes. Future research should focus on compositional optimization, scaling-up synthesis techniques, and long-term cycling studies to further explore the practical potential of these materials in energy storage technologies.
Acknowledgements
The authors are grateful for the financial support of the Unit of the Scientific Research Projects of Erciyes University (FDK-2023-12458).
Declarations
Conflict of interest
The authors declare no competing interests.
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