Improving supercapacitor electrode performance with electrospun carbon nanofibers: unlocking versatility and innovation

Abstract

Electrode materials have attracted considerable attention as pivotal components for high-performance supercapacitors (SCs). In recent years, electrospun carbon nanofibers (ECNFs) have emerged as promising candidates for SC electrodes owing to their distinctive one-dimensional morphology, adjustable internal pore size, and heteroatom doping capability. The design of ECNFs tailored for superior SC performance is currently a focal point of research. This review delineates the preparation and design strategies of ECNFs for SCs electrodes, encompassing both conceptual frameworks and practical considerations. First, it presents an overview of ECNF preparation techniques and modifications. Then, it focuses on the structural design of ECNFs for SC electrodes, encompassing functional group configuration, pore structure optimization, and composite structuring. Lastly, it examines prevailing challenges and outlines future research directions in this domain. This review is expected to serve as a foundation for future research endeavors on ECNFs, thereby promoting advancements in the realm of SC technology.

---

Hailang Xu

Hailang Xu received his B.S. degree from XinYu University in 2022. He is currently a postgraduate student in the School of Materials and Chemistry, University of Shanghai for Science and Technology. His research focuses on the rational design of nanoporous polymeric materials and their application in the field of energy storage.

Bin Li

Bin Li is currently a lecturer at the College of Energy and Machinery, Dezhou University. He received his PhD degree from Shandong University in 2020 under the supervision of Prof. Xihua Zhang. His research interests include the design of new type of transition metal oxide materials, and their applications in energy conversion and storage.

Heng Zhang

Heng Zhang is currently a lecturer at the School of Materials Science and Engineering, Suzhou University of Science and Technology. He received his M.S. degree from Xinjiang University in 2019. He received his PhD degree in 2022 at Southwest University under the supervision of Prof. Chang Ming Li. His research interests include the design of new type of energy conversion and storage devices, controllable synthesis of heterostructures and their applications in energy conversion and storage.

Deng-Guang Yu

Deng-Guang Yu received his PhD from Huazhong University of Science and Technology in 2007. Now he is a professor in School of Materials and Chemistry, University of Shanghai for Science and Technology. His researches mainly focus on nanomaterials and electrohydrodynamic atomization methods, such as electrospinning, electrospraying, and 3D printing.

Wenliang Song

Wenliang Song received his PhD from BK21 PLUS Center for Advanced Chemical Technology, Department of Polymer Science and Engineering from Pusan National University. In 2020 he joined the University of Shanghai for Science and technology. He also conducted postdoctoral research at the Korea Advanced Institute of Science and Technology (KAIST) under the supervision of Prof. Bumjoon Kim and at the University of California, Riverside (UCR), under the supervision of Prof. Yadong Yin. His research focuses on various self-assembly approaches for morphology control, performance optimization and application of nanoporous polymer materials and porous carbon materials.

---

1. Introduction

The past century has witnessed a remarkable surge in social productivity, resulting in a sharp increase in fossil fuel consumption and consequently leading to the pressing issue of energy depletion.^1–4 The extensive reliance on fossil energy has exacerbated global warming, increasing the prominence of the greenhouse effect. Addressing these challenges necessitates a reduction in fossil fuel dependency,^5–8 albeit with potential impacts on the enhancement of people's living standards. Consequently, attention has shifted towards clean and renewable energy sources as a means to ensure sustained productivity growth.^9–12 Additionally, there is a critical need for research on efficient energy conversion and storage devices, aiming to minimize energy losses during conversion and storage processes while enhancing overall energy utilization efficiency.

In recent years, energy storage and conversion devices such as fuel cells, solar cells, lithium batteries (LIBs), and supercapacitors (SCs) have garnered significant attention.^13–18 Among these, SCs are extensively used in electric vehicles, aerospace, mobile electronics, power grids, and military services due to their high-power density, excellent cycle life, rapid charge and discharge rates, and safety features.^19–22 SCs, being electrochemical devices, combine the power supply capabilities of conventional capacitors with the energy storage capacity of traditional batteries, thus exhibiting a dual nature. However, compared to LIBs, SCs have limited applicability because of low energy density and high self-discharge rates, which constrain their large-scale commercial applications. Therefore, enhancing the energy density of SCs without compromising their power density and cycle life has emerged as a prominent research focus.^23,24 By optimizing the morphologies, surface chemistries, and electronic structures, the electrochemical performance of SCs can be enhanced, leading to improved efficiency and functionality.^24–26

Electrode materials must be carefully selected to ensure attributes such as large specific surface areas (SSAs), high electrical conductivity, and favorable mechanical properties to maximize efficiency and enhance the overall performance of SCs.^27,28 The primary categories of electrode materials include carbon-based materials, transition metal oxides, conducting polymers, and composite materials. Among these, carbon-based materials garner significant attention due to their affordability, large SSA, and good electrical conductivity, making them a focal point for researchers in the field of SC electrode materials.²³ Carbon, being a widely used commercial material, can be molded into various forms. Common carbon-based materials, such as graphene, carbon nanogels, carbon nanofibers (CNFs), and carbon nanotubes (CNTs), find extensive application as SC electrodes. CNFs, in particular, have garnered attention due to their high unidirectional electron transfer efficiency and facile preparation of nanodevices.^29,30 Furthermore, CNFs facilitate various modifications such as doping, activation of functional groups, and multi-structural composites.^31–33 Thus, the advantages of CNFs underscore their significance in SC development.

Various strategies, including electrospinning techniques, chemical vapor deposition, self-assembly methods, melt-blown methods, template-guided synthesis, and electrodeposition, have been developed to fabricate fibbers with one-dimensional structures. Among these techniques, electrospinning is considered one of the most straightforward and versatile methods for preparing one-dimensional nanomaterials.^34–36 The optimization of process parameters and precursor solutions for electrospinning enables the easy tuning of fiber morphologies and properties. This technique facilitates the fabrication of CNFs with uniform diameter distribution, controllable size, and customized structure.³⁷ In addition, electrospinning offers advantages such as increased porosity, precise control of orientation, and incorporation of multiple components within the nanofibers (CNTs, graphene, Mxene, etc.).^38,39 Compared with other one-dimensional nanomaterial preparation techniques, electrospinning can directly prepare self-supporting, binder-free, flexible electrode materials. Based on these properties, ECNFs are widely used in various energy storage devices.^40–42 Consequently, electrospun nanofibers present the potential for realizing high-performance electrodes for SCs.

Several reviews discuss the development of electrospinning technology and its applications.^43–45 There are also a number of literature reviews on the progress of electrospun nanofibers for SCs.^46–48 However, the significant advantages of electrospun CNF (ECNF) as an electrode material for SCs have not been fully investigated. Specifically, there is no review summarizing the preparation and design strategies of ECNF for electrodes for SCs, including the spinneret design, defects' construction, the optimization of the pore structure, and the design of the composite structure. It is also worth noting that the number of papers on ECNFs and supercapacitors has increased dramatically in the last decade due to the advantages of electrospun nanofibers and supercapacitors (Fig. 1, data from Web of Science). The preparation of ECNFs with good porosity, large SSA, and high electrical conductivity by electrospinning is crucial for improving the performance of SCs.⁴⁹ We also give a hotspot map of software predictions based on web of science data and analyze it effectively, at the same time, we use the fish-shaped picture to make a detailed compendium of possible risk challenges and solutions. Therefore, a comprehensive review of the research progress in the field of ECNFs for SC electrodes is necessary.

Fig. 1 Number of published papers regarding ECNFs and SCs in the past decade with the data derived from the Web of Science using the keywords “electrospinning” and “supercapacitor,” respectively.

As electrodes for SCs, electrode materials must possess desirable electrochemical and mechanical properties. Suitable ECNFs can be meticulously designed through electrospinning and subsequent post-processing modification. This review provides a succinct overview of the utilization of ECNFs in SCs from the perspective of the design-demand relationship (Fig. 2). It elucidates the complexities of ECNF preparation and modification, followed by an in-depth discussion on enhancing the surface chemistry of electrode materials through the design of functional group structures to ameliorate their capacitive properties. Significantly, the review scrutinizes the formation and processing of pores and their consequential impact on the performance of SCs based on ECNFs. Furthermore, it delves into the influence and enhancement of composite structure design on the overall performance of SCs. Finally, the paper offers invaluable insights into the existing challenges in the field and outlines promising avenues for future research.

Fig. 2 Schematic diagram illustrating the preparation and modification of electrospun carbon nanofibers for supercapacitors.

2. Route for constructing modified electrospun carbon nanofiber architectures

2.1. Preparation and structural design of electrospun carbon nanofibers

Electrospinning is an electrohydrodynamic process that utilizes high-voltage electric fields to fabricate nanofiber membranes from polymer solutions or melts. Illustrated in Fig. 3a, electrospun devices typically comprise a high-voltage Direct Current (DC) power supply, a spinning solution propeller, a spinneret (usually a medical syringe needle), and a collection device. Three types of spinnerets—uniaxial, coaxial, and side-by-side—are employed to produce nanofibers with solid, hollow, core-sheath, and Janus structures (Fig. 3b and c).

Fig. 3 (a) Schematic diagrams of the electrospinning method. (b) Representative spinneret structures. (c) Typical fiber structures.

During electrospinning, the morphologies, structures, and properties of ECNFs are affected by various conditions and parameters during the spinning process. Therefore, to obtain the optimal parameters of the electrospinning process, the effects of different factors during the electrospinning process need to be investigated. Proper control of these factors enables the preparation of nanofibers with desired structures. In addition, the properties of electrospun fibers vary under different calcination conditions. The effects of these factors on the fibers are shown in Table 1.^50,51

Table 1 Effect of different experimental conditions on the morphology and properties of ECNFs

	Factor	Impact degree	Regression	Effect
Electrospinning	Spinneret size	Strong	Linear	Fiber diameter increases
	Feed rate	Strong	Linear	Fiber diameter decrease
	Voltage	Minor	Linear	Fiber diameter decrease
	Collecting distance	Strong	Power-law	Fiber diameter decreases
	Solution viscosity	Strong	Linear	Fiber diameter increases
	Polymer concentration	Strong	Power-law	Fiber diameter increases
	Temperature	Strong	Linear	Fiber diameter increases
	Relative humidity	Strong	Linear	Pore diameter and pore size distribution both increase; minor effect on fiber diameter
Carbonization	Pre-oxidation time	Strong	—	Longer pre-oxidation may lead to adverse defects; short pre-oxidation time will decreased crystallinity
	Carbonization time	Strong	Linear	Graphitization increases

---

2.1.1. Increasing the yield of nanofibers. Various factors during the spinning process affect the morphological structures and properties of nanofibers.^45,52–54 These factors can be classified into three groups: precursor solution parameters, process parameters, and environmental parameters. Precursor solution parameters include viscosity, concentration, volatility, and electrical conductivity of the solution. Process parameters include applied voltage, needle diameter, distance between needle and collector (collection distance), flow rate, and collector type. Environmental parameters comprise ambient temperature and humidity. Hence, investigating the influence of different factors in the electrospinning process is crucial for obtaining optimal electrospinning process parameters. Adequate control of these factors facilitates the preparation of nanofibers with desired structures.⁵⁵ Despite substantial progress in electrospinning methods, enhancing productivity and reproducibility remains a significant challenge. Consequently, many researchers have endeavored to improve the process efficiency of electrospinning by modifying the setup to suit various applications. The following section discusses improvements made to the electrospinning technique to enhance nanofiber yield.

During solution electrospinning, the jet of polymer solution stretches, elongates, and thins because of unstable churning, whereas the solvent rapidly evaporates and solidifies into fibers. However, polymers such as polyethylene and polypropylene pose challenges because they are difficult to dissolve in solvents suitable for solution electrospinning, hence leading to the development of melt electrospinning.^56,57 A schematic diagram illustrating melt electrospinning is presented in Fig. 4a. In such cases, the lower conductivity and higher viscosity of the melt during electrospinning, compared to solution electrospinning, result in a low surface charge density on the jet, thereby substantially reducing the whipping instability of the jet.

Fig. 4 (a) Schematic diagram of melt electrospinning. Reproduced with permission from ref. 56. Copyright 2016 Elsevier. (b) Schematic diagram of AC electrospinning. Reproduced with permission from ref. 58. Copyright 2013, Royal Society of Chemistry. (c) Schematic diagram of near-field electrospinning. Reproduced with permission from ref. 59. Copyright 2022 Elsevier.

Conventional electrospinning is typically performed in far-field mode (H = 5–15 cm) with high voltage (10–20 kV) applied. The excessively long jet process complicates the precise control of fiber deposition positions. Near-field electrospinning is achieved by reducing the distance to 500 μm to 5 cm, lowering the applied voltage to a few hundred volts (typically 0.6–3 kV), and reducing the liquid flow rate to 0.01–1 mL h⁻¹.⁵⁸Fig. 4b depicts a schematic diagram of near-field electrospinning. Due to the short spinning distance, the bending instability is significantly reduced, leading to well-controlled fiber deposition in the linear phase.

According to the aforementioned DC electrospinning principle, unipolar charges can accumulate along the jet on the collector surface when a DC power supply is used. Subsequently, the jet and fibers experience significant charge repulsion, increasing the instability of the jet during the whipping process and affecting fiber deposition positions and microstructure. Introducing alternating current (AC) high voltage can address these issues.⁶⁰ Under the influence of an alternating electric field, charges migrate back and forth along the jet cyclically, reducing charge repulsion. This lowers the instability of the whipping motion of the jet and promotes the orderly deposition of electrospun nanofibers on the collector.^59,61Fig. 4c illustrates a schematic diagram of AC electrospinning. Electroblowing melding, which integrates a blow melding system with a conventional electrospinning unit, represents an innovative electrospinning technology. The rapid feed rate makes it nearly 28 times more productive than conventional electrospinning.

2.1.2. Structural design of electrospun nanofibers. In single-needle electrospinning, the fiber yield is typically low, and achieving different fiber structures poses challenges. Altering the number of spinnerets allows for obtaining diverse electrospun fiber structures and increasing their yield. Multi-needle electrospinning technology enhances the spinning jet by augmenting the number of spinning needles atop single-needle electrospinning, thereby elevating nanofiber yield and facilitating the production of membranes with superior multifunctionality.⁶² The rational design of the spinneret structure plays a pivotal role in achieving high-quality electrospinning technology.⁶³ Chang et al. devised a spinneret structure known as a sheath-separated core structure for guiding and organizing three fluids (Fig. 5a).⁶⁴ This novel composite structure comprises a common shell and two separate nuclei, paving the way for designing and producing innovative functional nanomaterials. However, in multi-needle electrospinning, the electric field between individual needles induces mutual repulsion, resulting in process instability and uneven nanofiber deposition. To address this issue, Wang et al. fabricated embedded Janus fibers using a specially designed spinneret (Fig. 5b).⁶⁵ In this nozzle, a metal tube with a side-by-side structure protrudes for a certain length to provide an additional electric field while ensuring complete access to the working solution. Needle-free electrospinning enhances spinning efficiency by saturating a specific area with solution and voltage to generate as many jets as possible. Shi et al. achieved fibers with desirable electrochemical properties through needle-free electrospinning, enabling scalable fabrication of SCs.⁶⁷

Fig. 5 (a) A spinneret for preparing fibers with a sheath-separated core structure and corresponding fibers. Reproduced with permission from ref. 64. Copyright 2020 Elsevier. (b) A spinneret for preparing embedded Janus fibers and corresponding fibers. Reproduced with permission from ref. 65. Copyright 2023 Elsevier. (c) Porous electrospun carbon fibers prepared by template method. Reproduced with permission from ref. 66. Copyright 2021, American Chemical Society.

Ordered multistage porous (macro/meso/microporous) flexible ECNFs can be fabricated by varying the ratio of sacrificial templates in the precursor solution. Xia et al. prepared multilayer porous ECNFs by electrospinning using PVA and water-based polyurethane (PU) carbon precursors, polytetrafluoroethylene (PTFE) microporous templates and boric acid (BA) crosslinker.⁶⁶ When the mass ratio of PU/PTFE was reduced from 6 : 0 to 5 : 1 and 3 : 3, the ECNFs diameter did not change significantly, but the ECNFs surface became rougher and the pore size gradually increased (Fig. 5c). Porous ECNFs can be customized by varying the amount of PU and PTFE.

2.2. Modification of electrospun carbon nanofibers

ECNFs generally are converted from polymeric nanofibers through two processes: electrospinning and calcination. There are many polymers available for spinning, some common examples are polyacrylonitrile (PAN), polyimide (PI), polymethylmethacrylate (PMMA), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), lignin and pitch.^68,69 Most of the spinnable polymer-derived ECNFs have high carbon yields, and desirable mechanical properties, but relatively low SSA and active sites limit their application in SCs. Therefore, it is necessary to rationally design its functional group structure, pore, and composite structure to gradually improve its physical and chemical properties. Despite the many excellent properties of ECNFs, modification of the ECNF surface is necessary to enhance its performance as an electrode for SCs. Many methods have been reported for the surface treatment of ECNFs, such as plasma treatment, heat-induced treatment, oxidative treatment, grafting, and UV treatment.^70–74 All these surface treatments are designed to improve the interfacial properties by improving the wettability, surface roughness, or surface chemical bonding. Surface chemical bonding can enhance the hydrophilicity of ECNFs and make good contact between ECNFs and electrolytes, thus improving the electrode conductivity. Surface roughness can result in a larger surface area to increase the number of active sites. In addition, to further enhance the comprehensive performance of ECNFs, it is often necessary to compound ECNFs with other materials. The popular methods include layer-by-layer, dip-coating, and in situ polymerization.^75–77

3. Electrospun carbon-based nanofibers for supercapacitors

SCs consist of electrodes, electrolytes, separators, and collectors. Among them, the electrode is the core component, which is responsible for the storage of charge.⁷⁸ The charge storage capacity of SCs is directly influenced by the performance of the electrode material. Electrodes are usually prepared using materials with high SSA, good electrical conductivity, and excellent electrochemical stability. ECNFs that can meet the requirements of electrode materials for SCs can be prepared using electrospinning techniques. Since Kim et al. first reported ECNFs for SCs, the field has been developing for nearly two decades, and various strategies have been devised to improve the performance of SCs (Fig. 6).^79–88 Most of the spinnable polymer-derived ECNFs have high carbon yields, and desirable mechanical properties, but relatively low SSA and active sites limit their application in SCs. Therefore, it is necessary to rationally design its functional group structure, pore, and composite structure to gradually improve its physical and chemical properties. In this section, we discuss the effect of the functional group structure of ECNFs on the electrochemical performance of electrodes. Then, an overview of electrospinning methods for the preparation of porous ECNFs is presented, focusing on multi-structural design to improve the performance of SCs.

Fig. 6 A brief summary of the development historical of ECNFs for SCs.

3.1. Design of functional group structures

In general, surface functional groups of carbon materials improve the physicochemical properties of ECNFs by altering the band gap cations, generating oxygen vacancies and increasing the carrier density. When heteroatom functional groups are introduced into ECNFs, the ordering of the carbon atoms surrounding the heteroatoms is disturbed, resulting in lattice distortion of the carbon material and thus the formation of defects. Heteroatoms in the carbon structure not only act as electron donors to increase the electrical conductivity of the material, but also modulate its surface chemistry, improve the wettability of the electrodes, and contribute to the enhancement of the adsorption/desorption of electrolyte ions. In addition, the heteroatoms can participate in Faraday redox reactions as active sites, providing pseudocapacitance for the electrodes.

PAN is a commonly used N-containing polymer, most ECNFs are N-doped ECNFs (NECNFs). NECNFs usually contain four bonding types: pyridine N (N-6), pyrrole N (N-5), quaternary N (N-4), and oxidized N. Different bonding types of N-doped structures have various functions in the energy storage process of ECNFs.^89–91 Typically, there are two doping methods, one is adding material containing heteroatoms to the precursor solution and carbonizing them directly after electrospun, named in situ doping. The other is doping with heteroatoms by subsequent treatment after electrospun, named post-doping. Multiple heteroatoms co-doped ECNFs can be synthesized through in situ doping or post-doping methods using additional elements, such as N/S, N/P, N/B, and so on. The multiple heteroatoms in the co-doped ECNFs can produce clever synergistic effects, which can further enhance the electrochemical performance of the electrodes. For example, N heteroatoms in carbon can enhance electron-donating properties and O functional groups can enhance electron-accepting properties. They usually cooperate more to significantly enhance the Faraday reaction that generates pseudocapacitance. In addition, co-doping of oxygen and nitrogen atoms may form alternating hydrophilic and hydrophobic nanodomains on the surface of the carbon material, capable of inducing surface amphiphilicity in aqueous and ionic liquid electrolytes. The synergistic effect of co-doping can modulate the crystal and electronic structure of electrode materials by maintaining the advantages of each dopant or suppressing the disadvantages of other dopants, thus enhancing the electrochemical performance of electrode materials. In N, S co-doped carbon, S doping can significantly increase the N-6 content. Due to the synergistic effect, the additional N-6 and S provide more active sites for ECNF.⁹²

N doping can be achieved by adding N-rich substances to the precursor solution. For example, using PAN as a spinning substrate and providing N source, templates such as PMMA, PVP, and PEG were added to modulate the pores and N content, and the NECNFs obtained all had high Cs.^93–96 As mentioned earlier, ECNFs with porous structures were obtained by introducing copolymers into PAN chains. Because PAN is a N-containing polymer, the ECNFs obtained by this method were also doped with certain N-atoms. Li et al. enhanced the ECNFs N doping further by introducing an O-rich monomer (itaconic acid, IA) into the PAN chain (Fig. 7a).⁸⁹ The abundance of pyridine-N and pyrrole-N/pyridine-N in ECNFs contributed pseudocapacitance to the electrodes derived from the N-doped material via Faraday reactions. The Cs was increased to 347 F g⁻¹ (0.5 A g⁻¹) when NECNFs were used as an electrode material for SCs. In addition, the prepared flexible and fine SCs have excellent flexibility and utility. Yan et al. carbonized and activated PAN/dicyandiamide composite nanofibers to obtain highly graphitized PAN-derived ECNFs with cross-linked structures and profuse N-atoms.⁹⁴ The capacitance can be increased, and electron transfer can be promoted through the positive charge density of quaternary N (N-Q). Correspondingly, NECNFs showed a high Cs of 323 F g⁻¹ (0.5 A g⁻¹) in the three-electrode system. In addition to adding N-rich substances to the precursor solution, N-atoms can also be doped into NECNFs by in situ polymerization or chemical grafting after electrospinning.^97–99 Liu et al. implanted polyaniline into PAN-based electrospun nanofibers by in situ polymerization, and the NECNF obtained was doped with 5.9 at% conductive and 4.73 at% of reactive N-atoms (Fig. 7b).⁹⁷ N-atoms are evenly distributed on the surface of the fiber. Due to the presence of N-atoms, NECNFs impose a dense pair of symmetrical redox peaks on a equirectangular curve, which can be observed even at high scan rates of 500 mV s⁻¹. It tested 90 000 cycles at a current density of 20A g⁻¹, showing exciting long-term cycle performance. The solid-state SCs made of these NECNFs can perform a specific energy and power density of 18.3 W h kg⁻¹ and 16.9 kW kg⁻¹, and it also lights up with LEDs. Its excellent electrochemical performance is derived from the synergistic effect of ultrahigh-synchronous conductive-active nitrogen doping and high SSA. NECNFs can be successfully synthesized during carbonation through N-rich substances.^100–102 Furthermore, NECNFs can also be doped after the carbonization of electrospun nanofibers. For example, chemical vapor deposition (melamine), hydrothermal method (urea, tetra-2-pyridinylpyrazine), in situ polymerization (polyaniline).^103–108

Fig. 7 (a) Schematic of the carbonization of P(AN-co-IA) copolymer and the properties of the prepared flexible devices. Reproduced with permission from ref. 89. Copyright 2021, American Chemical Society. (b) The scheme diagram of the polyaniline was synthesized in situ on electrospun carbon nanofibers. Reproduced with permission from ref. 97. Copyright 2022, Elsevier.

Apparently, single doping can improve the higher pseudocapacitance for ECNFs-derived electrode materials. However, the enhancement of electrochemical properties by single doping is limited. To further enhance the electrochemical performance, multiple elements (O, S, P, B) can be doped into the material. Single heteroatom with limited content doping is not as favorable as multi-heteroatom doping with synergistic effects for the overall electrochemical performance.

The O element present in electrospun nanofibers usually arises from insufficient combustion of the polymer in the nanofibers.^109–113 By pre-oxidizing electrospun nanofibers before carbonization, O-containing functional groups can be introduced into the fibers, leading to incomplete carbonization of the fibers. While the introduction of O-containing functional groups can enhance the wettability of ECNFs and increase the number of redox-active sites, excessive introduction of O atoms can reduce the cycle life of ECNFs. Despite being in the same main group as N and having a higher electron-donating capacity, the effect of P doping on the carbon structure and electrochemical mechanism remains unclear, unlike the extensively studied N doping. Liu et al. proposed that the doped P atoms have a positive impact on the capacitance rate performance of the materials.¹¹⁴ This is due to the P atoms readily reacting with the O-containing functional groups on the surface of electrospun ECNFs to create more stable P–O compounds, which are embedded in the carbon material's surface. During charging and discharging, the doped P atoms reduce the consumption of O-containing functional groups on the carbon material's surface, thereby enhancing its stability.^115,116

The electrochemical properties of ECNFs are similarly enhanced when doped with B atoms, especially at low B doping concentrations, where B has a catalytic effect on carbon oxidation, contributing to enhanced hydrophilicity and adding additional pseudocapacitance.^117,118 Although the doping of S and F in ECNFs equally enhances the performance of SCs with ECNFs as the electrode material, the S functionalization leads to a significant decrease in SSA of ECNFs, while F doping is understudied due to the limited F source and low doping level.^119–121

Apart from adding P, B, S, or F atoms individually to the ECNFs, simultaneously introducing two or more heteroatoms can also be effective.^122,123 Co-doping with multiple atoms can produce synergistic effects and lattice defects between heteroatoms, increasing electrochemical activity (Table 2).

Table 2 Electrochemical properties of heteroatom doped ECNFs

Heteroatoms	Elemental contents	Capacitance	Cycle performance (capacitance retention)	References
N, O	5.80, 5.60 (at%)	367.0 F g^−1 at 0.50 A g^−1	93.00% after 10 000 cycles at 10.0 A g^−1	100
N, O	3.40, 4.29 (at%)	261.0 F g^−1 at 1.00 A g^−1	94.00% after 10 000 cycles at 10.0 A g^−1	109
N, O	3.41, 7.68 (at%)	233.1 F g^−1 at 0.20 A g^−1	90.17% after 4000 cycles at 2.0 A g^−1	110
N, O	7.84, 11.43 (wt%)	320.0 F g^−1 at 1.00 A g^−1	94.50% after 5000 cycles at 1.0 A g^−1	113
N, O, P	10.91, 4.04, 0.37 (at%)	288.7 F g^−1 at 0.50 A g^−1	No attenuation after 5000 cycles at 2.0 A g^−1	114
N, O, B	—	309.0 F g^−1 at 0.25 A g^−1	94.00% after 10 000 cycles at 1.0 A g^−1	117
N, O, B	—	200.2 F g^−1 at 0.10 A g^−1	104.81% after 3000 cycles at 3.0 A g^−1	118
N, O, S	7.80, 5.50, 1.40 (at%)	396.0 F g^−1 at 1.00 A g^−1	107.00% after 3000 cycles at 15.0 A g^−1	120
N, O, P, S	—	584.8 F g^−1 at 0.20 A g^−1	93.50% after 10 000 cycles at 1.0 A g^−1	122

---

3.2. Design of pore structures

To improve the electrochemical properties of ECNFs, electrospun nanomaterials with different morphologies such as hollow, porous and core–shell have been successfully synthesized.^124,125 Electrode materials with hollow structures not only reduce the diffusion path of electrons and ions but also can store charges, which is beneficial to the electrochemical performance of the materials. Asare et al. found that the Cs of these ECNFs is mainly determined by the mesopore volume and total pore volume.¹²⁶ The total pore volume reflects the amount of charge that can be adsorbed, while the mesopore volume controls the electrolyte resistance. Therefore, the electrochemical performance of ECNFs is determined by a variety of factors and cannot be judged by the SSA alone. The diameter of ECNFs also affects their electrochemical properties.¹²⁷ Considering the mechanism of energy storage in SCs, the effect of different pore sizes on the process of energy storage in the electrodes may be different.^128–130 Micropores present a high SSA, which facilitates charge storage and determines the capacitive performance of the electrode material. Mesopores facilitate electrolyte transport and can provide a higher power density, while macropores facilitate electrolyte transport to micropores and mesopores, making adsorption easier. The ECNFs can be made to form a porous structure by introducing sacrificial templates into the precursor solution. In addition, PECNFs can be obtained not only by introducing gases such as water vapor, CO₂, and air during the calcination process, but also by using chemical reagents such as KOH, NaOH, and phosphoric acid. According to the different ways of pore generation, it can be divided into templating methods and etching methods. Templating methods can be further divided into inorganic templates and organic templates, while the etching methods mainly include physical etching and chemical etching.¹⁴¹

3.2.1. Templating methods. Many inorganic salts that are soluble in precursor solutions are often employed as templates, ionic compounds with high thermal stability like ZnCl₂ SnCl₂, KCl, and K₂S.^{131,132,143,144} Liu et al. first found K₂S to be compatible and electrically spinnable in polymer solutions and reactive with the formed ECNFs.¹⁴⁴ During electrospinning, K₂S is presented as sulfide and thiosulfate and is further converted to pyrosulfate and sulfate during pre-oxidation. During annealing and carbonization, the pyrosulfate and sulfate react with the carbon in ECNFs, resulting in a porous structure. The multistage mesoporous/microporous ECNFs not only exhibit a larger SSA (835.0 m² g⁻¹) than ECNFs with zero K₂S addition but also show a higher PAN carbonization yield (84.0 wt%). A reasonable micropore-to-mesopore ratio (3 : 2) of ECNFs facilitates the electrolyte ions to reach the interior surface of the porous carbon, thus enhancing the double-layer capacitance. The two-electrode system exhibits a high capacitance of 210.7 F g⁻¹ at 0.2 A g⁻¹ and boasts outstanding cycling stability. During the pyrolysis of carbon precursors, inorganic salts such as CaCO₃, NaHCO₃, Co(NO₃)₂, Mg(NO)₃ can decompose into metal oxides and can also serve as inorganic templates to create pores.^{133–135,142} Zhang et al. prepared multistage porous ECNFs using PAN as a carbon precursor and nano CaCO₃ as a template.¹⁴² As shown in Fig. 8a, Liu et al. prepared multistage porous structured ECNFs after the following steps: electrostatic spinning, carbonization and template removal. The electrode demonstrated excellent performance in a two-electrode system. Although metal oxides can also be used as inorganic templates, they require acids for removal and because they are insoluble in polymer solutions, they collect at the spinneret and clog the spinneret.^136,137

Fig. 8 (a) Schematic and electrochemical properties of electrospun porous carbon nanofibers prepared using CaCO₃ as a template in a two-electrode system reproduced with permission from ref. 142. Copyright 2016 Elsevier. (b) Steps of preparing electrospun porous carbon nanofibers by SiO₂ as a template, carbonization and etching after electrospinning, SEM images of fibers in corresponding steps and electrochemical performance in two-electrode system. Reproduced with permission from ref. 138. Copyright 2016 Elsevier.

SiO₂ is commonly used as an inorganic template for the preparation of layered porous nanomaterials due to its ease of tuning various morphologies and sizes.^{138–140,145} As shown in Fig. 8b, after carbonization, the uniformly dispersed SiO₂ in the fibers is efficiently removed by NaOH, and ECNFs with conjugated surfaces and interconnected macroporous structures were obtained.¹³⁸ Unexpectedly, the conjugated macroporous ECNFs showed excellent performance in the two-electrode system.

Despite this, there are relatively few reports on the use of porous nanomaterials prepared from SiO₂ for SCs electrodes, this is because they need to be removed with hydrofluoric acid or corrosive strong alkaline solutions. The summary of the inorganic templates for the modification of the pores of ECNFs is shown in Table 3.

Table 3 SSA and electrochemical properties of porous ECNFs prepared by inorganic templating methods

Materials	Templates	SSABET (m^2 g^−1)	Performance	References
HPECNFs	KCl/K2CO3	726.0	293.0 F g^−1 at 0.2 A g^−1	131
HPECNFs	K2S	835.0	210.7 F g^−1 at 0.2 A g^−1	132
PECNFs	Co(NO3)2	468.9	104.5 F g^−1 at 0.2 A g^−1	133
PECNFs	Mg(NO3)2	1140.0	248.0 F g^−1 at 0.2 A g^−1	134
Cross-linked ECNFs	ZnCl2	520.0	214.0 F g^−1 at 1.0 A g^−1	135
Inner PECNFs	SnO2	967.0	328.0 F g^−1 at 1.0 A g^−1	136
Porous ultrathin ECNFs	ZnO	588.0	155.0 F g^−1 at 10.0 A g^−1	137
Meso–microporous ECNFs	SiO2	60.0	242.0 F g^−1 at 0.2 A g^−1	138
PECNFs	SiO2	496.5	247.8 F g^−1 at 1.0 A g^−1	139
Bubbled ECNFs	SiO2	593.0	287.0 F g^−1 at 0.4 A g^−1	140

---

Organic templates can not only provide ECNFs with pores of different sizes but also provide them with a carbon source. Due to the difference in properties between the organic template and the spinning polymer, the organic template can be removed by carbonization or other methods. Commonly used organic templates are tetraethoxy orthosilicate (TEOS), organic polymers, and organic salts.

TEOS is soluble in spinning precursors, and undergoes conversion to SiO₂ upon heat treatment, making it a popular choice for the preparation of porous ECNFs^146–150 Kim et al.¹⁴⁷ obtained Si/ECNFs with a high SSA and high microporous structure with a surface area up to 1386.91 m² g⁻¹, energy density within 15.0–22.3 W h kg⁻¹, and power density within 0.4–100 kW kg⁻¹ by adding a PAN-based precursor solution acting as a pore-forming template with a PAN/TEOS weight ratio of 8/2. TEOS can be converted to its hydrated form (Si(OEt)₄·nH₂O) during spinning, and during stabilization at 280 °C (temperature increase rate 1 °C min⁻¹), the Si–OEt bond is hydrolyzed to produce silanol (Si(OH)_x), followed by further condensation leading to the formation of SiO_x and the escape of gases such as CO₂, H₂, H₂O etc.¹⁴⁸ Wang's team proposed a coaxial electrospun method to prepare double capillary carbon nanofibers (DCNFs) with porous structure using TEOS/ethanol solution for the external fluid and lignin/dimethylformamide (DMF) for the internal fluid, as shown in Fig. 9a.¹⁴⁹ The SSA of these DCNFs can reach 870 m² g⁻¹, which is larger than that of the porous ECNFs (568 m² g⁻¹) and ECNFs (164 m² g⁻¹) used as controls. Among the tested materials, when the current density was increased from 1 A g⁻¹ to 20 A g⁻¹, DCNFs stood out with an impressive capacitance retention of 74%. In comparison, porous ECNFs and ECNFs displayed retention rates of 54% and 40%, respectively. Furthermore, after 4000 cycles at a current density of 10 A g⁻¹, it was able to reach 94% of the initial capacitance. What is surprising is that the assembled flexible device shows excellent performance after bending. Cui et al. prepared bamboo-like ECNFs with ideal porosity using TEOS as a template, which could achieve excellent electrochemical performance.¹⁵⁰ As shown in Fig. 9b, the precursors of PAN and TEOS in DMF were formed into nanofibrous nets by electrospinning. Bamboo-like ECNFs were obtained by heat-treating TEOS/PAN nanofiber networks in H₂/Ar (5/95 volume fraction) atmosphere and then etching SiO₂ in HF aqueous solution. The SSA of the bamboo-like ECNFs is as high as ∼1912 m² g⁻¹ calculated in the range of 0.05–0.25 P/P₀ by the Brunauer–Emmett–Teller (BET) model and the total pore volume is ∼2.27 cm³ g⁻¹ evaluated at a single-point adsorption at P/P₀ = 0.98. The pore volume for micro- and mesopores is ∼1.87 cm³ g⁻¹. In addition, the assembled flexible electrode can be bent at will, showing excellent flexibility.

Fig. 9 (a) Schematic diagram of the fabrication process for double capillary carbon nanofibers, and CV curves of related flexible devices under normal and bending conditions. Reproduced with permission from ref. 149. Copyright 2016, Elsevier. (b) Schematic diagram of the fabrication process of the bamboo-like carbon nanofibers and demonstration of the flexibility of the corresponding flexible electrodes. Reproduced with permission from ref. 150. Copyright 2015, American Chemical Society. (c) The detailed sequential process of etching PVP and constructing porosity on the MPNFs, and cycling performance of flexible devices. Reproduced with permission from ref. 121. Copyright 2018, Royal Society of Chemistry.

Organic polymers such as polyacrylic acid (PAA), PMMA, polystyrene (PS), and polyethylene glycol (PEG) can be used as sacrificial templates due to their different properties.^96,151–155 When PVP is co-mingled with PAN, PVP is also commonly used as a sacrificial template.^{121,156–159} PVP is more hydrophilic than PAN and can therefore be extracted from PAN nanofibers during hydrothermal treatment to produce mesoporous PAN nanofibers. The formation mechanisms of the observed changes in nanofiber structure and porosity are described (Fig. 9c).¹²¹ During hydrothermal treatment, the residual PVP in the PAN fibers agglomerates with each other, with the agglomerated PVP and the fiber shell separating from the PAN core. Since the molecular weight of PVP is higher than that of PAN, an increase in PVP content increases the viscosity and surface tension of the electrospinning precursor solution, increasing the diameter of the fibers produced.^121,156 The ECNFs derived from PVP–PAN electrospun nanofibers have high porosity and SSA, and PVP is an N-rich polymer that provides a certain pseudocapacitance for the ECNFs, presenting excellent electrochemical properties.^90,158 Ultrasonic treatment of PVP–PAN electrospun nanofibers also removes PVP and can obtain ultrafine ECNFs with a porous multilayer structure.¹⁵⁹

Although ECNFs with porous structures can be prepared by blending polymers and sacrificing templates, the uniformity of pore size is difficult to control. Zhou et al. proposed a method for the synthesis of hierarchical ENCFs with highly controlled structures by introducing block copolymer self-assembly.¹⁵¹ The ECNFs exhibit hierarchically interconnected mesopores and micropores with a surface area of up to 503 m² g⁻¹ and an exciting superior electrochemical performance (Fig. 10a). Moreover, substances like coal, asphalt, and lignin also can be utilized as templates in the production of porous ECNFs.^161–164

Fig. 10 (a) SEM images of the electrospun carbon fibers from PAN/PMMA, and PAN-b-PMMA and electrochemical properties of electrospun carbon nanofibers. Reproduced with permission from ref. 151. Copyright 2019, The American Association for the Advancement of Science. (b) Schematic preparation of MOFs-based porous ECNFs and their application in flexible SCs. Reproduced with permission from ref. 160. Copyright 2023, Elsevier.

Organic salts, such as tetramethylammonium oxalate (TMAO), manganese acetate (Mn(OAc)₂), and copper acetate (Cu(OAc)₂) can act as pore-forming agents in a similar principle to the inorganic salt templates.^165–169 The difference is that most organic salts are well soluble in polymers and can avoid the inhomogeneous pore structure caused by the dispersion of the template. Among them, many organometallic salt templates require acids for removal, while organic ammonium salts can be pyrolyzed during carbonization and can reduce the environmental pollution from acids.¹⁷⁰

Metal complexes such as metal–organic frames (MOFs) and manganese acetylacetonate can also form pores on ECNFs after removal.^{140,160,171–173} Utilizing the self-templating properties of MOFs, MOFs were combined with electrospinning as a precursor for porous carbon for energy and environmental applications. In a study by Wang et al. Zn-based MOFs were embedded in lignin-based electrospun nanofibers and carbonized as an electrode for high-performance SCs, and offer potential applications for the development of portable wearable devices.¹⁶⁰ As shown in Fig. 10b, upon carbonization at elevated temperatures, elemental Zn in the MOFs particles is reduced to metallic Zn and evaporated at carbonization temperatures above its boiling point, ultimately forming hollow-based particles inside the ECNFs. In addition, the SCs they assembled possessed good electrochemical properties. Upon carbonization, pyrolysis of MOF could introduce abundant mesopores for ECNFs for fast electrolyte ion channels. The doping of ZIF-8 greatly not only enlarged the SSA and mesopore volume but also increased the use of certain templates can leave materials with pseudocapacitance to remain in the ECNFs. So, the choice of different templates has varying effects on the porosity and surface area of the ECNFs, as well as on the enhancement of their electrochemical performance (Table 4). Even the use of the same template can result in different outcomes.

Table 4 SSA and electrochemical properties of porous ECNFs prepared by organic templating methods

Materials	Templates	SSABET (m^2 g^−1)	Capacitance	Cycle performance (capacitance retention)	References
Bamboo-like ECNFs	TEOS	1912.0	236.0 F g^−1 at 5.0 A g^−1	95.0% after 5000 cycles at 1.0 A g^−1	146
ECNFs	TEOS	870.0	245.0 F g^−1 at 0.5 A g^−1	94.0% after 10 000 cycles at 4.0 A g^−1	149
Flexible ECNFs	TEOS	2292.0	274.0 F g^−1 at 0.1 A g^−1	∼100% after 5000 cycles at 10 A g^−1	150
PECNFs	PMMA	503.0	360.0 F g^−1 at 1.0 A g^−1	∼100% after 10 000 cycles at 100.0 A g^−1	151
PECNFs	PMMA	503.0	334.0 F g^−1 at 1.0 A g^−1	98.0% after 5000 cycles at 100.0 A g^−1	96
PECNFs mats	PMMA	683.5	140.8 F g^−1 at 0.5 A g^−1	92.0% after 20 000 cycles at 1.0 A g^−1	154
Mesoporous ECNFs	PVP	596.1	252.6 F g^−1 at 0.5 A g^−1	95.4% after 10 000 cycles at 10.0 A g^−1	121
Microporous ECNFs	PVP	406.4	200.0 F g^−1 at 0.5 A g^−1	93.0% after 1000 cycles at 1.0 A g^−1	158
ECNFs mat	Coal	616.8	227.4 F g^−1 at 1.0 A g^−1	104.3% after 10 000 cycles at 5.0 A g^−1	162
Cross-linked ECNFs	Pitch	643.0	187.0 F g^−1 at 1.0 A g^−1	84.0% after 3000 cycles at 5.0 A g^−1	164
ECNFs	Co(Ac)2	338.6	256.5 F g^−1 at 0.5 A g^−1	84.7% after 1500 cycles at 1.0 A g^−1	166
ECNFs	MOF	1440.0	396.0 F g^−1 at 1.0 A g^−1	107.0% after 3000 cycles at 15.0 A g^−1	120
HPECNFs	MOF	111.3	664.4 F g^−1 at 1.0 A g^−1	83.1% after 5000 cycles at 10.0 A g^−1	170
Micro–mesoporous ECNFs	Acetylacetonate	563.0	272.0 F g^−1 at 1.0 A g^−1	96.7% after 3000 cycles at 100 mV s^−1	173
HPECNFs	PMMA/TEOS	1126.0	207.0 F g^−1 at 0.2 A g^−1	∼99.0% after 8000 cycles at 1.0 A g^−1	91

---

3.2.2. Etching methods. The inclusion of soft templates, hard templates, or even double templates in the polymer precursor solution offers a significant advantage in the preparation of porous ECNFs with a well-controlled and desirable pore size distribution. It is usually necessary to consider the solubility and dispersion of the sacrificial substance in the polymer, and the generated pores are mostly mesoporous and microporous.^96,168 To enhance the proportion of micropores and increase the charge storage, the electrospun nanofibers can be activated by physical etching or chemical activation. Etching treatment can remove some easily removable materials from the surface of electrospun fibers, making the fiber surface form a porous structure. During carbonization, gases such as air, CO₂, and water vapor can be introduced to perform the so-called physical activation of the fibers. In this process, some carbon atoms are oxidized and release gases, which leads to the formation of pores.^174–180 Yan et al. developed a green and efficient H₂O₂ activation method (Fig. 11a) that can regulate the pore structure and the chemical composition of ECNFs.¹⁷⁴ The H₂O₂ etching reaction starts from the N/O-containing carbon sites and gradually extends outward, widening the existing nanopores and creating new nanopores in the CNFs.

Fig. 11 (a) Schematic diagram of the process for the preparation of porous electrospun carbon nanofiber membranes, including electrospinning, imidization, carbonization and H₂O₂ activation. Reproduced with permission from ref. 174. Copyright 2022, Elsevier. (b) Schematic diagram of CNFs preparation by electrospun, carbonization and KOH activation. Reproduced with permission from ref. 181. Copyright 2019, Elsevier. (c) Phosphoric acid-activated electrospun nanofibers with high SSA even after carbonization at 1600 °C. Reproduced with permission from ref. 115. Copyright 2022, Elsevier.

The symmetric SCs they made from these ECNFs were able to light up LEDs. ECNFs obtained by etching the electrospun nanofibers with water vapor have a desirable SSA and pore volume, especially the micropore volume, and exhibit excellent performances in the SCs system.¹⁷⁵ Abeykoon et al. activated electrospun fibers with CO₂ during carbonization.¹⁷⁶ Abeykoon et al. activated electrospun fibers with CO₂ during carbonization.¹⁷⁶ The resulting ECNFs have a high SSA of 3010 m² g⁻¹ and a total pore volume of 1.6077 cm³ g⁻¹, of which the microporous volume is 0.9205 cm³ g⁻¹, accounting for 57.2%. The size of the micropores/mesopores that are formed is very similar to the size of the electrolyte ions, which results in more sites for charge transfer.¹⁷⁸ The capacitance of the CO₂-activated ECNFs is enhanced from 97.0 F g⁻¹ to 136.7 F g⁻¹ (10 mV s⁻¹) and the energy density from 49 W h kg⁻¹ to 79 W h kg⁻¹ (10 kW kg⁻¹) compared with that of the normal ECNFs.¹⁸⁰ During CO₂ activation, the fiber surface is etched by CO₂, which leads to a decrease in fiber diameter while increasing SSA. This results in a narrow charge diffusion pathway and large porosity exhibited between fibers, finally causing an enhancement of electrochemical properties.^176,177 Although water vapor can also be used to activate ECNFs and lead to an increase in micropores, it leads to ECNFs with lower conductivity and lower SSA relative to etching with CO₂.^175,178 However, this did not result in a higher Cs of the CO₂-activated ECNFs. Because the H₂O-activated ECNFs have a suitable pore size to accommodate the electrolyte ions, allowing for a large capacitance.¹⁵⁴ Through the reaction between the gas and the carbon atoms on the surface not only generates new pores, but also has a pore-opening effect, and the volatilization of the gas during the heating process can expand the closed pores and enrich the pore structure. In addition, when the oxidizing gas reacts with part of the pores, it can also make the pores expand and thus increase the width of the pores.

Some common bases (KOH, NaOH) and acids (H₃PO₄, HNO₃) could corrode carbon structure of nanofibers, thereby allowing ECNFs to obtain a porous structure.^{115,164,181–188} Among them, KOH is the most used because it has low pyrolysis temperature requirements and can produce satisfactory porosity.^181–185 As shown in Fig. 11b, the nanofiber mats were first prepared by electrospinning, and ECNFs were formed after carbonization at a certain temperature, followed by mixing with KOH and activation at high temperature.¹⁸¹ The reactions during the activation process are as follows:^48,181

6KOH + 2C → 2K + 3H₂ + 2K₂CO₃ (1)

K₂CO₃ → K₂O + CO₂ (2)

CO₂ + C → 2CO (3)

K₂CO₃ + 2C → 2K + 3CO (4)

C + K₂O → 2K + CO (5)

Singh et al. obtained the nanofiber mats by electrospun a mixture of lignin and PVA, and the ECNFs were prepared by pre-carbonation at 600 °C followed by activation with KOH at 800 °C.¹⁸² The activated ECNFs were made into all-solid-state SCs devices due to high flexibility, independence, and excellent electrode performance. The devices exhibited a high energy density of 65.52 W h kg⁻¹ and a power density of 1036.27 W kg⁻¹ with high coulombic efficiency (99.6%) after 10 000 cycles. However, the usual activation with KOH requires the use of highly corrosive hydrochloric acid for cleaning. It was found that by using lignin as the spinning material and adding HNO₃ for activation (Fig. 11c), the resulting ECNFs not only possess a porous structure but also have the presence of P elements, which in turn enhances the performance of the electrode.¹¹⁵ The addition of phosphoric acid shortened the thermal stability time of the fiber, increased the carbonization yield, generated phosphorus functional groups (P content up to 3 wt%), and increased the BET SSA of carbon fiber from 840 m² g⁻¹ to 1143 m² g⁻¹. Even after high-temperature carbonization at 1600 °C, the 822 m² g⁻¹ of SSA was retained. Pore creation with acid and alkali requires lower temperature and shorter time than gas etching, which can avoid pore rupture due to too high temperature and too long time. On the other hand, it is also more activated, which allows the fabrication of ECNFs with unique pore structures but suffers from high cost, equipment corrosion, and activator residue. Moreover, the porosity and surface chemistry of ECNFs are greatly affected by the activation temperature.

3.3. Design of composite structures

Despite efforts to improve performance by using electrode materials with high SSA and heteroatom doping, this remains challenging due to the limitations of the electrical double-layer capacitance storage mechanism. When compounded with materials such as transition metal compounds and conducting polymers, a synergistic effect is achieved, proving to be an effective approach.^189–193 The physical modification of carbon materials is mainly reflected in the contrast surface area, pore structure, and the improvement of electrical conductivity.^194–196 Through surface/interface engineering, it is possible to realize the construction of composite structures system systems and multi-component synergistic effects of carbon materials with other materials, which is conducive to the enhancement of the conductivity of materials and the stability of energy storage devices.^197–199

In addition to the regulation of pore structure and the introduction of heteroatoms into ECNFs, many researchers have attempted to design multi-structural systems to achieve the complementary advantages of different structures and improve the overall electrochemical performance of electrode materials.^200,201 ECNFs obtained by adding other structured materials (carbon, MOFs, MXene or others) to the precursor solution are called internal structure designs. In contrast, the design of multilayered structures obtained by post-treatment of electrospinning is called external structural design, which includes treatment on single fibers or fiber membranes.

3.3.1. Design of internal structures. The mechanical strength of ECNFs after calcination tends to be weak, so it remains a challenge to fabricate flexible ECNFs to meet the growing demand for wearable devices. Thanks to the advantages of electrospinning, some insoluble materials can be blended into ECNFs as reinforcing phases, which in turn enhance the mechanical properties of ECNFs. Graphene is considered a promising carbon-based electrode material because of its high SSA, good electrical conductivity, and exhibits good capacitive properties. The composite design of graphene and its derivatives with different carbon structures in ECNFs can enhance the electrical conductivity of the fibers and alter their structural properties.^202–205 As shown in Fig. 12a, Zhao et al. reported a novel high surface area graphene quantum dots (GQDs) reinforced CNFs (AGRCNF) with good electrical conductivity and strong mechanical properties integrated by electrospun, carbonization, and chemical activation processes.²⁰⁴ GQDs play a vital bifunctional role in constructing the entire reinforcing phase and conducting network due to their strong cross-linking effect and high crystallinity. As a stand-alone SCs electrode, AGRCNF shows a high capacitance of 335 F g⁻¹ at 1 A g⁻¹ and extremely high capacitance retention of 77% at 100 A g⁻¹ and 45% at 500 A g⁻¹. Importantly, the symmetrical device can be charged to 80% capacitance in as short as 2.2 seconds and cycled over 10 000 times without significant capacitance degradation. In addition, the AGRCNF-3 fabric can return to its initial shape after more than 100 cycles of gripping, folding, and twisting, demonstrating its excellent flexibility and durability. Interestingly, the folded AGRCNF-34 fabric with a thickness of only 3 μm (700 mg) can pull up to 200 g, demonstrating its superior mechanical strength to most porous carbon fabrics. The excellent electrochemical properties and mechanical strength of ECNFs derive from the complete reinforcing phase and conductive network constructed within the fibers by highly crystalline GQDs.

Fig. 12 (a) Schematic diagram of area graphene quantum dots reinforced carbon nanofiber prepared through electrospinning, carbonization, and chemical activation, and mechanical properties of materials. Reproduced with permission from ref. 204. Copyright 2021, American Chemical Society. (b) Schematic diagram of the production of Ti₃C₂T_x MXene/ECNFs via electrospinning, and cycling performance of MXene/ECNFs electrodes. Reproduced with permission from ref. 206. Copyright 2019, Royal Society of Chemistry. (c) Schematic diagram and morphology of one-pot deposition of coaxial bilayer MnO₂/Ni(OH)₂ ECNFs. Reproduced with permission from ref. 207. Copyright 2022, Springer Nature Limited.

Similarly, the introduction of other carbon materials such as CNTs and carbon black into ECNFs through blending can also improve its physical properties as well as their structure, thereby thus enhancing its electrochemical properties.^208,209 Inspired by the natural phenomenon of “lotus root may break, but the fiber stays connected”, Zhu et al. prepared hollow porous carbon nanofibers bridged by carbon nanotubes using coaxial electrospun technique.⁷⁷ The addition of multi-walled CNTs prevent the porous structure of the fibers from damaging their mechanical properties.

In addition to the introduction of carbon-based materials into ECNFs, many compounds were also used to design the internal structure of ECNFs as well.^210,211 Levitt et al. added metal carbides and nitrides (MXene) to the spinning solution to prepare MXene/ECNFs (Fig. 12b).²⁰⁶ The MXene/ECNFs electrode and a pure carbon electrode were cycled for 10 000 cycles, revealing stable performance at 50 mV s⁻¹. Metal compound with relatively low reduction temperature and excellent ductility is a good candidate as a reinforcing phase for ECNFs.²¹² Zhao et al. reported a highly flexible ECNFs that introduce Sn particles.²¹³ Due to the introduction of ductile Sn, the ECNFs were able to be repeatedly wound on glass rods without breaking. In addition, the reinforced ECNFs can be directly used as electrodes with a specific capacitance of 342.05 F g⁻¹ at 0.5 A g⁻¹ current density. In addition, the metal salts and MOF templates talked about in Section 3.2 in not only bring the ideal pore structure for ECNFs, but the metal oxides left behind after pyrolysis can also enhance the capacitance of ECNFs.

Embedding the reinforcing phase with excellent mechanical strength into ECNFs can prevent their pore collapse during calcination. On the other hand, the reinforcing phase is inserted into the ECNFs as if it were a nail, which makes it possible to maintain an intact membrane morphology when subjected to external forces. The relevant studies are summarized in Table 5.

Table 5 Impact of internal structure design on the performance of ECNFs

Materials	Electrochemical performance	Mechanical performance	References
Graphene-ECNFs	283.0 F g^−1 at 0.2 A g^−1	—	202
GQDs-ECNFs	319.0 F g^−1 at 0.5 A g^−1	Young's modulus value is 226.0 MPa	203
GQDs-ECNFs	335.0 F g^−1 at 1 A g^−1	Can pull up a weight of 200 g	204
CNTs-ECNFs	164.6 F g^−1 at 1.0 A g^−1	Tensile strength of 54.8 MPa	208
Carbon black-ECNFs	166.0 F g^−1 at 0.1 A g^−1	—	209
Va/Co oxides-ECNFs	1.83 F cm^−2 at 8.0 mA cm^−2	5.5% Cs degradation of assembled flexible devices after 180 degrees of bending	211
MXene-ECNFs	205 mF cm^−2 at 50.0 mV s^−1	Ability to wrap around vials	206
W2C-ECNFs	256.0 F g^−1 at 1.0 A g^−1	—	212

---

3.3.2. Design of external structures. Since the electrospinning technique gives the ECNFs membrane properties, this will help other materials to adhere better to the ECNFs. The presence of carbon, pores, and heteroatoms in the substrate improves the conductivity of the electrode properties, this will help other materials to adhere better to the ECNFs. The presence of carbon, pores, and heteroatoms in the substrate improves the conductivity of the electrode. However pure ECNFs are characterized by low specific capacitance and low energy density. Rate capability, power density, and stability are also suboptimal for pseudocapacitor materials, despite the abundance of redox chemistry and unique properties. Therefore, the use of ECNFs as the backbone (substrate) of pseudocapacitive materials in composite nanostructures may be an excellent way to prepare high-performance stand-alone SCs electrodes. There are two general categories for the design of external structure, one is the design of the shell and core structure of ECNFs, and the other is the design of multilayer ECNFs membranes.^77,214,215 For the first category, ECNFs can be loaded with conductive organic substances or other materials on the surface. For example, polyaniline was synthesized on the surface of ECNFs by in situ polymerization,²¹⁶ synthesis of NiCoO, CoS, and NiMoO₄ by hydrothermal method.^164,217,218 As for graphene, Zheng et al. prepared ECNFs/PEI/GO by a layer-by-layer assembly strategy.²¹⁹ This unique structure gives the electrodes excellent energy storage capacity with excellent durability and fast response to temperature. This strategy provides a simple and effective idea for preparing green and sustainable nanostructured energy storage devices. Nie et al. anchored coaxial bilayer MnO₂/Ni(OH)₂ nanosheets on ECNFs by a green one-pot method.²⁰⁷ As shown in Fig. 12c, Ni(OH)₂ is attached to ECNFs through MnO₂ and exhibits a layered morphology. MnO₂/Ni(OH)₂ ECNFs demonstrate exceptional rate and cycling capabilities, and can successfully light up commercial light-emitting diode.

As the second category, electrodes for high-performance stand-alone SCs can be prepared by preparing multilayer ECNFs membranes through multiple electrospun.^220–222 Kim et al. designed a sandwich structure that stabilizes the MnO₂ particles encapsulated in the middle layer as active sites to provide better electrochemical performance for the electrode.²²⁰ In another of their studies, urea was introduced as a nitrogen source into the bottom (or top) layer of the sandwich structure. The doping of the N-based functional groups led to an increase in the hydrophilicity of the surface of the ECNFs, which in turn increased their electrical conductivity. This sandwich structure is designed not only to avoid the exposure of pseudocapacitive materials to the surface of ECNFs, which would lead to reduced stability but also to have an excellent conductive network that provides short ion diffusion for ions and electrons. In addition, the method of designing both internal and external structures simultaneously has been used. Du et al. fabricated an ECNFs composite based on the concept of “maximizing structure utilization”, in which NiS particles were encapsulated internally by electrospinning, while the external structure was achieved by a simple impregnation-sulfurization process in which NiS nanosheets were uniformly embedded on the surface of interconnected carbon fibers.²²³ This structure simultaneously realizes the dual introduction of Faraday capacitance and the assurance of the conductivity of the electrodes. The NiS nanoparticles encapsulated inside the ECNFs not only prevent severe aggregation but also act as redox activators to increase the pseudocapacitance. The NiS nanosheets anchored on the outer surface of the ECNFs can mitigate the decrease in stability caused by buildup and facilitate the transfer of electrolyte ions through a large accessible surface area. By incorporating materials with various structures into ECNFs to create multi-structured ECNFs, these ECNFs can deliver high capacitance and outstanding cycling performance, while also enhancing their flexibility.

3.4. Discussion on the structural design of electrospun carbon fibers for supercapacitors

The structural design of ECNFs has three main aspects: functional group design, pore structure design, and composite structure design.

(1) Introducing unique functional groups into the carbon structure of ECNFs can enhance the conductivity of the electrodes, thus providing a certain pseudocapacitance. Commonly used doping elements in the structure of ECNFs are N, S, P, etc. According to the mechanism raised by Li et al.,²²⁴ the uniformly distributed N atoms in ECNFs provide high concentration of N-doped active sites and endow with good wettability,²²⁵ in addition, the quaternary N and pyrrolic N also could improve the capacitance by promote the interaction with electrolyte ions and pseudocapacitive contributions, respectively.²²⁶ S doping enhances ion diffusion and charge transfer at high speeds under high current densities. This rapid mass transfer across the electrode promotes both pseudocapacitive and electric double-layer capacitive behaviors.²²⁷ B doping can significantly increase surface hydrophilicity and wettability, thereby enhancing both electric double-layer capacitance and pseudocapacitance through more efficient ion penetration and interaction in carbon electrodes.²²⁸ P doping can provide more active sites for strong ion adsorption to improve electrochemical performances.²²⁹ The O doping mainly attributed to the special type, such as carbonyl (–C=O) type oxygen, –SO_x (x = 1, 2), C–O–PO₃ and (C–O)₃PO.^230,231 It should be noted that the doping of different elements results in different synergistic effects. Heteroatoms not only enhance redox pseudocapacitance but also prevent irreversible changes in oxygen-containing functional groups on the surface of carbon materials during charge and discharge.²³² Due to the additive effects of triple doping, the Faraday interaction between the heteroatoms or functional groups of the electrode and the electrolyte will result in significant pseudocapacitance.²³³ It should be noted that the synergistic effects of two- and multi-element doping are generally considered to be complex. Most notably, the synergistic effects of several doping species can boost the specific capacitance of carbon and create stronger active areas by multi-element co-doping. More defects in the graphene lattice may be generated by the multi-element co-doping, which would activate the related C atoms and lower the activation energy needed for redox processes (resulting in a high pseudocapacitance). Moreover, a suitable arrangement and blend of heteroatoms on the surface of carbon-based materials might mitigate the early breakdown of electrolyte, leading to a greater voltage window in both organic (3.5 V) and aqueous (1.2 V) electrolytes.^234,235

(2) ECNFs with multistage pores can be obtained by activation, templating, and etching, The enlarged pore diameters, especially meso and macro pores, can enhance the specific surface area of ECNFs, which in turn enhances the specific capacitance based on electric double-layer capacitance behavior. Simultaneous calcination of a certain ratio of ECNFs and KOH is a common activation method to prepare porous ECNFs this method mainly generates the micro pore. Surfactant or degradable polymers are often used as templates because they can decompose upon calcination, thus leaving mainly mesopores in the ECNFs. The templating method such as SiO₂, PS, could provide meso and macro pores in ECNFs compared to the activation method. The dispersible powder structure (high packing density), hierarchical porous structure (rapid ion diffusion) are usually responsible for the enhancement for supercapacitors. It should be noted that the micropores or mesopores in the ECNFs mainly provide abundant storage active sites.²³⁶ Furthermore, the macropores created by the stacking and curling of ECNFs can be utilized as a quick conduit for the movement of electrolytes.^237–239

(3) The composite structure of ECNFs not only enhances the electrochemical properties of ECNFs, but also prevents the pore collapse of ECNFs during calcination, which in turn enhances the mechanical properties of the electrodes. The direct contribution of the 3D structure to the capacitance of the electrode material is limited, mainly because it only changes the morphology and macroporous structure. The composite morphology has a weak effect on the performance of the electrode material, and the composite structure plays an auxiliary and fundamental role in the capacitance performance. The advantage of the composite structure is that it may provide more possibilities for the preparation of free standing and binder free supercapacitor mats, as well as providing convenience for the preparation of the next generation of more flexible, wearable supercapacitors.¹⁴²

At the same time, we also need to point out that the three points listed above are also possible directly synergistic with each other, such as the exposed mesopores and macropores doped with heteroatoms, which contribute greatly to supercapacitor's performance.

4. Conclusions

Electrospun nanomaterials are promising electrode materials for SCs owing to their superb electronic conductivity, electrolyte accessibility, rich porosity, and structural stability. This paper reviews the improvements introduced to the design of the pore structures and composition of ECNFs to enhance SCs performance. It delves into the evolution of ECNFs within the supercapacitor domain, emphasizing the symbiotic relationship between design and demand. In addition to discussing the influence of ECNFs' pore structure on electrode performance, this paper assesses pore formation techniques, including templating and etching methods. Lastly, the effects of doping treatments of ECNFs and multi-architecture designs on the performance of SCs based on ECNFs are discussed.

To summarise and study the cross-section of electrospinning and supercapacitors in more depth, we searched the web of science for ‘electrospinning’ and ‘supercapacitor’ for all the literature in the past 10 years, and we selected the top 500 highly cited articles as the key data to create a research hotspot map based on the VOSviewer software (Fig. 13). We observe that the cross-cutting areas between the two researchers focus more on application performance, energy, in fact nanoparticles, nanocomplexes, fibers, fibrous membranes, and finally, some wearable, pore size, polyaniline, electrical conductivity, and mechanical properties are all topics as well. From these keywords, we believe that the application of electrospinning carbon fibers in supercapacitors includes the following main aspects:

Fig. 13 Research hotspots at the intersection of electrospinning and supercapacitors based on VOSviewer Software analysis.

(1) The one-dimensional morphologies of ECNFs allow for facile adjustment and control, facilitating the preparation of porous ECNFs via templating or etching methods, as well as intricate structural designs. These attributes endow electrodes with unidirectional electron flow paths, excellent connectivity, high conductivity, and shortened electrolyte ion diffusion paths, thereby enhancing electrochemical performance. (2) Electrospinning provides a straightforward avenue for integrating multiple active components into a single nanofiber. The large-area fiber membrane obtained by electrospinning can be easily attached by other materials to prepare composite structures. Leveraging the synergistic effects, optimization of the interface between two or more components can yield exceptional electrochemical properties. (3) Lastly, electrospinning enables the fabrication of binderless, flexible, and wearable SC devices, paving the way for their integration into innovative applications.

5. Current challenges and outlooks

Although the advantages of this ECNFs have led to great progress in their application in the field of SCs, there are still many problems and challenges to be explored for electrospinning technology, ECNFs and SCs to reach maturity. As shown in the Fig. 14, we summarise the current challenges in the field of electrospinning carbon fibres for supercapacitors.

Fig. 14 Summary and outlook on the preparation of supercapacitor materials by electrospinning.

(1) SCs, as environmentally friendly energy storage devices, necessitate the adoption of green processes for the preparation of ECNFs, a critical electrode material. In electrospinning, the evaporation of toxic and corrosive solvents presents inherent biological and environmental risks. Encouraging the use of eco-friendly solvents such as water and ethanol is imperative. Additionally, post-electrospinning, the residual spinning solution mandates the selection of sustainable, biodegradable, and cost-effective polymers and solvents.

(2) ECNFs-based SCs are storing charges through EDLC behavior, a mechanism that endows ECNFs-based SCs with high cyclic stability. However, high cycling performance is still the focus for commercially applied SCs. The cycling performance of EDLC behavior is mainly affected by two factors: the stability of the carbon structure and the pore size that matches the size of the electrolyte ions. Designing ECNFs with stable structures for SCs and providing suitable electrolytes for ECNFs are the focus of enhancing the stability of SCs. It is crucial to provide more interfaces for electrochemical reactions through interface engineering, defect construction, and electronic structure modulation. Although increasing electrochemical reaction sites may enhance SCs capacitance, it usually impairs cycling stability. Therefore, designing ECNFs with both high specific capacitance and robust stability is a major challenge. Realizing unique structural designs on the outside of ECNFs may introduce a certain amount of pseudocapacitance. However, practical challenges such as delamination of pseudocapacitive materials, poor interfacial bonding between pseudocapacitive materials and ECNFs, and impeded electron transfer must be addressed. According to most of the literature, it is known that the test standards for cycling stability of electrode materials are not uniform, which makes the evaluation basis of cycling stability of electrode materials lack of comparability. Therefore, the test conditions should be standardized to monitor the change of stability and study the decay mechanism.

(3) Attention must be directed toward the alteration of metrics during ECNFs design to enhance the electrochemical performance of SCs. Modifying the structure and morphology of nanofibers can be achieved by refining needle structure and needle arrangement and incorporating additional structures into the spinning solution to introduce more active sites for energy storage and facilitate electron transfer. Further, the commercial production of ECNFs as electrodes for SCs is pivotal, highlighting its significance. Transitioning from lab-scale production to the commercialization of ECNFs poses a significant challenge. While electrospinning has seen some level of industrialization, further exploration is needed for industrializing post-processing techniques to optimize the performance of ECNFs.

(4) Energy density, power density, internal resistance, leakage current, and output voltage are critical metrics for quantifying SCs performance. Additionally, exploring the energy storage mechanism of designed ECNFs using advanced in situ microscopy and spectroscopy techniques is crucial for enhancing the electrochemical performance of SCs. Leveraging electrospinning and molding technology, ECNFs exhibit superior mechanical properties compared to other carbon fibers, particularly in flexible energy storage devices. Through astute structural design and preventive measures to mitigate fiber collapse during carbonization, which can diminish mechanical properties, a maximal balance between mechanical and electrochemical properties can be achieved. While existing studies predominantly analyze device flexibility based on electrochemical properties under various bending conditions, there is a dearth of quantifiable metrics. Hence, designing more quantifiable performance metrics and test methods is imperative to evaluate the performance of ECNFs in flexible SCs.

(5) To advance the application scenarios of SCs, ECNFs-based energy storage systems should possess additional characteristics. For example, activities like space missions and outdoor adventures necessitate novel energy storage technologies with excellent temperature and corrosion resistance, capable of operating efficiently for extended periods. Consequently, enhancing the performance of ECNFs in extreme environments becomes imperative. Moreover, designing appropriate test methods and performance indicators may improve the relevant performance metrics.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

Hailang Xu: conceptualization, writing – original draft. Bin Li: writing – review & editing, project administration, funding acquisition. Zeyu Wang: writing – review & editing. Qian Liao: writing – review & editing. Lingcong Zeng: writing – review & editing. Heng Zhang: writing – review & editing. Deng-Guang Yu: writing – review & editing. Xiaoliang Liu: writing – review & editing. Wenliang Song: supervision, writing – review & editing, project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Shanghai Sailing Program (21YF1431000), and the scientific research allowance of Dezhou University (Grant No. 2022xjrc303).

Notes and references

1. P. Achakulwisut, P. Erickson, C. Guivarch, R. Schaeffer, E. Brutschin and S. Pye, Nat. Commun., 2023, 14, 5425.
2. A. Philip and A. Ruban Kumar, Renewable Sustainable Energy Rev., 2023, 182, 113423.
3. S. Kandpal, T. Ghosh, C. Rani, A. Chaudhary, J. Park, P. S. Lee and R. Kumar, ACS Energy Lett., 2023, 8, 1870–1886.
4. C. Zhang, Y. Yang, X. Liu, M. Mao, K. Li, Q. Li, G. Zhang and C. Wang, Innovation, 2023, 4, 100518.
5. D. Shindell and C. J. Smith, Nature, 2019, 573, 408–411.
6. A. Slameršak, G. Kallis and D. W. O'Neill, Nat. Commun., 2022, 13, 6932.
7. Y. Bai, L. Luo, W. Song, S. Man, H. Zhang and C. M. Li, Adv. Sci., 2024, 11, 2308668.
8. W. L. Song, Y. X. Tang, B. Y. Moon, Q. Liao, H. L. Xu, Q. Hou, H. Zhang, D. G. Yu, Y. Z. Liao and I. Kim, Green Chem., 2024, 26, 2476–2504.
9. D. Coles, B. Wray, R. Stevens, S. Crawford, S. Pennock and J. Miles, Appl. Energy, 2023, 334, 120686.
10. Y. Bai, Y. Qin, J. Hao, H. Zhang and C. M. Li, Adv. Funct. Mater., 2023, 34, 2310393.
11. C. Li, X. Wang, E. Bi, F. Jiang, S. M. Park, Y. Li, L. Chen, Z. Wang, L. Zeng, H. Chen, Y. Liu, C. R. Grice, A. Abudulimu, J. Chung, Y. Xian, T. Zhu, H. Lai, B. Chen, R. J. Ellingson, F. Fu, D. S. Ginger, Z. Song, E. H. Sargent and Y. Yan, Science, 2023, 379, 690–694.
12. T. Oderinwale and C. R. McInnes, Appl. Energy, 2022, 317, 119154.
13. X. Li, Y. Chen, H. Huang, Y.-W. Mai and L. Zhou, Energy Storage Mater., 2016, 5, 58–92.
14. L. Xu, R. Iqbal, Y. Wang, S. Taimoor, L. Hao, R. Dong and Z. Sun, Innovations Mater., 2024, 2(1), 100060–100061.
15. Y. Zhang, Q. Zhou, W. Ma, C. Wang, X. Wang, J. Chen, T. Yu and S. Fan, Nano Res., 2023, 16, 9519–9529.
16. J. Wei, X. Wu and S. Xing, Green Chem., 2023, 25, 3940–3947.
17. C. Han, Y. Cao, S. Zhang, L. Bai, M. Yang, S. Fang, H. Gong, D. Tang, F. Pan, Z. Jiang and J. Sun, Small, 2023, 19.
18. H. Zhang, J. Wu, Z. Zou, Y. Bai, C. Wu, Q. Zeng, F. Liu, W. Shen, J. Jiang and C. M. Li, Energy Environ. Mater., 2023, 6, e12377.
19. M. Zhong, M. Zhang and X. Li, Carbon Energy, 2022, 4, 950–985.
20. P. Li, Y. Bräuniger, J. Kunigkeit, H. Zhou, M. R. Ortega Vega, E. Zhang, J. Grothe, E. Brunner and S. Kaskel, Angew. Chem., Int. Ed., 2022, 61, e202212250.
21. Q. Li, M. Liu, F. Huang, X. Zuo, X. Wei, S. Li and H. Zhang, Chem. Eng. J., 2022, 437, 135494.
22. G. Pacchioni, Nat. Rev. Mater., 2022, 7, 844.
23. W. Ma, Y. Zhang, S. Pan, Y. Cheng, Z. Shao, H. Xiang, G. Chen, L. Zhu, W. Weng, H. Bai and M. Zhu, Chem. Soc. Rev., 2021, 50, 7009–7061.
24. C. K. Maity, S. De, A. Panigrahi, S. Acharya, K. Verma, M. J. Kim and G. C. Nayak, Composites, Part B, 2023, 264, 110887.
25. T. Kim, S. Subedi, B. Dahal, K. Chhetri, T. Mukhiya, A. Muthurasu, J. Gautam, P. C. Lohani, D. Acharya, I. Pathak, S. H. Chae, T. H. Ko and H. Y. Kim, Adv. Sci., 2022, 9, 2200650.
26. P. Li, X. Ma, F. Liu, Y. L. Zhao, Y. Ding and J. Yang, Microporous Mesoporous Mater., 2020, 305, 110283.
27. J. Zhu, Q. Zhang, Y. Zhao, R. Zhang, L. Liu and J. Yu, Carbon, 2023, 202, 13–25.
28. B. Li, X. Zhang, C. Hu, J. Dou, G. Xia, P. Zhang, Z. Zheng, Y. Pan, H. Yu and C. Chen, J. Colloid Interface Sci., 2019, 556, 239–248.
29. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797.
30. W. Song, Y. Zhang, C. H. Tran, H. K. Choi, D.-G. Yu and I. Kim, Prog. Polym. Sci., 2023, 142, 101691.
31. S. Chen, L. Qiu and H.-M. Cheng, Chem. Rev., 2020, 120, 2811–2878.
32. W. Ma, W. Li, M. Li, Q. Mao, Z. Pan, J. Hu, X. Li, M. Zhu and Y. Zhang, Adv. Funct. Mater., 2021, 31, 210019.
33. W. Ma, W. Li, M. Li, Q. Mao, Z. Pan, M. Zhu and Y. Zhang, J. Mater. Chem. A, 2020, 8, 25355–25362.
34. W. Zhou, H. Shi, Y. Gao, D. Gui, D. Xie, Y. Liu and Y. Liu, Appl. Catal., A, 2024, 119847.
35. H. Liu, Y. Zhu, C. Zhang, Y. Zhou and D. G. Yu, Nano Today, 2024, 55, 102161.
36. H. Lv, Y. Liu, J. Zhou, Y. Bai, H. Shi, B. Yue, S. Shen and D.-G. Yu, Chem. Eng. J., 2024, 484, 149514.
37. D.-G. Yu and J. Zhou, J. Pharm. Sci., 2023, 112, 2719–2723.
38. Y. Sun, J. Zhou, Z. Zhang, D. G. Yu and S. W. A. Bligh, Int. J. Pharm., 2024, 658, 124180.
39. K. R. S. Dinuwan Gunawardhana, R. B. V. B. Simorangkir, G. B. McGuinness, M. S. Rasel, L. A. Magre Colorado, S. S. Baberwal, T. E. Ward, B. O'Flynn and S. M. Coyle, ACS Nano, 2024, 18, 2649–2684.
40. Y. Fang, S. Zhang, Z. Wu, D. Luan and X. Lou, Sci. Adv., 2021, 7, eabg3626.
41. Y. Fang, D. Luan, S. Gao and X. W. Lou, Angew. Chem., Int. Ed., 2021, 60, 8515–8520.
42. Y. Fang, Y. Zeng, Q. Jin, X. F. Lu, D. Luan, X. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2021, 60, 8515–8520.
43. D. Ji, Y. Lin, X. Guo, B. Ramasubramanian, R. Wang, N. Radacsi, R. Jose, X. Qin and S. Ramakrishna, Nat. Rev. Methods Primers, 2024, 4, 1.
44. Y. Li, J. Zhu, H. Cheng, G. Li, H. Cho, M. Jiang, Q. Gao and X. Zhang, Adv. Mater. Technol., 2021, 6, 2100410.
45. J. Xue, T. Wu, Y. Dai and Y. Xia, Chem. Rev., 2019, 119, 5298–5415.
46. J. Liang, H. Zhao, L. Yue, G. Fan, T. Li, S. Lu, G. Chen, S. Gao, M. Asiri Abdullah and X. Sun, J. Mater. Chem. A, 2020, 8, 16747.
47. B. Joshi, E. Samuel, Y.-i. Kim, A. L. Yarin, M. T. Swihart and S. S. Yoon, Coord. Chem. Rev., 2022, 460, 214466.
48. P. Fan, C. Ye and L. Xu, Diamond Relat. Mater., 2023, 134, 109803.
49. X. Lu, C. Wang, F. Favier and N. Pinna, Adv. Energy Mater., 2016, 7, 1601301.
50. X. Yang, Y. Chen, C. Zhang, G. Duan and S. Jiang, Composites, Part B, 2023, 249, 110386.
51. Q. Gao, S. Agarwal, A. Greiner and T. Zhang, Prog. Mater. Sci., 2023, 137, 101139.
52. W. Song, Y. Wen, Z. Wang, H. Xu, Q. Liao, Y. Tang, D.-G. Yu and I. Kim, Langmuir, 2024 DOI:10.1021/acs.langmuir.4c01041.
53. L. Xu, Q. Li, H. Wang, H. Liu, D.-G. Yu, S. A. Bligh and X. Lu, Chem. Eng. J., 2024, 492, 152359.
54. W. Gong, W. Yang, J. Zhou, S. Zhang, D.-G. Yu and P. Liu, Nanocomposites, 2024, 10, 240–253.
55. A. Rogina, Appl. Surf. Sci., 2014, 296, 221–230.
56. T. D. Brown, P. D. Dalton and D. W. Hutmacher, Prog. Polym. Sci., 2016, 56, 116–166.
57. T. M. Robinson, D. W. Hutmacher and P. D. Dalton, Adv. Funct. Mater., 2019, 29, 1904664.
58. Y. Huang, N. Bu, Y. Duan, Y. Pan, H. Liu, Z. Yin and Y. Xiong, Nanoscale, 2013, 5, 12007–12017.
59. Z. Zhang, H. Liu, D. G. Yu and S. W. A. Bligh, Biomolecules, 2024, 14(7), 789.
60. S. Maheshwari and H. C. Chang, Adv. Mater., 2009, 21, 349–354.
61. T. Kalous, P. Holec, R. Jirkovec, O. Batka, P. Zabka, P. Pokorny, P. Stepanova and J. Chvojka, J. Environ. Chem. Eng., 2023, 11, 109543.
62. Y. Hu, H. Qi, H. Shao, L. Yang, Q. Ma, Y. Sheng, Y. Xie, W. Yu, D. Li and X. Dong, Compos. Sci. Technol., 2023, 233, 109923.
63. W. L. Song, Y. X. Tang, C. Qian, B. J. Kim, Y. Liao and D.-G. Yu, Innovation, 2023, 4, 100381.
64. S. Chang, M. Wang, F. Zhang, Y. Liu, X. Liu, D.-G. Yu and H. Shen, Mater. Des., 2020, 192, 108782.
65. M. Wang, R. Ge, P. Zhao, G. Williams, D.-G. Yu and S. Bligh, Mater. Des., 2023, 226, 111652.
66. S. Xia, Y. Zhang, Y. Zhao, X. Wang and J. Yan, ACS Appl. Mater. Interfaces, 2021, 13, 44768–44776.
67. K. Shi and K. P. Giapis, ACS Appl. Energy Mater., 2018, 1, 296–300.
68. A. Al-Dhahebi, J. Ling, S. Krishnan, M. Yousefzadeh, N. Elumalai, M. Saheed, S. Ramakrishna and R. Jose, Appl. Phys. Rev., 2022, 9, 011319.
69. S. Jiang, Y. Chen, G. Duan, C. Mei, A. Greiner and S. Agarwal, Polym. Chem., 2018, 9, 2685.
70. M. Dufay, M. Jimenez and S. Degoutin, ACS Applied Bio Materials, 2020, 3, 4696–4716.
71. P. Feng, G. Song, X. Li, H. Xu, L. Xu, D. Lv, X. Zhu, Y. Huang and L. Ma, J. Colloid Interface Sci., 2021, 583, 13–23.
72. J. Zhang, M. Yu and S. Tao, Nano Res., 2024 DOI:10.1007/s12274-024-6753-8.
73. X. Li, Q. Zhao, X. Du, Y. Li, X. Tian and Y. Cui, Energy Fuels, 2022, 36, 4616–4624.
74. P. Y. Zhao, H. Y. Wang, B. Cai, X. B. Sun, Z. L. Hou, J. T. Wu and G. S. Wang, Nano Res., 2022, 15(9), 7788–7796.
75. Y. Xue, Z. Li, J. Luo, X. Yang, Z. Yang, Y. Shi, Y. Liu, Y. Ma, H. Zhang and J. Yu, Composites, Part B, 2024, 274, 111286.
76. C. Chen, Y. He, G. Xiao, Y. Wu, Z. He and F. Zhong, Prog. Org. Coat., 2018, 125, 420–431.
77. J. Zhu, Q. Zhang, L. Guo, Y. Zhao, R. Zhang, L. Liu and J. Yu, Chem. Eng. J., 2022, 434, 134662.
78. S. Lyu, H. Chang, L. Zhang, S. Wang, S. Li, Y. Lu and S. Li, Composites, Part B, 2023, 264, 110888.
79. C. Kim and K. S. Yang, Appl. Phys. Lett., 2003, 83, 1216.
80. Z. Tai, X. Yan, J. Lang and Q. Xue, J. Power Sources, 2012, 199, 373–378.
81. B.-H. Kim, K. Seung Yang and H.-G. Woo, Mater. Lett., 2013, 93, 190–193.
82. C. Ma, Y. Li, J. Shi, Y. Song and L. Liu, Chem. Eng. J., 2014, 249, 216–225.
83. P. Ramakrishnan, S.-G. Park and S. Shanmugam, J. Mater. Chem. A, 2015, 3, 16242–16250.
84. X. Li, J. Wang, F. Ge, S. Komarneni and Z. Cai, Chem. Commun., 2016, 52, 2691–2694.
85. J. Yan, K. Dong, Y. Zhang, X. Wang, A. A. Aboalhassan, J. Yu and B. Ding, Nat. Commun., 2019, 10, 5584.
86. R. Zhang, J. Yan, L. Wang, W. Shen, J. Zhang, M. Zhong and S. Guo, J. Power Sources, 2021, 513, 230559.
87. I. Pathak, D. Acharya, K. Chhetri, P. Chandra Lohani, S. Subedi, A. Muthurasu, T. Kim, T. H. Ko, B. Dahal and H. Y. Kim, J. Mater. Chem. A, 2023, 11, 5001–5014.
88. G. Wang, G. Hu, J. Lan, F. Miao, P. Zhang and G. Shao, J. Colloid Interface Sci., 2024, 653, 148–158.
89. J. Li, M. Yang, Y. Zhao, W. Zhang, L. Huo, X. Zhang, J. Gao, H. Pang and H. Xue, ACS Appl. Energy Mater., 2021, 4, 5830–5839.
90. X. Li, Y. Zhao, Y. Bai, X. Zhao, R. Wang, Y. Huang, Q. Liang and Z. Huang, Electrochim. Acta, 2017, 230, 445–453.
91. J. Zeng, Q. Cao, X. Wang, B. Jing, X. Peng and X. Tang, J. Solid State Electrochem., 2015, 19, 1591.
92. H. Zhang, Z. Lin, Z. Sun, M. Cai, W. Liu, W. Ye, H. Gao, J. Han, Y. Cheng, Q. Zhang and M. Wang, Adv. Funct. Mater., 2023, 33, 2300769.
93. D. Xuan, J. Liu, D. Wang, Z. Lu, Q. Liu, Y. Liu, S. Li and Z. Zheng, Energy Fuels, 2020, 35, 796–805.
94. S. Yan, C. Tang, H. Zhang, Z. Yang, X. Wang, C. Zhang and S. Liu, Nanotechnology, 2020, 31, 025402.
95. Y. Zheng, D. Ni, N. Li, W. Chen and W. Lu, Microporous Mesoporous Mater., 2021, 316, 110972.
96. Z. Zhou, T. Liu, A. U. Khan and G. Liu, Mol. Syst. Des. Eng., 2020, 5, 153–165.
97. M. Liu, X. Li, C. Shao, C. Han, Y. Liu, X. Li, X. Ma, F. Chen and Y. Liu, Energy Storage Mater., 2022, 44, 250–262.
98. F. Miao, C. Shao, X. Li, K. Wang and Y. Liu, J. Mater. Chem. A, 2016, 4, 4180–4187.
99. X. Zhao, G. Nie, Y. Luan, X. Wang, S. Yan and Y.-Z. Long, J. Alloys Compd., 2019, 808, 151737.
100. T. P. Mofokeng, Z. N. Tetana and K. I. Ozoemena, Carbon, 2020, 169, 312–326.
101. Q. Xu, X. Yu, Q. Liang, Y. Bai, Z. Huang and F. Kang, J. Electroanal. Chem., 2015, 739, 84–88.
102. L. Zhao, Y. Qiu, J. Yu, X. Deng, C. Dai and X. Bai, Nanoscale, 2013, 5, 4902–4909.
103. Y. Cheng, L. Huang, X. Xiao, B. Yao, L. Yuan, T. Li, Z. Hu, B. Wang, J. Wan and J. Zhou, Nano Energy, 2015, 15, 66–74.
104. L. Gao, Y. Wang, Y. Liu and L. Xu, Colloids Surf., A, 2023, 663, 131056.
105. Q. Jiang, M. Liu, C. Shao, X. Li, H. Liu, X. Li and Y. Liu, Electrochim. Acta, 2020, 330, 135212.
106. C.-T. Lo, K.-W. Lin, T.-P. Wang, S.-M. Huang and C.-L. Lee, Electrochim. Acta, 2021, 381, 138225.
107. H. Sun, S. Li, Y. Shen, F. Miao, P. Zhang and G. Shao, Appl. Surf. Sci., 2020, 501, 144001.
108. L. Sun, D. Miyagi, Y. Cai, A. Ullah, M. K. Haider, C. Zhu, M. Gopiraman and I. S. Kim, J. Energy Storage, 2023, 61, 106738.
109. Y. Xiao, Y. Xu, K. Zhang, X. Tang, J. Huang, K. Yuan and Y. Chen, Carbon, 2020, 160, 80–87.
110. Q. Li, W. Xie, D. Liu, Q. Wang and D. He, Electrochim. Acta, 2016, 222, 1445–1454.
111. W. Xu, Y. Lu, H. Chi, H. Zhao, J. Hu and G. Xiao, J. Energy Storage, 2022, 47, 103672.
112. T. He, X. Li, Y. Wang, T. Bai, X. Weng and B. Zhang, J. Electroanal. Chem., 2020, 878, 114597.
113. Z. Dai, P.-G. Ren, W. He, X. Hou, F. Ren, Q. Zhang and Y.-L. Jin, Renewable Energy, 2020, 162, 613–623.
114. C. Liu, G. Shi, G. Wang, P. Mishra, S. Jia, X. Jiang, P. Zhang, Y. Dong and Z. Wang, RSC Adv., 2019, 9, 6898–6906.
115. F. J. García-Mateos, J. M. Rosas, R. Ruiz-Rosas, J. Rodríguez-Mirasol and T. Cordero, Carbon, 2022, 200, 134–148.
116. K. Chen, J. Liu, H. Bian, W. Wang, F. Wang and Z. Shao, J. Electroanal. Chem., 2020, 878, 114473.
117. M. Wu, X. Zhao, J. Gao, J. Guo, J. Xiao and R. Chen, Surf. Interfaces, 2022, 31, 101983.
118. X. Zhang, G. Yan, H. Li, Z. Li, J. Chen, L. Wang and Y. Wu, J. Alloys Compd., 2023, 943, 169188.
119. X. Li, X. Du, Y. Li, X. Tian, H. Zheng and X. Li, J. Mater. Sci., 2022, 57, 9357–9369.
120. Y. Li, G. Zhu, H. Huang, M. Xu, T. Lu and L. Pan, J. Mater. Chem. A, 2019, 7, 9040–9050.
121. W. Na, J. Jun, J. W. Park, G. Lee and J. Jang, J. Mater. Chem. A, 2017, 5, 17379–17387.
122. M. W. Ahmad, S. Anand, B. Dey, A. Fatima, D. J. Yang and A. Choudhury, ACS Appl. Energy Mater., 2021, 4, 12177–12190.
123. Y. Ma, X. Zhang, Z. Liang, C. Wang, Y. Sui, B. Zheng, Y. Ye, W. Ma, Q. Zhao and C. Qin, Electrochim. Acta, 2020, 337, 135800.
124. G.-T. Park, H. Kim and Y. G. Jeong, Mater. Sci. Eng., B, 2023, 291, 116376.
125. X. Zhang, L. Cao, Y. Liao, Z. Qin, Z. Yang, R. Sun, W. Zhang, H. Li and G. Yan, J. Alloys Compd., 2022, 920, 165983.
126. K. Asare, M. F. Hasan, A. Shahbazi and L. Zhang, Surf. Interfaces, 2021, 26, 101386.
127. S. Bhattacharya, I. Roy, A. Tice, C. Chapman, R. Udangawa, V. Chakrapani, J. L. Plawsky and R. J. Linhardt, ACS Appl. Mater. Interfaces, 2020, 12, 19369–19376.
128. L. Yu, S. Zhang, J. Yin, C. Bai, J. Zhang, Y. Li, Y. Yang, Z. Ge, M. Zhang, L. Wei, M. Ma, Y. Ma and Y. Chen, Carbon, 2017, 124, 64–71.
129. S. Xu, S. Yan, X. Chen, H. Huang, X. Liang, Y. Wang, Q. Hu, G. Wei and Y. Yang, Chem. Eng. J., 2023, 459, 141528.
130. H. Zhang, G. Cao, Y. Yang and Z. Gu, Carbon, 2008, 46, 30–34.
131. G.-H. An and H.-J. Ahn, Carbon, 2013, 65, 87–96.
132. C. Zhan, Q. Xu, X. Yu, Q. Liang, Y. Bai, Z.-H. Huang and F. Kang, RSC Adv., 2016, 6, 41473.
133. K. K. Karthikeyan and P. Biji, Microporous Mesoporous Mater., 2016, 224, 372–383.
134. Y. Liu, J. Zhou, L. Chen, P. Zhang, W. Fu, H. Zhao, Y. Ma, X. Pan, Z. Zhang, W. Han and E. Xie, ACS Appl. Mater. Interfaces, 2015, 7, 23515–23520.
135. C. Ma, Z. Li, J. Li, Q. Fan, L. Wu, J. Shi and Y. Song, Appl. Surf. Sci., 2018, 456, 568–576.
136. W. Xie, X. Jiang, T. Qin, H. Yang, D. Liu and D. He, Electrochim. Acta, 2017, 258, 1064–1071.
137. L. Wang, G. Zhang, X. Zhang, H. Shi, W. Zeng, H. Zhang, Q. Liu, C. Li, Q. Liu and H. Duan, J. Mater. Chem. A, 2017, 5, 14801–14810.
138. L. Fan, L. Yang, X. Ni, J. Han, R. Guo and C. Zhang, Carbon, 2016, 107, 629–637.
139. S. Ghosh, W. D. Yong, E. M. Jin, S. R. Polaki, S. M. Jeong and H. Jun, Korean J. Chem. Eng., 2019, 36, 312–320.
140. A. Gopalakrishnan, P. Sahatiya and S. Badhulika, ChemElectroChem, 2017, 5, 531–539.
141. G. Nie, X. Zhao, Y. Luan, J. Jiang, Z. Kou and J. Wang, Nanoscale, 2020, 12, 13225–13248.
142. L. Zhang, Y. Jiang, L. Wang, C. Zhang and S. Liu, Electrochim. Acta, 2016, 196, 189–196.
143. Q. Jiang, X. Pang, S. Geng, Y. Zhao, X. Wang, H. Qin, B. Liu, J. Zhou and T. Zhou, Appl. Surf. Sci., 2019, 479, 128–136.
144. H. Liu, W. Song and A. Xing, RSC Adv., 2019, 9, 33539–33548.
145. D. Lee, J.-Y. Jung, M.-J. Jung and Y.-S. Lee, Chem. Eng. J., 2015, 263, 62–70.
146. C. Ma, S. Ruan, J. Wang, D. Long, W. Qiao and L. Ling, J. Colloid Interface Sci., 2018, 531, 513–522.
147. B.-H. Kim and K. S. Yang, J. Electroanal. Chem., 2014, 92, 714–715.
148. B.-H. Kim, K. S. Yang and H.-G. Woo, Electrochem. Commun., 2011, 13, 1042–1046.
149. J. Wang, J. Tang, Y. Xu, B. Ding, Z. Chang, Y. Wang, X. Hao, H. Dou, J. H. Kim, X. Zhang and Y. Yamauchi, Nano Energy, 2016, 28, 232–240.
150. Y. Sun, R. B. Sills, X. Hu, Z. W. Seh, X. Xiao, H. Xu, W. Luo, H. Jin, Y. Xin, T. Li, Z. Zhang, J. Zhou, W. Cai, Y. Huang and Y. Cui, Nano Lett., 2015, 15, 3899–3906.
151. Z. Zhou, T. Liu, A. Khan and G. Liu, Sci. Adv., 2019, 5, eaau6852.
152. J. M. Serrano, G. A. Spiering, Z. Xu, Z. Croft, D. Guo, K. Cao, R. B. Moore and G. Liu, ACS Appl. Polym. Mater., 2022, 4, 4980–4992.
153. B. Zhang, X. Weng, X. Yu, Z. Wang, X. Liu and T. He, Mater. Sci. Eng., B, 2023, 288, 116169.
154. G. He, Y. Song, S. Chen and L. Wang, J. Mater. Sci., 2018, 53, 9721–9730.
155. E. Jo, J.-G. Yeo, D. K. Kim, J. S. Oh and C. K. Hong, Polym. Int., 2014, 63, 1471–1477.
156. G. Wang, B. Qian, Y. Wang, Q. Dong, F. Zhan and J. Qiu, New J. Chem., 2016, 40, 3786–3792.
157. T.-S. He, X.-D. Yu, T.-J. Bai, X.-Y. Li, Y.-R. Fu and K.-D. Cai, Ionics, 2020, 26, 4103–4111.
158. C. Liu, Y. Tan, Y. Liu, K. Shen, B. Peng, X. Niu and F. Ran, J. Energy Chem., 2016, 25, 587–593.
159. T.-S. He, Q. Su, Z. Yildiz, K. Cai and Y. Wang, Electrochim. Acta, 2016, 222, 1120–1127.
160. M. Zhou, P. Wang, Y. Yu, W. Ma, Z. Cai, F. Ko, M. Li and Q. Wang, Energy, 2023, 278, 127705.
161. Y. He, L. Wang and D. Jia, Electrochim. Acta, 2016, 194, 239–245.
162. J. Jia, Z. Qin, X. Yang, B. Gu, M. Yin and Z. Lin, J. Taiwan Inst. Chem. Eng., 2022, 132, 104233.
163. G.-T. Park, H.-B. Jeon, S.-Y. Kim, H.-E. Gang and Y. G. Jeong, Mater. Sci. Eng., B, 2022, 275, 115530.
164. X. Tian, Y. He, Y. Song, T. Yang, X. Li and Z. Liu, Energy Fuels, 2020, 34, 14975–14985.
165. S. Perananthan, J. S. Bonso and J. P. Ferraris, Carbon, 2016, 106, 20–27.
166. G. Nie, X. Lu, M. Chi, M. Gao and C. Wang, J. Colloid Interface Sci., 2018, 509, 235–244.
167. N. Cai, J. Fu, V. Chan, M. Liu, W. Chen, J. Wang, H. Zeng and F. Yu, J. Alloys Compd., 2019, 782, 251–262.
168. T. Zhou, Q. Jiang, L. Wang, Z. Qiu, Y. Liu, J. Zhou and B. Liu, Appl. Surf. Sci., 2018, 456, 827–834.
169. Y. Luan, G. Nie, X. Zhao, N. Qiao, X. Liu, H. Wang, X. Zhang, Y. Chen and Y.-Z. Long, Electrochim. Acta, 2019, 308, 121–130.
170. Z. Li, J. Bu, C. Zhang, L. Cheng, D. Pan, Z. Chen and M. Wu, New J. Chem., 2021, 45, 10672–10682.
171. P. Á. Szilágyi and A. J. Sobrido, J. Energy Chem., 2022, 73, 348–353.
172. X. Liu, M. Naylor Marlow, S. J. Cooper, B. Song, X. Chen, N. P. Brandon and B. Wu, J. Power Sources, 2018, 384, 264–269.
173. Y. Li, W. Ou-Yang, X. Xu, M. Wang, S. Hou, T. Lu, Y. Yao and L. Pan, Electrochim. Acta, 2018, 271, 591–598.
174. B. Yan, J. Zheng, L. Feng, Q. Zhang, J. Han, H. Hou, C. Zhang, Y. Ding, S. Jiang and S. He, Diamond Relat. Mater., 2022, 130, 109465.
175. K.-H. Jung and J. P. Ferraris, Carbon, 2012, 50, 5309–5315.
176. N. C. Abeykoon, V. Garcia, R. A. Jayawickramage, W. Perera, J. Cure, Y. J. Chabal, K. J. Balkus and J. P. Ferraris, RSC Adv., 2017, 7, 20947–20959.
177. N. C. Abeykoon, S. F. Mahmood, S. B. Alahakoon, M. Wunch, R. A. Smaldone, D. J. Yang and J. P. Ferraris, ACS Appl. Energy Mater., 2021, 4, 8988–8999.
178. C. H. Kim, J.-H. Wee, Y. A. Kim, K. S. Yang and C.-M. Yang, J. Mater. Chem. A, 2016, 4, 4763–4770.
179. C. Kim, C. Yang, Y. Kim and K. Yang, Appl. Surf. Sci., 2019, 497, 143693.
180. S. D. Panangala, C. Karunaweera, R. Jayawickramage, K. J. Balkus and J. P. Ferraris, ACS Appl. Polym. Mater., 2019, 1, 3203–3209.
181. Y.-J. Heo, H. I. Lee, J. W. Lee, M. Park, K. Y. Rhee and S.-J. Park, Composites, Part B, 2019, 161, 10–17.
182. M. Singh, A. Gupta, S. Sundriyal, P. Dubey, K. Jain and S. R. Dhakate, J. Energy Storage, 2022, 49, 104193.
183. Z. Dai, P.-G. Ren, Y.-L. Jin, H. Zhang, F. Ren and Q. Zhang, J. Power Sources, 2019, 437, 226937.
184. M. F. Hasan, S. Mantripragada, S. Gbewonyo, S. Xiu, A. Shahbazi and L. Zhang, Int. J. Energy Res., 2022, 46, 22867–22882.
185. C. Ma, R. Wang, Z. Xie, H. Zhang, Z. Li and J. Shi, J. Porous Mater., 2017, 24, 1437–1445.
186. Y. Liu, Q. Liu, L. Wang, X. Yang, W. Yang, J. Zheng and H. Hou, ACS Appl. Mater. Interfaces, 2020, 12, 4777–4786.
187. F. J. García-Mateos, R. Ruiz-Rosas, J. María Rosas, E. Morallón, D. Cazorla-Amorós, J. Rodríguez-Mirasol and T. Cordero, Sep. Purif. Technol., 2020, 241, 116724.
188. X. Li, X. Tian, T. Yang, Y. He, W. Liu, Y. Song and Z. Liu, ACS Sustainable Chem. Eng., 2019, 7, 5742–5750.
189. K. Donthula, R. Vooradi, V. S. Patnaikuni and M. Kakunuri, J. Electrochem. Soc., 2023, 170, 030508.
190. Y. Wu, G. Xu, W. Zhang, C. Song, L. Wang, X. Fang, L. Xu, S. Han, J. Cui and L. Gan, Carbohydr. Polym., 2021, 267, 118116.
191. A. I. Abdel-Salam, S. Y. Attia, S. G. Mohamed, F. I. El-Hosiny, M. A. Sadek and M. M. Rashad, Int. J. Hydrogen Energy, 2023, 48, 5463–5477.
192. G.-H. An, H.-J. Ahn and W.-K. Hong, J. Power Sources, 2015, 274, 536–541.
193. X. Ni, Y. Jiang, H. Chen, K. Li, H. Chen, Q. Wu and A. Ju, Chin. Chem. Lett., 2021, 32, 2448–2452.
194. T. Zhang, P. Liu, Y. Zhong, J. Zheng, K. Deng, X. Lv, H. Li, W. Tian and J. Ji, Carbon, 2022, 198, 91–100.
195. S.-F. Tseng, J.-Y. Lin and J.-Y. Lin, Energy, 2024, 291, 130365.
196. F. Song, G. Yang, L. Pan and Q. Chen, Composites, Part B, 2023, 267, 111025.
197. T. Guan, Z. Li, D. Qiu, G. Wu, J. Wu, L. Zhu, M. Zhu and N. Bao, Adv. Fiber Mater., 2023, 5, 896–927.
198. G. Wu, X. Wu, X. Zhu, J. Xu and N. Bao, ACS Nano, 2022, 16, 10130–10155.
199. Y. Zhu, W. Peng, Y. Li, G. Zhang, F. Zhang and X. Fan, J. Mater. Chem. A, 2019, 7, 23577–23603.
200. H. Zhao, W. Han, W. Lan, J. Zhou, Z. Zhang, W. Fu and E. Xie, Electrochim. Acta, 2016, 222, 1931–1939.
201. A. R. Selvaraj, I. S. Raja, D. Chinnadurai, R. Rajendiran, I. Cho, D. W. Han and K. Prabakar, J. Energy Storage, 2022, 46, 103731.
202. X. Qiu, Z. Xiao, L. Wang and L.-Z. Fan, Carbon, 2018, 130, 196–205.
203. R. Zhang, W. Shen, M. Zhong, J. Zhang and S. Guo, J. Phys. Chem., 2020, 125, 143–151.
204. J. Zhao, J. Zhu, Y. Li, L. Wang, Y. Dong, Z. Jiang, C. Fan, Y. Cao, R. Sheng, A. Liu, S. Zhang, H. Song, D. Jia and Z. Fan, ACS Appl. Mater. Interfaces, 2020, 12, 11669–11678.
205. M. Pourmohammad, J. Ling, M. Yousefzadeh and R. Jose, Energy Fuels, 2022, 36, 15268–15278.
206. A. S. Levitt, M. Alhabeb, C. B. Hatter, A. Sarycheva, G. Dion and Y. Gogotsi, J. Mater. Chem. A, 2019, 7, 269–277.
207. G. Nie, Z. Zhang, Y. Liu, J. Wang, C. Fu, H. Yin, J. Chen, L. Zhao and Z. Pan, Adv. Fiber Mater., 2022, 4, 1129–1140.
208. J. Han, S. Wang, S. Zhu, C. Huang, Y. Yue, C. Mei, X. Xu and C. Xia, ACS Appl. Mater. Interfaces, 2019, 11, 44624–44635.
209. C. Ma, L. Wu, L. Zheng, R. Gan, Q. Fan, Y. Song and J. Shi, J. Nanopart. Res., 2019, 21, 33.
210. U. Kurtan, U. Sahinturk, H. Aydın, D. Dursun and A. Baykal, J. Inorg. Organomet. Polym. Mater., 2020, 30, 3608–3616.
211. G. Nie, X. Zhao, J. Jiang, Y. Luan, J. Shi, J. Liu, Z. Kou, J. Wang and Y.-Z. Long, Chem. Eng. J., 2020, 402, 126294.
212. J. Tian, Y. Shi, W. Fan and T. Liu, Compos. Commun., 2019, 12, 21–25.
213. C. Zhao, N. Sun, M. Li, W. Cai, W. Jiang and C. Zhao, Energy Fuels, 2023, 37, 13467–13475.
214. S.-C. Lin, Y.-T. Lu, Y.-A. Chien, J.-A. Wang, P.-Y. Chen, C.-C. M. Ma and C.-C. Hu, J. Power Sources, 2018, 393, 1–10.
215. Y. Liu, D. Wang, C. Zhang, Y. Zhao, P. Ma, W. Dong, Y. Huang and T. Liu, Adv. Fiber Mater., 2022, 4, 820–831.
216. M. Yanilmaz, M. Dirican, A. M. Asiri and X. Zhang, J. Energy Storage, 2019, 24, 100766.
217. M. El-Shafei, A. El-Deen, A. El-Moneim and A. Hessein, J. Mater. Sci.: Mater. Electron., 2021, 32, 15882–15897.
218. W. Liu, F. Qiao, J. Yang, J. Yuan, Y. Xie, T. Wang, J. Hu, J. Zheng, R. Ren, X. Kang, Y. Zhao and J. Zhang, Nano Res., 2023, 16, 10401–10411.
219. H. Zheng, Q. Wei, X. Han, X. Yu, Y. Li and J. Zhou, ACS Appl. Nano Mater., 2021, 5, 455–463.
220. H. C. Lee, Y. A. Kim, H.-J. Lee and B.-H. Kim, Carbon, 2023, 207, 154–161.
221. E. S. Kim, H.-J. Lee and B.-H. Kim, Electrochim. Acta, 2022, 406, 139883.
222. H. C. Lee, Y. A. Kim and B.-H. Kim, Carbon, 2022, 196, 78–84.
223. D. Wu, X. Xie, J. Zhang, Y. Ma, C. Hou, X. Sun, X. Yang, Y. Zhang, H. Kimura and W. Du, Chem. Eng. J., 2022, 446, 137262.
224. Z. Li, J. Lin, B. Li, C. Yu, H. Wang and Q. Li, J. Energy Storage, 2021, 44, 103437.
225. D. Wang, F. Li, L. Yin, X. Lu, Z. Chen, I. R. Gentle, G. Lu and H. Cheng, Chem.–Eur. J., 2012, 18, 5345–5351.
226. J. Dong, G. Lu, F. Wu, C. Xu, X. Kang and Z. Cheng, Appl. Surf. Sci., 2018, 427, 986–993.
227. J.-S. M. Lee, M. E. Briggs, C.-C. Hu and A. I. Cooper, Nano Energy, 2018, 46, 277–289.
228. P. Du, L. Liu, Y. Dong, W. Li, J. Li, Z. Liu and X. Wang, Electrochim. Acta, 2021, 370, 137801.
229. Y. Zhou, Z. Jia, L. Shi, Z. Wu, B. Jie, S. Zhao, L. Wei, A. Zhou, J. Zhu, X. Wang and Y. Fu, Chem. Eng. J., 2020, 385, 123858.
230. W. Wei, W. Liu, Z. Chen, R. Xiao, Y. Zhang, C. Du, L. Wan, M. Xie, J. Chen and Z. Tian, Appl. Surf. Sci., 2020, 509, 144921.
231. J. Zhou, H. Shen, Z. Li, S. Zhang, Y. Zhao, X. Bi, Y. Wang, H. Cui and S. Zhuo, Electrochim. Acta, 2016, 209, 557–564.
232. Z. Song, L. Li, D. Zhu, L. Miao, H. Duan, Z. Wang, W. Xiong, Y. Lv, M. Liu and L. Gan, J. Mater. Chem. A, 2019, 7, 816–826.
233. Y. Chang, H. Shi, X. Yan, G. Zhang and L. Chen, Carbon, 2020, 170, 127–136.
234. R. Kumar, S. Sahoo, E. Joanni, R. K. Singh, K. Maegawa, W. K. Tan, G. Kawamura, K. K. Kar and A. Matsuda, Mater. Today, 2020, 39, 47–65.
235. D. Qiu, M. Li, C. Kang, J. Wei, F. Wang and R. Yang, Small, 2020, 16, 2002718.
236. Y. Li, Z. Li and P. K. Shen, Adv. Mater., 2013, 25, 2474–2480.
237. M. Kotal, H. Kim, S. Roy and I.-K. Oh, J. Mater. Chem. A, 2017, 5, 17253–17266.
238. L. Hao, Y. Gao, A. W. Robertson and Z. Sun, Innovations Mater., 2024, 2(1), 100050.
239. W. Zhang, H. Cao and P. Chen, Innovations Mater., 2023, 1(1), 100006–1000011.

---