A facile hydrothermal synthesis of a reduced graphene oxide modified cobalt disulfide composite electrode for high-performance supercapacitors

Abstract

In this study, a 3D reduced graphene oxide modified CoS₂ composite electrode (CoS₂/RGO) is synthesized by a facile hydrothermal approach. The dimensions of the CoS₂ nanoparticles in CoS₂/RGO are effectively reduced due to the geometric confinement of RGO, and a novel, large-scale wave-like structure is formed. This leads to an enlarged specific surface area and improved conductivity and could thus be favourable for both fast electron and ion transport. As a consequence, CoS₂/RGO displays better electrochemical properties than the pure individual components. When the mass ratio of CoCl₂·6H₂O and GO as the raw materials is 1 : 2, the obtained CoS₂/RGO composite electrode (CoS₂/RGO-2) delivers the highest capacitance of 930.3 F g⁻¹ at 2 A g⁻¹ and retains a capacitance as high as 677.9 F g⁻¹ as the current density increases up to 20 A g⁻¹. Moreover, in order to obtain high energy and power densities, a high-voltage asymmetric supercapacitor has been designed and constructed using the optimized CoS₂/RGO-2 composite electrode as the positive electrode and activated carbon (AC) as the negative electrode material. Such a device with an operational voltage of 1.6 V can achieve a remarkable energy density of 45.7 W h kg⁻¹ at a power density of 797.0 W kg⁻¹, in addition to the superior rate capability and prominent stability towards long time charge–discharge cycles.

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Introduction

The development of energy storage devices with high energy and power outputs, long lifetimes, and short charging times is quite significant and desirable for use in many important areas such as plug-in electric vehicles, overnight power storage and energy storage for wind and large industrial equipment.^1–3 Supercapacitors are among the most important types of energy storage devices and have received tremendous attention due to their many features, which include a high power density, long cycle life, fast charge/discharge rate, and safe operation mode compared to traditional batteries.^4–6 However, the current commercial supercapacitors usually suffer from relatively lower energy densities as compared to rechargeable lithium batteries and fuel cells, and this has seriously restricted their practical applications.^7,8 Consequently, from the viewpoint of future practical applications, advanced high-performance supercapacitors have to be further developed with higher energy densities without significantly sacrificing on power delivery and cycle life.^2,8 More recently, asymmetric supercapacitors that perfectly incorporate the advantages of both Li-ion batteries and supercapacitors with enhanced energy densities have been considered attractive. The attractive features of the asymmetric configuration lie in the greatly improved specific capacitance arising from the pseudocapacitive electrode and the wide operation voltage, which favor a significantly increased energy density. Moreover, it has been well established that the electrochemical performance of such devices is directly affected by the properties and structures of the chosen electrodes and electrode materials. The main challenge to fabricating high-energy density asymmetric supercapacitors still lies in the selection and exploitation of advanced electrodes and electrode materials.

With the purpose of enhancing the electrochemical performance of the asymmetric supercapacitor, numerous efforts have been devoted to investigating extensively pseudocapacitive transition metal oxides, hydroxides, chalcogenides and conducting polymer materials. These can in principle offer higher specific capacitance and superior energy densities than typical carbonaceous materials with electric double layer capacitance.^9–11 In recent years, considerable research efforts have been devoted to preparing metal sulfides, including MoS₂,¹² WS₂,^12,13 CoS¹⁴ and VS₂,¹⁵ which show superior electrochemical performances as electrode materials for supercapacitors. One important type of transition metal chalcogenides for potential application in supercapacitors, lithium-ion batteries, alkaline rechargeable batteries, magnetic materials and catalysts are cobalt sulfides, including CoS₂, Co₃S₄, CoS and Co₉S₈, due to their excellent physical, chemical, electronic and optical properties.^16–20 In particular, cobalt disulfide (CoS₂), a key member of the semiconductor family, has been considered as one of the most promising materials for great potential applications in supercapacitors²¹ due to its high redox activity as well as multiple convertible valence states.²² However, previous attempts to fabricate supercapacitors based on CoS₂ or/and CoS₂/carbonaceous materials have encountered several major problems such as relatively low capacitance,^22,23 poor cycling stability¹¹ and rate capability.²⁴ The main causes for the unsatisfying performance can be attributed to the following four main aspects.^25,26 (1) The single CoS₂ component is intrinsically unstable. (2) The use of insulating polymer binders has an adverse effect on the electric conductivity of the fabricated electrode. (3) The traditional strategy of coating the slurry layer onto the electrode in general leads to the peeling of active material during cycling (4) random assembly of CoS₂ is not favorable for electron conduction during electrode reactions. Therefore, an effective synthesis strategy for direct growth of the CoS₂ active material on a conductive support to fabricate an integrated electrode has great potential to simultaneously resolve the abovementioned problems, and thus, drastically improve its electrochemical performance.

In this study, a facile hydrothermal approach for in situ growth of CoS₂ combined with reduced graphene oxide (RGO) on a Ni foam support to fabricate an integrated composite electrode (CoS₂/RGO) has been reported. Such a method could effectively enhance the specific capacitance of the product due to the spatial confinement of graphene and the synergetic effect between the CoS₂ component and the graphene component.²⁷ The influence of the mass ratios of CoS₂ and RGO in the composite electrode on the electrochemical performance has also been systematically investigated by tuning the addition of the raw materials. The test results showed that CoS₂ was uniformly wrapped by RGO sheets to form a novel large-scale wave-like structure on the surface of the Ni foam. Such a binder free, highly conductive and structure optimized CoS₂/RGO electrode exhibited a high specific capacitance of 930.3 F g⁻¹ at 2 A g⁻¹. In addition, a high performance asymmetric supercapacitor was successfully fabricated using the optimized CoS₂/RGO composite electrode as the positive electrode and activated carbon (AC) electrode as the negative electrode, which exhibited excellent rate capability (70.8% retention at initial current density of 1 A g⁻¹), high energy density (45.7 W h kg⁻¹), and superior cycling ability (∼90% after 6000 cycles).

Experimental

Materials preparation

All materials were of analytical grade and were used without further purification. Graphite oxide (GO) was synthesized by an improved Hummers' method, as reported previously.²⁸ The CoS₂/RGO composite electrode was prepared by a facile hydrothermal process, as illustrated in Scheme 1 (on the left). First, the Ni foam substrate (1 × 1 × 0.1 cm) was pretreated with 6 M HCl, deionized (DI) water and ethanol to remove the NiO film and impurities from its surface. Second, an appropriate amount of GO was dispersed into 35 ml DI water and 35 ml ethanol by ultrasonication for 2 h to obtain a homogeneous exfoliated GO solution. Third, 2 mmol CoCl₂·6H₂O and 2 mmol Na₂S₂O₃·5H₂O were added into the GO solution. Finally, the reaction mixture and Ni foam were transferred to a Teflon-lined autoclave and hydrothermally treated at 150 °C for 10 h. After washing with water and ethanol several times followed by drying, the CoS₂/RGO composite electrode was obtained. For comparison purposes, the pure CoS₂ electrode was also prepared by a similar hydrothermal method without GO.

Scheme 1 Schematic of the synthetic process (left) and structure (right) of the CoS₂/RGO composite electrode.

For further detailed experiments, the mass ratios of CoCl₂·6H₂O and GO were controlled at 1 : 0, 1 : 1, 1 : 2 and 1 : 3, and the as-prepared composite electrodes were named CoS₂, CoS₂/RGO-1, CoS₂/RGO-2 and CoS₂/RGO-3, respectively. The mass loading of the active material on Ni foam was about 1.5 mg.

Characterization

The crystal structures of the products were characterized by X-ray diffraction (XRD) analyses using a D/max-γB X-ray diffractometer (Rigaku, Japan) with a Cu Kα source. Raman spectra were obtained using a Raman spectrometer (Renishaw in Via, Germany) under laser excitation at 532 nm. The Brunauer–Emmett–Teller (BET) nitrogen adsorption measurements were obtained on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer (Micromeritics, Norcross, GA, USA). Morphologies and sizes of as-prepared samples were obtained by field-emission scanning electron microscopy (FESEM, Hitachi SU8000, Japan), transmission electron microscopy (TEM, Hitachi S-7650, Japan) and high resolution transmission electron microscopy (HTEM, JEM-2100, Japan). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, America) measurements were obtained using the monochromatic Al Kα radiation (1486.6 eV) to evaluate the elemental compositions and chemical status of the sample. In the XPS analysis, we used the carbon C 1s peak at 284.8 eV as a reference for charge correction.

Asymmetric supercapacitor devices

To construct the asymmetric supercapacitor, the optimized CoS₂/RGO-2 composite electrode and AC electrode were used as the positive and negative electrodes with 2 M KOH as the electrolyte. The negative electrode was fabricated by dispersing AC as the active material, acetylene black as the conductive agent and PTFE as the binder with a mass ratio of 85 : 10 : 5.²⁹ A small amount of distilled water was added to the abovementioned electrode material to form a uniform slurry, followed by plastering the slurry onto the Ni foam current collector within a certain area. Furthermore, the fabricated electrode was pressed and dried under vacuum at 60 °C for 8 h to obtain the final AC electrode. Based on a series of comparative experiments, 4.5 mg of AC was chosen to balance the capacitance of the two electrodes. Therefore, the total mass of the active electrode materials was 6.0 mg.

Electrochemical measurements

In the three-electrode electrochemical measurement system, the synthesized electrode was used as the working electrode, the platinum wire was used as the counter electrode and the saturated Ag/AgCl electrode was used as the reference electrode. The electrochemical performances of the samples were determined on an electrochemical workstation system (CHI660E, Shanghai ChenHua Co., Ltd, China). The electrochemical performances of supercapacitors were evaluated by characterizing their cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). The voltage window was from −0.1 to 0.6 V vs. Ag/AgCl for the CoS₂/RGO composite electrode, −1.0 to 0 V vs. Ag/AgCl for the AC electrode, and 0 to 1.8 V for the asymmetric CoS₂/RGO-2//AC cell. Their GCD tests were measured at different current densities. The EIS measurement was conducted using a superimposed 5 mV sinusoidal voltage in the frequency range from 100 kHz to 10 mHz at open circuit potential. The parameters of the equivalent circuit were calculated and analysed by computer simulations using the ZSimpWin software. The specific capacitance (C_s, F g⁻¹) and equivalent series resistance (ESR, Ω) in the three-electrode configuration were calculated from GCD curves on the basis of the following equation:^9,29

C_s = I × Δt/(ΔV × m) (1)

ESR = iR_drop/(2 × I) (2)

where I (A), Δt (s), ΔV (V) and m (g) are designated as the discharging current, the total discharging time, the discharging voltage excluding potential drop (iR_drop) and the mass of the electroactive materials, respectively. iR_drop (V) is defined as the electrical potential difference between the two ends of a conducting phase during charging–discharging.

The CV and GCD tests of the asymmetric electrochemical capacitors were performed in a two-electrode cell. The specific capacitance (C_cell, F g⁻¹), the energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) were calculated from GCD curves by the following equations:^8,30

C_cell = I × Δt/(ΔV × M) (3)

E = 0.5 × C_cell × (ΔV)²/3.6 (4)

P = E × 3600/(Δt) (5)

where C_cell (F g⁻¹) and M (g) are the capacitance of the asymmetric supercapacitor and the total mass of the electroactive materials on both the positive and negative electrodes.

In addition, the specific capacitance of the electrode can be calculated from the CV curves on the basis of the following equation:^9,31

C_s = (∫IdV)/(vm_acV) (6)

where I (A), V (V), v (mV s⁻¹) and m_ac (g) are the response current, the potential, the potential scan rate and the mass of the electroactive material in the electrode, respectively.

Results and discussion

Characterization of electrode materials

The crystallographic phase of CoS₂ and different CoS₂/RGO composites grown on the Ni foam was first characterized by XRD analysis. Because of the strong background of the Ni foam substrate on the XRD peak signals, the CoS₂ and different CoS₂/RGO composites had to be scratched from the Ni foam for XRD analysis. Fig. 1a shows the typical XRD patterns of GO, RGO, different CoS₂/RGO composites, CoS₂ and standard CoS₂ (JCPDS card no. 65-3322). The XRD pattern of GO obtained through an improved Hummers' method presents a main diffraction peak at 2θ = 11.2° with an interlayer spacing of 0.79 nm, which is similar to those of many other previously reported GO because of the introduction of oxygen-containing functional groups between the graphene sheets.³² In contrast, a weak diffraction peak at around 25.8° is distinguishable for RGO, which indicates the highly amorphous nature of RGO, and the results suggest that GO was reduced to RGO through hydrothermal treatment at 150 °C for 10 h. In addition, for CoS₂ and different CoS₂ composites, all Bragg peaks at 28.0°, 32.4°, 36.5°, 40.0°, 46.5°, 55.1°, 60.3° and 62.8° were indexed to the reflections from (111), (200), (210), (211), (220), (311), (230) and (321) crystal planes of the cattierite CoS₂ crystalline structure (JCPDS Card no. 65-3322),¹⁷ respectively, evidently indicating a perfect crystallinity. Moreover, in the XRD patterns of the CoS₂/RGO composites, only the CoS₂/RGO-3 composite was observed with a weak broad peak at about 25.6° corresponding to the (002) peak of RGO owing to its relatively higher RGO content. These results also indicate that the CoS₂ and different CoS₂/RGO composite electrodes were successfully prepared by our facile hydrothermal method.

Fig. 1 (a) XRD patterns of GO, RGO, different CoS₂/RGO composites and CoS₂. (b) Raman spectra of RGO, CoS₂/RGO-2 and CoS₂ composite.

Raman spectroscopy is a noninvasive technique that has been a widely employed tool for the characterization of carbonaceous materials, especially for characterizing their ordered and disordered crystal structures.⁴⁰ Fig. 1b shows the Raman spectra of the RGO, CoS₂ scratched from Ni foam and CoS₂/RGO-2 scratched from Ni foam. In the Raman spectrum of the CoS₂/RGO-2 composite, there is a D peak at approximately 1360 cm⁻¹ associated with defects in the graphite structure and a G peak at about 1590 cm⁻¹, corresponding to the first-order scattering of the E_2g mode.³² These two peaks are also observed in the RGO spectrum. In the Raman spectrum of CoS₂/RGO-2 composite, there are D peak (∼1350 cm⁻¹) and G peak (∼1590 cm⁻¹) of RGO. The additional peak located at about 380 cm⁻¹ is attributed to CoS₂, since it is also seen in the Raman spectrum of pure CoS₂ prepared in the absence of GO under the same conditions. Based on these results, it can be concluded that both RGO and CoS₂ are present in the as-prepared composite.

The X-ray photoelectron spectroscopy (XPS) measurements were performed to further confirm the oxidation state of Co in the CoS₂RGO-2 composite (Fig. 2). Two distinct peaks at binding energies of 779.9 and 794.7 eV were found in the high resolution spectrum of Co 2p (the inset). These two peaks could be attributed to Co 2p_3/2 and Co 2p_1/2, which is the characteristic of Co²⁺ in the CoS₂RGO-2 composite, and consistent with that of CoS₂ reported previously.^33–35 Moreover, the wide XPS spectrum of the composite exhibited that the binding energies of O 1s, S 2s and S 2p were determined to be 531.4 eV, 230.8 eV and 162.7 eV,³⁴ respectively, with the reference binding energy at 284.8 eV for the C 1s peak. Thus, XPS analysis further demonstrated that the state of Co within the composite was positive bivalent.

Fig. 2 XPS survey of the CoS₂/RGO-2 composite with high resolution of the XPS Co 2p spectrum.

The images and SEM images of the Ni foam, CoS₂ electrode and CoS₂/RGO-2 composite electrode are shown in Fig. 3a–f. The images of the Ni foam (left) and CoS₂ (right) are shown in Fig. 3a. The color of Ni foam changes to black after the hydrothermal reaction, which is indicative of the formation of CoS₂. Fig. 3b and d clearly show that the as-prepared particles are fully deposited on the surface of the Ni foam, indicating that the present hydrothermal system is favorable for growing the particles on conductive substrates. Similar to the previous reports,^36–38 a porous structure with an interconnected 3D scaffold of the Ni foam serves as a direct conductive substrate for the growth of materials. As shown in Fig. 3c, the pristine CoS₂ without GO, displays a severe aggregation of CoS₂ particles. However, in the presence of GO, it is revealed that the surface of the CoS₂/RGO-2 composite obtains a novel large-scale wave-like structure with CoS₂ nanoparticles uniformly wrapped by the RGO sheets, and the agglomeration of CoS₂ was effectively alleviated at the same time (Fig. 3e). The high-magnification SEM (Fig. 3f) image revealed that the CoS₂ particles had an equable size distribution ranging from 200 to 300 nm. Prevention of CoS₂ and RGO agglomeration results from the effect of steric hindrance between CoS₂ and RGO, which is advantageous for reducing the diffusion and migration distance of the ions and increasing the effective contact area between active material and electrolyte. In addition, the exclusion of an insulating binder, direct contact with the current collector (Ni foam) and facile electron conduction along the novel large-scale wave-like structure all enhance the electrochemical performance of the CoS₂/RGO composite electrode for the supercapacitor.

Fig. 3 (a) The image of the Ni foam and the CoS₂ electrode. (b and c) SEM images of the CoS₂ electrode. (d–f) SEM images of the CoS₂/RGO-2 composite electrode. (g) Nitrogen adsorption/desorption isotherms of CoS₂ and the CoS₂/RGO-2 composite. (h) Pore-size distribution curves of CoS₂ and the CoS₂/RGO-2 composite.

The mesoporous nature of the as-synthesized products was verified by surface area and pore size distribution analysis. Fig. 3g displays the N₂ adsorption–desorption isotherms of the CoS₂ and CoS₂/RGO-2 composite. The BET specific surface area of the CoS₂/RGO-2 composite was calculated to be 93 m² g⁻¹, which was much higher than that of CoS₂ (21 m² g⁻¹). Interestingly, based on the BJH model (Fig. 3h), both samples indicated the existence of mesopores. With respect to CoS₂, the higher surface area of CoS₂/RGO-2 is due to the introduction of RGO, resulting from the fact that RGO not only acts as the support for the deposition of CoS₂ nanoparticles but also prevents CoS₂ from aggregating. Moreover, combined with the CoS₂/RGO-2 mesoporous feature, it is advantageous to provide a short and fast transport pathway, thus enhancing the faradaic reaction.

To study the effects of different concentrations of GO, the SEM images of CoS₂/RGO-1 and CoS₂/RGO-3 are shown in Fig. 4. As shown in Fig. 4a and b, CoS₂ nanoparticles within the CoS₂/RGO-1 composite were mostly wrapped by the RGO sheets, but some CoS₂ particles still agglomerated on the surface without being in contact with the RGO sheets at the same time, which was attributed to the lower RGO content. Moreover, the CoS₂/RGO-3 composite (Fig. 4c and d) also displayed a large-scale wave-like structure with CoS₂ nanoparticles uniformly wrapped by the RGO sheets; however, a mild aggregation between RGO sheets was observed because of a relatively higher RGO content (see Fig. 5). The SEM images of GO and RGO are also shown in Fig. S1.†

Fig. 4 (a and b) SEM images of the CoS₂/RGO-1 composite electrode. (c and d) SEM images of the CoS₂/RGO-3 composite electrode.

Fig. 5 (a and b) TEM images of the CoS₂ electrode at different magnifications. (c and d) TEM images of the CoS₂/RGO-1 composite electrode at different magnifications. (e and f) TEM images of the CoS₂/RGO-2 composite electrode at different magnifications. (g and h) TEM images of the CoS₂/RGO-3 composite electrode at different magnifications. (i and j) TEM and HTEM images of the CoS₂/RGO-2 composite electrode, suggesting CoS₂ nanoparticles are well wrapped by RGO. (k) HTEM image of the CoS₂/RGO-2 composite electrode corresponding to the yellow-boxed area in (j), revealing well-crystallized CoS₂ nanoparticles. (l) FFT pattern of the HTEM images corresponding to the yellow-boxed area in (j). (m) HAADF-STEM image and elemental mapping results of the CoS₂/RGO-2 composite.

To further investigate the morphologies and particle sizes of the CoS₂/RGO composites with different concentrations of the raw GO material, TEM/HTEM images of the samples were obtained. From the TEM images (Fig. 5a and b) of the CoS₂ electrode at different magnifications, it can be seen that CoS₂ particles exhibit a non-uniform size distribution from 400 nm up to 500 nm. As shown in Fig. 5c and d, CoS₂ particles with sizes of 200 to 300 nm are wrapped by RGO sheets; however, to some extent, the CoS₂ particles are agglomerated together. TEM images of the CoS₂/RGO-2 composite electrode at different magnifications are shown in Fig. 5e and f. It can be seen that CoS₂ particles are uniformly wrapped by RGO sheets over a broad area to form the composite materials. Clearly, the size of these CoS₂ particles is reduced by about 100 nm compared to the pure CoS₂ particles. Moreover, Fig. 5g and h illustrate that CoS₂ particles are also homogeneously wrapped by RGO sheets, but the RGO aggregates slightly as well. These results are in good agreement with the abovementioned SEM analysis.

To further clarify the crystal structure of the CoS₂/RGO-2 composite electrode, TEM/HTEM images are shown in Fig. 5i and j. As displayed in Fig. 5i and f, CoS₂ nanoparticles are well wrapped by RGO. Moreover, from Fig. 5k, the widths of the lattice fringes are about 2.25 and 4.95 Å for CoS₂/RGO-2, corresponding to the (211) and (210) planes of CoS₂ crystals, which is in good agreement with the XRD analysis mentioned above. The fast Fourier transform (FFT) pattern (Fig. 5l corresponding to the yellow-boxed area in Fig. 5j) reveals that CoS₂ prefers to form single crystals with high crystallinity. To further confirm the elemental composition of the CoS₂/RGO-2 composite, elemental mapping was carried out using the high-angle annular dark-field scanning TEM (HAADF-STEM) (Fig. 5m). The resulting image from elemental mapping shows that the Co and S distributions are quite uniform with a trapezoidal shape, suggestive of CoS₂ nanoparticles, whereas C is also well distributed throughout, indicative of RGO homogeneously wrapping the CoS₂ nanoparticles.

The schematic of the structure of the CoS₂/RGO composite electrode is shown on the right in Scheme 1. In view of the CoS₂ nanoparticles growing directly on the Ni foam and being simultaneously wrapped by RGO sheets, ensuring that CoS₂ nanoparticles get electrons from all directions, the polarization phenomenon of the electrode could be further alleviated.³⁹ Hence, such a highly efficient and stable mixed (electron and OH⁻¹) conducting network could provide superior electronic and electrolyte contact between the CoS₂ nanoparticles and facilitate fast electron transport during the charge/discharge processes, which would be more effective in enhancing the electrochemical properties of CoS₂/RGO in comparison with CoS₂.

Electrochemical tests for positive electrode materials

The CV curve of the Ni foam substrate is similar to a straight line (Fig. 6a), confirming the electrochemical performance of Ni foam. Moreover, the GCD curve of the Ni foam (Fig. 6d) is also in good agreement with the CV test. Therefore, the contribution of Ni foam is negligible for the total electrode. In order to obtain the optimum CoS₂/RGO composite electrode, the effects of the mass ratios of CoCl₂·6H₂O and GO on the electrochemical properties of the composites were studied. The electrochemical performances of the RGO, CoS₂ and different CoS₂/RGO composite electrodes were firstly investigated by the CV method. Fig. 6a shows that the CV curves are measured under the potential window from −0.1 to 0.6 V in 2 M KOH at a scan rate of 10 mV s⁻¹ in a three-electrode system. Clearly, the CV curve of RGO is close to a quasi-rectangular shape, differing from the CV curves of CoS₂ and CoS₂/RGO composite electrodes. In contrast, all the CV curves of the CoS₂ and CoS₂/RGO composite electrodes consist of a couple of strong redox peaks, indicating that the capacitance characteristics are mainly governed by faradaic redox reactions. The mechanism for charge-storage of CoS₂ based electrodes in an alkaline solution might be explained by the following redox reaction:^16,22 CoS₂ + 2OH⁻ ↔ CoS₂O + H₂O + 2e⁻.

Fig. 6 (a) CV curves of supercapacitors based on Ni, RGO, CoS₂ and CoS₂/RGO composite electrodes with different ratios at a scan rate of 10 mV s⁻¹. (b) CV curves of a supercapacitor based on the CoS₂/RGO-2 composite electrode at different scan rates ranging from 2 to 100 mV s⁻¹. (c) The variation of the resultant anodic peak current density as a function of the scan rate. (d) GCD curves of supercapacitors based on Ni, RGO, CoS₂ and different CoS₂/RGO composite electrodes at a constant current density of 2 A g⁻¹. (e) GCD curves of a supercapacitor based on the CoS₂/RGO-2 composite electrode at different current densities. (f) C_s of a supercapacitor based on the CoS₂/RGO-2 composite electrode at different current densities.

The areas of the curves increase with increasing the mass ratios of CoCl₂·6H₂O and GO from 1 : 3 to 1 : 2. When the mass ratios of CoCl₂·6H₂O and GO further increase to 1 : 1, the area of the curves decrease; as is well known, the electrode specific capacitance is proportional to the area of the curves. On the other hand, the CoS₂/RGO-2 composite electrode has a higher electrode specific capacitance than of CoS₂ and other CoS₂/RGO composite electrodes at the same scan rate. Fig. 6b exhibits the typical CV curves of the CoS₂/RGO-2 composite electrode-based supercapacitors with various sweep rates ranging from 5 to 100 mV s⁻¹. It is seen that all the CV shapes have a pair of obvious peaks, which is remarkably different from the closely ideal rectangular CV shape for an electric double-layer capacitor.³³ Moreover, all the redox peaks are symmetrical with different scan rates, implying a desirable reversibility of the redox reaction at/near the CoS₂/RGO electrode surface. Simultaneously, the area and the peak current densities both apparently increase with the increase of the scan rate, possibly resulting from the large surface area of the CoS₂/RGO-2 and the good reversibility of the fast charge–discharge response. To further study the CV characteristics of the CoS₂/RGO-2 composite electrode, the relationship between anodic peak current (i_p) and scan rate (v) can be observed in Fig. 6c. The i_p vs. v^1/2 curve shows a reasonably linear relationship, which indicates that the redox reaction of the CoS₂/RGO-2 electrode is diffusion-limited, as noted in other reports.^36–41 These results demonstrate the excellent electrochemical properties of the CoS₂/RGO-2 composite electrode, such as a good rate capability and remarkable reversibility, which can be ascribed to the large surface area of the wave-like structure of the CoS₂/RGO-2 composite electrode and the fast ionic/diffusion rate within the composite electrode.

Fig. 6d shows the charge/discharge curves of the Ni foam, RGO, CoS₂ and different CoS₂/RGO composite electrodes tested at a constant current density of 2 A g⁻¹. The GCD curve of RGO is highly linear and symmetrical, revealing an ideal capacitive character. In addition to Ni and RGO curves, the shapes of the other GCD curves show that the capacitance mainly results from pseudocapacitance, which is in accordance with the CV measurements. As expected, the CoS₂/RGO-2 composite electrode has a much longer discharging time than pure CoS₂ and other CoS₂/RGO composite electrodes, suggesting that the CoS₂/RGO-2 composite electrode material exhibits much higher specific capacitance values than the other materials. Furthermore, the potential–time profiles exhibit almost symmetrical charge/discharge features, indicating their good pseudocapacitive character. On the other hand, based on their GCD measurements, the C_s of the electrodes can be evaluated through eqn (1). The C_s of RGO, CoS₂ electrode, and CoS₂/RGO-1, 2 and 3 composite electrodes are 140.2, 624.1, 830.0, 930.3 and 679.5 F g⁻¹, respectively. Furthermore, by GCD tests, the equivalent series resistances (ESR) can be calculated based on eqn (2). Their ESR values are 1.2, 0.9, 0.8 and 1.1 Ω, respectively, which demonstrates that the RGO can reduce the ESRs of supercapacitors, leading to the enhancement of C_s. However, when the mass ratio of CoCl₂·6H₂O and GO is 1 : 3, the C_s decreases as the GO content increases, mainly attributed to the production of lower pseudocapacitance from CoS₂. To further explain the rate capability of CoS₂/RGO-2, its GCD curves are measured at different current densities from 2 to 20 A g⁻¹ under an applied potential between 0 and 0.6 V (Fig. 6e). The charge curves have voltage plateaus and are highly symmetric with their corresponding discharge counter parts at various current densities from 2 to 20 A g⁻¹. This demonstrates that the CoS₂/RGO-2 composite electrode-based supercapacitor has a typical pseudocapacitance behavior and a rapid I–V response. The CoS₂/RGO-2 composite electrode shows a high capacitance of 930.3 F g⁻¹ at a current density of 2 F g⁻¹, whereas the C_s remains at 677.9 F g⁻¹ at a current density of 20 A g⁻¹, suggesting that the CoS₂/RGO-2 composite electrode has an excellent electrochemical reversibility. The C_s as a function of applied current density of the CoS₂/RGO-2 composite electrode is exhibited in Fig. 6f. In general, with the increase in the current densities, the C_s gradually decreases. It can be found that the C_s (at 20 A g⁻¹) is still around 72.9% retention of its initial values (at 2 A g⁻¹), revealing the excellent rate capability of the supercapacitor based on the CoS₂/RGO-2 composite electrode. All these results excellently demonstrate the synergistic effect of the combination of CoS₂ and RGO on the electrochemical performance. In comparison with the CoS₂ electrode and other CoS₂/RGO composite electrodes, the CoS₂/RGO-2 composite electrode exhibits the highest specific capacitance, mainly based on three reasons: (i) the RGO prevents the agglomeration of CoS₂ nanoparticles to obtain higher specific surface area between CoS₂/RGO and the electrolyte; (ii) lower resistance facilitates faster electron transport during the charge/discharge processes. (iii) The right amount of RGO not only forms an effective conductive network but also does not sacrifice much of the capacity of CoS₂.^42,48

In order to investigate the charge and ionic transfer characteristics of the electrodes, EIS measurements were carried out. Fig. 7 depicts the Nyquist plots of RGO, the CoS₂ electrode and the CoS₂/RGO-2 composite electrode. Notably, these plots exhibit a depressed arc at the high-frequency region, followed by a straight line at the low-frequency region. The enlargement of the EIS spectra at the high-frequency region and the equivalent circuit proposed to fit their EIS spectra are presented in the inset of Fig. 7, where R_e is the internal resistance, R_ct is the faradaic interfacial charge-transfer resistance, C_dl represents the electric double-layer capacitance, and W is the Warburg resistance.⁴³ R_e, which is attributed to the ionic resistance of the electrolyte, the intrinsic resistance of the electrode material, and the contact resistance at the interface of the electrode material, is counted from the point of intersection with the x-axis in the high frequency region. R_ct, which is related to the process taking place at the electrode/electrolyte, is calculated from the span of the single semi-circle along the x-axis from the high to low frequency region. As illustrated in Fig. 7, the inner resistance (R_e) of the CoS₂/RGO-2 composite electrode (0.6 Ω) is lower than that of the CoS₂ electrode (0.9 Ω), but higher than that of the RGO electrode (0.5 Ω). Moreover, the CoS₂/RGO-2 composite electrode also has smaller charge transfer resistance (R_ct, 0.2 Ω for CoS₂/RGO-2 and 0.5 Ω for CoS₂, respectively), meaning the CoS₂/RGO-2 composite electrode has easier electron transport because of the presence of RGO.

Fig. 7 Nyquist plots of supercapacitors based on RGO, the CoS₂ electrode and the CoS₂/RGO-2 composite electrode.

Asymmetric supercapacitor

Considering the abovementioned results, the CoS₂/RGO-2 composite electrode was chosen to fabricate an asymmetric supercapacitor to further assess its application value. Fig. 8a shows CV curves of the as-assembled CoS₂/RGO-2//AC asymmetric supercapacitor at different voltage windows in 2 M KOH aqueous electrolyte solution at 20 mV s⁻¹. It can be clearly seen that the assembled asymmetric supercapacitor displays distinct peaks beginning with 1.0 V, and the peaks gradually become stronger while the area also increases with the increase of the potential window. In Fig. 8b, the specific capacitance for the asymmetric supercapacitor is calculated to be as high as 143.0 F g⁻¹ (at the operation potential of 1.6 V) and 30.4 F g⁻¹ (at the operation potential of 0.8 V), meaning that the stored energy can be significantly improved based on the equation of the energy density. As a result, the overall performance of the supercapacitor is remarkably improved. However, for potential windows >1.8 V, the CV curves display a distortion and a slight hump around 1.8 V, indicating some irreversible reactions when the potential window is higher than 1.8 V,²⁵ while there is clearly a serious limitation related to H₂ evolution at the negative electrode. In our subsequent measurements, we chose a potential window from 0 to 1.6 V in 2 M KOH solution to further investigate the overall electrochemical characteristics of the as-fabricated asymmetric supercapacitor.

Fig. 8 (a) CV curves of the CoS₂/RGO-2//AC asymmetric supercapacitor measured at different potential windows in 2 M KOH aqueous solution at a scan rate of 10 mV s⁻¹. (b) C_cell of the asymmetric supercapacitor as a function of potential window.

Fig. 9 exhibits the electrochemical performance of the CoS₂/RGO-2//AC asymmetric two-electrode supercapacitor. Fig. 9a presents the CV curves of the CoS₂/RGO-2//AC asymmetric supercapacitor tested at various scan rates from 5 to 100 mV s⁻¹ in the potential range of 0–1.6 V. A pair of distinct peaks can be observed in the CV curves of the CoS₂/RGO-2//AC electrode. In addition, with the increase in the scan rates, the shapes of the CV curves are retained well.

Fig. 9 (a) CV curves of the CoS₂/RGO-2//AC asymmetric supercapacitor at different scan rates. (b) GCD curves and (c) C_cell of the CoS₂/RGO-2//AC asymmetric supercapacitor at different current densities. (d) Cyclic performances of the AC//AC symmetric supercapacitor, and the CoS₂//AC and CoS₂/RGO-2//AC asymmetric supercapacitor at current density of 10 A g⁻¹.

Furthermore, galvanostatic discharge curves at various charge/discharge current densities were obtained from Fig. 9b to further assess the rate capability of the asymmetric supercapacitor. As shown in Fig. 9b, the GCD curves of the CoS₂/RGO-2//AC asymmetric supercapacitor give an excellent linear variation with potential, suggesting superior electrochemical reversibility and capacitive behavior. Fig. 9c presents calculations of the C_cell based on the discharge curves of the CoS₂/RGO-2//AC asymmetric supercapacitor. C_cell is 129.3, 117.5, 107.9, 100.2, 96.6 and 91.6 F g⁻¹ at current densities of 1, 3, 5, 10, 15 and 25 A g⁻¹, respectively. This is substantially higher than the values reported previously for hierarchical porous NiO//AC (38 F g⁻¹ at 1 mV s⁻¹),⁴⁴ MnOOH/RGO//AC (74.4 F g⁻¹ at 1 A g⁻¹),⁷ and Ni₃S₂/MWCNTs//AC (55.8 F g⁻¹ at 1 A g⁻¹).⁴⁵ The results also show the decrease of C_cell with the increase of current density. However, the asymmetric supercapacitor reveals high rate capability because the C_cell still remains at 91.6 F g⁻¹ at a current density of 25 A g⁻¹ (70.8% retention at initial current density of 1 A g⁻¹), which is ascribed to high rate properties of both CoS₂/RGO-2 composite and AC.

A long-term cycle stability of the as-fabricated asymmetric supercapacitor was conducted by repeating the GCD test at current density of 10 A g⁻¹ over 6000 cycles. Fig. 9d shows that the overall specific capacitance can still be maintained at about 90% of the initial available capacitance after 6000 consecutive charge/discharge tests, which is comparable to that of AC//AC (92% after 6000 cycles) and CoS₂//CoS₂ (83% after 6000 cycles). Clearly, the asymmetric supercapacitor shows good stability towards long time usages. AC with excellent cycling stability^46,47 as well as the CoS₂/RGO-2 with synergistic effect both contribute to the superior cycling stability.

Power density and energy density are the two key parameters characterizing the performance of the electrochemical supercapacitors.^49,50 Although it is a challenge to compare the performances of all types of supercapacitors due to different measurement conditions such as potential window, charge–discharge rates and material mass loadings, a rough comparison between the developed electrode and documented work can still be made. Ragone plots (Fig. 10) were made based on GCD data calculated by eqn (4) and (5) to illustrate the energy and power properties of CoS₂/RGO-2//AC asymmetric supercapacitors. The asymmetric device presents the maximum energy density of 45.7 W h kg⁻¹ at a power density of 797.0 W kg⁻¹ and maintains a high energy density of 21.8 W h kg⁻¹ at a high power density of 16.4 kW kg⁻¹. These values are significantly improved compared to those of the CoS₂/RGO-2//CoS₂/RGO-2 and AC//AC symmetric supercapacitors and a CoS₂//AC asymmetric supercapacitor. The improved energy and power densities of the constructed CoS₂/RGO-2//AC asymmetric supercapacitor are mainly attributed to two reasons: on the one hand, the synergistic effect between the positive and negative electrodes to form high cell specific capacitance; on the other hand, the wide operation voltage window (1.6 V). A table showing comparisons of literature values of capacitance, working voltage window, stability, energy density and power density of supercapacitors based on CoS₂ and CoS₂/Graphene is shown in ESI (Table 1†).

Fig. 10 Ragone plots of symmetric and asymmetric supercapacitors based on AC, CoS₂/RGO-2 and CoS₂/RGO-2//AC.

Conclusions

In conclusion, we demonstrated a general paradigm for fabricating 3D CoS₂ electrodes and CoS₂/RGO composite electrodes by directly growing active materials on a Ni foam with strong adhesion for high-performance supercapacitors by a facile hydrothermal approach. When the mass ratio of CoCl₂·6H₂O and GO is 1 : 2, the CoS₂/RGO-2 composite electrode exhibits the highest capacitance of 930.3 F g⁻¹ at 2 A g⁻¹ and shows excellent rate capability. This could probably be attributed to the positive synergistic effect between RGO and CoS₂, the exclusion of insulating binder, direct contact with the current collector (Ni foam) as well as the facile electron conduction along the novel large-scale wave-like structure. More importantly, the asymmetric supercapacitor device has been fabricated using the CoS₂/RGO-2 composite electrode as the positive electrode and AC as the negative electrode. This can result in remarkable electrochemical performance, including the C_cell of 91.6 F g⁻¹ at a high current density of 25 A g⁻¹ in a wide voltage window of 1.6 V, as well as outstanding rate capability and superior cyclic durability (90% retention after more than 6000 charge/discharge cycles). Furthermore, the asymmetric device delivers a maximum energy density of 45.7 W h kg⁻¹ at a power density of 797.0 W kg⁻¹. It is believed that this strategy provides a perfect platform for further electrochemical applications in high energy-power density storage systems.

Acknowledgements

This study was supported financially by the National Natural Science Foundation of China (No. 50974045).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25665h

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