Xiaosa Xu,Yuqian Qiu,Jianping Wu,Baichuan Ding,Qianhui Liu,Guangshen Jiang,Qiongqiong Lu,Jiangan Wang,Fei Xu,,Hongqiang Wang,

1 State Key Laboratory of Solidification Processing,Center for Nano Energy Materials,School of Materials Science and Engineering,Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU),Xi’an 710072,China

2 Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V.,Helmholtzstr 20,01069 Dresden,Germany

Keywords:Energy Electrochemistry Nanomaterials Hollow carbon nanofibers Freestanding electrode Lithium-ion batteries

ABSTRACT One-dimensional porous carbons bearing high surface areas and sufficient heteroatom doped functionalities are essential for advanced electrochemical energy storage devices,especially for developing freestanding film electrodes.Here we develop a porous,nitrogen-enriched,freestanding hollow carbon nanofiber (PN-FHCF) electrode material via filtration of polypyrrole (PPy) hollow nanofibers formed by in situ self-degraded template-assisted strategy,followed by NH3-assisted carbonization.The PN-FHCF retains the freestanding film morphology that is composed of three-dimensional networks from the entanglement of 1D nanofiber and delivers 3.7-fold increase in specific surface area (592 m2﹒g-1) compared to the carbon without NH3 treatment (FHCF).In spite of the enhanced specific surface area,PNFHCF still exhibits comparable high content of surface N functionalities (8.8%,atom fraction) to FHCF.Such developed hierarchical porous structure without sacrificing N doping functionalities together enables the achievement of high capacity,high-rate property and good cycling stability when applied as self-supporting anode in lithium-ion batteries,superior to those of FHCF without NH3 treatment.

1.Introduction

Porous carbon nanomaterials have greatly contributed to the advances for energy storage-related applications owing to their unique structural features including high specific surface areas,tailored surface functionalities,controllable morphologies,good electric conductivity and excellent physicochemical stability [1–16].Among various porous carbons,one-dimensional carbon nanofibers have been demonstrated to be one of the promising candidates,especially as self-supporting carbon electrode that can avoid the use of metal current-collectors and binders,which not only simplifies the fabrication process but also reduces the electrochemical inert part for high performances[17–20].Development of porous structure and incorporation of heteroatoms like N are both desirable for enhanced electrochemical performances [21–24].For example,as anode materials for Li-ion storage,the developed porosity with high specific surface areas and hierarchical pore structures facilitates fast mass transport for achieving higher rate capability and better cycling performance [25,26],while the presence of N functionality could promote the Li-ion storage capacity through enhancing the reactivity,electrical conductivity and improving wettability of carbon [27,28].Therefore,it is desirable and urgent to develop novel one-dimensional carbon-based freestanding electrode with simultaneously high specific surface areas,hierarchical open pore structures and sufficient N functionalities.

As compared to their powdery carbon counterparts,most of the freestanding carbons show relatively limited surface areas[29–31],because the extensive creation of pores will undoubtedly give rise to the collapse of freestanding structure.Moreover,too much increase of specific surface areas will also lead to the decomposition of heteroatom-doped functionalities like N,which is a wellknown trade-off phenomenon for porous carbons [32].For example,for freestanding carbons,increasing the carbonization temperatures within a range would lead to higher specific surface areas(756 m2﹒g-1),however,the nitrogen content decreases to 2.36%[33].The activation of freestanding carbon with water can cause higher specific surface areas(713 m2﹒g-1),but significantly reduce N content of 0.5% [34].Vice versa,N content can be increased via urea treatment,but the specific surface areas decreased accordingly [35].Consequently,it is interesting but challenging to engineer well-balanced freestanding porous carbons with both developed porosity and high content of N functionality to realize maximum performances.

Herein,we fabricate both high porous and nitrogen-enriched freestanding carbon films(PN-FHCF)by a simple vacuum filtration of self-degraded template-assisted prepared polypyrrole(PPy)hollow nanofibers,followed by NH3-assisted carbonization.The asobtained PN-FHCF exhibits freestanding morphology with three dimensional(3D)hierarchical carbon skeletons,bears high specific surface areas of 592 m2﹒g-1which is 3.7-fold higher than carbon without NH3treatment(FHCF,158 m2﹒g-1),and shows a comparable high content of surface N functionalities (8.8%,atom fraction)to FHCF.Thus,the simultaneous achievement of both high porous structures and sufficient N surface functionalities shows their effectiveness for enhanced Li-ion storage performances in terms of superior Li capacity,high rate performances and extended cycling stability.

2.Materials and Methods

2.1.Preparation of PN-FHCF

The preparation of PPy was followed by previous literatures[36].In a typical procedure,0.62 g FeCl3was dissolved in 40 ml ultrapure water in the presence of 0.12 g methyl orange (MO),and then the solution was stirred for 30 min.After that,0.14 ml pyrrole monomer was added to the solution drop by drop.The reaction was carried out for 4 h.After reaction,a freestanding PPy film was obtained by vacuum filtration of the above reaction suspension,and washed by diluted hydrochloric acid,absolute alcohol and ultrapure water.The film was peeled off from the filtration membrane and dried at 60 °C for 12 h.Afterwards,the as-prepared PPy film was cut into circular disks with diameter of 12 mm.Subsequently PPy film was heated to 550°C with ramping rate of 2 °C﹒min-1and kept for 1 h in a mixture gas of N2and NH3(1:1 in volume).Then the temperature was raised to 800 °C with 5 °C﹒min-1and kept for 1 h under N2flow.PN-FHCF was yielded after cooling down under N2flow.

2.2.Preparation of FHCF

The controlled group (FHCF) was carbonized under the same conditions except that NH3was not used.

2.3.Cell fabrication and measurements for Li-ion batteries

Electrochemical measurements were conducted using a 2032 type coin cell with Li metal as the counter electrodes and separator(Celgard 2400) at room temperature.The carbonized polypyrrole films were used as electrode directly.Cell assembly was carried out in an argon filled glove box with both moisture and oxygen contents below 0.0001‰.For LIBs,the electrolyte was composed of 1 mol﹒L-1LiPF6in a 1:1 (in volume) mixture of EC/DMC.The amount of electrolyte added in each battery was 60 μl.The galvanostatic charge/discharge cycles were conducted employing a LAND battery test system at different current densities with a voltage window between 0.01 V and 3 V vs.Li/Li+.The mass loading of the freestanding electrode was～3.0 mg﹒cm-2,and further increasing the mass in electrode is important for practical application.Cyclic voltammetry (CV) and Electrochemical impedance spectra(EIS) measurements were carried out using a CHI660 potentiostat.CV was carried out in the voltage window between 0.01 V and 3 V vs.Li/Li+with scanning rate of 0.1 mV﹒s-1.EIS was performed over the range from 100 kHz to 0.01 Hz.

2.4.Material characterization

Scanning electron microscopy(SEM)images were taken using a FEI Nova 450.X-ray diffraction(XRD)patterns were collected with a PANalytical powder X-ray diffractometer X’Pert PRO MPD(Netherlands),using Cu Kα radiation(60 kV,60 mA).Transmission electron microscopy(TEM)measurement was conducted on a Tecnai F30 G2 (FEI,USA) with operation voltage of 300 kV,equipped with an energy dispersive X-ray (EDX) detector.Nitrogen sorption isotherms were measured on a Micromeritics ASAP 2020 analyzer at 77 K.All the samples were degassed at 250 °C for 6 h under dynamic vacuum.The specific surface area was determined by Brunauere-Emmette-Teller (BET) method and the pore size distribution was calculated by the density functional theory (DFT)method.Raman spectrum was conducted on a Renishaw inVia Laser Micro-Raman spectrometer using laser excitation at 532 nm.Surface elemental analyses of samples were performed by X-ray photoelectron spectroscopy(XPS),using a Thermo SCIENTIFIC ESCALAB 250Xi spectrometer.

3.Results and Discussion

3.1.Material fabrication and characterization

The preparation of PN-FHCF was based on the simple solution polymerization strategy combined with NH3-assisted carbonization,as illustrated in Fig.1.The carbon precursor PPy with 1D hollow nanofibers (Fig.S1),was firstly obtained by polymerization of pyrrole with ferric chloride as oxidant in the presence of methyl orange via reactive self-degraded template strategy.A fibrillar complex of ferric chloride and methyl orange is instantaneously formed and results in the direct growth of PPy on its surface,during which time the complex template itself automatically degrades owing to the reduction of Fe3+[37–39].After polymerization,the freestanding PPy nanofiber films were obtained by vacuum filtration.The corresponding carbon film PN-FHCF was obtained by carbonization of the above PPy film at 550°C for 1 h under an NH3and N2mixture atmosphere,and further 800 °C under N2flow for another 1 h.For comparison,FHCF carbonized under only inert N2atmosphere was also fabricated with the same procedure.Despite of using NH3treatment,the freestanding film structure was maintained (the inset of Fig.2a),similar to that of FHCF (the inset of Fig.S2a).As shown in the scanning electron microscope(SEM) image in Fig.2a,it can be seen that the as-synthesized PNFHCF product is composed of a number of entangled and winding nanofibers,forming unique 3D interconnected network porous structures.A magnified SEM image in Fig.2d shows that the diameter and length of these PN-FHCF nanofibers are estimated to be about 280–450 nm and 7–22 μm,respectively.Such structures are inherited from its polymeric precursor PPy,which exhibits similar 3D interconnected fibrillar morphology (Fig.S1).As a control,FHCF obtained without NH3treatment shows similar fibrillar morphology with diameter of 250–490 nm (Fig.S2b,d).Furthermore,PN-FHCF reveals hollow nanotube structure from Transmission Electron Microscopy (TEM) (Fig.2b).Furthermore,a high resolution TEM(HRTEM)shows that the shell is microporous with amorphous structure and its thickness is measured to be about 50 nm(Fig.2c).The as-obtained PN-FHCF and FHCF contain N-doped functionalities derived from the PPy precursor.The uniform distribution of C and N was observed by elemental mappings (Fig.2e,f and Fig.S3).

Fig.1.Schematic illustration for the preparation of porous,nitrogen-enriched,freestanding hollow carbon nanofiber PN-FHCF via NH3-assisted carbonization,while the preparation of FHCF by carbonization under inert N2 gas was shown for reference.

Fig.2.SEM and TEM images of PN-FHCF with different magnifications.SEM images of (a,d) PN-FHCF,and the inset in (a) showing the digital photo of PN-FHCF film;TEM images of PN-FHCF with (b) low and (c) high magnifications;the corresponding elemental mappings of (e) carbon and (f) nitrogen.

To further characterize the porosity of the PN-FHCFs,N2adsorption–desorption tests were carried out (Fig.3a and Table S1).As shown in Fig.3a,the N2adsorption-desorption isotherm of PN-FHCF shows a sharp nitrogen uptake at low relative pressure(P/Po),revealing the existence of micropores located inside the nanofibers [40].This result is in agreement with the observation in magnified TEM image (Fig.2c).In contrast,FHCF displays less nitrogen uptake at low P/Po,indicative of less developed micropores thus corresponding to lower specific surface areas.The obtained BET surface area of PN-FHCF is up to 592 m2﹒g-1,almost 3.7-fold higher than that of FHCF(i.e.,158 m2﹒g-1)and an improved total pore volume from 0.11 to 0.50 cm3﹒g-1(Table S1).This result shows that the use of NH3treatment does help to create micropores for enhanced specific surface areas,which can be explained by the preferential reactions between the NH3-generated species and carbon matrices for etching out carbon fragments significantly,leaving behind the new generated nanopores [41,42].Then the adsorption amount increases progressively and still does not reach a plateau near the P/Poof 1.0,illustrating the presence of meso-and macropores (Fig.3a).Such a hierarchical pore structure characteristic in PN-FHCF can be also validated by DFT pore size distribution in Fig.3b:(1) micropores (<2 nm) concentrated at 0.5–0.68 and 1.27 nm;(2) mesopores (2–50 nm) with a maximum peak at 27 nm;and (3) macropores (50–120 nm) with maximum peak at 117 nm.Compared with that of FHCF,it can be concluded that new micropores appeared in PN-FHCF at 0.5–0.68 nm,and the content of meso-and macropores also increased.The CO2adsorption further confirmed that PN-FHCF is rich in micropores with pore size at～0.35 nm and 0.48–0.82 nm (Fig.S4).Together with the SEM images of Fig.2 and S2,it can be concluded that the 3D meso-and macroporous network structure results from the entanglement of intertwined carbon nanofibers.The micropores exist inside the carbon nanofibers,which is a result of loss of many non-carbon elements and carbon-containing compounds during pyrolysis and the disordered packing of the resulting carbon sheets and clusters.The increase in micropore is beneficial to provide more active energy storage sites,while the enhancement of mesoporous and macroporous structures is useful for facilitating the diffusion of ions by providing multiple pathways,making it suitable for high-rate performance.

Fig.3.Pore structure and composition analysis.(a)N2 adsorption–desorption isotherms;(b)DFT pore size distribution curves;(c)Raman spectra;(d)C1s and(e)N1s spectra;(f) the chemical configurations of N in form of pyridinic,pyrrole and quaternary types in PN-FHCF and FHCF.

Besides the creation of developed 3D hierarchical porous structure,it is recognized that NH3-assisted carbonization also changes surface functionalities,such as N.The increased pore structure generally gives rise to the decrease of N content [43],owing to the elimination of N moieties under severe conditions for producing nanopores.The N content was determined by both combustion elemental analysis method and X-ray photoelectron spectroscopy(XPS) measurement (Fig.3e).Interestingly,the bulk N content for PN-FHCF(8.3%)is very close to that of FHCF(9.0%).Similar to combustion analysis,the surface N determined by XPS is also up to 8.8 at%,just a slightly lower than 9.4 at% of FHCF (Table S3),even though there is a significantly 3.7-fold increase in SBET,revealing the good N fix ability of PPy nanofiber.By fitting the C1score level spectra (Fig.3d),C=C/C—C,C—N and C—O peaks can be observed.The N1sspectrum of PN-FHCF can be deconvoluted into three peaks at 400.9 eV (N-Q),398.9 eV(N-5) and 398.1 eV(N-6),respectively(Fig.3e),in which N-Q,N-5 and N-6 represent the quaternary,pyrrole and pyridine-type N atoms in graphite-like structure,respectively.The N-Q,N-5 and N-6 content are calculated to be 72.9%,9.1% and 18.0% in PN-FHCF,similar to is 74.3%,10.0% and 15.7%in FHCF,respectively (Fig.3f).It can be concluded that different N types in both PN-FHCF and FHCF are almost equal,indicating that NH3treatment either does not decrease the N content considerably or change its chemical bonding configuration.This is different from previous reports that the N types changed upon NH3treatment [44].

The NH3-assisted carbonization results in amorphous microstructures with disordered packing of turbostratic carbon sheets and clusters,as was verified by Raman and wide-angle Xray diffraction(XRD)tests(Fig.3c,S5,and Table S2).The intensity ratio of the band D(disordered structure)to G(graphitic structure)is employed to estimate the disorder degree of carbon framework[45,46].The microcrystalline plane crystal size obtained for PN-FHCF is 0.831 nm,as shown in Table S2,very close to that of FHCF (0.829 nm),indicating that the disordered microstructure of the obtained PN-FHCF also does not change upon NH3treatment in contrast to previous report that increased structural defects were induced by NH3treatment.To further characterize the framework structure of the carbon materials,XRD test was performed(Fig.S5).The PN-FHCF and FHCF show similar two broad peaks at around 25° and 44°,corresponding to(0 0 2) and(1 0 0) crystal planes,also revealing the similar amorphous microstructures,consisting with the results from Raman spectra.From these results,it can be clearly shown that NH3-assisted carbonization with PPybased nanofibers permits considerably improved surface areas and porosity but without sacrificing N moieties and altering its surface N chemical state and the carbon framework microstructure.Taking into account of the unique structural combination of developed porous structures,well-maintained N functionalities and freestanding morphology with 3D interconnected structures,PNFHCF would be advantageous for boosting the performances in electrochemical applications.As a demonstration,the application of PN-FHCF as anode materials in lithium ion batteries (LIBs) was carried out below.

3.2.Li-ion batteries

Fig.4.Lithium-ion storage performance.(a)CV curves of PN-FHCF at 0.1 mV﹒s-1 for the first three cycles;(b)discharge–charge curves at 100 mA﹒g-1 at different cycles;(c)discharge–charge rate curves from 100 to 2000 mA﹒g-1;(d) cycle stability for 100 cycles at 100 mA﹒g-1 and (e) rate capability of PN-FHCF,FHCF and rGO electrodes.

The Li storage behaviour of the PN-FHCF was investigated with half coin cells using Li plates as counter electrodes.Fig.4a shows the cyclic voltammetry(CV)of PN-FHCF at a scan rate of 0.1 mV﹒s-1between 0.01–3.0 V (vs.Li/Li+).A strong cathodic peak at～0.8 V is displayed in the first scan but vanishes in the following scans,indicating that the insertion and extraction of Li ions during the first cycle is irreversible.This is attributed to the decomposition of electrolyte and the formation of solid electrolyte interphase(SEI)films[25,26].Moreover,a pair of redox peaks are observed at lower potentials,representing the Li insertion potential close to 0 V and the extraction in the range of 0.2–0.3 V [34].A broad oxidation peak appears around 1.5 V,corresponding to the deintercalation of Li ions from the structure of PN-FHCF,for instance,nanopores,vacancies,and corner of layers[47].And the subsequent CV curves are basically coincident with peak current density and integrated area intensity,reflecting the good electrochemical reversibility and stability of the electrode material.As shown in Fig.S6,the semicircle-like plot of PN-FHCF representing charge transfer resistance in Nyquist plots greatly reduced from 463 to 85 Ω after three cycles,suggesting the outstanding electrons transportation capability during cycling.Constant discharge–charge test was performed at a current density of 100 mA﹒g-1from 0.01 to 3.0 V(Fig.4b).During the first discharge,PN-FHCF shows a less sloping shape around～0.8 V,consistent with the redox behaviour in the first CV curve.The first discharge and charge capacities of PNFHCF are 931 mA﹒h﹒g-1and 423 mA﹒h﹒g-1,corresponding to an initial Coulombic efficiency of 45.4%.The initial irreversible capacity can be assigned to the formation of a SEI on the surface of electrode and the irreversible insertion of Li ions into PN-FHCF [48–50].In subsequent cycles,PN-FHCF electrode shows obvious sloping charge–discharge curves and an indistinct plateau below 0.1 V.Like the CV measurements,the overlapping charge–discharge profiles reveal the stable and reversible capacity after the first cycle,indicating a good cycling stability and reversibility of PN-FHCF in LIBs.

To show the advantageous structures in PN-FHCF,FHCH and rGO without N doping serve as references.After 100 cycles,the reversible capacity of PN-FHCF attains to 462 mA﹒h﹒g-1,while only 257 mA﹒h﹒g-1and 112 mA﹒h﹒g-1were left for FHCF and rGO,respectively(Fig.4d).The rate capability of PN-FHCF was evaluated by using stepwise current density from 100 to 2000 mA﹒h﹒g-1with 10 cycles at each rate.Typically,the discharge–charge profiles of PN-FHCFs at different current rates are easily distinguished under various rates,shown in Fig.4c,exhibiting excellent rate performance.PN-FHCF delivers stable capacities of 449,358,291,253,240 and 157 mA﹒h﹒g-1at the current densities stepwise from 100,200,500,800,1000 and 2000 mA﹒g-1,respectively.The cell still delivers capacity of 430 mA﹒h﹒g-1,approximate to the pristine capacity when the current density is recovered back to 100 mA﹒h﹒g-1(Fig.4e).The electrochemical performance of freestanding electrodes is significantly improved after NH3treatment.For example,FHCF only shows capacities of 309,247,198,165,151 and 96 mA﹒h﹒g-1at the current densities from 100,200,500,800,1000 and 2000 mA﹒g-1,highlighting the importance of developed porosities and surface areas for Li-ion storage.Moreover,rGO freestanding film with less porosity and no N doping was tested as control,which always delivers the lowest capacities under the similar conditions,for instance,only negligible capacity of 48 mA﹒h﹒g-1was left at high current rate of 2000 mA﹒g-1.The high surface areas and more developed hierarchical pore structure are beneficial to enlarge the contact area between electrolyte and electrode materials,and increase the diffusion rate of lithium ions and the reactive sites.At the same time,the nitrogen content of PN-FHCFs still maintains at a high level with similar high content of pyridinic nitrogen,guaranteeing the increasing reactive sites for enhanced the electrochemical capacity.

4.Conclusions

In conclusion,we prepared PN-FHCF with high specific surface areas,high N content,freestanding film structure to achieve excellent performance of LIBs.Experimental results demonstrate that by using the NH3-assisted carbonization,the as-prepared PN-FHCF shows a much higher specific surface area (592 m2﹒g-1) and more developed hierarchical porosity,well-retained N content (8.8%,atom fraction) with similar surface chemistry,as compared to FHCF by direct carbonization without NH3treatment.Therefore,PN-FHCF simultaneously shows both large specific surface areas and high content of surface N functionalities,while keeping the 3D freestanding morphology constructed by one-dimensional hollow carbon nanofibers.With such rational structural integration of significantly enhanced porosity with hierarchical structures,wellretained N functionalities and free-standing film morphology,PN-FHCF showed their significance and effectiveness in enhancing the electrochemical performances in LIBs,as compared with FHCF without NH3treatment and rGO with less surface area and no N doping.It is thus expected that our protocol not only reveals important insights into achieving of both highly porous structures and sufficient surface functionalities in free-standing carbon materials,but also proposes a new platform for the development of high-performance energy storage performances.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51972270,51702262,51911530212,51872240,51672225,61805201),the China Postdoctoral Science Foundation(2018T111093,2018M643732,2018BSHYDZZ57),the Natural Science Foundation of Shaanxi Province (2020JZ-07),the Key Research and Development Program of Shaanxi Province(2019TSLGY07-03),the Fundamental Research Funds for the Central Universities (3102019JC005 and 3102019ghxm004),and the Research Fund of the State Key Laboratory of Solidification Processing (NPU),China (2019-QZ-03).H.Wang acknowledges support from the 1000 Youth Talent Program of China.We would like to thank the Analytical&Testing Center of Northwestern Polytechnical University for XPS and TEM characterizations.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.055.