Nitrogen-enriched porous carbon nanofiber mat as efficient flexible

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Research Article pubs.acs.org/journal/ascecg

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*,∥ †

Department of Chemical Engineering, Birla Institute of Technology, Mesra, Ranchi 835-215, India School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Bukgu, Gwangju 500-757, Republic of Korea § 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 West Campbell Road, Richardson, Texas 75080, United States

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‡

ABSTRACT: Freestanding nitrogen-doped porous carbon nanofiber (NCNF) mats were prepared by electrospinning 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 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 10 000 cycles. The excellent capacitive performances of the NCNF85:15 were attributed to high effective surface area, high content of mesoporosity, good conducvity, and high fraction of heteroatom-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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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 three-dimensional metal oxide 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 coelectrospinning of a PAN/polyvinylpyrrolidone/SiO2 blend solution, followed by a pyrolysis process. Recently, functional nanostructured carbon materials have been proven to improve capacitive performance owing to their unique combination of properties such increased electrical conductivity, more ordered pore structure, and improved wettability between electrode and electrolyte in aqueous electrolyte solutions.15−18 In addition, the presence of

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 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 physico-chemical properties of the electrode materials. Therefore, current research has been heavily focused on the development of 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 kinetics and consequently yield inferior specific capacitance and low power densities. In order to solve these problems, the mesoporous carbon nanofibers 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 © 2017 American Chemical Society

Received: August 27, 2016 Revised: January 5, 2017 Published: February 8, 2017 2109

DOI: 10.1021/acssuschemeng.6b02031 ACS Sustainable Chem. Eng. 2017, 5, 2109−2118

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heteroatom groups on CNF surfaces induces pseudocapacitance that originates from the faradaic interactions between the ions of electrolytes and the carbon electrode surface. Generally, nitrogen-containing carbon nanofibers 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. In a comparison to post-treatment processes, pyrolysis of nitrogencontaining polymer precursors such as polyacrylonitrile,16,21 polyvinylpyridine,22 and polybenzoxazine23 is supposed to be a simpler and more 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 nanofibers via electrospinning combined with a postcarbonization process has received increasing attention. The method can efficiently produce freestanding NCNF mats with good mechanical integrity, large usable surface area, and controlled pore structure/pore size distribution, which could improve 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 properties of N-doped CNFs derived from the polyaniline-based precursor,24−26 while PmAP was not explored as a precursor to fabricate carbon materials for capacitance applications. The main difficulty for the production of conducting-polymer-based nanofibers via an electrospinning process is the inferior solubility and high conductivity of these polymers. Therefore, a polymer carrier needs to be used during electrospinning to produce precursor nanofibers. Many research groups have used polyacrylonitrile-based binary polymer blends as a precursor to fabricate carbon nanofibers.27−30 The PAN can be used as a carrier to disperse PmAP to obtain electrospun multiphase nanofibers 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. To date, there is no study that 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 binder-free nitrogen-doped CNFs via electrospinning of PAN/PmAP precursor along with subsequent carbonization. Dimethyl sulfoxide (DMSO) was used as a common solvent to produce homogeneous blend solution with desired viscosity and conductivity for the 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 not only were fabricated via simple electrospinning and subsequent carbonization process but also exhibited excellent electrochemical performances in both two- and three-electrode systems.

Research Article

EXPERIMENTAL SECTION

Preparation of Carbon Nanofibers. At first, PmAP was synthesized through chemical oxidation of m-aminophenol (m-AP, Sigma-Aldrich 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.) and as-prepared PmAP were dissolved in 30 mL of DMSO (Duksan Chemical Co., Korea) at 80 °C under vigorous stirring for 4 h followed by constant stirring at room temperature for another 24 h. 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 nanofiber 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 the electrospinning process. The electrospun nanofiber 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 nanofibers 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. Characterization of CNFs and NCNFs. Raman spectroscopic analysis of prepared CNFs and NCNFs was carried out with a JobinYvon HORIBA Raman spectrometer with a 632.8 nm He−Ne laser as the excitation source. The elemental and chemical compositions of the nanofiber samples were examined by X-ray photoelectron spectroscopy (XPS, MULTILAB 2000 SYSTEM). The surface morphologies of CNFs/NCNFs were characterized by fieldemission scanning electron microscopy (FE-SEM, 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). The specific surface area of nanofiber 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. 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 an AutoLab electrochemical workstation. The cyclic voltammetry (CV) measurements were carried out in a threeelectrode 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 disc-shaped sample (diameter 5 mm; weight ∼1.7 mg) was cut and directly fixed to the surface of a glassy carbon electrode using 5 μL of 0.5 wt % Nafion solution. The measurements were conducted in the potential range −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 0−1.0 V at a current density 0.5−20 mA cm−2. The specific capacitance (Csp) was calculated from the discharge curve area using eq 132,33 Csp =

2×I m × (ΔV /Δt )

(1)

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-electrode 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 kg−1) 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 2110

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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 N 1s peaks of (b) CNF100:0 and (c) NCNF85:15 and (d) O 1s peaks of NCNF85:15. (e) Digital images of NCNF85:15. (f) The forms of nitrogen possibly present in NCNFs. from the equation P = E/td.6,34 Electrochemical impedance spectra (EIS) were obtained in the frequency range 10 mHz to 100 kHz with an ac amplitude of 0.2 V.

prepared CNF and NCNFs. The ID/IG values, i.e., degrees of structural disorder, of the NCNF samples are found to be higher than that of pristine CNF. The higher ID/IG values together with more broadened Raman peaks indicate the presence of a 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. Figure 1b demonstrates the X-ray diffraction (XRD) patterns of pure CNFs and different formulated NCNFs. The XRD

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RESULTS AND DISCUSSION Raman spectroscopic analysis was executed to analyze the chemical structure of the as-prepared carbon nanofibers. 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 the 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 2111

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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 fiber samples. TEM images of CNF100:0 (f) and NCNF85:15 (h).

solution. The deconvoluted high-resolution N 1s XPS spectra of CNF100:0 and NCNF85:15 show several peaks with different binding energies (Figure 2b,c). 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 N 1s spectra of CNF100:0 show two main nitrogen component peaks, corresponding to pyridinic N (12.4%) and quaternary N (87.6%). The deconvoluted O 1s spectra of NCNF85:15 consist 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 (COH/COC), carboxylic (OCO), and chemisorbed oxygen or water, respectively. This finding is in good agreement with the Raman spectroscopic results which indicated the existence of a 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. The FE-SEM images of the CNF and NCNF webs, presented in Figure 3, show a straight fibrous structure with a circular cross-section. The morphology and structure of Ndoped carbonized nanofibers 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 fiber diameters (Figure 3a), whereas the N-doped CNFs (NCNFs) display aggregation of fiber strings (Figure 3b−d). With increasing the PmAP concentration in precursor blend, the surface of the NCNF webs become progressively more undulated and wrinkle together with large variation of fiber diameter. The liberation of heteroatoms like hydrogen, oxygen, nitrogen, etc., during pyrolysis of electrospun PAN/PmAP fibers produces porous nonwoven nanofibrous structure with retention of nitrogen functionalities. The cross-section SEM images in the inset of Figure 3 reveal a higher diameter of PAN/PmAP derived NCNFs as compared to that for PAN-based CNF. The crosssectional diameter of CNF and NCNFs are ∼223 nm for CNF100:0, ∼323 nm for CNF95:05, ∼331 nm for CNF90:10, and

pattern of pure CNFs consists of a broad diffraction peak centered at 2θ = 25.8° and a small peak in the 2θ region of 41− 45°, which correspond to (002) and (100) graphite planes 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 those of 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 a lower level of graphitization in the NCNFs compared to that for pure CNFs, which was previously confirmed by Raman data. In the case of NCNFs, the degree of graphitization decreased upon increasing the PmAP content in electrospun precursor fibers as is 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 those for pure CNFs, indicating higher interlayer spacing between the graphite layers. The interlayer spacing (d002) was calculated using the Bragg equation (nλ = 2d sin θ). 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 as-prepared 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 an increase of the PmAP content from 0 to 15 wt % in blend precursor. The carbon content of the NCNFs is found to be lower than that of pure CNFs, which could be attributed to the replacement of carbon by nitrogen-containing carbon generated by the cracking reaction of PmAP chains during carbonization. The high-resolution N 1s 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 2112

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ACS Sustainable Chemistry & Engineering Table 1. Surface Area and Pore Volume of Pure PAN and PAN/PmAP-Derived Carbon Nanofibersa CNF100:0 NCNF95:05 NCNF90:10 NCNF85:15 a

surface area (m2 g−1)

TPV (cm3 g−1)

Vmicro (cm3 g−1)

Vmeso (cm3 g−1)

% micropores

% mesopores

micropores/mesopores ratio

486.3 602.6 741.7 1031.4

0.1995 0.8169 0.9689 1.3433

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

TPV: total pore volume.

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 differently 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.

∼342 nm for CNF85:15. At the higher content of PmAP in the precursor blend, the fast release of a large number of heteroatoms, together with a substantial number of structure rearrangements during the pyrolysis process, perhaps impedes the densification of the carbon shell and consequently produces thicker nanofibers. The large diameter of NCNFs could be beneficial to achieve better electrochemical performances. TEM images exhibit an ordered crystal (graphitic) structure for the CNFs’ surfaces (Figure 3f) and relatively disordered crystal (graphitic) structure in the NCNFs (Figure 3h). These results

are in good agreement with Raman and XRD data. Figure 2e shows excellent flexibility for the prepared NCNF85:15. This improved mechanical property could be attributed to a uniquely ordered pore structure (micropore/mesopore ratio ≈1, as shown in Table 1)38 along with an interconnected fiber network structure in the NCNF85:15, which could facilitate fast stress distribution and subsequently resist the mechanical failure upon bending. In contrast, the pure CNFs are brittle in nature and obviously break down upon bending. 2113

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

content from 0 to 15 wt % in the precursor blend solution. The incompatibility between PAN and PmAP might promote enlargement of small pores during the carbonization process, which results into the production of mesoporous structure.30 However, the variation of PmAP content in the blend compositions leads to a change in the size and shape of the dispersed phases and thus controls 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 m2 g−1 was achieved for the NCNF85:15. The benefit of a 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 setup 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 double-layer capacitor behavior. In contrast, the CV curve of NCNFs shows a significant distortion of the rectangular shape with notable redox peaks indicating excellent capacitive behavior and the presence of a pseudocapacitance 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 pseudocapacitance. The integrated CV area of the NCNFs is found to be significantly higher than that of pure CNFs, which clearly demonstrates a greatly enhanced charge-storage capacity of NCNF electrodes compared to that of pure CNFs. It is

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 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, and indicates the presence of a relatively high mesoporous distribution. In contrast to NCNFs, the CNF100:0 possesses 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 was 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 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 size smaller than 1.4 nm occupy a 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 microand mesopores, and their pore volume ratio are summarized in Table 1. The mesoporosity in carbon nanofiber surfaces can be significantly enhanced by using PAN/PmAP blend as the carbon fiber precursor. The mesopore volume fraction was increased from 0.0178 to 0.6356 cm3 g−1 with increasing PmAP 2114

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Table 2. Capacitive Performances of N-Doped Carbon Nanofibers in This Work and Some Previously Reported Similar Materials precursor

freestanding

N-content (at. %)

SBET (m2 g−1)

specfic capacitance (F g−1)

ref

PAN/PANI PANI/CNF PPy/CNF PANI/bacterial cellulose PANI/graphitized CNFs PANI nanowires PAN/PmAP

yes no no no yes no yes

8.96 10.9 7.22 3

410 331 563 1326 422 516 1031.4

335 at 0.5 A/g 210 at 5 mV/s 202 at 1 A/g 296 at 2 mV/s 177 at 0.5 A/g 327 at 0.1 A/g 347.5 at 0.5 mA/cm2

43 44 45 46 47 48 this work

16.6 9.56

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 electrodes of pure CNF and different NCNFs.

discharge process. The presence of nitrogen and oxygen functional groups on the surfaces of NCNFs leads to a pseudocapacitance. Among the NCNF samples, the discharge time of NCNF85:15 is significantly longer at both lower and higher current densities (Figure 6a,b) 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 indicate the dual storage mechanisms, i.e., EDLC and pseudocapacitance. 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 indicate that the presence of high specific surface area and heteroatom-doping level of the NCNF significantly promoted the degree of reversible redox reactions and subsequently induce additional pseudocapacitance, and thus enhanced the specific capacitance. The NCNF85:15 exhibits the highest specific capacitance among all of the investigated

noticed that both current density and integrated CV area for NCNF electrodes are increased with an increase in the Ndoping level in the NCNFs. Moreover, the redox peaks become more prominent 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 pseudocapacitive performance. Figure 5b−d presents 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 an increase in the 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,b. The CD curves of all samples exhibit a slightly distorted triangular shape indicating good capacitive behavior. However, the distortion of CD curves is more pronounced for the NCNF samples, which could be increased from pseudofaradaic reactions during the charge− 2115

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ACS Sustainable Chemistry & Engineering

efficiency of the NCNF85:15 was 84% at 1 mA cm−2, which significantly increased upon cycling and reached 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 cycling progress. Similar behavior was also observed in some carbon materials as reported in the literature.53−55 Regone plots for CNF and different formulated NCNF samples are shown in Figure 7c. The N-doped carbon nanofibers exhibit an obvious improvement in energy density compared to that of CNF. The NCNF85:15 electrode yields its 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 N-doped 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.

samples, which could be attributed to the highest specific surface area, a major fraction of mesoporosity, and the 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 promotes 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 reduced electrochemical exploitation of the carbon electrodes. However, the NCNF85:15 exhibits 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 reduced, 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 relatively low densities and large surface area.49 Figure 7a shows the Nyquist plots of all CNF- and NCNFbased electrode samples. The plots of NCNFs exhibit semicircular shapes with a Warburg slope of 45° at the lowfrequency region between the semicircle and the straight line. In a comparison to results from CNF, the more vertical lines in the low-frequency region are observed in the case of NCNFs implying good capacitive performance. The diameter of a 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 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) values of NCNFs are lower than those 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 the electrochemical process.50 In a comparison of the results for CNF, the smaller semicircle of NCNFs implies lower charge transfer resistance (Rct). The NCNF85:15 shows 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 pseudocapacitance performance.51,52 The long-term cyclic stability of CNF and NCNF electrodes was determined via charge−discharge measurement up to 11 000 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 pseudocapacitance. 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 10 000 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 10 000 charge−discharge cycles, respectively. The above results clearly indicate better cyclic stability behavior of an NCNF85:15 electrode compared to a CNF electrode. The initial Coulombic

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CONCLUSIONS In summary, the electrochemical performances of N-doped carbon nanofibers 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 nanofiber mats. The resulting mat was then thermally stabilized and carbonized to obtain freestanding porous nitrogen-doped carbon nanofibers. 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 4 times higher than that of pristine CNF. The NCNF85:15 electrode exhibited good cycling stability with 90.5% capacitance retention after 10 000 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 of mesopores. High capacitance and excellent cyclability endorsed this new N-doped porous carbon nanomaterial as a potential candidate for use in energy storage systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +91 9430 732461. Fax: +91 651 2276184. E-mail: [email protected]. ORCID

Duck-Joo Yang: 0000-0003-3392-750X Notes

The authors declare no competing financial interest.

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