C hybrid nanofibers for supercapacitor application

Electrochimica Acta 176 (2015) 402–409

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Synergistic enhancement of electrochemical performance of electrospun TiC/C hybrid nanofibers for supercapacitor application Yaqi Ren, Jie Dai, Bo Pang, Xiang Liu, Jie Yu* Shenzhen Engineering Lab for Supercapacitor Materials, Shenzhen Key Laboratory for Advanced Materials, Department Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 May 2015 Received in revised form 3 July 2015 Accepted 5 July 2015 Available online 9 July 2015

Isotropically conductive TiC/C hybrid nanofibers (TCCNFs) have been prepared by electrospinning for supercapacitor application for the first time. By changing the atmosphere of stabilization process, the TCCNFs with different TiC contents were successfully synthesized with uniform morphology and average diameter of about 100 nm. The TCCNFs stabilized in Ar contain much more TiC than those stabilized in air. The specific capacitance of the TCCNFs stabilized in Ar and air were measured to be 77.8 F g
1 and 130.0 F g
1 at the current density of 0.1 A g
1, respectively, which were much higher than the pure TiC nanoparticles and reported carbon materials with similar specific surface area. The charge storage mechanisms were discussed by analyzing the capacitive and diffusion-controlled contributions to the total capacitance. Reversible valance change of Ti atoms was observed during charge/discharge process, indicative of the occurrence of pseudoreaction. The experimental results support that the higher specific capacitance of the TCCNFs may be caused by a synergistic enhancing effect between TiC and carbon. The capacitance retention reaches 98.9% and 93.0% for the TCCNFs stabilized in Ar and air after 25 000 cycles, respectively, showing excellent stability. The present work provides a novel conductive electrode material for supercapacitors with possible pseudocapacitance, worthy of further investigation. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: titanium carbide nanofibers supercapacitors pseudocapacitance

1. Introduction At present, supercapacitors are considered as one of the most promising energy storage devices because of the advantages of high power capability, long cycle life, low cost, and environmental friendliness [1,2]. Electrode material is the key part in supercapacitors and much effort has been devoted to the research of electrode materials for improving electrical conductivity, increasing specific capacitance, and developing novel materials [2–10]. According to the charge storage mechanism, supercapacitors can be classified into electrical double layer (EDL) capacitors by absorbing ions from electrolyte solutions onto the electrode surface and pseudocapacitors based on Faradaic redox reactions between electrode and electrolyte [2]. As the most widely used electrode materials, carbon-based materials with EDL mechanism exhibit good stability and higher power capability but lower specific capacitance [3,5,11–14]. Transition metal oxides with both EDL and Faradaic mechanism such as RuO2 [15] and MnO2 [3] are characterized by higher specific capacitance but poor cycling performance and electrical conductivity [11]. Currently, it is still

* Corresponding author. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.electacta.2015.07.025 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

necessary to explore novel electrode materials for obtaining improved properties for supercapacitor application. Titanium (Ti)-based compounds are attractive materials for energy application as exemplified by previous reports [16,17]. For example, TiO2 and TiN have been used for lithium-ion batteries and electrocatalysts, respectively [17–23]. Recently, several studies have been reported about using TiO2 and TiN as electrode materials for supercapacitors [16,24–29]. Ramadoss et al. [16] prepared TiO2 nanorod arrays by hydrothermal method for supercapacitor application, achieving an areal specific capacitance of 85 mF cm
2 in 1 M Na2SO4 electrolyte. Although TiO2 and TiN have been investigated for supercapator application [16,24–29], to the best of our knowledge, another type of Ti-compound TiC with a cubic crystal structure (Fm3m) has not been reported for supercapacitor application. TiC with excellent isotropical electrical conductivity (30  106 (ohm cm)
1), high corrosion resistance, and good stability [30–32]may be a candidate electrode material for supercapacitors. Since chemisorption phenomenon was observed in TiN there may also exist pseudocapacitance in TiC with a similar cubic crystal structure and bonding type but better stability. In this paper, we prepared TiC/C hybrid nanofibers (named as TCCNFs) by electrospinning and investigated their electrochemical properties for supercapacitor application for the first time. The TiC contents in

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the nanofibers have been well controlled by changing the preparation conditions. The TCCNFs exhibit improved electrochemical performance comparing with its carbon counterpart, which is dependent on the TiC content in the nanofibers. Interestingly, we found that TiC may have pseudocapacitance and a synergistic effect enhancing the electrochemical performance between TiC and C was observed. The present work provides a novel isotropically conductive electrode material with possible pseudocapacitance for supercapacitors apart from carbon materials. 2. Experimental The precursor fibers were electrospun by a conventional electrospinning setup. In a typical process, 1 g polyvinylpyrrolidone (PVP, Aladdin, Mw = 1300 000) and 0.5 g acetic acid (HAc) were dissolved in 10 mL dimethylformamide (DMF) to form a transparent solution. Then tetrabutyl titanate (TBT) with different amounts (0.6 mL, 1 mL, and 1.4 mL) were dropwise added into the PVP solution under vigorous stirring to prepare a transparent solution. The electrospinning was carried out at a fixed voltage of 8 kV and the fibers were collected by a piece of graphitic paper. The collected composite fibers were heated at 310  C for 3 h with a heating rate of 3  C min
1 in Ar atmosphere or air for stabilization and then the samples were heated at 1400  C for 3 h with a heating rate of 5  C min
1 in Ar atmosphere. For comparison, carbon nanofibers were prepared by electrospinning a solution with 0.5 g polyacrylonitrile (PAN, Aldrich, Mw = 150 000) in 10 mL DMF, subsequent stabilization at 310  C for 2 h in air, and carbonization at 1400  C for 3 h in Ar atmosphere. Commercial TiC nanoparticles with an average diameter of 50 nm purchased from Aladdin were also investigated for comparison. All electrochemical measurements were performed in a threeelectrode system in 6 M KOH solution, where the obtained samples coated on glassy carbon, Pt wire, and Ag/AgCl in 3 M KCl serve as working, counter, and reference electrode, respectively. The working electrode was prepared by mixing the active materials, acetylene black, and polytetrafluoroethylene (PTFE) in ethanol according to the weight percentage of 85%, 10%, and 5% to form slurry, which was then spread onto the glassy carbon. The area of the glassy carbon is 7 mm2 and the loading amount of the active materials is 30.1 mg cm
2. Cyclic voltammogram (CV) curves, galvanostatic charge/discharge (CD) curves, electrochemical impedance spectra (EIS), and the cyclic stability were measured on an electrochemical workstation (CHI760C, Shanghai Chenhua Instrument Co. Ltd., China). The stability of the samples was tested by measuring the CD curves at a current density of 10 A g
1. The frequency range and potential amplitude for the EIS measurements were 100 kHz – 10 mHz and 5 mV, respectively. All the properties of the samples were tested after the stabilization of electrode for 50 cycles.

403

Scanning electron microscopy (SEM, HITACHI S-4700), transmission electron microscopy (TEM, JEM-2010), and X-ray diffraction (XRD, Rigaku D/Max 2500/PC) were used to characterize the structures of the samples. Thermogravimetric (TG) curves were measured by a TG analyzer (STA449F3, Jupiter). Nitrogen sorption isotherms were measured at 77 K by a Micromeritics Tristar 3000 analyzer (USA) to determine the specific surface area (SSA) of the samples. Before measurements, the samples were degassed in vacuum at 200  C for 6 h. The SSA was calculated by the Brunauer–Emmett–Teller (BET) method. The pore sizes (Dp) were calculated from the adsorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 system, VG) was used to detect the bonding states and composition of the samples. 3. Results and discussion The TCCNFs were prepared from TBT/PVP fibers by stabilization in different atmosphere (Ar and air) at 310  C and carbonation at 1400  C in Ar. Fig. 1a shows the SEM image of the TBT/PVP fibers prepared at the TBT:PVP mass ratio of 1:1, which are 100-200 nm in diameter. It is observed that the introduction of TBT do not change the morphology of the electrospun fibers and the TBT/PVP fibers electrospun at different TBT:PVP mass ratios possess about similar morphology. Fig. 1b shows the SEM image of the TiC/C hybrid nanofibers stabilized in Ar (named as TCCNFs-Ar). The average diameter is about 100 nm and the fiber surface is rough. It is found that the morphology of the products is dependent on the TBT:PVP mass ratio. At the TBT:PVP mass ratio of 0.6:1 the fiber morphology can not be maintained for the products while at 1.4:1 the products possess about similar morphology to those prepared at the TBT: PVP mass ratio of 1:1. Fig. 1c shows the SEM image of the TiC/C hybrid nanofibers stabilized in air (named as TCCNFs-air). The diameters are from 50 to 150 nm and the fiber surface is much smoother than the TCCNFs-Ar. For comparison, the diameter of carbon nanofibers is about 80 nm. XRD patterns were measured to investigate the structure of the samples, as shown in Fig. 2. All the patterns exhibit five characteristic diffraction peaks at 36.0 , 41.9 , 60.7, 72.7, and 76.5 , corresponding to the diffraction of (111), (200), (220), (311), and (222) planes of cubic TiC (JCPDS 65-8808), respectively. However, the sample with TBT:PVP mass ratio of 1.4:1 also shows other weak diffraction peaks corresponding to rhombohedral Ti2O3 (JCPDS 10-0063). This is because the carbon source from the PVP is not sufficient for converting the intermediate TiOx [33,34] to TiC at this high TBT:PVP mass ratio. For the TCCNFs-air the XRD pattern shows a broad peak ranging from 20 to 30 corresponding to (002) diffraction of turbostratic carbon [2,35,36] besides the peaks from TiC, indicating high carbon content in the sample. This is because the reactions are different when heated in Ar and air, resulting in different structure change of the PVP. As well known, O2 is necessary for stabilization of polymers. When heated in air the

Fig. 1. SEM images of the samples prepared at the TBT:PVP mass ratio of 1:1: (a) electrospun precursor TBT/PVP nanofibers, (b) TCCNF-Ar, (c) TCCNF-air.

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Fig. 2. XRD patterns of the TCCNFs prepared at different TBT:PVP mass ratio and stabilization atmosphere: (I) 0.6:1, Ar, (II) 1:1, Ar, (III) 1.4:1, Ar, (IV) 1:1, air.

linear macromolecular chain of PVP can be turned to heat-resistant trapezoidal frame under the reaction of O2, which can be converted to carbon during subsequent carbonization at high temperature. However, when heated in Ar the PVP molecules can not be stabilized effectively in the absence of O2. In this case, the PVP will decompose at high temperature rather than converting to carbon. Consequently, the TCCNFs-air contain much more carbon than TCCNFs-Ar. Fig. 3 shows the TG curves of the TCCNF samples and commercial TiC nanoparticles. All the samples exhibit a mass increase starting at around 400  C with increasing the temperature. This mass increase corresponds to the oxidation of TiC to TiO2. For the TCCNF samples the mass decreases after a vertex with further increasing the temperature, which is caused by the oxidation of carbon contained in the samples. It is observed that the amount of mass decrease is higher for the TCCNFs-air than that for the TCCNFs-Ar while no obvious mass decrease appears for the commercial TiC nanoparticles, indicating the different carbon content in the different samples. Except the influence of oxygen and other elements in TiC and only considering the ratio of Ti:C, according to the TG curves the mole ratios of Ti:C were calculated to be 1:2.1, 1:8, and 1:1.3 for the TCCNFs-Ar, TCCNFs-air, and the commercial TiC nanoparticles, respectively, consistent with the XRD results. Fig. 4a shows the low magnification TEM image of the TCCNFsair. It is shown that the TCCNFs contain some nanoparticles with diameters of 10-30 nm, which are dispersed in the carbon matrix in the nanofibers. From the high resolution TEM image the lattice

Fig. 3. TG curves of the TCCNF samples and commercials TiC nanoparticles.

fringe spacing of the nanoparticles was measured to be 0.25 nm, which corresponds to interplanar spacing of the TiC (200) planes (Fig. 4c), indicating that the nanoparticles are TiC. Fig. 4b shows the TEM images of the TCCNFs-Ar. It is observed that the TCCNFs-Ar contain much more TiC nanoparticles than the TCCNFs-air. The TiC nanoparticles of 10-30 nm in size are densely packed in the TCCNFs-Ar, leading to many TiC/TiC interfaces and less TiC/carbon interfaces (Fig. 4d). On the contrary, for the TCCNFs-air most of the TiC nanoparticles are surrounded by the carbon matrix, forming the TiC/carbon interfaces (Fig. 4c). Obviously, the TiC content in the hybrid nanofibers can be effectively controlled by changing the stabilizing atmosphere. Fig. 5a shows the N2 adsorption/desorption isotherms of the different samples. The abrupt rise of the isotherm at very low relative pressure is caused by the adsorption of the micropores while the subsequent gradual increase with increasing the relative pressure originates from the adsorption of the mesopores and macropores [13]. The nitrogen adsorption-desorption isotherms of all the samples show type-IV curves with an evident hysteresis loop arising from capillary condensation, indicative of mesopores and macropores [37]. Due to the introduction of carbon, the adsorption capacity increased remarkably compared to the commercial TiC nanoparticles. It is also found that the adsorption capacity of the TCCNFs is much higher than the pure carbon nanofibers with smaller diameters, indicating the great increase of the SSA due to the introduction of the TiC nanoparticles. The calculated SSAs of the carbon nanofibers, TCCNFs-Ar, and TCCNFsair, and commercial TiC nanoparticles are 68.6, 116.5, 340.1, and 22.6 m2 g
1, respectively. The higher SSAs of the TCCNF samples may be mainly caused by the presence of the TiC/carbon interfaces, where pores are generated because of the different properties of the TiC and carbon phases. As indicated in the TEM images (Fig. 4 c and d), the TiC/carbon interfaces are defect–enriched and the TiC/ TiC interfaces are compact. The higher SSA for the TCCNFs-air is because more TiC/carbon interfaces are present. Pore size distribution curves of the samples are shown in Fig. 5b, which indicates that the proportion of the mesopores increases after introducing the TiC nanoparticles in the nanofibers. Electrochemical performance of the samples was evaluated in 6 M KOH solution with a conventional three-electrode system. Fig. 6a compares the CV curves of the carbon nanofibers, TCCNFsAr, and TCCNFs-air, and commercial TiC nanoparticles. The CV curves of the carbon nanofibers and commercial TiC nanoparticles exhibit quasi-rectangular shape. However, the CV curves of the TCCNFs are asymmetric with the current density higher at lower voltage than higher voltage [25]. The current density of the TCCNFs-Ar is higher than that of the TCCNFs-air, both of which are much higher than that of the carbon nanofibers. It can be seen that the TCCNFs-air has the highest specific capacitance among the three samples. Fig. 6b and c show the CV and CD curves of the TCCNFs-Ar tested at different scan rate and charging/discharging current density. The CD curves present nearly triangular shape, suggesting good charge propagation across the electrodes [11]. The specific capacitances at 10, 5, 2, 1, 0.5, 0.2, and 0.1 A g
1 were calculated to be 57.8, 60.6, 64.2, 67.6, 69.4, 73.7, and 77.8 F g
1, respectively. The capacitance retention reaches 74.3% when increasing the discharge current density from 0.1 to 10 A g
1. Fig. 6d and e show the CV and CD curves of the TCCNFs-air. From the CD curves the specific capacitances of the TCCNFs-air were calculated to be 74.9, 80.0, 96.1, 97.5, 109.9, 120.4, and 130.0 F g
1 at the current densities of 10, 5, 2, 1, 0.5, 0.2, and 0.1 A g
1, respectively. The capacitance retention is 57.6% when increasing the current density from 0.1 to 10 A g
1. Fig. 6f shows the specific capacitances of the carbon nanofibers, commercial TiC nanoparticles, and the TCCNF samples at different charging/discharging current densities. The specific capacitance of the TCCNFs-air is

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Fig. 4. TEM images of the TCCNFs-air (a and c) and TCCNFs -Ar (b and d).

considerably higher than that of the TCCNFs-Ar, both of which are much higher than that of the carbon nanofibers (only 3.2 F g
1 at 1 A g
1) and the commercial TiC nanoparticles (25.6 F g
1 at 1 A g
1). There are two kinds of charge storage mechanism in supercapacitors. One is EDL capacitors depending on the SSA and pore size of active materials [38,39], and the other is pseudocapacitors storing charge by redox reactions [40]. From the above results, it seems that coexistence of TiC and carbon generates higher specific capacitance and appropriate Ti:C ratio is beneficial for increasing the specific capacitance. As mentioned above, the SSAs of the TCCNFs-Ar and TCCNFs-air are 116.5 and 340.1 m2 g
1 and the capacitances are 67.6 and 97.5 F g
1 at 1 A g
1, respectively. However, according to the previous reports the specific capacitances of the EDL materials are generally much lower than the present TCCNFs at similar SSA (Table 1) [41–45]. Therefore, we consider that there exists a synergistic enhancing effect between TiC and carbon related to pseudoreaction in charge storage process. It has been reported that electrochemical reaction occurs during charge storage of TiN [10,26]. Because of the similarity in crystal structure and bonding type between TiC and TiN it is reasonable to consider that pseudocapacitance may also contribute to the total capacitance of TiC in KOH solution. To investigate the charge storage mechanism in TCCNFs, we analyzed the capacitive contribution to the current response by using the voltammetric scan rate dependence of the current [46–48]. The current response (i) at a fixed potential (V) can be described as the combination of two separate mechanisms, namely capacitive effects (k1n) (linearly diffusion process) and diffusion-controlled insertion (k2n1/2) (semi-infinite diffusion processes), with the following formula: iðVÞ ¼ k1 n þ k1 n1=2 where n is the sweep rate and k1 and k2 are constants at a particular sweep rate. By determining k1 and k2, we can use the above equation to distinguish the current arising from diffusion processes and that from capacitive effects.

Fig. 5. Nitrogen sorption isotherms (a) and pore size distribution (b) of the different samples.

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Fig. 6. (a) CV curves of different samples at 100 mV s
1. (b, c) CV curves and CD curves of TCCNFs-Ar. (d, e) CV curves and CD curves of TCCNFs-air. (f) Specific capacitances versus discharge current density of different samples.

Fig. 7 compares the capacitive contribution (shaded region) and pseudocapacive contribution to the current response of the CV curves of the TCCNFs calculated by the above formula. The capacitive contributions are 92.5% and 84.8% for the TCCNFs-Ar and TCCNFs-air, respectively. As discussed above, we can attribute the total current to be primarily from capacitive current contribution. In addition, the relative capacitive response to the total stored charge of TCCNFs-air is less than that of TCCNFs-Ar. The other small part of current contribution is whether electrostatic or pseudocapacitive. Further, if there are also redox contributions from changes in the oxidation states of surface Ti atoms during the charge/discharge process, the redox process are not diffusioncontrolled, and thus the diffusion-controlled contribution can be determined to be pseudocapacitive [48]. Though the contribution of pseudocapacitive is lower than that of capacitive, the application potential can be explored by deeper researches like controlling the structure or other elements. To further confirm the existence of pseudocapacitance, XPS were used to test the surface valence changes of Ti atoms during charge/discharge process, and the results are shown in Fig. 8. It is observed that the valence of Ti atoms changes regularly during the charge/discharge process. In the figure, the peaks at 455.5 eV and 461.3 eV (red) can be attributed to Ti 2p3/2 and 2p1/2 of Ti-C bond, respectively [49]. The peaks at 458.8 and 464.6 eV (dark yellow) belong to the Ti in Ti-O bond (from TiO2), which is always present on the TiC surface due to air oxidization [50]. The peaks at Table 1 Reported specific capacitance of carbon materials with different SSAs. Carbon materials

SSA / m2 g
1 Specific capacitance / F g
1 Reference

Single-wall nanotubes Reduced graphene oxide Carbon nanotubes Carbon nanofibers Carbon nanotubes

410 801

40 41.5

41 42

107.4 348 230 440

25 <10 20 40

43 44 45 Fig. 7. CV curves of the TCCNFs-Ar (a) and TCCNFs-air (b). Shaded region corresponds to the capacitive contribution.

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Fig. 8. Ti 2p XPS spectra of as-prepared (left), charged (middle), and discharged (right) the commercial TiC nanoparticles (a-c), TCCNFs-Ar (d-f), and TCCNFs-air (g-i).

456.2 and 462.2 eV (blue) correspond to Ti-C-O bond [50]. For commercial TiC nanoparticles, Ti 2p peaks do not change during charge/discharge process. Interestingly, for the TCCNFs-Ar and TCCNFs-air, it is found that the Ti-C peak decrease after charging and recover after discharging, revealing that reversible chemical reaction occurs during the charge/discharge process. By comparing the peak area of the Ti-C bond of the samples after charging and after discharging the proportion of the Ti atoms participating in the reversible reaction was calculated to be 2.2% and 3.8% for the TCCNFs-Ar and TCCNFs-air, respectively. Based on these results we consider that the pseudoreaction occurs at the interface of TiC and carbon, which needs the participation of the adjacent carbon atoms. As more TiC/carbon interfaces are present in the TCCNFs-Ar they possess higher specific capacitance. Therefore, it is expected that the pseudoreaction proceeds following the equation below. C
Ti½III þ OH

e
Ð C
Ti½IV
OH Long-term cycling performance of the TCCNFs was examined for 25000 cycles at a current density of 10 A g
1, as shown in Fig. 9a and c. The TCCNFs-Ar and TCCNFs-air exhibit excellent operation stabilities with the capacitance retention reaching 98.9% and 93%, respectively. It is noted that the stability of the

TCCNFs in alkaline electrolyte is comparable or superior to most carbon materials [44,51–54] and other titanium-based materials such as TiN [25] and TiO2 [16,24], being advantageous for supercapacitor application. Nyquist plots of the TCCNFs measured with a frequency range from 100 kHz to 0.01 Hz are shown in Fig. 9b and d after discharging. All the plots comprise one semicircle and an inclined linear tail. The intercept of the semicircle with the real axis at high frequency side corresponds to the internal resistance (Rs). The semicircle results from the charge transfer resistance (Rct) at the electrode/electrolyte interface. The low frequency straight line is associated with the capacitive behavior of the supercapacitors, which should exhibit a vertical shape for an ideal capacitor. The deviation from the vertical direction is due to the diffusion resistance of the ions in the electrode structure [2]. Before longterm cycling test the Rs and Rct were measured to be 3.8 V and 3.4 V for the TCCNFs-air and 3.5 V and 3.6 V for the TCCNFs-Ar, respectively, indicating excellent electrical conductivity of the samples. After 25000 cycles the Rs keep almost constant for the two samples and the Rct increased to 5.6 V and 4.4 V for the TCCNFs-air and TCCNFs-Ar, respectively. In addition, the low frequency straight line changes very little with increasing the cycling number for the TCCNFs-Ar while for the TCCNFs-air the

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Fig. 9. (a) Long-term cycling performance and (b) Nyquist plots of TCCNFs-Ar at 10 A g
1. (c) Long-term cycling performance and (d) Nyquist plots of TCCNFs-air at 10 A g
1.

straight line became much more inclined. These results indicate that the TCCNFs possess excellent structure and operation stability and the presence of TiC is in favor of improving the stability. 4. Conclusion In summary, isotropically conductive TiC/C hybrid nanofibers (TCCNFs) have been prepared for supercapacitor application. The TiC:C ratio in the hybrid nanofibers can be effectively controlled by changing the stabilization atmosphere, where stabilization in Ar generates higher TiC content than in air. The TCCNFs possess much higher specific capacitance than the pure TiC nanoparticles and the reported carbon materials with similar specific surface area. A synergistic enhancing effect between TiC and carbon accounts for the higher specific capacitance of the TCCNFs. Both EDL and pseudocapacitance contribute to charge storage. This work provides a novel isotropically conductive electrode material for supercapacitors besides carbon materials. The possible pseudoreaction for the TCCNFs deserves further study and the TCCNFs may be promising for supercapacitor application. Acknowledgements

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