N-doped porous hollow carbon nanofibers fabricated using electrospun polymer templates and their sodium storage properties

Abstract

N-doped hollow porous carbon nanofibers (P-HCNFs) were prepared through pyrolyzation of hollow polypyrrole (PPy) nanofibers fabricated using electrospun polycaprolactone (PCL) nanofibers as a sacrificial template. When used as anode material for NIBs, P-HCNFs exhibit a reversible capacity of 160 mA h g⁻¹ after 100 cycles at a current density of 0.05 A g⁻¹. An improved rate capability is also obtained at even higher charge–discharge rates. When cycled at a current density of 2 A g⁻¹, the electrode can still show a reversible capacity of 80 mA h g⁻¹. The N-doped sites, one-dimensional nanotube structure, and functionalized surface of P-HCNFs are capable of rapidly and reversibly accommodating sodium ions through surface adsorption and redox reactions. Therefore, P-HCNF is a promising anode material for next-generation NIBs.

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1. Introduction

Owing to almost unmatched volumetric energy density, Li-ion batteries (LIBs) have dominated the portable electronics industry for the past 20 years.¹ However, the limited abundance of Li in the Earth's crust, its uneven geographic distribution, and difficulties in recycling Li resources have raised concerns about large-scale application of LIBs.^2–4 Recently, Na-ion batteries (NIBs) have been considered as candidates for large-scale stationary energy storage due to the advantages of sodium (e.g. high abundance, low cost, and very suitable redox potential at only 0.3 V above that of lithium).^2,5–10 However, practical application of NIBs is greatly hampered by poor cyclability and low rate capability, which results from the poor kinetics of the Na ion insertion–extraction reaction caused by the relatively larger ionic radius (102 pm) of the Na cation than that of the Li cation (76 pm).^6,11–14 Substantial effort has been made to develop high performance NIBs including cathodes, anodes, and electrolytes.^{2,5,10,15–24} The search for cathode materials for NIBs has proven to be fruitful, while the development of anode materials is still in its infancy.

Most recent work has focused on carbonaceous materials as anodes for NIBs due to their large interlayer distances and disordered structures, which facilitate Na-ion insertion–extraction.^25–31 First, Doeff et al. investigated the electrochemical insertion of sodium into petroleum-coke carbon, observing a reversible capacity of 85 mA h g⁻¹.³² Tirado's group studied reversible insertion of sodium in amorphous carbon black and delivered a capacity of 200 mA h g⁻¹ considering only the discharge.²⁵ However, they reported the results of only a few cycles. In order to further improve the electrochemical performance of carbonaceous material anodes, one approach is to tune their microstructures. Following this method, carbonaceous materials with various morphologies (nanofibers, nanotubes, nanospheres, nanosheets, carbon nanofoam) have been fabricated as anodes for NIBs.^{17,20,27,28,33} For example, Tang et al. fabricated hollow carbon spheres as anode for NIBs, displaying a reversible capacity of 150 mA h g⁻¹.²⁰ Wang et al. demonstrated the use of 2D porous carbon sheets as an ideal high-rate anode material for NIBs.^27,34 Among those morphologies, one-dimensional (1D) carbon nanostructures are considered to be attractive due to their good electronic conduction along the 1D nanostructure, short lithium ion diffusion distance, and high interfacial contact area with electrolytes.^17,35,36 Moreover, N-atom incorporation has been proven as an additional effective strategy for improving the electrochemical performance of carbon-based Na-storage anodes.³⁴ Nitrogen as a dopant to modify the carbon structures can generate extrinsic defects and hence improve capacity,³⁷ enhance surface wettability of the materials,³⁸ and increase electronic conductivity.³⁹ Therefore, many works using N-doped carbon materials as electrodes for NIBs have been reported.^34,39,40 However, these methods suffer from limitations such as the requirement of toxic precursors, the use of special instruments, and low yield. Therefore, the preparation of N-doped carbon materials by a simple, facile, and scalable method remains a challenge.⁴¹

Electrospinning is a simple, scalable, and versatile technique that has gained substantial attention in both academic research and industrial applications. This method provides a straightforward and low-cost fabrication route to prepare 1D fiber with a diameter ranging from several tens of nanometers to a few micrometers.^42–44 In this work, we report the formation of N-doped hollow porous carbon nanofibers by carbonization–activation hollow PPy nanofibers using electrospun PCL fibers that serve as sacrificial templates for subsequent in situ polymerization of pyrrole coating on the fibers. As anode materials for NIBs, they exhibit high reversible capacity and good rate performance and deliver a reversible capacity of 160 mA h g⁻¹ after 100 cycles at 0.2 C and excellent rate capability of ∼80 mA h g⁻¹ at 8 C. The improved rate capability and cyclability can be attributed to the hollow porous structure and high-level N doping.

2. Experimental

2.1. Electrospinning of PCL nanofibers

In a typical process of electrospinnig PCL (M_n = 70 000–90 000, Aldrich) nanofibers, a solution of 1 g PCL was dissolved in 10 ml dichloromethane (DCM; 99%, Aldrich) that contained 0.0072 g of cetyltrimethylammonium bromide (CTAB, Aldrich). The obtained precursor solution was loaded into a 10 ml syringe with a 19-gauge blunt tip, and the fibers were spun at approximately 15.0 kV. The syringes were pushed by a syringe pump with a feeding rate of 4.02 μl min⁻¹. The distance between the tip of needle and the rotating cylinder was maintained at 23 cm.

2.2. Formation of hollow PPy nanofibers

The obtained PCL nanofiber paper was cut in to several small pieces (6 cm × 5 cm). 10 pieces of nanofibers were immersed in a 0.08 M aqueous solution of pyrrole (200 ml). The polymerization of pyrrole (Py) and the coating of PPy on the PCL nanofibers in ice bath conditions were initiated by dropping 0.07 M aqueous solution of FeCl₃ (200 ml) for 24 h with continuous stirring. The collected PCL (core)–PPy (sheath) fibers were washed with deionized water for several times to remove the residual FeCl₃. Subsequently, the PCL (core)–PPy (sheath) fibers were immersed in DCM for an additional 24 h to extract the PCL core, and the hollow PPy nanofibers were obtained. Finally, the hollow PPy nanofibers were washed with deionized water several times to remove the DCM and were dried in an oven at 80 °C.

2.3. Preparation of N-doped porous hollow carbon nanofibers (P-HCNFs)

The porous hollow carbon nanofibers were synthesized by pyrolysis of hollow PPy nanofibers that were previously chemically activated with KOH. Typically, a mixture of hollow PPy nanofibers (0.3 g) and KOH (0.9 g) at a 1 : 3 weight ratio was added into 20 ml deionized water, and the mixture was stirred for 12 h. Then the product was filtered and dried at 80 °C in an oven. Next, the PPy was chemically activated at 700 °C with a heating rate of 3 °C min⁻¹ and was kept for 0.5 h under nitrogen atmosphere. The product was then washed with deionized water until the filtrate was neutral and was dried in an oven at 80 °C. Finally, the N-doped porous hollow carbon nanofibers (P-HCNFs) were obtained. The N-doped hollow carbon nanofibers (HCNFs) was prepared according to a similar treatment but without the activation procedure.

2.4. Structural analysis and electrochemical measurements

X-ray powder diffraction (XRD) patterns of the products were obtained on a Philips X'Pert PRO SUPER X-ray diffractometer with Cu Kα radiation. A scanning electron microscope (JEOL, Tokyo, Japan) was used to perform field-emission Scanning electron microscopy (FESEM) and a transmission electron microscope (JEOL, Tokyo, Japan) was used to perform transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to study the morphology and microstructure of P-HCNFs and HCNFs. Raman spectroscopy was used to study the degree of graphitization. An ASAP 2020 accelerated surface area and porosimetry instrument was used to measure the nitrogen adsorption/desorption isotherms. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the nature of the nitrogen species at the surfaces of the P-HCNFs.

The working electrodes were prepared by mixing active materials (P-HCNFs or HCNFs) at 90 wt% and poly(vinylidene) fluoride (PVDF) at 10 wt% in N-methyl-2pyrrolidone (NMP) to form a slurry, which was coated on copper foil, dried at 80 °C under a vacuum overnight, and cut into pellets with diameters of 14 mm. The obtained discs were used as the electrodes of electrochemical cells consisting of P-HCNF (or HCNF)/Na with NaClO₄ (1 M) in propylene carbonate (PC) as the electrolyte. Glass fiber (Whatman) was used as a separator film. Pure sodium foil (99.9%, Aldrich) was used as counter electrode and reference electrode. The cells were assembled in an argon-filled glove box (MBRAUN LABMASTER 130) in which both moisture and oxygen levels were maintained below 1 ppm. Electrochemical experiments were performed using coin cells (CR2032). Galvanostatic charge–discharge tests were conducted at a voltage interval of 0–2.8 V on a battery test system (Neware BTS-610). Cyclic voltammetry (CV) was conducted by using a CHI 660D electrochemical workstation at a scan rate of 0.2 mV s⁻¹ (Chenhua Instrument Company, Shanghai, China).

3. Results and discussion

The processing steps of our fabrication procedure are shown schematically in Fig. 1. The precursor solution of PCL–CTAB/DCM was electrospun into fibers (Fig. 1A). As shown in Fig. 1B, the in situ polymerization reaction for PPy involved the use of Fe³⁺ as an oxidant and Cl⁻ as dopant. The CTAB served as a surfactant to self-assembled Py on PCL fibers.⁴⁵ Then, the obtained PCL fibers were used as a sacrificial template to create hollow PPy fiber. The hollow PPy fibers were subsequently chemically activated with KOH at N₂ atmosphere, resulting in N-doped porous hollow carbon nanofibers (P-HCNFs) with large surface areas.

Fig. 1 (A) and (B) Schematic illustration of the synthesis process for P-HCNFs.

Fig. 2A shows a field emission scanning electron microscopy (FESEM) image of the as-electrospun PCL–CTAB fibers. The as-spun fibers display a continuous fibrous morphology with uniform diameter distribution. The inset image in Fig. 2A shows that these nanofibers have smooth surfaces and an average diameter of ∼300 nm. Fig. 2B displays the FESEM image of PPy nanotubes after extraction of the PCL cores by DCM. The broken part of the fiber shows that a tubular structure was obtained after dissolution of the PCL. Obviously, PPy nanotubes replicating the morphology of the as-spun PCL fibers were produced. The surfaces of PPy nanotubes are rough. The outer diameter of the PPy nanotubes varied between approximately 500 nm and 800 nm. After activation with KOH under N₂ atmosphere, the PPy nanotubes maintained their nanotube morphology and transformed into porous hollow carbon nanotubes (P-HCNFs) (Fig. 2C). The outer diameters of the obtained P-HCNFs are approximately 500 nm, slightly less than those of the PPy nanotubes, displaying a shrinkage that can be ascribed to the decomposition of organic components of PPy during the calcination step. Fig. 2D reveals the morphology of the HCNFs, which is similar to that of the P-HCNFs.

Fig. 2 FESEM micrographs of as-electrospun PCL-CTAB nanofibers (A), PPy nanotubes (B), P-HCNFs (C) and HCNFs (D). The inset pictures are corresponding high magnification images.

The microstructures of P-HCNFs and HCNFs were further studied by transmission electron microscopy (TEM). Fig. 3A confirms the hollow structure of the P-HCNFs. The inner diameters of the P-HCNFs are ∼370 nm, and their wall thicknesses are approximately 65 nm. The HRTEM image of the P-HCNFs (Fig. 3B) reveals that large quantities of micropores are homogeneously distributed within the HCNFs, indicating its amorphous structure. These micropores resulted from the loss of hydrogen and nitrogen in PPy and the reaction between PPy and KOH during the carbonization process and can provide space for reversible accommodation of sodium ions.³⁴ Owing to these special structure characters, the material may have an excellent electrochemical performance when used as anode material for NIBs. Fig. 3C shows the hollow structure of HCNFs. The HRTEM image in Fig. 3D shows the formation of porous carbon.

Fig. 3 TEM image (A) and HRTEM image (B) of P-HCNFs. TEM image (C) and HRTEM image (D) of HCNFs.

To further examine the surface and characterize the pore size of the P-HCNFs and HCNFs, the two samples were studied by nitrogen adsorption–desorption tests. Fig. 4A and B exhibit type-IV nitrogen adsorption–desorption isotherms with an H4-type hysteresis loop. The specific Brunauer–Emmett–Teller (BET) surface area of P-HCNFs and HCNFs are 868 m² g⁻¹ and 134 m² g⁻¹, respectively. Obviously the BET surface area of P-HCNFs is much higher than that of HCNFs. The insets in Fig. 4A and B show the pore size distribution of P-HCNFs and HCNFs, respectively. The P-HCNFs possess small micropores (peak at 1.27 nm), and while the HCNFs possess both mesopores (peak at 2.52 nm) and micropores (peak at 1.59 nm and 0.73 nm).

Fig. 4 Nitrogen adsorption–desorption isotherms and PSD (inset) of P-HCNFs (A) and HCNFs (B).

X-ray photoelectron spectroscopy (XPS) is used to investigate the nature of nitrogen species at the surfaces of P-HCNFs and HCNFs. It has been demonstrated that PPy precursor has only one type of nitrogen atom denoted N-5 within a pentagonal ring.⁴⁶ However, three peaks at 398.2, 399.9, and 401 eV can be observed to correspond to pyridinic-N (N-6), pyrrolic-N (N-5) and graphite-N (N-Q), respectively, in the high-resolution N 1s peaks (Fig. 5A) after carbonization–activation of PPy with KOH.^34,46 Obviously, during the carbonization–activation process, parts of the nitrogen atoms within the pentagonal ring of polypyrrole are converted into two types of nitrogen, N-6 and N-Q. It has been proved such nitrogen-containing species (pyridinic-N and graphite-N) can improve the electrochemical activity of nitrogen-doped carbon materials.³⁴ For HCNFs, such N 1s peaks are also observed (Fig. 5B).

Fig. 5 N 1s XPS spectra of P-HCNFs (A) and HCNFs (B).

The phases of P-HCNFs and HCNFs were checked by XRD (Fig. 6A). Two characteristic peaks of carbon appear at 2θ = 25° and 43°, corresponding to the (002) diffraction and (100) diffraction of hexagonal carbon material (JCPDS, card no. 75-1621), respectively.⁴⁶ The intensity of the two peaks of P-HCNFs is slightly lower than that of HCNFs, suggesting a lower graphitic crystallinity of P-HCNFs due to more graphene fragments and defects generated when KOH is added to the chemical activation.⁴⁶ The Raman spectra of P-HCNFs and HCNFs (Fig. 6B) show two broad peaks at approximately 1580 cm⁻¹ (G-band) and 1350 cm⁻¹ (D-band) corresponding to the E_2g2 graphitic mode and the defect-induced mode, respectively.³⁵ The disorder degree of the carbon is estimated by the index R (R = I_D/I_G).⁴⁷ The R values for P-HCNFs and HCNFs are 1.01 and 0.95, respectively. This result indicates that the two materials are in a low degree of graphitization with a high disorder structure, which is consistent with the XRD patterns and HRTEM results.⁴⁸

Fig. 6 XRD patterns (A) and Raman patterns (B) of P-HCNFs and HCNFs.

The electrochemical performances of the P-HCNF and HCNF electrodes were evaluated by using a half-cell with sodium metal. Fig. 7 shows cyclic voltammetry (CV) curves of the P-HCNFs and HCNFs in the range of 0–2.6 V. A sharp cathodic peak (Fig. 7A) appears at 0.42 V of the first cycle, which is due to the decomposition of the electrolyte and the formation of solid electrolyte interface (SEI).^34,49 This peak disappears after the first cycle because the anode is isolated from the electrolyte after the formation of SEI in the first cycle. A pair of redox peaks near 0 V can be attributed to sodium ion insertion into/extraction from the micropores.² During the subsequent cycles, the CV curves almost overlap, which indicates the P-HCNFs display good capacity retention. In the case of the HCNFs (Fig. 7B), an irreversible peak occurs at 0.97 V during the reduction process in the first CV curve, which also can be ascribed to decomposition of electrolytes and the SEI formation. Additionally, a pair of small and broad redox peaks are observed in a wide potential range of 0.2–1.2 V, which is attributed to sodium ion insertion into hard carbon.

Fig. 7 Cyclic voltammograms of a P-HCNFs (A) and HCNFs (B) electrode between 0 and 2.6 V at a potential sweep rate of 1 mV s⁻¹.

Fig. 8A and B show the charge/discharge profiles of the P-HCNF and HCNF electrodes at a current density of 0.05 A g⁻¹ (0.2 C) in the voltage range of 0–2.8 V. The initial discharge and charge capacities of the P-HCNFs (Fig. 8A) are 993 and 319 mA h g⁻¹, respectively, showing a 32% initial Coulombic efficiency. The large irreversible capacity loss is caused by the decomposition of electrolytes and the formation of the SEI layer.^34,49 The P-HCNF electrode shows a sloping voltage profile and an indistinct plateau near 0 V in subsequent cycles, which results from Na ion insertion into nanopores of hard carbon.³⁴ In the case of HCNFs, the voltage profile displays similar behaviour, showing a discharge and charge capacity of 423 mA h g⁻¹ and 174 mA h g⁻¹, respectively, according an initial Coulombic efficiency of 41%. This improvement in initial Coulombic efficiency is related to the formation of less SEI. It has been demonstrated that a larger contact surface area between the electrode and electrolyte will result in more SEI and higher initial capacity loss.⁵⁰

Fig. 8 Voltage profiles of P-HCNFs (A) and HCNFs (B) electrodes at a current density of 50 mA g⁻¹. (C) Cycle performances of P-HCNFs and HCNFs electrodes at a current density of 50 mA g⁻¹. (D) Discharge capacity of P-HCNFs and HCNFs electrodes as a function of charge–discharge cycles at different charge–discharge current densities of 0.05 (0.2 C), 0.1 (0.4 C), 0.25 (1 C), 0.5 (2 C), 1 (4 C), and 2 (8 C) A h g⁻¹.

Fig. 8C compares the cycle performances of P-HCNF and HCNF electrodes at a current density of 0.05 A g⁻¹. Obviously, the cycling performance of the P-HCNF electrode is much better than that of the HCNF electrode. The P-HCNF electrode retains a reversible capacity of 160 mA h g⁻¹ after 100 cycles, corresponding to 50.2% of the initial charge capacity. However, the HCNF electrode delivers only a reversible capacity of 40 mA h g⁻¹ after 100 cycles, corresponding to 22.4% of the initial charge capacity. To further illustrate the improvement in the electrochemical performance of P-HCNFs, the rate capability of both electrodes was investigated (Fig. 8D). The reversible capacity of the P-HCNF electrode is 190 mA h g⁻¹, 160 mA h g⁻¹, 120 mA h g⁻¹, 86 mA h g⁻¹, 80 mA h g⁻¹, and 80 mA h g⁻¹ when cycled at a current densities of 0.05 A g⁻¹, 0.1 A g⁻¹, 0.25 A g⁻¹, 0.5 A g⁻¹, 1 A g⁻¹, and 2 A g⁻¹, respectively. After the current density was tuned back to 50 mA g⁻¹, the reversible capacity of the P-HCNF electrode also recovered to 180 mA h g⁻¹. These results demonstrate excellent cycle stability. For the HCNF electrode, however, the rate capability is significantly worse. When cycled at 1 A g⁻¹, it delivers a capacity of only 4 mA h g⁻¹.

The improvement in electrochemical performance of the P-HCNF electrode can be explained as follows. (i) The 1D nanotube structure can provide a short ion transfer length and continuous electron transportation. Moreover, the homogeneous distributed porous channels in P-HCNFs can enable effective short lengths for both sodium ions and electrons. (ii) The nitrogen doping in P-HCNFs can improve the electrochemical reactivity and electronic conductivity of the material.⁵¹

4. Conclusions

In summary, we fabricated N-doped hollow porous carbon nanofibers via a two-step method. First, hollow polypyrrole (PPy) nanofibers were fabricated via a combination of electrospinning and aqueous polymerization. Specifically, nanofibers electrospun from poly(ε-caprolactone) (PCL) were employed as a sacrificial template to generate hollow nanofibers of PPy via in situ polymerization. Second, the obtained PPy nanotubes were carbonized and further chemically activated with KOH to prepare the P-HCNFs. The P-HCNFs displayed hollow tubular structures and a surface area as high as 868 m² g⁻¹. When used as an anode material for NIBs, P-HCNFs electrode delivered a reversible capacity as high as 160 mA h g⁻¹ after 100 cycles at a current density of 0.05 A g⁻¹. The results demonstrate that P-HCNFs are promising candidates for the construction of low-cost sodium ion battery systems.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21171015, no. 21373195), the “1000 plan” from Chinese Government, program for New Century Excellent Talents in University (NCET), the Fundamental Research Funds for the Central Universities (WK2060140016 & WK2060140014) and Sofja Kovalevskaja award from the Alexander von Humboldt Foundation.

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