Introduction

Energy storage devices with high energy/power density and long cycling life, such as lithium-ion batteries (LIBs), lithium–air batteries (LABs), lithium–sulfur batteries (LSBs), sodium-ion batteries (NIBs) or supercapacitors (SCs) [1,2,3], are urgently needed to meet the ever-growing requirements of large-scale electric energy storage and of electric vehicles. Among the various materials suited for electrochemical energy storage, nanostructured materials with high surface-area-to-volume ratios play an increasingly important role [4]. These materials can increase the interface areas between solids and liquids or solids and gases, while at the same time shortening the diffusion distance, thereby facilitating electron or ion transport and improving electrochemical performance.

Ordinary nanoparticles, however, tend to aggregate and thereby minimize surface energy, which leads to reduction in the surface-area-to-volume ratio for nanostructured materials. One-dimensional (1D) nanomaterials, such as nanowires, nanorods, nanofibers (NFs) and nanotubes, are attracting a great deal of interest from developers of energy storage applications, as these materials have better electrical and physicochemical properties than nanoparticles [5,6,7,8,9,10]. These 1D nanomaterials can be prepared by a variety of methods, including self-assembly [11, 12], hydrothermal synthesis [13,14,15], template-assisted synthesis [16, 17], vapor-phase approaches [18], wet spinning [19], centrifugal spinning [20,21,22,23,24] or electrospinning [25,26,27]. Among these methods, electrospinning has special significance for the fabrication of NFs, as this technique allows facile and cost-effective fabrication of continuous, ultrathin NFs.

Electrospinning is a process that produces polymer NFs under the influence of an external electric field. This fiber production method was first proposed in [28], and it has since been widely applied for energy storage systems. As shown in Fig. 1, an electrospinning apparatus consists of a high-voltage supply, a grounded collector and a spinneret. The viscous electrospinning solution in the syringe is extruded by a feeding pump at a constant rate. Next, a high voltage is applied to the tip of the spinneret, and the solution inside the needle is charged. As the applied voltage increases, the hemispherical surface of the polymer solution is elongated to form a conical shape known as the Taylor cone. When the voltage is sufficiently high, the electrostatic forces overcome the surface tension of the liquid drop. A charged jet of the solution is then ejected from the tip of the Taylor cone and splits into many tiny jets between the tip and the collector. As the solvent evaporates, solidified continuous fibers form on the collector [29].

Figure 1
figure 1

Schematic illustration of electrospinning apparatus and various methods applicable to preparing porous NFs for electrochemical energy storage devices

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The morphologies and structures of electrospun NFs are greatly affected by a range of experimental parameters, such as the electric field [30], the polymer, solvent [31], nozzle [32], temperature [33] or humidity [34]. This variability of the NFs characteristics makes it feasible to properly adjust the electrospinning and to acquire electrospun NFs with various structures [35, 36]. For example, by properly selecting controllable parameters, one can fabricate porous, hollow or core–shell fibers, and fiber-in-tube or tube-in-tube structures [37, 38]. In addition, by controlling the post-treatment process (e.g., by heating as-spun NFs under different atmospheres), one can obtain various types of NFs such as metal or metal oxide NFs, carbon nanofibers (CNFs) or composite NFs [25, 39,40,41,42].

Electrospun NFs with porous structures may find productive applications in the field of energy storage, because they exhibit high specific surface areas and abundant pore structures that can improve the transmission dynamics of electrons and ions, thereby offering enhanced performance in electrochemical energy storage. The void spaces in the porous NFs can effectively buffer changes in the volume of electrode materials during the charge and discharge processes. Due to these advantages, many methods have been established based on the electrospinning technique for preparing highly porous NFs.

The huge number of currently available publications about porous NFs provides a good opportunity to summarize the present knowledge about electrospinning techniques for preparing porous NFs and to thereby provide a reference for future studies. Therefore, in this review, we focus on several kinds of methods based on electrospinning for preparing porous NFs. An emphasis is placed on the electrospinning techniques that use polymer as the template, and on the hard template, solvent-induced, nonsolvent-induced, and activation routes. At the same time, we also summarize the potential applications of porous NFs and their composites in electrochemical energy storage systems such as LIBs, LABs, LSBs, NIBs and SCs.

Electrospinning with polymer as the template

In the electrospinning process, the polymer usually acts as the matrix of the NFs, but it can also act as a template to produce porous structures. For example, with a single polymer as the template, porous metal oxide NFs can be obtained by removing the polymer that surrounds the nanoparticles. In addition, various blended polymers with different properties can be used as templates to prepare porous carbon nanofibers (PCNFs) and composites.

Electrospinning with a single polymer template to prepare porous metal (oxide) nanofibers

With polymer/inorganic nanoparticles or corresponding precursor solutions as the starting materials for electrospinning and subsequent calcination, hierarchical porous NFs composed of numerous nanoparticles can be confined in the polymer fibers and assembled into 1D structure. The resultant porous fiber structures can be prevented from aggregating, thereby better enabling the function of the nanoparticles. The void spaces among the nanoparticles and the gaps between the fibers serve to increase the specific surface area, and they facilitate the penetration of liquid electrolytes. This permeability, along with the porous structure’s capacity to buffer expansion in the volume of electrode materials during the charge/discharge process, contributes to improving the capacity for electrochemical energy storage. The single polymer template electrospinning method is mainly used to synthesize metal oxide NFs, for which the calcination process is conducted in the air atmosphere. In this process, the oxygen in the air allows complete combustion of the polymeric carbon, thereby adding to the porous structure of the as-fabricated metal oxide NFs, and enhancing energy storage.

Polyvinylpyrrolidone (PVP), with its high spinnability and good solubility in various solvents [such as N,N-dimethylformamide (DMF), water or ethanol], is often used as a template for fabricating neat metal oxide fibers. For example, Lu et al. [43, 44] prepared PVP/SnCl2 NFs via electrospinning. By heating the PVP/SnCl2 NFs under an air atmosphere to remove the PVP, these researchers obtained porous SnO2 nanotubes. In addition, they found that the concentration of SnCl2 had a major influence on the formation of SnO2 nanotubes and NFs. With a low concentration of SnCl2 in a PVP matrix, porous SnO2 nanotubes could be obtained, because the scattered SnCl2 molecules tended to be diffused to the surfaces of the NFs and then decomposed to form SnO2 nanoparticles. However, if the concentration of SnCl2 in the PVP matrix was high, larger number of SnCl2 molecules in the fibers decomposed to form SnO2 nanoparticles, which were uniformly distributed in the NFs.

Kundu and Liu [45] dissolved Ni(CH3COO)2, C6H8O7, and PVP in deionized water for electrospinning, and they collected the as-spun NFs with a Ni foam. After heat treatment, porous NiO NFs were obtained and directly used in SCs without any binder. These as-fabricated SCs exhibited a capacitance retention as high as 570 F g−1 even at a current density of 40 A g−1. The excellent performance of these SCs can be attributed to the highly porous structure of the electrodes, which facilitated the transport of ions and electrons. Porous NiO NFs prepared by electrospinning exhibited good electrochemical performance when applied in LIBs [46,47,48]. In addition, Fe2O3 [49], Co3O4 [50] and V2O5 [26, 51,52,53,54,55] NFs prepared by the same method exhibited similarly excellent electrochemical properties.

Actually, porous metal NFs can also be prepared by a single polymer template electrospinning method, because reducing the electrospun metal oxide NFs affords metallic NFs [56, 57]. With colloidal SiO2 and poly(acrylic acid) as the starting materials, Lee et al. [56] fabricated 3D mesoporous silicon NFs by using a simple technique involving electrospinning and the magnetic-isothermic reduction in silica. The constituent Si nanoparticles formed mesoporous, interconnected nanostructures that permitted the fibers not only to accommodate large changes in volume during battery operation, but also to achieve greater access to Li+ ions. The 3D mesoporous silicon NFs exhibited a high reversible capacity of 2846.7 mAh g−1 at a current density of 0.1 A g−1, a stable capacity retention of 89.4% after 100 cycles at a 1 C rate (2 A g−1) and a rate capacity of 1214 mAh g−1 even at 36 A g−1.

In the presence of multicomponent precursors of metal oxides, binary metal oxide NFs, such as ZnFe2O4 [58], ZnCo2O4 [59], ZnMn2O4 [60], NiFe2O4 [61], Li4Ti5O12 [62] or LiNb3O8 [63] NFs, can be fabricated as the anode materials for LIBs. For example, Li et al. [64] fabricated CaSnO3 NFs and nanotubes for LIBs by using SnCl2 and Ca(NO3)2 as the starting materials. They found that the morphology of as-fabricated CaSnO3 was largely dependent on the calcination temperature (Fig. 2). CaSnO3 nanotubes showed a better rate capability and cyclability than CaSnO3 NFs or CaSnO3 ruptured nanobelts. The excellent electrochemical performances of these nanotubes were attributed to their unique porous and hollow interior structures, which enabled the charge/discharge process of LIBs.

Figure 2
figure 2

a Schematic diagram illustrating the formation process of CaSnO3 nanofibers, nanotubes and ruptured nanobelts. b Cyclability of electrospun CaSnO3 nanomaterials (current density = 60 mA g−1). c Capacity of CaSnO3 nanomaterials at different current densities. Diagrams reprinted with permission from Ref. [64]. Copyright 2012, American Chemical Society

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By combining a gradient electrospinning method with a controlled pyrolysis method, Niu et al. [65] synthesized various mesoporous metal oxide nanotubes as cathode materials for LIBs and NIBs. Moreover, electrospinning could also be adopted to prepare porous CuCo2O4 [66] NFs and ZnCo2O4 [67] nanotubes used in SCs. Table 1 shows the electrochemical performances for several kinds of metal oxide NFs used in LIBs.

Table 1 Porous metal oxide nanofibers for LIBs
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Electrospinning can also be combined with calcination to deposit a carbon coating layer on porous metal oxide NFs, thereby significantly improving their electrochemical performance. Zhu et al. [68] prepared porous α-Fe2O3 NFs by electrospinning. Then, they dispersed the porous α-Fe2O3 NFs into PVP and calcined the mixture under a nitrogen atmosphere to obtain α-Fe2O3/C NFs. The as-obtained α-Fe2O3/C NFs exhibited a high reversible capacity of 842 mAh g−1 after 50 cycles of charge/discharge at a current density of 50 mA g−1.

Xu et al. [69] added the precursor of Li4Ti5O12 (LTO) to an ethanol solution of PVP and obtained composite fibers by electrospinning. They further calcined the as-spun composite fibers in an Ar/H2 atmosphere to obtain LTO/C fibers. After being heated in air, the resultant LTO/C fibers were transformed into porous LTO/C (PLTO/C) composite NFs, which exhibited a high specific capacity of 143 mAh g−1 even at a high rate of 30 C. Furthermore, the PLTO/C composite NFs were applicable for asymmetric hybrid SCs.

These studies demonstrated that the introduction of a carbon coating on the surfaces of porous metal oxide NFs serves to greatly improve the conductivity. Meanwhile, the carbon shell can limit the growth of crystallite in heating process and effectively buffer the volume expansion of the anode materials during the charge/discharge processes.

LABs show promising potential as energy storage devices because they exhibit a very high theoretical energy density, and their metal oxides can act as catalysts to facilitate the oxygen reduction reaction and promote oxygen evolution. Li et al. [77] dissolved La(NO3)3, Sr(NO3)2, Co(CH3COO)2 and PVP in DMF and obtained a homogeneous viscous solution for electrospinning. The as-prepared electrospun NFs were transformed into porous La0.5Sr0.5CoO2.91 NFs after being calcined in air. Thanks to their porous structures, the as-fabricated La0.5Sr0.5CoO2.91 NFs exhibited an initial discharge capacity of 7205 mAh g−1, at a current density of 100 mA g−1 (the voltage platform being around 2.66 V). In addition, Li et al. [78] synthesized porous NiCo2O4 nanotubes for LABs by electrospinning and acquired excellent cycling stability over 110 cycles of charge/discharge.

In these studies, the authors drew a common conclusion that hierarchical porous structures can provide channels for electron and oxygen transport, and such structures can also provide a large contact area of electrode–electrolytes to ensure a highly efficient catalytic reaction, thereby significantly improving the electrochemical performance of LABs.

Electrospinning with a polymer blend as the template

Polymers with different physical and chemical properties can be blended to compose templates for fabricating porous carbon NFs and composite NFs by electrospinning. This approach is the so-called polymer blend template electrospinning method. The polymer blend consists of two types of polymers with different degrees of thermal stability. The polymer with less thermal stability undergoes pyrolysis upon thermal decomposition, which creates pores in the carbon matrix. These pores are formed through the carbonization of the thermally stable polymer. The size and structure of the pores can be controlled by manipulating the thermal stability and compatibility (or solubility) of the various polymers, and greater differences in their compatibility lead to larger sized pores [79]. Table 2 lists a variety of the PCNFs that can be prepared via polymer blend template electrospinning method.

Table 2 Various PCNFs prepared via the polymer blend template electrospinning method
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To obtain porous carbon NFs through the polymer blend template electrospinning method, researchers often blend polyacrylonitrile (or PAN, a common carbonizing polymer) with other polymers such as polymethyl methacrylate (PMMA) [80,81,82,83,84,85,86] or polystyrene (PS) [87,88,89,90]. As PAN exhibits poor compatibility with PMMA (the sacrificial phase), island-like PMMA is formed in the PAN matrix when the PAN/PMMA solution undergoes electrospinning. During calcination in an inert atmosphere, the PAN NFs are carbonized to afford a carbon matrix, and PMMA is completely pyrolyzed to form pores in the carbon NFs.

Usually, PAN acts as the continuous phase in the PAN/PMMA NFs, and PMMA acts as the dispersed phase. However, Wei et al. [81] combined the electrospinning of a PAN/PMMA solution (consisting of PAN as the dispersed phase and PMMA as the continuous phase, and having a PAN/PMMA molar ratio of 3:7) with a calcination process, to prepare highly flexible PCNFs. When these as-fabricated, highly flexible PCNFs were used as the electrode materials for SCs, they could deliver a specific capacitance of 86 F g−1.

Peng and Lo [82] formulated PAN/PMMA solutions with differing compositions for electrospun and fabricated PCNFs. When directly used as an LIB anode without any binder, those PCNFs (prepared at a PAN/PMMA molar ratio of 5:5) exhibited greatly increased porosity and a high discharge capacity of 446 mAh g−1 under a current density of 150 mA g−1, with a favorable cycle stability (discharge capacity 354 mAh g−1) after 100 cycles at a current density of 200 mA g−1 (Fig. 3).

Figure 3
figure 3

SEM micrographs of carbonized electrospun fibers of a PAN/PMMA = 10:0; b PAN/PMMA = 7:3; c PAN/PMMA = 5:5; and d their rate capacities at different current densities when used as anodes for LIBs. Images reproduced from Ref. [82], with permission of Springer

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Kim et al. [84] reported that graphene could increase the electrical conductivity and specific surface area of PCNFs prepared by the electrospinning of PAN/PMMA. The resultant graphene-doped PCNFs exhibited a much higher specific capacitance (128 F g−1) than the undoped PCNFs. As reported elsewhere, these researchers also considered that the superior electrochemical performance of PAN/PMMA-derived carbon NFs could be attributed to their hierarchical pores and their high specific surface areas, which provided favorable contact between the electrodes and electrolytes and facilitated the transport of electrolyte ions.

PS [87, 89, 90], poly-l-lactic acid (PLLA) [91], poly(ethylene oxide) [92], Nafion [93], F127 [94], polyethylene glycol–polysiloxane–polyethylene glycol [95], polyphenylsilane [96] and polymethylhydrosiloxane [97] can also be used as the sacrificial templates for preparing PCNFs. Zhang et al. [98] formulated PAN/PVP solutions with differing molar ratios and obtained porous PAN NFs by electrospinning in association with follow-up treatment in water to remove PVP. The pore sizes and pore distributions of the porous PAN NFs could be easily controlled by varying the content of PVP in the composite polymer precursor. Also, the porous PAN NFs obtained at a PAN/PVP molar ratio of 0.8:1 could be carbonized in N2 atmosphere to form PCNFs with a specific surface area of 571.47 m2 g−1.

Using styrene-co-acrylonitrile (SAN)/DMF core/emulsified PAN-SAN/DMF shell solution as a precursor, Lee et al. [99] prepared porous hollow carbon nanofibers (pHCNFs) by coaxial electrospinning. During heat treatment, the SAN was burnt out, and the PAN was carbonized to form PCNFs (Fig. 4). The researchers found that the spacing between the graphene layers in the pHCNFs increased, owing to the hundred layer-sequential mismatches which occurred through pores by formation. This expanded structure promoted lithium-ion insertion and extraction. The resultant PCNFs, when used as the anode materials of LIBs, exhibited a high initial capacity of 1003 mAh g−1 at a current density of 50 mA g−1.

Figure 4
figure 4

a Schematic diagram illustrating the electrospinning process of SAN core/PAN-SAN shell nanofibers. b FE-SEM images of porous hollow carbon nanofibers. Images reprinted with permission from Ref. [99]. Copyright 2012, American Chemical Society

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In addition to neat PCNFs, PCNFs containing inorganic nanoparticles can also be prepared by mixing polymer blends and the precursors of inorganic nanoparticles for electrospinning. For example, by combining the PAN/PMMA polymer blend template with the precursor of inorganic nanoparticles, one can conduct electrospinning to obtain inorganic nanoparticles coated by PCNFs. The resultant PCNF-coated inorganic nanoparticles, such as SnSb [100, 101], TiO2 [102, 103], RuO2 [104, 105], Si [106, 107], P [108] or Sn [109, 110], exhibit good long-term cyclic stability and high rate capability in energy storage devices, because the PCNFs derived from PAN/PMMA can effectively buffer the large volume changes during the cyclic process and thereby improve the conductivity.

Yu et al. [109] conducted electrospinning of PMMA–PAN–tin octoate in DMF solution. By heating the resultant PAN-coated tin octoate in air at 250 °C, they obtained PAN-coated SnO2 nanoparticles. The carbonization of these PAN-coated SnO2 nanoparticles under an Ar/H2 atmosphere afforded multichannel PCNFs with Sn nanoparticles (Fig. 5). The as-fabricated multichannel PCNFs with Sn nanoparticles, when used as the anode material for LIBs, exhibited a reversible discharge capacity as high as 648 mAh g−1 even after 140 cycles. With PLLA [111, 112] and F127 [113] as the sacrificial polymer templates, PCNFs/Si nanoparticles could be prepared as the composite anode material for LIBs.

Figure 5
figure 5

a Cross-sectional images and b, c TEM micrographs of Sn encapsulated in porous multichannel carbon nanofibers (SPMCTs); d cyclability of SPMCTs and commercial Sn nanopowders with similar sizes (~200 nm) at a cycling rate of 0.5 C (current density 100 mA g−1). Note the inset displays the discharge capacity of the SPMCTs electrodes as a function of the discharge rate at 2 and 10 C. Images reprinted with permission from Ref. [109]. Copyright 2009, American Chemical Society

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In a similarly way, Qin et al. [88] added Fe3O4 nanoparticles to a PAN/PS solution for electrospinning and obtained Fe3O4/PCNFs by calcination. The porous carbon framework was able not only to buffer the volume expansion of Fe3O4 and to facilitate transfer of electrons and ions, but also to accelerate the formation of a stable SEI film outside the composite, thereby preventing the electrolyte from contacting the Fe3O4 directly. The as-fabricated Fe3O4/PCNFs exhibited a reversible capacity of 596 mAh g−1 at 2.0 A g−1 and a slow fade in capacity after 200 cycles of charge/discharge (541 mAh g−1).

It is widely accepted that using PCNF as the shell can prevent the volumetric expansion of metal (oxide) during the charge/discharge processes, and it can improve the electrical conductivity (as the channels of PCNFs allow for rapid transport of ions and electrons). More importantly, the pore sizes and structures of PCNFs can be easily controlled by adjusting the compositions and proportions of the polymer blends. Therefore, it is important to screen suitable solvents and formulate stable solutions of the polymer blends for electrospinning as a means to fabricate PCNFs with excellent electrochemical performance.

In the process of electrospinning with a polymer blend, one polymer is used as the template inside another polymer to form holes after heating. The sacrificial polymer acts as the pore-making template to form pores in the NFs. Meanwhile, the pore formation can be adjusted by manipulating the polymer compatibility in the polymer blend [79].

Hard template method based on electrospinning

Nanoparticles can be used as the hard templates to prepare porous carbon materials. The inorganic nanoparticles, added as hard templates in the polymer precursors for electrospinning, can be removed by corrosive acids or alkalis to afford uniform pores in the NFs. The pore diameter can be controlled by adjusting the size of the nanoparticles.

Table 3 lists a series of PCNFs prepared by hard template electrospinning, in which metals or metal oxides, such as SiO2 [86, 114,115,116,117], Sn [118, 119], ZnO [120, 121], Zn [122], Fe [123] or Ni [114, 124, 125], are presented as the materials most commonly used as hard templates for preparing electrospun PCNFs.

Table 3 A series of PCNFs prepared via the hard template method
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Ji et al. [115] directly added SiO2 nanoparticles into a PAN/DMF solution for electrospinning. They conducted carbonization of the electrospun PAN/SiO2 composite NFs in a nitrogen atmosphere, followed by immersion in an HF acidic solution to remove the SiO2. The resultant PCNFs had an increased specific surface area and an improved capacity as anode material for LIBs. Nan et al. [116] used tetraethoxysilane (TEOS) and polyamic acid (PAA) to fabricate electrospun NFs. In the process of carbonization, the TEOS decomposed to form SiO2, which was homogeneously dispersed in the carbon NFs. After etching with HF acid, the SiO2 nanoparticles were eliminated, and a PCNF fabric with a large specific surface area (950 m2 g−1) and a reversible capacity of 445 mAh g−1 (at the 50th cycle under 50 mA g−1) was obtained.

Some metal nanoparticles can act not only as the hard template, but also as the catalyst to promote the graphitization of PCNFs. Zhang et al. [123] prepared PAN fibers containing numerous Fe particles by electrospinning. The carbonization of the resultant Fe-doped PAN fibers in N2 atmosphere and the etching of the carbonized PAN fibers with an HNO3 solution afforded PCNFs in which the Fe nanoparticles not only served as a sacrificial phase for the production of pores, but also acted as the catalyst to promote the graphitization of the carbon NFs. Moreover, etching with HNO3 could introduce oxygenated groups that acted as the active sites for Li-ion storage. As a result, the as-prepared PCNFs exhibited an exceptionally high capacity of 983 mAh g−1 at a current density of 100 mA g−1.

Chen et al. [125] conducted coaxial electrospinning of a PAN/nickel acetate (Ni(Ac)2)/PMMA composite solution and obtained carbon nanotube/carbon nanofiber hybrid material in which Ni(Ac)2 was decomposed to form Ni nanoparticles and PMMA was decomposed to produce pores during calcination (Fig. 6). After activation, the final carbon nanotube/carbon nanofiber hybrid material exhibits a high specific surface area (1840 m2 g−1). This characteristic was achieved because the Ni nanoparticles could be removed to produce pores, and the Ni could act as a catalyst to enhance the growth of carbon nanotubes. When the as-prepared hybrid material was applied as the anode for LIBs, it exhibited a reversible capacity of ~1150 mAh g−1 after 70 cycles at a current density of 0.1 A g−1, and a capacity retention of more than 80% after 3500 cycles of charge/discharge at a current density of 8 A g−1.

Figure 6
figure 6

a Schematic diagram showing the design of an activated hollow carbon nanotube/carbon nanofiber hybrid material and b its charge–discharge voltage profiles as the anode for LIBs. Images reprinted with permission from Ref. [125]. Copyright 2013, American Chemical Society

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Certain varieties of inorganic salts such as sodium dodecyl sulfate (SDS) [126], CaCO3 [127] or NaHCO3 [128] can also be used as hard templates for preparing PCNFs by electrospinning. Nagamine et al. [126] fabricated PCNFs from a PVA solution containing diammonium hydrogen phosphate (DAP) and SDS by electrospinning. During stabilization and carbonization, the DAP and SDS were decomposed to form salt particles, which served as the templates, and these templates could be removed by leaching with water to create spherical pores.

Similarly, some organics can act as the templates to prepare PCNFs, and in some way, these templates can work better than inorganic salts, as organics can be easily removed during carbonization. Using g-C3N4 nanosheets as both the sacrificial template and the N-doping source, Liang et al. [129] prepared N-enriched mesoporous carbon nanofibers (NMCNFs). After the g-C3N4 nanosheets were totally removed during calcination to leave mesopores, the resultant NMCNFs film served as the electrode for an SC which exhibited a high specific capacitance of 220 F g−1 at a current density of 0.2 A g−1.

It should be emphasized that any metals or metal oxides used as the templates for preparing porous carbon fibers via the electrospinning need to be removed with acids. However, the inorganic salts and organics used as the sacrificial templates can be easily removed with water or by calcination. Therefore, using inorganic salts and organics as the sacrificial templates enable better environmental acceptance, cost-effectiveness and greater prospects of application than using metals and metal oxides as the templates.

In addition to PCNFs, porous composite NFs can also be prepared through the hard template method. For example, silicon- and tin-based materials are considered promising candidates as anode materials for LIBs, because they have a high theoretical capacity. However, these materials undergo large changes in volume during charge/discharge processes, which result in the pulverization of the particles and electrical disconnection with the current collector, causing poor cyclability. Therefore, it is important to combine the Si- and Sn-based materials with PCNFs that are prepared by electrospinning and contain massive pores, because the porous PCNFs can provide void spaces to cushion the volume variation of the anode materials during the cyclic process.

In this respect, the studies conducted by several groups of researchers from China are worth special attention. To describe a few of these studies, Zhou et al. [130] calcined Si nanoparticles in air and obtained a layer of SiO x on the surface of Si. They added the as-obtained Si@SiO x nanoparticles into a PAN/DMF solution for electrospinning and finally obtained Si@PCNFs after carbonization in association with HF etching. The as-prepared Si@PCNFs, when used as the anode material for LIBs, maintained a reversible capacity of 1104 mAh g−1 after 100 cycles at 0.5 A g−1, thereby showing excellent cycling performance.

Wang et al. [131] formulated a mixed solution of PVP, SnCl2 and TEOS in DMF and obtained hybrid NFs by electrospinning. The resultant NFs were annealed in air at 350 °C to create SiO x and SnO2, and the PVP was slightly carbonized. After the SiO x was etched with an HF solution, SnO2/C porous NFs were obtained. SnO2/PCNFs and Sn/PCNFs were finally obtained after SnO2/C porous NFs were calcined in an N2 atmosphere at 500 and 800 °C, respectively. Due to the small size of the SnO2 nanoparticles, the as-prepared SnO2/PCNFs exhibited a high reversible lithium storage capacity at a low current density. The Sn/PCNFs also exhibited good cycle performance at a high current density, due to the good conductivity of Sn.

Zou et al. [132] used ice as the hard template and prepared PCNFs with Sn/SnO x nanoparticles encapsulated in their pores as LIB anode materials. As the void spaces around the Sn/SnO2 nanoparticles were able to buffer the volumetric changes in Sn during cyclic process, the as-prepared PCNFs-Sn/SnO x exhibited a specific capacity of 510 mAh g−1 after 40 cycles of charge/discharge, at a current density of 30 mA g−1.

Zhou et al. [133] fabricated PCNFs with PVP as the carbon source, with F127 and SiO2 as the sacrificial templates. By immersing the PCNF substrate into a neutral KMnO4 solution, they obtained freestanding MnO2/PCNFs with an ultrahigh surface area (1814 m g−1). As a flexible electrode in an SC, the MnO2/PCNFs exhibited a high specific capacitance of 520 F g−1 at 0.5 A g−1 and showed superior cycling stability (92.3% of the capacitance was maintained after 4000 cycles of charge/discharge).

McCormac et al. [134] prepared porous Si/TiO2 composite NFs for LIBs by a sulfur-templating method. The porous TiO2 NFs acted as the lithium storage materials, and these NFs could therefore efficiently compensate for the expansions in volume of Si nanoparticles during the charge/discharge processes. The porous Si/TiO2 NFs exhibited an initial capacity of 839 mAh g−1 and a capacity retention of 50% after 180 cycles of charge/discharge (the specific capacity of Si NFs was reduced to 17 mAh g−1 after 100 cycles of charge/discharge).

It is generally recognized that in porous carbon hybrid NFs, PCNFs can provide a sufficient void space to buffer the volumetric expansion of the anode materials and thereby prevent the pulverization of the electrodes. Also, the hierarchical porous structure of the PCNFs facilitates the conductivity of the electrolyte and the lithium-ion diffusion. The size and morphology of the pores can be handily controlled by selecting the proper nanoparticles as the templates. Unfortunately, the hard template method is expensive and environmentally unfriendly, because it requires the use of acid and involves post-treatment.

Solvent- and nonsolvent-induced methods based on electrospinning

Solvent-induced method based on electrospinning

The solvent-induced method is another method that is applicable for preparing porous fibers, and it allows rapid phase separation in the volatile solvents during the electrospinning process. The solvent-rich phase is transformed into pores, and the polymer-rich phase is solidified into the matrix through flash evaporation of the solvent, resulting in the formation of pores on the fiber surfaces [135,136,137,138,139,140,141]. The porous structure of the resultant NFs can be adjusted by choosing different volatile solvents.

Lin et al. [136] dissolved PS in a THF/DMF mixed solvent and formulated an electrospinning precursor solution. By varying the weight ratio of THF/DMF, they obtained final fibers with differing microporous morphologies (Fig. 7). Li et al. [138] systematically studied the influence of various solvents (dichloromethane (DCM), acetone, chloroform, THF, ethyl acetate and DMF) on the structure of electrospun PMMA fibers. They found that dense cycloidal nanopores were distributed on the surface of fibers obtained from PMMA/DCM and PMMA/ethyl acetate solutions, but elliptical nanopores formed on the fibers produced from PMMA/chloroform and PMMA/DMF/DCM solutions. Huang et al. [142] dissolved PAN in a mixed DMF/chloroform solvent for electrospinning. They then collected the electrospun fibers with a glass dish filled with ice water and obtained PCNFs after carbonization. The as-prepared PCNFs, when used as the anode material for an LSB, delivered a high reversible discharge capacity of 340 mAh g−1 after 100 cycles.

Figure 7
figure 7

Cross-sectional FE-SEM images of PS fibers electrospun from 20 wt% PS solutions with different weight ratios of THF/DMF: a 4/0, b 3/1, c 2/2, d 1/3 and e 0/4. Images reprinted with permission from Ref. [136]. Copyright 2010, American Chemical Society

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Nonsolvent-induced method based on electrospinning

The construction of electrospun NFs is affected by various parameters, one of them being ambient humidity, which not only affects the spinnability of the precursor but also influences the hierarchical structure of the NFs [33, 34]. Casper et al. [141] used PS/THF solution for electrospinning and found that dense pores were gradually formed on the surface of the fibers with the increase in ambient humidity. These researchers suggested that the formation of such pores could be explained by the concept of breath figures. In other words, as the solvent evaporated, the surface of the fibers cooled, and water from the air condensed on the surface of the fibers. After the liquid was evaporated completely, and the water droplets were eliminated, leaving uniformly distributed dense pores.

Kongkhlang et al. [143] prepared polyoxymethylene (POM) NFs using a hexafluoroisopropanol (HFIP)-based solvent. The as-obtained POM NFs had a nanoporous structure at 55 and 75% relative humidity. These researchers suggested that the formation of the nanopores might be based on vapor-induced phase separation. In the electrospinning process, the HFIP rapidly evaporated, and the moisture (acting as a nonsolvent) penetrated into the fiber, resulting in phase separation. Finally, the solvent-rich phase formed the pores, and the polymer-rich phase formed the matrix.

Fashandi et al. [144] prepared PS/DMF and PS/THF solutions for electrospinning at different levels of humidity. They found that the vapor-induced phase separation gave the final fibers a porous structure, and these pores formed inside the fiber when a low volatility solvent (DMF) was used. Unlike in the above-mentioned studies, the pores in this case tended to develop on the fiber surfaces when a highly volatile solvent (THF) was used.

Water, as a nonsolvent, can also be directly added into the polymer solution to fabricate porous NFs. Yu et al. [145] prepared porous fibers by electrospinning a PAN/DMF/H2O solution. Here, water acted as the nonsolvent to induce phase separation and to generate a porous structure. The specific surface area and the diameter of the porous fibers expanded with increases in water content and in polymer concentration.

Shen et al. [146] applied mineral oil as both the nonsolvent and the template to fabricate Sn nanoparticle-loaded PCNFs by electrospinning. When used as the anode material for an LIB, the Sn nanoparticle-loaded PCNFs exhibited a higher discharge capacity (774 mAh g−1 over 200 cycles, at a high current density of 0.8 A g−1) than the Sn-CNF composite prepared without mineral oil.

The solvent-induced phase separation method for fabricating porous electrospun fibers is facile and cost-effective, because it does not require additive or post-treatment. By varying the composition of the solvent mixtures or importing nonsolvents, we can obtain porous NFs with different morphologies. But this method is less powerful in that the pores always form on the fiber surfaces due to the flash vaporization of the solvent.

Activation method based on electrospinning

The activation method is traditionally used for preparing porous carbon materials, and this approach involves either a physical or chemical activation method. After activation, the obtained electrospun carbon NFs present an abundance of micropores and a large specific surface area, which is favorable for transport of electrons and ions. Table 4 lists a series of activated electrospun PCNFs that have been applied in electrochemical energy storage devices.

Table 4 A series of PCNFs obtained via activation
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Physical activation based on electrospinning

The physical activation method normally uses oxidizing gases such as steam [84, 105, 147,148,149,150,151,152,154], CO2 [155,156,157] or air [158,159,160] to react with carbon. The disordered carbon is oxidation-etched to generate pores and develop microporous structures within the carbon materials. The activation efficiency of disordered carbon is relatively low, because it relies on the oxidation of carbon atoms to form a porous structure, and the temperature of carbonization and activation is very high (generally above 800 °C) [151, 153, 156,157,158].

Steam is often used to activate carbon NFs. Kim and Yang [147] used PAN as the carbon source and fabricated NFs by electrospinning in association with carbonization and activation by 30 vol% steam under a nitrogen flow. The as-obtained PCNFs, after being activated at 700 °C, provided a highest specific surface area of 1230 m2 g−1, and a maximum specific capacitance of 173 F g−1 when used as the electrode for an SC at low current density (10 mA g−1). This result was mainly due to the high volume fraction of micropores. However, at the high current density of 1000 mA g−1, the PCNFs activated at 800 °C exhibited a maximum specific capacitance of 120 F g−1, because of the enhanced volume fraction of the mesopores. This outcome could be explained by the tendency of ions in the mesopores to respond faster at a higher current density than the ions in the micropores, which would lead to a higher specific capacitance.

In addition to the water in steam, the oxygen in air can serve as a simple activator. Li et al. [159] introduced air into the Ar flow during the carbonization process for preparing highly porous carbon NFs. The as-obtained porous carbon NFs were directly used as the anode material for an LIB, which delivered a reversible capacity of 1780 mAh g−1 after 40 cycles at 50 mA g−1. When applied in LABs [157], the CO2-actived carbon nanofibers (ACNF) network provided a novel pathway for O2 across the cathode, and the mesopores introduced by activation acted as additional nucleation sites for Li2O2 formation, thereby leading to an increased discharge capacity (Fig. 8). After discharge, the ACNF cathode showed a layer-like morphology, and the nonactivated CNF showed a rugged surface. This outcome reflected the influence of numerous mesopores, which allowed Li2O2 to be homogeneously adsorbed onto the surface of the ACNF, while the CNF showed a relatively smooth surface that allowed easy migration of Li2O2 on the surface, thereby producing poorer cycling stability.

Figure 8
figure 8

a Schematic illustration of a CO2-activated carbon nanofibers network used in LABs. The SEM images are of CNF cathodes (b, e), ACNF cathodes (c, f) and BP 2000 composite cathodes (d, g) before and after discharge. Images reprinted with permission from Ref. [156]. Copyright 2016, American Chemical Society

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The oxidizing gases can react not only with carbon atoms, but they can also act as nitrogen sources to produce nitrogen-doped carbon materials. Nan et al. [153] prepared fiber networks by electrospinning a PAN/melamine solution. The electrospun fiber networks were carbonized in nitrogen and activated by ammonia to obtain a nitrogen-enriched porous carbon nanofibers (NPCNFs) networks with a specific surface area of 1198 m2 g−1, which was higher than that of steam-activated NPCNFs (1056 m2 g−1). When directly used as an LIB anode without a binder, the NPCNFs exhibited a reversible charge capacity of 1323 mAh g−1 in the first cycle and 1150 mAh g−1 in the 50th cycle (50 mA g−1).

Chemical activation based on electrospinning

The chemical activation method generally uses chemicals such as KOH [125, 161] and ZnCl2 [162] as the activators. These activators need to be removed by washing with acid or water. Ma et al. [161] fabricated phenolic-based activated carbon NFs paper by KOH activation. First, the as-spun phenolic fibers were immersed in KOH solutions of various concentrations for KOH coating. Then, carbon NFs paper was obtained by a one-step heat treatment. The treatment with KOH not only increased the porosity of the carbon NFs, but it also stimulated the surface oxygen functional groups to act as electrochemically active species. As a result, the as-obtained material, when used as the flexible electrode for an SC exhibited a high specific capacitance (362 F g−1).

The chemical activator had a strong capability for etching the carbon precursor, and it could bring about uniform, abundant micropores, thereby leading to a high specific surface area and promoting the surface oxygen-containing functionality. As a result, the large specific surface area and the increased conductivity attributed to high graphitization effectively facilitated the electrochemical storage performance of the electrospun fibers [163].

Conclusions and challenges

The electrospinning technique is a low-cost and scalable method for producing 1D continuous NFs, and it can be adopted to fabricate 1D porous NFs for electrochemical energy storage devices. The as-fabricated porous NFs exhibit large specific surface areas, and they allow rapid transport of electrons and ions. In addition, the void spaces in the porous NFs can effectively buffer the expansion of volume during the process of charging and discharging the electrode material. There are several methods for preparing 1D porous materials based on electrospinning, including the polymer template method, the hard (nanoparticles) template method, the solvent-induced and nonsolvent-induced methods, and the activation method. All of these methods can be applied to fabricate both PCNFs and composite NFs. The polymer template method, however, is only applicable for fabricating porous metal oxide NFs that are confined in 1D polymer fibers after removing the polymer template. By properly combining these methods with calcination and post-treatment, one can obtain highly porous NFs with high specific surface areas and unique porous structures. The as-obtained porous NFs, when used as the anode materials for LIBs, LABs, LSBs, NIBs or SCs, exhibit excellent electrochemical performance. However, some challenges still remain in developing 1D porous fibers and their composite structures:

  1. 1.

    It is important to develop PCNFs films with a naturally hierarchical porous structure, as this type of structure is needed to fabricate electrodes for energy storage devices without a binder. It is also important to broaden the range of porous PCNFs films prepared by electrospinning to oil–water separation, catalysis and tissue engineering.

  2. 2.

    It is imperative to further explore the potential for combining electrospinning techniques with other methods such as calcining or activation, as a means to acquire novel 1D porous fibers and their composite structures with significantly improved electrochemical performances.

  3. 3.

    More in-depth researches are necessary to realize better control in the preparation of porous NFs, thereby promoting the greater application of electrospun porous NFs in engineering.