A Comparative Study of the Influence of Nitrogen Content and Structural Characteristics of NiS/Nitrogen-Doped Carbon Nanocomposites on Capacitive Performances in Alkaline Medium

Supercapacitors (SCs) have been regarded as alternative electrochemical energy storage devices; however, optimizing the electrode materials to further enhance their specific energy and retain their rate capability is highly essential. Herein, the influence of nitrogen content and structural characteristics (i.e., porous and non-porous) of the NiS/nitrogen-doped carbon nanocomposites on their electrochemical performances in an alkaline electrolyte is explored. Due to their distinctive surface and the structural features of the porous carbon (A-PVP-NC), the as-synthesized NiS/A-PVP-NC nanocomposites not only reveal a high wettability with 6 M KOH electrolyte and less polarization but also exhibit remarkable rate capability (101 C/g at 1 A/g and 74 C/g at 10 A/g). Although non-porous carbon (PI-NC) possesses more nitrogen content than the A-PVP-NC, the specific capacity output from the latter at 10 A/g is 3.7 times higher than that of the NiS/PI-NC. Consequently, our findings suggest that the surface nature and porous architectures that exist in carbon materials would be significant factors affecting the electrochemical behavior of electrode materials compared to nitrogen content.


Introduction
Recent technological innovations have increased the widespread application of various electrochemical energy storage (EES) fields. Furthermore, scientists have developed numerous EES devices to store more energy with a long-life cycle. Among the existing EES devices, there is a complementary relationship between metal-ion batteries (MIBs) and supercapacitors (SCs) since the former has a high energy density, but the latter can deliver extreme power density [1][2][3][4][5]. It is recognized that increasing the power energy of the MIBs to as high as that of the SCs is intrinsically challenging, owing to the insertion/extraction of the metal ions from the corresponding electrode materials being principally essential, that is, the energy storage mechanism [6][7][8][9][10]. On the contrary, it has been demonstrated that Aldrich, St. Louis, MO, USA), carbon black (Super P ® , Timcal Ltd., Bodio, Switzerland) and Nafion ® perfluorinated resin solution (5 wt. %, Sigma-Aldrich, St. Louis, MO, USA) were employed without further purification. Deionized (DI) water produced from a Milli-Q ® Integral water purification system (Millipore Ltd., Burlington, MA, USA) was used throughout the experiments.

Preparation of Polymer-Derived Nitrogen-Doped Carbon Materials
To prepare the polyimide-derived carbon (PI-NC) sample, 5 g of PI powders were thermally decomposed in the tube furnace at 900 • C for 8 h under an argon atmosphere with a flow rate of 200 mL/min. The as-prepared PI-NC powders were collected when they were naturally cooled down to room temperature and thoroughly ground using pestle and agate mortar. As for the porous carbon, 4 g of polyvinylpyrrolidone powders were well-dissolved in the DI water (solution 1). Afterwards, the same mass ratio of the KCl and the K 2 CO 3 aqueous solutions were sequentially added to solution 1. Here, the former was adopted as the template, while the latter served as the activator [34]. The viscous mixture had been continuously stirred for 3 h before it was completely dried in the oven at 120 • C for 12 h. The resultant residues were then pyrolyzed in the tube furnace using the same temperature, atmosphere and flow rate as the PI-NC but only for 2 h. Following this, repeatedly rinsing with DI water to remove the KCl, drying and grinding procedures were carried out in order. At this point, the loose powders-the so-called activated polyvinylpyrrolidone-derived carbon (A-PVP-NC)-can be obtained eventually.

Microwave-Assisted Synthesis of NiS/Nitrogen-Doped Carbon Nanocomposites
For synthesizing the NiS/nitrogen-doped carbon nanocomposites, the typical processes were described as follows. Firstly, the nitrogen-doped carbon powders (0.05 g A-PVP-NC or 0.5 g of PI-NC) were homogenously dispersed within the 20 mL of HOC 2 H 4 OH by ultrasonication for 1 h (solution 1). Another 20 mL of the emerald green aqueous solution composed of the ingredients (equal molar of Ni(NO 3 ) 2 ·6H 2 O, CH 3 CSNH 2 , and C 6 H 8 O 6 ) were then mixed with solution 1 under magnetically stirring for 1 h. Subsequently, the precursor solution was carefully transferred to a microwave reactor (Discover ® System, CEM Corporation, Matthews, NC, USA). The output power, reaction temperature and period were set as 100 W, 150 • C and 1 h, respectively. During the microwave heating procedure, the precursor solution was continuously stirred to ensure a homogeneous reaction. The product was collected by filtration and repeated rinsing with DI water, C 2 H 5 OH and drying in the air (60 • C for 12 h). The as-synthesized NiS/A-PVP-NC or NiS/PI-NC were acquired after being thoroughly ground using pestle and agate mortar.

Characterizations
The crystalline structures of A-PVP-NC, PI-NC, NiS/A-PVP-NC and NiS/PI-NC were identified using a powder X-ray diffractometer (PXRD, Bruker D2 PHASER) with a Cu target (λ = 1.541 Å) that was excited at 30 kV and 10 mA. In addition, the corresponding PXRD patterns of NiS/A-PVP-NC and NiS/PI-NC recorded in the range of 2θ from 10 • to 100 • at a scanning rate of 2.5 sec/step were further refined via Rietveld analysis using TOPAS 4.2 software (Bruker AXS Inc., Karlsruhe, Germany). The Raman spectrum was collected by a Raman microscope (inVia, Renishaw, UK) equipped with a 633 nm laser source. The elemental mappings were examined by a scanning electron microscope (SEM, S-2600H, Hitachi, Ltd., Tokyo, Japan) connected with energy-dispersive X-ray spectroscopy (XFlash ® EDX Detector 3001, Bruker AXS Mikronalysis GmbH, Karlsruhe, Germany). For morphological observations, a transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan), operating at an accelerating voltage of 200 kV, was utilized. N 2 adsorptiondesorption isotherm was recorded at 77 K on a surface area and porosity analyzer (ASAP 2020 V3.00, Micromeritics Instrument Corporation, Norcross, GA, USA) after the A-PVP-NC had been degassed in a vacuum at 160 • C for 8 h. An elemental analyzer (FLASH 2000, Thermo Fisher Scientific Inc., Waltham, MA, USA) was applied for determining the per-centage of carbon, nitrogen and oxygen within the polymers, nitrogen-doped carbon and NiS/nitrogen-doped carbon nanocomposites. The chemical environments were analyzed with X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe III, ULVAC-PHI, Inc., Kanagawa, Japan) with a beam size of 100 um under Al Kα radiation (λ = 8.3406 Å). Their corresponding spectra were analyzed by the Gaussian-Lorentzian fitting method using an XPSPEAK 4.1 software. The compatibility between each NiS/nitrogen-doped carbon nanocomposite and electrolyte was carefully added to 0.5 mL of 6 M KOH solution in a glass vial that contained 0.1 g of NiS/nitrogen-doped carbon powder, in order to statically check their wettability in ambient conditions.

Electrochemical Measurements
The electrochemical tests throughout this study were conducted in a standard threeelectrode system, which was controlled by a multichannel electrochemical workstation (VMP3, Bio-Logic, Seyssinet-Pariset, France) in ambient conditions. To prepare the working electrode, 8 mg of active material (NiS/A-PVP-NC or NiS/PI-NC) and 2 mg of carbon black were well-dispersed within the solution (0.5 mL of C 2 H 5 OH, 0.25 mL of DI water, and 0.25 mL of 5 wt. % Nafion ® solution) by ultrasonication for 30 min. Then, 5 µL of the homogenous suspension was added dropwise onto the surface of a glassy carbon electrode (d = 3 mm, #CHI104, CH Instruments, Inc., Austin, TX, USA), resulting in the mass loading of about 0.566 mg/cm 2 of the active material. A graphite rod and a saturated calomel electrode (SCE, Hg/Hg 2 Cl 2 (sat. KCl), #CHI150, CH Instruments, Inc.) served as the counter and reference electrodes, respectively. The cyclic voltammograms (CVs), galvanostatic charge-discharge (GCD) profiles, and electrochemical impedance spectroscopy (EIS) spectra were accomplished under an N 2 -saturated 6 M KOH electrolyte. The potential and scanning rates used for CVs were in the range of 0 V and 0.5 V (vs. SCE) and from 10 mV/sec to 100 mV/sec, respectively. Regarding the conditions applied for GCD results, the electrodes were evaluated from the potential of 0 V to 0.4 V (vs. SCE), adopting the current densities of 1 A/g to 10 A/g. The corresponding specific capacity (C s ) can be calculated through the GCD curve according to the equation of C s (C/g) = I∆t/m, where I, ∆t, and m represent the current (A), discharge time (sec.) and the mass of the active material (g) [35,36]. The EIS spectra were recorded at open circuit potential (OCP) from 100 kHz to 0.01 Hz with an AC potential amplitude of 5 mV. Figure 1 compares the normalized PXRD patterns, fitted Raman spectra and lowmagnification TEM micrographs of A-PVP-NC and PI-NC. First of all, two predominant peaks representing the (002) and (100) crystallographic planes of carbon were reflected in Figure 1a,b, confirming the successful preparation of polymer-derived carbon materials with high purity. However, it is obvious that the characteristic peaks of PI-NC were more intense than those of the A-PVP-NC, indicating less crystallinity for the A-PVP-NC. This result might be correlated with the presence of KCl (template) and K 2 CO 3 (activator), leading to the porous and loosely structural features after high-temperature pyrolysis and rinsing procedures. This phenomenon is typically observed from the activated carbon materials [34].

Characterizations of A-PVP-NC and PI-NC
As for the Raman characterization, two distinct peaks assigned to the D (~1327 cm −1 ) and G (~1583 cm −1 ) bands were undoubtedly displayed in each spectrum (Figure 1c,d). On the other hand, it is known that the intensity ratio of the D and G bands (I D /I G ) is generally adapted to evaluate the degree of defects in the graphite material. Thus, an increase in the I D /I G ratio suggests decreased graphite integrity, signifying that many defects and/or highly disordered degrees have existed. This is also normally observed in carbon materials containing many functional groups [37,38]. As revealed, the high I D /I G ratio for the A-PVP-NC (1.18) and PI-NC (1.13) could be attributed to the presence of heteroatoms (i.e., N and O) and less crystallinity as evidenced by PXRD patterns. Furthermore, the corresponding Raman spectra were sequentially deconvoluted into four peaks (labeled peaks (1)-(4)) since the integrated area ratio of sp 3 to sp 2 (A sp 3 /A sp 2 ) has been demonstrated to deliver helpful information about the nature of carbon, for example, a low A sp 3 /A sp 2 ratio indicates that a large amount of carbon exists as the sp 2 type [39,40]. As depicted, the peaks of (2) and (4) are related to sp 2 -type carbon, while the others are associated with sp 3 -type carbon.
The calculated values of the A sp 3 /A sp 2 ratio were 0.26 for A-PVP-NC and 0.19 for PI-NC, inferring that the PI-NC exhibited high proportions of sp 2 -type carbons.
adapted to evaluate the degree of defects in the graphite material. Thus, an increase in the ID/IG ratio suggests decreased graphite integrity, signifying that many defects and/or highly disordered degrees have existed. This is also normally observed in carbon materials containing many functional groups [37,38]. As revealed, the high ID/IG ratio for the A-PVP-NC (1.18) and PI-NC (1.13) could be attributed to the presence of heteroatoms (i.e., N and O) and less crystallinity as evidenced by PXRD patterns. Furthermore, the corresponding Raman spectra were sequentially deconvoluted into four peaks (labeled peaks (1)-(4)) since the integrated area ratio of sp 3 to sp 2 (Asp 3 /Asp 2 ) has been demonstrated to deliver helpful information about the nature of carbon, for example, a low Asp 3 /Asp 2 ratio indicates that a large amount of carbon exists as the sp 2 type [39,40]. As depicted, the peaks of (2) and (4) are related to sp 2 -type carbon, while the others are associated with sp 3 -type carbon. The calculated values of the Asp 3 /Asp 2 ratio were 0.26 for A-PVP-NC and 0.19 for PI-NC, inferring that the PI-NC exhibited high proportions of sp 2 -type carbons. Owing to the A-PVP-NC being achieved by incorporating the KCl and K2CO3 then following through a high-temperature pyrolysis and rinsing procedures, it is reasonably believed that highly porous architectures could be revealed. As expected, numerous hierarchical pores were shown, as seen in Figure 1e. In contrast, a non-porous structure was revealed from the PI-NC ( Figure 1f). To accurately determine the classification of pores and specific surface area (SSA) of the A-PVP-NC, the nitrogen adsorption-desorption isotherm measurement was conducted. Figure S1 illustrates a typical isotherm curve, referred to as type I, and a predominant pore diameter less than 2 nm was detected, indicating a microporous characteristic of the A-PVP-NC [34]. Calculated by the Brunauer-Emmett-Teller (BET) method, the SSA value was as high as 1628 m 2 /g. Moreover, it is noticed that the SSA value of micropores was 1483 m 2 /g, whereas that for mesopores was just 52 m 2 /g. Accordingly, the total pore volume collected at P/P0 = 0.995 was 0.79 cm 3 /g, while the pore size distributions of ultramicropores, micropores and mesopores were also measured to be 0.25, 0.62, and 0.17 cm 3 /g, respectively (Table S1). The fascinating properties described above would be beneficial for absorbing more electrolytes, thus facilitating ionic transportation and energy storage [41].
To further understand the composition, elemental analyses of A-PVP-NC and PI-NC were carried out to determine the percentage of C, O and N. Prior to the pyrolysis, the nitrogen (oxygen) content was determined to be 11.3% (32.6%) and 6.0% (24.4%) for PVP and PI, respectively. This difference is reasonably ascribed to the molecular weight of the former (1,300,000) being much higher than that of the latter. It is found that the proportion Owing to the A-PVP-NC being achieved by incorporating the KCl and K 2 CO 3 then following through a high-temperature pyrolysis and rinsing procedures, it is reasonably believed that highly porous architectures could be revealed. As expected, numerous hierarchical pores were shown, as seen in Figure 1e. In contrast, a non-porous structure was revealed from the PI-NC (Figure 1f). To accurately determine the classification of pores and specific surface area (SSA) of the A-PVP-NC, the nitrogen adsorption-desorption isotherm measurement was conducted. Figure S1 illustrates a typical isotherm curve, referred to as type I, and a predominant pore diameter less than 2 nm was detected, indicating a microporous characteristic of the A-PVP-NC [34]. Calculated by the Brunauer-Emmett-Teller (BET) method, the SSA value was as high as 1628 m 2 /g. Moreover, it is noticed that the SSA value of micropores was 1483 m 2 /g, whereas that for mesopores was just 52 m 2 /g. Accordingly, the total pore volume collected at P/P 0 = 0.995 was 0.79 cm 3 /g, while the pore size distributions of ultramicropores, micropores and mesopores were also measured to be 0.25, 0.62, and 0.17 cm 3 /g, respectively (Table S1). The fascinating properties described above would be beneficial for absorbing more electrolytes, thus facilitating ionic transportation and energy storage [41].
To further understand the composition, elemental analyses of A-PVP-NC and PI-NC were carried out to determine the percentage of C, O and N. Prior to the pyrolysis, the nitrogen (oxygen) content was determined to be 11.3% (32.6%) and 6.0% (24.4%) for PVP and PI, respectively. This difference is reasonably ascribed to the molecular weight of the former (1,300,000) being much higher than that of the latter. It is found that the proportion of carbon in both materials was the same, at 86%, whereas that of oxygen is 8.0% and 5.7% in A-PVP-NC and PI-NC in turn. The primary influential factor is the percentage of nitrogen; the value of PI-NC was 3.4%, six times as much as its presence in A-PVP-NC. Kim et al. monitored the pyrolysis reaction of the two employed polymers and discovered that PVP completely decomposes between 350-450 • C, whereas PI pyrolyzes at 550-650 • C [42]. The main reason for the high stability of the PI structure is the presence of resonance between the nitrogen atom and two carbonyl groups in ortho positions in the same ring, while the resonance in PVP is between the nitrogen atom and one carbonyl group [43]. Notably, the pores of PI-NC are sealed by deposited carbon, resulting from the pyrolysis process [44]. Consequently, the surface of PI-NC is non-porous, and it impedes the removal of nitrogen from the structure. On the contrary, the PVP is utilized as a pore-forming agent due to gas emissions, resulting in the elimination of most of the nitrogen products as gases during the pyrolysis process [45]. Moreover, it is also reported that the nitrogen species contained in the carbon precursor/char were preferentially removed during chemical activation with K-based salts [34]. These would support the result of the higher nitrogen percentage for PI-NC after pyrolysis, as opposed to A-PVP-NC. According to the results discussed above, it is rationally anticipated that the A-PVP-NC, thus prepared, could contribute to more homogeneous growth of the NiS and provide sufficient pores as the electrolyte reservoir for enhancing the electrochemical performances more than those of the PI-NC.  [46]. In addition, a broad peak at 2θ = 26.3 • with less intensity was detected, mainly referring to the (002) plane of carbon materials. Interestingly, another peak corresponding to the (300) plane of NiS@carbon materials appeared at 2θ = 31.4 • [47]. These results suggest that the as-prepared carbon materials from two different sources did not contribute to the crystalline phase of NiS.
Because of the high SSA of, and more oxygen in, the A-PVP-NC, the NiS particles with a size of less than 10 nm were deposited onto the A-PVP-NC (Figure 2c). They were even better dispersed in comparison with the NiS/PI-NC, which is revealed in Figure 2d. This distribution would be beneficial for increasing the electrochemically active surface area of NiS for the redox reactions [47]. On the other hand, it is shown that fewer carbon signs were detected by the energy-dispersive X-ray spectroscopy (EDX) mapping in the NiS/PI-NC ( Figure S2b2) as compared with the NiS/A-PVP-C ( Figure S2a2). This observation would be attributed to the severe agglomeration of the NiS grown onto the PI-NC's surface, which was consistent with the micrograph shown in Figure 2d. As for the other elements (i.e., N, O, Ni, S), they were distributed throughout the resultant NiS/nitrogen-doped carbon nanocomposites. To clarify the effect of the microwave irritation on the percentage of N and O that existed in the NiS/nitrogen-doped carbon nanocomposites, the EA results were examined. It is observed that the nitrogen content for the NiS/A-PVP-NC and NiS/PI-NC was 0.33% and 3.1%, respectively, showing no significant change as compared to the pristine nitrogen-doped carbon samples (A-PVP-NC: 0.47% and PI-NC: 3.4%). These results reasonably suggest that the nitrogen contents detected in the resultant nanocomposites originated from the A-PVP-NC and PI-NC, implying that no extra nitriding occurred under 150 • C of the microwave irradiations.
To identify detailed information about the chemical environments of our NiS/nitrogendoped carbon nanocomposites, XPS analysis was conducted, while the corresponding spectra analyzed using the Gaussian-Lorentzian fitting method are illustrated in Figure 3 and Figure S3. It has previously been confirmed that the nitrogen atoms were incorporated into the NiS-based nanocomposites by an elemental analyzer. Here, we did not especially highlight the C-N bonding in C 1s spectra for better reading. From the C 1s spectra plotted in Figure 3a and Figure S3a (4) at 289.7 eV) deconvoluted from a higher energy level were also observed, which was attributed to the contribution of sp 3 -type carbon [39]. For the S 2p spectrum (Figure 3b), the constitute peaks ((1)~(3)) between the binding energy of 161 eV and 163 eV are assigned to S 2− within the NiS. However, more peak intensity and area (around 167.8 eV, peak 4) generated from the NiS/A-PVP-NC were noticed, which is likely due to more sulfate ions resulting from surface oxidation [46]. For the O 1s spectrum (Figure 3c), the peaks at the binding energies of 530.7 eV (1) and 532.2 eV (3) are associated with the Ni 3+ OOH and Ni 2+ SO 4 [48], whereas the peaks at the binding energies of 531.5 eV (2) and 533.5 eV (4) could be owed to the O=C−O bond and C=O bond, respectively [49]. Due to the surface oxidation evidenced in Figure 3b,c, the mixing valence states (Ni 2+ : 852.7 eV and Ni 3+ : 855.4 eV) were observed from Figure 3d and Figure S3d. Worth mentioning is that the proportion of Ni 3+ in the NiS/A-PVP-NC is higher than that of the NiS/PI-NC, that is, 82% vs. 53%, which could provide rich redox reactions of NiS [50]. For the N 1s spectrum, the typical fingerprints corresponding to pyridinic-N, pyrrolic-N and graphitic-N were revealed in the 397.6 eV, 399.2 eV and 400.1 eV [51]. The above results not only clearly identify the binding configurations within the NiS/nitrogen-doped carbon nanocomposites, but also help us to better understand the functions of nitrogen and oxygen. To identify detailed information about the chemical environments of our NiS/nitrogendoped carbon nanocomposites, XPS analysis was conducted, while the corresponding spectra analyzed using the Gaussian-Lorentzian fitting method are illustrated in Figure 3 and Figure  S3. It has previously been confirmed that the nitrogen atoms were incorporated into the NiSbased nanocomposites by an elemental analyzer. Here, we did not especially highlight the C-N bonding in C 1s spectra for better reading. From the C 1s spectra plotted in Figures 3a and S3a, a predominant peak assigned for the sp 2 -type carbon with C=C bond (1) appeared at 284.8 eV. Moreover, three peaks (C−O bond (2) at 286.3 eV, C=O bond (3) at 287.9 eV and O=C−O bond (4) at 289.7 eV) deconvoluted from a higher energy level were also observed, which was attributed to the contribution of sp 3 -type carbon [39]. For the S 2p spectrum (Figure 3b), the constitute peaks ((1)~(3)) between the binding energy of 161 eV and 163 eV are assigned to S 2− within the NiS. However, more peak intensity and area (around 167.8 eV, peak 4) generated from the NiS/A-PVP-NC were noticed, which is likely due to more sulfate ions resulting from surface oxidation [46]. For the O 1s spectrum (Figure 3c), the peaks at the binding energies of 530.7 eV (1) and 532.2 eV (3) are associated with the Ni 3+ OOH and Ni 2+ SO4 [48], whereas the As discussed earlier, the hierarchically porous architectures could provide a variety of properties, enlarging the electrochemical performances of the EES devices [52]. Hence, the compatibility between the NiS/nitrogen-doped carbon nanocomposites and alkaline electrolytes is worth confirming. As presented in Figure 4, it is shown that the NiS/A-PVP-NC was wetted immediately, while the NiS/PI-NC resisted the electrolyte, as highlighted by the white circle. Although the high nitrogen content increases wettability [53], the oxygen-containing functional groups and porous structures deliver more significant contributions [41,54,55]. As a result, the NiS-A-PVP-NC possessed superior wettability, with a 6 M KOH electrolyte, than the NiS/PI-NC. This can also be supported by elemental analysis, since the NiS/A-PVP-NC possessed a much higher oxygen content than the NiS/PI-NC, that is, 7.5% vs. 5.1%. As discussed earlier, the hierarchically porous architectures could provide a variety of properties, enlarging the electrochemical performances of the EES devices [52]. Hence, the compatibility between the NiS/nitrogen-doped carbon nanocomposites and alkaline electrolytes is worth confirming. As presented in Figure 4, it is shown that the NiS/A-PVP-NC was wetted immediately, while the NiS/PI-NC resisted the electrolyte, as highlighted by the white circle. Although the high nitrogen content increases wettability [53], the oxygen-containing functional groups and porous structures deliver more significant contributions [41,54,55]. As a result, the NiS-A-PVP-NC possessed superior wettability, with a 6 M KOH electrolyte, than the NiS/PI-NC. This can also be supported by elemental analysis, since the NiS/A-PVP-NC possessed a much higher oxygen content than the NiS/PI-NC, that is, 7.5% vs. 5.1%.   As discussed earlier, the hierarchically porous architectures could provide a variety of properties, enlarging the electrochemical performances of the EES devices [52]. Hence, the compatibility between the NiS/nitrogen-doped carbon nanocomposites and alkaline electrolytes is worth confirming. As presented in Figure 4, it is shown that the NiS/A-PVP-NC was wetted immediately, while the NiS/PI-NC resisted the electrolyte, as highlighted by the white circle. Although the high nitrogen content increases wettability [53], the oxygen-containing functional groups and porous structures deliver more significant contributions [41,54,55]. As a result, the NiS-A-PVP-NC possessed superior wettability, with a 6 M KOH electrolyte, than the NiS/PI-NC. This can also be supported by elemental analysis, since the NiS/A-PVP-NC possessed a much higher oxygen content than the NiS/PI-NC, that is, 7.5% vs. 5.1%.

Electrochemical Studies
To explore the influence of nitrogen content and structural characteristics on the electrochemical performances of as-synthesized NiS/nitrogen-doped carbon nanocomposites, the cyclic voltammetry was carried out in the potential range between 0 V and 0.5 V (vs. SCE) at a scan rate of 10 mV/sec to 100 mV/sec under an N 2 -saturated 6 M KOH electrolyte. The corresponding cyclic voltammograms (CVs) depicted in Figure 5a,b show that a couple of well-defined redox peaks in each NiS/nitrogen-doped carbon nanocomposite were detected. The resulting curves are distinguishable from the rectangular shape for electric double-layer capacitors, indicating that the energy storage of the NiS electrode is mainly attributed to the reversible Faradic redox reaction of NiS + OH − ↔ NiSOH + e − [56]. Besides, all CV graphs have stable shapes even as the scan rate increased to 100 mV/sec, showing the outstanding reversible reaction and rapid redox processes [57]. Especially highlighted is that current densities outputted from the NiS/A-PVP-NC were superior to those of the NiS/PI-NC among the various scan rates, that is, 29.1 A/g vs. 20.4 A/g at 40 mV/sec ( Figure S4a). However, it is apparent that there is a slight shift at oxidation and reduction peaks towards the positive and negative directions of potential when the scan rate rose, indicating that the charge transfer from the electrolyte to electrode materials and vice versa is the rate-determining step at a high scan rate [57]. Considering the potential difference (∆V) between anodic and cathodic peak positions, the ∆V values of NiS/A-PVP-NC shown in Figure S4b were smaller and even better than those of the NiS/PI-NC, especially at 100 mV/sec, that is, 197 mV vs. 227 mV (vs. SCE). Table S2 also summarizes the detailed values for both samples. The results discussed above can be recognized in the uniform distribution of the NiS onto A-PVP-NC and the better wettability between the NiS/A-PVP-NC and 6 M KOH electrolyte, as proven in Figures 2c and 4. Supported by the positive results disclosed in the CVs, the galvanostatic charge-discharge (GCD) tests were conducted in the potential range of 0 V to 0.4 V (vs. SCE) at current densities between 1 A/g to 10 A/g. As illustrated in Figure 6a,b, the charge-discharge plateaus were evidently displayed, and were consistent with the redox potentials obtained in the CV curves. In comparison with the NiS/PI-NC, the specific capacities reflected from the NiS/A-PVP-NC at all current densities were greater. The value recorded at 10 A/g was 3.7 times more than that of NiS/PI-NC, that is, 74 C/g vs. 20 C/g (Figure 6c). This considerable enhancement would not only be supported by a uniform distribution of the NiS onto A-PVP-NC and a larger proportion of the Ni 3+ observed from the XPS evidence (Figure 3d), but would also be correlated with the better wettability between the NiS/A-PVP-NC and alkaline electrolyte, and with the lower electronic resistance [46] confirmed by EIS spectra (Figure 6d). Furthermore, it is apparent that a semi-circuit at medium frequency was observed from the NiS/PI-NC, which is correlated with the presence of double-layer capacitance due to its low hydrophilic nature [62]. This causes higher charge-transfer resistance as compared with the NiS-A-PVP-NC. In addition, the small diffusion current appeared in the NiS/PI-NC at low frequency as a straight line, which is likely due to the absence of pores and poor wettability between the material surface and the electrolyte, leading to a shorter diffusion pathway [63]. Table 1 compares the capaci- For further understanding of the storage mechanism, their peak currents as functions of the square root of the scan rates are plotted in Figure 5c,d. Obviously, a shift of the anodic and cathodic peaks to higher (oxidation reaction) and lower (reduction reaction) potentials appeared as the scan rate gradually increased. This refers to the rate-determining step being controlled by the diffusion of electrolyte ions, which implies that both NiS/nitrogendoped carbon nanocomposites have the capability to store charges as battery-type electrode materials [58]. Encouragingly, the slope of NiS/A-PVP-NC is greater than that of NiS/PI-NC, suggesting rapid diffusion of the electrolyte ions. Moreover, Figure S5 shows the relationship between current density and scan rate based on the following equation: i = kν b , where i is the current density (A/g), v is the scan rate (V/sec), and k and b are adjustable parameters. The value of b provides information about the charge storage kinetics. b = 1 represents an ideal capacitive behavior, while b = 0.5 suggests a diffusion controlled process. Such values can be calculated by plotting the logarithm of the absolute current density against the logarithm of the scan rate, that is log i = b log v + k [59,60]. The obtained b values of NiS/A-PVP-NC and NiS/PI-NC were calculated to be 0.62 and 0.58 from the cathodic (reduction) peaks, signifying that the dominant process is diffusion controlled. This confirmed again that both electrodes process a battery-like behavior. Once the NiS nanoparticles had contact with the KOH electrolyte, the electroactive species, NiSOH, were produced accordingly. Meanwhile, the amount of such species depends on the contact area of the NiS nanoparticles. Therefore, rapid diffusion of the electrolyte ions could be helpful for a significant increase in capacitance [61].
Supported by the positive results disclosed in the CVs, the galvanostatic chargedischarge (GCD) tests were conducted in the potential range of 0 V to 0.4 V (vs. SCE) at current densities between 1 A/g to 10 A/g. As illustrated in Figure 6a,b, the chargedischarge plateaus were evidently displayed, and were consistent with the redox potentials obtained in the CV curves. In comparison with the NiS/PI-NC, the specific capacities reflected from the NiS/A-PVP-NC at all current densities were greater. The value recorded at 10 A/g was 3.7 times more than that of NiS/PI-NC, that is, 74 C/g vs. 20 C/g (Figure 6c). This considerable enhancement would not only be supported by a uniform distribution of the NiS onto A-PVP-NC and a larger proportion of the Ni 3+ observed from the XPS evidence ( Figure 3d), but would also be correlated with the better wettability between the NiS/A-PVP-NC and alkaline electrolyte, and with the lower electronic resistance [46] confirmed by EIS spectra (Figure 6d). Furthermore, it is apparent that a semi-circuit at medium frequency was observed from the NiS/PI-NC, which is correlated with the presence of double-layer capacitance due to its low hydrophilic nature [62]. This causes higher charge-transfer resistance as compared with the NiS-A-PVP-NC. In addition, the small diffusion current appeared in the NiS/PI-NC at low frequency as a straight line, which is likely due to the absence of pores and poor wettability between the material surface and the electrolyte, leading to a shorter diffusion pathway [63]. Table 1 compares the capacitive performances of various metal sulfide/carbon nanocomposites. The specific capacities reported in the present study may not be as high as most of those announced in the literature. However, it is worth mentioning that the capacity retention for the NiS/A-PVP-NC was above that of most cases, for example, CoS/g-C 3 N 4 [45], NiS/N-doped carbon [57], CuS/rGO [64], VS 4 /CNTs/rGO [65], NiS/N-doped graphene [66], Ni 3 S 2 /CNFs [67], and NiS/rGO [68].

Conclusions
In summary, this study presents a facile and efficient microwave-assisted approach for synthesizing NiS/nitrogen-doped carbon nanocomposites to explore the influence of nitrogen content and structural characteristics of polymer-derived nitrogen-doped carbon materials on the capacitive performance of alkaline supercapacitors. As indicated, the high purity, great crystallinity and hierarchically porous architectures of NiS/A-PVP-NC were successfully acquired by a microwave reactor at 150 • C for 1 h. Nevertheless, the dispersity of NiS nanoparticles varied considerably depending on the different textural properties obtained from the polymer-derived nitrogen-doped carbon materials. Due to the fascinating properties and synergistic features, that is., uniform distribution of the NiS, a higher proportion of Ni 3+ , better wettability, and superior electronic conductivity of NiS/A-PVP-NC, a smaller potential difference and excellent reversibility, rate capability and capacity retention were achieved accordingly. The results shown in this study suggest that the oxygen-containing functional groups and porous architectures within the carbon materials have more dramatic and positive effects on the electrochemical performances of alkaline supercapacitors than does the nitrogen content.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.