Highly active N, S Co-Doped Ultramicroporous Carbon for High-Performance Supercapacitor Electrodes

N, S-doped ultramicroporous carbons (NSUC-x) with a high nitrogen/sulfur content and a narrow pore-size distribution of around 0.55 nm were firstly prepared using L-cysteine as a nitrogen and sulfur source. The phase, graphitization degree, morphology, specific surface area, pore structure and surface condition of NSUC-x are investigated to analyze the key role in electrochemical performance. Such an ultramicroporous structure and N, S doping not merely provide a high-specific surface area and a suitable pore size, but also induce a good wettability for the fast transport and adsorption of electrolyte ions. Due to the above strategies, the typical NSUC-0.4 exhibits a high gravimetric capacitance of 339 F g−1 at 0.5 A g−1 as well as a capacity retention of 91.6% after 10,000 cycles in a three-electrode system using a 6 M KOH electrolyte. More attractively, a NSUC-0.4-assembled symmetrical supercapacitor delivers an energy output of 7.4 Wh kg−1 at 100 W kg−1 in 6 M KOH as well as a capacity retention of 92.4% after 10,000 cycles, indicating its practical application prospect. Our findings open up new prospects for the design and electrochemical application of N, S-doped ultramicroporous carbons.


Introduction
With the fast economic development and the excessive consumption of fossil fuels, the energy dilemma and environmental pollution have made it essential to develop renewable and clean energy devices for a sustainable society [1][2][3]. As promising energy storage devices, supercapacitors have drawn increasing attention due to their merits of being low cost and having long-cycle stability, fast charge-discharge rates, superior power density to batteries and higher energy density than traditional capacitors [4][5][6]. Based on the energy storage mechanism, supercapacitors are commonly classified into two types: (1) electric double-layer capacitors (EDLCs) store energy by the electrostatic accumulation of charges at the electrode-electrolyte interfaces and (2) pseudo-capacitors store energy by fast Faradaic reactions between the electrode and electrolyte [7][8][9].
Carbon materials, including activated carbon, carbon nanotubes and carbon aerogels, are mainly applied as EDLC electrodes due to their high specific surface area and designable pore structure [10][11][12][13]. For microporous carbons, the <1 nm pores have a relationship with an anomalous increase in electrochemical performance [14,15]. Moreover, owing to the small hydrated radius of K + (3.3 Å), Na + (3.6 Å), OH − (3.0 Å) and SO 4 2− (3.8 Å), the >0.5 nm micropores are usually considered to be electrochemically available in aqueous electrolytes [16][17][18]. Bandosz et al. used palm date pits as precursors to prepare ultramicroporous carbon with a regular pore size of around 0.55 nm and a capacitance of 114 F g −1 at 1 mV s −1 in 6 M KOH [19]. They proved that not only <2 nm pores govern the electrochemical performance in aqueous electrolytes, but also that the differences in the carbon materials, 10 wt.% polytetrafluoroethylene (PTFE) and 10 wt.% graphite, pressing on nickel foam and dried at 60 • C for 24 h. A voltage range from −1 to 0 V was applied in cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests, whose specific capacitance (C, F g −1 ) were calculated according to the Formulas (1) and (2), respectively: (1) Moreover, a coin cell (NEWARE CR2032) was assembled with two identical NSUC-0.4 electrodes, 6 M KOH electrolyte, a piece of filter paper separator to analyze the twoelectrode symmetric system. The capacitance of a single electrode (C two , F g −1 ), the energy densities (E, Wh kg −1 ) and power densities (P, W kg −1 ) of the device were obtained according to Formulas (3)-(5): In Formulas (1)-(5), m (g) is the mass of all active materials, ∆V (V) is the operation potential window, ∆t (s) is the discharging current time, r (V s −1 ) is the scan rate and I (A) is the response current (Formula (1)) and the discharging current (Formulas (2) and (3)).

Results and Discussion
In Figure 1a, the XRD patterns of NSUC-x in the range from 10 to 80 • exhibit two broad diffraction peaks around 23 and 44 • , which are ascribed to the (002) and (001) lattice facets of amorphous carbon [30]. The graphitization degrees of all samples were further detected by Raman analysis. As depicted in Figure 1b, all spectra are deconvoluted into four Gaussian-Lorentzian peaks according to the literature [31,32]. Such four peaks around 1222, 1353, 1489 and 1595 cm −1 correspond to I band (caused by the impurities near carbon atoms), D band (derived from the defects/disorders in sp3 structure), D band (ascribed to the defects from stacked graphene layers) and G band (attributed to the vibration of graphitic sp 2 -type carbon), respectively [33,34]. The intensity ratio of the D band to G band (I D /I G ) reflects the indicator of concrete graphitization degree. Apparently, the relative I D /I G in NSUC-0.4 is the lowest value (0.81) among all samples and implies the highest graphitization degree, which has a positive feedback on electronic conductivity.
The N 2 physisorption isotherms of NSUC-x presented in Figure 2a belong to the type I curve according to the classification of IUPAC. As noted in all isotherms, the steep increase at low relative pressure (P/P 0 < 0.1) and the flat plateau (0.1 ≤ P/P 0 ≤ 1.0) are ascribed to abundant micropores. Moreover, the pore size distribution curves of NSUC-x depicted in Figure 2b exhibit unimodal and sharp peaks around 0.55 nm, indicating the presence of a highly ultramicroporous structure. As listed in Table 1 to compare the porous structure parameters, the largest surface area (891 m 2 g −1 ) is achieved by NSUC-0.4, which also has the largest microporous specific surface area and total pore volume. The addition of L-cysteine has an important influence on the specific surface area of NSUC-x. When L-cysteine is not sufficient in the reaction, phloroglucinol cannot react with L-cysteine thoroughly, resulting in a decrement in the crosslinking density between phloroglucinol and L-cysteine. The region with a low crosslinking density pyrolyzes into gaseous products and leads to the collapse of the carbon matrix, resulting in the lower specific surface area in NSUC-0.3. On the otherwise, increasing the mass of L-cysteine beyond the optimum value increases the contents of N and S in the carbon samples. Due to the lower covalent band energy of C-N compared with C-C, the release of abundant nitrogen and sulfur atoms during the carbonization results in the partial collapse of the carbon framework, thus leading to the decrement in the specific surface area in NSUC-0.5. The N2 physisorption isotherms of NSUC-x presented in Figure 2a belong to the type I curve according to the classification of IUPAC. As noted in all isotherms, the steep increase at low relative pressure (P/P0 < 0.1) and the flat plateau (0.1 ≤ P/P0 ≤ 1.0) are ascribed to abundant micropores. Moreover, the pore size distribution curves of NSUC-x depicted in Figure 2b exhibit unimodal and sharp peaks around 0.55 nm, indicating the presence of a highly ultramicroporous structure. As listed in Table 1 to compare the porous structure parameters, the largest surface area (891 m 2 g −1 ) is achieved by NSUC-0.4, which also has the largest microporous specific surface area and total pore volume. The addition of L-cysteine has an important influence on the specific surface area of NSUC-x. When Lcysteine is not sufficient in the reaction, phloroglucinol cannot react with L-cysteine thoroughly, resulting in a decrement in the crosslinking density between phloroglucinol and L-cysteine. The region with a low crosslinking density pyrolyzes into gaseous products and leads to the collapse of the carbon matrix, resulting in the lower specific surface area in NSUC-0.3. On the otherwise, increasing the mass of L-cysteine beyond the optimum value increases the contents of N and S in the carbon samples. Due to the lower covalent band energy of C-N compared with C-C, the release of abundant nitrogen and sulfur atoms during the carbonization results in the partial collapse of the carbon framework, thus leading to the decrement in the specific surface area in NSUC-0.5.   The N2 physisorption isotherms of NSUC-x presented in Figure 2a belong to the type I curve according to the classification of IUPAC. As noted in all isotherms, the steep increase at low relative pressure (P/P0 < 0.1) and the flat plateau (0.1 ≤ P/P0 ≤ 1.0) are ascribed to abundant micropores. Moreover, the pore size distribution curves of NSUC-x depicted in Figure 2b exhibit unimodal and sharp peaks around 0.55 nm, indicating the presence of a highly ultramicroporous structure. As listed in Table 1 to compare the porous structure parameters, the largest surface area (891 m 2 g −1 ) is achieved by NSUC-0.4, which also has the largest microporous specific surface area and total pore volume. The addition of L-cysteine has an important influence on the specific surface area of NSUC-x. When Lcysteine is not sufficient in the reaction, phloroglucinol cannot react with L-cysteine thoroughly, resulting in a decrement in the crosslinking density between phloroglucinol and L-cysteine. The region with a low crosslinking density pyrolyzes into gaseous products and leads to the collapse of the carbon matrix, resulting in the lower specific surface area in NSUC-0.3. On the otherwise, increasing the mass of L-cysteine beyond the optimum value increases the contents of N and S in the carbon samples. Due to the lower covalent band energy of C-N compared with C-C, the release of abundant nitrogen and sulfur atoms during the carbonization results in the partial collapse of the carbon framework, thus leading to the decrement in the specific surface area in NSUC-0.5.   S BET : specific surface area (m 2 g −1 ); S Micro : micropore surface area (m 2 g −1 ); V tol : total pore volume (cm 3 g −1 ).
The surface chemical conditions of NSUC-x were investigated by XPS techniques. All spectra presented in Figure 3a exhibit signals for the binding energies of C 1s, O 1s, N 1s and S 2p, indicating the presence of four elements: C, N, S and O [35]. As tabulated in Table 1, the N and S content increased from 4.91 and 2.14 at.% to 8.11 and 3.01 at.% as the addition of L-cysteine increased. N 1s state involves four overlapping peaks of pyridine nitrogen oxide (N-X), graphitic nitrogen (N-Q), pyrrolic N (N-5) and pyridinic nitrogen (N-6). Usually, the N-5 with good electro-donor characteristics and high charge mobility is considered to enhance carbon catalytic activity in electron-transfer reactions and thus effectively improves electrochemical performance [36]. Meanwhile, the N−6 provides a pair of electrons for conjugation with the π-conjugated rings and thus can bring in electron-donor properties to carbon materials and efficiently enhance capacitances [37]. The N−Q can significantly boost the electron transfer and improve the conductivity of nitrogen-doping carbon materials [38]. The signals for S 2p state can be identified as three resolved peaks at 163.5 eV, 164.7 eV and 167.4 eV, being ascribed to S 2p 3/2 , S 2p 1/2 and oxidized sulfur moiety (−SO n −). The two prominent peaks correspond to the S 2p 3/2 and S 2p 1/2 of the C-S-C covalent bond of the thiophene-S caused by spin-orbit coupling [39]. N, S co-doping can comprehensively improve the electrochemical performance of carbon electrodes by optimizing the surface condition, electronic conductivity and Faradaic reactivity. and S 2p, indicating the presence of four elements: C, N, S and O [35]. As tabulated in Table 1, the N and S content increased from 4.91 and 2.14 at.% to 8.11 and 3.01 at.% as the addition of L-cysteine increased. N 1s state involves four overlapping peaks of pyridine nitrogen oxide (N-X), graphitic nitrogen (N-Q), pyrrolic N (N-5) and pyridinic nitrogen (N-6). Usually, the N-5 with good electro-donor characteristics and high charge mobility is considered to enhance carbon catalytic activity in electron-transfer reactions and thus effectively improves electrochemical performance [36]. Meanwhile, the N−6 provides a pair of electrons for conjugation with the π-conjugated rings and thus can bring in electron-donor properties to carbon materials and efficiently enhance capacitances [37]. The N−Q can significantly boost the electron transfer and improve the conductivity of nitrogen-doping carbon materials [38]. The signals for S 2p state can be identified as three resolved peaks at 163.5 eV, 164.7 eV and 167.4 eV, being ascribed to S 2p3/2, S 2p1/2 and oxidized sulfur moiety (−SOn−). The two prominent peaks correspond to the S 2p3/2 and S 2p1/2 of the C-S-C covalent bond of the thiophene-S caused by spin-orbit coupling [39]. N, S co-doping can comprehensively improve the electrochemical performance of carbon electrodes by optimizing the surface condition, electronic conductivity and Faradaic reactivity. All morphologies of NSUC-x observed in SEM images (Figure 4a-c) show a large bulk structure with a coarse surface, which becomes smooth with the increasing addition of L-cysteine. The morphology and porous structure of NSUC-0.4 were further investigated by the TEM method (Figure 4d). It can be noted that NSUC-0.4 shows a disordered ultramicroporous structure, which is consistent with the above pore size distribution. As shown in Figure 4e, NSUC-0.4 shows a small water contact angle of 19°, implying that heteroatom doping can ameliorate the surface hydrophilicity of the carbons [40]. In this All morphologies of NSUC-x observed in SEM images (Figure 4a-c) show a large bulk structure with a coarse surface, which becomes smooth with the increasing addition of L-cysteine. The morphology and porous structure of NSUC-0.4 were further investigated by the TEM method (Figure 4d). It can be noted that NSUC-0.4 shows a disordered ultramicroporous structure, which is consistent with the above pore size distribution. As shown in Figure 4e, NSUC-0.4 shows a small water contact angle of 19 • , implying that heteroatom doping can ameliorate the surface hydrophilicity of the carbons [40]. In this regard, the aqueous electrolyte ions have a good pore accessibility in the NSUC-0.4 electrode and can readily saturate into the inner surface, facilitating the electrochemical capacitance performance.
The CV curves of NSUC-x in Figure 5a exhibit a quasi-rectangular shape with a deformation, indicating the EDLC and pseudocapacitance energy storage. As displayed, the capacitance of NSUC-0.4 calculated from CV curves is much higher than those of the other two electrodes. For the NSUC-0.4 electrode in Figure 5b, an unobvious current hump occurs at −0.37 V as the scan rate decreases to 5 mV s −1 , indicating the presence of pseudocapacitance induced by N, S co-doping. The capacitive contribution of NSUC-0.4 (Figure 5c,d) was decoupled as the equation of i = k 1 v + k 2 v 1/2 , where k 1 v and k 2 v 1/2 equal to the current density associated with fast-and slow-kinetic process [41,42]. The fast-kinetic capacitance of NSUC-0.4 is 133 F g −1 , which is mainly contributed to EDLC.
Micromachines 2022, 12, x FOR PEER REVIEW 6 of 13 regard, the aqueous electrolyte ions have a good pore accessibility in the NSUC-0.4 electrode and can readily saturate into the inner surface, facilitating the electrochemical capacitance performance. The CV curves of NSUC-x in Figure 5a exhibit a quasi-rectangular shape with a deformation, indicating the EDLC and pseudocapacitance energy storage. As displayed, the capacitance of NSUC-0.4 calculated from CV curves is much higher than those of the other two electrodes. For the NSUC-0.4 electrode in Figure 5b, an unobvious current hump occurs at −0.37 V as the scan rate decreases to 5 mV s −1 , indicating the presence of pseudocapacitance induced by N, S co-doping. The capacitive contribution of NSUC-0.4 ( Figure  5c,d) was decoupled as the equation of ⁄ , where k1v and k2v 1/2 equal to the current density associated with fast-and slow-kinetic process [41,42]. The fast-kinetic capacitance of NSUC-0.4 is 133 F g −1 , which is mainly contributed to EDLC. The GCD profiles of NSUC-x at 1.0 A g −1 are shown in Figure 6a. In the manner of CV curves, all GCD profiles exhibit a quasi-triangular distribution with a little distortion, implying satisfactory electrochemical capacitive properties with a pseudocapacitive procedure. Meanwhile, the specific capacitance (283 F g −1 ) of the NSUC-0.4 electrode integrated from GCD is dramatically higher than those of NSUC-0.3 (218 F g −1 ) and NSUC-0.5 (189 F g −1 ). In Figure 6b, NSUC-0.4 exhibits an ultrahigh capacitance of 339 F g −1 at 0.5 A g −1 as well as remains 189 F g −1 with a triangle shape up to 10 A g −1 , signaling a high-rate chargedischarge capability. The charging-discharging kinetics of NSUC-0.4 were also discussed according to the formula ⁄ , where CT, CE and t are the total capacitance, EDLC and discharging time, respectively [43,44]. As shown in Figure 6c, the CE of NSUC- The GCD profiles of NSUC-x at 1.0 A g −1 are shown in Figure 6a. In the manner of CV curves, all GCD profiles exhibit a quasi-triangular distribution with a little distortion, implying satisfactory electrochemical capacitive properties with a pseudocapacitive procedure. Meanwhile, the specific capacitance (283 F g −1 ) of the NSUC-0.4 electrode integrated from GCD is dramatically higher than those of NSUC-0.3 (218 F g −1 ) and NSUC-0.5 (189 F g −1 ). In Figure 6b, NSUC-0.4 exhibits an ultrahigh capacitance of 339 F g −1 at 0.5 A g −1 as well as remains 189 F g −1 with a triangle shape up to 10 A g −1 , signaling a high-rate charge-discharge capability. The charging-discharging kinetics of NSUC-0.4 were also discussed according to the formula C T = C E + kt 1/2 , where C T , C E and t are the total capacitance, EDLC and discharging time, respectively [43,44]. As shown in Figure 6c, the C E of NSUC-0.4 is 155 F g −1 and close to the value (133 F g −1 ) fitted by CV curves. Thereby, the ultramicropores and N, S doping, which mainly contributed to the slowkinetic capacitance, are crucial to the enhancement of the electrochemical energy storage. The 10,000 consecutive charge-discharge cycles in Figure 6d were tested on the NSUC-0.4 electrode with a capacity retention of 91.6% at 2 A g −1 , indicating an excellent cycling stability. The electrochemical performance of NSUC-0.4 is compared with that of other carbon materials in Table 2.   Nyquist plots of NSUC-x electrodes measured in 6 M KOH aqueous solution in a frequency range of 0.01 to 10 5 Hz are shown in Figure 7a. Obviously, NSUC-0.4 electrode not only exhibits the equivalent series resistance (R s , 1.24 Ω) and the charge-transfer resistance (R ct , 1.06 Ω) with the smallest quasi-semicircle in the high-frequency region. Abundant micropores, especially ultramicropores, can benefit the fast transportation and diffusion of electrolyte ions to enhance the electrochemical performance. At the same time, the existence of nitrogen and sulfur elements can improve the surface properties and electric conductivity. Meanwhile, the relaxation time (τ) and the diffusive resistance (σ) of NSUC-0.4 obtained from Figure 7b,c are 1.96 s and 1.11 Ω s −1/2 , respectively, which were lower than those of NSUC-0.3 and NSUC-0.5 [54]. These ESR, τ and σ values imply the low resistance and the fast electrolyte/ion diffusion achieved by the NSUC-0.4 electrode.      Figure 8 shows the electrochemical performance of NSUC-0.4 measured in twoelectrode system using a 6 M KOH as electrolyte. In Figure 8a, all curves from 5 to 100 mV s −1 exhibit the rectangular shapes, implying the rapid and reversible chargedischarge procedure. Moreover, GCD curves detected with 0.2 to 10 A g −1 (Figure 8b) show symmetrical triangle shapes, further confirming the EDLC performance. The rate performance in Figure 8c demonstrates the high reversibility of the NSUC-0.4 electrode. As shown in the Ragone plots in Figure 8d, the KOH-loaded device displays a high energy density of 7.4 Wh kg −1 at 100 W kg −1 with excellent long cycle life (retention of 92.4%) and coulombic efficiency (98.5%) after 10,000 cycles (Figure 8e). Nyquist plots in Figure 8f further compared the resistance of the NSUC-0.4 electrode before and after 10,000 cycles. Both of the plots are nearly parallel to the imaginary axis in the low-frequency range, 45 • diagonal lines in the intermediate frequency region and semicircles in the high-frequency area. The R s (0.64 Ω) and R ct (0.66 Ω) of the 10,000th cycle are slightly larger than those of the 1 st cycle (R s = 0.52 Ω and R ct = 0.42 Ω), meaning a low electrochemical resistance in the KOH-loaded device with a good stability.

Conclusions
N, S co-doped ultramicroporous carbons were fabricated by applying the L-cysteine as a nitrogen and sulfur source. The high nitrogen (7.35 at.%)/sulfur (2.37 at.%) content as well as the ultramicroporous structure enhance the electrochemical performance of NSUC-0.4 in the following aspects: (i) heteroatom doping provides additional pseudocapacitance and superior hydrophilic surface, and (ii) the superior hydrophilic surface and high graphitization degree make the high specific surface area (891 m 2 g −1 ) to be effectively employed in electrochemical processes. Thereby, NSUC-0.4 exhibits an excellent specific capacitance of 339 F g −1 at 0.5 A g −1 . The assembled symmetrical supercapacitor based on the NSUC-0.4 electrode delivers an energy output of 7.4 Wh kg −1 at 100 W kg −1 in 6 M KOH with a long cycle life. Our results firmly show a new avenue that the heteroatoms-doped ultramicroporous carbons provide for advanced energy storage devices with a high electrochemical performance.

Conclusions
N, S co-doped ultramicroporous carbons were fabricated by applying the L-cysteine as a nitrogen and sulfur source. The high nitrogen (7.35 at.%)/sulfur (2.37 at.%) content as well as the ultramicroporous structure enhance the electrochemical performance of NSUC-0.4 in the following aspects: (i) heteroatom doping provides additional pseudocapacitance and superior hydrophilic surface, and (ii) the superior hydrophilic surface and high graphitization degree make the high specific surface area (891 m 2 g −1 ) to be effectively employed in electrochemical processes. Thereby, NSUC-0.4 exhibits an excellent specific capacitance of 339 F g −1 at 0.5 A g −1 . The assembled symmetrical supercapacitor based on the NSUC-0.4 electrode delivers an energy output of 7.4 Wh kg −1 at 100 W kg −1 in 6 M KOH with a long cycle life. Our results firmly show a new avenue that the heteroatomsdoped ultramicroporous carbons provide for advanced energy storage devices with a high electrochemical performance.

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