# Composite g-C3N4/NiCo2O4 with Excellent Electrochemical Impedance as an Electrode for Supercapacitors

Authors: Danfeng Cui; Zheng Fan; Yanyun Fan; Hongmei Chen; Penglu Li; Xiaoya Duan; Shubin Yan; Hongyan Xu; Chenyang Xue
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

For the development of supercapacitors, electrode materials with the advantages of simple synthesis and high specific capacitance are one of the very important factors. Herein, we synthesized g-C3N4 and NiCo2O4 by thermal polymerization method and hydrothermal method, respectively, and finally synthesized NiCo2O4/g-C3N4 nanomaterials by mixing, grinding, and calcining g-C3N4 and NiCo2O4. NiCo2O4/g-C3N4 nanomaterials are characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The microscopic morphology, lattice structure, and element distribution of NiCo2O4/g-C3N4 nanomaterials were characterized by scanning electron microscopy (SEM), transmission electron microscopy, high resoultion transmission electron microscopy, and mapping methods. The electrochemical performance and cycle stability of NiCo2O4/g-C3N4 were tested in a 6 M KOH aqueous solution as electrolyte under a three-electrode system. Due to the physical mixing structure of g-C3N4 and NiCo2O4 nanomaterials, the electrochemical energy storage performance of NiCo2O4/g-C3N4 supercapacitor electrodes is better than that of NiCo2O4 supercapacitor electrodes. At a current density of 1 A/g, the capacitances of NiCo2O4 and NiCo2O4/g-C3N4 are 98.86 and 1,127.71 F/g, respectively. At a current density of 10 A/g, the capacitance of NiCo2O4/g-C3N4 supercapacitor electrode maintains 70.5% after 3,000 cycles. NiCo2O4/g-C3N4 electrode has excellent electrochemical performance, which may be due to the formation of physical mixing between NiCo2O4 and g-C3N4, which has broad application prospects. This research is of great importance for the development of materials in high-performance energy storage devices, catalysis, sensors, and other applications.


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## Body

## 1. Introduction

Supercapacitor is an energy storage device, which is different from battery and capacitor [1, 2]. It has the advantages of fast charging speed [3], long service life [4], high electricity conversion efficiency [5], high power density [6], high safety factor, and green friendliness. Supercapacitors are used in many fields such as wind power generation systems, heavy-duty machinery, and hybrid vehicles. According to different principles, supercapacitors are classified as double-layer capacitors and pseudo-capacitors. Pseudo-capacitors [2], as a kind of supercapacitors, have been studied due to their advantages of higher discharge time and larger stored power. The supercapacitor electrode is the most crucial part of the storage capacity of the supercapacitor. In previous studies, pseudo-capacitance electrodes are mainly composed of oxides of elements such as Co [7–9], Fe [10, 11], Ru [12–14], Mn [15, 16], Ni [17, 18], W [19, 20], and Zn [21, 22]. These metal oxides have relatively high theoretical capacitance values [23, 24], and the synthesis method is simple. In recent years, multimetal oxides [25–27] have gradually attracted attention because of their excellent theoretical capacitance, high-power density, and outstanding cycling characteristics. Among them, the theoretical capacitance value of NiCo2O4 is 890 mAh/g [28], and it has good electrical conductivity due to the presence of bimetallic elements. These advantages provide support for NiCo2O4 to become a promising electrode material for supercapacitors [29]. Li et al. [30] facilely synthesized and electrochemically tested NiCo2O4 with different crystal structures. By controlling the ratio of CO(NH2)2 and NH4F composition, the crystal growth structure of NiCo2O4 was controlled. Among them, the NiCo2O4 with the best mass-specific capacitance is 1,710.9 F/g. However, a noncomposite bimetal oxide as a supercapacitor electrode usually has a large electrochemical impedance. By using suitable materials to compound with bimetallic oxides, it will usually help the electrode to have good electrochemical impedance.Graphite carbon nitride (g-C3N4) [31–33] is a widely used carbon-based material. g-C3N4 is a flexible layered structural material with good chemical stability, nontoxic, nonpolluting, and low cost [34–36]. Due to the presence of pyrrole nitrogen hole defects in the crystal lattice and the reduced distance between the edge covalent nitrogen atoms, the material exhibits a higher rate capability. Moreover, the porous heptazine and sp2 hybrid nitrogen also provide coordination sites [37]. With the synthesis of bimetallic transition metal element oxides dispersed on the g-C3N4 grid (with improved redox sites), its conductivity, electrochemical performance, hydrophilicity, and surface polarity have been enhanced. This allows supercapacitor electrodes to obtain excellent cycle performance and high-rate performance [38, 39]. Rabani et al. [37] studied the compound-prepared method and electrochemical performance of Co3O4@g-C3N4. The experimental results show that the capacitance of the Co3O4@g-C3N4 supercapacitor reaches 457.2 F/g, and it maintains 92% of the capacitance after 5,000 cycles. Thiagarajan et al. [40] synthesized NiMoO4/g-C3N4 by hydrothermal method and tested its electrochemical energy storage performance. The supercapacitor electrode NiMoO4/g-C3N4 reached 510 F/g, and maintained 91.8% capacity after 2,000 cycles.In this study, g-C3N4 is synthesized by thermal polymerization, NiCo2O4 is synthesized by hydrothermal method and thermal oxidation method. g-C3N4/NiCo2O4 nanomaterial is synthesized by fully grinding and calcining g-C3N4 and NiCo2O4. It can be observed that physical mixing structures of g-C3N4 and NiCo2O4 are formed in the g-C3N4/NiCo2O4 nanomaterial using TEM. Due to the existence of the physical mixing, the g-C3N4/NiCo2O4 nanomaterial has a higher mass-specific capacitance compared with NiCo2O4 when used as a supercapacitor electrode. By studying the electrochemical impedance spectroscopy (EIS) of the materials, the g-C3N4/NiCo2O4 nanomaterial has very low electrochemical impedance in the low-frequency response, showing good electrochemical energy storage performance.


## 2. Experimental

### 2.1. Preparation of the NiCo2O4/g-C3N4 Nanomaterial

At first, 20 g of urea was placed in a corundum crucible and placed in a tube furnace. Then set the heating rate of the tube furnace to 10°C/min. The tube furnace temperature was maintained at 550°C for 180 min. After waiting for the complete natural cooling to room temperature, the sample was taken out and fully ground to obtain g-C3N4 powder. Then, the NiCo2O4 was prepared by combining with hydrothermal method and annealing. To get the solution for the preparation of NiCo2O4 nanoparticles, the reagent with the precise molar ratio of NiCl2·6H2O : CoCl2·6H2O : CO(NH2)2 : NH4F = 1 : 2 : 6 : 15 was dissolved in 30 mL of deionized water (DI water). After magnetic stirring for 15 min, the evenly mixed aqueous solution was moved into the polytetrafluoroethylene lining of the high-pressure reactor, heated at 150°C for 8 hr, and then cooled and centrifuged to get the NiCo2O4 precursor. And NiCo2O4 precursors were annealed in a tubular furnace at 400°C for 2 hr to get NiCo2O4 crystals. Finally, 2% g-C3N4 and NiCo2O4 were mixed and fully pulverized, then calcined in a tubular furnace at 550°C for 180 min to get 2% g-C3N4/NiCo2O4 powder.


### 2.2. Preparation of the Electrodes

First, nickel foam was cleaned with ultrasonic DI water and dried at 60°C for 6 hr. After that, NiCo2O4/g-C3N4 trituration was mixed with acetylene black and polyvinylidene fluoride at a mass ratio of 0.8 : 0.15 : 0.05. A few drops of alcohol and 5% polytetrafluoroethylene were added and stirred. The foam was coated with nickel foam and the electrodes were dried for 24 hr for electrochemical test.


### 2.3. Characterization of the NiCo2O4/g-C3N4 Nanomaterial

The micromorphology of the nanomaterial surface was investigated by SEM (FEI Quanta FEG 250). The lattice structure of the NiCo2O4/g-C3N4 nanomaterial was investigated by a transmission electron microscope (Tecnai G2 F20). The NiCo2O4/g-C3N4 nanomaterial was confirmed by X-ray diffraction (XRD) (Rigaku Ultimate IV). The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi).


### 2.4. Electrode Performance Test Method

The electrode performance test was carried out under the three-electrode system of working electrode (foamed nickel with NiCo2O4/g-C3N4 nanomaterial), counter electrode (platinum electrode), and reference electrode (saturated calomel electrode). Cyclic voltammetry (CV), galvanostatic current charging–discharging (GCD), EIS, and cycling ability were tested in a 6 M KOH aqueous solution through an electrochemical workstation (Metrohm Multi Autolab M204).


## 2.1. Preparation of the NiCo2O4/g-C3N4 Nanomaterial

At first, 20 g of urea was placed in a corundum crucible and placed in a tube furnace. Then set the heating rate of the tube furnace to 10°C/min. The tube furnace temperature was maintained at 550°C for 180 min. After waiting for the complete natural cooling to room temperature, the sample was taken out and fully ground to obtain g-C3N4 powder. Then, the NiCo2O4 was prepared by combining with hydrothermal method and annealing. To get the solution for the preparation of NiCo2O4 nanoparticles, the reagent with the precise molar ratio of NiCl2·6H2O : CoCl2·6H2O : CO(NH2)2 : NH4F = 1 : 2 : 6 : 15 was dissolved in 30 mL of deionized water (DI water). After magnetic stirring for 15 min, the evenly mixed aqueous solution was moved into the polytetrafluoroethylene lining of the high-pressure reactor, heated at 150°C for 8 hr, and then cooled and centrifuged to get the NiCo2O4 precursor. And NiCo2O4 precursors were annealed in a tubular furnace at 400°C for 2 hr to get NiCo2O4 crystals. Finally, 2% g-C3N4 and NiCo2O4 were mixed and fully pulverized, then calcined in a tubular furnace at 550°C for 180 min to get 2% g-C3N4/NiCo2O4 powder.


## 2.2. Preparation of the Electrodes

First, nickel foam was cleaned with ultrasonic DI water and dried at 60°C for 6 hr. After that, NiCo2O4/g-C3N4 trituration was mixed with acetylene black and polyvinylidene fluoride at a mass ratio of 0.8 : 0.15 : 0.05. A few drops of alcohol and 5% polytetrafluoroethylene were added and stirred. The foam was coated with nickel foam and the electrodes were dried for 24 hr for electrochemical test.


## 2.3. Characterization of the NiCo2O4/g-C3N4 Nanomaterial

The micromorphology of the nanomaterial surface was investigated by SEM (FEI Quanta FEG 250). The lattice structure of the NiCo2O4/g-C3N4 nanomaterial was investigated by a transmission electron microscope (Tecnai G2 F20). The NiCo2O4/g-C3N4 nanomaterial was confirmed by X-ray diffraction (XRD) (Rigaku Ultimate IV). The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi).


## 2.4. Electrode Performance Test Method

The electrode performance test was carried out under the three-electrode system of working electrode (foamed nickel with NiCo2O4/g-C3N4 nanomaterial), counter electrode (platinum electrode), and reference electrode (saturated calomel electrode). Cyclic voltammetry (CV), galvanostatic current charging–discharging (GCD), EIS, and cycling ability were tested in a 6 M KOH aqueous solution through an electrochemical workstation (Metrohm Multi Autolab M204).


## 3. Results and Discussion

Figure1 shows the XRD spectrum of g-C3N4, NiCo2O4, and NiCo2O4/g-C3N4 composite nanomaterials. The blue line of g-C3N4 in Figure 1 has an obvious (002) peak. The green line in the figure represents the XRD line of NiCo2O4 nanoparticles. All the peaks in the line can correspond to the cubic crystal orientation of NiCo2O4 PDF#20-0781 (111) (220) (311) (222) (400) (511) (440), and there are no other obvious peaks [30, 41]. Therefore, the green line in the XRD spectrum proves that we have prepared NiCo2O4 nanoparticles with higher purity. The red line in Figure 1 represents the XRD spectrum of the NiCo2O4/g-C3N4 composite nanomaterial. In the red line, the (220) (311) (222) (511) (440) peaks correspond to the NiCo2O4 in the NiCo2O4/g-C3N4 composite nanomaterial. The XRD spectrum shows that the (111) lattice peak of nickel cobalt oxide is shifted. And the appearance of the (002) peak [40] represents the successful composite of NiCo2O4 nanoparticles and g-C3N4 nanomaterials, and the NiCo2O4/g-C3N4 nanomaterials were successfully prepared.Figure 1
XRD graphs of g-C3N4, NiCo2O4, and g-C3N4/NiCo2O4.Figure2 shows the SEM images of g-C3N4, NiCo2O4, and NiCo2O4/g-C3N4. Figures 2(a) and 2(b) are SEM images of g-C3N4. It could be observed from Figure 2(a) that there are 200–300 nm pores between the g-C3N4 nanoparticles. It can be observed from Figure 2(b) that the micron morphology of the g-C3N4 nanoparticles is porous floc. Such a structure significantly increases the effective boundary area between the nanomaterial and the electrolyte. Furthermore, the porous floc can improve the electrochemical energy storage contact interface to improve electrochemical energy storage performance. Figures 2(c) and 2(d) are SEM pictures of NiCo2O4 nanoparticles. It can be observed from Figure 2(c) that the dimension of NiCo2O4 nanoparticles is 50–100 nm. The particle size of NiCo2O4 matches the size of the nanopores of g-C3N4, which can better complete the composite between nanomaterials. Figures 2(e) and 2(f) are SEM images of NiCo2O4/g-C3N4 composite material. It can be seen in Figure 2(e) that NiCo2O4 nanoparticles are partially embedded in the nanopores of g-C3N4. NiCo2O4 nanoparticles and g-C3N4 nanomaterials are effectively combined through the method shown in Figure 2(e) to improve the electrochemical performance of NiCo2O4/g-C3N4 composite nanomaterials.Figure 2
SEM pictures of (a, b) g-C3N4; (c, d) NiCo2O4; (e, f) g-C3N4/NiCo2O4 samples.Figures3(a)–3(d) is TEM images of g-C3N4, NiCo2O4, and NiCo2O4/g-C3N4 composite nanomaterials, respectively. We can observe from Figure 3(d) that NiCo2O4 and g-C3N4 are in close contact to form a NiCo2O4/g-C3N4 composite nanomaterial. As shown in Figures 3(a) and 3(b), NiCo2O4 has smaller particles and structures than g-C3N4. The g-C3N4/NiCo2O4 nanomaterials are synthesized by grinding and calcining g-C3N4 and NiCo2O4. As shown in Figure 3(c), the larger area of g-C3N4 forms a physical mixing structure with smaller particles of NiCo2O4. Figure 3(d) is a larger magnification TEM image of g-C3N4/NiCo2O4 nanomaterials, which clearly shows the physical mixing structure formed by NiCo2O4 nanoparticles on the larger structure of g-C3N4. And comparing with the size of NiCo2O4 particles in Figure 3(b), the black particles in Figure 3(d) are NiCo2O4 particles after forming a composite with g-C3N4. Figure 3(e) is the high resoultion transmission electron microscopy (HRTEM) image of NiCo2O4/g-C3N4. In the figure, the distance between the crystal orientations of NiCo2O4 (220) and (311) is marked, which matches the XRD analysis result. Figure 3(f) shows the element mapping of NiCo2O4/g-C3N4 composite nanomaterials. In Figure 3(f), it can be clearly seen that C, N, O, Co, and Ni are uniformly distributed in the NiCo2O4/g-C3N4 composite nanomaterial, which proves the successful composite of NiCo2O4/g-C3N4 material.Figure 3
TEM pictures of (a) g-C3N4; (b) NiCo2O4; (c, d) g-C3N4/NiCo2O4 samples; (e) HRTEM picture of g-C3N4/NiCo2O4; (f) elements mapping of g-C3N4/NiCo2O4.Figure4 shows the XPS spectrum of the g-C3N4/NiCo2O4 composite nanomaterial. It can be clearly observed in Figure 4(a) that composite nanomaterials have elements such as C, N, O, Co, and C. In Figure 4(b), the peak splitting and fitting of Ni 2p can be clearly observed. Due to the existence of the spin orbits of Ni3+ and Ni2+, the two main peaks are 873.6 and 856.4 eV, respectively. Among them, the two weaker peaks are due to the weaker accompanying peaks produced by Ni3+ and Ni2+, which are also related to Ni3+ and Ni2+. The O 1s spectrum in Figure 4(d) shows three oxygen peaks, concentrated at 531.12, 532.57, and 529.66 eV. This is due to the formation of oxides by the OH− and O2− of Ni and Co, which correspond to these three peaks. The C 1s spectrum in Figure 4(e) has three carbon peaks at ∼284.8, 286.03, and 288.16 eV, which are related to the formation of carbon–carbon bonds and carbon–nitrogen bonds [35]. The N 1s spectrum in Figure 4(f) shows three nitrogen peaks, concentrated at 399.27, 400.19, 401.32, and 403.31 eV [39].Figure 4
(a) g-C3N4/NiCo2O4 XPS spectrum and high-resolution XPS spectra of (b) Ni 2p; (c) Co 2p; (d) O 1s; (e) C 1s; (f) N 1s.
(a)(b)(c)(d)(e)(f)First, we tested NiCo2O4 and g-C3N4/NiCo2O4 supercapacitor electrodes by CV. Figures 5(a) and 5(b) are the CV test curves of NiCo2O4 and g-C3N4/NiCo2O4 supercapacitor electrodes. NiCo2O4 and g-C3N4/NiCo2O4 supercapacitor electrodes were tested with different scan rates (5–100 mV/s) under the voltage of 0–0.45 V. Figure 5(a) clearly shows the redox peak of NiCo2O4 nanomaterials, indicating that NiCo2O4 supercapacitor electrodes have pseudo-capacitance characteristics. When the scan rate increases, the redox peak of the NiCo2O4 supercapacitor electrode shifts to a higher or lower voltage value, which is caused by the internal resistance of the electrode and the tortuous diffusion path of H+ ions in the supercapacitor electrode material. It can be clearly observed in Figure 5(b) that, compared with the NiCo2O4 supercapacitor electrode, the redox peak of the g-C3N4/NiCo2O4 supercapacitor electrode has a higher peak value. This shows that the g-C3N4/NiCo2O4 supercapacitor electrode has a higher working voltage window for electrochemical energy storage and is more suitable for high-voltage energy storage applications. It can be clearly observed in Figure 5(b) that the g-C3N4/NiCo2O4 supercapacitor electrode has a larger integration area compared to the NiCo2O4 supercapacitor electrode. This means that the g-C3N4/NiCo2O4 supercapacitor electrode has a higher electrochemical energy storage capacity. And the CV curve in Figure 5(b) is more symmetrical, indicating that the electrode of the g-C3N4/NiCo2O4 supercapacitor can undergo a more complete reversible reaction. In addition, the reaction equation represented by the redox peaks in Figures 5(a) and 5(b) should be(1)NiCo2O4+OH−+H2O⟷NiOOH+2CoOOH+e−(2)CoOOH+OH−⟷CoO2+H2O+e−.Figure 5
CV curves of (a) NiCo2O4; (b) g-C3N4/NiCo2O4; GCD curves of (c) NiCo2O4; (d) g-C3N4/NiCo2O4; (e) specific capacitance change trend; (f) cycle properties of g-C3N4/NiCo2O4 under the three-electrode system.
(a)(b)(c)(d)(e)(f)Figures5(c) and 5(d) are the GCD test diagrams of NiCo2O4 supercapacitor electrode and g-C3N4/NiCo2O4 supercapacitor electrode. The GCD test is to characterize the electrochemical energy storage capacity of the electrode more conveniently by using a constant current to charge and discharge the electrode. Mass-specific capacitance and area-specific capacitance can be calculated by the following two formulae:(3)Cs=I×ΔtΔV×sandCg=I×ΔtΔV×m.In Formula (3), Cs is the area-specific capacitance, I is the current during constant current discharge, t is the discharge time, V is the potential difference during discharge, s is the electrode area of the supercapacitor, and Cg is the mass-specific capacitance, m is the material quality of the supercapacitor loaded by the electrode. The loading weights of the NiCo2O4 and g-C3N4/NiCo2O4 composite material on the electrode used in the electrochemical energy storage test were 14.9 and 8.9 mg, respectively. It can be clearly observed from Figures 5(c) and 5(d) that the discharge time of the g-C3N4/NiCo2O4 supercapacitor electrode is much longer than that of the NiCo2O4 supercapacitor electrode under the same charge and discharge current. Calculated by Formula (3), when the charge and discharge current is 1–8 A/g, the mass-specific capacitance content of NiCo2O4 supercapacitor electrode is 98.86, 82.86, 69.43, 50, and 32 F/g. The area-specific capacitance of NiCo2O4 supercapacitor electrodes is 1.4829, 1.2429, 1.04145, 0.75, and 0.48 F/cm2, respectively. However, when the charge and discharge current is 1–10 A/g, the mass-specific capacitance content of the g-C3N4/NiCo2O4 supercapacitor electrode is 1,127.71, 1,031.43, 947.14, 811.43, 637.71, and 517.14 F/g. The area-specific capacitances of the g-C3N4/NiCo2O4 supercapacitor electrodes are 16.92, 15.47, 14.21, 12.17, 9.57, and 7.7571 F/cm2, respectively. Compared with the mass-specific capacitance of the NiCo2O4 supercapacitor electrode, the mass-specific capacitance of the g-C3N4/NiCo2O4 supercapacitor electrode has been significantly improved due to the synergistic effect of the g-C3N4/NiCo2O4 composite nanomaterial. Figure 5(e) is the comparison of mass-specific capacitance between NiCo2O4 supercapacitor electrode and g-C3N4/NiCo2O4 supercapacitor electrode under different current charging and discharging conditions. It can be seen in Figure 5(e) that at higher operating currents, g-C3N4/NiCo2O4 supercapacitor electrodes have better rate characteristics. Figure 5(f) shows the retention of the mass-specific capacitance of the g-C3N4/NiCo2O4 supercapacitor electrode at a current of 10 A/g after 3,000 cycles. After 3,000 cycles of the g-C3N4/NiCo2O4 electrode, the mass-specific capacitance of the comparison electrode was maintained at 70.5% before the cycle test, which has an acceptable capacitance retention. The decrease in the capacitance value of the composites may be caused by the dissolution effect of the alkaline electrolyte on the nickel cobaltate combined with the minor structural instability of the physical mixing [42].(4)E=C×V22andP=Et.Equation (4) is the calculation formula of energy density and power density, respectively. Among them, E, C, V, P, and t in Equation (4) are energy density, capacitance, potential, power density, and discharge time, respectively. Calculated by the formula, the highest energy density of the composite material is 69.07 Wh/kg, and the power density is 603.54 W/kg.EIS is a nondestructive parameter measurement and an effective method for determining the dynamic behavior of electrochemical energy storage devices [43, 44]. We apply a sinusoidal signal with a weak amplitude to the supercapacitor electrodes in the three-electrode supercapacitor system to obtain the change in the ratio of the excitation voltage to the response current, which is the impedance spectrum of the electrochemical system. The electrochemical impedance curve of electrodes made of NiCo2O4 and g-C3N4/NiCo2O4 is measured in the frequency range of 100,000–0.1 Hz. Figures 6(a) and 6(b) are the AC impedance spectrum curves of NiCo2O4 and g-C3N4/NiCo2O4. The intersection of the AC impedance spectrum curve and the x-axis is the solution resistance (Rs) at the interface between the electrolyte and the electrode. The partly circular AC impedance spectrum curve in the high-frequency region is mainly dominated by the charge transfer of the electrode material [45, 46]. The slope of the low-frequency region shows the diffusion coefficient of the material, which is mainly dominated by the material transfer of the electrode material. Rs in electrochemical impedance spectroscopy of NiCo2O4 is 0.46 Ω, Rs in electrochemical impedance spectroscopy of g-C3N4/NiCo2O4 is 0.374 Ω. This proves that the g-C3N4/NiCo2O4 composite nanomaterial has better conductivity than the single NiCo2O4. The argument of Q (CPE) has nothing to do with frequency and is called constant phase angle element. Generally, when the Q parameter n is between 1 and 0.5, there is a dispersion effect on the pole surface. When n = 0.5, CPE can be used to replace the Warburg element of the finite diffusion layer, and CPE can also simulate the high-frequency part of the Warburg element of infinite thickness. The linear slope of the g-C3N4/NiCo2O4 electrode in the low-frequency region is higher than that of the NiCo2O4 electrode. It shows that the mobility of electrolyte ions on the surface of g-C3N4/NiCo2O4 electrode is higher than that on the surface of NiCo2O4 electrode. In Figure 3(d), it can be clearly observed that g-C3N4 forms a physical mixing structure with NiCo2O4. From the above discussion, it can be seen that the electron mobility in the g-C3N4/NiCo2O4 composite can be improved, and the electrochemical impedance spectroscopy shows a lower electrochemical impedance.Figure 6
Nyquist diagram of (a) NiCo2O4 and (b) g-C3N4/NiCo2O4 electrode, illustrated with the fitted equivalent circuit and enlarged diagram.
(a)(b)Table1 shows the comparison of mass-specific capacitance values of metal oxides and carbon nitride composite nanomaterials. The g-C3N4/NiCo2O4 prepared in this study exhibited high mass-specific capacitance.Table 1
Specific capacitance comparison with CNs and metal oxides.
No.ElectrodesElectrolyteCurrent density (A/g)Supercapacitors capacitance (F/g)Ref.1g-C3N4/TNS4 M KOH0.25332[31]2Functionalized g-C3N4/CNF/TNS4 M KOH0.25817[31]3MnO2@pg-C3N4–1348.4[32]4Ce-SnO2@g-C3N42 M KOH1274[33]5Co3O4-rGO-gC3N40.5 M H2SO41675[35]6Co3O4@g-C3N43 M KOH1457.2[37]7NiMoO4/g-C3N46 M KOH1510[40]8g-C3N4/NiCo2O46 M KOH11,127.71This work


## 4. Conclusions

In this paper, we synthesized g-C3N4 and NiCo2O4 by thermal polymerization method and hydrothermal method, respectively, and finally synthesized NiCo2O4/g-C3N4 nanomaterials by fully mixing, grinding, and calcining g-C3N4 and NiCo2O4. Due to the effective combination of g-C3N4 and NiCo2O4 nanomaterials, the electrochemical energy storage performance of NiCo2O4/g-C3N4 supercapacitor electrodes is better than that of NiCo2O4 supercapacitor electrodes. At a current of 1 A/g, the mass-specific capacitances of NiCo2O4 and NiCo2O4/g-C3N4 are 98.86 and 1,127.71 F/g, respectively. At a current of 10 A/g, the NiCo2O4/g-C3N4 supercapacitor electrode maintains 70.5% of capacitance after 3,000 cycles. Moreover, NiCo2O4/g-C3N4 electrode shows an excellent electrochemical impedance compared with single NiCo2O4 electrode. NiCo2O4/g-C3N4 electrode has excellent electrochemical performance, which may be due to the formation of physical mixing between NiCo2O4 and g-C3N4, which has broad application prospects. This research is of great importance for the development of materials in high-performance energy storage devices, catalysis, sensors, and other applications.



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