Synthesis of Hierarchical Porous Ni1.5Co1.5S4/g-C3N4 Composite for Supercapacitor with Excellent Cycle Stability

In this work, the hierarchical porous Ni1.5Co1.5S4/g-C3N4 composite was prepared by growing Ni1.5Co1.5S4 nanoparticles on graphitic carbon nitride (g-C3N4) nanosheets via a hydrothermal route. Due to the self-assembly of larger size g-C3N4 nanosheets as a skeleton, the prepared nanocomposite possesses a unique hierarchical porous structure that can provide short ions diffusion and fast electron transport. As a result, the Ni1.5Co1.5S4/g-C3N4 composite exhibits a high specific capacitance of 1827 F g−1 at a current density of 1 A g−1, which is 1.53 times that of pure Ni1.5Co1.5S4 (1191 F g−1). In particular, the Ni1.5Co1.5S4/g-C3N4//activated carbon (AC) asymmetric supercapacitor delivers a high energy density of 49.0 Wh kg−1 at a power density of 799.0 W kg−1. Moreover, the assembled device shows outstanding cycle stability with 95.5% capacitance retention after 8000 cycles at a high current density of 10 A g−1. The attractive performance indicates that the easily synthesized and low-cost Ni1.5Co1.5S4/g-C3N4 composite would be a promising electrode material for supercapacitor application.


Introduction
Supercapacitor has attracted great attention in recent years, due to its high-power density, excellent cycling stability, fast charge-discharge and environmental friendliness [1]. The electrode materials for supercapacitor application mainly include carbon materials [2], metal oxides [3], conductive polymers [4], transition metal sulfides [5,6], and their composites [7]. Among various electrode materials, transition metal sulfides have a broad application prospect because of its inherent characteristics and excellent electrochemical performance [8]. Compared with oxide counterparts, the transition metal sulfides possessed better electrical conductivity, richer electrochemical activity and higher theoretical capacitance. Furthermore, ternary Ni-Co-S sulfides such as NiCo 2 S 4 and Ni 2 CoS 4 have been demonstrated to be more attractive than corresponding binary Ni or Co sulfides (e.g., NiS, CoS, Ni 3 S 4 , ect.) [9][10][11][12][13], thanks to their rich redox reaction sites and the advantage in terms of electronic conductivity [14]. Recently, several groups have reported that the atomic ratio of nickel and cobalt plays an important role in optimizing the electrochemical performance of electrodes [15][16][17]. The nonstoichiometric Ni 1.5 Co 1.5 S 4 showed a higher specific capacitance, attributing to the synergistic effects of nickel species and cobalt species.
Graphitic carbon nitride (g-C 3 N 4 ) is a two-dimensional graphite structure composed of sp 2hybridzed carbon and nitrogen atoms [18,19]. The presence of high content nitrogen in g-C 3 N 4

Preparation of Samples
The g-C 3 N 4 nanosheets were prepared through a simple improved calcination method as reported in the literature [27]. In brief, 1 g of melamine and 3 g of ammonium chloride were mixed and ground thoroughly in an agate mortar. Then the mixtures were put into a quartz boat and heated at 550 • C with a heat rate of 10 • C min −1 for 4 h in a tube furnace. After cooling to room temperature, the yellow g-C 3 N 4 was obtained. Finally, the g-C 3 N 4 were washed with deionized water and absolute ethanol several times, and ground into powders for further use.
The Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composites were prepared through a modified one-step hydrothermal method as described in our previous paper [28]. Typically, 3 mmol of NiCl 2 6H 2 O, 3 mmol of CoCl 2 6H 2 O and 20 mmol of CS(NH 2 ) 2 were dispersed in a mixture solution of 30 mL water and 50 mL ethylene glycol. Then, 60 mg of g-C 3 N 4 nanosheets was added to the above solution and stirred magnetically for 30 min. The pH value of the mixed solution was adjusted to 11 using NaOH. Afterwards, the mixed solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave (Xi'an Changyi Instrument Equipment Co., Ltd, Xian, China) and reacted at 200 • C for 24 h. After cooling to ambient temperature, the black precipitates were collected, washed with deionized water and ethanol several times, and dried at 60 • C for 12 h. The preparation process is shown in Figure 1.
CoCl26H2O and 20 mmol of CS(NH2)2 were dispersed in a mixture solution of 30 mL water and 50 mL ethylene glycol. Then, 60 mg of g-C3N4 nanosheets was added to the above solution and stirred magnetically for 30 min. The pH value of the mixed solution was adjusted to 11 using NaOH. Afterwards, the mixed solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave (Xi'an Changyi Instrument Equipment Co., Ltd, Xian, China) and reacted at 200 °C for 24 h. After cooling to ambient temperature, the black precipitates were collected, washed with deionized water and ethanol several times, and dried at 60 °C for 12 h. The preparation process is shown in Figure 1.

Characterizations of Samples
The X-ray diffraction (XRD) patterns of the samples obtained on an X-ray diffractometer (Bruker D8 ADVANCE, Bruker Daltonics Inc., Bruker, Germany) instrument. The X-ray photoelectron spectra (XPS) were collected using a spectrometer (Escalab 250XI, Thermo Fisher Scientific Inc., Walsham, MA, USA) with monochromatic aluminum target. The morphologies of the samples were observed using a field-emission scanning electron microscope (FESEM, JSM-6701F, JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 5 kV, and a transmission electron microscope (TEM, JEM2010, JEOL Ltd., Tokyo, Japan), respectively. The Brunauer-Emmett-Teller (BET) surface area and Barret-Joyner-Halenda (BJH) pore size distribution of the samples were measured by nitrogen adsorption-desorption isotherms at 77 K using a gas sorption analyzer (Micromeritics ASAP 2020, Micromeritics Instrument Inc., Atlanta, GA, USA).

Electrochemical Measurement
A three-electrode system and two-electrode system were used to test the electrochemical performance of the samples on a CS350H electrochemical workstation with 2 M KOH aqueous as electrolyte. The working electrode was prepared via mixing the active material (2.0 mg, 80 wt.%), Super P conductive carbon black (10 wt.%) and polyvinylidene fluoride binder (10 wt.%). Then, the slurry was coated on a piece of nickel foam current collector (1 cm × 1 cm), and dried at 60 • C for 12 h under vacuum. Finally, the working electrode was fabricated by pressing nickel foam loaded with active material at a pressure of 10 MPa. Platinum plate and saturated Ag/AgCl were used as counter electrode and reference electrode, respectively. An asymmetric supercapacitor (ASC) cell was assembled by using Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 as the positive electrode and commercial AC as the negative electrode. The electrochemical performance of the electrodes was characterized by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) methods. The specific capacitances (C) of the electrodes are calculated based on the GCD curves according to the following Equation [29].
where I is the constant discharging current (mA), ∆t is the discharge time (s), the potential window (∆V), and m is the mass of active materials in the electrode (mg). For two-electrode testing, the mass of active materials includes the electroactive materials of both the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 and AC.

Result and Discussion
The phase purity and crystal structure of the samples were analyzed using XRD patterns, and the results are shown in Figure 2. Six diffraction peaks can be perfectly indexed to the (111), (220), (311), (400), (511) and (440) of spinel structured NiCo 2 S 4 (JCPDS# 20-0782) or Ni 2 CoS 4 (JCPDS# 24-0334), respectively. In addition, no other metal sulfides such as NiS and Ni 3 S 2 were observed in the pattern, which indicates the pure spinel structure. Figure S1 shows the XRD pattern of the prepared g-C 3 N 4 . Two diffraction peaks at around 13.1 • and 27.3 • in g-C 3 N 4 correspond to the in-plane structure packing of aromatic systems of (100) plane and the interlayer stacking of conjugated aromatic systems of (002) Nanomaterials 2020, 10, 1631 4 of 12 plane, respectively [30,31], which reveals that the prepared g-C 3 N 4 nanosheets is the typical graphitic structure. No diffraction peaks of g-C 3 N 4 were found in the XRD pattern of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 , which is probably due to weak scattering intensity and relatively low content of g-C 3 N 4 . In order to determine the chemical bonds of the corresponding elements in the Ni1.5Co1.5S4/g-C3N4 composite, the XPS spectra of the sample are shown in Figure 3. The XPS survey spectrum ( Figure 3a) shows the presence of Ni, Co, S, C, N, and O elements in the sample. The O 1s peak is mainly attributed to contamination when the sample is exposed to ambient air. The high-resolution XPS spectra of Ni 2p, Co 2p, S 2p, C 1s, N 1s are fitted with Gaussian functions to acquire detail information of chemical bonding. For Ni 2p spectrum, the fitting peaks at 853.3 and 856.2 eV are assigned to Ni 2+ and Ni 3+ , respectively. For Co 2p spectrum, the fitting peaks at 778.7 and 780.9 eV are assigned to Co 3+ and Co 2+ , respectively. Moreover, two satellite peaks can be observed in each highresolution Ni 2p and Co 2p spectra. Obviously, the low-valent and high-valent metal ions coexist in the Ni1.5Co1.5S4/g-C3N4 composite, which is similar to previous reports [28]. Chen et al. believed that the easily valence-changed nickel can contribute the most faradaic capacity of the active materials, while the low-valent cobalt can offer the high electronic conductivity and assist the charge-transfer process in the binary metal sulfides based active materials [17]. Two peaks S 2p ( Figure 3d) located at binding energy of 161.4 and 162.5 eV are typical of metal-sulfur bonds [32,33]. The C 1s spectrum ( Figure 3e) is fitted into three peaks which could be attributed to sp 2 C-C (284.8 eV), C-O (286.5 eV) and N-C=N or C-(N)3 (288.5 eV) bonds, respectively [22]. Figure 3f shows the three different kinds of chemical states of nitrogen species in the g-C3N4. According to the literature [34][35][36], the peaks at binding energy of 398.4, 399.8 and 401.3 eV are assigned to sp 2 nitrogen in carbon containing triazine rings (C=N-C), bridged graphitic tertiary nitrogen bonded with carbon atom (N-(C)3), and amino functional groups (C-N-H), respectively. These peaks are agreement with the characteristics of nitrogen species in g-C3N4. In order to determine the chemical bonds of the corresponding elements in the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite, the XPS spectra of the sample are shown in Figure 3. The XPS survey spectrum (Figure 3a) shows the presence of Ni, Co, S, C, N, and O elements in the sample. The O 1s peak is mainly attributed to contamination when the sample is exposed to ambient air. The high-resolution XPS spectra of Ni 2p, Co 2p, S 2p, C 1s, N 1s are fitted with Gaussian functions to acquire detail information of chemical bonding. For Ni 2p spectrum, the fitting peaks at 853.3 and 856.2 eV are assigned to Ni 2+ and Ni 3+ , respectively. For Co 2p spectrum, the fitting peaks at 778.7 and 780.9 eV are assigned to Co 3+ and Co 2+ , respectively. Moreover, two satellite peaks can be observed in each high-resolution Ni 2p and Co 2p spectra. Obviously, the low-valent and high-valent metal ions coexist in the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite, which is similar to previous reports [28]. Chen et al. believed that the easily valence-changed nickel can contribute the most faradaic capacity of the active materials, while the low-valent cobalt can offer the high electronic conductivity and assist the charge-transfer process in the binary metal sulfides based active materials [17]. Two peaks S 2p (Figure 3d) located at binding energy of 161.4 and 162.5 eV are typical of metal-sulfur bonds [32,33]. The C 1s spectrum (Figure 3e) is fitted into three peaks which could be attributed to sp 2 C-C (284.8 eV), C-O (286.5 eV) and N-C=N or C-(N) 3 (288.5 eV) bonds, respectively [22]. Figure 3f shows the three different kinds of chemical states of nitrogen species in the g-C 3 N 4 . According to the literature [34][35][36], the peaks at binding energy of 398.4, 399.8 and 401.3 eV are assigned to sp 2 nitrogen in carbon containing triazine rings (C=N-C), bridged graphitic tertiary nitrogen bonded with carbon atom (N-(C) 3 ), and amino functional groups (C-N-H), respectively. These peaks are agreement with the characteristics of nitrogen species in g-C 3 N 4 . Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 13  Figure 4(a,b) shows the morphology of the Ni1.5Co1.5S4/g-C3N4. The as-prepared composite is composed of g-C3N4 nanosheets and Ni1.5Co1.5S4 nanoparticles. Compared with the Ni1.5Co1.5S4 ( Figure  S2), some macroporous structure is clearly observed in the Ni1.5Co1.5S4/g-C3N4 due to self-assemble of larger size g-C3N4 nanosheets as skeleton. The Ni1.5Co1.5S4/g-C3N4 has higher porosity which is also confirmed by the gas sorption experiments in Figure 5. It is seen from TEM images (Figure 4c,d) that a large number of Ni1.5Co1.5S4 nanoparticles (30-60 nm) were anchored on the on the surface of g-C3N4 nanosheets (0.8-2.0 μm). The selected area electron diffraction (SAED) pattern displays two sets of diffraction rings that can be indexed to the graphic structure g-C3N4 (yellow rings) and the spinel structure Ni1.5Co1.5S4 (blue rings), respectively. The high-resolution transmission electron microscope (HRTEM) image shows the formation of the distinct nanoparticle-on-nanosheet heterostructure.  Figure 4a,b shows the morphology of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 . The as-prepared composite is composed of g-C 3 N 4 nanosheets and Ni 1.5 Co 1.5 S 4 nanoparticles. Compared with the Ni 1.5 Co 1.5 S 4 ( Figure S2), some macroporous structure is clearly observed in the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 due to self-assemble of larger size g-C 3 N 4 nanosheets as skeleton. The Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 has higher porosity which is also confirmed by the gas sorption experiments in Figure 5. It is seen from TEM images (Figure 4c,d) that a large number of Ni 1.5 Co 1.5 S 4 nanoparticles (30-60 nm) were anchored on the on the surface of g-C 3 N 4 nanosheets (0.8-2.0 µm). The selected area electron diffraction (SAED) pattern displays two sets of diffraction rings that can be indexed to the graphic structure g-C 3 N 4 (yellow rings) and the spinel structure Ni 1.5 Co 1.5 S 4 (blue rings), respectively. The high-resolution transmission electron microscope (HRTEM) image shows the formation of the distinct nanoparticle-on-nanosheet heterostructure.  The pore structures of the Ni1.5Co1.5S4 and the Ni1.5Co1.5S4/g-C3N4 were tested by nitrogen adsorption-desorption at 77 K. As shown in Figure 5a, the samples display type IV isotherm with typical H1 hysteresis loop at a relative pressure of 0.8-1.0, which is characteristic for mesoporous materials [37]. The BET specific surface area of the Ni1.5Co1.5S4/g-C3N4 is 22.5 m 2 g −1 , which is much higher than that of the Ni1.5Co1.5S4 (15.2 m 2 g −1 ). It is seen from Figure 5b that two samples are mainly composed of mesoporous and macrospores, suggesting a hierarchical porous structure (Table S1). The BJH desorption cumulative volume of pores between 1.7 nm and 300.0 nm notably increases from 0.100 cm 3 g −1 for Ni1.5Co1.5S4 to 0.124 cm 3 g −1 for the Ni1.5Co1.5S4/g-C3N4, while the average pore diameter slightly decreases from 25.5 nm to 24.9 nm. These results indicate that the addition of g-C3N4 can not only increase the specific surface area, but also optimize the structure of pores. Consequently, an increase of the mesoporous channels in the Ni1.5Co1.5S4/g-C3N4 is more beneficial for the fast ion transportation to improve the electrochemical activity of the electrodes.  Figure 6 shows electrochemical properties of the samples tested through the three-electrode system. The CV curves was performed at a scan rate of 50 mV s −1 within potential window of −0.4-0.6 V. As shown in Figure 6a, the redox peaks of the Ni1.5Co1.5S4/g-C3N4 is similar to those of the Ni1.5Co1.5S4, which can be attributed to the reversible process of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ associated with the insertion and extraction of OH − anions to and from the electrode materials [38]. The integral area of the CV loop of the Ni1.5Co1.5S4/g-C3N4 is larger than that of the Ni1.5Co1.5S4, indicating superior electrochemical performance. This result can be further confirmed by GCD tests in Figure 6b. Figure  6(c,d) show the CV and GCD curves of the Ni1.5Co1.5S4/g-C3N4 at different scan rates and current The pore structures of the Ni 1.5 Co 1.5 S 4 and the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 were tested by nitrogen adsorption-desorption at 77 K. As shown in Figure 5a, the samples display type IV isotherm with typical H1 hysteresis loop at a relative pressure of 0.8-1.0, which is characteristic for mesoporous materials [37]. The BET specific surface area of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 is 22.5 m 2 g −1 , which is much higher than that of the Ni 1.5 Co 1.5 S 4 (15.2 m 2 g −1 ). It is seen from Figure 5b that two samples are mainly composed of mesoporous and macrospores, suggesting a hierarchical porous structure (Table S1). The BJH desorption cumulative volume of pores between 1.7 nm and 300.0 nm notably increases from 0.100 cm 3 g −1 for Ni 1.5 Co 1.5 S 4 to 0.124 cm 3 g −1 for the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 , while the average pore diameter slightly decreases from 25.5 nm to 24.9 nm. These results indicate that the addition of g-C 3 N 4 can not only increase the specific surface area, but also optimize the structure of pores. Consequently, an increase of the mesoporous channels in the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 is more beneficial for the fast ion transportation to improve the electrochemical activity of the electrodes. Figure 6 shows electrochemical properties of the samples tested through the three-electrode system. The CV curves was performed at a scan rate of 50 mV s −1 within potential window of −0.4-0.6 V. As shown in Figure 6a, the redox peaks of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 is similar to those of the Ni 1.5 Co 1.5 S 4 , which can be attributed to the reversible process of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ associated with the insertion and extraction of OH − anions to and from the electrode materials [38]. The integral area of the CV loop of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 is larger than that of the Ni 1.5 Co 1.5 S 4 , indicating superior electrochemical performance. This result can be further confirmed by GCD tests in Figure 6b. Figure 6c,d show the CV and GCD curves of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 at different scan rates and current densities. They almost maintain the symmetric shape without visible distort, suggesting that the electrode has excellent pseudocapacitive behavior and high coulombic efficiency. The anodic peak current shows a linear relationship with the square root of scan rate (Figure 6e), which indicates that the electrochemical kinetics is a diffusion-controlled process. The specific capacitances of the samples were calculated at the current densities ranging from 1 A g −1 to 20 A g −1 according to the GCD curves. The Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite exhibits a high specific capacitance of 1827 F g −1 at a current density of 1 A g −1 (Figure 6f), which is 1.53 times that of the Ni 1.5 Co 1.5 S 4 (1191 F g −1 ). Even if the current density increases 20 times, the specific capacitance still reaches to 1348 F g −1 , demonstrating a good rate performance. This result is superior to those of the most recently reported composites such as Ni-Co-S/graphene and NiCo 2 S 4 @g-C 3 N 4 composites [36,39]. Moreover, the CV and GCD curves of pure g-C 3 N 4 nanosheets is shown Figure S3 for a comparation. The specific capacitance of g-C 3 N 4 nanosheets is only 11 F g −1 at a current density of 1 A g −1 , which is far lower than that of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite. In order to further explore the effect of g-C 3 N 4 content on the electrochemical properties, the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composites with different g-C 3 N 4 content were also prepared and evaluated, shown in Figure S4. When the amount of g-C 3 N 4 is 60 mg, the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite shows the highest specific capacitance, owing to maximizing synergetic effects of Ni 1.5 Co 1.5 S 4 nanoparticles and g-C 3 N 4 nanosheets. However, the specific capacitance decreases when 90 mg of g-C 3 N 4 is introduced. This superior supercapacitive performance of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 can be mainly ascribed to two reasons. On the one hand, g-C 3 N 4 nanosheets can increase the specific surface area and mesoporous number, which provides more active sites for interface reaction and shortens the pathway of the electrolyte ion diffusion. On the other hand, g-C 3 N 4 nanosheets can improve electrical conductivity of the Ni 1.5 Co 1.5 S 4 , which facilitates for electron transport. As shown in Figure S5, the impedance plots imply that the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite possesses smaller internal resistance, faster ion diffusion process and lower charge transfer resistance during the faradic reaction.  Figure 7 shows the performance of the Ni1.5Co1.5S4/g-C3N4//AC supercapacitor. The working voltage window of the device was extended to 1.6 V (Figure 7a), because the potential window of the Ni1.5Co1.5S4/g-C3N4 and AC is in the range of −0.4 to 0.6 V and −1 to 0 V, respectively. Apparently, the capacitance of the device comes from the combined contribution of pseudocapacitive and electrical double behaviors. Furthermore, the charge-discharge curves are good symmetric with a coulombic efficiency of over 98.0% at different scan rate, demonstrating its high electrochemical reversibility (Figure 7b). The specific capacitance of the device is calculated to be 138 F g −1 at 1 A g −1 , and it still retains 76 F g −1 even at a high current density of at 20 A g −1 (Figure 7c). Figure 7d shows the Nyquist plot of device in the frequency range of 10 −2 to 10 5 Hz. The equivalent series resistance (Rs) and the charge transfer resistance (Rct) are as low as 0.73 and 1.55 Ω, respectively, which are considered to be good for improved charge-discharge rate and power density of the device. The impedance phase angle of the device is approximately −52.16° at a frequency of 0.01 Hz, and reaches −45° at a frequency of 0.04 Hz (Figure 7e). The resistance and reactance of the capacitor have equal magnitudes at the phase angle of −45°, so the frequency at this point is convenient for comparison [40]. This frequency of the Ni1.5Co1.5S4/g-C3N4//AC device is comparable to that of an activated carbon-based electric double-layer capacitor (0.05 Hz) [41]. Figure 7f shows the cycling stability of the device at a current density of 10 A g −1 . After 8000 cycles, the capacitance retention and the columbic efficiency still kept about 95.5% and 98.4%, respectively, indicating outstanding long-term stability. Energy density (E) and power density (P) are used as two major parameters to evaluate the performance of  Figure 7 shows the performance of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 //AC supercapacitor. The working voltage window of the device was extended to 1.6 V (Figure 7a), because the potential window of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 and AC is in the range of −0.4 to 0.6 V and −1 to 0 V, respectively. Apparently, the capacitance of the device comes from the combined contribution of pseudocapacitive and electrical double behaviors. Furthermore, the charge-discharge curves are good symmetric with a coulombic efficiency of over 98.0% at different scan rate, demonstrating its high electrochemical reversibility (Figure 7b). The specific capacitance of the device is calculated to be 138 F g −1 at 1 A g −1 , and it still retains 76 F g −1 even at a high current density of at 20 A g −1 (Figure 7c). Figure 7d shows the Nyquist plot of device in the frequency range of 10 −2 to 10 5 Hz. The equivalent series resistance (Rs) and the charge transfer resistance (Rct) are as low as 0.73 and 1.55 Ω, respectively, which are considered to be good for improved charge-discharge rate and power density of the device. The impedance phase angle of the device is approximately −52.16 • at a frequency of 0.01 Hz, and reaches −45 • at a frequency of 0.04 Hz (Figure 7e). The resistance and reactance of the capacitor have equal magnitudes at the phase angle of −45 • , so the frequency at this point is convenient for comparison [40]. This frequency of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 //AC device is comparable to that of an activated carbon-based electric double-layer capacitor (0.05 Hz) [41]. Figure 7f shows the cycling stability of the device at a current density of 10 A g −1 . After 8000 cycles, the capacitance retention and the columbic efficiency still kept about 95.5% and 98.4%, respectively, indicating outstanding long-term stability. Energy density (E) and power density (P) are used as two major parameters to evaluate the performance of supercapacitor in practical applications [31]. Figure 8 shows a Ragone plot of energy density and power density. The Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 //AC supercapacitor delivers high energy density of 49.0 Wh kg −1 at a power density of 799.0 W kg −1 . These values surpass those of previously reported symmetric and asymmetric supercapacitors based on g-C 3 N 4 composites, such as g-C 3 N 4 @Ni(OH) 2 [24], ZnS/g-C 3 N 4 [42] and porous g-C 3 N 4 [43][44][45].

Conclusions
In summary, we have prepared the hierarchical porous Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite by growing Ni 1.5 Co 1.5 S 4 nanoparticles on g-C 3 N 4 nanosheets using a hydrothermal method. Compared with pure Ni 1.5 Co 1.5 S 4 , the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite possesses larger surface area and optimized porous structures. The specific capacitance of the composites is strongly depended on the content of g-C 3 N 4 nanosheets. When the adding amount of g-C 3 N 4 is 60 mg, the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 composite exhibits the highest specific capacitance of 1827 F g −1 at a current density of 1 A g −1 , which is 1.53 times that of pure Ni 1.5 Co 1.5 S 4 . The enhancement in specific capacitance could be attributed to maximizing synergetic effects of Ni 1.5 Co 1.5 S 4 nanoparticles and g-C 3 N 4 nanosheets. A Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 //AC asymmetric supercapacitor exhibits a high energy density of 49.0 Wh kg −1 at a power density of 799.0 W kg −1 , and outstanding cycle stability with 95.5% capacitance retention after 8000 cycles at a current density of 10 A g −1 .
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/9/1631/s1, Figure S1: XRD pattern of g-C 3 N 4 , Figure S2: SEM images of pure Ni 1.5 Co 1.5 S 4 , Figure S3: Electrochemical properties of the Ni 1.5 Co 1.5 S 4 @g-C 3 N 4 with different content of g-C 3 N 4 : (a) CV curves, (b) GCD curves, Figure S4: Electrochemical properties of the Ni 1.5 Co 1.5 S 4 /g-C 3 N 4 with different content of g-C 3 N 4 : (a) CV curves, (b) GCD curves, (c) the specific capacitance at different current densities, and (d) the dependence of specific capacitance on g-C 3 N 4 content, Figure S5: Nyquist plot of pure Ni 1.5 Co 1.5 S 4 and Ni 1.5 Co 1.5 S 4 @g-C 3 N 4 , Table S1: The cumulative pore volume of Ni 1.5 Co 1.5 S 4 and Ni 1.5 Co 1.5 S 4 @g-C 3 N 4 .