Novel NiCo₂S₄@reduced graphene oxide@carbon nanotube nanocomposites for high performance supercapacitors

Abstract

We present a simple one-step solvothermal method for in situ growth of nickel cobalt sulfide (NiCo₂S₄) nanoparticles on the reduced graphene oxide (rGO) sheets and carbon nanotube (CNT) without adding any surfactant. The NiCo₂S₄@rGO@CNT structure was fabricated successfully with the hydrothermal method. When serving as an electrochemical supercapacitor electrode material, the NiCo₂S₄@rGO@CNT delivers a highly improved specific capacitance of 1242.51 F g⁻¹ at the current density of 2.0 A g⁻¹, as compared with that of pure NiCo₂S₄ (519.51 F g⁻¹) counterpart. Its durability is mainly owing to the synergistic effects of NiCo₂S₄, rGO and CNT, further presenting potential applications in energy storage.

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Introduction

Environmental pollution and the depletion of traditional energy force people to find clean and renewable energy sources.¹ Electrochemical capacitors (ECs), also called supercapacitors (SCs), are a new type of energy storage equipment and have attracted a great deal of attention because of their potential applications owing to their large power density, fast charging and discharging process, long cycling life and high reliability.^2–5

Although significant advances have been made in SCs, the practical applications of SCs are still held back by the really inferior electrochemical performances of the electrode materials, such as high cost of typical transition metal oxides, low specific capacitance of carbon-based materials and poor cycling stability of conductive polymers.^6,7

As an important kind of electrode material, transition metal oxide has been widely exploited for high performance supercapacitors due to their low cost, low toxicity and high theoretical specific capacity.^8,9 Unfortunately, its low conductivity impedes the application of transition metal oxides in high rate capability demanding fast electron transport. Stimulated by the remarkable electrochemical properties of NiCo₂O₄, spinel ternary nickel cobalt sulfides (NiCo₂S₄) is being explored more and more deeply because of its striking electrochemical performance. The research of NiCo₂S₄ has become a new hotspot recently.¹⁰ Theoretically, replacing oxygen with sulphur may create a more flexible structure with the elongation of chemical bonds, thus making it easier for electrons to transport in this tuned structure, which can further contribute to the enhancement of electrochemical performances of SCs.¹¹ Lately, transition metal sulfides have attracted more and more attention owing to their higher conductivity, comparing with the transition metal oxides and promising applications in high-performance supercapacitors.¹² Especially, NiCo₂S₄ can provide much higher electrochemical activity and specific capacitances than mono-metal sulfides, but also possesses 100 times higher conductivity than that of NiCo₂O₄.^13,14 In this sense, nanostructured NiCo₂S₄ is deemed to be a kind of promising candidate as an electroactive material for energy storage.^15,16

Generally, the serious aggregation of nanoparticles is likely to be caused during its preparation and application, which reduces its performance significantly.^17,18 An effective way to overcome this problem is building hierarchical nanoarchitecture.^19–21 Reduced Graphene Oxide (rGO) has large surface area and good electrical conductivity, superior mechanical property and good electrochemical stability, which make it an ideal support for nanocomposites.²² It has been demonstrated that the performances of supercapacitors can be improved significantly by constructing nanocomposites with rGO and transition metal oxides or sulfides.^7,23,24 On the other hand, carbon nanotube (CNT) has got significant attention since its great adaptability in building the highly conductive network like conductive paper and sponge.²⁵ It puts up with several promising characteristics, for example, excellent electrical conductivity, high surface area, durability, flexibility, high chemical stability and great mechanical strength. Notably, multi-walled carbon nanotubes (MWCNTs) permit a conducting pathway to support a fast electrochemical kinetic process during charge/discharge process at high a current density in a hybrid supercapacitor. A variety of electrode materials, such as metal oxide, metal sulfide and conducting polymers fabricated on MWCNTs have exhibited superior electrochemical behaviours.^26–29 The advantages of fabrication of metal oxide on CNT network in supercapacitor applications have been well demonstrated by many researchers.^30,31

To this end, CNT provides high electrical conductivity, because of the low interfacial resistance along the electron-transfer path way results in low energy loss during the charge/discharge cycles.^32,33 Graphene possesses numerous functional groups and edge planes, resulting in a high electro chemical activity of the electrode.^34–37 In addition, the network structure of rGO@CNT provides ion channels and electron-transfer pathways.^38,39 The main challenge is that these materials are difficult to be produced in quantity. Thus, in this work, we intend to synthesis a kind of NiCo₂S₄@rGO@CNT composite to reduce the defects of every component and increase the charge/discharge rate and cycling performance when the composite is used in supercapacitors.

Experiments

Synthesis of NiCo₂S₄ and NiCo₂S₄@rGO@CNT

Graphite oxide (GO) was obtained by modified Hummers method. In the typical synthesis of NiCo₂S₄@rGO@CNT, 150 mg of GO was mixed with 50 mL of deionized water. The mixture was ultrasonicated in a beaker for 120 min in order to fully exfoliate the GO to give homogeneous GO aqueous dispersion. After that, 1 mmol of Ni(NO₃)₂·6H₂O, 2 mmol of Co(NO₃)₂·6H₂O and 9 mmol of thiourea were added and dissolved in GO aqueous dispersion. 1 mL of ammonia, 25 mg of CNT and 30 mL of deionized water were added. This solution was magnetically stirred for 2 h. Later on, the mixture was transferred into a 100 mL Teflon-lined autoclave, sealed and heated in an oven at 180 °C for 24 h. The product was filtered and washed by deionized water and ethanol several times each. Then, it was set to dry in a vacuum at 60 °C for 12 hours. For comparison, pure NiCo₂S₄ was prepared by the same method without adding GO aqueous dispersion or CNT.

Characterization

The structures of synthetic samples were characterized by X-ray diffraction (XRD) (Bruker D8 Advance X-ray diffractometer, Cu Kα radiation). CelRef program was used to calculating the lattice parameter. The structure, morphologies and the size distribution of synthetic samples were observed by scanning electron microscope (SEM, Tescan MAIA3 XMH), transmission electron microscopy (TEM, JEOL JEM-2100 LaB6). X-ray photoelectron spectroscopy (XPS) was measured by Thermo Scientific ESCALAB 250Xi XPS instrument (Thermo Fisher Scientific, USA).

The electrochemical properties of the samples were tested with cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) technique by a three-electrode system using Autolab PGSTAT302N electrochemical workstation (Metrohm AG, Switzerland).

The electrochemical studies of individual electrodes were carried out in a three-electrode cell with 6 M KOH aqueous solution. The working electrodes were fabricated by mixing active materials (80 wt%), conductive material (acetylene black, 10 wt%) and binder (polytetrafluoroethylene, 10 wt%). All of the mixtures were first coated onto the surface of Ni foam sheet, and followed by drying at 120 °C for 12 h to obtain the working electrodes. The mass of the NiCo₂S₄ electrode is 9.41 mg, while that of NiCo₂S₄@rGO@CNT nanocomposites is 13.24 mg. A platinum rod and an Hg/HgO electrode were used as counter and reference electrodes, respectively.

The specific capacitance can be calculated by the following equation (eqn (1))

where C is the specific capacitance (F g⁻¹), I is the current of the charge/discharge, Δt is the discharging time, and m is the mass of the electroactive materials.

Results and discussion

Fig. 1 demonstrates the XRD pattern of as synthesized NiCo₂S₄@rGO@CNT nanocomposites and pure NiCo₂S₄. The characteristic peaks at 2θ of 16.3, 26.8, 31.6, 38.3, 50.5 and 55.3° can be respectively indexed into (111), (220), (311), (400), (511) and (440) planes of the cubic structured NiCo₂S₄ phase with a space group of Fd3m (JCPDS card no. 20-0782).⁴⁰ Compared with the assigned to a cubic NiCo₂S₄ phase, all the diffraction peaks of as prepared NiCo₂S₄ in the XRD patterns can be correctly indexed to the cubic NiCo₂S₄ phase. Furthermore, there are not any other peaks assigned to impurity observed, which confirms that the gained samples owned high purity. Besides, it can be found that the NiCo₂S₄ and the composite are similar with each other, which indicates that adding rGO and CNT does not influence the crystal lattice of NiCo₂S₄. After the calculation by the CelRef, the actual value of the NiCo₂S₄ is confirmed to be 9.3873 Å and the actual value of the composite is 9.3713 Å. In contrast, theoretical value of the a-lattice parameter (a) of NiCo₂S₄ is 9.4240 Å. These calculated results shows that the a-lattice parameter values of synthesized samples are a little different from the theoretical value, so it can prove that our samples exist a certain degree of crystal defects. As a matter of fact, there also appears to be peak shifting in the composite compared to that of the NiCo₂S₄, so the actual values demonstrate delicate difference.

Fig. 1 XRD patterns of NiCo₂S₄ and NiCo₂S₄@rGO@CNT.

XPS spectra of NiCo₂S₄@rGO@CNT are demonstrated in Fig. 2a–d. Fig. 2a shows a full-scan survey XPS spectrum of the sample in the range of 0–1000 eV. It can be clearly seen that the elements of Co, Ni, S and C were detected. Fig. 2b shows the Ni 2p spectrum of NiCo₂S₄@rGO@CNT, with two doublets main peaks and two shake-up satellites. The binding energy at 852.7 eV in Ni 2p_3/2 and 869.8 eV in Ni 2p_1/2 agrees well with the spin–orbit characteristic of Ni²⁺, while the binding energy at 856.2 eV in Ni 2p_3/2 and 874.5 eV in Ni 2p_1/2 are characteristic of Ni³⁺.⁴¹ As shown in Fig. 2c, the Co 2p spectrum can be properly fitted with two spin–orbit doublets and two shake-up satellites by using a Gaussian fitting method. The first doublet (at 777.9 and 793.0 eV) and the second doublet (at 780.6 and 797.2 eV) can be assigned to Co³⁺ and Co²⁺, respectively.⁴² As for the S 2p spectrum shown in Fig. 2d, the peaks at 161.9 and 162.9 eV correspond respectively to the S 2p_3/2 and S 2p_1/2, and the binding energy at 168.7 eV can be assigned to the shake-up satellite.⁴³ According to these XPS analysis results above, the NiCo₂S₄@rGO@CNT holds chemical composition of Co²⁺, Co³⁺, Ni²⁺, Ni³⁺ and S²⁻ in its near-surface area.

Fig. 2 (a) XPS survey spectra of NiCo₂S₄@rGO@CNT, (b–d) high resolution XPS of Ni 2p, Co 2p and S 2p for NiCo₂S₄@rGO@CNT nanocomposites.

The morphologies and microstructural features of the NiCo₂S₄ and NiCo₂S₄@rGO@CNT are characterized by SEM, TEM and EDS. As showed in Fig. 3a, the as-formed NiCo₂S₄ nanosheets are interconnected with each other and an open framework is exhibited, which is beneficial to the electrolyte wetting for better penetration. In Fig. 3b, the NiCo₂S₄ nanosheets grow on the rGO and the CNTs twine with them. In this structure, rGO acts as a growth substrate and a current collector for NiCo₂S₄ arrays. Especially, the special interconnected network structure not only provides a convenient ion and electron diffusion channel, but also increases efficient contact between the ions and the active material. Both of them are seen as promising electrode materials for supercapacitors.

Fig. 3 SEM image of (a) NiCo₂S₄ and (b) NiCo₂S₄@rGO@CNT nanocomposites.

The EDS elemental mapping of NiCo₂S₄ and NiCo₂S₄@rGO@CNT are shown in Fig. 4 and 5 all elements offer good dispersibility, corresponding to the SEM image. Besides, the three elements ratio confirms the formation of pure NiCo₂S₄. In Fig. 5, the element of carbon can be detected in large quantities, the EDS analysis demonstrates that the Ni, Co, and S elements are uniformly and continuously distributed on the graphene nanosheets. Since the nanosheets are very thin, the density of elements is lower than the pure NiCo₂S₄.

Fig. 4 (a) SEM image of the NiCo₂S₄ sample; (b–d) elemental mapping images of nickel, cobalt, and sulphur elements.

Fig. 5 (a) SEM image of the NiCo₂S₄@rGO@CNT; (b–d) elemental mapping images of nickel, cobalt, and sulphur elements.

The morphologies of NiCo₂S₄@rGO@CNT nanocomposites and pure NiCo₂S₄ were revealed in detail by TEM observation. Fig. 6 shows representative TEM images of two samples from low to high magnification. The low-magnification TEM images (Fig. 6a and c) indicate that the NiCo₂S₄ nanoparticles are composed of several thin layers. Its microscopic morphology is demonstrated to be nanosheet, same as SEM image. Besides, the composite is that the NiCo₂S₄ grows on the graphene and CNTs densely. The high-resolution TEM (HRTEM) image (Fig. 6b and d) confirms further close contacts in rGO, CNT and NiCo₂S₄ nanosheets, implying a formation of strongly coupled NiCo₂S₄@rGO@CNT. In addition, rGO sheets not only play a crucial role in providing nucleation sites for NiCo₂S₄ crystals growth, but also confine the increase of nanoparticles size by the restraint of its space environment. The d-spacing of 0.54 nm, 0.28 nm and 0.33 nm corresponds to the (111), (311) and (220) plane of NiCo₂S₄, respectively.⁴⁴ Moreover, its polycrystalline nature is demonstrated in SAED pattern with those typical diffraction rings representative of certain lattice planes.

Fig. 6 (a) and (c) Low magnification TEM images of the NiCo₂S₄ and NiCo₂S₄@rGO@CNT. (b) and (d) HRTEM image of NiCo₂S₄ and NiCo₂S₄@rGO@CNT, selected-area electron diffraction (SAED) patterns for a large parts of samples consisting of many NiCo₂S₄ nanoparticles.

To evaluate the electrochemical performance of the NiCo₂S₄ and NiCo₂S₄@rGO@CNT, the electrochemical properties of as-prepared NiCo₂S₄ and NiCo₂S₄@rGO@CNT nanocomposites were firstly investigated by using a three-electrode cell. Fig. 7a shows typical cyclic voltammetry (CV) curves of the electrodes made with the materials and tested in 6 M KOH electrolytes at 5 mV s⁻¹. NiCo₂S₄@rGO@CNT nanocomposites show a much higher current in a potential window of −0.1 to 0.4 V (vs. Hg/HgO), which reveals that the electrochemical performance characteristics mainly resulted from faradaic pseudocapacitance related to Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ redox couples.

Fig. 7 (a) CV curves of free NiCo₂S₄ nanoparticles and NiCo₂S₄@rGO@CNT nanocomposites at a scan rate of 5 mV s⁻¹. (b) Galvanostatic charge/discharge curves of NiCo₂S₄@rGO@CNT nanocomposites electrodes at various current densities. (c) Specific capacitance of NiCo₂S₄ nanoparticles and NiCo₂S₄@rGO@CNT nanocomposites at different current densities. (d) Cycling stability at 10 A g⁻¹ galvanostatic charge/discharge.

The integrated CV area for the NiCo₂S₄@rGO@CNT electrode is significantly larger than the NiCo₂S₄ electrode, confirming that the NiCo₂S₄@rGO@CNT material facilitates electron transport and enhances the electrochemical utilization of rGO@CNT. The reason can be assigned to the unique structure of the present electrode. First, the ultrathin and interconnected network structure characteristics of NiCo₂S₄@rGO@CNT endowed a very high surface area, which could provide numerous electroactive sites for redox reactions. Secondly, the open space between these ultrathin layers can be served as robust reservoir for ions, thus enhancing the electrolyte penetration within the electrode greatly. Third, the mesoporous structure of the electrode surface ensured efficient contact between the surface of the active materials and the electrolyte. In addition, the direct growth of NiCo₂S₄@rGO@CNT with good intrinsic electrical conductivity and robust adhesion could allow the rapid transportation of electrons.^15,45

The galvanostatic charge/discharge curves of this NiCo₂S₄@rGO@CNT electrode displayed a highly symmetry at different current densities (Fig. 7b). Fig. 7c displayed the specific capacitance of the NiCo₂S₄@rGO@CNT and the NiCo₂S₄ calculated by eqn (1). The specific capacitances of NiCo₂S₄@rGO@CNT nanocomposites are 1242.51, 768.77, 746.02, 720.02, 676.04 and 645.05 F g⁻¹ at current densities of 2, 3, 4, 5, 8 and 10 A g⁻¹, respectively, which are much higher than that (519.51, 465.77, 347.02, 253.77, 144.04 and 90.05 F g⁻¹) of the NiCo₂S₄ nanoparticles. By calculating, the theoretical specific capacitance of NiCo₂S₄ is 1899.61 F g⁻¹. Relatively, the specific capacitance of the samples almost meet the expectations. Fig. 7d reveals the cycle performance of the NiCo₂S₄@rGO@CNT and NiCo₂S₄ electrodes measured at a high current density of 10 A g⁻¹ for 2000 cyclic charging/discharging measurements. After 2000 cycles, the NiCo₂S₄@rGO@CNT and NiCo₂S₄ electrodes retained, respectively, 53.91% and 68.84% of their initial specific capacitances, indicating that the NiCo₂S₄ electrode material had excellent cycle stability and good reversibility. However, the whole specific capacitance of NiCo₂S₄@rGO@CNT is still much higher than NiCo₂S₄. When testing the supercapacitor of the composite material at a lower current density, we did the cyclic voltammetry and galvanostatic charge/discharge experiment by a three-electrode cell with 6 M KOH aqueous solution. After that, the test of cycling stability was further conducted. During these tests, the electrode varied with time. For example, the microstructure of the NiCo₂S₄@rGO@CNT collapse, the NiCo₂S₄@rGO@CNT didn't attach to the Ni foam sheet tightly and in this three-electrode cell system, the oxidation–reduction reaction is not reversible completely. The results confirm that the electrochemical stabilities of the as-synthesized hybrid nanocomposites need to be enhanced in the applications as electrode materials for energy storage.

The highly improved capacitances of NiCo₂S₄@rGO@CNT nanocomposites maybe attributed to their distinctive structure. Firstly, the strong chemical attachment and electrical coupling among the NiCo₂S₄ nanoparticles, rGO and CNT afford facile electron transport among CNT, rGO and the NiCo₂S₄ nanoparticles. In addition, the ultra-dispersed NiCo₂S₄ nanoparticles exceedingly increase the quantity of electroactive sites. Furthermore, rGO sheets overlapped with each other to provide a three-dimensional structure and substantially facilitate transport of the electrolyte. CNTs make the composites be less compact with large specific surface area, and originate the electric double layer capacitance with high conductivity.

Fig. 8 shows the comparison of CV curves, charge–discharge curves and the specific capacitance of the as-prepared samples. In Fig. 8a, the specific capacitance of NiCo₂S₄, NiCo₂S₄@rGO@CNT nanocomposites, NiCo₂S₄@rGO, NiCo₂S₄@CNT and rGO@CNT are 481.99, 1236.75, 362.68, 865.12 and 43.25 F g⁻¹ at a scan rate of 5 mV s⁻¹. At the same time, we can calculate the capacitance by the GCD curves in Fig. 8b, which are 519.51, 1242.51, 468.51, 845.01 and 31.51 F g⁻¹ at 2 A g⁻¹, respectively. Therefore, we can draw a conclusion that the specific capacitance of NiCo₂S₄@rGO@CNT is much higher than others. Obviously, we can see that the specific capacitance delivered by NiCo₂S₄@rGO@CNT is always higher than others under different current densities in Fig. 8c. That evaluates the great performance of the NiCo₂S₄@rGO@CNT furtherly.

Fig. 8 (a) CV curves of free NiCo₂S₄ nanoparticles, NiCo₂S₄@rGO@CNT nanocomposites, NiCo₂S₄@rGO, NiCo₂S₄@CNT and rGO@CNT at a scan rate of 5 mV s⁻¹. (b) Galvanostatic charge/discharge curves of free NiCo₂S₄ nanoparticles, NiCo₂S₄@rGO@CNT nanocomposites, NiCo₂S₄@rGO, NiCo₂S₄@CNT and rGO@CNT at current density of 2 A g⁻¹. (c) Specific capacitance of free NiCo₂S₄ nanoparticles, NiCo₂S₄@rGO@CNT nanocomposites, NiCo₂S₄@rGO, NiCo₂S₄@CNT and rGO@CNT at different current densities.

The specific surface area and porosity measurement of the as prepared NiCo₂S₄ and NiCo₂S₄@rGO@CNT samples were investigated by N₂ adsorption–desorption measurements (Fig. 9). On the basis of the adsorption–desorption isotherm, the Brunauer–Emmett–Teller (BET) surface area of the NiCo₂S₄ and NiCo₂S₄@rGO@CNT were calculated to be as 9.86 m² g⁻¹ and 19.63 m² g⁻¹. Fig. 9b and (d) present the pore size distribution calculated from the N₂ adsorption–desorption isotherm using the Barrett–Joyner–Halenda (BJH) method. The porosity measurement of NiCo₂S₄ and the composites are 14.34 nm and 15.64 nm, respectively. The maximum pore size distribution of the sample mainly centres in the range of 2–20 nm. Such a pore size distribution is favourable for electrolyte penetration, allowing reaction species to access the electrode quickly and contributes to excellent electrochemical behaviours. The surface area of NiCo₂S₄@rGO@CNT is twice than that of the NiCo₂S₄. The porosity measurement is similar, while the composite is a little bigger. It can explain the electrical conductivity of the NiCo₂S₄@rGO@CNT is better than the NiCo₂S₄.

Fig. 9 (a) and (c) N₂ adsorption–desorption isotherm of NiCo₂S₄ nanoparticles and NiCo₂S₄@rGO@CNT nanocomposites. (b) and (d) BJH adsorption pore size distribution of NiCo₂S₄ nanoparticles and NiCo₂S₄@rGO@CNT nanocomposites.

Conclusions

In this work, the composite of NiCo₂S₄@rGO@CNT was synthesized for the first time. It reveals a high specific capacitance of 1242.51 F g⁻¹ (2 A g⁻¹) and good cycle stability (53.91% after 2000 cycles), which can be attributed to the great electrochemical performance of NiCo₂S₄@rGO@CNT to its unique nanostructure consisting of a highly conductive rGO@CNT network and relative stable three-dimensional architecture. As a result, it instructs dramatic improvements in capability. In summary, this work also shows the importance of the synergetic effect among ternary components and the rational design and synthesis for advanced electrode material. In future, what we need to work out is enhancing the cycle stability. If we get it, the excellent electrochemical performance will make the NiCo₂S₄@rGO@CNT nanocomposites a promising electrode material for the electrochemical energy storage devices in the future.

Notes and references

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