A. Morphologies and chemical composition characterization

A schematic illustration of our synthetic strategies is depicted in Fig. 1(a). The polyaniline nanofibers (serving as the carbon precursors) were first fabricated by cyclic voltammetry from −0.3 to 0.8 V versus SCE at a scan rate of 50 mV/s in a three-electrode cell filled with 0.1 M aniline and 0.5 M H₂SO₄ as the electrochemical plating electrolyte, as shown in the left side of Fig. 1(a). The carbon cloth, saturated calomel electrode (SCE), and graphite foil were used as the working electrode, reference electrode, and counter electrode, respectively. After the electrochemical deposition, polyaniline nanofibers with a diameter of 80–100 nm were uniformly deposited on the carbon fiber surface [Fig. 1(b)]. These nanofibers are interconnected with each other, forming a highly porous 3D network, which is beneficial for ion and charge transport. The N-containing polymer, such as polyaniline, was widely used as the carbon precursor to fabricate N-doped porous carbon materials, benefiting from their high content of heteroatoms (N and O) and the intrinsic porous architectures. Therefore, polyaniline fibers were used as the precursor to further conduct the annealing treatment. After annealing in N₂ for 1 h, the fiber-based structure was mostly preserved with some observable partial aggregations that can be attributed to the melting of polyaniline at such a high temperature [Fig. 1(c)]. These aggregations may decrease the ion-accessible surface area and block the ion diffusion in the porous carbon matrix, which can lead to a lower rate performance. Interestingly, a highly porous spider web-shaped structure can be seen after annealing the polyaniline precursor directly in ammonia atmosphere for 1 h [Fig. 1(d)]. Each carbon filament is interconnected with each other to build up the 3D hierarchical porous structures. As can be seen from the SEM images, these carbon webs are full of mesopores and macropores. The TEM image was also collected to confirm the mesoporosity in PA700. [Fig. 1(d) inset]. The mesopores ranging from 25 to 40 nm in PA700 were highlighted by the dashed lines. This observation provides direct evidence for the existence of mesopores in PA700. This bi-modal porosity can act as an ion-buffering reservoir to shorten the electrolyte ion diffusion distance to the inner part of the electrode surface therefore ensuring a superior rate capability for the material compared to its nonporous counterparts.^{Reference Song, Liu, Yao, Li, Kou, Huang, Feng, Wang, Tong, Liu and Li12} In addition, the interconnected carbon filaments afford multidirectional fast charge transfer expressways to largely reduce the charge transfer resistance of the electrode.^{Reference Zhang, Liu, Hou, Kou, Yue, Guan and Li21} This morphology transformation from the fiber-like structure to the novel spider web-shaped structure could be attributed to the melting and the burn off of the polyaniline in ammonia at such a high temperature. It is reported that nitrogen atoms can be doped into carbon in ammonia at a temperature as low as 550 °C.^{Reference Luo, Wang, Heron, Allen, Morre, Maier, Stickle and Ji31} These incorporated N sites can further react with NH₃, introducing micro and/or mesopores and therefore inducing the morphology change.^{Reference Luo, Wang, Heron, Allen, Morre, Maier, Stickle and Ji31} In addition, ammonia can also react with the oxygen functional groups in the polymer chain to introduce N doping and etch the carbon matrix, leading to multiscale porosities.

FIG. 1.

(a) Schematic illustration of the major synthesis procedures for the nitrogen-doped spider-web-shaped carbon networks supported on carbon cloth. (b) The SEM image of the as-synthesized polyaniline fibers. (c) The SEM image of PN700 and (d) The SEM image of PA700. Inset shows the TEM image for PA700; the mesopores in PA700 are highlighted by the dashed lines.

To testify our hypothesis that the pyrolysis of polyaniline under ammonia atmosphere could increase the ion-accessible surface area, the UV-vis absorption spectra were collected for the original MB solution and the MB solutions after inserting and immersing samples of PANI, PN700, and PA700 for 24 h, respectively. The dye-adsorption method is widely used to qualitatively estimate the ion-accessible surface area of porous materials containing mesopores and macropores.^{Reference Feng, Song, Huang, Xu and Liu4,Reference Wang, Wang, Lu, Ling, Yu, Zhai, Tong and Li32} In a typical protocol, the sample is completely submerged into an 8 mg/L MB aqueous solution for 24 h. Afterward, a certain amount of dye molecules will adsorb on the sample surface, leading to a reduced color intensity of the MB solutions. Consequently, the MB concentration lessens the absorbance at the maximal absorption wave length of MB, which can be directly gauged by a UV-vis spectrometer. As can be seen from Fig. 2(a), all the UV-vis absorption spectra exhibit the typical pattern of MB, with the highest absorption peak observed at 665 nm. Among all the spectra, the maximal absorbance of the one measured after interacting with PA700 is the lowest. This observation confirms the hypothesis that PA700 has the largest surface area because it adsorbed the most MB molecules. This enhancement should be attributed to the highly porous structure derived from the pyrolysis of polyaniline in ammonia. The web carbon networks with multiscale pores can facilitate the ion diffusion into the interior parts of the materials and therefore are expected to achieve prominent rate capability and capacitance.

FIG. 2.

(a) UV-vis absorption spectra collected from the original MB solution (8 mg/L in water) and the MB solutions absorbed by the samples (PANI, PA700, and PN700). (b) The N 1s core-level XPS spectrum for PANI. (c) The N 1s core-level XPS spectrum for PN700. (d) The N 1s core-level XPS spectrum for PA700. (e) The O 1s core-level XPS spectrum for PN700. (f) The O 1s core-level XPS spectrum for PA700.

The chemical composition of all the synthesized samples is revealed by XPS and the results are summarized in Table I. XPS measurements were conducted by an XPS spectrometer (ESCALAB 250Xi, Thermo Scientific Escalab) with Al K_α radiation (λ = 8.34 Å) as the excitation source. All the XPS data were calibrated using the C 1s peak at 284.6 eV as the reference. Table I displays the element content of the samples derived from the XPS survey spectra, which are not shown here. This table presents the following points that are worth discussing. First, the heteroatom content (both N and O) of PN700 and PA700 is lower than that of PANI, as high-temperature carbonization processes are known to partly eliminate the heteratoms.^{Reference Yao, Zhang, Kou, Song, Liu and Li18} The oxygen atoms are expected to come from inherent dopants (e.g., sulfate ions in our case) and –OH or quinone groups in the over-oxidized polymer chain of PANI that is introduced during the electrochemical deposition process. It is reported that the electrochemical over oxidation of PANI produced benzoquinone/hydroquinone and p-aminophenol/benzoquinoneimine, which contain a large amount of oxygen functional groups.^{Reference Zhang, Zhou, Wang, Wu and Bai33} Second, the atomic content of N in PA700 is 9.72 at.%, higher than that of PN700 (7.72 at.%). The increment in the N content can be rationalized by some previous studies showing that ammonia can react with the oxygen functional groups anchored on the carbon surface at a temperature as low as 550 °C, leading to additional N doping.^{Reference Luo, Wang, Heron, Allen, Morre, Maier, Stickle and Ji31} Third, it is intuitive to conclude that the oxygen content of PA700 should be lower than that of PN700 as we claim that oxygen functionalities are prone to be converted to nitrogen functionalities by ammonia. However, we actually observed that the O content of PA700 slightly increased as compared to PN700. We need to emphasize here that it is not contradictory to our claim, as the carbon content of PA700 is appreciably lower than that of PN700. The “abnormal” observation suggests that ammonia preferably etched more carbon atoms rather than converting oxygen atoms, given the dominated concentration of carbon. In addition, the pyrolysis of PANI in N₂ also burned off the oxygen functionalities on the carbon surface to produce CO₂ evolution.

TABLE I.

The element content of the samples derived from the XPS survey spectra.

To further identify the types of various functionalities varied by the ammonia annealing, XPS N 1s and O 1s core level spectra were collected. Figure 2(b) shows the N 1s core level spectrum of PANI, which is deconvoluted into three synthetic peaks associated with three different types of N functionalities: imine nitrogen (–N=, 398.7 eV), amine nitrogen (–NH–, 399.5 eV), and positively charged nitrogen (–N⁺–, 401.1 eV).^{Reference Luo, Kong, Jia, Luo, Lu, Zhang, Qiu, Li and Yu34,Reference Wang, Yao, Bi, Huang, Wang and Chen35} After the pyrolysis of PANI in N₂ for 1 h, the surface functionalities were changed. As shown in Fig. 2(c), the N 1s core-level XPS spectrum of PN700 can be deconvoluted into three different types of N, i.e., pyridinic N (N-6, 398.3 eV), pyrrolic N (N-5, 399.8 eV), and quaternary N (N-Q, 400.8 eV).^{Reference Huang, Liu, Song, Li and Liu36–Reference Yang, Zhong, Bao, Zhou, Wei and Zhou38} Pyrrolic N (N-5) groups refer to the N atoms incorporated in the five-membered heterocyclic rings bonded with two adjacent carbon atoms. They contribute two p electrons to the C–N–C π system. N-6 groups exist at the edges of the carbon planes, linked with two adjacent carbon atoms and donating one p electron to the aromatic system. Quaternary N (N-Q) refers to the N atoms that are doped in a co-planar fashion into the graphite layers to substitute the carbon atoms in the hexagonal π system. The N-5 and N-6 groups generally locate at the edge of the graphite plane, while N-Q could be either at the edge or in the bulk. As shown in Fig. 2(d), the N 1s spectrum of PA700 can also be deconvoluted into three peaks, i.e., N-6 (398.4 eV), N-5 (399.9 eV), and N-Q (400.8 eV), respectively. However, the concentration ratio (evaluated by integrating the peak area of the corresponding synthetic peak and dividing it with the total peak area of all the synthetic peaks) of N in PA700 is in sharp contrast to that of PN700. PA700 shows a much higher content of N-Q (37.8%) than that in PN700 (18.8%). Since N-5 and N-6 are p-electron donors to the π system, they are expected to increase the electrical conductivity of the doped network. N-Q with positive charges serves as an electron acceptor, which facilitates electron transfer.^{Reference Yang, Zhong, Bao, Zhou, Wei and Zhou38,Reference Li, Xu, Wang, Ding, Zahiri, Holt, Tan and Mitlin39} Therefore, the different concentration ratios of the nitrogen functionalities introduced during the pyrolysis of PANI in ammonia or in nitrogen can induce different physical and chemical properties and thus affect the electrochemical properties of the as-prepared carbon materials.

The O 1s core-level XPS spectra of PN700 and PA700 were collected and can be deconvoluted into three synthetic peaks: quinone group at the binding energy of 530.6 eV, hydroxyl group (C–OH) at the binding energy of 531.9 eV, and the ether-type oxygen atoms in the esters and anhydrides (C–O–C) at the binding energy of 533.4 eV.^{Reference Huang, Liu, Song, Li and Liu36} As shown in Figs. 2(e) and 2(f), the atomic ratio of these oxygen functionalities in PN700 and PA700 are substantially different. These oxygen functionalities in carbon materials can react with NH₃, forming the N-containing groups.^{Reference Luo, Wang, Heron, Allen, Morre, Maier, Stickle and Ji31} PA700 exhibited a much lower content of quinone group (21.8%) than that of PN700 (37.8%). This result indicates that ammonia may be more likely to remove quinone groups during the annealing treatment. In addition, it is reported that these oxygen functional groups can enhance the wettability of the electrode surface, which can facilitate the electrolyte infiltration into the interior parts of the porous electrode.^{Reference Huang, Liu, Song, Li and Liu36,Reference Li, Xu, Wang, Ding, Zahiri, Holt, Tan and Mitlin39} Moreover, the oxygen groups can also provide pseudocapacitance and, therefore, further improve the capacitive performance of the materials.^{Reference Song, Liu, Xu, Feng, Yao, Kou, Liu and Li40} Taking together, ammonia annealing brings the effect of enhancing N contents, in addition to improving the surface area as we discussed in the above paragraph. Both factors in principal improve the capacitive performance by rendering the enlarged surface area for charge accumulation, improved pseudocapacitance, as well as boosted water wettability and reduced charge transfer resistance.

B. Electrochemical performances of PANI, PN700 and PA700 electrodes

The electrochemical performances of the electrodes were tested in a three-electrode cell containing the 1 M H₂SO₄ electrolyte using SCE and graphite foil as the reference and counter electrodes, respectively. To optimize the annealing temperature in ammonia, PANI nanofibers were annealed at 500 °C, 600 °C, 700 °C, and 800 °C in a tube furnace for 1 h. The as-prepared samples were denoted as PA500, PA600, PA700, and PA800, respectively. Figure 3(a) exhibits the cyclic voltammetry curves of these samples at a scan rate of 100 mV/s. PA700 shows the significantly larger area enclosed by the CV curve than that of PA500, PA600, and PA800, indicating it achieved the largest capacitance at the tested scan rate. This may be attributed to the higher carbonization degree of the PANI precursor at a higher temperature (700 °C), which leads to better electrical conductivity.^{Reference Huang, Liu, Song, Li and Liu36} However, when the temperature was further increased from 700 to 800 °C, the content of the O and N functionalities would be decreased, leading to a decreased pseudocapacitive contribution.^{Reference Luo, Wang, Heron, Allen, Morre, Maier, Stickle and Ji31,Reference Huang, Liu, Song, Li and Liu36} Hence, we chose PA700 as the electrode to conduct the following experiments. Figure 3(b) shows the cyclic voltammetry curves of PA700 at different scan rates from 5 to 200 mV/s. These near-rectangular CV curves collected at all the scan rates evidence the excellent rate capability of PA700, which can realize fast charging and discharging without compromising too much of the electrode’s performance. Comparison of the CV curves of PA700 and PN700 at a scan rate of 100 mV/s is shown in Fig. 3(c). The enclosed CV area of PA700 is substantially larger than that of PN700, meaning its higher electrochemical activities. In addition, a couple of reversible redox peaks were observed in the cyclic voltammetry profile of PA700, whereas that of PN700 shows none. This echoes the fact that different chemical compositions (i.e., with different contents of heteroatom doping) influences the electrochemical behavior. The higher N doping level of PA700 compared with that of PN700 may be the reason for the discernible redox peaks. The gravimetric capacitances of PA700 and PN700 at different scan rates are shown in Fig. 3(d). PN700 shows a medium gravimetric capacitance of 218 F/g at a scan rate of 2 mV/s. When the scan rate was increased 250 times to 500 mV/s, only 32% of the capacitance retention (70 F/g) can be retained. However, PA700 can deliver high specific capacitances of 266 F/g, 253 F/g, 245 F/g, 242 F/g, 230 F/g, 218 F/g, 200 F/g, and 164 F/g at scan rates of 2 mV/s, 5 mV/s, 10 mV/s, 20 mV/s, 50 mV/s, 100 mV/s, 200 mV/s, and 500 mV/s, respectively. PA700 also exhibits an excellent rate performance with a capacitance retention of 61% when the scan rate increased from 2 to 500 mV/s (250 times). These high gravimetric capacitances and excellent rate performances are much better than those of the recently reported N-doped carbon materials, such as a nitrogen-doped activated carbon CA800 (235 F/g at current density of 1 A/g, and 67% capacitance retention when the current density increased 11 times from 1 to 11 A/g),^{Reference Li, Liu, Li, Yang, Shen, Huang, Xiang and Li41} N-enriched carbon nanowires NE-CNWs-700 (∼210 F/g at current density of 2 A/g, and 61% capacitance retention when the current density increased 50 times from 0.1 to 5 A/g),^{Reference Yuan, Zhou, Zhou, Zou, Mo and Xia22} N-doped hollow carbon spheres PNHCS (213 F/g at current density of 0.5 A/g, and 55.6% capacitance retention when the current density increased 20 times from 0.5 to 10 A/g),^{Reference Han, Xu, Ding, Pan, Dou and MacFarlane27} heteroatom-doped carbon NOC-500 (141 F/g at current density of 0.43 A/g, and 57% capacitance retention when the current density increased 100 times from 0.43 to 43 A/g),^{Reference Huang, Liu, Song, Li and Liu36} N-doped porous carbon nanofibers N-CNFs-900 (202 F/g at current density of 1 A/g, and 81% capacitance retention when the current density increased 30 times from 1 to 30 A/g)², Lignin-derived hierarchical porous carbon LHPC (165 F/g at current density of 0.05 A/g, and 74% capacitance retention when the current density increased 200 times from 0.05 to 10 A/g),^{Reference Zhang, Lin, Lin, Yin, Lu, Liu and Zhao20} and hierarchical porous carbon foams HPCFs (206 F/g at current density of 1 A/g, and 88% capacitance retention when the current density increased 10 times from 1 to 10 A/g).^{Reference Zhang, Liu, Hou, Kou, Yue, Guan and Li21} These results undoubtedly confirm that our PA700 exhibits great potential as a high-performance supercapacitor electrode capable of being charged and discharged quickly.

FIG. 3.

(a) CVs of PANI annealed at different temperatures (500, 600, 700, and 800 °C) in ammonia. (b) CVs of PA700 at different scan rates from 5 to 200 mV/s. (c) CVs of PA700 and PN700 at a scan rate of 100 mV/s. (d) Specific capacitance of PA700 and PN700 as a function of the scan rate (rate capability performance).

Additionally, we also carried out EIS, which can give more information about the enhanced capacitive performance of PA700. Therefore, the EIS data of PA700 and PN700 were collected at an open circuit potential with a perturbation of 5 mV in a the frequency range from 10 mHz to 20 kHz using the CHI 660D electrochemical workstation. EIS tests were also conducted in a conventional three-electrode cell, using PN700 and PA700, SCE electrode, as well as graphite foil as the working electrode, reference electrode, and counter electrode, respectively. Nyquist plots of both electrodes are shown in Fig. 4. Both plots display a semicircle in the high-frequency region, followed by linear line leans at a scant of 45° in the medium-frequency region and a near-vertical linear spike in the low-frequency domain. These characteristics represent the near-ideal capacitive performance of the tested electrodes.^{Reference Song, Liu, Yao, Li, Kou, Huang, Feng, Wang, Tong, Liu and Li12,Reference Song, Xu and Liu42} The intercept at the horizontal axis in the high-frequency region refers to the equivalent series resistance of the electrode.^{Reference Song, Xu and Liu42} PA700 exhibits a negligible equivalent series resistance of ∼0.48 Ω as a result of its excellent electrical conductivity. This value is smaller than that of some other reported carbon-based materials, such as N-doped activated carbon CA800 (0.75 Ω),^{Reference Li, Liu, Li, Yang, Shen, Huang, Xiang and Li41} hetero-atom doped carbon NOC-500 (0.96 Ω),^{Reference Huang, Liu, Song, Li and Liu36} carbonized eggshell membrane CESM-300 (2.1 Ω),^{Reference Li, Zhang, Amirkhiz, Tan, Xu, Wang, Olsen, Holt and Mitlin43} and 3D graphene foam 3DGH (3.8 Ω).^{Reference Zhai, Lu, Wang, Wang, Mathis, Liu, Li, Tong and Li6} The charge transfer resistance (R _ct) of the electrode can be evaluated from the diameter of the semicircle in the real part.^{Reference Song, Liu, Yao, Li, Kou, Huang, Feng, Wang, Tong, Liu and Li12} PA700 also shows substantially smaller charge transfer (the diameter of the semi-circle) resistance than that of PN700, indicating its faster charge transfer kinetics on the electrode surface. This should be attributed to the ammonia treatment which introduced a higher content of N-Q, as shown in Figs. 2(c) and 2(d). N-Q can exhibit positive charge properties and induce electron acceptors, which could facilitate the charge transfer of the carbon materials.^{Reference Yang, Zhong, Bao, Zhou, Wei and Zhou38,Reference Li, Xu, Wang, Ding, Zahiri, Holt, Tan and Mitlin39} Therefore, these EIS data further confirm the advantages of our ammonia treatment in improving the capacitive performance of the porous carbon materials.

FIG. 4.

EIS study of PA700 and PN700 electrodes: Nyquist plots for PA700 and PN700. EIS was carried out at open circuit potentials with a perturbation of 5 mV in a frequency range from 10 mHz to 20 kHz.

Cycling stability is one of the most importance factors that affects the practical applications of the supercapacitor electrodes. Therefore, to evaluate the practicable application of PA700, cyclic voltammetry was conducted at a fast scan rate of 200 mV/s for 5000 cycles in a three-electrode system. As shown in Fig. 5, PA700 exhibits a good cycling stability of ∼90% capacitance retention after 5000 charging and discharging cycles. The slightly decreased capacitance could be ascribed to the partial loss of the O and/or N functionalities on the carbon matrix, which can also be observed in the other reported heteroatom-doped carbon materials.^{Reference Yu, Lin, Feng, Zeng, Tong and Lu7,Reference Huang, Liu, Song, Li and Liu36} The inset of Fig. 5 shows the cyclic voltammetry curves of the first cycle and the 5000th cycle for PA700 during the cycling test. The enclosed area of the cyclic voltammetry curves is similar, further confirming its good cycling stability. In addition, after the long-term cycling stability tests, the reversible redox peaks at about 0.3 V versus SCE still can be observed in the CV curve of the cycled sample (the inset in Fig. 5), indicating that the introduced heteroatoms in the carbon matrix of PA700 was electrochemically stable.

FIG. 5.

Cycling stability of PA700 at a scan rate of 200 mV/s during 5000 charge–discharge testing cycles. Inset shows the CV curve collected at the first cycle and the 5000th cycle.

To sum up, we ascribe the excellent capacitive performance of PA700 for the following reasons. First, the nitrogen-doped “spider webs” carbon materials can provide multiscale pore structures (e.g., macropores and mesopores). These interconnected hierarchical pores can act as ion-buffering reservoirs to shorten the ion diffusion distance to the interior part of the electrode surface. Second, the pyrolysis of polyaniline under ammonia without the structural aggregation can realize a higher ion-accessible surface area than that of the conventional nitrogen-treated sample. Third, the additional introduced N-containing functionalities via the ammonia treatment can provide stable pseudocapacitive properties for the electrode. In addition, the enhanced content of N-Q during ammonia treatment that can exhibit positive charge properties and induce electron acceptors reduces the charge transfer resistance of the carbon materials.