Nitrogen-doped activated carbon nanotubes derived from polyaniline for high-performance supercapacitors

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Various characterizations reveal that the ACNTs display high specific surface area, narrow pore size distribution and high nitrogen content, and the ACNTs interweave together to form a dimensional conductive network. Among the ACNTs materials, the ACNTs-700 sample with a specific surface area of 2988 m 2 g − 1 exhibits the largest capacitance (474 F g − 1 at the current density of 0.1 A g − 1 ) and best rate capability (335 F g − 1 at the density of 10.0 A g − 1 ), as well as the superior durability with a capacity retention of 96% after 2000 charge/discharge cycles, which is mainly attributed to its unique conductive network, large surface area and moderate nitrogen-doping content. Nitrogen-doped ACNTs Polyaniline Conductive network Supercapacitors Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction Supercapacitors have been widely used in the new energy field for its high-power density and long-cycle life [ 1 – 5 ]. Supercapacitors include pseudocapacitor with reversible redox reactions and electric double layer capacitance (EDLC) without reversible redox reactions in the energy storage processes based on the different energy storage principles [ 6 ]. The supercapacitor using carbon materials as electrode materials is EDLC. To date, numerous carbon materials are studied as energy storage materials, such as activated carbon (AC), carbon nanotubes (CNTs), graphene and mesoporous carbons [ 7 – 12 ]. Among the carbon materials, CNTs are often regarded as promising energy storage materials because of the unique open channel network and excellent conductivity. However, the low specific surface area of CNTs has limited its applications in supercapacitors or lithium ion batteries [ 13 , 14 ]. As a conductive macromolecule, polyaniline (PAN) has great potential in energy storage systems due to its easy synthesis, good environmental durability and unique doping mechanism [ 15 ]. Besides, PANs with a variety of morphologies such as nanofibers and nanotubes can be prepared by adjusting the synthesis conditions, and PANs with specific morphology were maintained after high temperature treatment in inert gas [ 16 , 17 ]. Thus, PANs may be employed as an excellent precursor to prepare carbon materials for energy storage [ 18 ]. For example, Yuan et al. [ 19 ] synthesized one kind of carbon nanowires showing a 325 F g − 1 at the current density of 0.1 A g − 1 by directly carbonizing nanowire-shaped PAN. Dou et al. [ 20 ] prepared one kind of nitrogen-doped hollow carbon spheres showing a 213 F g − 1 at the current density of 0.5 A g − 1 by directly carbonizing sphere-shaped PAN. However, the capacitance of these PAN-based carbon materials with small specific surface area is too low to meet the growing energy density. Increasing the specific surface area seems to be an effective way to enhance the activities of carbon materials, which is usually conducted by chemical activation method [ 21 ]. Up to date, few reports concerned the preparation of N-doped activated CNTs using PAN as the precursor with high capacitance [ 22 ]. In this work, one kind of nitrogen-doped activated carbon nanotubes (ACNTs) was fabricated by carbonizing and activating a polyaniline precursor with nanotube morphology at 600°C-800°C in Ar atmosphere. Among the ACNTs, ACNTs-700 with a specific surface area of 2989 m 2 g − 1 exhibits the largest capacitance (465 F g − 1 at the current density of 0.1 A g − 1 ) and best rate capability (335 F g − 1 at the density of 10.0 A g − 1 ), as well as superior durability with a capacity retention of 96% after 2000 charge/discharge cycles. 2 Experimental 2.1 Preparation of PAN-based CNTs PAN-based CNTs were prepared based on the reference [ 23 ]. Typically, 1.5 mL of aniline (AC) was added into 90.0 mL ultrapure water to form A. Meanwhile, 5.25 g of ammonium persulfate (APS) was poured into 90.0 mL ultrapure water to form B. Then B was mixed with A under moderate agitation and reacted at 4°C for 20 h. The resultant sample was filtrated and washed with adequate water and ethanol, and dried at 40°C for 48 h for reservation. The preoxidation process was operated at 200°C for 2 h in air and then the carbonization processes were conducted at 600°C-800°C for 3 h in Ar atmosphere, respectively. Finally, the activated process were conducted by blending CNTs and KOH with a mass ratio of 1:5 and carbonized at 600°C-800°C for 3 h in Ar atmosphere, respectively. The obtained mixtures were filtrated and washed with adequate water and ethanol, and dried at 40°C for 48 h. The activated samples at 600°C-800°C were denoted as ACNTs-600, ACNTs-700 and ACNTs-800, respectively. 2.2 Characterization of Materials X-ray diffraction patterns was performed on Rigaku smart lab X ray diffraction instrument using Cu Kα radiation (scanning rate: 5°/2θ min − 1 , scanning range 2θ = 5–70°). The morphology and structure of ACNTs-700 was carried out by scanned electron microscopy (SEM, Zeiss Supra 55), transmission electron microscopy (TEM, JEOL JEM-2012) and high resolution (HRTEM, JEOL-3010). The specific surface area and pore size analysis was performed on the ASAP-2020 expedited analysis system by operating nitrogen sorption isotherms at 77 K. 2.3 Electrochemical Measurements The ACNTs-700 sample, graphitized carbon black and polyvinylidene fluoride (PVDF) as binder, were blended with a quality ratio of 8:1:1 and added into 2.0 mL N-methyl-pyrrolidone (NMP) with moderate agitation. Then the resulting mixture was coated onto the nickel foam substrate, which was followed by drying at 120°C for 12h in a vacuum oven. The mass of each electrode is about 2.5 mg containing conducting agent and binder and the surface area of each electrode is 100 mm 2 . All electrochemical tests were conducted in a standard three-electrode system: nickel foil electrode with the active material as the working electrode, nickel hydroxide electrode as a counter electrode, and an Hg/HgO as a reference electrode, and 6.0 M KOH aqueous solution was employed as the electrolyte. The galvanostatic charge/discharge test was operated by CT2001A battery program controlling test system within the voltage of -0.8 V to 0.0 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 760E electrochemical working station. For the CV measurements, the sweep rate ranged from 1 to 50 mV/s within the voltage of -0.8 V to 0.0 V. For the EIS measurements, the frequency range was from 1 Hz to 10 kHz. 3 Results and discussion 3.1 Structure and morphology of PAN, CNTs and ACNTs-X The schematic illustration of ACNTs preparation was displayed in Fig. 1 . In the chemical oxidation polymerization process, aniline monomers were aggregated to form tubular micelles in APS solution under moderate ultrasound, and the tubular micelles were gradually grown into polyaniline nanotube at 4°C with the reaction time prolonged. Then polyaniline nanotube was preoxidized at 200°C for 2 h in air to remove the unstable volatiles in the carbon skeleton. Then the carbonization processes were conducted at 600°C-800°C for 3 h in Ar atmosphere, respectively. Finally, the activated processes were operated by blending the PAN-based CNTs with KOH and carbonized at 600°C-800°C for 3 h in Ar atmosphere, respectively. The morphology of polyaniline precursor was determined by SEM before the carbonization and activation treatment. It can be seen from Fig. S1 a that the prepared polyaniline precursor has typical nanotube morphology with uniform size in length, and is intertwined and connected together. The further enlarged SEM images in Fig. S1 (b - c) can clearly observe that the outer wall of the nanotube is not smooth, and the upper surface of the nanotube wall with one end opening is covered with a layer of small nanoparticles. As seen the TEM image in Fig. S1 d , it is further determined that the polyaniline precursor has a hollow structure with an outer diameter of 150–200 nm and an inner hollow diameter of 20 nm, and the nanotube ends of the nanotubes are open and cross connected. The prepared porous polyaniline-based carbon nanotubes were analyzed by X-ray diffraction as displayed in Fig. 2 . The ACNTs-X curves display a large and wide peak of the (002) crystal plane of graphite structure at around 23°C, which is a typical amorphous carbon structure. The intensity of XRD peak becomes stronger with the activation temperature rises from 600°C to 800°C, indicating that the crystallinity of the porous carbon nanotubes has a trend of improvement with the rise of activation temperature. Polyaniline, as a precursor for the preparation of carbon materials, is treated at high temperature under the protection of inert gas, will basically maintain its original morphology [ 24 ]. During the carbonization and activation process, it usually happens condensation and oxidation, and carbon skeleton is retained with the transformation of unstable groups into small molecules. The SEM images of ACNTs-X were shown in Fig. 3 (a - b) , and the original morphology of polyaniline precursor with the slight changes in the pipe diameter is still preserved after activation treatment at 600°C and 700°C. However, the morphology of ACNTs-800 in Fig. 3 c has changed for the reason that many nanotubes have stacked together and have the trend of soldering. The TEM image of ACNTs-700 is displayed in Fig. 3 d, which further proves the activation sample can maintain the hollow tubular structure with the nanotubes cross connected and one end opening. It can be inferred that KOH can enter the carbon skeleton for oxidation during the activation process from the changes of nanotube wall, and the open tubular structure can offer the buffer layer of ions to shorten the transmission distance, which is conducive to the storage of ions [ 25 – 27 ]. The HRTEM images of ACNTs-700 in Fig. 4 (a - b) show the typical amorphous structure with many discrete carbon nanosheets protruded out of the cylindrical nanotubes. The prepared porous carbon nanotubes have shorter pore length/ion transport distance, which is conducive to the diffusion and migration of electrolyte ions compared with other traditional porous materials [ 28 , 29 ]. Figure 4 (c - d) display the N 2 -adsorption/sorption isotherms and the corresponding BJH pore size distribution curves of the ACNTs-X samples, which reveals typical IV isotherms with the characteristics of micropore and mesopore. The detailed surface parameters of the ACNTs-X samples are summarized in Table 1 . Apparently, the specific surface area was greatly improved with the activation temperature increased from 600°C to 700°C, and slowly improved with the activation temperature increased from 700°C to 800°C. There is strong nitrogen adsorption at low relative pressure of P/P 0 < 0.1, which corresponds to the micropore filling process, and the slow increase of adsorption curve at medium relative pressure of 0.1 < P/P 0 0.8 corresponds to the capillary condensation of mesopores/macropores and multilayer molecular adsorption. The pore distribution curves of ACNTs-X in Fig. 4 d show that all the samples have micropores, mesopores and macropores meanwhile, and the pore size distribution range is relatively narrow, and the concentrated distribution is 1–4 nm. It can be seen from the pore distribution curve that the amount of mesopores is increasing with the increase of activation temperature, which is beneficial to both ion storage and transmission, and the micropore content in ACNTs-600 is the highest, and the micropore content in ACNTs-800 is the lowest. Table 1 The pore structure parameters of ACNTs-X. samples S BET (m 2 /g) S meso (m 2 /g) Vtatal (cm 3 /g) ACNTs-600 1775 953 1.16 ACNTs-700 2988 1579 1.93 ACNTs-800 3026 1765 1.98 The XPS element content results of ACNTs-X are shown in Table 2 . The content of N elements in ACNTs ranges from 3.52 at % minus as low as 1.16 at %, and the content of O elements decreases from 16.05 at% to 9.89 at%. The content of heteroatoms in the ACNTs-X decreases gradually with the increase of activation temperature, which is conducive to improve the conductivity of carbon based materials. However, heteroatoms in carbon based materials can provide pseudocapacitance and change the electron distribution on the surface of carbon atoms, which is conducive to increasing the wettability of samples in electrolyte [ 30 – 32 ]. Table 2 The elemental composition of ACNTs-X from XPS tests. samples C (at %) N (at %) O (at %) N + O (at %) ACNTs-600 80.03 3.52 16.05 19.97 ACNTs-700 84.32 2.97 11.71 15.68 ACNTs-800 88.95 1.16 9.89 11.05 3.2 The electrochemical performance of ACNTs-X samples Cyclic voltammetry (CV) test is one of the important means to evaluate the energy storage performance of electrode materials. The ACNTs-X samples as active substances are prepared electrode materials for three electrode supercapacitor test system, and the test voltage range is -0.8 V-0.0 V. Figure 5 (a - c) show the capacity voltage scanning curves calculated by the cyclic voltammetry test method with the rate of 20 mV/s, 50 mV/s, 100 mV/s and 200 mV/s, respectively. The CV curves of ACNTs-600 and ACNTs-700 are similar, and a relatively wide peak appears at low voltage, which corresponds to the reversible redox reaction during the CV process [ 33 ]. However, the cyclic voltammetry curve of ACNTs-800 is quasi rectangular, indicating that there is no obvious redox reaction during the CV process. The pseudocapacitance performance of ACNTs-600 and ACNTs-700 is significantly higher than that of ACNTs-800, which relates to the results of XPS elemental analysis. The CV tests of the ACNTs samples increased from 20 mV/s to 200 mV/s, and the CV curves did not change, indicating that the ACNTs samples has high cycle stability, which should be related to the developed pore structure of the sample. The developed pore structure can shorten the transmission distance of electrolyte, facilitate the infiltration of electrolyte and promote the rapid charge and discharge process of capacitor [ 34 ]. The capacity of ACNTs-700 is significantly higher than that of the other two samples, which mainly attributes to the specific surface area and heteroatom content. Although ACNT-600 has the highest heteroatom content, its specific surface area is 1775 m 2 /g, which is far lower than ACNTs-700 and ACNT-800. While the surface area of ACNTs-800 is higher than the other two samples, the heteroatom content is the lowest. The capacity performance of ACNTs-700 is excellent due to the combination of the two factors. Then the electrochemical behavior of porous ACNTs-X was studied by galvanostatic charge-discharge test with the voltage range from − 0.8 to 0 V, and the current densities are 0.1 A g -1 , 0.2 A g -1 , 0.5 A g -1 , 1.0 A g -1 , 2.0 A g -1 , 3.0 A g -1 and 10.0 A g -1 , respectively, and 500 cycles are for the low current density ( 1.0 A g -1 ). Figure 5 d shows the time-voltage curve of ACNTs-X at 0.1 A g -1 for electric double-layer capacitors, and the time-voltage curve of ACNTs-X is not a symmetrical triangle, which can be attributed to the effect of pseudocapacitance introduced by heteroatoms in the ACNTs-X samples. Obviously, the charge/discharge time of ACNTs-700 is longer than that of ACNTs-600 and ACNTs-800, indicating its capacity is higher than those of the other two samples. Figure 5 e shows the capacity curve of ACNTs-X under different current densities, and the specific capacitances of ACNTs-X at a current density of 0.1 A g -1 are 392 F g -1 , 474 F g -1 and 372 F g -1 , respectively. ACNTs-700 sample shows better capacity than the other two samples, which can be attributed to its high specific surface area and high heteroatom content. The capacity retention rates of current density from 0.1 A g -1 to 10.0 A g -1 are 63.2%, 70.2% and 71.6%, respectively, and the trend of retention rate is inversely proportional to the content of heteroatoms, that is, the higher the content of heteroatoms, the lower the retention rate. Figure 5 f shows the cyclic stability test of ACNTs-X at the current densities of 1.0 A g -1 and 10.0 A g -1 . It is basically stable for ACNTs-700 and ACNTs-800 after 20 cycles, and ACNTs-600 has a small downward trend during the cycles. However, it is basically stable for the ACNTs-X in the next 1000 cycles of 10.0 A g -1 . The electrochemical performance of ACNTs is better than traditional carbon nanotubes, which can be mainly attributed to three aspects: (I) the prepared ACNTs-X nanotubes intertwine together can form 3D conductive network; (II) the prepared ACNTs-X have high specific surface area, narrow pore distribution and highly developed chemical pore structure where micropores can provide ion buffer space and mesopores can improve the channel of ion transmission;(III) the prepared ACNTs-X have high heteroatom content, which can not only generate reversible redox reactions in the charge/discharge process, but also improve the electron distribution of carbon material and the wettability of the material to the electrolyte, and then effectively increase the active surface of the electrode [ 35 ]. AC impedance measurement is another effective way to study the internal resistance of electrode materials. The AC impedance test of this experiment is performed on the electrochemical workstation with the frequency of 1Hz to 10 kHz. The radius of the arc in the high frequency region reflects the interfacial resistance of the electrode /electrolyte, and it is found that the radius of the arc of ACNTs-600 is the largest, that is, the worst conductivity compared with the other two samples, which mainly due to the highest content of heteroatoms in the carbon material. Observing the inclination curve of 45° in the intermediate frequency region, it is found that the diffusion resistance of ACNTs-700 is the smallest, mainly because the pore volume of ACNTs-700 is much higher than that of ACNTs-600, which is conducive to the diffusion of ions, and the appropriate amount of heteroatom content and suitable pore structure are also the reasons for the highest capacitance performance of ACNTs-700. 4 Conclusions In summary, we have successfully fabricated a series of porous ACNTs-X samples by pyrolysis and activation treatment of polyaniline precursors with nanotube morphology. The ACNTs-X samples display high specific surface area, narrow pore size distribution and high nitrogen content, and interweave together to form a dimensional conductive network. Interestingly, the electrochemical performance of ACNTs-X samples are in connection with the nitrogen-doped content and pore structure. Among the ACNTs-X samples, ACNTs-700 with a specific surface area of 2988 m 2 g − 1 exhibits the largest capacitance (474 F g − 1 at the current density of 0.1 A g − 1 ) and best rate capability (335 F g − 1 at the density of 10.0 A g − 1 ), as well as the superior durability with a capacity retention of 96% after 2000 charge/discharge cycles, which is mainly attributed to its unique conductive network, large surface area and moderate nitrogen-doping content. This result highlights the opportunities of utilizing unique porous CNTs as promising electrode materials for the practical applications of supercapacitors. Declarations Author Contribution Fei Zhang and Pu Feng wrote the main manuscript text ,and Lixia Wang prepared figures 1-6, and Xiaodong Jia revised the manuscripts ,and Linsen Zhang provides support. All authors reviewed the manuscript. Acknowledgement This work is supported by This work is supported by the Science and Technology Project of Henan Province (No.232102240070, No.222102240058, 232102231051), the Joint Funds of the Technology Research and Development Program of Henan Province (No. 225200810100), Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 24HASTIT024), National Natural Science Foundation of China (No. 22075255).(No.232102240070, No.222102240058, 232102231051), the Joint Funds of the Technology Research and Development Program of Henan Province (No. 225200810100), Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 24HASTIT024), National Natural Science Foundation of China (No. 22075255). Data Availability The authors declared that they have no conflicts of interest to this work. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7719192","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":540496584,"identity":"bf6a9206-bbbc-43c8-9f92-a7e53c5b7860","order_by":0,"name":"Xiaodong 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Industry","correspondingAuthor":false,"prefix":"","firstName":"Linsen","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2025-09-26 07:53:49","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7719192/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7719192/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":95343062,"identity":"45e639bc-6ce2-4c1e-a517-d422f056fe17","added_by":"auto","created_at":"2025-11-07 02:30:21","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":68380,"visible":true,"origin":"","legend":"","description":"","filename":"Manuscript1.docx","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/9753da2a0d29a50bd900c68a.docx"},{"id":95525259,"identity":"d9c2483c-6129-4154-ba88-e04793be71ac","added_by":"auto","created_at":"2025-11-10 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1","display":"","copyAsset":false,"role":"figure","size":1025035,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/e91ee761814fa3b0ef66fb34.png"},{"id":95343067,"identity":"f7e0a0c2-4bea-4b2c-b86f-3f609261bd70","added_by":"auto","created_at":"2025-11-07 02:30:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1906301,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the preparation of the ACNTs-X samples.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/82c19690f60418cb170835f1.png"},{"id":95343061,"identity":"b6d748a1-4e2c-46d3-8c54-dd75fa4d1f08","added_by":"auto","created_at":"2025-11-07 02:30:21","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1081201,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction patterns of the ACNT-X samples.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/911ac3bd9db5b4da8246ecfe.png"},{"id":95524088,"identity":"42f7a22e-8225-48d7-b39f-77815f0e202f","added_by":"auto","created_at":"2025-11-10 10:02:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2790770,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the ACNT-X samples (a-c) and TEM image of the ACNT-700(d).\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/c8efaeea8a22ea5341b0dff8.png"},{"id":95343070,"identity":"50f2ff38-7bc5-424a-b275-2dc99e4ad4c3","added_by":"auto","created_at":"2025-11-07 02:30:21","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2186174,"visible":true,"origin":"","legend":"\u003cp\u003eHRTEM images of ACNTs-700 in Fig. 4 (a-b); the N\u003csub\u003e2\u003c/sub\u003e-adsorption/sorption isotherms (c) and the corresponding BJH pore size distribution curves (d) of the ACNTs-X samples.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/b8fcb95c1e95342594736d22.png"},{"id":95343078,"identity":"c4651664-dd2a-481a-9fc4-8bd5eb89f82f","added_by":"auto","created_at":"2025-11-07 02:30:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2195561,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c) Capacity voltage scanning curves of the ACNT-X samples calculated by the CV test method with the rate of 20 mV/s, 50 mV/s, 100 mV/s and 200 mV/s, respectively; (d) time-voltage curves of the ACNTs-X samples at 0.1 A g\u003csup\u003e-1\u003c/sup\u003e for electric double-layer capacitors; (e)\u003cstrong\u003e \u003c/strong\u003eCapacity curves of the ACNTs-X samples under different current densities, respectively; (f)\u003cstrong\u003e \u003c/strong\u003eCyclic stability tests of ACNTs-X at the current densities of 1.0 A g\u003csup\u003e-1\u003c/sup\u003e and 10.0 A g\u003csup\u003e-1\u003c/sup\u003e, respectively.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/77f36c1c9e6fada82be4fd60.png"},{"id":98777045,"identity":"1a1f3e35-7efe-4a94-8d9a-4cd982f1f69f","added_by":"auto","created_at":"2025-12-22 12:25:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":12310016,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7719192/v1/587771d5-a860-457a-9952-20d6bf64acba.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Nitrogen-doped activated carbon nanotubes derived from polyaniline for high-performance supercapacitors","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eSupercapacitors have been widely used in the new energy field for its high-power density and long-cycle life [\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Supercapacitors include pseudocapacitor with reversible redox reactions and electric double layer capacitance (EDLC) without reversible redox reactions in the energy storage processes based on the different energy storage principles [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The supercapacitor using carbon materials as electrode materials is EDLC. To date, numerous carbon materials are studied as energy storage materials, such as activated carbon (AC), carbon nanotubes (CNTs), graphene and mesoporous carbons [\u003cspan additionalcitationids=\"CR8 CR9 CR10 CR11\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Among the carbon materials, CNTs are often regarded as promising energy storage materials because of the unique open channel network and excellent conductivity. However, the low specific surface area of CNTs has limited its applications in supercapacitors or lithium ion batteries [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAs a conductive macromolecule, polyaniline (PAN) has great potential in energy storage systems due to its easy synthesis, good environmental durability and unique doping mechanism [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Besides, PANs with a variety of morphologies such as nanofibers and nanotubes can be prepared by adjusting the synthesis conditions, and PANs with specific morphology were maintained after high temperature treatment in inert gas [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Thus, PANs may be employed as an excellent precursor to prepare carbon materials for energy storage [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. For example, Yuan et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] synthesized one kind of carbon nanowires showing a 325 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.1 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e by directly carbonizing nanowire-shaped PAN. Dou et al. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] prepared one kind of nitrogen-doped hollow carbon spheres showing a 213 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.5 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e by directly carbonizing sphere-shaped PAN. However, the capacitance of these PAN-based carbon materials with small specific surface area is too low to meet the growing energy density. Increasing the specific surface area seems to be an effective way to enhance the activities of carbon materials, which is usually conducted by chemical activation method [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Up to date, few reports concerned the preparation of\u003c/p\u003e\u003cp\u003eN-doped activated CNTs using PAN as the precursor with high capacitance [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this work, one kind of nitrogen-doped activated carbon nanotubes (ACNTs) was fabricated by carbonizing and activating a polyaniline precursor with nanotube morphology at 600\u0026deg;C-800\u0026deg;C in Ar atmosphere. Among the ACNTs, ACNTs-700 with a specific surface area of 2989 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e exhibits the largest capacitance (465 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.1 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and best rate capability (335 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the density of 10.0 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), as well as superior durability with a capacity retention of 96% after 2000 charge/discharge cycles.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Preparation of PAN-based CNTs\u003c/h2\u003e\u003cp\u003ePAN-based CNTs were prepared based on the reference [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Typically, 1.5 mL of aniline (AC) was added into 90.0 mL ultrapure water to form A. Meanwhile, 5.25 g of ammonium persulfate (APS) was poured into 90.0 mL ultrapure water to form B. Then B was mixed with A under moderate agitation and reacted at 4\u0026deg;C for 20 h. The resultant sample was filtrated and washed with adequate water and ethanol, and dried at 40\u0026deg;C for 48 h for reservation.\u003c/p\u003e\u003cp\u003eThe preoxidation process was operated at 200\u0026deg;C for 2 h in air and then the carbonization processes were conducted at 600\u0026deg;C-800\u0026deg;C for 3 h in Ar atmosphere, respectively. Finally, the activated process were conducted by blending CNTs and KOH with a mass ratio of 1:5 and carbonized at 600\u0026deg;C-800\u0026deg;C for 3 h in Ar atmosphere, respectively. The obtained mixtures were filtrated and washed with adequate water and ethanol, and dried at 40\u0026deg;C for 48 h. The activated samples at 600\u0026deg;C-800\u0026deg;C were denoted as ACNTs-600, ACNTs-700 and ACNTs-800, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Characterization of Materials\u003c/h2\u003e\u003cp\u003eX-ray diffraction patterns was performed on Rigaku smart lab X ray diffraction instrument using Cu Kα radiation (scanning rate: 5\u0026deg;/2θ min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, scanning range 2θ\u0026thinsp;=\u0026thinsp;5\u0026ndash;70\u0026deg;). The morphology and structure of ACNTs-700 was carried out by scanned electron microscopy (SEM, Zeiss Supra 55), transmission electron microscopy (TEM, JEOL JEM-2012) and high resolution (HRTEM, JEOL-3010). The specific surface area and pore size analysis was performed on the ASAP-2020 expedited analysis system by operating nitrogen sorption isotherms at 77 K.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Electrochemical Measurements\u003c/h2\u003e\u003cp\u003eThe ACNTs-700 sample, graphitized carbon black and polyvinylidene fluoride (PVDF) as binder, were blended with a quality ratio of 8:1:1 and added into 2.0 mL N-methyl-pyrrolidone (NMP) with moderate agitation. Then the resulting mixture was coated onto the nickel foam substrate, which was followed by drying at 120\u0026deg;C for 12h\u003c/p\u003e\u003cp\u003ein a vacuum oven. The mass of each electrode is about 2.5 mg containing conducting\u003c/p\u003e\u003cp\u003eagent and binder and the surface area of each electrode is 100 mm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAll electrochemical tests were conducted in a standard three-electrode system: nickel foil electrode with the active material as the working electrode, nickel hydroxide electrode as a counter electrode, and an Hg/HgO as a reference electrode, and 6.0 M KOH aqueous solution was employed as the electrolyte. The galvanostatic charge/discharge test was operated by CT2001A battery program controlling test system within the voltage of -0.8 V to 0.0 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 760E electrochemical working station. For the CV measurements, the sweep rate ranged from 1 to 50 mV/s within the voltage of -0.8 V to 0.0 V. For the EIS measurements, the frequency range was from 1 Hz to 10 kHz.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Structure and morphology of PAN, CNTs and ACNTs-X\u003c/h2\u003e\n \u003cp\u003eThe schematic illustration of ACNTs preparation was displayed in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. In the chemical oxidation polymerization process, aniline monomers were aggregated to form tubular micelles in APS solution under moderate ultrasound, and the tubular micelles were gradually grown into polyaniline nanotube at 4\u0026deg;C with the reaction time prolonged. Then polyaniline nanotube was preoxidized at 200\u0026deg;C for 2 h in air to remove the unstable volatiles in the carbon skeleton. Then the carbonization processes were conducted at 600\u0026deg;C-800\u0026deg;C for 3 h in Ar atmosphere, respectively. Finally, the activated processes were operated by blending the PAN-based CNTs with KOH and carbonized at 600\u0026deg;C-800\u0026deg;C for 3 h in Ar atmosphere, respectively.\u003c/p\u003e\n \u003cp\u003eThe morphology of polyaniline precursor was determined by SEM before the carbonization and activation treatment. It can be seen from \u003cstrong\u003eFig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003ea\u003c/strong\u003e that the prepared polyaniline precursor has typical nanotube morphology with uniform size in length, and is intertwined and connected together. The further enlarged SEM images in \u003cstrong\u003eFig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e (b\u003c/strong\u003e-\u003cstrong\u003ec)\u003c/strong\u003e can clearly observe that the outer wall of the nanotube is not smooth, and the upper surface of the nanotube wall with one end opening is covered with a layer of small nanoparticles. As seen the TEM image in \u003cstrong\u003eFig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003ed\u003c/strong\u003e, it is further determined that the polyaniline precursor has a hollow structure with an outer diameter of 150\u0026ndash;200 nm and an inner hollow diameter of 20 nm, and the nanotube ends of the nanotubes are open and cross connected.\u003c/p\u003e\n \u003cp\u003eThe prepared porous polyaniline-based carbon nanotubes were analyzed by X-ray diffraction as displayed in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The ACNTs-X curves display a large and wide peak of the (002) crystal plane of graphite structure at around 23\u0026deg;C, which is a typical amorphous carbon structure. The intensity of XRD peak becomes stronger with the activation temperature rises from 600\u0026deg;C to 800\u0026deg;C, indicating that the crystallinity of the porous carbon nanotubes has a trend of improvement with the rise of activation temperature.\u003c/p\u003e\n \u003cp\u003ePolyaniline, as a precursor for the preparation of carbon materials, is treated at high temperature under the protection of inert gas, will basically maintain its original morphology [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. During the carbonization and activation process, it usually happens condensation and oxidation, and carbon skeleton is retained with the transformation of unstable groups into small molecules. The SEM images of ACNTs-X were shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e \u003cstrong\u003e(a\u003c/strong\u003e-\u003cstrong\u003eb)\u003c/strong\u003e, and the original morphology of polyaniline precursor with the slight changes in the pipe diameter is still preserved after activation treatment at 600\u0026deg;C and 700\u0026deg;C. However, the morphology of ACNTs-800 in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec has changed for the reason that many nanotubes have stacked together and have the trend of soldering. The TEM image of ACNTs-700 is displayed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed, which further proves the activation sample can maintain the hollow tubular structure with the nanotubes cross connected and one end opening.\u003c/p\u003e\n \u003cp\u003eIt can be inferred that KOH can enter the carbon skeleton for oxidation during the activation process from the changes of nanotube wall, and the open tubular structure can offer the buffer layer of ions to shorten the transmission distance, which is conducive to the storage of ions [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e]. The HRTEM images of ACNTs-700 in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cstrong\u003e(a\u003c/strong\u003e-\u003cstrong\u003eb)\u003c/strong\u003e show the typical amorphous structure with many discrete carbon nanosheets protruded out of the cylindrical nanotubes. The prepared porous carbon nanotubes have shorter pore length/ion transport distance, which is conducive to the diffusion and migration of electrolyte ions compared with other traditional porous materials [\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cstrong\u003e(c\u003c/strong\u003e-\u003cstrong\u003ed)\u003c/strong\u003e display the N\u003csub\u003e2\u003c/sub\u003e-adsorption/sorption isotherms and the corresponding BJH pore size distribution curves of the ACNTs-X samples, which reveals typical IV isotherms with the characteristics of micropore and mesopore. The detailed surface parameters of the ACNTs-X samples are summarized in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Apparently, the specific surface area was greatly improved with the activation temperature increased from 600\u0026deg;C to 700\u0026deg;C, and slowly improved with the activation temperature increased from 700\u0026deg;C to 800\u0026deg;C. There is strong nitrogen adsorption at low relative pressure of P/P\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1, which corresponds to the micropore filling process, and the slow increase of adsorption curve at medium relative pressure of 0.1\u0026thinsp;\u0026lt;\u0026thinsp;P/P\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.8 indicates the existence of mesoporous structure, and the sharp rise of adsorption curve at high relative pressure of P/P\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.8 corresponds to the capillary condensation of mesopores/macropores and multilayer molecular adsorption. The pore distribution curves of ACNTs-X in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed show that all the samples have micropores, mesopores and macropores meanwhile, and the pore size distribution range is relatively narrow, and the concentrated distribution is 1\u0026ndash;4 nm. It can be seen from the pore distribution curve that the amount of mesopores is increasing with the increase of activation temperature, which is beneficial to both ion storage and transmission, and the micropore content in ACNTs-600 is the highest, and the micropore content in ACNTs-800 is the lowest.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eThe pore structure parameters of ACNTs-X.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003esamples\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS \u003csub\u003eBET\u003c/sub\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS \u003csub\u003emeso\u003c/sub\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVtatal (cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1775\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e953\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.16\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-700\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e2988\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1579\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.93\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e3026\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1765\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.98\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eThe XPS element content results of ACNTs-X are shown in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The content of N elements in ACNTs ranges from 3.52 at % minus as low as 1.16 at %, and the content of O elements decreases from 16.05 at% to 9.89 at%. The content of heteroatoms in the ACNTs-X decreases gradually with the increase of activation temperature, which is conducive to improve the conductivity of carbon based materials. However, heteroatoms in carbon based materials can provide pseudocapacitance and change the electron distribution on the surface of carbon atoms, which is conducive to increasing the wettability of samples in electrolyte [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e].\u0026nbsp;\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eThe elemental composition of ACNTs-X from XPS tests.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003esamples\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eC (at %)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eN (at %)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eO (at %)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eN\u0026thinsp;+\u0026thinsp;O (at %)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e80.03\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e3.52\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e16.05\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e19.97\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-700\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e84.32\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e2.97\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e11.71\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e15.68\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eACNTs-800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e88.95\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.16\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e9.89\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e\u003cstrong\u003e11.05\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 The electrochemical performance of ACNTs-X samples\u003c/h2\u003e\n \u003cp\u003eCyclic voltammetry (CV) test is one of the important means to evaluate the energy storage performance of electrode materials. The ACNTs-X samples as active substances are prepared electrode materials for three electrode supercapacitor test system, and the test voltage range is -0.8 V-0.0 V. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e \u003cstrong\u003e(a\u003c/strong\u003e-\u003cstrong\u003ec)\u003c/strong\u003e show the capacity voltage scanning curves calculated by the cyclic voltammetry test method with the rate of 20 mV/s, 50 mV/s, 100 mV/s and 200 mV/s, respectively. The CV curves of ACNTs-600 and ACNTs-700 are similar, and a relatively wide peak appears at low voltage, which corresponds to the reversible redox reaction during the CV process [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. However, the cyclic voltammetry curve of ACNTs-800 is quasi rectangular, indicating that there is no obvious redox reaction during the CV process. The pseudocapacitance performance of ACNTs-600 and ACNTs-700 is significantly higher than that of ACNTs-800, which relates to the results of XPS elemental analysis.\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eThe CV tests of the ACNTs samples increased from 20 mV/s to 200 mV/s, and the CV curves did not change, indicating that the ACNTs samples has high cycle stability, which should be related to the developed pore structure of the sample. The developed pore structure can shorten the transmission distance of electrolyte, facilitate the infiltration of electrolyte and promote the rapid charge and discharge process of capacitor [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. The capacity of ACNTs-700 is significantly higher than that of the other two samples, which mainly attributes to the specific surface area and heteroatom content. Although ACNT-600 has the highest heteroatom content, its specific surface area is 1775 m\u003csup\u003e2\u003c/sup\u003e/g, which is far lower than ACNTs-700 and ACNT-800. While the surface area of ACNTs-800 is higher than the other two samples, the heteroatom content is the lowest. The capacity performance of ACNTs-700 is excellent due to the combination of the two factors.\u003c/p\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eThen the electrochemical behavior of porous ACNTs-X was studied by galvanostatic charge-discharge test with the voltage range from \u0026minus;\u0026thinsp;0.8 to 0 V, and the current densities are 0.1 A g\u003csup\u003e-1\u003c/sup\u003e, 0.2 A g\u003csup\u003e-1\u003c/sup\u003e, 0.5 A g\u003csup\u003e-1\u003c/sup\u003e, 1.0 A g\u003csup\u003e-1\u003c/sup\u003e, 2.0 A g\u003csup\u003e-1\u003c/sup\u003e, 3.0 A g\u003csup\u003e-1\u003c/sup\u003e and 10.0 A g\u003csup\u003e-1\u003c/sup\u003e, respectively, and 500 cycles are for the low current density (\u0026lt;\u0026thinsp;1.0 A g\u003csup\u003e-1\u003c/sup\u003e) and 1000 cycles are for the high current density(\u0026gt;\u0026thinsp;1.0 A g\u003csup\u003e-1\u003c/sup\u003e). Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed shows the time-voltage curve of ACNTs-X at 0.1 A g\u003csup\u003e-1\u003c/sup\u003e for electric double-layer capacitors, and the time-voltage curve of ACNTs-X is not a symmetrical triangle, which can be attributed to the effect of pseudocapacitance introduced by heteroatoms in the ACNTs-X samples. Obviously, the charge/discharge time of ACNTs-700 is longer than that of ACNTs-600 and ACNTs-800, indicating its capacity is higher than those of the other two samples.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee shows the capacity curve of ACNTs-X under different current densities, and the specific capacitances of ACNTs-X at a current density of 0.1 A g\u003csup\u003e-1\u003c/sup\u003e are 392 F g\u003csup\u003e-1\u003c/sup\u003e, 474 F g\u003csup\u003e-1\u003c/sup\u003e and 372 F g\u003csup\u003e-1\u003c/sup\u003e, respectively. ACNTs-700 sample shows better capacity than the other two samples, which can be attributed to its high specific surface area and high heteroatom content. The capacity retention rates of current density from 0.1 A g\u003csup\u003e-1\u003c/sup\u003e to 10.0 A g\u003csup\u003e-1\u003c/sup\u003e are 63.2%, 70.2% and 71.6%, respectively, and the trend of retention rate is inversely proportional to the content of heteroatoms, that is, the higher the content of heteroatoms, the lower the retention rate.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ef shows the cyclic stability test of ACNTs-X at the current densities of 1.0 A g\u003csup\u003e-1\u003c/sup\u003e and 10.0 A g\u003csup\u003e-1\u003c/sup\u003e. It is basically stable for ACNTs-700 and ACNTs-800 after 20 cycles, and ACNTs-600 has a small downward trend during the cycles. However, it is basically stable for the ACNTs-X in the next 1000 cycles of 10.0 A g\u003csup\u003e-1\u003c/sup\u003e. The electrochemical performance of ACNTs is better than traditional carbon nanotubes, which can be mainly attributed to three aspects: (I) the prepared ACNTs-X nanotubes intertwine together can form 3D conductive network; (II) the prepared ACNTs-X have high specific surface area, narrow pore distribution and highly developed chemical pore structure where micropores can provide ion buffer space and mesopores can improve the channel of ion transmission;(III) the prepared ACNTs-X have high heteroatom content, which can not only generate reversible redox reactions in the charge/discharge process, but also improve the electron distribution of carbon material and the wettability of the material to the electrolyte, and then effectively increase the active surface of the electrode [\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eAC impedance measurement is another effective way to study the internal resistance of electrode materials. The AC impedance test of this experiment is performed on the electrochemical workstation with the frequency of 1Hz to 10 kHz. The radius of the arc in the high frequency region reflects the interfacial resistance of the electrode /electrolyte, and it is found that the radius of the arc of ACNTs-600 is the largest, that is, the worst conductivity compared with the other two samples, which mainly due to the highest content of heteroatoms in the carbon material. Observing the inclination curve of 45\u0026deg; in the intermediate frequency region, it is found that the diffusion resistance of ACNTs-700 is the smallest, mainly because the pore volume of ACNTs-700 is much higher than that of ACNTs-600, which is conducive to the diffusion of ions, and the appropriate amount of heteroatom content and suitable pore structure are also the reasons for the highest capacitance performance of ACNTs-700.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn summary, we have successfully fabricated a series of porous ACNTs-X samples by pyrolysis and activation treatment of polyaniline precursors with nanotube morphology. The ACNTs-X samples display high specific surface area, narrow pore size distribution and high nitrogen content, and interweave together to form a dimensional conductive network. Interestingly, the electrochemical performance of ACNTs-X samples are in connection with the nitrogen-doped content and pore structure. Among the ACNTs-X samples, ACNTs-700 with a specific surface area of 2988 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e exhibits the largest capacitance (474 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.1 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and best rate capability (335 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the density of 10.0 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), as well as the superior durability with a capacity retention of 96% after 2000 charge/discharge cycles, which is mainly attributed to its unique conductive network, large surface area and moderate nitrogen-doping content. This result highlights the opportunities of utilizing unique porous CNTs as promising electrode materials for the practical applications of supercapacitors.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eFei Zhang and Pu Feng wrote the main manuscript text ,and Lixia Wang prepared figures 1-6, and Xiaodong Jia revised the manuscripts ,and Linsen Zhang provides support. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work is supported by This work is supported by the Science and Technology Project of Henan Province (No.232102240070, No.222102240058, 232102231051), the Joint Funds of the Technology Research and Development Program of Henan Province (No. 225200810100), Program for Science \u0026amp; Technology Innovation Talents in Universities of Henan Province (No. 24HASTIT024), National Natural Science Foundation of China (No. 22075255).(No.232102240070, No.222102240058, 232102231051), the Joint Funds of the Technology Research and Development Program of Henan Province (No. 225200810100), Program for Science \u0026amp; Technology Innovation Talents in Universities of Henan Province (No. 24HASTIT024), National Natural Science Foundation of China (No. 22075255).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eY. Zhang, H-X. Mei, Y. Cao, X-H. Yan, J Yan, H-L. Gao, H-W. Luo, S-W. Wang, X-D. Jia, J Yang, S-C. Xue, C-G. Zhou, L-X Wang, Y-H. Gui. Recent advances and challenges of electrode materials for flexible supercapacitors, Coord. Chem. Rev. 438 (2021) 213910.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eY. Zhang, X. Jing, X-H.Yan, H-L. Gao, K-Z. Gao, Y. Cao, S. Hu, Y-Y. Zhang. Rational design of NiMn-based electrode materials for high-performance supercapacitors. Coord. Chem. Rev. 499 (2024) 215494.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eH Fang, L Zhang, Y Xing, et. al., Nanostructured manganese oxide films for high performance supercapacitors, Int. J. Electrochem. 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Commom. 122 (2020) 108284.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Nitrogen-doped ACNTs, Polyaniline, Conductive network, Supercapacitors","lastPublishedDoi":"10.21203/rs.3.rs-7719192/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7719192/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHerein, one kind of nitrogen-doped activated carbon nanotubes (ACNTs) was fabricated by carbonizing and activating a polyaniline precursor with nanotube morphology at 600\u0026deg;C-800\u0026deg;C in Ar atmosphere. Various characterizations reveal that the ACNTs display high specific surface area, narrow pore size distribution and high nitrogen content, and the ACNTs interweave together to form a dimensional conductive network. Among the ACNTs materials, the ACNTs-700 sample with a specific surface area of 2988 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e exhibits the largest capacitance (474 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.1 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and best rate capability (335 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the density of 10.0 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), as well as the superior durability with a capacity retention of 96% after 2000 charge/discharge cycles, which is mainly attributed to its unique conductive network, large surface area and moderate nitrogen-doping content.\u003c/p\u003e","manuscriptTitle":"Nitrogen-doped activated carbon nanotubes derived from polyaniline for high-performance supercapacitors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-07 02:30:16","doi":"10.21203/rs.3.rs-7719192/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4f54816e-94b1-4876-8a99-6db9f94bee33","owner":[],"postedDate":"November 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-22T01:39:08+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-07 02:30:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7719192","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7719192","identity":"rs-7719192","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}