Nitrogen-Enriched Porous Carbon Nanofiber Mat as Efficient Flexible

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Nitrogen-enriched porous carbon nanofiber mat as efficient flexible electrode material for supercapacitors Arup Choudhury, Ji Hoon Kim, Susanta Sinha Mahapatra, Kap Seung Yang, and Duck Joo Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02031 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Sustainable Chemistry & Engineering

Nitrogen-enriched porous carbon nanofiber mat as efficient flexible electrode material for supercapacitors

Arup Choudhurya, Ji-Hoon Kimb, Susanta Sinha Mahapatrac, Kap-Seung Yangb* and Duck-Joo Yangd* a

Department of Chemical Engineering, Birla Institute of Technology, Mesra, Ranchi 835-215, India

b

School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Bukgu, Gwangju 500-757, Republic of Korea c

d

Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi 835-215, India

Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, 800 W. Campbell Road, Richardson, TX 75080, United States

*

Corresponding author. Tel.: +91 9430 732461; fax: +91 651 2276184; E-mail: [email protected] (Duck-Joo Yang) [email protected] (Kap-Seung Yang)

1 ACS Paragon Plus Environment


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Abstract: Freestanding nitrogen-doped porous carbon nanofibres (NCNFs) mats were prepared by electrospinning of polyacrylonitrile/poly(m-aminophenol) (PAN/PmAP) precursor blends with different polymeric compositions followed by thermal stabilization and carbonization. The morphology, pore structure and surface elemental compositions of as-prepared NCNFs were characterized by different techniques such as scanning electron microscopy, transmission electron microscopy, N2 adsorption, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The charge-storage capability of the fabricated NCNFs was investigated in KOH electrolyte. The electrochemical performances of NCNFs were evaluated by varying the PmAP loading in the blend compositions. The highest specific capacitance of 347.5 F g-1 at 0.5 mA cm-2 together with a capacitance retention of 173.2 F g-1 at 20 mA cm-2 was achieved for the PAN:PmAP (85:15 w/w) NCNFs (NCNF85:15). The volumetric capacitance of 200.8 F cm-3 at 0.5 mA cm-2 was recorded for the NCNF85:15. The NCNF85:15 showed the maximum energy density of 12.1 Wh kg-1 at 0.093 kW kg-1 and good cycling stability with 90.5% capacitance retention after 10000 cycles. The excellent capacitive performances of the NCNF85:15 was attributed to high effective surface area, high content of mesoporosity, good conducvity and high fraction of hetero-atom doped carbon, which result in both electrochemical double layer and Faradaic capacitance contributions.

Keywords: Poly(m-aminophenol); Polyacrylonitrile; Electrospinning; Carbon nanofibers; Specific capacitance; Supercapacitors


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1. Introduction In the past decade, porous carbonaceous materials have attracted great attention in the field of high-performance energy storage due to their unique properties like such as high specific surface area, large pore volume, chemical inertness, good mechanical stability, high environmental resistance and low cost [1-6]. However, the capacitive performance of supercapacitors intimately depends on the phsico-chemical properties of the electrode materials. Therefore, current research has been heavily focused on the development suitable electrode materials for high-performance supercapacitors. According to earlier reports, the most porous carbon based electrochemical capacitors (ECs) still suffer from inferior ion-transport kinetic and consequently yield inferior specific capacitance and low power densities. In order to solve these problems, the mesoporous carbon nanofibres have been extensively explored as electrode materials for electrochemical double layer capacitors (EDLCs) because of its accessible specific surface area and tunable pore sizes offer a minimum resistance and shorter diffusion pathways and subsequently enhance iontransport/diffusion rate [7-11]. Recently, Kim et al. [12] achieved high energy density (246 W h kg-1 at 3.5 V) for polyacrylonitrile (PAN)/tetraethyl orthosilicate based carbon nanofibers with finely tailored pore structure in an ionic liquid system. Niu et al. [13] constructed threedimensional metal oxides core-shell heterostructures as the electrodes for high-performance electrochemical energy storage application via electrospinning method combined with hydrothermal approach. Fun et al. [14] reported a specific capacitance of 242 F/g at 0.2 A/g with nitrogen-doped

carbon

nanofibers

prepared

through

co-electrospinning

of

a

PAN/polyvinylpyrrolidone/SiO2 blend solution, followed by pyrolysis process. Recently, functional nanostructured carbon materials have been proved to improve capacitive performance owing to their unique combination of properties such increased electrical



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conductivity, more ordered pore structure and improved wettability between electrode and electrolyte in aqueous electrolyte solutions [15-18]. In addition, the presence of heteroatom groups on CNF surfaces induces pseudo-capacitance originate from the faradaic interactions between the ions of electrolytes and the carbon electrode surface. Generally, nitrogen-containing carbon nanofibres could be prepared through post-treatment of CNFs with ammonia, pyridine, urea, amines etc [19,20] but the resultant N-doped CNFs showed low N-doping level with poor electrochemical properties. Compared to post-treatment, pyrolysis of nitrogen containing polymer precursors such as polyacrylonitrile [16,21], polyvinylpyridine [22], Polybenzoxazine [23] is supposed to be more simple and efficient route to produce N-doped CNFs. However, these N-containing carbon materials showed improved electrochemical performances to some extent but still needed further enhancement of their capacitance, power density and rate capability. The reason for this unsatisfactory performance was poor conductivity of the electrode due to the addition of polymeric binders or conductive additives. These binders can reduce the ion diffusion rate and electrical conductivity of electrodes and thus lower the specific capacitance. In recent years, the preparation of N-enriched carbon nanofibres via electrospinning combined with post carbonization process have been received increasing attention. The method can efficiently produce free-standing NCNFs mats with good mechanical integrity, large accusable surface area and controlled pore structure/pore size distribution, which could be improved the specific capacitance of supercapacitors together with high power and energy densities. Poly(m-aminophenol), which is an important conducting polymer containing both nitrogen and oxygen functionalities in the backbone chain, can be used as a precursor to produce heteroatom-doped carbon materials. A little attention has been focused on the charge storage


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properties of N-doped CNFs derived from polyaniline-based precursor [24-26], while PmAP did not explored as precursor to fabricate carbon materials for capacitance applications. The main difficulty to produce conducting polymer based nanofibres via electrospinning process is the inferior solubility and high conductivity of these polymers. Therefore, a polymer carrier is needed to be use during electrospinning to produce precursor nanofibres. Many research groups have used polyacrylonitrile based binary polymer blends as precursor to fabricate carbon nanofibres [27-30]. The PAN can be used as a carrier to disperse PmAP to obtain electrospun multiphase nanofibres due to its excellent electrospinnability. Most significantly, the N-doping level into the carbon matrix can easily be controlled by adjusting the PmAP content in the precursor solution. Till date, there is no study reported on electrochemical performances of nitrogen-doped CNFs derived from PAN/PmAP precursor blends. In the present investigation, we report a promising approach to produce flexible binderfree nitrogen-doped CNFs via electrospinning of PAN/PmAP precursor along with subsequent carbonization. Di-methyl sulfoxide (DMSO) was used as a common solvent to produce homogenous blend solution with desired viscosity and conductivity for electrospinning process. The effects of blend composition on electrochemical performances of the NCNFs are investigated. The electrochemical properties were correlated with N-doping level and microstructures of different NCNFs. With respect to previous studies, the present N-doped CNFs were not only fabricated via simple electrospinning and subsequent carbonization process but also exhibited excellent electrochemical performances in both two- and three-electrode systems.



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2. Experimental 2.1 Preparation of carbon nanofibres At first, PmAP was synthesized through chemical oxidation of m-aminophenol (m-AP, SigmaAldrich India Ltd.) using the procedure described in our earlier report [31]. Ammonium persulfate (Sigma-Aldrich India Ltd.) was used as an oxidant. Polyacrylonitrile (PAN; Mw = 160 000, Pfaltz & Bauer Chemical Co., USA) and as-prepared PmAP were dissolved in 30 ml DMSO (Duksan Chemical Co., Korea) at 80 °C under vigorous stirring for 4 hrs followed by constant stirring at room temperature for another 24 hrs. The weight percentage of polymer (PAN/PmAP) in DMSO was kept constant at 10 wt%, whereas the weight ratio of PAN and PmAP was varied as 100:0, 95:5, 90:10 and 85:15 (w/w). The PAN/PmAP nanofibre mats were fabricated by electrospinning of different formulated polymer blend solutions. An applied voltage of 25 kV to the positively charged capillary and a tip-to-collector distance of 17.5 cm were maintained during electrospinning process. The electrospun nanofibre mat was stabilized at 280 °C in air atmosphere, and then carbonized at 800 °C in a horizontal tube furnace under nitrogen flow at a heating rate of 5 °C min-1. The carbon nanofibres derived from PAN, PAN/PmAP (95:5), PAN/PmAP (90:10) and PAN/PmAP (85:15) are denoted as CNF100:0, NCNF95:05, NCNF90:10 and NCNF85:15, respectively. 2.2 Characterization of CNFs and NCNFs Raman spectroscopic analysis of prepared CNFs and NCNFs was carried out with a JobinYvon HORIBA Raman spectrometer with 632.8 nm He-Ne laser as the excitation source. The elemental and chemical compositions of the nanofibre samples were examined by X-ray photoelectron spectroscopy (XPS, MULTILAB 2000 SYSTEM). The surface morphologies of CNFs/NCNFs were characterized by field-emission scanning electron microscopy (FE-SEM,


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Hitachi, S-4700, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM2100F, JEOL, Japan). Nitrogen adsorption/ desorption isotherms were analyzed on an ASPS 2020 Physisorption Analyzer (Micromeritics, USA). The specific surface area of nanofibre samples was determined using the Brunauer–Emmett–Teller (BET) method, whereas pore size distribution (particularly mesoporosity) was calculated using the Barrett–Joyner–Halenda (BJH) method. X-ray diffraction patterns were recorded on a Rigaku Ultima-III X-ray diffractometer equipped with a Cu target at 30 kV and 100 mA. 2.3 Electrochemical analysis of CNFs and NCNFs The electrochemical performances of the different formulated CNF and NCNF electrodes were performed in both three- and two electrode configurations using a AutoLab electrochemical workstation. The cyclic voltammetry (CV) measurements were carried out in a three-electrode cell using a platinum wire as counter electrode, a saturated calomel electrode (SCE) as reference electrode and CNF/NCNF samples as the working electrode. A disk-shaped sample (diameter: 5 mm; weight: ~1.7 mg) was cut and directly fix to the surface of glassy carbon electrode using 5 µL of 0.5 wt% Nafion solution. The measurements were conducted in the potential range of -1.0 to 0 V at five different scan rates (10, 25, 50, 75 and 100 mV s-1) using 6 M aqueous KOH electrolyte. The charge-discharge measurements were performed with a two-electrode configuration, which was fabricated with two symmetric electrodes (area: 1.5 × 1.5 cm2) using nickel-foil as the current collector. The measurements were carried out in the potential range of 0 to 1.0 V at a current density of 0.5–20 mA cm-2. The specific capacitance (Csp) was calculated from the discharge curve area using the equation (1) [32,33], =

2× 1 × ∆⁄∆



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where I, t, V and m are denoted as current, discharge time, voltage and mass of the electrode materials, respectively. A factor of 2 is integrated due to the series capacitance formed in a two electrodes system. The volumetric capacitance (Cv) was obtained from the equation Cv = Csp × ρ, where ρ is the density of the sample (0.578 g cm-3 for NCNF85:15). The energy density (E, Wh kg1

) was calculated from the galvanostatic charge-discharge test using the equation of E = [(0.5 ×

Csp × V2], where power density (P, W kg-1) was determined from the equation of P = E / td [6,34]. Electrochemical impedance spectra (EIS) were obtained in the frequency range of 10 mHz ~ 100 kHz with an AC amplitude of 0.2 V.

3. Results and discussion Raman spectroscopic analysis was executed to analyze the chemical structure of the as-prepared carbon nanofibres. In Figure 1a, two peaks centered at 1353 cm-1 (D band) and 1588 cm-1 (G band) are observed for both pure CNF and N-doped CNFs. The G band is characteristic of the ordered graphitic structure, while D band is associated with the disordered carbon structure, i.e., defective and distorted graphite sheets [35]. The intensity ratio of D and G bands (ID/IG) was determined to measure the mass of disordered or amorphous carbon in the prepared CNF and NCNFs. The ID/IG values, i.e., degree of structural disorder, of the NCNF samples are found higher than that of pristine CNF. The higher ID/IG values together with more broadened Raman peaks indicates the presence of higher fraction of disordered amorphous carbon phases in the NCNFs. In addition, this result is also implying the development of mesoporous structure in the NCNFs. The ID/IG ratio of NCNFs increased with increasing PmAP content in precursor solution and attains a highest value of 1.053 for the NCNF85:15. These findings are well corroborated with the results of X-ray diffraction analysis.


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Figure 1b demonstrates the X-ray diffraction (XRD) patterns of pure CNFs and different formulated NCNFs. The XRD pattern of pure CNFs is consist of a broad diffraction peak centered at 2θ = 25.8° and a small peak in the 2θ region of 41-45°, which are corresponding to (002) and (100) graphite plane of carbon, respectively [36,37]. The diffraction patterns of NCNFs are nearly identical to those of pure CNFs. However, the relative peak intensities of NCNFs are significantly reduced compared to pure CNFs. This could be attributed to the presence of in-plane imperfections such as defects and heteroatoms of the graphitic lattice of the disordered sp2-carbon. The lower intensities of diffraction peaks might also indicate lower level of graphitization in the NCNFs compared to pure CNFs, which is previously confirmed by Raman data. In the case of NCNFs, the degree of graphitization decreased upon increasing the PmAP content in electrospun precursor fibres as evident by the decrease of diffraction peak intensity. It is also noticed that the diffraction peaks of NCNFs are slightly shifted toward lower 2θ values compared to pure CNFs, indicating higher interlayer spacing between the graphite layers. The interlayer spacing (d002) was calculated using the Bragg equation (nλ = 2dsinθ). The d002 values of CNF100:0 and NCNF85:15 are determined as 3.39 and 3.47 Å. The higher d002 value of NCNFs indicates lower order graphite staking at the mesopore walls. The surface chemical compositions of PAN-based CNFs and PAN/PmAP-derived NCNFs were determined by XPS analysis. The XPS results clearly confirmed the inclusion of nitrogen functional groups into the carbon skeleton of NCNFs. The wide-scan XPS spectra of asprepared CNFs and NCNFs are presented in Figure 2a. As shown in Figure 2a inset Table, the surface nitrogen concentration increased from 4.76 to 11.53 at% with increasing the PmAP content from 0 to 15 wt% in blend precursor. The carbon content of the NCNFs is found lower than that of pure CNFs, which could be attributed to the replacement of carbon by nitrogen-



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containing carbon generated by the cracking reaction of PmAP chains during carbonization. The high-resolution N1s spectra of CNFs and NCNFs are shown in Figure 2b, which indicates the presence of two main nitrogen species, i.e., pyridinic N (~ 398.3 eV) and quaternary N (~ 401.1 eV). The NCNFs exhibit higher pyridinic-to-quaternary nitrogen ratio (Npy/NQ) compared to pure CNFs, which suggests the formation of the higher fraction of pyridinic N compared to other nitrogen species during carbonization of PmAP. Moreover, the Npy/NQ ratio of NCNFs increases with increasing PmAP content in the precursor solution. The deconvoluted high resolution N1s XPS spectra of CNF100:0 and NCNF85:15 show several peaks with different binding energies (Figure 2b and 2c). The peaks of NCNF85:15 were fitted into four components centered at around 398.3, 400.2, 401.1 and 403 eV, corresponding to pyridinic N, pyrrolic/pyridonic N, quaternary N and pyridinic-N-oxide, respectively. The quantitative analysis demonstrates that the approximate fractions of different nitrogen species are: 26.7% of pyridinic N, 23% of pyrrolic/pyridonic N, 33.2% of quaternary N and 17% of pyridinic-N-oxide, respectively. On the other hand, the deconvoluted N1s spectra of CNF100:0 shows two main nitrogen component peaks, corresponding to pyridinic N (12.4%) and quaternary N (87.6%). The deconvoluted O1s spectra of NCNF85:15 consists of four component peaks at around 531, 532.4, 533.5 and 536.3 eV (Figure 2d), corresponding to carbonyl and or quinone (C=O), phenolic, hydroxyl and ether (COH/C-O-C), carboxylic (O-C=O) and chemisorbed oxygen or water, respectively. This finding is in good agreement with the Raman spectroscopic results indicated the existence of higher level of disordered carbon in NCNFs. The presence of nitrogen and oxygen functional groups on NCNF surfaces could effectively facilitate the electron transfer and thus make our material a potential candidate for supercapacitor applications.


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The FE-SEM images of the CNF and NCNF webs, presented in Figure 3, show straight fibrous structure with a circular cross-section. The morphology and structure of N-doped carbonized nanofibres were significantly influenced by varying the amount of PmAP loading in the precursor blend compositions. The SEM image of pure CNFs (CNF100:0) exhibit uniform distribution of fibre diameters (Figure 3a), whereas the N-doped CNFs (NCNFs) display aggregation of fibre strings (Figure 3b-d). With increasing the PmAP concentration in precursor blend, the surface of the NCNF webs become progressively more undulate and wrinkle together with large variation of fibre diameter. The liberation of hetero-atoms like hydrogen, oxygen, nitrogen etc. during pyrolysis of electrospun PAN/PmAP fibres produces porous nonwoven nanofibrous structure with retention of nitrogen functionalities. The cross-section SEM images in the inset of Figure 3 reveal higher diameter of PAN/PmAP derived NCNFs compared to PANbased CNF. The cross-sectional diameter of CNF and NCNFs are: ~223 nm for CNF100:0; ~323 nm for CNF95:05; ~331 nm for CNF90:10 and ~342 nm for CNF85:15. At the higher content of PmAP in the precursor blend, the fast release of large number of hetero-atoms together with substantial number of structure rearrangement during pyrolysis process perhaps impede the densification of carbon shell and consequently produce thicker nanofibres. The large diameter of NCNFs could be beneficial to achieve better electrochemical performances. TEM images exhibit ordered crystal (graphitic) structure of CNFs surface (Figure 3f) and relatively disordered crystal (graphitic) structure in the NCNFs (Figure 3h). These results good in agreement with Raman and XRD data. Figure 2e show excellent flexibility of the prepared NCNF85:15. This improved mechanical property could be attributed to unique ordered pore structure (micropore/mesopore ratio ≈ 1, as shown in Table 1) [38] along with interconnected fibre network structure in the NCNF85:15, which could facilitate fast stress distribution and subsequently resist the mechanical



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failure upon bending. In contrast, the pure CNFs are brittle in nature and obviously break down upon bending. The nitrogen adsorption/desorption isotherms for CNFs and different formulated NCNFs are presented in Figure 4a. As seen for NCNFs, the volume of adsorbed nitrogen is slowly increased with increasing relative pressure (P/P0) in the range of P/P0 = 0.2-0.8 representing a typical type II isotherm of mesoporous adsorption. The highest positive slope was observed in the isotherm of NCNF85:15, indicates the presence of relatively high mesoporous distribution. In contrast to NCNFs, the CNF100:0 possess rapid saturation of nitrogen adsorption at low relative pressure (P/P0 < 10-2) indicating the presence of a major fraction of micropores on carbon surfaces [39,40]. The desorption isotherms of NCNFs are found to lie slightly above the adsorption isotherms indicating the presence of slit-shaped pores [41]. The influence of PAN/PmAP blend composition on specific surface area and pore texture of the NCNFs were studied. The pore size distributions of different CNF and NCNF samples were determined by the BJH and MP methods (Figure 4b). The results indicate that the NCNFs possess a high mesoporosity with a broad mesopore size distribution in the range of 10-50 nm. These mesopores could provide short and easy diffusion pathways for electrolyte ions to enhance their electrochemical performance, and they can also promote intimate contact between electrolyte ions and electrode [42]. The micropores with the size smaller than 1.4 nm occupies greater fraction in total pore volume of CNFs and relatively smaller fraction in total pore volume of NCNFs. The BET surface area, total pore volume, volume fraction of micro and mesopores, and their pore volume ratio are summarized in Table 1. The mesoporosity in carbon nanofibre surfaces can be significantly enhanced by using PAN/PmAP blend as the carbon fibre precursor. The mesopore volume fraction was increased from 0.0178 to 0.6356 cm3 g-1 with increasing


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PmAP content from 0 to 15 wt% in precursor blend solution. The incompatibility between PAN and PmAP might promote enlargement of small pores during carbonization process, which results into the production of mesoporous structure [30]. However, the variation of PmAP content in the blend compositions leads to change the size and shape of the dispersed phases and thus control the pore size on NCNF surfaces. As shown in Table 1, the specific surface area of the NCNFs is significantly larger than that of CNFs. The highest specific surface area of 1031.4 m2g-1 was achieved for the NCNF85:15. The benefit of large fraction of mesoporosity together with high specific surface area for the present N-doped CNFs is extremely attractive for supercapacitor applications. The electrochemical capacitive performances of as-prepared CNFs and NCNFs were evaluated in a three-electrode set-up using 6 M KOH aqueous electrolyte solution. Typical CV curves of pure CNFs and different formulated NCNFs at a scan rate of 10 mV s-1 are presented in Figure 5a. The CV curve of pure CNFs exhibits a rectangular-shaped profile without any obvious redox peaks indicating a double-layer capacitor behavior. In contrast, the CV curve of NCNFs show a significant distortion of the rectangular shape with notable redox peaks indicating excellent capacitive behavior and presence of pseudo-capacitance effect. The redox peaks are associated with the redox reactions of electrochemically active functional groups on the surface of NCNFs. Among these surface functionalities, the pyridinic N and quaternary N were supposed to be greatly responsible for the improvement of pseudo-capacitance. The integrated CV area of the NCNFs is found significantly higher than that of pure CNFs, which clearly demonstrate a greatly enhanced charge-storage capacity of NCNF electrodes compared to pure CNFs. It is noticed that both current density and integrated CV area for NCNF electrodes are increased with increasing the N-doping level in the NCNFs. Moreover, the redox peaks become more prominent



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as the N-doping concentration increases. The high heteroatom functionalities and specific surface area of the NCNFs could promote both electrical double layer capacitance (EDLC) as well as pseudo-capacitive performance. Figure 5b-5d present CV curves of CNF100:0, NCNF90:10 and NCNF85:15 at different scan rates, i.e., 10, 25, 50, 75 and 100 mV s-1. It is observed that both CV loop area and current density increased with increasing scan rate from 10 to 100 mV s-1. The large loop area of the CV curve at high scan rate indicates the limited interaction between the electrolyte ions and the electrode surface. The charge-discharge (CD) cycling curves for CNF and different NCNF samples at current density of 0.5 and 20 mA cm-2 are presented in Figure 6a and 6b. The CD curves of all samples exhibit slightly distorted triangular shape indicating good capacitive behavior. However, the distortion of CD curves is more pronounced for the NCNF samples, which could be raised from pseudo-faradaic reactions during charge/discharge process. The presence of nitrogen and oxygen functional groups on the surfaces of NCNFs leads to a pseudo-capacitance. Among the NCNF samples, the discharge time of NCNF85:15 is significantly longer at both lower and higher current densities (Figure 6a and 6b) implying its higher capacitive performance, which agrees well with the CV results. Figure 6c shows the charge-discharge curves for the NCNF85:15 over a range of current densities and their asymmetrical features, which further indicating the dual storage mechanisms, i.e., EDLC and pseudo-capacitance. At a current density of 0.5 mA cm-2, the specific capacitances are determined to 86.7, 161.2, 279.3, 347.5 F g-1 for CNF100:0, NCNF95:05, NCNF90:10 and NCNF85:15, respectively. It is worth noting that the specific capacitance of NCNF85:15 in this work is very competitive to previously reported results as listed in Table 2. The results clearly indicating that the presence of high specific surface area and heteroatom doping level of the NCNF significantly promoted the


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degree of reversible redox reactions and subsequently induce additional pseudo-capacitance, and thus enhanced the specific capacitance. The NCNF85:15 exhibit highest specific capacitance among the all investigated samples, which could be attributed to the highest specific surface area, major fraction of mesoporosity and maximum level of nitrogen/oxygen functionalities in the NCNF85:15. The doping of N- and O-functionalities also increases the polarity and hydrophilicity of NCNF electrode surfaces and subsequently promote the wettability between electrolyte and electrode. The variation of specific capacitance as a function of current density is presented in Figure 6d. The specific capacitance of all CNF and NCNFs decreased with increasing current density, which could be ascribed to limited diffusion time of the electrolyte ions into the entire pore surfaces at high current density that led to reduce electrochemical exploitation of the carbon electrodes. However, the NCNF85:15 exhibit a high Csp value of 173.2 F g-1 even when the current density increased to 20 mA cm-2. The variation of volumetric capacitance as a function of current density is presented in Figure 6d. In contrast to gravimetric capacitance, the volumetric capacitance values of NCNF85:15 are significantly dropped, showing maximum values of 200.8 F cm-3 at a current density of 0.5 mA cm-2. The decreased volumetric capacitance could be attributed to the relative low densities and large surface area [49]. Figure 7a shows the Nyquist plots of all CNF and NCNF-based electrode samples. The plots of NCNFs exhibit semicircular shapes with a Warburg slope of 45º at the low frequency region between the semicircle and the straight line. Compared to CNF, the more vertical lines in the low frequency region are observed in the case of NCNFs implying a good capacitive performance. The diameter of semicircle is significantly decreased for the NCNFs compared to CNFs, which indicates that the contact area at the electrode-electrolyte interface is increased for the NCNF electrodes due to better wettability and development of interconnected



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micro/mesoporous structure. The functional groups on the surface of pore walls improved the wettability of the NCNF electrodes and subsequently promote the mobility of electrolyte ions through carbon matrix. It can be seen that the equivalent series resistance (Rs) of NCNFs are lower than that of CNFs. The smallest Rs value of 1.04 Ω was recorded for the NCNF85:15. The lower Rs values of NCNFs is associated with good electrical conductivity and better pore accessibility by the electrolyte ions during electrochemical process [50]. Compared to CNF, the smaller semicircle of NCNFs is implying lower charge transfer resistance (Rct). The NCNF85:15 show the smallest Rct value, which is beneficial for the charge transfer between the interface of electrode surface and electrolyte. The good conductivity together with rapid charge transfer could promote fast redox reactions, i.e., superior pseudo-capacitance performance [51,52]. The long-term cyclic stability of CNF and NCNF electrodes was determined via chargedischarge measurement up to 11000 cycles at 1 mA cm-2, and the results are demonstrated in Figure 7b. It can be seen that the capacitance of NCNF85:15 gradually reduced during this period, which could be ascribed to partial loss of pseudo-capacitance. However, the loss of specific capacitance is about 3.42% after 5000 cycles and still remains at about 245 F/g (about 90.5% of the initial capacitance) after 10000 cycles. On the other hand, the CNF electrode showed significant deterioration in specific capacitance, which is about 13.9% and 24.9% after 5000 and 10000 times charge-discharge cycles, respectively. The above results clearly indicate better cyclic stability behavior of NCNF85:15 electrode compared to CNF electrode. The initial coulombic efficiency of the NCNF85:15 was 84% at 1 mA cm-2, which significantly increased upon cycling and reached to 95% after 8 cycles and 98% after 17 cycles (Figure 7b). The increase in coulombic efficiency with cycling could be attributed to the activating process of the porous NCNF anode. It was observed that the charging rate is significantly improved as the


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cycling progress. Similar behavior was also observed in some carbon materials as reported in literature [53-55]. Regone plots for CNF and different formulated NCNF samples are shown in Figure 7c. The N-doped carbon nanofibres exhibit obvious improvement in energy density compared to CNF. The NCNF85:15 electrode yields highest energy density of 12.1 Wh kg-1 and power density of 2.36 kW kg-1 among all formulated NCNFs. It is noteworthy that the energy density of the present NCNF85:15 electrode is higher than those of the previously reported Ndoped porous carbon materials obtained from different types of N-enriched precursors including conducting polymers [43,56,57]. The choice of porous carbon material with a high surface area and good hydrophilicity are the imperative factors in achieving an improved energy density and power density.

4. Conclusions In summary, the electrochemical performances of N-doped carbon nanofibres were correlated with composition of PAN/PmAP precursor blends. The precursor solutions with different PAN/PmAP blend compositions viz., 100:0, 95:5, 90:10, 85:15, were electrospun to form flexible nanofibre mats. The resulting mat was then thermally stabilized and carbonized to obtain freestanding porous nitrogen-doped carbon nanofibres. The NCNF85:15 as electrode material showed excellent electrochemical performances with a highest specific capacitance of 347.5 F/g at a current density of 0.5 mA cm-2 in 6 M KOH electrolyte, about four time higher that of pristine CNF. The NCNF85:15 electrode exhibited good cycling stability with 90.5% capacitance retention after 10000 cycles. The energy density value of 12.1 Wh/kg at power density of 0.093 kW/kg was achieved for the NCNF85:15 capacitor. The outstanding electrochemical performances of NCNF85:15 are a result of its highest effective surface area, highest surface fraction of nitrogen, good electrical conductivity, well-balanced microporosity and mesoporosity, and highest content 17 ACS Paragon Plus Environment


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of mesopores. High capacitance and excellent cyclability endorsed this new N-doped porous carbon nanomaterial to be a potential candidate for use in energy storage systems.


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Table 1. Surface area and pore volume of pure PAN and PAN/PmAP-derived carbon nanofibres Vmicro Vmeso % % mesopores Micropores/ Surface area TPV 2 -1 3 -1 3 -1 3 -1 mesopores ratio (m g ) (cm g ) (cm g ) (cm g ) micropores CNF100:0 486.3 0.1995 NCNF95:05 602.6 0.8169 NCNF90:10 741.7 0.9689 NCNF85:15 1031.4 1.3433 TPV: Total pore volume

0.1817 0.5541 0.5721 0.7077

0.0178 0.2628 0.3968 0.6356

91.08 67.83 59.04 52.68

8.92 32.17 40.96 47.32

10.2 2.10 1.44 1.11

Table 2. Capacitive performances of N-doped carbon nanofibres in this work and some previously reported similar materials Precursors

Freestanding

PAN/PANI

Yes

N-content (at%) 8.96

SBET (m2 g-1) 410

Sp. capacitance (F g-1) 335 at 0.5 A/g

References

PANI/CNF

No

10.9

331

210 at 5 mV/s

44

PPy/CNF

No

7.22

563

202 at 1 A/g

45

PANI/bacterial

No

3

1326

296 at 2 mV/s

46

Yes

-

422

177 at 0.5 A/g

47

PANI nanowires

No

16.6

516

327 at 0.1 A/g

48

PAN/PmAP

Yes

9.56

1031.4

347.5 at 0.5 mA/cm2

This work

43

cellulose PANI/graphitized CNFs



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Captions for Figures Figure 1. (a) Raman spectra and (b) X-ray diffraction patterns of pure CNFs and different formulated NCNFs Figure 2. (a) Wide scan XPS spectra of CNF and NCNFs. High resolution XPS spectra of deconvoluted N1s peaks of (b) CNF100:0 and (c) NCNF85:15 and (d) O1s peaks of NCNF85:15. (e) Digital images of NCNF85:15. (f) The forms of nitrogen possibly present in NCNFs Figure 3. SEM images of CNF100:0 (a,e), NCNF95:05 (b), NCNF90:10 (c) and NCNF85:15 (d,g). Insets show the cross-section images of the fibre samples. TEM images of CNF100:0 (f) and NCNF85:15 (h). Figure 4. (a) Nitrogen sorption isotherms and (b) pore size distribution of pure CNFs and different formulated NCNFs. Figure 5. (a) CV curves of pure CNFs and different formulated NCNFs at a scan rate of 10 mV s-1. CV curves of (b) CNF100:0, (c) NCNF90:10 and (d) NCNF85:15 at different scan rates, i.e., 10, 25, 50, 75 and 100 mV s-1. Figure 6. Charge/discharge (CD) cycling curves for pure CNF and different NCNF samples at current density of (a) 0.5 and (b) 20 mA cm-2. (c) CD curves for the NCNF85:15 over a range of current densities. (d) Variation of gravimetric specific capacitance and volumetric capacitance of all samples at different current densities. Figure 7. (a) Nyquist plots of pure CNF and different NCNF samples, (b) Cyclic performance of CNF100:0 and NCNF85:15 at current density of 1 mA cm-2. Coulombic efficiency of NCNF85:15 at current density of 1 mA cm-2 as a function of cycle number and (c) Ragone plots of pure CNF and different NCNFs electrodes.


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Figure 1. (a) Raman spectra and (b) X-ray diffraction patterns of pure CNFs and different formulated NCNFs 254x190mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Figure 2. (a) Wide scan XPS spectra of CNF and NCNFs. High resolution XPS spectra of deconvoluted N1s peaks of (b) CNF100:0 and (c) NCNF85:15 and (d) O1s peaks of NCNF85:15. (e) Digital images of NCNF85:15. (f) The forms of nitrogen possibly present in NCNFs 254x190mm (96 x 96 DPI)


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Figure 3. SEM images of (a) CNF100:0, (b) NCNF95:05, (c) NCNF90:10 and (d) NCNF85:15. Insets show the cross-section images of the fibre samples. TEM images of CNF100:0 (e,f) and NCNF85:15 (g,h). 251x127mm (120 x 120 DPI)



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Figure 4. (a) Nitrogen sorption isotherms and (b) pore size distribution of pure CNFs and different formulated NCNFs. 254x190mm (96 x 96 DPI)


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Figure 5. (a) CV curves of pure CNFs and different formulated NCNFs at a scan rate of 10 mV s-1. CV curves of (b) CNF100:0, (c) NCNF90:10 and (d) NCNF85:15 at different scan rates, i.e., 10, 25, 50, 75 and 100 mV s-1. 254x190mm (96 x 96 DPI)



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Figure 6. Charge/discharge (CD) cycling curves for pure CNF and different NCNF samples at current density of (a) 0.5 and (b) 20 mA cm-2. (c) CD curves for the NCNF85:15 over a range of current densities. (d) Variation of gravimetric specific capacitance and volumetric capacitance of all samples at different current densities. 254x190mm (96 x 96 DPI)


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Figure 7. (a) Nyquist plots of pure CNF and different NCNF samples, (b) Cyclic performance of CNF100:0 and NCNF85:15 at current density of 1 mA cm-2. Coulombic efficiency of NCNF85:15 at current density of 1 mA cm-2 as a function of cycle number and (c) Ragone plots of pure CNF and different NCNFs electrodes. 254x190mm (96 x 96 DPI)



Title: Nitrogen-enriched porous carbon nanofiber mat as efficient flexible electrode material for supercapacitors Authors: Arup Choudhury, Ji-Hoon Kim, Susanta Sinha Mahapatra, Kap-Seung Yang and Duck-Joo Yang Synopsis: Nitrogen-doped carbon nanofibers were fabricated via electrospinning of PAN/PmAP precursor along with subsequent carbonization, with excellent electrochemical properties for supercapacitor applications. Keywords: Poly(m-aminophenol); Polyacrylonitrile; Electrospinning; Carbon nanofibers; Specific capacitance; Supercapacitors

Table of Content (TOC) CNF85:15 250 CNF90:10 CNF100:0 CNF85:15

3

CNF95:05

400

200

300

150

200

100

100

50

0

0 0

4

8

12

16

20

2

Current density (mA/cm )


Volumetric capacitance (F/cm )

500

Gravimetric specific capacitance (F/g)

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Nitrogen-Enriched Porous Carbon Nanofiber Mat as Efficient Flexible

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