4.1.1 CNFs

Carbon nanofibers have attracted widespread attention in EMI shielding due to their excellent electrical properties, environmental friendliness, chemical inertness, and ease of preparation.¹¹¹ PAN is a commonly adopted precursor for preparing carbon micro/nanofibers due to its high carbon formation rate. Electrospun PAN micro/nanofibers display a high specific surface area and excellent conductivity, making them one of the preferred materials for EMI shielding. Xu et al.¹¹⁵ prepared high-performance copolymerized PAN (coPAN) nanofibers using copolymerization and electrospinning techniques, increasing tensile strength from 304 to 595 MPa. Subsequently, Ma et al.¹¹⁶ obtained CNF by high-temperature carbonization of PAN and introduced it into the upper and lower layers of carbon fiber reinforced composites (CFRCs). Incorporating CNF does not affect the composite material's interlaminar shear strength (ILSS) and significantly improves its shielding performance by 35%. Im et al.¹¹⁷ fabricated a carbon-based EMI shielding material via the same approaches. They found that the material's dielectric constant increases with increasing carbonization temperature, resulting in a high EMI shielding performance.

Additionally, the experimental analysis shows that the EMI shielding effectiveness of this material could reach an average value of 31.79 dB in the frequency range of 800–8500 MHz. Bayat et al.⁹⁶ used the same process to prepare multifunctional Fe₃O₄/carbon nanofiber composites and analyzed their corresponding EMI shielding performance. The results show that the electrical conductivity of the nanofibers increased from 0.05 ± 0.004 S/cm to 2.6 ± 0.9 S/cm as the carbonization temperature increased from 700 to 900°C. Accordingly, the nanofibers' shielding effectiveness (SE) increases from 0 to 15 dB. All these studies indicate that electrospun CNFs can be widely applied in the EMI shielding field.

The aforementioned studies have all obtained carbon micro/nanofibers for EMI by carbonization of electrospun pure polymers. However, the EMI capabilities of those materials cannot meet the requirements of modern society. People started to add diverse fillers to enhance their EMI shielding effectiveness. Kang et al.¹¹⁸ prepared a carbon composite nanofiber web containing BaTiO₃ and Fe₃O₄ using electrospinning and carbonization, achieving a maximum SE of 23 dB. Deeraj et al.¹¹⁹ also prepared composite nanofiber materials doped with ZrO₂ via electrospinning, which exhibits an excellent EMI shielding ability in the Ku band (12–18 GHz). Additionally, the composites own an SE exceeding 90 dB by stacking this composite fibrous pad up to the 8th layer (thickness less than 0.8 mm). Li et al.⁵⁷ also prepared a composite film by coating a large area of electrospun CNF and silicone resin on the surface of an alternating multi-layer structure (AMS) for EMI application. The test results show that the SE of this AMS film with a thickness of about 0.6 mm is approximately 100 dB, displaying a fascinating EMI shielding performance. Subsequently, Wang et al.⁵⁸ reported a one-dimensional N-doped carbon nanofiber (Figure 4a,b), which encapsulated hollow Co₃SnC_0.7 nanocubes in the electrospun fibrous cavities. A strong impedance matching is achieved through the synergistic effect Co₃SnC_0.7 in conjunction with the carbon webs. At 800°C, this material with a thickness of 2.3 mm shows a necklace-like Co₃SnC_0.7/CNF with a low loading of 5%. It exhibits a strong absorption capability of −51.2 dB in the frequency of 14.56 GHz and an effective absorption bandwidth of 7.44 GHz with a matching thickness of 2.5 mm (Figure 4c,d). The multiple EMW reflections and interfacial polarization between the fibers and the fibers internally contribute to attenuating the EMW, as shown in Figure 4e.

4.1.2 GN

Graphene not only possesses comparable stability and physicochemical properties similar to CNF but also shows a unique feature of repairing these defects of EMWs transmission and absorption, making it one of the most attractive candidates for designing EMI shielding materials. In addition, GN can significantly enhance the electrical conductivity of electrospun micro/nanofibers without affecting their mechanical flexibility and structure integrity.¹²⁰ Graphene has a higher tensile strength than CNTs and a more significant thermal conductivity than any other carbon-based fillers. Meanwhile, the design and production of innovative, flexible, lightweight, and robust materials having outstanding EMI sheltering characteristics is now an imperative mission.^{121, 122} It is a good choice as a filler for electrospun polymer nanocomposites for EMI shielding applications. GN, a wonder material, exhibits great promise in various fields with its unique electronic structure and excellent physical, chemical, electrochemical, thermal, and mechanical properties.^{123, 124}

Many studies have explored an optimal preparation strategy to preserve GN's inherent physicochemical characteristics and create high-performance EMI shielding materials. Guo et al.¹²⁵ prepared polystyrenes (PS) based composites containing interconnected and oriented graphite nanoplatelets (GNPs) via electrospinning, cold-pressing, and hot-pressing. As shown in Figure 5a, the obtained composite materials endow with oriented distribution and random dispersion of GNPs/PS, which display an excellent EMI shielding effectiveness due to the formation of thermally and electrically conductive networks. Figure. 5b–d presents the EMI SE values of the random and oriented GNPs/ PS composites at 8.2–12.4 GHz (X-band) frequency. From Figure. 5e, the SE_T, SE_A, and SE_R all enhance with the increasing addition of GNPs and the SE_T, SE_A, and SE_R of the oriented GNPs/PS composites are all higher than that of random GNPs/PS composites, and the SE_A of all the GNPs/PS composites is higher than SE_R. Besides, the oriented GNPs/PS composites have stratified structures. The EMWs will have multi-reflection and absorption during transferring multi-layer GNPs (Figure. 5f). Moreover, the multi-reflection and absorption effects of the EMWs inner random GNPs/PS composites are also reduced due to poor conductive networks (Figure. 5g). And the SE of composite materials with 35 wt% GNPs and a thickness of 3 mm are found to be 33 dB. Subsequently, Liang et al.⁶⁰ used electrospinning to prepare flexible PVDF membranes with alternating GN/Ni nanochain orientations for EMI shielding and heat dissipation applications, as shown in Figure 5h. The heterogeneous alternating multilayer structure (HAMS) of the flexible PVDF membrane can form a practical electron-conducting pathway, resulting in an optimal electrical conductivity of 76.8 S/m. In particular, the unique structure can simultaneously enable coordinated electromagnetic losses and multi-stage electromagnetic reflections of EMWs, as shown in Figure 5m,n. The results show that HAMS membrane with a thickness of 0.5 mm can exhibit a high EMI SE of 43.3 dB (Figure 5i,k), which can be further improved to 51.4 dB when the thickness increases to 0.7 mm (Figure 5j,l). reveals the specific ratio of SE_A in SE_T of more than 50%. These results suggest the absorption dominant shielding mechanism for HAMS films. Both oriented distribution and HAMS could interconnect with each other to form effective conductive networks, thereby significantly enhancing the shielding performance of EMI materials.

Subsequently, Song et al.⁵⁹ prepared a new flexible all-carbon network by electrospinning a blend of PAN and graphene, followed by high-temperature carbonization to eliminate the insulating framework of the composite material and form a CNF-GN-CNF heterostructure. These heterostructures are important interface modifiers that can significantly improve the conductivity of the obtained matrix, thus highly enhancing its EMI SE, and the composite networks with 31.9 wt% GN, this material with a thickness of about 0.22–0.27 mm show a relatively superior SE of 25–28 dB. Afterward, Bian et al.¹²⁶ introduced the reduced graphene oxide (rGO) into electrospun CF networks via a similar method to prepare a novel graded carbon fiber/reduced graphene oxide/nickel (CF/rGO/Ni) composite textile. The EMI shielding performance indicated that the graded composite textile possesses a strong shielding effect greater than 61 dB, demonstrating a fabulous advantage over a conventional polymer shielding composite film. Similarly, Huang et al.⁸⁶ electrospun a multi-scale double-continue conductive network (TiO₂/SiO₂@PPy@rGO). The sandwich structure works as an effective dissipative medium, leading to a high SE of approximately 30 dB in the X band.

4.1.3 CNTs

Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNTs) and MWCNTs.¹²⁷ SWCNTs are more difficult and expensive to prepare compared to MWCNTs. Therefore, MWCNTs are usually chosen as conductive reinforcing fillers. Additionally, the excellent electrical conductivity (104 S/cm) and thermal conductivity (over 200 W/mK) of MWCNTs make them one of the most promising fillers in EMI shielding applications.¹²⁸ In terms of electrospun MWCNT/polymer composites, several studies have shown that they exhibit excellent physicochemical properties, including lightweight, high porosity, large surface area, and strong stability.²⁹ Especially those composed materials with low MWCNT loadings also own outstanding conductivities and high EMW absorption capabilities.^{129, 130} Those facts addressed that electrospun MWCNT/polymer membranes are preferred for constructing high-performance EMI shielding materials.

Commonly, the interconnection between MWCNTs can form a relatively complete conductive network, resulting in a high enhancement of EMI shielding abilities for electrospun micro/nanofibers embedded with them. For instance, Im et al.⁷⁸ prepared PANI-based fibers with MWCNT and polyethylene oxide (PEO) by electrospinning and employed them as EMI shielding materials. To improve the EMI shielding performance of the material, their surfaces were modified by fluorination treatment, leading to the dispersion and adhesion of MWCNT in PANI/PEO fiber. The test results established that the composite fibers with the MWCNT load of 82 wt% display an excellent conductivity (4.8 × 10³ S/m) and superior SE of about 42 dB. In contrast, Quanamnia et al.¹³¹ fabricated polystyrene (PS) nanofibers incorporated MWCNTs at varying concentrations ranging from 0 to 10 wt% via electrospinning. They found an opposite result: the addition of MWCNTs induces a decrease in the material's electrical conductivity due to the disorientations of MWCNT with increasing concentrations. Subsequently, Nasouri et al.⁶¹ fabricated electrospun lightweight conductive MWCNT/polyvinyl alcohol (PVA) composite nanofibers and studied the effects of several factors, including MWCNT contents, composite fiber thickness, and EMW frequency on their EMI shielding performance. They found that the composites with a thickness of 3 mm and MWCNT loading of 7.7 wt% show a low reflection loss of 8.8 dB and a maximum absorption loss of 31.5 dB at 12GHz frequency. Shoushtari et al.^{132, 133} prepared MWCNT/PVP composite nanofibers with different contents of MWCNT via electrospinning and measured their EMI shielding performance in the X-band (8.2–12.4 GHz) frequency range. They found that their SE can be as high as 42 dB. Chen et al.¹³⁴ fabricated CNT/PAN composite nanofibers with uniform sizes through electrospinning and carbonization. Subsequently, they tested the EMI shielding performance of the obtained materials in the frequency range of 300 MHz to 3GHz. The results present that the thin carbon fiber sheets can even achieve more than 15 dB. When its thickness increases to 150 μm, the SE of the composite material can reach more than 30 dB. Similarly, Jiang et al.¹³⁵ prepared PAN/MWCNT composite nanofibers via electrospinning and the test showed that the EMI shielding efficiency reached over 90% in the frequency range of 1–15 MHz when the MWCNT content was above 10%.

As mentioned, MWCNT shows a high porosity and specific surface area, which can offer a good shielding effect on EMWs in the lower frequency band. For example, through electrospinning, Salimbeygi et al.¹³⁶ prepared PVA sodium dodecyl sulphate (SDS)/MWCNT composite nanofibers. It was found the 1 mm thick PVA/SDS/MWCNT nanofibrous film with 10 wt% MWCNT loading exhibits a maximum reflection loss of 15 dB at 8 GHz, wherein the average diameter of the nanofibers is 224 ± 30 nm. Soon afterward, Chen et al.⁶⁵ applied magnetic-assisted electrospinning to produce CNT-poly(2-hydroxyethyl methacrylate) (CNT-PHEMA) composite material with arranged CNTs. The obtained materials present an awesome EMI shielding absorption performance and self-healing ability. Uniquely, the effective EWM absorption of the material is also largely due to the ordered CNT arrangements and high porosity. The SE of the 2 mm thick textile with very low CNT loading (0.17 wt%) is even 20.42 dB at a frequency of 11.3 GHz. Kim et al.⁶⁴ prepared an ultrathin and lightweight MWCNT buckypaper/electrospun PAN nanofiber (BP/NF) film by electrospinning, thermal-rolling, and vacuum filtration. The BP/NF-130 (thermal-press at 130°C) composite film with a thickness of 100 μm possesses a good conductivity (6.177 S/cm) and an EMI SE of 23.3 dB. Ramírez-Herrera et al.¹³⁷ developed interlayered composites consisting of MWCNT-filled polypropylene (PP) composite and carbon nanofiber mats to improve the conductivity and EMI shielding efficiency. The SE of assembled materials increases from 17 to 52 dB as the layer number changes from signal-layer to eight-layer at a frequency of 900 MHz. Sung et al.¹³⁸ electrospun MWCNT and polyurethane (PU) to prepare (MWCNT-PU) nets, which show a remarkable SE up to 25 dB in the frequency range of 50 MHz–10 GHz.

4.2.1 Metal nanoparticles

Metal nanoparticles with nanoscale structures and excellent conductivities are effective fillers for preparing high-performance EMI shielding materials.⁷⁵ Goude et al.¹⁴² synthesized carboxymethyl cellulose nanofibers (CMC-NFs) doped with different metal nanoparticles (Cu, Fe, Zn, Cd, and Co) via electrospinning. They evaluated their EMI shielding properties and found that the types of loading metals significantly influence the conductivity and EMI shielding performance of the resulting fibrous mats. Building upon this work, Kim et al.⁶⁸ prepared PVA composite nanofiber webs decorated with Fe₂O₃ and Ag nanoparticles using a combination of electrospinning and metal deposition. They studied the EMI shielding performance of various composite nanofiber webs with different thicknesses. Experimental results indicate that Ag-coated nanofibers with 10% Fe₂O₃ show an exceptional SE of up to 45.2 dB at a frequency of 10 GHz. Wei et al.¹⁴³ prepared polyethersulfone PES/PDA/Ag nanofibers with good electric conductivity by electrospinning and in situ polymerization.

In contrast to regular PES micro/nanofibers with 2.1 × 10⁹ Ω/cm, the electrical resistivity of PES/PDA/Ag decreased to only 202 Ω/cm via the optimized AgNPs preparation process. Furthermore, the EMI shielding efficiency of metal-deposited nanofiber mats rises proportionally with the thickness of the deposited layer, which is also true in both bi-metallic and multi-metallic deposition systems.⁶⁷ Zeng et al.⁶⁶ designed a novel cell membrane structure based on a facile PDA-assisted metal (copper or silver) deposition on electrospun polymer nanofibers, which exhibits superior electrical conductivity, mechanical flexibility, and an ultrahigh EMI shielding performance. The analysis of their EMI performance unveils that the prepared membrane, merely 2.5 μm thick at a density of 1.6 g/cm³, shows an EMI SE exceeding 53 dB across a broad frequency range. Additionally, the corresponding EMI SE of this film remains high at 44.7 dB as its thickness decreased to 1.2 um.

The EMI shielding effectiveness of metal-based shielding materials tends to increase with the amount of loading fillers.¹⁴⁴ Chiscan et al.¹⁴⁵ reported polyvinyl chloride (PVC)/Fe composite nanofibers via depositing Fe nanoparticles with varying concentrations onto the surfaces of PVC nanofibers with sizes ranging from 100 to 500 nm. The EMI shielding performance of the resulting composite fibers in the X-band is evaluated, and the results demonstrate that their EMI shielding effectiveness can be effectively tuned by controlling the concentrations of Fe nanoparticles. Ji et al.⁴² synthesized highly efficient cross-linked polyacrylonitrile (CPAN) composite films incorporating Ag, Cu, and Ni via electrospinning and chemical deposition. Experimental analyses show that the CPAN/Ag hybrid membrane (with a thickness of 53 μm) displays an excellent SE up to 90 dB, significantly higher than that of the hybrid films made of the other two metal nanoparticles. Li et al.⁷¹ used a similar technique to deposit Ag onto an electrospun fiber membrane. Then, they endowed a core-shell structure to the nanofiber via a simple wet-chemical deposition process. The research showed that this material exhibits a high electrical conductivity (approximately 17,788 S/m) and a strong EMI SE (approximately 82 dB). Plus, the composite film has a certain hydrophobicity and acid-alkali resistance properties. Yang et al.⁷³ also chemically deposited Ag on electrospun polymer nanofibers, which are then modified with stearic acid (SA) to obtain a porous blend film with spatially distributed Ag coating. This composed film achieves an excellent conductivity of 57,319 S/cm and an EMI SE up to 90.14 dB in the ultra-wideband frequency range. Subsequently, Xia et al.¹⁰⁰ chemically plated Cu on electrospun polyurethane (PU) nanofibers (Figure 6a), which can change from conducting to insulating when the stretch deformation reaches 50% and then back to conducting as the stress is released. The deformation causes repeated separation and contact of Cu nanoparticles on the surfaces of PU, thereby realizing effective shielding of EMWs (Figure 6b). This conductive fabric PU@Cu with stretching ability can dissipate EMWs by changing its structural properties. When the thickness of the materials is approximately 15.7 μm (Figure 6c), its EMI SE can be experimentally varied from 0 to 52.31 dB in real-time (Figure 6d).

Electrospun TPU usually exhibits remarkable properties, such as high tensile strength, good flexibility, and so forth, so it is often selected as the polymer substrate. Zhai et al.⁸¹ prepared multifunctional and highly stretchable EMI shielding fiber membranes using electrospun thermoplastic TPU fiber membranes, PDA, Ag nanoparticles (AgNPs), and MXene nanosheets. The TPU/PDA/Ag/MXene (TAMF) fiber membrane with a thickness of 150 nm has an ultra-high conductivity of 95,238 S/m and excellent EMI SE of 108.8 dB in the X-band. Subsequently, Yang et al.⁸² prepared a TPU/PDA/Ag composite material with superior EMI shielding capabilities by combining electrospinning, chemical deposition, and pressing techniques. When the thickness of the composite film is 45 μm, its SE is 109 dB in the X-band. This study proposed a unique fibers-induced segregated-like structure strategy for preparing EMI shielding materials, where a continuous Ag layer composed of a distinct quasi-segregation structure endows the composite film with a successive thermal conduction path. When such materials are applied in high-power LED modules, they can ensure a certain level of EMI SE and present a good thermal management ability. This study is significant for developing high thermally conductive and EMI shielding composite materials.

Similarly, Yang et al.⁷⁵ introduced the AgNWs layer into the electrospun TPU fibrous film decorated by Fe₃O₄@MWCNT. The unique asymmetric conductive network with an interconnected thermally conductive path was successfully constructed, composed of the bottom TPU/Fe₃O₄@MWCNT fibrous film for the EMW absorption and the upper AgNWs layer for the EMI shielding. The resultant film showed remarkable conductivity (2.22 × 10⁵ S/m), excellent SE (up to 78.48 dB), and super-high in-plane thermal conductivity (7.83 Wm⁻¹ K⁻¹).

In other studies, Huang et al.¹⁴⁶ fabricated a multifunctional electrospun PU fiber with a unique multidimensional conductive network structure. Then, they deposited Ag onto its surface using an Ag adsorptive reduction process to achieve high-performance EMI shielding. Jiang et al.⁷⁰ also prepared a fibrous membrane with a silver-carbon nanotube (AgNT) network structure by electrospinning and physical deposition techniques, as shown in Figure 7d, which also exhibits an excellent EMI shielding effectiveness and achieved thin-layer R_s of 1.0 Ω/sq and an EMI SE of 35 dB (Figure 7c,d). Figure 7b shows the shielding mechanism of AgNTs. Subsequently, Zhang et al.⁸⁰ proposed a method to get highly conductive and durable nanofiber composites assisted with PVP. PVP-assisted preparation of the PVP/Ag@PU is schematically described in Figure 7e, which acted as an interface adhesive to significantly improve the interface interactions between Ag and the electrospun base material. It can be seen from Figure 7g,h that the SE_A, SE_R, and SE_T all increase with the increase of the PVP concentration.

Furthermore, the PVP can form a protective layer to prevent oxidation of the Ag and enhance the mechanical properties of the polymer nanofibers. The prepared nanofiber composites have excellent EMI shielding performance (a conductivity of 245.7 ± 30.6 S/cm and an EMI SE of up to 96.9 dB). The EMI shielding mechanism is in Figure 7f. The facile fabrication approach for core-shell structures enables the preservation of multi-functionality to a significant extent for EMI materials, thereby endowing them with substantial potential in industrial applications. Wang et al.⁷² constructed a ternary PAN/W₁₈O₄₉/Ag composite nanofiber membrane through electrospinning and post-treatment. The conductivity of this membrane is as high as 30,400 S/cm⁻¹. When the thickness of the membrane is 0.11 mm, its highest EMI SE in the frequency range of 8–26.5 GHz can reach 100.9 dB. Zhang et al.¹⁴⁷ used electrospinning technology to obtain thermally stable and highly ductile EMI shielding membranes with sandwich structures. Their interlayers consist of two identical layers of Ag/polyimide (PI) nanofiber mats and the signal layer of very conductive MXene nanosheets. After hot pressing, the obtained membrane is endowed with excellent mechanical properties and EMI shielding performance (the highest SE in the X-band is 40.73 dB). Ju et al.⁷⁴ obtained TCCNF by embedding TiN and Co in CNF through electrospinning and annealing. The EMI shielding test results show that TCCNF demonstrates an ultra-high SE of 93 dB in the 8.2–12.4 GHz frequency range. Kazushige et al.¹⁴⁸ reported that Cu was deposited on the electrospun PVA composite nanofibers using the metal deposition technique. The Cu-deposited composite nanofibers showed a maximum EMI SE of about 25.1 dB at the frequency range from 0.5 to 18 GHz.

4.2.2 Metal oxide

Metal oxide is another powerful dopant in metal-based fillers for preparing EMI shielding materials.⁹⁶ Much research has been conducted on combining metal oxide NPs, including iron, copper, and other metal oxides, with electrospun polymers for high EMI shielding fields.

Fe₃O₄ is a great filler for EMI shielding owing to its advantages of a great filler with low toxicity and a small permeability threshold. And Fe₃O₄-based composite materials with a certain loading content can also achieve high EMI shielding performance, thus attracting lots of attention in recent years.^{96, 149} Chiscan et al.¹⁵⁰ deposited Fe₃O₄ nanoparticles on electrospun PVC to obtain a PVC/Fe₃O₄ composite nanofiber. They found that its EMI absorption property in the X-band can be turned by adjusting the loading amount of Fe₃O₄ in composite materials. Specifically, the composite nanofibers exhibit a transmission loss below −16 dB in the microwave frequency range of the X-band. Kim et al.⁶⁸ incorporated Fe₂O₃ and Ag into electrospun PVA nanofiber webs through the same method. The experimental results show that the EMI shielding effectiveness of the composite fiber correspondingly increases with increasing the thickness of the metal layer on the fiber surface. The SE of the obtained material with 10% Fe₂O₃ content could reach 45.2 dB in the frequency range of 10 GHz. Bayat et al.¹⁵¹ developed an EMI shielding composite nanofibers using electrospinning and heat treatment. They investigated the effect of Fe₃O₄ nanoparticle size and content on the microstructure and electromagnetic properties of the composite. The diameter and conductivity of the composite nanofiber become larger with bigger Fe₃O₄ nanoparticles, leading to a high EMI shielding. Bayat et al.⁹⁶ electrospun multifunctional Fe₃O₄/carbon nanofiber composite materials and tested their EMI shielding effectiveness in the X-band by examining Fe₃O₄ loading amount, carbonization temperature, and fiber thickness.

The test results indicate that a composite fiber with a 5 wt% Fe₃O₄ loading and a thickness of 0.7 mm exhibits the highest EMI shielding effectiveness (67 dB). Subsequently, Im et al.⁹⁷ prepared Fe₃O₄/BaTiO₃/MWCNT composite materials with the highest SE of 37 dB by electrospinning and heat treatment. The metal oxide additives preserve their original characteristics during the heat treatment process, and the introduction of MWCNTs increased the conductivity of the composite material. Building on this work, Chen et al.⁸³ prepared TPU/PAN/Fe₃O₄ composite nanofiber membranes by depositing Fe₃O₄ onto a unique self-supporting electrospun TPU/PAN film via electrospinning and dip-coating processes. The obtained film shows a high magnetization intensity of 18.9 emu/g, and its SE in the X-band exceeds 30 dB. Nasouri et al.⁹² prepared a sandwich-structured composite nanofiber for EMI shielding by dispersing Fe₃O₄ nanoparticles in electrospun PVP fiber using ultrasonic technology. The resulting materials with a 4 wt% Fe₃O₄ loading realize an SE of approximately 22 dB in the X-band.

ZrO₂ is also a popular additive in EMI shielding applications due to its high dielectric constant, low thermal conductivity, chemical inertness, and high hardness.⁶⁵ Deeraj et al.¹¹⁹ fabricated a ZrO₂-doped PAN film via electrospinning and carbonization. Then, they impregnated it with epoxy resin to form ZrO₂/CNF-epoxy laminates. The EMI shielding testing revealed that the eight-layer epoxy laminate displays a great reflection performance over −90 dB at the Ku frequency range. Lee et al.¹⁵² demonstrated a ZnO nanorod (NR)-decorated PVDF-MXene composite nanofibers for the EMI shielding application. The electrospun PVDF-MXene composite provided superb flexibility, while MXene encapsulated in PVDF and hydrothermally decorated ZnO NRs around the nanofibers offered multiple internal absorption sites. The experiments suggested that the obtained materials demonstrated a high EMI SE of 61 dB in the X-band. Cheng et al.⁸⁴ utilized polysilazane (PCS) and zirconium acetylacetonate (Zr(acac)₃) as precursors to successfully prepare lightweight and flexible ZrC/SiC composite nanofiber mats through electrospinning and high-temperature pyrolysis. The study demonstrated that a three-layer ZrC/SiC nanofiber mat with a thickness of 1.8 mm could achieve an SE of 18.9 dB. When the density of the materials is lower than 0.06 g/cm³, its SSE is up to 315 dB/cm³/g. Notably, the SE of the lightweight EMI shielding material using ZrC/SiC nanofiber mats at 600°C could be elevated to 20.1 dB. This indicates that those materials show considerable potential for applications in high-temperature environments.

Besides specific metal oxide fillers incorporated with electrospun micro/nanofiber can improve the EMI shielding performance, altering the structure of electrospun composite materials with metal oxide filler can also enhance the absorption and reflection of EMWs. Zhang et al.⁶⁹ employed an electrospinning-layering-hot pressing technique to prepare sandwich-structured EMI shielding nanocomposite films with PVA as a polymer matrix (Figure 8a). Fe₃O₄/PVA composite fibers are located on the top and bottom layers. In contrast, Ti₃C₂T_x/PVA composite fibers are positioned in the middle layer (Figure 8b), as indicated by the Fe and Ti elements shown in EDS images (Figure 8b₁,b₂). Figure 8c–f shows the EMI shielding performance of the sandwich-structure EMI shielding nanocomposite film. When the contents of Ti₃C₂T_x and Fe₃O₄ are 13.3 and 26.7 wt%, respectively, the SE of the 75 μm-thick fiber film is about 40 dB. Where after, Wu et al.⁶³ fabricated a composite film using a mixture of polydimethylsiloxane (PDMS)/carboxyl-functionalized MWCNTs and magnetic iron oxide nanoparticles (MWCNTs-COOH/Fe₃O₄ MFs), which is employed as the top and bottom layers of a sandwich structure with Ag-deposited electrospun TPU fiber membrane as the middle layer to achieve an excellent low reflection and good durability. It exhibits outstanding EMI shielding properties (electrical conductivity of 361 S/cm and EMI SE of 85.4 dB). This obtained film can effectively function as an absorber layer and act as a micro-nano scale superhydrophobic surface. Zhang et al.⁸⁵ prepared Fe₃O₄/polyamic acid (PAA) nanofiber membranes by in-situ polymerization and electrospinning (Figure 8h). Then, they obtained a Janus Ti₃C₂T_X-(Fe₃O₄/PI) film by depositing Ti₃C₂T_x nanosheets on the surface of the fiber membrane, followed by thermal imidization (Figure 8i). The two sides of this film present completely different properties. The Fe₃O₄/PI side exhibits excellent hydrophobicity and insulation.

In comparison, the Ti₃C₂T_x side displays outstanding hydrophilicity and electrical conductivity (Figure 8m,n). When the mass fraction of Ti₃C₂T_x reaches 80 wt%, the Janus Ti₃C₂T_x-(Fe₃O₄/PI) composite shows remarkable EMI shielding properties (SE of approximately 66 dB), and mechanical features (tensile strength of 114.5 MPa, Young's modulus of 5.8 GPa). Figure 8j–l shows the EMI shielding performances of Ti₃C₂T_x-(Fe₃O₄/PI) composite films in the X-band. As the EMWs are incident from both sides of the Janus film, the two sides of the film present different EMI shielding effects. Compared to the EMI shielding composite film prepared via a blending-electrospinning-hot pressing process, the sandwich-structured EMI shielding membrane is more conducive to forming magnetic, electrical, and thermal conduction paths, thereby improving their loss absorption toward EMWs.

In other works, Jin et al.⁷⁶ utilized electrospinning to fabricate CoFe₂O₄-containing PAN composite fiber membranes, which exhibit a SE of about 3.9 dB and a magnetization intensity of 4.7 emu/g. Nakhaei et al.¹⁵³ prepared novel TiO₂/SiO₂ (TS) composite nanofibers using electrospinning. The XRD spectrum of the TS sample displayed both hexagonal SiO₂ and rutile TiO₂ structures within the polymer matrix. The EMI shielding testing demonstrated that the TS core-shell nanofibers possess excellent microwave absorption properties. Li et al.⁹³ prepared recycled polyethylene terephthalate nanofiber web (RPET NFs) embedded using electrospinning from a bath-sonication solution. Compared with pure RPET NFs, the EMW reflection coefficient, absorption coefficient, and EMI shielding performance of the nanofiber web were significantly enhanced owing to the addition of Fe₃O₄ nanoparticles. Sharma et al.⁷⁷ developed a novel lightweight carbon foam (CF) decorated with zinc chloride nanofibers (ZONF) for high-performance EMI shielding via electrospinning. The impedance matching and interface polarization provided by ZONF can effectively improve the CF's ability to absorb EMWs. The experimental results showed that the composite material with a thickness of 2 mm exhibits a great shielding performance in the X-band (SE up to 58.6 dB). Notably, the density of the composite is only 0.28 g/cm³, making it an ideal material for aerospace and next-generation smart electronic devices.

Wang et al.⁷² constructed PAN/W₁₈O₄₉/Ag composite nanofiber membranes through electrospinning and post-treatment methods, which show outstanding EMI shielding effectiveness against multi-band EMWs. In the infrared band (15–150 THz), the fiber membrane with a thickness of 0.11 mm exhibits a low infrared emissivity (0.26–0.6). It also presents a 99% attenuation rate for 30 keV x-rays as its thickness increases to 1.92 mm. Some studies have focused on the impact of metal oxide types and electrospun polymer solution concentrations on the EMI shielding effectiveness. For example, Jamil et al.¹⁵⁴ prepared PVA/Bi₂O₃ and PVA/WO₃ nanofiber mats with various Bi₂O₃/WO₃ loading contents (0–40 wt%) and diverse PVA concentrations (10 or 15 wt%). The results show that the obtained materials with 35 wt% Bi₂O₃ loading or 35 wt% WO₃ loading in 15 wt% PVA indicate great shielding effectiveness against x-rays, respectively.

4.4.1 MXene laminates

The MXene layered films, including single-metal Ti₃C₂T_x, ordered dual-metal MoTiC₂T_x, and Mo₂Ti₂C₃T_x MXene, have been extensively studied for years.¹⁶³ The excellent EMI shielding performances of Ti₃C₂T_x MXene films are mainly attributed to their outstanding electrical conductivity and the layer thickness and uniform arrangements of their microstructures. Many efforts have been made to obtain a high EMI performance material.

A recent study investigated the effect of MXene layered film thickness on its EMI shielding capability. For instance, Yun et al.¹⁷⁰ assembled single-layer MXene films by interface self-assembling MXene flakes and then prepared multi-layer films by repeat stacking single-layer films. They investigated the influence of MXene layered film thickness on the material's shielding ability. They found that a single-layer Ti₃C₂T_x MXene film with an average thickness of 2.3 nm exhibited outstanding flexibility, over 90% transmission rate, and a thin-layer resistance of 1056 Ω/cm². As the number of Ti₃C₂T_x MXene layers increased, the thin-layer resistance correspondingly decreased, and the corresponding EMI shielding performance increased. Test results showed that the SE of the assembled single-layer Ti₃C₂T_x MXene film was 1 dB. In comparison, the SE of the multilayer film with a thickness of 55 nm was 20 dB, effectively shielding 99% of electromagnetic radiation.

Generally, the mechanical properties of MXene layered films are poor, which has led to an increasing popularity in developing robust and durable MXene-based composite materials. Ye et al.⁴³ first reported a highly flexible and stretchable electrospun PU nanofiber covered with Ti₃C₂T_x MXene. The sandwich-structured MXene/PU material is wrinkled and exhibits stable mechanical properties, demonstrating a high EMI shielding effectiveness (SE > 20 dB) within a deformation range of 30%. Subsequently, Guo et al.¹⁷¹ prepared a flexible composite film (d-Ti₃C₂T_x/r-CNFs) via electrospinning and vacuum-assisted filtration. The 15 μm thick composite film exhibits excellent EMI shielding performance (SE up to 42.7 dB) in the 2–18 GHz frequency range. Wei et al.¹⁴⁶ used a weight ratio 1:2 for CNT and MXene and twice Ag absorption reduction processes. 0D silver nanoparticles are designed to be loaded on 2D MXene sheets and combined with 1D CNT in the ternary conductive matrix. This material possesses remarkable strain sensitivity and stability and demonstrates outstanding EMI shielding performance. Wang et al.⁸¹ prepared a TPU/PDA/Ag composite fibrous membrane by spray coating and electrospinning techniques, which was then laminated with Ti₃C₂T_x MXene nanosheets to obtain a multifunctional ultra-drawing TPU/PDA/Ag/MXene. MXene enhances the composite film's EMI shielding performance and strain ability among them. The experimental results revealed that the composite material with a thickness of 150 μm displays excellent EMI shielding effectiveness (SE of ~108.8 dB) and ultra-high conductivity (95,238 S/m) in the X-band. Wu et al.¹⁴⁷ fabricated a highly thermally stable and tough Ag NW-PI/MXene/AgNW-PI composite with a sandwich structure via electrospinning, where its interlayer consisted of one layer of AgNW PI and one layer of highly conductive MXene nanosheets. The resulting membrane possesses outstanding mechanical properties and EMI shielding performance. Yu et al.¹⁷² successfully embedded MXene into electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers to form a composite fabric that demonstrated both superhydrophobicity and outstanding EMI SE of up to 160 dB/mm.

4.4.2 MXene hybrids

Exception of Mxene, MXene hybrids also contain other conductive and magnetic components, which can not only improve the stability and mechanical properties of the material itself but also lead to multiphase synergistic effects to enhance the EMW absorption inside the material, thereby further enhancing its shielding effectiveness. Many studies have demonstrated that constructing Mxene hybrids from different two-dimensional nanostructures is an effective way to achieve high-performance materials for microwave absorption. Meng et al.⁸⁹ prepared a mixed aerogel microsphere of graphene oxide (GO) and MXene by rapid freezing-assisted electrospinning. Due to differences in conductivity between MXene and GO and the newly generated heterogeneous interface and enriched surface groups, MXene@GO mixed aerogel microspheres (M@GAMS) exhibit a remarkable impedance matching and microwave absorption property. The 1.2 mm thick M@GAMS with a loading of 10% presents a reflection loss of 49.1 dB at a frequency of 14.2 GHz and also shows a high EMI shielding effectiveness in the microwave band. Jiang et al.⁹⁰ designed a multi-dimensional conductive framework loaded with TaC nanoparticles via electrospinning and high-temperature pyrolysis. The preparation process is shown in Figure 11a. The experimental results reported that the TaC nanoparticles can interconnect at a fixed concentration, resulting in a conductivity of up to 10.4 S/cm (Figure 11e).

Furthermore, they found that the X-band EMI SE of the composite fabric with a thickness of 0.2 mm is 37.7 dB, as shown in Figure 11d, and the SE_T, SE_R, and SE_A of fabrics were displayed in Figure 11f. They also performed an experimental demonstration to illustrate the EMI shielding properties of these flexible TaC/C composite fabrics in the practical application (Figure 11b,c). Subsequently, Guo et al.⁹¹ reported a TaC/Fe₃C-Fe-loaded composite using a similar method, as depicted in Figure 11g, which possesses superior features of a low density (0.34 g/cm³), excellent conductivity (approximately 15.4 S/cm) (Figure 11l), and saturated magnetization (13.3 emu/g). The experimental measurements addressed that the SE of the obtained material with a thickness of 0.18 mm reaches 46.4 dB (Figure 11k). The EMI shielding performance of electrospun composite fabrics with different weight ratios of TaC/Fe₃C–Fe at a frequency range of X-band is shown in Figure 11m, and the shielding mechanism is shown in Figure 11h. Overall, incorporating multiple spatial-scale conductive frameworks containing tantalum carbide (TaC) nanoparticles provides a simplified and highly effective means for fabricating lightweight and thin composite textiles with intricate conductive networks. At the same time, they show a high EMI shielding performance. Mi et al.¹⁷³ obtained a multilayer MXene/PLA film by alternatingly assembling MXene and electrospun poly(lactic acid) (PLA) nanofiber mats, wherein the MXene helped to retain the electromagnetic transmission pathway. Due to the effective conductivity of the MXene layer and the multiple reflection and absorption effects, the composite film realizes an optimal SE of 55 dB. Gu et al.⁸⁵ deposited MXene nanosheets onto electrospun Fe₃O₄/PAA nanofibers to produce Janus Ti₃C₂T_x (Fe₃O₄/PI) composite films. When the mass fraction of Ti₃C₂T_x was 80 wt%, the resulting composite film performed excellent EMI shielding abilities (the SE up to 66 dB) and mechanical properties (tensile strength of 114.5 MPa and Young's modulus of 5.8 GPa). Wang et al.¹⁷⁴ reported a bifunctional electrospun nanofiber membrane decorated with MXene nanosheets and Fe₃O₄ nanoparticles. The porous structure of the fiber, the magnetic loss effect of Fe₃O₄, and the dielectric loss effect of MXene result in outstanding EMI shielding performance of the composite fiber membrane with a thickness of 33 μm (SSE_t = 9212.1 dB cm²/g). Chen et al.⁸³ produced a unique TPU/ PAN self-supporting film via electrospinning and dipping processes. Subsequently, Fe₃O₄ nanoparticles and MXene were deposited onto its surface to form a hybrid membrane. Due to the hydrogen bond interaction between the PAN macromolecular chain and the surface functional group (T_x) of MXene, the interfacial attraction between MXene and TPU/PAN/Fe₃O₄ is enhanced, endowing the composite with a high stability and excellent EMI SE of over 30 dB in the X-band.