Application of Electrospun Fibers in Supercapacitors

Xiangye Li; Tianjiao Bai; Xin Weng; Bing Zhang; Zhenzhen Wang; Tieshi He

doi:10.7536/PC200718

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1159-1174 DOI: 10.7536/PC200718

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Application of Electrospun Fibers in Supercapacitors

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Abstract

The increasing demand for high-performance supercapacitor has promoted the rapid development of separators and electrode materials. Recently, electrospun nanofibers have been widely used as separators and electrode materials of supercapacitor, which is due to the high porosity, high electrochemical activity, large specific surface area and good structural stability. In this survey, the recent research progress in separator membranes and electrode materials of supercapacitor is reviewed. The discussion focuses on obtaining electrode materials for supercapacitor by electrospinning and other post-processing methods, including carbon nanofibers, carbon-based composite nanofibers, conductive polymer-based composite and metal oxide nanofibers. These investigation demonstrate that pore structure construction, activation treatment, and heteroatom doping can improve the specific surface area, electrochemical activity, wettability, and graphitization degree of carbon nanofibers, furthermore the electrochemical properties of electrode materials are enhanced. Moreover, combining carbon nanofibers with metal oxides, conductive polymers via blending, chemical deposition, electrochemical deposition, etc., can also improve capacitance, rate performance, and cycling stabilities of electrode materials. In addition, the existing problems of the regarding studies are pointed out. Finally, the future developments of electrospun nanofiber materials in supercapacitor is prospected.

Contents

1 Introduction

2 Separators

3 Electrode materials

3.1 Electrode materials of electric double layer capacitors

3.2 Electrode materials of pseudocapacitors

4 Conclusion and outlook

Graphical abstract

Keywords

electrospinning / supercapacitor / nanofiber / separator / electrode

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Xiangye Li, Tianjiao Bai, Xin Weng, Bing Zhang, Zhenzhen Wang, Tieshi He. Application of Electrospun Fibers in Supercapacitors. Progress in Chemistry, 2021, 33(7): 1159-1174 DOI:10.7536/PC200718

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1 Introduction

With the global energy crisis and the increasing environmental pollution caused by the excessive use of fossil fuels, the demand for clean and sustainable energy is increasing day by day, and effective energy storage and conversion devices such as solar cells, fuel cells, supercapacitors and lithium-ion batteries have attracted much attention in recent years^[1⇓⇓~4]. In many forms of energy storage and conversion systems, supercapacitors have the advantages of fast charging and discharging, high power density and long cycle stability, which are widely used in smart grid, electric vehicles, electronic communications, transportation and other fields, and become one of the most promising energy storage devices^{[5⇓⇓⇓~9]}. According to the different mechanisms of charge storage, supercapacitors are generally divided into two categories. One is the electric double layer capacitor represented by carbon materials. The appropriate pore structure and effective surface area of the material determine the specific capacitance. The rapid physical adsorption/desorption of ions on the electrode and electrolyte makes the capacitor show good cycle stability and rate characteristics^[10]. The other is pseudocapacitors represented by metal oxides and conductive polymers, which realize energy storage based on reversible electron exchange redox reaction, ion intercalation/deintercalation and potential deposition between electrolyte and metal oxides or conductive polymers^[11]. Pseudocapacitors can react both on the surface and in the interior of the electrode, and there will also be the formation of electric double layers, while electric double layer capacitors can only store charges on the surface of the electrode in contact with the electrolyte, so pseudocapacitors have higher energy density and specific capacitance under the same electrode area^{[12⇓⇓⇓⇓⇓⇓~19]}. However, in the process of energy storage, the pseudocapacitor is easy to cause volume expansion, which eventually leads to lower power density and cycle stability^{[19⇓⇓~22]}. Electrodes, electrolytes and separators are the key materials affecting the electrochemical performance of supercapacitors. The electrode affects the specific capacitance of the capacitor, and the separator affects the diffusion of electrolyte ions, thus affecting the specific power of the capacitor^[23,24]. Therefore, the preparation of supercapacitors with high power, high energy density and high capacitance requires not only suitable electrolytes, but also excellent separators and electrode materials. Hydrothermal method, sol-gel method, chemical vapor deposition method and electrospinning method are all methods to obtain nano/micro fiber structured separators and electrode materials^{[23,25⇓⇓~28]}. Among them, the nanofibers prepared by electrospinning have unique advantages such as high aspect ratio, large specific surface area and simple process, which have become the focus and focus of current research, and are likely to synthesize high-performance separators and electrode materials for supercapacitors.

It is well known that electrospun porous nano/micro structured electrode membrane materials have the characteristics of high porosity and fast ion migration. In the preparation of polymer-ionic liquid gel electrolyte, the polymer network is required to have high liquid holdup, suitable pore size and developed pore structure^[29]. Almost all of the commonly used electrode separator materials are non-porous or microporous nanofibers, but the ionic liquid electrolyte can not be charged and discharged at high current because of its large molecular volume, high viscosity, and the inability of ions to diffuse rapidly and fully infiltrate the separator and electrode^[30]. However, the nanofiber separator with meso/macroporous structure prepared by electrospinning has higher porosity and liquid holdup, which can significantly improve the electrochemical performance of supercapacitors^[31].

As the key component of supercapacitors, the specific surface area, electrochemical activity and stability of electrode materials directly affect the capacity, rate capability and cycle stability of supercapacitors, and determine their practical application prospects^[32]. Interfacial polymerization, chemical vapor deposition, sol-gel method, hydrothermal method and other methods for preparing nanofiber active electrodes have the problems of complex operation, low yield, high cost and difficult to avoid the agglomeration of nanoparticles^[33]^[34]^[28]^[35]. The most important thing is that when preparing the active material, it is necessary to grind it into powder and add carbon black and binder to enhance the overall conductivity of the material and the binding force with the current collector^[32]. Commonly used polymer binders are mainly fluoropolymers, rubbers and polyolefins, which do not provide charge storage and charge transport, but only play a role in electrode formation. Additionally, that bind reduces the energy storage density of the electrochemical energy storage device, increase its internal resistance, and reduces its cycle life. In recent years, the successful application of binderless carbon nanofibers (CNFs) based on polymer electrospun fibers as self-supporting electrode materials not only avoids the above problems and simplifies the manufacturing process,Moreover, due to its high surface area, high electrochemical activity and good structural stability, it shows high capacitance, high rate capability, high energy density and excellent cycle stability in supercapacitors. At present, especially in the field of supercapacitors, with the in-depth study of more and more researchers, the electrode materials with binderless self-supporting structure based on electrospun fibers have been more and more widely used.

2 Preparation of Membrane Materials by Electrospinning

Electrode separator is one of the key materials of supercapacitors, which is located between the positive and negative electrodes to avoid physical contact between the positive and negative electrodes and prevent electrons from passing through the supercapacitor, while ensuring the free passage of electrolyte ions. The performance of electrode separator has a direct impact on the electrochemical performance and safety of supercapacitors, so excellent separator materials are one of the keys to obtain high-performance supercapacitors. Most of the membrane materials prepared by traditional methods have the problems of complex operation, low porosity, poor ion permeability, high cost and uneven pore size distribution, which limit their application in the field of high-power energy storage^[36]. Generally, the following aspects should be considered in the selection of separator materials for supercapacitors: high porosity and liquid absorption and retention properties; Sufficient chemical and electrochemical stability; Sufficient mechanical strength and isolation performance; While taking into account production costs and applications^[37]. In view of the above factors, electrospinning is undoubtedly the best choice for the preparation of supercapacitor separator materials. First, electrospinning can continuously prepare nanofiber membranes having a nano/micro diameter. Secondly, the nanofiber film has higher porosity and uniform pore size distribution by adjusting and controlling the composition of the precursor, process parameters and post-treatment process, thereby ensuring that the film has higher liquid holding capacity and better ion permeability. In addition, the impedance of the energy storage device is reduced, and the safety and power performance of the supercapacitor are improved.

Polymers such as polyacrylonitrile (PAN), polyethylene (PE), polymethyl methacrylate (PMMA), polyimide (PI), polyvinylidene fluoride (PVDF) and cellulose are often used to prepare electrospun fibrous membranes^{[23,24,38⇓~40]}. Our group has reported a method to prepare polymer nanofiber membranes using polyvinylpyrrolidone (PVP) as a template^[41]. Hydrophobic PVDF and hydrophilic PVP powder are dissolved in a mixed solution of N, N '-dimethylformamide and acetone, and the PVDF/PVP composite electrospun fiber is obtained by electrospinning the mixed precursor solution. Due to the phase separation caused by the inaffinity between PVDF and PVP, PVP microdomains are formed in the composite electrospun fiber, and after the PVP microdomains are filtered out in water, the PVD-based ultrafine porous fiber membrane with a three-dimensional network structure and high porosity is obtained. The higher the PVP content in the PVDF/PVP electrospun fiber, the larger the pore size of the membrane. However, when the mass ratio of PVDF/PVP is more than 2 ∶ 1, due to the higher PVP content in the precursor solution, there will be a partial phase inversion, resulting in the formation of connected macropores and supermacropores. By comparing the electrospun composite fibers with different mass ratios of PVDF to PVP, it was found that the optimal mass ratio of PVDF to PVP was 2 ∶ 1. It has a high electrolyte absorption (360 wt%) and a high specific surface area (111.5 m²/g). In addition, PAN-based ultrafine porous fibrous membranes with network structure and high porosity were successfully prepared by our group^[42]. The PVP/PAN composite nanofibers were obtained by electrospinning the mixed spinning precursors of PVP and PAN with different ratios, and the PAN-based porous nanofiber membranes were obtained by aqueous phase separation based on the solubility difference between PVP and PAN in water. It was found that the optimal mass ratio of PAN-PVP was 2 ∶ 1, and the membrane had low resistance (0. 82 Ω), high specific surface area (355.2 m²/g) and high liquid holdup (400 wt%). Jabbarnia et al. Added PVP and different amounts of carbon black nanoparticles into PVDF solution, and obtained PVDF/PVP-based nanofiber membranes with different mass fractions of carbon black nanoparticles after electrospinning^[43]. The addition of PVP in the polymer solution not only solves the conductivity limitation caused by the high crystallinity of PVDF, but also increases the electrospinnability of the polymer solution. The results showed that the PVDF/PVP nanofibers combined with 4 wt% carbon black had better physical and chemical properties, and the dielectric constant of the membrane was increased from 1. 59 to 3. 97 compared with the pure PVDF/PVP fiber membrane. In addition, the wettability and thermal stability of the separator material are improved. Peng et al. Prepared a low-cost, environmentally friendly and degradable PAN @ SDBS (sodium dodecyl benzene sulfonate) nanofiber membrane as a separator material for supercapacitors by coaxial electrospinning^[24]. The resistance of the PAN @ SDBS nanofiber membrane with 10 wt% SDBS is as low as 0.45Ω. The results show that the separator can inhibit the self-discharge behavior of the supercapacitor without sacrificing the inherent capacitance of the supercapacitor, which is due to the negative charge on the electrolyte surface and the inhibition of ion diffusion driven by concentration gradient. The nanofiber membrane material prepared by the method has good practical application value in the field of supercapacitors, and opens up a new method for the research of self-discharge inhibition of supercapacitors. Nanofiber membranes prepared by electrospinning provide the possibility for the preparation of high performance supercapacitors.

3 Preparation of electrode materials by electrospinning

Electrode materials usually play a key role in supercapacitors, which determine the main performance parameters of supercapacitors, and are also an important entry point for improving and optimizing the electrochemical performance of supercapacitors^[22]. At present, the research on supercapacitors is mainly focused on electrode materials, especially the electrode materials with high specific capacity, high power density and long cycle life. Electrospinning provides a simple, green and effective technology for the preparation of unbonded self-supporting electrodes. On the one hand, it can be combined with carbon-based materials, metal oxides, conductive polymers and other excellent conductive materials to give full play to their respective advantages. Meanwhile, the pore structure and the specific surface area of the electrode material are controlled by combining pre-oxidation, high-temperature carbonization, activation, acid/alkali etching and the like. On the other hand, binderless electrode materials with various structures, such as hollow, porous or nanobelts, can be constructed by controlling the composition of precursors, process parameters and post-treatment processes^[32].

3.1 Electrospun Carbon Nanotubes as Electrodes for Electric Double Layer Capacitor

The electrospun CNFs not only have high specific surface area and abundant pore structure, which can improve the transmission power of electrons and ions and improve the adsorption and desorption capacity of the CNFs to the electrolyte, but also are easy to form a flexible film with a self-supporting structure, thereby avoiding the defects of increased interface resistance and reduced mass transfer efficiency caused by the use of a binder^[44]. CNFs are generally obtained by carbonizing carbon-rich polymer nanofibers at high temperature in an inert atmosphere. They are ideal electrode materials for electric double layer capacitors because of their controllable fiber diameter, specific surface area, surface chemical properties and graphitization degree. The specific surface area, conductivity and pore structure of electrospun CNFs affect the specific capacitance, rate capability and cycle stability of electric double layer capacitors, so the composition and structure of electrospun CNFs need to be adjusted to improve their electrochemical performance. The conductivity of electrospun CNFs can be improved by increasing the carbonization temperature, and the high specific surface area and pore structure can be achieved by activation, templating and heteroatom incorporation^[45⇓~47].

3.1.1 Porous CNFs

Many methods have been established to prepare polymer-based porous CNFs. Among them, the introduction of pore-forming templates into polymers is a common method to obtain rich porous structures. As a pore-forming template for porous CNFs electrode materials, the following conditions are often required: (1) the template exists uniformly and stably in the spinning precursor solution, which is the basis for the formation of uniform CNFs; (2) The template can be embedded into the electrospun fiber, which is an important prerequisite for the formation of porous structure; (3) The template can be pyrolyzed during the heat treatment of the electrospun fibers, or removed by etching under acid/alkali conditions after the electrospun fibers are cured and formed, which is a necessary condition for the formation of porous CNFs. At present, the synthesis of porous CNFs electrode materials by template method is very active.

Poly (ethylene glycol) (PEG), PMMA, PVP, etc. Are typical soft pore-forming templates because of their low carbon production and easy decomposition during high temperature carbonization^{[30,48⇓⇓~51]}. They are often blended with carbon-producing polymers such as PANs and PIs to prepare porous CNFs^[52⇓~54]. Using PVP as a soft template, our group has prepared flexible self-supporting binderless polyamic acid (PAA) -based porous CNFs by electrospinning PAA/PVP mixed precursor solution, thermally induced phase separation and high temperature carbonization^[55]. In the carbonization process, PAA with high carbonization rate was converted into carbon skeleton, and PVP with low carbonization rate was used as porogen. The pore size of CNFs can be controlled by adjusting the PVP content.When the mass ratio of PAA to PVP is 5 ∶ 2, CNFs have well-balanced pore structure, higher specific surface area (743.5 m²/g), larger pore volume (0.126 cm³/g) and suitable average pore diameter (about 3. 1 nm). The high proportion of mesoporous/macroporous structure in CNFs is conducive to fast ion transport, high energy storage density and the formation of electric double layers.The electrode material used as supercapacitor electrode material shows high specific capacity (210 F/G), high power density (2021 W/kg) and excellent cycle stability (98. 9%). Using the same template, we also prepared PAN-based porous CNFs as electrode materials for supercapacitors^[42]. On the one hand, the PAN-based porous nanofiber membrane is obtained through water phase separation by utilizing the solubility difference of PVP and PAN in water; on the other hand, the flexible porous PAN-based CNFs are obtained through high-temperature carbonization by utilizing the carbonization rate difference of PVP and PAN. The content of PVP determines the pore size, specific surface area, porosity and ionic liquid holdup of the separator and electrode materials, and then determines the electrochemical performance of supercapacitors. The results showed that when the mass ratio of PAN-PVP was 2 ∶ 1, the nanofibers showed higher specific surface area (355.2 m²/g) and larger pore volume (0.134 cm³/g), which was the best condition to obtain the membrane and electrode materials. A supercapacitor was assembled by using PAN-derived porous nanofibers as separator and electrode, and ionic liquid as electrolyte. Electrochemical tests showed that the electrode and separator exhibited high specific capacity (248. 6 F/G), low internal resistance (0. 82 Ω) and excellent charge-discharge efficiency (99. 8%) in supercapacitor. Le et al. Obtained self-supporting binderless PI-based porous CNFs by blending polymer PI/PVP precursor solution and heat treatment^[51]. The specific surface area of CNFs increased with the increase of PVP content. When the mass ratio of PVP to PI precursor was 1 ∶ 1, the specific surface area of CNFs reached the maximum of 802 m²/g, which was 1. 3 times of that of CNFs without PVP. The synergistic effect of the pore size after heat treatment, the chemical properties of the material surface and the wettability of electrolyte make the porous CNFs have good electrochemical performance. When the current density is 0.2 A/G, the specific capacitance of the electrode material is 215 F/G, and the power density is 20.0 kW/kg. After 1000 charge-discharge cycles, the specific capacitance can still maintain 92% of the initial value. In addition, Wang et al obtained porous CNFs by electrospinning a mixed precursor solution of PAN-polysulfone (PSF) and carbonizing it at high temperature^[56]. PSF is used as a template, and the mesopore content, specific surface area, graphitization degree, connection between fibers and conductivity of CNFs are controlled by adjusting the content of PSF. The scanning electron microscope of PAN/PSF composite nanofibers with different ratios and their CNFs is shown in Figure 1. The obtained CNFs with different ratios were used as electrode materials for supercapacitors, and the CNFs derived from 20% PAN-PSF exhibited the best performance, with a specific capacitance of 289 F/G at a scan rate of 10 mV/s and 257 F/G at a current density of 0. 25 A/G. In addition, it has good cycle stability (100% capacity retention after 6000 cycles) and large energy capacity (36 Wh/kg).

The porous structure of CNFs can be controlled by carbonizing other polymer blend fibers such as PAN/PMMA and PAN/cellulose acetate (CA) in addition to PAN/PVP and PI/PVP blend fibers^[30,51,57]^{[50,58⇓~60]}.

Compared with the soft template, the hard template has the advantages of easy control of pore size, clear morphology, no chemical reaction between the template and the carbon structure, and the adjustment of fiber diameter^[61]. ZnCl₂, CaCO₃, Co(NO₃)₂ and SiO₂ are commonly used as hard templates for electrospinning CNFs. Jiang et al. Used ZnCl₂ as template to electrospin the precursor solution of PAN/ZnCl₂, and obtained three-dimensional cross-linked PAN-based porous CNFs after pre-oxidation, carbonization and acidification^[62]. The ZnCl₂ not only acts as a template to increase the specific surface area of the nanofibers, but also acts as a cross-linking agent to simultaneously interconnect the individual nanofibers. Due to the synergistic effect of the enlarged specific surface area and the cross-linked structure, the obtained CNFs not only have more charge accumulation, but also can rapidly transfer and diffuse electrons and ions at the interface of the electrolyte and the electrode. The specific surface area of the acidified CNFs is as high as 520 m²/g. When the acidified CNFs are used as electrode materials for electric double layer capacitors, they exhibit a high specific capacitance of 214 F/G at a current density of 1.0 A/G, and a capacitance retention of 97.3% of the initial value after 60 000 charge-discharge cycles, indicating that they are ideal electrode materials for electric double layer capacitors. Zhang et al. introduced CaCO₃ nanoparticles into a polymer precursor solution in a two-component solvent (DMF and tetrahydrofuran (THF)), and obtained one-dimensional layered porous CNFs after electrospinning, heat treatment, and hydrochloric acid acidification. The preparation process and characterization are shown in Figure 2^[63]. The binary solvent is used for the electrospinning precursor solution, which is favorable for uniformly dispersing the CaCO₃ in the composite nanofiber.

During the high temperature carbonization process of the electrospun fibers, the nano-CaCO₃ thermally decompose and release CO₂ to form micropores and mesopores on the surface of the fibers, and the CaO nanoparticles are removed by acidification with hydrochloric acid to form macropores. The electrode material combines the advantages of hierarchical porosity and high specific surface area, and shows high specific capacitance (251 F/G) at a current density of 0.5 F/G when used as an electrode material for a supercapacitor, and also has good rate capability, showing a specific capacitance of 160 F/G at a current density of 20 A/G. Gopalakrishnan et al. prepared a PAN-based CNFs binderless high-performance electrode material based on surface porous bubbling by using SiO₂ as a template^[64]. The removal of template SiO₂ by high concentration of sodium hydroxide solution caused the formation of bubbles on the surface of CNFs, which provided abundant microporous/mesoporous structure for CNFs and increased their specific surface area from 15.34 m²/g to 593 m²/g. The specific capacitance of the supercapacitor electrode was 287 F/G at the current density of 0. 4 A/G.

The results show that the electrochemical performance of the electrode material made of pure polymer CNFs is not ideal because of its small specific surface area and almost no pore structure, so it is necessary to optimize its structure. Among them, the CNFs with porous structure obtained by template method have high specific surface area and abundant pore structure, which greatly enhance the adsorption and storage capacity of electrode materials for charges in electrolytes and improve the ion migration rate, thus improving the specific capacitance and rate performance of supercapacitors. In addition, the voids in the porous electrospun CNFs can effectively buffer the change of the internal structure of the electrode material during the charge and discharge process, thereby having high cycle stability.

3.1.2 Activated CNFs

Activation is also an effective method to improve the electrochemical performance of electrospun CNFs. Continuous chemical reactions occur during the heat treatment of electrospun fibers by physical or chemical means, and porous structures are formed on the surface of CNFs. CNFs are generally activated during heat treatment with an oxidizing gas (water vapor, carbon dioxide, air, etc.) Or a chemical activator (sodium hydroxide, potassium hydroxide, phosphoric acid, etc.)^{[65⇓⇓⇓⇓⇓⇓~72]}. CNFs are activated by a chemical activating agent, and the essence is that after the CNFs and the chemical activating agent are mixed and impregnated, the chemical agent is embedded in the internal structure of the nanofiber, and a porous structure is generated by heat treatment under the protection of inert gas. Among the chemical activators, KOH is the most widely used to activate CNFs. After the nanofibers were impregnated with KOH, micropores were formed at first, and then the micropores gradually changed into mesopores and macropores at high temperature above 700 ℃^[73⇓~75]. Water vapor and CO₂ can be activated to obtain porous CNFs, because oxidizing gases such as water vapor and CO₂ react thermochemically with carbon atoms in CNFs during heat treatment^[76,77]. In general, the activation of water vapor and CO₂ often requires higher temperature and longer time, but too high temperature and too long time will cause the collapse of the pore structure. In addition, water vapor and CO₂ react rapidly at the entrance of the pores and are difficult to diffuse into the pores at too high a temperature. Compared with the two activation methods, the activation degree of potassium hydroxide is stronger than that of water vapor and CO₂, and it is easy to produce meso/macroporous structure without too high temperature and too long time, but it also has the problems of high cost, equipment corrosion and activator residue.

Jayawickramage et al obtained PAN-based active CNFs with rich porous structure by electrospinning lignin/PAN mixed precursors with different ratios, pre-oxidation, carbonization and CO₂ activation^[78]. During carbonization at high temperature, part of lignin was decomposed, which contributed to the formation of high specific surface area and porous structure of CNFs. The results showed that when the mass ratio of PAN/lignin was 7 ∶ 3 and the current density was 1 A/G, the electrode showed higher specific capacitance and energy density, which were 128 F/G and 59 Wh/kg, respectively. Ma et al. Successfully prepared phenolic resin binderless flexible porous activated carbon nanofiber paper (PCNP) with high specific surface area on the surface of electrospun thermosetting phenolic resin nanofiber paper (PNP) after high temperature chemical etching and acid treatment^[69]. The larger the mass ratio of KOH/PNP is, the more significant the chemical etching effect is. When the mass ratio of KOH/PNP is more than 1:5, the flexibility of PCNP can not be maintained and PCNP can not be used as a binderless material. Compared with the traditional activation method, the amount of KOH is much less, the activation effect is good, and the porosity and specific surface area are large. The amount of micropores and mesopores of PCNP increased significantly after activation, and its specific surface area and pore volume were up to 1007 m²/g and 0.363 cm³/g, respectively. In addition, the content of oxygen-containing functional groups on the surface of the electrode is increased, so that the wettability of the electrode is enhanced. The prepared PCNP was directly used as the electrode material of supercapacitor, and the electrode had high specific capacitance and high rate capability. The maximum specific capacitance was 288 F/G at the current density of 0.2 A/G, and 204 F/G at the current density of 20 A/G. Shi et al. used industrial hemp straw and polyacrylonitrile as raw materials to obtain composite nanofibers with different proportions by electrospinning method, and obtained biomass-based PAN-based porous CNFs after pre-oxidation, carbonization, KOH activation and hydrochloric acid acidification^[79]. The specific surface area of CNFs with a mass ratio of 9 ∶ 1 was up to 2348.7 m²/g, which was much higher than that of traditional PAN-based CNFs (298.3 m²/g). In addition, the biomass-based porous CNFs had abundant mesoporous/macroporous structures, and the total pore volume was up to 1.14 m³/g, which was much higher than that of pure PAN-based CNFs (0.14 m³/g). When the composite CNFs were used as electrode materials for supercapacitors, the specific capacitance of the electrode was as high as 244. 8 F/G at a current density of 1 A/G, and the specific capacitance remained 96% of the initial value after 3 000 charge-discharge cycles at a current density of 2 A/G. Liu et al. Prepared hollow CNFs with layered pores by activating PVP/polysilazane (PSN) composite electrospun fibers with NaOH^[80]. By adjusting the carbonization temperature, porous hollow CNFs with different specific surface areas and total pore volumes can be obtained, which are used as electrode materials for supercapacitors. It is found that when the carbonization temperature is 800 ℃, the specific surface area of the obtained CNFs is the highest, which is 2628.10 m²/g, and the maximum pore volume is 2.32 cm³/g, so it shows the best supercapacitor performance, and its electrochemical performance is shown in Figure 3. It also showed a high specific capacity of 259. 86 F/G and excellent cycle stability at the current density of 1 A/G, with a specific capacity retention of 95. 3% even after 10 000 charge-discharge cycles.

CNFs with different degrees of porous structure can be obtained by activation. After activation, not only the specific surface area and pore structure of CNFs are greatly improved, but also the surface functional groups are increased to enhance the wettability of CNFs. Compared with the unactivated CNFs, the activated CNFs are not only beneficial to the adsorption/desorption of electrolyte ions, but also beneficial to the rapid diffusion of ions and electrons on the surface of the porous structure, and are respectively used as an electrode material of a supercapacitor, and electrochemical tests show that the specific capacitance, rate capability and cycle stability of the activated CNFs are remarkably improved.

3.1.3 Heteroatom-doped CNFs

Doping CNFs with heteroatoms (N, O, B, S, etc.) Is also an effective way to significantly improve the electrode performance^[32]. The surface functionalization of heteroatoms on the electrode can not only change the surface polarity and electron distribution of carbon, affect the wettability and conductivity of the constructed electrode materials, but also act as active sites for oxidation/reduction reactions and provide pseudocapacitance^[81]^[82]. In general, CNFs doped with nitrogen atoms are the most common. The main reasons are as follows: First, the radius of nitrogen atoms is close to that of carbon atoms, so it is relatively easy to replace carbon atoms in the lattice during the activation process, which can significantly improve the conductivity of CNFs^[83⇓~85]. Secondly, the nitrogen atoms in CNFs can form hydrogen bonds in the aqueous electrolyte, which helps to improve the wettability of CNFs with the electrolyte. Finally, the nitrogen atoms on the surface of CNFs can also act as active sites for oxidation/reduction reactions and form electron donors. Nitrogen-doped CNFs generally have four bond types: pyridine nitrogen (N-6), pyrrole nitrogen (N-5), quaternary nitrogen (N-4) and oxidized nitrogen. Nitrogen-doped CNFs with different bond types play different roles in the energy storage process of CNFs. Negatively charged pyrrole nitrogen and pyridine nitrogen have redox activity and can contribute to higher capacitance. The positively charged quaternary nitrogen (N-4) and oxidized N can promote the electron transport in the carbon lattice^{[86⇓⇓~89]}. There are two ways to dope CNFs with nitrogen atoms. One is by high temperature treatment in a gaseous atmosphere, such as thermal annealing in a NH₃^[90]. The other is to introduce a nitrogen source into the electrospinning solution, such as melamine, urea, polyaniline, cyanamide, etc. Pyrolysis of these solids can produce active species for nitrogen atom doping, which can be easily controlled by the treatment temperature^{[91⇓⇓⇓~95]}^[96]. In addition, PAN and PVP are also typical nitrogen-rich polymers, which can be used to prepare N-doped CNFs by electrospinning^[97].

Jiang et al. Obtained PVP-based porous CNFs from PVP and graphite carbonitride (g-C₃N₄) by electrospinning, pre-oxidation and carbonization^[96]. Graphite carbonitride (g-C₃N₄) was pyrolyzed to heat treat its CNFs. The pyrolysis products of the g-C₃N₄ contain a high concentration of active nitrogen species, which can be easily and efficiently incorporated into the CNFs. CNFs obtained at 800 ℃ showed the highest nitrogen content, and the nitrogen content in CNFs was adjusted from 6. 37 at% to 13. 72 at%. Due to the synergistic effect of the contribution of the pyridine nitrogen (N-6) state to the capacitance and the contribution of the quaternary nitrogen (N-4) state to the conductivity, the electrode material has ideal chemical properties.

Electrochemical measurements showed that the electrode exhibited a high specific capacitance of 265 F/G at a current density of 1 A/G, which was about 10 times higher than that of pure PVP-based CNFs. After 5 000 charge-discharge cycles, the capacitance retention of the electrode exceeded 100% due to the activation of the pseudocapacitance process. Li et al. Successfully prepared CNFs electrode materials with nitrogen-doped and porous structure by electrospinning different proportions of PAN/PVP mixed precursors through pre-oxidation, carbonization and activation^[98]. PAN was used as carbon precursor and PVP was used as nitrogen-increasing precursor and pore-forming agent. The specific surface area and porosity of the porous CNFs can be easily controlled by adjusting the mass ratio of PVP to PAN. When the mass ratio of PAN-PVP was 2 ∶ 3, the specific surface area was 452.5 m²/g and the nitrogen content was 4. 70%, which was the best condition for the preparation of CNFs. The specific capacitance of the electrode was 148 F/G at the current density of 1 A/G, which was about 6 times higher than that of pure PVP-based CNFs. After 5000 cycles, the specific capacitance was 104. 8% of the initial value. Zhang et al. introduced different mass of graphene into the lignin/PAN mixed electrospinning precursor solution, and successfully prepared active CNFs with three-dimensional network structure modified by N and S co-doped graphene after electrospinning, pre-oxidation, carbonization and activation. The synthesis steps and characterization results are shown in Figure 4^[99]. PAN is used as a nitrogen source and a spinning aid to improve its plasticity and introduce nitrogen because of its excellent flexibility and fiber forming ability. As a sulfur/nitrogen fixing agent, graphene can successfully capture HCN, NH₃ and SO₂ released from lignin and PANs during carbonization, and trap the gases in the fibers, thus increasing the content of N and S heteroatoms in the active CNFs. When the mass fraction of graphene is 0. 30 wt%, the prepared active CNFs have the largest specific surface area (2439 m²/g) and the highest heteroatom content (N: 11. 9 wt%, S: 0. 702 wt%).When the active CNFs are used as electrode materials for supercapacitors, the specific capacitance of the electrode is as high as 267. 3 F/G at current density of 1 A/G, and the capacitance retention rate is 96. 7% of the initial value after 5000 charge-discharge cycles. In addition, it has higher energy density and power density, which are 9.28 Wh/kg and 493 W/kg respectively. Tian et al. Successfully prepared nitrogen-rich porous CNFs by electrospinning, carbonization and NH₃ using water-soluble phenolic resin/polyvinyl alcohol blend solution as precursor^[100]. After NH₃ treatment, the CNFs have a high specific surface area and are 763 m²/g, and the surface nitrogen and oxygen contents are 5. 26 at% and 4. 37 at%, respectively. When CNFs were used as electrode materials for supercapacitors, the specific capacitance of the electrode reached 251. 2 F/G at the current density of 0. 1 A/G, which was about 25% higher than that of CNFs treated with argon, and the capacity retention rate was as high as 99% after 2000 charge-discharge cycles, showing excellent cycle stability. In addition, the specific capacitance is still 193 F/G at the current density of 20 A/G, with a high rate capability of 77%. The analysis shows that the appropriate pore size distribution, effective high surface area and the synergistic effect of a certain amount of nitrogen-containing and oxygen-containing functional groups make the electrode material show excellent electrochemical performance.

The results show that CNFs can be effectively doped with nitrogen atoms by introducing nitrogen source into the spinning precursor solution, pyrolyzing at high temperature during carbonization, or introducing NH₃ during the carbonization of nanofibers, and all of them have a certain pore-forming function. In general, the introduction of nitrogen source into the spinning precursor solution can disperse the doping elements uniformly in the fiber. The carbonization process introduces the method of NH₃, and the doping elements penetrate into the fiber from the outside to the inside. By adjusting the surface chemistry and electronic structure of CNFs, nitrogen doping can not only improve the surface wettability of the electrode, make the electrolyte fully contact with the surface of CNFs, and promote the rapid adsorption and desorption of ions; It can also introduce additional pseudocapacitance characteristics, in which pyridine nitrogen (N-6) and pyrrole nitrogen (N-5) with redox activity can contribute to higher pseudocapacitance. In addition, the lone pair electrons in the nitrogen atom can also increase the conductivity of the electrode material. By comparing the CNFs doped with nitrogen atoms with the CNFs without doping nitrogen atoms, it is found that the electrochemical performance of CNFs doped with nitrogen atoms is significantly improved, especially the specific capacitance and cycle stability.

3.1.4 CNFs/carbon-based composite electrode

Carbon nanomaterials, such as carbon nanotubes (CNT), graphene and derived carbon, have been proved to be common electrode materials for supercapacitors because of their low cost, large specific surface area, high conductivity, easy absorption by electrolyte and good cycle stability^[101~106]. However, in addition to the limited contribution of specific surface capacitance, carbon-based materials also have some shortcomings, such as low packing density and unsatisfactory volume performance^[107]. The combination of carbon-based materials and electrospun CNFs can greatly improve their electrochemical properties by changing the microstructure of the fibers, increasing the active components and improving the degree of graphitization, which is generally achieved by electrospinning polymer precursors containing carbon nanomaterials and carbonizing them at high temperature^[32].

Deng et al. Prepared multi-walled carbon nanotubes (MWNT)/CA-based active CNFs by electrospinning, deacetylation and high temperature carbonization^[108]. It was found that the specific surface area of active CNFs increased from 870 m²/g to 1120 m²/g when 6 wt% MWNT was added. The specific capacitances of pure CA/CNFs and 6 wt% MWNT/CNFs were 105 F/G and 145 F/G, respectively, when the current density was 10 A/G. The results show that the increase of specific capacitance of MWNT/CNFs is due to the change of MWNT/CNFs structure, which leads to the increase of specific surface area and conductivity. Dong et al sprayed graphite oxide during the preparation of PAN nanofibers, and obtained CNFs/graphene porous composite electrode materials after pre-oxidation and carbonization^[109]. PAN-based CNFs act as a framework to maintain graphene, which helps to prevent the agglomeration of graphene and provide high conductivity.Their porous structure and the synergistic effect of graphene and CNFs show better electrochemical performance than pure PAN-based CNFs. When the current density was 1 A/G, the specific capacitance was 183 F/G, which was 1. 6 times of that of pure PAN-based CNFs, and the specific capacitance could still reach about 92% of the initial value after 4 500 charge-discharge cycles. Guo et al. prepared porous CNFs/C composites by electrospinning, preoxidation and high temperature carbonization after introducing low carbonization polymer PVA into acidified coal dispersion^[110]. Coal is the main component of the carbon component, providing a carbon fiber skeleton structure and good electrical conductivity. PVA is not only a sacrificial template for carbon production, but also enhances the spinnability of the electrospinning precursor solution. It was found that the porosity and specific surface area of CNFs could be controlled by changing the mass ratio of PVA/coal in the precursor dispersion. When the mass ratio of PVA/coal is 1 ∶ 1, the composite has abundant pore structure and large specific surface area. When it is used as the electrode material of supercapacitor, the electrode shows a specific capacitance of 170 F/G at a current density of 1 A/G, and the specific capacitance is almost unchanged after 20 000 charge-discharge cycles.

Carbon nanomaterials have the advantages of good conductivity, large surface area and good cycle stability. However, when it is used as the electrode material of supercapacitor, its contribution to capacitance is limited, its packing density is low, its volume performance is not ideal, and the agglomeration of particles can not be avoided.The most important thing is that the binder is required to enhance the binding force with the current collector, which will reduce the energy storage density of the electrochemical energy storage device, increase its internal resistance and reduce its cycle life. The electrospun CNFs/carbon-based binderless composite electrode material can be obtained by introducing the carbon-based material into the nanofibers on the premise of ensuring the continuity of the nanofibers by utilizing good compatibility of the spinning solution and sufficient gaps among the nanofibers. In addition, the preparation of CNFs/carbon nanomaterials has the characteristics of environmental friendliness, controllable composition, simple operation and low cost, so the composite electrode has broad application prospects in the field of supercapacitors.

3.2 Preparation of pseudocapacitor electrode materials by electrospinning nanofiber

In a pseudocapacitance supercapacitor, electrical energy is stored by charge transfer between the electrode and the electrolyte, usually through a rapid, reversible oxidation/reduction reaction at the electrode. In the past decades, metal oxides and conducting polymers have been extensively studied, and the specific capacitance of these two types of electrode materials often exceeds that of carbon materials using double-layer charge storage. The key to increase the specific capacitance of the pseudocapacitor is to improve the utilization efficiency of the active material during the redox reaction charge/discharge process, which can be easily achieved by electrospinning technology^[111].

3.2.1 CNFs/metal oxide composite electrode

Although most transition metal oxides have high theoretical specific capacitance, their low conductivity and unstable structure limit their application in energy storage. In addition, metal oxides generally have the characteristics of agglomeration tendency and poor cycle performance^[112]. In order to avoid the above problems, a nanofiber composite electrode is obtained by combining a metal oxide and a polymer through electrostatic spinning, which not only has high redox activity, structural stability and conductivity,It can also construct a multi-level structure to promote the rapid transmission of electrons and ions, so that the specific capacitance, cycle stability, charge-discharge efficiency, power density, energy density and rate performance of the electrode material are greatly improved^[112~114]. At present, the main metal compounds studied are MnO₂, RuO₂, V₂O₅, NiO, Co₃O₄, Fe₂O₃ and their multi-component metal oxides^[115~120].

RuO₂ is one of the best transition metal oxide electrode materials, because its theoretical specific capacitance is more than 700 F/G, but its low abundance, high cost and low yield limit its wide application^[121~124]. An et al. Introduced RuO₂ into PAN spinning solution, and synthesized surface modified RuO₂/CNFs composites by electrospinning, carbonization and acid treatment^[125]. RuO₂ nanoparticles with the size of 2 ~ 3 nm were uniformly embedded in the surface-modified CNFs. After acid treatment, the number of oxygen-containing functional groups (— COOH) on the surface of CNFs increased by 10. 9%.When the CNFs were used as electrode materials for supercapacitors, the electrode exhibited a high specific capacitance of 224. 6 F/G at current density of 0. 2 A/G, and the capacitance retention was 90% of the initial value after 3 000 charge-discharge cycles. In addition, it has good rate capability, with a specific capacitance of 179. 2 F/G at a current density of 10 A/G. It is believed that the excellent electrochemical performance of CNFs is due to the synergistic effect of the uniform distribution of RuO₂ nanoparticles in CNFs and the improvement of wettability due to the increase of oxygen-containing functional groups on the surface of CNFs. Jun et al. And Kim et al. Also prepared a layered porous activated RuO₂/CNFs composite nanofiber electrode by a similar method, which also showed good electrochemical performance^[126]^[127].

V₂O₅ is particularly suitable as an electrode material for supercapacitors due to its ease of preparation, low cost, multiple oxidation States, and high power and energy density^[128~130]. Choudhury et al successfully prepared flexible V₂O₅/CNFs nanocomposite paper by in-situ hydrothermal deposition of V₂O₅ on the surface of PAN-based CNFs^[117]. CNFs have good electrical conductivity and fibrous network structure, which can be used as an effective matrix for metal oxides to deposit V₂O₅ on their surfaces.An efficient nanocomposite material with high specific surface area of metal oxide can be obtained, which not only has high redox activity, structural stability and conductivity, but also can construct a multi-level structure to promote the rapid transmission of electrons and ions. It was found that the specific surface area of the V₂O₅/CNFs nanocomposite paper was as high as 573.65 m²/g after in-situ hydrothermal deposition of V₂O₅. The specific capacitance of the electrode is 227 F/G at the current density of 1 A/G, and the specific capacitance retention rate is 80% of the initial value after 1000 charge-discharge cycles. Li et al. Prepared a highly flexible porous V₂O₅/PAN composite electrode based on electrospun CNFs by electrospinning combined with solvothermal method^[131]. The composite electrode exhibited high energy density (22. 3 Wh/kg), high power density (1 500 W/kg) and good cycling stability (specific capacitance decreased by 10. 7% after 10 000 cycles) when used as the electrode material of supercapacitor.

MnO₂ is one of the most promising transition metal oxides for pseudocapacitor electrode materials due to its high theoretical specific capacitance of 1370 F/G, low cost and environmental friendliness^[22,132,133]. Electrospun CNFs loaded with MnO₂ is a common way to prepare its composite electrodes.Guo et al. obtained its composite nanofibers by electrospinning PVA/lignin mixed precursor solution, and then put the composite nanofibers in KMnO₄ solution for 72 H, dried, preoxidized and carbonized to achieve the controllable preparation of flexible porous electrospun CNFs loaded with MnO₂^[134]. In the process of high temperature carbonization, polyvinyl alcohol provides a framework, a carbon source and a pore-forming agent for the composite electrode material, and the polyvinyl alcohol is used as a matrix of the composite electrode material loaded with the MnO₂. The results show that after carbonization at 1000 ℃, the specific surface area of the composite fiber is 385.1 m²/g, the pore volume is 0.12 cm³/g, and the content of Mn element is as high as 19. 8%. When the current density was 0. 3 A/G, the specific capacitance of the electrode was 131. 28 F/G, the energy density was 14. 7 Wh/kg, and the power density was 135. 0 W/kg. In addition to the loading method, blending is also a common way to prepare MnO₂/CNFs composite electrodes. Jeong et al. prepared a flexible porous highly conductive MnO₂/CNFs composite electrode material by adding low-carbon polymer PMMA to the mixed precursor solution of PAN/MnCl₂, electrospinning and heat treatment^[135]. PMMA enhances the conductivity and porosity of the MnO₂/CNFs composite, and improves its specific capacitance and rate capability. When the mass ratio of PAN/PMMA is 9∶1,MnCl₂ and the content of PAN/PMMA is 3 wt%, the specific surface area of the composite is up to 717 m²/g and the conductivity is up to 13.5 S/cm. The specific capacitance of the electrode is 183 F/G when the current density is 1 mA/cm², and the capacitance retention rate is 84% when the current density is increased from 1 mA/cm² to 20 mA/cm², showing high rate capability. In addition, it has a high power density and energy density of 10 000 W/kg and 25 Wh/kg, respectively. Youe et al. prepared a MnO₂/CNFs composite material by a method similar to that described above, which also showed excellent electrochemical properties^[136].

NiO and Co₃O₄ are potential active electrode materials because of their high specific capacitance^[137,138]. Vidhyadharan et al introduced NiAc into the aqueous solution of PVA to form a spinning precursor, and obtained the NiO/C composite electrode after electrospinning and heat treatment^[118]. During the carbonization process, the thermal decomposition of the polyvinyl alcohol provides a carbon skeleton and a pore-forming agent for the composite electrode material. In addition, the composite electrode with the one-dimensional porous structure has high crystallinity, and the improvement of the crystallinity can improve the cycle stability and the coulombic efficiency. As the electrode material of supercapacitor, the specific capacitance of the electrode is as high as 670 F/G and the coulombic efficiency is as high as 98% at the current density of 1 A/G. After 1000 charge-discharge cycles, the capacitance is almost unchanged. For Co₃O₄, Kebabsa et al. Impregnated electrospun PVA nanofibers into CoAc solution, dried after impregnation, and obtained Co₃O₄/PVA composite electrodes with high porosity and self-supporting structure after heat treatment^[139]. The results show that the hybrid nanofibers exhibit highly porous microstructure and enhanced crystallinity due to the catalytic graphitization of the Co₃O₄ nanoparticles, which results in the excellent electrochemical properties of the hybrid nanofibers. The specific capacitance of 137 F/G was obtained at the current density of 1 A/G, which was three times higher than that of pure PVA-based CNFs, and the capacitance retention was 91% of the initial value after 5 000 charge-discharge cycles. Abouali et al. added a Co source to the electrospinning precursor solution, and the Co₃O₄ nanoparticles were uniformly encapsulated in the continuous PAN-based CNFs by electrospinning and high temperature carbonization^[140]. The optimal content of Co₃O₄ nanoparticles in the final electrospun CNFs was determined by changing the mass ratio of CoAc to PAN. The electron microscopy and electrochemical properties of the final electrospun CNFs are shown in Figure 5. The formation of an onion-like graphite layer around the Co₃O₄ nanoparticles not only improves the conductivity of the electrode, but also prevents the nanoparticles from separating from the carbon matrix. Electrochemical tests showed that the best electrochemical performance was obtained when the mass ratio of PAN-CoAc was 4 ∶ 3. At the current density of 1 A/G, the electrode material showed a very high specific capacitance (586 F/G). It is believed that the high specific capacitance of the electrode material is mainly due to the uniform dispersion of the Co₃O₄ in the CNFs matrix and the synergistic effect of the pseudocapacitance provided by the Co₃O₄ and the electric double layer capacitance provided by the CNFs.

The electrochemical performance of metal oxide composite CNFs electrode materials is not only related to the morphology and structure, but also affected by the chemical composition^[97]. Compared with single-component metal oxides, multi-component metal oxides, such as NiCo₂O₄, CuCo₂O₄, ZnCo₂O₄ and MnCo₂O₄, have attracted much attention due to their higher theoretical specific capacitance, more abundant redox active sites and more stable structure^[141~146]. Yu et al. Successfully prepared ZnCo₂O₄/C composite porous nanofiber electrode material by electrospinning PVP/ZnAc/CoAc mixed precursor solution, followed by pre-oxidation and high temperature carbonization. The average diameter of the material is 300 ~ 500 nm, the average pore size is 3.83 nm, and the specific surface area is 199.0 m²/g^[147]. PVP was used as a sacrificial template to produce carbon during the heat treatment, and the porous structure was formed after pyrolysis, and the ZnCo₂O₄ provided the framework structure of the nanofibers. The electrochemical performance of the electrode was compared with that of the ZnCo₂O₄ powder electrode prepared by sol-gel method. The specific capacitance of the ZnCo₂O₄/C composite nanofibers was 327. 5 F/G and 90. 6 F/G, which was 3. 5 times higher than that of the ZnCo₂O₄ powder at the current density of 0. 5 A/g.After 1000 charge-discharge cycles, the ZnCo₂O₄/C nanofibers and the ZnCo₂O₄ powder retained 125% and 91% of their initial specific capacitance, respectively. In addition, the ZnCo₂O₄/C nanofiber electrode has a smaller resistance of 0.622Ω, while the ZnCo₂O₄ powder electrode has a resistance of 0.757Ω. The above results indicate that the performance of ZnCo₂O₄/C composite nanofibers as a pseudocapacitor electrode is much better than that of ZnCo₂O₄ powder electrode, mainly because the fiber morphology of the composite electrode can effectively inhibit the destruction of the microstructure of ZnCo₂O₄ nanoparticles during charge and discharge cycles.

It is believed that the electron transport rate and electrochemical reaction kinetics rate of metal oxides are often slow during charge and discharge, which makes the power performance of electrode materials poor. In addition, in the process of energy storage, it is easy to cause volume expansion, resulting in poor cycle stability. In order to improve the above problems, the most commonly used method is to compound with polymer CNFs. Metal oxide/CNFs not only make up for the shortcomings of single material to some extent, but also retain the advantages of each component. For example, metal oxide has high redox activity, which can provide high pseudocapacitance. Polymer CNFs have good electrical conductivity and nanoscale network structure, which can provide stable electric double layer capacitance in addition to being an effective matrix for metal oxides. However, there are still some problems in their composite electrodes. Firstly, the high cost of most metal oxides limits their wide application in supercapacitors. Therefore, the preparation of metal oxides with good electrochemical performance and low price is also the key to research. Secondly, although the metal oxide/CNFs composite electrode is helpful to improve its conductivity, it can not completely solve the phenomenon of lattice volume expansion and contraction during charge and discharge.

3.2.2 CNFs/conductive polymer composite electrode

Conducting polymers, such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTH), have the advantages of high charge density, low cost, high theoretical specific capacitance and low internal resistance. Although conducting polymers have high theoretical specific capacitance, their poor cycle stability limits their practical application in supercapacitors^[148]. The electrochemical activity, cycle stability and conductivity of composite electrode materials based on conductive polymers are related to the inherent properties of conductive polymers and the synergistic effect of each component. At present, the research mainly focuses on the composite of conductive polymer and polymer CNFs, which can be obtained by in-situ polymerization, electrochemical deposition, vapor deposition, aqueous deposition and so on combined with electrospinning^[22]. In the preparation process, electrospun nanofibers (polymers or CNFs) are used as a carrier, and a conductive polymer is coated on the surface of the electrospun fibers by appropriate polymerization to form a nanostructured surface, which brings a high specific surface area to the composite fiber electrode.

Lectrically conductive polymer are generally not elastic enough to be spun into fibers, so this present a significant obstacle to electrospinning. Blending PANI with other high molecular weight carrier polymers will increase the viscosity of the polymer and enhance the spinnability^[149,150]. Bhattacharya et al. applied secondary doping to electrospun fibers for the first time, and prepared PANI/polyethylene oxide (PEO) conductive nanofibers with multiple branches to promote electron conduction by electrospinning and blending low-volatility and high-volatility mixed precursors, with a conductivity of 1.73 S/cm^[150]. The specific capacitance of the PANI/PEO electrospun fiber was as high as 312.1 F/G at a current density of 0.1 A/G, which was the highest value reported so far. Silas et al. Prepared supercapacitor electrodes based on high-purity polyaniline nanofibers by electrospinning technology^[151]. The ultrahigh molecular weight PEO is blended with PANI, so that the electrospun fiber has enough chain entanglement, and the preparation of the electrospun nanofiber with high PANI content (93 wt%) is possible. In order to further improve the conductivity and stability of the electrode, a small amount of CNTs was added to the PANI/PEO precursor solution before electrospinning to generate PANI/CNT/PEO nanofibers. The electrochemical performance is shown in Figure 6. The PANI-CNT-based electrode obtained a high specific capacitance of 385 F/G at a current density of 0.5 A/G, and the specific capacitance retention rate of the PANI-CNT electrode was 81.4% after 1000 charge-discharge cycles.

It is believed that the good electrochemical performance of the electrode is due to the interconnected nanostructure of the non-woven fabric, the interconnected nanofiber network promotes efficient electron conduction, and the porosity between and within the fibers makes the electrolyte have good permeability in the polymer matrix, so that ions can be quickly transported to the active site. In addition, the conductive polymer can be deposited on the surface or inside of the polymer nanofiber by in-situ polymerization to prepare the PANI-based binderless electrode. Sun et al. introduced the PAN-based nanofibers obtained by electrospinning into the solution containing PANI, and obtained the PAN @ PANI composite electrode after preoxidation and high temperature carbonization by means of electrospinning combined with in-situ polymerization^[152]. It has a high surface area, fast electron/ion transport dynamics, and good mechanical flexibility. Meanwhile, PANI is also an ideal nitrogen-rich precursor, and in-situ nitrogen doping by PANI pyrolysis can improve the surface wettability and induce additional pseudocapacitance to increase the effective specific capacitance. It was found that the electrode material with PANI monomer volume of 1 mL had the best performance, which had a capacitance of 275 F/G at a current density of 0. 5 A/G and a capacity retention of 95. 3% after 10 000 charge-discharge cycles at a current density of 2 A/G. In addition, it also shows high energy density and power density, which are 8. 9 Wh/kg and 2700 W/kg, respectively. In addition to blending and in-situ polymerization, electrochemical deposition is also a common method to prepare conductive polymer electrodes. Simotwo et al successfully prepared a flexible porous CNFs/PANI composite binderless electrode material by loading PANI on the surface of CNFs through electrochemical deposition to form a core-shell structure^[153]. The π-π interaction between PANI and carbon on the porous CNFs promotes the rapid charge transfer at the PANI/carbon interface. CNFs with high specific surface area (2218 m²/g) contribute to higher electric double layer, and PANI supported on porous CNFs provides higher pseudocapacitance.Under the synergistic effect of both, the assembled supercapacitor has a specific capacitance of 320 F/G at a scan rate of 20 mV/s. When the current density is 2 A/G, the capacity retention is 75% of the initial capacity after 4500 charge-discharge cycles. In addition, Chen et al. Also prepared CNFs @ PPy @ rGO-based binderless high-performance electrode materials by the same method^[154].

PANI, PPy and PTH are a kind of conductive polymers with high specific capacitance, but their structures tend to swell and shrink during charging and discharging, which makes them unstable during cycling. It is found that the capacity retention of pure conductive polymer electrode materials is only 70% after 1000 cycles^[155~157]. In addition, the agglomerates of the conductive polymer have a relatively tightly packed structure, which prevents the penetration of electrolyte ions. The polymer CNFs is used as an effective carrier of the conductive polymer, and the conductive polymer is loaded on the surface of the electrospun nanofiber by a proper method to show the following advantages: (1) the interfacial bonding strength of the conductive polymer and the polymer CNFs is improved, and the interfacial electron transmission performance of an electrode material is effectively improved, thereby improving the rate capability; (2) the agglomeration among the conductive polymers is avoided, and the contact area between the electrode material and the electrolyte is increased; (3) the volume effect of the conductive polymer in the charging and discharging process is effectively inhibited, the structural stability of the composite electrode material is obviously improved, and the service life of the material is prolonged; (4) the problems of insufficient elasticity, poor fiber-forming ability and difficult morphological structure control of the conductive polymer are also solved; And (5) not only that preparation proces of the electrode is simplified, but also the addition of the bind is avoided, thereby improving the energy storage density of the whole electrode. Therefore, the combination of conductive polymer and polymer CNFs provides infinite possibilities for the preparation of high-performance supercapacitor electrode materials.

4 Conclusion and prospect

Electrospinning is widely used as separator and electrode materials for supercapacitors because of its unique advantages such as simple operation, good continuity and strong controllability. In the application of separator, electrospinning technology can prepare high-strength nanofiber non-woven membrane with high porosity and high specific surface area, which greatly reduces the resistance of energy storage devices. In the application of electrode materials, researchers have successfully prepared binderless electrode materials such as electrospun carbon-based nanofibers, metal oxide-based nanofibers, conductive polymer-based nanofibers and their composites, and the morphology and structure of the materials are complex and diverse, including solid fibers, hollow fibers, porous fibers, nanobelts, nanotubes and so on. At present, the optimization and improvement of electrospun fiber electrode materials mainly focus on the construction of porous structure, heteroatom doping and composite with other electrode materials. Polymer based CNFs with porous structure can significantly improve the transport dynamics of electrons and ions, and improve the adsorption and desorption ability of polymer based CNFs to electrolyte. Heteroatom-doped polymer-based CNFs can not only change the surface polarity and electron distribution of CNFs, thereby improving the wettability and conductivity of the material, but also introduce additional pseudocapacitance characteristics to increase the specific capacitance of the constructed electrode material due to the formation of nitrogen groups between the doped components and carbon atoms in CNFs. Polymer CNFs are compounded with carbon-based materials, metal oxides, conductive polymers and other electrode materials to achieve complementary advantages, so that the overall electrochemical performance of electrode materials is improved. Although great progress has been made in related research. However, many challenges remain. The main problems are as follows: (1) The preparation of nanofibers by electrospinning has not been industrialized so far. In addition to the equipment, the structure of electrode materials is difficult to control accurately. (2) There is still a certain gap between the electrochemical and mechanical properties of electrospun fiber electrode materials and the actual application requirements. (3) The electrochemical performance of most electrospun fiber electrode materials is significantly improved, but the energy density is low, which limits their wide application in supercapacitors.

In order to obtain better separator and electrode materials, the following points should be considered in the future research: (1) to obtain the relevant equipment suitable for large-scale preparation, constantly explore the appropriate production conditions, optimize the relevant process, and realize the continuous, stable and mass production of high-performance nanofibers; (2) to further study the relevant principles, explore the appropriate parameters and conditions, and combine with spinning experience to ensure the stability of the jet and the precise control of the nanostructure as far as possible; (3) Continuous development and utilization of new materials, which can be compounded with MOFs (metal-organic framework compounds), MXene (metal carbon/nitride with two-dimensional layered structure), COFs (covalent organic framework materials), metal nitrides and their high-performance polymers to make up for the shortcomings of a single component; (4) The combination of components with good mechanical properties or the improvement of calcination and post-treatment processes to enhance the mechanical properties of the material. (5) designing and preparing a nanostructure capable of promoting rapid diffusion of ions and electrons, such as a tube and sleeve structure, a nanobelt, a cross-linked structure, a hollow porous structure and the like; And (6) matching the pseudocapacitor electrode material and the electric double layer electrode material to construct an asymmetric capacitor, and achieving the purpose of improving the energy density by improving the potential window of the whole system. (7) The synergetic mechanism of the composite electrode was studied to provide a theoretical basis for the design and preparation of high-performance composite electrode materials. It is believed that through the continuous efforts of researchers, we can overcome the difficulties and prepare ultra-high performance supercapacitors with high capacity, high energy density and long service life, and gradually move towards practical application.

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Funding

National Key R&D Program of China(2020YFF0413818)

Natural Science Foundation of Liaoning Province, China(0518XN011)

Natural Science Foundation of Liaoning Province, China(0519BS014)

National Natural Science Foundation of China(21671025)

National Natural Science Foundation of China(21471021)

Innovation and Entrepreneurship Training Project of Liaoning Province, China(202010167019)

Innovation and Entrepreneurship Training Project of Liaoning Province, China(S202010167045)

Innovation and Entrepreneurship Training Project of Liaoning Province, China(S202010167046)

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Cite this article

1 Introduction

2 Preparation of Membrane Materials by Electrospinning

3 Preparation of electrode materials by electrospinning

3.1 Electrospun Carbon Nanotubes as Electrodes for Electric Double Layer Capacitor

3.1.1 Porous CNFs

3.1.2 Activated CNFs

3.1.3 Heteroatom-doped CNFs

3.1.4 CNFs/carbon-based composite electrode

3.2 Preparation of pseudocapacitor electrode materials by electrospinning nanofiber

3.2.1 CNFs/metal oxide composite electrode

3.2.2 CNFs/conductive polymer composite electrode

4 Conclusion and prospect

References

Funding

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