Hierarchical Porous Structured PVDF-Based Nanofiber Membranes Containing Alloy-Based Porous Nanospheres Derived from CoCuZn-MOFs for Electromagnetic Shielding

TL;DR

This paper introduces a new flexible and lightweight electromagnetic shielding material made from nanofiber membranes with a hierarchical porous structure.

Contribution

The novel contribution is the fabrication of alloy-based porous nanospheres from CoCuZn-MOFs and their integration into PVDF nanofiber membranes for enhanced electromagnetic shielding.

Findings

The MPPA membrane achieved an average SSE of 12,017.01 dB·cm²·g⁻¹ at 2 wt.% CCZ-C content.

The hierarchical porous structure improved electromagnetic attenuation and impedance matching.

The material is lightweight and flexible, suitable for modern electronic and military applications.

Abstract

Electromagnetic shielding (EMS) materials play an important role in modern technology and industry, especially in electronic equipment, communication technology, military applications and so on. With the continuous progress of technologies and the increasing demands for functional materials, EMS materials are expanding towards flexibility and being lightweight. Recently, metal–organic frameworks (MOFs) have garnered significant attention in the EMS field due to their unique structure and adjustable properties. In this paper, alloy-based porous nanospheres (CCZ-C) were fabricated by heat-treatment using CoCuZn-MOFs as precursors, and then electrospun CCZ-C/PVDF nanofiber membranes (NFMs) were prepared in a large-quantity by blending them with PVDF. Afterwards, a hierarchical porous structured NFM (MPPA) was obtained by loading a highly conductive Ag nanolayer on the surface of CCZ-C/PVDF nanofibers using pDA as a binder. By adjusting the CCZ-C content, it was determined that the EMS performance of MPPA was highest when the CCZ-C content was 2 wt.%, with an average SSE of 12,017.01 dB·cm2·g−1. This was because the hierarchical porous structure formed by adding an appropriate amount of CCZ-C further improved the electromagnetic attenuation and impedance matching of MPPA.

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Full text

1. Introduction

With the popularization and development of communication systems and electronic equipment, issues of electromagnetic interference (EMI) are becoming increasingly apparent [1,2,3,4]. The development of lightweight electromagnetic shielding (EMS) materials is of great significance in military and aerospace fields [5,6]. They can effectively block the detection signals of radar and communication equipment, helping to improve operational efficiency and tactical advantage in the battlefield [7,8]. They can also be used to isolate and protect the electronic equipment of aircrafts and spacecrafts, prevent external EMI, and guarantee their safe operation and communication capabilities. Traditional EMS materials, such as metals and conductive polymers, can provide EMS effects to a certain extent, but often have limitations such as high weight, easy corrosion and poor flexibility [9,10]. Because of their lightweight and flexible characteristics, electrospun nanofiber membranes (NFMs) can be easily used to manufacture EMS materials of diverse sizes and shapes, adapt to the needs of different equipment and environments, and improve the design flexibility and production efficiency of products [11,12,13].

For further enhancing the EMS performance of electrospun NFMs, a variety of functional nanoparticles (such as conductive nanoparticles and magnetic nanoparticles) are added to them. Metal–organic frameworks (MOFs) have complicated components and various structures, gradually becoming promising precursors for building functional composites with dielectric and magnetic losses, and have great potential in EMI shielding application [14,15,16]. Due to their high specific surface area and porosity, MOFs offer opportunities for multiple reflections and scattering of electromagnetic waves (EMWs), thereby achieving absorption-dominated EMS performances [17]. MOF derivatives obtained through high-temperature pyrolysis (such as carbon-based materials, metals or metal oxides) can significantly increase their electrical conductivity, making them more effective in absorbing and reflecting EMWs during EMI shielding. Meanwhile, their more porous structures and higher specific surface area can also provide a large number of surfaces to interact with EMWs, thereby increasing EMI shielding efficiency (SE) [18,19,20].

Lv et al. [19] prepared TiO_2_/Co/C composites originated from Ti_3_C_2_T_x_/Co-MOF through in situ solvothermal method and pyrolysis. The minimum reflection loss (RLmin) at a thickness of 3.0 mm was 50.45 dB. Ma et al. [20] soaked woods in the mixed solutions of Co/Zn-MOF (ZIF-67/8) and water-based epoxy resin (EP), and obtained the carbonized wood composite film (CWF/EP/Co) through hot pressing and carbonization. It had EMI SE (73 dB) at 200 μm thickness. Bai et al. [21] also synthesized Co_X_Fe_1_-_X_OOH nanoneedles by hydrothermal growth and fixed them on cotton fabrics. Then, CoCu-MOFs were grown in situ on the nanoneedles, and CoFe/CoCu alloy-carbon fibers with core–shell structures were fabricated by annealing and encapsulating with PDMS, whose average EMI SE reached 73.46 dB. In summary, the strong magnetism of Co effectively increased the magnetic losses of the MOF, while the high conductivity of Cu helps the MOF to conduct EMWs more effectively and to rapidly consume electromagnetic energy. Meanwhile, the stability and structural adjustment ability of Zn can enable CoCuZn-MOF to provide high EMI shielding ability. Therefore, CoCuZn-MOF derivatives not only have inherent merits in structure, but also have multi-functions of different components, which offers a novel idea and possibility for the advancement of efficient EMI shielding materials.

In this paper, based on a self-made spherical section free surface electrospinning (SSFSE) apparatus [22,23], a large number of hierarchical porous structured PVDF-based NFMs containing alloy-based porous nanospheres (CCZ-C) with excellent EMS performance were efficiently fabricated. First, CoCuZn-MOFs were prepared by solvothermal method, and their derivatives (CCZ-C) were obtained through high-temperature carbonization. Afterwards, electrospun CCZ-C/PVDF NFMs were efficiently prepared by blending CCZ-C with PVDF using SSFSE apparatus. Finally, a hierarchical porous structured NFM (MPPA NFM) was obtained by using pDA as a binder to load a highly conductive Ag nanolayer on the surface of CCZ-C/PVDF nanofibers. Due to the magnetic properties of CCZ-C itself and the hierarchical porous structure formed by their appropriate addition, MPPA NFM had superior EMI shielding performance, and their mechanical strength also met the basic requirements of NFMs for EMI shielding.

2. Results and Discussion

2.1. Fabrication Process

Figure 1 shows the fabrication process of hierarchical porous structured xMPPA (x = 1, 2, 3) NFMs with different CCZ-C contents (1 wt.%, 2 wt.%, and 3 wt.% of spinning solution separately) for EMI shielding. Firstly, CoCuZn-MOFs were prepared by solvothermal method using 2,5-H_2_BDC as ligands, and their derivatives (CCZ-C) were obtained by carbonization. Then, SSFSE was used to blend CCZ-C and PVDF to obtain high-yield CCZ-C/PVDF NFMs. Finally, a Ag nanolayer was grown on the surface of CCZ-C/PVDF nanofibers using pDA as a binder to obtain hierarchical porous structured xMPPA NFMs with excellent EMI shielding properties.

2.2. Characterization of CCZ-C

Figure 2 shows the morphology of CoCuZn-MOF and its derivative CCZ-C. The shape of CoCuZn-MOF was spherical, with a smooth surface and a particle size of about 300 nm (Figure 2A). After heat treatment, CCZ-C remained the same spherical shape and its surface became rough and porous, which was caused by the decomposition of organic ligands in MOF (Figure 2B). The internal microstructure of CCZ-C was characterized by TEM, and it could be seen that the CoCu alloys were evenly distributed in the carbon matrices in the form of nanoparticles (Figure 2C,D), which was further confirmed by the elemental mappings in Figure 2F. In the high-resolution TEM (HRTEM) picture of CCZ-C, the crystal lattice with a spacing of 0.209 nm corresponded to the (111) crystal face of CoCu alloy [24,25], while the crystal lattices with spacings of 0.208 nm and 0.204 nm corresponded to the (111) crystal faces of Cu and Co, respectively [26,27]. In addition, it was observed that the crystal lattice spacing of 0.345 nm was assigned to the (002) crystal face of C. Here, the absence of Zn was due to its low boiling point, which was removed during the thermal decomposition process [28]. Moreover, it was observed from Figure 2E that CCZ-C nanoparticles were evenly distributed in 2MP nanofibers.

The elemental states and chemical compositions in CCZ-C were characterized using XPS, as exhibited in Figure 3. The XPS full spectrum (Figure 3A) further confirmed the presence of C, Co, and Cu in CCZ-C, without the presence of Zn. The high-resolution C 1s spectrum displayed peaks at 284.8 eV, 285.8 eV, 290.07 eV, belonging to C-C, C-O, and O-C=O bonds, separately (Figure 3B) [29,30]. Figure 3C shows a high-resolution spectrum of Co 2p, where the peaks at 778.2 eV and 794.06 eV were correlated with Co^0^, and the peaks at 780.8 eV and 796.1 eV were correlated with Co^2+^. The two peaks of 784.8 eV and 801.8 eV were satellite peaks of Co 2p_3/2_ and Co 2p_2/1_, respectively [31]. Figure 3D displays a high-resolution spectrum of Cu 2p with two Cu^2+^ correlated peaks (934.59 eV and 954.29 eV), two Cu^0^ correlated peaks (932.6 eV and 952.33 eV) observed, as well as three satellite peaks (Cu 2p_3/2_ and Cu 2p_2/1_) appeared at 940.7 eV, 944.1 eV and 962.7 eV [32,33]. This agreed with the HRTEM analysis result.

2.3. Characterization of NFMs

Figure 4 shows the surface morphologies of all prepared NFMs. It was found that the nanofiber surfaces of 1MP, 2MP, and 3MP NFMs were smooth and that their fiber diameter distributions were uniform (Figure 4A–C). Moreover, as the content of CCZ-C increased, the diameter of nanofibers gradually thickened because of the enhanced solution viscosity [34,35]. In Figure 4D–F, due to the self-polymerization of DA, a thin film was formed on the surface of nanofibers to uniformly wrap them, and some nanoparticles appeared on the surface, resulting in a slight rise in the fiber diameter. Under the binding action of pDA, Ag nanoparticles grew uniformly on the fiber surface (Figure 4G–I).

Figure 5 shows the XRD patterns of all NFMs. As illustrated in Figure 5A, the diffraction peak with a 2θ of 20.9° observed in each NFM corresponded to the (110) crystal face of PVDF [36,37]. The diffraction peaks at 2θ = 44.2°, 51.2° and 75.8° corresponded to the (111), (200) and (220) crystal faces of Co (PDF#15-0806), respectively [38,39]. According to the standard card of Cu (PDF#04-0836), the diffraction peaks at 2θ = 43.3°, 50.4°, and 74.1° belonged to the (111), (200), and (220) crystal faces of Cu, respectively [24,40]. Figure 5B shows the XRD patterns of 1MPPA NFM, 2MPPA NFM, 3MPPA NFM and CCZ-C. Due to the surface of nanofibers being covered with Ag nanoparticles, no diffraction peaks of the metals Co and Cu were observed in xMPPA (x = 1, 2, 3) NFMs. The diffraction peaks at 2θ = 38.1°, 44.2°, 64.4° and 77.4° corresponded to the (111), (200), (220) and (311) crystal faces of Ag (PDF#04-0783), respectively [41,42].

According to Figure 5A, peak fitting was performed on the XRD patterns of PVDF, 1MP, 2MP and 3MP to estimate the relative proportion of β crystalline phase. As shown in Figure 6, the main diffraction peak of the α crystal form was at 2θ = 18.4°, while the main diffraction peak of the β crystal form was at 2θ = 20.6°, belonging to the (020) and (110) crystal planes of PVDF, respectively [43,44]. Table 1 demonstrates that compared to PVDF NFM, 1MP NFM with a lower CCZ-C content (1 wt.%) had a weaker effect on enhancing the β crystalline phase. When the CCZ-C content reached 2 wt.%, the β peak area of 2MP reached 65%. However, as the CCZC content further increased to 3 wt.%, the β peak area of 3MP significantly decreased. This was because excessive CCZ-C nanoparticles tended to agglomerate, which was not conducive to the formation of β crystalline phase. Therefore, an appropriate amount of nanoparticles significantly induced the conversion of PVDF from α phase to β phase.

Figure 7A shows the mechanical performances of NFM samples. It was observed that the mechanical performances of NFMs gradually improved as the content of CCZ-C increased. When the CCZ-C content was enhanced from 1 wt.% to 3 wt.%, the tensile strength of NFMs increased from 5.22 ± 0.12 MPa to 6.10 ± 0.08 MPa, and the elongation at break increased from 32.28 ± 1.05% to 38.47 ± 1.79%. This was due to the fact that the uniform dispersion of CCZ-C in the nanofibers prevented the crack development and absorbed local stresses [45]. The mechanical performances of NFMs reduced slightly when the binder pDA was loaded on the fiber surface. This might be because the brittleness of pDA itself would reduce the overall extensibility of nanofibers, cut down their deformability, and made the NFMs more prone to fracture under the action of external forces [46]. In addition, the uneven distribution of Ag nanoparticles on the fiber surface caused stress concentration in some parts of the nanofibers, resulting in cracks or local fractures of nanofibers more easily under external forces, thereby reducing the mechanical properties of NFMs. Fortunately, 3MPPA NFM loaded with 3 wt.% CCZ-C still maintained a tensile strength of 4.06 ± 0.08 MPa and an elongation at break of 26.07 ± 1.32%, meeting the practical application requirements of NFMs for EMS.

The pore structure inside the shielding materials provides more reflection paths for absorbing EMWs, which has a good effect on improving their EMI shielding performance [47,48]. Figure 7B and Table 2 show the pore size distribution of different NFMs. The larger fiber diameters caused by the higher CCZ-C content enabled NFMs to have a larger pore size and higher pore counts, which was found from the pore size distributions of 1MP, 2MP and 3MP NFMs. The same conclusion could also be drawn from the pore size distributions of xMPPA NFMs. However, the loading of Ag nanoparticles would cover some of the pores around the nanofibers, reducing the pore size and count of NFMs. In general, compared to 1MPPA and 3MPPA NFMs, 2MPPA NFM had a suitable pore size and pore count, which exhibited a greater positive impact on the EMS effect.

In addition, the multilevel pore structure inside the single nanofiber can increase its specific surface area and offer more reflection paths for absorbing EMWs, which can further improve the EMS performances of NFMs. The pore-size distributions and specific surface areas of PPA and 2MPPA NFMs were studied by N_2_ adsorption–desorption isotherms. Figure 8A shows that the N_2_ adsorption–desorption isotherms of the two samples were typical type IV adsorption/desorption curves with obvious hysteresis loops, indicating the existence of micropores and mesopores in the samples. Figure 8B further illustrates that both samples had a porous structure consisting of micropores and mesopores. Table 3 summarizes the specific surface areas and pore-size distributions of the samples, showing that the addition of CCZ-C significantly increased the specific surface area of the samples. It can be seen that the specific surface area of the 2MPPA sample reached 8.96 m^2^/g. This was because the porous structure of CCZ-C provided more surface area and pores for the samples, thereby increasing the adsorption sites. However, when the content of CCZ-C was 3 wt.%, agglomeration or accumulation occurred inside the nanofibers. This agglomeration could block some pores, thereby reducing the effective specific surface area, and thus the specific surface area of 3MPPA dropped to 4.41 m^2^/g.

2.4. EMI Shielding Effectiveness of NFMs

The commonly used index to characterize the shielding properties of materials is EMI SE, which is positively correlated with the shielding properties of materials [49,50]. The S-parameters of NFMs were measured by a VNA, and its reflection (R), transmission (T) and absorption (A) were calculated by the following Formulas (1)–(3) [51].

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$

The total shielding efficiency (SE_T_) involves reflection (SE_R_), absorption (SE_A_) and multi-reflection shielding efficiency (SE_M_) (Formula (4)). When SE_A_ is greater than 10 dB, SE_M_ is typically disregarded (Formula (5)).

$$[eqn]$$ $$[eqn]$$

where

$$[eqn]$$ $$[eqn]$$

Considering the balance between the weight (M_w_, g), area (M_a_, cm^2^) and shielding effect of different materials, SSE (SE/surface density) is used to more accurately evaluate the shielding properties of samples, and its calculation formula is as follows:

$$[eqn]$$

Figure 9 shows the SSE values of 1MPPA, 2MPPA and 3MPPA NFMs in the X-band frequency. It could be observed that when the CCZ-C load was 1 wt.%, the average SSE value of 1MPPA was only 11,451.61 dB·cm^2^·g^−1^. And with the increase in CCZ-C content, the SSE values of MPPA NFMs were first enhanced and then decreased. When the CCZ-C load increased to 2 wt.%, due to the better synergistic effect between CCZ-C inside the fiber and the Ag nanolayer on the fiber surface, as well as the more reasonable pore structure inside the fiber, the average SSE value of 2MPPA reached 12,017.01 dB·cm^2^·g^−1^. However, when the CCZ-C load was 3 wt.%, excessive CCZ-C nanoparticles caused the inhomogeneity in the internal structure of fibers, and even the aggregation of Ag nanoparticles on the fiber surface, which would affect the absorption and multi-reflection efficiency of EMWs in the NFM, leading to a drop in the average SSE value of 3MPPA to 4717.66 dB·cm^2^·g^−1^. As exhibited in Table 4, the EMS performance of 2MPPA was equivalent to or higher than reported EMS materials based on electrospun NFMs. It is worth noting that the high specific surface area and porosity of these hierarchical porous structured NFMs provided more opportunities for multiple reflections and internal scattering of EMWs, thus exhibiting EMS performance mainly based on absorption (Figure 9B).

2.5. EMS Mechanism of NFMs

The EMS mechanism of xMPPA (x = 1, 2, 3) NFMs was shown in Figure 10. From a nanoscale perspective, porous structured CCZ-Z had a large specific surface area and conductivity, which increased the loss of EMWs by interacting with it; this manifested as conduction loss, multiple reflections, and interfacial polarization, etc. Meanwhile, the CoCu alloy loaded on it also enhanced the magnetic dielectric loss, thus improving its EMI shielding effect (electromagnetic attenuation). Moreover, highly conductive Ag nanoparticles further promoted conduction loss. From a microscale perspective, adding CCZ-C nanoparticles to PVDF nanofibers not only introduced a multi-level pore structure, but also increased the specific surface area of fibers, resulting in multiple reflections and scattering of EMWs within the fibers, as well as interface polarization, thereby improving the interaction between EMWs and fibers and further enhancing the EMI shielding effect. Meanwhile, the highly conductive Ag nanoparticle layer coated on the fiber surface significantly increased the interface area between particles–fiber matrix, fibers–fibers, and particles–air, strengthening the interfacial polarization loss. Therefore, CCZ-C and Ag nanoparticles cooperate inside and outside the fibers to achieve impedance matching. Notably, the better the dispersion of nanoparticles, the more significant the interface loss. From a macroscale perspective, after EMWs were incident on a hierarchical porous structured composite NFM, a small amount was first reflected on its surface, and most of them entered the NFM. The porous interconnect structure of NFM allowed EMWs to withstand multiple reflections and scattering, prolonging the propagation paths and fully utilizing electrical/magnetic losses. Moreover, its high specific surface area further amplified interfacial polarization, while the porous structure optimized impedance matching to enhance absorption type shielding efficiency.

3. Materials and Methods

3.1. Materials

N,N-dimethylformamide (DMF) (Analytical Reagent), ethyl alcohol (Analytical Reagent) and Acetone (AC) (Analytical Reagent) were afforded by Shanghai Qiangsheng Chemical Co., Ltd. (Shanghai, China). Poly(vinylidene fluoride) (PVDF, M_w_ = 50 w) was offered by Shanghai D&B Biological Science and Technology Co., Ltd. (Shanghai, China). Tris(hydroxymethyl) aminomethane (Tris, ≥98%) was provided by Shanghai Haohong scientific Co., Ltd. (Shanghai, China). Co(NO)3·6H_2_O, D-(+)-Glucose and dopamine hydrochloride (DA-HCl) (98%) were provided by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). AgNO_3_ standard solution (0.1 mol/L) was supplied by Quanzhou Yiyantang technology Co., Ltd. (Quanzhou, China). Zn(NO)3·6H_2_O, 2, 5-dihydroxyterephthalic acid (2,5-H_2_BDC) and NH_3_·H_2_O (Guaranteed Reagent, 25–28%) were offered by Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China). Cu(NO)3·3H_2_O was supplied by Shanghai Hushi Co., Ltd. (Shanghai, China). All reagents were used directly without any treatment.

3.2. Preparation of CCZ-C

Zn(NO)3·6H_2_O, Co(NO)3·6H_2_O and Cu(NO)3·3H_2_O with a molar ratio of 3:1:1 were respectively dissolved in a mixed solvent of 120 mL DMF and ethanol with a volume ratio of 7:3, and then completely dissolved at room temperature to get solutions A, B and C. Then, 90 mmol of 2,5-H_2_BDC was dissolved in 360 mL of the above mixed solvent (DMF:ethanol = 7:3) to prepare solution D. Ultimately, solutions A, B and C were added into solution D in turn and stirred evenly. The molar ratio of metal ligand to organic ligand in the whole mixed solution was 1:1. The mixed solution was shifted to a Teflon reactor and heated at 135 °C for 12 h. The substances obtained after the reaction were rinsed three times with ethanol and dried at 60 °C to obtain CoCuZn-MOFs. Afterwards, CoCuZn-MOFs were put into a tube furnace and calcined at 900 °C for 3 h in N_2_ atmosphere, with a heating rate set to 5 °C/min. The final products fabricated were named as CCZ-C.

3.3. Fabrication of Hierarchical Porous Structured NFMs

Firstly, DMF and AC in a ratio of 6:4 were used as solvents to prepare a PVDF solution with a mass fraction of 10 wt.%. Then, CCZ-C with mass fractions of 1 wt.%, 2 wt.%, and 3 wt.% were separately added and stirred for 24 h at 25 °C until uniformly mixed. CCZ-C/PVDF spinning solutions with different CCZ-C contents were obtained. These spinning solutions were separately transferred to SSFSE apparatus for efficient spinning at a spinning voltage of 50 kV, a collector speed of 300 r/min and a receiving distance of 180 mm. These SSFSE processes were performed at 25 °C and 60% relative humidity. The electrospun CCZ-C/PVDF NFMs with different CCZ-C contents obtained using SSFSE apparatus were labeled as 1MP, 2MP and 3MP, respectively.

According to our previous work [23], 0.2 g DA-HCl and 0.12 g Tris were dissolved in 100 mL of deionized water to obtain a Tris-DA solution with a pH of about 8.35. The 1MP, 2MP and 3MP NFMs were immersed in Tris-DA solution for 6 h, washed clean in deionized water, and dried for 6 h at 60 °C. The prepared samples were labeled as 1MPP, 2MPP and 3MPP, respectively. Then, 58.8 mL of 0.1 mol/L AgNO_3_ standard solution was taken out and diluted to 100 mL with deionized water. NH_3_·H_2_O was added dropwise to the diluted AgNO_3_ solution until the solution clarified, thereby obtaining Tollens’ reagent. The 1MPP, 2MPP and 3MPP NFMs were immersed in the prepared Tollens’ reagent for 30 min respectively, and then slowly added into 100 mL of 20 g/L glucose solution for in situ reaction for 2 h. After rinsing with deionized water and drying at 60 °C, 1MPPA, 2MPPA and 3MPPA NFMs were obtained, respectively.

3.4. Characterization

High-resolution field emission scanning electron microscopy (SEM, Regulus8100, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, HT7700, Hitachi, Tokyo, Japan) were utilized to observe and analyze the surface morphology of samples. The crystal structure of samples was characterized by X-ray diffractometer (XRD, D8Advance, Bruker, Karlsruhe, Germany) in the range of 5–80° with a step width of 0.03°. The mechanical performances and pore size distributions of NFMs were measured by universal material testing machine (Instron-5967, Instron, Canton, OH, USA) and Capillary Flow Porometer (CFP-1500a, Porous Materials Inc., Pittsburgh, PA, USA), respectively. The elemental states and chemical compositions of CCZ-C were determined by X-ray spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA). The specific surface areas of samples were measured by a specific surface area and aperture analyzer (ASAP-2460, Micrometritics, Norcross, GA, USA). The EMI SE values of NFMs with a size of 22.86 mm × 10.16 mm and a thickness of 100 μm in the X-band (8.2–12.4 GHz) were measured utilizing a vector network analyzer (VNA, E5071C, Agilent, Santa Clara, CA, USA) combined with a BJ-100 rectangular waveguide by the waveguide method (GB/T 30142-2013 [65]).

4. Conclusions

In summary, CoCuZn-MOFs were prepared by solvothermal method and used as precursors to obtain the MOF derivatives (CCZ-C) through heat treatment, which were alloy-based porous nanospheres. Then, high-yield electrospun CCZ-C/PVDF NFMs with different CCZ-C contents were obtained using SSFSE technology. Afterwards, hierarchical porous structured NFMs were obtained by in situ polymerization of a Ag nanolayer on the surface of CCZ-C/PVDF nanofibers with pDA as binder. The Ag nanolayer with high conductivity formed an intact conductive path on the surface of the nanofibers, which made the NFMs have a good impedance matching. CCZ-C could bring more interface and structural complexity to improve the electromagnetic attenuation of the composites. By adjusting the content of CCZ-C, it was found that as its content increased, the SSE of MPPA NFM first enhanced and then reduced. When the content of CCZ-C was 2 wt.%, the average SSE value of 2MPPA was the highest, reaching 12,017.01 dB·cm^2^·g^−1^. This indicated that the optimized hierarchical porous structure formed by adding an appropriate amount of CCZ-C further improved the electromagnetic attenuation and impedance matching of NFM, thus significantly improving its EMI shielding effect.