2.1 Characterization by XRD
Figure 2 shows XRD spectra of MXene, Ti3AlC2 and MXene/NiS2-1:2-180 composite. As observed, peaks at 2θ = 9.58°, 19.25°, 34.09°, 36.7°, 39°, 41.8°, 60.36° and 65.6° correspond to MAX-phase diffraction peaks of Faces (002), (004), (101), (103), (104), (105), (110) and (1011) of MXene and MXene/NiS2-1:2-180, respectively. The XRD spectra of MXene etched by LiF/HCl were consistent with previous studies [14]. Compared with standard comparison card (PDF standard card, JCPDS:52–0875) of Ti3AlC2, the (002) peak further shifted to the small angle. This can be attributed to the increased interlayer spacing caused by embedment of Li+ in layers of MXene during etching by LiF/HCl. Additionally, the specific angle of (002) and (004) peaks were smaller than that of the MAX phase and a peak corresponding to Face (104) was observed at 2θ = 39° in the spectra of MXene. This may be attributed to the incomplete removal of Al. As shown in the XRD spectra of MXene/NiS2-1:2-180, additional diffraction peaks were observed at 2θ = 29.89°, 31.88°, 35.36°, 40.14° and 48.50°, which corresponding to Faces (111), (200), (210), (211) and (311), respectively. Comparison with the standard comparison card of NiS2 (JCPDS NO.083–0575) [15] denoted diffraction peaks of NiS2. Herein, diffraction peaks of Faces (111), (210) and (311) shifted to the small angle. This can be attributed to the increased interlayer spacing of MXene induced by addition of NiS2 [16]. In summary, the diffraction peaks of MXene and MXene/NiS2-1:2-180 suggested good crystallinity.
2.5 Testing of electrochemical properties
In order to determine the optimized preparation conditions, composites with different Ni/S ratios were prepared under different synthesis temperature. CV and GCD testing of the as-prepared composites were conducted in 6 mol/L KOH. Figure 6 (a) shows CV curves of MXene/NiS2 with different Ni/S ratios. The testing voltage range and scanning rate were 0 ~ 0.6 V and 10 mV·S− 1, respectively. According to the shapes of CV curves, MXene/NiS2 was exposed to redox reaction in the range of 0 ~ 0.6 V, which is different from conventional double-layer capacitors, indicating that MXene/NiS2 exhibits pseudocapacitance. As observed, peak area of the composite with a Ni/S ratio of 1:2 was significantly larger than those of other samples, indicating that the specific capacitance is maximize at a Ni/S ratio of 1:2. A possible reason is that the various active sites on the surface of MXene/NiS2-1:2 readily combine with ions in the electrolyte. Too many or too few sulfur sources would reduce the active sites of the composite [25]. Figure 6 (b) shows GCD curves of composites with different NiS ratios under a voltage window of 0 ~ 0.5 V and a current density of 1 A·g− 1. As observed, GCD curves of composites with different NiS ratios are approximately symmetric, suggesting intensive redox reactions of the corresponding composites during charging/discharging [26]. Additionally, calculations verified that conclusions: with Ni/S ratio = 1:2, MXene/NiS2-1:2 exhibited maximum specific capacitance (898.2 F·g− 1).
Figure 7 (c) shows the CV curves under different growth temperatures (scanning rate = 10 mV·S− 1, voltage window = 0 ~ 0.6 V). At a growth temperature of 180 ℃, the composite exhibited maximum integral area. A possible reason is that the reaction rate is slow and the NiS2 crystallinity is low, resulting in incomplete reaction; if the growth temperature is over-high, the reaction rate is over-high and NiS2 stacking may be observed, resulting in reduced specific surface area and degraded electrochemical properties. At a growth temperature of 180 ℃, NiS2 is exposed to effective crystal growth and nano sheets are connected with each other, resulting in a porous spherical structure. The pores are enlarged and the specific surface area increases, facilitating diffusion of electrolyte ions and its electronic conductivity. Ultimately, the electrochemical properties of the composites are improved. Figure 7 (d) shows the GCD curves of the three samples at a current density of 1 A·g− 1. As indicated, specific capacitances at growth temperatures of 160 ℃, 180 ℃ and 200 ℃ were 593.2, 898.2 and 516.2 F·g− 1, respectively. The results based on CV curves were consistent. In summary, Ni/S ratio of 1: 2 and growth temperature of 180 ℃ are the optimized conditions. In other words, MXene/NiS2-1:2-180 is the optimized composite.
Figure 7 illustrates electrochemical properties of MXene, NiS2 and MXene/NiS2-1:2-180. Figure 7 (a) shows the CV curve at a scanning rate of 10 mV·S− 1. The samples have voltage windows of 0 ~ 0.6 V and redox peaks. The detailed redox reaction is shown as follows [27]:
NiS + OH−⇌NiSOH + e−
Ti3C2(OH)X+OH−⇌Ti3C2(OH)X+1+e−
Ti3C2Oy + 2OH−⇌Ti3C2Oy + 1+e−+H2O
The redox peaks may be attributed to adsorption and desorption of ions on active substances or intercalation or delamination of cations (H+ or Li+) [28]. Meanwhile, considerable functional groups (-OH, -F, -O) are observed on the surface of MXene [29]. Additionally, the presence of NiS2 prevents collapse and agglomeration of MXene layers, resulting in increased metallic active sites of MXene. Ni2+ and Ni3+ accelerate electrochemical adsorption and desorption and promotes ion transport. Figure 7 (b) shows GCD of reference materials at current density of 1 A·g− 1 and voltage window of 0 ~ 0.5 V. As observed, the discharging time of MXene/NiS2-1:2-180 was significantly larger than those of MXene and NiS2 at constant current density. A possible reason is that the unique nano-structure of MXene/NiS2-1:2-180 prevents agglomeration of MXene, reduces the diffusion distance of ions in electrolyte, and increases the contact area of active substances and electrolyte. Figure 7 (c) illustrates the rate performances of samples under different current densities, which were obtained by calculation using Eq. (1). At a current density of 1 A·g− 1, specific capacitances of MXene, NiS2 and MXene/NiS2-1:2-180 were 157.4, 616 and 898.2 F·g− 1, respectively. At a current density of 10 A·g− 1, their specific capacitances decreased to 122, 320 and 478 F·g− 1, respectively. This can be attributed to the incomplete reaction caused by reduced time of redox reaction as a result of increased current density. Figure 7 (d) shows EIS curves of different samples at room temperature, an open-circuit voltage of 5 mV, and a frequency of 100 kHz ~ 0.01 Hz. The EIS curves consist of sectors at high frequency and lines at low frequency. The former is attributed to charge transfer, while the latter is related to mass transfer [30]. The line slopes represent the ion diffusion rates [31]. Herein, the intersection of high frequency region and real axis are essentially the internal resistances of the electrode (RS), namely internal resistances between active substances and internal resistance between active substances and nickel foam [32]. As observed, internal resistances of MXene, NiS2 and MXene/NiS2-1:2-180 are 0.62, 0.66 and 0.60 Ω, respectively. At low frequency, the slopes of composites are larger than those of NiS2, while MXene/NiS2-1:2-180 exhibits low Warburg impedance. Therefore, MXene/NiS2-1:2-180 exhibited low resistance and good conductivity compared with MXene and NiS2. During redox reaction, the ion diffusion rate is high.
Figure 8 (a) shows CV curves of MXene/NiS2-1:2-180 at different scanning rates. As observed, unlike conventional double-layer capacitors, MXene/NiS2-1:2-180 were exposed to redox reactions, whose reaction mechanism is described by Eq. (3). Meanwhile, CV curves under different scanning rates exhibited negligible differences in shape, suggesting that this electrode material exhibits ideal pseudocapacitance performance [33]. Figure 8 (b) shows GCD curves of MXene/NiS2-1:2-180 at different current densities. These curves are nonlinear and exhibit significant charging/discharging platforms, which are characteristics of battery electrodes [34]. As shown in Fig. 8 (c), b value obtained by Eq. 2.3 was determined according to the slopes of log i and log v. Under a strong redox peak potential, GCD is mainly controlled by diffusion and the composites exhibit pseudo capacitance behaviors when b approaches 0.5. When b approaches 1, GCD is mainly controlled by non-diffusion mechanisms and the composites exhibit double-layer behaviors. With b = 0.5, MXene/NiS2-1:2-180 exhibits pseudo capacitance behavior.
Figure 9 (a) - (e) show electrochemical properties of MXene/NiS2//SC. In order to verify practical applications of the as-prepared electrode material, an asymmetric supercapacitor (ASC) was developed with MXene/NiS2-1:2-180 in 6 M KOH electrolyte with as the anode and SC as the cathode based on charge balance. As shown in Fig. 9 (a), CVs of cathode and anode exhibit a rectangle shape and two significant redox peaks, respectively. As shown in Fig. 9 (b), the ASC device works at a voltage window of 0 ~ 1.5 V. Under different scanning rates, CVs consist of an approximate rectangle and a pair of redox peaks, indicating that the composites have both double layer and pseudocapacitance behaviors. As the scanning rate increases from 10 to 100 mV·S− 1, oxidation and reduction peaks shift slightly, indicating good reversibility of this device. As shown in Fig. 9 (c), the GCD curves are symmetrical, indicating that this device exhibits highly reversibility and effective energy-storage by charges.
At a current density of 1 A·g− 1, the specific capacitance is 102.3 F·g− 1. Figure 9 (d) shows the cyclic stability of MXene/NiS2//SC. As observed, specific capacitance of MXene/NiS2//SC decreased slightly as the charging/discharging cycles increased. At a current density of 2 A·g− 1, the capacitance retention was 85.74% after 3000 cycles. Figure 9 (e) shows the Ragone figure of MXene/NiS2//SC. The power densities of MXene/NiS2//SC at current densities of 1, 2, 4, 6, 8 and 10 A·g− 1 were 749.9, 1500.2, 3002.8, 4502.5, 6016 and 7548.4 W·kg− 1, respectively, while its energy densities were 32.1, 13.5, 10.8, 8.9, 7.5 and 6.5 Wh·kg− 1, respectively. Compared with double-electrode supercapacitors reported elsewhere, including MoS2-Ti3C2Tx (5.1 Wh·kg− 1 and 300 W·kg− 1) [35], CNFs/PANI (4.4 Wh·kg− 1 and 103 W·kg− 1) [36], CoNi2S4 (1.34 Wh·kg− 1 and 105 W·kg− 1) [37], MXene/NiS2//SC exhibited improved energy density, indicating that its excellent performance as asymmetric supercapacitor electrode.