Fabrication of 2D/2D nanosheet heterostructures of ZIF-derived Co3S4 and g-C3N4 for asymmetric supercapacitors with superior cycling stability

Weiwei Li a, Youjing Li a, Cui Yang b, Qingxiang Ma c, Kai Tao *acd and Lei Han *ad
aSchool of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China. E-mail: taokai@nbu.edu.cn; hanlei@nbu.edu.cn
bInstitute of Drug Discovery Technology, Ningbo University, Ningbo, Zhejiang 315211, China
cState Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China
dState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China

Received 7th July 2020 , Accepted 18th August 2020

First published on 18th August 2020


Metal sulfides with high activity are favorable electrode materials for supercapacitors. However, their relatively inferior electronic conductivity and poor stability in alkaline electrolyte solutions impede their applications. To overcome these drawbacks, herein, 2D/2D nanosheet heterostructures of Co3S4 and g-C3N4 have been successfully fabricated by a facile method that involves the in situ growth of 2D Co-based zeolitic imidazolate framework (Co-ZIF-L) crystals on g-C3N4 nanosheets followed by subsequent sulfurization. The as-prepared Co3S4/g-C3N4-10 exhibits a largely enhanced specific capacity (415.0 C g−1 at 0.5 A g−1) in comparison with solitary g-C3N4 (18.9 C g−1) and Co3S4 (194.4 C g−1) derived from Co-ZIF-L. Furthermore, it also displays good rate capability (54.5% retention at 10 A g−1). The asymmetric supercapacitor fabricated from Co3S4/g-C3N4-10 and activated carbon electrodes exhibits an outstanding energy density of 35.7 W h kg−1 at a high power density of 850.2 W kg−1. Most importantly, the asymmetric supercapacitor demonstrates an ultrahigh cycling durability with only 1.9% capacitance loss after 10[thin space (1/6-em)]000 cycles at 10 A g−1. This superior electrochemical performance can be attributed to the unique 2D/2D nanosheet heterostructures providing rich active sites, short ion diffusion pathways, fast charge transfer as well as improved conductivity and mechanic stability. This work may pave the way for a rational design of the heterostructures of metal sulfides and g-C3N4 for electrochemical energy storage devices with a long cycling lifespan.


1. Introduction

With increasing environmental problems caused by the consumption of a large amount of fossil resources, huge research efforts have been initiated on the conversion and storage of clean and renewable energy sources.1 Due to the advantages of large power density, rapid charge/recharge capability and long cycling lifespan, supercapacitors have stood out among numerous energy storage devices and attracted widespread attention.2 However, because of their low energy density, the large-scale application of supercapacitors has been hindered. The energy density of supercapacitors mainly depends on electrode materials. Therefore, exploring new electrode materials and rationally designing delicate nanostructures for existing materials seem to be effective methods to overcome this drawback.3

Taking all transition metal sulfide electrode materials into consideration, cobalt sulfide has been widely studied due to its higher electrical conductivity, better mechanical strength and richer redox reactions.4 Particularly, cobalt sulfide based electrode materials synthesized by using zeolitic imidazolate frameworks (ZIFs) as sacrificial templates or precursors have aroused great research interest recently. For example, Lou et al. constructed a NixSy@CoS double-shelled polyhedron nanocage from a single ZIF-67 template as the supercapacitor electrode and electrocatalyst, which showed excellent electrochemical performance.5 However, the metal sulfides have poor electronic conductivity and cycling stability in alkaline electrolytes, which have greatly limited their electrochemical performance and practical applications. Carbon material is recognized as a good conductive agent.6–8 Combining metal sulfides with carbon materials can not only improve electronic conductivity but also maintain the morphology of nanostructures during repeated cycles, thereby improving electrochemical performance significantly.9,10 However, there are still some problems with the carbon material itself, such as unstable electrical conductivity, low specific capacitance and energy density.11,12 The incorporation of nitrogen in carbon materials can effectively solve these problems.13,14 A graphite carbonitride (g-C3N4) polymer with a graphite-like sp2-bonded C–N structure has emerged as a hotspot material owing to its easy synthesis, low cost, fascinating structures and exceptional performance.15–19 Therefore, the composite of g-C3N4 and metal sulfide would display enhanced electrochemical performance. For example, Guo et al. synthesized a porous g-C3N4 nanosheet (2D)/NiCo2S4 nanoparticle (0D) composite with a large surface area, rich active sites and desirable conductivity, which exhibited a high specific capacitance of 1557 F g−1 at a current density of 1 A g−1 along with good cyclic performance (92.6% capacitance retention after 10[thin space (1/6-em)]000 cycles).20 Considering the unique properties and advantages of 2D materials, it is highly desirable to construct a 2D/2D heterostructure of metal sulfide and g-C3N4 as electrode material for supercapacitors. However, such heterostructure has rarely been achieved by a cost-effective and simple method.

Herein, we first report a 2D/2D nanosheet heterostructure of Co3S4 and g-C3N4 fabricated by a facile two-step synthesis including the in situ growth of Co-ZIF-L on g-C3N4 at room temperature followed by sulfurization. Taking the synergistic advantages of 2D structures, MOF and g-C3N4, Co3S4/g-C3N4 exhibits a high specific capacitance of 415.0 C g−1 at 0.5 A g−1, which is superior to that of blank g-C3N4 (18.9 C g−1) or Co3S4 (194.4 C g−1) derived from direct sulfurization of Co-ZIF-L. Besides, Co3S4/g-C3N4 also shows an enhanced cycling stability compared to solitary Co3S4. Moreover, an asymmetric supercapacitor (ASC) fabricated from Co3S4/g-C3N4-10 and activated carbon (AC) displays outstanding energy density (35.7 W h kg−1 at a high power density of 850.2 W kg−1) and remarkably high cyclic durability (98.1% of capacitance retention after 10[thin space (1/6-em)]000 cycles), demonstrating great promise for electrochemical energy storage devices.

2. Experimental section

2.1. Chemicals

All solvents and reagents were of analytical grade that could be used without purification. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole (2-MeIm), thioacetamide (TAA) and absolute ethanol were provided by Sinopharm Chemical Reagent Co., Ltd. Urea (CH4N2O) was obtained from Shanghai Macklin Biochemical Co., Ltd. The deionized water used in all experiments was produced by a water purification system.

2.2. Synthesis of g-C3N4

10 g of urea was ground in a mortar for half an hour. The resultant powder was placed in a porcelain boat, and heated up to 550 °C in a tubular furnace under nitrogen atmosphere at a ramp rate of 5 °C min−1 and kept for 4 h. After the furnace was cooled to room temperature, the light yellow g-C3N4 was obtained.

2.3. Synthesis of a Co-ZIF-L/g-C3N4 nanosheet heterostructure

10 mg of the as-prepared g-C3N4 was dispersed in 40 mL of deionized water under ultrasonication for 2 h to exfoliate the bulk g-C3N4 into 2D g-C3N4 nanosheets. Then, 1.313 g of 2-MeIm was added into the above solution, and the mixture was sonicated for 1 h to form solution A. 0.582 g of Co(NO3)2·6H2O was dispersed in 40 mL of deionized water to form solution B. Then, two solutions were combined, and the resulting mixture was stirred for 10 min and then kept static at room temperature for 4 h. The Co-ZIF-L/g-C3N4 nanosheet heterostructure was collected by centrifugation, rinsed with deionized water three times, and finally dried at 60 °C for 24 h. Co-ZIF-L was fabricated through the same process but without adding g-C3N4 powder.

2.4. Synthesis of a Co3S4/g-C3N4 nanosheet heterostructure

The obtained Co-ZIF-L/g-C3N4 (80 mg) was soaked into an ethanol solution (40 mL) containing 0.12 g of TAA. The mixture was then transformed into a Teflon-lined autoclave and heated at 120 °C for 4 h. After the reaction, the product was collected by centrifugation, rinsed with ethanol three times and dried in an oven at 60 °C for 24 h. The sample was denoted as Co3S4/g-C3N4-10. For comparison, Co3S4, Co3S4/g-C3N4-5, Co3S4/g-C3N4-15 and Co3S4/g-C3N4-20 were also synthesized by the same procedure except that the amounts of g-C3N4 were adjusted to 0, 5, 15 and 20 mg, respectively.

2.5. Characterization

X-ray diffraction (XRD) patterns were collected on a Bruker AXS D8 Advance diffractometer using a Cu Kα (1.5406 Å) radiation operating at a voltage of 40 kV and a current of 40 mA. Fourier transformation infrared spectra (FTIR) were acquired on a NICOLET 6700 spectrometer. Raman spectra were acquired on a Renishaw inVia spectrometer. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) spectra were obtained from a field emission scanning electron microscope (FESEM, Hitachi S-4800). Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images, selected area electron (SAED) pattern, and elemental mapping were acquired on a FEI Tecnai TF20 transmission electron microscope operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo Scientific ESCA-Lab-200i-XL spectrometer (Waltham, MA) with an Al source. The porosity of the samples was analyzed by N2 physisorption (Micrometrics ASAP-2020 M). The surface area was determined by the multiple Brunauer–Emmett–Teller (BET) method, and the pore diameter distribution was measured by the Barrett–Joyner–Halenda (BJH) method using the desorption branch of the isotherm.

2.6. Electrochemical measurements

Electrochemical measurements were conducted on a three-electrode system at room temperature. A platinum foil and a saturated calomel electrode (SCE) acted as the counter and reference electrodes, respectively. The working electrodes were prepared by mixing the active materials (80 wt%), acetylene back (10 wt%), polyvinylidene fluoride (PVDF, 10 wt%) with suitable 1-methyl-2-pyrrolidinone (NMP) to form a slurry, which was coated onto the nickel foam (NF, 1 cm × 1 cm) substrate. The resulting electrode was pressed at 10 MPa and dried at 60 °C for 24 h before use. The active materials attached to the working electrode were about 2.0 mg. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured by a CHI660E electrochemical workstation (Shanghai Chenhua) in a 3 M KOH electrolyte. The specific capacity (Cs, C g−1) was calculated by the following equation:
 
image file: d0dt02400g-t1.tif(1)
where I (A) is the discharge current, Δt (s) is the discharge time, and m (g) is the mass of active materials.

The asymmetric supercapacitor (ASC) was assembled by using the Co3S4/g-C3N4-10 electrode as the positive electrode and an activated carbon (AC) as the negative electrode in 3 M KOH solution. In order to achieve the charge balance between the positive and negative electrodes, the optimal mass ratio of active materials on two electrodes was calculated by the following equation:

 
image file: d0dt02400g-t2.tif(2)
where C+ (C g−1) is the specific capacitance of Co3S4/g-C3N4-10, C (F g−1) is the specific capacitance of AC, and ΔV (V) is the potential window of AC.

The energy density (E) and power density (P) were calculated on the basis of the total mass of active materials on both electrodes according to the following equations:

 
image file: d0dt02400g-t3.tif(3)
 
image file: d0dt02400g-t4.tif(4)
where C (F g−1) is the specific capacitance, ΔV (V) is the operating voltage window, and Δt (s) is the time for full discharge of ASC.

3. Results and discussion

The preparation process of the Co3S4/g-C3N4 2D/2D nanosheet heterostructure composite material is schematically illustrated in Scheme 1. First, g-C3N4 was prepared by thermal decomposition of urea at 550 °C in an inert atmosphere. Then, bulk g-C3N4 was exfoliated to layered g-C3N4 nanosheets with the assistance of sonication. Secondly, the Co-ZIF-L/g-C3N4 composite was synthesized via in situ assembly of cobalt ions and 2-MeIm on g-C3N4 nanosheets in aqueous solution at room temperature. Finally, through a simple solution sulfurization process using TAA as the sulfur source, Co3S4/g-C3N4 was obtained. A Co3S4 nanosheet was also synthesized by direct sulfurization of Co-ZIF-L for the purpose of comparison.
image file: d0dt02400g-s1.tif
Scheme 1 Schematic illustration showing the construction of a Co3S4/g-C3N4 nanosheet heterostructure.

The surface morphology and structure of g-C3N4, Co-ZIF-L/g-C3N4 and Co3S4/g-C3N4 were characterized by SEM. As shown in Fig. 1a and b, the bulk g-C3N4 derived from urea presents chiffon-like ripples and wrinkles, which is consistent with previous work.21 After being exfoliated by sonication and coming into contact with the synthesis solution of Co-ZIF-L, the g-C3N4 sheet is wrapped firmly by Co-ZIF-L crystals with different orientations, forming a 2D/2D nanosheet heterostructure (Fig. 1c). The enlarged view in Fig. 1d reveals that the Co-ZIF-L nanosheets present a smooth surface and large lateral size with a thickness of ∼360 nm. It can be seen that the overall 2D/2D nanosheet heterostructure of Co-ZIF-L/g-C3N4 is preserved after sulfurizing with TAA (Fig. 1e), and there is an intimate interaction between Co3S4 and g-C3N4, which is favorable for boosting the electrochemical performance of the composite. Compared with pristine Co-ZIF-L, the Co3S4 nanosheets are slightly curled, and their thickness is reduced to 190 nm (Fig. 1f). For comparison, the SEM images of the Co-ZIF-L and Co3S4 nanosheets are shown in Fig. S1.


image file: d0dt02400g-f1.tif
Fig. 1 SEM images of (a and b) g-C3N4, (c and d) Co-ZIF-L/g-C3N4 and (e and f) Co3S4/g-C3N4.

The crystal phase of g-C3N4 and the Co3S4/g-C3N4 composite was checked by XRD. As shown in Fig. 2, two obvious diffraction peaks are located around 12.6 and 27.3°, corresponding to the (100) and (002) crystal planes of pure g-C3N4.22,23 The XRD pattern of the Co-ZIF-L/g-C3N4 precursor (Fig. S2) matches well with that of simulated Co-ZIF-L, indicating the successful fabrication of the Co-ZIF-L/g-C3N4 heterostructure. For the Co3S4 and Co3S4/g-C3N4 composites, the main peaks observed at 16.5, 31, 47.3 and 55.0° are assigned to the (111), (311), (422) and (440) planes of cubic Co3S4 (JCPDS No. 42-1448).24 Two minor reflections at 33.5 and 35.2° belong to the (100) and (002) planes of CoS.25 The weak peak located at 20.3° can be ascribed to the (080) plane of sulfur (JCPDS No. 24-1251), arising from TAA decomposition.26 It should be noted that the characteristic peaks of g-C3N4 are hardly discernible in the Co3S4/g-C3N4-10 sample due to the low content of g-C3N4. With the increase of g-C3N4 loading, the peak located at 27.3° gradually increases. The above results show that the Co3S4/g-C3N4 heterostructure composites are successfully fabricated.


image file: d0dt02400g-f2.tif
Fig. 2 XRD pattern of g-C3N4, Co3S4, Co3S4/g-C3N4-10 and Co3S4/g-C3N4-20.

The successful preparation of the Co3S4/g-C3N4 heterostructure composites was further proved by FTIR and Raman measurements. The FTIR spectra of g-C3N4, Co3S4 and Co3S4/g-C3N4-10 are shown in Fig. 3a. The FTIR spectrum of Co3S4/g-C3N4-10 displays the superimposition of the characteristic peaks of both g-C3N4 and Co3S4. The peaks in the range of 1250 to 1600 cm−1 are associated with CN heterocycles, while the bands at 1108 and 621.4 cm−1 are attributed to the stretching vibrations of cobalt–sulphur bonds.27,28Fig. 3b compares the Raman spectra of Co3S4/g-C3N4-10 and Co3S4. The Raman spectra of both Co3S4/g-C3N4-10 and Co3S4 display bands at 192.3, 464.5, 507.4, 605.5 and 667.9 cm−1, which belong to the Ag, Eg, F2g, F2g and A1g modes, characteristic of Co3S4.29 Besides, Co3S4/g-C3N4-10 presents an additional broad band at 1100–1700 cm−1, which can be attributed to the C–N stretching vibrations, further proving the successful formation of the Co3S4/g-C3N4 heterostructure.27


image file: d0dt02400g-f3.tif
Fig. 3 (a) FTIR spectra of g-C3N4, Co3S4, Co3S4/g-C3N4-10 and (b) Raman spectra of Co3S4, Co3S4/g-C3N4-10.

The XPS measurement was conducted to analyze the valence state and the surface chemical compositions of Co3S4/g-C3N4-10, and the results are shown in Fig. 4. The C 1s spectrum of Co3S4/g-C3N4 can be fitted to four major peaks at 284.2, 284.6, 285.4 and 288.4 eV, corresponding to four different bonding states of carbon. The peak at 284.6 eV corresponds to the sp2 C–C bond, and the peaks at 284.2, 285.4 and 288.4 eV are assigned to the coordination of C[double bond, length as m-dash]C, C–N and C–O, respectively.30 In Fig. 4b, the N 1s spectrum presents three peaks at 398.3, 399.8 and 404.0 eV. The most obvious peak at 398.3 eV can be indexed to C[double bond, length as m-dash]N–C. The other two weak peaks can be assigned to the tertiary N bonded to carbon atoms and the π excitation.31 High resolution XPS spectrum of Co 2p (Fig. 4c) can be fitted with two spin–orbit doublets, which are characteristic of Co2+ and Co3+. The strong peaks located around 779.8 and 781.8 eV of Co 2p3/2 correspond to Co3+ and Co2+, respectively. Similarly, the peaks at the higher binding energy around 793.7 and 796.7 eV are attributed to Co3+ and Co2+ of 2p1/2 core level, respectively. Two additional peaks at 786.4 and 803.0 eV (satellite) further indicate the presence of both Co2+ and Co3+. The S 1s XPS spectrum of the sample is presented in Fig. 4d. The peak at 161.5 eV (S 2P3/2) is associated with the metal–sulfur bonds. In addition, two peaks centered at 162.8 and 164.0 eV correspond to S 2p1/2 of Co3S4. The satellite peak observed at 168.9 eV can be assigned to oxidized S species due to contact with air.32,33


image file: d0dt02400g-f4.tif
Fig. 4 High-resolution XPS spectra of Co3S4/g-C3N4-10 in (a) C 1s, (b) N 1s, (c) Co 2p, and (d) S 2p regions.

N2 adsorption–desorption measurements were carried out to understand the porosity and texture properties of the samples. The isotherms of g-C3N4, Co3S4 and Co3S4/g-C3N4-10 are presented in Fig. 5a. It can be seen that all samples present type IV isotherms, which indicate the presence of mesopores (2–50 nm).34 Based on the isotherms, the estimated BET surface area of Co3S4/g-C3N4-10 (28.7 m2 g−1) is larger than that of Co3S4 (16.3 m2 g−1). Therefore, it is inferred that Co3S4/g-C3N4-10 possesses more electrochemically active sites than Co3S4. The pore size distributions of the samples are shown in Fig. 5b. Co3S4/g-C3N4-10 and Co3S4 display bimodal pore size distributions with a peak at 5.3 nm and a broad peak centered around 60 nm. The bimodal porous structure is conducive to the exposure of more active sites and diffusion of electrolyte ions.


image file: d0dt02400g-f5.tif
Fig. 5 (a) Nitrogen absorption–desorption isotherms and (b) pore size distribution curves of Co3S4/g-C3N4-10, Co3S4 and g-C3N4.

In order to learn more about the Co3S4/g-C3N4-10 composite, TEM was utilized to observe its detailed structure. The 2D/2D nanosheet heterostructure of Co3S4/g-C3N4-10 is unambiguously confirmed by Fig. 6a. The Co3S4 nanosheet displays a porous leaf-like structure and is randomly deposited on the g-C3N4 nanosheet. The HRTEM image (Fig. 6b) shows obvious lattice fringes with an interplanar spacing of 0.285 nm, corresponding to the (311) crystal plane of Co3S4. The SAED pattern in Fig. 6c suggests that the sample is polycrystalline. Furthermore, EDS (Fig. S3) suggests the co-existence of C, N, Co and S, and the elemental mappings (Fig. 6d) confirm the homogenous distribution of these elements in the Co3S4/g-C3N4-10 heterostructure.


image file: d0dt02400g-f6.tif
Fig. 6 (a) TEM image, (b) HRTEM, (c) SAED and (d) elementary mapping of Co3S4/g-C3N4-10.

To evaluate the supercapacitive performance of g-C3N4, Co3S4 and Co3S4/g-C3N4 composites, the as-prepared electrode materials were tested in a three-electrode cell using 3 M KOH as the electrolyte. Fig. 7a shows the comparative CV curves of Co3S4, g-C3N4 and Co3S4/g-C3N4 composites at the same scan rate of 50 mV s−1. All CV curves show distinct redox peaks, indicating that the charge storage is associated with faradaic reactions. It can be found that the CV curve of the Co3S4/g-C3N4 composites have a larger integral area than that of Co3S4 and g-C3N4, implying that the Co3S4/g-C3N4 composites display higher charge storage capability. Among these Co3S4/g-C3N4 composites, Co3S4/g-C3N4-10 shows the highest charge storage capability. Thus, Co3S4/g-C3N4-10 was studied in detail by CV curves (Fig. 7b) at various scan rates and GCD curves (Fig. 7c) at different current densities. As the scan rate increases from 5 to 100 mV s−1, the peak current increases accordingly, and the profile of the Co3S4/g-C3N4-10 composite is well preserved, indicating that the faradaic reactions are highly reversible. The reversible faradaic reactions in the alkaline electrolyte over the Co3S4/g-C3N4-10 electrode can be expressed as follows:24

 
Co3S4 + OH ↔ Co3S4(OH) + e(5)
 
Co3S4OH + OH ↔ Co3S4O + H2O + e(6)


image file: d0dt02400g-f7.tif
Fig. 7 (a) CV curves of Co3S4, g-C3N4 and Co3S4/g-C3N4 composites at a scan rate of 50 mV s−1. (b) CV curves of Co3S4/g-C3N4-10 at various scan rates. (C) GCD curves of Co3S4/g-C3N4-10 at different current densities. (d) Specific capacities of Co3S4, g-C3N4 and Co3S4/g-C3N4 composites as a function of current density. (e) Nyquist plots of Co3S4, g-C3N4 and Co3S4/g-C3N4-10 along with an equivalent circuit. (f) Cycling performance of Co3S4 and Co3S4/g-C3N4-10 at a current density of 10 A g−1.

The GCD curves of Co3S4/g-C3N4-10 present distinct plateaus, further confirming the faradaic characteristic of the electrode. Moreover, the GCD curves are symmetric, demonstrating high coulombic efficiency and high reversibility of the electrode. For comparison, the CV and GCD curves of the g-C3N4, Co3S4 and Co3S4/g-C3N4 composites are shown in Fig. S4–S6. The specific capacity of the electrodes calculated from GCD curves by eqn (1) is shown in Fig. 7d. As expected, the Co3S4/g-C3N4 composites possess an enhanced capacity compared with g-C3N4 or Co3S4, and Co3S4/g-C3N4-10 exhibits the highest electrochemical activity. The specific capacity of Co3S4/g-C3N4-10 is 415.0 C g−1 at 0.5 A g−1, which is nearly two times higher than that of Co3S4 (194.4 C g−1) and twenty times that of g-C3N4 (18.9 C g−1), demonstrating a good synergy effect between Co3S4 and g-C3N4. Co3S4/g-C3N4-10 also exhibits superior rate capability; it retains 54.5% capacitance after a 20-fold increase in current density. Compared with other cobalt sulfide based electrode materials, Co3S4/g-C3N4-10 exhibits favorable capacity (Table S1). It should be noted that the electrochemical activity increases first and then decreases with the increase in g-C3N4 content because inadequate or excessive g-C3N4 will result in the aggregation of nanosheets. EIS was employed to study the redox reaction kinetics and ion diffusion. The Nyquist plots of electrodes with equivalent circuit diagram are shown in Fig. 7e. Generally, the Nyquist plot consists of a semicircle and a sloped line. The intercept with the Z′ axis and the diameter of the semicircle represent the equivalent series resistance (Rs) and the charge transfer resistance (Rct), respectively. Notably, the Rs (0.64 Ω) and Rct (0.52 Ω) of Co3S4/g-C3N4-10 are smaller than those of Co3S4 (Rs = 1.07 Ω, Rct = 0.88 Ω), revealing better electronic conductivity and faster charge transfer kinetics of Co3S4/g-C3N4-10. In the low-frequency region, the steep line implies small Warburg impedance and a rapid diffusion of ions. The electrochemical durability of the electrodes was evaluated by repeated GCD cycles at a current density of 10 A g−1, as shown in Fig. 7f. It can be observed that the Co3S4/g-C3N4-10 electrode displays better cycling stability than Co3S4. After 5000 cycles, the heterostructure composite still retains 75.6% of its initial capacitance, while the capacitance retention is only 61.9% for the Co3S4 electrode.

The charge storage mechanism was analysed by using the following formula:35

 
i = b(7)
where i is the current density, ν is the scan rate, and a and b are constants. The slope of the fitted linear line (log(i) vs. log(ν)) is b. When the b-value equals 1, the charge storage is controlled by a capacitive process, while the b-value equals 0.5, indicating diffusion-controlled charge storage. As shown in Fig. 8a, the b-values are 0.53 and 0.60 for the anodic and cathodic processes, indicating that the charge storage is dominated by diffusion. The contributions of capacitive (k1ν) and diffusion-controlled (k2ν1/2) processes to the total current (i) are described as follows:
 
i = k1ν + k2ν1/2(8)


image file: d0dt02400g-f8.tif
Fig. 8 (a) The logarithm of peak current vs. the logarithm of scan rate. (b) The relative contributions of the capacitive and diffusion-controlled processes.

By linear fitting i/ν1/2vs. k1ν1/2, k1 and k2 are obtained from the slope and intercept of the fitted linear line. Fig. S7 presents the representative CV curve at 10 mV s−1, showing 62.6% capacitance contributed by a diffusion-controlled process. However, the contributions of the diffusion-controlled process decline with the increase in the scan rate (Fig. 8b), because there is not enough time for electrolyte ions to diffuse into the inner electrode materials.

To further explore the potential applications of Co3S4/g-C3N4-10, a Co3S4/g-C3N4-10//AC ASC device was assembled in a 3 M KOH electrolyte. The electrochemical performance of individual AC in a three electrode system is shown in Fig. S8. Based on the charge balance rule, 1.5 mg of Co3S4/g-C3N4-10 and 5.3 mg of AC are required for the fabrication of the ASC device corresponding to a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]3.5 according to eqn (2). Fig. 9a shows the CV curves of the individual Co3S4/g-C3N4-10 and AC electrodes at a sweeping rate of 20 mV s−1 in three-electrode systems. It is indicated that Co3S4/g-C3N4-10 and AC electrodes have stable voltage windows of 0 to 0.65 V and −1 to 0 V, which suggests that the assembled ASC can afford an operating potential window up to 1.7 V. The CV curves of the ASC at different potential windows are displayed in Fig. S9. When the potential window is increased to 1.8 V, the CV curve has an apparent distortion. Therefore, the operating voltage window of the ASC was set at 1.7 V. As shown in Fig. 9b, with the increase of the scan rate from 5 to 100 mV s−1, the CV curves change slightly, implying the fast charge transfer and good electrochemical performance of the ASC device. Fig. 9c shows the GCD curves of ASC at various current densities. At 1 A g−1, the specific capacitance of ASC is estimated to be 93.9 F g−1, as depicted in Fig. 9d. The relationship between energy density and power density, also known as Ragone plots, is shown in Fig. 9e. The ASC can output a maximum energy density of 37.7 W h kg−1 at a power density of 850.2 W kg−1. The long-term lifespan is the most important factor for the practical applications of supercapacitors. Remarkably, the capacitance loss is only 1.9% after 10[thin space (1/6-em)]000 cycles (Fig. 9f), demonstrating a ultrahigh cycling stability of Co3S4/g-C3N4-10//AC, which is superior to many metal sulfide based ASCs such as Co9S8@C//APDC (14%, 10[thin space (1/6-em)]000 cycles at 5 A g−1),36 CF@NiCoZnLDH/Co9S8-QD//CNS-SCN (4.7%, 8000 cycles at 3 A g−1),37 Zn–Co–S//AC (29%, 1000 cycles at 10 A g−1),38 Ni–Co–S//AC (3.8%, 1000 cycles at 10 A g−1)39 and Ni–Co–Mn sulfide//AC (1.8%, 6000 cycles at 10 A g−1).40 The superior stability is also confirmed by the SEM image of Co3S4/g-C3N4-10 after cycling (Fig. S10), where the 2D/2D nanosheet heterostructure is well kept. The above results reveal that the Co3S4/g-C3N4 heterostructure is a desirable candidate electrode material for supercapacitors. The outstanding electrochemical performance could be attributed to the following factors: firstly, the 2D Co3S4 and g-C3N4 nanosheets can provide more active sites and facilitate the diffusion of electrolyte ions. Secondly, the N-rich g-C3N4 improves the wettability and conductivity of the electrode. Finally, in the heterostructure, the intimate interaction between Co3S4 and g-C3N4 not only benefits the fast charge transfer but also prevents the aggregation and pulverization during the charge–discharge process, giving rise to outstanding cycling stability.


image file: d0dt02400g-f9.tif
Fig. 9 (a) CV curves of Co3S4/g-C3N4-10 and AC at a scan rate of 20 mV s−1 in a three-electrode system. (b) CV curves at different scan rates, (c) GCD curves at various current densities, (d) specific capacitances at different current densities, (e) Ragone plots and (f) cycling performance at a current density of 10 A g−1 of the assembled Co3S4/g-C3N4-10//AC ASC.

4. Conclusions

In this study, 2D/2D nanosheet heterostructures of Co3S4 and g-C3N4 have been fabricated by utilizing 2D Co-ZIF-L as the precursor and template, and used as electrode materials for supercapacitors. Compared with Co3S4 and g-C3N4, the Co3S4/g-C3N4-10 composite demonstrates greatly improved electrochemical performance in terms of high specific capacity (415.0 C g−1at 0.5 A g−1) with good rate capability (54.5% retention at 10 A g−1). The assembled Co3S4/g-C3N4-10//AC can afford a large energy density of 37.7 W h kg−1 at a power density of 850.2 W kg−1. Most importantly, Co3S4/g-C3N4-10//AC demonstrates an ultrahigh cycling stability retaining 98.1% of its initial capacitance after 10[thin space (1/6-em)]000 cycles at 10 A g−1. This outstanding electrochemical performance is ascribed to the unique 2D/2D nanosheet heterostructure providing abundant electroactive sites, short ion diffusion pathways, fast charge transfer as well as improved conductivity and mechanic stability. This work has demonstrated that the electrochemical activity and stability of metal sulfide can be greatly enhanced by a rational design of the heterostructures of metal sulfide and g-C3N4. This work may pave the way for developing ASCs with exceptional stability for practical applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This research was supported by the Zhejiang Provincial Natural Science Foundation of China under Grant No. LY20E020005, the NSFC (51572272 and 21971131), the NSF of Ningbo (2019A610003), the Open Foundation of State Key Laboratory of Structural Chemistry (20200022), and the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2018-K06). The assistance in N2 isotherms and XPS measurements from the Shiyanjia Lab (http://www.shiyanjia.com) is greatly appreciated.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/D0DT02400G

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