Comparative electromagnetic shielding performance of Ti₃C₂T_x-PVA composites in various structural forms: compact films, hydrogels, and aerogels

Abstract

The structural design of light-weight MXene–polymer composites has attracted significant interest for enhancing both electromagnetic interference (EMI) shielding performance and mechanical strength, which are critical for practical applications. However, a systematic understanding of how various structural configurations of MXene composites affect EMI shielding is lacking. In this study, light-weight Ti₃C₂T_x-PVA composites were fabricated in three structural forms, hydrogel, aerogel, and compact film, while varying the Ti₃C₂T_x areal density (14 to 20 mg cm⁻²) to elucidate the role of structural design in X-band EMI shielding and mechanical properties. The EMI shielding performance depends on the structural configuration and areal density of the MXene in Ti₃C₂T_x-PVA composites. The shielding effectiveness increases with increasing Ti₃C₂T_x content in each configuration. At a fixed Ti₃C₂T_x areal density of 0.02 g cm⁻², the Ti₃C₂T_x-PVA hydrogel demonstrated the highest shielding effectiveness (SE = 70 dB at 10 GHz), attributed to strong dipole polarization and efficient ionic conduction behavior, followed by the compact film (40 dB) and then the aerogel (21 dB). Notably, the aerogel achieved the highest absorption coefficient (A = 0.89) due to the improved impedance matching and pronounced internal reflections, whereas the hydrogel and compact film exhibited reflection-dominated shielding. Furthermore, the incorporation of PVA polymer molecules into Ti₃C₂T_x MXenes significantly enhanced their mechanical properties across all configurations: the hydrogel achieved high stretchability (636%), the aerogel displayed superior compressive strength (0.215 MPa), and the compact film reached a tensile strength of 56 MPa, each surpassing the performance of its pristine Ti₃C₂T_x MXene counterpart. Overall, tailoring the structural configuration into a hydrogel, aerogel, or compact film offers versatile routes for optimizing both EMI attenuation and mechanical performance of MXene–polymer composites.

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Introduction

Rapid advances in mobile electronics and telecommunication technologies have led to widespread electromagnetic interference (EMI) issues such as device malfunctions, signal distortion, data loss, and even system failures in highly integrated compact electronic systems.^1–3 Traditionally, highly conductive metals such as aluminum, copper, and silver have been used for EMI shielding due to their high electrical conductivity and resulting strong shielding capabilities.^4–6 However, the related high density and challenges in applying thin uniform coatings on unevenly shaped devices hinder their use as light-weight shielding materials. On the other hand, light-weight conductive carbon-based materials such as graphene and carbon nanotubes require relatively high thickness and filler loadings to achieve satisfactory EMI shielding performance due to their low electrical conductivity.^7–11

Recently, MXenes, a large family of two-dimentional (2D) transition metal carbides, nitrides, and carbonitrides, have emerged as promising light-weight shielding materials. MXenes offer excellent metallic conductivity (exceeding 24 000 S cm⁻¹),¹² tunable composition,¹³ low apparent density, and outstanding solution processability.¹⁴ This was first demonstrated in 2016, when the Ti₃C₂T_x MXene exhibited remarkable EMI shielding effectiveness (SE), achieving 92 dB at a thickness of 45 μm.¹⁵ Furthermore, a 40 μm thick Ti₃CNT_x MXene film achieved an EMI SE of 116 dB, the highest value reported for any synthetic material.¹⁶ These findings highlight the potential of MXenes to outperform metals and carbon-based materials in EMI shielding at comparable thicknesses.

Despite their promising shielding performance, MXenes face challenges such as limited mechanical strength, dimensional stability, and long-term oxidation resistance, which are essential for practical applications. To address these challenges, interest has been growing in MXene-based polymer composites, which combine the excellent shielding properties of MXenes with the mechanical strength and structural stability of polymers.

The abundant surface terminal groups of MXenes, such as hydroxyl groups, provide easy solution processability, enabling the development of MXene–polymer composites in various structural configurations, including compact systems (films), porous structures (aerogels), and ionic species inclusions (hydrogels).¹⁷ These configurations have been investigated for their EMI shielding capabilities, and several studies have shown significant performance.

For instance, the first report of a Ti₃C₂T_x MXene compact composite film combined with sodium alginate (SA) achieved an EMI SE of 57 dB at a thickness of 8 μm.¹⁵ Since then, MXene composite films have been incorporated into a wide range of natural and synthetic polymers, including non-conductive materials such as polyethylene (PE), polymethyl methacrylate (PMMA), cellulose nanofibers (CNFs), polyvinyl alcohol (PVA), polyurethane (PU), polycarbonate (PC), polyvinylidene fluoride (PVDF), polyimide (PI), and polystyrene (PS), as well as conductive polymers like polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). These composite films have exhibited EMI SE ranging from 45 to 90 dB at thicknesses between 11 and 400 μm.^18–27 Porous structures, such as aerogels, benefit from multiple interphase interfaces, which promote multiple reflections and internal scattering of EM waves, indicating them as promising light-weight materials for EMI shielding. PVA-based Ti₃C₂T_x aerogels exhibited EMI SE values of 26–33 dB at thicknesses of 3.4–3.9 mm, whereas Ti₃C₂T_x-polyamide aerogels achieved a much higher SE of 80 dB at 210 μm.^28,29 Similarly, Ti₃C₂T_x-SA aerogels reached an EMI SE of 70 dB at 2 mm, and Ti₃C₂T_x-PU foam exhibited an EMI SE of 72 dB at the same thickness.^23,30 Moreover, Ti₃C₂T_x and water-soluble polymers can be effectively integrated into soft composite hydrogels; polar water molecules and mobile ions in the hydrogel configuration can significantly improve the attenuation of EM waves through polarization loss.³¹ For example, Ti₃C₂T_x-PEDOT:PSS hydrogels achieved an EMI SE of 51.7 dB at a thickness of approximately 0.3 mm,³² and Ti₃C₂T_x-PVA hydrogels exhibited an EMI SE of 31 dB at 1 mm thickness,³³ with another reported material reaching an EMI SE of 48.6 dB at 1.5 mm.³⁴

Despite these advances, a comprehensive study of shielding mechanisms across different structural types, such as hydrogels, aerogels, and compact films, remains a gap in the literature. A thorough exploration of these variations is essential for gaining a deeper understanding of how structural configuration affects the EMI shielding performance.

Therefore, this study investigates the EMI shielding performance of Ti₃C₂T_x-PVA composites in three distinct structural configurations: hydrogels, aerogels, and compact films. High-crystallinity Ti₃C₂T_x is used as a highly conductive MXene filler, and water-soluble PVA is chosen as the polymer matrix due to its excellent compatibility with the hydrophilic surface of Ti₃C₂T_x, enabling fabrication of homogeneous MXene–polymer composites in three structural configurations. By exploring the influence of structural configuration on EMI SE, this study provides a deeper understanding of how the arrangement and concentration of MXenes in polymer matrices can influence the EMI shielding performance. This study highlights the critical role of structural configuration in optimizing the EMI shielding performance of Ti₃C₂T_x-PVA composites.

Results and discussion

Preparation of hydrogels, aerogels, and compact films

To investigate the effects of structural configurations on EMI shielding, Ti₃C₂T_x-PVA composites were fabricated as hydrogels, aerogels, and compact films (Fig. 1a). Initially, the Ti₃C₂T_x MXene was synthesized via selective etching of Al from highly crystalline Ti₃AlC₂ MAX phase crystals (Fig. S1a†) using the modified minimally intensive layer delamination (MILD) etching method.³⁵ Successful etching was confirmed using the X-ray diffraction (XRD) pattern, where the (00l) peak shifted from 8.57° to 7.26°, indicating an increase in d-spacing from 1.03 nm to 1.22 nm (Fig. 1b, red spectra). The absence of characteristic Ti₃AlC₂ MAX peaks further confirmed complete removal of Al from Ti₃AlC₂ and successful synthesis of the Ti₃C₂T_x MXene.³⁶ Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns (Fig. 1c) revealed a single Ti₃C₂T_x MXene sheet with a highly crystalline hexagonal lattice structure. Scanning electron microscopy (SEM) images also confirmed the well-delaminated morphology of the MXene sheet, which appeared to be partially electron transparent (Fig. S1b†). A characteristic peak at 760 nm in the UV-visible spectrum (Fig. 1d) and the bright greenish color of the Ti₃C₂T_x dispersion (inset, Fig. 1d) further support the successful synthesis and delamination of the high-crystallinity Ti₃C₂T_x MXene.

Fig. 1 (a) Schematic illustrations of Ti₃C₂T_x-PVA-based hydrogels, aerogels, and compact films. (b) XRD patterns of a Ti₃C₂T_x compact film, Ti₃C₂T_x aerogel, Ti₃C₂T_x-PVA compact film, and Ti₃C₂T_x-PVA aerogel at an areal density of 20 mg cm⁻¹ and of the Ti₃AlC₂ MAX powder. (c) TEM image of a single Ti₃C₂T_x sheet (inset: SAED pattern showing highly crystalline hexagonal symmetry). (d) UV-visible spectrum of Ti₃C₂T_x dispersion in water (1 mg mL⁻¹). Digital photographs of (e) a Ti₃C₂T_x-PVA hydrogel (inset: photograph of an elongated hydrogel), (f) Ti₃C₂T_x-PVA aerogel (inset: SEM image), (g) Ti₃C₂T_x-PVA compact film (inset: SEM image), (h) Ti₃C₂T_x aerogel (inset: SEM image), and (i) Ti₃C₂T_x film (inset: cross-sectional SEM). All samples have the same MXene areal density of 20 mg cm⁻².

Due to the excellent dispersibility of Ti₃C₂T_x and the solubility of PVA in deionized water, a homogeneous Ti₃C₂T_x-PVA aqueous solution formed readily, enabling the fabrication of hydrogels, aerogels, and compact films at various Ti₃C₂T_x areal densities (14–20 mg cm⁻²). Ti₃C₂T_x-PVA hydrogels were fabricated by adding sodium tetraborate to the Ti₃C₂T_x-PVA solution. The crosslinking process occurs in two stages. In the first stage, the tetrahydroxy borate anion reacts with a diol group on the PVA chain, forming a monodiol complex. In the second stage, the remaining hydroxyl groups attached to the boron atom in the borate react with adjacent diol groups, leading to the formation of a three-dimensional elastic gel network,³⁷ as illustrated by the reaction equation in Fig. S2a.† The corresponding changes in bonding interactions within the hydrogel are schematically depicted in Fig. S2b and c.† This process is known as di–diol complexation and was confirmed by the presence of asymmetric stretching vibrations from the B–O–C bond at approximately 1420 cm⁻¹ and 1334 cm⁻¹, as observed in Fourier-transform infrared (FTIR) spectroscopy, shown in Fig. S3.†^38,39 Ultimately, a soft and elastic hydrogel was successfully formed (Fig. 1e and its inset). For comparison, a control sample of PVA hydrogel was prepared, and it demonstrated opaque and elastic behavior, as presented in Fig. S4a–c.†

Ti₃C₂T_x-PVA aerogels with a uniform porous structure were produced by directionally freeze-drying Ti₃C₂T_x-PVA solutions (14 mg cm⁻² to 20 mg cm⁻²). The temperature gradient created between the copper plate and the Teflon mold facilitated the formation of a uniform porous structure (details in the Experimental section) in the XY direction, as shown in Fig. 1f and its inset.^40,41 Although the color shifted from white for pristine PVA to light grey and black with increasing MXene content (Fig. S5a–e†), the overall porosity and long-range order remained constant, as shown in Fig. S6a–d.†

The Ti₃C₂T_x-PVA compact films (Fig. S7a–e†) were fabricated by vacuum-filtering Ti₃C₂T_x-PVA solutions, resulting in a dense and nonporous structure (Fig. 1g and Fig. S8a–d†). Unlike the optically transparent PVA films, the Ti₃C₂T_x-PVA films appeared opaque (Fig. S7a–e†). For comparison, pristine Ti₃C₂T_x aerogels and compact films were also fabricated from Ti₃C₂T_x dispersions (Fig. 1h and i). All three Ti₃C₂T_x-PVA composite configurations exhibited very low density and among them, the aerogels had the lowest density, as low as 0.10 g cm⁻³ (Table S1†).

The XRD analysis of the Ti₃C₂T_x-PVA aerogel and compact film revealed the (002) peak characteristic of Ti₃C₂T_x MXene sheets, along with a broad halo peak at approximately 20°, corresponding to the amorphous PVA peak (Fig. 1b). A slight increase in the d-spacing was observed compared with the Ti₃C₂T_x film, which can be attributed to the intercalation of PVA into the galleries of MXene sheets. For instance, the Ti₃C₂T_x-PVA compact film exhibited an increased d-spacing of 1.56 nm compared with 1.22 nm in the Ti₃C₂T_x film. In contrast, an opposite trend was detected in the Ti₃C₂T_x-PVA aerogel, where the d-spacing decreases to 1.85 nm from 1.65 nm. The reduction in d-spacing can be attributed to the increased compactness of the Ti₃C₂T_x-PVA aerogel (inset of Fig. 1f) compared to that of the Ti₃C₂T_x aerogel (inset of Fig. 1h). This increased compactness is likely driven by the interfacial interactions between the hydrophilic PVA and Ti₃C₂T_x, which promote the formation of hydrogen bonds.⁴² Additionally, the presence of anions in the system may further facilitate hydrogen bonding between Ti₃C₂T_x and PVA, leading to a reduction in interlayer spacing (Fig. 1b, green spectra).⁴³ Note that an XRD analysis of the Ti₃C₂T_x-PVA hydrogels was not feasible on the Bruker D8 XRD instrument used in this study.

Mechanical properties

Mechanical properties were evaluated using a universal testing machine (Fig. 2a). The PVA hydrogel exhibited excellent stretchability, up to 450%, which can be attributed to the dynamic diol–borate crosslinking between the borax and PVA chains that effectively dissipated energy under stress (Fig. 2b). These crosslinks enhanced the hydrogel elasticity and mechanical resilience. Incorporating the Ti₃C₂T_x MXene into the hydrogel further enhanced its elasticity, enabling an elongation of 636% at an areal density of 14 mg cm⁻² (Fig. 2b) and increasing the tensile strength from 2.48 MPa to 4.83 MPa. These enhancements are ascribed to robust hydrogen bonding and diol borate complexation between the Ti₃C₂T_x surface hydroxyl groups and PVA chains.⁴⁴ Specifically, the MXene surface chemistry facilitates the formation of reversible hydrogen bonding and debonding interactions with PVA, serving as a mechanism for energy absorption and distribution under stress.^45,46 The formation of hydrogen bonds can be confirmed by slight broadening of the –OH stretching peak at 3300 cm⁻¹ in the Ti₃C₂T_x-PVA system (Fig. S3†). Additionally, the high surface area of well-delaminated Ti₃C₂T_x MXenes reinforces the composite structure, enabling efficient stress transfer and contributing to improved mechanical performance. The synergistic effect of these interactions produces a mechanically resilient Ti₃C₂T_x-PVA hydrogel. However, further increasing the Ti₃C₂T_x content to 20 mg cm⁻² reduced elongation and fracture stress to approximately 500% and 3.44 MPa, respectively. This decrease can be attributed to impaired stretchability and increased rigidity at higher Ti₃C₂T_x loadings, leading to diminished energy dissipation and the formation of stress concentration points that negatively affect the overall mechanical performance.^47,48

Fig. 2 (a) Digital photograph showing the tensile test of the Ti₃C₂T_x-PVA hydrogel. (b) Stress–strain curves of PVA and Ti₃C₂T_x-PVA hydrogels. (c) Digital photograph of the Ti₃C₂T_x-PVA aerogel supporting a load exceeding 1500 times its own weight without collapsing. (d) Stress–strain curves of PVA, Ti₃C₂T_x, and Ti₃C₂T_x-PVA aerogels at various Ti₃C₂T_x areal densities. (e) Stress–strain curves of PVA, Ti₃C₂T_x, and Ti₃C₂T_x-PVA compact films at various Ti₃C₂T_x areal densities.

Fig. 2c shows the compression resistance of the Ti₃C₂T_x-PVA aerogel with a Ti₃C₂T_x areal density of 20 g cm⁻²; it supported more than 1500 times its own weight without disintegrating under 30% compression. The compressive stress–strain responses of neat PVA and Ti₃C₂T_x-PVA aerogels are shown in Fig. 2d. Each aerogel was compressed at a displacement rate of 1.5 mm min⁻¹ up to 50% strain. For the PVA aerogel, the stress barely increased until 20% strain and then gradually rose to 129 kPa at 50% strain.⁴⁹ In contrast, the Ti₃C₂T_x aerogel exhibited a negligible stress response to the applied strain, indicating complete collapse under even minimal compressive strain due to its brittleness. In the Ti₃C₂T_x-PVA aerogels, the compressive stress increased linearly without collapse. The Ti₃C₂T_x-PVA aerogel at 14 mg cm⁻² areal density achieved the highest compressive strength of 215 kPa, approximately 200 times higher than that of the Ti₃C₂T_x aerogel and 50% greater than that of the PVA aerogel. However, at a higher Ti₃C₂T_x areal density (20 mg cm⁻²), the aerogel's compression stress decreased to 114 kPa. The initial compressive moduli of Ti₃C₂T_x-PVA aerogels at 14 mg cm⁻² and 20 mg cm⁻² increased by nearly 1000-fold (63 kPa) and 400-fold (24 kPa), respectively, compared with the Ti₃C₂T_x aerogel (0.06 kPa) (Table S1†). Additionally, a similar trend was observed in the recovery response after removing compressive stress. The Ti₃C₂T_x aerogel exhibited negligible strain recovery, almost 1%, showing poor resilience as it collapsed entirely under compression. In contrast, the PVA aerogel demonstrated a 56% strain recovery, reflecting moderate resilience. The Ti₃C₂T_x-PVA aerogels exhibited higher strain recovery, with around 79% and 73% for areal densities of 14 mg cm⁻² and 20 mg cm⁻², respectively (Fig. S9†). This indicates significantly improved resilience, attributed to the enhanced ability of Ti₃C₂T_x-PVA aerogels to recover from compression without permanent deformation. The strain recovery was calculated based on the changes in the aerogel thickness before and after compression, as illustrated in Fig. S10.†

The PVA compact film exhibited substantial elongation at break (110%), with a fracture strength of 31 MPa and Young's modulus of 1.6 GPa. In contrast, the Ti₃C₂T_x compact film demonstrated brittle behavior, showing only 0.6% elongation at break, a fracture strength of 29 MPa, and a notably high modulus of 9.2 GPa. Incorporating a small amount of Ti₃C₂T_x (14 mg cm⁻²) into the PVA matrix initially enhanced the mechanical properties of the Ti₃C₂T_x-PVA compact film, yielding a maximum fracture strength of 55 MPa almost twice that of the Ti₃C₂T_x film (Fig. 2e). This enhancement is attributed to strong hydrogen-bonding interactions between Ti₃C₂T_x and PVA, which promote efficient load transfer and increase the mechanical strength.⁵⁰ However, at a higher Ti₃C₂T_x areal density (20 mg cm⁻²), the ductility decreased due to disruption of the PVA matrix, which impaired efficient load transfer and resulted in brittle behavior. Consequently, the fracture stress and strain decreased to 32 MPa and 14%, respectively (Fig. 2e). Meanwhile, the Young's modulus consistently increased with the Ti₃C₂T_x content, reaching 2.1 GPa at 14 mg cm⁻² and 6.1 GPa at 20 mg cm⁻².

These results underscore the distinct mechanical properties of the Ti₃C₂T_x-PVA composites by structural form. The hydrogel demonstrated outstanding elongation (650%), the aerogel offered enhanced load-bearing capacity with a maximum compressive strength of 215 kPa, and the compact film achieved a high modulus (6.1 GPa) and good fracture strength (55 MPa). Overall, incorporating the PVA matrix significantly improved the mechanical performance of the Ti₃C₂T_x-PVA composites in each form compared with their pristine Ti₃C₂T_x MXene counterparts.

Electrical and ionic conductivities

The structure and composition of hydrogels, aerogels, and compact films critically influence their electrical and ionic conductivities. Generally, hydrogels and dispersions exhibit high ionic conductivity due to the presence of water and ions, whereas aerogels and compact films primarily rely on electrical conductivity. In this study, Ti₃C₂T_x-PVA hydrogels and Ti₃C₂T_x dispersions (with varying MXene areal densities ranging from 14 to 20 mg cm⁻²) were characterized by electrochemical impedance spectroscopy at ambient temperature (Fig. 3a–c and Fig. S11a, b†). The Nyquist plots for the Ti₃C₂T_x-PVA hydrogels (Fig. 3a and Fig. S11a†) show a slight increase in impedance as the Ti₃C₂T_x areal density increased. Conversely, in the Ti₃C₂T_x dispersions (Fig. S11b†), impedance decreased as the Ti₃C₂T_x areal density increased. As a result, the ionic conductivity of the Ti₃C₂T_x-PVA hydrogels decreased marginally with increasing MXene content, and the ionic conductivity of the Ti₃C₂T_x dispersion increased (Fig. 3b and c).

Fig. 3 (a) Nyquist impedance plots of Ti₃C₂T_x-PVA hydrogels at various Ti₃C₂T_x areal densities. (b) Electrical conductivity of Ti₃C₂T_x-PVA films and aerogels and the ionic conductivity of Ti₃C₂T_x-PVA hydrogels plotted as a function of Ti₃C₂T_x areal density. (c) Real permittivity (ε′) of Ti₃C₂T_x-PVA hydrogels at various Ti₃C₂T_x areal densities in the X-band (8.2–12.4 GHz). (d) Electrical conductivity of Ti₃C₂T_x films and aerogels and ionic conductivity of Ti₃C₂T_x dispersions at various Ti₃C₂T_x areal densities. Schematic illustration of (e) water, (f) Ti₃C₂T_x, (g) PVA hydrogel, and (h) Ti₃C₂T_x-PVA hydrogel dispersion under an alternating electric field of electromagnetic waves.

In the Ti₃C₂T_x dispersions, the reduction in impedance and increase in ionic conductivity (Fig. 3d) are attributed to the formation of a continuous electron-transport network by the highly conductive MXene sheets (Fig. 3e and f).⁵¹ In contrast, gelation of the PVA matrix plays a pivotal role in the Ti₃C₂T_x-PVA hydrogels. The slight increase in impedance with increasing Ti₃C₂T_x areal density in these hydrogels can be attributed to mobile sodium ions (Na⁺) from sodium borate, which can coagulate in Ti₃C₂T_x sheets (Fig. S12a and b†) and disrupt the conduction pathway.⁵² Additionally, Na⁺ ions can become trapped between Ti₃C₂T_x layers, further hindering electron transport (Fig. 3g and h).^52,53 Nonetheless, the Ti₃C₂T_x-PVA hydrogels maintained relatively high ionic conductivity, likely due to the presence of ionic species within the hydrogel that facilitate ion transport.

Dielectric properties also play a significant role in EMI shielding. The real part of the permittivity (ε′) of the Ti₃C₂T_x-PVA hydrogels measured in the X-band range (8.2–12.4 GHz) increases along with the Ti₃C₂T_x areal density (Fig. 3c). This increase is primarily attributed to enhanced interfacial polarization between the Ti₃C₂T_x sheets and PVA chains.⁵⁴ Additionally, polar water molecules can contribute to dielectric polarization under an electric field of electromagnetic waves.³¹ Dielectric polarization is frequency-dependent, so ε′ decreases with increasing frequency because the dipoles and ions have less time to align with the alternating field at high frequency.⁵⁵

In the Ti₃C₂T_x-PVA aerogel and compact film, the electrical conductivity increased as the Ti₃C₂T_x content increased (Fig. 3b). Although PVA is inherently insulating (∼1 × 10⁻¹³ S cm⁻¹), the high-crystallinity Ti₃C₂T_x MXene is highly conductive (∼1.0 × 10⁴ S cm⁻¹). Consequently, the aerogel reached a maximum conductivity of 0.002 S cm⁻¹ at the highest MXene areal density of 20 mg cm⁻², and the compact film attained a maximum conductivity of 0.032 S cm⁻¹. This conductivity improvement is attributed to the formation of a percolative network of conductive MXene sheets within the insulating PVA matrix, resulting in continuous conductive pathways.

The Ti₃C₂T_x-PVA compact film achieved significantly higher conductivity than the aerogel at the same MXene areal density, primarily due to its dense structure. Ti₃C₂T_x sheets stack tightly, creating an interconnected conductive network structure that follows power-law behavior in electrical conductivity. In contrast, the aerogel's porous structure introduces large air gaps that impede electron transport, limiting its overall electrical conductivity. A similar trend was observed in Ti₃C₂T_x compact film and aerogel, with compact film exhibiting higher electrical conductivity (10 000 S cm⁻¹) compared to aerogel (1.1 S cm⁻¹) as shown in Fig. 3d and Table S1.†

EMI shielding performance

The EMI shielding performance was examined at the X-band (8.2–12.4 GHz) using a waveguide measurement setup (Fig. 4a). A customized sample holder was designed to accommodate Ti₃C₂T_x-PVA hydrogels and Ti₃C₂T_x dispersions (details in the Experimental section). The total EMI shielding effectiveness (SE_T) and its contributions from reflection (SE_R) and absorption (SE_A) were determined using eqn (2)–(8) as shown in the Experimental section below. Fig. 4b–d and Fig. S13a–f† show the X-band range EMI SE_T, SE_R, and SE_A of Ti₃C₂T_x-PVA hydrogels, aerogels, and compact films across various Ti₃C₂T_x areal densities. To assess the influence of Ti₃C₂T_x areal density on these three configurations, Fig. 4e–g present the SE_T, SE_R, and SE_A values at a midrange frequency of 10.3 GHz. These values exhibit trends similar to those of the average EMI SE_T, SE_R, and SE_A (Fig. S14a–i†)_. As the Ti₃C₂T_x areal density increased, SE_T improved in all three configurations: hydrogel (red), aerogel (yellow), and film (blue) (Fig. 4e). Among them, the hydrogel achieved the highest EMI SE_T, followed by the film and then the aerogel. Specifically, the Ti₃C₂T_x-PVA hydrogel showed a gradual increase in SE_T from 41.3 dB to 66.5 dB as the Ti₃C₂T_x areal density increased from 14 mg cm⁻² to 20 mg cm⁻². In contrast, the Ti₃C₂T_x-PVA aerogel exhibited a modest increase from 1.5 dB to 2.6 dB up to 17 mg cm⁻² before surging to 19 dB at 20 mg cm⁻². Meanwhile, the Ti₃C₂T_x-PVA compact film demonstrated a more pronounced increase in SE_T once the areal density surpassed 14 mg cm⁻², increasing from 2.6 dB (14 mg cm⁻²) to 19.2 dB (16 mg cm⁻²) and ultimately reaching 37.8 dB at 20 mg cm⁻².

Fig. 4 (a) Schematic illustration of the customized experimental setup used to measure the EMI total shielding effectiveness (SE_T) of the hydrogels and dispersions, using a holder with a transparent polyimide film support. The EMI SE_T of the (b) Ti₃C₂T_x-PVA hydrogels, (c) Ti₃C₂T_x-PVA aerogels, and (d) Ti₃C₂T_x-PVA compact films as a function of Ti₃C₂T_x areal density. (e) The EMI SE_T, (f) SE_R, and (g) SE_A values measured at 10.3 GHz as a function of Ti₃C₂T_x areal density. (h) The EMI SE_T, (i) SE_R, and (j) SE_A values at a fixed Ti₃C₂T_x areal density of 20 mg cm⁻¹ for Ti₃C₂T_x-PVA hydrogels, aerogels, and films. Notably, the hydrogel and aerogel have the same thickness of 5 mm, whereas the compact film has a thickness of approximately 150 μm, as listed in Table S1.†

The total EMI (SE_T) comprises both the reflection contribution (SE_R) and the absorption contribution (SE_A), as shown in Fig. 4f and Fig. 4g, respectively. As the Ti₃C₂T_x areal density increased, the Ti₃C₂T_x-PVA hydrogel showed a moderate increase in SE_R from 4.95 dB (14 mg cm⁻²) to 6.95 dB (20 mg cm⁻²). The Ti₃C₂T_x-PVA aerogel displayed a minimal increase from 0.15 dB (14 mg cm⁻²) to 2.12 dB (20 mg cm⁻²). In contrast, the Ti₃C₂T_x-PVA compact film exhibited a sharp increase in SE_R, from 0.89 dB (14 mg cm⁻²) to 8.23 dB (17 mg cm⁻²), ultimately reaching 20 dB (20 mg cm⁻²). Fig. 4g shows that SE_A is the dominant contributor to SE_T, following a trend similar to that of SE_T: the hydrogel achieved the highest EMI SE_A, followed by the film and then the aerogel. Specifically, the Ti₃C₂T_x-PVA hydrogel showed a gradual increase in SE_A from 36.2 dB to 59.52 (14–20 mg cm⁻²); the aerogel SE_A remained low (0.48–1.39 dB) until 17 mg cm⁻² and then jumped to 17.5 dB; the film SE_A steadily increased from 1.6 dB to 26.5 dB.

At a fixed areal density of 20 mg cm⁻² (Fig. 4h–j), the Ti₃C₂T_x-PVA hydrogel exhibited the highest EMI SE_T, followed by the compact film and then the aerogel. SE_A followed the same trend as SE_T, with the hydrogel showing the highest value (Fig. 4j). Meanwhile, the SE_R values differed only slightly among the samples, with the compact film exhibiting the highest result (Fig. 4i).

The superior EMI shielding of the Ti₃C₂T_x-PVA hydrogel arises from several factors. First, the dipolar nature of water molecules promotes polarization loss. Because a water molecule has an electronegative oxygen atom (partial negative charge) and two hydrogen atoms (partial positive charge), it aligns and rotates in response to an external electric field, leading to strong attenuation of EM wave energy.³¹ Second, the uniform distribution of Ti₃C₂T_x sheets in the insulating PVA matrix facilitates the formation of microcapacitors in the hydrogel, where MXene sheets act as electrodes, inducing significant capacitive loss under the AC electric field of EM waves.⁵⁶ Third, mobile ions (e.g., Na⁺ and Br⁻) generate an internally induced electric field that leads to ionic conductivity losses and can partially depolarize water molecules, further enhancing microwave absorption.^57–59 As a result, the EMI shielding performance of the Ti₃C₂T_x-PVA hydrogel improved as the Ti₃C₂T_x areal density increased and surpassed that of the Ti₃C₂T_x dispersions at the same Ti₃C₂T_x areal density (Fig. S15a–c†). For instance, at 20 mg cm⁻², the Ti₃C₂T_x dispersion achieved an SE_T of 48.6 dB (SE_A 40.9 dB), whereas the Ti₃C₂T_x-PVA hydrogel attained an SE_T of 66.5 dB (SE_A 59.5 dB), even though the dispersion exhibited a slightly higher SE_R (7.7 dB). Without PVA, the Ti₃C₂T_x dispersion lacks the microcapacitor effect, resulting in a lower absorption coefficient (A = 0.18) and a higher reflection coefficient (R = 0.82) than those of the Ti₃C₂T_x-PVA hydrogel (A = 0.24, R = 0.76) (Fig. 5a and b).

Fig. 5 Absorption coefficient (A) and reflection coefficient (R) of the (a) Ti₃C₂T_x-PVA-based hydrogel, aerogel, and film and (b) Ti₃C₂T_x-based dispersion, aerogel, and film at a fixed Ti₃C₂T_x areal density (20 mg cm⁻²).

The EMI shielding performance of the Ti₃C₂T_x-PVA aerogel is primarily based on a conductive Ti₃C₂T_x network embedded within a porous PVA matrix. Although the incorporation of PVA reduced the electrical conductivity from 0.85–1.0 S cm⁻¹ in the pristine Ti₃C₂T_x aerogel to 2.63 × 10⁻⁸–1.98 × 10 S cm⁻¹ in the Ti₃C₂T_x-PVA aerogel, it aided in impedance matching with free space, increasing EM wave penetration and absorption. Additionally, the conductive Ti₃C₂T_x network supplied abundant free electrons for reflection, and the porous structure fostered multiple internal reflections by creating numerous interfaces. As a result, although the Ti₃C₂T_x-PVA aerogel exhibited a slightly lower SE_T than the pristine Ti₃C₂T_x aerogel at the same Ti₃C₂T_x areal density (Fig. S15d–f†), it was more absorption-dominated (A ∼0.73, R ∼0.26 in Fig. 5a) than the pristine Ti₃C₂T_x aerogel (A ∼0.22, R ∼0.78 in Fig. 5b) at the same areal density of 20 mg cm⁻².

In contrast, the Ti₃C₂T_x-PVA compact film primarily benefited from the dense conductive Ti₃C₂T_x network within the PVA matrix, offering substantial attenuation of incident EM waves via reflection. The high impedance mismatch with free space limited the penetration of EM waves, and any wave that did penetrate was subsequently absorbed within the layered Ti₃C₂T_x-PVA structure, with reflection via the dominant EMI shielding mechanism, as demonstrated by the high reflection coefficient (R = 0.89) and low absorption coefficient (A = 0.11) at 20 mg cm⁻² (Fig. 5a). As with the Ti₃C₂T_x-PVA aerogel, the presence of insulating PVA reduced the conductivity of the Ti₃C₂T_x-PVA compact film compared with the pristine Ti₃C₂T_x film. Because of its high conductivity, the pristine Ti₃C₂T_x film showed superior EMI shielding in the X-band (SE_T = 70 dB, SE_R = 17.5 dB, SE_A = 52.5 dB) at 20 mg cm⁻² (Fig. S15g–i†), along with the highest reflection coefficient (R = 0.99) and lowest absorption coefficient (A = 0.01) among all configurations (Fig. 5a and b).

Fig. 6a–c illustrate the dominant shielding mechanisms for Ti₃C₂T_x-PVA hydrogels, aerogels, and films, respectively, with Ti₃C₂T_x areal densities of 14–20 mg cm⁻². In the hydrogel (Fig. 6a), most incident EM waves are reflected, and those that penetrate are strongly attenuated by the robust dielectric polarization of water molecules and ions, internally induced electric fields, and efficient charge conduction through the Ti₃C₂T_x-PVA hydrogel, yielding the highest shielding performance. In contrast, the porous structure of the Ti₃C₂T_x-PVA aerogel (Fig. 6b) and its lower electrical conductivity reduced reflection but enhanced internal scattering, leading to greater absorption with minimal reflection and transmission. Meanwhile, the Ti₃C₂T_x-PVA compact film (Fig. 6c) exhibits reflection-dominated shielding due to its dense structure and high electrical conductivity, yielding the highest absolute specific shielding (Fig. S16†).

Fig. 6 Schematic illustrations of the EMI shielding mechanism in (a) Ti₃C₂T_x-PVA hydrogels, (b) Ti₃C₂T_x-PVA aerogels, and (c) Ti₃C₂T_x-PVA compact films. The diagrams highlight the key factors contributing to EMI shielding performance reflection, absorption, and internal multiple reflections and their respective roles in attenuating electromagnetic waves.

Conclusion

In this study, Ti₃C₂T_x-PVA composites were fabricated in three structural forms hydrogels, aerogels, and compact films at various Ti₃C₂T_x areal densities (14–20 mg cm⁻²) to investigate the effects of structural configuration on EMI shielding and mechanical properties. Each configuration demonstrated a distinct attenuation mechanism: hydrogels leveraged dipolar polarization, aerogels used multiple reflections and moderate conductivity, and compact films relied on dense stacking and reflection-dominated shielding. Notably, the Ti₃C₂T_x–PVA hydrogel achieved the highest shielding effectiveness (70 dB at 10 GHz) at a fixed Ti₃C₂T_x areal density of 20 mg cm⁻², attributed to strong dipolar polarizations and ionic charge conduction. In contrast, the aerogel exhibited robust absorption-dominating shielding with a high absorption coefficient (A ∼0.73) due to its porous architecture and enhanced multiple internal reflections. Moreover, the incorporation of PVA into Ti₃C₂T_x MXenes significantly enhanced the mechanical strength across all configurations, surpassing the performance of their pristine Ti₃C₂T_x MXene counterparts in all configurations. This work underscores the importance of the interplay among structural design, Ti₃C₂T_x areal density, and PVA incorporation to optimize EMI shielding and mechanical performance. These findings offer insights valuable for strategically designing light-weight MXene–polymer composites in the form of hydrogels, aerogels, or compact films to enhance both EMI shielding and mechanical performance in practical applications.

Experimental section

Materials

Powders of graphite (99.8%; avg particle size, 45 μm), lithium fluoride (LiF, 98.5%), Ti (99.8%; avg particle size, 45 μm), and Al (99.8%; avg particle size, 45 μm) were purchased from Alfa Aesar. TiC powders, synthesized from a TiO₂ precursor via carbothermal reduction, were used to synthesize the high-crystallinity Ti₃AlC₂ MAX phase. Hydrochloric acid (HCl, 37% H₂O) and hydrofluoric acid (HF, ACS reagent grade 48 wt%) were acquired from Sigma Aldrich. Polypropylene transparent tape was purchased from 3M (ID:70005276194). Poly(vinyl alcohol) and sodium tetraborate hexahydrate (pure) were purchased from Sigma-Aldrich. All chemicals were used as received.

Synthesis of Ti₃AlC₂ MAX

The high-crystallinity Ti₃AlC₂ MAX phase was synthesized following a previously reported procedure.^35,60,61 In brief, TiC powders were mixed with Ti and an excess of Al in a molar ratio of 2 : 1.2 : 2.2, followed by ball-milling at 100 rpm for 12 h. The resulting mixture was pressed into a 50 mm diameter disk under 4000 psi and heat-treated at 1450 °C for 3 h under an argon atmosphere.⁶² The resulting Ti₃AlC₂ MAX disk was pulverized and treated with 9 M HCl for 12 h to remove excess aluminum and impurities and then thoroughly rinsed with deionized water. After vacuum drying at 60 °C, the product was sieved through a 400-mesh sieve to obtain a fine, high-crystallinity Ti₃AlC₂ MAX powder suitable for subsequent Ti₃C₂T_x MXene synthesis.

Synthesis of the Ti₃C₂T_x MXene

The Ti₃C₂T_x MXene was synthesized via the MILD method to selectively etch Al atoms from the parent Ti₃AlC₂ MAX phase.³ Briefly, LiF (1.6 g) was dissolved in a mixture of 20 mL of 9 M HCl and 4 mL of 48 wt% HF in a polypropylene container. The Ti₃AlC₂ MAX powder (1 g) was gradually added to the LiF/HCl solution under continuous stirring with a magnetic stirrer at 320 rpm. The etching process was carried out for 24 h at 35 °C under continuous stirring. The resulting product was washed multiple times with deionized water by centrifuging at 5000 rpm for 5 min until the supernatant reached pH 6. The delaminated Ti₃C₂T_x flakes obtained were subsequently used to prepare a 10 mg mL⁻¹ Ti₃C₂T_x dispersion.

Fabrication of Ti₃C₂T_x PVA hydrogels, aerogels, and films

To prepare the Ti₃C₂T_x-PVA hydrogels, PVA solution was prepared by first dissolving 3 g of PVA in 60 mL of deionized water at 90 °C with stirring (100 rpm) for 1 hour. Predetermined amounts of Ti₃C₂T_x, corresponding to different Ti₃C₂T_x areal densities (14, 16, 17, and 20 mg cm⁻²), were added to 3 mL of the cooled PVA solution to form a homogeneous mixture. Subsequently, 2 mL of a 0.026 M sodium tetraborate decahydrate solution was added dropwise, and the mixture was gently stirred until gelation occurred. Borate ions interact with the hydroxyl groups of PVA via hydrogen bonding and di-diol borate complexation, forming an elastic gel network. The resulting 5 mm thick Ti₃C₂T_x-PVA hydrogels with various Ti₃C₂T_x areal densities were used for further experiments. The Ti₃C₂T_x-PVA aerogels were obtained by freezing the Ti₃C₂T_x-PVA solution (with the predetermined Ti₃C₂T_x areal density) using directional freezing at −110 °C and 0.1 Pa for 48 hours. The solution was poured into a Teflon mold (23 mm × 11 mm × 45 mm) equipped with a copper plate. The Teflon mold provided thermal insulation, and the copper plate provided a temperature gradient. Similarly, the compact Ti₃C₂T_x-PVA films were fabricated by vacuum-filtering Ti₃C₂T_x-PVA solutions (with the predetermined Ti₃C₂T_x areal density) under ambient conditions. As control samples, PVA hydrogels, aerogels, and compact films were prepared following the same procedures, but without Ti₃C₂T_x.

Fabrication of Ti₃C₂T_x dispersions, aerogels, and films

Ti₃C₂T_x dispersions with various MXene areal densities (14–20 mg cm⁻²) were prepared from a 10 mg mL⁻¹ stock solution by diluting with deionized water. The Ti₃C₂T_x hydrogels, aerogels, and compact films were prepared from Ti₃C₂T_x dispersions at predetermined MXene areal densities following the same procedures used for the Ti₃C₂T_x-PVA hydrogels, aerogels, and films but without adding PVA.

Material characterization

The surface and cross-sectional morphologies of the MXene flakes and nanocomposites were examined using a field-emission SEM (JSM-7600F, Japan) equipped with an EDS system. The crystal structure was investigated using XRD (D8, Bruker, USA) with Cu Kα radiation. The XRD scans spanned a 2θ range of 4°–80° at a rate of 2° min⁻¹ and a window slit dimension of 10 × 10 mm². High-resolution TEM (JEM-2100F, Japan) at an acceleration voltage of 200 kV was used for the atomic-scale structural analysis of individual Ti₃C₂T_x flakes. Electrical conductivity measurements were performed using a four-point probe (MCP-TP06P PSP) connected to a Loresta-GP meter (Model MCP-T610, Mitsubishi Chemical, Japan).

Measurements

Tensile and compression testing. Uniaxial tensile mechanical tests were performed under ambient conditions using a universal testing machine (Shimadzu AGS-X) equipped with a 2 KN load cell and a crosshead speed of 1 mm min⁻¹. The sample dimensions were 30 mm × 10 mm × ∼200 μm (length × width × thickness). A paper support was used in the Ti₃C₂T_x films, and a tape support was used for hydrogels. Compression tests were carried out with a 2K N load cell at a displacement rate of 1.5 mm min⁻¹. The strain recovery was calculated using the changes in thickness of the aerogel before and after compressive stress.

Ionic conductivity. The ionic conductivities (σ_i) of the hydrogels and dispersions in the relaxed state were determined by AC impedance spectroscopy using an electrochemical workstation system (CHI660E) with a three-electrode setup. The voltage amplitude was set to 10 mV, and the frequency was swept from 100 kHz to 0.01 Hz. Each hydrogel sample was placed between two stainless-steel plate electrodes. The ionic resistance was obtained by extrapolating the impedance curve to the x-axis, and ionic conductivity was calculated according to eqn (1).⁶³where σ_i is the ionic conductivity, L is the distance between two electrodes, A is the electrode surface area, and R is the resistance.

Permittivity and EMI shielding. EMI-shielding measurements were performed over the X-band (8.2–12.4 GHz) using a two-port vector network analyzer (VNA, N5222B, Keysight, USA) coupled with a rectangular waveguide. A custom sample holder, with inner dimensions of 23 mm × 11 mm × 10 mm, was designed for both the dispersions and hydrogels. After the hydrogel sample was placed into the holder chassis, a transparent polyimide support sealed the holder to prevent any hydrogel leakage.

The permittivity of the dispersions and hydrogels was calculated in a similar manner, with calibration performed based on air, short-circuit, and deionized water measurements at room temperature prior to the actual measurements. The complex relative permittivity (ε_r) is given by eqn (2).

ε_r = ε′ − jε′′ (2)

where ε′ (the real part) represents the dielectric constant and ε′′ (the imaginary part) represents a loss factor.

The scattering parameters (S_mn), including S₁₁, S₂₂, S₂₁, and S₁₂, were obtained using the two-port VNA. The subscripts m and n represent the transmitting and receiving ports, respectively. R and T can be calculated using the respective eqn (4) and (5).

The total shielding effectiveness (SE_T) of a material at a given single impedance was calculated as the combined effects of reflection (SE_R) and absorption (SE_A).

SE_T = SE_R + SE_A (9)

Author contributions

S.M.N. and C.M.K. conceived the idea. S.M.N. designed and conducted the experiments. S.M.N., T.H., A.I., S.Z., S.C., N.H., X.K., Z.K., Z.H., and C.M.K. discussed the results and analyzed the data. S.M.N. and C.M.K. authored the manuscript with input from all the authors. C.M.K. supervised the study.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This study was supported by grants from the Basic Science Research Program (2021M3H4A1A03047327 and 2022R1A2C3006227) through the National Research Foundation of Korea, funded by the Ministry of Science, ICT, and Future Planning, Republic of Korea; and the National Research Council of Science & Technology (NST), funded by the Korean Government (MSIT) (CRC22031-000). This research was partially supported by the Ministry of Trade, Industry, and Energy (MOTIE) and the Korean Institute for Advancement of Technology (KIAT) through the International Cooperative R&D program (P0028332). This study was partially supported by a grant from Hyundai Mobis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nr00450k

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