3D reticulate Co_xNi_3−xS₂ nanostructure on nickel foam as a new type of electroactive material for high-performance supercapacitors

Abstract

3D reticulate Co_xNi_3−xS₂ nanostructures were grown on nickel foam using a simple precursor sulfuration route. Compared with monometallic Ni₃S₂, the introduction of an appropriate content of Co could markedly improve electrochemical properties. Also, the electrochemical performance of as-prepared Co_xNi_3−xS₂ nanostructures could be tuned by the Co content added to the Ni₃S₂. The investigation showed that 3D Co_xNi_3−xS₂ nanostructures prepared from the system with initial Co²⁺/Ni²⁺ molar ratio of 2/1 (labeled as S2/1) exhibited the best energy storage properties. At a current density of 2 mA cm⁻², the areal capacitance of S2/1 reached 3030 mF cm⁻²; and even at a current density of 30 mA cm⁻², the areal capacitance reached 2268 mF cm⁻¹. After 6500 cycles at a current density of 5 mA cm⁻², the capacitance retention retained 92.8% of the initial value, exhibiting excellent cycling stability. After 3D reticulate S2/1 nanostructures were fabricated into an asymmetrical supercapacitor with activated carbon, the energy density achieved 73.5 W h kg⁻¹ at a power density of 600 W kg⁻¹; and at high power density of 3024.06 W kg⁻¹, the energy density could still reach 50.04 W h kg⁻¹, which presents potential applications for energy storage as a high performance electrode material.

---

1. Introduction

The rapid development of industrialization means that demand for energy resources is dramatically increasing. However, fossil fuels such as oil and coal are gradually being exhausted. Therefore, it has become extremely urgent to explore environmentally friendly and alternative new energy sources. Although clean energy sources such as wind, solar, and geothermal are attractive, they strongly depend on natural conditions, which limits their work efficiency. Capacitors with high energy density would be useful to promote application of such clean energy sources. There has been extensive research interest in recent years on supercapacitors (SCs) with long cycle-life, high power density, and fast charge/discharge capability.^1–3

Electrode materials are the most important component of supercapacitors. Hence, in attempts to achieve excellent electrochemical performance, great efforts have been focused on identifying new electrode materials with high capacity or/and designing optimal electrode architectures.⁴ As one of the most prominent candidates of electrode materials for lithium ion batteries (LIBs) and SCs, transition metal sulfides have attracted much attention as they have higher electrical conductivity than oxides, larger specific capacitance than carbon-based electrodes, and longer cycle performance than conducting polymers.⁵ Among various transition metal sulfides, nickel or cobalt sulfides such as Ni_xS_y,^6–9 Co_xS_y,^10–12 and some emerging metal sulfides such as MoS₂,¹³ VS₂,¹⁴ and bimetallic NiCo₂S₄,^15,16 offer varieties of compositions, low cost, and excellent electrochemical performance. In particular, Ni₃S₂ has attracted increased interest because of its high theoretical specific capacitance in high-performance SCs¹⁷ and LIBs.^18,19 When used as an electrode material, however, Ni₃S₂ bears poor electrical conductivity and rate capability, which limits the charge–discharge rate of SCs.²⁰ To improve electrical conductivity and rate capability, series of composites have been formed using Ni₃S₂ and highly conductive materials such as carbon-based materials, metals, and metal oxides.^21–23 For example, Dai and coworkers hydrothermally synthesized hierarchical Ni₃S₂/carbon nanotube composites and investigated their electrochemical performances. The as-obtained composites exhibited a high specific capacitance of 800 F g⁻¹ and great cycling stability at a current density of 3.2 A g⁻¹.²⁴ Xing et al. prepared ZnO@Ni₃S₂ arrays through an electrodeposition route and found that the as-prepared arrays displayed high specific capacitance of 1529 F g⁻¹ at a current density of 2 A g⁻¹, and good rate capability and cycling stability.¹⁹

It is well known that the property of a material can be enhanced by proactively introducing certain impurities. When Ni element in Ni₃S₂ is partly substituted by other metal elements, a new freedom degree is endowed for optimization of the electrical conductivity and electrochemical activity.²⁵ Thus, doping offers another useful approach to improve the electrochemical properties of Ni₃S₂. In this study, a simple precursor sulfuration route was designed to successfully prepare 3D reticulate Co_xNi_3−xS₂ nanostructures grown on nickel foam. It was found that the electrochemical performances of Co_xNi_3−xS₂ nanostructures varied with changes in the amount of cobalt introduced into the Ni₃S₂. 3D Co_xNi_3−xS₂ nanostructures prepared from the system with initial Co²⁺/Ni²⁺ molar ratio of 2/1 (labeled as S2/1) presented the best energy storage properties. The areal capacitance reached 3030 mF cm⁻² at a current density of 2 mA cm⁻²; and even at a current density of 30 mA cm⁻², the areal capacitance reached 2268 mF cm⁻¹. After cycling for 6500 times at a current density of 5 mA cm⁻², there was 92.8% capacitance retention (against the initial value), exhibiting an excellent cycling stability. After assembling an asymmetrical supercapacitor with 3D reticulate S2/1 and activated carbon, the energy density achieved 73.50 W h kg⁻¹ at a power density of 600 W kg⁻¹, and even at a power density of 3024.06 W kg⁻¹, the energy density achieved 50.04 W h kg⁻¹. Compared with some previous reports,^23,26,27 the present asymmetrical capacitor possesses bigger energy density and power density. Also, its potential window was 1.6 V, higher than those of MoS₂ (0.8 V)²⁸ and NiCo₂S₄ (1.5 V).²⁹

2. Experimental

Materials

All reagents and chemicals were analytically pure, bought from the Shanghai Chemical Company and used without further purification.

Synthesis of 3D reticulate Co_xNi_3−xS₂ nanostructures on nickel foam

The synthesis of Co_xNi_3−xS₂ on nickel foam was realized through a two-step hydrothermal method. Before hydrothermal reactions, a piece of nickel foam (2 cm × 3 cm) was treated with 3 M HCl for 15 min to remove the oxide layer and then washed thoroughly with deionized water and ethanol. In a typical procedure, CoSO₄·7H₂O and NiSO₄·6H₂O with varying molar ratios of 3 : 0, 2 : 1, 1.5 : 1.5, 1 : 2, and 0 : 3 were first dissolved in a mixed solvent of 28 mL deionized water and 7 mL ethanol. Then, 10 mmol urea (CO(NH₂)₂) and 0.05 g PVP were added under magnetic stirring. After the as-obtained pale-pink solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 50 mL, the nickel foam was placed into the autoclave. Thereafter, the system was heated at 100 °C for 12 h. Nickel foam with pink matter on its surface was taken out and washed with deionized water and ethanol several times. Subsequently, nickel foam loaded by the precursor was transferred to another 50 mL autoclave with 35 mL 0.05 mol L⁻¹ of Na₂S solution. The sulfuration reaction was carried out at 140 °C for 8 h. After the autoclave was cooled to room temperature naturally, nickel foam with black matter on the surface was taken out and washed with deionized water and ethanol several times; and finally dried in a vacuum at 60 °C for 10 h. The as-prepared samples were separately denoted S3/0, S2/1, S1.5/1.5, S1/2, and S0/3. The masses of Co_xNi_3−xS₂ loading the surface of Ni foam (1.0 cm × 1.0 cm) were, in turn, 1.4, 1.5, 1.7, 2.0, and 2.2 mg.

Characterization

The X-ray powder diffraction patterns of the products were carried out on a BRUKER D8 ADVANCE X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154060 nm), employing a scanning rate of 5 °s⁻¹ and 2θ ranges from 10° to 80°. Transmission electron microscopy (TEM) images were carried out on a Hitachi HT7700 transmission electron microscope, employing an accelerating voltage of 100 kV. SEM images and EDS analysis of the product were obtained using a Hitachi S-4800 field emission scanning electron microscope, employing an accelerating voltage of 5 kV and 15 kV, respectively. X-ray photoelectron spectroscopy (XPS) of the product was obtained on a Thermo ESCALAB 250 instrument, employing monochromic Al Kα (hν = 1486.6 eV) at a power of 150 W.

Electrochemical performance

For the electrochemical measurements, Co_xNi_3−xS₂ loaded on nickel foam at a size of 1.0 cm × 1.0 cm directly acted as the working electrode, and a platinum wire and an Hg/HgO electrode acted as the counter and reference electrodes, respectively. A 3 M KOH aqueous solution acted as the electrolyte.

To fabricate an all-solid-state asymmetric supercapacitor, Co_xNi_3−xS₂ loaded on nickel foam at a size of 1.0 cm × 1.0 cm was directly used as the positive electrode. The negative electrode was prepared by mixing active carbon, acetylene black, and poly-tetrafluoroethylene (PVDF) in a mass ratio of 80 : 10 : 10 to obtain a slurry.²³ Then the slurry was pressed onto a piece of nickel foam (1.0 cm × 1.0 cm) and dried at 60 °C for 12 h. The positive and negative electrodes were put together and separated by a piece of cellulose paper separator that had been soaked in 3 M KOH aqueous solution.

3. Results and discussion

The surface shape changes of Ni foam before and after depositing the precursor were observed by SEM technology. As shown in Fig. S1,† the surface of pure Ni foam is smooth; however, after the precursor had been deposited, the surface of Ni foam had a porous structures (Fig. 1a). Further enlargement in Fig. 1b showed that the porous structures were reticulate and constructed by abundant nanosheets. Although different initial molar ratios of Co/Ni were used, all of the as-obtained precursors exhibited similar reticulate structures. After the precursors had been sulfurized by Na₂S, the reticulate structures could still be maintained (see Fig. S2†). Fig. 1c shows a high magnification SEM image of the product after the precursor had been sulfurized by Na₂S. The surfaces of the final product are slightly rougher than those of the precursor (see Fig. 1b). Fig. S3† shows the XRD patterns of five samples. Three strong diffraction peaks, centered at 44.6°, 52.0°, and 76.4°, come from metallic Ni. The other weak ones separately match with data of cubic Co₃S₄ (for S3/0, PDF card files no. 47-1738) and rhombohedral Ni₃S₂ (for S2/1, S1.5/1.5, and S1/2, PDF card files no. 44-1418). As cubic Co₃S₄ and rhombohedral Ni₃S₂ have close diffraction patterns, EDS technology was employed for composition analysis of S2/1, S1.5/1.5, and S1/2 samples. As shown in Fig. S4,† Co and Ni peaks can be detected simultaneously in the three samples. The integrated Co content in S2/1, S1.5/1.5, and S1/2 is, in turn, 31.7%, 26.9%, and 14.5%. S2/1 bears the highest Co content. Fig. 2 compares the diffraction patterns of S2/1 and S0/3 after removing the Ni foam. The two samples have the same peak sites but different diffraction intensities, implying that introduction of cobalt ions does not markedly vary the phase of Ni₃S₂. Fig. 3a gives a typical TEM image of S2/1, on which wrapped nanosheets can be seen. A HRTEM image is shown in Fig. 3b. Some lattice fringes can be seen clearly, implying good crystallinity of nanosheets. The ED pattern in the inset of (b) displays clear concentric rings, indicating the polycrystalline nature of nanosheets. Furthermore, the d-spacings of neighboring planes are measured to be 0.29 nm and 0.24 nm, which are separately close to 0.2873 nm of the (110) plane and 0.23793 nm of the (003) plane of rhombohedral Ni₃S₂. These results prove that the phase of Ni₃S₂ is not markedly changed by integration of Co(II), which should be attributed to the close ion radius of Co²⁺ and Ni²⁺.

Fig. 1 (a) A typical SEM image of Ni foam loading the precursor on its surface, (b) an enlarged SEM image of the precursor, and (c) a representative SEM image of the product obtained after sulfuration of the precursor.

Fig. 2 XRD patterns of S2/1 and S0/3 after Ni foam was removed.

Fig. 3 (a) TEM image and (b) HRTEM image of S2/1 prepared with original Co²⁺/Ni²⁺ molar ratio of 2 : 1. The inset in (b) is the SAED pattern of S2/1.

It is well known that urea can react with water to produce NH₃ and CO₂ (see eqn (1)) under hydrothermal conditions. Subsequently, CO₃²⁻ ions are formed rapidly because of reaction between the produced NH₃ and CO₂ (see eqn (2)). When some metal ions exist in the solution, it is probable that precipitates of metal carbonates are formed owing to their low solubility in water.^30,31 Therefore, under the present hydrothermal conditions, the precursor could be obtained because of co-precipitation of cobalt carbonate and nickel carbonate (see eqn (3)). As Ni foam was present in the system, the precursor gradually nucleated and grew on the surface of the Ni foam, and, with the assistance of PVP, reticulate structures were finally formed.

CO(NH₂)₂ + H₂O = CO₂ + 2NH₃ (1)

CO₂ + 2NH₃ + H₂O = CO₃²⁻ + 2NH₄⁺ (2)

Co²⁺/Ni²⁺ + CO₃²⁻ = Co(Ni)CO₃ (3)

Electrochemical performance

To investigate the electrochemical performance of products prepared from systems with various original Co²⁺/Ni²⁺ molar ratios, the CV curves of S3/0, S2/1, S1.5/1.5, S1/2, and S0/3 were measured at a scan rate of 10 mV s⁻¹. As shown in Fig. 4a, when only Ni²⁺ or Co²⁺ ion existed in the system, a pair of marked redox peaks separately appeared at 0.3–0.44 V for S0/3 and 0.29–0.48 V for S3/0, implying the presence of a reversible faradaic reaction. When Ni²⁺ or Co²⁺ ion coexisted in the system, two pairs of redox peaks appeared, attributed to redox reactions between Ni²⁺/Ni³⁺, Co²⁺/Co³⁺, and Co³⁺/Co⁴⁺.^32,33 Compared with S3/0 and S0/3, S2/1, S1.5/1.5, and S1/2 displayed better electrochemical property. Among them, S2/1 showed the biggest peak currents and CV area. Fig. 4b shows the charge–discharge curves of five samples at 10 mA cm⁻² and the correlations between specific capacity and current density (see the inset in (b)). One can clearly see that S2/1 also presents the best charge–discharge capacity and the highest specific capacity. These results confirm that S2/1 shows the best electrochemical performance. In this study, the loading masses on nickel foam of 1.0 cm × 1.0 cm were 1.4 mg (S3/0), 1.5 mg (S2/1), 1.7 mg (S1.5/1.5), 2.0 mg (S1/2), and 2.2 mg (S0/3). The loading mass increased with decreasing Co/Ni molar ratio in the solution. Generally, the loading mass has positive influence on the electrochemical performance of the electrode. However, although S2/1 had lower loading mass than S1.5/1.5, S1/2, and S0/3, and higher loading mass than S3/0, it displayed the best areal capacitance of all the samples. Clearly, loading mass was not the main factor affecting electrochemical performance of the electrode. Recently, Li and coworkers suggested that coexistence of different elements with varied chemical valence states could cause self-doping to occur to optimize the electrical conductivity and electrochemical activity.³⁴ In Ni–Co sulfides, Ni and Co species with higher valence states can be formed under electrochemical conditions. Theoretically, the presence of Ni³⁺ provides extra electrons as n-type doping whereas the presence of Co³⁺ will result in extra holes as p-type doping.³⁵ Therefore, more Ni³⁺ and Co³⁺ will lead to higher electrical conductivity for electrons and better electrochemical performance. Therefore, the synergistic effect of cobalt and nickel in the present work is likely to be the main factor affecting the electrochemical performance of the electrode.

Fig. 4 (a) CV curves of S3/0, S2/1, S1.5/1.5, S1/2, and S0/3 at 10 mV s⁻¹, and the inset in (a) shows the galvanostatic charge–discharge plots of S3/0, S2/1, S1.5/1.5, S1/2, and S0/3 at a current density of 10 mA cm⁻². (b) Effects of the current density of various samples on their specific capacitances.

Fig. 5 shows high resolution Ni 2p and Co 2p XPS analyses of S2/1. Utilizing the Gaussian fitting method, the Ni 2p spectrum is well fitted with two spin–orbit doublets and two satellite peaks (see Fig. 5a). The Co 2p spectrum presents similar results (see Fig. 5b). In the Ni 2p spectrum, the peaks at 855.4 eV and 873.0 eV, as well as satellite peaks at 861.2 and 879.1 eV, are separately attributed to Ni 2p_3/2 and Ni 2p_1/2. In the Co 2p spectrum, the binding energies at 777.9 and 796.3 eV of the Co 2p peaks are assigned to Co³⁺, and the binding energies at 780.9 and 802.2 eV to Co²⁺.³⁴ The above results are similar to those in Li's report,³⁴ indicating the presence of Ni²⁺, Ni³⁺, Co²⁺, and Co³⁺ in the product.

Fig. 5 High resolution Ni 2p (a) and Co 2p (b) XPS spectra of S2/1.

Fig. 6a shows the CV curves of S2/1 in the potential window from −0.2 to 0.7 V at scan rates from 5 to 50 mV s⁻¹. With increasing scan rate, the redox currents increase. The oxidation and reduction peaks separately shift towards higher and lower potentials, which is attributed to the increase of the internal diffusion resistance.^36,37 Fig. 6b depicts the galvanostatic charging–discharging curves of S2/1 at a current density of 2–20 mA cm⁻². The corresponding specific capacitances can be calculated according to the below equations:³⁸

where C_a (mF cm⁻²) and C_m (F g⁻¹) separately stand for the areal and mass specific capacitance; I (A) and Δt (s) are in turn the discharge current and time; S (cm²) and m (g) represent the areal and mass of the active material; and ΔV (V) is the potential window for the charge–discharge process. Based on eqn (4) and (5), the areal/mass specific capacitance of S2/1 reaches 3030 mF cm⁻²/2020 F g⁻¹ at a current density of 2 mA cm⁻², and 74.8% of the capacity is maintained with increase of the current density from 2 to 30 mA cm⁻², implying good rate capability. Fig. 6c exhibits the change of the specific capacitance with cycle time. The specific capacitance increases with increase of the cycle time from 0 to 1200, which is ascribed to the activating process of the electrode. Subsequently, the specific capacitance of the electrode decreases with increase of the cycle time to 6500. Herein, the capacitance is retained at 92.8% of the initial value, indicating that S2/1 has outstanding capacitance retention. Furthermore, the inset given in Fig. 6c shows the Nyquist plots of S2/1 at the first cycle and after 200 cycles in a frequency range of 100 000–0.01 Hz. The similar diameters of the semicircles indicate high stability of the electrode, implying that S2/1 is a promising material for application in electrochemical energy storage.

Fig. 6 (a) CV curves of S2/1 at scan rates from 5 mV s⁻¹ to 50 mV s⁻¹, (b) the GCD plots of S2/1 at current densities between 2 and 20 mA cm⁻², and (c) the cycling performance of S2/1 at a constant current density of 5 mA cm⁻². The inset in (c) is Nyquist plots of S2/1 at the 1^st and the 200^th cycles.

To explore practical applications of the as-prepared S2/1, an asymmetric supercapacitor was fabricated using S2/1 sample as the positive electrode and activated carbon as the negative one in a 3 M KOH solution. Mass loading of active materials on the positive electrode and negative electrode was 1.6 mg and 10.2 mg, respectively. The mass ratio (m₊/m₋) of the positive and negative materials was calculated to 0.1568. According to the charge balance mechanism,³⁹ the loading mass of materials on positive and negative electrodes can be controlled precisely. The corresponding formula is shown in eqn (6):

where m_+/− are the loading masses of materials on positive and negative electrodes; C_+/− and ΔE_+/− stand for mass specific capacitances and potential windows of the positive and negative electrodes. In the current work, activated carbon (AC) is used as the negative material and shows a specific capacitance of 185 F g⁻¹ at a current density of 1 A g⁻¹ (see Fig. S5†).

Fig. 7a depicts the CV curves of S2/1 and activated carbon at a scan rate of 50 mV s⁻¹ in a three-electrode system. The potential windows of activated carbon and S2/1 are, in turn, 1.0 V and 0.7 V. According to the specific capacitances and potential windows of CV curves, the mass ratio (m₊/m₋) of the positive and the negative materials is 0.156, which is in good agreement with the practical mass ratio. Simultaneously, the working voltage of S2/1-AC cell can be expanded to 1.6 V because of different working potentials of activated carbon and S2/1. Fig. 7b shows the CV plots of the S2/1-AC asymmetric supercapacitor at different scan rates. The CV curves are located in the range of 0–1.6 V, which is also confirmed by GCD curves at different current densities (see Fig. 7c). According to the present GCD curves, however, a low coulombic efficiency is obtained. This could be caused by electrolysis of the solvent under high potentials. In Fig. 7c, a platform appears in the region of 1.5–1.6 V, resulting from electrolysis of the solvent. After tuning the potential window to 0–1.5 V, the platform disappears and the coulombic efficiency can be markedly increased (see Fig. S6†). The specific capacitance values are calculated to be 827, 806, 739, 638, and 563 F g⁻¹ at current densities of 3, 5, 7, 10, and 15 A g⁻¹ (see Fig. 7d). The inset shown in Fig. 7d shows the cycle performance of the S2/1-AC asymmetric supercapacitor at a current density of 15 A g⁻¹. After cycling 2000 times, the specific capacity is retained at ∼71%, implying good stability of the present S2/1-AC asymmetric supercapacitor. In the present work, the 3D porous material was directly grown on the Ni foam as binder-free electrodes. Thus, highly accessible electro-active sites, fast diffusion of the electrolyte ions, and access of fast ion transport could be easily achieved.^40,41 Furthermore, the synergistic effect of cobalt and nickel in the active material also promoted the electrochemical properties. Hence, good stability was obtained. The corresponding energy density and power density of the asymmetric supercapacitor can be calculated using the eqn (7) and (8):^42,43

where C represents the specific capacitance of the asymmetric supercapacitor, ΔV is the operating voltage of the device, and t is the discharge time. According to the GCD measurement, S2/1-AC asymmetric supercapacitors present a high energy density of 73.50 W h kg⁻¹ at a power density of 600 W kg⁻¹, and even at a power density of 3024.06 W kg⁻¹, the energy density still reaches 50.04 W h kg⁻¹. Compared with some previous reports (see Table 1), the present hybrid supercapacitors deliver higher energy density than some asymmetric supercapacitors based on Ni–Co sulfides.

Fig. 7 (a) CV curves of activated carbon and S2/1 at a scan rate of 50 mV s⁻¹ in a three-electrode system. Electrochemical measurements of the S2/1-AC asymmetric supercapacitors: (b) CV curves at various scan rates; (c) charge–discharge curves at different current densities; (d) effects of the current density on the specific capacitance. The inset in (d) is the cycle performance of the asymmetric supercapacitor at a current density of 15 A g⁻¹.

Table 1 Comparison of the electrochemical performances of the present work with some previous reports

Materials	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)	Ref.
CoNi2S4–AC	762 F g^−1 at 10 mA cm^−1	33.9	409	1
NiCo2S4–RGO–AC	197 F g^−1 at scan rate of 5 mV s^−1	31.5	156.6	23
NiCo2S4@Ni3V2O8//AC	116.7 F g^−1 at 1 A g^−1	42.7	200	26
NiCo2S4//AC	1471 F g^−1 at 1 A g^−1	44.8	401	27
NiSrGO–AC	79.7 F g^−1 at 0.2 A g^−1	18.7	124	44
Ni3S2@CoS–AC	270 F g^−1 at 4 mA cm^−1	28.24	134.46	45
CoxNi3−xS2–AC	827 F g^−1 at 3 A g^−1	73.5	600	This work

---

Fig. 8 depicts the EIS of the as-assembled asymmetric capacitor before and after 2000 cycles. Each Nyquist plot consists of a semicircle in the high-frequency region and a straight line in the low-frequency one. Although the Nyquist plots have obviously changed before and after 2000 cycles, they have similar equivalent circuits (see the inset of Fig. 8), composed of a bulk solution resistance (R_s), a charge-transfer resistance (R_ct), a pseudocapacitive element (C), and a constant phase element for the double-layer capacitance (CPE). The R_ct before and after 2000 cycles is calculated to be 0.52 Ω and 1 Ω, respectively, with the increase in the R_ct attributed to structure collapse of the sample.

Fig. 8 Electrochemical impedance spectra (EIS) of the asymmetric capacitor assembled by S2/1 and activated carbon before and after 2000 cycles. The inset is the equivalent circuit.

4. Conclusion

In summary, Co_xNi_3−xS₂ with different Ni and Co ratios was synthesized by a facile precursor sulfuration method on Ni foam. It was found that introduction of Co²⁺ ion into Ni₃S₂ improved its specific capacity, rate performance, and cyclic stability. The excellent electrochemical performance of Co_xNi_3−xS₂ is attributed to its 3D reticulate structure and the synergistic effect between Ni and Co ions in sulfides. In particular, Co_xNi_3−xS₂ obtained from the system containing Co²⁺/Ni²⁺ molar ratio of 2/1 showed the best electrochemical performance, the highest specific capacitance, and excellent rate capability and cycling stability. An asymmetric supercapacitor prepared by S2/1 and activated carbon exhibited high specific capacitance values of 827, 806, 739, 638, and 563 F g⁻¹ at current densities of 3, 5, 7, 10, and 15 A g⁻¹, respectively; and good cycle stability. After cycling 2000 times at a current density of 15 A g⁻¹, the specific capacity retained ∼71%. Furthermore, the as-prepared asymmetric supercapacitor had a high energy density of 73.50 W h kg⁻¹ at a power density of 600 W kg⁻¹, and even at a power density of 3024.06 W kg⁻¹, the energy density still reached 50.04 W h kg⁻¹. The as-obtained 3D reticulate Co_xNi_3−xS₂ superstructures have potential application in energy storage as high performance electrode materials.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21571005) and High School Leading talent incubation program of Anhui province (gxbjZD2016010) for the funding support.

References

1. W. Hu, R. Q. Chen, W. Xie, L. L. Zou, N. Qin and D. H. Bao, ACS Appl. Mater. Interfaces, 2014, 6, 19318–19326.
2. Y. F. Zhang, C. C. Sun, H. Q. Su, W. Huang and X. C. Dong, Nanoscale, 2015, 7, 3155–3163.
3. C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868.
4. G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828.
5. B. H. Qu, Y. J. Chen, M. Zhang, L. L. Hu, D. N. Lei, B. G. Lu, Q. H. Li, Y. G. Wang, L. B. Chen and T. H. Wang, Nanoscale, 2012, 4, 7810–7816.
6. Z. Zhang, Z. Y. Huang, L. Ren, Y. Z. Shen, X. Qi and J. X. Zhong, Electrochim. Acta, 2014, 149, 316–323.
7. Y. J. Ruan, J. J. Jiang, H. Z. Wan, X. Ji, L. Miao, L. Peng, B. Zhang, L. Lv and J. Liu, J. Power Sources, 2016, 301, 122–130.
8. L. N. Wang, J. J. Liu, L. L. Zhang, B. S. Dai, M. Xu, M. W. Ji, X. S. Zhao, C. B. Cao, J. T. Zhang and H. S. Zhua, RSC Adv., 2015, 5, 8422–8426.
9. C. W. Lee, S. D. Seo, H. K. Park, S. B. Park, H. J. Song, D. W. Kim and K. S. Hong, Nanoscale, 2015, 7, 2790–2796.
10. S. J. Peng, X. P. Han, L. L. Li, Z. Q. Zhu and F. Y. Cheng, Small, 2016, 12(10), 1359–1368.
11. S. G. Liu, C. P. Mao, Y. B. Niu, F. L. Yi, J. K. Hou, S. Y. Lu, J. Jiang, M. W. Xu and C. M. Li, ACS Appl. Mater. Interfaces, 2015, 7, 25568–25573.
12. W. H. Shi, J. X. Zhu, X. H. Rui, X. H. Cao, C. L. Chen, H. Zhang, H. H. Hng and Q. Y. Yan, ACS Appl. Mater. Interfaces, 2012, 4, 2999–3006.
13. X. Y. Chia, A. Y. S. Eng, A. Ambrosi, S. M. Tan and M. Pumera, Chem. Rev., 2015, 115, 11941–11966.
14. J. Feng, X. Sun, C. Z. Wu, L. L. Peng, C. W. Lin, S. L. Hu, J. L. Yang and Y. Xie, J. Am. Chem. Soc., 2011, 133, 17832–17838.
15. M. Sun, J. J. Tie, G. Cheng, T. Lin, S. M. Peng, Z. Deng, F. Ye and L. Yu, J. Mater. Chem. A, 2015, 3, 1730–1736.
16. J. W. Xiao, L. Wan, S. H. Yang, F. Xiao and S. Wang, Nano Lett., 2014, 14, 831–838.
17. X. W. Ou, L. Gan and Z. T. Luo, J. Mater. Chem. A, 2014, 2, 19214–19220.
18. D. Li, X. W. Li, X. Y. Hou, X. L. Sun, B. L. Liu and D. Y. He, Chem. Commun., 2014, 50, 9361–9364.
19. Z. C. Xing, Q. X. Chu, X. B. Ren, C. J. Ge, A. H. Qusti, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, J. Power Sources, 2014, 245, 463–467.
20. W. Zhou, W. Chen, J. Nai, P. Yin, C. Chen and L. Guo, Adv. Funct. Mater., 2010, 20, 3678–3683.
21. J. L. Zhu, Y. Y. Li, S. Kang, X. L. Wei and P. K. Shen, J. Mater. Chem. A, 2014, 2, 3142–3147.
22. W. J. Zhou, X. H. Cao, Z. Y. Zeng, W. H. Shi, Y. Y. Zhu, Q. Y. Yan, H. Liu, J. Y. Wang and H. Zhang, Energy Environ. Sci., 2013, 6, 2216–2221.
23. W. T. Wei, L. W. Mi, Y. Gao, Z. Zheng, W. H. Chen and X. X. Guan, Chem. Mater., 2014, 26, 3418–3426.
24. C. S. Dai, P. Y. Chien, J. Y. Lin, S. W. Chou, W. K. Wu, P. H. Li, K. Y. Wu and T. W. Lin, ACS Appl. Mater. Interfaces, 2013, 5, 12168–12174.
25. Y. X. Xu, Z. Y. Lin, X. Zhong, X. Q. Huang, N. O. Weiss, Y. Huang and X. F. Duan, Nat. Commun., 2014, 5, 5554–5556.
26. L. Y. Niu, Y. D. Wang, F. P. Ruan, C. Shen, S. Chan, M. Xu, Z. K. Sun, C. Li, X. J. Liu and Y. Y. Gong, J. Mater. Chem. A, 2016, 4, 5669–5677.
27. Y. L. Xiao, Y. Lei, B. Z. Zheng, L. Gu, Y. Y. Wang and D. Xiao, RSC Adv., 2015, 5, 21604–21613.
28. K. Krishnamoorthy, P. Pazhamalai, G. K. Veerasubramani and S. J. Kim, J. Power Sources, 2016, 321, 112–119.
29. W. Kong, C. C. Lu, W. Zhang, J. Pu and Z. C. Wang, J. Mater. Chem. A, 2015, 3, 12452–12460.
30. N. N. Xiang, Y. H. Ni and X. Ma, Chem.–Asian J., 2015, 10, 1972–1978.
31. H. S. Zheng, Y. H. Ni, F. Y. Wan and X. Ma, RSC Adv., 2015, 5, 31558–31565.
32. G. Yang, L. W. Mi, W. T. Wei, S. Z. Cui, Z. Zheng, H. W. Hou and W. H. Chen, ACS Appl. Mater. Interfaces, 2015, 7, 4311–4319.
33. H. C. Chen, J. J. Jiang, L. Zhang, H. Z. Wan, T. Qi and D. D. Xia, Nanoscale, 2013, 5, 8879–8883.
34. X. M. Li, Q. G. Li, Y. Wu, M. C. Rui and H. B. Zeng, ACS Appl. Mater. Interfaces, 2015, 7, 19316–19323.
35. X. Wang, C. Y. Yan, A. Sumboja, J. Yan and P. S. Lee, Adv. Energy Mater., 2014, 4, 1301240–1301246.
36. J. Y. Ji, L. L. Zhang, H. X. Ji, Y. Li, X. Zhao, X. Bai, X. B. Fan, F. B. Zhang and R. S. Ruoff, ACS Nano, 2013, 7, 6237–6243.
37. U. Patil, K. Gurav, V. Fulari, C. Lokhande and O. S. Joo, J. Power Sources, 2009, 188, 338–342.
38. H. C. Chen, J. J. Jiang, L. Zhang, D. D. Xia, Y. D. Zhao, D. Q. Guo, T. Qi and H. Z. Wan, J. Power Sources, 2014, 254, 249–257.
39. J. H. Zhu, J. Jiang, Z. P. Sun, J. S. Luo, Z. X. Fan, X. T. Huang, H. Zhang and T. Yu, Small, 2014, 14, 2937–2945.
40. W. B. Fu, Y. L. Wang, W. H. Han, Z. M. Zhang, H. M. Zha and E. Q. Xie, J. Mater. Chem. A, 2016, 4, 173–182.
41. G. K. Veerasubramani, K. Krishnamoorthy and S. J. Kim, J. Power Sources, 2016, 306, 378–386.
42. Y. D. Wang, C. Shen, L. Y. Niu, R. Z. Li, H. T. Guo, Y. X. Shi, C. Li, X. J. Liu and Y. Y. Gong, J. Mater. Chem. A, 2016, 4, 9977–9985.
43. W. Li, S. L. Wang, L. P. Xin, M. Wu and X. J. Lou, J. Mater. Chem. A, 2016, 4, 7700–7709.
44. F. Cai, R. Sun, Y. R. Kang, H. Y. Chen, M. H. Chen and Q. W. Li, RSC Adv., 2015, 5, 23073–23079.
45. R. Li, S. L. Wang, J. P. Wang and Z. C. Huang, Phys. Chem. Chem. Phys., 2015, 17, 16434–16442.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20502j

---