Nitrogen, Phosphorus and Sulfur Co-Doped Pyrolyzed Bacterial Cellulose Nanofibers for Supercapacitors

Heteroatom doping is an effective way to raise the electrochemical properties of carbon materials. In this paper, a novel electrode material including nitrogen, phosphorus, and sulfur co-doped pyrolyzed bacterial cellulose (N/P/S-PBC) nanofibers was produced. The morphologies, structure characteristics and electrochemical performances of the materials were investigated by Scanning electron microscopy, Fourier transform infrared spectra, X-ray diffraction patterns, X-ray photoelectronic spectroscopy, N2 sorption analysis and electrochemical measurements. When 3.9 atom% of nitrogen, 1.22 atom% of phosphorus and 0.6 atom% of sulfur co-doped into PBC, the specific capacitance of N/P/S-PBC at 1.0 A/g was 255 F/g and the N/P/S-PBC supercapacitors’ energy density at 1 A/g was 8.48 Wh/kg with a power density of 489.45 W/kg, which were better than those of the N/P-PBC and N/S-PBC supercapacitors. This material may be a very good candidate as the promising electrode materials for high-performance supercapacitors.


Introduction
The remarkable progress in portable electronics achieved in recent decades has undoubtedly been driven by rechargeable energy storage systems such as supercapacitors, lithium-ion batteries, and others [1]. Supercapacitors is an essential reversible storage and releaser of electricity technology [2]; it can be divided into an electrochemical double-layer capacitor and pesudocapacitor, while the pesudocapacitor usually reveals a higher capacitance because of the Faradaic process from redox reaction [3]. Carbon-based supercapacitors have been considered as preferable alternative devices to replace traditional energy storage systems on account of their multiple advantages, including low weight and cost, high power delivery, and long cycling stability [4][5][6]. Heteroatom-doped carbon materials also have been an efficient method to enhance the pseudocapacitance effects, and the fabrication process is relatively easy to handle, versatile, and general [7].

Preparation of Heteroatom-Doped PBC
The purified BC pellicles was firstly neutralized with deionized water and then sliced into roundness (d = 20 mm) with a puncher. After this, the slices of BC were soaked in 100 mL NH 4 H 2 PO 4 , (NH 4 ) 2 SO 4 and NH 4 H 2 PO 4 /(NH 4 ) 2 SO 4 aqueous solution with different concentration, respectively. After that, the BC slices were oscillated at room temperature for 10 h and frozen in refrigerator. Subsequently, the slices were freeze-dried in a freeze dryer to evaporate the solvent. Finally, the as-obtained BC slices were heated in a nitrogen atmosphere at 2.0 • C/min to 520 • C and kept at this temperature for 1.0 h and then at 5.0 • C /min to 800 • C, and kept at that temperature for 1.0 h to form the pyrolyzed bacterial cellulose products (PBCs).
The doping content of heteroatom in the PBCs is controlled by adjusting the molarity of the NH 4 H 2 PO 4 or (NH 4 ) 2 SO 4 aqueous solution. The designation of samples was listed in Table 1. Table 1. The designation of samples prepared in this work.

Sample Codes Concentration of NH 4 H 2 PO 4 Solution (mol/L)
Concentration of (NH 4

Characterization of Materials
SEM images were performed on a Hitachi S-4800 field-emission scanning electron microanalyzer (Hitachi, Tokyo, Japan). The Fourier transform infrared spectra (FTIR) measurements were conducted on a Thermo Scientific Nicolet iS10 (Thermo Fisher, Shanghai, China). The phase structures of samples were analyzed by a Rigaku D/MAX-2500 (Rigaku, Shoshima, Tokyo, Japan). The X-ray photoelectronic spectroscopy (XPS) was collected on a Themno Fisher K-Aepna X-ray photoelectron spectrometer (Themno Fisher, Waltham, MA, USA) with Al Kα (1361eV) source. N 2 sorption isotherms of samples were analyzed by a Quantachrome instruments autosorb-iQ (Quantachorme, Shanghai, China) at 77 K.

Electrochemical Measurements
The electrochemical experiments were tested by cyclic voltammentry (CV) and galvanostatic charge-discharge in 2 mol L −1 H 2 SO 4 solutions, which were performed on a CHI 760D electrochemical workstation (Themno Fisher, Waltham, MA, USA) in a two-electrode system. The supercapacitors were prepared by employing the PBCs as free-standing electrode, a PP/PE membrane as the separator and a couple of stainless steel sheets as current collectors. In addition, the mass loading of PBCs was 2.5 mg (diameter about 16 mm).

Results
The primitive BC exhibited water-rich morphological characteristics, showing its strong hydrophilicity, which is due to the abundant hydrophilic groups in its network structure (Figure 1a). In the process of freeze-drying, ice directly sublimates into water vapor, which can prevent the collapse of the gel-network at the most extent [27]. The BC is constituted of mutual connective nanofibrils and cross-linked pores, and the average diameter of these nanofibers is about 33.21 nm (Figure 1b). The as-obtained BC aerogels were then pyrolyzed under flowing nitrogen at 800 • C to form PBC, which is a black, superlight conductive aerogel and reserves the inherent highly multihole and ultrafine network nanostructure of BC [28], with an average diameter of 28.85 nm (Figure 1c,d). The decrease in the quality is due to the carbonization of the BC and evaporation of volatiles such as CO, CO 2 , methanol and acetic acid [29]. In the process of impregnation, the high concentration of NH4H2PO4 or (NH4)2SO4 aqueous solution would make the as-prepared bacterial cellulose/NH4H2PO4, bacterial cellulose/(NH4)2SO4 and bacterial cellulose/NH4H2PO4-(NH4)2SO4 slices thin, which made PBCs bond together on the surface and break in inner ( Figure 2). In general, the distributions of N/P-PBC, N/S-PBC and N/P/S-PBC nanofibers were uniform and presented an almost perfect network structure ( Figure 3). Such nanofibers can increase the interfacial area, and the special network architecture would enable the fast transport of ion and electron in its three-dimensional directions [30].  In the process of impregnation, the high concentration of NH 4 H 2 PO 4 or (NH 4 ) 2 SO 4 aqueous solution would make the as-prepared bacterial cellulose/NH 4 H 2 PO 4 , bacterial cellulose/(NH 4 ) 2 SO 4 and bacterial cellulose/NH 4 H 2 PO 4 -(NH 4 ) 2 SO 4 slices thin, which made PBCs bond together on the surface and break in inner ( Figure 2). In general, the distributions of N/P-PBC, N/S-PBC and N/P/S-PBC nanofibers were uniform and presented an almost perfect network structure ( Figure 3). Such nanofibers can increase the interfacial area, and the special network architecture would enable the fast transport of ion and electron in its three-dimensional directions [30]. In the process of impregnation, the high concentration of NH4H2PO4 or (NH4)2SO4 aqueous solution would make the as-prepared bacterial cellulose/NH4H2PO4, bacterial cellulose/(NH4)2SO4 and bacterial cellulose/NH4H2PO4-(NH4)2SO4 slices thin, which made PBCs bond together on the surface and break in inner ( Figure 2). In general, the distributions of N/P-PBC, N/S-PBC and N/P/S-PBC nanofibers were uniform and presented an almost perfect network structure ( Figure 3). Such nanofibers can increase the interfacial area, and the special network architecture would enable the fast transport of ion and electron in its three-dimensional directions [30].    The functional groups in the sample were identified by FTIR spectrum. As Figure 4a shows, the freeze-drying of the BC samples showed several typical vibration bands at 1057 cm −1 (skeletal vibrations involving C-O stretching), 2894 cm −1 (C-H stretching of CH2 groups) and 3345 cm −1 (O-H stretching vibration). After impregnating, samples showed several important peaks at 3231 cm −1 (stretching of N-H), 1279 cm −1 (stretching of P=O) and 1405 cm −1 (stretching of -SO2-). These results showed that the solvent (NH4H2PO4 or (NH4)2SO4) was combined with the large number of functional groups (-OH or C=O) of BC [31]. After carbonization, the heteroatoms such as H and O were volatilized because of the broken of the C-H, C-O and O-H bonds. Hence, the peaks of more than 3000 cm −1 disappeared and the intensity of other peaks was weakened ( Figure 4b). In the spectra of N/P-PBC, N/S-PBC and N/P/S-PBC, a peak appeared at 2115 cm −1 corresponding to the -NH stretching vibration, which demonstrated the successful introduction of ammonium salt.  The functional groups in the sample were identified by FTIR spectrum. As Figure 4a shows, the freeze-drying of the BC samples showed several typical vibration bands at 1057 cm −1 (skeletal vibrations involving C-O stretching), 2894 cm −1 (C-H stretching of CH 2 groups) and 3345 cm −1 (O-H stretching vibration). After impregnating, samples showed several important peaks at 3231 cm −1 (stretching of N-H), 1279 cm −1 (stretching of P=O) and 1405 cm −1 (stretching of -SO 2 -). These results showed that the solvent (NH 4 H 2 PO 4 or (NH 4 ) 2 SO 4 ) was combined with the large number of functional groups (-OH or C=O) of BC [31]. After carbonization, the heteroatoms such as H and O were volatilized because of the broken of the C-H, C-O and O-H bonds. Hence, the peaks of more than 3000 cm −1 disappeared and the intensity of other peaks was weakened ( Figure 4b). In the spectra of N/P-PBC, N/S-PBC and N/P/S-PBC, a peak appeared at 2115 cm −1 corresponding to the -NH stretching vibration, which demonstrated the successful introduction of ammonium salt.  The functional groups in the sample were identified by FTIR spectrum. As Figure 4a shows, the freeze-drying of the BC samples showed several typical vibration bands at 1057 cm −1 (skeletal vibrations involving C-O stretching), 2894 cm −1 (C-H stretching of CH2 groups) and 3345 cm −1 (O-H stretching vibration). After impregnating, samples showed several important peaks at 3231 cm −1 (stretching of N-H), 1279 cm −1 (stretching of P=O) and 1405 cm −1 (stretching of -SO2-). These results showed that the solvent (NH4H2PO4 or (NH4)2SO4) was combined with the large number of functional groups (-OH or C=O) of BC [31]. After carbonization, the heteroatoms such as H and O were volatilized because of the broken of the C-H, C-O and O-H bonds. Hence, the peaks of more than 3000 cm −1 disappeared and the intensity of other peaks was weakened ( Figure 4b). In the spectra of N/P-PBC, N/S-PBC and N/P/S-PBC, a peak appeared at 2115 cm −1 corresponding to the -NH stretching vibration, which demonstrated the successful introduction of ammonium salt.     The crystallinity and the graphitization degree of samples were characterized by XRD ( Figure 5). The three main peaks located at 14 • , 16.3 • and 22.1 • could be assigned to the (110), (110) and (200) diffraction planes of cellulose I structure, respectively [15]. In the XRD patterns of PBCs, a wide and weak diffraction peaks appeared at 23.5 • , which may be ascribed to the (002) facets of hexagonal graphitic structure, showing that the sample was amorphous carbon with a lower graphitization degree in the carbonization temperature at 80 • C [32]. Nanomaterials 2020, 10, x 6 of 13 The crystallinity and the graphitization degree of samples were characterized by XRD ( Figure   5). The three main peaks located at 14°, 16.3° and 22.1° could be assigned to the (11 -0), (110) and (200) diffraction planes of cellulose I structure, respectively [15]. In the XRD patterns of PBCs, a wide and weak diffraction peaks appeared at 23.5°, which may be ascribed to the (002) facets of hexagonal graphitic structure, showing that the sample was amorphous carbon with a lower graphitization degree in the carbonization temperature at 80 °C [32]. It can be seen from X-ray photoelectron spectra (XPS) survey spectra that N, P and S were indeed co-doped into the carbon framework in as-prepared sample (Figure 6a). Here, the survey spectra of N/P/S-PBC is demonstrated and its binding environment is elucidated by the high-resolution XPS spectra (Figure 6b-d). The deconvolution of the high-resolution N1s peak could be separated into two kinds of nitrogen functional groups, pyridinic (N-6, 398.1 eV) and pyrrolic/pyridine (N-5, 400.9 eV), and the high proportion of available N species (N-5) would provide abundant active sites to improve the energy density of supercapacitor ( Figure 6b, Table 2) [33]. Furthermore, the highresolution P2p peak spectrum could be divided into three peaks, 132.6, 133.5 and 134.2 eV, which correspond to P-C binding, P-N binding and P-O binding, respectively (Figure 6c) [34]. In addition, the high-resolution S2p peak spectrum yielded two peaks, 164.0 and 165.2 eV, which correspond to S2p3/2 and S2p1/2, which illustrated that S atom combined with the C atoms to form C-S-C binding at the edge and defects of the carbon frameworks ( Figure 6d) [19]. Besides, the XPS results indicated that 3.9 atom% of nitrogen, 1.22 atom% of phosphorus and 0.6 atom% of sulfur were doped into the N/P/S-PBC (Table 3). It is believed that the N-5, P-C and S-C functionalities can provide more active sites to improve its capacitance, which was confirmed by the remarkable electrochemical performance of those reported materials [35,36].  It can be seen from X-ray photoelectron spectra (XPS) survey spectra that N, P and S were indeed co-doped into the carbon framework in as-prepared sample (Figure 6a). Here, the survey spectra of N/P/S-PBC is demonstrated and its binding environment is elucidated by the high-resolution XPS spectra (Figure 6b-d). The deconvolution of the high-resolution N1s peak could be separated into two kinds of nitrogen functional groups, pyridinic (N-6, 398.1 eV) and pyrrolic/pyridine (N-5, 400.9 eV), and the high proportion of available N species (N-5) would provide abundant active sites to improve the energy density of supercapacitor ( Figure 6b, Table 2) [33]. Furthermore, the high-resolution P2p peak spectrum could be divided into three peaks, 132.6, 133.5 and 134.2 eV, which correspond to P-C binding, P-N binding and P-O binding, respectively (Figure 6c) [34]. In addition, the high-resolution S2p peak spectrum yielded two peaks, 164.0 and 165.2 eV, which correspond to S2p 3/2 and S2p 1/2 , which illustrated that S atom combined with the C atoms to form C-S-C binding at the edge and defects of the carbon frameworks (Figure 6d) [19]. Besides, the XPS results indicated that 3.9 atom% of nitrogen, 1.22 atom% of phosphorus and 0.6 atom% of sulfur were doped into the N/P/S-PBC (Table 3). It is believed that the N-5, P-C and S-C functionalities can provide more active sites to improve its capacitance, which was confirmed by the remarkable electrochemical performance of those reported materials [35,36].  Table 3. The contents of nitrogen, phosphorous, sulfur and oxygen element (atom ratio, derived from XPS analysis) of different PBCs (the deviation of all test data is within 5%).

Sample
N1s P2p S2p O1s The porous structures of PBCs were investigated by nitrogen adsorption-desorption isotherms (Figure 7a). In the adsorption curves at a high relative pressure of 0.95, a following sharp increase was observed, resulting from the multilayer adsorption of nitrogen in macropores formed among the carbon nanofibers [32]. The pore size distribution of all the PBCs became narrower with the peaks centered around 3.5 nm (Figure 7b). The BET surface area and pore structure parameters of PBCs calculated from the isotherms are listed in Table 4. The N/P/S-PBC shows a maximum specific surface area of 498 m 2 /g, which attributed to the electric double-layer capacitance. Its pore volume and average pore diameter can reach up to 0.61 cm 3 /g and 3.13 nm. The result reveals that the macropores  The porous structures of PBCs were investigated by nitrogen adsorption-desorption isotherms (Figure 7a). In the adsorption curves at a high relative pressure of 0.95, a following sharp increase was observed, resulting from the multilayer adsorption of nitrogen in macropores formed among the carbon nanofibers [32]. The pore size distribution of all the PBCs became narrower with the peaks centered around 3.5 nm (Figure 7b). The BET surface area and pore structure parameters of PBCs calculated from the isotherms are listed in Table 4. The N/P/S-PBC shows a maximum specific surface area of 498 m 2 /g, which attributed to the electric double-layer capacitance. Its pore volume and average pore diameter can reach up to 0.61 cm 3 /g and 3.13 nm. The result reveals that the macropores of network-like structure, mesopores and micropores of N/P/S-PBC will contribute to the diffusion of the electrolyte in the aqueous electrochemical capacitors. Nanomaterials 2020, 10, x 8 of 13 of network-like structure, mesopores and micropores of N/P/S-PBC will contribute to the diffusion of the electrolyte in the aqueous electrochemical capacitors.  Therefore, after a simple, low-cost solution impregnation method, three elements (N, P and S) were introduced to the bacterial-cellulose-based carbon nanofibers and increased the surface area, which will contribute to the pseudocapacitance and electric double-layer capacitance, respectively. This may be a good way to introduce a functional group to carbon materials.
The electrochemical performances of PBCs were investigated in aqueous electrolyte of 2.0 M H2SO4 with a two-electrode system. The supercapacitors constructed include pure PBC supercapacitors, N/P-PBC-based supercapacitors, N/S-PBC-based supercapacitors and N/P/S-PBCbased supercapacitors. Their galvanostatic charge-discharge curves at 1.0 A/g show that the specific supercapacitance Cs (Equation (S1), Supporting Information) of N/P/S-PBC-based supercapacitors is higher than the other supercapacitors (Figure 8a-c). Meanwhile, compared with other N/P/S-PBCbased supercapacitors, the N/P/S-PBC supercapacitors shows a higher value of Cs (Figure 8d,e), and find that increasing the concentration of the (NH4)2SO4/NH4H2PO4 aqueous solution from 0.05/0.025 to 0.1/0.025 and 0.05/0.05, then to 0.1/0.05, will lead to a change in Cs from 215.8 to 255.0 and 250.5, then to 233.7 F/g at 1.0 A/g, respectively (Figure 8f). These enhanced electrochemical properties could be ascribed to the appropriate amounts of nitrogen, phosphorus and sulfur incorporated into carbon network and the increased BET surface area. However, the decrease in Cs may be due to the corrosion of the high concentration aqueous solution which damage the network structure of nanofiber ( Figure  2b). Hence, the performance of the N/P/S-PBC supercapacitor was further studied.  Therefore, after a simple, low-cost solution impregnation method, three elements (N, P and S) were introduced to the bacterial-cellulose-based carbon nanofibers and increased the surface area, which will contribute to the pseudocapacitance and electric double-layer capacitance, respectively. This may be a good way to introduce a functional group to carbon materials.
The electrochemical performances of PBCs were investigated in aqueous electrolyte of 2.0 M H 2 SO 4 with a two-electrode system. The supercapacitors constructed include pure PBC supercapacitors, N/P-PBC-based supercapacitors, N/S-PBC-based supercapacitors and N/P/S-PBC-based supercapacitors. Their galvanostatic charge-discharge curves at 1.0 A/g show that the specific supercapacitance C s (Equation (S1), Supporting Information) of N/P/S-PBC-based supercapacitors is higher than the other supercapacitors (Figure 8a-c). Meanwhile, compared with other N/P/S-PBC-based supercapacitors, the N/P/S-PBC supercapacitors shows a higher value of C s (Figure 8d,e), and find that increasing the concentration of the (NH 4 ) 2 SO 4 /NH 4 H 2 PO 4 aqueous solution from 0.05/0.025 to 0.1/0.025 and 0.05/0.05, then to 0.1/0.05, will lead to a change in C s from 215.8 to 255.0 and 250.5, then to 233.7 F/g at 1.0 A/g, respectively (Figure 8f). These enhanced electrochemical properties could be ascribed to the appropriate amounts of nitrogen, phosphorus and sulfur incorporated into carbon network and the increased BET surface area. However, the decrease in C s may be due to the corrosion of the high concentration aqueous solution which damage the network structure of nanofiber (Figure 2b). Hence, the performance of the N/P/S-PBC supercapacitor was further studied. In Figure 9a, with the scan rate increasing from 50 to 200 mV/s at a potential window of 1.0 V, the CV curves of N/P/S-PBC supercapacitors maintain quasi-rectangular shape, indicating its highrate capability and good capacitive behavior. The charge-discharge curve of the N/P/S-PBC supercapacitor exhibits a nearly symmetrical triangle and slightly nonlinear sloping potential profiles in a larger current density, which indicates that the supercapacitor has the properties of fast chargedischarge response and low internal resistance, and the redox reactions occur on the surface of the N/P/S-PBC electrode (Figure 9b) [37]. Electrochemical impedance spectroscopy (EIS) can give vast quantities of information regarding the internal resistance of the electrode material and resistance between the electrode and electrolyte. A Nyquist impedance spectrum of the N/P/S-PBC supercapacitor is presented in Figure 9c; it can be clearly seen that a semicircle is observed at the high-frequency region and an erect slant line at the low-frequency region. It is apparent that the In Figure 9a, with the scan rate increasing from 50 to 200 mV/s at a potential window of 1.0 V, the CV curves of N/P/S-PBC supercapacitors maintain quasi-rectangular shape, indicating its high-rate capability and good capacitive behavior. The charge-discharge curve of the N/P/S-PBC supercapacitor exhibits a nearly symmetrical triangle and slightly nonlinear sloping potential profiles in a larger current density, which indicates that the supercapacitor has the properties of fast charge-discharge response and low internal resistance, and the redox reactions occur on the surface of the N/P/S-PBC electrode (Figure 9b) [37]. Electrochemical impedance spectroscopy (EIS) can give vast quantities of information regarding the internal resistance of the electrode material and resistance between the electrode and electrolyte. A Nyquist impedance spectrum of the N/P/S-PBC supercapacitor is presented in Figure 9c; it can be clearly seen that a semicircle is observed at the high-frequency region and an erect slant line at the low-frequency region. It is apparent that the charge-transfer resistance (Rct) for N/P/S-PBC is about 1.54 Ω, which is crucial for enhancing the rate capability of the supercapacitors [38]. Furthermore, it also can see from Figure 9d that about 74% of the initial specific capacitance retention ratio is obtained at 10 A/g, which is consistent with the results of the EIS.
charge-transfer resistance (Rct) for N/P/S-PBC is about 1.54 Ω, which is crucial for enhancing the rate capability of the supercapacitors [38]. Furthermore, it also can see from Figure 9d that about 74% of the initial specific capacitance retention ratio is obtained at 10 A/g, which is consistent with the results of the EIS. The power density (P) and energy density (E) are calculated to estimate the performance of the N/P/S-PBC supercapacitor (Equations (S2,S3), Supporting Information). In Figure 9e, the N/P/S-PBC supercapacitor shows a high energy density about 8.48 Wh/kg with a power density of 489.45 W/kg, which is higher than other heteroatom-doped, carbon-based supercapacitors ( Figure 10). The high energy density is mainly attributed to the suitable co-doped of nitrogen/phosphorus/sulfur and The power density (P) and energy density (E) are calculated to estimate the performance of the N/P/S-PBC supercapacitor (Equations (S2,S3), Supporting Information). In Figure 9e, the N/P/S-PBC supercapacitor shows a high energy density about 8.48 Wh/kg with a power density of 489.45 W/kg, which is higher than other heteroatom-doped, carbon-based supercapacitors ( Figure 10). The high energy density is mainly attributed to the suitable co-doped of nitrogen/phosphorus/sulfur and increased surface area in the N/P/S-PBC electrode, which contribute to the pseudocapacitance and electric double-layer capacitance, respectively. The cycling stability is also a vital characteristic and was performed by a repeating galvanostatic charge-discharge test at 1.0 A/g. The result in Figure 9f shows that its specific capacitance value exhibits a slight change after 3500 cycles, which confirms that the as-prepared surpercapacitor has the merit of practical applications.
increased surface area in the N/P/S-PBC electrode, which contribute to the pseudocapacitance and electric double-layer capacitance, respectively. The cycling stability is also a vital characteristic and was performed by a repeating galvanostatic charge-discharge test at 1.0 A/g. The result in Figure 9f shows that its specific capacitance value exhibits a slight change after 3500 cycles, which confirms that the as-prepared surpercapacitor has the merit of practical applications.

Conclusions
N/P/S-PBC was successfully manufactured via employing a low-cost, eco-friendly bacterial cellulose as carbon nanofiber. When 3.9 atom% of nitrogen, 1.22 atom% of phosphorus and 0.6 atom% of sulfur co-doped into PBC, the specific capacitance for N/P/S-PBC at 1.0 A/g is about 255 F/g and the N/P/S-PBC supercapacitor's energy density is about 8.48 Wh/kg with a power density of 489.45 W/kg. The excellent electrochemical properties of N/P/S-PBC are due to the perfect three-dimensional structure of the PBC aerogels and the synergistic interaction of heteroatoms when co-doped, which contribute to the electric double-layer capacitance and pseudocapacitance, respectively. Therefore, the simple and convenient method could be scaled up for industrial applications in large-scale supercapacitor electronics.

Conclusions
N/P/S-PBC was successfully manufactured via employing a low-cost, eco-friendly bacterial cellulose as carbon nanofiber. When 3.9 atom% of nitrogen, 1.22 atom% of phosphorus and 0.6 atom% of sulfur co-doped into PBC, the specific capacitance for N/P/S-PBC at 1.0 A/g is about 255 F/g and the N/P/S-PBC supercapacitor's energy density is about 8.48 Wh/kg with a power density of 489.45 W/kg. The excellent electrochemical properties of N/P/S-PBC are due to the perfect three-dimensional structure of the PBC aerogels and the synergistic interaction of heteroatoms when co-doped, which contribute to the electric double-layer capacitance and pseudocapacitance, respectively. Therefore, the simple and convenient method could be scaled up for industrial applications in large-scale supercapacitor electronics.

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