Design and synthesis of MOF-derived CuO/g-C₃N₄ composites with octahedral structures as advanced anode materials for asymmetric supercapacitors with high energy and power densities

Abstract

In this study, two-component composites consisting of transition metal copper oxide (CuO) and graphite carbon nitride (g-C₃N₄) were successfully synthesized. Firstly, a typical metal–organic framework (MOF) material, namely CuBTC, was prepared by the hydrothermal method. Then, the hollow porous CuO/g-C₃N₄ composite with an octahedron structure was obtained by introducing a certain amount of g-C₃N₄ into the system using ultrasonic treatment and rapid thermal annealing. The synthesized CuO/g-C₃N₄ not only retains the unique octahedral morphology of the CuBTC template, but also is coated by a uniform carbon layer. This CuO/g-C₃N₄ displays a high reversible specific capacity (1530.4 F g⁻¹, 2 A g⁻¹) as an anode material. In addition, an asymmetric supercapacitor assembled with self-made NiCoMOF as the cathode and CuO/g-C₃N₄ as the anode achieves an excellent energy density of 50.8 W h kg⁻¹ at a power density of 800 W kg⁻¹ and stable cycling performance (70.1% capacity retention over 3000 cycles). These outstanding electrochemical properties are attributed to the synergistic effect between transition metal oxides and carbon-based materials as well as the unique structure of the composites, which also provides a facile design and synthesis idea to construct high-performance transition metal compounds with a unique hollow structure and C-coating for developing asymmetric supercapacitors with high energy and power densities.

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1. Introduction

In recent years, with the development of wearable electronic devices and the innovation of energy technology, the demand for energy storage devices is more and more urgent.¹ Among them, supercapacitors have shown great development potential because of their fast charging and discharging speed, long cycle life and high power density.^2–6 However, in order to meet the increasing energy density requirements of the next generation of electronic devices, the energy density of ultracapacitors needs to be further improved. Compared with symmetric supercapacitors (SSCs), asymmetric supercapacitors (ASCs) exhibit excellent electrochemical performance due to their wider potential window which improves the energy density.^7–12 For the past few years, the development of cathode materials for hybrid capacitors has been rapid, while the development of anode materials is relatively slow. Carbon materials are widely used as anode materials in hybrid supercapacitors due to their high specific surface area, excellent electrical conductivity and high power density.^13,14 However, the low specific capacitance (CSP) properties of carbon materials severely limit the energy density of ASCs. Therefore, seeking high-performance negative materials is still a hot and difficult topic in the future work.

Owing to the high theoretical specific volume, being rich in resources, inexpensiveness, and eco-friendliness, transition metal oxides (TMO) are widely used as electrode materials for supercapacitors (SCs).¹⁵ In the anode materials, TMOs are getting more attention because of their high theoretical specific capacity, good chemical and thermal stability, easy synthesis, abundant resources and environment friendliness.^16–18 However, the volume expansion of TMOs is relatively serious during their repeated cycling, which leads to poor conductivity and cycling performance. Hence, it is feasible to effectively improve the electrochemical performance from two aspects of increasing the conductivity and inhibiting the volume expansion.^19,20

At present, the introduction of carbon materials is the most commonly used electrode material strategy. Various types of carbon materials, for instance, zero-dimensional carbon quantum dots and carbon microspheres, one-dimensional carbon nanotubes and carbon nanofibers, two-dimensional graphene and graphene oxide, three-dimensional graphene foam, etc., have been successfully combined with transition metal oxides to construct excellent performance electrode materials.^21–28 Among them, graphite carbon nitride (g-C₃N₄) has appealed to more and more researchers, on account of its low cost, good chemical stability and mechanical stability.²⁹ The lone pair electrons of the N atom in g-C₃N₄ can strengthen the polarity of the molecule, which is beneficial to increase the transport efficiency of the charge carriers.³⁰ In addition, as a soft polymer, it is easy for g-C₃N₄ to compound with other materials, and its unique lamellar structure is conducive to the horizontal transfer of charges.^31–33 However, it is most critical for the direct introduction of an external carbon source to form a stable and firm interfacial bond between the active material and the carbon matrix.

In addition to optimizing the composition of the electrode material, it is significant to regulate the morphology and structure of the materials during the design process for their electrochemical performance.^34–36 It is well known that the hollow structure can improve the performance of the materials, due to their excellent physical and chemical properties such as lower-density, higher specific surface area, good penetrability and so on, which results in more active sites and more rapid capacity for charge transfer during redox reactions.^37–43 Moreover, the hollow structure can also alleviate volume expansion during charge and discharge, forming a great interface area between the electrode and electrolyte, which is convenient for charge transfer. Therefore, it is a major challenge to design and develop a kind of hollow porous electrode material with a special chemical composition and morphology. It is well known that as a self-sacrificing template, the metal organic frameworks (MOFs) can be used to synthesize active electrode materials with special morphologies because of their adjustable structure, variable composition and porosity.^44–46 For example, Guan et al. obtained NiCoO_x materials with a multi-shell structure by rapid thermal oxidation using NiCoMOF as a precursor.⁴⁷ Xu et al. synthesized hollow spherical NiSe using NiBTC as a template.⁴⁸ Herein, we fabricated a hollow octahedral structured CuO/g-C₃N₄ composite with a uniform carbon coating by using the CuBTC as the precursor. By optimizing the heat treatment temperature and dosage of g-C₃N₄, the synthesized composite finally obtains unique advantages in structure and composition, and displays an excellent reversible specific capacity at a large current density (1530.4 F g⁻¹, 2 A g⁻¹) as an anode material. Subsequently, an ASC assembled with self-made NiCoMOF as the cathode and CuO/g-C₃N₄ as the anode achieves an excellent energy density of 50.8 W h kg⁻¹ at a power density of 800 W kg⁻¹ and stable cycling performance (70.1% capacity retention over 3000 cycles). Our strategy provides a facile design and synthesis idea to combine transition metal compounds with a special hollow structure and C-coating for exploitation of high-performance ASCs.

2. Experimental

2.1. Materials

p-Benzenedicarboxylic acid (PTA), Cu(NO₃)₂·6H₂O, 1,3,5-benzenetricarboxylic acid, Ni(NO₃)₂·6H2O, CoCl₂·6H₂O, melamine and ammonium bicarbonate were purchased from Aladdin Reagents Company (China). N,N-Dimethylformamide and ethanol were purchased from Liaoning Quanrui Reagent Co., Ltd. All reagents were of analytical grade and no further purification was required. The deionized water in this work was used.

2.2. Preparation of precursors of CuBTC

CuBTC was prepared by a simple hydrothermal method without further purification. CuBTC was first dissolved in 1.932 g (8 mmol) Cu(NO₃)₂·6H₂O in 17 mL deionized water. Subsequently, 0.841 g (4 mmol) 1, 3, 5-benzenetricarboxylic acid was dissolved in the mixture of 17 mL DMF and 17 mL ethanol and stirred evenly. Then, the Cu(NO₃)₂ clarifying solution was laxly transferred into the homopolitriformic acid at a rate of 3 mL min⁻¹, and continued to be stirred for 6 h. All the mixtures were poured into a 100 mL Teflon-lined stainless steel reactor and reacted at 100 °C for 4 h. After natural cooling to room temperature, they were washed with DMF and anhydrous ethanol 3 times respectively. Finally, they were centrifuged and dried for 12 h under 60 °C vacuum conditions to get CuBTC.

2.3. Preparation of CuO/g-C₃N₄ composites

Using synthesized CuBTC as the template, the hollow porous CuO/g-C₃N₄ composites were prepared by ultrasonic composite incorporation and rapid annealing treatment. Firstly, g-C₃N₄ was prepared according to the reported method. After 1 g melamine powder and 0.3 g cyanamide carbonate powder were evenly mixed, the material was calcined at 600 °C for 4 h under a N₂ atmosphere at a heating rate of 5 °C min⁻¹ to prepare g-C₃N₄. Then, the synthesized CuBTC and g-C₃N₄ were uniformly mixed at a mass ratio of 1 : 0.5 and treated with ultrasound for 30 min. Finally, the hollow porous CuO/g-C₃N₄ composite was obtained by rapid annealing of the mixed powder in air at 500 °C, and the annealing time is 10 min at the same heating rate. In contrast, all steps for the synthesis of pure CuO were similar except that g-C₃N₄ was not added. CuBTC was annealed directly to obtain pure CuO.

In order to obtain the best preparation method, the control variable method was used to adjust the experimental parameters. First of all, for the best conditions of CuO conversion, calcination time is determined to be 10 min, adjusting the calcination temperature range of 300–600 °C, labeled as CuO₃₀₀, CuO₄₀₀, CuO₅₀₀ and CuO₆₀₀. Through the electrochemical test, CuO₅₀₀ performance is the best. Then, in order to determine the best proportion of the compound, the mix proportion of g-C₃N₄ was adjusted at the best calcination temperature, which was 0.25, 0.5, 0.75, and 1.0, respectively. It is denoted as CuO/g-C₃N₄-0.25, CuO/g-C₃N₄-0.5, CuO/g-C₃N₄-0.75, and CuO/g-C₃N₄-1.0.

2.4. Materials characterization

The morphology and structure of the products were characterized by scanning electron microscopy (SEM; XL-30 FEG, FEI) and transmission electron microscopy (TEM; TECNAI F20, FEI). The as-prepared samples were tested by powder X-ray diffraction patterns, and observed with a Rigaku SmartLab Auto X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the range of 2–70. Thermogravimetric analysis (TGA) was adopted on an SDT Q600 (TA Instruments). The X-ray photoelectron spectroscopy (XPS) measurements were obtained with a Thermo Fisher Scientific EscaLab 250Xi XPS with monochromatic Al Kα radiation. The N2 adsorption–desorption isotherms were detected with a Micromeritics Tristar 3020 instrument in static mode, and specific surface area and pore structure were derived from the BET equation and Barrett–Joyner–Halenda (BJH) method.

2.5. Preparation and test of the electrode

The electrochemical properties of the as-synthesized samples were tested with a CHI 660E electrochemical workstation in 2 M KOH aqueous solution. All tests were performed in a traditional three-electrode system, consisting of a working electrode, a reference electrode (Hg/HgO electrode) and a counter electrode (platinum plate electrode). In detail, all of the working electrodes were made by the following steps: a certain amount of the active material, acetylene black and polytetrafluoroethylene (PTFE) emulsion were scattered in ethanol at a mass ratio of 8 : 1 : 1 to come into being a uniform black slurry, which was subsequently coated on nickel foam (1.0 cm × 1.0 cm) under 10 MPa pressure for 30 s. In the end, the nickel foam with active substances was perched at 60 °C for 12 h. According to the galvanostatic charge–discharge results, the specific capacitance of the anodes was calculated on the basis of the following formula:where C, I, Δt, m and ΔV are the specific capacitance (F g⁻¹), discharge current (A), discharge time (s), mass of the active material (g) and potential window (V), respectively.

2.6. Preparation of asymmetric supercapacitors

The first is the preparation of the cathode material. The NiCoMOF cathode material was obtained by the method reported in the literature.⁴⁹ The mass of 0.192 g Ni(NO₃)₂·6H₂O, 0.1178 g CoCl₂·6H₂O and 0.332 g (PTA) dispersed in 40 mL DMF, and the whole mixture for 1 h. Then, 4 mL 0.4 M sodium hydroxide is added drop by drop slowly to the above solution. The resulting mixture was converted into 100 mL Teflon-lined stainless steel autoclaves and maintained at a temperature of 100 °C for 8 h. After natural cooling, the precipitation was washed with DMF and anhydrous ethanol. Finally, the NiCoMOF cathode material was centrifuged and obtained by drying at 60 °C for 12 h.

Asymmetric supercapacitors were prepared using hollow porous CuO/g-C₃N₄ as the anode material, NiCoMOF as the cathode material, filter paper as the diaphragm and 6 M KOH as the electrolyte.

In addition, the energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) are two key indicators to assess the potential of supercapacitors in practical applications, which can be calculated by the following formulas:⁵⁰

where C (F g⁻¹), ΔV (V) and Δt (s) are the specific capacitance, the battery working potential and the discharge time of the assembled asymmetric supercapacitor, respectively.

3. Results and discussion

3.1. Structure and morphology characterization

The CuBTC template was obtained through the solvothermal method by copper nitrate and 1,3,5-benzenetricarboxylic acid. Fig. S1a (ESI†) shows the SEM image of CuBTC. It can be seen that solid CuBTC as a precursor is octahedral with a smooth surface and an average side length is approximately 6 μm. In addition, the diffraction peaks of the synthesized CuBTC are consistent with the simulated pattern (Fig. S1b, ESI†) Thermogravimetric analysis (TGA) curves of CuBTC obtained under an oxygen atmosphere are seen in Fig. S2 (ESI†). It is found that CuBTC is weightless at 300 °C, indicating that ligand pyrolysis occurs. When the temperature rises to 500 °C, the mass does not change, indicating that the ligand has been completely pyrolyzed.

Hollow porous CuO and CuO/g-C₃N₄ were prepared by annealing using CuBTC as a sacrificial template. XRD analysis of the two products was performed as shown in Fig. 1a. It is noticed that the diffraction peaks of the products calcined at 500 °C are all copper oxide peaks (PDF45-0973). At the same time, there are no other impurity peaks, indicating that CuBTC is completely converted to CuO with a hollow structure. After adding g-C₃N₄, the characteristic peaks of CuO are obviously wider than that before recombination, because of the interaction of two different components, crystalline and amorphous. In addition, the product presents two diffraction peaks at 27.8° and 32.1°, which correspond to the (110) and (200) crystal faces of g-C₃N₄, respectively, and indicate the successful incorporation of g-C₃N₄. The SEM analysis of the products obtained shows that the ligand thermolysis of CuBTC results in the formation of CuO with a rough surface and a hollow octahedral structure (Fig. 1b and c). The effects of different calcination temperatures on the samples are shown in Fig. S3a–c (ESI†). In the temperature range of 300–500 °C, the higher the temperature extends, the rougher the surface and the more obvious the porous structure becomes. When the temperature reaches 600 °C, the octahedral structure collapses due to the residual stress generated by the transitional pyrolysis of ligands, indicating that calcination temperature has an important influence on the pyrolysis degrees of ligands during the oxidation of CuBTC to CuO. The SEM diagram of the composite prepared at 500 °C shows that the roughness of the porous CuO surface is weakened due to the uniform coating of g-C₃N₄ on the surface (Fig. 1d). At the same time, the g-C₃N₄ coated on the surface of the composite material is relatively stable to maintain the original state under this temperature condition because its thermal decomposition begins above 550 °C.⁵¹ Subsequently, in order to further research the effect of the dosage of g-C₃N₄ on the morphology of the samples, the SEM measurements of CuO/g-C₃N₄ are characterized and the results are shown in Fig. S4 (ESI†). With the increase of g-C₃N₄ content, the agglomeration of the carbon layer on the surface of the product will be more serious. When the mass ratio of CuBTC to g-C₃N₄ is 1 : 1, it is difficult to observe the octahedral structure.

Fig. 1 (a) XRD patterns of CuO/g-C₃N₄-0.5, (b and c) the SEM images of CuO₅₀₀, and (d) the SEM images of CuO/g-C₃N₄-0.5.

The structure of the CuO/g-C₃N₄-0.5 composite with a hollow octahedral structure is further detected by TEM, and the typical TEM images are displayed in Fig. 2a. It is clearly seen that the inner side and edge of the sample exhibit significant contrast between light and shade, which well proves the hollow octahedral structure of the CuO/g-C₃N₄-0.5 composite. The average thickness of the shell is 100 nm, which corresponds to the results of SEM. Fig. 2b is a local magnified view of the composite material, indicating that g-C₃N₄ successfully grows on the surface of CuO. Fig. 2c clearly shows the lattice fringe of the CuO/g-C₃N₄ composite material, and the measured lattice spacing is 0.24 nm, pointing to the (111) crystal plane of CuO. Furthermore, Fig. 2d–g are the EDS mapping analysis results of elements corresponding to the CuO/g-C₃N₄ composite. The four elements Cu, O, N and C are uniformly distributed in a single octahedral surface, further demonstrating the successful synthesis of the CuO/g-C₃N₄ composite.

Fig. 2 (a)–(c) HAADF-STEM image of CuO/g-C₃N₄-0.5, and (d)–(g) elemental mapping of CuO/g-C₃N₄-0.5 for Cu, O, N, and C.

The chemical composition of the samples before and after synthesis was investigated by X-ray photoelectron spectroscopy (XPS). The high-resolution Cu 2p spectra of CuO and CuO/g-C₃N₄ are shown in Fig. 3a. The image is composed of two spin–orbit double peaks and two satellite peaks. The fitting energy bands of characteristic peaks of Cu 2p_3/2 and Cu 2p_1/2 for CuO are 933.6 and 953.7 eV. Furthermore, the satellite peaks at 941.7, 944.1 and 962.1 eV are attributed to three shake-up peaks of Cu at high binding energy sides of Cu 2p_3/2 and Cu 2p_1/2 edges, respectively. It is noteworthy that the binding energy of the Cu 2p_3/2 characteristic peak of CuO/g-C₃N₄ is slightly shifted, indicating a strong interaction between CuO and g-C₃N₄.⁵²Fig. 3b shows the O 1s spectra of two samples. The O 1s spectra present two characteristic peaks at the binding energies of 529.9 and 531.8 eV, confirming the presence of the Cu–O bond and C=O bond. The C 1s spectrum and the core level C 1s spectrum of CuO/g-C₃N₄ could be divided into two bonds. Among them, one peak at 284.6 eV corresponds to the sp² C–C bond originating from the adventitious carbon in species of sample, while the other at 285.1 eV belongs to the C=C bond of the benzene ring in the ligand (Fig. 3c). For CuO/g-C₃N₄, the C=N bond at 288.1 eV may be attributed to g-C₃N₄. The peak strength corresponding to the C–C bond and C=O bond is stronger than before the compound g-C₃N₄, and the peak of C=C is slightly shifted, indicating a high interaction between CuO and g-C₃N₄.⁵³ Two characteristic peaks at 398.8 and 400.8 eV for CuO/g-C₃N₄ correspond to C–N=C and C–N–H, respectively, while the N 1s spectrum for pure CuO has no obvious characteristic peak (Fig. 3d).

Fig. 3 XPS spectra of three prepared samples: (a) Cu 2p, (b) O 1s, (c) C 1s and (d) N 1s.

The measurement of specific surface area and mesoporous characteristics of electrode materials is of great significance to evaluate their electrochemical properties. Fig. 4a describes the nitrogen adsorption/desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH, Fig. 4a inset) result of the CuO/g-C₃N₄-0.5 sample. The hollow porous octahedral CuO/g-C₃N₄-0.5 composite with a cladding g-C₃N₄ shell has a specific surface area of up to 61.3211 m² g⁻¹. The calculated BET surface area of the CuBTC, CuO₅₀₀ and g-C₃N₄ (Fig. 4b) is only 8.2442, 45.8526 and 30.5337 m² g⁻¹, respectively, indicating that the composite of the two different materials can significantly enhance the specific surface area of the material. The larger specific surface area and mesoporous structure can afford more active sites to promote the electron transfer between the electrolyte and the electrode, and also improve the charge storage efficiency. In addition, the adsorption isotherm of the CuO/g-C₃N₄-0.5 composite belongs to the typical IV-H3 hysteresis ring, indicating that the material is a mesoporous material. BJH results show that the composite exhibits a layered hollow structure with the range of pore sizes from 3 nm to 30 nm.⁵⁴ These layered pores make the contact area between the active material and the electrolyte large and sufficient, effectively reducing the self-aggregation of the hybrid structure in the course of electrochemical charge–discharge.

Fig. 4 (a) N₂ adsorption/desorption isotherms and pore size distribution of CuO/g-C₃N₄-0.5; (b) CuBTC, CuO₅₀₀, g-C₃N₄ and CuO/g-C₃N₄-0.5.

The feasible mechanism of CuO/g-C₃N₄ formation as a result of a hollow porous structure is as follows (Scheme 1). First, the CuBTC template with an octahedral structure was synthesized by a facile hydrothermal means. The uniform distribution of Cu²⁺ in the synthesized octahedral precursors provided a metal source for the further formation of CuO. The CuBTC template was then heat-treated in air, resulting in evenly arranged CuO and hollow structures on the surface of CuBTC templates due to heterogeneous shrinkage and ligand pyrolysis during heating treatment. The introduced g-C₃N₄ lamellar structure was heated to the surface of the octahedral structure. The coating uniformity can be effectively regulated through adjusting the amount of g-C₃N₄ and the temperature of heat treatment. The excellent electrochemical performance of composite materials can be attributed to the synergistic effect of CuO and g-C₃N₄, as follows: firstly, the vertical charge transfer path of g-C₃N₄ improves the conductivity of the CuO/g-C₃N₄ composite, which is conducive to rapid electron diffusion during charge and discharge processes. Secondly, the CuO octahedral structure can play a supporting role, prevent excessive aggregation of materials, and facilitate contact with the electrolyte. Finally, due to the hollow structure of the CuO/g-C₃N₄ composite, more active sites are exposed and electrochemical reactions are promoted. In conclusion, the CuO/g-C₃N₄ composite is considered to be an ideal combination of electrodes for supercapacitors.

Scheme 1 Schematic illustration of the fabrication process of the CuO/g-C₃N₄ composite with a hollow octahedral structure.

3.2. Electrochemical properties of electrode materials

For investigation of the electrochemical process and kinetic properties, cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were further conducted through a three-electrode system. Fig. S5a (ESI†) shows the GCD curves of CuO at different calcination temperatures at 2 A g⁻¹, showing that the proper calcination temperature can make CuO have a hollow octahedral structure to improve the electrochemical performances. Fig. S5b (ESI†) displays the CV comparison diagram of CuBTC, CuO₅₀₀ and CuO/g-C₃N₄-0.5 at a scanning speed of 10 mV s⁻¹. Both CuO and CuO/g-C₃N₄ have obvious redox peaks, confirming that reversible redox reactions have occurred on the electrode surface, proving that the materials have obvious pseudocapacitance properties. In addition, by calculating the specific capacitance of the integrated area of CV curves, the specific capacitance of the composite material is significantly increased, because the formation of a g-C₃N₄ coated-shell layer is conducive to ion/electron transport and provides more electrochemical active sites. For the detailed description of the effects of g-C₃N₄ dosage on the electrochemical properties, GCD tests were carried out on CuO/g-C₃N₄-0.25, CuO/g-C₃N₄-0.5, CuO/g-C₃N₄-0.75, and CuO/g-C₃N₄-1.0 at 2 A g⁻¹ (Fig. S5c, ESI†). The specific capacitance of CuO/g-C₃N₄-0.5 is 1530.4 F g⁻¹ at a charge–discharge current density of 2 A g⁻¹, which is more than those of CuO/g-C₃N₄-0.25 (1211.7 F g⁻¹), CuO/g-C₃N₄-0.75 (934.2 F g⁻¹) and CuO/g-C₃N₄-1.0 (769.7 F g⁻¹). This is due to the shell structure of CuO/g-C₃N₄-0.5 products being more uniform and stable than other ratios.

The CV curves of CuO/g-C₃N₄-0.5 at disparate scanning rates are revealed in Fig. 5a. With the increase of scanning speed, the redox peaks lag to both sides, which may be attributed to internal resistance and limitation of the charge transfer. The shape similarity of CV curves indicates remarkable stability of the sample. Anode peaks (A1 and A2) at −0.27 V and 0.19 V correspond to the oxidation of Cu to Cu(I) and Cu(I) to Cu(II), respectively. Similarly, cathode peaks (C1) at −0.90 V are derived from the reduction of Cu(II) to Cu. The possible reactions are as follows:^55,56

Fig. 5 (a) CV curves of CuO/g-C₃N₄-0.5 at scan rates of 10–50 mV s⁻¹; (b) GCD curves of CuO/g-C₃N₄-0.5 at different current densities; (c) the linear relation between the anodic/cathodic peak currents and the scan rates; (d) specific capacitance versus various current densities for CuO/g-C₃N₄-0.5 and CuO₅₀₀; (e) Nyquist plots of CuO/g-C₃N₄-0.5, and the inset shows the equivalent circuit; (f) cycling stability performance of CuO/g-C₃N₄-0.5 at a current density of 10 A g⁻¹.

A1: 2Cu + 2OH⁻ − 2e⁻ ↔ Cu₂O + H₂O

A2: Cu₂O + 2OH⁻ + H₂O − 2e⁻ ↔ 2Cu(OH)₂

C1: Cu + 2OH⁻ − 2e⁻ ↔ Cu(OH)₂

In general, the capacitance effect is determined by analyzing the volt-ampere response at different scanning speeds. The formula is as follows:

i_p = av^b (4)

where I_p represents the current density, v represents the scanning surface rate, a and b are constants, b is 1 corresponding to the nondiffusive-controlled surface redox reaction (capacitance effect), and b = 1/2 corresponds to the diffusive-controlled surface redox process (battery effect). As shown in Fig. 5c, after linear fitting of peak current and scanning speed, b values corresponding to three peaks are approximately equal to 0.5, which proves to be a typical diffusion control process between the electrode material and electrolyte, and CuO/g-C₃N₄-0.5 is a representative electrode material.⁵⁷Fig. 5b shows the GCD curve of CuO/g-C₃N₄-0.5 at 2–10 A g⁻¹. The obvious nonlinear curve further proves the pseudo-capacity characteristic. The specific capacitance calculated based on the charge–discharge curves is shown in Fig. 5d. The specific capacitance is 1530.4 F g⁻¹ at 2 A g⁻¹. Even though the current density raises to 10 A g⁻¹, the capacitance retention rate is about 69% (1054.5 F g⁻¹). However, the capacitance retention of CuO₅₀₀ without g-C₃N₄ is only 57%, which further demonstrates that the introduction of g-C₃N₄ can strengthen the stability of the composites.

For the sake of further investigation of the electrochemical characterization, electrochemical impedance (EIS) was adopted in the frequency range from 100 k to 0.01 Hz. As known in Fig. 5e, the Nyquist plot of CuO/g-C₃N₄-0.5 consists of semicircles in the high frequency region and linear components in the low frequency region. The real axis intercept is the equivalent series resistance (R_s) with a value of 0.68 Ω, which is composed of the ionic resistance of the electrolyte, the inherent resistance of the active material, and the contact resistance between the active material and the collector. The diameter of the semicircle indicates that the internal resistance of charge transfer (R_ct) of the system is 1.47 Ω. The smaller the R_ct value is, the faster the rate of charge transfer is, because of the formation of the g-C₃N₄ coated-shell to accelerate the charge transport capacity in the vertical direction. In the low frequency region, the slope of the line is Warburg impedance (Z_W), which is due to the diffusion along the octahedron surface at the electrolytic solution. Low R_s, R_ct and Z_W indicate that the hollow porous CuO/g-C₃N₄ has a larger active specific surface area and higher conductivity, which can extend the electrode reaction zone.⁵⁸ It makes the electrode material come into contact with the electrolyte more sufficiently during charge–discharge processes, thus improving its specific capacitance. Furthermore, CuO/g-C₃N₄ as an anode material has good cycling ability (Fig. 5f). After 3000 cycles the specific capacitance of the CuO/g-C₃N₄-0.5 electrode material can still retain 66.3% of the initial value at 10 A g⁻¹, which fully proves that the CuO/g-C₃N₄ electrode material has certain application potential. After the cycling test, the material was analyzed by SEM to explore the material morphology before and after the reaction (Fig. S7, ESI†). After the cycling test, the CuO/g-C₃N₄ material can still maintain the octahedral structure, and part of g-C₃N₄ on the surface is off, which may be the reason for the decrease of its specific capacitance.

3.3. Electrochemical performances of ASCs assembled by NiCoMOF//CuO/g-C₃N₄

The ASC was prepared through NiCoMOF as the positive electrode (Fig. 6a), and its electrochemical properties were investigated by CV and GCD tests in a three-electrode system (Fig. S6a and b, ESI†). In the potential window of 0–0.5 V, the CV curves of NiCoMOF show typical pseudocapacitive characteristics caused by the pseudocapacitive materials. According to the GCD curve, the specific capacitance of NiCoMOF is 828.50 F g⁻¹ at 0.5 A g⁻¹. On the base of the charge–discharge balance principle, the mass ratio of positive and negative materials can be determined to be 6.53 : 1. To determine its operating voltage range, Fig. S8 (ESI†) shows the CV diagrams of the NiCoMOF//CuO/g-C₃N₄ ASC in different voltage ranges (0–1.5 V, 0–1.6 V, 0–1.65 V, 0–1.7 V, and 0–1.8 V). Since there is significant polarization at 0–1.7 V and no polarization at 0–1.6 V, the working voltage is located at 0–1.6 V. Fig. 6b shows the CV curves of the NiCoMOF//CuO/g-C₃N₄ ASC device in the potential range of 0–1.6 V. At all of the scanning rates, the redox peaks of pseudocapacitance can be obviously observed, demonstrating the excellent stability. Subsequently, the basic symmetry of charge and discharge curves illustrates that ACS devices have good reversibility of charge storage (Fig. 6c). The specific capacitance calculated by the GCD curve is described in Fig. 6d. The specific capacitance of the NiCoMoF//CuO/g-C₃N₄ ASC device is 181.3 F g⁻¹ at 0.5 A g⁻¹. The specific capacitance is 91.3 F g⁻¹ and the capacitance retention rate is 50.3% even at 10 A g⁻¹. Therefore, the device has good rate performance. And the capacitance can still retain 70.1% of the initial value after 3000 cycles at 5 A g⁻¹ (Fig. 6e). The fast capacitance decay in the first 500 cycles is observed in both samples, followed by a very stable state. The relatively rapid capacitance deterioration is probably attributed to the structure collapse, the reduction of active surface area, the phase transformation and an increase of the resistance during charge–discharge processes in alkali environments.⁵⁹ The Ragone diagram of the NiCoMOF//CuO/g-C₃N₄ ASC device obtained according to the discharge curve is found in Fig. 6f. As the power density is 800 W kg⁻¹, the energy density of the NiCoMOF//CuO/g-C₃N₄ ASC device is 50.8 W h kg⁻¹, and the device maintains an energy density of 32.4 W h kg⁻¹ even at 8000 W kg⁻¹. All these clearly demonstrate the excellent performance of NiCoMOF//CuO/g-C₃N₄ ASC devices. For a better comparison, the electrochemical properties of many other hybrid ultracapacitors are also shown in the Ragone diagram.^60–65 In order to verify its actual electrochemical performance, two prepared NiCoMOF//CuO/g-C₃N₄ ASC devices were connected in series to power 2.0–2.2 V red, green and yellow LEDs, respectively. The results show that the LEDs can be lit for up to 30 min.

Fig. 6 (a) Schematic illustration of the NiCoMOF//CuO/g-C₃N₄ ASC; (b) CV curves; (c) GCD curves of the NiCoMOF//CuO/g-C₃N₄ ASC at different current densities of 0.5–10 A g⁻¹; (d) specific capacitance versus various current densities for the NiCoMOF//CuO/g-C₃N₄ ASC; (e) cycling performance of the NiCoMOF//CuO/g-C₃N₄ ASC at a current density of 10 A g⁻¹; (f) Ragone plot related to the energy and power densities of the NiCoMOF//CuO/g-C₃N₄ ASC and the comparisons with other research studies; the inset shows the photos of the LEDs lit up by the as-fabricated NiCoMOF//CuO/g-C₃N₄ ASC.

4. Conclusions

In summary, based on the unique precursor CuBTC, two-component CuO/g-C₃N₄ composites were designed through an ultrasonic treatment and rapid thermal annealing. By optimizing the temperature and the content of g-C₃N₄ in thermal annealing reaction, a hollow CuO with a uniform carbon coating displayed high electrochemical performance as an anode material in a three-electrode system and ASC, respectively. This work provides a feasible thought to effectively control the electronic structure and morphology of transition metal compounds with carbon materials to acquire excellent performance and expand a great application on the base of the structural integrity and compositional advantages.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51972049 and 52073010), and the Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ma00766a

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