Co-doped vanadium nitride/carbon composite fabricated from cobalt–vanadium metal–organic framework precursors for supercapacitors

Abstract

Vanadium nitride (VN) is a good candidate electrode material owing to its high theoretical specific capacitance and wide operational voltage window. However, its poor cycle life and dissolution in alkaline electrolytes hinder its practical application. To address these challenges, this study proposes melamine-assisted pyrolysis with cobalt-doped vanadium-based metal–organic framework (MOF) precursors to prepare a carbon/cobalt/vanadium nitride (C/Co/VN) composite. Electrochemical evaluation demonstrated that the optimized composite achieved a remarkable specific capacitance of 242.2 F g⁻¹ at 0.5 A g⁻¹. Furthermore, the assembled asymmetric supercapacitor device with the structure Ni(OH)₂//C/Co_x/VN could reach a high energy density of 19.08 Wh kg⁻¹ with a corresponding power density of 377.72 W kg⁻¹. Remarkably, it retained 97% of its initial capacitance after 20 000 cycles at 4 A g⁻¹.

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1. Introduction

New energy-storage devices have attracted increasing attention due to the excessive consumption of non-renewable energy for supercapacitors, lithium-ion batteries, and zinc-ion batteries. Among these, supercapacitors (SCs) are regarded as particularly important storage devices owing to their outstanding high power density, fast charge and discharge capability, and ultralong cycle stability.^1–3 Their excellent electrochemical performances are determined by the electrode material utilized, which include carbon materials,⁴ transition metal oxides,⁵ hydroxide,⁶ nitrides,^7–9 sulfides,¹⁰ and conductive polymers.¹¹ Vanadium nitride (VN), as one of transition metal nitrides (TMNs) based on pseudocapacitors’ energy-storage mechanism, has been widely studied because of its extremely high theoretical specific capacitance up to approximately 1340 F g⁻¹, broad potential window extending from −1.2 to 0 V, and remarkable conductivity of approximately 1.67 × 10⁶ S m⁻¹.^12,13 However, its dissolution in alkaline electrolytes has been proven to cause rapid capacitance decay, hindering its further development.

In recent years, researchers have proposed applying a carbon layer coating (VN@C)^14–16 or integrating VN into carbon materials (such as graphene or carbon nanotubes)^17–19 to suppress the dissolution of vanadium or agglomeration of VN nanoparticles, thereby improving electrical conductivity. For instance, Zhang et al. proposed growing carbon-coated VN nanowires on carbon nanotubes, which achieved an outstanding surface capacitance of 715 mF cm⁻² at 1.0 mA cm⁻², while the specific capacitance of a device assembled with these VN nanowires increased from 74 to 133.8 mF cm⁻².¹⁶ Wu et al. fabricated VN quantum dots (VNQDs) embedded into a carbon nanofiber framework (VNQD/CNF), which exhibited an increase in specific capacitance of 406.5 F g⁻¹ at 0.5 A g⁻¹ compared with those of pure CNF (86.7 F g⁻¹) and VN/C (106.2 F g⁻¹).¹⁹ Liu et al. prepared VN nanoparticles distributed in N-doped graphene (VN/NGr), which achieved a superior specific capacitance of 255 F g⁻¹ at 10 mV s⁻¹, which was higher than those of pristine NGr (155 F g⁻¹) and bare VN (150 F g⁻¹).²⁰ These typical studies have proven that good electrochemical performance can be achieved through suppression of oxide layer dissolution and improved electrical conductivity; however, the synthesis of vanadium nitrides still requires the use of ammonia gas, which poses environmental and safety issues that need to be resolved.

Metal–organic frameworks (MOFs), which are coordinately assembled from inorganic metal nodes and multifunctional organic ligands, have become the focus of advanced materials research because of their substantially high specific surface area, which endows them with superior molecular adsorption capabilities and enhanced catalytic activity, making them prime candidates for next-generation nanoporous material applications, architecturally tunable pore systems, and programmable structural topologies at the molecular scale.^21,22 These features render MOFs as optimal sacrificial templates for the synthesis of nanomaterials featuring large surface areas and customized structures.²³ Thus, it is also possible to use vanadium-based MOFs (V-MOFs) as a precursor to prepare VN materials. For instance, Liu et al. applied a hydrothermal process to produce a V-MOF precursor, which they subsequently sintered in a mixed nitrogen and ammonia atmosphere to synthesize vanadium nitride (VN) with a surface containing VO₂. This material exhibited a specific capacitance of 149 F g⁻¹ within a broad potential window of 1.2 V with 85% capacitance retention.²⁴ In addition, metal-doped V-MOF or double-metal-doped MOFs can also be prepared using other energy devices. Compared to MOFs with single metals, bimetallic MOFs demonstrate significantly enhanced electrochemical capacitance, attributable to their increased number of redox-active sites and improved electronic conductivity. This performance enhancement can be achieved through the strategic incorporation of a secondary transition metal with multivalent states to partially substitute the host metal centers, making the construction of bimetallic MOFs a promising approach for electrochemical applications.²⁵ Cobalt (Co), a transition metal, has an outer electron configuration of 3d⁸4s¹, which shares a similar coordination environment with vanadium (V). This enables the conjugation of Co to V metal sites, resulting in the formation of a redox-active center. Consequently, the electrochemical performance can be significantly enhanced.²⁶

In general, from a synthesis method point of view, most metal nitrides preparation methods require the existence of ammonia gas at high temperature. This raises safety and environmental issues, and so a new method is desired that can avoid the need for ammonia gas. In this regard, Qi et al. proposed an innovative dual-precursor strategy through the rational coordination of melamine (as a nitrogen source) and polyvinylpyrrolidone (a carbon precursor), and achieved the controllable synthesis of three-dimensional (3D) hierarchical VN microsheets uniformly encapsulated within nitrogen-doped carbon matrices (VN@NC), which demonstrated exceptional electrochemical performance in flexible solid-state asymmetric supercapacitors (ASCs). The conformal NC coating served dual functions: effectively preventing VN dissolution during electrochemical cycling, and maintaining the structural integrity of the nanosheets. The optimized VN@NC-based ASC device delivered exceptional performance metrics, achieving an impressive energy density of 65.3 Wh kg⁻¹ at 800 W kg⁻¹ power density, coupled with exceptional cycling durability, with 92% capacity retention over 4000 charge–discharge cycles, and remarkable mechanical flexibility (0–180° bending range) without performance degradation.²⁷ Joo et al. utilized a one-pot chemical method with melamine as a nitrogen source combined with precursor materials to in situ synthesize nanostructures of VN deposited on carbon nanobelts (VN@C), and reported this could achieve exceptional supercapacitance. After 2000 cycles at a current density of 30 A g⁻¹, the VN@C electrode retained 98% of its original capacity. Furthermore, when fabricated into a VN@C//AC device, it showed a significant specific capacitance of 102 F g⁻¹. Additionally, the device achieved an outstanding energy density of 30 Wh kg⁻¹ and power density of 5608 W kg⁻¹, respectively.²⁸

Based on the pervious literature, this work aimed to prepare a cobalt-doped VN composite with carbon material using Co-doped V-MOF as the vanadium precursor, with melamine as a nitrogen and carbon source through pyrolysis under a nitrogen atmosphere. The V : Co ratios were fine-tuned to study their effects on the electrochemical performance, achieving a high specific capacitance of 242.2 F g⁻¹. Moreover, the asymmetric supercapacitor configuration, employing C/Co_0.025/VN as the negative electrode and Ni(OH)₂ as the positive electrode, reached a maximum energy density of 19.08 Wh kg⁻¹ at a power density of 377.72 W kg⁻¹. It also demonstrated an excellent capacitance retention of 97% after 20 000 cycles. This research puts forward a straightforward and viable strategy to effectively mitigate the dissolution of vanadium nitride, presenting broad application prospects in advanced energy-storage systems.

2. Experimental

2.1. Synthesis of carbon/cobalt/vanadium nitride (C/Co_x/VN) composite

2.1.1. Synthesis of the cobalt-doped V-MOF precursor (Co_x–V-MOF). The V-MOF was prepared based on the previous literature²⁹ with some modifications. First, 6 mmol of vanadium oxysulfate (VOSO₄) and an equal molar ratio of p-phthalic acid (PTA, 6 mmol) were dissolved in 60 mL N,N-dimethylformamide (DMF) with stirring until the solution was clear and became deep blue. The homogeneous precursor solution was then transferred to a 100 mL autoclave. Subsequently, it underwent a programmed hydrothermal synthesis process at a temperature of 160 °C for a duration of 48 h. Then, it was gradually cooled to room temperature and the resulting dark yellow precipitate was collected via high-speed centrifugation (8000 rpm, 5 min) followed by washing five times with absolute ethanol until a clear supernatant was obtained. After that, the product was dried at 60 °C for 24 h to obtain the resulting V-MOF.

Co_x–V-MOF was obtained by adding different ratios of cobalt sulfate heptahydrate (CoSO₄·7H₂O) to vanadium oxysulfate (X = 0.005, 0.01, 0.025, 0.05, 0.1) in the hydrothermal process.

2.1.2. Synthesis of carbon/cobalt/vanadium nitride (C/Co_x/VN) composite. The as-synthesized precursor (0.1 g) was precisely blended with melamine in a 1 : 10 mass ratio and mechanically milled, followed by pyrolysis at 700 °C for 2 h under a continuous nitrogen flow. The samples were obtained as carbon/cobalt/vanadium nitride (C/Co_x/VN) composites.

For comparison, pristine V-MOF was pyrolyzed without melamine to prepare a carbon material, marked as C_V-MOF. In addition, V-MOF mixed with melamine was pyrolyzed to prepare carbon/vanadium nitride, marked as C/VN.

The same method mentioned above was used to prepare C/Co_0.025/VN composites at different temperatures (600, 700, 800 and 900 °C), labeled as C/Co_0.025/VN-Y (Y = 600, 700, 800, 900).

2.1.3. Synthesis of Ni(OH)₂. Ni(OH)₂ was prepared based on the previous literature.³⁰ First, 1.46 g Ni(NO₃)₂·6H₂O and 0.25 g sodium dodecyl sulfate (SDS) were dissolved in a mixed solution of 25 mL deionized water and 25 mL ethanol, respectively. Subsequently, 3.0 g urea was added with continuous stirring. Then the solutions were transferred into100 mL Teflon-lined stainless autoclaves and held at 110 °C for 15 h. After cooling down to room temperature naturally, the precipitates were collected by centrifugation and washed several times with deionized water and absolute ethanol. Finally, the products were obtained after drying at 60 °C.

2.2. Characterizations

The microstructural characteristics of the electrode materials were systematically investigated using scanning electron microscopy (SEM, SEM5000) and high-resolution transmission electron microscopy (TEM, JEM-F200). Crystalline phase identification was performed by X-ray diffraction (XRD, Smart Lab, Cu Kα). The elemental distribution and morphology were analyzed through energy-dispersive X-ray spectroscopy (EDS) mapping. The surface chemical composition and oxidation states were determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific™ ESCALAB™ Xi+).

2.3. Electrode preparation

The active material (80 wt%), acetylene black (7.5 wt%), and conductive graphite (7.5 wt%) were weighed and mixed. Then, one drop of polytetrafluoroethylene solution (PTFE, 5 wt%) was added to form a slurry. The prepared slurry was then coated on nickel foam as a current collector with a geometric area of 1 cm². Then, the electrode was dried at 60 °C for 12 h to eliminate any residual solvent and was then pressed under 10 MPa for 15 s.³¹ The final active material mass loading was 4 mg cm⁻².

2.4. Electrochemical measurements

A three-electrode cell configuration was utilized for the electrochemical characterizations. Specifically, a saturated calomel electrode (SCE) served as the reference electrode, platinum foil acted as the counter electrode, and the fabricated composite electrode functioned as the working electrode. All the experiments were conducted in 6 M KOH aqueous electrolyte at 25 °C using a CHI660E electrochemical workstation. The cyclic voltammetry (CV) measurements were performed at scan rates ranging from 5 to 50 mV s⁻¹, while the galvanostatic charge–discharge (GCD) tests were carried out at current densities between 0.5 and 5 A g⁻¹. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 0.01 to 10⁵ Hz using a Land CT3001A battery tester to assess the long-term stability of the electrodes.

The specific capacitance C_s (F g⁻¹) was calculated according to GCD curves using eqn (1):³²

C_s = IΔt/mΔV, (1)

where I denotes the discharge current (A), Δt represents the discharge duration (s), m corresponds to the mass of active material (mg), and ΔV indicates the operational potential window (V).

2.5. Preparation of an asymmetric supercapacitor (ASC)

The ASC device was assembled using C/Co_0.025/VN as the negative electrode, Ni(OH)₂ as the positive electrode in 6 M KOH as the electrolyte. Electrochemical characterizations were carried out in a two-electrode system within an optimized electrochemical stability window from 0 to 1.5 V. The mass of the active material for each electrode was calculated using the following formula:

m₋/m₊ = C_s+ × ΔV₊/C_s− × ΔV₋, (2)

where m is the mass of the electrode, C_s is the specific capacitance, ΔV is the voltage window, and “+” and “−” are the positive and negative electrode, respectively.

The specific capacitance C_s (F g⁻¹) was calculated by GCD curves using eqn (1) or by CV curves using eqn (3):³³

C_s = A/(2 × ΔV × m × k), (3)

where C_s is the specific capacitance, A is the integrated area of the CV curve, ΔV is the voltage window, m is the sum of the mass of the positive and negative loads, and k refers to the different scan rates, respectively.

The energy density (E) and power density (P) were calculated by applying the following fundamental formulas:

E = 0.5C_sΔV²/3.6 (4)

P = E × 3600/Δt (5)

3. Results and discussion

Fig. 1 displays a schematic representation of the synthesis of the C/Co_x/VN composites. V-MOF was synthesized using vanadium oxysulfate as the metal ion source and p-phthalic acid as the organic ligand. During this process, cobalt ions were introduced to partially substitute vanadium ions, forming Co_x–V-MOF. Subsequently, the Co_x–V-MOF was thermally treated with melamine at high temperature. The decomposition of melamine generated ammonia, which reacted with vanadium atoms derived from the MOF framework to form VN. Simultaneously, carbon matrices were formed through the thermal decomposition of both the melamine and the organic framework. Cobalt species were also released during the framework decomposition. Consequently, the final product comprised carbon-supported vanadium nitride with embedded cobalt nanoparticles, designated as C/Co_x/VN.

Fig. 1 Schematic of the synthesis of the C/Co_x/VN composites.

Fig. 2a and Fig. S1† present the SEM images of the V-MOF, which exhibited a cluster structure composed of okra-like short rods with a smooth surface.²⁹ After carbonization, rod-like V-MOF was decomposed and destroyed and showed a rough surface, caused by gas production from C_V-MOF, as shown in Fig. 2b and Fig. S2.† When heating mixtures of the cobalt–vanadium organic framework and melamine, the morphologies of the samples at different mass loadings of cobalt (Fig. 2c–g) showed obvious variations, revealing the presence of broken rods with some sheet-like morphologies. Further, TEM analysis of the C/Co_0.025/VN composite (Fig. 2h) revealed an amorphous carbon matrix, while the elemental mapping demonstrated a homogeneous spatial distribution of C, N, V, and Co species. The plane spacing of 0.235 nm well agreed with the (111) crystal plane of VN,³⁴ as shown in Fig. 2i, while limited cobalt species was detected due to its lower dopant concentration.

Fig. 2 SEM images of (a) V-MOF, (b) C_V-MOF, (c) C/Co_0.005/VN, (d) C/Co_0.01/VN, (e) C/Co_0.025/VN, (f) C/Co_0.05/VN, and (g) C/Co_0.1/VN; (h) TEM, element mapping and (i) HRTEM images of C/Co_0.025/VN.

Fig. 3 presents the XRD patterns of the C/VN and C/Co_x/VN composites. A broad diffraction peak could be observed centered at around 25°, which was characteristic of the (002) graphitic plane in amorphous carbon. Three distinct diffraction peaks could be observed at 37.7°, 43.8°, and 63.7°, well corresponding to the (111), (200), and (220) crystal planes of cubic vanadium nitride (VN, JCPDF 35-0768).³⁵

Fig. 3 XRD pattern of C/Co_x/VN and C/VN at a temperature of 700 °C.

Fig. 4 shows the high-resolution spectra of V 2p, N 1s and Co 2p, respectively. As the Co loading content increased, the fitting peaks of V 2p turned from two pairs of peaks to three pairs of peaks (Fig. 4a). The pair of peaks at ∼515 and ∼523 eV corresponded to V–N–O, while the pair of peaks at ∼517 and 524.54 eV corresponded to V–O, respectively, for C/Co_0.005/VN. Another pair of peaks at ∼516 and ∼523 eV corresponded to V–N–O. Meanwhile, the pair of peaks at ∼517 and 523.34 eV corresponded to V–O, respectively, for C/Co_0.01/VN, while another pair of peaks at ∼513.25 and ∼521.42 eV corresponded to V–N for C/Co_0.025/VN, which were not fitted and appeared due to the lower Co contents in C/Co_0.005/VN and C/Co_0.01/VN. In fact, the XRD peak of VN also was not clear, which agreed with this result. The peaks around 514 and 521, 516 and 523, and 517 and 524 eV belonged to V–N, V–N–O and V–O, respectively.³⁶ The presence of V–N–O and V–O indicated the partial oxidation of the VN surface. The high-resolution spectra of N 1s are shown in Fig. 4b, for which the peaks at around 398, 398.9, 400, and 401 eV could be assigned to N–V bonds, pyridine N, pyrrole N and graphite N, respectively, which confirm the VN and N-doped in the carbon material.¹⁵Fig. 4c shows the Co 2p XPS spectrum. There was no obvious Co peak in C/Co_0.005/VN due to its lower Co content. The fitted peaks at ∼778 and ∼795.63 eV were assigned to Co⁰. The characteristic peaks at 783, 797.81 and 780.7, 795.79 eV corresponded to Co²⁺ and Co³⁺, respectively. As the Co mass increased, the peaks shifted a little. The introduction of Co could regulate the surface properties of C/Co_x/VN. The strategic electronic coupling between the metallic cobalt (Co) nanoparticles and vanadium nitride (VN) components precisely modulated the adsorption energy of the critical oxygen-containing intermediates, as evidenced by theoretical calculations.³⁷ Furthermore, the engineered Co/VN heterostructure interfaces within the C/Co_x/VN framework synergistically optimized mass transport of the reactive species through the hierarchically porous architecture. This multifunctional integration collectively elevated the overall electrochemical performance, particularly in oxygen-involving redox reactions. The C 1s spectrum (Fig. S3a†) displays several fitted peaks, as per the following: 284.67 eV (C–C/C=C), 285.41 eV (C=N), 286.14 eV (C–O), 287.38 eV (O–C=O), and 289 eV (C=O). Interestingly, C–O gradually disappeared as the amount of cobalt doping increased. The high-resolution O 1s spectrum (Fig. S3b†) could be deconvoluted into multiple components, as per the following: 530.4 eV (V–O), 531.56 eV (O–C=O), and 532.92 eV (C–O). The detected V–O signatures in both the survey and high-resolution V 2p spectra demonstrated the formation of surface vanadium oxide layers, reflecting VN's propensity for spontaneous oxidation under atmospheric conditions.

Fig. 4 XPS spectra of C/Co_x/VN: (a) V 2p, (b) N 1s, (c) Co 2p.

Fig. 5 shows a systematic comparison of the electrochemical performances of the C_V-MOF, C/VN, and C/Co_x/VN composites through CV, GCD curves, specific capacitance analysis, and EIS. Related tests for other cobalt doping levels (C/Co_0.005/VN, C/Co_0.01/VN, C/Co_0.05/VN and C/Co_0.1/VN) are detailed in Fig. S4–S7†. Fig. 5a shows the CV profiles at 10 mV s⁻¹, with all the samples showing a small hump peak around −0.8 V, indicating pseudocapacitive behaviors. Additionally, the closed area of the CV curve of C/Co_0.025/VN was larger than that of the other samples (C/Co_x/VN, C/VN and C_V-MOF), indicating its higher specific capacitance. The GCD curves of all samples at 0.5 A g⁻¹ are presented in Fig. 5b, exhibiting symmetrical triangular-shaped characteristics. The C/Co_0.025/VN composite exhibited the longest discharge time, which was indicative of its higher specific capacitance compared to the other composites. As the scan rate increased, all the CV curves showed a good shape retainability, revealing their good rate ability (Fig. S4a–S7a†). Further, all the GCD curves showed close symmetrical triangular shapes with current density increasing, revealing their good rate ability, which agreed with the results shown by the CV curves (Fig. S4b–S7b†). Fig. 5c systematically compares the specific capacitance derived from the GCD measurements for all the cobalt-doped composites. When the current density was 0.5 A g⁻¹, the optimized C/Co_0.025/VN demonstrated a superior specific capacitance of 242.2 F g⁻¹, higher than C/Co_0.005/VN (171.3 F g⁻¹), C/Co_0.01/VN (209.6 F g⁻¹), C/Co_0.05/VN (184.8 F g⁻¹) and C/Co_0.1/VN (164.5 F g⁻¹), respectively. All the specific capacitances of the C/Co_x/VN composites were enhanced and higher than those of C_V-MOF (43.1 F g⁻¹) and C/VN (119.5 F g⁻¹) owing to the introduction of Co. Moreover, as the Co content increased, the specific capacitance initially increased and then decreased. When the current density increased up to 5 A g⁻¹, the specific capacitances were 88.1, 122.0, 156.4, 105.7 and 122.8 F g⁻¹ for C/Co_0.005/VN, C/Co_0.01/VN, C/Co_0.025/VN, C/Co_0.05/VN and C/Co_0.1/VN, respectively, which were still higher than those of C_V-MOF (7.0 F g⁻¹) and C/VN (55.6 F g⁻¹). The Nyquist curves of all the electrode materials are presented in Fig. 5d. All the materials exhibited characteristic features comprising a near-vertical curve at low frequency and a semicircle at high frequency. These semicircles suggest similar charge-transfer resistance characteristics across the samples. The charge-transfer resistances (R_ct) were 0.37, 0.16, 0.08, 0.19 and 0.06 Ω for C/Co_0.005/VN, C/Co_0.01/VN, C/Co_0.025/VN, C/Co_0.05/VN and C/Co_0.1/VN, respectively, which were much smaller than those of C_V-MOF (15.95 Ω) and C/VN (0.44 Ω), indicating their faster electrochemical kinetics.

Fig. 5 Electrochemical performances of C/Co_x/VN samples: (a) comparative CV profiles recorded at 10 mV s⁻¹, (b) GCD curves of different samples at 5 A g⁻¹, (c) calculated specific capacitance derived from the GCD data, (d) nyquist plots with equivalent circuit modeling shown in the inset.

The sintering temperature plays a crucial role in influencing the electrochemical performance. Therefore, the effect of temperature on the electrochemical properties was investigated. The CV curves in Fig. 6a exhibited rectangle-like shapes with small hump peaks. However, the smallest closed area was observed for C/Co_0.025/VN-600 among all the samples, revealing its smallest specific capacitance. This was likely because melamine does not decompose completely at 600 °C. Additionally, the GCD curves in Fig. 6b displayed symmetrical triangular shapes, reflecting the good reversibility of the composites. The C/Co_0.025/VN-700 composite exhibited the highest specific capacitance, as evidenced by the largest closed area in its CV curve (Fig. 6a) and longest galvanostatic charge–discharge duration compared to the other samples. Fig. 6c compares the specific capacitances of the C/Co_0.025/VN composites synthesized at varying annealing temperatures (600–900 °C). At 0.5 A g⁻¹, the sample treated at 700 °C achieved the highest capacitance of 242.2 F g⁻¹, while the samples sintered at other temperatures of 600 °C (4.0 F g⁻¹), 800 °C (207.1 F g⁻¹), and 900 °C (127.3 F g⁻¹) exhibited progressively diminished performance. As the current density increased, the specific capacitance gradually decreased for each sample, as presented in Fig. 6c. The EIS plots in Fig. 6d reveal the underlying charge-transfer characteristics of the samples. The charge-transfer resistances (R_ct) were 1.085, 0.123, 0.080 and 0.209 Ω for C/Co_0.025/VN-600, C/Co_0.025/VN-700, C/Co_0.025/VN-800 and C/Co_0.025/VN-900, respectively. The intrinsic resistances (R_s) were 0.49, 0.687, 0.695 and 0.728 Ω for C/Co_0.025/VN-600, C/Co_0.025/VN-700, C/Co_0.025/VN-800 and C/Co_0.025/VN-900, respectively. The smallest R_ct for C/Co_0.025/VN-700 among the samples indicated its fastest electrochemical kinetics. The corresponding SEM images are shown in Fig. 6e to help explain the specific capacitance variation. At a pyrolysis temperature of 600 °C, the composite was composed of a large number of nanoclusters with an irregular morphology. When the pyrolysis temperature increased to 700 °C, pyrolysis of the nanoclusters led to their transition into nanosheets, with a larger contact area and active sites, but the overall morphology was not obviously changed. As the pyrolysis temperature further rose to 800 °C and 900 °C, the irregular nanoclusters were clearly pyrolyzed into fragments, and the morphology of the nanosheets was destroyed.

Fig. 6 Electrochemical performances of C/Co_0.025/VN-Y (Y = 600, 700, 800, 900 °C) samples: (a) CV curves for different temperatures at 5 mV s⁻¹, (b) GCD curves at 0.5 A g⁻¹, (c) specific capacitances at different current densities, (d) EIS plots and (e) SEM images.

Fig. 7a shows the CV curves of C/Co_0.025/VN at varying scan rates from 5 to 50 mV s⁻¹. When the current density increased to the high scan rate of 50 mV s⁻¹, the CV shape was still similar to that at 5 mV s⁻¹, revealing the good rate ability of the sample. Fig. 7b shows the GCD curves of C/Co_0.025/VN at varying current densities from 0.5 to 5 A g⁻¹, which showed good symmetry, agreeing with the CV results. Fig. 7c presents the capacitance control and diffusion control based on the CV curve at 5 mV s⁻¹, showing the 53.9% capacitive contribution. Fig. 7d shows the contribution of diffusion control and capacitance control at different scan rates. As the scan rate increased from 5 to 50 mV s⁻¹, the contribution of capacitance control increased from 53.9% to 78.3% owing to the small resistance of the composites.

Fig. 7 Electrochemical analysis of C/Co_0.025/VN: (a) CV profiles at varying scan rates (5–50 mV s⁻¹), (b) GCD curves at varying current densities (0.5–10 A g⁻¹), (c) capacitive and diffusion-controlled contributions at 5 mV s⁻¹, (d) evolution of the charge-storage mechanism with increasing scan rates from 5 to 50 mV s⁻¹.

To evaluate the practical applicability of the C/Co_0.025/VN composite, an ASC device (Ni(OH)₂/C/Co_0.025/VN) was constructed using C/Co_0.025/VN as the anode, Ni(OH)₂ as the cathode, and 6 M KOH as the electrolyte. Fig. 8a presents the CV profile of the ASC at 5 mV s⁻¹, exhibiting a typical pseudocapacitive behavior. Fig. 8b presents the CV curves in a 0–1.5 V operational window across multiple scan rates (5–50 mV s⁻¹), predominantly resulting from the redox reactions of nickel hydroxide within the alkaline electrolyte, which proceeded in a reversible manner. The electrochemical consistency of the device was further verified through GCD measurements (Fig. 8c), which exhibited exceptional congruence with the CV observations. Quantitative analysis of this device revealed specific capacitances obtained by GCD curves of 60.8, 44.4, 33.9, 28.8, 25.0, and 22.8 F g⁻¹ at current densities of 0.5–5 A g⁻¹, while according to the CV curves, the specific capacitances at 5, 10, 20, 30, 40, and 50 mV s⁻¹ were 61.0, 51.7, 41.7, 36.3, 32.7 and 30.1 F g⁻¹, respectively. The Ragone plot, as shown in Fig. 8d, displays the energy-power relationship for the Ni(OH)₂//C/Co_0.025/VN asymmetric supercapacitor, exhibiting a characteristic inverse correlation, whereby the energy density diminished with increasing the power density. The device achieved a maximum energy density of 18.94 Wh kg⁻¹ at a power density of 375.05 W kg⁻¹ (as determined from the GCD curves) and 19.08 Wh kg⁻¹ at a power density of 377.72 W kg⁻¹ (as determined from the CV curves), which were higher than or corresponding to the values for VN/CNPs//VN/CNPs³⁸ (8 Wh kg⁻¹@575 W kg⁻¹), PCNs@VNNP//NiO³⁹ (16 Wh kg⁻¹@800 W kg⁻¹), CF@VN//AC⁴⁰ (8.24 Wh kg⁻¹@400 W kg⁻¹), VN/PC//Ti₃C₂T_x⁴¹ (12.81 Wh kg⁻¹@985.8 W kg⁻¹), VN@C//AC²⁸ (17 Wh kg⁻¹@701 W kg⁻¹), AuNP@PC@VN-0.3//NiO⁴² (18.9 Wh kg⁻¹@180 W kg⁻¹), and Ni/VN/NCs//Ni/VN/NCs⁴³ (13.4 Wh kg⁻¹@600 W kg⁻¹). Finally, the device could still achieve a high specific capacitance retention of 97% after 20 000 cycles (Fig. 8e). Further, two-coin cells connected in series could light up an LED bulb (Fig. S9†), demonstrating its potential for practical application.

Fig. 8 Ni(OH)₂//C/Co_0.025/VN electrochemical properties: (a) CV curves at 5 mV s⁻¹, (b) CV curves at different scan rates, (c) GCD curves at different current densities, (d) Ni(OH)₂//C/Co_0.025/VN energy density curves, (e) capacitance retention rate and coulombic efficiency at a current density of 4 A g⁻¹.

4. Conclusions

A C/Co_x/VN composite was prepared successfully by melamine-assisted pyrolysis of cobalt–vanadium metal–organic frameworks under an insert atmosphere. After optimizing the process (i.e., the mass loading and temperatures), the C/Co_0.025/VN composite showed a maximum specific capacitance of 242.2 F g⁻¹ at 0.5 A g⁻¹ at a heating temperature of 700 °C. Furthermore, an assembled asymmetric supercapacitor device of Ni(OH)₂//C/Co_0.025/VN could reach a high energy density of 19.08 Wh kg⁻¹ with a corresponding power density of 377.72 W kg⁻¹ and a capacitance retention of 97% after 20 000 cycles at 4 A g⁻¹.

Author contributions

Yongtao Tan: writing – review & editing, visualization, validation, supervision, investigation. Fengwei Tuo: experimental design, software, validation, drawing, data curation, writing – original draft. Yuan Zhang: validation, software, methodology.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China (No. 22202111) and the Sixth Group Youth Science and Technology Talents Project of Ningxia Hui Autonomous Region.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5dt01059d

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