Interfacial synergy in NiCo₂O₄@g-C₃N₄ heterostructures for advanced supercapacitors and selective organic pollutant degradation

Abstract

The development of efficient electrode materials is an important research area in advanced energy-storage technologies. In this study, a NiCo₂O₄@g-C₃N₄ heterostructure was synthesized via a facile hydrothermal method and investigated as a bifunctional material for supercapacitor and photocatalytic applications. The structural, morphological, and chemical properties of the synthesized materials were analysed using various characterization techniques. Electrochemical studies revealed that the NiCo₂O₄@g-C₃N₄ hybrid electrode exhibited enhanced charge-storage performance, delivering a high specific capacitance of 1776.4 F g⁻¹ at 2.4 A g⁻¹ in 3 M KOH electrolyte. The fabricated asymmetric supercapacitor demonstrated good energy-storage capability, retaining 94.48% of its capacitance after 6500 charge–discharge cycles, and produced a stable voltage sufficient to illuminate a red LED. Furthermore, the NiCo₂O₄@g-C₃N₄ nanocomposite exhibited superior visible-light-driven photocatalytic activity, achieving 96.17% degradation of methylene blue (MB) and 96.19% degradation of crystal violet (CV) within 150 min. The enhanced bifunctional performance is attributed to the synergistic interaction between NiCo₂O₄ and g-C₃N₄, which facilitates charge transfer, promotes efficient charge separation, and improves photocatalytic activity. These findings demonstrate the potential of NiCo₂O₄@g-C₃N₄ as a promising material for multifunctional energy-storage and photocatalytic applications.

Multifunctional nanostructures offer promising prospects for electrochemical energy storage and light-driven degradation of pollutants.

1. Introduction

Energy shortages and environmental pollution remain two of the most critical challenges affecting the quality of human life on Earth. As the world's population grows, the demand for renewable energy resources becomes increasingly critical. This has led researchers to focus on developing advanced energy storage devices to address the shortage of sustainable energy.¹ Conventional fossil fuels, including coal, oil, and natural gas, are becoming increasingly scarce and expensive, making them less viable as long-term energy solutions. The combustion of these fuels continues to release harmful gases like carbon dioxide and methane, which contribute to the greenhouse effect and disrupt the Earth's ecological balance.² To address resource scarcity, energy storage challenges, and environmental pollution, various energy storage and conversion technologies, including electrochemical capacitors, secondary batteries, fuel cells, and solar cells, have been explored.³ Sun-derived energy has also proven effective in applications such as photocatalysis, which purifies organic pollutants in industrial wastewater, reduces CO₂ to hydrocarbon fuels, and produces hydrogen gas through photocatalytic water splitting.⁴ Nanomaterials, characterized by their multifunctional behavior and integration in catalysis, sensing, electrochemical conversion, and storage applications, have drawn widespread research interest for the development of next-generation energy technologies. Consequently, the role of EC devices has become increasingly vital in supporting sustainable and alternative energy sources.⁵ Based on the charge storage behavior, supercapacitors are generally grouped into three types: (i) Electrochemical Double-Layer Capacitors (EDLCs), (ii) pseudocapacitors (PCs), and (iii) hybrid capacitors. In EDLCs, charge storage occurs through electrostatic separation at the electrode–electrolyte interface without involving faradaic charge transfer processes. In PCs, the charge is stored via reversible and rapid redox reactions occurring at the surface of the electrode, typically utilizing electroactive materials like conductive polymers and transition metal oxides.⁶ Hybrid supercapacitors combine both mechanisms (electrostatic and redox processes) and are further divided into three categories based on their electrode configurations: asymmetric, composite, and battery-type. Recent advancements in electrode material design, nanostructure engineering, and hybridization strategies have significantly contributed to improving the electrochemical performance and stability of supercapacitor devices.⁷ Therefore, the development of novel electrode materials with enhanced conductivity, active sites, and structural stability is essential for improving energy storage and catalytic performance.⁸ Co₃O₄ has garnered significant attention among transition metal oxides in pseudocapacitor due to its promising performance, especially in industries that produce harmful organic dyes. Research has explored substituting cobalt with less expensive and environmentally friendly elements to form spinel structures, potentially improving performance while addressing environmental concerns.⁹ Among various spinel-type mixed metal oxides, NiCo₂O₄ has attracted considerable attention due to its superior electrical conductivity and rich redox chemistry. The morphology-controlled synthesis of NiCo₂O₄, such as nanorod-like architectures prepared through PVP-assisted hydrothermal processing, has been demonstrated to enhance electrochemical performance by providing abundant active sites and facilitating ion transport.¹⁰ However, many of these materials exhibit limited capacitance and unstable long-term cycling performance during the charge–discharge process. Additionally, cobalt-based oxides can be expensive and toxic. Efforts have been made to replace cobalt in Co₃O₄ with cheaper, more environmentally friendly elements, without compromising electrochemical effectiveness or catalytic efficiency for organic dye removal. In spinel structures (AB₂O₄), the ‘A’ ions occupy a fraction of the tetrahedral sites, while ‘B’ ions occupy half of the octahedral sites. In inverse spinels, the ‘A’ ions are in octahedral sites, with ‘B’ ions distributed across both octahedral and tetrahedral positions.¹¹ The coexistence of two different cations in the crystal structure leads to unique properties, and partial substitution of transition metals (e.g., Mn, Fe, Co, Ni) in the spinel structure can enhance catalytic activity, stability, and resistance to catalytic or electrocatalytic poisoning.¹² Ternary compounds, such as MnCo₂O₄, NiCo₂O₄, CuCo₂O₄, and ZnCo₂O_4, have been usually studied as materials for supercapacitor electrodes and photocatalytic degradation due to their improved conductivity, electrochemical performance, and superior degradation efficiency compared to individual metal oxides.¹³ Replacing cobalt with cheaper and more sustainable elements has also been explored without sacrificing electrochemical performance or environmental benefits. For example, redox reactions in materials like NiCo₂O₄ involve both cobalt and nickel ions, resulting in high specific capacitance (exceeding the values for NiO and Co₃O₄ by two orders of magnitude). Despite its promise, NiCo₂O₄ suffers from capacity decline as large volume changes during redox cycling.¹⁴ Despite its limitations, NiCo₂O₄ still offers considerable potential for pseudocapacitor applications owing to its large theoretical capacitance (3560 F g⁻¹) and its lower charge compared to many other metal oxides. The behavior of materials in both electrochemical and photocatalytic applications is significantly influenced by factors such as the band gap, size, surface area, charge carrier mobility, and morphological features of the prepared samples.¹⁵ As evidenced by, decreasing the size of nanomaterials, their surface-to-volume ratio and the number of active sites lead to improved electrochemical and photocatalytic properties. Materials with semiconductor-level band gaps exhibit better electrical conductivity and enhanced light absorption ability. It is well established that structure plays a critical role in boosting both electrochemical and photocatalytic efficiency. These factors collectively contribute to higher energy storage capacity and greater photodegradation efficiency.¹⁶ In general, nanomaterials, due to their reduced grain size and increased structural interfaces, facilitate better charge transfer, resulting in improved electrochemical performance of materials like NiCo₂O₄. Various nanostructures of NiCo₂O₄, such as mesoporous microrods & microballs,¹⁷ nanopins,¹⁸ nanoflakes,¹⁹ nanosheets,²⁰ and microflowers,²¹ have been reported, T. Arun et al., recorded the synthesis of Co₃O₄ hexagonal platelets via low-cost chemical method and at 1 A g⁻¹ (current density), a highest capacitance of 718 F g⁻¹, even after 4000 cycles, 96% cyclic retention indicates that the hexagonal Co₃O₄ platelets possess strong potential as an electrode material for energy storage devices.²² V. Venkatachalam et al., using a hydrothermal synthesis approach, used NiCo₂O₄ as an electrode material. It delivers a specific capacitance of 767.5 F g⁻¹ at a current density of 0.5 A g⁻¹ in 3 M KOH, with outstanding cyclic performance, maintaining 94% stability after 1000 cycles at 5 A g⁻¹. It serves as a highly prospective material for high-efficiency supercapacitor systems.²³ The electrochemical performance of these materials is highly dependent on their nanostructures. Despite significant advancements, these materials still face limitations such as low intrinsic conductivity, inadequate surface area, poor cycling stability, and low energy and power densities during charge–discharge cycles.²⁴ These include fabricating nano-engineered structures and utilizing hybrid materials with synergistic effects. Nanostructured heterogeneous materials are particularly well-suited for both supercapacitor and catalytic applications.²⁵

To overcome these challenges, a promising approach is the development of a composite nanoarchitecture with high electrical conductivity and surface area. Studies have reported that morphology-based composites can greatly enhance the performance of cobalt-based materials. Moreover, recent advances in g-C₃N₄-based photocatalysts have demonstrated that structural modification strategies, including doping and heterostructure construction, can effectively improve charge separation efficiency and photocatalytic activity by tuning the electronic structure of g-C₃N₄.²⁶ I. Rabani et al. reported that Co₃O₄@g-C₃N₄ hybrid was produced using an eco-friendly mechanochemical one-pot approach, which exhibits both a supercapacitor electrode and photocatalyst. At 1 A g⁻¹, the material delivered a specific capacitance of 457.2 F g⁻¹, which is greater than that of pure Co₃O₄ when tested in 3 M KOH electrolyte, and maintained 92% capacitance retention after 5000 cycles. Furthermore, its photocatalytic activity was performed using a rhodamine B (RhB) dye, and 95% of the RhB dye was decomposed within 60 min under UV irradiation via its reduction mechanism.²⁷ S. H. S. Pai et al. reported that NiO@g-C₃N₄ nanocomposites were fabricated using a simple solvothermal method followed by heat treatment for enhanced supercapacitor applications. In a three-electrode configuration using different electrolytes, including 1 M Na₂SO₄ and 3 M KOH, the maximum C_s of 161.3 F g⁻¹ at a current density of 1 A g⁻¹ and 338.68 F g⁻¹ at a scan rate of 2 mV s⁻¹ .²⁸ Recently, the integration of multiple transition metal oxides has been explored as an effective strategy to improve electrical conductivity, active sites, and charge storage capability. A microwave-assisted in situ synthesized NiMn₂O₄ nanoparticle-embedded NiCo₂O₄ nanosheet electrode grown directly on nickel foam demonstrated enhanced electrochemical performance due to its binder-free architecture and synergistic effect between mixed metal oxides.²⁹ Furthermore, NiCo₂O₄-based carbon composite electrodes have been investigated to improve charge transport and electrochemical stability. A hydrothermally synthesized NiCo₂O₄@activated carbon composite exhibited enhanced electrochemical performance owing to the synergistic contribution of the redox-active NiCo₂O₄ phase and conductive carbon framework.³⁰ B. Guan et al. reported that electrochemical evaluation revealed that nanoneedle-shaped NiCo₂O₄/g-C₃N₄ delivers a specific capacitance of 253 F g⁻¹ at 2 A g⁻¹. In contrast, the nanosheet-based NiCo₂O₄/g-C₃N₄ exhibited enhanced cycling durability, retaining 101.4% of its initial capacitance over 1000 cycles in a three-electrode setup.³¹ J. Liu et al. synthesized NiCo₂O₄ nanodots, which grew on g-C₃N₄ heterojunction for the photocatalytic hydrogen evolution reaction under simulated sunlight. Outstanding activity was exhibited by the NiCo₂O₄/g-C₃N₄ (NC-6%) with a hydrogen production rate of 462.4 µmol g⁻¹ h⁻¹, which is an increase of 2.2 times compared to pristine g-C₃N₄ and nearly 400 times higher than that of NiCo₂O₄, respectively.³² Nitrogen-containing carbon-based materials are particularly useful in energy storage, energy conversion, and photocatalysis. The lone-pair electrons on nitrogen contribute negatively charged to the carbon-based material, improving its electronic conductivity.³³ The g-C₃N₄ is a cost-effective material that has found applications in supercapacitors, energy conversion, biomedical, and photocatalysis. Because of its high surface area, electronic structure, appropriate band gap, excellent chemical and thermal stability, and environmental friendliness, the g-C₃N₄ is valued.³⁴ Its surface polarity, electron-donor properties, and improved electron transport properties lead to enhanced electrochemical performance. In photocatalysis, the material's photo-excited charge transfer, efficient light utilization, reduced band gap, and large surface area all contribute to enhanced catalytic performance.⁴ Recent studies have demonstrated that constructing Z-scheme heterojunctions based on g-C₃N₄ can effectively promote interfacial charge migration, suppress electron–hole recombination, and improve photocatalytic degradation efficiency under light irradiation.³⁵ Combining n-type and p-type semiconductors is an effective strategy for improving catalytic ability. The partially filled Ni and Co ions in the cubic spinel NiCo₂O₄ can readily accept electrons from g-C₃N₄, facilitating the creation of an electron–hole pair on the nanocomposite surface.³⁶ This prolonged electron–hole separation accelerates the rate of photocatalysis. Reactive species, such as hydroxyl and superoxide radicals, generated by heterogeneous photocatalysts under suitable light illumination, effectively degrade environmental pollutants.³⁷

Based on previous reports, hydrothermally synthesized NiCo₂O₄@g-C₃N₄ heterostructures have shown potential for both electrochemical energy-storage and photocatalytic applications, motivating further investigation of their structure–property relationships. In this study, we investigated the phase purity, crystal structure, optical properties, morphology, and electrochemical characteristics of the synthesized nanocomposites. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to examine the electrochemical activity of the synthesized pristine NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ nanocomposites. The photocatalytic activity of the prepared samples was assessed by exposing them to visible light using methylene blue (MB) and crystal violet (CV) dyes as organic pollutants. The present work focuses on the hydrothermal fabrication of NiCo₂O₄@g-C₃N₄ and the evaluation of its combined electrochemical energy-storage and visible-light photocatalytic performance.

2. Experimental

2.1. Chemicals

Sigma-Aldrich purchased the chemicals Nickel chloride hexahydrate (NiCl₂·6H₂O), Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), graphitic carbon nitride (g-C₃N₄), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP), acetone (CH₃COCH₃), ethanol (C₂H₅OH), N-methyl pyrrolidine (C₅H₁₁N), polyvinylidene fluoride (PVDF), nickel foam, and potassium hydroxide (KOH, 99.9%).

2.2. Synthesis of NiO and Co₃O₄

Nickel oxide (NiO) and cobalt oxide (Co₃O₄) were synthesized through a simple hydrothermal route. At first, 50 mL of deionized water was added to 0.1 M NiCl₂·6H₂O and stirred until a clear solution was formed. Separately, a solution of 50 mL with 1 g of PVP (40 000 MW) in deionized water was prepared and then mixed with the earlier solution. Subsequently, a dropwise addition of 1 M NaOH dissolved in 50 mL of deionized water was carried out. Following one hour of stirring, the mixture was transferred to a stainless-steel autoclave lined with Teflon and heated to 180 °C for 12 hours. The obtained precursor was then washed and dried for 8 hours at 180 °C in a vacuum oven, and subsequently calcined at 350 °C for 3 h to yield NiO and Co₃O₄ nanoparticles.

2.3. Synthesis of NiCo₂O₄ and NiCo₂O₄@g-C₃N₄ nanocomposites

The NiCo₂O₄ and NiCo₂O₄@g-C₃N₄ nanocomposites were synthesized using a hydrothermal method. First, 1 g of g-C₃N₄ nanoparticles was suspended in 50 milliliters of double-distilled water and ultrasonicated for 60 minutes. Meanwhile, 50 mL of deionized water was continuously stirred at ambient temperature to make 0.1 M aqueous solutions of NiCl₂·6H₂O and Co(NO₃)₂·6H₂O. Separately, 1 gram of PVP (40 000 MW) was dissolved in 50 mL of deionized water and subsequently incorporated into the metal precursor mixture. The prepared g-C₃N₄ dispersion was then added to the above solution and stirred for an additional 30 minutes. A 1 M sodium hydroxide solution (50 mL) was added gradually while stirring continuously. The prepared suspension was kept in a Teflon-lined stainless-steel autoclave and kept at 180 °C for a 12 hours hydrothermal process. After cooling, the precipitate was collected and dried at 180 °C for 8 h in a vacuum oven. The dried product was subsequently calcined at 350 °C for 3 h in air to obtain crystalline NiCo₂O₄@g-C₃N₄. To obtain NiCo₂O₄ nanoparticles, the same procedure, including the calcination step at 350 °C for 3 h, was followed, excluding the addition of g-C₃N₄.

2.4. Fabrication of electrode and electrochemical characterization

To examine their capacitive behavior, NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ hybrid nanostructures were assessed through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques. All measurements were carried out in a three-electrode configuration using a Bio-Logic SP-300 electrochemical workstation with 3 M KOH as the electrolyte. The synthesized NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ hybrid nanostructures coated on nickel foams (NFs) served as the working electrodes, while a silver/silver chloride (Ag/AgCl) and a Pt wire were used as the reference electrode and counter electrode, respectively. The working electrode was fabricated using a slurry containing the active electrode material, carbon black, and PVDF (8 : 1 : 1) ratio, dispersed in NMP. The prepared slurry was uniformly coated onto nickel foam substrates measuring 3 × 1 cm², and the active material loading was maintained at approximately 5 mg per electrode, corresponding to a mass loading of 1.67 mg cm⁻² across all electrodes. CV and GCD probed the electrochemical behavior in a potential window of 0–0.8 V and 0–0.5 V, while EIS measurements were carried out in a frequency window of 100 kHz to 0.1 Hz. Eqn (1) was applied to calculate the specific capacitance of the prepared electrode materials from the GCD curves.^38,39

1

2

where C_s and Q_s are the specific capacitance (F g⁻¹) and specific capacity in C g⁻¹, respectively; I is the current density (A g⁻¹), Δt denotes the discharge time (s), ΔV indicates the potential range (V), and m stands for the mass (g) of the electrode materials.

2.5. Asymmetric supercapacitor fabrication

The asymmetric supercapacitor device was assembled using NiCo₂O₄@g-C₃N₄ as the positive electrode and rGO as the negative electrode. The electrodes were prepared by following the same procedure described above. The mass ratio of the two electrodes was determined from the charge balance consideration, and the corresponding active material loadings were 2.25 mg and 5.75 mg for NiCo₂O₄@g-C₃N₄ and rGO electrodes, respectively. Therefore, the total active material loading of the assembled device was 8 mg. The gel electrolyte was prepared by dissolving 6.25 g of polyvinyl alcohol (PVA) in 80 mL of deionized water under gentle stirring, followed by the addition of 8 g of KOH to obtain a homogeneous PVA–KOH gel electrolyte. The resultant mixture solution was stirred in a 150 °C water bath until a transparent gel formed. After cooling at room temperature, filter paper was used as separator and impregnated with the gel and inserted between a NiCo₂O₄@g-C₃N₄ cathode and an rGO anode to assemble the asymmetric supercapacitor. The electrochemical behavior of the device was systematically analyzed through CV and GCD characterization within a potential window of 0 to 1.2 V. EIS measurements were carried out in the frequency range of 100 kHz to 0.01 Hz. The corresponding electrochemical parameters were calculated using the following equations:⁴⁰

3

4

where C_s, V, E, P, and Δt, stands for specific capacitance, potential window, energy density, power density, and discharge time, respectively. A schematic diagram (Scheme 1) has been included to visually summarize the fabrication route of the asymmetric supercapacitor, providing a clear overview of the assembly configuration and component arrangement.

Scheme 1. Schematic illustration of the fabrication of the NiCo₂O₄@g-C₃N₄//rGO asymmetric supercapacitor (ASC), device configuration, and its electrochemical performance.

2.6. Photocatalytic activity test

Crystal violet (CV) and methylene blue (MB) were used as organic pollutants to assess the photocatalytic performance of pure NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ hybrid nanostructures. The nanoparticles were evaluated for their effectiveness in degrading MB and CV dyes exposed to visible light. A 25 mL (20 mg L⁻¹) dye solution was mixed with 25 mg of the catalyst for experimental investigation. To achieve adsorption–desorption equilibrium, the mixture was put in the dark for half an hour. After that, the mixture was exposed to radiation with visible light for 150 min, and 3 mL of the aqueous dye suspension was extracted at regular intervals for analysis. The maximum absorbance of MB and CV was recorded, and UV-vis spectroscopy was employed to evaluate the degradation efficiency.

The efficiency of photocatalytic degradation was calculated using the following formula:⁴¹

5

where C₀ denotes the dye's initial concentration, while C is dye concentration after irradiation.

The photocatalytic reaction kinetics were analysed using the Langmuir–Hinshelwood (L–H) model through the following eqn (2);

−ln(C/C_o) = kt 6

Where, k represents the pseudo-first-order rate constant, and t corresponds to the reaction time.

2.7. Characterization

X-ray diffraction (XRD) analysis was performed using a BRUKER D8 Advance with Cu Kα radiation (λ = 1.54056 Å) to identify the crystal phase. The textural structure of the composite was analyzed using a Field-Emission scanning electron microscope (FE-SEM, Zeiss Ultra-55). The binary metal oxide composite with g-C3N4 exhibited a hexagonal-like morphology, with anticipated element compositions of Ni, Co, O, and N, as evidenced by EDX analysis. High-resolution transmission electron microscopy (HR-TEM, JEOL Japan's JEM-2100 Plus) with a 200 kV was carried out to obtain structural, morphological, and elemental composition information. X-ray photoelectron spectroscopy (XPS), an X-ray radiation source (PHI5000 version Probe III) were used to determine the valence states of the elements in the studied samples. The nitrogen adsorption–desorption techniques were performed using Quantachrome Instruments and Autosorb IQ series to assess the sample surface area and porosity. The absorbance spectra of the produced photocatalysts were recorded using a UV-vis spectrophotometer (SYSTRONICS, Double beam spectrometer 2203).

3. Results and discussion

3.1. X-ray diffraction (XRD) analysis

The XRD spectra of NiO, Co₃O₄, NiCo₂O₄, and the NiCo₂O₄@g-C₃N₄ composite nanomaterials are shown in Fig. 1(a–d). The XRD pattern of NiO shows all peaks corresponding to the crystallographic orientations of the JCPDS card (04-0835) at (111), (200), (220), (311), and (222), without any additional noticeable peaks.⁴² The Co₃O₄ corresponds to the JCPDS card (78-1969) at (111), (220), (311), (400), (511), and (440), with no additional peaks.⁴³ The XRD pattern of NiCo₂O₄ nanoparticles shows all peaks match the cubic crystal structure as per JCPDS card (20-0781) at (111), (220), (311), (440), and (533), confirming the successful synthesis of high-purity NiCo₂O₄ nanoparticles.⁴⁴ The XRD patterns of the NiCo₂O₄@g-C₃N₄ composites show two characteristic peaks at 13.16° and 34.45°, corresponding to the (001) and (102) planes of g-C₃N₄, respectively. The composites also exhibit the characteristic diffraction peaks of NiCo₂O₄, confirming that NiCo₂O₄ has been successfully loaded onto the g-C₃N₄ nanosheets. The intensity of the XRD peaks varies with the g-C₃N₄ content. Moreover, the NiCo₂O₄@g-C₃N₄ samples display peak broadening and slight peak shifts compared to pure NiCo₂O₄, suggesting the incorporation of g-C₃N₄ and NiCo₂O₄. The average crystallite sizes of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄, calculated from their XRD patterns, were found to be 24.3 nm, 20.7 nm, 20.2 nm, and 14.32 nm, respectively.

Fig. 1. Presents the XRD pattern of (a) NiO, (b) Co₃O₄, (c) NiCo₂O₄, and (d) NiCo₂O₄@g-C₃N₄ hybrid nanostructure.

3.2. Surface morphological analysis

3.2.1. SEM and EDS characterization

The surface morphology of the synthesized NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ materials was investigated using scanning electron microscopy (SEM), as shown in Fig. 2(a–d). The pristine NiO in Fig. 2(a) exhibits an irregular and highly agglomerated morphology with a rough surface texture, whereas Co₃O₄ in Fig. 2(b) displays an agglomerated morphology composed of fine, irregular, flake-like features. In comparison, NiCo₂O₄ in Fig. 2(c) exhibits relatively well-defined hexagonal/platelet-like structures with interconnected assemblies, indicating a distinct morphological evolution upon formation of the bimetallic oxide. After incorporation of g-C₃N₄, the NiCo₂O₄@g-C₃N₄ composite in Fig. 2(d) exhibits a more heterogeneous irregular morphology, in which the well-defined hexagonal features observed in pristine NiCo₂O₄ become less distinct and are accompanied by irregular flake-like and agglomerated structures. The incorporation of g-C₃N₄ alters the surface morphology of NiCo₂O₄, resulting in a heterogeneous architecture characterized by interconnected features and interparticle spaces. Such structural characteristics may improve electrolyte penetration and provide pathways for ion diffusion during the electrochemical process.

Fig. 2. SEM images of (a) NiO, (b) Co₃O₄, (c) NiCo₂O₄, (d) NiCo₂O₄@g-C₃N₄, and (e) EDS spectrum of the NiCo₂O₄@g-C₃N₄ composite.

The elemental composition of the NiCo₂O₄@g-C₃N₄ composite was further examined by energy-dispersive X-ray spectroscopy (EDS), as shown in Fig. 2(e). The EDS spectrum confirms the presence of Ni, Co, O, C, and N elements, with corresponding weight percentages of 26.01, 46.25, 24.40, 2.10, and 1.24 wt%, respectively. No additional impurity peaks are observed, confirming the high purity and homogeneous distribution of elements in the synthesized material.

3.2.2. HR-TEM analysis

High-resolution transmission electron microscopy (HRTEM) was used to study the morphology and mesostructural features of the synthesized NiCo₂O₄@g-C₃N₄ heterostructure, illustrated in Fig. 3(a–l). The TEM images at low magnification in Fig. 3(a–h) show that the NiCo₂O₄ nanoparticles are well-dispersed and strongly coupled with the embedded g-C₃N₄ phase, indicating the formation of a homogeneous NiCo₂O₄@g-C₃N₄ heterostructure. The composite exhibits a hexagonal morphology surrounded by a large number of ultrafine nanoparticles, creating an outer ring-like appearance. This structural configuration provides abundant interfacial contact between NiCo₂O₄ and g-C₃N₄, which is advantageous for charge transport and electrolyte accessibility. The average particle size of NiCo₂O₄ nanoparticles is estimated to be approximately 19 nm, indicating fine nanoscale dispersion and intimate interfacial contact with the embedded g-C₃N₄ phase. The higher-magnification TEM images in Fig. 3(f–i) clearly show that the NiCo₂O₄ nanoparticles possess a porous and loosely packed surface texture. Such porosity increases surface area, providing more active sites for electrochemical interactions. The intimate contact between the NiCo₂O₄ nanoparticles and g-C₃N₄ forms a continuous conductive network, suggesting efficient electron transport pathways within the heterostructure. This interconnected framework enables synergistic interactions between the two components, resulting in enhanced electrochemical and photocatalytic performance. The lattice-resolved HRTEM images in Fig. 3(j and k) further confirm the crystalline nature. The (111) plane of NiCo₂O₄ is represented by the measured interplanar distance of 0.46 nm, whereas the (102) plane of g-C₃N₄ matches the spacing of 0.26 nm, indicating the coexistence of both phases within the composite. The visible lattice fringes and sharp interfacial contrast provide direct evidence of the strong link between NiCo₂O₄ and g-C₃N₄, confirming the establishment of a stable heterojunction. The nature of spinel NiCo₂O₄ is confirmed by the selected area electron diffraction (SAED) pattern in Fig. 3(l), which displays discrete concentric diffraction rings corresponding to the (111), (220), (311), (400), and (440) planes. The well-defined diffraction rings indicate that the prepared NiCo₂O₄@g-C₃N₄ composite possesses excellent crystallinity and homogeneous phase distribution. The composite is a promising electrode material for high-capacity energy storage and photocatalytic applications because of these structural advantages.

Fig. 3. TEM images of NiCo₂O₄@g-C₃N₄ at different magnifications (a–i), HRTEM images with lattice fringe analysis (j and k), and the corresponding SAED pattern (l) of the composite.

3.3. X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to examine the surface elemental composition and oxidation states of the materials. As shown in Fig. 4(a–e), the spectra reveal distinct peaks associated with Ni 2p, Co 2p, O 1s, and C 1s. Notably, in the survey spectrum, Fig. 4(a), a weak N 1s signal appears at approximately 395 eV, indicating the presence of nitrogen, although its intensity is relatively low. The Ni 2p spectra confirm the presence of Ni²⁺, Fig. 4(b), supporting information, which displays two main peaks at 854.9 eV (Ni 2p_3/2) and 872.2 eV (Ni 2p_1/2), together with satellite peaks at 861.6 eV and 880.8 eV.⁴⁵ The Co 2p spectrum further verifies the presence of mixed Co³⁺ and Co²⁺ oxidation states. As illustrated in Fig. 4(c), the main peaks located at 780.04 eV (Co 2p_3/2) and 795.3 eV (Co 2p_1/2), along with the corresponding satellite features at 789.7 eV and 804.2 eV, support this observation. In addition, the O 1s spectrum shown in Fig. 4(d) can be deconvoluted into three components at around 529.5 eV, 531.1 eV, and 532.4 eV, which are attributed to lattice oxygen (O_latt), adsorbed oxygen species, and surface hydroxyl groups, respectively.⁴⁶ These observations confirm the presence of oxygen species contributing to the electronic interactions between NiCo₂O₄ and g-C₃N₄. The C 1s spectra of NiCo₂O₄@g-C₃N₄, Fig. 4(e), reveal significant changes compared to pristine g-C₃N₄. In g-C₃N₄, two distinct peaks appear at 287.2 eV and 284.9 eV. The peak at 287.2 eV corresponds to sp²-hybridized carbon (C N) within the aromatic framework of the g-C₃N₄ structure, whereas the peak at 284.9 eV is attributed to the C–C bond.⁴⁷ Upon hybridization with NiCo₂O₄, noticeable changes in the C 1s peaks suggest electronic interactions between g-C₃N₄ and NiCo₂O₄.

Fig. 4. Shows the XPS spectra of NiCo₂O₄@g-C₃N₄ nanoparticles, including (a) the full survey spectrum and the high-resolution spectra of (b) Ni 2p, (c) Co 2p, (d) O 1s, and (e) C 1s.

3.4. Brunauer–Emmett–Teller

Fig. 5 illustrates the nitrogen adsorption–desorption isotherm of the NiCo₂O₄@g-C₃N₄ composite as relative pressure (p/p₀) versus adsorbed volume (STP, cm³ g⁻¹). The isotherm corresponds to a type IV profile with a characteristic H3-type hysteresis loop, according to IUPAC classification, signifying mesoporous structures with slit-like pores, typically associated with aggregates of hexagonal-like particles. Incorporating polyvinylpyrrolidone (PVP) during synthesis played a crucial role as a structure-directing agent, controlling particle growth and preventing agglomeration. PVP also acted as a soft template, facilitating the development of pore-rich channels and enhancing pore connectivity, which is reflected in the observed porous architecture.⁴⁸ BET analysis revealed a total pore volume of 6.777 cm³ g⁻¹ and a specific surface area of 20.62 m² g⁻¹, indicating a well-developed porous texture of the NiCo₂O₄@g-C₃N₄ composite. The relatively high pore volume, despite moderate surface area, indicates the presence of larger mesopores and minor macro-porosity, supporting efficient ion/electrolyte diffusion. The presence of pore diameters from 1.70 nm to 70.6 nm, with dominant peaks at 2.80 nm, 4.77 nm, and 6.17 nm, confirms that hierarchical meso-porosity is revealed by the BJH pore size distribution, as shown in the inset of Fig. 5. A small contribution of macropores (>50 nm), likely from interparticle voids, was also observed. The highest pore volume contributions were found near 4.77 nm (0.00492 cm³ g⁻¹) and 6.17 nm (0.00614 cm³ g⁻¹), indicating well-developed mesopores. The textural characteristics presented in this study are based on the experimentally obtained BET results for the NiCo₂O₄@g-C₃N₄ composite, whereas the morphological characteristics of the pristine NiO, Co₃O₄, and NiCo₂O₄ electrodes are discussed based on their corresponding SEM images. This optimized porous structure, attributed partly to the templating effect of PVP, is highly advantageous for supercapacitor applications, where mesopores enhance ion transport and charge storage.⁴⁹ The mesoporous channels (2.8–6.17 nm) provide sufficient pathways for electrolyte ion diffusion, including hydrated K⁺ and OH⁻ ions, during electrochemical reactions. These interconnected pores facilitate electrolyte accessibility to the active sites and reduce ion transport resistance, contributing to improved charge storage kinetics. Similarly, in photocatalytic systems, the hierarchical porosity enhances light harvesting, provides abundant surface-active sites, and facilitates faster reactant diffusion, thereby improving overall photocatalytic performance. Collectively, these structural features validate the material's potential for energy storage and environmental remediation applications.

Fig. 5. N₂ adsorption–desorption isotherm of NiCo₂O₄@g-C₃N₄ (inset – BJH pore-size distribution).

3.5. UV-vis spectroscopy

The optical absorption characteristics of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ were examined using UV-vis spectroscopy, as illustrated in Fig. 6. The UV-vis absorption spectra in Fig. 6(a) exhibit an absorption onset in the range of 250–750 nm for the NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ nanocomposites. Using the Tauc equation, the energy band gap (E_g) of the photocatalysts was determined from the absorption data,⁵⁰

(αℏν)^1/2 = β(ℏν − E_g) 7

where β is a constant associated with the indirect bandgap, α corresponds optical absorption coefficient, and ℏν represents the photon energy.

Fig. 6. (a) UV-visible absorption spectra and (b) corresponding Tauc plots of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ samples for the determination of optical band gap energies.

Fig. 6(b) illustrates 2.58 eV for NiO, 2.18 eV for Co₃O₄, 1.92 eV for NiCo₂O₄, and 1.61 eV for NiCo₂O₄@g-C₃N₄ as the estimated bandgap energy. The NiCo₂O₄@g-C₃N₄ nanocomposite exhibited the narrowest bandgap among all samples, which may contribute to its higher conductivity.

3.6. Electrochemical analysis

To evaluate the electrochemical behavior of the NiO, Co₃O₄, NiCo₂O₄, NiCo₂O₄@g-C₃N₄ hybrid in a three-electrode setup, CV measurements were performed within a potential window of 0–0.8 V in 3 M KOH electrolyte, while GCD tests were conducted over a 0–0.5 V potential window. Redox peaks are clearly seen in the CV curves in Fig. 7(a–d), indicative of the reversible faradaic processes associated with transition-metal oxides (MO; M = Ni or Co). The pseudocapacitive behavior of NiCo₂O₄ is further enhanced by the synergistic interaction between Ni and Co species, thereby improving electrochemical activity. The Ni²⁺/Ni³⁺ and Co³⁺/Co⁴⁺ transitions involve the reversible faradaic reactions corresponding to two distinct pairs of peaks. The following reaction represents the electrochemical processes occurring in the alkaline electrolyte:¹²

NiCo₂O₄ + OH⁻ + H₂O ↔ NiOOH + 2CoOOH + e⁻ 8

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ 9

Fig. 7. Presents the cyclic voltammetry (CV) curves of (a) NiO, (b) Co₃O₄, (c) NiCo₂O₄, and (d) NiCo₂O₄@g-C₃N₄ nanocomposite.

Redox peaks are observed in the CV scan rates between 5 to 100 mV s⁻¹, indicating that the faradaic reaction occurs during the energy storage process and exhibits obvious pseudocapacitive behavior. As the scan rate rises, the anodic peaks move to higher potentials, while the cathodic peaks shift toward lower potentials, indicating accelerated ion diffusion and rapid redox kinetics. To examine the mechanism by which the electrochemical performance of NiCo₂O₄@g-C₃N₄ is obtained. The capacitive (k₁ν) and diffusion-controlled (k₂ν^1/2) aids of charge storage were investigated for the three electrode materials at 5, 20, and 40 mV s⁻¹, as shown in Fig. 8(a–c). Both k₁v and k₂v^1/2are included in the current response (i):⁵¹

i = k₁ν + k₂ν^1/2 10

Fig. 8. (a–c) Capacitive and diffusive contributions at different scan rates, (d) corresponding contribution percentage.

The shaded regions represent the capacitive contribution, while the remaining area corresponds to the diffusion-dominated redox processes. When measured at a low scan rate of 5 mV s⁻¹, the electrode exhibits a predominantly diffusion-controlled response with only 12.02% capacitive contribution. With increasing scan rates of 20 and 40 mV s⁻¹, the capacitive fraction gradually increases to 21.47% and 27.88%, respectively, indicating faster surface redox kinetics and improved ion transport at higher potentials. Fig. 8(d) summarizes the quantitative distribution of capacitive and diffusive contributions at scan rates ranging from 5 to 100 mV s⁻¹. The capacitive contribution increases progressively from 12% to 38%, confirming that the NiCo₂O₄@g-C₃N₄ heterostructure exhibits a storage mechanism dominated by diffusion-controlled faradaic reactions at lower scan rates and surface-controlled at higher scan rates.

An enhanced high-rate charge–discharge capability is a key criterion for assessing the suitability of electrode materials in supercapacitor applications. To investigate this, GCD measurements were performed on the NiCo₂O₄@g-C₃N₄ hybrid electrode, along with pristine NiO, Co₃O₄, and NiCo₂O₄, at a current density within the potential window of 0 to 0.5 V, shown in Fig. 9(a–d). Unlike electric double-layer capacitors, which exhibit nearly linear discharge curves, all electrodes exhibit nonlinear discharge curves, indicating pseudocapacitive behavior arising from reversible redox reactions at the electrode and electrolyte interface. In comparison to the pristine metal oxide electrodes, the NiCo₂O₄@g-C₃N₄ hybrid electrode exhibits the longest discharge time, indicating better charge-storage capacity. To further assess rate performance, GCD profiles of pristine NiO, Co₃O₄, and NiCo₂O₄ electrodes under the same conditions are provided in Fig. 9(a–c). The corresponding GCD curves of the NiCo₂O₄@g-C₃N₄ hybrid electrode were recorded at different current densities, 2.4, 3, 3.6, 4, and 4.4 A g⁻¹, as shown in Fig. 9(d). The C_s and Q_s values, calculated using eqn (1) and (2), are presented in Table 1. As expected, increasing the current density (CD) leads to a progressive drop in C_s for all electrodes, which can be ascribed to the reduced accessibility of electroactive sites and lower utilization efficiency of the active material under high-rate conditions. Nevertheless, the NiCo₂O₄@g-C₃N₄ hybrid electrode retains an exceptionally high capacity of 154 mAh g⁻¹ even at a high current density of 4.4 A g⁻¹, which is greater than that of the pristine electrodes. The hybrid electrode superior pseudocapacitive nature and efficient charge-storage kinetics are demonstrated by the excellent rate capability, making it a promising electrode for supercapacitors with excellent performance.

Fig. 9. Shows the GCD curves of (a) NiO, (b) Co₃O₄, (c) NiCo₂O₄, and (d) NiCo₂O₄@g-C₃N₄ nanocomposite.

Table 1. Specific capacitance and specific capacity of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ electrodes at different current densities.

Current density (A g−1)	Specific capacitance (F g−1)				Specific capacity (mAh g−1)
	NiO	Co3O4	NiCo2O4	NiCo2O4@g-C3N4	NiO	Co3O4	NiCo2O4	NiCo2O4@g-C3N4
2.4	572.4	991.4	1096.5	1766.4	79.5	137.7	152.2	245.3
3	452.4	556.2	921.6	1545.18	62.8	77.2	128	214.6
3.6	366.9	409.9	788.4	1348.05	50.9	56.9	109.5	187.2
4	318.64	274.88	702.3	1208	44.2	38.1	97.5	167.7
4.4	276.05	212.69	646.97	1109.4	38.3	29.5	89.8	154

The CV curves of NiO, Co₃O₄, NiCo₂O₄, NiCo₂O₄@g-C₃N_4, and NF electrodes recorded at 5 mV s⁻¹ are presented in Fig. 10(a). The bare NF displays a substantially smaller integrated CV area and a markedly lower current response compared with the active electrodes, demonstrating its limited contribution to the overall electrochemical charge-storage response. The NiCo₂O₄@g-C₃N₄ composite exhibits the largest enfolded area and the most pronounced redox peaks, indicating improved faradaic activity and higher charge-storage capability. Compared to the mono-metal oxides, the bimetallic NiCo₂O₄ exhibits a higher current response, and the incorporation of g-C₃N₄ further enhances the kinetics of redox by improving conductivity and increasing the number of accessible active sites. The GCD curves and C_s values obtained at various CD are shown in Fig. 10(b). Among all the samples, the NiCo₂O₄@g-C₃N₄ electrode exhibits the highest specific capacitance due to its longest duration at each CD. The synergistic activity of the Ni–Co species integrated with the conductive porous g-C₃N₄ matrix, which permits effective ion diffusion and charge transport, is responsible for this better performance.

Fig. 10. (a) CV curves of NiO, Co₃O₄, NiCo₂O₄, NiCo₂O₄@g-C₃N_4, and NF electrodes. (b) Specific capacitance as a function of current density. (c) Nyquist plots of the prepared electrodes. (d) Cycling stability of the NiCo₂O₄@g-C₃N₄ electrode over 5000 cycles, with the inset showing the last 10 GCD cycles.

Fig. 10(c) displays the Nyquist plots of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄. The resistive and transport characteristics of the electrode systems were clarified using EIS. A Nyquist plot exhibits an almost vertical line in the low-frequency region, representing ion diffusion behavior, along with a depressed semicircle in the high-frequency region related to charge-transfer resistance. The intercept at high frequency on the real axis (Z′) denotes the equivalent series resistance (R_s), which arises from the combined effects of the electrolyte resistance, the inherent resistance of the electrode material, and the contact resistance between the electrode and the current collector. The semicircle diameter yields the charge-transfer resistance (R_ct),⁵² an essential measure of interfacial kinetics. Quantitatively, the R_ct values follow the trend: NiO (7.07 Ω) > Co₃O₄ (6.21 Ω) > NiCo₂O₄ (2.66 Ω) > NiCo₂O₄@g-C₃N₄ (1.33 Ω). The low R_ct of the NiCo₂O₄@g-C₃N₄ electrode highlights its accelerated faradaic reaction kinetics and superior electrical conductivity, arising from the synergistic coupling between NiCo₂O₄ and the conductive g-C₃N₄ network. In the low-frequency region, the NiCo₂O₄@g-C₃N₄ electrode exhibits the steepest capacitive line, signifying minimal ion-diffusion resistance and highly efficient mass transport. The magnified inset of Fig. 10(c) reinforces these trends: the NiCo₂O₄@g-C₃N₄ electrode displays the smallest Z′ intercept and the most compressed semicircle. These features confirm reduced charge transfer and bulk electrolyte resistance. Collectively, the EIS demonstrates that the integration of g-C₃N₄ enhances the electrical conductivity, ion-diffusion pathways, and charge-transfer efficiency of NiCo₂O₄, rendering the NiCo₂O₄@g-C₃N₄ nanocomposite a highly competitive electrode suitable for future high-performance supercapacitors. Although the GCD curves show an initial IR drop, it includes additional ohmic contributions such as electrolyte, contact, and ion-transport resistances and is therefore not inconsistent with the low R_ct value.

Fig. 10(d) shows the cycle-life performance of the electrodes. The NiCo₂O₄@g-C₃N₄ electrode retains 91.82% of its initial capacitance after 5000 successive charge–discharge cycles at a high current density (CD) of 4.4 A g⁻¹ in a three-electrode configuration, demonstrating good long-term cycling stability and electrochemical durability. This stable electrochemical performance can be associated with the mechanical robustness of the g-C₃N₄ framework and the buffering effect provided by the interlayer voids within its hexagonal lamellae, which may accommodate structural changes associated with repeated faradaic redox reactions. The electrochemical characteristics are assessed relative to previously reported NiCo₂O₄-based materials, as shown in Table 2. The comparison highlights the competitive electrochemical behavior of the NiCo₂O₄@g-C₃N₄ composite, particularly its high capacitance retention and good rate capability at elevated current densities.^53–59 These results substantiate the potential of the NiCo₂O₄@g-C₃N₄ architecture for durable supercapacitor applications.

Table 2. Comparison of the specific capacitance, synthesis method, potential window, and electrolyte of reported NiCo₂O₄-based electrodes with the present work.

Electrode material	Specific capacitance	Method	Potential window	Electrolyte	Reference
NiCo2O4@CNT	1191F g−1 at 1 A g−1	Sonication-assisted hydrothermal method	0 to 0.5 V	1 M KOH	53
NCO nanoflowers	1030F g−1 at 1 A g−1	Hydrothermal method	0 to 0.5 V	2 M KOH	54
Hollow NCO@G	1238 F g−1 at 1 A g−1	HM by 2step annealed in N2/air	0 to 0.5 V	2 M KOH	55
NCO nanoplatelet	1362 F g−1 at 1 A g−1	Chemical bath coprecipitation	0 to 0.5 V	2 M KOH	56
G@NCO nanorods	1684 F g−1 at 1 A g−1	Solvothermal method	0 to 0.5 V	2 M KOH	57
Rgo wrapped NCO	1185 F g−1 at 1 A g−1	Hydrothermal method	0 to 0.4 V	—	58
NCO@rgo@nanowire	1050 F g−1 at 1 A g−1	Hydrothermal method	0 to 0.5 V	2 M KOH	59
NiCo2O4@g-C3N4	1766.4 F g−1 at 2.4 A g−1	Hydrothermal method	0 to 0.5 V	3 M KOH	This work

3.7. Asymmetric device of electrochemical analysis

The overall electrochemical behavior of the asymmetric supercapacitor (ASC), fabricated with NiCo₂O₄@g-C₃N₄ as the positive electrode and rGO as the negative electrode, was evaluated in a two-electrode configuration. The fabrication steps and device structure are illustrated in Scheme 1. Fig. 11(a) shows the CV profiles obtained at 20 mV s⁻¹. The ASC delivers a stable operating 1.2 V, indicating the successful pairing of the faradaic-type NiCo₂O₄@g-C₃N₄ with the electric-double-layer-type rGO. The NiCo₂O₄@g-C₃N₄ electrode exhibits pronounced redox behaviour, whereas rGO retains a nearly rectangular shape, confirming its capacitive behaviour.⁶ The combined CV profile of the ASC reflects the coexistence of both pseudocapacitive and EDLC charge-storage mechanisms. Fig. 11(b) further illustrates the influence of the voltage window and scan rate. When the potential increases from 0.4 V to 1.2 V, the enclosed CV area expands proportionally, confirming enhanced faradaic activity at higher polarization. The asymmetric supercapacitor device CV response was recorded at scan rates between 5 and 100 mV s⁻¹. Fig. 11(c) displays a predominantly EDLC-type response, as reflected by its quasi-rectangular morphology and the absence of prominent redox peaks. Importantly, the CV profiles maintain their overall shape across the entire range of scan rates, indicating ideal capacitive performance, fast ion transport, and superior rate capability of the device. Although a slight distortion becomes noticeable at high scan rates, this variation is attributed to the surface-controlled faradaic reactions associated with the NiCo₂O₄@g-C₃N₄ electrode, which coexist with the dominant double-layer charge-storage process. Together, these features confirm the efficient synergy between EDLC and pseudocapacitive contributions, along with the rapid ion diffusion and robust electrochemical kinetics of the asymmetric device. The GCD profiles were acquired at different CD and V. Fig. 11(d and e) exhibit the characteristic non-linear charge–discharge behaviour, confirming the dominant pseudocapacitive contribution of the ASC device. As the potential window expands, the discharge time increases noticeably and remains stable in each CV analysis. Furthermore, the GCD curves display a nearly triangular shape, indicative of efficient redox kinetics and excellent pseudocapacitive performance. According to the GCD curves shown in Fig. 11(f), the C_s of 237.7 F g⁻¹ (79.2 mAh g⁻¹) at 1.6 A g⁻¹ gradually decrease to 163 F g⁻¹ (54.3 mAh g⁻¹) at 2.7 A g⁻¹. The intermediate values are 229.7, 208, 189.5, and 174.3 F g⁻¹ at 1.7, 2.0, 2.2, and 2.5 A g⁻¹. The device capability to sustain strong performance at increasing current densities is ascribed to the synergistic effects of the g-C₃N₄ conductive scaffold, which supports efficient electron transport, and the porous NiCo₂O₄ nanostructure, which provides numerous ion-accessible sites for rapid diffusion and faradaic charge storage. The Ragone plot in Fig. 11(g) highlights the outstanding energy and power characteristics of the ASC device. It achieves a peak E of 47.5 Wh kg⁻¹ at a P of 975 W kg⁻¹, and retains 32 Wh kg⁻¹ at a high P of 1650 W kg⁻¹. The specific capacitance values calculated at different CD are summarized in Table 3, clearly showing the comparison of electrode performance. This balance between high energy and high power demonstrates superior performance compared to typical carbon-based supercapacitors, confirming the synergistic electrochemical contribution of the hybrid NiCo₂O₄@g-C₃N₄. The asymmetric configuration offers practical advantages over conventional symmetric supercapacitors by integrating two complementary electrode materials with different charge-storage mechanisms. This design enables a wider operating voltage window, resulting in enhanced energy-storage capability while maintaining good power performance. Therefore, the NiCo₂O₄@g-C₃N₄//rGO asymmetric supercapacitor configuration is more promising for practical energy-storage applications. Table 4 presents a comparative summary of the electrochemical performance of NiCo₂O₄-based nanocomposites reported in previous studies.^60–66

Fig. 11. (a) CV curves of rGO and NiCo₂O₄@g-C₃N₄ electrodes. (b) CV curves at different potential windows. (c) CV curves at different scan rates. (d and e) GCD curves at different potential windows and current densities. (f) Specific capacitance versus current density. (g) Ragone plot. (h) Nyquist plot with equivalent circuit fitting. (i) Cycling stability and coulombic efficiency (inset: last 10 GCD cycles).

Table 3. Performance metrics of the NiCo₂O₄@g-C₃N₄//rGO asymmetric supercapacitor at various current densities.

Sample	Current Density	Specific capacitance (F g−1)	Specific capacity	Energy density	Power density
NiCo2O4@g-C3N4//rGO	1.6	237.7	79.2	47.55	975.07
	1.7	229.7	76.5	45.95	1050.08
	2	208	69.3	41.60	1200.09
	2.2	189.5	63.1	37.90	1350.10
	2.5	174.3	58.1	34.87	1500.12
	2.7	163	54.3	32.60	1650.13

Table 4. Comparison of the electrochemical performance of reported NiCo₂O₄-based asymmetric supercapacitors with the present work.

Sample	Potential window (V)	Current density (A g−1)	Specific capacitance(F g−1)	Energy density (Wh Kg−1)	Power density (W Kg−1)	Reference
NiCo2O4/rGO//AC	1.6	1	91	32.38	797	60
NiCo2O4//Cocoa AC-700	1.5	0.5	168.7 mAh g−1	47.7	430	61
NiCo2O4@CNFs//AMCNFs	1.6	2	107	38.5	1600	62
rGO/NiCo2O4@ZnCo2O4//rGO	1.4	1	260	71	980	63
NiO//AC	1.6	0.5	147.5	52.4	800	64
NiCo2O4/rGO/CF//AC	1.7	1	61.2	24.6	850.3	65
NiCo2S4@PPy-50//AC	1.6	5 mA cm−2	97.37	34.62	120.19	66
NiCo2O4@g-C3N4	1.2	1.6	237.7	47.55	975.07	This work

Electrochemical impedance spectroscopy (EIS) of the assembled NiCo₂O₄@g-C₃N₄//rGO asymmetric supercapacitor Fig. 11(h) provides essential insight into its internal resistance, ion-transport characteristics, and charge-transfer performance. In the Nyquist plot, the high-frequency intercept with the ESR (R_s) remains very low (0.76 Ω), indicating minimal contact resistance, efficient ionic conduction within the PVA/KOH gel electrolyte, and excellent electrode current collector integration. The semicircle in the high-frequency region reflects the charge-transfer resistance (R_ct), with a fitted value of 2.26 Ω, demonstrating fast interfacial redox kinetics at the NiCo₂O₄@g-C₃N₄ and rGO electrode surfaces. In the low-frequency region, the nearly vertical line signifies ideal capacitive behaviour and low ion-diffusion resistance, confirming rapid electrolyte penetration and well-developed ion-transport channels throughout the device architecture. The equivalent circuit shown in the inset, employed to fit the impedance data, comprises three resistive components (R₁, R₂, R₃) along with two constant phase elements (Q₂ and Q₃), each describing a specific electrochemical process. R₁ represents the ohmic resistance (R_s = 0.76 Ω), which involves the intrinsic ionic resistance of the gel electrolyte, the electronic resistance of the electrode, and contact resistance at the electrode and current-collector interfaces. R₂ corresponds to the charge-transfer resistance (R_ct = 2.26 Ω) accompanying the redox reactions appearing at the NiCo₂O₄@g-C₃N₄ with the electrolyte interface. R₃ accounts for the remaining internal resistance arising from ion migration and redistribution within the porous electrode network, including pore-constriction effects, irregular ion diffusion pathways, and interparticle contact resistance. The constant phase elements Q₂ and Q₃ represent non-ideal double-layer and pseudocapacitive responses from the rGO and NiCo₂O₄@g-C₃N₄ electrodes, respectively. Collectively, the low R₁, R₂, and R₃ values confirm highly efficient ion transport, rapid interfacial charge transfer, and minimal polarization. Fig. 11 (i) illustrates the cycling stability of the ASC device, which was investigated through repeated GCD measurements at a CD of 2.5 A g⁻¹. The device retained 94.48% of its initial capacitance even after 6500 cycles, while maintaining nearly 100% coulombic efficiency, indicating excellent reversibility and stable charge storage behavior. The outstanding cycling performance can be attributed to the structural stability and conductive network provided by the g-C₃N₄-based electrode, which facilitates efficient electron transport and minimizes degradation during repeated redox reactions. Furthermore, as a preliminary proof-of-concept demonstration, the charged ASC device was used to power a red light-emitting diode (LED), as illustrated in Scheme 1. The successful illumination of the LED indicates that the assembled device can deliver electrical energy; however, large-scale device fabrication and systematic optimization of electrode mass loading using different loading values were not investigated in the present study.

3.8. Photocatalytic activity

CV and MB are toxic organic pollutants that present significant environmental and health concerns due to their carcinogenic nature and persistence in water. As illustrated in Scheme 2, the visible light-driven photocatalytic reaction on NiCo₂O₄@g-C₃N₄ involves electron–hole pair separation, resulting in the formation of reactive radicals (˙O₂⁻ and ˙OH), which play a vital role in degrading these dye molecules. To address this problem, the visible-light photocatalytic efficiency of the prepared catalysts NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ was evaluated. Control experiments carried out under visible light without catalyst (blank) showed negligible dye degradation, while the dark adsorption experiments exhibited only limited dye removal, confirming that direct photolysis and adsorption contribute minimally to the observed photocatalytic degradation. As shown in Fig. 12(a–f), the UV-visible absorption spectra of CV progressively decreased with increasing irradiation time, showing the gradual breakdown of the chromophoric structure. After 150 minutes of visible-light exposure, the monometallic oxides NiO and Co₃O₄ achieved degradation efficiencies of 66.21% and 67.66%, respectively. Rapid recombination of photogenerated charge carrier and limited absorption of visible light are the main causes of their comparatively low activity. The binary oxide NiCo₂O₄ exhibited an enhanced degradation efficiency of 71.72%, due to the synergistic charge-transfer behavior of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ ion pairs, which improve charge transport and extend light absorption. However, partial charge recombination still limited its overall performance. A remarkable improvement was observed for the NiCo₂O₄@g-C₃N₄ composite, which achieved 96.19% CV degradation within 150 minutes of visible-light irradiation. This improved performance is attributed to enhanced charge separation and charge transfer in the NiCo₂O₄@g-C₃N₄ heterostructure. The g-C₃N₄ is well dispersed on the NiCo₂O₄ surface, providing effective interfacial contact that facilitates charge separation and transfer between the two semiconductors. The heterojunction structure is expected to enhance visible-light absorption and provide a larger surface area with numerous active sites for dye adsorption and oxidation. This may facilitate improved charge separation and reduced electron–hole recombination, thereby enhancing photocatalytic efficiency and stability.

Scheme 2. Schematic illustration of the visible-light-driven photocatalytic degradation mechanism of MB and CV dyes over the NiCo₂O₄@g-C₃N₄ photocatalyst.

Fig. 12. (a–d) UV-vis absorption spectra of CV dye degradation over NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ under visible-light irradiation. (e) Photocatalytic degradation profile (C/C₀) of CV dye. (f) Degradation efficiency of the prepared photocatalysts at different irradiation times.

A degradation was observed for MB under the same conditions. The monometallic oxides NiO and Co₃O₄ achieved moderate degradation efficiencies of 70.95% and 80.24%, respectively. The binary NiCo₂O₄ demonstrated a significantly enhanced efficiency of 88.67%, indicating its improved charge mobility and redox activity compared to individual oxides. The NiCo₂O₄@g-C₃N₄ composite exhibited optimal performance, achieving degradation of 96.17% MB within a duration of 150 minutes. The nearly identical efficiencies for CV (96.19%) and MB (96.17%) demonstrate the broad applicability and strong visible-light response of the NiCo₂O₄@g-C₃N₄ catalyst. The enhanced photocatalytic performance of the composite is associated with (i) interfacial interaction between NiCo₂O₄ and g-C₃N₄, (ii) improved visible-light absorption, (iii) effective separation and migration of photo-induced electron–hole pairs, and (iv) increased surface area providing additional reactive sites for dye oxidation. The analysis indicates that the NiCo₂O₄@g-C₃N₄ heterojunction photocatalyst exhibits good stability and high photocatalytic efficiency for both CV and MB dye degradation under visible light. These results indicate the potential of NiCo₂O₄@g-C₃N₄ as a practical photocatalytic material for visible-light-driven dye degradation.

Fig. 12(e) illustrates the initial concentration of CV dye, where Fig. 13(e) displays the corresponding MB dye concentration, both observed with and without the photocatalyst under continuous light exposure across various time intervals (t). The NiCo₂O₄@g-C₃N₄ nanocomposite exhibits superior degradation performance. Additionally, the pseudo-first-order kinetic analyses revealed in Fig. 14(a and b) indicate degradation rate constants for NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ toward CV dye as 0.0153 min⁻¹, 0.0171 min⁻¹, 0.0172 min⁻¹, and 0.0465 min⁻¹, respectively. Similarly, the rate constants for MB dye degradation are 0.0198 min⁻¹, 0.0247 min⁻¹, 0.0331 min⁻¹, and 0.0447 min⁻¹ for NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄, respectively. Table 5 presents the calculated rate constants (k) and R² values based on the kinetic evaluations.

Fig. 13. (a–d) UV-vis absorption spectra of MB dye degradation over NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ under visible-light irradiation. (e) Photocatalytic degradation profile (C/C₀) of CV dye. (f) Degradation efficiency of the prepared photocatalysts at different irradiation times.

Fig. 14. (a and b) Pseudo-first-order kinetic plots for CV and MB dye degradation. (c) Reusability of the NiCo₂O₄@g-C₃N₄ photocatalyst over five cycles. (d) Active species trapping experiments for CV and MB degradation using different scavengers.

Table 5. Comparison of the photocatalytic degradation performance of NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ toward CV and MB dyes.

Catalyst	Degradation efficiency (%) after 150 min		K (min−1)		R 2
	CV	MB	CV	MB	CV	MB
NiO	66.21	70.95	0.0153	0.0198	0.970	0.983
Co3O4	67.66	80.24	0.0171	0.0247	0.976	0.995
NiCo2O4	71.72	88.67	0.0172	0.0331	0.955	0.991
NiCo2O4@g-C3N4	96.19	96.17	0.0465	0.0447	0.94	0.95

3.8.1. Effect of radical scavengers on photocatalytic activity

In photocatalytic systems, the catalyst typically produces reactive species such as (h⁺), (˙OH), and (˙O₂⁻). Identifying which of these species actively participates in dye degradation is essential for understanding the reaction mechanism. To clarify the degradation pathway of dyes over the NiCo₂O₄@g-C₃N₄ system, specific scavengers were introduced into the reaction mixture: EDTA to capture (h⁺),⁶⁷ methanol⁶⁸ to quench (˙OH) radicals, and benzoquinone (BQ)⁶⁹ to suppress (˙O₂⁻) radicals. The effects of these scavengers on dye degradation are presented in Fig. 14(c). Along with BQ and EDTA markedly decreases the degradation efficiency, indicating that (˙O₂⁻) superoxide ions and photogenerated holes (h⁺) are involved in the degradation of both MB and CV dyes. In contrast, the addition of methanol caused only a slight reduction in efficiency, suggesting that hydroxyl radicals (˙OH) contribute minimally to the overall degradation. These results confirm that (˙O₂⁻) plays the predominant role, supported by the secondary involvement of (h⁺).

The photocatalytic removal of CV and MB dyes over the NiCo₂O₄@g-C₃N₄ heterostructure follows a multi-step redox mechanism by the generation and utilization of reactive species.⁷⁰

(i) Excitation and charge-carrier generation

NiCo₂O₄@g-C₃N₄ + hν → e⁻ (CB) + h⁺ (VB) 11

Upon visible-light illumination, the NiCo₂O₄@g-C₃N₄ composite absorbs photons and produces electron–hole pairs.

(ii) Formation of hydroxyl radicals (˙OH)

h⁺ + OH⁻ → ˙OH 12

h⁺ + H₂O → ˙OH + H⁺ 13

Photogenerated holes oxidize surface hydroxyl groups or water molecules, resulting in the production of highly reactive hydroxyl ions.

(iii) Generation of superoxide radicals (˙O₂⁻)

e⁻ + O₂ → ˙O₂⁻ 14

Electrons reduce molecular oxygen, producing superoxide ions.

(iv) Oxidative decomposition of dye molecules

Dye + ˙OH + ˙O₂⁻ → degradation products 15

The combined action of ˙OH and ˙O₂⁻ radicals initiates successive oxidative attacks on the CV and MB dye molecules, breaking chromophoric bonds and converting them into smaller, non-toxic intermediates and final mineralized products.

In this work, the NiCo₂O₄@g-C₃N₄ photocatalyst exhibits markedly superior activity compared with pristine NiO, Co₃O₄, and NiCo₂O₄. This enhanced performance can be mainly ascribed to two key factors: (i) its reduced band-gap energy and (ii) its heterostructure design. Notably, the NiCo₂O₄@g-C₃N₄ composite presents a narrower band gap of 1.61 eV, which substantially promotes visible-light absorption and suppresses charge-carrier recombination. Furthermore, the strongly coupled NiCo₂O₄-g-C₃N₄ interface provides a beneficial morphology that improves the adsorption and interaction of dye molecules.

The stability of the NiCo₂O₄@g-C₃N₄ composite was evaluated through five successive photocatalytic cycles. Fig. 14(d) presents the reusability and stability profile of the NiCo₂O₄@g-C₃N₄ photocatalyst for the degradation of both dyes. In this study, the catalyst was collected after each cycle by centrifugation, washed with water and ethanol, and dried naturally under ambient conditions. Even after five consecutive cycles, the photocatalyst retained approximately 90% of its activity for MB and 89% for CV, demonstrating good stability and high photocatalytic efficiency. The slight decrease in activity can be attributed primarily to unavoidable catalyst loss during repeated washing and handling. These findings clearly highlight the strong potential of NiCo₂O₄@g-C₃N₄ for practical wastewater treatment dye degradation. Table 6 summarizes recently reported photocatalytic studies, highlighting differences in catalyst composition, operating parameters, target pollutants, and degradation efficiencies for comparison.^71–78 Even under the time durations of visible-light exposure, NiCo₂O₄@g-C₃N₄ demonstrates outstanding photocatalytic activity, establishing it as an effective and promising candidate for the rapid degradation of CV and MB organic pollutants.

Table 6. Comparison of the photocatalytic performance of reported catalysts for organic dye degradation, including the present work.

Catalyst	Dye	Catalyst dosage	Dye concentration	Light source	Efficiency (%)	Time (min)	Reference
NiO	CR	5 mg	20 ppm	Sunlight	43	160	71
NiCo2O4/NiO	RhB	5 mg	5 ppm	Sunlight	83.66	120	72
NiCo2O4	MR	10 mg	10 mg L−1	Visible	95.1	120	73
NiCo2O4@CFs	MB	20 mg	10 ppm	Visible	91.3	120	74
NiCo2O4@WO3	RhB	50 mg	10 mg L−1	Visible	96.4	160	75
NiCo2O4@MWCNTs	RR (120)	10 mg	30 ppm	UV	89.8	180	76
g-C3N4/N-doped CeO2	MB	300 mg	30 ppm	Sunlight	97	120	77
ZnO/g-C3N4	MB	40 mg	10 mg L−1	Visible	97	60	78
NiCo2O4@g-C3N4	MB	30 mg	20 ppm	Visible	96.17	150	This work
NiCo2O4@g-C3N4	CV	30 mg	20 ppm	Visible	96.19	150	This work

4. Conclusion

In summary, NiO, Co₃O₄, NiCo₂O₄, and NiCo₂O₄@g-C₃N₄ hybrid nanocomposites were successfully synthesized through a facile hydrothermal route and evaluated for energy-storage and photocatalytic applications. The incorporation of g-C₃N₄ significantly enhanced the electrochemical performance of NiCo₂O₄ by improving charge-transfer characteristics and charge-storage capability. The NiCo₂O₄@g-C₃N₄ electrode exhibited superior specific capacitance, while the assembled asymmetric supercapacitor demonstrated excellent energy-storage performance with good cycling stability and practical voltage output. Furthermore, the hybrid composite showed enhanced visible-light-driven photocatalytic degradation of CV and MB dyes compared with the individual oxide catalysts. The improved multifunctional performance is attributed to the synergistic interaction between NiCo₂O₄ and g-C₃N₄, which promotes efficient charge separation and utilization of photogenerated carriers. Overall, the NiCo₂O₄@g-C₃N₄ heterostructure demonstrates potential for multifunctional energy-storage and photocatalytic applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study are available within the article. Additional data supporting the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra05905h.